242 10 13219 https://digitalcollections.lakeheadu.ca/files/original/0db73ce293c8674f66874392c1ce55d2.pdf 288aabd621a6c1c067a07e901099871b PDF Text Text 56TH ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY INTERNATIONAL FALLS, MINNESOTA MAY 19-22, 2010 PROCEEDINGS VOLUME 56 PART 1 – PROGRAMS AND ABSTRACTS ��INSTITUTE ON LAKE SUPERIOR GEOLOGY 56TH ANNUAL MEETING MAY 19-22, 2010 INTERNATIONAL FALLS, MINNESOTA PETER HOLLINGS, PETER HINZ, MARK SMYK, MARK JIRSA, AND TERRY BOERBOOM Co-Chairs Proceedings Volume 56 Part 1 – Program and Abstracts Edited by Terrence J. Boerboom, Minnesota Geological Survey Cover Photos: Conglomerate in Seine Group (Field Trip 6) �56TH INSTITUTE ON LAKE SUPERIOR GEOLOGY CONTENTS OF PROCEEDINGS VOLUME 56: PART 1: PROGRAM AND ABSTRACTS PART 2: FIELD TRIP GUIDEBOOK TRIP 1: MINERAL DEPOSITS OF THE RAINY RIVER AREA (CAN) TRIP 2: GEOLOGY OF AN ARCHEAN SUCCESSION AT ATIKOKAN (CAN) TRIP 3: STRUCTURAL GEOLOGY ALONG THE QUETICO FAULT (CAN) TRIP 4: ARCHEAN GEOLOGY OF VOYAGEURS NATIONAL PARK AND LITTLE AMERICA GOLD MINE (US) TRIP 5: ASH RIVER NEUTRINO DETECTOR LABORATORY AND THE ARCHEAN VERMILION GRANITIC COMPLEX (US) TRIP 6: TRANSECT THROUGH THE QUETICO-WABIGOON SUBPROVINCE BOUNDARY (US) TRIP 7: MINERAL DEPOSITS OF THE MINE CENTRE – RAINY LAKE AREA (CAN) TRIP 8: GEOLOGY AND ENVIRONMENTAL ISSUES OF THE STEEP ROCK MINE (CAN) Reference to material in Part 1 should follow the example below: Boerboom, T. J., 2010, Latest bedrock geologic map of Carlton County, east-central Minnesota and revisions to the southern edge of the Animikie basin [abstract]: Institute on Lake Superior Geology Proceedings, 56th Annual Meeting, International Falls, MN, v. 56, part 1, p. 1-2. Published by the 56th Institute on Lake Superior Geology and distributed by the ILSG Secretary: Peter Hollings Department of Geology Lakehead University Thunder Bay, ON P7B 5E1 CANADA peter.hollings@lakeheadu.ca ILSG website: http://www.lakesuperiorgeology.org ISSN 1042-9964 ii �TABLE OF CONTENTS PROCEEDINGS VOLUME 56 PART 1— PROGRAM AND ABSTRACTS Previous Institutes on Lake Superior Geology, 1955-2010 ...................................................... iv Sam Goldich and the Goldich Medal .............................................................................. vi Past Goldich Medalists and the 2010 Goldich Medal Recipient .................................... viii Goldich Medal Committee ........................................................................................... viii Citation for 2010 Goldich Medal Recipient .................................................................... ix ILSG Student Research Fund ........................................................................................ xii Student Paper Awards .................................................................................................. xiii Eisenbrey Student Travel Awards ................................................................................. xv Report of the Chairs of the 55th Annual Meeting .......................................................... xvi 2010 Board of Directors ................................................................................................................. xix 2010 Session Chairs ..................................................................................................... xix 2010 Student Paper Awards Committee ....................................................................... xix 2010 Meeting Sponsors ................................................................................................. xx 2010 Banquet Speaker .................................................................................................. xxi Program ..................................................................................................................... xxiii List of Poster Presentation Abstracts .......................................................................... xxx Summary of Oral Presentation Schedule ................................................................... xxxii Abstracts .................................................................................................................. xxxiii Location map for Frances Memorial Sports Center (starting point for several field trips ............................................................... 75 Location map for Backus Community Center (banquet location) ................................... 76 iii �PREVIOUS INSTITUTES ON LAKE SUPERIOR GEOLOGY, 1955-2009 ILSG YEAR PLACE CHAIRS 1 1955 Minneapolis, Minnesota C.E. Dutton 2 1956 Houghton, Michigan A.K. Snelgrove 3 1957 East Lansing, Michigan B.T. Sandefur 4 1958 Duluth, Minnesota R.W. Marsden 5 1959 Minneapolis, Minnesota G.M. Schwartz and C. Craddock 6 1960 Madison, Wisconsin E.N. Cameron 7 1961 Port Arthur, Ontario E.G. Pye 8 1962 Houghton, Michigan A.K. Snelgrove 9 1963 Duluth, Minnesota H. Lepp 10 1964 Ishpeming, Michigan A.T. Broderick 11 1965 St. Paul, Minnesota P.K. Sims and R.K. Hogberg 12 1966 Sault Ste. Marie, Michigan R.W. White 13 1967 East Lansing, Michigan W.J. Hinze 14 1968 Superior, Wisconsin A.B. Dickas 15 1969 Oshkosh, Wisconsin G.L. LaBerge 16 1970 Thunder Bay, Ontario M.W. Bartley and E. Mercy 17 1971 Duluth, Minnesota D.M. Davidson 18 1972 Houghton, Michigan J. Kalliokoski 19 1973 Madison, Wisconsin M.E. Ostrom 20 1974 Sault Ste. Marie, Ontario P.E. Giblin 21 1975 Marquette, Michigan J.D. Hughes 22 1976 St. Paul, Minnesota M. Walton 23 1977 Thunder Bay, Ontario M.M. Kehlenbeck 24 1978 Milwaukee, Wisconsin G. Mursky 25 1979 Duluth, Minnesota D.M. Davidson 26 1980 Eau Claire, Wisconsin P.E. Myers 27 1981 East Lansing, Michigan W.C. Cambray iv �28 1982 International Falls, Minnesota D.L. Southwick 29 1983 Houghton, Michigan T.J. Bornhorst 30 1984 Wausau, Wisconsin G.L. La Berge 31 1985 Kenora, Ontario C.E. Blackburn 32 1986 Wisconsin Rapids, Wisconsin J.K. Greenberg 33 1987 Wawa, Ontario E.D. Frey and R.P. Sage 34 1988 Marquette, Michigan J. S. Klasner 35 1989 Duluth, Minnesota J.C. Green 36 1990 Thunder Bay, Ontario M.M. Kehlenbeck 37 1991 Eau Claire, Wisconsin P.E. Myers 38 1992 Hurley, Wisconsin A.B. Dickas 39 1993 Eveleth, Minnesota D.L. Southwick 40 1994 Houghton, Michigan T.J. Bornhorst 41 1995 Marathon, Ontario M.C. Smyk 42 1996 Cable, Wisconsin L.G. Woodruff 43 1997 Sudbury, Ontario R.P. Sage and W. Meyer 44 1998 Minneapolis, Minnesota J.D. Miller, Jr. and M.A. Jirsa 45 1999 Marquette, Michigan T.J. Bornhorst and R.S. Regis 46 2000 Thunder Bay, Ontario S.A. Kissin and P. Fralick 47 2001 Madison, Wisconsin M.G. Mudrey, Jr. and B.A. Brown 48 2002 Kenora, Ontario P. Hinz and R.C. Beard 49 2003 Iron Mountain, Michigan L.G. Woodruff and W.F. Cannon 50 2004 Duluth, Minnesota S.A. Hauck and M. Severson 51 2005 Nipigon, Ontario P. Hollings and M.C. Smyk 52 2006 Sault Ste. Marie, Ontario R.P. Sage and A.C. Wilson 53 2007 Lutsen, Minnesota L.G. Woodruff and J.D. Miller, Jr. 54 2008 Marquette, Michigan T.J. Bornhorst and J.S. Klasner 55 2009 Ely, Minnesota J.D. Miller, Jr., G.J. Hudak, D.M. Peterson 56 2010 International Falls, Minnesota P. Hollings, P. Hinz, M. Smyk, M. Jirsa, and T. Boerboom v �SAM GOLDICH AND THE GOLDICH MEDAL Sam Goldich received an A.B. from the University of Minnesota in 1929, a M.A. from Syracuse University in 1930, and a Ph.D. from the University of Minnesota in 1936. During World War II Sam worked for the U.S. Geological Survey in mineral exploration. In 1948, Sam returned to the University of Minnesota, and became Professor and Director of the Rock Analysis Laboratory the following year. He rejoined the U.S. Geological Survey in 1959 and was appointed as the first Branch Chief of the Branch of Isotope Geology. Sam returned to academia in 1964 when he went to Pennsylvania State University. He left PSU in 1965 and moved to the State University of New York at Stony Brook, where he stayed for 3 years. Restless yet again, he moved to Northern Illinois University in 1968 where he was a professor until his retirement in 1977. Sam‘s final move was to Denver where he became an emeritus at the Colorado School of Mines. Sam died in 2000, less than a month before his 92 nd birthday. In the late 1970‘s, Geological Society of America Special Paper 182, which included seminal geochronological studies by Sam Goldich and coworkers on the Archean rocks of the Minnesota River Valley, was nearing completion. At this time various ILSG regulars began discussing the possibility of recognizing Sam for his pioneering work on the resolution of age relationships and thus the geology of Precambrian rocks in the Lake Superior region. Three members, R.W. Ojakangas, J.O. Kalliokoski and G.B. Morey, presented the idea to the ILSG Board of Directors in 1978. The Board approved the creation of an award, provided funding could be obtained. It was suggested that collecting one or two dollars at registration for a dedicated account would provide resources for striking the medal. A general request was made to the ILSG membership for donations and Sam himself offered a challenge grant to match the contributions. In total $4,000 was collected and thus began the work of creating the Goldich Medal. The initial Goldich Award was presented to Sam by G.B. Morey in 1979 and consisted of a large paper proclamation. For the actual medal, G.B. Morey consulted with the foundry on production details, while Dick Ojakangas and Jorma Kalliokoski worked on the design of the award, suggesting that it be given for ―outstanding contributions to the geology of the Lake Superior region.‖ Simultaneously, a committee of J.O. Kalliokosi, W.F. Cannon, M.M Kehlenbeck, G.B. Morey, and G. Mursky developed the Award Guidelines that were approved by the ILSG Board. By 1981 all the elements of the Goldich Award had come together, and the second recipient, Carl E. Dutton, Jr., received the Goldich Medal for 50 years of significant contributions to the understanding of the geology of the Lake Superior region. Since the beginning, the Awards Committee has consisted of individuals representing industry, government and academia, with each member of the Committee serving for three years. The medal is now awarded every year at the annual ILSG meeting. Reference: Morey, G.B. and Hanson, G.N. (editors). 1980. Selected studies of Archean gneisses and Lower Proterozoic rocks, southern Canadian Shield. Geological Society of America, Special Paper 182, 175 p. Prepared by various Goldich Medal Awardees, 2007 vi �INSTITUTE ON LAKE SUPERIOR GEOLOGY GOLDICH MEDAL vii �PAST GOLDICH MEDALISTS 1979 Samuel S. Goldich 1995 Gene La Berge 1980 not awarded 1996 David L. Southwick 1981 Carl E. Dutton, Jr. 1997 1982 Ralph W. Marsden 1998 Zell Peterman 1983 Burton Boyum 1999 Tsu-Ming Han 1984 Richard W. Ojakangas 2000 John C. Green 1985 Paul K. Sims 2001 John S. Klasner 1986 G.B. Morey 2002 Ernest K. Lehmann 1987 Henry H. Halls 2003 Klaus J. Schulz 1988 Walter S. White 2004 Paul Weiblen 1989 Jorma Kalliokoski 2005 Mark Smyk 1990 Kenneth C. Card 2006 Michael G. Mudrey 1991 William Hinze 2007 Joseph Mancuso 1992 William F. Cannon 2008 Theodore J. Bornhorst 1993 Donald W. Davis 1994 Cedric Iverson 2009 L. Gordon Medaris, Jr. Ronald P. Sage 2010 JOINT GOLDICH MEDAL RECIPIENTS William D. Addison and Gregory R. Brumpton Thunder Bay, Ontario GOLDICH MEDAL COMMITTEE Serving for the meeting years shown in parentheses Terry Boerboom (2007-2010) Government representative Allan MacTavish (2008-2011) Industry representative Mary Louise-Hill (2009-2012) Academic representative viii �CITATION FOR GOLDICH MEDAL RECIPIENTS William D. Addison and Gregory R. Brumpton 2010 Goldich Medal Recipients Bill Addison and Greg Brumpton are the first joint recipients of the Goldich Medal of the Institute on Lake Superior Geology. It is my privilege and pleasure to make this presentation in recognition of their remarkable contributions to the geology of the Lake Superior region. Bill and Greg worked as an inseparable team for more than a decade in the search for, and eventual discovery of the Sudbury impact layer, a bed containing material ejected from the crater near Sudbury, Ontario by the giant impact event at 1850 Ma. Their discovery and resulting publications constitute a fundamental contribution to the geology of the region and have ignited a renewed interest by many colleagues in studies of the classic geology of the iron ranges of the region where extensions of their original discoveries have since been documented. Greg Brumpton (left) and Bill Addison in their natural habitat near Thunder Bay. Before further discussing their scientific contributions I would like to present some biographical information on each of our medalists. Unlike so many of our career-long members, Bill and Greg appeared on our scene with a bang within the past decade, as mature, fully-formed researchers with something truly important to tell us. In assembling this biographical sketch it became apparent that virtually no one in the geologic community knew much of their pre-impact existence and they were truly international men of mystery to all of us. Even Google failed to provide enlightenment. ―William D. Addison‖, for instance, yields 18,800 hits suggesting that Bill clearly has multiple personae. So I was forced to go directly to the source and ask Bill and Greg themselves to give some pertinent information. Here are their abridged autobiographies. Bill Addison was born at an undisclosed location on April 27, 1939. His lifelong fascination and love of nature was instilled at an early age by his parents through many outdoor activities and their encouragement to not only enjoy nature but understand what he was experiencing in the outdoors. Bill‘s formal education was at the University of Toronto where he received a B.Sc. in forestry in 1963 and M.Sc. in fish physiology in 1966. Bill married his wife, Wendy, in 1966 and promptly treated her to a long honeymoon wilderness adventure in a remote corner of the Northwest Territories. Bill‘s early career was with the Ontario Department of Lands and Forests as a walleye research biologist. In 1970 he began his 28 year career as a science teacher as Westgate High School in Thunder Bay. ix �Greg Brumpton was born in Windsor, Ontario on July 16, 1941. His family‘s business in horticulture lead to an interest in agriculture and biology and Greg received his undergraduate degree in agriculture from the Ontario Agricultural College in Guelph, Ontario. He then studied at the University of Wisconsin-Madison, where he received his M.S. degree in entomology in 1966. Greg became a high school teacher in Windsor, Ontario in 1966 and married his wife, Carol Ann, in 1968. In 1970 Greg and Carol Ann moved to Thunder Bay and Greg continued his science teaching career at Westgate High School, arriving coincident with Bill Addison, thus beginning their lifelong collegial partnership. Greg, like Bill, retired in 1998 and the two soon took up their hobby of revolutionizing our understanding of the Paleoproterozoic history of the Lake Superior region. The discovery by Bill and Greg of the ejecta-bearing bed produced by the Sudbury impact, documented in their Geology paper in 2005, was a revelation to many of us with long experience in the geology of the Lake Superior region, as well as to the broader community of impact geologists. But it was a quiet labor of dedication and perseverance by our two medalists that spanned more than a decade of frustrations and false leads before their ultimate success and their sudden appearance in our midst with their remarkable discovery. Greg became interested in the topic of impact-induced mass extinctions in the 1980‘s and by 1991 had recognized that, in the Thunder Bay area, the highly fossiliferous Gunflint Formation was overlain by the nonfossiliferous Rove Formation and that the Gunflint/Rove boundary might be the same age as the Sudbury impact. He proposed therefore to examine that boundary for signs of ejecta and a possible mass extinction event. The hunt proved more difficult than it might have seemed because of sparse exposures and few drill holes to provide samples from the critical horizon. But perseverance ultimately paid off with discoveries of ejecta both in outcrop and drill cores. For many of us in the ILSG community who have followed Bill and Greg‘s discovery and subsequent investigations for the past five years, its importance to the geology of our favorite region is clear. To a great extent ―You heard it here first‖ because they have honored the Institute by presenting their findings through a series of papers at ISLG meetings. But, we also need to appreciate how their work has been received by the broader community of impact researchers. Bevan French, one of the most eminent researchers in the field of impact geology for many decades, and a pioneer in documenting the Sudbury impact itself, wrote the following in support of Bill and Greg‘s nomination. ―I consider Addison and Brumpton's study to be one of the most exciting discoveries in geology in recent years. It has provided one of those rare and unexpected major insights which permanently change our existing picture of geology, and, at the same time, indicate new and unexpected directions of research.” You may have noticed that I have not mentioned the word ―geology‖ anywhere in Bill‘s and Greg‘s professional training or experience. This is not an oversight. These two remarkable gentlemen are entirely self taught in our field of research. Their curiosity and intellectual enthusiasm have brought them to a level of geologic knowledge such that even close colleagues might not realize this lack of formal training were it not for their modesty in frequently pointing out this ―flaw‖ in their backgrounds. I believe that never in the history of ILSG have two self-taught individuals had such a profound impact on understanding the geologic history of the Lake Superior region or stimulated as much new research as our two medalists have done. x �It was my honor to have nominated Bill Addison and Greg Brumpton for the 2010 Goldich Medal. But I also would like to acknowledge the strong support of that nomination provided by Klaus Schulz and Laurel Woodruff of the USGS, Phil Fralick of Lakehead University, John Klasner of Marquette, Michigan, Jorma Kalliokoski of Houghton, Michigan, David Kring of the Lunar and Planetary Institute in Houston, Texas, and Bevan French, of NASA (retired) and scientist emeritus at the Smithsonian Institution Museum of Natural History in Washington, DC. On behalf of these supporters, and with great personal pleasure, I congratulate William D. Addison and Gregory R. Brumpton as recipients of the Goldich Medal of the Institute on Lake Superior Geology. William F. Cannon May, 2010 xi �ILSG STUDENT RESEARCH FUND The 2005 Board of Directors established the ILSG Student Research Fund with $10,000 US from the Institute‘s general fund to encourage student research on the geology of the Lake Superior region. A minimum of two awards of $500 US each for research expenses (but not travel expenses) will be made each year. Students are expected to present their research orally or during a poster session at an ILSG meeting. The award winners will also be automatically eligible for the Eisenbrey Travel Awards. To allow the fund to grow, the Fund will receive onehalf of any additional proceeds from each annual meeting, after all other commitments and expenses are covered. The ILSG Board of Directors will be responsible for selecting a minimum of two awards each year. The ILSG Treasurer will issue the awards. The ILSG Student Research Fund is available for undergraduate or graduate students working on geology in the Lake Superior region. The applications are due to the ILSG Secretary by August 31 st of each year Awards will be made by October 1st of each year. Names of the award recipients will be announced at the next annual meeting and posted on the ILSG website. Details of the application process can be found on the ILSG web site. The proposal will need to be signed by researcher‘s supervisor. In 2009 the ILSG Board of Governors awarded two $500 awards from the Student Research Fund. The winners are listed below, along with the title of the presentation they will be giving this year. Amanda van Lankvelt (Lawrence University) – Baraboo-interval quartzite breccias. Shelby Frost (University of Minnesota – Duluth) – Effects of contact metamorphism by the Duluth Complex on Proterozoic footwall rocks in northeastern Minnesota xii �STUDENT PAPER AWARDS Each year, the Institute selects the best of the student presentations and honors presenters with a monetary award. Funding for the award is generated from registrations of the annual meeting. The Student Paper Committee is appointed by the annual meeting Chair in such a manner as to represent a broad range of professional and geologic expertise. Criteria for best student paper— last modified by the Board in 2001—follow: The contribution must be demonstrably the work of the student. The student must present the contribution in-person. The Student Paper and Poster Committee shall decide how many awards to grant, and whether or not to give separate awards for poster vs. oral presentations. In cases of multiple student authors, the award will be made to the senior author, or the award will be shared equally by all authors of the contribution. The total amount of the awards is left to the discretion of the meeting Chair in conjunction with the Secretary, but typically is in the amount of about $500 US (increase approved by Board, 10/01). The Secretary maintains, and will supply to the Committee, a form for the numerical ranking of presentations. This form was created and modified by Student Paper and Poster Committees over several years in an effort to reduce the difficulties that may arise from selection by raters of diverse background. The use of the form is not required, but is left to the discretion of the Committee. The names of award recipients shall be included as part of the annual Chair's report that appears in the next volume of the Institute. Student papers are noted on the Program. (continued next page) xiii �STUDENT PAPER AWARDS (CONTINUED) In 2009 the ILSG Student Paper Committee presented several awards from the ILSG Student Paper Fund. Each of the following recipients received awards ranging from 80 to 100 dollars: Best oral presentations: Benedek Gal (Eötvös Lorând University, Budapest)) – Magmatic vs. hydrothermal processes in the South Filson Creek mineralization, South Kawishiwi Intrusion, Duluth Complex. Natalie Pietrzak (University of Western Ontario) – Ore textures and Mineral Chemistry within the oxide-carbonate-silicate flotation ores at the Cliffs Natural Resources’ Tilden Mine, Michigan Best poster presentations: Ryan Dayton (University of Minnesota-Duluth) – Quantifying assimilation vs. fractional crystallization using Sm-Nd, Lu-Hf and Pb isotope systems: The geochemical evolution of the Sonju Lake Intrusion, Finland, MN James Hiller (California State University) – Detailed petrographic analysis of anthraxolite morphology in the Biwabik Iron-Formation, northern Minnesota Andrew Jansen (University of Wisconsin-Oshkosh) – Lithogeochemical evaluation of Neoarchean mafic volcanic rocks comprising the footwall of the Soudan Member of the Ely Greenstone Formation, northeastern Minnesota Kyle Makovsky (Minnesota State University-Mankato) – Fluid movement through the Mesabi Iron Range, Minnesota Dan Costello (University of Minnesota-Duluth) – Geology of the Tuscarora Intrusion, northeastern Minnesota and its relationship to the anorthositic series of the Duluth Complex xiv �EISENBREY STUDENT TRAVEL AWARDS The 1986 Board of Directors established the ILSG Student Travel Awards to support student participation at the annual meeting of the Institute. The name "Eisenbrey" was added to the award in 1998 to honor Edward H. Eisenbrey (1926-1985) and utilize substantial contributions made to the 1996 Institute meeting in his name. "Ned" Eisenbrey is credited with discovery of significant volcanogenic massive sulfide deposits in Wisconsin, but his scope was much broader—he has been described as having unique talents as an ore finder, geologist, and teacher. These awards are intended to help defray some of the direct travel costs of attending Institute meetings, and include a waiver of registration fees, but exclude expenses for meals, lodging, and field trip registration. The annual Chair in consultation with the Secretary-Treasurer determines the number of awards and value. Recipients will be announced at the annual banquet. The student travel award application is available on the ILSG website. The following general criteria will be considered by the annual Chair, who is responsible for the selection: The applicants must have active resident (undergraduate or graduate) student status at the time of the annual meeting of the Institute, certified by the department head. Students who are the senior author on either an oral or poster paper will be given favored consideration. It is desirable for two or more students to jointly request travel assistance. In general, priority will be given to those in the Institute region who are farthest away from the meeting location. Each travel award request shall be made in writing to the annual Chair, and should explain need, student and author status, and other significant details. Successful applicants will receive their awards during the meeting. In 2009 the ILSG awarded 12 travel awards from the ILSG Eisenbrey Student Travel Fund, to students: Dan Costello Michael DeAngelis Adam Fage Nathan Forslund University of Minnesota – Duluth University of Tennessee – Knoxville Lakehead University Lakehead University Benedek Gal James Hiller Angela Hull Andrew Jansen Maura Kolb Cara Leitheiser Natalie Pietrzak Victoria Stinson Eötvös Lorând University California State University Chico Kent State University University of Wisconsin – Oshkosh Lakehead University University of Minnesota – Duluth University of Western Ontario Lakehead University xv �REPORT OF THE CHAIRS OF THE 55TH ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY ELY, MINNESOTA The Precambrian Research Center (PRC) of the University of Minnesota Duluth hosted the 55 th Annual Institute on Lake Superior Geology on May 5 – 10, 2009 at the Grand Ely Lodge in Ely, Minnesota – Gateway to the Boundary Waters Canoe Area Wilderness. Despite this area being well known for its classic Precambrian geology, this was the first time that the ILSG has been held in Ely. We are pleased to report that this was one of the best attended meetings in the 55 year history of ILSG with a total of 234 registrants, including 45 students. The organizing committee for the meeting was comprised of the three PRC directors: Jim Miller (meeting and field trip logistics, promotion, fundraising, and registration), George Hudak (technical program, student travel, and best paper awards), and Dean Peterson (field trip guidebook). Julie Ann Heinz, an executive office administrator at the UMD Natural Resources Research Institute, provided valuable assistance with meeting registration and planning. During the meeting, Julie Ann and Ginny Aldag handled on-site registration and other institute business. The two-day technical session held at the Grand Ely Lodge on Thursday and Friday (5/7 and 5/8) included 21 talks and 24 posters presentations, including 4 oral and 14 poster presentations by students. The meeting opened with a remembrance of Joe Mancuso who passed away in April of 2009. Joe was a professor of economic geology at Bowling Green State University (OH) and a long-time ILSG member (attended the 1st meeting in 1955). He received the Goldich medal in 2007. This year‘s Goldich medal recipient was L. Gordon Medaris of the University of Wisconsin-Madison. Gordon was recognized for his long and productive career studying the Paleoproterozoic of Wisconsin, in particular the Baraboo interval, and for his many contributions to the Institute. Gordon was presented the medal at the annual banquet by Bob Dott, his renowned colleague at UW Madison. The evening banquet talk was presented by Marvin Marshak, a physics professor at the University of Minnesota – Twin Cities and founder of the Soudan Underground Physic Laboratory. Dr. Marshak gave a very humorous and informative talk on the search for subatomic particles entitled “The Deep Underground Sky”. The meeting offered seven field trips that highlighted various aspects of the geology and ore deposits in the Ely area. Most trips were filled to capacity with a cumulative total of 325 field trip attendees. Pre-meeting field trips included a very popular two-day trip on the CU-NI DEPOSITS OF THE DULUTH COMPLEX led by Rich Patelke (Polymet Mining), Mark Severson (Natural Resources Research Institute, UMD), Dean Peterson (Duluth Metals Ltd.), Tim Jefferson (Teck American), and Ernie Lehmann (Franconia Minerals) and a one-day trip on GLACIAL GEOLOGY OF THE VERMILION MORAINE led by Phil Larson (Cliffs Natural Resources) and Howard Mooers (Department of Geological Sciences, UMD). On Friday afternoon following the completion of the technical session, two field trips were offered. One gave participants an underground tour of THE SOUDAN IRON MINE AND PHYSICS LAB led by Dean Peterson (Duluth Metals Ltd.), James Pointer – (Minnesota Department of Natural Resources, Parks and Recreation) and Marvin Marshak (Department of Physics, University of Minnesota). The other had Mark Jirsa (Minnesota Geological Survey) lead a PIONEER MINE CANOE EXCURSION around the perimeter of Miners Lake, just across the road from the meeting site in Ely. xvi �Several post meeting trips completed the meeting. A one-day trip on the GEOLOGY AND METAMORPHISM OF THE EASTERN MESABI RANGE led by Dick Ojakangas (Department of Geological Sciences, UMD), Mark Severson (Natural Resources Research Institute, UMD), Doug Halverson, Jeff Bird, Tom Campbell, Jarred Lubben, and Peter Jongewaard (Cliffs Natural Resources), and William Everett (Mesabi Nugget). A two-day trip on the ARCHITECTURE OF AN ARCHEAN GREENSTONE BELT was led by Dean Peterson (Duluth Metals Ltd.), Mark Jirsa (Minnesota Geological Survey), and George Hudak (Department of Geology, University of Wisconsin Oshkosh). Jim Miller (Dept. of Geological Sciences, UMD) led a twoday canoe excursion into the BWCA on the GEOLOGY OF THE LAKE ONE TROCTOLITE. The student paper committee comprised of Tom Fitz (Northland College), John Gartner (Prime Meridian Resources), and Dorothy Campbell (Ontario Geological Survey) once again had a difficult job of selecting among the 18 student oral and poster presentations. More than half of the students received financial aid to defray travel and registration expenses. A total of $5400 was distributed to 23 students from 13 different schools. This generous aid was provided by funds contributed by meeting registrants and several corporations and organizations. Specifically, meeting registrations surcharges generated $2000 and the ILSG Eisenbrey Fund contributed $1000. The American Institute of Professional Geologists (AIPG-Minnesota Chapter) contributed $1000, the Minnesota Mineral Resource Education Foundation (affiliated with Mining Minnesota) contributed $500, Northland Securities gave $500, and the Mesabi Range Geological Society contributed $500 (to UMD students). In addition to these contributions for student support, the meeting benefitted greatly by a $2000 contribution from Duluth Metals Ltd., which allowed the guidebook to be printed in color. Also, Barr Engineering contributed $1000 that went to sponsor the welcoming reception on Wednesday evening. See the 55th Program and Abstract volume for more details on student funding. . The Institute‘s Board of Directors met on May 7, 2009. The meeting was called to order by Chair George Hudak. The meeting was attended by the three co-chairs of the Ely meeting (Miller, Hudak, and Peterson), Treasurer Mark Jirsa, Secretary Peter Hollings, and Board Members Ted Bornhorst (2008 chair) and Ann Wilson (2006 chair). Secretary Hollings took the minutes of the Board meeting that are as follows: 1. Report of the Chairs (Bornhorst and Klasner) for the 54th ILSG, Marquette, Michigan was received and accepted. 2. Received, discussed, and accepted 2008-2009 ILSG Financial Summary (Jirsa). 3. Received, discussed, and accepted 2008-2009 ILSG Secretary‘s report (Hollings) 4. Appointed meeting chair (George Hudak) as on-going ILSG Board Member. 5. Discussed application process for ILSG Student Research Awards. Hollings to prepare a two-page form and circulate to the board 6. International Fall, MN was approved as the site for the 2010 (56 th Annual) meeting location with Mark Jirsa (MGS) and Pete Hollings (Lakehead) serving as co-chairs, and Terry Boerboom (MGS) and Mark Smyk (OGS) serving on the organizing committee.. xvii �7. Approved replacement of Richard Ojakangas as ―academic member‖ on Goldich Committee (end of term 2009) with Mary Louise Hill (Lakehead U). 8. Discussed moving toward advertising of the annual meetings exclusively by email and website. Miller gave a positive report on how this approach worked for the Ely meeting. Miller reported compiling an email list of over 750 contacts. 9. It was agreed that Jirsa would order 10 new Goldich medals. It was also agreed that Hollings would prepare a letter that could be sent to new Goldich committee members explaining the process. This letter will be distributed to the Board prior to being sent out 10. Hudak suggested that a notice page be added to the web site to be used to post job and graduate opportunities. It was agreed that the proceedings of the institute would now be posted immediately after each meeting. The hosting of the ILSG web site was discussed, Miller to investigate the possibility of hosting the site through the Precambrian Research Center. 11. The board thanked Mudrey and Hollings for their work on digitizing the volumes. These have proved a useful resource for researchers The co-chairs would like to thank the participants, especially the students, for their enthusiasm, the field trip leaders for their hard work, the presenters for their informative talks and posters, Marvin Marshak for his classic banquet talk, the session chairs and subcommittee members for their important contributions, and the meeting sponsors for their generosity. A big thank you to Julie Ann Heinz for her cheerfulness and organization in handling the registrations. Thanks also to Sue Marturano, Sue Salveson, and Denise Endicott for handling the budgeting. Thanks to our wives, Ginny Aldag, Rachel Pastoor and Deb Rausch, for their support and understanding. Thanks to Barb Lyke, Tara Akeman, and the entire staff of the Grand Ely Lodge for being accommodating, flexible and very professional. Although there were some things that we could have improved upon (e.g. a projector that showed true colors), we are pleased with how the meeting went. We are particularly gratified by the near record attendance, the popularity of the field trips, and the generally favorable comments we receive about the technical meeting. It made the planning, organizing, and oversight of the meeting worth the effort. You all should try it sometime…really. See you all in International Falls. Respectfully submitted, Jim Miller, George Hudak, and Dean Peterson Co-Chairs, 55th Institute on Lake Superior Geology xviii �2010 BOARD OF DIRECTORS Board appointment continues through the close of the meeting year indicated in parantheses, or until a successor is selected Peter Hinz, Co-Chair, 56th Meeting Ontario Geological Survey Peter Hollings, Co-Chair, 56th Meeting Lakehead University, Thunder Bay, ON Mark Smyk, Co-Chair 56th Meeting Ontario Geological Survey Mark Jirsa, Co-Chair, 56th Meeting Minnesota Geological Survey, St. Paul, MN Terry Boerboom, Co-Chair, 56th Meeting Minnesota Geological Survey, St. Paul, MN Jim Miller (2010) University of Minnesota – Duluth, MN Peter Hollings – Secretary (2010) Lakehead University, Thunder Bay, ON Theodore J. Bornhorst (2011) Michigan Technological University, MI Mark A. Jirsa – Treasurer (2011) Minnesota Geological Survey, St. Paul, MN George Hudak (2012) Precambrian Research Center and Natural Resources Research Institute 2010 SESSION CHAIRS Peter Hinz Ontario Geological Survey Mark Smyk, Ontario Geological Survey Peter McSwiggen, McSwiggen and Associates Charlie Blackburn, Blackburn Geological Leonard Espinosa, Retired, Michigan Department of Natural Resources 2010 STUDENT PAPER AWARDS COMMITTEE Graham Wilson (Chair), Turnstone Geological Services Ltd. Jim Miller, University of Minnesota – Duluth Anthony Pace, Ontario Geological Survey xix �2010 MEETING SPONSORS The organizers wish to sincerely thank the several companies and organizations who have contributed financial support and support in kind to various components the meeting. American Institute of Professional Geologists (AIPG) – Minnesota Atikokan Mineral Development Initiative Brett Resources Canadian Institute of Mining, Metallurgy, and Petroleum (CIM) – Thunder Bay Mesabi Range Geological Society Metalcorp Minnesota Geological Survey Numax Ontario Geological Survey Prime Meridian Resources Q-Gold Rainy River Resources Society for Mining, Metallurgy, and Exploration (SME) – Minnesota Voyageurs National Park William Miller – University of Minnesota School of Physics and Astronomy NOvA project xx �2010 BANQUET SPEAKER The Importance of Being Cratered: The New Role of Meteorite Impact as a „Normal‟ Geological Process Dr Bevan M. French Adjunct Professor Smithsonian Institution National Museum of Natural History Planetary geologist, petrologist and author xxi �xxii �PROGRAM xxiii �WEDNESDAY MAY 19, 2010 8:30 a.m. FIELD TRIP 1: MINERAL DEPOSITS OF THE RAINY RIVER AREA (CAN) Wally Rayner – Rainy River Resources CJ Baker – Rainy River Resources Craig Ravnaas – Ontario Geological Survey 77 passenger tour bus Return to rec centre approximately 6:00 p.m. TRIP LEAVES FROM AND RETURNS TO THE FORT FRANCES MEMORIAL SPORTS CENTER*, FORT FRANCES, ONTARIO *SEE MAP ON PAGE 75 IN BACK OF THIS VOLUME FOR LOCATION --------------------------------------------------------------------------------------------------------------------8:30 a.m. FIELD TRIP 2: GEOLOGY OF ARCHEAN SUCCESSION AT ATIKOKAN Phil Fralick – Lakehead University 77 passenger tour bus Return to White Otter Inn approximately 5:00 p.m. TRIP LEAVES FROM AND RETURNS TO THE WHITE OTTER INN, ATIKOKAN, ONTARIO --------------------------------------------------------------------------------------------------------------------8:30 a.m. FIELD TRIP 3: STRUCTURAL GEOLOGY ALONG THE QUETICO FAULT Dyanna Czeck – University of Wisconsin - Milwaukee Howard Poulsen – Consultant 9 – passenger vans Return to rec centre approximately 6:00 p.m. TRIP LEAVES FROM AND RETURNS TO THE FORT FRANCES MEMORIAL SPORTS CENTER*, FORT FRANCES, ONTARIO *SEE MAP ON PAGE 75 IN BACK OF THIS VOLUME FOR LOCATION 4:00 p.m. - 10:00 p.m. REGISTRATION AT RAINY RIVER INN**, INTERNATIONAL FALLS, MN 7:00 p.m. - 10:00 p.m. ICE BREAKER AND POSTER SESSION **THE RAINY RIVER INN IS THE FORMER HOLIDAY INN xxiv �THURSDAY MAY 20, 2010 A.M. 8:00 a.m. - 12:00 noon REGISTRATION 8:30 a.m. INTRODUCTORY REMARKS Mark Smyk and Peter Hinz, Ontario Geological Survey TECHNICAL SESSION I Session Chairs: Mark Smyk and Peter Hinz, Ontario Geological Survey 8:40 a.m. Miller, James D., and Stifter, Eric Cyclical Phase Layering in the Duluth Complex at Duluth – Evidence for Periodic Magma Venting from a Shallow Staging Chamber? 9:00 a.m. Hoaglund, S.A.*, Miller, J.D., Crowley, J.L., Schmitz, M.D. U-Pb zircon geochronology of the Duluth Complex and related hypabyssal intrusions: investigating the emplacement history of a large multiphase intrusive complex related to the 1.1 Ga Midcontinent Rift 9:20 a.m. Cundari, Robert*, Hollings, Peter, and Smyk, Mark Geology and geochemistry of the Rove basalts, a possible Midcontinent-Rift related extrusive unit south of Thunder Bay, Ontario 9:40 a.m. COFFEE BREAK AND POSTER SESSION 10:20 a.m. Magnus, Seamus*, Kissin, Stephen Assimilation and petrogenesis in the Navilus and Terry Fox sills, Thunder Bay, Ontario 10:40 a.m. Puchalski, Raya*, Hollings, Peter, and Smyk, Mark The petrology and geochemistry of the Riverdale Sill 11:00 a.m. Hollings, Peter, and Smyk, Mark Radiogenic isotope characteristics of Midcontinent Rift-related intrusions south of Thunder Bay 11:20 a.m. Lunch Break (2010 ILSG Board Meeting - by invitation) xxv �THURSDAY MAY 20, 2010 P.M. TECHNICAL SESSION II Session Chair: Peter McSwiggen, McSwiggen and Associates 1:00 p.m. van Lankvelt, Amanda*, and Bjornerud, Marcia Revisiting the Baraboo breccias 1:20 p.m. Pietrzak, Natalie*, Duke, Norm, Scott, Glenn, and Lukey, Helene, Hydrothermal Overprint by Greenstone Sills within the Tilden Pit, Michigan 1:40 p.m. Noah Planavsky*, Andrey Bekker, Olivier J. Rouxel, Balz Kamber, Axel Hofmann, Andrew Knudsen, Timothy W. Lyons The rare earth element and yttrium composition of Archean and Paleoproterozoic iron formations revisited: A new perspective on significance and mechanisms of iron formation deposition 2:00 p.m. O’Hare, Sean* and Fralick, Philip Sedimentological study of the glaciogenic Bruce Formation, Huronian Supergroup, in the North Shore area, Ontario 2:20 p.m. COFFEE BREAK AND POSTER SESSION 3:00 p.m. Jirsa, Mark A. Stratigraphy of Sudbury “impactite‟ near Gunflint Lake, NE Minnesota 3:20 p.m. Weiblen, Paul, W., Jirsa, Mark A., and McSwiggen, Peter Sudbury impact ejecta – amphiboles in samples from the vicinity of the Gunflint Trail, Minnesota 3:20 p.m. Cannon, William F., and Slack, John F. Global effects of the Sudbury bolide impact at 1850 Ma: Potential link to the evolution of marine mineral deposits ----------------------------------------------------------------------------------------------------------------6:00 p.m. ICE BREAKER – MIXER – CASH BAR 7:00 p.m. ANNUAL BANQUET AND AWARD PRESENTATION AT BACKUS COMMUNITY CENTER (SEE MAP ON PAGE 76 IN BACK OF THIS VOLUME FOR LOCATION) • Announcement of 57th Annual Meeting Location • 2010 Goldich Award Presentation to: • William D. Addison and Gregory R. Brumpton • 2010 Banquet Address by Dr. Bevan M. French, Smithsonian Institution All regstered participants are welcome to the banquet address xxvi �FRIDAY MAY 21, 2010 A.M. 8:30 a.m. INTRODUCTORY REMARKS Mark Jirsa, 2010 ILSG Co-Chair TECHNICAL SESSION III Session Chair: Charlie Blackburn, Blackburn Geological 8:40 a.m. Kolb, Maura J.* and Hill, Mary Louise A microstructural study of gold mineralization at Musselwhite Mine and Hammond Reef shear-zone-hosted gold deposits 9:00 a.m. Stinson, Victoria R.*, and Hill, Mary Louise Structural control at Hammond Reef gold deposit north of Atikokan, Ontario 9:20 a.m. Siemieniuk, Steven*, Hollings, Pete, McCuaig, Cam, and Porwal, Alok Gold in the Wabigoon Subprovince – GIS-based Mineral Potential Mapping 9:40 a.m. Scott, Robert J.* and Hill, Mary Louise Quartz eyes of the Moose Lake Porphyry Complex, Hemlo, Ontario 10:00 a.m. COFFEE BREAK AND LAST POSTER SESSION 10:40 a.m. Gilbert, Paul Geology and geochemistry of arc and ocean-floor volcanic rocks in the Northern Flin Flon Belt, Manitoba 11:00 a.m. Fralick, Philip Anatomy of an Archean carbonate platform: Geology of the Steep Rock Group, Canada 11:20 a.m. Smyk, Mark, Stott, Greg, and Atkinson, Brian The Ring of Fire: An Overview of the Geology, Mineral Deposits and Exploration History of the McFaulds Lake Area 11:40 p.m. LUNCH BREAK AND MEETING OF THE STUDENT PAPER COMMITTEE xxvii �FRIDAY MAY 21, 2010 P.M. TECHNICAL SESSION IV Session Chair: Leonard Espinosa Retired - Michigan Department of Natural Resources 1:00 p.m. Boerboom, Terrence J. Latest bedrock geologic map of Carlton County, east-central Minnesota and revisions to the southern Animikie Basin edge 1:20 p.m. Chandler, Val W.. The Search for Magnetic Basement in Minnesota: Sliding Along with the Euler Equation 1:40 p.m. Waggoner, T.D. Post Penokean Age of Mineralization on the Marquette Range, Michigan 2:00 p.m. PRESENTATION OF THE STUDENT TRAVEL AND BEST PAPER AWARDS 2:20 p.m COMPLETION OF 2010 ILSG TECHNICAL SESSIONS AND FINAL STATEMENTS BY THE 2009 ILSG CO-CHAIRS 3:00 p.m. FIELD TRIP 4: ARCHEAN GEOLOGY OF VOYAGEURS NATIONAL PARK AND LITTLE AMERICA GOLD MINE (US) Chris Hemstad – Boreal Explorations Return to Rainy River Inn approximately 6:00 p.m. TRIP LEAVES FROM AND RETURNS TO THE RAINY RIVER INN**, INTERNATIONAL FALLS, MN 3:00 p.m. FIELD TRIP 5: ASH RIVER NEUTRINO DETECTOR LABORATORY AND THE ARCHEAN VERMILION GRANITIC COMPLEX Mark Jirsa – Minnesota Geological Survey Ash River laboratory staff Return to Rainy River Inn approximately 6:00 p.m. TRIP LEAVES FROM AND RETURNS TO THE RAINY RIVER INN**, INTERNATIONAL FALLS, MN **THE RAINY RIVER INN IS THE FORMER HOLIDAY INN xxviii �SATURDAY MAY 22, 2010 8:30 a.m. FIELD TRIP 6: TRANSECT THROUGH THE QUETICO-WABIGOON SUBPROVINCE BOUNDARY (US) Mark Jirsa – Minnesota Geological Survey Chris Hemstad – Boreal Explorations 47 passenger tour bus Return to Rainy River Inn approximately 5:30 p.m. TRIP LEAVES FROM AND RETURNS TO THE RAINY RIVER INN**, INTERNATIONAL FALLS, MN **THE RAINY RIVER INN IS THE FORMER HOLIDAY INN --------------------------------------------------------------------------------------------------------------------8:30 a.m. FIELD TRIP 7: MINERAL DEPOSITS OF THE MINE CENTRE – RAINY LAKE AREA (CAN) Peter Hinz – Ontario Geological Survey Other industry geologists 9 – passenger vans Return to rec centre approximately 6:00 p.m. TRIP LEAVES FROM AND RETURNS TO THE FORT FRANCES MEMORIAL SPORTS CENTER*, FORT FRANCES, ONTARIO *SEE MAP ON PAGE 75 IN BACK OF THIS VOLUME FOR LOCATION --------------------------------------------------------------------------------------------------------------------8:30 a.m. FIELD TRIP 8: GEOLOGY AND ENVIRONMENTAL ISSUES OF THE STEEP ROCK MINE (CAN) Andrew Conly – Lakehead University Rob Purdon – Ontario Ministry of Natural Resources Mini-vans Return toWhite Otter Inn, Atikokan, approximately 6:00 p.m. TRIP LEAVES FROM AND RETURNS THE WHITE OTTER INN, ATIKOKAN, ONTARIO _____________________________________________ 56TH ANNUAL INSTITUTE ON LAKE SUPERIOR GEOLOGY ENDS xxix �POSTER PRESENTATIONS Blakely, S., Brown, A., Foley, D., Rowland, A., Stifter, E., and Miller J.D., Jr., Mesoproterozoic Bedrock Geology of the Homer Lake – Vern Lake Area, Cook County, Northeastern Minnesota Boerboom, T. J., Magnuson, A., Creighton, Dalyice, McGinn Mapping of Mesoproterozoic bedrock in the Tait Lake quadrangle, Northeastern Minnesota Buchholz, T. W., Falster, A. U. and Simmons, W. B. Additions and New Data on the Mineralogy of the Keweenaw Copper District, MI. Bultman, M. W. Preliminary analysis of ground-based magnetic profile data aimed at understanding the concealed mineral resource potential of the Wisconsin magmatic terranes Fage*, A. and Hollings, P. Geology and Geochemistry of the Hemlo East Property, Schreiber-Hemlo Greenstone Belt, Ontario Fahrenkrog, Brooke, Totenhagen, Mike, Jaret, Steven, Watson, Tabitha, and Jirsa, Mark A. Geologic mapping of Archean and Proterozoic rocks near Bingshick Lake, Boundary Waters Canoe Area Wilderness, by students of the Precambrian Research Center‟s 2009 field camp Fralick, P., Brumpton, G.R., Jirsa, M.A., Kissin, S.A., Severson, M.J. Sedimentology of the Paleoproterozoic ejecta layer from the Sudbury impact event Frost*, S. J Effects of contact metamorphism by the Duluth Complex on Proterozoic footwall rocks in northeastern Minnesota Hudak, G., Diedrich, T., Monson-Geerts, S., Schreiber, M., Zanko, L., and Schwanke, A. Taconite-derived mineral dust in population centers on Mesabi Iron Range: an update Markwood*, Levi W. and Zieg, Michael J., Textural analysis of a simple igneous intrusion, Nipigon, Ontario Medaris, L. G. Jr. and Koellner, S. E. Ferromagnesian Minerals in the Stettin Syenite Complex, Marathon County, Wisconsin: Compositions and Contrasts with the Wolf River Batholith Mulvey, Lucy, Ross, Cabin, Zeitler, Joseph, Pendleton, Matthew, McCarthy, Andrew, Copp, Lee, Nowak, Robert, Hudak, George, and Peterson, Dean, Bedrock geologic map of the Disappointment Lake area, Lake County, Northeastern Minnesota Patelke, R.L., and Severson, M.J. Virginia Formation outcrop (New outcrops versus old concepts) Ryan*, A. J. and Zieg, M. J. Petrographic and geochemical analysis of a Nipigon diabase sill xxx �Triplett*, J., Saini-Eidukat, B., Feit, S., and Dolezal, D. Identification and Characterization of Fibrous Zeolites in Western North Dakota Weiblen, Paul W. New ways to study old problems. White, Chris R., and Albers, Paul B. Geologic map of the central Seine Bay/ Bad Vermilion Lake Intrusion with accompanying airborne Geophysics, relating to Numax Resources, Inc., Mine Centre Property, Mine Centre, Ontario xxxi �Thursday a.m. Thursday p.m. Technical Session I Session chair: Mark Smyk and Peter Hinz Technical Session II Session chairs: Peter McSwiggen Session 1a. Midcontinent Rift 8:30 AM introductory remarks 8:40 AM Miller 9:00 AM Hoaglund* 9:20 AM Cundari* Session 2a. Misc. Paleoprot. 1:00 PM VanLankveldt* 1:20 PM Pietrzak* 1:40 PM Planavsky* 2:00 PM Ohare* 9:40 AM Coffee break and poster session 2:20 PM Coffee break and poster session Session 1b. MCR Mafic Sills 10:20 AM Magnus* 10:40 AM Puchalski* 11:00 AM Hollings Session 2b. Sudbury Impact 3:00 PM Jirsa 3:20 PM Weiblen 3:40 PM Cannon 11:20 AM Lunch break 2010 ILSG Board Meeting Friday a.m. Friday p.m. Technical Session III Session chairs: Charlie Blackburn Technical Session III Session chairs: Leonard Espinosa Session 3a. Archean Econ. Geol. 8:40 AM Kolb* 9:00 AM Stinson* 9:20 AM Siemieniuk* 9:40 AM Scott* Session 4a. Archean Geology 1:00pm Boerboom 1:20 PM Chandler 1:40 PM Waggoner 2:00 PM Student paper, poster, and travel awards 10:00 AM Coffee break and poster session Session 3b. Proterozoic 10:40 AM Gilbert 11:00 AM Fralick 11:20 AM Smyk Session 4b. Afternoon Field Trips 3:00 PM depart for field trips 11:40 PM Lunch break xxxii �ABSTRACTS xxxiii �xxxiv �MAPPING OF MESOPROTEROZOIC BEDROCK IN THE TAIT LAKE QUADRANGLE, NORTHEASTERN MINNESOTA Boerboom, T.J., Magnuson, A., Creighton, D., and McGinn, K. Precambrian Research Center, University of Minnesota Duluth, Duluth, MN 55812 The Precambrian Research Center (PRC) field camp culminates in several ‗capstone‘ mapping projects located in different geographical areas/geological terranes in northeastern Minnesota. One of the 2009 PRC capstone projects was focused on mapping an area within the North Shore Volcanic Group that is intruded by components of the Beaver Bay Complex. Compared to the other capstone projects, which were largely canoe-based and in areas of plentiful and clean bedrock exposures, this map area was just the opposite – no lakeshore outcrops, overgrown clearcuts, and relatively small and scattered outcrops located mostly near the apex of topographic knobs. As such, the participants in this camp gained a different perspective of ‗real-life‘ field geology: the reality of field work in most northern forested areas – thick brush, insects, heat, and lack of water. Nevertheless, the mappers persevered and were able to provide significant refinements to our understanding of the bedrock geology of the southwestern Tait Lake quadrangle. Prior to the PRC effort, the only mapping in the area was reconnaissance outcrop studies by John Green from the University of Minnesota-Duluth. This information was used in the construction of a regional, 1:200,000-scale geologic map, which portrayed the bedrock units as largely undifferentiated volcanic rocks (Miller et al., 2001). Based on the results of the PRC capstone project, within the chosen map area we have defined a southeast-dipping series of volcanic rocks that includes quartz- and feldspar-phyric rhyolite, sparsely plagioclase-porphyritic andesite, and porphyritic basalt. These volcanic rocks are intruded by the Lake Clara diabase, a southeastdipping semi-conformable sill which was known only in a general fashion from prior work. Our new mapping has identified a thin ferrodiorite layer that is apparently semi-conformable to the base of the diabase sill, which may be the product of partial melting of the rhyolite below the sill and contamination of the adjacent diabase. Tentative plans for the 2010 capstone project are to continue mapping in the northwest corner of the Tait Lake 7.5‘ quadrangle, in a zone that transitions from the Eagle Mountain granophyre (1098.6±3.6 Ma; Vervoort and others, 2007) into volcanic rocks of the North Shore Volcanic Group. This mapping would be continuous with prior detailed mapping to the north in the Brule Lake quadrangle (Davidson and Burnell, 1977). However, difficult access may present a serious impediment to this plan in which case work may proceed elsewhere. References: Davidson, D.M., Jr., and Burnell, J.R., 1977, Reconnaissance geologic map of the Brule Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map M29, scale 1:24,000. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.E., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota. Miscellaneous Map Series, M-119, scale 1:200,000, 2 sheets. Vervoort, J.D., Wirth, K., Kennedy, B., Sandland, T., and Harpp, K.S., 2007, The magmatic evolution of the Midcontinent rift: New geochronologic and geochemical evidence from felsic magmatism, Precambrian Research 157 (2007) 235–268. 1 �LATEST BEDROCK GEOLOGIC MAP OF CARLTON COUNTY, EAST-CENTRAL MINNESOTA AND REVISIONS TO THE SOUTHERN ANIMIKIE BASIN EDGE Boerboom, Terrence J., Minnesota Geological Survey (boerb001@umn.edu) As part of the Minnesota Geological Survey‘s ongoing County Geologic Atlas Program, a revised bedrock geologic map has recently been released for Carlton County, in east-central Minnesota. As with other county atlases produced by the Minnesota Geological Survey, this atlas was funded by a combination of state and county funds. Carlton County straddles the southern edge of the Animikie basin (Figure), which contains strata dominated by graywacke-slate, and includes the Virginia Formation to the north and the Thomson Formation to the south. To the north the Virginia Formation overlies the Biwabik Iron Formation and the Pokegama Quartzite, the latter of which was deposited directly on Archean basement as well as eroded ca. 2.1 Ga Kenora-Kabetogama dikes. In contrast, the Thomson Formation at the southern margin unconformably overlies metamorphosed sedimentary and igneous rocks of the Paleoproterozoic Mille Lacs Group. This paper focuses on the Thomson Formation at the southern margin of the Animikie basin. The bedrock geology of east-central Minnesota has been mapped numerous times, each version taking into account the latest geologic concepts and newly-acquired geophysical and drill-hole data. In early regional compilations the Thomson Formation included some rocks now assigned to the Mille Lacs Group, and the Cuyuna North and South Range strata were correlated to those in the lower part of the Animikie basin. However, more recent geologic interpretations associate the Cuyuna ranges with the older Mille Lacs Group, as part of a belt that was deformed and uplifted during the Penokean Orogeny, and upon which the Animikie strata were deposited in a foreland basin setting. This scenario is supported by the ca. 1,850 Ma metamorphic ages obtained in both the Mille Lacs Group and the Cuyuna North Range, and ca. 1,800 Ma detrital zircons in the Thomson Formation. Figure showing extent of Animikie basin (gray) in Minnesota. Twice-deformed Thomson in dark gray. Carlton County outlined by heavy line. 2 �Holst (1982) documented two structural domains in the Thomson Formation, separated by what is colloquially known as ‗Holst‘s line‘. Slate and graywacke south of this line (southern domain) contain a subhorizontal S1 foliation overprinted by regional F2 folds and accompanying S2 crenulation cleavage, whereas rocks north of this line (northern domain) were deformed and folded only once by the regional D2 deformation. Winchell (1899) described an outcrop of conglomerate with clasts of metagraywacke, slate, and black chert, in the vicinity of what is now ‗Holst‘s line‘. Although this outcrop has apparently been buried by road construction, its reported location and lithology imply a possible unconformity at the base of the northern domain. A second conglomerate, intersected by a deep drill hole near the base of the southern domain, contains clasts of granitoid rocks, metagraywacke, chert, and marble; this may represent a second, lower unconformity between the base of the southern Thomson domain and the older Mille Lacs Group. A possible scenario is that the southern domain may represent an early protoAnimikie depositional basin marked by a basal conglomerate, which was deformed and uplifted to provide sediment to the main, northern domain of the Animikie basin, the base of which is marked by a second conglomerate. Based on examination of drill cores and outcrops, multiple east-west trending, low-amplitude aeromagnetic anomalies over the southern domain of the Thomson Formation are considered to be caused by sulfidic intervals spatially associated with thin to thick beds of chert as well as local metagabbro sills. The aeromagnetic anomalies, which coincide with electromagnetic and gravity anomalies, continue west into areas formerly designated as Cuyuna South Range. It is proposed that much of what has mapped as Cuyuna South Range is actually infolded remnants of the lowermost Animikie basin strata. In this case the lower Animikie basin would include a substantial proportion of mafic volcanic rocks and hypabyssal intrusions. Geophysical modeling implies that the magnetic source to these anomalies is a north-dipping sheet, consistent with the inferred geometry of the Animikie basin. These sulfidic horizons may be analogous to thin oxide- and sulfide-facies iron-formations in the lower part of the Michigamme Formation in Michigan, which is correlative with this part of the Animikie basin (Rossell, 2008; Ware and others, 2008). Metamorphic ages in the Mille Lacs Group and the Cuyuna North Range show a Penokean metamorphic event (ca. 1,850 Ma), and a later Yavapai metamorphic overprint (ca. 1,760 Ma). However no metamorphic ages have been obtained from the Cuyuna South Range or from the Thomson Formation; an older ca. 1850 Ma age in the Cuyuna South Range would disprove the proposed correlation of the south range with the Animikie basin. The once-deformed Thomson Formation at its type locality contains detrital zircons as young as 1,800 Ma, the same age as the Hillman tonalite to the south. If that were the source for the zircons, deep erosion may have accompanied Yavapai uplift and deformation. References: Holst, T.B., 1982, Evidence for multiple deformations during the Penokean Orogeny in the Middle Precambrian Thomson Formation, Minnesota: Canadian Journal of Earth Sciences v. 19, p. 2043-2047. Rossell, D., 2008, Trip 7: Geology of the Keweenawan BIC intrusion, in 54th Annual Institute on Lake Superior Geology Proceedings Vol. 54, part 2 – Field Trip Guidebook; Bornhorst, T.J., and Klasner, J.S., eds., p. 181-199. Ware, A., Cherry, J., and Ding, X., 2008, Trips 4 and 8: Geology of the Eagle Project: in 54th Annual Institute on Lake Superior Geology Proceedings Volume 54, part 2 – Field Trip Guidebook; Bornhorst, T.J., and Klasner, J.S., eds., p. 87-115. Winchell, N.H., 1899, The geology of the Carlton plate, in Winchell, N.H., and others, Geology of Minnesota: volume 4 of the final report, 1896-1898: Minnesota Geological and Natural History Survey, p. 550-565. 3 �ADDITIONS AND NEW DATA ON THE MINERALOGY OF THE KEWEENAW COPPER DISTRICT, MI. Buchholz, Thomas W., 1140 12th Street North, Wisconsin Rapids, Wisconsin 54494, Falster, Alexander U. and Simmons, Wm. B., Department of Geology and Geophysics, University of New Orleans, New Orleans, Louisiana 70148 Recent work on material from several former mines on the Keweenaw Peninsula, MI has revealed the presence of several minerals not previously reported from the district. Heinrich & Robinson (2004) mentioned the possible occurrence of anilite (Cu 7S4) near Painesdale, MI. In an attempt to search for possible anilite and associated minerals, carbonate matrix was partially removed from samples of chalcocite-bearing vein material from the nearby Baltic Mine in order to better expose the sulfides. Small grains of a soft, deep blue sulfide showing excellent cleavage and evidence of multiple twinning were noted on a few samples. XRD analysis (Gandolfi camera) of a small crystal fragment yielded a pattern showing good agreement with anilite. The anilite seems restricted to quartz-rich portions of thin carbonatequartz-sulfide veinlets, is intimately associated with chalcocite, and is usually spatially associated with late cross-cutting fractures. Associated minerals in the veinlets include native copper, metallic grains of Cu-As alloys showing highly variable Cu-As ratios and spongy appearing masses of Cu-arsenide phases that seem to be composed of fine-grained domains with widely variable compositions. Work continues to attempt to characterize these phases. Several of the samples from the Baltic Mine dump revealed crude wires of native silver associated with chalcocite, quartz and minor barite. Small patches of a Cu-Ag sulfide were noted closely associated with the native silver during examination of this material using SEM/EDS. The Cu-Ag sulfide is identical in overall appearance to chalcocite and was analyzed via electron microprobe. The result, [(Cu1.024 Fe0.012Mn0.001)Ʃ1.037(Ag0.955) (S0.998 As0.009)Ʃ1.007)], is in good agreement with stromeyerite, CuAgS, and is the first report of this mineral for Michigan. It has been suggested that alteration of silver-bearing sulfides and sulfosalts may promote the growth of wire silver; if correct, it seems possible that Ag released by stromeyerite alteration supported the growth of these wire silvers. A small sample from the Wolverine #2 mine contained silvery inclusions in calcite that were confirmed as arsenopyrite by EMP: (Fe0.990 Cu0.003 Co0.001)Ʃ0.994 As0.997 S1.009), in good agreement with the accepted formula FeAsS. The arsenopyrite crystals were perched on a silvery to dark gray colored matrix. Portions of the matrix were analyzed by EMP and are an intergrowth of cobaltite, [(Co0.771 Fe0.158 Ni0.047 Cu0.023)Ʃ0.999 As0.994 S1.007) (analyzed) vs CoAsS], skutterudite, [(Co0.825 Fe0.189 Cu0.006 Ni0.003)Ʃ1.023 (As2.800S0.177)Ʃ2.977) (analyzed) vs CoAs3], and other As-S bearing phases. Minute well-formed crystals of probable safflorite, [(Co 0.587 Fe0.378 Cu0.059 Ni0.002)Ʃ1.026 (As1.850S0.119Se0.004)Ʃ1.973 (analyzed) vs CoAs2], are exposed on surfaces of the matrix and exhibit good orthorhombic morphology. Work continues to characterize additional phases in the matrix. Small grains and crystals of silver (0.12 wt% Hg, 0.21 wt% Se) are present throughout and upon the matrix. Neither arsenopyrite, cobaltite nor safflorite have been previously reported from the Keweenaw Peninsula, though undefined Co-arsenides have been reported from the Mohawk #2 mine, part of the same mineralized trend as the Wolverine mines (Moore, 1962; Heinrich & Robinson, 2004; Butler & Burbank, 1929). 4 �References: Heinrich, E. W., Robinson, George W. (2004) Mineralogy of Michigan. A. E. Seaman Mineral Museum Publishing, MI. 252p. Moore, P. B. (1962) Copper arsenides at Mohawk, Michigan. Rocks & Minerals 37:24-26. Butler, B. S., and Burbank, W. S. (1929) The Copper Deposits of Michigan. U S Geological Survey Professional Paper 144, 238p. 5 �PRELIMINARY ANALYSIS OF GROUND-BASED MAGNETIC PROFILE DATA AIMED AT UNDERSTANDING THE CONCEALED MINERAL RESOURCE POTENTIAL OF THE WISCONSIN MAGMATIC TERRANES BULTMAN, Mark W., U.S. Geological Survey, 520 N. Park Ave, Suite 355, Tucson, AZ, 85719 The Wisconsin magmatic terranes host numerous volcanogenic massive sulfide (VMS) mineral deposits ranging in size from uneconomic to the world class Crandon deposit with 72.5 million tons resource (DeMatties, 1994). While there has been extensive exploration in the terranes, an understanding of the total VMS resource is hindered by thick glacial deposits and/or relatively thin Cambrian sandstones that cover most of the area where permissive host rocks might occur. No medium size VMS deposits (10 to 60 million tons resource) are known in the area, and it is likely that some undiscovered deposits in this size range occur here (DeMatties, 1994). All known economic VMS deposits are associated with syngenetic and epigenetic strata-bound to stratiform mineralization within, along the flanks of, or near the top of felsic volcanic centers (DeMatties, 1994). Felsic centers are found within more mafic metavolcanic rocks creating a bimodal assemblage of volcanic rocks. The ability to distinguish concealed felsic centers from more mafic metavolcanic rocks would contribute to knowledge of the potential distribution and extent of these deposits and serve as an exploration tool. In order to discriminate concealed felsic centers from more mafic concealed metavolcanic lithologies, Earth‘s total intensity magnetic field profile data were acquired by a truck-mounted cesium vapor magnetometer. The magnetometer is mounted at 3.5 meters height and acquires data at measurement intervals of 1.5 to 2.0 meters. These data include information on the spatial distribution of near surface magnetic dipoles that in many cases are unique to a specific lithology. These magnetic signatures have distinct textures (the physical appearance of the data, especially groups of anomalies) that can be compared visually and characterized by several descriptive statistics. Ground-based magnetic profile data have been used successfully in the Basin and Range Province of North America for the identification of bedrock lithology concealed by 100 meters or more of Cenozoic basin fill (Bultman, 2009). In the Basin and Range Province, candidate lithologies (lithologies that may be found concealed by basin fill sediments) often outcrop in the mountain ranges directly adjacent to the basin. This allows the magnetic signature of a candidate lithology to be field calibrated and precisely defined. In areas with thick (10‘s of meters) continuous cover of glacial sediments, it is impossible to acquire the magnetic signature of a candidate lithology. As felsic rocks are generally less magnetic, they display a low amplitude signal in the ground-based magnetic profile data. The mafic metavolcanic rocks displays a much higher amplitude signal that is easily visible in profile data and is easily discriminated from felsic center profile data. Figure 1 displays approximately 6 km of ground-based magnetic profile data plotted along the road over which it was acquired. Numerous cultural features are identified within the profile, which is also plotted on a background image of the aeromagnetic data (Daniels and Snyder, 2002). A significant change in texture is indicated on the profile data and defines the location of a concealed 6 �Figure 1: Ground-based magnetic profile data (thin black line) acquired from west to east along County Roads I and V (black line), Rusk County, Wisconsin plotted on top of the Wisconsin aeromagnetic map (Daniels and Snyder, 2002). Location of a concealed contact between more mafic concealed lithology to the west and a more felsic concealed lithology to the east is indicated along with anomalies due to cultural features. Mapped contacts (Sims, 1989) are shown by white lines. Mapped units are: Cs= Upper Cambrian Mount Simon Sandstone and Xtg=Early Proterozoic felsic volcanic and volcanogenic rocks (after Sims, 1989). contact between more mafic rock to the west and felsic rock to the east. There is no indication of this change in concealed lithology on the aeromagnetic image, displayed in the background of Figure 1. This change in lithology can also be identified using statistical analysis of the groundbased magnetic profile data. Using this technique, known felsic centers have been identified and partially mapped at the Flambeau, Bend, Schoolhouse, Lynne, and Crandon deposits. In addition, several areas with very low amplitude magnetic signals in the over 1000 km of ground-based magnetic profile data acquired to date may represent additional concealed felsic centers. Presently, it is impossible to differentiate felsic metasediments from felsic centers, and it may be difficult to reconcile this issue. Future work includes the addition of more ground-based magnetic profile data in the Wisconsin magmatic terranes, acquisition of data on thin Cambrian sandstones that overlie the western end of the terrane, and acquisition and analysis of data over other terranes concealed by glacial drift in the upper Midwest. References: Bultman, M.W., 2009, The utility of ground-based magnetic profile data in concealed mineral resource investigations. Technical session abstract book: Northwest Mining Association Annual Meeting, Reno, Nevada, Nov. 29-Dec. 4, 2009, p. 43. Daniels, D.L. and Snyder, S.L., 2002, Wisconsin aeromagnetic and gravity maps and data, a web site for distribution of data, U.S. Geological Survey Open-File Report 02-493. http://pubs.usgs.gov/of/2002/of02-493/ DeMatties, T.A. , 1994, Early Proterozoic Volcanogenic Massive Sulfide Deposits in Wisconsin: An Overview: Economic Geology, Vol. 89, 1994, pp. 1122-1151. Sims, P.K., 1989, Geologic map of Precambrian rocks of Rice Lake 1x2° quadrangle, northern Wisconsin: U.S. Geological Survey Map I-1924. 7 �GLOBAL EFFECTS OF THE SUDBURY BOLIDE IMPACT AT 1850 MA: POTENTIAL LINK TO THE EVOLUTION OF MARINE MINERAL DEPOSITS Cannon, William F., and Slack, John F., U.S. Geological Survey, Mail Stop 954, Reston, VA 20192 The bolide impact at Sudbury, Ontario at 1850 Ma is of comparable magnitude to the Chicxulub impact that produced profound and well-documented global biological and environmental changes at the K-P boundary. Like Chicxulub, Sudbury probably produced global effects and we have speculated that it caused changes in ocean chemistry and structure which, in turn, influenced the nature of marine mineral deposits (Slack and Cannon, 2009). Several studies (Figure 1) show that circa 1850 Ma was a time of substantial changes in the occurrence and global abundance of sedimentary manganese deposits, volcanogenic massive sulfide (VMS) deposits, sedimentaryexhalative (SEDEX) sulfides, and deep-water (Cu-rich) VMS-related exhalites, in addition to Superior-type banded iron-formations (BIF), which is our focus here. From these changes it appears that 1850 Ma was a pivotal moment in marine metallogeny. The deposition of BIF required a stratified water body in which deep anoxic waters allowed high concentrations of dissolved ferrous iron, which was then deposited by oxidation to insoluble ferric iron where the chemocline intersected the shallow seafloor. Deposition of BIFs on continental shelves and in foreland basins implies that the entire ocean was stratified for the long periods of Earth history during which BIFs were deposited (Fig. 1). We suggest that the disappearance at 1850 Ma of BIFs (and granular iron formations) from open-marine settings marks the end of anoxic deep water conditions. The change from mainly reduced (sulfide) facies to oxidized (hematite, magnetite) facies in deep-water exhalites at approximately this time is consistent with loss of the deep mass of anoxic seawater. Secular correspondence of several fundamental changes in marine metallogeny shown in Figure 1 and the Sudbury impact is striking, a timing that we suggest is not coincidental. We propose that the Sudbury impact disrupted the previously stratified ocean and mixed deep anoxic water with shallow oxygenated water thus ending the requisite conditions for BIF mineralization. At 1850 Ma the Earth‘s surface (Fig. 2) contained a single supercontinent, Columbia, surrounded by a single ocean (Zhao, et al., 2004), a situation that is critical for evaluating global effects of the Sudbury impact into an epicontinental sea near the margin of Columbia. We propose three mechanisms that might have broken down the preexisting ocean stratification: 1-Giant tsunami: Impacts of large bolides into oceans produce giant tsunamis. Impacts may excavate a crater in the ocean water with a diameter 1.5 times that produced in the underlying crust; the crater is caused by both vaporization of water and physically impelling the water laterally. Tsunami waves result both from lateral expulsion of water and return flow into the crater. Models indicate that in open oceans with depths typically greater than 4 km, these waves can be hundreds of meters high, capable of disturbing the entire water column, and scouring the sea floor over large regions. The Sudbury impact into continental crust, although probably covered by an epicontinental sea, may have produced tsunami waves of lower amplitude. Data to estimate maximum wave heights from the Sudbury impact are lacking. Thus, the capability of Sudburyrelated waves to mix the global ocean cannot be predicted accurately by models. 2-Shelf collapse and debris flows: Earthquakes generated by the Chicxulub impact caused massive failure of much of the outer continental shelf of eastern North American, and spread a layer of debris as much as several meters thick across the western North Atlantic seabed (Norris and Firth, 8 �2002). Shelf failure was common for at least 4,000 km from the impact site; lesser failure is known at distances up to 7,000 km. A 4,000 km circle centered on Sudbury (Fig. 2) indicates that virtually the entire continental margin of Columbia was likely susceptible to shelf failure from the Sudbury impact. Debris flows radiating out from Columbia may have stirred and mixed ocean waters, creating a new suboxic state for deep seawater by mixing with oxygenated surface water. Antipodal effects: The massive earthquake caused by the Sudbury impact may have had global consequences, but the phenomenon of antipodal focusing of seismic energy should be considered in addition to simple hemispheric transmission and attenuation of seismic energy. Axial focusing of seismic energy and the delivery of a strong seismic signal at the antipode of an earthquake‘s origin is widely observed and well understood in theory. Large-scale deformed terranes are known on both Mercury and the Moon that are antipodal to major impact sites. We suggest that the intuitive notion of decreased seismicity away from Sudbury needs to be modified and the possibility of significant antipodal effects thoroughly evaluated. For instance, if the Sudbury impact produced major surface deformation at its antipode, probably on the deep seabed, significant tsumanis could have been generated there that were transmitted back toward the Columbia landmass, with physical mixing of ocean waters occurring at the antipode as well as closer to Sudbury. We propose that these three phenomena, all likely consequences of the Sudbury bolide impact, possibly acting in unison, resulted in profound changes in the nature of the 1850 Ma global ocean and in the numerous observed changes in the character of marine mineral deposits at that time. References Norris, R.D., and Firth, J.V., 2002, Mass wasting of the Atlantic continental margin following the Chicxulub impact event: Geological Society of America Special Paper 356, p. 79-95. Slack, J.F., and Cannon, W.F., 2009, Extraterrestrial demise of banded iron formations 1.85 billion years ago: Geology, v. 37, p. 1011-1014. Zhao, G., Sun, M., Wilde, S.A., and Sanzhong, L., 2004, A Paleo-Mesoproterozoic supercontinent: Assembly, growth, and breakup: Earth-Science Reviews, v. 67, p. 91-123. 9 �THE SEARCH FOR MAGNETIC BASEMENT IN MINNESOTA: SLIDING ALONG WITH THE EULER EQUATION Chandler, V.W., Minnesota Geological Survey, 2642 University Ave. St. Paul, MN, chand004@umn.edu Among the most commonly derived estimates from aeromagnetic data are depths to the top of magnetic sources, or ―magnetic basement‖. Originally these estimates were done graphically using profile records or contour maps, but in recent decades a variety of computer-based techniques have been developed. One particularly robust approach is based on Euler‘s homogeneity equation; which is iteratively solved within windowed subsets of gridded aeromagnetic data, thereby providing estimates of source locations in three-dimensional space (Reid and others, 1990). The Euler method requires no a priori information regarding source magnetization, and only requires selecting a structural index (SI), which is the power of the rate of field change that is sensitive to source-type. For example a SI=1 is consistent with a thin sheet (dike or sill) whereas a SI of 0.5 is commonly used to approximate contact solutions. The Euler Method is used here to investigate the Precambrian geology and upper crustal structure of Minnesota and adjacent areas. This study employs the ―located‖ Euler approach (Geosoft Inc.), in which the analytical signal is used to focus the Euler analysis to areas maximum anomaly gradients, where solutions are most likely to be accurate. Euler analysis of an aeromagnetic grid that has been continued to a relatively low level of 300 meters yields abundant information about sources at or near the Precambrian bedrock surface. In addition to providing thousands of estimates of depth, the Euler solutions form numerous chains of solutions that can facilitate geologic mapping. Not surprisingly, areas of near-surface Precambrian rocks are usually associated with noticeable concentrations of shallow depth estimates, but other solutions in these and in deeper bedrock areas indicate that some anomaly sources may actually lie 100‘s of meters below the Precambrian surface. Euler analysis of aeromagnetic data that were smoothed by upward continuing to a level of 1000 meters was useful for investigating deeper (1000-5000 meters) features, such as the Paleoproterozoic Animikie basin and the upper Keweenawan basins of the Mesoproterozoic Midcontinent Rift. Euler analysis of the Animikie basin using a structural index of 1 yields results that are comparable to earlier magnetic-based estimates, and indicates that the deeper parts of the Animikie basin contain 2500 to 4500 meters of non-magnetic strata. The deepest part of the Animikie basin lies to the east, along the basal contact of the Duluth Complex, perhaps reflecting a regional tilting related to Keweenawan rifting. Estimate of dip along this apparent slope averages around 5 degrees. A shelf along the northwestern margin of the Animikie basin corresponds to the west-southwestward extension of the Virginia Horn. Along the Midcontinent Rift, Euler analysis with a structural index of 0.5 indicates that sedimentary basins east and west of the St. Croix Horst typically contain 3-5 km and 2-3 km thick packages of strata, respectively (Figure 1), whereas the basins atop the horst typically have 1-2 km (Ashland Syncline) and 3-5 km (ancestral Twin Cities basin) of strata. These estimates are generally consistent with those using seismic data (Chandler, 1989; Allen, 1994), and they provide some new insights on those parts of the rift that lack seismic data. For example, the Euler results imply an unusually deep (>5000 meters) basin in south-central Minnesota along the Belle Plaine Fault (Figure 1), and further investigation is warranted. 10 �The results of this study indicate that the Euler method is a viable geophysical tool that will help in understanding Minnesota‘s geology in all three dimensions. References Allen, D.J., 1994, An integrated geophysical investigation of the Mid-continent Rift System: Western Lake Superior, Minnesota and Wisconsin: Lafayette, Ind., Purdue University, Ph.D.Dissertation, 267 p. Chandler, V.W., McSwiggen, P.L., Morey, G.B., Hinze, W.J., and Anderson, R.R., 1989, Interpretation of seismic reflection, gravity and magnetic data across Middle Proterozoic Mid-Continent Rift System, Northwestern Wisconsin, eastern Minnesota, and central Iowa: American Association of Petroleum Geologists Bulletin, v. 73, p. 261-275. Mooney, H. M., Farnham, P. R., Johnson, S. H., Volz, G., and Craddosck, C., 1970, Seismic studies over the Midcontinent Gravity High in Minnesota and northwestern Wisconsin: Minnesota Geological Survey Report of Investigations 11. Reid, A. B., Allsop, J. M., Gassner, H., Millet, A. J., and Somerton, I. W., 1990, Magnetic interpretation in three dimensions using Euler deconvolution: Geophysics, v. 55, p. 80-91. Figure 1. Depth to magnetic basement of the Midcontinent Rift in Minnesota and Wisconsin, based on Euler analysis of 1000 meter level aeromagnetic data with a structural index of 0.5. Contour interval is 1000 meters, heavy lines represent seismic refraction profiles by Mooney and others (1970), and lighter lines represent seismic reflection lines used by Allen (1994). SCH, and BPF designate the approximate location of the St. Croix Horst, and the Belle Plaine Fault, respectively. 11 �GEOLOGY AND GEOCHEMISTRY OF THE ROVE BASALTS, A POSSIBLE MIDCONTINENT RIFT-RELATED EXTRUSIVE UNIT SOUTH OF THUNDER BAY, ONTARIO CUNDARI, Robert, HOLLINGS, Peter Department of Geology, Lakehead University 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada, and SMYK, Mark Ontario Geological Survey, Ministry of Northern Development, Mines and Forestry, Suite B002, 435 James St. South Thunder Bay, ON P7E 6S7 Canada A unit of mafic rock in Devon Township, south of Thunder Bay, was mapped by Tanton (1931) and was termed Rove Formation Basalt. Geul (1970) mapped this unit as a Logan diabase sill. The primary objective of this study was to characterize the unit and test whether it is of intrusive or volcanic origin. This was achieved by mapping the unit at a scale of 1:20 000 (Fig. 1), and through petrographic and geochemical analysis. The unit is exposed on a plateau 7 km long and 0.8 to 1.0 km wide. The unit is 4 to 6 m thick and is in apparent conformable contact with the underlying shales of the Paleoproterozoic Rove Formation, where a pronounced chilled margin consists of variolitic material up to 20 cm thick. The flow-top also exhibits a variolitic texture up to 15 cm thick. The presence of ropy flow top and amygdules, as well as quench textures, support a volcanic origin. Figure 1: Map of the Study Area at 1:20000 scale (modified after Tanton, 1931) Major element chemistry reveals a tholeiitic, intermediate composition with samples plotting in the basaltic andesite to andesite fields as well as in the basaltic trachy-andesite to trachy-andesite fields on a TAS diagram. The unit typically has an intergranular texture consisting of randomly oriented plagioclase laths with interstitial chlorite, an alteration product of primary augite. Most samples contain minor serpentine (after olivine), opaque minerals, secondary quartz, oxides, pyrite and calcite. Amygdules are present in most samples and are infilled with some combination of calcite, quartz, chlorite and pyrite. Lower flow contacts and flow-tops are typically glassy with abundant spherulites that sometimes coalesce into bands. Rare-earth element geochemistry shows the unit to be relatively enriched in both HREEs and LREEs, similar to the ultramafic sills of the Nipigon Embayment as well as the Riverdale Sill 12 �(Hollings et al., 2007, 2009) (Fig. 2). A primitive mantle-normalized REE plot shows that the volcanic unit is characteristic of an Ocean-Island Basalt (Fig. 3), but with a negative Nb anomaly, most likely the result of lower crustal contamination. This evidence is further supported by an epsilon Nd(T=1100Ma) of -3.48, which also suggests contamination of the unit by a lower crustal source. The trace element characteristics of the volcanic unit suggest an origin in Keweenawan time as they are geochemically similar to volcanic units of the Midcontinent Rift (Hollings et al., 2007) rather than Paleoproterozoic volcanic units of the Gunflint Formation. Figure 2: Gd/Ybn versus La/Smn showing comparison between the study unit and other Keweenawan Midcontinent rift-related units. Fields from Hollings et al., (2007, 2009). Figure 3: Comparison of primitive mantlenormalized plots from the study unit (A) with Ocean Island Basalt (OIB) (B) and Gunflint volcanic rocks (C). OIB data from Hollings et al. (2007), and Gunflint data from Fralick (unpublished). Field, petrographical and geochemical analysis suggests that the unit is of volcanic origin with emplacement likely during Keweenawan Midcontinent rifting. Thus, we are proposing that the unit be renamed the Devon Volcanics. References Geul, J.J.C., 1970. Geology of Devon and Pardee Townships and the Stuart Location; Ontario Department of Mines, Geological Report 87, 52 p. Hollings, P., Hart, T., Richardson, A. And MacDonald, C.A., 2007a. Geochemistry of the mid-Proterozoic intrusive rocks of the Nipigon Embayement, northwestern Ontario. Canadian Journal of Earth Sciences 44: 1087-1110. Hollings, P., Smyk, M., Halls, H. and Heaman, L. 2009. Mesoproterozoic Midcontinent Rift-related mafic intrusions in northwestern Ontario: continuing geochemical, paleomagnetic, petrographic and geochronologic studies; in Institute on Lake Superior Geology, Proceedings and Abstracts, v.54, part 1, p.42-43. Tanton T.L., 1931. Pigeon River area, Thunder Bay District; Geological Survey of Canada, Sheet 1, Map 354A, scale 1:63360. 13 �GEOLOGY AND GEOCHEMISTRY OF THE HEMLO EAST PROPERTY, SCHREIBER-HEMLO GREENSTONE BELT, ONTARIO FAGE, Adam and HOLLINGS, Pete, Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1, Canada The Hemlo East property, owned by Metalcorp Limited, is located approximately 50 kilometres east of the town of Marathon, Ontario and is situated within the Schrieber-Hemlo greenstone belt of the Wawa subprovince of the Archean Superior Province. Hemlo East has been extensively explored over the past 40 years, mainly being examined for Hemlo-type gold deposits. The property is sandwiched between the Cedar Lake Pluton (2688 – 2687 Ma, Corfu and Muir, 1989) to the north and the Pukaskwa Batholith (2719 and 2688 Ma, Corfu and Muir, 1989) to the south. The geology is dominated by generally east trending and north dipping metasedimentary rocks with mafic to intermediate metavolcanic rocks and minor felsic metavolcanic rocks. High level intrusive dykes and Proterozoic diabase dykes crosscut all lithologies. The entire SchreiberHemlo Greenstone belt has been affected by lower to mid amphibolite facies metamorphism (Pan and Fleet, 1993). Tholeiitic basalts are the prevalent volcanic rock type. Mg# varies from 64 to 36 over a SiO 2 range of 45-54 wt%. Cr and Ni correlate with Mg# while Fe, Ti, Zr and all REE have a negative correlation (Fig. 1). These rocks are characterized by a near-flat pattern on a primitive mantle normalized plot and have been described as having a plume-derived oceanic plateau association by Polat (2008). Tholeiitic to calc-alkaline basalts, andesites, rhyodacites, and rhyolites make up a minor portion of the volcanic rocks and are characterized by LREE enrichment as well as Nb depletion; relative to Th and La (Fig. 1). Rhyolites also exhibit HREE depletion. These rocks posess a subductionderived oceanic island arc association (Polat, 2008). Preliminary zircon 206Pb/238U age dating on a rhyolite yielded an age of 2694.5±1 Ma. Depleted initial Nd isotopic compositions (εNd =+1.76 to +2.11) are consistent with long-term depleted heterogeneous mantle sources (Polat, 2008). The presence of both oceanic plateau and island arc associations of the volcanic rocks can be explained with a subduction zone at the edge of an oceanic plateau which led to rifting and the formation of a backarc basin; this is consistent with the findings of Polat (2008). 14 �Rock/Primitive Mantle 1000 Ocean Plateau Basalts Andesites Ocean Island Arc Basalts Rhyolites and Rhyodacites 100 10 1 0.1 Th Nb La Ce Pr Nd Zr Hf Sm Eu Ti Gd Tb Dy Y Ho Er Tm Yb Lu Al V Figure 1. Primitive mantle normalised diagram for average compositions of volcanic rocks of the Hemlo East property (normalising values from Sun and McDonough, 1989). References: Corfu, F. and Muir, T.L., 1989. The Hemlo-Heron Bay greenstone belt and Hemlo Au-Mo deposit, Superior Province, Ontario, Canada. I: Sequence of igneous activity determined by zircon U-Pb geochronology. Chemical Geology. 79:183-200. Fleet, M.E. and Pan, Y.,, 1991. Metamorphic Petrology of the White River Gold Prospect, Hemlo Area. Ontario Geological Survey, Grant 305, Final Report. 47 pp. Sun, S.-s., and McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In Magmatism in the ocean basins. Geological Society, Special Publication No.42: 313-345. Polat, A., 2008. The geochemistry of the Neoarchean (ca. 2700Ma) tholeiitic basalts, transitional to alkaline basalts, and gabbros, Wawa Subprovince, Canada: Implications for petrogenetic processes. Precambrian Research. 168:83-105. 15 �GEOLOGIC MAPPING OF ARCHEAN AND PROTEROZOIC ROCKS NEAR BINGSHICK LAKE, BOUNDARY WATERS CANOE AREA WILDERNESS, BY STUDENTS OF THE PRECAMBRIAN RESEARCH CENTER’S 2009 FIELD CAMP FAHRENKROG, Brooke1, TOTENHAGEN1, Mike, JARET1, Steven , WATSON1, Tabitha, and JIRSA2, Mark A. 1 2009PRC Students 2 Minnesota Geological Survey The Precambrian Research Center—a branch of the University of Minnesota, Duluth—conducted its third season of field camp in 2009. After 5 weeks of field training, students were assigned ―Capstone Projects‖ that provide an opportunity to create new geologic maps in areas of poorly understood geology. The students mapped Archean and Proterozoic bedrock in the Bingshick Lake area (Figure 1) that was impacted by forest fires in 2006 and 2007. The fires greatly improved access to and visibility of geologic features, allowing detailed mapping of rock units and illuminating complex contact relationships. Figure 1. Regional geology (simplified from Jirsa and Starns, 2008) showing map area of 2009 PRC students. Listed below, from youngest to oldest, are the principle map units: Mesoproterozoic Heterogeneous troctolite of the Tuscarora Intrusion of Duluth Complex: Light grey to grey, medium to coarse grained, heterogeneous troctolite with localized pegmatitic patches and ophitic clinopyroxene. The 16 �highly variable unit contains localized zones of augite-troctolite, mela-troctolite, and troctolite, with sulfide-rich patches occurring locally. Light orange oxidation staining occurs on weathered surfaces. Diabase dike (age uncertain): Consists of fine- to medium-grained, dark-colored mafic rock, with aphanitic chilled contacts against country rocks. Paleoproterozoic Gunflint Iron Formation: Grey to white, fine to coarse grained, Superior-type banded iron-formation. The unit is weakly to strongly magnetic. It consists of alternating medium regular-bedded slaty magnetite rich beds and massive cherty beds locally containing magnetite halos, diffuse structures and intraclasts of slate. Coarse magnetite and quartzite grains were observed near baked zones along the contact with younger troctolite. Red oxidation surfaces occur along the baked zone and on weathered surfaces. Rove Formation: Light to medium gray, brecciated or conglomeratic, biotite-rich schist. Neoarchean Trachyandesite of the Jasper Lake sequence (of Jirsa and Starns, 2008): Tan, fine to medium grained, heterogeneous trachyandesitic volcanic conglomerate and breccia. Angular to subangular fragments (up to 1 foot in diameter) ranging widely in composition from felsic to intermediate to mafic. Interbedded layers (as much as to 1 foot in thickness) of siltstone occur locally. Light chlorite and actinolite alteration occur throughout unit. Metagabbro of the Paulsen Lake sequence (of Jirsa and Starns, 2008): Grey, medium-grained, subophitic metagabbro. The unit displays what appears to be chlorite-actinolite alteration that locally replaced as much as 40% of precursor pyroxene. Metabasalt of the Paulsen Lake sequence: Light green to grey, aphanitic to fine-medium grained, metabasalt, with heavy chlorite and quartz-epidote alteration present throughout. The unit is weakly foliated and alternates between pillowed, massive, and pillow breccia flows. The most significant findings of our field research involved more accurate positioning of previously defined contacts, and investigating those contacts to shed light on the apparent age relationships between units. The main findings are as follows: 1) Discovery of an angular unconformity between pillowed and autobrecciated mafic-ultramafic flows of the Archean Paulsen Lake sequence and younger, but also Archean volcanic conglomerate of the Jasper Lake sequence. Some of the alteration in the older volcanic strata beneath the unconformity may be the result of paleoweathering during the hiatus. These attributes are similar to those described by Jirsa and Driese, 2009 (ILSG) for the post-2689 Ma erosional unconformity between the Ogishkemuncie conglomerate and Saganaga Tonalite. 2) Discovery of breccia at the stratigraphic top of Gunflint Iron Formation that may be related to 1850 Ma Sudbury meteorite impact event. The breccia occurs within a few hundred feet of the basal Duluth Complex, and therefore is highly metamorphosed. 3) The observation at the western edge of the map area that cherty units of the Gunflint Iron Formation lie directly on eroded Archean bedrock, with no intervening clastic horizon. This contrasts with areas to the east near the Gunflint trail and west on the Mesabi Iron Range where clastic units (Kakabeka Conglomerate and Pokegama Quartzite) occur at the unconformity. REFERENCES Jirsa, M.A., and Driese, S.G., 2009, Neoarchean weathering and atmospheric pO2 inferred from paleosaprolite between granite-greenstone and superjacent conglomerate in the Boundary Waters Canoe Area, NE Minnesota: Institute on Lake Superior Geology Proceedings 55:50-51. Jirsa, M.A., and Starns, E.C., 2008, Preliminary bedrock geologic map of the 2006 Cavity Lake fire area, parts of Ester Lake, Gillis Lake, Munker Island, and Ogishkemuncie Lake 7.5 minute quadrangles, northeastern Minnesota: Minnesota Geological Survey Open-File Report OF08-05, scale 1:24,000. 17 �SEDIMENTOLOGY OF THE PALEOPROTEROZOIC EJECTA LAYER FROM THE SUDBURY IMPACT EVENT Fralick, Philip, Department of Geology, Lakehead University, Thunder Bay, ON, P7B 5E1 Brumpton, G.R., 211 Henry St., Thunder Bay, ON, P7E 4Y7 Jirsa, M.A., Minnesota Geological Survey, 2642 University Ave., St. Paul, MN, 55114-1057 Kissin, S.A., Department of Geology, Lakehead University, Thunder Bay, ON, P7B 5E1 Severson, M.J., Natural Resources Research Inst., 5013 Miller Trunk Hwy., Duluth, MN 55811 Assistance in the field was provided by: W.D. Addison, W.F. Cannon, K.J. Schulz and L.G. Woodruff At 1850 Ma an object, probably in excess of ten kilometers in diameter, struck the edge of Superior Craton in the area that is now Sudbury, Ontario. The impact produced the second largest known crater on the surface of the Earth, propelling an immense amount of material into the atmosphere. The area four to seven hundred kilometers west of the impact site consisted of a peneplained terrain that was sub-aerial in its northern extremity, with lithified Gunflint Formation bedrock, and possibly flooded with very shallow marine conditions in its central and southern portions. The ejecta cloud swept across this setting, entraining locally derived material. In places the impact layer deposited by this surge was altered by possible tsunami activity, sub-aerial reworking and diagenesis, and fifteen million years later was buried by sediment of the transgressing ocean. Addison et al. (2005) first discovered this layer, which has since been described at other locations by Pufahl et al. (2007), Cannon et al. (2010) and Addison et al. (in press). To date over twenty outcrop and core sections through the layer have been found. It is extremely laterally variable, ranging from totally absent in some sections to tens of meters in thickness in others. Sections within a few hundred meters of one another can be composed of different material with contrasting textures and sedimentary structures. Common components of the deposits are: 1) pebbles to boulders of predominantly locally derived chert and carbonate, 2) pebble to sand-sized devitrified glass, 3) 3 to 20mm accretionary lapilli, 4) unshocked quartz and feldspar grains, and 5) quartz with planar deformation features (PDFs). Where pebble-sized devitrified glass is common accretionary lapilli are usually absent. Conversely, sections with lapilli in grain-support commonly do not contain devitrified glass. Massive carbonate and silica replacement has occurred. In sub-aerially exposed sections blocky calcite cements developed first, then this was overprinted (including lapilli) by quartz and iron carbonate. This lithified material occasionally locally collapsed forming boulder piles along what were probably fault scarps. The two most typical types of successions are: 1) up to 7 meter thick boulder conglomerates with devitrified glass +- lapilli, and 2) thin (averaging approximately 50 cm) graded sand- to granule-sized material with lapilli. The thick successions commonly overlie fractured to shattered bedrock with large rotated blocks. In other areas the boulder conglomerates overlie what had been water-saturated sediment that liquefied allowing sinking of interstratified, brecciated, chert layers. Models suggest this area would have experienced an earthquake of approximately magnitude 10, quite capable of producing these effects. The overlying disorganized pebble to boulder conglomerate contains clasts up to 3.5 meters in length that are commonly in matrix support, but in some portions of the sections are in clast support. Pebbles of devitrified glass can be mixed throughout the matrix or may appear only in the upper half of the conglomerate where they are commonly small pebble- to granule-sized. In some sections the conglomerates are organized as a series of stacked lenses that decrease in size upwards with some small lenses near the top filled with the first appearance of accretionary lapilli. In other sections lapilli are scattered between the disorganized boulders of the massive conglomeratic deposit. At some locations, approximately one meter of clast-supported, parallel laminated to trough cross-stratified lapilli, with possible chute and pool structures, interlayered with sand- to granule-sized material, overlies the conglomerate. Alternatively, at some locations it is overlain by a massive layer of devitrified glass 18 �pebbles or a thick assemblage of massive or large-scale cross-stratified, sand-sized material. These thicker successions appear to fill depressions. The thinner sections of impact related debris mostly overlie intact bedrock, which apparently was swept clean of the earthquake rubble. They commonly show grading, consisting of either: 1) one or more successions that are normally graded from sand and granules, with lapilli, to silt, or 2) are reverse then normally graded with the granules and lapilli concentrated in the middle of the tens of centimeters thick unit. Without modern analogues interpretation of these successions is difficult. The presence of areas with slickensides and chute and pool structures in some of the outcrops denotes deposition by a high velocity base surge, whereas multiple graded units in some drill-holes are possibly the product of tsunami activity. It is unlikely some other sections represent tsunami deposits as the accretionary lapilli and glass fragments would have arrived in the area via ballistic trajectories and the surge cloud prior to arrival of the tsunami. Thus, any tsunami deposit should have some of this material incorporated throughout it. However, in many sequences the glass and lapilli are not present near the base, making tsunami deposition of sediment in these sections unlikely. Alternatively, in base surges, steam with few solid particles forms the leading front of the cloud followed by the ejecta-rich portion. It is reasonable to visualize the earthquake-generated loose debris being initially redeposited in hollows by the steam-surge, with glass and lapilli deposited from the main surge cloud mixed with the local sediment capping this. As the velocity of the debris cloud decreased and the terrain developed at least a partial covering of impactrelated sediment the abundance of locally derived chert and carbonate grainstone clasts diminished and better sorted layers rich in either glass or accretionary lapilli developed. A further decrease in the velocity of the blast cloud resulted in a transition from pebble- and granule-sized material to sand-sized glass fragments and other debris. Tsunami reworking of these successions into more chaotic masses in some areas is possible. Deposition on a locally fault brecciated erosional surface with topographic relief led to rapid lateral changes in lithofacies and thickness. It is likely that the thicker, coarser basal units are confined to topographic lows, as, in places, they can be seen to pile-up against bedrock cliffs facing the impact site. The thin, reverse-graded ejecta layers are typical of small volume base surges. These probably formed from the lower density tail of the flow on topographically higher areas that were erosively swept clean by the main flow. Reworking of some of the deposits in whole or in part by gravity sliding or possibly fluvial activity is also indicated. References Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Davis, D.W., Kissin, S.A., Fralick, P.W. and Hammond, A.L., 2005. Discovery of distal ejecta from the 1850 Ma Sudbury impact event. Geology, v. 33, p. 193-196. Addison, W.D., Brumpton, G.R., Davis, D.W., Fralick, P.W. and Kissin, S.A., in press. Debrisites from the Sudbury impact event in Ontario, north of Lake Superior, and a new age constraint: are they base surge deposits or tsunami deposits? In, eds. W.U. Reimold and R.L. Gibson, Large Meteorite Impacts and Planetary Evolution IV, Geological Society of America Special Paper 465. Cannon, W.F., Schulz, K.J., Horton, J.W. and Kring D.A., 2010. The Sudbury impact layer in the Paleoproterozoic iron ranges of northern Michigan, U.S.A. Geological Society of America Bulletin, v. 122, p. 50-75. Pufahl, P.K., Hiatt, E.E., Stanley, C.R., Morrow, J.R., Nelson, G.J. and Edwards, C.T., 2007. Physical and chemical evidence of the 1850 Ma Sudbury impact event in the Baraga Group, Michigan. Geology, v. 35, p. 827-830. 19 �ANATOMY OF AN ARCHEAN CARBONATE PLATFORM: GEOLOGY OF THE STEEP ROCK GROUP, CANADA FRALICK, Philip, Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada, philip.fralick@lakeheadu.ca And RIDING, Robert, Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN, USA Carbonate platforms capping oceanic islands and plateaus are not uncommon in the modern ocean. However, they are rare in the rock record of the Archean. The 200 to possibly 500 meter thick succession of calcium carbonate rocks present at the long abandoned Steep Rock iron mine, Ontario, Canada, is one of the few such examples of an extensive Mesoarchean-Neoarchean limestone platform. The approximately 2.8 Ga sedimentary assemblage is dominated by limestones that unconformably overlie 3.0 Ga tonalitic gneiss incised by a paleochannel network backfilled with fluvial sandstones and conglomerates. The lower meters of the limestone contains scattered tonalitic sand and gravel, but the remainder has very low amounts of siliciclastic material. Wilks and Nisbet (1985, 1988) have provided excellent descriptions of the Steep Rock platform succession; from Stratifera-like and Irregularia-like structures that pass upward to laterally linked hemispherical stromatolites, branching walled and unwalled columnar forms and giant domes. They also noted the presence of crystal fans, which are probably the deposits originally described as the supposed fossil Atikokania by Walcott (1912). The present study adds details of lithotypes, and suggests the presence of grainstones in the interval underlying the giant domes and possibly interlayered with some domes. Mudcrack-like structures were observed on the upper surface of one dome. Giant domes dominate the upper 70 m of the succession. They are persistently elongated, suggesting current-influence, and range up to 5 m in length, 2 m in width and 1.4 m in height. Inter-dome sediment is usually lacking. The giant domes are distinctly banded and composed of centimeter-decimeter layers of crystal fans alternating with cuspate fenestral fabric (fenestrate microbialite, Sumner, 2000; Sumner and Grotzinger, 2000). Large domes are not developed in lithologies that lack cuspate fenestral fabric, and these show a variety of centimetric interlayers that include wavy, irregularly fenestral, and possibly granular fabrics, together with thin sheet cracks. Twenty-seven samples from various lithofacies were analyzed for major and trace elements and carbon isotope ratios. Carbon varies from delta 0 to 2.7, with the lightest values consistently present in the columnar stromatolites and the heaviest in the crystal fans. There is a progression from lighter carbon isotope values at the base of the section upward to heavier values at the lowest zone of crystal fans. Above this the values rapidly decline to near 0 where the columnar stromatolites are located, and then gradually rise again to the heavy carbon enriched upper third of the section dominated by the crystal fans. Strontium and barium concentrations have similar trends, whereas the concentrations of iron, and to a lesser extent manganese, behave in an inverse manner. Iron carbonate present in iron formation overlying the limestone has low levels of Sr and Ba, and contains significant amounts of Fe and to a lesser degree Mn. Its carbon isotopic ratio is -5.6, which is similar to values derived from other iron formations of similar age. 20 �Fralick et al. (2008) suggested that the oceanic plateau was drowned during isostatically driven subsidence. It is clear that carbonate deposition was able to keep pace with sea-level during most of the subsequent load-driven subsidence of the carbonate platform. The dominance of calcite rather than an iron carbonate in the sequence indicates lower levels of Fe+2 dissolved in the water over the shallow plateau than deeper portions of the plateau where siderite was accumulating. This possibly reflects a sub-oxic, shallow water zone created by photosynthetically released oxygen together with restricted circulation above a chemocline. Heavier carbon isotopic ratios could reflect evaporitic loss of C12 leading to aragonite crystal fan precipitation. Low Sr contents indicate that the non-crystal fan carbonates were deposited as calcite, except for the columnar stromatolites that are dolomite and may have been deposited as dolomite in the low sulfur Archean ocean. These stromatolites also have the lowest C isotopic ratio and high concentrations of iron and manganese. This suggests less saline conditions and better connection with the open ocean during deposition of the columnar stromatolites. A possible relationship between stromatolite morphology and salinity further indicates the organisms that constructed these features may not have been very salt tolerant. Conversely, they may have been iron or manganese oxidizers that could not survive when water on the shallow portions of the plateau became more stagnant and concentrations of Fe and Mn in solution dropped. The emerging picture is of an oceanic platform where carbonate deposition outpaced subsidence, sustaining very shallow water conditions that limited incursion of Fe- and Mn-rich open ocean water, increased salinity, and promoted precipitation of aragonitic fans. Carbonate deposition ended when the balance controlling water level switched in favor of relative sea-level rise. Deeper water conditions allowed the chemocline to advance over the area and iron formation deposition ensued References Fralick, P., Hollings, P. and King, D., 2008. Stratigraphy, geochemistry and depositional environments of Mesoarchean sedimentary units in western Superior Province: Implications for generation of early crust. In, Ed. K.C. Condie and V. Pease, When did Plate Tectonics Begin on Planet Earth? G.S.A. Spec. Paper 440, p. 77-96. Sumner, D.Y., 2000. Microbial vs environmental influences on the morphology of late Archean fenestrate microbialites. In, Ed. R.E. Riding and S.W. Awramik, Microbial Sediments, Springer, Berlin, p. 307-314. Sumner, D.Y. and Grotzinger, J.P., 2000. Late Archean aragonite precipitation: petrography , facies associations, and environmental significance. In, Ed. J.P. Grotzinger and N.P. James, Carbonate Sedimentation and Diagenesis in the Evolving Precambrian World. SEPM Spec. Pub. 67, p. 123-144. Walcott, C.D., 1912. Notes on fossils from limestones of Steeprock Series, Ontario. Geol. Sur. of Can., Mem. 28, p. 16-23. Wilks, M.E. and Nisbet, E.G., 1985. Archean stromatolites from the Steep Rock Group, northwestern Ontario, Canada. Can. Jour. of Earth Sc., v. 22, p. 792-799. Wilks, M.E. and Nisbet, E.G., 1988. Stratigraphy of the Steep Rock Group northwestern Ontario: A major Archean unconformity and Archean stromatolites. Can. Jour. of Earth Sc., v. 25, p. 370-391. 21 �EFFECTS OF CONTACT METAMORPHISM BY THE DULUTH COMPLEX ON PROTEROZOIC FOOTWALL ROCKS IN NORTHEASTERN MINNESOTA FROST, Shelby J.1, GOODGE, John W.1, SWENSON, John B.1, (1) Department of Geological Sciences, University of Minnesota – Duluth, 1114 Kirby Drive, Duluth, MN 55812, Frost113@d.umn.edu The Duluth Complex is composed of numerous mafic intrusions that were emplaced in northeastern Minnesota during formation of the Midcontinent Rift approximately 1.1 Ga (Miller et. al., 2002). When it intruded, the heat of this igneous body significantly affected the wall rocks around it, including those in the Animikie Group, and created a contact metamorphic aureole. The purpose of this research is to understand crustal conditions associated with emplacement of a large igneous complex, and develop a better idea of the thermal state of the crust during the time of rifting. To understand crustal conditions we must determine the effects that intrusion of the Duluth Complex had on adjacent wall rocks, in particular the Thomson and Virginia formations of the Animike Group. These effects include the extent and grade of metamorphism. Optical petrography was used to determine mineralogical and textural relationships between minerals. The Animikie Group was subjected to two metamorphic events based on sedimentary features such as relict sedimentary bedding, regional metamorphic features such as disrupted bedding, and contact metamorphic features such as porphyroblasts and mortar texture. Scanning electron microscopy and EDS analysis helped identify major metamorphic minerals including rare high-K cordierite. A thermal model was constructed to model heat flow from the Duluth Complex into the wall rocks, and mimic a thermal gradient similar to the one observed in nature. The two-dimensional model was a very simplified version of the parameters of the intrusion observed in nature, and ltimately we obtained discordant results that we can‘t explain. Based on this petrographic study, the contact metamorphic aureole extends from the Duluth Complex into the Animikie Group for approximately 30 m. This coincides with estimates made by Severson (1995) and Duchesne (2004). The aureole is so thin because the wall rocks were fairly cool when the Duluth Complex intruded, and probably had a temperature around 75 C. The peak metamorphic mineral assemblage includes minerals such as ferrosilite, gedrite, and cordierite, which occur at temperatures around 600-700 C and a pressures around 2.5 Kbar (Duchense, 2004). References Duchesne, L., (2004). Fusion partielle et microstructures associees dans l‟aureole de contact du complexe igne de Duluth, Minnesota. University of Quebec Masters thesis, 217 p. Miller, J. D. Jr., and Severson, M. J., (2002) Geology of the Duluth Complex in Miller, J. D. Jr., Green, J. C., Severson, M. J., Chandler, V. W., Hauck, S. A., Peterson, D. M., and Wahl, T. E., (2002). Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota. University of Minnesota Report of Investigations 58. 207 p. Severson, M. J., (1995). Geology of the southern portion of the Duluth Complex. Natural Resources Research Institute Special Report, 217 p. 22 �GEOLOGY AND GEOCHEMISTRY OF ARC AND OCEAN-FLOOR VOLCANIC ROCKS IN THE NORTHERN FLIN FLON BELT, MANITOBA. GILBERT, Paul Manitoba Geological Survey (360-1395 Ellice Ave. Winnipeg Manitoba R3G 3P2) The Flin Flon Belt is one of the most prolific mining districts in Canada, with combined reserves and production to 2007 of 183 million tonnes of Cu-Zn-Au-Ag ore from 26 different deposits. The Flin Flon Belt has a protracted magmatic history spanning approximately 60 million years (Stern et al., 1999). Juvenile arc magmatism (1.90–1.88 Ga) in an intra-oceanic setting was characterized by tholeiitic to calcalkaline and late alkaline (shoshonitic) volcanism, accompanied by rifting and subsequent back-arc basin development. Collisional tectonism, development of a 1.87–1.84 Ma ‗successor arc‘, and late brittle deformation resulted in the present complex structural geometry of the belt. Detailed geological mapping and numerous research projects have focused largely on the central part of the belt where almost all of the ore deposits are located. This paper is based on mapping of the northern Flin Flon Belt (NFFB), in which an approximately 15 km by 35 km area contains more than 20 tectonically distinct blocks or fault slices of diverse volcanic, related intrusive and subordinate sedimentary rocks. Most of these components are akin to modern analogues of arc and MORB volcanic rock types (Figure 1) and are part of the Flin Flon arc assemblage (Stern et al., 1995a). Mid-ocean ridge (MORB)–like volcanic rocks are structurally intercalated with juvenile arc rocks, and an extensive area of depleted-MORB-type rocks (Dismal Lake Terrane) extends along and across the northern boundary of the Flin Flon Belt, where it is locally in fault contact with paragneiss and orthogneiss of the Kisseynew Domain to the north (Gilbert, 2004). Arc-type rocks in the NFFB are assumed to be associated with subduction at a former oceanic arc, whereas MORB and depleted-MORB types are interpreted to have been associated with rifting and emplacement in a back-arc basin environment. Volcanic rocks of the Flin Flon arc assemblage are 1890 to 1878 Ma in age (Lucas et al., 1994; Stern et al., 1999; N. Rayner, pers. comm., 2009). MORB-type rocks in the NFFB may be roughly coeval with the arc assemblage, based on the 1.90 Ga age of synvolcanic intrusions in MORB-type volcanic rocks elsewhere in the Flin Flon Belt (Elbow-Athapapuskow ocean-floor assemblage; Stern et al., 1995b). Six lensoid enclaves of turbiditic sedimentary rocks are structurally intercalated with the volcanic fault blocks in the NFFB. These turbidite deposits, apparently devoid of volcanic interlayers, are distinguished from lithologically similar but older, fine-grained sedimentary rocks that occur within 1890-1878 Ma, juvenile arc volcanic sequences. Detrital zircon data from greywacke within the westernmost turbidite enclave suggest the depositional age likely does not exceed 1860 Ma (N. Rayner, pers. comm., 2009) and that the turbidites are thus of successor-arc age. Turbidite fault slices elsewhere in the NFFB may also be of successor-arc age or relatively older, penecontemporaneous with juvenile arc volcanism (e.g., the 1885 Ma Vick Lake tuff; Stern et al., 1993). Fluvial-alluvial conglomerate and cross-bedded sandstone (Missi Group) at the west margin of the Flin Flon Belt are the youngest known supracrustal rocks in the NFFB. These sedimentary deposits and associated volcanic rocks are unconformable or in fault contact with volcanic rocks of the Flin Flon arc assemblage and were deposited between 1845 and 1835 Ma (Ansdell et al., 1999). 23 �Figure 1: Th-Hf-Nb diagram showing the distinctive fields of arc and MORB-type volcanic rock suites in the northen Flin Flon Belt. Compositional fields of modern volcanic rocks after Wood (1980). Abbreviations: E-MORB, enriched mid-ocean ridge basalt; N-MORB, normal mid-ocean ridge basalt. References Ansdell, K.M., Connors, K.A., Stern, R., and Lucas, S.B., 1999; Coeval sedimentation, magmatism, and fold- thrust belt development in the Trans-Hudson orogen: Geochronological evidence from the Wekusko Lake area, Manitoba, Canada: Canadian Journal of Earth Sciences, vol. 36, p. 293-312. Gilbert, H.P. 2004: Geological investigations in the northern Flin Flon Belt, Manitoba (parts of NTS 63K13NE and 63K14NW); in Report of Activities 2004, Manitoba Industry, Economic Development and Mines, Manitoba Geological Survey, p. 9–23. Lucas, S.B., Stern, R.A., Syme, E.C., Reilly, B.A. and Thomas, D.J. 1996: Intraoceanic tectonics and the development of continental crust: 1.92–1.84 Ga evolution of the Flin Flon Belt, Canada; Geological Society of America Bulletin, v. 108, p. 602–629. Stern, R.A., Lucas, S.B. Syme, E.C., Bailes, A.H., Thomas, D.J., Leclair, A.D., and Hulbert, L. 1993. Geochronological studies in the NATMAP Shield Margin Project area, Flin Flon Domain: results for 1992–1993. In Radiogenic age and isotopic studies: Report 7. Geological Survey of Canada, Paper 93-2, p. 59-70. Stern, R.A., Machado, N., Syme, E.C., Lucas, S.B. and David, J. 1999: Chronology of crustal growth and recycling in the Paleoproterozoic Amisk Collage (Flin Flon Belt), Trans-Hudson Orogen, Canada. Canadian Journal of Earth Sciences, v. 36, p. 1807-1827. Stern, R.A., Syme, E.C., Bailes, A.H. and Lucas, S.B. 1995a: Paleoproterozoic (1.90–1.86 Ga) arc volcanism in the Flin Flon Belt, Trans-Hudson Orogen, Canada; Contributions to Mineralogy and Petrology, v. 119, p. 117–141. Stern, R.A., Syme, E.C. and Lucas, S.B. 1995b: Geochemistry of 1.9 Ga MORB- and OIB-like basalts from the Amisk Collage, Flin Flon belt, Canada: evidence for an intra-oceanic origin; Geochimica et Cosmochimica Acta, v. 59, p. 3131–3154. Wood, D.A. 1980: The application of a Th-Hf-Ta diagram to problems of tectonomagmatic classification and to establishing the nature of crustal contamination of basaltic lavas of the British Tertiary Volcanic Province. Earth and Planetary Science Letters, v. 50, p. 11-30. 24 �U-PB ZIRCON GEOCHRONOLOGY OF THE DULUTH COMPLEX AND RELATED HYPABYSSAL INTRUSIONS: INVESTIGATING THE EMPLACEMENT HISTORY OF A LARGE MULTIPHASE INTRUSIVE COMPLEX RELATED TO THE 1.1 GA MIDCONTINENT RIFT Hoaglund, S.A.a*, Miller, J.D.a, Crowley, J.L.b, and Schmitz, M.D.b a Department of Geological Sciences, University of Minnesota Duluth, Duluth, MN 55812 USA b Department of Geosciences, Boise State University, Boise, ID 83725 USA Previous geochronology of mafic intrusive magmatism associated with the 1.1 Ga Midcontinent Rift in NE Minnesota by Paces and Miller (1993) established, with high precision, the main intrusive periods that created the Duluth Complex and related intrusions. This study did not, however, resolve differences in emplacement ages within and between major intrusive units due to a small number of dated samples. We present new high-precision isotope dilution U-Pb baddeleyite and CA-TIMS zircon ages from four mafic intrusions that build on the work of Paces and Miller (1993). Three of these are from troctolitic intrusions (Partridge River, Bald Eagle, Tuscarora) belonging to the layered series of the Duluth Complex (DC) that, based on field relationships, appear to represent the entire range of layered series intrusive activity. They yielded irresolvable weighted mean Pb207/Pb206 ages between 1098.8 ± 0.3 and 1098.0 ± 0.7 Ma (Table 1). These ages fall in line with the 1098.6 ± 0.5 Ma and 1099.3 ± 0.3 Ma ages reported by Paces and Miller for two layered series intrusions (Figure 1). Moreover, these ages are irresolvable from the ages of two DC anorthositic series samples analyzed by Paces and Miller (1099.0 ± 0.6 Ma and 1099.1 ± 0.5 Ma) and recently confirmed by unpublished data from the Boise State University isotope laboratory. An early intrusion from the Beaver Bay Complex (BBC), the Houghtaling Creek troctolite, was also dated in this study to test the implication from Paces and Miller‘s study that a 3 Ma magmatic hiatus exists between emplacement of the deeper seated DC and the more hypabyssal BBC. The Houghtaling Creek sample yielded an age, which is irresolvable from the Duluth Complex ages and thus implies that BBC and DC magmatism likely overlaps. The evidently synchronous emplacement of layered series and anorthositic series intrusions indicate that voluminous main stage magmatism occurred during an extremely short interval of within 1 Ma. Reasonable estimates of the geometry of the dozen or so layered series intrusions that comprise the DC indicate that over 15,000 km3 of mafic magma intruded during emplacement of the layered series alone. The total volume of intruded magma is likely much higher when erosion is factored in and rocks of the anorthositic series and early BBC are included. 25 �Table 1. Summary of high-precision U-Pb zircon/baddeleyite data for the Duluth Complex and Beaver Bay Complex intrusions. All errors are reported at the 95% confidence interval. Sample Namea WLFG HCT BEI PRI TI ES Stratigraphic Unit Rock Type Early Phase (BBC) Early Phase (BBC) Late Phase (DC) Early Phase (DC) Early Phase (DC) Misc. intrusion ferrogabbro augite troctolite olivine gabbro augite troctolite troctolite augite troctolite Grain Type zrc./bad. zrc. zrc. zrc. bad. zrc. Pb207/Pb206 age (Ma) 1095.75 ± 0.92 1098.62 ± 0.50 1097.97 ± 0.72 1097.98 ± 0.37 1098.81 ± 0.32 not defined Pb206/U238 age (Ma) 1091.88 ± 0.35 1095.31 ± 0.25 1095.64 ± 0.19 1095.94 ± 0.18 not defined not defined a WLFG = Wilson Lake ferrogabbro; HCT = Houghtaling Creek troctolite; BEI = Bald Eagle intrusion; PRI = Partridge River intrusion; TI = Tuscarora intrusion; ES = Endion sill Figure 1. Summary of weighted mean Pb207/Pb206 ages for Duluth Complex and Beaver Bay Complex magmatism from Paces and Miller (1993) and this study (modified from Miller, 2008, unpublished figure). 26 �RADIOGENIC ISOTOPE CHARACTERISTICS OF MIDCONTINENT RIFTRELATED INTRUSIONS SOUTH OF THUNDER BAY HOLLINGS, Pete, Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1, Canada, SMYK, Mark C., Ontario Geological Survey, Ministry of Northern Development, Mines and Forestry, Suite B002, 435 James St. South, Thunder Bay, ON P7E 6S7 Canada. New geochronological, geochemical and paleomagnetic data for Mesoproterozoic Midcontinent Rift-related dikes and sills located in and south of Thunder Bay, Ontario demonstrate the presence of three sill suites: an earlier, spatially restricted mafic to ultramafic unit termed the Riverdale sill, the predominant Logan sills and Nipigon sills in the northern part of the study area (Hollings et al., 2010). In addition, three predominant dike sets are recognized: the northeasttrending Pigeon River swarm, the northwest-trending Cloud River dikes and the Mt. Mollie dike (Fig. 1). Figure 1: Geological map of the study area showing the location of major intrusive suites. Modified after Pye and Fenwick (1965). Thirty-four Rb-Sr and Sm-Nd isotope analyses were undertaken on the intrusive units south of Thunder Bay with the full data set reported in Smyk and Hollings (2009). Initial 87Sr/86Sr ratios (Sri) for the Logan Sills range from 0.7048-0.7115 and epsilon Nd(T=1100Ma) from -1.4 to +0.4, whereas the Riverdale Sill is characterised by smaller range of Sr i (0.7037-0.7055) and more negative epsilon Nd(T=1100Ma) (-1.6 to -1.9). The epsilon Nd values for both suites are broadly similar to those of the Nipigon sills reported by Hollings et al. (2007) but the Logan sills are characterised by significantly higher Sri values (Fig. 2). TDM values for the Logan and Riverdale sills are in the range of 1750-1850 Ma which are considerably younger than the generally Archean TDM values of the sills of the Nipigon embayment. The younger TDM and the elevated 27 �Sri of the southern sill suites are consistent with contamination by Rove shales which have Sr i of up to 0.7207 and epsilon Nd(T=1100Ma) of -9.3 (Fig. 2). All three dike suites are characterised by highly variable Sr i and epsilon Nd(T=1100Ma) values with epsilon Nd ranging from positive to negative in all three suites (Fig. 2). Samples with strongly negative epsilon Nd values are also characterised by strongly enriched Th abundances, negative Nb anomalies and older (~2200 Ma) TDM ages, consistent with contamination by a deeper and older source, likely Archean basement. The dikes with less negative epsilon Nd values have similar major and trace element geochemistry to the strongly contaminated samples suggesting they were derived from the same mantle source but were emplaced after limited interaction with the underlying crust. This supports geochronological data presented by Hollings et al. (2010) which suggested that the dike swarms were emplaced over an extended period. Figure 2: 87 Sr/86Sr i and eNd(T=1100Ma) for intrusive Midcontinent Rift rocks south of Thunder Bay. Fields from Hollings et al. (2007). References Hollings, P., Richardson, A., Creaser, R., and Franklin, J., 2007. Radiogenic isotope characteristics of the Midproterozoic intrusive rocks of the Nipigon Embayment, Northwestern Ontario. Canadian Journal of Earth Sciences, 44, 1111-1129. Hollings, P., Smyk, M., Halls, H., and Heaman, L., 2010. The geochemistry, paleomagnetism and geochronology of the dykes and sills associated with the Midcontinent Rift near Thunder Bay, Ontario, Canada. Precambrian Research, in press, doi:10.1016/j.precamres.2010.01.012. Pye, E. G., and Fenwick, K. G., 1965, Atikokan-Lakehead sheet, Kenora, Rainy River and Thunder Bay districts; Ontario Department of Mines, Geological Compilation Series, Map 2065, scale 1 inch = 4 miles. Smyk, M. and Hollings, P., 2009. Project Unit 08-021. Mesoproterozoic Midcontinent Rift–Related Mafic Intrusions Near Thunder Bay: Update. Summary of Fieldwork and Other Activities 2009, Ontario Geological Survey, Open File Report 6240, p. 11-1 to 18-5. 28 �TACONITE-DERIVED MINERAL DUST IN POPULATION CENTERS ON MESABI IRON RANGE: AN UPDATE HUDAK, George, DIEDRICH, Tamara, MONSON GEERTS, Stephen, SCHREIBER, Megan, ZANKO, Larry, and SCHWANKE, Alanna, Natural Resources Research Institute, 5013 Miller Trunk Highway, Duluth, MN, 55811 The Natural Resources Research Institute (NRRI) is conducting a detailed characterization of dust that is produced from mining and processing Biwabik Iron Formation ore. This three-year long study is an effort to evaluate the effects of past and present emissions from taconite mining and processing on air quality throughout the Mesabi Iron Range (MIR). NRRI‘s research will characterize airborne particulate matter within taconite operations, in communities surrounding taconite operations on the Mesabi Iron Range (MIR) and in population centers in other regions of northeastern Minnesota, as well as particulate matter deposited in lake sediments. NRRI‘s sampling and characterization work represents the community/environmental component of a larger University of Minnesota (U of M) effort that is addressing long-standing questions regarding the impact of dust derived from mining and processing of taconite (iron ore) on human health. The U of M School of Public Health (SPH), with whom NRRI is collaborating, is responsible for the human health- and exposure-related components of that effort, which include: 1) an occupational exposure assessment; 2) a mortality study; 3) a cancer incidence study; and 4) a respiratory health survey of taconite workers and spouses. Air sampling is performed within taconite operations and communities by NRRI scientists during both winter and summer seasons. MIR community sampling sites include: 1) Keewatin Elementary School; 2) Hibbing High School; 3) the Virginia Courthouse; 4) the Babbitt Municipal Building; and 5) Silver Bay High School. Population centers in northeastern Minnesota include: 1) the Ely Fernberg site; and 2) the Natural Resources Research Institute in Duluth, MN. Sampling sites within taconite operations include crushers, magnetic separators/concentrators, agglomerators/ball drums, and the kiln/pellet discharge area. Taconite processing plant sites, from west to east, include: 1) U.S. Steel‘s Keewatin Taconite (Keetac); 2) Cliff‘s Natural Resources‘ Hibbing Taconite (Hibtac); 3) U.S. Steel‘s Minntac; 4) Cliff‘s Natural Resources‘ United Taconite (UTAC); 5) ArcelorMittal‘s Minorca Mine; and 6) Cliff‘s Natural Resources‘ Northshore Mine. Particulate matter from air samples is collected using a microorifice uniform deposit impactor (MOUDI) (Marple et al., 1991) in conjunction with a total suspended particulate filter (TSP). Size-fractionated samples collected from the MOUDI, as well as the total filter, are evaluated via gravimetric analysis and subsequently subjected to comprehensive particulate matter characterization that includes: 1) scanning electron microscopy (SEM) imaging; 2) energy dispersive spectroscopy (EDS); 3) electron backscattered diffraction (EBSD); 4) proton induced x-ray emission (PIXE); 5) the Minnesota Department of Health‘s 852 Method Transmission Electron Microscopy (TEM) Analysis for Mineral Fibers in Air; and 6) the International Standard Organization‘s Method 10312 Ambient air – Determination of Asbestos Fibres – Direct-Transfer TEM Method (ISO 10312, 1995). 29 �Additional analyses are completed on relevant metamorphosed and unmetamorphosed Biwabik Iron-Formation diamond drill core. Analyses include: 1) optical and electron microscope studies of polished thin sections; 2) TEM studies of liberated elongated mineral particles produced by the modified elutriator method (Berman and Kolk, 2000); and 3) x-ray diffraction evaluations of crushed material. The lake sediment component of NRRI‘s work relies on studying cores collected from the eastern and western regions of the MIR to offer insights into historic production of mineral dust in the MIR. Core samples obtained from ―North-of-Snort‖ Lake near Babbitt and Silver Lake near Virginia are being analyzed for elongated mineral particles and are being age dated utilizing the 210Pb method to ensure that the oldest sediment samples pre-date MIR mining activities. The NRRI has completed numerous study tasks over the past year-and-a-half. Community air sampling is nearly completed and includes forty-eight sampling events in MIR communities and seven sampling events in the NE Minnesota communities. All operating taconite processing facilities are being sampled, and additional sampling events are planned for the mid-2010, including taconite mine road dust, drilling dust, and mine blasts. Figure 1: Locations of taconite processing plants on the Mesabi Iron Range being sampled during this study (after Oreskovich and Patelke, 2006) References Berman, D. W., and Kolk, A. J., 2000, Draft: Modified elutriator method for the determination of asbestos in soils and bulk materials Revision 1: Submitted to the U.S. Environmental Protection Agency Region 8, May 23, 2000. ISO 10312, 1995, Ambient air – determination of asbestos fibres – direct transfer transmission electron microscopy method, 51p. Marple, V. A., Rubow, K. L., and Behm, S. M., 1991, A microorifice uniform deposit impactor (MOUDI): description, calibration, and use: Aerosol Science and Technology, v. 14, p. 434-446. Oreskovich, J. A., and Patelke, M. M., 2006, Historical use of taconite byproducts as construction aggregate materials in Minnesota: A Progress Report: Natural Resources Research Institute Report of Investigation NRRIRI-2006-02, 10 p. 30 �STRATIGRAPHY OF SUDBURY ―IMPACTITE‖ NEAR GUNFLINT LAKE, NE MINNESOTA Jirsa, Mark A., Minnesota Geological Survey (jirsa001@umn.edu) The impactite near Gunflint Lake is sandwiched between underlying, gently dipping, parallel-bedded Gunflint Iron Formation (~1878 Ma), and overlying slate and siltstone of the Rove Formation. The term ―impactite‖ is a very general one used informally here to include all facies of sedimentary rocks inferred to have been formed or deformed in response to the ca. 1850 Ma Sudbury meteorite impact. By that definition, it includes autochthonous material interpreted to be seismically folded and shattered ironformation (ejecta-absent), and overlying strata composed largely of allochthonous material derived from the impact site (ejecta-bearing) (Fig. 1). This discussion uses stratigraphic observations from recent mapping to devise a sequence of depositional events that is consistent with experimental and empirical evidence of impact processes (e.g., www.lpl.arizona.edu/impacteffects). It builds on—and in some cases should supersede—preliminary results presented in 2008 by Jirsa and others (ILSG v. 54). The impactite locality at Gunflint Lake is one of several similar exposure areas in the Lake Superior region (see Fralick and others, and Cannon and Slack; this volume). All of these deposits exhibit extreme lithologic variability from place to place within each exposure area, and between exposure areas, which is not surprising given the chaotic nature of impact processes. It should not be inferred, therefore, that the interpretations presented here necessarily apply to successions in other areas. Figure 1. Stratigraphic framework derived from 8 exposures along a 2 mile strike-length near Gunflint Lake; hung from the contact between ejecta-bearing and ejecta-absent facies of the impactite horizon (bold dashed line). 31 �Because the impactite deposits near Gunflint Lake lie within the contact metamorphic aureole of Mesoproterozoic intrusions, much of the original mineralogic features are obscured (see Weiblen and others, this volume). Despite this, macroscopic textures and sedimentological details are sufficiently preserved to convey information about protolith and depositional mechanisms. Figure 1 depicts the apparent stratigraphic relationships. In no single outcrop are all facies present; nevertheless, an approximation of temporal relationships can be inferred from the juxtaposition of two or more facies in individual outcrops. In the following section, facies are described in apparent stratigraphic order from oldest to youngest: Ejecta-Absent Contorted iron-formation facies: The uppermost layers of iron-formation are chaotically folded and exhibit both ductile and brittle behavior in close proximity at the scale of individual outcrops. The rheologic response depends on the apparent rigidity of material at the time of deformation. Silica-rich layers display brittle, shattered to semi-ductile, boudinage-like textures. By contrast, much of the ironsilicate mudstone layers behaved in a ductile fashion, locally showing evidence of fluidization and injection into superjacent strata. Folds are non-systematic in trend and style, and multiple hinge detachments occur. These attributes counter-indicate a regional tectonic origin, and instead are best viewed in the context of impact as the result of impact-generated seismicity imposed on semi-lithified substrate. Parautochthonous breccia facies: At several locations, straight-bedded iron-formation passes laterally along strike into irregular zones in which the silica-rich layers have been broken and disheveled, while still retaining some semblance of jigsaw-puzzle fit. Megabreccia facies: This term is used for breccia composed of unsorted slabs (as large as 5 m), blocks, and smaller fragments of iron-formation. The fragments are angular and in most places have random orientations. Fragments of iron-silicate mudstone typically show some evidence of semi-ductile behavior, and locally this material was fluidized to form irregular matrix. Ejecta-Bearing Mesobreccia facies: This is fragmental rock containing angular, subrounded, and amoeboid clasts (up to 5 cm long) of dark green material and scattered accretionary lapilli. Petrography shows that much of the original structure of the clasts has been metamorphically recrystallized and annealed; however, relicts of amygdaloidal and fluid-looking textures remain—implying a glassy protolith. Lapillistone-gritstone facies: Accretionary lapilli as large as 1.5 cm occur as irregular masses and layers interbedded with sandy to silty gritstone. In areas least affected by metamorphism, several grains of shocked quartz with planar deformation features have been identified in thin section. Spherule, pellet, small lapilli facies: The upper parts of ejecta horizons locally contain layers, lenses, and interbeds of accretionary or relict glass grains that are smaller than typical lapilli and generally lack concentric zonation. These may represent waning ejecta plume deposition. Ejecta-bearing conglomerate facies: In a few localities, the uppermost part of impactite deposits consist of conglomerate containing subrounded fragments of iron-formation (in contrast to breccias described above), and matrices containing variably abraded lapilli. Nearly all contacts between individual facies are gradational, with one very important exception—in all exposures, the boundary between ejecta-absent and ejecta-bearing facies is extremely sharp. This is interpreted to reflect a fundamental shift in geologic process from intense seismic perturbation of uppermost iron-formation represented by the ejecta-absent facies, to deposition by the passing ejecta plume. The uppermost conglomerate facies represents mixing of local and exotic detritus, presumably by tsunamis or post-impact fluvial or marine processes. This interpretation was strongly influenced by field discussions with William Cannon, William Addison, Phillip Fralick, and Christian Koeberl, and the pioneering work of Bevan French. 32 �GEOCHEMICAL CHARACTERISTICS OF CADMIUM-RICH SOILS OF THE PEMBINA ESCARPMENT, NORTH DAKOTA JYOTI, Vijaya and SAINI-EIDUKAT, Bernhardt, Department of Geosciences and Environmental & Conservation Sciences Program, North Dakota State University, Fargo, ND 58108, HOPKINS, David and DESUTTER, Tom, Department of Soil Science, North Dakota State University, Fargo, ND 58108. Investigations of topsoils in northeastern North Dakota (Hopkins et al., 1999) revealed that trace element levels of cadmium (Cd), are 10 to 20 times higher than the baseline level of 0.3 mg/kg total Cd established by Garrett (1994) for northern prairie soils of Canada and the adjoining states of United States. North Dakota produces 96% of the nation‘s flaxseed, 50% of all durum wheat, and is the top U.S. producer of confectionary sunflower (NDASS, 2008). All these crops are known to bioaccumulate Cd. Naturally occurring Cd concentrations were determined for soils on the Pembina Escarpment in Cavalier County, NE North Dakota. These soils overlie the Cretaceous Pierre Formation and the glacial till parent materials include considerable shale lithic fragments. Surface and core sampling locations from both residual and transported soils included an active agricultural field, a field currently in the Conservation Reserve Program (CRP), and former agricultural fields that are now grassland fields in a North Dakota State Wildlife Management Area (WMA). Samples were analyzed for Cd and other trace elements using nitric acid digestion followed by ICP–OES at NDSU. Results revealed marked differences in measured Cd values between different sampling sites, e.g., values for WMA fields range from 0.3 mg/kg to 7.0 mg/kg (n=139) for the 0-30 cm depth, whereas values for the agricultural field and CRP area range from 3.0 mg/kg to 16.4 mg/kg (n=25), for samples ranging in depth from surface soils to 1.5 m. Values for core samples from the WMA measured at 15 cm intervals down to 225 cm depth range from 0.01 mg/kg (detection limit) to 1.9 mg/kg (n=188). The measured values for Cd concentrations in several samples in this study confirm earlier results showing significantly higher concentrations of Cd compared to the ‗environmental baseline‘ value of 0.3 mg/kg reported by Garrett (1994). Chemical analyses for cores were grouped by soil horizon and lithology and correlation matrices were calculated for each class. A subset of results illustrates the correlation of Cd concentrations with other analytes for each sample class (Table 1). In bedrock material, the highest positive correlations for Cd are with Ca, Fe, Mn, organic matter, and electrical conductivity, while a weaker positive correlation is seen for Cd-Ni. Cd-pH shows a weak negative correlation. In sand and silt layers, Cd is strongly positively correlated with As. Interestingly, most of these Cd-analyte pairs (with the exception of organic matter) are negatively correlated in topsoil. References Garrett, R.G., 1994. The distribution of cadmium in A horizon soils in the prairies of Canada and adjoining United States. In Current Research, B1994, 73-82. Geological Survey of Canada. Hopkins, D.G., Norvell, W.A., Wu, J., 1999. Formation and distribution of trace-element-enriched soils near the Pembina Escarpment, Cavalier County, North Dakota. Proc. 91st ND Acad. Sci., Grand Forks, ND. NDASS, 2008. North Dakota Agricultural Statistics Service. USDA. Fargo, ND. 33 �Table 1. Correlations of Cd with other analytes in soil horizons and lithologic units of cores taken from formerly cropped fields on the Pembina Escarpment in eastern Cavalier County, ND. Top value: Pearson correlation coefficients; second value: Prob > |r| under H0: Rho=0. Analyte pairs with p < 0.05 are in bold. EC: Electrical Conductivity; OM: Organic Matter; n: no. of observations. Argillic Shale/ Claystone Contact Shale Bedrock Carbonate -0.4520 0.1401 -0.0024 0.9872 -0.0332 0.9273 -0.0270 0.8076 0.2902 0.5769 -0.7948 0.0588 0.4462 0.1265 0.8443 0.0006 0.0029 0.9848 -0.3489 0.3231 0.2248 0.0398 0.2970 0.5677 -0.5338 0.2754 Ca -0.7071 0.0069 -0.0559 0.8630 -0.0596 0.6940 0.2294 0.5239 -0.1151 0.2970 0.9326 0.0067 0.6525 0.1602 Co -0.0347 0.9103 -0.4021 0.1951 -0.0232 0.8781 -0.0143 0.9688 -0.2762 0.0110 0.4110 0.4183 -0.2300 0.6610 Cr -0.4866 0.0917 0.3111 0.3250 -0.0537 0.7231 -0.1552 0.6685 -0.0036 0.9739 -0.5916 0.2161 -0.8484 0.0327 Cu -0.5146 0.0720 0.5872 0.0447 0.1572 0.2968 -0.2217 0.5382 0.1411 0.2004 0.5623 0.2454 -0.6847 0.1335 Fe 0.0130 0.9664 -0.0642 0.8430 0.0677 0.6549 -0.2225 0.5367 0.2971 0.0061 0.9542 0.0031 -0.8845 0.0192 Mn -0.5024 0.0802 -0.3479 0.2679 -0.3838 0.0085 0.0972 0.7894 -0.3030 0.0051 0.9724 0.0011 0.3275 0.5264 Mo 0.0323 0.9165 0.7907 0.0022 0.1808 0.2292 -0.2604 0.4675 0.0565 0.6099 0.2543 0.6267 -0.5448 0.2636 Ni -0.4427 0.1298 -0.3163 0.3165 0.0312 0.8372 0.21408 0.5526 -0.3201 0.0030 0.7446 0.0895 -0.5448 0.2636 Zn -0.3397 0.2562 -0.2677 0.4003 0.5441 <.0001 -0.08486 0.8157 -0.0158 0.8863 0.4436 0.3783 -0.9208 0.0092 OM 0.3361 0.2615 0.4367 0.1558 0.4244 0.0033 0.1346 0.7109 0.3723 0.0005 0.8919 0.0169 -0.9449 0.0045 pH -0.4932 0.1231 -0.4500 0.1177 0.0225 0.8876 0.5565 0.0948 -0.1676 0.1324 -0.9958 0.0587 0.9069 0.0126 EC -0.7882 0.0040 0.4638 0.1508 -0.2525 0.1067 0.2002 0.5791 -0.2152 0.0521 0.9976 0.0441 0.4759 0.3400 13 12 46 10 84 6 6 Topsoil Sand & Silt Al 0.5223 0.0671 As n Support from NIH grant P20 RR016471 from the INBRE program of the National Center for Research Resources is gratefully acknowledged. 34 �A MICROSTRUCTURAL STUDY OF GOLD MINERALIZATION AT MUSSELWHITE MINE AND HAMMOND REEF SHEAR-ZONE-HOSTED GOLD DEPOSITS Kolb, Maura J. and Hill, Mary Louise, Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1 Canada, maurajoy@gmail.com Musselwhite Mine and Hammond Reef are shear-zone-hosted gold deposits located in the Western Superior Province of the Canadian Shield. Like other ―lode-gold‖ or ―orogenic gold‖ depostis these two deposits are proximal to major regional shear-zones. Musselwhite Mine is located adjacent to the North Caribou-Totogan Lake shear zone, while Hammond Reef is adjacent to the Marmion shear zone. The importance of deformation and metamorphism in shear-zone-hosted gold systems is investigated by comparing microstructures associated with gold mineralization from two very different gold deposits. Musselwhite Mine is hosted by amphibolite facies metamorphosed banded iron formation while Hammond Reef is hosted by greenschist facies metamorphosed tonalite. For both deposits gold mineralization can be seen associated with deformational structures where nearby minerals demonstrate very different responses to strain due to differing competency. Examples include brittle failure of competent minerals while matrix minerals deform ductilely, as well as strain shadows formed around these competent minerals Strain shadows around garnet crystals are commonly observed at Musselwhite Mine. At Hammond Reef gold mineralization is often associated with strain fringes around pyrite crystals. These strain fringes show varying degree of recrystallization due to ductile deformation after formation. Quartz textures observed within these recrystallized strain fringes are consistent with textures observed in quartz within the greenschist facies of metamorphism. Gold inclusions have been seen within the metamorphic minerals grunerite and garnet at Musselwhite Mine as well as muscovite at Hammond Reef. This demonstrates that gold mineralization occurred during or before metamorphism. Gold mineralization is also commonly associated with sulphide minerals in both deposits (pyrrhotite at Musselwhite Mine and pyrite at Hammond Reef). Many similar microstructures host gold mineralization at Musselwhite Mine and Hammond Reef. Finding gold mineralization within common structures indicates the importance of deformation and metamorphism in the formation of shear-zone-hosted gold deposits. Textures and mineral assemblages observed in this study support the interpretation that gold mineralization occurred as an ongoing event for both Musselwhite Mine and Hammond Reef. 35 �ASSIMILATION AND PETROGENESIS IN THE NAVILUS AND TERRY FOX SILLS, THUNDER BAY, ONTARIO MAGNUS, Seamus, KISSIN, Stephen, Department of Geology, Lakehead University, 955 Oliver Rd., Thunder Bay, ON, Canada, P7B 5E1, seamusmagnus@hotmail.com The numerous diabase sills in the Thunder Bay area were called the Logan sills by A.C. Lawson at the end of the 19th century. They were first studied in detail by Blackadar (1956), particularly with regard to the pink granophyre associated with some of the sills. He attributed this pink granophyre, especially in the case of the Navilus sill, located immediately east of Thunder Bay, to in situ assimilation of granitic rock. Differentiation of diabase was not considered capable of producing the large volumes of granophyre present in some of the Logan sills. Textural evidence suggests, as discussed by Philpotts (1978), that on cooling, the magma separated into two immiscible liquids, one iron-rich and one silica-rich. This has resulted in a zone of quartzo-feldspathic granophyre containing minute hematitic inclusions, which produces the pink colour seen in hand sample. However, some reaction with xenoliths is also present as seen in the rounding of immiscible carbonate xenoliths and the formation of granophyric zones surrounding quartzo-feldspathic xenoliths. Geochemical evidence indicates that late-stage differentiation by fractionation is the primary cause of variation within the Navilus sill. The uppermost samples of the Navilus sill, which typically contain xenoliths, display this late fractionation through enrichment of light REEs and depletion of heavy REEs (Figure 1a). A Nb-Ta trough on the chondrite-normalized spider diagram, as well as enrichment in LILs, indicates assimilation of crustal material has most likely occurred. (a) (b) Figure 1 – Chondrite-normalized REE diagram (a) and chondrite-normalized spider-diagram (b) for the Navilus Sill. Note the clockwise rotation of uppermost samples (squares) due to fractionation. The Terry Fox sill, located 4 km west of the Navilus sill, shows REE and Spider patterns almost identical to the normal diabase in the Navilus sill. Although the sample of the basal contact between the Terry Fox sill and Rove shale shows some variability in the REE and Spider diagrams, a more detailed analysis is required to determine whether they are the effects of in-situ assimilation or fractionation. 36 �Figure 2 – Plot of Gd/Yb vs La/Sm for several Keweenawan Intrusive Suites, with data from Hollings et al. (2007) and Hart (2002). Note that the Navilus and Terry Fox samples plot within the Nipigon Sill Complex samples. Comparison of the Navilus and Terry Fox sills to other Keweenawan intrusions and intrusive complexes in the Thunder Bay area using trace-element discrimination diagrams (Figure 2) indicates that these sills are grouped with the Nipigon sills. In summary, textural evidence has shown that assimilation of granitic xenoliths is not likely the cause of the formation of the pink granophyre, but the development of magmatic immiscibility has produced this feature. Geochemical evidence indicates that assimilation of crustal material has most likely occurred, though chemical variations seen within the sills appear to have been caused by in-situ fractionation. A more detailed study on the possible effects of assimilation at the base of assimilation at the base of the Terry Fox sill is required, although assimilation of xenoliths has possibly contributed to geochemical differences as seen in the Navilus sill spider diagram (Figure 2). References Blackadar, R.G., (1956). Differentiation and Assimilation in the Logan Sills, Lake Superior District, Ontario. American Journal of Science, v. 254, p. 623-645. Hart, T.R. (2002). Proterozoic Volcanic and Intrusive Whole Rock Geochemical Data Associated with the Keweenawan Midcontinent Rift, Lake Superior Area, Ontario. Ontario Geological Survey, Miscellaneous Release of Data 114. Hollings, P., Hart, T., Richardson, A., and Macdonald, C.A., (2007). Geochemistry of the Mesoproterozoic intrusive rocks of the Nipigon Embayment, northwestern Ontario: evaluating the earliest phases of rift development. Canadian Journal of Earth Science, v. 44, p. 1087-1110. Philpotts, A.R., (1978). Textural Evidence for Liquid Immiscibility in Tholeiites. Mineralogical Magazine, v. 42, p. 417-425 37 �TEXTURAL ANALYSIS OF A SIMPLE IGNEOUS INTRUSION, NIPIGON, ONTARIO MARKWOOD, Levi W. and ZIEG, Michael J., Department of Geography, Geology, and the Environment, Slippery Rock University, Slippery Rock, PA, 16057. lwm9100@sru.edu, michael.zieg@sru.edu In this study, I examined the textural variations in a small (1.4 m) diabase sill from Nipigon, Ontario, which was apparently formed by a single, instantaneous injection event, in order to develop a baseline to test against other sills with unknown injection histories. With an understanding of the textural characteristics of a simple intrusion, we can recognize the variations in more complex intrusions with complicated thermal histories and make comparisons between observed textural variations and the predictions of theoretical models. Such textural criteria are particularly important when the magma composition remained constant through several injections, in which case reinjection events would not be reflected in the chemical composition or modal mineralogy of the rocks. Petrographic examination of the rocks in this sill focused on opaque oxides, as they can be characterized easily using automated image processing techniques. Samples were collected from a 1.4 meter thick diabase sill in Nipigon, Ontario spanning the entire vertical cross-section of the sill. Thin sections were prepared at 5 cm intervals throughout the sill, yielding 28 thin sections between chill margins. Randomly oriented test lines were used to determine mean crystal length using the intercept method discussed by Higgins (2006): Lmean= VV/PL, where VV is the modal fraction of the mineral of interest, as determined by automated image analysis, and P L is the number of crystals intersected along the test line. To determine P L, a hundred test lines were randomly oriented in the field of view of a petrographic microscope for each sample and grain intersections along the test lines were averaged. To determine V v, digital threshholding was used to determine the average area of opaque minerals present in each sample and divided by the entire area of the slide. Once Vv and PL are known, Lmean can be determined for any position in the sill. Figure 1 shows the modal abundance of opaque oxides with varying position in the sill and Figure 2 shows the variations in L mean through the sill. Based on the observed textural variations, namely a smooth transition from smaller grains at the chilled margins to larger grains in the sill‘s center, this sill was almost certainly emplaced in a single injection event. However, questionable data points and the large statistical uncertainties in calculate mean lengths prevent us from using this intrusion as a baseline for confidently identifying complex injection histories. Further investigation and refinement of this textural profile could ultimately make it useful as a null hypothesis for testing formation history: unless the textural profile in the intrusion of interest differs significantly from the profile in this sill, we must assume that it was formed as a single injection. 38 �Figure 1: Modal abundance of opaque minerals. Figure 2: Mean crystal lengths of opaque minerals. References Higgins, M.D., 2006. Quantitative Textural Measurements in Igneous and Metamorphic Petrology, p. 34-35. 39 �BEDROCK GEOLOGIC MAP OF THE DISAPPOINTMENT LAKE AREA, LAKE COUNTY, NORTHEASTERN MINNESOTA MULVEY, Lucy, ROSS, Cabin, ZEITLER, Joseph, PENDLETON, Matthew, McCARTHY, Andrew, COPP, Lee, NOWAK, Robert, HUDAK, George, and PETERSON, Dean, Precambrian Research Center, Natural Resources Research Institute, University of Minnesota Duluth, 5013 Miller Trunk Highway, Duluth, MN 55811 The ―capstone‖ project for the Precambrian Research Center (PRC) summer field camp encompasses one week of detailed field mapping in small groups with faculty from the field camp. During the fifth and sixth weeks of the 2009 field camp, seven students mapped Neoarchean and Mesoproterozoic rocks in the vicinity of Disappointment Lake (located in the Boundary Waters Canoe Area Wilderness) under the direction of PRC faculty Dr. Dean Peterson and Dr. George Hudak. Geological maps of this area were originally published by Van Hise (1901) and Gruner (1941). This capstone mapping project sought to: 1) define and characterize the nature of the contact between the Mesoproterozoic Duluth Complex and Neoarchean supracrustal strata; 2) to better understand the lithological components of the Duluth Complex and the Neoarchean supracrustal strata in this area; 3) to obtain a better understanding of geological structures and their orientations within the Neoarchean supracrustal stata; and 4) to identify and map out regions of Mesoproterozoic (Duluth Complex associated) and Neoarchean (Snowbank Lake Granite-associated (Miller et al., 2001; Jirsa and Miller, 2004)) contact metamorphism within Neoarchean strata. Field mapping was completed at 1:5,000 scale, with the final map being published at 1:10,000 scale. Mapping was completed by means of lakeshore mapping from canoes and traverses through the bush. The north-central and northeastern regions of the field area are composed principally of Neoarchean intrusive and supracrustal strata. The Neoarchean supracrustal sequence is interpreted to represent the Knife Lake Group (Jirsa and Miller, 2004), and is composed, from oldest to youngest units, of: a) massive and pillowed basalt lava flows; and b) thick-bedded to massive rhyodacitic to dacitic tuffs locally interbedded with matrix- to nearly clast-supported, medium-bedded to massive polymict volcaniclast-dominated epiclastic deposits. Regional D2 deformation led to the development of localized west-northwest – east-northeast-trending chlorite schist zones which can be traced for at least 300 meters. Southwest of the chlorite schist zones, greenschist facies metamorphosed Timiskiming-type interbedded polymict conglomerates, conglomeritic sandstones, and mudstones interpreted as the Ogishke Conglomerate (Jirsa and Miller, 2004) appear to unconformably overlie the mafic lava flows and felsic volcaniclastic rocks and associated epiclastic sedimentary rocks described above. These sedimentary strata have been deformed into a series of relatively tightly folded anticlines and synclines characterized by moderately- to steeply- dipping (60°-90°) fold limbs with westnorthwest – east-southeast trending fold axes. Folding predated the intrusion of medium- to coarse-grained granite and hornblende granite (the Snowbank Lake Granite (Jirsa and Miller, 2004) and associated aplitic to pegmatitic dikes which occur in the westernmost one-third of the field area. Locally, xenoliths of sedimentary rocks, and sedimentary rocks along the margin of this intrusion, have been recrystallized into fine-grained to medium-grained granoblastic to granular hornblende hornfels. 40 �The southeastern one-third of the field area comprises Mesoproterozoic intrusive rocks that comprise the Duluth Complex. A marginal phase composed of medium- to coarse-grained oxide gabbro and poikilitic olivine gabbro transitions into medium- to coarse-grained troctolite that locally displays modally-layered subhedral olivine crystals and ophitic augite crystals. In the northeastern part of the field area, a southeast- northwest-trending wedge-shaped marginal phase of diorite and hornblende diorite occurs in an area roughly 1.5 km long by 1.0 km wide. Three distinctive zones of Mesoproterozoic thermal metamorphism have also been identified and mapped in the field area. Neoarchean sedimentary strata have been thermally metamorphosed to the pyroxene hornfels facies for up to 250 north of the Duluth Complex contact to produce fineto coarse-grained norite. Basalt hornfels xenoliths are locally present in the troctolitic phase of the Duluth Complex. In the east-central region of the field area, approximately 1 to 3km northwest of the contact between the Duluth Complex and Neoarchean supracrustal strata, a southeast - northwest-trending 1.5km by 2.0km zone of hornblende hornfels facies metamorphosed Neoarchean conglomerate, sandstone, and mudstone occurs. This southeast – northwest-trending zone has the same orientation as the wedge-shaped zone of marginal diorite and hornblende diorite phase of the Duluth Complex. The spatial distribution of the hornblende hornfels facies metamorphosed rocks, as well as the marginal hornblende diorite phase of the Duluth Complex, suggest the former presence of a shallowly-dipping southeast- northwest trending sill of Duluth Complex that has subsequently been eroded away. References Chandler, V. W., 1991, Aeromagnetic anomaly map of Minnesota: Minnesota Geological Survey State Map Series S-17, scale 1:500,000. Gruner, J. W., 1941, Structural geology of the Knife Lake area of northeastern Minnesota: Geological Society of America Bulletin, v. 52, p. 1577-1642. Jirsa, M. D., and Miller, J. D., 2004, Bedrock geology of the Ely and Basswood Lake 30‘ by 60‘ quadrangles, northeast Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-148, scale 1:100,000. Miller, J. D., Jr., Green, J. C., Severson, M. J., Chandler, V. W., and Peterson, D. M., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-119, scale 1:200,000, two sheets. Van Hise, C. R., 1901, The iron-ore deposits of the Lake Superior region: 21st Annual Report of the U. S. Geological Survey, Part III. 41 �FERROMAGNESIAN MINERALS IN THE STETTIN SYENITE COMPLEX, MARATHON COUNTY, WISCONSIN: COMPOSITIONS AND CONTRASTS WITH THE WOLF RIVER BATHOLITH MEDARIS1, L. Gordon Jr. and KOELLNER2, Susan E. 1 Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706 2 20509 Hillside Trail, Lindale, TX 75771 medaris@geology.wisc.edu, susankoellner@gmail.com The Stettin syenite complex is the northernmost of four overlapping, concentric intrusions, which are located immediately west of Wausau in central Marathon County, Wisconsin (Myers et al., 1984). The intrusions decrease in age from north to south, with zircon U-Pb ages of 1565 ± 4 Ma in the Stettin complex, 1522 ± 6 Ma in the Wausau complex, and 1506 ± 3 Ma in the Ninemile complex (Van Wyck et al., 1984; Dewayne and Van Schmus, 2007). Previously, these intrusions were thought to be associated with geon 14 Wolf River magmatism, but their ages are significantly older than the 1468-1484 Ma Wolf River batholith and preclude a cogenetic relation between them. The Stettin complex consists of concentrically arranged syenite bodies, the most important of which are syenite, tabular syenite, nepheline syenite, and quartz syenite (Myers et al., 1984). The latter two units illustrate the divergent silica differentiation trends in the complex, i.e. silica undersaturation in nepheline syenite and silica oversaturation in quartz syenite. Values for the Thornton-Tuttle Differentiation Index (normative qtz+or+ab+ne) range from 71 to 94 in nepheline syenite and from 82 to 87 in quartz syenite. The complex is markedly Fe-rich, with Fe#'s ranging from 89.3 to 99.8. Stettin syenites are more alkaline than Wolf River granites, A/CNK = 0.84 ± 0.08 (n=10) vs. 1.00 ± 0.08 (n=38), have higher Agpaitic Indices, (Na+K)/Al = 1.44 ± 0.10 vs. 1.21 ± 0.10, and are more sodic, Na/K = 1.77 ± 0.15 vs. 0.88 ± 0.22 (Myers et al., 1984; Anderson and Cullers, 1978). Another important distinction is the presence of titaniferous magnetite in the Stettin complex and ilmenite in the Wolf River batholith. 42 �Clinopyroxene occurs in all four major syenite units in the Stettin complex, ranging in composition from Di55Hd44Acm1 to Di8Hd90Acm2 in syenite, from Di12Hd70 Acm18 to Di6Hd39Acm55 in tabular syenite, from Di11Hd81Acm8 to Di10Hd35Acm55 in nepheline syenite, and from Di32Hd64Acm4 to end-member acmite in quartz syenite. Overall, clinopyroxene in the Stettin complex displays one of the most pronounced agpaitic compositional trends reported among alkaline complexes worldwide. Olivine occurs in syenite and nepheline syenite, ranging in composition from Fa 77 to 97, and having a consistently higher Fe# than coexisting clino-pyroxene. Amphibole in syenite, nepheline syenite, and tabular syenite is largely hastingsitic, but in quartz syenite reaches near end-member riebeckite, which has been stabilized under igneous conditions by the presence of F (>2 wt.%). Amphibole in the Wolf River batholith consists of ferroedenite to hastingsite (Anderson, 1980), devoid of the marked alkaline trend in Stettin amphibole. Biotite approaches annite (or ferriannite) in composition and is relatively poor in Al. Many samples contain insufficient Si+Al to fill the tetrahedral position, which is a common feature in biotite from alkaline rocks. Wolf River biotite is also relatively Fe-rich, but is more aluminous than Stettin biotite (Anderson, 1980). The compositions of Stettin and Wolf River ferromagnesian minerals are dependent in part on intensive crystallization conditions, e.g. relatively reducing conditions, at or below that of the FMQ buffer, allow the crystallization of hedenbergite and fayalite. However, the occurrence of sodic pyroxene and amphibole in the Stettin complex, as in many other alkaline complexes, is more a function of bulk composition, i.e., a relative increase in Na, combined with an increase in Fe3+/Fe2+, during differentiation. The contrasts in Stettin and Wolf River amphibole and biotite compositions are also due to differences in bulk composition between the two bodies, reflecting their different degrees of alkalinity. References Anderson JL (1980) American Journal of Science, v. 280, 289-332. Anderson JL and Cullers RL (1978) Precambrian Research, v. 7, 287-324. Dewane TJ and Van Schmus WR (2007) Precambrian Research, v. 157, 215-234. Koellner SE (1974) M.S. thesis, University of Wisconsin-Madison, 155 pp. Myers PE et al. (1984) Institute on Lake Superior Geology, v. 30, Field Trip #3, 58 pp. Van Wyck N et al. (1984) Institute on Lake Superior Geology, v. 30, Part 1 Program and Abstracts, 81-82. 43 �CYCLICAL PHASE LAYERING IN THE DULUTH COMPLEX AT DULUTH – EVIDENCE FOR PERIODIC MAGMA VENTING FROM A SHALLOW STAGING CHAMBER? Miller, James D., and Stifter, Eric, Department of Geological Sciences, University of Minnesota Duluth, Duluth, MN 55812 Igneous layering is an almost ubiquitous feature in mafic intrusive bodies and yet remains an enigma. One of the more intriguing forms of layering in the 1.1 Ga Duluth Complex of NE Minnesota is cyclical phase layering exhibited by the Layered Series at Duluth (DLS). The DLS is a well-exposed, well-differentiated, north-south striking, eastward-dipping, sheet-like intrusion. Its cumulate stratigraphy ranges from troctolite (Pl+Ol) upward to olivine oxide gabbro (Pl+Cpx+Ol+Ox) and ultimately to intermediate and felsic rock types as final differentiates. Detailed field mapping by Miller and Green (2008) separated the DLS into five distinct zones: the basal contact zone, the troctolitic zone, the cyclic zone, the gabbroic zone, and the upper contact zone. The cyclic zone is composed of least 5 macrocycles wherein troctolitic (Pl+Ol) cumulates grade upward into olivine gabbroic (Pl+Aug+Ol+Ox) cumulates. The 4phase gabbros are then abruptly overlain by troctolitic cumulate marking the base of the next macrocycle. A conventional interpretation of such layering may be that it formed by progressive differentiation followed by magma recharge. However, cryptic layering of olivine and pyroxene across the cumulate regressions marking each macrocycle boundary show no significant increase in mg#, which such a model would predict. Instead, Miller (Miller and Ripley, 1997; Miller et al.. 2002) proposed that decompression due to magma venting at shallow depths might better explain this layering by significantly changing phase equilibrium without changing magma composition. Exposed in the Spirit Mountain ski area and within the second macrocycle of the cyclic zone is a large outcrop exhibiting meter-scale mesocyclic layering that mimics the textural and mineralogical changes seen in the larger scale macrocycles (Fig. 1A & B). Detailed field, petrographic, and geochemical studies of these mesocycles were performed to further test the plausibility of magma recharge as the mechanism responsible for the cyclic phase layering. Analyses of olivine and augite revealed that mg# (=Mg/(Mg+Fe)*100) remained within 2% of the mean throughout individual cycles and throughout the entire mesocyclic system of 16 cycles (Fig. 2). Despite this minor variation, there is a systematic difference in the mg# of olivine, such that the olivine in troctolite cumulates is consistently more fosteritic than gabbroic cumulates. Some cycles show a similar shift in augite, though this is less consistent. This may at first glance seem to argue for recharge of more primitive magma as the cause for the phase layering. However, the shift in mg# in olivine may also reflect a shift in the Fe-Mg equilibrium between melt and olivine when augite becomes a cumulus phase. Trace element data, particularly REE, show a significant difference between the troctolite and gabbro layers, but this can be attributed to differing partition coefficients for the main cumulate phases. In conclusion, the field, petrographic, mineral chemistry, and lithochemical attributes of the mesocyclic layering observed as Spirit Mountain does not unambiguously favor either a magma recharge or decompression venting model for its origin. More data and evaluation are needed to definitely determine the origin of the mesocyclic and larger macrocyclic layering of the Cyclic Zone of the Duluth Layered Series. 44 �References Miller, J.D. Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.E., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota. Minnesota Geological Survey Report of Investigations 58, 207p. Miller, J.D., Jr., and Green, J.C., 2008, Bedrock geology of the West Duluth and eastern portion of the Esko quadrangles, St. Louis County, Minnesota. Minnesota Geological Survey Misc. Map M-183, scale 1:24,000 Miller, J.D., Jr., and Ripley, E.M., 1996, Layered intrusions of the Duluth Complex, Minnesota, USA. in Cawthorne, R.G. (ed.), :Layered Intrusions: Amsterdam, Elsevier, p. 257-301. Olivine Augite Figure 1. Mesocyclic layering of troctolite (Ol+Pl) and Ox-Ol gabbro (Pl+Cpx+Ol+Ox) cumulates in outcrop (A) and in thin section (B). Figure 2. Stratigraphic changes in the physical and compositional characteristics of mesocycles in the Spirit Mtn area. 45 �MESOPROTEROZOIC BEDROCK GEOLOGY OF THE HOMER LAKE – VERN LAKE AREA, COOK COUNTY, NORTHEASTERN MINNESOTA Jim Miller, Sam Blakely, Amy Brown, Dan Foley, Aaron Rowland, and Eric Stifter Precambrian Research Center, University of Minnesota Duluth, Duluth, MN 55812 As one of the capstone mapping projects for the 2009 Precambrian field camp, a crew of four students (Blakely, Brown, Foley, and Rowland), a teaching assistant (Stifter), and an instructor (Miller) conducted five days of field mapping bedrock geology of the Homer Lake and Vern Lake areas of northeast Minnesota. This area comprises intrusive and volcanic rocks formed during the 1.1 Ga Midcontinent Rift in northeastern Minnesota. This mapping project expanded on a capstone mapping project conducted in 2007 (Frost et al., 2007). A 1:12,000-scale bedrock geologic map that integrates the previous mapping and new mapping from last season will be displayed as a poster presentation. A pdf version of this and other Precambrian field camp capstone geologic maps can be downloaded from the PRC website: www.d.umn.edu/prc/fieldcamp/capstone. Prior to the detailed mapping conducted by the PRC field camp students, the general geology of the Homer Lake area was poorly known. Township-scale mapping by Grout et al. (1959) showed the area to be dominantly composed of gabbroic to felsic intrusive rocks, mafic volcanic rocks and minor volcaniclastic sedimentary rocks. They noted that the gabbroic rocks are well layered and portions are particularly rich in Fe-Ti oxide. Curiously, Davidson and Burnell (1977) did not integrate Grout et al.‘s (1959) mapping into their reconnaissance geologic map of the Brule Lake 7.5‘ quadrangle, which contains Homer Lake. Showing only one outcrop on Homer Lake, they portrayed the geology as being composed of exclusively gabbroic and anorthositic rocks. The regional geologic map of northeastern Minnesota focused on the geology of the Duluth Complex (Miller et al., 2001) incorporates Davidson and Burnell‘s (1977) interpretation, which current mapping shows to be largely incorrect. In fact, Grout et al.‘s (1959) mapping has been shown to be quite accurate. Capstone mapping in August of 2007 by Shelby Frost, Natalie Juda, and Jim Miller and reconnaissance mapping in 2006 by Miller resulted in a geologic map of the Homer Lake area (Frost et al., 2007). This map interpreted the area to be composed of two main gabbroic suites separated by a thin screen of basaltic and sedimentary hornfels that runs along the north shore of Homer Lake (and noted by Grout et al., 1959). The gabbroic suites and the hornfels units have an east-west strike and gentle (15-25°) dips to the south. The basaltic hornfels has some remnant volcanic features such as flow top breccias and amygdaloidal zones and the sedimentary hornfels commonly displays cross-bedding. These units are interpreted to be basalt flows and interflow sediments of the North Shore Volcanic Group. Gabbroic rocks north of (below) this hornfels screen include olivine diabase, oxide gabbro, ferromonzodiorite, and granophyric gabbro, which were collectively termed the Axe Lake gabbroic sequence. South of the hornfels units are a series of oxide gabbros that were termed the Homer Lake gabbroic sequence. Two textural variants of the oxide gabbros were distinguished – a medium-grained, well-foliated and intergranular rock type and a coarse-grained, poorly foliated, and ophitic to intergranular lithology that can locally contain up to 25% interstitial granophyre. These two variants were found to be interlayered in 4-5 cycles over a stratigraphic thickness of about 600 meters. Capstone mapping during the 2009 field camp extended the mapping of Frost et al. (2007) to the south and west of Homer Lake. This mapping expanded the total map area from 3 square miles to about 10 square miles. The screen of volcanic and sedimentary hornfels was traced across the map area, though at its western extent where it crosses Vern Lake, the hornfels occurs as inclusions in the base of the Homer Lake sequence. The Axe Lake sequence was traced to the west and east of Axe Lake The more intermediate lithologies recognized in the Axe Lake area during 2007 mapping were found to persist to the east, but were not observed to the west. West of Axe Lake, the main rock type observed is olivine 46 �gabbro to augite troctolite. Exposures along Vern Lake at the western extent of the map area, troctolite and olivine gabbro contain abundant inclusions of anorthositic gabbro. Mapping to the south of Homer Lake to East Pipe and Pipe Lakes revealed that the oxide gabbros that compose most of the Homer Lake gabbroic sequence grade upward into an apatitic oxide gabbro, a ferromonzonite, and then into a granophyric leucogranite. In addition to recognition of these new map units, the 2009 map simplified the multiple coarse and medium grained oxide gabbro units of Frost et al. (2007) into two main alternating cycles. Also, the lowermost coarse oxide gabbro unit of Frost et al. (2007) was renamed as the heterogeneous oxide gabbro to highlight the variable texture and granophyre content of this unit. One the more interesting discoveries was the occurrence of nearly pure layers of FeTi oxide in the upper oxide gabbro cycle in the vicinity of Vern River. Grout (1949-50) noted such layers of nearly pure ilmenite in this area and to the west, which he speculated may be an economic Ti resource. Although too little of the Axe Lake sequence has been mapped to adequately speculate as to its petrogenesis, the Homer Lake sequence has been revealed as a rather complete mafic to felsic differentiated sequence. In contrast to other mafic layered intrusions in the Duluth and Beaver Bay complexes, however, the differentiation of the Homer Lake sequence appears to start from a very evolved oxide gabbro (Pl+Cpx+Ox) cumulate rather than more primitive troctolitic (Ol+Pl) cumulates. Moreover, granophyre capping the Homer Lake sequence is part of the massive Eagle Mountain granophyre (Miller et al., 2001). Therefore, this much granophyre could not have fractionated from the oxide gabbro. Rather, as found with other granophyre masses in the Duluth and Beaver Bay complexes, the felsic rocks likely served as a density barrier to the mafic magmas of the Homer Lake sequence. Underplating of the Eagle Mountain granophyre by the Homer Lake mafic magmas would have caused partial melting of the lower part of the granophyre, which may have assimilated with the upper part of the Homer Lake sequence to create the ferromonzonite unit. If this scenario is correct, then the recent U-Pb age of 1098.6±3.6 Ma reported by Vervoort et al. (2007) for the Eagle Mountain granophyre gives a maximum age for the Homer Lake sequence. A geochemical study through the Homer Lake sequence into the overlying Eagle Mtn granophyre is necessary to discern the extent to which this apparent differentiation sequence is the result of fractional crystallization or contamination by the granophyre. Plans for the 2010 capstone mapping project in this area are to follow the stratigraphy of the Homer Lake sequence to the west. Aeromagnetic data indicates that the sequence should not only persist to the west, but should also thicken. References Davidson, D.M. and Burnell, J.R., 1977, Brule Lake Quadrangle, Cook County, Minnesota: Minnesota Geological Survey, Miscellaneous Map Series, M-29, 1:24,000. Frost, S.J., Juda, N.A., and Miller, J., 2007, Bedrock Geology Map of Homer Lake and Adjacent Areas; Cook County, Northeastern Minnesota: University of Minnesota Duluth, Precambrian Research Center, PRC/MAP-2007-02, 1: 12,000. Grout, F.F., 1949-50, The titaniferous magnetites of Minnesota: Department of Iron Range Resources and Rehabilitation, 117 p. Grout, F.F., Sharp, R.P., and Schwartz, G.M., 1959, The Geology of Cook County Minnesota: Minnesota Geological Survey Bulletin 39, 163p. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.E., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota. Miscellaneous Map Series, M-119, scale 1:200,000, 2 sheets. Vervoort, J.D., Wirth, K., Kennedy, B., Sandland, T., and Harpp, K.S., 2007, The magmatic evolution of the Midcontinent rift: New geochronologic and geochemical evidence from felsic magmatism. Precambrian Research 157, p. 235-268. 47 �SEDIMENTOLOGICAL STUDY OF THE GLACIOGENIC BRUCE FORMATION, HURONIAN SUPERGROUP, IN THE NORTH SHORE AREA, ONTARIO O’HARE, Sean and FRALICK, Philip Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada The Bruce Formation is a diamictite succession located in the lower-most portion of the Paleoproterozoic Quirke Lake Group, of the Huronian Supergroup. The Formation is exposed on the north shore of Lake Huron, and covers an area east of Sault Ste. Marie to east of Sudbury, Ontario, Canada. This assemblage of rocks represents deposition on a passive margin after initial rifting and volcanism at approximately 2450 Ma (Fralick and Miall, 1989). The passive margin succession is composed of three glaciogenic cycles beginning with outwash sandstones, which are overlain by glacial diamictites and capped by marine deposits. The Bruce Formation is situated above the Mississagi Formation, which commonly shows evidence of extensive soft sediment deformation of its cross-stratified sandstones (Fig. A). These are also more rarely intruded by clastic dykes (Fig. B), orientated perpendicular to bedding and filled with diamictite. The upper contact with the Espanola Formation is sharp and conformable. The Espanola represents the oldest cap carbonate known in the worldwide geologic record and may be evidence of a snowball Earth event. The Bruce Formation was studied in detail in an attempt to understand the ambient conditions that were present at the time of its deposition. The study included measuring and describing 16 detailed sections in the field, plus petrological and geochemical analysis of samples collected from the field areas. Glaciogenic deposits can be either terrestrial or marine and marine deposits can be further divided into three lithofacies zones: 1) the grounding ice line zone, 2) the floating ice-shelf zone, and 3) the proximal iceberg zone. Deposits in each of these environments have distinctive characteristics that, where present, can be used to identify the depositional setting. Most outcrop areas in the sections studied consisted of massive diamictite that contained considerable quantities of sand and only scattered pebbles and cobbles. The thickness of the assemblage of diamictites making up the Formation varied drastically from section to section, with three less than 10 meters thick and others hundreds of meters in thickness. Within these massive diamictites are areas with more distinctive features. Lenses and layers of moderately well-sorted sandstone are not uncommon. In other portions of the section mud layers and mud wisps highlight layering in the diamictite, and at one location a fine-grained succession within the diamictite contained a dropstone (Fig. C). If other dropstones exist in areas of the diamictite they are commonly impossible to recognize without layering to deform. However, where clay-rich layers and wisps are present in the diamictite they can be seen to bend under and be terminated by penetration of dropstones. Layers of pebbles and cobbles in clast support were more rarely present (Fig. D). These represent deflation lags produced by current activity removing the finegrained material. Rarest of all were flat bottomed and convex-up mounds of pebbles and cobbles in clast-support approximately one meter across, although a larger deposit of this type may also have been present in one section. Similar features are produced in modern environments by uneven melting of icebergs causing flipping and dumping of melt-out debris on their surface. In Pleistocene successions diamictite deposited near the grounding ice line commonly contains subaqueous outwash sand layers and lenses. The diamictite deposited below floating ice shelves is generally massive but has features, such as mud-rich layers and sand-rich layers, which indicate current activity. Dropstones and deflation lag layers can also develop in this zone. The 48 �proximal iceberg zone can also be dominated by massive diamictite, which has a lower clast concentration, and also contains dropstones. The most conclusive evidence of the iceberg zone is the presence of mound shaped iceberg dumps, which are created when an iceberg overturns. This implies that large segments of the glaciogenic succession were deposited under floating ice with open water present in the basin. The open water is necessary to develop current activity. Although this data does not negate the development of a snowball earth at some point during this glacial period it does not support it. 49 �VIRGINIA FORMATION OUTCROP (New Outcrops Versus Old Concepts) Patelke, Richard L. (PolyMet Mining, 6500 County Highway 666, Hoyt Lakes, MN 55750) and Severson, Mark J., Natural Resources Research Institute, 5013 Miller Trunk Highway, Duluth, MN, 55811 In their 1983 study of Virginia Formation stratigraphy, Lucente and Morey noted the absence of natural exposures of the formation and the temporary nature of a few Virginia Formation exposures related to iron mining. Their stratigraphic study was based solely on four deep core holes that were drilled as part of the ―Mesabi Deep Drilling Project.‖ The statement (taken on faith?) that the Virginia Formation doesn‘t outcrop is a truism that may actually have discouraged geologists from looking for outcrop. However, several areas of actual outcrop have been found in the last 15 years. While not technically on the Iron Range, there are long known exposures of weakly to strongly partially melted Virginia Formation in close proximity to the Duluth Complex at Linwood Lake (Severson 1995). These particular outcrops were originally found by a minerals exploration company in the 1970s and their rumored occurrence was eventually followed up. As part of a mapping project for the Minnesota Geological Survey, Severson and Miller, (1999) routinely visited areas as part of outcrop searches and recorded a small area of Virginia Formation outcrop in the area of the Siphon Fault (related to structure) at the east end of the LTV taconite mine in the Allen Quadrangle. Recent low water in that same area has expanded the outcrop area to about 200 ft. by 1200 ft of scattered exposure, ranging from intact outcrop, inplace blocks, and a large area of frost shattered angular blocks. About one and a half miles east of this location, along the Dunka Road, is another area of frost shattered angular boulders of Virginia Formation that was discovered in early April, 2010. Again, these subcrops were recently exposed by low water and presumably represent outcrop. In the fall of 2009, upon the insistence of some of PolyMet employees, who are residents of Hoyt Lakes and Aurora, we canoed into an area on the St Louis River, just south of Whitewater Lake, that the locals call ―The Slates‖. Here we found flaggy outcrop of Virginia Formation in floor of the river, exposures up to 12 ft. high at the riverside, and exposures in the woods near the river. The area extends about 750 ft. along the stream. Unlike the other areas of outcrop, metamorphism is not readily apparent. Sedimentary features include bedding, cross-bedding, ripple marks, and load casts. Rock types are siltstones intermixed with graywackes. Compass issues precluded quality measurements, but overall strike is SW to NE, with gentle dips to the southeast. Sulfide (pyrite) is present, but rare, in these exposures. Another area to the south of Hoyt Lakes reported by the same locals has yet to be visited. This experience would indicate the value of pursuing that lead. In summary, a reported paucity of outcrop of any unit should never be taken at face value. Outcrops can be found in some of the most unlikely areas if enough effort is expended in systematic searches. This search should also include conversations with local outdoor enthusiasts, woodsmen, loggers, and hunters. 50 �References: Lucente, M.E., and Morey, G.B., 1983, Stratigraphy and Sedimentology of the Lower Proterozoic Virginia Formation, Northern Minnesota, Minnesota Geological Survey Report of Investigations 28, 28 pages. Severson, M.J., 1995, Geology of the southern portion of the Duluth Complex: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report, NRRI/TR95/26, 185 p. Severson, M.J., and Miller, J.D. Jr., 1999, Bedrock Geologic Map of Allen Quadrangle: Minnesota Geologic Survey, Miscellaneous Map —91, 1:24,000. 51 �HYDROTHERMAL OVERPRINT BY GREENSTONE SILLS WITHIN THE TILDEN PIT, MICHIGAN PIETRZAK, Natalie, University of Western Ontario, DUKE, Norm University of Western Ontario, SCOTT, Glenn, Cliffs Natural Resources and LUKEY, Helene, Cliffs Natural Resources The Negaunee Iron Formation in the Paleoproterozoic Marquette Range Supergroup contains numerous greenstone sills and dykes. Relict gabbroic textures within these bodies identify their mafic igneous origin. Direct dating has been unsuccessful due to the pervasive retrogression of the primary silicate mineralogy. Schneider et al., (2002) suggests the Negaunee and ~1.8 Ga Hemlock volcanics interfinger and therefore date the Negaunee between 1880 and 1860 Ma. However, Hans and Runnegar (1992) consider the Negaunee to be 2.1 Ga based on Sm-Nd isotopes and identification of fossils. The lowest greenstone sill exposed in the Tilden Pit is termed the ―Pillar Intrusive‖ and this separates the Martite Domain from the overlying Hematite Domain (Lukey et al., 2007). Three strategic drill cores have been studied to document detailed petrography from the base of the pillar to the lowest Negaunee clastics. Microscopic examinations supported by microprobe analyses reveal that primary iron formation textures have been hydrothermally overprinted up to 100 meters below the pillar. The chilled margin of the pillar is mineralogically complex and comprises chlorite, sericite, albite and veinlets of carbonate, quartz, minor rutile and apatite: Fe-oxides are absent. Proximal to the chilled margin, iron formation textures are overprinted by massive recrystallized quartz with trace to minor disseminated Fe-oxides and K-feldspar. Vugs, occasionally filled with specularite and commonly associated with goethite patches, increase towards the sill contact. With increasing depth away from the sill, Fe-oxides increase in size and abundance whereas the goethite patches decrease in size and abundance. Within the sill chilled margin chlorites are fine grained and show moderately high iron content (Figure 1A). Chlorite and carbonate are absent in the recrystallized quartz zone and reappear in veinlets cross-cutting the primary oolitic-chert textures 100 meters below the pillar. Below the recrystallized aureole the chlorite shows progressive increasing iron content from low iron in the oolitic-chert lithologies to higher iron contents in the lower iron formation bands and basal clastic units (Figures 1B-D). Carbonate shows a similar compositional trend as chlorite. Within the chilled margin the carbonate is fine grained calcite (Figure 2A), while ankerite-dolomite species occur in distal oolitic-chert. Within the lower iron formation bands and basal clastic lithologies the iron content of the carbonate increase dramatically to form siderite-magnesite species (Figures 2B-D). The emplacement of greenstone sills and related dykes was accompanied by hydrothermal overprinting of the enveloping iron formation as well as the mafic intrusions. The intense chloritization of the sills and dykes suggest they intruded into still hydrous sediments and autohydration resulted in near-complete chlorite replacement of the chilled margins. Domains of chlorite schist that develop along the contact clearly demonstrate hydrothermal alteration predated Penokean folding. The recrystallized massive quartz aureole, with vuggy textures, represents a dehydration zone that underwent loss of iron and CO2. The increase in porosity and permeability proximal to the sill allowed ingress of cooler fluids, giving rise to the goethite patches. Evidence for direct involvement of dehydration of enveloping sediments and pervasive retrogression of the greenstones indicates the sills are likely of similar age to the deposition of the Negaunee Iron Formation. 52 �Figure 1: Chlorite chemistry of grains defined by ellipses: A) the chilled margin; B) oolitic-chert unit; C) chertcarbonate- banded unit; D) clastic unit. Figure 2: Carbonate chemistry defined by ellipses from: A) the chilled margin; B) oolitic-chert unit; C) chert-carbonate banded unit; D) clastic unit. References Hans, T. and Runnegar, B. 1992. Megascopic Eukaryotic Algae from 2.1 Ga Negaunee Iron Formation: Science 257: 323-235 Lukey, H.M., Johnson, R.C., and Scott, G.W. 2007. Mineral Zonation and Stratigraphy of the Tilden Haematite Deposit, Marquette Range, Michigan, USA: Proceedings of Iron Ore (AusIMM:Melbourne) Schneider, D., Bickford, M., Cannon F., Schulz, K. and Hamilton, M. 2002. Age of volcanics and syndepositional iron formations, Marquette range Supergroup: implications for the tectonic setting of the Paleoproterozoic iron formations of the Lake Superior region: Canadian Journal of Earth Sciences 39: 999-1012 53 �THE RARE EARTH ELEMENT AND YTTRIUM COMPOSITION OF ARCHEAN AND PALEOPROTEROZOIC IRON FORMATIONS REVISITED: A NEW PERSPECTIVE ON SIGNIFICANCE AND MECHANISMS OF IRON FORMATION DEPOSITION Noah Planavsky1,2*, Andrey Bekker3, Olivier J. Rouxel2,4, Balz Kamber5 Axel Hofmann6, Andrew Knudsen7, Timothy W. Lyons1 1 Department of Earth Sciences, University of California, Riverside, Riverside, CA 92521, USA 2 Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institute, Woods Hole, MA 02543, USA 3 Department of Geological Sciences, University of Manitoba, Winnipeg, MB R3T 2N2, Canada 4 University of Brest, European Institute for Marine Studies, Technopôle Brest-Iroise, Place Nicolas Copernic, 29280 Plouzané - France 5 Department of Geology, Laurentian University, Sudbury, ON P3E 2C6, Canada 6 School of Geological Sciences, University of Kwazulu-Natal, Durban, 4000, South Africa 7 Department of Geology, Lawrence University, Appleton, WI 54912, USA The ocean and atmosphere were largely anoxic in the early Precambrian, which dictated a dramatically different iron cycle than today. Extremely iron-rich sedimentary deposits, iron formations, are the most conspicuous evidence of a disparate iron cycle in the early oceans. Rare Earth Element (REE) systematics have long been used as a tool to understand the genesis of iron formations and the chemistry of ancient oceans. However, many earlier REE studies of iron formations have drawn ambiguous conclusions, partially due to analytical limitations and sampling from severely altered formations. We present new chemical analyses of iron formation samples from 18 units, but with a focus on Lake Superior iron formations. These new analyses allow a reevaluation of the mechanisms and significance of Precambrian iron formation deposition. There are several temporal trends in our REE and Y dataset that we propose to reflect shifts in marine redox conditions. In general, Archean iron formations do not display significant shale-normalized negative Ce anomalies and only iron formations younger than 1.9 Ga display prominent positive Ce anomalies. Low Y/Ho ratios and high shale-normalized light to heavy REE (LREE/HREE) ratios are also present in ca. 1.9 Ga and younger iron formations but are essentially absent in their Archean counterparts. These marked differences in REE+Y patterns of late Paleoproterozoic and Archean iron formations can be explained in terms of varying water column REE cycling. Similar to modern redox-stratified basins, the REE+Y patterns in late Paleoproterozoic iron formations record evidence of a shuttle of metal and Ce oxides across the redoxcline from oxic shallow seawater. Oxide dissolution— mainly of Mn oxides—in an anoxic water column lowers the Y/Ho ratio, raises the light to heavy REE ratio, and increases the concentration of Ce relative to neighboring REE (La and Pr) values/concentrations. In contrast, Archean iron formations do not display REE+Y patterns indicative of an oxide shuttle, which implies an absence of a distinct Fe-Mn redoxcline prior to the rise of atmospheric oxygen in the early Paleoproterozoic. These results question classical models for deposition of Archean iron formations that invoke oxidation at or above a redoxcline. In contrast, we suggest that metabolic iron oxidation is a more likely oxidative mechanism for these iron formations, implying that the iron distribution of Archean oceans could have been controlled by microbial iron uptake and ecosystem stratification rather than the oxidative potential of shallow-marine environments. 54 �55 �THE PETROLOGY AND GEOCHEMISTRY OF THE RIVERDALE SILL PUCHALSKI, Raya, HOLLINGS, Pete Department of Geology, Lakehead University 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada, and SMYK, Mark Ontario Geological Survey, Ministry of Northern Development, Mines and Forestry, Suite B002, 435 James St. South Thunder Bay, ON P7E 6S7 Canada The Riverdale Sill is located within the southern city limits of Thunder Bay. Although the sill consists mainly of gabbronorite, olivine gabbro can be found in a number of localities. The intrusive contact between the sill and the Paleoproterozoic Rove Shale is sharp and the chilled margins are only a few centimetres thick. The sill is exposed over an area approximately 6 km long and 2 km wide but the true thickness is unknown because the upper contact is not visible. In comparison to other Midcontinent Rift-related intrusions, the Riverdale Sill is geochemically similar to ultramafic intrusions such as the Hele and Disraeli (Hollings et al. 2007). Although it is found south of Thunder Bay it most closely resemble the Jackfish Sill of the Nipigon Embayment rather than the Logan and Nipigon Sills which surround it. The Riverdale Sill crops out in a quarry on West Riverdale Road where it has an exposed thickness of 10 m. Detailed sampling was carried out every metre up through the cross section of the sill to investigate geochemical variations within it. There is no geochemical evidence for fractionation within the sill (Fig. 1) which is supported by the absence of cumulate textures. Fig. 1 Geochemical data from a transect through the Riverdale Sill. SiO2, TiO2 and MgO in weight percent Cr and Ni in ppm. Geochemical evidence for contamination of the sill is localized to within less than 1m of the contact with Rove Shale, as shown by the higher SiO 2 values in the gabbronorite at the contact (Fig. 1). On a primitive mantle-normalised plot, a number of samples of the Riverdale Sill broadly resemble Ocean Island Basalts (OIB; Fig. 2). However, some samples within the sill have negative niobium anomalies which are best interpreted to be the result of crustal contamination at depth. The less-contaminated samples are typically found towards the center of the sill. The geochemistry of the center of the sill varies slightly from that near the sill margins. Samples of gabbronorite taken adjacent to a shale xenolith within the sill do not display a negative Nb 56 �anomaly. The lack of this anomaly, combined with the lack of contamination above the contacts in the sill at the quarry, supports a model in which contamination is occurring at depth rather than during emplacement. ЄNd(T=1100Ma) values of -1.6 to -1.9 for the Riverdale Sill are consistent with this model (Smyk and Hollings, 2009). Fig. 2 Rare earth element primitive-mantle normalised plot (Modified after Hollings et al. 2007 and Sun et al.1989) Geochemical variations within the sill can be interpreted as the result of two pulses of magma, with the first more-contaminated pulse intruded by the second, less-contaminated magma shortly after the emplacement within the shale. The less-contaminated magma may have pushed the contaminated magma to the edges of the sill, leaving a less-contaminated core. References Hollings, P., Hart, T., Richardson, A. and MacDonald, C.A., 2007. Geochemistry of the mid-Proterozoic intrusive rocks of the Nipigon Embayment, north western Ontario. Canadian Journal of Earth Sciences 44: 1087-1110 Smyk, M. and Hollings, P., 2009. Project Unit 08-021. Mesoproterozoic Midcontinent Rift-Related Mafic Intrusions Near Thunder Bay: Update. Summary of Field Work and Other Activities 2009, Ontario Geological Survey, Open File Report 6240, p. 11-1 to 18-5. Sun, S.-S. and McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalt: implications for mantle composition and processes. Magmatism In the Ocean Basins. Geological Society, London: Special Publication No. 42, pp. 313-345. 57 �PETROGRAPHIC AND GEOCHEMICAL ANALYSIS OF A NIPIGON DIABASE SILL RYAN, Andrew J. and ZIEG, Michael J., Department of Geography, Geology, and the Environment, Slippery Rock University, Slippery Rock, PA 16057 ajr2727@sru.edu; michael.zieg@sru.edu In this study, we present preliminary results of a detailed stratigraphic analysis of a Nipigon diabase sill. This sill has been sampled in a drill core (BSE-07-01) that has been made available for study by RPT Uranium Corp and is archived by the Ontario Geological Survey. The sill is intersected over 266 m in the core. In this preliminary investigation, we report on the stratigraphic variations in modal mineralogy (by point counting), geochemical composition (by XRF), and plagioclase composition (by optical methods). The ultimate goal of this study is to examine the relationships between mineralogical, chemical, and textural variations in mafic sills as indicators of emplacement and differentiation processes. In a companion study (Markwood and Zieg, this volume), textural and mineralogical variations are examined in a much smaller (~1.5 m) sill. The small sill was emplaced in a single injection event, while we are hypothesizing that the larger (266 m) sill was emplaced in a series of smaller injections (as was originally proposed for the petrographically similar Logan sills by Blackadar, 1956). By comparing the variations in these two sills, we hope to develop techniques to better recognize reinjection events and characterize the petrologic evolution of mafic intrusions. Of particular interest in the differentiation of this sill is the evidence for post-injection settling of entrained olivine phenocrysts. The settling of phenocrysts to form discrete olivinerich horizons is well-known from, e.g., the Palisades sill (Gorring & Naslund, 1995). In this sill, the distribution of olivine-rich zones (Fig. 1) is similar to the variation trends in plagioclase composition (Fig. 2), for example between 75 and 110 m above the basal contact. Based on this petrographic evidence, it is proposed that this sill is a composite intrusion, built up by a sequence of at least three separate injections. This type of composite intrusion has been recognized in the Beacon sill of Antarctica (Zieg, 2007) that was emplaced without phenocrysts, and in this study we are specifically interested in examining the additional effects of phenocryst settling on the geochemical and textural variations. Future work will focus on further refining the spacing and timing of these reinjection events through additional sampling of olivine-rich zones and textural analysis of potential reinjection horizons. References Blackadar, R.G., 1956. Differentiation and assimilation in the Logan sills, Lake Superior District, Ontario. American Journal of Science, 254, 623-654. Gorring, M.L. and Naslund, H.R., 1995. Geochemical reversals within the lower 100 m of the Palisades sill, New Jersey. Contributions to Mineralogy and Petrology, 119, 263–276. van der Plas, L. and Tobi, A.C., 1965. A chart for judging the reliability of point counting results. American Journal of Science 263, 87-90. Zieg, M.J., 2007. Geochemical, mineralogical, and textural characterization of the Beacon Sill, McMurdo Dry Valleys, Antarctica. EOS, Transactions of the American Geophysical Union, 88(52), 2007 Fall Meeting Supplement, Abstract #V43A-1119. 58 �Figures Figure 1. Stacked modal abundances of major minerals at ~10 m intervals in stratigraphic height. Abundances determined by 1000-1100 grain point count, yielding maximum error of ~ ±3% (van der Plas & Tobi, 1965). Figure 2. Plagioclase composition as a function of stratigraphic height in sill, determined using Carlsbad-Albite method. 59 �QUARTZ ―EYES‖ OF THE MOOSE LAKE PORPHYRY COMPLEX, HEMLO, ONTARIO SCOTT, Robert J. and HILL, Mary Louise, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, P7B 5E1 rjscott@lakeheadu.ca. The Hemlo gold deposit, Ontario, Canada is located in the Archean Schreiber-Hemlo greenstone belt, Wawa subprovince, of the Superior Province. The deposit has been subjected to a complex history of deformation, metamorphism and intense hydrothermal alteration. The host lithology to most of the gold at Hemlo is the Moose Lake Porphyry Complex (MLPC), a felsic orthogneiss, characterized by the occurrence of quartz ―eyes‖. These quartz ―eyes‖ are actually deformed quartz porphyroclasts. The matrix of the MLPC is mylonitic, composed of quartz, microcline, minor biotite and white mica. The micas define the weak to strong foliation. Other minerals present are calcite, plagioclase, and pyrite, with trace gold, sphene and tourmaline. The porphyroclasts of quartz range in size from 1mm to 5mm and are typically elongated to form the characteristic eye shape. Rarely, feldspar porphyroclasts are present in addition to quartz porphyroclasts. Although common in association with gold deposits, quartz ―eyes‖ are generally rare. These occurrences of quartz porphyroclasts in a mylonitic quartz/feldspar matrix is curious and challenging to explain. Microscopic observations reveal the brittle deformation and alteration of feldspar porphyroclasts in spatial association with ductile deformation of quartz porphyroclasts. The existence of quartz porphyroclasts in a mylonitic matrix could be dependent on the protolith of the MLPC, which is the subject of some debate. Was the protolith a volcanic rock with quartz phenocrysts or is the MLPC a deformed mylonitized granitic intrusive rock? 60 �GOLD IN THE WABIGOON SUBPROVINCE – GIS-BASED MINERAL POTENTIAL MAPPING SIEMIENIUK, Steven, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, P7B 5E1, sesiemie@lakeheadu.ca HOLLINGS, Pete, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, P7B 5E1, pnholling@lakeheadu.ca McCUAIG, Cam, University of Western Australia, Centre for Exploration Targeting – M006, 35 Stirling Highway, Crawley, Western Australia, 6009, Australia PORWAL, Alok, University of Western Australia, Centre for Exploration Targeting – M006, 35 Stirling Highway, Crawley, Western Australia, 6009, Australia Using a Geographical Information System (GIS) it is possible to analyze multiple large datasets to identify and quantify spatial relationships. The rationale behind this project is to identify areas of potential gold mineralization in the Wabigoon Subprovince using automated empirical and conceptual techniques in a GIS environment. Specific objectives include defining empirical gold associations in the Wabigoon Subprovince; creating a template for conceptual gold exploration using a mineral systems approach; and generating empirical and conceptual maps of gold mineralization potential. While referred to as the traditional Wabigoon Subprovince, the chosen study area was defined as being the Archean rocks between the English River and Quetico sedimentary basins. The study area excludes the Proterozoic rocks of the Nipigon Basin, and is bounded on the west by the Manitoba border and on the east by the Paleozoic rocks of the James Bay Lowlands (Figure 1). Figure 1: 1:250,000 bedrock geology with study area shaded in black. (Modified from Ontario Geological Survey, Miscellaneous Release Data 126 – Revised) 61 �Mineral potential mapping is applied using either empirical (data driven) or conceptual (knowledge driven) techniques. Recent developments in mathematical geological modeling are beginning to combine the benefits of both techniques but are outside the scope of this project. Weights of Evidence (W of E) and Neural Networks will be used to perform the empirical analyses, with fuzzy logic being the technique applied to do the conceptual targeting. The mineral systems approach that will be used to do the conceptual targeting allows for expert input into both defining the critical and constituent processes in gold mineralizing environments and their subsequent ranking. The mineral systems approach to characterizing ore deposition processes stems from the oil and gas industry and the widely applied source rock-migration-trapseal methodology in petroleum exploration plays. Applied to gold mineralization, this would translate to source-active pathway-physical trap-chemical trap. While many small low-grade small-tonnage gold deposits exist, it is believed that each of the above 4 processes (source-active pathway-physical trap-chemical trap) must be present and active to generate a large gold deposit. At the scale of the Wabigoon Subprovince it is important to select datasets that have relatively uniform coverage across the study area so as to not introduce bias. To achieve this goal, publically available geosciences datasets were examined and various layers representing proxies to critical gold mineralization processes were determined with a mineral systems framework in mind. Once the data compilation and generation stage is complete, data analysis and mineral potential mapping can begin. It is important to realize that these techniques will not provide drillable targets, but rather allow for a quantifiable and justifiable means to examining multiple geosciences datasets in a common framework. Mineral potential mapping forces the examination of the spatial relationships between objects, documents the reasoning behind expert opinions and helps to eliminate ―armwaving‖ in the decision making process. Useful not only in mineral exploration, mineral potential mapping and the techniques used during its application can be useful in environmental risk assessments, land-use planning, and many other related disciplines. References Joly, A., McCuaig, C., Porwal. A., and Bagas, L. (2010) 4D integrative-approach for target generation, GranitesTanami Orogen, Western Australia, Public Lecture, Lakehead University Ontario Geological Survey (2006) 1:250 000 Scale Bedrock Geology of Ontario, Ontario Survey, Miscellaneous Release Data 126 - Revised. Geological Peterson, D. M. (2001) Development of Archean Lode-Gold and Massive Sulfide Deposit Exploration Models using Geographic Information System Applications: Targeting Mineral Exploration in Northeastern Minnesota from Analysis of Analog Canadian Mining Camps, Unpublished PhD Thesis, University of Minnesota, 503 pages. Porwal, A. K. (2006) Mineral Potential Mapping with Mathematical Geological Models, Published PhD Thesis, Utrecht University, 289 pages. 62 �THE RING OF FIRE: AN OVERVIEW OF THE GEOLOGY, MINERAL DEPOSITS AND EXPLORATION HISTORY OF THE MCFAULDS LAKE AREA SMYK, Mark, Resident Geologist Program, Ontario Geological Survey, Thunder Bay ON STOTT, Greg, Precambrian Geoscience Section, Ontario Geological Survey, Sudbury ON, and ATKINSON, Brian, Resident Geologist Program, Ontario Geological Survey, South Porcupine ON The McFaulds Lake area of northern Ontario has been the focus of intense exploration activity since 2002, already resulting in the discovery of a remarkable number of varied metallic mineral deposits, including: Volcanogenic massive sulphide (VMS) copper-zinc-silver; o e.g. McFaulds #3 deposit (Indicated: 802 000 t @ 3.75% Cu, 1.1% Zn) o e.g. McFaulds #1 deposit (Inferred: 839 700 t @ 1.03% Cu, 2.38% Zn) Orthomagmatic copper-nickel-PGE; o e.g. Eagle‘s Nest deposit (Indicated: 6.9 Mt @ 2.04% Ni, 0.95% Cu, 1.3 g/t Pt, 3.4 g/t Pd; Inferred: 4.3 Mt @ 1.42% Ni, 0.87% Cu, 0.8 g/t Pt, 3.4 g/t Pd) Orthomagmatic chromium + iron + vanadium + titanium + PGE; o e.g. Black Thor deposit (Inferred Mineral Resource: 69.5 Mt @ 31.9% Cr2O3 ; 25% Cr2O3 cut-off); and Lode gold o e.g. Triple J occurrence: (>1.0 km strike length, depth >300 m.; grades ranging from 0.3 to 30.0 g/t Au) The arcuate area termed the ―Ring of Fire‖ is underlain by the folded Neoarchean McFaulds Lake greenstone belt and subvertically dipping mafic to ultramafic intrusions, at least some of which are layered and crosscut the western portion of the belt. An altered, feldspar-phyric dacite associated with the VMS mineralization returned an age of 2737 + 7 Ma (Rayner and Stott 2005). Limited radiometric dating, field constraints indicate that there might be at least two main ages of layered intrusions preserved in this region: an older, Mesoarchean suite, ca. 2808 Ma (e.g. Highbank Lake intrusion), close to the southwestern boundary of the Oxford-Stull domain, and a younger, Neoarchean suite of intrusions (e.g. Ring of Fire Intrusion, 2735 Ma; Gowans et al. 2010) hosting the copper-nickel-PGE and chromite deposits. This latter intrusive event is synchronous with local VMS mineralization and may represent the synvolcanic heat source for the VMS system. Over 40 companies have been involved in exploration in the Ring of Fire, spending approximately $100 million since 2002. Approximately 32 000 claim units, totaling over 5000 km2, were staked. The scarcity of outcrop and Paleozoic cover in the eastern portion of the area have created a reliance on detailed geophysical surveys and diamond drilling to identify exploration targets, delineate deposits and elucidate Archean basement geology. Gowans, R., Spooner, J., San Martin, A.J., and Murahwi, C. 2010. Technical report on the Mineral Resource Estimate for the Blackbird chrome deposits, James Bay Lowlands, northern Ontario, Canada; unpublished company report, Noront Resources Limited, 188p. Rayner, N. and Stott, G.M. 2005. Discrimination of Archean domains in the Sachigo subprovince: a progress report on the geochronology; in Summary of Field Work and Other Activities 2005; Ontario Geological Survey, Open File Report 6172, p.10-1 to 10-21. 63 �STRUCTURAL CONTROL AT HAMMOND REEF GOLD DEPOSIT NORTH OF ATIKOKAN, ONTARIO STINSON, Victoria R. and HILL, Mary Louise, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, P7B 5E1; vrstinso@lakeheadu.ca Hammond Reef is located 22km north of Atikokan, Ontario, within the Marmion Lake batholith adjacent to the Finlayson Lake greenstone belt. Gold mineralization at Hammond Reef is commonly found associated with sulphides, such as pyrite, and in its native form. The main ore zones, and historical mine workings, are primarily composed of muscovite schist with greater than 20cm quartz veins within a northeast trending shear zone. The main ore zone exhibits ductile, brittle-ductile, and brittle deformation, chloritization, sericitization, and carbonitization. Intensely sheared muscovite schist hosts gold when adjacent to lithons which provide competency contrasts. Areas outside of the main ore zones also show prospect for mineralization as they too underwent similar deformation, metamorphism, and alteration histories. In granitoids the pyrite mineralization is found most abundantly in areas of rheological contrast, such as adjacent to competent lithons. The areas with the greatest amount of pyrite and gold mineralization have the largest and most intense ductile shear zones and the greatest abundance of lithons. In pegmatites the sulphide mineralization and alteration is generally absent but gold is found within structures similar to the gold-bearing granitoid. In the field, areas of highest competency contrast that are the most prospective for gold can be identified based on intensity, size, and spacing of ductile shear zones and lithons. At Hammond Reef there is a correlation between width of shear bands and intensity of ductile strain. Strain can be described as weak if shear bands are greater than 5cm wide, moderate with shear bands 5 to 2cm wide, strong with shear bands 2cm to 2mm wide, and intense with shear bands less than 2mm wide. Lithons are described based on size, lithology, composition, and characteristics which make the rock more competent than the surrounding shear zone, such as differing lithology, composition, folding, increased quartz veins, etc. Where outcrop is scarce, or pyrite is absent in areas of intense deformation adjacent to lithons, it is difficult to locate mineralized zones and other structures are required to locate areas of highest strain. Structures to be noted in the field which indicate high strain ductile and brittle-ductile deformation, and therefore the greater potential for mineralization, are pinch-and-swell, boudinage, foliation, lineation, pressure shadows of amphibole, quartz, and pyrite, drag and sheath folds, lithons, and S-C fabrics. Low-lying areas and rivers are also geomorphological indicators of eroded shear zones at Hammond Reef. Structures that have undergone the greatest amount of deformation, and therefore areas that have a history of deformation in a ductile, brittle-ductile, and brittle manner, have the greatest potential for mineralization at Hammond Reef when formed adjacent to a competent lithon. Long deformation histories result in the greatest amount of porosity and permeability, however brief, and therefore gold mineralization. Ductile structures which have been crosscut by brittleductile and brittle structures are the best indicators for mineralization as they have been reopened repeatedly through time. Prospecting along the splays of the regional shear zone utilizing knowledge of competency contrasts that have undergone brittle-ductile and ductile deformation histories should be noted in the Finlayson Lake greenstone belt and Marmion batholith area. 64 �IDENTIFICATION AND CHARACTERIZATION OF FIBROUS ZEOLITES IN WESTERN NORTH DAKOTA TRIPLETT, Jason1, jason.triplett@ndsu.edu, SAINI-EIDUKAT, Bernhardt1,2, FEIT, Sharon1, and DOLEZAL, Dillon1 1Department of Geosciences, 2Environmental and Conservation Sciences Program, North Dakota State University, Fargo, ND 58108-6050. The naturally occurring, non-asbestos, fibrous zeolite mineral erionite is a concern due to its potential for causing pleural malignant mesothelioma (PMM) in humans (Baris et al., 1987). PMM is a rare cancer usually associated with exposure to fibrous asbestos minerals. Studies have shown that low-level environmental exposure to erionite-bearing altered volcanic bedrock may be the explanation for a high PMM rate in certain locations (Metintas et al., 1999). Epidemiologic studies suggest erionite could potentially be more active and more toxic than some forms of asbestos (Metintas et al., 1999). The presence of erionite in North Dakota was reported by Forsman (1986) as occurring in the Killdeer Mountains (KDM), one of a number of buttes in western North Dakota. It was discovered in volcanic tuffs of the late Oligocene to early Miocene Arikaree Formation. Other formations throughout western North Dakota, South Dakota, and Montana have been recognized as erionite-bearing (Goodman and Pierson, 2010). Due to the possible health risks associated with inhalation of the fibers, hazard mapping (Forsman, 2006) was undertaken by the North Dakota Department of Health (NDDoH), in cooperation with the North Dakota Geological Survey (NDGS) and the Environmental Protection Agency (EPA). These investigations led to gravel quarry restrictions, gravel use restrictions, dust control measures, and guidance plans to control and reduce the overall exposure by businesses and private landowners working in close proximity to the bedrock formations and/or gravel quarries which potentially contain erionite (NDDoH 2009). For the study reported here, samples were collected from regions in ND where erionite is known or suspected to be present. A total of 37 rock and/or soil samples were taken from North and South Killdeer Mountains in Dunn County, West and East Rainy Buttes in Slope County, and White Butte, also in Slope County. Sample preparation consisted of breaking down each sample and placing it in a water column to separate any zeolite fibers from larger size fractions. Suspended materials, including the zeolite fibers, were collected from the water column and vacuum filtered. All 37 filtered samples were analyzed using powder X-ray diffraction (XRD). Of those, the 8 which showed the strongest zeolite patterns were analyzed using scanning electron microscopy (SEM). Visual inspection of the samples was conducted (Fig. 1), as well as determination of a preliminary chemical composition of the suspected zeolite fibers using SEM/EDS. The remaining 29 samples, which did not show as strong XRD indications of erionite or offretite, were also scanned by SEM to identify any other potential zeolite containing samples. The SEM/EDS chemical composition data of fibers were plotted for comparison with data from Forsman (1986) and Passaglia et al. (1998) (Fig. 2). The results of this study are consistent with those of Lowers and Meeker (2007), who analyzed fibers separated from ND soil and roadbed samples. SEM/EDS data plot in both regions of erionite and offretite. Ongoing and future work includes stratigraphic correlation, additional powder XRD and SEM analysis, and electron microprobe analysis. 65 �Figure 1: Concentrated zeolite fibers from South KDM. Left: sample jwt080602-01. Right: sample jwt080603-03 Figure 2: Comparison of SEM/EDS analyses. Passaglia et al. (1998): erionite (open diamond) and offretite (open box). Forsman (1986): erionite (bold, open box). This study: erionite/offretite (filled box). References Baris, I., Artvinli, M., Saracci, R., Simonanto, L., Pooley, F., Skidmore, J., and Wagner, C., 1987, Epidemiological and environmental evidence of the health effects of exposure to erionite fibers: A four-year study in the Cappadocian region of Turkey. International Journal of Cancer, 39:1, 10-17. Forsman, N.F., 1986, Documentation and diagenesis of tuffs in the Killdeer Mountains, Dunn County, North Dakota. Report of Investigation No. 87, North Dakota Geological Survey, 13 p. Forsman, N.F., 2006, Erionite in tuffs of North Dakota: The need for erionite hazard maps. GSA Abstracts with Programs, 38:7, 366. Goodman, B.S. and Pierson, M.P., 2010, Erionite, a naturally occurring fibrous mineral hazard in the tri-state area of North Dakota, South Dakota, and Montana. GSA Abstracts with Programs, 42:3, p. 5. Lowers, H.A., and Meeker, G.P., 2007, Denver microbeam laboratory administrative report 14012007: U.S. Geological Survey Administrative Report, 11 p. Metintas, M., Hillerdal, G., and Metintas, S., 1999, Malignant mesothelioma due to environmental exposure to erionite: follow-up of a Turkish emigrant cohort. Eur. Respiratory Journal 13, 523-526. North Dakota Department of Health, http://www.health.state.nd.us/EHS/Erionite/. Accessed Aug. 17, 2009. Passaglia, E., Artioli, G., and Gualtieri, A., 1998, Crystal chemistry of the zeolite erionite and offretite. Amer. Mineral. 83, 577-589. Support from NIH grant P20 RR016471 from the INBRE program of the National Center for Research Resources is gratefully acknowledged. 66 �REVISITING THE BARABOO BRECCIAS VAN LANKVELT, Amanda, and BJØRNERUD, Marcia, Geology Department, Lawrence University, 711 E Boldt Way, Appleton, Wisconsin 54911 USA. Many outcrops of Baraboo-interval quartzites in Wisconsin contain brecciated zones with angular quartzite fragments in a network of quartz veins. The formation of the breccias has generally been assumed to coincide with the folding and deformation of the quartzite at 1630 Ma (Holm et al 1998), but muscovite in some of the quartz veins yields well-defined 40 Ar/39Ar ages of ~1450-1460 Ma, approximately coeval with the emplacement of the Wolf River Batholith (Medaris et al 2002; 2003). By re-examining the textures in the breccias, we seek to resolve some of the ambiguity about their formation. Samples of breccia and quartz veins were collected from five sites: three in the Baraboo Syncline itself (Ableman‘s Gorge and Williams Quarry on the north limb; Devil‘s Lake on the south limb); and two from other Baraboo-interval quartzites (Necedah Mound and Waterloo). Muscovite grains in samples from Williams Quarry, Devil‘s Lake and Waterloo have yielded ‗Wolf River‘ ages (Medaris et al. 2002; 2003). Dateable muscovite is not found in the veins at Ableman‘s Gorge and is present but has not been analyzed at Necedah Mound. The outcrop-scale geometry of the breccias and veins is not the same at every site, and their relationship to other structures is not always clear. The brecciated zone at Ableman‘s Gorge is tens of meters thick, traceable for km along strike, and generally bedding-parallel. The breccias from Williams Quarry and Devil‘s Lake, in contrast, are relatively localized and have more complex geometries not obviously related to bedding or to other secondary structures. The breccias at Necedah and Waterloo are meters thick and can be traced for up to 100 m, but their relationship with primary layering, which is difficult to discern in the massive quartzite, is not known. Internally, the breccias are quite similar to each other. The breccia fragments are angular and appear to be purely dilational, since there is no evidence for cataclasis or shear offset. The fragments of quartzite have a ‗jigsaw‘ appearance and seem to have been separated almost isotropically through the growth of vein materials. The breccia vein material is composed primarily of coarse-grained quartz with small amounts of muscovite and/or kaolinite. Primary void space is present in some of the breccias. Several types of textural evidence point to multiple stages of mineral growth in the veins. Pronounced differences in the size and shape of quartz grains within individual samples suggest distinct episodes of growth, and the coarsest, commonly euhedral, quartz crystals are optically zoned, with growth bands marked by planes of fluid and mineral (mainly quartz and muscovite) inclusions. In optical cathodoluminescence microscopy, the zoned vein quartz, as well quartz grains in the host rock, are entirely dark, indicating either that the all of the quartz is compositionally uniform and/or that no ‗activator‘ trace elements are present. The muscovite grains from which Ar ages have been obtained have generally grown epitaxially on the vein quartz although in some cases (e.g., Devil‘s Lake) the textural relationships point to multiple, possibly alternating, episodes of quartz and muscovite growth. In samples from all localities, some or most of the vein quartz exhibits undulose extinction and quasi-ductile microfractures, indicating that it was subjected to significant deviatoric stress (> ~100 MPa), but small finite strains, some time after its formation. 67 �Provisional Interpretations: The dilational nature of the breccias and textural evidence for multiple, distinct episodes of mineral growth points to a sustained period of elevated fluid pressures that transiently and repeatedly reached supralithostatic values. While the vein quartz could easily have formed from locally derived silica-rich fluids, there is no source of potassium in the mineralogy of the supermature host rock (Medaris et al, 2003) to allow formation of the observed muscovite. This indicates that some of the fluids responsible for the veining came from a remote source. Although some of the breccias do occur along layer-parallel zones, this is unlikely to reflect fluid flow along primary, stratigraphically controlled channels of high permeability since the breccias formed during or after the folding event. It is more likely that flexural slip between stratigraphic layers during folding created layer-parallel zones of increased permeability that provided channels for vein-forming fluids. The textural and deformational observations in combination with the tectonic history of the Midcontinent provide several possible interpretations. One is that the breccias and most of the vein fillings formed during the Baraboo event at 1630 Ma, and that the plastic deformation in the vein quartz represents stresses late in the orogenic cycle. In this interpretation, the muscovite must have formed later, probably from fluids related to the emplacement of the Wolf River Batholith (WRB). This scenario requires some mechanism for generating elevated and fluctuating fluid pressures, and large fluid pressure gradients, to create a pervasive ‗Wolf River‘ signature in Baraboo-type quartzites (including the Sioux Quartzite of SW Minnesota; Naymark et al., 2001). Another permissible (if radical) interpretation is that the Baraboo deformation occurred at 1450 Ma, and that the WRB was not ‗anorogenic‘ as traditionally believed, but emplaced at the end of an orogenic cycle. This is consistent with evidence for a 1.4 Ga tectonic event in New Mexico (Daniel and Pyle, 2006) and would provide a source for both the K-rich fluids and the larger deviatoric stresses needed for deformation of the vein quartz. A third possible interpretation is that the vein-forming fluid flow events occurred in Wolf River time but that the vein quartz was deformed by far-field stresses from the Grenville orogeny at ca. 1 Ga. References cited Daniel, C. and Pyle, J., 2006. Monazite-xenotime thermochronometry and Al2SiO5 reaction textures in the Picuris Range, northern New Mexico: New Evidence for a 1450-1400 Ma Orogenic event. Journal of Petrology 47, 97-118. Holm, D., Schneider, D., & Coath, C., 1998. Age and deformation of Early Proterozoic quartzites in the southern Lake Superior region: Implications for extent of foreland deformation during final assembly of Laurentia. Geology 26, 907-910. Medaris, L.G., Jr., Singer, B.S., Brown, P.E., Jicha, B.R. & Smith, M.E., 2002. Wolf River-age brecciation in the Baraboo Quartzite, Wisconsin: Implications for Proterozoic tectonics in the Lake Superior region. Proceedings of the Institute on Lake Superior Geology 48, 24-25. Medaris, L.G., Jr., Singer, B.S., Dott, R.H., Jr., Naymark, A., Johnson, C.M. & Schott, R.C., 2003. Late Paleoproterozoic climate, tectonics and metamorphism in the southern Lake Superior region and Proto-North America: Evidence from Baraboo interval quartzites. Journal of Geology 111, 243-257. Naymark, A., Singer, B., Medaris, L.G., Jr., 2001. Recognition of post-1630 Ma fluid-driven metamorphism in Baraboo interval quartzites by means of laser probe geochronology. Proceedings of the Institute on Lake Superior Geology 47, 66-67. 68 �POST PENOKEAN AGE OF MINERALIZATION ON THE MARQUETTE RANGE, MICHIGAN T. D. Waggoner, Geological Consultant Email: thomaswaggonergeo@hotmail.com The Champion Mine located on the south limb of the Marquette Syncline in Western Marquette County, Michigan is noted for the diverse hydrothermal mineral assemblage that includes molybdenite, native gold, boron and bismuth. The molybdenite is found associated with greisen and with the magnetite ore. Moly occurs as coarse flakes and books that can be readily separated from the gangue. Test work was conducted under contract by David Selby in England at Durham University. The technique utilized is found in the reference listing below. With the advent of the age determination technique using isotopic Rhenium (Re) and Osmium (Os) it was natural that molybdenite would be tested as it contains an abundance of both. The decay constant of 187 Re to 187Os is the key to the age determination. Re-Os date delineation utilizes the isochron dating method assuming 187Os did not exist at the time of emplacement. The isochron plots the ratio of radiogenic 187Os to non-radiogenic 188Os against the ratio of the parent isotope 187Re to the non-radiogenic isotope 188Os. It was long suspected the alteration minerals were emplaced at the end of the Negaunee iron formation ~1874 Ma or at least no younger than the Humboldt granite (~1760 Ma) located five miles to the southeast of the Mine. It then came as some surprise when the molybdenite associated with the greisen showed an age of 1672+or-7 Ma and the molybdenite in the magnetite yielded an age of 1570+or-7 Ma. It may be that a continuum of geological events is present in the Superior Province of the Canadian Shield. Further age dating of the sulfides associated with the hard ores on the Animikie Basin will help further define the hydrothermal events. References Stein, H.J., 1999, The Status of the Re – Os Chronometer for Dating Sulfides and Oxides, in Mineral Deposits: Processes to Processing, Stanley et al. (Eds), p. 1291-1294. Selby, D. et al., 2001, Re – Os Geochronology and Systematics in Molybdenite from the Endako Porphyry Molybdenite Deposit, British Columbia, Canada, Econ. Geol. v. 96, p. 197-204. Selby, D. et al., 2001, Late and Mid-Cretaceous Mineralization in the Northern Canadian Cordillera: Constraints from Re – Os Molybdenite Dates, Econ.Geol. v. 96, p. 1461-1467. Selby, D. et al, 2009, Re – Os Sulfide (Bornite, Chalcopyrite and Pyrite) Systematics of the Carbonate-Hosted Copper Deposits at Ruby Creek, Southern Brooks Range, Alaska, Econ. Geol. v. 104, p. 437-444. 69 �NEW WAYS TO STUDY OLD PROBLEMS WEIBLEN, Paul W., Minnesota Geological Survey, pweib@umn.edu This poster shows how two recently developed instruments, the Electric-Pulse Disaggregator, and the Tabletop scanning electron microscope, provide new methods to characterize the textures, minerals, and mineral compositions of rock samples See http://www.marinereef.com and http://www.ndsu.edu/pubweb/~sainieid/PPD/Curator_ms.shtml. Samples from the Paleoproterozoic Sudbury Ejecta Layer are used as examples. The scanning electron microscope (SEM) and the electron microprobe (EMP) are widely used to augment the petrographic microscope as tools to characterize rocks. The tabletop scanning electron microscope (TSEM) provides a new and simpler tool than the SEM for obtaining images and energy dispersive x-ray (EDX) spectra (including more accurate analyses of Carbon). Since it is not necessary to evaporate a carbon film on polished thin sections or mineral grains, samples can be transferred directly from a binocular, or petrographic microscope. The convenience and time saved is significant when studying many samples (Figs. 1 and 2). Electric pulse disaggregation (EPD) provides a mechanism for liberating individual mineral grains while preserving their three dimensional surfaces. This method of mineral separation has several additional advantages over conventional mineral separation methods. First, the separated grains provide clean samples (Figs. 3 and 4) for energy dispersive x-ray, electron microprobe and bulk chemical analysis (for the two first methods boundary effects of adjacent phases in polished thin sections are eliminated). Secondly, EPD products provide samples in which interesting minerals of low concentration, e.g., zircon, and platinum group minerals can be efficiently concentrated and recovered for further study. Thirdly, concentrates of EPD products can provide samples of minerals of very low concentration or small grain size that may not be recognized in thin section or only postulated on the basis of bulk chemistry. Fig. 1. Photomicrograph of Sudbury Ejecta containing glassy fragments and spherules in a carbonate matrix. Cross polarized transmitted light. Fig. 3. EPD products of sample OB KS GTP. Note the glass spherules are cleanly separated from the carbonate matrix. 70 Fig. 2. Photomicrograph of fine-grained matrix in iron formation breccia, that cannot be resolved optically. Plain polarized light. Fig. 4. EPD Products of Sample G 652. The fine-grained matrix actually consists of euhedral crystals. Some as small as 0.050 mm. �SUDBURY IMPACT EJECTA – AMPHIBOLES IN SAMPLES FROM THE VICINITY OF THE GUNFLINT TRAIL, MINNESOTA WEIBLEN, Paul W., Minnesota Geological Survey, pweib@umn.edu JIRSA, Mark, Minnesota Geological Survey, jirsa001@umn.edu MCSWIGGEN, Peter, McSwiggen Associates, PMcS@McSwiggen.com Over two dozen outcrops of Sudbury Meteorite Impact Ejecta have been found and mapped in the vicinity of the Gunflint Trail in Minnesota (Fig. 1). A generalized stratigraphic column has Fig. 1. Geologic map depicting the Sudbury Meteorite Impact Layer, in the Gunflint Trail area. The layer lies within the contact aureole of the Mesoproterozoic intrusive rocks. Hence contact metamorphism combined with weathering (See Figs. 4 and 7) make comparison of mineral assemblages, textures, and compositions with Sudbury ejecta elsewhere problematic. been derived from these exposures (See Jirsa, this volume). The complete column is not exposed at any of the exposures. We have studied the occurrence of amphibole in the four samples noted on Fig. 1. See Figs. 2 -4 for photos. These samples provide a first look at differences in occurrence, textures, and compositions of amphibole in the basal breccia (G652), the zone of cmsized accretionary lapilli (G033 and G498, same outcrop). and an upper zone of smaller (< 3mm), pellets (G508) that are generally not zoned except for finer-grained outer rims. Optical and scanning electron microscopy (SEM), electron microprobe analysis (EPA), x-ray diffraction (XRD), and electric-pulse disaggregation (EPD) (See Weiblen this volume) have been used to characterize textures and mineral compositions. Chemical analyses have been obtained on select samples. A sampling of the results thus far is presented in Figs. 5-11. Fig. 2. Outcrop photo G 652. Fig. 3. Outcrop photo G033 Fig. 4. Sample photo G508 The photomicrographs below are all cross polarized light images. Fig. 5. G652Z-Porphyroblast Fig. 6. G498-Lapilli Matrix interface 71 Fig. 7. G508A-Pellet in Matrix �Fig. 9. Back Scatter Electron Image of EPD products from G498 Z. Euhedral crystals as small as 0.005 mm are found. Fig. 8. A portion of a 1.5 mm electron microprobe traverse across a matrix pellet interface in G 508A.. See text for discussion. G 652 Amphibole-Pyroxene Traverse Mole Fraction 1 0.8 Si 0.6 Ca 0.4 Fe Mg 0.2 142 143 135 139 140 144 0 Selected Points Fig. 10 A portion of a 1.5 mm electron microprobe traverse from the center of one accretionary lapilli across matrix into the center of an adjacent lapilli in G 498Z.. See text for discussion. Fig. 11 Selected analyses across the Amphibole (144-139)-Pyroxene (135-142) interface shown in Fig. 5. We have found it useful to distinguish the small ~< 3mm, generally unzoned ―lapilli‖ with the term ―pellet‖ (Figs. 4, 7 & 8). Unaltered/weathered pellets are just discernible in the alteredweathered rim of sample G508 A (Fig. 4). We have separated intact pellets from the matrix of altered-weathered rims by EPD. Since the pellets and the matrix have the same quartzcarbonate-amphibole mineral assemblage, it is curious that the pellets have remained impervious to alteration-weathering in contrast to the matrix. The data in Fig. 8 indicates negligible Ca in matrix amphibole in contrast to the Ca bearing pellet amphibole. Is this difference a metamorphic or an alteration-weathering effect? EPD traverses across accretionary lapilli indicate the zoning is defined by variations in quartz, carbonate, and amphibole. If amphibole formed from a Mg,Fe carbonate as found in lapilli elsewhere, we thought it remarkable that the original zoning would be preserved. However we find that the reaction products are fine-grained euhedral crystals (Fig. 8) and the zoning is preserved on a scale of 0.010-0.030 mm! The products of the reaction Mg carbonate + quartz + water + oxygen are Mg,Fe amphibole + calcite + carbon dioxide. [ 7((Mg,Fe)Ca(CO3)2) + 8SiO2 +H2O+7O2 = (Mg,Fe)7Si8O22(OH)2 +7CaCO3 +14CO2 ] This reaction could be the origin of some of the relatively pure calcite found in Gunflint ejecta samples (Incidentally, thermodynamic data indicate that dolomite + quartz + water break down to tremolite + calcite at ~ 190 oC and 1 atm.). Another example of contact metamorphism in the Gunflint area is the occurrence of porphyroblasts in some samples (Fig. 5). This preliminary data indicates that the ejecta samples contain much information on the contact aureole in the Gunflint area. For further examples of how our studies augment petrographic examination (Figs.2-4) are presented in an accompanying poster session (See Weiblen, this volume). 72 �GEOLOGIC MAP OF THE CENTRAL SEINE BAY/ BAD VERMILION LAKE INTRUSION WITH ACCOMPANYING AIRBORNE GEOPHYSICS, RELATING TO NUMAX RESOURCES, INC., MINE CENTRE PROPERTY, MINE CENTRE, ONTARIO WHITE, Chris R., 1776 W. Chig-A-Big Rd., Ely, MN 55731, white812@d.umn.edu, and ALBERS, Paul B., 3925 Willowwood St. SW, Prior Lake, MN 55372, pbalbers@gmail.com Numerous massive oxide (Fe-Ti-V) outcrops were discovered in 2004 by Numax Resources, Inc. on their Mine Centre property, which is located in northwestern Ontario. This was followed up by prospecting, mechanical trenching, channel sampling, two ground geophysical surveys (walking magnetic and VLF electromagnetic), and three diamond drill holes over the next five years. Numerous channel and drill core samples assayed greater than 60% Fe2O3, 15% TiO2, and 3000 ppm V. This encouraging data prompted Numax Resources, Inc. to requisition a detailed (1:5000 scale) geologic map of the property. Along with the property map, ten previously excavated trenches were mapped in greater detail (1:400 scale). Numerous 1 to 30 m-wide continuous to semi-continuous, massive to semi-massive oxide layers (Figs. 1 and 2) trend in an east-west to northeast-southwest orientation across the 12.5 km-long Mine Centre property (Bernatchez, 2009). These massive and semi-massive oxide layers correlate well with magnetic highs discovered during the Ontario Geological Survey airborne geophysical survey (OGS, 2009). The massive oxide layers are bound and interlayered with various medium- to coarse-grained mafic intrusive rocks consisting of gabbro, pyroxenite, and anorthosite (Albers and White, 2010). In the western portion of the property, the rocks are subvertical and dip steeply to the north. In the eastern part of the property, the rocks dip 30-60 degrees to the northwest. Tops are interpreted to the north based on graded modal layering of silicates. The northern contact of the Seine Bay/Bad Vermillion Lake intrusion is bound by felsic intrusive rocks consisting of granodiorite, quartz diorite, tonalite, and trondhjemite; while the southern contact consists of mafic volcanic rocks including massive, amygdaloidal, and plagioclase-phyric basalt. Locally, subvertical shear zones are adjacent to major lithologic contacts and massive oxide layers, though individual massive oxide layers appear to be largely undeformed. Upcoming work includes systematically-spaced diamond drilling of high grade Fe-Ti-V layers, petrographic analysis through numerous sections of the mafic intrusion, electron microprobe study of the massive oxide mineralization, 3D geologic modeling, and continued detailed geologic mapping. 73 �Figure 1. One of numerous massive oxide outrops (1.5 m wide) within sheared and/or lineated gabbro in Trench L28. Figure 2. Hematite alteration of massive oxide in Turtle Trench. Selected References Albers, P.B., and White, C.R. 2010. Report on the Geology and Mineral Potential of the Seine Bay/Bad Vermilion Lake Intrusion, Mine Centre Property, Mine Centre, Ontario. Unpublished report prepared for Numax Resources, Inc. Bernatchez, R.A. 2009. A Report on the Fe-Ti-V and Cu-Ni-PGM and other Mineral Potential for the Mine Centre Property. Unpublished report prepared for Numax Resources, Inc. Poulsen, H.K. 2000. Precambrian Geology and Mineral Occurrences, Mine Centre – Fort Frances area; Ontario Geological Survey, Map 2525, scale 1:50,000. Ontario Geological Survey. 2009. Ontario airborne geophysical surveys, Magnetic and Electromagnetic Data, Grid and Profile Data (ASCII and Geosoft formats) and Vector Data, Mine Centre Area; Ontario Geological Survey. Geophysical Dataset 1061a ISBN 978-1-4435-0854-4 [DVD] ISBN 978-1-4435-0855-1 [ZIP FILE]. 74 �Map showing location of Frances Memorial Sports Centre, starting point for several of the field trips. 75 �Map showing location of Backus Community Center in Relation to the Rainy River Inn (formerly the Holiday Inn). The banquet will be at the Community Center, located at 900 5th Street, International Falls. 76 � https://digitalcollections.lakeheadu.ca/files/original/32399adc6c38eb74de1ea21245ef73a3.pdf c2bc448b184da4557bd192d8d0e7624f PDF Text Text 56TH ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY INTERNATIONAL FALLS, MINNESOTA MAY 19-22, 2010 PROCEEDINGS VOLUME 56 PART 2 – FIELD TRIP GUIDEBOOK ��INSTITUTE ON LAKE SUPERIOR GEOLOGY 56TH ANNUAL MEETING MAY 19-22, 2010 INTERNATIONAL FALLS, MINNESOTA PETER HOLLINGS, PETER HINZ, MARK SMYK, MARK JIRSA, AND TERRY BOERBOOM Co-Chairs Proceedings Volume 56 Part 2 – Field Trip Guidebook Edited by Terrence J. Boerboom, Minnesota Geological Survey Cover photo: Moderately deformed Seine conglomerate containing metavolcanic and granitoid clasts, with clast tiling due to dextral deformation. Hwy 11, 1 km east of Horsecollar Junction �56TH INSTITUTE ON LAKE SUPERIOR GEOLOGY CONTENTS OF PROCEEDINGS VOLUME 56: PART 1: PROGRAM AND ABSTRACTS PART 2: FIELD TRIP GUIDEBOOK TRIP 1: MINERAL DEPOSITS OF THE RAINY RIVER AREA (CAN) TRIP 2: GEOLOGY OF AN ARCHEAN SUCCESSION AT ATIKOKAN (CAN) TRIP 3: STRUCTURAL GEOLOGY ALONG THE QUETICO FAULT (CAN) TRIP 4: ARCHEAN GEOLOGY OF VOYAGEURS NATIONAL PARK AND LITTLE AMERICA GOLD MINE (US) TRIP 5: ASH RIVER NEUTRINO DETECTOR LABORATORY AND THE ARCHEAN VERMILION GRANITIC COMPLEX (US) TRIP 6: TRANSECT THROUGH THE QUETICO-WABIGOON SUBPROVINCE BOUNDARY (US) TRIP 7: MINERAL DEPOSITS OF THE MINE CENTRE – RAINY LAKE AREA (CAN) TRIP 8: GEOLOGY AND ENVIRONMENTAL ISSUES OF THE STEEP ROCK MINE (CAN) Reference to material in Part 1 should follow the example below: Published by the 56th Institute on Lake Superior Geology and distributed by the ILSG Secretary: Peter Hollings Department of Geology Lakehead University Thunder Bay, ON P7B 5E1 CANADA peter.hollings@lakeheadu.ca ILSG website: http://www.lakesuperiorgeology.org ISSN 1042-9964 ii �TABLE OF CONTENTS PROCEEDINGS VOLUME 56 PART 2— PROGRAM AND ABSTRACTS TRIP 1: MINERAL DEPOSITS OF THE RAINY RIVER AREA (CAN)………………. .................... 1 TRIP 2: GEOLOGY OF AN ARCHEAN SUCCESSION AT ATIKOKAN (CAN) ………………. ..... 14 TRIP 3: STRUCTURAL GEOLOGY ALONG THE QUETICO FAULT ((CAN) ………………. ........ 47 TRIP 4: ARCHEAN GEOLOGY OF VOYAGEURS NATIONAL PARK AND LITTLE AMERICA GOLD MINE (US) ............................................... ……………….76 TRIP 5: ASH RIVER NEUTRINO DETECTOR LABORATORY AND THE ARCHEAN VERMILION GRANITIC COMPLEX (US) ………………........................................... 79 TRIP 6: TRANSECT THROUGH THE QUETICO-WABIGOON SUBPROVINCE BOUNDARY (US) ………………. ................................................... 81 TRIP 7: MINERAL DEPOSITS OF THE MINE CENTRE – RAINY LAKE AREA (CAN) .. ………….91 TRIP 8: GEOLOGY AND ENVIRONMENTAL ISSUES OF THE STEEP ROCK MINE (CAN)……….126 The editor (Terry Boerboom) would like to sincerely thank all of those who contributed to this field trip guidebook. The time and effort that went into the preparation of the field trip stops and write-ups are greatly appreciated by all. iii �. iv �Field Trip 1 Mineral Deposits of the Rainy River area, Ontario N.W. (Wally) Rayner and staff Rainy River Resources Ltd. Suite 303 – 1620 West 8th Avenue Vancouver, BC V6J 1V4 Visible gold in drill hole NR09-446 (returned 1088.45 g/t Au over 1.5 m), courtesy www.rainyriverresources.com 1 �Field Trip 1 MINERAL DEPOSITS OF THE RAINY RIVER AREA, ONTARIO B.W. (Wally) Rayner, Rainy River Resources, Ltd. INTRODUCTION Rainy River Gold Deposit, Rainy River Resources Limited (modified from: http://www.rainyriverresources.com/The-Project/Rainy-RiverOverview/default.aspx) Overview The Rainy River Gold Project (RRGP) is an advanced-stage gold exploration project situated in the southern half of Richardson Township, approximately 50 km northwest of Fort Frances (Fig. 1). In June 2005, Rainy River Resources Limited acquired a 100% interest in the project from Nuinsco Resources Limited and has since added approximately 4 million ounces in total gold resources to the deposit. The property has year-round road access, power lines in close proximity and a railway 21 km to the south. With over $65 million in its treasury, Rainy River plans to carry out aggressive exploration in 2010. In addition to over 70,000 m of planned drilling, the company will be completing a Preliminary Economic Assessment in the third quarter of 2010. The company is also conducting Metallurgical Testing and Environmental Baseline and Geotechnical Studies. (Portions of the following text have been directly extracted from SRK Consulting‘s Mineral Resource Evaluation Report dated July 10, 2009.) History of the Rainy River Gold Project (RRGP) The Rainy River Project has attracted exploration interest since 1967. Various companies including Noranda, International Nickel Corporation of Canada (INCO), Hudson‘s Bay Exploration and Development and Mingold Resources operated in the area centered on the RRGP between 1967 and 1989. The Ontario Geological Survey undertook geological mapping in 1971 and again in 1987-88 in conjunction with and a rotasonic overburden drilling program. Nuinsco undertook exploration activities between 1990 and 2004, with Rainy River continuing from 2005 onwards. Nuinsco drilled a series of widely spaced reverse circulation drill holes from 1994 to 1998, defining a 15 km long ―gold-grains-in-till‖ dispersal train emanating from a 2 �thickly overburden-covered, 6 km2 "gold-in-bedrock" anomaly. Nuinsco completed a series of diamond drilling programs to assess the mineral potential of the above anomalies which led to the initial discovery of the 17 Zone in 1994. Nuinsco subsequently discovered the 34 Zone in 1995 and 433 Zone in 1997. Between 1994 and 1998, Nuinsco drilled 597 reverse circulation holes and 217 diamond drill holes (49,515 m). These were mostly in the Richardson area. The 34 Zone was further drill-tested between 1999 and 2004. In June 2005, Rainy River completed the acquisition of a 100% interest in the project from Nuinsco. In the same year Rainy River re-logged key sections of the historical core drilled on the property and then input all of the data into a GIS database. Rainy River subsequently drilled in excess of 100 reverse circulation holes in three phases to better define the ―gold-in-till‖ and ―gold-in-bedrock‖ anomalies. In 2007, Rainy River discovered the ODM Zone, which is correlated as being the western extension of the 17 Zone. Rainy River completed a fourth phase of exploratory reverse circulation drilling at this time. Between 2005 and 2007 an additional 209 diamond drill holes for 95,340 m were drilled. In April 2008, a mineral resource estimate for the RRGP was completed. At a cut-off grade of 0.5 g/t Au, an Indicated resource of 34,238,000 t at 1.26 g/t Au and an Inferred resource of 67,564,000 t at 1.03 g/t Au were reported. Figure 1. Location of the Rainy River property (from http://www.rainyriverresources.com/Theme/RainyRiver/files/Technical_Reports/July%2010,%2 02009.pdf) 3 �In 2008, Rainy River drilled an additional 112 diamond drill boreholes for 59,719 m and completed a fifth phase of reverse circulation drilling near the resource area totaling 47 holes. During this time a better understanding of the deposit was gained with additional zones of mineralization recognized, including the HS zone. The addition of 124 diamond drill holes (68,453 m) from the 2009 drilling campaign has resulted in an overall increase in gold resources to 2.37 M ounces gold in the indicated category and 2.66 M ounces gold in the inferred category. The company‘s focus of defining mineralization below the limits of the proposed open pit has successfully increased total underground gold resources to 935,000 ounces in all classes, a 298% increase over the 2009 NI 43-101-compliant estimate. Regional and Local Geology The Rainy River property falls within the 2.7 Ga Rainy River Greenstone Belt (―RRGB; Fig. 2‖) which forms part of the Wabigoon Subprovince. The Wabigoon Subprovince is a 900 km long east-trending area of komatiitic to calc-alkaline metavolcanic rocks which are in turn succeeded by clastic and chemical sedimentary rocks. Granitoid batholiths have intruded into these rocks, forming synformal structures in the supracrustal rocks which often have shear zones along their axial planes. Figure 2. Regional geology of the Rainy River Gold Property (RRGP) (modified from http://www.rainyriverresources.com/Theme/RainyRiver/files/Technical_Reports/July%2010,%2 02009.pdf and from OGS Map 2443) 4 �The Wabigoon Subprovince is host to the Sturgeon Lake volcanogenic massive sulphide (VMS) deposits to the northeast in addition to several other, but smaller, orebodies. It is a very prospective terrain in which to conduct exploration. The Wabigoon Subprovince was locally overlain by Mesozoic (Jurassic and Cretaceous) sedimentary rocks and were subjected to deep lateritic weathering followed by Quaternary glaciation. Limited preservation of the Mesozoic cover sediments and saprolite occurs in localized palaeolows. The Wabigoon basement rocks and remnant Mesozoic sedimentary rocks are overlain by Labradorian till of northeastern provenance. This till has been found to contain anomalous concentrations of gold grains, auriferous pyrite and Cu-Zn-sulphides. It is overlain by a glaciolacustrine clay and silt horizon and by argillaceous and calcareous Keewatin till of western provenance. The Rainy River area therefore was covered successively by the Labradorean and Keewatin ice sheets. The Property is centered on Richardson Township with the Sabaskong granitoid batholith to the north and the Black Hawk Stock to the east. A package of metasedimentary rocks is found south of Richardson Township. Wedged in between these lithologies are a conformable series of tholeiitic mafic and overlying calc-alkalic intermediate to felsic metavolcanic rocks. These strike almost east and dip to the south (Fig. 3). Figure 3. Bedrock geological interpretation for the area surrounding the Richardson caldera (from: http://www.rainyriverresources.com/Theme/RainyRiver/files/ Technical Reports/July%2010,%202009.pdf) 5 �Intermediate metavolcanic rocks (dacites) host most of the Rainy River gold mineralization. A well-defined penetrative fabric is commonly observed on a regional scale. This foliation is approximately parallel to the trend of the metavolcanic rocks on the Rainy River Property, which strike at approximately 300° and dip 50° to 70° to the south. A steep, southwest-plunging stretching lineation is recorded in all lithologies within the main mineralized zones, with kinematic indicators suggesting south-over-north, reverse-sinistral deformation. The regional-scale, east-trending Quetico Fault has been interpreted to not extend as far west as the Richardson Township area, not being detected in diamond and reverse circulation drilling programs to date. No other thoroughgoing shear zones have been identified; significant shearing appears to be restricted to gold-bearing rocks that were weakened by the earlier volcanogenic alteration. Late, broadly north-trending, brittle faulting produces small offsets in the mineralized horizons. Deposit Types and Mineralization Early exploration work on the Rainy River Project was based on the premise that the gold mineralization was of a shear-hosted, epigenetic type. As exploration activities progressed, a volcanogenic massive sulphide model was proposed. A volcanogenic caldera model (termed the ‗Richardson Caldera‘ model) was proposed in 1997 to explain the gold and sulphide mineralization observed at Rainy River. Recent studies of all available exploration data support a model of sulphide and gold mineralization being of early volcanogenic rather than later epigenetic (shear-hosted) origin, belonging to the gold-rich subclass of the volcanogenic massive sulphide spectrum although the sulphides are mainly disseminated rather than massive. Several mineralized zones have been delineated by exploration on the RRGP to date (Fig. 4). Gold mineralization is found in the southern Cap Zone, the central ODM/17, Beaver Pond and West Zones and the northern HS and 433 Zones. Recent studies suggest that there are at least two stages of gold mineralization within the Rainy River Project, an early pervasive disseminated stage as well as a later stage of cross-cutting veinlets that often contain visible gold. The lowest-grade, disseminated gold mineralization is mostly associated with deformed volcaniclastic (permeable) dacites. Zones of higher gold mineralization are often associated with strong silicification and finely layered, foliation-parallel sphalerite and pyrite. Visible gold is typically associated with narrow (<2 cm thick) quartz veinlets, narrow pyrite veins or a sphalerite- / pyrite-rich breccia matrix. Detailed mineralogy studies have revealed that gold and electrum occur as inclusions in pyrite or in close association with sphalerite, carbonates (ankerite) and fine-grained silicates. The gold-bearing pyrite occurs as relict, anhedral grains, whereas the gold-rich sphalerite represents late-stage sulphides that formed rims on or infilled fractures in pyrite. Magmatic Ni-Cu-Co-PGE-Au-Ag mineralization is found in the 34 Zone which is associated with a tubular, late-stage pyroxenite-gabbro intrusion that crosscuts the ODM/17 Zone. The magmatic sulphides vary from massive to net-textured and disseminated. 6 �Figure 4. Resource zones location map with Digital Elevation Model backdrop (http://www.rainyriverresources.com/Theme/RainyRiver/files/images/Plan_map_april.jpg) 7 �FIELD TRIP STOPS UTM Coordinates are nad ‘83 Zone 15 Rainy River Gold Project – Richardson Twp Ontario ** Please refer to map at end of field trip stop descriptions** MAFIC UNITS: *Stop 1A 1B UTM- 422272E/5411907N UTM- 422222E/5411941N Local Information: outcrop mapped by Blackburn (1976) is adjacent to reverse circulation (RC) drill hole #358 and identified as andesite, where as bedrock in RC holes #357 and #358 was identified as dacite, indicating that this first stop is very close to a lithological contact. Lithology: definite mafic rock containing (>40 % actinolite, chlorite). Southern part of outcrop is thick lava flow while the northern part is partly pillowed. Flows face southwestward and foliation trend is 055deg dipping 85 deg SE. Comments: The orientation of the lava flow is compatible with the regional strike of the predacite mafic units on the east limb of the Dearlock syncline as shown by Blackburn (1976). *Stop 2 UTM-425740E/5411640N Local information: Isolated exposure on the north side of sand and gravel pit. Lithology: Medium grained mafic rock that can be either part of thick lava flow or intrusion. Pillowed mafics present. Mafics are intruded by several generations of porphyritic, felsic to intermediate dykes up to 2 m wide. Comments: Part of the pre-dacite mafic basement sequence on the east side of the eastern inner caldera fault. Several 10-20 cm wide fault zones marked by brecciation and increased schistosity trending 040-050 degrees. No stripped outcrop. Stop 3A 3B UTM 427048E/5411103N UTM 426607E/5411108N Local Information: Exposures on Roen pasture. Lithology: Medium grained mafic rock that could be either part of thick lava flow or intrusions. Pillows present locally trend NE and face SE. Minor porphyritic felsic to intermediate dykes. Comments: Part of the pre-dacite basement mafic sequence, east of the eastern inner caldera fault. The base of the dacite is inferred to be 150m above to the SE of the outcrop area. 8 �*Stop 4A 4B 4C UTM 425580E/5409350N UTM 425423E/5409413N Local Information: These series of stripped areas follow the strike of a mafic lava flow sequence and metagabbro occurrences within the dacite caldera sequence. 4A was stripped by Nuinsco as a possible site for a portal for underground exploration. 4B and 4C were stripped as part of a Nuinsco exploration program. Lithology: 4A- mafic lava flows vary between porphyritic and non-porphyritic and have been cut by fine grained quartz phyric intermediate dykes. Flow top breccia zones up to 50 cm thick are observed in the trenches. 4B- mafic lava flow with quartz carbonate and quartz carbonate sulphide veining host the Cap mineralization. Vein sulphides are primarily pyrite along with chalcopyrite and sphalerite. 4C- mafic lava flow in contact with dacite, possible fault contact. Comments: This mafic sequence is an inter-caldera flow that may have formed a cap on the main dacite sequence hosting the Rainy River Gold deposit. Within the mafic sequence are sills of metagabbro. *Stop 5 UTM-423174E/5410172N Local Information: Mafic outcrop exposed on Highway 600 toward Dearlock. Lithology: North trending ophitic metagabbro sill (possible thick mafic flow) 100 m thick, 400m strike length. Western contact not exposed. On the east, the metagabbro is in sharp contact with massive and pillowed mafic lava flows. Pillow shapes indicate that the flows face westward. Comments: Attitude of the mafic flows is compatible with the regional structural trend of Blackburn(1976) but is not compatible with the rock units deduced from bedrock encountered by RC drilling by ODM (Averill 1997). INTERMEDIATE UNITS: *Stop 6 UTM-426650E/5410563N Local Information: Davis Pasture, partly cleaned outcrops. Lithology: Dominantly a subaqueous, felsic-intermediate lava flow characterized by white weathered flow lobes > 10 meters thick, 50 m long at 035 degrees. In the NW corner of the outcrop there is a contact between a felsic to intermediate dyke on the SE and brecciated dacite on the NW at 030 degrees. Comments: Rusty matrix is probably an autoclastic breccia or hyaloclastite produced by quench fragmentation of felsic magma during the advance of the lava flow under water. The intermediate dyke is only sparsely porphyritic and phenocrysts are generally > 2 mm in size. 9 �*Stop 7 UTM-426148E/5410326N Local Information: These outcrops occur behind Alvin McClain‘s residence and on strike with the units at Station #6. Lithology: The lithologies exposed are complicated by a number of faults. Volcaniclastic conglomerates occur at the north end of the outcrop up to 5 m thick and comprised of 75-90 % sub-rounded to rounded felsic to intermediate clasts in a coarse sandy matrix. Rare fuchsite and chlorite rich pebbles are observed. Several brecciated lava flows occur in sequence; these units contain rounded-angular lapilli and blocks from 5mm to 10‘s of cm. Quartz crystal rich dacite is most abundant lithology in this outcrop. Comments: this is a key outcrop as it exposes a number of lithologies and relationships between them. The brecciated units and the quartz crystal rich dacite may represent a fall deposit or pyroclastic flow. Alternately this area may represent similar quench fragmentation of subaqueous dacite flows as seen at Stop 6 or fragmented lava domes within the caldera complex. *Stop 8 UTM- 426273E/5409497N Local Information: A well exposed, cleaned, discontinuous outcrop area south of the drill road. ATV outcrop. Lithology: Most of the outcrop is quartz crystal bearing dacite that contains about 5% medium grained rounded quartz crystals as well as rare rounded rock fragments. Within this unit are discontinues beds of sandstone and conglomerate. The massive dacite shows well developed late jointing and minor displacement faulting. The mafic dyke that strikes E-W across the top of the outcrop shows the style of displacement that is observed in drill core along main litho contacts as well as in the surface projection of the ODM/17 zone mineralization. Comments: this outcrop represents the upper part of the thick dacite unit which is the hanging wall to the main ODM/17 zone mineralization. Based in the presence of sparse rock fragments and the variability in crystal content across the unit, Ayres interpreted this quartz bearing dacite unit as a pyroclastic flow deposit. Drill hole NR97-43 is in this outcrop. Stop 9 UTM-426074E/5408994N Local Information: Large lichen covered outcrop west of Highway 600 north of the Teeple farm drainage ditch. Lithology: Relatively uniform quartz crystal bearing dacite that contains 5% quartz and 15-20% plagioclase crystals of similar size. The unit is intruded by several metagabbro dykes. Comments: Intermediate unit of upper dacite that occurs stratigraphically above the capping mafic unit to the northwest. This dacite is probably of crystal tuff fall deposit or pyroclastic flow unit. 10 �*Stop 10 UTM-426230E/5409875N Local Information: West of Highway 600, opposite the junction with Roen Road. Lithology: Relatively uniform quartz crystal poor dacite. No clast or internal features were observed in this exposure. In the NE corner of the outcrop, there is a 2 m wide zone with strong planar fabric trending 140 degrees. This fabric is parallel to the trend of the inferred east boundary caldera fault. Comments: Rocks are part of the thick eastern dacite sequence on the east site of the eastern inner caldera fault, but they are relatively close to the inferred location of the fault. This unit is a dacite tuff fall or pyroclastic flow. Stop 11 UTM-426320E/5409850N Local Information: Large, lichen covered outcrop east of Highway 600 Lithology: This large outcrop area contains lithologies that are mainly dacite quartz crystal tuff and or flows. Locally pebble to cobble conglomerate units contain rounded felsic to intermediate clasts. The pyroclastic rocks appear to be pumice rich. The dacite is intruded by a number of metagabbro (mafic) dykes that have diverse trends and cut by a number of small scale faults. The faults are in places several metres across and are hosts to diversely orientated quartz veins. Comments: This unit weathers with positive relief and may represent the up lift part of the eastern caldera block. Stop 12 UTM-426900E/5410000N Local Information: Large lichen covered outcrop immediately east of station 11 and east of the junction with the Roen Road. Lithology: Quartz crystals are ubiquitous and variable abundance, exposures are similar to the outcrop at stop 11. Outcrops in this area weather with higher relief, due to less alteration and the presence of metamorphic biotite and hornblende. Comments: These rocks probable fall within the metamorphic halo resulting from the emplacement of the Blackhawk Stock. Stop 13 UTM-427536E/5409453N Local Information: Large outcrop area north and east of the Teeple farm. Lithology: Outcrops are mainly quartz and feldspar porphyritic flow and tuffs and autoclastic breccias interbedded with lens of cobble to boulder conglomerate, locally with exotic clasts of unknown origin. Comments: Ayres mapping in 2005 indicated that conglomerate beds range up to 120m thick, this unit was not located in subsequent mapping of this area. 11 �Stop 14 UTM-427300E/5410175N Local Information: Series of poorly exposed small outcrops along a highlevel swamp within the same outcrop area as stop 13. Access is from the Clark Road that runs along the east side of the Teeple Farm. Lithology: Pyritic mafic tuff or flow, possibly hyaloclastite about 50 m thick. Comments: Strong AEM conductor drilled by Nuinsco. Both pyrite and pyrrhotite occur in this unit and explains the conductive nature of the lithology. This unit is either a mafic flow within the dacite or part of the underlying basement sequence displaced into its current position by unrecognized fault. FELSIC UNITS: *Stop 15 UTM-426632E/5408608N Local Information: Small outcrop in Teeple pasture about 30m north of Highway 600. Lithology: Quartz and plagioclase crystal bearing unit with no primary structures. Possible felsic lava flow northeast of the dyke. Comments: Outcrop cut by northwest trending diabase dyke. Tour note: The stops mark with * are recommended the other are of general interest. Lunch break for the tour group will be at the core shack where core from the central part of the ODM/17 zone will be on display as well as other polished core of high grade mineralization. 12 �13 �Field Guide Addendum Rainy River Resources OFF Lake STOP 1: Cunningham Farm (PRIVATE PROPERTY) Two outcrops in the south part of the OLFC contain extensive quartz vein systems. The vein systems were discovered by Dr. Lorne Ayers in 2006 during the course of geological mapping. The host rock in both outcrops is a suite of white to pale grey, to pale brown, to rusty weathering, quartzphyric felsic dikes that contain 1 to 3%, 1- to 4-mm, quartz phenocrysts. Groundmass of the dikes has been recrystallized and is now fine grained. Dikes vary from foliated to massive. The two outcrops may be part of a single vein system. A. Northern (Lorne’s) Outcrop (436670E; 5412250N) The vein system in the northern outcrop (Figure 1), was mechanically cleaned and washed in October, 2007, and has since been channel sampled. This is a relatively small, subcircular outcrop, the cleaned part of which is 35 m (east-west) by 25 m (north-south). On this outcrop, there is evidence of at least three intrusive events, three vein injection events, several periods of deformation and sulphide mineralization. Felsic dike complex The first intrusive event was emplacement of felsic dikes. Because of early deformation of the dikes, resulting in a well developed fabric, injection of voluminous quartz veins, and other stages of intrusion, vein injection and deformation, it was not possible to identify more than one phase of dike emplacement on the northern outcrop. However, based on observations made elsewhere (Ayres, 2007), including the larger southern outcrop, dike emplacement was a multiple intrusive event with some dike emplacement possibly postdating mineralization. The early fabric, which trends 000° and has a vertical dip, is a combination of sericitic foliation, fractures and <0.5-cm-wide quartz veins. Mineralization The quartz-phyric, felsic dikes contain as much as 10% pyrite and minor chalcopyrite. The sulphide minerals occur as disseminated grains and aggregates, which are as much as 1 cm in diameter and in narrow, sericitic, shear zones. Anomalous gold values occur in a sample collected in 2006 (Ayres, 2007). As noted below, sulphide mineralization also occurs in early quartz veins. Early quartz veins At the northern outcrop, the felsic dike complex was injected by an early stockwork of pale-grey, glassy, quartz veins that contain <5% pyrite and have localized malachite staining. Most veins are 1 to 5 cm wide and are typically 5 to 30 cm apart, but locally veins are 40 to 120 cm wide; the wider veins appear to be discontinuous pods and/or the result of amalgamation of veins. Although the veins are interconnected to form a stockwork, there is, in most places, a preferred orientation to many veins. Vein orientation ranges from 100 to 160°, and the veins dip between 45° SW and 90°; connector veins are as much as 90° divergent to this trend. Variations in vein trend are a function of both differences in orientation from place to place across the outcrop and warping of veins. The preferred orientation of veins may be a function of a dominant fracture system and associated subsidiary fracture systems, flattening, or a combination of these factors. Some vein deformation is suggested by localized warping of the veins. In most of the northern outcrop, the early quartz veins range in abundance from 10 to 70%, although there are local areas where only a few veins were observed. Veins are most abundant in the central 14 �part of the outcrop and decrease in abundance toward the southwest and northeast margins. Where veins are abundant, the quartz-phyric, felsic, host rock forms angular, elongated fragments surrounded by quartz. Where veins are less abundant, vein habit is variable, and this variation is, at least in part, a function of vein abundance: 1. in some places, particularly where vein width is variable, the host rock has a brecciated appearance with rounded to angular, more equidimensional fragments separated by 1- to 5cmwide veins; <1cm-wide, subsidiary veins extend into, and across the larger fragments; and 2. in other places, particularly where vein abundance is only 10 to 15%, veins have a relatively consistent trend, and few veins deviate more than 20° from this trend; these veins are partly interconnected and, in places, bifurcate and rejoin, surrounding lenticular rock areas, but there is no obvious stockwork or breccia pattern. On the southwest side of the outcrop, there is a relatively abrupt boundary, with a general trend of 120°, between an area that contains abundant quartz veins on the northeast and an area with only sparse veins on the southwest. This boundary is gradational over 1-2m, and the trend of the boundary is irregular; the irregularities have amplitudes of several metres. From this boundary, the vein zone extends northeastward for 23 m to the edge of the cleaned exposure. On the northeast side of the exposure, vein abundance is less than in the central part of the stockwork zone, but quartz veins are still present in a small outcrop, which was not cleaned, about 15 m northeast of the cleaned exposure. Near the east side of the outcrop, there are two fault-bounded blocks that are 1 to 3 m wide and contain only sparse, early quartz veins. Boundary faults truncate early quartz veins in adjacent parts of the outcrop. These blocks are wedge-shaped on horizontal outcrop surfaces, consist of the same quartz-phyric, felsic host rock as elsewhere on the is 4 m long and 1 m wide; it has a general northerly trend, subparallel to the well developed foliation. A second northerly trending block at least 3 m wide is incompletely exposed in the southeast corner of the outcrop. Early faults and veins The early quartz veins are offset along faults that typically trend 020 to 030°; in most places the offset is only a few centimetres. The faults vary from ductile to brittle structures. On the outcrop surface, ductile faults lack obvious fault lines whereas brittle faults have obvious fault lines, some of which are filled by relatively straight, quartz + chlorite veins that are as much as several centimetres wide. Locally, early quartz veins have been dragged and rotated against faults that are now filled by quartz + chlorite veins. The dragging indicates a left-lateral sense of movement and some dragged veins now almost parallel the early faults. Late quartz veins On the northwest side of the outcrop, several, white to locally rusty, quartz veins as much as 40 cm wide transect both the early quartz veins and the quartz + chlorite veins; no sulphide minerals were observed in these veins. These late veins have an average trend of 045°, and dip ranges from 40°NW to 70°SE. Vein margins vary from straight to irregular with narrow vein offshoots extending as much as several metres subparallel to the main veins and 40 cm perpendicular to the main veins. Late granitoid dikes Two distinct ages of granitoid dikes were intruded into the late quartz veins and all older structures and rock units. The granitoid dikes are distinguished by colour and trend, and they comprise an earlier, texturally variable, pink- to pale-grey-weathering phase and a younger pale-grey- to cream- 15 �weathering phase. Dikes of both phases extend completely across the outcrop. These dikes have not been metamorphosed, and they are probably related to the late tectonic Finland stock. As mapped by Blackburn (1976), the margin of this stock is 600 m west of the northern outcrop. Alternatively, the dikes could be related to the Fleming batholith, the western edge of which is 1200 m east of this outcrop. Three, pink- to grey-weathering, leucocratic dikes, ranging in width from 1 to 15 cm, were observed; the dikes are slightly sinuous, but the average trend is 060°, and the dip is 75°NW. The dikes contain at least 25% quartz and <1% mafic minerals, and, texturally, they vary from aplitic to pegmatitic. The pegmatitic dike, which ranges in width from 1 to 3 cm, has a central, 5- to 10-mm-wide, quartzmuscovite zone with a grain size of 5 to 8 mm. The younger, pale-grey- to cream-weathering dikes have straight to sinuous to zigzag habits, and they trend 020° to 030°. They include a >3-m-wide dike that is incompletely exposed at the southwest corner of the exposure, and two, closely spaced, 10- to 20-cm-wide dikes in the centre of the outcrop; one of the narrow dikes bifurcates, but the second branch terminates within 2 m. These dikes have a grain size of 1 to 2 mm, and they contain 25 to 30% quartz and 5% biotite. The wider dike contains sparse, 1- to 2-cm-wide, unmineralized, quartz veins, and it is weakly foliated parallel to the contact, which is a 2-mm-wide, pinch and swell shear zone. B. Southern outcrop: (436648E; 5411951N) In this outcrop, which is considerably larger than the northern outcrop, the quartz vein system occurs along the relatively poorly exposed northwest edge; the vein system has been stripped in a number of places by Joe Hackl and the author. The quartz vein system is very similar to that in the better exposed northern outcrop. Pale-grey, quartz veins that are generally <5 cm wide, but locally are as much as 50 cm wide, occur in foliated, rustyweathering, quartz-phyric, felsic dikes. Most veins preferentially trend between 030° and 100°, but the veins are interconnected to form a stockwork within an in situ breccia. The vein system is at least 30 m wide, and it was traced about 100 m to the northeast, along a trend of 045°, before disappearing in an area of poor exposure about half way across the outcrop. To the northeast, the vein system appears to decrease in width, and the system here is less obvious because there is less rusty weathering. On the southeast side of the quartz-injection zone, foliated, rusty-weathering, quartz-phyric, felsic dikes that contain the vein system are in sharp contact with a more massive, quartz-phyric, felsic phase that lacks quartz veins and rusty weathering. The contact is covered by a 1m width of overburden, and it was not actually observed. However, there is good control on the contact location, and the contact trends 030°, subparallel to the vein system. The more massive phase could be a younger intrusion, although still part of the Off Lake felsic dike complex. The vein system in this outcrop was intruded by two dikes that may be related to the late tectonic, Finland stock. These include: 1. a 30cm-wide, pink-weathering, feldspar-phyric, granitoid dike that contains sparse, 1cm-long, feldspar phenocrysts in a medium-grained groundmass, and 2. a 20cm-wide, zigzag, pegmatite dike.The granitoid dike trends 030° and the average trend of the pegmatite is 060°. In the southern outcrop, the veins are on the west side of the outcrop and appear to form a generally northerly trending zone as much as 10 m wide; within this zone, individual veins are as much as 1 m wide and have diverse trends. 16 �Genesis of the vein systems If the pale-grey vein system in the southern outcrop is projected along the apparent trend of the system, as established by the dominant trend of the veins and tracing of the system, it would be close to the early vein system in the northern outcrop, although the vein system in the northern outcrop has a different average trend. This spatial relationship in addition to the lack of vein systems on outcrops farther north suggest that the two vein systems are genetically related. The early quartz veins predate several generations of later quartz veins, both ductile and brittle faults, and two periods of granitoid intrusions. The early age of the veins and the sharp contact on the southern outcrop between foliated, quartz-phyric, felsic rocks containing the quartz stockwork and a more massive, quartz-phyric, felsic unit that lacks quartz veins suggests that the vein system may be related to emplacement of the Off Lake felsic dike complex. The only similar quartz stockwork observed to date in the area is associated with the Off Lake fault. The vein system on the Cunningham option has a distinctly different trend to that of both the Off Lake fault, the inferred extension of which is about 800 m to the east, and the Potts fault, which is about 1 km to the south. However, the quartz stockwork on the Cunningham option may be related to an undiscovered early fault in the southern part of the Off Lake felsic dike complex. 17 �Field Trip 2 GEOLOGY OF THE STEEP ROCK GROUP: ANATOMY OF AN ARCHEAN CARBONATE PLATFORM Philip Fralick Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada, P7B 5E1 philip.fralick@lakeheadu.ca Part A – Geology of the Steep Rock Group: Anatomy of an Archean Carbonate Platform. Part B – Description of Hammond Reef Gold Deposit, Brett Resources Inc. INTRODUCTION The Steep Rock Group is located near Atikokan, Ontario where it lies unconformably on Mesoarchean tonalite (Figures 1 and 2). It was first described by Smyth (1881) as consisting from bottom to top of: 1) Conglomerate, 2) Lower Limestone, 3) Ferruginous Formation, 4) Interbedded Crystalline Traps, 5) Upper Calcareous Greenschist, 6) Upper Conglomerate, 7) Greenstones, 8) Agglomerate, and 9) Dark Grey Slate. Lawson (1913) reduced this to four units by combining the upper six into one unit that he described as interbedded crystalline traps. Jolliffe (1966) applied the term Group to the succession, thus changing its status from a Series (chronostratigraphic unit), which it had been termed before, to a lithostratigraphic unit. He also separated the volcanic rocks into a lower Ashrock and an upper assemblage of flows, tuffs and sedimentary rocks. Wilks and Nisbet (1988) were the first to formally define the Steep Rock Group according to the procedures put forward by the International Commission on Stratigraphic Classification outlined in Hedberg (1972, and subsequent publications of the ICSC). Their formal classification remains in place and should be followed when referring to the rock units that were defined. From bottom to top the Steep Rock Group consists of (Wilks and Nisbet, 1988): 18 �Wagita Formation – predominantly conglomerate and sandstone with some pelite (mudstone). It is 0 to 150 m thick and appears to occupy depressions (paleovalleys) in the unconformably underlying Marmion Complex. It is overlain conformably by the Mosher Carbonate Mosher Carbonate – This unit is composed mainly of calcite with minor amounts of dolomite, ferronian dolomite (ankerite), quartz, pyrite and carbon (in the form of kerogen, Rothpletz 1916; Hayes et al, 1983). It is up to 500 meters thick with an irregular upper surface. Jolliffe Ore Zone – The lower member consists of the unconsolidated, earthy, Manganiferous Paint Rock Member. It is composed of fragments of goethite, hematite, chert and quartz with a matrix consisting of these minerals plus kaolinite, illite, calcite, gibbsite and pyrolusite (Huston, 1956). It is 100 to 300 meters thick and averages 3.8 % manganese. The Goethite Member overlies the Manganiferous Paint Rock Member. It is composed of goethite (67%) and hematite (21%) with accessory quartz and kaolinite. Where the ore is not brecciated and altered it resembles banded iron formation. This unit is 50 to 100 meters thick and is overlain by the Dismal Ashrock. Dismal Ashrock – This unit is predominantly a mafic to ultramafic pyroclastic rock. Most clasts are rounded and less than 1 centimeter in diameter but fragments up to seven meters occur. Thin volcanic flows interlayer with the pyroclastics. Lenses of massive bedded pyrite and carbonaceous chert also occur in this Formation and the Jolliffe Ore Zone. The ashrock is 100 to 400 meters thick and is possibly structurally overlain by the Witch Bay Formation. Witch Bay Formation – Mafic and intermediate volcanic and sedimentary rocks make up this unit. It is extensively deformed with a minimum, and poorly constrained, thickness of 1000 meters. Its lower contact may be a structural discontinuity and it is only tentatively included in the Steep Rock Group. Its upper contact is either intrusive or structural. All units in the above description have been metamorphosed and the prefix meta is implied to apply to all rock names. Kusky and Hudleston (1999) believe that both the Dismal Ashrock and the Witch Bay Formation are allochthonous, having been tectonically transported to their present positions. They interpret the Dismal Ashrock as a mélange with a ductile high strain zone at its top. They further believe that deposition of the Steep Rock Group began at 3.0 Ga as the Marmion Gneiss Complex rifted, the Jolliffe Ore Zone was formed by the sub-aerial exposure and weathering of the Mosher Carbonate and the Steep Rock Group may have formed a portion of a linear array of 3.0 Ga platform sequences along with the Sachigo and La Grande River Subprovinces. The highly oxidized state of the Jolliffe Ore Zone and its forming under an Archean anoxic atmosphere is difficult to reconcile. This combined with the huge amount of limestone that would need to be dissolved to form a terra rosa up to 300 meters thick make their model for the formation of the Jolliffe Ore Zone untenable. In addition, age determinations obtained since this paper was published throw other portions of their conclusions into serious doubt. The age of the Steep Rock succession remained conjectural for most of its history in the literature. The first step towards understanding the age relationships occurred in 1988 when 19 �Davis and Jackson published a U-Pb zircon age of approximately 3000 Ma for the Marmion Batholith. This age was refined to 3001.6 1.7 Ma by Tomlinson et al. (2003). A U-Pb zircon age of 2780.4 1.4 Ma was also obtained by Tomlinson et al. (2003) for the Dismal Ashrock, though the age of this zircon may reflect inheritance. Based on correlations between the Steep Rock area and the Finlayson and Lumby areas to the north, which were put forward by Fralick and King (1996), Fralick et al. (2008) bracketed the age of the Steep Rock assemblage between 2828 Ma, the age of volcanics in the Lumby Lake belt that underlie the sedimentary rocks, and 2780 Ma, the youngest zircon in the overlying Dismal Ashrock. Recently age determinations on detrital zircons obtained from basal siliciclastics of the Wagita Formation provided a youngest zircon age of 2779 22 Ma (Denver Stone, O.G.S., personal communication). Unlike age determinations on igneous rocks, where the statistically most likely age is given as the probable age, ages on detrital zircons indicate the unit is probably younger than a specific age and, therefore, the older age of the range is quoted. Thus, the Wagita Formation is likely younger than 2801 Ma. This agrees well with the age put forward by Fralick et al. (2008) of between 2828 and 2780 Ma. REGIONAL SETTING The following is taken largely from Fralick et al. (2008). Examining the Steep Rock Group in isolation, without considering its place within the framework of correlative units in the area does not allow a regional picture to emerge. The Steep Rock Group forms a portion of a once continuous rock assemblage that has been dissected by northeast trending faults (Figures 1 and 2) (Fralick and King, 1996; King, 1998; Wyman and Hollings, 1998; Tomlinson et al., 1999). Figure 1. Generalized geology map of Superior Province showing the belt structure and the location of the Atikokan area on the southern edge of Wabigoon Subprovince. 20 �Figure 2. Geology of the Steep Rock region, including correlative units in the Finlayson and Lumby Lake greenstone belts. 21 �Figure 3. Stratigraphy of the volcanic and sedimentary units in the Atikokan area (ages in millions of years). Consistency of younging indicators throughout the stratigraphic successions is supported by U-Pb zircon age determinations and appropriate stratigraphic position of correlative units in adjacent areas. Units have been tectonically flattened. For sources of age determinations see Fralick et al. (2008). 22 �Sedimentary rocks of the Finlayson Lake Group and Upper Lumby Lake Group overly a thick volcanic succession that began erupting before 3014 Ma (Tomlinson et al. 2003) (Figure 3). Tomlinson et al. (1999) proposed that the mantle source of the Mesoarchean volcanic assemblages in the Steep Rock and Lumby Lake areas was geochemically similar to the source of basaltic rocks that form the Cretaceous Ontong Java oceanic Plateau and that eruption probably occurred in a continental setting following rifting of the crust. Wyman and Hollings (1998) had previously proposed that most of the volcanic units in the Lumby Lake belt have an ocean plateau affinity. The combination of interpretation of depositional environments with sediment geochemistry and precise U-Pb geochronology of volcanic units enables reconstruction of the paleogeography for this area spanning a 230 m.y. period from the Mesoarchean to the Neoarchean (Fralick et al., 2008). Thick successions of tholeiitic basalt older than 2997 Ma. are present at the base of the Lumby and Little Falls belts. This implies that a sub-aqueous lava plateau existed in the area by 3.0 Ma (Figure 4). Figure 4. Mafic volcanism in the Steep Rock–Lumby Lake area prior to 3.0 Ga begins building a basaltic plateau. At 3000 Ma tonalitic magmas intruded the tholeiitic basaltic plateau. In both the Lumby and Little Falls areas, Marmion magmas erupted at surface, resulting in felsic volcanism and deposition of clastic aprons surrounding the volcanic centers (Little Falls Group; Figure 5). 23 �Ongoing mafic volcanism throughout this time interval resulted in ash layers from this source interlayered with the felsic volcanic debris. From 3000 to 2920 Ma mafic volcanism continued to thicken the volcanic pile. Periodic bursts of felsic volcanism linked to tonalitic intrusions also occurred (Figure 6). Throughout the next 100 m.y. mafic volcanism sporadically continued with felsic volcanic pulses, such as the one that erupted the 2898 Ma rhyodacites in the Lumby Lake belt and supplied the sub-aqueous clastic apron forming the Lower Lumby Lake Group. With the cessation of volcanism throughout the area sometime after 2828 Ma, minor block faulting, possibly triggered by adjustment to thermal decay in the region, created localized depocenters for epiclastic sediment in the northern Finlayson area and possibly upraised the tonalities of the Steep Rock area (Figure 7). These tonalities and mafic volcanic country rocks underwent erosion, and sediment was channeled through paleovalleys to the shoreline where deltaic deposits accumulated in the shallow areas and mass-flows off the deltas dominated the deeper regions. Subsidence of the area, due to isostatic adjustment to the weight of the ocean plateau, led to submergence of abandoned portions of the delta top, as seen in the upper section of the Finlayson Lake Group. As subsidence and erosion lowered the source area to base-level, the bedrock channel system backfilled, forming the basal Steep Rock clastics of the Wagita Formation, and starved the now largely submerged Finlayson deltaic complexes of sediment. Continued subsidence led to the development of carbonates in the shallower areas, which was succeeded by cherts and iron formation as the area continued to deepen as it isostatically adjusted (Figure 8). Previously, the sedimentary rocks in the Steep Rock – Little Falls – Finlayson – Lumby Lake areas have been ascribed to a rift setting (Wilks and Nesbet, 1988; Thurston and Chivers, 1990; Tomlinson et al., 1996). The major problem with this interpretation is that some of the volcanic units and associated sedimentary aprons are as old as the supposedly rifted tonalitic basement. This still allows the younger, upper sedimentary units to be rift related. However, recent investigations of the igneous geochemistry have indicated that the mafic to komatiitic volcanic pile associated with the sedimentary rocks was erupted in an ocean-plateau setting (Wyman and Hollings, 1998; Hollings et al., 1999; Hollings and Wyman, 1999). An excellent modern analogue for the evolution of the regional geological setting through time is the Cretaceous of the Kerguelen Plateau off Antarctica. As volcanism ceased in the Kerguelen Plateau 120 m.y. ago (Schlich, 1989) the sub-aerial portion remained above sea-level for 40 m.y. leading to erosion of up to 8000 meters of igneous rock (Coffin et al., 1990; Coffin, 1992). Similar erosive downstripping in the Steep Rock area would account for exposure of the Marmion Batholith. The basal units of the sedimentary succession capping the Kuerguelen Plateau are composed of fluvial units analogous to the Wagita Formation. These are overlain by algal limestones, which developed in shallow marine environments (Barron et al, 1989) as sub-aerial deposition failed to keep pace with subsidence driven by isostatic adjustment to the thick volcanic pile. The clastic succession at Steep Rock is capped by stromatolitic limestones deposited after base-level rise flooded source areas. As load and thermal decay induced subsidence continued to outpace sedimentation on the Kurguelen Plateau, the calcareous shallow water successions were succeeded by marine chalk and marl. Deep-water cherts and iron formation record the same 24 �Figure 5. At 3000 Ma tonalitic magmas, possibly generated by partial melting at depth of the thick plateau assemblage, intruded into the Steep Rock-Lumby Lake ocean plateau. The resultant surface volcanism built positive relief features from which sediment aprons composed of eroded felsic volcanic debris spread out over the plateau. Figure 6. The Steep Rock-Lumby Lake ocean plateau continued to grow during the 3000 to 2920 Ma time period. Sporadic tonalitic intrusion fed minor intermediate to felsic volcanic events, but the dominant volcanism erupted tholeiitic basalt. 25 �Figure 7. Volcanism ceases on the Steep Rock-Lumby Lake ocean plateau between 2820 and 2780 Ma, and the major sedimentary sequence is deposited. Localized relief produces slope failures and resultant agglomerates in the north Finlayson area. These are overlain by a deltaic clastic wedge prograding from the incised channels of the Steep Rock area. Figure 8. The siliciclastic system depicted in Figure 7 is succeeded by chemical/biochemical deposits as transgression causes clastic sediment starvation. With flooding of the source terrain stromatolitic carbonates developed in shallower areas and iron formation, which had dominated the deeper regions to the north, migrated first over the adjacent siliciclastic deposits and then blanketed the carbonate assemblage as well. 26 �process on the Archean Steep Rock – Finlayson – Lumby Lake Plateau. The striking analogies between the sedimentary units at Steep Rock and the sediments of the Kerguelen Plateau reinforce interpretations based on igneous petrochemistry (Wyman and Hollings, 1998; Hollings and Wyman, 1999; Hollings et al., 1999) that this assemblage represents an oceanic plateau. THE CARBONATE PLATFORM Carbonate platforms capping oceanic islands and plateaus are not uncommon in the modern ocean. However, they are rare in the rock record of the Archean. This may be the result of low preservation potential. However, Phanerozoic carbonate platforms suffer from this problem due to their oceanic location and there are still plentiful examples of these preserved in the geologic record. Thus, the Steep Rock carbonate succession represents one of the few opportunities to decipher the paleoecology and paleohydrology of an Archean carbonate platform. Wilks and Nisbet (1985, 1988) have provided an excellent description of the paleoecology of the Steep Rock platform. The following is largely taken from their work. Layering within the Mosher Carbonate exhibits a systematic change in form upward through the unit (Figure 9). The physical and chemical changes in the water mass that must have governed this transition in environments may have been related to a deepening trend from the shoreline environments represented by carbonates on top of fluvial conglomerates and sandstones to the overlying iron formation that was deposited in quite, deeper water (Figure 10) (Fralick et al., 2008). Alternatively, environmental change may have been related to periods of open verses more restricted circulation of water on the plateau leading to evaporation driven precipitation. We will discuss these possibilities in the field. -------------------------------------Figure 9. Variation in carbonate lithosomes with height in the section. 27 �Figure 10. A possible model for organization of carbonate deposition on the Steep Rock ocean plateau. Transgression produced a vertical stacking of the lithofacies. The upward changes in lithosomes present in Figure 9 may be caused by shifts in salinity rather than the lateral variation combined with transgression depicted here. The carbonate succession begins with an interlayered to sheared originally conformable contact with the underlying fluvial, valley-fill Wagita Formation or the Mesoarchean tonalite gneiss. The best representation of this contact is in an exposure near the bottom of the pit that is now under more than a hundred meters of water. Here decimeter-scale carbonate layers are interbedded with sandstone layers, with similar thickness, over a vertical distance of approximately half a meter. At the first major carbonate outcrop we will visit the basal contact with the 3001 Ma tonalite is sheared, however, pieces of weathered tonalite are present in the carbonate. If the contact is traced laterally it is quite evident that the limestones are onlapping the basement and the tonalitic debris was not carried very far away from the gneiss into the nearshore. The carbonates near the base of the succession have fairly flat layering. The layering is highlighted by changes in organic carbon (kerogen), iron oxides (recent weathering of iron carbonates?) and chert content. What follows, with the exception of the Figures, which are new, is the description provided by Wilks and Nisbet (1985) of the stromatolite succession: ―Near the base of the unit, simple Stratifera-like stratiform structures are characteristically present (Figure 11). These have flat to undulatory laminae, occasionally with the development of pseudocolumnar, laterally linked structures, which occur throughout the carbonate member. These Irregularia-like pseudocolumnar structures become more common upwards, and columns obtain heights of up to 2 cm (Figure 12A). Above this are hemispherical, laterally linked stromatolites (Figure 12B-D). These cumulate linked structures are similar to those figured by Hofmann (1971) as Cryptozoon walcotti. Laminae are wavy and 0.5-3.5 mm thick, and the structures are 5-15 cm high and in basal diameter. 28 �Figure 11. Stratiform layering, which dominates the lowest portion of the section. A) Undulatory, disrupted layering is further disrupted by the small faults. B) This sample has more consistent layering. Note the presence of white cement following some layers. C) Close-up of B. White calcite cement filling void with a flat bottom and undulating top. This shape is typical of gas bubbles that form in present day bacterial mats. D) Laterally consistent layering with small pseudocolumnar stromatolites. 29 �Figure 12. Moving stratigraphically up-section from Figure 11 the stromatolites increase in relief. A) Well developed pseudocolumnar stromatolites. B) Next hemispherical stromatolites begin to appear. C) Close-up of B. Successive hemispherical stromatolites are easily visible growing on top of one another. D) Laterally linked hemispheres in places resemble digitate forms. 30 �Figure 13. Columnar and branching columnar forms, such as those depicted here, are striking but are only developed in a vertically limited area. 31 �In places these ―cryptozoon‖ structures are succeeded by branching walled and unwalled columnar forms (Figure 13 and 14A). Branching furcate columns with ragged margins occur with a y style of parallel branching (Walter 1972). Height is up to 20 cm. Above these, simple, small conical structures occur, with diameters of 2-10 cm.‖ Wilks and Nesbit (1985). The 3.5 meters below the branching columnar stromatolites contains the first appearance of layers containing the outlines of fans of crystals. In some layers these crystal fans are growing upwards in others they are growing downwards (Figure 14B). The following is a description of crystal fans by Sumner (2000): Figure 14. A) Cross-sectional views of columnar forms in outcrop often appear as oncolite-like in shape. B) The first appearance of crystal fans immediately below the columnar stromatolites. Both upward and downward radiating fans are present. ―Microbialites commonly are cross-cut by fibrous mineral pseudomorphs. These pseudomorphs are replaced by calcite, but preserve their primary fibrous character with blunt crystal bundle terminations which suggests an aragonite primary mineralogy (e.g. Sandberg, 1985). The psuedomorphs form fanning bundles of crystals that grew upward. Laterally, neighboring fans of crystals interfered with each other‘s growth (Figure 14). Wavy bedding in the microbialites is due to doming of microbialites over areas with abundant fibrous pseudomorphs. This geometry suggests that fibrous mineral precipitation and microbialite growth were contemporaneous (Sumner and Grotzinger, 2000).‖ Sumner (2000). The upper half of the carbonate succession contains ―giant stromatolites‖ (Figure 15). These are composed of 1 to 15 cm thick layers of crystal fans commonly radiating upwards (Figure 16 A and B). They are overlain by darker layers of fenestrate microbialite. The microbialites consist of stacked, concave-up thin dark lines, which give the appearance of stacks of upside-down fingernails (Figure 16C and D). In some layers the dark filaments forming the stack are supported and separated by thin support columns (Sumner and Grotzinger, 2000; Sumner, 2000). 32 �Figure 15. Giant domes in the upper portion of the carbonate succession. A) Giant domes in plan view. Elongation may reflect current activity (elongate parallel to current). B) Giant domes in cross-sectional view. C) Crystal fans dominate this succession and the domal structure, as seen here in cross-section, is very subdued. D) Close-up of silicified crystal fans in C. The spaces between the microbialite layers are filled with white calcite. Sumner (1997, 2000) believed that the cuspate laminae and the support structures were formed by two different microbial communities. Where early calcification of the support structures occurred, the open, net-like morphology with white calcite cement in the pores was preserved (Figure 16D, upper 33 �portion). Where the support structures were not mineralized during formation the structure was Figure 16. A and B) Crystal fans underlain by a thin dark layer (iron- and/or organic-rich?). In A the lowest layer appears to be an open-structure microbialite but in B it may be a coarse grainstone. C) Finestrate microbialite with a collapsed structure. D) The lower portion is a finestrate microbialite with a collapsed structure, overlain by one with an open structure. compacted and the darker layers dominated by cuspate layering were formed (Figure 16C and D lower portion) (Sumner, 2000). The combination of crystal fans and in some layers cuspate layering, which appears in places to be draping the fans, produces irregular bed surfaces that have an appearance similar to an egg carton (Figure 17A). In other less common areas, the presence of hemispherical stromatolites in the upper layer of the giant domes creates an upper surface composed of small elongate mounds (Figure 18A). More rarely, some surfaces are similar to small mudcracks with turned-up edges (Figure 17B). The dominance of the crystal fans in the layers forming the large domes indicates that these structures probably should not be referred to as stromatolites. They do appear to have layering formed through microbial processes, but whether these layers cause the shape of the structures is not known. In areas where microbial layers are not present and crystal fans dominate domes are not well developed (Figure 15C and D), indicating that possibly the organically produced layers are the determining factor in the development of domes. 34 �Figure 17. Plane views of bedding surfaces in giant domes. A) The upper ends of crystal fans cause an irregular egg carton-like pattern on most surfaces. B) Much more rarely surfaces have what appear to be mud-cracks (photo is approximately 15 cm across). Grainstones have not been previously described from the Steep Rock limestones. However, clastic carbonate layers with grainsizes from micrite to granules and small pebbles are present (Figure 18C). They probably dominate the succession between the columnar stromatolites and the large domes. They are also found interlayered with the large domes and forming the base on which some of the fans in the domes are growing. The micrite and grainstone layers are 1 to tens of centimeters thick, massive to parallel laminated and in one instance trough cross-stratified. The clastic beds commonly appear as non-descript, massive continuous, parallel-sided layers and this blandness may have contributed to their non-recognition in the past by researchers primarily interested in the stromatolites. Though metamorphism and structural deformation in the Steep Rock Group is not intense the carbonates have been subjected to post-depositional alteration. Wegenast (1954), Jolliffe (1955, 1966), Wilks (1986), Kusky and Hudleston, (1999) and Sumner (2000) believed that a karst topography developed on the upper surface of the Mosher Carbonate during sub-aerial exposure. They view the manganiferous paint rock as a residual soil composed of goethite, gibbsite, hematite, and kaolinite with blocks of carbonate and lenses of pisolitic ferruginous bauxite, which formed during this interval. The Jolliffe Ore Zone would represent an iron formation that was deposited on top of the thick soil as marine transgression once again flooded the area. Alternatively, Kimberley and Sorbara (1976) believed that aqueous fluids acidified by the sulfide-rich dismal ashrock were responsible for the alteration. Some serious problems exist with the former more widely held hypothesis. 1) The manganiferous paint rock is consistently described as unconsolidated and earthy, yet authors who believe it is a residual soil that formed upon exposure of the Mosher Carbonate require it to have gone through the Kenoran Orogeny. It is difficult to believe that it was metamorphosed to greenschist facies but not lithified. All other paleosols of this age in the Canadian Shield are lithified and deformed. 2) This same problem applies to the karst collapse breccias as well. Although Kusky and Hudleston (19999) stated that these were weakly deformed the numerous breccias I have examined are not (Figure 18B). The delicate cements formed surrounding the breccia blocks show no evidence of deformation or 35 �Figure 18. A) Where hemispherical stromatolites are present near the surface of the domes the surface acquires an elongate bulbous appearance. B) Non-deformed breccia with two generations of cement. C) Cross-stratification in a grainstone tempestite layer. D) Iron content increasing towards the top of a layer. This type of iron enrichment is not uncommon. 36 �metamorphism making their existence before the Kenoran Orogeny unlikely. 3) Alteration zones in the carbonate appear to be vertical and some are not associated with the contact. These ironenriched altered areas are stratigraphically overlain by unaltered carbonate, but the alteration does continue vertically where outcrop allows observation. 4) The type of alteration implies highly oxidizing fluids. As well, the manganiferous paint rock ―soil‖ is up to 300 meters thick. Where did the large concentrations of oxygen come from in the Archean? Certainly not the atmosphere, as other Archean weathering profiles exhibit little to no oxidation. 5) The same succession of siliciclastics with conglomerates and sandstones overlain by carbonates and in turn overlain by iron formation is present in the Lumby Lake area, as previously described. There is no evidence of a period of exposure between the carbonate and iron formation present there. 6) The iron formation overlying the Mosher Carbonate is also brecciated and altered (it has goethite whereas hematite and/or magnetite are standard for oxide facies iron formation in Superior Province). Late mafic and lamprophyre dikes that cut lower units are involved in this brecciation (Shklanka, 1972). Thus, if the paint rock is a soil on a karst two episodes of brecciation substantially separated in time are needed. 7) Lastly, pieces of wood have been found in with the ferruginous pisoliths (R. Bernatchez, personal communication, in Wilks and Nisbet, 1988), and Machado (1987) believed that there was evidence of termite activity during formation of the ore. This placed the age of the ore as younger than Jurassic (Machado, 1987). This evidence overwhelmingly does not support the dissolution and alteration occurring in the Archean, but rather indicates it occurred in approximately the last 100 Ma. The scenario for the development of the carbonate platform is summarized in Figure 10. Here Walthers Law has been used to juxtaposed horizontally the lithofacies that succeeded one another vertically. This is based on the alteration in the upper carbonate and overlying iron formation developing in the Phanerozoic and the succession representing a transgression culminating in deeper water where the iron formation was deposited. This may not be accurate as the up-section change in lithosomes in the limestone may reflect open circulation evolving to more closed evaporitic conditions leading to precipitation of the crystal fans, but it does reflect a possible scenario. In this depiction the grainstones are in the higher energy areas of the platform close to the deeper water conditions where shoaling waves would be expected. Also, upwelling of deeper water in this zone may have lead to the precipitation of the crystal fans. The stromatolite-dominated lithofacies are in the platform interior where quieter water conditions should have prevailed. Eventually the entire assemblage subsided below the photic zone and deposition of iron formation replaced carbonate precipitation. 37 �FIELD TRIP STOPS When looking at the carbonate outcrops it is essential to get up close to them. There is a reason sedimentologists usually have their face within a few decimeters of the rock. You will miss most of the detail if you do not do this. As well the outcrops on this trip will seem to be repetitive if you do not look at the detail, and think about what it might be inferring about how the rock formed. Do not use hammers on outcrops. You can beat and sample loose material all you want. And, most importantly, be careful and always mindful of your surroundings. We will be walking on slopes and near cliffs all day. BRING YOUR CAMERA Stop #1: Overlooking northeast end of Hogarth Pit. The basement tonalite is visible at this stop. We will take a few minutes to look at the tonalite and discuss the overall setting of the Steep Rock Group. Special emphasis will be placed on the igneous and sedimentary history of the area from 3.0 to 2.8 Ga, prior to deposition of the Steep Rock Group. This will develop the context of the paleogeographic setting as deposition began in this area. Stop #2: One quarter of the way down the east side of Hogarth Pit from stop #1 (locations #3, 4 and 5 of Wilks and Nesbit, 1985, 1988). We will probably be at this location from one to two hours. After disembarking from the bus we will walk down a hill towards the pit for approximately 15 minutes. Notice how the small stream following the path changes from precipitating white material to orange material. The white precipitate is a mixture of calcium carbonate with minor aluminum hydroxide. The stream is spring fed and as the water flows onto the surface CO2 is degassed, which leads to a decrease in acidity by driving the formula: H 2O + CO2 --- HCO3- + H+ to the right. This, in turn, causes supersaturation of the Ca and Al in solution and their precipitation. As the stream continues to flow down the slope the decrease in acidity and the presence of oxygen result in the precipitation of iron hydroxides, causing the orange colour. A large cliff with boulder debris is present near the bottom of the hill. The rocks forming this cliff are mostly composed of carbonate collapse breccia fragments with calcite or dolomite cement. Some of the cement filling voids is intricately banded. Are there examples of miniature stalactites and stalagmites or dripstone indicating carbonate precipitation above the water table? Is this breccia deformed, i.e. did it go through the Kenoran Orogeny? Stay back from the cliff as you examine the boulders as loose material periodically falls. 38 �Next we will walk across the large flat area to the high ground to the south. We need to make our way up the sand and gravel slope to the glacially polished outcrop. The contact between the limestone, which forms most of this outcrop, and the tonalite is exposed to the left (NE). A weak schistosity is developed along it, but if you get up close to it tonalitic debris is visible in the limestone. Actually, the top 60 cm of the tonalite is not tonalite. It is sandy tonalitic debris. If you follow the contact 50 meters up the hill you will notice that it is at an angle to the layering in the limestone and the limestone layers were progressively burying this paleohill. The presence of tonalite debris in the basal limestone and a regolith in the upper tonalite strongly infers that this contact is original not a later structural discordance. Moving up-section in the lower portion of the outcrop you will be in the stratiform stromatolitic layers for the first 5 meters. Then there is an area where chert replacement has preserved thumbsized columnar stramatolites growing upwards from the flat-mat forms. Bulbous forms the size of an orange start to appear in this zone. Some of the flat mat layers and bulbous forms have white calcite cement filled vugs that have flat bottoms and arched tops. These are the classic shape of gas bubbles that form and are trapped in leathery mats as the microorganisms rot. They are an indication that flat layering is stromatolitic in origin. The stratiform stromatolites and small mounds continue for 40 meters from the base of section. The first crystal fans appear at approximately the 40 meter mark. They average 5 cm high and are along certain horizons, with the fans in some layers orientated upwards and in other layers downwards. The zone containing columnar stromatolites overlies the area with the crystal fans. These are obliquely growing out towards us so they are truncated forming nests of elipses. They appear to be composed of ferruginous dolomite (ankerite) or dolomite with hematite. The spaces between columns are filled with white cement, grainstone? and draping microbial laminations. (Grainstone is carbonate sandstone.) This is the only known layer of large columnar forms and can be used as a marker horizon. Probable grainstones are present approximately 10 meters above the columnar forms. The finegrained grainstones are approximately 5 cm thick and the granule layers average 10 cm thick. After approximately 20 meters of probable grainstones the layering is disrupted by a zone of iron alteration and brecciation. The breccia varies from clast to matrix support. How did this breccia form? Is it related to Archean karst on the upper surface of a then horizontal Mosher Carbonate or did it form after folding and metamorphism? Now we will walk around the hill and pick up the section approximately 50 meters higher. Stay away from the drop-off and aware of where you are relative to hazards at all times. The outcrop begins with approximately 14 meters of grainstone. A layer approximately 6 meters above the base of this section has possible trough cross-stratification. The large carbonate domes lie on top of the grainstone. Notice that the 1 to 15 cm thick layers are composed of dark (organic-rich), radiating crystal fans, with light calcite cement between individual crystals, overlain by fenestrae microbialite composed of thin dark lines resembling stacks of upside-down fingernails next to one another. These linked concave-up structures are not easy to see. In other fenestrae microbialite the dark lines form a small net design with light calcite cement between the lines. These latter ones display more of the original form as they have not collapsed due to early compaction. Why are the top surfaces of the layers so irregular? From here we will proceed back to the bus. 39 �Stop #3: Southeast end of Hogarth Lake (South Roberts Pit, Site #7 of Wilks and Nisbet, 1988). This is our lunch stop. We will eat our lunch in the clearing next to the road. A path goes to the lowest bench where large domes are exposed, but internal structure is poorly preserved. We will not spend much time at this outcrop. Stop #4: About 100 meters down the hill from stop #3 a field strewn with boulders and debris piles is down a slope to the left (east). It is a difficult walk down this slope and across the field to the path on the other side but that‘s where we are going. The path leads to benches overlooking Errington Pit. Once on the path the walking is easy. Take the right fork and after approximately ten minutes an outcrop will appear on your right. The first 45 meters consists of centimeter thick layers of dark and light banded limestone. Then there is a 6 meter thick chlorite-rich diabase dike. This is followed by 9 more meters of the no-stramatolitic, banded limestone, then 15 meters of large (0.75 m high by 1.5 to 2 m across) domes. Some delicate laminations are preserved here. The next 10 meters is sheared with a dike following the fault zone. The rock on the other side of the fault appears to be a repetition of the lower portion of this section with approximately 50 meters of cm- to dm-thick layers of dark limestone with white calcite cement void fills. There is a four meter wide dike in the upper portion of this assemblage. As you go around the curve in the road keep away from the far edge where there is a cliff going down and do not walk close to the end of the road where it has collapsed into the pit! Seventy meters of large (5m x 2m x 1.4m high) domes caps this succession. We will need to go to a lower bench to see the top of this assemblage. Smaller cabbage-sized stromatolites and flat layering with white cement lenses are also common in this upper unit. One of the large domes has an overturned edge. After looking at this section we will walk up the road the way we came in to the flagging tape. Those with a reasonable fitness level can cut through the woods here to the lower road. Those wishing not to climb down a steep slope should continue up the road to the fork and take the fork on your right that we have not yet been down. As we walk through the woods nearing the road to a lower bench the slope of the ground will increase. We will reach a point where we have to carefully pick our way and here a glacially polished outcrop will open up before us. This consists of what were originally fairly flat, sharp-sided, continuous layers. Some of these have silicified upright crystals that are leaning slightly due to deformation. They give the appearance of layer upon layer of 10 to 15 cm tall crystals growing from the bottom. Be very, very careful at this exposure as the polished surface has small pebbles on it that make it very easy to slip over the cliff below your feet!! Do not go near this outcrop unless you are holding a tree or something similar that will keep you in place if you lose your footing. The layers curve at one spot showing that relief was developed. Abundant styolites are also present here. Have a look at the large boulder sitting next to the base of the outcrop as well. It appears that the ankeritic area dropped into the rest bending the layering. Is there another explanation? The group will reform at the base of this hill on the lower road. We will continue along this road and cut down to a still lower road when we near the pit. Walk with caution through this area as the various benches all have drop-offs at their outer edges and all eventually end in drop offs. Do not roll boulders, etc., over the edges, you do not know if someone is below. These lower benches provide access to the best preserved large domes. Here the internal crystal fan 40 �structure is easily visible. Of particular interest is the three dimensional relationship between the domes. Look at where they come together. Are the layers continuous from one to the other? Do the layers in one dome truncate those in another or do the domes bud off one another? Some of the crystal fans are growing from clastic layers of grainstone and rudstone (pebble conglomerate). The fenestral microbialite layers also appear to be rare here, with crystal fans in places overlain by dark micrite. Where microbial layering is present in the upper portion of a layer in a dome note how it controls the surface topography of the bed. Some very good samples of crystal fans in the loose material can be obtained here. From here we will walk back along the lower road and across the boulder field and up the slope to the bus. Stop #5: We will drive to the confluence of the roads at the south end of Hogarth Lake, park and walk a very short distance around the bottom of the lake to the south western corner. Here debris of the Dismal Ashrock litters the ground. This unit may be an in situ ultramafic pyroclastic conformable overlying the iron formation or it may be a highly tectonised ultramafic that was caught in the zone of shearing during overthrusting of the Witch Bay Volcanics. From here we will go back to Atikokan. REFERENCES Barron, J., et al., 1989. Proceedings of the Ocean Drilling Program, Initial Reports, Volume 119, 942 p. Coffin, M.F., 1992. Subsidence of the Kerguelen Plateau: The Atlantis concept. In, ed. by S.W. Wise et al., Proceedings of the Ocean Drilling Program, Scientific Results, v. 120, p. 945-949. Coffin, M.F., Munschy, M., Colwell, J.B., Schlich, R., Davies, H.L. and Li, Z.G., 1990. Seismic stratigraphy of the Aggat Basin on the southern Kerguelen Plateau: Tectonic and paleooceanographic implications. Geological Society of America Bulletin, v. 102, p. 563-579. Davis, D.W. and Jackson, M.C., 1988. Geochronology of the Lumby Lake greenstone belt: A 3 Ga complex within the Wabigoon subprovince, northwest Ontario. Geological Society of America Bulletin, v. 100, p. 818-824. Fralick, P.W. and King, D., 1996. Mesoarchean evolution of western Superior Province: Evidence from metasedimentary sequences near Atikokan. In, ed. by R.M. Harrap and H. Helmstaedt, Lithoprobe Report 53, Lithoprobe Secretariat, University of British Columbia, p. 29-35. Fralick, P.W., Hollings, P. and King, D., 2008. Stratigraphy, geochemistry and depositional environments of Mesoarchean sedimentary units in western Superior Province: Implications for generation of early crust. In, ed. by K.C. Condie and V. Pease, When Did Plate Tectonics Begin on Planet Earth? Geological Society of America Special Paper 440, p. 77-96. 41 �Hayes, J.M, Kaplin, I.R. and Wedeking, K.W., 1983. Precambrian organic chemistry, preservation of the record. In, ed. by J.W. Schopf, Earth‘s Earliest Biosphere, its Origins and Evolution. Princeton University Press, p. 93-132. Hedberg, H.D., 1972. Summary of an international guide to stratigraphic classification, terminology and usage. Lethaia, v. 5, p. 297-323. Hofmann, H.J., 1971. Precambrian fossils, pseudofossils, and problimatica in Canada. Geological Survey of Canada Bulletin 189. Hollings, P. and Wyman, D., 1999. Trace element and Sm-Nd systematics of volcanic and intrusive rocks from the 3 Ga Lumby Lake greenstone belt, Superior Province: Evidence for Archean plume-arc interaction. Lithos, v. 46, p. 189-213. Hollings, P., Wyman, D. and Kerrich, R., 1999. Komatiite-basalt-rhyolite volcanic associations in northern Superior Province greenstone belts: Significance of plume-arc interaction in the generation of the proto-continental Superior Province. Lithos, V. 46, p. 137-161. Huston, W.J., 1956. The Steep Rock manganiferous footwall paint. M.Sc. thesis, Queen‘s University, Kingston, Ontario. Jolliffe, A.W., 1955. Geology and iron ores of Steep Rock Lake. Economic Geology, v. 50, p. 373-398. Jolliffe, A. W., 1966. Stratigraphy of the Steep Rock Group, Steep Rock Lake, Ontario. In, ed. by A.M. Goodwin, Precambrian Symposium, Geological Association of Canada, Special Paper Number 3, p.75-98. Kimberley, M.M. and Sorbara, J.P., 1976. Post-Archean weathering of Steep Rock Group iron formation. Proceedings, 1976 Geotraverse Conference, University of Toronto, Toronto, Ontario, p. 128-136. King, D., 1998. Depositional environments of the 3.0 Ga Finlayson and Lumby Lake greenstone belts, Superior Province, Canada. M.Sc. thesis, Lakehead University, Thunder Bay, 200p. Kusky, T.M. and Hudleston, P.J., 1999. Growth and demise of an Archean carbonate platform, Steep Rock Lake, Ontario, Canada. Canadian Journal of Earth Sciences. v. 36, p. 565-584. Lawson, A.C., 1913. The Geology of Steep Rock Lake, Ontario. Geological Survey of Canada, Memoir 28, p. 7-15. Machado, A. de B., 1987. On the origin and age of the Steep Rock buckshot ore. Chemical Geology, v. 60, p. 337-349. Sandberg, P., 1985. Aragonite cements and their occurrence in ancient limestones. In, ed. by N. Schneidermann and P.M. Harris, Carbonate Cements, S.E.P.M. Speceal Publication 36, p. 33-57. Schlich, R., et al., 1989. Principal results and summary. In, ed. by Schlich et al., Proceedings of the Ocean Drilling Program, Initial Reports, Volume 120, p. 73-85. Shlanka, R., 1972. Geology of the Steep Rock Lake area, District of Rainy River. Ontario Department of Mines and Northern Affairs, Geology Report 93. Smyth, H.L., 1891. Structural geology of Steep Rock Lake, Ontario. American Journal of Science, v. 43, 3rd Series, p.317-331. Sumner, D.Y., 1997. Late Archean calcite-microbe interactions: Two morphologically distinct microbial communities that affected calcite nucleation differently. Palaios, v. 12, p. 300-316. Sumner, D.Y., 2000. Microbial vs environmental influences on the morphology of late Archean fenestrate microbialites. In, ed by R.E. Riding and S.M. Awramik, Microbial Sediments, Springer-Verlag, Berlin, p. 307-314. 42 �Sumner, D.Y. and Grotzinger, J.P., 2000. Late Archean aragonite precipitation: petrography, facies associations and environmental significance. In, Carbonate Sedimentation and Diagenesis in the Evolving Precambrian World, S.E.P.M. Special Publication 67, p.123-144. Thurston, P.G. and Chivers, K.M., 1990. Secular variations in greenstone sequence develop-ment emphasizing Superior Province, Canada. Precambrian Research, v. 46, p. 21-58. Tomlinson, K.Y., Thurston, P.C., Hughes, D.J. and Keays, R.R., 1996. The central Wabigoon region: Petrogenesis of mafic-ultramafic rocks in the Steep Rock, Lumby Lake, and Obonga Lake greenstone belts (continental rifting and drifting in the Archean). In, ed. by R.M. Harrap and H. Helmstaedt, Lithoprobe Report 53, Lithoprobe Secretariat, University of British Columbia, p. 65-73. Tomlinson, K.Y., Hughes, D.J., Thurston, P.C. and Hall, R.P., 1999. Plume magmatism and crustal growth at 2.9 to 3.0 Ga in the Steep Rock and Lumby Lake area, western Superior Province. Lithos, v. 46, p. 103-136. Tomlinson, K.Y., Davis, D.W., Stone, D. and Hart, T.R., 2003. U-Pb age and Nd isotope evidence for Archean terrain development and crustal recycling in the south-central Wabigoon subprovince, Canada. Contributions to Mineralogy and Petrology, v. 144, p. 684-702. Walter, M.R., 1972. Stromatolites and the biostratigraphy of the Australian Precambrian and Cambrian. Special Papers in Palaeontology, Number 11. Wegenast, W.G., 1954. The footwall rocks of the Steep Rock ore zone. M.Sc. thesis, Queens University, Kingston, Ontario. Wilks, M.E., 186. The geology of the Steep Rock Group, northwestern Ontario: A major Archean unconformity and Archean stromatolites. M.Sc. thesis, University of Saskatchewan, Saskatchewan, Canada, 206 p. Wilks, M.E. and Nisbet, E.G., 1985. Archean stromatolites from the Steep Rock Group, northwestern Ontario, Canada. Canadian Journal of Earth Science, v. 22, p. 792-799. Wilks, M.E. and Nisbet, E.G., 1988. Stratigraphy of the Steep Rock Group, northwestern Ontario: A major Archean unconformity and Archean stromatolites. Canadian Journal of Earth Sciences, v. 25, p. 370-391. Wyman, D. and Hollings, P., 1998. Long-lived mantle plume influence on an Archean protocontinent: geochemical evidence from the 3.0 Ga Lumby Lake greenstone belt, Ontario, Canada. Geology, v. 26, p. 719-722. 43 �Hammond Reef Gold Deposit, Brett Resources Inc. Mark Smyk, Ontario Geological Survey (modified from http://www.brettresources.com/s/HammondReef.asp) INTRODUCTION The Hammond Reef project is the flagship project of Brett Resources Inc.. In 2006, Brett agreed to earn a 60% option on the property from Kinross Gold Corp. Brett management recognized that this project has significant upside potential. Prior to agreeing to the option, Kinross Gold Corp. outlined a 1.8 million ounce inferred resource on the property. Subsequent to earning their 60% interest in 2008, Brett acquired the remaining 40% of the project from Kinross. In the summer of 2009, Brett announced their second resource estimate for the property - 155 Mt at 1.04 g/t Au in the Inferred category, using a 0.6 g/t Au cut-off, for a base case of 5.2 million ounces of gold. The majority of the ground surrounding the known deposit remains untested by modern exploration methods. Small-scale mining took place on the property directly to the northeast of the current deposit, the Manley area, at the turn of the 19 th century and again in the 1930's. Small-scale mining focused only on the free gold found in quartz veins, which records indicate had grades of approximately 0.3 ounce Au per ton. The Hammond Reef property hosts widespread gold mineralization within a 100 to 300 m wide, northeasterly-trending corridor. Gold mineralization is associated with quartz vein stockworks. Mineralization defined in the current deposit is relatively flat-lying, dipping between 35° to 45° and seems to flatten out at depth. The favourable geometry of the deposit and the fact that 97% of the inferred resource is found within 300 m of surface, makes the deposit amenable to openpit mining. LOCATION AND INFRASTRUCTURE The Hammond Reef Project is located in the Sawbill Bay-Marmion Reservoir area of the Thunder Bay Mining District, approximately 170 km west of Thunder Bay, and 23 k northeast of Atikokan. The property can be accessed both by water and by gravel road and there is a power line approximately 10 km to the west of the property. GEOLOGY AND MINERALIZATION The Hammond Reef property is predominantly underlain by the Marmion Batholith granitoid suite, characterized by fresh to intensely altered tonalite-trondhjemite; subordinate, unaltered granitoid gneiss; and minor mafic lenses (typically highly altered). Minor pegmatite dykes and pegmatite segregations are present. Quartz stockwork overprints all phases but is only weakly developed in the mafic lenses. The quartz stockwork hosts the gold mineralization. The stockwork is confined to a broad, anastomosing envelope of alteration measuring up to 6000 m wide on surface exposures and has a northeast strike over a length of approximately 40 km. The trend of the alteration system, major quartz veins, gneissic enclaves, and mafic lenses parallel the dominant east-northeast-trending regional fabric. This alteration comprises a gradual increase in thicknesses of halos of saussurite surrounding fractures. The gradually coalescing halos of alteration, with increased frequency of quartz veining, control the development of pervasively altered granitoid. Gold values show a gradual increase once weak but continuous areas of veining and alteration are observed. 44 �Strong alteration and foliation develops into a schist zone without appreciable veining (Hammond Reef Schist Zone-HRSZ). A discrete zone of strongly foliated Fe-carbonate with variable amounts of sericite, chlorite, hematite, limonite and pyrite lenses is easily mapped and has previously been described as highly foliated, tectonized granitoid breccia or breccia zone. Veining can be grouped into two styles -- 5 cm to 50 cm wide, straight and generally undeformed "leader" veins and millimetre- to centimetre-thick, densely spaced, randomly oriented "stockwork" veins -- and suggests extensional dilation during formation. The veins are coeval, with no clear crosscutting features or consistent overprinting observed. The Hammond Reef property exhibits a number of similarities to the Fort Knox deposit in Alaska. Hosted within the Fort Knox Pluton which intrudes the Fairbanks schist, gold occurs in the margins of stockwork veins and veinlets, quartz-filled shear zones, and along fractures within the granite. RESOURCES Brett announced an updated resource estimate for the Hammond Reef Deposit (July 23, 2009 news release). Subsequently, on November 12, 2009, Brett announced the results of a Preliminary Assessment Study which uses the new Base Case Inferred Mineral Resource of 259.4 Mt at 0.80 g/t Au, totaling 6.70 million ounces of gold at a 0.3 g/t Au cut-off. The Preliminary Assessment Study was prepared by Scott Wilson RPA under the supervision of Mr. Richard J. Lambert, P.E., a registered professional engineer and an independent Qualified Person under the standards set forth by National Instrument 43-101. *The Study is preliminary in nature, it includes Inferred Mineral Resources that are considered too speculative geologically to have the economic considerations applied to them that would enable them to be categorized as Mineral Reserves, and there is no certainty that the Preliminary Assessment will be realized. A table of the Inferred Resource at various cut-off grades can be found below. Cut-off Tonnes Grade Gold Oz Au g/t (000,000) Gold g/t (000,000) 1.00 60.2 1.46 2.83 0.90 77.1 1.35 3.34 0.80 98.4 1.24 3.93 0.70 124.6 1.14 4.56 0.60 155.0 1.04 5.19 0.50 188.5 0.95 5.78 0.40 227.0 0.87 6.34 0.30 259.4 0.80 6.70 0.20 281.9 0.76 6.89 45 �METALLURGY In September 2009, Brett announced the results of a metallurgical test program conducted at SGS-Lakefield Research on samples from both the "A" and "41" Zones. Head grades of the test composites varied from 0.60 to 3.4 g/t Au. The result of the program was a new flowsheet that is simple and will reduce capital and operating costs while improving recoveries: Coarse primary grind of 180 microns Bulk sulphide flotation to pre-concentrate the gold to a stream of less than 10% by weight of the plant feed with a gold recovery of over 95% Fine regrinding of the flotation concentrate Cyanidation of the reground flotation concentrate, recovering 97.8% of the contained gold Overall gold recovery 93% All test composites were found to be likely non-acid generating as a result of Acid Base Accounting (ABA) and Net Acid Generation (NAG) analyses. The sulphide sulphur content of the deposit is low, and there appears to be adequate carbonate to neutralize any potential acid generation. EXPLORATION HISTORY The Sawbill Bay Gold District has been the focus of intermittent gold exploration since the original discovery of the Hammond Reef property in 1895. Small-scale underground mining and milling occurred between 1896 and 1900, and again from 1937-1941, with the property changing hands several times. Active "modern" exploration programs were conducted at Hammond Reef by Falconbridge Gold in the 1980's and by Pentland Firth in the 1990's. This work resulted in the delineation of two separate low-grade gold mineralized zones, the "A" and "41" Zones, delineated by a total of 83 diamond drill holes (18,060 m). A mineral inventory was prepared by Pentland Firth which indicated that the two zones contained an estimated 88 Mt grading 0.93 g/t Au, for a total of 2.64 million troy oz. gold*. (*This is a historic, non- NI 43-101-compliant resource provided for informational purposes only.) Since first optioning the property from Kinross in March 2006, Brett Resources has used systematic exploration to expand the known areas of mineralization at the Hammond Reef Project. In 2006, Brett started their work with a geological exploration/sampling program, including geological mapping, prospecting, systematic sampling of outcrops, soil sampling, core sampling for geochemical signatures, and magnetometer surveys. Initial success came as soil sample results outlined three strong new northeast-trending anomalous gold trends west of Snail Bay area, northeast of the claims known as the Manley Patents. The Manley Patents are the site of at least nine historic shafts mined by small-scale mining in the early 1900's, but were until recently privately held, and have not been subject to modern exploration methods. A trenching program was conducted over the new anomalous gold area and results outlined three separate mineralized zones indicating the presence of a new wide gold mineralized horizon open to the north, and more than 2 km from the A and 41 Zone areas. An additional 50 to 60 m wide mineralized zone was exposed over 450 m of strike length through trenching on the optioned Sande & Stewart property. It remains open and bears the same orientation and appearance as the mineralized zone north of Mitta Lake. 46 �From the end of 2006 to the end of 2008 Brett completed 34,145 m of drilling at Hammond Reef. Step-out drilling around the A Zone encountered significant new mineralization to the southwest, southeast and northeast. Of particular interest were new intersections to the northeast, indicating that mineralization continued along strike towards the 41 Zone. The continuity both at depth and laterally was found to be remarkably good. In-fill drilling in the 41 Zone confirmed the continuity of the zone on a 50 m spacing, as well as expanded mineralization westward. By the middle of 2008, Brett had sufficient new drill intersections to complete a NI 43-101compliant resource estimate for the Hammond Reef deposit, incorporating the contiguous A, 41 and Mitta Lake zones of mineralization. The result was a resource of nearly 5 million ounces of gold in the inferred category (see details in Resources section), nearly double the estimate by previous operators. Eight holes drilled late in 2008 were not included in the resource estimate, and some holes intersected well developed gold mineralization in areas that will add to the gold inventory in future resource estimates. The first phase of the 2009 diamond drilling program started in February, with 16 step-out drill holes for a total of approximately 5000 m of core drilling. Results of the spring drilling program were reported in late May, with the program having successfully extended the Hammond Reef resource in three directions: to the west; down-dip by over 150 m; and northeastward by confirming that the strike extent of the gold-bearing structures extend past the Manley Patents by more than 2 km northeast from the currently defined resource. Previous exploration identified broad low-grade mineralization in two surface trenches located in the Snail Bay area, northeast of the Manley Patents and more than 2 km along strike to the northeast of the current gold resource. One of the primary objectives of the spring drilling program was to test the depth extension of the mineralization in these trenches. Two holes were drilled and both holes returned mineralized intersections. These new discoveries of gold mineralization in the Manley area have the same style of mineralization found in the A Zone and remain open along strike and at depth. No modern exploration techniques have been applied to this area and that was one of the main objectives of the exploration program in the summer and fall of 2009. A second objective of the 2009 spring drilling program was to provide information for a scoping study expected to be completed during the fourth quarter of 2009. In particular, drilling was to test the mineralization up- and down-dip of the currently defined resource area. The holes stepped out from 50 to 275 m along sections and tested both deep and shallow targets and extended the resource at depth by as much as 150 m down-dip to the south. The program has also intersected additional shallow gold-bearing structures that are not in the current resource. These new structures have excellent potential to allow Brett to expand the current resource and positively impact the strip ratio of the pit. The scoping study will incorporate an updated Hammond Reef resource. The best results from the drilling program were from hole BR 126 which intersected 40.5 m of 1.95 g/t Au and from hole BR 127 which returned 159 m of 0.97 g/t Au. Brett's scoping study will evaluate the economic viability of the Hammond Reef gold deposit. A major part of the study will require engineering reviews of mining methods, pit design and location and processing plant design. 47 �Brett conducted widespread field exploration work at Hammond Reef in 2009. With a combination of geophysics, soil sampling, prospecting, mapping and trenching. Brett hopes to detect new anomalous gold mineralization and to identify new exploration targets for drill investigation. Brett recently announced that Osisko Mining Corporation mailed shareholders of Brett and filed with the Canadian securities regulatory authorities and the U.S. Securities and Exchange Commission, the offer and take-over bid circular, formally commencing Osisko's friendly offer to acquire all of the outstanding common shares of Brett on the basis of 0.34 of an Osisko common share and $0.0001 in cash for each common share of Brett (Brett Resources Inc., news release, April 13, 2010). Osisko and Brett entered into a support agreement on March 21, 2010 providing for the terms of the Offer and the agreement by the board of directors of Brett to recommend that Shareholders accept the Offer. The consideration under the Offer represents a premium of 52.5% using the 20-day volume weighted average prices of Osisko and Brett quoted on the Toronto Stock Exchange and TSX Venture Exchange, respectively, for the 20 trading day period ending March 16, 2010. 48 �Hammond Reef property location and regional geology (from http://www.brettresources.com/i/maps/091202_Regional_Geology.jpg) 49 �General geology of the Hammond Reef property, showing location of altered and goldmineralized zones (from http://www.brettresources.com/i/pdf/2009-11-27_HR_NI43-101.pdf). Each UTM square is 5 km wide. 50 �Field Trip 3 DEFORMATION IN THE RAINY LAKE REGION: A FABULOUS DISPLAY OF STRUCTURES CONTROLLED BY RHEOLOGICAL CONTRASTS Dyanna M. Czeck University of Wisconsin-Milwaukee Howard Poulsen Consultant, Ottawa Photo: Flattened Seine conglomerate 51 �1. Introduction The Rainy Lake area has been the locus of geologic studies for more than a century, starting with the pivotal early work by Lawson (1887; 1913). Over the decades, it has seen its share of controversy from Lawson‘s classic arguments for the existence of crystalline rock that was demonstrably younger than supracrustal rocks (Lawson, 1887) to the long-lived ―SeineCoutchiching Problem‖ stemming from cross-border disputes about relative ages of sedimentary sequences (Van Hise et al., 1905; Van Hise and Leith, 1911; Lawson, 1913; Grout, 1925; Tanton, 1927; Hawley, 1930; Gill and Hawley, 1931; Merritt, 1934; Ojakangas, 1972; Wood, 1980). The stratigraphic arguments have largely ceased due to an understanding of the dynamic tectonic framework responsible for the juxtaposition of the disparate rock types (Poulsen et al., 1980; Davis et al., 1989; Poulsen, 2000a) and the application of high precision U-Pb dating to key units (Davis et al., 1989; Fralick and Davis, 1999). During our fieldtrip, we will focus on analyzing key structural features that will help us understand the kinematics of the regional and local deformation. As the region straddles a significant boundary within the Superior Province, the kinematic history of the area may be used to help elucidate Archean tectonic styles and patterns of continental growth. The complex juxtaposition of various lithological units makes for intriguing geology in general, and structural geology in particular. The various rock units provide a study in contrasts where the structural styles and strain intensities are largely controlled by the rheological contrasts of adjacent rock types. When a region is tectonically deformed, deformation is preferentially localized in weaker rocks, leaving stronger rocks to remain relatively undeformed with weak to nonexistent deformation fabrics (analogous to embedding a marble into modeling clay and squishing the whole package- all of the deformation is in the modeling clay and the marble stays undeformed). In the field, we can see these rheologic contrasts at a variety of scales, from cm scale pebbles with contrasting strengths in deformed conglomerates, to km scale strong plutonic rocks surrounded by weaker volcanic rocks. Indeed, we can even see evidence for strength inversions. For example, when granites intrude, they are molten and weaker than their host rocks, but as they cool, they gradually become stronger than their hosts, resulting in an inversion in the relative strengths over time. A general theme amongst the stops on this trip will be to evaluate the relative strength, or competence, of adjacent rock types and evaluate how the competence contrasts are manifested into diverse styles of structures. 2. Geologic Setting and Geochronology The large Archean Superior Province (Fig. 1) can be divided into subprovinces based on lithostratigraphy and metamorphic grade (Card and Ciesielski, 1986). The subprovinces were assembled by repeated island arc, microcontinent collisions. Several pieces of evidence are consistent with the island-arc accretion model. First, the microcollisions are evidenced by rocks that can be interpreted as arc sequence subprovinces (metavolcanic rocks of the greenstone belts) and their corresponding accretionary prism subprovinces (metasedimentary rocks) (Langford and Morin, 1976; Hoffman, 1989; Percival and Williams, 1989; Card, 1990; Hoffman, 1990). Second, the ages in the greenstone belts are generally similar along strike, but differ systematically across strike (Hoffman, 1989). Third, the deformation characteristics found 52 �Figure 1. Superior Province based on divisions by Card and Ciesielski (1986). across the province are best explained by contractional deformation. A history of subduction and southward accretion was responsible for the juxtaposition of Superior Province subprovinces (Langford and Morin, 1976; Percival and Williams, 1989; Card, 1990). The Rainy Lake zone (Fig. 2) lies on the western part of the boundary between two lithotectonic subprovinces of the Superior Province: the Wabigoon granite-greenstone terrane to the north and the Quetico metasedimentary terrane to the south (Card and Ciesielski, 1986). Oblique collision between the Wabigoon and Wawa volcanic-plutonic terranes during the late Archean approximately 2.69-2.7 Ga (Stockwell, 1982; Davis et al., 1989; Fralick and Davis, 1999) most likely formed the tectono-metamorphic features of this boundary zone (e.g. Poulsen, 1986; Percival and Williams, 1989). The metavolcanic subprovinces (Wabigoon, Wawa) were subsequent island arcs that collided due to ongoing subduction, and the metasedimentary subprovince (Quetico) is comprised from sedimentary basin deposits between the arcs (Percival, 1989; Blackburn et al., 1991; Williams, 1991). 53 �54 Figure 2. Simplified Rainy Lake region geologic map modified from Davis et al., (1989). Full detailed map is Poulsen (2000c). �Poulsen (1986) recognized the Rainy Lake boundary area as a complex transpressive deformation zone. The area is bounded by two major structures, the Quetico and the Rainy Lake-Seine River faults, which have most likely behaved as both dextral faults and shear zones during their deformation histories (Fig. 2). The two faults/ shear zones converge to the east into a single structure and delineate a wedge-shaped area between them (Poulsen, 1986, 2000a, b, c). Within the wedge, a complex pattern of anastomosing subvertical shear zones and faults wrap around gneissic domes and granitoid plutons (Poulsen, 1986, 2000a, b, c; Druguet et al., 2008). Flattening fabrics combined with the evidence for dextral shearing indicate an overall dextral transpressive tectonic regime with subhorizontal NW-SE shortening (Poulsen, 1986; Tabor and Hudleston, 1991; Borradaile and Dehls, 1993; Borradaile et al., 1993; Poulsen, 2000a, b; Czeck and Hudleston, 2003; Druguet et al., 2008; Czeck et al., 2009). The main lithological units in the study area can be summarized based on dates obtained by Davis et al. (1989) and Fralick and Davis (1999), as follows: (1) Keewatin Group tholeiitic and calcalkaline metavolcanic rocks. These form interlayered sequences of ultramafic to felsic compositions, although basalts and andesites are most common. Crystallization ages range from 2728+4/-3 to 2725±2 Ma. (2) Metagabbroic rocks with metadolerites and metaanorthosites. They are found as sills and larger intrusions of variable dimensions, generally showing compositional and textural layering. Includes the Seine Bay- Bad Vermillion Complex (2728+/-2 Ma for the trondhjemitic Mud Lake component) and the Grassy Portage Sill (2727+/-2 Ma). (3) Gneisses. Most gneisses in the region are orthogneisses corresponding to synvolcanic ―Laurentian‖ plutonic rocks, about 2725±2 Ma that vary compositionally from granitic to tonalitic and form the core of the Rice Bay dome and the gneisses near Black Sturgeon Bay. (4) Coutchiching Group metasedimentary rocks. These are the sedimentary rocks located south of the Quetico Fault and north of the Seine-River Rainy Lake Fault and are bracketed in age between 2704±3 and 2692±2 Ma by detrital zircon and a crosscutting intrusion. They contain metagreywackes and metapelites, especially biotite schists. They locally contain porphyroblasts of garnet, andalusite and staurolite. (5) Quetico metasedimentary rocks. The Quetico turbiditic metasediments are found to the south of the Seine River-Rainy Lake Fault. The age of the youngest analyzed detrital zircon is 2699±1 Ma. (6) Seine River Metasedimentary Group. Deposited partly during and after the nearby Coutchiching metasedimentary rocks. The depositional age is bracketed between 2693±1-2692±1 Ma, (Fralick and Davis, 1999). This unit formed as one of the Superior Province‘s molasse basin-fill units (Corcoran and Mueller, 2007) and includes the Seine River conglomerates, well developed near Mine Centre (Wood, 1980; Frantes, 1987; Czeck, 2001; Czeck and Fralick, 2002; Czeck and Hudleston, 2003; Fissler, 2006; Czeck et al., 2009). (7) ―Algoman‖ calk-alkaline granitoids. These are relatively unmetamorphosed and poorly deformed granite, granodiorite, and quartz monzonite rocks that form several individual plutons (age of the Ottertail pluton: 2686+2/-1 Ma). They were previously interpreted as 55 �postkinematic and their age of emplacement was used as a constraint for the ending of regional deformation (Davis et al., 1989; Poulsen, 2000a). However, by means of field relationships, microstructural analysis, anisotropy of magnetic susceptibility (AMS), and gravity inversion techniques, Czeck et al. (2006), demonstrated that these plutons formed syntectonically, late within the Archean transpressional event. 3. Structural Framework Many authors have found evidence for a three phase (D1, D2 and D3) deformational history (Poulsen, 1986; Davis et al., 1989; Tabor and Hudleston, 1991; Poulsen, 2000a; Druguet et al., 2008). All of these phases are associated with oblique convergence related to the island arc collisions. While the structures are similar in style and sequence to other features found throughout the central Superior Province, the phases are unlikely to temporally correlate across subprovince boundaries, and may not even correlate within the Rainy Lake region due to the dynamic nature of the tectonic setting. D1 is characterized by early recumbent folding that produced the inversion of the stratigraphic sequence and S1, the first regional schistosity (Poulsen et al., 1980). The combination of several observations including the present-day juxtaposition of adjacent right-way-up and overturned folds (Poulsen et al., 1980; Borradaile, 1982; Poulsen, 2000a), suggest that the original orientations of many of the folds were nappe-like. S1 is generally subparallel to the layering in the metavolcanic and metasedimentary rocks. A gneissic foliation attributed to D 1 is well developed in the gneiss domes. D1 is likely to have included significant thrust or oblique faulting that juxtaposed allochthonous units. There is less direct evidence for thrusting, due to a general lack of sedimentary markers that would enable one to recognize duplicated sequences of rock. However, the hypothesis of early faulting is corroborated by the strain patterns within this and other Archean belts, which are generally too low to account for the now largely steeply dipping units (e.g. Schultz-Ela, 1988; Hudleston and Schwerdtner, 1997). This early history of thrusting and recumbent folding can be thought of as ―stacking‖ (Davis et al., 1989; Czeck, 2001; Czeck and Fralick, 2002). As the crust was ―stacked‖ in D1, rocks that were once near the surface became buried. In these rocks, D1 deformation was followed by D2, which is characterized by folding and dextral shearing under a transpressional regime. F2 folds resulted in steep to subvertical bedding subparallel to a regional subvertical S2 foliation with a mean ENE-WSW strike. The Seine River Group was deposited in a fault-related basin during early D2 (Poulsen, 1986, 2000a), which explains their strong D2 fabrics and minor amounts of internal folding (Czeck, 2001; Czeck and Hudleston, 2003; Czeck et al., 2009). However, it is important to note that the transition of D 1 to D2 was not necessarily contemporaneous across the Rainy Lake region and the vertical bedding in the Seine is likely a result of initial internal stacking followed by D 2 deformation styles. The Algoman plutons were emplaced and cooled during late stages of D 2 (Czeck et al., 2006). Many leucocratic and quartz veins were emplaced throughout D2 (Druguet et al., 2008). D2 was responsible for the penetrative foliation and lineation fabrics and much of the recorded strain throughout the region. In general, the foliation is subvertical (Fig. 3a), and where strain 56 �Figure 3. Equal area stereonets representing structural fabric in the Rainy Lake region from Czeck and Hudleston (2003). (a) Stretching (mineral) lineations. (b) Poles to foliation. can be measured, such as within the Seine conglomerates, strain is largely of a flattening type (Czeck, 2001; Czeck and Hudleston, 2003; Fissler, 2006; Czeck et al., 2009). Dextral shear sense indicators in the form of asymmetric folds, conglomerate strain shadows, clast tiling, and other features are often observed on subhorizontal planes regardless of lineation orientation (Czeck and Hudleston, 2003). Most of these fabric features along the boundary are consistent with a type of ductile transpression first described by Sanderson and Marchini (1984). This style of transpression is a mathematical description of a specialized case of ―transpression‖ that in its simplest form just means oblique convergence (Harland, 1971). The style of transpression described by Sanderson and Marchini (1984) involved homogeneous deformation consisting of orthogonal simple shear and pure shear components with constant volume in a vertically bounded zone (Fig. 4a). Such an idealized scenario has since been termed ―monoclinic transpression‖ (e.g. Lin et al., 1998) and is perhaps most likely to correspond to strain in deep, vertical ductile shear zones during oblique convergence. The structural fabrics for this model include vertical foliations, flattening fabrics, and asymmetric shear sense indicators on the horizontal plane (Fossen and Tikoff, 1993) (Fig. 5). The consistencies of fabrics observed along the Wabigoon–Quetico boundary with the homogeneous transpression model suggest that at least the Wabigoon–Quetico boundary as a whole has undergone quasi homogeneous transpression. Figure 4. Transpression Models. a) Homogeneous monoclinic transpression (Sanderson and Marchini, 1984). b) Triclinic transpression with oblique simple shear component (Lin et al., 1998). c) Triclinic transpression model with horizontal simple shear component and oblique extrusion direction (Czeck and Hudleston, 2003, 2004). 57 �Figure 5. Schematic deformation fabrics and strain predicted for monoclinic transpression (Fossen and Tikoff, 1993). Planes shown are foliations, lines shown are stretching lineations, and schematic ―pebble‖ indicating flattening strain and asymmetric strain shadows from Czeck and Hudleston (2003). The lineation orientation (Fig. 3b) is one major aspect of the structural fabric in the Rainy Lake region that does not mesh with the monoclinic transpression model, which predicts either vertical or horizontal lineations (Fossen and Tikoff, 1993). The orientations of the stretching lineations, as defined by the preferred orientation of metamorphic minerals, varies at the hectometric scale; they plunge between 0-90° in both east and west directions (Czeck and Hudleston, 2003). The subhorizontal asymmetric shear sense indicators combined with the greatly variable stretching lineation orientations prove to be difficult to explain with most kinematic models. In shear zones that deformed via simple shear, we would expect to find asymmetric shear sense indicators on the horizontal plane, but the lineations should be horizontal. In shear zones that deformed via homogeneous monoclinic transpression models (Sanderson and Marchini, 1984; Fossen and Tikoff, 1993), we would expect to find asymmetric shear sense indicators on the horizontal plane, but the lineations should be only vertical or horizontal depending on the amount of pure shear versus simple shear and the strain magnitude. In many triclinic transpression models that can explain obliquely plunging lineations (Jiang and Williams, 1998; Jones and Holdsworth, 1998; Lin et al., 1998), the simple shear component is no longer horizontal, so the asymmetric shear sense indicators should be found on a corresponding oblique plane (Fig. 4b). For these reasons, most kinematic models for shear zones are incompatible with structures found in the region. The best model proposed for D2 that explains the subhorizontal asymmetric shear sense indicators and the highly variable stretching lineation orientations is a triclinic transpression with horizontal simple shear and variable extrusion direction (Czeck and Hudleston, 2003, 2004) (Figs. 4c, 6). In the original monoclinic transpression and most subsequent triclinic models (Sanderson and Marchini, 1984; Fossen and Tikoff, 1993; Robin and Cruden, 1994; Dutton, 1997; Jones and Holdsworth, 1998; Lin et al., 1998), material was assumed to extrude vertically upwards. This assumption is logical because, in general, one would expect the direction towards the earth‘s surface to provide the least resistance for material movement. However, rocks at depth may have other boundary conditions imposed by anastomosing shear zone geometries or large scale rheology contrasts due to a diverse lithological assemblage that cause localized deviations in the extrusion direction. Certainly, the Rainy Lake region has both complex three58 �Figure 6. Schematic deformation fabrics and strain predicted for transpression with oblique extrusion from Czeck and Hudleston (2003). Planes shown are foliations, lines shown are stretching lineations, and schematic ―pebble‖ indicating flattening strain and asymmetric strain shadows. (a) Simple view of transpression with nonvertical extrusion. (b) Schematic view of transpression with overall bulk vertical extrusion and localized zones of nonvertical extrusion. Dark colored ellipses represent schematic ‗hard‘ zones that influence local extrusion directions. The relative location of (a) is indicated. dimensional anastomosing shear zones and lithologies with strong competence contrasts that could account for local variation in the extrusion directions. Domains of higher strain are localized in the relatively incompetent metavolcanic and metasedimentary rocks and channeled around the competent gneissic domes, metagabbros, and Algoman plutons. Two-dimensional strain was estimated using folded and boudined dikes, and the areas of lowest strain are located on the ―strain shadow‖ flanks of the gneiss domes and plutons (Castaño, 2007; Druguet et al., 2008). As a result, the dominant, composite S1+S2 foliation displays a complex heterogeneous pattern both in orientation and intensity. Dextral transpression continued as the rocks were exhumed. It is evidenced by retrograde metamorphism and D3 structures (Poulsen et al., 1980; Poulsen, 2000a). During D3, strain localized along major mylonitized dextral shear zones and faults such as the Quetico and Seine River-Rainy Lake Faults, narrow mesoscale shear zones, and crenulation bands. This late stage of deformation is more brittle-ductile in nature than D2, and thus represents rock exhumation. Progressive D2 to D3 dextral transpressional deformation was likely controlled by the distribution of lithological units with contrasting competence, the previous structure, and the emplacement of the Algoman plutons. Many of the late quartz veins may have been emplaced during D 3, but this inference needs to be more rigorously tested. Some authors note that the final D3 stage of deformation involved amplification of strike-slip motion along wrench zones (Bauer et al., 1992). The latest brittle faulting has largely been 59 �described as dextral strike-slip motion (Kennedy, 1984), but local late-stage brittle faults associated with N-S shortening also have been observed (Tabor and Hudleston, 1991). In all likelihood, the last stages of deformation associated with the oblique convergence may have been strongly partitioned with the major strike-slip faults accommodating the boundary-parallel motion and the domains between the major faults accommodating the boundary-perpendicular motion (Tabor and Hudleston, 1991). 4. Field Trip Stops We have arranged the stops in a sequence that will allow us to unpeel the stages of deformation. We will look at the least deformed, youngest rocks first (the Ottertail Pluton), followed by syndeformational metasedimentary rocks (the Seine Conglomerates), followed by older rocks that have seen most/ all of the deformation and, in many cases, are intruded by many generations of syntectonic veins. In this way, we will see rocks with progressively more complicated (and exciting!) structures. There are a few bonus stops at the end in case we have time on our way back to Fort Frances. The locations of most stops can be found on Figure 2. Stops 3 and 4 can be found on Figure 14 which is a map drawn for the entire Seine Basin (overlaps with map on Fig. 2). Stop 5 can be found on both Figs. 2 and 14. Stops are located by UTM coordinates based on NAD 83, UTM Zone 15 Stop 1: Ottertail Pluton Margin From the Recreation Center in Fort Frances, drive east along Hwy 11 for approximately 45 km. Outcrop located on both sides of road. (0506850E, 5396300N) The Ottertail Pluton is one of the calcalkaline granitoids of the Algoman type according to Lawson (1913) that range in composition from diorite-granite. More recent studies have linked them to sanukitoid magma suites (Shirey and Hanson, 1984). They are compositionally distinct from the older orthogneisses in the area, such as in the core of the Rice Bay Dome, which typically have a tonalitic composition (Poulsen, 2000a). All of the Algoman type intrusions have relatively discrete discordant boundaries and are generally semi-elliptical on map view. Although the Ottertail is obviously relatively late as judged simply by looking at the map, the details of how late is a bit more complicated and requires a look at both its margins and interior. The Ottertail itself is mostly granite to granodiorite. The granites are two-feldspar subsolvus granites, containing both K-feldspar and plagioclase along with quartz, hornblende, and biotite. Across its extent, it has a heterogeneous composition and grain size distribution (Czeck et al., 2006). At this stop, we will view agmatitic breccias that are common at the margins of the Ottertail (Poulsen, 2000a). Agmatites are defined by breccia- like mesosomes intruded by a leucosome. The breccia like character implies hydraulic fracturing at the pluton edge 60 �(Kornprobst, 2002). The xenolithic blocks within the breccia are foliated country-rock remnants that were likely deformed during D1 and D2. The Ottertail pluton‘s composition near the boundaries has significantly higher percentages of chlorite, biotite, and hornblende, presumably input from the country rock. Much of the chlorite is a retrograde reaction product of either biotite or hornblende. Together, the percentages of these three minerals range from 2%–15% in the center of the pluton, grading to 34%–58% along the eastern and western margins (Czeck et al., 2006). Feldspars are more sericitized in these boundary regions (Czeck et al., 2006). Stop 2: Ottertail Pluton Continue driving east on Hwy 11 for approximately 4 km. Approximately 1 km east of Pearson’s Road turnoff, outcrop is located on both sides of Hwy 11. (0510221E, 5397524N) Here we are located in the center of the Ottertail Pluton with the typical granitic chemistry. The rocks appear largely unmetamorphosed in outcrop, but were subjected to lowgrade, often hydrous metamorphism (Fig. 7a). Chlorite is the primary metamorphic mineral present, which formed by retrograde metamorphic reactions of biotite and hornblende (Czeck et al., 2006). Trace evidence of other metamorphic minerals including epidote, clinozoisite, and zoisite are often present (Czeck et al., 2006). The pluton is not highly deformed, but a range of deformation microstructures can observed throughout, either to subtle or intense degrees (Fig. 7b, c). The amount of grain scale deformation varies non-systematically throughout the pluton. In most samples, quartz microstructures, such as undulose extinction and subgrain formation, indicate that some solidstate deformation accommodated by dislocation creep mechanisms was active (Czeck et al., 2006). In the samples with the most deformation microstructures, a weak to moderate S–C fabric, quartz ribbons, fractured feldspars, and kinked biotites have also been observed (Czeck et al., 2006). Variations in the microstructural assemblages are most likely due to strain localization or possibly different cooling histories. The microstructures indicate that the granite was emplaced and cooled enough to deform in the solid state (in at least some portions) during deformation. Well-defined planar anisotropy of magnetic susceptibility (AMS) fabrics within the Ottertail Pluton are roughly coincident with the steep foliations measured in the surrounding rocks, indicating that the AMS fabrics were most likely caused by regional deformation (Czeck et al., 2006). Gravity inversion analysis indicates that the pluton is shallow, with a maximum depth of approximately 4.5 km (Czeck et al., 2006). The shallow nature of the pluton supports the conclusion that the steep internal fabrics are unlikely to be purely flow from a feeder zone, but rather a result of the regional strain. The pluton was likely syntectonic, late in D 2. Its geometry is consistent with intrusion into a transpressional setting, possibly into shallow rhombochasms created by en echelon P shears related to the major shear zones or along oblique thrusts (e.g. Tikoff and Teyssier, 1992). 61 �Figure 7. Photomicrographs of Ottertail Pluton from Czeck et al. (2006). a) Typical thin section with low-grade metamorphic mineralogy roughly consistent with regional greenschist-facies metamorphism (cross-polarized light). b) Undulose extinction in quartz (cross-polarized light). c) Subgrain formation in quartz (cross-polarized light). Stop 3: Moderately strained Seine Metaconglomerates Continue east along Hwy 11 for approximately 33 km to Horsecollar Junction . Outcrop located along north side of Highway 11. (0542130E, 5398538N) The Seine Group was interpreted to have been deposited in a fluvial system by Lawson (1913), Wood (1980) and subsequent workers (e.g. Czeck and Fralick, 2002). This formation undergoes a gradual transition from conglomerate dominated near its base to sandstone dominated near its top. The lithofacies found within the Seine match various environments of braided stream systems including channels and bars. Conglomerate dominated longitudinal bar-channel sequences are the most common. However, sandier channel sequences are not rare and gain importance higher in the formation (Czeck and Fralick, 2002). Lawson (1913) suggested the Seine Group was deposited at the site of an ancient fault zone, the strike of which is approximated by the current distribution of the conglomerate and sandstone. 62 �Within the conglomerates, asymmetric shear sense indicators are prevalent. In general, these are either in the form of asymmetric pressure shadows at the ends of clasts, wrapped foliation indicating rotation of the most rigid clasts, and clast tiling. All of these shear-sense indicators suggest dextral shear and are most evident on the subhorizontal plane, regardless of lineation orientation. The best model proposed for the deformation in the metaconglomerates that explains the subhorizontal asymmetric shear sense indicators, the dominant flattening fabrics, and the highly variable stretching lineation orientations is a transpression with horizontal simple shear and variable extrusion direction (Czeck and Hudleston, 2003, 2004). The specific clast strain combined with the other structural evidence should allow for a more rigorous test to this model (Fernández et al., in preparation). The metaconglomerates within the Seine Group provide an excellent opportunity to observe competence contrasts and their effects on deformation. The deformation here is typical of much of the Seine: the granitoid clasts being fairly undeformed while the volcanic clasts reflect significant flattening. Minor amounts of quartzite, BIFs, and other lithologic clast types are also observable. Czeck et al. (2009) used the clast shapes on the three-dimensional road outcrops to calculate strain within the Seine‘s various clast populations using the Rf/ method for two-dimensional strain (Ramsay, 1967; Dunnet, 1969; Lisle, 1985), a bootstrap statistical technique (McNaught, 2002; Mulchrone, 2005; Yonkee, 2005), and best-fit three-dimensional ellipsoid calculations (Launeau and Robin, 2003, 2005) (Fig. 8). In most cases, the calculated long axes of the strain ellipsoid match the mineral lineation and short axes are normal to the foliation, indicating that primary clast fabrics have not significantly affected the results. The strain magnitude is strongly dependent on lithology and varies greatly and nonsystematically throughout the region. In general, the mafic and felsic clast populations have similar strain magnitudes at a particular outcrop and granitoid clasts have consistently smaller strain magnitudes. The regional-scale variations in strain magnitude cannot be linked to changes in the concentrations of clast types within the outcrops (Fissler, 2006), but are likely related to spatial proximity to the minor shear zones. Of the 26 outcrops evaluated, 19 have an overall flattening strain, five exhibited a constrictional strain, and two exhibited plane-strain. Flattening dominates throughout the field area while constriction exists in the western and easternmost stations. Strain magnitude does not correlate to strain shape. At this location, long/short strain axes ratios were approximately 2:1 for granitoid clasts, 4:1 for felsic clasts, and 6:1 for mafic clasts. Czeck et al. (2009) also used detailed finite strain measurements of the clasts to calculate effective viscosity contrasts using established methods (Treagus and Treagus, 2002). The two volcanic clast populations have similar effective viscosities, regardless of strain magnitude, with ratios of felsic/mafic ranging from 0.27 to 2.12 and average ratio of 1.19. The granitoid clasts are significantly more competent than the mafic clasts with effective viscosity ratios ranging from 2.35 to 12.39 and average ratio of 5.61. The results are consistent with a qualitative competence hierarchy of granitoid > felsic ≥ mafic, although the quantitative effective viscosity ratios change with strain magnitude: for small strains, granitoid >> felsic ≥ mafic, but for large strains, granitoid > felsic ≥ mafic. The variations in these ratios throughout the field area indicate that the effective viscosity contrast between clast types is strain dependent and possibly deformation path dependent (Czeck et al., 2009). 63 �Figure 8. Plots indicating a) strain magnitudes ( s west to east across basin (stations A-Z) from Czeck et al. (2009). 64 �Stop 4- Highly strained Seine Metaconglomerates Continue east along Hwy 11 for approximately 19 km . Outcrop located along north side Highway 11. (0559619E, 5398914N) The Seine here is extremely deformed (amongst the highest strain found within this unit), reflected in the strained shapes of both the volcanic and plutonic clasts. Many clasts are deformed beyond recognition, giving the rock a striped appearance. Is it possible to accurately estimate strain in a rock that is this deformed? Most likely, all strain estimates are minimum estimates because only the least deformed clasts can be measured. Czeck et al. (2009) estimated strain for the various clast types at this outcrop and found the most disparate strain measurements in their study. Long/short strain axes ratios were approximately 3:1 for granitoid clasts, 11:1 for mafic clasts, and 136:1 for felsic clasts! The errors on the best fit ellipsoids were high, leading to doubts on the veracity of these results (don‘t put much faith in these numbers!). The lineation plunges ~44°E. While this lineation orientation is the approximate ―average‖ stretching lineation for the Seine, it cannot be thought of as ―typical‖ because the lineation orientations are so inconsistent. This outcrop is a key location to demonstrate that the lineation orientation is not directly related to the amount of strain (Czeck, 2001; Fissler, 2006; Czeck et al., 2009). This is important because most transpression models predict subvertical-vertical lineations in highly strained outcrops, and outcrops such as this one allow us to rule out many kinematic models. Again, the dextral asymmetric indicators are most prominent on the subhorizontal plane which allows us to rule out many triclinic transpression models. The combination of subhorizontal asymmetric shear indicators and variable lineation orientations makes a deformation model of quasi homogeneous transpression with a variable extrusion direction most likely (Czeck and Hudleston, 2003, 2004). This outcrop also displays some crenulation cleavage. In general, crenulations formed in the Seine when the clasts are stained enough to create a layering effect. Therefore, crenulations formed relatively late within the strain history, probably during D 3. Crenulations of both dextral and sinistral senses can be observed, indicating that there was likely still a pure shear component to the strain at the time of crenulation formation. The extensive silica veining and carbonate alteration in this rock suggests that fluids infiltrated the rock during deformation. The relative timing of the carbonate fluid and quartz veins is ambiguous in thin section, suggesting that they were at least at times, coeval (Czeck, 2001). Fluid flow in shear zones is often associated with the possibility of preferential volume loss of soluble material, although the largely constrictional strains measured here (Czeck et al., 2009) are not consistent with typical volume loss apparent strains which fall in the flattening field (Ramsay and Graham, 1970). There was probably a symbiotic relationship between fluid localization and enhanced deformation. With such instances of fluid infiltration in highly deformed zones, people often ask the ―chicken vs. egg‖ question of whether the fluid caused the high deformation through softening of the rock or whether the high deformation caused paths for fluids to infiltrate. We will leave the ―chicken vs. egg‖ question open for discussion. 65 �Stop 5 – Two-dimensional horizontal view of Seine Metaconglomerates Continue on Hwy 11, turn south on to Forest Tour Road (dirt track overgrown with weeds). (0536650E, 5398850N) This outcrop allows us to see a large two-dimensional view of the Seine Metaconglomerates with many beautiful examples of clast tiling, asymmetric strain shadows, and other kinematic indicators. Many sandy beds and channels, often with graded bedding, can be located within this outcrop. Is it possible to determine stratigraphic facing here? As a note of historical interest, A.C. Lawson recognized cross-beds in the Seine sandstones as early as 1911, only a year or so after their significance as a ―way-up‖ indicator was appreciated in younger, flat-lying rocks elsewhere. Lawson‘s work in the Seine appears to have been the first use of observed reversals in stratigraphic facing to define traces of folds in a Precambrian setting. Stop 6- Pseudoboudins Return west on Hwy 11 for approximately 66 km. Outcrop is located at junction of Hwy 11 and dirt road turnoff (south side of Hwy). Also small outcrop on north side of Hwy. (0497963E, 5393628N) At this outcrop of pelitic–psammitic schists, we can see fine examples of segmented pegmatite veins resembling boudin trains. Some of these segmented veins are indeed boudins, and they can be distinguished based on: 1) clearly defined edges, often with straight sides formed by fracturing that are oriented at high angle to the host rock fabric and 2) clear fabric deflection of the host rock into the boudin neck regions. However, within other segmented veins, we can see distinct examples of apparent boudinage or ―pseudoboudinage‖ (Bons et al., 2004; Bons and Druguet, 2007; Druguet et al., 2008) (Fig. 9). In pseudoboudinage, a string of pegmatite beads is oriented at a low angle to the foliation and layering. Although resembling boudinage, this structure more likely formed by irregular inflation and collapse during vein intrusion (Bons et al., 2004) or intrusion into dilational jogs (Bons and Druguet, 2007). This interpretation is based on: 1) the irregular ―cauliflower‖ shape of the individual segments and 2) the large amount of extension required to form these structures by boudinage, an amount that is incompatible with strain determined from nearby boudins (Druguet et al., 2008). Subsequent vein-parallel tectonic extension may have produced further separation between segments. Good examples of pseudoboudinage can be seen on the outcrop off of the south side of the highway. Good examples of folded and boudinaged veins can be seen on the small outcrop on the north side of the highway. 66 �Figure 9. Schematic illustration of geometrical differences between apparent boudinage and actual boudinage from Bons et al. (2004). a) Collapse and inflation of a pegmatite dyke upon emplacement, without any dike-parallel stretching. b) Boudinage of a solidified pegmatite dyke, due to dike-parallel stretching. Stop 7- Shear zones within the Grassy Portage Sill Continue west along Hwy 11 for approximately 4 km to the junction with Hwy 502. Turn right and drive north along Highway 502 for approximately 2 km. Turn east along unnamed road and drive for approximately 4.5 km. Outcrop is a small mound on north side of road. (0498897E, 5399146N) The Grassy Portage intrusion is a large metamorphosed layered gabbroic sill, which is exposed approximately 20 km along its strike. It has been overturned so that it dips steeply to the northwest (Poulsen, 2000a). Within the sill, layering is formed by variations in mineralogy and chemistry, both regionally (km scale) and locally (cm scale) (Poulsen, 2000a). Compositions range from melagabbro to anorthosite (Poulsen, 2000a). The margins have been exploited for disseminated chalcopyrite and pyrrhotite mineralization (Poulsen, 2000a). We will observe the sill along its base (northwest margin) on a subhorizontal glaciated surface. This lower portion of the sill is dominated by compositions of gabbroic and anorthositic rocks that either may have formed by gravitational setting or autointrusion (Poulsen, 2000a). The metagabbroic rocks range from leucogabbro to gabbro in composition and contain hornblende and plagioclase with minor amounts of chlorite and biotite (Poulsen, 2000a). Most of the gabbros have a poikilitic texture with hornblende megacrysts, but some have an equigranular texture. 67 �Within the sill, the anorthositic rocks are often found as lenticular pods within the gabbros that range in size up to 50 cm maximum diameter. They have adcumulate textures of fine-grained andesine (Poulsen, 2000a). At this particular outcrop, the pods have adcumulate textures, but are composed of fine-grained zoisite. The zoisite textures indicate that they were formed by hydrous alteration of plagioclase as evidenced by some relict plagioclase grains with quartz and minor amounts of calcite (Carreras et al., in press). The metagabbros contain a wavy pre-shearing weak foliation formed by aligned and linked amphiboles, which strikes approximately 110° with steep-subvertical dip. The orientation of this local foliation is consistent with the prevailing foliation in the immediate region, which is deflected from the typical NE-SW regional strike to a WNW-ESE orientation due to deflection around the Rice Bay Dome. Both the gabbros and anorthosites are affected by subsequent small-scale (cm-dm) shear zones, defined by deflection and intensification of this prevailing foliation and studied in detail by Carreras et al. (in press) (Fig. 10). They are narrow and discrete bands, with a maximum width of a few centimetres, that define an anastomosed pattern with orientations dominated by NNWSSE striking, moderately to steeply dipping strands (Carreras et al., in press). Inside the shear zones, the rocks are well-foliated and fine-grained mylonites. The shear zones are preferentially within the gabbros and often localize at the gabbro/pod margins. This prevalence of ductile deformation within the gabbros indicates that the gabbros were less competent than the anorthosites during much of the deformation. However, the shear zones also cut across some pods where they are defined by mylonitic banding with finer grain size and segregated compositional banding. Through detailed kinematic analysis including foliation deflection patterns and relative timing criteria outlined in their paper, Carreras et al. (in press) determined evolution of the shear zones including the relative timing of initiation and subsequent rotation of the features (Fig. 11). They distinguished two main sets of shear zones, dextral and sinistral, which each exhibit a range of orientations. The final angular pattern between dextral and sinistral shears is not an original feature. Dextral and sinistral shears formed together at nearly right angles, and the angles progressively opened towards the extension direction as a result of increasing strain. The obtuse angles were achieved by the combined effects of continued shearing on newly forming shears and internal deformation of the lozenge-shaped domains of lesser deformed rock. Through time, there was an increasing prevalence of dextral shears over sinistral ones . The studied pattern and sequential analysis indicate that bulk deformation was noncoaxial with a deformation regime evolving from a pure shear-dominated dextral transpression to a higher vorticity dextral transpression (Carreras et al., in press). 68 �Figure 10. Detailed outcrop map of the shear zones within the Grassy Portage Sill from Carreras et al. (in press). 69 �Fractures are the latest structures, cutting both the foliation and shear zones. They are located exclusively in the anorthosite (zoisite) pods and are chiefly oriented transverse to the mylonitic foliation. The regularity of the fracture orientations suggests that they formed in response to the final state of instantaneous strain because one would only expect them to be subparallel if they formed without any subsequent rotation. They are interpreted as extensional (Mode I) fractures. The incidence of the fractures exclusively within the pods indicates that the pods were relatively more competent than the surrounding gabbros at this final stage of deformation. Stop 8- Folds and syntectonic veins within mafic metavolcanics Return to Hwy 502 and drive north for approximately 3 km. Outcrop is located along west side of Highway 502. (0496149E, 5401196N) At this stop, we are located in relatively incompetent mafic metavolcanic rocks that are channeled between more competent units: the Black Sturgeon Bay gneiss dome to the northwest, the Rice Bay gneiss dome to the southwest, and the Baseline Bay (Algoman) pluton to the east. The foliation at this outcrop is roughly striking NE/SW, in an orientation that is consistent with the foliation in the incompetent units being channeled around the more competent units. Here, the banded mafic metavolcanic rocks are isoclinally folded (F2). A sequence of leucocratic veins are intruded into the host rock. The leucocratic veins are syntectonic with D2 because many such veins that cross-cut F2 folds are folded and co-planar with them (axial planes in veins are subparallel to those in the metasedimentary rocks), although in a more open geometry. This relationship reveals that the host rocks underwent folding prior to vein intrusion, but both vein and host rock underwent additional D2 folding. Druguet et al. (2008) used veins such as these to estimate two-dimensional strain in metavolcanic and metapelitic host rocks surrounding the competent Rice Bay gneiss dome. This outcrop is located in one of the highly strained zones (Fig. 12). The veins and their host rocks underwent a competence inversion. When the veins were first intruded, they were less competent than the host rock, but as they cooled, they eventually became more competent. We can see evidence for the early less competent veins with cuspate margins and irregular veins boundaries. We can see evidence for the late more competent veins with folding. 70 �Figure 11. Two-dimensional (horizontal) kinematic model for shear zone set formation from Carreras et al. (in press). a) Mean orientation of the early and late dextral (D) and sinistral (S) sets of shears and corresponding Rose diagrams. The finite bulk shortening direction faces the obtuse angle between the sets. In both stages, there are more dextral shears than sinistral ones. The orientation and corresponding Rose diagram of the late fractures (F) is also included. b) Model for the sequential development of shears, drawn on the XZ plane with north upwards. In addition to shears, the orientation of the bulk X and Z directions, the assumed shear plane and the trend of the pre-existing foliation (dashed lines) have been drawn. It is assumed that the initial angle that faced the bulk shortening direction between sets was ~90°. As deformation progressed, the two sets rotated in opposite directions, and the angle between them increased. The later development of new dextral and sinistral sets led to complex coalescences and crosscutting relationships. 71 �Stop 9- Folding and syntectonic veins in alternating pelitic and psammitic schists Turn around and return south on Hwy 502. Outcrop is located along south side of Hwy 11 immediately east of Hwy 502 turnoff. (0495680E, 5396171N) At this stop, we are located in relatively incompetent metasedimentary rocks that are channeled between more competent units: the Rice Bay gneiss dome to the northwest and the Grassy Portage gabbroic sill to the east. The foliation at this outcrop is striking approximately NE/SW, parallel with the map expression of this channeled metasedimentary unit. The rock consists of alternating pelitic and psammitic schists with F2 isoclinal folds easily seen in the metapsammitic layers and early quartz veins. There are multiple veins ranging from quartz to leucocratic in a variety of orientations. We can see folding and boudins in the veins depending on their orientation. The folds in the veins have a tighter geometry in the pelitic schists compared to the psammitic schists due to the higher competence contrast between the veins and the metapelites (e.g. Ramsay, 1967; Cruikshank and Johnson, 1993). Based on two-dimensional strain analysis of vein deformation with assumed constant volume, this outcrop is located in a moderately strained zone with long/short axial strain ratios of approximately 2:1 and plane strain (Druguet et al., 2008) (Fig. 12). Stop 10- Decimeter scale upright folds Located along north side of Hwy 11 immediately west of Hwy 502 turnoff. (0495180E, 5396173N) The next stop is right near the last within the layered metasedimentary rocks. Here we can observe decimeter scale low amplitude folds within the sedimentary layers. Note that the F2 folds plunge eastward and display an asymmetry consistent with the location on the south-east flank of the Rice Bay Dome. Two outcrop localities convincingly demonstrate downward facing in this metasedimentary (metagreywacke) unit: near Pocket Pond (pillow lavas) and Bear Pass Boat Launch (graded beds) (Poulsen et al., 1980). In both cases, the metagreywackes are structurally beneath, but stratigraphically above the metavolcanics, a relationship that is supported by zircon ages (Davis et al., 1989). Given that these metasedimentary rocks are younger than both the mafic-ultramafic rocks to the east and the felsic rocks in the core to the dome to the northwest, there is a problem in interpreting their configuration prior to development of the F2 ―dome.‖ The possible solutions are shown in Fig. 13, all resulting in ―stacked‖ tectonostratigraphy. Therefore, in the big-picture view, down-plunge and eastward from this locality is an antiformal syncline. 72 �Figure 12. Schematic structural map of the study area from Druguet et al. (2008) with localities where strain estimations were performed (A–G). Diagrams of azimuth distribution of veins from four different areas. Pale grey: shortened veins; black: extended veins and, where possible; grey: shortened and extended veins. (LSZ = from a low strain zone, MSZ= from a moderate strain zone, HSZ = from a high strain zone.). Fieldtrip Stop 8 is from the HSZ at location 5e in the north-central portion of the map. Fieldtrip Stop 9 is from the MSZ at location 8d in the northcentral portion of the map. Fieldtrip Stop 11 is from the HSZ at location 5a in the western portion of the map. Figure 13. Schematic illustration showing alternative explanations for stacking of tectonic slices in Rice Bay – Bear Pass area. From Davis et al., (1989). 73 �74 Figure 14. Simplified map of Seine Basin including Stops 3-5. Modified from (Czeck, 2001; Czeck and Hudleston, 2003; Czeck et al., 2009) based on maps from (Wood et al., 1980b, a; Stone et al., 1997a, b) . �Stop 11- Four consecutive sets of strongly deformed leucocratic veins within a foliated metasedimentary rock Drive west along Hwy 11 for approximately 17 km to junction with Rocky Inlet Road. Outcrop is located on the SE corner of the intersection. (0479790E, 5391262N) This outcrop is a strongly foliated metasedimentary rock of the Coutchiching Group located between various Algoman granitic intrusions to the north and south. Four consecutive sets of strongly deformed leucocratic veins intrude the host rock and are boudinaged and folded depending on orientation. When veins intruded at angles higher than 30° with respect to the foliation, they were folded. When intruded at lesser angles, they were boudinaged or folded then boudinaged. There is an increasing measurable strain in progressively older veins, which indicates multiple synkinematic D2 intrusions (Druguet et al., 2008). Based on two-dimensional strain analysis of vein deformation with assumed constant volume, this outcrop is located in a highly strained zone with long/short axial strain ratios of approximately 5.5:1 and flattening strain (Druguet et al., 2008) (Fig. 12). In this highly strained zone, the folds are tighter than at lesser deformed outcrops. The folds are tight to isoclinal and classified as Class 1C and 2 folds characterized by relative thickening of the hinge zones, which is compatible with flattening strain. Lastly, the veins are offset by discrete brittle faults, primarily ductile, that presumably occurred during D3. Bonus Stop 12 If we have time for this stop, it is located approximately 4 km east of Stop 11 on north side of Highway 11, immediately next to convenience store with Bear statues. (0484020E, 5393045N) Here, we can observe metabasalts with prominent pillows in three-dimensions. Stretching lineations defined by alignment of mafic minerals and long axes of pillows are near vertical. Strain appears to be constrictional, although detailed strain measurement has not been performed. Leucocratic intrusions have cuspate structures that preserve magmatic relationships and weak internal fabrics, suggesting syntectonic intrusion. Bonus Stop 13 From Bonus Stop 12, drive west along Hwy 11 for approximately 8 km to the western lookout point on north side of Noden Causeway, Hwy 11, immediately under powerlines. (0477336E, 5388940N) Here, we can observe well-foliated granitoid rock with large K-feldspars that has been intruded by many felsic and mafic dikes. Fabrics include a foliation oriented approximately N86W, 88NE defined by alignment of plagioclase and a steep (80°W pitch) mineral lineation. The granitoid contains many semi brittle- semi ductile shear zones. 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Geology 20, 927-930. Treagus, S. H., Treagus, J. E., 2002. Studies of strain and rheology of conglomerates. Journal of Structural Geology 24, 1541-1567. Van Hise, C. R., Adams, F. D., Bell, R., Lane, A. C., Leith, C. K., Miller, W. G., 1905. Report on the special committee for the Lake Superior region. Journal of Geology 13, 89-104. Van Hise, C. R., Leith, C. K., 1911. The geology of the Lake Superior region. United States Geological Survey Monographs 52. Williams, H. R., 1991. Quetico subprovince. In: Thurston, P. C., Williams, H. R., Sutcliffe, R. H. & Stott, G. M. (Ed.), Geology of Ontario. Ontario Geological Survey, Special Volume 4, Part 1, 383-403. Wood, J., 1980. Epiclastic sedimentation and stratigraphy in the North Spirit Lake and Rainy Lake areas; a comparison. Precambrian Research 12, 227-255. Wood, J., Dekker, J., Jansen, J. G., Keay, J. P., Panagapko, D., 1980a. Mine Centre Area (Eastern Half), District of Rainy River. Ontario Geological Survey Preliminary Map P. 2202, scale 1:15840. Wood, J., Dekker, J., Jansen, J. G., Keay, J. P., Panagapko, D., 1980b. Mine Centre Area (Western Half), District of Rainy River. Ontario Geological Survey Preliminary Map P. 2201, scale 1:15840. Yonkee, A., 2005. Strain patterns within part of the Willard thrust sheet, Idaho-Utah-Wyoming thrust belt. Journal of Structural Geology 27, 1315-1343. 79 �FIELD TRIP 4 ARCHEAN GEOLOGY OF VOYAGEURS NATIONAL PARK AND THE LITTLE AMERICAN GOLD MINE Chris Hemstad (Boreal Explorations) and Mark Jirsa (Minnesota Geological Survey) This boat trip through a very small part of Voyageurs National Park highlights the complex Archean geology of the southern Wabigoon and northern Quetico subprovinces of Superior Province (Fig. 4.1). The tour begins at the landing located within metasedimentary rocks of the Quetico subprovince. The boat travels northward, up Black Bay to Little American Island, then northeastward to Dryweed Island. Due to limited time and docking capabilities, the only actual stop will be at the historic Little American Gold Mine—the first and only operating gold mine in Minnesota. Elsewhere the geology will be viewed and interpreted ―over the gunnels.‖ Little American Island lies within the broad zone of dextral shear associated with the Rainy Lake-Seine River Fault, shown by wavy dashed lines on Figure 4.1. Gold-bearing quartz veins on the island were emplaced into variably deformed metabasalt and mafic schist of the Wabigoon subprovince. Dryweed Island, to the northeast, forms a lozenge of comparatively less deformed rocks between the Rainy Lake-Seine River and Tilson Bay fault zones. Most of the exposures on Dryweed Island are components of the Seine Group, a sequence of quartzofeldspathic sandstone, conglomerate, and minor felsic tuff. Stratigraphic facing in the Seine is consistently southward, so this exposure appears to represent a fault-bounded, southward-tilted block. See Jirsa and Hemstad, Field Trip 6, this volume for more details. Figure 4.1—Geology of Little American Island and Dryweed Island in Rainy Lake (modified from 1:24,000-scale maps of Hemstad and others, 2000, 2001). 80 �Figure 4.2—Portion of an historic geologic map showing Little American Island (circled), and the town site of Rainy Lake City on the mainland to the southeast (cross-hachured). Figure 4.3—Plat map of Rainy Lake City and historic photo (from Perry, 1993) 81 �Gold was discovered on Little American Island in 1893 by George Davis, a prospector en route to exploration in Canada, who camped on the well-used spot (Fig. 4.2). There he discovered a quartz vein, cracked some off, crushed and panned it. Later assays returned estimates of gold contents in the range of $50 to $100 per ton. The rush was on! Investors from Duluth and elsewhere quickly amassed $300,000 to capitalize a mining venture. By 1894, Rainy Lake City was platted on the mainland to the southeast, where a mill for stamping and processing the ore was constructed. In a short time, the city had several sawmills, 8 dry-goods stores, 8 grocers, three hotels and restaurants (Fig. 4.3), seventeen saloons, two newspapers, a livery barn, three laundries, a school, a bank, a doctor, and a lawyer (Perry, 1993). The mine produced $4,635 worth of gold in 1893 from the 2 meter wide composite quartz vein. However, its operation was plagued with episodes of failing capitol and production calamities. An incompetent amalgamator sent $10,000 worth of gold into the lake, and the collapse of a loading dock sent a season‘s ore to the bottom (Perry, 1993). Mining continued intermittently, and the mine eventually reached a depth of 210 feet. At today‘s prices, an estimated total of $45,000 worth of gold was extracted during its operation, but the immense cost of transportation and milling brought the mine to a close by 1895. Several attempts to reopen it were made until 1898, when the County seized the property. By that time, the lure of the Klondike sapped much of the excitement from this remote endeavor. The town site of Rainy Lake City was abandoned and dismantled. By contrast, other mines in what was called the "Rainy Lake Gold Region" were very productive, especially those in the nearby Mine Center area of Canada. Acknowledgement: This boat tour is courtesy of the National Park Service, Voyageurs National Park. We extend our thanks to their staff, including Mike Ward-Park Superintendent and Tawnya Schoewe-Head Park Interpreter, for making it possible. REFERENCES Hemstad, C.B., Ojakangas, R.W., and Southwick, D.L., 2000, Bedrock geologic map of the Island View quadrangle, Koochiching County, north-central Minnesota: Minnesota Geological Survey Miscellaneous Map M-105, scale 1:24,000. Hemstad, C.B., Ojakangas, R.W., Southwick, D.L., and Nemitz, M., 2001, Bedrock geologic map of Cranberry Bay quadrangle, Koochiching and St. Louis Counties, north-central Minnesota: Minnesota Geological Survey Miscellaneous Map M-110, scale 1:24,000. Perry, David E., 1993, Gold town to ghost town: Boom and bust on Rainy Lake: Lake States Interpretive Association, HCR 9, Box 600, International Falls, MN 55649. 82 �FIELD TRIP 5 ASH RIVER NEUTRINO DETECTOR LABORATORY (NOVA) AND THE ARCHEAN VERMILION GRANITIC COMPLEX Mark Jirsa (Minnesota Geological Survey*) and William Miller (University of Minnesota School of Physics and Astronomy) *This manuscript was not reviewed to conform to editorial standards of the Minnesota Geological Survey This field trip visits the NOvA project, under construction off the Ash River Trail southeast of International Falls. The project is part of a wide ranging effort to understand the fundamental make-up of matter. The acronym ―NOvA‖ stands for NuMI, Off-axis, electron neutrino (ve), Appearance. ―NuMI‖ refers to Neutrinos at the Main Injector facility at Fermi National Accelerator Laboratory (Fermilab) in Chicago. More than 180 scientists from 28 national laboratories and institutions around the world are working on the NOvA neutrino experiment that will take place at the Ash River site. The experiment will investigate the role of subatomic particles called neutrinos in the origin and shaping of matter in the universe. Fermi National Accelerator Laboratory, which manages the project, will generate a beam of neutrinos to send to a 15,000-ton detector at the new facility. Despite traveling through the earth‘s crust (Fig. 5.1), the particles will complete the 500 mile interstate trip in less than a hundredth of a second. Scientists will study changes that the particles undergo as they travel. Figure 5.1—Schematic diagram of neutrino trajectories. 83 �The $278 million NOvA project is funded by the Office of Science of the U.S. Department of Energy and Fermi National Accelerator Laboratory. The NOvA experiment at Ash River will be operated by the University of Minnesota School of Physics and Astronomy. The American Recovery and Reinvestment Act provided $40.1 million to the project. For more information, contact Bill Miller via e-mail at Miller@soudan.umn.edu or visit the website at wwwnova.fnal.gov/fermilab_nova.pdf. Excavations and outcrops near the NOvA site expose leucogranite-rich migmatite of the Vermilion Granitic Complex in the heart of the Quetico subprovince of Superior Province. See Jirsa and Hemstad, Field trip 6, stop 6-10 this volume for more details. The narrow, darker colored unit shown on the Figure 5.2 near the site is biotite-hornblende diorite. Figure 5.2—Geologic map of the NOvA site (modified from Southwick and Ojakangas, 1979). REFERENCE Southwick, D.L., and Ojakangas, R.W., 1979, Geologic map of Minnesota, International Falls sheet: Minnesota Geological Survey, scale 1:250,000. 84 �FIELD TRIP 6 TRANSECT THROUGH THE QUETICO-WABIGOON SUBPROVINCE BOUNDARY Mark Jirsa (Minnesota Geological Survey*) and Chris Hemstad (Boreal Exploration) *This manuscript was not reviewed to conform to editorial standards of the Minnesota Geological Survey GEOLOGIC SETTING This trip examines the geology along the boundary between two subprovinces of the Archean Superior Province—the volcanoplutonic Wabigoon subprovince to the north, and the largely metasedimentary and migmatitic Quetico subprovince to the south. The boundary is somewhat transitional, as it is marked by a broad zone of shear associated with the Rainy Lake-Seine River Fault and several other fault structures. The two terranes have contrasting lithologic and metamorphic attributes. The Wabigoon portion of the area consists of tightly folded volcanic rocks and metagraywacke, unconformably overlain by and structurally interleaved with conglomerate and quartzite of the Seine Group and metamorphosed to greenschist facies. The Quetico portion contains biotite schist of graywacke protolith and migmatitic plutonic rocks that are multiply deformed, metamorphosed to amphibolite facies, and intruded by younger granite. Although the subprovince boundary is clearly a metamorphic one, work by Devaney and Williams (1989) on exposures along strike to the northeast implies that the coincident lithologic shift reflects a depositional transition during Neoarchean terrane accretion. The Wabigoon represents oceanic plateau and island arc deposition; the Quetico represents the transition arc to submarine margin and trench-fill deposition. Seismic reflectivity indicates north-dipping structures are present at depth along the subprovince boundary in Canada (Percival and others, 2006). These structures may represent the accretionary boundary that has been over-steepened and segmented locally by subsequent north-south crustal shortening and dextral wrench faulting. The Seine Group, which lies within a fault- and unconformity-bounded block, appears to represent continental sedimentation into one or more successor basins developed along the subprovince boundary. All of these Archean rocks were intruded by diabase dikes of the Kenora-Kabetogama swarm at about 2067 Ma (Wirth and others, 1995). 85 �WABIGOON SUBPROVINCE Only a sliver of the subprovince is exposed on the Minnesota side of the border, and therefore, much of what is known about it is based on work to the north. Volcanism in the subprovince as a whole occurred in the range of 2745-2720 Ma (Corfu and Davis, 1992); with rare older and younger volcanic sequences locally. Metavolcanic rocks exposed on the mainland and islands in Minnesota are variably schistose and include pillowed, massive, and tuffaceous basalt; felsic tuff, and hypabyssal intrusions. Schistose metasedimentary rocks of apparent turbiditic origin are infolded with the volcanic strata. Plutonic rocks vary from synvolcanic batholiths consisting of tonalite, diorite, and gabbro; to younger granodiorite batholiths and plutons (~2710 Ma), monzodiorite (sanukitoid) plutons (~2698-2690 Ma), and monzogranite (~2690-2660 Ma). Younger clastic successions, including the Seine Group described below, sit with local unconformably on older volcanic strata and typically have ages in the range of 27112686 Ma. Seine Group The Seine Group exposed in Minnesota (or Seine conglomerate, as it is referred to in Canada) consists of interbedded conglomerate, quartzofeldspathic and lithic sandstone, and minor tuffaceous strata. Although moderately to strongly deformed, sedimentary structures are locally preserved and include trough cross-bedding and normal and reversed grading. Collectively, these characteristics indicate fluvial deposition. Conglomeratic beds contain pebble- to cobble-size, rounded to subrounded, polymictic clasts that include fragments of subjacent rocks and a variably porphyritic volcanic component of intermediate to felsic composition. The contrast between the relatively deep water, pillowed volcanic and turbiditic rocks, and the superjacent quartzose and conglomeratic strata of the Seine is striking and implies that significant uplift by tectonic shortening must have occurred. The unit trends discontinuously northeastward across the mainland and islands and continues into Canada. In Minnesota, it appears to be bounded on the north by the Tilson Bay Fault, and stratigraphic younging is consistently southward. Additional details about the sedimentology and deformation of the Seine conglomerate can be found in Czeck and Fralick, 2002; and Poulsen and Czeck, field trip 3, this volume. The Seine Group is one of a series of similar deposits throughout the Superior Province represented by subaerial unconformities overlain by fluvial and lacustrine or marine basin-fill sediments. The relative temporal framework of these deposits indicates that they are timetransgressive, generally younging to the south, which is consistent with arc collisional events (Corcoran and Mueller, 2007). Czeck and Fralick 2002 suggested that the Seine conglomerate was deposited following collision of the southern Wabigoon terrane with the Wawa and intervening Quetico terranes. The age of the Seine is bracketed between about 2696 Ma, the age of a clast, and 2686 Ma, the age of an intrusion which cuts the Seine (Davis and others, 1990). QUETICO SUBPROVINCE The Quetico is one of a number of east-trending, largely metasedimentary subprovinces in the Superior province. It is bounded on the south by the Wawa volcanoplutonic subprovince, and on the north by the Wabigoon subprovince. It consists of schist derived from turbiditic sedimentary rocks and a complex suite of granitic intrusions and associated migmatite. In northeastern Minnesota, the subprovince displays a roughly symmetrical distribution of metasedimentary rocks on the north and south, that grade irregularly through zones of schist-rich 86 �migmatite, to a central axial zone composed largely of polyphase granitoid migmatite and younger granite. Metamorphic grade mimics this symmetry, with generally higher grade rocks in the central axis and lower grade near the bounding volcanoplutonic subprovinces. The accretionary prism model of Williams (1990) implies sediment deposition by submarine fans and abyssal turbidites. Timing of deposition is constrained by youngest detrital zircons at about 2698 Ma, and intrusion ages of about 2688 Ma (Davis and others, 1990). The migmatitic and plutonic rocks in the axial zone are known collectively in Minnesota as the Vermilion Granitic Complex (Southwick and Sims, 1980). The migmatite is divided into schist-rich and granite-rich components, depending on the ratio of paleosome to neosome. These can be equated generally with the terms metatexite (low degree of partial melting) and diatexite (nearly complete fusion), respectively, of Sawyer, 2008. Field relationships within the complex (Southwick, 1991) indicate that earliest granitoid phases are leucogranite, granodiorite, and trondhjemite, which make up the leucosome of a broad area of migmatite. The migmatite is interlayered at all scales with paleosomes of biotite schist and amphibolite, and is cut by dikes, sills, and irregular masses of two mica leucogranite with accessory garnet, and biotite granite that contains minor magnetite. The latter forms a large intrusion known as the Lac La Croix granite, and apophosial intrusions that produce aeromagnetic anomalies highlighting broad fold structures. The earlier migmatite was derived from partial melting of a metasedimentary protolith. Southwick (1991) suggested that the younger Lac La Croix-type granite is a further distillation of granitic liquid from partial melting of the older migmatite and metasedimentary rocks. The apophosial intrusions of Lac La Croix granite generally decrease in thickness and continuity westward, suggesting that the western portion of the complex may represent the roof zone of a batholith cored by massive granite. DEFORMATION AND METAMORPHIC HISTORY Both terranes have undergone a complex deformation history. As described by Day (1990), three main phases of deformation are recognized. D1 produced tight to isoclinal, northeasttrending folds plunging to the southwest and locally overturned to the southeast. This event may locally have produced recumbent folds over a broad region (Bauer, 1985; Poulsen and others, 1980). It occurred shortly after deposition and involved burial to produce metamorphic conditions of moderate pressure and temperature (Valli and others, 2004). D 2 deformation was synchronous with peak regional metamorphism to upper greenschist facies in the Wabigoon subprovince and amphibolite facies in the adjacent Quetico subprovince. This is considered lowpressure, moderate temperature conditions by Valli and others (2004); however there is a progressive increase in metamorphic conditions across the Quetico subprovince from 0.2 GPa and 500 C on the west near International Falls, to 0.5-0.6 GPa and 780C in the east. D2 deformation produced small-scale F2 folds, and an axial-planar cleavage and schistosity that strikes to the east-northeast. Folds and other asymmetric features indicate that D 2 deformation was a dextral transpressional event, which probably initiated movement along the major faults. D3 deformation is recorded in the youngest movement along major fault zones. Vertical offset during D3 or D4 events, and subsequent differential erosion produced the contrasting metamorphic grades across the Rainy Lake-Seine River Fault. Combining structural, metamorphic, and published geochronologic data, Valli and others (2004) infer ages of tectonometamorphic events in the Quetico subprovince as follows: D1 Moderate pressure and temperature metamorphic event shortly after deposition of turbidites in the range of 2698-2690 Ma 87 �D2-3 Biotite-sillimanite-garnet peak metamorphism (0.6 GPa 700C) during transpression which began ~2689 Ma D4 Low pressure, moderate temperature metamorphism in the range of 2671-2667 Ma According to Fralick and others (2006), clastic deposition in the northern Quetico began about 2698 Ma during D1 and continued to <2696 Ma, supporting the interpretation of a transition from accretionary wedge to foreland basin deposition. Subsequent deformation during D 1 was associated with accretion of these sediments onto the active Wabigoon arc. Deformation during D2 and D3 may represent docking of the Wawa arc from the south. FIELD TRIP STOP DESCRIPTIONS This field guide is based on mapping by Hemstad and others (2000, 2001, 2002) and Day (1990), and an earlier guide produced for the Institute on Lake Superior Geology (Ojakangas and others, 1982). Some of the stops described below were also visited during the 1982 meeting and are described in more detail in that guide book—for reference, prior stop numbers are shown in [brackets]. Because of road layout, the order of stops is not necessarily geochronologic. Stops 1-6 and 9 lie within the Wabigoon subprovince (Fig. 6.1); Stops 7, 8, 10, and 11 are within the Quetico subprovince to the south (Figs. 6.1 and 6.2). All UTM coordinates are given in NAD 83, zone 15. Figure 6.1—Geologic map of the Rainy Lake area, modified from Day, 1990, showing locations of field trip stops 6-1 through 6-9. 88 �STOP 6-1 Archean monzonite cut by Paleoproterozoic diabase dikes Location: 0.65 miles east of US Hwy 53 on MN Hwy 11; ~500 feet north of Hwy 11 (UTM 471,295E / 5,383,386N). Description: Pink, coarse-grained, porphyritic hornblende-quartz monzonite. Rock displays a pronounced trachytoid foliation that trends N70E. Unit contains irregular autoliths of monzodioritic to dioritic composition, and rare xenoliths of schistose metasedimentary rocks, which form the country rock into which the intrusion was emplaced. The long axes of inclusions are typically parallel to foliation, implying that foliation is largely magmatic; however, tectonic and metamorphic overprint is also likely. Note that some of the clasts in conglomerate of the Seine Group at STOP 6-6 have textures similar to this intrusion. Several fine-grained diabase dikes, having varied thicknesses and a northerly trend, cut the monzodiorite. STOP 6-2 Archean metagraywacke and slate Location: Just east of village of Rainier; 3 miles east of US Hwy 53 on MN Hwy 11; ~400 feet N of Hwy 11 (UTM 474,888E / 5,384,188N) Description: This outcrop exposes fine-grained plagioclase-biotite schist of graywacke and mudstone protolith, metamorphosed to upper greenschist facies. Bedding is locally visible and parallels cleavage and schistosity at N68E, dipping 75N. Stratigraphic younging is northward, based on graded bedding and cross-cutting relationships of basal turbidite beds. This is consistent with the outcrop position on the north side of an east-trending, westward plunging anticline that is overturned slightly to the south. Recessed aligned depressions on the outcrop surface generally parallel the foliation and may represent boudinaged carbonate-rich horizons or zones of concretions. The rock is cut by a number of quartz veins emplaced along small-scale faults. NE-trending veins show sinistral offset, NW-trending ones are dextrally offset. Late silicification ―veinlets‖ trending N25W create a ribbed relief on the outcrop surface. The temporal relationship between metagraywacke exposed here and similar but more highly metamorphosed schist in the Quetico subprovince to the south is unresolved. STOP 6-3 Schistose feldspathic-lithic quartzite of the Seine Group [1982 stop #2] Location: Approximately 2.5 miles east of Rainier on MN Hwy 11 to County Road 109; turn south on 109 and follow it south and east for about 1.2 miles. Ridge of outcrop trends east from here. (UTM 478,990E / 5,382,630N) Description: This outcrop ridge extends more or less parallel to bedding and foliation. The rock is medium-grained, light gray to tan. Framework grains consist of quartz and feldspar and are angular to subrounded and slightly flattened in the regional foliation. Matrix consists of sericite, chlorite, and quartz. Beds range in thickness from 5 cm to 1 meter, planar lamination and trough cross stratification are common. This outcrop lies within an irregular belt of quartzite and conglomerate trending to the east-northeast into adjacent Canada. It appears to be fault-bounded on the north by the Tilson Bay Fault, and stratigraphic younging is consistently southward. Though not in evidence here, this unit and that of STOP 6-6, are inferred to unconformably overlie metasedimentary and metavolcanic strata of the Wabigoon subprovince. STOP 6-4 89 �Metavolcanic and meta-igneous rocks [1982 stop #3] Location: Return on County Road 109 to MN Hwy 11, proceed east 1.0 mi. to outcrop on north side of highway (UTM 479,745E / 5,383,009N) Description: At least two rock types are exposed here; a schistose green to gray chloritic tuff containing iron-carbonate, and a coarser grained meta-igneous rock composed of what may have been pyroxenes, now pseudomorphed by amphibole. Both rock types display S 2 schistosity. Though strongly deformed, this exposure is typical of the local greenstone. STOP 6-5 Tectonite in the Rainy Lake-Seine River fault zone Location: Continue east on Mn Hwy 11 for 2 mi., crossing Tilson Creek to outcrops and boat landing on north side of highway (UTM 482,770E / 5,382,850N) Description: These outcrops lie within a broad zone of shearing associated with the Rainy LakeSeine River fault that separates Wabigoon and Quetico subprovinces. The zone continues eastward to Little American Island, where Minnesota‘s only gold mine (to date) operated from 1893 to about 1898—see Hemstad and Jirsa, Field Trip 4, this volume for more details. The main part of the fault is represented topographically by the northeast-trending swamp visible just to the south. Outcrops consist of amphibolitic and chloritic schist, inferred to be of basaltic protolith, and minor chert-magnetite iron-formation. Both carry a strong, vertically dipping, N50E-trending cleavage and abundant detached quartz veins in the subparallel shear fabric. The rocks contain minor amounts of pyrite. Some exploration drilling was done near here in the late 1980s (Newmont), to investigate the possibility of more significant mineralization, presumably at junctions of the regional schistosity (S 2) and local shear structures. STOP 6-6 Conglomerate and quartzite of the Seine Group Location: Continue east on MN Hwy 11 for 1.2 mi. to Gold Shores Road (County Rd 138), turn NE on 138 and follow it around (one-way) for about 1 mile to roadcuts (UTM 485,876E / 5,383,470N) Description: This large area of outcrop includes polymictic, clast-supported conglomerate, underlain to the north and locally interbedded with feldspathic lithic quartzite similar to that at STOP 6-3. The conglomerate contains rounded pebbles, cobbles, and boulders of felsic plutonic rocks, mafic to felsic volcanic rocks, chert, biotite schist, and iron-formation. Plutonic clasts are subspherical to elliptical, whereas volcanic clasts tend to be moderately elongated in the regional lineation. The texture of some plutonic clasts is similar to monzonite exposed at STOP 6-1. The schistose matrix is composed of fine- to medium-grained biotite- or chlorite-rich lithic arenite that contains angular to subrounded granules of volcanic rock fragments, intrusive rock fragments, plagioclase, quartz, and chert. Bedding is crude to massive. Subtle, fining-upward to coarsening-upward sequences are common in the conglomerate. Note that some volcanic clasts display pronounced concentric zonation, manifest by color and surface relief. It is unclear whether this represents original mineralogic zonation or metamorphically modified weathering. Similar observations have been made of clasts in other ―Timiskaming-type‖ or successor-basin conglomerates near Virginia, Minnesota (Jirsa, 2000) and in the Ogishkemuncie conglomerate in the Boundary Waters Canoe Area (Jirsa and Starns, 2008). 90 �STOP 6-7 Voyageurs National Park Visitor Center Location: Return to MN Hwy 11 and continue east for 0.8 mi. to south-trending road to Black Bay visitor center and boat landing (UTM 488,050E / 5,381,191N) Description: Visitor center displays and restrooms should be open. Several roadcuts along this trail and lakeshore outcrops near the visitor center expose biotite schist of the northern-most Quetico subprovince. This will be the subject of STOP 6-8. STOP 6-8 Quarry and gravel pit in Quetico metasedimentary rocks Location: Return to MN Hwy 11 and turn west, follow Hwy 11 ~0.8 miles to Gold Shores Road and walk south to large quarry and pit (UTM 484,630E / 5,382,325N) Description: Outcrops of fine- to medium-grained, light- to dark-gray schist derived from rhythmically bedded, texturally graded graywacke and shale. The rock is composed dominantly of plagioclase, quartz, biotite, and muscovite; and minor garnet, sillimanite, and hornblende in beds of appropriate bulk composition. Individual beds are typically 5–20 centimeters thick; some sandy beds are as thick as two meters. Outcrop displays folded discontinuous layers of boudinaged concretions, sandy beds, or quartz veins. En echelon gash quartz veins occur locally. STOP 6-9 Wabigoon subprovince metabasalt and iron-formation [1982 stop #10] Location: Continue west on MN Hwy 11 for about 9 mi. to MN Hwy 332 near the east edge of International Falls. Drive south and east on Hwy 332 for 2.5 mi. to large road cuts (UTM 472,350E / 5,380,800N) Description: Schistose rocks of basaltic and iron-formation protolith are exposed on both sides of the highway. Rock is strongly foliated amphibolitic schist, with lenses and layers of tightly folded magnetite-, chert-, garnet- and sulfide-bearing iron-formation. The strong foliation nearly obliterates bedding and other features. Fabric asymmetry indicates much of the fabric is a product of dextral transpression attributed in part to proximity to the Rainy Lake-Seine River Fault just to the south (as at STOP 6-5). STOP 6-10 Quetico subprovince—granite-rich migmatite of the Vermilion Granitic Complex [1982 stop #15] Location: Continue south on Hwy 332 for about 1.7 mi. to US Hwy 53. Follow US Hwy 53 south and east for 19.5 miles to large road cut (UTM 493,730E / 5,361,730N) Note: this is 5.4 miles east of Ray at jct. Hwys 53 and 217, and 1.6 miles west of hwy 122 entrance to Kabetogama landing. Description: This complex and photogenic outcrop is typical of granite-rich migmatite in the Vermilion Granitic Complex (Fig. 6.2). Overall, the unit mapped by Southwick and Ojakangas (1979) as granite-rich migmatite contains between 5% and 25% paleosomatic inclusions. In this area where granite is dominant, paleosome blocks and slabs up to several meters in size are rafted in neosome. Some paleosome blocks have sharp boundaries characteristic of agmatic migmatite, some have distinct flow fabric of schlieren migmatite, and some have diffuse boundaries representing partial assimilation typical of nebulitic migmatite (vernacular of Mehnert, 1968). The neosome is coarse-grained, light gray to pink, muscovite-biotite granite of the Lac La Croix-type with weak foliation. The paleosome includes biotite schist, amphibolite, tonalite, and early migmatite displaying variable degrees of deformation. The early migmatite 91 �has paleosomes of amphibolite and biotite schist, and neosome of tonalite or granodiorite. Much of the amphibolite has a relict porphyritic texture pseudomorphed by biotite, resembling lamprophyric intrusions exposed in adjacent greenstone terranes. Figure 6.2—Geologic map of the International Falls area, modified from Southwick and Ojakangas, 1979 (original map scale 1:250,000), showing locations of field trip stops 6-9 to 6-11. STOP 6-11 Quetico subprovince—schist-rich leucogranite migmatite of the Vermilion Granitic Complex Location: Continue south on US Hwy 53 for about 18.4 miles to road cuts; just southeast of Ash Lake (UTM 507101E / 5,339,900N) Description: These road cuts expose shallowly dipping schist-rich migmatite of the Vermilion Granitic Complex. According to the mapping convention of Southwick and Ojakangas (1979), the map unit is defined by a leucogranitic neosome content of 10% and 15%. The paleosome consists largely of biotite schist that displays strong metamorphic foliation (S 2?) that was deformed by D3 in broad open folds. No petrography has been conducted from samples of this exposure, so details of neosome composition and metamorphic mineral content are unknown. Nevertheless, it is apparent the schist is cut by a variety of leucogranitic intrusions that are strongly boudinaged and crumpled in planes of S 2 foliation and refolded. 92 �REFERENCES Bauer, R.L., 1985, Correlation of early recumbent and younger upright folding across the boundary between an Archean gneiss belt and greenstone terrane, northeastern Minnesota: Geology, 13:657660. Czeck, D., and Fralick, P., 2002, Structure and sedimentology of the Seine conglomerate, Mine Centre area, Ontario; Field Trip 3 in Institute on Lake Superior Geology Proceedings v. 48, pt. 2., p. 37-67. Corcoran, P.L., and Mueller, W.U., 2007, Time-transgressive Archean unconformities underlying molasse basin-fill successions of dissected oceanic arcs, Superior Province, Canada: Journal of Geology 115:655-674. Corfu, F., and Davis, D.W., 1992, A U-Pb geochronological framework for the western Superior Province, Ontario: in Geology of Ontario, Thurston, P.C., Williams, H.R., Sutcliffe, R.H., and Stott, G.M., eds., Ontario Geological Survey Special Vol. 4, Part 2, p. 1335-1346. Davis, D.W., Pezzutto, F., and Ojakangas, R.W., 1990, The age and provenance of metasedimentary rocks in the Quetico subprovince, Ontario, from single zircon analyses: implications for Archean sedimentation and tectonics in the Superior Province: Earth and Planetary Science Letters 99: 195205. Davis, D.W., Poulsen, K.H., and Kamo, S.L., 1989, New insights into Archean crustal development from geochronology in the Rainy Lake area, Superior Province, Canada: Journal of Geology 97:379-398. Day, Warren, C., 1990, Bedrock geologic map of the Rainy Lake area, northern Minnesota: U.S. Geological Survey Miscellaneous Investigation Series I-1927, scale 1:50,000. Devaney, J.R. and Williams, H.R., 1989. Evolution of an Archean subprovince boundary: A sedimentological and structural study of part of the Wabigoon-Quetico boundary in northern Ontario: Canadian Journal of Earth Sciences, 26:1013-1026. Fralick, P., Purdon, R.H., and Davis, D.W., 2006, Neoarchean trans-subprovince sediment transport in southwestern Superior Province: sedimentalogical, geochemical, and geochronological evidence: Can. J. Earth Sci. 43:1055-1070. Hemstad, C.B., Ojakangas, R.W., and Southwick, D.L., 2000, Bedrock geologic map of the Island View quadrangle, Koochiching County, north-central Minnesota: Minnesota Geological Survey Miscellaneous Map M-105, scale 1:24,000. Hemstad, C.B., Ojakangas, R.W., Southwick, D.L., and Nemitz, M., 2001, Bedrock geologic map of Cranberry Bay quadrangle, Koochiching and St. Louis Counties, north-central Minnesota: Minnesota Geological Survey Miscellaneous Map M-110, scale 1:24,000. Hemstad, C.B., Southwick, D.L., and Ojakangas, R.W., 2002, Bedrock geologic map of Voyageurs National Park and vicinity: Minnesota Geological Survey Miscellaneous Map M-125, scale 1:50,000. Jirsa, M.A., 2000, The Midway sequence: a Timiskaming-type, pull-apart basin deposit in the western Wawa subprovince, Minnesota: Can. J. Earth Sci. 37:1-15. Jirsa, M.A., and Starns, E.C., 2008, Preliminary bedrock geologic map of the 2006 Cavity Lake fire area, parts of the Ester Lake, Gillis Lake, Munker Island, and Ogishkemuncie Lake 7.5 minute quadrangles, northeastern Minnesota. Minnesota Geological Survey Open-File Report OF08-05, scale 1:24,000. Ojakangas, R.W., Day, W.C., and Southwick, D.L., 1982, Archean geology of the International FallsKabetogama area, Minnesota: Field Trip 2 in Institute on Lake Superior Geology Proceedings 28:89-139. 93 �Percival, J.A., Sanborn-Barrie, M., Skulski, T., Stott, G.M., Helmstaedt, H., and White, D.J., 2006, Tectonic evolution of the western Superior Province from NATMAP and Lithoprobe studies: Can. J. Earth Sci., 43: 1085-1117. Poulsen, K.H., Borradaile, G.J., and Kehlenbeck, M.M., 1980, An inverted Archean succession at Rainy Lake, Ontario: Canadian Journal of Earth Sciences 17:1358-1369. Sawyer, E.W., 2008, Atlas of migmatites: Canadian Mineralogist, Special Pub. 9, NRC Research Press, Ottawa, Ontario, Canada, 371p. Southwick, D.L., 1991, On the genesis of Archean granite through two-stage melting of the Quetico accretionary prism at a transpressional plate boundary: Geological Soc. of America Bulletin 103:1385-1394. Southwick, D.L., and Ojakangas, R.W., 1979, Geologic map of Minnesota, International Falls sheet: Minnesota Geological Survey, scale 1:250,000. Southwick, D.L., and Sims, P.K., 1980, The Vermilion Granitic Complex—A new name for old rocks in northern Minnesota: U.S. Geological Survey Professional Paper 1124A, p. A1-A11. Valli, F., Guillot, S., and Hattori, K.H., 2004, Source and tectono-metamorphic evolution of mafic and pelitic metasedimentary rocks from the central Quetico metasedimentary belt, Archean Superior Province of Canada: Precambrian Research, 132:155-177. Williams, H.R., 1990, Subprovince accretion tectonics in the south-central Superior Province: Can. J. of Earth Sci. 27:571-581. 94 �Field Trip 7 MINERAL DEPOSITS OF THE MINE CENTRE – RAINY RIVER AREA, ONTARIO Peter Hinz Mines Group Ontario Ministry of Northern Development, Mines and Forestry 435 James Street South, Suite B002 Thunder Bay, ON P7E 6S7 Chris R. White Numax Resources, Inc., Ely, Minnesota & Paul B. Albers Numax Resources, Inc., Prior Lake, Minnesota Delio Tortosa Geological Consultant Q-Gold Resources Ltd., Fort Frances, Ontario Photo of the Golden Star Mine, Mine Centre area, circa. 1898 from the Report of the Bureau of Mines, Volume VII, First Part, 1898. 95 �FOREWORD This trip will examine sites containing a variety of mineral deposits in the Mine Centre Area. Historically gold was the primary focus of mineral exploration, development and extraction. However in recent years sub-economic deposits of iron, titanium, copper, nickel and zinc have been identified and are being evaluated by junior exploration companies. The trip will visit four mineral deposit types in the Mine Centre area: lode gold; magmatic copper-nickel, PGEs; magmatic iron-titanium; and pyroclastic-hosted diamonds. REGIONAL SETTING Introduction and Tectonic Setting (taken from Czeck and Fralick, ILSG 2002) The central portion of the Superior province is characterized by alternating subprovinces of metavolcanic-plutonic and metasedimentary natures (Fig. 1). Figure 1. The Superior Province. From Card and Ciesielski (1986). One popular tectonic interpretation for the central portion of the Superior Province is of repeated island arc, microcontinent collisions. The collisions are evidenced by rocks that can be interpreted as arc sequence subprovinces (metavolcanics) and their corresponding accretionary prism subprovinces (metasediments) (Langford and Morin, 1976; Hoffman, 1989; Percival and 96 �97 Figure 2. Simplified geological map of the Mine Centre – Rainy Lake area with field trip stops. �Williams, 1989; Card, 1990; Hoffman, 1990). In general, the ages in the greenstone belts are similar along strike, but differ systematically across strike (Hoffman, 1989). This is consistent with the island-arc accretion model. A history of southward accretion has been proposed to explain the juxtaposition of Superior Province terranes (Langford and Morin, 1976; Percival and Williams, 1989; Card, 1990). The Seine Conglomerate, located in the Rainy Lake region of the western Superior Province, was deposited along the boundary between the Wabigoon metavolcanic/plutonic subprovince and the Quetico metasedimentary subprovince (Fig. 2). The structural observations along the Wabigoon–Quetico boundary are consistent with an oblique island-arc microplate collision circa 2.7 Ga. In this part of the Superior Province, it seems likely that first the Quetico acted as a subduction prism during accretion of the Wawa to Wabigoon. Then, it was effectively shifted from the subduction prism setting to a back-arc setting, as subduction shifted (Percival and Williams, 1989). In the Rainy Lake region, a series of lithostratigraphic terranes were assembled together along structurally controlled, stratigraphically discordant boundaries during the collisions. The boundary between the Wabigoon and Quetico Subprovinces in this region is divided into three primary blocks by dextral wrench faults (Poulsen, 1986). Each of the small terranes and subterranes may have undergone a somewhat unique history of formation and deformation. The Quetico Fault forms the northern boundary, separating the granite-greenstone terrain of Wabigoon Subprovince from the Coutchiching Group argillites and the Seine Conglomerate. The Seine River-Rainy Lake Fault forms the southern boundary of these sedimentary sequences with the Quetico turbiditic metasediments [sic] to the south. The wedge-shaped area lying between the two major fault systems is itself dissected by splays off the major east-west [sic] faults, which isolate the lithic units, destroying stratigraphic integrity. This problem has resulted in historical speculation on the lateral equivalency of the Seine and Coutchiching sediments (Merritt, 1934) and the Coutchiching and Quetico (Lawson, 1913). STRUCTURAL GEOLOGY (taken from Poulsen, Geological Report 266) The 2 most prominent structural features in the study area are the Quetico and the Rainy Lake– Seine River faults. Both truncate distinctive lithologic units which merge at a low angle with the faults in a manner that suggests a dextral sense of displacement (Map 2525) [sic]. The Quetico Fault, up to 1 km wide, strikes east and contains heterogeneous rock types possessing an intense steep foliation that parallels the fault. Mylonite is common and pseudotachylite occurs locally. The mylonite itself is commonly folded into minor folds of dextral asymmetry. Felsic plutonic rocks are the most probable protoliths of mylonites, and protomylonites containing centimetresized feldspar porphyroclasts are similar to porphyritic quartz monzonites that are common phases of the Algoman intrusions. This observation, plus the marked truncation of the Ottertail Lake pluton, indicate clearly that the fault postdates the emplacement of the late-stage Algoman granitoid suite. The rocks of the Rainy Lake–Seine River fault zone contrast lithologically with those of the Quetico Fault. The sharp contact with the Quetico metasedimentary rocks is marked by a zone as little as 50 m wide, composed of chlorite schist and phyllonite. Some of these schists have been 98 �mapped historically as mafic volcanic rocks, but detailed examination reveals that they are commonly heterolithic with enclaves of conglomerate, iron formation and carbonaterich schist. The nature of this zone is well-illustrated by the changes that occur in metagabbro along the fault near the Scott Islands. Outcrops of gabbro to the north of the fault contain local chloritic shear zones that parallel the trace of the main fault. The width and abundance of these zones increase southward to the main schistose zone, which marks the fault. Small folds superimposed on the steep schistosity and sigmoidal quartz veins in the vicinity of the fault near Neil Point, Minnesota, 3 km southwest of Red Pine Island, indicate a right-hand sense of displacement. Figure 3. Schematic diagram illustrating structural features of Rainy Lake Wrench Zone. Short solid arrows identify downward facing units. From Poulsen (1986). Structure The rocks of the Mine Centre–Fort Frances area contain evidence of progressive deformation involving folds, ductile shear zones and faults. While some of the above features likely formed contemporaneously, there is evidence of a continued transition from ductile to brittle deformation. The attitudes of most structures appear to be dominated by incremental shortening about a subhorizontal axis oriented west-northwest. This imparts a dominant northeasterly trending structural ―grain‖ to the rocks of the area. Folds, Cleavage And Lineation Variations in the distribution, attitudes and facing of mappable lithologic units attest to the existence of large folds within the study area. One of these, the Rice Bay Dome, is not a simple structure but can be related to the development of 3 distinctive fold sets (Poulsen, Borradaile and Kehlenbeck 1980). New lithologic mapping of the magnetic ultramafic unit exposed in this area has resulted in further definition of these folds. Early folds (F1) were recumbent and involved substantial inversion of the stratigraphic succession. An early foliation (S1), recognized locally 99 �by extreme flattening of pillow lavas, is probably related to D1. Refolding about axes (F2) that are confined to an east-northeast-striking axial surface (S2) produced a complex interference structure (Figure 8). This superposition results in F2 folds that face structurally downward onto the dome. D2 structures are common and small F2 folds are locally coaxial with pronounced lineations (L2) which result from crystallographic and dimensional orientation of metamorphic minerals. Cleavage (S2) that is axial planar to F2 folds is generally well developed. A third fold set (F3) is accompanied by the development of kink bands and a crenulation cleavage (S3) that strikes northwest. D3 minor structures are particularly abundant in the Bear Passage area. Away from the Rice Bay Dome, similar sets of minor structures can be recognized. Rarely, 3 discrete cleavages have been observed in a single outcrop. While direct temporal correlation of such cleavages with similar structures in the Rice Bay Dome is unwarranted, the persistence of east-northeast-and northwesterly striking sets throughout the area suggests a genetic relationship to a west-northwest- oriented axis of shortening. These cleavages are commonly axial planar to small folds but their direct relationship to big folds is more ambiguous. In some cases, they appear to transect large structures defined by younging reversals, implying the possible existence of early fold sets such as those mapped at Rice Bay. Minor fold axes and lineations have variable attitudes, but shallow plunges on east-northeast and west-southwest trends are common. Nearvertical stretching lineations are well-developed west of the Rice Bay Dome. Shear Zones And Faults The attitude of minor fold axes and cleavage are clearly controlled by proximity to the Quetico and Rainy Lake–Seine River faults. The sigmoidal pattern of cleavage orientation suggests that these faults involve a zone of ductile deformation in which rotation of early-formed structures has taken place. Deflection of marker units indicates right-hand components of displacement for both faults so that the intervening terrane can be considered to be a dextral wrench zone (Figure 3). The orientations and senses of mesoscopic ductile shear zones across the area support this interpretation. Three common orientations exist: 2 sets of right-hand shear zones parallel to each of the major faults can be distinguished from a northwesterly striking, left-hand conjugate set (see Figure 3). The interpreted direction of regional shortening (see Figure 3) is consistent with that indicated by the folds. 100 �Mineral Deposit Types Poulsen (2000a) proposed the mineral deposit classification shown in Table 1, deposit Type 5, Diamond-bearing ultramafic pyroclastic rocks has been added by the author. The presence of diamonds in the ultramafic pyroclastic rocks had not been identified at the time of Poulsen‘s field work. Table 1. Mineral deposits classification, Mine Centre–Fort Frances area Type 1 Stratabound Mineralization Hosted by Felsic to Mafic Metavolcanic Rocks A. Sphalerite-galena-chalcopyrite associated with siliceous volcanic rocks B. Sphalerite-chalcopyrite associated with intermediate to mafic amygdaloidal volcanic flows, tuffs and breccias C. Sphalerite-chalcopyrite associated with iron formation D. Lean iron formation (mainly chert-magnetite and massive pyrite-pyrrhotite, minor chalcopyrite common) Type 2 Mineralization Hosted by Layered Gabbroic Intrusions A. Chalcopyrite associated with gabbro and leucogabbro near the base of sills (Northrock) B. Disseminated chalcopyrite associated with siliceous phases of intrusions C. Ilmenite-magnetite-apatite-rutile lenses in the upper portions of the intrusions (Barber Lake property) (Stop 3) Type 3 Vein Mineralization A. Quartz-gold-sulphide veins in shear zones and cleavage-parallel dilatant zones (Foley Mine, McKenzie-Gray; Stop 1) B. Quartz-molybdenite-pyrite veins and disseminations in unmetamorphosed granitoid rocks Type 4 Disseminated chalcopyrite-pyrrhotite mineralization hosted by ultramafic metavolcanic rocks (Northrock) (Stop 4) Type 5 Diamond-bearing ultramafic pyroclastics (Grassy Portage Ultramafic Pyroclastic) (Stop 5) 101 �FIELD TRIP STOPS (Stops are located by UTM co-ordinates based on NAD 83, UTM Zone 15) Depart from the Fort Frances Memorial Sports Centre and drive east on Highway 11 for approximately 65 km to the hamlet of Mine Centre. Continue an additional 1.3 km and turn right (south) onto the Shoal Lake Road. Drive south along Shoal Lake Road for approximately 8.5 km to the entrance of the Foley Mine property, turn right onto the access road. (0526320E 5395406N) STOP 1– Vein Mineralization - Lode Gold Q-Gold Resources Limited (Foley Mine and McKenzie-Gray Prospect) Delio Tortosa This portion of the trip will be led by Delio Tortosa, Consulting Geologist, Q-Gold Resources Ltd. Q-Gold Resources Ltd. has mineral rights over a large portion of the historical Mine Centre Mining Camp (Figure 4). Past-producing gold mines and numerous mineral prospects occur in quartz and quartz-carbonate fissure veins oriented north-northwest and dipping steeply. The predominant host rock is the Bad Vermilion Felsic Intrusion that ranges in composition from trondhjemite to tonalite and granodiorite. The intrusion has a sigmoidal shape that is characteristic of the rock units and major faults that occupy the area between the Quetico Fault to the north and the Rainy River – Seine River Fault to the south. Typically, the mineralized veins are laminated and occur within shear zones with a left-hand sense of movement. The quartz contains pyrite, chalcopyrite, arsenopyrite, galena and sphalerite. Free gold is commonly associated with the sulphide-rich zones and displays a strong ‘nugget’ effect along the vein systems. STOP 1a: Foley Mine Area The Foley Mine is the best-developed mine in the area with 2.5 kilom of drifting on seven levels to a depth of 260 m. Initial development and mining took place in the 1890’s, and the mine was further developed to greater depth in the 1920’s, with extensive drifting but little gold production. A total of 5,267 ounces of gold was produced between 1893 and 1934. There are two main veins related to the Foley Mine on surface: the Bonanza Vein, which is currently overburden-covered and where the Foley Shaft is situated, and the nearby Jumbo Vein (Figure 5). The Jumbo Vein is well-exposed west and south of the Foley Mine over a distance of 200 m with a north-northwest strike and steep dip. The wall rocks consist of sheared trondhjemite with the shear fabric displaying a left-hand sense of movement. The Bonanza Vein appears to be a splay off the Jumbo Vein. All the vein systems in the area appear to be interconnected either along strike and/or at depth. 102 �Chip sampling by Q-Gold Resources along the Jumbo Vein resulted in a weighted average of 5 g/t Au, 15 g/t Ag, and 0.33% Zn. Two 25-ton bulk samples taken by Nipigon Gold Resources in 1992 returned 0.5 oz/t Au and 0.05 oz/t Au – reflecting the ‘nugget effect’ for gold in these vein systems. STOP 1b: McKenzie Gray Prospect A substantial amount of mineral exploration and bulk sampling was carried out on the McKenzie Gray Prospect between 1984 and 1995. In 1992, Nipigon Gold Resources Ltd. completed 15 diamond drill holes and estimated a mineral resource of 63,000 tons grading 0.17 oz/ton Au, 1.13 oz/ton Ag, 0.25% Cu, and 3.05% Zn (not compliant with NI 43-101). In 1984 the Mine Centre Joint Venture Group processed a 28-ton bulk sample through a local mill resulting in an average grade of 7.2 g/t Au, 112.1 g/t Ag, 10.18% Zn, and 0.11% Pb. A 1995 report by Nipigon Gold Resources, reported that a 204-ton bulk sample from the open pit processed through their on-site test mill returned an average grade of 0.17 oz/ton Au. In 2009 Q-Gold Resources Ltd. completed a diamond-drilling program consisting of 12 NQ holes spaced on sections 15 m apart to evaluate the northwestern extension of the McKenzie Gray vein system. The drill results indicated that the vein ranges in width from less than 1 m up to 7 m wide with weighted assay values of 17 g/t Au, 21 g/t Ag, and 2% Zn. The weighted average grade of all mineralized intersections was 4.7 g/t Au, 30.8 g/t Ag, and 2.9% Zn. The McKenzie Gray Prospect can be subdivided into two major vein systems: the McKenzie Gray Vein and the East Vein (Figure 6). The McKenzie Gray Vein is thought to be a crack-seal textured vein that crosscuts the East Vein, an extension-type vein (Glidden, 1990). The McKenzie Grey Vein contains much of the Au-Ag-Zn-Cu-Pb mineralization, while the East Vein is noted for its high silver content (possibly argentite). The McKenzie Gray Vein has a strike of 310° to 315°, dipping 75° southwest to vertically. The vein has an anastomosing character, splitting into two veins and then joining into a wider single vein along strike and down dip. The vein consists of reddish quartz that is bounded by and contains strongly foliated/sheared and altered sections of trondhjemite with the foliation generally parallel to the vein direction. The McKenzie Gray vein is mineralized with varying concentrations of sulphides that carry significant amount of gold and silver. The most prevalent minerals are: sphalerite, pyrite, chalcopyrite, galena, and a gray metallic mineral (possibly argentite). Gold occurs as 10-micron grains within the sphalerite and as 10-40 micron grains within the quartz (Larouche, 1995). The East Vein is situated immediately east and on the footwall side of the McKenzie Gray vein. It averages between 3 and 5 m wide and has an irregular shape trending sub-parallel to the McKenzie Gray vein and dipping 60° to 80° southwest. The vein is composed of white to gray quartz and is weakly mineralized, primarily with pyrite and minor amounts of chalcopyrite, and a gray metallic mineral (possibly argentite). The quartz vein contains numerous inclusions of altered trondhjemite. One of the wider sections of the East Vein intersected by the Q-Gold drilling program consisted of a 6.7 m-wide zone containing 54 g/t Ag, 0.2 g/t Au, and 0.33% Zn. The weighted average grade of all intersections of the East Vein was 35 g/t Ag. 103 �Figure 4: Regional Geology, Gold Mines and Prospects (Internal Company Report, Q-Gold Resources Ltd.) 104 �Figure 5: Foley Mine area shafts and veins (Internal Company Report, Q-Gold Resources Ltd.) 105 �Figure 6: McKenzie Gray Prospect, Detailed Geology (Internal Company Report, Q-Gold Resources Ltd.) 106 �STOP 2 – Historical Stop – Arastra (Optional) Peter Hinz Return to Highway 11 turn left and drive approximately 7.1 km west, pull off to the north side of the road. (0522191E 5400245N) If conditions are favourable walk around the perimeter of the wetland to the low concrete structure at the edge of the open water. The Mine Centre arastra (a.k.a. arrastra) is a historical oddity in northwestern Ontario. An arastra, also known as a drag-stone mill or ‗chili mill‘ is a pre-industrial form of a crusher used in South and Central America for crushing vein material. Local residents have told the author that this is one of three known in the local Mine Centre area, it is believed these are the only known arastras in existence in Ontario. A small pile of quartz-carbonate vein material adjacent to the arastra could have come from either the Olive Mine located 2.5 km to the northwest or the Stellar Mine located 1.7 km, both of which host gold in quartz-carbonate veins. Figure 7. An arastra from the U.S. midwest, circa. 1890’s. Image from http://www.calisphere.universityofcalifornia.edu/ Figure 8. The author beside the Mine Centre arastra. 107 �STOP 3 – Magmatic Iron-Titanium Mineralization – Numax Resources Property Paul Albers & Chris White Continue west on Highway 11 for approximately 3.5 km to the Barber Lake Road,, turn south and drive 15.4 km (0509484E 5390837N). There is a gate located at approximately 10.6 km. This portion of the trip will be led by Chris White and Paul Albers, Numax Resources Ltd. Introduction Numax Resources, Inc. (Numax) holds a 100% interest in the Mine Centre Fe-Ti-V Property. The property comprises 45 contiguous mineral claims totaling 5,188 ha in the Kenora Mining Division and is situated along Seine Bay of Rainy Lake and Bad Vermilion Lake (Fig. 1). The Mine Centre Property has been the focus of Numax‘s exploration efforts dating back to the summer of 2004. The property was previously explored by Hunter in 1911, Goodwin from 1917-1918, Butler Brothers in 1943, and Stratmat Limited from 1956-1958. While relatively inactive at the Mine Centre Property for numerous decades, Numax commenced field work in 2004 that included grid line cutting, two ground geophysical surveys (magnetic and electromagnetic), extensive trench excavation, channel sampling of trenches, field sampling, diamond drilling, and prospecting. During the fall of 2009, geologic mapping (Scale 1:5000) was conducted across a majority of the property. Along with this, ten trenches were mapped in detail (Scale 1:400). Figure 9: Location of the Numax Resources, Inc., Mine Centre Property, modified from Poulsen, 2000. 108 �Local Geologic Setting The igneous stratigraphy of the Seine Bay/Bad Vermilion Lake intrusion features interlayered anorthosite, gabbro, melagabbro, pyroxenite, semi-massive oxide, and massive oxide. The intrusion has been rotated such that layers are now oriented sub-vertically in the west, and dipping at 30-60° in the east. Layers generally strike easterly in the west, and northeasterly in the east, with a bend in the intrusion near the Snake Lake area. Graded modal layering of silicate minerals indicates that layers top to the north and northwest, respectively. Zones of massive and semi-massive oxide demonstrate the geologic expression of, and correlate with magnetic anomalies discovered during Ontario Geological Survey airborne geophysical surveys (2009, 1980), and a ground magnetometric survey conducted by Numax (Simoneau, 2008). These massive and semi-massive oxide zones are present as discrete, continuous to semicontinuous layers of magnetite, ilmenite, and titaniferous magnetite, covering a strike length of at least 10 km, and likely extend in both to the east and west. The Seine Bay/Bad Vermilion Lake intrusion is uniquely situated between two major fault zones (the Quetico Fault to the north and the Rainy Lake-Seine River Fault to the south; Fig. 1). Though the gabbros and anorthosites of the intrusion appear to be largely undeformed, numerous discrete (metre-scale) shear zones exist within the intrusion, typically trending in a general easterly direction. It is quite possible that deformation within the intrusion has been largely accommodated by zones of massive oxide as the contacts between massive oxides and gabbroic rocks locally exhibit an obliquely trending (to the contact) mineral lineation emanating from the massive oxide into the gabbro, which dominantly features Fe-Ti-oxide smearing into the gabbro. Additionally, several large shear zones have been identified within the intrusion; a fault displaces barren anorthositic rocks in contact with oxide- and sulfide-bearing gabbroic rocks. In the area of Numax‘s Mine Centre Property, the Seine Bay/Bad Vermilion Lake intrusion is sandwiched between metamorphosed felsic intrusive rocks to the north, and mafic meta-volcanic rocks to the south, which largely underlie the northern shore of Seine Bay. However, mafic intrusive rocks of the Seine Bay/Bad Vermilion Lake are again present to the south of Seine Bay (Poulsen, 2000). The exact natures of both the upper and lower contacts of the intrusion are not completely understood at this time. The felsic intrusive rocks to the north display a distinct equigranular texture with spherical quartz eyes. Occurrences of quartz eyes (1-3 mm in size), can be observed locally in the mafic intrusive rocks of the Seine Bay/Bad Vermilion Lake intrusion, proximal to the upper contact. Additionally, interlayers of felsic and mafic intrusive rocks can be found locally near the upper contact. The mafic meta-volcanic rocks exposed on the northern shore of Seine Bay could be a very large inclusion block within the intrusion, or more likely could be related to the regional faulting in the area. The contact between mafic metavolcanic rocks and mafic intrusive rocks appears intrusive, as it is extremely heterogeneous and features pegmatitic gabbro intermixed with mafic metavolcanic rocks. Closest to the mafic meta-volcanic rocks, apophyses of gabbroic melt migrate downward into the mafic volcanic rocks; while closer to the gabbro above, blocks (1-3 m) of mafic metavolcanic rocks occur as inclusions within the gabbro. 109 �In the eastern part of the property a shear zone displaces pegmatitic gabbroic rocks adjacent to anorthositic rocks. This shear zone is within 10-20 m of the lower contact with mafic metavolcanic rocks. Quite possibly, this shear zone has displaced the far western extent of the intrusion eastward, causing the intrusion to become ―stacked,‖ and displacing a portion of the mafic meta-volcanic footwall to its current position within the intrusion. Description of Map Units The lithologic units present in Numax‘s Mine Centre Property are remarkably correlative in outcrop and exposed trenches, generally along an easterly strike. Figure 2 displays a simplified geologic map of the central Seine Bay/ Bad Vermilion Lake intrusion as it relates to Numax‘s Property. The lithologic units present on the property include: Felsic volcanic undivided (FVu) – Felsic volcanic rocks (undivided), typically interpreted as inclusions or groups of inclusions within the Seine Bay/Bad Vermilion Lake intrusion, typically very fine-grained, fresh and weathered surfaces white to light pinkish – beige, locally contains fine- to medium-grained disseminated chalcopyrite up to 1% found in both the Heat-Sink Trench and the Trenches on Line 28. Massive and Semi-massive Oxide (MOX) – Massive (90-100% Fe-Ti oxide) and Semi-massive (30-90% Fe-Ti oxide) mineralization of iron – titanium oxide, typically hosted by medium- to coarse-grained pyroxenite, fresh surfaces are metallic black to dark purple, weathered surfaces are rusty orange to brown, magnetic strength varies from moderate to very strong, typically contains trace to 2% disseminated and massive sulfide (1-2 cm thick) consisting of pyrrhotite + chalcopyrite + pyrite, sulfides are typically fine- to very fine-grained, oxides are typically medium- to coarse-grained. Mafic Intrusive, oxide-bearing (MIox) – Oxide-bearing (5-30% Fe-Ti oxide) mafic intrusive rocks, typically consisting of homogeneous, medium- to coarse-grained gabbro, melagabbro, pyroxenite, and rare leucogabbro, fresh surfaces are medium grey, and weathered surfaces are medium grey to brown, magnetic strength typically moderate to strong, contains lenses and layers of massive oxide that are <1 m thick, clinopyroxene is locally ophitic, but is typically intergranular, locally contains rare to trace disseminated sulfide mineralization, alteration dominantly includes moderate chlorite after pyroxene. Mafic Intrusive (MI) – Non-oxide-bearing (<5% Fe-Ti oxide) mafic intrusive rocks, typically consisting of homogeneous, coarse- to medium-grained anorthosite and leucogabbro, and locally gabbro, fresh and weathered surfaces are light grey, locally displays modal layering between plagioclase and clinopyroxene, typically consisting of layers <1m thick that top to the north-northwest. Mafic and Felsic Intrusive undivided (MFIu) – Undivided mafic and local felsic intrusive rocks, assumed from Poulsen, 2000. 110 �Figure 10: Simplified geologic map of the central Seine Bay/Bad Vermilion Lake intrusion, modified from Albers and White, 2010. 111 �Description of Map Units, continued Felsic Intrusive (FI) – Felsic intrusive rocks dominantly consisting of granodiorite, quartz diorite, tonalite, and trondhjemite, typically medium- to coarse-grained, these rocks typically display the effects of contact metamorphism largely demonstrated by the texture of quartz, forms the hanging wall to the Seine Bay/Bad Vermilion Lake intrusion in the map area. Mafic Volcanic undivided (MVu) – Undivided mafic volcanic rocks, typically consisting of homogeneous, massive, fine- to very fine-grained basalt, typically displays contact metamorphism to hornfels, locally plagioclase-phyric, locally displays pillows, local sulfide mineralization, occurs both as inclusions within the Seine Bay/Bad Vermilion Lake intrusion, and as an extensive ―finger‖ occurring across the length of the intrusion in the map area, an extremely heterogeneous intrusive contact is observed in the Discovery Zone and Discovery Zone North Trench between the basalt ―finger‖ and the Seine Bay/Bad Vermilion Lake intrusion in which prominent sulfide and oxide mineralization is displayed. Massive and Semi-massive Oxide assumed (MOXa) – Massive and Semi-massive oxide, assumed from Ontario Geological Survey (2009) airborne geophysical survey (approximate). Field Trip Stop The field trip consists of one stop, which will traverse a complex of trenches excavated on section line 28. The trenches include Beaver Dam, Buck, and Doe trenches. Figure 3 displays a geologic map of these trenches with accompanying Fe2O3 and TiO2 assay data profiles, as well as a profile of the ground magnetometric geophysical survey data (Simoneau, 2008). Ten distinct lithologies, which display correlative stratigraphy through the complex of trenches and outcrops on section line 28, and with adjacent trenches and outcrops at least 600 m to the east and west, are displayed. Detailed lithologic descriptions that correspond to Figure 3 are documented below (from north to south). A) Melagabbro: Medium- to fine-grained, weakly to non-magnetic, weak chlorite alteration, locally verges on pyroxenite; silicate modal mineralogy is as follows: 68-88% clinopyroxene, 515% plagioclase, and 5-15% olivine, trace disseminated and veinlets of very fine-grained chalcopyrite + pyrrhotite (<0.5%), <2% iron oxide, olivine seems to occur as clusters with plagioclase and weathers to rusty brown creating a pock-marked appearance, weathered surface is medium slate grey, fresh surface dark slate grey, cut by a dike of felsic to intermediate composition which is very fine-grained, non-magnetic, and contains tiny phenocrysts of white alkali feldspar, clear to white quartz, and 1-2 mm needles of hornblende, weathered surface is white, fresh surface is medium blue/grey, upper contact buried beneath overburden/swamp, lower contact gradational, at least 33 m thick. 112 �Figure 11: Geologic map (Scale 1:400) of the Trenches on Line 28 with associated Fe2O3 and TiO2 profiles from channel samples, and walking magnetometric geophysical survey profile. 113 �B) Melagabbro (layered): Similar in composition to the melagabbro above (A), but contains less olivine and is medium- to coarse-grained, displays prominent modal layering between plagioclase and pyroxene, layers are typically <1 m thick and in many instances thinner layers (<10 cm thick) stack on top of thicker layers, plagioclase-rich tops are typically 1-4 cm thick, layers are very consistent with sharp contacts and local fault off-sets (<10 cm), layers dip subvertically and strike at ~245° with a topping to the northwest, lower contact is gradational over ~3 m and displays a zone with thin fractures or shears (1 mm to 2 cm thick) filled with chlorite and skirted by plagioclase-rich gabbro that abruptly dissipates back to melagabbro, fractures are irregularly spaced (1 cm to 0.5 m apart) with a subvertical dip and strike of ~240°, ~43 m thick. C) Oxide Pyroxenite: Fine-grained, weakly to non-magnetic, strong chlorite alteration, ~5% iron oxide which is clumped together with very fine-grained plagioclase, trace very fine-grained chalcopyrite + pyrrhotite, weathered and fresh surfaces are pale green and purplish, lower contact abrupt, ~8 m thick. D) Oxide Melagabbro to Oxide Pyroxenite: Medium-grained, weakly to moderately magnetic, strong chlorite alteration, <3% very fine-grained granular olivine, 5-15% iron oxide, trace finegrained disseminated chalcopyrite + pyrrhotite, local patchy veining or fracture fillings of chlorite <0.5 m thick and discontinuous with pinching and swelling, local very distinct change in weathering: broken-up and sheared with a strong foliation dipping sub-vertically and a general southwest trend (too magnetic to obtain strike), near the bottom of this unit is a thin subvertically orientated band of fibrous-fanning amphibole (2 mm to 1 cm thick) generally striking southwest (too magnetic to obtain strike), lower contact is sharp to gradational over 3-4 m, ~25 m thick. E) Massive Oxide: Medium-to fine-grained, strongly magnetic, strong chlorite alteration, hosted by pyroxenite, rare fine-grained disseminated chalcopyrite, vague localized modal layering displaying as thin bands (1 mm to 3 cm thick) of chlorite (very soft with blue-green color) semicontinuous and flat-lying, sub-vertical generally trending southwest (too magnetic to obtain strike), spaced 10-20 cm apart commonly occurring as pairs with a thinner layer (2-4 cm thick) overlying a thicker (10-20 cm thick) layer, 4-9 m thick, lower contact is sharp with a 0.5 m thick lens or dike locally cross-cutting the contact between this massive oxide and the gabbro below, dike contains 1% fine-grained disseminated chalcopyrite and stringers of chlorite, weathered and fresh surfaces are white (felsic volcanic?), ~4-10 m thick. F) Oxide Gabbro: Medium-grained, weakly to non-magnetic, strong chlorite alteration, ~5% iron oxide, rare fine-grained disseminated chalcopyrite + pyrrhotite, thin sinuous and continuous laminations of <1 mm thick plagioclase and chlorite occur in within 0.5 m of upper contact, contains local layers (<10 cm thick) of massive oxide oriented normal to strike of upper contact, lower contact is abrupt over ~1 m, with lineated iron oxide emanating obliquely to contact (~30°) into oxide gabbro from massive oxide below showing a somewhat boudinage appearance, dipping sub-vertically (too magnetic to obtain strike), 3-25 m thick. G) Gabbro (lineated/sheared/folded): Fine- to medium- to coarse-grained, locally weakly magnetic, strong chlorite alteration, pyroxene and iron oxide are typically lineated and stretched 114 �in direction of lineation (too magnetic to obtain trend), local pervasive folding with up to nine folds observed in a single structure, typically folds are rimmed with chlorite (2 mm to 2 cm thick) and dominantly composed of quartz and plagioclase with local iron oxide, locally strongly foliated dipping sub-vertically and striking southwest (too magnetic to get strike), but foliation is normal to mineral (iron oxide) lineation at contact above, numerous layers (10 cm to 1.5 m thick) of massive oxide dipping sub-vertically with strike normal to upper contact, contacts between massive oxide and gabbro are sharp and irregular, massive oxide is hosted by pyroxenite with local olivine and displays sub-vertically oriented laminations (1-2 mm thick) of chlorite striking normal to contacts, locally grades to oxide gabbro toward lower contact, lower contact sharp, up to 45 m thick. H) Massive Oxide: Medium- to fine-grained, strongly magnetic, strong chlorite alteration, hosted by pyroxenite, weathered surface is brown/rusty-orange to dark-deep-reddish/purple, fresh surface is metallic dark-purple/black, local silver-white staining generally as small (several cm) sinuous patches, local discontinuous sinuous bands of massive pyrrhotite + pyrite + chalcopyrite (<2 cm thick), local lenses that form a discontinuous layer of strongly chloritized sheared gabbro (1 m thick x <3 m long), lower contact consists of a 1-2 m cliff face plunging beneath the overburden, at least 7-10 m thick. I) Felsic Volcanic: Strongly metamorphosed and altered, massive appearance, weathered surface is pinkish/orange to beige with intermixed patchy blue-green, fresh surface is generally pale blue-green-grey, mafic phase is strongly altered to chlorite, local pale-yellow epidote alteration, local thin (<1 mm thick) folded bands that show a somewhat preferred orientation with strike (~270°) of contact, local foliation and shearing toward top of zone, rare fine-grained disseminated pyrite and possibly chalcopyrite, could be the same as dikes above, lower contact buried beneath overburden and standing water, at least 37 m thick. J) Dikes and/or Felsic Volcanic Inclusions: Very fine-grained, non-magnetic, locally contains up to 1% fine-grained disseminated chalcopyrite, local phenocrysts (1-2 mm in size) of white alkali feldspar, clear to white quartz, and needles of hornblende, local irregular-shaped clasts of beige to light reddish/orange quartz (1-15 cm long), weathered surface is white to light blue-grey, fresh surface is medium to light blue-grey, contacts are generally sharp, 1-4 m thick. Iron-titanium-oxide geochemical data collected from continuous channel sampling through the trenches display very distinct profiles that correlate remarkably well with the geology observed in the trenches (Fig. 3). Restricted (<1.5 m) zones or layers of massive oxide within gabbroic rocks show spikes in Fe2O3 and TiO2, while felsic to intermediate dikes display corresponding decreases in Fe2O3 and TiO2 values. The profiles are most interesting through units C, D, E, and F. Here the profiles display a steady increase from north to south through the pyroxenite and oxide pyroxenite, hitting a peak of roughly 60% Fe 2O3 and 25% TiO2 in unit E (massive oxide). The profile of the ground magnetometric geophysical survey (Simoneau, 2008) along section line 28 is also quite correlative with the observed geology (Fig. 3). The survey profile displays spikes of >80,000 gammas in zones of massive oxide, and corresponding decreases in unmineralized gabbro and felsic rocks to <60,000 gammas. As discussed above and presented in Figure 3, the geology, geochemistry, and geophysics of the trenches on line 28 are remarkably correlative. The igneous stratigraphy of the Seine Bay/Bad 115 �Vermilion Lake intrusion throughout the Numax Mine Centre property also correlates remarkably well, with both geologic mapping, and airborne geophysical surveys (Ontario Geological Survey, 2009), showing continuous to semi-continuous layers of massive oxide through the property for a strike length of at least 10 km, and likely up to 15 km. An extensive drilling program focused on confirming and defining this enormous Fe-Ti prospect is both warranted and forthcoming. Return to Hwy 11 turn left and drive west for 24.4 km, turn left (south) on the bush road and drive 2.25 km to the Y-intersection. Take the right branch and continue for 1.95 km to the Beaverpond Zone (0495043E 5394021N) STOP 4 – Magmatic Copper-Nickel-PGE Mineralization – MetalCORP Limited – North Rock Property Peter Hinz NOTE: This portion of the field trip guide is taken from the MetalCORP website www.metalcorp.ca. MetalCORP Limited‘s 100%-owned North Rock Property is located approximately 25 km east of Fort Frances and is easily accessed via Highway 11 and then 4 km via a gravel road. The property consists of 370 claims or 5,920 ha (14,800 acres). Figure 12. Property outline and generalized geology, North Rock property, MetalCORP Resources Ltd. From MetalCORP website www.metalcorp.ca. 116 �The Property is underlain by the 20 km long Grassy Portage layered mafic intrusion and hosts four known zones of magmatic copper-nickel sulphide mineralization; the Beaver Pond, the Main South, the East and the West zones all of which occur at or near the base of the intrusion, along its western contact. GENERAL GEOLOGY: The property is underlain by mafic and ultramafic metavolcanic rocks, which are in contact with the Grassy Portage gabbroic intrusion (Figure 20). The ultramafic rocks are tuffs and lapilli tuffs that are generally actinolite-rich and intensely magnetic. Metabasaltic flows overlie these to the south, and pillow shapes indicate southward facing. The contact of the pillow lavas with the Grassy Portage intrusion is sharp, and is occupied by a 20 m thick unit of coarse-grained, hornblende-rich melagabbro. The melagabbro grades south-eastward to medium- to coarsegrained gabbro composed of subequal proportions of hornblende and plagioclase. Layers of anorthosite composed of andesine intrude the gabbro at the southern margin of the property, where they are spatially related to masses of magnetite-actinolite amphibolite. Numerous mafic to intermediate dikes cut the gabbroic rocks. Copper-Nickel Mineralization The most significant of the zones is the Beaver Pond Zone, which was discovered by Noranda in 1958 and subsequently explored from underground via a 90 metre deep shaft and one drift on the 70 m level. Resource estimates for the Beaver Pond Zone, reported by Bergman (1973) range (depending on the cut-off grade) from 1,020,458 tons grading 1.17% Cu to 265,230 tons grading 2.08% Cu. The resource estimate was calculated to a depth 90 m and the deposit appears to remain open to depth. One drill hole intersected 2.21% Cu over 36.5 feet at a vertical depth of 175 m. (Note that these historical resource estimates are not current and should not be relied upon as they were not prepared in compliance with National Instrument 43-101.) Other significant drill intersections reported by Noranda from within the Beaver Pond Zone include: 4.3% Cu over 8.1 m, 4.1% Cu over 7.8 m, 3.4% Cu over 8.0 m, 1.5% Cu over 21.3 m, and 1.0% Cu over 55.5 m. (These Noranda assays have not been verified and as such should not be relied upon). There is no record of any further mineral exploration work having been conducted on or near the Beaver Pond Zone, or on any of the formerly leased claims in the area, since the underground program was concluded in 1973. Very little historical data exist regarding the abundance and distribution of nickel, cobalt, platinum group metals (PGM) or gold in the magmatic sulphide deposits on the North Rock Property. However, recent grab sampling of a 10,000 ton surface stockpile of material recovered from underground development of the Beaver Pond Zone yielded assays of up to 8.9% Cu, 0.8% Ni, 0.05% Co, 1.6 g/t Pt, 0.7 g/t Pd and 0.7 g/t Au. Although grab sample assays are typically not representative of the overall grade, these results do confirm the presence of significant nickel, cobalt, PGM and gold values associated with the copper mineralization. 117 �Figure 13. Mineralized zones and assays, North Rock property, MetalCORP Limited. From MetalCORP website www.metalcorp.ca. The three other known sulphide zones are less-explored and occur over a 1.5 km strike length along the base of the intrusion extending northeast from Beaver Pond Zone. These areas, as well as several other sulphide occurrences higher up in the intrusion, are all considered highly prospective for hosting additional magmatic nickel-copper (+/- PGE's, Co) sulphide mineralization. MetalCORP conducted surface exploration, an airborne magnetometer/electromagnetic survey and three phases of drilling on the Property from 2005 to the end of 2007. The first phase of drilling was carried out in 2005 and consisted of 14 diamond drill holes totaling 3,906 m. Phase 1 drilling was designed to test the depth extension of the known copper mineralization at the Beaver Pond Zone. All drill holes intersected widespread, varied amounts of net-textured copper-nickel sulphide mineralization similar to that reported by previous drilling by Noranda. Hole 4 appears to have intersected the core of the zone returning 1.5% copper over 13.7 m. The drilling also intersected a hanging wall Platinum Group Metal (PGM) zone that can be traced for 500 m along strike and to a depth of 200 m. Assay values as high as 3.7 g/t Pt + 7.1 g/t Pd over 0.8 m were reported. The Company completed an airborne magnetic/EM geophysical survey over the property which detected several anomalies with a 1 km long anomaly coincident with the East zone which is 1.2 km northeast and along strike of the Beaver Pond zone. Mapping and sampling confirmed the 118 �presence of PGM mineralization at the East zone and a Phase 2 drilling program was set up to test the mineralization. The Company drilled 21 holes totaling approximately 4,000 m with several holes intersecting PGM mineralization. Hole 20 intersected 3.7 m (2.5 m true width) from 151.9 to 155.6 m grading 1.2 g/t Pt, 0.1 g/t Pd, 0.1 g/t Au, 0.6% Cu and 0.2% Ni (see March 6 and April 19, 2006 news releases) www.metalcorp.ca. In June 2007 the Company announced a Phase 3 diamond drilling program of 25 holes totaling approximately 8,000 m to test several airborne EM conductors over a 20 km strike length on the entire Property. Drilling in the Belacoma area, approximately 4.0 km northeast of the Beaver Pond Zone, intersected platinum and palladium mineralization as well with the following values: 3.5 g/t Pd over 2.8 m, 1.2 g/t Pt and 1.0 g/t Pd over 1.0 m, 2.9 g/t Pd over 0.9 m and 1.6 g/t Pd over 0.9 m. A number of other airborne EM conductors were tested, with the majority explained by pyrrhotite veins and/or sulphide iron formation. The sulphide iron formation in the Nickel Lake area was found to be zinc-rich, with the best assay from hole #57 of 0.5% Zn over 19.6 m. Return to Hwy 11, turn left and drive for 2.7 km, pull off to the right into the small bush lane. Walk through the bush 120 m to the northwest to the outcrops on the south side of the beaver pond (497137E/5396262N) STOP 5 – Diamonds in the Grassy Portage Ultramafic Pyroclastic – MetalCorp – GUP Property The Grassy Portage Ultramafic Pyroclastic (GUP) is located approximately 33 km east-northeast of Fort Frances. The property straddles highways 11 and 502 and extends out into Grassy Portage Bay, Rainy Lake (Figure 14). Portions of the property are accessible by road or boat. The property is held by MetalCORP Limited under option from Messrs. R. Cousineau, L. Cousineau and K. Desjardins, all of Fort Frances. The general geology of the area is described by Poulsen (2000): The rocks of the Rainy Lake area are part of the Archean Superior Province and form a fault-bounded wedge between 2 subprovinces, the Wabigoon granitegreenstone terrane to the north and the Quetico metasedimentary terrane to the south. The Quetico and Rainy Lake-Seine River faults define this wedge, which is interpreted to be a dextral wrench zone and which displays distinct stratigraphic, structural and metamorphic relationships. Poulsen (2000) also described the GUP: A unique ultramafic unit occurs in the Redgut Bay-Grassy Portage Bay area. Rocks from this unit have distinctive textures as well as a magnesian chemical composition; samples plot as basaltic komatiites. Locally, the rock consists of angular to subrounded fine-grained clasts, up to 5 cm in diameter, set in a fine-grained chlorite-tremolite matrix. The rocks are moderately to strongly magnetic and, where visible fragments are not present, they are fine-grained magnetic tremolite schists. As a whole, this ultramafic unit strongly resembles the "ashrock" unit (Joliffe 1955) that overlies the iron ore horizon of the Steep Rock Group north of the Quetico Fault at Atikokan. 119 �The author collected 15 samples from various parts of the property during the 2000 field season; all samples plotted at the boundary between basaltic komatiite (BK) and picritic (ultramafic) komatiite (PK) on a Jensen Cation plot (Jensen 1976) (Figure 15). A plot of total alkalis (Na2O + K2O) versus silica (SiO2) (Le Maitre 1989) shows that the majority of samples plot within the komatiite or meimechite field (see Figure 16). The differentiation between the two rock types is based upon titanium oxide content, meimechite having greater than 1% TiO2, and komatiite, less than 1% TiO2. Meimechite is defined as, "…the extrusive equivalent of kimberlite" (Mitchell, 1985). Schaefer and Morton (1991) conducted research on both the GUP and the Dismal Ashrock of the Steep Rock Group. Their work indicated that "GUP is divided into komatiitic lapilli tuff and komatiitic volcanic breccia. Both pyroclastic units contain cored and composite lapilli, evidence of explosive volcanism. Locally, some of the lapilli fragments are highly vesicular (up to 30% by volume), greater than reported for any other komatiite." A variation in clast composition was noted at various locations in the GUP. Cored clasts up to 10 cm across were noted on the shoreline (495819E/5395232N) and islands (499902E/5394822N) of Grassy Portage Bay. At the Beaver Pond Occurrence (497137E/5396262N) clast-dominant GUP is seen to be in contact with a fine-grained matrix-dominant GUP. The contact between the two units suggests that the younging direction is roughly to the east. This is based on what is interpreted, by the author, to be scouring of the fine-grained tuffs during passage of a pyroclastic debris flow. The majority of clasts appear to be ultramafic in composition with <2% of exotic origin. At all locations the clasts range from subangular to subrounded, with a maximum dimension of 15 cm. Rocks of the GUP contain only trace amounts of sulphide minerals (pyrite +/- pyrrhotite). Some sections contain appreciable amounts of euhedral magnetite, visible to the naked eye. A bronzecoloured mica was observed on a small island in Grassy Portage Bay and identified as phlogopite in the field; X-ray diffraction analysis confirmed the presence of phlogopite/biotite. Talc was also noted in some sections. Shaefer and Morton (1991) and Poulsen (2000) concurred that the GUP has undergone amphibolite-grade metamorphism. Only late-stage, weak carbonatization was noted by the author. Schaefer and Morton (1991) commented on the structure of the area: "GUP outcrops form an arcuate fold interference pattern, are strongly deformed …". However, outcrops visited by the author display very little deformation, the only structural fabric was a weak foliation observed at the Beaver Pond occurrence. This apparent lack of deformation is supported by observed clast morphology, which ranges from subangular to well-rounded. The property and surrounding area has undergone extensive exploration at various times since the late 1800s for copper, zinc, nickel, PGM, gold and iron. Prior to 2000, no record of diamond exploration is documented in the area. Under an agreement with the property owners, INCO Ltd. conducted a sampling program on the property in 2003, with samples being analysed by Lakefield Research (L. Cousineau, Prospector, personal communication, 2003). One sample collected by INCO was reportedly identified as a hypabyssal kimberlite. 120 �Figure 14. Geology of the Grassy Portage ultramafic pyroclastic (modified from Schaefer and Morton 1991). Rectangles represent sample areas. (From Lichtblau et al. 2004). 121 �BK – Basaltic komatiite CA – Calc-alkaline andesites CB – Calc-alkaline basalts CD – Calc-alkaline dacites CR – Calk-alkaline rhyolites HFT – High iron tholeiites HMT – High – magnesium tholeiites PK – Picritic komatiite TA – Tholeiitic andesites TD – Tholeiitic dacites TR – Tholeiitic rhyolites Figure 15. Jensen cation plot (Jensen 1976) for samples collected from the Grassy Portage ultramafic pyroclastic. Triangles represent sample plots, this study. Figure 16. Alkali versus silica plot (LeMaitre 1989) for samples collected from the Grassy Portage ultramafic pyroclastic. Diamonds represent sample plots, this study. 122 �As part of its ongoing exploration program MetalCORP announced in February 2008 the discovery of micro-diamonds on the property in the Grassy Ultramafic Pyroclastic. The Company took a 100 kg sample and sent it for processing by Kennecott Canada Exploration Inc. ("Kennecott") in its Mineral Processing Laboratory, located in Thunder Bay, Ontario. A total of six diamonds were recovered with one sitting on the 0.212 mm sieve, three were on the 0.106 mm sieve and two were less than the 0.106 mm sieve size. In December 2008 Kennecott and MetalCORP signed an option agreement to explore for diamonds on the property but in September 2009 MetalCORP was advised by Kennecott that it had completed processing a 1,200 kg sample taken during its summer exploration program and, based on the results, concluded that the diamond potential is very low. Accordingly, Kennecott terminated its program and its option and returned the property to MetalCORP. Figure 17. SEM images of four of the six diamonds recovered by MetalCORP from the Grassy Portage Ultramafic Pyroclastic. 123 �REFERENCES Albers, P.B., and White, C.R. 2010. 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A classification of igneous rocks and glossary of terms; Blackwell, Oxford, 193p. Lichtblau, A., Hinz, P., Ravnaas, C., Storey, C.C., Kosloski, L. and Raoul, A. 2004. Report of Activities 2003, Resident Geologist Program, Red Lake Regional Resident Geologist Report: Red Lake and Kenora Districts; Ontario Geological Survey, Open File Report 6127, 104p. Merritt, P.I., 1934. Seine-Coutchiching problem. Geol. Soc. Am. Bull. 45, pp 333-374. Mitchell, R.S. 1985. Dictionary of rocks; Van Nostrand Reinhold Company, New York, 222p. Ontario Geological Survey, 2009, Ontario airborne geophysical surveys, Magnetic and Electromagnetic Surveys, Grid and Profile Data (ASCII and Geosoft Formats) and Vector Data, Mine Centre Area; Ontario Geological Survey. Geophysical Dataset 1061a ISBN 9781-4435-0854-4 [DVD] ISBN 978-1-4435-0855-1 [ZIP FILE]. Percival, J. A., Williams, H. R., 1989. Late Archean Quetico accretionary complex, Superior province, Canada. Geology 17, pp 23-25. Poulsen, K. H., 1986. Rainy Lake Wrench Zone: An example of an Archean Subprovince boundary in Northwestern Ontario. In: de Wit, M. J., Ashwal, L. D. (Eds.), Tectonic evolution of greenstone belts Technical Report 86-10, pp. 177-179. Poulsen, K. H., 2000b. Archean metallogeny of the Mine Centre - Fort Frances area. Ontario Geological Survey Report 266, 121p. Poulsen, H. K., 2000, Precambrian geology and mineral occurrences, Mine Centre – Fort Frances area; Ontario Geological Survey, Map 2525, scale 1:50,000. Poulsen, K. H., Borradaile, G. J., Kehlenbeck, M. M., 1980. An inverted Archean succession at Rainy Lake, Ontario. Canadian Journal of Earth Sciences 17, pp 1358-1369. Schaefer, S.J. and Morton, P. 1991. Two komatiitic pyroclastic units, Superior Province, northwestern Ontario: their geology, petrography, and correlation; Canadian Journal of Earth Sciences, v.28, pp 1455-1470. Severson, M.J., and Hauck, S.A., 1990, Geology, Geochemistry, and Stratigraphy of a portion of the Partridge River Intrusion, NRRI/GMIN-TR-89-11, 236 p. Simoneua, P., 2008, Magnetometric (Walking Mag) and Electromagnetic VLF survey on Mine Centre Property. Unpublished report prepared for Numax Resources, Inc. Stone, D., Hallé, J., and Murphy, R. 1997a, Precambrian geology, Mine Centre area; Ontario Geological Survey, Preliminary Map P. 3372, scale 1:50,000. Stone, D., Hallé, J., and Murphy, R. 1997b, Precambrian geology, Mine Centre area; Ontario Geological Survey, Preliminary Map P. 3373, scale 1:50,000. Tabor, J. R., Hudleston, P. J., 1991. Deformation at an Archean subprovince boundary, northern Minnesota. Canadian Journal of Earth Sciences v28, pp 292-307. Wilcox, R. E., Harding, T. P., Seely, D. R., 1973. Basin wrench tectonics. The American Association of Petroleum Geologists Bulletin 57, 74-96. Wood, J., Dekker, J., Jansen, J. G., Keay, J. P., Panagapko, D., 1980a. Mine Centre Area (Eastern Half), District of Rainy River. Ontario Geological Survey Preliminary Map P. 2202, scale 1:15 840. Wood, J., Dekker, J., Jansen, J. G., Keay, J. P., Panagapko, D., 1980b. Mine Centre Area (Western Half), District of Rainy River. Ontario Geological Survey Preliminary Map P. 2201, scale 1:15 840. 125 �Field Trip 8 GEOLOGY AND ENVIRONMENTAL ISSUES OF THE STEEP ROCK MINE Andrew Conly Department of Geology, Lakehead University 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada Rob Purdon Ontario Ministry of Natural Resources, Thunder Bay, ON P7E 6S7 Canada Lindsay Moore Department of Geology, Lakehead University 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada View from north rim of the Hogarth Pit during the late 1970’s. Courtesy J. Baechler) 126 �Introduction Flooding since 1979 of the decommissioned Steep Rock iron mines open pits north of Atikokan, Ontario has lead to the formation of the joint Hogarth-Roberts‘, Errington and Caland pit lakes (Fig. 1). Currently, the two largest pit lakes, Hogarth and Caland, have maximum water depths of 200 m, with waters levels expected to rise another 90 m, reaching the overflow elevation of 390 m (the original lake water level elevation). Overflow has been estimated to occur as soon as 2030 (Van Crook, 2005), but other estimates indicate overflow will take place around 2060 or later (Jackson, per. comm., 2007; Mikkeleson, unpublished data). Consequently, the overflow of the pit lakes is predicted to flow into the West Arm and possibly into the Seine River system, which flows into the International Boundary Waters of Rainy Lake and Lake of the Woods. The main issue with the waters from the pit lakes is their elevated sulphate contents (up to 1500 mg/L SO42-), which have been shown to be acutely to chronically toxic for aquatic life (McNaughton; 2001; Goold, 2008; Godwin, 2009). In addition to water quality issues, it is anticipate that Crown, municipal and private lands and roads, a section of Provincial Highway 622 and portions of the Ontario Power Generation Atikokan Generating Station site will undergo varying degrees of flooding. Consequently, Steep Rock is currently one of the top three hazardous environmental sites in Ontario (Laderoute, pers. comm., 2008). Figure 1. Map showing the locations of pit lakes at the former Steep Rock iron mine site (image from Google Earth). 127 �In 1988 the Steep Rock mine site was returned to the Province of Ontario where the Ministry of Natural Resources (MNR) has managed the risks to public safety and the environment in accordance with the 1991 MNR Steep Rock Management Plan. However, this plan did not anticipate the water quality issues, which were identified by researchers from Lakehead University, nor did it anticipate a recent proposal to use pit lakes for tailings disposal. Remediation work conducted on the site has been sporadic. However, in 2000 Ministry of Northern Development Mines and Forests (MNDMF) spent $650,000 to remove industrial buildings under the Province‘s Abandoned Mine Hazard Abatement Program. Furthermore, all PCB materials were removed from the site by 2008 for treatment at approved waste disposal facilities, as well as the removal of some of the buried metal waste. Owing to the environmental concerns of the Steep Rock site, in 2008 the MNR received approval to commence the development of a long-term management strategy that will address issues related to surface and ground water quality and hazardous lands. The Steep Rock Mine Rehabilitation Project, a multi-year effort to develop a sustainable long-term management plan for the site has been initiated. This project is governed by a Project Steering Committee with director-level participation from: Ministry of Natural Resources (MNR), Ministry of Northern Development Mines and Forests (MNDMF), Ministry of Energy and Infrastructure (MEI), Ministry of Municipal Affairs and Housing (MMAH), Ministry of the Environment (MOE), Ministry of Transportation (MTO), and Cabinet Office. As part of the Steep Rock Mine Rehabilitation Project, the potential for contaminant release and/or discharge of impaired water from the pit lakes will be examined and remedial strategies developed to ensure that the downstream waters of the Seine River and Rainy Lake remain protected. History Preceding mining, Steep Rock Lake, host of the Steep Rock Mine, was a large, 2050 ha, Mshaped lake located north of the town of Atikokan in northwestern Ontario (Fig. 2). Although iron was discovered as early as 1891, several engineering challenges owing to the nature of the ore body, being located beneath the lake, prevented its extraction. However, following the increasing demands for iron ore during World War II, the Canadian Government declared the development of the Steep Rock Iron Mine as a project ―essential to the war effort‖ under ―The War Measures Act‖. To mine the ore, it was necessary to divert the Seine River that flowed through Steep Rock Lake around the middle and eastern arms of the Steep Rock Lake (Fig. 3). This large river, draining a watershed of approximately 5000 square km, was diverted north around the mine through Finlayson Lake to the northern end of the West Arm of Steep Rock Lake that ultimately retained the flow of the river. This diversion required the construction of two dams that served to separate the West Arm from the rest of Steep Rock Lake. Following the construction of these two dams, work to expose the ore body began. This required draining of the middle and east arms of Steep Rock Lake and the removal of approximately 15 million m3 of lake-bottom silt and varved clay that were ultimately pumped into the West Arm. 128 �Figure 2. Steep Rock Lake prior to mining and diversions (courtesy of the Ministry of Natural Resources). In 1944, the first ore was mined from Errington Pit with subsequent ore bodies having been developed in the Hogarth and Roberts’ pits ca. 1950. During the removal the lake bottom sediment of the latter two ore bodies, a massive release of approximately 4 million m3 of colloidal silt and clay into the Seine River system and ultimately into Rainy Lake (located 145 km downstream) prompted a second diversion of the Seine River system. This diversion required the transformation of the West Arm of Steep Rock Lake into a closedsystem settling basin (Fig. 4) where the once 60 m deep lake now has an average depth of 3 m owing to the deposition of nearly 90 million m 3 of silt and clay sediment. In 1955, a second major dredging project was underway to access the ore body located along the east arm that was still under 30 m of water and 120 m of fine glacial silt and clay. Marmion Lake, now divided in half through the construction of several new dams, served as catchment of the approximately 120 million m 3 of dredged sediment from the dredging project. The newly accessible deposit became the Caland Pit, which started production in 1960. The Steep Rock ore was predominantly hematite, and at 63% iron, could be shipped directly to steel mills and loaded into blast furnaces following the reduction of the material into 6-inch chunks and smaller. A more highly processed, finely crushed ore rolled into marble-sized balls and fired to produce transportable pellets later replaced the previously used ―direct shipping‖ of the ore. No chemical processing was used on the Steep Rock mine site. 129 �Figure 3. Seine River flow following first Steep Rock diversion (1940‘s). Dash line shows the original flow pattern through Steep Rock Lake (courtesy Ministry of Natural Resources). Figure 4. Seine River flow following the second Steep Rock diversion (1950‘s). Dash line shows the flow pattern of the 1940 diversion (courtesy Ministry of Natural Resources). 130 �Figure 5. Location of open pits (1955-1979; courtesy Ministry of Natural Resources). Open pit mining continued until 1979 when both Steep Rock (operating the Errington, Roberts‘, and Hogarth Pits) and Caland reached the bottom of their conical pits and could no longer reach the deeper ores (Fig.5). Following the termination of mining, Steep Rock and Caland had removed 53 million tons and 35 million tons of ore, respectively. Following the closure of the Caland open pit, Caland Ore returned their leased lands to Steep Rock Iron mines. By 1985, Steep Rock Mines determined that no further development was expected, informing the Ministry of Natural Resources (MNR) of their intention to return their mining lands to the Province. At the time of surrender there were no specific regulations regarding mine closures and hence Steep Rock Iron mines were under no legal obligations regarding site remediation. However, Steep Rock Iron mines paid nearly a half million dollars to the MNR to defray any current and immediate future costs associated with mine site management as well as ownership and responsibility of two PCB sites (currently under Control Orders issued by the Ministry of the Environment). Over the years, obsolete mining equipment was either removed or buried with hundreds of thousands of dollars spent on removing structures and materials that the Province did not want. On April 1, 1988, Steep Rock Iron mines and the MNR signed the Surrender Agreement at which point the Province of Ontario regained ownership and management of the Steep Rock site. Following the surrender, the Steep Rock site was largely left alone, allowing the pits to slowly fill with water where maximum water depths currently exceed 200 m. Between 1988 and 2009, the Caland pit hosted the Snow Lake Fish Farm and today the site is largely utilized as an unofficial recreation facility for the residents of Atikokan. 131 �Geology The Steep Rock area is in northwestern Ontario within the Superior province, on the southern border of the Wabigoon Subprovince adjacent to the Quetico Subprovince (Kusky and Hudleston, 1999; Shklanka, 1972; Card, 1990; Stone et al., 1992). The rocks in the area are structurally repeatedly folded and faulted. Three major metamorphic events resulted in three major periods of deformation. These coincide with the major orogenic events known as Kenorian, Hudsonian, and Grenville (Shklanka, 1972). The geology of the Steep Rock area is comprised of the Marmion Gneiss Complex, the Steep Rock Group, the Dismal Ash Formation and the Witch Bay Formation (Kusky and Hudleston, 1999; Fig. 5). Figure 5. Geological cross-section of Errington Pit (after Kusky and Hudleston, 1999). The Marmion Complex is part of a regional belt of arc-type plutons and volcanics in the central Wabigoon Province (Kusky and Hudleston, 1999). It consists of several different units with ages between 3001 Ma and 2928 Ma (Stone et al., 1992). These units to consist of mafic tonalitic gneiss, a more leucocratic tonalite containing amphibolitized remnants of mafic rocks, and several units of felsic and intermediate tuffaceous rock, all intruded by granodiorite and gabbro dykes (Kusky and Hudleston 1999; Stone et al., 1992). Overlying the Marmion Gneiss Complex is the Steep Rock Group, consisting of the Wagita Formation, the Mosher Carbonate Formation and the Jolliffe Ore Zone. The Wagita Formation consists of metamorphosed siliciclastic rocks ranging from argillaceous rocks to conglomerate (Shklanka, 1972; Wilks and Nisbet, 1988; Stone et al., 1992; Kusky and Hudleston, 1999). It is 0 to 150 m thick and not well exposed on the Steep Rock property (Kusky and Hudleston, 1999). The Mosher Formation of the Steep Rock Group overlies the Wagita Formation and is up to 500 m in thickness. It was deposited at ca. 3.0 Ga on the rifted arc margin of the Marmion Complex (Kusky and Hudleston, 1999). The formation and consists of primarily of dolostone and 132 �limestone, which is often brecciated, and in some localities is characterized by geologically significant and well developed, giant stromatolite mounds (Wilks and Nisbet, 1988; Kusky and Hudleston, 1999). The Jolliffe Ore Zone is 100 to 400 m thick and overlies the Mosher Formation and is composed of (in ascending stratigraphic order): the Paint Rock Member, Geothite Member and Pyrite Member (Kusky and Hudleston, 1999; Jolliffe, 1966). The Manganiferous Paint Rock Member has large blocks of carbonate rock, while the brecciated Geothite Member resembles a banded iron formation (Tanton, 1926, 1941a, 1941b, 1946; Bartley, 1939; Smith, 1942; Wilks and Nisbet, 1988). The Dismal Ashrock Formation unconformably overlies Jolliffe Ore Zone and consists of mafic to ultramafic volcanic and pyroclastic rocks, which range in thickness from 100 to 400 m (Schaefer and Morton, 1991; Kusky and Hudleston, 1999). The Dismal Ashrock Formation occurs as a series blocks and lenses of komatiitic volcanic breccia, mafic pillow basalt, komatiitic volcaniclastic rock, carbonate, tonalitic gneiss, and goethite-hematite-rich rock surrounded by a matrix of fragmental komatiitic volcaniclastic rock (Kusky and Hudleston, 1999). Overlying the Dismal Ash Formation is the Witch Bay Formation, which consists of a complex assemblage of metamorphosed mafic volcanic and volcanoclastic rocks that are approximately 2932 Ma in age (Kusky and Hudleston, 1999). The Witch Bay Formation displays physical characteristics of island-arc deposits while being geochemically similar to mid-ocean ridge basalts (Kusky and Hudleston, 1999; Tomlinson et al., 1995). These rocks are correlative in terms of age and lithology to meta-volcanic rocks in the Finlayson belt (Kusky and Hudleston, 1999). Previous Environmental Studies Lakehead University undergraduate and graduate students have been studying the two pit lakes since 1998, and in cooperation with the Ministry of Natural Resources from 2002 to 2009. Work by Lakehead University has largely involved seasonal sampling for water quality assessment of Hogarth and Caland pit lakes, and it was through these studies that the water quality issues were first identified. McNaughton (2001) conducted a limnological study of the two pit lakes to determine the potential of developing an aquaculture facility on Hogarth similar to the one on Caland. Vancook (2005) used GIS mapping to predict the path and timing of overflow of Caland into Hogarth and subsequent outflow into the Seine River system. Through the integration of mineralogy, geochemistry and stable isotope geochemistry, MacDonald (2005) determined that the high sulphate levels originated largely from inflowing ground waters rather than contaminated surface waters. Cockerton (2007) explored the influence of water-rock interactions in the two pits and found that the main cause of the difference in water chemistry between the two pits is the apparent higher pyrite content of the Jolliffe ore zone below Hogarth. Goold (2008) determined that the toxicity of Hogarth is due largely to the elevated levels of dissolved sulphate and less so to heavy metals which are present in very low concentrations in both lakes. Perusse (2009) conducted the first groundwater survey, investigating water quality issues of two tailings impoundments. Godwin (2009) used experimental water-rock interaction experiments coupled with toxicity identification and evaluation studies in order to predict water chemistry and toxicity at the time of outflow. Current research continues with a complete hydrodynamic 133 �model being developed by L. Mikkelsen and a bioremediation investigation into the use of permeable reactive barriers to promote sulfate reduction being conducted by S. Shankie. Water Quality – Pit Lakes Caland exhibits classic meromictic conditions with a dichotomy in water quality versus depth forming a mixolimnion of oligotrophic water for a depth of 20 m and a sulphate-saline, anoxic monimolimnion for the remaining 180 m. Sulphate levels in the monimolimnion reached up to 450 mg/L. Freshwater inflow has created the thinning mixolimnion. Hogarth has uniform water quality throughout its 200 m depth except for a very shallow, recently forming, freshwater surface layer. Sulphate levels in Hogarth often exceed 1500 mg/L. The entire Hogarth water column is sulphate-saline and aerobic. McNaughton (2001) determined that water from Hogarth was acutely toxic and there were no apparent signs of aquatic life in the lake; however, a recent study (Goold, 2008) found that it now appears to be chronically toxic. Snails and minnows were observed in shallow littoral zones of the lake during the summer of 2006 (Godwin, 2009). In 2004, Hogarth merged with Roberts‘ pit at the south end and appears to have a very similar chemical profile to Hogarth. The concentration of dissolved ions in Caland is lower than that of Hogarth particularly with respect to major cations (Ca2+, Mg2+, Na2+, K+) and dissolved sulfate. All of these parameters increase with depth in each lake. Goold (2008) found that chronic toxicological affects produced by water from the Hogarth pit were likely caused by the elevated levels of dissolved ions especially sulfate (SO42-). As a consequence, Hogarth has higher concentrations of dissolved ions compared to Caland, and subsequently higher conductivity, hardness, and TDS (total dissolved solids) levels. Likewise, these parameters all increase with depth in both pit lakes. The pH of the two lakes is circum neutral to slightly basic. The pH in Caland is on average 8.05 at 2 m and decreases with depth to an average of 7.50 near bottom, whereas Hogarth has an average pH of 7.80 at 2 m and decreases with depth to 7.10 near bottom. The alkalinity is greater is Caland (130-173 mg/L CaCO3) than in Hogarth (94-126 mg/L CaCO3) and values increase with depth in both lakes. As a consequence of the Snow Lake Fish Farm on Caland, levels of certain organic constituents are greater throughout the water column in Caland as compared to Hogarth (Godwin, 2009). Caland water has higher dissolved organic carbon (DOC) concentrations than Hogarth (Caland: 2.9-3.2 mg/L; Hogarth: 1.0-1.3mg/L). Curiously the maximum DOC contents in Caland occur at 18 m whereas, at the same depth, Hogarth displays the minimum value. However, the average total nitrogen (Caland: 0.42-0.50 mg/L; Hogarth 0.40-0.75 mg/L) and total phosphorus (Caland: 0.032-0.037 mg/L; Hogarth: 0.029-0.037 mg/L) values from 2005-2008 do not differ greatly between the two pit lakes. The isotopic composition of dissolved sulphate is depleted relative to pyrite from the submerged ore zone and surface waste rocks ( 34S = -1.5 to 10‰) and other pit wall rocks ( 34S = -1.6 to 4.9‰), consistent with oxidation of ore zone pyrite and subsequent fractionation due to precipitation of saturated sulphate minerals from the water column. The oxygen isotopic composition of dissolved sulphate indicates that Hogarth sulphate is produced by anaerobic oxidation of pyrite, whereas the oxygen isotope composition of Caland sulphate is consistent with aerobic oxidation of pyrite (Fig. 6a). 134 �The near neutral pH of both pit lakes is the result of interactions with carbonate wall rocks. The carbon isotopic composition of DIC from Hogarth is similar to that isotopic composition of the carbonate wall rocks ( 13C = -4.0 to 0.6‰; Fig. 6b). However, DIC in Caland pit water is more depleted and suggests either isotopic exchange with isotopically depleted DOC generated by fish farming activities or atmosphere exchange. Figure 6. a) The 34S and d18O isotopic composition of dissolved sulphate, and b) DOC composition of Hogarth and Caland pit lakes (from Conly et al., 2008). 13 CDIC and Water Quality – Ground Water Figure 7 shows the average composition of major anions and cations, Fe, Mn, total metals and pH for all ground water monitors and selected surface waters. Shallow ground waters contained within pyrite-quartz and goethite-hematite-quartz tailings/waste rock are sulphate-dominant, whereas the one regional ground water is chlorine-dominant. Interactions with pyritic waste rock produces ground waters with elevated sulfate (>3000 mg/L), low pH (<6) and high Fe (>95 mg/L). Fe-oxide + quartz tailings produce pH-neutral ground waters with moderate to high concentrations of sulphate (200-1000 mg/L) and are Fe-depleted (<1 mg/L). The difference in pH, sulphate and Fe levels reflects the following: i) the relatively insoluble behaviour of goethite and hematite in pH-neutral waters; ii) possible occurrence of soluble Fe-sulphates; and, iii) the relative abundance of pyrite and the rate at which it is oxidized. Surface waters parallel the observed trends for ground waters. However, for both sites anion and cation abundances are typically lower, and likely reflect dilution from surface run-off and precipitation. 135 �Figure 7. Schoeller diagrams, with pH (inset), showing the average compositions for ground water and surface water from sites 1 (A) and 2 (B). Abbreviations: Alk – alkalinity; AGW – Atikokan ground water (from Conly et al., 2009). 136 �Figure 8 shows the variation in carbon and sulphur isotopes for Steep Rock ground water and surface water. 34S ratios for ground water from both sites are within the reported range for the two pit lakes, and indicate that the dissolved sulphate is produced from the oxidation of pyrite. 13 C, and is within the range reported for the two pit lakes, and reflects exchange with atmospheric CO 2 and carbonate 13 dissolution. C of ground water from Fe-oxide tailings is both more depleted and variable. Such values are interpreted to reflect isotopic exchange with CO2 released from buried organic-rich (bog) soils. Ground waters that are both highly depleted 13C and enriched 34 S reflect methanogenesis and bacterial sulfate reduction, arising from interaction with organic-rich (bog) soils from below. Figure 8. Variation in 13C and 34S of ground water and surface water (from Conly et al., 2009). Data for other waters (excluding regional water; unpublished) from Conly et al. (2008). 137 �Field Trip Stops Stop Number 1 Location 2 Caland Lookout 3 Red, Green and Moore‘s Ponds 4 Errington Pit 5 6 B-2 Shaft Building Roberts Pit 7 ―Party Point‖ Hardy Dam Details Largest earthen dam owned by the Province of Ontario Originally constructed in 1944 Holds back Rawn Reservoir, former flow into Steep Rock Lake directed into Atikokan River through a series of tunnels to the southwest Highway 622 will be under approximately 5 m water. Triangulation Island Snow Lake Fish Farms (now defunct) Caland Flats (flooding) Water quality, sulphate, TDS, meromictic conditions Rock slope stability AMD causing low pH pH buffered by carbonate host rocks elevated sulphate, high TDS pyritic waste rock piles slope stability First open pit mined Pit floor approximately 180m below current water level ―Green Pond shows evolution of water quality ―Yellow Creek‖ Fe pptn on creek bed PCB Storage site Forced entry, public safety Pit lake 180 m deep Water quality Slope stability Narrows Dam, inundation scenarios Slope stability Water quality, sources of impacts 138 UTM Coordinates Zone 15, nad’83 606391E 5403980N 603135E 5408000N 600055E 5405046N 600133E 5405282N 599167E 5405329N 599206E 5405782N 600118E 5407376N �Stop Number 8 Location 9 A-2 Shaft Remnants 10 B-1 Shaft/Vault 11 Narrows Dam 12 West Arm Dam SR Pelletizing Plant Details Soil impacts (As, metals) Relict structures Closure activities Underground openings Public safety Unauthorized salvage operations in underground workings Unknown infrastructure Overlook of pit, ―Road to Nowhere‖ Water Quality Slope stability Original diversion Flooding/inundation Pyrite rich material Mobilisation of sediments in West Arm Repairs 2007 Changes to Steep Rock Lake Settling Basin Ongoing risk 139 UTM Coordinates Zone 15, nad’83 600365E 5408468N 599630E 5408685N 598614E 5408105N 598931E 5407490N 597854E 5402450N �Figure 9. LIDAR ortho map showing field trip stops and other points of interest. 140 �References Bartley, M.W., 1939. Iron deposits of the Steeprock Lake area. Ontario Department of Mines, Annual Report, v. 48, p 419-421. Card, K.D., 1990. A review of the Superior Province of the Canadian Shield. Geoscience Canada, v. 13, p. 5-13. Cocketon, S., 2007. An experimental water-rock interaction study into the origin of the chemical characteristics of the Hogarth and Caland pit lakes, Steep Rock Lake, Atikokan. H.B.Sc. Thesis, Lakehead University, Thunder Bay, Ontario, 46 p. Conly, A.G., Lee, P.F., Goold, A. and Godwin, A., 2008. Geochemistry and stable isotope composition of Hogarth and Caland Pit lakes, Steep Rock iron mine, northwestern Ontario, Canada. In Rapantove N. and Hrkal, Z. (eds), Mine Water and the Environment Proceedings v. 10, p. 555-558. Godwin, A., 2009. Geochemical and toxicological investigation of the Hogarth and Caland Pit Lakes, former Steep Rock iron mine site. M.Sc. Thesis, Lakehead University, Thunder Bay, Ontario, 105 p. Goold, A.R., 2008. Water quality and toxicity investigations of two pit lakes at the former Steep Rock iron mines, near Atikokan, Ontario. M.Sc. Thesis, Lakehead University, Thunder Bay, Ontario, 90 p. Hayes, J.M., Kaplan, I.R., and Wedeking, K.W., 1983. Precambrian organic geochemistry, preservation of the record. In Earth‘s earliest biosphere, its origin and evolution. Edited by J.W. Schopf. Princeton University Press, Princeton, N.J. p. 93-132. Joliffe, A.W., 1966. Stratigraphy of the Steeprock Group, Steep Rock Lake, Ontario. GAC Special Paper No. 3, p. 75-96. Kusky, T.M. and Hudleston, P.J. 1999. Growth and demise of an Archean carbonate platform, Steep Rock Lake, Ontario, Canada. Canadian Journal of Earth Science. v. 36, p. 565-584. MacDonald, J.C., 2005. An integrated mineralogy, geochemistry, and stable isotope investigation into potential sulphate sources of the Hogarth Pit Lake, Steep Rock Iron Mine, Atikokan. B.Sc. Thesis, Lakehead University, Thunder Bay, Ontario, 52 p. McNaughton, K.A., 2001. The limnology of two proximal pit lakes after twenty years of intense flooding. M.Sc. Thesis, Lakehead University, Thunder Bay, Ontario, 78 p. Perusse, C., 2009. Hydrogeological and geochemical assessment of two tailings impoundments at the former Steep Rock iron mine Atikokan, Ontario. H.B.Sc. Thesis, Lakehead University, Thunder Bay, Ontario, 39 p. Schaefer, S.J., and Morton, P., 1991. Two komatiitic pyroclastic units, Superior Province, northwestern Ontario: their geology, petrology, and correlation. Canadian Journal of Earth Science, v. 28, p. 1455-1470. Shklanka, R., 1972. Geology of the Steep Rock Lake area, District of Rainy River. Part 1. Geology of Steep Rock Lake. Ontario Department of Mines and Northern Affairs, Geological Report 93, p. 1-80. 141 �Smith, F.G., 1942. Notes on the iron ores of Steeprock Lake, Ontario. University of Toronto Studies, Contributions to Canadian Mineralogy, v. 47, p. 71-75. Stone, D., Kamineni, D.C., and Jackson, M.C., 1992. Precambrian geology of the Atikokan area, northwestern Ontario. Geological Survey of Canada, v. 405, 106 p. Tanton, T.L., 1926. Mineral deposits of Steeprock Lake map-area, Canada. In Summary Report 1925, Part C. Geological Survey of Canada, p. 1–11. Tanton, T.L., 1941a. Origin of the hematite deposits at Steeprock Lake, Ontario. Transactions of the Royal Society of Canada, Section 4: Geological Sciences, v. 35, p. 131–141. Tanton, T.L., 1941b. Areas in the vicinity of Steeprock Lake, Rainy River District, Ontario. Geological Survey of Canada, Paper, p. 41-13. Tanton, T.L. 1946. The iron ore at Steeprock Lake. Transactions of the Royal Society of Canada, Section 4: Geological Sciences, v. 40, p. 103-110. Tomlinson, K.Y., Hall, R.P., Hughes, D.J., and Thurston, P.C., 1995. The Archean Steeprock greenstone belt, NW Ontario: geochemistry of platformal komatiitic volcanics and overlying tholeiitic lavas — evidence for secular and tectonic separation. Precambrian 95, Abstracts, p. 273. Vancook, M., 2005. The limnology and remediation of two proximal pit lakes in Northwestern Ontario. M.Sc. Thesis, Lakehead University, Thunder Bay, Ontario, 81 p. Wilks, M.E. and Nisbet, E.G., 1988. Stratigraphy of the Steep Rock Group, northwestern Ontario: a major Archaean unconformity and Archaean stromatolites. Canadian Journal of Earth Sciences, v. 25, p. 370-391. 142 � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Proceedings, 2010 Description An account of the resource Institute on Lake Superior Geology. International Falls, Minnesota. May 19-22, 2010. Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology Date A point or period of time associated with an event in the lifecycle of the resource 2010 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English https://digitalcollections.lakeheadu.ca/files/original/73d8517270988c319ee40c8061c3d887.pdf 5ed6d41d45acb966d9a218591d343814 PDF Text Text 57TH ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY ASHLAND, WISCONSIN MAY 18-21, 2011 PROCEEDINGS VOLUME 57 PART 1 – PROGRAMS AND ABSTRACTS ��INSTITUTE ON LAKE SUPERIOR GEOLOGY 57TH ANNUAL MEETING MAY 18-21, 2010 ASHLAND, WISCONSIN HOSTED BY: NORTHLAND COLLEGE TOM FITZ Chair Proceedings Volume 57 Part 1 – Program and Abstracts Edited by: Tom Fitz, Allison Mills, Kristi Wilson, Cassandra Bodette, and Drew Cramer Cover Photo: Intrusive breccia in the Mellen Intrusive Complex, Mellen, WI �57TH INSTITUTE ON LAKE SUPERIOR GEOLOGY CONTENTS OF PROCEEDINGS VOLUME 57: PART 1: PROGRAM AND ABSTRACTS PART 2: FIELD TRIP GUIDEBOOK TRIP 1: IGNEOUS STRATIGRAPHY OF THE LAYERED SERIES AT DULUTH - TYPE INTRUSION OF THE DULUTH COMPLEX TRIP 2: MIDCONTINENT MICROCOSM TRIP 3: GEOLOGY OF THE BAYFIELD PENINSULA: KEWEENAWAN BAYFIELD GROUP AND PLEISTOCENE DEPOSITS TRIP 4: GEOLOGY AND REMEDIATION AT THE ASHLAND/NORTHERN STATES POWER SITE TRIP 5: BAD RIVER WATERSHED CULVERT RESTORATION PROGRAM TRIP 6: GEOLOGY OF COPPER FALLS STATE PARK TRIP 7: GEOLOGY OF THE MONTREAL RIVER MONOCLINE TRIP 8: THE ARCHEAN/PALEOPROTEROZOIC UNCONFORMITY NEAR DENHAM, MINNESOTA TRIP 9: GRANITIC, GABBROIC, AND ULTRAMAFIC ROCKS OF THE KEWEENAWAN MELLEN INTRUSIVE COMPLEX Reference to material in Part 1 should follow the example below: LaMaskin, T.A., 2011, Testing models of late Paleoproterozoic Penokean orogenesis in the Great Lakes Region, U.S.A. using sedimentary provenance: planning the investigation [abstract]: Institute on Lake Superior Geology Proceedings, 57th Annual Meeting, Ashland, WI, v. 57, part 1, p. 53-54. Published by the 57th Institute on Lake Superior Geology and distributed by the ILSG Secretary: Peter Hollings Department of Geology Lakehead University Thunder Bay, ON P7B 5E1 CANADA peter.hollings@lakeheadu.ca ILSG website: http://www.lakesuperiorgeology.org ISSN 1042-9964 ILSG 2011 ii Program and Abstracts �TABLE OF CONTENTS PROCEEDINGS VOLUME 57 PART 1— PROGRAM AND ABSTRACTS Previous Institutes on Lake Superior Geology, 1955-2010........................... iv Sam Goldich and the Goldich Medal............................................................. vi Past Goldich Medalists and the 2010 Goldich Medal Recipient ................. viii Goldich Medal Committee........................................................................... viii Citation for 2010 Goldich Medal Recipient .................................................. ix ILSG Student Research Fund ........................................................................ xi Student Paper Awards ................................................................................... xii Eisenbrey Student Travel Awards ............................................................... xiii Report of the Chairs of the 56th Annual Meeting ......................................... xv 2011 Board of Directors ............................................................................. xvii 2011 Session Chairs .................................................................................... xvii 2011 Student Paper Awards Committee ..................................................... xvii 2011 Banquet Speaker ............................................................................... xviii Program ........................................................................................................ xix Schedule of Events ....................................................................................... xx Schedule of Talks and Posters .................................................................... xxi Abstracts .................................................................................................. xxviii Author Index ................................................................................................. 97 ILSG 2011 iii Program and Abstracts �PREVIOUS INSTITUTES ON LAKE SUPERIOR GEOLOGY, 1955-2010 ILSG YEAR PLACE CHAIRS 1 1955 Minneapolis, Minnesota C.E. Dutton 2 1956 Houghton, Michigan A.K. Snelgrove 3 1957 East Lansing, Michigan B.T. Sandefur 4 1958 Duluth, Minnesota R.W. Marsden 5 1959 Minneapolis, Minnesota G.M. Schwartz and C. Craddock 6 1960 Madison, Wisconsin E.N. Cameron 7 1961 Port Arthur, Ontario E.G. Pye 8 1962 Houghton, Michigan A.K. Snelgrove 9 1963 Duluth, Minnesota H. Lepp 10 1964 Ishpeming, Michigan A.T. Broderick 11 1965 St. Paul, Minnesota P.K. Sims and R.K. Hogberg 12 1966 Sault Ste. Marie, Michigan R.W. White 13 1967 East Lansing, Michigan W.J. Hinze 14 1968 Superior, Wisconsin A.B. Dickas 15 1969 Oshkosh, Wisconsin G.L. LaBerge 16 1970 Thunder Bay, Ontario M.W. Bartley and E. Mercy 17 1971 Duluth, Minnesota D.M. Davidson 18 1972 Houghton, Michigan J. Kalliokoski 19 1973 Madison, Wisconsin M.E. Ostrom 20 1974 Sault Ste. Marie, Ontario P.E. Giblin 21 1975 Marquette, Michigan J.D. Hughes 22 1976 St. Paul, Minnesota M. Walton 23 1977 Thunder Bay, Ontario M.M. Kehlenbeck 24 1978 Milwaukee, Wisconsin G. Mursky 25 1979 Duluth, Minnesota D.M. Davidson 26 1980 Eau Claire, Wisconsin P.E. Myers 27 1981 East Lansing, Michigan W.C. Cambray ILSG 2011 iv Program and Abstracts �28 1982 International Falls, Minnesota D.L. Southwick 29 1983 Houghton, Michigan T.J. Bornhorst 30 1984 Wausau, Wisconsin G.L. La Berge 31 1985 Kenora, Ontario C.E. Blackburn 32 1986 Wisconsin Rapids, Wisconsin J.K. Greenberg 33 1987 Wawa, Ontario E.D. Frey and R.P. Sage 34 1988 Marquette, Michigan J. S. Klasner 35 1989 Duluth, Minnesota J.C. Green 36 1990 Thunder Bay, Ontario M.M. Kehlenbeck 37 1991 Eau Claire, Wisconsin P.E. Myers 38 1992 Hurley, Wisconsin A.B. Dickas 39 1993 Eveleth, Minnesota D.L. Southwick 40 1994 Houghton, Michigan T.J. Bornhorst 41 1995 Marathon, Ontario M.C. Smyk 42 1996 Cable, Wisconsin L.G. Woodruff 43 1997 Sudbury, Ontario R.P. Sage and W. Meyer 44 1998 Minneapolis, Minnesota J.D. Miller, Jr. and M.A. Jirsa 45 1999 Marquette, Michigan T.J. Bornhorst and R.S. Regis 46 2000 Thunder Bay, Ontario S.A. Kissin and P. Fralick 47 2001 Madison, Wisconsin M.G. Mudrey, Jr. and B.A. Brown 48 2002 Kenora, Ontario P. Hinz and R.C. Beard 49 2003 Iron Mountain, Michigan L.G. Woodruff and W.F. Cannon 50 2004 Duluth, Minnesota S.A. Hauck and M. Severson 51 2005 Nipigon, Ontario P. Hollings and M.C. Smyk 52 2006 Sault Ste. Marie, Ontario R.P. Sage and A.C. Wilson 53 2007 Lutsen, Minnesota L.G. Woodruff and J.D. Miller, Jr. 54 2008 Marquette, Michigan T.J. Bornhorst and J.S. Klasner 55 2009 Ely, Minnesota J.D. Miller, Jr., G.J. Hudak, D.M. Peterson 56 2010 International Falls, Minnesota P. Hollings, P. Hinz, M. Smyk, M. Jirsa, and T. Boerboom ILSG 2011 v Program and Abstracts �SAM GOLDICH AND THE GOLDICH MEDAL Sam Goldich received an A.B. from the University of Minnesota in 1929, a M.A. from Syracuse University in 1930, and a Ph.D. from the University of Minnesota in 1936. During World War II Sam worked for the U.S. Geological Survey in mineral exploration. In 1948, Sam returned to the University of Minnesota, and became Professor and Director of the Rock Analysis Laboratory the following year. He rejoined the U.S. Geological Survey in 1959 and was appointed as the first Branch Chief of the Branch of Isotope Geology. Sam returned to academia in 1964 when he went to Pennsylvania State University. He left PSU in 1965 and moved to the State University of New York at Stony Brook, where he stayed for 3 years. Restless yet again, he moved to Northern Illinois University in 1968 where he was a professor until his retirement in 1977. Sam‘s final move was to Denver where he became an emeritus at the Colorado School of Mines. Sam died in 2000, less than a month before his 92nd birthday. In the late 1970‘s, Geological Society of America Special Paper 182, which included seminal geochronological studies by Sam Goldich and coworkers on the Archean rocks of the Minnesota River Valley, was nearing completion. At this time various ILSG regulars began discussing the possibility of recognizing Sam for his pioneering work on the resolution of age relationships and thus the geology of Precambrian rocks in the Lake Superior region. Three members, R.W. Ojakangas, J.O. Kalliokoski and G.B. Morey, presented the idea to the ILSG Board of Directors in 1978. The Board approved the creation of an award, provided funding could be obtained. It was suggested that collecting one or two dollars at registration for a dedicated account would provide resources for striking the medal. A general request was made to the ILSG membership for donations and Sam himself offered a challenge grant to match the contributions. In total $4,000 was collected and thus began the work of creating the Goldich Medal. The initial Goldich Award was presented to Sam by G.B. Morey in 1979 and consisted of a large paper proclamation. For the actual medal, G.B. Morey consulted with the foundry on production details, while Dick Ojakangas and Jorma Kalliokoski worked on the design of the award, suggesting that it be given for ―outstanding contributions to the geology of the Lake Superior region.‖ Simultaneously, a committee of J.O. Kalliokosi, W.F. Cannon, M.M Kehlenbeck, G.B. Morey, and G. Mursky developed the Award Guidelines that were approved by the ILSG Board. By 1981 all the elements of the Goldich Award had come together, and the second recipient, Carl E. Dutton, Jr., received the Goldich Medal for 50 years of significant contributions to the understanding of the geology of the Lake Superior region. Since the beginning, the Awards Committee has consisted of individuals representing industry, government and academia, with each member of the Committee serving for three years. The medal is now awarded every year at the annual ILSG meeting. Reference: Morey, G.B. and Hanson, G.N. (editors). 1980. Selected studies of Archean gneisses and Lower Proterozoic rocks, southern Canadian Shield. Geological Society of America, Special Paper 182, 175 p. Prepared by various Goldich Medal Awardees, 2007 ILSG 2011 vi Program and Abstracts �INSTITUTE ON LAKE SUPERIOR GEOLOGY GOLDICH MEDAL ILSG 2011 vii Program and Abstracts �PAST GOLDICH MEDALISTS 1979 1980 1981 1982 1983 1984 Samuel S. Goldich not awarded Carl E. Dutton, Jr. Ralph W. Marsden Burton Boyum Richard W. Ojakangas 1995 Gene La Berge 1996 David L. Southwick 1997 Ronald P. Sage 1998 Zell Peterman 1999 Tsu-Ming Han 2000 John C. Green 1985 1986 1987 1988 1989 1990 1991 Paul K. Sims G.B. Morey Henry H. Halls Walter S. White Jorma Kalliokoski Kenneth C. Card William Hinze 2001 2002 2003 2004 2005 2006 2007 1992 William F. Cannon 1993 Donald W. Davis 1994 Cedric Iverson John S. Klasner Ernest K. Lehmann Klaus J. Schulz Paul Weiblen Mark Smyk Michael G. Mudrey Joseph Mancuso 2008 Theodore J. Bornhorst 2009 L. Gordon Medaris, Jr. 2010 William D. Addison and Gregory R. Brumpton 2011 GOLDICH MEDAL RECIPIENT Dean M. Rossell Kennecott Exploration Company Salt Lake City, Utah GOLDICH MEDAL COMMITTEE Serving for the meeting years shown in parentheses Al MacTavish, Industry member - Chair (2008-2011) Mary Louise Hill, Academic member (2009-2012) Laurel Woodruff, Government member (2010-2013) ILSG 2011 viii Program and Abstracts �CITATION FOR 2011 GOLDICH MEDAL RECIPIENTS Dean M. Rossell It is my privilege and great pleasure to introduce Dean Rossell as this year's Goldich Medal recipient. Dean has been an ILSG contributor and stalwart attendee of its meetings and field trips since his student days many years ago. Dean grew up in Minneapolis, and graduated from the University of Minnesota-Duluth in 1979, studying under some of the early recipients of the Goldich Medal (Ralph Marsden, Dick Ojakangas, and John Green). Immediately after graduation he got a temporary job in Nevada, and that summer, by tracing boulders up dry washes, was the co-discoverer of what soon thereafter became the Trout Creek barite mine. But he promptly returned to the Lake Superior region, graduating from Michigan Technical University in 1983, studying under the aegis of another Goldich medalist, Ted Bornhorst. His M.Sc. thesis was entitled "Alteration of the Deer Lake Peridotite in the Vicinity of the Ropes Mine, Marquette County, Michigan." This grew out of his summer student work at that old gold mine, which Callahan Mining Corp. was putting back into production after nearly a century of abandonment. After graduating from MTU, he spent the next seven years as an exploration geologist for Resource Exploration, Inc. out of Marquette, working for Bill Bodwell. His wife Karen also worked in the Resource Exploration office in those days as a draftsperson. Dean himself never spent much time in the office, though. His work for clients during this period included: re-logging all drill core and re-interpreting the Keweenawan Western Syncline sediment-hosted chalcocite deposit, Michigan; mapping, sampling and drilling VMS base metal and shear zone-hosted gold targets in the Archean Ramsay-Lake Gogebic greenstone belt, Michigan; initiating and carrying out field exploration and drilling on numerous grassroots-developed gold targets in the Archean greenstones and felsic intrusives of the Virginia Horn, Minnesota; and drilling for platinum group elements near the Reserve Mining property in the Duluth Complex, Minnesota. In short, he came up the old fashioned way, doing the field work in numerous exploration programs, mapping, sampling and drilling rocks around the Lake Superior region. Beginning in 1991 Dean worked as a contract geologist for Kennecott Exploration Company out of Crystal Falls, Michigan, initially doing mapping and field sampling in the search for Proterozoic sedex base metal deposits. It was during this time that he alertly found a nickelcopper sulfide-bearing boulder in a logging roadcut that led to a complete change in direction of that program. A few months later, he was the first to discover nickel-copper mineralization in the BIC layered mafic intrusion, and he strongly advocated an investigation of the Yellow Dog peridotite. In 1995 Dean gained permanent status with Kennecott and carried on with his tireless and imaginative efforts to explore for base metals in Keweenawan Rift-related terranes on both sides of the border. His investigations eventually led to the discovery of the Eagle nickelcopper-PGE deposit at the Yellow Dog peridotite in 2002, the result of a nearly singlehanded effort on his part. This very high grade Michigan deposit has now received its permits for underground mining, and promises to be an important contributor to the economy of the Upper Peninsula. ILSG 2011 ix Program and Abstracts �In the early days of 21st Century exploration for nickel-copper in the Black Sturgeon district of Ontario, he acquired and tested one of the known ultramafic intrusions (Kitto), and tested others in elsewhere in Ontario that he is still not authorized to talk about. He carried his ideas into Minnesota, leading Kennecott to search for mineralization in buried ultramafic intrusions in the Animikie Basin. Again with the imagination and persistence that are his hallmarks, this work eventually led to discovery of the Tamarack deposit in Carlton County, Minnesota. Several years of drilling turned up only anomalous Ni-Cu values at Tamarack, but Dean's careful persistence eventually paid off with Kennecott's announcement in June 2008 of a long drill intercept of very high grade nickel-copper. This deposit is currently being intensively evaluated by Kennecott, and Tamarack has the promise of being a larger version of the Eagle deposit in Michigan. Dean and his family are now based in Salt Lake City, but he still doesn't spend much time in the office. That's not where the rocks are. He has been to nearly every part of the world where one can see the right rocks. In fact, his title in Kennecott's Project Generation Group is "Global Nickel Specialist", and he's now internationally regarded as an expert on exploration for ultramafic-hosted Ni-Cu-PGE deposits. Despite the competitive secrecy that surrounds such efforts, Dean has shared his expertise and knowledge with others in the geologic community when able. As is the case with most company geologists, Dean has had to be restrained in publishing his work, but has made the scientific aspects of it known at ILSG meetings: Duluth, 2004 (Eagle deposit presentation and abstract), Nipigon 2005 (Eagle deposit poster session). He also published on the Eagle deposit at the AIME-SEG meeting in Salt Lake City in 2005 (presentation and abstract), and prepared and led the field trip to the BIC intrusion at the Marquette ILSG in 2008. He has also contributed more indirectly, getting Kennecott to provide access and thesis research funds for graduate students studying the geology of the Eagle and Tamarack deposits, and he has been an important supporter of student involvement in the ILSG. Although his base is now in Utah, he still spends most of his time in the Lake Superior region, something not likely to change anytime soon. Dean Rossell is a standout in the ranks of exploration geologists: a real minefinder, and one who gives back to the community of his peers (especially the ILSG community), as well. Please join me in congratulating Dean as the 2011 recipient of the Goldich Medal of the Institute on Lake Superior Geology. Doug Duskin April, 2011 ILSG 2011 x Program and Abstracts �ILSG STUDENT RESEARCH FUND The 2005 Board of Directors established the ILSG Student Research Fund with $10,000 US from the Institute‘s general fund to encourage student research on the geology of the Lake Superior region. A minimum of two awards of $500 US each for research expenses (but not travel expenses) will be made each year. Students are expected to present their research orally or during a poster session at an ILSG meeting. The award winners will also be automatically eligible for the Eisenbrey Travel Awards. To allow the fund to grow, the Fund will receive one-half of any additional proceeds from each annual meeting, after all other commitments and expenses are covered. The ILSG Board of Directors will be responsible for selecting a minimum of two awards each year. The ILSG Treasurer will issue the awards. The ILSG Student Research Fund is available for undergraduate or graduate students working on geology in the Lake Superior region. The applications are due to the ILSG Secretary by August 31st of each year Awards will be made by October 1st of each year. Names of the award recipients will be announced at the next annual meeting and posted on the ILSG website. Details of the application process can be found on the ILSG web site. The proposal will need to be signed by researcher‘s supervisor. In 2010 the ILSG Board of Governors awarded one $500 award from the Student Research Fund. The winner is listed below, along with a description of his research project. Scott Scheiner (Kent State University) – Source regions of sediments in the Animikie Basin Scott is working with Dan Holm (Kent State University) and Terry Boerboom (Minnesota Geological Survey) studying the geochemistry and clast mineralogy the greywacke and turbidite sequences of the Animikie basin to determine the source of the sediments. ILSG 2011 xi Program and Abstracts �STUDENT PAPER AWARDS Each year, the Institute selects the best of the student presentations and honors presenters with a monetary award. Funding for the award is generated from registrations of the annual meeting. The Student Paper Committee is appointed by the annual meeting Chair in such a manner as to represent a broad range of professional and geologic expertise. Criteria for best student paper—last modified by the Board in 2001—follow: The contribution must be demonstrably the work of the student. The student must present the contribution in-person. The Student Paper and Poster Committee shall decide how many awards to grant, and whether or not to give separate awards for poster vs. oral presentations. In cases of multiple student authors, the award will be made to the senior author, or the award will be shared equally by all authors of the contribution. The total amount of the awards is left to the discretion of the meeting Chair in conjunction with the Secretary, but typically is in the amount of about $500 US (increase approved by Board, 10/01). The Secretary maintains, and will supply to the Committee, a form for the numerical ranking of presentations. This form was created and modified by Student Paper and Poster Committees over several years in an effort to reduce the difficulties that may arise from selection by raters of diverse background. The use of the form is not required, but is left to the discretion of the Committee. The names of award recipients shall be included as part of the annual Chair's report that appears in the next volume of the Institute. Student papers are noted on the Program. STUDENT PAPER AWARDS The student paper committee comprised of Graham Wilson (Chair, Turnstone Geological Services Ltd.) Jim Miller (University of Minnesota – Duluth) and Anthony Pace (Ontario Geological Survey) yet again had a difficult job of selecting among the 16 student oral and poster presentations. The following students were granted awards of $200 each, in recognition of their excellence of research and presentation: Andrew Ryan Petrographic and geochemical analysis of a Nipigon diabase sill Ryan, Andrew J. and Zieg, Michael J., Department of Geography, Geology, and the Environment, Slippery Rock University, Slippery Rock, PA ILSG 2011 xii Program and Abstracts �Steve Hoaglund U-Pb zircon geochronology of the Duluth Complex and related hypabyssal intrusions: investigating the emplacement history of a large, multiphase intrusive complex related to the 1.1 Ga Midcontinent Rift Hoaglund, S.A.a*, Miller, J.D.a, Crowley, J.L.b, and Schmitz, M.D.b a Department of Geological Sciences, University of Minnesota Duluth, Duluth, MN, b Department of Geosciences, Boise State University, Boise, ID Victoria Stinson Structural control at Hammond Reef gold deposit north of Atikokan, Ontario Stinson, Victoria R. and Hill, Mary Louise, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario EISENBREY STUDENT TRAVEL AWARDS The 1986 Board of Directors established the ILSG Student Travel Awards to support student participation at the annual meeting of the Institute. The name "Eisenbrey" was added to the award in 1998 to honor Edward H. Eisenbrey (1926-1985) and utilize substantial contributions made to the 1996 Institute meeting in his name. "Ned" Eisenbrey is credited with discovery of significant volcanogenic massive sulfide deposits in Wisconsin, but his scope was much broader—he has been described as having unique talents as an ore finder, geologist, and teacher. These awards are intended to help defray some of the direct travel costs of attending Institute meetings, and include a waiver of registration fees, but exclude expenses for meals, lodging, and field trip registration. The annual Chair in consultation with the Secretary-Treasurer determines the number of awards and value. Recipients will be announced at the annual banquet. The student travel award application is available on the ILSG website. The following general criteria will be considered by the annual Chair, who is responsible for the selection: The applicants must have active resident (undergraduate or graduate) student status at the time of the annual meeting of the Institute, certified by the department head. Students who are the senior author on either an oral or poster paper will be given favored consideration. It is desirable for two or more students to jointly request travel assistance. In general, priority will be given to those in the Institute region who are farthest away from the meeting location. Each travel award request shall be made in writing to the annual Chair, and should explain need, student and author status, and other significant details. Successful applicants will receive their awards during the meeting. ILSG 2011 xiii Program and Abstracts �The following 28 students were awarded travel grants to help defray the cost of meeting attendance. Award amounts varied by travel arrangement and distance from $75-$300. We think this is a record number of annual awards, thanks largely to generous contributions from corporate and individual sponsors. Robert Cundari—Lakehead University, Thunder Bay, ON Adam Fage—Lakehead University, Thunder Bay, ON Maura Kolb—Lakehead University, Thunder Bay, ON Seamus Magnus—Lakehead University, Thunder Bay, ON Sean O’Hare—Lakehead University, Thunder Bay, ON Raya Puchalski—Lakehead University, Thunder Bay, ON Robert Scott—Lakehead University, Thunder Bay, ON Steve Siemieniuk—Lakehead University, Thunder Bay, ON Victoria Stinson*—Lakehead University, Thunder Bay, ON James Blount—Lawrence University, Appleton, WI Rachel Carver—Lawrence University, Appleton, WI Suzanne Craddock—Lawrence University, Appleton, WI Sarah Ehlinger—Lawrence University, Appleton, WI Meaghan Gallagher—Lawrence University, Appleton, WI Megan Luedtke—Lawrence University, Appleton, WI Sarah Shay—Lawrence University, Appleton, WI Amanda Van Lankvelt*—Lawrence University, Appleton, WI Levi Markwood—Slippery Rock University, PA Andrew Ryan*—Slippery Rock University, PA Dan Cervin—University of Minnesota, Duluth, MN Avery Cota—University of Minnesota, Duluth, MN Dan Foley—University of Minnesota, Duluth, MN Shelby Frost**—University of Minnesota, Duluth, MN Steve Hoaglund*—University of Minnesota, Duluth, MN Cabin Ross—University of Minnesota, Duluth, MN Stephanie Theriault—University of Minnesota, Duluth, MN Michael Totenhagen—University of Minnesota, Duluth, MN Natalie Pietrzak—University of Western Ontario, London, ON *Also recipient of 2010 STUDENT PAPER AWARD **Also recipient of 2009 STUDENT RESEARCH GRANT ILSG 2011 xiv Program and Abstracts �REPORT OF THE CHAIRS OF THE 56TH ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY INTERNATIONAL FALLS, MINNESOTA The 56th Annual Institute on Lake Superior Geology on May 19-22, 2010 was a cross-border meeting with an international team of co-chairs. The meeting in International Falls was chaired by Mark Jirsa, Terry Boerboom, Mark Smyk, Peter Hinz and Pete Hollings. Despite this the meeting was a great success with a total of 171 registrants, including 30 students. The two-day technical session held at the Holiday Inn on Thursday and Friday included 23 talks and 17 posters presentations, including 11 oral and 5 poster presentations by students. This year‘s Goldich Medal recipients were Bill Addison and Greg Brumpton whose tireless work in the Thunder Bay area led to the first recognition of the ejecta layer from the Sudbury impact. Bill and Greg were presented their medals at the annual banquet by another Goldich Medalist, Bill Cannon from the USGS. Fittingly, the evening banquet talk was presented by Dr. Bevan M. French of the Smithsonian Institution who discussed his work on meteor impacts. The meeting offered eight field trips that highlighted various aspects of the geology and ore deposits of two countries. Pre-meeting field trips included Mineral deposits of the Rainy River area led by Wally Rayner and C.J. Baker (Rainy River Resources), and Craig Ravnaas (Ontario Geological Survey), a one-day trip on Geology of the Archean succession at Atikokan led by Phil Fralick (Lakehead University) and Denver Stone (OGS) and Structural geology along the Quetico fault led by Howard Poulsen (Consultant) and Dyanna Czeck (UW-Milwaukee). On Friday afternoon following the completion of the technical session, two field trips were offered. The first took participants on a boat trip through the Archean geology of Voyageurs National Park and Little America gold mine led by Chris Hemstad (Boreal Explorations) while Mark Jirsa (MGS) led a group to the Ash River neutrino detector laboratory and the Archean Vermilion Granitic Complex. Post-meeting trips included a Transect through the Quetico-Wabigoon subprovince boundary led by Mark Jirsa (Minnesota Geological Survey) and Chris Hemstad (Boreal Exploration), Mineral deposits of the Mine Centre – Rainy Lake area led by Peter Hinz (OGS) and last, but not least, Geology and environmental issues of the Steep Rock mine led by Andrew Conly (Lakehead University) and Rob Purdon (Ontario Ministry of Natural Resources). The Institute‘s Board of Directors met on May 20, 2010. The meeting was called to order by Chair Peter Hinz. The meeting was attended by George Hudak, Ted Bornhorst, Jim Miller, Mark A. Jirsa - Treasurer (2011), Pete Hollings, Terry Boerboom and Mark Smyk. Secretary Hollings took the minutes of the Board meeting that are as follows: 1. Accepted report of the Chairs for the 55th ILSG, Ely, Minnesota; as printed in the Proceeding Volume (Miller), and minutes of last Board meeting, May 7, 2009 (Hollings) 2. Received, discussed, and accepted 2009-2010 ILSG Financial Summary (Jirsa). ILSG 2011 xv Program and Abstracts �3. Received, discussed, and accepted 2009-2010 report of the Secretary (Hollings). 4. Approved Hollings to continue as Institute Secretary (this was later presented to the membership and approved) 5. Approved Peter Hinz as on-going ILSG Board member 6. Discussed application process for ILSG Student Research Awards. Hollings to revise the application to be a two-page form, not including references, figures or tables. Application dates to remain unchanged 7. Approved Ashland, Wisconsin at the site for the 57th annual ILSG meeting. The meeting will be hosted by Tom Fitz with help from Jim Miller, Laurel Woodruff and Bill Cannon. 8. Discussed and approved replacing Terry Boerboom as the ―member from government‖ on Goldich Committee (end of term 2010) with Laurel Woodruff 9. It was agreed that the Institute would adopt the following policy ―The Institute will not distribute email advertisements for ‗for-profit‘ organizations or individuals. The institute will post links for such organizations on the Institute web site and will include a logo and brief notice in the annual proceedings in return for a donation‖. Hollings to prepare a new page on the web site highlighting institute sponsors and donors. The co-chairs would like to thank all the sponsors for their support of the meeting, all those who helped us organize and run the meeting, the Backus Community Center who put together a magnificent banquet on very short notice using community volunteers, and to all presenters and participants without whom the meeting could not have happened. We never cease to be amazed by the patience, enthusiasm, and passion of the membership of the ILSG. Respectfully submitted, Pete Hollings, Mark Smyk, Peter Hinz, Terry Boerboom and Mark Jirsa Co-Chairs, 56th Institute on Lake Superior Geology ILSG 2011 xvi Program and Abstracts �2011 BOARD OF DIRECTORS Board appointment continues through the close of the meeting year indicated in parentheses, or until a successor is selected. Tom Fitz, Chair (2011-2014) Northland College Ashland, Wisconsin Peter Hinz, (2010-2013) Ministry of Northern Development, Mines and Forestry Thunder Bay, Ontario George Hudak, (2009-2012) NRRI, Duluth Ted Bornhorst, (2008-2011) Michigan Technological University Houghton, Michigan Mark A. Jirsa, Treasurer (2008-2011) Minnesota Geological Survey St. Paul, Minnesota Peter Hollings, Secretary (2010-2013) Lakehead University Thunder Bay, Ontario 2011 SESSION CHAIRS Marcia Bjørnerud, Lawrence University Bill Cannon, U.S. Geological Survey Todd LaMaskin, Wisconsin Geological Survey Mark Jirsa, Minnesota Geological Survey George Hudak, Natural Resources Research Institute Jim Miller, University of Minnesota – Duluth Peter Hollings, Lakehead University Mark Smyk, Ontario Geological Survey 2011 STUDENT PAPER AWARDS COMMITTEE Elizabeth Gordon, University of Wisconsin – Parkside (Chair) Laurel Woodruff, U.S. Geological Survey James Shannon, MMG – Minerals and Metals Group ILSG 2011 xvii Program and Abstracts �Thursday Evening Awards Banquet with Keynote Address by: Dr. Huifang Xu (University of Wisconsin – Madison) Banded Iron Formations: Unique Products from the Interactions between Komatiites and Seawater of the Early Earth ILSG 2011 xviii Program and Abstracts �PROGRAM ILSG 2011 xix Program and Abstracts �SCHEDULE OF EVENTS TUESDAY, MAY 17 4:00 p.m. – 9:00 p.m.: Registration 4:00 p.m. – 9:00 p.m.: Cash bar is open WEDNESDAY, MAY 18 (ALL TRIPS DEPART AND RETURN TO THE AMERICINN) 7:30 a.m. – 6:00 p.m. Field Trip 1 – Duluth Layered Series trip (Rendezvous in Duluth at 9:00) 8:00 a.m. – 6:00 p.m. Field Trip 2 – Midcontinent Microcosm Field Trip 3 – Bayfield Peninsula 4:00 p.m. – 10:00 p.m.: Registration continues 7:00 p.m. – 10:00 p.m.: Poster Session, Ice-breaker social at AmericInn THURSDAY, MAY 19 8:00 a.m. – 12:00 p.m.: 8:10 a.m. – 11:40 a.m.: 11:40 a.m. – 1:15 p.m.: 1:15 p.m. – 4:40 p.m.: 6:00 p.m. – 7:00 p.m.: 7:00 p.m. – 9:30 p.m.: Registration Continues Technical Session I Lunch break. Lunch on your own. Technical Session II Social mixer and cash bar Banquet, awards ceremony, and keynote address FRIDAY, MAY 20 (ALL TRIPS DEPART AND RETURN TO THE AMERICINN) 8:00 a.m. – 11:40 a.m.: Technical Session III 11:40 a.m. – 1:15 p.m.: Lunch break. Lunch on your own. 1:15 p.m. – 2:40 p.m.: Technical Session IV 3:00 p.m. – 6:00 p.m.: Field Trip 4 – Ashland Lakefront Site Field Trip 5 – Bad River Watershed Culverts Field Trip 6 – Copper Falls Hike 6:30 p.m. – 9:00 p.m.: Barbecue at Northland College. Shuttle vans to/from AmericInn provided. SATURDAY, MAY 21 (ALL TRIPS DEPART AND RETURN TO THE AMERICINN) 8:00 a.m. – 5:00 p.m.: Field trip 7 – Montreal Monocline Field Trip 8 – Unconformity at Denham, MN Field Trip 9 – Mellen Complex End of meeting. ILSG 2011 xx Program and Abstracts �SCHEDULE OF TALKS AND POSTER SESSIONS THURSDAY, MAY 19 Technical Session I: Regional geology and geophysics, Archean stratigraphy and structure. Session Co-chairs: Marcia Bjørnerud and Bill Cannon 8:10 Introductory comments 8:20 Bruce A. Brown and Peter Schoephoester Preserving Wisconsin’s mining history: Building a GIS-based mined-lands inventory as a resource for planning, environmental management, future mineral exploration, and historical research 8:40 Val W. Chandler and Richard S. Lively Application of the horizontal-to-vertical spectral ratio (H/V) passive seismic method in Minnesota 9:00 Mark A. Jirsa, Terrence J. Boerboom, Val W. Chandler, John H. Mossler, Anthony C. Runkel, and Dale R. Setterholm Highlights of the new Bedrock Geologic Map of Minnesota 9:20 John M. Esch Bedrock topography, glacial drift thickness and bedrock outcrop maps, Upper Peninsula of Michigan 9:40 Coffee Break and Poster Session 10:00 H. Paul Gilbert Stratigraphic investigation of the Neoarchean Bird River Belt, Southeast Manitoba, Canada 10:20 Paul Geller*, Phil Fralick, Mary Louise Hill, and Patricia Gillies A review of the Western Superior Lithoprobe Line 3 10:40 Brittney Swoffer* and Mary Louise Hill A deformation history of the Ivanhoe Lake Fault 11:00 Robert J. Scott*, Maura J. Kolb, and Mary Louise Hill Comparison of microscale and regional scale structural control on gold mineralization in the North Caribou greenstone belt, Superior Province, northwestern Ontario 11:20 Victoria Stinson* and Mary Louise Hill Microstructural control on gold mineralization in greenschist to amphibolite facies metamorphism, Greenstone, Ontario ILSG 2011 xxi Program and Abstracts �11:40 a.m. – 1:15 p.m.: Lunch break. Lunch on your own. Technical Session II: Paleoproterozoic geology and mineral deposits Session Co-chairs: Todd LaMaskin and Mark Jirsa 1:15 Introductory Comments 1:20 Amanda Van Lankvelt*, David Schneider , Keiko Hattori, and John Biczok Neoarchean magmatism in the NW Superior Craton: Granitoids of the North Caribou Terrane 1:40 Michael A. DeVasto*, Dyanna M. Czeck, and Prajukti Bhattacharyya Quantifying deformation fabrics and chemistry across small granitic ductile shear zones from Mountain, Wisconsin 2:00 Thomas W. Buchholz, Al Falster, and William B. Simmons Compositional trends in chevkinite group minerals from the Wausau Complex, Marathon County, WI. 2:20 Todd A. LaMaskin Testing models of late Paleoproterozoic Penokean orogenesis in the Great Lakes Region, U.S.A. using sedimentary provenance: planning the investigation 2:40 Coffee Break and Poster Session 3:00 Michael R. Easton, and Larry M. Heaman Detrital zircon geochronology of Matinenda Formation sandstones (Huronian Supergroup) at Elliot Lake, Ontario: Implications for uranium mineralization 3:20 Phillip Larson, John Swenson, and Marsha Patelke The Biwabik Iron Formation: geochemical and textural evidence for deposition of iron-formation in a Paleoproterozoic epeiric sea 3:40 Steph Theriault*, Jim Miller, Mike Berndt, and Ed Ripley The mineralogy, spatial distribution, and isotope geochemistry of sulfide minerals in the Biwabik Iron Formation 4:00 Michael Totenhagen*, Penny Morton, and Phil Larson Characterization of gangue minerals in lower cherty ores of the Biwabik Iron Formation at United Taconite LLC 4:20 Thomas Waggoner, Doug Duskin, Musa Karakus, and John Gartner Pyroclastic Magnetite Bombs in the Hemlock Formation, Iron County, Michigan 6:00 – 7:00 ILSG 2011 Poster Session during Social Mixer xxii Program and Abstracts �FRIDAY, MAY 20 Technical Session III: Paleoproterozoic to Mesoproterozoic geology and mineral deposits. Session Co-chairs: George Hudak and Jim Miller 8:10 Introductory comments 8:20 Breanne Beh* and Phil Fralick Depositional Processes Operating on the Paleoproterozoic Gowganda Ice Margin: an example from the Marquette, Michigan area 8:40 Philip Fralick and Kamil Zaniewski Sedimentology of a wet, pre-vegetation floodplain assemblage: the Mesoproterozoic Outan Island Formation, Sibley Group, Ontario, Canada 9:00 Carl Christian*, Peter Hollings, and Mark Smyk Chemistry and petrology of Midcontinent Rift-related intrusive rocks of the Sibley Peninsula, Ontario 9:20 Robert Cundari*, Peter Hollings, and Mark Smyk Compilation and reevaluation of the geochemistry of Midcontinent Rift-related intrusive rocks around Thunder Bay, Ontario 9:40 Coffee Break and Poster Session 10:00 Mark Smyk, Peter Hollings, and Robert Cundari The Pillar Lake volcanics: new insights into an enigmatic Mesoproterozoic volcanic suite near Armstrong, Ontario 10:20 Dayton N. Ryan*, James D. Miller, and Jeffery D. Vervoort The geochemical evolution of the Sonju Lake intrusion: an assimilation-fractional crystallization model 10:40 Dan Cervin*, Penny Morton, Jim Miller, and Richard Patelke Characterization of precious metal occurrences in the NorthMet deposit of the Partridge River Intrusion, Duluth Complex, Minnesota, USA 11:00 Daniel J. Foley*, and James D. Miller Petrology and Cu-Ni-PGE mineralization of the Bovine Igneous Complex, Baraga County, Northern Michigan 11:20 Erik R. Tharalson* and Thomas Monecke Geology and mineralization of the Serpentine Cu-Ni deposit, Duluth Complex Minnesota 11:40 – 1:15: Lunch break. Lunch on your own. ILSG 2011 xxiii Program and Abstracts �Technical Session IV: Midcontinent Rift mineral deposits Session Co-chairs: Peter Hollings and Mark Smyk 1:15 Introductory comments 1:20 Brian D. Goldner and James D. Miller Petrology of the Ni-Cu-PGE-mineralized Tamarack Intrusion, Aitkin and Carlton Counties, Minnesota 1:40 Allan D. MacTavish, Geoffrey J. Heggie, V. Roland Goodgame, Justin R. Johnson, Anthony E. Beswick, William E. Stone, and Keith P. Watkins Platinum-Palladium-Copper-Nickel Mineralization at the Thunder Bay North Deposit, Ontario 2:00 Dean Peterson, Phil Larson, Gabriel Sweet, and Jack Gibbons The Mineral Exploration Trifecta 2:20 Presentation of student awards * Indicates eligibility for Student Paper Award. If presenter is not the first author, then the name of the presenter is underlined. POSTER PRESENTATIONS Regional Geology and Geophysics 1) Val W. Chandler, and Richard S. Lively Upgrade of the gravity and rock properties databases at the Minnesota Geological Survey 2) Mark A. Jirsa, Terrence J. Boerboom, and Val W. Chandler Bedrock Geologic Map of Minnesota—Precambrian Geology Archean and Paleoproterozoic Geology 3) Jakob Wartman*, Ron Morton, George Hudak, and Cory Hercun Physical volcanology and hydrothermal alteration of the Rainy River Gold Project, Northwestern Ontario 4) Ryan Rague and Steven Losh Fluid flow events in the Biwabik Iron Formation, Minnesota ILSG 2011 xxiv Program and Abstracts �5) Brittany Saylor*, Jonathan Stencil, Michael DeVasto, and Prajukti Bhattacharyya Whole rock geochemical analyses of sheared granitic rocks from Mountain, Wisconsin 6) Jolene T. Traut* and Dyanna M. Czeck Using cleavage refraction and microstructural evidence to determine the relative rheology of naturally deformed quartzites and phyllites from Baraboo, Wisconsin Midcontinent Rift Rocks 7) Terrence J. Boerboom and John C. Green Bedrock geologic maps of the Grand Marais and Kadunce River Quadrangles, North Shore of Minnesota 8) Eric J. Carlson, Terrence J. Boerboom, Corey J. Holton, Kyle W. Kubitza, Lucy Mulvey, and Eric Scheurer Mesoproterozoic bedrock in the Kadunce River Quadrangle, NE Minnesota— Precambrian Research Center capstone 9) Katherine Cummings* and Marcia Bjørnerud Possible eukaryotic macrofossils in the 1.1 Ga Copper Harbor Formation, Michigan 10) Evgeniy V. Kulakov, A.V. Smirnov, and J.F. Diehl Paleomagnetism and Paleointensity as recorded by 1.08 GA Lake Shore Traps (Keweenaw Peninsula, Upper Michigan) 11) Elisa J. Piispa, Aleksey V. Smirnov, and Lauri J. Pesonen Paleomagnetism of Midcontinent Rift rocks from the northern shore of Lake Superior (Ontario, Canada): Preliminary results Midcontinent Rift Mineral Deposits 12) Laetitia Amisse* and Sarah J. Barnes Role of Te-As-Bi-Sb (TABS) in the distribution of PGE and formation of PGM: Application to the Duluth Complex 13) Matthias Queffurus* and Sarah J. Barnes The use of S/Se ratios in magmatic Ni-Cu-PGE sulfide deposits and its implications for exploration: Example of the Duluth Complex, Minnesota, USA 14) Cabin Ross*, George Hudak, Ron Morton, Tom Quigley, and Bob Mahin Preliminary stratigraphy and physical volcanology associated with the Paleoproterozoic Backforty VMS deposit, Menominee County, Michigan ILSG 2011 xxv Program and Abstracts �15) Jillian E. Votava and Theodore J. Bornhorst Towards a geochemical characterization of native copper ores of the Keweenaw Peninsula, Michigan Quaternary Geology and Geological Education 16) Maverick Deschamp* and Jennifer McGuire The uptake of lead by Brassica rapa; implications for hunting in agricultural fields 17) Corrie T. Floyd*, Kent M. Syverson, and Christina M. Hupy Using LiDar data and ArcGIS to evaluate subtle glacial landforms associated with the early Chippewa and Emerald Phase ice-margin positions, Barron County, Wisconsin 18) George Hudak, Stephen Monson Geerts, Larry Zanko, April Severson, Allison Severson, and Bryan Bandli The Minnesota Taconite Workers Health Study: Environmental study of airborne particulates - 2011 update 19) William Bajjali, Cole Holstrom, and Dan Fuller Comparing sources of high salinity of two Urban Streams and their effects on Superior Bay in City of Superior 20) Stephen Mattox Using Credit-by-Exam to Connect Advanced High School Geology Courses to University Geology Programs: Lessons Learned from a State-wide Pilot Study in Michigan Precambrian Research Center Field Camp 21) Ryan Birkmeier, Tyler Boley, Brittnee Brannan, Ryan Doucette, Mark Jirsa, and Aubrey Lee Geologic mapping of Neoarchean rocks near Ogishkemuncie Lake, by students of the Precambrian Research Center’s 2010 field camp 22) Jim Miller, Ben Brooker, Max Hadley, Levi Markwood, Jeff Olson, and Alex Tomlinson 2010 Precambrian Field Camp Mapping in the Jack Lake Area, Cook County, Northeastern Minnesota ILSG 2011 xxvi Program and Abstracts �23) Amy L. Radakovich, Charlie T. Parent, , Molly E. Partridge, Andrew D. Ritts, Rita Pierce, and George J. Hudak Reconnaissance Bedrock Geological map of the northern part of Sudan Underground Mine State Park and the northwestern part of Lake Vermilion State Park, St. Louis County, Minnesota 24) Alexandra M. Vallowe, Ernest J. Thalhamer, Damon L. Rhoades, and Dean M. Peterson Surface and subsurface geologic maps of the Soudan Underground Mine State Park, St. Louis County, Northeastern Minnesota * Indicates eligibility for Student Paper Award ILSG 2011 xxvii Program and Abstracts �ILSG 2011 xxviii Program and Abstracts �ABSTRACTS ILSG 2011 xxix Program and Abstracts �ILSG 2011 xxx Program and Abstracts �Role of Te-As-Bi-Sb (TABS) in the distribution of PGE and formation of PGM: Application to the Duluth Complex Laetitia Amisse and Sarah J. Barnes Sciences de la Terre, Université du Québec à Chicoutimi, 555 Bld. de l’Université, Saguenay, Québec, Canada G7H 2B1laetitia.amisse1@uqac.ca and SarahJane_Barnes@uqac.ca Platinum-group elements (PGE) are commonly present in minerals such as pyrrhotite, pentlandite and chalcopyrite (BMS) crystallized from sulfide liquid. However, recent studies of deposits have shown that while much of the Re, Os, Ir, Ru, Rh are in solid solution in BMS, Pt and in some cases Pd are absent of these phases. The general observation shows that Pt and Pd are present in platinum-group minerals (PGM) with elements like tellurium, arsenic, bismuth and antimony (TABS). In some cases the PGM are closely associated with BMS but there are some examples where they are not. This raises the question of what role these elements play in collecting PGE. More and more authors think that TABS have an effect on the behavior of Pt and Pd. Some authors (Dare et al., 2010) suggest that PGE may crystallize as PGM directly from the sulfide liquid. Others authors (Hanley, 2007; Holwell and McDonald, 2010) suggest that TABS are derived from country rocks and formed an immiscible TABS liquid where collected PGE. However, one difficulty in testing these hypotheses is that TABS especially Bi and Te are present at very low concentrations in crustal rocks and are volatiles, which makes them difficult to determine. Some authors (Hall and Pelchat, 1997) introduce a more sensitive routine method for determining TABS based on hydride generation and inductively coupled plasma-mass spectrometry (HG-ICP-MS). This study will consist of two parts. The first will consist developing the HG-ICP-MS at laboratory of ―Université du Québec à Chicoutimi‖ to obtain better detections limits. This second part will investigate whether TABS have redistributed or collected PGE at the Dunka Road deposit of the Duluth Complex. REFERENCES Dare, S.A.S., Barnes, S.J., and Prichard, H.M., 2010a, The distribution of platinum group elements (PGE) and other chalcophile elements among sulfides from the Creighton Ni– Cu–PGE sulfide deposit, Sudbury, Canada, and the origin of palladium in pentlandite: Mineralium Deposita, doi 10.1007/s00126-010-0295-6. Hall, G.E.M., and Pelchat, J.C., 1997, Determination of As, Bi, Sb, Se, and Te in Fifty Five Reference materials by Hydride Generation ICP-MS: Geostandards Newsletter, v. 21, p.85–91. Hanley, J.J., 2007, The role of arsenic-rich melts and mineral phases in the development of high-grade Pt–Pd mineralization within komatiite-associated magmatic Ni–Cu sulfide horizons at Dundonald Beach South, Abitibi subprovince, Ontario, Canada: Economic Geology, v.102, p. 305–317. Holwell, D.A., and McDonald, I., 2010, A review of the behavior of platinum group elements within natural magmatic sulfide ore systems: the importance of semimetals in governing partitioning behavior: Platinum Metals Review, v. 54, p. 26–36. ILSG 2011 1 Program and Abstracts �Comparing sources of high salinity of two urban streams and their effects on Superior Bay in City of Superior William Bajjali, Cole Holstrom, and Dan Fuller University of Wisconsin-Superior, Department of Natural Sciences, Superior, WI A study of water quality was conducted for Newton and Faxon Creeks that run through an urban area in the city of Superior and discharge into Hog Island Inlet (HII) and Superior Bay, respectively. Faxon Creek was monitored for comparison purposes and recorded salinity concentrations much higher than that of Newton Creek. The chloride concentration was greater than that of Newton Creek and much higher than the secondary maximum contaminant level established by the Environmental Protection Agency. Water salinity of the creeks is attributed mainly to the chloride and sodium concentrations. Both Newton and Faxon Creeks revealed different chemical makeups. Faxon Creek reveals that all ion concentrations in its water are higher than in Newton Creek with the exception of sulfates and nitrates. The average chloride:sulfate ratio is 13.5 times higher in Faxon Creek than in Newton Creek. Nitrate concentrations in Newton Creek are almost 4 times the natural abundance of nitrate in natural water and the phosphorous is 3 to 4 times the recommended concentration value set by EPA. A geochemical model reveals that the water of HII is the result of a mixing ratio of about 90 % of Lake Superior water and 10 % of Newton Creek water. The source of water contaminations in Newton Creek originate mainly from the discharge of treated wastewater from Murphy Oil with minimum contributions from surface runoff. The source of contamination in Faxon Creek is mainly due to urban surface runoff that is influenced by road deicing practices. ILSG 2011 2 Program and Abstracts �Depositional processes operating on the Paleoproterozoic Gowganda ice margin: an example from the Marquette, Michigan area Breanne Beh and Philip Fralick Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1, philip.fralick@lakeheadu.ca Glacial sediments of the Huronian Supergroup outcrop along the north shore of Lake Huron and were likely deposited on what is thought to have been a divergent continental margin (Fralick and Miall 1981; 1989). The glaciogenic Enchantment Lake Formation, part of the Chocolay Group of the Marquette Range Supergroup, is chronostratigraphically correlated to the Huronian Supergroup based on U-Pb age determination on detrital zircon, 2317 6 Ma, and diagenetic xenotime, 2133 11 Ma (Vallini et al. 2005). As the Gowganda is the thickest and most commonly preserved of the three Huronian glacial events it is reasonable to assume it correlates to the Enchantment Lake Formation. Two metamorphosed stratigraphic sections were logged on the north side of the Dead River Basin located near Marquette, Michigan. According to the mapping discussed in a United States Geological Survey professional paper (Puffett 1974), the sections are identified as the Enchantment Lake Formation of the Marquette Range Supergroup. Puffett‘s (1974) map shows that the Enchantment Lake is underlain by Archean rocks to the north, indicating the sections young to the south. The logged sediments were grouped into ten lithofacies: 1) Shale, 2) Siltstone, 3) Siltstone with coarse grains, 4) Diamictite, 5) Fine sandstone, 6) Hummocky cross-stratified sandstone, 7) Coarse sandstone, 8) Granule to small pebble conglomerate, 9) Pebble conglomerate and 10) Boulder lense. These lithofacies are probably a subaqueous glacial outwash fan. The presence of an iceberg dumped boulder mound (Fig 1A) as well as hummocky cross-stratification (Fig 1B) indicates that deposition on the fan was occurring in an open water setting. Siltstone is the most common sediment throughout the logged section. It makes up the matrix of the diamictite as well as the conglomerates. In some locations, it is also interbedded with fine- and coarse-grained sandstone. The fact that the siltstone occurs in conjunction with the majority of the other lithofacies indicates that it is likely the background sediment deposited from suspension settling of subaqueous outwash material. Resedimentation events such as small slumps and debris flows are thought to account for the incorporation of coarse-grained lithofacies in the predominantly siltstone succession. In particular, these resedimentation events probably produced the siltstone with coarse grains lithofacies as these layers represent a mixing of two separate grain-size populations in a manner not compatible with traction current deposition (Fig 1C). Throughout the section, stringers of coarse-grains as well as small lag deposits indicate that current reworking of the sediments has occurred by removing the finer grains (Fig 1D). Currents created by the glacial outwash would have been incapable of creating this winnowing effect because they were sediment ladened. The erosive currents were likely geostrophic currents generated in the open water environment during storm events. ILSG 2011 3 Program and Abstracts �A B C D Figure 1. A) A cobble from the boulder mound compressing underlying sediments. B) Hummocky cross-stratification. C) Siltstone with coarse grains indicated by the arrow. D) A small lag deposit of granule to small pebble conglomerate. REFERENCES Fralick, P.W., and Miall, A.D. 1981, Grant 84: Sedimentology of the Matinenda Formation, in Pye, E.G., ed., Geoscience Research Grant Program, Summary of Research 1980 – 1981: Ontario Geological Survey, Miscellaneous Paper 98, p. 80-89. Fralick, P.W., and Miall, A.D., 1989, Sedimentology of the Lower Huronian Supergroup (Early Proterozoic), Elliot Lake area, Ontario, Canada: Sedimentary Geology, v. 63, p. 127-153. Puffett, W.P., 1974, Geology of the Negaunee quadrangle, Marquette County, Michigan: U.S. Geological Survey, Professional Paper 788, 53 p. Vallini, D.A., Cannon, W.F., and Shulz, K.J. 2005, New age data for the Chocolay Group, Marquette Range Supergroup: implications for the Paleoproterozoic evolution of the Lake Superior and Lake Huron regions: Institute on Lake Superior Geology 51st Annual Meeting, Proceedings and Abstracts, v. 1, p. 64. ILSG 2011 4 Program and Abstracts �Geologic mapping of Neoarchean rocks near Ogishkemuncie Lake, by students of the Precambrian Research Center’s 2010 field camp Ryan Birkmeier1, Tyler Boley1, Brittnee Brannan1, Ryan Doucette1, Mark Jirsa2, and Aubrey Lee1 1 2010 Field Camp Students, Precambrian Research Center, Natural Resources Research Institute, University of Minnesota Duluth, 5013 Miller Trunk Highway, Duluth, Minnesota 55811 2 Minnesota Geological Survey (MGS), University of Minnesota, 2642 University Avenue W., St. Paul, Minnesota 55114 This presentation shows the results of mapping Neoarchean bedrock in the Ogishkemuncie Lake area (Fig. 1) by 5 students of the Precambrian Research Center field camp. Figure 1. Simplified geology of the Cavity Lake fire area (modified from Jirsa and Starns, 2008), showing location of 2010 mapping near Ogishkemuncie Lake. The Precambrian Research Center—a branch of the University of Minnesota, Duluth— conducted its 4th season of field camp in 2010. The final phase of training is a ―Capstone Project‖ that provides students an opportunity to create new bedrock geologic maps in areas of poorly understood geology. The students were charged with mapping the area to help unravel the complexity and construct a sequence of depositional and deformation events ILSG 2011 5 Program and Abstracts �Based on prior mapping, Ogishkemuncie Lake lies along a complex unconformity that is marked locally by paleosaprolite and dissected by faults. The unconformity is overlain by the Ogishkemuncie Lake sequence, consisting of conglomerate and other strata containing sedimentary structures indicating deposition by alluvial fan, fluvial, and fluvial-lacustrine (or marine) processes, and syn- and post-depositional faulting. The unconformity is underlain by the Paulson and Jasper Lake sequences composed of metabasalt, metagabbro, trachyandesite and trachyandesitic volcanic conglomerate. An offset at the south end of the lake between the trachyandesite and volcanic conglomerate units, combined with a shear fabric containing sigma-shaped clasts indicates a component of right-lateral displacement along an anastamosing fault zone in and adjacent to Ogishkemuncie Lake. Carbonate alteration and multiple cycles of quartz veining are pervasive along the fault zone. The exposure of the trachyandesite unit in the southwest and northeast of the lake are evidence of a large synform. The Ogishkemuncie conglomerate contains clasts derived from the Paulson Lake and Jasper Lake sequences, together with abundant clasts of Saganaga Tonalite, recently dated at ~2690 Ma (Driese and others, 2011). The clast-supported, moderately-sorted deposits in the Ogishkemuncie sequence are indicative of braided stream deposition. The sequence also contains lenses of matrix-supported debris flow deposits indicative of alluvial fans. The conglomeratic strata are locally interlayered with units of graded mudstone and sandstone that may represent deposition in standing water in a fluvial-lacustrine system. These findings are consistent with the evolution of a pull-apart basin in which braided streams flowing along the basin axis were temporally dammed by transgressing alluvial fans (Fig. 2). Figure 2. Model representing deposition of the Ogishkemuncie Lake sequence prior to regional deformation and metamorphism (from Driese and others, 2011); north is toward upper right of figure. REFERENCES Driese, S.G., Jirsa, M.A., Ren, M., Sheldon, N.D., Brantley, S.L., Parker, D., and Schmitz, M., 2011, Neoarchean paleoweathering of tonalite and metabasalt: Implications for reconstructions of 2.69 Ga early terrestrial ecosystems and paleoatmospheric chemistry: Precambrian Research, in press. Jirsa, M.A., and Starns, E, Geologic map of the Cavity Lake Fire Area: MGS Open-File Report 08-05. ILSG 2011 6 Program and Abstracts �Bedrock geologic maps of the Grand Marias and Kadunce River Quadrangles, North Shore of Lake Superior, Minnesota Terrence J. Boerboom1 and John C. Green2 1 Minnesota Geological Survey, boerb001@umn.edu 2 University of Minnesota-Duluth, jgreen@d.umn.edu The Minnesota Geological Survey has continued ongoing mapping of the bedrock geology of 7.5‘ quadrangles adjacent to the shore of Lake Superior as part of the USGS STATEMAP program, resulting to date in eighteen published 1:24,000 scale maps from Duluth to beyond Grand Marais, in addition to 10 quadrangles already published under the former USGS COGEOMAP program. The Grand Marais and Kadunce River quadrangles (Boerboom and Green, 2010; Boerboom and Green, 2011) are the most recent of these geologic maps (figures 1 and 2). All maps in this series are available as plotter printed maps, or as PDF and GIS shapefiles at the MGS website (www.mngs.umn.edu). Outcrop mapping was augmented by over 100 sets of high-quality water well cutting samples, collected at 10 foot intervals by McKeever Well Drilling of Little Marais, Minnesota. These provided a crucial glimpse of the volcanic stratigraphy in the third dimension as well as information where the bedrock is poorly exposed. The area of this map lies in the central portion of the northeast limb of the 7-10 km thick North Shore Volcanic Group (NSVG), which include abundant felsic volcanic rocks (e.g. the Devil Track, Maple Hill, Kimball Creek, Devil‘s Kettle, Trout Lake, and unnamed units of rhyolite, and the Kadunce River icelandite) as well as andesitic to mafic lava flows (e.g. the Red Cliff basalts and the Marr Island Lavas). In addition there are several thin hypabyssal diabase dikes and sills emplaced into the volcanic rocks, and near the northern margin of the maps the volcanic rocks are intruded by largely felsic rocks related to the Duluth Complex (e.g. Pine Mountain granophyre). This mapping has refined the volcanic stratigraphy of the NSVG in this area, working down-section towards the base of the northeast limb of the NSVG. This work has also shown that the lavas in and near Grand Marais have been pervasively intruded by diabase dikes, likely along fault planes, that now separate much of the volcanic rocks into isolated blocks which are disheveled and rotated to varying degrees. Minor amounts of hybrid felsic magmas, likely related to emplacement of the diabase, are also present. In keeping with prior work, the NSVG is subdivided into informal lithostratigraphic packages separated by major compositional changes, by intrusions or faults across which correlation is tenuous, or where thick flows or flow sequences form mappable units. The informal lithostratigraphic packages shown on these maps include the Grand Marais felsites (rhyolite and icelandite), the Croftville lavas (which includes the Pincushion Mountain trachybasalt of Green, 2002), the Devil Track and Maple Hill rhyolites, the Red Cliff basalts, the Kimball Creek rhyolite, the Kadunce icelandite, the Marr Island lavas, and the Brule River lavas. The Brule River lavas contain a substantial proportion of felsic volcanic rocks including the Devil‘s Kettle rhyolite (1,097.7 ±1.7 Ma: Davis and Green, 1997), and the newly named Trout Lake rhyolite. The Devil Track and Kimball Creek rhyolites are interpreted as rheoignimbrites with large aspect ratios, i.e. long strike length compared to flow thickness (Green and Fitz, 1993). The aphyric Devil Track rhyolite flow, the largest felsic flow in the NSVG, is approximately 250 meters thick and extends at least 40 km west and inland from the mouth of Devil Track River (and an unknown distance to the east, beneath Lake Superior). This flow is inferred to have been either a hot superliquidus lava flow or possibly a thick, hot rheoignimbrite that flowed and underwent complete recrystallization after deposition (Green and Fitz, 1993). The Kimball Creek rhyolite is estimated to be the second largest felsite flow in the North Shore Volcanic Group, with an aspect ratio similar to the Devil Track rhyolite. It exhibits fiamme textures and tightly folded flow layering which are restricted to the top and bottom of the unit, but the majority of the exposures are fine-grained uniform rhyolite that do not contain recognizable flow structures (Green and Fitz, 1993). ILSG 2011 7 Program and Abstracts �Intrusive rocks in these map areas consist of thin dikes and subconformable sills most likely related in timing to the Beaver Bay Complex, and deeper-seated granophyric rocks that are part of the Pine Mountain granophyre (1,095.3 ±3.8 Ma; Vervoort and others, 2007). Although this long-term mapping project is on hold for the 2011 field season, the Minnesota Geological Survey has plans to resume mapping in 2012, working east up the shore towards the Canadian border. Figure 1 (left). Location map showing the extent of the Keweenawan Midcontinent Rift System in Minnesota, and the locations of the Grand Marais and Kadunce River quadrangles (dark gray boxes). Figure 2 (right). Simplified geologic map of the Grand Marais and Kadunce River quadrangles, showing the names of the major lithostratigraphic units discussed in the text REFERENCES Boerboom, T.J., and Green, J.C., 2010, Bedrock geology of the Grand Marais quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series Map M-189, scale 1:24,000. Boerboom, T.J., and Green, J.C., 2011, Bedrock geology of the Kadunce River quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series Map M-190, scale 1:24,000. Davis, D.W., and Green, J.C., 1997, Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution: Canadian Journal of Earth Science, Volume 34, No. 4, April 1997, p. 476-488. Green, J.C., and Fitz, T.J. III, 1993, Extensive felsic lavas and rheoignimbrites in the Keweenawan Midcontinent Rift plateau volcanics, Minnesota: petrographic and field recognition: Journal of Volcanology and Geothermal Research, v. 54, p. 177-196. Green, J.C., 2002, Volcanic and sedimentary rocks of the Keweenawan Supergroup in northeastern Minnesota, in Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., and Wahl, T.E., Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, p. 94-102. Vervoort, J.D., Wirth, K., and Kennedy, B., 2007, The magmatic evolution of the Midcontinent rift. New geochronologic and geochemical evidence from felsic magmatism: Precambrian Research, vol. 157, no. 1-4, p. 235-268. ILSG 2011 8 Program and Abstracts �Preserving Wisconsin’s mining history: Building a GIS-based mined-lands inventory as a resource for planning, environmental management, future mineral exploration, and historical research Bruce A. Brown and Peter Schoephoester Wisconsin Geological and Natural History Survey, 3817 Mineral Point Road, Madison, WI 53705 A comprehensive statewide inventory of metallic and nonmetallic mining sites has never been compiled for Wisconsin. A GIS-based inventory, including current and historic sites would be useful to planners, developers, and environmental regulators as a guide to potential hazards such as improperly abandoned shafts and drill holes, areas of potential subsidence, sites that may require additional reclamation, and possible sources of surface and groundwater contamination. Access to documents and maps of historical lead-zinc and iron mining areas would also be of interest to historians. A complete inventory of metallic and nonmetallic mining sites could provide a guide to undeveloped resources for the future. Many of the pieces necessary to assemble a statewide inventory already exist or are in various stages of preparation. The WGNHS recently completed scanning, editing, and georeferencing more than 1400 section maps from the Wisconsin Mineral Development Atlas. This data set contains maps of hundreds of underground zinc mines and crevice lead mines dating from the 1820s to 1978. More than 28,000 exploration drilling logs were recently scanned and added to this data set. A Mines, Pits, and Quarries database (MPQ) currently under construction will contain locations, geologic information, and engineering test data for current and recently active aggregate and industrial mineral mines. It will also incorporate the historic WGNHS Road Materials Survey, which provides a record of aggregate mining and exploration activity back to the early 1900s. Historic nonmetallic and metallic sites compiled by the U.S. Geological survey in the Mineral Resource Data System (MRDS) and by the former U.S. Bureau of Mines in the Mineral Availability System and Mineral Industry Location System (MAS/MILS) have been integrated into the MPQ GIS. WGNHS is also in the process of constructing a locational database for the mineral exploration and engineering cores stored at the sample repository in Mt. Horeb, WI. Several important historical data sets still need to be inventoried and evaluated. Examples that will require significant work are the extensive collection of documents and paper maps related to Iron mining and exploration in the Gogebic Range, Florence District, Baraboo District, and the Mayville Ironstone mines. This material has significant local and statewide historical value, and contains locations of long forgotten potential hazards a well as valuable geologic data. We are beginning to sort and evaluate this material to prioritize documents that need to be scanned or digitized to include in the statewide inventory. The age and fragile condition of many of these documents requires that they be handled carefully in the scanning process. The WGNHS does not have staff and facilities for archiving, and after essential data is captured, many original documents are being transferred to the UW Archives or appropriate regional archive centers of the State Historical Society where they can be preserved while still available for research. ILSG 2011 9 Program and Abstracts �Compositional trends in chevkinite group minerals from the Wausau Complex, Marathon County, WI. Thomas W. Buchholz 1, A. U. Falster 2, and W. B. Simmons2 1 1140 12th Street North, Wisconsin Rapids, Wisconsin 54494, 2 Department of Earth and Environmental Sciences, University of New Orleans, New Orleans, Louisiana 70148 The Wausau Syenite Complex (WSC) is composed of four plutons, from the oldest (1565 Ma +3-5 )(Van Wyck, 1994), most alkalic Stettin Pluton, through the younger and progressively less alkalic and more silicic Wausau and Rib Mountain Plutons, to the youngest (1505.9 ± 2.7 Ma) (Dewane & Van Schmus, 2007), most silicic Nine Mile Pluton. Over the last several years chevkinite group minerals have been identified from three sites within the Wausau Complex. Chevkinite group minerals are complex titanosilicates, most of which contain Light Rare Earth Elements (LREE, La to Eu), Ca and Fe as essential components. Extensive substitution often allows the incorporation of significant Nb, Ta, Th, HREE and Zr, among other elements. Recently chevkinite group minerals (probably either chevkinite-(Ce) or perrierite-(Ce) have been identified from three sites within the Wausau Complex. The first occurrence noted is at a site just north of the Aspirus Hospital in Wausau, where phases of the Wausau Pluton near a contact with a large quartzite xenolith were exposed by quarrying to cut back a hillside. Small pegmatoidal veins and clots are exposed in the approximate center of the cut and are largely composed of feldspars, quartz, an unidentified amphibole, zircon and locally abundant grains of a usually more-or-less altered chevkinite-group mineral. Accessory minerals include apatite, monazite and allanite-(Ce). Both mafic and more felsic phases of the veins carry chevkinite-group minerals. The grains generally present a mottled, altered appearance that is probably the result of post-emplacement hydrothermal alteration. These altered grains range in color from dark yellow through reddish hues to dark glassy black. Highly altered grains are chalky white in color. Careful examination of heavy mineral separates nonetheless resulted in finding unaltered material suitable for analysis. The second occurrence is in a long-abandoned and overgrown quarry or borrow pit located near the SE corner of CTH O and U in the Stettin Pluton. The small, poorly exposed pegmatites at this site include sparse, small glassy brown prismatic crystals of a chevkinitegroup mineral associated with quartz, microcline, albite, biotite, aegerine, ilmenite, probable rhabdophane-(Ce), unusually heavy-rare-earth-element enriched xenotime-(Y), an unidentified amphibole, fayalite, an euxenite-like Y-Nb oxide phase, and magnetite. The third occurrence is in heavy mineral separates prepared from a fine-grained, miarolitic phase of a locally megacrystic mafic dike cutting Nine Mile granite in the Ladick quarry. Clearly younger than the host granite, the mafic dike may be similar in age to a dike of similar material in the Koss quarry, classified as meladiorite, with a K-Ar age of 1307.2 +/- 41 Ma (Cordua, 2004). Associated minerals include amphiboles, biotite, chlorite, fluorapatite, and very rare columbite- and monazite-group species. Most samples examined are metamict, so no XRD determination could be made as to whether the chevkinite-group minerals are chevkinite-(Ce) or perrierite-(Ce), two chemically similar members. However, the samples were analyzed by electron microprobe (EMP), and when CaO vs. FeO is plotted as suggested by MacDonald & Belkin (2002) the compositions ILSG 2011 10 Program and Abstracts �from all three sites clearly fall into the chevkinite field, providing strong evidence that the mineral in question in all three cases is probably chevkinite. EMP analysis also shows that chevinikite-group minerals from each site exhibit a distinct chemical composition based on the rations of Nb + Ta versus Ti + Fetotal (Fig. 1). The most Nb + Ta-depleted samples are from the ―Ladick meladiorite‖. Samples most enriched in Nb + Ta are from the ―Aspirus felsic syenite phase‖, whereas those from the ―Aspirus melasyenite phase‖ and the ―Stettin syenite‖ exhibit intermediate Nb + Ta contents. While the range in chemical variation is relatively small, it likely reflects the abundance of these elements in the respective melts, thus the degree of fractionation. A chondrite-normalized plot of LREE contents shows overall relatively small variations in the LREE; all sites track rather closely with the exception of Eu. The observed greater Eu anomaly for the Stettin and Ladick samples is likely due to greater substitution of Eu2+ in plagioclase. Hence, the ―Ladick meladiorite‖ dike, though probably significantly younger and not part of the WSC, is chemically similar and thus may have originated from a similar source as the intrusive centers of the WSC. REFERENCES Cordua, William S., 2004, Enigmatic 1300 – 1400 Ma Mafic Pluton from the Koss Pit, Marathon County, WI [abstract]: Institute on Lake Superior Geology Proceedings, 50th Annual Meeting, Duluth, MN, v. 50, part 1, Program and Abstracts, p. 48-49 Dewane, T. J., Van Schmus, W. R., 2007, U-Pb geochronology of the Wolf River batholith, north-central Wisconsin: Evidence for successive magmatism between 1484 Ma and 1468 Ma: Precambrian Research, v. 157, p. 215-234. Macdonald, R., Belkin, H.R., 2002, Compositional variation in minerals of the chevkinite group: Mineralogical Magazine, v. 66, p. 1073-1098. Van Wyck, N., 1994, The Wolf River A-type magmatic event in Wisconsin: U/Pb and Sm/Nd constraints on timing and petrogenesis. [abstract]: Institute on Lake Superior Geology, 40th Annual Meeting, Houghton, MI, v. 40, part 1, Program and Abstracts, p. 81-82. ILSG 2011 11 Program and Abstracts �Geochemistry and petrology of Midcontinent Rift-related intrusive rocks of the Sibley Peninsula, Ontario Christian Carl1, Peter Hollings1, and Mark Smyk2 1 Department of Geology, Lakehead University 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada 2 Ontario Geological Survey, Ministry of Northern Development, Mines and Forestry, Suite B002, 435 James St. South Thunder Bay, ON P7E 6S7 Canada The 300 km2 Sibley Peninsula projects south into Lake Superior approximately 30 km east of Thunder Bay, Ontario. The geology of the peninsula consists of Southern Province assemblages, including intrusive rocks of the ~ 1.1Ga Midcontinent Rift. Due to their location within Sleeping Giant Provincial Park, the Sleeping Giant Sill (SGS) and the numerous dikes on the Sibley Peninsula have not been geochemically sampled nor classified. With permission from Ontario Parks, this study was undertaken to classify the Sibley Peninsula dikes and the SGS based on their geochemistry and to determine relative ages of the SGS and dikes of various orientations. Figure 1: Map of sample locations (Hollings et al., 2010). Thirty-four dike samples and five SGS samples were collected and analyzed for whole rock geochemistry at the Geoscience Laboratories (Fig. 1). Dikes with three different orientations were sampled: 1) numerous ~070˚-striking dikes, which represent the majority of ILSG 2011 12 Program and Abstracts �the dikes on the peninsula, 2) two 110˚-striking dikes and 3) two 029˚-striking dikes. The ~070˚-striking dikes were classified as Pigeon River Dikes based on Mg# versus TiO2 and Gd/Ybn versus La/Smn discrimination diagrams (Fig. 2). The 110˚-striking dikes plotted near or within fields defined for ultramafic intrusions of the MCR (Hollings et al., 2007). However, due to their orientation, they are unlikely to be feeders to any of the ultramafic assemblages found in the Nipigon Embayment, northeast of the study area. The 029˚striking dikes also plotted in the field defined for mafic / ultramafic sills and intrusions on the Mg# versus TiO2 plot and in the field defined for Nipigon Sills on the Gd/Ybn versus La/Smn diagram (Fig. 2). Although these dikes are geochemically similar to Osler volcanic rocks of the neighbouring Black Bay Peninsula, they were just 18 and 20 cm wide, respectively, and tapered eastward, making them unlikely candidates as feeders to Osler volcanic rocks. Discrimination diagrams show that the SGS is of Logan affinity and now represents the easternmost recognized Logan Sill (Fig. 2). The thickness of the SGS was determined to be 194 m, making this the thickest known Logan Sill. (A) (B) Figure 2. (A) Plot of Gd/Ybn versus La/Smn. Fields from Hollings et al (2007). Normalizing values of Sun and McDonough (1989). (B) Plot of Mg# versus TiO2. Fields from Hollings et al. (2007). Cross cutting relationships indicate that the ~070˚-striking dikes are younger than both the SGS and the 029˚-striking dikes. The SGS and the 029˚-striking dikes had lower silica and higher magnesium weight percentages than the ~070˚-striking dikes, which may suggest that more primitive magmas were locally emplaced earlier. The ~070˚-striking dikes displayed the largest negative niobium anomalies which could indicate an increase in contamination of magma sources over time, since these dikes were the youngest intrusions observed in this study. REFERENCES Hollings, P., Hart, T., Richardson, A., and MacDonald, C., 2007. Geochemistry of the Midproterozoic intrusive rocks of the Nipigon Embayment, Northwestern Ontario. Canadian Journal of Earth Sciences, 44: 1087-1110. Hollings, P., Smyk, M.C. and Carl., C. 2010. Geochemistry of Midcontinent Rift–Related Dikes on the Sibley Peninsula, Thunder Bay: A Preliminary Report; Summary of Field Work and Other Activities 2010, Ontario Geological Survey, Open File Report 6260, p.10-1 to 10-3. Sun, S. and McDonough, W., 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. Magmatism in the Ocean Basins. Geological Society, Special Publication No. 42, pp. 313–345. ILSG 2011 13 Program and Abstracts �Mesoproterozoic bedrock in the Kadunce River Quadrangle, NE Minnesota—Precambrian Research Center capstone Eric J. Carlson, Terrence J, Boerboom, Corey J. Holton, Kyle W. Kubitza, Lucy Mulvey, and Eric Scheurer Precambrian Research Center, University of Minnesota-Duluth, Duluth, MN 55812 The Precambrian Research Center (PRC) field camp culminates in several ‗capstone‘ mapping projects located in different geological terranes in northeastern Minnesota. One of the 2010 PRC capstone projects was focused on mapping an area within the Midcontinent rift system, mainly in lava flows that are part of the North Shore Volcanic Group, intruded by thin diabase dikes and sills likely related to the Beaver Bay Complex. The mapping crew consisted of 4 students (Carlson, Holton, Kubitza, and Scheurer), one teaching assistant (Mulvey) and an instructor (Boerboom). After a day of orientation the students/TA/instructor were broken into three pairs who mapped independently, and the pairings were switched on a daily basis. Since this capstone had the luxury of staying indoors, all outcrop data (outcrop areas, structure, lithology, etc.) was entered into a GIS database file each evening. This capstone project was chosen to augment a concurrent mapping project by the Minnesota Geological Survey in the Kadunce River 7.5‘ quadrangle, which is located on the shore of Lake Superior just east of Grand Marais (figure 1). The information and geologic interpretations obtained by this capstone project were incorporated into a geologic map published by the Minnesota Geological Survey (Boerboom and Green, 2011). Most of the exposures within this capstone project area are located either on the shore of Lake Superior, or in the many small streams which are incised into the adjacent highlands. Bedrock is well exposed in these streams, particularly on the slopes that face Lake Superior, affording an excellent opportunity to view flow features within the strata of the lava flows. Outside of the stream valleys outcrops are restricted to scattered exposures in topographically high areas, which are typically overgrown clearcuts of various generations. All of the students had a turn locating not only the sporadic outcrops, but also ourselves, in the highland bush country. The map area is dominated by felsic lava flows, including the Devil Track and Maple Hill rhyolites, the Kimball Creek rhyolite, and the Kadunce icelandite. In addition to these felsic flows, the map area included the entire thickness of the Red Cliff basalt flows, and the uppermost basalt flow of the Marr Island lavas, a thick series dominated by volcanic rocks of andesitic composition that also includes some thinner flows of tholeiitic basalt. The stratigraphy of the volcanic pile is more or less perpendicular to the trend of the stream valleys, which enabled us to correlate map units from one stream to the next. One of the challenges was the seemingly simple task of differentiating the Kadunce icelandite from the overlying Kimball Creek rhyolite, since both have very similar macroscopic textures with nearly identical phenocryst assemblages and only subtle variations in the textures of the matrix. As mappers we learned to identify volcanic flow features such as amygdaloidal flow tops, pipe vesicles, etc., and subsequently identified several flow contacts within the individual units. During the map compilation stage we were supplied aeromagnetic data that was used to infer the extension of small diabase dikes exposed in the river channels; however we were slightly overzealous in using the magnetic data and did end ILSG 2011 14 Program and Abstracts �up accidentally incorporating some areas of known volcanic exposures into the diabase dike units. Data collected during field work, including outcrop locations, lithology, structure, flow contacts, etc., was digitized using Arcmap software, which was also used for construction of the final geologic map. Figure 1 (left). Location map showing the extent of the Keweenawan Midcontinent Rift System in Minnesota, and the locations of the capstone mapping project (Kadunce River quadrangle). REFERENCE Boerboom, T.J., and Green, J.C., 2011, Bedrock geology of the Kadunce River quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series Map M-190, scale 1:24,000. ILSG 2011 15 Program and Abstracts �Characterization of precious metal occurrences in the NorthMet deposit of the Partridge River Intrusion, Duluth Complex, Minnesota, USA Dan Cervin1, Penny Morton1, Jim Miller1, and Richard Patelke2 1 Department of Geological Science, University of Minnesota Duluth, Duluth, MN, 55812 2 PolyMet Mining, PO Box 475, County Road 666: Hoyt Lakes, MN 55750 The NorthMet deposit is a Cu-Ni-PGE magmatic sulfide ore body located along the northwestern margin of the Partridge River Intrusion (PRI), which is part of the 1.1 Ga Duluth Complex. PolyMet Mining Company is currently seeking a permit to develop an open pit mine at the site, which is about 7 miles south the town of Babbitt, MN. During pilot-plant test runs by PolyMet, approximately 75% of the total mass of precious metals (mostly Pd, Pt, Au) known to exist from assay data were recovered; total sulfide recovery was 90%. In a sulfide flotation beneficiation process, it is assumed that precious metals are contained within sulfide minerals as small (micron-sized) platinum group minerals (PGM), as Au-Ag minerals and alloys, or in solid solution. The 75% recovery implies that many precious metal mineral and alloy phases (PMM) may not be hosted by sulfide minerals. Building on a summary report of PGM occurrences in the Duluth Complex by Severson and Hauck (2003), this study seeks to more completely characterize the mineralogical and textural occurrences of PMM in the NorthMet ore feed and concentrates. This information is not only of importance to the beneficiation of NorthMet ores, but also to the understanding of the metallogenesis of PGE-Au in magmatic sulfide deposits. The energy dispersive spectrometer-equipped scanning electron microscope at University Minnesota Duluth was used in backscatter electron composition mode (BEC) to conduct detailed compositional scanning of polished thin sections to locate PMM. As the 75% precious metal recovery would predict, NorthMet PMM primarily occur in association with sulfide minerals. Of the 347 PMM investigated in this study, 234 (67%) were hosted by sulfide minerals (mostly chalcopyrite and pentlandite), either as inclusions or at sulfide grain boundaries (Fig. 1). The remaining 33% (133) of PMM were found in a variety of primary silicates, secondary silicates, and apatite (Fig. 1). Forty-four percent of sulfide-hosted PMM are located at sulfide grain boundaries, 56% occur as inclusions in sulfide. However, 3D studies by Godel et al. (2010) indicate that most PGM observed as inclusions in sulfides in 2D studies are in fact boundary occurrences. Boundary PMM are susceptible to isolation from a sulfide host by silicate alteration minerals, these isolated PMM are presumably unrecoverable by sulfide flotation beneficiation processes. The preliminary conclusions of this study are: 1) The lower recoveries of precious metals relative to base metals in NorthMet ores is largely due to PMM boundary occurrences being lost to tailings because they are isolated in gangue minerals or are broken off of sulfide grains during comminution. 2) The occurrence of PMM in secondary silicates (chlorite, amphibole, sericite) that appear to be replacing sulfide minerals implies that PGM-Au-Ag can be isolated from a sulfide host by post-magmatic deuteric fluids causing sulfide dissolution. 3) PMM that appear to be hosted in primary silicates occur in very close association with fine-grained, interstitial sulfide minerals (generally <0.5 mm distant). Many PMM that appear to be hosted in primary silicates in SEM BEC images most likely have a sulfide association that is not always apparent in 2D SEM imagery. A preliminary spot check of primary silicate PMM occurrences with transmitted and reflected light microscopy indicates ILSG 2011 16 Program and Abstracts �that sulfides, which are not visible in SEM images, are visible as opaque minerals below the surface of the polished thin sections. Additional petrographic study is needed to confirm whether PMM actually occur in primary silicates without associated sulfide or secondary minerals, for if this is the case, this would have profound implications for the metallogenesis of PGE. Figure 1: Frequency of PMM occurring in various types of sulfide and silicate minerals in NorthMet ores; cp=chalcopyrite, pn=pentlandite, cb=cubanite, tn=talnakhite, bn=bornite; High Plag= An >60, Low Plag=An <60, cpx=clinopyroxene, and opx=orthopyroxene. REFERENCES Godel, B., Barnes, Sara J., Barnes, Stephen J., Maier, W.D., 2010, Platinum Ore in Three Dimensions: Insights from High Resolution X-ray Computed Tomography; Geology, v. 38, no. 12, pp.1127-1130. Severson, M., Hauck, S., 2003, Platinum Group Elements and Platinum Group Minerals in the Duluth Complex, Natural Resources Research Institute Technical Report NRRI/TR-2003/37,University Minnesota Duluth, 312 p. ILSG 2011 17 Program and Abstracts �Application of the horizontal-to-vertical spectral ratio (H/V) passive seismic method in Minnesota Val W. Chandler and Richard S. Lively Minnesota Geological Survey, 2642 University Ave., St. Paul, MN 55114 chand004@umn.edu Determining the thickness of unconsolidated materials over bedrock is a common and sometimes difficult geologic problem. It is especially difficult in areas of Minnesota where glacial deposits may be over 200 meters thick and few drill holes penetrate to bedrock. Test drilling in such deep sediment can be prohibitively expensive, seismic soundings can also be slow and costly, and most electrical resistivity methods do not penetrate deeply enough. Recent technological advances in seismometers has opened a window for passive seismic data acquisition, also known as the horizontal-to-vertical -spectral ratio method (HVSR or H/V). This method relies solely on ambient seismic noise, and shows great promise for rapidly and inexpensively estimating the thickness of unconsolidated deposits over bedrock. The H/V method uses continuous, background vibrations in the subsurface to estimate the fundamental resonant frequency of a layer of unconsolidated materials. Data collection and processing was developed (largely in Europe) to delineate areas of high-risk for earthquake damage (SESAME, 2004). If the acoustic impedance (density*seismic velocity) at the overburden-bedrock contact differs by a factor of at least 2, the thickness (Z) of the unconsolidated materials can be estimated by the relationship: Z=af0b Where f0 is the fundamental resonant frequency (for shear waves) of the unconsolidated sequence, and a and b are parameters that are empirically determined for a given region. Although the theory behind the H/V method is still developing, it appears that the behavior of shear waves (transversely vibrating body waves) and Rayleigh waves (backwardrolling surface waves) near the fundamental resonant frequency of the overburden layer are the principal components. At this frequency, the horizontal component of shear waves tend to maximize and the vertical component of Rayleigh waves tend to minimize. When the average horizontal spectrum of seismic noise is divided by the corresponding vertical spectrum, a pronounced peak results at the resonant frequency. This resonant frequency is used in the above equation to compute an overburden thickness. In Minnesota, passive seismic data have been acquired at over 180 sites in northeastern, east-central, and southeastern parts of the state, with encouraging results. Through analysis of data from 34 sites with bedrock control from drill holes, we have obtained parameters for the above equation (a=130.96, b=-1.3304) specific to Minnesota. Estimates from the data of depth to bedrock have an error of 15-20% over depths ranging from a few meters to greater than 150 meters. We have found that the H/V method must be used with appropriate caution. For example, some relatively poor H/V results are obtained in parts of southwestern Minnesota, where thick regolith and unusually dense tills may complicate the simple, sharply contrasting 2-layer model that is preferred for this kind of analysis. Although, the H/V method cannot match seismic refraction profiling for overall accuracy and amount of derived information ILSG 2011 18 Program and Abstracts �per line, the advantage of passive seismic is seen in rapid data collection, low field deployment costs, and ease of data analysis. For example, at a bedrock depth of around 500 feet, H/V depth estimates are obtained within 20 minutes, and are accomplished by a single individual and possibly a vehicle. In comparison, only one or two seismic refraction soundings per summer day would be possible at these depths, and this usually with a crew of 3-4 individuals, at least two large vehicles, and an energy source. Finally, a 3 component, broadband seismograph that is designed for H/V surveying is small, light weight and costs a fraction of that required for a conventional multi-channel seismic profiling system. All things considered the H/V method should prove to be a powerful tool for geologic and depth to bedrock studies, and geo-engineering investigations in the Lake Superior region. ACKNOWLEDGEMENTS Professor Justin Revenaugh at the University of Minnesota, and the IRIS consortium generously supported the pilot stages of this study in various ways and information on equipment and techniques from John Lane of the U.S. Geological Survey is greatly appreciated. This study was supported by the Minnesota Legislature through the County Geologic Atlas Program of the Legislative and Citizens Commission on Minnesota Resources. Additional support was through the State Special Appropriation of the Minnesota Geological Survey. REFERENCES SESAME (Seismic Effects assessment using Ambient Excitations), 2004, Guidelines for the implementation of the H/V spectral ratio technique on ambient vibrations. Measurements, processing and interpretation, WP12 European commission - Research general directorate project no. EVG1-CT-2000-0026 SESAME, report D23.12, 62 pp. ILSG 2011 19 Program and Abstracts �Upgrade of the gravity and rock properties databases at the Minnesota Geological Survey Val W. Chandler and Richard S. Lively Minnesota Geological Survey, 2642 University Ave., St. Paul, MN 55114 chand004@umn.edu Gravity and rock property data have been a significant resource for geological investigations in Minnesota. Used in conjunction with the upgraded, aeromagnetic database, the gravity and rock property data at the Minnesota Geological Survey (MGS) have been foundational to mapping of Precambrian bedrock geology. During 2009-2011 the MGS has upgraded these databases, with significant additions of new and preexisting data that have become available over the last decade as well as improving the location information for better usage in a digital environment. Much of the upgrade of the gravity database involved relocating stations with subsequent recalculation of the Bouguer Anomaly. Many of the older stations had location errors of 200 meters or more made when the data was originally digitized from the field maps. Field numbers and other logistical information were lost during this early digitization. To remedy this, the original gravity location data were imported into an ArcGis environment. The points were compared with locations shown on the original field maps, which were usually located at spot elevations. The point locations were moved to match what was shown on the field maps. The original station numbers were also restored and, if available, other information was added, including observers, date of acquisition, and the meter that was used. If the field materials were missing, the stations were at least moved to their most likely position along roads and trails. The re-computed Bouguer anomaly values equate to differences of up to 0.15 milligals for most of the pre-existing stations, although differences up to 0.25 milligal error exist for some of the oldest stations. In addition 2,000 new gravity stations were added to the database. Of these, 878 stations were acquired at spacings of 100250 meters in 1979 along seismic reflection lines in west-central Minnesota. The remaining stations were acquired in 2003-2010 as part of various Statemap and County Atlas mapping in northeastern and southwestern Minnesota. Many of these latter stations lie within large coverage gaps in the earlier data. The rock properties database has been expanded from 4161 to over 8800 entries. The new database includes determinations for over 5125 densities, 6890 magnetic susceptibilities, and 980 measurements of Natural Remnant Magnetization (NRM). Most entries are based on individual measurements from hand or core samples. The new density data also include 739 determinations that were based on seismic refraction velocities. These produce density estimates for materials that were underrepresented in the previous release, including glacial deposits, Cretaceous rocks, Paleozoic rocks, and Keweenawan sandstones. In addition, results from cuttings and core samples acquired by previous investigators resulted in several thousand magnetic susceptibility and several hundred density measurements being added to the database. Finally, the location of many rock property sample sites have been improved through GIS overlays with detailed topographic bases and precisely mapped locations of outcrop. ILSG 2011 20 Program and Abstracts �The upgraded gravity and rock property databases, which are web-accessible, represent a significant improvement over the previous available versions. They should serve the needs of the MGS and the broader geological community for many years to come. ACKNOWLEDGEMENTS University of Minnesota students Jack Barland and Roger Morrison provided valuable assistance thought much of the data compilation. This study was supported by the Minnesota Legislature through the Minerals Coordinating Committee and the Minnesota Department of Natural Resources. ILSG 2011 21 Program and Abstracts �Possible eukaryotic macrofossils in the 1.1 Ga Copper Harbor Formation, Michigan Katherine Cummings and Marcia Bjørnerud Geology Department, Lawrence University, 711 E Boldt Way, Appleton WI 54911 bjornerm@lawrence.edu Unusual and possibly biogenic cm-scale features have been found on the bedding plane of a siltstone layer within the ca. 1.1 Ga Copper Harbor Formation, the lowermost of the rift-filling sedimentary units of the Midcontinent Rift. The Copper Harbor Formation includes cobble- to boulder-conglomerates, sandstones, siltstones and mudstones, representing alluvial fan, braided stream, and ephemeral lake environments within the axial valley of the Midcontinent Rift (Elmore 1984). Calcified cabbage-sized stromatolites occur at several horizons in the upper part of the Copper Harbor formation; some of these grew on top of a substrate of mud, while others encrusted cobbles and boulders (Elmore, 1094; Planavsky and Bjørnerud, 2006). The newly discovered features were found in a siltstone layer between two stromatolitic horizons at Horseshoe Harbor, near Copper Harbor, Keweenaw County, Michigan. In plan view, the enigmatic features are circular to slightly elliptical with a diameter of 0.3-1.0 cm (Fig.1). Many have a transecting lenticular element or septum that cuts across them in a ‗theta‘ geometry. These septa lie at random orientations in the ca. 25 x 30 cm bedding plane slab in which more than 50 of the features were found. In cross section, the features are funnel-shaped, narrowing downward to a blunt tip. In several of the features, the funnel terminates in a mm-scale crystal of calcite (Fig 2). Finer calcite cements the fine-grained material in the interior of the features and also defines the edges of the funnel shapes. Petrographic and XRD analyses indicate that the rest of the interior material is silt-sized quartz and feldspar together with minor clay (halloysite) and iron oxides, the same minerals found in the surrounding rock. SEM imaging of the material inside and outside the features similarly revealed no obvious differences in grain size or texture. Some of the funnel-shaped elements have a crude layering defined by iron oxides, but it is not easily traceable from the enclosing rock. The layering could either be primary or a product of post-depositional pressure dissolution. These features do not resemble any common sedimentary or diagenetic structures. They are unlikely to be dewatering structures since there is no evidence of ‗soft-sediment‘ distortion of the surrounding layering. Their consistent and unusual shapes make it seem unlikely that they are concretions or similar structures. A National Academy of Sciences workshop (Knoll, 1999) proposed the following criteria for identifying microbial fossils in Precambrian rocks: 1) Provenance [the source and geologic context of the parent rock must be firmly established]; 2) Age [the parent rock must be have a reasonably well constrained age]; 3) Indigenousness [the features must be embedded in the rock matrix]; 4) Syngenicity [the features must have been formed at the same time as the parent rock]; and 5) Biogenicity – the features must be definitively biological. The features from the Copper Harbor Formation meet the first four of these criteria; the last, of course, is the most difficult to establish unambiguously. ILSG 2011 22 Program and Abstracts �Fig. 1: The siltstone slab with unusual bedding plane features. Lower right: Close-up of one feature, highlighting the transecting element. Fig. 2: Thin section (cross-polarized light) of one of the features in cross section, showing calcite crystal at terminal end of the funnel (arrow). If they are biogenic, it seems most likely that the features represent either body or trace fossils of a eukaryotic organism (Andrew Knoll, personal communication, 2010). They are far too large to have been formed by single prokaryotes, and their symmetry and consistent geometry would seem improbable for colonies of individuals. Eukaryotic organisms were diverse and well-established by Mesoproterozoic time in the marine realm, but there are fewer reports of eukaryotic terrestrial biota (Knoll et al. 2006). Fedonkin and Yochelson (2002) described fossils of colonial eukaryotes on bedding surfaces of freshwater siltstones within the 1.5 Ga Appekunny Fm. of the Belt Series in Montana. In a study of the 1.1-1.2 Ga Torridonian sequence of Scotland, similar both in age and depositional setting to the Copper Harbor Formation, Prave (2002) argued for a diverse terrestrial fluvial and lacustrine ecosystem, based on extensive and well-preserved microbial crusts and evidence for unusually cohesive sandy sediment. The Copper Harbor stromatolites indicate that in spite of the high-energy sedimentary environment, there was an active lacustrine ecosystem within the valley of the Midcontinent Rift at ca. 1.1 Ga, perhaps seeded via wind transport of dormant reproductive structures (Planavsky and Bjørnerud, 2002). The newly discovered features described here may indicate an even greater degree of biodiversity than previously recognized. The Midcontinent Rift is already renowned for its unparalleled volcanic effusion rates and worldclass copper deposits; perhaps it should also be re-examined for clues to the early colonization of the continents by Life. REFERENCES Elmore, R., 1984. The Copper Harbor Conglomerate: A late Precambrian fining-upward alluvial fan sequence in northern Michigan. Geol. Soc. America Bulletin 95, 610-617. Fedonkin, M., & Yochelson, E., 2002. Middle Proterozoic 1.5 Ga Horodyskia moniliformis: Oldest Known Tissue-Grade Colonial Eucaryote. Smithsonian Contrib. Paleobiology 94. Knoll, A., ed., 1999. Size Limits of Very Small Organisms: Proceedings of a Workshop. Washington, D.C: National Academy Press. 164 pp. Knoll, A.., Javaux, E., Hewitt, D., and Cohen, P., 2006. Eukaryotic organisms in Proterozoic oceans. Philosophical Transaction of the Royal Society 361, 1023-1038. Planavsky, N., and Bjørnerud, M., 2004. Blowing in the Wind: The Copper Harbor stromatolites revisited. Proceedings of the Institute on Lake Superior Geology 50, 137-138. Prave, A., 2002. Life on land in the Proterozoic: Evidence from the Torridonian rocks of northwest Scotland. Geology 30, 811-814. ILSG 2011 23 Program and Abstracts �Compilation and reevaluation of the geochemistry of Midcontinent Rift related intrusive rocks around Thunder Bay, Ontario Robert Cundari1, Peter Hollings1, and Mark Smyk2 1 Department of Geology, Lakehead University 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada 2 Ontario Geological Survey, Ministry of Northern Development, Mines and Forestry, Suite B002, 435 James St. South Thunder Bay, ON P7E 6S7 Canada Recent geochemical, geochronological, and geophysical data have provided further insight into the relationship of Midcontinent Rift (MCR)-related intrusive units south of Thunder Bay (e.g. Hollings et al., 2010). These data have shown that the intrusive relationships are more complicated than previously thought and further work is needed in order to understand these relationships and put them in context within the MCR. The initial phase of this study will: 1) identify the issues surrounding the intrusive units within the study area and how they may potentially be resolved; and 2) present preliminary findings of a geochemical compilation for MCR rocks in Canada, with particular focus on indices of fertility for Ni-Cu-PGE deposits. Mapping focused around the Arrow River has revealed an extension of the recently recognized Devon basalts (Cundari, 2010). Outcrops on the western bank of the Arrow River exhibited an amygdaloidal texture and a cracked ―elephant skin‖ flow-top texture similar to Devon basalt outcrops observed ~4 km to the west, suggesting that the unit may be significantly more extensive than previously recognized. A La/Sm cn ratio of 2.98 and a Gd/Yb cn ratio of 3.13 are well within the range (La/Sm cn = 2.6 to 3.1; Gd/Yb cn = 3.0 to 3.4) for the unit (Cundari, 2010), supporting this interpretation. Digital elevation models will also be generated in order to help correlate laterally extensive volcanic units and sills. Additional geochemistry and reinterpretation of geological maps suggest that a broadly northeast-trending dyke swarm, hitherto collectively termed Pigeon River dykes, may in fact comprise two distinct trends: one trending northeast and the other trending eastnortheast. Dating by Heaman et al. (2007) revealed a considerable range of ages for Pigeon River dykes (i.e. east-northeast-trending 1078±3 Ma Arrow River dyke and the northeasttrending 1141±20 Ma Rita Bolduc dyke), suggesting that it may be possible to subdivide the Pigeon River swarm. Although preliminary data suggest that there are no significant geochemical variations between the two orientations, closer examination of the geochemistry, as well as additional detailed field work, is planned to further investigate this question. A detailed compilation of existing data sets, comprising ~2400 data points of spatially referenced samples with whole rock geochemical analyses (including ~800 samples with fire assay PGE data), has been undertaken. This database represents all major MCR units in Canada and is currently being analyzed in order to investigate geochemical variations in these rocks. Only preliminary interpretations of these data sets have been undertaken (e.g. Hollings et al., 2007a, 2010). Detailed study of these data sets will be conducted, particularly in regard to PGE mineralization. Preliminary observations suggest that Logan sills display broadly higher Cu/Pd ratios than those of the Nipigon sills, the Osler volcanic rocks and the ultramafic intrusions of the Nipigon Embayment (Figure 1). Samples from a detailed traverse through the Osler Group stratigraphy undertaken by Hollings et al. (2007b) have been ILSG 2011 24 Program and Abstracts �reanalyzed for PGE in order to look for detailed variation in one of the few MCR-related sequences in Canada where the stratigraphy is well-understood. This study will allow for investigations in PGE distribution over a ~3 million-year interval and the implications for possible Cu-Ni-PGE mineralization. Figure 1: Age versus Cu/Pd graph. Median age dates were taken from Heaman et al. (2007). REFERENCES Cundari, R. 2010. The Geology and Geochemistry of the Devon Volcanics, South of Thunder Bay, Ontario. Unpublished honours thesis. Lakehead University, 62p. Heaman, L.M., Easton, R.M., Hart, T.R., Hollings, P., MacDonald, C.A., Smyk, M., 2007. Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon Region, Ontario. Canadian Journal of Earth Sciences 44, p.1055–1086. Hollings, P., Hart, T., Richardson, A., MacDonald, C.A., 2007a. Geochemistry of the Mesoproterozoic intrusive rocks of the Nipigon Embayment, northwestern Ontario: evaluating the earliest phases of rift development. Canadian Journal of Earth Sciences 44, p.1087–1110. Hollings, P., Fralick, P., Cousens, B., 2007b. Early history of the Midcontinent Rift inferred from geochemistry and sedimentology of the Mesoproterozoic Osler Group, northwestern Ontario. Canadian Journal of Earth Sciences 44, p.389–412. Hollings, P., Smyk, M., Heaman, L.M., and Halls, H., 2010. The geochemistry, geochronology, and paleomagnetism of dikes and sills associated with the Mesoproterozoic Midcontinent Rift near Thunder Bay, Ontario, Canada. Precambrian Research, v.183, Issue 3, p.553- 571. ILSG 2011 25 Program and Abstracts �The geochemical evolution of the Sonju Lake intrusion: an assimilationfractional crystallization model Dayton, R.N.1*, Miller, J.D.1, and Vervoort, J.D.2 1 Department of Geological Sciences, University of Minnesota Duluth, Duluth, MN 55812 USA School of Earth and Environmental Sciences, Washington State University, Pullman, WA 99164, USA 2 The Sonju Lake Intrusion, located within the Beaver Bay Complex near Finland, MN, is the most completely differentiated intrusion related to the Midcontinent Rift System (Stevenson, 1974; Wieblen, 1982). The Sonju Lake intrusion exhibits a cumulate stratigraphy consistent with closed system differentiation of tholeiitic magma by fractional crystallization (Stevenson, 1974; Miller et al., 1993; Miller and Ripley, 1996). The Finland granite, composed of micrographic-textured ferromonzonite to leucogranite, forms the hanging-wall to the Sonju Lake intrusion. Field relationships from outcrop and drill core through the Sonju Lake intrusion and the overlying Finland granite show a cyclic to irregularly gradational contact. This relationship, the smooth compositional variations across the felsic-mafic contact, and the parallel zonation of the two subunits of the Finland granite with the strike of the mafic cumulates of the Sonju Lake intrusion are consistent with the Finland granite being a late stage felsic differentiate of the Sonju Lake intrusion (Miller and Green, 2002; Miller and Ripley, 1996). However, geophysical modeling of gravity and aeromagnetic data implies a volume of granophyre that approaches that of the Sonju Lake intrusion and therefore greatly exceeds the volume of felsic material that could be accounted for by differentiation of a mafic body of that size (Miller et al., 1990). Miller and Ripley (1996) suggested that the Finland granite was emplaced first and acted as a density barrier to the upward movement of the mafic Sonju Lake magma. Since underplating of the mafic magma would be expected to cause melting in the lower portions of the Finland granite, the lithologically and chemically gradational contact between the two bodies might instead represent a mixing zone between the mafic magma of the upper Sonju and the partially melted base of the Finland granite. The major and minor element geochemistry of the Finland granite resembles an extreme differentiate of the Sonju Lake intrusion and precludes using lithogeochemical data to evaluate the extent of mixing between mafic and felsic magmas. However, because the granite has an isotopic signature that is distinct from the mafic rocks of the Sonju (Vervoort et al., 2007, and this study), a radiogenic isotope study has a better chance of assessing the extent of assimilation of the partially melted Finland granite by the Sonju Lake intrusion and the effect of this assimilation on the compositional evolution of the well differentiated Sonju Lake intrusion. The Finland granite and Sonju Lake intrusion have distinct and identifiable isotopic compositions of Nd. Samples from the leucogranite range from -3 to -3.6 εNdat 1096 Ma with an average of -3.4 εNd for five samples (Vervoort and Green,1997, Vervoort et al., 2007, this study). Samples from the Sonju Lake intrusion, below the cumulus arrival of apatite, range from 1.47 to -1.62 εNd. The rocks above and including the uppermost cumulate layer of the Sonju Lake intrusion consistently exhibit decreasing values of εNd with increasing stratigraphic height closer to the Finland granite. If the consistent isotopic signature of the leucogranite(-3 to -3.6 εNd) is taken as the initial εNd of the Finland granite as a whole, it is clear that the underlying quartz ferromonzodiorite, with εNd compositions of 1.7 to -2, contains a more radiogenic Nd component attributable to the Sonju Lake intrusion. ILSG 2011 26 Program and Abstracts �The quartz ferromonzodiorite may be a late-stage differentiate of the Sonju Lake intrusion that assimilated non-radiogenic Nd from the Finland granite. An assimilation-fractional crystallization model is presented to evaluate Nd isotopes in the upper portions of the Sonju Lake intrusion and overlying Finland granite (Fig. 1).Anomalously negative εNd values for samples from the SLI-1 core that profiles the felsic-mafic contact west of the main exposure area are enigmatic. Figure1. Concentration of Nd vs. εNd(1096) showing AFC model curves and simple mixing curve for identical end members. Ticks on mixing curve are 10% increments. REFERENCES Miller, J.D., Jr, Schaap, B.D., and Chandler, V.W., 1990, The Sonju Lake intrusion and associated Keweenawan rocks: Geochemical and geophysical evidence of petrogenetic relationships. 36th Annual Institute on Lake Superior Geology, p. 66–68. Miller, J.D., Jr., Green, J.C., Chandler, V.W., and Boerboom, T.J., 1993, Geologic map of the Finland and Doyle Lake quadrangles, Lake County, Minnesota. Minnesota Geological Survey Miscellaneous Map Series M-72, 1:24,000 scale. Miller, J.D., Jr., and Ripley, E.M., 1996, Layered intrusions of the Duluth Complex, Minnesota, USA. in Cawthorne, R.G. (ed.):Layered Intrusions: Amsterdam, Elsevier, p. 257-301. Miller, J.D., Jr., Green, J.C., 2002. Geology of the Beaver Bay Complex and related hypabyssal intrusions. in Miller, J.D. Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.E., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota. Minnesota Geological Survey Report of Investigations 58, pp. 106-143 Stevenson, R.J., 1974. A mafic layered intrusion of Keweenawan age near Finland, Minnesota. M.S. Thesis, University of Minnesota Duluth, 160 pp. Vervoort, J.D. , Wirth, K., Kennedy, B., Sandland, T. , and Harpp, K.S., 2007. The magmatic evolution of the Midcontinent rift: new geochronologic and geochemical evidence from felsic magmatism, Precambrian Research157, p. 235– 268 Vervoort, J.D., Green, J.C., 1997. Origin of evolved magmas in the Midcontinent rift system, northeast Minnesota: Ndisotope evidence for melting of Archean crust. Canadian Journal of Earth Science. 34, p. 521–535. Weiblen P.W., 1982, Keweenawan intrusive igneous rocks. Geological Society of America Memoir 156, p. 57-82 ILSG 2011 27 Program and Abstracts �The uptake of lead by Brassica rapa; implications for hunting in agricultural fields Maverick Deschamp and Jennifer McGuire Department of Geology, University of Saint Thomas, 2115 Summit Ave, St Paul, MN 55105 According to the Center for Disease Control and Prevention, about 1 in 22 children in America have high levels of lead in their blood. Research has shown that the long-term effects of elevated concentrations of lead in the blood of children can be severe including learning disabilities, decreased growth, hyperactivity, impaired heading, and brain damage. In 1987 Minnesota banned the use of lead shot for hunting waterfowl because it concluded that the use of lead shot was killing waterfowl and poisoning their predators. However, lead is used in other forms of hunting and thus may also be significant in the environment suggesting it should also be removed from use. To test this hypothesis, I grew Brassica rapa, a fast growing wild mustard plant, exposed to varying amounts of lead in its soil. The contamination was lead shot that would be commonly used in hunting and recreational shooting. In total, 10 plants were grown to maturity with lead ranging from 0-20 grams per pot. The plants and the soil from which they were grown, with remaining lead shot removed, were then digested using EPA method 3050B, a rigorous hydrochloric and nitric acid digestion that accesses all environmentally available elements, and analyzed using an ICPAES. The data shows that the plants and soil increased in total lead corresponding with the amount of lead per pot. This result shows that with increased lead present, the plants and soil become enriched in lead and potentially pose a greater risk to consumers. ILSG 2011 28 Program and Abstracts �Quantifying deformation fabrics and chemistry across small granitic ductile shear zones from Mountain, Wisconsin Michael A. DeVasto1, Dyanna M. Czeck1, Prajukti Bhattacharyya2 1 Dept. of Geosciences, University of Wisconsin-Milwaukee, P.O. Box 413, Milwaukee, WI 53201. Dept. of Geography and Geology, University of Wisconsin-Whitewater, Upham 119, Whitewater, WI 53190 2 INTRODUCTION Deformation processes in ductile shear zones can be profoundly affected by the presence of fluids (Wawrzyniec et al., 1999). Fluid-rock interaction also has potential for causing volumetric changes in shear zones, which are important to consider during kinematic analyses (Ramsay and Graham 1970). The purpose of our study is to develop a way to systematically and quantitatively describe textural and chemical changes across small-scale ductile shear zones, which may be indicative of the presence of fluids during deformation. The small (cm scale) shear zones in deformed granite of the Mountain Shear Zone (MSZ) in Mountain, Wisconsin are ideal for this study for a variety of reasons. 1) Sims et al. (1989) conducted a detailed study of the MSZ, allowing for a more detailed structural analysis. 2) Granite is an excellent rock to study microtextures due to its isotropy and simple mineralogy and chemistry. 3) The scale of these shear zones is ideal for a high resolution sampling method in order to achieve a complete geochemical dataset through the strain gradient. GEOCHEMISTRY Tracking geochemical changes across the strain gradient can act as an indicator of fluid-rock interactions during deformation and volume loss (Newman and Mitra, 1992; Yonkee et al., 2003). Bialek (1999) summarizes the inconsistency in results between many studies, which indicate that ductile shear zones may or may not have significant fluid input and volume change. These inconsistencies may be due to lack of resolution in the chemistry data, which typically are only collected in the protolith and shear zone. Therefore, , we plotted individual elemental chemistry, determined through x-ray fluorescence, with respect to distance from the shear zone (Yonkee et al. 2003). This method provides more information than the commonly used isocon method, which lacks spatial relevance and may misrepresent elements with small changes in concentration. Our granitic shear zones have a variety of geochemical gradients. Preliminary results in one shear zone show relatively no major element chemical changes. Preliminary results in another shear zone from the same outcrop show enrichments in Fe2O3, MgO, Al2O3 and K2O with depletion in SiO2 and Na2O. CaO, P2O5, and TiO2 concentrations vary along the transect, but show no obvious trend with respect to the shear zone. In comparison, the isocon method indicates minor depletions/enrichments in SiO2, Fe2O3, MgO, and Al2O3, whereas other analyzed elements remained ―immobile‖. A GIS TECHNIQUE TO QUANTIFY FABRIC FORMATION Microstructures may be indicative of fluid interaction and localized volume loss, offering a different, yet largely descriptive, approach to studying fluid-rock interaction during ILSG 2011 29 Program and Abstracts �deformation. To completely convey the importance of microstructures there needs to be a method to quantify various features. Li et al. (2008) developed a semi-automatic digitizing model within ArcGIS to improve the ease of tracing crystal grain boundaries. We have expanded the use of this model by applying it to deformed polymineralic rocks. Because this model works within a GIS, we can build a spatial database of microstructural information, including mineralogy and deformation fabrics, which can be queried for statistical and geospatial information. The average nearest neighbor tool is a good example of what even a simple spatial analysis can do. Running this tool yields a plethora of information, most notably the nearest neighbor index (figure 1), which will indicate whether the distribution of grains is statistically either clustered or dispersed. Our data show that quartz and plagioclase are both significantly dispersed outside of the shear zone, yet abruptly change to a ―less-dispersed‖ pattern inside the shear zone. The significance of this spatial Figure 2: ArcGIS graphical output for distribution still needs to be determined. This the calculated Nearest Neighbor Index. study will help to establish a quantitative A) corresponds to inside the shear zone. method for thin section fabric analysis. B) corresponds to samples outside of Moreover, insight from this information will the shear zone. allow us to find a potential link between textural and chemical analyses in deformed rocks. REFERENCES Bialek, D. 1999. Chemical changes associated with deformation of granites under greenschist facies conditions: the example of the Zawidów Granodiorite (SE Lusatian Granodiorite Complex, Poland). Tectonophysics 303, 251-261. Li, Y., Onasch, C. M. and Guo, Y. 2008. GIS-based detection of grain boundaries. Journal of Structural Geology 30, 431-443. Newman, J. and Mitra, G. 1993. Lateral variations in mylonite zone thickness as influences by fluidrock interactions, Linville Falls fault, North Carolina. Journal of Structural Geology 15, 849863. Ramsay, J. G. and Graham, R. H. 1970. Strain variation in shear belts. Canadian Journal of Earth Sciences 7, 786-813. Sims, P. K., Klasner, J. S. and Peterman, Z. E. 1990. The Mountain Shear Zone, Northeastern Wisconsin –A Discrete Ductile Deformation Zone Within the Early Proterozoic Penokean Orogen. Geologic Survey Bulletin, Precambrian Geology of Lake Superior Region, A1-A15. Wawrzyniec, T., Selverstone, J. and Axen, G. J. 1999. Correlations between fluid composition and deep-seated structural style in the footwall of the Simplon low-angle normal fault, Switzerland. Geology 27, 715718. Yonkee, W. A., Parry, W. T. and Bruhn, R. L. 2003. Relations between progressive deformation and fluid-rock interaction during shear-zone growth in a basement-cored thrust sheet, Sevier orogenic belt, Utah. American Journal of Science 303, 1-59. ILSG 2011 30 Program and Abstracts �Detrital zircon geochronology of Matinenda Formation sandstones (Huronian Supergroup) at Elliot Lake, Ontario: Implications for uranium mineralization R.M. Easton1 and L.M. Heaman2 1 Precambrian Geoscience Section, Ontario Geological Survey, Sudbury, Ontario P3E 6B5 mike.easton@ontario.ca 2 Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, T6G 2E3 larry.heaman@ualberta.ca Between 1956 and 1996, 12 mines in the Elliot Lake camp produced 138,500 tonnes of uranium metal at an average grade of 763 ppm U (900 ppm U3O8). The uranium ore was hosted in quartz pebble conglomerate beds within the lower part of the Matinenda Fm near the base of the Paleoproterozoic Huronian Supergroup. The Matinenda Fm is composed almost entirely of trough and planar crossstratified, greenish to buff, coarse-grained sandstones deposited in a braded fluvial system (Fralick 2003). The ore-conglomerates occur mainly in NW-oriented valleys (Roscoe 1969), with current direction from the NW (McDowell 1957; Pienaar 1963). Numerous workers (see Roscoe 1969 for list) have suggested a paleoplacer origin for the mineralized conglomerates, however the location and distance of the source region is problematic. McDowell (1957) suggested a source 225 to 400 km to the W-NW of Elliot Lake (roughly the Wawa greenstone belt), but previous detrital zircon studies of the Matinenda Fm near Sault Ste. Marie and Espanola (Rainbird and Davis 2006, Easton and Heaman 2008) indicate a dominant zircon population ranging from 2650 and 2670 Ma, not the >2700 Ma zircons expected for a Wawa greenstone belt source. Due to increased exploration interest in the camp, in 2009 the Ontario Geological Survey began compilation mapping in the camp (Easton 2009, 2010). As part of this study, 4 Matinenda Fm sandstones were collected to better understand its provenance. U-Pb isotopic analyses were carried out using laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) at the University of Alberta. The samples were collected from a 180 m thick section of the Matinenda Fm exposed along a hydro transmission line west of Pecors Lake on the south limb of the Quirke Lake syncline. Two samples are from the Ryan Member, one 5 m below and the other 7 m above the mineralized Main Conglomerate Bed (09RME-0344 and 09RME-0343, respectively). A third sample (09RME-0342) is from the lower part of the Stinson Member, roughly 75 m above the Main Conglomerate Bed. The fourth sample (09RME-0129) is from the uppermost Stinson Member, roughly 10 m below the unexposed contact with the overlying McKim Formation. The Ryan Member samples adjacent to the Main Conglomerate Bed (09RME-0344 and 09RME-0343) give near identical weighted average 207Pb/206Pb ages of 2648.9±4.3 (MSWD=2.1, n=100) and 2648.5±6.3 (MSWD=2.2, n=118), respectively. The youngest concordant grains are 2620 and 2634, and the oldest grains are 2776 and 2684 Ma, respectively. Sample 09RME-0344 contained 4 concordant grains between 2702 and 2776 Ma. Sample 09RME-0343 contained a significantly higher proportion of discordant grains, perhaps due to metamict effects related to the underlying uranium mineralization. Sample 09RME-0342, located roughly 85 m above the base of the formation, has a weighted average 207Pb/206Pb age of 2638.5±5.2 (MSWD=0.55, n=72), but contains younger grains (2546 Ma) and 3 Mesoarchean grains which give an upper intercept age of 2836±31Ma. The stratigraphically highest sample, 09RME-0129, has peaks at 2657 and, with a minor peak at 2708, suggesting a slightly more diverse provenance. The grains between 2640 to 2710 Ma give a weighted average 207Pb/206Pb of 2667.4±4.0 (MSWD=8.9, n=79). The youngest concordant grain is 2580 Ma and there are 4 concordant grains between 2706 and 2760 Ma. These results are consistent with the recently obtained 2687±2 Ma age of felsic metavolcanic rocks at Elliot Lake (Easton 2010), and with the ages of Archean granitic rocks NE of Elliot Lake that indicate emplacement between 2620 and 2670 Ma (J. Ayer, Ontario Geological Survey, personal communication 2011), all consistent with local sourcing. The age and geochemical data are consistent with nearby Archean granites being an important source component, as suggested by Roscoe (1969). The ages are younger than those typical of the Abitibi or Wawa greenstone belts and contrast with those from other sandstone units in the Huronian Spg which show a more diverse provenance consisting of mixed ILSG 2011 31 Program and Abstracts �<2700 and >2700 populations and a variety of Mesoarchean grains (Rainbird and Davis 2006; Easton and Heaman 2008). Geochemistry of the sandstones (Table 1) confirms the observations of Fralick (2003). The presence of monazite is reflected in LREE enrichment and the presence of negative Eu anomalies throughout the Matinenda Fm. This contrasts with the lack of Eu anomalies in units immediately overlying the Matinenda Fm. The increase in SiO2 and decrease in Al2O3 and K2O in the better sorted upper Stinson Member is consistent with the observed decreased clay mineral content in this unit, however, Th and total REE content do not decrease, indicating that heavy mineral content was unchanged. The low K2O and high CIA value for sample 09RME-0129 may indicate that this sample underwent minor metasomatism compared to sample 09RME-0341, located only 5 m higher in the section. The Huronian metavolcanic rocks in the area lack any diagnostic trace element signature (such as high Cr) that could be detected in the sandstones. In summary, local sourcing of the Matinenda Fm means that the character of the adjacent Archean basement needs to be accounted for in exploration. Table 1. Summary of geochemical data for Matinenda Formation samples along the study transect. All samples are sandstones, except for sample 09RME-0318 which is an uraniferous quartz-pebble conglomerate bed. Geochronology samples are indicated in bold. Ratios listed are those used by Fralick (2003). Data from GeoLabs, Ontario Geological Survey, Sudbury. Abbreviations: CIA = chemical index of alteration. Sample 09RME- SiO2 wt.% 0341 5.79 5.11 0129 - 4.17 0342 - 8.67 0343 0318 7.16 - 0344 9.88 Al2O3 wt.% K2O wt.% Zr ppm 9 .50 9 .02 8 .28 7 2.87 8 .13 7 2.04 2 .42 3 .37 9 .04 1 .64 5 .32 1 .28 1 2 0 1 6 3 6 7 2 70 6 7 Th ppm 7 9 8 4 3 6 1 112 9 7 Total REE (ppm) 4 89 6 71 4 9 9 6 > 759 1 1 La/ Sm Gd/ Yb 1 .7 2 .8 2 .3 6 3.8 1 3.4 9 0.3 9 2.1 9 2.9 9 1.8 1 4.9 1 2.4 1 6 Al/ K Zr/ Hf 3 .8 1 5 3 .2 8 1 1 .9 1 9 2 3 15 6 09 0 3 .2 2 41 8 8 8 6 5 6 3 7 4 6 1 3 1 5 2 3 .3 6 9 25 7 1 .9 2 3 7 3 10 .8 5 .2 3 1 CIA 1 20 .0 4 Zr/ TO2 3 .8 6 .5 Ce/ Th 6 REFERENCES Easton, R.M. 2009. Compilation mapping, Pecors-Whiskey Lake area, Superior and Southern provinces; in Summary of Field Work and Other Activities, 2009, Ontario Geological Survey, Open File Report 6240, p.10-1 to 10-21. Easton, R.M. 2010. Compilation mapping, Pecors-Whiskey Lake area, Superior and Southern provinces; in Summary of Field Work and Other Activities, 2010, Ontario Geological Survey, Open File Report 6260, p.8-1 to 8-12. Easton, R.M. and Heaman, L.M. 2008. Detrital zircon geochronology of Huronian Supergroup sandstones located within the Vernon structure, north of Espanola, Ontario; 54th Institute on Lake Superior Geology, Proceedings, v.54, pt.1, p.21-22. Fralick, P.W. 2003. Geochemistry of clastic sedimentary rocks: ratio techniques; in Geochemistry of sediments and sedimentary rocks: Evolutionary considerations to mineral deposit-forming environments, D.R. Lentz, editor; Geological Association of Canada, GeoText 4, p.8-103. Pienaar, P.J. 1963. Stratigraphy, petrology, and genesis of the Elliot Group, Blind River, Ontario, including the uraniferous conglomerate; Geological Survey of Canada, Bulletin 83, 140p. Rainbird, R.H. and Davis, W.J. 2006. Detrital zircon geochronology of the western Huronian Basin; Institute on Lake Superior Geology, Proceedings, 52, pt.1, Programs and Abstracts, 55-56. Roscoe, S.M. 1969. Huronian rocks and uraniferous conglomerates in the Canadian Shield; Geological Survey of Canada, Paper 68-40, 205p. ILSG 2011 32 Program and Abstracts �Bedrock topography, glacial drift thickness and bedrock outcrop maps, Upper Peninsula of Michigan John M. Esch, Michigan Dept. of Environmental Quality, Office of Geological Survey, P.O. 30256, Lansing, MI 48909, eschj@michigan.gov Few studies exist concerning the thickness of the glacial drift, the underlying bedrock surface topography or the distribution of outcrops in Michigan. Newly assembled bedrock topography, glacial-drift thickness and bedrock outcrop maps were constructed using traditional depth to bedrock data from water, oil and gas, and mineral wells. Due to the sparse well control over much of the Upper Peninsula, nontraditional bedrock elevation sources such as bedrock shorelines and outcrops, SSURGO county soil data, waterfalls, quarries, aerial photos and General Land Office surveyor‘s plats were also used (Esch, 2011). In the Upper Peninsula, the bedrock surface consists of high relief resistant bedrock highlands and cuestas as well as lowlands and deep bedrock valleys. The bedrock surface in places has well defined drainage networks with tributaries as well as long linear valleys. The bedrock surface has been sculpted by hundreds of millions of years of preglacial erosion and by paleo-river channels cut into it during the numerous glacial ice advances over the last 2.5 million years. There may also be local relationships between of certain bedrock valleys and scarps with jointing and deeper bedrock structures. The Upper Peninsula is characterized by two distinct bedrock surface regions. In the western Upper Peninsula, Precambrian crystalline bedrock forms large, high relief bedrock uplands with bedrock elevations generally greater that 1000 feet above mean sea level (amsl). Archean Granite and Gneiss generally form bedrock highlands. In fact Mount Arvon consisting of Archean Granite and Gneiss in eastern Baraga County has the highest land surface and bedrock elevation in the state at 1979 feet amsl. The Portage Lake Volcanics and the Copper Harbor Conglomerate form a prominent bedrock and land surface ridge along their extent. This contrasts with the eastern Upper Peninsula where bedrock consists of less resistant Paleozoic sedimentary rocks with elevations generally less than 1000 feet amsl and lower relief. Cuestas and bedrock valleys are the dominant bedrock features in the eastern Upper Peninsula. Bedrock valleys are most numerous in eastern Chippewa County. The Niagaran cuesta and Cambro-Ordovician Sandstone cuestas are especially prominent bedrock features. Locally, the Stonington Formation/Big Hill Dolomite and Black River Formation form lower relief and less extensive cuestas. The Mackinaw Breccia forms localized bedrock highlands north of the Mackinaw Straits. Broad bedrock lowlands generally occur over the Jacobsville Sandstone except at the northern part of Sugar Island in Chippewa County. One of the lowest bedrock locations in the Upper Peninsula at 200 feet amsl occurs in a deep bedrock valley at Rudyard in Chippewa County. This bedrock valley runs from Brimley on Lake Superior past Rudyard to St. Martin Bay on Lake Michigan. It has some of the thickest drift in the Upper Peninsula at 435 feet thick. Parts of the Waiska and the Pine Rivers overlay this bedrock valley. Another deep bedrock valley underlies the Sturgeon River valley in Baraga and Houghton Counties near Pelkie. The bottom of the bedrock valley is around 200 feet amsl and drift is 400 feet over it. It is unclear if there is any relationship between the deep bedrock valleys in the eastern Upper Peninsula and the deep north-south troughs on the floor of the east side of Lake Superior which are at a much lower elevation. ILSG 2011 33 Program and Abstracts �Drift thickness ranges from nonexistent at bedrock outcrops to 450 feet. It averages 40 feet thick but can change thickness abruptly over short distances. Drift is locally thicker overlying bedrock valleys, broad bedrock lowlands, along the back cuesta lowlands and over end moraines. This contrasts with the Lower Peninsula where drift averages 270 feet thick and is up to 1,320 feet thick which is the thickest glacial drift on land in North America. The land surface topography in the Upper Peninsula generally mimics the underlying bedrock topography. Modern drainage often overlies broad bedrock lowlands, bedrock valleys and back cuesta lowlands. In eastern Menominee, western Delta and southwestern Marquette counties the bedrock is a surprisingly smooth downward sloping surface to the southeast. The bedrock formations are Trenton, Black River and Prairie du Chien. In parts of this area the land surface mimics this southeast sloping surface. These maps will assist geologists in understanding glacial lobe dynamics, in seismic data processing for oil and gas exploration, bedrock geology mapping, in aggregate and mineral exploration. These maps should be especially helpful for ground-water exploration in the highly variable drift and generally limited aquifer areas over crystalline bedrock in the western Upper Peninsula and for finding potential glacial aquifers in bedrock valleys. Figure 1: Upper Peninsula of Michigan bedrock topography and major bedrock valleys REFERENCES Esch, John, M., 2011, Michigan bedrock topography, glacial drift thickness and bedrock outcrop maps, Geological Society of America Abstracts with Programs, Vol. 43, No. 1, p. 56 ILSG 2011 34 Program and Abstracts �Using LiDAR and ArcGIS to evaluate subtle glacial landforms associated with the Early Chippewa and Emerald phase ice-margin positions, Barron County, Wisconsin Corrie T. Floyd1, Kent M. Syverson1, and Christina M. Hupy2 1 Department of Geology, 2Department of Geography and Anthropology, University Wisconsin-Eau Claire, Eau Claire, WI 54702-4004, floydct@uwec.edu Glacial landforms in Barron County, Wisconsin, are a result of at least four glacial advances during the Pleistocene Epoch (2.58 to 0.012 Ma) (Johnson, 1986; Syverson and others, 2011). During the early Chippewa and Emerald Phases (Late Wisconsin Glaciation, 31-17 ka), till of the Pokegama Creek and Poskin Members of the Copper Falls Fm. were deposited over till of the River Falls Fm. (Illinoian Glaciation, >130 ka). These tills are all reddish-brown, sandy, and lithologically indistinguishable. Johnson (1986) was unable to map the location of the Emerald Phase ice margin using glacial geomorphology because these till surfaces do not display obvious glacial landforms in the field. Johnson used Late Wisconsin lake sediment in the northwest-flowing Fourmile Creek valley as evidence for the Emerald Phase ice-margin position (Fig. 1). In addition, Fourmile Creek makes a 90degree bend, and Johnson (1986) attributed this to flow along the Emerald Phase ice margin. LiDAR data obtained from the Barron County Land Information Office was used to evaluate the early Chippewa and Emerald Phase ice-margin positions proposed by Johnson (1986). High-resolution terrain models have been generated using the LiDAR data. The point spacing of the data is about 3 ft, and Figure 1. Elevation model of the study area and the compared to ground truth points, the root proposed ice-margin positions of the early Chippewa and Emerald Phases. Johnson (1986) mapped the mean square error (RMSE) of the survey is Emerald ice margin based on the sharp bend in 0.29 ft. Even with LiDAR‘s high resolution, Fourmile Creek downstream (NW) from a broad, flat primary glacial landforms are lacking on till valley filled with Late Wisconsin lake sediment (crosssurfaces in the 90 sq km study area. hatched area). Each ―x‖ marks meltwater features discovered by LiDAR terrain analysis. The river valley However, ArcMap‘s 3D analyst sub-parallel to the proposed Emerald Phase ice margin tools and ArcScene‘s 3D visualization might have been incised along a recessional icecapabilities reveal three distinct fluvial marginal position. channels incised in the Poskin and Pokegama Creek till surfaces (Fig. 1). Longitudinal and cross-sectional profiles were generated to study channel morphology. These meltwater channels are sinuous, 900 to 1000 ft long, and reach maximum depths of 20 ft. They are different from modern channels because they cut across drainage divides, are sub-parallel to the land contour in some places, and typically appear abruptly on the landscape with little area for water catchment. Channel MC-1 (Figs. 1, 2) is an example of a lateral meltwater channel formed during deglaciation from the early Chippewa Phase ice maximum. Overall, these channels are evidence for a younger, lessmodified landscape impacted by the Late Wisconsin Glaciation as compared to the River ILSG 2011 35 Program and Abstracts �Falls Fm. till surface from the Illinoian Glaciation which lacks fresh glaciofluvial and glacial landforms. LiDAR data did not reveal a sharp landform-assemblage difference between the Late Wisconsin and Illinoian till surfaces. Figure 2. (A) 3D view of lateral meltwater channel MC-1 generated with ArcScene 10. Note: Vertical elevation is exaggerated 4x and the arrows represent water-flow direction. (B) A 2D map view of MC-1 created in ArcGIS 10. Near the midpoint of the channel, modern drainage is toward the northeast. This valley must have been filled with ice to force the water flow to the east during deglaciation. REFERENCES Johnson, M.D., 1986, Pleistocene Geology of Barron County, Wisconsin: Wisconsin Geological and Natural History Survey Information Circular 55, 42 p. Syverson, K.M., Clayton, L., Attig, J.W., and Mickelson, D.M., eds., 2011, Lexicon of Pleistocene Stratigraphic Units of Wisconsin: Wisconsin Geological and Natural History Survey Technical Report 1, 180 p. ILSG 2011 36 Program and Abstracts �Petrology and Cu-Ni-PGE mineralization of the Bovine Igneous Complex, Baraga County, Northern Michigan Daniel J. Foley1 and James D. Miller1 1 Department of Geological Sciences, University of Minnesota Duluth, Duluth, MN 55812 The Bovine Igneous Complex (BIC), located 8 km southeast of the town of L‘Anse, Michigan, is a small basin-shaped mafic/ultramafic intrusion emplaced in the southwestern part of the Baraga Basin. Although age dating of the intrusion has so far been unsuccessful, the BIC intrusion was very likely emplaced during the early magmatic stage of Midcontinent Rift development, given its similarities to other early stage intrusions, such as Tamarack (MN) and Eagle (MI). Investigated by Kennecott as a possible Cu-Ni-PGE prospect, the intrusion has undergone extensive exploration drilling since 1995. This work has shown the intrusion to be weakly to moderately mineralized with Cu-Ni-PGE-enriched sulfides. Metal tenors provided by initial drilling averaged less than .5% Cu and Ni, and less than 350 ppb Pt and Pd (Rossell, 2008). For this study, which is part of Dan Foley‘s MS thesis, two drill cores that profile the BIC (08BIC044 and BIC0101) were investigated for their petrographic attributes, cryptic mineral compositions, and whole rock geochemistry. A detailed (1:6,000) re-mapping of the BIC was also conducted for this study. Preliminary field and petrographic studies by Rossell (2008) interpreted the intrusion to be a simple three unit system composed of a basal wehrlite/melagabbro, overlain by a clinopyroxenite/gabbro, and finally an oxide gabbro. Field mapping, core logging, and petrography conducted for this study have found that the lithostratigraphy of the BIC is a somewhat more complicated. The stratigraphy can be subdivided into three main zones – a lower ultramafic zone, an upper ultramafic zone, and a gabbro zone, each of which can be further subdivided by cumulate mineralogy. As profiled in core 08BIC044 (Fig. 1), the lower ultramafic zone is in sharp contact with a footwall of granitic gneiss at about 670m. A medium fine-grained feldspathic wehrlite (Ol cumulate with intercumulus Cpx and Pl) occurs at the basal contact and gradually coarsens up section and becomes less feldspathic. At about 525m, augite abruptly increases in mode and becomes granular to create a feldspathic olivine pyroxenite (Cpx+Ol cumulate with intercumulus Pl). The contact with the base of the upper ultramafic zone, at about 500m, is marked by the abrupt reappearance of feldspathic wehrlite that is vari-textured and contains abundant inclusions of chert and carbonate. Several fine-grained mafic dikes cut the lower 70 meters of this heterogeneous wehrlite. Above the uppermost dike, a more homogeneous, medium-grained feldspathic wehrlite (Ol cumulate with intercumulus Cpx and Pl) persists up to about 205m, at which point cumulus augite reappears and the modal rock type becomes a feldspathic olivine clinopyroxenite (Cpx+Ol cumulate with intercumulus Pl). At about 75m, an abrupt increase in the Fe-Ti oxide mode to about 10% and a loss of olivine generates a feldspathic oxide clinopyroxenite (Cpx+Ox±Ol cumulate with intercumulus Pl). Soon thereafter (~ 60m), plagioclase becomes abundant (>50%) and lath-shaped to create an oxide gabbro (Pl+Cpx+Ox cumulate). Apatite becomes a cumulus phase at about 50m to create an uppermost cumulate of Pl+Cpx+Ox+Ap. Outcrops of apatitic oxide gabbro, at presumably higher stratigraphic levels than seen in drill core, contain patches of interstitial granophyre. Assuming upward-directed crystallization, this igneous stratigraphy implies a cumulus paragenesis of: Ol Cpx+Ol // Ol Cpx+OlCpx+Ox±Ol Pl+Cpx+OxPl+Cpx+Ox+Ap. The cumulus regression evident at the lower and upper ultramafic zone contact and the heterogeneous nature of the basal upper ultramafic zone strongly implies that this contact demarks two major magma emplacement events. Further evidence of two episodes of magma emplacement come from mineral chemical data on olivine and augite. Cryptic variations of Fo and En components through core 08BIC044 ILSG 2011 37 Program and Abstracts �(Fig. 1) show trends that are consistent with two major episodes of emplacement followed by fractional crystallization. The base of each ultramafic zone is characterized by decreased En content of postcumulus augite which is consistent with chilling of a parental magma. Fo content of olivine in the lower ultramafic zone remains elevated, which is consistent with chilling of primocrystic olivine. Olivine at the base of the upper ultramafic zone shows a decrease in Fo suggesting reequilibration of a new magma pulse with the resident magma. As both the lower and upper ultramafic zone wehrlites transition into olivine clinopyroxenites, both Fo and En decrease, which is consistent with progressive iron enrichment due to fractional crystallization. Interestingly, the upper ultramafic zone and overlying gabbro zone progress to more evolved cumulates, but the cryptic variation is more muted than in the lower ultramafic zone. Noting that the upper ultramafic cumulates are more adcumulate (i.e. contain less postcumulus minerals) than the lower ultramafic zone cumulates, the more subdued cryptic variation of the upper cumulates may be due to a lower trapped liquid shift. A suite of 27 samples have been submitted for lithogeochemical and assay analyses, but the results were not available at the time of this writing. We hope to report on the geochemical data at the meeting. The whole rock geochemistry will be used to evaluate whether the two magma pulses involved similar parental magma composition. Analyses of wehrlite from the base of the lower ultramafic zone and mafic dikes from the base of the upper ultramafic zone will be evaluated as potential candidates for chilled parent magma compositions. The geochemical data will also be used to evaluate the history of sulfide saturation and metallogenesis during the crystallization of the BIC magma(s). Figure 1. Lithostratigraphy and cryptic variation of Fo in olivine and En in augite in DDH 08BIC044. Unit abbreviations are fWERfeldspathic wehrlite, fOCPfeldspathic olivine clinopyroxenite, Db-diabase, fOxCP – feldspathic oxide clinopyroxenite, OxGB – oxide gabbro, AOxGB – apatitic oxide gabbro ILSG 2011 38 REFERENCES Rossell, D., 2008. Geology of the Keweenawan BIC Intrusion. 54th Annual Institute on Lake Superior Geology, Part II: Field Trip Guidebook, pp. 181-193. Program and Abstracts �Sedimentology of a wet, pre-vegetation floodplain assemblage: the Mesoproterozoic, Outan Island Formation, Sibley Group, Ontario, Canada Philip Fralick1 and Kamil Zaniewski2 1 Department of Geology, Lakehead University, P7B 5E1, Thunder Bay, Ontario, Canada philip.fralick@lakeheadu.ca 2 Department of Geography, Lakehead University, P7b 5E1, Thunder Bay, Ontario, Canada Kamil.Zaniewski.lakeheadu.ca Descriptions of fluvial systems operating prior to significant terrestrial macrophyte vegetation have concentrated on assessing the impact of plant evolution on channel style. This is probably in part due to the scarcity of well-developed floodplain successions in fluvial assemblages of these ages. Those studies that do exist are concerned with depositional process on semi-arid and arid floodplains, which can be modeled by comparison with the recent. This investigation adds to the knowledge base on wet, pre-vegetation floodplain deposits and processes with a description of a continuous succession of drill-core through an extensive, 1,400 Ma, delta-top channel-floodplain assemblage. This succession forms the upper portion of the Outan Island Formation in the central portion of the one kilometer thick Sibley Group, present in northwestern Ontario, Canada. The sub-aerial floodplain deposits are dominated by flaser to lenticular bedded, fine-grained sands to shales, with abundant small shale rip-up clasts. Iron oxide-rich, millimeter-thick shale laminae denote exposure surfaces. Soft-sediment deformation of these units is ubiquitous with loading and injection features the most prominent. Where soft sediment deformation is minor the layering appears lensy, with the ripple laminated, fine-grained sandstones able to maintain their appearance. However, significant sections of the core suffer more severe deformation with the layering developing a wispy appearance. Thicker medium-grained sandstones, representing crevasse splays, commonly have protocols (very immature soils) developed in their upper portions. Geochemistry indicates downward movement of meteoric water through the upper portions of the crevasse splays. Well-laminated sediments with wave ripples and only rarely containing rip-up clasts and soft-sediment deformation were deposited in floodplain ponds. These sub-aqueous units are commonly orange or green rather than the ubiquitous red of the sub-aerial deposits. Reduction spots centered on specks of carbon, which is probably of organic origin, indicate that life did exist on the sub-aerial surface, but the highly oxidizing nature of the sedimentary environment led to little preservation of this material other than the prolific, transported carbon specks. These deposits differ from post-vegetation floodplain sediments in having: 1) better preservation of layering without rootlet bioturbation; 2) dominance of rippled sand on the floodplain, probably due to lack of vegetation induced baffling; 3) large-scale generation of small, intraformational clasts, possibly as a result of less rootlet binding; 4) desiccation crack fills consisting of soil peds and locally derived intraclasts, probably delivered due to erosion of the floodplain surface during rainfall events; 5) ubiquitous soft-sediment deformation features in sub-aerial deposits; and, 6) welllaminated, oxidized sediment that accumulated in floodplain ponds. ILSG 2011 39 Program and Abstracts �Figure 1. Diagram showing the types of layering developed in various sub-environments of the depositional system. A- Main Channel; B and C- Levee; E and F- Proximal to Distal Crevasse Splay; H- Wave reworked Crevasse Splay; D and G- Floodplain; I, J and KFloodplain Pond. ILSG 2011 40 Program and Abstracts �A review of the western Superior lithoprobe line 3 Paul Geller, Phil, Fralick, Mary Louise Hill, and Patricia Gillies Lakehead University Department of Geology 955 Oliver Road, P7B 5E1, Thunder Bay, Ontario, Canada. pgeller@lakeheadu.ca The Lithoprobe project was a Canada-wide project that had a multi-disciplinary approach to mapping the deep crust. Line 3 of the Western Superior transect crosses from the Eastern Wabigoon Subprovince into the Quetico Subprovince. The transect passes through two terranes of the Eastern Wabigoon Subprovince: the Onaman-Tashota Terrane and the Beardmore-Geraldton Belt. The Onaman-Tashota Terrane is a patchwork of igneous intrusive and extrusive rocks. The Beardmore-Geraldton Belt is a collection of interlayered metasedimentary and metaigneous units. The Quetico Subprovince is metasedimentary, primarily composed of metaturbidites intruded by felsic igneous rocks and migmatized in places. The seismic section does not follow a straight-line but instead has an east-west jog to the otherwise north-south trend (Figure 1). This, as well as the related areas of low data confidence, has important ramifications for the interpretation. Gravity data was used in collaboration with surface geology to add an extra degree of robustness to the investigation of structures at depth. A structure is present, dipping at approximately 12° to the south at the boundary between the Quetico Subprovince and the Wabigoon Subprovince. Structures inferred to be the Paint Lake Fault and the Jellicoe River Fault appear below the surface expression of these features, with dips of approximately 86°. Other features that seem to appear in the section must be dismissed at present as they occur in areas of low confidence. Figure 1. A 3D representation of the seismic line from surface to approximately 48 km depth, with associated geology and interpreted geology. ILSG 2011 41 Program and Abstracts �Stratigraphic investigation of the Neoarchean Bird River Belt, SE Manitoba, Canada H. Paul Gilbert Manitoba Geological Survey 360-1395 Ellice Ave. Winnipeg Manitoba Canada R3G 3P2 The Neoarchean Bird River Belt (BRB) is located in the Bird River Subprovince within the southwestern Superior Province, between flanking older cratonic blocks — the 3.0-2.87 Ga North Caribou Superterrane to the north and the 3.4-2.8 Ga Winnipeg River Subprovince to the south (Percival et al., 2006). The BRB consists of diverse volcanic and sedimentary rocks in two structural panels (north and south), separated by the relatively younger turbiditic Booster Lake Formation that extends through the centre of the belt. The calcalkaline north panel rocks (ca. 2733 Ma) are akin to modern subduction-related rocks at active continental margins, whereas the tholeiitic sequence in the south panel (ca. 2725 Ma) documents incipient rifting in an extensional tectonic regime (Fig. 1). The arc–type rocks are flanked to both north and south by mid-ocean-ridge basalt (MORB)–type sequences that are interpreted as relatively older than the arc-type rocks, and may be associated with early arc rifting in a back-arc setting (Gilbert et al., 2008). The north and south panel rocks, although distinguished by different composition and age, have broadly similar internal stratigraphies. The south panel contains a lower felsic volcanic sequence overlain by voluminous pillowed mafic flows and an upper, diverse volcano-sedimentary assemblage. The north panel is also interpreted to consist of a lower felsic volcanic sequence – Peterson Creek Formation (PCF), overlain by varied sedimentary and volcanic rocks – the Diverse Arc assemblage (DAA), which includes a pillowed amygdaloidal andesite member at the base. The PCF is a felsic volcanic complex composed mainly of volcanic fragmental rocks and subordinate rhyolite flows. A spherulitic zone within massive flows that extends laterally within the PCF for over 20 km represents a stratigraphic break where rapid chilling was associated with alteration and penecontemporaneous devitrification (Fig. 2). Compositionally distinctive, intrusive (and possibly extrusive) sanukitoid-type rocks (Fig. 1) are interpreted as late- or post-DAA in age. Whereas these rocks intrude the DAA as well as Booster Lake Formation rocks, sporadic clasts of apparent sanukitoid type within some DAA conglomeratic deposits suggest the deposition of DAA rocks and Booster Lake Formation turbidites may have been, in part, contemporaneous. Detrital zircon U-Pb data that indicate maximum depositional ages for the DAA (2706 ±23 Ma) and Booster Lake Formation (2712 ±17 Ma) are consistent with this model (Gilbert, 2008). Overlap of sanukitoid-type magmatism with late volcanism is also supported by the occurrence of synvolcanic sanukitoid massive to fragmental volcanic rocks with an inferred igneous age between 2715 Ma and 2723 Ma in the contiguous Rice Lake belt, north of the BRB (Anderson, 2008). ILSG 2011 42 Program and Abstracts �Figure 1. Th/Ta vs Yb plot of mafic to intermediate volcanic rocks in the Bird River Belt north and south panels, and late sanukitoid-type intrusive rocks. OA = Oceanic arc; ACM = Active continental margin; WPVZ = Within-plate volcanic zone. Figure 2. Chondrite-normalized REE plot of massive and spherulitic rhyolite (PCF). REFERENCES Anderson, S. D. 2008. Geology of the Rice Lake area, Rice Lake greenstone belt, southeastern Manitoba (parts of NTS 52L13, 52M4); Manitoba Science, Technology, Energy and Mines, Manitoba Geological Survey, Geoscientific Report GR2008-1, 97 p. Gilbert, H.P., 2008: Stratigraphic investigations in the Bird River greenstone belt, Manitoba (part of NTS 52L5, 6); in Report of Activities 2008, Manitoba Science, Technology, Energy and Mines, Manitoba Geological Survey, p.121-138. Gilbert, H.P., Davis, D.W., Duguet, M., Kremer, P.D., Mealin, C.A. and MacDonald, J. 2008: Geology of the Bird River Belt, southeastern Manitoba (parts of NTS 52L5, 6); Manitoba Science, Technology, Energy and Mines, Manitoba Geological Survey, Geoscientific Map MAP2008-1, scale 1:50 000 (plus notes and appendix). Percival, J.A., McNicoll, V. and Bailes, A.H. 2006: Strike-slip juxtaposition of ca. 2.72 Ga juvenile arc and >2.98 Ga continent margin sequences and its implications for Archean terrane accretion, western Superior Province, Canada; Canadian Journal of Earth Sciences, v. 43, p. 895–927. ILSG 2011 43 Program and Abstracts �Petrology of the Ni-Cu-PGE-mineralized Tamarack Intrusion, Aitkin and Carlton Counties, Minnesota Brian D. Goldner*1 and James D. Miller2 1 Department of Geological Sciences, University of Minnesota Duluth, Duluth, MN 55812 *Present Address: Rio Tinto Exploration, 224 N 2200 W Salt Lake City, UT, 84116 gold0334@umn.edu 2 Department of Geological Sciences, University of Minnesota Duluth, Duluth, MN 55812 mille066@umn.edu The Tamarack intrusion is an unexposed mineralized ultramafic intrusion located near the town of Tamarack, about 50 miles west of Duluth, Minnesota. Rio Tinto Exploration (previously Kennecott Exploration) has been conducting exploration drilling of the Tamarack intrusion for Cu-Ni-PGE sulfide deposits since 2001. The intrusion was emplaced into black slates of the Paleoproterozoic Animikie Basin during the early magmatic stage of the 1.1Ga Midcontinent Rift and a new U-Pb baddeleyite age reported here yields an age of 1105.6 ± 1.3 Ma. Drilling and geophysical data indicate that the Tamarack intrusion has a tadpole-like shape that is about 13 km long and is between 1 and 4 km wide (Fig. 1). The narrow tail area of the intrusion, which is the site of greatest exploration drilling, is composed of exclusively of ultramafic rock types. The wider ―body‖ area at the southeastern end of the intrusion is composed of a wider variety of rock types ranging from lherzolite to granophyric gabbronorite. Core logging, petrographic observations, mineral chemical analyses, lithogeochemical analyses, and XRF scanning of drill core were employed on four drill cores from the Tamarack intrusion to evaluate its emplacement and crystallization history. Core logging and petrography in three cores from the tail area shows that this part of the intrusion can be subdivided into two texturally and modally distinct units of olivine cumulates – a lower Feldspathic Lherzolite Unit composed of coarse-grained olivine mesocumulates with poikilitic pyroxenes and plagioclase, and an upper Lherzolite Unit composed of medium-grained olivine mesocumulates to adcumulates. The contact between the two lherzolite units was investigated all three cores. In one, the contact occurs across a meter-thick zone of intense alteration; in another, it shows the two lherzolite lithologies irregularly interlayered; and in a third, the feldspathic lherzolite is contains gabbroic inclusions of unknown origin. The one 268-m-long drill core investigated from the body area ends in an olivine mesocumulate similar to the upper Lherzolite Unit of the tail area. At 225m, the lherzolite gives way up section to a 10m-thick interval of intergranular olivine websterite (Cpx+Opx+Ol cumulate) which in turn gives way to an intergranular lherzolite (Ol+Cpx+Opx cumulate). At 210m, plagioclase abruptly increase in mode (~50%) and changes from poikilitic to granular resulting in a gabbronorite. Granular augite and orthopyroxene persist into the gabbronorite, oxide increase to about 5 vol. %, but olivine disappears. This gabbronorite is interpreted to a Pl+Cpx+Opx ±Ox cumulate. The upper 50 meters of the core (below a 59m-thick cap of till) contains interstitial to irregular segregations of granophyre and shows the reappearance of olivine. Disseminated Ni-Cu-PGE sulfide mineralization is present throughout most of the lithologies studied here, but is particularly abundant in the tail area at the basal contact of the ILSG 2011 44 Program and Abstracts �Feldspathic Lherzolite unit and in a zone straddling the contact between the two lherzolite units. Whereas mineral chemical and whole rock data within the feldspathic lherzolite and lherzolite units found in the tail of the intrusion show little cryptic variation, the chemical attributes of lithologies in the body areas show evidence extreme differentiation. Olivine composition ranges from Fo84 in the lowermost lherzolite and becomes progressively evolved to Fo10 in the uppermost granophyric gabbronorite. Other mineral and whole rock data both show smooth gradations from the lherzolite to the gabbronorite which are consistent with these lithologies having formed from one parental magma by fractional crystallization in a closed system. The main petrologic conclusions of this study are: 1) The two lherzolitic units in the tail area formed from a similar high-Mg olivine tholeiitic parent magma. The composition of this parent magma is estimated from subtracting 30% Fo89 olivine phenocryst composition from a chilled margin found at the basal contact of the Feldspathic Lherzolite unit. The resulting composition is comparable to other picritic compositions found at the base of MCR-related Mamainse Point Volcanics. 2) The emplacement of the lower feldspathic lherzolite preceded that of the upper lherzolite in the tail area. The differences in texture and modal mineralogy between the two lherzolite units are attributed to the more rapid cooling of the earlier feldspathic lherzolite creating an orthocumulate in contrast to the more adcumulate upper lherzolite. 3) The sulfide mineralization straddling the lherzolite contact in the tail area is attributed to country rock assimilation and S contamination of the leading edge of the lherzolite parent magmas during the two main episodes of emplacement. The sulfide mineralization in upper part of the Feldspathic Lherzolite is thought to be related to downward infiltration of sulfide liquid from the overlying Lherzolite unit magma upon its emplacement into the semi-molten core of the Feldspathic Lherzolite. 4) Finally, the well differentiated lithologic sequence comprising the body area is interpreted to have resulted from closed-system fractional crystallization of the second magma pulse that created the upper Lherzolite unit in the tail area (Fig. 1). Tai l Body till Gn Lh fLh Figure 1. Possible correlation of lithologies between the tail and body of the Tamarack intrusion. fLh – feldspathic lherzolite unit; Lh – lherzolite unit; Gn – gabbronorite unit ILSG 2011 45 Program and Abstracts �The Minnesota Taconite Workers Health Study: Environmental study of airborne particulates - 2011 update George Hudak1, Stephen Monson Geerts1, Larry Zanko1, April Severson1, Allison Severson1, Bryan Bandli2 1 Natural Resources Research Institute, 5013 Miller Trunk Highway, Duluth, MN, 55811 2 Department of Geological Sciences, University of Minnesota Duluth, 229 Heller Hall, 1114 Kirby Drive, Duluth, MN 55812 Since 2008, the Natural Resources Research Institute (NRRI) has been conducting a detailed characterization of mineral dust in northeastern Minnesota. The purpose of this research is to evaluate the effects of past and present emissions from taconite mining and processing on air quality throughout the Mesabi Iron Range (MIR) by characterizing airborne particulate matter within taconite operations, in communities surrounding taconite operations on the MIR, in population centers in other regions of northeastern Minnesota, as well as particulate matter deposited in lake sediments (Figure 1). NRRI‘s sampling and characterization work represents the community/environmental component of the Minnesota Taconite Workers Health Study, a broad University of Minnesota (UM) research effort investigating long-standing questions regarding the impact of dust derived from mining and processing of taconite (iron ore). The UM School of Public Health (SPH), with whom NRRI is collaborating, is responsible for the human health- and exposure-related components of that effort, which include: 1) an occupational exposure assessment; 2) a mortality study; 3) a cancer incidence study; and 4) a respiratory health survey of taconite workers and spouses. Figure 1. Locations of taconite processing plants on the Mesabi Iron Range being sampled during this study (after Oreskovich and Patelke, 2006) Air sampling is performed within taconite operations, MIR communities, and nonMIR communities by NRRI scientists during both winter and summer seasons. Sampling at taconite operations takes place at four locations: 1) secondary crushers; 2) magnetic ILSG 2011 46 Program and Abstracts �separators/concentrators, agglomerators/ ball drums, and the kiln/pellet discharge area. Sampling within MIR communities takes place on the rooftops of public buildings, whereas sampling in non-MIR communities occurs on rooftops or in remote locations so that background air quality can be evaluated. Airborne particulate are collected using a microorifice uniform deposit impactor (MOUDI) (Marple et al., 1991), which enables sizefractionated particulate collection, and a total suspended particulate filter (TSP). Particulate samples are evaluated via gravimetric analysis and subsequently subjected to comprehensive particulate matter characterization that includes: 1) scanning electron microscopy (SEM) imaging; 2) energy dispersive spectroscopy (EDS); 3) electron backscattered diffraction (EBSD); 4) proton induced x-ray emission (PIXE); 5) the Minnesota Department of Health‘s 852 Method Transmission Electron Microscopy (TEM) Analysis for Mineral Fibers in Air; and 6) the International Standard Organization‘s Method 10312 Ambient air – Determination of Asbestos Fibers – Direct-Transfer TEM Method (ISO 10312, 1995). Over the past year, the NRRI has completed particulate sampling within MIR taconite operations, MIR communities, and non- MIR communities. This includes 14 sampling events at taconite operations and 67 sampling events at locations within communities in northeastern Minnesota, as summarized in Table 1. As well, continued analysis of lake sediment samples from ―North of Snort Lake‖ indicates collection of sediment dating back to ~1840, which pre-dates iron mining on the MIR. At Silver Lake, logging activities in the early part of the 20th century disrupted the sediments; however, we believe we will have good post-1915 lake sediment data, which will mark the period where the transition from natural ore to taconite mining took place. Continued analysis, interpretation and reporting will take place in 2011. Taconite Facility United Taconite In-Plant Sampling Events Sampling Events Taconite Facility 2 events while active Keetac Hibbing Taconite Sampling Events 1 event while active, 1 event while inactive 1 event while active, 3 events while active 3 events while active 1event while active, Northshore 1 event while inactive Minntac 1 event while active Minorca Community Sampling Events Community Sampling Location Sampling Events and Number per Season Silver Bay High School 11 Events (4 Winter / 7 Summer) Virginia Court House 9 Events (4 Winter / 5 Summer) Hibbing High School 9 Events (4 Winter / 5 Summer) Keewatin Elementary School 6 Events (3 Winter / 3 Summer) Babbitt Municipal Building 15 Events (7 Winter / 8 Summer) Duluth NRRI Rooftop 10 Events (4 Winter / 6 Summer) Ely Fernberg Site 7 Events (4 Winter / 3 Summer) Table 1. Summary of in-plant and community sampling events REFERENCES ISO 10312, 1995, Ambient air – determination of asbestos fibers – direct transfer transmission electron microscopy method, 51p. Marple, V. A., Rubow, K. L., and Behm, S. M., 1991, A microorifice uniform deposit impactor (MOUDI): description, calibration, and use: Aerosol Science and Technology, v. 14, p. 434-446. Oreskovich, J. A., and Patelke, M. M., 2006, Historical use of taconite byproducts as construction aggregate materials in Minnesota: A Progress Report: Natural Resources Research Institute Report of Investigation NRRI-RI-2006-02, 10 p. ILSG 2011 47 Program and Abstracts �Bedrock geologic map of Minnesota—Precambrian geology Mark A. Jirsa, Terrence J. Boerboom, and Val W. Chandler Minnesota Geological Survey (MGS), 2642 University Ave., St. Paul, MN, (www.mngs.umn.edu) Digital products associated with the new bedrock geologic map of Minnesota (MGS State Map Series S-21) allow removal of Cretaceous, Paleozoic, and some parts of Mesoproterozoic strata to reveal an interpretation of the underlying Precambrian bedrock (simplified on Figure 1). Corresponding GIS data tables permit attribute searches based on age, terrane, and lithologic and nomenclatural subdivisions; which allow users to create attribute-based thematic maps. Companion data containing an outcrop database and all known geochronologic analyses linked to sample locations were published in 2010 as MGS Open-File Report OF10-02. Collectively, these products provide a new interpretation of Minnesota‘s Precambrian terranes, summarized here. Much of the Precambrian geology on S-21 is depicted in the context of major orogenic, rifting, and meteorite impact events. The understanding of these events in the Lake Superior region has evolved considerably in recent years by the works of many authors. The Archean rocks are subdivided approximately by their apparent temporal relationship to three major periods of deformation; the first (D1) at about 2695 Ma, which may be equated with the Shebandowanian orogeny in adjacent Ontario; the second (D2) at about 2680 Ma occurred during the Minnesotan orogeny and produced regional transpressive fabrics, folds, and metamorphism to greenschist-amphibolite grade (orogenic nomenclature via Percival and others, 2006). The former may represent collision of the Wawa subprovince with the composite Superior superterrane to the north. The latter can be attributed to oblique collision of the Minnesota River Valley subprovince with the Superior superterrane along a north-dipping suture known as the Great Lakes Tectonic Zone. A D3 event is manifest in the Quetico subprovince as broad folds involving D2 fabrics, and elsewhere by faulting. Structures bounding major components of the Superior Province are locally inferred to be thrust faults that initially formed during terrane assembly. Although little is known in detail about their vergence and offset history, the interpretation here builds on seismic surveys in Minnesota, and extrapolation from results of the Western Superior Lithoprobe and NATMAP Transects in adjacent areas of Canada (Percival and Helmstaedt, 2006). The Proterozoic rocks are the products of 4 orogenic and rifting events. The Paleoproterozoic rocks contain evidence for a cycle of rifting and continental collision during the Geon 18 Penokean orogeny. An ejecta blanket resulting from the 1850 Ma Sudbury meteorite impact forms a definitive time-line within the Animikie Group at the stratigraphic top of Gunflint and Biwabik Iron Formations. Crust in the southern half of the state was deformed, metamorphosed, and plutonized to varying degrees during Geon 17 Yavapai orogenesis. The Yavapai Province in extreme southeastern Minnesota contains older continental crust deformed during Geon 17 and intruded by granitic rocks. Although deposition of strata assigned to the Animikie Basin began during the late stages of Penokean orogenesis, it is likely that sedimentation continued during the Yavapai event. The effects of later orogenic events in Minnesota are unclear. The Geon 16 Mazatzal orogeny is well delineated in adjacent Wisconsin; however, only scant evidence of metamorphic overprint is documented in the McGrath Gneiss in Minnesota, and alteration of the Sioux Quartzite is assigned to a Geon 14 hydrothermal event. Mesoproterozoic rocks result from Geon 11 continental rifting, producing volcanic and sedimentary rocks of the Keweenawan Supergroup and plutonic rocks of the Midcontinent Rift Intrusive Supersuite. Mafic dike swarms of inferred Paleoproterozoic and Mesoproterozoic age represent significant episodes of crustal extension. ILSG 2011 48 Program and Abstracts �Figure 1. Terrane map of Precambrian bedrock showing subprovinces of the Archean Superior Province (caps), significant components of Paleoproterozoic bedrock, the Mesoproterozoic Midcontinent rift, and mafic dikes (gray) of Paleoproterozoic and Mesoproterozoic age. REFERENCES Percival, J.A., and Helmstaedt, H., 2006, The Western Superior Lithoprobe and NATMAP transects: Introduction and summary: Can. Jour. Earth Sci. 43:743-747 and articles therein. Percival, J.A., Sandborn-Barrie, M., Skulski, T., Stott, G.M., Helmstaedt, H., and White, D.J., 2006, Tectonic evolution of the western Superior Province from NATMAP and Lithoprobe studies: Can. Jour. Earth Sci. 43:1085-1117. ILSG 2011 49 Program and Abstracts �Highlights of the new bedrock geologic map of Minnesota Mark A. Jirsa, Terrence J. Boerboom, Val W. Chandler, John H. Mossler, Anthony C. Runkel, and Dale R. Setterholm Minnesota Geological Survey (MGS), 2642 University Ave., St. Paul, MN, (www.mngs.umn.edu) The latest bedrock geologic map of Minnesota—MGS State Map Series S-21*—is a new construct that incorporates existing geologic maps where prior mappers had adequate ground control, and new interpretations based on drill hole, geophysical, and unpublished data where they did not. The interpretation differs significantly from previous maps to reflect the new data and accommodate scale. It portrays our current geologic understanding of the temporal and geographic distribution of units within major Precambrian terranes (Jirsa and others, this volume) and of Phanerozoic strata. The previous state-wide compilation of bedrock geology, S-20, was created in 2000, modified in 2003, and produced at a scale of 1:1,000,000. Although S-20 was a suitable image for its time, the new map shows considerably more detail at twice the scale; it incorporates recent detailed mapping and previously unpublished data; and its interpretations are reconciled with reprocessed, highresolution geophysical data. Digital files associated with this map allow removal of Cretaceous, Paleozoic, and some parts of Mesoproterozoic strata to reveal an interpretation of the underlying Precambrian bedrock. Corresponding GIS data tables permit attribute searches based on age, terrane, and lithologic and nomenclatural subdivisions. Ancillary files, published in 2010 as MGS Open-File Report OF10-02*, include new state-wide maps of outcrops, bedrock topography, depth to bedrock, and geochronologic data. Major areas of improvement include the depiction of Precambrian rocks beneath Phanerozoic strata, and the subdivision of Archean units in a broad area of subdued magnetic character north of the Great Lakes Tectonic Zone that was formerly referred to as ―the Quiet Zone.‖ Mafic dike swarms are the remnants of significant crustal extension events during the Paleoproterozoic and Mesoproterozoic. Despite this significance, and the prominent magnetic fabric created by dike anomalies, this map is the first attempt at state-wide depiction. Improvements in Phanerozoic geology include a much more accurate western erosional edge of Paleozoic bedrock, subdivision of Cambrian strata into two units, and the use of modern stratigraphic nomenclature that is more broadly correlative with adjacent jurisdictions. The Phanerozoic geology was compiled from mapping that required contouring of bedrock surfaces using test hole and water well data. Much of the Cretaceous depiction, for example, is taken from grid-subtraction of contoured elevations of top and bottom of units. The new map provides a jumping-off point for future scientific and exploration endeavors. It should be noted that the geologic depiction is based solely on geophysical maps and scant drill hole information in many areas. In addition, the creation of S-21 did not involve logging of all drill core and MGS well-cuttings sets, targeted geophysical modeling, and test drilling, as would be conducted for larger-scale mapping. The lack of ground-truth in much of central and western Minnesota is a call to new drilling. Recent studies in other regions have shown that both temporal and tectonic considerations are needed to identify greenstone belts with enhanced base and precious metal prospectivity. In light of this, the dearth of high-precision geochronologic and lithogeochemical data for Precambrian supracrustal rocks in Minnesota clearly needs to be addressed. Work on the topography of major bounding surfaces, and the depiction of geology beneath them, is on-going. *MGS Open-File Report OF10-02 and State Map Series S-21 were prepared and published with the support of the Minnesota Legislature, as administered by the Minerals Coordinating Committee. ILSG 2011 50 Program and Abstracts �Paleomagnetism and Paleointensity as recorded by 1.08 GA Lake Shore Traps (Keweenaw Peninsula, Upper Michigan) Evgeniy V. Kulakov1, A.V. Smirnov1, J.F Diehl1 1 Department of Geological and Mining Engineering and Sciences, Michigan Technological University,1400, Townsend Drive, Houghton, Michigan, 49931, USA. Here we report new results of paleomagnetic and paleointensity investigations carried out on samples collected from a sequence of lava flows (12 lava flows) interbedded within the Copper Harbor Conglomerate on the Keweenaw Peninsula of Michigan. This sequence, known as the Lake Shore Traps (LST), represents the latest stage of volcanic activity associated with North American Mid-Continent Rift (MCR). Our sampling localities were on Silver Island (10 lava flows) situated along the western shore of Keweenaw Peninsula north of Eagle Harbor, MI and on the mainland (two lava flows) directly east of Silver Island. The Silver Island lava flows are equivalent to the upper lava flows of the Middle LST at the tip of the Peninsula while the mainland lava flow are equivalent to lower lava flows of the middle LST (Diehl and Haig, 1994; Diehl et al., 2009). At the tip of the Peninsula, the upper and lower sections of the middle LST are separated by a 27 m thick conglomerate layer. The LST flows exposed along the western coast of Keweenaw Peninsula and at its northern tip are characterized by different strike and dip angles. That allows us to use the paleomagnetic directional data to test the conclusion by Hnat et al. (2006) that the curvature of MCR in the Keweenaw Peninsula, where structural trends vary from NE/SW to E/W to NW/SE, is a primary rift structure (as opposed to a secondary feature caused by oroclinal bending of the rift related rocks). Characteristic remanent magnetizations (ChRM) were isolated by thermal or alternating field demagnetization, or by combination of both techniques. ChRM was interpreted as primary magnetization, acquired during the original cooling of these lavas. Mean paleomagnetic directions were combined with the results from the previous study (Diehl and Haig, 1994) of the Lower and Upper Middle LST, sampled at northern tip of Keweenaw Peninsula. The group means (N=2) for Upper and Lower Middle LST from the western sites coincide at 95 per cent confidence with those reported for same flows exposed on the northern tip of the peninsula. This provides further support to the conclusion that the curvature of MCR is primary in its origin. We also performed determinations of the strength of Earth magnetic field (paleointensity) recorded by the Lake Shore Traps. The paleointensity database for the Precambrian is very limited and needs to be significantly extended. Determinations of absolute paleointensity were conducted using Thellier method (Thellier and Thellier, 195?) on 107 samples from nine lower lava flows and ten upper flows. 43 per cent (46 samples) of determinations passed our reliability criteria and have been accepted. Paleointensity (36 μT) recorded by the oldest lower flows is relatively high and comparable with the modern field strength. Paleointensity decreases to ~13.2 μT toward in the younger lower flows. All upper flows are characterized by low values of paleofield with mean of ~13.1 μT. Our paleointensity values are less than 50% the values reported by Pesonen and Halls (1983). We suggest that their values are biased because of the use of low-temperature demagnetization data for calculating paleointensity. ILSG 2011 51 Program and Abstracts �REFERNECES Diehl, J.F., T.D. Haig, 1994, A paleomagnetic study of the lava flows within the Copper Harbor Conglomerate, Michigan: new results and implications, Canadian Journal Sciences, 31, 369-380. Diehl, J. F., A.J. Durant, C. Schepke, 2009, Paleomagnetism of Silver Island, Keweenaw Peninsula, Michigan: Additional Support (?) for the Primary Curvature of the MCR, Eos Trans. AGU, 90(22), Jt. Assem. Suppl., Abstract GP11E-01. Hnat, J.S., B.A. van der Pluijm, R. van der Voo, 2006, Primary curvature of the MidContinent Rift: Paleomagnetism of the Portage Lake Volcanics (northern Michigan, USA), Tectonophysics, 425 Pesonen, L. J., H.C. Halls, 1983, Geomagnetic field intensity and reversal asymmetry in late Precambrian Keweenawan rocks. Geophysical Journal. Royal Astronomical Society, 73, 241-270. Thellier, E., and O. Thellier, 1959, Sur l‘intensite´ du champ magne´tique terrestre dans le passe´ historique et ge´ologique: Annales Ge´ophysique, v. 15, p. 285–378. ILSG 2011 52 Program and Abstracts �Testing models of late Paleoproterozoic Penokean orogenesis in the Great Lakes Region, U.S.A. using sedimentary provenance: planning the investigation Todd A. LaMaskin Department of Environmental Science, University of Wisconsin -Extension & Wisconsin Geological and Natural History Survey, 3817 Mineral Point Road, Madison, Wisconsin 53705, lamaskin@wisc.edu Aspects of the overall plate-tectonic setting and regional controls on crustal deformation during late Paleoproterozoic time in the Lake Superior region of the U.S. midcontinent remain debated. Synthesis of the geology and tectonic history of the region suggests that both marginal-basin regime and Cordilleran-style plate-margin tectonic processes were well established in late Paleoproterozoic time; however, the driving forces that produced crustal deformation and coeval subsidence are not fully understood. During Paleoproterozoic time, the southern periphery of Laurentia was a convergent margin that experienced episodic southward continental growth through accretion of both arc and micro-continent lithosphere (Fig. 1). Tectonic "events" during the Penokean orogenic interval (ca. 1.9–1.75 Ga) represent the initiation of this long-lived stage of accretionary southward growth of the North American craton. The Penokean Orogenic Province (POP; Fig. 1) of the Lake Superior region provides an excellent testing ground for evaluating the style and timing of lithospheric accretion in the Proterozoic growth of Laurentia. The POP contains Archean and Paleoproterozoic rocks that represent two distinct magmatic arcs and their associated sedimentary basins (Marshfield and Pembine-Wasau terranes, respectively), as well as two temporally distinct, spatially overlapping sedimentary successions: (1) turbidite iron-formation basins of the Menominee Group, and (2) a collisionrelated foreland fold-thrust complex (Baraga-Animikie Groups). The original composition of weathered source rocks is a dominant control on the makeup of terrigenous sediments, therefore, geographic and stratigraphic variations in sediment provenance in both successions can provide important constraints on the tectonic evolution of the region. Investigation of sedimentary successions in the POP is planned using U-Pb and Hf detrital zircon analysis, mudrock trace-element and Sm-Nd geochemistry, and (U-Th)/He thermochronology. The sampling program is designed to test specific predictions of proposed and new models for the tectonic setting of individual basins, and for the tectonic evolution of the region as a whole. ILSG 2011 53 Program and Abstracts �Figure 1. Map showing major Precambrian crustal provinces in the United States prior to accretion of the Mazatzal Province and formation of Mesoproterozoic igneous and tectonic provinces. ILSG 2011 54 Program and Abstracts �The Biwabik Iron Formation: geochemical and textural evidence for deposition of iron-formation in a Paleoproterozoic epeiric sea Phillip Larson1, John Swenson2, and Marsha Patelke3 1 Duluth Metals Limited, 306 W Superior Street #610, Duluth MN 55802, USA 2 Department of Geological Sciences, 1114 Kirby Drive, University of Minnesota Duluth, Duluth MN 55812, USA 3 Natural Resources Research Institute, 5013 Miller Trunk Highway, Duluth MN 55811, USA The Paleoproterozoic Biwabik Iron Formation of northeastern Minnesota, USA, consists of interbedded ‗cherty‘ granular iron-formation (GIF) and ‗slaty‘ banded ironformation (BIF) lithofacies. Advances in geochronology have demonstrated that the 1.85 Ga Biwabik IF is the chronostratigraphic correlative of other Paleoproterozoic iron-formations preserved along the margins of the 2.7 Ga Superior craton, including the Gunflint, Ironwood, Vulcan, Negaunee, Temiscamie, Sokomon, and Kipalu iron formations. The Biwabik and Gunflint Ifs are essentially undeformed flat-lying platform sequences, at present extending several 100 km inboard of the craton margin. However, in contrast the majority of the circum-Superior iron-formations are preserved as deformed rocks in orogenic belts at the craton margin. In common, all these iron-formations, ranging in thickness from 10s to several 100 m, are sandwiched within clastic sequences. In the case of the Biwabik IF, chemically precipitated Fe occurs in both the footwall and hangingwall clastic sequences, indicating that the trigger for iron-formation accumulation was not the commencement of Fe precipitation, but rather the cessation of clastic input into the basin. While the origin of GIF is contested (shallow-water oolite vs. reworked BIF), the association with high energy environments is clear. The preponderance of GIF in the Biwabik IF, as well as the occurrence of Mn-rich bioherms interbedded with GIF, suggests a predominantly shallow-water depositional environment. Mineralogy of the Biwabik IF is dominated by Fe-oxides, silica, and a variety of Febearing silicate and carbonate species. Lithofacies are characterized by distinct mineralogical assemblages, suggesting a strong control imposed on mineralogy by the depositional environment. Textural evidence indicates essentially all mineral species present are the product of diagenetic reactions, chiefly silica addition, recrystallization of primary precipitates into oxide, carbonate, and silicate species, and conversion of ferric Fe-bearing precipitates to ferrous Fe-bearing minerals. Magnetite strongly correlates with carbonatebearing iron-formation, and magnetite enrichment suggests significant ferrous Fe mobility during late diagenesis. Wrapping of bedding around early chert concretions in BIF indicates significant volume and mass loss subsequent to deposition. Stylolites are a common feature in both silicate-carbonate BIF and GIF; they too indicate significant mass loss during early diagenesis, likely from carbonate-rich iron-formation precursor sediments. A significant fraction of the precipitates deposited on the seafloor seem to have remobilized soon after deposition. This evidence indicates that iron-formation accumulation is strongly controlled by physical and chemical processes in the benthic zone, and is largely independent of the processes controlling Fe precipitation in the water column. The most abundant elements in iron-formation – Fe, Si, Mg, Mn, Ca, and P – were deposited as chemical precipitates. In contrast, Al, Ti, and to a lesser extent K, were deposited as terrigenous sediment. In the Biwabik IF, Al and Ti strongly correlate, suggesting ILSG 2011 55 Program and Abstracts �they are ultimately sourced from a long-lived, homogeneous reservoir. We suggest this reservoir was ultimately either windblown or suspended clay and silt. Al2O3 concentration in the Biwabik IF averages 0.44%. Concentrations are lowest in GIF, and highest in BIF, ranging from 0.10% in the coarsest GIF to 1.84% in the Intermediate Slate. Assuming PAAS-like compositions, these translate into terrigenous input contributions of 0.4% to 9.5%, averaging 2.5%. In comparison, the insoluble residue of modern Bahamian periplatform carbonates is 1-2% (Boardman et al. 1986). We infer that the bulk of Biwabik IF sedimentation therefore occurred in an environment similarly isolated from terrigenous input as the modern Bahamas platform. These lines of evidence suggest that the Biwabik IF and its correlative ironformations were deposited in a single continental-scale depositional system. The bulk of the iron-formation accumulated in shallow-water depositional environments in conjunction with carbonate, likely at very low rates. Indeed, a lack of any significant terrigenous input may have been the key factor responsible for accumulation of significant thicknesses of ironformation. The clear textural, geochemical, and mineralogical evidence for co-precipitation of ferric iron and carbonate suggests a depositional environment essentially analogous to Paleozoic epeiric sea carbonates, if not modern platform carbonates. We suggest that deposition of the Biwabik IF and its correlative iron-formations occurred in an epeiric sea (and adjacent continental margins) that nearly, if not completely, inundated the Superior craton through much of Biwabik IF time. Iron-formation a was a ubiquitous feature of the Paleoproterozoic sea, and accumulation on the Superior craton only ceased when renewed orogenic activity along the craton margin (e.g. Penokean orogeny) significantly increased the flux of terrigenous sediment onto the craton, with consequent dilution of iron-formation accumulation. REFERENCES Boardman, M.R., Neumann, A.C., Baker, P.A., Dulin, L.A., Kenter, R.J., Hunter, G.E., and Kiefer, K.B. 1986. Banktop responses to Quaternary fluctuations in sea level recorded in periplatform sediments. Geology 14: 28-31. ILSG 2011 56 Program and Abstracts �Platinum-Palladium-Copper-Nickel mineralization at the Thunder Bay North Deposit, Ontario A.D. MacTavish1, G.J. Heggie1, V.R. Goodgame2, J.R. Johnson1, A.E. Beswick3, W.E. Stone4, and K.P. Watkins5 1 Magma Metals (Canada) Limited, 1004 Alloy Drive, Thunder Bay, Ontario, Canada P7B 6V1 2 Excelsior Mining Corp., Suite 1240, 1140 West Pender Street, Vancouver, British Columbia, Canada V6E 4G1 3 Consulting Geoscientist, 11 Little Panache Road, Whitefish, Ontario, Canada P0M 3E0 4 Magma Metals Limited, Suite 2700, Brookfield Place, 161 Bay Street, Toronto, Ontario, Canada M5J 2S1 5 Magma Metals Limited, Level 3, 18 Richardson Street, West Perth, Western Australia WA6005 The drill discovery of the Thunder Bay North (TBN) Pt-Pd-Cu-Ni deposit by Magma Metals (Canada) Limited (Magma) in late 2006 was the result of the initial 2001 discovery of glacially transported mineralized peridotite boulders on the west shore of Current Lake. The TBN deposit is located ~50 km northeast of the city of Thunder Bay, Ontario. Since December 2006 Magma has drilled 630 holes (120,000 m) that have defined 9.5 million tonnes (Mt) indicated at 2.3 gpt Pt Equivalent (Pt-Eq) and 0.27 Mt inferred at 2.8 gpt Pt-Eq. Conceptually, 90% of the defined deposit can be mined by an open pit. Recoverable metals include Pt, Pd, Cu, Ni, Ag, Au, Rh and Co. The TBN deposit occurs within the 1120±23 Ma Current Lake Intrusive Complex (‗CLIC‘) which crosscuts Archean rocks of the Quetico Sub-province. The CLIC is one of three mafic-ultramafic intrusive complexes which are part of a network of magma conduits associated with the Keweenawan age Mid-Continent Rift. The conduits comprise narrow, nearly flat-lying, sinuous tubes and small tabular bodies that are classified as chonoliths, rather than sills or dykes. The emplacement of these conduits is controlled by pre-existing sub-horizontal and sub-vertical structures. Local conduit morphology is controlled by the rheology of the enclosing country rocks. The Archean country rocks consist of variably deformed fine wackes and siltstones metamorphosed to lower amphibolite-grade, and granitoid rocks typified by foliated granodiorite and tonalite. Conduits occurring within the metasedimentary rocks form tabular bodies, whereas those within granitoid rocks form tubes. The CLIC is undeformed, only weakly altered, and has been preserved in almost the same orientation as it formed. The earliest CLIC intrusive phases are a complex series of variably inclusion-rich, hybridized gabbro to diorite rocks that exhibit a wide variety of textures. The hybrids intrude along preexisting sub-horizontal and sub-vertical structures, are hematized, and contain carbonate ocellae. The mineralized mafic-ultramafic rocks are emplaced through these earlier hybrid rocks, and comprise two or more pulses of sulphide-bearing peridotite and olivine melagabbro. The mafic-ultramafic rocks exhibit chemical differentiation, but lack layering, and all internal contacts are gradational. The upper portions of the conduits contain diffuse zones of varitextured (taxitic) gabbro. Serpentinization of olivine is observed near the conduit margins and is attributed to late magmatic (deuteric) processes, rather than postmagmatic hydrothermal activity and metamorphism. The host rocks immediately adjacent to ILSG 2011 57 Program and Abstracts �the CLIC are hematized for 5 to 10m. Metasedimentary hanging-wall rocks exhibit strong in-situ brecciation, carbonatization, epidotization and sericitization, and contain up to 5% finely disseminated pyrite. This brecciation and alteration is less prominent in metagranitoid hanging-wall rocks. Mineralization within the TBN deposit is dominated by thick zones of disseminated pyrrhotite, chalcopyrite, pentlandite, and minor cubanite. Disseminated mineralization commonly fills the entire conduit within the Current Lake and Bridge zones of the deposit. Small, lenticular, very high-grade zones of net-textured and massive sulphides occur within troughs and depressions at the base of the conduit within the western Beaver Lake Zone. Most of the mineralization within the central and eastern Beaver Lake Zone is disseminated to blebby and is also confined to basal troughs and depressions. In addition to the basal mineralization, the ‗Cloud Zone‘ forms a diffuse zone containing <2% very finely disseminated chalcopyrite that occupies the upper third of the conduit in the northern Beaver Lake area. Sub-horizontal and sub-vertical sulphide veins and veinlets, up to 3 cm thick, are common within the Current Lake and Bridge zones, but rare within the Beaver Lake Zone. Composite sulphide blebs, up to 2 cm in diameter, are common throughout the wellmineralized portions of the deposit. Angular to sub-angular, fractured or broken sulphide fragments, up to 3 cm in length, are minor, but observed throughout the deposit. There is a strong positive correlation of sulphide abundance to base and precious metal grades. A very strong positive correlation of Pt, Pd, Cu and Ni indicates a pristine magmatic system with little re-distribution of metals. The Pt:Pd ratio value is 1.07; the Ni:Cu ratio value is approximately 0.5:1.0; the PGE and base metals tenor is relatively consistent throughout most of the deposit; and the chalcophile metal pattern is fractionated. The magmatic conduit system presently defined within the TBN area is comparable in size to other sulphide-rich magma conduits elsewhere. The CLIC shares a number of common features with the conduits observed at Noril‘sk-Talnakh (Russia): 1) the mineralized intrusive rocks form chonoliths; 2) both systems exhibit sill-like marginal mafic rocks; 3) taxitic (varitextured) phases are observed in both; 4) both systems contain both basal and internal mineralization; 5) both magmatic systems were dynamic, 6) both are closely associated with large igneous provinces, and 7) both show evidence of marked wall rock alteration. The CLIC is part of a much larger magmatic system which formed from a deep sourced magma, focused magma flux, and considerable wall-rock assimilation. The sulphides within the TBN deposit are strongly fractionated (PGE and Cu-rich), which reflects entrained sulphides being transported to their present location from a crystallizing and fractionating Ni-rich sulphide trap located deeper within the system. This scenario strongly suggests there may be one or more Ni-rich, massive sulphide deposits located magmatically upstream from the TBN Deposit. Magma continues to explore along strike from the defined TBN deposit and has identified low-grade mineralization at 1040 m depth, 2 km to the southeast, within the downplunge extension of the CLIC. Exploration also continues on the two other conduit systems identified within The Thunder Bay North land package. The most advanced work is at the Steepledge Lake Intrusive Complex (SLIC), located 3 km west of the CLIC, where drilling has intersected rocks and disseminated Pt-Pd-Cu-Ni mineralization similar to that present within the CLIC. ILSG 2011 58 Program and Abstracts �Using Credit-by-Exam to connect advanced high school geology courses to university geology programs: Lessons learned from a state-wide pilot study in Michigan Stephen Mattox1 and Sandra Rutherford2 1 Department of Geology, Grand Valley State University, Allendale, MI 49401-9403, mattoxs@gvsu.edu 2 Geography and Geology Department, Eastern Michigan University, Ypsilanti, Michigan 48197, srutherf@emich.edu To address the national shortage of geologists and to diversify the geoscience workforce we are constructing a seamless path from rigorous high school geology classes taught by well-trained teachers to geoscience programs at state universities and colleges. This program is modeled on an existing high school – university collaboration that has added a significant number of students to the career pipeline. We have obtained Memorandums of Understanding (MOU) from school districts with high populations of diverse students that have administrators and teachers ready to join the program. Discussions and visits with department chairs and faculty at numerous state universities and colleges resulted in a second set of Memorandums of Understanding to award college credit to students that successfully pass a rigorous credit by exam. Although geologists have vigorously lobbied for an AP course and exam in geology, the College Board has resisted because they perceive that demand will be low. The lack of an AP course has made it difficult for those high schools nationwide that offer advanced (i.e., college-level content) geology courses to obtain appropriate recognition for their students‘ accomplishments. Mattox designed a test, with input from my GVSU peers, that includes 60 multiple choice, 10 essay questions, a map test with skills and landforms, and a rock and mineral exam (essentially what we use in our physical geology introductory course). We give the exam over 5-6 hours on two different days. About 70-80% of students from a local high school pass. With the same exam at a different high school no students passed, although several were close. This exam is prepared, reviewed, and administered by geology department faculty. The authors received a NSF planning grant titled ―Collaborative Project: Collaborations for Building Michigan Geology Talent‖ and significant accomplishments include: Mattox visited eight state universities and colleges and discussed credit-by-exam and the high school advanced geology exam with geology department faculty and university administrators. A template MOU was drafted for high schools and for universities. To date, seven signed MOUs to award credit-by-exam and be involved with the high school advanced geology exam have been received from EMU, GVSU, Lake Superior State University, MTU, Wayne State University, Western Michigan University, and Hope College (a private college). Geology faculty at two other universities (Central Michigan University and University of Michigan-Dearborn) are guiding MOUs through administrative review at their institutions. Michigan State University is interested but currently focused on curriculum restructuring of their geology program. Mattox is currently in dialogue with ILSG 2011 59 Program and Abstracts �faculty at the University of Michigan and Northern Michigan University to establish site visits and visits with administrators. By the end date of this planning grant at least 7 of 10 state universities with B.S. geology degree programs will have signed agreements to award credits to students that pass an approved high school advanced geology exam. Recruiting targeted high schools with underrepresented groups throughout Michigan has been initiated in our planning grant. Rutherford received MOUs from school districts, including the Detroit school district with 30 high schools and diverse student populations. Administrators and teachers are interested in adding the geology course. If funding continues, we plan to support existing highly trained teachers that are ready to offer the course. Teachers that need additional training will be supported to earn a M.S. in geoscience education from EMU. Our proposal also includes funds for teaching materials. The MOU asks the high school to return the materials they were provided and releases the obligation to pay for the teachers‘ Masters if the high school does not teach the advanced geology class. Additional insights from this project include: We have developed a poster to be displayed in high school classrooms where the advanced geology course and exam are being offered. Administrators were surprising receptive to and supportive of a rigorous high school geology credit by exam course. Numerous teachers are excited at the prospect of earning a M.S. degree to be better prepared to teach the geology credit by exam course. Many university faculty were unaware that credit by exam was already available at their university. This mechanism made establishing a high school credit by exam easier. After reviewing the existing exam most faculty immediately embraced the idea of credits that pass the exam. We have demonstrated the feasibility of establishing a statewide network of universities to award college credit for passing a credit by exam during a high school geology course. This material is based upon work supported by the National Science Foundation under OEDG Grant No. NSF 08-605 1006652. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. ILSG 2011 60 Program and Abstracts �2010 Precambrian field camp mapping in the Jack Lake area, Cook County, Northeastern Minnesota Jim Miller, Ben Brooker, Max Hadley, Levi Markwood, Jeff Olson, and Alex Tomlinson Precambrian Research Center, University of Minnesota Duluth, Duluth, MN 55812 As part of the 2010 Precambrian field camp, a crew of five students and an instructor (Miller) conducted five days of field mapping bedrock geology in the Jack Lake area of northeast Minnesota. This area is composed of intrusive and volcanic rocks formed during the 1.1 Ga Midcontinent Rift (MCR) in northeastern Minnesota. This year‘s mapping project expanded on capstone mapping projects conducted in 2007 (Frost et al., 2007) and 2009 (Blakely et al., 2009). These mapping projects have focused on an as yet unnamed, well differentiated layered mafic intrusion that forms the western part of the Brule Lake –Hovland gabbro of Miller et al. (2001). A 1:12,000-scale bedrock geologic map was generated of a 4 mile by 1 mile area that profiles the igneous stratigraphy of the intrusion. A pdf version of this and other Precambrian field camp capstone geologic maps can be downloaded from the PRC website: www.d.umn.edu/prc/fieldcamp/capstone. Prior to the detailed mapping conducted by the PRC field camp students, the general geology of this area was poorly known. Township-scale outcrop mapping by Grout et al. (1959) showed the area to be dominantly composed of gabbroic to felsic intrusive rocks, mafic volcanic rocks and minor volcanogenic sedimentary rocks. They noted that the gabbroic rocks are well layered and portions are particularly rich in Fe-Ti oxide. Reconnaissance mapping by Davidson (1972) designated the mafic intrusion as an olivine gabbro unit that he interpreted as an early intrusion relative to granophyric and anorthositic rocks in the area (Davidson, 1977; Davidson and Burnell, 1977). The regional geologic map of northeastern Minnesota, which focused on the geology of the Duluth Complex (Miller et al., 2001), incorporated Davidson‘s (1972) mapping of the area, but his olivine gabbroic unit is now interpreted to be a relatively late intrusion that is roughly equivalent to the Beaver Bay Complex to the south. 2010 capstone mapping was conducted along a north-south-trending chain of lakes and rivers that included Kelly Lake, Jack Lake, Weird Lake, Mama Bear Lake, and the Temperance River. Exposures along the shorelines of these waterways provided a nearly complete profile across the moderately south-dipping internal structure of the unnamed mafic intrusion and the overlying Eagle Mountain granophyre. The igneous stratigraphy of the approximately 2 km-thick intrusion generally defines a progressively differentiated sequence. However, it is not clear whether this differentiation took place entirely by in situ fractional crystallization. The lowest unit exposed in the north of the map area, along the Temperance River, is a complex mixture of medium-grained, augite troctolite to olivine gabbro and fine-grained, intergranular gabbroic rocks of uncertain origin – diabase intrusions or hornfels basalt inclusions? Overlying this mixed rock unit, the small Mama Bear Lake area, is an augite troctolite (Pl+Ol cumulate) unit that is ophitic to subophitic, poorly to moderately foliated, and medium- to coarsegrained. The augite troctolite grades into an interval that contains abundant hornfels inclusions of basalt and volcanogenic sandstone. The rock hosting the inclusions ranges from augite troctolite to olivine gabbro. This inclusion-rich unit, which is exposed at the north end of Weird Lake, is overlain by a very thick sequence of foliated, intergranular oxide gabbro to olivine oxide gabbro (Pl+Cpx+Ox±Ol cumulate). This unit locally contains cm- to m-thick layers rich in Fe-Ti oxides and occasional inclusions of gabbroic anorthosite. The lower section of this oxide gabbro unit is intruded by a complex network of intermingled diabase, ferromonzondiorite, and quartz monzonite that mostly appear to be sill-like. These mixed intrusion also occur higher in the sequence where they appear to be more dike-like. In the northern part of Jack Lake, the oxide gabbro abruptly grades into an apatite oxide gabbro with the appearance of 3-5% apatite prisms (Pl+Cpx+Ox+Ap cumulate). The apatite ILSG 2011 61 Program and Abstracts �oxide gabbro unit is homogeneous, medium-grained, and well foliated. The oxide gabbro and the apatite oxide gabbro units are each about 800 m thick and comprise about 80% of the intrusion stratigraphy. At the south end of Jack Lake, the apatite oxide gabbro grades into a prismatic ferromonzodiorite, which grades into a ferromonzonite, and which in turn grades into micrographic leucogranite of the Eagle Mountain granophyre. The transition from apatite oxide gabbro to leucogranite occurs over a 200 m-thick interval and appears to represent a zone of mixing and assimilation between the younger mafic intrusion and the older granophyre body. The Jack Lake sequence generally correlates with the igneous stratigraphy mapped to the east during 2007 and 2009 capstone projects. The main correlation horizon that links the Jack Lake sequence with areas to the east is the interval rich in hornfels inclusions of basalt and volcanogenic sandstone. This interval can be traced for over six miles from the north end of Weird Lake, to the north end of Vern Lake, and along the north shore of Homer Lake. This volcanic screen was actually identified by Grout et al. (1959), but curiously is not shown on Davidson‘s maps (Davidson, 1977; Davidson and Burnell, 1972). Below the hornfels-rich horizon, the troctolite-dominant lithologies of the Temperance River /Mama Bear Lake area generally correlate with what Frost et al. (2007) and Blakely et al. (2009) termed the Axe Lake sequence to the east. The oxide gabbros and apatite oxide gabbros of the Jack Lake area generally correlate with what is called the Homer Lake sequence to the east. One notable difference is that the 800m-thick apatite oxide gabbro unit in the Jack Lake area dramatically thins by an order of magnitude to the east. Plans are to return to the area for the 2011 capstone and continue mapping to the west. In addition, Ben Brooker will be conducting more mapping throughout the area next summer, as well as petrographic and mineral chemical studies, to better understand the emplacement and crystallization history of this intrusion for his MS thesis at UMD. A major objective of Ben‘s study is to evaluate the genetic relationship between the lower troctolitic cumulates and the upper oxide gabbroic cumulates. Although these rock types could represent the progressive differentiation of a mafic magma, their separation by a screen of volcanic and interflow sedimentary rocks suggests that they may represent two discreet intrusions formed by different parental magmas. Ben also promises to name the intrusion. REFERENCES Blakely, S., Brown, A., Foley, D., Rowland, A., Stifter, E., and Miller, J., 2009, Bedrock Geology Map of Homer Lake and Adjacent Areas; Cook County, Northeastern Minnesota: University of Minnesota Duluth, Precambrian Research Center, PRC/MAP-2009-01, 1: 12,000. Davidson, D.M., Jr., 1972. Eastern part of Duluth Complex. In: Sims, P.K. & Morey, G.B. (eds.) Geology of Minnesota - A Centennial Volume. Minnesota Geological Survey, p. 354-360 Davidson, D.M., Jr., 1977, Cherokee Lake Quadrangle, Cook County, Minnesota: Minnesota Geological Survey, Miscellaneous Map Series, M-30, 1:24,000. Davidson, D.M., Jr., and Burnell, J.R., 1977, Brule Lake Quadrangle, Cook County, Minnesota: Minnesota Geological Survey, Miscellaneous Map Series, M-29, 1:24,000. Frost, S.J., Juda, N.A., and Miller, J., 2007, Bedrock Geology Map of Homer Lake and Adjacent Areas; Cook County, Northeastern Minnesota: University of Minnesota Duluth, Precambrian Research Center, PRC/MAP-2007-02, 1: 12,000. Grout, F.F., Sharp, R.P., and Schwartz, G.M., 1959, The Geology of Cook County Minnesota: Minnesota Geological Survey Bulletin 39, 163p. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.E., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota. Miscellaneous Map Series, M-119, scale 1:200,000 ILSG 2011 62 Program and Abstracts �The Mineral Exploration Trifecta Dean Peterson, Phil Larson, Gabriel Sweet, and Jack Gibbons Duluth Metals Limited, 306 West Superior Street, Suite 610, Duluth, MN 55802 Mineral exploration is the research activity of the mining industry. At its foundation are ideas, visions, or intuitive geologic thoughts that are evaluated through repeated phases of data gathering (geological, geochemical, and geophysical studies), interpretation, and drilling. Mineral deposit models have arisen as mental paradigms that act as guidebooks to help geologists process information. Effective mineral deposit models increase discovery rates and decrease exploration and mining costs. However, it is becoming increasingly apparent that analog deposit targeting models are decreasing in their effectiveness. Adherence to these decreasingly effective analog deposit targeting models has created a misalignment between future societal mineral resource needs and the ability of the mineral exploration industry to meet those needs. Over the last several decades, companies have focused on brownfields exploration around existing mines with a trend toward lower grade open-pit resources at the expense of greenfields exploration for the next generation of high-quality deposits. In addition, we have moved into an energy-constrained and environmentally-concerned world and global society‘s demands that the mining industry reduce its physical, social and environmental footprint are translating into legislation. Duluth Metals recognizes these challenges and has implemented a three-part fundamental philosophical shift in our mineral exploration programs to support the discovery of new, high-quality ore deposits. We at Duluth Metals have coined this shift the ―Mineral Exploration Trifecta‖, which utilizes (1) the mineral system approach, (2) innovative technology, and (3) trained people. Mineral System Approach A mineral system approach to exploration attempts to understand ore deposits as expressions of multi-scale systems encompassing the Earth that focus mass and energy transfer to ultimately form ore deposits. This approach to mineral exploration leads to systematic, scale-dependent targeting models and allows for the recognition of the largestscale footprints of ore-forming systems. Of particular importance to us at Duluth Metals is the recognition of the physical magmatic processes in the Duluth Complex intrusions we are exploring. We view all these processes from the perspective of the initial conditions of the intrusion in general (and the target area in detail) and attempt to understand the chemical processes that are intimately paired with these physical processes. We ask each other simple questions like: (1) Where did magmas in intrusion X behave in a manner that was conducive for the concentration of ore minerals? (2) Where are their changes in the geometry of intrusions Y, i.e., areas of flow expansion, and areas of flow focusing? Technology The quantity and quality of data available to exploration geologists increases greatly with each passing year. Great innovations in geophysical techniques, geochemical analyses, and geospatial data management occur seemingly every month and a sound mineral exploration program must create and utilize these data in inventive ways into their ore ILSG 2011 63 Program and Abstracts �deposit targeting models. Duluth Metals is fully engaged in the innovations being developed for the minerals exploration industry and have: (1) acquired state-of-the-art airborne geophysical data over a 160,000 acre survey area and have seemingly innumerable 2D and 3D datasets of magnetics, gravity, and conductivity; (2) set in motion a large geochemical survey that will utilize QEMSCAN & MLA technologies as well as in-the-field geochemical analyses via a hand-held XRF; and (3) have built an impressive computer system with stateof-the-art software (ArcMap GIS, Oasis Montaj, Encom PA) to integrate all of these data into a single exploration platform. People Duluth Metals recognizes that the science of mineral exploration is more than just applied economic geology. It is a distinct discipline in its own right that embraces aspects from many other fields including; probability theory, risk-management, economics, and decision science. Most importantly, mineral exploration depends on well trained people who know geology and know how to think big, utilize technology, and protect the environment. Duluth Metals is a proud sponsor of the Precambrian Research Center and has other research ties to the University of Minnesota to help answer focused earth system questions (e.g. AMS to understand magmatic flow directions ILSG 2011 64 Program and Abstracts �Paleomagnetism of Midcontinent Rift rocks from the northern shore of Lake Superior (Ontario, Canada): Preliminary results Elisa Piispa1, Aleksey Smirnov1, Lauri Pesonen2 1 Department of Geological and Mining Engineering and Sciences, Michigan Technological University, 630 Dow, ESE Building, 1400 Townsend Drive, Houghton, MI 49931, USA 2 Department of Physics, University of Helsinki, P.O. Box 64, FIN-00014 Helsinki, Finland We report the preliminary results of our paleomagnetic and rock magnetic investigations of a suite of extrusive and intrusive rocks representing the Mesoproterozoic Midcontinent Rift system along the northern shore of Lake Superior in Ontario, Canada. Specifically we investigated: (1) The abundant ~1144 Ma lamprophyre dykes near Marathon. These dykes, coeval with the Abitibi diabase dyke swarm, possibly represent the first magmatic stage of the Midcontinent Rift; (2) The ~1100 Ma copper-nickel-PGE-mineralized layered intrusion named Crystal Lake Gabbro South of Thunder Bay; (3) The ~1115 Ma Logan Sills predominating in the Southern Thunder Bay; (4) At least three different dyke sets: the NE –trending Pigeon River dykes with two potential magmatic episodes at ~1079 and ~1141 Ma, the ~1109 Ma NW-trending Cloud River dykes, and the ~1109 Ma Mount Mollie dyke; (5) The rocks of ~1108 Ma Coldwell Complex which represents the largest alkaline intrusion related to the Midcontinent Rift; and (6) Newly discovered Devon Township basaltic outcrops NE of Thunder Bay with an unknown age. A preliminary set of 220 independently oriented samples of these rocks were subjected to a detailed paleomagnetic study using both alternating field and thermal demagnetization, baked contact tests, and the measurements of magnetic hysteresis and other rock magnetic properties. In combination with the recently published high precision ages, we plan to use these paleomagnetic data to test the models of the apparent geomagnetic reversal asymmetry during the Precambrian as well as to further refine both the Apparent Polar Wander Path between 1144 and 1079 Ma and the evolution of the Midcontinent rifting sequence. ILSG 2011 65 Program and Abstracts �The use of S/Se ratios in magmatic Ni-Cu-PGE sulfide deposits and its implications for exploration: Example of the Duluth Complex, Minnesota, USA Matthias Queffurus and Sarah J. Barnes Université du Québec à Chicoutimi, DSA, matthias.queffurus@uqac.ca S/Se ratios have been used in the study of magmatic Ni-Cu-PGE sulfide deposits as petrogenetic indicator of ore forming processes (e.g., Eckstrand et al., 1989). Despite the utility of S/Se ratios, no study has been dedicated specifically to the characteristics of the processes that could modify these ratios. We have compiled S, Se, δ34S and metals data from more than fifty magmatic Ni-Cu and PGE deposits and added some new Se data for the country rocks from the Duluth Complex with the aims of: a) characterizing the deposits, b) better constrain the application of S/Se and, c) considering a wider range of processes than have been discussed in the past. Se concentrations were determined by TCF-INAA technique (Thiol Cotton Fiber - Instrumental Neutron Activation Analysis), developed by Savard et al. (2006). Our results indicate that some processes, other than those mentioned in the past, i.e. the assimilation of country rocks and the remobilization of S post-cumulate, could modify the S/Se ratios and these include: a) changes in silicate to sulfide liquid ratio, R-factor (Thériault and Barnes, 1998); b) the segregation of the sulfide liquid (Barnes et al., 2009); c) a fractionation between MSS and ISS. These processes influence the concentrations of S and Se in mineralized rocks, i.e. S/Se ratio, and may overprint the original source of S and the interpretation of the genesis of the deposit. At the Duluth Complex, where country rocks assimilation is believed to play a key role in the genesis of the mineralization, the preliminary results of our work show that most of the Virginia Formation samples have S/Se ratios lower than the ore. Only the samples close to the contact of a particularly S-rich layer, known as the Bedded Pyrrhotite Unit (BPU) and the xenoliths, have S/Se ratios higher to than sulfidebearing igneous rocks. The concentration of S, high S/Se ratio and sulfur isotopic values of rocks samples from the BPU strongly indicate that this layer would be the main source of S assimilated by the mafic magma. REFERENCES Eckstrand, O.R., Grinenko, L.N., Krouse, H.R., Paktunc, A.D., Schwann. P.L., and Scoates, R.F., 1989, Preliminary data on sulphur isotopes and Se/S ratios, and the source of sulphur in magmatic sulphur in magmatic sulphides from the Fox River Sill, Molson Dykes and Thompson nickel deposits, northern Manitoba. In: Current Research, Part C. Geological Survey of Canada v. 89-1C, p. 235-242. Barnes, S.-J., Savard, D., Bédard, L.P., and Maier, W.D., 2009, Selenium and sulfur concentrations in the Bushveld Complex of South Africa and implications for formation of the platinum-group element deposits. Mineralium Deposita, v. 44, p. 647-663. Savard, D., Bédard, L.P., and Barnes, S.-J., 2006, TCF selenium preconcentration in geological materials for determination at sub-μg.g-1 with INAA (Se/TCF-INAA): Talanta v. 70, p. 566-571. Thériault, R.M., and Barnes, S.-J., 1998, Compositional variations in Cu–Ni–PGE sulfides of the Dunka Road deposit, Duluth Complex, Minnesota: the importance of combined assimilation and magmatic processes: Canadian Mineralogist v. 36, p. 869-886. ILSG 2011 66 Program and Abstracts �Reconnaissance Bedrock Geological map of the northern part of Sudan Underground Mine State Park and the northwestern part of Lake Vermilion State Park, St. Louis County, Minnesota Amy L. Radakovich, Charlie T. Parent, Molly E. Partridge, Andrew D. Ritts, Rita Pierce, and George J. Hudak Precambrian Research Center, Natural Resources Research Institute, University of Minnesota Duluth, 5013 Miller Trunk Highway, Duluth, MN 55811 The ―capstone‖ project for the Precambrian Research Center (PRC) summer field camp encompasses detailed field mapping and map production in small groups with faculty from the field camp. During the fifth and sixth weeks of the 2010 field camp, five PRC field camp students, under the direction of PRC Faculty member George Hudak, mapped Neoarchean rocks in the northeastern part of Soudan Underground Mine State Park and the northwestern part of Minnesota‘s newest state park, Lake Vermilion State Park (Radakovich et al., 2010). This capstone mapping project sought to: 1) identify the lithologies and stratigraphy of the Neoarchean supracrustal strata in this area; 2) define and characterize the nature of the contact between various units of the Neoarchean supracrustal strata and intrusive rocks; 3) obtain a better understanding of geological structures and their orientations within the area; and 4) to make a comprehensive bedrock geological map for future use by the new Lake Vermilion State Park. Prior to mapping, detailed 1:5000 scale laminated field mapping sheets were produced. One side of each field mapping sheet consisted of a georeferenced air photo, and the other side of the mapping sheet consisted of a corresponding georeferenced topographic map. Mapping was completed by means of lakeshore mapping from canoes, as well as numerous traverses through the bush. Following each day of field mapping, students and faculty transferred their field data to a master map, enabling the generalized geology of the region to be established by the middle of the fifth week of field camp. During the sixth week of field camp, students produced a digital version of the field map utilizing a variety of software (ArcView, AutoCad, Surfer, Adobe Illustrator). Neoarchean supracrustal rocks consist of a NE/SW striking, NW facing homoclinal sequence comprising three distinctive rock sequences. Up-section, these sequences are: (1) the Central Basalt Sequence of the Lower Member of the Ely Greenstone Formation, which is composed of medium green to dark green, aphyric to sparsely plagioclase-phyric, sparsely amygdaloidal sheet flow- and pillowed facies basalt lava flows which are locally strongly quartz-epidote altered; (2) the Soudan Member of the Ely Greenstone Formation, an interbedded sequence comprising: a) laminated to medium-bedded, planar-bedded, locally chaotically folded, dark gray to red-brown, Algoma-type oxide facies iron formation; b) medium- to dark green, aphyric to locally sparsely plagioclase-phyric, massive to moderately amygdaloidal andesite to basalt sheet flow facies lava flows; and c) localized gray to gray green, quartz ± plagioclase-phyric rhyodacite tuffs and polymict lapilli tuffs which are generally less than 10m in stratigraphic thickness. (3) the Gafvert Lake Member of the Lake Vermilion Formation, which is dominantly composed of intermediate to felsic volcaniclastic rocks comprising: a) a basal unit composed of massive, light gray, quartz- and plagioclase-phyric polymict dacite to ILSG 2011 67 Program and Abstracts �rhyodacite tuff; b) interbedded light gray, massive, quartz- and plagioclase-phyric dacite to rhyodacite tuff and pumiceous lapilli tuff; c) epiclastic deposits comprising light gray to brownish-gray, polymict volcaniclastistic conglomerates and sandstones derived primarily from felsic rocks; and d) white to gray, thinly-bedded to massive, fine- to locally coarse-grained dacite to rhyodacite tuff breccias which locally exhibit graded bedding. Chemical sedimentary rocks composed of: a) light gray to black, laminated to very thickly bedded chert deposits; and b) light gray to dark gray, laminated to medium-bedded Algoma-type iron formation, occur in two distinctive stratigraphic horizons up to 300 meters thick in the central and northeastern parts of the field area. Intrusions into the supracrustal assemblages include feldspar ± quartz porphyritic dacite to rhyodacite dikes and sills, medium-grained granodiorite dikes, and medium-grained gabbro sills. An east-west striking shear zone in the northeastern part of the field area is composed of highly foliated basalt or chlorite schist. Our analysis leads us to interpret the following sequence of events for the development of the bedrock geology visible today in this part of the Vermilion District: 1) deposition of submarine basalt lava flows, comprising both sheet flows and pillowed facies, in a relatively deep water (>500 meters water depth) setting (Peterson and Patelke, 2003; Hudak et al., 2007); 2) deposition of Algoma-type iron formations of the Soudan Member of the Ely Greenstone Formation as a result of hydrothermal activity during volcanically quiescent periods, (Peterson, 2003) and deposition of interbedded mafic and felsic volcanic and volcaniclastic strata associated with intermittent volcanism; 3) the onset of voluminous explosive dacitic to rhyodacitic felsic volcanism, alternating with periods of volcanic quiescence characterized by continued chemical sedimentation and localized clastic sedimentation; 4) synvolcanic to post-volcanic intrusion of mafic to felsic sills and dikes; and 5) structural deformation, probably associated with regional D2 deformation (Peterson and Patelke, 2003), that formed the sheared mafic rocks and chlorite schists that occur in the northeastern part of the field area. REFERENCES Hudak, G. J., Hoffman, A. T., Peterson, D. M., and Heine, J., 2007, Recent developments understanding the volcanic, magmatic, tectonic and metallogenic evolution of the Ely Greenstone Formation, Vermilion District, NE Minnesota. Institute on Lake Superior Geology, Proceedings Volume 53, Part 1 – Programs and Abstracts, p. 42-43 Peter, J. M., 2003, Ancient iron formations: their genesis and use in the exploration for stratiform base metal sulphide deposits, with examples from the Bathurst Mining Camp: in Lentz, D. R., ed., Geochemistry of Sediments and Sedimentary Rocks: Evolutionary Considerations to Mineral Deposit-Forming Environments: Geological Association of Canada, GeoText 4, p. 145-176. Peterson, D. M., and Patelke, R. L., 2003, National Underground Science and Engineering Laboratory (NUSEL): Geological Site Investigation for the Soudan Mine, NE Minnesota: Natural Resources Research Institute, Technical Report NRRI/TR-2003/29, 88 p. Radakovich, A. L., Parent, C. T., Partridge, M. E., Ritts, A. D., Pierce, R., and Hudak, G. J., 2010, Reconnaissance Bedrock Geological Map of the northern part of Soudan Underground Mine State Park and the northwestern part of Lake Vermilion State Park, St. Louis County, Minnesota: Precambrian Research Center Map Series, PRC/MAP-2010/04, 1:5000 scale. ILSG 2011 68 Program and Abstracts �Fluid flow events in the Biwabik Iron Formation, Minnesota Ryan Rague and Steven Losh Dept. of Chemistry and Geology, FH 241, Minnesota State University, Mankato MN 56001 Fluids have interacted with banded iron formation rocks of the Paleoproterozoic (~1.85 Ga) Biwabik Fm, exposed in the Mesabi range, during diagenesis and dissolution/oxidation. Burial diagenesis likely took place shortly after deposition at temperatures of around 150° – 200° C based on mineral assemblages and published oxygen isotope thermometry (Perry et al., Econ. Geol. 68, 1110-1125), whereas dissolution of silicates and carbonates and oxidation of iron to hematite (forming ‗natural ore‘) has long been interpreted as a much younger supergene phenomenon associated with intense chemical weathering, perhaps during the Cretaceous. However, fluid inclusion and bulk geochemical analysis (specifically rare earths) of veins and whole-rock samples, respectively, from the Hibbtac, Thunderbird, Thunderbird South, Fayal, and LTV#6 pits, shows that oxidation may have taken place over a range of burial depths and temperatures, hence over a protracted period of time. Quartz vein samples from a variety of paragenetic settings (mainly from unoxidized low-angle faults, high-angle veins (+/- calcite +/- minnesotaite) without oxidation, and veins in high-angle faults (+/- Fe oxides) associated with oxidation) show considerable overlap of fluid inclusion homogenization temperatures (mean = 154° +/- 74° C, n= 467) and salinity (9.5 +/- 5.3 wt % NaCl equivalent, n=166) in the sample set. These temperatures are at the low end of those inferred for diagenesis; addition of a temperature correction due to fluid pressure, estimated at ~ 50° C corresponding to ~ 3-4 km burial depth, improves the correlation. Notably, fault breccia samples associated with oxidized (red hematitic) iron formation in the Hibbtac and Thunderbird South pits show elevated temperature and salinity, not consistent with supergene alteration. Rare earth element analysis of samples of unoxidized and oxidized iron formation and veins from nearby faults indicate that fluids that precipitated quartz in the fault zones had locally interacted with a large amount of iron formation, acquiring its rare earth element ‗fingerprint‘ in terms of positive europium, thulium, and/or cerium anomalies (relative to NASC). In the Fayal pit, ‗natural ore‘ and associated quartz / calcite + hematite fault breccia are both characterized by strong cerium anomalies consistent with near-surface oxidation. In the Hibbtac pit, both oxidized iron formation and quartz + hematite fault breccia associated with oxidized ore show no cerium anomaly. Instead, both rock types show pronounced Tm anomalies (as does unoxidized iron formation at that location), indicating local control on the chemistry of fluids associated with oxidation and absence of large volumes of externallyderived supergene fluids during oxidation of iron formation adjacent to the sampled fault. Thus an early oxidation event, separate from late supergene ‗natural ore‘ formation, may have tapped a mixture of diagenetic and oxidizing meteoric waters during uplift shortly after burial. ILSG 2011 69 Program and Abstracts �Preliminary stratigraphy and physical volcanology associated with the Paleoproterozoic Back Forty VMS deposit, Menominee County, Michigan Cabin Ross1, George Hudak2, Ron Morton1, Tom Quigley3, and Bob Mahin3 1 Department of Geological Sciences, University of Minnesota Duluth, Duluth, MN 55812 2 Precambrian Research Center, NRRI, University of Minnesota Duluth, Duluth, MN 55811 3 Aquila Resources Inc., E807 Gerue Street, Stephenson, MI 49887 Aquila Resources‘ Back Forty volcanogenic massive sulfide (VMS) deposit is the most recent VMS discovery within the Wisconsin Magmatic Terrane. The deposit is located approximately 200 meters east of the Menominee River, approximately 25 kilometers west of Stephenson in Michigan‘s Upper Peninsula. This research, which represents the first detailed volcano logical and hydrothermal alteration studies of the Back Forty VMS deposit, aims to: 1) establish the physical, chemical, and spatial attributes of the lithologies associated with the Back Forty deposit; 2) establish the stratigraphy in the vicinity of the Back Forty deposit and its relationship with VMS mineralization; 3) establish the physical, chemical, and spatial attributes of metamorphosed hydrothermal alteration mineral assemblages associated with the Back Forty deposit; 4) evaluate the spatial relationships of the alteration mineral assemblages to the Back Forty deposit mineralization; 5) identify chemical variations associated with alteration mineral phases at the Back Forty deposit, and evaluate the spatial relationships of these mineral compositional variations with the Back Forty mineralization; and 6) develop a coherent geological model of the volcanic setting and mineralizing environment associated with the genesis of the Back Forty VMS deposit. Our study involved a multi-faceted approach including geological mapping, diamond drill core logging, petrographic, lithogeochemical, and mineral chemistry analyses. Geological mapping of the Back Forty deposit and surrounding area was completed at a 1:100 or 1:500 scale. Information mapped at each outcrop included descriptions of lithology, alteration minerals present, alteration intensity, mineralization, structural features and measurements, and other unique features (i.e. weathering patterns, magnetism, etc.). Regional mapping was limited to outcrop sampling with reference to previous work by Greenberg and Brown (1984). The drill cores investigated were selected based on location within mineralized zones, and previously defined stratigraphy and alteration types (Quigley et al., 2008). Twenty-three diamond drill core were selected throughout the mineralized zone with an additional 22 cores evaluated for regional lithology and alteration zone characteristics (a total of approximately 10,000 meters of drill core). 170 samples were selected for petrographic analysis based on lithological distribution, alteration mineral representation, variable spatial relationships to massive sulfides zones (i.e. above, below, or away from), and volcanic textures. Seventy-seven samples were submitted to Activation Laboratories Ltd. (ACTLABS, Ancaster, Ontario) for whole rock lithogeochemical major and trace element analysis. Preliminary stratigraphy of the Back Forty deposit includes four main tuff units: A) massive, coarse-grained, quartz-phyric, rhyolitic lapilli tuff with 7-10%, <1-5mm euhedral to subhedral quartz crystals and angular quartz crystal fragments in a coarse, inequigranular quartz-rich matrix containing possible pumice and glass fragments; B) massive, quartzfeldspar-phyric, rhyolitic-rhyodacitic tuff with 5-7%, <1-5mm, subhedral quartz crystals and angular quartz crystal fragments and 1-3%, <1-2mm subhedral feldspar crystals and feldspar ILSG 2011 70 Program and Abstracts �crystal fragments in a variably coarse- to fine-grained, quartz-rich matrix; C) massive, finegrained, quartz-phyric, rhyolitic-rhyodacitic tuff breccia with 5-7%, <1-4mm quartz crystal fragments and quartz shards in a fine-grained, quartz-rich matrix that contains fragments of unit A; D) massive, coarse-grained, aphyric, rhyolitic-rhyodacitic tuff to tuff breccia containing quartz-phyric tuff fragments in an aphyric, coarse-grained, quartz-rich matrix. Units A-C are interbedded with laminated ash and tuffaceous sedimentary rocks. Compositional classification (Winchester and Floyd, 1977) and tectonic affinity (Pearce at al., 1984) based on immobile trace elements indicate that host rocks for the Back Forty mineralization are arc-associated FII-type rhyodacites and rhyolites. Alteration mineral assemblages and alteration zoning are still under review. Initial studies indicate the presence of four primary alteration minerals of various abundances: sericite, Mg-chlorite, iron-rich chlorite and quartz. Mineralization at the Back Forty is generally bounded by laminated tuffaceous sedimentary rocks within the massive tuff units and could indicate conformable massive sulfide zones. However, remnant textures in massive sulfide zones suggest replacement-style mineralization (Doyle and Allen, 2003). To date, drilling at the Back Forty deposit has outlined a combined NI 43-101 compliant measured and indicated resource (open pit and underground development potential) of 18.1 million tonnes grading 0.19% copper, 2.48% zinc, 1.63 grams/tonne gold, and 20.04 grams/tonne silver (Aquila Resources‘ Press Release, October 15, 2010; www.aquilaresources.com). Preliminary stratigraphic interpretations place tuff units A-D from oldest to youngest, respectively. Further analysis is needed to more completely define these relationships. Detailed physical volcanological environment is under investigation. Thick sequences of felsic volcanic rocks may be consistent with a caldera setting; however, more widespread regional stratigraphic studies will be needed to further test this interpretation. REFERENCES Doyle, M.G. and Allen, R.L., 2003, Subsea-floor replacement in volcanic-hosted massive sulfide deposits: Ore Geology Reviews, 23, 183-222. Greenberg, J.K. and Brown, B.A., 1984, Bedrock Geology of Wisconsin, Northeast Sheet: Wisconsin Geological and Natural History Survey Map 84-2, Scale 1:250,000. Pearce, J.A., Harris, N.B.W. and Tindle, A.G., 1984, Trace element discrimination diagrams for the tectonic interpretation of granitic rocks: Journal of Petrology, 25, 956-983. Quigley, T., Mahin, B., and Aquila Field Office Geologic Staff, 2008, Back Forty Geology and Mineralization: 54th Annual Institute on Lake Superior Geology, Field Trip #3. Winchester, J.A. and Floyd, P.A., 1977, Geochemical discrimination of different magma series and their differentiation products using immobile elements: Chemical Geology, 20, 325-343. ILSG 2011 71 Program and Abstracts �Whole rock geochemical analyses of sheared granitic rocks from Mountain, Wisconsin Brittany J. Saylor, Jonathan C. Stencil, Michael A. DeVasto, and Prajukti Bhattacharyya University of Wisconsin – Whitewater, 800 W. Main St,. Whitewater, WI 53190 Ductile deformation deep within the earth‘s crust and mantle are localized within narrow bands called shear zones. The purpose of our research is to gain a better understanding of how chemistry and mineral alignment patterns change across shear zones. We conducted our research on samples collected from around the Mountain Shear Zone exposed near the town of Mountain, Oconto County, Northern Wisconsin (Figure 1a-b: Sims et al. (1989a, b, 1991)) concluded that rocks in the study area belong to the eastern part of the Pembine-Wausau Terrane, which was accreted to the Superior craton during the Penokean Orogeny approximately 1.8 billion years ago. The study region was affected by ductile shearing about 25-35 million years after the main continent-arc collision event of the Penokean Orogeny (Sims et al., 1990, 1991). Most of the shear zones of comparable age in Wisconsin are buried under glacial deposits, so the exposed Mountain Shear Zone provides an excellent opportunity to study rock deformation processes that have taken place deep within the earth. Here, we present our preliminary observations on whole rock geochemistry, variation of magnetic susceptibility values along and across deformed samples collected from the study area. In order to see whether whole rock chemistry changes with shear deformation we collected both deformed and undeformed granitic rock samples from the study area (Figure 1b), and recorded their locations using a hand-held GPS unit. We analyzed the whole rock geochemistry of selected samples using X-Ray Fluorescence (XRF) at the UW Milwaukee Campus to see how the whole rock chemistry differs in deformed and undeformed rocks. We also used the Magnetic Susceptibility Meter SM 30 to measure how the magnetic susceptibility values change within the same deformed samples. The major minerals in the studied samples are quartz, potassium feldspar, plagioclase, biotite, and hornblende, with normative magnetite. Based on the QAP diagram, the analyzed samples can be classified as grandodiorite. The studied samples can be classified as peraluminous S-type granite common in orogenic belts. We used samples M0513-O, M0513-Q, and M1003 as examples of undeformed rocks, and M1002-1 and M1002-A was taken from different parts of a sample showing a shear zone. The results of the chemical analysis show that the chemical compositions of even undeformed samples are different from each other. One explanation for this might be that there was more than one episode of minor granitic intrusion after the shearing event. Observations of rock texture in sample M1002-AB seems to support this hypothesis. The magnetic susceptibility values per unit area of rock surface changes with distance in deformed samples, but seems to show no correlation with mineral alignment patterns. This variation could be due to higher concentration of magnetic minerals like magnetite in some parts of the analyzed samples as indicated by normative mineralogy. We need to conduct detailed geochemical analyses including REE and trace element analyses to see if we can identify different generations of granite in the study area. We also ILSG 2011 72 Program and Abstracts �need to conduct detailed mineralogical and textural analyses using an optical microscope to study the effects of shearing on our samples. Figures Figure 1a Figure 1b Figure 1. (a). A simplified regional geologic map modified after Sims, (1990) showing the study area outlined in red. (b). Detailed geologic map of the area around Mountain Shear Zone, modified after Sims, (1989a). REFERENCES Sims, P.K. 1989a. Geologic map of Proterozoic rocks near Mountain, Oconto County, Wisconsin: U.S. Geological Survey Miscellaneous Investigations Series map I-1903, scale 1:24,000 Sims, P.K., Van Schmus, W.R.., Schulz, K.J., and Peterman, Z.E., 1989b, Tectonostratigraphic evolution of the Early Proterozoic Wisconsin magmatic terranes of the Penokean Orogen. Canadian Journal of Earth Sciences, 26, 2145-2158. Sims, P.K., Klasner, J.S., and Peterman, Z.E. 1990. The Mountain shear zone, northeastern Wisconsin – a discrete ductile deformation zone within the early Proterozoic Penokean Orogen. U.S. Geological Survey bulletin,1904, Contributions to Precambrian geology of Lake Superior region Sims, P.K., Klasner, J.S., Day, W.C., and Peterman, Z.E. 1991. Mountain shear zone, Oconto County, Wisconsin – A post-Penokean discrete ductile deformation zone. Field trip guide, 37th Annual Meeting of Institute on Lake Superior Geology, Eau Claire, Wisconsin. ILSG 2011 73 Program and Abstracts �Comparison of microscale and regional scale structural control on gold mineralization in the North Caribou Greenstone Belt, Superior Province, Northwestern Ontario Robert Scott1, Maura Kolb1, and Mary Louise Hill1 1 Dept. of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, P7B 5E1, Canada; rjscott@lakeheadu.ca The North Caribou greenstone belt is bounded on the north and east by the North Caribou-Totagan Lake shear zone that divides the North Caribou terrane from the Island Lake domain. The greenstone belt is host to the gold deposit at Musselwhite Mine, where gold is found in banded iron formation metamorphosed to the lower amphibolite facies. Microstructural analysis shows that gold is commonly found in the strain shadows of garnet where strain is lower. Strain shadows at the regional scale may also provide an area of lower strain that serves as a preferential location for gold mineralization. Understanding regional structural control on gold mineralization will provide an important tool in the exploration for gold. On the microscopic scale, less competent matrix minerals (including quartz, biotite and grunerite) deform in a ductile manner while more competent minerals (including garnet and arsenopyrite) are more rigid and deform in a brittle manner. The competency contrast between garnet and the matrix creates strain shadows around the garnet; these areas of low strain are favourable locations for gold mineralization. Similarly, the lithological competency contrast between rigid granitic plutons and the more ductile metasedimentary and metavolcanic greenstone belt creates areas of low strain on the regional scale. The North Caribou batholith shows microstructural evidence of solid state deformation by dislocation creep along its eastern margin, indicating that it was crystallized and solid before being deformed by the North Caribou-Totogan Lake shear zone system. Although the batholith is more competent and therefore strain is localized in the more ductile greenstone belt, the edge of the batholith is affected by the deformation. At Libert Lake, a splay off the main North Caribou-Totagan Lake shear zone appears to anastomose around the Southern pluton. In this area there is evidence for recovery through grain boundary area reduction (GBAR) in addition to evidence for dislocation creep. GBAR suggests a significant decrease in strain to allow for recovery at the elevated temperatures of metamorphism. There is a common spatial relationship between gold deposits and strain shadows on the regional scale, some of which are major gold camps including Hemlo, Red Lake and Musselwhite Mine, where the gold deposits are situated in greenstone belts proximal to several granitic plutons. The low strain (strain shadow) areas are produced by competency contrast between plutons and greenstone belts and in these cases host significant gold mineralization. Regional structural control of gold mineralization is similar in geometry to the microstructural control, and lithological competency contrast may be a useful tool for predicting the occurrence of other economic orogenic gold deposits. ILSG 2011 74 Program and Abstracts �The Pillar Lake Volcanics: new insights into an enigmatic mesoproterozoic volcanic suite near Armstrong, Ontario Mark Smyk1, Peter Hollings2, and Robert Cundari2 1 Ontario Geological Survey, Ministry of Northern Development, Mines and Forestry, Suite B002, 435 James St. South Thunder Bay, ON P7E 6S7 Canada 2 Department of Geology, Lakehead University 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada The Pillar Lake volcanic suite was first recognized and mapped by Macdonald (2004) and is the subject of an ongoing study by Magee (in progress). Initial observations suggested that it largely consisted of a sequence of flat-lying pillowed and massive flows and autoclastic and hyaloclastic breccias. The volcanic rocks unconformably overlie the Badwater gabbro (~1599 Ma) and Badwater syenite (~1590 Ma) and are capped by Keweenawan Inspiration diabase sills (1159 + 33 Ma), yielding an apparent thickness of 20 to 40 m (Hart and Macdonald 2007; Heaman et al. 2007). Titanite in an andesitic unit yielded a 207Pb/206Pb age of 1129.0 + 4.6 Ma; no zircon nor baddeleyite was recovered in the sample (Heaman et al. 2007). Reinvestigation of these volcanic rocks in 2010 was prompted by a new exposure south of Armstrong that had been created during ballast quarry development. The quarry face exposes a ~15 m section of thin (0.5 to 2 m), flat-lying, variably altered, columnar-jointed, basaltic andesite flows, capped by a diabase sill. Individual flows may persist over the 130 m length of the exposure while others bifurcate and terminate as thin tendrils in flow breccia. Autobrecciated zones are rubbly weathering and occupy the spaces between thin, pinching flows. Thin flow top breccias separate flows. Massive flows contain zones of pipe amygdules at their bases and tops. The morphology and disposition of these flows is suggestive of an intercalated, subaerial pahoehoe and a‘a flow succession. Ropy flow tops were noted at a nearby location for the first time during this investigation. Ellipsoidal features seen in some sections that were initially considered to be pillow forms may now be reinterpreted as pahoehoe toes. These volcanic rocks are variably altered along fractures, flow contacts and within brecciated zones. This hydrothermal alteration is typically manifested as a beige to pink discolouration of the dark grey-black flows resulting from the destruction of primary ferromagnesian minerals and the introduction of alkali feldspar, sericite and quartz. Void spaces along joints and fractures and in vesicles has been occupied by large (< 3 cm), black, euhedral actinolite crystals. Small acicular asbestiform crystals of edenite were also identified by x-ray diffraction analysis. Alteration is characterized by increases in Al 2O3, K2O, Na2O, and SiO2 and decreases in CaO, Fe2O3, MgO, P2O5 and TiO2. The trace element geochemistry of the sill that caps the quarry face is identical to that of the Inspiration sill identified by Hart and Macdonald (2007) and we propose that this sill is part of that intrusive suite. Samples of the volcanic rocks display REE enrichment and negative Nb anomalies comparable to the range of samples of the Pillar Lake volcanic suite reported by Magee (personal communication, 2011). Samples of the Inspiration sills are geochemically indistinguishable from the Pillar Lake volcanic rocks and to least-altered pillowed samples with the lowest LOI values. The similarity between these two enigmatic ILSG 2011 75 Program and Abstracts �suites suggests that they may be derived from the same source. MacDonald and Tremblay (2005) distinguished the Inspiration sill, which exclusively overlies the Pillar Lake rocks, on the basis of its normal polarity and distinct geochemistry. Primary clinopyroxene in the Inspiration sill was commonly replaced by actinolite (Schandl, 2004). MacDonald et al. (2005) also noted a remarkable lack of chilled margins on the Inspiration sill and suggested that it was older than the Nipigon sills. Later work by Heaman et al. (2007) determined an age of 1159±33 Ma for the Inspiration sill, which is consistent with the early period of normal polarity. We propose that the Pillar Lake basalts may in fact be may be coeval with the Inspiration sill which, in turn, may represent either a subvolcanic intrusion or possibly a massive, ponded flow / lava lake. This may account for the extensive alteration in the Pillar Lake basalts with the sill acting as an impermeable cap to hydrothermal fluids, which were concentrated in the underlying volcanic flows. The similarity of the Pillar Lake and Inspiration sill magmatism to other magmas of the MCR, including the Nipigon sills and Osler volcanic rocks (Hollings et al., 2007), suggests that these rocks may represent a very early extrusive to subvolcanic phase of rift activity. The location of these rocks close to the ~1599 Ma Badwater gabbro, the ~1590 Ma Badwater syenite and the ~1540 Ma English Bay anorogenic granite, which have been interpreted as evidence for a long-lived crustal weakness (Hollings et al. 2004), may offer an explanation for how these magmas were erupted so early in the rift history. REFERENCES Hart, T.R., MacDonald, C.A., 2007. Proterozoic and Archean geology of the Nipigon Embayment: Implications for emplacement of the Mesoproterozoic Nipigon diabase sills and mafic to ultramafic intrusions. Canadian Journal of Earth Sciences 44, 1021–1040. Heaman, L.M., Easton, R.M., Hart, T.R., Hollings, P., MacDonald, C.A., Smyk, M., 2007. Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon Region, Ontario. Canadian Journal of Earth Sciences 44, 1055–1086. Hollings, P., Hart, T., Richardson, A., MacDonald, C.A., 2007a. Geochemistry of the Mesoproterozoic intrusive rocks of the Nipigon Embayment, northwestern Ontario: evaluating the earliest phases of rift development. Canadian Journal of Earth Sciences 44, 1087–1110. Hollings, P., Fralick, P., Kissin, S., 2004. Geochemistry and geodynamic implications of the Mesoproterozoic English Bay Granite-Rhyolite complex, northwestern Ontario. Canadian Journal of Earth Sciences 41, 1329–1338. MacDonald, C.A. 2004. Precambrian geology of the south Armstrong-Gull Bay area, Nipigon Embayment, northwestern Ontario; Ontario Geological Survey, Open File Report 6136, 42p. MacDonald, C.A. and Tremblay, E. 2005. Lake Nipigon Region Geoscience Initiative, Bedrock Mapping Project: Geology of the northwest Nipigon Embayment; unpublished poster, Ontario Geological Survey. MacDonald, C.A., Tremblay, E. and Easton, R.M. 2005. Precambrian geology of the west-central map area, Nipigon Embayment, northwestern Ontario, Lake Nipigon Region Geoscience Initiative; Ontario Geological Survey, Open File Report 6164, 49p. Magee, A. (in progress). Geology and geochemistry of the Pillar Lake volcanic sequence, northwestern Ontario; unpublished M.Sc. thesis, Carleton University, Ottawa ON. Schandl, E.S. 2004. Petrographic data from the northwest Nipigon Embayment, Lake Nipigon Region Geoscience Initiative (LNRGI); Ontario Geological Survey, Miscellaneous ReleaseData 141. ILSG 2011 76 Program and Abstracts �Microstructural control on gold mineralization in greenschist to amphibolite facies metamorphism, Greenstone, Ontario Victoria R. Stinson and Mary Louise Hill Dept. of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, P7B 5E1 Canada; vrstinso@lakeheadu.ca In Greenstone, Ontario, north of Lake Superior in the Superior Province, gold mineralization is associated with regional shear zones within the Beardmore-Geraldton and Onaman-Toshota greenstone belts. Metamorphic grade increases from greenschist facies near Jellicoe, Ontario to amphibolite facies in Longlac, Ontario. Gold is hosted within various lithologies, including iron formation, mafic volcanics, and granodiorite, typically in association with ductile and brittle-ductile structures such as deformed quartz veins. On the microscopic scale, gold is found along quartz-quartz and quartz-muscovite grain boundaries, as inclusions in porphyroblasts of pyrite and albite, and in pressure shadows around competent porphyroblasts. Gold is present in association with similar microstructures regardless of lithology and metamorphic grade. Mafic volcanics at the Prodigy Gold Incorporated Hercules and Castlewood properties north of Jellicoe have been metamorphosed to the greenschist facies with stable metamorphic mineral assemblages that include chlorite and albite. Gabbro at the Prodigy Gold Inc. Milestone property in Longlac has been metamorphosed to amphibolite facies with a stable metamorphic mineral assemblage of hornblende and plagioclase. Within shear zones at Milestone, epidote and chlorite replace hornblende indicating retrograde greenschist facies metamorphism during deformation after peak amphibolite facies metamorphism. All samples with gold mineralization from both Jellicoe and Longlac have microstructures characteristic of ductile deformation at temperatures of greenschist facies metamorphism or higher including quartz grains with irregular grain boundaries, undulose extinction in quartz, and subgrains in quartz. In samples from Longlac with gold mineralization, plagioclase has undulose extinction consistent with ductile deformation at temperatures of amphibolite facies metamorphism or higher. In all samples, gold is found along subgrain boundaries in quartz and along grain boundaries, commonly within muscovite. Pyrite commonly has strain fringes of quartz that have been rotated, indicating non-coaxial strain. Inclusions of gold are within porphyroblasts of pyrite and albite, and also within pressure shadows around the porphyroblasts. Gold is present in similar microstructures regardless of metamorphic grade. ILSG 2011 77 Program and Abstracts �A deformation history of the Ivanhoe Lake Fault Brittney Swoffer and Mary Louise Hill Lakehead University, Department of Geology, 955 Oliver Road, P7B 5E1, Thunder Bay, Ontario, Canada. bswoffer@lakeheadu.ca Ivanhoe Lake is located 220km east of Wawa, Ontario, Canada. The Ivanhoe Lake Fault runs northeast to southwest and is the boundary between the high grade metamorphism of the Kapuskasing Structural Zone and the low grade metamorphism of the Abitibi Subprovince. This difference in grade of metamorphism is what makes the Ivanhoe Lake Fault so interesting to study. Through analysis of the microstructures seen in thin section a long deformation history was revealed. Ductile deformation characteristic of deep burial as well as deformation at or near the brittle-ductile transition zone were evident. This affects a wide area that extends to either side of the fault. This was followed by brittle fracturing at a shallower depth of burial. The brittle deformation is confined to a narrow zone found along Ivanhoe Lake. Examples of ductile deformation within the thin sections include: undulatory extinction in quartz, subgrains in quartz and feldspars, deformation twins in plagioclase and microcline, as well as irregular grain boundaries for all grains. The brittle-ductile transition zone is documented by the existence of pseudotachylites. These glassy rocks are characteristic of large faults. The brittle history is documented by quartz or calcite filled fractures found at both the outcrop and microscopic scale. These brittle structures commonly cross cut the ductile features. This cross-cutting relationship suggests that the fault has had a long history which began with ductile deformation and progressed into brittle deformation. ILSG 2011 78 Program and Abstracts �Geology and mineralization of the Serpentine Cu-Ni deposit, Duluth Complex Minnesota Erik R. Tharalson and Thomas Monecke Department of Geology and Geological Engineering, Colorado School of Mines, 1516 Illinois Street, Golden, Colorado 80401, USA, etharals@mines.edu The Serpentine Cu-Ni deposit is located approximately 7km southeast of Babbitt, MN and is hosted within the South Kawishiwi intrusion, in the 1.1 Ga Duluth Complex (Fig. 1). The footwall rocks to the deposit are comprised of metagreywacke and banded iron ores of the Paleoproterozoic Virginia and Biwabik Iron Formations. Figure 1: Generalized geologic map showing the locations of deposits within the Duluth Complex (modified from Severson and Hauck, 2008). The South Kawishiwi intrusion has been studied in detail and an igneous stratigraphy has been established by Severson (1994). However, the identified igneous stratigraphy is not recognizable in the area of the Serpentine deposit, possibly due to the occurrence of a local magmatic feeder intruding the stratigraphy or even the occurrence of separate intrusion. The Serpentine deposit is located immediately adjacent to the Grano fault (Fig. 1), which is believed to have been instrumental in controlling the location of a feeder for the Bathtub intrusion (Severson & Hauck, 2008) and the Local Boy massive sulfide deposit (Severson, 1994). Drill core logging in the Serpentine area has identified a heterogeneous mix of troctolitic rocks with minor norites. Igneous textures are characterized by subhedral to euhedral plagioclase with intergranular olivine, augite, and oxides. Plagioclase is locally enclosed in subophitic to ophitic augite. The Serpentine deposit was discovered in 1958 by AMAX following up an EM survey (Kulas, 1979). The deposit consists of two resources: (1) a disseminated sulfide resource yielding 430 million tons of 0.41% Cu and 0.14% Ni, and (2) a semi-massive sulfide resource of 7 million tons of 0.88% Cu and 0.30% Ni (Kulas, 1979). Disseminated mineralization occurs as thick intervals (>50 feet) of interstitial sulfides making up <5% by volume. Pyrrhotite is the dominant sulfide, but chalcopyrite and cubanite locally occur in ILSG 2011 79 Program and Abstracts �equal amounts. The semi-massive and massive sulfides are pyrrhotite-rich with up to 7% copper sulfides and minor pentlandite. The massive sulfide intervals display sharp and gradational contacts and can be up to 19 feet thick. They often display variable textures; including rounded pyrrhotite with interstitial copper sulfides and pegmatitic pyrrhotite with wormy, crosscutting copper sulfides. Most of the semi-massive to massive sulfide occurs within the footwall Virginia Formation or at the basal contact of the Virginia Formation with the South Kawishiwi intrusion. Sulfur isotope values for the Virginia Formation were analyzed by Arcuri and others (1998) and found to range from 8.1 to 29.1 per mil, with most values above 15 per mil. New sulfur isotope values for massive sulfides ranging from 13.1 to 16.3 per mil are consistent with the Virginia Formation representing the principal sulfur source. Understanding the deposit geology and reasons for the accumulation of massive sulfide at Serpentine will help to better focus exploration in developing this deposit. Continued research will establish whether the deposit occurs in proximity to a local magmatic feeder. REFERENCES Arcuri, T., Ripley, E.M., and Hauck, S.A., 1998, Sulfur and oxygen isotope studies of the interaction between pelitic xenoliths and basaltic magma at the Babbitt and Serpentine Cu-Ni deposits, Duluth Complex, Minnesota. Kulas, J.E., 1979, Serpentine reserve; MINNAMAX Project Babbitt, Minnesota: Internal AMAX Company Report, 17p. Severson, M.J., 1994, Igneous stratigraphy of the South Kawishiwi intrusion, Duluth Complex, northeastern Minnesota: Natural Resources Research Institute, Technical Report NRRI/TR-93/34, 210 p., 15 pls. Severson, M.J., and Hauck, S.A., 2008, Finish logging of Duluth Complex drill core (and a reinterpretation of the geology at the Mesaba (Babbitt) deposit): Natural Resources Research Institute, Technical Report NRRI/TR-2008/17, 68 p., 94 pls. ILSG 2011 80 Program and Abstracts �The mineralogy, spatial distribution, and isotope geochemistry of sulfide minerals in the Biwabik Iron Formation Steph Theriault1, Jim Miller1, Mike Berndt2, and Ed Ripley3 1 Department of Geological Sciences, University of Minnesota-Duluth, Duluth, MN 55812 2 Division of Lands and Minerals, MN Department of Natural Resources, St. Paul, MN 55155 3 Department of Geological Sciences, Indiana University, Bloomington, IN 47405 Sulfate cycling in the St. Louis River Watershed (SLRW) requires knowledge of the distribution of sulfide minerals undergoing oxidation in the region, including those from the Biwabik Iron Formation (BIF). The BIF occurs along the Mesabi Range in northeastern Minnesota and continues to be extensively mined for taconite. Associated tailings basins and waste rock piles containing BIF and the overlying Virginia Formation are located along the northern portions of the SLRW. Although the BIF is not considered a sulfide-bearing formation, it contains measurable amounts of sulfide minerals, primarily in the form of pyrite and pyrrhotite (Gundersen and Schwartz, 1962). Three notable previous studies have been conducted on the geochemistry of primary sulfide minerals in the Animikie Basin sediments. The sulfur isotope (δ34S) values cited in each of the studies suggest that primary sulfide minerals of the Animikie Group sediments are the result of bacterial reduction of late Paleoproterozoic seawater sulfate (Poulton et al., 2010; Johnston et al., 2006; Carrigan and Cameron, 1991). However, these studies principally focused on primary sulfide minerals which formed during or soon after the deposition of the BIF. This study, which is being conducted in conjunction with the Minnesota Department of Natural Resources sulfate cycling study of the SLRW, seeks to delineate the mineralogy, distribution, and sulfur isotope geochemistry of sulfide minerals in the BIF. This includes minerals formed by secondary processes, such as the thermal metamorphism of the 1.1 Ga Duluth Complex and the formation of natural ores. These data can be used to better define the major source components of sulfate to the SLRW and to help determine how sulfate, once released, behaves in the environment. Samples containing visible sulfide minerals were collected from five well-spaced drill cores across the Mesabi Range (Fig. 1). Each sulfide-bearing stratigraphic subunit of the BIF (upper slaty, upper cherty, lower slaty, intermediate slate, lower cherty) was sampled in each drill core. Commonly multiple samples were collected when various sulfide mineral habits were encountered in a given unit. In all, 123 samples were collected; 63 were prepared for sulfur isotope analysis and 26 were made into polished thin sections for petrographic study of mineral paragenesis and mineral identification. Results were compared to sulfur isotope data for dissolved sulfate in streams leading from the mining operations. Preliminary results show a complex relationship between geographic location, stratigraphic position, and type of sulfide occurrence. A large range of δ34S values were obtained from samples collected for this study, spanning from +80.37‰ to -36.11‰ (see Fig. 1). Also, four different sulfide mineral habits were noted in the BIF: euhedral cubes, euhedral framboids or spheroids, anhedral ―blebs,‖ and veins. Primary sulfides, in this study, appear mainly as anhedral ―blebs‖ but also occurred as euhedral cubes. They are characterized by a limited range of slightly heavy δ 34S values, from +2‰ to +13‰, similar to those cited in the studies mentioned above. In this study, for example, the primary sulfide isotope values were observed throughout the intermediate slate layer and in the eastern-most, metamorphosed portion of the Mesabi Range. Metamorphism, including the transition from pyrite to pyrrhotite, does not appear to have affected sulfur isotopic composition of the sulfide minerals. However, it is possible that the δ34S values were homogenized during metamorphism and now artificially appear primary. Sulfide minerals formed through secondary processes occur in veins or as euhedral framboids, spheroids, or cubes. They correspond to the more extreme δ34S values, which presumably formed through processes such as weathering and hydrothermal alteration. ILSG 2011 81 Program and Abstracts �In conclusion, both primary and secondary processes are evident in the formation of sulfide minerals throughout the BIF, with secondary sulfide values spanning a broader range than the limited, +2‰ to +13‰, values associated with primary sulfides. Sulfur isotope values for sulfates in stream water samples collected close to mining operations have a relatively narrow range of +4‰ to +9‰, as well. This suggests that primary sulfides within the BIF may contribute most of the sulfate found in the streams. The veins and other noted sparse occurrences sampled in this study may also contribute to the sulfate in area streams, but it is possible their contribution is negligible compared to that of the primary sulfides or there is a balance where the average values from secondary sulfides are, coincidentally, isotopically similar to that of the primary sulfides. However, because sampling was conducted only for visible sulfide minerals in this study, it is possible this correlation is only coincidental. Additional sulfur isotope analyses in the region will help to further delineate all possible sources of sulfate to the SLRW. Figure 1. Distribution of sulfur isotope (δ34S) values at each core location across the Mesabi Range in northeastern Minnesota. Values are reported in per mil (‰) from sulfide mineral occurrences taken throughout the Biwabik Iron Formation and lower most portions of the Virginia Formation. REFERENCES Carrigan, W.J. and Cameron, E.M., 1991, Petrological and stable isotope studies of carbonate and sulfide minerals from the Gunflint Formation, Ontario: evidence for the origin of early Proterozoic iron-formation, Precambrian Research, v. 52, p. 347-380. Gunderson, J.N. and Schwartz, G.M., 1962, The Geology of the Metamorphosed Biwabik Iron-Formation, Eastern Mesabi District, Minnesota. Johnston, D.T., Poulton, S.W., Fralick, P.W., Wing, B.A., Canfield, D.E., and Farquhar, J., 2006, Evolution of the oceanic sulfur cycle at the end of the Paleoproterozoic, Geochimica et Cosmochimica Acta, v. 70, p. 5723-5739. Poulton, S.W., Fralick, P.W., and Canfield, D.E., 2010, Spatial variability in oceanic redox structure 1.8 billion years ago, Nature Geoscience, v.3, p. 486-490. ILSG 2011 82 Program and Abstracts �Characterization of gangue minerals in lower cherty ores of the Biwabik Iron Formation at United Taconite LLC Michael Totenhagen1, Penny Morton2, and Phil Larson3 1 M.S. Candidate Department of Geological Sciences, University of Minnesota Duluth, 229 Heller Hall 1114 Kirby Drive, Duluth, MN 55812 2 Associate Dean Swenson College of Science and Engineering, Associate Professor Department of Geological Sciences, University of Minnesota Duluth, 229 Heller Hall 1114 Kirby Drive, Duluth, MN 55812 3 Senior Geologist Duluth Metals, 306 West Superior Street, Suite 60, Duluth, MN 55802 Gangue minerals in lower cherty ores of the Biwabik Iron Formation at United Taconite's Thunderbird mine in Eveleth Minnesota cause an observed decrease in plant water quality during processing. Cliffs Natural Resources (CNR) thinks this problem may be related to a particular gangue mineral staying suspended in plant water. CNR has identified this mineral as a Fe-rich talc occurring in the LC4 ore unit, a cherty thick to thin bedded ore as defined by United Taconite. This project focuses on the gangue minerals in lower cherty ore at United Taconite‘s Thunderbird mine through petrography, X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and microprobe analysis, to describe both the mineral abundance and composition in diamond drill core. Fe-rich talc, minnesotaite, stilpnomelane and Fe-Al rich talc display the most intense variations in abundance, occurrence and composition in comparison with carbonates, oxides and quartz. The total abundance of these minerals varies between 10-70%, with the highest abundance occurring in the LC4 and LC2 ore units. There is no apparent spatial variation along strike (Fig 1. A). However, the LC4 and LC2 ore units contain the majority of Fe-Al rich talc and the LC5 ore unit contains a high concentration of Mg-rich stilpnomelane (Fig 1. B). Some of the Fe-rich talc contains elevated amounts of Al as shown by both SEM and microprobe analysis (Figure 1. A & B). This suggests that there might be an unknown mineral (Fe-Al rich talc) occurring with Fe-rich talc, minnesotaite and stilpnomelane. This Fe-Al rich talc appears to be an early stage mineral as it is commonly overprinted by minnesotaite and stilpnomelane but overprints early quartz and carbonate. ILSG 2011 83 Program and Abstracts �A. B. Figure 1. A. Compositional variation by drillhole of talc-minnesotaite, Al rich talc, and stilpnomelane in the LC4 ore unit. B. Stratigraphic compositional variation in drill hole 1081001. These results are consistent with CNR observations of stratigraphic variation in Ferich talc, however we have identified a Fe-Al-rich talc which has not been previously reported. Both of these minerals might be responsible for the decrease in water quality during processing of some LC4 ore. Further research is needed to follow the character of these minerals through the concentrating process to confirm if the observed variation in abundance, composition, and distribution are responsible for variable results during processing. ILSG 2011 84 Program and Abstracts �Using cleavage refraction and microstructural evidence to determine the relative rheology of naturally deformed quartzites and phyllites from Baraboo, Wisconsin Jolene T. Traut and Dyanna M. Czeck Department of Geosciences, University of Wisconsin–Milwaukee, P.O. Box 413, Milwaukee, WI 53201 Understanding the rheology of rocks is a first-order question in tectonic studies where we aim to understand how rocks flow in response to stress. Rock rheology is largely studied through experimental work, but cannot be easily applied to natural deformation because of the long time-scales involved and the complications derived from rocks containing multiple minerals, all with different strengths. The Baraboo syncline near Baraboo, WI is an ideal test case to compare natural deformation to rheologic experimental results conducted on quartz because the main lithology at Baraboo is a quartzite derived from a clean quartz arenite. Within the Baraboo syncline, the variation in lithology (quartzite and phyllite) results in a difference in how these rocks responded to deformation and metamorphism during the regional contraction from the Mazatzal orogeny. We conducted a detailed analysis of the cleavage refraction through a graded contact between the quartzite and phyllite layers in several areas around the syncline (Figure 1) and combined those results with mineralogy determined by X-ray diffraction (XRD). The angle between bedding and cleavage varies between approximately 89° in the most competent quartzite layers (approximately 99% quartz and 1% pyrophyllite) and 15° in the least competent phyllite layers (approximately 82% quartz and 13% pyrophyllite). Due to the increase in pyrophyllite, cleavage readily forms in the phyllite causing a smaller cleavage/bedding angle than what can be seen in the more competent quartzite. Where possible, we determined effective viscosity ratios using the Treagus (1999) cleavage refraction method. Preliminary cleavage/bedding angle data suggest that the effective viscosity ratios range between 1.0 – 3.7, which are values comparable to other studies. Microstructural evidence indicates that varying degrees of recrystallization of quartz occurred during deformation, and it is possible that the amount of smaller, recrystallized grains respond differently than the original depositional grains. By combining point counting methods and the total quartz abundance determined through XRD, we will be able to extract whether these recrystallized grains impact the overall rheology. ILSG 2011 85 Program and Abstracts �Figure 1: Field photos of cleavage refraction in a graded contact between quartzite and phyllite located in the Williams Quarry near Baraboo, WI on the north limb of the syncline. The black lines represent the orientation of bedding, and the white lines represent the orientation of cleavage as it refracts through the two lithologies. REFERENCES Treagus, S., 1999. Are viscosity ratios of rocks measurable from cleavage refraction? Journal of Structural Geology, 21, 895-901. ILSG 2011 86 Program and Abstracts �Surface and subsurface geologic maps of the Soudan Underground Mine State Park, St. Louis County, Northeastern Minnesota Alexandra M. Vallowe1, Ernest J. Thalhamer2, Damon L. Rhoades3, and Dean M. Peterson4 University of Minnesota Duluth, Precambrian Research Center 1 Virginia Polytechnic Institute and State University 4044 Derring Hall (0420) Blacksburg, VA 24061, USA 2 SUNY College: University at Buffalo, Buffalo, NY 14260-1608 3 University of Central Missouri, Warrensburg, MO 64093 4 University of Minnesota Duluth, University of Minnesota Duluth, Duluth, MN 55812 Three Precambrian Research Center field camp students, under the guidance of Dean M. Peterson, completed an underground and surface mapping project at the Soudan Mine, northeastern Minnesota. The purpose of the mapping was to provide the Soudan Underground Mine State Park an updated geological map of the surface and subsurface geology of the Soudan Mine for use in geologic tours and scientific research. In addition, the underground geologic map of the 27th level has been, and continues to be used extensively in numerous geobiology studies. These maps are the result of five days field mapping by the authors in August 2010. The surface geology (Map 1) was mapped for several hours in the afternoon over a period of five days. The authors worked as individuals to cover the mapping area. The authors recorded notes and observations, collected samples, took structural measurements (bedding orientations, lineation, foliation), and plotted data on a 1:5,000 scale field map. Location was determined using a handheld GPS unit, and structural measurements were taken using a Brunton Pocket Transit. Surface outcrops were exposed by peeling back moss layers and sweeping up remaining soils with whisk brooms. Open pits and other ―manmade‖ exposures were also available within the mapping area, but with limited access. The surface bedrock map displays sheared rocks composed of chlorite (5c) and sericiteschists (5s), intrusive rocks such as granite (Gr) and quartz feldspar porphyry (Qfp), and the Upper Sequence rocks which include oxide facies iron formation (4a), lapilli tuff (2e), and greywacke (3a). The surface map also shows two major shear zones which are truncated by the Soudan fault to the east, as well as a minor shear zone. Our assumptions and interpretations of the surface field data were influenced by previously existing map data. Information from Peterson and Patelke‘s 2003 surface map, as well as historical drill core data (circa 1954) from the Oliver Mining Division of US Steel Corporation was incorporated into building the final map. The geology of the 27th level west drift was mapped by the authors over a period of five nights (5:00 PM – Midnight). The authors worked as a team to locate contacts, collect samples, take structural measurements (cleavage, lineation, fault orientations, etc.), record notes and observations, and plot important features on a field map. Geologic information from horizontal drill holes were used to interpret the geology adjacent to the drift. Locations in the mine were determined by measuring tape. Structural data were measured using a Brunton Pocket Transit; there was no deviation of local magnetic field observed. In contrast to the surface mapping, the drift provided complete outcrop exposure with rare exceptions where the drift walls and ceilings were obscured by pipes and other man-made structures. ILSG 2011 87 Program and Abstracts �The drift map (Map 2) includes rock units that correlate to units on Map 1, however, there are two below ground units, gabbro (Gb) and fragmented schist (5f), that do not show a surface expression. Map 2 includes the geology of the 27th level east drift which was previously mapped by Peterson and Patelke (2003). Map 2 also includes annotations by the authors which indicate the locations of significant drift features that may be relevant for mine employees, visitors, and scientists. REFERENCES Oliver Iron Mining Company., 1953-1954. Soudan Mine Area Worksheet. Peterson, D. M. and Patelke, R.L., 2003.Bedrock Geologic Map of the Soudan Mine Area, St. Louis County, Northeastern Minnesota: Natural Resources Research Institute Technical Report NRRI/TR-2003/29. Scale 1:5,000. Peterson, D.M. and Patelke, R.L., 2003. Geologic Map of the 27th Level East Drift, Soudan Mine. Peterson, D.M. and Patelke, R.L., 2003. National Underground Science and Engineering Laboratory (NUSEL) Geological Site Investigation for the Soudan Mine, Northeastern Minnesota. Economic Geology Group, Natural Resources Research Institute, University of Minnesota Duluth. ILSG 2011 88 Program and Abstracts �Neoarchean magmatism in the NW Superior Craton: Granitoids of the North Caribou Terrane Amanda Van Lankvelt1, David Schneider1, Keiko Hattori1, and John Biczok2 1 Department of Earth Sciences, University of Ottawa, Ottawa, ON K1N 6N5 Canada Goldcorp Canada Ltd., Musselwhite Mine, P.O. Box 7500, Thunder Bay, ON, P7B 6S8 Canada 2 The North Caribou Terrane (NCT), located in northwestern Ontario and eastern Manitoba, is the core of the Archean Superior Province. Like many Archean terranes, the NCT is characterized by central greenstone belts of strongly deformed metavolcanics and metasediments surrounded by variably deformed granitoids. We are investigating these external granitoids that comprise the larger batholiths surrounding the North Caribou Greenstone belt in the NCT in an attempt to understand the magmatic history and processes involved in batholith formation. Mapping and petrologic work identified two distinct groups of granitoids in the region: voluminous, massive to gneissic tonalite, trondhjemite, granodiorite (TTG) batholiths and massive pegmatitic to aplitic K-feldspar-rich granites, which intrude the margins of the greenstone belt and are also ubiquitous throughout the TTG suites. These rocks cross-cut all rock types and occur mostly as convoluted, brittle-ductile dykes (Figure 1). There are two types of pegmatitic rocks: an undated suite consisting primarily of K-feldspar and quartz with minor oxides, including magnetite, and S-type intrusions, containing muscovite, biotite, garnet, and less K-feldspar. Notably, zircon and titanite are absent from pegmatites. The TTG batholiths have a wider variety of compositions, ranging from tonalite to granite (sensu stricto). Important accessory minerals include titanite, zircon, epidote, and hornblende. Epidote tends to occur in cross-cutting quartz veins, which suggests a late- to post-magmatic metasomatic event. This thermal episode was accompanied by moderatetemperature strain as evidenced by undulose extinction and bulging recrystallization in quartz and planar alignment of biotite and amphibole. Migmatitic textures in discrete parts of the batholiths indicate that partial melting of, or melt segregation within, the TTG batholiths could be the source for the K-feldspar-rich pegmatites. We performed Al-in-hornblende geobarometry on several batholith samples spanning granitic, granodioritic and tonalitic compositions; emplacement depths are variable across the batholith, ranging from pressures of 3.0±0.5 kbar (~13 km) to the southeast (tonalite) and 6.5±0.5 kbar (~20 km, granodiorite) to the north of the belt. To complement our geobarometric data, we also carried out U-Pb geochronology on igneous titanites and zircons using LA-ICP-MS methods to determine crystallization ages. The oldest zircons have a ca. 3.0 Ga inherited core, but the majority of grains have well-defined concordant ages of ca. 2850 Ma or ca. 2730 Ma. The former age signature is more prominent in the north of the belt, in contrast to the ca. 2730 Ma age that is more localized in the south-centre margin of the belt. In one sample, zircons have 2850 Ma cores and 2730 Ma rims. Titanite ages are similar to those from zircon. These new U-Pb ages are in accord with the few previously reported ages from the NCT (e.g. Breaks et al. 2001, Klipfel 2002, Wyman et al. 2010). The original core of the Superior Province, recorded in the inherited zircons in this study and detrital and volcanic zircons from elsewhere in the NCT (Percival 2007, Lin et al. 2006) had formed by 3.0 Ga. This crust was subsequently deformed and intruded by, or incorporated into, 2850 Ma granitoids, which form the large batholiths that comprise much of ILSG 2011 89 Program and Abstracts �the NCT. At this stage, the granitoids of tonalitic composition were emplaced at midcrustal levels. Later magmatism, at 2730 Ma, was more evolved with granodioritic to granitic composition. This stage of magmatic activity took place at shallower, mid- to uppercrustal depths. The most recent magmatic activity (granitic pegmatites) was post-deformation, occurred near the brittle-ductile transition, and has a highly evolved composition. The discrete magmatic events recognized in the NCT shows a long history of magmatism, over 300 m.y., and re-working of existing crust in the interior of the craton. Figure 1: Pegmatite intruding strongly lineated amphibolite (Amp). Notice brittle and ductile structures (cracks parallel to lineation and pinched-out amphibolite rafts), suggesting pegmatites were intruded near the brittle-ductile transition. REFERENCES Breaks, F., Osmani, L., DeKemp, E. 2001. Geology of the North Caribou Lake Area, Northwestern Ontario, Ontario Geological Survey, Open File Report 6023. 80 pp. Klipfel, P. 2002. Musselwhite U-Pb zircon and Ar-Ar dates: synthesis and interpretation. Internal report. 23 pp. Lin, S., Corkery, M., Bailes, A., and Davis, D. 2006. Geological evolution of the northwestern Superior Province: Clues from geology, kinematics, and geochronology in the Gods Lake Narrows area, Oxford-Stull terrane, Manitoba. Canadian Journal of Earth Sciences. v. 43. pp. 749-765. Percival, J. 2007. Geology and Metallogeny of the Superior Province, Canada. in Goodfellow, w., ed., Mineral Deposits of Canada: A Synthesis of Major Deposit Types, District Metallogeny, the Evolution of Geological Provinces, and Exploration Methods: Geological Association of Canada, Mineral Deposits Division, Special Publication No. 5, pp. 903-928. Wyman, D.A., Hollings, P., Biczok, J. 2011. Crustal evolution in a cratonic nucleus: Granitoids and felsic volcanic rocks of the North Caribou Terrane, Superior Province Canada. Lithos. in press. ILSG 2011 90 Program and Abstracts �Towards a geochemical characterization of native copper ores of the Keweenaw Peninsula, Michigan Jillian E. Votava and Theodore J. Bornhorst A. E. Seaman Mineral Museum, Michigan Technological University, Houghton, MI 49931 The Keweenaw Peninsula is well known for its world-class native copper deposits. These deposits have been subject to many geologic studies yet the native copper ores are poorly characterized in terms of bulk geochemical composition. This study represents a first step towards a geochemical characterization of the native copper ore. The samples for this study came from the Caledonia native copper mine near Mass, MI since there is underground access. The Caledonia is part of the southern extension of the main native copper district where native copper is hosted in several different mineralized basalt lava flow tops, collectively known as the Evergreen Series. At Caledonia the native copper is hosted by two brecciated flow tops and is accompanied by secondary minerals filling amygdules and spaces between fragments. The secondary minerals include: quartz, feldspar, pumpellyite, chlorite, calcite, and epidote. The Caledonia mine has yielded ~3.1 million kg of refined copper with an average grade of 1.24%. Native copper mineralization is typical of that found elsewhere in the district. For this study, 16 underground grab samples were taken at thirteen locations and 1 background sample of massive, visually unaltered basalt from the surface rock pile. Underground mineralized samples were focused on areas with high and variable degrees of alteration. The goal was to obtain a sufficient number of samples with different amounts of copper that bracketed the average copper in the ore. Thereby, the median abundance of constituents in the suite of mineralized samples will be used to represent the geochemical composition of the native ore. Representative subsets of each grab sample were prepared and analyzed for 64 elements by Actlabs using 4-Acid digestion with ICP or ICP-MS finish, INAA, and Hg by cold vapor FIMS. The Caledonia background sample was compared to fresh tholeiitic basalt (BCR-1) to insure Caledonia background values were reasonable and if not, BCR-1 was used for the background value. Ratios of enrichment for each element were calculated using the median of the mineralized samples. Using these ratios each element was placed into one of 6 categories: enriched, depleted, potentially enriched or depleted, same as background, and indeterminate. Ratios greater than 1.0 were considered to be enriched while ratios less than 1.0 were considered depleted. Ratios around 1.0 were generally considered same as the background (no change). Elements with moderate variance or with some (less than half) of the samples less than the detection limit were placed into the appropriate potentially depleted or potentially enriched category. Those with high variance or with more than half the samples below the lower detection limit were considered indeterminate. The native copper ore is enriched in Cu, Ag, Hg, As, Te, Sr, Ca, Cd, and Se while depleted in Ba, Rb, Na, Nb, K, and V. Each of these elements was plotted against copper and the highest correlations are with Hg and Ag (ρ= 0.597 and 0.635, respectively). Strikingly, when rank correlations between all of these are calculated, Hg and Ag had a rank correlation coefficient of 0.966 and a log-log plot illustrates this high correlation (Fig. 1). Heretofore, the native copper ores were well known for the association of Ag and As with Cu. The abundance of calcite as an alteration mineral is consistent with the enrichment of Ca and Sr. The enrichment of Cd, Te, and Se and potential enrichment of Au is previously ILSG 2011 91 Program and Abstracts �not documented. The strong enrichment of Hg is also previously not documented although geochemical studies of stamp sands suggested this enrichment (Kerfoot, personal communication). That Hg so strongly correlates with Ag suggests a synchronous timing of deposition. The combination of Hg, Se, Te, and Au are often found associated with gold deposits elsewhere and their genetic connection to the native copper deposits is unknown at this time. This study highlights the value in characterizing the geochemical composition of the native copper ores. Additional sampling is needed to validate the results presented here since they only represent one mine. Table 1: Chemical characterization of native copper ore from Caledonia Mine, MI. Values represent amount of element at the median Cu level (1.6%). Categories described in text. Composition of Native Copper Ore with 1.6% Cu at Caledonia Mine Enriched Depleted Ag 9.68 ppm 65 ppb H g As 4.5 ppm Te 0.4 ppm Sr 837 ppm Ca 12.8 % 0.9 ppm C d Se 0.7 ppm Potentially Enriched Pb 25 ppm 1 ppb A u Ba Rb 14 ppm 2.70 ppm Na Nb K V 0.1 % 0.6 ppm 0.38 % 120 ppm Potentially Depleted Zn 52 ppm Cr 140 ppm Same as Background Al 7.79 % Be 0.6 ppm Same as Background Nd 18 ppm P 0.045 % Ce Dy Er Eu Fe 36 ppm 4 ppm 2.3 ppm 1.57 ppm 7.35 % Pr Sb Sc Sm Tb Ge Gd Hf Ho 0.3 ppm 4.6 ppm 1.9 ppm 0.8 ppm La Lu Mn 18 ppm 0.3 ppm 1040 ppm Indeterminate Co Li Re Sn 4.5 ppm 0.2 ppm 21 ppm 4.4 ppm 0.7ppm Mg Ti Tl In Ir W Ta S Mo Bi Th Tm U Y 1.5 ppm 0.3 ppm 0.3 ppm 23 ppm Ga Ni Br Cs Yb Zr 2 ppm 65 ppm 10000 Hg (ppb) 1000 Figure 3: Plot of Ag versus Hg and the apparent relationship between silver and mercury in the samples from Caledonia Mine. 100 10 1 0.1 0.01 1 100 10000 Ag (ppm) ILSG 2011 92 Program and Abstracts �Pyroclastic Magnetite Bombs in the Hemlock Formation Iron County, Michigan Thomas Waggoner1*, Doug Duskin2, Musa Karakus3 and John Gartner4 1 141 Chippewa Negaunee, MI 49866 thomaswaggonergeo@hotmail.com 2 210 Union St. Camden, SC 29020 3 Cliffs NR 550 E. Division Ishpeming, MI 49849 4 PMR 723 Riverside Plaza Iron River, MI 48935 The Mt. Hemlock Stratovolcano has a footprint of at least 2000 square miles and reaches a height of 30,000 feet without inclusion of the two Kiernan Sills. Recent age dating (Schneider, 2002) places a single rhyolite unit age at 1874+-7 Ma. The volcanic pile includes massive/vesicular/pillowed basalts, agglomerates/hyaloclastites, rhyolite flows/tuffs, basic volcanoclastics, ash beds, slates and at least two bands of carbonate chert iron formation. Limited outcrop exposure and drilling has prevented better definition of the detail lithology so each new drill hole offers additional useful information. Basalt outcrops on the west side of Mt. Hemlock (Johnson, 1975; Dann, 1978; Foose, 1981) indicate a high magnetite content and samples from the southwest flank show the iron content doubles in the top 6,000 feet of the Hemlock volcanics. The increase in iron oxide content is associated with two banded iron formations of limited areal extent that formed on the inclined flank of the volcano. In 2007 Prime Meridian Resources drill tested a 2,000 nT total field magnetic anomaly over outcrops that showed traces of chalcopyrite. Located in the NE ¼ NE 1/4 Sec. 32 T. 43N. R. 31 W., inclined NQ diamond core hole KS-102-2 was drilled on an angle to the east designed to intercept the volcanics and test the metal content. The hole penetrated thin cherty zones alternating with thin amygdaloidal basalts near the top followed by a tuff unit, amygdaloidal basalt and ending in pyroclastics consisting of rounded and flattened vesicular bombs of basalt and magnetite resting in fine volcanoclastic sediments. SEM-EDS, XRF and XRD were employed to identify the rock constituents. Iron values ranged from 8 to 25%, mostly as magnetite, while titanium oxide values ranged from 2-6.5%. Both metals are present in unusually high amounts. The TiO2/FeO ratio plotted against FeO illustrates the Hemlock is quite different from other basalts. Even though titanium is elevated, it does not match the larger increase in iron oxide. Basaltic units of the Hemlock Formation have already been reported to contain high percentages of titanium free magnetite (Johnson, 1974, Dann, 1978). Magnetite in some bombs account for up to 80% of the groundmass. The magnetite is confined to the ground mass and does not fill the vesicles indicating primary crystallization. The drill hole is located between the basal pyroxenite and the lower gabbro zone of the differentiated West Kiernan sill. Some metasomatism of the proximal Hemlock occurred during emplacement of the sill. Both the Hemlock and West Kiernan sills have undergone sodium and potassium metasomatism. Original minerals including pyroxene, calcium feldspars and ilmenite were replaced by epidote, albite, titanite, Fe-chlorite, biotite, quartz, potassium minerals, calcite, ilmenite, chalcopyrite, pyrite and calcium REE fluorocarbonate + barite in biotite. Many of these minerals can be found as amygdaloidal fillings. The sodium, silica and potassium alteration has converted the pyroxenes, feldspar and ilmenite to albite, chlorite, biotite and titanite leaving the original magnetite and apatite ILSG 2011 93 Program and Abstracts �unaltered. The subhedral and euhedral magnetite quickly crystallized from the magma and migrated toward the vesicle openings that formed quickly by rapid de-gassing. The fine magnetite crystals range from 3-50 microns in size and are generally devoid of any inclusions, including titanium minerals. It is postulated the gravity separation took place in a quiescent magma chamber where iron oxide rich segregations pooled. The subsequent violent eruption of a gas laden magma allowed for creation of vesicular magnetite pyroclastic bombs that varied in size from pebbles up to more than 8 inches that quickly solidified and deposited in water on the slope of the volcano in a random pattern. High magnetite tholeiitic basalts and magnetite rich bombs are very rare in extrusive volcanic rocks of any age. One example is the high grade magnetite bombs preserved in the 2 Ma El Laco deposit located in Chile (Henriquez, 1998) where they are part of magnetite flows. We can speculate that near cessation of active volcanism an iron oxide rich magma was tapped by sea water and formed a large hydrothermal plume that vented distal to the central vent on the sea bottom instantly forming fine crystals of magnetite or specularite depending on piping, distance or chemistry at the instant of entry into a reduced temperature/pressure gradient. The Fence River iron formation on the east flank of the Amasa uplift exhibits both magnetite and specularite lithologies. Drilling, mapping and analytical examination have shown that parts of the Hemlock Formation originated from unusually rich iron oxide magma that resulted in the creation of vesicular magnetite rich pyroclastics. REFERENCES Dann, J.C., 1978, Major-Element Variation within the Emperor Igneous Complex and the Hemlock and Badwater Volcanic Formations, MTU MS Thesis, 198 pages. Foose, M.P., 1978, Geologic Map of the Ned Lake Quadrangle, Iron and Baraga Counties, Michigan, USGS Map I-1284, Scale: 1:62,500. Henriquez, F., Nastrom, J.O., 1998, Magnetite Bombs at El Laco Volcano, Chile, GFF V. 120 p. 269-271. Johnson, D.J., 1975, Petrology of a Portion of the Hemlock Formation, Iron County, Michigan. MTU MS Thesis, 55p. Schneider, D.A., et al, 2002, Age of Volcanic Rocks and Syndepositional Iron Formation, Marquette Range Supergroup; Implications for the Tectonic Setting of Paleoproterozoic Iron Formations of the Lake Superior Region, Canadian Journal of Earth Sciences, V. 39, p. 999-1012. ILSG 2011 94 Program and Abstracts �Physical volcanology and hydrothermal alteration of the Rainy River Gold Project, Northwestern Ontario Jakob Wartman1, Ron Morton1, George Hudak2 and Cory Hercun3 1 Dept. of Geol. Sciences, Univ. of Minnesota Duluth, Duluth, Minnesota 55812 2 Precambrian Research Center / NRRI, Duluth, Minnesota 55811 3 Rainy River Resources Ltd., P.O. Box 3, Emo, Ontario P0W 1E0 The Rainy River Gold Project (RRGP)is located 75km northwest of Fort Frances, Ontario, within the Rainy River greenstone belt. This advanced stage exploration project has a NI43-101 compliant gold resource of 3.42 Moz indicated and 3.17Moz inferred (from Rainy River Resources Press Release, February 2011) represented by low grade (<2g/t), lowmoderate grade (2-10g/t), and high grade (>10g/t) gold mineralization. The nature of the gold mineralization in this deposit has been the subject of controversy, and several competing models have been proposed to explain its genesis. Initial exploration in 1967 suggested that the deposit wasa shear zone-hosted resource. However, recently completed exploration drilling has now defined large, diffuse zones of gold mineralization in dacitic volcanic and volcaniclastic rocks, suggesting, in part, a syn-genetic genesis for the gold mineralization. While previous studies have examined structural regimes and timing of gold mineralization, this research focuses on the physical volcanology and hydrothermal alteration associated with the deposit. Field mapping is difficult due to a paucity of outcrop, and geological correlations are complicated by polyphase deformation, hydrothermal alteration, and both regional, and locally contact, metamorphism. This study included comprehensive fieldwork involving mapping of all available outcrops at a 1:24,000 scale and exploration drill core logging along three sections totaling ~7,400 m of core. Fieldwork, supplemented with 210 thin sections and 73 geochemical samples, has enabled distinction of stratigraphy, volcanic facies, and hydrothermal alteration assemblages. Drill core is locally intensely altered and deformed, resulting in many of the units having a false pyroclastic appearance. Despite this, strata associated with the RRGP contain some well-preserved primary textures. These primary textures indicate that the volcanic facies in the deposit include coherent dacitic flows and associated syn-volcanic intrusions with autoclastic breccias, hyaloclastites, peperites, and syn- to post-depositional resedimented volcaniclastic deposits. The coherent dacitic flows are massive, range in thickness up to 150 m, and in lateral extent for 2500 meters. Coherent dacite flows grade into a heterogeneous facies, characterized by pods and lobes of coherent dacite, enveloped by autoclastic breccia and hyaloclastite. Flows are interspersed with strongly altered volcaniclastic sediments that are locally punctuated by peperites. Volcanic facies reconstruction indicates the presence of lobe-hyaloclastite dome/flow complex fed, and locally intruded by, synvolcanic dacite hypabyssal intrusions. An apparent feeding fissure is centered to the west of the main area of mineralization. Hydrothermal alteration is widespread throughout the deposit and is marked by silicification, chloritization, sericitization, and carbonitization (as well as minor epidote and local biotite). Quartz, sericite and chlorite are ubiquitous in the deposit. Grant‘s (1986) isocon method and Large et al.‘s (2001) box plot where applied to the rocks at the RRGP and quantify the chemical alteration resulting from the hydrothermal alteration. Alteration ILSG 2011 95 Program and Abstracts �assemblages are dominantly stratabound, and related to original rock permeability, with flow tops, autoclastic breccias, and volcaniclastic sediments being most strongly altered. Shearzones also preserve stronger alteration intensities. Gold mineralization appears to have occurred within a synvolcanic, low-sulfidation (Simmons et al., 2005) epithermal system. Elevated gold values are strongly correlated with highly permeable units and increased alteration intensity, suggesting enhanced mineralization in areas that experienced higher water: rock ratios. Post-volcanic remobilization of the gold appears to have occurred, as the highest gold values in the deposit are spatially related to shear-zones and associated quartz-carbonate-epidote veins. REFERENCES Grant, J. A., 1986, The Isocon diagram—A simple solution to Gresen‘s equation for metasomatic alteration: Economic Geology, v. 81, p. 1976–1982. Large, R.R., Gemmell, J.B., Paulick, H., and Huston, D., 2001, The alteration box plot: A simple approach to understanding the relationship between alteration mineralogy and lithogeochemistry associated with VHMS deposits: Economic Geology, v. 96, p. 957– 972. Simmons, S.F., White, N.C., and John, D.A., 2005, Geological characteristics of epithermal precious and base metal deposits: Economic Geology, 100th Anniversary Volume, p. 485–522. ILSG 2011 96 Program and Abstracts �AUTHOR INDEX PROCEEDINGS VOLUME 57 PART 1— PROGRAM AND ABSTRACTS Amisse, L. and Barnes, S.J .................................................................................................................... 1 Bajjali W., Holstrom C., and Fuller, D. ...............................................................................2 Beh, B. and Fralick, P ..........................................................................................................3 Birkmeier, R., Boley, T., Brannan, B., Doucette, R., Jirsa, M., and Lee, A. .......................5 Boerboom, T.J. and Green, J.C. ...........................................................................................7 Brown B.A. and Schoephoester, P. ......................................................................................9 Buchholz, T.W., Falster A., and Simmons, W. B. .............................................................10 Carl, C., Hollings, P., and Smyk, M. .................................................................................12 Carlson E.J., Boerboom T.J., Holton C.J., Kubitza, K.W., Mulvey, L., and Scheurer, E. ................................................14 Cervin, D., Morton, P., Miller J., and Patelke, R. ..............................................................16 Chandler, V.W. and Lively, R.S. .......................................................................................18 Chandler, V.W. and Lively, R.S. .......................................................................................20 Cummings, K. and Bjørnerud, M.......................................................................................22 Cundari, R., Hollings, P., and Smyk, M. ...........................................................................24 Dayton, R.N., Miller, J.D., and Vervoort, J.D. ..................................................................26 Deschamp, M. and McGuire, J. .........................................................................................28 DeVasto, M.A., Czeck D.M., Bhattacharyya, P. ...............................................................29 Easton, R.M. and Heaman, L.M. .......................................................................................31 Esch, J.M............................................................................................................................33 Floyd, C.T., Syverson, K.M., and Hupy, C.M. ..................................................................35 Foley, D.J., and Miller, J. D. ..............................................................................................37 Fralick, P. and Zaniewski, K. .............................................................................................39 Geller, P., Fralick, P., Hill, M.L., and Gillies, P. ...............................................................41 Gilbert, H.P. .......................................................................................................................42 Goldner B.D. and Miller J.D..............................................................................................44 Hudak, G., Monson Geerts, S., Zanko, L., Severson A., Severson, A., and Bandli, B. ....46 Jirsa, M.A., Boerboom, T.J., and Chandler, V.W. .............................................................48 Jirsa, M.A., Boerboom, T.J., Chandler, V.W., Mossler, J.H., Runkel, A.C., and Setterholm., D.R......................................50 Kulakov, E.V., Smirnov, A.V., and Diehl, J.F. .................................................................51 LaMaskin, T.A. ..................................................................................................................53 Larson, Phillip, Swenson, J., and Patelke, M. ....................................................................55 MacTavish, A.D., Heggie, G.J., Goodgame, V.R., Johnson, J.R., Beswick, A.E., Stone, W.E., and Watkins, K.P. ...................57 Mattox, S. ...........................................................................................................................59 Miller, J., Brooker, B., Hadley, M., Markwood, L., Olson, J., and Tomlinson, A. ...........61 Peterson, D., Larson, P., Sweet, G., and Gibbons, J. .........................................................63 Piispa, E.J., Smirnov, A.V., Pesonen, L.J. .........................................................................65 ILSG 2011 97 Program and Abstracts �Queffurus, M. and Barnes, S.J. ..........................................................................................66 Radakovich, A.L., Parent, C.T., Partridge, M.E., Ritts, A.D., Pierce, R., & Hudak G.J. ..67 Rague, R. and Losh, S. .......................................................................................................69 Ross, C., Hudak, G., Morton, R., Quigley, T., Mahin, B. .................................................70 Saylor, B.J., Stencil J.C., DeVasto, M.A., and Bhattacharyya, P. .....................................72 Scott, R. J., Kolb, M.J., and Hill, M.L. ..............................................................................74 Smyk, M., Hollings, P., and Cundari, R. ...........................................................................75 Stinson, V. and Hill, M.L. ..................................................................................................77 Swoffer, B., and Hill, M.L. ................................................................................................78 Tharalson, E.R., and Monecke, T. ....................................................................................79 Theriault, S., Miller, J., Berndt, M., and Ripley, E. ...........................................................81 Totenhagen, M., Morton, P., and Larson, P. ......................................................................83 Traut, J.T. and Czeck, D.M. ...............................................................................................85 Vallowe, A.M., Thalhamer, E.J., Rhoades, D. L., and Peterson, D.M. .............................87 Van Lankvelt, A., Schneider, D., Hattori, K., and Biczok, J. ............................................89 Votava, J.E. and Bornhorst, T.J .........................................................................................91 Waggoner, T., Duskin, D., Karakus, M., and Gartner, J....................................................93 Wartman, J., Morton, R., Hudak, G., and Hercun, C. ........................................................95 ILSG 2011 98 Program and Abstracts � https://digitalcollections.lakeheadu.ca/files/original/3d0c78b21ab22e0ec00084aad9293a15.pdf 09d9f20af1d143b5a95f8a7098f16c65 PDF Text Text 57TH ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY ASHLAND, WISCONSIN MAY 18-21, 2011 PROCEEDINGS VOLUME 57 PART 2 – FIELD TRIP GUIDEBOOK ��INSTITUTE ON LAKE SUPERIOR GEOLOGY 57THANNUAL MEETING MAY 18-21, 2010 ASHLAND, WISCONSIN HOSTED BY: NORTHLAND COLLEGE TOM FITZ Chair ProceedingsVolume 57 Part 2 – Field Trip Guidebook Edited by: Tom Fitz, Allison Mills, Kristi Wilson, Cassandra Bodette, and Drew Cramer Cover Photo: Copper Falls at Copper Falls State Park near Mellen, Wisconsin – T.Fitz �57TH INSTITUTE ON LAKE SUPERIOR GEOLOGY CONTENTS OF PROCEEDINGS VOLUME 57: PART 1: PROGRAM AND ABSTRACTS PART 2: FIELD TRIP GUIDEBOOK TRIP 1:IGNEOUS STRATIGRAPHY OF THE LAYERED SERIES AT DULUTH - TYPE INTRUSION OF THE DULUTH COMPLEX TRIP 2:MIDCONTINENT MICROCOSM TRIP 3:GEOLOGY OF THE BAYFIELD PENINSULA: KEWEENAWAN BAYFIELD GROUP AND PLEISTOCENE DEPOSITS TRIP 4:GEOLOGY AND REMEDIATION AT THE ASHLAND/NORTHERN STATES POWER SITE TRIP 5:BAD RIVER WATERSHED CULVERT RESTORATION PROGRAM TRIP 6:GEOLOGY OF COPPER FALLS STATE PARK TRIP 7:GEOLOGY OF THE MONTREAL RIVER MONOCLINE TRIP 8:THE ARCHEAN/PALEOPROTEROZOIC UNCONFORMITY NEAR DENHAM, MINNESOTA TRIP 9:GRANITIC, GABBROIC, AND ULTRAMAFIC ROCKS OF THE KEWEENAWAN MELLEN INTRUSIVE COMPLEX Reference to material in Part 2 should follow the example below: Bjørnerud, M., and Cannon, W.F., 2011, Midcontinent microcosm: Geology of the Atkins Lake–Marengo Falls area: Institute on Lake Superior Geology, 57th Annual Meeting, Ashland, WI, v. 57, part 2, p. 31-46. Published by the 57th Institute on Lake Superior Geology and distributed by the ILSG Secretary: Peter Hollings Department of Geology Lakehead University Thunder Bay, ON P7B 5E1 CANADA peter.hollings@lakeheadu.ca ILSG website: http://www.lakesuperiorgeology.org ISSN 1042-9964 ILSG 2011 ii Field Trip Guidebook �TABLE OF CONTENTS PROCEEDINGS VOLUME 57 PART 2: FIELD TRIPS TRIP 1:IGNEOUS STRATIGRAPHY OF THE LAYERED SERIES AT DULUTH – TYPE INTRUSION OF THE DULUTH COMPLEX LEADER: JIM MILLER 1 TRIP 2:MIDCONTINENT MICROCOSM: GEOLOGY OF THE ATKINS LAKE – MARENGO FALLS AREA LEADERS: MARCIABJØRNERUD&WILLIAM F. CANNON 31 TRIP 3:GEOLOGY OF THE BAYFIELD PENINSULA: KEWEENAWAN BAYFIELD GROUP AND PLEISTOCENE DEPOSITS LEADERS: DICK OJAKANGAS, DREW CRAMER, & TOM FITZ 49 TRIP 4:GEOLOGY AND REMEDIATION AT THE ASHLAND/NORTHERN STATES POWER SITE LEADER: JAMES R. DUNN 79 TRIP 5:BAD RIVER WATERSHED CULVERT RESTORATION PROGRAM LEADERS: MICHELE WHEELER & CASSANDRA BODETTE 87 TRIP 6:GEOLOGY OF COPPER FALLS STATE PARK LEADERS: ALLISON MILLS, DREW CRAMER, & TOM FITZ 97 TRIP 7:GEOLOGY OF THE MONTREAL RIVER MONOCLINE LEADER: WILLIAM F. CANNON 111 TRIP 8:THE ARCHEAN/PALEOPROTEROZOIC UNCONFORMITY NEAR DENHAM, MINNESOTA LEADER: TERRY BOERBOOM 127 TRIP 9:GRANITIC, GABBROIC, AND ULTRAMAFIC ROCKS OF THE KEWEENAWAN MELLEN INTRUSIVE COMPLEX LEADER: TOM FITZ 163 ILSG 2011 iii Field Trip Guidebook �Locations of ILSG 2011field trips. Numbers on the map correspond to field trip numbers. The lead editor, Tom Fitz, would like to thank the field trip leaders, the reviewers, and the dedicated team of editors who have contributed their time and effort to creating this field trip guidebook. Their efforts are greatly appreciated by everyone who will use this book. We hope the guidebook will help foster understanding and appreciation of the remarkable geology the Lake Superior region. ILSG 2011 iv Field Trip Guidebook �57TH ANNUAL INSTITUTE ON LAKE SUPERIOR GEOLOGY FIELD TRIP 1 IGNEOUS STRATIGRAPHY OF THE LAYERED SERIES AT DULUTH – TYPE INTRUSION OF THE DULUTH COMPLEX Rocks of the Duluth Layered Series -J. Miller ILSG 2011 1 Field Trip 1 �ILSG 2011 2 Field Trip 1 �Field Trip 1 Igneous Stratigraphy of the Layered Series at Duluth – Type Intrusion of the Duluth Complex James D. Miller Department of Geological Sciences and Precambrian Research Center University of Minnesota Duluth This trip guide is modified from a guidebook entitled Field Guide to the Geology and Mineralization of Mafic Layered Intrusions of the Duluth and Beaver Bay Complexes, Northeastern Minnesota (Miller and Severson, 2009). The guidebook was developed for a field trip associated with the Precambrian Research Center Professional Workshop on Field, Petrographic and Mineralization Characteristics of Mafic Layered Intrusions held October 4-10, 2009 GEOLOGY AND PGE MINERALIZATION OF THE LAYERED SERIES AT DULUTH The well-exposed gabbroic rocks forming the escarpment above the city of Duluth have long been recognized as the type section of the Duluth Complex. While early surveys recognized the presence of two distinct rock types in the Duluth area (Winchell, 1899), Grout was the first to interpret the layered gabbros as a product of convection and magma differentiation (Grout, 1918a-e). Taylor (1964) produced the first detailed-scale (1:24,000) geologic map of the complex in the Duluth area and defined the main distinctions between the layered and anorthositic series. Moreover, based on field, petrographic, and very limited geochemical data, Taylor recognized basic similarities between the layered series at Duluth and the Skaergaard intrusion, which Wager and Deer (1939) had established as the classic example of a mafic intrusion formed by closed-system fractional crystallization of a tholeiitic magma. More recently, detailed mapping in the Duluth area has delineated much more about the structure and cumulate stratigraphy of the layered series (Miller and Green, 2008a & b; Green and Miller, 2008). The petrology of the layered series was summarized by Miller and Ripley (1996) and Miller and Severson (2002). The Layered Series at Duluth (DLS) is a well-differentiated, 3- to 4.5-km-thick, eastdipping sheet-like mafic layered intrusion that is the southernmost intrusion of the Duluth Complex. Exposure is very good along the 600'-high escarpment above Lake Superior and the St. Louis River estuary, but is spotty inland. Aeromagnetic data show that the DLS has a north-south strike length of about 60 kilometers long and eventually pinches out into the Boulder Lake intrusion. The hanging wall of the DLS consists of olivine gabbroic and troctolitic anorthosite of the anorthositic series, which according to U-Pb dating was emplaced just prior to DLS intrusion at 1099 Ma (Paces and Miller, 1993). The footwall of ILSG 2011 3 Field Trip 1 �the DLS at its southern end is reversed polarity lavas of the Ely's Peak basalts (Fig. 1-1). Layering and foliation in the DLS, which dip 20º-40º to the east, and modeling of aeromagnetic and gravity data indicate that the basal contact is not conformable with shallow-dipping (~15°) Keweenawan basalt flows in the footwall, but instead dips at a greater than 35° angle (Miller and Green, 2008b). Aeromagnetic data further imply that the basalt is cut out by the DLS to the north, whereupon the footwall becomes graywacke and slates of the Thomson Formation. Figure 1-1. Geology of the Duluth Complex at Duluth showing the field trip stops. ILSG 2011 4 Field Trip 1 �Figure 1-2. Igneous stratigraphy and cryptic variation of olivine, pyroxene and plagioclase composition through the Layered Series at Duluth. mg# values of olivine and pyroxene and An content of plagioclase are based on multiple microprobe analyses (error bars indicate one standard deviation). From Miller and Severson, 2002, Figure 6.8.Note that the cumulate codes for augite are A/a rather than the C/c (clinopyroxene) used in the text. The DLS is divided into five major zones based on dominant rock type (Fig. 1-1). The basal contact zone is composed of coarse-grained, taxitic olivine gabbro and augite troctolite (Stop 1). The lowermost cumulate sequence is the troctolite zone (Stops 1, 2A & 2B). It is 1 to 2 kilometers thick, consists mostly of homogeneous foliated troctolitic (PO) cumulates, and locally displays modal and textural layering, especially in its lower third. The cyclic zone (Stops 3, 4, & 5) forms the medial section of the DLS and is characterized by cyclical variations in cumulus mineralogy between troctolitic and gabbroic (PCFO) cumulates (Fig. 1-4). The persistent occurrence of gabbroic cumulates defines the gabbro zone. Gabbroic cumulates, in turn, grade upward into unlaminated (noncumulate) apatitic ILSG 2011 5 Field Trip 1 �Figure 1-3. Geology of the Cyclic Zone of the DLS showing field stops 1-3 to 1-5. Note that the cumulate codes for augite are A/a rather than the C/c (clinopyroxene) used in the text. ILSG 2011 6 Field Trip 1 �Figure 1-4. Cryptic variation of Fo and En in outcrop samples from the cyclic zone taken along two profiles north and south of Interstate 35 (Fig. 1-3). Note that the cumulate codes for augite are A/a rather than the C/c (clinopyroxene) used in the text. ILSG 2011 7 Field Trip 1 �quartz DLS ferromonzodiorite, which composes most of the upper contact zone (Stop 6). This quartz ferromonzodiorite complexly mixes with a fine-grained biotitic ilmenite ferrodiorite, which ultimately forms a "chilled" contact with anorthositic series rocks (Stop 7). A body of melanogranophyre that irregularly cuts through the anorthositic series probably represents the uppermost differentiate of the DLS (Stop 8). This igneous stratigraphy is complimented by cryptic layering of cumulus mineral compositions (Fig. 1-2) and together these imply the generally formed by bottom-up fractional crystallization of a moderately evolved, olivine tholeiitic parent magma. Although the DLS is overall, a well-differentiated intrusion, the repeated progression from troctolitic to gabbroic cumulates in the cyclic zone indicates that it did not fractionally crystallize as a closed system. The cyclic zone consists of at least five macrocycles (each 50200 m thick) within which troctolitic cumulates grade upward to gabbroic cumulates (Figs. 13, 1-4). Macrocycle boundaries are marked by the abrupt regression in the cumulus mineralogy from gabbroic back to troctolitic cumulates. The gabbroic parts of the macrocycles commonly contain inclusions of anorthositic series rocks and the very uppermost parts of the macrocycles locally have discontinuous layers of fine-grained gabbroic adcumulate (microgabbro). This cyclicity in phase layering does not correspond to a complimentary cryptic variation in mineral chemistry (Fig. 1-4). Based on these characteristics, Miller and Ripley (1996) suggested that the macrocyclic phase layering is predominantly related to devolatilization and decompression that attended magma venting events from a shallow (< 5 km) chamber, with magma recharge possibly having played a secondary role. Geochemical data were acquired from 83 hand samples that profile the general stratigraphy of the DLS. The sampling focused on specific horizons, particularly in the cyclic zone, that contain visible sulfide mineralization or appear to represent perturbations to the magmatic system (Fig. 1-5, 1-6). Most of the samples have Pt+Pd concentrations below 15 ppb; 14 samples have concentrations between 15 and 150 ppb and five samples have concentrations >150 ppb. Whole rock S and Cu concentrations (Fig. 1-5) imply that the DLS magma 1) was consistently sulfide-undersaturated during troctolite-zone crystallization; 2) achieved intermittent saturation during crystallization of the cyclic zone; and 3) was fairly consistently saturated as the gabbro zone accumulated. In contrast to the singular large increase in Cu abundance and Cu/Pd ratio observed in the Sonju Lake intrusion (See Day 3 description), the DLS shows erratic variability in these parameters (Fig. 1-5). Such variability probably reflects the openness of the DLS system to magmatic recharge. Nevertheless, a two to three order-of-magnitude range in the Cu/Pd ratio through the cyclic zone indicates some level of efficiency in the ability of sulfide melt to extract PGE from silicate magma (e.g., Maier and others, 1996). Six hand samples that contain the most enriched Pt+Pd and sulfide concentrations come from the upper portions of macrocycles I and II (Figs. 1-5 & 1-6) and the two highest values are associated with gabbro-microgabbro interfaces. The model of magma venting to explain the phase layering of the cyclic zone (Miller and Ripley, 1996) also can explain the elevated concentrations of sulfide and PGE at cyclic zone boundaries of cumulus regression. Experimental data at low pressures (1-2 kb) suggest a positive correlation between pressure and sulfide solubility in hydrous tholeiitic magmas (Carroll and Rutherford, 1985). If valid, this implies that magma decompression due to venting from shallow differentiated magma chambers may trigger sulfide saturation (or oversaturation in a saturated magma). ILSG 2011 8 Field Trip 1 �Devolatilization resulting from venting of a volatile-rich magma would also cause an abrupt increase in fO2 , thereby having a compounding effect on reducing sulfur solubility (Poulson and Ohmoto, 1990). The attractiveness of a magma venting as a trigger for sulfide saturation in terms of PGE enrichment is that it would promote chamber-wide sulfide segregation if the system was well mixed, or a rain of sulfide out of the roof zone if the magma chamber was compositionally zoned. Both situations would promote high R factors, though the second scenario would required enough overproduction of sulfide so as to survive the descent to the cumulate floor. Figure 1-5. Stratigraphic variation in sulfur, copper, copper/palladium, and palladium + platinum through the Layered Series at Duluth (Fig. 8.11, RI-58). ILSG 2011 9 Field Trip 1 �Figure 1-6. Geology of the cyclic zone of the DLS showing locations and concentration ranges of Pt+Pd from outcrop samples. (Modified from Fig. 8-12, Severson et al., 2002). ILSG 2011 10 Field Trip 1 �FIELD STOP DESCRIPTIONS STOP 1-1: Basal Contact Zone and Lower Troctolite Zone, Duluth Layered Series and Ely’s Peak Basalts Location: Railroad Grade of the former Duluth, Winnipeg, and Northern Railway, Bardon Peak, Duluth. West Duluth 7.5' quadrangle (T49N, R15W, Secs. 33 SE & 34 SW; approx. NAD 83 UTM – Start: 557740E, 5170300N; End: 558460E, 5170040N). Duration: 90 min. Description: This stop traverses about 400 meters of stratigraphic section of the lower part of the Layered Series at Duluth (DLS). The exposures will be accessed by walking the railroad grade from the western side of Ely‘s Peak. Along the way, several extensive exposures of Ely‘s Peak basalts (including a tunnel) will be encountered. The Ely‘s Peak basalts form the footwall of the Duluth Complex and show the effects of intense thermal metamorphism by the gabbro. The Ely‘s Peak basalts are comprised of about 1.5 kilometers of gently (10-15 ) dipping, tholeiitic basaltic lavas and represent the earliest volcanic eruptions into the Midcontinent Rift. Starting at the basal contact, which trends ~N-S in a valley between Ely‘s and Bardon‘s peaks, a variety of rock types belonging to the basal contact zone and the lower part of the troctolitic zone of the DLS are exposed in roadcuts and outcrops along the abandoned railroad grade as it arcs around Bardon Peak. Several macrocyclic layers grading in mode and texture are apparent in the lower part of the troctolitic zone and are thought to represent major recharge and differentiation/cooling episodes associated with the early integration stage of DLS emplacement. The exposures on the north side of the tracks are schematically shown in Figure 1-7 and areas of particular interest are flagged along the route and described below. A. Basal Contact The traverse starts at the irregular contact between strongly hornfelsed mafic volcanics of the Ely‘s Peak basalts and taxitic olivine gabbro and augite troctolite of the basal contact zone (Flag A). Granophyre dikes cut both rock types, but display lobate contacts with the coarse gabbro suggesting two-liquid mixing. Grout (1918e) cited this mixing of granophyre and gabbro as evidence of silicate immiscibility. An alternative explanation is that the granophyre represents anatectic melts from the footwall that were not completely assimilated into the mafic magma. Contacts between the hornfels basalt and the gabbro are commonly characterized by coarse augite prisms oriented perpendicular to the contact. Roadcuts on the downslope side of the railroad grade (Flag A‘) show a small, irregular intrusion of coarse gabbro into hornfels basalt. B. Basal Contact Zone This exposure of taxitic olivine gabbro to augite troctolite with its variable textures (medium-grained to pegmatitic, subophitic to ophitic, nonfoliated to poorly foliated) and variable modal compositions are characteristic of the basal contact zone. In contrast to other layered series intrusions along the northwestern margin of the Duluth Complex that locally contain abundant Cu-Ni(-PGE) sulfide (see Day 2 field stops), the basal contact zone of the DLS is devoid of significant sulfide mineralization. ILSG 2011 11 Field Trip 1 �ILSG 2011 12 Field Trip 1 Photo 1-1. Strong sheet jointing in feldspathic dunite formed from intense serpentine veining. Area 1G’ Photo 1-2. Trough layering at Area 1-I. Note thickening of leucocratic layers toward the axis of the trough. Figure 1-7. Outcrop exposed along the north side of the Duluth, Winnipeg and Northern railroad grade showing variability in rock type across the basal contact zone (BCZ) and several macrocycles in the lower troctolitic zone (TZ1-3). �C. Lower Macrocyclic Unit of the Troctolite Zone Exposed here is a modally layered, medium-grained, moderately foliated, ophitic augite troctolite. This foliated POcf cumulate marks the base of the troctolitic zone and the first of three macrocyclic layers exposed along this traverse. At the east end of this exposure, the rock grades to a medium coarse-grained, augite troctolite to olivine gabbro. D. Upper Part of Lower Macrocycle This exposure of medium coarse- to very coarse-grained, non-foliated to moderately foliated, ophitic to subophitic olivine oxide gabbro characterizes the upper part of the lower macrocycle (TZ1, Fig. 1-7). The exposure at D‘ is coarser grained and generally nonfoliated. E. Contact between Lower and Medial Macrocycles Down from the grade is a sharp contact between coarse-grained non-foliated olivine oxide gabbro in a lower exposure (similar to outcrop D‘) and a coarse-grained feldspathic, biotitic oxide peridotite above. Following this contact upslope to the west, the peridotite gives way to a medium-grained melatroctolite overlying coarse olivine oxide gabbro. This abrupt transition from coarse olivine gabbro to medium melatroctolite is interpreted to mark a major recharge event into the DLS magma chamber. The origin of the coarse-grained oxide peridotite is unclear (see Stop 1-2A for a discussion). The oxide peridotite commonly contains trace amounts of sulfide (note local Cu staining) and has higher PGE concentrations (10-30 ppb) than surrounding rock types. F. Middle Section of Medial Macrocycle Medium- to medium coarse-grained, moderately foliated, locally layered augite troctolite is exposed in small outcrops on either side of trail leading north of tracks. These exposures occur in the midsection of the second macrocycle (TZ2, Fig. 1-7). G. Contact between Medial and Upper Macrocycles Exposures nearest the north side of the railroad grade are of massive, biotitic coarsegrained oxide peridotite which form the upper part of the medial macrocycle (as at E). Following the flagging to an outcrop ledge, an abrupt contact is observed between the oxide peridotite and medium fine-grained, well foliated, feldspathic dunite to melatroctolite. The orientation of the contact between the feldspathic dunite and the oxide peridotite is locally irregular. It is unclear if this irregularity is due to intrusion of the peridotite into the dunite or due to scouring of the dunite into the peridotite. Evidence for both processes are found elsewhere in this area. Here and in exposures along this ledge, the feldspathic dunite/melatroctolite display a very pronounce sheet jointing formed by intense serpentine veining (Photo 1-1). Following the flagging over the top of this ledge leads to another 5meter-high ledge outcrop that displays lenticular interlayering of troctolite and melatroctolite. Lenses of troctolite consistently thicken to the west suggesting that they may denote channel or trough layering. A similar type of lenticular (channel?) layering is more completely displayed at I, which is approximately at the same stratigraphic level. Also evident at the eastern end of this upper exposure is steeply inclined serpentine veining that refracts through the different layers. ILSG 2011 13 Field Trip 1 �H. Lower Part of Upper Macrocycle. At the beginning of a very deep cut is well layered melatroctolite grading up from the feldspathic dunite observed at G and forming the lower part of the uppermost macrocycle (TZ3, Fig. 1-7). The melatroctolite grades upward into a medium-grained, well foliated and intermittently layered, ophitic augite troctolite. At two locations (flagged as H‘), the troctolite is cut by irregular steeply oriented bodies of oxide olivine gabbro pegmatite. Some have suggested that this pegmatite and the oxide peridotite are related late magmatic features. I. Layered Troctolite of the Upper Macrocycle At the highest part of the cut is an exceptional exposure of lenticular layered melatroctolite and leucotroctolite. Similar to the upper exposure at G, trough/channel structures are seemingly implied by the bowing of the layering and the consistent thickening of leucocratic layers in the axis of the trough. Several trough structures have been recognized in the Troctolitic Zone in the Bardon Peak area. The axis of the troughs tend to trend easterly (i.e., in the down dip direction of regional layering). Stop 1-2A: lower troctolite zone, Duluth Layered Series, Duluth Complex (OPTIONAL) Location: Skyline Parkway near Bardon Peak, Duluth. West Duluth 7.5' quadrangle (T49N, R15W, Sec. 34, SW of NW; NAD83 UTM (Area E) 558540E, 5170280N). Duration: (60 min.) Description: This stop examines rocks comprising the lower part of the Duluth Layered Series in roadcuts and barren knob exposures at the south end of Skyline Parkway. This outcrop area is approximately 500 m above the moderately dipping (~45°) basal contact of the Duluth Complex against shallow-dipping (<15°) basalt flows, which form Ely‘s Peak (rocky hill to the west). This area is situated in the lower part of the troctolitic zone, but upsection of the upper exposures observed in the traverse at Stop 1-1. As seen in that section, this part of the Troctolite Zone continues macrocyclic layering of melatroctolite grading upward to coarser augite troctolite. Because the layering is similar to that seen below, it is unlikely that we will be able to visit this exposure. It is included here for those who which to return at a later date. Figure 1-8 shows the geology of the area with eight areas of interest (A-H) noted and generally described below. The PO to OP cumulate rocks exposed at this stop include ophitic olivine gabbro, augite troctolite, troctolite, melatroctolite, and feldspathic dunite. In general, the more olivine-rich melatroctolites and dunites tend to be finer grained than the more augite-rich troctolites and olivine gabbros. These rocks display a variety of types and scales of layering. Macrocyclic layering of rock types is evident on a meter to decimeter scale—typically finer grained troctolite, melatroctolite, and feldspathic dunite upward to from coarser grained augite troctolite to olivine gabbro. Cryptic differences in olivine composition are evident with melatroctolite to feldspathic dunite being more Mg-rich (Fo62-65) compared to augite troctolite-olivine gabbro (Fo48-60). Contacts between macrocycles are abruptly (<10 cm) to narrowly gradational (<1 m) (areas C, D, and G, Fig. 1-8). Within macrocycles, various type of finer-scale layering is common. Subtle grain-size layering locally occurs in the mediumto coarse-grained augite troctolite to olivine gabbro intervals (area E). Centimeter-scale isomodal layering and decimeter-scale graded modal layering is very common in the more melatroctolitic rocks and locally has very rhythmic alternations (area F). Trough layering is locally implied by variable orientations of modal layering (areas A and C) and the tendency ILSG 2011 14 Field Trip 1 �for the more melatroctolitic layers to pinch out along strike. However, some of what appears to be pinch out may be due to faulting along ENE-trending structures through the area (Fig. 1-8). Very abrupt reorientation and steepening of lamination in Area G suggests that some faulting may be contemporaneous with crystallization. Deeply weathered, coarse-grained, biotitic oxide dunite to peridotite, similar to that seen at Stop 1-1 (areas E & G), occurs in two locations here (areas B and G, Fig. 1-8). Petrographic studies show the rock to be composed of 50-90% granular olivine, 3-20% Fe-Ti oxide (mostly ilmenite), 2-25% ophitic augite, 1-4% biotite, and 0-20% interstitial plagioclase. Primary minerals are commonly altered to chlorite, serpentine, talc, and actinolite. Olivine compositions in these bodies are Fo50-59 and therefore are similar to the augite troctolite and olivine gabbro cumulates in the area. The limited aerial extent of these oxide ultramafic bodies suggest that many may occur as small pipes or dike-like bodies. However, as seen at Stop 1-1, they also occur as stratabound, sub-conformable lenses at olivine gabbro-melatroctolite contacts. Figure 1-8: Outcrop geology of the Bardon Peak area denoting areas of interest at Stop 1-2A. Dashed lines are inferred faults. Narrow lines are topographic contours portraying 50 foot intervals. Ross (1985) suggested that these small oxide plugs and lenses are metasomatic replacement bodies formed by volatile fluxing out of the footwall basalts. Severson (1995; Severson and Hauck, 1990) found similar oxide ultramafic (OUI) bodies along the western and northwestern margin of the Duluth Complex to be spatially related to iron-formation inclusion and suggested that partial melting and assimilation of these inclusions may have generated the bodies. Another possible explanation is that they formed by mobilization of ILSG 2011 15 Field Trip 1 �interstitial volatile-rich magma extracted from lower cumulates in the basal contact zone which became ponded beneath fine-grained melatroctolite/dunite layers at the base of macrocyles. More study, especially isotopic data, are clearly needed to resolve the origin of these bodies. STOP 1-2B: troctolite zone, Duluth Layered Series and Overview of Duluth Complex. Location: Skyline Parkway, Bardon Peak, Duluth. West Duluth 7.5' quadrangle (T49N, R15W, Sec.34 SW; approx. NAD 83 UTM - 5170450N, 558740E). Duration: 15 min. Description: This brief stop, which corresponds to Area H in Figure 1-8, provides a panoramic view of the breadth of the Duluth Complex at Duluth. A 5 kilometer-thick stratigraphic section of the layered series and the overlying anorthositic series are exposed along the 200 meter high, and 15 kilometer long escarpment above the St. Louis River estuary. The top of the Duluth Complex on the rise just above downtown Duluth which is marked by a cluster of radio towers and the Enger Tower landmark. This is where our trip will end today (Stop 9). As exemplified by the layering of the troctolitic rocks exposed at this overlook, the general internal structure of the DLS dips 20-25 to the east, thus the northeast trending escarpment cuts an acute angle across the intrusion. The anorthosite series overlying the DLS can be recognized by its higher topographic expression. STOP 1-3: Cyclic Zone, Duluth Layered Series Location: Spirit Mountain Ski Resort, Duluth. West Duluth 7.5' quadrangle (T49N, R15W, Sec 23 NW; approx. NAD 83 UTM - 5174250N, 559952E). Duration: 30 min. Description: About 100 m east of the north side of the ski lodge, near the top of the northernmost chairlift is series of outcrop ledges that display over a 10-meter-thick section about 16 successive meter-scale mesocycles (Fig. 1-9) that mimic the decameter-scale macrocycles that characterize the medial Cyclic Zone of the DLS (Fig. 1-3). Each mesocycle is composed of a medium fine grained troctolite at its sharp base with the cycle below. About 1/3 of the way up, the troctolite develops small clots (~5mm) of subpoikilitic augite and lesser oxide, which then grades on a cm-scale to a medium-grained, intergranular oxide olivine gabbro (Photos 1-3 and 1-4). All rocks display well developed foliation of plagioclase (in troctolite) and plagioclase and augite (in oxide olivine gabbro). Although the thickness of each mesocycle gradually increases from 30 cm to 100 cm upsection, the proportion of gabbro to troctolite comprising each cycle remains generally constant at about 2:1 (Fig. 1-9). The mesocycles fundamentally show a rapid progression from a two-phase (PO) to a four-phase (PCOF) cumulate. The mesocyclic boundaries represent even more abrupt regressions from four-phase back to two-phase cumulates. ILSG 2011 16 Field Trip 1 �4 5 Photo 1-4. Close-up of contact relationships between oxide olivine gabbro (OG) and troctolite 6 (Tr). 1-3 1-4 Photo 1-3. Mesocyclic phase layering at Stop 1-3. Cycles 4 and 5 and the upper part of Cycle 6 are visible in this photo. Microprobe analyses of olivine and augite in samples collected across mesocycles 1 through 4 and 10 through 13 (Fig. 1-9) show subtle but systematic variations in mg# (MgO/(MgO+FeO), mole%). Olivine (and less consistently augite) show a 2-3% increase in mg# in the troctolite intervals compared to the gabbro intervals. The mg# increase can be recognized on the scale of a single thin section at the sharp mesocyclic boundaries. The maximum mg# typically is found in the central part of the troctolite intervals then gradually decreases as subophitic augite clots appear. The mg# stays remarkably constant through the gabbro intervals. The average mg# of each mesocyclic zone is also remarkably constant across the multiple cycles investigated (Fig. 1-9). ILSG 2011 17 Field Trip 1 �Figure 1-9. Stratigraphic variations in mesocycle thickness, volume % of gabbro composing each mesocycle, and mg# (MgO/(MgO+FeO), mole%) of olivine and augite through the mesocycles exposed at Stop 1-3. White parts of the mesocycles indicate troctolite intervals (PO cumulate), yellow indicates subophitic augite troctolite (POcf cumulate), and orange indicate olivine oxide gabbro intervals (PCFO cumulate). Figure after Miller and Stifter (2010). This may at first glance seem to argue for recharge of more primitive magma as the cause for the phase layering. However, the shift in mg# in olivine may also reflect a shift in the Fe-Mg equilibrium between melt and olivine when augite becomes a cumulus phase. Trace element data, particularly REE, also show a significant difference between the troctolite and gabbro layers, but this can be attributed to differing partition coefficients for the main cumulate phases (Miller and Stifter, 2010). Collectively, the field, petrographic, mineral chemistry, and lithochemical attributes of the mesocyclic layering observed here does not unambiguously favor either a magma recharge or decompression venting model for its origin. More data and evaluation are needed to definitely determine the origin of the mesocyclic and larger macrocyclic layering of the Cyclic Zone of the Duluth Layered Series. Stop 1-4: Cyclic Zone, Duluth Layered Series Location: North of I-35, west of Boundary Ave., slope north of Northern Engine & Supply Co. and Spirit Mtn. Red Roof Motel, Duluth. West Duluth 7.5' quadrangle (T49N, R15W, Sec 15 SE, NAD 83 UTM - 5175100N, 559350E (area E)) Duration: 60 min. Description: This stop examines the first macro-scale transition from troctolite cumulates to olivine gabbro cumulates that defines the base of the Cyclic Zone (Fig. 1-3). ILSG 2011 18 Field Trip 1 �From behind the Northern Engine and Supply Co., head north and uphill to outcrops of medium-grained ophitic augite troctolite (A, Fig. 1-10). This POcf cumulate contains about 7-10% interstitial Fe-Ti oxide and augite, with the latter commonly forming 2-4 cm oikocrysts. Progressing about 70 m to the east over intermittent outcrop ledge, the augite troctolite gradually becomes coarser grained and more enriched in augite and oxide (B, Fig. 1-10). Figure 1-10: Outcrop geology and cryptic variation along south facing slope at Stop 1-4. Igneous foliation and layering dip at 15-25° to the east. Exposure corresponds to the top of macrocyclic unit I marking the base of the cyclic zone (Fig. 1-3). Troctolitic rocks 120 m east of the powerline behind the motel indicate the presence of an unexposed cumulus reversal. Crossing a 35-m gap in exposure, the next outcrop is of a medium- to very coarse grained, subophitic to ophitic olivine gabbro (PcOf; C, Fig. 1-10). In one area of the exposure, this rock type is in sharp, discordant, but unchilled contact with a coarse-grained, moderately laminated gabbroic anorthosite that is probably an inclusion of the anorthositic series. Beyond an 8-m gap is a 10-m-high outcrop knob that grades from a medium-grained, subophitic augite troctolite with 1/2-cm high density augite oikocrysts near the base (D, Fig. 1-10) to a texturally layered, medium- to fine-grained, intergranular oxide olivine gabbro at the top (E. Fig. 1-10). Textural layering on top of knob is defined by 2- to 20-cm-thick, laterally discontinuous layers of fine grained, moderately laminated, intergranular oxide olivine gabbro (microgabbro) that intermittently occurs within medium-grained, well-laminated, intergranular oxide olivine gabbro. Contacts between the medium gabbro and the microgabbro range from sharp to gradational on a scale of centimeters. In some areas, the textural layering drapes over football-sized blocks of a coarser grained, subophitic olivine gabbro, similar to the rock type seen just to the west (down section). Gossan of sulfidebearing areas are locally present. If time allows, we will follow flagging tape about 200 meters from the powerline (Fig. 1-10) to the parking lot behind the Red Roof motel. Exposed in an 80m-long roadcut and pavement exposure is medium-grained, well foliated, locally layered augite troctolite, thus implying that a cumulus reversal was crossed between site E and here. This rock is interpreted to form the lower part of the second macrocycle unit (Fig. 1-3). Exposure of this macrocycle boundary to the south shows that the texturally layered oxide olivine cumulate ILSG 2011 19 Field Trip 1 �interval (E, Fig. 1-10) is only locally present and appears to be lenticular in form (Fig. 1-3). In some exposures, the boundary between macrocyles I and II is marked by a coarse-grained ophitic olivine gabbro with anorthositic series inclusions (as seen at C, Fig. 1-10) in sharp contact with troctolite. With the exception of the medium-grained, subophitic augite troctolite at D, cryptic variations through this sequence show a general decrease in An, Fo, and En (Fig. 1-10). Plagioclase, olivine, and augite compositions in the troctolite above the layered gabbro define a regression to more primitive values. Mineral compositions are also more primitive in the microgabbro layers compared to the enclosing medium-grained gabbro. Across the cumulus reversal from oxide olivine gabbro to troctolite, Fo, En and An increase slightly. Again a straightforward interpretation of this sequence is that it represents progressive crystallization differentiation leading to multiple saturation in plagioclase, olivine, augite, and ilmenite, followed by recharge of a more primitive magma causing a regression to saturation in just plagioclase and olivine. The occurrence of anorthosite inclusions and the microgabbro layers suggest however that any such recharge event may have been preceded by an eruption event from the chamber. Miller and Ripley (1996) interpreted the microgabbro lenses to have formed by decompression quenching of magma due its reaching water saturation in the roof zone (see Stops 1-5 and 1-7 for further discussion). Decompression may also have triggered sulfide saturation which caused local stratiform enrichment of PGE at this and other cumulus regression. Although some enrichment in PGE above background is noted from samples taken from site E, more pronounced sulfide and PGE enrichment are evident at this macrocycle interface to the south and at the interface between the second and third macrocycles (Figs. 1-5, 1-6). Stop 1-5: Cyclic Zone, Duluth Layered Series Location: Roadcut on Skyline Parkway at Thompson Hill Rest Area. West Duluth 7.5' quadrangle (T49N, R15W, Sec 14, center, NAD 83 UTM - 5175350N, 560700E) Duration: 90 min (including lunch). Description: In the layered sequence of gabbroic rocks exposed along this 200-mlong roadcut (Fig. 1-11), a reversal in cumulus paragenesis can be seen that is characteristic of higher levels in the cyclic zone. The west end of the roadcut begins with a coarse-grained, moderately laminated, subophitic olivine gabbro, which locally is intergranular and elsewhere is leucocratic (sample A, Fig. 1-11). Augite in this rock is marginally cumulus but becomes definitely so quickly upsection where it consistently has an anhedral granular to subprismatic habit. This coarse-grained, intermittently layered (locally graded), olivine-poor (<5%) oxide gabbro (sample B) classifies as a PCFO cumulate. Beyond a poorly exposed interval, this gabbro includes a 2-m-thick interval wherein minor olivine becomes subpoikilitic and concentrated in layers (sample C). Another 3 m above this, the coarse gabbro passes into a medium-grained, well-laminated, subpoikilitic olivine-bearing oxide gabbro (samples D and D') which displays layering of olivine oikocryst concentration and elsewhere isomodal layers rich in Fe-Ti oxide and pyroxene. The very strong foliation and subhedral to euhedral habit of cumulus phases (plagioclase, pyroxene, and ilmenite) impart an adcumulate texture to this rock. Over a poorly exposed interval about 15 m long is an altered, coarse-grained, ophitic gabbroic anorthosite (sample E) that is texturally and mineralogically identical to rocks in the anorthositic series. At the beginning of the next well-exposed section of roadcut, several similar gabbroic anorthosite inclusions are found in ILSG 2011 20 Field Trip 1 �a coarse-grained, subophitic to intergranular olivine gabbro (sample F), which gradually grades upward into a more consistently subophitic texture over the remainder of the roadcut (sample G). This rock type closely resembles that at the west end of the roadcut and indicates a downgrading in the cumulus status of pyroxene (and oxide?) and a reemergence of cumulus olivine. Figure 1-11: Geology and cryptic variation along Skyline Parkway roadcut near Thompson Hill rest area; Stop 1-5. View is to the north. Dip of lamination and layering is exaggerated; averages about 20° to the east. Contacts between units are gradational over thicknesses of 10 cm to 1 m. The cryptic variation of En and Fo across the cumulus regression exposed in this section (Fig. 1-11) is the reverse of that expect from magma recharge. Although there is a general decrease in these parameters below the cumulus regression as would be expected due to progressive differentiation, a sharp reversal takes place at samples D and D' – the most differentiated samples based on phase mineralogy. The increase in Fo and En in these samples may reflect their adcumulate nature (i.e., low trapped-liquid component) or perhaps reflect a shift in the equilibrium compositions of mafic silicates due to the cumulus crystallization of Fe-Ti oxide. Nevertheless, the expected increase in Fo and En above the recharge horizon is not observed in either the pyroxene or the olivine. Indeed, the lowest Fo olivine in the section occurs above the cumulus regression. Again, an alternative explanation to magma recharge is that the textural and compositional variations across this interval reflect decompression of the chamber due to eruption to the surface. Decompression of a volatile-enriched magma would cause supercooling and multiple saturation of the magma and thereby explain the abrupt decreases in grain size and cumulus phase changes without much compositional variation. Magma expulsion through the roof of the layered series would also explain the occurrence of a gabbroic anorthosite inclusion and elsewhere throughout the cyclic zone. The hydrothermally altered nature of the gabbroic anorthosite is consistent with a volatile-rich environment in the anorthositic series cupola of ILSG 2011 21 Field Trip 1 �the DLS magma chamber. The cumulus reversal to ophitic olivine gabbro without an increase in mg# could be explained by repressurization of a devolatized magma. STOP 1-6: Gabbro Zone, Duluth Layered Series and Anorthositic Series Location: Roadcuts on Skyline Parkway above Oneota Cemetery. Duluth Heights 7.5' quadrangle (T49N, R15W, Sec 1, SE, approx. NAD 83 UTM - 5177970N, 562995E) Duration: 30 min. Description: In roadcuts along a 0.5-km section of Skyline Parkway, rocks forming the contact between the layered series and the anorthositic series may be examined. Exposed in low, deeply weathered outcrops at the west end of the section is a poorly to well foliated, intergranular, apatitic olivine ferrodiorite cumulate (PCFOAp ). It is composed mostly of plagioclase (zoned An38-50, avg. An44), augite (avg En33Fs27Wo40), and bladed ilmenite. In addition, as much as 10% olivine (Fo30), 5% orthopyroxene (En45), 5% apatite, 3% biotite, and 5% granophyric mesostasis is present. Generally medium-grained, it locally varies in grain size from fine to coarse. Medium- to fine-grained variants have well-developed foliation, whereas coarser grained ferrodiorite tends to be poorly foliated to nonfoliated. Coarser ferrodiorite tends to be more granophyric, as well. By being locally well-laminated, olivine-bearing, and lacking ferromonzodioritic component, the ferrogabbro here is not typical of the upper contact zone of the layered series; relationships more characteristic of the upper contact zone will be seen at the next stop. Rather, this rock is more typical of gabbroic cumulates of the gabbro zone. About 35 meters east of the ferrogabbro, a prominent roadcut displays several common variants of anorthositic rocks that compose the anorthositic series. The western end of the roadcut is composed of a poorly to moderately laminated, poikilitic olivine gabbroic anorthosite (OGA) that contains olivine (Fo42-46) oikocrysts up to 6 cm across, as well as interstitial augite and oxide. Progressively eastward, this rock type grades into a subpoikilitic to granular olivine gabbroic anorthosite. At the high point in the roadcut, this granular OGA contains an inclusion of olivine (Fo60)-bearing anorthosite (Photo 1-5). Whereas the enclosing OGA contain about 80-85% plagioclase (avg. An62), this inclusion contains more than 95% plagioclase (unzoned An65). The contact between OGA and the anorthosite inclusion is sharp with some concordant alignment of plagioclase in OGA. Photo 1-5. Olivine-bearing anorthosite inclusion in subpoikilitic olivine gabbroic anorthosite as Stop 1-6. Hammer at contact. ILSG 2011 22 Field Trip 1 �The inclusion relationships and the outcrop-scale variability in internal structure and lithology (mode, texture, olivine habit, and lamination development) of the anorthositic rocks here are ubiquitous features of the anorthositic series. These complexities led Taylor (1964) to characterize the anorthositic series as an igneous breccia. He and later Miller and Weiblen (1990) concluded that these rocks formed by repeated intrusions of plagioclase crystal mush. This stop is at the western end of a westward projection of the layered series-anorthositic series contact (Fig. 1-2). A possible explanation for the irregular trace of the contact is that it reflects the original shape of the DLS roof. This would seem to be a difficult shape to maintain if the anorthositic series rocks were hot and lighter than the DLS magma. Rather, it seems more likely that the westward projection of the anorthositic series represents a large inclusion that detached from the roof zone and settled down to the cumulate floor of the DLS at the time of gabbro zone crystallization. Evidence for this is suggested by a truncated cryptic variation in the DLS leading up to this contact compared to the thicker DLS section to the north and the atypical character of the ferrodiorite in contact with the anorthositic series here (compare with next stop). Stop 1-7: DLS ―Chill‖, Granophyre, and Anorthositic Series Location : Skyline Parkway, Duluth Heights 7.5' quadrangle (T50N, R14W, Sec 32, SE, NAD 83 UTM - 5179675N, 564880E). Duration: 30 min. Description: Exposed at the northeast end of this roadcut is a fine-grained mafic rock with intermingled granophyre that together cut coarse-grained olivine gabbroic anorthosite of the anorthositic series. Since early mapping by Grout in the Duluth area in 1912, this rather innocuous exposure has been a keystone outcrop in interpreting the intrusive history of the Duluth Complex. This fine-grained mafic rock can be traced up over the ridge to the west where it merges into the upper contact zone of Layered Series. Grout (1918a) and later Taylor (1964) saw this exposure as evidence that the anorthositic series was considerable older and cold when the layered series was intruded. This paradigm was accepted by all subsequent workers on the Duluth Complex up through the 1980‘s. It came therefore as a shock when high resolution U-Pb age dating (Paces and Miller, 1993) showed that the anorthositic series and the layered series were virtually identical in age (within 0.5 Ma relative to the 22 Ma span of Midcontinent Rift magmatic activity). This precipitated a major paradigm shift in the perception of the intrusive relationships between these two series here and throughout the Duluth Complex. A closer look at this DLS ―chill‖ reveals that it is not a thermal quench of the Layered Series at all. This rock type is found at the contact with the anorthositic series throughout most this area and has a remarkably homogeneous composition with an mg# of about 37 (Table 1-1). In thin section, it is a subprismatic biotitic oxide ferrodiorite. Applying its composition to the MELTS routine indicates that it should be in equilibrium with augite, ilmenite, and plagioclase with compositions comparable to gabbroic cumulates found in the cyclic zone and gabbro zones of the DLS. In sum, this rock is much too evolved to have produced the troctolitic rocks of the layered series. Rather than this being a thermal quench, Miller and Ripley (1996) have interpreted this rock to represent decompression quenching of an evolved, water-saturated DLS magma during venting at a time when the cyclic zone was crystallizing. Whereas decompression of a water-undersaturated magma should result in superheating and a suspension of crystallization (or at least a significant change in phase ILSG 2011 23 Field Trip 1 �equilibrium), decompression under water-saturated conditions should cause supercooling and quenching. This model is fits nicely with the explanation for the cyclic zone with which this composition is apparently comagmatic. Photo 1-6. Lobate contacts between fine-grained oxide ferrodiorite (fd) and medium-grained melanogranophyre (mg), which is in sharp contact with coarsegrained gabbroic anorthosite (ga) at Stop 1-7. fd mg gaga The lobate contacts between the irregular masses of medium-grained granophyre and the fine-grained ferrodiorite host (Photo 1-6) gives the appearance of two magma mixing. The ferrodiorite is not of a composition that would indicate that these two liquid formed by immiscibility. An alternative explanation is that these felsic magmas were derived from anatectic melting of various inclusion carried into the DLS chamber. Because of their high silica content and low density, these felsic melts did not readily mix or assimilate with mafic melt, but rather rose to the roof zone where they ponded beneath the anorthositic series cupola. During magma venting from the chamber, the felsic melts became entrained and irregularly mixed with the mafic magmas. While decompression under water-saturated conditions caused rapid crystallization of the mafic magma, the felsic melt became irregularly entrapped in the quenched mafic host and cooled more slowly to a mediumgrained texture. ILSG 2011 24 Field Trip 1 �Table 1-1. Compositions of six fine-grained ferrodiorite samples collected at the DLS-AS contact. Stop 1-8: Ferrodiorite/Melanogranophyre Composite Intrusion in Anorthositic Series Location : Roadcuts on Skyline Parkway on the south and north sides of Twin Ponds swimming area. Duluth 7.5' quadrangle (T50N, R14W, Sec 21, E central; NAD 83 UTM 5180870N, 566980E). Duration: 30 min. Description: Outcrops and roadcuts north and south of Twin Ponds expose contact relationships between very altered anorthositic series rocks and an irregular composite body of intermediate to felsic rocks. Exposed in the roadcut south of the T-junction is an altered, coarse-grained, granophyric gabbroic anorthosite. All ophitic pyroxene has been replaced by fibrous amphibole, and a granophyric mesostasis locally composes 5-15% of the rock. The altered and granophyre-enriched character of anorthositic rocks is common over most of the upper part of the anorthositic series in the Duluth area (Fig. 1-1). ILSG 2011 25 Field Trip 1 �About 15 m east of the T-junction, an abrupt but unchilled contact is exposed between the gabbroic anorthosite and a medium-grained, apatitic olivine ferromonzodiorite. As seen in thin section, all the olivine and most of the clinopyroxene has been replaced by amphibole, chlorite, and iron oxide, and plagioclase is commonly mantled by dusty Kfeldspar which becomes intergrown with quartz to form a granophyric mesostasis. Locally the ferromonzodiorite is moderately laminated, and in one area it displays a streaky modal layering that is moderately inclined to the southeast. About 75 m east of the junction, a very complex contact between the ferromonzodiorite and a plagioclase-porphyritic quartz ferromonzodiorite is encountered which appears to be a hybrid mixture of gabbroic anorthosite and ferromonzodiorite. Over a 25 m interval, this hybrid rock grades into variably altered and granophyric gabbroic anorthosite. With the exception of small irregular bodies of quartz monzodiorite about 140 m from the junction, the remainder of the roadcut is altered and granophyric gabbroic anorthosite. On the north side of the Twin Ponds, a roadcut exposes a pink, mafic quartz monzonite (or melanogranophyre) cut by a 2-m-wide tholeiitic diabase dike. The melanogranophyre is composed of about 50% subhedral plagioclase, 5-10% skeletal prisms of Fe-pyroxene and amphibole, and 5% granular Fe-oxide in a felsic matrix of micrographically intergrown dusty K-feldspar and quartz. This rock type makes up 80% of the irregular composite body label as melanogranophyre in Figure 1-1. Uphill (north) from the roadcut in intermittent pavement outcrops, the melanogranophyre can be observed to grade into more mafic compositions (quartz ferromonzodiorite to ferrodiorite) as sharp but unchilled contacts with gabbroic anorthosite are approached. The alteration, contact relationships, and variety of rock types observed here indicate that evolved magma and hydrothermal fluids emanating from the underlying DLS passed through the anorthositic series over a protracted period of time and at a variety of scales. The very altered and granophyre-rich character of anorthositic rocks observed here and over much of the upper part of the anorthositic series suggests that late-stage melts and hydrothermal fluids fluxed through the intercumulus pore spaces of the (partially molten?) anorthositic series over a large area. In addition to this widespread percolation, the composite melanogranophyre body appears to represent a discrete conduit through which DLS-originating magmas passed and partially crystallized on their way to higher level intrusions or surface eruptions. The crude zonal distribution of composition more ferrodioritic at the margins and more quartz monzonitic in the interior—is consistent with this conduit being open during crystallization of at least the upper third of the DLS. Moreover, the hybridization between the composite body and the gabbroic anorthosite host suggest that the anorthositic series was partially molten at least in the vicinity of this conduit. Stop 1-9: Upper Contact of Duluth Complex and North Shore Volcanic Group. Location : Radio Tower Hill. East of corner on gravel road corresponding to 5th Ave and 9th Street. Duluth 7.5' quadrangle (T50N, R14W, Sec 28, NE; NAD 83 UTM 5182260N, 567650E). Duration: 45 min. Description: The east side of Radio Tower Hill overlooking downtown Duluth approximates the slope of the upper contact of the Duluth Complex with the overlying lava flows of the North Shore Volcanic Group, which now underlie all the lower ground to the northeast along the shore. As observed at the southeastern crest of the hill, the rock exposed ILSG 2011 26 Field Trip 1 �over most of this slope is a medium- to fine-grained, plagioclase-porphyritic (5-15%), subophitic to ophitic olivine gabbro that contains many decimeter- to meter-sized basaltic hornfels inclusions. In thin section, the gabbro typically displays intense hydrothermal alteration resulting in sericitic breakdown of plagioclase and uralitic, chloritic, and oxide replacement of pyroxene and olivine, though its primary subophitic to ophitic texture is typically preserved. The basaltic inclusions are locally intensely recrystallized and the gabbro shows little to only a weak chill around the inclusions. Less commonly, an inclusion of gabbroic anorthosite is present in the gabbro. To the west (downsection), basaltic inclusions become less common and plagioclase phenocrysts become more abundant, but in a nonsystematic way. The contact with the main suite of gabbroic anorthosite is not well exposed but seems to be abrupt and may actually finger into the anorthositic series as dikelike apophyses (Fig. 1-12). Figure 1-12: Outcrop geology of Radio Tower hill area, showing the starting and ending locations of Stop 1-9. Open circles indicate the location of radio towers. Along a telephone line down the slope to the northeast, the gabbro gradually becomes finer grained and less porphyritic. At the base of the slope, the gabbro abruptly grades into a pink, medium-grained, intergranular to micrographic hornblende quartz monzonite (Fig. 112). The quartz monzonite is composed of ~15% amphibole and ferroaugite, 5% granular Fe-oxide, 40% plagioclase, 30% dusty K-feldspar, and 10% graphic to granular quartz. Abundant miarolitic cavities indicate a shallow depth of crystallization. The contact relationship between the gabbro and the quartz monzonite is best seen in a roadcut along the driveway to 415 W. Skyline Parkway. Here, the two phases, along with some basaltic inclusions, occur in a complex mixture over a 15-m-wide zone. The melanogranophyre ILSG 2011 27 Field Trip 1 �probably formed by partial melting of intermediate to felsic lavas of the overlying North Shore Volcanic Group which form the hanging wall of the Duluth Complex . A strongly recrystallized porphyritic felsic (icelandite) volcanic rock can be observed in low roadcuts about 40 m northeast of Skyline Parkway on W. 8th St. A possible interpretation of the porphyritic gabbro is that it represents the upper border phase of the anorthositic series that formed by flow differentiation of plagioclase crystals away from the contact with overlying volcanic rocks. An alternative explanation is that this body represents a discrete intrusion, perhaps comagmatic with the layered series, which was emplaced at the anorthositic series-volcanic contact. Except for its highly altered state, its primary mineral compositions and texture are similar to the basal contact zone rocks of the layered series. REFERENCES Carroll, M.R., and Rutherford, M.J., 1985, Sulfide and sulfate saturation in hydrous silicate melts: Journal of Geophysical Research, v. 90C, p. 601-612.Grout, 1918a-c Green, J.C., and Miller, J.D., 2008, Bedrock geology of the Duluth quadrangle, St. Louis County, Minnesota. Minnesota Geological Survey Miscellaneous Map M-182, scale 1:24,000. Grout, F.F., 1918a, Internal structures of igneous rocks; their significance and origin with special reference to the Duluth Gabbro. Journal of Geology 26, 439-458. Grout, F.F., 1918b, Two-phase convection in igneous magmas. Journal of Geology 26, p. 481-499. Grout, F.F., 1918c, A type of igneous differentiation. Journal of Geology 26, 626-58. Grout, F.F., 1918d, The lopolith, an igneous form exemplified by the Duluth gabbro. American Journal of Science, series 4, vol. 46, p.516-522. Grout, F.F., 1918e, The pegmatites of the Duluth Gabbro. Economic Geology, v. 13, p.185197. Maier, W.D., Barnes, S.J., De Klerk, W.J., Teigler, B., Mitchell, A.A., 1996, Cu/Pd and Cu/Pt of silicate rocks in the Bushveld Complex: implications for platinum-group element exploration: Economic Geology, v. 91, p. 1151-1158. Miller, J.D., and Green, J.C., 2008a, Bedrock geology of the Duluth Heights and eastern portion of the Adolph quadrangles, St. Louis County, Minnesota. Minnesota Geological Survey Miscellaneous Map M-181, scale 1:24,000. Miller, J.D., and Green, J.C., 2008b, Bedrock geology of the West Duluth and eastern portion of the Esko quadrangles, St. Louis County, Minnesota. Minnesota Geological Survey Miscellaneous Map M-183, scale 1:24,000. Miller, J.D., and Ripley, E.M., 1996, Layered intrusions of the Duluth Complex, Minnesota, USA. in Cawthorne, R.G., ed., Layered Intrusions: Amsterdam, Elsevier Science, p. 257-301. Miller, J.D., and Severson, M.J., 2002, Chapter 6: Geology of the Duluth Complex, in Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota. Minnesota Geological Survey Report of Investigations 58, p. 106-143. ILSG 2011 28 Field Trip 1 �Miller, J.D., and Stifter, E, 2010, Cyclical phase layering in the Duluth Complex at DuluthEvidence for periodic magma venting from a shallow staging chamber?. Annual Institute on Lake Superior Geology, Proceedings Volume 56, Part 1- Program and Abstracts, International Falls, MN. p. 44-45. Miller, J.D., and Weiblen, P.W., 1990, Anorthositic rocks of the Duluth Complex: Examples of rocks formed from plagioclase crystal mush. Journal of Petrology 31, p. 295-339. Paces, J.B., and Miller, J.D., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: geochonological insights to physical, petrogenetic, paleomagnetic and tectono-magmatic processes associated with the 1.1 Ga Midcontinent rift system: Journal of Geophysical Research, v. 98, no.B8, p. 13,99714,013. Poulson, S.R. and Ohmoto, H., 1990, An evaluation of the solubility of sulfide sulfur in silicate melts from experimental data and natural samples: Chemical Geology, v. 85, p. 57-75. Ross, B.A., 1985. A petrologic study of the Bardon Peak peridotite, Duluth Complex. Unpublished M.S. thesis, University of Minnesota, Minneapolis, 140 p. Severson, M.J., 1995, Geology of the southern portion of the Duluth Complex: Natural Resources Research Institute, University of Minnesota-Duluth, Technical Report NRRI/TR 95/26, 185 p. (with plates). Severson, M.J., and Hauck, S.A., 1990, Geology, geochemistry, and stratigraphy of a portion of the Partridge River intrusion: Natural Resources Research Institute, University of Minnesota-Duluth, Technical Report, NRRI/GMIN-TR-89-11, 236 p. (with plates). Severson, M.J., Miller, J.D., Peterson, D.M., Green, J.C., and Hauck, S.A., 2002, Mineral potential of the Duluth Complex and related intrusions, in Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota. Minnesota Geological Survey. Report of Investigations 58, p. 164-200. Taylor, R. B., 1964. Geology of the Duluth Gabbro Complex near Duluth, Minnesota. Minnesota Geological Survey Bulletin 44, 63 p. Wager, L.R., and Deer, W.A., 1939, Geological investigations in East Greenland, Part III. The petrology of the Skaergaard intrusion, Kangerdlugssuaq, East Greenland. Meddr. Gronland. 105, 352 p. Winchell, N.H., 1899, The Geology of Minnesota. Geological and Natural History Survey of Minnesota, Final Report v. 4, 354p. w/100 plate ILSG 2011 29 Field Trip 1 �ILSG 2011 30 Field Trip 1 �57TH ANNUAL INSTITUTE ON LAKE SUPERIOR GEOLOGY FIELD TRIP 2 MIDCONTINENT MICROCOSM: GEOLOGY OF THE ATKINS LAKE –MARENGO FALLS AREA Photomicrograph of ultracataclasite from the north side of the Marengo River below Marengo Falls. The protolith is probably basalt of the Siemens Creek Volcanic series. The field of view is about 2 mm. – M. Bjørnerud ILSG 2011 31 Field Trip 2 �ILSG 2011 32 Field Trip 2 �Field Trip 2 Midcontinent Microcosm: Geology of the Atkins Lake –Marengo Falls area Marcia Bjørnerud1 and William F. Cannon2 1 2 Lawrence University, 115 South Drew Street,Appleton, WI U.S. Geological Survey, 12201 Sunrise Valley Drive,Reston, VA ___________________________________________________________________________ OVERVIEW OF FIELD TRIP STOPS 1. Paleoproterozoic Ironwood Iron-formation, metadiabase sills, and breccias near Atkins Lake 2. Mesoproterozoic Mellen-type granite near Atkins Lake 3. Mesoproterozoic Siemens Creek Volcanics contact metamorphosed to pyroxene hornfels and granophyre 4. Penokean shear zone in Paleoproterozoic Palms Formation, Marengo River 5. Paleoproterozoic Bad River Dolomite: Stromatolites, Penokean deformation, contact metamorphism, Grandview Quarry, Marengo River 6. Marengo Falls, South side: Neoarchean Puritan Batholith; fault breccias, cataclasites and pseudotachylyte along the Atkins Lake-Marenisco Fault 7. Marengo Falls, North side: Ultracataclasite along the Atkins Lake-Marenisco Fault INTRODUCTION Archean and Proterozoic rocks exposed over about 16km2 between Atkins Lake and Coffee Lake in southeastern Bayfield County (Fig. 1) chronicle almost all of the major Precambrian geologic events in the history of the southern Superior Craton. The oldest rocks are part of a locally gneissic quartz monzonite complex, the Puritan Batholith, with an igneous Rb-Sr age of 2710+140 Ma (Sims et al., 1977). At the regional scale, this complex is part of one of the youngest Archean granite-greenstone belts in the Superior Province, and it intrudes greenstones of the Neoarchean Ramsay Formation. In the Atkins Lake – Marengo River area, the Puritan Batholith is nonconformably overlain by the Paleoproterozoic (ca. 2200 Ma) Bad River Dolomite. The Bad River Dolomite is in turn separated by an unconformity from rocks of the ca. 1875 Ma Menominee Group (Palms Formation and Ironwood Iron-formation), which locally contain mafic volcanic rocks and diabase sills (Cannon et al., 2008). These Paleoproterozoic rocks provide insight into climate and biogeochemical cycles during the transition to an oxidizing atmosphere (Bekker et al., 2006) and have deformational fabrics (folds, strong cleavage, local mylonite zones) that record the ca. 1850 Ma Penokean Orogeny. The youngest rocks in the area are Mesoproterozoic ILSG 2011 33 Field Trip 2 �basaltic lava flows (Siemens Creek Volcanics, ca. 1110 Ma) and a layered mafic complex (the Mineral Lake Intrusion, also ca. 1100 Ma), both related to the Mid-continent Rift. All of the stratified units show static contact metamorphic textures near their contacts with the Mineral Lake Intrusion. Thus the area constitutes a microcosm of the regional bedrock geology, and the cross-cutting relationships among the units provide clear constraints on the relative timing of different phases of deformation and magmatism (Cannon etal., 2008, Bjørnerud, 2010a). The rocks in the Atkins Lake-Marengo Falls area are part of a regional structure called the Montreal River Monocline (Fig. 2), a northward-dipping sequence of Archean through Mesoproterozoic rocks that exposes a 35-km thick section of the crust as it existed at the time of the closure of the Midcontinent Rift (see also chapter 7 of this guidebook). The monocline extends almost 100 km along strike, from Gogebic County in northern Michigan to Bayfield County, Wisconsin (Schmidt, 1976). The regional northerly tilt of the strata is thought to have been caused by south-directed post-rifting reverse displacement on a system of lithosphere-scale, north-dipping listric faults, including the Atkins Lake-Marinesco and Pelton Creek Faults, which may have formed initially as normal faults during the development of the rift (Cannon et al., 1993). Because the rift-filling lavas and sediments formed a much thicker stratigraphic section to the north of these normal faults, the subsequent reactivation of the normal faults as reverse faults caused the Mesoproterozoic riftrelated rocks to be thrust over older, Archean and Paleoproterozoic rocks (in contrast to the usual older-over-younger stratigraphic pattern caused by reverse faulting in a simple ‗layer cake‘ sedimentary sequence). The Atkins Lake-Marenisco fault cuts higher in the crustal section on its western terminus, where Paleoproterozoic rocks form the footwall, than in the east, where it transects Archean crust (Cannon et al., 2008). Apart from the major faults bounding the Montreal River Monocline on the south, there is no evidence for significant stratigraphic disruption or repetition within the exposed section in north-central Wisconsin. Therefore, the steeply dipping strata of the monocline provide a complete transect of the crust at ca. 1000 Ma. The listric shape of the Atkins Lake-Marenisco fault seems well constrained by the outcrop patterns in the hanging wall (north side): the northward-shallowing, northerly dips of Proterozoic strata, and the fanning pattern of southward dips seen in (originally vertical) Midcontinent rift-related dikes intruded into Archean rocks (Fig. 2) (Cannon et al., 1993, 2008). Also, in the eastern half of the monocline, where the fault cuts through Archean basement rocks, there is a well-defined biotite Rb-Sr age front as one approaches the fault from the north. The Archean rocks north of this line preserve Archean and Paleoproteorozoic (Penokean) ages, showing that they have remained below the 270° C Rb-Sr blocking temperature for biotite since at least early Proterozoic time. South of the front, the Archean rocks yield anomalous Mesoproterozoic Rb-Sr ages (ca. 1040-1060 Ma), indicating that they cooled through the blocking temperature at the time of closure of the Midcontinent rift and were brought up from mid-crustal depths by rotational displacement along the curved surface of the Atkins Lake-Marenisco fault (Cannon et al., 1993; Holm et al., 2007). The Rb-Sr age front thus represents an exhumed Mesoproterozoic isotherm. These regional relationships indicate that the Atkins Lake-Marenisco fault was a major structure that accumulated many kilometers of reverse displacement in Mesoproterozoic time (Aldrich, 1929; Cannon et al., 1993, 2008). ILSG 2011 34 Field Trip 2 �ILSG 2011 35 Field Trip 2 �Figure 2. Schematic cross sections through the Montreal River Monocline in northern Michigan and Wisconsin (Modified from Cannon et al., 1993). Sections (a) and (b) depict eastern and western parts of the monocline, respectively, as indicated in the inset. The major reverse faults originated as riftbounding normal faults, creating a thicker stratigraphic section to the north. MCR =Midcontinent rift; Paleoprot = Paleoproterozoic metasedimentary rocks; PCFZ = Pelton Creek Fault Zone (inferred). Dashed lines indicate the positions of isotherms prior to closure of the rift. Thin lines in Archean rocks in upper section represent orientations of Midcontinent rift-age mafic dikes. Filled star shows location of Marengo Falls (Stops 6, 7); open star shows inferred depth of formation of pseudotachylytes at Stop 6. Stop 1. Paleoproterozoic Ironwood Iron-formation, metadiabase sills, and breccias (Text modified from Klasner and LaBerge, ILSG Field Guide 1996) Numerous exposures of the Ironwood Iron-formation and the Palms Formation, both part of the Menominee Group, occur in a 4,000 m long northeast-trending zone from Atkins Lake to the Marengo River (Fig. 1), where the Early Proterozoic units are truncated by the Atkins Lake-Marenisco fault. The Palms Formation consists of an upper massive quartzite member and a lower argillaceous quartzite member, similar to exposures elsewhere on the Gogebic Range. (Due to time limitations, we will not visit exposures of the Palms Formation at this stop.) The Ironwood Iron-formation consists of magnetic wavy-bedded and laminated cherty iron-formation and argillite, some of which is also magnetic. In places, mafic igneous bodies—either sills or flows—occur in contact with iron-formation. ILSG 2011 36 Field Trip 2 �A group of closely spaced outcrops along a ridge east of Atkins Lake (N46.27904, W 91.02187) shows typical metamorphosed Ironwood Iron-formation, unusual intraformational breccias, and gabbro sills (Fig. 3). These outcrops are a good representation of the Ironwood in the western extremity of the Gogebic Range, and of the igneous and tectonic overprints that are more pronounced here than in the central part of the range. At this stop, enigmatic breccia units are spatially associated with the upper and lower contacts of a concordant mafic body in the Ironwood Iron-formation (Fehrer and Flood, 1995). These breccias contain angular to sub-rounded clastsof banded siliceous sedimentary rock,ranging from cm to several m in length, within a matrix of a fine-grained, massive mafic igneous rock. There is a marked gradation in grain size in the mafic igneous material, from basalt-like at the contacts with the iron-formation, where the breccia occurs, to diabasic and gabbroic away from the contacts (Fig. 3). Clasts within the breccia are mainly siliceous argillite;a minority is magnetic. Puzzlingly, the population of clasts appears to be more siliceous than the rock in the immediately adjacent iron-formation. Most clasts, regardless of size and composition, are elongate and tabular. Bent or folded clasts are not uncommon. The argillite clasts consist of alternating layers of recrystallized quartz, fine-grained chlorite, biotite or actinolite, garnet, and variable amounts of magnetite. The fine-grained mafic matrix contains conspicuous vesicles, especially around clasts. The matrix is composed of medium-grained amphiboles and plagioclase laths, probably a relict ophitic texture. Plagioclase phenocrysts are present locally. Limited geochemical analysis indicates a tholeiitic composition for the matrix. It should be noted that the mineralogy and, to some extent, the textures of these rocks have likely been modified by Mesoproterozoic contact and regional metamorphism. The origin of the breccias is problematic. Formation by debris flows, faulting or impact ejecta can be ruled out because the matrix is igneous and not fragmental. Outcropscale relationships are arguably consistent with either intrusive or extrusive magmatic activity. The presence of breccia units at both the upper and lower margins of the igneous body seems most simply interpreted as evidence that they formed during intrusion of a sill. The presence of angular, and locally deformed, blocks of country rock within mafic materialcould be evidence for intrusion into semi-consolidated sediments at shallow depth -or could indicate that the clasts were hot enough to deform ductilely in a mobile magma that was shearing against the walls of more rigid host rock. Alternatively, the breccia and mafic rocks could represent an extrusive basaltic flow that incorporated ‗rip-up‘ clasts from the sediments over which it flowed. Sometime later, this stratigraphic horizon may have become the site for the concordant intrusion of a mafic sill that separated the clast-bearing basalt flow into two parts. Features consistent with this origin include: 1) perfectly concordant contacts with thinly laminated sediments at several outcrops, and the lack of any observed crosscutting contacts; and 2) the marked textural contrast between the fine-grained breccia matrix and presumably intrusive coarse-grained, inclusion-free metadiabase and gabbro. Whether the breccias are the result of extrusive or shallow intrusive igneous activity, they do indicate that mafic magmatism occurred during or soon after iron-formation deposition in the western part of the Gogebic Range. Field relationships at this site are similar to those in the eastern part of the Gogebic Range, where the Emperor Volcanic Complex is interlayered with the Ironwood Ironformation (Klasner et al., 1998). In both areas, platformal sediments, including the Bad River Dolomite, the Palms Formation, and the Sunday Lake Quartzite in the east, are similar to ILSG 2011 37 Field Trip 2 �equivalent rocks in the central part of the Gogebic Range. However, evidence for igneous activityduring the time of deposition of the Ironwood Iron-formation is much scarcer in the central part of the range, limited to a few minor ash beds (R. G. Schmidt, personal communication, 1995). The larger volume of igneous rock on either end of the Gogebic Range indicates a far more volcanically active environment in these regions. Figure 3. Detailed map (Klasner and LaBerge, unpublished) of the area east of Atkins Lake showing outcrops and geologic relationships among iron-formation, breccia units, diabase and gabbro. Descriptions of the numbered localities are provided below. Locality 1. This outcrop consists of layered magnetic, greenish, actinolitic iron-formation with metadiabase, and a breccia zone with clasts of iron-formation in a fine-grained metadiabase matrix. Bedding orientations at this outcrop are consistent with orientations of bedding elsewhere in the area, indicating that the outcrop is in place and not a detached raft of iron-formation within the igneous rock.Walk northeast about 60 m to the top of a hill. Locality 2. At this location, large thin slabs of argillite lie in various orientations in a matrix of locally vesicular metadiabase. Some of the slabs are contorted or folded, and one is rolled into a ball with a central layered portion surrounded by a concentric zone of smaller ILSG 2011 38 Field Trip 2 �clasts in the metadiabase matrix. Slabs range in length from less than 1 cm to more than 1 m.Walk east along the crest of the hill about 90 m to a small NNW-trending valley. When in the valley, follow the edge of the hill toward the SSW for about 60 m. Locality 3. Strongly magnetic, wavy-bedded, granular cherty iron-formation. Here lenticular chert beds are up to 15 cm thick. This bedding style is typical of wavy-bedded iron-formation farther east along the Gogebic Range.Walk north-northeast along edge of hill about 25 m. Locality 4. Thin-bedded, non-magnetic chert-argillite or argillaceous ironformation.Some cherty layers appear boudinaged. The argillite unit is about 50 m thick and grades into a more argillaceous unit with less cherty layers to the north.Walk northeast about 55 m. Locality 5. The top of the argillaceous iron-formation layer isin sharp contact here with the overlying breccia unit, in which argillite clasts aresurrounded by metabasalt matrix. Clasts in the breccia tend to become largerhigher in the section (northward from the contact). Remnants of what appears to be a thin slab of iron-formation lie along the top of the breccia unit both here and at locality 6.Walk east-southeast about 10 m. Locality 6.Exposed here is the southeast continuation of the thin slab of iron-formation that lies along the contact between the breccia on the southwest and massive metagabbro on the northeast.Walk northeast about 50 m. Locality 7.Massive metadiabase/metagabbro.Walk northwest about 35 m. Locality 8. Contorted clasts, mainly argillitic, in a vesicular metabasaltmatrix. Stop 2. Mesoproterozoic (?) Mellen-type granite A small undated granite body is exposed in several outcrops east of Atkins Lake, in the open woods south of Forest Service Road 379 (N46.27739, W91.02542). The granite is pink and white, with a medium- to coarse-grained equigranular to porphyritictexture. Principal minerals are potassium feldspar, plagioclase, biotite, and quartz. The granite intrudes Paleoproterozoic metasediments of the Ironwood Iron-formation, and the lack of metamorphic effects and or deformational fabric within the granite suggests that it is Mesoproterozoic in age. This granite is very similar in appearance to the well-known Mellen granite, which is a phase of the Mellen Intrusive Complex, a major magmatic unit of the Midcontinent Rift exposed over a large area more than 20 km to the northeast of this site. In addition to the Mellen granite, the Mellen Intrusive Complex is made up of two large, layered gabbroic bodies -- the Potato River and the Mineral Lake intrusions -- and several related igneous bodies, including the peridotitic Rearing Pond intrusion, and a group of gabbro and granophyre sills. A granophyre within the Mineral Lake intrusion has yielded a U-Pb zircon age of 1102.0 ± 2.8 Ma (Zartman et al., 1997). An indistinguishable U-Pb zircon age (within error) of 1100.9 ± 2.8 Ma has been determined for the Mellen granite, which intrudes and brecciates gabbro of the Mineral Lake intrusion (Zartman et al., 1997). ILSG 2011 39 Field Trip 2 �Stop 3. Mesoproterozoic Siemens Creek Volcanics contact metamorphosed to pyroxene hornfels and granophyre(text from Cannon et al., ILSG Field Guide 1996) The Siemens Creek Volcanics, the lowermost formation within the Powder Mill Group, is the oldest volcanic unit in the Midcontinent Rift in northern Wisconsin. It consists of a sequence of flood basalts erupted as relatively thin flows. These Keweenawan rocks are thrust southward over Paleoproterozic rocks (such as those seen at Stops 1, 4, and 5) along the Atkins Lake-Marenisco fault (Figs. 1 and 2). The northward tilt caused by the faulting has exposed the basal contact of the Mineral Lake intrusion together with a structurally lower sequence of intruded and metamorphosed volcanic rocks. At this stop (N46.2805, W91.0268) an unusually highly metamorphosed section of the Siemens Creek Volcanics is exposed on an isolated hill about 100 m north of Forest Service Road 210 (Club Lake Road), just north of Atkins Lake. The Atkins Lake-Marenisco fault lies under the low ground between the road and the hill. No indications of faulting have been found in the outcrops we will examine, but other low outcrops just south of the road (on private land) contain zones of markedly sheared basalt. The Siemens Creek Volcanics at this locality are intensely recrystallized to pyroxene hornfels. Most original textures are obliterated. Locally, textures possibly representing relict amygdular flow tops can be seen, but these occur discontinuously, making it difficult to recognize distinct lava flows. Small bodies of pink, granophyric granitic rock are common and appear to represent segregations of magma generated by partial melting of the basalt during high-grade metamorphism. These intensely metamorphosed and partly melted rocks can be traced for about 10 km along strike, beyond which they grade both east and west into less metamorphosed equivalents. This unit is up to about 1 km thick and is bounded on the north by the western extension of the Mineral Lake intrusion and related intrusive rocks. Although it is tempting to ascribe the intense metamorphism at this stop to contact effects of the Mineral Lake intrusion, the grade and geographic extent of the metamorphism seen here are much greater than those in the border zone of the gabbro in other areas. Additional conductive heating during the time that these rocks resided deep in the crust, at the base of the thick sequence of lavas and sediments that filled the Midcontinent Rift, may have contributed to the unusually high metamorphic grade. Stop 4. Penokean shear zone in Paleoproterozoic Palms Formation, Marengo River On the south bank of the Marengo River immediately downstream from the private bridge off Forest Road 198 (N46.28965, W90.99405), laminated argillite of the lower part of the Palms Formation is exposed in several subdued outcrops. The rockshere have been intensely sheared within a NE-trending zone that is at least 60 m wide. This shear zone, presumed to be of Penokean age, is essentially bedding parallel and now strikes50° NE and dips 50°NW. It separates a thick metadiabase sill in the footwall from the upper Palms Formation and thinner metadiabase sills in the hanging wall. At this site, bedding in the Palms is tightly to isoclinally folded and transposed into a mylonitic foliation. The folds are commonly dismembered, with limbs so strongly attenuated that only relict fold hinges can be seen. These small folds typically plunge about 50 degrees to the northwest and their axes lie within the plane of the mylonitic foliation. Some of the relict fold hinges apparently acted like rigid porphyroclasts during shearing and display sigma-type tails that indicate dextral shear (in the present orientation). ILSG 2011 40 Field Trip 2 �We interpret this fault to have formed as a bedding-parallel thrust during the Penokean Orogeny. The current northwestward dip of the Paleoproterozoic strata and the fault are a result of regional tilting that occurred during inversion of the Midcontinent Rift at about 1060 Ma. If the strata are restored to their attitude prior to this rotation, they are nearly flat lying and the fault likewise is a nearly flat surface across which the upper plate was thrust northward. Thus, the shear zone seen here is, to our knowledge, the best exposed example of a Penokean thrust fault in this part of the Lake Superior region. Stop 5. Paleoproterozoic Bad River Dolomite: Stromatolites, Penokean deformation, contact metamorphism, Grandview Quarry, Marengo River Please note that Stops 5-7 are on private land owned by the S.C. Johnson family, who have been generous in making these outcrops accessible to geologists for educational purposes. Steep slopes and loose rocks at these sites pose hazards, and field trip participants are responsible for their own safety. The oldest Paleoproterozoic unit in the western Gogebic Range, the Bad River Dolomite of the Chocolay Group, is exposed here in an abandoned quarry that was developed on a natural glaciated bedrock outcrop (N46.28509, W90.98608). In this area, the Bad River Dolomite lies nonconformably and discontinuously on top of the Neoarchean Puritan Batholith (Stop 6), but in the eastern Gogebic range it overlies the Sunday Quartzite, interpreted as a shallow marine tidally influenced unit. At this site, the Bad River Dolomite strikes approximately N50°E and dips 50°-60° NW, concordant here with the overlying Palms Quartzite (Stop 4), but regional relationships indicate that the contact with the Palms is disconformable (Cannon et al., 2008). The Bad River Dolomite is a fine-grained chert-bearing dolostone in which massive, meter-scale strata give way up-section to laminated dolostone with horizons of silicified stromatolites. The stromatolites are well exposed in cross section on the glacially scoured surface at the rim of the quarry and are typically 20-30 cm in diameter. Along the Marengo River, the Bad River Dolomite experienced metamorphism as a result of the emplacement of the nearby Mineral Lake intrusion, and radiating bundles of pale green tremolitic amphibole can be seen in and around chert nodules and within stromatolitic layers, where silica and dolomite are interbedded at the centimeter scale. These relationships record the classic calcsilicate reaction: Dolomite + SiO2 + H2O Tremolite + Calcite + CO2, which occurs either 1) in the presence of very CO2-rich fluids under low pressure conditions (ca. 1-2 kbar) or 2) over a wider range of fluid compositions at higher pressures (>3 kbar) (Spear, 1993). The isotropically radiating clusters of tremolite indicate nucleation-limited growth and static metamorphism (i.e., under low deviatoric stress). In places the tremolite is very fibrous, even asbestoform. Fine mineralogical specimens can be found in the waste rock piles in the old quarry. The quarrying also exposed several meter-scale Keweenawan dikes that cut discordantly across the tilted beds. In a detailed geochemical and isotopic study of carbonate rocks from the Chocolay Group across the Lake Superior region, Bekker et al., (2006) found that the Bad River Dolomite (where it is less metamorphosed) has a 13C signature distinct from that of other Chocolay Group carbonates (e.g., the Kona Dolomite in the Marquette Range and the Gordon Lake Formation in Ontario). They concluded that the Bad River is slightly younger than these units, deposited after the early Proterozoic (Gowganda) glaciations but just before the ILSG 2011 41 Field Trip 2 �global biogeochemical upheaval recorded by the large positive carbon isotope excursion known as the Lomagundi event. Stop 6. Marengo Falls, S side: Neoarchean Puritan Batholith; fault breccias, cataclasites and pseudotachylyte along the Atkins Lake-Marenisco Fault The deeply incised valley of the Marengo River in southeastern Bayfield County, Wisconsin (sections 14 and 15, T44N, R5W) provides a 0.5 km transect through the Atkins Lake-Marenisco fault zone and is one of the few places within the Midcontinent Rift where rocks along a major rift-closing fault are well exposed (Fig. 4). At the falls of the Marengo River (N46.286, W90.964), the Puritan Batholith is juxtaposed across the Atkins LakeMarenisco Fault with the Ironwood Iron-formation and Paleoproterozoic diabase sills, and also with both basaltic lava flows (Siemens Creek Volcanics) and a layered mafic complex (the Mineral Lake Intrusion) of the Midcontinent Rift. Based on regional outcrop relationships, discussed in the Introduction, the vertical throw on the Atkins Lake-Marenisco Fault is probably on the order of 15 km (Fig.4). The river flows northward over Archean rocks until it intersects the fault zone, marked by a 15 m high waterfall near the southern boundary of section 14 (Fig. 4). At that point, the river bends sharply to the west-northwest, subparallel to the fault zone, and cuts through rocks in the core of the zone. Along this transect, the Puritan Quartz Monzonite can be traced from an undeformed condition above the falls to an ultracataclastic state a few hundred meters downstream. Rocks exposed between these two locations preserve intermediate textures that record the temporal evolution of the fault zone (Bjørnerud 2010a). The nearly undeformed Puritan Quartz Monzoniteis a medium- to coarse-grained (0.5-2.0 cm), locally pegmatitic granitoid with consertal igneous texture in thin section. It is comparatively poor in mafic minerals and those present (mainly biotite, some hornblende) are largely altered to chlorite. The feldspars, principally albitic plagioclase and microcline, are commonly sericitized, and this becomes more pronounced near the fault zone. Away from the fault, feldspars show little evidence of internal deformation, although in places where a weak gneissic fabric is developed, a fine-grained (0.1 mm) polygonalized texture indicates partial annealing following ductile deformation, presumably during Neoarchean time (Schmidt, 1976). The earliest fault-related microstructures, found in rocks immediately downstream from Marengo Falls, are quasi-ductilely deformed albite and microcline grains with kinked or bent twin planes and patchy extinction. In some samples, very small (ca. 0.05 mm) subgrains have developed along the most deformed parts of these crystals. These textures are overprinted by more brittle features. In the steep cliffs on the south side of the river, the quartz monzonite has been broken into heterogeneous breccias and cataclasites. Theserocks lack any prominent planar fabric, slickenlines or other indications of slip sense; they are simply pervasively fragmented. ILSG 2011 42 Field Trip 2 �Figure 4.Geologic map of the Atkins Lake-Marenisco Fault zone in the Marengo Falls area, Bayfield County, Wisconsin (modified from Cannon et al., 2008).Star marks location of Marengo Falls (Stops 6 & 7). The location of Stop 5 is just west of the area marked Xb. The non-parallelism of section lines reflects the difficulty early surveyors had in mapping areas with extensive outcrops of magnetite-bearing iron-formation. In a series of outcrops of the Puritan quartz monzonite on the south side of the river, extending about 200 m immediately west from an abandoned hydroelectric station, breccias and cataclasites are complexly interlaced with very dark veins, 0.2 to 6 cm thick, of what appears to be devitrified (mainly chloritized) pseudotachylyte (Bjørnerud, 2007, 2010a). Several types of textural evidence support this interpretation. First, the chlorite in this material is texturally isotropic, occurring as uniformly spaced radial bundles 0.1-0.2 mm in diameter. This suggests that it nucleated in a homogeneous and isotropic medium – i.e., glass or on primary microlitic spherules in glass sometime after the deformation had ceased. Second, this chloritic material typically includes relatively large (0.5-2 mm), angular clasts from the protolith (including quasi-ductilely deformed feldspar grains), while finer clasts are conspicuously absent. The absence of fines is consistent with preferential melting of smaller rock fragments, a phenomenon documented in natural and experimental pseudotachylytes (e.g., Tsutsumi, 1999). Third, the contacts between some of the chloritic material and the host rock are distinctly scalloped, a partial melting texture observed in pseudotachylyte injection veins (Swanson, 2006). Fourth, in some of the thicker veins, a disproportionate number of the surviving clasts are embayed grains of titanite, a refractory accessory mineral in the protolith that could have withstood temperatures sufficient to melt the major minerals (quartz and alkali feldspar). The irregular, cuspate edges of these crystals are again suggestive of partial assimilation by a melt phase. Finally, some of the chloritic material ILSG 2011 43 Field Trip 2 �preserves ‗pseudomorphs‘ of apparent flow textures similar to those observed in younger, still-glassy pseudotachylytes. Collectively, these observations support the interpretation that much of the black chloritic material in the fault zone originated as pseudotachylyte. The spatial relationships among the various types of fragmented rock and the pseudotachylytes shed light on the dynamic processes in the fault zone over time. In thin sections of some pseudotachylyte-bearing samples, a continuous gradient in grain size can be observed from cataclasite to ultracataclasite to pseudotachylyte, suggesting that the textures formed in a single slip event. More commonly, though, fine-grained cataclasites or microbreccias transect and/or include scattered pieces of the pseudotachylytic material, indicating that these granulated rocks are generally younger than the pseudotachylytes. One group of samples shows two distinct generations of pseudotachylyte, which in turn are both incorporated as clasts in a cross-cutting breccia. Some of these microbreccias appear to have dilational and intrusive (fracture Mode I), rather than shearing (Mode II), cross-cutting relationships with the pseudotachylyte and the surrounding rock. As such, they are not cataclasites in the strict sense of having been produced only by grinding and frictional wear. They occur as dike-like fingers that ‗explosively‘ disaggregate the pseudotachylyte and make inroads into fractures in the host rock. These microbreccia veins are typically matrix- rather than clast-supported and contain parallel bands of uniform grain size, commonly fining inward from wall to interior, possibly a result of dynamic sorting of fluidized granular material. Such textures may record thermal pressurization of fluids during a seismic event (Otsuki et al., 2003; Bjørnerud, 2010b). Stop 7.Marengo Falls, N side: Ultracataclasite along the Atkins Lake-Marenisco Fault The most extreme comminution of grains has occurred in rocks exposed in prominent bluffs on the north side of the river. In outcrop, these ultracataclasites are dark gray, massive, and transected by anastomosing veins of chlorite, quartz and zeolites. In thin section, one can discern very fine (<0.1 mm) angular feldspar grains in a still finer matrix of chlorite with a weak preferred orientation. These rocks are so modified by deformation and mineralization that their protolith is unclear; they are likely derived from the basaltic lava flows of the Siemens Creek Formation, which crops out to the north of the river below the falls(Fig. 4). In the very first geologic description of these rocks, C.E. Wright (1879), who mapped the Penokee Gap under the direction of T.C. Chamberlin in 1876, noted their unusual character, giving them the non-genetic designation ―chloro-silicious rocks‖ (Fig. 5). These fault rocks are overprinted by multiple generations of vein networks, with a wide array of orientations, indicating a protracted period of fracture-hosted fluid flow that continued after the main period of faulting and cataclasis had ceased. Many of these veins have a fibrous texture, indicating a ‗crack-seal‘ mechanism of formation, and some appear to record almost isotropic dilation of the fault rock. Vein minerals include chlorite, zeolites, quartz, and calcite. These same minerals are common as vein fillings and slickenfibers in Midcontinent Rift basalts in northern Wisconsin (Garbowicz and Bjørnerud, 2003) and in the Upper Peninsula of Michigan, where they occur together with hydrothermal native copper deposits known to have formed between about 1060 and 1050 Ma (Bornhorst et al., 1988). Summary: The AtkinsLake-Marenisco fault zone of north-central Wisconsin provides a rare window into faulting processes along the flanks of the Midcontinent Rift during its inversion at ca. 1060 Ma. The presence of quasi-ductilely deformed feldspars and cm-thick ILSG 2011 44 Field Trip 2 �pseudotachylyte fault veins indicate that faulting began at mid-crustal levels and that the fault slipped in one or more large earthquakes. Cross-cutting cataclasites and matrix-supported microbreccias record progressive comminution and intermittent fluidization of rocks in the fault zone. Elevated fluid pressures almost certainly played a role throughout the life of the fault. Reverse slip on the steeply dipping AtkinsLake-Marensico fault and other rift bounding faults coincided in time with widespread hydrothermal mineralization, including native copper deposition, in rocks of the Midcontinent Rift. The Atkins Lake- Marenisco fault is a thick-skinned lithosphere-scale structure, and its characteristics support the hypothesis (Cannon, 1994) that large deviatoric stresses were transmitted far inland during the Grenville Orogeny. Figure 5. The first geologic map of the Marengo Falls area, by C.E. Wright (1879), based on field work done in the summer of 1876. The ultracataclasites along the Atkins LakeMarenisco fault zone are mapped as ―Chloro-silicious rocks‖; the Puritan Quartz Monzonite as ―Granite‖ and Siemens Creek Volcanics as ―Greenstone‖. Only the strike of the Ironwood Iron-formation (―Magnetic schist‖) is inaccurate (shown as NW rather than NE). ILSG 2011 45 Field Trip 2 �REFERENCES Aldrich, H. R., 1929. The geology of the Gogebic iron range of Wisconsin. Wisconsin Geological and Natural History Survey Bulletin 71. Bekker, A., Karhu, J. and Kaufman, A., 2006. Carbon isotope record for onset of the Lomagundi carbon isotope excursion in the Great Lakes area, North America. Precambrian Research 148, 145-180. Bjørnerud, M., 2007.Evidence for paleoseismic events during closure of the Midcontinent Rift, Atkins Lake-Marenisco fault zone, northern Wisconsin. Proceedings of the Institute on Lake Superior Geology 53, 6-7. Bjørnerud, M., 2010a. Evidence for Grenville-age seismicity and thick-skinned deformation in northern Wisconsin, USA. Journal of Geology 188,45-58. Bjørnerud, M., 2010b. Rethinking conditions necessary for pseudotachylyte formation: Observations from the Otago schists, S Island, New Zealand. Tectonophysics 490, 6980. Bornhorst, T., Paces, J., Grant, N., Obradovich, J., and Huber, N. K., 1988. Age of native copper mineralization, Keweenaw Peninsula, MI. Economic Geology 83, 619-625. Cannon, W.F., 1994. Closing of the Midcontinent Rift: A far-field effect of Grenvillian compression. Geology 22, 155-158. Cannon, W.F., Woodruff, L.G., Nicholson, S.W., and Hedgman, C.A., 1996, Bedrock geologic map of the Ashland and the northern part of the Ironwood 30‘x60‘ quadrangles, Wisconsin and Michigan: U.S. Geological Survey, Miscellaneous Investigation Series Map I-2556, scale 1:100,000. Cannon, W.F., LaBerge, G., Klasner, J., and Schulz, K., 2008. The Gogebic Iron Range – A sample of the northern margin of the Penokean fold and thrust belt. U.S. Geological Survey Professional Paper 1730. Cannon, W.F., Peterman, Z., and Sims, P. K., 1993, Crustal-scale thrusting and origin of the Montreal River monocline – a 35-km thick cross section of the mid-continent rift. Tectonics 12, 728-745. Feher, L. and Flood, T., 1995. Vesicles and breccia due to injection of mafic magma into partially lithified sediments of the early Proterozoic Ironwood Iron Fm., Gogebic Range, NW Wisconsin. Proceedings of the Institute on Lake Superior Geology, 41, 1315. Garbowicz, A. and Bjørnerud, M., 2003. Paleostress inferences from slip vectors in the eastern part of the Wisconsin segment of the Midcontinent rift, Proceedings of the Institute on Lake Superior Geology 49, 18. Holm, D., Schneider, D., Rose, S., Mancuso, C., McKenzie, M., Foland, K. and Hodges, K., 2007, Proterozoic metamorphism cooling in the southern Lake Superior Region of North America and its bearing on crustal evolution. Precambrian Research 157, 106126. Klasner, J.S., LaBerge, G.L., and Cannon W.F., 1998, Geologic map of the eastern Gogebic iron range, Gogebic County, Michigan: U.S. Geological Survey Geologic Investigations Series map I-2606, scale 1: 24,000. Klasner, J, and LaBerge, Gene L., 1996, Lake Namekagon and Penokee Gap areas, west Gogebic Range, Wisconsin. Proceedings of the Institute on Lake Superior Geology, Part 2: Guidebooks to Field Trips, Field Trip 5, p. 83-108. ILSG 2011 46 Field Trip 2 �Otsuki, K., Monzawa, N., and Nagase, T., 2003. Fluidization and melting of fault gouge during seismic slip: Identification in the Nojima fault zone and implications for focal earthquake mechanisms. Journal of Geophysical Research108 B4, doi: 1029/2001JB001711. Schmidt, R., 1976. Geology of the Precambrian W (lower Precambrian) rocks in western Gogebic County Michigan. U.S. Geological Survey Bulletin 1407. Sims, P. K., Peterman, S., and Prinz, W., 1977. Geology and Rb-Sr age of Precambrian W Puritan quartz monzonite, northern Michigan. U.S. Geological Survey Journal of Research 31, 185-192. Spear, F., 1993, Metamorphic phase equilibria and pressure-temperature-time paths: Monograph 1, Mineralogical Society of America, Washington, D.C. Swanson, M., 2006. Pseudotachylyte-bearing strike-slip faults in mylonitic host rocks, Fort Foster brittle zone, Kittery, Maine. In Abercrombie, R., McGarr, A., Kanamori, H. and DiToro, G., eds. Earthquakes: Radiated Energy and the Physics of Faulting. Geophysical Monograph 170. Washington D.C., American Geophysical Union, 166179. Tsutsumi, A., 1999. Size distribution of clasts in experimentally produced pseudotachylytes. Journal of Structural Geology 21, 305-312. Wright, C.E., 1879, The Huronian Series west of Penokee Gap, Chap. IV in T.C. Chamberlin, ed., Geology of Wisconsin, Survey of 1873-1879, Volume III. Madison WI: Wisconsin Commissioners of Public Printing. Zartman, R.E., Nicholson, S. W., Cannon, W .F., and Morey , G.B., 1997. U-Th-Pb zircon ages of some Keweenawan Supergrouprocks from the south shore of Lake Superior. Canadian Journal of Earth Sciences 34, 549 -561. ILSG 2011 47 Field Trip 2 �ILSG 2011 48 Field Trip 2 �57TH ANNUAL INSTITUTE ON LAKE SUPERIOR GEOLOGY FIELD TRIP 3 GEOLOGY OF THE BAYFIELD PENINSULA: KEWEENAWAN BAYFIELD GROUP AND PLEISTOCENE DEPOSITS Bayfield Peninsula – K.Wilson ILSG 2011 49 Field Trip 3 �ILSG 2011 50 Field Trip 3 �Field Trip 3 Geology of the Bayfield Peninsula: Keweenawan Bayfield Group and Pleistocene deposits Richard Ojakangas1, Drew Cramer2, and Tom Fitz2 1 University of Minnesota – Duluth, 229 Heller Hall, 1114 Kirby Drive, Duluth, MN 55812 E-mail: rojakang@d.umn.edu 2 Northland College, 1411 Ellis Avenue, Ashland, Wisconsin 54806 E-mail: tfitz@northland.edu THE KEWEENAWAN BAYFIELD GROUP Introduction In the Lake Superior region of the Midcontinent Rift System (MRS), as much as 20,000 m of dominantly basaltic sub-aerial flood lavas are overlain by 10,000 m of sedimentary rocks (Figs. 1, 2) (Behrendt et al, 1988; Cannon et al, 1989). The exposed volcanic rocks range in age from 1109-1086 Ma; the greatest bulk was extruded during a magnetically reversed interval around 1108-1105 Ma, and during a magnetically normal interval around 1097-1094 Ma (Green, 1995). The overlying, unmetamorphosed, sedimentary rocks, comprise the Oronto Group and the younger Bayfield Group (Fig. 3). Although we will not see the Oronto Group rocks on this field trip, a short description is provided here to set a sedimentological and petrographic background for the Bayfield Group rocks that we will see. Cessation of volcanism was followed by the deposition of the Oronto Group, which includes the Copper Harbor Conglomerate, the Nonesuch Formation, and the Freda Sandstone (Daniels, 1982). On the Keweenaw Peninsula of Michigan, the Copper Harbor Conglomerate, which is as thick as 1340 m, contains basaltic flows that represent some of the last volcanism within the basin. The Lakeshore Traps in the Copper Harbor have been dated at 1087.2 +/-1.6 Ma (Davis and Paces, 1990), and an intrusive on Michipicoten Island in eastern Lake Superior at has been dated at 1086 Ma (Palmer and Davis, 1987). The Copper Harbor is largely a coarse, clast-supported conglomerate; the clasts are dominantly volcanic and were derived from a variety of volcanic rocks. Sandstones, which are also largely composed of volcanic detritus, are common, especially in the upper half of the formation; generally, they are lithic arenites. Siltstones and shales are less abundant, at least in the exposed portion of the formation, but may be more abundant at depth nearer the axis of the basin. Deposition was on alluvial fans and associated braided alluvial plains; rare stromatolite horizons formed in standing bodies of water during pauses in sedimentation. Aprobable correlative unit is the subsurface Solor Church Formation of southeastern ILSG 2011 51 Field Trip 3 �Figure 1. Generalized geologic map of the Midcontinent Rift System in the Lake Superior region showing the study area. From Adamson, 1977, modified from Dickas, 1986. Figure 2. Aeromagnetic map of Northern Wisconsin. From Nicholson et al., 2006. ILSG 2011 52 Field Trip 3 �Figure 3. Stratigraphic correlation chart for Keweenawan Supergroup rocks in Minnesota, Wisconsin, and Michigan. From Morey and Van Schmus, 1988. ILSG 2011 53 Field Trip 3 �Minnesota. The Nonesuch Formation, which is as thick as 200 m, is noted for its copper content (mainly chalcocite) and the presence of petroleum seeps in the now-closed White Pine Copper Mine. The Nonesuch is largely a fine-grained sequence of gray to black siltstone with minor thin shales and carbonates as well as local, graded sandstone beds and conglomerates. The fine sediment was deposited in a restricted lacustrine basin,where organic material could accumulate as a result of a reducing chemical environment (Elmore et al, 1988; Suszek, 1995). The Freda Sandstone, as thick as 1525 m, is comprised dominantly of red to buff sandstone with intercalated red siltstone and shale; the sandstones are more mature than those lower in the group. The environment of deposition fits a braided stream model (Fig. 4). Figure 4. Generalized reconstruction of what the Midcontinent Rift System may have looked like after cessation of volcanism about 1086 Ma. Note the streams entering the rift from the adjacent highlands and the presence of lakes (white). From Ojakangas and Matsch, 1982. The overlying Bayfield Group, 2100 m thick, is comprised from the base upwards of the Orienta Sandstone, the Devils Island Sandstone, and the Chequamegon Sandstone. All three units are best exposed on the Bayfield Peninsula and the Apostle Islands. Correlative units are found in Minnesota, Michigan, and Ontario (Ojakangas and Morey, 1982). The ILSG 2011 54 Field Trip 3 �contact between the subhorizontal Bayfield Group and the steeply dipping underlying Oronto Group is not exposed, but field and geophysical data can be interpreted as indicating an unconformable relationship (Morey and Ojakangas, 1982). Figure 5 shows the distribution of the three formations in the Bayfield Group. Figure 5. Distribution and attitudes of the three Bayfield Group formations. From Adamson, 1997. Orienta Sandstone The Orienta, and its equivalent several miles to the west in Minnesota (the Fond du Lac Formation), are comprised of red feldspatho-lithic arenite, red siltstone, and shale units with fining-upward sequences that are typical of deposition in meandering river systems;large-scale trough cross-beds are common andparting lineation and current ripple marks are also present. Devils Island Sandstone The white,buff, and orange Devils Island Sandstone is about 100 m thick. It is a quartz arenite, as is its equivalent unit,the Hinckley Sandstone, to the west in Minnesota, (Tryhorn and Ojakangas, 1972). Symmetrical ripples suggest a lacustrine environment as the final site of deposition, but eolian processes must have been an important factor in both the textural and the mineralogical maturation of the sand; creating a quartz arenite from felspatho-lithic sands (Fig. 6). The discovery of adhesian ripples and other eolian features by Johnson et al (2001) in the Hinckley support this suggestion, although the latter works also suggest the presence of a braided stream environment. More recently, Galston and Havholm (2008) have identified sedimentary features that they interpret as eolian dunes, adhesian ripples and wind ripples that were deposited on a floodplain with braided streams. An ILSG 2011 55 Field Trip 3 �equivalent to the east in Michigan may be an upper portion of the Jacobsville Sandstone (Kalliokoski,1982) as exposed on the Middle Branch of the Ontonogan River on Michigan Highway 28. Quite likely, the maturation of the sand on the vegetation-less surface of that time was due to eolian processes, which are the most effective agents of such maturation. Figure 6. Compositional triangle showing the relative abundances of quartz, feldspar, and lithics (rock fragments) in the formations of the Oronto and Bayfield Groups. The Chequamegon Formation is outlined with the dashed line. Note the increasing compositional maturity upwards from the base of the Oronto Group through the Bayfield Group. From Adamson, 1977, after Ojakangas, 1986. Chequamegon Sandstone This feldspatho-lithic unit resembles the Orienta Sandstone, but overlies the Devils Island. The similarity and stratigraphic relationship has long been questioned, largely because the formations are subhorizontal and exposures are scattered. However, Adamson (1997), on the basis of both field and well data, concluded that the Chequamegon does indeed overlie the Devils Island. Another question involving the Bayfield Group is whether the unfossiliferous units could be nonmarine Cambrian deposits (e.g., Hamblin, 1958; Ostrom, 1967; personal communication, A.C. Runkel).Paleocurrent data from several sources show that the dominant currents moved toward the north or northeast, toward the basin axis ILSG 2011 56 Field Trip 3 �in both sedimentary groups (Fig. 7).Whereas intrabasinal volcanogenic detritus is dominant in the Oronto Group, extrabasinal granitic detritus dominates in the younger Bayfield Group. Figure 7. Generalized paleocurrent trends in the Bayfield Group of Wisconsin, the Fond du Lac Formation and the Hinckley Sandstone of Minnesota, and the Jacobsville Sandstone of Michigan and Ontario. The stippled areas in southeastern Minnesota and southwestern Wisconsin are subsurface occurrences. The dashed line represents the axis of the Lake Superior Syncline. From Ojakangas and Morey, 1982. Beginning in 1869, the Chequamegon was extensively quarried for ―brownstone‖ on the Bayfield Peninsula as well as on several of the Apostle Islands. There were also quarries in the Orienta and in the equivalent Fond du Lac Formation near Duluth; in total, there were 12 quarries. The stone was shipped to Chicago, Milwaukee, Detroit, Toledo, Cincinnati, St. Paul and Kansas City, as well as used locally in lighthouse construction and in buildings such Washburn‘s main street bank building (now a museum) and Northland College‘s Wheeler Hall. ILSG 2011 57 Field Trip 3 �Apostle Islands Because of both time and uncertain weather, it was decided that a boat trip to the Apostle Islands was not possible as an ILSG field trip. However, we have included a short summary of the islands. (Take an excursion boat trip on your own some day from Bayfield.) Why the name? Presumably, the early French explorers concluded there were 12 islands, and named them after the 12 apostles of the Bible. Actually, there are 22 islands, 21 of which are in the Apostle Islands National Lakeshore, established in 1970, that also includes part of the Bayfield Peninsula north of the Red Cliff Indian Reservation (Fig. 8). The bedrock on 19 of the islands is the Chequamegon Sandstone;not so surprisingly, the bedrock on Devils Island, the northernmost island, is the Devils Island Sandstone, and Sand Island, the north-westernmost island, consists of the Orienta Sandstone. Well-exposed sedimentary structures are common. Glacial deposits of the Miller Creek Formation blanket most of the islands, but red to buff bedrock is visible in cliffs, sea caves, sea stacks, arches and elsewhere along the shorelines where erosion has exposed the bedrock. Several excellent beaches and a few tombolos are present. Locally, there are sand dunes. The islands look quite pristine, but they were extensively logged for white pine, hemlock, yellow birch and maple between 1850 and 1970. Brownstone quarries were established on Basswood Island in 1869, and later on Hermit and Stockton Islands. There are six lighthouses on the islands, built between 1857 and 1891. Figure 8. The Apostle Islands. From Nuhfer, 2004. ILSG 2011 58 Field Trip 3 �PLEISTOCENE GEOLOGY Introduction The Bayfield Peninsula has been repeatedly glaciated over the last 2.4 million years, but the most recent glaciations have wiped out evidence of previous glaciations, leaving a record of only the last 30,000 years. Glaciers scoured down to bedrock, then deposited many tens of meters of till and outwash as they melted away. Glacial lakes intermittently occupied the margins of the ice sheets where deposits of lacustrine sediment accumulated, much of which was reworked by minor glacial advances. These sediments have been divided into two units: the sandy Copper Falls Formation deposited as till and outwash before 11,500 BP, and the younger, finer-grained Miller Creek Formation consisting of lacustrine and glacially reworked lacustrine sediments deposited between 11,500 and 9,500 years BP (Fig. 9) (Clayton, 1984). F igure 9. Glaciations in Wisconsin. From Syverson and Colgan, 2004. ILSG 2011 59 Field Trip 3 �Glaciation The Bayfield Peninsula was most recently glaciated by the Superior Lobe and the Chippewa Sub-lobe of the Laurentide Ice Sheet, during the Wisconsin Glaciation from 25,000 BP to 9,500 BP. Deposits from several glacial advances have been identified in northern Wisconsin, the youngest of which is referred to as the Lake View Advance (Fig. 10). The ice-margin positions have been determined by a combination of lithostratigraphic evidence, ice-margin landforms, and inferred connections in areas where no evidence remains (Clayton, 1984). Figure 10. Furthest extent and flow direction of the Lake View Advance, between 10,000 BP and 9500 BP. From Clayton, 1984. Surficial Units Copper Falls Formation The Copper Falls Formation lies above pre-Pleistocene rock types and is generally composed of sandy till derived from Keweenawan sandstones in the Superior basin (Fig. 11). The Copper Falls Formation is at the surface in large areas of northern Wisconsin, and is prevalent at elevations higher than 1000 feet where it has not been covered up by younger deposits of lacustrine and reworked lacustrine sediments. The type section for the Copper Falls Formation is a river cutback near Copper Falls. Although the Copper Falls Formation is primarily composed of sandy till, it contains other sediment types including common proglacial stream deposits. These proglacial stream deposits are often interbedded with lithologically similar till. The formation on average is 40-80% sand, 15-50% silt, and 2-20% clay. The Copper Falls is slightly calcareous and the clay component is mostly composed of illite and smectite (Clayton, 1984). ILSG 2011 60 Field Trip 3 �Figure 11. The stratigraphic relationship between the Keweenawan rocks, the Copper Falls Formation and the Miller Creek formation. From Clayton, 1984. Miller Creek Formation The Miller Creek Formation, overlying the Copper Falls Formation, was deposited between 11,500 and 9,500 BP, and is composed of offshore clayey sediment and clayey till derived from fine-grained offshore sediment. The Miller Creek Formation has two subunits on the western half of the Bayfield Peninsula: the Hanson Creek Member and the overlying the Douglas Member. It is the majority of the subsurface material between 1000 feet and lake level (602 feet). The Hanson Creek Member is generally unlaminated and is 45-70% clay, 20-45% silt, 3-20% sand, trace pebbles, cobbles, and boulders. The silt and clay particles are typically 10% carbonates. The Douglas Member generally is 45-85% clay, 1040% silt, 3-20% sand, trace pebbles, cobbles, and boulders. Where it overlies sand it may contain up to 60% sand. It is usually dull reddish brown and calcareous. East of the Bayfield Peninsula the Miller Creek Formation is not divided into named members. It is 30-65% silt, 10-60% clay, 5-30% sand, trace pebbles, cobbles, and boulders. It is usually reddish brown and where it overlies sand may be more sandy (Clayton, 1984). The presence of wellpreserved laminations in some parts of the Miller Creek Formation, and their absence in other places, is indicative of the variable depositional environments and post-depositional histories of the sediments. Some parts are undisturbed lake sediments, while other parts were reworked, mixed with coarser sediment, and deformed by glacial action by later ice advances (Clayton, 1985; Shumway and Iverson, 2008). FIELD TRIP STOPS Head west out of Ashland on U.S. Highway 2. Just outside of town, we are driving to a young beach ridge built up at the mouth of Fish Creek (Fig. 12). This formed as the result of very rapid sedimentation from Fish Creek at the shallow southwest end of Chequamegon Bay. Three miles west of Ashland, past the junction of U.S. Highway 2 and Wisconsin Highway 13, is the Northern Great Lakes Visitor Center, which has excellent displays of natural and cultural history, including some that are geologic. Between the junction with Wisconsin Highway 13 and County Road E in Ino, a distance of 12 miles, we are driving on ILSG 2011 61 Field Trip 3 �the Pleistocene Miller Creek Formation. The glacial topography has been greatly modified by Glacial Lake Duluth. From just west of Ino, traveling 20 miles to Brule, , we are driving on the older Pleistocene Copper Falls Formation. Figure 12. Map of field trip stops. Stop 1: Brule River Outlet of Glacial Lake Duluth In Brule, turn south on Wisconsin Highway 27 and proceed for 3 miles to of Winneboujou. Winneboujou is located at the bottom of a steep-walled outlet channel, presently 100 feet deep, about one-mile-wide, that was cut when Glacial Lake Duluth drained via the Bois-Brule River outlet into the St. Croix River, which enters the Mississippi River (Figs. 13, 14). Figure 13. Generalized map showing the BoisBrule outlet that carried waters southward from Glacial lake Duluth to the Sta. Croix River and hence to the Mississippi River. The heavy line near the bottom represents the southernmost advance of the Late Wisconsin glaciers. From Dott and Attig, 2004. ILSG 2011 62 Field Trip 3 �When Glacial Lake Duluthbreached this outlet, at an elevation of about 1100 feet, the Bois-Brule flowed southward, cutting this deep and wide channel. The present underfit Brule River now flows northward into Lake Superior, with its headwater 30 miles to the south. Native Americans, early explorers, and fur traders came up the Mississippi and St. Croix Rivers and then traversed a two-mile portage from the St. Croix River into the present Brule River. This divide is located a few miles north of Solon Springs. Figure 14. Landscape image of the Bois-Brule outlet of Glacial Lake Duluth just south of the village of Brule. Note the Glacial Lake Duluth plain. From Dott and Attig, 2004. ILSG 2011 63 Field Trip 3 �Between Brule and Maple, a distance of 8 miles, we are again driving on the younger Miller Creek Formation. At Maple, we descend over several low, sandy beach ridges (elevations about 1080 feet) onto lake-modified glacial topography, the bed of Glacial Lake Duluth. Three miles west of Maple is the village of Poplar, located on this lake bed. Proceed about 5 miles and turn right on Country Road U. Follow County Road U a short distance to the Amnicon State Falls Park entrance on the left. Stop 2A: Amnicon Falls State Park: Orienta Sandstone, Keweenawan Volcanics, and the Douglas Fault. The Douglas Fault has thrust Middle Keweenawan basalt (Chengwatana Volcanics) northward over the Late Keweenawan Orienta Sandstone (Fig. 15). A waterfall exists as the Amnicon River flows over the basalt into the gorge cut into the much softer, subhorizontal red Orienta Sandstone. The basalt flows dip at 30-40 to the southeast whereas the Orienta dips at about 5 . These relationships are easily seen after crossing the bridge and observing the east bank from the west side of the river. Note that the Orienta has been dragged into a near-vertical attitude adjacent to the few-meter-thick gouge and breccia zone between the fault and the Orienta. Prior to a collapse of the side of the gorge several years ago, at the base of the short stairway on the east bank, steep slickensides were visible on the sandstone beneath the breccia. Figure 15. Sketch of the Douglas Fault as exposed at the base of the main falls in Amnicon Falls State Park. The arrows show the relative motion along this fault. From Dickas, 1989, adapted after Thwaites, 1912. ILSG 2011 64 Field Trip 3 �You can walk on the Orienta Sandstone a short distance downstream from the bridge on the northeastern bank. Note trough cross-bedding and parting (current) lineation.The Douglas Fault defines the northwestern edge of a major feature of the Midcontinent Rift, the St. Croix Horst. (see Fig. 2). It has been traced by geophysical methods from the Twin Cities through northwestern Wisconsin and into Lake Superior, and may have 3000 m of displacement (Craddock, 1972). There is now some question about whether it connects with the Isle Royale Fault beneath Lake Superior. The southeastern side of the horst is marked by the Lake Owen-Keweenawan Fault system, also reverse faults. It is generally assumed that these faults were originally normal faults during the formation of the rift, but became reverse faults later due to compressional forces perhaps caused by the collision of the Grenville micro-continent with eastern North America. A paleomagnetic study by Watts (1981) estimated the age of the compressive event at 950-1040 Ma. Stop 2B: (Optional) Quarry in Keweenawan Volcanics Exit the park and return to U.S.Highway 2. Turn right (west) and drive a short distance to an entrance road into a quarry operated by Northwoods Paving of Ashland. Drive a short distance behind the crushed rock piles to basalt exposures on the quarry wall. Nearby is gabbro of the Amnicon Intrusion (Nicholson et al, 2006) that has altered and deformed these flows. Return to County Road U on which the entrance to Amnicon Falls State Park is located, and proceed north for 5 miles to Wisconsin Highway 13. Proceed eastward for about 35 miles to Port Wing. Note that we are driving this entire distance on the flat bed of Glacial Lake Duluth (the Lake Superior Lowland), and that this flat area (the Douglas Member of the Miller Creek Formation) is deeply dissected by numerous rivers and smaller streams, all flowing northward into Lake Superior (Fig. 16).Locally some of the rivers have cut down to the underlying Pleistocene Copper Falls Formation and to the Bayfield Group sandstones. Figure 16. Lake–modified glacial topography intersected by incised stream valleys. U.S. Geological Survey Poplar NE Quadrangle (7.5-minute, topographic, 10-ft contour interval. T. 48 N.. R. 12 W. From Clayton, 1984. ILSG 2011 65 Field Trip 3 �Stop 3: Orienta Sandstone at Twin Falls Park in Port Wing Upon entering Port Wing, pull off to the right into Twin Falls Park. Follow the path a few hundred feet to an observation deck above the falls. Note that the Flag River has cut a deep gorge through the glacial beds and into the Orienta Sandstone. Stop 4: Devils Island Sandstone at Siskiwit Falls, Cornucopia In Cornucopia, turn right (south) on Superior Avenue and continue for one block. Turn left (east) on County Road C and continue 0.4 miles to Siskiwit Falls Road. Turn left and continue for one block. Park on shoulder just before the bridge over the Siskiwit River. The falls, really a rapids, formed by water passing over a series of sandstone beds,is just upstream from the bridge. This, unfortunately, is the only easily accessible exposure of the quartz arenite of the Devils Island Sandstone. At low water, a total thickness of about 6 meters can be seen. Look for current ripple marks and cross-bedding. If the water is high, the only observation possible may be a close look at the composition of loose pieces of the formation under a handlens—99% fine-grained quartz. Good rounding of grains may be visible in coarser-grained pieces. Transect Across the Bayfield Peninsula Stops 5 through 10 constitute a transect across the peninsula, from the highest elevations in the middle of the peninsula down to the lakeshore on the east side (Fig. 17). All of the stops are on public land, with the exception of Stop 7, which is on private land. Locations are given in public land survey grid, and UTM coordinates (Zone 15T), datum NAD 1927. The transect crosses the transition zone between sandy Copper Falls sediments and clay-rich Miller Creek sediments on both sides of the Peninsula (Fig. 18). From Siskiwit Falls, go east on Siskiwit Falls Road, then turn right (south) on Stage Road, which becomes Star Route Road. Take Star Route road southeast for approximately three miles uphill to where the road levels off and turns east. After passing over the top of the hill, go left on Settlement Dump Road. Stop 5 is at the end of this road. The road has a steep grade and loose sand, so it may not be passible at all times of the year. Figure 17. Map showing locations of field trip stops on the Bayfield Peninsula. ILSG 2011 66 Field Trip 3 �Figure 18. Surficial geologic map showing field trip stops 4-10. Miller Creek Formation: gl—lake-modified glacial topography (teal on map), gw—wave-planed topography (green), gh—valley sides(darkest green), b—shoreline sediment (orange); Copper Falls Formation: suc—uncollapsed proglacial stream sediment (Valhalla surface) (reddish orange), sc— collapsed proglacial stream sediment (red), sg—hummocky stream sediment overlain by silty material (light green). Modified from Clayton, 1985. Stop 5: Glacial Outwash and the Glacial Lake Duluth Beach Location: Gravel pit at the north end of Settlement Dump Road; UTM coordinates: 0649710 mE, 5185261 mN; T50N, R5W, Sec 18, south-central. (Fig. 19) Stop 5A: Glacial Outwash Above the Lake Duluth Beach Travel up Settlement Dump Road watching for exposures of sediment; the best exposures are in a gravel pit at the top of the hill (UTM coordinates listed above). This is glacial outwash of the Copper Falls Formation deposited by meltwater rivers flowing out of the Superior Glacial Lobe to the west and the Chippewa Lobe to the east. Here in the central part of the Bayfield Peninsula, at elevations above 1100 feet, outwash of the Copper Falls Formation is gravelly sand with few cobbles and boulders. There is a wide variety of clasts types, which is typical of the Copper Falls Formation. The hilltop to the north of the pit is the highest point on the Bayfield Peninsula, with an elevation of 1426 feet – which is 824 feet above the present surface of Lake Superior and well above all the glacial lake beaches. The sandy soils and relatively high topographic relief in the area make for excessively well drained soils. These soils support a sparse forest consisting of only drought-tolerant trees such as jack pine, red pine, and scrubby red oak. Be careful of glass and debris in this gravel pit. ILSG 2011 67 Field Trip 3 �Stop 5A Stop 5B Figure 19. Topographic map showing location of Stop 5 area (U.S.G.S., 1964a). Go back down Settlement Dump Road and go right (west) on Star Route Road. In 0.25 miles stop on the flat stretch where the road turns from west to northwest. You are now on a beach of Glacial Lake Duluth – stop 5B. Stop 5B: Glacial Lake Duluth Beach Terrace Location: On Star Route Road; UTM coordinates: 0647828 mE, 5180050 mN; T50N, R5W, Sec. 19, north-central (Fig. 19). There is a distinct terrace at an elevation of 1100 feet throughout this part of the southern Lake Superior region, and is especially well defined here on the Bayfield Peninsula. This beach terrace was created 9500 BP when Lake Duluth was at its highest level (Fig. 20). The terrace at 1100 feet is a distinct step in the landscape and an important transition zone from very sandy sediments on and above the beach level to finer sediments on the slopes below. The sandy outwash seen in stop 5A underlies all areas above the beach – deep deposits of gravelly sand with no silt or clay. The beach terrace goes all the way around this hill, indicating that at one time it was an island within Glacial Lake Duluth. Continue northwest on Star Route Road back toward Siskiwit Falls. In 0.7 miles, turn left (west) on Mountain Road and travel two miles to County Road C. Turn south on County Road C and go 3.5 miles to North Boundary Road. Turn left (east) on North Boundary Road East, which is also labeled FR 439, and head toward the Bayfield County Sportsman‘s Club. Stop 7 is one mile east on this road. ILSG 2011 68 Field Trip 3 �Figure 20. Cross section of a beach terrace. Letter a represents the lake level, b is the outer edge of the wave-built terrace, and c is the base of the wave-built terrace From Clayton, 1984. Stop 6: Pitted Outwash Plain Location: North Boundary Road East; UTM coordinates: 0647828 mE, 5180050 mN; T50N, R6W, Sec 36, south-central (Fig. 21). Stop 6 67 Figure 21. Topographic map of Stop 6 area (U.S.G.S., 1964a). Areas above the 1100-foot beaches have a variety of landforms including outwash plains, eskers, outflow channels, and kettles of all sizes. These features were created during melting of the ice of the Airport Advance, the last glacial advance to cover the high interior of the Bayfield Peninsula, about 12,300 BP (Clayton, 1984). The large flat outwash plain, at an elevation of 1250-1280 feet, in the middle of the Peninsula is the Valhalla Surface. ILSG 2011 69 Field Trip 3 �Numerous large blocks of ice were left behind in some areas as the ice margin retreated, and subsequently the blocks were buried in thick deposits of sandy outwash. As the ice blocks melted, kettles formed in the otherwise flat outwash plain. These pits are especially abundant in Sections 30, 31, and 36 (T50N, R6W), some up to 180 feet deep and 3000 feet across with very steep slopes (Figure 22). Here at stop 6 is a relatively small pit, the southern-most kettle in a large kettle field. Just to the east is a road that goes north into the heart of the kettle field if time permits, we will walk the road north into the kettle field. The absence of surface water here is very revealing about the high permeability of the sands of the Copper Falls Formation. There are no rivers in this high interior part of the Bayfield Peninsula because all water infiltrates and is drained by groundwater. The presence of kettles nearly 200 feet deep that are dry on the bottom is evidence of the great depth of the water table. Return to County Road C and turn south. Travel 7.5 miles to Church Corner Road and turn right (south). In 0.5 miles stop at the crest of the hill. This is stop 8. Stop 7: Beach Terrace and Former Bay Location: Church Corner Road; UTM coordinates: 0653786 mE, 5173208 mN; T49N, R5W, Sec. 27, west-central (Fig. 22). This is private land so do not explore far off the road without seeking permission from land owners. Stop 7 Figure 22. Topographic map of Stop 7 area (U.S.G.S., 1964b). This stop is on the terrace of the highest Lake Duluth beach, the 1100-foot beach level visited on stop 5B. This terrace marks the highest and longest lived lake level of the numerous lake levels that existed in the Superior Basin during deglaciation, thus it is the topographically most distinct terrace and the only one which has significant sandy beach deposits (Farrand, 1969). Below this terrace the topography was planed by waves of at least ILSG 2011 70 Field Trip 3 �five short-lived lower lake levels, so lower beaches are less well defined. On the east side of the road, there is an exposure of pebbly sand typically present on this terrace. The sand of this beach is now used as a resource for road construction and other uses, and several sand pits exist along contours of this hill. The view to the north is of the Fourmile Creek valley and the Bayview ridge beyond. At the time that the Glacial Lake Duluth shoreline was here, about 9500 bp, this was a large bay closing to the west. The hydrology changes dramatically below 1100 feet, where the sand of the Copper Falls Formation is overlain by silt and clay of the Miller Creek Formation. There is typically a transition zone between these two areas, below which fine sediments and soils prevent much infiltration of precipitation. As a result, nearly all the rivers on the Bayfield Peninsula start at the edge of the fine sediments just below an elevation of 1000 feet. This stop provides an excellent view of the transition zone where the sediment changes from sand on the hill slopes to mud on the bottom of the valley. The transition is rapid on steep slopes such as this one, with clay-rich sediments of the Miller Creek Formation dominating below elevations of about 950 feet. At that elevation the hydrology and soils change, so there is a corresponding change in forest types and ecosystems, from the dry jack pine and red oak forests above to wet-tolerant species at lower elevations. Although the differences in the forest types are much less pronounced today than they were prior to the forests being cut over in the late 19th century, the differences are still apparent. At the time of first settlement, the ―Lake Superior clay plain‖ was occupied by spruce, fir, and cedar, whereas today it is primarily red maple, aspen, and birch. Travel 0.5 mile and turn left (east) on Maple Hill Road. Travel 1 mile to County Road C and turn right (east) toward Washburn. In 2 miles, turn left (north) on Big Rock Road. Travel 1.5 miles to Big Rock Park and stop at the parking lot just north of the bridge over Sioux River -- this is stop 9. Stop 8: Chequamegon Sandstone and Miller Creek Formation Location: The parking lot at Big Rock Park; UTM coordinates: 0658541 mE, 5174862 mN; T49N, R5W, Sec. 24, east-central (Fig. 23) Figure 23. Topographic map of Big Rock Park area at Stop 8 (U.S.G.S., 1964b). Stop 8 ILSG 2011 71 Field Trip 3 �Stop 8A: Chequamegon Sandstone Location: The Sioux River valley downstream of Big Rock Park; UTM coordinates: 0658541 mE, 5175068 mN (Fig. 23). From the parking lot hike north on the trail along the Sioux River for about ¼ mile to two long outcrops of Chequamegon Sandstone. The northern outcrop is located at 0658541 mE, 5175068 mN. Pleistocene deposits have been eroded away in the Sioux River valley to expose the Chequamegon Sandstone, the stratigraphically highest formation in the Bayfield Group. The Chequamegon Sandstone here is ared feldspathic sandstone that has enough clay on weathered surfaces to give it a chalky white appearance. It is poorly cemented and friable, so it is not very resistant to erosion. Although it is composed primarily of sand-sized clasts, there are sparse quartz pebbles and some mud chips; the chips are soft red mud, which easily erodes out of the sandstone, leaving deep holes in the surface of the outcrop. Some of the mud chips in this outcrop are up to 25cm in length. Bedding here is horizontal and has large cross beds 0.5 to 1m in height, with dip of cross strata indicating the flow was in a northerly direction, similar to paleocurrent indicators throughout the Bayfield Group. Some of the bedding is disturbed by various soft-sediment deformation features, including at least one excellent ball-and-pillow structure exposed in the southern outcrop. The sediment along the base of the cliff is sand derived from weathering of the friable sandstone immediately above. However, about 2m away from the cliff face the sediment is hard sandy mud. Most of the Sioux River valley is filled with this red mud – sediment typical of the Miller Creek Formation. The presence of Miller Creek mud deep within a bedrock gorge indicates that the gorge was carved into the sandstone prior to the deposition of the mud. Thus, it is likely that the Sioux River gorge was carved during a Pleistocene interglacial time, then filled with mud during deposition of the late Pleistocene Miller Creek Formation, then re-excavated to create the river valley present today. This situation is similar to that of Fish Creek west of Ashland, and Sand River on the northern side of the Bayfield Peninsula (Nuhfer, 2004). Hike back out the trail, past the parking lot, south along the road to the banks on the east side of Big Rock Road – this is stop 9B. Stop 8B: Fine mud of the Miller Creek Formation Location: East side of Big Rock Road, just south of the bridge over Sioux River. UTM coordinates: 0658570 mE, 5174755 mN (Fig. 23). The bank is composed of red mud of the Miller Creek Formation. The Bayfield County Highway Department has attempted to stabilize the bank, with moderate success; the clay of the Miller Creek Formation is notorious for its instability, especially where oversteepened by erosion or excavation. The mud here in the Sioux River valley is especially fine grained for the region, consisting of approximately 60% clay, 35% silt, and 5% sand, with few, if any, pebbles. Although no laminations have been seen in this exposure, there are excellent examples of horizontal, undeformed, thin laminations in the mud upstream along west side of the Fourmile Creek valley (just downstream of the Hwy C bridge). ILSG 2011 72 Field Trip 3 �Drive south on Big Rock Road, back to Hwy C. Go left (east) on C into Washburn. At the intersection of C and Hwy 13, go straight across the intersection (south) onto South 8th Avenue West. In about 0.5 mile enter Thompson‘s West End Park. The artesian well is on the right (west) as you first enter the park – stop 10. Stop 9: Flowing Artesian Well Location: Thompson‘s West End Park in Washburn; UTM coordinates: 0660194 mE, 5170054 mN; T48N, R4W, Sec. 6, SE1/4 (Fig. 24). Stop 10 Stop 9 Figure 24. Topographic map of Washburn and stops 9 and 10 (U.S.G.S., 1964b). The fine-grained sediments of the Miller Creek Formation create a confining layer over more-permeable sediments of the Copper Falls Formation. The very permeable sands of the Copper Falls Formation exposed at the surface in the middle of the Bayfield Peninsula – areas above the 1100-foot-elevation beaches – serve as the recharge area for this confined aquifer. This situation creates high enough artesian pressure that the piezometric surface is above the land surface and flowing wells can exist. This hydrogeologic condition exists all around Chequamegon Bay, and there are numerous flowing artesian wells in the area. There are also natural springs, although none near here that have high flow rates. The artesian wells are popular sources of drinking water, and often there is a line of people waiting to fill their bottles. This is the historic Sprague Well: drilled in 1903, it was the first artesian well in Bayfield County; it is 119 feet deep and currently produces 43 gallons per minute (gpm). ILSG 2011 73 Field Trip 3 �The water has a constant temperature of 45o F and total dissolved solids concentration of 104 ppm. The interpretive sign at the well says that it originally produced 224 gpm from a fourinch pipe and was ―so free of minerals that it was piped directly into the sawmill boilers.‖ Return to Hwy 13 and turn left (east). Go east on Hwy 13 to downtown Washburn, then go right (southeast) on Central Avenue toward Lake Superior. Just before the town dock there is a parking lot on the left (north) and a small beach – stop 10. Stop 10: View of Chequamegon Bay Location: Washburn beach, near the town dock; UTM coordinates: 0661718 mE, 5170422 mN; T49N, R5W, Sec. 24, NE ¼ (Figs.24, 25) Long Island Washburn Oak Point Figure 25. Satellite image of Chequamegon Bay (image from UW SSEC and Wisconsin View). From the beach you can look out over Chequamegon Bay, which has served as a safe harbor to vessels on Lake Superior for centuries because it is so well protected from the wind ILSG 2011 74 Field Trip 3 �and waves of the open lake. This protection is provided by the Apostle Islands and Bayfield Peninsula to the north, and Long Island to the northeast (Figure 25). Long Island is a large barrier spit that has been created by deposition of sediment carried by longshore transport from the southeast. The shore of Lake Superior southeast of the bay has high bluffs of Miller Creek Formation which supply large volumes of sediment to longshore currents. The spit has grown since the lake level stabilized at its current level about 2000 years ago. Smaller spits first grew in the area where Oak Point is located today, and when the White River captured the Bad River 800 bp, the sediment supply increased, eventually leading to the development of the spit in its current position (Meeker, unpublished data). The power of the waves continues to deliver sediment and reshape the spit today. In historic times, it had been a series of unconnected barrier islands, thus its name Long Island (Bona, 1990; Collie, 1901). But deposition and erosion has caused it to alternate between being a barrier spit connect to the mainland, to being a barrier island. The last time it changed dramatically was in November 1975 when a large storm, the same storm that sank the Edmond Fitzgerald, caused deposition of enough sand to connect the sand bars into the large barrier spit present today. REFERENCES Adamson. Kent F., 1997, Petrology, Stratigraphy, and sedimentation of the Middle Proterozoic Bayfield Group, Northwestern Wisconsin, Unpublished M.S. Thesis, University of Minnesota Duluth, 203 p. Behrendt, J.C., Green. A.G., Cannon, W.F., Hutchinson, D.R., Lee, M.W., Milkereit, B., Agena, W.F., and Spencer, C., 1988, Crustal structure of the Midcontinent rift system: Results from GLIMPCE deep seismic reflection profiles: Geology v. 16, p. 81-85. Bona, L.J 1990, Geomorphology and Holocene geologic history of Long Island, Apostle Islands National Lakeshore, Wisconsin [abs.]: Geological Society of America, Abstracts with Programs, v. 22, no. 5, p. 4. Cannon, W.F., et al, 1989, The North American Midcontinent Rift beneath Lake Superior from GLIMPCE seismic reflection profiling: Tectonics, v. 8, p. 305-332 Clayton, L., 1984, Pleistocene geology of the Superior Region, Wisconsin, Informational Circular 46, Wisconsin Geological and Natural History Survey, Madison, Wisconsin, p. 40. Clayton, L., 1985, Pleistocene geology of the Superior Region, Wisconsin: Wisconsin Geological and Natural History Survey, scale 1:250 000, 1 sheet. Clayton, L., Attig, J.W., Mickleson, D.M., Johnson, M.D., and Syverson, K.M., 2006, Glaciation of Wisconsin, Wisconsin Geological and Natural History Survey, Madison, Wisconsin, Educational Series 36, Third Edition. Collie, G.L., 1901, Wisconsin shore of Lake Superior: Geological Society of America Bulletin, v. 12, p. 197 – 216. Craddock, C., 1972, Regional geologic setting, in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota: A Centennial Volume, Minnesota Geological Survey, p. 281291. ILSG 2011 75 Field Trip 3 �Daniels, P.A., 1982, Upper Precambrian sedimentary rocks: Oronto Group, MichiganWisconsin,in Wold, R.J. and Hinze, W.J., Geology and Tectonics ofthe Lake Superior Basin, Geological Society of America Memoir 156, p. 107-133. Davis, D.P and Paces, J.B., 1990, Time resolution of geological events on the Keweenaw Peninsula and implications for development of the Midcontinent Rift System: Earth and Planetary Science letters, v. 97, p. 54-64. Dickas, A.B., 1986, Seismological analysis of arrested stage development of the Midcontinent Rift: Geoscience Wisconsin, v. 11, p. 45-562. Dickas, A.B, 1989, Lake Superior Basin segment of the Midcontinent Rift System, 28th International Congress Field Trip Guidebook T344, 62p. Dott, R.H. Jr., and Attig, J.H., 2004, Roadside Geology of Wisconsin, Mountain Press Publishing company, Missoula, MT, 345 p. Elmore, R.D., Milavec, G.J., Imbus, S.W., and Engel, M.H., 1989, The Precambrian Nonesuch Formation of the North American Mid-continent rift, sedimentology and organic geochemical aspects of lacustrine deposition: Precambrian Research, v. 43, p. 191-213. Farrand, W.R., 1969, The Quaternary history of Lake Superior: Proceedings of the 12th Annual Conference on Great Lakes Research, p. 181-197. Galston, Lynn, and Havholm, K.G., 2008, Re-evaluation of the Proterozoic Devils Island Sandstone, Keweenawan Rift, Northern Wisconsin (Abs.), North-Central Section 42nd Annual Meeting 24-25 April, 2008, Geological Society of America Abstracts with Programs, v. 40, No. 5, p. 76. Green, J.C., 1995, Volcanic rocks of the Midcontinent rift system: A review, in Ojakangas, R.W., Dickas, A.B., and Green, J.C., eds., Basement Tectonics 10, Kluwer Academic Publishers, Dordrecht/Boston/London, p. 65-67. Hamblin, W. K. 1958, Cambrian sandstones of northern Michigan, Michigan Geological Survey Publication 57, 149 p. Johnson. A.D., Beaster, K.F., Kohn,J.D., and Havholm, K.G., 2001, Braided-stream/eolian environment of Proterozoic Hinckley Sandstone, Keweenawan rift, East-Central Minnesota (Abs.), Geological society of America Annual Meeting, Nov. 5-8. 2001. Kalliokoski, J., 1982, Jacobsville Sandstone, in Wold, R.J. and Hinze, W.J., eds., Geology and Tectonics of the Lake Superior Basin. Geological Society of America Memoir 156, p. 147-155. Meeker, J., unpublished data: Northland College, Ashland, WI. Morey, G.B. and Ojakangas, R.W., 1982, Keweenawan sedimentary rocks of eastern Minnesota and northwestern Wisconsin, in Wold, R.J., and Hinze, W.J., eds., Geology and Tectonics of the Lake Superior Basin, Geological Society of America Memoir 156, p. 135-146. Morey, G.B. and Van Schmus, W.R., 1988, Correlation of Precambrian rocks of the Lake Superior Region, United States, in Harrrison, Jack and Peterman, Zell, eds., U. S. Geological Survey Professional Paper 1241-F, 31p. Nicholson, S. W., Cannon, W.F., Woodruff, L. G., and Dicken, C.L., 2006, Bedrock Geologic map of the Port Wing, Solon Springs, and part of the Duluth and Sandstone 30‘ X 60‘ quadrangles, Wisconsin and Minnesota.: U.S. Geological Survey, Scientific Investigations Map 2869. ILSG 2011 76 Field Trip 3 �Nuhfer, E.B., 2004, A Guidebook to the Geology of Lake Superior‘s Apostle Islands National Lakeshore and nearby areas of the Bayfield Peninsula of Wisconsin, with poetry by Mary P. Dalles: Eastern National, Fort Washington, PA, 141 p. Ojakangas, R.W., 1986, Reservoir characteristics of the Keweenawan Supergroup, Lake Superior Region: Geoscience Wisconsin, v. 11, p. 25-31. Ojakangas, R. W. and Matsch, C. L., 1982, Minnesota‘s Geology: University of Minnesota Press, Minneapolis, Minnesota, 255 p. Ojakangas, R.W., and Morey, G.B., 1982, Keweenawan sedimentary rocks of the Lake Superior Region: a summary, in Wold, R.J. and Hinze, W.J., eds., Geology and Tectonics of the Lake Superior Basin, Geological society of America Memoir 156, p. 157-164. Ostrom, M.E., 1967, Paleozoic stratigraphic nomenclature of Wisconsin: Wisconsin Geological and natural History Survey Information circular 8. Chart and txt. Palmer, H.C. and Davis, D.W., 1987, Paleomagnetism and U-Pb geochronology of volcanic rocks from Michipicoten Island, Lake Superior, Canada: Precise correlation of the Keweenawan polar wander track: Precambrian Research. v. 37, p. 157-171. Shumway, JR., and Iverson, NR., 2008, Magnetic fabrics of the Douglas Till of the Superior lobe: exploring bed-deformation kinematics, Quaternary Science Reviews, Elsevier, p. 107-119. Suszek, Thomas, 1995, Petrography and sedimentation of the Middle Proterozoic (Keweenawan) Nonesuch Formation, Western Lake Superior Region: Midcontinent Rift System, in Ojakangas, R.W., Dickas, A.B.,, and Green, J.C., eds., Basement Tectonics 10: Kluwer Academic Publishers, Dordrecht/Boston/London., p. 25-28. Syverson, K.M. and Colgan, P.M., 2004, The Quaternary of Wisconsin: a review of Stratigraphy and glaciation history, in: Ehlers, J. & Gibbard, P.L. (eds.), Quaternary Glaciations -- Extent and Chronology, Part II: North America, Amsterdam, Elsevier Publishing, 295-311. Thwaites, F.T., 1912, Sandstones of the Wisconsin coast of Lake Superior: Wisconsin Geological and Natural History Survey Bulletin 25, 117 p. Tryhorn, A.D., and Ojakangas, R.W., 1972, Sedimentation and petrology of the Upper Precambrian Hinckley Sandstone of north-central Minnesota: in Sims, P.K, and Morey, G.B., eds., Geology of Minnesota: A Centennial Volume, Minnesota Geological Survey, p. 431-435. U.S. Geological Survey, 1964a, Cornucopia quadrangle, 7.5-minute series topographic map: scale 1:24,000. U.S. Geological Survey, 1964b, Washburn quadrangle, 7.5-minute series topographic map: scale 1:24,000. Watts D.R., 1981. Paleomagnetism of the Fond du Lac Formation and the Eileen and Middle River sections with implications for Keweenawan tectonics and the and the Grenville problem: Canadian Journal of Earth Sciences v. 18, p. 829-841. ILSG 2011 77 Field Trip 3 �ILSG 2011 78 Field Trip 3 �57TH ANNUAL INSTITUTE ON LAKE SUPERIOR GEOLOGY FIELD TRIP 4 GEOLOGY AND REMEDIATION AT THE ASHLAND/NORTHERN STATES POWER SITE Ashland Lakefront circa 1900 ILSG 2011 79 Field Trip 4 �ILSG 2011 80 Field Trip 4 �Field Trip 4 Ashland/Northern States Power Company Lakefront Superfund Site James R. Dunn1 1 Hydrogeologist, Wisconsin Department of Natural Resources E-mail: james.dunn@wisconsin.gov As with many cities of the late 1800‘s, Ashland Wisconsin‘s lights and heat were supplied by manufactured gas. From 1887 into the 1940‘s a manufactured gas plant (MGP) operated under various owners, making gas and producing wastes. The Ashland MGP produced mainly carbureted water gas from feed stocks of coal and oil. MGP wastes included oil tars (coal tar) and wastewater as well as boiler ash, retort brick, and purifier box wastes. The City of Ashland sits on the shores of Lake Superior‘s Chequamegon Bay. The MGP was located less than a block from the historic shoreline transected by a ravine that discharged to the bay. Over time the ravine was filled with wastes from the MGP and the lakebed area was filled mainly with wood waste from a series of saw mills that operated up to 1930. Regional geology in the Ashland area consists of unconsolidated glacial deposits overlying Precambrian aged sedimentary bedrock. Unconsolidated deposits consist of the Miller Creek Formation, which overlies the Copper Falls Formation. Precambrian-aged sandstone of the Oronto Group, is the uppermost bedrock unit in the Ashland area. Thickness of the sandstone unit has not been determined. The Oronto sandstone is most likely underlain by Precambrian basalt. Surficial soils in the Ashland area are underlain by red clay and silt deposits of the Miller Creek Formation, the predominant surficial lithologic unit in the Ashland area. The Miller Creek Formation is a fine-grained clayey silt to silty clay. It consists of lacustrine deposits and glacial till deposited during the last major advance of glacial ice. The thickness of the Miller Creek Formation in the Ashland area ranges from approximately 15 to 50 feet based on local well logs. The Miller Creek Formation encountered at the Site consists of clays and silts and range in thickness from 5.5 to 40 feet. The thinnest portion of the Miller Creek Formation is found at the base of the bluff where the former lake shoreline was located. This thin zone likely resulted from wave erosion of the Miller Creek Formation, forming the ―bench‖ shaped bluff and former lake shoreline. Data from soil borings and downhole geophysics confirm that the Miller Creek Formation thickens to the north of the bluff. Sand and gravel layers interbedded with silty clay lenses have been encountered near the contact of the Miller Creek Formation and the underlying Copper Falls Formation. The Copper Falls Formation consists of interbedded glacial clays, sands, and gravels. In addition to unconsolidated glacial deposits, fill soil units are present beneath the upper bluff area and Kreher Park. Fill soil encountered in the upper bluff area was placed in a former ravine that dissected the Miller Creek Formation in the vicinity of the former MGP ILSG 2011 81 Field Trip 4 �facility. Historic documents indicate that the ravine was filled by 1909. Kreher Park consists of fill material that was used to fill the former lakebed. Historic documents indicate that the former lakebed was filled beginning in the late 1800‘s, and continued to be altered by filling activities until the 1950‘s. The ravine fill unit consists of silty clay fill material mixed with solid and liquid MGP wastes, ash, cinders, slag, and fragments of concrete and brick, glass bottles, scrap steel, and wire. Fill material at Kreher Park consists predominantly of silty clay soil mixed with rocks, bricks, concrete, wood, and miscellaneous debris. A buried clay berm is also located along the shoreline on the northeast side of the Site near the former wastewater treatment plant WWTP. This fill unit ranges in thickness from 3 to 7 feet. A wood waste layer consisting of wood planks was encountered beneath the fill soil. Native soil underlying the fill unit includes a beach sand layer (predating filling of the lakebed) that ranges in thickness from 0 to 5.5 feet at Kreher Park. The Miller Creek Formation was encountered below the fill and beach sand and ranges in thickness from 7 feet at the MW-7 well nest to 40 feet at the MW-26 well nest. Offshore geology of the Chequamegon Bay inlet area consists of a discontinuous layer of submerged wood chips and debris of varying size on the lake bottom underlain by variably fine to medium grained sediments. The sediments are underlain by silts and clays of the Miller Creek Formation. The Copper Falls Formation was not encountered during investigation of the offshore sediments. Consequently, the thickness of the Miller Creek Formation below the bay is unknown. Regional hydrogeologic units correspond to regional geologic units. The three aquifers that occur in the Lake Superior Basin in the vicinity of Ashland include the following: • The Pleistocene sand and gravel aquifer (Copper Falls Formation); • The Precambrian sandstone aquifer (Oronto Formation); and • The Precambrian basalt aquifer. The Copper Falls aquifer occurs between 25 to 55 feet below ground surface in the Ashland area. Based on deep piezeometer boring information, the Copper Falls aquifer is over 150 feet thick Sandy till units within the aquifer yield low volumes of water (5 to 10 gallons per minute gpm), while sand and gravel lenses can yield up to 100 gpm. The Copper Falls aquifer is confined by the overlying Miller Creek Formation near the Chequamegon Bay shoreline where the Miller Creek Formation has a higher plasticity and clay content. The low permeability Miller Creek Formation functions as an aquitard for the underlying Copper Falls aquifer in this area. Based on a series of investigations of the site, MGP wastes including free product oils and tars are present within the filled ravine, Miller Creek Formation, Copper Falls Formation, Kreher Park (filled lakebed) fills, and approximately 17 acres of Chequamegon Bay. The contaminants within the sediments manifest themselves as slicks during windy conditions. A pilot test, pump and treat system has operated in the Copper Falls Formation since 2001. In 10 years of operation over 11,000 gallons of oil/tar free product has been collected and over 2 million gallons of contaminated groundwater treated. ILSG 2011 82 Field Trip 4 �In consultation with the Wisconsin Department of Natural Resources, the USEPA has approved the Record of Decision for the remediation of contaminated soil, sediment and groundwater at the Ashland/NSP Superfund site. EPA‘s final cleanup plan includes: Removing soil from the most contaminated areas of Kreher Park and the Upper Bluff/Filled Ravine that overlooks the park area, treating the soil on-site and reusing it after treatment. Using barriers to contain and stop the movement of contaminants in ground water, treating the ground water in-place, and adding wells to extract and treat ground water. Digging up wood waste and contaminated sediment near and along the shore of Chequamegon Bay and dredging contaminated sediment in the bay, covering excavated and dredged sections nearshore and offshore with six inches of clean material, and treating contaminated sediment after removal. The estimated cost for this cleanup plan is between $84 million and $98 million. Photo 1: Product slick on Chequamegon Bay, Ashland, Wisconsin.Photo by Ed Monroe ILSG 2011 83 Field Trip 4 �Photo 2: Product slick on Chequamegon Bay, Ashland, Wisconsin.Photo by Ed Monroe REFERENCES URS, 2007, Remedial Investigation Report – Ashland/Northern States Power Lakefront Superfund Site USEPA, 2010, Record of Decision, Ashland/Northern States Power Lakefront Superfund Site ILSG 2011 84 Field Trip 4 �57TH ANNUAL INSTITUTE ON LAKE SUPERIOR GEOLOGY FIELD TRIP 5 BAD RIVER WATERSHED CULVERT RESTORATION PROGRAM Hagger Road west culvert – before replacement. -T. Fitz ILSG 2011 85 Field Trip 5 �ILSG 2011 86 Field Trip 5 �Field Trip 5 Bad River Watershed Culvert Restoration Program Michele Wheeler1 and Cassandra Bodette2 1 Bad River Watershed Association P.O. Box 875 Ashland WI 54806 2 Northland College 1411 Ellis Ave, Ashland WI 54806 INTRODUCTION The Bad River watershed (BRW), a sparsely populated, largely forested area of northern Wisconsin, covers 700,000 acres in the Lake Superior basin. It is one of the largest watersheds in the Great Lakes basin. The BRW encompasses parts of Ashland, Bayfield and Iron Counties, draining approximately 1,094 square miles. With 1,140 miles of streams, the BRW supports one of the most diverse fish assemblages in the Lake Superior basin. Watershed Geology The BRW is divided into three primary drainage and topographic zones: the upper basin, transition zone and clay plain (Figure 1). Figure 1. General watershed zones in the Bad River system The upper basin is characterized by sandy glacial till with frequent rock outcroppings and a poorly developed drainage network with no valleys (Fitzpatrick, 2005). The lower portion of the watershed is known as the Lake Superior clay plain. This area is dominated by ILSG 2011 87 Field Trip 5 �clay till soils deposited across the landscape by glaciers and glacial Lake Duluth. Streams are often found in entrenched, alluvial valleys in this zone. The middle portion of the watershed that separates these two zones is known as the transition zone. This area is characterized by shallow sandy soils, as well as mixed sand and clay soils. The topography is characterized by steep unstable stream banks, entrenched valleys with steep ravines, with many springs and seeps (Fitzpatrick, 2005). Culvert Program Description Background When culverts are employed to cross streams with roads, fish habitat may be significantly compromised in at least two ways. First, culverts can indirectly reduce habitat quality by accelerating sedimentation. Sediment can come from roadside ditches that empty directly into stream channels, in stream erosion associated with high water velocities, and bank erosion caused by misaligned crossings. In addition to these regular, smaller scale inputs, major crossing failures deliver large volumes of sediment into waterways. As a result of sediment loading from ―blow outs,‖ stream channels can gradually fill in, further affecting fish habitat. As the stream substrate becomes covered with sand and fine sediment, both food and water quality can be diminished, and spawning gravels can be covered. The result can affect the growth and survival of fish. In addition, the Bad River watershed is the largest producer of sediment into Lake Superior from Wisconsin waters (Figure 2), largely as the result of non-point sources like failed crossings. Figure 2. Mouth of the Bad River, and a destroyed culvert after a blowout event. Second, culverts may block the ability of fish to move up and downstream. In this case, juvenile fish may not be able to reach food or find cover from predators in critical rearing habitats. Adult fish may not be able to reach spawning habitats. Fish may not be able ILSG 2011 88 Field Trip 5 �to retreat to tributary streams during floods or find relief in cold water during the heat of summer. Consistent with the mission of the United States Fish and Wildlife Service (USFWS) and the Bad River Watershed Association (BRWA) to maintain the integrity of the Bad River Watershed, the culvert project was initiated to build community support and capacity to address problems at road stream crossings. The Culvert Program includes inventory, prioritization, coordination, restoration and monitoring elements in the overall goals to restore naturally functioning stream channels in the Bad River watershed. Inventory Inventory protocols were developed to identify sites that are likely fish barriers and/or significant contributors of sediment passage problems. Protocols included recording dimensions of the culvert, measurements from the stream channel, and conditions at the crossing that reflect how the stream and culvert interact. Partners throughout the region combined efforts to conduct surveys with local citizen volunteers, Ashland and Iron counties, US Forest Service, Northland College and the Bad River tribe contributing to survey efforts. To date, 996 of the 1,154 crossings in the watershed have been inventoried. Prioritization Site specific criteria have been developed for both fish passage and sedimentation concerns. Existing fisheries criteria include a drop greater than 6 inches and in-pipe velocity greater than 1 foot/second. Existing sedimentation criteria include sites with steep, unvegetated and eroding embankments. Erosion of stream banks above and below crossings is use to estimate alignment issues. Town road crew knowledge of eroding or regularly washing out sites was also considered as a sedimentation criteria. Results are summarized into this Needs Assessment that outlines problem sites by sub basin. The Needs Assessment shows that blockages to fish migration and sedimentation problems at road stream crossing are widespread and identifies sites with the most significant problems. There are 164 crossings on the priority list that were identified using these criteria, 69 of which are on trout streams. Strategic Plan The primary purpose of this document is to provide local road and fish managers with information that can be used as a planning tool for crossing replacements. Restoration In the past 5 years, about 10 culvert replacement projects in the watershed have been designed and funded by fishery interests to ensure proper fish passage and to reduce excess sediment inputs into the system. Monitoring Assumptions have been made about the benefits of these projects, based on past experience, design standards and scientific literature: More fish pass through the culverts after replacement than before and this is a positive thing for fishery populations and fish communities. Erosion from sites has decreased immediately and in the long-term. ILSG 2011 89 Field Trip 5 �Sediment from upstream head-cuts (created when new culverts are placed into channel beds rather than perched above them) move effectively through the culvert and are carried downstream, improving upstream and within-culvert habitat. Mobilized sediment from culvert replacement is not negatively impacting downstream habitat in gentle-sloped channels or coastal wetlands, such as the Bad River Slough, even though the influence of excess sediment load to downstream habitat quality is not known. The BRWA and USFWS led an effort to test these assumptions by initiating a monitoring program of culvert replacements in the Bad River watershed. Companion fish, habitat, and channel stability protocols were being developed as a result of a coordinated monitoring workshop held in December 2008. The purpose of the workshop was to identify monitoring protocols that can be used to evaluate how well restoration work in the BRW is achieving program goals. At the workshop, participants (listed in Appendix A) developed specific recommendations for Culvert Program monitoring in the Bad River watershed. These recommendations included objectives, critical questions to be answered, parameters, and metrics for both physical and biological aspects of a monitoring program for the Culvert Program. Specific protocols followed are provided in Appendix B. Monitoring Objectives and Questions Objective 1: Reconnect artificially fragmented stream channels. Question: Have we restored fish passage? Objective 2: Determine changes in species assemblages associated with culvert replacements. Question: Are there differences in species richness between upstream and downstream reaches of culverts prior to restoration? Question: Are there differences between treatment and references reaches prior to restoration? Question: Are treatment reaches more similar to reference reaches post restoration? Objective 3: Restore or improve instream habitat & prevent accelerated erosion and sedimentation Questions: Has the channel morphology, slope, and sediment characteristics improved or restored (relative to reference reach), upstream, downstream and within the culvert and does this result in quality habitat? This question relates to the following sub-questions: Is there deposition of fine sediment and bar formation? Is the composition of streambed sediment appropriate for biotic health? Is there habitat complexity in the channel geomorphic features? (riffles/pools) Can bedload move appropriately through the culvert? Is the stream aggrading/degrading or widening/narrowing? Is the channel morphology similar to a preferred reference reach? Are the banks eroding? ILSG 2011 90 Field Trip 5 �Education Although many culverts that pose fisheries and sedimentation problems have been replaced in the Bad River watershed by natural resource professionals, road managers from towns, counties and state agencies are involved in a far greater number of replacements each year. Investment by road managers in fish friendly culvert installation techniques is essential to adequately restore connectivity and reduce sedimentation in the watershed. Therefore, the BRWA has invested in education and outreach activities to transportation managers to encourage consideration of stream channel integrity as much as road maintenance at road stream crossings. Thanks to BRWA and partner sponsored events, elected officials and/or road crew members from all 15 townships in the watershed have attended fish friendly culvert installation trainings. BRWA staff have prepared township specific summaries of culvert inventory data, and have worked with townships to strategically plan culvert replacements. Outreach sheets summarizing problems at existing culverts, restoration solutions and monitoring results have been prepared and distributed to all streamside landowners and town officials. Regular press releases and articles in local newsletters inform the general public of restoration efforts to conserve fish and their habitats, building public support for future restoration projects. FIELD TRIP STOPS We will be visiting three to four sites within the transition zone of the Bad River Watershed. At each stop, we will describe conditions at the crossing prior to restoration and compare the 3 different treatment techniques. Stop 1: Baffled culverton a Marengo Tributary Location: Hager Road West - Latitude 46.398877 N Long. 90.931833 W (Fig. 3) Description: The shallow, fast-moving water and drop at the culvert exit prevented fish from moving upstream. If a new pipe was set sufficiently low enough to eliminate the drop, the stream would have headcut substantially. Therefore the new culvert was installed at a 3.75% slope (based on natural channel conditions) with baffles throughout the pipe to prevent a velocity barrier (Figs. 4 and 5). !( 2011 Troutmere Cr. Restoration Site Transition Zone Control Site 2010 Hager Rd Restoration sites Transistion Zone streams Perennial Streams Intermittent Streams !( Troutmere Creek Control site Hager Road West ILSG 2011 Hager Road East 91 Figure 3. Map showing locations of Hagar Road and Troutmere Creek sites. Field Trip 5 �Figure 4. Hagar Road west culvert; the photo on the left shows the old culvert before replacement, the photo on the right shows the new culvert. Before restoration Figure 5. Longitudinal profile of the unnamed tributary of the Marengo River showing the stream profile before and after replacement of the Hagar Road west culvert. ILSG 2011 92 Field Trip 5 �Stop 2: Culvert Replacement on a Marengo Tributary Location: Hager Road East -Latitude 46.399013 N Longitude 90.925979 W Description: The culvert was old and needed to be replaced (Figure 6). There was no sand, gravel, or rock within the culvert to mimic a natural stream bottom. The shallow, fast moving water and plunge at the culvert exit prevented smaller fish from moving upstream. The old culvert was replaced with a new, larger culvert set into the stream bottom to simulate a natural channel. Larger culverts prevent erosion by allowing water to pass through during high flow events. Larger culverts also slow the water down, making it easier for fish to swim up through the pipe. Figure 6. The Hagar Road east culvert before replacement (top photo), and after replacement (bottom photo). ILSG 2011 93 Field Trip 5 �Stop 3: Raising the grade at Troutmere Creek Location: County C - Latitude 46.406291 N Longitude 90.911559 W Description: The pipe was in pretty good shape at this crossing (Figure 7). The 10inch drop at the end of the culvert prevented many fish from accessing the 2 miles of stream above. Instead of replacing the culvert, the stream channel will be built up by placing large boulders in the stream below the culvert. This will cause sediment to fill in around the boulders, raising the stream bed and eliminating the 10-inch drop at the outlet of the culvert. It is anticipated that the boulders and sediment will form a terrace-like stream bottom similar to a staircase. Figure 7. Culvert at Troutmere Creek. Stop 4 (Time allowing): Control for Hager Road sites Location: Midway (County Line) Road; Lat. 46.406302 N Long. 90.916723 W Description: This site is a control site for transition zone restoration sites. Discussion Evaluation of the monitoring results calls for clarification and refinement of what we intend to achieve with our Culvert Program. Overall, monitoring set out to characterize site scale and broad scale population response to culvert replacement restoration. Movement protocols served to establish if passage was restored and if habitat was affected (site scale), and fish community metrics served to provide insights to population response (catchment scale). For the later to be effective, a better understanding of current fisheries habitat, distribution, status and limiting factors is required. In addition, a desired outcome, or fish population target, that is agreed upon among agencies is paramount to success. At the outset of the program, natural resource managers recognized that these building blocks of effective management are currently lacking for the watershed. Monitoring protocols intended to improve understanding of population response to restoration given these constraints. Recommendations for the monitoring program are provided for watershed ILSG 2011 94 Field Trip 5 �scale context in evaluating restoration projects (catchment scale) and specifically for future culvert restoration projects (site scale). Recommendation for monitoring at catchment level Increase examination of habitat-fisheries linkages Improve catchment level assessments as context for restoration benefits Continue fisheries monitoring to examine efficacy of various treatment sites Monitoring protocols were developed with the intention of considering subwatershed scale effects of culvert replacements. Evaluation of monitoring program data has shown a good assessment of reach scale effects, however only a small proportion of the watershed has been sampled. At present, the status of fisheries habitat, fish distribution and status is not well known. In order to consider culvert restoration projects in a landscape scale context, additional data is needed. Existing modeling efforts may help provide this context. Some work is underway through U.S. Fish and Wildlife Service (USFWS) initiatives to provide landscape level analysis on these topics. The USFWS Great Lakes Basin Fish Habitat Partnership is in the process of conducting habitat condition assessments for several species. Fish habitat condition can be estimated and changes in habitat conditions from restoration and protection actions could be tracked. The USFWS is also currently involved in classifying catchments in the Lake Superior basin that maintain self-sustaining brook trout populations. Researchers have identified subwatershed metrics found useful in identifying changes in brook trout status.By compiling data from state and federal partners, this effort will describe the distribution, status, and stressors of brook trout throughout the region, including in the Bad River watershed. This landscape scale modeling work will include data inputs to ground truth predicted brook trout status in areas where status is unknown. Upon review of monitoring program results, local managers recommend focusing local monitoring on efficacy of different structure types, in different zones of the watershed. In particular, the steep erosive channels in the transition zone area of the watershed raise question about longer term channel adjustments following culvert restoration. In particular, continued monitoring of sites with different culvert techniques (in pipe baffles, grade control below, or a larger pipe embedded into the stream bottom) will provide insights into the longer term efficacy of these techniques. This is a research oriented approach to evaluate culvert replacements in the different zones and different types of culvert designs. This program has demonstrated that collaboration among groups is essential to implementing responsible watershed restoration programs over the long term. This collaboration assists all partners in achieving their respective objectives, in a way that maximizes watershed integrity. ACKNOWLEDGEMENTS The BRWA‘s Culvert Project has been part of an integrated effort in concept and structure from its inception. We sincerely thank all partners who have contributed to this project, including: ILSG 2011 95 Field Trip 5 �Bad River Band of LakeSuperior Chippewa Indians, Natural Resource division, the Nature Conservancy, Sigurd Olson Environmental Institute, BRWA volunteer culvert inspectors, Ashland County Land and Water Conservation, Iron County Land and Water Conservation, Bayfield County Land and Water Conservation, U.S. Forest Service, Wisconsin Department of Natural Resources, U.S. Fish and Wildlife Service, U.S. Geologic Survey, Trout Unlimited, local townships, Natural Resources Conservation Service, and landowners adjacent to culvert sites. Funding and resources for the BRWA Culvert Project have been provided by: Wisconsin Environmental Education Board, US Fish and Wildlife Service, National Fish and Wildlife Foundation, Pri-Ru-Ta, ABDI Land Conservation Department, Wisconsin Department of Natural Resources, the Great Lakes Aquatic Habitat Network and Fund, Wisconsin Coastal Management Program, Great Lakes Basin Fish Habitat Partnership, Army Corps of Engineers Section 154 funds, and U.S. Forest Service Resource Advisory Committee REFERENCES Bad River Watershed Association (BRWA). 2007. Culvert Program Needs Assessment. Ashland, WI. 174 p. Bad River Watershed Association (BRWA). 2008. Culvert Program Strategic Plan. Ashland, WI. 24 p. Fitzpatrick, F. 2005. Project Update - Investigation of Erosion, Sedimentation, Channel Migration, and Streamflow Trends for the Bad River, Wisconsin. U.S.D.I. Geological Survey and Bad River Tribe of Lake Superior Chippewa Natural Resources Department, project update/no publication number. ILSG 2011 96 Field Trip 5 �57TH ANNUAL INSTITUTE ON LAKE SUPERIOR GEOLOGY FIELD TRIP 6 GEOLOGY HIKE AT COPPER FALLS STATE PARK Devils Gate at Copper Falls State Park -T. Fitz ILSG 2011 97 Field Trip 6 �ILSG 2011 98 Field Trip 6 �Field Trip 6 Geology hike at Copper Falls State Park Allison Mills, Drew Cramer, and Tom Fitz Northland College, 1411 Ellis Avenue, Ashland, WI 54806 ___________________________________________________________________________ COPPER FALLS GEOLOGIC SETTING Introduction Copper Falls State Park, near Mellen, Wisconsin, is 2,676-acre park with hiking trails, scenic waterfalls, excellent bedrock exposures, and beautiful surficial geologic features (Figure 1). The Bad River flows from south to north though the park, and has cut through glacial sediments, erodingdeeply into bedrockto form the Copper Falls gorge. After a 12foot drop at Copper Falls,the Bad River travels about twomilesthrough the gorge. Downstream, theTyler Forks River plunges 30 feet over volcanic ledges to join the Bad River before flowing throughthe narrow passage of Devil‘s Gate. The gorge has excellent exposures of Keweenawan volcanic and sedimentary rocks overlain by Pleistocene glacial deposits. This guidebook explains the geology along the1.7- mile Doughboys‘ Nature Trail that takes hikers around the top of the gorge (WDNR, 2009). Copper Falls State Park was established in 1929, aided by the Civilian Conservations Corps and the Works Progress Administration. Several buildings at the park are from this era and contain some nice gabbro and slate building stones. While Copper Falls State Park was preserved for its natural beauty, people once sought native copper at the site. Though no significant deposits were discovered, mine shafts can still be found within the park. In the early 1900s, the Bad River was diverted to prevent flooding in the mine shafts; the original channel is visible along the trail at stop 8 (WDNR, 2009). Bedrock Geology The bedrock exposed in the park is part of the 1100 Ma Keweenawan Midcontinent Rift (MCR) sequence. The MCR was created when the Laurentian craton began to pull apart, resulting in huge outpourings of flood basalts withinmajor rift valleys. Many thousands of feet of volcanic rock accumulated in the rift‘s central graben, but the tectonic activity stopped before an ocean basin could form. Volcanic activity was most voluminous between 1109-1094 Ma, and, with waning extension and volcanism, the rift filled with sediments. The MCR then underwent tectonic compression, and the central graben was partially inverted. As a result rocks along the southern arm of the rift now dip steeply to the north.One volcanic and three sedimentary bedrock units related to the MCR can be seen in Copper Falls State Park: the upper part of the Kallander Creek Volcanics, and the Copper Harbor Conglomerate, the Nonesuch Formation, and the conglomeritic facies of the Freda Sandstone,all part of the Oronto Group (Figure 2). ILSG 2011 99 Field Trip 6 �Figure 1. Location of Copper Falls State Park. These rock units are situated on the southern flankof the Montreal monocline –a large structure created when the rocks of the central graben of the MCR were tipped north during compression of the rift about 1060 Ma ago(Figure 3). Erosion of the monocline has exposed more than 35 km of crustal rocks. Layeringin the monocline strikes northeast-southwest and dips steeply north (Cannon et al., 1996; Cannon et al., 1993b; and Chapter 7 of this guidebook volume). The Kallander Creek Volcanics consist of mostly basalt, with some andesite and rhyolite—the lowermost flows are flood basalts, while the upper sections are dominated by andesites and rhyolites—erupted from a large central volcano that was probably located in the region of Copper Falls Park (Cannon et al., 1993a). This central volcano acted as a topographic high during the early phase of volcanism of the MCR at the time, and somewhat after, the Kallander Creek eruptions. Overlying bedrock units pinch-out in the Mellen area and thicken to the east away from this area. The intrusive Mellen Complex is interpreted to have been emplaced within the volcano late in its magmatic activity. The time of igneous activity is known from U-Pb zircon ages—rhyolite in the middle of the Kallander Creek Volcanics has an age of 1107 +/- 2 Ma, and granite in the Mellen Complex has an age of 1102 +/- 2 Ma (Cannon et al., 1993a). Thus, the central volcano was probably active from about 1108 Ma to 1102 Ma ago (Cannon et al., 1993a). ILSG 2011 100 Field Trip 6 �Magnetic Polarity Formation Age (Keweenawan Rocks) MellenIntrusions (1102 Ma) Kallander Creek Volcanics (~1108 - 1099Ma) Siemens Creek Volcanics Bessemer Formation Tyler Ironwood Palms Bad River Dolomite Puritan Batholith (2750 Ma) Ramsey Formation Bergland Group Powder Mill Group Portage Lake Marquette Range Supergroup Freda Nonesuch Copper Harbor Pleistocene Oronto Group Copper Falls / Miller Creek Normal Mesoproterozoic Reversed Normal Reversed Paleoproterozoic Neoarchean Figure 2. Stratigraphic table of formations in the Copper Falls area. The Kallander Creek through Copper Falls Formations are visible in Copper Falls Park. Note that the table shows rocks by age, not position – the Mellen Intrusions were mostly emplaced between the Siemens Creek and Kallander Creek Volcanics (after Green, 1982; Nicholson et al., 1997). The eastward thickening of the units above the Kallander Creek is referred to as ―The Wedge‖ on interpretive signs within the park. In fact, the Portage Lake Volcanics, just above the Kallander Creek Volcanics and below the Oronto Group, pinches out east of Copper Falls State Park and is not visible on this field trip. Therefore, going up-section, the sedimentary units above the Kallander Creek arethe Copper Harbor Conglomerate, the Nonesuch Formation, and the Freda Sandstone, all of which are much thicker to the east in Michigan (Figure 2). The shale and siltstone of the Nonesuch is overlain by interbedded red and brown conglomerate and sandstone of the Freda. These units record the waning of volcanic activity in the MCR and the subsequent filling of the rift with sediments. Rocks of the Mellen Complex are located nearby, butare not exposed in the area of the Copper Falls gorge. The Mellen Complex consistsprimarilyof gabbro and granite intrusions that were emplaced along the base of the MCR volcanic pile. The southern part of Copper Falls park is underlain by granite of the Mellen Complex, and a small gabbro quarry ILSG 2011 101 Field Trip 6 �can be seen on the west side of State Highway 169 south of the park, and along State Highway 13 north of Mellen (seeChapter 9 of this guidebook volume). F E D C B F B A G Figure 3. Bedrock geologic map of the Copper Falls area (from Cannon et al., 1996). Formations are: A, Yk) Kallander Creek andesite and basalt; B) Kallander Creek rhyolite; C) Portage Lake Volcanics, D) Copper Harbor Conglomerate; E) Nonesuch Formation; F) Freda Sandstone; G) Mellen Granite; Ymgp) Granophyre of Potato River Intrusion in Mellen Complex. The inset box shows the area of this field trip. Surficial Geology Bedrock exposed in the park is overlain by Pleistocene glacial deposits of the Copper Falls and Miller Creek Formations that record the retreat of the last of the Wisconsin glacial lobes prior to 9,500 BP (Clayton, 1984). The Copper Falls Formation contains sandy till and outwash deposited by the Chippewa Lobe prior to 11,500 BP, whereas the younger Miller Creek Formation is comprised of clay-rich deposits associated with Glacial Lake Duluth that were deposited between 11,500 and 9500 BP (Figure 3). Areas occupied by these two different deposits are often separated by a ribbon of sand – part of the Miller Creek Formation – at an elevation of 1100 feet that marks the highest beach of Glacial Lake Duluth. Deposits of sandy till of the Copper Falls Formation cover the bedrock in the southern part of the park and can be seen in the banks of the Bad River along the field trip route. Here the Copper Falls Formation is a mix of sand and silt with some gravel and boulders but has considerable local variability. The topography in the vicinity of the park is primarily controlled by the form of bedrock hills that have little or no glacial sediment on top, separated by intervening valleys where the Copper Falls Formation is tens of feet thick (Figure 4). Fine-grained sediments of Miller Creek Formation form large banks just downriver of the field trip route. ILSG 2011 102 Field Trip 6 �Soils that developed atop these sediments on topographic highs, such as in the landscape of the park, are moderately well-drained. These soils support a mixed forest with abundant white pine, sugar maple, and hemlock. In spite of the permeable sediments in the region, topographic lows are poorly drained and have a high water table because of the deranged drainage pattern. In these wetter areas, soils support spruce, fir, and cedar forests. There are also numerous kettle lakes and bogs that have thick accumulations of peat. Downstream (north) of the gorge the bedrock is covered by clay-rich sediment of the Miller Creek Formation. It is the distinctive red clay of the Lake Superior ―clay plain‖ in this region. The red clay is hard and stable when dry, yet soft and unstable when wet, which results in the erosion of very steep banks in the meanders of the Bad River immediately north of the gorge and Doughboys‘ Nature Trail. Figure 3. Surficial geologic map of the Copper Falls area (from Clayton, 1984). The area of this field trip is shown in the inset box. Areas represented in green are the Copper Falls Formation; those in blue-green are Miller Creek Formation. The strip of orange represents a beach deposit from Glacial Lake Duluth. Hydrology The Bad River starts at Caroline Lake, southeast of the park and the town of Mellen. The land in the headwaters of the Bad River is poorly drained and has numerous bogs. The water flowing from this headwater region is rich in tannins and organic acids derived from organic material in the peaty bogs. This gives the Bad River its characteristic brown color and is the source of the name ―Copper Falls‖. ILSG 2011 103 Field Trip 6 �From Caroline Lake the river flows southwest through a large area of poor drainage, then turns north and goes through the bedrock gorge of the Penokee Gap. The Bad River then flows through the City of Mellen, continuing north into the park. Within the park, the river meanders across a floodplain in a relatively flat area probably held in place by the nick point defined by the hard basalt at the head of the Copper Falls Gorge. The river then plunges over the ledge at Copper Falls and flows through the deep gorge. The presence of the gorge is likely due to nonresistant rock, which may be due to a combination of poor cementing in sedimentary rocks and closely spaced fractures in the felsite. Also, the rock may be highly fractured due to faulting. Figure 4. Topographic map of the Copper Falls area. From U.S.G.S. maps of the Mellen quadrangle (1967) and the High Bridge quadrangle (1984). FIELD TRIP STOPS This field trip guide is for a walking tour of the Copper Falls State Park Doughboys‘ Nature Trail starting and ending at the parking lot of the concessions building near the river. The stops are listed in order going around the Copper Falls trail loop in a clockwise direction, although the hike can be done in either direction (Figure 5). Helpful interpretive signs mark many of the significant geologic features along the trail and will be mentioned here. Also, bring along binoculars; some of the outcrops are inaccessible by foot. This area is represented on the Mellen and Highbridge topographic quadrangle maps(U.S.G.S.), on the Pleistocene map of Clayton (1984), and the bedrock geologic maps of Cannon et al. (1996) and Sims (1992). All locations are given in UTM coordinates (Zone 15 T), NAD 1927.Maps and brochures are also available at the park; some of which are accessible on-line through the Wisconsin DNR website. ILSG 2011 104 Field Trip 6 �Figure 5. Field trip stops 1-9 along the Doughboys‘ Nature Trail. Map courtesy of the Wisconsin Department of Natural Resources. Stop 1. Bridge over the Bad River Location: UTM coordinates: 681444 mE, 5137895 mN The bridge is located past the concessions building and marks the trailhead for the Doughboys‘ Nature Trail. Upstream of the bridge, thereis a large bank of silty sand and some gravel of the Copper Falls Formation. Boulders eroded out of the slope make up the channel and banks of the river. There is a widevariety of lithologies in these boulders, reflecting the diverse bedrock geology of the region. Rock exposed under the bridge is reddish gray basalt of the Kallander Creek Volcanics. The rock is slightly amygdaloidal and extensively fractured. The amygdules and fractures are filled with calcite stained red with hematite. During the Keweenawan eruptions, these flows were deeply buried by younger lava flows and sedimentary rocks, deep enough to undergo zeolite facies metamorphism. Oxidation of ferromagnesian minerals created abundant hematite that gives the rocks their dark reddishtint. No lava flow contacts are ILSG 2011 105 Field Trip 6 �visible in this part of the section, butthe lava flows seen from the bridge are tipped to near vertical, dipping steeply to the north. The bridge, built in the 1930s by the CCC, sits on a foundation constructed with Mellen gabbro. During mining activities in the early 1900s, the Ruggles Mining Company blasted the basalt exposed belowthe bridge in order to remove a small bedrock ridge to prevent flooding in the mine shafts. This allowed the river to flow directly east before turning north into the gorge; prior to blasting, the river flowed south around a big meander bend, visible at Stop 8. Stop 2. Copper Falls Overlook Location: UTM coordinates: 681510 mE,5138011 mN Along bend in trail with good view of the falls; history-focused interpretive sign nearby. In 1902, Copper Falls was reportedly nearly 30-feet high, but several large bedrock pieces have broken off, and today the falls is about 12-feet high. During spring floods, the water can rise high enough to flow over the basalt island separating the channels. The rock of the falls is basalt of the Kallander Creek Volcanics, similar to that seen at Stop 1. Look carefully to see if you can see any lava flow contacts or interflow sedimentary units. The interpretive sign here says that the gorge formed because of a fault—what are your thoughts? Stop 3. Brownstone Falls Overlook Location: UTM coordinates: 681682 mE, 5138203 mN Along trail in bend that offers view of Brownstone Falls; interpretive sign nearby. Brownstone Falls marks the confluence of Tyler Forks River and Bad River and reveals a contact between the mafic and felsic rocks of the Kallander Creek Volcanics.Felsic rocks are visible along the west side of the gorge, just left of the falls, apparent by theirreddish color and closely spaced fractures. This quartz- and plagioclase-phyric unit is informally named the Sheep Farm rhyolite and is the uppermost unit of the Kallander Creek Volcanics. It is at least 300 m thick and is likely the product of a single eruption. It has been dated at 1099 Ma, and lies just above the horizon of the principle magnetic reversal of the Keweenawan from reversed to normal polarity (Zartman et al., 1996; Cannon, chapter 7 of this guidebook volume). Don‘t climb over the fence, but examine the rocks alongside the path, and see what you can find. Along the trail, watch for some rare Northern Wisconsin vegetation: cutoversurviving white pines, eastern hemlock, and ravine-hugging cedars. The gorge, with its steep sides, has offered protection for the hemlock and cedar from over-browsing by deer.The cedars also show evidence of soil creep by the way their trunks curve down slope. Stop 4. Devils GateOverlook Location: UTM coordinates: 681506 mE,5138285mN Overlookalong the trail above Devils Gate Despite the name, this part of the field trip is really quite tame and the river‘s gradient has decreased. From this vantage, several formations of the Oronto Group can be seen across the river. The beds strike about 24 degreesnortheast-southwestand dipnorthwest at 86 degrees. The Copper Harbor Conglomerate, characterized by mainly red and brown conglomerate, sandstone, and siltstone, is straight across the gorge;there is some crude ILSG 2011 106 Field Trip 6 �bedding apparent. These rocks represent a prograding alluvial fan complex in the MCR (Daniels, 1982). The Nonesuch Formation—up-section and to the left—is characterized by mostly gray to black shale, siltstone, and mudstone. These fine-grained sediments were derived from local sources and deposited in alternating lacustrine and fluvial environments (Cannon et al., 1993a). The conglomeritic facies of the Freda Sandstone, the farthest outcrop visible to the left, is characterized by red and brown interbedded sandstone and conglomerate with thick bedding. The clasts of this facies are approximately equal proportions of immature, subangular to well-rounded, and poorly sortedpebbles offelsic and mafic rocks (Cannon et al., 1996). These clasts indicate that volcanoes were still serving as the main source of sediment in the rift. Stop 5. View of Devils Gate from Bridge Location: UTM coordinates: 681389 mE, 5138371mN Looking upstream into Devil‘s Gatefrom the bridge is another view of the Oronto Group members. The name ‗Devil‘s Gate‘ may come from the wicked turbulence and power of the Bad River in this constriction, especially notable during floods. This stop is unique as three units of the Oronto Group are visible. The different units here are relatively thin as they pinch-out towards Mellen;the thinning is especially apparent when these same formations are compared at different locations, particularly as they thicken northeastwards into the Michigan Upper Peninsula. The varying slopes indicate different rock resistance: the Nonesuch Formation has clearly eroded back, while the conglomerates and sandstones of the Copper Harbor and Freda have formed more resistant outcrops protruding into the river. The Copper Harbor Conglomerate is the farthest upstream, while the Freda Sandstone is closest to the bridge. From the next overlook along the trail, there is a good close-up view of the siltstones and sandstones of the Freda Sandstone. Stop 6. Overlook of Tyler Forks River Location: UTM coordinates: 681742 mE, 5138243mN Follow the wooden bridge path down to the farthest overlook, which is almost directly over Brownstone Falls. From this vantage, north of the confluence, we are looking upstream into the Bad River gorge with Tyler Forks River on the left, just to the right, Brownstone Falls plunges over a ledge of the Kallander Creek Volcanics.Felsic rock is visible on the right; a close-up of the outcrops seen at Stop 3. To the left is Tyler Forks Cascades—a series of short turbulent fallstumbling over felsite. A better view of the cascades can be seen at the overlook a short distance back along the wooden walkway. Stop 7. Brownstone Falls: a Different Perspective Location: UTM coordinates: 681723 mE, 5138192 mN Past the bridge, along the trail, is another good view of Brownstone Falls from the east side of the confluence. At this stop, there isanother view of Brownstone Falls:to the right, the Tyler Forks River flowsin from the northeast, joins the Bad River below the falls, andcontinues northwest through Devil‘s Gate. Across the gorge, looking towards Brownstone Falls, an overflow channel can be seen carved into the rock;this abandoned channel funnels water into the Bad ILSG 2011 107 Field Trip 6 �River during floods. More mafic and felsic rocks of the Kallander Creek Volcanics are visible—do you see any evidence of a contact? Stop 8. Old Bad River meander Location: UTM coordinates: 681477 mE, 5137861 mN Along trail, just before the first bridge; old channel is to the left. In the early 1900s, the Ruggles Mining Company put in several shafts near the current concessions building, prospecting for copper. To prevent flooding, workers blasted a new channel so the Bad River could be redirected. Today, the old meander bend is still visible, though trees and other vegetation have successively reclaimed the channel. As for the mining, although some of these same formations are rich in copper father east, this endeavor proved unsuccessful. Stop 9. Flagstones in the path at the concession building Locally quarried slate flagstones in the sidewalk near the concessions buildings are from the Paleoproterozoic Tyler Formation. They show good examples of thin bedding, graded beds, concretions, and other sedimentary structures. Additionally,more slate and gabbro were used in the bridges, buildings, and along the trails, particularly on the steps. Around the corner from the first waypoint, on the west side of the concessions building, is a large chimney constructed of anorthositic gabbro of the Mellen Complex. Stop 9a. Location: UTM coordinates: 681383 mE, 5137847 mN Along the paved path, near the southwest corner of the concessions building kitchen. This particular location, along the south wall of the concessionsbuilding, has several flagstones that show thinly interbedded mudstone and sandstone. Look for interesting sedimentary structures in other flagstones.The angle of slaty cleavage varies from being parallel to bedding to being perpendicular to bedding, with many angles in between. Stop 9b. Location UTM coordinates: 681330 mE, 5137846 mN Along the paved path, closer to the parking lot. Thislocation, closer to the parking lot, has some good examples of graded bedding. All of this slate comes from the Paleoproterozoic Tyler Formation. The Tyler Formation crops out sporadically south and east of the park,and is considerablydown-section of the Keweenawan rocks found in Copper Falls State Park. ACKNOWLEDGEMENTS Thanks to Laurel Woodruff for her astute editing. ILSG 2011 108 Field Trip 6 �REFERENCES Cannon, W.F., Woodruff, L.G., Nicholson, S.W., and Hedgman, C.A., 1996, Bedrock geologic map of the Ashland and the northern part of the Ironwood 30- x60-minute quadrangles, Wisconsin and Michigan: U.S. Geological Survey Miscellaneous Investigation Series Map I-2556, scale 1:100,000. Cannon, W.F., Nicholson, S.W., Zartman, R.E., , Peterman, Z.E., and Davis, D.W., 1993a, Kallander Creek Volcanics—a remnant of a Keweenawan central volcano centered near Mellen, Wisconsin [abs.] Institute on Lake Superior Geology, 39 th Annual Meeting, Proceedings and Abstracts, p. 20-21. Cannon, W.F., Peterman, Z.E., and Sims, P.K., 1993b, Crustal-scale thrusting and the origin of the Montreal River monocline—a 35-km-thick cross section of the midcontinent rift in northern Michigan and Wisconsin: Tectonics, v. 12, p. 728-744. Clayton, L., 1984, Pleistocene geology of the Superior Region, Wisconsin: Wisconsin Geological and Natural History Survey, Information Circular no. 46, 40 p., map scale: 1:250,000. Daniels, P.A., Jr., 1982, Upper Precambrian sedimentary rocks: Oronto Group, MichiganWisconsin, inWold, R.J., and Hinze, W.J., eds., Geology andtectonics of the Lake Superior basin: Geological Society of America Memoir 156, p. 107-134 Green, J.C., 1982, Geology of Keweenawan extrusive rocks,inWold, R.J., and Hinze, W.J., eds., Geology andtectonics of the Lake Superior basin: Geological Society of America Memoir 156, p. 57-82. Nicholson, S.W., Shirey, S.B., Schulz, K.J., Green, J.C., 1997, Rift-wide correlation of 1.1 Ga Midcontinent rift system basalts: Implications for multiple mantle sources during rift development: Canadian Journal of Earth Sciences, v. 34, p. 504-520. Sims, P.K., 1992, Geologic map of Precambrian rocks, southern Lake Superior region, Wisconsin and northern Michigan: U.S. Geological Survey Miscellaneous Investigation Series Map I-2185, scale 1:500,000. U.S. Geological Survey, 1984, High Bridge, Wisconsin quadrangle, 7.5-minute topographic map: photorevised 1980, scale 1:24,000, 1 sheet. U.S.Geological Survey, 1967, Mellen, Wisconsin quadrangle, 7.5-minute topographic map: photorevised 1980, scale 1:24,000, 1 sheet. Wisconsin Department of Natural Resources, 2009, Copper Falls State Park, http://dnr.wi.gov/Org/land/parks/specific/copperfalls/ (January 2011). Zartman, R.E., Nicholson, S.W., Cannon, W.F., and Morey, G.B., 1996, U-Th-Pb zircon ages of some Keweenawan Supergroup rocks from the south shore of Lake Superior: Canadian Journal of Earth Sciences, v. 34, p. 549-561. ILSG 2011 109 Field Trip 6 �ILSG 2011 110 Field Trip 6 �57TH ANNUAL INSTITUTE ON LAKE SUPERIOR GEOLOGY FIELD TRIP 7 GEOLOGY OF THE MONTREAL RIVER MONOCLINE Steeply dipping Freda Sandstone at the mouth of the Montreal River -W.F. Cannon ILSG 2011 111 Field Trip 7 �ILSG 2011 112 Field Trip 7 �Field Trip 7 Geology of the Montreal River Monocline William F. Cannon U.S Geological Survey MS 954 Reston, Virginia 20192 Note: This fieldtrip was originally run for the 42nd Annual Institute on Lake Superior Geology in 1996. At that time, poor road conditions including deep snow on some back roads, ―vehicular traction issues‖, and high water in streams prevented a complete examination of all intended localities. New developments in the past 15 years and hopes for better field conditions in 2011 lead us to rerun the trip for the 57th Annual Meeting. The Field Guide is based largely on the original publication in 1996 (Cannon, 1996) with a few modifications based on new exposures and recent studies. INTRODUCTION South of Lake Superior, near the Wisconsin-Michigan border, a remarkable monoclinal sequence of rocks exposes, at the present land surface, what had been the upper 35 km of the crust (Figure 1) prior to uplift and monoclonal tilting during inversion of the Midcontinent rift. This section is seen in a sequence of steeply to vertically dipping and north-facing volcanic and sedimentary units of Paleoproterozoic and Mesoproterozoic age and underlying Neoarchean volcanic rocks. Figure 1. Cross section of the Montreal River monocline showing the stratigraphic position of field trip stops and approximate burial depths. ILSG 2011 113 Field Trip 7 �This structure, named the Montreal River monocline by Cannon and others (1993), resulted from southward-directed thrusting on a northward-dipping listric thrust of crustal scale, the Atkings Lake-Marenisco fault (Figure 2). The age of thrusting is known to be about 1040-1060 Ma, the date of reset biotite Rb-Sr ages in Archean rocks that were upthrust and cooled on the upper plate above the fault (Cannon and others, 1993). Figure 2. Schematic cross section of the Montreal River monocline showing the inferred crustal-scale structures responsible for its formation. Figure 2 shows the regional structural context of the Montreal River monocline. The section extends approximately from the Bayfield Peninsula on the northwest to Mercer, Wisconsin on the southeast, a distance of roughly 100 km. To the northwest, a prominent basement high, White‘s Ridge, limited the depositional extent of much of the Keweenawan volcanic section and remained a positive topographic feature until the close of the extensional phase of the rift. As more widespread thermal subsidence supplanted extension, the ridge was buried beneath the Oronto and Bayfield Groups, which remain nearly flatlying. Southeastward from White‘s Ridge to the present outcrop belt of the Keweenawan volcanics, the volcanic section becomes progressively thicker. This geometry suggests that the deepest parts of the rift in this area may actually have been to the south of the preserved volcanic rocks, in an area since uplifted and deeply eroded. The present steeply northdipping attitude of the rocks of the Montreal River monocline is believed to be the result of crustal-scale ramping on the Atkins Lake-Marenisco fault. The Atkins Lake-Marenisco fault is interpreted to penetrate the entire crust and to have thrust rift units and their Paleoproterozoic and Archean basement southward. The listric geometry of the fault surface caused the upper plate to tilt northward, after which erosion has exposed the present crustalscale cross section. The involvement of Archean rocks in the monoclonal structure is shown by the southerly dip of originally vertical diabase dikes and by Mesoproterozoic Rb-Sr ages of biotite, which mark the time of uplift and cooling. The 270o isotherm, the Rb-Sr blocking temperature of biotite, was located by determining the Rb-Sr age of biotite in Arcehan rocks. Rocks north of the isotherm (originally above the isotherm) remained cooler than 270o throughout Keweenawan burial and retained ages reflecting Paleoproterozoic and Archean conditions. Rocks between the isotherm and the Atkins Lake-Marenisco fault were heated above 270o by Keweenawan burial and the Rb-Sr ages were reset. In this area biotite ages ILSG 2011 114 Field Trip 7 �range from 1040-1060 Ma and date the time of uplift, erosion, and cooling caused by thrusting along the Atkins Lake-Marenisco fault. South of the fault Mesoproterozic burial was not deep and biotite ages continue to reflect Paleoproterozoic thermal events. The trip begins at the mouth of the Montreal River (Figure 3) where extensive exposures of vertically dipping Freda Sandstone, the youngest unit exposed in the monocline, form bluffs along the Lake Superior shore. Based on projections of seismic data from Lake Superior these strata may have been buried by as much as 5 km of overlying clastic rocks now preserved beneath the lake. The trip progresses southward for about 25 km through successively older and steeply dipping to vertical strata of the Mesoproterozoic Keweenawan Supergroup, the Paleoproterozoic Marquette Range Supergroup, and into Neoarchean metavolcanic rocks. The Paleoproterozoic and Neoarchean rocks seen at stops 8 and 9 have been exhumed from lower crustal depths, but lack the expected high metamorphic grade and deformation typical of rocks from such depths. The generally low-grade metamorphism and lack of Mesoproterozoic penetrative deformation attest to a short residence time at these depths. Rapid subsidence during rifting and rapid uplift during rift inversion brought these rocks to great depth and returned them to shallower depths at a rate that did not allow conductive heating to achieve typical deep crustal temperatures. ILSG 2011 115 Field Trip 7 �Figure 3. Geologic map of the field trip area showing location of stops. Geology generalized from Cannon and others (1996). Stop 1: Freda Sandstone at the mouth of the Montreal River (46.566, -90.4169)From the parking area at the top of bluff overlooking Lake Superior walk down the trail to exposures along the lakeshore. The Freda Sandstone is the youngest unit exposed in the Montreal River monocline. The rocks here, representing the middle to upper parts of the Freda, may have been buried by as much as 3-5 km under younger parts of the Freda and younger sediments of the Bayfield Group. The sandstone is well exposed in high continuous bluffs along the Lake Superior shoreline. It is mostly a well bedded and commonly crossbedded feldspathic sandstone. Dips are essentially vertical. Volcanic rock fragments are a major constituent, indicating that riftderived volcanic rocks composed much of the source area. Muscovite is also a common detrital mineral in many layers indicating that an Archean basement terrane was also exposed that contributed a substantial part of the detritus. Stop 2: Middle part of Oronto Group along Parker Creek (46.5241, -90.4199) These exposures are best reached by walking west from Highway 122 to Parker Creek. The coordinates are approximately at the contact between the Copper Harbor Conglomerate and Nonesuch Formation. These exposures are on private land. The top of the Copper Harbor Conglomerate, the Nonesuch Formation, and the lower part of the Freda Sandstone can be seen along the incised bed of Parker Creek. The Parker Creek section is the most complete exposure of the Nonesuch and adjacent units in the Lake Superior region. Beds dip from 75°N to vertical. Detailed stratigraphy of the Parker Creek section was described by Suszek (1991) on whose work the following generalized description is based. The upper part of the Copper Harbor Conglomerate consists of massive to crudely bedded clast-supported conglomerate with cross-bedded, pebbly sandstone interbeds. Clasts consist mostly of basalt and rhyolite with lesser granite and gabbro, all probably derived from slightly older Keweenawan rocks. Banded iron-formation, jasper, quartzite, and chert clasts are a minor component but are widespread, indicating that pre-Keweenawan basement was also exposed to the south within the drainage basin. The Nonesuch Formation is about 130 m thick and comprised of mostly gray to pale brown siltstone, sandstone, and minor shale. It is generally thinly bedded to laminated, but thicker massive beds also occur. Small-scale trough crossbedding is common. There is an overall coarsening-upward trend to the formation, although many individual units display fining-upward textures. The fine-grained rocks of the Nonesuch Formation lie on coarse conglomerate of the Copper Harbor. A transitional bed, one meter thick, contains reddishbrown, trough-crossbedded sandstone at the base and grades upward into gray-brown calcareous siltstone. The upper contact of the Nonesuch with the Freda Sandstone is gradational over about 20 m, in which gray siltstone and shale are interbedded with reddishbrown, medium to coarse sandstone. ILSG 2011 116 Field Trip 7 �Stop 3: Top of the Porcupine Volcanics at SaxonFalls (46.5380, -90.3751) Drive to the end of the road to Saxon Falls dam. The exposures are along the river bed below the dam. Basalt flows and interflow conglomerate and sandstone of the uppermost part of the Porcupine Volcanics are exposed along the Montreal River below the dam at Saxon Falls flowage. The uppermost exposed unit is thin-bedded red sandstone and siltstone, which is underlain by a massive basalt flow about 100 m thick. This flow is underlain by about 30 m of interbedded conglomerate and sandstone. The lowest exposed unit is another basalt flow with an amygdular, partly brecciated top. About the upper half meter of the flow shows typical rubbly flow-top breccia, and the upper three meters are coarsely amygdular. The most abundant amygdule-filling mineral is thomsonite, which occurs in masses of coarse radiating blades in the larger amygdules. The predominance of this low temperature zeolite indicates that rocks at this level of burial were only weakly heated before uplift. Stop 4: Rhyolite at top of Kallander Creek Volcanics (46.5108, -90.3265) This exposure is on private land on an active farm. Permission for access should be requested from the land owner. The uppermost unit of the Kallander Creek Volcanics is a thick, mostly massive, quartz- and plagioclase-phyric rhyolite. We informally designate this unit the Sheep Farm rhyolite and this stop is our informal type locality. The unit is at least 300 m thick and appears to be the product of a single eruptive event. It can be traced from this locality westward for about 30 km to Copper Falls State Park near Mellen, where it is the resistant rock that holds up Brownstone Falls on the Tyler Forks River. At that locality, we have located by field measurement the horizon of the principal magnetic reversal recorded in the Keweenawan section. The reversal lies a few flows beneath the rhyolite. The rhyolite has been dated at 1099 Ma (Zartman and others, 1996), so the age of the magnetic reversal must also be approximately 1099 Ma. At this stop, the exposures reveal the basal part of the flow, which contains numerous blocks of rhyolite up to about 20 cm across. These blocks have a slightly different texture and phenocryst content than the host rhyolite. Stop 5: Icelandite in Kallander Creek Volcanics (46.4936, -90.3292) The exposures to be examined are in a gravel pit to the northeast of the intersection of Lakehead Road and US Highway 2 The upper member of the Kallander Creek Volcanics contains abundant rocks of intermediate composition in addition to basalt and rhyolite. We have proposed that the upper part of the Kallander Creek Volcanics represents the partly eroded remains of a broad central volcano, and that the Mellen Complex is the crystallized remnants of a magma chamber that was intruded into the volcanic edifice late in its eruptive history (Cannon and others, 1993). ILSG 2011 117 Field Trip 7 �At this locality our data show that the rock knob in the gravel pit is a fine-grained reddishbrown icelandite (SiO2=64-66 wt%; Ti02=1.26 wt%; CeN/YbN=4.5) that contains small plagioclase phenocrysts. The rock has a strong primary foliation, perhaps indicating that it is the basal zone of a flow, and weathers into platy fragments parallel to the nearly vertical foliation. Small outcrops nearby show a coarser, more massive icelandite. The icelandite is overlain by a basalt (Si02=50.24wt%; Ti02=3.78 wt%; CeN/YbN=3.7) that is exposed in low outcrops along the power-line north of the gravel pit. Stop 6: Upson Lake area—Siemens Creek Volcanics, Bessemer Quartzite, and Tyler Formation (46.3867, -90.4442) Drive road along the south shore of Upson Lake to the small parking area at the west end of the lake. Walk north and east along logging road about 300 meters and then north to exposures on higher part of the south-facing slope. At this locality the base of the Keweenawan section is exposed in contact with the underlying Tyler Formation. At the top of the prominent bluff north of Upson Lake the lowermost basalt flows of the Siemens Creek Volcanics are exposed. They are pillowed and brecciated as a result of eruption into a shallow lake in which the Bessemer Quartzite had been deposited. The flows have a distinctive chemical composition marked by low A1203 (812 wt%), and high Cr, Ni, and MgO. In addition, rare earth element (REE) patterns are quite steep (CeN/YbN = about 15) and heavy REEs have low abundances. These chemical characteristics indicate that the magmas formed by a small degree of partial melting of a deep, garnet-bearing enriched mantle source (postulated to be a mantle plume), and that the magma erupted directly to the surface without undergoing significant crystal fractionation. Flows of this chemical character can be recognized in the field by the presence of small phenocrysts of clinopyroxene. These clinopyroxene-phyric flows have been traced from near Bessemer, Michigan, to just west of this locality. They apparently represent a unique magma composition that was erupted only at the very outset of volcanism. Identical flows have been recognized at the base of the North Shore Volcanics in Minnesota, both at Ely's Peak and at Pigeon Point and Grand Portage (Green, 1977). An interflow sedimentary bed is exposed between the first and second flows. The sediments are, in part, an algal laminite and are now substantially metamorphosed so that original calcareous layers are a fine-grained mat of wollastonite. The metamorphism was caused, at least in part, by emplacement of the Potato River Gabbro about 1 km up-section. The Siemens Creek Volcanics are underlain by the Bessemer Quartzite, the oldest unit in the Keweenawan Supergroup. Scattered exposures of white to gray laminated quartzite are on the slope below the basalt exposures. Passing down-section, the Bessemer contains basal beds of coarse conglomerate composed largely of Paleoproterozoic metasedimentary clasts, including iron-formation. Small garnet porphyroblasts are common in the matrix. A small exposure near the base of the slope shows an angular unconformity between basal conglomerate of the Bessemer and thin-bedded argillite of the Tyler Formation. This is the only exposure known to us where the unconformable relationship between the Keweenawan Supergroup and the Marquette Range Supergroup can be seen. Stop 7: Tyler Formation (46.3500, -90. 5051) Exposure is a roadcut on the north side of High 77. ILSG 2011 118 Field Trip 7 �This roadcut exposes typical thin-bedded graywacke and argillite of the Paleoproterozoic Tyler Formation. Although dips are nearly vertical, the lack of penetrative fabric indicates little structural effect of the Penokean orogeny on the Tyler in this area. The highest grade metamorphic mineral is biotite, which is common throughout the matrix. Stops 8 and 9: Mt. Whittlesey area The prominent ridge of Mt. Whittlesey (Figure 4) is held up by the Ironwood Ironformation. An unusually great thickness of iron-formation here resulted from structural repetition by thrust faults and folds produced during the Penokean orogeny. Mt. Whittlesey lies at the easternmost extent of the Penokean (Paleoproterozoic) foreland fold and thrust belt. Here, a series of thrusts repeats the Paleoproterozoic section, having detached it from Archean rocks along a basal decollement. The thrusts all rise in the section to the east along a series of lateral ramps. Displacement along the basal decollement diminishes eastward as splays from the decollement successively rise in the section. East of Stop 9 the Paleoproterozoic section lies unconformably on Archean rocks. ILSG 2011 119 Field Trip 7 �Figure 4. Geologic map of the Mt. Whittlesey area showing the locations of Stops 8, and 9. Stop 8: Ironwood Iron-formation in Mt. Whittlesey (46.30226, -90. 61633) Vehicle access is via a poorly maintained road that extends eastward from County Road MM. Some steep sections of road are susceptible to washouts and may be impassable at times. From the Berkshire mine ruins walk eastward on an ATV trail up the slope to large bedrock exposures. Excellent exposures of the Ironwood Iron-formation occur on the north face of the ridge west of the summit, and along a railroad grade near the Berkshire mine ruins. At the former locality, a large area was cleaned of overburden in the 1920s in anticipation of open pit mining. Only a small pit was developed near the base of cleared area; the remainder provides an excellent, glacially polished exposure in which many details of sedimentary ILSG 2011 120 Field Trip 7 �features are extraordinarily well displayed. At the Berkshire mine ruins an abandoned railroad grade provides a continuous exposure of several hundred meters of section and a tight fold is exposed in the cut. The iron-formation contains several lithologic variations. The classic five member internal stratigraphy defined to the east (Hotchkiss, 1919; Huber, 1959) consists of alternating members of even-bedded, carbonate iron-formation (slaty iron-formation) and irregularly-bedded hematitic jasper. This stratigraphy becomes more complex and irregular westward along the iron range; we have not been able to apply it west of Mt. Whittlesey. The iron-formation exposed here characteristically has many lithologic variations on a small scale so that individual exposures may show a complete range of lithologies interbedded on a scale of meters or less. The following detailed description of the Ironwood exposed on the northwest flank of Mt. Whittlesey was prepared by Philip Fralick as part of an informal guidebook for the Workshop on Geology, Mineralogy, and Genesis of Precambrian Iron-Formations held at the University of Minnesota-Duluth Precambrian Research Center in October 2010. It is based largely on a detailed study of these exposures reported in Pufahl and Fralick (2004). Scattered outcrops in the vicinity of Mt. Whittlesey define three upward-coarsening, asymmetric cycles composed of a basal unit of slaty iron-formation that is gradational into an upper unit of trough cross-stratified cherty iron-formation. The middle cycle provides an unparalleled opportunity to investigate cycle sedimentology as it is completely stripped of overburden to appraise the Fe resources of this area. Like the cycles above and below, it consists of ~14 m of slaty iron-formation gradationally overlain by ~16 m of cherty ironformation, which is capped by a ~1 m thick unit of convolute-bedded, slaty iron-formation. Pufahl and Fralick (2004) conducted a detailed description of a 40x44 m glacially polished exposure through 1.5 cycles combined with a reconnaissance study of poorly exposed strata above and below the logged section. This area provides an excellent opportunity to investigate the depositional controls governing the accumulation of ironformation facies. Their research was augmented with geochemical analysis of iron-formation facies in order to elucidate the chemical controls on iron-formation precipitation. This approach has yielded important insights into the relative roles tectonics, fluctuating hydraulic regime, and changing water chemistry played in governing the distribution of iron-formation facies. ILSG 2011 121 Field Trip 7 �Figure 5. Schematic map of vertically dipping asymmetric cycles developed in Superior-type iron-formation of the study area. The slaty unit forming the lower portion of cycle 1 overlies a slumped unit of convolutedly-bedded chemical mudstones and is gradational into a succession of cherty grainstone lenses. These coarse-grained beds are sharply overlain by another slumped chemical mudstone unit. The base of a second coarsening upward cycle (cycle 2) forms the upper portion of the outcrop. ILSG 2011 122 Field Trip 7 �Grainstone lenses become progressively thicker upwards in cycles with the largest at cycle tops, where they are sharply overlain by a unit of slumped chemical mudstone. Cycles formed through progradation when offshore-directed storm currents transported progressively greater quantities of chert sand intraclasts that were formed in nearshore settings, outboard into middle and distal shelf environments (Fig. 5). Abrupt subsidence events, probably resulting from normal faulting associated with extensional tectonism, repeatedly terminated shallow-water chert grainstone accumulation and may have also generated the slumped units at cycle boundaries. The episodic storm currents are also interpreted to have transported biologically oxygenated waters over the shallow-water, inner shelf outboard along the seafloor into otherwise anoxic bottom waters of the strongly stratified distal shelf (Fig. 6). The consequence of such transport and mixing was rapid deposition of chemical mud, mainly as precipitated Fe-oxide. In many cases, the resultant decrease of Fe2+ in the water column, together with pelagic inorganic precipitation of SiO2 and rainout of terrigenous clays, resulted in submillimeter- to millimetre-thick, chemically graded laminae. The concomitant decreasing Fe2+/Mn2+ ratio also led to increasing Mncompound precipitation and enrichment in the upper portions of some chemically graded laminae (Fig. 7). The outcrop is mostly two-dimensional and glacially polished so that sedimentary features are well displayed. It is sub-parallel to the trend of the paleoshoreline but oblique enough so that paleocurrent trends can be ascertained. Figure 6. Physical depositional model for an asymmetric iron-formation cycle. Rapid transgression was followed by formation of a shoaling upwards, prograding cycle. Offshore flow during or immediately after major storm events built shelf sand sheets from chert and Fe-oxide intraclasts which progressively migrated into more distal environments as water depth decreased. ILSG 2011 123 Field Trip 7 �Figure 7. (1) Chemical depositional model for an asymmetric iron-formation cycle. Low free oxygen levels allowed Fe and Mn to remain dissolved in a water mass with near-neutral pH. The main O2 production took place near-shore where light penetration to the seabed allowed cyanobacteria to flourish. (2) Storm-generated offshore flows, which were responsible for generating hummocky cross-stratification in more shore-proximal locations, transported the oxygenated coastal waters to the mid and outer shelf where they mixed with Fe- and Mn-bearing offshore waters, with resultant chemical precipitation. Storm-wave mixing of the stratified water body may have also played a role in this process, although an O2-bearing surface layer is not necessary. The succession represented by Column A reflects the formation of thick Fe-oxide laminae overlying grainstone lenses. The succession illustrated in Column B would have developed in a more distal shelf position resulting in thinner and finer grained sand lenses and thinner chemical layers than in Column A. The assemblage which developed furthest offshore is shown in Column C. ILSG 2011 124 Field Trip 7 �Figure 8. Progressive formation of a millimetre-scale chemical mud laminae. Chemical grading is interpreted to be the result of changes in water chemistry driven by the precipitation process. (A) Offshore currents generated during storm events delivered nearshore oxygenated water to deeper portions of the shelf, where it mixed with normal (Fe 2+-, Mn2+- and PO43--bearing) ocean water. (B) Precipitation proceeded relatively rapidly as the abundant Fe2+ and O2 combined to probably form FeOOH‘s. (C) As Fe content decreased in seawater precipitation rate slowed and ambient clastic and SiO2 deposition gained in importance. (D) As the Fe/Mn ratio in the aqueous phase decreases, Mn2+ more effectively combined with oxygen. A progressively decreasing Fe2+/PO43- ratio results in more scavenging of PO43- by the FeOOH‘s. Carbon is delivered to the sediment by the rainout of phytoplankton from the surface ocean. (E) Diagenesis and low-grade metamorphism resulted in the mineral assemblage present today. ILSG 2011 125 Field Trip 7 �Stop 9A: Palms Formation and Ironwood Iron-formation along Ballou Creek, east end of Mt. Whittlesey (46.31083, -90.57756) The exposures are in a long road cut along the west side of Lake Road. Recent road improvement along Lake Road has produced a set of roadcuts along the west side the valley of Ballou Creek that provide a cross section of most of the Palms Formation and part of the lower Ironwood Iron-formation. Starting at the south, at the lower part of the section, a nearly continuous exposure, about 75 m long, of laminated argillite and sandstone illustrates the lower part of the Palms Formation. Proceeding north, up-section, along the outcrop many typical sedimentary structures of the Palms are well exposed. These features have been interpreted to indicate sedimentation on a tidal flat or tidally influenced lagoon. Farther north, a covered interval about 75 m wide is probably underlain by the upper part of the laminated lower Palms Formation and quartzite of the upper Palms Formation. Next, to the north, is a 30 m exposure of the upper part of the Palms Formation quartzite and the Ironwood Iron-formation. The uppermost quartzite is coarse-grained and cross-bedded. The transition to the Ironwood Iron-formation takes place in a 0.5 m interval containing reworked fragments of the Palms surrounded by magnetite. The overlying Ironwood Ironformation varies from even-bedded magnetite-rich material to irregularly bedded jasper at a scale of a meter or less. Stop 9B. Contact of Paleoproterozoic strata and Archean metavolcanic rocks on the southeast flank of Mt. Whittlesey (46.30834, -90.58632) Walk west from Lake Road about 600 meters along the base of the ridge. Low outcrops, near the base of a prominent ridge supported by the Palms Formation, reveal the basal contact of Paleoproterozoic strata with underlying icelandite breccia of Neoarchean age. The breccia has a penetrative structural fabric, best expressed by the prominent elongation of clasts. Long axes plunge moderately southward, but must have plunged moderately to steeply northward prior to tilting of the rocks into the monoclinal structure. The basal Paleoproterozoic unit is a chert breccia that locally is the basal unit of the Palms Formation. We have interpreted this breccia to be residual concentration of chert produced by dissolution of the Bad River Dolomite, a cherty carbonate rock that is preserved sporadically beneath the breccias. The chert fragments were reworked during the transgression of the Palms Formation over a long-lived erosion surface that separates the Bad River and Palms. The upper meter or two of the icelandite breccia is strongly sheared parallel to the contact with the chert breccia, and the Archean fabric typically is completely overprinted by this secondary fabric. Shearing is weak to absent in the chert breccia. To our knowledge, these outcrops are the only ones in the western part of the Gogebic Range where a basal detachment between the Paleoproterozoic strata and Archean basement rocks can be directly observed. We believe the Paleoproterozoic strata were thrust northward along a nearly flatlying basal contact during the Penokean orogeny. Because of later tilting of the Montreal River monocline, the present apparent displacement reflects a down-to-the-north normal fault. This fault can be traced westward, and about 1 km to the west of these exposures it passes up- ILSG 2011 126 Field Trip 7 �section so that the Bad River-Palms-Ironwood sequence of the upper plate is juxtaposed over Ironwood Iron-formation in the lower plate, producing the unusually wide outctrop belt of Ironwood along higher parts of Mt. Whittlesey. Just east of this stop, the basal thrust appears to pass upward into the Paleoproterozoic strata. Exposures just east of Ballou Creek show that the base of the Paleoproterozoic rocks lies unconformably on Archean volcanic rocks without an intervening shear zone. The Archean rocks seen at Stop 9B are the structurally lowest parts of the monocline that are seen on the field trip. But the trace of the Atkins LakeMarerinsco fault, the master thrust that formed the Montreal River Monodline, lies about 8 km farther to the south. Thus, all of the stratigraphic section to the north that is seen on the trip, plus an additional 8 km of Archean rocks to the south, form the upper plate of the Atkins Lake-Marenisco fault, a coherent structural unit that was uplifted and tilted late in the Mesoproterozoic history of the region. REFERENCES Cannon, W.F., 1996, Geology of the Montreal River monocline: a traverse through 25 km of the crust: Proceedings of the Institute on Lake Superior Geology, v. 42, part 3, Fieldtrip guidebook, p. 49-63. Cannon, W.F., Woodruff. L.G, Nicholson, S. W., and Hedgeman, C.A., 1996, Bedrock geologic map of the Ashland and northern part of the Ironwood 30‘x60‘ quadrangles, Michigan and Wisconsin: U.S Geological Survey Miscellaneous Investigations map I-2566, scale 1:100,000. Cannon, W.F., Peterman, Z.E., and Sims, P.K., 1993, Crustal-scale thrusting and origin of the Montreal River monocline - a 35-km-thick cross section of the Midcontinent rift in northern Michigan and Wisconsin: Tectonics, v. 12, p. 728-744. Green, J.C., 1977, Keweenawan plateau volcanism in the Lake Superior region, in Baragar, W. R. A., and others, (eds.), Volcanic regimes in Canada: Geological Association of Canada Special Paper 16, p. 407-422. Hotchkiss, W.O., 1919, Geology of the Gogebic Range and its relation to recent mining developments: Engineering and Mining Journal, v. 108, p. 443-452, 501-507, 537541, 577-582. Huber, N.K., 1959, Some aspects of the origin of the Ironwood Iron-formation of Michigan and Wisconsin: Economic Geology, v.54, p.82-118. Pufahl, P.K., and Fralick, P.W., 2004, Depositional controls on Paleoproterozoic iron formation accumulation, Gogebic range, Lake Superior region, USA: Sedimentology, v. 51, p. 791-808. Suzek, T.J., 1991, Petrology and sedimentation of the Middle Proterozoic (Keweenawan) Nonesuch Formation, western Lake Superior region, Midcontinent rift system: Duluth, Minn., University of Minnesota, Duluth, unpublished M.S. thesis, 198 p. Zartman, R.E., Nicholson, S.W., Cannon, W.F., and Morey, G.B., 1996, U-Th-Pb zircon ages of some Keweenawan Supergroup rocks from the south shore of Lake Superior: Canadian Journal of Earth Sciences, v. 34, p. 549-561. ILSG 2011 127 Field Trip 7 �ILSG 2011 128 Field Trip 7 �57TH ANNUAL INSTITUTE ON LAKE SUPERIOR GEOLOGY FIELD TRIP 8 THE ARCHEAN/PALEOPROTEROZOIC UNCONFORMITY NEAR DENHAM, MINNESOTA Pillowed basalt flow near Denham, Minnesota - T. Boerboom Field Tri ILSG 2011 129 Field Trip 8 �ILSG 2011 130 Field Trip 8 �Field Trip 8 Transect from Archean basement to the Animikie basin, East-central Minnesota Terry Boerboom Minnesota Geological Survey boerb001@umn.edu INTRODUCTION This field trip runs parallel to the western margin of the Mesoproterozoic Midcontinent Rift System, where the continental crust is dominated by the geon 18 (1,800 – 1,900 Ma) Penokean Orogen and is intruded by rocks of Yavapai age (1,800 – 1,772 Ma; geon 17). Together, these orogenic components form a major curvilinear orogenic belt that spans an area in Minnesota from the Mesabi Iron Range to central Minnesota, with outlying intrusions as far south as the Iowa border (Figure 1). The first part of the trip will start on Archean basement rocks (the McGrath Gneiss dome) which are non-conformably overlain by the Paleoproterozoic Denham Formation, which in turn is overlain by the eastern extension of the overlying Little Falls Formation. The Denham Formation consists of basal coarse clastics interlayered with pillow basalts that were erupted into a shallow-water environment, overlain by a thick unit of dolomitic marble, in turn overlain by a thick sequence of turbidites correlative with the Little Falls Formation. The Denham Formation and the lower part of the Little Falls Formation have been metamorphosed to staurolite-zone amphibolite facies. The remainder of the trip will examine outcrops of turbiditic sediments of progressively lower metamorphic grade, ending in the low-grade Thomson Formation near Carlton, Minnesota. The stops will examine the structural attributes of the rocks and the gradual decrease in metamorphic grade from south to north, highlighting the difficulty of defining a boundary between the Animikie basin to the north and the older rocks of the Penokean orogen to the south. One of the stops will be in an area traditionally mapped as part of the older rocks south of/underneath the Thomson Formation, but which are now considered to be part of the Thomson Formation. We will also discuss the different generations of deformation evident in the rocks, and the likelihood that the later deformation may be related to the geon 17 (1,700 – 1,800 Ma) Yavapai Orogeny. The reader is greatly encouraged to make use of the numerous references listed, which provide detailed studies that tie together the various components of the Penokean Orogen as well as the effects of the Yavapai and Mazatzal orogens in Minnesota, Wisconsin, Michigan, and Ontario. A special volume of Precambrian Research (Volume 157 , August 2007) contains the latest interpretations of the Paleoproterozoic history of the Lake Superior region. ILSG 2011 131 Field Trip 8 �Figure 1. Simplified geologic map of southern two-thirds of Minnesota showing extent of eastcentral Minnesota batholith and other intrusions of known and inferred Yavapai age in the Minnesota ILSG 2011 132 Field Trip 8 �River Valley subprovince; also extent of Yavapai-interval intrusive rocks in central Minnesota (stippled pattern). TECTONIC ELEMENTS OF EAST-CENTRAL MINNESOTA Early workers (e.g. Marsden, 1972, Morey and others, 1981) correlated components of the Mesabi Iron Range (Pokegama, Biwabik, and Virginia Formations) with those of the Cuyuna North and South Ranges (Mahnomen, Trommald, Rabbit Lake Formations), and considered the Thomson Formation (equivalent to the Virginia Formation) to include the higher metamorphic grades south of Thomson Dam, e.g.stops 8-4 and 8-5 of this trip. They considered the Denham Formation to be part of an older sequence termed the Mille Lacs Group, which also included such units as the Trout Lake dolomite north of the Cuyuna Range, the Little Falls Formation, the Glen Township Formation, and other mafic volcanic and high-grade metasedimentary units. Subsequently Southwick and others (1988) reinterpreted the entire Penokean Orogen in terms of a plate-tectonic model, with the aid of high-resolution aeromagnetic data that was unavailable to earlier workers. They considered the Cuyuna Range and everything to the south (Figure 1) to be part of an older series of folded and thrust-faulted panels that increased in metamorphic grade (hence greater degrees of exhumation), from north to south, culminating in the southern-most plutonic terrane near St. Cloud. In this model, northwarddirected thrust-stacking created uplift along the southern margin of the Animikie basin, and the basin developed to the north of the uplifted crust in response to flexural loading. All of the components were considered to be of Penokean age. The next step in refining our understanding of the Penokean Orogen in Minnesota was and is the acquisition of age dates. The first significant studies began in the 1990‘s, through multiple geochronological studies initiated by Daniel Holm at Kent State University, and later by others. Ar-Ar ages define widespread ca. 1,760 Ma metamorphism throughout the Penokean Orogen (e.g. Holm and others, 1993; Holm andLux, 1996; Holm and others, 1998). Extensive dating of various components of the east-central Minnesota batholith (Holm and others, 2005) show that nearly all of the intrusions in the heart of the ‗Penokean Orogen‘ internal zone are actually between 1,772 – 1,800 Ma (Yavapai), and only the Bradbury Creek granodiorite (1,858 – 1,877 Ma) is Penokean in age. More recently, in-situ U-Pb geochronological studies of hydrothermal xenotime and metamorphic monazite have further documented metamorphic and hydrothermal events related to both the Penokean and Yavapai orogens (e.g. Schneider and others, 2004; Vallini and others, 2007; Holm andothers, 2007). Detrital zircon studies (e.g. Wirth and others, 2006) have provided ages for source materials for many of the units. Ongoing U-Pb zircon dating of thin ash layers and interbedded volcanic rocks within the lower parts of the Animikie basin and the Baraga basin in Michigan (e.g. Schneider and others, 2002; Fralick and others, 2002; Addison and others, 2005) and the important recognition of the Sudbury ejecta layer (e.g. Addison and others, 2005; Jirsa and others, 2008; Cannon and others, 2010) has allowed for confident correlation of the major depositional basins in the region and allowed for a more thorough interpretation of the Paleoproterozoic evolution of the Lake Superior region. For example, Schulz and Cannon (2007), using newly available dates for igneous and metamorphic events in combination with the Penokean-wide Sudbury impact marker layer, have compiled a robust interpretation of the geologic history of the Penokean and Yavapai ILSG 2011 133 Field Trip 8 �events in the Lake Superior region. In their synopsis, the Penokean orogeny began ca. 1,880 Ma via docking of the Pembine Wausau terrane to the southern margin of the Superior craton, resulting in subduction jumping to the south and the development of back-arc basins in both the Superior craton edge and the arc. Extension and subsidence of the back arc basins produced volcanic grabens within a broad, shallow sea into which the major iron formations (i.e. Mesabi-Gunflint, Ironwood, and Negaunee) were formed. The iron formations at the south edge of the basin (e.g. Marquette and Gogebic ranges) are coeval with volcanic rocks whereas deposition at the north edge of the basin (e.g. Mesabi and Gunflint ranges) was distal to the volcanic activity. At around 1,850 Ma (a time for which the Sudbury impact layer provides a basin-wide marker horizon; Figure 2), a fragment of Archean crust (Marshfield terrane in Wisconsin) arrived at the subduction zone and converged with the Superior craton, causing subsidence in front of the collision zone, and forming a broad foreland basin. Some parts of the basin saw uplift and possible subaerial exposure of the iron formation for a time, prior to inundation and burial by a transgressive sediment sequence (Animikie and Baraga basins). Sedimentation into the southern part of the Animikie foreland basin began at 1,850 Ma and ended by 1,833 Ma, whereas in the northern part of the basin sedimentation did not start until ca. 1,835 Ma and continued until at least 1,770 Ma. Tectonic thickening produced medium-grade metamorphism of the sediments, which continued at the south edge of the basin until ca. 1,830 Ma when the deformed sediments and accreted arc terranes were intruded by post-tectonic granitic rocks (Van Schmus, 1976), marking the end of Penokean orogenesis. Younger tectonic and thermal events that overprint the Penokean orogen, previously ascribed to the Penokean orogeny, are now known from metamorphic and thermochronologic studies to be Yavapai-interval events that post-date the Penokean by tens of millions of years (e.g. Holm and others, 2007; Tohver and others, 2007). Renewed Yavapai-interval activity that peaked in mid-geon 17 produced doming of the Archean crust (e.g. McGrath Gneiss dome in Minnesota and other gneiss domes in Michigan – the ‗gneiss dome corridor), accompanied by mid-crustal level emplacement of the east-central Minnesota batholith, and widespread resetting of metamorphic ages in the rocks of east-central Minnesota. Detrital zircons as young as 1,770 Ma (Heaman and Easton, 2005) in the northern reaches of the Animikie basin (Rove Formation in Ontario) show that deposition continued into the Yavapai-interval accretion event, at least in the upper portions of the basin. Deformation at the southern margin of the Animikie basin may have begun in the late stages of Penokean orogenesis, but likely continued during geon 17 (Scheiner, Boerboom, and Holm, 2011). ILSG 2011 134 Field Trip 8 �Figure 2. Correlation diagram of Paleoproterozoic strata across Minnesota, Wisconsin, and Michigan, showing the 1,850 Ma Sudbury impact layer timeline. From Cannon and others, 2010. Figure 3. General geologic map of eastcentral Minnesota showing the major subdivisions of the Penokean Orogen. Red box outlines field trip traverse. ILSG 2011 135 Field Trip 8 �LITHOLOGIC UNITS Animikie Basin Foredeep The Animikie basin is conceptualized as a foreland basin that developed in front (north) of the fold and thrust belt (everything south of the Animikie basin in Figures 1 and 3). It includes the main depositional basin as well as two outliers that probably represent erosional remnants of a larger, formerly continuous area. Strata of the Animikie basin unconformably overlie both Archean crust and deformed rocks of the Penokean Orogen fold and thrust belt (e.g. the Cuyuna North Range). The strata at the north edge of the Animikie basin are essentially undeformed, with a gentle south dip, whereas at the southern margin they are folded into a series of generally upright and open, east-west trending folds (stop 89). Slaty cleavage associated with folding first appears about 15 km south of the Mesabi Iron Range, and increases in strength southward in conjunction with progressively tighter folds (Southwick and others, 1998). A narrow belt along the southern edge of the basin contains an early bedding-parallel metamorphic foliation that is folded into upright folds which are apparently coaxial with folds related to the single deformation to the north. The Animikie Basin is composed of a thin lower unit of quartzite and siltstone (e.g. the Pokegama quartzite on the Mesabi Iron Range; Figures 1, 2, 3, 5) overlain by the Biwabik and Gunflint Iron Formations, and a thick upper sequence of turbidites and shale (the Virginia-Thomson Formations and related outliers, and the Rove Formation). The Pokegama quartzite onlaps ca. 2,700 Ma Archean granite-greenstone terrane to the north, and contain only locally-derived 2.9 – 2.6 Ga detrital zircons eroded from adjacent Neoarchean crust (Wirth and others, 2006). In Ontario, the 1,850 Ma Sudbury ejecta layer (Addison and others, 2005) lies above an 1,878±1.3 Ma volcanic ash layer near the top of the Gunflint Formation (Fralick and others, 2002) and below an 1,836±5 Ma ash layer near the base of the Rove Formation (Addison and others, 2005). In addition an ash layer near the base of the Virginia Formation on the Mesabi Iron Range in Minnesota yields a U-Pb zircon age of 1832±3 Ma (Addison and others, 2005). The nearly 40 m.y. hiatus at the north side of the basin is ca. 15 m.y. greater in duration than the same hiatus associated with the same unconformity at the south side of the basin in Michigan, which was more proximal to the Penokean tectonic front. (Shulz and Cannon, 2009). The position of the Mille Lacs Group and Cuyuna Range strata in Minnesota relative to deposition of the Animikie basin fill is still unresolved, but geophysical evidence implies that the Cuyuna North Range Group continues beneath, and is unconformably overlain by, strata of the Animikie Group (Figures 3 and 5). Detrital zircon studies to date from turbiditic sediments at the southern edge of the Animikie basin in the Thomson Formation (e.g. stop 8-9 of this trip) yield ages mostly in the 2.05 – 1.80 Ga range (Wirth and others, 2006). They report similar ages for detrital zircons from the Rove Formation in northeastern Minnesota and from the Tyler Formation in northern Wisconsin. Heaman and Easton (2005), obtained detrital zircon ages as young as 1,777 Ma from near the top of the Rove Formation in Ontario, approximately 400 m above the base (Maric and Fralick, 2005). This age overlaps with the age of the east-central Minnesota batholith in Minnesota (Holm and others, 2005), and indicates that sedimentation, at least in the uppermost strata of the Rove portion of the Animikie basin, was synchronous with Yavapai magmatic activity to the south. Using the interpretation that the Penokean orogeny ended at ca. 1,830 (Schulz and Cannon, 2009), these data imply that much of the deposition into the basin may have occurred during Yavapai time. ILSG 2011 136 Field Trip 8 �Thomson Formation The Thomson Formation is correlative with the Virginia Formation, but is situated at the southeastern margin of the Animikie Basin (Figures 1, 3, and 5). As reported by Morey (1978), the type locality of the Thomson Formation (stop 8-9) is composed of 34-46 percent graywacke, 27-39% siltstone, and 27% slate. The Thomson Formation has been metamorphosed to the greenschist facies, and is cut by a series of northeast-trending diabase dikes that are presumed to be associated with the Midcontinent rift system. The bulk of the Thomson Formation is deformed into a series of open, east-trending folds with axes that dip vertical to steeply south and plunge shallowly to the east and west. However, Holst (1984) recognized a ‗southern structural terrane‘ which contains an early, near bedding-parallel, S1 foliation that is inferred to be parallel to poorly exposed recumbent isoclinal folds, and the entire area is proposed to be on the upper limb of a large recumbent fold. This early fabric is refolded along open and upright folds having axial traces that plunge gently both east and west, which has crenulated the S1 cleavage and produced an S2 cleavage. The second generation of folds has the same style and orientation as those to the north, where only one generation of folding (regional D2) has produced a single slaty cleavage. The boundary between the once- and twice-deformed rocks has been interpreted by Holst (1984) as the edge of a thrust nappe, south of which the rocks contain two deformations and north of which the rocks were only affected by the second deformation. However, recent mapping (e.g. Boerboom, 2009) across this transition shows that there seems to be a progressive decrease in metamorphic grade from south to north, and the transition between twicedeformed and once-deformed rocks (e.g. Figure 3) is not easily demarcated. The twicedeformed ‗southern structural terrane‘ includes rocks interpreted to be part of the Penokean fold and thrust belt as well as the southern edge of the Thomson Formation/Animikie basin, without a clear-cut structural front to separate the two entities. An alternate explanation, based on magnetic and petrofabric studies, proposes that the early subhorizontal S1 cleavage in the southern zone formed at the same time as the single subvertical cleavage in the northern zone (Sun and others, 1995 and references therein). Their paleomagnetic studies showed that the natural remnant magnetism of the Thomson Formation record the field during deposition and compaction, and was not reset during deformation. This field is recorded by ferromagnetic minerals (e.g. hematite). Anisotropy of magnetic susceptibility, recorded primarily by the preferred orientation of metamorphic chlorite, records minima perpendicular to the single upright cleavage in the northern zone (i.e. subhorizontal), whereas it is normal to the early S1 cleavage in the southern zone (i.e. subvertical: Figure 4). They invoke a model, based on studies of other foreland orogens, involving simple shear on subhorizontal planes (south) transitioning into pure shear with subhorizontal shortening (north). Late-stage regional shortening produced the second deformation, which shows up preferentially in the southern zone due to favorable orientation there of the S 1 cleavage perpendicular to compression, whereas in the northern zone the later deformation is not apparent because the preexisting fabric was already oriented subparallel to the later compressional event. This model may be a means to explain the transitional nature observed between the northern and southern (once- and twice-deformed) panels of the Thomson Formation (e.g. between stops 8-8 and 8-9 of this trip), where the strength of S2 cleavage in the southern zone seems to progressively weaken to the north. However, the samples for the paleomagnetic ILSG 2011 137 Field Trip 8 �study were confined to the immediate area of the transition, and do not include the highergrade metamorphic facies to the south. The promising use of future paleomagnetic studies should incorporate samples from the higher-grade terrane to the south. Figure 4. Alternative model for explaining the contrasting deformational history of the northern vs. southern zones of the Thomson Formation. From Sun and others, 1995. On prior geologic interpretations, the southern margin of the Thomson Formation was considered to be the point at which the large area of featureless aeromagnetic signature associated with the Animikie basin ends, and where characteristically ‗busy‘ signatures associated with the Mille Lacs Group begins. However, based on remapping and relogging of drill cores and cuttings, the southern margin of the Thomson Formation is now considered to include an east-west belt marked by moderately high gravity in combination with an eastwest belt of discontinuous linear aeromagnetic anomalies (Figure 5). This linear zone wraps southwest into apparently tightly-folded strata between rocks of the Cuyuna South Range and the Mille Lacs Group, and the argument could be made that the entire Cuyuna South Range is the continuation of this southern-most belt now mapped as Thomson Formation. The discontinuous lenses shown as ‗sulfidic horizons‘ in the base of the Thomson Formation on Figure 3 are characterized by large cubic pyrite porphyroblasts. Most of these zones are confined to the very base of the Thomson, but some lenses occur as far as 4 km north of the ILSG 2011 138 Field Trip 8 �base, well within classic Thomson Formation. In addition to the sulfidic horizons, this lower unit includes generally thin units of chert, and minor amounts of mafic hypabyssal intrusions (Boerboom, 2009). These sulfidic horizons may be similar to those in the base of the Baraga basin in Michigan, e.g. the Bijiki Iron Formation. Figure 8-5. First-vertical derivative aeromagnetic anomaly map superimposed on secondverticalderivative gravity. Note the moderate gravity high that along the southern margin of the Animikie basin that extends southwest into rocks currently mapped as Cuyuna South Range. Compare to Figure 3. Fold and Thrust Belt The fold and thrust belt (Figure 3) shows decreasing metamorphic grade and structural complexity (i.e. a decreasing depth of tectonic burial) from south to north. It contains a complex assemblage of thrust-stacked, folded and faulted metasedimentary and metavolcanic rocks and associated hypabyssal mafic sills, which overall are of low to moderate metamorphic grade except for the southern margin which have been metamorphosed to the amphibolite facies. The medial zone includes the South range of the Cuyuna district and the Mille Lacs Group and associated rocks (Figure 1, 3 and 5), which encompass a broad area that extends from the Moose Lake area south of the Animikie basin to as far southwest as Stearns County. Iron-formations in the medial zone are closely associated with mafic volcanic rocks and euxinic shale. The external zone, which forms the ILSG 2011 139 Field Trip 8 �northwest-most panel and includes the Cuyuna district North range, is composed of folded and weakly metamorphosed strata including the Mahnomen, Trommald, and Rabbit Lake Formations, but also includes a substantial proportion of mafic volcanic rocks. Ironformations (mainly the Trommald) are associated with fine-grained argillaceous rocks as well as mafic volcanic rocks. Aeromagnetic data (Figure 5) and geophysical models clearly indicate that tightly folded strata of the Cuyuna North range district continue to the east beneath unconformably overlying strata of the Animikie group, a relationship that has been verified by geophysical models (Carlson, 1985). Also clearly visible on Figure 5 is the truncation of magnetic anomalies associates with the Mille Lacs Group and Cuyuna South Range by the overthrust Little Falls Formation, which is magnetically quiet. Denham and Little Falls Formations The Denham Formation (Morey, 1978) is composed of a heterogeneous mixture of interbedded pelitic to carbonate-cemented, arenitic arkosic sedimentary rocks, calc-alkaline pillow basalt, dolostone, and fragmental volcanic rocks. This sequence nonconformably overlies the Archean McGrath Gneiss, and has been subjected to amphibolite-facies metamorphism. Drill holes (cores and cuttings) from north and east of the Denham locality show that the Denham is transitional upward into graywacke now inferred to be the eastern extension of the Little Falls Formation (Figure 1, 3, and 5), with the transition marked by a 1m thick zone of graphitic argillite. Aeromagnetic and widely scattered outcrop data indicate that the Denham Formation extends westward along the north edge of the McGrath Gneiss to Mille Lacs Lake, and the Little Falls Formation continues west and south a considerable distance past Mille Lacs Lake. Old drilling records and corroborating aeromagnetic anomalies also indicate that infolded remnants of the Denham Formation are present within the McGrath Gneiss (Figures 3 and 5). Stops 8-1 through 8-3 of this field trip will cover the transition from the McGrath Gneiss to the base of the Little Falls Formation. The age of the Denham Formation is constrained by the underlying McGrath Gneiss (2,557 15 Ma; Figure 6; Holm and others, 2005). The Denham Formation and overlying Little Falls Formation are interpreted as a panel thrust northwest over the Mille Lacs Group, which is cut by a granite stock (Mille Lacs granite; Figure 3) that yields a U-Pb zircon age of 2,009 Ma (Holm and others, 2005). Volcanic rocks in the Denham Formation have provided a Sm-Nd isochron age of 2197 39 Ma (Beck, 1988). Thus, the age of the Denham Formation is poorly constrained to somewhere between 2,550 to possibly as young as 2,009 Ma, depending upon the relationship to the Mille Lacs Group. Monazite U-Th-Pb singlespot electron microprobe analyses and single-spot ion microprobe 207Pb/206Pb analyses of samples from the Little Falls Formation yield mainly 1,741 – 1,776 Ma Yavapai-interval ages, and a sample from stop 8-3 of this trip also yielded an age of 1,844 7 Ma in addition to a 1,788 4 age, thus this location records both Penokean and Yavapai metamorphic events (Schneider and others, 2004; Holm and others, 2007). ILSG 2011 140 Field Trip 8 �Figure 6. Locations of various types of age date determinations in east-central Minnesota. Base map is same as in figure 8-3. Rocks of the Denham and Little Falls Formations have undergone regional amphibolite-grade metamorphism and at least two periods of deformation attributed to Penokean and Yavapai Orogenic events. The first deformation event was synchronous with metamorphism to the garnet zone of the amphibolite facies (Holm, 1986). It produced an early S1 foliation that is typically bedding-parallel, and a locally strong, shallowly plunging, stretching lineation (Figure 7a). S1 foliation and S0 bedding were subsequently folded along steeply dipping axes (Figure 7b), concurrent with or followed by ca. 1,760 Ma peak metamorphism that produced staurolite. In the Denham valley (stop 8-2), the stratigraphic sequence dips variably north, having local F2 folds with overturned limbs. North of the valley in the overlying Little Falls Formation (stop 8-3) bedding and S1 in metagraywacke are nearly horizontal, and are deformed into open F2 folds with local crenulation features. Farther north the bedding dips mostly south, defining a broad, regional-scale, upright F2 syncline. ILSG 2011 141 Field Trip 8 �Figure 7. Equal area, lower hemisphere projections of structural elements of the Denham Formation. A. Measured lineations; B. Poles to measured foliations. Open diamonds represent the main, early foliation, which has been refolded by a second deformation that produced only a weak north-dipping foliation (crosses). Despite deformation and metamorphism, the stratigraphy of the Denham Formation forms a coherent package that is shown schematically in Figures 8. Omitting the prefix 'meta' for clarity, the base of the Denham Formation consists of interbedded siltstone and cross-stratified pebble conglomerate. This is overlain by coarse-grained and locally conglomeratic arkose that apparently pinches out laterally. The primary clastic grains in the dolomitic arkose are well preserved owing to the abundance of dolomite in the matrix, which accommodated most of the deformation. The arkose is interbedded with amygdaloidal basalt flows that contain well-preserved primary macroscopic flow features. The flows grade stratigraphically upward (northward) from massive bases to pillowed interiors to fragmental upper portions. The volcanic flows are thickest at the eastern outcrop limit, where at least four flows of nearly 1000 feet total thickness were recognized, and thin westward to two flows of 300 feet total thickness. An upper sequence of mafic fragmental volcanic rocks nearly merges to the east with the pillow basalts (see Figure 10 under stop 8-2), possibly implying that later stages of volcanic activity were more explosive in nature and that both volcanic horizons emanated from a common center to the east, which may presently be buried beneath the Mesoproterozoic Fond du Lac Formation. Arkosic and pelitic strata (now staurolite-garnet-mica schist) between the flows and fragmental volcanic sequence pinch out to the east where the flow package thickens, and are not present in drill holes to the north and east of the Denham valley. The northern-most outcrops in the valley consist of very pure dolomite, now marble, which contains ptygmatically folded and strongly lineated quartz veins. Drill cores show that the dolomitic marble is at least 500 feet thick, and is overlain by graywacke (e.g. stop 8-3) that is exposed discontinuously to the north for some distance. A thin layer of graphitic argillite marks the contact between the dolomite and overlying graywacke. ILSG 2011 142 Field Trip 8 �Figure 8. Schematic stratigraphic section of the Denham Formation, showing relative location of field trip stops. Field and petrographic observations imply that clastic detritus in the Denham Formation was derived in large part from a weathering residuum on the subjacent McGrath Gneiss. Near the contact with the Denham Formation, the McGrath grades abruptly from granite gneiss containing quartz, orthoclase, plagioclase, and biotite to strongly foliated, quartz- and sericite-rich schist that contains orthoclase, but no plagioclase. The arkosic parts of the Denham Formation similarly lack plagioclase, and are composed of quartz and orthoclase grains, together with small clasts of granitic gneiss. Studies of saprolite developed beneath Cretaceous sedimentary rocks on Precambrian crystalline rocks in southwestern Minnesota may provide an analog (Setterholm and others, 1989). These studies demonstrate that plagioclase and mafic minerals such as biotite and hornblende are the first minerals to alter to clay during the weathering process, and that orthoclase and quartz are the most resistant to weathering. The basal Cretaceous strata locally consist of reworked saprolite, including beds of cross-stratified sandstone and nearly pure kaolinitic shale. Exposures of basal Cretaceous sedimentary rocks locally contain detrital orthoclase and quartz derived by slight reworking of weathered granite grus, set in a carbonate matrix. Similar processes apparently occurred in the Paleoproterozoic by erosion and reworking of weathered McGrath Gneiss into beds of calcareous arkose and kaolinitic shale. These were subsequently metamorphosed to produce recrystallized dolomitic arkose and staurolite-garnet-sericite ILSG 2011 143 Field Trip 8 �schist. Slight weathering of orthoclase may have liberated potassium for the inferred conversion of kaolinite to sericite during metamorphism. The Denham Formation is interpreted to represent a rift-margin assemblage deposited during the Paleoproterozoic, similar to and perhaps temporally equivalent to the Chocolay Group in Michigan. In this setting, the McGrath Gneiss was part of the continental margin that was weathered and eroded to provide detritus to an evolving rift basin undergoing active, shallow water volcanism. Interbedded arkose and dolomite higher in the stratigraphic section represent foundering of the shelf and deepening water, possibly by subsidence of localized grabens. The paucity of clastic material in the upper, dolomite-dominated part of the sequence indicates that deposition of coarse detritus was restricted to the shallow, nearshore environment adjacent to the McGrath Gneiss. The stratigraphically upward sedimentological gradation of dolomite to graywacke (aka Little Falls Formation) indicates further deepening water and associated turbidite deposition. The deformation of the Denham Formation is inferred to be the product of basin closure during the Penokean Orogeny, later influenced by deformation related to Yavapai orogenic activity which produced doming of the underlying McGrath Gneiss. McGrath Gneiss Dome (gneiss dome/plutonic terrane) The Neoarchean McGrath Gneiss occurs within the internal zone of the Penokean orogen as discussed prior (Southwick and others, 1988). The McGrath Gneiss is exposed in several localities between Denham and Mille Lacs Lake to the west. Although this unit locally contains layered gneissic structure, it is best characterized as a metamorphically foliated porphyritic granite. The 2557 Ma McGrath Gneiss is only slightly younger than the undeformed 2600 Ma Sacred Heart Granite of the Minnesota River Valley in southwestern Minnesota (Doe and Delevaux, 1991). Holm and Lux (1996) interpreted the basement-cover contact of the McGrath gneiss dome to be a detachment fault because of an apparent 50 m.y. cooling age difference between basement and cover rocks. However, more recent detailed Ar ion laser data show that the basement and cover rocks have essentially the same lower temperature thermal history (Figure 9; McKenzie, 2004). Also, as discussed under stop 8-1, field and petrographic evidence (Boerboom and others, 2001) indicates that the McGrath Gneiss had a saprolite developed on it at the time of deposition of the overlying Denham Formation. Together, these data indicate that the boundary may simply be a domed nonconformable contact. Further evidence of this lies in the inlier of Denham Formation that is infolded into the top of the McGrath Gneiss dome (e.g. Figure 3). The McGrath Gneiss dome and adjacent Denham Formation represent the culmination of the north to south increase in metamorphic grade, from greenschist facies at the base of the Animikie basin to upper amphibolite facies. This southward increase in metamorphic grade is considered to reflect an increase in depth of exposure from the foreland basin at the north to the deeply eroded crust of the Penokean orogen at the south, related to geon 18 accretion. Plutonic activity in east-central Minnesota between 1,800 – 1,772 Ma, likely due to Yavapai-interval convergence (Holm and others, 2005) was more or less concurrent with formation of the McGrath Gneiss dome, and with widespread regional ca. 1,750 Ma regional metamorphism. ILSG 2011 144 Field Trip 8 �Figure 9. Spot age data (in Ma) of muscovite grains from Ar ion laser data (McKenzie, 2004). A. McGrath gneiss basement near stop 8-1; B. metapelite cover rocks at stop 8-3. Malmo Structural Discontinuity (MSD) The internal zone of the Penokean Orogen (the gneiss dome – plutonic terrane and the Denham-Little Falls Formations) are separated from the Penokean fold and thrust medial zone (e.g. Mille Lacs Group) by the Malmo structural discontinuity (originally termed the Malmo thrust), inferred to be a major, south-dipping fault (Wunderman and Young, 1987). The western end of the Malmo structural discontinuity (MSD: figure 1, 3, and 5) separates relatively low metamorphic grade sedimentary and volcanic rocks to the north from relatively high-grade metamorphosed granitic and sedimentary rocks and Yavapai intrusive igneous rocks to the south. The western portion of the MSD is clearly marked by the sharp truncation of linear aeromagnetic anomalies associated with thin iron-formations in the Cuyuna south range and Mille Lacs Group, indicating that the ‗internal‘ zone containing the Little Falls panel was thrust west-northwest over lower-grade rocks of the medial zone. Also, recent metamorphic geochronology indicates that the ages of metamorphism differ across the MSD (geons 17 and 18 to the north and dominantly geon 17 to the south). These data all imply that the MSD is a post-Penokean geon 17 structure along which the gneiss dome – plutonic terrane (i.e. the Penokean orogen internal zone and the McGrath Gneiss dome) was uplifted and subsequently exhumed, most likely during or after emplacement of the Yavapai-interval east-central Minnesota batholith. East of Mille Lacs Lake, the location of the MSD is more obscure due to the lack of contrast in geophysical data. Historically the eastern end of the MSD has been placed at the northern margin of the McGrath Gneiss. However, more recent mapping (e.g. Boerboom, 2009) indicates that the MSD may lie a short distance north of the McGrath Gneiss (Figure 3). The graywacke unit (stop 8-3), which overlies the Denham Formation (stop 8-2), contains the same metamorphic and lithological attributes as the Little Falls Formation, and if this correlation is correct the MSD must lie north of the Denham and Little Falls Formations, an observation supported by outcrop and drill core lithologies. South of where the MSD is currently mapped the rocks are composed of only metagraywacke and related rocks with no interbedded volcanic rocks. North of where the MSD is mapped, the bedrock types are a ILSG 2011 145 Field Trip 8 �diverse assemblage of carbonate, iron formation, graphitic schist and phyllite, and mafic intrusive and volcanic rocks. West-northwest-directed thrusting may have also caused more displacement and hence a greater degree of exhumation to the west, whereas to the east the MSD fades into more of a strike-slip regime, with less vertical displacement and hence less contrast in metamorphic grade. FIELD TRIP STOPS The general geologic setting and locations of stops are shown on Figure 1. A detailed location map is included with each stop description, with a base made at scale from a portion of the appropriate 7.5‘ quadrangle map. The UTM coordinates are reported in NAD83, zone 15. The regional setting of the various rock units is given in the introduction, and only facts pertinent to each specific stop are given below. --From the parking area near Carlton go west on Highway 210 through Carlton then south on I-35 to the Willow River exit (no. 205) – a distance of approximately 34 miles. Go west to Willow River, turn north on Highway 61 for 3 miles to intersection with State Road 52, go west on 52 for approximately 6.2 miles (road changes from pavement to gravel). The road makes a right angle turn to the north towards the town of Denham, but instead turn south and walk or drive on small dirt path to gate. Proceed past gate into open pasture until road crosses a gentle rise and west end of outcrop (stop 8-2a). Outcrop is 0.65 miles south of 52. ILSG 2011 146 Field Trip 8 �Road map showing stop locations Stop 8-1: McGrath Gneiss and contact with overlying Denham Formation Location: T.45N., R.21W., Sec. 36 Denham 7.5’ quadrangle UTM 503979 easting; 5132367 northing HIGHLIGHTS: ARCHEAN MCGRATH GNEISS AND METAMORPHOSED SAPROLITE (AMG), CONGLOMERATE AT BASE OF PALEOPROTEROZOIC DENHAM FORMATION (PPDS). Note:Stops 8-1, 8-2, and 8-3 are shown in apparent stratigraphic context on Figure 8. These stops will examine in order the north-facing transition from the Neoarchean McGrath Gneiss (stop 8-1), to the overlying Paleoproterozoic Denham Formation (stop 8-2), and then to graywacke of the Little Falls Formation that stratigraphically overlies the Denham Formation (stop 8-3). ****Stops 8-1 and 8-2 are on PRIVATE PROPERTY. Please procure permission from landowner before entering**** At this locality, the northernmost outcrops of the McGrath Gneiss (metamorphosed porphyritic granite) grade abruptly from the typical mineral assemblage of quartz, orthoclase, plagioclase, and biotite to strongly foliated, quartz- and sericite-rich schist that contains coarse relict orthoclase, but no plagioclase. The latter assemblage, present just below the base of the Denham Formation, may be the product of metamorphism of partially weathered granite, in which the plagioclase feldspar had weathered to clay, but quartz and potassium feldspar remained fresh, analogous to the weathering patterns noted in younger saprolites that underlie Cretaceous rocks in Minnesota. A sample of McGrath Gneiss from 20 miles southeast of this locality yielded a U-Pb zircon age of 2557±15 Ma. (Holm and others, 2005). Detrital zircons from the metasedimentary rocks just above the McGrath Gneiss contact (see next paragraph) yield ages as young as 2,072±17 Ma and as old as 3,447±17 Ma, with spikes around 2,625, 2,900, and 3,300 Ma. Detrital zircons from stratigraphically higher dolomitic arkose (stop 8-2) yield ages as young as 2,173±11 Ma and as old as 3,505 ±8 Ma, with groups around 2,600, 2,800, and 3,360 Ma (Wirth and others, 2006). Just north of the McGrath gneiss is a thin layer of very poorly exposed biotite schist of sedimentary protolith which contains irregular lenses of coarse conglomerate with abundant pebbles of quartz and pink orthoclase, but no plagioclase (unit Ppds onFigure 10). Similarly, ILSG 2011 147 Field Trip 8 �the clastic component of the arkosic beds stratigraphically higher in the Denham Formation (stop 8-2b) is comprised almost entirely of quartz and orthoclase feldspar with essentially no plagioclase feldspar. Together, these observations are consistent with the interpretation that the McGrath gneiss was weathered to the point that plagioclase and mafic minerals had altered to clay minerals, and this weathered residuum acted as a source of detritus for the overlying Denham Formation. Figure 10. Bedrock geologic map of the Denham Formation type locality, showing stop locations. Black circles are locations of exploratory drill holes and gray areas are mapped outcrops. McGrath Gneiss-Amg; Denham Formation-Ppds (metasiltstone), Ppda (metadolomitic arkose), Ppdb (metabasalt), Ppdg (metapelite), Ppdv (meta-fragmental mafic volcanic rocks), Ppdm (dolomitic marble); Little Falls Formation equivalent metagraywacke – Ppgs. --Return to vehicles. Drive back east 0.75 miles on State road 52 to a small gated trail to the south. Walk south down trail for approximately 0.5 miles following edge of ravine to south-most outcrop on west side of ravine (see inset map). This stop will traverse from south to north over outcrops located along west edge of ravine. There are also outcrops on the east side of the ravine that show parts of the stratigraphy not exposed on the western valley traverse. **Private Property ** ILSG 2011 148 Field Trip 8 �Stop 8-2: Denham Formation Location: T.45N., R.21W., Sec. 25, SE 1/4 Denham 7.5’ quadrangle UTM Start: 505406 easting; 5132575 northing UTM end: 505208 easting, 5133330 northing HIGHLIGHTS: INTERBEDDED BIOTITE SCHIST(SILTSTONE), PILLOW BASALT, DOLOMITIC ARKOSE, AND DOLOMITIC MARBLE. From bottom to top, the Denham Formation consists of fine-grained metamorphosed siltstone (Ppds), dolomite-cemented arkosic arenite with rare cobbles of McGrath gneiss (Ppda), pillowed basalt flows (amphibolite; Ppdb), shale (staurolite-garnet-mica schist; Ppdg), interbedded dolomitic arkose and dolostone (Ppda), fragmental basaltic volcanic rocks (amphibolite; Ppdv), and at the top of the section, pure dolostone (marble; Ppdm). See Figure 8 (repeated below) for stratigraphic section of the Denham Formation. A. (South most). Gray biotite schist interpreted as metamorphosed siltstone (unit Ppds), overlain by brownish-tan weathered dolomitic arkose (unit Ppda). B. Pillow basalt flows (unit Ppdb). Classic basalt flow sequences, with massive bases that grade upward (north) into amygdaloidal pillow basalt, which in turn grades to coarsely fragmental or blocky flow tops. The fragmental and amygdaloidal character of the flows indicates eruption into a shallow-water environment. Two distinct flows are present in the valley, but in hills to the east where the unit thickens, as many as four flow sequences were identified. Strong stretching lineation evident in fragmental portions of flow is accentuated by differential weathering between clasts and matrix. Basalt geochemistry shows slight calc-alkaline tendencies (Southwick and others, 2001). Outcrop gap is inferred to represent garnet-staurolite-mica schist (unit Ppdg) that is exposed both east and west of this gap. C. Dolomitic arkose (unit Ppda). Weathered reddish-tan color due to abundant dolomite in matrix. Coarse sand-sized detrital grains of orthoclase and quartz, and rare cobbles identical to the McGrath Gneiss. The original clastic grains in the arkosic rocks are well preserved despite metamorphism, largely because strain has been partitioned into the comparatively more ductile dolomitic matrix. As discussed at the McGrath gneiss stop (8-1), the arkosic rocks contain almost no plagioclase, presumably due to sediment derivation from a weathered McGrath Gneiss source. ILSG 2011 149 Field Trip 8 �Figure 11. Photomicrograph of dolomitic arkose from Stop 8-2C. D. Fragmental volcanic rocks (unit Ppdv). Dark green amphibolitic clasts and scattered clasts of white cherty material are strongly flattened and lineated. In map view, this fragmental unit nearly merges to the east with the underlying pillow basalt, possibly indicating a common volcanic source located to the east beneath the Keweenawan Fond du Lac Formation. Holm and others (1993) obtained Ar/Ar plateau ages of 1,756 ±16 Ma on a hornblende separate, and 1754 ±13 Ma on a biotite separate from this amphibolitic unit. E. Dolomitic marble (unit Ppdm). Tan weathered, dark gray fresh. In drill holes (cores and cuttings) to the east marble changes from gray to white with depth, with intercepts of massive dolomite as much as 500 feet. ILSG 2011 150 Field Trip 8 �Figure 7 repeated. Stratigraphic section of the Denham Formation, showing relative locations of field trip stops. Age of McGrath gneiss from nearby outcrop, and age ranges of detrital zircons shown in boxes. ---Return to vehicles. Drive east approximately 0.75 miles on State road 52 to a “T” intersection with gravel road to the north. Go north on gravel road 0.65 miles to junction of the Soo Line Trail – a former railroad grade now utilized as an ATV trail. Park at trail junction and walk northeast on trail about 500 feet to outcrops and low road cuts. Stop 8-3: Metagraywacke and argillite Location: T.45N., R.20W., Sec. 19, SE 1/4 Denham 7.5’ quadrangle UTM: 506343 easting; 5134441 northing HIGHLIGHTS: GRADED BEDS, METAMORPHIC ASSEMBLAGE ILSG 2011 151 Field Trip 8 �Railroad cuts and flat outcrops of metagraywacke with pelitic beds. Pelitic units are micaceous and contain small garnet and staurolite crystals, although the latter are more abundant further to the northeast. Abundant 1 mm diameter garnets are synkinematic with respect to S1, and have a well-developed internal schistosity defined by quartz and ilmenite inclusions (Figure 12). At this locality, inclusion-rich cores are surrounded by haloes of inclusion-free garnet, and staurolite has overgrown both the schistosity and the crenulation cleavage. Garnet rim analyses give final equilibration temperatures of 520-590 degrees C and a pressure of ca. 6 kbar (Holm and Selverstone, 1990). Chemically homogeneous monazite grains at this locality give a prominent metamorphic age domain of ca. 1788 4 Ma and a less prominent age domain of 1840 Ma (McKenzie, 2004). In the series of outcrops to the northeast of this stop, bedding and S 1 cleavage are gently folded along a series of shallow east-plunging, northeast-striking F2 fold axes which are oriented similar to those at the last stop but are more open and upright. A series of 10-30 cm-thick graded beds and scoured crossbedding visible in the flat outcrop north of the trail give a sense of younging to the north. Figure 12. Photomicrograph of rolled garnet from stop 83. G – garnet; S – staurolite; B- biotite, matrix is quartz and plagioclase. Drill cores from approximately 1 mile southeast of this stop (locations shown on Figure 10) show that this metagraywacke conformably overlies the marble to the south (Stop 8-2). The transition between the graywacke and marble is marked by a thin (1 meter) interval of carbonaceous argillite. ---Return to vehicles. Take State road 52 back east to Highway 61. Turn left (north) on Highway 61 and go approximately 7.5 miles to junction with Highway 27 west at stop light in Moose Lake. Turn left (west) on Highway 27 a short distance to the Soo Line ATV trail; just before trail turn left into parking lot. Walk north along Soo Line trail for approximately 0.2 mile to outcrops along trail. ILSG 2011 152 Field Trip 8 �Stop 8-4: Schist at Moose Lake Location: T.46N., R.19W., Sec. 20, NE 1/4 Moose Lake 7.5’ quadrangle UTM start: 517815 easting; 5144635 northing UTM end: 518205 easting, 5145203 northing HIGHLIGHTS: GARNET-GRADE METAGRAYWACKE AND PELITIC SCHIST; FOLDED BEDDING AND S1 FOLIATION This pelitic schist is part of the same belt of rocks as the last stop, but here is north of the staurolite isograd (McSwiggen, 1987). A well-developed, bedding-parallel, moderately south-dipping S1 foliation is folded by steeply to moderately south-dipping F2 folds which are accompanied by an axial-planar crenulation cleavage (Figure 13). Internal inclusion trails in small (0.5 mm diameter) garnets show as much as 160 degrees of rotation. These garnets give final equilibration temperatures of 440-500 degrees C. The age of metamorphism here is ca. 1830 Ma based on monazite ages obtained from a similar grade rock south of this locality. Figure 13. Photomicrograph of crenulated micaceous phyllitic schist from this stop. Fine-grained micas define S1 fabric which is parallel to bedding; both are folded by D2 deformation. An east-northeast trending diabase dike of presumed Mesoproterozoic age may be visible towards the north end of this set of outcrops, but the exposure has become quite overgrown. The dike is well exposed farther north along this trail but this trip will not go there. ILSG 2011 153 Field Trip 8 �---Return to vehicles and drive back east to Highway 61. Turn left (north) on Highway 61 for approximately 4.5 miles to Barnum, turn right (west) on County Highway 6 for a short distance and park at the Willard Munger trail parking lot. Stop 8-5: Phyllitic schist at Barnum Location: T.46N., R.19W., Sec. 1, NW ¼ and Sec 2, NE ¼. Barnum 7.5’ quadrangle UTM: 523180 easting; 5149900 northing HIGHLIGHTS: BIOTITE-GRADE METAGRAYWACKE AND PELITIC SCHIST; FOLDED BEDDING AND S1 FOLIATION. Fine-grained, phyllitic, silvery micaceous schist metamorphosed to biotite isograd; this location is north of the garnet isograd (McSwiggen, 1987). Contains a strong subhorizontal to south-dipping, approximately bedding parallel foliation that is weakly folded along small east-plunging (5-10 degrees) fold axes. A weak S2 cleavage parallels the fold axes; in thin section a strong S2 crenulation cleavage is obvious (Figure 14). Micaceous minerals are dominantly chlorite and sericite with minor biotite. Figure 14. Photomicrograph of crenulated micaceous phyllitic schist from this stop. Earlier S1 foliation is strongly overprinted and transposed by S2 fabric; S1 is refracted through a sandy bed, in upper right part of photograph. --Return to Highway 61. Turn left (north) on Highway 61 for 6 miles to Mahtowa, continue northeast for another 0.5 miles. Veer left (north) on Brandt Road for 0.8 miles and turn left(west) onto Boundary Road/Township Road 85 after the first curve and proceed to second driveway on the ILSG 2011 154 Field Trip 8 �left (3260 Boundary Road). **Private property permission must be obtained before entering** From farmhouse walk south on trail across pasture to powerline, then east across low ground to knob of white quartz. Stop 8-6: Base of Thomson Formation, mafic intrusion Location: T.47N., R.18W., Sec. 4, NW ¼ and Sec 5, NW ¼. Barnum 7.5‘ quadrangle UTM start: 528370 easting; 515950 northing UTM end: 527540 easting, 5159462 northing Highlights: Chert, pyrite porphyroblasts, metagabbro. This stop is just south of what was ‗traditionally‘ considered to be the south edge of the Animikie basin, but is now considered to be the lowermost part of it, based on remapping and analogues to Michigan. There are a variety of rock types here including recrystallized chert, metagabbro/diorite, and pyritic slate. We will first walk east along the powerline to point ‗A‘ on the above figure, then will go back west to examine outcrops of phyllitic slate (B), and then to outcrops of sheared, porphyritic metagabbro (C). On the index map above the three black dots are the locations of core holes drilled in the 1980‘s by Rocky Mountain Energy. Summaries of those drill core logs are listed at the end of this stop. Substop A: This is a small knob composed mainly of recrystallized black chert that is flooded by white to pinkish-colored, pre-cleavage, quartz veins. A smaller outcrop to the west contains abundant small cubes of oxidized pyrite and is stained yellow and red. Petrographically this chert is composed of fine-grained saccharoidal quartz, minor muscovite, and (where black) abundant submicroscopic dusty opaque inclusions that are not identifiable by optical methods. Scattered rhombic shapes imply that there were dolomite porphyroblasts, which are now replaced by fine-grained quartz. McSwiggen (1987) identified thin intervals of dark gray chert in the drill cores drilled adjacent to this outcrop as being phosphatic. Other drill cores obtained along this lower, basal-most unit of the Animikie basin further west also intersect thin to thick beds of chert interbedded with pyritic slate. Substop B. Between substops A and C are some small outcrops of dark gray, finegrained phyllitic slate/schist in which bedding and S1 cleavage are kinked by a later S2 fabric. Scattered 2-3 cm square pits represent weathered-out cubes of pyrite. Also visible are small dark green clots that may be retrograde-altered porphyroblasts formed during an earlier prograde metamorphic event. The dark retrograded porphyroblasts and the pyrite cubes are both found elsewhere at this same stratigraphic horizon, for example near Kalevala township and near the Tamarack intrusion to the west (Figure 3). The retrograded ILSG 2011 155 Field Trip 8 �porphyroblasts (possibly after garnet or andalusite) contain an earlier, rotated S1 metamorphic foliation; pressure shadows are also evident (see Figure 15). Figure 15. A). Photomicrograph of drill core samples showing rotated, retrograde-altered porphyroblasts similar to those present at this stop. Bedding-parallel S1 foliation is highlighted by black lines and S2 fabric by white lines). B.) Close-up of rotated porphyroblasts. C.) Photograph of drill core from approximately 15 km west of this stop from the same stratigraphic horizon, showing characteristic pyrite porphyroblasts. Substop C: Green, variably shear-foliated metagabbro metamorphosed under greenschist-facies conditions. In less deformed lozenges 5-10% white plagioclase phenocrysts up to 3cm in size are preserved, but most of the phenocrysts are broken and strung out in the foliation plane and appear as flaser-like grains. Schist is composed of sericite, quartz, carbonate, chlorite, sphene/leucoxene, and minor sulfides, and is locally overprinted by rhombic carbonate and rarely, coarse-grained muscovite porphyroblasts. The foliation dip varies from 25 degrees east at the north end of the unit to 60 degree east at the south end; this is at a high angle to the regional fabric and is likely the result of cleavage refraction through this more competent unit. In thin section the main (presumably S1) fabric is weakly folded by a later D2 crenulation cleavage. Summary of drill core logs; locations of core holes shown on location map above: Drill Core MLCH-9 (vertical hole) 39-53 – Thinly laminated graphitic argillite. 53-430 – Strongly sheared/mylonitic, porphyritic metagabbroic rock. Flattened plagioclase phenocrysts give the rock a flaser-gneiss like appearance. Below 395 is generally less deformed. 430-550 – Thinly laminated tightly folded graphite-rich interbedded slate and metagraywacke with pyrite cubes, ankerite clots, and some bedding-parallel pyritic layers. Last 5 feet is strongly graphitic. ILSG 2011 156 Field Trip 8 �Drill Core MLCH-11 (0, -60) 9-36 – Dark gray sooty graphitic phyllitic argillite, variably pyritic, with pyrite cubes and local thin layers semi-massive sulfides. 36-42 – Fine-grained silicic graywacke with white siliceous lapilli-like fragments. 42-45 – Dark grayish-black chert, similar to nearby outcrop. 45-83 – Meta-argillite and graywacke, variably sulfidic and graphitic, minor chert. 83-137 – Variably shear-foliated metagabbroic rock. 137-242 – Fine-grained thinly laminated argillite with carbonate spots, locally graphitic, local layers rich in disseminated sulfides. 242-354 – Variably shear-foliated, porphyritic metagabbroic rock. Drill Core ML-1 (azimuth and inclination unknown; core incomplete) 9-105 – Weakly crenulated phyllitic slate, argillite, and graywacke, locally graphitic with minor stratiform sulfides. ---Return to Highway 61. Turn left (north) on Highway 61 for 6.6 miles to Gillogly Road. Turn right (south) on Gillogly Road and proceed 0.64 miles around curve to small outcrop just north of the crest of the hill, on the east edge of the road.--Stop 8-7: Weakly D2 – crenulated slate and graywacke Location: T.48N., R.17W., Sec. 21, NW 1/4 Iverson 7.5’ quadrangle UTM Start: 537374 easting; 5164270 northing HIGHLIGHTS: S1 FOLIATION AT A HIGH ANGLE TO BEDDING, BOTH AFFECTED BY D2 FOLDING. This stop is near the northern edge of the twice-deformed portion of southern edge of the Animikie basin. Graded beds strike roughly N60W, 60°N, with younging directions upright to the northeast. Bedding can easily be traced by following layers of carbonate concretions. This is one of the rare places where S1 foliation (developed mainly in the upper pelitic portions of the graded beds) is observed at a high angle to bedding. F2 fold axes plunge approximately 15° to N85E. This is one of the key outcrops used by Holst (1984; Figure 16) to conclude that the entire area of twice-deformed metasedimentary rocks is on the upper limb of a large, north-directed thrust nappe. The following sketch is from Holst (1984), showing the relationships between bedding (S0), early S1 cleavage, and later S2 cleavage. ILSG 2011 157 Field Trip 8 �Figure 16. Sketch showing relationships between bedding (S0), early S1 cleavage, and later S2 cleavage for stop 8-7. From Holst, 1984). --Go back to Highway 61 and continue north approximately 3 miles to the junction with Highway 210. Turn right (east) on Highway 210 and go approximately 2 miles into town, then take a right on County Highway 3/1 for 0.6 miles then stay left (east) on County Highway 1 Follow Highway 1 for 1.3 miles then turn left (east) on small road and park at Munger Trail adjacent to Highway 1. Walk north approximately 400 feet on the Munger trail to rock cuts on both sides of trail. --- Stop 8-8: Slate and graywacke with minimal S1 foliation Location: T.48N., R.17W., Sec. 21, NW 1/4 Cloquet 7.5’ quadrangle UTM Start: 545770 easting; 5166742 northing HIGHLIGHTS: VERY WEAK S2 FOLIATION, KEWEENAWAN DIABASE DIKE This stop is the northern-most location within the southern, twice-deformed terrane, and at this point the S1 fabric which is obvious to the south is nearly absent. On the index map above, the dashed line is the break between one deformation (north of the line) and two deformations (south of the line), as mapped by Clark (1985). Here the dominant cleavage is subvertical and parallel to gentle east-west trending F2 fold hinges; the earlier S1 cleavage is very weak and difficult to see. Thin sections show a very weak bedding-parallel foliation cross-cut by the stronger S2 cleavage (Figure 17). Bedding varies from thinly bedded slategraywacke sequences to thicker sandy graywacke beds up to 3 meters thick. Due to the nature of the outcrop the bedding can be hard to observe and a good way to see it is by ILSG 2011 158 Field Trip 8 �observing patterns of cleavage refraction through pelitic and sandy beds, and in a few places graded beds can be seen which show upright younging directions. Carbonate concretions are weakly flattened in the slaty cleavage plane. This stop and the last are both within classic Animikie basin-type sediments, i.e. low-grade graywacke-slate sequences, but unlike the bulk of the Animikie basin to the north, these have been twice-deformed, in the same manner as the phyllites and schists to the south which show progressively higher grades of metamorphism from north to south, as seen on earlier trip stops. The metamorphic grade and style of deformation here is virtually identical to that of the Thomson Formation at the type locality at Thomson Dam, which will be the last stop of this trip. Figure 17. Photomicrograph of slate from stop 8-8 showing very weak early S1 foliation (highlighted by black lines; parallel to bedding which is not visible in this photo), cut by S2 cleavage that is parallel to D2 fold axes (highlighted by white lines). Diabase Dike: Towards the north end of this outcrop the graywacke is cut by a near vertical, 6mwide diabase dike that is oriented approximately N45E. The dike has strongly chilled margins and in the center is fine-medium grained. Petrographic examination shows the chilled margin to be devitrified glass with small spherulitic or variolitic structures; the dike interior is composed of plagioclase, sub-prismatic augite, opaque Fe-Ti oxides, and minor olivine. Approximately 30% of the rock in the dike interior is turbid, opaque-dusted mesostasis that is likely devitrified glass which contains abundant small prismatic pyroxene grains. This dike is part of the Carlton dike swarm; see stop 8-10 for further description. ILSG 2011 159 Field Trip 8 �Stop 8-9: Slate and graywacke of Thomson Formation; Keweenawan diabase dikes Location: T.48N., R.16W., Sec. 5, SW 1/4 Cloquet 7.5’ quadrangle UTM Start: 546612 easting; 5168119 northing HIGHLIGHTS: VARIETY OF SEDIMENTARY STRUCTURES, FOLDS, DIABASE DIKES This is the type locality for the Thomson Formation, which is southern equivalent of the Virginia Formation along the Mesabi Iron Range. The Virginia Formation overlies the Biwabik Iron Formation, and collectively the Virginia/Thomson Formations cover a broad area (the Animikie basin) that extends south from the Mesabi Iron Range to a few miles south of this locality (Figure 1). Deformation of the Animikie strata increases to the south, in closer proximity to the Penokean fold and thrust belt. Here, near the south edge of the Animikie basin, the strata are folded into a series of gently east- and west-plunging, open symmetric to locally asymmetric, slightly overturned folds with near-vertical axial-planar cleavage. Several northeast-trending diabase dikes of Mesoproterozoic age cut the Thomson Formation. These dikes are part of the Carleton County dike swarm, which is one of a number of dike swarms that flank the Midcontinent rift system. The dikes here were emplaced along northeast-trending joints in the graywacke, and they exhibit well-developed subhorizontal columnar cooling joints and have chilled margins. The approximately 2 meter wide dike just below the parking area is reversely polarized, and like most of the dikes in the Carlton County swarm has a composition of high Fe-Ti continental tholeiite (Green and others, 1987). Overall, dikes of the Carlton swarm range from centimeters to greater than 60 meters in width. Narrow contact metamorphic albite-epidote hornfels are found adjacent to the contacts of only the thicker dikes. Green and others (1987) have summarized the geologic, geochemical, and paleomagnetic data on the dikes, and has demonstrated that these swarms are most likely Mesoproterozoic in age. ---Return to your vehicles in the parking lot to end the trip.--- ILSG 2011 160 Field Trip 8 �REFERENCES Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Davis, D.W., Kissin, S.A., Fralick, P.W., and Hammond, A.L., 2005, Discovery of distal ejecta from the 1850 Ma Sudbury impact event: Geology, v. 33, p. 193-196. Beck, J.W., 1988, Implications for Early Proterozoic tectonics and the origin of continental flood basalts, based on combined trace element and neodymium/strontium isotopic studies of mafic igneous rocks of the Penokean Lake Superior belt, Minnesota, Wisconsin, and Michigan: Minneapolis, University of Minnesota Ph.D. dissertation, 273 p. Boerboom, T.J., 2001, Bedrock geologic map and sections, pl. 2 of Boerboom, T.J., project manager, Geologic Atlas of Pine County, Minnesota: Minnesota Geological Survey County Atlas C-13, pt. A., 7 pls., scale 1:100,000. Boerboom, T.J., 2009, Plate 2 – Bedrock Geology, pl. 2 of Setterholm, D.R., project manager, Geologic Atlas of Carlton County, Minnesota, Minnesota Geological Survey County Geologic Atlas Series C-19, pt. A, 7 pls, scale 1:100,000. Boerboom, T.J., Runkel, A.C., and Chandler, V.W., 2001, Bedrock geology of Pine County, in Boerboom, T.J., project manager, Contributions to the geology of Pine County, Minnesota: Minnesota Geological Survey Report of Investigations 60, p. 1-18. Boerboom, T.J., Southwick, D.L., and Severson, M.J., 1999, Bedrock geology of the Mille Lacs Lake 30 x 60 minute quadrangle, east-central Minnesota: Minnesota Geological Survey Miscellaneous Map M-100, scale 1:100,000. Cannon, W.E., Schulz, K.J., Horton, J.W. Jr., and Kring, D.A., 2010, The Sudbury impact layer in the Paleoproterozoic iron ranges of northern Michigan, USA: GSA Bulletin v. 122, no. 1/2, p. 50-75. Carlson, K.E., 1985, A combined analysis of gravity and magnetic anomalies in east-central Minnesota: Minneapolis, Minn., University of Minnesota, M.S. Thesis, 138 p. Clark, R.C., 1985, The structural geology of the Thomson Formation: Cloquet and Esko quadrangles, east-central Minnesota: M.Sc. Thesis, University of Minnesota-Duluth, 114 p. Doe, B.R., and Delevaux, M.H., 1991, Lead-isotope investigations in the Minnesota River Valley-- late-tectonic and posttectonic granites, in Morey, G.B., and Hanson, G.N., eds., Selected studies of Archean gneisses and Lower Proterozoic rocks, southern Canadian Shield: Geological Society of America Special Paper, v. 182, p. 105-112. Fralick, P., Davis, D.W., and Kissin, S.A., 2002, The age of the Gunflint Formation, Ontario, Canada: Single zircon U-Pb age determinations from reworked volcanic ash: Canadian Journal of Earth Sciences, V. 39, p. 1085-1091. Green, J.C., Bornhorst, T.J., Chandler, V.W., Mudrey, M.G., Jr., Myers, P.E., Pesonen, L.J., and Wilband, J.T., 1987, Keweenawan dykes of the Lake Superior region: Evidence for evolution of the Middle Proterozoic midcontinent rift system of North America: in Halls, H.C., and Fahrig, W.F., Mafic Dyke Swarms, Geol. Assoc. Can. Spec. Pap. 34, pp. 289302. Heaman, L.M., and Easton, R.M., 2005, Proterozoic history of the Lake Nipigon area, Ontario: constraints from U-Pb zircon and baddeleyite dating (Abs.): Institute on Lake Superior Geology 51, Part 1 – Program and Abstracts, p. 24-25. ILSG 2011 161 Field Trip 8 �Holm, D.K., 1986, A structural investigation and tectonic interpretation of the Penokean Orogeny: east-central Minnesota: M.S. Thesis, University of Minnesota – Duluth, 105 p. Holm, D.K., Holst, T.B., and Lux, D.R., 1993, Postcollisional cooling of the Penokean orogen in east-central Minnesota: Canadian Journal of Earth Sciences v. 30, p. 913-917. Holm, D.K., and Selverstone, J., 1990, Rapid growth and strain rates inferred from synkinematic garnets, Penokean orogeny, Minnesota: Geology, v. 18, p. 166-169. Holm, D.K., and Lux, D., 1996, Core complex model proposed for gneiss dome development during collapse of the Paleoproterozoic Penokean orogen, Minnesota: Geology, v. 24, p. 343-346. Holm, D.K., Darrah, K., and Lux, D., 1998, Evidence for widespread ~1760 Ma metamorphism and rapid crustal stabilization of the Early Proterozoic Penokean Orogen, Minnesota: American Journal of Science v. 298, p. 60-81. Holm, D., Holst, T., and Ellis, M.A., 1988, Oblique subduction, footwall deformation and imbrication: a model for the Penokean orogeny, Minnesota: Geological Society of America Bulletin, v. 100, p. 1811-1818. Holm, D.K., Van Schmus, W.R., MacNeill, L.C., Boerboom, T.J., Schweitzer, D., and Schneider, D., 2005, U-Pb zircon geochronology of Paleoproterozoic plutons from the northern mid-continent, U.S.A.: Evidence for subduction flip and continued convergence after geon 18 Penokean orogenesis; Geological Society of America Bulletin, March/April, v. 117, no. 3/4, p. 259-275 Holm, D.K., Schneider, D.A., Rose, S., Mancuso, C., Mckenzie, M., Foland, K.A., and Hodges, K.V., 2007, Proterozoic metamorphism and cooling in the southern Lake Superior region, North America and its bearing on crustal evolution: Precambrian Research v. 157, p. 106-126. Holst, T.B., 1984, Evidence for nappe development during the Early Proterozoic Penokean Orogeny, Minnesota: Geology, v. 12, p. 135-138. Jirsa, M.A., Weiblen, P.W., Vislova, T., and McSwiggen, P.L., 2008, Sudbury impactite layer near Gunflint Lake, NE Minnesota: Institute on Lake Superior Geology Volume 54, Part 1 – Programs and abstracts, p. 42-43. Maric, M., and Fralick, P., 2005, Sedimentology of the Rove and Virginia Formations and their tectonic significance: Institute on Lake Superior Geology 51, Part 1 – Program and abstracts, p. 41-42. Marsden, R.W., 1972, Cuyuna district, in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota: A centennial volume: Minnesota Geological Survey, p. 227-239. McKenzie, M.A., 2004, Age pattern and nature of Late Paleoproterozoic metamorphism of the Penokean crust, east-central Minnesota: M.S. thesis, Kent State University, 105 p. McSwiggen, P.L., 1987, Geology and geophysics of the Denham-Mahtowa area, east-central Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-63, scale 1:48,000. Morey, G.B., 1978, Lower and Middle Precambrian stratigraphic nomenclature for eastcentral Minnesota: Minnesota Geological Survey Report of Investigations 21, 52 p. Morey, G.B., Olsen, B.M., and Southwick, D.L., 1981, Geologic map of Minnesota, eastcentral Minnesota, bedrock geology: Minnesota Geological Survey, scale 1:250,000. Scheiner, S.W., Boerboom, T.J., and Holm, D.K., 2011, Reinterpretation of Paleoproterozoic sedimentation and deformation in east-central Minnesota: Geological Society of ILSG 2011 162 Field Trip 8 �America Abstracts with Programs, Vol. 43, No. 1, p. 167; Northeastern (46th Annual) and North-Central (45th Annual) Joint Meeting (20–22 March 2011). Schneider, D.A., Bickford, M.E., Cannon, W.F., Schulz, K.J., and Hamilton, M.A., 2002, Age of volcanic rocks and syndepositional iron formations, Michigan and Wisconsin: U.S. Geological Survey Bulletin 1460, 96 p. Schneider, D.A., Holm, D.K., O‘Boyle, C., Hamilton, M., and Jercinovic, M., 2004, Paleoproterozoic development of a gneiss dome corridor in the southern Lake Superior region, USA, in Whitney, D.L., Teyssier, C, and Siddoway, C.S., eds., Gneiss domes in orogeny: Geological Society of America Special Paper 380, p. 339-357. Schulz, K.A., and Cannon, W.E., 2007, The Penokean orogeny in the Lake Superior region: Precambrian Research, V. 157, p. 4-25. Setterholm, D.R., Morey, G.B., Boerboom, T.J., and Lamons, R.C., 1989, Minnesota kaolin clay deposits—A subsurface study in selected areas of southwestern and east-central Minnesota: Minnesota Geological Survey, Information Circular 27, 99 p Southwick, D.L., Morey, G.B., and McSwiggen, P.L., 1988, Geologic map (scale 1:250,000) of the Penokean Orogen, central and eastern Minnesota, and accompanying text: Minnesota Geological Survey Report of Investigations 37, 25 p. Southwick, D.L., Boerboom, T.J., and McSwiggen, P.L., 2001, Geochemical characterization of Paleoproterozoic volcanic and hypabyssal igneous rocks, east-central Minnesota: Minnesota Geological Survey Report of Investigations 57, 33 p. Sun, WeiWei, Hudleston, P.J., and Jackson, M., 2005, Magnetic and petrofabric studies in the multiply deformed Thomson Formation, east-central Minnesota: Tectonophysics, vol. 249, iss. 1-2, p. 109-124. Tohver, E., Holm, D.K., van der Pluijm, B.A., Essene, E.J., and Cambray, F.W., 2007, Late Paleoproterozoic (geon 18 and 17) reactivation of the Neoarchean Great Lakes Tectonic Zone, northern Michigan, USA: Evidence from kinematic analysis, thermobarometry, and 40Ar/39Ar geochronology: Precambrian Research vol. 157, Nos. 1-4, p. 144-148. Vallini, D.A., Cannon, W.F., Schulz, K.J., and McNaughton, N.J., 2007, Thermal history of low metamorphic grade Paleoproterozoic sedimentary rocks of the Penokean orogen, Lake Superior region: Evidence for a widespread 1786 Ma overprint based on xenotime geochronology: Precambrian Research V. 157, p. 169-187. Van Schmus, W.R., 1976, Early and middle Proterozoic history of the Great Lakes area, North America: Philos. Trans. R. Soc. Lond., Ser A 280 (1298), p. 605-628. Wirth, K.R., Vervoort, J., Craddock, J.P., Davidson, C., Finley-Blasi, L., Kerber, L., Lundquist, R., Vorhies, S., and Walker, E., 2006, Source rock ages and patterns of sedimentation in the Lake Superior region: results of preliminary U-Pb detrital zircon studies: Institute on Lake Superior Geology, 52nd Annual Meeting, Sault Ste Marie, Ontario, Part 1 – Programs and abstracts, p. 69-71. Wunderman, R.L., and Young, C.T., 1987, Evidence for widespread basement decollement structures and related crustal asymmetry associated with the western limb of the Midcontinent Rift [abs.]: Institute on Lake Superior Geology, 33rd Annual Meeting, Wawa, Ontario, Proceedings and Abstracts, v. 33, pt. 1, p. 85-86. ILSG 2011 163 Field Trip 8 �ILSG 2011 164 Field Trip 8 �57TH ANNUAL INSTITUTE ON LAKE SUPERIOR GEOLOGY FIELD TRIP 9 GRANITIC, GABBROIC, AND ULTRAMAFIC ROCKS OF THE KEWEENAWAN MELLEN INTRUSIVE COMPLEX Intrusive breccianear Mellen, Wisconsin – T. Fitz ILSG 2011 165 Field Trip 9 �ILSG 2011 166 Field Trip 9 �Field Trip 9 Granitic, gabbroic, and ultramafic rocks of the Mellen Intrusive Complex in northern Wisconsin Tom Fitz Northland College 1411 Ellis Avenue Ashland, Wisconsin 54806 tfitz@northland.edu INTRODUCTION The Mellen Intrusive Complex (MIC) is a group of Keweenawan Midcontinent Rift (MCR) intrusions underlying 380 km2 in northern Wisconsin (Fig. 1). The Complex was intruded as large sill-like bodies into the lower part of the Keweenawan volcanic sequence 1102 Ma ago (Zartman et al., 1997). The rocks of the MIC, along with rest of the rocks in the region, were tilted steeply north during the compression and folding that created the Lake Superior Syncline (Cannon et al., 1993a). Subsequent erosion has exposed a complete cross section of this magmatic system – from the feeder dikes that brought magma from depth, to the subvolcanic magma chambers, and the overlying volcanic sequence (Woodruff, 2005). The MIC is bi-modal in composition, meaning mafic and felsic rocks dominate and there is a relatively small volume of intermediate rocks. This is similar to the rest of the igneous rocks of the MCR, and is typical of continental rifts. Anorthositic gabbro and granite are the two most common lithologies in the MIC, but there is a wide variety of rock types. This variety of rocks, as well as the fact that they are mostly pristine and well exposed, makes the MIC an excellent showcase of igneous petrology. The goal of this trip is to see this variety of rocks. The Mellen Complex is one of four major intrusive complexes of the MCR – these are: the Duluth Complex in northeastern Minnesota; the Nippigon sills near Lake Nippigon in Ontario; the Coldwell Complex on the north shore of Lake Superior in Ontario; and the Mellen Complex in Wisconsin (Miller, 2007). Another MCR layered mafic intrusion, the relatively small Clam Lake Intrusion, occurs at depth in the Clam Lake area 25 km southwest of the MIC. It is not exposed at the surface but is known from its magnetic and gravity signatures, and from two drill cores (Mudrey et al., 2003). Midcontinent rift magmatism produced an estimated 1,000,000 km3 of igneous rock over a period spanning 29 Ma. (Klewin and Shirey, 1991; Miller, 2007). It occurred in three main pulses: an early episode from about 1115 to 1107 Ma; the main phase that produced a huge volume of volcanic and plutonic rocks from about 1102 to 1092 Ma; and a third, less prodigious phase, from 1092 to 1086 Ma (Paces and Miller, 1993; Miller, 2007). Magnetic ILSG 2011 167 Field Trip 9 �polarity reversals occurred during rifting, so the remnant magnetism of the rocks has been important in deciphering the history of the MCR and making correlations between rocks in different parts of the rift: The early pulse of magmatism occurred during a time of reverse magnetic polarity, followed by a switch to normal polarity before the main phase of activity (Halls and Pesonen, 1982). Area of this field trip. Figure 1. Location map showing the Mellen Intrusive Complex in black, and the area of this field trip outlined in the white box. From the Bedrock Geologic Map of Wisconsin (Mudrey et al., 1982) The age of the MIC is known from U/Pb dating of zircon crystals from granite which yield an age of 1100.9 +/- 1.4 Ma, and from granophyre that give an age of 1102 +/- 2.8 Ma (Zartman et al, 1997). These dates, along with the contact relationships within the MIC, indicate the magma was intruded over a relatively short period of time very early in the main pulse of MCR magmatism. Its age is nearly the same as the Kallander Creek Volcanic rocks which it intruded, so the intrusions of the MIC probably represent crystallized magma chambers in the subvolcanic plumbing system of a large volcanic center located near what was is now Mellen, Wisconsin (Cannon, 1993b). INTRUSIONS OF THE MELLEN COMPLEX ILSG 2011 168 Field Trip 9 �The MIC consists of four intrusions– from west to east these are: the Mineral Lake Intrusion, the Rearing Pond Pluton,the Mellen Granite in the central region, and the Potato River Intrusions in the east (Fig. 2). The Mineral Lake and the Potato River Intrusions are each made up of two or more intrusions, whereas the Rearing Pond and Mellen Granite appear to be single intrusive bodies (Olmsted, 1979; Klewin, 1990; Seifert et al., 1992). Mellen Granite Rearing Pond Intrusion Mellen, Wisconsin 0 10 20 Miles N Figure 2. Map of the four main intrusions of the Mellen Intrusive Complex. The intrusions were emplaced into the lower parts of the volcanic sequence and likely fed magma upward to the surface where it was extruded as lava flows. In the eastern part of the Potato River intrusion, the sill-like gabbro bodies are nearly concordant and are within the lower part of the 2- to 4-km-thick Kallander Creek Volcanics(Klewin, 1990; Seifert et al., 1992).The intrusions are not exactly concordant however, and the level of emplacement gets progressively deeper toward the west (Fig. 3). The bottom of the MIC cuts through the Kallander Creek Volcanics and the underlying Siemens Creek Volcanics in the area of Mellen, Wisconsin. West of Mellen, the intrusions are in contact with the Paleoproterozoic Tyler Formation, and completely separate overlying volcanic rocks from the basement rocks on which they were deposited. In the area of Mineral Lake, the MIC crosses the Tyler and into the underlying Ironwood Formation. Then, finally, at the western end of the MIC the intrusions cutcross Archean rocks, so the entire Paleoproterozoic and lower Keweenawan volcanics are missing. In some areas, the intrusions occur at multiple levels and are separated by screens of volcanic rock preserved between parallel sill-like intrusive bodies. The upper intrusions must have been emplaced at relatively shallow levels since miarolitic cavities are quite common. The total thickness of the MIC varies from about 1 km to just over 5 km. ILSG 2011 169 Field Trip 9 �Strong igneous lamination defined by parallel alignment of plagioclase crystals is very apparent in many of the gabbroic rocks. Additionally, the large mafic bodies have largescale Figure 3. layering, with more mafic, olivine-rich gabbroic rocks near the base, which transition upward to a main section of anorthositic gabbro, which grades upward into intermediate to Geologic map of the western part of the Mellen Intrusive Complex. Field trip stops are show in white boxes. Formations are labeled on the map and rocks of the Mellen Complex have these symbols: Mineral Lake Intrusion (shown light green and dark green): Ymo: olivinebearing gabbroic rocks; Ymg: gabbro, anorthositic gabbro, gabbroic anorthosite; Ymf: ferrrodiorite; Ympg: granophyre. Rearing Pond Intrusion (shown in blue): Yrp: olivine gabbro; Yrs: serpentinized peridotite. Mellen granite ILSG 2011 (shown in medium Fre da San dsto ne 1 Gab bro, An orth osit e 0 1 2 3 4 5 M ile s Kal lan der Cre ek Vol cani cs 6 2 3 5 Puri tan Bat holi th Oli vin e Ga bbr o 4 Tyl er For mat ion 12 7 N 9 8 170 10 , 11 Kal lan der Cre ek Vol cani cs Con glo mer ate of Fre da For mat ion Field Trip 9 �felsic rocks near the top. The cumulate texture and broad compositional layering indicates the magmas were emplaced as crystal mushes that underwent crystal settling and fractional crystallization, and possibly some contamination, to create the variety of rocks present today (Seifert et al., 1992). However, well-defined small-scale compositional layering is not common. Some of the granitic intrusions have sharp chilled contacts against the gabbroic rocks, which means that not all of the felsic magma was created by fractionation within the preserved intrusions. Rocks of the Mellen Complex were extensively quarried for building stone starting in the early 20th century and some quarrying activity continues today. The black, coarsegrained gabbro, which has the trade name ―black granite‖, was widely used for decorative stone and monuments. It serves as structural stone in many historic buildings in the area, including numerous buildings in the town of Mellen and at Copper Falls State Park. In 1963, a monument made of Mellen ―black granite‖ was shipped to Arlington National Cemetery for the tombstone of the late President John F. Kennedy. Although no cutting and polishing operations remain in the area, several quarries still extract gabbro and granite for ballast rock and rip rap. Small abandoned quarries are common in the area and provide excellent bedrock exposures. The importance of quarrying in the local history is evident in the name of the Mellen school athletic teams – the ―Granite Diggers‖. Mineral Lake Intrusion The Mineral Lake Intrusion (MLI) consists of two intrusive bodies separated by a screen of Kallander Creek Volcanics (Seifert et al., 1992). The lower intrusion is 4.5 km thick and has a basal olivine gabbro, a thick central section of anorthositic gabbro, and an upper ferrodiorite zone. The composition of the plagioclase changes gradually upward in the intrusion from a modal value of An64 near the base to An23 near the top (Seifert et al., 1992). Cumulate texture is common and Seifert et al (1992) concluded that trapped intercumulus liquid constituted 25–35% of the volume of the accumulated crystal mush. The central portion of the intrusion consists of a large volume of coarse-grained anorthositic gabbro composed of plagioclase feldspar, augite, and minor amounts of olivine and magnetite. The igneous lamination defined by the alignment of large plagioclase crystals is distinct in most of this zone. The lamination dips about 70o northwest and is generally parallel to the regional structure, which indicates the crystals are aligned parallel to the floor of the intrusion, probably due to crystal settling. The central zone grades upward into gabbroic rocks with hornblende instead of olivine and augite, and eventually into diorite with small amounts of quartz and alkali feldspar. The diorite is iron rich and has a relatively high proportion of magnetite, a characteristic special enough to warrant the name ―ferrodiorite‖. It is a dark-colored rock with an appearance similar to gabbro, but close inspection reveals that its mineralogy is distinctly different than that of gabbroic rocks – it is composed of plagioclase feldspar, hornblende, magnetite, in some places biotite, and minor amounts of quartz and alkali feldspar. It does not contain olivine or augite as do most of the gabbroic rocks lower in the MLI. Based on REE data, Olmsted et al. (1986) concluded that the ferrodiorite is the product of fractional crystallization and that final crystallization took place at a temperature of 850 o– 750oC. Granophyre is abundant in the upper parts of the MLI, especially in the upper part of the higher of the two intrusions. The granophyre is brick red and usually fine to medium ILSG 2011 171 Field Trip 9 �grained for a granitic rock. It has spectacular micrographic intergrowths of quartz and microperthitic orthoclase feldspar that often penetrates the entire rock. Although it is usually visible only at the microscopic scale, it is this textural characteristic that distinguishes it from granite. Leighton (1954) found that the micrographic texture is well developed only in rocks in which the An content of the plagioclase is within the feldspar exsolution lamellae is within a restricted range. Klewin (1990) concluded that the roof-zone granophyres were residual liquids intruded along the upper margin of the MLI. Rearing Pond Intrusion The Rearing Pond Intrusion (RPI) is a relatively small intrusion mostly contained within the larger MLI. It is interpreted to have intruded as a single magma body into the MLI, but the nature of the contact between the two bodies is unknown because of poor exposure. The RPI is the most mafic intrusion of the MIC, being dominated by Mg-rich olivine gabbro and troctolite. Cumulate layers change upward from an olivine-rich peridotite layer near the base of the intrusion, through olivine gabbro in the middle, to gabbro near the top. The composition of plagioclase changes vertically as well, from An80 near the base to An60 near the top. This sequence is interpreted to have been created by fractional crystallization of olivine, followed by olivine, plagioclase, and clinopyroxene (Olmsted, 1979). Although the basal peridotitelayer is partially serpentinized, it locally has wellpreserved olivine crystals and igneous textures. Potato River Intrusion The Potato River Intrusion (PRI) is similar to the MLI in that it has a lower zone of olivine gabbro overlain by a thick section of anorthositic gabbro that has conspicuous igneous lamination, and granophyre near the top (Klewin, 1990; Tabet and Mangham, 1978). It differs from the MLI in that it has a larger volume of anorthosite. Mellen Granite The Mellen Granite (MG) occupies the central portion of the MIC and is in contact with the MLI on the west and the PRI on the east (Fig. 3). It is a medium-grained,grey to pink biotite granite. It includes xenoliths of gabbro and ferrodiorite that closely resemble those rock types elsewhere in the MIC and are probably locally derived. The lack of significant chilled margins in the contact zone indicates the granite was intruded while the mafic rocks were still relatively hot, and the presence of miarolitic cavities indicates the granite was emplaced at a depth of less than about 6 km. Together those facts attest to the rapid succession of emplacement of the MIC high in the Keweenawan volcanic sequence. The granite is interpreted to be a stock intruded near the end of the igneous activity that created the MIC and Kallander Creek Volcanics (Cannon et al., 1993b). In some areas on the east and west sides of the MG, the contact between the granite and the gabbroic country rocks contains a zone of extremely brecciated gabbro xenoliths (Sikkila, 2002). The gabbro country rocks were complexly intruded by the granite,creating intrusive breccia comprised of angular blocks that appear to have been shattered by the intrusion. Additionally, there are often xenoliths of several different types of gabbroic rocks and ferrodiorite mixed together in the breccia zone, suggesting that the brecciation was followed by mixing of the inclusions within the granite (Cervin, 2009; Goscinak, 2010). The granite in some breccia zones has highly variable texture showing a complex arrangement of ILSG 2011 172 Field Trip 9 �pegmatite, graphic granite, and hypidiomorphic granular granite. The combination of a diverse range mafic rock xenoliths, complex brecciation, and highly variable granitic textures makes this a very distinctive and intriguing intrusive breccia. FIELD TRIP STOPS The field trip starts near Sanborn, Wisconsin and goes through the Chequamegon National Forest east to Mellen, Wisconsin on gravel roads maintained by the U.S. Forest Service (Fig. 4). The roads are generally well maintained but may be impassible during mud season. The U.S.G.S. topographic quadrangle maps covering the area of this field trip are, from west to east: Marengo Lake, Mineral Lake, and Mellen, Wisconsin. All locations in this chapter are given in UTM coordinates (Zone 15 T) with NAD 1927, which is the datum for all the local topographic maps. This guide focuses on the western part of the MIC– specifically the MLI, the RPI, and the MG. It has one stop in the very western part of the PRI and a dike that penetrated up through the underlying Tyler Formation into the PRI. A detailed description of the eastern part of the Mellen Complex, with numerous stops in the PRI, can be found in the 1989 ILSG field guide chapter by Klewin, et al. In this chapter, rock names for the gabbroic rocks are based on the classification system of Miller et al., 2002, and the felsic and intermediate rocks are classified according to the IUGS classification system (Middlemost, 1991; Streckeisen, 1976). C County Rt. C Co. Lin e Rd. Ba ss La ke R oa d C FR 18 7 Stop 4 Stop 1 Rt. 13 C Stop 12 County Rt. C Stop 3 FR-189 Spring Brook Rd. Stops 9, 10, 11 Stop 2 Mellen FR-199 Quarry Road English Lake Stop 7 Rt. 13 Mineral Lake Stop 5 Stop 6 0 Stop 8 GG FR-187 1 2 3 4 5 Miles N Figure 4. Road map showing location of field trip stops. ILSG 2011 173 Field Trip 9 �Stop 1: Granophyre at Morgan Falls Location: 0659762 mE, 5134883 mN;T45N, R4W, Sec. 30, west-central. Park at the Forest Service parking lot for Morgan Falls and St. Peter‘s Dome located on the east side of Forest Service road FS-199(Fig.5). The UTM coordinates listed above are for the parking lot. From the parking lot, walk east on the trail to St. Peter‘s Dome for approximately 0.25 miles, then bear right (south) at the fork and take the trail to Morgan Falls. This area has good exposure of gabbro and granophyre in the upper part of the MLI. These intrusions were emplaced as large sills concordant with lava flows of the Kallander Creek Volcanic rocks, which are exposed on the hills across the valley to the north. Screens of Kallander Creek Volcanics are preserved between the sills but are not well exposed because they are relatively non-resistant in comparison to the hard intrusive rocks. In the area of this stop, volcanic rocks of the Kallander Creek served as the roof rocks, below which is a relatively thin roof zone of gabbro, below which is a large granophyre sill that is exposed at Morgan Falls and on St. Peter‘s Dome (Cannon et al., 1996). Stop 1 Parking Stop 1 Figure 9. Topographic map showing the area of Stop 1. Stop 1-A: Gabbro of the Mineral Lake Intrusions Location: Near Morgan Falls, on the west side of trail; 0660363 mE, 5134684 mN. The first outcrops encountered along the trail are just south of the bridge over Morgan Creek, on the west side of the trail. These outcrops are all in gabbro of the MLI. This is the uppermost intrusion of the MLI, and this outcrop is approximately 200 – 300 m below the roof of the intrusion. The rock is gabbro,or oxide gabbro, containing approximately 80% plagioclase feldspar, 15% oxide mineral, at least some of which is magnetite, 5% ILSG 2011 174 Field Trip 9 �intercumulus augite and hornblende, and a trace of quartz. This lithology is typical of this part of the MLI. Stop 1-B: Granophyre at Morgan Falls Location: At the end of the Morgan Falls trail; 0659762 mE, 5134884 mN. Scenic Morgan Falls is a 70-foot cascade in a narrow rock canyon where Morgan Creek falls over dark red granophyre. The joint pattern in the rock is distinct and exerts a strong control on the form of the rock face and the flow of the water over the falls. The rock at the falls is a dark red, medium-grained granophyre. It is composed almost entirely of quartz and microperthitic orthoclase feldspar crystals intergrown in a micrographic texture. The micrographic texture is not readily apparent in hand sample, especially in finer-grained samples, but is very distinctive in thin section (Fig. 6). The contact between the granophyre and the overlying gabbro is exposed in the sides of the valley to the west, but the slopes are extremely steep and treacherous so we will not be visiting the outcrop. The contact is sharp and the granophyre has a distinct chilled margin against the medium-grained gabbro, indicating the granophyre intruded into the gabbro after the gabbro had cooled. Figure 6. Photomicrograph of granophyre from Morgan Falls at Stop 1. The image was taken with plane transmitted light, and is shown here in black-and-white. The rock is composed primarily of alkali feldspar (dark grey) and quartz (light grey) intergrown in micrographic texture. The field of view is 2 mm. ILSG 2011 175 Field Trip 9 �Stop 2: Anorthositic Gabbro of the Mineral Lake Intrusion Location: South side of road FS-189; T45N, R4W, Sec. 30, west-central; 0666203 mE, 5133192 mN. There are several large outcrops in the woods immediately south of the road. The rock at this location is coarse-grained anorthositic gabbro with a strong parallel alignment of plagioclase feldspar defining a lamination that dips 70o northwest. This rock is very typical of the central zone of the MLI, and is the most common rock type in the intrusion overall. The rock is composed of 80–85% plagioclase, the rest being intercumulusferromagnesian silicate minerals, now biotite and hornblende. The southern part of the outcrop has some small-scale modal layering. Stop 3: Olivine Melagabbro and Peridotite of the Rearing Pond Intrusion Location: T45N, R4W, Sec. 26, SW1/4; start of hike: 066638 mE, 5133660 mN; hilltop destination: 666713 mE, 5134266 mN. This stop involves a 0.4-mile bushwhack north from FS-189 to a hilltop with exposures of olivine melagabbro and peridotite. From the starting location hike N10oW to the ridge, then N65oE to the end of the ridge where there are several large outcrops of olivine melagabbro. There is a concrete foundation and detritus from a work camp on the highest point. Stop 3 Start of hike to Stop 3 Stop 2 Figure 7. Topographic map of the area of Stops 2 and 3. ILSG 2011 176 Field Trip 9 �The rock exposed on the hilltop is olivine melagabbro composed of approximately 50% plagioclase feldspar, 25% olivine, 20% augite, and 5% oxide mineral. There is no lamination, which is typical of the RPI. Hike northwest, downhill a short distance to outcrops of peridotite. The rock is black, medium-grained, olivine-rich, partially serpentinized peridotite. The outcrop extends down the steep hill to the valley floor – hike carefully down this slope and then return to FR-189. There are few outcrops of the peridotite, but Klewin et al., 1987 describe another outcrop of this unit about 1 km northeast of this stop. Perhaps the best exposures are located in the ledges above the Brunsweiler River in T45N, R4W, Sec. 23 SW1/4; 0667514 mE, 5135895 mN. We will not be visiting these outcrops on this trip because of their remote location. At that location the rock is slightly coarser grained and less serpentinized. It is composed of approximately 50% olivine, 15% orthopyroxene, 10% opaque oxide mineral, and 25% serpentine(Fig 8). The large orthopyroxene crystals enclose euhedral olivine crystals. Some of the serpentine contains thin veins of parallel-aligned acicular crystals oriented perpendicular to the margins of the vein, which is typical of serpentinites. Figure 8. Photomicrographs of serpentinized peridotite of the Rearing Pond Intrusion. The upper image was taken with plane light, the lower image is the same view, but with crossed polars. The rock is composed primarily of olivine (highly birefringent), magnetite (opaque), and serpentine (fine-grained, low birefringence). Field of view is 2 mm. This rock is not from the outcrop at Stop 3, but is from an outcrop of the same unit at 0667514 mE, 5135895 mN. ILSG 2011 177 Field Trip 9 �Stop 4: Ferrodiorite in the Upper Part of the Mineral Lake Intrusion Location: T45N, R4W, Sec 25, north-central,0668365 mE; 5134955 mN The rock at this stop is within the thin ferrodiorite unit at the top of the MLI. The dark-colored ferrodiorite somewhat resembles gabbro, but it‘s mineral composition is distinctly different than that of the underlying gabbroic rocks. The rock is composed of plagioclase, hornblende, magnetite, and minor amounts of alkali feldspar and quartz. The proportion of alkali feldspar and quartz increases to the north, toward the top of the unit. Stop 5: Olivine Gabbro of the Mineral Lake Intrusion Location: T44N, R4W, Sec. 13, NW1/4; Park along the road near the entrance to the Mineral Lake boat ramp (0668420 mE, 5129159 mN). This stop includes one outcrop a short distance into the woods on the west side of Mineral Lake Road (Stop 4A), and several outcrops on along the ridge on the east side of the road (Stop 4B) (Fig. 9). Stop 5 Figure 9. Topographic map showing the area of Stop 5. Stop 5A: Coarse-Grained Gabbro Location: T44N, R4W, Sec. 13, NW 1/4; 0668392 mE, 5129146 mN. This outcrop is located in the woods on west side of Mineral Lake Road. The rock is coarse-grained gabbro composed of approximately 80% plagioclase feldspar, 20% intercumulus pyroxene that is now mostly actinolite, 5% oxide minerals, some of which is magnetic, some of which is not. It does not show laminations or compositional layering. ILSG 2011 178 Field Trip 9 �Stop 5B: Troctolite and olivine melagabbro Location: 0668468mE, 5129193 mN. The outcrops of this stop are located in the woods a short distance east of Mineral Lake Road. The low ridge trending northeast toward Potter Lake has numerous exposures of the olivine gabbro, troctolite, and melatroctolite of the basal zone of the MLI. The rock varies in grain size and proportion of olivine, with some of the finer-grained rocks containing up to 50% olivine. There is a trace of sulfide mineral present. The map of Cannon et al, 1996 shows a Cu-Ni zone here, and Bakheit (1981) studied the petrography of Cu-Ni mineralization in this area. It is likely the rockshere are compositionally layered, although it is not apparent in individual exposures because of the relatively small size of most outcrops. These rocks are within 200 m of the base of the MLI. The basal contact zone is not exposed in the area of Mineral Lake, but it is exposed in one place approximately 2.5 km east of here near English Lake. This field trip will not be visiting that basal contact exposure because of difficulties with access, but the site is described in the Klewin et al., 1989ILSG guidebook chapter (their Stop 7). The exposed contact has a chill zone where fine-grained gabbro of the MLI is in contact with contactmetamorphosed rocks of the Tyler Formation. Stop 6: Metamorphosed Iron Formation Below the Mineral Lake Intrusion Location: T44N, R4W, Sec 14, NE 1/4; 0667764 mE, 5128518 mN. This outcrop is located in the woods just south of County Road GG. Use caution while parking along the road because the shoulder is narrow; there a small pull-off on the shoulder on the north side of the road not far west of the outcrop that has adequate room for one car. This outcrop is in the upper part of the Ironwood Iron Formation where iron formation has been contact metamorphosed by the MLI. Although the contact is not exposed, olivine gabbro of the MLI is not far to the north. The rock here is black, dense, metamorphosed siliceous iron formation. It is composed of approximately 50% dark green to black olivine, 30% black amphibole, and 20% quartz. This is a relatively rare lithology – rocks containing both quartz and olivine are not common and are restricted to just a few geologic settings, one of which is where siliceous iron formation has undergone hightemperature metamorphism. The situation here, where the Ironwood Iron Formation was contact metamorphosed by the MLI, hadone of these special circumstances in which olivine crystals could grow in a silicic metamorphic rock. The fact that the olivine is in an iron-rich rock and occurs with quartz – which is not stable with Mg-rich olivine – suggests it is ironrich (fayalitic) olivine. Stop 7: Quarry in Coarse-Grained Anorthositic Gabbro of the Mineral Lake Intrusion Location: T44N, R3W, Sec. 4, NW 1/4; 0672101 mE, 5132430 mN. This abandoned quarry is in very coarse-grained anorthositic gabbro. The rock is composed of plagioclase feldspar, hornblende, some augite, and non-magnetic oxide minerals. The quarry dump contains large blocks of freshly broken gabbro displaying a wide range of textures, including some pegmatitic gabbro. Hornblende is more common than augite as the ferromagnesian mineral in these rocks, which is common in upper parts of the MLI. ILSG 2011 179 Field Trip 9 �Stop 8: Feeder Dike at Mellen Location: T44N, R2W, Sec. 6, NE 1/4; 0680368 mE, 5132423 mN; near the intersection of Fayette Avenue and Highway 77 in Mellen, Wisconsin. This outcrop is fine-grained olivine gabbro within a dike that cuts across the Tyler Formation. It lies immediately below rocks of the PRI and is likely a feeder dike approximately 100 – 200 m wide that carried magma upward into the base of the PRI. This rock is considerably finer grained than most of the gabbroic rocks of the MIC becausethe relatively small size of this intrusion caused it to cool quite rapidly. The dike creates a resistant ridge that trends south from here, down-section, into the area of less-resistant slate of the Tyler Formation. Stop 9: Anorthositic Gabbro of the Potato River Intrusion Location: East side of Wisconsin Highway 13, 0.5 miles north of Mellen, Wisconsin; T45N, R2W, Sec. 31, NE ¼; 0680249 mE, 5133847 mN. This is a large highway road cut outcrop of anorthositic gabbro in the western part of the PRI. It is composed of plagioclase feldspar with a strong parallel alignment that dips north, and minor amounts of augite and magnetite. At the south end of the outcrop there is a 5-cm-wide basalt dike that dips south, cutting across the lamination. Further north along the outcrop, the rock surface is not freshly broken gabbro, but instead is joint and minor fault surfaces covered with epidote and calcite. In a few areas where freshly broken gabbro is exposed, there are tabular zones approximately 2 cm wide of very fine-grained light-colored rock interpreted to be gabbroic mylonite created along minor faults. The next outcrop to the north along the highway is similar in that its surface is mostly fine-grained epidote. The area apparently has numerous small-scale faults, along which the rock broke when it was blasted during highway construction, so what is exposed on the surface is mostly epidote. Stop 10: Intrusive Breccia of the Mellen Granite – South Outcrop Location: T45N, R2W, Sec. 30, SW ¼; 0679689 mE, 5134810 mN. This is a large highway road cut on the west side of Wisconsin State Highway 13, 1.3 miles north of Mellen. The rocks on the upper slopes of this outcrop are unstable so climbing would be hazardous, and if knocked out of place, the blocks would pose a significant hazard to anyone below. Thus, do not climb the outcrop, examine it from ground level. This outcrop is in the contact zone of the MG; the main body of the MG lies to the west, and the PRI lies to the east. Grey granite of the MG has intruded and brecciated mafic and intermediate rocks of the MIC, creating an intrusive breccia that is volumetrically about half inclusions and half granite. The granite is grey, medium-grained hypidiomorphic granite typical of the MG. The xenoliths are composed of anorthositic gabbro, biotite-rich gabbro, ferrodiorite, minor troctolite, and probably others as well. The ferrodiorite inclusions tend to be finer grained than other inclusions. The biotite-rich gabbro, informally called the ―biotite cluster gabbro‖, tends to be the coarsest grained of the inclusions. Cervin‘s (2009) study of this outcrop and that of Stop 11 included whole-rock chemical analyses of the MG and fivedifferent xenolithic rocks from the breccia zone (Table 1). The dark-colored mafic- and intermediate-rock xenoliths have been broken into a complex array of angular fragments, some of which are not completely fragmented but have thin intrusions of granite that nearly penetrate the xenolith (see photograph on the cover page ILSG 2011 180 Field Trip 9 �of this chapter). In some xenoliths, the granite is finer grained within the fractures than in the surrounding granite. This is interpreted to be the result of filter pressing of the magma – liquid was forced into the cracks but larger feldspar crystals in the crystal mush could not travel into the small nascent fractures. No rocks other than MIC lithologies are present as inclusions, so they are probably all locally derived. However, the inclusions constitute a wide variety of rock types. Some of this heterogeneity may be due to intrusion of the granite into a zone of the PRI that had a complex mixture of rock types in one area, but at least some of the compositional heterogeneity is likely the result of entraining and mixing of inclusions from different sources within the MIC. Table 1. Whole-rock chemical analyses of Mellen Granite and selected xenoliths in the intrusive breccias. Sample 8-10-13 8-10-11 8-10-16 8-10-1 8-10-14 8-10-15 Rock Mellen Ferro- Anortho Gabbro Hbl-rich Fn-grain Granite Diorite Gabbro Fe-Dior Troctolite SB NB SB NB SB SB SiO2 72.22 55.82 51.24 48.07 46.95 43.44 % 0.01 Al2O3 13.45 12.74 22.81 20.72 11.14 12.11 % 0.01 Fe2O3(T) 1.9 13.09 6.71 9.48 20.35 20.08 % 0.01 MnO 0.02 0.174 0.076 0.11 0.236 0.229 % 0.001 MgO 0.25 3.01 1.97 6.61 4.83 18.08 % 0.01 CaO 1.13 5.67 10.29 9.53 9.1 5.57 % 0.01 Na2O 3.47 3.29 4.3 2.76 3 1.64 % 0.01 Location Units Detection Limit Analyte K2O 4.9 2.02 0.62 0.62 0.75 0.21 % 0.01 TiO2 0.161 2.461 1.28 0.682 3.439 0.16 % 0.001 P2O5 0.03 0.47 0.25 0.06 0.54 0.03 % 0.01 LOI 0.52 0.7 0.39 0.94 -0.01 -0.99 % Total 98.04 99.44 99.94 99.58 100.3 100.6 Ba 2449 594 197 123 211 62 ppm 2 Sr 102 171 395 376 176 199 ppm 2 Y 77 72 20 6 61 2 ppm 1 % 0.01 Sc 1 25 10 9 35 6 ppm 1 Zr 626 518 117 36 321 35 ppm 2 Be 4 3 1 <1 3 <1 ppm 1 V 11 266 213 93 441 31 ppm 5 Locations: SB=South Breccia outcrop; NB=North Breccia outcrop Method: FUS-ICP Actlabs Report Date: 14/04/2009 Also reported in: Cervin, 2009 ILSG 2011 181 Field Trip 9 �Stop 11: Intrusive Breccia of the Mellen Granite – North Outcrop Location: T45N, R2W, Sec. 31, SW ¼; 0679517 mE, 5135384 mN. This is a low road-cut outcrop on the west side of Highway 13, 1.8 miles north of the town of Mellen. The rock at this outcrop is similar to that of Stop 9 – intrusive breccia with granite enclosing angular inclusions of a variety of mafic and intermediate lithologies of the MIC. The granite is different here though – it is pink granite with a wide variety of textures. Some is granitewith hypidiomorphic granular texture typical of the MG, but this grades into zones of graphic granite that in turn grade inward to pegmatitic zones. Some of the pegmatitic zones have concentric zoning around central miarolitic cavities that contain euhedral quartz crystals, others are filled with calcite. There is also zoning of the granite outward from inclusions (Fig. 10), suggesting the inclusions influenced the crystallization of the granite. The textures of this granite indicate complex crystallization processes in the granite of the breccia zone. Sikkila (2002) concluded there was significant potassium metasomatism here, which altered plagioclase to potassium feldspar and caused alteration of ferromagnesian minerals in both the granite and the xenoliths. The complex variation in textures and the potassium metasomatism indicate that water was important in petrologic processes the during and after crystallization of this rock. Figure 10. Intrusive breccia at ―the north breccia outcrop‖ of Stop 11. The rock has darkcolored, angular inclusions of ferrodiorite enclosed in pegmatitic granite. Quarter for scale. Goscinak‘ s (2010)detailed mapping of this outcrop revealed that the lithology of the inclusions defines three zones within the outcrop (Fig. 11). Ferrodiorite is the dominant rock type of inclusions in the central section, whereas ―biotite-cluster gabbro‖ occurs at the north and south ends of the outcrop. If, as the evidence suggests, the intrusive breccia was created by forceful injection of granite at a shallow level, then perhaps the magma exploded to the surface as a pyroclastic eruption. This hypothesis is consistent with the presence of large rhyolite units in the Kallander Creek Volcanics, so perhaps there is a connection between the intrusive breccias and the older rhyolites. ILSG 2011 182 Field Trip 9 �Figure 11. Zones of xenolith composition in the ―north breccia outcrop‖ of Stop 10. From Goscinak, 2010. Stop 12: Fault in Mellen Granite Location: T45N, R3W, Sec. 27, west-central; 0674116 mE, 5134740 mN. This quarry is owned and operated by Milestone Materials Corporation. Permission must be obtained from Milestone Materials before venturing into the quarry. This is an active quarry with significant hazards such as loose rock on quarry walls and in rock piles. Use great caution while in the quarry; do not climb on any rock faces, and do not even approach the tall faces on the south side of the quarry. Piles of freshly broken rock in the quarry allow detailed examination of the granite, which is typical pink granite of the MG. Inclusions of ferrodiorite are common, some of which have gradational contacts with the granite. The outstanding feature at this stop is the large fault surface on the east side of the quarry. The fault surface is green due to a neomineral coating of epidote on and within the fault zone. Slickenlines and fault mullions on the surface clearly indicate strike-slip motion. The fault is not a single surface but instead is a zone of nearly parallel surfaces that enclose lens-shaped bodies of largely undeformed granite. The fault zones are 1–2 cm wide and contain angular fragments of granite enclosed within the fine-grained epidote-rich cataclasite. The potassium feldspar in the granite along the fault is slightly darker red, evidently due to metasomatism. The timing of faulting and the amount of displacement on the fault are unknown, but since the rock is identical on both sides of the fault, the offset is not on the order of kilometers. Ferrodiorite is exposed along both sides of the access road into the quarry. There is an especially good outcrop of the ferrodiorite on the west side of the road: 0674139 mE, 5134942 mN. The contact between the ferrodiorite and the granite is not exposed here, but based on their relationships elsewhere, the granite is probably intrusive into the ferrodiorite. ACKNOWLEDGEMENTS Dan Cervin and Chris Goscinak studiedthe ―north breccia outcrop‖ of Stop 11 for their senior capstone research projects at Northland College. Their companionship during our exploration of that outcrop and many others within the MIC is greatly appreciated. Their ILSG 2011 183 Field Trip 9 �work has both answered and raised many questions about the origin of the intrusive breccias. I am grateful to Bill Cannon for his suggestions for field trip stops and for his time in the field showing me some especially interesting rocks in and around the MIC. REFERENCES Bakheit, A.K., 1981, Petrography of Cu-Ni mineralization in Mineral Lake area, Ashland County, Wisconsin: Madison, Wisconsin, University of Wisconsin—Madison, M.S. thesis, 104 p. Cannon, W.F., Woodruff, L.G., Nicholson, S.W., and Hedgman, C.A., 1996, Bedrock geologic map of the Ashland and the northern part of the Ironwood 30‘x60‘ quadrangles, Wisconsin and Michigan: U.S. Geological Survey, Miscellaneous Investigation Series Map I-2556, scale 1:100,000. Cannon, W.F., Nicholson, S.W., Zartman, R.E., Davis, D.W., 1993a, Kallander Creek Volcanics—a remnant of a Keweenawan central volcano centered near Mellen, Wisconsin [abs.], Institute on Lake Superior Geology, 39th Annual Meeting, Thunder Bay, Ontario, Proceedings, p. 20-21. Cannon, W.F., Peterman, S.E., and Sims, P.K., 1993b, Crustal-scale thickening and the origin of the Montreal River Monocline—a 35-km-thick cross section of the Midcontinent Rift in Northern Michigan and Wisconsin: Tectonics, v. 12, p. 728-744. Cervin, D., 2009, Geology of igneous breccia in the border zone of the Mellen Granite near Mellen, Wisconsin: Northland College, Ashland, Wisconsin, senior thesis, 21 p. Goscinak, C., 2010, Lithologies and intrusive relationships of the igneous breccia of the Mellen Igneous Complex near Mellen, Wisconsin: Northland College, Ashland, Wisconsin, senior thesis, 10 p., 1 plate. Halls, H.C., and Pesonen, L.J., 1982, Paleomagnetism of Keweenawan rocks, in Wold, R.J., and Hinze, W.J., eds., Geology and tectonics of the Lake Superior Basin: Geological Society of America Memoir 156, p. 173-202. Klewin, K.W., 1990, Petrology of the Proterozoic Potato River Layered Intrusion, Northern Wisconsin, USA: Journal of Petrology, v. 31, p. 1115-1139. Klewin, K.W., and Shirley, S.B., 1992, The igneous petrology and magmatic evolution of the Midcontinent rift system: Tectonophysics, v. 213, p. 33-40. Klewin, K.W., Olmstead, J.F., and Seifert, K.E., 1989, Rock types and relationships of the Mellen Igneous Complex: Institute on Lake Superior Geology, 35th Annual Meeting, Duluth, MN, Field Trip Guidebook, p. C1-C15. Leighton, M.W., 1954, Petrogenesis of a gabbro-granophyre complex in northern Wisconsin: Bulletin of the Geological Society of America, v. 65, p. 401-442. Middlemost, E.A.K., 1991, Towards a comprehensive classification of igneous rocks and magmas: Earth-Science Reviews, v. 31, no. 2, p. 73-87. Miller, J., 2007, The Midcontinent Rift in the Lake Superior Region: A 1.1 Ga large igneous province: Large Igneous Provinces, http://www.largeigneousprovinces.org/07nov.html Miller, J.D. Jr., Green, J.C., and Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.E., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex ILSG 2011 184 Field Trip 9 �and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, 207 p. w/ CD-ROM. Mudrey, M.G., Brown, B.A., and Greenberg, J.K., 1982, Bedrock geologic map of Wisconsin: Wisconsin Geological and Natural History Survey, scale 1:1,000,000. Mudrey. M.G., Ervin, C.P., and Olmsted, J.F., 2003, Middle Keweenawan Basin evolution inferred from geophysical analysis of a strongly magnetic intrusion, Clam Lake, Wisconsin: Wisconsin Geological and Natural History Survey, Open-File Report 2003-2004, 15 p. Olmsted, J.F., 1979, Crystallization history and textures of the Rearing Pond gabbro, northwestern Wisconsin: American Mineralogist, v. 64, p. 844-855. Olmsted, J.F., Windom, K.E., and Seifert, K.E., 1986, Mineralogy and chemistry of the ferrodiorite of the Mineral Lake Intrusion, Mellen, Wisconsin [abs.]: Institute on Lake Superior Geology, 32nd Annual Meeting, Wisconsin Rapids, WI, Proceedings, p. 65. Paces, J.B., and Miller, J.D., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: geochronological insights to physical, petrogenic, paleomagnetic, and tectonomagmatic processes associated with the 1.1 Ga Midcontinent rift system: Journal of Geophysical Research, v. 98, no. B8, p. 13,997-14,013. Seifert, K.E., Peterman, Z.E., and Theiben, S.E., 1992, Possible crustal contamination of Midcontinent Rift igneous rocks: examples from the Mineral Lake Intrusions, Wisconsin: Canadian Journal of Earth Sciences, v. 29, p. 1140-1153. Sikkila, K., 2002, Description of a pegmatite occurrence on the east margin of the Mellen Granite, State Highway 13, Ashland County, Wisconsin [abs.]: Institute on Lake Superior Geology, 48th Annual Meeting, Kenora, Ontario, v. 48, part 1, p. 48. Streckeisen, 1976, To each plutonic rock its proper name: Earth-Science Reviews, v. 12, no. 1, p. 1-33. Tabet, D.E., and Mangham, J.R., 1978, The geology of the eastern Mellen Intrusive Complex, Wisconsin: Geoscience Wisconsin, v. 3, p. 1-19. Vervoort, J.D., Wirth, K., Kennedy, B., Sandland, T., and Harpp, K.S., 2007, The magmatic evolution of the Midcontinent rift: New geochronologic and geochemical evidence from felsic magmatism: Precambrian Research, v. 157, p. 235-268, doi: 10.1016/j.precamres.2007.02.019. Woodruff, L.G., 2005, Geology of the Midcontinent rift in western Lake Superior [abs.], Geological Society of America, Abstracts with Programs, North-Central Section 39th Annual Meeting. Zartman, R.E., Nicholson, S.W., Cannon, W.F., and Morey, G.B., 1997, U-Th-Pb zircon ages from some Keweenawan Supergroup rocks from the south shore of Lake Superior: Canadian Journal of Earth Sciences, v. 34, p. 549-561. ILSG 2011 185 Field Trip 9 �Northland College Van #1 on the verge of meeting its deserved demise at a ―black granite‖ quarry near Mellen, Wisconsin. ILSG 2011 186 Field Trip 9 � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Proceedings, 2011 Description An account of the resource Institute on Lake Superior Geology. Ashland, Wisconsin. May 18-21, 2011. Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology Date A point or period of time associated with an event in the lifecycle of the resource 2011 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English https://digitalcollections.lakeheadu.ca/files/original/aee6ad921cb93985dfe1d1a5fddfcefc.pdf 40a74911abcb44b762a3664bbc18cf2d PDF Text Text 58th Annual Meeting Institute on Lake Superior Geology Thunder Bay, Ontario - May 16-20, 2012 Part 1 – Proceedings and Abstracts �58th Annual Meeting Institute on Lake Superior Geology May 16-20, 2012 Thunder Bay, Ontario HOSTED BY: Pete Hollings Chair Lakehead University Proceedings - Volume 58 Part 1 – Proceedings and Abstracts Edited by Pete Hollings (Lakehead University) Cover Photos: Top - Lac des Iles open pit, Middle - Kakabeka Falls in flood, notice light coloured tuff units on right, Bottom - the Sea Lion in 1985, Silver Islet (photos courtesy of Al MacTavish and Bill Addison). �Proceedings of the 58th ILSG Annual Meeting - Part 1 58th Institute on Lake Superior Geology Volume 58 consists of: Part 1: Program and Abstracts Part 2: Field Trip Guidebook Trip 1 & 13: Sudbury Impactoclastic Debrisites at Thunder Bay Trip 2: Geology of the Sibley Peninsula Trip 3: Lac des Iles mine Trip 4: Shebandowan Mine Area Trip 5: Geology of the Thunder Bay area Trip 6: Thunder Bay Amethyst Mine Trip 7: building stone tour of Downtown Port Arthur, Thunder Bay Trip 8: Highway 527 Transect Trip 9: Rehabilitation of the Past-Producing Shebandowan and North Coldstream Mine Sites Trip 10: Geoarchaeology of Thunder Bay Trip 11: Midcontinent rift intrusions Trip 12: Musselwhite mine Reference to material in Part 1 should follow the example below: Baumann, S., and Cummings, K., 2012. Bedrock Geology of Copper Falls State Park, Ashland County, Wisconsin. In; Hollings, P. (Ed.), Institute on Lake Superior Geology Proceedings, 58th Annual Meeting, Thunder Bay, Ontario, Part 1 - Proceedings and Abstracts, v. 58, part 1, 1-2. Published by the 58th Institute on Lake Superior Geology and distributed by the ILSG Secretary: Pete Hollings - ILSG Secretary Department of Geology Lakehead University 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada Email: peter.hollings@lakeheadu.ca ILSG website: www.lakesuperiorgeology.org ISSN 1042-9964 -i- �Proceedings of the 58th ILSG Annual Meeting - Part 1 Table of Contents Institutes on Lake Superior Geology, 1955-2012.............................................................. iii Sam Goldich and the Goldich Medal...................................................................................v Goldich Medal Guidelines................................................................................................ vii Goldich Medalists.............................................................................................................. ix 2012 Goldich Medal Recipient.......................................................................................... ix Goldich Medal Committee ............................................................................................... ix Citation for Goldich Medal Recipient..................................................................................x Eisenbrey Student Travel Awards..................................................................................... xii Student Paper Awards....................................................................................................... xii ILSG Student Research Fund........................................................................................... xiii Student Paper Awards Committee.................................................................................... xiii Session Chairs . ............................................................................................................... xiii Board of Directors............................................................................................................ xiv Local Committee.............................................................................................................. xiv Banquet Speaker.............................................................................................................. xiv Report of the Chair of the 57th Annual Meeting ..............................................................xv Sponsors.......................................................................................................................... xvii Program.......................................................................................................................... xviii Abstracts..............................................................................................................................1 Author index....................................................................................................................101 - ii - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Institutes on Lake Superior Geology, 1955-2012 # Date Place Chairs 1 1955 Minneapolis, Minnesota C.E. Dutton 2 1956 Houghton, Michigan A.K. Snelgrove 3 1957 East Lansing, Michigan B.T. Sandefur 4 1958 Duluth, Minnesota R.W. Marsden 5 1959 Minneapolis, Minnesota G.M. Schwartz & C. Craddock 6 1960 Madison, Wisconsin E.N. Cameron 7 1961 Port Arthur, Ontario E.G. Pye 8 1962 Houghton, Michigan A.K. Snelgrove 9 1963 Duluth, Minnesota H. Lepp 10 1964 Ishpeming, Michigan A.T. Broderick 11 1965 St. Paul, Minnesota P.K. Sims & R.K. Hogberg 12 1966 Sault Ste. Marie, Michigan R.W. White 13 1967 East Lansing, Michigan W.J. Hinze 14 1968 Superior, Wisconsin A.B. Dickas 15 1969 Oshkosh, Wisconsin G.L. LaBerge 16 1970 Thunder Bay, Ontario M.W. Bartley & E. Mercy 17 1971 Duluth, Minnesota D.M. Davidson 18 1972 Houghton, Michigan J. Kalliokoski 19 1973 Madison, Wisconsin M.E. Ostrom 20 1974 Sault Ste. Marie, Ontario P.E. Giblin 21 1975 Marquette, Michigan J.D. Hughes 22 1976 St. Paul, Minnesota M. Walton 23 1977 Thunder Bay, Ontario M.M. Kehlenbeck 24 1978 Milwaukee, Wisconsin G. Mursky 25 1979 Duluth, Minnesota D.M. Davidson 26 1980 Eau Claire, Wisconsin P.E. Myers 27 1981 East Lansing, Michigan W.C. Cambray 28 1982 International Falls, Minnesota D.L. Southwick 29 1983 Houghton, Michigan T.J. Bornhorst 30 1984 Wausau, Wisconsin G.L. LaBerge 31 1985 Kenora, Ontario C.E. Blackburn 32 1986 Wisconsin Rapids, Wisconsin J.K. Greenberg 33 1987 Wawa, Ontario E.D. Frey & R.P. Sage - iii - �Proceedings of the 58th ILSG Annual Meeting - Part 1 # Date Place Chairs 34 1988 Marquette, Michigan J. S. Klasner 35 1989 Duluth, Minnesota J.C. Green 36 1990 Thunder Bay, Ontario M.M. Kehlenbeck 37 1991 Eau Claire, Wisconsin P.E. Myers 38 1992 Hurley, Wisconsin A.B. Dickas 39 1993 Eveleth, Minnesota D.L. Southwick 40 1994 Houghton, Michigan T.J. Bornhorst 41 1995 Marathon, Ontario M.C. Smyk 42 1996 Cable, Wisconsin L.G. Woodruff 43 1997 Sudbury, Ontario R.P. Sage & W. Meyer 44 1998 Minneapolis, Minnesota J.D. Miller & M.A. Jirsa 45 1999 Marquette, Michigan T.J. Bornhorst & R.S. Regis 46 2000 Thunder Bay, Ontario S.A. Kissin & P. Fralick 47 2001 Madison, Wisconsin M.G. Mudrey & Jr., B.A. Brown 48 2002 Kenora, Ontario P. Hinz & R.C. Beard 49 2003 Iron Mountain, Michigan L. Woodruff & W.F. Cannon 50 2004 Duluth, Minnesota S. Hauck & M. Severson 51 2005 Nipigon, Ontario M. Smyk & P. Hollings 52 2006 Sault Ste. Marie, Ontario A. Wilson & R.Sage 53 2007 Lutsen, Minnesota L. Woodruff & J. Miller 54 2008 Marquette, Michigan T. Bornhorst & J. Klasner 55 2009 Ely, Minnesota J. Miller, G. Hudak & D. Peterson 56 2010 International Falls, Minnesota M. Jirsa, P. Hollings, T. Boerboom, P. Hinz & M. Smyk 57 2011 Ashland, Wisconsin T. Fitz 58 2012 Thunder Bay, Ontario P. Hollings - iv - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Sam Goldich and the Goldich Medal Sam Goldich received an A.B. from the University of Minnesota in 1929, a M.A. from Syracuse University in 1930, and a Ph.D. from the University of Minnesota in 1936. During World War II Sam worked for the U.S. Geological Survey in mineral exploration. In 1948, Sam returned to the University of Minnesota, and became Professor and Director of the Rock Analysis Laboratory the following year. He rejoined the U.S. Geological Survey in 1959 and was appointed as the first Branch Chief of the Branch of Isotope Geology. Sam returned to academia in 1964 when he went to Pennsylvania State University. He left PSU in 1965 and moved to the State University of New York at Stony Brook, where he stayed for 3 years. Restless yet again, he moved to Northern Illinois University in 1968 where he was a professor until his retirement in 1977. Sam’s final move was to Denver where he became an emeritus at the Colorado School of Mines. Sam died in 2000, less than a month before his 92nd birthday. In the late 1970’s, Geological Society of America Special Paper 182, which included seminal geochronological studies by Sam Goldich and coworkers on the Archean rocks of the Minnesota River Valley, was nearing completion. At this time various ILSG regulars began discussing the possibility of recognizing Sam for his pioneering work on the resolution of age relationships and thus the geology of Precambrian rocks in the Lake Superior region. Three members, R.W. Ojakangas, J.O. Kalliokoski and G.B. Morey, presented the idea to the ILSG Board of Directors in 1978. The Board approved the creation of an award, provided funding could be obtained. It was suggested that collecting one or two dollars at registration for a dedicated account would provide resources for striking the medal. A general request was made to the ILSG membership for donations and Sam himself offered a challenge grant to match the contributions. In total $4,000 was collected and thus began the work of creating the Goldich Medal. The initial Goldich Award was presented to Sam by G.B. Morey in 1979 and consisted of a large paper proclamation. For the actual medal, G.B. Morey consulted with the foundry on production details, while Dick Ojakangas and Jorma Kalliokoski worked on the design of the award, suggesting that it be given for “outstanding contributions to the geology of the Lake Superior region.” Simultaneously, a committee of J.O. Kalliokosi, W.F. Cannon, M.M Kehlenbeck, G.B. Morey, and G. Mursky developed the Award Guidelines that were approved by the ILSG Board. By 1981 all the elements of the Goldich Award had come together, and the second recipient, Carl E. Dutton, Jr., received the Goldich Medal for 50 years of significant contributions to the understanding of the geology of the Lake Superior region. Since the beginning, the Awards Committee has consisted of individuals representing industry, government and academia, with each member of the Committee serving for three years. The medal is now awarded every year at the annual ILSG meeting. Reference: Morey, G.B. and Hanson, G.N. (editors). 1980. Selected studies of Archean gneisses and Lower Proterozoic rocks, southern Canadian Shield. Geological Society of America, Special Paper 182, 175 p. Prepared by various Goldich Medal Awardees, 2007 -v- �Proceedings of the 58th ILSG Annual Meeting - Part 1 Institute on Lake Superior Geology Goldich Medal - vi - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Goldich Medal Guidelines (Adopted by the Board of Directors, 1981; amended 1999) Preamble The Institute on Lake Superior Geology was born in 1955, as documented by the fact that the 27th annual meeting was held in 1981. The Institute’s continuing objectives are to deal with those aspects of geology that are related geographically to Lake Superior; to encourage the discussion of subjects and sponsoring field trips that will bring together geologists from academia, government surveys, and industry; and to maintain an informal but highly effective mode of operation. During the course of its existence, the membership of the Institute (that is, those geologists who indicate an interest in the objectives of the ILSG by attending) has become aware of the fact that certain of their colleagues have made particularly noteworthy and meritorious contributions to the understanding of Lake Superior geology and mineral deposits. The first award was made by ILSG to Sam Goldich in 1979 for his many contributions to the geology of the region extending over about 50 years. Subsequent medallists and this year’s recipient are listed in the table below. Award Guidelines 1) The medal shall be awarded annually by the ILSG Board of Directors to a geologist whose name is associated with a substantial interest in, and contribution to, the geology of the Lake Superior region. 2) The Board of Directors shall appoint the Goldich Medal Committee. The initial appointment will be of three members, one to serve for three years, one for two years, and one for one year. The member with the briefest incumbency shall be chair of the Nominating Committee. After the first year, the Board of Directors shall appoint at each spring meeting one new member who will serve for three years. In his/her third year this member shall be the chair. The Committee membership should reflect the main fields of interest and geographic distribution of ILSG membership. The out-going, senior member of the Board of Directors shall act as liaison between the Board and the Committee for a period of one year. 3) By the end of November, the Goldich Medal Committee shall make its recommendation to the Chair of the Board of Directors, who will then inform the Board of the nominee. 4) The Board of Directors normally will accept the nominee of the Committee, inform the medallist, and have one medal engraved appropriately for presentation at the next meeting of the Institute. 5) It is recommended that the Institute set aside annually from whatever sources, such funds as will be required to support the continuing costs of this award. Nominating Procedures 1) The deadline for nominations is November 1. Nominations shall be taken at any time by the Goldich Medal Committee. Committee members may themselves nominate candidates; however, Board members may not solicit for or support individual nominees. 2) Nominations must be in writing and supported by appropriate documentation such as letters of recommendation, lists of publications, curriculum vita’s, and evidence of contributions to Lake Superior geology and to the Institute. 3) Nominations are not restricted to Institute attendees, but are open to anyone who has worked on and contributed to the understanding of Lake Superior geology. - vii - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Selection Guidelines 1) Nominees are to be evaluated on the basis of their contributions to Lake Superior geology (sensu lato) including: a) importance of relevant publications; b) promotion of discovery and utilization of natural resources; c) contributions to understanding of the natural history and environment of the region; d) generation of new ideas and concepts; and e) contributions to the training and education of geoscientists and the public. 2) Nominees are to be evaluated on their contributions to the Institute as demonstrated by attendance at Institute meetings, presentation of talks and posters, and service on Institute boards, committees, and field trips. 3) The relative weights given to each of the foregoing criteria must remain flexible and at the discretion of the Committee members. 4) There are several points to be considered by the Goldich Medal Committee: a) An attempt should be made to maintain a balance of medal recipients from each of the three estates— industry, academia, and government. b) It must be noted that industry geoscientists are at a disadvantage in that much of their work in not published. 5) Lake Superior has two sides, one the U.S., and the other Canada. This is undoubtedly one of the Institute’s great strengths and should be nurtured by equitable recognition of excellence in both countries. - viii - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Goldich Medalists 1979 Samuel S. Goldich 1996 David L. Southwick 1980 not awarded 1997 Ronald P. Sage 1981 Carl E. Dutton, Jr. 1998 Zell Peterman 1982 Ralph W. Marsden 1999 Tsu-Ming Han 1983 Burton Boyum 2000 John C. Green 1984 Richard W. Ojakangas 2001 John S. Klasner 1985 Paul K. Sims 2002 Ernest K. Lehmann 1986 G.B. Morey 2003 Klaus J. Schulz 1987 Henry H. Halls 2004 Paul Weiblen 1988 Walter S. White 2005 Mark Smyk 1989 Jorma Kalliokoski 2006 Michael G. Mudrey 1990 Kenneth C. Card 2007 Joseph Mancuso 1991 William Hinze 2008 Theodore J. Bornhorst 1992 William F. Cannon 2009 L. Gordon Medaris, Jr. 1993 Donald W. Davis 2010 William D. Addison & Gregory R. Brumpton 1994 Cedric Iverson 2011 Dean M. Rossell 1995 Gene La Berge 2012 Goldich Medal Recipient Jim Miller University of Minnesota Duluth, Duluth, Minnesota Goldich Medal Committee Serving through the meeting year shown in parentheses Mary Louise Hill (2012) Lakehead University Laurel Woodruff (2013) United States Geological Survey Graham Wilson (2014) Turnstone Consulting Mary Louise Hill, as out-going senior member of Institute Board of Directors, is liaison between Goldich Medal Committee and the Board through the 2012 meeting. - ix - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Citation for Goldich Medal Recipient Dr. Jim Miller is indeed a worthy recipient of the 2011 Goldich Medal. Jim exemplifies the best of what this award represents on the basis of his contributions to our understanding of Lake Superior geology and his longstanding involvement with the Institute on Lake Superior Geology. Most of us know Jim for his work in the Midcontinent Rift in Minnesota over more than 25 years. The major emphasis of Jim’s research with the Minnesota Geological Survey and the University of Minnesota since 1985 has been field, petrologic, and metallogenic studies of the igneous rocks of this 1.1 billion-year-old rift. His work has focused on producing bedrock geologic maps of the Duluth Complex and related intrusive terranes of the Rift in northeastern Minnesota and on studying the petrology and crystallization history of various intrusions within it. Jim has authored or co-authored dozens of peer-reviewed reports, papers and maps and is regarded world-wide as an expert in the Duluth Complex and the Midcontinent Rift. He has 54 contributions in ILSG publications, including both abstracts and field trip guidebooks. Jim’s work is not purely academic, however, his research and mapping has led to the evaluation of the coppernickel-platinum group element potential of many Midcontinent Rift intrusions. One of the first collaborations that Jim and I worked on was a field trip for the International Geologic Correlation Program looking at mafic intrusions in Ontario and Minnesota, a trip that we reprised in 2002 and 2010 for the International Platinum Symposium. Jim has had a long-standing history with this symposium, presenting papers, workshops and field trips on the economic potential of the Lake Superior region. Jim is highly regarded by geologists in the mineral exploration industry and frequently takes the opportunity to present his research and ideas at industry-based symposia. From 1989 to 2005, Jim was the principal investigator for a series of six biennial grants from the Minnesota State Legislature’s Minerals Coordinating Committee. Specific projects included: • Shallow drilling project, central Duluth Complex; • Geologic mapping in the Duluth area; • Petrology and metallogenesis of the Duluth Complex at Duluth; • Geologic mapping in the Allen quadrangle; • Digital geologic mapping of the central Duluth Complex; • PGE potential of the Sonju Lake Intrusion; • Geology and mineral potential of the Duluth Complex; • Geology and PGE potential of the Greenwood Lake Intrusion and PGE evaluation of mafic intrusions outside the Duluth Complex. Jim’s research, as well as that of the many undergraduate and graduate students he has supervised, has contributed to the understanding of the general tectonic and magmatic history of the Midcontinent Rift and its mineral deposits. This has led to new ideas in rift development and metallogeny. Jim’s ideas and discoveries have led to industry interest and investment, including the drilling of a recently discovered PGE-bearing reef in the Sonju Lake intrusion. In addition to his research, Jim has also worked with the Minnesota Department of Natural Resources to produce geological displays for North Shore State Parks. He continues to be an advocate of public education and works with researchers in different disciplines such as health, public policy and the environment in making informed decisions involving geology and mineral development. -x- �Proceedings of the 58th ILSG Annual Meeting - Part 1 Another major focus of Jim’s work involves educational outreach. Jim frequently speaks to school groups, teaches non-credit classes and leads field trips in an attempt to introduce Minnesota geology to non-geologists. He has been actively involved in planning and implementation of workshops for kindergarten to Grade 12 earth science teachers for many years. Jim has taught Compleat and Practical Scholar Courses, non-credit classes on Minnesota’s geology sponsored by the University of Minnesota’s General College. Course offerings have included: • Drifting Continents/Expanding Oceans: An Introduction to the Dynamic Earth (1994-98); • The Making of Laurentia – Minnesota’s Geologic History (1994-99); • What’s this Rock? An Introduction to the Geology of Minnesota’s North Shore (2003-05). Since 2006, Jim has taught non-credit, one-day summer field classes at the University of Minnesota’s College of Continuing Education’s Curiosity Camp entitled “A Geology Tour of the Twin Cities”. The Teacher Inquiry-based Minnesota Earth Science (TIMES) Project was a federally funded program sponsored by Hamline University that organized two week-long summer field courses for middle school earth science teachers in Minnesota. Jim served as guest lecturer and assisted on field trips for projects between 2000 and 2009 and was the lead instructor for the 2008 program. The Minnesota Mineral Education Workshop began in 1997. Jim served as an instructor, field trip leader and program coordinator for this annual three-day workshop for K-12 earth science teachers that offers short courses and field trips on the geology and mineral resources of Minnesota. He has also lectured on the geology of Minnesota for the national Elderhostel Program since 1986. Jim, now a professor at the University of Minnesota-Duluth, continues to enthusiastically teach geology and encourage students. He is a staunch advocate of economic geology and tirelessly lobbies administration, government and industry for their ongoing support of the geology program. Most recently, Jim has spearheaded the creation and administration of the Precambrian Research Center (PRC), which was established to satisfy an urgent, long-term need for geologists trained in the mapping and study of Precambrian rocks and their mineral deposits. As a director, Jim continues to canvass the mineral industry for sponsorships, thesis support and ideas on maintaining the relevance of the PRC. The PRC organizes professional workshops and is planning to take a major role in managing the popular Minnesota Minerals Education Workshop for teachers. Jim is a member of the Geological Society of America (GSA), Prospectors and Developers Association of Canada (PDAC), Society for Mining, Metallurgy, and Exploration (SME) and the Society of Economic Geologists (SEG). He served in 1995 as Co-chair of UNESCO-sponsored International Geologic Correlation Program (IGCP) Project 336 on the topic of mineral deposits associated intracontinental rift systems. From 2003 to the present, Jim has served as principal investigator for one-year grants from the U.S. Geological Survey’s EDMAP program, which go toward funding student field research. He is currently Vice Chair of Operations, Minnesota Center for Minerals Resource Education and was Field Trip Chairman for the 2011 Geological Society of America meeting in Minneapolis. Jim has co-chaired the annual meeting of the Institute on Lake Superior Geology three times and has remained a staunch supporter of the Institute for over 30 years (1979-present). His numerous contributions to our understanding of Lake Superior geology are well-regarded by colleagues, peers and clients alike. He is a vocal advocate of education in the earth sciences. He continues to be captivated by the geology and mineral potential of the Lake Superior region. He exemplifies the dedication and enthusiasm that are the hallmarks of previous Goldich Medal recipients. Jim is indeed a worthy addition to this list. Submitted by Mark Smyk, P.Geo. Ontario Geological Survey, Thunder Bay - xi - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Eisenbrey Student Travel Awards The 1986 Board of Directors established the ILSG Student Travel Awards to support student participation at the annual meeting of the Institute. The name “Eisenbrey” was added to the award in 1998 to honor Edward H. Eisenbrey (1926-1985) and utilize substantial contributions made to the 1996 Institute meeting in his name. “Ned” Eisenbrey is credited with discovery of significant volcanogenic massive sulfide deposits in Wisconsin, but his scope was much broader—he has been described as having unique talents as an ore finder, geologist, and teacher. These awards are intended to help defray some of the direct travel costs of attending Institute meetings, and include a waiver of registration fees, but exclude expenses for meals, lodging, and field trip registration. The number of awards and value are determined by the annual Chair in consultation with the Secretary and Treasurer. Recipients will be announced at the annual banquet. The following general criteria will be considered by the annual Chair, who is responsible for the selection: 1) The applicants must have active resident (undergraduate or graduate) student status at the time of the annual meeting of the Institute, certified by the department head. 2) Students who are the senior author on either an oral or poster paper will be given favored consideration. 3) It is desirable for two or more students to jointly request travel assistance. 4) In general, priority will be given to those in the Institute region who are farthest away from the meeting location. 5) Each travel award request shall be made in writing to the annual Chair, and should explain need, student and author status, and other significant details. Successful applicants will receive their awards during the meeting. Student Paper Awards Each year, the Institute selects the best of the student presentations and honors presenters with a monetary award. Funding for the award is generated from registrations of the annual meeting. The Student Paper Committee is appointed by the annual meeting Chair in such a manner as to represent a broad range of professional and geologic expertise. Criteria for best student paper—last modified by the Board in 2001—follow: 1) The contribution must be demonstrably the work of the student. 2) The student must present the contribution in-person. 3) The Student Paper Committee shall decide how many awards to grant, and whether or not to give separate awards for poster vs. oral presentations. 4) In cases of multiple student authors, the award will be made to the senior author, or the award will be shared equally by all authors of the contribution. 5) The total amount of the awards is left to the discretion of the meeting Chair in conjunction with the Secretary, but typically is in the amount of about $500 US (increase approved by Board, 10/01). 6) The Secretary maintains, and will supply to the Committee, a form for the numerical ranking of presentations. This form was created and modified by Student Paper Committees over several years in an effort to reduce the difficulties that may arise from selection by raters of diverse background. The use of the form is not required, but is left to the discretion of the Committee. 7) The names of award recipients shall be included as part of the annual Chair’s report that appears in the - xii - �Proceedings of the 58th ILSG Annual Meeting - Part 1 next volume of the Institute. Student papers will be noted on the Program. ILSG Student Research Fund The 2005 Board of Directors established the ILSG Student Research Fund with $10,000 US from the Institute’s general fund to encourage student research on the geology of the Lake Superior region. A minimum of two awards of $500 US each for research expenses (but not travel expenses) will be made each year. Students are expected to present their research orally or during a poster session at an ILSG meeting. The award winners will also be automatically eligible for the Eisenbrey Travel Awards. To allow the fund to grow, the Fund will receive one-half of any additional proceeds from each annual meeting, after all other commitments and expenses are covered. • The ILSG Board of Directors will be responsible for selecting a minimum of two awards each year. The ILSG Treasurer will issue the awards. • The ILSG Student Research Fund is available for undergraduate or graduate students working on geology in the Lake Superior region. • The applications are due to the ILSG Secretary by August 31st of each year Awards will be made by October 1st of each year. • Names of the award recipients will be announced at the next annual meeting and posted on the ILSG website. • Details of the application process can be found on the ILSG web site. • The proposal will need to be signed by the researcher’s supervisor. In 2011 the ILSG Board of Governors awarded two $500 awards from the Student Research Fund. The winners were Evgeniy Kulakov from the Department of Geological and Mining Engineering and Sciences, Michigan Technological University for his work on the “Paleomagnetism of the Geordie (Coubran) Lake Basalts” and Ernest Thalhamer from the University of Wisconsin - Milwaukee for his work on “Shear Zones and Strain in the Rainy Lake area”. Student Paper Awards Committee Ryan Weston - Magma Metals (Canada) Ltd. Gordon Medaris Jr. - University of Wisconsin – Madison Michael Easton - Ontario Geological Survey Session Chairs Mary Louise Hill - Lakehead University George Hudak - Precambrian Research Centre Mark Severson - Natural Resources Research Institute Mark Smyk - Ontario Geological Survey Ann Wilson - Ontario Geological Survey Laurel Woodruff - US Geological Survey - xiii - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Board of Directors Board appointment continues through the close of the meeting year shown in parentheses, or until a successor is selected Mark Smyk - General Chair 2012 meeting (2015) Tom Fitz (2014) - Northland College, Ashland, Wisconsin Peter Hinz (2013) - Ontario Geological Survey George Hudak (2012) - NRRI, Duluth Pete Hollings - Secretary (2013) - Lakehead University, Thunder Bay, Ontario Mark A. Jirsa - Treasurer (2014) - Minnesota Geological Survey Local Committee Chair Pete Hollings - Lakehead University, Thunder Bay, Ontario Volume Editors Pete Hollings & Bill Addison - Lakehead University, Thunder Bay, Ontario Allan MacTavish - Magma Metals (Canada) Ltd., Thunder Bay, Ontario Organising Committee Bill Addison - Thunder Bay, Ontario Dorothy Campbell - Ontario Geological Survey, Thunder Bay, Ontario Peter Hinz - Ontario Geological Survey, Thunder Bay, Ontario Al MacTavish - Magma Metals (Canada) Ltd. Mark Smyk - Ontario Geological Survey, Thunder Bay, Ontario Banquet Speaker Dr. David Overstreet Human Adaptation to Late Pleistocene Landscapes - A View from Southeastern Wisconsin Study of mammoth bone piles in Kenosha County Wisconsin has provided evidence of human-mammoth interaction between 13,500-12,500 radiocarbon years before present. Chipped stone tools in association with two of the bone piles and micro-wear analyses support the thesis that the carcasses were butchered. Bone modification analyses also support this conclusion. Whether or not the mammoth remains represent active or moribund prey is open to debate and thus the results do not speak to the role of human predation in Late Pleistocene extinctions. - xiv - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Report of the Chair of the 57th Annual Meeting Ashland, Wisconsin The 57th ILSG was held in Ashland, Wisconsin on May 18-21, 2011. Hosted by Northland College and chaired by Tom Fitz, the meeting was attended by a total of 247 people, with most people also attending field trips. Seventy-one students attended, 30 of who received financial support through the Eisenbrey scholarship fund. The two-day technical session began on Thursday morning with oral presentations on regional geology and then proceeded in chronological order from Archean topics through Friday afternoon’s presentations on Quaternary geology. A total of 30 talks were given, 16 of which were presented by students. The poster sessions were also arranged chronologically from Archean through recent, with a total of 24 posters, 18 of which were presented by student authors. The 2011 Goldich Medal was awarded to Dean Rossell of Kennecott Exploration Company. Doug Duskin presented the award during the annual banquet and supplied numerous examples of how Dean’s research has expanded our understanding of ore deposits in the region. Huifang Xu of the University of Wisconsin – Madison gave the keynote address, talking about the origin of banded iron formations by interaction between komatiite and seawater. The meeting offered nine field trips that highlighted the geology of the southwestern Lake Superior region. Three pre-meeting trips were run on Wednesday, including Jim Miller’s trip “Igneous Stratigraphy of the Layered Series at Duluth – Type Intrusion of the Duluth Complex”; Marcia Bjørnerud and Bill Cannon’s “Midcontinent Microcosm: Geology of the Atkins Lake – Marengo Falls Area”; Dick Ojakangas, Drew Cramer, and Tom Fitz’s trip “Geology of the Bayfield Peninsula: Keweenawan Bayfield Group and Pleistocene Deposits”. Friday’s technical sessions and the presentation of student awards were followed by three late-afternoon field trips: “Geology and Remediation at the Ashland/Northern States Power Site” led by Jamie Dunn; “Bad River Watershed Culvert Restoration Program” led by Michele Wheeler and Cassandra Bodette; and “Geology of Copper Falls State Park” led by Allison Mills, Drew Cramer, and Tom Fitz. Three field trips were run on Saturday: “Geology of the Montreal River Monocline” led by Bill Cannon; “The Archean/Paleoproterozoic Unconformity near Denham, Minnesota” led by Terry Boerboom; and “Granitic, Gabbroic, and Ultramafic rocks of the Keweenawan Mellen Intrusive Complex” led by Tom Fitz. On Friday evening Northland College hosted a beer and barbeque social event for ILSG participants outside the college’s student center. A variety of door prizes were given away and a good time was had by all. The Institute’s Board of Directors met on May 19, 2011 to discuss business and preliminary plans for future Institutes. The meeting was attended by Ted Bornhorst, Peter Hinz, George Hudak, Mark Jirsa (Treasurer), Pete Hollings (Secretary), and Tom Fitz (2011 Chair). Secretary Hollings took the minutes of the meeting, that are as follows: 1. Accepted report of the Chairs for the 56th ILSG, International Falls, Minnesota; as printed in the Proceedings Volume (Hinz), and minutes of last Board meeting, May 20, 2010 (Hollings). 2. Received, discussed, and accepted 2010-2011 ILSG Financial Summary (Jirsa). 3. Received, discussed, and accepted 2010-2011 report of the Secretary (Hollings). 4. Approved Jirsa to continue as Institute Treasurer until 2014 (this was later presented to the membership and approved). - xv - �Proceedings of the 58th ILSG Annual Meeting - Part 1 5. Approved Tom Fitz as on-going ILSG Board member. 6. Approved Thunder Bay, Ontario at the site for the 58th annual ILSG meeting. The meeting will be hosted by Pete Hollings with help from Mark Smyk and Peter Hinz. Ted Bornhorst offered to host the 59th Annual Meeting in Houghton. 7. Discussed and approved replacing Al MacTavish as the “member from industry” on Goldich Committee (end of term 2011) with Graham Wilson. 8. There was some discussion as to whether or not presentations from the meeting should be posted on the ILSG web site or included on a CD given to participants. 9. It was agreed that a copy of the DVD with all proceedings to date would be included with the volumes mailed out to our standing orders. The Chair would like to thank the student committee made up Northland College students Kristi Wilson, Allison Mills, Cassie Bodette and Drew Cramer for their hard work in organizing and running the meeting. Appreciation also goes to the field trip leaders and session chairs, to the sponsors of the meeting, to the ILSG Board and the many people who worked to make the 2011 Institute a successful, educational, and enjoyable meeting. Respectfully submitted, Tom Fitz Chair, 57th Institute on Lake Superior Geology - xvi - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Sponsors The following organizations made generous contributions to the 58th Annual Meeting. We thank them for their commitment to the Institute on Lake Superior Geology. For the past 50 years this organization has thrived as a result of the interest of individuals, corporations, universities and government agencies. The dedication to an exchange of scientific ideas and a passion for field trips has enabled the Institute to provide one of its primary objectives – to promote better understanding of the geology of the Lake Superior region. - xvii - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Program Wednesday May 16 7:30 a.m. Field Trip 3: Lac des Iles Pd mine Leader: John Corkery (North American Palladium Ltd.) 8:00 a.m. Field Trip 1: Sudbury Impactoclastic Debrisites at Thunder Bay Leaders: Bill Addison and Greg Brumpton 8:00 a.m. Field Trip 2: Geology of the Sibley Peninsula Leaders: Dr. Philip Fralick (Lakehead University) Mark Smyk & Riku Metsaranta (Ontario Geological Survey) 8:00 a.m. Field Trip 4: The Shebandowan greenstone belt Leaders: Alan Aubut (Sibley Basin Group Geological Consulting Services Ltd.) and Dorothy Campbell (Ontario Geological Survey) 6:00 p.m. Return of Trips 1-4 4:00 p.m. - 8.00 p.m. Registration (Barcelona Room, Travelodge Airlane) 7:00 p.m. - 9.00 p.m. Ice Breaker Social (Barcelona Room, Travelodge Airlane), Poster Setup (Tiberio Room, Travelodge Airlane) and Core Shack (Courtyard, Travelodge Airlane) Thursday May 17 7:30 a.m. - 4:00 p.m. Registration (Barcelona Room, Travelodge Airlane) 8:20a.m. - 8:30 a.m. Introductory Remarks - Peter Hollings, Chair Technical Session I NOTE: Asterisk * denotes a student eligible for a Best Student Paper Award Session Chairs: Ann Wilson (Ontario Geological Survey) and Mary Louise Hill (Lakehead University) 8:30 a.m. Gilbert, H. Stratigraphy and tectonic setting of Neoarchean arc volcanic rocks in the Bird River Belt, Manitoba, Canada 8:50 a.m. Chaffee, M.*, Miller, J., Hollings, P., Heggie, G., MacTavish, A., and Bandli, B. Petrographic and geochemical study of the hybrid rock unit associated with the Current Lake Intrusive Complex, Magma Metals’ Thunder Bay North Property 9:10 a.m. Brooker, B.* and Miller, J. Geology and petrology of a Mesoproterozoic layered mafic intrusion in portions of the Brule Lake and Cherokee Lake 7.5’ Quadrangles, northeastern Minnesota 9:30 a.m. Good, D., McLean, K., Epstein, R., Linnen, R., and Samson, I. Geology of the Layered Series, Coldwell Alkaline Complex, Ontario 9:50 a.m. Cundari, R.*, Hollings, P. and Smyk, M. Petrogenesis and crustal contamination of the Nipigon sills: a geochemical and spatial reevaluation 10:10 a.m. - 10:40 a.m. Coffee Break, Poster Session and Core Shack - xviii - �Proceedings of the 58th ILSG Annual Meeting - Part 1 10:40 a.m. Rousell, D., Petrus, J., Easton, R., Tinkham, D. and Napoli, M. The tectonometamorphic, magmatic and mineralization history of the Wanapitei Complex, Grenville Front tectonic zone, Ontario 11:00 a.m. Berkley, J. New York’s Adirondack Mountains: Window to the Mesoproterozoic Grenville Orogen 11:20 a.m. Van Lankvelt, A.*, Schneider, D., Hattori, K. and Biczok, J. Anatomy of a Mesoarchean batholith 11:40 a.m. Kuzmich, B.*, Hollings, P. and Campbell, D. Geochemistry and petrology of the Dog Lake Granite Chain, Quetico Basin, Northwestern Ontario 12:00 p.m. - 1:30 p.m. Lunch Break, Poster Session and Core Shack (ILSG Board Meeting by invitation) Technical Session II Session Chair: Mark Severson (Natural Resources Research Institute) and Laurel Woodruf (USGS) 1:30 p.m. Horner, S.* and Fralick, P. Chert stromatolites in the basal Gunflint Formation, Kakabeka Falls: Primary precipitation or silicification? 1:50 p.m. Phillips, B., Dean, F. and Zaniewski, K A catastrophic breach and inter-lake flow through the Marks Moraine via Brule Creek and Cedar Creek, Thunder Bay Region, Ontario. A small but significant detail of the eastern outflow history of Lake Agassiz 2:10 p.m. Beh, B.* and Fralick, P. Depositional processes operating on the Paleoproterozoic Gowganda ice margin: an example from the Espanola area, Ontario 2:30 p.m Kerkermeier, L.* and Fralick, P. Ferromanganese precipitates in lacustrine environments of Northwestern Ontario and Nova Scotia: Effects on arsenic concentration 2:50 p.m. - 3:20 p.m. Coffee Break, Poster Session and Core Shack 3:20 p.m. Frederiksen, A., Deniset, I., Bollman, T., Van der Lee, S. and Darbyshire, F. Erosion of Archean lithosphere by subduction and rifting: a tomographic image of central North America 3:40 p.m. Pesonen, L. and Veikkolainen, T. Supercontinents during the Proterozoic – A paleomagnetic view with Keweenawan data (Lake Superior) as an example 4:00 p.m. Swanson-Hysell, N., Burgess, S., Maloof, A., and Bowring, S. Temporal context of the Mamainse Point succession: a record of magmatic activity and fast plate motion across the “latent stage” of Midcontinent Rift development 4:20 p.m. Easton, R.M. The source of the Elliot Lake uranium ores: The neodymium isotope story 4:40 p.m. Medaris, G., Driese, S., Boerboom, T. and Jirsa, M. Granitic saprolites in the Lake Superior region: K-metasomatism, intensity vs. magnitude of weathering, and estimates of atmospheric pCO2 - xix - �Proceedings of the 58th ILSG Annual Meeting - Part 1 7:00 p.m Annual Banquet and Award Presentation (Barcelona Room, Travelodge Airlane) Announcement of 59th Annual Meeting Location 2012 Goldich Award Presentation to Jim Miller 2012 Banquet Address - Dr. D. Overstreet Meeting participants not registered for the banquet are welcome to attend the address Friday May 18 9:00 a.m. - 12:00 p.m. Registration Technical Session III Session Chairs: George Hudak (Precambrian Research Centre) and Mark Smyk (OGS) 8:30 a.m. Stinson, V.* and Hill, M.L. Regional to microstructural control of gold mineralization along the Quetico-Wabigoon subprovince boundary 8:50 a.m. Thalhamer, E.* and Czeck, D. Analyzing ductile shear zone network geometries in the Grassy Portage Sill, Rainy Lake Region, Northwestern Ontario, Canada 9:10 a.m. Heggie, G., MacTavish, A., Johnson, J., Weston, R. and Ma, L. Structural control on the emplacement of the TBN-Igneous Complex 9:30 a.m. Bailes, A., Galley, A., Paradis, S.and Taylor, B. Using large synvolcanic alteration zones to explore for VMS deposits at Snow Lake, Manitoba, Canada 9:50 a.m. Scott, G., Karakus, M., Shinkle, D. and Mitchell, A. Exploration history, mineralogy and genesis of the Black Thor chromite deposit, Ring of Fire Intrusion, Northwestern Ontario 10:10 a.m. - 10:40 a.m. Coffee Break, Poster Session and Core Shack 10:40 a.m. Bornhorst, T. and Williams, W. The Copperwood sediment-hosted stratiform copper deposit, Upper Peninsula, Michigan 11:00 a.m. Blaske, A., Braun, G., Leier-Engelhardt, P., Mottl, R., Anderson, D. and Bornhorst, T Glacial geology of the Copperwood Project, Gogebic County, Michigan, and environmental and engineering implications for site development 11:20 a.m. Waggoner, T and Karakus, M. The Little Commonwealth exploration - an example of a SEDEX Deposit 11:40 a.m. Mikkelsen, L. and Conly, A. Predictive water level and preliminary hydrodynamic models of Caland and Hogarth pit lakes, Atikokan, Ontario 12:00 p.m Presentation of Best Student Paper Award and Eisenbrey Awards 1:00 p.m. Field Trip 5: Geology of the City of Thunder Bay Leader: Mark Smyk (Ontario Geological Survey) 1:00 p.m. Field Trip 6: Panorama Amethyst Mine Leader: Steve Kissin (Lakehead University) - xx - �Proceedings of the 58th ILSG Annual Meeting - Part 1 1:00 p.m. Field Trip 7: Port Arthur building stone walking tour Leader: Peter Hinz (Ontario Geological Survey) 6:00 p.m. Return of Trips 5-7 7:00 p.m BBQ (Whitewater Golf Club - bus service provided leaving from the Travelodge Airlane beginning at 6.30 p.m.) Poster Presentations Baumann, S. and Cummings, K. Bedrock Geology of Copper Falls State Park, Ashland County, Wisconsin Boerboom, T. Bedrock geologic map of Morrison County, Central Minnesota Bollmann, T.*, Van der Lee, S., Frederiksen, A., Lou, X. Preliminary results of P- and S-wave delay times in the Superior Region from transportable array and SPREE stations Buchholz, T., Falster, A. and Simmons, W. Accessory minerals of a roadside pegmatite, Orr, Minnesota Craddock, S.* and Craddock, J. Strain variations in carbonates across the Proterozoic Grenville Orogen Cundari, R.*, Hollings P., Scott, J. and Campbell, D. Geology and geochemistry of the Coubran Lake Basalts, a Midcontinent Rift-related sequence within the central Coldwell Complex, Marathon, Ontario Gaspar, B*, and Hill, M.L. The black line faults of the Red Lake gold mines, Ontario Gasparotto, M.*, and Hill, M.L. A microstructural study of the Otjikoto Deposit, Namibia: Inferences from gold mineralization in the Superior Province Goscinak, C.* and Hansen, V. Quartz Fabric analysis of the Kawishiwi Shear Zone, NE Minnesota Heim, N., Scott, H., Kilduff, R., Rahtz, C., Vial, A., Young, S., Mahr, C., and Hudak, G. Preliminary bedrock geology map of the Eastern part of Lake Vermilion State Park, St. Louis County, NE Minnesota Hudak, G., Monson Geerts, S., Zanko, L., Severson, A., Severson, A. and Bandli, B. The Minnesota Taconite Workers Health Study: Environmental Study of Airborne Particulates - 2012 Update Jirsa, M. Bedrock geologic map of the Crane Lake and Brule Narrows 30’X60’ quadrangles, Quetico subprovince, northern Minnesota Jirsa, M., Baggetto, L., Eliason-Johnson, G., Hansen, D., Hoxsie, E., Kilpatrick, K. Reconnaissance geologic mapping of Neoarchean rocks in the central Boundary Waters Canoe Area Wilderness by students of the Precambrian Research Center’s 2011 field camp. - xxi - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Johnson, T.*, Shannon, J., Boerboom, T., Wendlandt, R. Geochemistry of mafic dikes from the Carlton Dike Swarm in Minnesota Kern, A.*, Kulakov, E., Smirnov, A. and Diehl, J. Paleomagnetism of the alkaline Coldwell Complex: New Results, new Insights. Koroscil, J.*, Hill, M.L. and Fralick, P. Thrust faulting in the Gunflint Formation north of Lake Superior Kulakov, E.*, Smirnov, A. and Diehl, J. Paleomagnetism of the Geordie Lake and Silver Mountain basalts Lee, A.*, Albers, P., Miller, J., Severson, M. and Deen, T. Bedrock Geologic map of the Seine Bay/Bad Vermilion Lake intrusion, Mine Centre, Ontario McCormick, K. and Paterson, C. Mafic intrusions along the southern boundary of the Superior Craton, SD: What is their potential for NiCu-PGE mineralization? Miller, J., Brooker, B., Asp, K., Leu, A., Parisi, A. and Sletten, D. 2011 Precambrian Field Camp Mapping in the Sawbill Lake Area, Cook County, Northeastern Minnesota Pietrzak-Renaud, N. A Metallogenic Model and Geometallurgical Treatment of the Basal Iron Ores within the Negaunee Iron Formation, Tilden Mine, Michigan Piispa, E.*, Smirnov, A., Pesonen, and Diehl, J. Paleomagnetic, rock magnetic and geochemical data from the Keweenawan dykes and baked rocks from the Little Mountain and Sugar Loaf areas, Michigan, USA Roe, C.*, and Bjørnerud, M. The ca. 1650 Ma Freedom Formation, Baraboo Range, Wisconsin: A late iron formation in the Lake Superior Region Santaguida, F., Lappalainen, M., Jones, S., Voipo, T., Siikaluoma, J. and Ylinen, J. Geologic Setting of the Kevitsa Ni-Cu-PGE Deposit, Central Lapland Greenstone Belt, Finland Selagi, J.*, and Hill, M.L. Microstructural analysis of porcellaneous nepheline syenite, Coldwell Complex, Ontario Shahabi Far, M.*, Samson, I., Gagnon, J., Linnen, R., Good, D. Textural and compositional variation in apatite and plagioclase in the Marathon PGE-Cu deposit, Northwestern Ontario; implications for fluid-rock interaction Taylor-Hollings, J. Quartz quarry sites in northwestern Ontario: A new ‘prospect’ for geoarchaeologists investigating lithic raw material sources Van der Lee, S., Frederiksen, A., Bollmann, T. and Spree Team. SPREE: Field experiment to study deep structure of the Midcontinent Rift Vaughan, A.*, Swanson Hysell, N. and Feinberg, J. Paleomagnetic data in stratigraphic context from 1.1 Ga Osler Group basalt flows on Simpson Island, Ontario: Evidence for rapid plate motion of Laurentia in the late Mesoproterozoic - xxii - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Wilson, G. and McCausland, P. The curious meteorite harvest of the Lake Superior Region, I. Overview. Wilson, G. and Kissin, S. The curious meteorite harvest of the Lake Superior Region, II. Gems in the Rough NOTE: Asterisk * denotes a student eligible for a Best Student Paper Award Saturday May 19 6:00 a.m. Field Trip 12: Musselwhite gold mine (participants should check in at the Wasaya Airlines counter at Thunder Bay Airport by 6 AM for a 7 AM departure) Leaders: John Biczok (Musselwhite Mine) 8:00 a.m. Field Trip 8: Highway 527 Transect Leaders: Mark Smyk (OGS) and Philip Fralick (Lakehead University) 8:00 a.m. Field Trip 9: Rehabilitation of the Past-Producing Shebandowan and North Coldstream Mines Leader: Mark Puumala (OGS) 8:00 a.m. Field Trip 10: Geoarchaeology of Thunder Bay Leaders: Scott Hamilton (Lakehead University) 8:00 a.m. Field Trip 11: Midcontinent rift intrusions Leaders: Rob Cundari & Pete Hollings (Lakehead University) and Mark Smyk (Ontario Geological Survey) 8:00 a.m. Field Trip 13: Sudbury Impactoclastic Debrisites at Thunder Bay Leader: Greg Brumpton 6.00 p.m. Return of Trip 8 to Armstrong, overnight there 6.00 p.m. Return of Trips 9-11 & 13 7.45 p.m. Return of Trip 12 to Thunder Bay Airport Sunday May 20 8:00 a.m. Field Trip 8: Highway 527 Transect Leaders: Mark Smyk (OGS) and Philip Fralick (Lakehead University) 6.00 p.m. Return of Trip 8 - xxiii - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Using large synvolcanic alteration zones to explore for VMS deposits at Snow Lake, Manitoba, Canada BAILES, Alan, Bailes Geoscience, 6 Park Grove Drive, Winnipeg, MB, R2J3L6, Canada, GALLEY, Alan, Exploration Research Director, CMIC, 2-250-155 Queens Street, Ottawa ON, Canada, PARADIS, Suzanne, GSC, P.O. Box 6000, 9860 West Saanich Road, Sidney, BC V8L 4B2, Canada and TAYLOR, Bruce, GSC, 601 Booth Street, 7th Floor, Room 702, Ottawa, ON K1A 0E8, Canada Due to its rich mineral endowment, excellent outcrop exposure and large domains of hydrothermally altered rocks, the juvenile 1.89 Ga Snow Lake arc volcanic assemblage is an important natural laboratory for applying modern volcanic massive sulphide theories to ancient deposits that are well exposed in cross section. For 25 years, a series of multidisciplinary investigations have examined regionally metamorphosed products of subseafloor hydrothermal events at Snow Lake (Galley, 1993; Galley et al, 1993; Bailes and Galley 1996, 2007; Galley at al., 2002). This presentation will summarize some of the important findings of these studies and place the VMS mineralization, including the recently discovered 30 million tonne Au-rich Lalor Lake VMS deposit, into a stratigraphic-hydrothermal model. Alteration zones at Snow Lake are characterized by extensive zones with anomalous 1.82 Ga metamorphic mineral assemblages, including porphyroblasts of garnet, staurolite, amphibole, biotite, gahnite and/or kyanite, produced from altered rocks created during pre-metamorphic, 1.89 Ga, synvolcanic, hydrothermal, fluid-rock interaction. Three separate episodes of hydrothermal alteration (Fig. 1) are recognized that span evolution of the host volcanic rocks from a primitive (Anderson sequence) to mature arc (Chisel sequence) geotectonic setting. Figure 1. Schematic cross section of large-scale zones of alteration affecting rocks of the Snow Lake arc assemblage. The alteration zones comprise those associated with the Anderson sequence VMS deposits (Anderson alteration zone), those associated with a formational pyrrhotite-pyrite zone (Foot-Mud alteration zone) and those associated with the Chisel-Lalor VMS deposits (Chisel-Lalor alteration zone). The alteration pipe west of Chisel Lake VMS deposit (C) is the footwall pipe for the Lalor Lake VMS deposit (not shown). -1- �Proceedings of the 58th ILSG Annual Meeting - Part 1 The geological, geochemical, mineralogical and isotopic attributes of the zones indicate that they include VMSrelated and VMS-unrelated, and were produced at high- and low- temperatures and in seafloor and sub seafloor (intra-stratal) environments. Semi-conformable alteration zones at Snow Lake are up to 20 km in strike length and 0.8 km wide. Their large exploration ‘footprint’ compared to associated ‘pipe-like’ alteration zones and VMS deposits means that such zones provide a useful target to ‘vector-in’ exploration to VMS depositional settings within volcanic belts. The VMS-related semi-conformable alteration zones at Snow Lake display diagnostic variations in intensity and style of alteration along strike towards VMS deposits, are stratigraphically underlain by altered portions of synvolcanic intrusions, are cut by discordant zones of more intensely altered rocks, and can be demonstrated to have formed by interaction with high temperature(>350°C) hydrothermal fluids. References Bailes, A.H, and Galley, A.G., 1996. Setting of Paleoproterozoic volcanic-associated massive sulphide deposits, Snow Lake, Manitoba, in Bonham-Carter G.F., Galley A.G., Hall, G.E.M. (eds.) EXTECH I: A multidisciplinary approach to massive sulphide research in the Rusty Lake and Snow Lake greenstone belts, Manitoba, Geological Survey of Canada, Bulletin 426, p. 105-138. Bailes, A.H., and Galley, A.G., 2007. Geology of the Chisel–Anderson lakes area, Snow Lake, Manitoba (NTS areas 63K16SW and west half of 63J13SE); Manitoba Science, Technology, Energy and Mines, Manitoba Geological Survey, MAP Geoscientific map 2007-1, 1 colour map with accompanying notes, scale 1:20 000. Galley, A.G., 1993. Charactersitics of semiconformable alteration zones associated with volcanogenic massive sulphide districts, Journal of Geochemical Exploration, v. 48, p. 175-200. Galley, A.G., Bailes, A.H., and Kitzsler, G., 1993- Geological setting and hydrothermal evolution of the Chisel Lake and North Chisel Zn-Pb-Ag-Au massive sulphide deposit, Snow Lake, Manitoba, Exploration Mining Geology, v. 2, p. 271295. Galley, A.G., Bailes, A.H., Hannington, M., Holk, G., Katsube, J., Paquette, F., Paradis, S., Santaguida, F., and Taylor, B, compiled by B. Hillary, 2002. Database for CAMIRO Project 94E07: interrelationships between subvolcanic intrusions, large-scale alteration zones and VMS deposits, Ontario, Manitoba, Quebec and Sweden, Geological Survey of Canada Open File 4431, 1-CD-ROM. -2- �Proceedings of the 58th ILSG Annual Meeting - Part 1 Bedrock Geology of Copper Falls State Park, Ashland County, Wisconsin BAUMANN, Steven D.J. and CUMMINGS, Katy E., Midwest Institute of Geosciences and Engineering, Geology Section, Chicago, Illinois, email: steveb@migeweb.org The geology of Copper Falls State Park in Wisconsin has been a point of geologic and economic interest since the 19th century. The Park contains both Precambrian sedimentary rocks that have been tilted vertical and Precambrian igneous intrusions. The sedimentary rocks are comprised of the Oronto Group and are about 1060 Ma to about 1088 Ma. Beneath the sedimentary rocks are the basalt and andesite flows of the Powder Mill Group, which includes the local “Sheep Farm Rhyolite”. The Powder Mill Group ranges in age from about 1100 Ma to 1109 Ma old. The Powder Mill Group is intruded by the 1102 Ma Mellen Intrusive Complex which is a mix of granites, ferrodiorites, gabbros, and granoporphyries. In June of 2011, the authors mapped the Park at a scale of 1:14,400 and an area of about 6 mi2. The result was a bedrock geology map with new information on the structure and geologic history of the Park. If you tour the Park you will see yellow signs depicting a fault between the Powder Mill and Oronto Groups. The Powder Mill Group does show signs of breccia within, but this is more likely the cause of “reworked” igneous flows. Plus there is no evidence for faulting anywhere else in the Park. If faulting had occurred, it most likely would have affected the softer Oronto Group sediments. There is evidence on top of the Sheep Farm Rhyolite of a thin “weathered” zone. The field evidence suggests that the Powder Mill-Oronto Group contact is erosional and not faulted. There are four distinct alternating flows of basalt and andesite of the Powder Mill Group within the Park. The individual flows were not named but contain an andesite at the base and basalt at the top and are part of the Kallandar Creek deposits. These four main flows show internal signs of brecciation and minor sedimentary deposits. There are likely more flows within the four larger flows, but their lateral extent is difficult to determine with the existing exposures in the Park. The Oronto Group shows some internal variations previously unnoted. The Freda Sandstone exhibits an intertonguing relationship between sandstone and shale, with two large sandstone tongues. The Sandstone facies also exhibits sharp glauconitic contacts with the finer shale facies at the Horseshoe Outcrop (Fig. 1A). The two basal Oronto Formations, the lower Copper Harbor Conglomerate and the younger Nonesuch Shale show a complex intertonguing relationship at Devil’s Gate, where they are excellently exposed (Fig. 1B). The two basal formations are also at their thinnest in the park and pinch out near the west center of the Park. This is due to their deposition on significant paleotopography of the Powder Mill Group. The overall structure of the geologic units in the Park is vertical bedding from Brownstone Falls to Devil’s Gate. As you head north the beds begin to deviate from the vertical and dip about 80° to the northwest and the finer facies of the Freda Sandstone dominates. The Powder Mill and Oronto Groups were deposited nearly flat and deformed into their near vertical position by the Mellen Creek Intrusions to the south and southeast. Like the Copper Harbor Conglomerate and Nonesuch Shale, the Sheep Farm Rhyolite pinches out within the Park (Fig. 1C). -3- �Proceedings of the 58th ILSG Annual Meeting - Part 1 A C B Figure 1. A) Contact between the sand and shale facies at the Horseshoe Outcrop. B) Inter tonguing beds within the Nonesuch Shale at Devil’s Gate. C) The Sheep Farm Rhyolite on the east cliffs near Devil’s Gate References Baumann, S.D.J. and Cummings, K.E., 2011. Bedrock Geologic Map of Copper Falls State Park, Ashland County, Wisconsin. Midwest Institute of Geosciences and Engineering, Map M-082011-1A. Cannon, W.F., 1997. Bedrock Geologic Map of the Ashland and Northern Part of the Ironwood 30’ X 60’ Quadrangle, Wisconsin and Michigan. United States Geologic Survey Miscellaneous Information Series, Map I-2566. Fitz, T. (Editor), 2011. 57th Institute on Lake Superior Geology’s Annual Meeting May 18-21, Ashland Wisconsin. Institute on Lake Superior Geology’s 57th Meeting, Part 2. Ostrom, M.E., 1973. Guidebook to the Precambrian Geology of Northeastern and North Central Wisconsin. 19th Institute on Lake Superior Geology’s Annual Meeting. -4- �Proceedings of the 58th ILSG Annual Meeting - Part 1 Depositional Processes Operating on the Paleoproterozoic Gowganda Ice Margin: An example from the Espanola Area, Ontario BEH, Breanne and FRALICK, Philip, Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1, bbeh@lakeheadu.ca Glacial sediments of the Huronian Supergroup outcrop along the north shore of Lake Huron and were likely deposited on what is thought to have been a divergent continental margin (Fralick and Miall, 1981; 1989). The rocks of the Gowganda Formation record one of three glacial events preserved in the Supergroup and are therefore of interest for developing further glaciomarine models (Puffett, 1974) and furthering understanding of this early stage in Earth’s history. Data has been collected in five main study areas in an attempt to cover as much of the ancient continental margin as possible. The Espanola study area consisted of the most extensive and well exposed outcrops of the Gowganda Formation, with more than a kilometer of nearly continuous exposure, therefore it will be used as an example to show how the depositional history can be interpreted. Two metamorphosed stratigraphic sections were logged on McGregor Bay and one on Iroquois Bay, which are located near Espanola, Ontario. The logged sediments were grouped into six lithofacies associations: 1) Planar Cross-Stratified Sandstone Lithofacies Association, 2) Diamictite Lithofacies Association, 3) Interlayered Siltstone and Fine-Grained Sandstone Lithofacies Association, 4) Slump Lithofacies Association, 5) Fine- to Coarse-Grained Sandstone Lithofacies Association and 6) Quartz-Rich Sandstone Lithofacies Association. These lithofacies associations (LA) likely represent a sequence of depositional environments on a shallow continental shelf. Initially, the shelf was dominated by what were likely large-scale, low-angle sandwaves (Fig. 1A) interbedded with successions of wavy bedding and possible hummocky cross-stratification indicating an Figure 1. A) Large-scale, low-angle planar cross-stratified sandstone. B) A dropstone compressing underlying laminations. C) A mud-block conglomerate of slumped prodelta deposits. D) Herringbone cross-stratified sandstone. -5- �Proceedings of the 58th ILSG Annual Meeting - Part 1 open-water setting with tidal and storm processes reworking the sediments. The shelf then gradually evolved into an environment dominated by diamicite layers. The diamictite layers have dropstones as well as evidence of current activity indicating outsized clasts were likely being introduced into the environment as ice-rafted debris (Fig. 1B). Resedimentation events in the form of debris flows are thought to account for conglomeratic layers that are common in the Diamictite LA. The Interlayered Siltstone and Fine-Grained Sandstone LA, along with the Slump LA (Fig. 1C), closely resemble deposition from suspension in a prodelta setting where large slump events are common. The gradual transition into a more sandstone dominated LA, with an abundance of current related sedimentary structures, is indicative of the shallowing and coarsening upwards succession common to deltaic deposits. A final transition into the Quartz-Rich Sandstone LA indicates a return to a sandy, current-dominated open continental shelf environment with abundant tidally generated sedimentary structures such as herringbone cross-stratification (Fig. 1D). The southernmost sections contain only mass-flow diamictite with no massive diamictite. Thus, the likelihood of an ice shelf in this area is remote indicating the Gowganda Formation does not represent snowball Earth conditions. References Fralick, P.W., and Miall, A.D., 1981. Grant 84: Sedimentology of the Matinenda Formation, in Pye, E.G., ed., Geoscience Research Grant Program, Summary of Research 1980 – 1981: Ontario Geological Survey, Miscellaneous Paper 98: 80-89. Fralick, P.W., and Miall, A.D., 1989. Sedimentology of the Lower Huronian Supergroup (Early Proterozoic), Elliot Lake area, Ontario, Canada: Sedimentary Geology, 63: 127-153. Puffet, W.P., 1974. Geology of the Negaunee quadrangle, Marquette County, Michigan: U.S. Geological Survey, Professional Paper 788: 53 p. -6- �Proceedings of the 58th ILSG Annual Meeting - Part 1 New York’s Adirondack Mountains: Window to the Mesoproterozoic Grenville Orogen Berkley, Jack, Department of Geosciences, Houghton Hall, SUNY Fredonia, Fredonia, NY 14063 USA The Adirondack Mountains occupy most of the northeastern lobe of New York State, with associated Grenville metamorphic rock exposures and structures extending north through the Laurentian Lowlands and into Ontario, Canada (Fig. 1). Isotopic data give whole-rock ages of roughly 1.3 to 0.9 Ga corresponding to the late Mesoproterozoic, thus are generally coeval with the Midcontinent Rift system exposed prominently in the Lake Superior region. Orogenic evolution of the Grenville Orogeny was complex, and arose from at least two major continent-continent collisions producing the eastern (including the Adirondacks) and southern chains (including Texas and Mexican exposures) during the assembly of the supercontinent Rodinia. The Adirondack region reveals evidence for Himalayan-magnitude mountains that were part of a nearly continuous chain stretching from present-day Scandinavia through North America, Africa, Australia and Antarctica. Major Grenville exposures in North America include the (1) Grenville Province (Labrador-Quebec-Ontario), which includes the AdirondackNY-Laurentian Lowland region, (2) inliers within the Green, Taconic, Blue Ridge Appalachian chain, (3) the Llano County, Texas granite-gneiss/schist terrain, and (4) the Oaxacan Complex, southern Mexico. Figure 1. Generalized geology of the Adirondacks with ages of principal units. ANTanorthosite; HSRG-Hyde School & Rockport granites; HWK-Hawkeye granite; LMG-Lyon Mtn. granite; MCG-mangerite, charnockite, granite; RDAG-Rossie diorite & Antwerp diorite; RMTG-Royal Mtn. tonalite & Granodiorite; CCZ-CarthageColton mylonite shear zone; HL-Highlands; LL-Lowlands. After McLelland, 2008 The overall timeframe of Grenville deformation, metamorphism and plutonism began with the Elzevirian Orogeny (from 1240-1220 Ma), followed by the Shawinigan event (1190-1140 Ma), the Ottawan (1090-1020 Ma), and the 1010-980 Ma Rigolet Orogeny (Rivers, 2008). Grenville tectonic history – as exemplified in the Adirondacks – consisted of major convergent folding events (Shawinigan and Ottawan) with intermittent relatively quiescent periods featuring rifting episodes and erosion-deposition of sedimentary units in rift basins and shelf areas. These quiet spans also spawned the extensive intrusion into countryrock of AMCG (anorthositemangerite-charnockite-granite) plutonic suites, well-represented in the Adirondacks. Evidence gleaned from extensive modern studies, including recent data from TIMS and SHRIMP single-grain isotopic determinations (e.g., McLelland, 2008) shows that subduction with formation of island arcs long preceded collision events, with magmatic arc intrusive and volcanic rocks eventually riding over continental margins on overlying NW-vergent thrust sheets. The closing of the Iapetus seaway during the Neoproterozoic-Paleozoic Appalachian orogenies some 700 million years later presents parallels to Grenville tectonic mechanisms. -7- �Proceedings of the 58th ILSG Annual Meeting - Part 1 Exposed areas in the Grenville orogen consist of three litho-structural “belts”, the Gneiss Belt, Metasedimentary Belt, and the Granulite Terraine. Relatively narrow shear zones separate each belt. In the Adirondacks the Metasedimentary Belt occupies most of the Laurentian Lowlands terrain consisting of marble, calcsilicates, rare gypsum-anhydrite layers (mostly in mine excavations) along with various gneiss and amphibolite terrains. The Carthage-Colton shear zone separates the Metasedimentary Belt from the Granulite Terrain, which makes up the bulk of geology within the Adirondack State Park. As its name implies, most rocks in the Granulite Terrain are assigned to the granulite metamorphic facies, while Metasedimentary Belt assemblages predominantly plot in the amphibolite facies. Greenschist and lower P-T assemblages are absent in the Adirondacks. Various models have been advanced seeking to establish a genetic and regional kinematic connection between the Grenville orogen and the Midcontinent rift system (for a synopsis, see Hauser, 1996). Early ideas postulated dextral strike-slip movement along the Grenville front causing opening of the Midcontinent rift (e.g., Anderson and Burke, 1983), however, these models were fraught with problems. Principal among these is that isotopic dates along the Canadian Grenville front are consistently younger than Keweenawan terrains, also shown by cross-cutting truncation of the eastern rift arm by the Grenville front near Detroit. Current thinking posits a primary role of an uprising mantle plume in producing the midcontinent rift at around 1.10 Ga (e.g., Nicholson and Shirey, 1990), but with some interplay with the Grenville belt. For example, creation of horst ridges during later convergent rift inversion may have been influenced by Grenville shortening during early Ottawan times, around 1.08 Ga (McLelland, 2008). Also, strike-slip offsets along the southern extent of the western rift arm may be influenced in some manner by the southern (Texas-Mexico) Grenville front (Hauser, 1996). References Anderson, S. and Burke, K., 1983. A Wilson Cycle approach to some Proterozoic problems in eastern North America, in Medaris, L.G. et al. eds., Geological Society of America Memoir, 161: 75-93. Hauser, E., 1996, Midcontinent rifting in a Grenville embrace, in van der Pluijm, B. and Catacosinos, P., eds., Geological Society of America Special Paper, 308: 67-75. McLelland, J., 2008, Geologic setting and characteristics of Adirondack anorthosite and related mangerite-charnockitegranite (AMCG suite), Field Trip Guidebook, New York State Geological Association, 80th meeting: 1-18. Nicholson, S. and Shirey, S., 1990, Midcontinent rift volcanism in the Lake Superior region: Sr, Nd and Pb isotopic evidence for a mantle plume origin. Journal of Geophysical Research, 95: 10, 851-10,868. Rivers, T., 2008, Assembly and preservation of lower, mid, and upper orogenic crust in the Grenville Province – Implications for the evolution of large, hot, long-duration orogens. Precambrian Research, 167: 237-259. -8- �Proceedings of the 58th ILSG Annual Meeting - Part 1 Glacial Geology of the Copperwood Project, Gogebic County, Michigan, and Environmental and Engineering Implications for Site Development BLASKE, Allan R., 401 S. Washington Square, Lansing, Michigan 48933, allan.blaske@aecom.com, BRAUN, Gary, AECOM, 11425 W. Lake Park Dr., Milwaukee, Wisconsin 53224, LEIER-ENGELHARDT, Paula, AECOM, 1035 Kepler Drive, Green Bay, Wisconsin 54311, MOTTL, Robert, AECOM, 1035 Kepler Drive, Green Bay, Wisconsin 54311, ANDERSON, Dave, Orvana Resources US Corp., 10199 Lake Road, Ironwood, MI 49938 and BORNHORST, Theodore J., A. E. Seaman Mineral Museum, Michigan Technological University, Houghton, MI 49931. The Copperwood Project is located in the northwestern portion of Gogebic County, Michigan, between the Black River and the Presque Isle River, on the west side of the Porcupine Mountains. In 2008, Orvana Minerals Corporation began development of a stratiform copper deposit at the base of the Nonesuch Shale in the Western Syncline. Data on the glacial overburden were collected through rotosonic soil borings, monitoring well installations, test pits, grain size analyses, and hydrogeological testing methods. AECOM collected data necessary for an Environmental Impact Assessment pursuant to Michigan’s Non-Ferrous Metallic Mining Regulations (Part 632) funded by Orvana Minerals Corporation. The broader context of the data provided here is provided in the publically available Part 632 Mine Permit application for the Copperwood Project. Research regarding the glacial deposits in the western Upper Peninsula of Michigan is generally scattered and not especially detailed. Most available information is on the regional scale, rather than a local scale. Farrand and Bell (1982) mapped the area west of the Porcupine Mountains as “lacustrine clay and silt.” Hack (1965) described the Ontonagon Plain (east of the Porcupine Mountains) to be underlain by reddish-brown glacial lake sediments and till. He described three units within the glacial deposits – lower, intermediate, and upper units. Thin lacustrine deposits related to Glacial Lake Duluth are described as a veneer of strongly-laminated clay, silt and sand of variable thickness. More recently, Petersen (1985 and 1986) described the glacial deposits of the area as thin drift over bedrock, generally less than 30 feet thick, composed of calcareous red till of fine sand, silt, and clay with a small percentage of angular clasts. Detailed investigation at the Copperwood site indicated the unconsolidated materials consist primarily of a reddish-brown glacial till. The glacial section most closely matches that described by Hack (1965). The overburden material was interpreted as subglacial till based on the following evidence: • Uniform composition and consistently found to be massive, matrix-supported diamicton with little evidence of interbedded sorted sediments; • Little structure and no observed stratification, lamination or fining upward or downward trends; • The material is widespread and consistent over several square miles of the project area; • Grain-size distribution curves are typical of those associated with subglacial tills; and • The matrix is uniform based on USDA grain size distribution plots. The average composition of the cohesive (silt/clay) samples encountered at the site was found to be approximately 50% silt, 20% clay, and 30% sand. The gravel/cobble portion of the material was approximately 10 to 20% and consisted primarily of dark reddish-brown sandstone (Freda Sandstone). The fine fraction (<200 sieve size) were determined to be predominantly quartz, with very minor amounts of clay minerals. The matrix also contains carbonate (fizzes when exposed to dilute hydrochloric acid). Layers of coarser (non-cohesive) material were observed throughout the site in various borings. These soil types generally consisted of fine to medium-grained sand with varying amounts of silt and clay. Granular deposits are thin and isolated, with none greater than 10 feet thick. These deposits are not laterally extensive, and for the most part cannot be correlated between adjacent borings. They are interpreted to be lacustrine, intra-till sands, or subglacial meltwater deposits. These deposits are typically stratigraphically between the upper and lower till -9- �Proceedings of the 58th ILSG Annual Meeting - Part 1 units, but some are located at the base of the overburden, lying directly on the top of the bedrock surface. Thin, surficial, lacustrine deposits related to Lake Duluth are present across the site, as wave-cut benches and beach deposits. The beach deposits are generally less than 10 feet thick and less than 200 feet wide. These beach deposits consist of sand and gravel deposits adjacent to the wave-cut bluffs that parallel topography. The unconsolidated overburden at Copperwood is interpreted to be the result of least two advances of the Ontonagon lobe. The first advance moved south out of the Lake Superior basin, and deposited the lower till unit, as well as the granular deposits on the bedrock surface. The ice then retreated, depositing the lacustrine/subglacial meltwater deposits which are located between the upper and lower till units, where present. The youngest glacial advance occurred as the ice moved out of the western Lake Superior basin and overrode previous deposits, and influenced the orientation of surface drainage features. Glacial Lake Duluth occupied the area after final retreat of the Ontonagon lobe. This lake had an elevation as high as approximately 1,200 feet msl. Drainage of this lake continued as ice continued to retreat. Several Lake Duluth shorelines are present across the site between elevation 650 feet msl and 1,125 feet msl. The Lake Nipissing shoreline (elevation 615 feet msl) is not present at the site and likely has been destroyed by erosion along the current Lake Superior shoreline. The characteristics of the glacial material have significant implications for site development. The very low hydraulic conductivity of the till limits surface water infiltration producing streams that are runoff-dominated and flashy. Streams are present in deeply incised, closely-spaced parallel valleys, impacting space for site development. Abundant wetlands are present across the site, due to the poor infiltration, and are not hydraulically connected to groundwater. The very low hydraulic conductivity also leads to very slow movement of groundwater. Hydraulic testing and migration modeling suggests that a tailings disposal facility can be constructed in the clay till soil thus using a native material as natural liner. Orvana Minerals Corporation generously allowed publication of this abstract. References Farrand, W. R. and Bell, D. L., 1982, Quaternary Geology of Northern Michigan, Michigan DNR, Geologic Survey Division. Hack, J. T., 1965, Postglacial Drainage Evolution and Stream Geometry in the Ontonagon Area, Michigan, USGS Professional Paper 504-B. Peterson, W. L., 1985, Surficial Geologic Map of the Iron River 1 degree x 2 degree Quardangle, Michigan and Wisconsin, USGS Miscellaneous Investigations Series, Map I-1360-C. Peterson, W. L., 1986, Late Wisconsinan Glacial History of Northeastern Wisconsin and Western Upper Michigan, USGS Bulletin 1652. - 10 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Bedrock Geologic map of Morrison County, Central Minnesota BOERBOOM, Terrence J., Minnesota Geological Survey, boerb001@umn.edu Morrison County, located in the center of Minnesota, has recently been remapped with help from the USGS STATEMAP mapping program, and eventually it will become integrated into the MGS County Atlas Program. Although the basic geologic framework was well established prior to this mapping effort, much has been learned by logging of all available drill cores, integration of old exploration data, and outcrop examinations. Scattered bedrock outcrops are present throughout the southern two-thirds of the county, and more than 200 drill holes targeted at iron ore were placed in the northern part of the county prior to the 1960’s. Although the core from many of these was not preserved, drill logs, including iron assay data, locations, and some cross-sections, are available for all. The only iron mine to come out of this effort was the tiny Gorman mine (U of M 1954 Mining directory reports operation from 1951-1952, with 116,000 tons shipped). More recently, about 100 drill holes were placed into other parts of the county; these are a mixture of diamond and mineral exploration, scientific research, and other borings. About 50 of these have useful core or cuttings available and the rest have descriptions deemed reliable for use. In total 85 cores and cuttings were logged. In areas between drill holes and bedrock outcrops, the bedrock geology is inferred, with varied degrees of confidence, from geophysical data. Morrison County is located along the southern Margin of the Superior Province – Wawa subprovince, and also straddles the Penokean Orogen (Fig. 1). Archean bedrock is inferred to be primarily felsic gneisses and metamorphosed supracrustal rocks intruded by felsic to mafic plutons. The rest of the county is composed of Paleoproterozoic bedrock that includes, from north to south, basal quartzite and turbidites of the Animikie basin, the Cuyuna North and South Ranges (external and medial zones of the fold-and-thrust belt, Fig. 1), the Little Falls Formation, and the east-central Minnesota batholith (both within the internal zone; Fig. 1). The external and medial zones of the Penokean fold-and-thrust belt are composed of phyllitic muddy to quartzose sedimentary rocks interbedded with mafic flows and sills and thin iron-formations, all metamorphosed under lower greenschist-facies conditions. In contrast, the Little Falls Formation, within the internal zone, is composed only of turbidites metamorphosed under amphibolite-facies conditions, and ultramafic to felsic intrusive plugs and plutons. The break between low-grade and high-grade rocks, the Malmo structural discontinuity (MSD), is modeled as a southdipping thrust fault. The MSD is clearly recognizable on aeromagnetic data, where sharp linear magnetic trends associated with iron-formations to the north are truncated by low, uniform aeromagnetic patterns to the south. South of the MSD, the metamorphic grade of the Little Falls Formation increases in proximity to the ECMB, from biotite at the northwest, through a narrow band of garnet, into a wide zone of sillimanite-garnetstaurolite to the southeast. Cordierite occurs around the margins of what are inferred to be gneiss domes. Adjacent to mafic intrusions staurolite has been thermally metamorphosed to an   - 11 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 assemblage of hercynite and corundum. The ECMB batholith is composed of several separate intrusions that range from tonalite to granite. Some of these include the approximately 1,800 Ma Hillman tonalite, the Pierz granodiorite (1796±19 Ma), the Foley granite (1779±4 Ma non-magnetic phase and 1774±1 Ma moderately magnetic phase), the Freedhem granodiorite (ages of 1775±3 to 1776±7 Ma; intruded by 1773±2 granite dike) and unnamed granites, one of which is 1766±8 Ma and another that is 1784±8 Ma (all U-Pb zircon ages reported in Holm et al., 2005). Small dominantly mafic to ultramafic, but also felsic, plug-like intrusions are abundant. Fourteen of these have either been drilled or are exposed, and the rest are mapped on the basis of small, sharp aeromagnetic anomalies. Those drilled or exposed consist predominantly of porphyritic gabbronorite to norite, and others include ultramafic olivine-pyroxene hornblendite, hornblendite, hornblende gabbro, and diorite to tonalite. The gabbronorite contains euhedral phenocrysts of augite, variably altered olivine, and rare plagioclase in a fine-grained, weakly trachytoid groundmass composed of lath-shaped plagioclase, intergranular bronzite hypersthene and varied proportions of augite, and lesser poikilitic biotite/phlogopite, interstitial alkali feldspar, and apatite. One of these bodies gives an Ar-Ar plateau age of 1791±8 Ma (Jirsa et al., 2006). Only two of the intrusions sampled are ultramafic; these are coarse-grained, phlogopitic pyroxene-olivine hornblendite with 3040% poikilitic, pargasitic Mg-hornblende, 30-35% variably altered olivine, 15-20% augite, 2-10% phlogopite, 1-15% secondary actinolitic amphibole, 3-6% secondary oxides, and minor chlorite, chromite, and apatite. These intrusions contain high Cr, Ni, and MgO. A second distinct type of hornblendite is dark green, medium-grained, and composed mainly of deep green equant hornblende. This unit forms plugs, is observed as a 3m-wide dike that cuts the Little Falls Formation, and as local phases in the gabbronorite intrusions. A third distinct type of hornblende-rich intrusion is composed medium- to coarse-grained, strongly prismatic-foliated (magmatic), weakly plagioclase-phyric, hornblende gabbro. This intrusion is substantially larger than the other plug-like bodies and yields an Ar-Ar plateau age of 1770±6 Ma (Jirsa et al., 2006). Thin sills of dioritic rocks within the Little Falls Formation wallrock adjacent to the mafic rocks, as well as irregular zones within the mafic and ultramafic intrusions, are identical to small, discrete dioritic plugs. These felsic members contain strongly zoned plagioclase phenocrysts, and petrographic textures indicate that they are likely the product of contamination by melting of the adjacent Little Falls Formation. Geochronological studies show that supracrustal rocks in the Penokean fold and thrust belt were metamorphosed as recently as 1,760 – 1,770 Ma, which compares with emplacement ages of 1,800 – 1,776 Ma for the ECMB, indicating that the metamorphism may be attributed to widespread crustal thickening and heating related to pluton emplacement. These metamorphic and pluton emplacement ages correspond to the Yavapai Orogeny (e.g. Holm and others, 2007, and references therein). Intrusions within the ECMB show mafic/felsic magma mingling textures, and the ECMB is likely related to the small outboard plugs but emplaced at a deeper crustal level in the heart of the orogenic belt where there was greater crustal thickening and wholesale crustal melting. References Holm, D.K., Van Schmus, R.V., MacNeill, L.C., Boerboom, T.J., Schweitzer, D., and Schneider, D., 2005, Evidence for subduction flip and continued convergence after geon 18 Penokean orogenesis, Geological Society of America Bulletin, v. 117, p. 259-275. Holm, D.K., Schneider, D.A., Rose, S., Mancuso, C., McKenzie, M., Foland, K.A., and Hodges, K.V., 2007, Proterozoic metamorphism and cooling in the southern Lake Superior region, North America and its bearing on crustal evolution: Precambrian Research, v. 157, nos. 1-4, p. 106-126. Jirsa, M.A., Miller, J.D., Jr., Severson, M.J., and Chandler, V.W., with contributions by Boerboom, T.J., Lively, R.S., Keatts, M.J., and Holm, D.K., 2006, Final Report on the geology, geochemistry, geophysical attributes, and platinum group element potential of mafic to ultramafic intrusions in Minnesota, excluding the Duluth Complex: Project Summary: Minnesota Geological Survey Open File Report OF-06-3, 49 p. - 12 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Preliminary results of P- and S-wave delay times in the Superior Region from transportable array and SPREE stations BOLLMANN, Trevor, VAN DER LEE, Suzan, Department of Earth and Planetary Sciences, Northwestern University, 1850 Campus Dr., Evanston, IL 60208, FREDERIKSEN, Andrew, Department of Geological Sciences, University of Manitoba, 125 Dysart Rd., Winnipeg, MB R3T 2N2 and LOU, Xiaoting, Department of Earth and Planetary Sciences, Northwestern University, 1850 Campus Dr., Evanston, IL 60208 The Superior Province is cut by a Proterozoic structure named the Midcontinent Rift (MCR). It is one of the most striking gravity anomalies in North America on account of its length and height in mgals. The anomaly is caused by large masses of mafic/ultra mafic rock deposited along the rift axis around 1.1. Ga. To investigate the deep structure of this region we measured delay times of teleseismic waves recorded at 242 seismic stations from the Earthscope Transportable Array (TA) and the Superior Province Rifting Earthscope Experiment (SPREE). To date, 6144 P-wave and 3013 S-wave delay times were measured from 33 teleseismic earthquakes. Using a new, interactive automated delay time assessment interface, we could measure absolute delay times, relative to global seismic velocity model iasp91. Interesting features found by mapping the average delay times are: a region of negative delays around Lake Nipigon and regions of positive delays in central Minnesota into Wisconsin, and in south-central Iowa (P-wave only). These delay times are most affected by the structures in the upper mantle due to the fact that the lithospheric portion of the delay time is less than 5 percent of the whole delay time. Since deeper structures cause larger variations with distance and azimuth it is not a surprise that the delay times vary between individual events. Looking at delay time maps of individual events show patterns of planar wave arrivals. These patterns will be investigated to discern their origins and if they are related to their azimuth or distance. More measurements will be added as new TA data is downloaded from the IRIS Data Management Center and as SPREE servicing runs are completed in the next 1.5 years. References Kennett, B.L.N., E.R Engdahl. 1991. Travel times for global earthquake location and phase identification, Geophysical Journal International, v. 105, p. 429–465 Lou, X., S. Lloyd, S. Van der Lee. 2012. A Python/Matplotlib Tool for Measuring Teleseismic Body Wave Arrival Times. (In Preparation) Bedle, H., and S. van der Lee. 2006. Fossil flat-slab subduction beneath the Illinois basin, USA, Technophysics, v. 424(1-2): p. 53-68 - 13 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 The Copperwood Sediment-Hosted Stratiform Copper Deposit, Upper Peninsula, Michigan BORNHORST, Theodore J., A.E. Seaman Mineral Museum, Michigan Technological University, 1404 E. Sharon Avenue, Houghton, MI 49931 and WILLIAMS, William C., Orvana Minerals Corporation, 181 University Avenue, Suite 1901, Toronto, Ontario M5H 3M7 The Copperwood copper deposit is located in Gogebic County, Upper Peninsula, Michigan within the Porcupine Mountains sediment-hosted copper district. It was discovered in 1956 by United States Metal Refining Company. In 2008, Orvana Minerals Corporation began exploration and development work at Copperwood. The company is planning first production in 2014, which would be the first Upper Peninsula copper mine since 1996. The copper mineralization occurs as very fine-grained chalcocite hosted by siltstones and shales of the lowermost Nonesuch Formation, which is underlain by red-bed sandstones, conglomerates, and minor siltstones of the Copper Harbor Formation. The mineralized beds are stratigraphically equivalent to the lower copper mineralized units at the now closed White Pine Mine about 30 kilometers to the northeast. These rocks are part of a thick section of clastic sedimentary rocks that comprise the upper fill of the Mesoproterozoic (1.1 to 1.0 Ga) Midcontinent rift. The copper deposit is a tabular body averaging 2.5m thick, lacks significant basal undulations, and dips 8° to 12° northward on the southwest limb of the Western Syncline, an open fold with a shallow plunge to the northwest. Copper mineralization occurs as 5 to 50 micron disseminated grains of chalcocite and as concentrations along laminations. The deposit is relatively undeformed with only one shallow, north-dipping reverse fault recognized; it’s displacement is as much as 7m. Gangue minerals in the ore body are quartz, clinochore, muscovite/illite, plagioclase, K-feldspar, calcite, and hematite. Reported Canadian National Instrument compliant Proven and Probable Reserves, fully diluted, are 27.42 million metric tonnes averaging 1.41 % Cu and 3.6 ppm Ag for contained metal of 852 million pounds of copper and over 3.2 million ounces of silver. A model for sedimentation and mineralization was based on the logging of core from nearly 200 drill holes at Copperwood and over another 100 drill holes throughout the Western Syncline. The host rocks were likely deposited under mostly anoxic conditions along the margins of a lake. In these conditions, reduced organic carbon-bearing muds with very fine-grained, syngnetic pyrite were deposited in the copper-bearing sequence. Red intervals within the otherwise black to gray sequence suggest periodic oxic conditions. During diagenesis and incipient lithification, copper-bearing, saline basinal fluids passed vertically through the sequence and formed chalcocite after pyrite; copper was likely leached from the underlying red-bed paleoaquifer. The stratigraphic sequence was folded during the Grenvillian compression of the rift strata long after lithification. In addition to the formation of the Western Syncline and recognized reverse faulting, calcite veinlets were emplaced at Copperwood. The Copperwood deposit has the characteristics of a reduced-facies, sediment-hosted stratiform copper deposit, or Kupferscheifer-type. Although the mineralized sequence is stratigraphically equivalent to that of the White Pine deposit, it lacks the structural complexity and hydrothermal overprint characteristic of that deposit. The simple mineralogy and geochemistry, single copper-mineralizing event, and minimal deformation distinguishes the Copperwood deposit from other more complex, reduced-facies sediment-hosted stratiform copper deposits throughout the world. Orvana Minerals Corporation has granted permission to publish this abstract. The results and interpretations herein are the responsibility of the authors. Certain statements constitute forward-looking statements or forwardlooking information within the meaning of applicable securities laws. Readers are cautioned not to put undue reliance on forward-looking statements. - 14 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Geology and petrology of a Mesoproterozoic layered mafic intrusion in portions of the Brule Lake and Cherokee Lake 7.5’ Quadrangles, northeastern Minnesota BROOKER, Ben, and MILLER, Jim, Department of Geological Sciences, University of Minnesota Duluth, Duluth, MN 55812, USA The Duluth Complex and related Mesoproterozoic intrusions of northeastern Minnesota comprise one of the largest igneous complexes on Earth (Miller and Severson, 2002). These intrusions, and the comagmatic volcanics into which they were emplaced, are part of the 1.1 Ga Midcontinent Rift (MCR). Many areas of northeastern Minnesota have sufficient bedrock exposure to be mapped at a detailed scale (e.g., 1:24,000). Some areas, however, are underlain by the Duluth Complex and related MCR rocks, particularly those contained within the Boundary Water Canoe Area Wilderness, have been only cursorily investigated or mapped at a reconnaissancescale. One such area of incomplete mapping, which has been a particular curiosity since the acquisition of high resolution aeromagnetic data over northeastern Minnesota (Chandler, 1983), is located in the Sawbill Lake area of Cook County. Part of what is generalized as the Brule Lake-Hovland gabbro on the M-119 map of NE Minnesota (Miller et al., 2001) is characterized by a strongly banded, tightly curved anomaly pattern. Reconnaissance mapping in the anomaly area by Grout et al. (1959) and Davidson (1977; Davidson and Burnell, 1977) indicates a predominance of well foliated and locally layered, oxide-rich olivine gabbro. Recently, several areas of the gabbro were mapped in detail by PRC field camp students (Frost et al., 2007; Blakely et al., 2009; Brooker et al., 2010; Asp et al., 2011). For the first author’s MS thesis project, he is focused on integrating this previous mapping with his own bedrock mapping to establish the geology of a previously unrecognized, well differentiated mafic layered intrusion, which we have named the Sawbill Lake intrusion (SLI). The detailed mapping of the SLI indicates that it is composed of troctolitic cumulates in its lower section, oxide gabbro cumulates in its medial section, and apatite ferrogabbro cumulates to ferromonzonite in its upper section. The footwall of the SLI is composed of a mix of intermediate to gabbroic rocks, anorthositic gabbros, and Pl-porphyritic diabase. The hanging wall is composed of leucogranites of the Eagle Mountain granophyre. The gradational nature of the contact between SLI ferromonzonite and Eagle Mtn granophyre implies that the SLI underplated and partially melted the granophyre body, as found elsewhere throughout the Duluth Complex (Miller and Severson, 2002). Petrographic analysis of samples collected along multiple profiles through the intrusion confirmed the cumulate stratigraphy of the SLI implied from field mapping, namely POaPCFaPCFOA. The majority of samples collected were concentrated along three main profile lines in different areas of the intrusion. One set of profiles was in the west end, one in the central, and the other in the eastern end of the SLI. One of the more striking features within the intrusion is the presence of an interval rich in subconformable blocks of basaltic and sedimentary hornfels, which were first recognized by Grout et al. (1959). The interval is located immediately above the transition between troctolite and oxide gabbro cumulates. Although the cumulate stratigraphy of the SLI shows a typical progression that could be related to progressive differentiation of a tholeiitic mafic magma, the presence of a persistent volcanic screen at the major transition in cumulate mineralogy suggests that this intrusion probably did not form by uninterrupted closed system fractional crystallization. Possible explanations for these relationships are 1) the hornfels interval separates two distinct intrusions, 2) the onset of oxides becoming cumulus caused the liquid in the magma chamber to become less dense and allowed volcanic rocks in the hanging wall to fall to the cumulate floor of the chamber, or 3) the hornfels blocks mark a significant venting event that depressurized the magma chamber and triggered the saturation (cumulus arrival) of pyroxene and oxide. To determine which of these scenarios is most likely, EDS analyses of olivines and pyroxenes were collected from the samples profiling the intrusion with the intention of evaluating whether the abrupt mineralogical change marked by the hornfels interval also corresponds to a compositional break. The mineral chemical data collected from over 68 samples does not unequivocally support any one theory for the emplacement and crystallization history of the SLI, but is does seem to favor the venting model best. Perhaps - 15 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 the most significant result of the mineral chemical data is that it shows a smooth cryptic variation throughout the entirety of the intrusion in both the olivine and pyroxene. The lack of any significant chemical break at the hornfels interval would seem to indicate that the SLI fractionally crystallized in situ from one major pulse of magma, thus supporting the density inversion and venting models. The problem with the density inversion model is that one would expect that if hornfels blocks fell to the crystallized floor due to the cumulus crystallization of oxide, one would expect to find inclusions of the volcanic rocks throughout the upper oxide gabbroic part of the intrusion. This is not the case. Alternatively, the singular occurrence of volcanic hornfels at horizon marked by the abrupt change in pyroxene habit from subophitic-ophitic below to granular above, but no significant mineral chemical change, seems more consistent with a significant venting model. While decompression attending a venting event could have an profound affect on the phase equilibrium of the system to where augite and oxide may become cumulus, it should have a minimal effect on the chemistry of the magma system. The presence of high density subophitic-ophitic pyroxenes below the screen indicate that the magma was very close to becoming saturated in augite such that venting could have triggered the cumulus arrival of augite and oxide. References Asp, K., Leu, A., Parisi, A., Sletten, D., Brooker, B., Miller, 2011, Bedrock geology of the Sawbill Lake area: University of Minnesota Duluth, Precambrian Research Center, PRC/MAP-2011-04, 1: 12,000. Blakely, S., Brown, A., Foley, D., Rowland, A., Stifter, E., and Miller, J., 2009, Bedrock geology map of Homer Lake and adjacent areas; Cook County, Northeastern Minnesota: University of Minnesota Duluth, Precambrian Research Center, PRC/MAP-2009-01, 1: 12,000. Brooker, B.P.,Hadley, M.L., Markwood, L.W., Olson, J., Tomlinson, A.P., and Miller, J.D.,2010, Bedrock geologic map of the Jack Lake and Weird Lake areas, Cook County, northeastern Minnesota: University of Minnesota Duluth, Precambrian Research Center, PRC/Map-2010-05, 1: 12,000. Chandler, Val W, 1983, Aeromagnetic map of Minnesota, Cook and Lake counties: Minnesota Geological Survey, Aeromagnetic Map Series, Map A-1, scale 1:250,000 Davidson, D.M., 1977, Reconnaissance geologic map of the Cherokee Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-30, scale 1:24,000 Davidson, D.M., Jr. and Burnell, J.R., Jr., 1977, Reconnaissance geologic map of the Brule Lake quadrangle, Cook County, Minnesota: Minnesota Geologic Survey Miscellaneous Map M-29, scale 1:24,000. Frost, S.J., Juda, N.A., and Miller, J., 2007, Bedrock Geology Map of Homer Lake and Adjacent Areas; Cook County, Northeastern Minnesota: University of Minnesota Duluth, Precambrian Research Center, PRC/MAP-2007-02, 1: 12,000 Grout, F.F., Sharp, R.P., and Schwartz, G.M., 1959, The geology of Cook County, Minnesota: Minnesota Geological Survey Bulletin 39, 163 p. Miller, J.D., and Severson, M.J. 2002. Geology of the Duluth Complex. In Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota, Minnesota Geological Survey Report of Investigations 58, p. 106-143. Miller, J.D., and Green, J.C., 2002, Geology of the Beaver Bay Complex and related hypabyssal intrusions. In Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota, Minnesota Geological Survey Report of Investigations 58, p. 144-163. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.E., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota. Miscellaneous Map Series, M-119, scale 1:200,000, 2 sheets. - 16 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Accessory minerals of a roadside pegmatite, Orr, Minnesota Buchholz, Thomas, 11140 12th Street North, Wisconsin Rapids, Wisconsin 54494, Falster, Alexander, and Simmons, William, Department of Earth and Environmental Sciences, University of New Orleans, New Orleans, Louisiana 70148 While returning from ILSG 2010, the first author took some samples from a pegmatite exposed on the west side of US 53 just north of Orr, MN. Interesting results from heavy mineral separates prepared from the samples encouraged a return visit in August 2011, in which samples were taken from discrete zones of the pegmatite. Heavy mineral separates were prepared at the University of New Orleans, mounted in epoxy, and examined under SEM/EDS. Selected grains were analyzed with the electron microprobe. Rock units in the vicinity of the pegmatite were mapped by Jirsa (2011) as Neoarchean biotite schist-rich migmatite intruded by Lac La Croix granite. The pegmatite is zoned, roughly horizontal, and with multiple alternating zones of aplite and pegmatite. The footwall is granititic with scattered pegmatite stringers, but the hanging wall is not well exposed; rafts or screens of biotite-rich schist in the uppermost pegmatite exposure suggest close proximity to the top of the dike. Zones are not continuous, but pinch and swell laterally. No discrete massive quartz core such as frequently observed in pegmatites is present, but pegmatite zones do have distinctive mineralogy, exhibiting varying degrees of fractionation; hence a more highly fractionated, core zone can be distinguished. Aplite zones throughout the dike are generally lacking in HFSE-bearing mineralization, though zircons are present in all phases. Lower pegmatite zones adjacent and near the lower contact carry Mn and Zn-bearing ilmenites, along with Th-rich allanite. Nb- rich titanite in these zones appears to accommodate Nb by charge balance using Al+Fe3+<-->Nb5++Ta5+. Magnetite is common in all pegmatite zones, and carries significant Zn and some Mn, reflecting pegmatite enrichment in these elements. The third pegmatite zone is perhaps the most highly fractionated, and is thusly considered to correspond to the core zone of the pegmatite. Mn-rich almandine garnet, magnetite, Th-rich monazite, possible chevkinite(Ce), and columbite-Fe have been noted. Sparse zircons exhibit significant Hf enrichment, ranging from about 2.1 wt% to about 5.3 wt% HfO2. Sparse columbite-Fe is somewhat enriched in Ta, with up to almost 16 wt% Ta2O5. Notably, detectable Sc is present, ranging from 0.021 to 0.031 Sc2O3. Some grains of columbite-(Fe) were found embedded in magnetite grains, and detectable Nb (in the range of 0.0X to 0.00X wt%) was found in the adjacent magnetite. Since Nb cannot be reasonably incorporated in the magnetite structure it is likely present as minute grains of Nb-bearing minerals, most likely columbite-(Fe). Heavy mineral separates from the biotite-rich metamorphic rock rafts in the upper portion of the pegmatite exposure carry abundant titanite and sparse zircons. The titanites carry Nb just barely above detection limit and exhibit variable composition suggesting alteration, likely the result of metasomatism via pegmatite fluids that introduced the Nb. Since it is generally accepted that pegmatites form from a single injection of relatively volatile-rich, fractionated melt, the interlayered aplite and pegmatite suggest fluctuating conditions of crystallization, from nucleation-suppressing volatile-rich conditions facilitating coarse pegmatite formation, to nucleation-enhancing less volatile-rich conditions facilitating aplite formation. The low abundance of volatile-rich phases, with the exception of biotite in pegmatite layers, suggests a relatively “dry” melt; pegmatite phase formation proceeded until volatile levels decreased to the point that mica formation and volatile-induced nucleation-inhibition ceased, and aplite formation proceeded until buildup of volatiles again inhibited nucleation and allowed pegmatite formation to resume. It is interesting to note that Grout (1926) noted abundant magnetite in pegmatites of the Vermillion Batholith, now the Lac La Croix granite. Evidently few analyses of the magnetite were made, but the one analysis included - 17 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 by Grout (magnetite in border phases of the Vermillion Batholith: 0.26 % Mn, p. 74) notes significant Mn contents. There is no evidence that Zn was looked for by these researchers, but it is possible that Zn-rich magnetites, noted in our study, may be more common in these pegmatites than has been recognized. References Jirsa, Mark A., 2011. Bedrock Geology of the Crane Land and Brule Narrows 30’ X 60’ quadrangles, Northern Minnesota, Miscellaneous Map Series Map M-192, University of Minnesota, Minnesota Geological Survey Grout, Frank F., 1926. The Geology and Magnetite Deposits of Northern St. Louis County, Minnesota, Bulletin No. 21, Minnesota Geological Survey, 220 pages. - 18 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Petrographic and geochemical study of the hybrid rock unit associated with the Current Lake Intrusive Complex, Magma Metals’ Thunder Bay North Property CHAFFEE, Matthew, MILLER, James, Department of Geological Sciences, University of Minnesota Duluth, 1114 Kirby Dr., Duluth, MN 55812, HOLLINGS, Peter, Department of Geology, Lakehead University, 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada, HEGGIE, Geoff, MACTAVISH, Allan, Magma Metals Ltd, 1004 Alloy Drive, Thunder Bay, ON P7B 6A5 Canada, BANDLI, Bryan, Department of Geological Sciences, University of Minnesota Duluth, 1114 Kirby Dr., Duluth, MN 55812 The Current Lake Intrusive Complex (CLIC) is one of several recently discovered ultramafic to mafic intrusions associated with the Midcontinent Rift (MCR) that host Ni-Cu-PGE deposits (Heggie, 2005; Ware et al., 2008; Rossell, 2008; Ding et al., 2010; Goldner, 2011; Foley, 2011). Largely unexposed in the Current Lake area about 50 kilometers northeast of Thunder Bay, Ontario, the CLIC was discovered when glacially transported ultramafic boulders containing disseminated sulfides were located along the shores of Current Lake (MacTavish and Smyk, 2010; Goodgame et al., 2010). Subsequent geophysical surveys and drilling have outlined a 3.6 kilometer long, tubular ultramafic chonolith that is composed mostly of wehrlite to dunite and is intruded into granitic and metasedimentary rocks of the Quetico subprovince. Over the course of drilling, it was discovered that an intensely altered and contaminated heterogeneous rock unit, termed the hybrid rock unit (HRU), commonly occurred at the margins of the CLIC. The focus of this study is to characterize the mineralogical, textural and geochemical attributes of the HRU in order to understand the origin of its heterogeneity and address the question of its petrogenetic relationship to the CLIC. Ten cores profiling two representative sections across the CLIC were re-logged and sampled. Six cores profile the narrow (30-50m) Current Lake zone where the CLIC intrudes granitic country rocks northwest of the Quetico fault. Four cores profile the wider (up to 600m) Beaver Lake zone southeast of the Quetico fault where the country rock is metasedimentary schists and minor granite. A total of 139 petrographic thin sections that are representative of the various textures and lithologies were studied for this project. 52 samples were submitted to Acme Laboratories in Vancouver, British Columbia for trace element and PGE fire assay analysis. XRF analyses of the ground pulps were conducted in the UMD Research Instrumentation Lab for major element chemistry. Having only recently completed the geochemical analyses, we will focus here mostly on the results of the petrographic study. Initial core logging conducted by Magma Metals distinguished two types of hybrid lithologies, which they termed red hybrid and grey hybrid by their obvious coloration differences. Typically, the red hybrid occurs on the margins of the CLIC or as narrow offshoots into the country rock. The grey hybrid typically occurs sandwiched between the marginal red hybrid and mineralized wehrlite/dunite. In places, the red hybrid can be missing and the grey forms the CLIC margin in direct contact with the country rock. Like the red, the grey can also occur as isolated offshoots into the country rock. Core logging and petrography of what has been identified as the red hybrid show it to be an intensely altered and hematized, texturally variable (prismatic to subophitic), fine to medium-grained, quartz-bearing gabbro to quartz diorite, with the dominant lithology being quartz leucogabbro with 2-5% sulfide (mostly iron sulfide). The rock is fine-grained at its contact with country rock, but is medium-fine grained near its abrupt contact with the grey hybrid. Occurrences of this rock type in the Current Lake and Beaver Lake zones are generally similar, with the exception that the Beaver Lake zone contains large, chlorite-mantled quartzite and granitic xenoliths and has more abundant interstitial quartz. A significant discovery is that some of what was logged as grey hybrid because of its color is mineralogically and texturally a quartz leucogabbro that lacks hematization. To steer clear of using secondary coloration as a primary criteria for classifying rock types, we refer to this lithology as the quartz leucogabbro unit (QLG) Petrographic investigations of rock types that have been logged as the grey hybrid are mineralogically and - 19 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 texturally distinct from the QLG unit and instead bear more resemblance to the CLIC ultramafic rocks. The average “grey hybrid” rock type is a moderately to intensely altered, fine to medium-fine grained, intergranular to subpoikilitic feldspathic wehrlite to melagabbro with 3-5% Ni-Cu sulfide. As such, we have renamed the grey hybrid, the feldspathic wehrlite (FW) unit. However, significant changes in mode and texture and alteration occur across the 1-3 meter wide FW unit from its outer contact with country rock or the QLG unit inward to the mineralized werhlite/dunite. Where the FW unit is adjacent to the QLG unit, the contact occurs over several centimeters width and is marked by 1) an abrupt decrease in grain size, 2) a rapid decrease in plagioclase mode and its change from lath shaped to subpoikilitic, and 3) a rapid increase in clinopyroxene mode and its change in habit from subophitic to anhedral granular. Progressing inward from the altered, fine-grained, intergranular melagabbro at the QLG-FW contact, several gradual changes occur within the FW unit: alteration decreases, grain size increases to medium-grained, subpoikilitic plagioclase mode decreases, clinopyroxene mode decreases and its texture changes from anhedral granular to subpoikilitic, and granular olivine quickly appears and become the dominant phase. In one Current Lake zone core where the FW unit is in direct contact with the granitic country rock, the rock grades over 1-3 meter wide interval from a very fine grained (chilled) altered gabbro at the contact to a subpoikilitic feldspathic wehrlite. The modal and textural changes across the FW unit are interpreted as indicating that this unit is a rapidly cooled marginal phase of the mineralized ultramafic intrusion. The contact relationships between the QLG and mineralized ultramafic intrusion are best explained by a twostage model of emplacement. The QLG unit was emplaced first, incorporated abundant country rock xenoliths, became strongly contaminated, and extensively hydrothermally altered. This precursor intrusion “set the table” for the main intrusive event by establishing a conduit system and preheating and devolitalizing the country rock. The emplacement of the main CLIC ultramafic magma generally followed the same path as the QLG unit probably by reaming out and inflating the semi-molten core of the precursor intrusion. The incorporation of silica, sulfides, and volatiles residing in the QLG precursor intrusion during the dynamic emplacement of the main chonolithic ultramafic intrusion resulted in high tenor sulfide mineralization. Preliminary evaluation of lithogeochemical data indicate that despite its intense felsic contamination and hydrothermal alteration, the trace element characterisitic of the QLG unit are consistent with its having similar parent magma as the ultramafic rocks of the CLIC. These results also indicate that the granitic and metasedimentary rocks of the Quetico subprovince are a reasonable source of contamination. Further geochemical modeling is underway in order to confirm the apparent genetic relationship between the QLG and the mineralized ultramafic CLIC. References Ding, X., Li, C., and Ripley, E.M., 2010, The Eagle and East Eagle sulfide ore-bearing mafic-ultramafic intrusions in the Midcontinent Rift System, upper Michigan: geochronology and petrologic evolution. G3-Geochemistry, Geophysics and Geosytems, Volume 11, Number 3, 22p. Foley, D.J., 2011. Petrology and Cu-Ni-PGE mineralization of the Bovine Igneous Complex, Baraga County, Northern Michigan. M.S. thesis. University of Minnesota Duluth, Duluth, MN. Goldner, B.D., 2011. Igneous petrology of the Ni-Cu-PGE mineralized Tamarack intrusion, Aitkin and Carlton Counties, Minnesota. M.S. thesis. University of Minnesota Duluth, Duluth, MN. Heggie, G., 2005, Whole rock geochemistry, mineral chemistry, petrology and Pt, Pd mineralization of the Seagull Intrusion, Northwestern Ontario. M.Sc. thesis, Lakehead University, Thunder Bay, ON MacTavish, A. and Smyk, M.C., 2010, Thunder Bay North Project, Magma Metals Limited. In Miller, J.D., Smyk, M.C. and Hollings, P.N. (eds.).Cu-Ni-PGE deposits in mafic intrusions of the Lake Superior region: A field trip for the 11th International Platinum Symposium; Ontario Geological Survey, Open File Report 6254, 166p. Rossell, D., 2008. Geology of the Keweenawan BIC intrusion: Institute of Lake Superior Geology. 54th Annual Meeting, Marquette, MI, Proceedings and Abstracts, part 2. p. 181-199. - 20 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Strain Variations in Carbonates across the Proterozoic Grenville Orogen Craddock, Suzanne D., Geology Department, Lawrence University, Appleton, WI, 54911, Craddock, John P., Geology Department, Macalester College, St. Paul, MN 55105 The Grenville orogeny involved the accretion of numerous terranes, likely with reversals of slab polarity, adding a width of ~800 km of new crust to Laurentia as part of the supercontinent Rodinia (Easton, 1991; Carr et al., 2000). Top-to-the-northwest thrusting is the common mechanism for accretion but there is a component of post-orogenic collapse (Rivers, 2012) documented by southeast trending kinematic indicators (Carlson et al., 1990). We report the results of a traverse across the Grenville orogen from Parry Sound, Ontario (NW) to Ft. Ann, New York (SE), including the younger, adjacent Taconic allocthon. Forty three carbonates were collected resulting in 57 strain analyses on mechanically twinned calcite with the following distribution: Parry Sound domain (1), Bancroft domain (20), Harvey-Cardiff domain (1), Belmont domain (4), Marzinaw domain (1), Frontenac domain (1), Adirondack Lowlands (5), Adirondack Highlands (8), and the Taconic allochthon (2). Within the regional sample suite there are two areas studied in detail, the Bancroft shear zone and a roadcut near Ft. Ann, NY. From northwest to southeast, the twinning strain fabric is dominantly a layer-parallel shortening fabric oriented N-S (Parry Sound), then becomes parallel to the Grenville thrust direction (NW-SE) across the Composite Arc Belt (Central Metasedimentary Belt) to the Adirondack Highlands where the sub-horizontal shortening strain becomes margin-parallel (SW-NE). The easternmost Adirondack Highlands preserve a vertical shortening strain in calcite veins that are cross-cut by veins found in the overlying Taconic allochthon; the Ordovician limestones and veins preserve a layer-parallel shortening strain oriented at 330°. Marbles from the Bancroft shear zone contain calcite grains with two sets of twin lamellae (e1 and e2). The better-developed e1 sets (n=406) record a LPS fabric oriented NW-SE whereas the younger e2 lamellae (n=146) preserve a marginparallel (SW-NE) LPS fabric. Both the e1 and e2 strains record an overprint strain (NEV) that records vertical shortening, perhaps related to the collapse of the Ottawan orogenic lid. References Carlson, K.A., van der Pluijm, B.A. and S. Hamner, 1990, Marble mylonites of the Bancroft shear zone: evidence for extension in the Canadian Grenville: Geol. Soc. Am. Bulletin 102, p. 174-181. Carr, S.D., Easton, R.M., Jamieson, R.A. and N.G. Culshaw, 2000, Geologic transect across New York and Ontario: Canadian J. Earth Sciences 37, p. 193-216. Easton, R.M., 1991, The Grenville Province and the Proterozoic history of central and southern Ontario: Geology of Ontario, Ontario Geologic Survey Special volume 4, part 2, Chapter 19, p. 715-906. Rivers, T., 2012, Upper-crustal orogenic lid and mid-crustal core complexes: signature of a collapsed orogenic plateau in the hinterland of the Grenville Province: Can. J. Earth Sc. 49, p. 1-42. - 21 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Petrogenesis and crustal contamination of the Nipigon sills: a geochemical and spatial re-evaluation CUNDARI, Robert, HOLLINGS, Peter, Department of Geology, Lakehead University, 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada, and SMYK, Mark, Resident Geologist Program, Ontario Geological Survey, Ministry of Northern Development and Mines, Thunder Bay, Ontario, P7E 6S7, Canada A compilation and re-evaluation of 2397 publically available, spatially referenced samples with associated whole-rock geochemistry has yielded previously unrecognized variation within the Midcontinent Rift-related Nipigon sills of the Nipigon Embayment. Nipigon diabase sills represent the most volumetrically significant Midcontinent Rift-related unit in Canada covering an area in excess of 20 000 km2 (Sutcliffe, 1991). 796 Nipigon sill samples have been investigated using Th/Yb ratios as a means of evaluating crustal contamination. This investigation revealed three distinct Nipigon sill types comparable to the three distinct suites of sills suggested by Hollings et al. (2007) based on radiogenic isotope data. This discrimination supports the isotopic signatures for each group as higher Th/Ybpm values are consistent with a more negative eNd(t=1000Ma) values. Ranges for the three Nipigon sill types are summarized in Table 1. Table 1. Summary of geochemical data for the Nipigon sills   Th/Ybpm   Nb/Ybpm   Nipigon  I   Nipigon  II   Nipigon  III   0.75  –  1.65   1.2  –  1.8   1.5  –  2.2   1.97  –  3.4   3.4  –  5.0   Nb/Nb*   0.425  –  0.65   0.35  –  0.55   Gd/Ybpm   1.0  –  1.9   1.0  –  1.9   La/Smpm   εNd(t=1000Ma)   1.2  –  1.8   1.60  –  2.0   -­‐0.5  to    -­‐1.5   1.5  to  3.0   5.0  –  6.5   0.3  –  0.5   2.2  –  2.6   1.0  –  1.9   >  5.0   Populations of more-contaminated samples (Nipigon types II and III) dominantly lie within two areas (Fig. 1); a north-trending, linear group in the southwestern Nipigon Embayment and an arcuate array in northern and eastern Lake Nipigon. Nipigon sill types II and III display higher Th/Ybpm values, lower Nb/Nb* values and more negative eNd(t=1000Ma) values when compared to Nipigon sill type I. These populations appear to be proximal to major structures such as the Black Sturgeon fault, southwest of Lake Nipigon, (Fig. 1). Type I Nipigon sills are located peripherally to Nipigon sill types II and III and display lower Th/Ybpm values, higher Nb/Nb* values and less negative eNd(t=1000Ma). Type I Nipigon sills do not appear to be related to any known major structures. Preliminary analyses show that Archean rocks of the Quetico Subprovince appear to control the Th/Ybpm values, whereas Sibley Group sedimentary rocks have a stronger control on Nb/Nb*. Type II and III Nipigon sills display higher Th/Ybpm values than type I sills, consistent with the model in which type II and III sills have assimilated metasedimentary rocks of the Quetico Subprovince. The higher Nb/Nb* values of type I Nipigon sills likely reflects a greater degree of interaction with the rocks of the Sibley Group suggesting shallower level contamination (as the Sibley is not present at depth within the Nipigon Embayment). The more crustalcontaminated nature of type II and III sills in conjunction with their proximity to major structures, suggest that magmas feeding these sills ascended through structures that provide more interaction with Quetico rocks at greater depths. This is supported by type II and III samples showing more negative eNd(t=1000Ma) values than - 22 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 type I samples, a signature that infers interaction with older continental crust. The source magma feeding type I Nipigon sills possibly exploited the same pathways yet had less interaction with continental crust at depth as the system was armoured by previously ascended magmas. As type I melts were laterally emplaced, they were contaminated by Sibley Group sedimentary rocks as displayed by elevated Nb/Nb* signatures. Figure 1. Map of the Nipigon Embayment showing the distribution of Nipigon sill types I (yellow circles), II (green squares) and III (red triangles). Major faults after Hart and MacDonald (2007). Figure 2. A) Nb/Ybpm vs. Th/Ybpm plot; B) eNd(t=1000Ma) vs. Th/ Ybpm Nipigon sill types I (yellow circles), II (green squares) and III (red triangles). Radiogenic isotope data from Hollings et al. (2007), Normalizing values from Sun and McDonough (1989). References Hart, T.R., and MacDonald, C.A., 2007. Proterozoic and Archean Geology of the Nipigon Embayement: implications for emplacement of the Mesoproterozoic Nipigon diabase sills and mafic to ultramafic intrusions. Canadian Journal of Earth Sciences 44: 1021-1040. Hollings, P., Richardson, A., Creaser, R. and Franklin, J., 2007. Radiogenic isotope characteristics of the mid-Proterozoic intrusive rocks of the Nipigon Embayment, northwestern Ontario. Canadian Journal of Earth Science 44: 1111-1129. Sun, S.S., and McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In Magmatism in the Ocean Basins. Geological Society, Special Publication No. 42, 313345. Sutcliffe, R.H. 1991. Proterozoic geology of the Lake Superior area. In Geology of Ontario. Edited by P.C. Thurston, H.R. Williams, R.H. Sutcliffe, and G.M. Stott. Ontario Geological Survey, Special Vol. 4, Part 1, pp. 405–484. - 23 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Geology and Geochemistry of the Coubran Lake Basalts, a Midcontinent Riftrelated sequence within the central Coldwell Complex, Marathon, Ontario CUNDARI, Robert, HOLLINGS, Peter, Department of Geology, Lakehead University 955 Oliver Road Thunder Bay, Ontario, Canada, P7B 5E1, SCOTT, John and CAMPBELL, Dorothy, Resident Geologist Program, Ontario Geological Survey, Ministry of Northern Development and Mines, Thunder Bay, Ontario, P7E 6S7, Canada The Coubran Lake Basalts occur as part of a large, preserved roof pendant of volcanic rocks in the center of the ~28 km diameter Mesoproterozoic Midcontinent Rift-related, alkalic Coldwell intrusive complex near Marathon, on the northern shore of Lake Superior. In one location, basalt is exposed in a 3 to 5 m wide by~350 m long stripped area. Thin (~2 m), amygdaloidal flows appear to be draped over the side of a hill and exhibit ropy flow tops, suggesting subaerial emplacement (Figs. 1 A and B). The volcanic pile is estimated to comprise five or six individual flows at this locality. The basalts are locally hornfelsed, altered (Fig. 1C) and are intruded by syenite dykes (Fig. 1D). They likely predate the main phase of syenitic magmatism that produced the central part of the Coldwell intrusive complex. Figure 1. Photos showing characteristic features of the Coubran Lake Basalts. Major element chemistry for all groups displays very little variation, with SiO2 values ranging from 49.33 to 51.98 wt%, TiO2 values ranging from 0.78 to 0.99 wt%, and MgO ranging from 4.77 to 7.34 wt%. However, three groups (types A, B, and C) can be distinguished based on their incompatible element abundances. Basalt type A lies towards the base of the unit, forming the lowermost 10% of the exposed sequence and displays strong LREE enrichment (La/Smn = 7.44 to 9.32) with HREE ratios comparable to that of types B and C (Gd/Ybn = 2.48 to 2.52). Basalt type B dominates the succession and displays light rare-earth element (LREE) enrichment (La/ - 24 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Smn = 2.45 to 4.77) and heavy rare-earth element (HREE) fractionation (Gd/Ybn = 1.96 to 2.39). Basalt type C comprises only three samples sporadically located through the sequence. It is characterised by LREE enrichment (La/Smn = 1.99 to 2.94) and HREE fractionation (Gd/Ybn = 1.97 to 2.10). It is distinguishable from basalt type A and B based on the absence of a negative niobium anomaly and lower abundances of large-ion lithophile elements (LILE). These samples appear to mark the tops of geochemically distinct flows. Midcontinent Rift-related volcanic rocks have been classified and correlated as five distinct groups (basalt types I through V) based on their major element, trace element and Nd isotopic analyses (Nicholson et al., 1997). Basalt type II represents a reversely polarized group of volcanic rocks deposited during an early phase of magmatism (>1105 Ma). They are characterized by LREE enrichment, HREE fractionation, as well as a negative niobium anomaly (Fig. 2). Primitive mantle-normalized diagrams show the Coubran Lake Basalts to be similar to basalt type II, which includes the upper Siemens Creek Volcanics, the central suite of the Osler Group and the recently recognized Devon Volcanics (Cundari, 2010). Furthermore, the Coubran Lake Basalts may be genetically related to the Two Duck Lake Gabbro of the Coldwell Complex based on preliminary evaluation of trace element abundances. Further correlative work, including radiogenic isotope analysis and paleomagnetism, is ongoing in order to place the Coubran Lake Basalts in context with the Coldwell Complex and other Midcontinent Riftrelated volcanic units. Figure 2. Primitive mantle normalized plots show distinctions between the four Coubran Lake Basalt types. Basalt type II data from Nicholson et al. (1997). Normalizing values from Sun and McDonough (1989). References Cundari, R., 2010. Geology and Geochemistry of the Devon Volcanics. South of Thunder Bay, Ontario. Unpublished HBSc thesis, Lakehead University. 68p. Nicholson, S.W., Shirey, S., Schulz, K., and Green, J., 1997. Rift-wide correlation of 1.1 Ga Midcontinent rift system basalts: implications for multiple mantle sources during rift development. Canadian Journal of Earth Sciences 34: 504-520. Sun, S.S., and McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In Magmatism in the Ocean Basins. Geological Society, Special Publication No. 42, 313345. - 25 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 The source of the Elliot Lake uranium ores: The neodymium isotope story EASTON, R.M., Precambrian Geoscience Section, Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, ON P3E 6B5 Canada, mike.easton@ontario.ca Uranium ore in the Elliot Lake camp is hosted in quartz pebble conglomerate beds within the lower part of the Matinenda Fm near the base of the Paleoproterozoic Huronian Supergroup. The Matinenda Fm is composed mainly of trough and planar cross-stratified, greenish to buff, coarse-grained sandstones deposited in a braded fluvial system (Fralick, 2003). Many workers (see Roscoe, 1969 for list) have suggested a paleoplacer origin for the mineralized conglomerates, but whether the source region was local (cf. Roscoe, 1969) or distal (cf. McDowell, 1969) has remained problematic. Based on a detrital zircon provenance study through the Matinenda Fm, Easton and Heaman (2011) suggested that the Matinenda Fm was locally sourced, especially the basal, mineralized, Ryan Member. This study reports neodymium isotopic data obtained from the Elliot Lake area, including the detrital zircon samples used by Easton and Heaman (2011). This data provides additional insights into the source rocks of the Matinenda Fm. Neodymium data are presented in Figure 1 and were obtained from the Isotope Geochemistry and Geochronology Research Centre at Carleton University. Archean felsic volcanic and granodiorite samples from the Whiskey Lake greenstone belt (samples -0108, -0180, -0202), all of which have been dated by U-Pb TIMS on zircon, have positive eNdT values close to the depleted mantle evolution curve. Paleoproterozoic rocks of the Thessalon Fm (sample -0042), and the East Bull Lake intrusive suite (sample -0335 and data from Prevec (1993)) have ΕNdT values ranging from 2.58 to -2.28, suggesting derivation from a primary magma originating from a depleted mantle source, which locally was affected by minor amounts of crustal contamination.   Figure 1. Epsilon neodymium versus age plot for Archean and Paleoproterozoic samples from the Elliot Lake area. Archean rocks indicated by circles, Paleoproterozoic mafic rocks by filled squares, Matinenda Fm samples by open squares. Numbers refer to sample numbers mentioned in the text. - 26 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 In contrast, the Matinenda Fm. sandstone samples all have negative eNdT, ranging from -0.52 to -9.21. Samples with the highest negative eNdT (-0129, -0344) are also enriched in Th, most likely due to the presence of monazite. The magnitude of the negative eNdT values in the Matinenda Fm indicates a negligible contribution from the volcanic and intrusive rocks of the Whiskey Lake greenstone belt, all of which have positive eNdT. The Matinenda Fm data can be explained if the sandstones contain a significant component derived from a suite of radiogenic granites located 30 to 40 km north and northwest of Elliot Lake which have eNdT of -6.19 (sample -1519). These radiogenic granites typically contain 8 to 33 ppm U and 30 to 50 ppm Th (Easton, 2010), thus they are also a potential source of uranium. In contrast, uranium contents of other Archean felsic intrusions in the Elliot Lake area are 1 to 4 ppm (Easton, 2010). It may be no coincidence that the most negative of the Matinenda Fm samples (-0344) was collected only a few metres below the mineralized Main Conglomerate Bed. In conclusion, the neodymium data, as well as the detrital zircon and geochemical data reported by Easton and Heaman (2011), are all consistent with a local source region that included radiogenic granites found north and northwest of Elliot Lake. The absence of similar radiogenic granites north of the Huronian Supergroup between Elliot Lake and Sault Ste. Marie may explain why the Matinenda Fm west of Elliot Lake contains no significant uranium occurrences. References Easton, R.M. 2010. Compilation mapping, Pecors-Whiskey Lake area, Superior and Southern provinces; in Summary of Field Work and Other Activities, 2010, Ontario Geological Survey, Open File Report 6260, p.8-1 to 8-12. Easton, R.M. and Heaman, L.M. 2011. Detrital zircon geochronology of Matinenda Formation sandstones (Huronian Supergroup) at Elliot Lake, Ontario: Implications for uranium mineralization; 57th Institute on Lake Superior Geology, Proceedings, v.57, pt.1, p.31-32. Fralick, P.W. 2003. Geochemistry of clastic sedimentary rocks: ratio techniques; in Geochemistry of sediments and sedimentary rocks: Evolutionary considerations to mineral deposit-forming environments, D.R. Lentz, editor; Geological Association of Canada, GeoText 4, p.8-103. McDowell, J.P. 1957. The sedimentary petrology of the Mississagi quartzite in the Blind River area; Ontario Department of Mines, Geological Circular 6, 31p. Prevec, S.A. 1993. An isotopic, geochemical and petrographic investigation of the genesis of early Proterozoic mafic intrusions and associated volcanism near Sudbury, Ontario; unpublished PhD thesis, University of Alberta, Edmonton, Alberta, 223p. Roscoe, S.M. 1969. Huronian rocks and uraniferous conglomerates in the Canadian Shield; Geological Survey of Canada, Paper 68-40, 205p. - 27 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Erosion of Archean lithosphere by subduction and rifting: a tomographic image of central North America FREDERIKSEN, Andrew, DENISET, Ian, Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, R3T 2N2, BOLLMANN, Trevor, VAN DER LEE, Suzan, Department of Earth and Planetary Sciences, Northwestern University, 1850 Campus Drive, Evanston, IL 60208-2150 and DARBYSHIRE, Fiona, Université du Québec à Montréal, Centre GEOTOP, CP 8888, succ. Centre-Ville, Montréal, Québec, H3C 3P8 The core of the North American continent was assembled by Precambrian accretionary processes, which are generally believed to also be responsible for the formation of the underlying continental lithosphere. Signatures of lithospheric accretion are preserved in the lithosphere in a form detectable by observations of seismic velocity and fabric. These signatures, however, are subject to later modification by later tectonic processes, such as orogeny, rifting, and mantle plume impingement. We examine the lithosphere of the region surrounding the southwest portion of the Archean Superior Province using a large teleseismic data set from Canadian and American sources. The western Superior is associated with high seismic P velocity and a strong, consistent eastwest pattern of shear-wave splitting. The western edge of the Western Superior velocity and splitting anomalies lies ca. 200 km east of the crustal contact with the Proterozoic Trans-Hudson Orogen, perhaps indicating that Trans-Hudson orogenic processes eroded the Superior lithosphere. To the southwest, the Superior lithosphere is disrupted by a low-velocity lithospheric channel ca. 250 km wide striking northwest through Minnesota and the Dakotas, which is also associated with a sharp weakening of anisotropy. The origin of this feature is enigmatic, as it does not correspond to any known crustal feature, though it appears to be truncated by the Mid-Continent Rift and so may represent a failed rift arm at the lithospheric level. The rift itself is only partially resolved, but shows indications of low velocities; resolution of the Mid-Continent Rift will improve over the next few years due to a combination of the eastward migration of the Earthscope Transportable Array and new data from the temporary Superior Province Rifting Earthscope Experiment (SPREE). - 28 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 The black line faults of the Red Lake gold mines, Ontario GASPAR, Brandon and HILL, Mary Louise, Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1 Canada Black line faults are a common feature in the Red Lake gold mines. Characterized by their distinctive black color, they are planar structures that cross-cut and displace various lithologies, dykes, and gold-bearing veins. They may have anomalous traces of gold but are usually barren. Although black line faults along the Red Lake mine trend vary in size, geometry and amount of displacement, they all share the same general composition of mostly tourmaline, chlorite, and interstitial quartz. They have previously been interpreted as minor faults associated with brittle late-stage deformation; however thin-section analysis indicates that some are overprinted by ductile deformation. Rather than representing a discrete late-stage event, they are better characterized as compressive shear fractures that formed during progressive high-temperature deformation that was dominantly ductile. Black line faults are significant in that they represent a time of brittle-ductile deformation and the presence of complex hydrothermal fluids, typically related to orogenic gold mineralization. - 29 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 A microstructural study of the Otjikoto Deposit, Namibia: Inferences from gold mineralization in the Superior Province GASPAROTTO, Marc and HILL, Mary Louise, Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1 Canada Shear-zone-hosted gold deposits in the Superior province of northwestern Ontario are characterized by heterogeneous deformation in high-temperature brittle-ductile shear zones. Since microstructure is critical to identifying this type of deformation, microstructural analysis may provide a vector to gold mineralization in orogenic gold deposits where scarce outcrop inhibits structural mapping. The Otjikoto project in Namibia is an example of such an orogenic gold deposit. It is situated in the province of Otjozondjupa, approximately 300 km north of the capital city of Windhoek. A 20-metre-thick layer of calcrete covers the exploration area, making the mapping of structural features difficult. Research for this Lakehead University Honours thesis was carried out through the microstructural analysis of thin sections (obtained from Auryx Gold) that were collected from drill core. The microstructures produced by deformation exhibited in the minerals of this deposit provide information about the relative timing of metamorphism, deformation, quartz veins and gold mineralization of the Otjikoto project. The metapelitic Okonguarri Formation, host to the Otjikoto gold mineralization, contains evidence for peak metamorphism at the temperatures and pressures of the sillimanite zone of the amphibolite facies. Grain size reduction of feldspar indicates that mylonitization also occurred at amphibolite facies temperatures. The formation also exhibits brittle deformation, specifically in the competent amphibole and garnet porphyroblasts, which occurs during synchronous metamorphism and ductile deformation. The mineralized quartz veins are synorogenic as evidenced by both brittle and ductile deformation under at least greenschist facies metamorphism. In particular, boudins of coarse-grained quartz (interpreted to be boundinaged quartz veins) indicate the mutual overprinting of brittle and ductile deformation which is characteristic of a shear-zone-hosted gold deposit. Based on evidence for the synchronous metamorphism, brittle deformation and ductile deformation, and the synorogenic mineralized quartz veins, a shear-zone-hosted gold deposit model should be considered for the Otjikoto project. - 30 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Stratigraphy and tectonic setting of Neoarchean arc volcanic rocks in the Bird River Belt, Manitoba, Canada. GILBERT, H.P., Manitoba Geological Survey (360-1395 Ellice Ave., Winnipeg, Manitoba R3G 3P2 The Neoarchean Bird River Belt (BRB) in the western Superior Province is currently the focus of geological and geochemical investigations that have led to a revised interpretation of its tectonic setting and geological history. A collaborative mapping project initiated in 2005 by the Manitoba Geological Survey is currently focused on detailed studies of economically important mafic-ultramafic intrusions and rare-element–bearing pegmatites. The BRB is part of a continental arc, juxtaposed against the north margin of the Winnipeg River Subprovince, which extends east for over 150 km, as far as Separation Lake in Ontario. The stratigraphy of this ca. 2.75 to 2.70 Ga belt documents a history of arc magmatism and associated sedimentation, as well as subsequent deformation that resulted from the convergence and ultimate collision of flanking older cratonic terranes to the north and south: North Caribou Terrane (NCT) and Winnipeg River Subprovince (WRS), respectively. BRB arc activity in the Neoarchean occurred during an interval of widespread magmatism in the central part of the NCT and along its north and south margins (Percival et al., 2006). In many cases the Neoarchean arc assemblages are contiguous with older, Mesoarchean assemblages, but the latter appear to be absent in the BRB. The oldest known rocks in the BRB are mid-ocean-ridge basalt (MORB)-like rocks flanking the arc volcanics along both north and south margins of the belt, and thought to represent back-arc and ocean-floor settings respectively. The northern MORB type is notable for the presence of mafic-ultramafic intrusions (2745 Ma Bird River Sill, Wang, 1993; Mayville intrusive complex), thought to be genetically related to their host rocks, and associated with Cu-Ni-Cr-PGE ore deposits (Mealin, 2006). North (2735-2731 Ma) and south (2725 Ma) panels of arc-type rocks in the BRB (Gilbert et al., 2008) are stratigraphically distinctive but they are probably part of an evolving arc volcanic succession that was associated with subduction of oceanic crust at a continental margin to the north. Sm-Nd isotopic data and crustal residence ages of the arc volcanic rocks indicate the source magma was influenced by older ca. 3.0 Ga continental crust. Crustal assimilation is also indicated by the fact that 70% of the mapped area of BRB arc volcanics consists of felsic to intermediate rock types. Remnants of the cratonic block north of the BRB may be represented by Mesoarchean (2.85 Ga) granitoid rocks within the multiphase Maskwa Lake Batholith, which also contains 2725 Ma granitoid intrusion(s) of synvolcanic age. Geochemical discrimination of incompatible elements and element ratios in the arc type rocks shows a systematic trend from least evolved compositions in the north panel, towards progressively more evolved compositions in the lower, main and upper parts of the south panel, consistent with a change from arc to arcrift (extensional) tectonic settings (Gilbert et al., 2008). The upper part of the north panel is a lithologically diverse, mainly volcaniclastic to epiclastic succession (Diverse arc assemblage, DAA), possibly representing an extensional basin. A youngest detrital zircon date of 2706 ±23 Ma for a turbidite deposit provides a rough estimate for the maximum depositional age of the DAA. Turbidite deposits (Booster Lake Formation, BLF) in the central part of the BRB are thought to represent an elongate rift basin that extends east between the north and south panels of arc rocks. The turbidites are interpreted as analogous with 2713-2704 Ma orogenic sedimentary rocks and related paragneiss in the English River Subprovince, commonly interpreted as a fore-arc basin that extends for 800 km along the south margin of the Uchi domain (Hrabi and Cruden, 2006). A 2712 ±17 Ma youngest detrital zircon date provides an approximate depositional age for the BLF. Fluvial-alluvial deposits of slightly younger age (2697 ±18 Ma) in the eastern part of the BRB structurally overlie the margin of the arc volcanic rocks. The coarse siliciclastic rocks are interpreted as synorogenic, more proximal equivalents of the BLF turbidites. Contacts between the various tectonic components (arc, back-arc and orogenic sedimentary rocks) appear to be invariably faulted. - 31 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Synvolcanic granitoid and gabbroic plutons (2723 Ma) intrude the south panel arc rocks, but similar plutons are absent in the north panel that contains, instead, relatively younger sanukitoid rocks. The light-rare-earthelement–enriched and strongly fractionated sanukitoid suite is represented by (1) clasts in DAA conglomerate, (2) dikes and sills in north panel arc volcanic rocks, and (3) minor intrusions in BLF turbidites. The sanukitoid magmatism thus spanned syn- and post-volcanic intervals. Terminal collision of the NCT and WRS resulted in regional folding, faulting and locally tectonic intercalation between BLF turbidites and arc volcanic rocks. Posttectonic (ca. 2660-2646 Ma) granitoid plutons of probable anatectic origin, and rare-element-bearing pegmatites (e.g. 2640 ±7 Ma Tanco pegmatite; Baadsgaard and Černý, 1993) are among the last manifestations of the sequence of events - initiated ca. 3.0 Ga - that were associated with the convergence and collision of Mesoarchean cratonic blocks in the western Superior Province. References Baadsgaard, H. and Černý, P. 1993: Geochronological studies in the Winnipeg River pegmatite populations, southeastern Manitoba; Geological Association of Canada–Mineralogical Association of Canada, Joint Annual Meeting, Program with Abstracts, v. 18, p. A5. Gilbert, H.P., Davis, D.W., Duguet, M., Kremer, P.D., Mealin, C.A. and MacDonald, J. 2008: Geology of the Bird River Belt, southeastern Manitoba (parts of NTS 52L5, 6); Manitoba Science, Technology, Energy and Mines, Manitoba Geological Survey, Geoscientific Map MAP2008-1, scale 1:50 000 (plus notes and appendix). Hrabi, R.B. and Cruden, A.R 2006: Structure of the Archean English River subprovince: implications for the tectonic evolution of the western Superior Province, Canada; Canadian Journal of Earth Sciences, v. 43, p. 947-966. Mealin, C.A. 2008: Geology, geochemistry and Cr-Ni-Cu-PGE mineralization of the Bird River Sill: evidence for a multiple intrusion model; M.Sc. thesis, University of Waterloo, 155 p. + 1 folded colour map. Percival, J.A., Sanborn-Barrie, M., Skulski, T., Stott, G.M., Helmstaedt, H. and White, D.J. 2006: Tectonic evolution of the western Superior Province from NATMAP and LITHOPROBE studies; Canadian Journal of Earth Sciences, v. 43, p. 1085–1117. Wang, X. 1993: U-Pb zircon geochronology study of the Bird River greenstone belt, southeastern Manitoba; M.Sc. thesis, University of Windsor, Windsor, Ontario, 96 p. - 32 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Geology of the Layered Series, Coldwell Alkaline Complex, Ontario GOOD, David, McLEAN, Katrina, EPSTEIN, Rachel, Stillwater Canada Inc, 11 Sydenham St., Dundas Ontario, Canada, L9H 2T5, LINNEN, Robert, Department of Earth Sciences, Western University, London, Ontario, Canada, N6A 3K7 and SAMSON, Iain, Department of Earth and Environmental Sciences, University of Windsor, 401 Sunset Avenue, Windsor, Ontario, Canada, N9B 3P4 The Eastern Gabbro forms the outer margin of the Coldwell Alkaline Complex, a large lopolith emplaced during the Mid Continent Rift Event at 1108 Ma (Heaman and Machado (1992) The Eastern Gabbro is up to 1500 meters thick and, as described by Shaw (1997), formed by multiple intrusions of evolved basaltic magma with a subalkaline parentage into a partial ring dike structure that cut the Archean country rock.     Figure 2. Ternary diagram for CIPW Norm abundances demonstrates the linear modal trends within the layered series. FH-08-05 Fly Hill samples contain a higher proportion of pyroxene and less plagioclase than the typical oxide augite melatroctolites. Figure 1. Eastern margin of the Coldwell Complex showing the Eastern Gabbro, Marathon Deposit, and Syenitic Rocks. Geochemical sampling locations are indicated by red symbols. The Eastern Gabbro is shown to be a composite intrusion made up of at least three distinctive components or gabbroic series including the early Fine Grained Series, the Layered Series and the Marathon Series. This study presents the results of detailed petrography of the Layered Series at five sites covering 24 of the 31 km strike length of the Eastern Gabbro (Fig. 1). The Layered Series makes up the majority of the Eastern Gabbro and is compositionally, geochemically - 33 - �Proceedings of the 58th ILSG Annual Meeting - Part 1   Figure 3. (a) Modal layering in the Layered Series shows cross bedding, gradational variation of plagioclase and clinopyroxene and strong contrast between cyclical layers. (b) Polished thin section of a plagioclase free interval within the oxide augite melatroctolite unit. and texturally similar for all sample locations. The Layered Series is dominated by massive to modally layered olivine gabbro (Fig. 3a), but also includes relatively thick units of weakly layered oxide augite melatroctolite (Fig. 3b). Contacts between these units are typically gradational. The oxide augite melatroctolite unit at the Fly Hill location contains iron rich clinopyroxene. CIPW Normative abundance of minerals are plotted on ternary diagrams (Fig. 2) to highlight the sequence of layered rocks that trend from gabbroic anorthosite to olivine melagabbro. The Mg number calculated for olivine gabbro displays a narrow range of values from 35 to 65 and is similar to the results of Shaw (1997). Samples of oxide augite melatroctolite at Fly Hill located near the top of the Layered Series represent the most fractionated rocks with an Mg number of 10 and samples from the Three Finger Lake area represent the most primitive gabbro with Mg numbers up to 65. References Heaman, L.M., Machado, N., 1992. Timing and origin of Midcontinent rift alkaline magmatism, North America: evidence from the Coldwell Complex. Contrib. Mineral. Petrol., 110: 289-303. Shaw C.S.J., 1997, The petrology of the layered gabbro intrusion, eastern gabbro, Coldwell alkaline complex, Northwestern Ontario, Canada: evidence for multiple phases of intrusion in a ring dyke, Lithos, v. 40, p.243-259. - 34 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Quartz Fabric Analysis of the Kawishiwi Shear Zone, NE Minnesota GOSCINAK, Christopher and HANSEN, Vicki, Department of Geological Sciences, University of Minnesota Duluth, 1049 University Drive, Duluth MN 55812 Structural fabrics within the Vermilion district, NE Minnesota, including metamorphic foliation and mineral lineation are well established and can be summarized as northeast striking, near vertical foliation containing oblique to vertical lineations (Sims, 1972, 1976; Hudleston 1976; Hudleston et al., 1988; Shultz-Ela & Hudleston, 1991; Bauer & Bidwell, 1990; Goodman, 2008; Karberg, 2009; Johnson, 2009; Erikson, 2010). The oblique to vertical lineations are present throughout the Vermilion district, yet areas of subhorizontal lineation orientations are also present locally. Two different interpretations emerge from previous studies: 1) dextral transpression associated with terrane accretion (e.g., Hudleston, 1976; Hudleston et al., 1988) and 2) lineation-parallel shearing consisting of regional dip-slip shearing and later, more focused, strike-slip shearing (e.g., Goodman, 2008; Karberg, 2009; Johnson, 2009; Erikson, 2010). The Kawishwi Shear Zone (KSZ), one of several Vermilion district shear zones, shows structural relationship particularly well. Foliation consistently strikes east-northeast and dips vertically. Lineations are broadly down-dip with discrete areas of subhorizontal. Goodman (2008) performed a structural and kinematic analysis of the KSZ and interpreted lineation-parallel shearing with dip-slip shearing followed by strike-slip shearing. However, previous studies do not specifically constrain flow within L-S tectonites relative to lineation. This study aims to characterize the kinematic pattern of flow through use of quartz fabric analysis of c- and a-axis petrofabrics. The data reveal the dominant slip planes and direction of flow during deformation, and also provide deformation temperature and strain geometry information. Oriented samples from the KSZ were analyzed. Sample KS7J contains a vertical lineation and quartz petrofabric data indicate flow nearly parallel to the lineation, thus dominantly dip-slip displacement. Quartz microstructures are consistent with greenschist deformation. Sample KS6UI, collected from a localized zone with strike-parallel lineation, displays Quartz petrofabric data indicative of slip along the prism <a> plane with flow parallel to lineation. However, quartz microstructures are inconsistent with this higher temperature slip plane; suggesting another mechanism to activate slip. One such mechanism, orthogonal reactivation, accounts for high temperature slip at lower temperatures due to shearing in two orthogonal directions (Oliver, 1996). In this model, initial shearing produces a crystallographic preferred orientation within quartz. Subsequent orthogonal shearing exploits weaknesses within the aligned quartz lattice and triggers slip along high temperature slip planes. Quartz petrofabrics, in particular a-axis data, support this model. References Bauer, R. L and Bidwell, M. E., 1990. Contrasts in the response to dextral transpression across the Quetico-Wawa subprovince boundary in northeastern Minnesota. Canadian Journal of Earth Sciences, 27, 1521-1535. Erickson, E., 2010. Structural and kinematic analysis of the Shagawa Lake shear zone, Superior Province, northern Minnesota: implications for the role of vertical versus horizontal tectonics in the Archean. Canadian Journal of Earth Sciences, 47, 1463-1479. Goodman, S., 2008. Structural and Kinematic Analysis of the Kawishiwi Shear Zone, Superior Province. M.S. Thesis, University of Minnesota Duluth, MN. Hudleston, P. J., 1976. Early deformational history of Archean rocks in the Vermilion district, northeastern Minnesota. Canadian Journal of Earth Science vol. 13, 579-592 Hudleston, P.J., Schultz-Ela, D., Southwick, D. L., 1988. Transpression in an Archean greenstone belt, northern Minnesota. Canadian Journal of Earth Sciences, vol 25, 1060-1068. Johnson, Thomas K., 2009, Structural, Kinematics, and Hydrothermal Fluid Investigation of the Gold-Bearing Murray Shear Zone, Northeastern Minnesota, M.S. Thesis, University of Minnesota Duluth, MN. Karberg, S M., 2009. Structural and Kinematic Analysis of the Mud Creek Shear Zone, Northeastern Minnesota. M.S. Thesis, University of Minnesota Duluth, MN. - 35 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Oliver, Douglas, 1996. Structural, Kinematic and Thermochronometric Studies of the Teslin Suture Zone, South-Central Yukon Territory. PhD dissertation, Southern Methodist University, TX. Schultz-Ela, D.D., Hudelston, P.J., 1991. Strain in an Archean greenstone belt of Minnesota. Tectonophysics, 190, 223-268. Sims, P. K., 1972. Vermilion district and adjacent areas. In Geology of Minnesota: a centennial volume, P. K. Sims & G. B. Morey editors. Minnesota Geological Survey 49-62 Sims, P.K., 1976. Early Precambrian tectonic-igneous evolution in the Vermillion district, northeastern Minnesota. Geol. Soc. Am. Bull. 87, 379-389. - 36 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Structural control on the emplacement of the TBN-Igneous Complex. HEGGIE, Geoff, MACTAVISH, Allan, JOHNSON, Justin, WESTON, Ryan, and MA, Leon, Magma Metals (Canada) Ltd. 1004 Alloy Drive, Thunder Bay, Ontario, 1gheggie@magmametals.ca The Lake Superior area was the focus of extensional and compressional deformation during the development of the Mesoproterozoic Midcontinent Rift (MCR) rift sequence. Age determination carried out around the MCR identify a rift evolution sequence spanning ~60 m.y. and subdivided into 4 stages (Heaman et al., 2007). It is within Stage 2 (1115-1105 Ma) that a series of mafic-ultramafic intrusions prospective for Ni-Cu-PGE were emplaced proximal to the main rift axes (e.g. Seagull, Kitto, Disraeli, Hele, Thunder Bay North, Tamarack and Eagle Intrusions). Pre-existing Archean structures are postulated to be a strong influence on the early rift architecture and are implicated to be a primary control on the distribution of these Stage 2 intrusions (Hollings et al., 2007; 2010). The Thunder Bay North (TBN)-igneous complex was emplaced during Stage 2 of the rift event. The complex comprises a series of contemporaneous layered intrusions, dykes, sills and chonoliths (Fig. 1). Magma Metals Ltd. has been actively exploring the TBN-complex since 2006 with the discovery of Pt-Pd-Cu-Ni mineralization hosted within the complex. To date the TBN-Complex hosts an indicated resource of 9.8 Mt at 2.34 g/t Pt-Eq for 741 000 contained Pt-Eq ounces and an inferred resource of 0.53 Mt at 2.87 g/t for 49 000 contained Pt-Eq ounces (MMW, Feb. 23, 2012). During the delineation of the mineral resource a number of lines of evidence were observed within the TBN-complex to support the hypothesis that pre-existing Archean structures controlled the spatial emplacement of intrusions, the morphology of igneous bodies, and consequently the distribution of mineralization. Structural controls on the TBN-complex can be divided into two groups: 1) macro-controls which are evident in the morphology of the intrusion, but not apparent in drill core, and 2) micro-controls which are readily seen Figure 1. Plan map overview of the TBN-Complex as defined by diamond drilling and airborne magnetics. Outline of the igneous complex shown in orange, mineral resource in red. - 37 -   �Proceedings of the 58th ILSG Annual Meeting - Part 1 in drill core. Macro-structural controls (Fig. 2) include the east-west trending Quetico fault which defines the boundary between Archean metasedimentary rocks to the south and anatectic granites to the north. This structure provides a zone of weakness for the intrusive complex to develop along. Exfoliation and horizontal joints in the Archean rocks provide vertical discontinuities for the lateral development of sills. Regional conjugate joint sets and faults in the texturally homogneous anatectic granite provide a network of rheological discontinuities that were exploited during the development of chonoliths. Of the micro-structural controls, fault zones and zones of decreased rock quality are identifiable in the host rocks. The development of invasive magma fingering into these incompetent structural breccias are strong evidence for the influence of pre-existing structure controlling the emplacement, progressive development and final morphology of the intrusions. The progressive development and morphology of the complex influence magma flow dynamics, which in turn effect particle settling and the distribution of fluid load, specifically the distribution of immiscible sulfides and crystal accumulation.   Figure 2. Idealized schematic cross-section showing differing structures controlling the morphology of intrusions. References Heaman, L.M., Easton, R.M., Hart, T.R., Hollings, P., MacDonald, C.A., Smyk, M., 2007. Further refinement to the timing of Mesoproterozoic magnetism, Lake Nipigon region, Ontario: Canadian Journal of Earth Sciences, v. 44, p. 1055-1086. Hollings, P., Smyk, M., Heaman, L.M., Halls, H., 2010. The geochemistry, geochronology and paleomagnetism of dikes and sills associated with the Mesoproterozoic Midcontinent Rift near Thunder Bay, Ontario, Canada: Precambrian Research, v. 183, p. 553-571. Hollings, P., Fralick, P., Cousens, B., 2007. Early history of the Midcontinent Rift inferred from geochemistry and sedimentology of the Mesoproterozoic Osler Group, northwestern Ontario: Canadian Journal of Earth Sciences, v. 44, p. 389-412. Magma Metals Ltd., Feb. 23, 2012. News release, Magma Metals increases mineral resources at TBN to 790 00 PlatinumEquivalent ounces. SEDAR, 5p. - 38 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Preliminary Bedrock Geology Map of the Eastern part of Lake Vermilion State Park, St. Louis County, NE Minnesota HEIM, Nick, SCOTT, Heather, KILDUFF, Rob, RAHTZ, Christine, VIAL, Andrew, YOUNG, Spencer, MAHR, Chris, and HUDAK, George, Precambrian Research Center, Natural Resources Research Institute, University of Minnesota Duluth, 5013 Miller Trunk Highway, Duluth, MN, 55811 Each year, students from the Precambrian Research Center (PRC) geology field camp complete “capstone” projects that encompass approximately one week of detailed field mapping followed by one week of mapmaking and map publishing. During the fifth and sixth weeks of the 2011 field camp, seven PRC field camp students, under the direction of PRC Faculty member George Hudak, mapped Neoarchean rocks in the eastern half of Minnesota’s newest state park, Lake Vermilion State Park (Heim et al., 2011). This capstone mapping project sought to: 1) identify the lithologies and stratigraphy of the Neoarchean supracrustal strata in this area; 2) define and characterize the nature of the contact between various units of the Neoarchean supracrustal strata and intrusive rocks; 3) obtain a better understanding of geological structures and their orientations within the area; and 4) to complete field mapping which will result in a comprehensive bedrock geological map for future use by employees of, and visitors to, Lake Vermilion State Park. Prior to mapping, detailed 1:5000 scale laminated field mapping sheets were produced. One side of each field mapping sheet consisted of a georeferenced air photo, and the other side of the mapping sheet consisted of a corresponding georeferenced topographic map. Mapping was completed by means of lakeshore mapping from canoes, as well as numerous traverses through the bush. Following each day of field mapping, students and faculty transferred their field data to a master map, enabling the generalized geology of the region to be established by the middle of the fifth week of field camp. During the sixth week of field camp, students produced a digital version of the field map utilizing a variety of software (ArcView, AutoCad, Surfer, Adobe Illustrator). In the area mapped, the Neoarchean supracrustal rocks consist of a NE/SW striking, NW facing homoclinal sequence comprising several distinctive volcanic and sedimentary sequences, synvolcanic and post-volcanic intrusive rocks, and sheared rocks adjacent to post-volcanic deformation zones. Up-section, these sequences are: (1) the Fivemile Lake Sequence of the Lower Member of the Ely Greenstone Formation, which is composed of; a) light bluish green to green, aphyric to sparsely plagioclase-phyric, highly amygdaloidal sheet flow - and pillowed facies andesite and basalt; b) medium-bedded to massive andesite to basalt tuffs and lapilli tuffs; c) grayish-green, aphyric to quartz-phyric coherent rhyolite lava flows and associated flow breccias; and d) green to tan, massive polymict breccias comprising up to 20cm diameter clasts of pumice, scoria, and/or amygdaloidal basalt/andesite; (2) the Central Basalt Sequence of the Lower Member of the Ely Greenstone Formation, which is composed of medium green to dark green, aphyric to sparsely plagioclase-phyric, sparsely amygdaloidal sheet flow - and pillowed facies basalt lava flows which are locally strongly quartz-epidote altered; (3) the Soudan Member of the Ely Greenstone Formation, an interbedded sequence comprising: a) laminated to medium-bedded, planar-bedded, locally chaotically folded, dark gray to red-brown, Algoma-type oxide facies iron formation; b) medium- to dark green, aphyric to locally sparsely plagioclase-phyric, massive to moderately amygdaloidal andesite to basalt sheet flow facies lava flows; c) gray to gray green, massive quartz ± plagioclase-phyric rhyodacite tuff; and d) light gray, massive polymict lapilli tuffs comprising up to 5% quartzand plagioclase-phyric coherent rhyodacite lapilli and 10-15% dark green chlorite-rich lapilli interpreted to be chloritized pumice; (4) the Gafvert Lake Member of the Lake Vermilion Formation, which is composed of; a) a basal unit - 39 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 comprising light gray, massive, polymict volcaniclastic conglomerates and sandstones; b) light gray, massive, quartz- and plagioclase-phyric polymict dacite to rhyodacite tuff and lapilli tuff containing up to 20% quartz- and plagioclase phyric coherent dacite and rhyodacite lapilli and up to 7% locally quartz- and plagioclase-phyric pumice lapilli; c) light gray, massive, quartz- and plagioclase-phyric tuff and lapilli tuff containing up to 15% subangular quartz- and plagioclase-phyric pumice lapilli; d) gray, thinly-bedded to massive tuff breccias with poorly developed graded bedding; e) red to dark gray, laminated to medium bedded oxide facies iron formation; and f) light gray to black laminated to very thinly bedded chert. Numerous sills and dikes intrude the supracrustal assemblage in the area. Intrusive bodies which appear to be synvolcanic include a) grayish green to black, medium-grained, locally ophitic gabbro; b) gray to gray-green, medium-grained diorite; and c) light gray, massive, quartz- and plagioclase-phyric diorite and quartz-diorite characterized by euhedral blue-gray quartz phenocrysts up to 1cm in diameter. Post-volcanic intrusive rocks identified include: a) pink to green-gray medium-grained hornblende diorite; b) light gray to pale green gray, quartz- and plagioclase-phyric diorite to quartz diorite sills and dikes; and c) light gray, fine- to medium-grained feldspar-phyric diorite. Locally, sericite-dominant schists and phyllites, chlorite-dominant schists and phyllites, and chlorite-rich sheared mafic volcanic rocks occur along discrete ductile to brittle-ductile structures. Our analysis leads us to interpret the following sequence of events for the development of the bedrock geology visible today in this part of the Vermilion District: 1) deposition of highly amygdaloidal sheet- and pillowed facies andesite and basalt lava flows, felsic lava flows, primary mafic tuffs and lapilli tuffs, and associated resedimented polymict volcaniclastic rocks in a shallow submarine setting (Hudak et al., 2007); 2) deposition of submarine basalt lava flows, comprising both sheet flows and pillowed facies, in a relatively deep water (>500 meters water depth) setting (Hudak et al., 2007); 3) deposition of Algoma-type iron formations of the Soudan Member of the Ely Greenstone Formation as a result of hydrothermal activity during volcanically quiescent periods and deposition of interbedded mafic and felsic volcanic and volcaniclastic strata associated with intermittent volcanism; 4) the onset of voluminous explosive dacitic to rhyodacitic felsic volcanism, alternating with periods of volcanic quiescence characterized by continued chemical sedimentation and localized clastic sedimentation; 5) synvolcanic to post-volcanic intrusion of mafic to felsic sills and dikes; and 4) structural deformation, probably associated with regional D2 deformation (Peterson and Patelke, 2003) that formed the sheared mafic rocks and chlorite schists that occur in the northeastern part of the field area. References Heim, N., Scott, H., Kilduff, R., Rahtz, C., Vial, A., Young, S., Mahr, C, and Hudak, G., 2011, Preliminary Bedrock Geology Map of the Eastern Part of Lake Vermilion State Park, St. Louis County, NE Minnesota: Precambrian Research Center Map Series, PRC/MAP-2011/01, 1:5000 scale. Hudak, G. J., Hoffman, A. T., Peterson, D. M., and Heine, J., 2007, Recent developments understanding the volcanic, magmatic, tectonic and metallogenic evolution of the Ely Greenstone Formation, Vermilion District, NE Minnesota. Institute on Lake Superior Geology, Proceedings Volume 53, Part 1 – Programs and Abstracts, p. 42-43 Peterson, D. M., and Patelke, R. L., 2003, National Underground Science and Engineering Laboratory (NUSEL): Geological Site Investigation for the Soudan Mine, NE Minnesota: Natural Resources Research Institute, Technical Report NRRI/ TR-2003/29, 88 p. - 40 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Chert Stromatolites in the Basal Gunflint Formation, Kakabeka Falls: Primary Precipitation or Silicification? HORNER, Simon and FRALICK, Philip, Department of Geology, Lakehead University, Thunder Bay, On, Canada, P7B 5E1, philip.fralick@lakeheadu.ca Modern-day stromatolites primarily form in carbonate-dominated environments through binding particulate matter to the sticky extracellular polysaccharides secreted by the cyanobacteria. Evidence of the operation of this process in constructing stromatolites extends back into the Phanerozoic. However, Precambrian stromatolites do not contain abundant silt- to sand-sized particles characteristic of this process. Instead they are primarily composed of material that has precipitated from seawater. This is commonly a carbonate phase, though chert stromatolites are common in some Proterozoic and Archean sequences. In recent years controversy has developed in the literature as to whether the chert stromatolites in the Gunflint Formation and similar rock units represent silicification of carbonate or are primary silica precipitates (see: Knoll and Simonson, 1981; Simonson, 1985; Knoll, 1985; Siever, 1992; Maliva et al., 2005). Some of this work utilized outcroppings of the Gunflint Formation that are highly diagenically altered, and thus ambiguous concerning primary mineralogy. Roadwork on Highway 588 near Kakabeka Falls exposed an excellent example of chert microbialites and stromatolites entombed in ankeritic grainstone with a sharp boundary between the two, providing an opportunity to add to the debate on the possible primary origin of the silica in chert stromatolites. The 1.878±1 Ga Gunflint Formation is a chemical sediment dominated shelf succession deposited on Archean basement. It records a number of transgressive-regressive cycles. Hofmann (1969) and Franklin et al. (1982) described stromatolites occurring at two different stratigraphic levels; at the base of the Gunflint growing on Archean units and the basal conglomerate and secondly, on a hard-ground in the middle of the Formation developed during regression. The basal biogenic sediment at Kakabeka Falls directly overlies Archean granodiorite and consists of a lower one meter thick, continuous chert microbialite. The millimeter-scale, crinkly layering is sub-horizontal, with organic carbon only giving the unit a dark colouration in limited places. A large stromatolitic head develops off the upper microbialite layers. It is the size of a beach ball, with a slightly narrower base. There appears to be a sharp contact between the microbialite-stromatolite and carbonate grainstone, which overlies the microbialite and abuts against the stromatolite. The layering in the stromatolite does not build on top of the lower grainstone laminae, indicating that the stromatolite was fully grown before the grainstone entered the environment. Samples of all lithologies were investigated with a petrographic microscope, a field emission scanning electron microscope with an energy dispersive spectrometer, inductively coupled plasma atomic emission spectrometry and mass spectrometry. The diagenetic history is more complex than it appears to the naked eye. The stromatolite contains abundant microquartz, megaquartz, chalcedony, and organic matter. The silica is rather clean and well developed with little to no carbonate inclusions or ghosts. Carbonate present in the stromatolite is seen replacing chalcedony fans. Distinct growth zones can be seen in the outer portions of the carbonate. Rarely, quartz crystals present in the stromatolite show carbonate overprinting, replacing them to varying degrees. Carbonate is visible replacing megaquartz grains and overprinting microquartz as very fine grained specular carbonate. Growth zoning is present in most of the carbonate grains. The carbonate grainstone unit overlying the stromatolite has been highly silicified. The quartz is texturally similar to the quartz in the stromatolite; microquartz and megaquartz are abundant with minor amounts of chalcedony. The megaquartz is seen in the intergranular spaces as space-filling cement. Higher magnification shows extinct carbonate grains that have been silicified with small amounts of carbonate remaining behind. However, further from the contact with the stromatolite the carbonate grainstone unit is silicified to a much lower degree. Some of the carbonate grains have experienced some silicification, - 41 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 but the rock is mainly composed of carbonate with some organic material. Silica in the form of microquartz is forming within the carbonate grains. A closer observation reveals that the quartz developing in the carbonate grains is itself being over-printed by carbonate growth. The carbonate grains have growth zoning in their outer portions. Rare earth element plots for the ankerite and chert are generally similar, though this probably reflects both inheriting the seawater pattern. The most striking difference between the two lithologies is the Ba/Sr ratio. The stromatolitic cherts have one consistent ratio, whereas the carbonates and known replacement cherts from the area have a different consistent ratio. All of the above is consistent with the microbial and stromatolitic cherts being a primary precipitate, but it is not conclusive. If the silica is primary new models will need to be developed to explain how Precambrian cherty stromatolites form. References Franklin, J.M., et al 1982. Proterozoic geology of the northern Lake Superior area. Geological Association of CanadaMineralogical Association of Canada Field Trip Guidebook, Trip 4. Hofman, H.J. 1969. Stromatolites from the Proterozoic Animikie and Sibley Groups, Ontario: Geological Survey of Canada, Paper 68-69. Knoll, A.H., 1985. Exceptional preservation of photosynthetic organisms in silicified carbonates and silicified peats: Philosophical Transactions of the Royal Society of London, v. B311, p. 111-122. Knoll, A.H., and Simonson, B., 1981. Early Proterozoic microfossils and penecontemperaneous quartz cementation in the Sokoman Iron Formation, Canada: Science, v. 211, p. 478-480. Maliva, R.G., Knoll, A.H., and Simonson B.M., 2005. Secular change in the Precambrian silica cycle: Insights from chert petrography. Geological Society of America, v.117; no.7/8; p. 835-845 Siever, R. 1992. The silica cycle in the Precambrian. Geochimica et Cosmochimica Acta Vol. 56, p. 3265-3272. Simonson, B.M., 1985. Sedimentology of cherts in the Early Proterozoic Wishart Formation, Quebec-New-foundland, Canada: Sedimentology, v. 32, p. 23-40. - 42 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 The Minnesota Taconite Workers Health Study: Environmental Study of Airborne Particulates - 2012 Update HUDAK, George, MONSON GEERTS, Stephen, ZANKO, Larry, SEVERSON, April, SEVERSON, Allison, Natural Resources Research Institute, 5013 Miller Trunk Highway, Duluth, MN, 55811, and BANDLI, Bryan, Department of Geological Sciences, University of Minnesota Duluth, 229 Heller Hall, 1114 Kirby Drive, Duluth, MN 55812 Since 2008, the Natural Resources Research Institute (NRRI) has been conducting a detailed characterization of mineral dust in northeastern Minnesota. The purpose of this research is to evaluate the effects of past and present emissions from taconite mining and processing on air quality throughout the Mesabi Iron Range (MIR) by characterizing airborne particulate matter within taconite operations, in communities surrounding taconite operations on the MIR, in population centers in other regions of northeastern Minnesota, as well as particulate matter deposited in lake sediments (Fig. 1). NRRI’s sampling and characterization work represents the community/environmental component of the Minnesota Taconite Workers Health Study, a broad University of Minnesota (UM) research effort investigating long-standing questions regarding the impact of dust derived from mining and processing of taconite (iron ore). The UMN School of Public Health (SPH), with whom NRRI is collaborating, is responsible for the human health- and exposure-related components of that effort, which include: 1) an occupational exposure assessment; 2) a mortality study; 3) a cancer incidence study; and 4) a respiratory health survey of taconite workers and spouses. Figure 1. Locations of taconite processing plants on the Mesabi Iron Range being sampled during this study (after Oreskovich and Patelke, 2006) Air sampling is performed within taconite operations, MIR communities, and non-MIR communities by NRRI scientists during both winter and summer seasons. Sampling at taconite operations takes place at four locations: 1) secondary crushers; 2) magnetic separators/concentrators, agglomerators/ ball drums, and the kiln/ pellet discharge area. Sampling within MIR communities takes place on the rooftops of public buildings, whereas sampling in non-MIR communities occurs on rooftops or in remote locations so that background air quality can be evaluated. Airborne particles are collected using 1) a micro orifice uniform deposit impactor (MOUDI) (Marple et al., 1991), which enables size-fractionated particulate matter collection, and 2) a total suspended particulate filter (TSP). Particulate matter is evaluated via gravimetric analysis and subsequently subjected - 43 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 to comprehensive particulate matter characterization that includes: 1) scanning electron microscopy (SEM) imaging; 2) energy dispersive spectroscopy (EDS); 3) electron backscattered diffraction (EBSD); 4) proton induced x-ray emission (PIXE); 5) the Minnesota Department of Health’s 852 Method Transmission Electron Microscopy (TEM) Analysis for Mineral Fibers in Air; and 6) the International Standard Organization’s Method 10312 Ambient air – Determination of Asbestos Fibers – Direct-Transfer TEM Method (ISO 10312, 1995). Over the past year, the NRRI has completed particulate matter sampling within MIR taconite operations, MIR communities, and non- MIR communities. This includes 14 sampling events at taconite operations and 79 sampling events at locations within communities in northeastern Minnesota (73) and Minneapolis (6), as summarized in Table 1. Continued analysis of lake sediment samples from “North of Snort Lake” indicates collection of sediment dating back to ~1840, which pre-dates iron mining on the MIR. At Silver Lake, logging activities in the early part of the 20th century disrupted the sediments; however, we believe we will have good post-1915 lake sediment data, which will mark the period where the transition from natural ore to taconite mining took place. Continued analysis, interpretation and reporting will take place in 2012. Table 1. Summary of in-plant and community sampling events Taconite Facility United Taconite In-Plant Sampling Events Sampling Events Taconite Facility 2 events while active Keetac Hibbing Taconite   Sampling Events 1 event while active, 1 event while inactive 1 event while inactive, 3 events while active 3 events while active 1event while active, Northshore 1 event while inactive Minntac 1 event while active Minorca Community Sampling Events Community Sampling Location Sampling Events and Number per Season Keewatin Elementary School 7 Events (3 Winter / 4 Summer) Hibbing High School 9 Events (4 Winter / 5 Summer) Virginia City Hall 10 Events (5 Winter / 5 Summer) Babbitt Municipal Building 16 Events (7 Winter / 9 Summer) Silver Bay High School 13 Events (4 Winter / 9 Summer) Ely Fernberg Site 6 Events (3 Winter / 3 Summer) Duluth NRRI Rooftop 12 Events (7 Winter / 5 Summer) UMN Mech. Eng. Rooftop 6 Events (3 Winter / 3 Summer) References ISO 10312, 1995, Ambient air – determination of asbestos fibers – direct transfer transmission electron microscopy method, 51p. Marple, V. A., Rubow, K. L., and Behm, S. M., 1991, A micro orifice uniform deposit impactor (MOUDI): description, calibration, and use: Aerosol Science and Technology, v. 14, p. 434-446. Oreskovich, J. A., and Patelke, M. M., 2006, Historical use of taconite byproducts as construction aggregate materials in Minnesota: A Progress Report: Natural Resources Research Institute Report of Investigation NRRI-RI-2006-02, 10 p. - 44 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Bedrock geologic map of the Crane Lake and Brule Narrows 30’X60’ quadrangles, Quetico subprovince, northern Minnesota JIRSA, Mark A., Minnesota Geological Survey (jirsa001@umn.edu) The Crane Lake-Brule Narrows map portrays a complex history of sediment deposition, multiple intrusive events, and several periods of deformation and metamorphism. It lies within the Quetico subprovince of the Archean Superior Province (Fig. 1, small inset). The bedrock consists of biotite-plagioclase schist, granitoid and minor mafic intrusions, and complex migmatite containing multiple paleosomatic and neosomatic components. The schist was derived from graywacke and pelitic sediments deposited ~2695 Ma in an accretionary prism during early stages of collision between the Wawa subprovince island arc to the south, and the evolving Superior superterrane to the north. Later stages of this D1 collisional event produced tilting, broad nappe structures, and Figure 1. Location and regional geologic setting of Crane Lake-Brule Narrows map (MGS miscellaneous map M-192). Adjacent 1:100,000-scale maps recently published by MGS are numbered: Bigfork (M-176); Vermilion Lake (M-141); ElyBasswood Lake (M-148). thrust imbrication of volcanoplutonic rocks in subprovinces north and south of the Quetico subprovince, and recumbent folding within the Quetico. An early suite of leucogranite, granodiorite, trondhjemite, and tonalite is interlayered on all scales with biotite schist. This early suite has compositional and approximate temporal similarities with the 2690 Ma Saganaga Tonalite. A second deformation event (D2) occurred at about 2680 Ma, and produced regional penetrative fabrics, folds, and prograde metamorphism to greenschist and amphibolite facies. A third deformation event (D3) is manifest in the Quetico subprovince as broad, east- and west-plunging folds of D2 fabrics (synforms, antiforms), and by faulting. At least part of this deformation was synchronous with migmatization and emplacement of 2-mica leucogranite and a slightly younger, typically red, variably magnetic biotite granite known as the Lac La Croix granite. A U-Pb zircon age of the latter in adjacent Canada - 45 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 is approximately 2666 Ma, which is consistent with published ages for metamorphism inferred to be related to D3. The metamorphic grade is more or less symmetrical along the axis of the Quetico subprovince, with greenschist grade at the margins and middle to upper amphibolite facies near the axis. Metamorphism was generally syntectonic with D2 and D3; and a contact metamorphic overprint occurs locally adjacent to the Lac La Croix Granite. Bedrock in the western part of the map area is cut by Paleoproterozoic diabasic dikes that constitute the easternmost extent of the ~2076 Ma, Kenora-Kabetogama dike swarm. The geologic interpretation is based on archived and newly acquired outcrop and geophysical data augmented with geophysical and digital land surface topographic maps. Nowhere in Minnesota is bedrock so pervasively exposed, and a strong correlation is apparent between topography and the structure and composition of bedrock (Fig. 2). For example, large areas of granitic bedrock in the south and east are topographically higher than areas of migmatitic and schist-rich rocks in the north and west. Differential weathering produced curvilinear topographic relief that crudely depicts foliation trends and reveals complex folding. Prominent fractures and faults in all rock types are manifest in linear topographic lows. The aeromagnetic data similarly reflect foliation, folding, and fracture-controlled oxidation. Ground magnetic susceptibility data acquired from lithologically diverse outcrops indicate that granitic components typically produce higher signatures. The new image created by combining these data sources mimics at the map scale the complex magmatic, migmatitic, deformation, metamorphic, and alteration features observed at the outcrop scale. Mapping was supported by the U.S. Geological Survey STATEMAP element of the National Cooperative Geologic Mapping Program. Figure 2. Image of digital land surface topography in the Crane Lake-Brule Narrows area. Highest elevations, international border, and lake names are white. - 46 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Reconnaissance geologic mapping of Neoarchean rocks in the central Boundary Waters Canoe Area Wilderness by students of the Precambrian Research Center’s 2011 field camp. JIRSA, Mark, Minnesota Geological Survey, University of Minnesota, 2642 University Avenue W., St. Paul, Minnesota 55114; jirsa001@umn.edu, BAGGETTO, Louis, ELIASON-JOHNSON, Garret, HANSEN, Darren, HOXSIE, Erin, and KILPATRICK, Kayla, 2011 Field Camp Students, Precambrian Research Center, University of Minnesota Duluth, 5013 Miller Trunk Highway, Duluth, Minnesota 55811 The Precambrian Research Center (PRC)—a branch of the University of Minnesota, Duluth—conducted its 5th season of field camp in 2011. The final phase of field training is a “Capstone Project” that provides students an opportunity to create new geologic maps in areas of poorly understood geology. In contrast to detailed mapping that is typical of Capstone exercises, the students in this group were charged with reconnaissancestyle mapping in parts of four 1:24,000-scale quadrangles along a 44 mile canoe route in the Boundary Waters Canoe Area Wilderness (BWCAW). The route traversed ~22 lakes (Fig. 1), and a great variety of primarily Archean metavolcanic, metasedimentary, and intrusive rocks. Students mapped shoreline outcrops by canoe, and traced pertinent contacts during short inland traverses and at portages. Much of the area hasn’t been mapped in significant detail since J.W. Gruner’s work 1927-1938 (published in 1941). Although that work was remarkably detailed for its time, base maps were rudimentary and thus, surface control was uncertain. This project was intended to help place Gruner’s work into an accurate geographic context, verify or refute his observations, understand the nature of contacts, and apply modern geological nomenclature and interpretations. The regional bedrock is part of the Neoarchean Wawa subprovince of Superior Province (Fig. 2). Two main suites occur; pillowed volcanic and volcaniclastic rocks of the Newton Lake Formation that are cut by Figure 1. Map of central BWCAW showing route of 2011 geologic mapping (dark dashed line). Some of the larger lakes along the route are labeled. - 47 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Figure 2. Generalized geologic map of northeastern Minnesota showing map area and geologic entities mentioned in text. felsic intrusions including the Basswood Lake granodiorite, and sedimentary rocks of the Knife Lake Group that locally contain fragments derived from the Newton Lake and Basswood Lake rocks, and thus are inferred to be younger. The geology is parceled into lozenges of strata having internally consistent stratigraphic relationships, separated by anastomosing shear and fault zones. Offset magnitude on those faults is largely unknown, and correlation of geologic units from one lozenge to another is not always possible. Significant discoveries from the reconnaissance mapping include: 1) The presence of iron-formation interlayered with volcaniclastic strata near Moose Lake in a sequence of rocks mapped regionally as the Newton Lake Formation. 2) The Basswood Lake granodiorite is compositionally and texturally identical with parts of the 2690 Ma Saganaga Tonalite, and wall-rock relationships are similar, suggesting temporal equivalence. 3) Identification of an unconformity and paleosaprolite between metagabbroic rocks likely equivalent to the Newton Lake Formation, and sedimentary strata of the Knife Lake Group. 4) Fault-bounded lozenges appear to represent slightly different crustal levels of exposure. For example, the lozenge exposed at Knife Lake was likely uplifted to expose the older, weathered and eroded metagabbroic floor of the Knife Lake Group depocenter. Geologic maps of all capstone projects can be downloaded or viewed at the PRC website: http://www.d.umn. edu/prc. - 48 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Geochemistry of mafic dikes from the Carlton Dike Swarm in Minnesota JOHNSON, Teresa, Department of Geology and Geological Engineering, Colorado School of Mines, 1516 Illinois Street, Golden, CO 80401, SHANNON, James, MMG, 390 Union Boulevard, Suite 200, Lakewood, CO 80228, BOERBOOM, Terrence, Minnesota Geological Survey, 2642 University Avenue W., St. Paul, MN 55114, and WENDLANDT, Richard, Department of Geology and Geological Engineering, Colorado School of Mines, 1516 Illinois Street, Golden, CO 80401 The Carlton Dike Swarm (CDS) is a reversely-polarized diabase dike swarm associated with the earlyreversal period of the ~1.1 Ga Midcontinent Rift System. The mafic dikes occur west of Duluth centered on Carlton County, MN and are generally oriented rift parallel. Previous characterization of the CDS has been completed by field mapping, petrography, geochemistry and aeromagnetics. Continued characterization of the CDS is being completed to delineate possible subgroups based on structural orientation, mineralogy, texture, geochemistry and geochronology. The four dike subgroups under consideration are (1) the standard CDS diabase dikes described by Green et al. (1987) and Reichhoff (1987), (2) a Ti-enriched set that may be related to the reversely polarized Esko intrusion, (3) an ultramafic-like set described by Boerboom (2009) and (4) the NWtrending orthogonal set. In addition two normally-polarized dikes in Carlton County are included to contrast temporal differences. Detailed characterization studies of rift-related dike swarm intrusions may elucidate temporal-spatial relationships to extrusive and intrusive rocks of the Midcontinent Rift including mineralized, magmatic Cu-Ni chonoliths and Duluth-type intrusions. Distinct petrographic differences in olivine modes, oxide compositions/modes, phenocryst phases/modes and quench textures are initially being used to delineate subgroups. Based on these criteria, NW-trending dikes have similar petrographic attributes to the standard CDS. The variations of ilmenite and magnetite exsolution and oxidation characteristics are being investigated further to use as a distinguishing factor. Quench textures of chain olivine, Ti-rich augite, plagioclase and ilmenite in the chilled margins of Esko-like dikes are similar to the mineralogy and quench textures found in the upper part of the Esko intrusion. The standard CDS dikes, NW-trending and normally-polarized dikes have chilled margins ranging from glassy to aphanitic. Further comparisons of the chilled margins among subgroups are in progress using whole rock geochemistry and electron microprobe analyses. Considering the difficulties in age determination for mafic rocks, QEMSCAN® is being used to identify possible uranium enriched zirconium minerals. Representative thin sections from each of the proposed subgroups are analyzed at 4 µm intervals with BSE imaging (Fig. 1). Points with a designated brightness (BSE95-150) are further analyzed with EDS to determine the mineralogy. The minerals identified with this technique are zircon, zirconolite, baddeleyite and monazite. For age determination, samples with zirconium minerals greater than 20 µm will be candidates for bulk analysis with ID-TIMS, and samples with grain sizes smaller than 20 µm will be considered for in-situ analysis with SIMS methods designed by Chamberlain et al. (2010). The final selection of grains for in-situ analysis is enhanced by additional high resolution BSE and CL imaging to characterize mineral associations, compositional zoning, intergrowths and overgrowths. - 49 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Figure 1. BSE image of NW-trending dike; white circles show locations of zirconium minerals References Boerboom, T.J., 2009, C-19 Geologic Atlas of Carlton County, Minnesota [Part A]: Minnesota Geological Survey, County Atlas Series. Chamberlain, K.R., Schmitt, A.K., Swapp, S.M., Harrison, T.M., Swoboda-Colberg, N., Bleeker, W., Peterson, T.D., Jefferson, C.W., and Khudoley, A.K., 2010, In situ U–Pb SIMS (IN-SIMS) micro-baddeleyite dating of mafic rocks: Method with examples: Precambrian Research, v. 183, no. 3, p. 379–387. Green, J.C., Bornhorst, T.J., Chandler, V.W., Mudrey Jr., M.G., Meyers, P.E., Pesonen, L.J., and Wilband, J.T., 1987, Keweenawan dykes of the Lake Superior region: evidence for evolution of the middle Proterozoic Midcontinent Rift of North America, in Mafic Dyke Swarms, Special Paper 34, Geological Association of Canada, p. 289–302. Reichhoff, J.A., 1987, Two Keweenawan Basaltic Dike Swarms in the Duluth Area, Minnesota: University of Minnesota, Duluth, 206 p. - 50 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Ferromanganese Precipitates in Lacustrine Environments of Northwestern Ontario and Nova Scotia: Effects on Arsenic Concentration KERKERMEIER, Leah and FRALICK, Philip, Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1, Canada Freshwater lakes in Northwestern Ontario and Nova Scotia have iron and manganese precipitates growing at the sediment-water interface. The precipitates appear as disc shaped nodules with ringed structures composed of alternating iron- and manganese-rich laminations surrounding a nucleus of a pebble or cobble (Fig. 1). On the surface of some of the structures, micro-columnar stomatolites are formed (Fig. 2). The nodules are often found covered with a thick bacterial mat. Precipitates collected from Lake Shebandowan and Sowden Lake in northwestern Ontario and Lake Charlotte in Nova Scotia have been analyzed. The nodules have revealed extreme naturally concentrated arsenic with values as great as 3500ppm.   Figure 1. A ferromanganese nodule collected from Sowden Lake, Northwestern Ontario Figure 2. Micro-columnar stromatolites found on the surface of a ferromanganese nodule To understand such an arsenic concentration, natural environments able to create reduction/oxidation boundaries are being examined. The precipitates are composed primarily of the redox sensitive metals iron and manganese allowing for speculation that arsenic concentration has a relationship with the redox potential of an environment. In a reduced environment, such as some groundwaters, iron and manganese can be transported in solution until reaching an oxidized environment, such as lake water. It is proposed that arsenic will behave similar to phosphorous in a lake environment due to its molecular similarities and valance state, therefore also having an attraction towards metals such as iron (Takamatsu et al., 1985). Two possible natural redox environments can occur in a lacustrine environment allowing for precipitation of redox sensitive metals. When reduced groundwater comes into contact with oxidized lake water by means of a spring or diffuse flow on a lake bottom, redox sensitive elements can oxidize and precipitate out of solution coating a nucleation point such as a pebble. This is hypothesized to be occurring in the study area in Lake Charlotte and areas of Lake Shebandowan. The concentration of arsenic, by means of this redox method, can be high due to the ability of groundwater to leach a large area and transport ions in solution for long distances, concentrating the dissolved material in the nodules. - 51 -   �Proceedings of the 58th ILSG Annual Meeting - Part 1 The second postulated redox environment involves upwelling of anoxic, deeper lake water carrying redox-sensitive elements to the oxic water in the shallows (Dean et al. 1981). In conjunction with this process, microorganisms concentrate dissolved manganese and iron and upon death are buried in the lake bottom sediment. The anoxic environment in the substrate allows for the reduction of iron and manganese, releasing the soluble reduced forms of these metals into sediment pore waters. These ions then diffuse upward into the lake water, until contact with photosynthetic microorganisms enables re-oxidization and precipitation onto a nucleus, often a pebble (Dean et al. 1981). Continued accretion of iron and manganese will form a nodule. This redox process probably creates ferromanganese precipitates that do not accumulate substantial amounts of arsenic due to lack of a large reservoir that can be leached. It is hypothesized that areas of Shebandowan and Sowden Lake fall into this category. References Dean, W.E., Moore W.S., Nealson, K.H. 1981. Manganese cycles and the origin of manganese nodules, Oneid Lake, New York, USA. Chemical Geology, 34: 53-64. Stevens, L.B., Investigation of ferromanganese nodule precipitation and arsenic uptake in modern biochemical sediments: Lake Charlotte, Nova Scotia. Unpub. B.Sc. thesis, Lakehead University, 70pp. Takamatsu,T., Kaswashima, M., and Koyama, M., 1985. The role of Mn2+- Rich Hydrous Manganese Oxide in the Accumulation of Arsenic in Lake sediments. Water Resource. 19 (8): 1029-1032. - 52 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Paleomagnetism of the Alkaline Coldwell Complex: New Results, New Insights. Kern, Ashley, Kulakov, E.V., Smirnov, A.V., and Diehl, J.F, Department of Geological and Mining Engineering and Sciences, Michigan Technological University,1400, Townsend Drive, Houghton, Michigan, 49931, USA. The alkaline Coldwell Complex is a ≈1.1 Ga intrusive complex which is thought to have been emplaced during three different episodes of magmatism. Previous paleomagnetic work by Lewchuk and Symonds (1990) has shown that rocks representing magmatic episodes I and III exhibit reversed (R) magnetizations with mean directions that are significantly different. The rocks representing episode II were characterized by normal (N) magnetizations which are symmetrical with respect to the R magnetizations. Lewchuk and Symonds (1990) interpreted these results as indicating the reversal asymmetry seen in other Keweenawan age rocks is not the effect of a long-standing non-dipole field but the effect of plate motion. To further investigate the reversal asymmetry question in Keweenawan-aged rocks, we have begun a new paleomagnetic study of the alkaline Coldwell Complex at the Earth Magnetism Laboratory of Michigan Technological University. We are also using the micro-baddeleyite method to refine the ages of the various episodes of Coldwell Complex magnetism, but are only reporting our preliminary paleomagnetic results at this meeting. Six to ten samples were collected from 32 sites along the Trans-Canadian HWY 17 (previously sampled and studied by Lewchuk and Symons (1990)). In addition, we sampled 11 sites north of Marathon and near Middleton. Characteristic remanent magnetizations (ChRM), in our study, have been isolated by thermal and alternating field demagnetization techniques. Mean paleomagnetic directions have been categorized according to the existing model of the three episodes of magmatism. The mean paleomagnetic directions for episodes I and III have been found to be statistically similar at 95 percent confidence, making these episodes indistinguishable with paleomagnetism unlike the Lewchuk and Symons’ study. We will discuss two possible scenarios of the geomagnetic field behavior during the formation of the Coldwell Complex: 1. Multiple reversals RaNaR, as recorded consecutively by episodes I, II and III; 2. Single reversal from R to N polarity, where episodes I and III were emplaced at the same time (single episode) or in pulses during a time interval insufficient to result in any difference between the primary ChRM directions. The normal polarity paleomagnetic direction of episode II appears to be antipodal to both reversed components as well as to the mean direction calculated from all reversely magnetized sites. The reversal test (McFadden, 1990) is positive in classification C for both RaNaR reversals and positive in the same classification for the RaN case. References. McFadden P. L., M. W. McElhinny, 1990. Classification of the reversal test in palaeomagnetism. Geophysical Journal International, 103, 3, pp 725–729. Lewchuk M.T., D.T.A. Symons, 1990. Paleomagnetism of the Late Precambrian Coldwell Complex, Ontario, Canada. Tectonophysics, 184, pp 73-86. - 53 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Thrust faulting in the Gunflint Formation north of Lake Superior KOROSCIL, Jesse, HILL, Mary Louise and FRALICK, Philip, Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, Canada P7B 5E1 New exposure of the Gunflint-Rove contact at the Terry Fox monument north of Thunder Bay reveals several thrust faults that cut through the Sudbury impact layer and into the overlying Rove Formation. This discovery has implications for the timing of thrust-fault deformation here and elsewhere in the area north of Lake Superior. Previously, thrust-fault deformation in the Gunflint Formation was interpreted to be associated with the Penokean orogeny (Hill and Smyk, 2005). However, new observations of thrust faults cutting the impact layer and overlying Rove Formation mean that this deformation is too young to be Penokean. The thrust faults exposed in this outcrop are mainly small discrete bedding plane faults with few kinematic indicators observed in the hanging wall or footwall. The faults are most visible where they cut across bedding; once identified they can be traced along the outcrop. Thrust faults within the impact layer are identified by slickenlines on fault plane surfaces as well as possible duplication of strata, resulting in localized thickening of the impact layer. The faults can be traced along strike until they are either covered by overburden or cut by a prominent normal fault which displaces all units in the hanging wall down to the south several meters. The timing of thrust-fault deformation can be constrained based on cross-cutting relationships and published ages of the 1878 Ma Gunflint Formation (Fralick et al., 2002), 1850 Ma Sudbury impact event (Krogh et al., 1984) and the 1832 Ma Rove Formation (Addison et al., 2005). References Addison, W.D, Brunpton, G.R., Valinni, D.A., McNaughton, N.J., Davis, D.W., Kissin, S.A., Fralick, P.W. and Hammond, A.L., 2005, Discovery of distal ejecta from the 1850 Ma Sudbury impact event: Geology, v. 33, p. 193-196. Fralick, P.W., Davis, D.W. and Kissin, S.A., 2002, The age of the Gunflint Formation, Ontario, Canada: Single zircon U-Pb age determinations from reworked volcanic ash: Canadian Journal of Earth Sciences, v. 38, p. 1085-1091. Hill, M.L. and Smyk, M.C., 2005, Penokean fold-and-thrust deformation of the Paleoproterozoic Gunflint Formation near Thunder Bay, Ontario [abstract]: in Institute on Lake Superior Geology Proceedings, 51st Annual Meeting, Nipigon, Ontario, v.51, part 1, p.26. Krogh, T.E., Davis, D.W. and Corfu, F., 1984, Precise U-Pb zircon and baddeleyite ages for the Sudbury area: in E.G. Pye et al. (eds.), The Geology and Ore Deposits of the Sudbury Structure, Ontario Geological Survey Special Volume 1, p. 431-446. - 54 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Paleomagnetism of the Geordie Lake and Silver Mountain basalts Kulakov, Evgeniy V., Smirnov, A.V. and Diehl, J.F., Department of Geological and Mining Engineering and Sciences, Michigan Technological University, 1400, Townsend Drive, Houghton, Michigan, 49931, USA. A paleomagnetic study of 17 basaltic lava flows exposed in the northern part of the Coldwell Complex, near the Geordie Lake (GL) (Ontario, Canada), and 13 lava flows that crop out on Silver Mountain (SM) (Baraga County, Northern Michigan) has been carried out in order to investigate the paleomagnetic directions of these rocks and to determine their relationship to other rocks of the North American Midcontinent Rift (MCR). The Geordie Lake basalts consist of a series of flat-lying lava flows that overlay gabbros of the northern part of the Coldwell Complex and are assumed to be similar in age (~1108 Ma) to these gabbros. The lava flows at Silver Mountain, a 300 foot-high glacial polished dome-shaped hill, are part of the Siemens Creek volcanics, the lowermost formation of the Powder Mill Group. This sequence of lava flows dip 15 degrees to north-west. Silver Mountain is surrounded by Cambrian and younger sediments as well as glacial deposits. Six to ten samples were collected from each lava flow at both locations. Well-defined characteristic remanent magnetizations (ChRM) were isolated using thermal demagnetization techniques. Both the GL and the SM CHRMs define a reversed (R) polarity magnetization. Mean directions for each flow as well as group means were calculated using standard Fisher statistics. Group mean paleomagnetic directions of GL (D=109.1°, I=-63.1°, α95=3.81°, K=89, N=17) and SM (D=107.9°, I=-60.4°, α95=2.43°, K=232, N=13) are statistically indistinguishable after the correction of SM directions for structural tilt. In addition, both the SM and GL mean directions are statistically similar to the published data for reversely magnetized Lower North Shore Volcanics (LNSV) (e.g. Books, 1968; Halls and Pesonen 1982) dated at 1107±1.9 Ma using the U-Pb method (Davis and Green, 1997). Paleomagnetic poles calculated from GL and SM paleomagnetic direction are located at lat=42.9°N, long=206.5°E (α95=3.6°) and lat=39.8°N, long=202.8°E (α95=3.4°), respectively. These poles fall directly on the eastern arm of the North American Precambrian apparent polar wonder path (APWP) (Logan Loop (Robertson and Fahrig, 1971)) and are similar to that calculated for 1107 Ma reversely magnetized LNSV (lat=41.5°N, long=202.7°E) at the 95 percent confidence level. Therefore, we conclude that rocks of the Geordie Lake and Silver Mountain represent the earliest stage of rift related volcanic activity. Similarity of paleomagnetic directions and paleomagnetic poles for these sequences suggests no clockwise rotation of Midcontinent Microplate after 1107 Ma, as proposed by Hauser (1996). References. Books, K., 1968.Magnetization of the lowermost Keweenawan lava flows in the Lake Superior area. USGS Processional Paper. 600-D, D248–D254. Campbell R.E., 1952 Geophysical Investigation of the Silver Mountain Are – Houghton County, Michigan, MS Thesis, Michigan college of Mining and Technology. Davis, D.W., Green, J.C., 1997. Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution. Canadian Journal of. Earth Sciences. 34 (4), 476–488. Halls, H.C., and Pesonen, L.J. 1982. Paleomagnetism of Keweenawan rocks. In Geology and tectonics of the Lake Superior Basin. Geological Society of America, Memoir 156, pp. 173 -201. Hauser E.C, 1996, Midcontinent rifting in a Grenville embrace, Geological Society of America, Special paper, 308, pp 67-75 Davis, D.W., Green, J., and Manson, M. 1995. Geochronology of the 1.1 Ga North American Mid-Continent Rift [abstract). Proceedings of the 41st Conference of the Institute on Lake Superior Geology, Marathon, Ont., May 1995, Part I, Program and Abstracts, pp. 9-10. Robertson, W.A., W.F. Fahrig, 1971. The great Logan paleomagnetic loop-the polar wandering path from the Canadian Shield rocks during the Neohelikian Era. Canadian Journal of Earth Sciences, 8, 1355-1372. - 55 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Geochemistry and Petrology of the Dog Lake Granite Chain, Quetico Basin, Northwestern Ontario KUZMICH, Ben, HOLLINGS, Pete, Department of Geology, Lakehead University, 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada, CAMPBELL, Dorothy, and Scott, John, Resident Geologist Program, Ontario Geological Survey, Thunder Bay, Ontario P7E 6S7 The Dog Lake Granite Chain is composed of six ovoid magnetite-bearing granitic intrusions within the Quetico Basin, Northwestern Ontario (Fig. 1). From east- to west the intrusions have been termed the Penasen Lake, White Lily, Barnum Lake, Trout Lake, Silver Falls, and Shabaqua intrusion. These granites have been mapped previously mapped as syn- to post-tectonic, massive to foliated granite/granodiorites (Ontario Geological Survey, 2006). Of the intrusions, only the Barnum Lake intrusion has been studied in detail and was classified as a homogenous quartz-monzonite with lesser amounts of syenite and gabbroic phases (Steinert, 1975; Kelhenbeck, 1977). The Dog Lake Granite Chain displays a linear trend, which parallels the tectonic boundary between the Wawa-Abitibi terrane to the south, and the Quetico Basin to the north. Figure 1. Aeromagnetic map of the Dog Lake Granite Chain (modified from Ontario Geological Survey, 1999) Petrologic and geochemical data has been used to classify the granites as both I- and S-type. The I-type granites can be classified into three broad groups a microcline-phyric monzonite/quartz-monzonite, syenite/quartzsyenite, and a monzodiorite. These granites are massive, silica poor, largely metaluminous, and characterized by a hornblende+ magnetite+ sphene +/-pyroxene assemblage. The microcline-phyric monzonite/quartz-monzonite and syenite/quartz-syenite groups are characterized by a positive εNd (+1.44 to +2.11). The I-type granites have - 56 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 been recognized within the Trout Lake, Barnum Lake, White Lily, and Penasen Lake intrusions. The S-type granites are typical of the Quetico Basin, and have a largely peraluminous affinity, variable εNd signatures (-1.44 to +1.09), and are characterized by a muscovite+ biotite+/- garnet assemblage. The S-type granites have been sampled within the Silver Falls, Trout Lake, and White Lily intrusions. The recognition of I-type granites within the Quetico Basin, which is predominantly composed of S-type granites requires a different model for the formation of this relatively rare rock type. It is suggested that the formation of I-type granites within the Quetico Basin involves the partial melting of the mantle wedge beneath the Wawa-Abitibi island arc. The mafic melt would have underplated the Archean lithosphere possibly near the contact between the Wawa-Abitibi terrane and the Quetico Basin. The mafic melts would then have evolved into granitic melts through fractionation, thus giving rise to the I-type granites. Small volumes of these melts were then emplaced within the Quetico Basin, possibly along weaknesses associated with lithosphere scale structures at the boundary between the Wawa-Abitibi terrane and Quetico Basin. The majority of the underplated melts may also have aided in the production of large S-type granites, which are typical of the Quetico Basin. These S-type melts formed from the melting of sedimentary rocks, and may have interacted with the I-type granites, producing variations in isotopic and geochemical signatures as seen within the S-type granites of this study. Although magnetite-bearing granitic intrusions within the Quetico Basin are not unique, the identification and classification of I-type granites has not been widely documented. The regional implication of I-type granites within the Quetico Basin is both profound and complex. It is suggested that other magnetite-bearing metaluminous granites within the Basin (e.g., the Vermillion complex, northern Minnesota) may have formed through a similar processes as the Dog Lake Granite Chain. References Kehlenbeck, M.M. 1977. The Bamum Lake pluton, Thunder Bay, Ontario; Canadian Journal of Earth Sciences, v.14, p.21572167. Ontario Geological Survey 1999. Single master gravity and aeromagnetic data for Ontario, GeoSoft® format; Ontario Geological Survey, Geophysical Data Set 1036. Ontario Geological Survey 2006. 1:250 000 Scale Bedrock Geology of Ontario; Ontario Geological Survey, Miscellaneous Release---Data 126-revised. Steinert, G. 1975. Structure and petrology of the Barnum Lake intrusive; unpublished BSc thesis, Lakehead University, Thunder Bay, Ontario, 67p. - 57 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Bedrock Geologic map of the Seine Bay/Bad Vermilion Lake intrusion, Mine Centre, Ontario LEE, Aubrey, UMD Department of Geological Sciences, Duluth, MN, ALBERS, Paul, Freeport-McMoRan Copper & Gold, Oro Valley, Tucson, Arizona, MILLER, Jim, UMD Department of Geological Sciences, Duluth, MN, SEVERSON, Mark, Natural Resources Research Institute, UMD, Duluth, MN, DEEN, Tobin, University of Minnesota Department of Earth Sciences, Minneapolis, MN The Seine Bay-Bad Vermilion Lake Intrusion (SB/BV) is a layered plutonic sequence of gabbroic rocks exposed along the shores of Seine Bay (Rainy Lake) and Bad Vermilion Lake in Northwest Ontario, Canada. It is approximately 14 km long and 1-3 km wide, stretching along the northern shores of Seine Bay in the southwest and Bad Vermilion Lake in the northeast with the town of Mine Centre marking its northeast tip. The SB/BV lithologically consists of a layered mafic intrusive package of anorthositic to gabbroic to pyroxenitic rocks with layers/lenses of semi-massive and massive oxides that have been significantly altered and possibly metamorphosed. The intrusion occurs between a hanging wall of felsic intrusive rocks at its northern contact and a footwall of mafic volcanic rocks at its southern contact. The nature of these contacts and the relative magmatic history of each unit are currently poorly known. Although gold has historically been the primary focus of mineral exploration in the Mine Centre area, deposits of iron, titanium, copper, nickel and zinc have been identified in recent years by various junior exploration companies (Hinz et al., 2010). Lode gold, magmatic Cu-Ni-PGE, magmatic Fe-Ti-V, and pyroclastichosted diamond deposits in the Mine Centre area are currently being evaluated for economic viability. Numax Resources, Inc. (Numax) holds 45 contiguous mineral claims totaling 5,188 ha in the Kenora Mining Division, covering the length of the SB/BV. While the Mine Centre property was somewhat inactive for decades, Numax’s 2004 field season saw a jump in activity including prospecting, diamond drilling, trenching, channel sampling,   Figure 1. Bedrock Geologic Map of the Seine Bay/Bad Vermilion Lake Intrusion, Numax Resources, Inc., Mine Centre, Ontario, Canada. Compiled and finalized by A. Lee, this map was generated with data collected by A. Lee during the summer of 2011 and by P. Albers and C. White during the summer of 2009. (Lee et al., 2011) - 58 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 field sampling, and two ground geophysical surveys (magnetic and electromagnetic). In 2009 Numax contracted Paul Albers and Chris White to conduct geologic bedrock mapping at 1:5000 scale and detailed trench mapping at 1:400 scale across the southwest portion of the property (Albers and White, 2010). In the summer of 2011, Aubrey Lee with assistance from Tobin Deen was recruited to map the northeast portion of Numax’s property at 1:5000 scale. A total of 83 hand-samples were collected during Lee’s mapping and have subsequently been geochemically assayed and cut into thin-sections. This field work and the sample suite constitute the basis for Aubrey Lee’s MS thesis at UMD. Recent geologic mapping of the SB/BV has illustrated that massive, semi-massive and disseminated oxide layers containing significant Fe-Ti mineralization exist within the intrusion (Lee et al., 2011). These layers are generally persistent along the strike of the intrusion, but may anastomose and bifurcate, as indicated by bedrock mapping, magnetic surveys, and drill core. The layers are typically hosted by pyroxenite and contain the oxides magnetite and ilmenite. Titaniferous magnetite may also be present. Oxide-rich layers range in width from a few centimeters to over 20 meters and number dozens. They dip sub-vertically and likely extend along with the host intrusion to a considerable depth. Magnetic surveys of the Mine Centre area reveal three distinct magnetic highs extending the length of the intrusion and merging in the northeast where the exposure of the intrusion narrows along Bad Vermilion Lake. It is also unknown how many massive Fe-Ti oxide-rich layers exist within the intrusion. Drill core from three holes drilled by Numax establish the lateral continuity of the layers along the strike of the intrusion. The principal objectives of this study are to characterize the lithologic, petrographic, and geochemical attributes of the SB/BV through analyses of hand samples, channel samples, and drill core from three widely spaced traverses across its stratigraphy. The research goals are three-fold: 1) to establish the lithologic and chemical stratigraphy of the intrusion so as to delineate the history of emplacement and crystallization within the magma chamber; 2) to fully characterize the mineralogic, textural, and chemical attributes of the Fe-Ti oxiderich layers with the goal of documenting their composition and understanding their origin; and 3) to assess the history of sulfide saturation in the intrusion so as to evaluate the potential for precious metal mineralization. More specifically, the research will address the following questions: 1) Recognizing that the formation is a gabbroic layered intrusion that has been significantly altered, deformed, and possibly metamorphosed, what are the original igneous rock types that comprise the intrusion?; 2) How many massive Fe-Ti oxide-rich layers exist within the intrusion, how do they compare in mineralogy, texture and chemistry, what is the nature of lateral and vertical variations of the layers, and which layers contain the highest, most homogeneous Fe, Ti, V, and P content?; 3) Was the cyclically layered gabbroic sequence formed in a closed system or was it affected by multiple magma pulses into the chamber, and what mechanism acted to concentrate multiple layers of massive oxides?; 4) What was the parental magma composition of the intrusion?; 5) What is the nature of the contact between the mafic intrusion and the felsic rocks at its top and the basaltic rocks at its base and what do these relationships reveal about the magmatic history of these rock units?; and 6) What is the potential for PGE reef-style mineralization? References Albers, P.B., and White, C.R. 2010. Report on the Geology and Mineral Potential of the Seine Bay/Bad Vermilion Lake Intrusion, Mine Centre Property, Mine Centre, Ontario. Unpublished report prepared for Numax Resources, Inc. Hinz, P., White, C.R., Albers, P.B., and Tortosa, D., 2010, Mineral Deposits of the Mine Centre – Rainy River Area, Ontario: Institute on Lake Superior Geology, 56th Annual Meeting, International Falls, MN, v. 56, part 2, p. 95-125. Lee, A., Albers, P., White, C., and Deen, T., 2011, Bedrock Geologic Map of the Seine Bay/Bad Vermilion Lake Intrusion, Numax Resources Inc., Mine Centre, Ontario, Canada. - 59 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Granitic saprolites in the Lake Superior region: K-metasomatism, intensity vs. magnitude of weathering, and estimates of atmospheric pCO2 MEDARIS, Gordon Jr, Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706, medaris@geology.wisc.edu, DRIESE, Steven, Department of Geology, Baylor University, Waco, TX 76798, Steven_Driese@baylor.edu, BOERBOOM, Terry, Minnesota Geological Survey, St. Paul, MN 55114, boerb001@umn.edu and JIRSA, Mark Minnesota Geological Survey, St. Paul, MN 55114, jirsa001@umn. edu. Paleosols provide insights into past climates, atmospheric compositions, and the role of terrestrial biomass in weathering processes. A geochemical comparison of nine granitic paleosols in the Lake Superior region (Table 1), ranging in age from 2690 to 100 Ma, has been undertaken to evaluate weathering conditions in the region over time. Table 1. Characteristics of selected granitic saprolites in the Lake Superior region Overlying Locality Age, Ma Formation Saganaga 2690 Ogishkemuncie Pronto 2450 Matinenda Ville Marie 2450 Lorrain McGrath ~2200 Denham Baraboo 1700 Baraboo Sibley 1500 Pass Lake Monticello 500 Mt. Simon EC MN 100 Cenomanian SW MN 100 Cenomanian Saprolite Saprolite max pCO2 paleo- 1 Protolith Depth, m PIA MW tonalite granite granite gneiss granite granite tonalite granite gneiss 70.5 97.7 99.6 93.0 96.5 90.3 94.9 97.9 98.8 <16 6.2 8.6 n.d. 4.3 2.3 2.1 >36 >22 <47.4 13.4 7.6 n.d. 7.9 5.6 2.2 >34.7 >33.8 2 3 K <2.2 2.0 1.1 n.d. 0.7 1.0 0.3 -2.3 -2.1 × PAL4 latitude <137 11 4.5 n.d. 6.4 12 2.5 6.8 6.8 25º N 40º N 40º N 10º N 50º N 25º N 10º S 40º N 40º N PIA, Plagioclase Index of Alteration: 100 × (Al2O3 – K2O)/(Al2O3 + CaO* + Na2O – K2O) Magnitude of Weathering: integrated loss of SiO2+MgO+CaO+Na2O+K2O (moles/cm2) 3 integrated change in K2O (moles/cm2); for Precambrian & Cambrian saprolites, relative to estimated weathered material (see text) 4 partial pressure of CO2 times pre-industrial level, calculated for 100,000 years of weathering n.d., not determined 1 2 In A-CN-K diagrams (Fig. 1), the compositions of Precambrian and Cambrian saprolites are displaced markedly from a “normal” weathering trend towards the K apex (except for the Barron saprolite), indicating a substantial degree of K-metasomatism. Such metasomatism precludes use of the Chemical Index of Alteration (CIA) as an indicator of the intensity of weathering. Instead, the Plagioclase Index of Alteration (PIA), which is a measure of plagioclase removal, may be used and yields values >90 for all but the Saganaga saprolite (Table 1). The % change of oxides was calculated for each saprolite relative to its protolith, taking Al2O3 as the immobile constituent. For metasomatised saprolites, K2O loss was taken to be 75% of Na2O loss, by analogy with the Cretaceous saprolites and modern day weathering profiles. The magnitude of weathering (MW) is a measure of mass removal and is defined as the loss of SiO2, MgO, CaO, Na2O, and K2O, integrated over the depth of weathering. Although most of the saprolites have comparably high intensities of weathering (PIA >90), the Cretaceous saprolites have appreciably larger magnitudes of weathering compared to the Precambrian and Cambrian saprolites, i.e. MW values of 22 to 36 moles/cm2 vs. 2 to 9 moles/cm2 (Table 1). The Saganaga - 60 - �Proceedings of the 58th ILSG Annual Meeting - Part 1   Figure 1. Compositions of protoliths and saprolites in terms of molar Al2O3, CaO*, Na2O, and K2O. saprolite is an exception to this pattern, most likely due to uncertainty in the depth of weathering. The mass of K2O added to the weathered Precambrian and Cambrian saprolites is 0.3 to 2.0 moles/cm2, an amount which is comparable to the mass of K2O removed by weathering from the Cretaceous saprolites, 2.1 to 2.3 moles/cm2 (Table 1). It is apparent that under high (= modern) atmospheric pO2, Fe is retained in most Cretaceous saprolites, whereas in Neoarchean and Proterozoic profiles Fe is generally lost, consistent with previous interpretations of lower pO2; the Cambrian Monticello saprolite shows a pattern for Fe loss with depth more like the Precambrian profiles. Following Sheldon’s method (2006, Precam. Res., v. 147, 148-155), pCO2 has been calculated for the Precambrian and Cambrian saprolites (Fig. 2, Table 1). Among the parameters involved, the MW and duration of weathering are most critical. The uncertainty in duration of weathering may preclude direct comparison of pCO2 among the individual saprolites, although the results in Figure 2 indicate a generally higher level of atmospheric pCO2 associated with the Precambrian and Cambrian saprolites compared to the pre-industrial level. Note that the depth of weathering and pCO2 estimate for the Saganaga saprolite are uncertain. Despite elevated concentrations of atmospheric CO2 and lowto mid-paleolatitudes, the Precambrian saprolites have significantly lower MW values than do the Cretaceous saprolites, perhaps reflecting enhanced weathering in Cretaceous time in response to organic compounds contributed by vascular land plants. References Boerboom 2011 ILSG Proc 57/2: 127-163; Driese & Medaris 2008 J Sed Res 78: 443-457; Driese et al. 2007 J Geol 115: 387-406; Driese et al 2011 Precam Res 189:1-17; Nedachi et al 2005 Chem Geol 214: 21-44; Rainbird et al 1990 J Geol 98: 801-822; Rogala et al 2007 Can J Earth Sci 44: 1131-1149; Setterholm et al 1989 Minn Geol Surv Inf Circ 99 pp - 61 -   Figure 2. pCO2 vs. duration of weathering �Proceedings of the 58th ILSG Annual Meeting - Part 1 Mafic intrusions along the southern boundary of the Superior Craton, SD: What is their potential for Ni-Cu-PGE mineralization? McCORMICK, Kelli A. and PATERSON, Colin J., Dept of Geology and Geological Engineering, South Dakota School of Mines and Technology, 501 E. St. Joseph St., Rapid City, SD 57701, USA A significant volume of mafic intrusive rocks is present along the southern margin of the Superior Craton, mainly within the Superior Boundary Zone, in southeastern South Dakota, northeastern Nebraska, and northwestern Iowa (McCormick, 2010). This region of the North America Craton experienced a number of orogenies in the Proterozoic, including the Trans-Hudson, Penokean, Yavapai, and Mazatzal. Most certainly at least three of these have significantly affected the southern margin of the Superior craton in this tri-state region. These intrusions of interest are typically 700 to 900 ft deep. Existing samples of these rocks are limited to the upper few feet of each body with the exception of a fairly well-studied 2890 Ma layered mafic-ultramafic intrusion in northwestern Iowa (Windom et al., 1993), a 1733 Ma metagabbro present just south of the Spirit Lake Tectonic Zone (Van Schmus et al., 2007) in Union County, South Dakota, and an undated mafic intrusion in Clay County, South Dakota near the town of Wakonda. The location of these mafic intrusions along a craton margin, the potentially elevated heat flow over an extended period of time due to the active orogenesis, and the subsequently strong possibility of a mantle contribution to these mafic bodies, suggests the potential for existence of an economic Ni-Cu-PGE deposit within the region. Our objective is to characterize and understand the regional tectonic and metallogenic framework of the southern margin of the Superior Craton in South Dakota and contiguous states in a number of integrated projects focusing on petrologic, geochemical, isotopic, and geochronological studies. We hope to initiate the research through the study of the two 1000 ft cores of the Wakonda intrusion drilled by WMC exploration in 2003/2004. References McCormick, K.A., 2010, Precambrian basement terrane of South Dakota: South Dakota Geological Survey Bulletin 41, 35 p. Van Schmus, W.R., Schneider, D.A., Holm, D.K., Dodson, S., and Nelson, B.K., 2007, New insights into the southern margin of the Archean-Proterozoic boundary in the north-central United States based on U-Pb, Sm-Nd, and Ar-Ar geochronology, in Holm, D.K., Schneider,D., and Chandler, V.W., eds., Proterozoic tectonic and crustal evolution of the Upper Great Lakes region, North America: Precambrian Research, 157, issues 1-4, 80-105. Windom, K.E., Van Schmus, W.R., Seifert, K.E., Wallin, E.T., and Anderson, R.R., 1993, Arch-ean and Proterozoic tectonomagmatic activity along the southern margin of the Superior Province in northwestern Iowa, United States: Canadian Jour. Earth Sci., 30, 1275-1285. - 62 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Predictive water level and preliminary hydrodynamic models of Caland and Hogarth pit lakes, Atikokan, Ontario. MIKKELSEN, Larissa1 and CONLY, Andrew, Department of Geology, Lakehead University, 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada. 1present address, True Grit Consulting Ltd. 1127 Barton Street Thunder Bay, Ontario P7B 5N3 Canada Recent limnology studies of the pit lakes on the former Steep Rock Iron Mines property (McNaughton, 2001; Gould, 2008) have shown that Caland and Hogarth pit lakes are meromictic and holomictic, respectively, and that, in general, measured concentrations of sulphate within the lakes are at or above Ontario Drinking Water Standards (ODWS) or Environmental Protection Agency (EPA) Secondary Maximum Contaminant Level (SMCL). Pit lake water quality can be influenced by hydrodynamics. For example, unexpected overturn of a meromictic pit lake can transport dissolved metal laden waters that are toxic to aquatic life to the surface, or where overturn in a holomictic pit lake transports dissolved oxygen down to submerged tailings producing acid mine waters where sulphide minerals exist. Changes in hydrodynamic behavior of Caland or Hogarth will influence whether the water released from the super pit lake will be of good quality or have elevated sulphate concentrations, potentially impacting downstream drinking water sources. Predictions of future water quality in the pit lakes are important to assess the type of rehabilitation efforts, if any, that may be required at the former Steep Rock Iron Mines property with respect to out flowing water quality (Conly et al., 2008; Godwin et al., 2010). This research develops a predictive analytical model of pit lake water level over time that is used to define water volume inputs for the hydrodynamic models. Preliminary hydrodynamic models of Caland and Hogarth pit lakes are designed to assess if the model can accurately simulate current conditions as well as future limnology, as flooding continues and Caland and Hogarth join forming a super pit lake before out flowing into the West Arm Reservoir. Future water quality is assessed by proxy using a linear relationship between water salinity and sulphate concentrations in the Steep Rock pit lakes and data deficiencies with respect to the models are discussed. The future water levels in the pit lakes are modeled using a void filling approach. The volume of Caland, Hogarth and South Roberts open pits and Fairweather Lake are calculated based on ultimate pit limits and contours based on 1986 aerial photography. The surface area of the pit lakes between the contours is calculated using linear interpolation at a 1 m interval. A water balance for each pit defines the volume of water entering the pits as the sum of precipitation landing on the lakes plus runoff and groundwater minus evaporation from the pit lake surfaces. The surface area of the pit lake and watershed area are variable depending on the accumulated volume in the pit lakes. The model predicts that water from Caland will flow into Hogarth in 2057 and the super pit lake will flow in the West Arm in 2077 or 45 years and 65 years from present day. Compared to the Regional Engineering model for the Steep Rock pit lakes (MNR, 1986), which has proven to be accurate, this study prediction is the same as the Regional Model except this study predicts it will take 8 years longer to completely fill. The methods used to construct the two different models are similar, but this study benefits from a longer data set for meteorological parameters and that the remaining volume capacity of Hogarth pit lake when the two lakes join is taken into account in subsequent water level predictions. This study was the first hydrodynamic modeling attempted for the Steep Rock pit lakes. The hydrodynamics of Caland and Hogarth are modeled using Dynamic Reservoir Simulation Model (DYRESM) from the Centre for Water Research at the University of Western Australia (www.cwr.uwa.edu.ca). DYRESM simulations of current conditions accurately portray the observed limnological characteristics of Caland and Hogarth pit lakes (McNaughton, 2001), including: i) that Caland is meromictic and has a lower salinity relative to Hogarth; and, ii) that Hogarth develops a temporary meromix. Simulations of the two pits joining indicate that Caland’s freshwater lens is maintained, but is thinner, and Hogarth pit lake develops a permanent meromix. Simulations of when the pit lakes outflow into the West Arm indicate that Caland maintains its upper freshwater lens and that a fresh water lens is seen in Hogarth is quickly lost when the pit lakes join. In most cases, variations in simulation parameters including additional inflows, alteration of the inflow salinities, and the use of a slower rebound rate to define the - 63 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 DYRESM water balance only produced minor changes in the simulation. Based on the linear trend between salinity and sulphate concentrations, the DYRESM salinity profiles suggest that: i) future outflow waters from Caland into Hogarth will have sulphate concentrations ranging from 0 mg/L to 100 mg/L; and, ii) waters that outflow from Hogarth will be toxic with sulphate concentrations ranging from 1700 mg/L to 1900 mg/L. In general, the sulphate concentrations in Caland are below maximum acceptable limit of the ODWS and EPA SMCL, while those in Hogarth exceeded these standards. Simulation results for when the pit lakes outflow suggest that the waters will be toxic. However, results may be influenced by the initial profile used to execute the model due to the short simulation period. Further work will address this issue by lengthening the simulation period and by using different initial profiles of the pit lake water column to assess their influence on the pit lake hydrodynamics. Other limitations to the model that should be addressed in future work include: the ill-defined inflow volumes and chemistry overtime which must be assumed for the length of the simulation period; the availability of information pertaining to site hydrogeology was limited; and, the light extinction coefficient should be measured because the presence of iron oxides influences light absorption in water. References Godwin, Amy Lynn, Lee, Peter Ferguson, Conly, Andrew George, Goold, Andrea R. (2010): Predicting toxicity of future combined pit lakes at the former Steeprock Iron Mine near Atikokan, Ontario. – In: Wolkersdorfer, Ch. & Freund, A.: Mine Water & Innovative Thinking, 343 – 347; Sydney, Nova Scotia (CBU Press), 343 – 347. Conly, A. G., Lee, P. F., Godwin, A., Cockerton, S. 2008. Experimental Constraints for the Source of Sulfate Toxicity and Predictive Water Quality for the Hogarth and Caland Pit Lakes, Steep Rock Iron Mine, Northwestern Ontario, Canada. – In: Rapantova, N. & Hrkal, Z.: Mine Water and the Environment. – Paper #139; Ostrava (VSB – Technical University of Ostrava). (http://www.imwa.info/docs/imwa_2008/IMWA2008 _139_ Conly.pdf Accessed December 8, 2011) Gould, A.R. 2008. Water quality and toxicity investigations of two pit lakes at the former Steep Rock iron mines, near Atikokan, Ontario. Unpublished MSc thesis, Lakehead University, Thunder Bay, ON, Canada, 122 p. McNaughton, K.A. 2001. The limnology of two proximate pit lakes after twenty years of flooding. Unpublished MSc. thesis, Lakehead University, Thunder Bay, ON, Canada, p. 85. MNR (Ministry of Natural Resources). 1986. Report on the Surrender of Mining Claims By Steep Rock Inc., Steep Rock Lake Atikokan District., Ontario. 146. - 64 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 2011 Precambrian Field Camp Mapping in the Sawbill Lake Area, Cook County, Northeastern Minnesota MILLER, Jim, BROOKER, Ben, ASP, Kris, LEU, Adam, PARISI, Andrew, and SLETTEN, Dan, Precambrian Research Center, University of Minnesota Duluth, Duluth, MN 55812 As part of the 2011 Precambrian field camp, a crew of four students, a teaching assistant (Brooker) and an instructor (Miller) conducted five days of field mapping bedrock geology in the Sawbill Lake area of northeast Minnesota. This area is composed of intrusive and volcanic rocks formed during the 1.1 Ga Midcontinent Rift (MCR) in northeastern Minnesota. This year’s mapping project expanded on three previous capstone mapping projects conducted to the east (Frost et al., 2007; Blakely et al., 2009; Brooker et al., 2010). These mapping projects have focused on a previously unnamed, well differentiated layered mafic intrusion that forms the western part of the Brule Lake –Hovland gabbro of Miller et al. (2001). This intrusion is now called the Sawbill Lake intrusion (SbLI) since the intrusion is centered and well exposed in and around the large lake. A 1:12,000-scale bedrock geologic map (Asp et al., 2011) generated of a 3 mile by 3 mile area profiles the igneous stratigraphy of the lower two-thirds of the intrusion and the footwall rocks. A pdf version of this and other Precambrian field camp capstone geologic maps can be downloaded from the PRC website: www.d.umn.edu/prc/fieldcamp/ capstone. Prior to the detailed mapping conducted by the PRC field camp students, the general geology of this intrusion was poorly known. Township-scale outcrop mapping by Grout et al. (1959) showed the area to be dominantly composed of gabbroic to felsic intrusive rocks, mafic volcanic rocks, and minor volcanogenic sedimentary rocks. They noted that the gabbroic rocks are well layered and portions are particularly rich in Fe-Ti oxide. Reconnaissance mapping by Davidson (1972) designated the mafic intrusion as an olivine gabbro unit that he interpreted as an early intrusion relative to granophyric and anorthositic rocks in the area (Davidson, 1977; Davidson and Burnell, 1977). The regional geologic map of northeastern Minnesota, which focused on the geology of the Duluth Complex (Miller et al., 2001), incorporated Davidson’s (1972) mapping of the area, but his olivine gabbroic unit is now interpreted to be a relatively late intrusion that is roughly equivalent to the Beaver Bay Complex to the south. 2011 capstone mapping was conducted in the northern half of Sawbill Lake, and reached to the west into Handle and Java lakes and to the north into Ada Lake. Exposures along the shorelines of these waterways provided a nearly complete profile of the lower two thirds of the SbLI as well as a footwall composed of ferrodiorite of unknown affiliation. The internal structure of the SbLI defines a southeast plunging synform with foliation and intermittent layering dipping moderately to the south at the east end and moderately east at the south end. The igneous stratigraphy of the approximately 1.5 to 3 km-thick intrusion generally defines a progressively differentiated sequence. Unlike the complex mixture of medium-grained, augite troctolite to olivine gabbro and fine-grained, intergranular gabbroic rocks observed at the basal contact during last summer’s mapping two miles to the east (Brooker et al., 2010), the basal contact exposed in the Ada Lake area shows well foliated and intermittently layered troctolite and melatroctolite in sharp contact with a well foliated ferrodiorite. This contact is also exposed along the western shoreline of Sawbill Lake, but here melatroctolite is rare. The troctolite grade upward into an medium-grained, moderately foliated augite troctolite (Pl+Ol cumulate) unit which contains 2-3 oikocrysts of ophitic to subophitic augite. Exposed in Ada Creek and through the north central part of Sawbull Lake is the gabbroic unit rich in hornfels inclusions of basalt and volcanogenic sandstone that has been traced over six miles between Sawbill and Homer lakes. This inclusion-rich unit, which is exposed at the north end of Weird Lake, is overlain by a very thick sequence of foliated, intergranular oxide gabbro to olivine oxide gabbro (Pl+Cpx+Ox±Ol cumulate) exposed along the shorelines of northeastern Sawbill Lake. The cm- to m-thick layers in Fe-Ti oxides found in this unit to east are not very abundant in the Sawbill Lake area. Some occurrences of intermingled intrusions of diabase, ferromonzondiorite, and quartz monzonite found to the east were observed in the Sawbill Lake however. that mostly appear to be sill-like. The cumulus arrival of apatite which characterizes - 65 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 the upper part of the SbLI was not observed in this map area, but was found to the south in the Burnt and Smoke Lake area (see Brooker and Miller, this volume). Following the 2010 capstone project, Ben Brooker chose to develop an MS thesis project at UMD that would integrated the previous mapping by Grout, Davidson, and the PRC field camp projects and conduct additional mapping to create a complete geological picture of the Sawbill Lake Intrusion. He has conducted petrographic studies of over 200 thin sections to characterize the mineralogic and textural attributes of the main rock types. To document the cryptic variation throught the SbLI, he has conducted mineral chemical analyses of olivine and pyroxene from a suite of samples collected along several profiles across the intrusion. As reported elsewhere in this volume (Brooker and Miller, 2012), the main topical objective of this study is to evaluate the genetic relationship between the lower troctolitic cumulates and the upper oxide gabbroic cumulates. Although these rock types could represent the progressive differentiation of a mafic magma, their separation by a screen of volcanic and interflow sedimentary rocks suggests that they may represent two discreet intrusions formed by different parental magmas. References Asp, K., Leu, A., Parisi, A., Sletten, D., Brooker, B., and Miller, J., 2011, Bedrock Geology of the Sawbill Lake area. Precambrian Research Center, PRC/MAP-2011-04, 1: 12,000. Blakely, S., Brown, A., Foley, D., Rowland, A., Stifter, E., and Miller, J., 2009, Bedrock Geology Map of Homer Lake and Adjacent Areas; Cook County, Northeastern Minnesota: University of Minnesota Duluth, Precambrian Research Center, PRC/MAP-2009-01, 1: 12,000. Brooker, B., Hadley, M., Markwood, L., Olson, J., Tomlinson, A., and Miller, J., 2010, Bedrock geology map of the Jack Lake and Weird Lake areas, Cook County, northeastern Minnesota. Precambrian Research Center, PRC/MAP-2010-05, 1: 12,000. Davidson, D.M., Jr., 1972. Eastern part of Duluth Complex. In: Sims, P.K. & Morey, G.B. (eds.) Geology of Minnesota - A Centennial Volume. Minnesota Geological Survey, p. 354-360 Davidson, D.M., Jr., 1977, Cherokee Lake Quadrangle, Cook County, Minnesota: Minnesota Geological Survey, Miscellaneous Map Series, M-30, 1:24,000. Davidson, D.M., Jr., and Burnell, J.R., 1977, Brule Lake Quadrangle, Cook County, Minnesota: Minnesota Geological Survey, Miscellaneous Map Series, M-29, 1:24,000. Frost, S.J., Juda, N.A., and Miller, J., 2007, Bedrock Geology Map of Homer Lake and Adjacent Areas; Cook County, Northeastern Minnesota: University of Minnesota Duluth, Precambrian Research Center, PRC/MAP-2007-02, 1: 12,000. Grout, F.F., Sharp, R.P., and Schwartz, G.M., 1959, The Geology of Cook County Minnesota: Minnesota Geological Survey Bulletin 39, 163p. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.E., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota. Miscellaneous Map Series, M-119, scale 1:200,000 - 66 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Supercontinents during the Proterozoic – A paleomagnetic view with Keweenawan data (Lake Superior) as an example. PESONEN, Lauri J. and VEIKKOLAINEN, Toni, Division of Geophysics & Astronomy, University of Helsinki, FI-00014 Helsinki, Finland The importance of supercontinents in understanding the geological evolution of the Earth has been recently discussed in several articles (e.g., Pesonen et al., 2012, and references therein). Geological processes linked to the existence of supercontinents include concepts such as large igneous provinces (LIPs), mantle superplume events, low latitude glaciations (“Snowball Earth”) and the high obliquity-theory of the rotation axis, carbon isotope excursions, fragmentation of continental dyke swarms, truncations of tectonic belts and major rifts, matching of conjugate orogenic belts, episodic nature of distributions of magmatic activities, discoveries of Precambrian ophiolites and the concept of true polar wander. Three supercontinent assemblies (Pangaea 350-165 Ma, Gondwanaland 550-400 Ma, Rodinia ~ 1050-750 Ma) have existed during or since the Neoproterozoic. The two younger assemblies have been constructed by sea floor magnetic data (Pangaea), and by palaeomagnetic and biostratigraphic results supported by geology (Gondwanaland). A current debate concerns the relative positions of the continents in supercontinent Rodinia, and the timing of its assembly and breakup. The consequences of supercontinents on geological evolution of the Earth have led some people to look also for pre-Rodinian supercontinents. In this paper, we use the updated palaeomagnetic database combined with new geological information, to define the positions of the continents during Paleoproterozoic (2.5 - 1.5 Ga) and Mesoproterozoic eras (1.5 - 1.04 Ga). We do the reconstructions relying mainly on isotopically (U-Pb) dated high quality palaeopoles since this method provides strict information of ancient latitudes of the cratons throughout the Proterozoic time. The new database is both geographically and temporally more abundant than any previous database and contains 2813 observations. Before putting the continents to a supercontinent assembly, we have tested the validity of the geocentric axial dipole model (GAD) of the Paleo-Mesoproterozoic geomagnetic field using four methods. The tests yield support to the GAD-model, but do not rule out a ca. 5% non-dipole (octupole) field. During the Proterozoic, the continents lie predominantly in low to intermediate latitudes. The sedimentological indicators of palaeoclimate are generally consistent with the palaeomagnetic latitudes, with the exception of the Early Proterozoic, when low latitude glaciations took place on several continents. The Proterozoic continental configurations are also in agreement with current geological models of the evolution of the continents. Three large continental landmasses existed during the Proterozoic. The oldest one is the Neoarchaean Kenorland. The protracted break up of Kenorland during the 2.45-2.10 Ga interval is manifested by mafic dykes and sedimentary rift-basins on many continents. The two Meso- to Neoproterozoic supercontinents are Columbia (known also as Nuna or Hudsonland) and Rodinia, respectively, which were predominantly in moderate to low paleolatitudes during their life span, although during the Paleoproterozoic some parts of Columbia, notably India (Dharwar craton) and Australia (Yilgarn craton), occupied polar latitudes. The pre-Columbia orogenies were due to a complex set of collisions, rotations and transform or strike slip faultings that caused the orogenic belts to appear obliquely. The final amalgamation of Columbia did not happen until ca. 1.53 Ga. Columbia broke up at ca. 1.18 Ga during several rifting episodes, followed by a short period of independent drift of most continents. The youngest assembly is the Meso-Neoproterozoic supercontinent Rodinia, which was formed by continentcontinent collisions during ~ 1.10-1.00 Ga and which involved most of the continents. A new model for its assembly and configuration is presented, which suggest that multiple Grenvillian age collisions took place during 1.10-1.00 Ga (Fig. 1). In building up the Rodinia model, the 1.2-1.0 Ga apparent polar wander paths from Laurentia (Keweenawan Lake Superior data), Kaapvaal (Umkondo Igneos Province), Yilgarn (intrusive), dyke data from Baltica and North China, as well data from Siberian sediments are strikingly similar containg a large - 67 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Figure 1. (a) Reconstruction of continents at 1.04 Ga showing the Rodinia configuration. Data available from Laurentia (L), Baltica (B), Amazonia (Am), Australia (A), Congo/SãoFrancisco (C-Sf), Siberia (S), India (I), North China (NC) and Kalahari (K). The Grenvillian age orogenic belts are shown in red. (b) Geologically made reconstruction of LaurentiaBaltica-Amazonia (= the 0.9 Ga SAMBA-assembly by Johansson, 2009). loop (“the Logan Loop”). The similarity suggests that the various cratons experienced very rapid drift during 1.15-1.10 Ga just before docking to Rodinia, or the whole Earth suffered a true polar wander, or “non-dipole” episode during that time. References Pesonen, L.J., Mertanen, S., Veikkolainen, T., 2012. Paleo-Mesoproterozoic Supercontinents - A Paleomagnetic View. GEOPHYSICA (in press). Johansson, Å., 2009. Baltica, Amazonia and the SAMBA connection-1000 million years of neighbourhood during the Proterozoic? Prec. Res., 175, 221-234. - 68 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 A catastrophic breach and inter-lake flow through the Marks Moraine via Brule Creek and Cedar Creek, Thunder Bay Region, Ontario: A small but significant detail of the eastern outflow history of Lake Agassiz. PHILLIPS, Brian, Department of Geography, Lakehead University, 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada, DEAN, Frederick, Research Technician, 3482 Rosslyn Road, Rosslyn, Ontario P7K 0P8, Canada, and ZANIEWSKI, Kamil, Department of Geography, Lakehead University, 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada The search for the eastern outlets of glacial Lake Agassiz began with John Elson’s pioneering aerial reconnaissance in Northwestern Ontario (Elson, 1957). His air photo interpretation revealed six possible outlets: Brule Creek, Dog River, Kaiashk River, Kaministiquia River, Pitikigushi River and Pillar Lake. Field studies followed in the summer of 1961 (Zoltai, 1963). Stephen Zoltai was tasked with ground truthing Elson’s work, a project that would lead to the publishing of “The Surficial Geology of Thunder Bay”, Map S265 (1965a, Zoltai, 1965b). Zoltai concluded that two outlets, the Brule Creek spillway and the Kaministiquia River spillway (and the associated Dog River tributary) were not related to glacial Lake Agassiz. He concluded that “the Kaministiquia –Dog River spillway was functioning to carry away meltwater from the area of the Kaiashk Moraine, while the Brule spillway follows closely the outer margin of the Marks Moraine” (Zoltai, 1963 p.110). The remaining spillways would become the well known “Nipigon outlets” (Zoltai, 1965a,b, 1967; Elson, 1967; Teller and Thorleifson, 1983). Interest in the two all-but-forgotten spillways waned until Thorleifson (1996) brought about their revival in his review of Lake Agassiz history. Work by Minning at al (1994) began to reveal that Lake Agassiz levels were still high south and west of Lake Nipigon and the eastern “Nipigon outlets” could not have been functioning to draw down early Lake Agassiz levels. This lead Thorleifson (1996) to propose that the Matawin-Shebandowan outlet (into the Kaministiquia River spillway) and the Savanne River outlet (into the Dog River spillway) were active. Research in the Fort Frances area by Bajc et al (2000) further supported high Lake Agassiz levels along the Ontario/Minnesota border, prompting Teller et al to publish “Alternative Routing of Lake Agassiz Overflow During the Younger Dryas: New Dates, Paleotopography and a Re-evaluation” (2005). This lead to renewed interest in Glacial Lake Kaministiquia, the Kaministiquia spillway and the chronology of glacial and deglacial events in the Thunder Bay area (Loope, 2006; Loope et al, 2006). In 2009, Lowell et al published “Radiocarbon Deglaciation Chronology of the Thunder Bay, Ontario area and Implications for Ice Retreat Patterns”, in which a key point made was that ice at the Brule Moraine blocked the Matawin-Shebandowan outlet. The low point between the Arctic and Great Lakes watersheds was determined as between Squeers and Watershed lakes, and since the Brule Moraine was not breached here, they concluded that no overflow had taken place (Lowell et al 2009). However, the conclusion was based on digital elevation maps, and is considered in error by the present authors. Squeers and Watershed lakes lie at the top of Squeers Creek at 484m (1588’) ASL, while the Squeers Creek spillway, mapped by Mollard and Mollard (1980), runs at the bottom of the drainage line from 472m to 453m (1550’ to 1486’) ASL. Bajc (2000) established the level of Glacial Lake Agassiz in this very area, based on the South Burchell Lake delta grading to 472m (1550’) ASL. At 472m (1550’) there is a 2km (6562’) breach in the Brule Moraine, leading to Sawmill Bay on Lake Shebandowan. Mollard and Mollard (1980) mapped the deepest incision of the Squeers Creek spillway to be over 500m (1640’) wide. This breach area was used before and after the Steep Rock readvance (Dean and Phillips, 2011) and flow was through Lake Shebandowan, down the Shebandowan River and into Glacial Lake Kaministiquia, also a part of the Dog-Kaministiquia spillway. Burwasser (1981) determined three distinct levels of Glacial Lake Kaministiquia: 1450’ (442m), 1500’ (457m) and 1550’ (472m) ASL. Earlier, Zoltai (1963) had noted the highest level as 1540’ (469m) ASL. With the sill between Kashabowie and Lac de Milles Lac lakes close to this high level, it must be concluded that the 1540-1550’ (468-472m) level was during the peak of the Marquette readvance - 69 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 when ice was holding up Lake Kaministiquia and before breaching the Marks Moraine and ultimately flowing south into a pro-glacial lake. Phillips and Fralick (1994), determined from a perched delta north of Thunder Bay, that a post Marquette level of Glacial Lake Kaministiquia was 1500’ (457m) ASL, and Bajc (2000) noted 1510’ (460m) ASL as a Marquette ice held level of Glacial Lake Kaministiquia. This would have been when red clays stopped flowing west out of Glacial Lake Kaministiquia through Lac de Milles Lac to be deposited in front of the Hartmann Moraine (Zoltai, 1965b; Teller and Thorleifson, 1983). Loope (2006) suggested that Glacial Lake Kaministiquia could have drained catastrophically following retreat of Marquette ice. This could have been when the level first dropped to 1500’ (457m) ASL (Phillips and Fralick, 1994). This level would have been the start of the Brule Creek spillway (Elson, 1957), running out of the Brule Creek valley, and along the Marks Moraine and the valley of Cedar Creek. There, based on the characteristics of a feature named in this paper as the “Cedar Creek Delta”, the authors present evidence of a catastrophic breaching of the Marks Moraine, waters of Glacial Lake Kaministiquia flowing south into “Glacial Lake Cedar Creek”, initially at the 442m (1450’) level. Subsequently, Glacial Lake Kaministiquia lowered to the 442m (1450’) level (Burwasser, 1981), before finally breaching the Marks Moraine along the Kaministiquia River valley at Kakabeka (Phillips and Fralick, 1994; Loope, 2006) and draining away into the Upper and Lower Beaver Bay levels of Lake Superior (Phillips and Fralick, 1994). References Bacj, A.F., 2000. Glacial History and Regional Till Sampling in the Archean Greenstone Belt, Institute on Great Lakes Geology, Proceedings of the 46th Annual Meeting, Thunder Bay, Ontario, 46, 32pp. Bacj, A.F., Schwert, D.P., Warner, B.G. and Williams, N.E., 2000. Canadian Journal of Earth Sciences, 37: 1335-1353. Burwasser, G.J., 1981. Ontario Geological Survey, Miscellaneous Paper 94, 10pp. With Map P2203, Scale 1:50,000. Dean, J.F. and Phillips, B.A.M., 2011. Pre and Early Agassiz Outlets to the Western Superior Basin, in Miller, J.D., et al., Eds. Archean to Anthropocene: Field Guides to the Mid-Continent of North America, Geological Society of America Field Guide 24, p.317-349, doi:10.1130/2011.0024(15). Elson, J.A., 1957. Lake Agassiz and the Mankato-Valders Problem, Science, 126, 3281, 999-1002. Elson, J.A., 1967. Geology of Glacial Lake Agassiz, in Mayer-Oakes, W.J. Ed. Life, Land and Water, Winnipeg, University of Manitoba Press, 37-95. Loope, H.M., 2006. Deglacial Chronology and Glacial Stratigraphy of the Western Thunder Bay Lowland, Northwestern Ontario, Canada, MSc. Thesis, University of Toledo, Ohio. Loope, H.M., Fisher, T.G., Lowell, T.V. and Hajdas, I., 2006. Geological Society of America Annual Meetings, Philadephia, Proceedings and Program, 38 (7) 72. Lowell, T.V., Fisher, T.G., Hajdas, I., Glover, K., Loope, H., and Henry, T. 2009. Quat. Science Reviews 28, 1597–1607. Minning, W.G, Cowan, W.R., Sharpe, D.R. and Warman, T.A., 1994, Geological Survey of Canada Memoir 436, 74p. Mollard, D.G. and Mollard, J.D., 1980. Lac de Mille Lacs Area (NTS 52B NE) Northern Ontario Engineering Geology Terrain Study, 56, 28p. with Database Map, Lac de Mille Lacs Area, Districts of Rainy River and Thunder Bay, Ontario Geological Survey, Map 5074, Scale 1:100000. Phillips, B.A.M. and Fralick, P.W., 1994. Journal of Great Lakes Research 20 (2), 390-406. Teller, J.T. and Thorleifson, L.H., 1983. The Lake Agassiz-Lake Superior Connection in Teller, J.T. and Clayton, C., Eds., Glacial Lake Agassiz, Geological Association of Canada Special Paper 26, 261-290. Teller, J.T., Boyd, M., Yang, Z., Kor, P.S.G. and Faro, A.M., 2005. Alternative Routing of Lake Agassiz during the Younger Dryas; New Dates, Paleotopography and Re-evaluation, Quaternary Science Reviews, 24, 1890-1905. Thorleifson, L.H., 1996. Review of Lake Agassiz History, in Teller, J.T., et al.,Eds., GAC Field Trip Guide Book for GAC/ MAC of Canada Joint Annual Meeting, Winnipeg, 55-84. Zoltai, S.C., 1963. Glacial Features of the Canadian Lakehead Area, The Canadian Geographer, 7, 3, 101-115. Zoltai, S.C., 1965a. Glacial Features of the Quetico-Nipigon Area, Canadian Journal of Earth Sciences, 2, 247-269. Zoltai, S.C. 1965b. Thunder Bay - Surficial Geology, 1:506,880, Map S265, Ontario Department of Lands and Forests. Zoltai, S.C., 1967. Eastern Outlets of Lake Agassiz, in Mayer-Oakes, W.J., Ed., Life, Land and Water, Winnipeg, University of Manitoba Press, 107-120. - 70 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 A metallogenic model and geometallurgical treatment of the Basal Iron Ores within the Negaunee Iron Formation, Tilden Mine, Michigan PIETRZAK-RENAUD, Natalie, Earth Sciences Department, Western University, 1151 Richmond Street, London, Ontario, N6A 3K7, Canada The complex paragenesis of the Tilden ores can be tied to the regional tectonic framework. Variable mineral chemistry and textures of basal Negaunee iron ores mined in the Main Tilden Pit have led to metallurgical difficulties. Core-logging and detailed petrography supported by microprobe investigations, identify three upward fining lithofacies within the Main Pit Carbonate and overlying Martite ore domains: 1) Basal Clastics; 2) Medial BIF; and 3) Granular Iron Formation. Growth fault related subsidence controlled deposition of the Basal Clastics, comprised of detrital quartz dispersed in a matrix of chlorite, cemented by initial ferri-hydrite, chert and Mg-siderite. Subsequent starvation of any clastic input led to cyclic iron-silica precipitation throughout the deposition of Medial BIF. Increasing wave action accompanied marine transgression caused deposition of granular rip-up clasts to form the Upper Granular Iron Formation. These domains are cross-cut by numerous chloritized feeder dykes and are capped by a greenstone Pillar, indicating overlapping mafic magmatism. Low grade regional metamorphism attending the 1850 Ma Penokean arc-continent collision led to magnetite growth, carbonate grain coarsening and chlorite crystallization. Metamorphic fluids facilitated martite replacement of magnetite and deformation led to developing local platy specularite schists. A late retrograde hydrothermal overprint post-dates peak thermal conditions that accompanied the development of the 1750 Ma Republic Metamorphic Node. This is expressed by high-Fe chlorite, Fe-dolomite/ankerite, zoned Mn-rich siderite with associated trace Cu-Fe sulphide and REE bearing fluro-apatite and monazite, diagnostic of an “IOCG” type signature. The development of a metallogenic model accounting for the multistage paragenetic sequence has shown that textures and mineral chemistries reflect a complex evolution. Bulk chemical analyses do not always reliably predict concentrate chemistries. Textural variations and mineral speciation of the different lithofacies can affect grinding and liberation and must be taken into account. Therefore, ore treatment processes are more effective when the genesis of the ore deposit is fully understood. This complex paragenetic history accounts for the unpredictable geometallurgical response of the basal Negaunee iron ores. Treatment difficulties relate to: 1) variable silica-iron separation that occurs due to liberation from detrital quartz versus massive mosaic and granular textured chert; 2) the bulk iron content is expressed not only in iron oxide but in iron carbonate and iron chlorite; and 3) the intense late IOCG overprint proximal to the Southern Shear Zone. - 71 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Paleomagnetic, rock magnetic and geochemical data from the Keweenawan dykes and baked rocks from the Little Mountain and Sugar Loaf areas, Michigan, USA PIISPA, Elisa, SMIRNOV, Aleksey, Department of Geological and Mining Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive, 630 DOW ESE Building, Houghton, MI 499311295, USA, PESONEN, Lauri, Department of Physics, Division of Geophysics and Astronomy, University of Helsinki, PO Box 64, FIN-00014 Helsinki, Finland, and DIEHL, Jimmy, Department of Geological and Mining Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive, 630 DOW ESE Building, Houghton, MI 49931-1295, USA Rock magnetic, paleomagnetic and geochemical data from the Keweenawan diabase dykes from the Little Mountain (Baraga County) and Sugar Loaf (Marquette County) areas are presented. The study areas are located in the Upper Peninsula of Michigan where the dykes intrude metasedimentary and metavolcanic rocks of Animikean age, forming part of the Marquette Range Supergroup. This supergroup unconformably overlies a dominantly granitic basement of Archean age. At least three Keweenawan dyke swarms crop out in the area. Most of the Middle Keweenawan units have dual polarities and there is a tendency of the R polarity units to be slightly older according to radioisotope dating, paleomagnetic studies, and stratigraphic evidence. Previous paleomagnetic studies have shown that the E-W trending Baraga and Marquette dykes are of reversed polarity, with minor but significant differences in their paleomagnetic and physical properties. Our study reveals the presence of normal polarity dykes in the study area. Based on cross-cutting relationships, they are younger than the R-polarity dykes. The top-hill exposure of the Little Mountain, where the cross-cutting of the dykes takes place, is complex: the rocks have been hit by lightning and the dykes and their baked contact rocks are of mixed polarity. In contrast, the down-hill sites, where the N and R dykes are more distant from each other, they reveal either N- or R-polarities (the NE-SW and the E-W trending dyke respectively). The R-polarity and N-polarity dykes have slightly different petrophysical, rock magnetic and paleomagnetic properties. Geochemical signatures of the two dykes are also different: typically the R-polarity dykes have higher Fe- and lower Al- and Mg-values than their N-polarity counterparts. In addition, the R-polarity dykes of the Baraga County are paleomagnetically, petrophysically and geochemically slightly different than the R-polarity Marquette dykes. The same is not true for N polarity dykes from the two areas. The differences between the R polarity dykes may reflect a small difference in age (Baraga dykes being older). The new paleomagnetic and geochemical data together with cross-cutting relationships also hint that an even older N-polarity Keweenawan dyke event is present in the Sugar Loaf area. These older dykes may represent the earliest stage of the intrusive activity associated with the Mid-Continental Rift. - 72 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 The ca. 1650 Ma Freedom Formation, Baraboo Range, Wisconsin: A late iron formation in the Lake Superior Region ROE, Carly and BJØRNERUD, Marcia, Geology Department, Lawrence University 711 E Boldt Way, Appleton WI 54911, USA, bjornerm@lawrence.edu Iron formation deposition in the Lake Superior region is typically considered to have ceased by about 1850 Ma, with the end of deposition of Animikie Group iron formations including the Gunflint, Biwabik, and Ironwood Formations in western Ontario, northern Minnesota, Wisconsin, and the Upper Peninsula of Michigan. However, a little-known iron formation occurs within the latest Paleoproterozoic (ca. 1650 Ma) Freedom Formation, part of the Baraboo sequence of southern Wisconsin. The Freedom Formation does not occur in outcrop, but was the target of iron mining in the early 1900s, and drill cores taken more than a century ago were recently acquired by the core repository of the Wisconsin Geological and Natural History Survey (WGNHS). The Freedom Formation, which is at least 300 m thick, lies conformably above the Seeley Slate, which in turn overlies the well-known Baraboo Quartzite (Dalziel and Dott, 1970). According to a 1904 WGNHS report by Samuel Weidman, the Freedom Formation includes interbedded ferruginous slates, ferruginous cherts, and iron-rich dolostones, and is made up almost entirely of quartz, dolomite, hematite, magnetite, kaolinite and chlorite. The lower half of the formation consists mainly of thinly bedded slates and cherts and lesser dolomite, as well as localized stratiform bodies of iron ore (mainly hematite) with up to 65% Fe. The upper half of the formation is predominantly dolomitic. We studied core samples originally drilled for the Cahoon Iron Mine (43.4470 N, 89.7565 W), on the south limb of the Baraboo syncline about 3 km northwest of Devils Lake. The cores had been stored for at least 80 years in an abandoned mine building before they were obtained by the WGNHS, and no related documents other than depth tags were preserved. As a result, we are not sure of the precise stratigraphic position of the cores we examined, but the lithologies present – primarily ferruginous slate and chert – are consistent with Weidman’s (1904) descriptions of the lower half of the Freedom Formation. Most of the rock in the cores shows clastic textures and mm-cm scale layers of quartz, magnetite, and lesser carbonate. In places, the magnetite is altered to hematite. The layers are somewhat irregular or lensoid, commonly varying in thickness even across the width of the 1.5 cm cores. In thin section, the magnetite is seen to occur in mm-scale clumps or granules that are subspherical but lack any obvious concentric banding that would indicate they formed as ooids. These textures are similar to those observed in classic Lake Superior granular iron formations (GIFs). Unlike banded iron formations (BIFs) sensu stricto, which represent laminated chemical muds, GIFs originated largely as well-sorted sands, most likely as a result of intrabasinal erosion and redeposition of pre-existing iron formation, and are younger than most BIFs (Simonson, 2003; Bekker et al., 2010). XRF analysis of trace metals in the Freedom Formation show Fe-normalized concentrations comparable to those in the Ironwood Iron Formation, a GIF-type deposit. In contrast, the chemistry, mineralogy and textures of the Freedom Formation do not resemble Phanerozoic ironstones, which tend to be highly oolitic, with chamosite, glauconite and/or pyrite as major mineral constituents (van Houten and Bhattacharyya, 1982). The geochemical factors leading to deposition of the Freedom Formation are not clear. The Baraboo Quartzite and Seeley Slate are both supermature sedimentary units deposited in a shallow marine environment adjacent to a rhyolitic/granitic landmass that had undergone intense and sustained tropical weathering (Medaris et al., 2003). The contact between the Seeley Slate and Freedom Formation seems to be conformable and gradational, suggesting that the Freedom Formation was deposited under similar conditions. While Phanerozoic ironstones are also thought to have accumulated in tropical settings with intense chemical weathering, they typically occur at the base of transgressive sequences and represent protracted periods of sea level lowstand (van - 73 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Houten and Bhattacharyya, 1982). This does not appear to have been the case for the Freedom Formation, which is underlain by more than 1000 m of uninterrupted marine sediments. The Freedom Formation therefore seems to be more closely related in genesis to other (earlier) Paleoproterozic GIFs than to Phanerozoic ironstones. If this is the case, its existence challenges models that point to an end for iron formation deposition at ca. 1850 Ma as a result of deep ocean euxinia (Canfield, 1998), the Sudbury impact (Slack and Cannon, 2009) or cessation of komatiitic volcanism (Wang et al., 2009). Instead, we suggest with Planavsky et al. (2011) that ferruginous ocean conditions and GIF deposition may have persisted well after early Paleoproterozoic time. Remaining questions about age of the Baraboo Sequence and the timing of its deformation are relevant to understanding the significance of the Freedom Formation. The youngest detrital zircons in the Baraboo Quartzite indicate that it can be no older than 1710 Ma (Medaris et al., 2003). There are, however, no direct constraints on the ages of the overlying units nor on the timing of the Baraboo folding event, although indirect arguments suggest that it occurred at ca. 1630 Ma (Holm et al., 1998). 40Ar/39Ar analyses of muscovite in breccias within the Baraboo quartzite consistently yield ages in the range of 1470-1450 Ma (Medaris et al., 2003), and it is possible that the deformational event could have occurred as late as this time (van Lankvelt and Bjørnerud, 2010). If so, the Freedom Formation could be as young as Mesoproterozoic and may indicate that Paleoproterozoic-type ocean chemistry prevailed much longer than previously thought. References Bekker, A., et al., 2010. Economic Geology, v. 105, p. 467-508. Canfield, D., 1998. Nature, v. 396, p. 450-453. Dalziel, I., and Dott, R. H. Jr., 1970, WGNHS Information Circular no. 14. Holm, D., Schneider, D., and Coath, C., 1998, Geology, v. 26, p. 907-910. Medaris, L. G. Jr. et al., 2003, Journal of Geology, v. 111, p. 243-257. Planavsky, N., et al., 2011, Nature, v. 477, p. 448-451. Simonson, B., 2003. Geological Society of America Special Paper 30, p. 231-241. Slack, J., & Cannon, W., 2009, Geology, v. 37, p. 1011-1014. van Houten, F. and Bhattacharyya, D., 1982, Ann. Rev. Earth Plan. Sci., v. 10, p. 441-457. van Lankvelt and Bjørnerud, 2010, ILSG Proceedings, v. 56, p. 67-68. Wang, Y., et al., 2009. Nature Geoscience, v. 2, p. 781-784. Weidman, S., 1904, WGNHS Bulletin XIII, Economic Series 8. - 74 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 The tectonometamorphic, magmatic and mineralization history of the Wanapitei Complex, Grenville Front tectonic zone, Ontario ROUSELL, D.H., PETRUS, J.A., Department of Earth Sciences, Laurentian University, Sudbury, ON P3E 2C6 Canada drousell@laurentian.ca, japetrus@gmail.com, EASTON, R.M., Precambrian Geoscience Section, Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, ON P3E 6B5 mike.easton@ontaro. ca, TINKHAM, D.K., Department of Earth Sciences, Laurentian University, Sudbury, ON P3E 2C6 Canada, dtinkham@laurentian.ca and NAPOLI, M.G.,Vale, Copper Cliff, ON mars.napoli@vale.com This presentation summarizes the results of new fieldwork and U-Pb geochronology from the Wanapitei Complex (WC), located 13 km ESE of Sudbury (Fig. 1). The WC is elliptical in plan view (6 x 2.5 km) and is located 400 m SE of the Grenville Front (Fig. 1). The WC is surrounded by a variety of gneissic rocks of the Grenville Front tectonic zone, including orthogneiss, paragneiss, amphibolite, migmatite, mylonite and breccia. All marginal contacts of the WC are sheared and no chilling is present near the margins. All new U-Pb data reported herein were obtained from the laser-ablation -ICP-MS facility at Laurentian University. The WC is divided into three segments each separated by a covered interval: a large NW and smaller SE and NE segments (Fig. 2). The WC was intruded twice by felsic dikes and twice by mafic dikes, subsequently underwent local folding and shearing, and finally was cut by late NWtrending pegmatite dikes (Napoli, 2003). The latter, dated regionally at 979±3 Ma (Easton et al., 1999), strike parallel to a major joint set and cut all rocks of the WC as well as a hornblende-plagioclase gneiss body in the SE segment. Within the SE segment a deformed, sill-like, hornblendeplagioclase gneiss body (4.4 x 0.36 km) trends NE and is in   sheared contact with the WC (Fig. 2). A cluster of concordant Figure 1. Regional setting of the Wanapitei Complex zircons from it yields a Concordia age (Ludwig 1998) of 2377±4 Ma (site 2, Fig. 2), similar to the age of the Creighton granite (Smith, 2002). This age represents a minimum age for emplacement of the body. The WC in the SE segment consists of olivine norite, norite and hornblende gabbro. Olivine and plagioclase occur as phenocrysts enclosed by hypersthene and augite, suggesting that the former two minerals are cumulates, subsequently enclosed in post-cumulus pyroxene. The olivine norite is also characterized by coronas around olivine and hypersthene, resulting from the reactions (1) olivine + plagioclase a bronzite + pargasite-spinel; (2) olivine + pyroxene a bronzite; and (3) pyroxene + plagioclase a pargasite-spinel. Other mafic rocks include norite and gabbro with varied amounts of secondary hornblende. Discordant zircon from a hornblende norite yielded an upper intercept age of 1747+12/-6 Ma (Prevec, 1993). In the NE segment of the WC, toward the margin of the complex, gabbro grades into recystallized gabbro and then into a breccia containing clasts of garnetiferous gabbro and infolded boudins of country rock. The bulk of the NW section of the WC consists of gabbro which was intruded by porphyritic quartz monzonite emplaced as large dikes and anastomosing vein systems which form an intrusion breccia. A sample of one of these dikes yielded a single population of concordant zircons with a Concordia age of 1707±4 Ma (site 1, Fig. 2), slightly younger than the age of 1747+12/-6 Ma from the same unit reported in Easton et al. (1999). To the NE, the breccia passes into garnet metagabbro, which has a Concordia age of 1735±3Ma (site 4, Fig. 2). Finegrained mafic dikes (68°/90°), typically 1 m wide with non-chilled contacts, are widely distributed in the WC. Locally garnetiferous mafic dikes with chilled margins cut injection breccia in the WC, and a sample yielded 2 - 75 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Figure 2. Geological map of the Wanapitei Complex showing the location of new U-Pb sample locations (sites 1 to 4). Map modified from Rousell and Trevisiol (1988).   clusters of concordant zircons, with Concordia ages of 1694±7 Ma and 1640±10 Ma, respectively (site 3, Fig. 2). Discordant zircons from the same sample define a Pb loss line trending toward 1000 Ma. Mineral exploration in the WC began in the 1940s, and is ongoing. The SE and NE segments of the WC contain 17 occurrences of sulphide mineralization (pyrrhotite, chalcopyrite, pentlandite, pyrite) all hosted by norite. The sulphide minerals are interstitial to the silicates, are inter-cumulus, and of primary magmatic origin. Although not all rocks of the WC are deformed, all are metamorphosed to upper amphibolite facies, with the possible exception of the olivine norite (do the coronas reflect subsolidus reactions or metamorphic overprinting?). Based on new and previous geochronology, the WC likely crystallized deep in the crust from multiple magma injections between 1747 and 1707 Ma during Killarney Belt magmatism. It was subsequently emplaced into its current position, in the solid state, along shear zones near the end of the Grenville Orogeny. During this emplacement, it was dismembered into three segments, with later independent tilting and/or rotation, and may be the remnant of a much larger intrusion. References (abbreviated titles) Easton, R.M., Davidson, A., and Murphy, E.I. 1999. Guidebook A2, GAC Sudbury’99, 52 p. Ludwig, K.R., 1998. On the treatment of concordant uranium-lead ages; GCA, v.62, p.665-676. Napoli, M.G. 2003. Geology of the Wanapitei Complex; MSc, Laurentian University, Sudbury. Prevec, S.A. 1993. Early Proterozoic mafic intrusions; PhD thesis, Univ Alberta, Edmonton, 223p. Rousell, D.H. & Trevisiol, D.D. 1988. Wanapitei complex; Mineralium Deposita; v.23, p.138-149. Smith, M.D. 2002. The timing and petrogenesis of the Creighton pluton; MSc, Univ Alberta, 123p. - 76 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Geologic setting of the Kevitsa Ni-Cu-PGE Deposit, Central Lapland Greenstone Belt, Finland SANTAGUIDA, F., LAPPALAINEN, M., JONES, S., VOIPIO, T., SIIKALUOMA, J. and YLINEN, J. First Quantum Minerals Limited. 1 Kaikutie, Sodankylä, Finland The Kevitsa Ni-Cu-Pt-Pd Mine is a new operation in northern Finland commencing full production in 2012. The current measured and indicated mineral resource (NI 43-101 compliant) for the deposit is 240Mt @ 0.30% Ni, 0.41% Cu, 0.21 gpt Pt, 0.15 gpt Pd, 0.11 gpt Au using a 0.1% Ni cutoff value. Additionally, an inferred resource of 34.7 Mt exists with comparable grade. The deposit is located within the Central Lapland Greenstone Belt; a Paleo-Proterozoic volcano-sedimentary sequence. Ultramafic intrusive and volcanic rocks are widespread and occur at several stratigraphic levels within the greenstone belt sequence (Hanski et al., 2001). The host ultra-mafic intrusion at Kevitsa has been dated at 2058 ± 4 Ma and is considered in general to be 2050 Ma in age based on dates from other cross-cutting dykes (Mutanen and Huhma, 2001). Mineralization consists largely of disseminated sulphides defined as Regular Ore, False Ore, Ni-PGE Ore and Contact Ore. Sulphide minerals are dominantly pyrrhotite, pentlandite, and chalcopyrite; although pyrite is also locally present. The Regular Ore constitutes approximately 95% of the resource. The deposit is hosted within a thick (> 1km) peridotite rock unit segregated as olivine pyroxenite, websterite and plagioclase-bearing pyroxenite (Figure 1). A ubiquitous amphibole overprint has replaced the primary minerals, affecting mainly clino-pyroxene, making rock identification locally impossible in hand specimen and in drill core. A crude internal layering is evident within the peridotite highlighted by horizons of sulphide mineralization. Individual sulphide lenses are 10s to 100s meters in thickness typically associated with the olivine pyroxenite layers, suggesting emplacement of magmas as pulses. Olivine-rich, “dunite”, xenoliths occur locally and are aligned within discrete zones discordant to the internal layering. In places, the dunite is seen as irregularshaped masses intercalated with the peridotite groundmass. The Ni-PGE Ore is spatially associated with these dunite zones. Dunitic rocks also occur below the peridotite unit, but are poorly defined. The peridotite unit is overlain by gabbro which forms a comparatively thin lithologic unit (typically < 200m) in the SW portion of the intrusion. The dunite, peridotite and gabbro geochemically occur as a fractionation series. A large, discrete, serpentinitized dunite body exists within the central portion of the intrusion that is geochemically distinct from the main Kevitsa body and is un-mineralized. Presently, this body is not well understood, but is considered to cut the Kevitsa peridotite unit. The immediate country rocks surrounding the Kevitsa Intrusion consist of mafic volcanic flows and tuffs interlayered with clastic metasediments and sulphide-rich carbonaceous schists. Continuing exploration to depth has improved the 3D geologic model of the deposit and of the intrusion. Both 2D- and 3D-seismic surveys have been utilized to delineate regional- and mine-scale structures as well as to define the basal Kevitsa Intrusion contact in un-drilled areas. Several new research initiatives have commenced recently to aid the exploration effort and in general will better position the Kevitsa deposit within the current Ni-Cu-PGE genetic models. - 77 - �Proceedings of the 58th ILSG Annual Meeting - Part 1     Figure 1. Bedrock geology of the Kevitsa Intrusion area defined by surface mapping and diamond drilling. The surface projection of the complete Ni-Cu-PGE orebody is shown by the dotted line. The geologic cross-section is drawn along the dashed line (drill section 12150N); note: the cross section is an expanded scale from the map. References Hanski, H., Huhma, H., Rastas, P., and Kamenetsky, V., 2001. The Paleoproterozoic komatiite-picrite association of Finnish Lapland. Journal of Petrology, 42: 855-876. Mutanen, T. and Huhma, H., 2001. U-Pb geochronology of the Koitelainen, Akanvaara and Keivitsa layered intrusions and related rocks. Geological Survey of Finland, Special Paper 33: 229-246. - 78 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Exploration history, mmineralogy and genesis of the Black Thor chromite deposit, Ring of Fire Intrusion, Northwestern Ontario Scott, G.W., Karakus, M., Shinkle, D. A. and Mitchell, A. R., Cliffs Natural Resources Inc., Thunder Bay, Ontario The McFaulds Lake Black Thor chromite deposit, an emerging world class chromite deposit, is located in northwestern Ontario approximately 550 km north of the City of Thunder Bay in the Ring of Fire greenstone belt. The initial discovery of the deposit was made by Freewest Resources Inc. in 2008 when a diamond drill hole intersected approximately 100 m of massive chromitite. Subsequent geophysical surveys and drilling have outlined a deposit amenable to open pit mining 400 m deep with a strike length of approximately 3 km, occurring in vertically dipping, sheet-like zones with mineable widths between 50 and 100 m. Chromite mineralization has been confirmed beyond the open pit zone to a depth of approximately 700 m and remains largely open downdip. Cliffs Natural Resources (CNR) acquired Freewest in 2010, and is currently proceeding with a definitive feasibility study and environmental assessment for the project. The chromite mineralization is hosted by a multi-phase layered ultramafic intrusion (Ring of Fire Intrusion or RFI) consisting of: peridotite, olivine cumulates, pyroxenite and gabbro occurring in three discrete fault bounded domains: the Southwest Domain, the Central Domain and the Northeast Domain. All domains feature some degree of layering and repetition of massive chromitite horizons. These features may be attributed a combination of primary magmatic and structural origins. Magma mixing, sulphide removal and sialic crustal contamination are possibilities with respect to the saturation of the magma in chromium. The volume of chromite mineralization in the deposits does not balance with the magma volume of the delineated extent of the RFI. Emplacement in a magma conduit has been proposed as a possible physical mechanism for the concentration of chromite mineralization of the magnitude represented by the deposits. Samples from drill holes in the Northeast Domain and from the Southwest Domain were studied at Cliffs Technology Laboratories to determine the bulk chemical composition, chemical composition of chromite, typical chromite textures, free magnetite content and gangue mineralogy. The Cr2O3 content of chromite grains increases from 45% in disseminated mineralization to over 52% in massive chromitite. Similarly, Cr/Fe ratio of chromite grains increases from 1.45 in disseminated mineralization to 1.96 in massive chromitite. Chromite mineral content ranges from as low as 7.8% in disseminated to more than 88% in massive chromitite. Semi-massive mineralization contains 35-63% chromite. The gangue minerals in disseminated mineralization are primarily sjoegrenite, lizardite and to a lesser extent antigorite, whereas clinochlore was the primary gangue mineral in massive chromitite. Disseminated mineralization zones were also characterized by the presence of magnetite and absence of talc-carbonate veins. Several distinct types of chromite textures are recognized. They are (#1) un-deformed and unaltered chromite, (#2) un-deformed chromite with slight surface alteration, (#3) deformed and fractured chromite but with preserved grain outline, (#4) intense cataclastic textured chromite, (#5) completely silicate altered chromite, (#6) chromite with many gangue or sulfide inclusions, and (#7) very fine precipitated chromite with very high chromia content. Types #1-#5 are the most common textures and can occur within the same drill core. The mean grain size of chromite grains in the massive type is slightly larger (200-220 µm) than in semi-massive (~160-167 µm) and in disseminated samples (180-190 µm). - 79 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Microstructural analysis of porcellaneous nepheline syenite, Coldwell Complez, Ontario SELAGI, Josh and HILL, Mary Louise, Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON Canada P7B 5E1 The Coldwell Complex is a large Proterozoic alkaline igneous complex on the north shore of Lake Superior west of Marathon, Ontario. From east to west across the complex, three distinct magmatic centres form an overlapping sequence; Center 2 is characterized by biotite-bearing alkaline gabbro and nepheline syenite (Mitchell and Platt, 1982). Within Center 2, a fine-grained porcelaineous variety of nepheline syenite is recognized as being chemically equivalent to the more typical coarse-grained nepheline syenite. This porcelaineous nepheline syenite outcrops in a narrow arcuate zone along the western shore of Redsucker Cove. Microstructural evidence for dislocation creep indicates that the porcelaineous syenite is a mylonite formed by localized solid-state deformation. As the Coldwell Complex has not been subjected to regional metamorphism, the high temperature required for ductile solid-state deformation in the porcelaineous syenite must be related to the magmatic activity. Localized high-temperature ductile strain within this arcuate zone could have led to catastrophic brittle failure along ring faults culminating in caldera collapse. This discovery may have implications for understanding the processes associated with caldera collapse in large alkaline igneous complexes. Reference Mitchell, R.H. and Platt, R.G., 1982, Mineralogy and petrology of nepheline syenites from the Coldwell Alkaline Complex, Ontario, Canada: Journal of Petrology, v. 23, p. 186-214. - 80 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Textural and compositional variation in apatite and plagioclase in the Marathon PGE-Cu deposit, Northwestern Ontario: Implications for fluid-rock interaction SHAHABI FAR, Maryam, SAMSON, Iain, GAGNON, Joel, Department of Earth and Environmental Sciences, University of Windsor, LINNEN, Robert, Department of Earth Sciences, Western University, and GOOD, David, Stillwater Canada Inc The Marathon PGE-Cu deposit is hosted within the Two Duck Lake gabbro (TDLG) of the Mesoproterozoic (1108 ± 1 Ma) Coldwell alkaline complex. Three zones of mineralization with different textural, mineralogical and geochemical characteristics have been identified: Footwall Zone, Main Zone, and W-Horizon. The relative roles of magmas and volatiles in Cu and PGE enrichment in this deposit remain a point of debate. The TDLG is coarse-grained to pegmatitic, and comprises plagioclase with variable amounts of interstitial ophitic clinopyroxene, olivine, and magnetite. Apatite, biotite and amphibole are common accessory minerals. Hydrothermal alteration is a common feature, especially in the Main and Footwall Zone, and is dominated by replacement of primary igneous minerals by amphibole, chlorite, biotite, and serpentine. Epidote and carbonate are also present as alteration minerals in some samples. Preliminary petrographic studies and SEM-CL imaging have revealed complex textures in both plagioclase and apatite, including evidence of dissolution and replacement, normal or reverse zoning, oscillatory zoning, and patchy zoning in plagioclase. Preliminary LA-ICP-MS analyses of various textural types of plagioclase and apatite have been conducted to evaluate any relationship between textural complexities and chemical variations. Magmatic plagioclase is characterized by a strong positive Eu anomaly, and has higher REE concentrations in the rims relative to the cores. Plagioclase with replacement rims occurs in the vicinity of quartz-feldspar granophyre patches, suggesting alteration by fluids exsolved from the granophyric melts. This is supported by the lower An content (An33-50) of the replacement plagioclase compared to the magmatic plagioclase that was replaced (An60-75). These rims also contain higher concentrations of Cu and Pb compared to magmatic plagioclase. Apatite occurs as euhedral to subhedral acicular and tabular prisms, and as anhedral crystals hosted by both altered and un-altered minerals. SEM-CL imaging shows that apatite crystals can exhibit growth (oscillatory) and replacement zoning, and EDS analyses indicate that the F and Cl contents are variable. In different rocks, apatite either predated (early apatite) or postdated plagioclase crystallization (late apatite). Apatite crystals from mineralized Main and Footwall Zone samples generally have high Cl contents, whereas apatite from the W-Horizon has low Cl contents, and generally have fluorapatite end-member compositions. Apatite crystals from the Footwall Zone (the lowest stratigraphic zone) have the highest Cl/F values, whereas those from the W-Horizon (the highest stratigraphic zone) show the lowest Cl/F. In addition, zoning, re-crystallization textures, and the presence of primary fluid inclusions are more common in apatite crystals from Main and Footwall Zone samples compared to the W-Horizon. Apatite consistently has a strong negative Eu anomaly, and early apatite has higher REE concentrations in the rims compared to the cores. The magnitude of the negative Eu anomaly, however, increases from core to rim. Late apatite has a REE abundance between that of the core and rim of early apatite. This suggests that late apatite probably crystallized from a new influx of magma rather than from continued crystallization of a resident, more fractionated magma. Replacement rims of apatite crystals show lower ∑REE, higher Cl, and higher transition metal (e.g., Ti, Fe, Cu) contents compared to early apatite. The similar REE core to rim evolution of the apatite and plagioclase suggests that early apatite and plagioclase coprecipitated; similarly, the sequestration of Eu by plagioclase resulted in the decrease in Eu* in apatite from core to rim. The observed spatial changes in Cl/F in apatite can be explained by compositionally distinct magma pulses, responsible for each mineralized zone, as most apatite crystals are euhedral to subhedral, formed early in the paragenetic sequence, and are magmatic. Intracontinental basaltic melts, however, (cf. Bushveld, and Stillwater complexes) are reported to be poor in Cl. Thus, apatite crystallizing from such a melt is expected to - 81 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 be fluorapatite. Much of the chalcopyrite in the Main Zone is reported to replace pyrrhotite and is intergrown with hydrous silicate minerals, which suggests that Cu was introduced into the system by fluids. This observation can be explained by a process in which fluids flux through the Footwall Zone, and transport Cu to the Main Zone during its upward migration. Therefore, a zone-refining process in which volatiles, derived from footwall country-rock dehydration, migrated up through the crystallizing gabbros is an attractive alternative by which Cl could be added to the system. The low Cl contents of apatite in the W-horizon can be explained if these fluids did not migrate upward into the W-Horizon, or the W-Horizon represents late-stage magma infiltration. Thus, the alteration of the primary igneous minerals, the re-crystallization textures, high Cl/F exhibited in apatite within the Footwall and Main Zones, and higher Cl and transition metal contents of the apatite replacement rims can be attributed to the interaction of a fluid phase at suprasolidus and/or subsolidus conditions. References Barrie, C.T. et al. 2002, Contact-type and magnetitite reef-type Pd-Cu mineralization in ferroan olivine gabbros of Coldwell Complex, Ontario, in Canadian Institute of Mining, Metallurgy and Petroleum, p. 321-337. Boudreau, A.E. & Hoatson, D.M., 2004, Halogen variations in the Paleoproterozoic Layered mafic-ultramafic intrusions of east kimberley, western Australia: Implications for platinum group element mineralization, Economic Geology, 99(5), p. 1015-1026. Boudreau, A.E. & Kruger, F.J., 1990, Variation in the composition of apatite through the Merensky cyclic unit in the western Bushveld Complex, Economic Geology, 85(4), p. 737-745. Boudreau, A.E. & McCallum, I.S., 1989, Investigations of the Stillwater Complex: Part V. Apatites as indicators of evolving fluid composition, Contributions to Mineralogy and Petrology, 102(2), p. 138-153. Boudreau, A.E., Mathez, E.A. & McCallum, I.S., 1986, Halogen geochemistry of the Stillwater and Bushveld Complexes: evidence for transport of the platinum-group elements by Cl-rich fluids, Journal of Petrology, 27(4), p. 967-986. Dahl, R., Watkinson, D.H. & Taylor, R.P., 2001, Geology of the Two Duck Lake intrusion and the Marathon Cu-PGE deposit, Coldwell Complex, Northern Ontario, Exploration and Mining Geology, 10(1-2), p. 51-65. Ginibre, C., Worner, G. & Kronz, A., 2007, Crystal zoning as an archive for magma evolution, Elements, 3(4), p. 261-266. Good, D.J. & Crocket, J.H., 1994a, Genesis of the Marathon Cu-platinum-group element deposit, Port Coldwell alkalic complex, Ontario; a Midcontinent rift-related magmatic sulfide deposit, Economic Geology, 89(1), p. 131-149. Good, D.J., 2010, Abstracts, 11th International Platinum Symposium, Applying multistage dissolution upgrading and 3d-GIS to exploration at the Marathon Cu-PGE deposit, Canada. p. 21-24. Heaman, L.M. & Machado, N., 1992, Timing and origin of midcontinent rift alkaline magmatism, North America: evidence from the Coldwell Complex, Contributions to Mineralogy and Petrology, 110(2), p. 289-303. Ruthart, R.G., Linnen, R.L., Samson, I.M. & Good, D.J., 2010, Abstracts, 11th International Platinum Symposium, Characterization of high-PGE low-Sulphur mineralization at the Marathon PGE-Cu deposit, Ontario. Samson, I.M., Fryer, B.J. & Gagnon, J.E., 2008, The Marathon Cu-PGE deposit, Ontario: Insights from sulphide chemistry and textures, Geochimica et Cosmochimica Acta Supplement, 72, p. 820. Watkinson, D.H. & Ohnenstetter, D., 1992, Hydrothermal origin of platinum group mineralization in the Two Duck Lake intrusion, Coldwell Complex, Northwestern Ontario, Canadian Mineralogist, 30, p. 121-136. - 82 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Regional to microstructural control of gold mineralization along the QueticoWabigoon subprovince boundary STINSON, Victoria R., and HILL, Mary Louise, Dept. of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, P7B 5E1 Canada Gold occurrences in Greenstone, Ontario are located along major regional shear zones, along the Quetico and Wabigoon subprovince boundary within the Superior province. On the regional scale, gold mineralization occurs within anastomosing ductile to brittle-ductile shear zones in pressure shadows around competent plutons or in ductilely deformed fractures within the plutons. In outcrop and drill core gold mineralization is observed in areas of heterogeneous deformation, especially at rheological or lithological contacts, such as along an amphibolite-granodiorite contact within the Klotz Lake shear zone east of Longlac, Ontario. Microstructure mimics the regional structure, as gold is found in fractures in competent minerals as well as in pressure shadows around the competent minerals, such as the gold found in fractured garnet west of Caramat, Ontario. The regional shear zones in Greenstone are transcurrent, ductile to brittle-ductile shear zones that typically trend east-west, parallel to the Quetico-Wabigoon subprovince boundary. Deviations from this eastwest trend occur where shear zones anastomose around older, competent granitoid plutons, like the Elmhirst meta-granodiorite pluton north of Jellicoe. Areas of highest strain along the pluton margins exhibit evidence of dislocation creep, such as quartz ribbons and grain size reduction, and host sporadic gold mineralization. No gold mineralization is present in the interiors of the plutons where strain fabric is minimal or absent. The highest gold mineralization is present immediately adjacent of the zones of highest strain and within the competent plutons, usually in ductiley-deformed fractures or along rheological contrasts. Significant gold mineralization is also present in pressure-shadows around the deformed plutons, where complex folding is typically located. Pressure shadows that host gold are present on all scales, including the microscopic scale. Gold is commonly found in pressure shadows around garnet, plagioclase, hornblende, and pyrite throughout the QueticoWabigoon subprovince boundary. Hetereogeneous deformation on the microscopic scale hosts gold mineralization within fractures of competent minerals, such as pyrite and garnet, while the less competent matrix minerals, like quartz, deform more ductilely. Gold is typically located at the grain boundary between competent and less competent minerals. Fractured meta-granitic plutons adjacent to shear zones that host gold are analogues to gold found in fractured garnet within a schistose fabric, although on much different scales. Exploration for gold mineralization along the Quetico-Wabigoon subprovince boundary should continue west of Lake Nipigon and east of Longlac and focus on heterogeneous deformation within shear zones on all scales. Heterogeneous deformation continues north into the Quetico and south into the Wabigoon and so should the exploration for new gold occurrences. - 83 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Temporal context of the Mamainse Point succession: a record of magmatic activity and fast plate motion across the “latent stage” of Midcontinent Rift development SWANSON-HYSELL, Nicholas L., Institute for Rock Magnetism, Department of Earth Sciences, University of Minnesota, 291 Shepherd Labs, 100 Union Street SE, Minneapolis, MN 55455, USA, BURGESS, Seth D., Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA, MALOOF, Adam C., Department of Geosciences, Princeton University, Guyot Hall, Washington Road, Princeton, New Jersey 08544, USA and BOWRING, Samuel A., Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA A popular model for the development of the Midcontinent Rift (MCR) proposes that there were four stages of magmatism: Early Stage (>1109-1106 Ma), Latent Stage (1106-1100 Ma), Main Stage (1100-1094 Ma) and Late Stage (1094-1086 Ma) (Miller and Vervoort, 1996; Vervoort et al., 2007). U-Pb dates of both extrusive (Davis and Green, 1997) and intrusive (Paces and Miller, 1993; Vervoort et al., 2007) sequences in the western part of the Lake Superior basin largely fall within either the early or main magmatic stages–leading to the interpretation that the period between them was a latent stage characterized by minimal eruptive activity. The early stage of MCR volcanism is characterized by magnetizations of reversed polarity while the main stage of MCR volcanism is characterized by magnetizations of normal polarity. In the Powder Mill Group and the North Shore Volcanic Group (NSVG), U-Pb dates on extrusive felsic units (Davis and Green, 1997, Zartman et al., 1997) reveal condensed stratigraphy during the time period of the latent magmatic stage accompanied by the switch from reversed to normal magnetizations. In contrast, the succession in the eastern Lake Superior basin exposed at Mamainse Point has been interpreted to represent a much more continuous record spanning nearly the entire duration of rift volcanism (Klewin and Berg, 1990). This interpretation of more continuous eruption in the eastern Lake Superior basin has stemmed, in part, from the presence of multiple geomagnetic reversals in the Mamainse Point succession (R-NR-N; first identified by Palmer, 1970) that have not been recognized in other extrusive successions in the rift. These reversals suggest that eruptive activity continued locally, albeit with some shorter duration lulls marked by conglomeratic units, while there was the hiatus of the latent magmatic stage elsewhere in the rift. Due to the large change in paleomagnetic inclination throughout the development of the MCR (Davis and Green, 1997, Swanson-Hysell et al., 2009), paleomagnetic data can be used to provide rough chronostratigraphic constraints through comparison of undated paleomagnetic poles to those with age control. Applying this approach with the paleomagnetic data set we have developed at Mamainse Point, we find that the lowermost flows at Mamainse Point correlate to those from the lower northeast sequence of the NSVG (the early magmatic stage with a U-Pb crystallization age of 1107.9±1.8 Ma determined from the intercept of a linear regression with the concordia curve; Davis and Green, 1997) and that the upper normal flows correlate with the upper southwest sequence of the NSVG (the main magmatic stage with concordia intercept U-Pb crystallization ages of 1096.6±1.7 Ma and 1098.4±1.9 Ma; Davis and Green, 1997). This result implies that the intervening stratigraphy at Mamainse Point spans the latent magmatic stage during which there were at least three reversals of the geomagnetic field. A U-Pb crystallization age of 1096.2±1.9 Ma for a felsic unit within the lower reversed zone at Mamainse Point was presented in an ILSG abstract and tentatively interpreted to be extrusive (Davis et al., 1995)—although the unit was mapped as a felsic intrusion by Giblin (1969). An extrusive interpretation implies that all the geomagnetic reversals within the succession occurred during the main magmatic stage—in conflict with the consistently normal magnetostratigraphy of the upper sequence of the NSVG and the Portage Lake Volcanics as well as the comparative analysis of paleomagnetic poles. New field observations document that this fine-grained unit with quartz phenocrysts has a cross-cutting relationship with the basalt flows such that the felsic unit both overlies and underlies the pahoehoe flow top of a single basalt flow. These observations indicate the unit is - 84 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 intrusive with its age providing a minimum rather than absolute age constraint on the lower reversed polarity zone at Mamainse Point. During a field season when fortuitously low lake levels led to excellent exposure of the stratigraphy in Flour Bay, we collected a crystal-rich tuff in close stratigraphic proximity to the base of the “Great Conglomerate” within the upper reversed polarity zone. We will present a new chemical abrasion TIMS U-Pb date from euhedral prismatic zircons from this tuff that anchors the Mamainse Point stratigraphy in absolute time and demonstrates that the record does indeed span much of the latent magmatic stage. These results strengthen the temporal framework of the Mamainse Point succession and allow for refined stratigraphic correlations across the rift. Furthermore, these results along with an expanded paleomagnetic data set support the interpretation that significant plate motion of North America was ongoing between the early and main magmatic stages that led to the decrease in the paleomagnetic inclination. References Davis, D., Green, J., and Manson, M., 1995. Geochronology of the 1.1 Ga North American Mid-Continent rift. Institute on Lake Superior Geology Proceedings, 41: 9-10. Davis, D. and Green, J., 1997. Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution. Canadian Journal of Earth Science, 34: 476–488. Giblin, P. E., 1969. Kincaid Township. Preliminary Geological Map, Ontario Department of Mines, 553. Klewin, K.W. and Berg, J.H., 1990. Geochemistry of the Mamainse Point volcanics, Ontario, and implications for the Keweenawan paleomagnetic record. Canadian Journal of Earth Sciences, 27: 1194-1199. Miller, J.D., Jr., and Vervoort, J.D., 1996. The latent magmatic stage of the Midcontinent rift: A period of magmatic underplating and melting of the lower crust. Institute on Lake Superior Geology Proceedings, 42: 33-35. Paces, J.B. and Miller, J.D., 1993. Precise and U-Pb ages of Duluth Complex and related mafic intrusions northeastern Minnesota: Geochronological insights to physical, petrogenetic, paleomagnetic and tectonomagmatic processes associated with the 1.1 Ga Midcontinent Rift system. Journal of Geophysical Research, 98: 13,997-14,013. Palmer, H.C., 1970. Paleomagnetism and correlation of some Middle Keweenawan rocks, Lake Superior. Canadian Journal of Earth Sciences, 7: 1410-1436. Swanson-Hysell, N.L., Maloof, A.C., Weiss, B.P., Evans, D.A.D., 2009. No asymmetry in geomagnetic reversals recorded by 1.1-billion-year-old Keweenawan basalts. Nature Geoscience, 2: 713–717. Vervoort, J.D., Wirth, K., Kennedy, B., Sandland, T., and Harpp, K.S., 2007. The magmatic evolution of the Midcontinent Rift: New geochronologic and geochemical evidence from felsic magmatism. Precambrian Research, 157: 235-268. Zartman, R.E., Nicholson, S.W., Cannon, W.F. and Morey, G.B., 1997. U-Th-Pb zircon ages of some Keweenawan Supergroup rocks from the south shore of Lake Superior, 34: 549-561. - 85 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Quartz quarry sites in northwestern Ontario: A new ‘prospect’ for geoarchaeologists investigating lithic raw material sources TAYLOR-HOLLINGS, Jill, Department of Anthropology, 13-15 HM Tory Building, University of Alberta, Edmonton, AB, T6G 2H4 Flintknapping is the process of creating stone tools by utilizing percussion and pressure flaking techniques on siliceous minerals. Preservation of lithic materials in the archaeological record provides some of the best evidence for reconstructing past human activities. In the boreal forest of northwestern Ontario, Aboriginal people began flintknapping about 9500 years ago, as evident from dated contexts of stone tools, cores, and resultant flaking debris (debitage). Despite this lengthy time frame, there is limited knowledge of archaeological sites in general and even less information about primary, secondary, or tertiary sources that Aboriginal people accessed to obtain useable siliceous material for stone tool manufacturing. Archaeological quarries are typically large sites with dense clusters of tools, debitage, and diverse processing areas. However, few quarries sites have been found in the Canadian boreal forest. Over the last seven years, rare opportunities enabled eight brief archaeological surveys to take place on the Bloodvein River in Ontario’s Woodland Caribou Provincial Park (now enlarged into a Signature Site); these projects are part of ongoing collaborations with Ontario Parks as well as Pikangikum, Lac Seul, and Little Grand Rapids First Nations in their traditional territories (e.g., Taylor-Hollings, 2006). The Ontario portion of this Canadian Heritage river is about 106 km long and all projects required fly-in transportation to this remote area. These surveys enabled data collection for the author’s PhD research as well as park and community-based land use planning. The first survey indicated that the most common lithic artifacts in that area consisted of various forms of quartz: rock crystal, white, yellow, and smoky. However, it was unknown where this quartz was sourced. As Bakken (2011:124) suggests, “quartz is a complicated issue in regional raw material studies, and one that remains to be resolved”. Thus, while looking for archaeological sites, it was decided to look for quartz sources, such as vein quarries or pebbles and cobbles, since the latter forms are known from northern Minnesota sites (Bakken, 2011). As part of the larger Canadian Shield, the Bloodvein River lies within the North Caribou Terrane, a collage of ~3-2.8 billion years old diverse lithotectonic assemblages within the northern Archean Superior Province (Thurston et al., 1991; Corfu and Stone, 1998). This part of the terrane was formerly known as the Berens River subprovince as defined by Corfu and Stone (1998). Geological mapping (Rickaby, 1923; Corfu and Stone, 1998) and research has been minimal in the Bloodvein River area. Published data is available for only one sample of a biotite tonalite from along that river system in Ontario (Corfu and Stone, 1998:2985). This is likely explained by the Bloodvein River being part of the Woodland Caribou Signature Site, where mineral prospecting has not been allowed since at least the 1980s, although possibly as early the 1940s when it first became a protected area (OMNR, 2004). Underlying most of the Bloodvein River in Ontario is a biotite granite suite consisting of a massive granodiorite to granite unit generally containing tonalite inclusions (Corfu and Stone, 1998). Within these types of bedrock outcrops along the river, there are a large number of quartz veins available for quarrying stone tool materials. However, sources of the most common quartz artifacts were previously unknown until this study. Although few archaeologists have looked for quartz vein quarrying previously in the boreal forest, the Woodland Caribou Signature Site presented an opportune locale to look for these sites where there was minimal mineral sampling known or evident. Due to the extensive amount of prospecting in northwestern Ontario since the last century, particularly in the nearby Red Lake area, it is important to consider carefully the evidence for precontact quartz quarrying as opposed to quartz sampled for mineral exploration. Three types of evidence for Aboriginal quarrying activities were found along the Bloodvein River during these surveys: (1) quartz veins - 86 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 located adjacent to other types of archaeological sites typically with quartz artifacts (n=9); (2) quarried quartz veins with artifacts found beside or in them (n=5); and (3) mined quartz veins with evidence of non-recent quarrying (n=7). All of the places considered to be the result of ancient quarrying activities were weathered and had lichen developed on them, sediments deposited and/or soil development, which had been disturbed to reveal the quartz vein. No pebble or cobble based quartz sources were found, but they may be present in the tills found in the Lac Seul moraine to the east of the park. Since locally derived quartz is evidently the most commonly found raw material in the Bloodvein River area, Aboriginal people from this region were selecting reliable, nearby quartz vein sources more often than typically higher quality exotic types from afar. There is a specific Ojibwe word for white quartz, wiininwaabik (fat rock), whereas most other minerals and rocks are generically known as asin. Perhaps this distinction stems from the longstanding importance of this material in lithic technology within Anishinaabeg territories. In addition, the Bloodvein River examples indicate that geoarchaeologists do need to search for these types of small-scale quartz vein quarry sites in the boreal forest where quartz artifacts are typically prevalent. References Bakken, K.E., 2011. Lithic raw material use patterns in Minnesota. Unpublished Ph.D. dissertation, Department of Anthropology, University of Minnesota. Corfu, F., and Stone D., 1998. The significance of titanite and apatite U-Pb ages: Constraints for the post-magmatic thermal-hydrothermal evolution of a batholithic complex, Berens River area, northwestern Superior Province, Canada. Geochimica et Cosmochimica Acta 62(17):2979-2995. Ontario Ministry of Natural Resources (OMNR), 2004. Woodland Caribou background information. Queens Printer, Ontario. Rickaby, H.C., 1923. Bloodvein River to Twelfth Base Line, 1922. 32nd Annual Report, Part II:49-59, Ontario Department of Mines. Clarkson W. James, Printer to the King’s Most Excellent Majesty, Toronto. Taylor-Hollings, 2006. Stage two archaeological research at Knox and Peisk Lakes in the Woodland Caribou Signature Site, northwestern Ontario. Research report submitted to the WCSS Park Superintendent, Ontario Ministry of Culture, Pikangikum First Nation, and research partners. Thurston, P.C., Osmani, I.A., and Stone, D., 1991. Northwestern Superior Province: review and terrane analysis; in Thurston, P., Williams, H., Sutcliffe, R., Stott, G. (Eds.), Geology of Ontario. Ontario Geological Survey, Special Volume 4, Part 1, 80-142. - 87 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Analyzing ductile shear zone network geometries in the Grassy Portage Sill, Rainy Lake region, Northwestern Ontario, Canada THALHAMER, Ernest, and CZECK, Dyanna M., Department of Geosciences, University of WisconsinMilwaukee, P.O. Box 413, Lapham Hall 366, Milwaukee, WI 53201 The Grassy Portage Sill (GPS) is a ~2.7 Ga metagabbroic sill located in the Rainy Lake region of northwestern Ontario (Fig. 1). The Rainy Lake region is located in the Superior Province between the metavolcanic Wabigoon subprovince to the north and the metasedimentary Quetico subprovince to the south (Card and Ciesielski, 1986). Two regional faults bound the region and intersect to the east, forming a wedge which defines the Rainy Lake zone (Poulsen, 1986). This area was regionally deformed due to dextral transpression resulting from the Kenoran Orogeny (~2.7 Ga) (Poulsen et al., 1980; Davis et al., 1989; Czeck and Hudleston, 2003; Druguet et al., 2008). The GPS is approximately 20 km long and 1-2 km wide, and has undergone heterogeneous strain along its length. This strain variation is a function of the competence contrast between the GPS, the gneissic Rice Bay Dome to the west, and the metavolcanic and metasedimentary units between the two (Druguet et al., 2008; Carreras et al., 2010). The GPS has a higher competence than the adjacent metavolcanic and metasedimentary units, but all have a lower competence than the Rice Bay Dome. Figure 1. Geologic map of the Grassy Portage Sill and surrounding region with regional foliations indicated by dashed lines. Modified from Druguet et al. (2008) from mapping by Poulsen (2000). Within the GPS, anastomosing ductile shear zone networks accommodated the bulk of deformation within the largely competent sill (Fig. 2; Carreras et al., 2010), although a penetrative foliation also formed locally. The network geometries vary along the length of the sill, apparently related to strain variations. At nearly all locations, steeply dipping dextral and sinistral sets of shear zones formed, presumably simultaneously as evidenced by mutually offsetting cross-cutting relationships where present. Conjugate shear zone sets initiated at a near perpendicular orientation, and through progressive deformation have rotated towards one another. At the least strained sites, the gabbro has a pervasive foliation, but few, if any, shear zones. The varying responses of the GPS to deformation may be explained by lithological heterogeneity. Where grain size is coarse, the rock is likely stronger and deformation is localized into discrete shear zones. Where grain size is fine, the rock is generally weaker and pervasive foliation forms. Regionally the bulk shortening direction is oriented NW-SE, due to the N-S oriented shortening accompanied by dextral transpression. Estimates of bulk shortening direction based on shear zone geometries within the GPS indicate wide variation along the length of the unit, but outcrops can be grouped into three general clusters. In - 88 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 the north, bulk shortening is predominantly oriented NE-SW. In the central part, shortening is oriented between N-S and NW-SE. In the south, the shortening direction is oriented NW-SE. Overall the shortening direction appears to “wrap” around the GPS, which is similar to the trend of the foliation variation within surrounding units. The variations seen at the unit scale can be attributed to a regional scale “strain shadow” imparted by the highly competent Rice Bay gneissic dome. Figure 2. Photograph of the anastomosing shear zone network within metagabbro at a highly strained outcrop. Shear zones have been traced, and are defined by narrow zones of intense foliation surrounding undeformed or less deformed lozenges. They define an anastomosing network pattern. References Card, K.D., Ciesielski, A., 1986. DNAG subdivisions of the superior province of the Canadian shield. Geoscience Canada 13, 5-13. Carreras, J., Czeck, D.M., Druguet, E., Hudleston, P.J., 2010. Structure and development of an anastomosing network of ductile shear zones. Journal of Structural Geology 32, 656-666. Czeck, D.M., Hudleston, P.J., 2003. Testing models for obliquely plunging lineations in transpression: a natural example and theoretical discussion. Journal of Structural Geology 25, 959-982. Davis, D.W., Poulsen, K.H., Kamo, S.L., 1989. New insights into Archean crustal development from geochronology in the Rainy Lake area, Superior Province, Canada. Journal of Geology 97, 379-398. Druguet, E., Czeck, D.M., Carreras, J., Castaño, L.M., 2008. Emplacement and deformation features of syntectonic leucocratic veins from Rainy Lake zone (Western Superior Province, Canada). Precambrian Res. 163, 384-400. Poulsen, K.H., Borradaile, G.J., Kehlenbeck, M.M., 1980. An inverted Archean succession at Rainy Lake, Ontario. Canadian Journal of Earth Sciences 17, 1358-1369. Poulsen, K.H., 1986. Rainy LakeWrench Zone: an example of an Archean Subprovince boundary in Northwestern Ontario. In: deWit,M.J., Ashwal, L.D. (Eds.), Tectonic Evolution of Greenstone Belts, Technical Report 86-10, pp. 177–179. Poulsen, K. H., 2000. Archean metallogeny of the Mine Centre - Fort Frances area. OGS Report 266. - 89 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 SPREE: Field experiment to study deep structure of the Midcontinent Rift VAN DER LEE, Suzan, Department of Earth and Planetary Sciences, Northwestern University, 1850 Campus Dr., Evanston, IL 60208, FREDERIKSEN, Andrew, Department of Geological Sciences, University of Manitoba, 125 Dysart Rd., Winnipeg, MB R3T 2N2, BOLLMANN, Trevor, Department of Earth and Planetary Sciences, Northwestern University, 1850 Campus Dr., Evanston, IL 60208 and SPREE Team By about 1 Ga North America’s midcontinent region completed a formation process broadly similar to the current American west, with convergence along the Grenville Orogeny and extension along the contemporaneous Mid-continent Rift (MR). Now buried under platform sediments, more than a half million cubic km of dense igneous rock was deposited in the MR. These rocks generate an elongated 60+ mgal Bouguer gravity anomaly and a correlated magnetic anomaly, which cuts curiously through major geologic units such as the Superior and Yavapai Provinces. Although the MR is the most striking surface geophysical anomaly in the midcontinent, no evidence has yet been found for current geologic activity or correlated anomalies in the mantle lithosphere. The installation this year of the first swath of Earthscope-USArray stations east of the Mississippi River allows us to shed light on this enigmatic anomaly. Specifically, our seismic field experiment SPREE (Superior Province Rifting Earthscope Experiment) aims to uncover important details such as the depth and lateral extent of crust and mantle structures related to rifting, rift cessation/inversion, or post-rift stabilization. In addition, SPREE can answer questions about the longevity and healing ability of Proterozoic continental lithosphere. SPREE is a collaboration between Canadian and US universities. Eighty-three Flexible-Array (FA) seismic stations were installed between April and June this year on and near the MR. The array configuration includes an extension of the US-based Transportable Array (TA) into Ontario north of Lake Superior as well as three lines of 10-km spaced stations along and across the MR west and east of the northern Mississippi. SPREE recorded the M2.5 western Minnesota earthquake (April 29), the M7.3 Fox Islands earthquake (June 24), the M5.8 Virginia earthquake, and an unusually high number of moderate earthquakes in the relatively stable continental interior, from eastern Ontario to Oklahoma. We aim to use the recorded earthquakes and ground motion noise to detect microseismicity as well as to construct a multi-scale, three-dimensional image of the seismic velocity and discontinuity structure of the study region’s lithosphere and underlying mantle. These investigations will reveal the deep structure of the mid-continent rift. Moreover, the central location of SPREE with respect to North America’s seismically active plate boundaries contributes optimally to the seismic imaging of the sub-continental mantle elsewhere in stable North America. - 90 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Anatomy of a Mesoarchean Batholith VAN LANKVELT, A., SCHNEIDER, D.A., HATTORI, K., Science de la Terre/ Earth Sciences, Université d’Ottawa/ University of Ottawa, 140 Louis-Pasteur Ottawa, ON K1N 6N5 Canada and BICZOK, J. Goldcorp Canada Ltd., Musselwhite Mine, P.O. Box 7500 Thunder Bay, ON P7B 6S8 Canada The North Caribou greenstone belt (NCGB) lies in the North Caribou Terrane (NCT) of the Superior Province at the northeastern boundary of the North Caribou Core and the Island Lake Domain (Stott et al. 2010). The NCGB hosts the Musselwhite gold mine, a banded-iron-formation-hosted orogenic lode gold deposit, and structural controls on the deposit have been linked to intrusions along the margins of the NCGB (Stott and Biczok 2010). The most prominent of these intrusions is the North Caribou Batholith (NCB), which forms the southern and western boundaries of the west-central half of the NCGB. The contact of the NCB with the greenstone belt is marked by a crescent of low magnetic susceptibility relative to the centre of the NCB, which has been interpreted by Stott and Biczok (2010) as a “crescent pluton,” like those found elsewhere in the Superior Province. These crescent plutons intrude along the contact of greenstone belts and older gneissic terranes and impose a strain aureole on the less-competent greenstone belts, causing them to form arcuate shapes. Since the margin of the NCB conforms to the structural and geometric constraints of crescent plutons, this area has been dubbed the North Caribou Pluton (NCP). This interpretation prompted an investigation of the NCP to determine if this pluton is a discrete, younger intrusion. New U-Pb ages from zircon and titanite, along with whole rock and zircon geochemistry, and amphibole and plagioclase thermobarometry illuminate the evolution and relationship between the NCP and the rest of the NCB. Rocks in the NCB range from tonalites to granites and have a wide variety of textures and compositions. Textures throughout the NCB include enclaves of isotropic, equigranular, weakly-deformed granitoids, strongly foliated and lineated gniesses, and migmatites. Some of the migmatites are partially-melted amphibolites, which occur near the northern margin of the intrusive complex, but there are also granitoids with schlieren and other textures suggesting either partial melting, melt segregation, or magma mixing. Late K-feldspar-rich pegmatites occur around the margins of the NCB, in both the NCP and the central batholith. U-Pb geochronology on zircon and titanite was conducted using LA-ICP-MS methods at the University of New Brunswick. Single zircon ages range from 3132-2693 Ma, with a majority of rock ages falling between 2870 Ma and 2830 Ma. These ages are consistent with those reported by previous workers in the NCB and elsewhere in the NCT (e.g. Biczok et al., 2012). Ages from titanite are similar to the younger zircon populations. In the central NCB, titanite ages from 2798-2791 Ma are found in rocks with zircon ages of 2834-2833 Ma. This 30 m.y. difference could represent cooling through the different closure temperatures of titanite and zircon (500 and 900 ºC, respectively). The titanite in the NCP, however, record ages between 2766 Ma and 2748 Ma, with a ~100 m.y. difference between the zircon and titanite ages. As the titanites from these rocks are roughly aligned with the foliation, and the ages match zircon ages in younger intrusions surrounding the NCGB, these ages likely reflect later tectonism. To further assess the relationship between the rocks in the NCP and those in the central NCB, depth and temperature of emplacement was estimated using the plagioclase-amphibole thermobarometer and the Ti-inzircon thermometer. Titanium concentrations in zircon were measured using LA-ICP-MS, and calculated temperatures range from 830-760 ºC in the NCP and 790-750ºC in the central NCB. Plagioclase-amphibole temperatures and pressures were calculated following Holland and Blundy (1994) and Anderson and Smith (1995). Only five samples from the NCB have the appropriate mineralogy for the thermobarometer, only one of which was in the NCP. The temperature/ pressure conditions for the NCP sample are 600 ± 20 ºC and 6.3 ± 0.2 kbar (~22 km). Since this sample also contained isotopically disturbed titanite, it is possible that the plagioclase and amphibole compositions record this later event, rather than the emplacement conditions. The PT conditions in the central part of the batholith range from 640-620 ºC and 7.5-6.7 kbar (~26-23 km). - 91 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 These data provide some evidence supporting the crescent pluton model of the NCP. The shape of the NCGB, the magnetic signature of the NCP, and the structural data from previous workers are similar to the observations of crescent plutons in other greenstone belts. The model requires an intrusive contact between the crescent plutons and the supracrustal rocks, and the partially-melted amphibolites at the contact between the NCP and the NCGB clearly demonstrate that this contact is intrusive. Major, minor, and trace element geochemistry from the NCP is less variable than that from the rest of the NCB, despite a wide range of SiO2 concentrations, and REE patterns in zircon in the NCP have distinct, shallower slopes than those in the NCB. This suggests that the rocks in the NCP are more similar to each other than those in the rest of the NCB. U-Pb ages from titanite in the NCP are also younger than those in the central NCB. There are, however, new data that do not support a late crescent pluton. Zircon U-Pb ages in the NCP range from 2870-2858 Ma, which is slightly older than the ages from the centre of the NCB, which are 2852-2833 Ma. There is also no difference between the textures in the pluton and the batholith, as both contain gneisses and isotropic granitoids. Temperatures and pressures are also indistinguishable across the contact between the NCP and the central NCB. The older zircon ages in the NCP suggest either that the NCP represents a chill-margin on the NCB or a discrete earlier intrusion. Since the NCB has a map area of ~2800 km2 and does not exhibit cumulate textures, it is unlikely the entire complex was emplaced as a single melt batch. It is more likely that the primary conduit for the magma was near the centre of the NCB, and that several intrusions emanating from this area were emplaced at roughly the same depth over ~40 m.y.. This model predicts older granitoids on the margins of the intrusive complex, which have been shouldered aside radially by continued magmatism at the centre of the complex. This is consistent with the aeromagnetic data and textures observed in the NCB, as well as the geochronology, geochemistry and thermobarometry. Since this study did not investigate fabrics and structures related to the intrusions, we cannot comment on whether the kinematics of this model are consistent with these data. Further studies of the NCB, including fabric analysis, could constrain the dynamics of the intrusions and improve the proposed model. References Anderson, J.L., Smith, D.R. 1995. The effects of temperature and fO2 on the Al-in-hornblende barometer. American Mineralogist, 80: 549-559. Biczok, J., Hollings, P., Klipfel, P., Heaman, L., Maas, R., Hamilton, M., Kamo, S., Friedman, R. 2012. Geochronology of the North Caribou greenstone belt, Superior Province Canada: Implications for tectonic history and gold mineralization at the Musselwhite mine. Precambrian Research, 192-195: 209-230. Holland, T., Blundy, J. 1994. Non-ideal interactions in calcic amphiboles and their bearing on amphibole-plagioclase thermometry. Contributions to Mineralogy and Petrology, 116: 433-447. Stott, G.M., Biczok, J. 2010. North Caribou Greenstone Belt: Gold and Its Possible Relation to the North Caribou Pluton Emplacement—A Belt-wide Contact-Strain Aureole? Summary of Field Work and Other Activities 2010, Ontario Geological Survey, Open File Report 6260: 22-1-22-12. Stott, G.M., Corkery, M.T., Percival, J.A., Simard, M., Goutier, J. 2010. A Revised Terrane Subdivision of the Superior Province. Summary of Field Work and Other Activities 2010, Ontario Geological Survey, Open File Report 6260: 201-20-2. - 92 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Paleomagnetic data in stratigraphic context from 1.1 Ga Osler Group basalt flows on Simpson Island, Ontario: Evidence for rapid plate motion of Laurentia in the late Mesoproterozoic VAUGHAN, Angus A., Department of Geology, Carleton College, 1 North College St., Northfield, MN 55057, SWANSON-HYSELL, Nicholas L.and FEINBERG, Joshua M., Institute for Rock Magnetism, Department of Earth Sciences, University of Minnesota, 291 Shepherd Labs, 100 Union Street SE, Minneapolis, MN 55455 The Osler Group represents the extrusive component of the early magmatic stage of the Midcontinent Rift in the northern Lake Superior region. The Osler Group overlies the epicontinental sediments of the Mesoproterozoic Sibley Group with angular unconformity. The lowest 100 m of stratigraphy of Osler Group contains rift-related sandstones and conglomerates (Hollings et al., 2007). In some locales, these basal sedimentary units are overlain by a quartz-feldspar porphyritic rhyolite for which a U-Pb zircon age of 1107.5 +4/-2 Ma has been reported (Davis and Sutcliffe, 1985). This age was obtained from an outcrop of rhyolite porphyry on Black Bay Peninsula, about 40 km to the west of Simpson Island. This unit was tentatively interpreted as extrusive (Davis and Sutcliffe, 1985; Lightfoot et al., 1991; Hollings et al., 2007), and the age has been interpreted as constraining the time at which Osler Group volcanism commenced. However, observations made during this study of a quartz-feldspar porphyry unit on Simpson Island, mapped as equivalent to the unit from which the age was obtained (Giguere, 1975), provide additional evidence for Giguere’s inference that this unit is actually intrusive. A thin veneer of basalt, ranging in thickness from several cm down to 1-2 mm, is variably present overlying the porphyry. The basalt veneer displays pahoehoe flowbanding, which is unlikely to have developed if the flow was originally this thin, implying that the felsic unit intruded into the basalt, cutting into an originally thicker flow. We also observed protrusions of porphyry surrounded by host basalt, providing further support for an intrusive relationship. This evidence suggests that the 1107.5 Ma age is a minimum age for the eruption of the first Osler basalt flows, rather than an absolute age for that point in the Osler Group stratigraphy. Above the felsic porphyry is a succession of relatively continuous tholeiitic basalt flows, with minor siltstone and conglomerate interbeds. The flows range in thickness from 5 cm to about 40 m thick; individual flows are recognized by a transition from massive basalt at the base of flows (occasionally with pipe vesicles in the basal 10-20 cm) to vesicular basalt towards the top, and by occasionally well-exposed pahoehoe flow tops. A rhyolite, interpreted as extrusive by Davis and Sutcliffe (1985), occurs near the top of the Osler Group basalt flows at Agate Point (stratigraphically higher than the highest flow on Simpson Island). Davis and Green (1997) obtained a U-Pb zircon age from the Agate Point Rhyolite of 1105 ± 2 Ma, which because of the extrusive nature of this rhyolite, is a robust age for that point in the Osler Group stratigraphy. It is worth noting that this age is statistically indistinguishable from the rhyolite porphyry discussed above. Here we report the first paleomagnetic investigation of the lower portion of the Osler Group stratigraphy, which builds on the work of Halls (1974) in the Nipigon Strait region. Oriented paleomagnetic cores were collected from 45 basalt flows in 5 stratigraphic sections along the eastern and southern shores of Simpson Island, spanning about 2500 m of Osler stratigraphy. Alternating field demagnetization, thermal demagnetization and low-temperature cycling experiments show that the majority of the remanent magnetization in Osler flows is held by magnetite and records a thermal remanent magnetization acquired at the time of eruption. Paleomagnetic data from the Osler Group suggest rapid plate motion during the early stages of the Midcontinent Rift. A preliminary paleomagnetic pole (λp = 220.6, φp = 44.2, A95 = 5.0) was calculated for 8 measured flows towards the bottom of the Osler Group and another preliminary pole (λp = 197.9, φp = 35.2, A95 = 6.3) was calculated for 9 flows towards the top of the reversely magnetized Osler flows. These two poles imply ~20° of latitudinal motion during the time the Osler Group was erupted. Significant latitudinal change during the lower reversed polarity zone of the Midcontinent Rift supports similar data from correlative stratigraphy at Mamainse Point, Ontario and represents the second place in the rift where such motion has been documented in - 93 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 the lower reversed polarity zone (Swanson-Hysell et al., 2009). These data add to the database of paleomagnetic poles from Keweenawan volcanics that together imply rapid plate velocities (>20 cm/yr) for Laurentia during the development of the Midcontinent Rift. References Davis, D. W., and Green, J. C., 1997, Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution: Canadian Journal of Earth Sciences, v. 34, p. 476-488. Davis, D. W., and Sutcliffe, R. H., 1985, U-Pb Ages from the Nipigon Plate and Northern Lake-Superior: Geological Society of America Bulletin, v. 96, p. 1572-1579. Giguere, J. F., 1975, Geology of St. Ignace Island and adjacent islands, District of Thunder Bay: Canada (CAN), Ontario Geological Survey, Toronto, ON, Canada (CAN), p. 35. Halls, H. C., 1974, Paleomagnetic Reversal in Osler Volcanic Group, Northern Lake Superior: Canadian Journal of Earth Sciences, v. 11, p. 1200-1207. Hollings, P., Fralick, P., and Cousens, B., 2007, Early history of the Midcontinent Rift inferred from geochemistry and sedimentology of the Mesoproterozoic Osler Group, northwestern Ontario: Canadian Journal of Earth Sciences, v. 44, p. 389-412. Lightfoot, P. C., Sutcliffe, R. H., and Doherty, W., 1991, Crustal Contamination Identified in Keweenawan Osler Group Tholeiites, Ontario - a Trace-Element Perspective: Journal of Geology, v. 99, p. 739-760. Swanson-Hysell, N. L., Maloof, A. C., Weiss, B. P., and Evans, D. A. D., 2009, No asymmetry in geomagnetic reversals recorded by 1.1-billion-year-old Keweenawan basalts: Nature Geoscience, v. 2, p. 713-717. - 94 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 The Little Commonwealth exploration - An example of a SEDEX deposit WAGGONER, Thomas, 141Chippewa Negaunee, MI 49866, thomaswaggonergeo@hotmail.com and KARAKUS, Musa, Cliffs NR 550 E. Division, Ishpeming, MI 49849 The Little Commonwealth Exploration (LCE) is located in Florence Co., Wisconsin. The LCE represents a subaqueous synsedimentary exhalative deposit where silica, iron and manganese rich minerals are deposited on an elevated mound that formed a barrier separating sand (quartzite) to the west from mud (slate) to the east. The entire deposit outcrops in a small area encompassing less than 80 acres. The surface geology is primarily Michigamme slates with minor quartzites, conglomerates and isolated hydrothermal banded iron formation deposits. The quartzite can be characterized as a vitreous white ortho-quartzite except at the LCE where banded magnetite chert and Mn-chlorite, Mn-stilpnomelane, Mn-garnet chert breccias were deposited. The garnet, chlorite, stilpnomelane and ilmenite are all enriched in manganese characteristic of mineralization associated with SEDEX/VMS deposits. The rock with abundant spessartine garnet has been called coticule. Beginning with the deposition of the quartzite unit within the Michigamme formation a restricted area was subjected to hydrothermal mineralization. Some minerals are replacement of existing sediments (i.e. Dunkel Exploration located one mile to the northwest) while others are subaqueous exhalites like the one found at the LCE. Hot fluids carried Fe, Si, Al, Mn, Mg, P, B and REE. Upon reaching the surface and experiencing a significant drop in the pressure and temperature some of the minerals immediately crystallized and were deposited directly on the slope of the mound. Specularite appears to be later as the crystal laths display a random orientation usually enclosing magnetite or ilmenite. Late addition of sulfur, Fe++, copper and arsenic is evidenced by the presence of small quantities of sulfides. The major minerals present are magnetite, chert, specularite, Mn-chlorite, MnStilpnomelane, Mn-garnet and apatite. Minor minerals include Mn-ilmenite, monazite, tourmaline, pyrite, chalcopyrite and arsenopyrite. The only carbonate identified was a few grains of kutnohorite and dolomite. The manganese oxide content in the garnet is quite variable (Table 1) while the manganese in the chlorite and stilpnomelane is about more constant at about 1.8%. Ilmenite exhibits MnO values of ~10%. Table 1. Garnet Composition Oxide SiO2 Al2O3 FeO MnO MgO CaO FC-103 @80' 35.00 18.62 31.41 7.38 0.47 3.62 FC-104 Breccia Breccia @ 250' 2A E. Sft 2B E. Sft 35.09 35.37 36.12 18.5 18.95 19.14 28.39 26.53 27.38 12.91 13.87 20.23 0 0 0.69 1.93 2.31 2.53   The rocks appear to have experienced low grade greenschist alteration as evidenced by replacement of the garnet by either Mn-chlorite or Mn-stilpnomelane. Some of the magnetite is believed to be the result of immediate crystallization on exit from the vent and has undergone some oxide overgrowth resulting in crystals that are +100 microns in size. The rare earth elements reside in monazite which yields a high LREE pattern and a very low HREE suite in a chondrite normalized plot (Fig. 1). A slight negative Eu value would support the presence of hydrothermal fluids but with abundant sea water dilution. The age of the LCE deposit is constrained between 1836 and 1833 Ma based on a tuff age in the basal Rove formation and the local intrusive of the Tobin Lake granite indicating the magma chamber that produced the iron oxide rich Hemlock volcanics and associated with the northward subduction zone was still viable near the end - 95 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 LITTLE COMMONWEALTH MARTITE CHONDRITE NORMALIZED REE LOG SCALE: SAMPLE/CHONDRITE 100.00 10.00 TOTAL REE 121.48 1.00 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu RARE EARTH ELEMENTS Figure 1. Chondrite normalized REE plot for Little Commonwealth martite of the Penokean Orogeny. When the Wisconsin Magmatic terrain moved northeast along a series of NW-SE major faults it resulted in three parallel allochthons that were tilted and rotated exposing repeat sections of Michigamme strata. There are abundant indications within the three fault blocks to indicate the SEDEX mineralization is not an isolated occurrence. A number of iron oxide rich showings have been noted in the Michigamme formation particularly from the quartzite unit upward. Replacement of existing sediments is indicated in a few of the shows like the Dunkel located one mile northwest of the LCE. The LCE is an excellent example of a synsedimentary exhalative deposit with both SEDEX and IOCG features situated adjacent to an active hydrothermal vent (not located) that produced iron oxides-chert-Mn silicates. A series of much larger and more active vents can easily be envisioned as a source for Lake Superior Type iron deposits found in the Animikie Basin. References Dutton, C.E., 1971, Geology of the Florence Area, Wisconsin and Michigan, USGS Prof. Paper 633, 54p. Johnson, R.W., 1958, Geology of the Little Commonwealth Area, Florence County, Wisconsin, USGS OFR 58-53, 98p. Schneider, D.A., et al, 2002, Age of Volcanic Rocks and Syndepositional Iron Formations, Marquette Range Supergroup: Implications of the Tectonic Setting of Paleoproterozoic Iron Formations of the Lake Superior Region, Can. J. Earth Sci. v. 39, p 199-1012. - 96 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 The curious meteorite harvest of the Lake Superior region I - Overview WILSON, Graham C., Turnstone Geological Services Limited, P.O. Box 1000, Campbellford, ON K0L 1L0 and McCAUSLAND, Phil J.A., Department of Earth Sciences, University of Western Ontario, London, ON N6A 3K7 Introduction The historic trove of discrete meteorite falls and finds in the region surrounding Lake Superior is 52 (the official total, on 10 April 2012, as recounted in the on-line database of the Meteoritical Bulletin, http://www. lpi.usra.edu/meteor/metbull.php?), plus five more in Manitoba awaiting official recognition. Current official tallies for large countries include: U.S.A. (1,633); Australia (646); Chile (206); India (136, plus 4 insufficiently documented); Argentina (74); and Canada (60, plus 13 “in progress” of certification). Our chosen region for this short survey includes the four Superior-bounding jurisdictions plus Manitoba (with 3 finds in and near Whiteshell Park, which is closer to Superior than the populous south of Ontario). For the three states the documented haul of meteorites corresponds to a cumulative mean density of 32 in 645,304 km2, or 1 meteorite recovered per 20,166 km2. For the two provinces the total is 25 in 1,724,192 km2, or 1 meteorite recovered per 68,968 km2 (in this instance, the cosmic odds appear 3.4 times better in the U.S.!). The two sub-regions have respective populations of 20.891 and 14.291 million, and mean population density of the three states is 3.9 times that of the two provinces (data from National Geographic Society, 2011). The table breaks down the 57 meteorites into their most basic families (see Norton and Chitwood, 2008, for a readable taxonomy of meteorites; Grady, 2000, may be the last global meteorite catalogue in print). Irons and the rarer stony-irons (the two regional examples are both the olivine-metal mixtures known as pallasites) are quite distinct from Earth rocks. The stony meteorites, both the more primitive, undifferentiated chondrites and the rare, igneous achondrites, vary in their properties but can be much harder to identify, especially when weathered. There are 29 irons, two stony-irons and just 26 stones, including 25 ordinary chondrites (43.9% of the total). The latter (Met.Bull., 10 April 2012) are 87.4% of all authenticated meteorites, irons just 2.5% (but 51% of the regional meteorite haul), stony-irons just 0.6% (the other 9.5% are a legion of achondrites and rare chondrites). Note also the rarity of achondrite recoveries: none in the region, depending on the classification of Mount Morris (Wisconsin), nor in the whole of Canada; and of the reduced enstatite chondrites (just one in Ontario – Blithfield – which being near the Ottawa valley lies far from Superior). Table 1. Meteorites of the Lake Superior region   State / province Ontario Manitoba Minnesota Wisconsin Michigan Irons 8 4 4 7 6 Stonyirons 1 1 0 0 0 Achondrites Chondrites 0 0 0 0 0 7 4 4 7 4 All meteorites 16 9 8 14 10 Impact structures 5 2 0 2 1 The volume of research on the meteorites varies greatly, as will be seen: although all the meteorites are in Grady (2000) and/or the Met.Bull. database, no other specific records were found* for eight of the 32 U.S. meteorites, while 21 of these were described by 1 to 10 citations each, and just three by 20-plus articles. A tabular review of Canadian meteorites is available at http://www.turnstone.ca/canamet4.pdf (update of April 2012, cf. Wilson and McCausland, 2012). * The more exact rendering of “found” is “accrued to the MINLIB database by 13 April 2012”: the above link further explains the methodology and serendipity involved. Note that the MINLIB coverage of Canadian meteorites will be more complete than for those recovered south of the border. - 97 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Ontario The significant meteorite haul in Ontario (n=16; seven irons, one stony-iron, six ordinary chondrites and an enstatite chondrite, plus a 2.7-kg iron named in, and for, Toronto but probably found in Quebec) has been won largely along the populous corridor extending from Ottawa W.S.W. towards Detroit. There are 11 finds and five falls, most recently the Grimsby (2009) fall (Brown et al., 2011). The sole meteorite from northern Ontario is the Osseo iron, recovered in the Temiskaming region. The Manitouwabing iron and Southampton pallasite (Kissin et al., 2012) were recovered east of Georgian Bay and along the eastern shore of Lake Huron, respectively. The only meteoritic calling-cards west of Sudbury, in fact, seem to be the Slate Islands impact structure and the debris layer from the 1850 Ma Sudbury impact, preserved around Thunder Bay (e.g., Cannon and Addison, 2007). References Brown, P., McCausland, P.J.A., Fries, M., Silber, E., Edwards, W.N., Wong, D.K., Weryk,R.J., Fries, J. and Krzeminski, Z. (2011). The fall of the Grimsby meteorite I: Fireball dynamics and orbit from radar, video, and infrasound records. Meteoritics & Planetary Science 46, 339 363. Cannon, W.F. and Addison, W.D. (2007) The Sudbury impact layer in the Lake Superior iron ranges: a time line from the heavens. Abs. 53rd Annual Meeting, Institute on Lake Superior Geology, vol.53 part 1, 89pp., 20 21, Lutsen, MN. Grady, M.M. (2000) Catalogue of Meteorites. Natural History Museum, London / Cambridge University Press, 5th edition, 690pp. plus CD ROM. Kissin, S.A., Macrae, N.D. and Keays, R.R. (2012) Southampton: Canada’s third pallasite. Can.J.Earth Sci., in press. National Geographic Society (2011) Atlas of the World. National Geographic Society, Washington, DC, 9th edition, 137+153pp. Norton, O.R. and Chitwood, L.A. (2008) Field Guide to Meteors and Meteorites. Springer Verlag London Limited, 287pp. Wilson, G.C. and McCausland, P.J.A. (2012) Canadian meteorites: a brief review. Abs. Geol.Assoc. Canada / Mineral. Assoc. Canada, St. John’s, Newfoundland / manuscript submitted to Can.J.Earth Sci. - 98 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 The curious meteorite harvest of the Lake Superior region II - Gems in the rough WILSON, Graham C., Turnstone Geological Services Limited, P.O. Box 1000, Campbellford, ON K0L 1L0 and KISSIN, Stephen A., Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5B From Ontario, we continue a counterclockwise tour of the meteorites closest to Lake Superior … Manitoba Three irons and two chondrites await certification, for an unofficial total of nine finds. The five “candidates” include an H4 chondrite plucked from a road by a grader operator and recognized for its unusual nature, and the informally-named Pinawa and Lone Island Lake irons. These last, plus Bernic Lake, were all retrieved from the southeast corner of the province by a single person, between 2002 and 2005. Minnesota Minnesota has two small (<10 g) meteorites of muddied provenance (like Toronto), an iron and a stone. To be a Minnesota meteorite is to be doomed to obscurity, except for three interesting irons. Redressing the balance of the tiny, misplaced finds is the 22.39-kg Turtle River IIIAB iron, largest find in the state. Best-studied is Arlington, one of the rare IIE irons that may strictly be non-magmatic, arising as melt pools on the surface of their parent body (Wasson and Wang, 1986). The Anoka iron has also been a test case in iron meteorite classification. Minnesota is home to one fall and seven finds. Wisconsin The Trenton iron, a 505-kg find collected in 1858 and subsequent years, remains the largest meteorite known in the five jurisdictions. At least 29 articles refer to it, 1961 onwards, making it, after Allegan, perhaps the bestresearched meteorite in the Upper Great Lakes States. Five falls and nine finds are recorded. The Hammond iron (24 kg) was ploughed up in a cornfield in St. Croix County (a synonym) in 1884. George Kunz etched a piece and noted troilite (FeS) nodules and a Widmanstatten pattern (Fisher, 1887). There is an intriguing possibility of kinship between the Pine River iron with silicate inclusions, and Mount Morris (Wisconsin), two murkilydocumented finds within 15 km of one another. The latter has been interpreted to be the remains of a large silicate inclusion in Pine River, and itself possesses the mineralogy of a winonaite, a rare achondrite (Bevan and Grady, 1988). The most recent meteorite fall in the Superior region was the Mifflin L5 chondrite on the evening of 14 April 2010. Michigan The closest known meteorite visitor to Lake Superior is Iron River, a IVA iron find in 1889, ~75 km south of the head of Keweenaw Bay. The best-researched meteorite in the Upper Great Lake States is probably Allegan (H5 chondrite fall of 1899) with 48 references on file, 1903 onwards (in 1903 just three meteorites were known in the state – the next 109 years have added but 7 more: six finds and four falls are known). Eight are described by Del Chamberlain (1968). The Rose City H5 chondrite is another interesting Michigan meteorite, a highly shocked, black chondrite (23 references known, third-most-studied local meteorite). As an impact melt breccia, with recrystallized olivines, melt veins and masses of metal (Rubin, 1995), it has been compared to the Portales Valley H6 stone (a 1998 fall in New Mexico), and has been exhibited at the Smithsonian and in the Arthur Ross Hall of Meteorites at the A.M.N.H. in New York City. Conclusions - 99 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 The modest meteorite trove of the region is easy to attribute to low population density, predominant lakes and forests, and large distances. Persons in the region are often transient or seasonal (e.g., hunters, loggers, cottagers) cf. farmers. The factors all limit the chances of tracking a fireball and making a recovery, thus finds are predominant (a regional total of 15 falls and 42 finds, cf. India, with at least 10 witnessed falls recovered in 2001-2008 alone: http://www.turnstone.ca/indmet.htm). In general, the best prospects for recovery of a fall are a well-tracked fireball crossing populous areas to land on farmland. It is surely not a coincidence that 9 of the 10 recovered falls in the three states are in Wisconsin and Michigan. At least five of the 32 US meteorites were ploughed up, and a further three or more found in fields. A Wisconsin farmer is more apt to find a meteorite than a logger in Minnesota or the U.P. of Michigan! The best prospects for finds may be discoveries in a zone of glacial stillstand. The three recent iron finds in southeast Manitoba may lie in such a concentration (Hildebrand et al., 2006). The predominance of iron finds in the region may speak to their unique nature, since they are relatively easy for the general public to recognize as something out of the ordinary. References Bevan, A.W.R. and Grady, M.M., 1988. Mount Morris (Wisconsin): a fragment of the IAB iron Pine River? Meteoritics 23, 349-352. Del Chamberlain,V., 1968. Meteorites of Michigan. Michigan Geol.Surv. Bull. 5, 20pp. Fisher, D., 1887). Description of an iron meteorite from St. Croix Co., Wisconsin. Amer.J.Sci. 134, 381-383. Hildebrand, A.R., Beech, M., Kissin, S.A. and Quade, G.B., 2006. A possible meteorite lag deposit after continental glaciation in southeastern Manitoba. Meteoritics & Planetary Science 41, A77. Rubin, A.E., 1995. Fractionation of refractory siderophile elements in metal from the Rose City meteorite. Meteoritics 30, 412 417. Wasson, J.T. and Wang, J., 1986. A nonmagmatic origin of group IIE iron meteorites. Geochim. Cosmochim. Acta 50, 725 732. - 100 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Author Index Albers, P.  Anderson, D.  Asp, K.  Baggetto, L.  Bailes, A.  Bandli, B.  Baumann, S.  Beh, B.  Berkley, J.  Biczok, J.  Bjønerud, M.  Blaske, A.  Boerboom, T.  Bollmann, T.  Bornhorst, T.  Bowring, S.  Braun, G.  Brooker, B.  Buchholz, T.  Burgess, S.  Campbell, D.  Chaffee, M.  Conly, A.  Craddock, J.  Craddock, S.  Cummings, K.  Cundari, R.  Czeck, D.  Darbyshire, F.  Dean, F.  Deen, T.  Deniset, I.  Diehl, J.  Driese, S.  Easton, R.  Eliason-Johnson, G.  Epstein, R.  Falster, A.,  Feinberg, J.  Fralick, P.  Frederiksen, A.  Gagnon, J.  Galley, A.  Gaspar, B.  Gasparotto, M.  Gilbert, H.  Good, D.  Goscinak, C.  58 9 65 47 1 43 3 5 7 91 73 9 11, 49, 60 13, 28, 90 9, 14 84 9 15, 65 17 84 24, 56 19 63 21 21 3 22, 24 88 28 69 58 28 53, 55, 72 60 26, 75 47 33 17 93 5, 41, 51, 54 13, 28, 90 81 1 29 30 31 33, 81 35 Hansen, D.  Hansen, V.  Hattori, K.  Heggie, G.  Heim, N.  Hill, M.  Hollings, P.  Horner, S.  Hoxsie, E.  Hudak, G.  Jirsa, M.  Johnson, J.  Johnson, T.  Jones, S.  Karakus, M.  Kerkemeier, L.  Kern, A.  Kilduff, R.  Kilpatrick, K.  Kissin, S.  Koroscil, J.  Kulakov, E.  Kuzmich, B.  Lappalainen, M.  Lee, A.  Leier-Engelhardt, P.  Leu, A.  Linnen, R.  Lou, X.  MacTavish, A.  Mahr, C.  Ma, L.  Maloof, A.  McCausland, P.  McCorkmick, K.  McLean, K.  Medaris, G.  Mikkelsen, L.  Miller, J.  Mitchell, A.  Monson Geerts, S.  Mottl, R.  Napoli, M.  Paradis, S.  Parisi, A.  Paterson, C.  Pesonen, L.  Petrus, J.  - 101 - 47 35 91 19, 37 39 29, 30, 54, 80, 83 19, 22, 24, 56 41 47 39, 43 45, 47, 60 37 49 77 79, 95 51 53 39 47 99 54 53, 55 56 77 58 9 65 33, 81 13 19, 37 39 37 84 97 62 33 60 63 15, 19, 58, 65 79 43 9 75 1 65 62 67, 72 75 �Proceedings of the 58th ILSG Annual Meeting - Part 1 Phillips, B.  Pietrzak-Renaud, N.  Piispa, E.  Rahtz, C.  Roe, C.  Rousell, D.  Samson, I.  Santaguida, F.  Schneider, D.  Scott, G.  Scott, H.  Scott, J.  Selagi, J.  Severson, Allison  Severson, April  Severson, M.  Shahabi Far, M.  Shannon, J.  Shinkle, D.  Siikaluoma, J.  Simmons, W.  Sletten, D.  Smirnov, A.  Smyk, M.  Stinson, V.  Swanson-Hysell, N.  Taylor, B.  Taylor-Hollings, J.  Thalhamer, E.  Tinkham, D.  Van der Lee, S.  Van Lankvelt, A.  Vaughan, A.  Veikkolainen, T.  Vial, A.  Voipo, T.  Waggoner, T.  Wendlandt, R.  Weston, R.  Williams, W.  Wilson, G.  Ylinen, J.  Young, S.  Zaniewski, K.  Zanko, L.  69 71 72 39 73 75 33, 81 77 91 79 39 24, 56 80 43 43 58 81 49 79 77 17 65 53, 55, 72 22 83 84, 93 1 86 88 75 13, 28, 90 91 93 67 39 77 95 49 37 14 97, 99 77 39 69 43 - 102 - � https://digitalcollections.lakeheadu.ca/files/original/48334c7f548fa7b9dc6dbb56d0f14bba.pdf 89a9b157235d7048264752ca3185ccee PDF Text Text 58th Annual Meeting Institute on Lake Superior Geology Thunder Bay, Ontario - May 16-20, 2012 Part 2 – Field Trip Guidebook �Sponsors The following organizations made generous contributions to the 58th Annual Meeting. We thank them for their commitment to the Institute on Lake Superior Geology. For the past 50 years this organization has thrived as a result of the interest of individuals, corporations, universities and government agencies. The dedication to an exchange of scientific ideas and a passion for field trips has enabled the Institute to provide one of its primary objectives – to promote better understanding of the geology of the Lake Superior Region. �58th Annual Meeting Institute on Lake Superior Geology May 16-20, 2012 Thunder Bay, Ontario HOSTED BY: Pete Hollings Chair Lakehead University Proceedings - Volume 58 Part 2 – Field Trip Guidebook Edited by Pete Hollings, Al MacTavish and Bill Addison Cover photos: Top - Neoarchean conglomerate in the Max Lake area, Hwy 527, Wabigoon Subprovince, Middle - Silver Islet Mine, Lake Superior, Right - Inspiration diabase sills, Chimney Lake near Armstrong (all photos courtesy of Mark Smyk). �58th Institute on Lake Superior Geology Volume 58 consists of: Part 1: Program and Abstracts Part 2: Field Trip Guidebook Trip 1 & 13: Sudbury Impactoclastic Debrisites at Thunder Bay Trip 2: Geology of the Sibley Peninsula Trip 3: Lac des Iles mine Trip 4: Shebandowan Mine Area Trip 5: Geology of the Thunder Bay area Trip 6: Thunder Bay Amethyst Mine Trip 7: building stone tour of Downtown Port Arthur, Thunder Bay Trip 8: Highway 527 Transect Trip 9: Rehabilitation of the Past-Producing Shebandowan and North Coldstream Mine Sites Trip 10: Geoarchaeology of Thunder Bay Trip 11: Midcontinent rift intrusions Trip 12: Musselwhite mine Reference to material in Part 2 should follow the example below: Addison, W., and Brumpton, G., 2012. Field trips 1 & 13 - Sudbury impactoclastic debrisites at Thunder Bay. In; Hollings, P., MacTavish, A. and Addison, W. (Eds.), Institute on Lake Superior Geology Proceedings, 58th Annual Meeting, Thunder Bay, Ontario, Part 2 - Field trip guidebook, v.58, part 2, 2-26. Published by the 58th Institute on Lake Superior Geology and distributed by the ILSG Secretary: Pete Hollings - ILSG Secretary Department of Geology Lakehead University 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada Email: peter.hollings@lakeheadu.ca ILSG website: www.lakesuperiorgeology.org ISSN 1042-9964 �Proceedings of the 58th ILSG Annual Meeting - Part 2 Table of Contents Introduction, safety considerations and acknowledgements................................................1 Field trips 1 & 13 - Sudbury Impactoclastic Debrisites at Thunder Bay . ..........................2 Field trip 2 - Geology of the Sibley Peninsula...................................................................27 Field trip 3 - Lac des Iles Mine . .......................................................................................56 Field trip 4 - Shebandowan Mine Area ............................................................................67 Field trip 5 - Guide to the Thunder Bay area ....................................................................74 Field trip 6 - Thunder Bay Amethyst Mine . .....................................................................82 Field trip 7 - Building stone tour of downtown Port Arthur, Thunder Bay, Ontario ........93 Field trip 8 - A geologic transect across the Western Superior Province and Nipigon Embayment, Thunder Bay to Armstrong, Ontario . ................................................101 Field trip 9 - Rehabilitation of the Past-Producing Shebandowan and North Coldstream Mine Sites ..............................................................................................................136 Field trip 10 - Geoarchaeology of the Thunder Bay area ..............................................150 Field trip 11 - Midcontinent Rift-Related Mafic Intrusions around Thunder Bay...........189 Field trip 12 - The Musselwhite Gold Deposit................................................................208 -i- �Proceedings of the 58th ILSG Annual Meeting - Part 2 Introduction, safety considerations and acknowledgements Pete Hollings Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada This volume is intended to serve not only as a guide for 58th ILSG field trip participants but also as a reference for those planning to revisit these areas at a later date. Consequently we have included UTM coordinates in the NAD 83 datum for the majority of stops, as well as instructions on how to reach them. For some of the stops on private land we have witheld the UTM coordinates to respect the privacy of the property owner. As some of the stops are on private and staked land, please be sure to obtain the land owners’ permission before entering their land. For upto-date information on land ownership please contact the Thunder Bay Resident Geologists’ Office (807 475 1331). Sample collection is prohibited at some stops on private land or in Provincial Parks. Many of the fieldtrips will be visiting stops along either major highways or busy logging roads. Please take care when crossing or parking along these roads. For those field trips that are visiting active mine sites personal protective equipment will be required. Please notify the field trip leaders if you have any medical conditions that may be of concern during the trip. Each trip leader is equipped with a first aid kit and satellite/ cell phone, so please notify them of any incident. We would like to thank all authors who contributed to this field guide and also all those who provided comments and assisted with the running of the field trips themselves. We appreciate the assistance and cooperation of the exploration and mining companies in providing us access and information concerning their properties. We are particularly grateful to the Musselwhite and Lac des Iles mines for running field trips on their properties. Figure 1. Map showing the general locations of field trips for the 2012 meeting. -1- �Proceedings of the 58th ILSG Annual Meeting - Part 2 Field trips 1 & 13 - Sudbury Impactoclastic Debrisites at Thunder Bay Bill (W.D.) Addison and Greg (G.R.) Brumpton Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada Abstract Eight outcrops of chaotic debrisite containing ejecta from the 1850 Ma Sudbury impact event have been identified in and near the city of Thunder Bay, Ontario, 650 km west of the center of the Sudbury crater. Ejecta features include devitrified vesicular impact glass, spherules, accretionary lapilli, microtektites and tektites, and shocked quartz grains containing relict planar features including planar deformation features. The original volume of ejecta has been significantly reduced by carbonate replacement and recrystallization, so that today ejecta only make up ~ 20 % of the debrisite volume. Major debrisite components include ripped up clasts of carbonate grainstones, stromatolites and chert of the 1878 Ma Gunflint Formation. These Gunflint boulder to coarse sand-sized clasts commonly fine upward, in marked contrast to the chaotic nature of the remainder of the debrisite. Seven of the eight sites have had the upper portion of the impact layer removed by glaciation. The eighth site shows a complete stratigraphic section from the Gunflint Formation, up through the ejecta bearing layer, and into the overlying 1832 Ma Rove Formation. The sequence of events deduced from these outcrops is as follows. 1. Mafic volcanic ash was deposited and reworked in a carbonate dominated, near-shore environment that supported microbial mat growth and stromatolites. 2. These areas were then subaerially exposed. 3. Upon impact, earthquakes fractured some stromatolites as well as the underlying Gunflint Formation chert and carbonate. 4. Impact-generated density currents (base surges) stripped the area of loose sediment and incorporated ripped up Gunflint chert-carbonate breccia clasts, before being deposited as a chaotic variable layer. 5. In an ensuing period of subaerial exposure lasting < 18 m.y., blocky, meteoric calcite cements formed in this material while weathering and erosive reworking modified the deposits. 6. The Rove Sea then transgressed the area depositing the overlying Rove Formation carbonaceous shale. Introduction In 2005, Addison et al. documented an ejecta layer formed by an 1850 Ma (Krogh et al., 1984) impact event in cores drilled north of Lake Superior in Ontario and Minnesota. Features in this Sudbury impact layer (SIL) included planar shock features, notably planar deformation features (PDF) in quartz grains; accretionary lapilli; devitrified microtektites and tektites; and devitrified vesicular impact glass (DVIG). The ejecta were linked to the 1850 Ma Sudbury impact by their presence between an 1878 Ma Gunflint Formation tuff (Fralick et al., 2002) approximately 105 metres below the ejecta and tuffs variously dated at 1827 ± 8 Ma, 1832 ± 3 Ma and 1836 ± 5 Ma from the Rove and Virginia Formations 5 to 6 metres above the ejecta (Addison et al., 2005). Sudbury is the only known terrestrial impact from this time interval (Earth Impact Database, 2012) and an oceanic impact would not have produced the quartz- and feldspar-rich, cratonsourced ejecta. The identification of the SIL has led to the discovery of about 30 additional ejecta-bearing drill core and outcrop sites (Fig. 1) in the Lake Superior region (Cannon and Addison, 2007; Pufahl et al., 2007; Jirsa et al., 2008; Cannon et al., 2010). Eight of them are in the northern Gunflint Formation outcrop area in and near Thunder Bay, Ontario (Fig. 1). Locations are supplied for all sites except one in a private yard which is omitted to protect the owner’s privacy. These new sites are 660-680 km from Sudbury. Using the crater radius of 130 km as determined by Spray et al. (2004), these new outcrops are 5.1-5.2r (crater radii) from the Sudbury crater center. The approximate boundary between proximal and distal ejecta is usually given as 5 crater radii (French, 1998) but this boundary is a transition zone, not a sharp line. The ejecta features seen on this field trip are consistently proximal, not distal. This field trip will examine outcrops for macroscopic ejecta and non-ejecta features (Table 1) and relate -2- �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 1. Approximate locations of some Sudbury impact layer (SIL) localities in the Lake Superior region. Concentric lines represent multiples of the final Sudbury impact crater radius of ~130 km as determined by Spray et al. (2004). them to the dynamics of the Sudbury impact, the second largest and fourth oldest impact known on Earth (Earth Impact Database, Jan. 10, 2012). A large impact results in a sequence of events at the impact site which subsequently played out in the Thunder Bay area. The Earth Impacts Effects Program (impact.ese. ic.ac.uk/ImpactEffects/) allows an estimate of the time of delivery and magnitude of events from the impact by inputting: 1) distance from impact (660 km); 2) projectile diameter; 3) projectile density; 4) projectile velocity; 5) impact angle and; 6) target rock type (crystalline – granitic at Sudbury). A velocity of 25 km/s with an impactor diameter of 23 km, along with the other variables noted above, produces a final radius of 134 km, very close to the actual value of Spray et al. (2004). It is interesting that the SIL thickness predicted by the model is not matched by reality at Thunder Bay. For instance, the maximum ejecta thickness seen is ~4 m at Hillcrest Park, where it was once thicker because the exposure top is erosively truncated. The only complete stratigraphic exposure of the SIL is 3.2 m. Likewise, complete SIL in drill cores BP99-2 and PR98-1 from ~35 km south of Thunder Bay do not Table 1. Major effects of a Sudbury-sized impact at Thunder Bay, 660 km from its epicenter, as predicted by the Earth Impacts Effects Program (impact.ese.ic.ac.uk/ImpactEffects/). The field trip outcrops will show the effects of earthquakes and some of the types of ejecta features generated at the impact site. Ambiguous evidence of air blast will be seen at one site. Effects from the fireball have not been identified so far. -3- �Proceedings of the 58th ILSG Annual Meeting - Part 2 exceed 0.8 m in thickness. All of these values are well short of the model’s predicted 12 m thickness which raises questions either about the model or about the SIL’s post-depositional history or both. Terminology and Features Like other branches of geology, the geology of large extraterrestrial impacts has its own rapidly evolving vocabulary which is not widely known in the larger geological community. Therefore terms and specialized impact features are best defined or described and illustrated before seeing and discussing the outcrops on this field trip. Ejecta Ejecta is a collective name for anything thrown out of the crater during the impact. It includes target rock breccia clasts of all sizes from µm- to km-scale. It includes melt which cooled to form glass clasts of various shapes and sizes, most of them now devitrified. It also includes dust and glassy spherules which condensed from rock vapour ejected high into Earth’s atmosphere and even above it. Ballistic Ejecta Curtain Many ejecta components initially travel outward as a curtain on a ballistic trajectory, most of it landing at about 2r from the crater center (French, 1998). There, this massive amount of material lands, severely abrading the landscape and incorporating the abraded material into an outward-rolling debris flow. Debrisite Shanmugam (2006) argues that tsunamite should not be used to describe tsunami deposits because it describes a process and does not deal with clast sizes like conventional sedimentary terminology e.g., sandstone, claystone, etc. He proposes the term “debrite” for tsunami deposits because of their wide variety of clast sizes. By inference, impact related deposits should not be called impactites. We choose debrisite as the best descriptor for these deposits which result from four sets of related energetic events: 1) the impact; 2) impact-induced earthquakes; 3) ejecta traveling in ballistic trajectories and; 4) ground-hugging density currents (called base surges in earlier literature). Thus, the SIL is composed of debris with clasts in the µm to metre size range and of variable origins. Even though all debrisite sites reported here contain ejecta, debrisite is not synonymous with ejecta because ejecta features only comprise about 20 % of debrisite while localized areas of some outcrops seemingly lack ejecta. Ground-hugging Density Currents (Base surges) Much of the early impact literature applied volcanic terminology to impact generated deposits. In fact, a number of the SIL deposits south of Lake Superior were identified by searching the literature for pyroclastic deposits, then checking to see if ejecta features were present (W. F. Cannon, personal communication, 2008). Base surge is one such borrowed term but we use both base surges and the newer impact literature term ground-hugging density currents interchangeably. Matrix (not ejecta) The largest debrisite component by volume is carbonate matrix (Figs. 2A-D), with calcite > dolomite > ankerite. Carbonate has partially replaced most ejecta features (Figs. 3A-D, F) and likely obliterated many more of them. It has also infilled vesicles (Figs. 3A-D). Pervasive carbonate recrystallization, with crystals up to 5 mm maximum dimension, has further destroyed ejecta features (Figs. 3A & F, 2C). There are areas in the matrix comprising angular submillimetre to millimetre carbonate clasts composed of crystals ≤ 20 µm in size (Fig. 2). These clasts may represent Gunflint carbonate ground up in the turbulent events leading to deposition of the SIL but, if so, most show at least minor recrystallization planes. Silica also replaces ejecta features (Fig. 3E). Silica is most visible as anastomosing chert at submillimetrescales in most thin sections as well as centimetre-scale bands in outcrops. Such silica is usually microcrystalline and is clearly a post-depositional feature. Today, the matrix comprises ~80 % of the debrisite volume leaving ejecta features at ~20 %. At the time of deposition the ratio of ejecta to matrix must have been a significant but unknown amount higher, judging from the volume of partially carbonate-replaced ejecta remnants. Stromatolites (not ejecta) Seven of the eight identified outcrop sites show: 1) debrisite lying directly on stromatolites (Fig. 4A) or microbialite mats and/or; 2) broken subcentimetre to decimetre-size stromatolite clasts within the debrisite. The stromatolite or microbialite clasts attest to violent events breaking them up and mixing them into the -4- �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 2. Splashform devitrified vesicular glass (DVIG) clasts in carbonate matrix. A – GTP site; plane polarized light (pp). B – GTP site; crossed polarizers (xp). C – Private yard site (pp). D – Atypical abundance of DVIG fragments, splashform or otherwise. Hwy 588 (pp). debrisite. Subrectangular Blocks of Upper Gunflint ChertCarbonate (not ejecta) Prior to highway reconstruction in 2011 the original Terry Fox exposure showed the upper 0.5 m of Gunflint chert-carbonate bedrock heavily fractured with the subrectangular blocks slightly separated from each other but still basically in situ (Fig. 5B). Subrectangular blocks of Upper Gunflint chertcarbonate, commonly exceeding 0.5 m maximum dimension (Fig. 4B), are found at or near the base of most debrisites (Table 1). They, along with fractured chert clasts of all sizes, show upward fining within the chaotic debrisite. The chert-carbonate blocks usually have sharply angled corners, except at the Private Yard (Fig. 4B), GTP and BB sites where some subrounded blocks exist among angular blocks. None of the blocks show weathering rinds. Anastomosing Silica and Agate with Mini-stalactites (not ejecta) Hillcrest Park, with its 3.5-4 m thick debrisite, shows extensive post-depositional anastomosing chert deposits, some of which include banded agate in localized zones. These chert deposits flow around debrisite clasts but never cut through them. In places, the chert and agate has been deposited in debrisite voids. Near the top of the Hillcrest Park deposit, two vugs contain silica and agate stalactites 1-3 cm long (Fig. 4E). The Banning Bluff and Baseball Central sites and the DVIG-rich, recessively weathering layer at the Terry Fox site also show anastomosing chert but on a smaller scale than Hillcrest Park. Micro-scale anastomosing chert is seen in thin sections from all sites. The agate layer at the top of the TF site shows digitate projections from both the base and top of the layer which may have been miniature stalactites and -5- �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 3. Varying degrees of vesicle deformation in devitrified vesicular impact glass (DVIG) clasts and spherules. A – undeformed vesicles infilled by calcite. Note recrystallized carbonate at top center and right; GTP Site; plane polarized light (pp). B – ovoid vesicles aligned subvertically and infilled with calcite. There is no evidence of lateral compression of the clast, so presumably the vesicles were deformed prior to deposition; GA site (pp). C – calcite-infilled, deformed spherules in cluster; Private Yard (pp). D – fibrous spherule rim, partially destroyed by carbonate replacement; GA site; crossed polarizers (xp). E – collapsed and partially collapsed spherules. Presumably the spherules were hollow prior to collapse. Hwy 588 (xp). F – fractured spherule rim bits and devitrified whole spherules. Note recrystallized carbonate at upper left and lower right; Hwy 588 site (pp). -6- �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 4. Various features of SIL sites. A – Gunflint Formation stromatolites exposed on a glacially truncated surface. While it is not recognizable in the photo, debrisite lies over stromatolites at upper right of the photo. Private Yard site. B – Angular to slightly subangular clast-supported Gunflint Formation breccia with a finer DVIG-rich and calcite-rich matrix, all of which lies directly on Gunflint stromatolites. The angular clasts suggest a short travel distance from their point of origin. C – DVIG clasts within a recrystallized calcite matrix. The silicate devitrification product supports growth of a black lichen, whereas calcite prevents lichen growth. The vesicles are calcite infilled. Private Yard site. D – Orange weathered accretionary lapilli in a recrystallized carbonate matrix. Hillcrest Park. E – Stalactites hanging from top of a vug with agate flowstone deposited on bottom of vug, an indicator of postdepositional subaerial exposure. Hillcrest Park. F – Ocean transgression sequence beginning with an iron-rich alteration profile at the bottom of the photo which marks the top of the debrisite. Above it are rip-ups composed of mudstones or clasts of the iron-rich alteration profile embedded in a carbonate matrix. The boundary between these two units marks a disconformity. The rip-up zone grades into siltstones of the lower Rove Formation at the top of the photo. Original Terry Fox site. Scale is graduated in centimetres. -7- �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 5. A – Earthquake fractured black Gunflint chert. The fractures are thought to have opened during passage of the dilational phase of the earthquake wave, allowing very fine-grained ankeritic sand (light gray) to fill the cracks preventing them from closing back up. Highway 588 site. B – Rectangular to subrectangular earthquake fractured uppermost Gunflint chert-carbonate clasts which delaminated along bedding planes (between dotted lines). These blocks are still more or less in situ, the base surge having failed to rip them up. The more blocky material on top of it is the SIL debrisite. This old Terry Fox site, was removed by highway reconstruction in 2011. stalagmites at one point but, if so, silica deposition continued until they were all encased in a solid agate mass. Unshocked Quartz and Feldspar Grains (some may be ejecta, some are not) Both angular and subrounded detrital quartz and feldspar grains are found in the debrisite at all eight sites, with some thin sections showing as many as 1020 grains per slide. The grains range in size from 40 µm to 800 µm, too small to be seen on this field trip. The angular grains tend to be at the small end of this size range, are shard-like and are likely ejecta. Subrounded grains are probably detrital sand picked up by the base surges flowing across the landscape. Neither grain type is seen in Gunflint Formation rocks. Planar Features in Quartz Grains (ejecta) A few quartz grains show planar features, some of them PDFs as defined by French (1998 and references therein). Nearly all of the planar features have been found within accretionary lapilli at the Hwy 588 site and Hillcrest Park (Figs. 6A-4D). Up to three intersecting sets of PDF are seen in quartz grains, which are typically 50-100 µm in size. A single quartz grain from the Hwy 588 site has planar fractures (Fig. 6D) with their characteristic wide spacing and thick lines (French, 1998). A quartz grain from the Terry Fox upper iron-rich alteration zone shows similar features. The dark lines of PDFs are isotropic quartz glass formed when the high pressure shock waves instantaneously destroy the quartz crystal structure without melting it. They are diagnostic of extremely high pressure shock waves, only obtained in nature by impacts, but they are microscopic and, thus, not seen on this field trip. Spherules (ejecta) Most spherules seen near Thunder Bay are frozen melt droplets ejected during the impact. Spherule sizes range from 50 µm to 1 mm. Except for very rare single spherules, all spherules are clustered, with cluster sizes ranging from 1 mm to 5 cm maximum dimension (Figs. 3E, 5F). Extrapolating from the two dimensions seen in thin section clusters to three dimensions, spherule numbers probably ranged from a few tens of spherules to perhaps 1,000 per cluster. Some spherule clusters contain quartz grains within them. Spherule clusters are the dominant ejecta feature by volume in these debrisites. Despite that, identifiable spherule clusters will not be seen on the field trip. Spherules show prominent rims (Figs. 3D-F). Many rims are round and complete; however, other rims are variously deformed, ranging from slightly ovoid, through ovoid, to totally collapsed where all that remains are two flat rim layers squeezed together -8- �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 6. Planar features in quartz grains within accretionary lapilli. A – two PDF sets; Hillcrest Park; plane polarized light (pp). B – single PDF set; Hwy 588 site; crossed polarizers (xp). C – two PDF sets, with the less distinct set along the right side of the grain; Hillcrest Park (xp). D – planar fractures; Hwy 588 (pp). (Fig. 3E), suggesting that these spherules were hollow. In some cases, a portion of the rim has fractured but remains attached to an otherwise almost intact spherule. Fractured rim pieces are also seen scattered amongst intact spherules, or they are randomly oriented in a cluster, presumably at the site of a former spherule (Fig. 3F). Sometimes the spherules in an entire cluster are collapsed, but in other cases, only a few spherules within a cluster are collapsed. Clusters showing spherules with little or no deformation generally show spherules with 1-3 contact points with adjacent spherules indicating that they have experienced little post depositional compaction (Simonson, 2009). replaced spherules are rimless or else the rims were destroyed during devitrification. Spherule rims range from amorphous features composed of unidentified clay minerals (Fig. 3F) to crystalline silica (Fig. 3E) and calcite rims, and crystalline rims of as yet unidentified minerals. Rims also vary in thickness. Some of the smallest clay- Carbonate replacement of spherules is pervasive. Judging from the remnants, we estimate that >50 % of the spherules have been replaced by recrystallized carbonate, most commonly calcite and less so, dolomite and rarely ankerite. The volume of material Spherules generally show one of two general core types. The first is featureless and composed of as yet unidentified clays. The second core type is crystalline, most commonly calcite, and less commonly silica. The cores are usually centered within the spherule but in some cases they are off-center. The outer boundary of the cores is typically smooth but some cores show botryoidal ingrowths from the core edge towards the center. In other cases carbonate replacement has produced uneven core boundaries. -9- �Proceedings of the 58th ILSG Annual Meeting - Part 2 replaced by carbonate is probably significantly higher than this because portions of thin sections are pure recrystallized carbonate, offering no clue as to what the original material was in those areas. Perhaps the most notable feature of the spherules and spherule clusters is their extensive morphological and compositional variability. Devitrified Glass (ejecta) There are three categories of devitrified glass: 1) devitrified vesicular impact glass (DVIG) clasts and, 2) rare microtektites (<1 mm in size) and tektites (>1 mm in size) and 3) spherules. It will be seen best at the Private Yard site (Table 1.1). DVIG clasts are usually irregularly shaped (Fig. 4C) but splashform (streamlined) shapes are also present (Figs. 2A-C). They range in size from 1-2 mm up to 5 cm. Vesicles in DVIG are usually infilled with carbonate, most commonly calcite. Vesicle shapes range from round (Fig. 3A) to ovoid (Figs. 3B-C). If most vesicles in a clast are ovoid, they show a preferred orientation along their long axes (Figs. 3B, C). Vesicle size, whether within a single clast or between clasts, is also variable (Figs. 3A-C). There are few positively identifiable microtektites and tektites in these deposits but carbonate-replaced microtektite and tektite shapes are more numerous. However, the Gunflint Formation has iron-rich chloritic granules that have many shapes in common with splashform microtektites and are the same size (average 0.8 mm). The two can only be distinguished if some remnant of their internal structure has not been replaced by carbonate. Microtektites and tektites show a blue-gray platy or granular fabric under crossed polarizers, whereas chloritic granules have a blotchy black appearance in plane polarized light. Thus, if a microtektite shape is totally carbonate replaced, there is no way of visually determining whether it was a microtektite or a Gunflint chlorite granule. Accretionary Lapilli (have ejecta and non-ejecta components) Accretionary lapilli consist of fine clasts of accreted target rock, usually quartz, some of which show shock induced planar deformation features (PDFs) and feldspar. The accretionary lapilli form in the base surge from the impact and as such incorporate non-target dust-sized particles picked up by the ground-hugging base surges. Impact-generated base surge dynamics are poorly understood and super-computers are not yet powerful enough to model these complex flows (N. Artemevia, personal communication, 2008). Accretionary lapilli and armored lapilli are found in outcrop only at Hillcrest Park and Hwy 588, and then only within localized areas of the larger exposure at each site. They range in size from 2-13 mm maximum dimension at Hillcrest Park and 5-25 mm maximum dimension at the Hwy 588 site. Lapilli show rounded to subrounded shapes (Figs. 7A-D). Lapilli fragments are present but rare, so they have undergone little breakage. The lapilli range from fairly uniformly gray accreted grains to ones with alternating dark gray, thick bands with thinner bands of very fine black amorphous material (Fig. 7B). The alternating dark gray and black laminations may be repeated up to two times in larger lapilli. The black laminations appear in ~ 35 % of lapilli. By volume, the most common feature in lapilli is 10-50 µm carbonate crystals. Larger carbonate clasts, up to 2.5 mm maximum dimension, form the cores of the few lapilli in which cores are visible. Lapilli show carbonate recrystallization, but where this has occurred, it has enlarged the original crystals only marginally. In contrast, large recrystallized carbonate crystals up to 5 mm in size are abundant in the debrisite outside the lapilli abutting their outer margins, so some unknown factor has inhibited large scale carbonate recrystallization within lapilli. All lapilli also contain 30-250 µm grains of quartz and feldspar, which are found scattered in the gray, coarser-grained areas of the lapilli. Quartz and feldspar grains comprise about 5% of the total lapilli grain population (carbonate grain clusters being the majority). Approximately 1% of the quartz grains exhibit PDFs, making them very difficult to find. The features in the Hillcrest Park lapilli are more poorly defined than those at Hwy 588. The Hillcrest Park lapilli thin sections were prepared from weathered rock, whereas the Hwy 588 lapilli thin sections were prepared from unweathered rock. Geologic Setting The Sudbury impact occurred during a period of tectonic activity along the southern margin of the Superior craton. Prior to the Sudbury event, two interpretations for the area’s geologic setting exist. Kissin and Fralick (1994), Hemming et al. (1995), Van Wyck and Johnson (1997) and Pufahl et al. (2004) see it - 10 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 7. Accretionary lapilli. A – ‘stack of cards’ type accretionary lapilli; Hwy 588 site. Terminology after Schumacher and Schmincke (1991, 1995). B – armored, banded accretionary lapillus. Black band is an extremely fine-grained unidentified black substance. The nucleus is a clast of fine-grained, angular, fractured carbonate; Hwy 588 site. C – accreted material resembling an accretionary lapillus but < 2mm in diametre. Yancey and Guillemette (2008) have called such structures sublapilli, a term which we adopt. Hwy 588 site. D – Accretionary and armored lapilli draped unconformably over a stromatolite, composed of silicified carbonate, which was abraded to its present configuration likely by a base surge immediately preceding the deposition of the lapilli; Hwy 588 site, polished surface. The gray component in all lapilli photos is primarily fine-grained, angular, fractured carbonate clasts whose individual crystals are usually < 10 µm maximum dimension. These clasts are typically < 50 µm in size but they may be as large as 500 µm. Quartz and feldspar grains are a minor component among the carbonate clasts within the lapilli. The black lapilli on the polished surface in D resemble those in A, B and C when seen in thin section. Lapilli from Hillcrest Park are not shown because they are heavily weathered and their features are less distinct. as a backarc basin formed on this margin as extension, possibly subduction roll-back had caused an area of the continental crust to subside and be flooded. An alternative explanation is summarized by Schneider et al. (2002) and Schulz and Cannon (2007) involving successive island arc collisions and the development of a foreland basin which subsided to receive the Gunflint Formation sediments on its northern margin. Pufahl et al. (2010) described the backarc basin evolving into a foreland basin. Chemical sediments (chert, iron oxides and iron carbonates) and volcaniclastics of the Gunflint Formation were deposited onto Archean basement rocks (Gill, 1926; Tanton, 1931; Moorehouse and Goodwin, 1960) in a nearshore marine setting and organized into fining- and coarsening-upwards successions (Fralick and Barrett, 1995) on this open, wave and tide dominated environment (Ojakangas, 1983; Fralick, 1988; Pufahl et al., 2000; Pufahl and Fralick, 2004). Shegelski (1982) interpreted - 11 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 8. A – general stratigraphy of the Gunflint and Rove Formations showing location of the Sudbury Impact Layer (SIL) and the locations of dated zircon. B – more detailed stratigraphy for 10 m above and below SIL. C – composite cartoon of debrisite features from all eight SIL sites. No site shows all features. stromatolites and carbonate at the top of the Gunflint Formation in the Thunder Bay area as a carbonate-rich lagoon environment marking the end of the Gunflint Formation. A depositional hiatus exists between the top of the 1878 Ma (Fralick et al., 2002) Gunflint Formation and the overlying 1832 Ma (Addison et al., 2005) basal Rove Formation, probably caused by the 1860-1835 Ma (Sims et al., 1989) Penokean Orogeny - 12 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 to the south, which resulted in crustal up-warping and withdrawal of the sea (Johnston et al., 2006; Cannon and Schulz, 2009). Alteration of this subaerial Gunflint surface, including development of meteoric calcite cement, silicification, and agate/pyrite veins and vugs, occurred during this time interval (Tanton, 1931; Fralick and Burton, 2008). The SIL lies on this stromatolitic, silicified carbonate surface at the top of the Gunflint Formation (Figs. 8, 9). The SIL is overlain by carbonaceous black shale and grainstone of the Rove Formation which records the end of the Penokean Orogeny and the beginning of crustal relaxation and flooding of the area. The Rove sediment was likely eroded from the Trans-Hudson Orogen to the northwest (Maric and Fralick, 2005; Johnston et al., 2006). Today, the Gunflint and Rove Formations lie on a homocline dipping southeast towards Lake Superior at an average of 5° (Gill, 1926). These rocks remain unmetamorphosed except for localized zones adjacent to diabase sills and dikes (Tanton, 1931). The Debrisite Outcrops Figure 9. Cartoon stratigraphic column as seen at the pre 2010 Terry Fox Lookout rock cut, Hwy 11-17. This is the only complete outcrop exposure extending from the Gunflint Formation, through the Sudbury impact layer and up into the Rove Formation in the Thunder Bay area. Disconformities exist at the Gunflint Formation-sheared debrisite contact and the alteration profile-dolomite contact. The outcrops located since 2005 (Fig. 10) contain ejecta features similar to those seen in the drill cores described by Addison et al. (2005; Fig. 1). However, there are significant variations in the particular ejecta features present from outcrop to outcrop (Table 2) and on a decimetre- to metre-scale within a single outcrop. Only the Terry Fox site (TF) displays a complete Figure 10. Debrisite containing Sudbury impact event ejecta in and near The City of Thunder Bay. Note: one site in a private citizen’s yard is not shown to protect their privacy. Sites 2-6 are either on private property or in city parks and, as such, are “No Hammer” and “No Collecting” zones. - 13 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 GTP abandoned railway rock cut Garden Avenue Quarry area Private yard, Thunder Bay Highway 11-17 at Terry Fox Lookout Hillcrest Park, Thunder Bay yes no yes yes no no no yes yes yes ?? yes yes no no no no ?? no yes yes yes no yes yes yes no no no no no no yes yes yes yes yes yes yes yes no no no no yes no no yes no no no yes no yes yes yes yes yes yes yes yes yes ?? yes yes ?? no ?? no no no no no no no ?? no yes yes no yes yes yes no no no yes no yes ?? no no ?? ?? no yes no yes yes no ?? yes no no yes yes yes yes yes yes yes yes no yes no ?? no no no yes no no no yes yes yes no yes 0.4 0.4 0.5 0.6 ~2 ~2 2.7 ~4 Site Feature Stromatolites or microbialite mats in situ below debrisite base Ripped up stromatolite or microbialite clasts in debrisite Subrectangular Gunflint blocks, >0.5 m maximum dimension in debrisite Fractured black chert or chertcarbonate more or less in situ below ejecta base Angular chert clasts and shards, sub-cm to 3 dm max. dimension in debrisite Post-depositional anastomosing chert in debrisite Alteration profile (possible paleosol) below base of debrisite Devitrified vesicular glass with carbonate in-filled vesicles Accretionary lapilli Microtektites in debrisite thin sections Gunflint Formation iron granules PDF in quartz or feldspar grains/shards Isotropic quartz containing crystallites Subrounded to angular quartz & feldspar grains in debrisite matrix Top of debrisite deposit visible Bottom of debrisite deposit visible Approximate debrisite thickness (m) - 14 - Banning St. bluff below Waverly Towers Baseball Central, Central Ave. Highway 588 ditches Table 2. Each debrisite site has a unique combination of ejecta and non-ejecta features that are summarized in Table1. Please note that “no” has two meanings: 1. the feature may not be present at all at that site and; 2. we have not found the feature but it may be present. For instance, more thin sections might show PDFs where none are currently found. “??” means that the evidence for this feature is weak and ambiguous. Again, more thin sections might resolve the ambiguity. �Proceedings of the 58th ILSG Annual Meeting - Part 2 stratigraphic column extending from the Upper Gunflint Formation, through the debrisite and up into the Rove Formation (Fig. 9). The other seven sites (Fig. 10) are all erosively truncated. Most of these sites are also briefly described in Jirsa et al. (2011). All of the ejecta-bearing debrisites, except the TF site, are seen primarily in plan view and range in area from as little as 10 m2 at the Highway 588 (Hwy 588) site to over 1000 m2 at Baseball Central and Garden Avenue. Most sites also show some portion of themselves in cross-section. Preserved debrisite thickness ranges from 0.4 m at the Hwy 588 and Grand Trunk Pacific Railway (GTP) sites to 3.5-4 m at Hillcrest Park (Table 2). Weathered accretionary lapilli are present and are confined to a localized area comprising <5 % of the total exposure face. The patch of 3-13 mm diameter lapilli (Fig. 7D) is located 1.5-2.5 m above the base of the exposure. Planar features and PDFs are present in quartz grains within accretionary lapilli (Figs. 4A, 4C). Planar features have not been found in quartz grains outside accretionary lapilli within the debrisite matrix. Scattered angular Gunflint chert and chert-carbonate rip-ups range in size from 0.5 m maximum dimension near the base of the deposit to 1-2 mm near the deposit top. One disintegrating heavily weathered round granitic boulder 33 cm across lies at the base of the debrisite. The SIL shows four major components: 1) a matrix of carbonates, commonly dolomite and calcite and least commonly ankerite; 2) Gunflint Formation clasts in the submillimetre to metre size range; 3) ejecta and; 4) minor components of uncertain origin such as subrounded quartz and feldspar grains. The debrisites are chaotic, showing large variations in the percentage of the various components both in surface and crosssectional exposures. Gunflint Formation clasts show upward fining when seen in cross-section thicknesses >1 m, whereas there is little evidence of upward fining in the other components. Carbonate-replaced microtektites may be present based upon size and shape of some features. However, carbonate-replaced Gunflint Formation chlorite granules have sizes and shapes similar to microtektites making it impossible to distinguish between the two if they are totally carbonate replaced as described previously. So far, only one confirmed microtektite has been observed at Hillcrest Park compared to tens of carbonate-replaced microtektite or granule shapes. N.B. The only sites on public land are Highway 588 and Terry Fox on Highway 11-17. Please respect “no hammering” and “no collecting” at all other sites. Spherules appear in clusters in which the spherules are frequently deformed or crushed. Many apparent spherule clusters are heavily altered by carbonate replacement making it difficult to determine whether the feature is carbonate-replaced DVIG or whether they are really spherules. Most ejecta features at Hillcrest Park are poorly preserved because of a combination of carbonate or silica replacement and weathering. Stop 1. Hillcrest Park UTM coordinates: NAD83; 16U 0334728E / 5366952N Hillcrest Park has debrisite exposures on a dip slope with a true thickness of 3.5-4 m, the thickest of all exposures. However, it was once thicker because it lacks a carbonate cap topped by shale, which marks the transition from the debrisite to the Rove Formation seen at the Terry Fox site and in drill cores (Addison et al., 2005). An intermittent, erosively truncated, 5-15 cm thick microbialite layer lying on Upper Gunflint chert-carbonate lies beneath the debrisite. The Hillcrest Park lane cliff face shows four chaotic, undulating, largely ungraded lenses with one lens displaying a prominent U-shaped channel. The lenses become thinner upwards. Each shows a heterogeneous mix of features and a chaotic patchiness at decimetreto metre-scales. Irregularly shaped DVIG clasts up to 2 cm maximum dimension are a common debrisite feature. Some DVIG vesicles are ovoid or totally flattened. This is the best site to view post depositional anastomosing black chert and light gray and black banded agate in the debrisite. The agate usually appears to have been deposited in vugs. Centimetre-sized silica stalactites occur in two vugs (Fig. 4E) near the top of the debrisite exposure. Stop 2. Private Yard (no UTM coordinates to protect owner’s privacy) A bedrock exposure, about 5 m by 15 m, in a private yard in Thunder Bay contains a spectacular debrisite exposure composed mainly of Gunflint chert-carbonate breccia (Fig. 4B) and ejecta, primarily DVIG, which is surrounded and partially replaced by blocky calcite - 15 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 cement (Figs. 4C, 3C, 2C). The debrisite remnant preserved here is 0-0.5 m thick and unconformably overlies stromatolites and chloritic grainstone of the uppermost Gunflint Formation (Fig. 4A). An iron-rich alteration zone exists approximately 30 cm below the erosive contact between the debrisite and the Gunflint bedrock. DVIG clasts are up to 2 cm across. Vesicles range from round to ovoid to nearly flat. Angular quartz and feldspar grains, chert shards, and chloritic granules are also present. Quartz grains with PDFs have not been found here. Stop 3. Banning Street Bluff (BB) UTM coordinates: NAD83; 16U 0335129E / 5367236N A bluff at the north end of Banning Street shows both SIL and Upper Gunflint Formation clast-supported breccia composed of cobble to boulder-size clasts as large as 3-4 m maximum dimension, separated in places by pyritic and carbonaceous black shale similar to the Rove Formation.. There are both subrectangular Gunflint Formation chert-carbonate blocks and manysided, nearly equidimensional chert blocks. As with Gunflint breccia and clasts at other sites, none show weathering rinds. Stromatolite clasts rest upside down and on their sides in the debrisite. The only ejectabearing debrisite lies at the base of the breccia pile, the inverse of the sequence at the GTP site. Other non-ejecta features include millimetre-scale, sharply angular chert fragments. One chert fragment contains chloritic granules similar in shape and size to microtektites. Three ejecta-bearing 26 mm by 45 mm thin sections showed only one subrounded quartz grain. Postdepositional anastomosing chert is present but it is not nearly as common as at Hillcrest Park. There is meager evidence of ejecta in the BB debrisite with DVIG clasts and both crushed and uncrushed spherule clusters being the most obvious ejecta features. A 250 µm clast showed one set of enigmatic planar features in a quartz crystal within it, plus a second crystal with two sets of possible relict PDFs. Microtektites were not observed in the BB thin sections. We interpret this site to be a slide deposit which occurred after the SIL layer was lithified and after transgression by the Rove Sea. Stop 4. Highway 11-17, Terry Fox Lookout (TF) UTM coordinates: NAD83; 16U 340112E / 5372511N The Terry Fox site today was created by Hwy 1117 reconstruction in 2011 (Fig. 11). The old rock face, now removed for fill, was about 40 years old and its weathered surface showed a number of faint features brought out by the weathering (Fig. 12). Had we not had the benefit of the weathered surface we probably would not have been able to interpret the new rock face (Fig. 11). The following description is based on the now-removed outcrop. Given another 3-5 decades, the new face will probably resemble the old one. This is the only outcrop showing a complete ~ 3 m cross-section of the ejecta-bearing debrisite layer extending from Gunflint chert-carbonate up into the basal Rove Formation, which is overlain in turn by a diabase sill (Figs. 9, 10). An iron-rich alteration profile, heavily replaced by secondary pyrite, lies ~ 1 m below the base of the debrisite and a few metres northeast of the main outcrop. The basal SIL is a recessively weathering, locally sheared, clastic layer about 0.5 m thick containing crushed spherule clusters, some of which are aligned subvertically instead of in the usual subhorizontal position. Several sets of subhorizontal slickensides, whose striae are aligned at a 140º azimuth, are found at various levels within this layer. Postdepositional anastomosing chert has replaced much of this basal sheared layer, obliterating considerable structural detail. Non-ejecta features include centimetre to millimetre-sized angular chert clasts and angular, subrounded to round Gunflint Formation iron carbonate clasts plus two rounded crystalline rocks with prominent alteration rinds. The presence of clasts with weathering rinds reinforces the idea that Gunflint clasts lacking such rinds were freshly fractured by impactgenerated earthquakes before being incorporated into the debrisite. The main body of the 2.2 m thick debrisite lies in sharp contact over the basal sheared clastic unit. It is so heavily replaced by recrystallized dolomite that any possible ejecta features are only seen as vaguely outlined shapes on weathered surfaces (Fig. 13) or in thin section. Almost all detail, including any vesicles in possible DVIG shaped clasts, has been destroyed. Tektites and microtektites may be present, based upon shape and rare faint devitrification textures. A single, polycrystalline, rounded quartz grain shows faint planar features. Both angular and rounded millimetre- - 16 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Ocean transgression sequence Pyritic (iron-rich) alteration profile Recessively weathering spherule-rich layer Ejecta-bearing SIL debrisite (2.5 m) SIL basal horizontally sheared zone Earthquake shattered Gunflint chert ccarbonate Figure 11. The current Terry Fox outcrop is a nearly featureless gray carbonate face. Given several decades of weathering, faint features within the debrisite should begin to appear as in Figure 12. White vertical scale is 1 m. Rusty area is weathered rock. scale chert clasts are also present, but not common. A 5-20 cm thick undulating, dark brown, recessively weathering, spherule-cluster-rich layer appears as a groove across the cliff face at the top of the dolomitereplaced debrisite. This mass of spherule clusters is much more concentrated than seen at any other location or than is suggested by faint shapes in the main dolomite-replaced layer immediately beneath it. These concentrated clusters seem to be the residuum of a thicker layer. Plentiful thin anastomosing post depositional chert strands weave through this spherulerich material but on a much finer scale than at Hillcrest Park. Red-brown agate 3-8 cm thick lies on top of the spherule-rich layer. Laterally discontinuous vertical digitate projections extend down from the top and project up from the base of this agate layer. They are similar in shape and size to the agate stalactites in vugs at Hillcrest Park (Fig. 4E), except that in this case the spaces between the projections were subsequently infilled by more agate. The red-brown colour is similar to that of the iron-rich alteration profile overlying it but it is a less saturated hue. An iron-rich alteration profile on top of the spherulerich layer, consisting of hematite has been largely replaced by secondary pyrite. Prominent deformed spherule clusters are locally present. The total thickness of all these ejecta-bearing layers is 3 m. The top of this iron-rich layer marks a return to carbonate deposition. The basal 10-15 cm of this 80100 cm thick carbonate zone is unstratified and shows dark, angular, commonly rectangular, millimetrecentimetre-size rip-up mudstone clasts and probable Gunflint Formations clasts (Fig. 4F). This is followed by millimetre- to centimetre-scale layered carbonate strata topped by a zone with a few poorly defined, - 17 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Diabase Sill Rove shale Ocean transgression sequence Iron-rich alteration profile & recessively weathering spherule layer Ejecta-bearing base surge debrisite (2.2 m) Earthquake shattered Gunflint bedrock-sheared debrisite contact zone Unshattered Gunflint chert carbonate Figure 12. The weathered Terry Fox outcrop on Highway 11-17, Dec. 26, 2010. The weathered surface brought out faint features that had been carbonate replaced, notably devitrified vesicular impact glass (DVIG) shapes and tektites. Pick is 0.9 m tall. laterally discontinuous beds containing centimetrescale, angular carbonate clasts. The carbonate then makes an abrupt transition to 1015 cm of gray siltstone and is overtopped by 10-15 cm of black, rusty weathering shale characteristic of the Rove Formation. The black shale is interrupted by 5 cm of chert before returning to 0.9-1.2 m of black, rusty weathering shale which is overlain in turn by a diabase sill more than 8 m thick. The shale is less friable than typical lower Rove shale, probably the result of low grade metamorphism induced by the overlying sill. Stop 5. Grand Trunk Pacific Railway Rock Cut (GTP) UTM coordinates: NAD83; 16U 0326399E / 5363836N A cut through an outcrop knob on the abandoned GTP right-of-way, approximately 0.5 km east of Mapleward Road and north of Highway 11-17, shows about 4 m composed of Upper Gunflint Formation clast- - 18 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 chloritic granules are abundant at this site but single granules are rare within the secondary carbonate cement. Stop 6. Highway 588 (Hwy 588) UTM coordinates: NAD83; 16U 0307539E / 5357977N Figure 13. Weathered carbonate-rich debrisite surface of pre-2011 Terry Fox rock cut. The rounder light features are likely microtektites or DVIG, however, they are sufficiently indistinct that this is not certain. Many of the more angular features are probably large recrystallized carbonate (likely dolomite) crystals. The reddish tinge is due to a light iron staining. supported, boulder-sized breccia lacking visible ejecta features topped by up to 0.4 m of DVIG-rich debrisite similar to that seen at the private yard. The Gunflint breccia blocks range from well rounded to angular. The largest clast is a rectangular calcite-cemented slab of grainstone 0.4 m thick by 5 m long containing an upside down stromatolite indicating that the slab is overturned. The small amount of matrix between blocks appears similar to the blocks but finer grained. The dip of the Gunflint beds increases westward, suggesting a nearby fault beneath overburden. The east side of the knob had exposures of fractured but in situ, sharp-cornered Gunflint chert-carbonate. The fractures remain closed and did not show any infilling of the ankeritic grainstone seen at Hwy 588. This exposure has now been destroyed by ATV traffic. The overlying ejecta-bearing debrisite is dominated by irregularly shaped and splashform DVIG clasts (Figs. 2A &B). Vesicle shape ranges from round (Fig. 3A) to ovoid (Fig. 3B) to crushed (Figs. 3E &F) depending on the clast examined. All vesicles but the crushed ones are infilled with calcite. A small number of deformed spherule clusters are present. A single 1.0 mm accreted sublapillus (pellet) composed of quartz and feldspar grains, similar to accretionary lapilli, is present but accretionary lapilli (>2 mm diametre) are absent. Rounded to angular sub millimetre quartz grains are present. Gunflint Formation clasts containing blotchy black When first observed in 2000, the Hwy 588 outcrop was a bedrock exposure in the ditch on the northwest side of the highway, 2.4 km southwest of the hamlet of Stanley. It was a glacially polished and striated surface showing erosively truncated stromatolites up to 0.5 m diametre, some of which were surrounded by accretionary lapilli 3-25 mm in diametre (Fig. 7D). Ankeritic grainstone and chloritic grainstone surrounded other stromatolites. This exposure was subsequently blasted to deepen the ditch and the blasted rock now lines the ditch slopes, giving a highly fragmented cross-section and plan view of the exposure. Since then we have exposed bedrock in the ditch about 50 m southwest of the first exposure. It shows a glacially striated surface of exposed stromatolites and shattered, but in situ black chert with an ankeritic grainstone filling in the cracks. The chert is assumed to have fractured during the compressional stage of impact-triggered earthquake waves with the fractures then opening during the dilational wave phase. Fine granular material then fell into the openings, preventing them from closing and subsequently the material was lithified (Fig. 5A). Thin sections prepared from the blasted material show a variety of ejecta features, the most obvious being accretionary lapilli (Figs. 7A-D) which have yielded quartz and feldspar grains showing planar deformation features (PDFs) and planar fractures (Figs. 6B &D). Planar features have not been found in larger subrounded and angular quartz and feldspar grains contained within the debrisite generally as opposed to within accretionary lapilli. This is the only site in which DVIG is not the most obvious ejecta feature within the debrisite. In fact, no DVIG has been observed, however carbonate and silica replaced clusters of spherules are present (Figs. 3E &F). Non-ejecta features include subrounded to round chert grains in carbonate cement, subcentimetre stromatolite fragments and mudstone and shale ripups. Chloritic, blotchy, black Gunflint Formation granules, similar in shape and size to microtektites, are present within the carbonate cement. Carbonatereplaced microtektite shapes are present but since - 19 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 they lack residual internal structure, it is impossible to determine if they were microtektites or carbonatereplaced Gunflint chlorite granules. Optional stops - Garden Avenue Quarry (GA) and Baseball Central (BC). These two outcrops will not be visited on the trip because their features are similar to the outcrops we will visit. However, should you wish to visit them their locations follow. Garden Avenue Quarry, accessed off Hwy 11-17: UTM Coordinates: NAD83; 16U 032695E / 5363209N. eroded lower Rove Formation plus the underlying SIL, leaving topographic depressions, most of which were subsequently post-glacially infilled by lakes, swamps or sediments. Where erosional surfaces have reached the Upper Gunflint Formation chert and chert-carbonate, these more resistant layers have provided a floor to further erosion which persists today. Thus, the few remaining ejecta-bearing debrisite outcrops are small erosional remnants of a once extensive SIL debrisite sheet exposed along the Rove-Gunflint contact. Fractured Gunflint Formation Bedrock and Gunflint Rip-ups These eight small outcrop areas appear anomalous given the large area over which debrisite must have been originally deposited. Initially, we anticipated finding the SIL along the entire exposed length of the contact between the Gunflint and Rove Formations. Not so. In Ontario the Upper Member of the Gunflint Formation is a widespread “very complex unit” with “beds of ferruginous carbonate and chert” (Moorehouse and Goodwin, 1960). The chert ranges from chalcedony to microcrystalline quartz. We hypothesize that this surficial or near-surficial Gunflint chert-carbonate was fractured by the powerful earthquakes which arrived in the study area approximately two minutes after impact (Marcus et al., 2000, Earth Impact Effects Program: http://impact.ese.ic.ac.uk/ImpactEffects/), providing much of the sharply angular chert breccia subsequently embedded in the overlying debrisite (Fig. 8B). However, other sub centimetre chert clasts embedded in the debrisite are quite rounded, yet their appearance is indistinguishable from Gunflint chert. This suggests that such clasts are Gunflint chert and that they underwent extended travel and abrasion within a ground-hugging density current. These detrital clasts may have been produced by conventional erosional processes but this type of detrital material has never been noted elsewhere in the Upper Gunflint Formation. The Gunflint Formation outcrops sporadically from Thunder Bay to the Slate Islands in Lake Superior 165 km east of Thunder Bay (Sage, 1991). Thus, it is possible for small chert clasts to have traveled as much as 150 km in the violent base surge environment, perhaps producing the rounded clasts from ripped up earthquake-fractured chert. The basal 9.8 m of the Rove Formation consists of siltstone and friable shale interspersed with forty-six, 2-15 cm thick, poorly lithified, greenish-gray tuff beds (Maric, 2006). Just a single year of weathering has reduced the tuff beds in exposed drill cores to flaky mush. Two further multi-layered tuffaceous zones occur in the 60 m above this basal zone. Thus, wherever the lower Rove Formation and ejecta-bearing debrisite were exposed, pre-glacial weathering, glacial gouging and post-glacial weathering have removed the easily The Hwy 588 (Fig. 5A) and Terry Fox (Fig. 5B) sites are alone in showing fractured but still in situ Gunflint Formation chert bedrock with ankeritic grainstone deposited in the cracks between the blocks. The depth of this widespread fracturing at the Hwy 588 site is unknown because no visible vertical section is present but, judging from blasted bedrock lining the ditch banks, fractures are estimated to have penetrated as much as 0.5 m based on what is seen in cross-section at the Terry Fox site. The GTP site also shows in situ Baseball Central, accessed off Central Avenue: UTM Coordinates: NAD83; 16U 332333E / 5364300N. Interpretation and Discussion Do All Eight Sites Contain Sudbury Ejecta? Only the Hillcrest Park and Hwy 588 outcrops contain shocked quartz with PDF sets indicating that their debrisites are of impact origin, which leaves the origin of the debrisites at the other six sites open to question. However, a variety of features (Table 1) indicate that all sites share a common origin, namely their common stratigraphic position, their chaotic nature, upward fining Gunflint Formation clasts, DVIG clasts, spherules, and iron-rich alteration profiles, to name the more obvious ones. Therefore, all eight outcrops are attributed to the Sudbury impact event. So Extensive a Deposit, So Few Outcrops - 20 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 fractures outlining rectangular blocks of Gunflint chert-carbonate, but the fractures remain closed and are not infilled. The fractures at these Gunflint sites are smaller in scale than those in Archean granite attributed to Sudbury event earthquakes at Silver Lake, Michigan (3.8r) by Cannon and Schulz (2008). There, Paleoproterozoic sediments were injected into the Archean granite fractures. The shattering of the Hwy 588 chert may well be similar to events at Silver Lake where it is suggested that the dilational phase of seismic waves opened fractures, allowing emplacement of overlying soft sediments into the openings (Cannon and Schulz, 2008). Bedrock fractures in the Barton Creek dolomite at Albion Island, Belize are similarly ascribed to seismic fracturing during the Chicxulub event by Ocampo et al. (1996). There is no evidence of in situ bedrock fracturing at other sites. However, there is ample evidence of fractured Gunflint carbonate and chert-carbonate and stromatolites in the form of rectangular blocks and clasts within the debrisite, at other sites. These blocks suggest that near-surface Gunflint Formation chertcarbonate and carbonate was seismically fractured and delaminated along bedding planes and that some of this fractured material was ripped up by base surges and incorporated into the debrisite. Similarly deposited angular to sub angular clasts are reported from the Chicxulub event in Belize (Kenkmann and Schönian, 2006). The lack of alteration rinds on either rounded or angular Gunflint breccia fragments at any site suggests that they were not derived from pre-impact features such as weathered talus. However, alteration rinds typical of weathered surfaces are present on a rounded granite boulder and on an unidentified crystalline cobble at the base of the debrisite at Hillcrest Park and on two crystalline cobbles in the basal shear zone at the Terry Fox site. The presence of weathering rinds on these non-Gunflint clasts supports the view that Gunflint clasts, all of which lack weathering rinds, were derived from freshly earthquake shattered Gunflint bedrock and subsequently ripped up and incorporated into the debrisite by the base surge. Deposits of Gunflint breccia and lapillistone are found between the Gunflint and Rove Formations at Gunflint Lake, Minnesota, 760 km (5.8r) from Sudbury (Jirsa et al., 2008, 2011). The areal extent of the 7 m thick Gunflint Lake debrisite is much greater than comparable Gunflint breccia zones at the BB and GTP sites at Thunder Bay. It is also nearly twice as thick as any Thunder Bay site. This contradicts the general westward thinning of the SIL (Addison et al., 2005). If the Gunflint Lake deposits were emplaced by base surges, and the base surges had lost sufficient energy by the time they reached the Gunflint Lake area, the entrained debris could have piled up into thick ‘ramparts’ as described for end-of-flow Martian base surge deposits (Kenkmann and Schönian, 2006; Osinski, 2006; Mouginis-Mark and Garbeil, 2007). Basal Shear Zone The slickensides seen at the base of the Terry Fox debrisite are similar to “highly chaotic shear planes often connected with polished and striated surfaces…” described for Chicxulub debris by Kenkmann and Schönian (2006) and Wigforss-Lange et al. (2007), and to “a thin basal shear zone” seen in the Stac Fada Member debrisite in Scotland (Amor et al., 2008). The slickenside striae are aligned at an azimuth of 140º, which supports the idea that the slickensides are related to drag shearing during deposition of the overlying fast moving base surge arriving from Sudbury, which lies at an azimuth of 108°. However, a fault just north of the new entrance to the Terry Fox Welcome Centre may also have produced these locally present slickensides. Peritidal or Subaerial Depositional Environment? It seems intuitive, with the chaotic SIL resting directly on microbialite mats and stromatolites, that it was deposited in peritidal lagoon environments that were subsequently reworked by tsunamis. As observations accumulated we were forced to reexamine this idea. Was the Area Subaerial Prior to Debrisite Deposition? The uppermost 30 m of the Gunflint Formation shows an upward-shoaling succession from thin fine grainstone layers in chemical and clastic mudstone to dominantly grainstone layers to thicker and coarser ripple-laminated and cross-stratified grainstones. These are overlain by the chert-carbonate, stromatolitic limestone, and grainstones, which are erosively terminated by the overlying debrisite. This falling stage sequence suggests that the area was nearly emergent prior to emplacement of the SIL, but it does not show that the area was subaerial at the time of SIL deposition. Moorehouse and Goodwin (1960) noted that the uppermost Gunflint Formation is composed of a thin, calcite-rich unit which they designated the Limestone - 21 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Member. This is the material that directly underlies the debrisite in many locations. It is composed of ironchlorite grainstone and iron-chlorite-rich layers in stromatolites that are cemented by blocky calcite. This calcitic cement is probably meteoric in origin (Fralick and Burton, 2008). This cement shows 100x vanadium and 10x uranium enrichment relative to lower Gunflint Formation background levels, indicative of a redox front in a subaerial environment (Fralick and Burton, 2008). Was the Area Subaerial During SIL Deposition? This is really a question of whether the SIL was deposited by base surges, by tsunamis, or by some combination of the two. Despite the top of the Gunflint Formation being a gently sloping, recently subaerial shallow lagoon environment with an ocean towards the south, the evidence for these being tsunami-emplaced deposits is weak. We see no evidence of tsunami lithologic couplets created by wave runup and backwash (Nishimura and Miyaji, 1995; Scheffers and Kelletat, 2004; Fujino et al., 2006). Nor is there any evidence of sand or other particle injection into the Gunflint substrate, a feature created by very high dynamic pressures of large tsunamis (Le Roux and Vargas, 2005). On the other hand, like the K/P boundary, these deposits are “sedimentologically complex, differing in architecture and composition from place to place” (Smit et al., 1996). However, in the SIL the complexity is very localized compared to the K/P deposits and it lacks the multi-unit stratigraphy described by Smit et al (1996). Thus, these SIL debrisites are different from the K/P deposits which are ascribed to large tsunamis even though both share some common ejecta. The heterogeneous distribution of devitrified glass clasts also argues against tsunami deposition of these deposits. These clasts range from tektites (once solid, non vesicular glass) with a density of about 2200 kg/ m3 to highly vesicular clasts, some of which may have been able to float on water. With this range in density, some clast sorting based on density would be expected during tsunami deposition, with the highly vesicular clasts being preferentially laid down towards the top of the deposit and with a higher proportion of the least vesicular, denser clasts deposited towards the bottom of the debrisite. This should be especially notable in the waning stages of tsunami wave recession. We observe no evidence of this. Tsunamis cannot be totally ruled out for the deposits described here. If such tsunamis were weak and reworked only the topmost freshly deposited debrisite, the record of such could have been erased during postdepositional subaerial exposure (discussed below). But, had a tsunami reworked the top of base surge deposited debrisite, water would presumably have settled into the hot, non-reworked debrisite and we would expect to see fluid or vapour escape pipes. We have seen none, suggesting that even weak tsunamis never reached these deposits. The absence of key tsunami features casts doubt on the SIL debrisite being deposited or reworked by tsunamis. Base surge deposit features are present. The TF site and Hillcrest Park are the only sites with sufficient thickness to see the structures typical of base surge deposits and the TF site is so heavily overprinted by dolomite recrystallization that structures within it are barely visible. U-shaped channels and massive bedding are among the typical volcanic base surge features (Hattson and Alvarez, 1973; Fisher and Schmincke, 1984; Gencalioğlu-Kuşcu et al., 2007; Branney and Brown, 2011). Volcanic base surge beds show distinct upward fining (Fisher and Schmincke, 1984; Dellino et al., 2004). Upward fining in these SIL beds is either absent or, at best, ambiguous, with the notable exception of upward fining of Gunflint clasts in these otherwise chaotic features. Like tsunami deposition, key features of base surge deposits are obscured or missing because of the small scale of the outcrops relative to the scale of the SIL, or the glacial truncation of most outcrops, or, in the case of the TF site, the massive carbonate recrystallization which obscures so much detail. The question of whether these debrisites were deposited by tsunamis or base surges cannot, at this stage, be unequivocally answered, but the evidence obtained to date supports base surge deposition of the SIL at Thunder Bay. Future work comparing features of the Thunder Bay sites to sites elsewhere in the Lake Superior region may help to answer the question more definitively. Did the Area Remain Subaerial After Debrisite Deposition? There is 5-6 m of sediment between the top of the 1850 Ma (Krogh et al., 1984) Sudbury impact event debrisite and dated zircons from three sites in the Rove Formation and one site in the correlative Virginia Formation with an age of 1832 Ma (Addison et al., 2005). This low sedimentation rate for an approximately 18 m.y. period suggests a depositional hiatus. The disconformity at - 22 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 the debrisite-Rove Formation contact seen at the TF site (Fig. 8F) also supports this view. In addition, the silica stalactites at Hillcrest Park (Fig. 8E) could only have been produced in a subaerial environment. There is a sequence of three lithofacies at the top of the TF debrisite that offers important support for a prolonged period of subaerial exposure: 1) The lowermost of the three lithofacies is the recessively weathering, spherule and DVIG-rich layer. It suggests that a significant but unknown thickness of the carbonate component of the debrisite was leached away, leaving behind a winnowed spherule-DVIG residuum. 2) The 2-8 cm thick layer of agate and chert with upward and downward pointing projections within it, which may have been stalactites and stalagmites before the void was totally infilled by agate and chert, seems to be the product of leaching of overlying material and redeposition at this lower level. 3) The 5-30 cm thick iron-rich alteration profile at the top of the TF site may be a paleosol. It also contains spherules, DVIG, and microtektites. This paleosol hypothesis will remain so until further work tests the idea, but it is in the zone where a paleosol would be expected and the concept is consistent with the other interpretations. There was a period of subaerial exposure after deposition of the SIL but its duration is unknown. The great mystery is how any unconsolidated debrisite survived such a long period of subaerial exposure. How Did Any Debrisite Survive? The reasons for debrisite survival are unknown but the Rove Formation suggests a possible mechanism for debrisite preservation. Its lower 9.8 m contains 46 tuff layers, which decrease in frequency from seven layers per metre at the base of the deposit to zero within that thickness (Maric, 2006). The combination of tuffs found in the Gunflint Formation below the SIL, combined with the 46 tightly spaced tuffs immediately above the SIL in the Rove Formation, suggests tuff deposition may have been ongoing during the depositional hiatus. If so, the tuffs may have borne the brunt of weathering during the period of subaerial exposure rather than the debrisite. The tuffs may also have provided silica leachate which was subsequently deposited as the anastomosing chert and agate throughout the middle to upper SIL seen at Hillcrest Park and at BB and BC sites, thus helping to preserve it. Sumary and Conclusions Eight SIL outcrops containing ejecta from the 1850 Ma Sudbury impact event have been identified in and near the city of Thunder Bay, Ontario, north of Lake Superior. The SIL was likely deposited by base surges on a subaerially exposed carbonate succession forming the top of the Gunflint Formation. The primary debrisite component by volume is recrystallized carbonate in which Gunflint chert and chert-carbonate breccia and ejecta are embedded. Today, ejecta are a minor component of the total debrisite volume, however, at the time of deposition, it was undoubtedly greater because carbonate replacement has destroyed many features, while recrystallization of carbonate further obscured features. Ejecta features include shocked quartz grains with relict planar features including PDFs and planar fractures, unshocked quartz and feldspar grains, spherules, DVIG clasts, rare microtektites and tektites and accretionary lapilli. Seven of the eight sites have had some portion of the Sudbury impact layer (SIL) removed by glaciation and subsequent weathering. The eighth site near Terry Fox Lookout shows a complete stratigraphic section from the Gunflint Formation, through the SIL and up into the overlying Rove Formation. Disconformities appear at both the base and top of the SIL. The study area has had a complex history, summarized as follows. 1. The upper Gunflint Formation shows an upward fining sequence ending with mafic volcanic ash being deposited and reworked into a carbonatedominated, nearshore environment supporting microbialite mat growth and stromatolites. 2. Regression of the Gunflint Sea was completed at some unknown time prior to the 1850 Ma Sudbury impact event. Prior to deposition of the SIL, blocky, meteoric calcite cements formed beneath the subaerial surface. 3. Approximately two minutes after the impact, violent earthquakes fractured and delaminated lithified portions of the Upper Gunflint Formation, as evidenced by still in situ fractured rock at the Terry Fox, Hwy 588 and GTP sites. 4. The earthquakes were followed by ground-hugging density currents (base surges) which stripped all unlithified material down to bedrock and ripped up, ground up and entrained some portion of the earthquake-fractured upper Gunflint Formation rock. The base surges then contained the following mixture of features: 1) clasts of fractured carbonate in the fine sand to fine gravel size range; 2) ripped up clasts of Gunflint fractured chert, chertcarbonate and stromatolites; 3) ejecta consisting of - 23 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 DVIG, spherules, accretionary lapilli, tektites and microtektites, and quartz and feldspar grains and shards, some of which show planar features and PDFs; and 4) small clasts of uncertain origin. 5. The sharply angular nature of most Gunflint chert and chert-carbonate clasts indicates a relatively short travel distance. Slightly rounded chert-carbonate clasts are less common and probably traveled only slightly further from their source than the angular ones. None of these clasts show weathering rinds. Some submillimetre- and millimetre-scale chert clasts are well rounded and could have traveled as much as 165 km from the furthest east Gunflint Formation known today at Slate Islands. 6. Accretionary lapilli formed within the base surges. Some accretionary lapilli passed through zones with varying water vapour concentrations, allowing them to accumulate alternating coarser-grained layers and finer-grained layers. Armored lapilli are also present. 7. The debrisites deposited by these base surges show chaotic patchiness with significant changes in clast sizes and composition over metre and even centimetre distances within the deposits. The one exception to this chaos is an upward fining of Gunflint clasts within the otherwise chaotic debrisite. 8. The SIL was subaerially exposed after deposition as evidenced by anastomosing chert and agate within the debrisite, centimetre-scale agate stalactites in debrisite vugs and the tri-level lithofacies of relict winnowed spherule clusters, agate, and iron-rich alteration profile at the top of the TF site. Silica and carbonate replacement and recrystallization probably began during this period of subaerial exposure. An unknown quantity of debrisite was removed during this period of subaerial exposure. 9. Tuffs deposited on top of the debrisite may have provided sufficient protection to allow survival of some of the debrisite. They could also have provided leachate which led to the extensive deposition of anastomosing chert in the SIL seen at both megascopic and microscopic levels. 10. The Rove Sea then transgressed over the area, first depositing about one metre of carbonate and siltstone before the lower Rove Formation organicrich mud began accumulating, intercalated with numerous volcanic ash layers. 11. Compaction and carbonate replacement during diagenesis probably continued destroying features in the debrisite. Silica replacement did the same to a lesser extent. Carbonate recrystallization further destroyed or obscured features. Acknowledgements The following have graciously assisted us and educated us: Bill Cannon, Don Davis, Phil Fralick, Mary Louise Hill, Pete Hollings, Mark Jirsa, Steve Kissin, Paul Knauth, Jon North, Dick Ojakangas, Rick Ruhanen, Klaus Schulz, Mark Smyk, Daniela Vallini, Paul Weiblen, and Laurel Woodruff. Our heartfelt thanks to all of the above and to Lakehead University for giving us access to its labs and instruments. Large portions of this guide have been imported from: Addison et al. (2010). 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Fralick, P.W., 1988, Microbial bioherms, lower Proterozoic Gunflint Formation, Thunder Bay, Ontario in Geldsetzer, H.H.J., James, N.P., and Tebbutt, G.E., (eds.), Reefs - Canada and Adjacent Areas, Canadian Society of Petroleum Geologists Memoir 13, p. 2429. Fralick, P.W., and Barrett, T.J., 1995, Depositional controls on iron formation associations in Canada in Plint, A.G. (ed.), Sedimentary Facies Analysis, International Association of Sedimentologists, Special Publication 22, p. 137-156. Fralick, P., Davis, D.W. and Kissin, S.A., 2002, The age of the Gunflint Formation, Ontario Canada: single zircon U-Pb age determinations from reworked volcanic ash. Canadian Journal of Earth Science, 39: 1085-1091. Fralick, P.W., and Burton, J., 2008, Geochemistry of the Paleoproterozoic Gunflint Formation carbonate: Implications for early hydrosphere-atmosphere evolution, Geochimica et Cosmochimica Acta, Special Supplement, v. 72, no.125, p. A280. French, B.M., 1998, Traces of Catastrophe: Lunar and Planetary Institute Contribution 954, 120 p. Fujino, S., Masuda, F., Tagomori, S., and Matsumoto, D., 2006, Structure and depositional processes of a gravelly tsunami deposit in a shallow marine setting: Lower Cretaceous Miyako Group, Japan: Sedimentary Geology, v. 187, p. 127-138 Gencalioğlu-Kuşcu, G., Atilla, C., Cas, R.A.F.., and Kuşcu, I., 2007, Base surge deposits, eruption history, and depositional processes of a wet phreatomagmatic volcano in Central Anatolia (Cora Maar): Journal of Volcanology and Geothermal Research, v.159, p. 198-209. Hemming, S.R., McLennan, S.M., and Hanson, G.M., 1995, Geochemical and Nd/Pb isotopic evidence for the provenance of the Early Proterozoic Virginia Formation, Minnesota: Implications for tectonic setting of the Animikie Basin, Journal of Geology, v. 103, p. 147-168. Jirsa, M.A., Weiblen, P.W., Vislova, T., and McSwiggen, P.L., 2008, Sudbury impactite layer near Gunflint Lake, NE Minnesota: Institute on Lake Superior Geology, Proceedings, v. 54, Part 1, p. 42-43. Jirsa, M.A., Fralick, P.W., Weiblen, P.W., and Anderson, J.L.B., 2011, The Sudbury impact layer in the western Lake Superior region, in Miller, J.D., Hudak, G.J., Wittkop, C., and McLaughlin, P.I., eds., Archean to Anthropocene: Field Guides to the Geology of the Mid-Continent of North America: Geological Society of America Field Guide 24, p. 147-169. Johnston, D.T., Poulton, S.W., Fralick, P.W., Wing, B.A., Canfield, D.E., and Farquhar, J., 2006, Evolution of the oceanic sulfur cycle at the end of the Paleoproterozoic. Geochimica et Cosmochimica Acta, v. 70, p. 5723-5739. Kenkmann, T., and Schönian, F., 2006, Ries and Chicxulub: Impact craters on Earth provide insights for Martian ejecta blankets: Meteoritics & Planetary Science, v. 41, p.1587-1603. Kissin, S.A., and Fralick, P.W., 1994, Early Proterozoic volcanics of the Animikie Group, Ontario and Michigan, and their tectonic significance, Institute on Lake Superior Geology, Proceedings, v. 40, Part 1, p. 18-19. Krogh, T.E., Davis D.W., and Corfu F. 1984, Precise U-Pb zircon and Baddeleyite ages for the Sudbury area, in The Geology and Ore Deposits of the Sudbury Structure, Ontario Geological Survey Special Volume 1, p. 431-446. Le Roux, J.P., and Vargas, G., 2005, Hydraulic behavior of tsunami backflows: insights from their modern and ancient deposits: Environmental Geology, v. 49, p.65-75. Maric, M., and Fralick, P.W., 2005, Sedimentology of the Rove and Virginia Formations and their tectonic significance, Institute on Lake Superior Geology, Proceedings, v. 51, Part 1, p. 41-42. Maric, M., 2006, Sedimentology and sequence stratigraphy of the Paleoproterozoic Rove and Virginia Formations, southwest Superior Province, M. Sc. Thesis, Lakehead University, v + 111 p., 2 CDs. Gill, J.E., 1926, Gunflint iron-bearing Formation, Ontario, Canada Department of Mines, Geological Survey, Summary Report, 1924, Part C, p. 28c-88c. Marcus, R., Melosh, H.J., and Collins, G., 2000, Earth Impact Effects Program: A Web-based computer program for calculating the regional environmental consequences of a meteoroid impact on Earth: http:// impact.ese.ic.ac.uk/ImpactEffects/ Hattson, P.H., and Alvarez, W., 1973, Base surge deposits in Pleistocene volcanic ash near Rome: Bulletin of Volcanology, v. 37, p. 553-572. Moorehouse, W.W., and Goodwin, A.M., 1960, Gunflint Iron Range in the vicinity of Port Arthur and Gunflint Iron Formation of the Whitefish Lake area: Ontario - 25 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Department of Mines, v. LXIX, Part 7, iv + 67 p., 8 maps. Mouginis-Mark, P.J., and Garbeil, H., 2007, Crater geometry and ejecta thickness of the Martian impact crater Tooting: Meteoritics & Planetary Science, v. 42, p. 1615-1625. Nishimura, Y., and Miyaji, N., 1995, Tsunami deposits from the 1993 Southwest Hokkaido earthquake and the 1640 Hokkaido Komagatake eruption, northern Japan: Pure and Applied Geophysics, v. 144, p.719733. Ojakangas, R.W., 1983, Tidal deposits in the Early Proterozoic basin of the Lake Superior region – the Palms and Pokegama Formations: Evidence for subtidal-shelf deposition of superior-type banded-iron formation in Medaris, L.G. (ed.), Early Proterozoic Geology of the Great Lakes Region, Geological Society of America Memoir 160, p. 49-66. Ocampo, A.C., Pope, K.O., and Fischer, A.G., 1996, Ejecta blanket deposits of the Chicxulub crater from Albion Island, Belize, in Ryder, G., Fastovsky, D., and Gartner, S., eds., The Cretaceous-Tertiary Event and Other Catastrophes in Earth History: Geological Society of America Special Paper 307, p. 75-88. Range Supergroup: implications for tectonic setting of the Paleoproterozoic iron formations of the Lake Superior region: Canadian Journal of Earth Science, v. 39, p. 999-1012. Schulz, K.J., and Cannon, W.F., 2007, The Penokean orogeny in the Lake Superior region: Precambrian Research, v. 157, p. 4-25 Schumacher, R., and Schmincke, H.-U., 1991, Internal structure and occurrence of accretionary lapilli – a case study at Lacher See volcano: Bulletin of Volcanology, v. 53, p. 612-634. Schumacher, R., and Schmincke, H.-U., 1995, Models for the origin of accretionary lapilli: Bulletin of Volcanology, v. 56, p. 626-639. Shanmugam, G., 2006, The tsunamite problem: Journal of Sedimentary Research, v. 76(5), p. 718-730. Shegelski, R.J., 1982, The Gunflint Formation in the Thunder Bay area, in Franklin, J.M., ed., Field Trip Guidebook 4: Geological Association of Canada, p. 14-31. Simms, M.J., 2003, Uniquely extensive seismite from the latest Triassic of the United Kingdom: Evidence for bolide impact?: Geology v. 31, p. 557-560. Osinski, G., 2006, Effect of volatiles and target lithology on the generation and emplacement of impact crater fill and ejecta deposits on Mars: Meteoritics & Planetary Science, v. 41, p. 1571-1586. Simonson, B.M., Sumner, D.Y., Beukes, N.J., Johnson, S., and Gutzmer, J., 2009, Correlating multiple Neoarchean-Paleoproterozoic impact spherule layers between South Africa and Western Australia, Precambrian Research, v. 169, p. 100-111. Pufahl, P.K., Fralick, P.W., 2000, Depositional environments of the Paleoproterozoic Gunflint Formation in Fralick, P.W. (ed.), Institute on Lake Superior Geology, Proceedings, v. 46, Part 2 – Field Trip Guidebook, 45 pp. Smit, J., et al., 1996, Coarse-grained, clastic sandstone complex at the K/T boundary around the Gulf of Mexico: Deposition by tsunami waves induced by the Chicxulub impact?: Geological Society of America Special Paper 307, p. 151-182. Pufahl, P.K., and Fralick, P.W., 2004, Depositional controls on Paleoproterozoic iron formation accumulation, Gogebic Range, Lake Superior region, USA: Sedimentology, v. 51, p.791-808. Spray, J.G., Butler, H.R., and Thompson, L.M., 2004, Tectonic influences on the morphometry of the Sudbury impact structure: Implications for terrestrial cratering and modeling: Meteoritics & Planetary Science, v. 39, p. 287-301. Pufahl, P.K., Hiatt, E.E., Stanley, C.R., Morrow, J.R., Nelson, G.J., and Edwards, C.T., 2007, Physical and chemical evidence of the 1850 Ma Sudbury impact event in the Baraga Group, Michigan: Geology, v. 35, p. 827–830. Pufahl, P.K., Hiatt, E.E., and Kyser, T.K., 2010, Does the Paleoproterozoic Animikie Basin record the sulfidic ocean transition?: Geology, v. 38, p. 659-662. Sage, R.P., 1991, Slate Islands: Ontario Ministry of Northern Development and Mines, Ontario Geological Survey Report 264: xi + 111 p., 1 map. Scheffers, A., and Kelletat, D., 2004, Bimodal tsunami deposits – a neglected feature in paleo-tsunami research in Schernewski, G., and Dolch, T., eds., Geographie der Meere und Küsten, Coastline Reports 1: p. 67-75. Schneider, D.A., Bickford, M.E., Cannon, W.F., Schulz, K.J., and Hamilton, M.A., 2002, Age of volcanic rocks and syndepositional iron formations, Marquette Tanton, T. L., 1931, Fort William and Port Arthur, and Thunder Cape map-areas, Thunder Bay District, Ontario: Geological Survey of Canada Memoir 167, 222 p. Van Wyck, N., and Johnson, C.M., 1997, Common lead, Sm-Nd, and U-Pb constraints on petrogenesis, crustal architecture, and tectonic setting of the Penokean orogeny (Paleoproterozoic) in Wisconsin, Geological Society of America Bulletin, v. 109, p. 799-808. Wigforss-Lange, J., Vajda, V., and Ocampo, A., 2007, Trace element concentrations in the Mexico-Belize ejecta layer: A link between the Chicxulub impact and the global Cretaceous-Paleogene boundary: Meteoritics & Planetary Science., v. 42, p. 1871-1882. Yancey, T. E., and Guillemette, R. N., 2008, Carbonate accretionary lapilli in distal deposits of the Chicxulub impact event: GSA Bulletin, v. 120, p. 1105-1118. - 26 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Field trip 2 - Geology of the Sibley Peninsula Philip Fralick Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada Mark Smyk Resident Geologist Program, Ontario Geological Survey, Ministry of Northern Development and Mines, Thunder Bay, Ontario, P7E 6S7, Canada Riku Metsaranta Ontario Geological Survey, Precambrian Geoscience Section, Sudbury, Ontario, Canada Introduction The Sibley Peninsula extends into Lake Superior, approximately 25 km east of the City of Thunder Bay (Fig. 1). The peninsula, approximately 52 km long and 10 km wide, separates Thunder Bay (the bay of Lake Superior, not the city) on the west from Black Bay on the east. It can be divided into two physiographic units, based largely on bedrock geology. Highlands or tablelands underlain by Mesoproterozoic Midcontinent Rift-related mafic sills intruding the Rove Formation and/or Sibley Group sandstones dominate the area west of Highway 587, rising as much as 380 m above Lake Superior at the Sleeping Giant. East of the highway, flat-lying Sibley Group siltstones and calcareous sedimentary rocks result in a relatively subdued topography. This field trip will examine a number of Paleoand Mesoproterozoic sedimentary and igneous units from the base of the Sibley Peninsula to its tip (c.f., Franklin et al., 1982; Fralick et al., 2000). Most of the stops are road-cuts so caution must be exercised. Do not stand on the paved portion of the road and be aware of vehicular traffic at all times. Sample collecting is not allowed without a collecting permit in Sleeping Giant Provincial Park. Archean basement below the Sibley Peninsula consists of metavolcanic and intrusive rocks of the Sibley Peninsula Figure 1. Regional geology east of Thunder Bay including the Sibley Peninsula south of Pass Lake. - 27 -   �Proceedings of the 58th ILSG Annual Meeting - Part 2 Wawa-Abitibi Subprovince. These are nonconformably overlain by the chemical-clastic sedimentary units of the 1878+-1 Ma Gunflint Formation (Fralick et al., 2002). The Animikie Basin, in which the Gunflint Formation was deposited, developed due to backarc spreading (Fralick et al., 2002) on the southern margin of Superior Province and forms a southwardthickening wedge sedimented on the shelf during transgressive-regressive-transgressive cycles (Fralick and Barrett, 1995; Pufahl, 1996; Pufahl and Fralick, 2000, 2004). The 1850 Ma (Krogh et al., 1984) Sudbury ejecta layer occurs near, or in places at, the top of the Gunflint Formation. These units will not be seen on this trip, as they form the subcrop. The 1835 Ma Rove Formation (Addison et al., 2005) disconformably overlies the Gunflint Formation. It consists of a lower siltstone dominated unit meters to 10 meters thick. This is overlain by approximately 100 meters of black shale representing a starved succession. The upper eight hundred meters of the formation is dominated by turbiditic, progradational parasequences outbuilding from distal deltaic bars to the north-northwest represented by lenticular to flaser bedded sandstones at the top of the preserved succession (Maric and Fralick, 2005; Maric, 2006). Sediment was probably derived from the Trans-Hudson Orogeny that was underway to the northwest. The Sibley Group (Fig. 2) lies disconformably on the Rove Formation, with the directly underlying shales showing the effects of Mesoproterozoic weathering. The age of the Sibley Group is poorly constrained. The best estimate is obtained from its polar wander position, which is the same as the 1450 to 1400 Ma Belt Supergroup (Elston et al., 1993, 2002; Evans et al., 2000). The Belt, which was deposited on the western side of North America, also records the same climatic fluctuations as the Sibley (Rogala et al., 2007). The Sibley basin originally developed as a down-sag accumulating conglomerates and sandstones filling topographic lows and then expanding to cover the area with sheet sandstones. The main sediment source was from the northwest, with Paleoproterozoic zircons dominating this population (Rogala et al., 2007). This indicates a probable source from the eroding TransHudson highlands. Lacustrine conditions developed throughout the area, with the lake becoming more saline with time. As the lake shrank, strand-line stromatolitic dolostone was deposited with a sub-aerial weathered upper surface and terra rosa (soil) development in places. Next a period of basin instability occurs as it down-tilts to the north, the basement collapses into   Figure 2. Stratigraphy of the Mesoproterozoic Sibley Group (From Rogala et al., 2007). a half-graben and within the basin, sub-aerial massflows are generated (Rogala, 2003; Rogala et al., 2005, 2007). Sediment feed is now from the southwest, comprising considerable Mesoproterozoic zircons. This indicates that the Penokean area was not a barrier to sediment transport from the south. The above is the highest stratigraphic unit we will see on this trip. The descriptions below include overlying units. The general geology of the Sibley Peninsula was most recently summarized by Carl (2011): - 28 - Gunflint Formation Gunflint Formation sedimentary rocks make up a chemical-clastic assemblage whose upper portion was deposited 1878Ma (Fralick et al., 2002). This formation crops out close to the northern limits of the Sibley peninsula near Pass Lake, Ontario. At this site, rare folding is present in Gunflint sedimentary layers. This �Proceedings of the 58th ILSG Annual Meeting - Part 2 folding is thought to be related to fold-andthrust belt deformation caused by Penokean compression (Hill and Smyk, 2005). Despite the presence of these compression-related folds, most Animikie Group sedimentary rocks in Ontario are undeformed (Sutcliffe, 1991). Animikie Group sedimentary rocks in Ontario have been classified as having a sub-greenschist metamorphic grade (Easton, 2000) and are frequently considered to be unmetamorphosed for convenience of interpreting depositional environments. northeastern shores of the Sibley Peninsula due to the southeastern dip direction of these rocks. On the southern tip of the Sibley peninsula, south of Perry Bay and Sawyer Bay, the Rove Formation is well represented. Here, sedimentary layers, consisting mostly of black shales, are present beneath the Sleeping Giant landform at elevations up to 369m. The occurrence of Rove Formation sedimentary rocks at such high elevations, when the south-easterly dip of this unit should cause it to be beneath the surface on the peninsula’s southern tip, can be explained by the Silver Islet fault described by William Logan as a transverse dislocation that lets down the succeeding formation by several hundred feet (Logan, 1847). This fault displaced Animikie Group sedimentary rocks as well as other assemblages and played a key role in the genesis of ore at the historic Silver Islet mine site (Horton, 1989). Eventually, the long-lived Animikie Basin closed and deposition of Rove Formation sedimentary rocks ceased. This resulted in a gap in the rock record on the Sibley Peninsula and allowed for erosion of Animikie Group sedimentary rocks. Rove Formation The Gunflint Formation was once thought to transition conformably into the overlying Rove Formation; however, a disconformity has been recognized between these two assemblages (Schulz and Cannon, 2007). The idea that the Gunflint and Rove Formations are discontinuous first became apparent when it was proposed that the Sudbury impact which occurred 1850Ma was a subaerial occurrence (Addison et al., 2005) in the Thunder Bay area (P. Fralick, pers. comm., 2011). This suggests there was a period during which no deposition was occurring that was coeval with the Sudbury impact (Schulz and Cannon, 2007). Using a volcanic ash layer near the base of the Rove Formation, Addison et al. (2005) determined an age of 1836Ma for basal Rove shales. This indicates deposition in the Animikie basin had resumed, and water was again present in this basin at 1836Ma. In the vicinity of the Sibley Peninsula, the Rove Formation has a shallow southeasterly dip (Horton, 1989) and a thickness greater than ~610m based on a diamond drill hole at Sibley Bay (Geul, 1973). Due to erosion, only the lower part of the Rove Formation can be found on the southern portion of the Sibley Peninsula. This basal portion of the Rove Formation consists mostly of black shales with interbeds of siltstone (Maric and Fralick, 2005). Sibley Group Sedimentary Rocks Large-scale subsidence following doming related to the intrusion of the approximately 1540Ma Mesoproterozoic English Bay Complex (Hollings et al., 2004) created an ovoid depression known as the Sibley Basin (Rogala, 2003). Located to the north of the Sibley Peninsula, the English Bay Complex is a granite-rhyolite assemblage (Hollings et al., 2004) with an age of 1537Ma (Davis and Sutcliffe, 1985). The formation of the Sibley Basin post dates igneous activity of the English Bay Complex, with deposition in this basin probably beginning slightly before 1500 Ma (Rogala, 2003). Sediments deposited in the Sibley Basin comprise the Sibley Group, a relatively flat, unmetamorphosed assemblage divided into five distinct formations (Franklin et al., 1980). The lowest three formations of the Sibley Group are found in abundance throughout Sleeping Giant Provincial Park and make up the majority of the surficial geology of the Sibley Peninsula. Rove Formation shales sporadically outcrop on the western shoreline of the peninsula from Sawyer Bay northwards and are never more than a few metres above the 183m elevation of Lake Superior. Also on the peninsula’s western shoreline, well-preserved Rove shale concretions can be seen next to the Kabeyun Trail between Clavet Bay and Hoorigan Bay. Similar Rove Formation sedimentary rocks are lacking on the Pass Lake Formation The oldest Sibley Group formation is the Pass Lake Formation which itself consists of two members: the Loon Lake Member and the Fork Bay Member (Cheadle, 1986). The Loon - 29 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Lake member is the lowermost assemblage of the Sibley Group and consists of conglomerate lenses that were deposited in depressions caused by erosion of the underlying rock (Franklin et al., 1980). This unit can be seen on the Sibley Peninsula directly adjacent to Pass Lake where it is in contact with the overlying sandstones of the Fork Bay Member. Cliff faces adjacent to Pass Lake are dominated by Fork Bay Member plane bedded sandstones which are also commonly seen in cliff faces close to the western shores of the Sibley Peninsula. In general, Fork Bay Member sedimentary rocks comprise sandstones which can be massive and well sorted, massive and poorly sorted, silty, laminated or rippled (Rogala et al., 2005) These sandstones are thought to have been deposited in a shallow, quiet, lacustrine environment (Franklin et al., 1980). evaporites has been interpreted to represent a clastic sabkha environment (Fralick et al., 2000). The dolomitic mudstones are at times overlain by sporadic stromatolitic chert-carbonate lithofacies which are indicative of shallow water, near-shore environments. These laterally discontinuous stromatolitic facies are part of the Middlebrun Bay Member and may represent migrating shorelines of partially restricted bays (Rogala, 2003). Above the Middlebrun Bay Member is the Fire Hill Member which is the uppermost member of the Rossport Formation. The Fire Hill Member can be extremely difficult to distinguish from the Channel Island Member (Rogala et al., 2005). For this reason, the previously characterized member assemblages (Cheadle, 1986) are now described as lithofacies associations (Rogala et al., 2005). The uppermost of these associations are primarily composed of a variety of conglomerates, sandstones, siltstones and mudstones. These uppermost sedimentary rocks were deposited onto a mudflat, with coarser, unsorted sediments and mud-chip conglomerates representing debris flows and slumping events (Rogala, 2003). Siltstones can be found at the top of the Rossport Formation which grade into the overlying Kama Hill Formation. Rossport Formation Fork Bay Member sandstones are overlain by the Rossport Formation, which consists of three members and ten facies associations. The Channel Island Member is the lowermost assemblage of the Rossport Formation and consists of dolomitic mudstones, which gradually increase in abundance as the Pass Lake Formation transitions into the Rossport Formation (Rogala, 2003). Many of the ten facies associations of the Rossport Formation contain siltstones and carbonates which provide clues to the types of environments that once existed when many rocks of the Sibley Peninsula formed. A unique cyclic siltstone-dolomite lithofacies association is present in the Rossport Formation, and has been interpreted to represent a shallow offshore environment where muds were deposited during wet periods and dolomite precipitated during periods of drought (Fralick et al., 2000). This suggests that the lake(s) in which sands of the Pass Lake Formation were deposited became progressively more saline and ultimately formed playa lakes (Rogala et al., 2005). This interpretation appears to be consistent with unpublished paleomagnetic results of G. Borradaile, which suggest at the time of sediment deposition, the Sibley Basin was near the Earth’s equator where arid conditions would have dominated (Fralick et al., 2000). Directly above the cyclic siltstone-dolomite layers, dolomitic mudstones containing mud cracks and gypsum nodules are present. The occurrence of these Kama Hill Formation The Kama Hill Formation is divided into four lithofacies associations. The first three consist of fine-grained sandstones and siltstones that are horizontally laminated, mud-cracked and rippled (Rogala et al., 2005). Horizontally laminated mudstones often cap these finegrained sandstone and siltstone units. These four lithofacies associations are thought to represent a floodplain system which periodically contained ponds. Thicker units of fine-grained sedimentary rocks which sometimes contain wave ripples, likely indicate the presence of long-lasting ponds (Rogala, 2003). Flooding events which covered the floodplains probably increased in occurrence until the formation of the subaqueous Outan Island Formation (Rogala et al., 2005). Outan Island and Nipigon Bay Formations Although not present on the Sibley Peninsula, the Outan Island and Nipigon Bay Formations represent important final stages in the depositional history of the Sibley Basin. The Outan Island Formation consists of mudstone, laminated - 30 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 streams), pebble to cobble conglomerate (ephemeral braided streams), trough crossstratified sandstone (ephemeral braided streams), massive cobble conglomerate (transgressive lag, reworking of braided stream deposits during lacustrine transgression), green sandstonesiltstone (wave and storm influenced fluvial dominated deltas), planar cross-stratified sandstone (nearshore migration of large sandwaves), and thinning upward sandstone (beach and storm remobilized nearshore sandstone sheets). sandstone/mudstone, siltstone, sandstone and conglomerate lithofacies associations (Rogala, 2003). The lower part of the Outan Island Formation has been interpreted as representing a deltaic environment, whereas its upper part represents a fluvial environment (Rogala, 2005). As described by Rogala (2003), the Nipigon Bay Formation is the uppermost formation in the Sibley Group that consists of a crossstratified sandstone lithofacies association and a horizontally laminated sandstone lithofacies association. These associations are thought to represent an ancient aeolian environment. This interpretation is supported by the high degree of sediment sorting seen in sandstones, as well as the presence of large-scale dune topography. The Nipigon Bay Formation was likely subjected to a semiarid to arid climatic regime and probably resembled a modern day desert. 3. The mixed siliciclastic-carbonate unit disconformably to conformably overlies the lower clastic unit and consists of the following lithofacies associations: red siltstone (nonsaline lake), red siltstone-dolostone (perennial saline lake, distal from clastic sources) and red siltstone-dolomitic sandstone (perennial saline lake, proximal to clastic sources). The Sibley Group has a minimum depositional age of 1339 Ma based on an Rb-Sr isochron constructed by Franklin (1978). This age is presently the youngest depositional age determined for the Sibley Group, with final deposition in the Sibley Basin probably occurring shortly after this date. The final formation of Sibley Group sedimentary rocks signaled the beginning of a roughly 200 million year hiatus in rock formation on the Sibley Peninsula. An expanded description of the lower Sibley Group was presented by Metsaranta (2006) in his M.Sc. thesis on the sedimentology, geochemistry and paleohydrology of these rock units: Based on the analysis of lithofacies associations and stratigraphy, the following conclusions can be made: 4. The upper clastic unit sharply overlies the mixed siliciclastic carbonate unit and consists of the sheet sandstone lithofacies association (ephemeral playa lake (?) or perennial lake with increased clastic supply with respect to underlying units), and the black chert-carbonate lithofacies association (shoreline). Subaerial exposure features are present at the top of the black-chert-carbonate lithofacies association and include the intraformational conglomerate lithofacies association (subaerial debris flows, intrusive and/or extrusive sedimentary breccias, terra rossa style soils, dissolution collapse breccias). 5. The mixed siliciclastic-carbonate-evaporite unit overlies the subaerial exposure surface at the top of the upper clastic unit. It consists of the massive dolostone (saline lake), the red siltstonesulfate (wet evaporate-rich mudflats around lake margins) and the fine-grained sandstone (dry, evaporate poor mud and sand flats around lake margins) lithofacies associations. 1. The portions of the Sibley Group studied (lithostratigraphic Pass Lake and Rossport Formations) contain a variety of distinct lithofacies associations. These lithofacies associations can be divided into 4 informally defined allostratigraphic units which roughly correspond to existing lithostratigraphic subdivisions. Based on the stable isotope, Sr isotope and trace element data, the following conclusions can be made: 2. The lower clastic unit forms the base of the Sibley Group and contains the following lithofacies associations representing distinct depositional settings: boulder conglomeratesandstone-calcrete (proximal ephemeral braided 1. Overall, the geochemical data supports a non-marine origin for the Pass Lake and Rossport Formations. 2. Low Sr isotope ratios from calcrete in the - 31 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 lower clastic unit suggest atmospheric deposition and weathering of Gunflint Formation carbonate bedrock was the primary source of cations for pedogenic carbonate rather than weathering of local silicate sources. Relatively 13C-rich calcrete carbon isotopic composition suggests little organic contributions to soil CO2. REE geochemistry suggests calcretes precipitated from oxidizing non-marine water. activity of the MCR and reached thicknesses in excess of 20km (Cannon et al., 1989). These basalts are generally tholeiitic in composition and are thought to be plume-sourced (Klewin and Shirey, 1992). Large mafic igneous bodies associated with the MCR are predominantly attributed to the upwelling of a mantle plume beneath the North American Continent (Burke and Dewey, 1973; Hollings et al., 2010a), with the most primitive magmas associated with the MCR being emplaced early in the rift’s history (Hart and MacDonald, 2007). This upwelling plume was responsible for the formation of numerous volcanic and intrusive units. The Logan and Nipigon Sills, Osler volcanic rocks, and what some have speculated to be Pigeon River dikes (Sutcliffe, 1991) dominate the MCR exposures proximal to the Sibley Peninsula. 3. S, Sr, REE and Y data for the mixed siliciclastic carbonate unit support a lacustrine origin for these rocks. Variations in S isotopic composition may be related to changes in the composition of sulfides weathering to supply sulfate to the system. MREE enriched PAAS normalized REE patterns for dolostone samples differ from those found in other carbonate lithofacies and this probably relates to more reducing conditions in lake waters relative to surface waters supplying the lake. Stratigraphic variations in C and O for this unit were created by evaporation and/or residence time effects. Logan Igneous Suite 4. Slightly enriched δ13C and δ18O values in stromatolitic units in both the upper siliciclastic unit and mixed siliciclastic-carbon-evaporite unit reflect a generally more arid evaporitic environment as compared to the mixedsiliciclastic unit. Shifts toward lighter δ13C in pedogenic carbonates from these units probably reflect a contribution of dissolved organic carbon. REE data for these units is consistent with a nonmarine, oxidizing depositional setting. Carl (2011) continued to describe the igneous units on Sibley Peninsula thus: Midcontinent Rift The Logan Sills and Nipigon Sills are part of what is now known as the Logan Igneous Suite (LIS; Hollings et al., 2007a). The LIS contains various diabase sill formations located north of Lake Superior (Hollings et al., 2007a). Using geochemical data, sills to the north of Thunder Bay have been classified as Nipigon Sills, with sills south of Thunder Bay referred to as Logan Sills (Hollings et al., 2007a). The diabase sill exposures on the Sibley Peninsula occur on the peninsula’s southern tip and make up the Sleeping Giant and Thunder Mountain landforms. For convenience, this sill will henceforth be referred to as the Sleeping Giant Sill (SGS). The SGS is located due east of Thunder Bay and has not been the subject of geochemical analysis in the past. Logan Sills At approximately 1.15 Ga, early stage mafic magmatism of the Midcontinent Rift (MCR) began (Heaman et al., 2007). Centered around Lake Superior, the MCR contains over one million cubic kilometres of mafic volcanic and plutonic rock (Klewin and Shirey, 1992). These rocks formed as the result of a major continental rifting episode (Shirey et al., 1994) that spanned at least 60 million years (Heaman et al., 2007). During this time, rifting of the Superior craton nearly resulted in the splitting of the North American continent and the formation of an ocean. This rifting event is mostly represented by flood basalts, which dominated the igneous Logan sills, such as those seen on Mt. McKay, commonly appear as mesas of the Nor’Wester Mountains found immediately south of Thunder Bay. These sills concordantly intrude sedimentary layers of the Rove Formation and have a gentle southwest dip (Sutcliffe, 1991). The Logan Sills have been classified as quartz-tholeiitic diabase that often contains labradorite, augite, pigeonite and iron-titanium oxides (Sutcliffe, 1991). Logan Sills are characterized by high TiO2 and Gd/Ybn when compared to Pigeon River dikes and Nipigon Sills (Hollings et al., 2010a). In hand sample, Logan Sills contain medium to coarse grains that are dark grey and ophitic-textured. Heaman et - 32 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 magmatic activity. This dike swarm is located on the northern shore of Lake Superior from Grand Portage, Minnesota, to the Black Bay Peninsula in Ontario (Osmani, 1991). Pigeon River Dikes can either cross-cut Logan Sills or terminate against these sills. Geochemical data suggest the Pigeon River Dikes could not have acted as feeders to the Logan Sills (Hollings et al., 2010a). Ages of Pigeon River Dikes range from 1078Ma for a dike near Arrow River in Devon Township to 1141Ma for a dike in Crooks Township (Heaman et al., 2007). This span of over 60 million years is appreciably longer than the duration of formation for most plume-derived large igneous provinces (Hollings et al., 2010a). Pigeon River Dikes have an olivine-tholeiitic composition (Sutcliffe, 1991) and can be distinguished from Logan Sills based on their low TiO2 and Gd/Ybn values (Hollings et al., 2010a). These values are broadly similar to the Nipigon Sills on Gd/Ybn versus La/Smn and Mg# versus TiO2 diagrams (Hollings et al, 2010a). Pigeon River Dikes generally have northeasterly strikes and very steep dips to the south (Osmani, 1991). Pigeon River Dikes have been cross-cut by the geochemically distinct Cloud River Dikes located south of Thunder Bay (Smyk and Hollings, 2007). On the Sibley Peninsula, numerous dikes having northeasterly strikes are present (Tanton, 1924). Dikes of the Sibley Peninsula have been shown to have both normal and reversed polarities suggesting a range of ages for these dikes (Pesonen and Halls, 1983). al. (2007) determined an age of approximately 1115 Ma for the sill that caps Mt. McKay, which presently serves as the only reliable dating of a Logan Sill (Heaman et al., 2007). Hollings et al. (2010a) reported small Nb anomalies and fractionated REE patterns in samples from Logan Sills. These characteristics along with elevated TiO2 values and low Mg# values are the best traits for distinguishing Logan Sills from other nearby sill suites (Smyk and Hollings, 2009). Nipigon Sills Located in the Nipigon Embayment (Sutcliffe, 1991), the Nipigon Sills represent a northern component of the MCR that may have formed in a failed arm of the rift (Richardson and Hollings, 2005). These sills may be up to 200m thick and intrude all other rocks in the area (Heaman et al., 2007). The Nipigon Sills are diabase comprised of medium-to-coarse-grained, lath-shaped, euhedral crystals of ophitic-textured plagioclase with abundant pyroxene as well as trace olivine and magnetite (Hart et al., 2005). The sills of the Nipigon Embayment were sometimes broadly referred to as Nipigon Sills and have recently been subdivided into five distinct sill suites based on geochemical analysis (Hollings et al, 2007a). These suites are the mafic Nipigon, Inspiration and McIntyre Sills and the ultramafic to mafic Jackfish and Shillabear Sills (Hollings et al., 2007a). The SGS is a mafic sill, and therefore only the mafic Nipigon, Inspiration, and McIntyre sills will be considered here. The Nipigon and Inspiration sills both have low La/Smn ratios with the Inspiration sills having elevated Gd/Ybn ratios compared to the Nipigon sills (Hollings et al., 2007a). The McIntyre sills are distinguished by their low La/Smn ratios and intermediate Gd/Ybn ratios compared to other sill suites (Hollings et al., 2007a). These suites can also be differentiated by plotting Mg# versus TiO2 with the McIntyre sills having TiO2 values elevated similarly to the TiO2 values recorded for Logan Sills (Fig. 2.3) near Thunder Bay (Hollings et al., 2007a). Sills of the Nipigon Embayment range in age from roughly 1106 Ma to perhaps as much as 1159 Ma and are considered to be amongst the oldest magmatic expressions of the MCR (Heaman et al., 2007). Osler Group The Osler Group consists primarily of mafic rocks found along the north shore of Lake Superior, representative of basaltic flows that occurred approximately 1108-1105 Ma (Hollings et al., 2007b). These basaltic flows reached a thickness of approximately 3 km and can be frequently found in outcrop to the northeast of the Sibley Peninsula. Osler Group basalts have chondrite-normalized La/Smn ratios that range from 1.5 to 3.9 and chondrite-normalized Gd/Ybn ratios ranging from 1.5 to 3.7 (Hollings et al., 2007b). Osler Group volcanic rocks are abundant on the Black Bay Peninsula, located east of the Sibley Peninsula. No volcanic rocks of any kind have been observed on the Sibley Peninsula. Pigeon River Dikes The Pigeon River Dikes occur as a northeast trending swarm that formed as a result of MCR - 33 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Road log for field tripFOR (NAD83, UTM Zone 16) ROAD LOG FIELD TRIP (NAD83, UTM Zone 16) STOP NAME Blende Creek Area: Gunflint Fm chert-carbonate Gunflint Fm chertcarbonate Rove Fm concretions Watson Site Kettle Brohm Site Pass Lake (Animikie / Sibley disconformity Pass Lake (basal conglomerate) Rossport Fm. Siltstones Rossport Fm. Lacustrine Sheet Sandstones Rossport Fm., Lacustrine Channel Sandstones Rossport Fm., Lacustrine Channel Sandstones STOP NO. LANDMARK (0 Km) DISTANCE (km) Junction Highways 1117 and 587 0.0 1A EASTING 0.6 5384392 369112 1.4 2.1 5383837 369581 3.5 5382426 370841 4.8 5.2 5382460 5381279 5380897 371737 371241 371236 4A 6.0 5380509 371853 4B 6.5 5380537 372294 5 7.7 5380075 373001 6 8.8 5378948 372877 7A 9.9 5377943 372842 10.1 5377728 372868 5376859 373095 1B CNR Trestle 2 3 optional optional Watson Site (turn-off) n.a. 7B Pass Lake Crossroad Rossport Fm., Subaerial Siltstone-Caliche NORTHING 8 3.8 10.6 10.9 Jakobsen Road 11.2 - 34 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Northwest-striking dykes in Rossport Fm. East-northeaststriking dyke in Rossport Fm 9A 9B 10 Sleeping Giant Provincial Park boundary Kay Lake Portage Drive Joe Creek trail Joeboy Lake Thunder Bay Lookout turnoff eastnortheaststriking dyke eastnortheaststriking dyke east-striking dyke Sifting Lake trailhead northeaststriking dyke Lake Marie Louise (north end) northeaststriking dyke Lake Marie Louise campground Plantain Lake trailhead Silver Islet loop junction Sawyer Bay trailhead eastnortheaststriking dyke Silver Islet General Store - 35 - 14.2 14.3 5374104 5374052 372312 372320 21.8 5368080 371382 22.6 5367443 370954 23.8 5366398 370595 23.9 5366287 370537 5365748 370280 5357977 366961 5355569 364889 14.4 16.9 17.2 17.8 19.5 21.1 24.2 24.6 29.8 34.1 34.2 34.8 35.7 37.3 37.5 38.5 �Proceedings of the 58th ILSG Annual Meeting - Part 2 Rove Fm and east-northeaststriking- diabase dyke east-northeasttrending dyke in Rove Fm. / Silver Islet view 11 12 Sibley Creek (bridge) Loop Tjunction Middlebrun Bay trailhead Loop Tjunction 39.0 5354955 365488 39.9 5355427 366232 40.0 40.3 40.7 42.1   Figure 3. Stop locations, northern Sibley Peninsula; north to left of image from Google Earth Stop Descriptions Stops 1A,B: Blende Creek area (Gunflint Fm.) UTM coordinates: NAD83; 16U A - 0369112E / 5384392N, B - 0369581E / 5383837N Along Highway 587, rock cuts display thinly bedded, generally flat-lying sedimentary rocks of the Gunflint Formation. The outcrops we have driven past are composed of ankerite and siderite grainstones (medium-grained sand sized iron carbonates) referred to as granular iron formation (GIF). These are common in the Thunder Bay region, dominating the near-shore of the Animikie Basin. The iron carbonate grains were produced by wave erosion of carbonate precipitates and represent storm deposits in the near-shore. The iron may have precipitated as a carbonate in this shore-proximal zone due to photosynthesizing bacteria removing CO2 from the water and thus increasing the pH and driving the carbonate phase into supersaturation. The outcrop we are looking at has these carbonate grainstones weathering orangey-brown alternating with white chert layers (Fig. 4). In places the chert can be seen - 36 - �Proceedings of the 58th ILSG Annual Meeting - Part 2   and faulted Gunflint strata on Highway 588, Stop 1B. Figure 4. Folded replacing the carbonate but other layers appear to be primary chert. In the older literature an outcrop such as this would be ascribed to deeper water due to less evidence of current activity. However, because of its shore proximal location it probably formed in a quieter water location near the strand-line, i.e., a sheltered lagoonal area behind on offshore bar. These exposures are somewhat unique in that the rocks are folded; elsewhere, they are undeformed. The hinge zones, where the majority of stress is focused, are commonly fractured (Fig. 4). These fractures may be occupied by quartz-calcite veins following the vertical axial plane. The outcrop to the west hosts numerous veins and vein breccias that strike between 40° and 45° and dip almost vertically to the southeast. These breccias contain sparry calcite, drusy quartz and also altered shale fragments, suggesting that these Rove Formation rocks likely occurred above this section during vein emplacement. A thin, northwest-dipping diabase dyke intrudes the Gunflint rocks at this location and is, in turn, cut by these veins. Recent examination of the Gunflint Formation near Pass Lake has led to the recognition of structures typical of Penokean (circa 1875 to 1835 Ma) fold-andthrust belt deformation (Hill and Smyk 2005). Discrete bedding-plane faults with locally developed gouge and breccia can be traced laterally into horizontal, hangingwall ramps with associated fault-bend folding. Foldand-thrust belt deformation is caused by regional compression. Previous workers had ascribed the folds to syn-sedimentary slumping and Keweenawan diabase sill emplacement and thought that they were attributable to local, rather than regional-scale, deformation. Displacement in fold-and-thrust belts tends to be localized along discrete bedding planes and not easily recognized. This may account for the perceived lack or absence of structures elsewhere in the Gunflint Formation (Hill and Smyk, 2005). Penokean structures on the northern side of Lake Superior represent the northward migration of thrust faults into the foreland (passive margin Archean basement + Gunflint Formation) caused by hinterland collision to the south. Stop 2: “Devil’s Flower Pots” (Rove Formation concretions) UTM coordinates: NAD83; 16U 0370841E / 5382426N Just north of Highway 587, a quarry face exposure of black, fissile Rove Formation shale displays lenticular and elliptical concretions, flattened along bedding planes (Fig. 5). These structures form during diagenesis, following initial compaction and dewatering of the sediments. They represent a concentration of a cementing agent (e.g., silica, calcite) focused during the migration of fluid through the sediments. They often are nucleated around a piece of organic material or other foreign object, which creates a perturbation - 37 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 5. Lenticular concretion in Rove Formation shale,  old quarry face, north of Highway 587, Stop 2. Concretion is almost 1 m in diameter. in fluid flow with a distinct chemistry. Because the cementing agent in this case is more resistant to weathering, these concretions stand out of the soft shale and may commonly completely detach form their host rock. Groundwater and surficial water flow through the shale has led to the dissolution and subsequent precipitation of a variety of low-temperature minerals (e.g. carbonates, sulphates, hydroxides) that occur as white and yellow encrustations on the bedrock surface. One of the more unusual of these secondary minerals is yellow magnesium aluminocopiaptite ((Mg,Al) (Fe,Al)4(SO4)6(OH)2.20H2O; Resident Geologist’s Files, Thunder Bay). Figure 6. 1835 Ma Rove shale, note bleached zone overlying oxidized zone, overlain by the approximately 1450 Ma basal sandstones and conglomerates of the Loon Lake Member,   Pass Lake Formation, Sibley Group, Stop 3. Stop 3: Watson Road section (Pass Lake and Rove formations) - private property; permission is required to access UTM coordinates: NAD83; 16U 0371737E / 5382460N A private access road extending up the mesa provides an excellent 150 m long section exposing the disconformity between the Rove Formation and the overlying basal conglomerate and sandstones of the Pass Lake Formation. The Rove shales immediately below the contact were subject to Mesoproterozoic weathering (Fig. 6).   Figure 7. Disrupted zone in the Rove shales underlying the Sibley Group, Stop 3. The origin of this structure is enigmatic, but may have been caused by fluid escape - 38 - �Proceedings of the 58th ILSG Annual Meeting - Part 2   Figure 8. Close-up of conglomerate shale contact, Stop 3. Note the strong oxidation of both units. Geochemical investigations have outlined an oxidized zone below the contact grading to a more reduced zone with abundant chlorite a few tens of centimeters lower in the section. In one area what may be a dewatering or degassing structure strongly deforms the shale (Fig. 7). Very immature, iron oxide-rich conglomerates and sandstones of the Loon Lake Member, Pass Lake Figure 9. Typical Loon Lake Member coarse-grained sediments at Stop 3. From the bottom to the top of the photograph: 1) matrix-supported conglomerate, probably a high-density mass-flow; 2) a one clast-thick, pebble-cobble lag developed at the top of this conglomerate through erosion. This may represent either an Aeolian deflation lag or one developed by water erosion of the fine-grained fraction. 3) a boulder-cobble, matrix-supported, mass-flow conglomerate. This was probably a very high-viscosity flow as the larger clasts were suspended near the top of the flow. 5) an upper flow regime parallel-laminated sandstone probably deposited by sheet-flood on the alluvial fans surface; 6) a clast-supported fluvial conglomerate. Formation, overlie the Rove (Fig, 8). The conglomerate and sandstone layers are laterally discontinuous (Fig. 9), with some conglomerates in clast-support (fluvial deposits) and some in matrix-support (sub-aerial debris-flow deposits). Successions such as this in the Sibley are typical of arid to semi-arid alluvial fans (Cheadle 1986), though this would have been a very small one. The abundant hematite probably denotes a deep water table. Clasts are locally derived from the erosion of underlying units. This is sharply overlain by mature, well-sorted, medium-grained sandstones of the Fork Bay Member, Pass Lake Formation (Fig. 10). Detrital zircon geochronology and paleocurrents (Cheadle 1986; Rogala et al. 2007) indicate that the major source of this sediment was the Trans-Hudson highlands. The travel distance accounts for its maturity compared to the locally derived underlying conglomerates. The sandstone was deposited as sheet flows into the shallow nearshore of a lacustrine system that had flooded the area (Cheadle 1986; Rogala 2003; Metsaranta 2006; Rogala et al. 2007). These sandstone layers are laterally continuous, massive to parallellaminated, in places with trough cross-stratified or rippled tops (Fig. 11). Rare, odd features are present both in cross-sectional and bedding plane views in this outcrop (Figs. 12, 13). These may be dewatering pipes.     Figure 10. Sharp contact between the hematite-rich conglomerates of the Loon Lake Member and the wellsorted, buff sandstones of the Fork Bay Member, Pass Lake Formation, Sibley Group, Stop 3. - 39 - �Proceedings of the 58th ILSG Annual Meeting - Part 2   Figure 11. Medium- to coarse-grained, well-sorted sandstone bed of the Fork Bay Member, Stop 3. The majority of the bed is upper flow regime parallel laminated, with a reworked, cross-stratified top. Stops (Optional): Kettle and Brohm Archaeological Site UTM coordinates: NAD83; 16U 0371241E / 5381279N and 371236E / 5380897N respectively Local archaeological sites are closely tied to paleo-shorelines, especially those associated with Lake Minong. When flooded to Minong levels (Fig. 14), Sibley Peninsula becomes a virtual island, connected to the mainland near Pass Lake with a series of baymouth bars, forming a spit between the sandstone cliffs (Geddes et al., 1987). The location of these sites may relate to the importance of this paleogeography in constricting the movement of caribou and other animals from the peninsula to the mainland. Although there is no organic preservation to confirm that these were caribou ambush and processing sites, it remains a compelling theory.   Figure 12. Bedding-plane view of odd concentric layering, Stop 3. The pattern is caused by erosion of the top of domed layers. These may be water escape structures with the basal portions of sand volcanoes preserved, or not.   Figure 13. Cross-section through a domal structure in a Fork Bay sandstone, Stop 3. - 40 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure  14. Reconstruction of shoreline near Pass Lake during Minong time (Hinshelwood 1990) A historic plaque on a roadside pull-off describes the archaeological discovery: In 1950, archeological investigations in this area uncovered a site which had been used as a workshop camp by a group of the earliest known people in this part of the Upper Great Lakes basin. Called Aqua-Plano Indians because they migrated from the western plains to fossil beaches of glacial and post-glacial lakes in - 41 - this region, they appeared about 9,000 years ago following the retreat of glaciers and the northward movement of plants and animals. They developed a distinctive tradition based primarily on large game hunting using weapons and specialized tools made of taconite, a stone that was obtained locally. Their way of life, which was closely related to the environment, disappeared as the climate grew warmer. �Proceedings of the 58th ILSG Annual Meeting - Part 2 Pass Lake section (http://www.ontarioplaques.com/Plaques_ STU/Plaque_ThunderBay17.html) This Quaternary geology of this area was also described by Geddes et al. (1987; Fig. 15 and 16): [Figure 15] shows the arrangement of bars which separated Pass Lake from open water about 9000 years B.P. The strait between Black Bay and Thunder Bay appears to have followed a subglacial channel scoured into the rock floor, and which forms a narrow trough along the length of Pass Lake. Geomorphological evidence of the environmental conditions at the time of habitation will be seen in gravel pit sections of the baymouth bars. Cryoturbated layers show a marked vertical alignment of platy shale fragments, suggesting that the bar surfaces were exposed to intensely cold conditions as they accumulated. The grounding of small icebergs, as evidences by the reorientation of bar features around depressions is also indicated [“Kettle” Stop]. Stop 4A: Disconformity between Pass Lake and Rove Fm.’s (UTM 371853E / 5380509N) UTM coordinates: NAD83; 16U 0371853E / 5380509N The rock cut adjacent to the railway at Pass Lake provides an excellent exposure of several types of flatlying sedimentary rocks and their contact relationships. This cliff is the type section for the Pass Lake Formation of the Sibley Group. Exposure is almost continuous for about 3.2 km along the tracks and gives a stratigraphic thickness of 50 m. The oldest rocks in the section, fine-grained, fissile shales of the Rove Formation (Animikie Group), are exposed at the northwestern end of the cliff exposure. Originally black, they were oxidized in pre-Sibley times, resulting in their purple and green colour. These rocks originated as 1835 Ma muds. The Animikie Group rocks are disconformably overlain by coarser-grained sedimentary rocks of the Sibley Group, similar to what we observed at the Figure  15. Baymouth bar complex in the vicinity of Pass Lake and Brohm archaeological site (Geddes et al. 1987). “Kettle” field trip stop corresponds to pond shown near the highway - 42 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 previous outcrop. Immediately above the AnimikieSibley disconformity, lenses of conglomerate comprise the base of the Pass Lake Formation (Fig. 17). This basal conglomerate contains angular to rounded pebbles, cobbles and boulders in a sandy matrix. These clasts are derived predominantly from erosion of Gunflint Formation chert and taconite; there are also clasts of quartz veins and Archean granite. (Gunflint rocks are exposed a few kilometres to the northwest on Highway 587). This conglomerate attains a thickness of approximately 3 m at the southeast end of the exposure, where it is sharply overlain by massive to parallel laminated, buff-coloured Pass Lake sandstone (quartz arenite). The constituent sand grains consist mainly of quartz, with minor chert and feldspar, with calcite cement lower in the cliff and silica cement higher up. The sandstones were deposited in the nearshore of the lacustrine system. Figure 16. Paleo-Indian sites, paleogeography and field trip stop locations (Hinshelwood, 2004) - 43 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 than those observed earlier. This opens the possibility that the conglomerates at this location were reworked by wave activity during initial lacustrine flooding. While examining the lithofacies in the field we will discuss the merits of each interpretation. The sandstone beds again represent sheet-floods forming sand-flats in the shallow lake. The thinning- and fining-upward sequence of sandstone beds is a classic example of a transgressive succession showing decreased sand supply through time as the shoreline moves further away from the area. Stop 5: Rossport Formation Siltstones UTM coordinates: NAD83; 16U 0373001E / 5380075N The dip of the strata to the south, probably developed due to block rotation during Mid-Continental rifting, allows us to observe higher levels of the Sibley Group as we drive down this stretch of the highway.   Figure 17. Disconformity between weathered Rove shales and Pass Lake basal conglomerate, Stop 4A Stop 4B: Pass Lake Formation A cliff on the far side of the railroad tracks contains the type section of the Pass Lake Formation. The basal conglomerate thins and thickens laterally, pinching down to pebbly sandstone in places. Clasts are generally surrounded and dominated by local Gunflint Formation lithologies. The matrix is poorly sorted. The conglomerates are overlain by a thinning upward sequence of sandstone beds (Fig. 18) capped by siltstones on the top of the cliff. Individual beds are reasonably laterally continuous though sometimes lense out. They are dominated by upper flow regime parallel lamination with occasional ripples and smallscale dunes on their tops. Both alluvial fan-braided fluvial and shallow lacustrine (Cheadle, 1986; Franklin et al., 1980 respectively) depositional environments have been proposed. The bedding organization of the conglomerates exposed here is somewhat different Figure 18. Thinning-upward sequence in Pass Lake sandstones, Stop 4B - 44 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 The fining- and thinning-upward trend of the sandstones in the last section culminates in the massive siltstones we see at this stop. Not much can be said about these structureless red siltstones. They appear to have formed from rainout sediment in the offshore portion of the lacustrine system. STOP 6: Rossport Formation Lacustrine Sheet Sandstones UTM coordinates: NAD83; 16U 0372877E / 5378948N Here we have another example of the offshore red siltstones, but with two sandstone layers in them (Fig. 19). The lower layer is actually composed of two amalgamated layers. These sheet sandstones probably represent large flood (storm) events during which the flow conditions were intense enough to transport the sand into the further offshore areas of the lake. Rare sedimentary structures, consisting of hummocky cross-stratification (Fig. 20), in the otherwise massive sandstones indicate storm generated currents deposited the sands. The presence of washed-out dunes at other locations indicates flow velocities were transitional from lower to upper flow regime, as opposed to the near shore sand sheets we looked at two stops back, which were deposited during upper flow regime conditions. The tops of the sand sheets were reworked by waves forming ripples.   Figure 20. Hummocky cross-stratification in a sheet sandstone, Stop 6. This type of layering is produced by the interaction of storm waves and an offshore flowing current carrying sand. These offshore currents, geostrophic flows, are produced by the storm surge draining away from land during the waning of the storm. Stop 7A (west side of road): Rossport Formation Lacustrine Channel Sandstones UTM coordinates: NAD83; 16U 0372842E / 5377943N This outcrop consists of somewhat chaotic layering (Fig. 21). It appears to have several steeply dipping faults running through it. On the southern (down-road) side of the outcrop the sandstone layers abruptly abut against red siltstone. Sandstone layers near the top of the outcrop appear to lever downwards. There are also two lithologies present here that we have not seen so far. The most evident of these is purple shale. It has an interesting mineralogy compared to the red siltstone. The red siltstone has abundant potassium-rich micas and clays. The purple shale does not contain these but instead has potassium feldspar (SEM-EDX and XRD analyses). This implies an alteration where potassium enrichment drove the standard hydrolysis weathering reaction backwards. The other new rock type is dolostone. Beds of it resemble the sandstone, but it is easily distinguished on fresh surfaces. Try to figure out what is causing the deformation of the layering. Stop 7B (east side of road): Rossport Fm., Lacustrine Channel Sandstones Figure 19. Sandstone sheets deposited in the off-shore during storm events, Stop 6. Fairweather deposition produced the siltstone between the sandstone sheets. UTM coordinates: NAD83; 16U 0372868E / 5377728N The layers strike across the road, meaning this section should be similar to the one we just looked at. - 45 -   �Proceedings of the 58th ILSG Annual Meeting - Part 2   Figure 21. Chaotic layering in an outcrop stratigraphically overlying and sheet sandstones and massive siltstones, Stop 7b. It is not. Before reading further look at the outcrop and see if you can figure out what is going on. The outcrop consists of three units. There are beds of sandstone at the top of the outcrop. Underlying these is a chaotic, poorly-sorted, jumble of siltstone, shale and dolomite clasts with a pebbly silt matrix. Underlying this is a thick layer of dolostone that has a very irregular upper surface (Fig. 22). In places the intraformational conglomerate extends down to ground level. Now that you possibly have a better idea what the lithologies are can you figure out what happened here? The answer: You are looking at a karst surface on the dolostone. It was buried by a sub-aerial massflow deposit, possibly triggered by a change in the basin slope at this time, from down to the southeast to down to the northwest. The sand-sheets that also come in at this stratigraphic interval are flowing from the southeast (Cheadle, 1986; Rogala, 2007). There is also the possibility that the chaotic unit represents a collapse breccia. Figure 22. Sub-aerial mass-flow deposits overlying a very irregular karst surface eroded into the dolostone at the bottom of the photo, Stop 7b. There is also a possibility that the intraformational conglomerate represents a collapse breccia. - 46 -   �Proceedings of the 58th ILSG Annual Meeting - Part 2 Stop 8: Rossport Formation., Subaerial SiltstoneCaliche STOPS 9A,B: Northwest-striking dykes in Rossport Fm. UTM coordinates: NAD83; 16U 0373095E / 5376859N UTM coordinates: NAD83; 16U 0372312E / 5374104N and 0372320E / 5374052N This outcrop represents the highest level of the Sibley Group that we will see on the peninsula. Again we have an outcrop of massive siltstone. However, the internal structuring here is very different than those previously examined (Fig. 23). This would have been more evident a few years ago when the outcrops were buried by small, centimeter and less, chunks weathered out from them. In the interim people, including PWF, discovered that this material can be used to make decorative garden paths and it has disappeared. If you have worked in the southwestern badlands you may have seen modern examples of this type of material coating the ground. The small chunks are soil peds that are baked in the sun. They commonly have clay coatings called cutans on their sides. Further evidence that what we are looking at represents arid to semi-arid soil is the presence of the light green layers. These are rich in dolomite and represent dolocrete layers which form in soils in semi-arid environments. They are created where evaporation is greater than precipitation and there is a net upward movement of water due to capillary action. Evaporation causes the soil fluid to be super-saturated and precipitate carbonate. We can see the peds and dolocrete layers in these exposures (Fig. 23), denoting the large, probably shallow, lake had gone from this area for good. Figure 23. Soils developed in the semi-arid environment of the Rossport Formation. The upper light greenish grey unit is a carbonate-rich horizon with some dolocrete. The red unit consists of soil peds, the small fragments it is disintegrating into. Two narrow, parallel, northwest-striking diabase dykes intrude Rossport Formation siltstones at these two locations. Sampling by Hollings et al. (2009) identified profound geochemical differences between northeast- and northwest-striking dykes on Sibley Peninsula. These dykes plot within, or very near the fields defined for mafic/ultramafic sills and intrusions on both Gd/Ybcn versus La/Smcn and Mg# versus TiO2 diagrams (Fig. 24). These fields are derived from mafic/   Figure 24. Gd/Ybcn versus La/Smcn plot and TiO2 versus Mg# plot for diabase dykes at Stops 9A,B and 10, as well as other mafic and ultramafic intrusions on the Sibley Peninsula and the northern Midcontinent Rift (Carl, 2011; Hollings et al., 2007a,b) - 47 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 ultramafic intrusions located in the Nipigon embayment northeast of the Sibley Peninsula (cf. Hollings et al., 2007a,b). The northwest strike directions make them spatially unsuitable candidates for feeders of any of the mafic/ultramafic intrusions located near Lake Nipigon, northeast of the Sibley Peninsula (Carl, 2011). REE patterns for these dikes do not display large negative niobium anomalies compared to the REE patterns of east-northeast-striking dikes. The small negative niobium anomalies suggest a more primitive source for these northwest-striking dikes compared to the eastnortheast striking dikes of the Sibley Peninsula. The SiO2 weight percentages of these dykes is 48.7 and 49.4, respectively. TiO2 and MgO weight percentages for these two dikes are roughly 3.35 and 3.65 and 7.74 and 7.19, respectively. No cross-cutting relationships were noted at the Highway 587 sample sites, and therefore a relative age relationship of these dikes to the east-northeast striking dikes (e.g., Stop 10) is unknown (Carl, 2011). Stop 10: East-northeast-striking dyke in Rossport Formation UTM coordinates: NAD83; 16U 0373382E / 5368080N At this location, a steeply dipping, east-northeaststriking diabase dyke crosscuts Rossport Formation siltstones (Fig. 25). There appear to be some localized contact metamorphic effects, including some hornfels and reduction of the iron in these hematite-rich sedimentary rocks. The geochemistry (Gd/Ybcn versus La/Smcn, TiO2 versus Mg#; Fig. 24) of east-northeaststriking dykes along Highway 587 falls into the field previously defined by Nipigon sills (Carl, 2011; Hollings et al., 2007a,b). They may be correlative with Pigeon River dykes of similar orientation south of Thunder Bay. STOP 11: Rove Formation and east-northeaststriking diabase dyke (UTM 5354955N, 365488E) UTM coordinates: NAD83; 16U 0365488E / 5354955N The unweathered Rove Formation can be seen here (Fig. 26). The dominance of siltstone raises the possibility that this is the lower portion of the Rove Formation. In fact the numerous siltstone layers imply that we may be looking at the lowest ten metres of the Rove Formation, as during initial transgression the more shore proximal location led to a more silt-rich interval. The fine parallel layering and lack of wave formed structures is interesting as it appears that no shallow water, coarser-grained lithofacies were deposited in the Rove. This may denote very rapid transgression or an arid climate limiting sediment delivery to the basin. Or Figure 25 . East-northeast-striking diabase dyke crosscutting Rossport Formation siltstones, Stop 10, south of Joeboy Lake. - 48 -   �Proceedings of the 58th ILSG Annual Meeting - Part 2   Figure 26. A grey siltstone dominated succession of the Rove Formation, Stop 11. This outcrop also contains thin dark shales and thicker, clay-rich sandstones. The Rove in this area either represents the basal siltstone-rich area or a portion of the turbiditic fan/ramp higher in the Formation. The diabase can be seen in the upper right of the photo. possibly, as paleocurrents indicate the Rove Formation represents foreland deposits associated with the TransHudson Orogeny, an early sediment starved phase was produced by development of a tectonic moat. This is common in other orogeny related black shale-turbidite sequences such as the Martinsburg Formation, which was deposited in the Taconic foreland. The other possibility is that this section is considerably higher in the Rove Formation. The presence of sandstones indicates that this may be a portion of the submarine Figure 27. Geology of the southern Sibley Peninsula (Tanton, 1924, 1931) showing stop locations. - 49 -   �Proceedings of the 58th ILSG Annual Meeting - Part 2 fan/ramp system that overlies the 100 meters of basal shales and siltstones. STOP 12: East-northeast-trending dyke in Rove Formation / Silver Islet view UTM coordinates: NAD83; 16U 0366232E / 5355427N This stop affords us the opportunity to look across the waters of Lake Superior to the former site of the Silver Islet Mine, 1.8 km to the south. This tiny speck, barely rising above the waves, was once home to the most famous silver mine in the world (Figs. 28, 29). The epic story of its discovery, development and legacy was summarized by Mohide (1985): The early regulations of the government with regard to the area of mining locations were, compared to modern ideas, generous to a fault. A mining location was required to be two miles in front by five miles in width, comprising an area of 6,400 acres. In 1856 a mining company had obtained from the Crown sixteen of these locations fronting at intervals on the north and northeast shores of Lake Superior, comprising in all 99,498 acres. The conditions required the grantee “to commence and bona fide carry on mining operations within a period of eighteen months”, under penalty of forfeiture of the lands. But the company did not comply with the conditions, neither did the government exact the penalty. The grant reserved all mines of gold and silver and imposed a royalty varying from 2 to 10 per cent of the value of the ore extracted. In, as we may suppose, its anxiety to start things moving, the government abandoned the reservations of gold and silver, repealed the royalties, and forgave one-half of the purchase money of 80 cents per acre. The only offset on the part of the government was to levy a tax of two cents per acre on all lands granted previous to 1868. All restrictions having been removed and facing an annual tax of approximately $3,140 the company apparently deemed it advisable to examine their lands for possible deposits of mineral. Their interest was probably quickened by the discovery of silver made by the McKellar brothers and others on the north shore of Lake Superior. The task of examination the company committed to Thomas MacFarlane, a well-known geologist and civil engineer. MacFarlane and his party set out in the spring of 1868 and cruised the locations one by one. On the Jarvis location they found a vein of silver, upon which considerable work was afterwards done and a quantity of silver recovered. Eventually the party arrived at the Woods location. He determined to make a complete study of the location and set his assistant, Gerald C. Brown, to survey the shore-line. While engaged in planting pickets on the islands in Lake Superior fronting the location, Brown landed on the tiny rock about the size of a ballroom to which Macfarlane afterwards gave the name of Silver Islet and here he noticed a vein carrying galena. Macfarlane thereupon visited the spot himself and put three of his men to work. On the north shore of the islet there was a vein having a width of 20 feet, which on the south divided into two branches, each seven to eight feet wide. On the 10th of July the first metallic silver was noticed by John Morgan, one of the exploring party, at the water’s edge on the east or hanging side of the west branch of the vein, in the form of small nuggets. A single blast was sufficient to detach all the vein rock carrying ore above the surface of the water, but the ore was traced some distance out into the lake, where instead of scattered nuggets of native silver, large patches of veinstone rich in galena were visible, intermixed with small particles and large nuggets of silver. The thickness of the rich part of the vein varied from a few inches to two feet and by working in the icy water with crowbars some rich pieces of ore were broken off. On the 15th of July three packages of the best specimens were shipped from Fort William, Thunder Bay, altogether 1,336 lbs. of ore having been obtained. This shipment was carefully weighed and sampled in the following December. Assays by Professor Chapman of Toronto, Dr. Hayes of Boston and Macfarlane himself, gave an average of 2,087 ounces Troy per long ton. Next year explorations were resumed on the rock, but winds and waves, together with the extreme coldness of the water, proved great hindrances. Nevertheless, by working with tongs and longhandled shovels in two to four feet of water, the party was able to raise and ship 46 half-barrels of good ore, weighing 9,455 Ibs. valued by Macfarlane on the basis of his assays at $6,751. Such results indicated a mine and amply justified further development. A shaft house and sleeping - 50 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 and dining rooms for the men were erected and strong barriers of two-inch plank built to protect them from the furious gales and sweeping winds of Lake Superior. Inflowing water slowed up work in the shaft and the contracted area of the mine severely hampered operations. Weather conditions, however, had their compensations for when the winter set in, the frozen surface of the lake provided solid footing for the men who managed to raise some nine tons more of the ore. The total quantity of ore recovered up to this time was 28,073 lbs. which realized after being smelted, the sum of $23,115. The mining company in March, 1870 sold not only the Woods tract, but all of its other locations, now 18 in number, to a new group, the price realized being $225,000. proved to be too large, for the amount realized was 5137,022 less than the value originally placed on the shipment. Frue had been succeeded as superintendent by Richard Trethewey, who decided to divert the vertical shaft to an incline following the line of the diorite dike, associated with the very rich ore of the earlier workings had been found. He carried it down to a further depth of 414 feet and drifts started at the bottom showed the vein to present highly promising conditions. Some very rich ore was encountered in a south drift. The hope was that the chimney of ore would again be tapped at the junction of the diorite and the slates, but although fugitive bunches of ore were met with, the mine failed to respond to expectations. Evidently the end was approaching. Minerals which frequently accompanied the silver, such as [zinc] blende, galena, pyrite, cobalt and nickel were found, but the silver was absent. Notwithstanding financial and mining difficulties, the work was continued until February, 1884. It was still intended to carry on, but a cargo of coal in charge of a drunken ship’s captain had failed to arrive before the close of navigation and no course was open but to allow the mine to fill with water and cease operations. The new company formed the Ontario Mineral Lands Company and selected Captain William B. Frue as manager, who at once set to work in September of that year. Frue was a man of remarkable quality and is worthy of some words of mention. Not only was he a skilled and experienced mine superintendent, but he seems to have been of heroic calibre. During the six years beginning with 1870 production of silver from the mine amounted to 1,561,882 fine ounces, but output was lessening year by year, due to unfavourable changes in the vein as the lower workings were reached. The company got into financial difficulties and facing a heavy deficit, sold all its holdings to a new concern known as the Silver Islet Consolidated Mining and Lands Company, with a capitalization of 51,000,000. The total production of the mine in ounces cannot be precisely stated, the records being incomplete, but the entire value is given at $3,500,000. At the average price of silver during the 16-year period of operations say $1.15 per ounce, this would represent a total output of about 3,044,000 fine ounces. To this may be added 16,769 ounces obtained during 1921 and 1922 by the Islet Exploration Company which removed a quantity of rich ore from the roof of the mine. The main production was from two very rich bonanzas, one of which was completely worked out in 1874, yielding over two million dollars. In shape this mass of ore resembled an irregular pear and consisted of arborescent silver. The second bonanza was found on the third level in 1878. It was remarkable for its width (5 feet solid across the breast) and for the occurrence of two previously unknown compounds [mixtures] of silver, huntilite and animikite. This deposit was phenomenal in its structure, the middle of the fourth level being sunk literally through solid silver, the metal projecting boldly from the four walls of the winze. In the breast it stood out in great arborescent masses in the shape of hooks The new company met with wonderful success, far exceeding their expectations. The first three levels were explored and much silver recovered from them. The mine, which had been allowed to fill up as far as the third level, was dewatered and work was carried on from the fourth to the tenth level. The results are thus described in the company’s report for 1878: “Silver of unparallelled riches was found in the winzes, in the drifts and in the stopes and rich stamp-mill rock abounded in all workings, the vein north of the shaft being particularly productive”. The year 1878 closed with an output of silver estimated at 724,632 ounces, of which 551,111 ounces was obtained from “packing ore” (i.e. one rich enough to be put up and shipped to the smelter in small packages) and 170,521 ounces from stamp mill concentrates. This estimate, however, - 51 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 28. Silver Islet ca. 1900 (top; www.canadiangeographic.ca) and today. Figure 29. Aerial view of present-day Silver Islet. Site plan of old mine from Barr (1988). Note that the original island consisted only of the small outcrop at the lower end of the present island. - 52 - �Proceedings of the 58th ILSG Annual Meeting - Part 2   Figure 30. Stop locations, southern part of Sibley Peninsula. Image from Google Earth. and spikes and in gnarled, drawn out and twisted bunches. The width of the deposit was over 10 feet and including the accompanying stamp rock, it yielded about 800,000 ounces of silver. Probably nowhere, or at any rate nowhere in Canada, had mining been carried on under conditions so difficult as at Silver Islet, the area of which before enlargement by protective works, was no larger than a good-sized ballroom. Wind and water conspired to prevent an invasion of the tiny spot. 159p. Burke, K., and Dewey, J., 1973. Plume generated triple junctions: Key indicators in applying plate tectonics to old rocks. Journal of Geology, 81: 406-433. Cannon, W.F., Green, A.G., Hutchinson, D.R., Lee, M., Milkereit, B., Behrendt, J.C., et al. 1989. The North American Midcontinent Rift beneath Lake Superior from GLIMPCE seismic reflection profiling. Tectonics, 8: 305–332 Carl, C.F.J. 2011. Geochemistry and petrology of intrusive rocks of the Sibley Peninsula; unpublished HBSc thesis, Lakehead University, Thunder Bay, 77p. Silver Islet not only rose to fame as a prolific silver producer, having contributed half of all the silver in Canada in 1870’s. The Frue vanner, still in use in different forms today, was developed at Silver Islet in 1872. The use of steam-powered diamond drills and the Burleigh piston-type, compressed air-powered rock drill, was pioneered there. The discovery of Silver Islet led to the development and prosperity of Port Arthur (now Thunder Bay) and spurred the exploration and settlement of northwestern Ontario (cf. Barr 1988). References Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Davis, D.W., Kissin, S.A., Fralick, P.W. and Hammond, A.L. 2005. Discovery of distal ejecta from the 1850 Ma Sudbury impact event; Geology,v.33, p.193-196 Barr, E. 1988. Silver Islet: Striking it rich in Lake Superior; Natural Heritage/Natural History Inc.,Toronto, ON, Cheadle, B.A. 1986. Alluvial-playa sedimentation in the lower Keweenawan Sibley Group, Thunder Bay District, Ontario. Canadian Journal of Earth Sciences, 23: 527–542. Davis, D.W. and Sutcliffe, R.H. 1985. U-Pb ages from the Nipigon Plate and northern Lake Superior; Bulletin of the Geological Society of America, v. 96, p. 15721579. Easton, R., 2000. Metamorphism of the Canadian shield, Ontario, Canada. II. Proterozoic metamorphic history. Canadian Mineralogist, 38: 319–344. Elston, D.P., Link, P.K., Winston, D. and Horodyski, R.J., 1993. Correlations of Middle and Late Proterozoic succession. In, ed. J.C. Reed et al., Precambrian: conterminous United States. The Geology of North America. Geological Society of America, Vol. C-2, 468-485. Elston, D.P., Enkin, R.J., Baker, J. and Kisilevsky, D.K., 2002. Tightening the Belt: paleomagneticstratigraphic constraints on deposition, correlation and deformation of the Middle Proterozoic (ca. 1.4 - 53 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Ga), Belt-Purcell Supergroup, United States and Canada. Geological Society of America Bulletin 114, 619-638. Evans, K.V., Aleinikoff, J.N., Obradovich, L.D. and Fuming, C.M., 2000. U-Pb geochronology of volcanic rocks, Belt Supergroup, western Montana: evidence for rapid deposition of sedimentary strata. Canadian Journal of earth Sciences, 37, 1287-1300. Fralick, P.W. and Barrett, T.J., 1995. Depositional controls on iron formation associations in Canada. In, ed. A.G. Plint, Sedimentary Facies Analysis. International Association of Sedimentologists Special publication 22, 137-156. Fralick, P., Smyk, M., and Mailman, M. 2000. Geology and stratigraphy of the Mesoproterozoic Sibley Group. Institute on Lake Superior Geology, 46th Annual Meeting, Thunder Bay, Ont., Proceedings, Vol. 46, part 2, Field Trip Guidebook, pp. 1–41. Fralick, P.W., Davis, D.W. and Kissin, S.A. 2002. The age of the Gunflint Formation, Ontario: single zircon U-Pb age determinations from reworked volcanic ash; Canadian Journal of Earth Sciences, v.39, no.7, p.1085-1091. Franklin, J.M. 1978. Uranium mineralization in the Nipigon area, Thunder Bay District, Ontario. In Current research, part A. Geological Survey of Canada, Paper 78-1A, pp. 275–282. Franklin, J.M., McIlwaine, W.H., Poulsen, K.H., and Wanless, R.K. 1980. Stratigraphy and depositional setting of the Sibley Group, Thunder Bay District, Ontario, Canada. Canadian Journal of Earth Sciences, 17: 633–651. Franklin, J.M., McIlwaine, W.H., Shegelski, R.J., Mitchell, R.H. and Platt, R.G., 1982. Proterozoic geology of the northern Lake Superior area; Field Trip Guidebook, GAC-MAC Annual Meeting, Winnipeg, 71p. Geddes, R.S., Kristjansson, F.J. and Teller, J.T. 1987. Quaternary features and scenery along the north shore of Lake Superior; XIIth International Union for Quaternary Research, Excursion guide book C-12, 62p. Geul, J., 1973. Geology of Crooks Township, Jarvis and Prince Locations and Offshore Islands, District of Thunder Bay. Ontario Division of Mines, Geological Report 102, 46p. Hart, T.R. and MacDonald, C.A. 2007. Proterozoic and Archean geology of the Nipigon Embayment: Implications for emplacement of the Mesoproterozoic Nipigon diabase sills and mafic to ultramafic intrusions; Canadian Journal of Earth Sciences, v.44, no.8, p.1021-1040. Hart, T.R., MacDonald, C.A., Hollings, P., and Richardson, A., 2005. Proterozoic intrusive rocks of the Nipigon Embayment and Midcontinent Rift. In, T.O. Tormanen and T.T Alapieti, 10th International platinum Symposium Extended Abstracts, Geology Survey of Finland, 365-368. Heaman, L.M., Easton, R.M., Hart, T.R., Hollings, P., MacDonald, C.A. and Smyk, M. 2007. Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon Region, Ontario. Canadian Journal of Earth Sciences, v.44, no.8, p.1055-1086. Hill, M-L. and Smyk, M.C. 2005. Penokean Fold-andthrust Deformation of the Paleoproterozoic Gunflint Formation near Thunder Bay, Ontario; 51st annual Institute on Lake Superior Geology, Program with abstracts, v.1, p.26. Hinshelwood, A. 1990. 1987 observations at the Brohm site (DdjE-1), Sibley Provincial Park, Conservation Archaeology North Central Region, Report 27, Ontario Ministry of Citizenship and Culture, Heritage Branch, Thunder Bay ON. Hinshelwood, A. 2004 Archaic Reoccupation of Late Palaeo-Indian Sites in Northwestern Ontario. In The Late Palaeo-Indian Great Lakes: Geological and Archaeological Investigations of Late Pleistocene and Early Holocene Environments. edited by Lawrence J. Jackson and Andrew Hinshelwood Mercury Series Archaeology Papers 165, Canadian Museum of Civilization, Gatineau. Hollings, P., Fralick, P. and Kissin, S., 2004. Geochemistry and geodynamic implications of the Mesoproterozoic English Bay Granite-Rhyolite complex, northwestern Ontario. Canadian Journal of Earth Sciences, 41, 1329-1338. Hollings, P., Hart, T., Richardson, A., and MacDonald, C.A. 2007a. Geochemistry of the Mesoproterozoic intrusive rocks of the Nipigon Embayment, northwestern Ontario: evaluating the earliest phases of rift development; Canadian Journal of Earth Sciences, v.44, no.8, p.1087-1110. Hollings, P.N., Smyk, M.C. and Hart. T. 2007b. Geochemistry of Midcontinent Rift-related mafic dykes and sills near Thunder Bay: New insights into geographic distribution and the geochemical affinities of Nipigon and Logan sills and Pigeon River and other dykes; 53rd Institute on Lake Superior Geology, Annual Meeting, Lutsen, Minnesota, May 2007, Proceedings Volume 53, Part 1, p.40-41. Hollings, P., Smyk, M., Halls, H. and Heaman, L. 2009. Mesoproterozoic Midcontinent Rift-related mafic intrusions in northwestern Ontario: Continuing geochemical, paleomagnetic, petrographic and geochronologic studies; in Institute on Lake Superior Geology 54th Annual Meeting, Proceedings and Abstracts, v.54, part 1, p.42-43. Hollings, P., Smyk, M., Halls, H. and Heaman, 2010a. L. in press. The geochemistry, paleomagnetism and geochronology of the dykes and sills associated with the Midcontinent Rift near Thunder Bay, Ontario, Canada; Precambrian Research, 2010a, doi:10.1016/j. precamres.2010.01.012. - 54 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Hollings, P., Smyk, M., and Carl, C. 2010b. Geochemistry of Midcontinent Rift–related dikes on the Sibley Peninsula, Thunder Bay: a preliminary report; in Summary of Field Work and Other Activities 2010, Ontario Geological Survey, Open File Report 6260, p.10-1 to 10-3. Horton, R., 1989. The Mining and Geologic History of the Silver Islet Mine, and a Conceptual Ore Genesis Model for the Deposit. Institute on Lake Superior Geology Proceedings, 35: 29-31. Klewin, K., and Shirey, S., 1992. The igneous petrology and magmatic evolution of the Midcontinent Rift system. Tectonophysics, 213: 33–40. Krogh, T.E., Davis, D.W. and Corfu, F., 1984. Precise U-Pb zircon and baddeleyite ages for the Sudbury area, In, ed. Pye et al., The Geology and Ore Deposits of the Sudbury Structure. Ontario Geological Survey, Special Volume 1, 431-446. Gogebic range, Lake Superior region, USA. Sedimentology, 51, 791-808. Richardson, A., and Hollings, P., 2005. Geochemical Variation within the Mesoproterozoic Nipigon Diabase Sills. Institute on Lake Superior Geology 51st Annual Meeting, Proceedings Volume 51, Part 1 – Program and Abstracts, 52-53. Rogala, B., 2003. The Sibley Group: a lithostratigraphic, geochemical and paleomagnetic study. Unpublished M.Sc. thesis, Lakehead University, Thunder Bay, ON, 254 pp. Rogala, B., Fralick, P.W. and Metsaranta, R.T, 2005. Stratigraphy and sedimentology of the Mesopreterozoic Sibley Group and related igneous intrusions, northwestern Ontario. Lake Nipigon Region Geoscience Initiative, Ontario Geological Survey, Open File Report 6174, 128 pp. Kustra, C.R., McIlwaine, W.H., Fenwick, K.G. and Scott, J.F. (1977) Proterozoic rocks of the Thunder Bay area, northwestern Ontario; Field Trip Guidebook, 23rd Annual I.L.S.G. Meeting, Thunder Bay, 47p. Rogala, B., Fralick, P.W., Heaman, L and Metsaranta, R.T., 2007. Lithostratigraphy and chemostratigraphy of the Mesoproterozoic Sibley Group, northwestern Ontario, Canada. Canadian Journal of Earth Sciences, 44, 1131-1149. Maric, M., 2006. Sedimentology of the Rove and Virginia Formations. Unpub. M.Sc. thesis, Lakehead University, Thunder Bay, ON. Schulz, K., and Cannon, W. The Penokean orogeny in the Lake Superior region. Precambrian Research, 157: 4–25. Maric, M. and Fralick, P.W., 2005. Sedimentology of the Rove and Virginia Formations and their tectonic significance. Institute on Lake Superior Geology, 51, 41-42. Shirey, S., Klewin, K., Berg, J., Carlson, R., 1994. Temporal changes in the sources of flood basalts: isotopic and trace element evidence from the 1100Ma old Keweenawan Mamainse Point Formation, Ontario, Canada. Geochimica et Cosmochimica Acta 58, 4475–4490. Metsaranta, R. 2006. Sedimentology and geochemistry of the Mesoproterozoic Pass Lake and Rossport Formations, Sibley Group; unpublished M.Sc. thesis, Lakehead University, Thunder Bay ON. Mohide, T.P. 1985. Silver; Ontario Ministry of Natural Resources, Mineral Policy Background Paper No. 20, 406p. Osmani. I.A. 1991. Proterozoic mafic dike swarms in the Superior Province of Ontario; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 1, 661-681. Pesonen, L.J. and Halls, H.C. 1983. Geomagnetic field intensity and reversal asymmetry in late Precambrian Keweenawan rocks. Geophysical Journal of the Royal Astronomical Society, 73: 241-270. Pufahl, P.K., 1996. Stratigraphic architecture of a Paleoproterozioc iron formation depositional system: the Gunflint, Mesabi and Cuyuna iron ranges. Unpub. M.Sc. thesis, Lakehead University, 167 pp. Pufahl, P.K. and Fralick, P.W., 2000. Field trip 4 Depositional environments of the Paleoproterozoic Gunflint Formation. In, ed. P.W. Fralick, Institute on Lake Superior Geology, Proceedings Volume 46, Part 2: Field Trip Guide Book. Smyk, M.C. and Hollings, P.N. 2007. Midcontinent Riftrelated mafic intrusions north of the international border; 53rd Institute on Lake Superior Geology, Annual Meeting, Lutsen, Minnesota, May 2007, Proceedings Volume 53, Part 2, Field Trip Guidebook, p.53-80. Smyk, M. and Hollings, P., 2009. Project Unit 08-021. Mesoproterozoic Midcontinent Rift–Related Mafic Intrusions Near Thunder Bay: Update. Summary of Fieldwork and Other Activities 2009, Ontario Geological Survey, Open File Report 6240, p. 11-1 to 18-5. Sutcliffe, R.H. 1991. Proterozoic geology of the Lake Superior area; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 1, p. 627658. Tanton, T.L. 1924. Thunder Cape, Lake Superior; Geological Survey of Canada, Publication 1902, scale 1:36 000. Tanton, T.L. 1931. Fort William and Port Arthur, and Thunder Cape map-areas, Thunder Bay District, Ontario; Geological Survey of Canada, Memoir 167, 222p. Pufahl, P.K. and Fralick, P.W., 2004. Depositional controls on Palaeoproterozoic iron formation accumulation, - 55 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Field trip 3 - Lac des Iles Mine Mark Smyk Resident Geologist Program, Ontario Geological Survey, Ministry of Northern Development and Mines, Thunder Bay, Ontario, P7E 6S7, Canada John Corkery North American Palladium, Ltd., Thunder Bay, Ontario, Canada Regional Setting of the Lac des Iles Mine The Lac des Iles Mine area is underlain by mafic to ultramafic rocks of the Neoarchean Lac des Iles Intrusive Complex (LDI-IC). The LDI-IC is part of the Lac des Iles Suite, whose mafic intrusive rocks generally range in age between 2686 and 2699 Ma (c.f. Stone, 2010). These rocks have intruded a variety of metamorphosed granitoid and supracrustal greenstone belt rocks (ca. 2.9 to 2.7 Ga in age) of the Wabigoon Subprovince of the Superior Province (Fig. 1). The LDI-IC lies immediately north of the boundary between the volcano-plutonic Wabigoon and metasedimentary Quetico subprovinces. The LDI-IC is the largest of a series of mafic and ultramafic intrusions that occur along the Wabigoon-Quetico boundary and which collectively define a 30 km diameter circular pattern (Fig. 1). There are three broad, temporally diverse settings for Archean platinum group element (PGE) mineralization west of Lake Nipigon (c.f. Smyk et al., 2002): (1) Pre-tectonic mafic to ultramafic subvolcanic(?) intrusions intimately associated and coeval with greenstone belts of various ages in the Wabigoon Subprovince; (2) Mafic intrusive rocks occurring within syntectonic to post-tectonic, diorite-monzodiorite-monzonite suites with sanukitoid affinity within the Wabigoon and Quetico subprovinces (e.g., Shelby Lake Figure 1. Regional setting of the Lac des Iles complex and related ultramafic and mafic intrusions within the Wabigoon Subprovince (from Lavigne and Michaud, 2002). - 56 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 2. Geologic setting of the Lac des Iles complex and related ultramafic and mafic intrusions (from Lavigne and Michaud) batholith, Stone et al., 2003; Roaring River Complex, Schnieders et al., 2002). (3) Posttectonic, mafic to ultramafic intrusions related to late plutonism in the Wabigoon Subprovince, hosted by gneissic tonalite-granodiorite (e.g., Lac des Iles suite). Pre-tectonic, deformed mafic to ultramafic intrusions and/or coeval komatiitic metavolcanic rocks within greenstone belt assemblages may host coppernickel-PGE mineralization. This broad classification is equivalent to the “komatiitic-associated” and “intrusions comagmatic with volcanic rocks” settings for coppernickel-PGE ± chromium mineralization described by Fyon et al. (1992). Such deposits are characterized by remobilized, deformed and annealed, net-textured to massive sulfides. Examples include the past-producing Shebandowan Mine in the Shebandowan greenstone belt, west of Thunder Bay (8.64 million tonnes mined, grading 1.92% Ni, 0.98% Cu, 2.62 g/t Pt+Pd; Resident Geologist’s Files, Thunder Bay South District, Thunder Bay), and in the Obonga Lake belt, the Core Zone gabbro (2733±7 Ma; Tomlinson et al., 1999) and the Puddy Lake serpentinite (both may contain indications of such mineralization). Lavigne et al. (1991) reported metal contents from the Puddy Lake serpentinite up to 5.02% Cu, 2.1% Ni, 415 ppb Au, 1500 ppb Pt and 3750 ppb Pd; cobalt values of 0.07% were reported from drilling in the 1960s (Lavigne et al., 1992). PGE mineralization is also associated with rocks of the sanukitoid suite (ca. 2688 to 2690 Ma, Davis et al., 1990; Kamo, 2004; cf. Stern et al., 1989). The Shelby Lake batholith, which consists of hornblende leucogabbro (diorite) to monzodiorite and hornblende granodiorite to granite, contains disseminated sulfide zones in thin units of hornblende gabbro distributed along its northwestern margin (e.g., Turtle Hill and Stocker occurrences). Similar, somewhat larger sulfide occurrences have been described (Stone et al., 2003) at Wakinoo Lake, Towle Lake (e.g., Powder Hill, Stinger and Vande occurrences) and Legris Lake (e.g., Main and Poplar occurrences). Diamond drill holes in hornblende gabbro at Legris Lake contained 2.04 g/t Pd, 0.41 g/t Pt, 0.71 g/t Au, 0.42% Cu and 0.13% Ni over 9.95 m (News Release, Avalon Ventures Ltd. and Starcore Resources Ltd., November 10, 2000). The Roaring River complex (Stern and Hanson, 1991) consists of a variety of plutonic rocks including diorite, monzodiorite, monzonite, quartz monzodiorite and granodiorite, all of sanukitoid affinity; gabbroic and - 57 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 pyroxenitic mega-inclusions occur in these phases. Grab samples of the Mere showing contain up to 1249 ppm Ni, 3159 ppm Cu and 1.1 g/t Pt+Pd+Au and the Leigh (boulder) occurrence returned up to 2067 ppm Ni, 1920 ppm Cu and 1.23 g/t Pt+Pd+Au (Schnieders et al., 2002 and references therein). Disseminated to locally net-textured chalcopyrite, iron sulfides, pentlandite and magnetite typically characterize PGE- mineralized zones, which are commonly associated with intrusive contacts, polyphase intrusive breccia, and sheared and hydrothermally altered zones. Mafic to ultramafic intrusions of the Lac des Iles suite (ca. 2686 to 2699 Ma; Stone, 2010 and references therein; Davis, 2003; Kamo, 2004) and their associated copper-nickel-PGE mineralization at North American Figure 3. Geology of the Northern Ultramafic and Mine Block intrusions of the Lac des Iles Complex (from Lavigne and Michaud , 2002). - 58 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Palladium Ltd.’s Lac des Iles mine were most recently described by Stone et al. (2003). This suite includes the Buck Lake, Dog River, Taman Lake, Demars Lake, Bullseye and Tib Lake intrusions (see Fig. 2), as well as the Northern Ultramafic intrusion and Mine Block intrusion at Lac des Iles (Fig. 3). These leucogabbro and gabbronorite intrusions (± anorthosite, peridotite) range from 1 to 10 km in diameter and are considered to represent a continuum of the Quetico suite of mafic to ultra mafic intrusions (cf. MacTavish, 1999; Pettigrew et al., 2000). Michaud (1998), Lavigne and Michaud (2002), and Lavigne et al. (2005) provided recent descriptions of the deposits in the Mine Block intrusion (Fig. 4). Platinum group elements are associated with disseminated Cu-Ni-sulfide minerals in the matrix of magmatic breccia, in varitextured to pegmatitic gabbroic rocks (which together represent the Breccia Zone), and also in pyroxenite that is part of the HighGrade Zone. Platinum group elements also occur with sulfide-poor, varitextured to pegmatitic gabbro in the Roby and North Roby zones and are locally associated with strong silicate alteration (e.g., Roby Zone and portions of the High-Grade Zone; Lavigne et al., 2005). The Roby Zone is the product of multiple stages of intrusion, alteration and mineralization (Lavigne et al., 2005). Hinchey et al. (2005) put forward a schematic model illustrating a deposit model for the history of mineralization at the southern Roby Zone (Fig. 5). The textures of the Lac des Iles deposit are similar to those of contact-type PGE deposits, but there are fundamental differences between the two. The Lac des Iles deposit is not localized near the contact between the host intrusion and the country rocks and evidence of the assimilation of the host rocks is lacking. Instead, the mineralization at Lac des Iles has many features in common with layered intrusion-hosted deposits, in which pulses of primitive magma introduced the PGE. Unlike the quiescent magma chambers of most Figure 4. Geology of the Mine Block intrusion showing the main ore zones (from Lavigne and Michaud, 2002). - 59 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 5. Schematic mineralization model for the Lac des Iles Mine (Hinchey et al., 2005) layered deposits, the magmas at Lac des Iles were intruded energetically, forming breccias and magmamingling textures. Magmas formed by a high degree of partial melting in a depleted mantle source (Fig. 5, A1) became enriched in Cu, Pt, and Pd through fractional crystallization of olivine, chromite, and high- - 60 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 temperature PGM (Fig. 5, A2), segregated sulfide melt that had low Cu/Pd ratios along the conduit and the base of the magma chamber (Fig. 5, A3), and solidified as the early leucocratic gabbros. A second episode of partial melting in the mantle source produced another batch of fertile magma. As with the early magma, this magma was enriched in Cu, Pt, and Pd through fractional crystallization (Fig. 5, A2). This magma incorporated the earlier sulfide melt and intruded forcefully into the partially crystallized leucocratic rocks (Fig. 5, B1), causing brecciation and magma mingling, and solidified as fertile melanocratic gabbro. Aqueous fluids that separated from the melanocratic magma percolated through the cumulates, partially dissolving Pd and concentrating it in the High Grade ore zone adjacent to barren East Gabbro (Fig. 5, B2). History of Exploration and Mining The Lac des Iles area was initially mapped by Jolliffe (1934) and later by Pye (1968), Watkinson and Dunning (1979), Sutcliffe and Sweeny (1985, 1986), Sweeny and Edgar (1987), and Stone et al. (2003). The regional geology has been summarized by Stone (2010). Economic interest in the area was sparked by the ground-truthed aeromagnetic anomalies. Significant palladium mineralization was first discovered in the Roby Zone in 1963 by a prospecting syndicate. Various exploration programs were undertaken over the next 25 years by a number of companies, including Gunnex Ltd., Anaconda Ltd., Texas Gulf Sulphur Co. Inc., and Boston Bay Mines Ltd. In 1990, Madeleine Mines Ltd., a precursor to North American Palladium Limited (NAPL), developed the property. After intermittent production and continuing capital expenditures, commercial open pit production of the Roby Zone was achieved in December 1993. NAPL was formed through corporate reorganization. In 2000, an expansion program began and a new mill was commissioned in the second quarter of 2001 to achieve its rated 15,000 t per day throughput in August 2002. From 1999 to 2001, an extensive drilling campaign identified mineralization at depth, below the ultimate pit bottom. The drilling identified 2 zones with potential for underground mining: the Roby Underground Zone and the Offset Zone. On July 31, 2003, a positive pre-feasibility study for underground mining of the Roby Underground Zone (down-dip extension of the open pit Main Zone) was completed, and was followed by a feasibility study for underground mining in 2004. Development on the Roby Underground Zone started in 2004, with the ramp developed and the zone accessed in late 2005. Development muck was delivered to the concentrator in December 2005 and underground commercial production began in March 2006. The Offset Zone, discovered in 2000, was historically subdivided into the Offset High Grade Zone and the adjacent Roby Footwall Zone. The Offset Zone is the fault-offset, down-dip extension of the Roby Underground Zone that was mined below the Roby open pit until October 2008. A number of surface and underground drilling programs have targeted the Offset Zone since 2001. In 2008, a surface drilling program focused on exploring targets on the Mine Block Intrusion and on Table 1. Production figures for Lac des Illes (MD&A, 2011) Unit Ore Mined Waste Mined – Open Pit Mill Throughput Pd Head Grade Pd Recovery Pd Produced Pt Produced Au Produced Ni Produced Cu Produced Tonnes Tonnes Tonnes g/t % Oz Oz Oz Lbs Lbs 2011 2010 1,830,234 615,926 1,689,781 3.70 78.34 146,624 9,143 7,267 816,037 1,596,185 649,649 6.06 80.80 95,057 4,894 4,023 395,622 658,013 2009 Mine closed * Added; Ore Mined - Underground + Ore Mined - Open Pit - 61 - 2008 *3,676,418 6,964,501 3,722,732 2.49 75.30 212,046 16,311 15,921 2,503,902 4,623,278 �Proceedings of the 58th ILSG Annual Meeting - Part 2 Table 2. Resource figures for Lac des Illes (MD&A, 2011) the Southeast Breccia Zone, situated adjacent to the southeastern corner of the open pit. The Cowboy Zone was discovered in June 2009 during infill drilling of the Offset Zone to support a prefeasibility study (news release, NAPL, June 25, 2009). It is located 30 to 50 m down-section to the west of the Offset Zone, extends for up to 250 m along strike and 350 m down-dip, and it remains open in all directions. Similar to the Offset Zone, the Cowboy Zone appears to consist of several mineralized subzones. Intersections include 5.10 g/t Pd over 4 m, 3.88 g/t Pd over 4 m, and 4.46 g/t Pd over 5 m. Open pit mining of the Roby Zone began in 1993. The open pit was operated by conventional truckandshovel mining, with low- and high-grade material stockpiled near the on-site concentrator. In May 2004, LDI collared a portal in the northwest wall of the pit and ramped down to access the Roby Underground Zone that continues down-dip from the Roby Zone hanging wall below the pit. LDI began processing development muck from the Roby Underground Zone in December 2005. The ramp was extended around the pit to the north and the new portal was opened in the east wall in 2006. The Roby Underground Zone reached commercial production at 2000 t per day in April 2006. Operations were suspended in October, 2008 due to the global economic downturn and depressed metal prices. Palladium production at Lac des Iles Mine resumed in April 2010 (news release, NAPL, April 14, 2010). NAPL expects to produce 140 000 ounces of palladium per year. Ore production from the Roby Underground zone is expected to increase to a target rate of 2600 t per day. A summary of ore mined is presented in Table 1. Since production began in 1993 at Lac des Iles Mine, almost 42 Mt of ore have been processed, and approximately 2.3 million ounces of palladium produced (see Table 1). A Mineral Resource Summary (December 31, 2008) is given in Table 2 (McCombe et al., 2009). Local and Property Geology Many of the following excerpts have been modified after McCombe et al. (2009). The mine lies in the southern portion of the Lac des Iles Intrusive Complex (LDI-IC) (see Figs. 2 & 3), in a roughly elliptical intrusive package measuring 3 km long by 1.5 km wide, termed the Mine Block Intrusive (MBI) (see Figs. 3 & 4). It hosts a number of PGE deposits and the most important of these is the Roby Zone with its three subzones: the North Roby Zone, the High Grade Zone, and the Breccia Zone. The MBI comprises rocks with a very wide range of textures and mafic and ultramafic compositions, ranging from anorthosite to clinopyroxenite, leucogabbronorite to melanonorite, and includes magnetite-rich gabbro. Textures include equigranular, fine- to coarsegrained, porphyritic and pegmatitic, varitextured - 62 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 units, and heterolithic gabbro breccias. These last three textural types are the most common host to PGE mineralization, including the Roby Zone. The MBI consists of two lithologically distinct domains. The oval-shaped domain immediately south of Lac des Iles is lithologically complex and contains widespread PGE mineralization, while the domain further to the south is dominated by massive, mediumgrained, PGE-barren gabbronorite (see Figs. 2 to 4). Extensive stripping has disclosed that the interior of the oval-shaped domain has an abundance of monolithic and heterolithic breccia with an average composition of gabbronorite. Within this area, individual lithological units are not laterally extensive and are chaotically distributed. The most laterally continuous unit is a massive, mediumgrained gabbro, referred to as East Gabbro (EGAB) (see Fig. 5). EGAB is adjacent to a varitextured gabbro “rim” to the west and more equigranular gabbronorite (GN) to the east. The varitextured rim is host to the Roby palladium deposit, where heterolithic gabbro breccia (HGABBX) commonly occurs as pipes and pods, and large blocks (~60 m) of varying composition. A pyroxenite unit (PYXT), at the contact between the EGAB and the HGABBX, is host to much of the High Grade Zone. The principal rock types in the Offset Zone area include the following: East Gabbro (EGAB) – is a well-known gabbro “marker unit” that is characteristically uniform and compositionally homogeneous. EGAB has very minor alteration, with local trace pyrite and epidote. It has no significant associated mineralization, and bounds the Roby Zone to the east. (i.e., hanging wall contact of the Roby Zone). Heterolithic Gabbro Breccia (HGABBX) – the principal host for the Roby Zone, consisting of a melanogabbro to gabbro matrix with variable clast composition, ranging from leucogabbro to pyroxenite. Clast percentage varies commonly from 15 to 60%. This unit comprises most of the economic ore grade material in the open pit and underground reserves. Varitextured Gabbro (VGAB) – the majority of rock types, excluding EGAB, have a varitextured counterpart. The VGAB varies from leucocratic to pyroxenitic, with grain sizes from fine to very coarse, to pegmatitic. The coarser-grained units form patches and “veinlets” within finer-grained counterparts. Gabbro (GAB) – the most common gabbros in the MBI are medium grained and equigranular, but range from fine to coarse grained and may locally be leucocratic to melanocratic. Magnetic Gabbro (MTGAB) – medium-grained, equigranular gabbro occurs within the MBI and contains black, fine-grained, interstitial magnetite (typically < 20%); magnetite content ranges from trace amounts to local, narrow layers of 60 to 95% magnetite. Pyroxenite (PYXT) – a steeply dipping, thin layer situated along the contact between the Heterolithic Gabbro and EGAB; it hosts the highest proportion of the High Grade Zone. This unit is responsible for much of the high PGE grades. Not all pyroxenites locally carry economic PGE grades. Gabbronorite (GN) – a 20 to 50 m thick, steeply dipping slab located along the northwestern contact of the EGAB; it is also a host unit of the High Grade Zone, although to a lesser degree than the PYXT. The gabbronorite appears to be a gradational extension of the pyroxenite to the northeast of the mine site. Gabbronorite Breccia (GNBX) – a palladiummineralized (Twilight Zone) heterolithic breccia, similar to the HGABBX but without pegmatitic phases or varitextured gabbro; it occurs as a roughly cylindrical pod, approximately 150 m in diameter, completely enclosed by the EGAB. Dikes – late, post-mineralization, mafic dikes vary from small, discrete bodies that occupy space within the modeled mineralized wire frames to large bodies that control the northern termination of the Offset Zone. A dike swarm approximately 30 m wide and trending approximately easterly was mapped at the southern extent of the Roby Zone. Two major faults have been interpreted to influence the Offset Zone. The Offset Fault structure displaces the High Grade Zone down and approximately 300 m to the west. This fault, easily picked out in diamonddrill core, is often marked by extensive fault gouge, fracturing, alteration of adjacent country rock, and infilling by mafic dikes. The B2 Fault has recently been recognized and interpreted from the underground Offset Zone diamond drilling. It lies approximately 20 to 40 m below and parallel to the westerly dipping Baker Fault and is marked by narrow intersections of fault gouge, fracturing and late mafic dikes. Mineralized Zones The Roby Zone is a bulk-mineable, PGE-enriched disseminated sulfide deposit with a minimum north to south length of 950 m, and a width of 815 m, including the Twilight Zone in the southwestern portion of the - 63 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 deposit. The Roby Zone consists of 3 distinct ore types: High Grade Ore (7.6% of volume), North Roby Ore (5.3% of volume), and Breccia Ore (87.1% of volume). The High Grade Ore is the primary ore type mined underground. High Grade Zone ore is hosted mainly within a 15 to 25 m thick unit of locally sheared pyroxenite/ melanogabbro. A host to high-grade PGE mineralization, it is located in the east-central portion of the Roby Zone, bounded by the barren EGAB hanging wall and HGABBX-hosted Breccia Ore to the west. The High Grade Zone is primarily confined to a 400 m long segment of the pyroxenite, although it does extend northward into the gabbronorite. The High Grade Zone, striking north-northwest to northnortheast, dips almost vertically near surface and flattens to nearly 45° at depth. Below the open pit, this zone is referred to as the Roby Underground Zone. The zone appears to be terminated down dip by a relatively shallow dipping fault, the Offset Fault. The Offset Zone, a higher grade zone similar to the High Grade Zone, is located below the Offset Fault structure, where it is displaced down and approximately 300 m to the west. The Offset Zone can be split into 3 horizons and has been divided into 3 subzones: the High Grade (HG) Subzone; the Mid (MID) Subzone; and the Footwall (FW) Subzone. High Grade Subzone mineralization is stratabound, along the contact between the EGAB and the mineralized HGABBX. Within the HGABBX, there is a high-grade core typically constrained to an easily recognized ultramafic unit, the pyroxenite. Width varies from 4 to 30 m, with an average of 15 m. Approximately 2% of the zone is occupied by late dikes (dilution). Less than 1% is occupied by shears and faults. The MID Subzone is proximal to the HG Zone, generally sharing a common boundary in the centre sections and then splitting away near the top and bottom areas. Palladium grades within the MID Subzone can approximate the high grades found within the HG Zone. Apparent widths can vary from 4 to 90 m, with an average of 15 m. Approximately 4% of the zone is occupied by late dikes (dilution). Less than 1% is occupied by shears and faults. The Footwall Subzone is a stand-alone band of higher grade mineralization that can be defined based on higher grade intersections within the Footwall varitextured gabbro mineralization. This subzone, located approximately 2 to 40 m from the MID Zone, was interpreted based on vertical continuity seen in the drill hole intersections. It is discontinuous and sinuous in plan and has less of a defined areal extent than the other zones. Apparent widths can vary from 4 to 20 m, with an average of 7 m. Approximately 1% of the zone is occupied by late dikes (dilution). Other mineralized zones present within the MBI, as shown in Figure 2-4, are described below: The Twilight Zone was removed with the mining of the open pit. The Baker Zone is located approximately 1 km northeast from the Roby and Twilight zones and contains similar rock types and textures. Gabbronorites/norites have been intruded by eastnortheast-trending, heterolithic melanogabbro breccia and lesser melanogabbro, leucogabbro breccia, varitextured gabbro and late pyroxenite dikes. Surface exploration has exposed the Baker Zone breccias and associated lithologies over a 150 by 55 m area. The heterolithic melanogabbro breccia hosts blebby to disseminated to narrow veinlets of sulphide with sporadic mineralization in the adjacent lithologies. The north-trending, shallowly westerly dipping Baker Fault appears to truncate the Baker Zone mineralization at depth. Extensive surface exploration by NAP occurred mainly from 1998 to 2001 and consisted of prospecting, stripping/trenching (including the main stripped area of approximately 200 by 120 m), channel sampling, geological mapping and ground induced polarization (IP) / resistivity surveys. Sixteen diamond-drill holes in 1998–99 tested the main portion of the Baker Zone over a 250 m strike length and to a maximum depth of 200 m. Subsequent exploration (trenching and diamond drilling) has tested possible strike extensions of the zone and the area below the Baker Fault. The Moore Zone is a low-grade, presently uneconomic, mineralized zone approximately 500 m south of the current Roby pit with similar lithologies and textures to other MBI breccias. The central area of interest is a small breccia pod measuring approximately 200 m long, varying from approximately 15 to 115 m wide, which occurs within the massive, medium-grained gabbronorite typical of the more southerly domain of the MBI. The main Moore Zone mineralization is located in the eastern portion of the breccia pod and appears to be structurally controlled (trending ~030°, dipping 70° east), ranging from 5 to 25 m thick. Prospecting, mapping, trenching, sampling and limited diamond drilling of the Moore Zone have indicated limited economic potential. - 64 - The Creek Zone is located approximately 2 km �Proceedings of the 58th ILSG Annual Meeting - Part 2 northeast of the Roby pit in the northeastern nose of the MBI, near the contact with the north LDI-IC. Surface trenching has exposed the main portion of the Creek Zone in an area 90 m long by 10 to 40 m wide. It is dominated by low-sulfide breccias that have intruded the varitextured gabbro rim of the MBI. The breccias consist of approximately 90% GBNR clasts and only approximately 10% MGAB matrix. Unlike the Roby Zone, mineralization is not dominantly hosted by the breccia matrix but seems to occur within the pegmatitic gabbronorite. Prospecting, mapping, trenching, sampling and limited diamond drilling of the Creek Zone have indicated limited mineralized potential. The platinum-group minerals at Lac des Iles Mine include the following (Lavigne and Michaud, 2001): Braggite (Pt,Pd)S Kotulskite Pd(Te,Bi)2 Isometrieite Pd11(Sb,Te)2As2 Merenskyite PdTe2 Moncheite PtTe2 Palladoarsenide Pd2As Sperrylite PtAs2 Stibiopalladinite Pd5Sb2 Stillwaterite Pd8As3 Mineralization Platinum group element and base metal mineralization at the Lac des Iles Mine appears to be dominantly stratabound along the contact between the EGAB and the mineralized HGABBX. Within the HGABBX, there is a high-grade core typically constrained to an easily recognized pyroxenite unit. Visible PGE mineralization is rare and its occurrence is difficult to predict. In general, economic PGE grades are anticipated within gabbroic to pyroxenitic rocks (in close proximity to the marker unit EGAB) that exhibit strong sausseritization of plagioclase feldspars, strong talcose alteration and association with either disseminated or blebby secondary sulfides. Higher PGE grades (mean – 7.89 g/t Pd, maximum – 55.95 g/t Pd) occur in those portions of the pyroxenite that are altered to an assemblage of amphibole (anthophylliteactinolite-hornblende)-talc-chlorite. The PGE tenor is not proportional to the sulfide content, and samples free of visible sulfide often contain more than 10 g/t Pd. The high-grade mineralization is located primarily within the western, highly altered portion of the pyroxenite, since much of the pyroxenite between the barren EGAB and the High Grade Zone is low grade. The higher grade “High Grade Ore” is not restricted to the pyroxenite as it commonly straddles the pyroxenite/ gabbro breccia contact to widths exceeding 250 m. The majority of platinum-group minerals occur either interstitially to sulfides as cumulus grains or are associated with sulfides at sulfide-silicate boundaries, occurring as discrete mineral inclusions within secondary silicates of altered rocks (Sweeny 1989; Lavigne and Michaud 2001). Palladium and platinum mineralization within the High Grade Zone consists primarily of fine-grained PGE sulfide, braggite and the telluride minerals merenskyite and kotulskite (Sweeny, 1989; Lavigne and Michaud, 2001). Vysotskite PdS Unnamed Ag4Pd3Te4 Unnamed Pd5As2 Melonite, gold, pentlandite Pd in solid solution Field Trip Stops Roby Zone open pit Baker Zone North VT Rim Trenches Exploration office and diamond drill core Directional drilling sites and Devico Unit Mill References Davis, D.W. 2003. U-Pb geochronology of rocks from the Lac des Iles area, northwest Ontario; Ontario Geological Survey, internal report, June 12, 2003. Davis, D.W., Pezzutto, F. and Ojakangas, R.W. 1990. The age and provenance of metasedimentary rocks in the Quetico Subprovince, Ontario, from single zircon analyses: Implications for Archean sedimentation and tectonics in the Superior Province; Earth and Planetary Science Letters, v.99, p.195-205. Fyon, J.A., Breaks, F.W., Heather, K.B., Jackson, S.L., Muir, T.L., Stott, G.M. and Thurston, P.C. 1992. Metallogeny of metallic mineral deposits in the Superior Province of Ontario; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, pt.2, p.1091-1174. Hinchey, J.G., Hattori, K.H. and Lavigne, M.J. 2005. Geology, petrology, and controls on PGE mineralization of the Southern Roby and Twilight Zones, Lac des Iles Mine, Canada; Economic Geology, v.100, p.43-61. Jolliffe, F. 1934. Block Creek map area, Thunder Bay - 65 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 District, Ontario; Geological Survey of Canada Summary Report 1933, pt.D, p.7-15. Kamo, S. 2004. U-Pb geochronological investigations of rocks from the Lac des Iles area, northwestern Ontario, the Michipicoten Greenstone belt, Wawa, and the Tomiko Terrane, Mattawa, Ontario; Ontario Geological Survey, internal report, July 2004. Lavigne, M.J. and Michaud, M.J. 2001. Geology of North American Palladium Ltd.’s Roby Zone Deposit, Lac des Iles; Exploration and Mining Geology, v.10, Nos. 1 and 2, p.1-17. Lavigne, M.J. and Michaud, M.J. 2002. Geology of North American Palladium Ltd.’s Roby zone deposit, Lac des Iles; Exploration and Mining Geology, v.10, p.117. Lavigne, M.J., Michaud, M.J. and Rickard, J. 2005. Discovery and geology of the Lac des Iles palladium deposits; in Exploration for Platinum Group Element Deposits, Mineralogical Association of Canada, Short Course Series, v.35, Oulu, Finland, p.369-390. Lavigne, M.J., Scott, J.F. and Sarvas, P. 1991. Thunder Bay Resident Geologist’s District; in Report of Activities, 1990, Resident Geologists, Ontario Geological Survey, Miscellaneous Paper 152, p.107-126. Lavigne, M.J., Scott, J.F. and Sarvas, P. 1992. Thunder Bay Resident Geologist’s District; in Report of Activities, 1991, Resident Geologists, Ontario Geological Survey, Miscellaneous Paper 158, p.87-105. MacTavish, A.D. 1999. The mafic-ultramafic intrusions of the Atikokan–Quetico area, northwestern Ontario; Ontario Geological Survey, Open File Report 5997, 154p. Management’s Discussion and Analysis and Consolidated Financial Statements Fourth Quarter 2011 For the year ended December 31, 2011 p. 11 http:// www.napalladium.com/Theme/NAP/files/Q4%20 2011%20MDA%20and%20FS-Final%20Feb23.pdf McCombe, D.A., Blakley, I.T., Routledge, R.E. and Cox, J.J. 2009. Technical report on the Lac des Iles Mine, North American Palladium Ltd., internal report, Scott Wilson Roscoe Postle Associates Inc., 130p. Michaud, M.J. 1998. The geology, petrology, geochemistry and platinum group element-gold-copper-nickel ore assemblage of the Roby Zone, Lac des Iles mafic-ultramafic complex, northwestern Ontario; unpublished MSc thesis, Lakehead University, Thunder Bay, Ontario, 183p. Pettigrew, N.T., Hattori, K.H. and Percival, J.A. 2000. Mafic-ultramafic intrusions of the central portion of the western Quetico subprovince, northwestern Ontario; in 2000 Western Superior Transect, 6th Workshop, Lithoprobe Report #77, University of British Columbia, p.104-110. Pye, E.G. 1968. Geology of the Lac des Iles area, District of Thunder Bay, Ontario Department of Mines, Geological Report 64, 47p. Schnieders, B.R., Scott, J.F., Smyk, M.C., Parker, D.P. and O’Brien, M.S. 2002. Report of Activities 2001, Resident Geologist Program, Thunder Bay South Regional Resident Report: Thunder Bay South District; Ontario Geological Survey, Open File Report 6081, 45p. Smyk, M.C., Mason, J.K., Schnieders, B.R. and Stott, G.M. 2002. A synopsis of Archean and Proterozoic platinum group element mineralization in the Thunder Bay District, Ontario; Extended Abstract Volume, 9th International Platinum Symposium, 25 July 2002, Billings, Montana, p.433-434. Stern, R.A. and Hanson, G.N. 1991. Archean highMg granodiorite: A derivative of light rare earth elementenriched monzodiorite of mantle origin; Journal of Petrology, v.32, pt.1, p.201-238. Stern, R.A., Hanson, G.N. and Shirey, S.B. 1989. Petrogenesis of mantle-derived, LILE-enriched Archean monzodiorites and trachyandesites (sanukitoids) in southwestern Superior Province; Canadian Journal of Earth Sciences, v.26, p.1688-1712. Stone, D. 2010. Precambrian geology, central Wabigoon Subprovince area, northwestern Ontario; Ontario Geological Survey, Preliminary Map P.2229, scale 1:250 000. Stone, D., Lavigne, M.J., Schnieders, B.R., Scott, J. and Wagner, D. 2003. Regional geology of the Lac des Iles area; in Summary of Field Work and Other Activities 2003, Ontario Geological Survey, Open File Report 6120, p.15-1 to 15-25. Sutcliffe, R.H. and Sweeny, J.M. 1985. Geology of the Lac des Iles complex, District of Thunder Bay; in Summary of Field Work and Other Activities 1985, Ontario Geological Survey, Miscellaneous Paper 126, p.47-53. Sutcliffe, R.H. and Sweeny, J.M. 1986. Precambrian geology of the Lac des Iles complex, District of Thunder Bay, Ontario; Ontario Geological Survey, Preliminary Map P.3047, scale 1:15 840. Sweeny, J.M. and Edgar, A.D. 1987. The geochemistry, origin and economic potential of platinum-group element bearing rocks of the Lac des Iles complex, northwestern Ontario; in Geoscience Research Grant Program, Summary of Research, 1986-1987, Ontario Geological Survey, Miscellaneous Paper 136, p.140152. Tomlinson, K.Y., Davis, D.W., Percival, J.A., Hughes, D.J. and Thurston, P.C. 1999. Neoarchean supracrustal development in the central Wabigoon Subprovince: Nd isotope data and U/Pb geochronology; in Western Superior Transect Fifth Annual Workshop, LITHOPROBE Report 70, p.147-152. Watkinson, D.H. and Dunning, G.R. 1979.Geology and platinum-group mineralization, Lac des Iles complex, northwestern Ontario; Canadian Mineralogist, v.17, p.453-462. - 66 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Field trip 4 - Shebandowan Mine Area Alan Aubut, P.Geo. Sibley Basin Group Geological Consulting Services Ltd. Dorothy Campbell, P.Geo Resident Geologist Program, Ontario Geological Survey, Ministry of Northern Development and Mines, Thunder Bay, Ontario, P7E 6S7, Canada Introduction This field trip will focus on two main aspects of the geology of the Lake Shebandowan area: the presence of a suite of metasediment and metavolcanic rocks usually described as being “Timiskaming-type” that unconformably overlies older Archean, “Keewatintype” rocks; and the presence of numerous komatiitic ultramafic bodies within the underlying Keewatin metavolcanic rocks that in part are spatially associated with a Timiskaming-age pull-apart basin (Figs. 1 and 2). Many of the Archean terrains within the Canadian Shield are host to a neo-Archean sequence of shoshonitic/ alkali metavolcanic and fluvial metasedimentary rocks that occupy pull-apart basins typically spatially related to major sub-province transcurrent boundary faults such as the Kirkland Lake-Cadillac Fault and the Porcupine-Destor Fault. This suite of rocks is commonly referred to as “Timiskaming-type” after the Timiskaming Group found within the Abitibi Terrain. Examples of “Timiskaming-type” include the Oxford Lake Group in Manitoba, the Hauy Formation in the southern and northern parts of the Abitibi, the Opemisca Group near Chibougamau, the Seine Group of the Wabigoon Subprovince, and the Shebandowan Group (Card, 1990).   Figure 1. Shebandowan field trip stops - 67 - �Proceedings of the 58th ILSG Annual Meeting - Part 2   Figure 2 – General Geology – Shebandowan area. From Pye and Fenwick (1965) Timiskaming-type rocks range in age from approximately 2700 Ma to about 2680 Ma (Card, 1990; Corkery et al., 2000). They unconformably overlie older metavolcanic sequences hereafter referred to as “Keewatin”. Timiskaming-type metavolcanic rocks usually are shoshonitic (high Al2O3 and K2O with TiO2 content < 1.3 wt.%) and are associated with fluvial meta-conglomerates and meta-sandstones. The Timiskaming-type rocks in the Lower Shebandowan Lake area consist of calc-alkaline metavolcanic rocks and alluvial-fluvial metaconglomerates and meta-sandstones. The metasedimentary rocks are immature, typically with cross-bedded arenites and conglomerates with some shale and ironstone. The metavolcanic rocks are calc-alkaline to alkali/shoshonitic subaerial volcanic rocks. They all display rapid facies changes and internal unconformities and typically only display late deformational and metamorphic events. have traditionally been considered intrusive bodies. While no spinifex textures, considered diagnostic of flow emplacement, have been found there are other features present that indicate they were deposited as flows. These include the fact that they are stratabound and commonly have ironstone or chert beds along one contact. In addition, generally accepted notions that ultramafics are intrusive does not take into consideration that the density of molten ultramafic rocks is too high to allow emplacement by density contrast and therefore must have relied more on processes such as over-pressure. This, combined with the fact these rocks are typically associated with extensional environments makes emplacement by intrusion, especially when they are conformable to local stratigraphy, extremely difficult to explain except by extrusive processes. The komatiitic bodies of the Shebandowan area - 68 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Road Log Field Trip Stop Descriptions Drive west from Thunder Bay on Highway 1117. At Shabaqua turn left onto Highway 11 and then drive 12.4 km to the Junction of Highway 11 and the Shebandowan Mine Road. Turn Left. Stop 1. Timiskaming-type metavolcanic debris flow with intercalated sediments. 0 km - Junction of Hwy 11 and Shebandowan Mine Road 0.7 km – bridge over Shebandowan River 1.9 km – Stop 1 3.1 km – Stop 2 5.7 km – Stop 3a 5.8 km – Stop 3b 5.9 km – Stop 3c 7.6 km – trail into Stop 4. Follow trail for 250 m, keeping to the left 15.9 km – gate barring entrance to Shebandowan mine site 16.4 km – junction with road to No. 1 shaft. 16.7 km – Stop 5. 17.1 km – Stop 6. Return to turn off to No. 1 Shaft. Reset trip meter. Turn Right (west). 1.7 km – Stop 7. 2.9 km – junction with Otto Lake road – keep to the right. 4.9 km – junction with road – keep to the right. 5.5 km – Stop 8. Return to Otto Lake road junction, reset trip meter and then turn right. 0.9 km – Stop 9. 1.2 km – Stop 10. Return to Gate house. Take the road to the right. Drive 325 metres and turn left and drive 120m to Stop 11. Return to Gate House and turn right and head east. 13.2 km – Junction with Duckworth Road – Turn Right. 17.6 – Junction – keep to the right. 18.5 – Junction – keep to the left. 19.0 – Junction with I Zone Road – turn right. 20.5 – Stop 12. UTM coordinates NAD83; 15U 0716408E / 5388223N The relatively undeformed metavolcanic debris flows at this stop consist of poorly sorted sub-angular to well-rounded fragments in a fine- to medium -grained tuffaceous matrix. A characteristic feature is the presence of hornblende phenocrysts in both the fragments and the matrix. Locally what appears to be graded bedding within the debris flows is present. One such locality is on the east side of the road where the debris flows are in contact with an intercalated greywacke unit. Here fragments within the debris flow fine to the south. On the west side of the road the same greywacke unit shows evidence of folding (fold axis plunging vertically) with graded bedding in the north limb indicating a synclinal structure. Compositionally the debris flows vary from basalt (<53% SiO2) to rhyolite (Brown, 1985). Shegelski (1980) has shown that they represent a typical calcalkaline volcanic suite with shoshonitic affinities. The presence of red pigmentation in these debris flows has led to much speculation as to their depositional environment. Pigmentation has resulted in clasts and matrix ranging from grey-green to red in colour. Locally grey-green clasts exhibit hematized rims while others are totally hematized. There are several possible processes that may have produced this red colouration: hydrothermal alteration after lithification; magmatic differentiation; deposition in an Archean oxygenated atmosphere resulting in red bed formation; or secondary oxidation during the Proterozoic or Phanerozoic. By plotting the ratios of ferric iron oxide (Fe2O3) to ferrous oxide (FeO) against SiO2 Shegelski (1980) ruled out magmatic differentiation. He then postulated that the red pigmentation was the product of red bed development. Due to the variability in red pigmentation doubt still remains as to whether it is a product of red bed development. In particular is the spectrum of pigmentation, including red fragments in a greygreen matrix, green fragments in a reddish matrix and fragments that are partially red and partially green. It’s due to this variability that has led others, such as Brown (1985), to believe the pigmentation is the product of varying degrees of hematization subsequent to deposition. - 69 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Stop 2. Timiskaming-type conglomerate. UTM coordinates NAD 83; 15U 0715383E / 5387509N At this location two facies of the epiclastic suite of Timiskaming-type rocks are exposed. The dominant rock-type is poorly sorted, highly foliated metaconglomerate (Fig. 3). Note the heterolithic nature of the fragments, including minor Keewatin-type red jasper fragments. This particular outcrop is highly deformed with the clasts being stretched, forming a well-developed lineation plunging steeply to the southeast. Note the abundant iron carbonate alteration within the sandy matrix. In fault contact with the meta-conglomerate to the west are meta-mudstone and meta-siltstone. Here we have near vertical mineral lineations normal to rolls on the bedding planes. This unit is finely bedded with grading, although present, obscured by the deformation. sheared unconformity between the Timiskaming-type metasedimentary rocks and the underlying Keewatin metavolcanic rocks (15U 0712637E / 5387160N). Relatively pristine Keewatin felsic metavolcanic rocks are exposed to the west (15U 0712548E / 5387139N). Note the intense deformation of the meta-conglomerate on the north side of the road with deformation intensity increasing to the south. On the south side we have exposed highly foliated and carbonatized rocks that may or may not be Timiskaming-type metasedimentary rocks in contact with relatively undeformed, possibly Keewatin fragmental rocks. On the north side of the road (15U 0712742E / 5387227N) there are bands of conglomerate with a significant proportion of barren sulphide pebbles and cobbles. Also present are several felsic intrusive rocks that cut through at a low angle to the stratigraphy and which are also strongly carbonatized. Stop 4. Timiskaming-type Monzonite. Stop 3. Timiskaming-Keewatin contact. UTM coordinates NAD 83; 15U 0710946E / 5386868N UTM coordinates NAD 83; 15U 712653E / 5387161N Here we will examine what may possibly be the On the north side of the mine road is an overgrown logging road. Follow it, keeping to the left, for 230 metres. At this location, and several other outcrops to the east, we have exposed a small intrusion within the Timiskaming debris flow pile. This intrusion is interpreted to be a high level magma chamber that was parent to the volcanic system that produced the debris flows. Macroscopically the rock resembles the fragments found in the debris flows; both possess hornblende phenocrysts and have the same mottled green to red colourization. This pigmentation within an intrusive environment indicates that the reddish colourization was not the product of exposure to an oxygenated atmosphere but is more likely related to oxidizing deuteric fluids. The restriction of this alteration to the Timiskaming-type igneous rocks, both intrusive and extrusive, limits other possible interpretations. Stop 5. Deformed debris flows. UTM coordinates NAD 83; 15U 0702659E / 5385962N Figure 3. Timiskaming Conglomerate at Stop 2 This outcrop consists of strongly deformed hornblende-phyric Timiskaming-type debris flows.   Note the highly foliated nature reflecting its proximity to the Crayfish Creek Fault, approximately 100 metres to the north. Though strongly foliated note the many - 70 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 similarities with the undeformed debris flows seen at Stop 1. Stop 6. Shebandowan Mine Ultramafic Rocks. UTM coordinates NAD 83; 15U 0702841E / 5386253N This stop is beside the now capped production shaft used to extract the nickel-copper sulphide ore from the Shebandowan deposit. During its operation it produced 9.4 million tonnes grading 1.7% Ni, 0.9% Cu and 1.56 g/t total precious metals (Pt + Pd + Au). The outcrop consists of serpentinized peridotite (see photo below) with numerous narrow zones of talccarbonate schist that form an anastomosing network. It is this unit that is host to the nickel-copper sulphides. On the north side of the outcrop is a feldspar porphyry dike, an apophysis of the Shebandowan Lake Stock to the north, cutting across the peridotite. Stop 7. Ultramafic with iron formation. iron formation. The south outcrop is serpentinized peridotite and the iron formation is on the north side. There is another iron formation on the south side of this ultramafic unit (Fig. 4). Geological mapping by Morton (1982) found that tops in this area are to the south. Just to the west there are a series of outcrops of massive mafic flows with flow top breccias that confirm tops are to the south. Stop 8. Discovery Point. UTM coordinates NAD 83; 15U 0701334E / 5386459N The first sign of nickel mineralisation of what eventually became the Shebandowan Mine was made in 1913 by Jules Cross. At the time he was mapping for the Ontario Department of Mines and while doing mapping along the shoreline noted signs of nickel and copper mineralisation. The original showing can still be seen in the form of several pits right at the water’s edge at Discovery Point. Stop 8a. UTM coordinates NAD 83; 15U 0701100E / 5385538N This rock cut is through the contact between an ultramafic massive flow and an oxide-facies UTM coordinates NAD 83; 15U 0701350E / 5386512N Walk east on the road 75 metres. On the left is the Stop 7 IF Ultramafic Stop 9 Figure 4. Detailed geology of the Southwest Bay area (Morton, 1982).   - 71 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 concrete cap over the No. 1 Exploration shaft that was used for initial access to the Shebandowan ore body. debris flow. Stop 8b Here we have several examples of angular pieces of black chert ripped up off the chert horizon capping the ultramafic (exposed just to the north on the other side of the old logging road) and incorporated into the base of the felsic debris flow unit. UTM coordinates NAD 83; 15U 0701323E / 5386451N Return to vehicles. Walk 90 metres to the west until you see an old road on the left that works its way east down and along the hillside. Continue 120 metres to the bottom where there was once a boat landing at the waters edge. Work your way along the edge of the bay east for 50 metres to several old pits very close to the water. Here the mineralisation is at the north edge of the mine peridotite body where it is in contact with sheared and banded mafic metavolcanic rocks. Look for sheared peridotite with signs of nickel bloom (Fig. 5). UTM coordinates NAD 83; 15U 0698906E / 5385040N Stop 11. Pillowed Mafic Feldspar Phyric Flows. UTM coordinates NAD 83; 15U 0703244E / 5385343N Here we have a unit that probably can be used as a stratigraphic marker horizon as it has been found at several locations over a 5 kilometre strike-length. It consists of obvious pillows with well developed selvages but with feldspar phenocrysts, some up to several centimetres across. Note how the crystals are smaller as you approach the pillow margins. Tops are consistently to the south. Stop 12. Iron formation hosting felsic dike with gold-bearing quartz ladder veins. UTM coordinates NAD 83; 15U 0714705E / 5382490N At this stop we have Timiskaming oxide-facies iron formation intercalated with argillite intruded by a later felsic dike (Fig. 6). This dike is host to gold-bearing quartz ladder veins (Fig. 7). Fractures opened up in the dike due to the ductility contrast of the enclosing iron-rich argillites and the felsic dike. Later auriferous   Figure 5. Cu-Ni Mineralisation at Discovery Point (Stop8b). Stop 9. Contact between ultramafic flow and overlying felsic fragmental unit. UTM coordinates NAD 83; 15U 0699131E / 5385139N This ultramafic flow unit is a fine-grained peridotite with well-developed polyhedral jointing. It is cut by a felsic dike. Locally the contact with the overlying felsic debris flow is exposed and is characterised by a thin, strongly foliated black chert horizon capping the ultramafic, evidence that this unit was deposited as a flow. Stop 10. Fragments of Black Chert in the base of the Figure 6. Felsic dike cutting iron formation at Stop 12. - 72 -   �Proceedings of the 58th ILSG Annual Meeting - Part 2 References Aubut, A., Lavigne Jr., M.J., Scott, J. And Kita, J. 1990. Metallogeny, Stratigraphy and Structure of the Shebandowan Greenstone Belt; Field Trip 3 Guide Book, Mineral De[posits of Central Canada, CIM Thunder Bay Branch. Brown, H. 1985. A Structural and Stratigraphic Study of the Keewatin Type and Shebandowan Type Rocks West of Thunder Bay, Ontario; Unpublished MSc. Thesis, Lakehead University. Card, K.D. 1990. A Review of the Superior Province of the Canadian Shield, a product of Archean Accretion; Precambrian Research, Vol. 48, p. 99-156. Corkery, M.T., Cameron, H.D.M., Lin, S., Skulski, T., Whalen, J.B. and Stern, R.A. 2000. Geological Investigations in the Knee Lake belt (Parts of NTS 53L); in Report of Activities 2000, Manitoba Industry, Trade and Mines, Manitoba Geological Survey, p. 129-136.   Figure 7. Auriferous quartz ladder veins within the dike at Stop 12. fluids, likely carrying gold as thio complexes reacted with the iron oxides resulting in the formation of pyrite and precipitation of native gold. If one looks carefully at the exposed contact of the dike, where the iron formation has been eroded away and looking for pyrite concentrations you commonly will also find fine- to coarse-grained native gold. Figure 8 (from Aubut et al., 1990) shows the simplified geology of this location. Morton, P. 1982. Archean Volcanic Stratigraphy and Chemistry of Mafic and Ultramafic Rocks, Chromite, and the Shebandowan Ni-Cu Mine, Shebandowan, Northwestern Ontario; Unpublished PhD. Thesis, Carlton University. Shegelski, R.J. 1980. Archean Cratonization, emergence and Red Bed Development, Lake Shebandowan Area, Canada; Precambrian Research, Vol. 12, p. 331-347.. Pye, E.G and Fenwick, K.G. 1965. Atikokan-Lakehead sheet, geological compilation series, Kenora, Rainy River and Thunder Bay districts; Ontario Ministry of Northern Development and Mines, Ontario Geological Survey. Map 2065, Scale 1: 253,440. Figure 8. Simplified geology of Stop 12. From Aubut et al. (1990) - 73 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Field trip 5 - Guide to the Thunder Bay area Mark Smyk Resident Geologist Program, Ontario Geological Survey, Ministry of Northern Development and Mines, Thunder Bay, Ontario, P7E 6S7, Canada Regional Geology Thunder Bay is situated on the northern margin of the Southern Province of the Canadian Shield. The Southern Province consists of Proterozoic rocks which unconformably overlie Archean basement rocks of the southern Superior Province (cf. Tanton, 1931; Pye, 1969). Archean basement rocks of the Wawa Subprovince are predominantly granitoid plutons and slivers of greenschist- to amphibolite-facies supracrustal (i.e. greenstone belt) rocks (Williams et al., 1991; Fig. 1). The Paleoproterozoic Animikie Group is represented locally by the Gunflint Formation and overlying Rove Formation. These dominantly sedimentary formations constitute a largely unmetamorphosed, undeformed, homoclinal succession which dips shallowly towards the center of the Mesoproterozoic Midcontinent Rift (MCR) to the southeast. The Gunflint Formation is a chemical-clastic assemblage which yielded a U-Pb age from reworked volcanic ash of 1878.3 ± 1.3 Ma (Fralick et al., 2002). These rocks grade upward into turbiditic sandstone and shales of the Rove Formation south of Thunder Bay. U-Pb zircon ages from ash beds in the basal Rove Formation yielded 1836+5 and 1832+3 Ma (Addison et al., 2005). A sandstone sample from the submarine fan portion of this succession yielded a youngest detrital zircon U-Pb age of approximately 1780 Ma (Heaman and Easton, 2006). The Sibley Group, exposed on the nearby Sibley Peninsula, has been subdivided into five formations; Figure 1. General geology of the Thunder Bay area, modified after Pye (1969) - 74 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 detailed descriptions of each formation have been reported previously (Franklin et al., 1980; Cheadle, 1986; Rogala, 2003; Rogala et al., 2005, 2007). The overall sedimentary environment indicates a fluctuating climatic scenario, in which the Sibley Group was deposited in a lacustrine system (Pass Lake Formation) that gradually evolved into a saline playa lake environment (Rossport Formation). As the climate progressively became drier, a sabkha-type environment developed (Kama Hill Formation). The Outan Island Formation represents the transition from subaqueous to subaerial conditions, and the Nipigon Bay Formation represents an aeolian environment (Rogala, 2003; Rogala et al., 2007). The depositional age for much of the Sibley Group is constrained between ~1340 and 1450 Ma. The northern margin of the Midcontinent Rift (MCR) is dominated by hypabyssal rocks of the Mesoproterozoic Midcontinent Rift Intrusive Supersuite (Miller et al. 2002), which intrude Paleoproterozoic Animikie Group and Mesoproterozoic Sibley Group sedimentary rocks and Archean basement (Fig. 2). Older hypabyssal rocks (1124 Ma Seagull intrusion, 1109-1113 Ma sills; Heaman et al., 2007) predominate in the Nipigon Embayment (cf. Hart and MacDonald, 2007). Volcanic and minor sedimentary rocks of the ca. 1108 to 1105 Ma Osler Group (Davis and Sutcliffe, 1985; Davis and Green, 1997) are exposed to the east on Black Bay Peninsula and on offshore islands in Lake Superior. Osler Group rocks are also intruded by mafic dykes and intrusive complexes (1095 Ma Moss Lake gabbro, Heaman et al., 2007; 1089 Ma St. Ignace Complex, Smyk et al., 2006) which represent the youngest local MCR magmatism. Ages in this part of the MCR range from ca. 1140 Ma (Heaman and Easton, 2007) to ages younger than the magnetic polarity reversal that occurred between 1105 and 1102 Ma (Davis and Green, 1997). Hollings et al. (2007a) proposed that the term Logan Igneous Suite, which would fall within the Midcontinent Rift Intrusive Supersuite of Miller et al. (2002), should be applied to all the diabase sills in the area north of Lake Superior, with subdivision into the informal terms, Nipigon sills for the sills north of Thunder Bay, and Logan sills to the south. Starting about 11,000 years ago (Ka), Wisconsinan ice melted back from its position in central Minnesota and Wisconsin, and quickly exhumed the Thunder Bay region, forming recessional moraines during brief stillstand periods (Phillips, 2004; Phillips et al., 1994).   Figure 2. Schematic block diagram illustrating local stratigraphy (Pye 1969). - 75 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Giant can be seen across the waters of Thunder Bay (Fig. 3). The 240 m high cliffs facing Thunder Bay are the highest in Ontario. The Sleeping Giant is capped by a Logan diabase sill which has intruded Rove Formation shale and sandstone. Other prominent mesas and cuestas include Pie Island and Mount McKay and the other hills of the Nor’Wester range to the south. All of these hills consist of Rove Formation sedimentary rocks capped by Logan sills. Isle Royale (Michigan), visible on the distant horizon, consists of Keweenawan basalt flows (Portage Lake Volcanics) associated with the Midcontinent Rift. Raised beaches, which represent former lake levels of glacial Lake Minong, are visible within the city and extend westward up the Kaministiquia River valley. The most prominent of these in downtown Thunder Bay North, extending along Algoma Street, is associated with the Nipissing Great Lakes stage (ca. 5500 years ago), approximately 20 m above presentday Lake Superior. The large bell at this lookout rests upon the “Upper Limestone” member of the Gunflint Formation. This   The Lake Superior basin was occupied by Early Lake Minong, the shoreline of which is found close to the 1400 foot (427m) contour in the borderland area. About 10 Ka, ice re-advanced from north of Lake Nipigon, sweeping across the Superior Basin (Marquette Readvance). As that ice began to melt, glacial lakes were formed between the moraines and the retreating ice margins. As Superior ice melted, water levels lowered, forming a series of shoreline features down-slope and depositing thick lacustrine clays. Superior ice withdrew to the north of Lake Nipigon around 9.5 Ka, and for the first time since the Marquette Re-advance, the Superior basin was occupied by a single lake, Lake Minong. This lake level extended up the Kaministiquia embayment to Rosslyn, where a large delta structure was built. The Minong shoreline runs through the upper part of the city, being particularly evident in Boulevard Park where river mouth bars and terraces of the Current River are seen. The Minong shoreline in the city is strongly associated with Palaeo-Indian sites, the Cummins Site being the best-known. It is likely that as water levels fell, these early people moved down from the Arrow-Whitefish Lakes area into the Kaministiquia embayment. Little remains of the toolkit of these people other than a variety of knapped lithic tools made from taconitic chert that occurs in the local Gunflint Formation (cf. Hamilton, 1996). A number of field guides (e.g., Pye, 1969; Kustra et al., 1977; Franklin et al., 1982) have covered the Thunder Bay area, including those most recently during the 46th Institute on Lake Superior Geology (e.g. Pufahl et al., 2000; Phillips et al., 2000). Stop Descriptions Stop 1A: Hillcrest Park area UTM coordinates: NAD83; 16U 0334689E / 5366961N On a clear day, Sibley Peninsula and the Sleeping   Figure 3. Panoramic view (ca. 130 º) of Thunder Bay from Hillcrest Park (image from http://www.360cities.net/image/ hillcrest-park-thunder-bay#106.03,-0.78,54.2) Map above depicts field of view from this vantage point. - 76 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 unit is now interpreted as carbonatized ejecta related to the Sudbury impact event (ca. 1850 Ma) [see Field Trip guide 1, this volume]. Proceed down the concrete stairs, turn right and walk along the laneway. The ejecta here is characterized by debrisite breccia and accretionary lapilli (Fig. 4). Calcareous beach rock contains algal bioherms and basal cryptalgal laminites with fenestrae fabric which are overlain by coarse-grained, poorly sorted breccia. Stop 1B: Debrisite Breccia UTM coordinates: NAD83; 16U 334163E / 5366301N (n.b. Private Property, ask for permission to access) Another spectacular debrisite breccia (Fig. 5a) is exposed at the corner of Markland and Hill streets. This outcrop was mapped in detail by Shegelski (1982; Fig. 5b) and later described in the context of impactrelated brecciation by Addison et al. (2010): A bedrock exposure, ~5 m by 15 m, in a private yard…contains a spectacular exposure of Gunflint chert-carbonate breccia and ejecta, primarily devitrified vesicular impact glass, which are surrounded and partially replaced by blocky calcite cement. The debrisite remnant preserved here is 0-0.5 m thick and unconformably overlies stromatolites and chloritic grainstone of the uppermost Gunflint Formation. An iron-rich alteration zone exists ~30 cm below the erosive contact between the debrisite and the Gunflint bedrock. The most common ejecta feature is devitrified vesicular impact glass clasts up to 2 cm across. Vesicles range from round to ovoid to nearly flat. Angular quartz and feldspar grains, chert shards and chloritic granules are also present. Our route then takes us west along Highway 1117 toward the community of Kakabeka Falls. The route follows the Kaministiquia River valley, which is dominated by fluvio-lacustrine deposits related to proglacial lakes (Burwasser, 1977). Tills associated with Superior lobe ice predominate north of the valley, extending southward from the rolling hills of exposed Archean rocks. In contrast, the landscape south of the highway consists of flat plains underlain by flat-lying Gunflint and Rove Formation sedimentary rocks and Quaternary sediments, punctuated by diabase-topped cuestas. Just west of the junction of Highway 11-17 and the Highway 588 (Stanley) turn-off, glacio-fluvial gravels and sands of the Stanley delta formed where the Kaministiquia River entered into Lake Beaver Bay ca. 9.7 Ka (Phillips, 2004; Phillips et al., 2004). A well-formed bluff, representing a lower Beaver Bay phase (260 m / 853 feet) extends along the north side of the highway. The present-day river has deeply incised the delta. The town of Kakabeka Falls is built on the floor (at 277 m ASL) of an old distributary of the Kaministiquia River which cut through a higher terrace level. This terrace (~300 m / 984 feet) represents the highest level of the Stanley delta. The Crane archaeological site is found on the west side of the river, where the old river entered Lake Beaver Bay (Phillips et al., 1994, 2000; Phillips, 2004).     Figure 4. Accretionary lapilli in debrisite, Hillcrest Park (Stop 1A). Figure 5a. Debrisite breccia, corner of Markland and Hill streets, Stop 1B. - 77 - �Proceedings of the 58th ILSG Annual Meeting - Part 2  Figure 5b. Detailed map of the debrisite breccia outcrop at Stop 1B by Shegelski (1982). Stop 2: Kakabeka Falls Provincial Park Stop 2A: Junction of Highways 11-17 and 590 UTM coordinates: NAD83; 16U 0305176E / 5364668N Walk uphill on the west side of Highway-590. Highway excavation has revealed the sharp unconformity between Archean, gneissic, felsic plutonic rocks and the overlying Paleoproterozoic Gunflint Formation. Large, domical stromatolites occur on “highs” on the eroded Archean basement (Fig. 6). The associated sedimentary rocks, representing intertidal, foreshore sedimentation, are laminated cryptalgal cherts overlain by a wavy bedded grainstone-micrite facies. The latter is capped by chaotic, slumped, laminated chemical sedimentary rocks which are probably cryptalgal. A few metres further uphill, overlying the previous sequence is a brecciated and slumped pyritic black chert. Gunflint conglomerate (Kakabeka Member), exposed in the river gorge a few hundred metres to the north, forms the basal member of the Gunflint Formation in this area. A major, northeasttrending fault has resulted in a down-dropping of the block to the southeast. The Kakabeka gorge, ~600 m to the east, exposes rocks much higher up in the Gunflint stratigraphy. As noted by Pufahl et al. (2000) the first set of rapids above the highway bridge are formed by Archean granitoids. The slow-water area to the south is underlain by the Gunflint Formation. Kakabeka conglomerate patchily overlies the Archean basement. Silicified stromatolites are developed on the conglomerate or directly on the granitic basement. This is the location from which samples collected in the 1950’s yielded the first Gunflint cyanobacteria described in the literature. The samples, from the silicified stromatolites, were described by Tyler and Barghoorn (1954). Figure 6. Unconformity between Archean granitoids and Gunflint Formation, Stop 2A. - 78 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Stop 2B: Kakabeka Falls UTM coordinates: NAD83; 16U 0305738E / 5364400N (n.b. Entry/parking fee is required in Kakabeka Falls Provincial Park. No sample collecting permitted) The park is dominated by a single, spectacular feature, Kakabeka Falls, which drops 39 m over sheer cliffs in Gunflint Formation sedimentary rocks. Kakabeka is an aboriginal word meaning “steep cliffs”. The age of the river gorge below the falls is still debated. If none of it existed prior to the glacial Lake Beaver Bay stage, then it is less than 9700 years old. The portage around the falls contains artifacts ranging from the Paleoindian to the historic (fur trade) periods. The falls owes its existence to the thin, lowermost chert-carbonate bed of the Gunflint Formation which forms a resistant cap rock to the softer underlying shales. Looking down the gorge, one can observe a lapilli-tuff member as a lighter grey unit near the base overlain by a thick sequence of black shales (Fig. 7). Note that shale is the predominant lithology in the Kaministiquia sections and this is, in fact, typical for the Gunflint Formation in general throughout the Thunder Bay region. As noted by Pufahl et al. (2000), this sequence represents the major volcaniclastic horizon present in the upper Gunflint Formation and is traceable to the south as the Biwabik Formation through the Mesabi Range. Basalts outcropping approximately 30 km to the southwest are probably correlative with this unit. The outcrop on the northern edge of the parking lot contains layers of banded chert-carbonate within black, fissile shale. The alternating, dark grey chert and brown siderite-ankerite layers display slump and soft-sediment deformation features. Microscopic examination of banded chert-carbonates reveals delicate lamination in the chert which resembles the “ribbon texture” of algal mats. The interlayered carbonate bands contain complex, microspherical structures which likely resulted by nucleation from a gel state. Local thick beds of carbonaceous siderite (2-3 wt% carbon) form carbonate iron formation; contemporaneous deposition of carbon and carbonate suggests biological activity during iron deposition (cf. Pufahl et al., 2000).   Figure 7. The gorge below Kakabeka Falls, developed in Gunflint Formation shales and tuffs (www.Audreyhansen.com) - 79 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 References no.8, p.1021-1040. Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Davis, D.W., Kissin, S.A., Fralick, P.W. and Hammond, A.L. 2005. Discovery of distal ejecta from the 1850 Ma Sudbury impact event; Geology,v.33, p.193-196. Addison W.D., Brumpton, G.R., Davis, D.W., Fralick, P.W. and Kissin. S.A. 2010. Debrisites from the Sudbury impact event in Ontario, north of Lake Superior, and a new age constraint: Are they base-surge deposits or tsunami deposits? Geological Society of America Special Papers, 2010, 465, p. 245-268. Burwasser, G. 1977. Quaternary geology of the City of Thunder Bay and vicinity; Ontario Geological Survey, Report 164, 70p. Cannon, W.F. and Addison, W.D. 2007. The Sudbury impact layer in the Lake Superior iron ranges: A time-line from the heavens; 53rd annual Institute on Lake Superior Geology, Lutsen, Minnesota, Proceedings volume with abstracts, v.1, p. 20-21. Cheadle, B.A. 1986. Alluvial-playa sedimentation in the lower Keweenawan Sibley Group, Thunder Bay District, Ontario. Canadian Journal of Earth Sciences, 23, p.527–542. Davis, D.W. and Green, J.C. 1997. Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution; Canadian Journal of Earth Sciences, v.34, p.476-488. Davis, D.W. and Sutcliffe, R.H. 1985. U-Pb ages from the Nipigon Plate and northern Lake Superior; Bulletin of the Geological Society of America, v. 96, p. 15721579. Fralick, P.W., Davis, D.W. and Kissin, S.A. 2002. The age of the Gunflint Formation, Ontario: single zircon U-Pb age determinations from reworked volcanic ash; Canadian Journal of Earth Sciences, v.39, no.7, p.1085-1091. Franklin, J.M., McIlwaine, W.H., Poulsen, K.H., and Wanless, R.K. 1980. Stratigraphy and depositional setting of the Sibley Group, Thunder Bay District, Ontario, Canada. Canadian Journal of Earth Sciences, v.17, p.633–651. Franklin, J.M., McIlwaine, W.H., Shegelski, R.J., Mitchell, R.H. and Platt, R.G. 1982. Proterozoic geology of the northern Lake Superior area; Field Trip Guidebook, GAC-MAC Annual Meeting, Winnipeg, 71p. Hamilton, J. S. 1996. Pleistocene landscape features and Plano archaeological sites upon the Kaministiquia delta, Thunder Bay District; Lakehead University Monograph in Anthropology #1, 112p. Hart, T.R. and MacDonald, C.A. 2007. Proterozoic and Archean geology of the Nipigon Embayment: Implications for emplacement of the Mesoproterozoic Nipigon diabase sills and mafic to ultramafic intrusions; Canadian Journal of Earth Sciences, v.44, Hart, T.R., MacDonald, C.A., Hollings, P., and Richardson, A., 2005. Proterozoic intrusive rocks of the Nipigon Embayment and Midcontinent Rift. In, T.O. Tormanen and T.T Alapieti, 10th International platinum Symposium Extended Abstracts, Geology Survey of Finland, 365-368. Heaman, L.M. and Easton, R.M. 2006. Preliminary U/ Pb geochronology results: Lake Nipigon Region Geoscience Initiative. Ontario Geological Survey, Miscellaneous Release-Data 191, 79p. Heaman, L.M., Easton, R.M., Hart, T.R., Hollings, P., MacDonald, C.A. and Smyk, M. 2007. Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon Region, Ontario. Canadian Journal of Earth Sciences, v.44, no.8, p.1055-1086. Hollings, P. and Smyk, M.C. 2008. Whatever happened to the Logan sills? Ongoing research into the geochemistry of Midcontinent Rift-related mafic intrusive rocks south of Thunder Bay: 54th Institute on Lake Superior Geology, Annual Meeting, Marquette, Michigan, May 2008, Proceedings Volume 54, Part 1, p.36-37. Hollings, P., Hart, T., Richardson, A., and MacDonald, C.A. 2007a. Geochemistry of the Mesoproterozoic intrusive rocks of the Nipigon Embayment, northwestern Ontario: evaluating the earliest phases of rift development; Canadian Journal of Earth Sciences, v.44, no.8, p.1087-1110. Hollings, P.N., Smyk, M.C. and Hart. T. 2007b. Geochemistry of Midcontinent Rift-related mafic dykes and sills near Thunder Bay: New insights into geographic distribution and the geochemical affinities of Nipigon and Logan sills and Pigeon River and other dykes; 53rd Institute on Lake Superior Geology, Annual Meeting, Lutsen, Minnesota, May 2007, Proceedings Volume 53, Part 1, p.40-41. Kustra, C.R., McIlwaine, W.H., Fenwick, K.G. and Scott, J.F. 1977. Proterozoic rocks of the Thunder Bay area, northwestern Ontario; Field Trip Guidebook, 23rd Annual I.L.S.G. Meeting, Thunder Bay, 47p. Miller, J.D., Green, J.C. and Severson, M.J. 2002. Terminology, nomenclature and classification of Keweenawan igneous rocks of northeastern Minnesota; in Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota; Minnesota Geological Survey, Report of Investigations Phillips, B. 2004. Of moraines, lake floors, deltas and shorelines: A brief summary of the deglaciation of the Kaministiquia embayment, Thunder Bay, Ontario; unpublished report, World Wide Website, http:// www.lakeheadu.ca/~geogwww/phillips/FOP%20 page_4.htm (accessed 2004). Phillips, B., Hill, C., Fralick, P. and Ross, B. 1994. Postglacial shorelines and Paleoindian migration along the northwestern shore of Lake Superior; Field Trip - 80 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Guidebook, 13th Biennial meeting of AMQUA, Minnepaolis, MN. Phillips, B., Stewart, J., Hamilton, S., Julig, P. and Ross, B. 2000. Geoarchaeology of the Thunder Bay area; 46th Institute on Lake Superior Geology, Thunder Bay, Ontario, Field Trip Guidebook, 40p. Pufahl, P., Fralick, P. and Scott, J.F. 2000. Geology of the Paleoproterozoic Gunflint Formation; in Institute on Lake Superior Geology, 46th Annual Meeting, Field Trip Guidebook. Pye, E.G. 1969. Geology and scenery, north shore of Lake Superior; Ontario Department of Mines, Geological Guidebook No.2, 148p. Rogala, B. 2003. The Sibley Group: a lithostratigraphic, geochemical, and paleomagnetic study. Unpublished M.Sc. thesis, Lakehead University, Thunder Bay, Ontario, 254 p. Rogala, B., Fralick, P.W., and Metsaranta, R. 2005. Stratigraphy and sedimentology of the Mesoproterozoic Sibley Group and related igneous intrusions, northwestern Ontario: Lake Nipigon Region Geoscience Initiative. Ontario Geological Survey, Open File Report 6174, 87 p. Rogala, B., Fralick, P.W., Heaman, L.M., and Metsaranta, R. 2007. Lithostratigraphy and chemostratigraphy of the Mesoproterozoic Sibley Group, northwestern Ontario. Canadian Journal of Earth Sciences, v.44. Shegelski, R.J. 1982. The Gunflint Formation in the Thunder Bay area; in Franklin, J.M. ed, Field Trip Guidebook 4: Winnipeg, Manitoba; Geological Association of Canada, p.14-31. Smyk, M.C., Hollings P. and Heaman, L.M. 2006. Preliminary investigations of the petrology, geochemistry and geochronology of the St. Ignace Island Complex, Midcontinent Rift, northern Lake Superior, Ontario; Institute on Lake Superior Geology, 52nd Annual Meeting, Sault Ste. Marie, ON, Program with Abstracts, v. 52, p.61-62. Sutcliffe, R.H. 1989. Mineral variation in Proterozoic diabase sills and dykes at Lake Nipigon, Ontario; Canadian Mineralogist, v.27, p.67-79. Tanton, T.L. 1931. Fort William and Port Arthur, and Thunder Cape map-areas, Thunder Bay District, Ontario; Geological Survey of Canada, Memoir 167, 222p. Tyler, S.A. and Barghoorn, E.S., 1954. Occurrence of structurally preserved plants in Precambrian rocks of the Canadian Shield; Science v. 199, p.606-608. Williams, H.R., Stott, G.M., Heather, K.B., Muir, T.L. and Sage, R.P. 1991. Wawa Subprovince; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, p.485-539. - 81 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Field trip 6 - Thunder Bay Amethyst Mine Stephen Kissin Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, P7B 5E1, Canada Introduction owing to the presence of Fe4+ , as originally shown by Cox (1977). Properties of Amethyst The proposed mechanism requires the coincidence of four geological conditions for the formation of amethyst: Amethyst, occurring in abundance in the Thunder Bay region, is a purple gemstone variety of quartz. It has been known for some time that an iron impurity in quartz is the underlying source of amethyst coloration (Holden, 1925). However, incorporation of iron of alone cannot account for the formation of amethyst, as many varieties of quartz contain trace amounts of iron, yet amethyst is relatively rare, and large deposits of amethyst are very rare. In a series of papers by Cohen and co-workers, culminating in a summary in Cohen (1989), a simultaneous sequence of reactions was proposed for the formation of amethyst. (1) (Al–O)- → (Al–O)° + e Ionizing radiation forms a hole center from oxidizing the substitutional Al-O bond. (2) Na+ + e- → Na° Electron from step 1 is trapped by an interstitial alkali metal ion. (3) Fe3+int → Fe4+int + e Induced ionizing radiation forms a trapped hole center via oxidizing the interstitial Fe3+. (4) (Al–O)°+e- → (Al–O)Trapped hole center is satiated as [AlO°] is reduced via gaining the electron from step 3. The presence of iron is positions interstitial with respect to the SiO4 framework was established by Adekeye and Cohen (1986), in noting its correlation with pervasive Brazil law twinning in colored sectors of amethyst crystals. Data on incorporation of the alkalis Na, K and Li and trivalent Al and Fe in quartz were reported by Deer et al. (1963), who further noted that the incorporation of Al3+ (and presumably Fe3+), is compensated by the incorporation of Na+ or Li+ into interstitial sites. The color of amethyst is produced by absorption of light in the visible region of the spectrum (1) The incorporation of Fe and Al, as well as Na or Li. This is not a limiting condition, as the small concentrations of these trace elements are readily available in hydrothermal solutions. (2) A source of ionizing radiation, either from U and Th or 40K in order to produce the defects in Fe and Al. (3) Deposition at generally rather shallow depth such that oxidizing conditions prevail and iron is in the form of Fe3+. (4) Deposition with a temperature range for the stability of Fe4+, the source of amethyst coloration. The mechanism proposed above is consistent with observed data and provides a logical mechanism for the formation of amethyst. However, Rossman (1994) noted that there are unestablished factors in the model such that its acceptance is tentative. Crystal forms expressed in amethyst are invariably simple, consisting only of combined positive {101 ̅1} and negative {011 ̅1} rhombohedra. The faces of one of the forms are generally largely and are designated as the major rhombohedron r, and the other form is designated as the minor rhombohedron z (Fig. 1). The only other form occasionally observed is the ditrigonal prism m (Frondel, 1962). Amethystine coloration is unevenly distributed in the crystal, generally with concentration in the major rhombohedral forms, in which Brazil law twins are also concentrated (Fig. 2; Frondel, 1962). The orientation of Brazil law twins in Figure 2 is typical of their occurrence in α-quartz; however, in amethyst the twins are polysynthetic with a typical width of 0.1 mm (McLaren and Pitkethly, 1982). The twin plane of the Brazil law is {101 ̅1}, which separates right-handed and left-handed orientations of quartz. McLaren and Pitkethly (1982) demonstrated that the composition plane of the Brazil law twin provides space for - 82 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 1. A typical amethyst crystal viewed perpendicular to the c-axis, illustrating the combination of positive {101 ̅1} and negative {011 ̅1} rhombohedra. - 83 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Garland (1994). Geology of the Thunder Bay amethyst mine Geologic Setting Figure 2. Etched basal α-quartz illustrating the typical occurrence of Brazil law twinning in which alternate bands contain left- and right-handed α-quartz (after Frondel, 1962). incorporation of Fe3+ and that iron is preferentially concentrated along this composition plane in amethyst. Amethyst Deposits in the Thunder Bay Area In this summary of the history of amethyst in the Thunder Bay area, Patterson (1985) reported that as early as 1642, Radisson described the use of “torquoise” as a gemstone by local indigenous peoples. However, it was not until 1862 that amethyst was commercially exploited in various mines associated with leadzinc and silver deposits. In the 1880s, amethyst was mined northeast of Thunder Bay in a place now called Amethyst Harbour. Interest in Thunder Bay amethyst declined around the turn of the century with the development of deposits of high quality, inexpensive amethyst from Brazil. In 1961, the construction of a road leading to a now abandoned fire tower revealed a very large amethyst deposit, which is now known as the Thunder Bay Amethyst Mine Panorama. In large vugs near the surface of the vein deposit, amethyst crystals of spectacular size were obtained. The development of the deposit with wide-spread sales and distribution of specimens revitalized interest in amethyst in the region (Sinkankas, 1976). The interest and activity in amethyst deposits in the Thunder Bay area led to the proclamation in 1975 designating amethyst as Ontario’s provincial gemstone (Patterson, 1985). A comprehensive report on amethyst deposits and mining activity in the Thunder Bay area was completed by The geological setting of the mine is complex, as an Archean and a Proterozoic record are preserved in the area. This record has been recently reviewed by Franklin et al. (1986) and will not be repeated in detail here. The Thunder Bay Amethyst Mine is hosted in the Archean Hilma Lake granite of McCrank et al. (1981). This pluton lies on the boundary of the Quetico Subprovince and the Shebandowan Subprovince, with typical greenstone lithologies on its southern margin and gneissic metasedimentary rocks on the northern margin. The Hilma Lake granite in the vicinity of the mine consists predominantly of monzonite, with compositional variation along the trend monzonite quartz monzonite - granite - granodiorite and pegmatite and pegmatitic textural variants (Jennings, 1985). Jennings’ study indicates that monzonite had been cut first by granodiorite, then by pegmatite, with some metasomatic alteration of early monzonite toward granodioritic composition. At the Greenwich Lake uranium occurrence, a vein-type occurrence located 10 krn to the northwest, Franklin (1978a) noted the presence of quartz monzonitic pegmatites containing 60 -100 ppm U in the form of uraninite. As these pegmatites are apparently comagmatic with the Hilma Lake granite, its uranium-rich character is likely a general feature. The Proterozoic rocks were deposited on the eroded Archean surface; however, the Animike Group (Gunflint and Rove formations) is missing in the vicinity of the amethyst mine. As indicated by Franklin et al. (1980), the Mesoproterozoic Sibley Group progressively onlaps Archean terrain in a northerly direction. The Sibley Group is presently absent in the vicinity of the Thunder Bay Amethyst Mine, although its presence as abundant fragments in mineralized breccias within the vein system indicates that these sediments were present as basement cover during the forming of the deposit. The significance of the Sibley Group is unclear in the face of contradictory evidence concerning its age and depositional setting. Franklin et al. (1980) noted that the Sibley Group is deposited at the location of a failed arm of an r-r-r triple junction, although they admitted to uncertainty as to the contemporaneity of sedimentation and rifting. Although some features of - 84 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 the Sibley Group are suggestive of a rift-filling deposit, the whole-rock Rb/Sr age of 1339 ± 33 Ma (Franklin, 1978b) is approximately 200 Ma prior to the main stage of rifting of the Midcontinent (Keweenawan) Rift (Van Schmus et al., 1982). Cheadle (1986), however, concluded on the basis of sedimentological studies that the Sibley Group was not deposited in a classical aulocogen, but represents a deposit on a sagging crust preceding rifting. The Sibley Group was more recently dated by U/Pb geochronology in zircons in a basal rhyolite unit at 1537 +10/-2 Ma (Davis and Sutcliffe, 1985). This timing makes a relationship with the Midcontinent Rift event unlikely, and Hollings et al. (2004) proposed that the Sibley Basin formed due to effects of a plume track that created an infracratonic basin. Other deposits located at or near the Sibley -Archean unconformity include the Dorion lead -zinc -barite veins (Fig. 3). The ore-depositing solution was considered to be a basinal, connate brine by Franklin and Mitchell (1977), an interpretation supported by the fluid-inclusion studies of Haynes (1988). As illustrated in Figure 3, there is a close spatial relationship between the lead-zinc -barite veins and the amethyst, and both are spatially related to the Sibley-Archean unconformity . Geological features of the mine The Thunder Bay Amethyst Mine is located within a first-order strike-slip fault, which strikes at 90-100º and dips steeply to the south. This fault is roughly parallel to one 2.1 km to the south, which strikes eastnortheasterly and has a vertical displacement of at least 125 m (Jennings, 1985), forming a major boundary to the Sibley Group’s depositional basin. The strike-slip fault hosting the amethyst deposit is offset by seven first-order strike-slip faults, five of which are illustrated in Figure 4, which strike 162 - 150° and dip vertically producing en echelon, pull-apart structures in the main fault. These structures are filled by breccias of granitic country rock and Sibley Group sedimentary rocks with large proportions of void space. The brecciated fault was subsequently mineralized by hydrothermal solutions. At least two periods of mineralization occurred, as an early generation of amethyst was clearly brecciated and subsequently coated by a second generation of amethyst. Figure 4 is an illustration of the state of the mine in 1987. At present, the main pit configuration is basically the same but has been deepened. In that year, an Figure 3. Local geology and location map of amethyst deposits and lead-zinc-barite deposits, showing the relationship of the former to the margin of the Sibley Group outcrop and the Hilma Lake granite. The location of the Thunder Bay Amethyst Mine is indicated by the star. Modified after Patterson (1985). extension of the vein system to the east was developed, offset to the north by a few metres by a strike-slip fault. Jennings (1985) subdivided the mineralization patterns into three basic types: (i) open fracture fillings, (ii) breccias with tectonic and collapse subtypes, and (iii) “honeycomb” veins. The strike directions of these veins are strongly clustered in two groups, one slightly west of north and parallel to the second stage of strike-slip faulting, and one easterly, parallel to the principal directions of - 85 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 4. Diagram of the main pit of the Thunder Bay Amethyst Mine. faulting. Open fracture fillings are common in the shallower zones of the deposit where low lithostatic pressure permitted the maintenance of open fissures following faulting. The veins are widest near the edges of collapsed breccias and at the intersections of oblique shears with the main fault zone. Fracture-fill mineralization occurred at the crystal-fluid interface as quartz crystals grew outward from the fracture walls. The crystals formed as parallel to radial growths with long crystal axes oriented perpendicular or subperpendicular to the growth surface. The crystal size invariably increases outward, and outward growth from opposite fractures resulted in an interlocking comb structure of euhedrally terminated crystals. This vein type may also contain vugs up to 2-3 m in diameter with large quartz crystals up to 10-15 cm in prism diameter. Tectonic breccias are here attributed to fault movement, as opposed to brecciation caused by collapse. Some breccia fragments are surrounded only by a later portion of the paragenetic sequence, suggesting that multiple fault motion during the mineralizing event has occurred. Breccia fragments of this type are invariably angular and may consist of fragments of earlier deposited vein material, which may have been thermally bleached. Collapse brecciation is not always differentiated from tectonic brecciation, and some collapse breccias have undergone subsequent tectonic brecciation and vice versa. Evidence of collapse brecciation is seen in the occurrence of Sibley Group lithologies not present in the mine area now, together with granite and diabase as breccia fragments. Sibley Group fragments are particularly abundant within channel- or pipe-like structures in which fluid transport and abrasion have produced subangular to subrounded fragments, which have undergone an appreciable degree of sorting. Collapse-breccia fragments are typically coated with successive layers of chalcedony, colorless quartz, and amethyst, producing a cockade structure. Vugs have developed in open space produced in the breccia in which crystals with prism diameters of up to 10 cm have grown. Honeycomb veins are the result of quartz crystallization that has occurred in all directions from small nuclei, usually chalcedony, hematite, or silicified granite fragments, rather than from a fracture wall. The amethyst and quartz are more massive than in the other types of veins, but the growth is chaotic. Mineralogy Amethyst and several varieties of quartz occur in the Thunder Bay Amethyst Mine, including colorless quartz; chalcedony; amethyst; the yellowish variety, citrine; and the greenish variety, prasiolite or “greened - 86 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Table 1. Paragenetic sequences observed in the veins of the Thunder Bay Amethyst Mine Stage Type Thickness (cm) Older sequence (observed as breccia fragments) 1 Chalcedony 0.1 -0.3 2 Colorless quartz 0.8-1.0 3 Chalcedony 0.1-0.2 4 Prasiolite 2.0-3.0 5 Prasiolite 2.0-7.0 Younger sequence (main vein deposits) 1 Chalcedony and hematite 0.1-0.3 2 Colorless to smoky quartz 0.5-1.0 2a Amethyst I (top of stage 2) 0.5 -0.8 3 Amethy st III - IV 2.0-3.0 4 Amethyst II - III 2.0- 12.0 5 Amethyst II -IV 2.0- 14.0 6 Black gem Up to 4.0 Notes: Variations observed include (i) late-stage greenish and yellowish-amethyst; (ii) late-stage smoky quartz; (iii) discontinuous hematitic and milky quartz capping to crystal terminations; and (iv) development of black gem in crystals, deposited in vugs. amethyst”. Smoky quartz has very limited development. The only variety of gemstone interest is amethyst, although the occurrence of the other varieties has aided in establishing the sequence of deposition. A grading system based on estimated intensity of coloration and clarity of specimens is in use at the mine, and this system has also aided in establishing the paragenetic sequence. Thus, the intensity of coloration may be from I (lightest) to IV (darkest) and clarity from a (clear) to f (opaque). Table 1 lists typical paragenetic sequences in an older sequence, which is present as breccia fragments in a younger sequence presently occupying the veins. The prasiolite in stages 4 and 5 of the older sequence appears to be thermally bleached amethyst on the basis of both its appearance and experimental evidence that heat-treated amethyst can be transformed to prasiolite (Lehmann and Bambauer, 1973). In the younger sequence, late-stage variations are noted, particularly as cappings to stage 5. A distinctive variety called black gem, a dark, brownish-black amethyst, is apparently characteristic of larger crystals grown in vugs in which iron-enriched, late-stage fluids were trapped. These frequently have final growth zone that contains abundant hematite inclusions, such that recent sales of such material has been called “Thunder Bay red”. It was this material, recovered in the early development of the deposit that led to the notorious statement by Sinkankas (1976, p. 204): “By far most of the amethyst is unsuited for lapidary purposes, with very little being free from flaws and hence useless for faceted gems or even baroques.” The compositions of specimens of amethyst by neutron activation analysis for selected trace elements (Table 2) revealed the presence of subequal concentrations of Fe and Al. As well, low concentrations of Ge were sought based on absorption spectra that indicated its presence. The low Ti concentrations are perhaps related to the spotty occurrence of rutile needles in the amethyst; needles occurring when concentrations are relatively greater. Other non-sulfide minerals. Barite is rare in the Table 2. Analyses of r-zones of amethyst for selected trace elements (in ppm; Kissin, 1997) Sample No. Fe Al Ge Ti DZS1* 217 447 0.5 n.d LZS2 102 393 0.5 0.01 BSS3 273 369 1.0 n.d. TPS4 368 249 0.5 n.d. ____________________ *DZS1 evidently contains solid inclusions, as high concentrations (in ppm) were noted; e.g. Ta 0.329, W 0.38, Eu 0.349, Sr 89.43, Zr 1.02, Nb 0.13, Ba 2985.26, La 3.85, Ce 0.35, U 0.387. All samples contain small, but detectable quantities of Co, Ni, Ga, Rb, Nb, Zr, Mo, Sn, Sb, Cs, La, Pr, Nd and U. - 87 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 veins at the Thunder Bay Amethyst Mine, although it is abundant in other amethyst mines of the district, where it follows the final stage of quartz deposition. It was not observed in the course of the present study, but has been noted in the mine. Calcite is fairly common in thin, monomineralic veins, but was not observed within the amethyst-bearing veins. The genetic link between the calcite veins and amethyst veins, if any, is unclear. Hematite is abundant as minute- (< 0.1 mm diam.) solid inclusions in stage 5 of amethyst deposition and occurs sporadically at other stages of deposition as well. Hematite occasionally occurs as a daughter mineral in fluid inclusions, particularly in stage 5 of crystallization. Rutile occurs as needles that transect the growth zones of the quartz in scattered locations within the mine. The orientations of the needles are apparently random; however, the possibility of crystallographically controlled growth directions has not been considered in detail. Native copper occurs in association with copper-iron sulfides. Sulfides. The common base-metal sulfides pyrite, chalcopyrite, galena, and sphalerite occur in small amounts throughout the vein succession and as veinlets and replacement bodies in altered granitic wall rock. Copper -iron sulfides, however, are predominant, and a sequence of the minerals cuprite-native copperchalcocite-covellite associated with hematite and pyrite was documented by McArthur et al. (1993). The copper -iron sulfides exhibit typical replacement textures (atoll structures, core-and-rim relationships) both in amethyst growth stages and in wall rock. The assemblages bornite + pyrite and chalcopyrite + pyrite and chalcopyrite + pyrite occur in wall rock only; however, spatial relations of wallrock sulfides to the veins do not reveal any pattern, owing in part to their scarcity. Malachite is present as a supergene product derived from these hypogene copper minerals. Wall-rock alteration mineralogy. Hematitization, chloritization, and kaolinitization are prominent in envelopes surrounding the veins within zones of brecciated granite; however, the alteration extends only a few centimetres into the granites outside of the zone of brecciation. Intense hematitization occurs fairly generally in altered rock nearest the amethyst veins. The strongly hematitized zone is generally only a few centimetres thick, but weaker hematitization is notable throughout the altered zone. Outward from the hematized zone is an irregular zone of highly chloritized rock ranging from a few to a few tens of centimetres thick. Sometimes associated with the chloritization is diffuse epidotization, which produced a pistachio green tint over zones up to a metre wide. Kaolinitization is widespread and pervasive through the breccia zone, imparting a chalky, white appearance to relict feldspars. Other clay minerals (e.g., montmorillonite and illite) may also be present; however, they have not been sought in a detailed examination. The pervasive kaolinitization has allowed weathering to penetrate into the brecciated zone, resulting in a soft and loosely aggregated matrix in which the near-surface exposures of the amethyst are contained. The nature of this matrix has enabled a good deal of the amethyst to be mined with a minimum of blasting. The hematite - chlorite - epidote alteration assemblages in the presence of ubiquitous quartz are characteristic of the propylitic alteration typical in many hydrothermal ore deposits. The kaolinite and other clay minerals are characteristic of the argillic alteration of hydrothermal ore deposits. The two alteration types are analogous at least in their relative timing to early peripheral propylitic alteration, which is overprinted by argillic alteration stemming from downward-infiltrating meteoric water. Genesis of the deposit The genesis of the deposits of the Thunder Bay Amethyst Mine were discussed in detail by McArthur et al. (1993). The conclusions of their study are given below; however, for details of the evidence, their paper should be consulted. Genetic speculations on the Thunder Bay Amethyst Mine are hampered at the outset by questions as to the timing of amethyst deposition, as discussed in the Introduction. The spatial and geochemical affinities of the amethyst deposits with the Dorion lead-zinc-barite veins and the relationships of both to the depositional margin of the Sibley Group sediments suggest that all three are interrelated. Franklin and Mitchell (1977) proposed that the lead -zinc -barite veins formed when, during diagenesis and settling of the Sibley Group sediments, metal-bearing brines were formed when expelled connate waters mobilized metals from the Sibley Group sediments and(or) weathered granitic basement rocks below the Archean-Proterozoic unconformity. The solutions thus formed would have hypothetically migrated through the basal Pass Lake Formation aquifer to escape at basin-marginal faults. Precipitation of sulfide, carried in chloride- and sulfate-bearing solution, occurred because of mixing of the relatively oxidized solution with H2S gas trapped at the Pass Lake Formation - 88 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 pinch-out. The amethyst deposits seem to be a variant of the lead-zinc-barite type of deposit in which the temperature was lower than that of the sulfide-rich lead-zinc-barite veins. The initially oxidizing to later reducing character of the solution is similar to that proposed for the lead-zinc- barite veins, but the relation to Pass Lake Formation pinch-outs is not present in most amethyst deposits. Rather, the amethyst deposits are generally hosted in granitic basement often with no Sibley Group sediments present. The amethyst deposits are richer in dissolved silica, having gained this component through the kaolinitization of feldspar during hydrothermal alteration of granitic country rock. As the amethyst deposits formed near the present or former unconformity with the Sibley Group, local reduction of the solution would have tended to occur as H2S was released during thermal breakdown of organic matter in the sediments. The quantity of sulfides precipitated would have been limited not only by the relatively small amount of H2S produced but also by the lower metal content of the solutions as compared with those depositing the Dorion lead-zinc-barite veins. The latter characteristic is inferred by a comparison of the results of this study with those of Haynes (1988) on the Dorion lead-zinc-barite veins. He found that fluid inclusions from these deposits are NaC1-CaC12 -H2O type on the basis of microthermometry and direct analysis of decrepitates. However, the fluid inclusions depositing sulfides are significantly more saline than those at the Thunder Bay Amethyst Mine in that they contain daughter salts. The more saline and higher temperature (105-203°C) fluid inclusions indicate that solutions that they represent would have had a better metal carrying capacity as chloride complexes. The similarity of the solution components to those at Thunder Bay Amethyst Mine lends support to the idea that the same event formed both types of deposits. The solutions depositing amethyst would have been cooler and less saline variants of those that formed the lead-zinc-barite veins. If the two types of deposit are genetically linked, both suffer from the problem of lack of knowledge of the timing of ore deposition. The maximum age of both is 1339 Ma, the whole rock Rb/Sr age of the Sibley Group (Franklin 1978b), as both types of veins cut Sibley Group rocks and contain breccia fragments of them. Franklin and Mitchell (1977) did not suggest a specific timing for formation of the Dorion lead-zinc-barite veins; however, their suggested mechanisms for creation of the deposit favor a timing soon after the deposition of the Sibley Group sediments. The expulsion of pore water would presumably occur during late diagenesis. However, as there is no evidence to suggest that the Sibley Group sediments have ever been deeply buried, the source of heat is a problem. If the timing of deposition were close to the formation of the Sibley depositional basin, it is possible that a thermal anomaly, perhaps augmented by seismic pumping, in the lower crust was responsible for both phenomena. Haynes (1988) suggested that the Dorion leadzinc-barite veins formed either in the environment of Keweenawan rifting or later, possibly in the Paleozoic. There is no geological evidence for activity in the Paleozoic in the western Lake Superior region, and the style of mineralization associated with Keweenawan events is different (silver deposits associated in part with Ni-Co arsenides; Franklin et al., 1986). Our preferred hypothesis is that the lead-zinc-barite veins and amethyst veins are associated with the timing of formation of and deposition in the Sibley basin. We, therefore, believe that these deposits are distinct from silver deposits in the Thunder Bay area and formed at a somewhat earlier time. The timing is, however, not at all certain, and some additional work is underway in an effort to resolve this remaining question. Summary Field and laboratory studies of the Thunder Bay Amethyst Mine reveal the following: (1) The vein system hosting amethyst deposits was formed by mineralization of an east-weststriking, steeply dipping strike-slip fault, opened into en echelon pull-apart structures by a series of later strike-slip faults, also dipping steeply and intersecting the first-formed fault at high angles. Much open space with brecciated and vuggy textures resulted. Breccia fragments include granitic host rock and Sibley Group sedimentary rocks, implying that the latter were present as a thin cover at the time of mineralization, although they are erosionally removed from the mine area at present. At least one early generation of amethyst is included as breccia fragments, indicating that fault movement continued during mineralization. (2) At least two phases of amethyst crystallization separated by a period of brecciation are present. The older sequence contains five stages of quartz growth, the latter two of which were originally amethyst, but were thermally bleached to prasiolite by the influx of hot solutions that deposited the younger sequence - 89 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 to the Dorion lead-zinc-barite veins. Both are believed to have been formed by solutions expelled and mobilized during diagenesis and compaction of the Sibley Group. The lead-zinc-barite veins formed in fractures at or near the margin of the Sibley depositional basin from solutions that were both hotter and more saline than those depositing amethyst. Amethyst-depositing solutions travelled longer distances in granitic basement, dissolving silica by alteration of feldspar. Although the amethyst-depositing solutions probably carried less metal as chloride complexes than did the solutions forming the lead-zinc-barite veins, less H2S at the site of deposition was probably the most significant factor causing a low sulfide content in the amethyst veins. of quartz. The younger sequence contains five and occasionally six stages of deposition, beginning with a stage of chalcedony and a stage of colorless quartz, followed by amethyst. Both sequences of deposition are traceable throughout the mine. (3) Sulfide minerals including pyrite, chalcopyrite, galena, and sphalerite accompany amethyst deposition as small mineral inclusions and occur, as well, as veinlets and replacement bodies in altered granitic wall rock. Copper and copper -iron sulfides are most abundant and, together with native copper and cuprite. Eh-pH relationships indicate that the solutions forming the deposit were initially rather oxidizing and weakly acidic. In the course of crystallization, the solution became more reducing and slightly more acidic. (4) Fluid-inclusion studies indicate that in the younger sequence of quartz deposition, homogenization temperatures range from 146.5 to 114.7°C (mean 132.1°C) as contrasted with 91.2-40.9°C (mean 68.4°C) for amethyst. Eutectic temperatures of frozen inclusions indicate that the solution was of the NaCl-CaC1-H2O system, with possible concentration of an additional halide salt component in late-stage fluids. Few inclusions contain daughter minerals, and those found are hematite and sphalerite in late-stage fluids. Final melting temperatures indicate a trend of decreasing salinity in later growth stages. (5) Oxygen isotopic determinations on quartz indicate a range of δ180 outside that of juvenile waters and end-member basinal brines. Progressive mixing of basinal brine with local meteoric water is suggested. (6) Sulfur isotopic analyses of pyrite yield δ34S of -0.4-0.6 ‰ and -1.4 ‰ in chalcopyrite. These volumes are consistent with derivation from H2S gas liberated by thermal action protection on organic material involving iron. The values are similar to those of the sulfur contained in sulfides in the Dorion lead-zinc-barite veins. (7) The presence Sibley breccia fragments cemented by quartz indicates that the veins cannot be older than 1339 Ma, the Rb/Sr age of the unit. However, a younger limit cannot be established at present. (8) On grounds of similarity in geological setting, proximity, composition of the ore-depositing solution, and sulfur isotopic composition, the amethyst veins are believed to be genetically related (9) The temperature conditions under which amethyst forms appear to have a high temperature limit; at the Thunder Bay Amethyst Mine this limit is no higher than approximately 115°C and may be as low as approximately 90°C. Temperatures as high as approximately 145°C but possibly as low as 115°C may be sufficient to thermally bleach earlier generations of amethyst in the influx of hot solutions. However, this theory of thermal bleaching has been recently criticized by Hebert and Rossman (2008), who attributed the development of greenish-grey to greenish quartz to the presence of H2O in the crystal. Our work (Klarner and Kissin, 201l) confirms the presence of water in IR absorption spectra; however, the water is largely contained in fluid inclusions, which are abundant and of secondary origin. Use of the highly focus beam of an FTIR microscope has shown that molecular water is of low and nearly identical concentration in both amethyst and “greened amethyst”. Work on this problem is continuing. Road Log Airlane Travelodge to Thunder Bay Amethyst Mine Leaving the Airline Travelodge, we will follow the portion of the Trans-Canada Highway 11-17 which is the Thunder Bay Expressway. The flat terrain is the remnant of the bottom of the Nipissing stage of ancestral Lake Superior, and proceeding northeasterly, we pass upward through strandlines of the receding Pleistocene lake. At 5.7 km is the intersection with Oliver Road, which leads to Lakehead University about 2 km to the east. Lakehead University itself is underlain by the Gunflint - 90 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Formation at or near the top of the unit. Shaly rocks near the top of the formation are exposed in the bed of the McIntyre River that flows through the campus; however, in recent years blocks of rock containing the Sudbury ejecta debrisite were excavated during construction of new student residences. These placed in various places around the campus as ornamentation or barriers to vehicular traffic. and steeply dipping Archean metavolcanic rocks was exposed on the left of the highway. Hopefully, new road construction will expose this unconformity once again. The hill is formed by the outcrop of the Mackenzie granite, an unmetamorphosed and undeformed, late Archean pluton. The highway continues on top the of granite, which contains occasional roof pendants of Archean metavolcanics. Continuing on at 6.7 km, outcrops of a Logan diabase sill are exposed on the left side of the expressway. These sills form the caps of the prominent mesas south of town and underlie the high ground in the northern section of Thunder Bay. Passing the junction of Red River Road (Highway 102) at 10.0 km, the expressway is on a level stretch marking the top of a Logan sill. After crossing the Mackenzie River at 35.7 km sparse outcrops of granite are replaced by poorly exposed Gunflint Formation until just past the junction with Highway 587 at 49.6 km. Here, well-bedded redstained carbonates of the Gunflint Formation crop out beside the highway. Passing onward to the East Loon Road at 55.2 km, turn left onto the road, then right on Bass Lake Road after 0.6 km. Continue to the turn off on the right to the private road to the mine. Proceeding along the mine road, it climbs steeply up from the Sibley basin onto the Archean Hilma Lake granite, ascending along a border fault surface. The expressway then passes downhill to the Current River at 15.9 km. In proceeding downhill outcrops of Logan sill diabase, Gunflint shale and Gunflint carbonate are successively exposed. The carbonate is ankeritic and is oxidized to yellowish orange. Climbing uphill from the Current River bridge, the expressway ends and the highway is again cutting into a diabase sill. A fault trends along the highway offsetting the sill on opposite sides of the highway. A few hundred metres further along the highway, the sill is dropped downward by a fault trending perpendicularly to the highway. Recent work has shown that this sill, known locally as the Terry Fox sill, is a Nipigon sill (Magnus and Kissin, 2010). Nipigon sills, which occur from here northeasterly to the Lake Nipigon area, are somewhat younger than Logan sills and can be distinguished on the basis of their trace element composition. Proceeding downhill and rounding a curve to the left, there is a high bluff on the left capped by a prominent diabase sill. The sill has intruded the top of the Gunflint Formation and the overlying Sudbury debrisite layer, which is capped by a thin remnant of Rove Formation shale. This is the only outcrop known in the Thunder Bay area that contains the complete debrisite layer. The east end of this outcrop is bounded by a fault that dropped down the section. Continuing onward, high ground on both sides of the highway are capped by sills; the sill on the right was extensively quarried for railway bed ballast and large stone for construction of the breakwall in the harbor. After the junction with Highway 527 at 20.1 km, the highway climbs the hill locally known as KOA hill. Prior to a widening of the highway about a decade ago, the angular unconformity between the Gunflint Formation At the top of the grade, there is a chance to view Lake Superior with Black Bay, the Black Bay Peninsula and the Sibley Peninsula, clear weather permitting. A few more kilometres brings the road to the mine. Amethyst Mine tour Note: Safety boots or shoes recommended. sandals or open-toed shoes. No The tour will pass through the operating mining area, which is not available to ordinary tourists. No collecting is allowed in this area. After visiting the mining area, there will be an opportunity to look for specimens in a designated collecting area. The charge for specimens is by weight. Hammering or chiseling is not permitted in the collecting area. Specimens are also for sale in the shop. References Adekeye, J.I.D., and Cohen, A.J., 1986, Correlation of Fe4+ optical anisotropy, Brazil twinning and channels in the basal plane of amethyst quartz, Applied Geochemistry, v. 1, p.153-160. Cheadle, B.A., 1986, Alluvial-playa sedimentation in the Lower Keweenawan Sibley Group, Thunder Bay District, Ontario. Canadian Journal of Earth Science, v. 23, p. 527-541. Cohen, A.J., 1989, New data on the cause of smoky and amethystine color in quartz. Mineralogical Record, v. 20, p. 365-367. - 91 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Cox, R.T., 1977, Optical absorption of the d4 ion Fe4+ in pleochroic amethyst quartz, Journal of Physics C: Solid State Physics, v. 10, p. 4631-4643. Davis, D.W., and Sutcliffe, R.H., 1985, U-Pb ages from the Nipigon Plate and northern Lake Superior. Geological Society of America Bulletin, v. 96, p. 1572-1579. Deer, W.A., Howie, R.A., and Zussman, J. 1963, RockForming Minerals,Vol. 4 Framework Silicates. John Wiley and Sons, Inc., New York, 435 p. Franklin, J.M. 1978a, Uranium mineralization in the Nipigon area,Thunder Bay District, Ontario. in Current Research, Part A. Geological Survey of Canada, Paper 78-lA, pp. 275-282. Report to Precious Purple Gemstones Ltd., Thunder Bay, Ont., 65 p. Kissin, S.A., 1997, Comprehensive research to colour enhance Canadian amethyst by heat treatment and irradiation. Final Report, Amsearch Colour Project, Northern Ontario Development Agreement SSC File #015SQ-2223440-2-9243, 36 p. and appendix. Klarner, J.M., and Kissin, S.A., 2011, Hydrothermal bleaching of amethyst at the Thunder Bay Amethyst Mine, Ontario. Geological Society of America Annual Meeting, Minneapolis, Paper No. 44-11. Lehmann, G., and Bambauer, H.U., 1973, Quartz crystals and their colors. Angewendtede Chemie International Edition, v. 12, p. 283-291. Franklin, J.M., 1978b, The Sibley Group, Ontario, in Rubidium strontrium isochron age studies report 2. Edited by R.K. Wanless and W.D. Loveridge. Geological Survey of Canada, Paper 77-14, p. 31-34. Magnus, S., and Kissin, S., 2010, Assimilation and petrogenesis in the Navilus and Terry Fox sills, Thunder Bay, Ontario; in Institute on Lake Superior Geology, Proceedings and Abstracts, v. 56, part 1, p. 36-37. Franklin, J.M., and Mitchell, R.H., 1977, Lead-zinc -barite veins of the Dorion area, Thunder Bay District, Ontario. Canadian Journal of Earth Sciences, v. 14, p. 1963-1979. McArthur, J.R., Jennings, E.A., Kissin, S.A., and Sherlock, R.L., 1993, Stable-isotope, fluid-inclusion, and mineralogical studies relating to the genesis of amethyst, Thunder Bay Amethyst Mine, Ontario. Canadian Journal of Earth Science, v. 30, p. 19551969. Franklin, J.M., McIlwaine, W.H., Poulsen, K.H., and Wanless, R.K., 1980, Stratigraphy and depositional setting of the Sibley Group, Thunder Bay District, Ontario, Canada. Canadian Journal of Earth Science, v. 17, p. 633-651. Franklin, J.M., Kissin, S.A., Smyk, M.C., and Scott, S.D., 1986, Silver deposits associated with the Proterozoic rocks of the Thunder Bay District, Ontario. Canadian Journal of Earth Sciences, v. 23, p. 1576-1591. Frondel, C. 1962, The System of Mineralogy, 7th edition, Vol. III Silica Minerals. John Wiley & Sons, New York and London, 334 p. Garland, M.I., 1994, Amethyst in the Thunder Bay area. Ontario Geological Survey, Open-file Report 5891, 197 p. Haynes, F.M., 1988, Fluid-inclusion evidence of basinal brines in Archean basement, Thunder Bay Pb-Zn-Ba district, Ontario, Canada. Canadian Journal of Earth Sciences, v. 25, p. 1884-1894. Hebert, L.B., and Rossman, G.R., 2008, Greenish quartz from the Thunder Bay Amethyst Mine Panorama, Thunder Bay, Ontario, Canada. Canadian Mineralogist, v. 46, p. 111-124. McCrank, G.F.D., Misiura, J.D., and Brown, P.A., 1981, Plutonic rocks in Ontario. Geological Survey of Canada, Paper 80-23, 171 p. McLaren, A.C., and Pitkethly, D.R., 1982, The twinning microstructure and growth of amethyst quartz. Physics and Chemistry of Minerals, v. 8, p. 128-135. Patterson, G.C., 1985, Amethyst in the Thunder Bay area of Ontario, Canadian Gemologist, V. 6, p. 104-116. Rossman, G.R., 1994, Colored varieties of the silica minerals, in P.J. Heaney, C.T. Prewitt, and G.V. Gibbs, eds., Silica: Physical Behavior, Geochemistry and Materials Applications, Mineralogical Society of America, Reviews in Mineralogy, v. 29, p. 433-467. Sinkankas, J., 1976, Gemstones of North America, Vol, II. D. Van Nostrand Company, Inc., New York, 494 p. Van Schmus, W.R., Green, J.C., and Halls, H.C., 1982, Geochronology of Keweenawan rocks of the Lake Superior region: A summary, in R.J. Wold and W.H. Hinze, eds., Geology and Tectonics of the Lake Superior Basin, Geological Society of America, Memoir 156, p. 165-171. Holden, E.F., 1925, The cause of color in smoky quartz and amethyst, American Mineralogist, v. 10, p. 203-252. Hollings, P., Fralick, P., and Kissin, S., 2004, Geochemistry and geodynamic implications of the Mesoproterozoic English Bay graniterhyolite complex, northwestern Ontario. Canadian Journal of Earth Science, v. 41, p. 1329-1338. Jennings, E.A., 1985, Geology of the Thunder Bay Amethyst Mine and Precious Purple Gemstone claims. - 92 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Field trip 7 - Building stone tour of downtown Port Arthur, Thunder Bay, Ontario Peter Hinz Ring of Fire Secretariat, Ontario Ministry of Northern Development and Mines, 435 James Street South, Suite B332 Thunder Bay, Ontario, P7E 6S7, Canada Foreward This walking tour will examine a number of buildings in the downtown core of the Port Arthur portion of Thunder Bay. The buildings of interest are constructed from a range of stones, the majority of which were produced in northwestern Ontario. The walking tour will consider the geological and architectural features of each building. Introduction (taken from Hinz et al.,1994) The dimension and monument stone industry in northwestern Ontario has a long history and is linked to the development and prosperity of the region. One of the earliest commercial operations was located on Vert Island in Nipigon Bay of Lake Superior. The Mesoproterozoic Sibley Group yielded an attractive red sandstone which was extracted by the ChicagoVerte Island Sandstone Company. The stone was shipped to Chicago, Winnipeg, southern Ontario and other U.S. cities for construction uses. Development of some of the earliest quarries in the Marathon and Nipigon areas was directly related to the construction of the Canadian Pacific Railway in the late 1880’s. Syenites surrounding Marathon and sandstones south of Nipigon were used in the construction of railway trestles to span the Black, Pic, Little Pic, Steel and Nipigon rivers. Today these trestles show very little wear and are a testament to the long-standing durability of the stones. Although markets for dimension stone decreased in the early 1900’s, production continued at the Simpson Island sandstone quarry (1900-1910) and at the Bannerman and Horne quarry (1912-15) near Ignace. The next period of quarry development took place during the late 1920’s to early 1930’s. Five small scale quarries operated northwest of Marathon along the Canadian Pacific Railway. Black and brown granites were extracted and shipped to customers in Toronto, Buffalo, Chicago and Detroit. In 1932, the last of these quarries closed due to the loss of a market. In 1948, the Vermilion Pink Granite Company opened a quarry approximately 12 km southwest of the town of Vermilion Bay. This highly popular pink granite began production in 1954 and continued sporadically under various names until 1991 when the quarry, now named Granite Quarriers (GQI) Inc., closed. In 1981, Nelson Granite Limited of Sussex, New Brunswick began production of an identical granite from a quarry immediately south of the highway from the Granite Quarriers Inc. site. This quarry has operated year round since that time and is still in production. Currently, 2012, Nelson Granite Limited is the only stone producer operating in northwestern Ontario. Nelson Granite produces a range of colours including pink, yellow, green, brown and white granite from four quarries located north of Kenora and west of Vermilion Bay. Northwestern Ontario stone is shipped around the world for a range of uses including: building stone for interior and exterior uses; monumental stone; and landscape uses including pavers, benches and accent pieces. Detailed descriptions of the historic quarries, their operations, geology and geotechnical test results are provided in Hinz et al. (1994). Descriptions of current producers are available in the Kenora portion of the Report of Activities 2010, (Lichtblau et al., 2011). A list of building stone quarries in northwestern Ontario is given in Table 1. Geologic setting Sandstones of the Sibley Group The red and buff coloured sandstones utilized as building stone throughout Thunder Bay were sourced from quarries in the Nipigon to Rossport area. The sandstones represent lithologies hosted within the Pass Lake Formation of the Sibley Group, the following description is taken from Fralick et al. (2000). “Sedimentary rocks of the Sibley Group discontinuously outcrop on the north shore of - 93 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Table 1. Building stone quarries of northwestern Ontario. The asterisk (*) denotes stones which will be viewed at the various field trip stops. Sibley Group Sediments Quarry Cooke Point George Point La Grange Island Nipigon River * Quarry Island Ruby Lake Simpson Island * Vert Island * Wolf River Rock Type Marble Grey sandstone Red sandstone Variegated marble White sandstone Variegated marble Buff sandstone Red sandstone Buff sandstone Years of Operation 1931-1940 (?) Late 1800’s (?) 1882-1883 (?) 1883-1910 (?) Late 1880’s (?) 1996-1998 1904-1912 1881-1912 1913-15; 1921-31 Granitic Rocks Quarry Bulter Grey * Cold Spring C.P.R. Docker Township * Forgotten Lake GQI Quarriers Inc. Peninsula Granite Pine Green Red Deer Lake Redditt Rock Type Grey granite Black granite Black granite Pink granite Yellow granite Pink granite Brown granite Green granite Brown granite White granite Years of Operation 1892-1943, 1946-52, 1989 1931-38(?) 1880’s (?) 1981-present 1997-present 1948-1991 1880’s-1927(?) 1992-present 1996-present 2010-present Note: The usage of the term “granite” in the building stone industry is used regardless of lithology (eg. granite, syenite, gabbro) Lake Superior and around Lake Nipigon (Fig. 1). The flat-lying to gently dipping, clastic-carbonate succession occupies a broad oval area disconformably to unconformably overlying Mesoproterozoic, Paleoproterozoic and Neoarchean rocks. Sibley Group rocks are located in the Southern Province of northwestern Ontario. in thickness and consists of basal conglomerate and upward-thinning beds of quartz arenite. It was deposited in a shallow lacustrine environment (Franklin et al., 1980).” The fluvial and lacustrine strata of the Sibley Group are divided into three subhorizontal formations: 1) the lowermost is the Pass Lake Formation; 2) the Rossport Formation overlies it, followed by 3) the Kama Hill Formation. “This marble consists of contact metamorphosed, Mesoproterozoic, Rossport Formation (Sibley Group) dolostone and other, calcareous sedimentary rocks in the contact metamorphic aureole of Keweenawan diabase sills. It has previously been termed Nipigon River marble and was quarried from 1883 to ca. 1910 The Pass Lake Formation varies from 0 to 50 m Marble of the Sibley Group (taken from Fralick et al., 2000) - 94 - �Proceedings of the 58th ILSG Annual Meeting - Part 2   Figure 1. Field trip stop locations: 1. Port Arthur Collegiate Institute; 2. Trinity United Church; 3. Masonic Temple; 4. Former Hymer’s Mens Wear; 5. Ontario Government Building. at a site on the eastern side of the Nipigon River, approximately 6 west of the Ruby Lake quarry (Hinz et al., 1994). A similar hornfelsed unit is described in more detail at Stop 2-6. Calcite, dolomite, epidote and opaque minerals were noted in thin section by Hinz et al. (1994) from the Nipigon River quarry.” Granites of the Wabigoon Subprovince (taken from Beakhouse et al., 1995) “The Wabigoon subprovince is a 150 kilometre wide volcanoplutonic domain that has an exposed strike extend of 700 kilometres, extending an unknown distance beneath Paleozoic strata at either end. The western Wabigoon region is characterized by interconnected, arcuate, metavolcanic ‘greenstone belts’ surrounding large elliptical batholiths. Granitoid rocks within the western Wabigoon region include large elliptical to multi-lobate batholiths that define the architecture of the greenstone belts as well as smaller stocks. Most of the large batholiths (Aulneau, Atikwa, Sabaskong) range compositionally from ultramafic to granitic but are predominantly tonalitic to granodioritic.” Both the Butler Grey and Vermilion Pink granites are sourced from intrusions within the Wabigoon subprovince. Storey (1986) described the geology of the Butler Grey quarry: “The rock is massive, light grey to white, biotite granite (approximately 5% biotite). There are local variations in grain size and resultant colour variations. There are a few minor patch pegmatites. A very weak foliation trends north-northwest.” Storey (1986) also described the Vermilion Pink granite: “The rock was classified as quartz monzonite by Mattison (1952) and granite by Pryslak (1976). A modal analysis from Mattison (1952) plots as granite in the Streckeisen (1976) classification.” Field trip stops Stops are located by UTM co-ordinates based on NAD 83, UTM Zone 16 The walking tour starts on the east side of the Port Arthur Collegiate Institute building. Park on Waverly Street on the south side of Waverley Park (0335224E 5367273N), walk to the east entrance of the Port Arthur Collegiate Institute. - 95 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Stop 1. Port Arthur Collegiate Institute (aka. P.A.C.I.; 401 Red River Road) UTM coordinates: NAD83; 16U 0335208E / 5367358N This building, constructed in 1909 of Simpson Island buff sandstone (Pass Lake Formation), is an example of the Queen Anne style that was in common use from the 1880s to the 1910s. When the school board made the initial planning for the building, it was decided that it should be “erected for posterity, and not be of the ‘shack’ order”, so they chose the stately Queen Anne style. The original P.A.C.I. can be seen in Figure 2. Alterations in 1925 resulted in the addition of four more classrooms, and more renovations to the north and south in 1953 and 1962 created other rooms. These alterations used Indiana Limestone in an attempt to blend into the original building. The younger, middle Mississippian (335-340 Ma) aged limestone is easily distinguished from the much older Mesoproterozoic Sibley sandstones by the prolific presence of marine fossils such as crinoids, bryozoa and gastropods (Fig. 3). A gymnasium planned in 1964 and completed in 1974, provoked controversy as its design was incompatible with the rest of the school. Walk back to Waverley Street to view several residential houses which incorporate Simpson and Vert island sandstones (eg. 369, 349 and 332 Waverley Street) walking east approximately 190 metres along Waverley Street to the Algoma Street corner. Stop 2. Trinity United Church (30 Algoma Street South) UTM coordinates: NAD83; 16U 0335432E / 5367226N This building, completed in 1906, was formerly known as the Trinity Methodist Church, and became the Trinity United Church after the United Church of Canada was formed in 1925. Constructed of rough cut, Simpson Island buff sandstone (Pass Lake Formation), this structure is an example of the Late Gothic Revival style that was popular from the 1890s to the 1940s (Fig. 4). The unusual tower features very narrow windows (lancets), four buttresses, each capped with a pyramid shaped finial, and an extremely sharp hexagonal spire. The rest of the building also features very steeply pitched roofs, and arched windows in the Gothic style. Walk north and cross Red River Road, approximately 90 metres, then cross Algoma Street and walk east approximately 125 metres. Figure 2. The Port Arthur Collegiate Institute ca. 1909, from http://images.ourontario.ca/ - 96 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 3. Left - Fossiliferous Indiana limestone, arrow points to a fan-like bryozoan. Right - Enlarged view of the fan-like bryozoan. Figure 4. Trinity United Church ca. 1930, from http://www.hpd.mcl.gov.on.ca/ - 97 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 5. Masonic Temple, Red River Road, Nipigon River marble displaying mat-like stromatolites. Stop 3. Masonic Hall ( 262-270 Red River Road) UTM coordinates: NAD83; 16U 0335596E / 5367166N Built in 1910 and also known as the Shuniah Lodge, this stone, brick and concrete building replaced the old Masonic temple that was destroyed by fire in 1909. The first floor is made of cut Nipigon River marble (Rossport Formation) and the entrance features carved marble pilasters and decorative panels (Fig. 5). Originally there was a dome on the roof over the entrance, which has since been removed. The central portion of the building has a Mansard roof of French design. The building’s windows are decorated with alternating round and triangular pediments above them. Commercial space occupies the ground floor, while the lodge is located above. Continue east along Red River Road, crossing Court, St. Paul and Cumberland streets, approximately 300 metres. Cross Red River Road and proceed south for 60 metres to Lorne Street where you will see a red sandstone wall. Stop 4. Former Hymer’s Men’s Wear (17 Cumberland Street South, Lorne Street wall) UTM coordinates: NAD83; 16U 0335826E / 5366930N The north wall of this building is constructed of Vert Island red sandstone, the building was constructed circa. 1900. The stone is a brick red sandstone which is part of the Mesoproterozoic Sibley Group, Pass Lake Formation (Fig. 6a). Syneresis cracks are evident on one block. These cracks are caused by subaqueous Figure 6a (left). Red sandstone wall on the north side of the former Hymer’s Men’s Wear. 6b (right) Reductions band in Pass Lake formation sandstone. - 98 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 6c (left). Intraformational conglomerate, Pass Lake formation. 6d (right). Syneresis cracks in Pass Lake formation sandstone. shrinkage of sediments without dessication (Fig. 6d). Also visible in many blocks are intraformational conglomerate, buff coloured reduction spots and bands, ripples (Fig. 6b & c). Walk 35 metres east along Lorne Street to the Ontario Government Building. Stop 5. Ontario Government Building (189 Red River Road) UTM coordinates: NAD83; 16U 0335847E / 5366925N Opened in 1990, the Ontario Government Building was built to incorporate granite building stone from quarries in the Ignace and Vermilion Bay of northwestern Ontario, the stones included Butler Grey and Vermilion Pink (Fig. 7). The stones were used in the exterior and interior of the building and include polished, honed and flame finishes. The Ontario Government Building is described in the Ontario Architecture website (http:// www.ontarioarchitecture.com/postmodern.htm): “It is a classic Post Modern building in that it uses traditional architectural vocabulary in a new and impressive way. The front colonnade is a good example. The columns have neither bases nor capitals, but a decorative band level with the first floor lintels. There is an exaggerated cornice atop the architrave which has three horizontal bands. There is no ornament, not even fluting on the columns, and instead of marble, the columns are metal. Behind the colonnade, the building is a cutain wall of glass with an open concept foyer. Winding around the colonnade is a balustrade leading to other portions of the building and a landscaped front.” References Beakhouse, G.P., Blackburn, C.E., Breaks, F.W., Ayer, J.A., Stone, D. and Stott, G.M. 1995. Western Superior Province Fieldtrip Guidebook; Ontario Geological Survey, Open File Report 5924, 94 p. Fralick, P., Smyk, M. and Mailman, M., 2000. Geology and stratigraphy of the mesoproterozoic Sibley Group (field trip guide): Institute on Lake Superior Geology Proceedings, 46th Annual Meeting, Thunder Bay, Ontario, v. 46, part 2, p. 5 Hinz, P., Landry, R.M. and Gerow, M.C. 1994. Dimension stone occurrences and deposits in northwestern Ontario; Ontario Geological Survey, Open File Report 5890, 191 p. Lichtblau, A.F., Ravnaas, C., Storey, C.C., Bongfeldt, J., McDonald, S., Lockwood, H.C., Bennett, N.A. and   Figure 7. Entrance of the Ontario Government Building with the flame-finished Butler Grey granite on the exterior and features of the Post Modern architecture. - 99 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Jeffries, T. 2011. Report of Activities 2010, Resident Geologist Program, Red Lake Regional Resident Geologist Report: Red Lake and Kenora Districts; Ontario Geological Survey, Open File Report 6261, 93p. Mattinson, C.R. 1952: A Study of Certain Canadian Building and Monumental Stones of Igneous Origin; Unpublished MSc Thesis, McGill University, Montreal, Quebec. Pryslak, A.P. 1976: Geology of the Bruin Lake-Edison Lake Area; District of Kenora; Ontario Division Mines, Geological Report 130, 61p. Storey, C.C. 1986. Building and Ornamental Stone Inventory in the Districts of Kenora and Rainy River; Ontario Geological Survey, Mineral Deposits Circular 27, 168p. Streckeisen, A. 1976: To Each Plutonic Rock Its Proper Name; Earth Science Reviews, Vol. 12, p. l-33. - 100 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Field trip 8 - A geologic transect across the Western Superior Province and Nipigon Embayment, Thunder Bay to Armstrong, Ontario Mark Smyk Resident Geologist Program, Ontario Geological Survey, Ministry of Northern Development and Mines, Thunder Bay, Ontario, P7E 6S7, Canada Philip Fralick Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada Introduction This field trip along Highway 527 between Thunder Bay and Armstrong describes a 250 km long transect through Archean rocks of the western Superior Province of the Canadian Shield and the Mesoproterozoic Nipigon Embayment. This transect extends from the Wawa Subprovince near Thunder Bay, across the entire width of the Quetico Subprovince and into the Wabigoon Subprovince, south of Armstrong (Fig. 1). The Wabigoon and Wawa (ca. 3.0 to 2.7 Ga) are volcano-plutonic subprovinces, containing a number of greenstone belts. The intervening Quetico Subprovince (ca. 2.7 Ga) consists of metamorphosed clastic sedimentary rocks, their high-grade metamorphic equivalents and derived granitic rocks. Mesoproterozoic rocks. Bear in mind that this trip marks the first time that many of these stops have been visited and described as part of a formal field trip. Please exercise caution when stopping and viewing roadside exposures. Stott (2009) noted that the terminology of crustal subdivisions across the Archean Superior Province is slowly evolving such that regional lithologic subdivisions as subprovinces (Card and Ciesielski, 1986) are currently being reassessed in terms of terranes and adjacent, typically autochthonous domains (e.g., Percival and Helmstaedt, 2006; Stott et al., 2007). Subdivisions in the field trip area are shown in Table 1. The most recent geological synopsis in the transect area was provided by Hart and MacDonald (2007): These subprovinces are intruded or unconformably overlain in this area by a variety of Mesoproterozoic rocks of the Nipigon Embayment (Southern Province). These Mesoproterozoic rocks include Sibley Group (ca. 1.3 Ga) sedimentary rocks, the Badwater intrusive complex (ca. 1.6 Ga), and the English Bay felsic intrusive-volcanic complex (ca. 1.54 Ga). Voluminous mafic to ultramafic intrusive rocks related to the Mesoproterozoic Midcontinent Rift (ca. 1.1 Ga) predominate. Day One of the trip will highlight representative Archean lithologies while Day Two will focus on The Nipigon Embayment is underlain, from north to south, by Archean rocks of the English River, Wabigoon, and Quetico subprovinces (Fig. 2). Much of the Embayment is underlain by a series of east-trending 2950 to 2700 Ma greenstone belts separated by 3000 to 2690 Ma intrusive rocks of the central and eastern Wabigoon subprovince (e.g., Blackburn et al., 1991). Tomlinson et al. (2004) proposed a north– south subdivision of the Wabigoon subprovince into the Winnipeg River and Marmion terranes based on isotopic data. The boundary between Table 1. Geological subdivisions in the field trip area Existing Nomenclature Wabigoon Subprovince Proposed Nomenclature Winnipeg River Terrane (north) Marmion Terrane (south) Quetico Subprovince Quetico Basin Wawa Subprovince Wawa-Abitibi Terrane - 101 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 1. General geology of the field trip transect area, from Smyk and Franklin (2007) the Wabigoon and Quetico subprovinces is the Blackwater Fault to the east, south of Beardmore, and is located south of Peevy Lake to the southwest. The metasedimentary rocks of the Quetico subprovince are intruded by “I-type” tonalite to granodiorite and “S-type” muscovite and two-mica leucogranites (e.g., Williams, 1991; Breaks et al., 2003), with minor mafic to ultramafic and syenitic bodies (e.g., MacTavish, 1999; Pettigrew and Hattori, 2006). A minimum the terranes is obscured by later granites west of Lake Nipigon, but is correlated with the Humboldt Bay high-strain zone to the east of the lake. The southern margin of the Wabigoon subprovince consists of a series of metasedimentary and metavolcanic belts separated from the granite– greenstone terrane by the Paint Lake Fault east of Lake Nipigon and the Max Creek Fault west of the lake (e.g., Williams and Stott, 1991; MacDonald et al., 2005). The boundary between - 102 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 2. Hart and MacDonald’s (2007) generalized geology of the Nipigon Embayment (modified from Ontario Geological Survey 1993; MacDonald 2004; Hart and Magyarosi 2004; MacDonald et al. 2005; Hart 2005) and the Archean rocks of the Wabigoon and Quetico subprovinces surrounding and underlying the Embayment. intrusions of the Lac des Iles area (e.g., Sutcliffe, 1987; Tomlinson et al., 2002; Stone et al., 2003). The various intrusions of the Lac des Iles area… have been collectively referred to as the Lac des Iles suite by Stone et al. (2003) and include the Northern Ultramafic and Mine Block intrusions of the Lac des Iles Complex (e.g., Hinchey et al., 2005). Previous workers have suggested that the intrusions of the Lac des Iles suite may be part of a contemporaneous magmatic event (e.g., age of deposition for the metasedimentary rocks is constrained by ages of 2665 ± 2 and 2653+3/4 Ma on leucogranites immediately southwest of the Nipigon Embayment (Percival and Sullivan, 1988), as well as the 2688+6/-5 Ma Samuels Lake bodies (Pettigrew and Hattori, 2006), and the 2689 ± 2 Ma Black Pic monzodiorite (Zaleski et al., 1999). There are a number of late- to posttectonic mafic to ultramafic intrusions in the central Wabigoon subprovince, including the - 103 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Stone et al., 2003; Pettigrew and Hattori, 2006). A number of intrusions identified as a result of mapping (e.g., Hart, 2000) and diamond drilling (e.g., MacDonald et al., 2005) may be part of the same magmatic event. diabase dyke was found that may correlate with the 1140 Ma Abitibi swarm (Ernst et al., 2006). Three Meso- to Paleoproterozoic [sic] lithologic units are located in the northwest portion of the Nipigon Embayment: the Badwater (Creek) intrusion, the Pillar Lake Volcanic rocks, and the English Bay Complex (Fig. 3). The mafic to felsic Badwater intrusion is unconformably overlain by flat-lying mafic pillowed volcanic rocks of the Pillar Lake volcanic unit (MacDonald, 2004). The English Bay volcanic–intrusive complex is located to the southeast, on the northwest shore of Lake Nipigon (e.g., Sutcliffe and Greenwood, A series of north-striking diabase dykes intrudes the Archean rocks of the Wabigoon and Quetico subprovinces. A paleomagnetic and geochemical study of the dykes located west of the Nipigon Embayment suggests that there are 2130– 2120 Ma reverse-magnetized and 2110– 2100 Ma normal-magnetized Marathon dykes (Ernst et al., 2006). A solitary northeast-trending Figure 3. Hart and MacDonald’s (2007) generalized geology of the Mesoproterozoic rocks of the Nipigon Embayment (modified from Ontario Geological Survey, 1993; MacDonald, 2004; Hart and Magyarosi, 2004; MacDonald et al., 2005; Hart 2005) - 104 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 1985a). All three units appear to be localized along major regional structures. Archean rocks (e.g., MacDonald, 2004; Hart, 2005). A lack of obvious textural, mineralogical, or geochemical variations within sill exposures hinders regional correlation of the sills and the development of a stratigraphic succession (e.g., Hollings et al., 2007). However, a recent airborne magnetic survey (Ontario Geological Survey, 2004), combined with new geochemistry and geochronology, has distinguished a number of distinctive sills with limited extents…, including the Inspiration sill (MacDonald 2004; Hollings et al., 2007a; Heaman et al., 2007) and the McIntyre sill (Richardson et al., 2005; Hollings et al., 2007; Heaman et al., 2007). Geological mapping suggests that the formation of the Nipigon Embayment was controlled by a series of north-, northwest- and northeast-trending faults that appear to correlate with prominent Archean basement structures (e.g., Hart, 2005; MacDonald et al,. 2005). Interaction between these faults formed an asymmetric basin or half-graben in the southwest portion of the Nipigon Embayment as originally defined by Coates (1972). Franklin et al. (1980) suggested that the faults defining parts of the Nipigon Embayment represented a failed arm of the Midcontinent Rift, and Sutcliffe (1987) and Lightfoot et al. (1991) proposed that the ultramafic intrusions and Nipigon diabase sills intruded along these faults. Alternatively, the faults may be the result of subsidence following an anorogenic thermal event as proposed by Fralick and Kissin (1995) and Hollings et al. (2004). Further work by Rogala et al. (2007) indicates that most fault activity related to halfgraben development was the result of broad subsidence that occurred more than 200 million years before the Midcontinent Rift, thus lending further credence to the interpretations of Fralick and Kissin (1995) and Hollings et al. (2004). Unconformably overlying the basement Archean and earlier Proterozoic rocks are the clastic and chemical sedimentary rocks of the Sibley Group. The Sibley Group is interpreted to have been deposited in a fluvial to shallowlacustrine environment with transitions to playa lake and sabkha environments followed by reflooding of the basin, recorded in the middle to upper stratigraphic parts of the sequence (e.g., Franklin et al., 1980; Cheadle, 1986; Rogala, 2003). The thickest accumulation of Sibley Group rocks in the western portion of the Nipigon Embayment is within a half-graben, defined by the faults in the area of the Black Sturgeon River, thinning toward the west (e.g., Coates, 1972). Rogala et al. (2007) interpret the initial period of fault activity as representing a change from broad subsidence to active basin formation in the Lake Nipigon area prior to 1339 ± 33 Ma, and prior to formation of the Midcontinent Rift. There are four sill-like mafic to ultramafic intrusions, with three (Disraeli, Seagull, and Hele) located to the south of Lake Nipigon and one (Kitto) located along the east side of the lake, emplaced into the Sibley Group and underlying Archean rocks (e.g., Sutcliffe, 1986, 1987; Hart and Magyarosi, 2004). Ultramafic-hosted PGE mineralization in the Seagull intrusion is located within discrete, laterally continuous zones, which a study by Heggie (2005), using wholerock geochemistry, isotope geochemistry, and mineral chemistry, suggests have been caused by sulphur saturation of the magma during initial stages of emplacement, with zones higher in the intrusion probably reflecting influxes of lessevolved magma. Other examples of fine-grained, massive mafic to ultramafic sills occur scattered through the Nipigon Embayment. The thickest are the Jackfish sill in the northwest corner of Lake Nipigon and the Shillabeer sill south of the lake (e.g., MacDonald, 2004; Hollings et al., 2007a). The current outline of the Nipigon Embayment is defined by a series of diabase sills estimated to cover an area in excess of 20 000 km2 (Sutcliffe, 1991). The shallow-dipping Nipigon diabase sills, ranging in thickness from <5 m to >180 m, intrude the mafic to ultramafic intrusions, Sibley Group, Pillar Volcanics, the English Bay Complex, and the underlying A synopsis of mineral deposits in the area was given by Smyk and Franklin (2007): A variety of metallic and non-metallic mineral deposit types occur within Archean and Proterozoic rocks in the area encompassing the Lake Nipigon Region Geoscience Initiative. Archean deposit types include: Algoma-type banded iron formation-hosted iron (e.g., Lake Nipigon iron range) ; volcanogenic massive sulphide copper-zinc (e.g., Onaman-Tashota belt); ultramafic intrusion-hosted chromium (e.g., Puddy-Chrome lakes); mafic to ultramafic - 105 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Table 2. Mapping of the Precambrian bedrock geology of the transect area by the Ontario Geological Survey Map Number P2984 ARM48C Scale 1:15 840 1:63 360 Year 1986 1939 Authors J.F. Scott, J.M. Seguin R.D. Macdonald 1:63 360 1971 M.E. Coates Eayrs Lake-Starnes Lake area 1:63 360 1969 L. Kaye Max Lake sheet 1:31 680 1967 E.G. Pye Southwest Portion of the Nipigon Embayment 1:100 000 2006 Hart, T.R. Lac des Iles Greenstone Belt 1:20 000 2001 Hart, T.R., MacDonald, C.A., Lepine, C.D. P3434 Heaven Lake Greenstone Belt 1:20 000 2001 P3560 Cheeseman-Black Sturgeon Lakes Area 1:50 000 2005 Kabitotikwia Lake Area 1:50 000 2005 Hart, T.R., MacDonald, C.A., Lepine, C.D. MacDonald, C.A., Tremblay, E., ter Meer, M. MacDonald, C.A., Tremblay, E., ter Meer, M. P3537 English Bay-Havoc Lake Area 1:50 000 2004 P3536 Waweig-Wabinosh Lakes Area 1:50 000 2004 Pashkokogan-Caribou lakes sheet 1:126 720 1974 M2235 M2172 M2136 P3580 P3435 P3559 P0962 Map Area MacGregor Township, west half Gorham Township and vicinity Disraeli Lake sheet intrusion-hosted copper-nickel-platinum group element (PGE) (e.g. Lac des Iles); and pegmatitehosted deposits of rare metals (Li, Ta, Be), uranium and molybdenum (e.g., Georgia Lake field; Black Sturgeon Lake; Anderson Lake, respectively). Mesothermal lode gold deposits are prominent in the Beardmore-Geraldton camp. MacDonald, C.A., ter Meer, M., Lepage, L., Préfontaine, S., Tremblay, E. MacDonald, C.A., ter Meer, M., Lepage, L., Préfontaine, S., Tremblay, E. Sage, R.P., Breaks, F.W., Stott, G., McWilliams, G., Bowen, R.P. Rift -related Osler Group volcanic and interflow sedimentary rocks. Native copper and Cusulphides occur in Mesoproterozoic Sibley Group sedimentary rocks, adjacent to ultramafic intrusions. These mafic to ultramafic intrusions, associated with Midcontinent Rift magmatism, host copper-nickel-PGE deposits (e.g. Seagull, Great Lakes Nickel). Silver-bearing veins occur in Paleoproterozoic Animikie Group sedimentary rocks in proximity to Midcontinent Rift-related Superior-type iron formation occurs in Paleoproterozoic Gunflint Formation. “Red-bed” copper occurs in Mesoproterozoic Midcontinent - 106 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 mafic intrusions (e.g., Silver Islet; Silver Mountain). Lead-zinc-barite veins, uraniumbearing veins and amethyst vein- and replacement -type deposits may be co-genetic and formed at or near the unconformity between Sibley Group basal sandstone and underlying Archean granitic basement (e.g., Dorion; Black Sturgeon Lake; McTavish Township). The hydrothermal systems that produced all of these veins were probably driven by heat associated with Midcontinent rifting. Many occur in structures related to riftbounding faults. Iron oxide-copper-gold deposits may occur near the English Bay intrusion. Embayment was undertaken as part of the Lake Nipigon Region Geoscience Initiative (Table 2). Stop descriptions Day One (Figs. 4 & 5) Stop 1-1: Pillowed Metavolcanic Rocks, Wawa Subprovince UTM coordinates: NAD83; 16U 0340852E / 5376037N Mapping of the Precambrian bedrock geology of the transect area by the Ontario Geological Survey (and its predecessors) ranges from detailed (e.g. 1:15 840) to reconnaissance-scale (1:250 000). Much of the detailed mapping focused on greenstone belts. Newer, comprehensive 1:50 000 mapping of the Nipigon This is a typical exposure of greenschist-facies, massive to pillowed, locally vesicular mafic metavolcanic rocks, foliated at 260/80. The unit is cut by small shear zones, mafic xenolith-bearing feldspar porphyry dykes and north-northeast-trending calcite veins. Similar veins to the northeast contain lead and zinc sulphides and may be related to a Mesoproterozoic   Figure 4. General geology of the transect area, showing the location of field trip stops along Highway 527 - 107 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 RoadLog Logfor for Field Field Trip Road TripStops Stops (N.B. Please exercise caution along highway and road right-of-ways.) (N.B. Please exercise caution along highway and road right-of-ways.) STOP NAME STOP NUMBER LANDMARK (0 Km) DISTANCE (km) NORTHING EASTING 5376037 340852 5378261 341252 5384413 344754 5389006 346350 5388319 346239 5391178 347109 DAY ONE Intersection of Hwy. 527 and Hwy. 11-17 Wawa – Pillowed Metavolcanic Rocks 1-1 2.8 Mt. Baldy Road Wawa Timiskaming Metasedimentary Rocks 2.9 1-2 5.1 Weigh Scales Compressor Station Road Wawa Penassen Lakes Stock 1-3 1-4A 1-4B 6.5 13.2 15.0 Beaverlodge Road (exit Highway 527 to access stops 1-4A, 1-4B) Beaverlodge Road, north spur, 1.3 km west off Hwy. 527 Beaverlodge Road, north spur, 1.0 km west off Hwy. 527 Kingfisher Lake Road Wawa – White Lily Lake Stock 5.2 12.4 Gibson's Road Magone Road Quetico Metasedimentary Rocks Wawa Feldspar-phyric granitoid 0.0 1-5 16.1 19.0 19.7 Escape Lake Road - 108 - 22.4 �Proceedings of the 58th ILSG Annual Meeting - Part 2 Quetico Pegmatite Quetico Fault in Metasedimentary Rocks Quetico Fault in Granitoid Rocks 1-6 Barnum Road 25.3 Bush Road 100 m west off Hwy. 527 26.8 South Current River Monday Lake Road 27.9 29.6 31.1 5401948 348057 1-7B 31.5 5402246 348117 5405692 348152 5407714 345756 5416579 339226 5448888 328624 Shallownest Road 1-8 Quetico Migmatites 1-9 35.2 100 m east of Hwy. 527 38.4 Orr's Place Store Dorion Cut-Off Pace Lake Road 1-10 40.0 45.2 49.9 51.1 Mott Lake Road DeCourcey Lake Eaglehead Lake Road Pipeline Fensom Lake Road Mawn Lake Road Max Lake Road Camp 45 Road Wabigoon Conglomerate 346508 1-7A Quetico Pegmatitic Granite Quetico Cordieritebearing granitoids 5398003 1-11 56.6 64.1 72.5 74.5 80.8 82.2 84.2 87.5 88.1 Max Creek 88.3 Wabigoon – Tuff-breccia 1-12 88.6 5449317 328280 Wabigoon Pillowed Mafic Metavolcanic Rocks 1-13 88.8 5449467 328159 - 109 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Wabigoon Brecciated Metavolcanic Rocks 1-14 90.7 5450297 326447 Wabigoon – Banded Iron Formation 1-15 92.5 5451015 324893 5573375 356068 5564137 345619 LaChapelle Creek 93.7 Lac Des Iles Road 93.8 Poshkokogan Lake Road 103.6 Whistle Lake Road 104.9 DAY TWO Pillar Lake Volcanics Alarie Quarry Pillar Lake Volcanics – T-Junction, Mattice Lake Road Intersection, CNR Tracks and Hwy. 527, West end of Armstrong 0.0 Intersection, CNR Tracks and King St., East end of Armstrong turn-off to quarry 3.0 3.3 2-1 5.0 Intersection, CNR Tracks and Hwy. 527, West end of Armstrong MacKenzie Lake Inn Clearwater Lake Road Frontier Road Mattice Lake Road 0.0 2.5 6.2 7.8 10.0 Intersection of Mattice Lake Road with Highway 527 Bridge Badwater Creek turnoff Bridge 0.0 0.8 3.8 4.5 2-2 5.4 - 110 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Badwater Gabbro – Badwater Creek Road Badwater Gabbro - East of Badwater Creek Road Intersection of Mattice Lake Road with Badwater Creek Road 2-3A 2-3B Pillar Lake Volcanics Chimney Lake 2-4 Pillar Lake Volcanics – Hwy. 527 2-5 Nipigon Diabase Sill, Highway 527 2-6 Sibley Group Sedimentary Rocks and Nipigon Diabase Sills 2-7 0.0 0.6 5564306 347047 (outcrops 30 m and 60 m east of road) 0.8 5564142 347043 Intersection of Mattice Lake Road with Highway 527 0.0 outcrops 300 m east of highway) 2.4 5563345 349738 3.3 5562320 349460 Northern end of Waweig Lake 5.5 Bridge on Hwy. 527 at Gull River 0.0 Kabi River Bridge 20.2 24.2 5499194 342613 (outcrops 160 m southeast of highway) 26.8 5493814 341205 mineralizing event. Stop 1-2: Timiskaming Metasedimentary Rocks, Wawa Subprovince UTM coordinates: NAD83; 16U 0341252E / 5378261N These steeply dipping, clastic metasedimentary rocks are massive, thickly to thinly bedded, quartz-rich arkosic sandstones with abundant mudstone fragments. Thinly bedded turbidites display graded bedding with tops to the south, indicating that this sequence (at 270/85°) has been locally overturned. The northern Wawa Subprovince contains five major packages of sedimentary rock associations. There are: 1) cherts and iron formations deposited as inter-flow sediments in basaltic successions, 2) resedimented andesitic volcanic eruptive material forming areas of graded beds interlayered in calc-alkaline successions, 3) thick turbidite sequences similar to the Quetico turbidites deposited when the Quetico trench was full and sediment gravity flows gained access to the Wawa ocean floor to the south (Fralick et al., 2006), 4) strandline to deep shelf deposits formed at 2692 Ma (unpublished U-Pb zircon age on volcanic ash) during a period of shoshonitic volcanism related to initial collision between a Wawa island arc and the Quetico accretionary complex, and 5) 2686 Ma fluvial conglomerates and sandstones shed into pull-apart basins during transpression during the main orogeny. The succession we are examining is reasonably nondescript, but most closely resembles units present in the shelf succession of the 2692 Ma assemblage. At other locations, hummocky cross-stratification present closer to shore in this system attests to the operation of geostrophic flows draining storm surges away - 111 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Stop 1-3: Subprovince Penassen Lakes Stock, Wawa UTM coordinates: NAD83; 16U 0344754E / 5384413N The Penassen Lakes stock is part of the Dog Lake granite chain, a linear series of at least six separate, possibly genetically related granitoid intrusions: Silver Falls, Trout Lake, Barnum Lake, Shabaqua, Penassen Lake and White Lily (Kuzmich et al., 2011; Fig. 7). They are emplaced in semi-pelitic to pelitic metasedimentary and gneissic rocks along the Quetico–Wawa subprovinces boundary north of Thunder Bay (Kehlenbeck, 1977). The intrusions form an approximately east-northeast-trending, evenly spaced chain spanning roughly 70 km, none of which have undergone geochronological analysis. The magnetic signature of these intrusions suggests that they may be distinct from the typical S-type granites found within the Quetico Subprovince. Detailed geochemical and petrographic studies of the granites by Kuzmich (in progress) and Kuzmich et al. (2011) will provide additional insights into the origins and petrogenesis, as well as the development of the Quetico Subprovince as a whole. The Penassen Lakes stock is a dark pink, massive, magnetic, medium-grained, quartz-monzodiorite to monzodiorite (Fig. 8). At this location, equigranular hornblende monzodiorite is cut by aplitic dykes and contains mafic xenoliths. Amethyst-bearing veins crosscut the stock at a locality approximately 600 m south along the highway.   Figure 5. General geology and field trip stops, Day One (Archean) from the coast. Below storm wave-base these flows deposited graded beds with the same appearance as slump induced turbidites. The lithologies present here are similar to these types of units, which are associated with hummocky cross-stratification and tidal flat deposits at other locations such as Finmark, west of Thunder Bay.   Figure 6. Overturned turbidites, Stop 1-2 - 112 - �Proceedings of the 58th ILSG Annual Meeting - Part 2   Figure 7. Aeromagnetic image of the area north of Thunder Bay, with Dog Lake intrusions labeled (Kuzmich et al., 2011) Stop 1-4A: Metasedimentary Rocks, Quetico Subprovince UTM coordinates: NAD83; 16U 0346350E / 5389006N This site displays typical, thinly to thickly bedded Quetico metaturbidites (Fig. 9). Relict graded beds indicate southward younging and may have load casts at their bases. The fine-grained, pelitic tops appear to have porphyroblasts of what was tentatively identified as andalusite. Quartzo-feldspathic dykes host quartz veins in the necks of boudins. These dykes are locally cut by quartz-feldspar-muscovite pegmatite dykes, which may represent neosome generated by partial melting of the metasedimentary rocks. This corresponds to low- to medium-grade metamorphic assemblages outlined by Seemayer (1992) in this area along the southern margin of the Quetico Subprovince. Bear in mind that Wawa metavolcanic rocks have been noted north of this location, suggesting that the subprovincial boundary here may consist of intercalated panels of metavolcanic and metasedimentary rocks, intruded by late granitoids.  Figure 8. QAP diagram for samples of the Penassen Lakes stock (Kuzmich, in progress) As described by Seemayer (1992), these lowgrade metasedimentary rocks generally consist of quartz, plagioclase and biotite, with minor amounts of muscovite and chlorite. Porphyroblastic muscovite, - 113 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 medium-grained, magnetic, dark pink, alkali feldsparrich phase; and a medium- to coarse-grained, massive, magnetic, dark pink phase (Figs. 10 & 11). At this location, the intrusion consists of medium- to coarsegrained, K-feldspar-phyric, amphibole-bearing monzonite with a high magnetic susceptibility. Some slickensided surfaces were noted. The southern contact of the intrusion with metasedimentary country rocks is exposed approximately 700 m south along the highway in the vicinity of the Kingfisher Lake Road turn-off. STOP 1-6: Pegmatite, Quetico Subprovince UTM coordinates: NAD83; 16U 0346508E / 5398003N  Figure 9. Graded metaturbidites and pegmatite dykes, Stop Strongly peraluminous, muscovite-, cordierite- and garnet-bearing pegmatitic granite dikes, with local black tourmaline, were found to occur widely in the Quetico Subprovince along the Highway 527 from Walkinshaw Lake north to DeCourcey Lake (Breaks et al., 2003). Approximately 150 m west of Highway 527 and 40 m south of an old logging road, quartz-feldsparmuscovite pegmatite is exposed in a large whale-back outcrop. This locality, along with other pegmatites and related granitoid rocks, were described by Breaks et al. (2003): 1-4A Rare-element mineralization was discovered by the current survey within an extensive swarm of pegmatitic granite dikes at Onion Lake near Thunder Bay. The lens-shaped dikes of this swarm, as seen in the area near the junction of Highway 527 and the Barnett Lake road, occur as northeast-striking, whale-back glacial erosional andalusite, garnet and cordierite are more common near contacts with granitoids and are attributed to contact metamorphism. Kehlenbeck (1977) noted hornfelsic textures in these contact zones. Stop 1-4B: Feldspar-phyric granitoid, Wawa Subprovince UTM coordinates: NAD83; 16U 0346239E / 5388319N Just south of Stop 1-4A, there are exposures of an undeformed, medium-grained, feldspar-phyric granitoid with recessively weathered biotite and a moderate magnetic susceptibility. It may be a smaller, isolated intrusion related to one of the neighbouring (White Lily or Penassen Lakes) granitoid stocks. STOP 1-5: White Lily Lake Stock, Wawa Subprovince UTM coordinates: NAD83; 16U 0347109E / 5391178N Kuzmich (in progress) has subdivided the White Lily intrusion into two separate phases: a fine- to   Figure 10. QAP diagram for samples of the White Lily Intrusion (Kuzmich, in progress) - 114 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 11. TAS diagram showing samples from Penassen Lakes (diamonds) and White Lily stocks (circles), from Kuzmich (in progress). remnants that achieve a maximum size of 100 by 300 m. The internal units comprise: • muscovite-rich potassic pegmatite • quartz-rich patches with blocky K-feldspar, coarse muscovite books and sparse beryl • fine-to medium-grained, garnet-biotitemuscovite granite • • potassic pegmatite and enclosed quartz-rich patches. Dikes and foliation-concordant peraluminous granites and pegmatites were emplaced into Quetico Subprovince metasedimentary rocks during at least three intrusive episodes characterized by the following rock types: • garnet-biotite-muscovite pegmatitic leucogranite grey, garnet-biotite granite, fine- to medium-grained • cordierite and garnet-cordierite granite garnet and muscovite-garnet aplite • sheets of pegmatitic leucogranite and associated quartz-rich patches The quartz-rich patches locally contain pale green beryl up to 1 by 16 cm, as at locality 01-FWB-107 at Onion Lake (UTM 346199E, 5397916N, Zone 16). Black, tantalum-oxide minerals (ferrocolumbite: 27.31 weight % Ta2O5), up to 3 by 3 by 5 mm, were discovered at locality 01-JBS-52 (UTM 346512E, 5398007N, Zone 16) [STOP 1-6; Fig. 12] and apparently associated with local albitization of potassium feldspar megacrysts. Blocky potassium feldspar megacrysts up to 50 cm in diameter and muscovite books up to 10 cm in thickness were noted in the Stop 1-7A: Quetico Fault in Metasedimentary Rocks UTM coordinates: NAD83; 16U 0348057E / 5401948N As we near the Quetico Fault from the south, low-grade metasedimentary rocks give way to wellfoliated schists which retain evidence of a layered sedimentary protolith but which may display incipient anatexis (Seemayer, 1992). They are characterized by a dominant, subvertical west-striking foliation. North of - 115 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 12. Geology and rare metal mineral occurrences along Highway 527 (after Breaks et al., 2003) the Quetico Fault, migmatites predominate. This section along Highway 527 was described by Seemayer (1992): Mylonitic and cataclastic rocks of the subvertical Quetico Fault zone cut the migmatites of Block C [higher-grade subdivision]. The fault rocks outcrop for 2.5 km along Highway 527. Feldspars in mylonitized leucosome show a characteristic brick-red alteration colour which makes the fault rocks easy to identify. Recognizable stromatic migmatites in the fault zone show well-developed C-S fabric and abundant shear planes in the leucocratic layers. The fact that the migmatites have been sheared shows that final movement on the Quetico Fault post-dates - 116 - the migmatization and peak metamorphism, although the fault may have been initiated during an earlier period of transpression. Mackasey et al. (1974) attributed a dextral displacement of 100 km to the Quetico Fault. Detailed examination of the fault from Rainy Lake to Highway 527 by Kennedy (1984) found evidence to support the dextral sense of strike-slip motion. Kennedy found that brittle deformation followed the predominantly ductile deformation within the fault zone. Purdon (1989) concluded that motion along the Quetico Fault northeast of Thunder Bay was of a complex nature. An early dip-slip component inferred from subvertical stretching lineations on foliation surfaces was �Proceedings of the 58th ILSG Annual Meeting - Part 2 overprinted by slickenfibres resulting from dextral strike-slip motion. It is not surprising, then, that metasedimentary rocks showing incipient metamorphic differentiation are adjacent to wellsegregated, stromatic migmatites separated by the fault, although the initial bulk composition of the rocks on either side of the fault was likely very similar. at a site where a bulk sample of feldspar was taken by local company, Thunderbrick Ltd. in 1981 (Assessment Files, Thunder Bay Resident Geologist’s Office). Geochemical analyses and results of testing of its suitability for ceramics applications are not available. A pervasive foliation (~250/70) is locally kinkbanded. Granitic dykes and anastomosing, black patches of what may be pseudotachylite cross-cut the high-grade metamorphic rocks (Fig. 13). Foliated pegmatitic rocks consist of quartz, feldspar, muscovite, garnet and biotite. Feldspar crystals may reach up to 60 cm in size. Garnet euhedra occur as disseminated crystals or trains of crystals. Biotite bands are also evident. Stop 1-9: Migmatites, Quetico Subprovince UTM coordinates: NAD83; 16U 0345756E / 5407714N Stop 1-7B: Quetico Fault in Granitoid Rocks UTM coordinates: NAD83; 16U 0348117E / 5402246N North of the ravine, a large rock cut displays red, coarse-grained to pegmatitic, K-feldspar granitoids. These granitoid rocks host numerous chlorite- and epidote-coated and slickensided joint surfaces and are locally well-foliated. Locally developed, shallowly west-plunging lineations and patchy pseudotachylite were also noted. Stop 1-8: Pegmatitic Granite, Quetico Subprovince UTM coordinates: NAD83; 16U 0348152E / 5405692N Graphic-textured, pegmatitic granitoids are exposed As noted by Seemayer (1992), stromatic (with lesser schlieric and agmatitic) migmatites predominate north of the Quetico Fault. Leucosome consists of quartz, plagioclase and perthite or microcline. Melanosome consists of biotite, quartz and plagioclase. Porphyroblasts of garnet, cordierite and sillimanite and mineral assemblages devoid of muscovite, but including alkali feldspar, reflect regional highgrade metamorphic conditions. Their occurrence is suggestive of diatexis of pelitic protoliths (Seemayer, 1992). At this location, “lit-par-lit” migmatites, consisting of equal proportions of leucosome and melanosome display a gneissosity at 255/80° and schollen structure Figure 13. Narrow   zones of cataclasite and pseudotachylite associated with the Quetico Fault, Stop 1-7A - 117 - �Proceedings of the 58th ILSG Annual Meeting - Part 2   Figure 14. Schollen and schlieric migmatite, Stop 1-9 muscovite, cleavelandite, quartz, brown black pyroxene and green fluorapatite). in large, rafted blocks. Quartzo-feldspathic, pegmatite neosome dykes cut the migmatites (Fig. 14). Garnet porphyroblasts up to 1 cm in diameter are also noted. Stop 1-10: Cordierite-bearing Granitoids, Quetico Subprovince STOP 1-11: Conglomerate, Wabigoon Subprovince UTM coordinates: NAD83; 16U 0328624E / 5448888N UTM coordinates: NAD83; 16U 039226E / 5416579N The 16 kilometer stretch of road that we have just Migmatitic rocks are intruded by cordierite-garnetmica pegmatites along this stretch of highway, where relict bands and schollen of biotitic schists are still evident. These pegmatitic rocks were described in detail by Breaks et al. (2003): Pegmatite sheets, at least 5 m thick, are evident on the Armstrong highway as at locality 01-FWB105 near Keelor Lake (UTM 339218E, 5416563N, Zone 16). These sheets consist of coarse-grained, garnet-muscovite-cordierite granite that contain 15 to 20% cordierite crystals pervasively altered to soft, dark green-black pseudomorphs (Fig. 15). The coarse [-grained] granite is gradational into muscovite-rich, miarolitic cavity-bearing, pegmatite patches (blocky potassium feldspar,   Figure 15. Cordierite crystals with incipient alteration along their margins, Stop 1-10 - 118 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure  16. Location of field trip stops 1-11 to 1-15 (Google earth image). Scale is 1.2 km long. traversed crosses a mainly metasedimentary terrain that is the western portion of the Beardmore-Geraldton area (Fig. 16). It is composed of three metasedimentary belts that are separated by metavolcanic belts. The northern metasedimentary belt is dominated by conglomerates and sandstones that were deposited by braided streams flowing from the Onaman-Tashota volcanic arc terrain to the north (Devaney, 1987; Fralick and Kronberg, 1997; Fralick et al., 1992; Fralick, 2003). The outcrop we will look at is part of this belt. The central metasedimentary belt to the east forms a northward younging, coarsening upward succession of ramp-fan turbidites (Barrett and Fralick, 1989) culminating in braid deltas interbedded with iron formation (Fralick and Pufahl, 2006). The same trend is present in the central metasedimentary belt here, though younging directions vary in the turbiditic portion due to isoclinal folding. The southern metasedimentary belt is composed of turbidites that are similar morphologically and geochemically to those in the central belt and the Quetico. This system served to deliver sediment to the Quetico trench as shown by these similarities and almost identical geochronology of their zircon populations (Fralick, 2003; Fralick et al., 2006). The main zircon population ranges from 2708 to 2698 Ma and represents erosion of synchronous calc-alkaline sub-areal volcanism occurring to the north. The clast composition reflects a source dominated by volcanic arc lithologies (mafic to felsic volcanics; diorite and other granitoids). Deep seismic profiling reveals that this succession was overthrust onto the volcanic arc rocks and was itself overthrust by the Quetico metasediments. This agrees with previous work that concluded the BeardmoreGeraldton belt represents a forearc basin between the Onaman-Tashota subareal volcanic arc to the north and the Quetico accretionary complex to the south (Barrett and Fralick, 1989; Eriksson et al., 1994, 1997). The outcrop we will examine forms a portion of the fluvial braided stream deposits that fed into the subaqueous portion of the basin to the south. Cobblepebble conglomerates represent longitudinal gravel bars with interbedded coarse- to medium-grained sandstones formed as waning flow sand sheets on the bar tails. Channels are represented by the thicker sandstones, commonly containing trough crossstratification, and some of the conglomeratic units. Small chute channels that cut into the bar tops during waning flow are represented by the sandstone lenses in the conglomerate. The rivers in this area were transitional to the south into sandy braided rivers, producing outcroppings of trough-cross-stratified, coarse- and medium-grained sandstone. This also formed in distributary mouth bars where the rivers flowed into the ocean to the south. The iron formations in the region are associated with this subaqueous, deltatop environment. In the marine basin to the south of the deltas turbidites accumulated in water that was shallow enough to allow storm reworking of the tops of these - 119 - �Proceedings of the 58th ILSG Annual Meeting - Part 2   Figure 17. Conglomerate, Stop 1-11 layers into dunes and ripples. Changes in river mouth position allowed muddy successions a few meters thick to develop during sediment starved intervals. Stop 1-12: Tuff-breccia, Wabigoon Subprovince The sequence has been tectonised as evidenced by the flattening of the clasts in the conglomerate (Fig. 18). The mafic clasts have been affected the most and in places are reduced to ribbon-like shapes. In contrast, the granitoid clasts behaved as rigid bodies, tending to fracture rather than deform plastically. A deformed intermediate tuff-breccia occurs within the dominantly mafic metavolcanic sequence near the intersection of Highway 527 and Kingdon Lake Road. Flattened, buff-coloured pyroclasts (lapilli to bombs) have aspect ratios ranging from 2:1 to >10:1 and define a strong foliation (050/50). Both pyroclasts and matrix are feldspar-phyric. UTM coordinates: NAD83; 16U 0328280E / 5449317N Stop 1-13: Pillowed Mafic Metavolcanic Rocks, Wabigoon Subprovince UTM coordinates: NAD83; 16U0328159E / 5449467N Pillowed mafic flows are best-exposed on the eastern side of the highway (Fig. 20). They are intensely foliated (055/52°) at the southern end of the outcrop. Light green-weathering bun and mattress pillows are locally vesicular/amygdaloidal and have darker green selvages; small re-entrants were noted. Pillows have been flattened into ovoid shapes with aspect ratios ranging from 5:1 to 10:1, precluding unequivocal “tops” determination.   Figure 18. Deformed conglomerate, showing rotation of competent granitoid cobbles and flattening of less-competent clasts, Stop 1-11 - 120 - �Proceedings of the 58th ILSG Annual Meeting - Part 2   Figure 19. Tuff-breccia, showing flattened pyroclasts, Stop 1-12 Stop 1-14: Brecciated Wabigoon Subprovince Metavolcanic Rocks, UTM coordinates: NAD83; 16U 0326447E / 5450297N perhaps both pyroclastic and autoclastic brecciation, is exposed on the eastern side of the highway. Flattened, buff (altered?) aphanitic fragments define a foliation at 050/55. An enigmatic volcanic unit, showing aspects of   Figure 20 Pillowed basalt flow, Stop 1-13 - 121 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 a focus of base metal exploration. Dome Exploration (Canada) Ltd. drilled this iron formation in 1972; no analytical results were reported (assessment files, Thunder Bay Resident Geologist’s Office, Thunder Bay). Day Two (Figs. 21 & 22) Stop 2-1: Pillar Lake Volcanic Rocks, Alarie Quarry UTM coordinates: NAD83; 16U 0356068E / 5573375N The Pillar Lake volcanic suite was first recognized and mapped by Macdonald (2004) and is the subject of an ongoing study by Magee (in progress). Initial observations suggested that it largely consisted of a sequence of flat-lying pillowed and massive flows and autoclastic and hyaloclastic breccias. The volcanic rocks unconformably overlie the Badwater gabbro (~1599 Ma) and Badwater syenite (~1590 Ma) and are capped by Keweenawan Inspiration diabase sills (1159 + 33 Ma), yielding an apparent thickness of 20 to 40 m (Hart and Macdonald, 2007; Heaman et al., 2007). Titanite in an andesitic unit yielded a 207Pb/206Pb age of 1129.0 + 4.6 Ma; no zircon nor baddeleyite was recovered in the sample (Heaman et al., 2007).   Figure 21. General geology and field trip stops, Day Two (Mesoproterozoic) Stop 1-15: Banded Iron Formation, Wabigoon Subprovince UTM coordinates: NAD83; 16U 0324893E / 5451015N A rusty-weathering sulphide-facies banded iron formation occurs within the volcanic succession. It is intercalated with tuffaceous and feldspathic, fragmental (pyroclastic?) rocks. The iron formation consists of pyrite, pyrrhotite + chalcopyrite and chert and has been Reinvestigation of these volcanic rocks in 2010 (Smyk et al., 2011) was prompted by a new exposure south of Armstrong that had been created during ballast quarry development (Fig. 23). The quarry face exposes a ~15 m section of thin (0.5 to 2 m), flat-lying, variably altered, columnar-jointed, basaltic andesite flows, capped by a diabase sill. Individual flows may persist over the 130 m length of the exposure while others bifurcate and terminate as thin tendrils in flow breccia (Fig. 24). Autobrecciated zones are rubbly weathering and occupy the spaces between thin, pinching flows. Thin flow top breccias separate flows. Massive flows contain zones of pipe amygdules at their bases and tops. The morphology and disposition of these flows is suggestive of an intercalated, subaerial pahoehoe and a’a flow succession. These volcanic rocks are variably altered along fractures, flow contacts and within brecciated zones. This hydrothermal alteration is typically manifested as a beige to pink discolouration of the dark greyblack flows resulting from the destruction of primary ferromagnesian minerals and the introduction of alkali feldspar, sericite and quartz. Void spaces along joints and fractures and in vesicles has been occupied by large (< 3 cm), black, eudhedral actinolite crystals. - 122 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure  22. Locations of stops 2-2 to 2-5 along Mattice Road and Highway 527 (Google earth image). Scale bar is 1452 m long.   Figure 23. Thin, amygdaloidal (a’a?) flows separated by interflow breccia, quarry face, Stop 2-1. The flow succession is overlain by an Inspiration diabase sill. - 123 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 24. Thin,  bifurcating flows separated by rubbly breccia, quarry face, Stop 2-1. Scale bar (left) is 1 m long Small acicular asbestiform crystals of edenite were also identified by x-ray diffraction analysis. Alteration is characterized by increases in Al2O3, K2O, Na2O, and SiO2 and decreases in CaO, Fe2O3, MgO, P2O5 and TiO2 (Fig. 25). The trace element geochemistry of the sill that caps the quarry face is identical to that of the Inspiration sill identified by Hart and Macdonald (2007), suggesting that this sill is part of that intrusive suite (Figs. 26 & 27). Samples of the volcanic rocks display REE enrichment and negative Nb anomalies comparable to the range of samples of the Pillar Lake volcanic suite reported by Magee (personal communication, 2011; Fig. 28). Samples of the Inspiration sills are geochemically indistinguishable from the Pillar Lake volcanic rocks and to least-altered pillowed samples with the lowest LOI values. The similarity between these two enigmatic suites suggests that they may be derived from the same source. MacDonald and Tremblay (2005) distinguished the Inspiration sill, which exclusively overlies the Pillar Lake rocks, on the basis of its normal polarity and distinct geochemistry. Primary clinopyroxene in the Inspiration sill was commonly replaced by actinolite Figure 25. Comparison of major element chemistry, unaltered and altered basaltic andesite, Stop 2-1 - 124 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 26. TAS plot for basaltic andesite (diamonds) and Inspiration diabase (circles), Stop 2-1 Figure 27. Plot of Mg# versus TiO2 for Pillar Lake volcanic rocks collected by Magee (in progress; squares) and at Stop 2-1 by Smyk et al. (2011; circles), compared to Inspiration sill and Osler Group volcanic rocks - 125 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 28. Chondrite-normalized REE plot for Pillar Lake volcanic rocks and Inspiration diabase (Smyk et al. 2011) (Schandl, 2004; Fig. 29). MacDonald et al. (2005) also noted a remarkable lack of chilled margins on the Inspiration sill and suggested that it was older than the Nipigon sills. Later work by Heaman et al. (2007) determined an age of 1159±33 Ma for the Inspiration sill, which is consistent with the early period of normal polarity. Smyk et al. (2011) proposed that the Pillar Lake basalts may, in fact, be coeval with the Inspiration sill which, in turn, may represent either a subvolcanic intrusion or possibly a massive, ponded flow / lava lake. This may account for the extensive alteration in the Pillar Lake basalts with the sill acting as an impermeable cap to hydrothermal fluids, which were concentrated in the underlying volcanic flows. The similarity of the Pillar Lake and Inspiration sill magmatism to other magmas of the Midcontinent Rift, including the Nipigon sills and Osler volcanic rocks (Hollings et al., 2007), suggests that these rocks may represent a very early extrusive to subvolcanic phase of rift activity. The location of these rocks close to the ~1599 Ma Badwater gabbro, the ~1590 Ma Badwater syenite and the ~1540 Ma English Bay anorogenic granite, which have been interpreted as evidence for a long-lived crustal weakness (Hollings et al., 2004), may offer an explanation for how these magmas were erupted so early in the rift history.  Figure 28.   Photomicrographs of basaltic andesite (left) and Inspiration diabase (right), crossed nicols, FOV = 4 mm in both images, Stop 2-1. - 126 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Stop 2-2: Pillar Lake Volcanic Rocks, T-Junction, Mattice Lake Road indicate that they are likely Proterozoic. The first sample …, a mafic volcanic flow, was collected in 2003 to directly determine the age of these volcanic rocks. A total of about 30 colourless irregular zircon grains were recovered, but none of these have the physical characteristics readily attributed to volcanic zircon (generally too large). Two single zircon grains were analyzed but did not yield consistent results (Heaman and Easton, 2006) and were not considered to provide a good constraint on the time of volcanism. Subsequent mapping in the McLaurin Lake area revealed the presence of a flat-lying stratigraphic succession consisting of a lower, pillowed mafic volcanic flow, a thin sandstone horizon …and an overlying sequence of mafic and andesitic flows …, all overlain by an Inspiration diabase sill ... Because of the well-preserved stratigraphic relationships, this section was sampled in detail for geochronology, to at least bracket the age of volcanism, even if it were not possible to directly obtain an age from the volcanic rocks themselves. UTM coordinates: NAD83; 16U 0345619E / 5564137N Several low-lying outcrops and large boulders cluster around a T-junction on the Mattice Lake Road. First discovered by MacDonald (2004), these Pillar Lake volcanic rocks display features indicative of submarine extrusion. Supporting evidence includes autoclastic breccias (flow/pillow breccia), hyaloclastite and pillow forms (Fig. 29). MacDonald (2004) described these rocks in this vicinity: An apparently flat-lying succession of mafic metavolcanic rocks crops out near the north end of Pillar Lake. The unit extends over an approximately 20 to 25 km2 area and consists of pillowed flow breccia, hyaloclastite breccia, pillowed and massive flows. Examination of outcrops from several locations would suggest that the apparently flat-lying unit has a thickness between 20 and 40 m. Lower portions consist of 3 or 4 alternating beds of pillowed flows with accompanying flow top breccia and or pillow breccia and hyaloclastite overlain by a 1 to 2 m thick layer of massive flow and/or sill. Locally, this unit displays slightly flattened 10 cm to 1 m diameter pillows with interflow hyaloclastite. Concentric jointing and tortoise shell-like cracks are common. Alteration of this unit typically consists of weakly to moderately pervasive to moderate patchy hematite alteration and local weak to moderate patchy sericite alteration. Alteration intensity increases with proximity to the north tip of Pillar Lake and along a creek leading between Mundell and Pillar lakes. The flat-lying and relatively undeformed nature of these flows, combined with the well-preserved nature of relatively delicate primary features such as hyaloclastite may suggest that these rocks are Proterozoic rather than Archean. A sample of fine-grained grey andesite from McLaurin Lake was collected to establish the age of the Pillar Lake volcanics in this region. There was no zircon or baddeleyite recovered from this sample; however, a modest amount of rutile and titanite was recovered. The multigrain fractions of rutile (1) and titanite (2) both have low uranium contents (8.3 and 7.6 ppm) and contrasting Th/U (0.222 and 27.818, respectively). The rutile fraction is concordant with a 206Pb/238U age of 1106.4 ± 2.8 Ma. The titanite fraction is also within error of concordia with a slightly North of Pillar Lake, drilling in 2004 showed that the Pillar Lake volcanic rocks (unconformably?) overlie the 1599 Ma Badwater gabbro. The ongoing uncertainty regarding the absolute age of the Pillar Lake volcanic rocks was summarized by Heaman et al. (2007): When first discovered in 2003, there was some question as to whether these volcanic rocks were Archean or Proterozoic in age, although their flat-lying character and alteration patterns   Figure 29. Autoclastic pillow breccia and hyaloclastite, Stop 2-2 - 127 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 older 207Pb/206Pb age of 1129.0 ± 4.6 Ma. The titanite age of 1129 Ma is interpreted to be the best constraint on the age of the McLaurin Lake andesite. It could be a minimum age or it could closely mark the age of volcanism. The 1106 Ma rutile age is similar to the age of the McLaurin Lake diabase (reported previously) and could reflect thermal resetting during emplacement of the diabase (rutile has a much lower closure temperature to Pb diffusion of ~400°C). Corporation in 2004 and 2008 indicated that gabbroic rocks underlie Pillar Lake volcanic rocks from at least west of Pillar Lake, east to McLaurin Lake (Middleton, 2004; Middleton and Bennett, 2008). Stop 2-3A: Badwater Gabbro, Badwater Creek Road At this locality, the gabbro is typically coarsegrained and unaltered. Anorthositic bands, variations in cumulus and intercumulus minerals, and igneous foliation are suggestive of igneous layering (Fig. 31). Thin section petrography cited by Middleton and Bennett (2008) for a gabbro drilled 2.3 km east of this location listed a modal mineralogy as approximately. Plagioclase (labradorite/bytownite) 55% Clinopyroxene (augite?) 25% UTM coordinates: NAD83; 16U 0347047E / 5564306N Biotite 10% Olivine (partly relict) 3% Stop 2-3B: Badwater Gabbro, East of Badwater Creek Road Talc/sericite, minor iddingsite (after olivine) 2% Amphibole (secondary, actinolitic) 2% UTM coordinates: NAD83; 16U 0347043E / 5564142N The Badwater gabbro is best-exposed north of Pillar Lake, where it is overlain by Pillar Lake volcanic rocks. It was dated by Heaman et al. (2007) at 1598.7 ± 1.1 Ma. It was intruded by the Badwater syenite at 1590.1 ± 0.8 Ma (Fig. 30). Drilling by East West Resource Opaque (magnetite?) 2% (pyrrhotite?) 1 % Clay? /sericite (after plagioclase) trace Plagioclase forms mainly euhedral crystals up to about 4 mm long, with random orientations, partly   Figure 30. Badwater gabbro xenoliths in feldspar-phyric syenite, eastern shore of Pillar Lake - 128 - �Proceedings of the 58th ILSG Annual Meeting - Part 2   Figure 31. Badwater gabbro, displaying igneous foliation and anorthositic lens/layer, Stop 2-3B. enclosing the mafic minerals. Pale brownish green clinopyroxene forms somewhat irregular, mainly subhedral crystals up to 4.5 mm long. Some crystals are partly altered, mainly around the rims, to minor secondary amphibole. Biotite forms ragged, irregular subhedral crystals mostly <2.5 mm in diameter, commonly wrapped around pyroxene, or interstitial to pyroxene and plagioclase. Olivine (or relict olivine) displays somewhat rounded or subhedral outlines up to almost 3 mm in diameter, commonly contained within pyroxene crystals or aggregates. The olivine is generally strongly fractured, with traces of minute secondary magnetite along the fractures. In places, the olivine is partly to locally completely replaced or pseudomorphed by very fine-grained talc/sericite or minor red-brown to greenish-brown iddingsite. Accessory opaque minerals appear to be mostly magnetite forming skeletal to irregular subhedral crystals up to 2 mm in diameter, interstitial to plagioclase, or associated with biotite and olivine or relict olivine. Sulfides (mostly pyrrhotite) form irregular subhedra up to 0.5 mm long and are also commonly interstitial to plagioclase and pyroxene, and are associated with biotite (Middleton and Bennett, 2008). STOP 2-4: Pillar Lake Volcanics, Chimney Lake UTM coordinates: NAD83; 16U 0349738E / 5563345N This outcrop on the northern shore of Chimney Lake is enigmatic (Fig. 32). It appears to be a volcanic breccia, but with variable clast compositions and what appears to be clastic matrix material. It could represent a mass-flow resulting from topography created by volcanism or fault movements related to magma recharge, though the lack of reaction rims makes a lahar improbable. Could it possibly be a “diatreme”? We will discuss its genesis in the field. A ~1m thick interflow sandstone unit described by Magee (in progress), approximately 1.5 km north of Chimney Lake consisted of basal sandstone, lithic arenite, and an upper quartz grain matrix breccia with locally derived basalt clasts. 50 detrital zircons from the interflow sandstone (Heaman et al., 2007) yielded a youngest concordant zircon of 1514 Ma. Dominant zircon populations fall between: 2700 to 2300 - 129 - �Proceedings of the 58th ILSG Annual Meeting - Part 2   Figure 32. Fragmental unit, Pillar Lake Volcanics, Chimney Lake, Stop 2-4 Ma; 1950 to 1900 Ma; 1880 to 1780 Ma. The basal sandstone geochemical signature is andesitic (Magee, in progress). STOP 2-5: Pillar Lake Volcanic Rocks, Highway 527 UTM coordinates: NAD83; 16U 0349460E / 5562320N This road cut on the eastern side of Highway 527 provides a ~3 m section through flat-lying basalt flows with small (10 to 30 cm) ellipsoidal features, suggestive of bun pillows. However, Smyk et al. (2011) suggested that these features may be pahoehoe toes in cross-section. The suggestion that these were subaerial flows is supported by the discovery of ropy flow top in a dislodged boulder in the ditch at the base of the exposure (Fig. 33). STOP 2-6: Nipigon Diabase Sill, Highway 527 UTM coordinates: NAD83; 16U 0342613E / 5499194N An 85 m long, 12 m high road cut provides an exceptional exposure of a Nipigon diabase sill at this locality (Fig. 34). The diabase is massive, homogeneous, fine- to medium-grained and locally feldspar-phyric. Irregular joint faces have been infilled with coarse, drusy quartz-chlorite-calcite-pectolite(?) + malachite veins. An orthogonal set of shallowly south-dipping and steeply north-dipping joints suggests that the sill may be shallowly south-dipping. The glacially polished surface on top of the road cut displays scattered pink, feldspathic (granophyric?) patches and pectolite(?)coated joint surfaces. STOP 2-7: Sibley Group Sedimentary Rocks and Nipigon Diabase Sills UTM coordinates: NAD83; 16U 0341205E / 5493814N Sibley Group sedimentary rocks are typically preserved in this area of the Nipigon Embayment below diabase sills which protect them from erosion. At this locality, straddling an overgrown logging road, a 3 to 4 m section of white-weathering, thinly bedded Rossport Formation dolomites are sandwiched between two thin Nipigon diabase sills (Fig. 35). As a result, the calcareous sedimentary rocks have been contact metamorphosed, resulting in the formation of metamorphic calc-silicates. Where Sibley Group rocks rich in carbonates, such as the Middlebrun Bay or Channel Island Members of the Rossport Formation, are proximal to thick sills the mineralogy consists of sodium- and potassium-rich varieties of pargasite, tremolite, talc, magnesium-rich clinoclore and calcite. - 130 - �Proceedings of the 58th ILSG Annual Meeting - Part 2   Figure 33. Cross-section through thin basalt flows, showing possible pahoehoe toes (left); ropy flow surface preserved in dislodged boulder (above); Stop 2-5.     Figure 34. Inspiration diabase sill, showing shallowly south-dipping joints, Stop 2-6 - 131 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Pargasite is common near the large sills, but tremolite, then talc, become important with distance away from the heat source (Rogala et al., 2005). This indicates temperatures in the 600°C dropping to 400°C range. The outcrop we are visiting appears to have initially been largely composed of dolomite. This combined with relict bedding and possible stromatolitic structures that are still evident indicates that this may represent the Middlebrun Bay Member. On the other (western) side of the road, a flat-lying outcrop displays well-developed polygonal jointing (aka “tortoise-shell” texture) that are characteristic of the upper chilled contacts of these sills (Fig. 36). Feldspathic alteration along these joints results in their raised appearance. Numerous, recessively weathering ovoid pits may be the remnants of fluid- and volatilerich pockets which migrated to the top of the cooling sill. Acknowledgements The authors wish to acknowledge the contributions of many people who provided suggestions and assistance in the development of this field trip and guidebook. John Scott, recently retired from the Resident Geologist Program, OGS, Thunder Bay, was an invaluable asset in suggesting sites and providing information. Assistance in the field during the scouting of field trip sites was provided by John Scott, Dorothy Campbell and Robert Cundari (RGP-OGS, Thunder Bay). The authors have benefited from field work and discussions in the field with Carole Ann MacDonald and Tom Hart (both formerly with Precambrian Geoscience Section, OGS), Dr. Peter Hollings (Lakehead University), Angelique Magee (Carleton University/Geological Survey of Canada) and Robert Middleton (formerly East West Resource Corporation). References Barrett, T.J. and Fralick, P.W., 1989. Turbidites and iron formations, Beardmore-Geraldton, Ontario: Application of a combined ramp-fan modelto Archean chemical and clastic sedimentation. Sedimentology, 36: 221-234. Blackburn, C.E., Johns, G.W., Ayer, J. and Davis, D.W. 1991. Wabigoon Subprovince; in Geology of Ontario; Ontario Geological Survey, Special Volume 4. pt.1, pp.302-381. Breaks, F.W., Selway, J.B. and Tindle, A.G. 2003. 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Unpublished Ph.D. thesis, The University of Western Ontario, London, Ontario, 325 p. Sutcliffe, R.H. 1987. Petrology of Middle Proterozoic diabases and picrites from Lake Nipigon, Canada. Contributions to Mineralogy and Petrology, 96: 201– 211. Sutcliffe, R.H. 1991. Proterozoic geology of the Lake Superior area. In Geology of Ontario. Edited by P.C. Thurston, H.R. Williams, R.H. Sutcliffe, and G.M. Stott. Ontario Geological Survey, Special Vol. 4, Part 1, pp. 405–484. Sutcliffe, R.H., and Greenwood, R.C. 1985a. Geological series, Precambrian geology, Lake Nipigon area, Kelvin Island sheet, District of Thunder Bay. Ontario Geological Survey, Preliminary Map P.2838, scale 1: 50 000. Tomlinson, K.Y., Davis, D.W., Percival, J.A., Hughes, D.J., Thurston, P.C., 2002. Mafic to felsic magmatism and crustal recycling in the Obonga Lake greenstone belt, western Superior Province: evidence from geochemistry, Nd isotopes and U–Pb geochronology. Precambrian Res. 114/3-4, 295–325. Tomlinson, K.Y., Stott, G.M., Percival, J.A. and Stone, D. 2004. Basement terrance correlations and crustal recycling in the western Superior Province: Nd isotopic character of granitoid and felsic volcanic rocks in the Wabigoon Subprovince, N. Ontario; Precambrian Research, v.132, pp.245-274. Williams, H.R. 1991. Quetico Subprovince; in Geology of Ontario; Ontario Geological Survey, Special Volume 4. pt.1, pp.383-403. Williams, H.R. and Stott, G.M. 1991. Subprovince accretion in the southern Superior Province; Geological Association of Canada-Mineralogical Association of Canada-Society of Economic Geologists, Field trip guidebook, 26p. Zaleski, E., Van Breemen, O., and Peterson, V.L. 1999. Geological evolution of the Manitouwadge greenstone belt and Wawa–Quetico subprovince boundary, Superior Province, Ontario, constrained by U–Pb zircon dates of supracrustal and plutonic rocks. Canadian Journal of Earth Sciences, 36: 945–966. - 135 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Field trip 9 - Rehabilitation of the Past-Producing Shebandowan and North Coldstream Mine Sites Mark Puumala Ministry of Northern Development and Mines, 435 James St. S., Suite B002, Thunder Bay, Ontario, P7E 6S7, Canada Introduction This trip will provide an overview of rehabilitation measures that have been implemented at two pastproducing mines that are located in the Shebandowan greenstone belt west of the City of Thunder Bay (Fig. 1). The Shebandowan Mine operated between 1971 and 1998, producing 9.29 million tonnes of ore grading 1.75% nickel, 0.88% copper, 0.063% cobalt, 0.0533 oz/ ton platinum group elements and 0.0575 oz/ton silver (Inco, 2001). The North Coldstream Mine operated between 1957 and 1967, producing approximately 2.5 million tonnes grading 1.97% copper, 0.012 ounces per ton gold and 0.22 ounces per ton silver (Golder Associates, 2002). Both mines produced significant quantities of acidgenerating tailings during their operational lives and provide a good illustration of historic and current mining waste management practices for base metal mines, and the technologies that are employed to prevent and mitigate adverse water quality impacts. Mine Rehabilitation Regulatory Framework Mineral exploration, mine development and mine rehabilitation in the Province of Ontario are regulated under the Mining Act. Part VII of the Mining Act deals principally with the rehabilitation of mines and mining lands and was proclaimed in 1991 (with the most recent significant amendments occurring on June 30, 2000). Under Part VII, proponents of all advanced exploration projects and operating mines are required to file a certified Closure Plan including financial assurance to indicate the method, schedule and cost of all rehabilitation to be conducted on the site once Figure 1. Location map illustrating Shebandowan and North Coldstream Mine sites relative to the City of Thunder Bay. - 136 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 closure commences. 1996: Mine Closure Plan accepted by MNDM. Closure Plans are not mandatory for historic mines that closed before 1991. However, all proponents of mining lands are responsible to ensure that any historic mine hazards on their property are progressively rehabilitated to prescribed standards. The minimum standards for mine rehabilitation are prescribed under Ontario Regulation 240/00 – Mine Development and Closure under Part VII of the Act. The Shebandowan Mine, which operated until 1998, is an example of a mine site that is being rehabilitated under a Mine Closure Plan, while the North Coldstream Mine, which closed before 1991, is currently undergoing progressive rehabilitation. 1998: Shebandowan Mine permanently ceased operation and work began to implement the Mine Closure Plan. Work completed to date includes flooding of tailings basin, waste rock relocation to tailings pond, infrastructure demolition and removal, capping of mine openings, closure of two landfills, and revegetation of disturbed areas. The Ministry of Northern Development and Mines (MNDM) is the government agency that is responsible for the administration and enforcement of the Mining Act. Shebandowan Mine Exploration and Development History The following summary of the historical exploration and development of the Shebandowan Mine property is based on information compiled from the files of the MNDM Mines and Minerals Division office in Thunder Bay. 1913: Nickel-copper ore found at Discovery Point by prospector Julian Cross. 1936: International Nickel Company (Inco) purchased property. 1936-1965: Various surface exploration programs were carried out. 1966-1967: No.1 development shaft completed, underground diamond drilling. 1968: Inco announced decision to develop mine and supporting facilities at Shebandowan. No.2 (production) shaft and mill commissioned. 1969-1974: Forest debris created by road and on-site construction removed, topsoil stockpiled for later usage and contouring, and revegetation began (hydro-seeding of grass with oats/rye, straw mulch; 15 000 seedlings / trees). 1973: Mine and mill complex officially opened June 28. 1986-1988, 1992-1995: Operations temporarily suspended due to economic conditions. 2001: Mine Closure Plan Amendment filed with MNDM. 2003: Option / joint-venture agreement signed between Inco (now Vale) and North American Palladium (NAP) to explore former mine property and environs. 2008: Underground exploration, ramp/decline advanced to collect bulk sample of ore from Shebandowan West deposit. This work was done under a separate advanced exploration Closure Plan filed by NAP. Operations were suspended due to the fall 2008 economic downturn. 2012: Vale continues to be responsible for the implementation of the Shebandowan Mine Closure Plan, while NAP is responsible for the Shebandowan West site. Denison Environmental has been contracted by Vale to carry out on-going site maintenance and rehabilitation activities. Deposit Geology The Shebandowan nickel-copper deposit is hosted in a serpentinized peridotite sill that forms part of a mafic metavolcanic rock-dominated sequence (Morin, 1973; Osmani 1997). The ore body is also located near the southern margins of the quartz diorite Shebandowan Lake Stock. The northwest-trending Crayfish Creek Fault (a regional-scale dextral transcurrent structure) is located immediately south of the ore body. Rocks to the south of the fault form a separate domain consisting largely of intercalated felsic, intermediate and mafic metavolcanic rocks (Osmani, 1997). The Shebandowan Mine ore body included three styles of mineralization: massive, breccia, and stringer ore (Osmani, 1997). Massive ore consisted of pyrrhotite, pentlandite, chalcopyrite, pyrite and magnetite. Breccia ore was comprised of pyrrhotite, chalcopyrite and pentlandite, and contains fragments of peridotite, mafic metavolcanics and granitic rock. Stringer ore occurred as stringers of chalcopyrite, pyrite and minor pyrrhotite and pentlandite in shear zones. The average width of the ore body was approximately 7.5 m (Inco, 2001) and was mined over a strike length - 137 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 of approximately 3.5 km to a maximum depth of approximately 1000 m. Field trip stops Stop 1: Discovery Point UTM coordinates NAD 83; 15U 0701365E / 5386525N Nickel-copper ore was first discovered 99 years ago along the shoreline of Lower Shebandowan Lake at Discovery Point. The initial underground exploration shaft was sunk at this location by Inco during the 1960s. The shaft location is now marked by a vented reinforced concrete cap and is located near the west end of the ore body (workings extend approximately 500 m further to the west). The underground mine workings are mostly located below the lake, and extend a further 3 km to the east. Rehabilitated vent raises are located on an island Photo 1. Reinforced concrete cap, Shaft No. 1 - Discovery Point   that can be seen from the outcrop near the shoreline immediately east of the shaft, while the No. 2 production shaft was located on a point behind the island. The mining method used at Shebandowan was cut and fill Figure 2. Satellite image of Shebandowan Mine site showing field trip stop locations. - 138 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 3. Shebandowan Mine longitudinal section from Closure Plan (Inco, 2001). Figure 4. Plan view showing area of underground workings (Inco, 2001). - 139 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 stoping, with the mine workings being backfilled with a cemented mixture of 60% tailings and 40% alluvial sand (30:1 ratio of backfill to Portland cement). The minimum crown pillar thickness beneath the lake is 33 metres and an engineering study has indicated that there are not likely to be any long-term rock stability issues. have the potential to generate acid, a lined ore pad was also constructed adjacent to the pond in order to ensure that any contaminated runoff was not discharged to the natural environment. As per the requirements of the Closure Plan (North American Palladium, 2008), all ore was shipped off site for processing before activities at the site were suspended. Stop 2: Shebandowan West Prospect Stop 3: No. 2 Shaft Area UTM coordinates NAD 83; 15U 0700710E/ 5386900N UTM coordinates NAD 83; 15U 0702850E / 5386235N The Shebandowan West prospect consists of three shallow Ni-Cu mineralized zones (West, Road and D Zones) located immediately west and along strike with the Shebandowan Mine ore body. North American Palladium (NAP) http://www.napalladium.com/ English/projects/reserves-and-resources/default.aspx reports measured and indicated resources of 1,292,000 tonnes grading 0.91% Ni, 0.62% Cu, 1.09 g/t Pd, 0.34 g/t Pt, and 0.23 g/t Au. During 2008, a ramp was advanced to collect a bulk sample for metallurgical testing. After the project was suspended, the portal and vent raise were backfilled to prevent inadvertent access to the underground workings. These are considered to be temporary measures that would need to be upgraded and certified if the proponent were to decide to permanently close-out the site. The clearing at this stop was previously the location of the Shebandowan Mine headframe and hoist structures. These were demolished and removed from the site in 2001. Similar to the No.1 shaft, the No. 2 shaft was subsequently capped with reinforced concrete. The Mine Rehabilitation Code of Ontario requires that disturbed areas of mine sites be revegetated to stabilize surface soils, improve aesthetics, establish sustainable vegetation growth and support the To the west of the portal, NAP constructed an engineered containment pond to collect and retain water pumped from the underground workings. The pond is lined with high density polyethylene (HDPE) to prevent seepage. Because the ore is considered to   Photo 2. Ore pad and containment pond at Shebandowan West project site  Photo 3. View of No. 2 Shaft area. Concrete caps and pump house are visible in centre of photo. - 140 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 designated end use of the site. The No. 2 shaft area was contoured and seeded after the completion of demolition activities. Vegetation growth has been successful, with some native species (e.g., trees) already beginning to colonize the area. The last remaining original building on the site is the former process water pumphouse located adjacent to the lake. This pumping equipment now serves as a source of water that is used during periods of low precipitation to ensure that the tailings pond remains saturated. Stop 4: Mill Area UTM coordinates NAD 83; 15U 0703230E / 5385400N All buildings associated with the former mine/mill complex were demolished and removed from the site in 2003. Similar to the headframe area, the mine/mill complex area was seeded, and vegetation growth has been occurring. Revegetation efforts in this area have also included the planting of trees. An acid-generating waste rock pile was previously located in the low-lying area at the southwest corner of the clearing. This waste rock was relocated to the tailings pond, and is now located under water. Residual groundwater quality impacts continue to be monitored in groundwater monitoring wells that have been installed to the south of the former waste rock area. A storm water collection pond is located at the east end of the mill area clearing. Water collected in this pond contains elevated concentrations of metals  Photo 5. Storm water collection pond. Pump barge can be seen in centre of pond. Water is pumped to tailings pond at Dam No. 4. (most notably nickel) leached from soils in the mill area. Groundwater in the vicinity of the pond is also impacted by residual petroleum hydrocarbons from a fuel spill that occurred in its vicinity during mine operations. Water from the storm pond continues to be pumped to the tailings pond for treatment. This will continue until the water quality meets regulatory requirements for direct discharge to the environment. Groundwater quality in the area downgradient of the storm pond also continues to be monitored for acid rock drainage (ARD) and petroleum hydrocarbon impacts. Groundwater monitoring must continue until geochemical stability has been demonstrated and there are no longer any significant risks of adverse impact to downgradient receiving water bodies. Stop 5: Tailings Dam No. 4 Seepage Collection Pond UTM coordinates NAD 83; 707780E, 5384550N   Photo 4. View of rehabilitated mill area The Shebandowan Mine tailings were deposited in a 115 ha impoundment located approximately 1.5 km southeast of the mine/mill complex. The tailings contain a significant proportion of sulphide minerals and are considered to be acid-generating. Data presented in the 1996 Closure Plan (Inco, 1996) indicated that unoxidized tailings contain up to 13.2% sulphur and have NP/AP (neutralization/acid generation potential) ratios of approximately 0.16. Values less than 1 indicate that a material is acid generating. As a result, the tailings impoundment closure design includes a permanent water cover to prevent sulphide oxidation and acid production. The tailings area is contained in a basin - 141 - �Proceedings of the 58th ILSG Annual Meeting - Part 2  Photo 6. Shebandowan Mine tailings pond. Dam No. 4 is located to right hand side of pond. Splitter dyke can be seen at centre of photo crossing pond. This dyke limits wave development in pond. that is bounded by natural topography (bedrock outcrop areas) and six engineered dam structures. Although the tailings dams are designed to retain water, some seepage occurs. Seepage that collects in a pond at the toe of Dam No. 4 contains elevated concentrations of iron and nickel. As a result, this seepage is pumped back into the tailings pond for treatment. Groundwater monitoring is carried out downgradient of all of the tailings dams to monitor the groundwater quality impacts resulting from seepage. Although there are elevated concentrations of iron, manganese and sulphate in these seepage plumes, there is no evidence of acidic drainage (Wesa, 2009).  Photo 8. Tailings pond spillway Discharges from the tailings basin to the natural environment are regulated by the Ontario Ministry of the Environment under an Industrial Sewage Works approval. Since mine closure, the water quality in the tailings pond has improved to the point where no active treatment is required to meet the applicable effluent limits. However, monitoring will continue to be required until water quality meets the more stringent Provincial Water Quality Objectives. Stop 6: Tailings Pond Spillway UTM coordinates NAD 83; 0706315E / 5384850N Discharges from the tailings pond occur through a spillway located at the east end of the impoundment. The spillway is excavated through solid bedrock near the north abutment of Dam No. 2. Discharge from the tailings pond is intermittent, and only occurs following times of significant precipitation or snow melt. When discharge is occurring, water quality monitoring must be carried out to demonstrate that the water quality meets effluent limits. The receiving water body is Gold Creek, which is part of the Matawin River watershed, which ultimately flows to Lake Superior. North Coldstream Mine Exploration and Development History   Photo 7. Dam No. 4 seepage collection pond and pump house The following summary of the historical exploration and development of the North Coldstream Mine property is based on information compiled from the files of the MNDM Mines and Minerals Division office in Thunder Bay. - 142 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 corporate amalgamation. 1870s: Copper mineralization discovered. 1951-1957: Coldstream Copper Mines Ltd. carried out exploration and development program. 1957: Production commenced. 1958: Operations suspended. 1959: Company re-organized and name changed to North Coldstream Mines Ltd. 1960-1967: Mine produced approximately 2.5 million tons of ore grading 1.97% copper, 0.012 ounces per ton gold and 0.22 ounces per ton silver. 1968: Mill and associated infrastructure and surface rights sold to Nelson Machinery. 1971: North Coldstream Mines changes name to Coldstream Mines. 1976: Coldstream goes into receivership. 1991: Nelson Machinery placed into receivership. 1992-1995: MNDM ordered Nelson Machinery to submit a Closure Plan for mill site infrastructure, and Conwest to submit Closure Plan for tailings, mill yard. Subject to appeals, Mining Commissioner ruled that Nelson was responsible for rehabilitation of mill area, Conwest responsible for tailings and mine openings. 1996: Conwest acquired by Alberta Energy Company, re-named AEC West. Mineral rights later sold subject to an agreement that AEC West would retain responsibility for rehabilitation of tailings and mine workings. AEC West was subsequently re-named EnCana West and EWL Management (current corporate identity). 1998-2000: Tailings areas rehabilitated. 1977: International Mogul acquired mineral rights. 1982: Conwest becomes mineral rights owner through 2000: MNDM rehabilitated mill site Abandoned Mines Rehabilitation Fund. Figure 5. North Coldstream Mine field trip stop locations. - 143 - through �Proceedings of the 58th ILSG Annual Meeting - Part 2 2000-2002: Several mine openings to surface rehabilitated, fencing erected around open stopes. 2008-2012: Crown pillar stability investigations, additional tailings rehabilitation activities, work towards development of long-term monitoring program. Deposit Geology The North Coldstream deposit is located within an inferred s-folded metavolcanic and gabbroic rock sequence (Osmani, 1997). The gabbro intrudes along the contact between felsic and mafic metavolcanic rocks. The northeast-trending Burchell Lake Fault is located immediately west of the site. The North Coldstream Mine ore body is a 120 x 300 m silicified zone located at the contact between the gabbro and mafic metavolcanic rocks. Osmani (1997) has interpreted the mineralized zone as silicified gabbro. The ore zone consists of a high density network of chalcopyrite and pyrite veinlets, and massive and disseminated mineralization within a siliceous host rock that resembles chert. Field trip stops Stop 7: TMA-1 Tailings Area   Photo 10. TMA-1 tailings area in 1991. chalcopyrite and roughly similar quantities of pyrite. Tailings sampling carried out by CANMET in 1994 indicated that the average sulphur content was 5.7%, and that they are acid generating (Burns et al., 1999). The majority of TMA-1 is located over permeable soil (sand and gravel) with a deep water table. This hydrogeologic setting has resulted in the development of a significant ARD plume in the groundwater immediately below and downgradient of the tailings. This contaminant plume has low pH and contains extremely high concentrations of sulphate and metals (e.g., Fe, Cu, Co, Mn, and Ni). Tailings Management Area 1 (TMA-1) was used for the deposition of North Coldstream Mine tailings until 1962 (Burns et al., 1999) and was the largest of two tailings deposition areas that were used during mine operations. The ore contained approximately 2 to 8% Groundwater from TMA-1 migrates in a westerly direction toward the Wawiag River, which is known to be a location of groundwater discharge from the overburden aquifer. The overburden aquifer is located in a west-trending bedrock depression that controls the ARD plume flow direction. The core of the ARD plume sinks toward the bottom of the aquifer between TMA-1 and the river. Prior to reaching the river, the   Photo 11. Embankment on west side of TMA-1 in 1998. UTM coordinates NAD 83; 0678515E, 5386325N Photo 9. Aerial view of mine/mill complex and southwest end of TMA-1 tailings area in 1991 - 144   - �Proceedings of the 58th ILSG Annual Meeting - Part 2   Wawiag   R.   Groundwater   flow   TMA -­ 1   Figure 6. Approximate groundwater flow path from TMA-1. deepest portions of the plume become confined below a silt confining layer. As a result, the most severely impacted groundwater does not discharge to the river. Nevertheless, measurable water quality impacts attributable to TMA-1 do occur in the Wawiag River (i.e., elevated levels of Fe and Co), especially during periods of low flow. Deep ARD-impacted groundwater beneath the confining layer changes flow direction and migrates in a southwest direction through another overburden-filled bedrock depression that parallels the Wawiag River toward Burchell Lake. The ultimate location where the deep ARD plume is believed to discharge to Burchell Lake is approximately 1.5 km offshore at a depth of 40 to 70 m (Golder Associates, 2011). To date, no significant impacts to the lake have been documented as a result of the TMA-1 tailings plume. In 1998-1999, a vegetated low permeability cover with a capillary break was placed over the TMA-1 tailings in an effort to reduce groundwater quality impacts. This work has resulted in reduced concentrations of sulphate and metals in the tailings impact plume. However, portions of the plume remain acidic and it is expected to take decades for the acid rock drainage plume to be fully rehabilitated. Between 2007 and 2010, EWL Management carried out several environmental and geochemical investigations to better characterize site conditions. Some of the key findings of this work are listed below. • Shallow groundwater in the northern half of TMA1 has neutral pH and lower sulphate and metal concentrations than in the southern half. As a result, the northern portion of the TMA-1 cover appears to be functioning as expected. • September 2010 data for MW38B: pH = 7.4, sulphate = 1440 mg/L, Fe = 54 mg/L, Mn = 2.3 mg/L, Cu = <0.01 mg/L, Co = 0.0005 mg/L (Golder Associates 2010). • Acidic groundwater continues to be present in the southern half of TMA-1, indicating that this portion of the cover was not performing as expected. Photo 12. Vegetated TMA-1 cover as it appeared in the  summer of 2005. - 145 - • September 2010 data for MW35B: pH = 3.3, sulphate = 3390 mg/L, Fe = 1000 mg/L, Mn = 11 mg/L, Cu = 3.6 mg/L, Co = 4.7 mg/L (Golder �Proceedings of the 58th ILSG Annual Meeting - Part 2  Photo 13. Photograph taken in October 2011 during placement of cover over relocated tailings at TMA-1. Clay layer is visible to left, with overlying granular cover layer to right Associates 2010) • Significant aquifer recharge was occurring at the southeast end of TMA-1 immediately following major rainfall events. This is likely to have been responsible for the continued ARD generation in the south half of TMA-1. • Storm drainage in the eastern perimeter spillway was continuing to show signs of acid generation. • Two previously unidentified “orphan” tailings areas were found north and west of TMA-1. These were most likely related to mobilization from TMA-1 during historic storm events. • Two small tailings areas located to the south of TMA-1 were identified as ongoing sources of ARD that required additional rehabilitation. Photo 14. Reconstructed eastern perimeter spillway. East   side of tailings relocation area can be seen to left. During 2011, additional rehabilitation work was done on the site to address the on-going ARD issues. This work included the relocation of the orphan and southern tailings areas to TMA-1 and the reconstruction of the eastern perimeter spillway. The relocated tailings were placed over the eastern half of TMA-1 during the winter of 2011, and a low permeability clay cover was placed over them during the summer and fall. The eastern perimeter spillway was also reconstructed in 2011 to more effectively convey storm drainage around TMA-1. A key design element was the installation of a geosynthetic clay layer on the western side of the spillway to isolate drainage from the TMA-1 tailings. It is expected that this additional rehabilitation work will significantly improve storm drainage quality, reduce infiltration at the southeast end of TMA-1, and reduce contaminant loadings to the overburden aquifer. Stop 8: TMA-2 Tailings Area UTM coordinates NAD 83; 0679185E / 5386600N From 1962 to the end of the mine life in 1967, approximately 500,000 tonnes of North Coldstream Mine tailings were deposited in Halet Lake, which is now known as TMA-2 (Burns et al., 1999). Prior to 1998, a 3 ha tailings beach was located at the former tailings discharge location at the southwest end of the lake. These tailings were generating acid and contributing metal loadings to the tailings pond and downstream receiving water bodies. The TMA-2 outlet drains north to Background Lake, which subsequently drains toward the Wawiag River and Burchell Lake. During the summer of 1998, the majority of the TMA-2 tailings beach was relocated below water, with approximately 4,000 tonnes relocated to TMA-1 (Burns et al., 1999). The goal of the tailings relocation was to completely submerge the tailings and prevent further oxidation and acid generation. Suspended sediment, acidity and metals were released into the TMA-2 pond during tailings relocation. A temporary dam was constructed to prevent downstream discharge, and the pond was limed to reduce acidity and metal concentrations. Since completion of the tailings rehabilitation, water quality has stabilized, with neutral pH and low metal concentrations in the TMA-2 discharge (although copper and cobalt concentrations do slightly exceed Provincial Water Quality Objectives). Following relocation, some of the tailings along the west margin of TMA-2 remain less than 2 m below the water surface. Tailings in the central basin are much - 146 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 during 2008 due to the construction of a beaver dam on the outlet stream that regulated the water level and kept it well above the tailings. During the winter of 2012, EWL Management planned to construct an engineered water level control structure on the outlet stream in order to permanently maintain higher water levels in TMA-2. A long-term water quality monitoring program will be carried out to monitor the success of the tailings rehabilitation efforts at the North Coldstream Mine. This monitoring program will include the sampling of surface water and groundwater monitoring wells.  Photo 15. View of TMA-2 (Halet Lake) from former tailings beach area. farther below surface, ranging in depth from 7 to 12 m (Golder Associates, 2011). Because some tailings are relatively close to surface, it is important that the water elevation is maintained at a level that maintains permanent saturation. Prior to 2008, water levels occasionally dropped during dry years to expose some of the tailings. However, water levels rose substantially Stop 9: Mine/Mill Area UTM coordinates NAD 83; 0678120E / 5386040N All surface structures on the mine/mill site were demolished and removed in 2000. By this time, the buildings had deteriorated to the point where they had become significant safety hazards. This work was performed by the Ministry of Northern Development and Mines utilizing the Abandoned Mines Fund. Since the completion of the building demolition Figure 7. Map of North Coldstream mine/mill area showing locations of mine openings to surface. The locations of the former buildings are also shown. - 147 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 investigation and rehabilitation plan pillar areas. It is additional fencing program. is currently developing a final for all potentially unstable crown likely that this plan will include and a rock stability monitoring Stop 10: Burchell Lake “Ghost Town” UTM coordinates NAD 83; 0677550E / 5386090N  Photo 16. Glory Hole project, EWL Management and its predecessor companies have been working toward completing the rehabilitation of the mine openings to surface and the underground mine workings. By 2002, most of the shafts and raises had been permanently rehabilitated (Golder Associates, 2002). Certified reinforced concrete caps were constructed over the Nos. 3 and 4 shafts, and the 250 and 257 vent raises. The No. 2 shaft was backfilled to surface and its long-term stability has been certified. Areas with open stopes and/or unstable crown pillars have been fenced. These include areas near the No. 1 shaft, and above the 2-4-49 W and 2-449 E stopes. An open stope (known as the Glory Hole) is present in the fenced area around the 2-4-49 E stope. This is a general interest stop to view the remains of the former town site of Burchell Lake. Although a number of larger buildings were removed following mine closure, many abandoned houses remain. Mine management homes were located in a separate development located further to the south. These buildings continue to be used as seasonal cottages. EWL Management has carried out a crown pillar  Photo 18. Abandoned houses in former town site of Burchell Lake. References Burns, R.C., Orava, D.A., Zurowski, M. and Mellow, R.J., 1999. A case study of the rehabilitation of sulphide tailings at the Coldstream mine tailings management area no. 2: in Proceedings of Sudbury ’99 Mining and the Environment Conference, p. 301-308. Inco Limited Ontario Division, 2001. Shebandowan mine closure plan part I of II: unpublished report, Ministry of Northern Development and Mines Thunder Bay Mines and Minerals Division office, 84p.   Photo 17. No. 4 Shaft and 250 Vent Raise are located below fenced vent pipes. Fencing was installed to protect against vandalism. Inco Limited Ontario Division, 1996. Shebandowan mine closure plan part I of II: unpublished report, Ministry of Northern Development and Mines Thunder Bay Mines and Minerals Division office, 120p. - 148 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Golder Associates Limited, 2011. Coldstream mine site 2010 surface water and groundwater monitoring report; unpublished report, Ministry of Northern Development and Mines Thunder Bay Mines and Minerals Division office, 51p. Golder Associates Limited, 2002. Closure report, Coldstream mine openings, Burchell Lake area, Northwestern Ontario; unpublished report, Ministry of Northern Development and Mines Thunder Bay Mines and Minerals Division office, 32p. Morin, J.A., 1973. Geology of the Lower Shebandowan Lake area, District of Thunder Bay; Ontario Division of Mines, Geological Report 110, 45 p. North American Palladium Limited, 2008. Shebandowan West advanced exploration project closure plan; unpublished report, Ministry of Northern Development and Mines Thunder Bay Mines and Minerals Division office, 62p. Osmani, I.A., 1997. Geology and mineral potential Greenwater Lake area, west-central Shebandowan greenstone belt; Ontario Geological Survey, Geological Report 296, 135 p. Wesa Incorporated, 2009. Groundwater characterization assessment of potential impacts to existing water supply wells, Shebandowan mine; unpublished report, Ministry of Northern Development and Mines Thunder Bay Mines and Minerals Division office, 60p. - 149 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Field trip 10 - Geoarchaeology of the Thunder Bay area Brian Phillips Department of Geography (Emeritus), Lakehead University, 955 Oliver Road, Thunder Bay, P7B5E1 Scott Hamilton Department of Anthropology, Lakehead University, 955 Oliver Road, Thunder Bay, P7B5E1 Bill Ross Ross and Associates/Department of Anthropology, Lakehead University, 955 Oliver Road, Thunder Bay, P7B5E1 Pat Julig Department of Anthropology, Laurentian University, Sudbury, Ontario Joe Stewart Department of Anthropology (Emeritus), Lakehead University, 955 Oliver Road, Thunder Bay, P7B5E1 Objectives The field trip focuses on the deglaciation and lake level history of Thunder Bay and the immediately surrounding area (Fig. 1). In particular, we will examine evidence of Palaeo-Indian occupation along abandoned shorelines, river mouths and deltas of the Lake Minong stage of Lake Superior (circa 9.5 ka BP.). The trip includes a visit to the former mouth of the Current River at the north end of the city, where the Simmonds and McDaid sites are located. We then travel east along Highway 11/17 to view several PalaeoIndian sites currently undergoing salvage excavation along the path of highway development. The tour then returns to Thunder Bay to visit the Cummins site, and also the nearby Neebing R. sites. Next we visit the Rosslyn delta, on the Kaministquia River, the western extent of Lake Minong. From there we will travel west to Kakabeka Falls, where earlier Lake Beaver Bay features will be examined. Finally, the trip will 1 2 3 9 4 5 6 7 8 10 11 15 14 12 13 N 1 2 3 4 5 7 km Figure 1 Thunder Bay area orientation map. - 150 - Dog Lake Moraine Mackenzie Moraine Intola Moraine Marks Moraine Brule Moraine 6 Pass Lake site cluster 7 Mackenzie site cluster 8 Hodder Ave Cluster 9 Current River Cluster 10 McIntrye River Cluster 11 Cummins/Neebing Cluster 12 Breukelman-Evergreen Cluster 13 Breukelman Farm Cluster 14 Drezecky-Pawlick Sites 15 Crane Site Cache. �Proceedings of the 58th ILSG Annual Meeting - Part 2 Cummins Mackenzie Brohm Figure 2 The spatial relationship of probable Plano and Archaic sites with Lake Minong shorelines (dashed line) and exposures of tool stone deriving from Gunflint Formation (hatchured lines). After Hinshelwood (2004:234). mount the Marks moraine, providing a view that will place the day’s observations in context with pre and post Marquette ice marginal events and Palaeo-Indian presence. Introduction In the Thunder Bay area there is a strong, though not exclusive, relationship between Palaeo-Indian habitation, bedrock exposures of the favoured tool stone within the Gunflint Formation, and the abandoned shores of post-glacial lakes of the Superior basin (Fig. 2). Of particular importance are shores of Lake Minong, established about 9.5 ka B.P. This spatial relationship is perhaps illusory since it is clear that these people occupied a number of habitats, some far from ancient lakeshores, but in geomorphological mapping of these shorelines (often in the context of urban development), it is the lakeshore sites that have been most commonly found and reported. Evidence accumulated since MacNeish’s (1952) excavations at the Brohm Site (Pass Lake) on the Sibley Peninsula (Fig. 2) by archaeologists (Fox, 1975, 1980; Dawson, 1983b; Julig, 1984; Ross, 1997; Hinshelwood 2004) suggest that Palaeo-Indians migrated northeast from the Dakotas and Minnesota, and into the culde-sac formed between Lake Agassiz, the lakes of the Superior basin, and the retreating margin of the Laurentide ice sheet (Fig. 3). Lake Agassiz covered most of Manitoba and parts of Northwestern Ontario at its maximum extent, and contributed significantly to the complex hydrological sequence affecting the Lake Superior basin (Fig. 4). As such Lake Agassiz played an important role in the initial peopling of the greater part of northwestern Ontario since its spatial expanse shifted north over time with glacial retreat, and conditioning when land would have been available for northward human occupation (Fig. 5). While some fluted projectile points (i.e. early Palaeo-Indian Clovis; Fig. 6) have been encountered as far north as central and perhaps northern Minnesota, none have been found yet in northwestern Ontario. Perhaps Clovis technology disappeared before the ice had retreated from the region, or that insufficient research has been done in the rugged uplands of the cul-de-sac that was first deglaciated (and therefore the most likely zone where such finds might be made). - 151 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 3 Proposed late Palaeo-Indian migration into the cul-de-sac formed between Lakes Agassiz and Minong and the Laurentide Ice Sheet (after Hamilton and Ross, 1997). In any case, the Thunder Bay area has yielded unfluted lanceolate projectile points, probably deriving from several late Palaeo-Indian cultures (often collectively referred to as Plano) (Fig. 6). These represent hunting groups who pursued game throughout the cul-de-sac at some time after the Marquette readvance (ca. 9,900 to 9,500 y BP) (Fig. 3). While a range of projectile point types have been recovered, Ross (1997) has proposed that these sites contribute to the “Interlakes Composite”- consisting of a series of inter-related local populations who jointly utilized the deglaciated landscape at some point after ca. 9,500 years ago. This material culture is characterized in part by paralleloblique flaked projectile points (Fig. 6a), and heavy use of siliceous stone deriving from the Gunflint Formation (i.e., Jasper Taconite, Gunflint Silica) that outcrops in the Thunder Bay area (Fig. 2). Other important raw materials include Knife Lake Siltstone, deriving from bedrock sources near the Minnesota/Ontario border, and a sparse array of non-local materials (including Hixton Silicified Sandstone from central Wisconsin and perhaps also Chalcedony from western North Dakota). Local Palaeo-Indian sites exhibit a strong preference for bedrock lithic sources, with only a minor percentage Figure 4 Repeated spills of water from Lake Agassiz into Lake Minong (Hamilton 1996:30 after Teller and Thorleifson 1983). - 152 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 5 Northeastward retreat of the Laurentide glacier, with related northward shift of Lake Agassiz waters. A sparse array of early and middle Holocene archaeological sites suggest the time-transgressive northward migration of human populations (Hamilton nd). of the raw materials suggesting use of cobble and pebble sources. This forms a sharp contrast to other cultures in the region dating to the Middle and Late Holocene. This resulted in some repeatedly used archaeological sites where exposures of suitable stone coincide with the ancient beaches of Lake Minong. The famous Cummins Site represents one such quarry/ workshop that has yielded thousands of discarded flakes, blocks, preforms and other debris from tool/ preform fabrication, but with a very low relative frequency of formal or informal tools. Other sites, that likely served as short-term camps, seasonal aggregation places, hunting/ambush sites, observation points, and other functions also dot the landscape. While they also yield much discarded stone, a somewhat higher relative frequency of lost, broken or discarded tools are recovered, suggesting more generalized site functions. While the shorelines were not the only areas utilized on the early Holocene landscape, it is clear that they were important (likely used for seasonal ingathering), no doubt because of the spatial convergence of valued resources. Some of this site function variability is addressed at the various sites visited during the tour. Deglaciation History The broad details of the deglaciation of the Superior basin are shown in Figure 7. As ice of the Rainy River and Superior lobes withdrew from central Minnesota, a series of recessional moraines were left in place. The Vermilion moraine, trending northwest to southeast, across northeastern Minnesota, was followed by - 153 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 1999). Unfortunately these surface finds have not yet been subjected to absolute dating. A major event in the deglaciation history of the Lake Superior basin was the Marquette readvance (9.9 ka B.P.), during which the basin and its margins were briefly reoccupied by ice (Fig. 8b). Recessional moraines northeast of the Brule moraine were destroyed and more recent features (i.e., Marks Moraine, etc) formed in their place. This event also has considerable archaeological implications, since any evidence of occupation in the path of the Marquette readvance was likely buried or destroyed by the new ice cover. Hudson Bay ice pushed southwestward into the Lake Superior basin, to halt on its southern shore (Drexler, Farrand and Hughes, 1983). Only in a small portion of Whitefish Bay near Sault Ste. Marie, in Figure 6 Pettipas’ (2011) now-obsolete Lake Agassiz temporal sequence with proposed relationship to the PaleoIndian cultural historical sequence. He revised the lake sequence in light of new data published by Leverington and Teller (2003) and Fisher (2005, 2008). We include it here because it offers a sense of archaeological conventional wisdom regarding the cultural sequence. This will soon be updated in light of ongoing research conducted at the Mackenzie I Site that has yielded a very large collection of projectile points. This will allow development of a more regionally relevant typology. the Steep Rock moraine on the Canadian side of the border, and the Brule moraine (Fig. 7). Guided by these ice marginal positions, Lake Agassiz found an early eastern outlet through the Arrow/Whitefish lakes corridor (circa 11 ka B.P.) and, shortly after, through the Shebandowan lake corridor, ultimately using the Lake Nipigon spillways around 10.4 ka B.P. (Fig. 8a) to enter Early Lake Minong (Teller, 1985) which occupied the Superior basin. There is no reason to believe that warming climate and biological regeneration upon the uplands unaffected by the Marquette readvance would not have enabled immigration of animal herds and people into the Thunder Bay region at this time. Only tenuous evidence of such early occupation has been reported (Phillips, 1993: Ross, 1994), though recent work in the Arrow/Whitefish corridor has confirmed a number of sites that may represent an earlier Palaeo-Indian presence (McLeod and Phillips, pers. communication, Figure 6a A sketch and photo of lanceolate projectile point style from the Brohm Site. Interlakes Composite sites generally yield a very small assemblage of projectile points reminiscent of a range of late Palaeo-Indian (unfluted) styles. However the Brohm and Mackenzie I Sites have yielded a high percentage of points with lanceolate blade form, basal indentation through repeated flaking, and edge grinding along the lower lateral portions of the blade. Particularly notable is the pattern of oblique parallel flaking that seems to be an important feature of the knapping strategy that appears unrelated to the functional utility of the tools. - 154 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 7 Map of Quetico-Nipigon area showing moraines and direction of ice movement during various phases (from Zoltai, 1965a). the southeast corner of the basin, did Early Lake Minong remain an open lake (Farrand and Drexler, 1985). The Marquette lobe pushed westward against the steep Minnesota shore and, where less obstructed, flowed northwest up the Kaministiquia valley to the Marks moraine (Figs. 1, 7, 8). This prominent ridge, rising to over 470 m (1550’) in places, curves from Lappe through Mokomon and around the northwest of Kakabeka towards the Pigeon River (Zoltai, 1963, 1965a, 1965b; Burwasser, 1977, 1980; Burwasser and Ferguson, 1980). Contemporaneously, to the west of the Lake Superior basin, the Patricia ice lobe pushed towards the Rainy River district and halted to deposit the Dog Lake moraine. This runs northwest from Lappe and holds up Hazlewood, One Island, Hawkeye and Dog lakes. East of Lappe, where the two moraines meet, a line of glacial debris known as the Mackenzie interlobate moraine can be traced through Pearl and on to the Black Bay peninsula (Figs. 1, 8). A glacial lake, Lake Kaministiquia, was formed between the two ice margins (Teller, 1985) and, at Lappe, a huge sand and gravel delta was built by sediment pouring off the interlobate moraine. This scenario is still the subject of debate, however, and there is some field evidence that the Marks and Dog Lake moraines may be older features that were simply reoccupied by ice of the Marquette advance (Julig, McAndrews and Mahaney, 1990; Tickle, 1996; Noble, pers.comm.1999). Lake Agassiz, deprived of its eastern outlets by the advance of the ice, expanded in area and depth (the Emerson phase), and flooded catastrophically through the Clearwater and Athabasca valleys in Saskatchewan - 155 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 8 The retreat of the glaciers in the Superior Region between 10,400 and 9,500 years ago (from Philips, 1993:95). and Alberta into the Mackenzie River and the Arctic Ocean (Smith and Fisher, 1993). The Marquette readvance obliterated evidence of the earlier phase of Lake Superior’s shoreline history in most of the basin. Shoreline sites that prehistoric people might have occupied in the area of Thunder Bay before 9.9 ka B.P. may lie buried beneath Marquette deposits. In the four hundred years between 9.9 and 9.5 ka B.P., a period about which much more is known, ice withdrew from the Lake Superior basin (Figure 8c/8d). Then, again, Palaeo-Indian people followed game up the Interlakes corridor and into the Thunder Bay region, this time to settle, in part at least, on the shores of Lake Beaver Bay (Stuart, 1993) and Lake Minong (Phillips, 1988). Shoreline History At the peak of Wisconsinan glaciation, the northeastern margin of the Lake Superior basin was depressed by the weight of ice (isostatic depression) to a greater degree than the less heavily ice-loaded southwestern side. As a result, “rebound” (isostatic recovery) since that time has been greater on the north shore of the Lake Superior basin than on the south shore. Lake shorelines which had been originally horizontal became progressively tilted along an approximate northeast axis, such that a shoreline of the same chronological age increases in altitude from southwest to northeast along the western shore of Lake Superior. A theoretical archaeological site at Grand Marais, Minnesota, found at 184 metres, just above the present storm beach, will be of similar age to another theoretical site in Terrace Bay, Ontario, at the 300 metre contour (117 metres above the present lake), with both sites lying on the same tilted shoreline (Fig. 9). As the Marquette ice lobe wasted back, the eastern and western sides of the basin were exposed as separate entities (Fig. 8c). On the eastern side, a fairly stable Lake Minong, controlled by the height of the St. Mary’s River outlet at Sault Ste. Marie, extended northwards up the Ontario shore and westwards along the Michigan shore as they were exposed by melting ice (Farrand and Drexler, 1985). In the enclosed western end of the basin, a distinct succession of ever larger but progressively lower level lakes formed (Lakes Duluth, Highbridge, - 156 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 9 Shoreline diagram for the Pigeon River/Thunder Bay area (Farrand, 1960). Moquah, Washburn, Manitou, and Beaver Bay), each extending further northeast up the Minnesota - Ontario shore and east along the Wisconsin shore (Farrand and Drexler, 1985). Along the land-based margins of these water bodies were formed various coastal features, such as bluffs and beaches, by which the shorelines can be traced today. As ice vacated the basin, the eastern and western lakes were united and the single shoreline of Lake Minong was formed around the basin about 9.5 ka B.P. (Figure 8d). For the next 1,500 years, waterlevels in the Lake Superior basin declined as the St. Mary’s sill was eroded down to bedrock. A staircase of Post-Minong shorelines were formed, the last and lowest of which was Lake Houghton, about 8.0 ka B.P. By 8.0 ka B.P., due to isostatic uplift, the rising levels of Lake Huron had flooded into the St. Mary’s River and reversed the flow (Larsen, 1987). This backflooding led to slowly rising water-levels in the Lake Superior basin and culminated in the Nipissing lake stage around 5,000 years B.P. Known as the preNipissing transgression, this rise in water-level was imposed upon a still-tilting basin. On the north shore, east of Dorion, the rate of isostatic uplift remained more rapid than the rising waters of the pre-Nipissing period, with the result being that the shoreline marking the Nipissing maximum level lies at a lower altitude than all older shorelines, including that of Lake Houghton. On the south shore the pre-Nipissing transgression was more rapid than isostatic recovery, and wave action “inherited” the features of older shorelines, modifying and destroying portions of them, including pre-existing shoreline archaeological sites (Phillips, 1977). This causes a chronological discontinuity which is at its greatest near Duluth and decreases towards Dorion where the Houghton shoreline appears above the present waterlevel (Fig. 10). In Thunder Bay, evidence of Palaeo-Indian activities would normally have been traced as a continuum from the high Minong shoreline to the Houghton (which lies just below present water-level), but the pre-Nipissing transgression reoccupied the lower and later part of that record and cut a prominent bluff on which the General Hospital, the Court House, St. Joseph’s Hospital, and Lakehead University are built. The Nipissing shoreline can be traced up the valley towards Mapleward road and across towards Mount McKay. Thus, the later part of the Palaeo-Indian record, the important transition into the Archaic period, and a good portion of the - 157 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 10 The pre-Nipissing transgression and the resulting loss of potential archaeological sites along the shores of Lake Superior (Phillips, 1993:100). early Archaic is missing in Thunder Bay. Hinshelwood (2004) offers the observation that these shifting lake levels into the mid-Holocene suggests that some of the much older Plano lithic quarry and workshop sites might have been re-occupied during Archaic times. Several sites that are thought to be of Archaic age have also been encountered along the pre-Nipissing transgression strandline were it cuts through Lakehead University and adjacent properties. Local History Figure 11 represents an interpretation of the detailed history of the withdrawal of Superior (Marquette) ice from the Thunder Bay area, based on currently known information, though subject to revision. As ice wasted back from the Marks Moraine, shorelines on the south side of the moraine show that a body of water collected between the moraine and the ice front (Fig. 11a). This proglacial lake has been named Lake Cedar Creek (Jahnke, 1993), and there is tenuous evidence that it connected with high level lakes to the south, perhaps of Lake Duluth equivalence. As ice withdrew from the Kaministiquia embayment a small readvance to the Intola Moraine occurred (Fig. 11b). The higher levels of Lake Beaver Bay have been traced into this moraine (Stuart, 1993), and it is likely that Palaeo-Indian peoples entered the area at about this time, probably using the Marks moraine as a causeway. As ice withdrew further and water level declined to the lower levels of Lake Beaver Bay (Fig. 11c), the possibility of Palaeo-Indian occupation increases, and by the time Lake Minong was established in the Kaministiquia embayment, there is plenty of evidence to prove their presence (Fig. 11d). The shoreline diagram (Figure 9) shows that only as ice withdrew from the Thunder Bay region did the sequence of post-glacial lakes extend into the area. Beaver Bay shorelines can be found at Kakabeka, but only the lower levels extend eastwards through the city towards the Mackenzie River. A few notable sites are: 1) the Simmonds and McDaid sites at the mouth of the Current River, 2) the Hodder and Naomi Sites overlooking the Current River Mouth, 3) a site cluster around a possible Minong embayment near the present Mackenzie River, 4) the Biloski Site at the outlet of the McIntyre River into Minong, 5) Catherine, Neebing River and Cummins sites along the north side of the Kam embayment, 6) the Irene Site on a high bedrock-controlled upland overlooking the Minong shore, and 7) a collection of sites on the Rosslyn delta at the mouth of the Kaministiquia River. While many of these sites are associated with Lake Minong shores, some landscape associations are less simple and deserve more analysis (Hamilton, 1996). Some Palaeo-Indian sites unrelated to shorelines occur at High Falls on the Pigeon river, on Dog Lake (McLeod, 1982), at Harstone Hill, near the junction of the Whitefish and Kaministiquia rivers and near Kakabeka Falls where the Kaministiquia River might have been most logically crossed. The Crane site near Kakabeka, found in a vegetable garden, provides a - 158 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 11 An interpretation of post-Marquette history in the Thunder Bay area (Phillips et al., 1994). rich cache of beautifully crafted bifaces in a location apparently unrelated to any topographic feature, though on the surface of one of the higher Beaver Bay terraces. Because of their antiquity and the acidic nature of the Boreal forest soils, only the lithic materials remain to be found at these sites, though it is very likely that many other natural materials would have been used. Field Excursion Stops After leaving the hotel, we will travel up Edward/ Golf Links Rd. This route takes us across the lower flats of the Kaministquia River delta, with the bluff forming the Nipissing Transgression strand line occurring at Stop 1. Stop 1 Golf Lines Road-Thunder Bay Golf and Country Club. UTM coordinates: NAD83; 16U 0331712E / 5365013N While we will not leave the van, this location provides a view of the bluffs forming the strand associated with the Pre-Nipissing Transgression. To the left of Golf Links Road on the lower slopes of this bluff a taconite lithic scatter was encountered. Figure 23 provides a view of this beach feature, with several sites within the Lakehed University campus thought to be Archaic age. We will be returning along this route after decending down the Marks Moraine uplands, over the Upper Beaver Bay strand and the Lake Minong Strand to show the elevation contrast between these various beaches. - 159 - �Boulivard Proceedings of the 58th ILSG Annual Meeting - Part 2 A series of taconite lithic scatters are reported in this bedrock controlled upland area. They are likely Plano. Hinshelwood also assessed a taconite lithic quarry along the north side of the highway right of way overlooking the Current River. Naomi Hodder 260 m Current R. Hodder Ave. climbs a slope defined by ‘steps’ representing a series of Minong Lake phases. McDaid Simmonds 220m 300m Artificial Lake Figure 12 The Current River ‘mouth’ into Lake Minong, with the Naomi and Hodder Sites located on uplands well above glacial lakeshores. Note that Boulevard Lake is an artifacial headpond for the dam and old hydro generating station on the Current River located downstream. Stop 2 - Hillcrest Park Lookout. then reboard to cross Black Bay bridge and turn left on Centennial Park Rd to visit the McDaid site (Stop 4), adjacent to the roadway. UTM coordinates: NAD83; 16U 0334707E / 5366973N We follow a route east along Oliver Road, and then north up High Street for a brief visit to Hillcrest Park from where the city can be seen in context with local topography and Lake Superior. Stop 3 and 4 - Boulevard Park, the Bluffs and Centennial Park Rd. Figure 12 UTM coordinates: NAD83; 16U 0337219E / 5371002N We travel northeast on High St., over the St. Joseph/ Hillcrest island of Minong times, left on Balsam St, right on Hudson Ave and onto Arundel St. from which a left turn before Black Bay bridge will lead to the Bluffs Scenic Lookout (Stop 3). Walk down to Simmonds site. This site has been severely disturbed by park development and repeated cart track disturbance. We The Simmonds (DcJh-4) and McDaid (DcJh-16) Sites The present Current River runs in a channel incised into a gently lakeward dipping shelf of Gunflint formation that fronts a bounding rock wall, a structural feature, which now forms the ‘bluffs’ scenic lookout (Figs. 12, 13). In the Minong period, the river carried much water and sediment from inland proglacial lake flows and here evidence supports a major ‘bar building’ phase of coastal history, a series of river mouth spits or bars being formed on both sides of the river as water level generally declined. The highest Minong shoreline lay against the bounding rock wall at approximately the 252 m (827’) level, but between 240 and 236 m (787-774’) a series - 160 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 13 The changing geography and present characteristics of the Simmonds and McDaid sites (from Phillips, 1988:135). of sand bars formed parallel to the wall on the west bank of the river and a matching series of bars on the east side curve sharply into the then river mouth from a source on the same rock wall east of the point where it is cut by the river. It appears that the river entered Lake Minong along the rock wall, forming offshore and beach bars which extend south-eastwards across the shallow McVickers embayment to the southwest. Simultaneously, longshore transport from the east built bars partially across the river mouth at times, only to be later truncated by fluvial action. The Simmonds site on the west side, occurring on the parallel bars at about 236 m (774’) is matched in elevation and position by the McDaid site on the eastern curving bars (Figs. 13, 14). Neither site is a long-term habitation site but show evidence of activity typical of a river mouth camping and fishing site. Interestingly, the major bar building episode appears to have been just subsequent to the occupation of these two sites, two large curving bars being formed at 231 and 227 m (758-745’) on the east bank, the latter flat topped one largely a subaqueous feature that was probably contemporaneous with the supra-aqueous ridge form of the first. On the west bank a very long bar, now unfortunately truncated at its river mouth end, runs south west, in places broadening to over 100 m (328’) in width. In almost text-like manner, the mean grain size and sorting characteristics along its length confirm that it prograded out from the mouth across the McVickers embayment, probably mostly in subaqueous form. Some evidence of Palaeo-Indian activity has been found on the crests of these newer bars but no sites equivalent to those named. Stop 5 Naomi and Hodder sites. Figure 12. UTM coordinates: NAD83; 16U 0338765E / 5372052N Upon departure from the McDaid Site, the bus travels to Hodder Ave where it turns north to climb a long slope to its intersection with Highway 11/17 - 161 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 14 Geomorphic details of McDaid Site. (Fig. 12). Hodder Ave crosses a series of ancient beach strands marking various phases of Lake Minong. At the top of this slope where Hodder intersects the highway, considerable construction is underway. We may not be able to park near the intersection nor leave the van, depending upon construction activity at the time of our visit. The construction intercepted two archaeological sites located high on the bedrock-controlled uplands overlooking the Current River to the west and the waters of Lake Minong to the south. Both of these sites are far removed from modern or ancient water sources, and both are located on the north (leeward) side of the upland. Both the Naomi and Hodder Sites (Fig. 12) were salvage excavated as part of highway expansion by Western Heritage (an archaeological consulting firm based in western Canada). This position may have been calculated to offer protection from winds blowing off the glacial lake, and would have also provided a panoramic view of the extensive Current River valley to the north. If the landscape was comparatively open (taiga-steppe) then perhaps these sites might have offered a viewscape useful for observing game. These sites offer important ‘cautionary tales’ against undue emphasis on the apparent spatial association of PalaeoIndian sites and former Lake Minong shorelines. Clearly Plano settlement and land use was much more complex than first appearances would suggest. The tour continues east along Highway 11/17 for about 20 minutes to the location of another cluster of Palaeo-Indian sites associated with the former outlet of the Mackenzie River into Lake Minong (Fig. 15). While two sites are found near the river bank (one on each high bank overlooking the Mackenzie River gorge), several other sites are associated with beaches and sand spits found along a shallow former embayment (Fig. 15). Stop 6- Mackenzie Site Cluster (Figs. 1, 15) UTM coordinates: NAD83; 16U 0355847E / 5377436N Twinning of Highway 11/17 triggered extensive archaeological salvage excavations at several sites associated with high Minong Lake phases. Western Heritage is conducting ongoing salvage excavations. Thus, interpretation of these deposits remains preliminary and tentative. Again, because of active highway construction, it is not certain how close to this cluster of sites we will be permitted (safety and liability issues). - 162 - While the main site (Mackenzie I) is likely located �Proceedings of the 58th ILSG Annual Meeting - Part 2 Upland (Mackenzie Moraine) Stevens Former L. Minong and foreshore? Hydro line N L. Superior DEM courtesy of T. Sapic 1 km Figure 15 B/W version of a Digital Elevation Model of Mackenzie Site area, showing locations of Palaeo-Indian sites identified along highway construction corridor. on a reworked deltaic deposit where the Mackenzie River channel entered Lake Minong, several others have been discovered upon beach strand features within a shallow embayment of Lake Minong (Fig. 15). Archaeological inspection of where the hydro transmission lines cross the Mackenzie River in the 1970s led to the discovery of a Plano projectile point and two flakes in disturbed context. This site was named the Newton Site, and likely marks the southern extreme of the Mackenzie I Site (Fig. 16). The main part of the Mackenzie I Site remained undiscovered within the forest land to the north of the hydro transmission corridor until exploratory testing by Archaeological Services Inc. in anticipation of the construction of the new Mackenzie River bridge. Several sites in this locality are found on sandy sediment, and are associated with bedrock knob exposures that might have once formed rocky coastal headlands. Longshore sediment accumulation from a Mackenzie River source seems to be the most likely explanation for these sandy beach features. Enhanced archaeological deposition (consistent with mixed function encampment) also seems to be associated with these protected zones. This is dramatically evident with the distribution of material culture at the Mackenzie I site where artifact processing and spatial analysis is the furthest advanced. Also of note are several small and ephemeral springs that bisect the beach strandlines. The most prominent encampment is the Mackenzie I Site, located on sandy sediments accumulated north and northwest of a bedrock exposure along the west side of the Mackenzie River gorge (Fig. 17). More than 2,500 square metres of this site has been excavated, making it one of the largest Palaeo-Indian excavations ever conducted in Canada (Fig. 18). Many thousands of pieces of debitage have been recovered. However, in sharp contrast to most other excavated local Palaeo-Indian sites that are dominated by debitage, the Mackenzie I site has also yielded a wide variety of tools, including upwards of 200 complete or fragmentary projectile points. This diversity of artifact types, coupled with the discontinuous clustering of material culture suggests a repeatedly used aggregation site where diverse activities were undertaken. Because of the uncharacteristically rich recovery of diagnostic projectile points, this site will be an important type site for tool typologies useful for assessing Interlakes Composite in relation to other late Palaeo-Indian cultures throughout North America. The rich and diverse artifact recovery is enabling graduate student research addressing a wide range of topics (tool typology, lithic reduction strategies, sedimentology - 163 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Hyd ro l ine Newton Figure 16 Oblique air photo of new bridge over Mackenzie River, with Mackenzie I and II sites on each end. Early Lake Minong shorelines coincide with cleared area, with a main bank defined by white line. and geoarchaeology, artifact patterning, activity areas and site function, etc. While the Mackenzie River gorge is likely deeply downcut from its former configuration, the dense site distribution suggests repeated use of the embayment during Palaeo-Indian times. Notably, nothing except Palaeo-Indian diagnostic material has been encountered along the highway construction corridor, but reconnaissance by K.C.A. Dawson in the 1960s and 70s along the lower Mackenze River valley revealed a succession of Archaic and Woodland era sites at successively lower outlets into Lake Superior. Continued research in the Highway construction corridor has led to discovery, assessment and salvage excavation of several other sites, of particular note, the Woodpecker I and II and RLF Sites (Fig. 19). These sites are positioned upon the top of a sandy bank overlooking an extensive area of muskeg and beaver ponds. These wetlands are interpreted to be the former shoreline and shoals of Lake Minong, with the sandy/ fine gravel bank forming the beach strand line. The Woodpecker I and II sites were located by ASI during preliminary assessment of the highway corridor. These sites are initially interpreted to be localized lithic scatters. Subsequent forest clearing and mapping greatly improves interpretative resolution in their paleo-hydrological context (Figs. 20 and 21). The gently curving high bank illustrated in Figure 19 is interpreted to be the Lake Minong strandline, which forms a shallow embayment dotted with bedrock knob exposures that acted as rocky headlands along the former shore. These headlands broke the wave velocity along the shore, and allowed longshore accumulation of sediment on their leeward side. This resulted in a high raised bank with subtle berms and swales defining storm beach features. Upon one such berm is the small encampment/flaking station called the RLF site. The Woodpecker Sites are more complicated, with a bedrock dome enabling the development of a long spit or ridge of sandy sediment to accumulate to the west of the bedrock. This beach feature is bisected in at least three places by small streams that flow through underfit gullies across the beach strand. At issue is whether these streams are contemporaneous with Lake Minong water levels, or whether they are of mid to late Holocene derivation. Given the small drainage basin and rather small stream budget, coupled with the fact that no alluvial fan sediment accumulation is noted - 164 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 17 25 cm contour map of Mackenzie I Site showing bedrock with sandy/gravelly sediments between exposures. The extent of the block excavation is not fully documented here (see Fig. 18). Note the configuration of the contour lines suggests beach berms and back-beach swales. Well-sorted gravel deposits on the north side of main bedrock dome suggests former stream bed. Deep stratigraphic sections reveal deltaic sedimentation (well-sorted lens of fine pebbles, sands, and silts) overlaid by aeolian reworked fine sediment. in the muskeg lowland below the beach strand, it is suspected that they are of early Holocene derivation. That is, larger stream-flows drained downslope off the Mackenzie Moraine uplands to the north, and either accumulated in the swales behind the storm beach, or down and through the beach, with eroded sediment being carried away by wave action along the lakeward side of the beach strand line. When examining Figure 20 and 21, the most densely occupied portion of the Woodpecker I and II sites are associated with the banks - 165 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 ³ Bedrock Bedrock MACKENZIE RIVER 1 SITE Legend 7.5 http://gis.westernheritage.ca/aspnet_client/ESRI/WebADF/Print... Cores Drills Knifes Units Not Excavated Retouched Flakes Units Excavated Scrapers 0 Metres References Bifaces Map 15 15 Project No. 10-064-02 Date Nov 23, 2011 NAD83 UTM Z16N Scale 1:650 GIS LGK Map Figure 18 Preliminary plots of various tool types from the Mackenzie I excavations (Courtesy of Western Heritage). The plotted objects represent a very small fraction of the assemblage (formal and informal tools), while the many thousands of debitage, core fragments and other discarded debris is still being catalogued. The upper left image shows the extent of excavation in each of the two years of work at the site. When compared to Figure 17, it is evident that an important area for settlement was on the flat sandy surfaces to the immediate northwest of the main bedrock dome. This supports the speculation that these domes were important for breaking wave velocity and facilitating longshore sediment accumulation along their lee sides. We also speculate that these bedrock exposures where important considerations for settlement as they provided shelter from winds blowing off the lake. Such notions assume that the site was occupied while Lake Minong was at its high level. Bedrock Points (continued) Points Points (continued) 2 peices Lateral Frag Tip Frag Base Mid and Tip Tip and Base Base and Tip Midsection Base? Preform Complete Preform? Lateral Edge Tip Tip/Base Not_Points Grid_Total_Units 0 2 1 of 2 11-11-21 1:50 PM - 166 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 19 View northeast from existing highway along the construction zone. Note the ‘bank’ in the clearing (left side of frame) that likely is the former Lake Minong strandline. In the foreground the current highway cuts through this former beach feature. Note the location of the Woodpecker I, II, and RLF sites along the beach strand line. The former is located on the northwest side of a bedrock knob, while the RLF site is a small flaking station located on a berm (perhaps a high storm beach) overlooking the main strand line. of one such stream (Woodpecker I), or on the protected leeward side of the bedrock dome (Woodpecker II). Thus, occupation was densest along the protected leeward side of the bedrock dome and along the banks of the now dry stream bed. The RLF Site was discovered after a bulldozer cleared the dense coniferous forest along the north highway lane, revealing taconite flakes. Subsequent test-pitting and block excavation revealed a series of small and localized lithic clusters that are interpreted to represent short-term camps or lithic reduction stations. Topographic mapping (Figure 22) facilitated interpretation of the site locality to represent a raised storm beach berm that overlooks a back-beach swale. This high storm beach also overlooks a second swale (to the south and lakeward) and a second slightly lower storm beach berm located on the top brink of the bank forming the most prominent Lake Minong strandline (Figure 22). The RLF site is at least 50 metres north (inland) form the main strandline and at least 100 metres removed from the nearest stream bed that bisects the former beach zone. While yielding much less artifact material than the others in the area, such sites remain scientifically important because their relatively simple and brief depositional history render them much more archaeologically interpretable. Stop 7 McIntryre River outlet and the Biloski Site. Figure 23 UTM coordinates: NAD83; 16U 0332310E / 5367746N Upon completion of our examination of the Mackenzie River area, we return to Thunder Bay, travelling down the 11/17 Expressway past a series of sites that have been located along the former shores of Lake Minong (and also shores representing the pre-Nipissing Transgression (Fig. 23). In the interest of time, we will just stop along the highway. On the north side the low Minong bluff is seen, with a housing development on top. The Biloski site occupies this surface and a small sand bar at the bluff foot, where a small embayment and secondary river channel once existed. This and several other nearby sites are likely Plano age, but the proximity of the pre-Nipissing Transgression strand line suggests the close spatial association of Plano and Archaic sites, consistent with Hinshelwood’s (2004) observations. As the Expressway curves to the south, the area on the right holds a lengthy baymouth bar on the end of which lies the Catherine site, unseen from the road (Fig. 23). On the summit of Rabbit Mt. (275m, 902’), which rises behind the embayment, lies the Irene site (Fig. 23), a lookout site. - 167 - �- 168 - Stream Stream Beach strand? RLF Site Figure 20 The Woodpecker and RLF sites are located upon sandy and fine gravelly sediments that form part of the Lake Minong strandline. Note the position of the sites in proximity to small relict stream beds that drain across the strand and into the projected Lake Minong. Noteably, no alluvial fan is associated with the lower end of either of the stream beds associated with the Woodpecker Sites, suggesting that it is not a recent stream that eroded through the pre-existing sandy ridge. Continued excavation at the Woodpecker site locality suggests a much more extensive deposit on the north and west side of the bedrock dome illustated in Figure 21. This suggests encampment on flat sandy beach deposits that accumulated on the lee side of the proposed bedrock headland adjacent to a small spring flowing across the beach. The RLF site was discovered while archaeologists were walking the cleared north lane centre lane. Subsequent excavation revealed a small encampment/flaking station upon a raised berm overlooking the primary Lake Minong Beach strandline. While somewhat removed from the site, also note the small underfit stream bed located about 100 metres east of the site. Stream Newly defined extent of Woodpecker II site. Proceedings of the 58th ILSG Annual Meeting - Part 2 �- 169 - Muskeg formerly occupied by glacial meltwater Sparse Recovery Woodpecker I Trail Bedrock Woodpecker II Figure 21 25 cm contour map of the Woodpecker Sites located on a raised sandy ‘ridge’ formed north and west of a bedrock exposure. Low muskeg ground to the south of sandy ridge likely contained Lake Minong waters. Several abandoned or underfit stream beds bisect the sandy ridge, and suggest streams flowing into Lake Minong across this raised beach formed by longshore sediment accumulation on the lee side of a bedrock headland protruding into Lake Minong. Ongoing geoarchaeological and OSL dating research is assessing the viability of this interpretation. Archaeological materials were initially discovered along the banks of one of these relict streams, and also in disturbed context along the cart trail that bisects the site. Continued archaeological investigation reveals an extensive deposit on the northwest or lee side of the bedrock and along the east bank of the small abandoned stream bed. underfit Creek underfit Creek Proceedings of the 58th ILSG Annual Meeting - Part 2 �- 170 - Swale Area tested by Western Heritage Creek Figure 22 25 cm contour interval map of the RLF Site. This site was accidentially discovered by archaeologists while walking along the bulldozed north lane of the proposed highway. Lithic debitage exposed during bulldozing down trees led to shovel test reconnaissance, followed by Magnetic Gradiometer survey (Grey squares), and block excavation. Very localized dense clusters of debitage were encountered that suggests localized encampments or flaking stations. Much of the material appears to be located on the south flank of a raised berm (perhaps a sandy storm beach) about 50 metres north of the most prominent Lake Minong beach strand line. A small underfit stream bed is located ca. 100 metres east of the site. Locating such small and ephemeral (but highly interpretable) encampment zones is very difficult in forested conditions, particularly using conventional site prospecting techniques (5 metre interval shovel test pits). Western Heriage and Lakehead U. have been collaborating in experimentation of multi-proxy approaches to site discovery. Former L. Minong Main L. Minong Strandline RLF High Berm Swale Proceedings of the 58th ILSG Annual Meeting - Part 2 �Proceedings of the 58th ILSG Annual Meeting - Part 2 Biloski Irene McIntrye R. L. Minong (approx.) LU Black dots: aceramic (Plano or Con. Archaic) sites College reported along McIntrye beach strand lines R. pre-Nipissing Transgression (approx) 700 m Figure 23 Archaeological sites near the McIntyre River. Some of these sites have confirmed or probable Plano affiliation, specifically those near the L. Minong Strandline. While not yielding diagnostic artifacts, it has been proposed that the sites near the pre-Nipissing Transgression are of Arcahic affiliation. Also note the Irene Site, located on top of Rabbit Mountain, overlooking the Lake Minong shoreline. - 171 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Stop 8 - Mapleward Road, the Cummins Site. Figure 24 UTM coordinates: NAD83; 16U 0325670E / 5364042N The route turns west on Oliver Road and within 2 km rises up the distinct Minong bluff. A left turn on Mapleward Road sees a gentle descent to the Minong shore again, and the Cummins site. The Cummins Site (DCJi-1) The best published Palaeo-Indian site in the region is the Cummins site (Fig. 24), that is one of a number of Plano sites in the area where the Neebing River flowed into Lake Minong. Reported by a prominent local collector (Hugh Cummins) in 1962, the site was most intensively excavated as part of Dr. Pat Julig’s Ph.D. research. Initially considered a typical surface site, Julig et al. (1986) concluded that it is a rare stratified Palaeo-Indian site, under continued use over a long period of changing environmental conditions. The basic tool kit is reminiscent of Plains Plano culture (Dawson, 1983; Julig, 1984), and the recoveries imply diverse activities, albeit dominated by stone quarrying and knapping. This includes woodworking, fishing and beaver trapping in addition to the regular hunting of caribou and perhaps bison (McAndrews, 1982). Newman and Julig (1989) also attempted to extract and characterize blood residues from tools from this site, and propose a diverse diet. The Cummins Site was a major regional preform-making centre and the presence of exotic lithic components implies a broad geographical interaction with other groups in the region (Julig, 1984). The Cummins Site occurs across the surface of several large sand and gravel bars which trend northeastwards from the then Neebing river mouth across a broad southerly gentle dip slope of local Gunflint shales, overlain by a thin water washed silty till (Figs. 24 and 25). In these shales occurs jasper taconite, the major source of tool making material. Figure 25 shows the fenced area of the site and the morphological details (Julig et al., 1990). Figure 26 shows the paleogeographic reconstruction of events believed to have formed the area (Phillips, 1982). Longshore transport from the Neebing river mouth built a series of progressive bars across the shallow water rock shelf, recurving into a minor river valley which formed a sheltered embayment to the west of a rock island. A further bar was built along the front of existing ones eventually crossed the embayment in tombolo-like form to enclose a small lagoon, the Cummins Pond, after which lower and later shoreline features mimicked the established plan shape. A pollen core taken in the pond suggests this enclosure took place before 8.1 ka BP. (Julig et al., 1986). The plan shape and structure of the bars is not compatible with a declining water level margin, indeed the accumulation and building up of these large features on a gentle shelf slope is unlikely without transgressive wave action. Even so, such action across a gentle shore slope would not ordinarily build up a large supraaqueous bar without some initial encouragement to accumulate sediments along a line rather than disperse them in a sheet. The key to the existence of these bars in this location is underlying bedrock control. While Julig (1984) determined through resistivity and ground radar studies that variation in sediment character and bedrock irregularities could be traced, a more simple reconstruction of the bedrock profile is also possible. By surveying down the exposed bedrock dip-slope north of the site, the trend (see inset, Fig. 26) showed that beneath the site must lie a marked rock step, typical of many that occur in the present topography of these flat-lying shales, and often sharpened by wave action. Accumulation of sediments took place firstly against this rock step and subsequently over the top of it. It is possible that some till remained in the angle of the step. To the west of Mapleward Road an exposure of coarse sand and gravel with angular shale inclusions at the rear of the bar in contact with bedrock suggests overwash of the type that would be expected in this scenario. Once the linear feature was established, longshore sediment supply would extend its length and add to the lakeward face. However, overwash and building in elevation is characteristic of wave action either in an extreme storm event or a transgressive lake margin. Again, a ‘bar building’ period seems evident at Cummins. Julig (1984) found confirmatory evidence of habitation of these bars during the process of their accumulation, a buried layer of water worn taconite artefacts being associated with a period of overwash and construction. This suggests occupation of the site ...“when a lakeshore location offered advantages of longshore access, lagoon fishing and perhaps water transport” (Dawson, 1983b). A cremation burial site was recovered from a sand quarry wall on the beaches marked by the triangle in Figure 25. This was erroneously cited as being located on the rock island by Dawson (1983) since Bill Ross (pers. comm.) reports information from J.V. Wright that suggests that a cartographic error resulted in its - 172 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 fenced area DcJi-1 DcJi-11 DcJi-16 Harbourview Extension (Highway 11/17) 244 m Figure 24 The Neebing River cluster contains a series of probable Plano and Archaic sites associated with shoreline features of either the Lake Minong Strandline, or the Nipissing Transgression strand. Perhaps the most important of this group is the Cummins Site (DcJi-1), a registered National Historic Site, a portion of which is provincially owned (within fenced area). Much of the balance of this site is slowly being destroyed by urban development. J.V. Wright, K.C.A. Dawson and P. Julig have conducted excavations at the Cummins Site. Julig’s geoarchaeological examination, coupled with geomorphic mapping by B.A.M. Phillips and P. Fralick have enabled geomorphic interpretation. A Hinshelwood’s salvage excavations at DcJi-16 demonstrate Archaic reoccupation of this Plano site (2004). - 173 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 25 Cummins Site, with major paleo-lacustrine features (from Julig, McAndrews and Mahaney, 1990 based on Phillips, 1982). Black triangle marks the approximate location of the cremated human burial recovered by Wright in 1963 that was radiocarbon dated to 8480 ±390 (NMC-1216). misplacement, thereby miss-informing Dawson. This small sample was subjected to AMS radiocarbon dating, and the low collagen yield resulted in its complete consumption. The resultant date was 8,480 ±390 years BP (NMC-1216). Note that the very large sigma associated with this date renders it a not particularly precise measure of the antiquity of the Cummins occupation, but it remains the oldest dated human remains recovered so far in Ontario. Julig (1984) found artefacts both just below the aeolian sands that characteristically modified the topography of the shoreline features once the offshore shelf was exposed by lower lake levels (circa 8.0 ka BP) and in peat that accumulated in Cummins Pond. Indications are that long after the lake ceased to lap the beach face, the site remained used, at least until 7.5 ka BP (Julig et al., 1986). A well-developed, beach deposit is exposed at the Cummins site. Figure 27 shows a schematic representation of the sandy cliff face where the strandline deposits are visible. The following quotation describes the sequence. Low-angle, planar cross-stratified sands of - 174 - the foreshore dominate the exposure. Individual laminae dipping 3° to 12° lakeward (original swash-backwash surfaces) are arranged into packets which erosively truncate one another at very low angles. The planar cross-stratified sands are transitional both laterally and vertically into massive sands through a bioturbated zone. The bioturbated area represents a sparsely vegetated backshore environment while the massive sands were deposited as aeolian dunes. The foreshore sands are erosively truncated in the eastern portion of the cliff by magnetiterich sands. Intemal structures indicate that they are also foreshore deposits. The magnetite-rich foreshore laminae were formed during regression when storm wave activity reworked the lower portion of the beach, erosively truncating the older deposits. During the storm events sand was removed and stored in offshore bars. In intervening periods of fair weather, sand moved from the offshore bars back onto the beach and was winnowing by small waves, producing the magnetite-rich lag deposits filling scour truncations. The beach assemblage is overlain �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 26 The changing geography and present characteristics of the Cummins Site (from Phillips, 1988:133). by erosively based, trough cross stratified sands. These were formed after subaereal exposure of the area. Major rainfall events caused streams to flow off the adjacent rock knoll dissecting and reworking the upper layer of beach deposits. DcJi-16, located at the former outlet of the Neebing River into Lake Minong (Figs. 24 and 28). This stop on the side of the highway is approximately within the middle of this former site. In fact, the highway runs up the middle of the site, likely destroying most of the rivermouth feature upon which it was located. While the salvage excavation report has not been published, some of the relevant data is included in Hinshelwood’s 2004 publication. Phillips, Fralick and Ross, 1987. From INQUA Field Guide C-12, Eds. Geddes, Kristjansson and Teller. Stop 9. The Neebing River Site (DcJi-16). (Fig. 28) UTM coordinates: NAD83; 16U 0324874E / 5363477N In anticipation of highway development west from Thunder Bay, Andrew Hinshelwood conducted extensive excavations in the vicinity of the Neebing River south of the Cummins Site. One such site is He identifies and excavates a series of habitation areas on the terraces that successively built up at the river mouth. This site strongly resembles the site context noted at the McDaid Site, but with an important new observation. Figure 28 reports the recovery of several Archaic affiliated objects in much the same context as the Plano materials. He proposes that this locality (and - 175 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 27 Schematic representation of the cliff face at the Cummins Site (Phillips, Fralick and Ross 1987). others) were first occupied by Plano people when Lake Minong waters were nearby, but that it was reoccupied during Archaic times, perhaps because of its sandy riverbank sedimentary context coupled with nearby exposures of taconite. He also makes the point that Nipissing Transgression water levels had reflooded the Kaministiquia River delta to within about 2 km of DkJi-16, and that probable Archaic age sites are to be found to the south near the new outlet of the Neebing River into the Lake Superior Basin (Fig. 24). Stop 10 (several) - The Rosslyn Delta. Figure 23:A. UTM coordinates: NAD83; 16U 0317690E / 5360523N The route continues west up the Harbour-view Expressway extension, past the Neebing River site (DcJi-16), and then southwest to once more intercept the bluff defining the Lake Minong shoreline at the Highway weigh scales station. Turning left off the highway, once on the terrace, the road leads south to several fields in which lithic material has been found. Figure 28 Contour map of DcJi-16, at the former outlet of the Neebing River into Lake Minong. Note its similarity to the placement of the McDaid Site (Fig. 14). Also note the mixed Plano and Archaic objects recovered on this site that Hinshelwood (2004) interpreted as evidence of re-occuaption of this location. - 176 - �- 177 - H E F I C G A B B D Approx Minong Figure 29 Overview map of selected sites within the upper Kaministquia River Delta. Letters reference archaeological sites detailed in some of the following images. Dashed lines representing Strandlines are only approximate. 1.5 km Crane Strand line Strand line Proceedings of the 58th ILSG Annual Meeting - Part 2 �Proceedings of the 58th ILSG Annual Meeting - Part 2 These sites, the Dairy Farm, Breukelman Evergreen and Halow A. B and C, will be viewed (Fig. 30) and then the route will follow the north side of the Kaministiquia River towards Stanley. Enroute the upper and lower Drezecky sites will be seen (Fig. 32), as well as the Pawlick site which can be seen on the south side of the river. From Stanley the route will climb the terraces of the Stanley delta and return to Hwy 11-17. This stop provides a view north across the fields that contain a series of small lithic clusters associated with low sandy knolls around a shallow swale (Fig. 30). This swale coincides with the 750 foot contour line and suggests a shallow embayment of Lake Minong. The following extract of text and figures derives from S. Hamilton (2000). The paper addresses the problem of developing archaeological predictive models in light of environmental transformation, and as yet incompletely understood past land use practises. Among the many challenges is the issue of development of temporally sensitive palaeo-environmental reconstructions, and also models of land use relevant to the cultures under consideration. The apparent correlation between relict shorelines and Plano archaeological sites is well known in the Thunder Bay area where landscape evolution has been the subject of some study (Fig. 14). While many Plano shoreline archaeological sites are documented, the best known ones are lithic quarry and workshop sites such as the Cummins, Biloski, Simmonds, and McDaid Sites (Julig 1984, 1994; MacNeish 1952; Hinshelwood 1990; Phillips 1988, 1993). These large sites are well removed from modern shorelines, but are associated with the late Pleistocene shorelines of Glacial Lake Minong and Gunflint Formation bedrock exposures that are suitable for lithic tool production (Fig. 14; see Julig, McAndrews and Mahaney, 1990; Phillips 1988, 1993). However, these special-purpose sites do not characterize the full Palaeo-Indian settlement pattern. Rather, the dense recoveries from, and ready visibility of, these sites has resulted in their overrepresentation in the published archaeological literature. Archaeological reconnaissance upon agricultural fields within the Kaministiquia River delta has revealed a number of Plano or probable Plano sites in a wide range of landscape contexts (Hamilton, 1996) (Fig. 12). Many of these sites are found at or near Lake Minong shoreline elevations (i.e. 750 feet or 228.6 metres A.S.L.). When placed in a hypothetical Lake Minong environmental context, they are found associated with: 1) springs flowing into sheltered coves of Lake Minong [Figs. 29:A, 30] (Breukelman Evergreen); 2) on points of land that protruded out into Lake Minong to the northeast and southeast of the Breukelman Evergreen site cluster [Figs. 29:B, 30]; 3) upon low, well-drained sandy knolls surrounded by poorly drained floodplain/deltaic sediments [Figs. 29:C, 30] Halow C); 4) along deltaic backwater channels [Figure 29:D, Figure 31] DbJi-8, DbJi-7, DcJi-28, DcJi32); or 5) on sandy storm beaches developed upon relict deltas [Figs. 29:D, Figure 31] DcJi-30, DcJi-31). 6) raised Pleistocene terraces overlooking the present Kaministiquia River channel [Figs. 29:E, 30] Pawlick); 7) on high valley rims that offer panoramic views [Figs. 29:F, 32] Drezecky E); 8) along draws leading down to the Lake Minong shores [Figs. 29:G, 30] Halow A and B); 9) along upland streams well removed from late Pleistocene shorelines [Figs. 29:H, 33] DcJj-12, DcJj-13); and 10) upon well-drained upland knolls on what likely were formerly discontinuous permafrost uplands [Figs. 29:H, 33] Breukelman Field: lithic scatters A to E). Probable Plano lithic scatters have also been found upon isolated bedrock controlled knolls offering panoramic views of floodplains adjacent to the Lake Minong shoreline, [Figs. 29:I, 34] and at the top of the high bluff overlooking the gorge outlet of the Kaministquia River into the shoreline floodplain of Lake Minong [Fig. 32]. This range of landscape characteristics is certainly not exhaustive, but is sufficient to demonstrate the diverse microhabitats frequented by Plano people. These sites are consistent with other Lakehead Complex sites in terms of a very strong preference for Gunflint Formation lithic materials (Hamilton 1996). However, virtually - 178 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 30 Some archaeological sites identified at A, B, C and G in Figure 29. Sites DcJi-23 to 26 are small lithic clusters discovered on localized sandy knolls in the middle of the field. If the 750 foot contour is used as a proxy of Minong Shorelines, then they are found within a shallow embayment, while two other sites (DcJi-12 and Unnamed) are on points of land extending into the glacial lake. The sites in Halow A and B fields are small clusters or single finds oriented along draws or gullies draining towards Lake Minong, while DcJi-20 consists of localized taconite clusters on a point bar feature overlooking the Kam River. - 179 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 31 These sites referenced as D in Figure 29 are all located on localized sandy knolls within the cultivated field. Some are interpreted to be occuaptions upon a former storm beach (DcJi-29 to 30), while the others may reflect use of localized well-drained knolls within a former deltaic wetland at or immediately below the 750 foot contour. all of these sites are well removed from bedrock exposures, are significantly smaller than the quarry-workshop sites (such as the Cummins Site), and yield many fewer artifacts. When compared to the quarry/workshop sites, these small sites also yield a much higher relative frequency of tools, preforms and utilized flakes compared to debitage (Hamilton, 1996). This indicates that the small sites represent encampments, hunting stands and food procurement sites, rather than lithic extraction and reduction stations. Such observations are hardly surprising, but they do serve as cautionary tales regarding the dangers of predicting site distribution on the basis of our current incomplete heritage inventory. These examples are also important in that they demonstrate that modelling ancient human behaviour requires ongoing refinement, an understanding of human forager behaviour, and a well-developed sense of the nature and structure of the ancient landscape and its microhabitats.” Stop 11 Roadside stop to view Lower Beaver Bay strandlines Figure 37. UTM coordinates: NAD83; 16U 0309392E / 5361819N As we leave the Rosslyn delta (Minong, 9.5 ka B.P.) - 180 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 32 Location labelled F and G in Figure 29. Two small lithic scatters were encountered on the top brink of the gorge edge within Field Drezecky E. These are interpreted as game scouting sites given their commanding view across the lower gorge and Kam Delta to the south and east. Figure 33 Location labelled H in Figure 29. The sites within the Breukelman and Meyer fields are found in localized clusters on top of sandy well-drained knolls surrounded by now-dry drainage channels. These sites are far removed from paleoshoreline contexts and are interpreted as localized encampment zones within the interior region removed from glacial lake edges. - 181 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 area covered by the waters of a pre-Marquette lake (Early Lake Minong), now approximated by the 1400’ contour. On the basis of some long known sites, both in the interior and on the Minnesota northshore, Phillips and Hill (1995) proposed that a Palaeo-Indian routeway along the Minnesota shore turned inland just east of Judge C. Magney State Park, towards North and South Fowl lakes and the Whitefish Lake region. A number of more recently discovered sites in the Whitefish - Arrow Lake corridor supports the contention that Palaeo-Indian peoples were present in the area perhaps before and during the Marquette advance, and that the corridor continued to be used into the post-Marquette periods of Lake Beaver Bay and Lake Minong. Though conjectural, there is the possibility that after the retreat of Marquette ice had begun, the Marks moraine itself provided a high ground routeway from the Whitefish Lake area along the north side of the Kaministiquia valley and into the area to the north and east of Thunder Bay. Stop 12 - Kakabeka Falls UTM coordinates: NAD83; 16U 0305771E / 5364428N Figure 34 Location I in Figure 29. This set of small lithic clusters is located on the top brink of a localized upland facing north. It is interpreted as a hunting viewing location, whereby occupants could have watched the surrounding plains along the shores of Lake Minong (defined perhaps by the 750 foot contour line). and move up the present Kaministiquia valley, one is in effect travelling back in time. The Stanley delta was built into Lake Beaver Bay (9.7 ka B.P.), the first Superior lake to occupy the area recently vacated by Marquette ice retreating from the Marks moraine. Though perhaps less conspicuous, Palaeo-Indian sites of this period are also present. The larger step back in paleogeography is to consider the area that lies to the west of the maximum position of the Marks moraine, an area which remained unglaciated by the Marquette readvance and which was freed from ice around 11.0 ka B.P. Figure 35 shows the projected maximum position of the Marquette lobe and the large Just west of the junction of Hwy 11-17 and the Stanley turn-off, the huge structure of the Stanley delta is seen (Fig. 36). This delta was formed as the Kaministiquia River entered Lake Beaver Bay. A well formed bluff representing a lower Beaver Bay phase of 260m (853’), runs along the north side of the main highway, and the surface below it (250-240m; 820787’) is heavily exploited by the sand and gravel industry. The present Kaministiquia river has incised deeply into the delta, creating a terraced valley side. Kakabeka village is built on the floor (277m; 909’) of an old distributary of the Kaministiquia river which cut through a higher terrace still (Fig. 36). Remnants of this terrace surface at around 300m (984’) overlook the village. They represent the highest level of the Stanley delta, as it formed into Upper Lake Beaver Bay, and on the one on the west side of the Kaministiquia River, the Crane site was found. Kakabeka Falls is a major scenic attraction. It is formed as a result of a very resistant chert bed that caps the underlying, softer shales. It was just this sort of rock material that was desired for tool making. Today, a fairly spectacular gorge lies below the falls (Fig. 37). There has been some debate concerning the age of the gorge, since unless inherited and exhumed from a previous stage of river incision prior to the Marquette readvance, there is only post Beaver Bay time for it to develop its current grandeur. - 182 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 35 Plano and probable Plano sites found in the interior uplands around Arrow and Whitefish Lakes. Phillips and Hill (1995) proposed that these sites might predate or be contemporaneous with the Marquette Re-advance (Hamilton, 2000). Figure 36 Strandline and other features between Stanley Corners and Kakabeka Falls. - 183 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 An interesting abandoned falls and plunge pool on the west side of the gorge lies a short walk from the Park Information Centre, and represents a wide ‘horseshoe’ fall that would have been more spectacular than the present falls. Stop 13 (several) - The Marks Moraine. UTM coordinates: NAD83; 16U 0300280E / 5368976N The route will turn on Hwy 590 just west of Kakabeka, run across the High Beaver Bay terrace remnant (look for the cemetery on the right) and due west until turning north towards the Marks Moraine. As the dirt road rises up the outer face of the Marks moraine it crosses two distinct benches at 426m (1400’) and 442m (1450’). These shorelines of pro-glacial Lake Cedar Creek have been traced all along the outer slope of the Marks moraine (Jahnke, 1993) and represent still stands in a lake earlier than and isolated from the lakes of the Superior basin. The Marks Moraine. Stopping on the crest around 460m (1510’), the view south over the Whitefish valley and the rugged borderland country of the mesa-like NorWesters is spectacular. It is not hard to envisage ice pushing its way up to the Marks moraine, nor is it hard to imagine pro-glacial Lake Cedar Creek occupying the narrow strip between the moraine and the retreating ice margin. The moraine is not a simple structure. Although characterised by a till which contains diagnostic pieces of Sibley red sedimentary rocks, along much its length the Marks moraine joins and smothers isolated rock outcrops which probably greatly influenced the extent to which the ice pushed inland. The top of the moraine is remarkably flat in many places and is pockmarked with kettles. Spillway channels cross the feature in places suggesting that at one point Lake Kaministiquia to the north was a few metres higher than Lake Cedar Creek to the south. The moraine appears to have briefly existed as a narrow ridge between these two water bodies, and a possible post-Marquette routeway for animals and Palaeo-Indian people, from the unglaciated (Marquette) Arrow/Whitefish region and Interlakes corridor. Figure 37 Kakabeka Falls and the Kaministiquia River gorge. - 184 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 38 Cross section exposure of Conmee Pit (Tickle 1996). The Conmee Pit and spillway. Continuing north across the moraine and then turning east brings the excursion to a distinct channel cutting from north to south across the moraine surface. Here Conmee Township has several gravel pits. Apart from the huge boulder beds that suggest high discharges down this spillway at times, the pit reveals another key fact in understanding local deglaciation. Figure 38 shows a rare occasion when the western pit face was cleanly exposed. Tickle (1996) interpreted the sequence as consisting of two tills separated by fluvioglacial sediments. However, while one till was of diagnostic Sibley origin, the underlying one was of a northern provenance typical of the Patricia or Rainy River lobes. Elsewhere, at several places along the Marks moraine a thin Sibley till is plastered over fluvioglacial sediments. The observation suggests that the bulk of the Marks moraine may in fact be pre-Marquette in origin, with thin Marquette ice reoccupying the moraine. This has been the growing opinion of another local field worker (T. Noble, pers. communication, 1999) and looking at Figure 5, the possibility of the Marks moraine being the result of pushing the eastward part of the former Brule moraine is not unreasonable. Several exposures of contorted till structures also support this idea. Marks Moraine scenic lookout. Turning left out of the pit and again at the crossroad brings one to a point in the road from which a view east over the Kaministiquia valley, Thunder Bay city and the distant Sleeping Giant is obtained. The stop is useful only to impress viewers with scale of things. It is easy to imagine ice grinding past Mt. McKay and flowing up the valley, just as it is easy to imagine that a slight tilt of the present lake would bring the margin of the lake right up into the Kaministiquia and Whitefish valleys. Returning to the crossroad, the route will turn left down the face of the Marks moraine. The road is straight, save for a jog around a kettle hole, and, lower down, rock is exposed in the fields and along the stream beds. Turning right at Hwy 11-17 brings the excursion back to Kakabeka Falls. From here the route will take Oliver Road back to the University. The road climbs out of Kakabeka over the Upper Beaver Bay terrace remnant and then runs due east to Lakehead University. One landform of note on the way is a crag and tail feature, showing the flow of ice up the Kaministiquia valley. The ‘Old Barn’ restaurant lies on the tail and on the crag is the farmhouse, open for bed and breakfast (another time, eh!). Just east of Murillo but to the north of Oliver Road is the Murillo drumlin field, a series of long narrow forms parallel to the strike of the Gunflint shales, suggesting a good deal of bedrock control. References and Bibliography Arthurs, D. 1979 An Archaic site on the western Lake Superior shore. Report on file with the Historic Planning and Research Branch, Ontario Ministry of Culture and Communication, Toronto, 35 pp. Bjorck, S. 1985 Deglaciation chronology and revegetation in northwestern Ontario. Canadian Journal of Earth Science 22, 850-871. Burwasser, G.J. 1977 Quaternary Geology of the City of Thunder Bay and Vicinity. Ministry of Natural Resources, Ontario Geological Survey Report GR164. Burwasser, G.J. 1980 Quaternary geology of the Onion Lake and Sunshine area, District of Thunder Bay. Ministry of Natural Resources, Ontario Geological Survey Report MP94. Burwasser, G.J. and Ferguson, A. 1980 Quaternary geology of the Onion Lake and Sunshine area, District of Thunder Bay. Ministry of Natural Resources, Ontario Geological Survey Preliminary Map P.2203, Geological Series, 1:50,000. Clayton, L. 1983 Chronology of Lake Agassiz Drainage to Lake Superior: in Teller, J.T., and Clayton, Lee, eds., - 185 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Glacial Lake Agassiz, Geological Association of Canada Special Paper 26,291-307. River Delta, Thunder Bay District, Lakehead University Monographs in Archaeology, 1. Clayton, L. 1984 Pleistocene geology of the Superior region, Wisconsin, Information Circular No. 46, Wisconsin Geological and Natural History Survey, Madison, Wisconsin. Hamilton, S. 2000 Archaeological Predictive Modelling in the Boreal Forest: No Easy Answers Canadian Journal of Archaeology Vol. 24, p.p. 41-76. Clayton, L. and Moran, S. 1982 Chronology of Late Wisconsin glaciation in middle North America, Quaternary Science Reviews, vol. 1, pp. 55-82. Dawson, K.C.A. 1983a Prehistory of Northern Ontario. Thunder Bay Historical Museum Society. Dawson, K.C.A. 1983b Cummins Site: A Late PalaeoIndian (Plano) Site at Thunder Bay, Ontario. Ontario Archaeology 39,3-31. Drexler, C.W, Farrand, WR. and Hughes, J.D. 1983 Correlation of Glacial Lakes in the Superior Basin with Eastward Discharge Events from Lake Agassiz ill Teller, J.T., and Clayton, Lee, eds., Glacial Lake Agassiz, Geological Association of Canada Special Paper 26,309-329. Dudzik, M.J. 1993 The Palaeo-Indian tradition in northwestern Wisconsin, State Archaeology Regional Program, Siren, Wisconsin. Farrand, WR. 1960 Former shorelines in western and northern Lake Superior Basin, unpublished Ph.D. Thesis, University of Michigan, Ann Arbor Farrand, WR. and Drexler, C. W 1985 Late Wisconsin and Holocene history of the Lake Superior basin, In Karrow, P.F. and Calkin, P.E. eds., Quaternary Evolution of the Great Lakes, pp.17-32, Geological Association of Canada, Special Paper 30. Fisher, T. 2005 Strandline Analysis in the Southern Basin of Glacial Lake Agassiz, Minnesota and North and South Dakota. Geological Society of America Bulletin 117(11/12):1481-1496. Fisher, T. 2008 Chronological Status of Glacial Lake Agassiz. Geological Society of America, Abstracts with Programs 40(5):4. Fox, WA 1975 The Palaeo-Indian Lakehead Complex, in Nunn, P., ed. ,Canadian Archaeological Association, Collected Papers, Historic Sites Branch, Ontario Ministry of Natural Resources, Archaeological Research Report 6, 29-53. Fox, WT. 1980 The Lakehead Complex: New Insight Archaeological Research Report 13, Historical Planning and Research Branch, Ontario Ministry of Citizenship and Culture, Toronto, 127-151. Grootenboer, J. 1972 Former Shorelines in the Kaministikwia Plain and the Geomorphology of the Kakabeka Falls-Stanley Area. H. B.A. Dissertation, Department of Geography, Lakehead University, Thunder Bay, Ontario. Hamilton, J.S. 1996 Pleistocene Landscape Features and Plano Archaeological Sites upon the Kaministiquia Hamilton, S. nd A World Apart? Archaeology, Culture and History of Ontario’s Canadian Shield. In Before Ontario: New Insights from the Archaeology of the Eastern Great Lakes. edited by M. Munson and S. Jamieson. Submitted for review to McGill-Queens University Press. Hinshelwood, A 1990 1987 Observations at the Brohm Site (DdJe-1), Sibley Provincial Park, Conservation Archaeology North Central Region Report 27, Ontario Ministry of Citizenship and Culture, Heritage Branch, Thunder Bay, Ontario. Hinshelwood, A. 2004 Archaic Reoccupation of Late Palaeo-Indian Sites in Northwestern Ontario. In The Late Palaeo-Indian Great Lakes: Geological and Archaeological Investigations of Late Pleistocene and Early Holocene Environments. edited by Lawrence J. Jackson and Andrew Hinshelwood Mercury Series Archaeology Papers 165, Canadian Museum of Civilization, Gatineau. Huber, J.K. 1992 An overview of the vegetational history of the Arrowhead region, northeastern Minnesota in Field trip guidebook for the glacial geology of the Laurentian divide area, St. Louis and lake Counties, Minnesota, Minnesota Geological Survey, Field Trip Guidebook 18, pp. 55-64. Huber, J.K. and Hill, C.L. 1987 A pollen sequence associated with Paleo indian presence in northeastern Minnesota, Current Research in the Pleistocene, vol. 4, p. 89-91. Jahnke, R. 1993. Initial evidence of an early Post-Marquette pro-glacial lake on the flanks of the Marks Moraine, Thunder Bay, Ontario. HBSc. dissertation, Dept. of Geography, Lakehead University, Thunder Bay, Ontario. Johnson, L. L. and M. Stright (eds.) 1991 Paleoshorelines and prehistory: an investigation of method, CRC Press, Boca Raton, Florida. Julig, P. 1984 Cummins Palaeo-Indian Site and its Paleoenvironment, Thunder Bay, Canada. in Gramly, R.M., ed., New Experiments upon the Record of Eastern Palaeo-Indian Culture, Archaeology of Eastern North America 12,192-209. Julig, P.J., McAndrews, J.H., and Mahaney, WC. 1986 Geoarchaeological Investigations at the Cummins Palaeo-Indian Site, Thunder Bay, Ontario in Paleoenvironments: Geosciences, Current Research in the Pleistocene 3, 79-80. Julig, P.J., McAndrews, JH and Mahaney, WC. 1990 Geoarchaeology of the Cummins site on the beach of proglacial Lake Minong, Lake Superior basin, Canada. in N.P. Lasca and J. Donahue, eds., - 186 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Archaeological Geology of North America, pp. 2150. Geological Society of America, Centennial Special Volume 4, Ch.2. Julig, P.J. 1994 The Cummins Site Complex and PalaeoIndian occupations in the Northwestern Lake Superior Region, Ontario Archaeological Reports 2, Ontario Heritage Foundation, Toronto. Landmesser, C.W, T.C. Johnson, R.J. Wold 1982 Seismic reflection study of recessional moraines beneath Lake Superior and their relationship to regional deglaciation, Quaternary Research, vol. 17, p. 173190. Larsen, CE 1987. Geological History of Glacial Lake Algonquin and the Upper Great Lakes. U.S. Geological Survey Bulletin 1801, 36pp. Lehr, J.D. and H.C. Hobbs 1992 Glacial geology of the Laurentian divide area, SI. Louis and Lake Counties, Minnesota, Minnesota Geological Survey Field Trip Guidebook 18, prepared for the 39th Midwest Friends of the Pleistocene Field Trip. Leverett, F. 1929 Moraines and shorelines of the Lake Superior basin, U.S.G.S. Professional Paper 154-A Leverett, F. and F.W Sardeson 1932 Quaternary geology of Minnesota and parts of adjacent states United States Geological Survey· Professional Paper 161, U.S. Government Printing Office, Washington D.C. Levson, V.M. DE Kerr, and T.R. Giles 1993 Recognizing Late Pleistocene paleo shoreline levels from geomorphic and stratigraphic records of glaciofluvial delta, Current Research in the Pleistocene, vol. 10, p. 82-84 MacNeish, R.S. 1952 A Possible Early Site in the Thunder bay District, Ontario, National Museum of Canada Bulletin 120, 23-47. Matsch, C. L. and A F. Schneider 1986 Stratigraphy and correlation of the glacial deposits of the glacial lobe complex in Minnesota and northwest Wisconsin in Sibrava, B., D.Q. Bowen, and G.M. Richmond (eds.), Quaternary glaciations in the Northern Hemisphere: Quaternary Science Reviews, vol. 5, p. 59-64, McAndrews, J.H. 1982 Holocene environment of a fossil bison from Kenora, Ontario, Ontario Archaeology, 37, 41-51. McLeod, MP. 1982 A Re-Evaluation of the PalaeoIndian Perception of the Boreal Forest. Manitoba Archaeological Quarterly, 6, 4, 107-116. Mulholland, S. and E. Dahl 1988 Two late Palaeo-Indian projectile points associated with a Glacial Lake Duluth beach, Carlton County, Minnesota, The Minnesota Archaeologist, vol. 47, no. 2, p. 41-50. Newman, M. and Julig, P. 1989. The Identification of Protein Residues in Lithic Artifacts from a Stratified Boreal Forest Site. Canadian Journal of Archaeology 13, 119-132 Nordeng, S.C., A.P. Ruotsala, SA Nordeng 1987 Buried bog dates the time of the establishment of post-glacial Lakes Houghton and Nipissing in the western arm of the Lake Superior basin, North-central Section Geological Society of America Abstracts with Program, vol. 19, no. 4, p. 237. Pettipas, Leo 2011 An Environmental and Cultural History of the Central Lake Agassiz Region, with special reference to southwestern Manitoba, 12,000 - 7,000 BP. Manitoba Archaeological Journal Vol. 21, no. 1 and 2. Phillips, BA M. 1977 Shoreline Inheritance in Coastal Histories, Science 195,11-16. Phillips, BAM. 1982 Morphological Mapping and Palaeogeographic Reconstruction of Former Shorelines between Current River and Rosslyn, Thunder Bay, Ontario, Including Cummins Site DcJi-1. Report on file with Historical Planning and Research Branch, Ontario Ministry of Citizenship and Culture, Toronto, Ontario. Phillips, BAM. 1988 Palaeogeographic reconstruction of shoreline archaeological sites around Thunder Bay, Ontario. Geoarchaeology: An International Journal, 3, 2, 127-138. Phillips, BAM. 1993 A Time-Space Model for the Distribution of Shoreline Archaeological Sites in the Lake Superior Basin. Geoarchaeology: An International Journal, 8, 2, 87-107. Phillips, BAM. and W.A. Ross 1993 The Glacial Period and Early Peoples. In Thunder Bay: From Rivalry to Unity. pp. 2-15 Thunder Bay Historical Museum Society Phillips, BAM. and Hill, C. 1994 The Geology, Glacial and Shoreline History and Archaeological Potential of the Minnesota North Shore of Lake Superior: a background paper for Geomorphologial and Archaeological Studies of Individual State Parks on the North Shore. Report for Minnesota Department of Natural Resources, Division of Parks and Recreation, SI. Paul, MN. Phillips, BAM. and Fralick, P.W 1994. A Transgressive Event on Lake Minong, Northshore of Lake Superior - Possible Evidence of Lake Agassiz Inflow, circa 9.5 KA BP. Canadian Journal of Earth Science, v. 31, pp. 1638-1641. Phillips, BAM., Hill, C.L., Fralick, PW and WA. Ross 1994 Post-Glacial Shorelines and Palaeo-Indian Migration along the Northwestern Shore of Lake Superior, Guidebook for Fieldtrip E, 13th Biennial Meeting of AMQUA, Minneapolis, Mn. Phillips, BAM. and Hill, C.L. 1995 The deglaciation history and geomorphological character of apart of the region of the ‘Interlakes Composite’ - some new evidence. Symposium on ‘Archaeology, geomorphology and Paleoenvironmet: Paleo indian Occupations of the Western Great Lakes, 60th Annual Meeting of the Society for American Archaeology, Minneapolis, - 187 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 P.K. and G.B. Morey, eds. Geology of Minnesota: a Centennial volume, Minnesota Geological Survey, p. 561-578. MN. May 3-7. Ross, W 1980 A Palaeo-Indian Refined Biface from the Oliver Lake Area. Wanikan, 80, 3. Thunder Bay Chapter, Ontario Archaeological Society. Ross, W 1997 The Interlakes Composite: a re-definition of the initial settlement of the Agassiz-Minong Peninsula. Wisconsin Archaeologist, 76, 3-4. Sharp, R.P. 1953 Shorelines of the glacial Great Lakes in Cook County, Minnesota: American Journal of Science, 251, 109-139. Smith, D.G. and Fisher, T.G. 1993 Glacial Lake Agassiz: The northwestern ‘outlet and paleoflood, Geology 21, 9-12. Stewart, J.D., Ross, WA. and B.A.M. Phillips 1984 Investigations at the McDaid Site (DcJh-16), Thunder Bay, Report to the Ontario Heritage Foundation. Stewart, J.D., Ross, WA Faykes, A and A.S. Kissin 1989 Two Isolated Biface Finds from the Thunder Bay Area, Wanikan, 89, 3, 4-6, Thunder Bay Chapter, Ontario Archaeological Society. Stuart, A.J. 1993 Paleogeographical reconstruction of Lake Beaver Bay raised shorelines with correlation to possible Palaeo-Indian settlement, Thunder Bay region, Ontario. HBSc. dissertation, Dept of Geography, Lakehead University, Thunder Bay, Ontario. Taylor, F.B. 1895 The Nipissing beach on the north Superior shore, American Geologist, vol. 15, p. 304-314. Teller, J.T 1985 Glacial Lake Agassiz and its Influence on the Great Lakes. in. Karrow, P.F., and Calkin, P. E. eds., Quaternary Evolution of the Great Lakes, Geological Association of Canada Special Paper 30, 1-16. Wright, H.E. 1972b Quaternary history of Minnesota, in Sims, P.K. and G.B. Morey, eds. Geology of Minnesota: a Centennial volume, Minnesota Geological Survey, p. 515-547. Wright, H.E. Tunnel valleys, glacial surges and subglacial hydrology of the Superior lobe, Minnesota, Geological Society of America Memoir 136, p. 251-276. Wright, H.E., C.L. Matsch, and E.J. Cushing 1973 Superior and Des Moines Lobe, Geological Society of America Memoir 136, p. 153-185 Wright, H.E., W.A. Watts 1968 Glacial and vegetational history of northeastern Minnesota, Minnesota Geological Society Survey Special Publication 11 . Wright, J. V. 1963 An Archaeological Survey along the North Shore of Lake Superior Anthropology Papers, National Museum of Canada 3, Department of Northern Affairs and National Resources, Ottawa, 9 pp . Zoltai, S.C. 1963. Glacial features of the Canadian Lakehead, Canadian Geographer 7, 101-115 Zoltai, S.C. 1965a. Glacial features of the Quetico-Nipigon area, Ontario. Canadian Journal of Earth Science 2, 247-269. Zoltai, S.C 1965b. Thunder Bay - Surficial Geology, 1:506,880, Map S265, Ontario Department of Lands and Forests. Teller, J.T. 2004 Controls, History, Outbursts, and Impact of Large Late-Quaternary Lakes in North America. In The Quaternary Period in the United States: Developments in Quaternary Science, Vol. 1, edited by A. Gelespie, S. Porter and B. Atwater, pp. 45-61, Elsevier Amsterdam Teller, J.T. and Thorleifson, L.H. 1983. The Lake Agassiz -Lake Superior connection. In J.T. Teller and L. Clayton, eds., Glacial Lake Agassiz, pp. 261-290, Geological Association of Canada, Special Paper 26. Tickle, R. 1996. Depositional Systems developed during Deglacialion: Evidence from a portion of the Marks Moraine, Conmee Township, ON., HBA. dissertation, Dept. of Geography, Lakehead University, Thunder Bay, Ontario. Van, Hise, C.R. and C.K. Leith 1911 The Geology of the Lake Superior Region, United States Geological Survey, Washington. Winchell, N.H. 1901, Glacial Lakes of Minnesota, Geological Society of America Bulletin, vol. 12, p. 108-128. Wright, H.E., 1972a Physiography of Minnesota, in Sims, - 188 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Field trip 11 - Midcontinent Rift-Related Mafic Intrusions around Thunder Bay, Ontario Robert Cundari and Pete Hollings Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada Mark Smyk Resident Geologist Program, Ontario Geological Survey, Ministry of Northern Development and Mines, Thunder Bay, Ontario, P7E 6S7, Canada Introduction This field trip covers an area that has been the focus of much recent research. The guidebook has benefited from geochronologic and geochemical studies conducted as part of the Lake Nipigon Region Geoscience Initiative (Heaman et al., 2007; Hollings et al., 2007a,b, 2010) as well as recent undergraduate theses (Puchalski, 2010; Cundari, 2010; Carl, 2011) and on-going Masters Theses (Cundari, in progress). These studies have elucidated the nature of magmatism in the northern part of the Midcontinent Rift (MCR) and have been used to augment and refine our previous understanding of these magmatic events. This trip focuses on a variety of mafic intrusions associated with the MCR in and around Thunder Bay. These intrusions encompass changes in the nature of early to mid-stage MCR magmatism over a span of ~20 million years. They include Nipigon sills, Logan sills, Pigeon River dykes and the Riverdale sill. Contacts with Paleoproterozoic Rove and Gunflint formations sedimentary rocks are well-exposed in this area and illustrate some of the mechanisms of dyke/sill emplacement, as well as magma-wallrock interactions. This guide builds upon those previously written and compiled by Franklin and Kustra (1972), Miller and Smyk (1995), Parker (2001), Miller et al. (2002) and Smyk and Hollings (2007). Bear in mind that when visiting exploration or private properties, permission must be granted by the property owner. Current ownership information can be obtained from the Resident Geologist’s Office, Ontario Geological Survey, in Thunder Bay. Please exercise caution along highway right-of-ways, near cliffs and along the lake shore. Regional Geology Situated within the Southern Province of the Canadian Shield, the field trip area is dominantly underlain by Paleoproterozoic Rove Formation clastic sedimentary rocks to the south of Thunder Bay and Gunflint Formation in and to the north of Thunder Bay (Animikie Group), both of which have been intruded by MCR-related mafic intrusions. Previous mapping has been conducted by Tanton (1936a,b), Geul (1970, 1973) and Smith and Sutcliffe (1989). Detailed mapping, geophysical surveys and diamond drilling undertaken by exploration companies have provided additional detail and much-needed information about sub-surface geology. Figure 1 shows the generalized geology of the Thunder Bay area. The field trip area is a rugged, upland area of diabasecapped mesas and ridges that occupies a 70 km by 30 km, northeast-trending topographic feature between Thunder Bay and the Minnesota border, termed the “Logan Basin” by North (2000). Logan Sills underlie and cap mesas that commonly rise 150 m above valleys consisting of deeply eroded,sub-horizontal, Rove Formation sedimentary rocks. Northwest of the Logan Basin, Archean granitoid rocks of the Superior Province form low, rolling hills. Southeast of the Logan Basin the topography is dominated by northeast-trending, linear ridges consisting of Pigeon River dykes. The area north of Thunder Bay displays less relief compared to the Logan Basin and has been described as peneplain by Tanton (1931). Mesoproterozoic diabase sills still provide the most dominant topographic features, as seen at the Silver Harbour Quarry (Stop 1), The Bluffs (Stop 2) and Mount McKay (Stop 3). Animikie Group Paleoproterozoic Gunflint and Rove sediments were deposited in the Animikie Basin, forming a southward-thickening wedge covering the southern margin of the Superior Province, which is truncated in east-central Minnesota and northern Wisconsin by Penokean magmatic terranes. Gunflint sedimentation - 189 - �Figure 1: Simplified geological map of the Thunder Bay area. Modified after Pye and Fenwick (1965) and Carter et al. (1973). Proceedings of the 58th ILSG Annual Meeting - Part 2 - 190 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 began approximately 2.1 Ga and ceased approximately 1.85 Ga, prior to, or during the Penokean orogeny. The nature of the sediment varies considerably, ranging from volcanic through clastic to chemical precipitates which form thick successions of iron formation. The Rove Formation is a turbidite-dominated shelf sequence, which overlies the Gunflint Formation (1878 +2 Ma; Fralick et al., 1998). It consists of a lower section of black, locally pyritic shales, which grades upwards into shales, interbedded with wackes. These clastic rocks were deposited by southeastward-moving turbidity currents, shed from the Archean craton to the north. The Rove has an approximate thickness of 500 to 600 m south and east of Thunder Bay, and thickens to the south. Rocks of the Rove Formation are flatlying or dip gently to the southeast. The shales are thinbedded, dark and fissile. Recent work by Amurawaiye (2001) and Maric and Fralick (2005) described a submarine ramp system in which the movement of coarse sediments into the deeper parts of the basin was mainly through the action of low- and high-density turbidity currents. Fair-weather and storm-generated currents dominated depositional activity at the edge of the basin. Amurawaiye (2001) concluded that approximately 70% of the Rove Formation locally consisted of organic shale. The lower 100 to 150 m of the Rove Formation and the correlative Virginia Formation in Minnesota, consist of alternating shale-siltstone and black, pyritiferous shale successions, probably reflecting fluctuations in sea level (Maric and Fralick, 2005). These successions, and especially the upper black shale, likely represent a major condensed interval deposited in water ~100 to 200 m deep. Lucente and Morey (1983) ascribed sedimentation of this interval to pelagic rainout of fine-grained sediment from dilute suspension or hemipelagic processes involving diffuse turbidity currents. The presence of abundant, submillimeter rip-up intraclasts also denotes the operation of sporadic bottom currents (Maric and Fralick, 2005). Tidal deposits present in correlative rocks to the south of Lake Superior confirms open connection to the ocean (Ojakangas et al., 2001). Above the upper, pure black shale interval, graded fine-grained sandstones are organized into a coarsening-upward succession approximately 100 m thick that is transitional into 400 m of medium-grained, sandstone-dominated, stacked parasequences (Maric and Fralick, 2005). This is overlain by lenticular to wavy bedded sandstones and shales with both wave and current ripples. The coarsening-upward to sandstone-dominated portion of the Virginia and Rove Formations has been interpreted as a submarine fan (Lucente and Morey, 1983; Maric and Fralick, 2005) with the uppermost ripple laminated succession representing progradation of distal distributary mouth bars of a delta (Maric and Fralick, 2005). A sandstone sample from the submarine fan portion of the succession yielded a youngest U-Pb detrital zircon age of approximately 1780 Ma (Heaman and Easton, 2006). The predominantly Paleoproterozoic zircon population and paleocurrents indicating sediment derivation from the north, strongly suggest the Trans-Hudson Orogen was the source of the detritus (Morey, 1973). Keweenawan Supergroup Mesoproterozoic intrusive, volcanic and minor sedimentary rocks associated with the MCR collectively constitute the Keweenawan Supergroup. On the northern margin of the MCR, Keweenawan rocks include a variety of intrusive rocks and Osler Group volcanic rocks, which represent some of the earliest magmatism in the MCR. As shown in Table 1, ages range from ca. 1140 Ma (Heaman et al., 2007) to ages younger than the magnetic polarity reversal that occurred between 1105 and 1102 Ma (Davis and Green, 1997). A tabulated synopsis is provided below; bolded units occur within the field trip area. The majority of mafic and ultramafic rocks in the Lake Nipigon and northern Lake Superior areas, including the Nipigon and Logan sills, appear to have been emplaced in a short, magnetically reversed, interval between ca. 1115 and 1100 Ma (Heaman et al., 2007). Emplacement of alkalic intrusions, such as the 1108 Ma Coldwell Complex (Heaman and Machado, 1992), and filling of much of the submerged part of the rift in Lake Superior, also occurred in this period. This was followed by a period of magnetically normal, waning mafic and felsic magmatism, between 1096 and 1085 Ma, that is preserved mainly along the Lake Superior shore by units such as the Crystal Lake (1099±1 Ma), Moss Lake (1095±2 Ma) and Blake (1095±2 Ma) gabbros, and a Pigeon River dyke near Arrow River (1093±3 Ma; Heaman et al., 2007). Hypabyssal Mafic Rocks Diabase sills, extending from the vicinity of Thunder Bay to east of Lake Nipigon, represent the northern remnants of the Midcontinent Rift, and have previously been referred to as the Logan sills (Stockwell et al., 1972), however, recent work suggests - 191 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Table 1: Geochronology data of MCR-related rocks in Northwestern Ontario Lithologic Unit St. Ignace Island Complex gabbro Arrow River Dyke Pigeon River Dyke Blake Gabbro Moss Lake Gabbro Crystal Lake Gabbro Mt. Mollie Dyke Cloud river Dyke Osler Osler Group rhyolite (central suite) Osler Group rhyolite (lower suite) St. Ignace Island Complex Rhyolite Coldwell Complex Logan Sills Nipigon Sills Ultramafic Intrusions Inspiration Sill Marathon lamprophyre dykes Locality / Age (Ma) St. Ignace Island / 1089.2 ±3.2 Reference(s) Smyk et al.(2006) Arrow River / 1078 ± 3 Rita Bolduc / 1141 ± 20 Blake Township / 1091.0 ± 4.5 Black Bay Peninsula / 1094.7 ± 3.1 Great Lakes Nickel / 1099.6 ± 1.2 1109.3±6.3 1109.2 ± 4.2 Heaman et al. (2007) Heaman et al. (2007) Heaman et al. (2007) Heaman et al. (2007) Heaman et al. (2007) Hollings et al. (2010) Hollings et al. (2010) Agate Point / 1105±2 Davis and Green(1997) Black Bay Peninsula /1107.4 +4/-2 Davis and Sutcliffe (1985) St. Ignace Island / 1107.2 ± 2.4 Smyk et al.(2006) Coldwell Complex / 1108 ± 1 Mt. McKay / 1114.7 ± 1.1 Nipigon Embayment / 1114-1110 Nipigon Embayment / 1124-1113 Lake Nipigon / 1141 ± 20 McKellar Harbour / 1145 +15, -10 Heaman and Machado (1987) Heaman et al. (2007) Heaman et al. (2007) Heaman et al. (2007) Heaman et al. (2007) Queen et al. (1996) a geochemical difference between the sills to the north and south of the City of Thunder Bay (Hart, 2003; Hart et al., 2005). Hollings et al. (2007a) proposed that the term Logan Igneous Suite, which would fall within the Midcontinent Rift Intrusive Supersuite (Miller et al., 2002), should be applied to all the diabase sills in the area north of Lake Superior, with subdivision into the informal terms, Nipigon sills for the sills north of Thunder Bay, and Logan sills to the south. Logan sills generally consist of fine- to coarsegrained, ophitic to intergranular, quartz tholeiitic diabase/gabbro (Smith and Sutcliffe, 1987; Geul, 1970, 1973). Coarse-grained, intergranular gabbro, locally rich in granophyric mesostasis, is common in the interior of the thicker sills. The upper sections of the diabase sills are commonly plagioclase-porphyritic, containing as much as 60% phenocrysts. Chilled margin and bulk compositions are iron-rich, quartz-tholeiitic basalt. Compositional and textural variation in sills has been noted by North (2000) and Beskar (2001) in Blake Township, where vari-textured, “taxitic” gabbro has been described. Logan sills are recognized by their reversed magnetic polarity and generally take the form of columnarjointed, thick sheets and sills whose geometry is strongly controlled by the subhorizontal bedding of the country rock. They form conspicuous erosional remnants that create mesa and cuesta topography. From the international boundary area to Thunder Bay as many as six diabase sheets were emplaced nearly conformably into Animikie sedimentary rocks (Weiblen et al., 1972; Smith and Sutcliffe, 1987, 1989). Diamond drilling has also shown that stacked sills exist in the subsurface. For example, Dumont Nickel Inc. reported intersecting 14 gabbroic sills in a 705 m deep drill hole in central Pardee Township (Assessment Files, Thunder Bay South Resident Geologist’s District, Thunder Bay). North of the border, Smith and Sutcliffe (1989) reported sills up to 50 m thick, whereas in Minnesota, Jones (1984) studied four sills ranging from 50 to 160 m in thickness. Rare exposures of feeder dykes to sills and preserved sill terminations have been noted. The textural stratigraphy, which varies from a lower, ophitic zone to an upper pegmatitic zone, indicates that in most cases, the sills cooled as single units, probably over a period of 200 to 500 years (Smith and Sutcliffe, 1989). Chilled contact zones are developed against sedimentary country rocks; sedimentary xenoliths are rare. - 192 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Nipigon sills are commonly massive, medium- to coarse-grained, olivine-tholeiitic diabase/gabbros (Sutcliffe, 1989; Hart and MacDonald, 2007). Nipigon sills are dominantly present throughout the Lake Nipigon area but have also been recognized in the Thunder Bay area (Hollings et al., 2007b). Nipigon sills are characterized by a massive, subophitic to ophitic, plagioclase and clinopyroxene texture with trace to 3% olivine and 1-2% modal magnetite (Hart et al., 2005). Nipigon sills display a reverse magnetic polarity and generally form thick, columnar jointed sheets. Sills commonly intrude Sibley group sedimentary rocks but also can be found in contact with Archean rocks of the Quetico subprovince and the Marmion and Winnipeg River terranes. Sills often intrude earlier emplaced ultramafic units of the Nipigon Embayment as well as the 1129.0 ± 2.3 Ma Pillar lake Volcanic rocks and the 1129.0 ± 2.3 Ma English Bay Complex (Heaman et al., 2007) providing evidence for their emplacement during the second main phase of magmatism (Hart and MacDonald, 2007). The shallow dipping Nipigon diabase sills are estimated to cover an area in excess of 20 000 km2 (Sutcliffe 1991) ranging in thickness from <5m to >180m (Hart and Macdonald, 2007). Pigeon River dykes trend east-northeast to northeast and dip steeply to the southeast (Geul, 1970, 1973; Smith and Sutcliffe, 1989). Displacement and warping of the Rove Formation is evident along many of the dykes.Composite intrusions are noted in several dykes. Dyke widths average between 50 and 70 m, but may be as much as 150 m across in Ontario (Smith and Sutcliffe, 1987) and 500 m in Minnesota (Green et al., 1987). Forming northeast-trending, linear ridges, dykes can be traced semi-continuously for 15 km along strike. As noted by many workers, some dykes clearly crosscut Logan sills. However, Geul (1973) and Smith and Sutcliffe (1987) noted that others display somewhat ambiguous crosscutting relationships. In these latter cases, dykes may appear to merge with sills, suggesting that they were contemporaneous or that sills impeded the upward migration of the dykes. The presence of multiple sets of horizontal columnar jointing suggests the development of multiple or composite dykes. The dykes typically consist of ophitic diabase that may be plagioclase-porphyritic. A typical, nonporphyritic olivine diabase consists of approximately 60% plagioclase (zoned labradorite; An55-70), 20% augite + hypersthene, up to 15% olivine and up to 5% magnetite, ilmeno-magnetite and sulphides (Geul 1970, 1973). Average whole rock compositions of Pigeon River dikes are moderately evolved (Mg# = 52) olivine tholeiitic basalt. The Riverdale sill was first characterized by Hollings et al. (2007b) as being geochemically and petrographically distinct from the surrounding Logan sills and the Nipigon sills to the north. Puchalski (2010) described the geochemical and petrographical characteristics. The Riverdale sill lies within the southern city limits of Thunder Bay, close to the northern boundary of the Logan basin. The unit displays a sill morphology exposed over an area approximately 6 km long and 2 km wide with true thickness unknown as the upper contact is not exposed (Puchalski, 2010). Exposures within a quarry on West Riverdale Road (Stop 4) display a thickness of 10 m where detailed sampling and subsequent geochemistry and petrography analyses were completed. Paleomagnetic work performed by Hollings et al. (2010) confirmed the unit to display a reverse polarity. The following is summarized from Puchalski (2010). Rocks comprising the Riverdale sill are dominantly gabbronorites with lesser olivine gabbro present towards the centre of the intrusion. The gabbronorites are generally fine-grained and display no cumulate textures within any of the samples. Plagioclase typically occurs as subhedral laths with euhedral orthopyroxene and lesser clinopyroxene and olivine. Minor alteration is present in most samples as chlorite replacing pyroxene and sericite replacing plagioclase. The olivine gabbro samples lying toward the centre of the unit are petrographically similar to the gabbronorite samples except for a higher modal percentage of anhedral to euhedral olivine. Olivine grains are typically finegrained but may range to medium grained.Variable amounts of serpentine is found replacing olivine. A unit of mafic rock in Devon Township, south of Thunder Bay, was mapped by Tanton (1931) and was termed Rove Formation Basalts, but was subsequently mapped as a Logan diabase sill (Geul, 1970). Cundari (2010) described the detailed geological, petrographical and geochemical characteristics of the unit. The unit is exposed on a plateau 7 km long and 0.8 to 1.0 km wide. The unit is 4 to 6 m thick and is in apparent conformable contact with the underlying shales of the Paleoproterozoic Rove Formation, where a pronounced chilled margin consists of variolitic material up to 20 cm thick. The flow-top also exhibits a variolitic texture ~15 cm thick.The presence of ropy flow top and amygdules as well as quench textures, support a volcanic origin. - 193 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Major element chemistry reveals a tholeiitic, intermediate composition with samples plotting in the basaltic andesite to andesite fields as well as in the basaltic trachy-andesite to trachy-andesite fields on a TAS diagram. The unit typically has an intergranular texture consisting of randomly oriented plagioclase laths with interstitial chlorite, an alteration product of primary augite. Most samples contain minor serpentine (after olivine), opaque minerals, secondary quartz, oxides, pyrite and calcite. Amygdules are often present and are infilled with some combination of calcite, quartz, chlorite and pyrite. Lower flow contacts and flow-tops are typically glassy with abundant spherulites that sometimes coalesce into bands. This unit is definitively related to Midcontinent rift magmatism and is now referred to as the Devon Volcanics. Contact Metamorphism There is a remarkable range in the reported intensity and nature of contact metamorphic effects in Rove sedimentary rocks at diabase dyke and sill contacts, owing mainly to the subjectivity of the mapper and the exposures in question. Geul (1973) noted that sedimentary hornfelsic rocks are restricted to a narrow zone of baking between 2 to 10 cm wide at diabase dyke contacts. Metamorphosed siltstone displays two stages: first: slight recrystallization of biotite aggregates in an incipient hornfelsic texture; and second, a more complete recrystallization of biotite, surrounded by pale sericitic aggregates, set in a quartzo-feldspathic matrix. Conversely, Franklin (1970) suggested that contact effects existed up to 8 m from sill contacts and possibly up to 23 m.They were manifested as microporphyroblasts of mica and chlorite (a.k.a. “spotted alteration”), graphite destruction and the conversion of pyrite to pyrrhotite. Geul (1973) noted that minute particles (< 0.01 mm) of oxide and sulphide minerals are locally abundant in the contact zone. Rove Formation sedimentary rocks may be deformed along dyke contacts. As noted by Geul (1973) beds appear to dip toward the dykes or are “up-dragged” along dyke contacts. Deformed and fractured sedimentary rocks have been noted near sill terminations. Narrow, parallel tension gashes filled with quartzo-feldspathic leucosome/neosome occur in metatectic, deformed siltstones. Assimilation of country rock Pink granophyric features have been welldocumented in Logan sills to the west of Thunder Bay which are attributed to in-situ assimilation of granitic material (Blackadar, 1956). This theory was reevaluated by Magnus (2010) who studied assimilation features in the Navilus and Terry Fox sills. Although these outcrops will not be visited on this field trip, they provide a sound explanation for some of the assimilation and sill-top features observed throughout the trip (i.e., Stops 2 and 5). Two zones were reported to consistently appear around xenoliths present in the Navilus sill: a zone of quartzo-feldpathic intergrowths, or “granophyre”, adjacent to xenoliths, followed by a zone of pyroxene grains present on the interface between normal diabase magma and the granophyric zone (Magnus, 2010). Late-stage granophyric formations are also found interstitially between plagioclase and pyroxene grains with iron-oxides, likely representing immisicibility of a late-stage silica- and iron-rich liquid exsolved from the magma (Magnus, 2010). This premature exsolution of silica and iron-rich liquids from the magma are attributed to the introduction of silicate-rich xenoliths to the already silica-saturated, quartz-tholeiitic magma (Magnus, 2010). Magnus (2010) further concluded that geochemical variation between diabase with inclusions and normal diabase was caused by late-stage fractionation during crystallization as noted by variable depletion in high field-strength elements (HFSEs) with LIL enrichment attributed to in-situ assimilation. Discussion of Geochemistry As part of the Lake Nipigon Region Geoscience Initiative, whole rock analyses were performed on a number of Midcontinent Rift-related intrusions south of Thunder Bay as well as the Lake Nipigon region. Subsequent research conducted at Lakehead University provided additional sampling of laterrecognized units of interest (i.e., the Riverdale sill and the Devon Volcanics). Data from these studies, as well as data from previous mapping endeavors conducted by the Ontario Geological Survey, have been compiled in a database totaling 2400 spatially referenced points with whole-rock geochemical analyses. This database is currently being reevaluated by Cundari (in progress) to detect variability within units as well as put many of the obscure units in the context of the MCR. Current discrimination of intrusive units associated with the MCR is through trace element patterns (e.g., primitive mantle-normalized spider plots, as well as measures of heavy and light rare-earth - 194 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 present within the city of Thunder Bay and north around Lake Nipigon. This geographic distribution may be a result of either tapping different source regions at depth or the presence of a major compositional boundary (Hollings et al., 2007a). Geochemical and petrographic data show no evidence for fractionation within the Riverdale sill, with only slight variation present towards the lower margin and the centre of the sill. Samples within 1 m of the contact display negative niobium anomalies interpreted to be a result of crustal contamination at depth, with samples towards the centre characterized as olivine gabbros. This suggests that the sill is composed of two pulses of magma, with the more-contaminated first pulse intruded by a less-contaminated second pulse (represented by the olivine gabbro pushing the contaminated magma towards the outer margins of the sill). The Riverdale sill is geochemically distinct based on its heavy rare-earth element abundances when compared to the surrounding Logan sills. Displaying a high Gd/Ybn ratio denotes heavy rare earth element fractionation indicative of a deep-seated mantle melt sourced from below the garnet-spinel stability field (>100 km). As the Riverdale sill displays a Gd/Ybn ratio of 3.0 – 3.5, it was likely sourced from this region suggesting it is genetically related to the ultramafic intrusions of the Nipigon Embayment (Puchalski, 2010). Figure 2: Discrimination diagrams for mafic and ultramafic intrusions near Thunder Bay. Data are from Hollings et al. (2007a) and Puchalski (2010).Normalizing values from Sun and McDonough (1989). element abundances displayed by the plot of La/Smn (LREE) and Gd/Ybn (HREE; Fig. 2). Major element abundances, i.e. Mg# vs. TiO2, also show distinct populations between units (Fig. 2; Table 2). Nipigon and Logan sills show broadly similar morphological characteristics but can be distinguished from each other based on TiO2 abundances. The Logan sills present to the south of Thunder Bay within the Logan Basin, (e.g., Mt. McKay, Stop 5) display higher TiO2 content than the Nipigon sills (Stops 1 and 2) When the data from the dyke swarms are compared to the regional data set generated for the sills and intrusions of the Lake Nipigon embayment Hollings et al. (2007a) showed that Pigeon River dyke swarm closely resembles the sills of the Nipigon suite than the ultramafic intrusions or the Logan sills. In contrast, the Mt. Mollie swarm appears to be transitional between Nipigon sills and Inspiration sills. Additional isotopic and geochronological studies will be required in order to further investigate the relationships between these MCR-related intrusions. Rare-earth element geochemistry of the Devon Volcanics show the unit to be relatively enriched in both HREEs and LREEs, similar to the ultramafic sills of the Nipigon Embayment as well as the Riverdale Sill (Hollings et al., 2007a, 2010). A primitive mantlenormalized REE plot shows that the volcanic unit is characteristic of an Ocean-Island Basalt, but with a negative niobium anomaly, most likely the result of lower crustal contamination. This evidence is further supported by an εNd(t=1100Ma) of -3.48, which also suggests contamination of the unit by a lower crustal source. - 195 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Table 2: Geochemical analysis of select intrusive rocks from the Thunder Bay area. Data from: A) Hart and Magyarosi (2004), B) Hollings et al. (2011). 1 3 4 5 6 7 8 Nipigon sill Hornfelsed Rove Riverdale sill Riverdale sill Pigeon River dykes Pigeon River dykes Upper sill Silver Harbor quarry Squaw Bay road quarry Olivine gabbro Gabbronorite Whiskeyjack Whiskeyjack point N point S 03TRH201 03TRH202 DB-167 DB-12 RP-8QB RP-5Q DB-10 DB-11 Intrusive unit Description Sample 2 Logan sills Lower sill (upper contact) Source A A B B B B B B SiO2 48.36 48.86 48.32 76.56 46.4 48.64 53.14 50.62 TiO2 3.65 3.59 0.91 0.45 2.04 2.94 1.23 1.22 A12O3 13.74 14.46 15.45 10.48 8.16 9.89 14.62 14.31 FeOt 14.62 15.00 11.73 4.88 12.96 13.94 10.90 11.32 MnO 0.21 0.18 0.19 0.02 0.16 0.2 0.18 0.19 MgO 4.4 4.05 7.88 1.48 12.53 7.14 5.49 6.58 CaO 7.12 7.45 10.85 0.2 9.24 8.51 9.75 8.65 Na2O 2.81 3.43 2.11 2.11 1.51 3.09 2.64 2.85 K2 O 1.35 1.27 0.4 1.52 0.41 0.8 0.91 1.57 P2 O5 0.37 0.42 0.09 0.08 0.21 0.24 1.15 0.12 Volatiles 1.44 0.37 0.57 2.78 3.87 3.29 1.15 1.74 Total 99.7 100.76 99.81 101.09 98.93 100.24 101.38 100.45 mg# 23.13 21.26 40.18 23.28 49.16 33.87 33.50 36.76 Cr 30 41 112 190 >1300 266 68 11 Co 34 36 62 12 93 67 45 47 Ni 81 88 131 35 406 92 95 95 (ppm) The trace element characteristics of the volcanic unit suggest an origin in Keweenawan time as they are geochemically similar to units of the MCR (Hollings et al., 2007a) rather than Paleoproterozoic volcanic units of the Gunflint Formation. Field trip stops Stop 1: Silver Harbour Quarry UTM coordinates: NAD83; 16U 0354388E / 5374970N Location: Quarry adjacent to road cut at Silver Harbour boat launch. Silver Harbour Road off Lakeshore Drive. Description: The first stop on the trip offers an excellent exposure of a Nipigon sill. Material was quarried from this locality to create many of the breakwalls along this portion of the bay. Two localities are of interest here: the first being the actual quarry which exposes a typical Nipigon diabase sill; the second is a road cut at the northern end of the quarry which displays some enigmatic, late-stage features. The road cut can be reached by a trail along the northwestern side of the clearing. The features observed in Figures 4 and 5 appear to represent magma injected into the still-crystallizing sill. They are typically finer-grained than the surrounding medium- to coarse-grained, subophitic diabase. They appear as blobby masses or dykes with undulating contacts appearing to propagate up through the sill from depth. The amorphous form suggests that the surrounding sill material was not fully crystallized when these features were emplaced but must have been partially solid, as distinct contacts are preserved. These features may represent an episode of immiscible magma interaction. - 196 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Field trip stops - Coordinates and Descriptions STOP NAME STOP NUMBER Description Silver Harbour Quarry sill 1 Quarry and adjacent road cut at Silver Harbour boat : launch The Bluffs 2 Flat-lying outcrop atop The Bluffs lookout; off Arundel St. 337266 E 5371048 N Baked Rove outcrop Optional Sandstone quarry 333765 E 5354546 N Pigeon River dykes (Chippewa park/Whiskeyjack point) 3 Two parallel, E-trending dykes cross-cutting Rove Formation sedimentary rocks; Whiskeyjack point, Lake Superior 336798 E 5355397 N Riverdale sill (Robin’s Donuts) Optional Easternmost exposure of Riverdale sill in Robin’s donuts parking lot; Highway 61 326487 E 5355715 N Riverdale sill Quarry 4 Riverdale sill exposed in contact with Rove sedimentary rocks; Candy Mountain Dr. 322410 E 5355212 N Mount McKay 5 Mount McKay lookout on top of lower sill; hike to upper sill via trail is optional 331126 E 5357384 N - 197 - NORTHING EASTING (NAD 83) 354388 E 5374970 N �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 3: Satellite image of the Thunder Bay area showing field trip stops. Figure 4: Photo of outcrop at Silver Harbour road cut showing immiscibility features. South side of road - 198 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 5: Photos of outcrop at Silver Harbour road cut showing immiscibility features. Left - dyke with offset on north side of road, Right - blob on south side of road. Samples SP-RC-016 and SP-RC-018 from the younger intrusion have higher silica contents as well as larger loss on ignition values (Table 3) suggesting that the late-stage material was contaminated in silica and hydrous minerals possibly from the surrounding sedimentary rocks of the Gunflint Formation. This is further supported by the elevated light rare-earth elements abundances of these samples compared to the host sill (Fig. 6). Alternatively, the later-stage material may represent a slightly more fractionated product of a typical Nipigon sill melt. This is supported by the pronounced negative europium and titanium anomalies (Fig. 6) representing plagioclase and magnetite fractionation, respectively. The most plausible scenario is that the material injected into the sill underwent both processes whereby the material has residency time in a shallow crustal magma chamber. Here it was able to fractionate plagioclase and magnetite as well as leach hydrous material including silica from the Gunflint Formation sedimentary rocks, resulting in the rareearth element enrichment, elevated silica content and loss on ignition. Stop 2: The Bluffs lookout UTM coordinates: NAD83; 16U 0337266E / 5371048N Location: The Bluffs lookout. Unmarked road off Arundel St. west of Lyon Blvd. W. Description: This stop provides a good vantage point for the city of Thunder Bay and Lake Superior, looking southeast. The pronounced topographic high Table 3: Major element chemistry for samples at Silver Harbour road cut. Data from Cundari (in progress) Sample SP-RC-016 SP-RC-017 SP-RC-018 Late-stage bleb Nipigon sill Late-stage dyke SiO2 TiO2 64.75 0.51 Al2O3 11.97 FeO 4.98 MnO 0.09 MgO 2.66 CaO 3.59 Na20 3.5 P205 0.11 LOI 5.6 Total 100.28 48.86 0.71 14.91 11.03 0.18 8.02 9.07 1.61 0.06 2.67 100.43 58.03 0.5 10.9 10.92 0.15 4.17 1.54 0.81 0.08 8.83 99.52 - 199 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 6: Primitive mantle-normalized trace element plots for samples from Silver Harbour quarry and road cut. Normalizing values from Sun and McDonough (1989). Data from Cundari (in progress). that forms the lookout is situated on a diabase sill of Nipigon affinity. The flat-lying outcrop to the east of the lookout/parking lot exposes the top a sill, commonly characterized by feldspar-phyric patches (Fig. 7). The likely source of the feldspar-phyric patches is earliercrystallized material or autoliths buoyantly rising to the top of the melt and crystallizing in situ. Stop (OPTIONAL): Hornfelsed Rove Formation Sandstone UTM coordinates: NAD83; 16U 0333765E / 5354546N Location: Clearing off Squaw Bay road. of hornfelsed sedimentary rocks resulting from Midcontinent Rift-related magmatism. In addition, a raised beach related to a higher stand (Minong or post-Minong stage?) of present-day Lake Superior is present towards the northeastern corner of the quarry (cf. Burwasser, 1977). A massive, ~4 m thick bed of sandstone displays a characteristic hornfelsed texture. From afar, this looks like a diabase sill but upon closer inspection can be identified as a thick sandstone bed that may have been metamorphosed by an overlying (and possibly underlying) intrusion (Fig. 8). The SiO2 content of this unit is 76.56 wt %. Description: This stop in Fort William First Nation provides excellent exposure of Rove Formation sandstone/wacke and one of the best examples Figure 7: Feldsapr-phyric patches present towards the top of a Nipigon sill at The Bluffs; Stop 2. - 200 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 8: Primitive mantle normalized trace element plots for hornfelsed Rove Formation sandstone as well as Gunflint formation sedimentary rocks. Data from Hollings et al. (2011). Normalizing values from Sun and McDonough (1989) Stop 3: Chippewa Park/Whiskeyjack Point UTM coordinates: NAD83; 16U 0336798E / 5355397N Location: Shoreline outcrops off Sandy Beach Rd. at Whiskeyjack Point Description: Along the shoreline of Lake Superior at Whiskeyjack Point, two east-northeast trending Pigeon River-style dykes crosscut Rove Formation shale. This stop also provides a panoramic view of Thunder Bay including the Sleeping Giant, which is capped by a Logan diabase sill (Carl, 2011), as is Pie Island and the mesas to the south along the shoreline. These dykes display a medium-grained, subophiticophitic texture. Jointing, measured here perpendicular to dyke trend, can be used as a proxy to define approximate trends in other dykes where contacts are not observed. Logan Basin. This contradicts the geochronology data as Pigeon River dykes have been dated at 1141 ± 20 Ma for the Rita Bolduc dyke (UTM 310563E 53247021N; NAD83) and 1078 ± 3 Ma (UTM 296694E 5324134N; NAD83) for the Arrow River dyke whereas Logan sills are dated at 1114.7 ± 1.1 Ma (Heaman et al., 2007). Further geochronological and paleomagnetic work is ongoing to resolve these issues. Stop (Optional): Riverdale sill at Robin’s Donuts, Highway 61 UTM coordinates: NAD83; 16U 0326487E / 5355715N Location: Outcrop in Robin’s Donuts parking lot, Highway 61. Geochemically, the Pigeon River dykes display broadly similar trace element patterns to those of the Nipigon sills suggesting they are genetically related, possibly representing the feeders to the Nipigon sills. However, the wide considerable distance between the two suggests that this is unlikely. An alternative explanation has been presented by Hollings et al (2010) who have suggested that the Pigeon River dykes tapped the same long-lived mantle reservoir as the Nipigon sills that were present throughout the Midcontinent rifting. Based on field relationships, Pigeon River dykes post-date Logan sills as several cross-cutting relationships have been documented throughout the Figure 9: Photo of a Pigeon River dyke outcrop at Whiskeyjack point; Stop 11-3. - 201 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 10: Primitive mantle-normalized trace element plots for Pigeon River dyes at Whiskeyjack Point, with Nipigon sill sample for comparison. Data from Hollings et al. (2011). Normalizing values from Sun and McDonough (1989) Description: This exposure of the Riverdale sill represents the easternmost expression of the unit as described by Puchalski (2010). This low-lying, moderately weathered outcrop of the Riverdale sill gabbronorite was the “discovery outcrop” for this unit (Smyk and Hollings, 2007). Stop 4: Riverdale sill in Quarry UTM coordinates: NAD83; 16U 0322410E / 5355212N Location: Quarry at east end of Candy Mountain Dr. Description: Sampling by Smyk and Hollings (2007) identified this as a Riverdale gabbronorite sill in Rove Formation shale, wacke and minor tuffaceous units (Fig. 11). Subsequent detailed petrographic and Figure 11: Photo of Riverdale sill quarry; Stop 4. - 202 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 12: Height vs. elemental abundance for Riverdale sill quarry samples. TiO2, SiO2 and MgO are in weight percent and Cr and Ni are ppm (Puchalski, 2010). geochemical analyses were carried out by Puchalski (2010). Samples were taken through stratigraphy at the quarry to investigate composition and contamination, as well as to test whether the sill had undergone differentiation. The following section provides a concise summary of those findings. The mafic intrusive rocks within the quarry are dominantly classified as gabbronorites with olivine gabbro present towards the centre of the sill. The gabbronorites are generally fine-grained with plagioclase occurring as subhedral laths. Orthopyroxene is present in greater abundance than clinopyroxene, occurring as anhedral to subhedral crystals. Varying degrees of alteration are manifested as sericitization of plagioclase and chloritization of pyroxene. The olivine gabbro is texturally similar to the gabbronorite, albeit with a higher modal percentage of fine-grained, anhedral to euhedral olivine. In most samples, olivine is replaced by serpentine, producing secondary quartz and calcite, as well as minor magnetite. Alteration is significantly greater in the narrow chilled margin at the contact. Pyrite occurs throughout the unit; minor chalcopyrite has also been noted. Sampling for whole rock major and trace element geochemistry was undertaken by Puchalski (2010) throughout the 10 m exposure at 1m intervals. Olivine gabbro samples display broadly similar trace element characteristics to those of the gabbronorite samples. Differences lie within the major element abundances; olivine gabbros are lower in SiO2 and elevated in MgO, Cr, Co, and Ni compared to the gabbronorite samples (Table 2). The sill does not display any evidence for differentiation as shown by the erratic trends of MgO, SiO2, TiO2, Cr and Ni through stratigraphy (Fig. 12). An olivine gabbro in the centre of the sill displays elevated MgO, Cr, and Ni values as well as a lower abundance of silica when compared to the surrounding samples. This is likely the result of a slightly more primitive magma intruding the centre of the sill. The lack of chilled margins between the olivine gabbro and the gabbronorite suggest that the sill had not fully crystallized when the second pulse intruded. A sample of a 60 cm wide north-trending diabase dyke which intrudes the sill near the western end of the quarry is geochemically comparable to the surrounding Logan sills. Contamination by the Rove shale is evident in samples taken from close to the contact (<1 m above the contact). These samples display higher SiO2 values as well as lower Nb/Nb* and Gd/Ybn values than the rest of the unit (Fig. 13). As the Rove shale displays significantly lower Nb/Nb* and Gd/Ybn values (Fig. 8) than that of the surrounding gabbronorite. The Rove shale is the likely source of this contamination signature. Two different pulses of magma are recognized within the Riverdale sill based on contamination signatures denoted by negative niobium anomalies. The lesscontaminated samples are typically found towards the core of the intrusion with rocks above and below displaying a greater degree of contamination (Fig. 14). Samples taken within 60 cm of a shale xenolith do not display a distinct negative niobium anomaly. This shows that the source of contamination responsible for the negative niobium anomaly is not the Rove shale but is likely a crustal component from depth. εNd(T=1100Ma) values of -1.6 to -1.9 for the Riverdale Sill are consistent with this model (Smyk et al., 2009). - 203 - Although the Riverdale sill is located near Logan �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 13: Height vs. elemental abundance (ppm) for Riverdale sill quarry samples. SiO2 is in weight percent (Puchalski, 2010). sills, it remains petrographically and geochemically distinct from them. Geochemical discrimination based on La/Smn (LREE) vs. Gd/Ybn (HREE) shows characteristics similar to those for the ultramafic units of the Nipigon Embayment (e.g., Disraeli, Kitto, Hele and Seagull), closely resembling the mafic to ultramafic Jackfish sill (Fig. 2). The Jackfish sill is finer-grained and displays a higher modal abundance of olivine than the Nipigon sills surrounding it (Hollings et al., 2007a). This suggests that the Riverdale sill may be genetically related to the ultramafic and mafic to ultramafic units of the Nipigon Embayment. This is consistent with the reversed polarity of the Riverdale sill (Hollings et al., 2010). Figure 14: Primitive mantle-normalized trace element plots for successive samples through stratigraphy at the Riverdale sill quarry (Puchalski 2010). Normalizing values from Sun and McDonough (1989) - 204 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 15: Primitive mantle normalized trace element plots for upper and lower sills at Mount McKay with Nipigon sill sample for comparison. Data from Hart and Magyarosi (2004) and Hollings et al. (2011). Normalizing values from Sun and McDonough (1989). Stop 5: Mount McKay outcrop (Fig. 16). UTM coordinates: NAD83; 16U 0331126E / 5357384N Location: Mount McKay scenic lookout, off Mission Rd. Description: This stop provides an exceptional view of the city of Thunder Bay as well as a great example of a stacked Logan sill sequence. The summit of Mount McKay at 482 m ASL is approximately 300 m higher than Lake Superior. Great exposures of Logan sills are abundant throughout the Logan basin south of Thunder Bay. From drill core, it has been reported that many sills are present at depth, as many at 14 as noted by Dumont Nickel Inc. who reported intersecting 14 gabbroic sills in a 705 m deep drill hole in central Pardee Township (Assessment Files, Thunder Bay South Resident Geologist’s District, Thunder Bay). It is inferred that many of the Logan sills are underlain by additional sills but exposures of this are rare. Mount McKay provides the best example of a stacked sill sequence in outcrop. The geochemistry of samples from the two sills is presented in Figure 15. The stop is centered on the lookout area, which represents the top of the lower sill at approximately 337 m ASL. Outcrop of the upper, ~60 m thick sill and adjacent, hornfelsed Rove wacke can be accessed by way of a hiking trail. If time permits, those interested in completing this hike to the upper sill may do so with extreme care. Feldspar-phyric patches similar to those observed at Stop 2 are present in an exposure of the fine-grained, upper, chilled contact of the lower sill found along a short trail to the west of the clearing next to a religious shrine. Polygonal jointing, characteristic of chilled contact zones, is well-developed in this References Amurawaiye, O. 2001. The Paleoproterozoic Rove Formation of northwestern Ontario: A turbidite-dominated shelf sequence; unpublished H.B.Sc thesis, Lakehead University, Thunder Bay, Ontario, 44p. Beskar, S. 2001. The Blake gabbro: A taxitic-textured gabbro sill south of Thunder Bay, Ontario; 47th Institute on Lake Superior Geology, Annual Meeting, Madison, Wisconsin, May 9-12, 2001, Proceedings Volume 47, Part 1, p.1 Blackadar, R.G. (1956). Differentiation and Assimilation in the Logan Sills, Lake Superior District, Ontario. American Journal of Science, vol. 254, p. 623-645. Burwasser, G.J. 1977: Quaternary geology of the City of Thunder Bay and vicinity, District of Thunder Figure 16: Photo showing polygonal jointing at the upper chill margin of the lower sill at Mount Mckay; Stop 5. - 205 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Bay; Ontario Geological Survey, Report 164, 70p. Accompanied by Map 2372, scale 1:50 000. Carl, C. 2011. Geochemistry and petrology of Midcontinent Rift-related intrusive rocks of the Sibley Peninsula, Ontario. Unpublished honours thesis. Lakehead University. Cundari, R. 2010. The geology and geochemistry of the Devon Volcanics, South of Thunder Bay, Ontario. Unpublished Lakehead University Honours Thesis. Davis, D.W. and Green, J.C. 1997. Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution; Canadian Journal of Earth Sciences, v.34, p.476-488. Davis, D.W. and Sutcliffe, R.H. 1985. U-Pb ages from the Nipigon Plate and northern Lake Superior; Bulletin of the Geological Society of America, v. 96, p. 15721579. Fralick, P.W., Kissin, S.A. and Davis, D.W. 1998. The age and provenance of the Gunflint lapilli tuff; 44th annual meeting, Institute on Lake Superior Geology, Proceedings Volume 44, Program and abstracts, p.6668. Franklin, J.M. 1970. Metallogeny of the Proterozoic rocks of the Thunder Bay District, Ontario; unpublished Ph.D. thesis, University of Western Ontario, London, 317p. Franklin, J.M. and Kustra, C.R. 1972. The Proterozoic rocks of the Lake Superior area, northwestern Ontario; in Field Excursion C34: The Precambrian rocks of the Atikokan-Thunder Bay-Marathon area, 24th International Geological Congress, Guidebook, p.2046. Geul, J.J.C. 1970. Geology of Devon and Pardee Townships and the Stuart Location; Ontario Department of Mines, Geological Report 87, 52 p. Geul, J.J.C 1973. Geology of Crooks Township, Jarvis and Prince Locations, and Offshore Islands, District of Thunder Bay; Ontario Department of Mines, Geological Report 102, 46 p. Green, J.C., Bornhorst, T.J., Chandler, V.W. et al. 1987. Keweenawan dikes of the Lake Superior region: evidence for evolution of the middle Proterozoic Midcontinent Rift of North America; in Mafic dike swarms, H.C. halls and W.F. Fahrig, eds., Geological Association of Canada, Special Paper 34, p.289-302. Hart, T.R. 2003. Keweenawan mafic and ultramafic intrusive rocks of the Lake Nipigon and Crystal Lake areas, northwestern Ontario; 49th Institute on Lake Superior Geology, Proceedings volume 49, Part 1, Programs and abstracts, p.21-22. Hart, T.R., MacDonald, C.A., Hollings, P., and Richardson, A., 2005. Proterozoic intrusive rocks of the Nipigon Embayment and Midcontinent Rift. In, T.O. Tormanen and T.T Alapieti, 10th International platinum Symposium Extended Abstracts, Geology Survey of Finland, 365-368. Hart, T.R., and MacDonald, C.A., 2007. Proterozoic and Archean Geology of the Nipigon Embayement: implications for emplacement of the Mesoproterozoic Nipigon diabase sills and mafic to ultramafic intrusions. Canadian Journal of Earth Sciences 44: 1021-1040. Hart, T.R. and Magyarosi, Z. 2004. Northern Black Sturgeon River–Disraeli Lake area, Nipigon Embayment, northwestern Ontario: lithogeochemical, assay and compilation data; Ontario Geological Survey, Miscellaneous Release—Data 133. Heaman, L.M. and Easton, R.M. 2006. Preliminary U/ Pb geochronology results: Lake Nipigon Region Geoscience Initiative. Ontario Geological Survey, Miscellaneous Release-Data 191, 79p. Heaman, L.M., Easton, M., Hart, T.R., Hollings, P., Macdonald, C.A. and Smyk, M., 2007. Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon region, Ontario. Canadian Journal of earth Sciences 44: 1055-1086. Heaman, L.M. and Machado, N. 1992. Timing and origin of the Midcontinent Rift alkaline magmatism, North America: Evidence from the Coldwell complex; Contributions to Mineralogy and Petrology, v. 110, p. 289-303. Hollings, P., Hart, T., Richardson, A. And MacDonald, C.A., 2007a. Geochemistry of the mid-Proterozoic intrusive rocks of the Nipigon Embayement, northwestern Ontario. Canadian Journal of Earth Sciences v. 44: 1087-1110. Hollings, P.N., Smyk, M.C., Hart, T., 2007b. Geochemistry of Midcontinent Rift-related mafic dikes and sills near Thunder Bay: New insights into geographic distribution and the geochemical affinities of Nipigon and Logan sills and Pigeon River and other dikes. 53rd Institute on Lake Superior Geology, Annual Meeting,Proceedings volume 53, Part 1. Lutsen, Minnesota, May 2007, pp. 40–41. Hollings, P., Smyk, M., Heaman, L.M., and Halls, H., 2010. The geochemistry, geochronology, and paleomagnetism of dikes and sills associated with the Mesoproterozoic Midcontinent Rift near Thunder Bay, Ontario, Canada. Precambrian Research. Precambrian Research, v. 183, iss. 3, p.553-571. Hollings, P., Cundari, R., Pulchalski, R. and Smyk, M.C. 2011. Geochemistry of Midcontinent Rift- related mafic intrusions, Thunder Bay area; Ontario Geological Survey, Miscellaneous Release—Data 261 – Revised. Jones, N.W. 1984. Petrology of some Logan diabase sills, Cook County, Minnesota; Minnesota Geological Survey, Report of Investigations 29, 40p. Lucente, M.E. and Morey, G.B., 1983. Stratigraphy and sedimentology of the lower Proterozoic Verginia Formation, northern Minnesota. Minnesota Geological Survey Report of Investigations 28, 28 p. - 206 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Magnus, S. 2010. An investigation of the assimilation hypothesis in the Navilus Sill, Thunder Bay, Ontario. Unpublished Lakehead University Honours Thesis. intrusive rocks of the Thunder Bay area: in Summary of Field Work 1987, Ontario Geological Survey, Miscellaneous Paper 137, p. 248-255. Maric, M. And Fralick, P.W., 2005. Sedimentology of the Rove and Virginia Formations and their tectonic significance. Institute on Lake Superior Geology, v. 51, p. 41-42. Smith, A.R. and Sutcliffe, R.H. 1989. Precambrian geology of Keweenawan intrusive rocks in the Crystal LakePigeon River area: Ontario Geological Survey, Map P.3139, scale 1:50 000. Miller, J.D. and Smyk, M.C. 1995. Gabbroic intrusions of the International Boundary area; in Field trip guidebook for the geology and ore deposits of the Midcontinent Rift in the Lake Superior region; International Geological Correlation Program Project 336, Minnesota Geological Survey, Guidebook 20, p.171-181. Smyk, M.C., Hollings P. and Heaman, L.M. 2006.Preliminary investigations of the petrology, geochemistry and geochronology of the St. Ignace Island Complex, Midcontinent Rift, northern Lake Superior, Ontario; Institute on Lake Superior Geology, 52nd Annual Meeting, Sault Ste. Marie, ON, Program with Abstracts, v. 52, 61-62. Miller, J.D., Smyk, M.C., Severson, M.J., Lavigne, M.J. and Middleton, R.S. 2002. PGE occurrences in mafic intrusions around western Lake Superior, USA and Canada; 9th International Platinum Symposium, Field Trip Guidebook, 135p. Smyk, M. and Hollings, P., 2007. Midcontinent rift-related mafic intrusion north of the international border. Proceedings of the Institute on Lake Superior Geology v. 53: 53-80. Morey, G.B. 1973. Stratigraphic framework of middle Proterozoic rocks in Minnesota. In, ed. G.M. Young, Huronian Stratigraphy and Sedimentation. Geological Association of Canada Special Paper 12, p. 211-249. North, J. 2000. Nature and distribution of Logan diabase sills and gabbro channels in the Keweenawan rift near Thunder Bay, Ontario: Brief comparison to Noril’sk; Abstract, 46th Institute on Lake Superior Geology, Annual Meeting, Thunder Bay, Ontario, Proceedings Volume 46, Part 1 (2000). Ojakangas, R.W., Morey, G.B. and Southwick, D.L., 2001. Palaeoproterozoic basin development and sedimentation in the Lake Superior region, North America; Sedimentary Geology, v. 141, p. 319-341. Parker, D.P. (ed.), Middleton, B., Schnieders, B.R., Smyk, M.C. and Scott, J.F. 2001. Intrusions of the Nipigon Basin; Superior PGE 2001, Canadian Institute of Mining and Metallurgy, Geological Society Field Conference, Thunder Bay, September 16-19, 2001, Field Trip Guidebook, 43p. Puchalski, R. 2010. The Petrology and Geochemistry of the Riverdale sill. Unpublished Lakehead University Honours Thesis. Queen, M., Heaman, L.M., Hanes, J.A., Archibald, D.A. and Farrar, E. 1996. 40Ar/39Ar phlogopite and U-Pb perovskite dating of lamprophyre dykes from the eastern Lake Superior region: Evidence for a 1.14 Ga magmatic precursor to Midcontinent Rift volcanism; Canadian Journal of Earth Sciences, v.33, p.958-965. Rosatelli, M.P., 2002. Assessment report on the 2002 lithogeochemical rock sampling program, Pigeon River block. McVicar Minerals. Lrd. BHP Billiton World Exploration Inc., and Falconbridge Limited; Assessment Files, Thunder Bay South District, Thunder Bay, FN 2.24485, 36p. Smyk, M.C., Hollings, P., Heaman, L.M., 2006. Preliminary Investigations of the Petrology, Geochemistry and Geochronology of the St. Ignace Island Complex, Midcontinent Rift, northern Lake Superior, Ontario. Institute on Lake Superior Geology, 52nd Annual Meeting, Sault Ste. Marie, ON, Program with Abstracts, v. 52, pp. 61–62. Stockwell, C.H., McGlynn, J.C., Emslie, R F., Sanford, B.V., Norris, A.W., Donaldson, J.A., Fahrig, W.F. and Currie K L. 1972. Geology of the Canadian Shield, in Geology and Economic Minerals of Canada, edited by R.J.W. Douglas, Geological Survey of Canada, Economic Geology Report 1, 838 p. Sun, S.S., and McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In Magmatism in the ocean basins. Geological Society, Special Publication No.42, 313-345. Sutcliffe, R.H. 1989. Mineral variation in Proterozoic diabase sills and dykes at Lake Nipigon, Ontario; Canadian Mineralogist, v.27, p.67-79. Tanton T.L., 1931. Pigeon River area, Thunder Bay District; Geological Survey of Canada, Sheet 1, Map 354A, scale 1:63360. Tanton T.L., 1936a. Pigeon River area, Thunder Bay District. Geological Survey of Canada, Sheet 1, Map 354A, scale 1:63,360. Tanton T.L., 1936b. Pigeon River area, Thunder Bay District. Geological Survey of Canada, Sheet 2, Map 355A, scale 1:63,360. Weiblen, P.W., Mathez, E.A. and Morey, G.B. 1972. Logan intrusions; in Sims, P.K. and Morey, G.B. eds., Geology of Minnesota: A centennial volume; Minnesota Geological Survey, p.394-410. Smith, A.R. and Sutcliffe, R.H. 1987. Keweenawan - 207 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Field trip 12 - The Musselwhite Gold Deposit John L. Biczok Goldcorp Canada Ltd., Musselwhite Mine, PO Box 7500, Thunder Bay, ON P7B 6S8 Summary History of the Property The Musselwhite gold mine in northwestern Ontario began production in the spring of 1997 with quoted reserves of 1.8 million ounces of gold. Exploration efforts since that time have successfully replaced mined reserves in most years and added substantially to them in several years. By the end of 2011 Musselwhite had produced 3.34 million ounces with remaining proven and probable reserves of 2.28 million ounces and a measured+indicated resource of 146,000 ounces and an inferred resource of 917,000 ounces. Recorded exploration in the mine area began in 1962 when brothers Harold and Alan Musselwhite of Kenpat Mines Ltd. discovered the small Kenpat gold showing in quartz veins on the north side of Opapimiskan Lake as well as several showings in the iron formations on the south side. Over the next 5 years they conducted mapping, trenching and diamond drilling (12 holes totalling 773m). The Musselwhites re-staked the ground in 1973 and were subsequently financed by a syndicate of Dome Exploration, Canadian Nickel Co., Esso Minerals Canada Ltd. and Lacana Mining Corp. From 1976-1983 a major drilling program was undertaken in the West Anticline area followed by underground development and exploration in 1984. The West Anticline zone proved to be uneconomic and work soon shifted to the East Bay Synform area where the T-Antiform Zone was discovered by 1986. Exploration work carried on for another ten years and eventually the syndicate was reduced to two partners, Placer Dome (68%) and Kinross (32%). The T-Antiform ore zones had failed two early feasibility studies and failed to meet Placer Dome’s economic thresholds in a third study. However, the project proponents used a risk analysis study to convince the company’s board of directors that there was a very high probability of much more ore being found once the mine was in production and the go-ahead for construction was given in 1996 (Lewis, 1998). In 2002-3 the PQ Deeps ore zones were discovered followed by the Lynx Zone in 2010. After a series of corporate takeovers, Placer Dome’s interest became the property of Goldcorp Canada Ltd. in 2006 who subsequently bought out Kinross’ interest in 2007 and now hold 100%. Musselwhite is considered an orogenic gold deposit, hosted by tightly folded banded iron formation dated at ~ 2.98 Ga. It is located at the northern edge of the North Caribou Terrane (NCT), which forms the core of the North Caribou Superterrane (NCST) and the western Superior Province itself. The mine is adjacent to the ~2.86 Ga suture between the NCT and the Island Lake Domain to the north. Mineralization has been dated at 2.69 Ga, an age very close to that of gold occurrences elsewhere along the northern margin of the NCST. Unlike many orogenic gold deposits, but like a number of major BIF-hosted deposits, no major fault or shear zone that might have served as a pathway for mineralizing fluids has been found at Musselwhite. The current model for the formation of this deposit involves the development of mineralized high-strain zones along the steep limbs of the folded iron formation created during the flattening and folding event. This tectonic event was likely a result of the collision of the NCT with the Island Lake Domain 75km north of the mine, and/or the collision of the Northern Superior Superterrane ~200km to the north. Field trip participants will have the opportunity to observe weakly deformed, shallow-dipping BIF at several outcrops west of the mine, followed by steeply dipping exposures of the BIF which hosts most of the ore at depth. Underground stops will be dependent on the faces available at the time of the tour but we hope to visit well mineralized, highly strained portions of the orebodies. The following description of the geology is taken in part from Biczok (2007) and Biczok et al. (2012). Regional Geology The Musselwhite property covers a portion of the northwest-trending North Caribou Lake greenstone belt (NCGB) which is located in the northern margin of the Archean North Caribou Superterrane (NCST), adjacent to its internal boundary with the Island Lake Domain (Fig. 1). The NCST is a continental block consisting of ~3.0 Ga juvenile plutonic and minor volcanic rocks which underwent two periods of rifting - 208 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 1. Tectonic setting of the Musselwhite gold deposit. After Rayner and Stott (2005) and related deposition of arc sequences at 2.98-2.85 Ga and 2.85-2.71 Ga, followed by extensive reworking by continental arc magmatism at 2.75-2.70 Ga (Percival, 2007, and references therein). The North Caribou greenstone belt is one of the earlier arc sequences. Volcanic rocks of this belt have been dated at ~29822868Ma and, more specifically, those in the mine area at 2.98-2.97 Ga (Breaks et al., 2001; Biczok et al., 2012; unpublished Musselwhite data). The North Caribou greenstone belt (Fig. 2) has been mapped at various times by Satterly (1941), Emslie (1962), Andrews et al., (1981), and most recently a three year, multi-disciplinary effort in the mid1980’s by Breaks et al. (2001). These latter authors identified four dominantly volcanic rock suites in the Musselwhite mine area and these make up the McGruer Assemblage (Fig. 3): North Rim Metavolcanic Suite (NRU): Occurs in the northeast corner of Opapimiskan Lake and extends northwest from there over 60km along the northern margin of the greenstone belt. It consists largely of mafic and lesser ultramafic volcanic rocks. A minor felsic volcanic unit within this sequence was recently dated at 2868 Ma on Musselwhite’s behalf, confirming a previous age of 2870 Ma (Davis and Stott, 2001). South Rim Metavolcanic Suite (SRU): Occurs on the northwestern side of Opapimiskan Lake and extends north and northwest from there more than 50km along the southern margin of the NCGB. Regionally it is dominated by fine- to medium-grained, massive to pillowed basaltic flows with minor felsic and intermediate units and rare ultramafic units. Due to the paucity of felsic volcanic rocks in the SRU, only one age-date has been undertaken on rocks ascribed to the SRU by the OGS, that from an intermediate volcanic unit on the north shore of Opapimiskan Lake and based on only two zircons. These rocks were dated at 2982 Ma, however, it is not certain that they actually belong to the SRU, they are more likely part of the OMU. A number of authors have interpreted the SRU as being the folded repetition of the NRU on opposite sides of a major synform forming the axial core of the NCGB. Recent lithogeochemical work at the University of Ottawa suggests that the NRU and SRU formed in different tectonic settings and are not - 209 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 2. General geology of the North Caribou greenston belt with recent ages (after Breaks et al., 1987). Figure 3. Geologic map of the Musselwhite mine area - 210 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 equivalent (J. Duff, pers. comm.). Drilling by the Musselwhite exploration department over the past 10 years has identified a thick sequence of felsic volcanic flows, tuffs and volcaniclastic units beneath Opapimiskan Lake and at depth below the mafic volcanic rocks exposed on the northwest shore of the lake. This felsic pile varies from coarse pyroclastic rocks to very fine-grained, massive units (flows or welded ash tuffs?) to biotite-rich volcaniclastic units. These features, and the sheer volume of the felsic units. indicate that there was a major felsic volcanic edifice at this location which overlies the OpapimiskanMarkop suite and lies in the area mapped as part of the South Rim unit. However, the continuation of intraformational iron formations common in the upper Opapimiskan-Markop suite into the lower section of the felsic volcanic rocks of the felsic suite implies, at the very least, that this is largely a conformable contact. Alternatively, the Opapimiskan-Markop volcanism may actually include the felsic and mafic rocks on the north side of Opapimiskan Lake. U-Pb age dating of these felsic rocks is currently underway and may shed light on this issue. Opapimiskan-Markop Metavolcanic Suite (OMU): Occupies the central portion of the NCGB in the Opapimiskan Lake area and is dominated by mafic to ultramafic flows with intercalated clastic and chemical sedimentary units, including the banded iron formations which host the Musselwhite gold deposit (described in more detail in the following section). The lower portion of the volcanic pile is dominated by ultramafic to high-Mg basalts, iron formation and lesser siliciclastic sediments, a “primitive” sequence common in many greenstone belts of the NCS and interpreted to be products of plume-related rifting by Hollings and Kerrich (1999). The upper portion of the OMU, above the main BIF horizons, consist of predominantly tholeiitic basalts. As noted above, it is currently unclear which unit the felsic volcanic strata located above these tholeiitic basalts (Fig. 4) should be assigned to, the OMU or SRU. Further geochronological and lithogeochemical work is ongoing to answer this Figure 4. Generalized cross-section of the Musselwhite mine area and ore zones - 211 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 question. Forester Lake-Neawagank Metavolcanic Suite: Dominated by mafic and ultramafic volcanic units and occurs in the southeastern extremity of the belt. Granitoid gneiss and intrusions The greenstone belt is bounded to the north by the ~2.86 Ga Schade Lake granitic gneiss complex and various poorly-defined granitoid plutons within it. To the southwest is the ~2.85 Ga North Caribou Pluton and to the southeast is a poorly documented granitic batholith region assumed to also be ~2.85-2.86 Ga, but intruded by at least two 2.72 Ga plutons south of Musselwhite. These younger plutons are similar in age to those formed in a back arc position to the Confederation arc in the Uchi subprovince 200km to the south along the southern margin of the NCT. They are also similar to those found north of Musselwhite as far as the Hudson Bay Lowlands and potentially related to subduction of the Northern Superior Superterrane under the NCT. Further work is required to determine which of these suites the young plutons at Musselwhite belong to. Locally abundant S-type pegmatitic granite dykes occur throughout the area, particularly within areas underlain by metasedimentary rocks. These granites contain muscovite, garnet and tourmaline and are assumed to have formed by the partial melting of the metasedimentary rocks at depth. They have been dated at 2716 to 2669 Ma and are the only intrusive rocks known in the area which overlap the age of the mineralization (Biczok et al., 2012). Structural Geology Three deformation events have been recognized in the NCGB by previous workers (Hall and Rigg, 1986; Breaks et al., 2001). While the time between each event may be uncertain, there is good field evidence for these three discrete episodes of variably oriented strain. The earliest event, D1, is typically manifested only by tight to isoclinal folds in the iron formations, which are typically refolded by D2/F2. An excellent exposure of a large refolded F1 fold occurs in the West Anticline area (Fig. 5) and a classic basin and dome interference fold pattern occurs within the BIF on “Grunerite Island” in Opapimiskan Lake (Fig. 6). D2 is by far the dominant deformation event in the area and has produced a near vertical, moderate to strong planar north-trending foliation throughout the area with variably developed lineations, boudinage, and mesoscopic folds (Breaks et al, 2001). Syn-D2 shearing is commonly developed parallel to F2 and locally produces well-developed rootless folds in the Figure 5. F1 fold refolded by F2, West Anticline area. - 212 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 more highly strained margins of the iron formations. D3 is a relatively weak and localized event evidenced by minor warping and crenulation cleavages. Figure 6. Dome and basin interference fold pattern, Grunerite Island, Opapimiskan Lake. In the mine area, the strata have been folded into a broad antiform known as the West Anticline and the adjacent broad synform known as the East Bay Synform (Fig. 4). The West Anticline is an open fold with a relatively flat, undulating crest ~ 1 km across, featuring a series of smaller gentle folds that plunge to the north at <5° to ~30°. Rocks in this area are commonly only weakly deformed and preserve a variety of soft-sediment features in the iron formations including slumping (Fig. 7). In contrast, the East Bay Synform is bounded by limbs that dip steeply between ~70-90° and the keel is host to two 2nd order antiforms, the “T-Antiform” and the “W-Fold”. The folds plunge fairly consistently at ~12° to the north. This is a considerably higher strain setting than the West Anticline and primary structures are rarely preserved here. Late shears and faults are common in the volcanic rocks coring this synform and these are typically pervasively biotitized and laced with 20-40% thin calcite veinlets. Mineralized high-strain zones are predominantly developed in the upper margin of the Northern Iron Formation within the steepest portions of the fold limbs. The T-Antiform, the adjacent synformal keel (known as the “PQ Deeps”), and the east (PQ) limb of the East Bay synform host the bulk of the gold mineralization at Musselwhite. A major sub-vertical fault at ~3° to the fold axes has sliced the eastern limb (known as the PQ limb) into two pieces over a 700m interval and displaced the western portion ~1.3km to the south, forming a very large tubular sheath fold. This fault has fortuitously juxtaposed the ore zones over an interval of several hundred metres. Metamorphism Sedimentary rocks intercalated throughout the volcanic pile in the mine area commonly contain garnet +/- staurolite and the area is therefore considered to be of amphibolite grade. The greenschist-amphibolite isograd is thought to be located at least 5km from the mine to the north. Mine Stratigraphy Figure 7. Soft sediment slump features in BIF. Rocks hosting the Musselwhite gold deposit belong to the Opapimiskan-Markop Metavolcanic Suite (OMU) and have been subdivided into a detailed stratigraphy that is relatively consistent over the property, although major facies changes are locally observed along and - 213 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 across strike. This “mine stratigraphy” is depicted in Figure 8. folded areas including the Ranger, Red Wing and Thunderwolves. The lower portion of the OMU consists mainly of komatiitic basalts and ultramafic flows/intrusions, with local high-Mg andesite flows. This presence of andesite, which is commonly bleached and highly biotitized in the mine area, is somewhat unusual in such a maficultramafic sequence. Lithogeochemical analysis suggests that it formed by fractional crystallization of komatiite melt contaminated with either crustal TTG melts or felsic volcanic magma (Hollings and Kerrich, 1999). This sequence is overlain by two major banded iron formations (BIF) separated by 10-30m of maficultramafic volcanics and local high-Mg andesite. The Northern Iron Formation (NIF) sits ~20-30m above the SIF and is the main ore host at Musselwhite. This is a complexly layered horizon typically ~40m thick in total and consists of seven different facies thought to reflect varying proportions of clastic and chemical sedimentation combined with variations in the Redox conditions. Not every facies is always present across the drilled extent of the NIF, but where they are, the following stratigraphy is observed from the base to the top of the formation. The Southern Iron Formation (SIF) is the lowermost BIF and is a relatively monotonous sequence of thinly laminated magnetite and chert. There is generally little or no silicate, sulphide, or other facies within this horizon in the mine area. The SIF commonly occurs in two principal horizons, 5 to 20m thick, separated by 5-10m of basalt. The SIF hosts a number of small mineralized zones in tightly Unit 4H: This unit is a sulphidic iron formation, composed of 10-80% syngenetic pyrrhotite in a dark grey to black cherty argillite (Fig. 9). The continuity of the 4H is poor and where present its thickness varies from <10cm to as much as10m. Unit 4A: This unit is fairly ubiquitous throughout the mine area and is the most common basal unit of the NIF. It is composed of pale grey, weak-moderately magnetic chert, interlayered with 30-50%, diffuse bands of fine-grained, light yellow grunerite. Figure 8. Detailed stratigraphy of the Northern Iron Formation including presumed Redox conditions during formation. - 214 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 9. 4H, massive pyrrhotite exhalite with chert fragments. Drill core sample 5.1cm (2”) wide. Unit 4B: Composed of thinly laminated to thickbanded (~1-2cm) chert-magnetite oxide-facies BIF, the 4B is typically 20-30m thick and forms ~3/4 of the NIF (and usually all of the SIF). It can be subdivided into two varieties: a lower, thick-banded, relatively pure chert-magnetite BIF (Fig. 10), and an upper, thinly laminated, more clastic-rich variety consisting of alternating intervals, typically <1-2cm thick, of thin, diffuse laminae of magnetic chert, and more homogeneous, medium-dark green, fine-grained amphibole-rich layers (Fig. 11). With increasing stratigraphic height these amphibole dominant bands become garnetiferous and may contain up to 25% pale red garnets <2mm in diameter. The amphibole+/-garnet layers are readily affected by hydrothermal alteration. Adjacent to mineralized zones and/or major quartz veins they are commonly altered to massive, finegrained black biotite, with an associated coarsening of the garnets. Figure 10. 4B, thick banded chert-magnetite BIF. Drill core sample 5.1cm (2”) wide. Figure 11. Laminated 4B (chert magnetite) below, clastic 4B with thin layers of green amphibole-garnet above. Drill core sample 5.1cm (2”) wide. Unit 4EA: Pristine 4EA is a silicate iron formation composed almost entirely of massive bands of garnet-grunerite with ~20-30% bands of pale grey, moderately magnetic chert <1-2cm thick (Fig. 12). The garnet-grunerite layers contain ~30-55% almandine garnets, 1-4mm across, in a fine-grained matrix of pale yellow grunerite and minor fine-grained disseminated magnetite. The 4EA forms the main orehost at Musselwhite and in mineralized zones it has undergone quartz flooding/veining, replacement of the original grunerite by green amphibole adjacent to the veins (hornblende or ferrotschermakite; Otto, 2002), significant coarsening of the garnets due to hydrothermal overgrowths, and pyrrhotite mineralization. There has been some debate over the years regarding the origin of the grunerite in the 4EA. Some have argued that it Figure 12. 4EA, garnet-grunerite silicate facies iron formation. Drill core sample 5.1cm (2”) wide. - 215 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 is the product of hydrothermal alteration of precursor magnetite + chert. While such alteration is locally observed on a small scale (mm to cm) in sheared, quartz veined portions of the 4B for example, it is difficult to envisage such a process producing the ~10m thick unit we see today that is continuous over many kilometers, always at the same stratigraphic heights, commonly has no evidence of hydrothermal alteration (such as quartz veins, calcic amphiboles, etc.), and displaying no gradation along strike into “fresh” magnetite+chert units. Perhaps most compelling is the nature of the BIF approximately 20km along strike to the north of Musselwhite in a greenschist grade region. Outcrops here are dominated by grunerite-chert layers with very little or no magnetite. What little magnetite is present occurs in very thin laminae, interbedded with grunerite, and delicately folded into complex patterns. It seems highly unlikely that the grunerite laminae here could have formed at the expense of the magnetite and still have preserved this delicate layering. The preferred explanation for the formation of the grunerite in the 4EA is that it is the product of metamorphism of the original iron-silica gels produced by seafloor hydrothermal vents. Unit 4F: The 4F is a garnet-biotite +/- staurolite schist with an average Fe2O3 content of ~25-30% and thus qualifies as an iron formation itself (Fig. 13). Typically it contains 30-55% subhedral garnets, 1-5mm across, in a fine-grained matrix of 60-70% biotite with lesser quartz, feldspar and magnetite. Intraformational 4F horizons within the basaltic pile commonly contain up to 30-40% anhedral, light yellow staurolite grains 1-2mm across. Unit 6: This is a thin, but semi-continuous unit, typically <1m thick, that occurs in the upper portion Figure 13. 4F, garnet-biotite-(Qtz-Fd)±staurolite schist; ferruginous metapelite. Drill core sample 5.1cm (2”) wide. Figure 14. 4E, garnetiferous amphibolite. Drill core sample 5.1cm (2”) wide. of the 4F sequence in the NIF. It is a light beige-grey, siliceous, fine-grained, equigranular, very homogeneous rock composed of 20-30% finely dispersed biotite and 70-80% quartz-feldspar. Lithogeochemical analyses indicate that this unit is very similar to the local felsic volcanic rocks but has relatively elevated levels of V, Mn and Ba. It is interpreted as a meta-sediment derived from a waterlain felsic ash tuff. Unit 4E: Where present, the 4E forms the uppermost unit of the NIF. It is generally a thin, <1m, fine-grained, massive, medium-dark green amphibolite containing 15-30% anhedral pale red garnets 2-4mm across (Fig. 14). It has little or no visible quartz or feldspar and averages 25% Fe2O3. The 4H, 4A, 4E and 4F all occur as discrete, “intraformational” horizons within the basaltic pile in addition to occurring within the NIF. These horizons are most commonly <2m thick but can swell to ten’s of metres within fold crests or keels. They are most abundant in the first 20-30m above the NIF and can be locally well mineralized. Overlying the NIF is a variable thickness of basalts ranging from <2m in the Esker fold area to 30-50m in the T-Antiform area. There are little or no komatiitic basalts or ultramafic units above the NIF. In the past, the various basalts in the mine area were designated as “BVol” (for “basic” or “basement” volcanics) or “2Vol” for the units near the NIF. Given the uncertainty of defining a “basement” in the mine area, these terms have been abandoned in recent years in favour of a generic “Unit 2” and its variants for all basalts and andesites. - 216 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Mineralization and Alteration Mineralization at Musselwhite is found largely within sub-vertical high-strain zones in the favourable iron formation units, primarily the silicate facies (4EA) and to a lesser extent the oxide facies (4B), where a number of the smaller ore zones are found (e.g., Jets, Ranger, Thunderwolves, lower portion of the PQ Deeps A-Block). Scattered small mineralized shears cut mafic volcanic rocks, however, to date these occurrences are of limited extent and uneconomic. Discrete mineralized shear zones up to 10m wide occur across a zone ~225m from the “Moose Zone” in the west through the T-Antiform and into the PQ Deeps in the east. Locally these shear zones coalesce into broad zones up to 40m wide (e.g. PQ Deeps C-Block). Individual mineralized shear zones tend to be quite persistent along strike, following the upper margin of the Northern Iron Formation and/or high strain zones within second order folds (e.g. the Jets Zone) for 2-3km. While the shear zones are primarily ductile, on both macroscopic and microscopic scales, there is evidence of associated brittle deformation that produced dilatant zones and allowed the infiltration of gold-bearing fluids. Gold mineralization within the 4EA is generally accompanied by substantial quartz veining or flooding, pyrrhotite formation, green amphibole (hornblendeferrotschermakite; Otto, 2002) replacement of the original grunerite-rich host (Fig. 15), a coarsening of the garnets, and local late-stage chlorite. The formation of pyrrhotite is thought to be a consequence of sulphidation reactions between the original gold-bearing bi-sulphide complexes and the iron-rich minerals of the BIF. The gold content is crudely proportional to the sulphur Figure 16. Reflected light photomicrograph of garnet cut by pyrrhotite-gold filled fracture. content of the mineralization in the ratio of 5 g/t Au for each 1% of sulphur. Gold occurs as free-milling native gold, most commonly in pyrrhotite-filled fractures in garnets (Fig. 16), with lesser amounts in the pyrrhotite, green amphibole, and rarely in quartz veins. The clastic 4B underlying mineralized 4EA is commonly highly altered itself. The original very fine-grained, green amphibole-rich laminae are replaced by massive finegrained biotite and 5-15% medium-grained secondary garnets; this alteration is especially common adjacent to quartz veins. In spite of the intensity of this alteration, it typically has nil to very low levels of gold or pyrrhotite. Surface Field Trip Stops Stop 1a: Trench #4, West anticline Lithology: This stripped outcrop on the west side of the exploration road exposes the Southern Iron Formation and the adjacent ultramafic rocks. The BIF here is dominated by chert with lesser beds of a light grey magnetite-amphibole-rich unit not seen outside the West Anticline area. Only along the margins of the outcrop does one see the chert-magnetite BIF which is predominant on the rest of the property. Figure 15. Photomicrograph of 4EA cut by quartz veins flanked by green amphibole replacement of grunerite (PPL). Comments: The change in the degree of strain is evident in this exposure as one crosses from the central, shallow-dipping area to the steeply-dipping, highly strained margins. The abundant small minor folds along the margins have an overall “Z” pattern with their axes plunging shallow to the (grid) north. On flat surfaces the tops of these folds appear as rootless or - 217 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 intrafolial folds. in these outcrops including: An unusual iron-rich chlorite schist, with a peculiar knobby or ovoid texture, appears to be locally interbedded with the cherty BIF as well as cross-cutting it. This unit averages ~40% Fe2O3, 15% Al2O3, 27% SiO2 and 6% MgO. It commonly contains intergrown magnetite and tourmaline grains and has been interpreted as an iron-rich metasediment that was locally injected as “dykes” through cracks in the more lithified BIF above. 1) The overall structural fabric is predominantly flattening rather than strike-slip movement or shearing. Stop 1b: Z-folded Chert-Magnetite BIF 3) The hanging wall basalts have a distinct pale pinkpurple color due to the pervasive fine-grained biotite alteration. Biotite alteration, bleaching of the amphiboles, and a strong foliation are typical of the basalts adjacent to the contact of the Northern Iron Formation and indicates that strain was partitioned (focused) at this contact. On the opposite (east) side of the road from the large stripped outcrop discussed above is a small exposure of 4B (well banded chert-magnetite BIF) that displays metre-scale Z-folds. Minor folds like these throughout the West Anticline can be related to their position on the series of undulating folds that make up the crest of the major antiform. Stop 2: Lakeshore Exposure of Gently Folded 4E/4EA Lithology: This small exposure near the shore of Opapimiskan Lake was only rediscovered in the Fall of 2011. What little of it was exposed at that time consisted of gently folded 4E and/or 4EA belonging to the Northern Iron Formation, plunging to the north. Comments: The outcrop was partially excavated in the Fall and will be power-washed in the spring prior to the field trip. It is expected to provide an excellent look at weakly deformed 4E / 4EA of the NIF, one of the few such exposures in the area. Stop 3: PQ Limb Section through the Northern Iron Formation Lithology: This is the only outcrop of the complete Northern Iron Formation found on the property and was created during overburden stripping operations related to the development of the PQ Shallows and the Ranger open pits. The exposures are part of the sub-vertical PQ limb, the eastern limb of the East Bay synform. The section begins with the eastern footwall which exposes deformed pillow basalts and passes through an almost complete section of the NIF, including a probable 4H, minor 4A, well-developed 4B, 4EA, 4F, Unit 6, and 4E. A small basaltic (“Bvol”) dyke is found within and roughly parallel to the trend of the 4EA. Comments: there is a wide range of features to note 2) There is a well developed zone of quartz veining and intense biotite alteration in the clastic 4B immediately below the 4EA. This type of alteration is typically barren and conversely biotite alteration such as this is relatively rare in well-mineralized 4EA. 4) There is a northeast trending series of small-scale folds and cleavages scattered throughout the exposures. These may be part of the D3 deformation event. References Andrews, A.J., Sharpe, D.R., and Janes, D.A. 1981. Preliminary Reconnaissance of the WeagamowNorth Caribou Lake Metavolcanic-Metasedimentary Belt, including the Opapimiskan Lake (Musselwhite) Gold Occurrence; in Summary of Field Work, 1981, Ontario Geological Survey, Miscellaneous Paper 100, p. 196-202. Biczok, J.L., 2007. 2006-2007 North Shore Project, Diamond Drilling Report. Assessment report filed with the Ontario Ministry of Northern Development and Mines on behalf of Goldcorp Canada Ltd. AFRI # 20000003409 Biczok, J.L., Hollings, P., Klipfel, P., Heaman, L., Maas, R., Hamilton, M., Kamo, S., and Friedman, R., 2012. Geochronology of the North Caribou greenstone belt, Superior Province Canada: Implications for tectonic history and gold mineralization at the Musselwhite mine. Precambrian Research, 192-195 (2012), p. 209-230. Breaks, F.W., Osmani, I.A., and deKemp, E.A. 1987. Precambrian Geology of the OpapimiskanNeawagank Lakes Area, Western Part (Opapimiskan Project), Kenora district (Patricia Portion); Ontario Geological Survey, Map P.3080, Geological SeriesPreliminary Map, scale 1:31,680. geology 1986. Breaks, F.W., Osmani, I.A., and deKemp, E.A. 2001. Geology of the North Caribou Lake area, northwestern Ontario; Ontario Geological Survey, Open File report 6023, 80p. - 218 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Davis, D.W., and Stott, G.M., 2001. Geochronology of several greenstone belts in the Sachigo Subprovince, northwestern Ontario, #18 Project Unit 89-7. In: Summary of Field Work and Other Activities 2001, Ontario Geological Survey Open File Report 6070, 18-1 – 18-13. deKemp, E.A., 1987. Stratigraphy, provenance and geochronology of Archean supracrustal rocks of western Eyapamikama lake area, northwestern Ontario. Unpublished M.Sc. thesis, Carleton University, Ottawa, Ontario, 98p. Emslie, R.F. 1962. Wunnummin Lake (NTS 53A), Ontario; Geological Survey of Canada, Map l -1962, scale l inch to 4 miles or 1:253 440. Geology 1962. Hall, R.S. and Rigg, D.M., 1986. Geology of the West Anticline Zone, Musselwhite Prospect, Opapimiskan Lake, Ontario, Canada. In: Macdonald, A. J. (Ed.), Gold ’86: An International Symposium on the Geology of Gold Deposits, Proceedings Volume, 124-136. Hollings, P., and Kerrich, R., 1999. Trace element systematics of ultramafic and mafic volcanic rocks from the 3 Ga North Caribou greenstone belt, northwestern Superior Province. Precambrian Research, v. 93, p. 257-279. Lewis, T.D., 1990. Musselwhite: An exploration success. CIM Bulletin Vol. 91, No. 1017, pp. 51-55. Klipfel, P., 2002. Musselwhite U-Pb Zircon and Ar-Ar Dates, Synthesis and Interpretation. Internal Placer Dome Exploration research report. gold deposit Musselwhite Mine, Ontario, Canada. Unpublished M.Sc. thesis, Freiberg University of Mining and Technology. 86p. Percival, J.A., 2007. Geology and Metallogeny of the Superior Province, Canada, in Goodfellow, W.D., ed., Mineral Deposits of Canada: A Synthesis of Major Deposit-Types, District Metallogeny, the Evolution of Geological Provinces, and Exploration Methods. Geol. Assoc. of Canada, Mineral Deposits Div., Spec Pub. No. 5, p. 903-928. Piroshco, D.W., Breaks, F.W., and Osmani, I.A., 1989. The Geology of Gold Prospects in the North Caribou Greenstone Belt, District of Kenora, Northwestern Ontario. Ontario Geological Survey, Open file Report 5698. Satterly, J. 1941. Geology of the Windigo-North Caribou Lakes Area, Kenora District (Patricia Portion); Ontario Department of Mines, Annual Report for 1939, v.48, Part 9, p. 1-32. Accompanied by Maps 48h and 49j. Stott, G., and Rayner, N., 2005. Discrimination of Archean domains in the “Sachigo Subprovince”, northwestern Ontario. Ontario Geological Survey, poster, Ontario Exploration and Geoscience Symposium, Toronto, Ontario, December 13-14. Thurston, P.C., Osmani, I.A., Stone, D., 1991. Northwestern Superior Province Review and Terrane Analysis: Geology of Ontario. Ontario Geological Survey, pp. 81–144 Otto, A., 2002. Ore forming processes in the BIF-hosted - 219 - � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Proceedings, 2012 Description An account of the resource Institute on Lake Superior Geology. Thunder Bay, Ontario. May 16-20, 2012. Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology Date A point or period of time associated with an event in the lifecycle of the resource 2012 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English https://digitalcollections.lakeheadu.ca/files/original/f89df4b1f66e16d34fb5de82344c88e6.pdf c23729b90c272dc16d576f9f09f8c61b PDF Text Text �������������������� Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Proceedings, 1957 Description An account of the resource Institute on Lake Superior Geology. Michigan State University, East Lansing, Michigan. May 6-8, 1957. Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology Date A point or period of time associated with an event in the lifecycle of the resource 1957 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English https://digitalcollections.lakeheadu.ca/files/original/1c7782936aa262c1fe4aef17e30d432a.pdf 5b46d679a8d6d3a763bca217af240a40 PDF Text Text Institute on Lake Superior Geology 59th Annual Meeting Houghton, Michigan May 8 - 11, 2013 Proceedings Volume 59 Part 1 - Program and Abstracts Editors: Allan R. Blaske and Theodore J. Bornhorst www.lakesuperiorgeology.org ��Institute on Lake Superior Geology 59TH ANNUAL MEETING MAY 8-11, 2013 HOUGHTON, MICHIGAN SPONSORED BY: A. E. Seaman Mineral Museum Michigan Technological University THEODORE J. BORNHORST AND ALLAN R. BLASKE Co-Chairs Proceedings Volume 59 Part 1 – Program and Abstracts EDITED BY ALLAN R. BLASKE AND THEODORE J. BORNHORST Cover Photo: Native copper from the Central Mine, Keweenaw Peninsula, Michigan. Collection of the A.E. Seaman Mineral Museum. Photograph by George Robinson. ��59TH INSTITUTE ON LAKE SUPERIOR GEOLOGY PROCEEDINGS VOLUME 59 CONSISTS OF: PART 1: PROGRAM AND ABSTRACTS PART 2: FIELD TRIP GUIDEBOOK TRIP 1: GEOLOGIC OVERVIEW OF THE KEWEENAW PENINSULA, MICHIGAN TRIP 2: CALEDONIA MINE, KEWEENAW PENINSULA NATIVE COPPER DISTRICT, ONTONAGON COUNTY, MICHIGAN TRIP 3: GEOLOGY OF SILVER MOUNTAIN, HOUGHTON COUNTY, MICHIGAN TRIP 5: GEOLOGY OF THE KEWEENAWAN SUPERGROUP, PORCUPINE MOUNTAINS, ONTONAGON AND GOGEBIC COUNTIES, MICHIGAN TRIP 6: GEOLOGY AND ENVIRONMENTAL SITE CONDITIONS OF THE COPPERWOOD DEPOSIT, GOGEBIC COUNTY, MICHIGAN Reference to material in Part 1 should follow the example below: Cannon, W. F. Woodruff, L. G., and Schulz, K.. J., 2013, The Hiawatha Graywacke of the Iron River-Crystal Falls district, Michigan: a megaturbidite triggered by seismicity related to the 1850 Ma Sudbury impact [abstract]: Institute on Lake Superior Geology Proceedings, 59th Annual Meeting, Houghton, MI, v. 59, part 1, p. 14-15. Published by the 59th Institute on Lake Superior Geology and distributed by the ILSG Secretary: Peter Hollings Department of Geology Lakehead University Thunder Bay, ON P7B 5E1 CANADA peter.hollings@lakeheadu.ca ILSG website: http://www.lakesuperiorgeology.org ISSN 1042-9964 i ��Table of Contents Institutes on Lake Superior Geology, 1955-2013 iii Sam Goldich and the Goldich Medal vi Goldich Medal Guidelines viii Goldich Medalists x 2013 Goldich Medal Recipient x Goldich Medal Committee x Citation for Goldich Medal Recipient xi Memorial to Glenn Morey xii Memorial to Paul Sims xiii Eisenbrey Student Travel Awards xiv Joe Mancuso Student Research Awards xv Doug Duskin Student Paper Awards xvii Student Paper Awards Committee xvii Board of Directors xviii Local Committee xviii Session Chairs xviii Banquet Speaker xix Report of the Chair of the 58th Annual Meeting xx Sponsors xxii Program xxiii Poster Presentations xxviii Abstracts 1-83 ii �Institutes on Lake Superior Geology, 1955-2013 95 o o 85 o Wabigoon subprovince90 o 80 48 o Wawa-Abitibi subprovince 48o Wawa-Abitibi subprovince o 45 45o Minnesota River Valley subprovince MEETING LOCATIONS Phanerozoic Mesoproterozoic Map by Mark Jirsa 95o Paleoproterozoic o 90 85o Archean Superior Province # Date Place Chairs 1 1955 Minneapolis, Minnesota C.E. Dutton 2 1956 Houghton, Michigan A.K. Snelgrove 3 1957 East Lansing, Michigan B.T. Sandefur 4 1958 Duluth, Minnesota R.W. Marsden 5 1959 Minneapolis, Minnesota G.M. Schwartz & C. Craddock 6 1960 Madison, Wisconsin E.N. Cameron 7 1961 Port Arthur, Ontario E.G. Pye 8 1962 Houghton, Michigan A.K. Snelgrove 9 1963 Duluth, Minnesota H. Lepp 10 1964 Ishpeming, Michigan A.T. Broderick 11 1965 St. Paul, Minnesota P.K. Sims & R.K. Hogberg 12 1966 Sault Ste. Marie, Michigan R.W. White 13 1967 East Lansing, Michigan W.J. Hinze 14 1968 Superior, Wisconsin A.B. Dickas 15 1969 Oshkosh, Wisconsin G.L. LaBerge 16 1970 Thunder Bay, Ontario M.W. Bartley & E. Mercy iii �# Date Place Chairs 17 1971 Duluth, Minnesota D.M. Davidson 18 1972 Houghton, Michigan J. Kalliokoski 19 1973 Madison, Wisconsin M.E. Ostrom 20 1974 Sault Ste. Marie, Ontario P.E. Giblin 21 1975 Marquette, Michigan J.D. Hughes 22 1976 St. Paul, Minnesota M. Walton 23 1977 Thunder Bay, Ontario M.M. Kehlenbeck 24 1978 Milwaukee, Wisconsin G. Mursky 25 1979 Duluth, Minnesota D.M. Davidson 26 1980 Eau Claire, Wisconsin P.E. Myers 27 1981 East Lansing, Michigan W.C. Cambray 28 1982 International Falls, Minnesota D.L. Southwick 29 1983 Houghton, Michigan T.J. Bornhorst 30 1984 Wausau, Wisconsin G.L. LaBerge 31 1985 Kenora, Ontario C.E. Blackburn 32 1986 Wisconsin Rapids, Wisconsin J.K. Greenberg 33 1987 Wawa, Ontario E.D. Frey & R.P. Sage 34 1988 Marquette, Michigan J. S. Klasner 35 1989 Duluth, Minnesota J.C. Green 36 1990 Thunder Bay, Ontario M.M. Kehlenbeck 37 1991 Eau Claire, Wisconsin P.E. Myers 38 1992 Hurley, Wisconsin A.B. Dickas 39 1993 Eveleth, Minnesota D.L. Southwick 40 1994 Houghton, Michigan T.J. Bornhorst 41 1995 Marathon, Ontario M.C. Smyk 42 1996 Cable, Wisconsin L.G. Woodruff 43 1997 Sudbury, Ontario R.P. Sage & W. Meyer 44 1998 Minneapolis, Minnesota J.D. Miller & M.A. Jirsa 45 1999 Marquette, Michigan T.J. Bornhorst & R.S. Regis 46 2000 Thunder Bay, Ontario S.A. Kissin & P. Fralick 47 2001 Madison, Wisconsin M.G. Mudrey & Jr., B.A. Brown 48 2002 Kenora, Ontario P. Hinz & R.C. Beard 49 2003 Iron Mountain, Michigan L. Woodruff & W.F. Cannon iv �# Date Place Chairs 50 2004 Duluth, Minnesota S. Hauck & M. Severson 51 2005 Nipigon, Ontario M. Smyk & P. Hollings 52 2006 Sault Ste. Marie, Ontario A. Wilson & R.Sage 53 2007 Lutsen, Minnesota L. Woodruff & J. Miller 54 2008 Marquette, Michigan T. Bornhorst & J. Klasner 55 2009 Ely, Minnesota J. Miller, G. Hudak & D. Peterson 56 2010 International Falls, Minnesota M. Jirsa, P. Hollings, T. Boerboom, P. Hinz & M.Smyk 57 2011 Ashland, Wisconsin T. Fitz 58 2012 Thunder Bay, Ontario P. Hollings 59 2013 Houghton, Michigan T. J. Bornhorst and A. R. Blaske v �Sam Goldich and the Goldich Medal Sam Goldich received an A.B. from the University of Minnesota in 1929, a M.A. from Syracuse University in 1930, and a Ph.D. from the University of Minnesota in 1936. During World War II Sam worked for the U.S. Geological Survey in mineral exploration. In 1948, Sam returned to the University of Minnesota, and became Professor and Director of the Rock Analysis Laboratory the following year. He rejoined the U.S. Geological Survey in 1959 and was appointed as the first Branch Chief of the Branch of Isotope Geology. Sam returned to academia in 1964 when he went to Pennsylvania State University. He left PSU in 1965 and moved to the State University of New York at Stony Brook, where he stayed for 3 years. Restless yet again, he moved to Northern Illinois University in 1968 where he was a professor until his retirement in 1977. Sam’s final move was to Denver where he became an emeritus at the Colorado School of Mines. Sam died in 2000, less than a month before his 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 vi �INSTITUTE ON LAKE SUPERIOR GEOLOGY GOLDICH MEDAL vii �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. viii �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. ix �Goldich Medalists 1979 Samuel S. Goldich 1996 David L. Southwick 1980 not awarded 1997 Ronald P. Sage 1981 Carl E. Dutton, Jr. 1998 Zell Peterman 1982 Ralph W. Marsden 1999 Tsu-Ming Han 1983 Burton Boyum 2000 John C. Green 1984 Richard W. Ojakangas 2001 John S. Klasner 1985 Paul K. Sims 2002 Ernest K. Lehmann 1986 G.B. Morey 2003 Klaus J. Schulz 1987 Henry H. Halls 2004 Paul Weiblen 1988 Walter S. White 2005 Mark Smyk 1989 Jorma Kalliokoski 2006 Michael G. Mudrey 1990 Kenneth C. Card 2007 Joseph Mancuso 1991 William Hinze 2008 Theodore J. Bornhorst 1992 William F. Cannon 2009 L. Gordon Medaris, Jr. 1993 Donald W. Davis 2010 William D. Addison & Gregory R. Brumpton 1994 Cedric Iverson 2011 Dean M. Rossell 1995 Gene La Berge 2012 Jim Miller 2013 GOLDICH MEDAL RECIPIENT Tom Waggoner Goldich Medal Committee Serving through the meeting year shown in parentheses. Laurel Woodruff (2013) United States Geological Survey Graham Wilson (2014) Turnstone Consulting Bernhardt Saini-Eidukat (2015) North Dakota State University x �Citation for Goldich Medal Recipient Tom helped Cliffs make the transition from underground mining of direct shipping ore to pellets, which are now the life blood of iron mining in the Lake Superior region. The success of his contributions is best measured by 40 years of continual production of pellets from the Empire and Tilden mines that together produce 13 to 14 million tons per year. The transition to pellets is public knowledge but much of the details are lacking. Before pellet production could begin iron formations had to be sampled for their recoverable iron content and ease of recovery. After mining, there had to be day to day adjustment in grinding time, depending on the hardness of taconite, and blending ores in order to control phosphorous contents. Tom’s contribution to preserving iron mining in the Lake Superior region has been documented by his colleagues because the skills of company geologists are seldom part of the public domain. During his long career with Cliffs (1965-1997), Tom actively participated in the teaching function of the Institute. He led 45 field trips for various organization and made numerous poster and oral presentations at annual meetings of the Institute. At his retirement in 1997, Tom was chief geologist. After his retirement he has continued to serve the mining industry. After the state of Michigan ended funding a curator for the core library at Harvey, Tom became the unpaid overseer of the facility and went looking for a building to assemble and preserve the large amount of additional core that exists at scattered location in the upper peninsula. Like the core library at Harvey it will be a repository for future exploration geologists who are essential for sustaining mining in the Lake Superior region. He found a building on the grounds of the closed K. I. Sawyer Air Force base that was suitable as a core library and has been actively seeking funding to acquire and maintain it. Submitted by Ronald E. Seavoy xi �Memorial to Glenn B. Morey 1935-2012 Glenn B. Morey, geologist, known to family, friends, and colleagues as “G.B.” or “Morey,” died at age 76 on August 2, 2012 in St. Paul, Minnesota. He spent the greater part of his 40-year career at the Minnesota Geological Survey in positions that ranged from junior geologist to associate director and chief geologist. He was a senior fellow of the Geological Society of America, a member of the Society of Economic Geologists, the History of Earth Sciences Society, and a lifelong member of the Mesabi Range Geological Society and the Institute on Lake Superior Geology. He received the institute’s Goldich Medal in 1986 for his contributions to geologic understanding of the Lake Superior region. G.B. Morey was raised in Proctor, a town on the Duluth, Mesabi, and Iron Range Railroad through which the iron ore and taconite from the Mesabi range passed on its way to the port of Duluth and steel mills of the lower Great Lakes. He learned of iron mining from his father who worked on the railroad, and he devoted much of his professional career to studies of the stratigraphy, mineralogy, and genesis of the Biwabik Iron Formation—the geological source of prosperity in the mining towns of the Mesabi Iron Range and communities along the routes between mine and mill. Morey completed an M.S. degree in 1960 and a Ph.D. in 1965, both at the University of Minnesota-Twin Cities under professor F.M. Swain. Sedimentology and stratigraphy were the principal foci of his graduate program and continued to be his primary geological interests throughout his professional career. His M.S. thesis entitled “Geology of the Keweenawan sediments near Duluth, Minnesota” was the first of many papers he authored or coauthored on the Mesoproterozoic sedimentary sequences within the Midcontinent Rift. Likewise, his Ph.D. thesis entitled “The sedimentology of the Precambrian Rove Formation in northeastern Minnesota” was the precursor to his many publications on Paleoproterozoic clastic rock units associated with iron-formation on the Biwabik, Cuyuna, and Gunflint iron ranges. Morey’s bibliography contains 100 refereed papers, 25 geologic maps, and many published abstracts and field trip guides—including those for the Institute on Lake Superior Geology. G.B. Morey’s professional accomplishments and adherence to high scientific standards are widely recognized and appreciated throughout the Great Lakes region. He mentored and critiqued many of us at the Minnesota Geological Survey and elsewhere, commonly playing the skeptic to extract the best from colleagues. G.B. was a credit to the survey, the University of Minnesota, and the geological profession as a whole. We who knew him celebrate his memory while we mourn our loss. David L. Southwick, Mark Jirsa, and Paul Weiblen xii �Memorial to Paul K. Sims 1917-2011 Paul K. Sims joined the field trip from which no geologist returns on October 29, 2011 in Denver, Colorado. Born in Newton, Illinois on September 8, 1918, Paul excelled in basketball, and entered University of Illinois Business School. The geology bug bit him and by 1940 he was actively engaged in his Master’s Degree (1942) based on rotary drilling in the coal beds of Illinois. After his degree, he began work with the USGS on zinc-lead deposits in Arizona and Washington before serving with distinction in the navy during World War II in the Pacific theater. Contacts at that time led him to Princeton University for his PhD (1950) on the Dover Magnetic District, New Jersey. He pursued many professional avenues while with the USGS, including international work, uranium geochemistry, editing, and geologic mapping. Lake Superior called, on September 1, 1961, he took leadership of the Minnesota Geologic Survey as Director. Within a year, PK initiated programs in all parts of the stratigraphic column, and developed an annual report series highlighting programs and accomplishments that were used to advance MGS activities. He identified geologic mapping as the backbone of research and he and his colleagues produced numerous reports of local to regional interest. After departing Minnesota in 1973, he continued his mapping and tectonic analysis in Wisconsin and Michigan before resuming his pursuit of the Precambrian in the Rocky Mountains. His geological mapping in the Lake Superior District and the western US led him to suggest that deformation during Archean and Proterozoic was not analogous to Phanerozoic plate-tectonic model, but instead consisted of oblique shortening and progressed from ductile to brittle. His work on Archean mantle gneiss domes resulted in his visualization of the Great Lakes Tectonic Zone north of which Archean volcanogenic assemblages prevail, and south of which Proterozoic continental collision and island arc basins dominated Significant professional accomplishments included President of the Society of Economic Geologists (and many committees), Secretary of the Subcommission of Precambrian Stratigraphy (IUGS). Numerous awards included SEG Thayer Lindsley Lecturer (1984-1985) and Ralph Marsden Award (1989); USGS Meritorious Service Award; and ILSG Goldich Medal (1985). PK was a reserved individual who was enormously unselfish and was quick to give credit to those with whom he collaborated. He encouraged all with whom he came in contact with. In his own work he was demanding and expected clear documentation of which page was fact and which was interpretation. He was a joy to be with, and always stimulated discussion. He was a credit to our profession, and we mourn his passing. Michael Mudrey xiii �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. xiv �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 onehalf of any additional proceeds from each annual meeting, after all other commitments and expenses are covered. • The ILSG Board of Directors will be responsible for selecting a minimum of two awards each year. The ILSG Treasurer will issue the awards. • The ILSG Student Research Fund is available for undergraduate or graduate students working on geology in the Lake Superior region. • The applications are due to the ILSG Secretary by August 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. xv �In 2012 the ILSG Board of Governors awarded five $500 awards from the Student Research Fund. The winners were: Jonathan E. Dyess University of Minnesota-Duluth Dept. of Geological Sciences, 229 Heller Hall, 1114 Kirby Drive, Duluth, MN 55812 Current degree program: PhD Candidate (Advisor: Vicki Hansen) Shagawa Lake shear zone Elisa Piispa Michigan Technological University 1400 Townsend Drive, 630 DOW, ESE Building, Houghton, MI 49931-1295 Current degree program: PhD in Geology Paleomagnetism of the ~1140 Ma lamprophyre dykes in Ontario, Canada: Implications for the mantle plume hypothesis for Mid-Continental Rift origin Evgeniy V. Kulakov Michigan Technological University Department of Geological and Mining Engineering and Sciences 617 Dow ESE Bldg, 1400 Townsend Drive, Houghton, MI 49931-1295 Current degree program: PhD Paleomagnetism and Geochemistry of the Porcupine Volcanics and Lake Shore Traps: Implications for the Midcontinent Rift evolution. Mark Leatherman 1001 E. 10th Street, Department of Geological Sciences, Bloomington, IN 47405 Current degree program: PhD The Eagle and Tamarack Deposits Craig Caton Department of Geological Sciences
 229 Heller Hall, 1114 Kirby Drive, University of Minnesota Duluth, Duluth, MN 55812 Current degree program: Masters Petrogenesis and Metallogenesis of the Southern Troctolite Zone of the Bald Eagle Intrusion, Duluth Complex, Northeastern MN 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 long-time friend of the Institute. The 2012 ILSG Board of Directors approved adding Doug’s name to the award to acknowledge his contributions, and distribute those donations in a manner that would have pleased him. The Duskin Student Paper Committee is appointed by the Meeting Chair. Criteria for best student paper—last modified by the Board in 2001—follow: 1) The contribution must be demonstrably the work of the student. 2) The student must present the contribution in-person. 3) The Student Paper Committee shall decide how many awards to grant, and whether or not to give separate awards for poster vs. oral presentations. 4) In cases of multiple student authors, the award will be made to the senior author, or the award will be shared equally by all authors of the contribution. 5) The total amount of the awards is left to the discretion of the meeting Chair in conjunction with the Secretary, but typically is in the amount of about $500 US (increase approved by Board, 10/01). 6) The Secretary maintains, and will supply to the Committee, a form for the numerical ranking of presentations. This form was created and modified by Student Paper Committees over several years in an effort to reduce the difficulties that may arise from selection by raters of diverse background. The use of the form is not required, but is left to the discretion of the Committee. 7) The names of award recipients shall be included as part of the annual Chair’s report that appears in the next volume of the Institute. Student papers will be noted on the Program. Student Paper Awards Committee Helene Lukey – Cliffs Natural Resources Tom Fitz – Northland College Milt Gere – Michigan DNR (retired) xvii �Board of Directors Board appointment continues through the close of the meeting year shown in parentheses, or until a successor is selected Allan R. Blaske (Co-Chair) (2013-2016) – AECOM Theodore J. Bornhorst (Co-Chair) (2013-2016) – Michigan Technological University Tom Fitz (2011-2014) – Northland College Peter Hinz (2010-2013) – Ontario Geological Survey Pete Hollings - Secretary (2010-2013) – Lakehead University Mark A. Jirsa - Treasurer (2011-2014) – Minnesota Geological Survey Local Committee Chair Theodore J. Bornhorst – A. E. Seaman Mineral Museum, Michigan Technological University Allan Blaske – AECOM Volume Editors Theodore J. Bornhorst – A. E. Seaman Mineral Museum, Michigan Technological University Allan Blaske – AECOM Robert Barron – Department of Geological and Mining Engineering and Sciences, Michigan Technological University Session Chairs Jack Berkley – SUNY Fredonia Marcia Bjørnerud – Lawrence University Terry Boerboom – Minnesota Geological Survey Paula Leier-Engelhardt – HydroGeo Solutions Joe Maki – Michigan Department of Environmental Quality Glenn Scott – Cliffs Natural Resources xviii �Banquet Speaker Dr. James W. Ashley Postdoctoral Research Associate Lunar Reconnaissance Orbiter Camera Science Operations Center School of Earth and Space Exploration Arizona State University Rusty Metal at the Martian Equator: The Search for Life on the Red Planet The pursuit of an answer to the ancient question "Are We Alone in the Universe?" leads scientists down many paths that cross a multitude of scientific disciplines. In the planetary sciences, the quest often results in the careful engineering of robotic spacecraft designed to answer specific questions about the planets they are sent to explore. Mars is a world that is both easily accessible at reasonable costs, and potentially habitable. We are interested in the roles that water may have played in Mars' geologic history because of its importance to life on Earth. The Mars Exploration Rover (MER) mission was designed to last for 90 days on Mars in 2004. One of the two rovers (Opportunity) continues exploring today more than nine years later. Among the many discoveries made during this mission are several large, iron meteorites that show dramatic signs of corrosion and other effects of water interaction. MER science team member Dr. James Ashley lead the meteorite discovery and assessment campaign on the mission, and will show how rusty meteorites on Mars are giving us new insight into climate conditions at the red planet's equator. Dr. James Ashley, a Grand Rapids native, earned his BS in geology at Grand Valley State University, his MS in geological science at Michigan State University, and his PhD under Philip Christensen at Arizona State University. He is currently a postdoctoral research fellow at the Lunar Reconnaissance Orbiter Camera (LROC) Science Operations Center at ASU, where he studies Earth's Moon using LROC instruments. His most recent work has focused on Si-rich volcanic complexes on the Moon, potentially cavernous systems beneath the lunar surface, and the understanding of impact melt on the lunar far side. He is the executive director of Minor Planet Research, Inc., a non-profit company dedicated to mitigating the asteroid impact hazard, and has made many appearances on the History and Discovery Channels discussing the threat from near-Earth asteroids. Prior to his work in the planetary sciences, Dr. Ashley worked full time as a consulting hydrogeologist, and is a member of the American Institute of Professional Geologists. xix �Report of the Chair of the 58th Annual Meeting Thunder Bay, Ontario The 58th ILSG was held in Thunder Bay, Ontario on May 16-20, 2012. The meeting was chaired by Pete Hollings (Lakehead University) with the considerable assistance of the local organizing committee (Bill Addison, Mark Smyk, Peter Hinz, Al MacTavish & Dorothy Campbell), the meeting was attended by a total of 240 delegates. Thanks to very generous support from our corporate sponsors (Goldcorp Inc. – Musselwhite Mine, Osisko Mining Corporation, Activation Laboratories Ltd. (Actlabs), Cliffs Natural Resources Inc., Magma Metals (Canada) Limited, MMG Limited, Rio Tinto, Stillwater Canada Inc., Fladgate Exploration, Thunder Bay CEDC, Mega Precious Metals Inc., Rainy River Resources Ltd., Metals Creek Resources Corp., Midwest Institute of Geosciences and Engineering (MIGE), Benton Resources Corp. & Rare Earth Metals Inc.) we were able to provide free registration to the 60 students who attended. The two-day technical session began on Thursday morning with oral presentations on regional geology and then proceeded in chronological order from Archean topics through Friday afternoon’s presentations on Quaternary geology. A total of 27 talks were given, 7 of which were presented by students. A total of 31 posters were displayed, 15 of which were presented by student authors. The 2011 Goldich Medal was awarded to Jim Miller from the University of Minnesota Duluth. Mark Smyk presented the award during the annual banquet and supplied numerous examples of Jim’s passion for the geology of the Lake Superior region. David Overstreet gave the banquet address, discussing Human Adaptation to Late Pleistocene Landscapes - A View from Southeastern Wisconsin. For the first time we ran a core shack during the meeting with core from the Ring of Fire chromite deposits provided by Cliffs, from the Current Lake Ni-Cu-PGE deposit provided by Magma Metals Ltd. and core from Musselwhite Mine provided by Goldcorp Inc. The meeting offered 13 field trips that highlighted the geology of Thunder Bay region. Four premeeting trips were run on Wednesday, including the Lac des Iles Pd mine led by John Corkery (North American Palladium Ltd.), the Sudbury Impactoclastic Debrisites at Thunder Bay led by Bill Addison and Greg Brumpton, the Geology of the Sibley Peninsula led by Dr. Philip Fralick (Lakehead University) Mark Smyk & Riku Metsaranta (Ontario Geological Survey) and the Shebandowan greenstone belt led by Alan Aubut (Sibley Basin Group Geological Consulting Services Ltd.) and Dorothy Campbell (Ontario Geological Survey). On the Friday afternoon three trips were offered, the Geology of the City of Thunder Bay led by Mark Smyk (Ontario Geological Survey), the Panorama Amethyst Mine led by Steve Kissin (Lakehead University) and the Port Arthur building stone walking tour led by Peter Hinz (Ontario Geological Survey). Following the meeting, Goldcorp Inc. flew a lucky group of individuals up to the Musselwhite Mine for a tour led by John Biczok (Goldcorp Inc., Musselwhite Mine). Mark Puumala (OGS) led a trip to examine the Rehabilitation of the Past-Producing Shebandowan and North Coldstream Mines. Scott Hamilton (Lakehead University) took a group to look at the Geoarchaeology of Thunder Bay. Rob Cundari & Pete Hollings (Lakehead University) and Mark Smyk (Ontario Geological Survey) led a trip to look at the Midcontinent Rift intrusions. Greg Brumpton led a reprise of the Ejecta trip and Mark Smyk (OGS) and Philip Fralick (Lakehead University) took a group on a two-day trip to examine the geology along the Highway 527 xx �Transect. Many of the trips sold out and all were well-attended. On Friday evening the organizers hosted a barbeque social event for ILSG participants at the Whitewater Golf Club. The Institute’s Board of Directors met on May 17 to discuss the business of the Institute. The meeting was attended by Al MacTavish, Peter Hinz, Mark Smyk, George Hudak, Bill Addison, Dorothy Campbell, Mark Jirsa and Pete Hollings. ILSG Secretary Hollings took the minutes of the meeting, that are as follows: 1. Accepted the report of the Chair for the 58th ILSG, Ashland, Wisconsin; as printed in the 2012 Proceedings Volume, and minutes of last Board meeting, May 19, 2011. 2. Accepted the 2011-2012 ILSG Financial Summary. 3. Accepted the 2011-2012 ILSG Secretary’s report. 4. Appointed Al MacTavish as the on-going ILSG Board Member. 5. Discussed and approved 2013 (59th annual) meeting location. 6. Replaced Mary Louise Hill as “academic member” on Goldich Committee with Bernhardt Saini-Eidukat. 7. Discussed and approved the naming of the student research award after Joe Mancuso and student paper awards after Doug Duskin The Chair would like to thank all those who assisted with the running of this year’s meeting either by chairing sessions, running field trips or helping with the Registration desk. He is particularly appreciative of the work of the local organizing committee who made his job much easier through their tireless efforts. Respectfully submitted, Pete Hollings Chair, 58th Institute on Lake Superior Geology xxi �Sponsors The following organizations made general contributions to the 59th Annual Meeting. We thank them for their commitment to the Institute on Lake Superior Geology. For the past 59 years this organization has thrived as a result of the interest of individuals, corporations, universities and government agencies. The dedication to an exchange of scientific ideas and a passion for field trips has enabled the Institute to provide one of its primary objectives – to promote better understanding of the geology of the Lake Superior region. EAGLE MINE xxii �PROGRAM WEDNESDAY MAY 8, 2013 8:00 a.m. FIELD TRIP 1: GEOLOGIC OVERVIEW OF THE KEWEENAW PENINSULA, MICHIGAN Ted Bornhorst, Michigan Tech 8:00 a.m. FIELD TRIP 2: CALEDONIA MINE, KEWEENAW PENINSULA NATIVE COPPER DISTRICT, ONTONAGON COUNTY, MICHIGAN Robert Barron, Michigan Tech Richard Whiteman, Red Metal Minerals Ted Bornhorst, Michigan Tech 8:00 a.m. FIELD TRIP 3: GEOLOGY OF SILVER MOUNTAIN, HOUGHTON COUNTY, MICHIGAN Evgeniy Kulakov, Michigan Tech 5:00 p.m. Return of Trips 1, 2, and 3 4:00 p.m. - 8:00 p.m. Registration at Franklin Square Inn, 7th Floor 7:00 p.m. - 9:00 p.m. Ice Breaker Social and Poster Session, Franklin Square Inn, 7th Floor THURSDAY MAY 9, 2013 Note: Asterisk * denotes a student eligible for Best Student Paper Award + denotes students which qualify for travel awards, but not Best Student Paper awards 8:00 a.m. - 2:40 noon REGISTRATION 8:20 a.m. INTRODUCTORY REMARKS Theodore J. Bornhorst and Allan R. Blaske, Co-Chairs, 2013 ILSG TECHNICAL SESSION I Session Chairs: Marcia Bjørnerud – Lawrence University Terry Boerboom – Minnesota Geological Survey 8:30 a.m. Peter Hollings and Mark Smyk Preliminary geochemical analysis of the Nipigon Bay granites, northern Lake Superior xxiii �8:50 a.m. Emily Smyk*, Pete Hollings, and John Biczok Geochemical and petrographic study of a Mesoarchean felsic metavolcanic unit near Musselwhite Mine, North Caribou greenstone belt, northwestern Ontario 9:10 a.m. Aubrey Lee* and Jim Miller The Igneous Stratigraphy of the Bad Vermilion Intrusion, Mine Centre, Ontario, Canada: Which Way is Up? 9:30 a.m. Skylar Schmidt* and Mary Louise Hill North American Palladium’s Lac des Iles mine: Evidence for high temperature deformation and possible control on Pd mineralization 9:50 a.m. Ben Kuzmich*, Pete Hollings, and Michel G. Houlé Preliminary Investigations of the Fe-Ti-V-P mineralization associated with the Thunderbird and Butler gabbroic intrusions within the McFaulds greenstone belt, Superior Province, Northern Ontario, Canada 10:10 a.m. to 10:30 a.m. COFFEE BREAK AND POSTER SESSION 10:30 a.m. Ian R. Dasti* and Stephen A. Kissin The Geochemistry and Mineralogy of the Sulfides within the Ni-Cu-PGE Shakespeare Deposit, Ontario 10:50 a.m. Erik Haroldson and Philip Brown Fluid Inclusion study of the Magino Archean Gold Deposit; Implications for Regional Mineralizing Systems 11:10 a.m. Sarah Canning, Zoran Madon, and Keith Wallace A Geological Model and Resource Update for the Hammond Reef Gold Deposit 11:30 a.m. LUNCH BREAK – 2013 ILSG Board Meeting (by invitation) TECHNICAL SESSION II Session Chairs: Paula Leier-Engelhardt – HydroGeo Solutions Glenn Scott – Cliffs Mining Service Company 1:00 p.m. Phillip Larson Chemostratigraphy of the Biwabik Iron Formation: Implications for Basin Longevity and Evolution 1:20 p.m. Christopher Yip*and Philip Fralick Sedimentology and Geochemistry of a Regressive Surface in the Chemical Sediments of the Paleoproterozoic Gunflint Formation xxiv �1:40 p.m. Gordon Medaris Jr., Terry Boerboom, Brian Jicha and Brad Singer The McGrath metasaprolite: viewing Paleoproterozoic weathering through a veil of metamorphism and metasomatism 2:00 p.m. Mark Puumala Natural Groundwater Geochemistry in Bedrock of the Thunder Bay Area 2:20 p.m. – 2:40 p.m. COFFEE BREAK AND POSTER SESSION 2:40 p.m. William F. Cannon, Laurel G. Woodruff, and Klaus J. Schulz The Hiawatha Graywacke of the Iron River-Crystal Falls district, Michigan: a megaturbidite triggered by seismicity related to the 1850 Ma Sudbury impact 3:00 p.m. Monica M. Karman* and Philip W. Fralick Sedimentology and Paleographic Reconstruction of the Strata Adjacent to the Sudbury Impact Layer in a Cored Drillhole 3:20 p.m. Daniel LaFontaine* and Philip Fralick Sedimentology and geochemistry of the Espanola Formation, Huronian Supergroup 3:40 p.m. Breanne Beh*and Philip Fralick Depositional Processes Operating on the Paleoproterozoic Gowganda Ice Margin 4:00 p.m. SESSION ENDS 6:00 p.m. ICE BREAKER – MIXER – CASH BAR 7:00 p.m. ANNUAL BANQUET AND AWARD PRESENTATION • Announcement of 60th Annual Meeting Location • 2013 Goldich Award Presentation to Tom Waggoner • 2013 Banquet Address by Dr. James W. Ashley, Postdoctoral Research Associate LROC, School of Earth and Space Exploration, Arizona State University All registered participants are welcome to the banquet address xxv �FRIDAY MAY 10, 2013 8:25 a.m. INTRODUCTORY REMARKS Theodore J. Bornhorst and Allan R. Blaske, Co-Chairs, 2013 ILSG TECHNICAL SESSION III Session Chairs: Jack Berkley – SUNY Fredonia Joe Maki – Michigan Department of Environmental Quality 8:30 a.m. Teresa Johnson*, Richard Wendlandt, and James Shannon Geochemistry of reversely-polarized intrusions along the SW limb of the Midcontinent rift system, Carlton County, Minnesota 8:50 a.m. Raymond Anderson and Ryan Clark The Northeast Iowa Intrusive Complex; a Duluth Complex analog? What we know as the investigation begins. 9:10 a.m. Benjamin Drenth, Raymond Anderson, Val Chandler, William Cannon, Klaus Schulz, Joshua M. Feinberg, Paul Bedrosian, and Andy M. Kass High-resolution, multi-method geophysical imaging of a portion of the Northeast Iowa Intrusive Complex 9:30 a.m. Robert Cundari, Peter Hollings, and Mark Smyk Geochemistry of the Logan Igneous Suite and implications for the magmatic evolution of the northern part of the Midcontinent Rift 9:50 a.m. Jack Berkley Lithospheric Delamination during Midcontinent Rifting 10:10 a.m. – 10:30 a.m. COFFEE BREAK AND END OF POSTER SESSION 10:30 a.m. Connor Mulcahy, Edward Hansen, Theodore Bornhorst, and Dieter Rhede Chemical Zoning in Calc-Silicate Minerals Associated with Native Copper from the Keweenaw Peninsula, Michigan 10:50 a.m. Alex C. Brown Brine viscosity vs. temperature: A key to copper deposition in the finest-grained basal Nonesuch Formation, White Pine-Presque Isle district, northern Michigan 11:10 a.m. Stanley L. Vitton Glacial Lake Ontonagon and the Development of Large Scale Landslides 11:30 a.m. Bruce A. Brown Hydrofrac Sand: A major mining boom in the upper Midwest xxvi �11:50 a.m. Val W. Chandler and Richard S. Lively Passive-aggressive geophysics: An update on using the horizontal-to-vertical spectral ratio (HVSR) passive seismic method for determining glacial deposit thickness in Minnesota 12:10 p.m. Presentation of Student Awards Student Travel Awards Best Student Paper Awards 12:30 p.m. LUNCH BREAK AND END OF TECHNICAL SESSIONS 2:00 p.m.–7:00 p.m. FIELD TRIP 4: OPEN HOUSE AT A. E. SEAMAN MINERAL MUSEUM, MICHIGAN TECH, 1404 E. SHARON AVENUE, HOUGHTON Ted Bornhorst, Museum Director FIELD TRIP 5 PRESENTATION AT 8:00 P.M., AMERICINN, SILVER CITY, MI SATURDAY MAY 11, 2013 8:00 a.m. FIELD TRIP 5: GEOLOGY OF THE KEWEENAWAN SUPERGROUP, PORCUPINE MOUNTAINS, ONTONAGON AND GOGEBIC COUNTIES, MICHIGAN William Cannon, Laurel Woodruff, Klaus Schulz, Suzanne Nicholson, USGS LEAVES FROM AMERICINN, SILVER CITY 7:00 a.m. FIELD TRIP 6: GEOLOGY AND ENVIRONMENTAL SITE CONDITIONS OF THE COPPERWOOD DEPOSIT, GOGEBIC COUNTY, MICHIGAN Ted Bornhorst, Michigan Tech Allan Blaske, AECOM Dave Anderson and Tom Repaal, Orvana Resources US Corp. MEETS AT ORVANA RESOURCES US CORP OFFICE IN IRONWOOD, MICHIGAN AT 8:30 AM CDT LEAVES HOUGHTON FRANKLIN SQUARE INN AT 7:00 EDT 5:00 p.m. Return of Trips 5 and 6 END OF 59TH ANNUAL INSTITUTE ON LAKE SUPERIOR GEOLOGY xxvii �POSTER PRESENTATIONS Steven D. J. Baumann, Alex B. Cory and David Wilson Precambrian Faulting in the Ripon Wisconsin Area and Its Impacts on Groundwater Contamination, Originating at Superfund Site Ripon NN/FF Landfill Craig Caton Crystallization of Chrome Spinel in the Southern Troctolite Zone of the Bald Eagle Intrusion, Duluth Complex, Northeastern MN Jonathan Dyess* and Vicki Hansen Application of LiDAR to resolving regional tectonic and glacial fabrics in glaciated terrane: An example from an Archean granite-greenstone belt in NE Minnesota Jonathan Dyess* and Vicki Hansen Structural and Kinematic Analysis of the Shagawa Lake Shear Zone and Snowbank Lake Stock, Superior Province, NE Minnesota Ellen Fehrs+, Edward Kenny+, John Kuchma+, Sarah Sauer+, William Sylvester+, and George Hudak Bedrock Geologic Map of the Putnam Lake Area, St. Louis County, NE Minnesota – Precambrian Research Center Capstone Project George Hudak, Stephen Monson Geerts, Larry Zanko, April Severson, Allison Severson, Stuart Kramer and Bryan Bandli The Minnesota Taconite Workers Health Study: Environmental Study of Airborne Particulate Matter - 2013 Update Mark A. Jirsa Bedrock geologic map of the western Gunflint Trail area, northeastern Minnesota Mark A. Jirsa, Dale R. Setterholm, and V. W. Chandler Minnesota River Valley subprovince as depicted on a new bedrock geologic map of Renville County, southwestern Minnesota Katrina Korman+, Suzanne Craddock+, Michael Doyle+, Jessica Walter+, Aubrey Lee+, and Mark Jirsa Geologic mapping of Neoarchean and Paleoproterozoic rocks near Ester Lake by students of the Precambrian Research Center's 2012 field camp – Precambrian Research Center Capstone Project xxviii �Mark Leatherman*, Edward Ripley, Dean Rossell, Andrew Ware, and Chusi Li Geochemistry and origin of slate-hosted massive sulfides of the Eagle Ni-Cu-PGE Deposit, northern Michigan: A preliminary study. Aubrey Lee*, and Jim Miller Field, Petrographic, and Geochemical Study of the Bad Vermilion Intrusion, Mine Centre, Ontario, Canada Adam Leu+, Lionel Djon+, Emily LaPietra+, Zech Martin+, Ricardo Martinez+, and Jim Miller 2012 Precambrian Field Camp Mapping in the Wilder Lake Intrusion, Lake County, Northeastern Minnesota – Precambrian Research Center Capstone Project Steven Losh and Ryan Rague Silica Remobilization in the Biwabik Iron Formation, Minnesota USA Brynley Nadziejka* and Marcia Bjørnerud Contrasting pressure-temperature-time paths for high-grade metamorphic rocks in the interior of the Penokean-Yavapai orogenic belt, southern Lake Superior region Matthew Schmus*, Prajukit Bhattacharyya, and David Hart Effects of Preexisting Fractures on Groundwater Flow Today Klaus J. Schulz, William F. Cannon, and Laurel G. Woodruff The Parent Lake Volcanics: Product of a phreatomagmatic eruption of basalt during deposition of the Michigamme Formation? Brent Trevisan*, Pete Hollings and Doreen Ames Petrology, mineralization, and alteration of the Thunder mafic to ultramafic intrusion, Midcontinent Rift, Thunder Bay Peter Voice, William Harrison, and Joyashish Thakurta A Preliminary Survey of the Geology of the Pre- Michigan Basin Rocks of the Southern Peninsula xxix �The Northeast Iowa Intrusive Complex; a Duluth Complex analog? What we know as the investigation begins. ANDERSON, Raymond and CLARK, Ryan Iowa Geological and Water Survey, 109 Trowbridge Hall, Iowa City, Iowa 52242-1319 The Northeast Iowa Intrusive Complex (NEIIC) is defined by a suite of gravity and magnetic anomalies that stretch from east-central Iowa to southeast Minnesota and are currently interpreted as mafic intrusions. They are characterized by a series of intersecting circular positive gravity anomalies and corresponding positive and negative aeromagnetic anomalies (Figs 1, 2). The NEIIC was described by Pals and Anderson (2011) who proposed that the complex was of Keweenawan age and analogous to the Duluth Complex. These anomalies were sampled by drilling in four locations (Figs 1, 2). Cores in Minnesota penetrated a "gabbroic rock" (B-1) and a metagabbro (BO-1) that was dated at 1760 Ma (Van Schmus et al., 2007) and may be country rock to the NEIIC. In northeast Iowa a 90 year-old oil test (Pioneer #1) penetrated 480 m of "troctolite" or "olivine gabbro" which yielded a Rb/Sr age of 1,130 Ma (Lidiak et al., 1966); no samples are currently available. A mineral exploration core recovered from northeast Iowa (A12) sampled 220 m of serpentenite and troctolite from a dike-like anomaly. Drill data and other interpretations suggest that the Precambrian basement surface lies about 350 m (north) to about 900 m (south) below the land surface. Many of the geophysical anomalies associated with the NEIIC have been surveyed and modeled by geology students (Fig 3). These anomalies have been interpreted as mafic intrusives, including lopoliths, dikes, and plug-like intrusions (see Dixt, 1984; Heathcote, 1979; Kittleson, 1975; Stepanek, 1978). Two of the modeled intrusives (Dixt, 1984; Heathcote, 1979) were interpreted as mafic lopoliths with maximum diameters of 47 km and 37 km with density contrasts of +0.3 g/cm3 with the felsic country rocks. Heathcote’s (1979) model displayed a Koeningsberger ratio (remanent to induced magnetism) of 6.78 and model remanent vectors consistent with Keweenawan directions and angles. Her model also featured a magnetic field reversal (normal to reverse) captured during the cooling of the intrusive (Fig 4). Kittleson’s (1975) analysis of the Osborne A1-2 core revealed an ultramafic composition composed of cyclic layers of olivine cumulates and olivine-plagioclase cumulates that constituted the upper portion of a dike-like intrusive with a width of about 300 m and a depth extent of about 4.8 km. The age of these intrusives is currently interpreted as Keweenawan based on several lines of evidence. The rocks into which the NEIIC was intruded were interpreted as Yavapai (geon 17) by Van Schmus and others (2007). The only subsequent major magmatism in the area was the widespread felsic anorogenic events (ca. 1,470 and 1,370 Ma) which produced low density plutons with gravity anomalies lower than regional values. Additionally, the 1,130 Ma age (Pioneer #1), the use of Keweenawan remanence vectors in pluton modeling (see above), and the trend of the NEIIC, subparallel to the Keweenawan Midcontinent Rift System, argue for a Keweenawan age. The U.S. Geological Survey, working with the state geological surveys of Iowa and Minnesota, has recently began investigation the potential of the NEIIC for Duluth Complex-like platinum group, nickel, and copper mineralization. Initial stages include acquisition of additional geophysical data and comprehensive modeling and interpretation, with possible core drilling to follow. 1 �Figure 1. Shaded relief total magnetic intensity map of NEIIC and Midcontinent Rift System (MRS) (http://www.mngs.umn.edu/nicegeo/niceimgs.htm) Figure 2. Bouguer gravity anomaly map of NEIIC (http://www.mngs.umn.edu/nicegeo/niceimgs.htm) Figure 3. Map identifying NEIIC intrusives that were modeled by geology students on Aeromagnetic Map of Iowa Figure 4. Magnetic model for north-south profile across Manchester Anomaly (Heathcote, 1979) References Cited Dixit, S.R., 1984, A geologic interpretation of the Vinton geophysical anomaly, in east-central Iowa: unpub. M.S. thesis, University of Iowa, 136 p. Heathcote, S.K.H., 1979, Geological interpretation of the Manchester geophysical anomaly, Delaware County, Iowa: unpub. M.S.. thesis, University of Iowa, 110 p. Kittleson, K.L., 1975, A gravity study of the Osborne magnetic anomaly, Clayton County, Iowa: unpub. M.S. thesis, University of Iowa, 81 p. Lidiak, E.G., Marvin, R.F., Tomas, H.H., and Bass, S.S.,1966, Geochronology of the Midcontinent Region No. 3: Journal of Geophysical Research, v. 71, p. 5,427-5,438. Pals, D.W., and Anderson, R.R., 2011, Reassembling Iowa: spatial and temporal evaluation of the mineral potential of the Iowa segment of the Micontinent Rift and related plutons: Geological Society of America Abstracts with Programs, v. 43, no. 5 p. 396. Stepanek, J.G., 1978, Geological interpretation of an aeromagnetic anomaly near Randalia, northeastern Iowa: unpub. M.S. thesis, University of Iowa, 105 p. Van Schmus, W.R., Schneider, D.A., Holm, D.K., Dodson, S., and Nelson, B.K., 2007, New insights into the southern margin of the Archean-Proterozoic boundary in the north-central United States based on U-Pb, Sm-Nd, and Ar-Ar geochronology: Precambrian Research, v. 157, p. 80-105. 2 �Precambrian Faulting in the Ripon Wisconsin Area and Its Impacts on Groundwater Contamination Originating at Superfund Site Ripon NN/FF Landfill BAUMANN, Steven D.J.1, CORY, Alex B.1 and WILSON, David2 1 Geology Section, Midwest Institute of Geosciences and Engineering, 2328 W. Touhy Ave. Chicago, IL 60645 2 Superfund Division, U.S. EPA Region 5, 77 West Jackson Blvd. Chicago, IL 60604 Faults are known to serve as conduits for the migration of contaminants, especially in the subsurface. One case of particular interest is the Superfund site designated as the Ripon NN/FF Landfill (Ripon Superfund), center of which is located 1,000 feet south-southeast of the junction of County Road FF and South Koro Road (GPS: 43.866850o, -88.870945o). This superfund site has historically shown vinyl chloride contamination in several off-site monitoring wells. The nearest potable well for the city of Ripon has also shown contamination. The contamination has spread in a manner that is inconsistent with traditional modeling techniques, suggesting that something unknown is occurring at the Precambrian-Cambrian contact. Thanks to information provided by David Wilson of the United States Environmental Protection Agency (U.S. EPA) new insight has been gained about the subsurface geology. The reason for the unusual migration of contaminants is most likely due to unmapped parallel faults in the Precambrian basement rock. A detailed bedrock geologic map was compiled by Steven Baumann in 2011 of Ripon and the surrounding area, to include the Ripon Superfund location. Faults in the basement rock were expected at the time the bedrock map was compiled, but could not be confirmed. In 1993 a deep borehole (P-107D) into the bedrock was drilled through the glacial cover, through the underlying Cambrian sands, and down into the Precambrian basement rock. During the compilation of the Ripon Wisconsin bedrock map (Baumann 2011), the superfund borings were unknown to the author. Even without the Superfund logs the surficial field work conducted in the Ripon area yielded several distinct and prominent structural features. No faults were observed extending up through the youngest surface rocks. Although faulting was suspected at the time of mapping, the available data did not provide direct evidence of the presence of faults. Based on the detailed log for boring P-107D (the only detailed log to significantly penetrate the Precambrian) the basement consists of a thin layer of purple quartzite on top of a thick red granitic or syenite sequence, similar to the rocks exposed in the Baraboo Wisconsin area. The Precambrian geology of Ripon is expected to be similar to that of Baraboo. At Baraboo, there is highly fractured Precambrian quartzite on top of igneous rocks, which in turn are covered by thick sequences of Cambrian and Ordovician rocks. The only real difference between Baraboo and Ripon is the thick cover of glacial outwash deposits present at Ripon. The western half of the Baraboo Precambrian exposures are part of the “Driftless Area” and were not glaciated. The similarities between Baraboo and Ripon, give justification for modeling structural features in the Ripon area in a similar manner to the structures observed in the Baraboo area. The presence of quartzite on the bedrock high penetrated by P-107D is a strong indication of local faulting. Based on the orientation of known local structures in the Cambrian and Ordovician a fault just south of P-107D is expected to trend N55W to N75W based on the orientation of the nearby Ripon Arch. However, near the theoretical north limit of the Ripon Arch (near Arcade Acres) does turn more north then west-northwest. The understanding of any local faulting in the Precambrian is of key importance due to the likelihood of the faults serving as a route of contamination, possibly leading to the City of Ripon’s #9 Supply Well (Wilson 2012). 3 �Any faults in the area are probably roughly parallel to each other and the Ripon Arch. The faults likely show fracturing 60 feet or so within the Precambrian-Cambrian (PC-C) boundary and most likely fade out within the Cambrian Wonewoc Formation. Although fracturing is very likely near the PC-C boundary, the faults are probably tight deeper than 30 feet below the PC-C boundary. The total displacement of the faults is likely on the order of 40 to 80 feet. This will greatly affect groundwater flow direction in the deepest hydrogeologic unit designated as “Layer 4” (Wilson 2012). “Layer 4” is the basal known confined aquifer in the Cambrian and it likely connects to the groundwater in the faults and fractures in the Precambrian basement. Weathering patterns at the PC-C boundary were not noted in the P-107D log. However, using the PC-C boundary at Baraboo, it is likely that the contact is significantly lithified, yet extremely porous. Although the groundwater in P-107D at the PC-C boundary is likely connected to “Layer 4” a definite connection cannot be ascertained without additional data. At present fault dynamics and contaminant migration cannot be definitively determined at present. Additional deep basement down gradient sentinel wells will need to be drilled in order to develop a good model and plan to stop additional contaminants from reaching the Ripon #9 Supply Well. References: Baumann, S.D.J., 2011. Geologic Bedrock Map of the Ripon Area, Green Lake and Fond du Lac Counties, Wisconsin U.S.A. Midwest Institute of Geosciences and Engineering M-102011-2A Baumann, S.D.J., 2011. Surficial Geologic Map of the Upper Narrows near Rock Springs, Sauk County, Wisconsin U.S.A. Midwest Institute of Geosciences and Engineering M-092011-3A Dalziel, I.D.W., Dott Jr., R.H., 1970. Information Circular No. 14: Geology of the Baraboo District Wisconsin. Wisconsin Geological and Natural History Survey Fassbender, J.L., Noel, M.R., Ronk, J.J. 1994. Remedial Investigation Report Ripon FF/NN Landfill Volumes I and II. Hydro-Search Inc. Contract SF-92-01 Wilson, D., 2012. Review of the Monitored Natural Attenuation for Ripon Landfill Site WI. U.S. EPA, Region 5, Superfund Division, Memorandum 4 �DEPOSITIONAL PROCESSES OPERATING ON THE PALEOPROTEROZOIC GOWGANDA ICE MARGIN BEH, Breanne1, and FRALICK1, Philip 1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1, bbeh@lakeheadu.ca Glacial sedimentary rocks of the Huronian Supergroup crop out along the north shore of Lake Huron and were likely deposited on what is thought to have been a divergent continental margin (Fralick and Miall, 1981; 1989). The rocks of the Gowganda Formation record one of three glacial events preserved in the Supergroup and are therefore of interest for developing further glaciomarine models (Puffett, 1974) and furthering understanding of this early stage in Earth’s history. Data has been collected in five main study areas in an attempt to cover as much of the ancient continental margin as possible. The study areas include Espanola; Elliot Lake; Thessalon; and Cobalt, Ontario; and Marquette, Michigan. There are two glaciogenic formations in the Marquette area of Paleoproterozoic age, the Reany Creek Formation and the Enchantment Lake Formation. The Enchantment Lake Formation has been chronostratigraphically correlated to the Huronian Supergroup based on U-Pb age determination on detrital zircon, 2317±6 Ma, and diagenetic xenotime, 2133±11 Ma (Vallini et al., 2006). As these formations are present in such close proximity to each other, and there are no Archean glacial events recorded in the rest of the Canadian Shield, it is reasonable to correlate them with the Gowganda Formation, the thickest and most commonly preserved of the three Huronian glacial events. Stratigraphic sections were compiled in each of the study areas and the sedimentary rocks were grouped into seven lithofacies associations (LA): 1) Planar Cross-Stratified Sandstone LA, 2) Basal Breccia LA, 3) Diamictite LA, 4) Interlayered Siltstone and Fine-Grained Sandstone LA, 5) Slump LA, 6) Heterogeneous Sandstone LA and 7) Quartz-Rich Sandstone LA. These lithofacies associations likely represent a sequence of depositional environments on a shallow continental shelf. Initially, the shelf was dominated by what were likely large-scale, low-angle sandwaves (Figure 1A), interbedded with successions of wavy bedding and possible hummocky cross-stratification indicating an open-water setting with tidal and storm processes reworking the sediments. The shelf then gradually evolved into an environment dominated by diamicite layers. The diamictite layers have dropstones (Figure 1B) as well as evidence of current activity indicating outsized clasts were likely being introduced into the environment as ice-rafted debris. Resedimentation events in the form of debris flows are thought to account for conglomeratic layers that are common in the Diamictite LA. The Interlayered Siltstone and Fine-Grained Sandstone LA, along with the Slump LA (Figure 1C), seem to indicate deposition from suspension in a prodelta setting where large slump events are common. The gradual transition into a more sandstone dominated LA, with an abundance of current-related sedimentary structures, is indicative of the shallowing and coarsening upwards succession common to deltaic deposits. A final transition into the Quartz-Rich Sandstone LA indicates a return to a sandy, current-dominated open continental shelf environment with abundant tidally generated sedimentary structures such as herringbone cross-stratification (Figure 1D). The Cobalt study area differs from this overall model in that evidence of grounded ice is present. Less exposure in the Marquette study area makes it difficult to draw overall conclusions on the evolution of the continental shelf but deposition in a subaqueous glacial outwash fan is hypothesized. 5 �Figure 1. A) Large-scale, low-angle planar cross-stratified sandstone. B) A dropstone compressing underlying laminations. C) A mud-block conglomerate of slumped prodelta deposits. D) Herringbone cross-stratified sandstone. REFERENCES FRALICK, P.W., AND MIALL, A.D., 1981. Grant 84: Sedimentology of the Matinenda Formation, in Pye, E.G., ed., Geoscience Research Grant Program, Summary of Research 1980 – 1981: Ontario Geological Survey, Miscellaneous Paper 98: 80-89. FRALICK, P.W., AND MIALL, A.D., 1989. Sedimentology of the Lower Huronian Supergroup (Early Proterozoic), Elliot Lake area, Ontario, Canada: Sedimentary Geology, 63: 127-153. PUFFETT, W.P., 1974. Geology of the Negaunee quadrangle, Marquette County, Michigan: U.S. Geological Survey, Professional Paper 788: 53 p. VALLINI, D.A., CANNON, W.F., AND SHULZ, K.J., 2006, Age constraints for Paleoproterozoic glaciation in the Lake Superior Region: detrital zircon and hydrothermal xenotime ages for the Chocolay Group, Marquette Range Supergroup: Canadian Journal of Earth Science, v. 43, p. 571-591. 6 �Lithospheric Delamination during Midcontinent Rifting BERKLEY, Jack, Department of Geosciences, Houghton Hall, SUNY Fredonia, Fredonia, NY 14063 USA The process of delamination, briefly defined as the physical peeling off and descent of a cold lithospheric slab into ascending hot mantle asthenosphere, has historically been applied mostly to convergent orogenic systems (Fig. 1a) involving continent-continent collision and subduction (e.g. Bird, 1979; Hamilton et al., 2004). Delamination has been invoked to explain perplexing tectonic-topographic expressions such as the Colorado Plateau, as well as massive collisional fold and thrust belts like the Himalayas, Alps, and more ancient terrains, notably the Mesoproterozoic Grenville Orogen (e.g., Wallner and Schmeling, 2010; Hamilton et al., 2004). Continental rift terrains (Fig. 1b), although dominated by extensional rather than convergent stress fields, are formed in the same general geophysical milieus as convergent systems (i.e., 30 km+ low-density crust plus upper mantle lithosphere -- overlying hot, low-viscosity asthenosphere). Rift terrains (Fig. 1b) are also subjected to upwelling heat sources with liberated volatiles (esp. water) that function to exacerbate the weakening of previously hyper-strained lithosphere. Given a few extenuating circumstances (see below), upper mantle delamination is predictable, or at least possible. (a) (b) Figure 1: (a) Convergent delamination of a mantle lithospheric root (gray) (from Schott and Schmeling,1998). (b) Extensional delamination in the Basin-Range province, USA (from Meissner and Mooney, 1998). If delamination played a controlling role in the production of magmatic suites of the Midcontinent Rift (MCR) specifically in the well-exposed Lake Superior region, what criteria can be used to detect that influence? Much depends on the precise sub-surface structural configuration underlying the rift environment, conditions that can vary widely from region to region – and, more importantly, along strike in the MCR. Recent and past studies show that the MR consists of two linear branches, the Western branch that extends northeast from Kansas to Lake Superior (active interior continental rift), and the Eastern branch (leaky transform fault) that extends southeast from Lake Superior to SE Michigan, and possibly as far as Kentucky. The bulk of igneous activity occurs in the Lake Superior basin due to the likely influence of a deep mantle plume (e.g., Merino et al., 2012; Davis and Green, 1997). Deep earth tomography reveals that early rift-induced normal faults have later transitioned to low-angle thrust faults in response to west-directed Grenville orogenic compression. In that light, conditions required for rift-induced delamination (“RID”, Wallner and Schmeling, 2010) include: (1) a thermal anomaly (e.g., plume), (2) low-strength lower crust, and (3) lateral density variations within the lower crust. 7 �Excess temperature and low yield stress at depth are especially critical parameters. The Lake Superior region meets the prima facie criteria required for delamination, although that fact does not guarantee that delamination has, in fact, occurred. On the other hand, the presence of late Grenville thrusting could provide a facilitating factor missing in most rift environments. Most confirmed or suspected delamination events occur during compressive, crust-thickening stress events, as in the relative nearby Grenville fold belt (as in the Adirondacks, e.g., Hamilton et al., 2004). Although not exhaustive, the following observations could be used to evaluate the role of delamination in the Lake Superior region: (1) Magmas produced by delamination would be primarily mafic (melting of hot mantle asthenosphere), but a variety of voluminous felsic magmas would also be expected from melting of Archean crust or differentiation of ponded mafic plutons, (2) Igneous rocks directly produced by delamination should show relatively young ages compared to, say, purely plume-generated magmas, and should be restricted in age, (3) Assuming a progressively peeling off and sinking lithospheric slab (or slabs) ages of pluton series may vary older-to-younger laterally across surface exposures, (4) Seismic studies show one or possibly two highly disrupted deep crustal zones under Lake Superior (Hamilton and Mereu, 1993) in which mantle material displays up-welling high up into crustal layers. Obviously, such areas should be considered prime targets for possible delamination effects on geographically associated igneous exposures. References Bird, P., 1979. Continental delamination and the Colorado plateau. Journal of Geophysical Research, 84: 7561-7571. Davis, D.W. and Green, J.C., 1997. Geochronology of the North American Mid-continent rift in western Lake Superior and implications for its geodynamic evolution. Canadian Journal of Earth Sciences, 34(4): 476-488. Hamilton, D.A. and Mereu, R.F., 1993. 2-D tomographic imaging across the North American Midcontinent Rift system; Geophysics Journal International. 112: 344-358. Hamilton, M.A., McLelland, J., and Selleck, B., 2004. SHRIMP U-Pb zircon geochronology of the Anorthosite-mangerite-charnockite-granite suite, Adirondack Mountains, New York: Ages of emplacement and metamorphism. Geological Society of America, Memoir 197: 337-355. Meissner, R. and Mooney, W., 1998. Weakness of the lower continental crust: a condition for delamination, uplift, and escape. Tectonophysics 296: 47-60. Merino, M., Keller, G., Stein, S. and Stein, C., 2012. Variations in mid-continent rift magma volumes consistent with microplate evolution. Pre-publication manuscript, Department Earth and Planetary Sciences, Northwestern University, Evanston, Ill. Schmeling, H., 2010. Dynamic models of continental rifting with melt generation. Tectonophysics, 480: 3347. Schott, B. and Schmeling, H., 1998. Delamination and detachment of a lithospheric root. Tectonophysics, 296: 225-247. Wallner, H. and Schmeling, H., 2010. Rift induced delamination of mantle lithosphere and crustal uplift: a new mechanism for explaining Rwenzori Mountains’ extreme elevation? International Journal Earth Science, 99: 1511-1524. 8 �Brine viscosity vs. temperature: A key to copper deposition in the finest-grained basal Nonesuch Formation, White Pine-Presque Isle district, northern Michigan BROWN, Alex C., 13250 rue Acadie, Pierrefonds, QC H9A 1K9, acbrown@polymtl.ca The sulfide-dominant main-stage stratiform copper (SSC) mineralization in the fine-grained carbonaceous Nonesuch Formation at White Pine, northern Michigan, has long been attributed to a vertical influx of oxidized low-temperature cupriferous brine from the underlying Copper Harbor Conglomerate aquifer during early Nonesuch diagenesis. However, despite the difficult influx of brine from a good aquifer into a fine-grained aquitard, the best-mineralized Nonesuch of the White Pine district is hosted by the finest-grained basal Nonesuch. This dilemma was addressed by White (1971) and Swenson et al. (2004) who showed, in 2-dimensional profiles, that brines driven southward out of the Lake Superior rift basin by compaction could have been forced upward into the basal Nonesuch where the Copper Harbor thins abruptly over a large volcanic dome (Porcupine Volcanics) located directly below the White Pine mine district. However, in 3-dimensions, those brines should have escaped largely to the east and west around the volcanic dome where the Copper Harbor is thick. Furthermore, compaction brines should have equilibrated with ferrous constituents of the aquifer and thus been too reducing to carry significant amounts of copper. On the other hand, copper may be carried in very significant amounts in oxidized lowtemperature brines such as generated during meteoric recharge (Brown, 2005). Furthermore, latent heat from a dormant or extinct resurgent caldera, represented here by the Porcupine Volcanics, could have locally increased the temperature of brine hosted by the Copper Harbor Conglomerate to ~100oC, the estimated maximum temperature experienced by the basal Nonesuch Formation at White Pine (Grigorita and Brown, 2002; Fig. 1). By analogy to modern calderas (e.g., the Valles caldera, New Mexico), heat may have emanated from the Porcupine Volcanics for more than one Fig. 1. a) Location map. b) A schematic illustration of resurgent caldera heat, from the recently dormant or extinct Porcupine Volcanics dome, inducing an exceptional brine infiltration into fine-grained basal Nonesuch beds in the White Pine-Presque Isle area. Lateral grain-size gradations within the Nonesuch are represented by shading: dark gray = very fine-grained, lighter gray = coarser grained. 9 �million years at rates (e.g., up to 2500 mW/m2; Goff and Gardner, 1994) far above the normal heat flow of intracontinental rifts (e.g., 60 to 120 mW/m2). Thermal blanketing by the fine-grained Nonesuch would have aided in the accumulation of anomalous heat in the Copper Harbor brine over the resurgent caldera. Heating of the brine from 20 to 100oC would have decreased its density by about 5% and thus encouraged upward buoyant circulation. More significantly, warming of the brine would have lowered its viscosity to ~30% of its viscosity at 20oC (Fig. 2). On the basis of density and viscosity changes alone, the vertical flux of brine into the basal Nonesuch in the area underlain by the resurgent caldera should have been ~3.5 times the flux in cooler distal areas. Applying Darcy’s law (Ingebritsen and Appold, 2012), a larger infiltration of brine into the basal Nonesuch is found even if the permeability of the finest-grained Nonesuch was 1/2 order of magnitude less than the permeability of more distal, coarser-grained Nonesuch. Furthermore, if brine infiltration into the basal Nonesuch was more rapid where it was warmed locally by a recently dormant or extinct resurgent caldera, replacement brine in the Copper Harbor aquifer should have been drawn laterally toward this area from surrounding areas (Fig. 3). This convergent supply of ore-forming brine in the Copper Harbor aquifer would have been more efficient than the ore-forming process of a linear meteoric recharge-driven scenario. Fig. 3. Sketch illustrating the convergence of meteoric recharge-driven brine (curved arrows) toward the base of the fine-grained Nonesuch Formation (due to anomalous heat accumulated there as a result of thermal blanketing) with a consequently high rate and focus of brine infiltration. Location of highland area is schematic only. Fig. 2. Viscosity (µ) vs. temperature from 20 to 100oC, for H2O and various brines. References Brown, A.C., 2005. Refinements for footwall red-bed diagenesis in the sediment-hosted stratiform copper deposits model. Economic Geology, 100: 765-771. Goff, F. and Gardner, J.N., 1994. Evolution of a mineralized geothermal system, Valles Caldera, New Mexico. Economic Geology, 89: 1803-1832. Grigorita, A. and Brown, A.C., 2002. A resurgent caldera model, rather than a compaction or gravity-driven model, for stratiform copper mineralization at White Pine, Michigan. Soc. Econ. Geol., Newsletter 50 (July 2002), p. 32. Ingebritsen, S.E. and Appold, M.S., 2012. The physical hydrogeology of ore deposits. Econ. Geology, 107: 559584. Swenson, J.B., Person, M., Raffensperger, J.P., Cannon, W.F., Woodruff, L.G. and Berndt, M.E., 2004. A hydro-geological model of stratiform copper mineralization in the Midcontinent Rift System, northern Michigan, USA. Geofluids, 4: 1-22. White, W.S., 1971. A paleohydrologic model for mineralization of the White Pine copper deposit, northern Michigan. Economic Geology, 66: 1-13. 10 �Hydrofrac Sand: A major mining boom in the upper Midwest BROWN, Bruce A. Badger Mining Corp., 409 South Church St., Berlin, WI 54923 The phenomenal growth of the hydrofrac sand industry in Wisconsin, Illinois, and Minnesota in the last five years has resulted in a mining boom the likes of which has not been seen since the discovery of lead-zinc and iron in the 19th century. In 2008 there were less than ten industrialsand mines in Wisconsin, including foundry sand mines. In 2012 the count was more than one hundred, almost exclusively hydrofrac sand operations. The sand boom is the direct result of the successful application of improved horizontal drilling techniques and hydraulic fracturing to hydrocarbon-rich shales that were previously too impermeable to develop by conventional vertical drilling. The hydraulic fracturing process involves applying high pressure to a well sufficient to fracture a hydrocarbon-bearing formation of low permeability. The pressure opens fractures around the well bore, and a proppant, usually sand grains, is injected into the well to prop the fractures open after the pressure is released, allowing oil or natural gas to flow into the well. To be useful as a proppant, sand must be pure quartz, of specific grain size, have high roundness and sphericity, and a high compressive strength. Hydrofrac sand has been produced in the region for over fifty years. The Jordan, Wonewoc, and Mount Simon sandstones of Cambrian age and the Ordovician St. Peter sandstone have long been recognized as excellent sources of proppant sand. Most of the new mines produce from the Wonewoc Formation, which is a good source of the finer (40-70) sand used for gas drilling. The Jordan and Mount Simon Formations are coarser and produce more of the 20-40 mesh sand favored for oil drilling. The St. Peter in Wisconsin and Minnesota is too fine for hydrofrac sand but is mined as a source for foundry sand. The rapid growth of natural gas drilling, particularly in Pennsylvania, Texas, and North Dakota resulted in an acute shortage of sand and high prices for proppant. Energy companies, established sand producers, and entrepreneurs with little experience in the industry rushed to the region to get into the proppant business. In late 2012 a surplus of natural gas resulted in a drop in demand for the finer grades of sand, just at the time that several large mines were coming on line. The industry is now facing the prospect of excess production capacity. The future of hydrofrac sand mining in the upper Midwest looks favorable. Hydraulic fracturing is the key to energy independence for the United States, and the Cambrian sandstones of Wisconsin and Minnesota are the best source of proppant sand in the country. The next few years will be interesting as the industry deals with overcapacity and with an ever-growing list of regulatory and land use issues that have resulted from rapid growth. 11 �A Geological Model and Resource Update for the Hammond Reef Gold Deposit CANNING, Sarah, MADON, Zoran, and WALLACE, Keith. Osisko Hammond Reef Gold Ltd, 101 Goodwin Street, Atikokan, ON P0T 1C0 The Hammond Reef Gold Deposit is located about 210 km west of Thunder Bay and 25 km north-east of Atikokan. On January 28, 2013, Osisko Mining Corporation released a new resource estimate of 7.2 Million ounces after completion of an aggressive definition drill program. The company drilled over 2,100 holes for a total of 629,000 m in the last 2 years and was able to upgrade 75% of the gold inventory into the Measured and Indicated category. Using a 0.5 g/t cut-off, the deposit contains 5.43 Million oz Measured+Indicated @ 0.86 g/t and 1.75 Million oz Inferred @ 0.72 g/t. Figure 1: Artist’s rendition of the Hammond Reef open pit and infrastructure. Permitting for mine development continues. The company recently submitted a Draft Environmental Impact Study/Environmental Assessment Report to both the Canadian Environmental Assessment Agency and the Ontario Ministry of Environment. The Project Feasibility Study is on schedule and expected to be finished by the second quarter of this year. Osisko has received letters of support from all Métis and First Nations communities affected by the project and continues its dialogue with them. 12 �Hammond Reef is situated in the Marmion batholith of the Wabigoon Subprovince near the north-east trending contact with the Finlayson greenstone belt. The Mesoarchean Marmion is a diverse assemblage of felsic intrusive rocks, predominantly tonalitic in composition that was later invaded by several intrusive pulses. These units are all variably sheared and altered along a 1 to 6 km wide anastomosing deformation corridor (Marmion Deformation Corridor - MDC) that is sub-parallel to the contact with the Finlayson volcanics. The MDC is the locus of numerous gold occurrences. As with many structurally-controlled gold deposits, Hammond Reef occurs within a significant flexure of the MDC. The MDC displays numerous characteristics of brittle-ductile deformation, including moderately to strongly sheared rocks, brecciation, veining and stockwork. The MDC is also characterized by a sericite-chlorite-ankerite-hematite alteration overprint. Gold was most likely introduced during this late Archean hydrothermal or metamorphic episode, along with pyrite and accessory sulfides and tellurides. Gold mineralization occurs in all lithological phases of the Marmion batholith, associated with fracture-controlled stockwork and pyrite. Native gold blebs are usually found within pyrite crystals – along py-py grain boundaries as well as healed fractures and inclusions. Free gold grains are found rarely on sericitic foliation planes. Accessory minerals include chalcopyrite, galena and hessite. Three main ore types were identified in the geological model that was developed for the Hammond Reef gold deposit. These include – 1) gold in structurally confined and pervasively altered tonalites, 2) gold impinging into partially altered tonalites, and 3) gold impinging into “unaltered” tonalites. 13 �The Hiawatha Graywacke of the Iron River-Crystal Falls district, Michigan: a megaturbidite triggered by seismicity related to the 1850 Ma Sudbury impact CANNON, W.F.1, WOODRUFF, L. G.2, SCHULZ, K.J1. 1 U.S. Geological Survey, Reston VA, 20191, wcannon@usgs.gov, kschulz@usgs.gov 2 U.S. Geological Survey, Mounds View, MN 55112, woodruff@usgs.gov The Hiawatha Graywacke is a coarse clastic unit, which includes breccias with fragments as large as a meter. It is underlain and overlain by chemical sediments and very fine clastic rocks of the Riverton Iron-formation and Stambaugh Formation respectively. The Hiawatha has long been interpreted as a submarine slump deposit generated by a strong earthquake (James and others, 1968). A connection to the 1850 Ma Sudbury impact event was established with recognition of shock metamorphic features (quartz grains with relict planar deformation features), as well as small fragments of devitrified glass (Cannon and others, 2009). The Hiawatha Graywacke was deposited in a backarc basin along the southern margin of the Superior craton and is the southernmost and deepest water occurrence of the Sudbury Impact Layer so far identified. It is about 20 m thick in the east, where it is almost entirely breccia, and 150 m or more thick in the west, where breccia is mostly near the base and is overlain by massive graywacke with interlayers of siltstone and breccia. The breccia is derived largely from the underlying Riverton Iron-formation. Clasts are chert and siderite, and rarely other sedimentary rocks. The matrix is predominantly fine-grained siderite. Sand-sized clasts of quartz are widespread and typically compose a few percent of the breccia matrix, indicating that material other than iron-formation also was incorporated into the breccias. Quartz grains that contain well preserved planar deformation features, indicative of impact shock, are widespread but generally sparse. A greater abundance of shocked quartz is found in the upper parts of the formation than in basal breccias. Massive siltstone beds in the upper part of the Hiawatha have the greatest abundance of shocked quartz that we have found in the Lake Superior region. This suggests that the breccias are largely locally derived by disaggregation and submarine slumping of the Riverton Iron-formation with incorporation of only minor amounts of other material, whereas the upper parts of the Hiawatha have a greater component of ejecta particles that arrived in the area while submarine slumping was in progress. In the western part of the district, a layer, as much as a few meters thick near the middle of the Hiawatha, is composed mostly of vesicular glass fragments with abundant shocked quartz, indicating that ejecta deposition locally overwhelmed deposition of terrestrial material. The Sudbury impact, about 550 km east of the Iron River-Crystal Falls area, generated an earthquakeof probably roughly magnitude 10.5 based on computer model results. This is about 30 times more powerful than the largest recorded earthquake. The first seismic wave arrived here about 1.5 minutes after the impact and disrupted the seabed, composed of partly consolidated chert and siderite. This material flowed down the paleoslope toward deeper parts of the basin. About 4 minutes later the first airborne ejecta arrived and settled through the water column to be incorporated to variable degrees into the still active submarine slumps. Thicker parts of the Hiawatha in the Iron River area have some stratigraphic variation in mineralogy suggesting that multiple sediment sources were sampled by multiple lobes of turbidites. A third catastrophic event caused by the impact should have been massive tsunami waves, arriving about an hour after the ejecta. We have not identified evidence of tsunami activity, but perhaps upper parts of the Hiawatha were reworked by tsunamis, followed by settling of suspended sediment from the water column to produce the more prominent bedding that characterizes the upper part of the formation. Accumulation of a large volume of slump debris in the western part of the basin requires a comparable amount of denudation elsewhere. Some areas in the southeastern part of the district are devoid of the Riverton Iron-formation and, more locally, of the underlying Dunn Creek Slate, 14 �so that the Hiawatha lies directly on older volcanic rocks. Perhaps these areas were the source for slump debris that was transported and deposited as the basal breccias farther west in the basin. Submarine slumps, well studied in many other parts of the world, commonly begin by mass wasting of large slabs of sediments, some of which become successively disaggregated and water-saturated, grade into debris flows, and eventually into true turbidites. We suggest that this process happened here as well and that there are unrecognized megaclasts of the Riverton within the western part of the basin. A peculiarity of the western part of the basin is a chaotic folding style shown both on maps and cross sections. The western folds are much more complex than in the eastern part of the district and all surrounding areas, where folding is intense but has a regular pattern of deformation. We suggest two possibilities for this unique structural style that might be related to massive slumping. (1) Parts of the Riverton in the western part of the basin may be megablocks of slump material that were folded during slumping. Extremely large blocks of this nature, some many kilometers in extent and hundreds of meters thick, are known elsewhere in the world (ref?). Perhaps part of the Riverton is such a block that was emplaced already folded, so that subsequent tectonic folding formed the uniquely complex pattern seen there. 2) The Hiawatha in the western part of the basin is a sedimentary mélange containing numerous megablocks of Riverton Iron-formation. Previous maps and cross sections of the area were interpreted on the assumption, perhaps unwarranted, of a continuous unit of Riverton and a coherent stratigraphy. By that assumption, all occurrences of iron-formation had to be connected to all others and stratigraphic relationships had to be maintained. To satisfy these requirements many folds had to be inferred. In contrast, if the Hiawatha is a sedimentary mélange containing megaclasts of the Riverton, many of these inferred folds are not required and the structural style, although still complex, may be simpler than previously interpreted. References Cannon, W.F., Schulz, K.J., Horton, J. Wright, Jr., and Kring, David A., 2009, The Sudbury impact layer in the Paleoproterozoic iron ranges of northern Michigan, USA: Geological Society of America Bulletin, v. 122, p. 50-75. Dutton, C.E., 1971, Geology of the Florence area, Wisconsin and Michigan: U.S. Geological Survey Professional Paper 633, 54 p. James, H.L., Dutton, C.E., Pettijohn, F.J., and Wier, K.L., 1968, Geology and ore deposits of the Iron River–Crystal Falls district, Iron County, Michigan: U.S. Geological Survey Professional Paper 570, 134 p. 15 �Crystallization of Chrome Spinel in the Southern Troctolite Zone of the Bald Eagle Intrusion, Duluth Complex, Northeastern MN Caton, Craig, Department of Geological Sciences, University of Minnesota Duluth, 114 Kirby Drive, Duluth MN 55812 Duluth Metals had acquired mineral rights to portions the Bald Eagle Intrusion (BEI), located in the Duluth Complex about 35 km southeast of Ely, MN as a target for Ni-Cu-PGE potential. Exploration drilling by Duluth Metals in the BEI, a funnel-shaped (Weiblen, 1965; Green et al., 1966), differentiated mafic layered intrusion within the 1.1 Ga Duluth Complex in northeastern Minnesota, has identified multiple stratiform intervals enriched in chromium spinel (Cr-spinel) mineralization. Centimeter-scale layers enriched in up to 50 vol. % Cr-spinel occur within troctolite host rocks, which compose most of the southern BEI (Weiblen, 1965). The principal objective of this study is to characterize the mineralogy and textural occurrence of these chromite layers in order to understand their genesis and the magmatic history of the BEI. This information will be used as an exploration tool for further exploration of the BEI and other mafic intrusions within the Duluth Complex. This poster presents the observations made of textures and stratigraphic occurrences of Cr-spinel enriched intervals found in the troctolitic cumulates of the Bald Eagle Intrusion as well as the chemical variations throughout the study area. These observations are focused on three Duluth Metals drill holes that have been logged and assayed for whole rock geochemistry. Using these data along with petrographic descriptions and mineral chemistry from SEM of Cr-spinel layers and adjacent lithologies, possible models for emplacement and crystallization can be theorized. Core logging and follow-up petrographic observations show a sharp basal contact to all semi-massive oxide intervals that denote the start of a cyclic unit. In general, a cycle consists of a lower semi-massive oxide interval followed by an ultramafic section that grades to troctolitic followed by anorthositic compositions. Variations of a cycle sequence may occur. Inclusions are often present in oxide units, however the origin of these inclusions under interpretation. These descriptions suggest that individual cycles could be the product of additional magma inputs with fractional crystallization, or a result of changes to the stability field at the contact between cycles. Research is ongoing and further interpretations will be made using data collected via SEM and microprobe to constrain petrogenesis of cyclical units and metallogenesis of Cr-spinel in the southern BEI. References Weiblen, P.W., 1965. A Funnel-Shaped, Gabbro Troctolite Intrusion in the Duluth Complex, Lake County, Minnesota: PH.D. Thesis University of Minnesota, Minneapolis, MN. Green, J.C., Phinney, W.C., and Weiblen, P.W., 1966, Geologic map of Gabbro Lake quadrangle, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, map M-2, scale 1: 31,680. 16 �Passive-aggressive geophysics: An update on using the horizontal-to-vertical spectral ratio (HVSR) passive seismic method for determining glacial deposit thickness in Minnesota Val W. Chandler and Richard S. Lively Minnesota Geological Survey, 2642 University Ave., St. Paul, MN 55114 chand004@umn.edu Considerable progress has been made on evaluating the horizontal-to-vertical-spectral ratio method (HVSR or sometimes H/V) for determining the thickness of glacial deposits in Minnesota. The HVSR method is used to estimate the primary resonant frequency (shear wave) of unconsolidated overburden. At this frequency, the horizontal components of oscillation are amplified relative to the vertical component. By dividing the averaged horizontal spectra by the vertical spectrum, a HVSR spectrum is produced, ideally with a pronounced peak at the primary resonant frequency. If the acoustic impedance (density*seismic velocity) at the overburdenbedrock contact differs by a factor of at least 2, the thickness (Z) of the unconsolidated materials can be estimated by the relationship: Z=af0b where f0 is the estimated primary resonant frequency, and a and b are parameters that are determined empirically for a given region from control points that have a range of known bedrock depths. The HVSR method has been used in several Minnesota Geological Survey (MGS) projects, with data acquired at over 675 sites state-wide. Roughly 40% of these were located at control points whose distribution allows the HVSR method to be evaluated in three different geologic environments; the seven-county Twin Cities Metropolitan area, south-central Minnesota, and the floodplains and terraces along major streams, including the Mississippi, Minnesota, and St. Croix Rivers. The 7-county Twin Cities Metropolitan area is particularly favorable for the HVSR method. Paleozoic strata comprising the bedrock surface are generally rigid, with little soft material, such as saprolith or Cretaceous sediment present and the glacial sequence is fairly simple, consisting chiefly of late Wisconsinan deposits. Analysis of 41 selected control points exhibiting single, high-amplitude (>3.5) HVSR peaks produced a fitted curve in the form of the above equation with an R value (Correlation Coefficient) = 0.968. Depth estimates at the 41control points have an average error of 13% and 90% are within +/-25% of the known bedrock depths. These figures serve as a proxy for expected error using HVSR peaks of similar quality in areas that lack well control. For low-quality peaks the error can be significantly greater; applying the same fitted curve to 20 control stations that exhibited flat-topped peaks, interfering multiple peaks, or low-amplitude (<3.5) peaks produced an average error of 26% and only 60% of the estimates were within +/-25% of known bedrock depth. Regardless of error for individual depth estimates however, the HVSR method is still very useful as a simple mapping tool, for example, it was highly effective for mapping the trace of a buried bedrock ravine in central Washington County which had been largely missed by drillhole and seismic data. The terrain of south-central Minnesota presents some sobering challenges to the HVSR method. Glacial deposits, which include both Wisconsinan and Pre-Wisconsinan materials, tend to be thick, complex, and have dense, over-consolidated tills in the lower parts of the section. These deposits may also overlie a bedrock surface composed of soft materials, such as saprolith or Cretaceous sediment. Hence, impedance contrasts that can produce a HVSR peak and depth information may actually occur within the glacial sequence and may miss the bedrock surface 17 �altogether. In some instances, the HVSR data may also produce a trough in addition to a peak (at 2 time the frequency of the peak) and occasionally the trough provides a better indication of bedrock depth than the peak, especially for depths of >100 meters. In other instances no useful HVSR signal can be confidently extracted from any of the data. The effect of soft bedrock was evaluated by comparing control points reporting either saprolith or Cretaceous strata against an idealized power-curve, based on 27 control points where these soft materials appeared to be either thin or absent. Correlation for this curve has an R value = 0.953. Use of this curve tends to overestimate the depth to saprolitic bedrock by as much as 30-50%, implying that the HVSR signal is picking up an interface below the bedrock surface, likely somewhere in the transition to fresh bedrock. For areas with a Cretaceous bedrock surface, the overestimate can be much worse, and it appears that the HVSR signal is actually responding to the Pre-Cretaceous surface rather than the Cretaceous bedrock surface. Overall the HVSR method must be applied cautiously in south-central Minnesota, and in other areas that are likely to have similar conditions. Nonetheless, useful information can still be derived at many locations. In contrast to south-central Minnesota the floodplains and terraces along major streams in eastern Minnesota provide an almost ideal environment for the HVSR method. The channels in this part of the state served as sluiceways for melt-water during the closing stages of the Wisconsinan glaciation, and the valley bottoms have been largely swept clean of soft materials such as Cretaceous strata, saprolith, or earlier glacial deposits, and replaced by poorly consolidated outwash, fluvial and lacustrine deposits. A strong acoustic impedance contrast is expected at the bedrock surface and the observed HVSR peaks, which usually have single, very high-amplitude (>5) signatures, are consistent with this. Using 37 control stations a curve was established with an R correlation value = 0.951. Curve-based depth estimates at the control points have an average percentage error of 20% and 82% are within +/-25% of observed bedrock depths. The greatest errors generally occur at depths less than 20 meters, and may reflect sloping or uneven bedrock surfaces near valley side-walls. In any case, the relationship derived here should be useful for rough estimates of bedrock depth along the bottoms and terraces of the major river valleys in eastern Minnesota and adjacent areas Although, the HVSR method does not quite match conventional seismic profiling for accuracy and derivative information, the advantages of passive seismic for determining depth to bedrock include rapid data collection (usually 16 minutes recording time), much lower equipment and field costs, relative ease of data analysis and large number of samples that can be collected within a given area. In addition, rough estimates of depth with a denser array of points are quite adequate for many kinds of geological applications. Finally, the HVSR method can be readily applied in areas of significant cultural noise, where conventional seismic profiling is difficult. ACKNOWLEDGEMENTS This study was supported by the Minnesota Legislature through the County Geologic Atlas Program of the Legislative and Citizens Commission on Minnesota Resources. Additional support was through The STATEMAP Program of the U. S. Geological Survey and the State Special Appropriation of the Minnesota Geological Survey. 18 �Geochemistry of the Logan Igneous Suite and implications for the magmatic evolution of the northern part of the Midcontinent Rift CUNDARI, Robert1,2, HOLLINGS, Peter2, and SMYK, Mark1 1 Resident Geologist Program, Ontario Geological Survey, Ministry of Northern Development and Mines, 435 James St. S., Suite B002, Thunder Bay, ON, P7E 6S7 Canada 2 Department of Geology, Lakehead University, 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada Recent field work and interpretation of geochemistry has focussed on defining the emplacement sequence of Midcontinent Rift (MCR)-related units and evaluating their geochemical sources and contamination characteristics so as to better understand the magmatic evolution of the Logan Igneous Suite (Cundari, 2012). New geochemical discriminators applied to the Nipigon sills (e.g. Cundari et al., 2012) have been applied to a larger data set in order to identify previously unrecognized relationships between units. The plot of Nb/Ybpm versus Th/Ybpm (Fig. 1) illustrates a mixing model between mantle-derived melts and crustal sources. Separate trends with distinct ranges of Nb/Ybpm scattering away from the field of mantlederived melt compositions towards higher Th/Ybpm suggests the presence of separate, geochemically distinct magma source types within the Logan Igneous Suite which have undergone separate crustal contamination histories. The plot of Th/Ybpm versus Nb/Thpm (Fig. 2) also delineates distinct primary source compositions, as separate trends curve away from the field of mantle-derived melt compositions towards lower Nb/Thpm and higher Th/Ybpm. The position of the ultramafic units (i.e. Hele, Disraeli, Seagull, Kitto), which plot within or towards the field of mantle-derived melts in Figures 1 and 2, support previous work suggesting the ultramafic units are the best indicators for the primary magma formed from a deep-seated mantle plume (Nicholson et al., 1997; Hollings et al., 2007). Based on these criteria, four distinct distributions are noted (Figs. 1 and 2): the Nipigon sill trend; the Jackfish, McIntyre, Inspiration and Logan trend (JMIL); the Dyke trend (including the Pigeon River dykes, Cloud River dykes, Mount Mollie dyke and the Crystal Lake gabbro); and the Devon volcanics and the Riverdale sill trend (DR). Field work in the Logan Basin has delineated the following timing sequence between units, from oldest to youngest: Riverdale sill; Devon volcanics; Logan sills; Pigeon River dykes; Cloud River dykes; Mount Mollie dyke and Crystal Lake gabbro. In light of these relative ages, the mantle source characteristics are shown to become more depleted in incompatible elements as rift development progresses, each trend having been derived from a geochemically distinct mantle source. The mantle source of the Devon volcanic and the Riverdale sill magmas is the most enriched. The mantle source of the Logan sills is relatively more depleted and the mantle source of all dyke sets is the most depleted of all units in the Logan Basin. Furthermore, although all three dyke sets display similar source characteristics, younger dykes display progressively stronger crustal contamination signatures (i.e., higher Th/Ybpm and more negative ƐNd(t=1100Ma)) consistent with the magma having spent progressively more time in the magma chamber. A similar incompatible element depletion trend in mantle sources, akin to that in the Logan Basin units, is also apparent in the Nipigon Embayment. It is widely accepted through geochronological data and field relationships that the Nipigon sills post-date the ultramafic units of the Nipigon Embayment (e.g. Heaman et al., 2007). Geochemical source characteristics for the Nipigon sills and the ultramafic units show that the ultramafic units were derived from a more enriched source when compared to a more depleted source for the Nipigon sills (Figs. 1 and 2). Logan sills of the Logan Basin and ultramafic units of the Nipigon Embayment all show similar source characteristics (Figs. 1 and 2), suggesting an overlapping magmatic history. From the geochemical data presented here, in conjunction with known emplacement sequences, it is proposed that magma which produced units of the Logan Igneous Suite were derived from 19 �geochemically distinct mantle sources showing a progressive depletion in incompatible elements as MCR development progressed. Figure 1: Diagram showing variations in Nb/Ybpm and Th/Ybpm ratios for Midcontinent Rift-related mafic rocks. Normalizing values from Sun and McDonough (1989). Figure 2: Diagram showing variations in Th/Ybpm and Nb/Thpm ratios for Midcontinent Rift-related mafic rocks. Normalizing values from Sun and McDonough (1989). References Cundari, R.M., 2012. Geology and geochemistry of Midcontinent rift-related igneous rocks. Unpublished M.Sc. thesis, Lakehead University, Thunder Bay, ON, 142 p. Cundari, R.M., Hollings, P.N. and Smyk, M.C., 2012. Petrogenesis and crustal contamination of the Nipigon sills: a geochemical and spatial re-evaluation; 58th Institute on Lake Superior Geology, Annual Meeting, Thunder Bay, ON, May 16-20, 2012, Proceedings Volume 58, Part 1, p.22-23. Heaman, L.M., Easton, M., Hart, T.R., Hollings, P., Macdonald, C.A. and Smyk, M., 2007. Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon region, Ontario; Canadian Journal of Earth Sciences 44: 1055-1086. Hollings, P., Hart, T., Richardson, A. and MacDonald, C.A., 2007. Geochemistry of the mid-Proterozoic intrusive rocks of the Nipigon Embayment, northwestern Ontario; Canadian Journal of Earth Sciences 44: 1087-1110. Nicholson, S.W., Shirey, S., Schulz, K., and Green, J., 1997. Rift-wide correlation of 1.1 Ga Midcontinent rift system basalts: implications for multiple mantle sources during rift development; Canadian Journal of Earth Sciences 34: 504-520. Sun, S. and McDonough, W. F. 1989. Chemical and isotopic systematics of oceanic basalts: Implications for mantle composition and processes. in A.D. Saunders and M.J. Norry (eds.) Magmatism in the Ocean Basins ; Spec. Publ. Vol. Geol. Soc. Lond. , No. 42, pp. 313-345. 20 �The Geochemistry and Mineralogy of the Sulfides within the Ni-Cu-PGE Shakespeare Deposit, Ontario DASTI, Ian R. and KISSIN, Stephen A. Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1 Canada The 2217 Ma Shakespeare deposit, located 70 km west of Sudbury, Ontario, is part of the 2.2 Ga Nipissing Gabbro suite and is hosted within the metasediments of the 2.45-2.2 Ga Huronian Supergroup. The past producing Ni-Cu -PGE Shakespeare Mine, which has probable reserves of 11.8 Mt grading 0.87 g/t PGE+Au, 0.33% Ni, and 0.35% Cu, is hosted in the Shakespeare deposit (Prophecy Platinum, 2013). The majority of the mineral reserves are in the form of disseminated sulfides located within the Shakespeare melagabbro. Other significant sulfide mineralization is in the form of 2-5 cm heavily disseminated blebs to net-textured and locally semi-massive sulfides immediately above the disseminated sulfides. The aforementioned sulfides are collectively known as “interconnected sulfides” and straddle the contact between the Shakespeare quartz gabbro and melagabbro. The Shakespeare deposit is situated below the Mississagi Quartzite and above the unmineralized Nipissing Gabbro. Sulfide mineralogy within the disseminated and heavily disseminated portions of the deposit is largely dominated by pyrrhotite, pentlandite, and chalcopyrite. A detailed SEM study has also led to the discovery of rarer minerals, such as molybdenite, several compounds containing tellurium and bismuth, a rhenium sulfide, argentopentlandite, and gersdorfite . Agentopentlandite seemed to show an affinity for chalcopyrite, forming discreet, euhedral grains along the edges of chalcopyrite crystals. Interestingly, the bismuth tellurides also rarely contain abundant silver, up to 20 percent, and antimony. The SEM study also showed that gersdorfite was rarely encountered in its pure form but rather was usually in solid solution with cobaltite and arsenopyrite . Platinum and palladium minerals were not observed, suggesting they could be present in concentrations lower than 2-3% or as nuggets that were not encountered. However, a rhenium sulfide nugget was encountered on a few occasions, and because Re, Pt, and Pd are expected to act similarly under the geochemical conditions present, it is likely that Pt and Pd occur as minerals in the form of nuggets. Previous workers (Sproule et al., 2005) have suggested a contamination event lead to sulfur saturation in the magmas of the Shakespeare deposit, but those works do not comment as to whether the contamination event contributed significant sulfur to the magmas . Nine samples were taken from various stratigraphic depths in the deposit for sulfur isotope analysis and returned δ34S values from 0.01 ‰ to 2.38 ‰, averaging 1.14 ‰ (table 1). Sulfur data obtained by LECO analysis and preliminary selenium data obtained by ICP-MS provide S:Se ratios between 1245 and 3271, averaging 1810 (table 2). The data strongly suggest that the sulfur source for the Shakespeare deposit is dominantly magmatic, with little to no input from crustal sources. Additionally, the rocks of the Huronian Supergroup ((Mississagi through Matinenda Formations) are devoid of a sulfur source with which to contribute sulfur to the magmas that led to the formation of the Shakespeare deposit. 21 �Table 1. Sulfur isotope data Sample* δ34S ‰ 119-304.5 2.38 119-324 1.83 119-346 1.05 119-360 1 119-378 0.5 122.396.2 1.43 122-417 0.01 122-437.7 1.38 122-464.7 0.72 mean 1.14 First three digits = drill-hole, Next digits = depth in metres Table 2. Sulfur and selenium data and S/Se ratios ratios for mineralized samples Sample Se (ppm) S % S/Se ratio (ICP-MS) LECO 119-340 3.3 0.53 1607 119-350 8.4 1.44 1714 119-366 10.2 1.27 1245 119-378.6 9.4 1.35 1436 122-435.7 5.2 0.88 1692 122-440.7 9.2 3.01 3272 122-442.7 11.1 2.37 2135 122-444.7 7.5 1.32 1760 122.451.7 14.9 2.92 1960 122-454.7 6.9 1.05 1522 122-455.7 11.3 1.77 1566 mean 1810 References Lightfoot, P.C., Conrod, D., Naldrett, A.J. and Evensen, N.M., 1987. Petrologic, chemical, isotopic, and economic-potential studies of the Nipissing Diabase, Grant 230 in Milne, V.G. (ed.) Geoscience Research Grant Program, Summary of Research 1986-1987, Ontario Geological Survey, p. 4-26. Prophecy Platinum (2013) Prophecyplat.com, Accessed March 22, 2013. Sproule, R.A., Sutcliffe, R., Tracanelli, H., Lesher, C.M., 2008. Palaeoproterozoic Ni-Cu-PGE mineralization in the Shakespeare Intrusion: A new style of Nipissing gabbro-hosted mineralization, Transactions of the Institution of Mining and Metallurgy B. Applied Earth Science, 116: 188-200. 22 �High-resolution, multi-method geophysical imaging of a portion of the Northeast Iowa Intrusive Complex DRENTH, Benjamin1, ANDERSON, Raymond2, CHANDLER, Val3, CANNON, William4, SCHULZ, Klaus4, FEINBERG, Joshua M.5, BEDROSIAN, Paul 1, and KASS, M. Andy1 1 U.S. Geological Survey, PO Box 25046, MS 964, Denver, CO, 80225 2 Iowa Geological and Water Survey, 109 Trowbridge Hall, Iowa City, IA, 52242 3 Minnesota Geological Survey, 2642 University Avenue W., St. Paul, MN, 55114-1032 4 U.S. Geological Survey, 12201 Sunrise Valley Dr., MS 954, Reston, VA, 20192-6320 5 Dept. Earth Sciences, Univ. Minnesota, 310 Pillsbury Dr. SE, Minneapolis, MN, 55455-0219 Numerous large amplitude regional aeromagnetic anomalies and ground gravity highs over northeast Iowa and southeast Minnesota (Fig. 1) suggest the presence of a buried intrusive complex made up of mafic/ultramafic rocks. This complex is known as the northeast Iowa Intrusive Complex (NE IIC) (e.g., Pals and Anderson, 2011). The NE IIC lies along the eastern margin of the Midcontinent Rift System (MRS) and occupies a minimum estimated area of 17,000 square kilometers, making it comparable in size to the Duluth Complex. Country rocks are thought to be accreted island arc terranes of the Paleoproterozoic Yavapai Province (1.7-1.8 Ga), implying at least a somewhat younger age for the NE IIC. While not yet directly dated, these considerations suggest that a Keweenawan (MRS) age for some or all of the NE IIC is possible and imply significant potential for undiscovered Ni-Cu-PGE deposits. Alternatively, the NE IIC could include Mesoproterozoic (~1450 Ma) gabbro-anorthosite-rapakivi granite intrusions like the Wolf River Batholith in Wisconsin. Only four boreholes are known to reach the complex, which is covered by 200-500 meters of Phanerozoic sedimentary rocks and sediments. Geophysical methods are thus critical to developing a better understanding of the fundamental nature and resource potential of the NE IIC. A high-resolution, multi-method geophysical mapping program was initiated in 2012 as a collaborative effort between the U.S. Geological Survey Mineral Resources Program, the Iowa Geological and Water Survey, and the Minnesota Geological Survey. An initial 3,333 line kilometer airborne data collection campaign in the region of Decorah, Iowa, included magnetic, gravity gradient (AGG), and time-domain electromagnetic (TDEM) data along flight lines spaced 400 m apart. At the time of this writing, only preliminary versions of these data are available for inspection. The preliminary data show numerous magnetic anomalies that are paired with AGG highs, indicating widespread strongly magnetized and dense rocks of likely mafic/ultramafic composition. In the Decorah region, a prominent horseshoe-shaped, 15 kilometer diameter magnetic- and gravity-field high is correlated with the occurrence of basement rocks that have been described as unaltered gabbro and troctolite, suggesting a ring-shaped anomaly source. A Yavapai age layered(?) metagabbro pluton (Van Schmus et al., 2007) is suspected to produce complex magnetic and gravity anomalies with different forms than the other basement rocks nearby. The TDEM data appear to image crystalline basement rocks in select locations, and it is expected that the data will ultimately provide important constraints on depths to the NE IIC and other Precambrian basement rocks. References Pals, D.W., and Anderson, R.R., 2011, Reassembling Iowa: spatial and temporal evaluation of the mineral potential of the Iowa segment of the Micontinent Rift and related plutons: Geological Society of America Abstracts with Programs, v. 43, no. 5 p. 396. Van Schmus, W.R., Schneider, D.A., Holm, D.K., Dodson, S., and Nelson, B.K., 2007, New insights into the southern margin of the Archean-Proterozoic boundary in the north-central United States based on U-Pb, Sm-Nd, and Ar-Ar geochronology: Precambrian Research, v. 157, p. 80-105. 23 �Figure 1: Regional aeromagnetic total-field (grayscale) and complete Bouguer gravity anomalies for the NE IIC region. Gravity contour (black lines) interval 10 milligals. White dots indicate wells that reach Precambrian rocks. Area of recent survey in region of Decorah, IA, shown as gray polygon. 24 �Application of LiDAR to resolving regional tectonic and glacial fabrics in glaciated terrane: An example from an Archean granite-greenstone belt in NE Minnesota DYESS, Jonathan and HANSEN, Vicki, Department of Geological Sciences, University of Minnesota Duluth, 1114 Kirby Drive, Duluth MN 55812 Regional tectonic fabrics define the broad structural architecture of an area and commonly have an associated topographic expression that may be identified via remote sensing (Chardon et al., 2002, 2008, 2009; Bedard et al., 2003). Glaciation commonly forms a geomorphologic fabric in the form of drumlins, flutes, valleys, eskers, and crag and tail features (Smith et al., 2006; Sharp, 1953). Overprinting of glacial fabrics, vegetation, and sediment cover commonly obscure topographic lineaments and hinder identification of lineament fabrics via high-resolution aerial photography and low to moderate resolution satellite imagery. Airborne LiDAR (light detection and ranging) systems provide high-resolution altimetry in vegetated areas (e.g., Haugerud et al., 2003). Although LiDAR altimetry is a useful tool for mapping small–scale geologic structures (e.g., Pavlis and Bruhn, 2011) and glacial geomorphologic features (e.g., Smith et al., 2006), previous studies do not constrain whether LiDAR altimetry may be used to differentiate between tectonic fabrics and overprinted glacial fabrics within the same area. In this study, we examine an Archean granite-greenstone terrane in NE Minnesota to illustrate the application of high-resolution LiDAR altimetry to mapping regional tectonic fabrics in glacially striated, forested areas. More specifically, we describe how to distinguish between tectonic and glacial fabrics and the effect of glaciation on the overall topographic expression of the tectonic fabric. A 1-m posted LiDAR derived bare-earth DEM (digital elevation model) collected as part of the Minnesota Elevation Mapping Project and shaded relief images constructed from the bare-earth DEM comprise the raw data for this study. Data processing and lineament mapping were done using ESRI ArcGIS software. Evaluation of the bare-earth DEM and shaded relief images revealed that shaded relied images provide the most potential for lineament mapping. In order to maximize the chance of mapping lineaments at all orientations, we constructed shaded relief images with a sun elevation of 45˚ and varying sun azimuth at 45˚ intervals. Using ESRI ArcScene, we draped shaded relief images over the bare-earth DEM to create a 3D perspective view of the field area and to visualize the topographic surface. Mapping revealed two suites of lineaments. Suite A consists of relatively short (1-2 km), discrete lineaments with a unimodal orientation distribution and a mean trend of 045. We recognize multiple striated deposits of sediment across the study area. Sediment deposits contain suite A lineaments only. Suite B consists of lineaments ranging in length from 1-30 km. Suite B lineaments are more continuous than suite A and have a quasi-bimodal orientation distribution. Suite B lineaments have a mean trend of 065 across the study area with local areas trending 090. In areas where suite A parallels suite B only one pervasive lineament set is visible. Where suite A and suite B are at high angles to one another, suite A lineaments are shorter and pervasive while suite B lineaments longer and spaced. We interpret suite A as a geomorphological fabric related to glaciation and suite B as the regional tectonic fabric. Field measurements of foliation trajectory (Goodman, 2008; Erickson, 2008; Dyess, unpublished field data) are largely consistent with suite B lineaments across the study area. Although not all suite B lineaments correlate to mapped structures, our analysis demonstrates that high-resolution LiDAR altimetry may be used to map regional tectonic fabrics in glaciated terrane. 25 �References Bedard, J.H., Brouillette, P., Madore, L., Berclaz, A., 2003. Archean cratonization and deformation in the northern Superior Province, Canada: an evaluation of plate tectonic versus vertical tectonic models. Precambrian Research, 127, 61-87. Chardon, D., Peucat, J., Jayananda, M., Choukroune, P., Fanning, C. M., 2002. Archean granitegreenstone tectonics at Kolar (South India): Interplay of diapirism and bulk inhomogeneous contraction during juvenile magmatic accretion. Tectonics, 21, no. 3, 1016. Chardon, D., Jayananda, M., Chetty, T.R.K., 2008. Precambrian continental strain and shear zone patterns: South Indian case. Journal of Geophysical Research, 113. Chardon, D., Gapais, D., Cagnard, F., 2009. Flow of ultra-hot orogens: A view from the Precambrian, clues for the Phanerozoic. Tectonophysics, 477, 105-118. Erikson, E., 2008. Structural and kinematic analysis of the Shagawa Lake shear zone, Superior Province, northeastern Minnesota. M.S. Thesis, University of Minnesota Duluth, MN. Goodman, S., 2008. Structural and Kinematic Analysis of the Kawishiwi Shear Zone, Superior Province. M.S. Thesis, University of Minnesota Duluth, MN. Haugerud, R.A., Harding, D.J., Johnson, S.Y., Harless, J.L., Weaver, C.S., Sherrod, B.L., 2003. High-resolution topography of Puget Lowland, Washington-A Bonanza for Earth Science: GSA Today, 13, no. 6, 4-10. Pavlis, T.L. and Bruhn, R.L., 2011. Application of LIDAR to resolving bedrock structure in areas of poor exposure: An example from the STEEP study area, soutern Alaska. GSA Bulletin, 123, 206-217. Sharp, R.P., 1953. Glacial Features of Cook County, Minnesota, American Journal of Science, 251, 855-883. Smith, M.J., Rose, J., Booth, S., 2006. Geomorphological mapping of glacial landforms from remotely sensed data: An evaluation of the principal data sources and an assessment of their quality. Geomorphology, 76, 148-165. 26 �Structural and Kinematic Analysis of the Shagawa Lake Shear Zone and Snowbank Lake Stock, Superior Province, NE Minnesota DYESS, Jonathan and HANSEN, Vicki, Department of Geological Sciences, University of Minnesota Duluth, 1114 Kirby Drive, Duluth MN 55812 The Archean (3.85-2.5 Ga) Superior Province, to a first approximation, consists of a series of east-west trending subprovinces of supracrustal rocks (greenstone belts) and granitoid rocks interpreted as a series of microcontinents, remnant arcs, oceanic terranes, and accretionary prisms that accreted to a growing continental block during dextral transpression driven by NW-directed oblique subduction (e.g., Percival et al., 2007, and references therein). Transpression platetectonics would predict the formation of faults/shear zones that record significant unidirectional strike-slip displacement at or near terrane boundaries (Sleep, 1992). The Vermillion District, southern Superior Province, is a Neoarchean (2.8-2.5 Ga) granite-greenstone terrane dominated by a series of NE-striking subvertical shear zones with ovoid to circular granitic bodies scattered throughout. Vermillion District shear zones have been interpreted as primarily dextral strike-slip shear zones formed during terrane assembly driven by NW oblique subduction (Hudleston et al., 1988; Bauer and Bidwell, 1990; Schultz-Ela and Hudleston, 1991). Others interpret Vermillion District shear zones as zones of dominantly oblique to dip-slip shear possibly formed during greenstone sagduction between rising granitoid diapirs (Erickson, 2008, 2010; Wolf, 2006; Goodman, 2008; Karberg, 2009). Differing interpretations of Vermillion District shear zones invoke different assumptions about displacement direction during non-coaxial shear. Displacement direction (flow direction) is genetically tied to foliation and elongation lineation orientation. Within the Shagawa Lake shear zone of NE Minnesota, displacement direction remains undetermined. Adjacent to the Shagawa Lake shear zone is the Snowbank Lake stock, a 30 km2 composite stock dominated by syenite and granodiorite (Sanders, 1929). Due to the proximity of the Shagawa Lake shear zone to Quetico and/or Wawa subprovince boundary, tectonic fabrics within the Shagawa Lake shear zone have implications for crustal assembly of the southern Superior Province. If the Shagawa Lake shear zone records significant unidirectional strike-slip displacement, then supported plate-tectonic models for Vermillion District formation will be further constrained. If the Shagawa Lake shear zone does not record significant unidirectional strike-slip displacement, then existing plate-tectonic and structural models of terrane amalgamation along the Vermillion District require reevaluation. We conducted a structural and kinematic analysis of the Shagawa Lake shear zone in three phases: 1) analysis of regional tectonic fabrics through Light Detection and Ranging altimetry data; 2) structural analysis of outcrop-scale structures through detailed field mapping; and 3) analysis of shear-sense indicators through kinematic analysis of thinsections. The Shagawa Lake shear zone contains a regional subvertical metamorphic foliation with an average strike of 065 but varies locally from 065 to 100. Near the Snowbank Lake stock foliation deviates from the regional trend and turns roughly parallel to the stock boundary. We recognize two types of elongation lineation within the Shagawa Lake shear zone. These include ridge-ingroove striations on C-foliation surfaces (Lc) as well as stretching lineations on S-surfaces (Ls) (Lin and Williams, 1992; Lin et al., 2007). The Shagawa Lake shear zone hosts unimodal Lc and Ls ranging from dominantly steep plunge to moderate plunge. Asymmetric fabrics occur in foliation-perpendicular, lineation-parallel planes and symmetric fabrics occur in foliationperpendicular, lineation-perpendicular planes, which is consistent with non-coaxial shear with lineation forming parallel to shearing. Therefore, in high pitch domains, displacement was vertical to oblique, and microstructures record both north-side-up and south-side-up displacement in different samples. Samples with oblique lineation record an apparent dextral strike-slip shearsense despite varying lineation orientation. Our data indicate the Shagawa Lake shear zone 27 �experienced both N-side-up and S-side-up dip- to oblique-slip with relatively minor apparent dextral strike-slip and does not record significant unidirectional strike-slip displacement. References Bauer, R. L and Bidwell, M. E., 1990. Contrasts in the response to dextral transpression across the Quetico-Wawa subprovince boundary in northeastern Minnesota. Canadian Journal of Earth Sciences, 27, 1521-1535. Erikson, E., 2008. Structural and kinematic analysis of the Shagawa Lake shear zone, Superior Province, northeastern Minnesota. M.S. Thesis, University of Minnesota Duluth, MN. Erickson, E., 2010. Structural and kinematic analysis of the Shagawa Lake shear zone, Superior Province, northern Minnesota: implications for the role of vertical versus horizontal tectonics in the Archean. Canadian Journal of Earth Sciences, 47, 1463-1479. Goodman, S., 2008. Structural and Kinematic Analysis of the Kawishiwi Shear Zone, Superior Province. M.S. Thesis, University of Minnesota Duluth, MN. Hudleston, P.J., Schultz-Ela, D., Southwick, D. L., 1988. Transpression in an Archean greenstone belt, northern Minnesota. Canadian Journal of Earth Sciences, vol 25, 1060-1068. Karberg, S M., 2009. Structural and Kinematic Analysis of the Mud Creek Shear Zone, Northeastern Minnesota. M.S. Thesis, University of Minnesota Duluth, MN. Lin, S., Williams, P.F., 1992. The origin of ridge-in-groove slickenside striae and associated steps in an S-C mylonite. Journal of Structural Geology 14, 315e321. Lin, S., Jiang, D., Williams, P., 2007. Importance of differentiating ductile slickenside striations from stretching lineations and variation of shear direction across a high-strain zone. Journal of Structural Geology, 29, 850-862. Percival, J.A., 2007, Geology and metallogeny of the Superior Province, Canada, in Goodfellow,W.D., ed.,Mineral Deposits of Canada:ASynthesis ofMajor Deposit-Types, District Metallogeny, the Evolution of Geological Provinces, and Exploration Methods: Geological Association of Canada, Mineral Deposits Division, Special Publication No. 5, p. 903-928. Sanders C.W., 1929, A composite stock at Snowbank Lake in northeastern Minnesota. Journal of Geology, 37, 135-149. Schultz-Ela, D.D., Hudelston, P.J., 1991. Strain in an Archean greenstone belt of Minnesota. Tectonophysics, 190, 223-268. Sleep, N., 1992. Archean plate tectonics: what can be learned from continental geology?. Canadian Journal of Earth Sciences, 29, 2066-2071. Wolf, D. E., 2006. The Burntside Lake and Shagawa/Knife Lake shear zones: Deformation kinematics, geochemistry and geochronology; Wawa Subprovince, Ontario, Canada. Masters Thesis, Washington State University. 28 �Bedrock Geologic Map of the Putnam Lake Area, St. Louis County, NE Minnesota FEHRS, Ellen1, KENNY, Edward1, KUCHMA, John1, SAUER, Sarah1, SYLVESTER, William1, and HUDAK, George1,2 1 Precambrian Research Center, Natural Resources Research Institute, University of Minnesota Duluth, 5013 Miller Trunk Highway, Duluth, MN, 55811 2 Minerals Division, Natural Resources Research Institute, University of Minnesota Duluth, 5013 Miller Trunk Highway, Duluth, MN 55811 Each year, students from the Precambrian Research Center (PRC) geology field camp complete “capstone” projects that encompass approximately one week of detailed field mapping followed by one week of mapmaking and map publishing. During the fifth and sixth weeks of the 2012 field camp, five PRC field camp students, under the direction of PRC Associate Director George Hudak, mapped Neoarchean rocks on the southwest side of the Tower-Soudan Anticline in the vicinity of Putnam Lake (Fehrs et al., 2012). This capstone mapping project sought to: 1) identify the lithologies and determine the detailed stratigraphy within the Neoarchean supracrustal strata in this area; 2) define and characterize the nature of the contacts between various units of the Neoarchean supracrustal strata and intrusive rocks; 3) obtain a better understanding of geological structures and their orientations within the area; 4) produce a detailed geological map in an area on the south side of the Tower-Soudan anticline, which has previously only been mapped at a regional scale; and 5) test the utility of LiDAR for mapping in heavily forested greenstone belt terranes. Prior to mapping, detailed 1:5000 scale laminated field mapping sheets were produced. One side of each field mapping sheet consisted of a georeferenced air photo, and the other side of the mapping sheet consisted of a corresponding georeferenced topographic map using recently released LiDAR- based elevation data available from the Minnesota Geospatial Information Office. Mapping was completed by means of numerous traverses through the bush, as well as traverses along the lakeshore of Putnam Lake. Following each day of field mapping, students and faculty transferred their field data to a master map, enabling the detailed geology of the region to be established by the middle of the fifth week of field camp. During the sixth week of field camp, students produced a digital version of the field map utilizing a variety of software (ArcView, AutoCad, Surfer, Adobe Illustrator). Previous mapping (Peterson and Jirsa, 1999; Peterson, 2005) established that the Neoarchean supracrustal rocks in this area comprise a NW/SE striking, SW facing homoclinal sequence that makes up the southern limb of the Tower-Soudan Anticline. This regional mapping indicated that the area consisted of rocks comprising the Lower Member of the Ely Greenstone Formation (Armstrong Lake / Central Basalt mafic-intermediate lava flows, dacite lava flows, and local Algoma-type oxide facies iron formations), the Soudan Member of the Ely Greenstone Formation (Algoma-type banded oxide-facies iron formation), and the Upper Member of the Ely Greenstone Formation (South Limb Basalts, comprising massive- to pillowed-facies basalt lava flows), as well as synvolcanic hypabyssal diabase/gabbro sills and dikes, and post-volcanic quartz-feldspar porphyry, hornblende-feldspar porphyry, and lamprophyre intrusions. The contact between the Soudan Member and Upper Member of the Ely Greenstone Formation was interpreted as a minor shear zone associated with regional D2 deformation (Peterson, 2005) In light of lithological characteristics obtained from mapping in the vicinity of Soudan Underground Mine State Park and Lake Vermilion State Park (Peterson and Patelke, 2003; Radakovich et al., 2010; Heim et al., 2011), we now believe that supracrustal rocks in this part of the Tower-Soudan anticline comprise part of the Soudan Member of the Ely Greenstone Formation. In this area, the Soudan Member is composed of: a) medium to light gray laminated to very thinly bedded chert; b) light gray to red to dark gray laminated to very thinly bedded interlayered horizons of chert, jasper, and magnetite-rich Algoma-type oxide facies iron formation; c) interbedded light green to medium green, sparsely amygdaloidal, sparsely plagioclase-phyric massive sheet flow- and pillowed-facies basalt lava flows locally interbedded with 0.10-1.0 meter thick laminated oxide facies iron formation horizons; d) light 29 �green-gray to medium green, sparsely amygdaloidal, sparsely plagioclase-phyric massive sheet flowfacies basalt lava flows; e) light green-gray to medium green, sparsely amygdaloidal, sparsely plagioclase-phyric pillow-facies basalt lava flows; and f) green-gray, locally amygdaloidal, plagioclasephyric dacite lava flows that are locally interbedded with chert and argillite horizons. Unfortunately, outcrop areal distribution (about 1% of the field area) was not sufficient to identify, or map out, individual lava flows with confidence. Locally, synvolcanic and post-volcanic sills and dikes intrude the supracrustal assemblage in the area. Synvolcanic diabase/gabbro sills and dikes are medium green-gray to dark green, fine- to medium grained, subophitic and locally plagioclase-phyric. Post-volcanic intrusions include: a) light gray to pinkish-gray hornblende- and plagioclase-phyric dacite sills and dikes; b) light gray to light pinkish-gray quartz- and plagioclase-phyric rhyodacite sills and dikes; and c) light to medium-gray, fine- to mediumgrained biotite-phyric lamprophyre dikes. Based on our mapping, the following sequence of geological events is believed to have formed the stratigraphic succession in this part of the Vermilion District: 1) early mafic volcanism dominated by massive sheet flow-facies lava flows; 2) deposition of several horizons (up to 50 meters thick) of Algomatype oxide facies iron formations during breaks in mafic volcanism; 3) resumption of extrusive mafic volcanism forming both massive sheet flow-facies and pillowed-facies basalt lava flows with local extrusion of dacite lava flows and minor, intermittent hydrothermal activity to form thin (less than 10 meters thick) Algoma-type banded iron formations; 4) extrusive mafic volcanism forming both massive sheet flow-facies and pillowed-facies basalt lava flows followed by local intrusion of diabase/gabbro sills and dikes; 5) intrusion of sills and dikes of both hornblende- and plagioclase-phyric dacite and quartzand plagioclase-phyric rhyodacite; 6) development of the Tower-Soudan anticline; 7) D2 deformation forming a penetrative east-west-trending foliation; and 8) intrusion of biotite-phyric lamprophyre intrusions. Based on the similarity of the basalt lava flows mapped in this area to the Soudan Basalts mapped in other parts of the Vermilion District, it appears that the Upper Member of the Ely Greenstone Formation may be absent on the southern limb of the Tower-Soudan anticline. More mapping, petrographic studies, and lithogeochemical studies will be necessary to further evaluate this preliminary interpretation. As well, we found that LiDAR-based topographic maps provided excellent base maps, and were extremely useful for identifying small topographic features in heavily forested areas that were later found to be outcrops that had never been previously mapped. References Fehrs, E., Kenny, E., Kuchma, J., Sauer, S., Sylvester, W., and Hudak, G., 2012, Bedrock Geologic Map of the Putnam Lake Area, St. Louis County, Northeastern Minnesota: Precambrian Research Center Map Series, PRC/MAP-2012/02, 1:5000 scale. Heim, N., Scott, H., Kilduff, R., Rahtz, C., Vial, A., Young, S., Mahr, C, and Hudak, G., 2011, Preliminary Bedrock Geology Map of the Eastern Part of Lake Vermilion State Park, St. Louis County, NE Minnesota: Precambrian Research Center Map Series, PRC/MAP-2011/01, 1:5000 scale. Peterson, D. M., 2005, Bedrock geologic and volcanogenic massive sulfide deposit mineral potential map of the Lower Ely Greenstone and Adjacent Areas: Soudan, Eagles Nest, and Bear Island 7.5” Quadrangles, St. Louis County, NE Minnesota: unpublished geologic map, 2005 North-Central Geological Society of America Field Trip 9, Minneapolis, MN May 2005, 1:10000 scale. Peterson, D. M., and Jirsa, M. A., 1999, Bedrock Geological Map and Mineral Exploration Data, Western Vermilion District, St. Louis and Lake Counties, Northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-98, scale 1:48,000. Peterson, D. M., and Patelke, R. L., 2003, National Underground Science and Engineering Laboratory (NUSEL): Geological Site Investigation for the Soudan Mine, NE Minnesota: Natural Resources Research Institute, Technical Report NRRI/TR-2003/29, 88 p. Radakovich, A. L., Parent, C. T., Partridge, M. E., Ritts, A. D., Pierce, R., and Hudak, G. J., 2010, Reconnaissance Bedrock Geological Map of the Northern Part of Soudan Underground Mine State Park and the Northwestern Part of Lake Vermilion State Park, St. Louis County, Minnesota: Precambrian Research Center Map Series, PRC/MAP-2010/04, 1:5000 scale. 30 �Fluid Inclusion study of the Magino Archean Gold Deposit; Implications for Regional Mineralizing Systems HAROLDSON, Erik1, BROWN, Philip1 1Department of Geoscience, University of Wisconsin-Madison, 1215 W. Dayton, Madison, WI, 53706 USA The Magino gold deposit is located approximately 45 km's Northeast of Wawa, Ontario, Canada in the Goudreau-Localsh area of the Michipicoten Greenstone Belt. The Archean Lode Gold deposit is a newly enlarged gold resource in a mining camp historically known to host deposits of ≤1 Moz. in total endowment. As of October 2012; the Magino resource is reported (NI 43-101) as having 6.25 Moz of gold, and there remains potential for expansion. Gold was first discovered on the Magino project in 1917. Production on the property has been from two generations of historical mining. The first generation was in the 1930’s and approximately 10,000 oz of gold were produced. The second more recent underground venture operated from 1987 to 1993 and an additional 105,000 oz of gold was produced. Recently Argonaut Gold Inc. has taken ownership of the property through the acquisition of Prodigy Gold Inc. Argonaut Gold is planning a Pre-Feasibility study for 2013 which will look to exploit the Magino deposit as an open pit mine. The objective of the study is to better understand the mineralizing system(s) Figure 1 shows the location of the Magino mine project in Northern responsible for forming this Ontario, Canada. Geology data is from OGS: Bedrock Geology of Ontario gold deposit and to put that 1:250,000 (Revised MRD 126) information in the context of the regional geology to better understand the economic potential of the deposit along with the surrounding greenstone belt. Fluid inclusion research will constrain the geologic conditions at the time of mineralization, as well as provide clues towards the water-rock interactions prior to and during ore deposition. Initial work involves microscopy of polished thick sections and microthermometry to establish fluid inclusion assemblages. To better aid the assignment to and interpretation of fluid inclusion assemblages, the fluid inclusion thick sections are being imaged in a Scanning Electron Microscope using variations in cathodoluminescence signatures to differentiate various hydrothermal quartz generations from earlier deformed primary quartz phenocrysts associated with the host trondhjemite intrusion. Raman spectroscopy will be 31 �utilized to interpret carbonic-rich fluid inclusions for CH4 content and to aid in pressure interpretations. The recent expansion of the Magino resource raises the exploration potential of the surrounding Michipicoten greenstone belt significantly. The opportunity exists to aid in discovery of new and expansion of known similar type gold deposits in the region. Fluid inclusion studies in the region have been somewhat inconclusive as to understanding the nature of the gold mineralizing system(s). In the Michipicoten Greenstone belt, fluid inclusion studies including my preliminary work on the Magino deposit have Figure 2 (A) shows a methane rich fluid inclusion at room temperature; notice small bright unknown solid and dark cluster of unknown solids near top. (B) shows the same inclusion after cooling to -100°C; notice a frozen CO2 solid (darker) and a Ch4 vapor bubble (lighter) which formed while cooling at ~ 98°C. hinted strongly at a regional genetic link between the various widespread study areas. Fluid inclusion study will form the foundation to more rigorous study of the Michipicoten Greenstone Belt evolution and most importantly the gold mineralization. REFERENCES Borthwick, R.W. 1987. The distribution and association of gold within quartz veins. Magino mine prospect, Wawa, Ontario; A thesis presented to the Department of Geological Sciences Brock University in partial fulfillment of the requirements for the degree Bachelor of Science with Honours in Geology, 49p Brown, P.E., and Hagemann, S.G., 1995, The program MacFlinCor and its application to geobarometry in Archean lode-gold deposits. Geochim Cosmochim Acta 59, 3943-3952. Brown, P.E., and Hagemann, S.G., 1994, MacFlinCor: A computer program for fluid inclusion data reduction and manipulation. In De Vivo and Frezzotti (eds) Fluid Inclusions in Minerals: Methods and Applications, VPI Press, 231-250. Götze, Jens. Application of Cathodoluminescence Microscopy and Spectroscopy in Geosciences. Microscopy and Microanalysis 18, no. 06 (2012): 1270–1284. Heather, K.B. and Arias, Z. 1992. Geological and structural setting of gold mineralization in the GoudreauLochalsh area, Wawa gold camp; Ontario Geological Survey, Open File Report 5832, 159p. Lu, Wanjun, I-Ming Chou, R.C. Burruss, and Yucai Song. A Unified Equation for Calculating Methane Vapor Pressures in the CH4–H2O System with Measured Raman Shifts. Geochimica et Cosmochimica Acta 71, no. 16 (August 15, 2007): 3969–3978. Sage, R.P. 1993. Geology of Augonie, Bird, Finan and Jacobson townships, District of Algoma; Ontario Geological Survey, Open File Report 5588, 286p Samson, I.M. Bulent, B., and Holm, P.E., 1997, Hydrothermal evolution of auriferous shear zones, Wawa, Ontario: Economic Geology, v. 92, p. 325-342. Studemeister, P.A., and Kilias, S., 1987, Alteration pattern and fluid inclusions of gold-bearing quartz veins in Archean trondhjemite near Wawa, Ontario, Canada: Economic Geology, v. 82, p. 429-439 Sutherland, K.S., 1987, A preliminary report on the Magino gold deposit, Wawa, Ontario, A report submitted to the Department of Geological Sciences in partial fulfillment of the requirements for the non-research Master’s of Science in Mineral Exploration, Queen’s University, Kingston, Ontario. 32 �Preliminary geochemical analysis of the Nipigon Bay granites, northern Lake Superior HOLLINGS, Peter1, and SMYK, Mark2 1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1 Canada 2 Resident Geologist Program, Ontario Geological Survey, Ministry of Northern Development and Mines, 435 James St. S., Suite B002, Thunder Bay, ON, P7E 6S7 Canada The granitic basement to exposed, overlying Mesoproterozoic Sibley Group sedimentary rocks in Nipigon Bay, northern Lake Superior, was first identified by diamond drilling in 1997 (Wells, 1997). A series of three, prominent magnetic anomalies delineates these magnetite-bearing granitoids in the subsurface (Fig. 1). These granitic intrusions occur at a confluence of three regional-scale structures: the northeast-trending Gravel River fault; the north-northeast-trending Jackpine River fault; and the west-southwest-trending North Shore fault, which forms the base of the Osler Group volcanic rocks of the Midcontinent Rift. The boundary between the Neoarchean Wawa and Quetico subprovinces has also been extrapolated beneath Proterozoic cover under Nipigon Bay (Williams, 1989). Sampling of drill core for geochemical and geochronologic analysis was recently undertaken in order to describe and characterize these intrusive rocks. Figure 1. A) Map of upper Great Lakes showing the location of the study area. B) Regional geology map showing the location of the Nipigon granites. Granitic rocks from beneath Nipigon Bay are characterised by enriched LREE (La/Smn = 2.9 to 7.7) and flat to weakly fractionated HREE (Gd/Ybn = 1.4 to 3.1) with pronounced negative Nb anomalies (Nb/Nb* = 0.1 to 0.2). The granites are metaluminous and plot within the field of volcanic arc granites on the granite discrimination diagrams of Pearce et al. (1984). When compared to other granite suites in the area, the Nipigon Bay granites most closely resemble the I-type granites of the Dog Lake chain (Kuzmich et al., 2012) rather than the S-type Neoarchean granites of the Pukaskwa batholith (Beakhouse et al., 2011) and nearby Georgia Lake area (Breaks et al., 2008), or the Mesoproterozoic anorogenic English Bay granites in the Nipigon Embayment (Hollings et al., 2004). 33 �The granites of the Dog Lake chain, 80 km west-southwest of Nipigon Bay, appear as a series of distinct aeromagnetic “highs” along the southern boundary of the Quetico subprovince. Kuzmich et al. (2011) interpreted the granites to have formed within a suprasubduction mantle and were subsequently emplaced along crustal-scale faults that form terrane boundaries. Similarities in geochemistry, magnetic signature and regional tectonic setting suggest that the Nipigon Bay and Dog Lake granites may have formed in a similar manner. Pending geochronologic and geochemical data for both the Dog Lake and Nipigon Bay intrusions will help to elucidate this possible relationship. References Beakhouse, G., Lin, S. and Kamo, S., 2011. Magmatic and tectonic emplacement of the Pukaskwa batholith, Superior Province, Ontario, Canada; Can. J. Earth Sci., 48, p.187–204. Breaks, F.W., Selway, J.B. and Tindle, A.G. 2008. The Georgia Lake rare-element pegmatite field and related S-type, peraluminous granites, Quetico Subprovince, north-central Ontario; Ontario Geological Survey, Open File Report 6199, 176p. Hollings, P., Fralick, P. and Kissin, S., 2004. Geochemistry and geodynamic implications of the Mesoproterozoic English Bay Granite-Rhyolite complex, northwestern Ontario; Canadian Journal of Earth Sciences, 41, p.1329-1338. Kuzmich, B., Hollings, P., Campbell, D. and Scott, J, 2012. Geochemistry and petrology of the Dog Lake granite chain, Quetico Basin, Northwestern Ontario; Institute on Lake Superior Geology Proceedings, 58th Annual Meeting, Thunder Bay, Ontario, Part 1 - Proceedings and Abstracts, v. 58, part 1, 56-57. Pearce, J., Harris, N. and Tindle, A., 1984. Trace element discrimination diagrams for the tectonic interpretation of granitic rocks; Journal of Petrology, 25, 956-983. Wells, K. 1997. Assessment report on the 1997 drilling program, Nipigon bay area; unpublished report 2.17382, Falconbridge Limited, assessment files, Thunder Bay South District, Ministry of Northern Development and Mines, Thunder Bay, 138p. Williams, H.R. 1989. Geological studies in the Wabigoon, Quetico and Abitibi-Wawa subprovinces, Superior Province of Ontario, with emphasis on the structural development of the Beardmore-Geraldton Belt; Ontario Geological Survey, Open File Report 5724, 189p. 34 �The Minnesota Taconite Workers Health Study: Environmental Study of Airborne Particulate Matter - 2013 Update HUDAK, George1, MONSON GEERTS, Stephen1, ZANKO, Larry1, SEVERSON, April1, SEVERSON, Allison1, KRAMER, Stuart1, BANDLI, Bryan2 1 Natural Resources Research Institute, 5013 Miller Trunk Highway, Duluth, MN, 55811 2 Department of Geological Sciences, University of Minnesota Duluth, 229 Heller Hall, 1114 Kirby Drive, Duluth, MN 55812 Since 2008, the Natural Resources Research Institute (NRRI) has been conducting a detailed characterization of mineral dust in northeastern Minnesota. The purpose of this research is to evaluate the effects of past and present emissions from taconite mining and processing on air quality throughout the Mesabi Iron Range (MIR) by characterizing airborne particulate matter within taconite operations, in communities surrounding taconite operations on the MIR, in population centers in other regions of northeastern Minnesota, and non-MIR locations, as well as particulate matter deposited in lake sediments (Figure 1). NRRI’s sampling and characterization work represents the community/environmental component of the Minnesota Taconite Workers Health Study, a broad University of Minnesota (UM) research effort investigating long-standing questions regarding the impact of dust derived from mining and processing of taconite (iron ore). The UM School of Public Health (SPH), with whom NRRI is collaborating, is responsible for the human health- and exposure-related components of that effort, which include: 1) an occupational exposure assessment; 2) a mortality study; 3) a cancer incidence study; and 4) a respiratory health survey of taconite workers and spouses. Figure 1. Locations of taconite processing plants on the Mesabi Iron Range being sampled during this study (after Oreskovich and Patelke, 2006) Air sampling is performed within taconite operations, MIR communities, and non-MIR communities by NRRI scientists during both winter and summer seasons. Sampling at taconite operations takes place at four locations: 1) secondary crushers; 2) magnetic separators/concentrators; 3) agglomerators/ ball drums; and 4) kiln/pellet discharge area. Sampling within MIR communities takes place on the rooftops of public buildings, whereas sampling in non-MIR communities occurs on rooftops or in remote locations so that background air quality can be evaluated. Airborne particles are collected using: 1) a micro orifice uniform 35 �deposit impactor (MOUDI) (Marple et al., 1991), which enables size-fractionated particulate matter collection; and 2) a total suspended particulate filter (TSP). Particulate matter is evaluated via gravimetric analysis and subsequently subjected to comprehensive particulate matter characterization that includes: 1) scanning electron microscopy (SEM) imaging; 2) energy dispersive x-ray spectroscopy (EDS); 3) electron backscattered diffraction (EBSD); 4) proton induced x-ray emission (PIXE); 5) the Minnesota Department of Health’s 852 Method Transmission Electron Microscopy (TEM) Analysis for Mineral Fibers in Air; and 6) the International Standard Organization’s Method 10312 Ambient air – Determination of Asbestos Fibers – Direct-Transfer TEM Method (ISO 10312, 1995). During the past year, the NRRI has been evaluating particulate matter physical data (including gravimetric data, and particulate matter morphology data), particulate matter mineralogical compositions, and particulate matter chemical compositions obtained from both MIR taconite operations, MIR communities, and non-MIR communities, including 14 sampling events at taconite operations and 79 sampling events at locations within communities and sites in northeastern Minnesota (73) and Minneapolis (6), as summarized in Table 1. Lake sediment analysis continues, and will provide important data regarding potential mineralogical inputs from iron mining and processing from ~1840 (which predates iron mining on the MIR) to the present, which includes the period where the transition from natural ore mining to taconite mining took place. Continued analysis, interpretation and reporting will take place in 2013. Taconite Facility United Taconite In-Plant Sampling Events Sampling Events Taconite Facility 2 events while active Keetac Hibbing Taconite Sampling Events 1 event while active, 1 event while inactive 1 event while inactive, 3 events while active 3 events while active 1event while active, Northshore 1 event while inactive Minntac 1 event while active Minorca Community Sampling Events Community Sampling Location Sampling Events and Number per Season Keewatin Elementary School 7 Events (3 Winter / 4 Summer) Hibbing High School 9 Events (4 Winter / 5 Summer) Virginia City Hall 10 Events (5 Winter / 5 Summer) Babbitt Municipal Building 16 Events (7 Winter / 9 Summer) Silver Bay High School 13 Events (4 Winter / 9 Summer) Ely Fernberg Site 6 Events (3 Winter / 3 Summer) Duluth NRRI Rooftop 12 Events (7 Winter / 5 Summer) UMN Mech. Eng. Rooftop 6 Events (3 Winter / 3 Summer) Table 1. Summary of in-plant and community sampling events. References ISO 10312, 1995, Ambient air – determination of asbestos fibers – direct transfer transmission electron microscopy method, 51p. Marple, V. A., Rubow, K. L., and Behm, S. M., 1991, A micro orifice uniform deposit impactor (MOUDI): description, calibration, and use: Aerosol Science and Technology, v. 14, p. 434-446. Oreskovich, J. A., and Patelke, M. M., 2006, Historical use of taconite byproducts as construction aggregate materials in Minnesota: A Progress Report: Natural Resources Research Institute Report of Investigation NRRI-RI-2006-02, 10 p. 36 �Bedrock geologic map of the western Gunflint Trail area, northeastern Minnesota JIRSA, Mark A. Minnesota Geological Survey (MGS), St. Paul, jirsa001@umn.edu Figure 1—Generalized map of northeast Minnesota showing geologic setting of map area along the western end of the Gunflint Trail (dashed line). The 2007 Ham Lake forest fire provided an opportunity for detailed mapping in a classic area of Precambrian geology along the western Gunflint Trail corridor into the Boundary Waters Canoe Area Wilderness (Fig. 1). Because the area is a favorite of campers and resorters, the map was designed somewhat differently from other products of the MGS to provide more “general interest” content, including highlights of the unique geologic features that lie along the many well maintained hiking trails (Fig. 2). The burn covers more than 120 mi2, but nearly 2/3 of it is in adjacent Canada—the Gunflint map encompasses most of the U.S. portion. Mapping was supported in part by U.S. Geological Survey STATEMAP element of the National Geologic Mapping Program, and the results were published in 2011 as MGS Miscellaneous Map M-191. The western part of the Gunflint trail provides a transect across well exposed rock units of Neoarchean, Paleoproterozoic, and Mesoproterozoic age. Archean granite-greenstone terrane of the Wawa subprovince of Superior Province is represented by a succession of metavolcanic rocks locally known as the Paulson Lake sequence (ca. 2720 Ma), and the Saganaga Tonalite (ca. 2690 Ma). Diabasic dikes of imprecisely known age cut the Archean bedrock. Both they and the Archean rocks are unconformably overlain by Paleoproterozoic metasedimentary strata of the Animikie Group (ca. 1870-1830 Ma), which includes the Rove Formation and Gunflint Iron Formation. The stratigraphic top of iron-formation is marked by a thin sequence of ejecta from a meteorite impact that occurred near Sudbury Ontario, ca. 1850 Ma. This sequence forms a discontinuous ejecta blanket that is present throughout the Lake Superior region in Ontario, Michigan, and elsewhere in Minnesota (Jirsa and others, 2011, and references therein). Mesoproterozoic rifting is manifest in hypabyssal dikes and sills of the Logan intrusions (ca. 1115 Ma), and several phases of the Duluth Complex (ca. 1100 Ma) emplaced into both Archean and Proterozoic rocks. Stratigraphic facing directions in Neoarchean supracrustal rocks, based on pillowed metabasalt flows, indicate that the sequence forms a northwest-trending, steeply south-dipping and younging homocline. Much of the temporal distinction between various geological elements in the Archean rocks is based on regionally consistent fabrics that resulted from three major phases of deformation, denoted D1, D2, and D3. All three deformation events are the result of NS- to NW-SE-directed compression. Regionally, D1 deformation folded, tilted, and thrust faulted large sections of the supracrustal rocks, but did not produce significant metamorphic mineral assemblages or cleavage and schistosity. The timing of D1 deformation is bracketed between deposition of the volcanic and clastic rocks at about 2722 Ma (Peterson and others, 2001), and emplacement of the Saganaga Tonalite at 2690.83 Ma (Driese and others, 2011). The effects of D2 deformation in this area include moderate to mild flattening of minerals and inclusions near the margins of the Saganaga Tonalite; and strong schistosity, metamorphism, folding of tonalite 37 �dikes, and flattening of pillow structures in the immediately adjacent supracrustal rocks. U-Pb dates of intrusions that bracket D2 place the regional deformation and metamorphic event between about 2674 Ma and 2685 Ma (Boerboom and Zartman, 1993). D3 deformation produced faults and shear zones. Much of the map’s apparent complexity in areas of Paleoproterozoic strata and Logan intrusions is a product of the shallow dip and differential erosion of these formations, local faults and shallowly plunging folds, and moderate to high topographic relief. Deformation associated with the Lookout Fault grades from a distinct fault structure on the west—where Paleoproterozoic iron-formation is folded and faulted against Archean rocks—to a sympathetic drape structure in the eastern part of the area that is dominated by higher stratigraphic levels of iron-formation and Rove slate. The drape structure is manifest in a shallowly east-plunging, anticline-syncline pair that shows progressive flattening of limbs toward the southeast. The depiction of the Mesoproterozoic Logan intrusions differs significantly from previous mapping that implied the intrusions were folded. New field work indicates that the intrusions are semi-concordant with adjacent strata where dips are gently southeastward, but are locally discordant, particularly along the north-dipping limb of the anticline-syncline pair. Although the sills generally mimic fold structures, their local discordance implies that much of the deformation associated with the Lookout Fault and related structures predated emplacement of Logan intrusions. Figure 2. Geology of the southern part of Gunflint map area showing sites of geologic interest. References Boerboom, T.J., and Zartman, R.E., 1993, Geology, geochemistry, and geochronology of the central Giants Range batholith, northeastern Minnesota: Can. J. of Earth Sci., 30: 2510-2522. Driese, S.G., Jirsa, M.A., Ren, M., Sheldon, N.D., Brantley, S.L., Parker, D., and Schmitz, M., 2011, Neoarchean paleoweathering of tonalite and metabasalt: Implications for reconstructions of 2.69 Ga early terrestrial ecosystems and paleoatmospheric chemistry: Precambrian Research, 189:1-17. Jirsa, M.A., Fralick, P.W., Weiblen, P.W., and Anderson, J.L.B., 2011, Sudbury impact layer in the western Lake Superior region, in Miller, J.D., Hudak, G.J., Wittkop, C., and McLaughlin, P.I., eds., Archean to Anthropocene: Field Guides to the Geology of the Mid-Continent of North America: Geological Society of America Field Guide 24, p. 147–169, doi:10.1130/2011.0024(08). Peterson, D.M., Gallop, Christina, Jirsa, M.A., and Davis, D.W., 2001, Correlation of Archean assemblages across the U.S.-Canadian border: Phase I geochronology: (abstract): Institute on Lake Superior Geology Proceedings, 47th annual meeting, Madison, WI, v. 47, Part 1, p. 77-78. 38 �Minnesota River Valley subprovince as depicted on a new bedrock geologic map of Renville County, southwestern Minnesota JIRSA, Mark A., SETTERHOLM, Dale R., and CHANDLER, V.W. Minnesota Geological Survey, 2642 University Ave., St. Paul, MN 55114-1032 (jirsa001@umn.edu) The bedrock geology of Renville County has a long history of geologic study related to the apparent antiquity of the bedrock, and commercial interest in quarrying, clay mining, and early speculation about coal resources. The bedrock ranges in age from Paleoarchean (~ 3500 Ma) to Late Cretaceous (~90 Ma), and records a complex history involving multiple igneous, sedimentary, metamorphic, tectonic, and weathering events. The interpretation presented here focuses on the Archean and Paleoproterozoic geology. The map derives from published and unpublished mapping, augmented with high-resolution geophysical data, scant drill core, and field work by the authors in a narrow strip of scattered exposure along the Minnesota River that forms the county’s southern border. Figure 1. Generalized geologic map of Minnesota showing Renville County (black) adjacent to the Minnesota River (bold line) and within block subdivisions of the Minnesota River Valley subprovince of the Archean Superior Province. 39 �The Minnesota River Valley subprovince is divided into largely fault-bounded blocks, each having distinctive attributes. Renville County straddles two of those blocks—the Montevideo block to the north, and the Morton block to the south (Fig.1). Both blocks are characterized by gneissic bedrock, but they differ from one another in composition and grade of metamorphism of the gneisses contained therein. The Montevideo block consists of variably layered granitic to gabbroic gneisses, metamorphosed under granulite facies conditions and collectively referred to as the Montevideo Gneiss. The Morton block contains tonalitic to granitic migmatitic gneiss with amphibole-rich rafts, metamorphosed under upper amphibolite facies conditions; and granitic neosome occurring as diffuse pods, lenses, and discrete intrusions. Collectively, these rock types comprise the Morton Gneiss. The two blocks are separated by the geophysically distinct Yellow Medicine shear zone. The complex intrusive, metamorphic, and deformation history of the gneisses and intrusions has been partially unraveled by recent high-precision U-Pb geochronologic studies. The apparently oldest components of the gneissic bedrock in both the Montevideo and Morton blocks range in age from 3535±4 Ma (Bickford and others, 2006), to 3422±1 Ma (Schmitz and others, 2006). Collectively, the ages indicate that gneisses formed in the Paleoarchean (3600-3200 Ma), were multiply intruded, deformed, and metamorphosed during Mesoarchean (3200-2800 Ma) and Neoarchean (2800-2500 Ma) time, and intruded by granitic magmas at ca. 2600Ma. Geologic mapping by the authors and structural analyses by previous workers indicate that the gneissic bedrock underwent two major periods of regional deformation (designated D1, D2), and two later events that can be distinguished locally. The D1 event produced high-grade metamorphism and gneissic foliation (S1). The D2 event folded this foliation into shallowly northeast-plunging antiforms and synforms, localized doubly-plunging dome and basin structures, and zones of semi-ductile shear. Much of the apparent complexity of map patterns is inferred to be the result of these open, shallowly plunging F2 folds of the early-formed, nearly horizontal D1 metamorphic fabric. D2 also produced a localized foliation (S2) subparallel to fold axes. Movement along the Yellow Medicine shear zone was likely complex and protracted, but at least some of the deformation is inferred to represent north-over-south thrusting associated with crustal shortening during the Neoarchean (~2680 Ma) Minnesotan Orogeny. Drill core from two holes in the northern part of the county indicate that primary foliation and localized shearing in and near the Yellow Medicine shear zone dips northward, which is consistent with geophysical modeling conducted for this study. It is likely that the Yellow Medicine shear zone was reactivated during the Paleoproterozoic (2500-1600 Ma) to juxtapose strata inferred to be part of the Little Falls Formation against older bedrock during the Penokean and/or Yavapai orogenies. The gneissic bedrock in both blocks is cut by a variety of intrusions, including granite, granodiorite, and quartz monzonite of likely Archean age; discretely bounded felsic to mafic intrusions that could be either Archean or Proterozoic; and diabasic dikes of Paleoproterozoic and perhaps Mesoproterozoic age. The Bedrock Geology of Renville County is Plate 2 of the Minnesota Geological Survey’s County Atlas C-28 (http://www.mngs.umn.edu), which was supported by the Renville County Board of Commissioners and the Department of Natural Resources Division of Waters. References Bickford, M.E., Wooden, J.L., and Bauer, R.L., 2006, SHRIMP study of zircons from Early Archean rocks in the Minnesota River Valley: Implications for the tectonic history of the Superior Province: Geological Society of America Bull. v. 118, p. 94-108. Schmitz, M.D., Bowring, S.A., Southwick, D.L., Boerboom, T.J., and Wirth, K.R., 2006, High-precision U-Pb geochronology in the Minnesota River Valley subprovince and its bearing on the Neoarchean to Paleoproterozoic of the southern Superior Province: Geological Society of America Bull., v. 118, p. 82-93. 40 �Geochemistry of reversely-polarized intrusions along the SW limb of the Midcontinent rift system, Carlton County, Minnesota JOHNSON, Teresa1, WENDLANDT, Richard1, SHANNON, James2, 1 Department of Geology and Geological Engineering, Colorado School of Mines, 1516 Illinois Street, Golden, CO 80401 2 MMG, 390 Union Boulevard, Suite 200, Lakewood, CO 80228 The early rifting phase (1115-1100 Ma) of the Midcontinent rift system (MCR) is characterized by the emplacement of reversely-polarized extrusive and intrusive basaltic rocks. Comprehensive studies of the basalts from this early rifting period have delineated three distinct compositions, type I, II and III, which can be correlated throughout the rift system (Nicholson et al., 1997). These three basalt types are recognized in the Ely’s Peak Basalts along the southwest limb of the rift near Duluth, Minnesota. The reversely-polarized intrusions in this location are the diabase dikes of the Carlton Dike Swarm (CDS), the evolved Fe-Ti rich gabbro of the Esko intrusion and the Ni-Cu-PGE mineralized ultramafic Tamarack Intrusion. This study evaluates the diabase dikes of the Carlton Dike Swarm and their petrogenetic relationship to the other MCR reversely-polarized rocks. Previously, the CDS was characterized as a Ti- and Fe-enriched quartz tholeiite (Green et al., 1987; Reichhoff, 1987). This study evaluates four geochemical subgroups of dikes, types A-D (Table 1). Within the main NE-trending Carlton Dike swarm, three compositional subgroups are observed. The majority of dikes (type A) are classified as high-TiO2 (2.9-4.6 wt%) and outcrop mainly along the St. Louis River and include the prominent columnar-jointed dike outcropping south of the Thomson Reservoir. The two other dike types include: a similar high-TiO2 dike (type B) with a greater percentage of hydrous minerals (5%), and the steepest HREE slopes; and a lowTiO2 dike (1.2-1.3 wt%) (type C) locally known as the Cloquet dike. The Cloquet dike is recognized as the longest dike (55 km) in the swarm based on its aeromagnetic signature. In addition, a smaller subset of NW-trending dikes (type D) is also distinguished by aeromagnetics, with the most prominent dike located approximately 50 km to the southwest of the main CDS. This dike has major and trace element compositions that are analogous to type A dikes. The Esko Intrusion is a circular-shaped aeromagnetic feature located on the NE corner of Carlton County. The intrusion is also geochemically similar to the high-TiO2 CDS with a greater abundance of hydrous minerals similar to the type B dikes. The CDS diabase dikes have geochemical similarities to type II and III basaltic compositions, which is consistent with the timing of CDS dike emplacement near the end of the early reversal period (Green et al., 1987). The higher Mg#’s associated with the basaltic composition type I are not found in the diabase dikes sampled during this study or in previous research (Table 1). The type C dike is comparable to type II basalts with a similar Mg# and TiO2 but has a nearly horizontal HREE pattern and more pronounced negative Nb and Ta anomalies (Figure 1). Type A, B & D dikes are most similar to type III basalts. Type A & D dikes have the most evolved Mg#’s like type III basalts, while type B is less evolved. The trace-element and REE patterns of type D dikes overlap with the type A field, and both are more enriched in HREE than type III basalts (Figure 1). A significant difference, arguably, is the depletion of Nb relative to Th in the type A dikes, which is interpreted as indicating late contamination by a similarly sourced magma. The type B dikes and type III basalts have similar slightly negative Nb anomalies although the dikes are characterized by a positive Eu anomaly and steeper HREE pattern (Table 1). 41 �Nicholson and Schulz (2011) propose that type I and III basaltic compositions are possibly related by fractionation and variable amounts of contamination. Other investigators (e.g. Hou et al., 2011) have modeled sources of high-Ti basalts by fractional crystallization from highMg, low-Ti basalts. To evaluate possible genetic relationships between different dike groups, fractional crystallization models involving the most primitive samples (e.g. type C) along with basalt types I, II and III are being used. Thermodynamic modeling of the origin of high-Ti basaltic compositions is enhanced by low fO2 and a high Mg#, but possible influences of assimilation and magma mixing remain a complexity. Figure 1: Chondrite normalized trace-element patterns. Normalization factors from McDonough & Sun (1995) except for P from Thompson et al. (1983). Data for basalt type I, II and III from Nicholson et al. (1997). Table 1: Chondrite normalized as indicated (McDonough & Sun, 1995); FeO=0.85FeOT; Mg# = Mg/Mg+Fe (mole percent); n, number of samples REFERENCES Green, J., Bornhorst, T., Chandler, V., Mudrey Jr., M., Meyers, P., Pesonen, L., and Wilband. J., 1987. Keweenawan Dykes of the Lake Superior Region: Evidence for Evolution of the Middle Proterozoic Midcontinent Rift of North America. In Mafic Dyke Swarms. Geological Association of Canada. Special Paper 34: 289–302. Hou, T., Zhang, A., Kusky, T., Du, Y., Liu, J., and Zhao. A., 2011. A Reappraisal of the high-Ti and low-Ti Classification of Basalts and Petrogenetic Linkage Between Basalts and Mafic–Ultramafic Intrusions in the Emeishan Large Igneous Province, SW China. Ore Geology Reviews, 41: 133–143. Nicholson, S., Schulz, K., Shirey, S., and Green, J., 1997. Rift-Wide Correlation of 1.1 Ga Midcontinent Rift System Basalts: Implications for Multiple Mantle Sources During Rift Development. Canadian Journal of Earth Sciences, 34: 504–520. Nicholson, S., and Schulz. K., 2011. Geochemical Evolution of 1.1 Ga Midcontinent Rift Magmatism. Geological Society of America, Abstracts with Programs, 43: 228. Reichhoff, J., 1987. Two Keweenawan Basaltic Dike Swarms in the Duluth Area, Minnesota. University of Minnesota, Duluth, 206 p. 42 �SEDIMENTOLOGY AND PALEOGRAPHIC RECONSTRUCTION OF THE STRATA ADACENT TO THE SUDBURY IMPACT LAYER IN A CORED DRILLHOLE KARMAN, Monica M.1, and FRALICK, Philip W.1 1Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1, mmkarman@lakeheadu.ca, pfralick@lakeheadu.ca Drill core BDQ-2, extracted adjacent to Highway 588 near Thunder Bay, Ontario, Canada revealed stratified layers of the 1878.3 1.3 Ma (Fralick et al., 2002) Gunflint Formation, the 1850 Ma (Krogh et al., 1984) Sudbury Impact Layer, and the overlying 1832 3 Ma (Addison et al., 2005) Rove Formation. A limestone unit in the drill core, situated just above the Sudbury Impact Layer, and immediately below the fine-grained siliciclastic sediment of the Rove Formation, was analyzed to determine its mode of formation. The approximately three meters of chemical sediment overlying the ejecta layer has evidence of silicification below the uppermost meter. The top meter is composed of euhedral and subhedral calcite crystals with thin wisps of dark material separating the crystals and forming a chicken-wire texture (Figure 1A). The crystals consist of, on average, CaO 46.1%, MgO 0.68%, FeO 0.54% and MnO 0.19%. Five points in the carbonate unit were analyzed: point 1 being at the top of the analyzed unit, and going sequentially downward, through to point 5 at the bottom of the analyzed unit. Point 1 is situated at a sharp contact with the Rove Formation (Figure 1B). This point contains the largest carbonate crystals in the unit, consisting of ~3-5mm euhedral and subhedral zoned calcite crystals, in encasing fine-grained material (Figure 1A). The size of the calcite crystals decreases down the drill core, in the direction of the Sudbury Impact Layer, and at point 5 are ~200 -1.0mm in size. Results from SEM-EDX analysis using elemental mapping show zoned calcite crystals (Figure 2A) with Fe enrichment (Figure 2B) adjacent to the Mg enrichment (Figure 2C). The finegrained sediment present between the calcite crystals was also analyzed and is an assortment of clay minerals and calcite. Results from ICP-AES and MS analysis show low vanadium and chromium abundances in samples from points 1 to 4, but with much higher values, two orders of magnitude and one order of magnitude respectively, for the sample from point 5. PAAS normalized rare earth element curves for points 1 to 4 (Figure 3) have similarities with curves for meteoric water, whereas the curve for point 5 is similar to Paleoproterozoic seawater. Somewhat oxygenated meteoric water is needed for the transport of the vanadium and it would have precipitated under more reduced conditions in the sub-aerially exposed marine carbonate sediments. The fine-grained siliciclastic sediments above this were more oxygenated resulting in no vanadium enrichment though calcite crystals were precipitated from the meteoric waters creating a chicken-wire fabric. No modern analogs to this type of diagenetic alteration exist. 43 �1A 1B 3 2A 2B 2C Figure 1A) Calcite crystal chicken wire texture. Figure 1B) Contact between Rove Formation and carbonate unit. Figure 2A) SEM image of zoned calcite crystal. Figure 2B) Fe enrichment. Figure 2C) Mg enrichment. Figure 3) REE's for Points 1-5 (PAAS). 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, n 3, p. 193-196. Fralick, P.W., Davis, D.W. and Kissin, S.A., 2002. The age of the Gunflint Formation Ontario, Canada: single zircon U-Pb age determinations from reworked volcanic ash. Canadian Journal of Earth Sciences, v. 39, p. 1085-1091. Krogh, T.E., Davis, D.W., and Corfu, F., 1984. Precise U-Pb zircon and baddeleyite ages for the Sudbury area. In, E.G. Pye ed., The Geology and Ore Deposits of Sudbury Structure. Ontario Geological Survey, Special Volume 1, p. 431-446. 44 �Geologic mapping of Neoarchean and Paleoproterozoic rocks near Ester Lake by students of the Precambrian Research Center’s 2012 field camp KORMAN, Katrina1, CRADDOCK, Suzanne1, DOYLE, Michael1, WALTER, Jessica1, LEE, Aubrey2, and JIRSA, Mark3 1 2012 Field Camp Students, Precambrian Research Center, Natural Resources Research Institute, University of Minnesota Duluth, 5013 Miller Trunk Highway, Duluth, Minnesota 55811 2 University of Minnesota Duluth, Department of Geological Sciences, 1114 Kirby Drive, Duluth, Minnesota 5581 3 Minnesota Geological Survey (MGS), University of Minnesota, 2642 University Avenue W., St. Paul, Minnesota 55114(jirsa001@umn.edu) The University of Minnesota-Duluth’s Precambrian Research Center conducted its sixth annual field camp in 2012, and this presentation is one of a series that show some of the results. During the fifth and sixth weeks of camp, teams of students participate in “capstone projects” that test student skills by creating new geologic maps in areas of poorly known geology. This capstone project involved mapping an area of the Boundary Waters Canoe Area Wilderness accessed by 12 lakes, with Ester Lake at its center (Fig. 1). The map provides details about the complex depositional and deformation history of a Neoarchean, largely metasedimentary terrane that is part of the Wawa subprovince of Superior Province. Figure 1. Generalized bedrock geologic map of northeastern Minnesota showing the Ester Lake capstone area. The unit labeled “Knife Lake Group” also encloses older volcanic sequences that are not delineated separately at this scale. Dashed line is the border of the Boundary Waters Canoe Area Wilderness. The Ester Lake map area lies along and west of the boundary between the Saganaga Tonalite (ca. 2690 Ma), and sedimentary strata of the Knife Lake Group that are inferred to have been derived in part from it. Both rock units were tilted, folded, faulted, and metamorphosed to low 45 �greenschist facies during a regional deformation event at about 2680 Ma, which provides an approximate minimum age for the Knife Lake Group. Our mapping demonstrated that strata of the Knife Lake Group in this area form a broad, northeast-trending synclinorium that is bounded by the Saganaga Tonalite on the east, and an apparently uplifted fault-block of metabasalt on the west (see cross section on poster). The limbs of this large structure are marked by smaller sympathetic folds, and are dissected by several faults and shear zones. Mapping to the east revealed an erosional unconformity at the contact between Saganaga Tonalite and basal Knife Lake strata. The tonalite appears to have been weathered and is overlain by sandstone containing both subrounded and angular quartz pebbles—approximately the same size as quartz phenocrysts in the tonalite—in a sandy matrix of altered plagioclase grains and fine grunge that resembles reworked granitoid saprolite. To the west, this basal unit grades stratigraphically upward into irregularly interbedded sequences of arkose; conglomerate containing amoeboid clasts of tonalite (“Fish Lake conglomerate”); hornblende-phyric, trachyandesite-bearing coarse pebble to cobble conglomerate; polymictic conglomerate with metabasalt clasts; gritstone composed of subangular sand grains of hornblende and plagioclase; and graywacke and slate containing rare, thin lenses of banded iron-formation. Collectively, this arrangement of stratigraphic facies indicates that Saganaga Tonalite and the rocks it intruded were uplifted and subaerially eroded to provide detritus to nearby basins, now manifest as Knife Lake strata. However, the diverse lithologic character of these strata indicates that a great variety of sediment source regions and depositional settings existed. The Saganaga Tonalite was likely weathered to form saprolite, which was eroded by slopewash, alluvial fan, and fluvial processes to produce the quartz pebble sandstone, and to contribute clasts to various conglomeratic layers. We attribute the several hundred-foot-thick arkosic unit to reworking of saprolite in a near-shore marine or lacustrine environment. The Fish Lake conglomerate, containing matrix-supported amoeboid (paleosaprolitic) tonalite clasts, is interpreted to be channelized alluvium deposited in a rapidly subsiding basin. Conglomerate containing hornblende-trachyandesite clasts likely formed from erosion off the informally named Jasper Lake volcanic sequence of Knife Lake Group, which lies southeast of the map area (Jirsa and Starns, 2008). Graywacke and slate are interpreted to represent deposition in a lacustrine or marine setting, and the interlayered coarser polymictic clastic strata may represent braided stream, alluvial fan, or subaqueous fan deposition of sediment shed off the uplifted flanks of the basin. The layered strata exhibit chaotic soft-sediment deformation features, local growth faults, and abrupt facies changes, suggesting that deposition was synchronous with episodic basin subsidence. Thin layers and lenses of jasper-bearing iron-formation that are associated with graywacke and slate are interpreted as chemical precipitates into what may have been a shallow marine environment. The lithologic diversity and structural complexity is consistent with a model of deposition in rhombochasms created during strike-slip or extensional faulting. The general fining-upward character of Knife Lake strata, albeit irregular, may represent increasing water depth over time within the depocenter. The western, fault-bounded metabasalt unit contains abundant tightly packed mattress-size pillows that lack amygdules, implying deposition in fairly deep water. These attributes, along with the vertical dip of metabasalt flows, are similar to parts of the ca. 2720 Ma Ely Greenstone and Newton Lake Formation, and represent a very different tectonic and depositional setting from that of the adjacent Knife Lake Group. A small unit of metadiabase identified during mapping may be part of a swarm of mafic intrusions seen elsewhere in the region that are inferred to be Paleoproterozoic in age. This and other maps produced by capstone projects can be viewed at www.d.umn.edu/prc. Reference Jirsa, M.A., and Starns, E.C., 2008, Preliminary bedrock geologic map of the 2006 Cavity Lake fire area, northeastern Minnesota: MGS Open-File Report OF08-05, scale 1:24,000. 46 �Preliminary Investigations of the Fe-Ti-V-P mineralization associated with the Thunderbird and Butler gabbroic intrusions within the McFaulds greenstone belt, Superior Province, Northern Ontario, Canada KUZMICH, Ben1, HOLLINGS, Pete1, HOULÉ, MICHEL G.2 1 Department of Geology, Lakehead University, 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada 2 Geological Survey of Canada, GSC-Quebec, 490 rue de la Couronne, Québec, Quebec G1K 9A9 The McFaulds Lake area (i.e., Ring of Fire) located in northern Ontario (Canada) has been the site of recent mineral exploration leading to the discoveries of several mineralization types such as chromite and nickel sulfide deposits. Although the majority exploration has been focused on chromium, this area also contains significant Fe-Ti-V-P mineralization associated with gabbroic intrusions, in which the Thunderbird and Butler occurrences are the best defined.   These gabbroic intrusions are widely distributed throughout the McFaulds Lake area and maybe grouped into two main types of occurrences: (1) large mafic-dominated intrusions and (2) subconcordant to slightly discordant mafic-dominated sills/dikes characteristic of the Thunderbird and the Butler intrusions respectively. These intrusions are composed of an evolved mafic suite termed the ‘Ferrogabbro’ characterized by the presence of Fe-Ti oxides. Through detailed core logging, it has been recognized that both intrusions are largely composed by very similar lithologies including iron-rich gabbros, leucogabbros, and anorthosites. Two types of mineralization occur in these intrusions: (1) Fe-Ti-V and (2) Fe-Ti-P mineralization. Fe-Ti-V mineralization occurred within both intrusions whereas the Fe-Ti-P mineralization have been only identified within the Thunderbird intrusion. The mineralization occurs dominantly as disseminated magnetite and ilmenite (1-10%), but also present as semi-massive (50-80%), to massive layers (>80%). These layers typically contain distinct sharp, stratigraphically lower contacts and gradational upper contacts typical of primary igneous layering. The massive oxide layers are composed of magnetite and ilmenite typically at a ratio of 10:1. The ilmenite occurs as anhedral to subhedral crystals and to a lesser extent, as very fine-grained exsolutions within anhedral magnetite grains. Vanadium grades range from 0.30% to more than 0.60% (V2O5) and titanium grades range from 2.5% to more than 4.5% (TiO2) whereas Vanadium grades, at Butler, range from 0.42% to more than 1.17% (V2O5) and titanium grades range from 0.46% to more than 11.3% (TiO2).   47 �Sedimentology
and
geochemistry
of
the
Espanola
Formation,
Huronian
 Supergroup
 LAFONTAINE,
Daniel
and
FRALICK,
Philip
 Department
of
Geology,
Lakehead
University,
955
Oliver
Road

Thunder
Bay,
ON
 P7B
5E1
Canada
 
 The
Huronian
Supergroup
is
a
southerly
thickening
wedge
of
Paleoproterozoic
 sediments
with
a
maximum
thickness
of
12
km
and
an
age
range
of
2450
Ma
to
2219
Ma
 (Bennett
et
al.,
1991).

The
area
is
hypothesized
to
be
a
divergent
continental
margin
with
 the
paleo‐ocean
directly
to
the
south
(Fralick
and
Miall,
1989).

The
Huronian
Supergroup
 contains
evidence
of
three
separate
glacial
related
formations,
including
the
Ramsay
Lake
 Formation,
Bruce
Formation
and
the
Gowganda
Formation.

The
immense
size
of
similar
 glacial
formations
and
their
sedimentary
deposits
at
tropical
paleo‐latitudes
led
geologists
 (ie.,
Paul
Hoffman)
to
put
forth
the
Snowball
Earth
theory.

Geochemistry
from
the
Espanola
 Formation
may
very
well
help
identify
possible
reflections
of
atmospheric
composition,
 specifically
related
to
carbon
dioxide
and
oxygen
content.

The
Espanola
Formation
is
a
lone
 cap
carbonate
sandwiched
between
the
Bruce
and
the
Gowganda
Formations.

It
is
 classically
divided
into
three
individual
members;
the
lower
Limestone
member,
the
middle
 Siltstone
member
and;
the
upper
Dolomitic
Cap.

The
sediments
of
the
Espanola
were
 deposited
on
a
carbonate
shelf
post‐glaciation
that
begin
to
reflect
a
shallowing
upwards
 sequence
later
in
stratigraphy.

This
can
be
attributed
to
post‐glacial
isostatic
adjustment.

 This
abstract
will
discuss
the
lithofacies
associations
of
each
member,
their
depositional
 environments
and
possible
oceanic
geochemical
signatures.

Sediments
deposited
consist
of
 a
terrigenious
sediment
source
with
a
carbonate
cement
as
well
as
carbonate
minerals
 precipitated
directly
from
seawater.

Partial
dissolutions
in
acetic
acid,
of
selected
samples,
 were
analyzed
by
ICP
MS
to
better
understand
the
geochemistry
of
the
carbonates.

Yttrium
 is
not
removed
from
seawater
like
it’s
geochemical
twin
Holmium
(due
to
differing
surface
 stabilities)
and
seawater
generally
displays
high
Y/Ho
ratios
that
may
range
between
44‐74
 (Nagarajan
et
al.,
2011).

The
Espanola
geochemical
results,
represented
by
Figure
1
in
a
 graph
of
Yttrium
verses
Holmium,
displays
a
low
ratio
of
~28
which,
is
indicative
a
 dominant
chondritic
terrigeneous
sediment
source
(Nagarajan
et
al.,
2011),
possibly
 attributed
to
river
run‐off
systems
fed
by
the
glacial
melt
waters.
 Using the same samples, the spider diagram in Figure 2 displays a distinct “hat-like” pattern of MREE enrichment. This is not consistent with carbonates precipitating from Paleoproterozoic seawater but analogous with the MREE enriched patterns of major modern river systems (Matsaranta, 2006). This MREE enrichment supports a non-marine water components in the near-shore waters from which the carbonates precipitated .

The
possibility
that
the
carbonate
precipitation
in
the
middle
 siltstone
member
was
simply
carbonate
cement
is
likely
with
the
high
amount
of
siliciclastic
 material,
while
the
upper
and
lower
members
are
dominated
by
carbonate
precipitates.

 Geochemical
evidence
does
not
indicate
a
stratigraphic
control
that
classically
divides
the
 Espanola
members
but
rather
suggests
a
lateral
geographic
change
away
from
the
shore‐ line
in
limestone/dolostone
precipitation.

The
Espanola
represents
a
period
of
minor
 increases
in
oxygen
content
and
future
thesis
work
will
hopefully
aid
in
drawing
 conclusions
regarding
paleo‐atmospheric
composition.
 
 48 �
 References
 Bennett,
G.,
Dressler,
B.O.,
and
Robertson,
J.A.,
1991.
The
Huronian
Supergroup
and
Associated
 Intrusive
Rocks.
Ontarion
Geologic
Survey,
v.4,
no.1,
549–586.
 Fralick,
P.
and
Miall,
A.D.,
1989.
Sedimentology
of
the
Lower
Huronian
Supergroup
(Early
 Proterozoic),
Elliot
Lake
area,
Ontario,
Canada.
Sedimentary
Geology,
v.63,
127–153.
 Metsaranta,
R.,
2006.
Sedimentology
and
Geochemistry
of
the
Mesoproterozoic
Pass
Lake
and
 Rossport
Formations,
Sibley
Group.
Unpublished
Masters
Thesis,
Lakehead
University,
p.217
 Nagarajan,
R.
et
al.,
2011.
Geochemistry
of
Neoproterozoic
limestones
of
the
Shahabad
Formation,
 Bhima
Basin,
Karnataka,
southern
India.
Geosciences
Journal,
v.15:
9–25.
 49 �Chemostratigraphy of the Biwabik Iron Formation: Implications for Basin Longevity and Evolution LARSON, Phillip Duluth Metals Limited, 306 W Superior Street #610, Duluth MN 55802 United States The Paleoproterozoic Biwabik Iron Formation (BIF), an ~200m thick iron-formation in northeastern Minnesota, USA, is traditionally subdivided into four conformable members: Lower Cherty (LC), Lower Slaty (LS), Upper Cherty (UC), Upper Slaty (US), comprised predominantly of granular (cherty) and banded (slaty) iron-formation. Basin geometry and depositional environment of the iron-formation in the BIF and other 1.88 Ga circum-Superior Craton ironformation has been a topic of speculation for decades. Recent work suggests deposition on a clastic sediment-starved stable platform is a viable model for explaining the sedimentological and geochemical characteristics of circum-Superior iron-formations, as well as their widespread distribution. Geochemical evidence from the BIF in the Virginia Horn area provides further evidence in support of this model, and offers additional insight into both basin longevity and basin evolution. The BIF is composed predominantly of chemically precipitated sediment, reflected in Fe, Si, Mn, P, Mg, and Ca concentrations. A minor detrital clastic component is reflected in Al, Ti, and K concentrations. Al2O3 (the predominant detrital component) concentration ranges from 0.03% in granular iron-formation to 3.79% in slaty iron-formation, averaging 0.35% (sd=0.38%) (Fig. 1). Assuming a PAAS shale-like composition (Taylor and McLennan 1985), this corresponds to a detrital component averaging 2.3%, comparable to the detrital component found in modern platform carbonates. In contrast to the variability in detrital content, the Al2O3:TiO2 ratio is remarkably consistent through the thickness of the BIF, with the exception of a break in ratio corresponding to the top of the ‘Variably-Bedded and/or Mottled Unit’ of the LC member of Severson and others (2009) (Fig. 2). Below this horizon, Al2O3:TiO2 in the lower LC is 29:1, while above this horizon Al2O3:TiO2 in the LC-LS-UC-LS is 10:1. These constant ratios suggest that detrital material input to the BIF was sourced from long-lived, homogeneous reservoirs, likely as fine-grained, windborne dust. The constant ratios also suggest that variation in detrital concentration is a function of variation in chemical precipitate accumulation rates, rather than variation in detrital deposition rates. The two Al2O3:TiO2 ratios suggest two fundamentally different detrital source areas contributed to the BIF; the 29:1 ratio is comparable to that of modern airborne particles sourced from mature continental sediment in the Sahara, while the 10:1 ratio may reflect a volcanic source. The abrupt transition between the high Al2O3:TiO2 lower LC and the overlying low Al2O3:TiO2 LC-LS-UCUS sequence suggests that this contact represents a significant depositional hiatus, accompanied by a fundamental reorganization of atmospheric circulation and (or) emergence of a new detrital source area. Modern windborne dust accumulation rates in south Florida are on the order of 1.25 g·m-2·yr-1.; assuming similar deposition rates during BIF accumulation, it is possible to calculate rates and the duration of iron-formation accumulation from detrital concentration. Assuming s.g. 3.34, instantaneous accumulation rates range from 1.5 to 189 m/m.y., averaging 32 m/m.y. (Fig. 3). Granular iron-formation subunits are notably characterized by significantly higher accumulation rates than banded iron-formation subunits, both in the LC and LS-UC-US members. 50 �Overall, for the BIF in the Virginia Horn area, 1.6 myr of accumulation in the 63 m thick lower LC and 13.6 myr of accumulation in the upper 150 m thick LC-LS-UC-LS are indicated. The presence of a significant disconformity internal to the LC member challenges the assumption that accumulation of iron-formation in the BIF (and in other circum-Superior iron-formations) is the product of a single conformable depositional sequence, and particularly suggests re-evaluation of the traditional four member (LC-LS-UC-LS) subdivision of the BIF is warranted. 125.0 Depth (ft) Relative to top of "Variably-Bedded and/or Mottled Unit" (Severson et al 2009) Depth (m) Relative to top of "Variably-Bedded and/or Mottled Unit" (Severson et al 2009) 125.0 100.0 75.0 50.0 25.0 0.0 -25.0 -50.0 75.0 50.0 25.0 0.0 -25.0 -50.0 0.0 1.0 2.0 3.0 Al2O3 (wt%) 0 Figure 1. Al2O3 in the Biwabik Iron Formation, Virginia Horn area, Minnesota. n=1341 30 60 90 120 150 Accumulation Rate (m/myr) Figure 3. Iron-formation accumulation rates in the Biwabik Iron Formation, Virginia Horn area, Minnesota. LC 0.14 LS/UC/US 0.12 TiO2 (wt%) 100.0 0.10 0.08 0.06 0.04 0.02 0.00 0.0 0.5 1.0 Al2O3 (wt%) 1.5 Figure 2. Al2O3 and TiO2 in the Biwabik Iron Formation, Virginia Horn area, Minnesota. n=917 51 �Geochemistry and origin of slate-hosted massive sulfides of the Eagle Ni-Cu-PGE Deposit, northern Michigan: A preliminary study. LEATHERMAN, Mark1, RIPLEY, Edward1, ROSSELL, Dean2, WARE, Andrew2, and LI, Chusi1 1 Department of Geological Sciences, Indiana University, 1001 E. 10th St., Bloomington, Indiana 47405 2 Kennecott Eagle Minerals-Rio Tinto, Ishpeming, Michigan 49849 The Eagle Ni-Cu-PGE sulfide deposit is located within the Baraga Basin in northern Michigan. It is also located just south of the 1.1 Ga Midcontinent Rift System axis. Sulfide mineralogy consists of pyrrhotite-chalcopyrite-pentlandite. Primary ore types consist of disseminated, semimassive (net-textured), and massive that are associated with the main feldspathic peridotitemelagabbro intrusion. Recent drilling has revealed three additional classes of sulfides that are hosted in black slates of the Michigamme Formation. Class 1 consists of massive sulfide analogous in appearance to that of igneous-related massive mineralization (Figure 1). Class 2 consists of sulfides with quartz veins thought to be hydrothermal in origin. Some of these occurrences are associated with localized soft-sediment deformation. Also, neither of these sulfide classes shows evidence of lobate margins and dissolution. Class 2 sulfides can further be divided based on their appearance. Class 2a sulfides have fragments of sulfide entrained in predominant quartz (Figure 2a), whereas class 2b has approximately equal quartz and fine-grained sulfide intermixed (Figure 2b). Class 3 consists of discrete lenses or pods of massive pyrrhotite (class 3a) or chalcopyrite (class 3b). Furthermore, class 3 sulfides are enigmatic due to there being no obvious emplacement mechanism within slate (Figure 3a, b). Sulfur isotope and petrographic evidence suggests a dual igneous-replacement and thermal-sedimentary origin for the slate-hosted sulfides. Class 1 sulfides show 34S = 1.2 – 3.9 and minimal quartz along the sulfide-slate contact (origin). Class 2 sulfides show  34S = 7.5 – 11.8 and sharp sulfide-slate contacts with no noticeable alteration mineralogy. Class 3a sulfides display  34S = 5.4 – 10.3 along with acicular muscovite forming along the sulfide-slate boundary, protruding into the former, and are distal from the igneous intrusion; whereas class 3b displays  34S = 1.6 – 2.9, acicular Mg-rich chlorite, and are located proximal from the intrusion. Class 1 sulfides are thought to be magmatic in origin with some minor interaction with crustal sediments at lower temperatures. Class 2a sulfides are hypothesized to form as a result of preEagle tectonic activity and remobilization whereas class 2b sulfides are inferred to have originated via aqueous fluids that were initiated by heat flow supplied by intrusives. The class 3b chalcopyrite is suggested to be magmatic in origin with some crustal interaction, produced as a result of fractional crystallization of an immiscible sulfide liquid. Minor crustal interaction is indicated given: 1) petrographic evidence showing quartz and other silicates entrained in sulfide, 2) presence of Mg-bearing phases along slate-sulfide contact, and 3) the development of lobate sulfide-silicate contact zones. A hypothesis on the emplacement mechanism of slate-hosted chalcopyrite is that they are replacements of older sedimentary sulfides (i.e. trace pyrite). The class 3a pyrrhotite is thought to be of a thermal-sedimentary origin based on the following: 1) moderately high  34S values, 2) lack of mafic alteration phases along the slate-sulfide contact, and 3) significant spatial separation from the intrusion. Sedimentary pyrite may be converted to pyrrhotite in C-rich sedimentary rocks via reactions such as: FeS2 + 3H2O+ 5/2C = FeS + H2S + 3/2CO2 + CH4. It is clear that multiple periods of sulfide generation occurred in the area of the Eagle intrusion, all related in some manner to thermal effects associated with rift-related magmatic activity. 52 �Figure 1 (top): Class 1 sulfide showing evidence of hydrothermal-sedimentary involvement. Figure 2a (middle-left): Class 2a sulfide – fragments in majority quartz thought to be from pre-Eagle tectonism. Figure 2b (middle-right): Class 2b sulfide intermixed with quartz showing localized soft-sediment deformation. Figure 3a (bottom left): Pod of massive pyrrhotite (class 3a sulfide) encased in slate with no noticeable sedimentary deformation. Figure 3b (bottom right): Bleb of massive chalcopyrite (class 3b sulfide) encased in slate. 53 �Field, Petrographic, and Geochemical Study of the Bad Vermilion Intrusion, Mine Centre, Ontario, Canada LEE, Aubrey, and MILLER, Jim Department of Geological Sciences, University of Minnesota Duluth, 1114 Kirby Drive, HH 229, Duluth, MN 55812. During the summer of 2011, Aubrey Lee was contracted by Numax Resources, Inc. to map the bedrock geology of the Bad Vermilion Intrusion (BVI) which lies within Numax subsurface claims in Mine Centre, Ontario, Canada (Fig. 1b, Lee et al., 2012). The objective was to establish continuity of massive oxide units along the northern shore of Bad Vermilion Lake which had previously been documented by White and Albers (2010) along the northern shore of Seine Bay (Rainy Lake) to the west. In addition to facilitating accurate predictions for future exploration targets, the field work was to provide data for a field, petrographic, and geochemical study of the BVI as Aubrey’s master’s thesis at the University of Minnesota, Duluth. This study seeks to characterize the lithologic, petrographic, and geochemical attributes of the BVI in order to evaluate the potential for economic concentrations of Fe-Ti oxides, as well as PGE reef mineralization. The map (Fig. 1b) was presented last year at the 58th annual ILSG meeting in Thunder Bay (Lee et al., 2012). This poster presentation displays geochemical data and petrographic observations which provides a more comprehensive depition of BVI geology. The BVI is a 14 km long and 1-3 km wide, sub-vertically dipping, sill-like, layered plutonic sequence of gabbroic rocks exposed along the shores of Seine Bay (Rainy Lake) and Bad Vermilion Lake in Northwest Ontario, Canada (Fig. 1b; White and Albers, 2010). The intrusion is situated at the boundary between the metavolcanicplutonic Wabigoon subprovince to the north and the metasedimentary Quetico subprovince to the south in the Superior Province of the Canadian Shield Precambrian Figure 1: a) Magnetic Anomaly Map and Oxide Unit Correlation of the BVI; terrain (Poulsen, 1986). Ontario Geological Survey (2008). b) Geologic Map with subsets showing sampling profiles, drill collar locations, and plan-view extents of drill holes. It occurs within a dextral 54 �wrench fault zone bounded by the Quetico fault to the north and the Rainy Lake-Seine River fault to the south. The BVI lithologically consists of a mafic layered intrusive package of anorthositic, gabbroic, pyroxenitic rocks with layers/lenses of semi-massive to massive Fe-Ti oxides that have been significantly altered and possibly metamorphosed. The intrusion is exposed between felsic intrusive rocks at its northern contact and mafic volcanic rocks at its southern contact. The nature of these contacts and the magmatic relationship between each unit is enigmatic. Magnetic surveys of the Mine Centre area reveal three distinct magnetic highs extending the length of the intrusion and merging in the northeast where the exposure of the intrusion narrows along Bad Vermilion Lake (Fig. 1a; Ontario Geological Survey, 2008). Correlation of bedrock mapping with magnetic surveys indicates that the anomalies correspond to massive, semi-massive, and disseminated Fe-Ti oxide layers which are generally persistent along the length of the intrusion, though they may pinch, swell, and anastomose. The economic importance of the BVI lies in these oxide layers which contain significant concentrations of iron, titanium, vanadium and/or phosphorous (White and Albers, 2010). Four Numax drill holes (Fig. 1b) in the BVI highlight Fe-Ti mineralization, though only two on the west end of the intrusion have been fully geochemically assayed. Two near the central and east areas will eventually be assayed and incorporated into this study. During 2011 mapping, samples were collected along three profiles in the west, central, and eastern areas to supplement core data. Geochemical assays of field samples and drill core have helped establish petrologic and mineralogic continuity along the strike of the intrusion and document the igneous stratigraphy of the BVI. The western drill holes both intersected a thick package of melagabbroic rocks with 10 cm – 30 m intervals of massive oxide among oxide-bearing melagabbros. There are at least four massive oxide intervals with continuity between the two holes (600 m apart along strike). It is likely that these units pinch and swell, evidenced by variations in unit thickness and strong shearing at oxide layer contacts. In addition, 200 thin sections from field samples and drill core are being viewed in both reflected and transmitted light to delineate mineralogy, texture, alteration and, most importantly, to establish oxide mineralogy. Oxide layers were originally thought to be hosted by pyroxenite or gabbro, but petrographic analysis shows that the rocks are significantly altered and metamorphosed. Chlorite, talc, and amphibole alteration is dominant, though quartz-carbonate veining is also common. Photomicrographs of samples taken from the “top” of the intrusion indicate that primary apatite is strongly associated with massive oxides in the upper zones of the BVI. The most common oxides are expected to be secondary magnetite and ilmenite after primary titanomagnetite which will be verified through reflected light petrography. REFERENCES Lee, A., Albers, P., Miller, J., Severson, M., Deen, T., 2012, Bedrock Geologic Map of the Seine Bay/Bad Vermilion Lake Intrusion, Mine Centre, Ontario. In: Hollings, P. (Ed.), Institute on Lake Superior Geology Proceedings, 58th Annual Meeting, Thunder Bay, Ontario, Part 1 – Proceedings and Abstracts, v. 58, part 1, 1-2. Ontario Geological Survey, 2009. Ontario airborne geophysical surveys, Magnetic and Electromagnetic Surveys, Grid and Profile Date (ASCII and Geosoft Formats) and Vector Data, Mine Centre Area;. Ontario Geological Survey, Geophysical Dataset 1061a ISBN 978-1-4435-0854-4 [DVD] ISBN 978-1-4435-0855-1[ZIP FILE]. Poulsen, K. H., 1984. The Geological Setting of Mineralization in the Mine Centre – Fort Frances Area, District of Rainy River; Ontario Geological Survey Open File Report 5512, 126 p.  White, C. R. & Albers, P. B., 2010. Report on the Geology and Mineral Potential of the Seine Bay/Bad Vermilion Lake Intrusion, Mine Centre Property, Mine Centre, Ontario, s.l.: Unpublished report prepared for Numax Resources, Inc.  55 �The Igneous Stratigraphy of the Bad Vermilion Intrusion, Mine Centre, Ontario, Canada: Which Way is Up? LEE, Aubrey, and MILLER, Jim Department of Geological Sciences, University of Minnesota Duluth, Duluth, MN 55812. The Bad Vermilion Intrusion (BVI) is a layered plutonic sequence of gabbroic rocks exposed along the shores of Seine Bay (Rainy Lake) and Bad Vermilion Lake in Northwest Ontario, Canada (White and Albers, 2010). The intrusion is the subject of a field, petrographic, and geochemical study as Aubrey Lee’s master’s thesis at the University of Minnesota-Duluth. The BVI is sill-like, approximately 14 km long and ~1 km wide, and stretches along the northern shores of Seine Bay in the southwest and Bad Vermilion Lake in the northeast with the town of Mine Centre marking its northeast extent. The BVI lithologically consists of a mafic layered intrusive package of altered and metamorphosed anorthositic, gabbroic, and pyroxenitic rocks with layers/lenses of semi-massive to massive oxides. Primary apatite occurs near the upper (northern) zones and is strongly associated with massive oxide units. Within the east-west elongate block bounded by the Quetico and Rainy Lake-Seine River fault zones (Poulsen, 1986), the BVI dips sub-vertically (85˚ N-NW shallowing to 60˚ N-NW in central region) between felsic intrusive and mafic volcanic rocks (Fig. 1). The Mud Lake Trondhjemite to the north and the Bad Vermilion Tonalite to the south (along Bad Vermilion Lake) are concordant with the BVI while the mafic volcanic rocks (chiefly basalt) to the south (along Seine Bay) are weakly discordant. The nature of these contacts and the magmatic relationship between the BVI and the country rocks is poorly known. It is also unclear as to whether the intrusion tops to the north or south. Bedrock mapping (Lee et al., 2011), magnetic surveys, and petrographic analysis helped to establish subdivisions within the intrusion (Fig. 1). This was also made possible by the fact that the sub-vertical dip of the BVI exposes the entire stratigraphy of the intrusion in near-true thickness. On a large scale, the region can be divided into a Mafic Layered Series, bounded on the northern side by the Mud Lake Trondhjemite, in the southeast by the Bad Vermilion Tonalite, and in the southwest by mafic volcanics. The Mafic Layered Series can be further subdivided into three zones; the Leucogabbroic Zone to the south, the central Gabbroic Zone, and the Melagabbroic Zone to the north. Although all zones contain oxide-bearing lithologies throughout, each zone contains an oxide “unit” that is composed of multiple layers of massive to semi-massive Fe-Ti oxides Figure 1: Idealized igneous stratigraphy of the BVI showing the main zones of the mafic layered concentrated in discrete intervals, typically toward the base of the zones. The oxide “units”, series and the approximate locations of the three which range from 20 – 200 m in thickness, all oxide units. Vertical scale is approximate. 56 �contain more than one layer/lens of massive oxide which are typically 10 cm – 1 m thick. However, one oxide unit within the Melagabbroic Zone is up to 30 m thick, as observed in core and field exposures. Though all three oxide units are evident in the western and central areas of the BVI (White and Albers, 2010), they thin and appear to coalesce toward the northeast (Lee et al., 2011). Geochemical and petrographic analysis will hopefully help to resolve how the eastern extent of the BVI correlates with the thicker sequences to the west. In White and Albers’ (2010) mapping of the central and western BVI, they interpreted the southern Mafic Volcanic Sequence and Bad Vermilion Tonalite to be the footwall and the Mud Lake Trondhjemite the hanging wall of the BVI. North topping was originally determined based on graded layering of mafic to feldspathic lithologies which occur throughout the intrusion. This is especially evident in the northernmost Melagabbroic Zone near the contact with the Mud Lake Trondhjemite, and can locally be traced for up to 400 m. Typically, the base of each layer is dominated by clinopyroxene with plagioclase abundance increasing upward and a sharp contact with the base of the next layer. Moreover, apatite abundance in the massive oxides of the Melagabbroic Zone indicates P-enrichment to the north. However, if the BVI does indeed top to the north, one would expect the abundance of plagioclase to increase from south to north on a large scale. Instead, the general south-to-north progression is from plagioclase-rich rocks (Leucogabbroic Zone) to mafic rocks (Melagabbroic Zone) and may indicate that the intrusion is slightly overturned and that the Mud Lake Trondhjemite is actually the footwall of the BVI. The nature of the contacts between the BVI and its bounding rocks are difficult to interpret. There is no evidence for faulted contacts. Moreover, the felsic intrusive rocks on either side of the BVI are clearly not upper differentiates given their tonalitic/trondhjemitic composition and the abrupt transition from mafic to felsic lithologies. Rather, the mafic volcanics at the southwestern contact of the BVI show effects of contact metamorphism and thus imply an intrusive contact (White and Albers, 2010). The contact between the BVI and the Bad Vermilion Tonalite to the southeast is concealed beneath Bad Vermilion Lake, but has also been considered intrusive by Poulsen (1984). The nature of the northern contact between the BVI and the Mud Lake Trondhjemite is more enigmatic. Although the trondhjemite is concordant with the BVI, grades into the melagabbro zone over a narrow interval of quartz gabbro, and has the same strike length as the BVI, it is petrologically unlikely that fractionation of the BVI would generate a trondhjemitic differentiate. Rather, given that the trondhjemite is bounded on the north by an extensive region of felsic volcanics (Poulsen, 1984), it is possible that the trondhjemite represents a partially melted zone generated by underplating of the hot BVI magma beneath the felsic volcanics. Narrow compositional grading from melagabbro to quartz gabbro to trondhjemite at the northern contact of the BVI is perhaps evidence for a gradational partially melted upper contact, but back-veining of the trondhjemite into the BVI supports the idea that the trondhjemite is actually the basal intrusive contact. This latter interpretation also fits better with the southward mafic to felsic progression of BVI zones. It is hoped that evaluation of geochemical cryptic layering through the BVI will help resolve which way is up. REFERENCES Lee, A., Albers, P., Miller, J., Severson, M., Deen, T., 2012, Bedrock Geologic Map of the Seine Bay/Bad Vermilion Lake Intrusion, Mine Centre, Ontario. In: Hollings, P. (Ed.), Institute on Lake Superior Geology Proceedings, 58th Annual Meeting, Thunder Bay, Ontario, Part 1 – Proceedings and Abstracts, v. 58, part 1, 1-2. Poulsen, K. H., 1984. The Geological Setting of Mineralization in the Mine Centre – Fort Frances Area, District of Rainy River; Ontario Geological Survey Open File Report 5512, 126 p.  White, C. R. & Albers, P. B., 2010. Report on the Geology and Mineral Potential of the Seine Bay/Bad Vermilion Lake Intrusion, Mine Centre Property, Mine Centre, Ontario, s.l.: Unpublished report prepared for Numax Resources, Inc.  57 �2012 Precambrian Field Camp Mapping in the Wilder Lake Intrusion, Lake County, Northeastern Minnesota LEU, Adam, DJON, Lionel, LaPIETRA, Emily, MARTIN, Zach, MARTINEZ, Ricardo, and MILLER, Jim Precambrian Research Center, University of Minnesota Duluth, Duluth, MN 55812 In the summer of 2012, the Precambrian Research Center of the University of Minnesota-Duluth held its sixth annual Precambrian field camp in northeastern Minnesota. As in years past, the fifth and sixth weeks of the camp are dedicated to student’s “capstone” mapping projects during which detailed geologic mapping is conducted in areas of poorly understood geology and digital geologic maps are generated. Three of last summer’s capstone projects focused on areas of the Duluth Complex affected by the Pagami Creek fire, which burned a 160-squaremile area within the Boundary Water Canoe area wilderness in the fall of 2011. The intense burn created a timesensitive opportunity to map inland exposures that were previous difficult to access due to thick woods and blowdown areas created from a 1999 windstorm. Our capstone mapping project focused an area centered on the Wilder Lake Intrusion (WLI), which had been previously reconnaissance mapped only along shoreline exposures (Phinney, 1972; Miller, 1986; Turnbull and Miller, 2004). When it was recognized that the most intense burn area of the Pagami Creek fire was centered in the Wilder Lake area, Jim Miller successfully applied for a USGS EDMAP grant to have a UMD graduate student map the newly created exposures of the WLI. He recruited Adam Leu, an alum of the 2011 PRC field camp, to make this mapping project the centerpiece of his MS thesis at UMD and to be a teaching assistant for the 2012 Precambrian field camp. The objective of the WLI capstone mapping project was to map now easily accessible inland areas of the western WLI. This new mapping would be intergrated with the previous shoreline mapping of Miller (1986) and Turnbull and Miller (2004) to create more complete and detailed geologic map of the western part of the WLI. This mapping would also serve as the anchor point for continued mapping of the WLI by Adam to the east. This area has no lake access and therefore could only be mapped by overland traverses. Adam completed much of the eastward mapping in the fall of 2012, and will return next summer to finalize the mapping. What will be presented at this poster presentation is the 1:24,000-scaled geological map created from the capstone mapping of a two square-mile area of the western end of the WLI (LaPietra et al., 2012). A pdf version of this and other Precambrian field camp capstone geologic maps can be downloaded from the PRC website: www.d.umn.edu/prc/fieldcamp/capstone. The Wilder Lake intrusion (WLI) is a mafic layered intrusion emplaced within the anorthositic series, and is part of the layered series of the Duluth Complex (Miller et al., 2002). The few studies conducted on the WLI show it to be one of the most distinctive intrusions of the layered series by virtue of its northward dip, emplacement entirely within the anorthositic series, reversed cryptic variation, and unique cumulate stratigraphy (oxide before augite) (Miller and Ripley, 1997; Miller et al., 2002). The WLI was first recognized by reconnaissance mapping by Phinney (1972), who documented exposures of well-foliated and layered gabbros and troctolites that extend from North Wilder Lake to the west and Arrow Lake to the east; a strike-length of about 10 kilometers. Phinney noted that internal layering and foliation dips to the north-northeast between 15° and 35°, which contrasts with the southerly to easterly (riftward) dip of most layered intrusions of the Duluth Complex. Miller (1986) conducted reconnaissance mapping of the western extent of the WLI in the Wilder Lake area and term the body the Wilder Lake gabbro. Miller and Ripley (1997) reported olivine and augite data which define a reversed cryptic variation in mg# for both phases. Unpublished mapping and geochemical data from Joy Turnbull 58 �acquired in 2002-04 verified the basic cumulate stratgraphy identified by Miller (1986) in the Wilder Lake area and the reversed cryptic variation defined by olivine and augite. Turnbull’s mapping in the South Wilder Lake area (Turnbull and Miller, 2004) revealed a variation in the thickness of cumulate units and the occurrence of a lower olivine gabbro unit that is not evident in the western part of the intrusion mapped previously by Miller (1986). 2012 capstone mapping verified the basic cumulate stratigraphy of the approximately 1-2 km-thick intrusion previously describe by Miller (1986), but added considerably more detail and insight to the origin of that stratigraphy.. As noted previously by Turnbull and Miller, (2004), the basal contact of the WLI is a varitextured olivine gabbro to augite troctolite in sharp contact with coarse-grained anorthositic series rocks. This taxitic basal unit grades into a thick sequence of troctolitic (Pl+Ol) cumulates. The troctolite progress from a lower ophitic augite interval, which locally shows well developed modal layering, into a more homogeneous troctolite containing abundant anorthositic series inclusions. Overlying the troctolite unit is a thin (20-30m-thick) oxide troctolite unit defined by the abrupt cumulus arrival of Fe-Ti oxide. This unit is then overlain by a well foliated, four-phase olivine oxide gabbro cumulate (Pl+Cpx+Ox+Ol). Miller (1986) noted that the four-phase cumulate abruptly gives way back to a troctolite and puzzled as to the significance of this cumulus regression. He speculated that it could represent a recharge event, or perhaps crystallization of troctolite from the roof zone down to a sandwich horizon represented by the four-phase olivine oxide gabbro. Detailed mapping this past summer showed unequivocally that the upper troctolite is a latter intrusive pulse that actually cut down into the four phase gabbro and is locally in contact with the oxide troctolite unit. Another revelation from last summer’s mapping is that the western contact of the WLI is not a fault as speculated by Miller (1986), but rather is an intrusive contact between the WLI and anorthositic series rock, wherein a taxitic margin is developed along the WLI. Adam Leu’s MS thesis will integrate this capstone map along with previous mapping by Phinney, Miller and Turnbull and new mapping of now well exposed areas in the eastern WLI in order to piece together a complete geological picture of this unique intrusion. This research aims to verify and evaluate the origin of some of its enigmatic petrologic features (early cumulus oxide arrival, reversed cryptic variation) by conducting detailed field mapping and sampling, petrographic analyses, mineral chemical analyses, and lithogeochemical analyses from a suite of samples collected along several profiles across the intrusion. References Cited LaPeitra, E., Martin, Z., Martinez, R, Leu, A., Djon, L., and Miller, J., 2012, Bedrock geologic map of the Wilder Lake intrusion, Lake County, Northeastern Minnesota. UMD Precambrian Field Camp Map Series, PRC Map 2012-4, 1:24,000. Miller, J.D., Jr., 1986, The geology and petrology of anorthositic rocks in the Duluth Complex, Snowbank Lake quadrangle, northeastern Minnesota. unpublished Ph.D. dissertation, University of Minnesota, Minneapolis, 280 p. Miller, J.D., Jr., 1999, Geochemical evaluation of platinum group element (PGE) mineralization in the Sonju Lake intrusion, Finland, Minnesota. Minnesota Geological Survey Information Circular 44, 32 p. Miller, J.D., Jr., and Ripley, E.M., 1996, Layered intrusions of the Duluth Complex, Minnesota, USA. In Cawthorne, R.G., ed., Layered Intrusions: Amsterdam, Elsevier Science, p. 257-301. Miller, J.D. Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.E., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota. Minnesota Geological Survey Report of Investigations 58, 207p. w/ CD-ROM Phinney, W.C., 1972, Northwestern part of Duluth Complex. In: Sims, P.K. & Morey, G.B. (eds.) Geology of Minnesota -A Centennial Volume. Minnesota Geological Survey, p. 335-345 Turnbull, J.A. and Miller, J.D., Jr, 2004, Preliminary geological map of the Wilder Lake Intrusion of the Duluth Complex, Lake County, Minnesota. Unpublished map submitted to USGS EDMAP program, 1:24,000. 59 �Silica Remobilization in the Biwabik Iron Formation, Minnesota USA LOSH, Steven and RAGUE, Ryan Dept. of Chemistry and Geology, FH 241, Minnesota State University, Mankato MN 56001 Excess silica in magnetite separated from iron formation in the Mesabi Range of northern Minnesota can diminish the quality of taconite pellets made there. Normally, silica comprises less than 5 wt% of magnetic concentrate, a desirable level, but it can exceed 10% in some instances. The source and nature of this silica has been enigmatic; it has been noted in and near oxidized zones and in particular stratigraphic units in the mined iron formation, but it has proven difficult to characterize in terms of its nature and distribution. To better understand silica behavior in magnetite ore, we applied petrographic, SEM, fluid inclusion, and bulk geochemical methods to samples collected from three mines, the Hibbing Taconite Mine, the Thunderbird Mine, and the Fayal Reserve Mine, to document the occurrence and origin of quartz that is likely included in magnetite in the separation process. We found that silica-filled microfractures and pits typically form in coarse magnetite within faulted iron formation near both low-angle and high-angle faults, the latter being everywhere associated with oxidation. Silica reprecipitation accompanied early oxidation, in which magnetite was variably oxidized to hematite (martite) and iron silicates were oxidized to goethite and recrystallized quartz near high-angle faults. In unoxidized magnetite in and near low-angle (bedding-parallel) faults, diagenetic inclusions and microfractures host quartz. For both highand low-angle faults, this microfracturing/silica remobilization event took place at diagenetic temperatures (150° – 200° C) and involved oxidizing, relatively high-salinity aqueous (late diagenetic) fluids. In contrast to quartz retention and recrystallization that is the hallmark of the early oxidation event, later supergene (lateritic) oxidation, the stage that produced the high-grade hematite/goethite ‘natural ores,’ extensively dissolved quartz. In magnetite grains in iron formation near faults, silica-filled microfractures are typically less than 5 microns in width; silica-filled pits are commonly less than 40 microns across, many less than 20. Ore is typically ground to -325 mesh (-44 microns), significantly coarser than these quartz inclusions. Image analysis shows that these features can comprise up to 11% of the magnetite by volume, potentially accounting for much of the excess silica in concentrate.   60 �The McGrath metasaprolite: viewing Paleoproterozoic weathering through a veil of metamorphism and metasomatism MEDARIS, Gordon Jr1, BOERBOOM, Terry2, JICHA, Brian1 and SINGER, Brad1 1 Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706 2 Minnesota Geological Survey, St. Paul, MN 55114 medaris@geology.wisc.edu, boerb001@umn.edu, bjicha@geology.wisc.edu, bsinger@geology.wisc.edu Paleosols are important indicators of ancient weathering processes and climatic conditions. However, many Precambrian paleosols have been metamorphosed and metasomatized, thereby obscuring their pedogenic features and modifying their original chemical compositions. A metamorphosed saprolite occurs in eastern Minnesota in the Archean McGrath Gneiss beneath the Paleoproterozoic Denham Formation. A detailed investigation has been undertaken of this metasaprolite to distinguish the effects of weathering from those of subsequent metamorphism and metasomatism and to evaluate the depth, characteristics, and magnitude of weathering. The McGrath Gneiss is granitic in composition, containing quartz, microcline, plagioclase, biotite, and muscovite. The upper ~400 cm of gneiss is devoid of plagioclase and rich in muscovite, consisting of quartz, microcline, muscovite, and minor biotite, and is interpreted to be a metasaprolite, from which plagioclase has been removed by weathering. The base of the Denham Formation is composed of metamorphosed siltstone, arkosic sandstone, and lenses of pebble conglomerate that contain abundant microcline grains and gneiss fragments derived from the underlying McGrath Gneiss. The age of weathering is thought to be ~2100 Ma, as constrained by the age of the McGrath protolith (2557 ± 15 Ma; Holm et al., 2005), the youngest age of detrital zircon from the base of the Denham Formation (2072 ± 17 Ma; Wirth et al., 2006), and the age of volcanic rocks in the Denham Formation (2197 ± 39 Ma; Beck, 1988). Metamorphism in the area, which reaches staurolite grade, was largely a Yavapai event, based on geon 17 U-Pb and 207Pb-206Pb ages of monazite and xenotime in the Little Falls Formation (Schneider et al., 2004; Holm et al., 2007), which overlies the Denham Formation. In a plot of molar Al2O3 (CaO+Na2O)K2O, a.k.a. ACNK, the compositions of metasaprolite deviate markedly from the trend expected for weathering of the McGrath Gneiss (Fig. 1). Such deviation is ascribed to K metasomatism, in which kaolinite formed by weathering has been transformed to muscovite by the introduction of K. Potassium metasomatism is a common phenomenon in paleosols and is exhibited by many Precambrian saprolites in the Lake Superior region (Medaris et al., 2012). Step heating of muscovite from the uppermost sample of metasaprolite (depth = 80 cm) yields a 40 Ar/39Ar spectrum with a well-defined plateau at 1742 ± 3 Ma, which includes 88% of 39Ar released. Thus, K metasomatism was probably associated with Yavapaiage metamorphism, although the 40Ar/39Ar result only places an upper limit on the age of muscovite growth, because metamorphism occurred at temperatures above the blocking temperature for Ar diffusion in muscovite. 61 �Potassium metasomatism precludes use of the Chemical Index of Alteration as an indicator of the intensity of weathering in the McGrath metasaprolite. Instead, the Plagioclase Index of Alteration [molar (Al2O3-K2O)/(Al2O3+CaO+Na2O+K2O)], which is a measure of plagioclase removal, may be used and yields values increasing from 67.4 at a depth of 370 cm to 93.5 at 80 cm. The % changes in selected oxides and elements with depth, relative to Al 2O3 in the mean McGrath Gneiss, are illustrated in Fig. 2, in which the compositional trends reveal that weathering extended to a depth of ~400 cm. The removal of CaO, Na2O, and Sr reflects the weathering of plagioclase, and the addition of K2O, Ba, and Rb is attributed to post-weathering metasomatism. SiO2 has been substantially removed over much of the weathering profile (10 to 20%), as has P2O5 (10 to 75%). The three protolith samples appear to be heterogeneous in their Fe 2O3 contents, with little significant change in Fe2O3 in the weathering profile, except for the uppermost sample. The magnitude of weathering of the McGrath metasaprolite, i.e. the mass removal of SiO2, CaO, and Na2O integrated over the depth of weathering, is 2.2 moles/cm 2. Assuming a duration of weathering of 100,000 years and following Sheldon's (2006) method, atmospheric pCO2 was 5 times that of the pre-industrial level. Although the McGrath metasaprolite represents an important episode of Paleoproterozoic weathering in the Lake Superior region, its degree of weathering was less than that of some other Paleoproterozoic paleosols. For example, the ~1700 Ma Baraboo paleosol has a larger magnitude of weathering, 4.3 vs. 2.2 moles/cm2, greater depth of weathering, 7.9 vs. 4 meters, and complete removal of potassium feldspar in addition to plagioclase, perhaps reflecting a warmer and wetter climate for Baraboo. References Beck JW (1988) University of Minnesota Ph.D. dissertation, 273 pp; Holm DK et al. (2005) Geological Society of America Bulletin 117, 259-275; Holm DK et al. (2007) Precambrian Research 157, 106-126; Medaris LG et al. (2012) 58th ILSG, Proceedings with Abstracts, 60-61; Schneider DA et al. (2004) Geological Society of America Special Paper 380, 339-357; Sheldon ND (2006) Precambrian Research 147, 148-155 62 �Chemical Zoning in Calc-Silicate Minerals Associated with Native Copper from the Keweenaw Peninsula, Michigan MULCAHY, Connor1, HANSEN, Edward1, BORNHORST, Theodore2, and RHEDE, Dieter3 1 Geological and Environmental Sciences Department, Hope College, Holland, Michigan, 49423 2 A.E. Seaman Mineral Museum, Michigan Technological University, Houghton, MI 49931 3 Helmholtz-Zentrum Potsdam, Deutsches GeoForschungsZentrum (GFZ), Telegrafenberg, D14473, Potsdam, Germany Calc-silicate minerals occur in clusters in hydrothermally altered basalts and rhyolite-pebble conglomerates associated with native copper deposits in the Keweenaw Peninsula. We studied chemical zoning in these minerals hosted by basalts from the Kearsage, Isle Royale, and Caledonia Mines and by rhyolite-pebble conglomerates and sandstones. All basalt and some conglomerate samples came from the research rock collection of the A.E. Seaman Mineral Museum, Michigan Tech. Additional conglomerate samples came from personal collecting from rock piles at the base of Bumbletown Hill. Samples were examined with the Scanning Electron Microscope at Hope College. Quantitative mineral analyses were carried out using a JEOL JXA-8500F field emission electron microprobe (hyperprobe) at the Deutsches GeoForschungsZentrum, Potsdam (GFZ). Calc-silicate masses in the basalt samples consist of pumpellyite, titanite, and epidote +/- prehnite. Veins of epidote cut pumpellyite, and pumpellyite occurs as inclusions in epidote, indicating that the formation of pumpellyite preceded the growth of epidote. Variations in brightness can be seen within epidote, prehnite and pumpellyite grains in SEM backscatter images (Figure 1, 2, and 3). Electron microprobe analyses indicate that the brighter regions are characterized by higher Fe/(Fe + Al) (fe ratio) (Figures 1, 2 and 3). In epidote these bright regions with relatively high fe ratios frequently exhibit regular geometric forms (Figure 1). Within these forms the fe ratio is relatively constant but decreases rapidly at the boundaries. Zones with regular geometric forms correspond to crystallographic faces (Figure 1) and areinterpreted as sector zoning in which the variation in fe ratios is due to the propensity of different crystal faces to incorporate different amounts of iron during mineral growth. However, some epidotes contain irregularly shaped bright patches and electron microprobe traverses across these patches show gradational changes in the fe ratio. This zoning may represent changes in the compositions of hydrothermal fluids during the growth of epidote. Zoning of rare earth elements (REE) in epidote was observed in samples from the Isle Royale mine where approximately rectangular patches are enriched in REE within larger epidote crystals (Figure 4). Hour glass zoning has been observed in pumpellyite grains (Figure 3) and this is also interpreted as sector zoning. Zoning in prehnite (Figure 2) is perpendicular to the elongated axes, and increases in the fe ratio may be a result of an increase in temperature, pH, the activity of Fe+3, or the activity of Ca+2 in the fluid phase. Calc-silicate clusters in conglomerates consist of epidote + titanite +/- andraditic garnet. Zoning was not detected in the garnet. The fe ratio zoning patterns in epidote hosted by conglomerate are very similar to patterns in epidote hosted by basalts. REE-zoning was found in epidote grains. Narrow zones enriched in REE outline crystal forms of epidote and are interpreted as a relatively brief increase in activities of REE elements during growth of the epidote (Figure 5). REE–enriched zones in epidote grains either at their margins or along fractures are interpreted as the replacement (by dissolution recrystallization) of ordinary epidote by REE-enriched epidote after the main period of hydrothermal epidote growth (Figure 6). This replacement could be due to a decrease in temperature, but could also be due to a decrease in the activities of F or Cl both of which form complexes with REE. The REE flurocarbonate synchysite was 63 �found from the Allouez mine. REE minerals appear to be more abundant in the conglomerate host rocks than in basalt host rocks suggesting that the REE may have been remobilized from the rhyolite clasts. Figures: Back-scatter-SEM images of calc-silicate minerals. The graphs below each image give either Fe/(Fe+Al) (fe ratios) or total REE concentrations (in wt%) across the traverse (A-B) marked in the image. Figure 1: Fe-Al sector zoning in epidote grains from the Kearsage mine. Figure 2: Fe-Al zoning parallel to growth direction in prehnite from the Kearsage mine. Figure 3: Fe-Al zoning in an epidote grain containing a pumpellyite inclusion with hourglass sector zoning from the Kearsage mine. The fe ratios of the two analyzed spots on the pumpellyite grain are given in white lettering directly on the image. Figure 4: REE-zoning in epidote from the Isle Royale mine. Figure 5: Epidote from a conglomerate showing narrow growth zones enriched in REE (bright lines). The broader zones (lighter to darker gray) with sharp rectilinear boundaries reflect variations in the fe ratios. Figure 6: Epidote grain from a conglomerate showing REE-enriched, BSE-bright veins following fractures. 64 �Contrasting pressure-temperature-time paths for high-grade metamorphic rocks in the interior of the Penokean-Yavapai orogenic belt, southern Lake Superior region NADZIEJKA, Brynley and BJØRNERUD, Marcia Geology Department, Lawrence University, Appleton, Wisconsin, 54911 USA Amphibolite-facies metamorphic rocks occur in a narrow band in the internal part of the Paleoproterozoic orogenic belt south of Lake Superior in Michigan, Wisconsin and Minnesota. Peak metamorphic temperatures were once thought to have been reached during Penokean tectonism (ca. 1830-1850 Ma), but recent monazite and 40Ar/39Ar dating has shown that these relatively high-grade rocks have a more complex thermal history that spans both the Penokean and Yavapai (ca. 1760-1740 Ma) events (Holm et al., 2007). Furthermore, the timing and causes of the thermal maxima appear to be different in different parts of this tectonic zone. The ‘metamorphic nodes’ in the Upper Peninsula of Michigan, such as the one near Republic, are classic examples of Barrovian metamorphism, with peak temperatures of 550-615°C and pressures of 0.2-0.3 GPa (Attoh & Klasner, 1989). The current consensus is that the nodes record post-Penokean development of gneiss domes and juxtaposition, via normal-sense shear zones, of sedimentary rocks against hot, remobilized Archean basement (Tinkham & Marshak, 2004). Gravitational instability may have been enhanced by tectonically thickened piles of dense iron formation. Monazite ages from the highest-grade rocks at Republic suggest that the thermal maximum occurred in Yavapai time at 1758-1768 Ma (Holm et al., 2007), consistent with petrographic observations that porphyroblasts overprint Penokean foliations. The full P-T-t path for these rocks would be clockwise, with Penokean burial preceding Yavapai heating. In east central Minnesota, rocks of the Denham Formation experienced comparable peak temperatures (520-590°C) but higher peak pressures (0.5-0.6 GPa) (Holm & Selverstone, 1990). Petrographic analyses point to one main period of porphyroblast growth, but monazite and zircon dates indicate distinct thermal pulses at ca. 1830, 1800, and 1780 Ma, with the third pulse apparently related to a combination of Yavapai doming and emplacement of the East-Central Minnesota batholith (Holm et al., 2007; Boerboom, 2010). The P-T-t path for these rocks is thus more complex than for the Republic node, but still consistent with an overall clockwise trend. The relatively high-pressure kyanite-bearing rocks of the Watersmeet Terrane in northern Wisconsin seem to have a significantly different metamorphic history from those in Michigan and Minnesota. In samples of schist taken near Powell, Wisconsin, the primary minerals are biotite, quartz, albitic feldspar, garnet, kyanite and rare staurolite. The rather coarse (2-3 mm) biotite grains define a crude planar fabric, with garnet, feldspar, and staurolite occurring in biotite-poor domains. Muscovite is notably absent, and quartz is less abundant than feldspar. The few staurolite crystals in the samples tend to be fragmented, and the feldspars typically have serrate grain boundaries, suggesting they have undergone partial recrystallization following ductile deformation. The garnets are relatively small (ca. 1 mm) and contain abundant inclusions, though not well-defined inclusion trails, of quartz, biotite, and in at least one case, staurolite. The kyanite crystals are as long as 4 cm and show a weak alignment. Most significantly, kyanite grains contain inclusions of quartz, biotite, feldspar and garnet and clearly overprint the planar fabric. Based on these textural relationships, we infer that staurolite and feldspar grew early in the metamorphic history, followed by garnet and finally kyanite. Biotite was likely an early phase that continued to grow over time as the later minerals formed. Most of the deformation occurred prior to the formation of both the garnet and especially the kyanite. Garnet-biotite geothermometry on rocks from the Powell area by Geiger and Guidotti (1989) placed Tmax in the range 630-680° C. Using the garnet-plagioclase-Al2SiO5-quartz barometer and the coexistence of sillimanite and kyanite in some specimens, they estimated Pmax at 0.75 GPa. Monazite dates from the Watersmeet terrane samples record two major thermal events at 1830 Ma and 1765 Ma (Holm et al., 2007). The fact that kyanite clearly postdates all the other metamorphic phases seems to preclude a clockwise P-T-t path for the rocks of the Watersmeet terrane. Unless 65 �nucleation and growth of kyanite had for some reason been kinetically suppressed earlier in the rocks’ history, the textural relationships require that at least one of the thermal maxima preceded the eventual pressure maximum. In addition, the combined geochronologic and textural constraints indicate that the pressure maximum post-dates Penokean time. These observations must be incorporated into orogenic models for the Penokean and Yavapai events. Schneider et al. (2004) proposed that orogenic ‘channel flow’ in Yavapai time could account for the exhumation of the high-pressure rocks of the Watersmeet terrane. They depict the channel as bounded by a south-dipping normal fault at the southern boundary of the terrane and a southdipping reverse fault at the northern boundary. Crustal thickening in Yavapai time could account for the late high-pressure metamorphism recorded by the petrographic relationships, but the kyanite-bearing rocks would initially have had to be in the footwall beneath the channel to experience elevated pressures, then subsequently incorporated into the channel, perhaps by northward migration of the lower channel boundary. Ductile extrusion of the Watersmeet terrane is predicted by the channel flow model, but the rocks from the Powell area show little evidence of deformation after the formation of the kyanite. However, deformation in channel flow is more concentrated at the boundaries, and it could be that the available outcrops represent the central part of the channel. The channel flow model for northern Wisconsin is also an imperfect match with the geometry of the ductile channel in the modern Himalaya, where the phenomenon was first recognized. In the Himalaya, the channel boundaries (Main Central Thrust, South Tibetan Detachment; Beaumont, et al. 2001) dip in the same direction as the subduction zone, while in the Watersmeet terrane, the proposed boundaries dip south while Yavapai subduction is thought to have been north-directed. Another requirement of the channel flow model is rapid erosion of the extruded rock mass. One interesting implication of the model could be the interpretation of the Baraboo Quartzite as the distal part of a clastic wedge formed by intense erosion of the Yavapai orogen. This is consistent with the ages of detrital zircons from the Baraboo and correlative quartzites (1782-1712 Ma) and with evidence that these units were deposited in a warm, humid climate (Medaris et al., 2003). Whether or not the channel flow model is appropriate for the Watersmeet terrane, there remains the question of why the P-T-t path for metamorphic rocks in northern Wisconsin differs so markedly from those in Michigan and Minnesota. Differences in the plate boundary configuration, location within the orogen, and nature of the Archean basement are among the factors that should be explored. Attoh, K. and Klasner, J., 1989. Teconic implications of metamorphism and gravity field in the Penokean orogen of northern Michigan. Tectonics, 8: 911-933. Beaumont, C., Jamieson, Ngyuen, M., and Lee, B., 2001. Himalayan tectonics explained by extrusion of a low-viscosity crustal channel coupled to facued surface denudation. Nature, 414: 738-742. Boerboom, T., 2010. Transect from Archean basement to the Animikie basin, east-central Minnesota. Institute on Lake Superior Geology Field Trip Guidebook, 57: 129-161 Geiger, C. and Guidotti, C., 1989. Precambirn metamorphism in the southern Lake Superior region and it bearing on crustal evolution. Geoscience Wisconsin, 13: 1-33. Holm, D., Schneider, D., Rose, S., Mancuso, C., McKenzie, M., Foland, K., and Hodges, K., 2007. Proterozoic metamorphism and cooling in the southern Lake Superior region, North America and its bearing on crustal evolution. Precambrian Research, 157: 106-126. Holm, D. and Selverstone, J., 1990. Rapid growth and strain rates inferred from synkinemative garnets, Penokean orogen, Minnesota Geology, 26: 166-169. Medaris, L.G., Singer, B. Dott, R., Naymark, A., Johnson, C., and Schott, R., 2003. Late Paleoproterozoic climate, tecotnics and metamorphism in the southern Lake Superior region and Proto-North America: Evidence form Baraboo interval quartzites. Journal of Geology, 111: 243-257. Scheider , D., Holm, D., O’Boyle, C., Hamilton, M., and Jercinovic, M., 2004. Paleoproterozoic development of a gneiss dome corridor in the southern Lake Superior region. GSA Special Paper 380: 339-357. Tinkham, D. and Marshak, S., 2004. Precambrian dome-and –keel structure in the Penokean orogenic belt of northern Michigan. GSA Special Paper 380: 339-357. 66 �Natural Groundwater Geochemistry in Bedrock of the Thunder Bay Area PUUMALA, Mark, Ontario Geological Survey, 435 James Street South, Suite B002, Thunder Bay, Ontario P7E 6S7 During 1978 and 1979, the Ontario Ministry of the Environment (MOE) carried out a groundwater quality sampling program in the Thunder Bay area (McMullen, 1985). This program involved the collection of 354 groundwater samples from private drinking water wells completed in both overburden (153 samples) and bedrock (201 samples). The data were used in the production of two groundwater resource evaluation reports for the privately-serviced rural areas of the City (McMullen, 1985; Trow Hydrology Consultants, 1988). Some significant geochemical variations between hydrogeologic units were noted in each report. However, limited work was done to understand the reasons for these differences. The purpose of this study was to take a more detailed look at the MOE data set to gain a better understanding of how the geology of the major bedrock formations in the Thunder Bay area influences groundwater geochemistry. The study area is located on the Canadian Shield, at the boundary between the Archean-age rocks of the Superior Province, and the Proterozoic-age rocks of the Southern Province. The northern half of the study area is underlain by Neoarchean-age metavolcanic, metasedimentary, and intrusive rocks (Brown, 1995), while the southern half is underlain by the flat-lying and relatively un-deformed Paleoproterozoic-age Animike Group sedimentary rocks of the Gunflint and Rove formations (Sutcliffe, 1991). These sedimentary rocks are intruded by Mesoproterozoic Logan diabase sills and dikes associated with the Midcontinent Rift (Sutcliffe, 1991). For the purposes of this study, the Archean-age rocks, Gunflint Formation, Rove Formation and diabase are considered to be four distinct hydrogeologic units. Although the Archean-age rocks include a diverse range of lithologies, they are grouped together because they are all sparselyfractured crystalline rocks with similar water-transmitting characteristics (McMullen, 1985). This study focussed on data from the 201 bedrock wells that were sampled by MOE in 1978-79. Each well was classified according to the 1:250 000 scale bedrock lithology mapped on the ground surface at that location (Ontario Geological Survey, 2011). The numbers of wells assigned to each hydrogeologic unit were as follows: 65 Archean; 81 Gunflint; 41 Rove; 3 Diabase. Eleven wells are in an area of deep overburden where the Gunflint/ Rove contact has not been defined and were classified as “Gunflint/Rove contact area” wells. All of the groundwater samples were analyzed for the following list of geochemical parameters: hardness, pH, colour, turbidity, conductivity, bicarbonate, sulphate, chloride, Na, K, Ca, Mg, Fe and Mn. 17 samples were also tested for the following additional list of parameters: Cd, Co, Cu, Zn, Pb, Ni, Hg and As. Data analysis for this study included the plotting of major ion data (Ca, Mg, Na, K, Cl, sulphate, bicarbonate) on Piper tri-linear diagrams, and the calculation of mean values and ranges for conductivity, hardness, chloride, sulphate, Fe and Mn. Both of these methods were used to evaluate the major ion geochemistry of each hydrogeologic unit, including variability and possible groundwater geochemical evolution trends. The results of this analysis showed that the Archean, Gunflint and Rove hydrogeologic units each have distinctive groundwater geochemical signatures that are attributable to differences in lithology. The data set for the diabase unit was too small to allow for any meaningful assessment of its groundwater geochemistry. 67 �Groundwater sampled from drinking water wells drilled into the Archean crystalline rocks typically had the lowest levels of dissolved solids and the least geochemical variability. The water is also typically of the Ca + Mg bicarbonate-type. The combination of low dissolved solids content and Ca + Mg bicarbonate dominated geochemistry is indicative of recently-recharged groundwater that has had limited time to interact with aquifer solids. This is consistent with what would be expected for a data set collected from relatively shallow wells in low permeability, competent bedrock units such as these. Although the major ion geochemistry of the Archean crystalline rock groundwater was relatively consistent regardless of lithology, there were some differences noted for iron and manganese, with higher mean concentrations in wells sourced from metasedimentary rock (possibly related to the weathering of biotite). Groundwater in the Gunflint Formation displays much more geochemical variability and has a much higher mean concentration of dissolved solids than in the Archean hydrogeologic unit. Although most groundwater in the Gunflint has a geochemical signature indicative of recent recharge, two important apparent geochemical evolution trends were noted. The dominant trend is from a Ca + Mg bicarbonate-type toward a Ca + Mg chloride-type groundwater. The second trend is toward a mixed cation chloride-type groundwater (i.e., higher relative proportion of Na). The second trend has a close spatial association with the argillite tuff horizon mapped by Moorhouse (1960), and is interpreted to be related to groundwater interaction with this stratigraphic unit. Because the Gunflint Formation is not known to contain evaporite minerals, the most likely source of chloride in this formation is connate brine that was trapped during diagenesis (Hem, 1985). Rove Formation groundwater has the highest mean dissolved solids content, and also shows significant geochemical variability. Similar to the Gunflint, there are two apparent geochemical evolution trends in the Rove. However, these trends are distinct from those seen in the Gunflint. The dominant trend is from Ca + Mg bicarbonate-type toward Ca + Mg sulphate-type groundwater, with a second trend toward sodium bicarbonate-type. The dominant trend toward a sulphate-type geochemistry is likely to be due to the weathering of pyrite, which is locally present in Rove Formation shale (Sutcliffe, 1991). The second trend is accompanied by elevated pH and may be related to the weathering of albite (Kehew, 2001). This trend also shows a close spatial association with diabase sills and appears to be characteristic of groundwater in altered/ metamorphosed contact zones between the Rove Formation and diabase. References Brown, G.H., 1995. Precambrian Geology, Oliver and Ware Townships. Ontario Geological Survey, Geological Report 294, 48p. Hem, J.D., 1985. Study and interpretation of the chemical characteristics of natural water. United States Geological Survey, Water-Supply Paper 2254, 263p. Kehew, A.E., 2001. Applied Chemical Hydrogeology. Prentice Hall Inc., Upper Saddle River, New Jersey, 368p. McMullen, R.F,. 1985. Groundwater potential in minimum service rural residential areas, City of Thunder Bay; Ministry of the Environment, 23p. Moorhouse, W.W., 1960. Gunflint Iron Range in the Vicinity of Port Arthur; Ontario Department of Mines, Volume 69, Part 7, 40p. Ontario Geological Survey, 2011. 1:250 000 scale bedrock geology of Ontario-revised. Ontario Geological Survey, Miscellaneous Release-Data 126-revision 1. Sutcliffe, R.H,. 1991. Proterozoic geology of the Lake Superior region: Geology of Ontario. Ontario Geological Survey, Special Volume 4, Part 1, p. 627-658. Trow Hydrology Consultants Ltd., 1988. A study of groundwater resources in the minimum service rural residential area designations of the City of Thunder Bay Official Plan, Thunder Bay, Ontario. unpublished report, 56p. 68 �North American Palladium’s Lac des Iles mine: Evidence for high temperature deformation and possible control on Pd mineralization SCHMIDT, Skylar and HILL, Mary Louise, Department of Geology, Lakehead University, 955 Oliver Road Thunder Bay, On, P7B5E1 Canada North American Palladium’s Lac des Iles Mine is located in a mafic to ultramafic intrusive complex just north of the Wabigoon-Quetico subprovince boundary. The Mine Block intrusion, host to the economic palladium mineralization, preserves evidence of a complex history of high-temperature deformation suggesting that the intrusive complex is pre- or syntectonic, not post-tectonic as commonly presumed. If so, deformation may be significant to mineralization and provide insight on future targets for exploration. The potential link between heterogeneous deformation and palladium enrichment will be further investigated. The Mine Block intrusion is elongate in a northeast-southwest direction, parallel to regional deformation and is composed of mainly gabbro-noritic rocks. Locations of investigation to date include the Baker zone, the North VT rim and the Sheriff zone. The Baker zone is the discovery outcrop of the property and is located near the centre of the intrusion as a topographic high. The North VT rim is located along the north edge of the intrusion and is characterized by variable grain size, and the Sheriff zone is an area of mineralization located to the south-east of the pit. In the Baker zone, northeast-striking mafic dikes are boudinaged and disaggregated, with narrow shear zones preserved along some of the boudin margins. In the Sherriff zone, felsic dikes and narrow ductile shear zones are mutually overprinting, indicating progressive brittle-ductile deformation. Economic Pd mineralization is commonly associated with chlorite-actinolite schist; some sulfide grains occur along cleavage planes in the metamorphic silicate minerals. The East Gabbro is mylonitized at the contact with the chlorite-actinolite schist and deformation decreases further from the contact. Microstructural analysis provides further evidence for deformation in the Mine Block intrusion. In the Baker zone, plagioclase has deformation twins and with some grains showing subsequent deformation of these twins. Subgrains in plagioclase, and small strain-free grains along margins of larger strained grains, provide evidence of dislocation creep in plagioclase in the Baker zone, Sherrif zone, and North VT rim. In the Baker zone, some sulfide grains have been fractured and healed, indicating subsequent deformation after initial formation. 69 �Effects of Preexisting Fractures on Groundwater Flow Today SCHMUS, Matthew1, BHATTACHARYYA1, Prajukti, and HART, David2 1 Department of Geography and Geology, University of Wisconsin-Whitewater, Whitewater, WI 53190 United States 2 Wisconsin Geologic and Natural History Survey, Madison, WI 53705 United States Crystalline rocks are not normally considered aquifers, but when joints and fractures are present, there is a very real chance for pathways to emerge as conduits for groundwater flow. Three bore holes were drilled in Pittsville, WI (Figure 1) within the Marshfield terrene. This area was the stage where orogeny and volcanism occurred when the Marshfield terrene collided with the Wausau terrene and Superior craton forming multiple episodes of deformation (Schulz and Cannon, 2007). Data from the boreholes was collected by Dr. David Hart, and was analyzed at University Wisconsin-Whitewater. The data includes orientations of five different surfaces, including (a), major and minor open joints or fractures, (b), partially open joints or fractures, (c), filled fractures or joints, (d), bedding, banding, or foliation planes, and (e), induced fractures. We investigated orientations of regional stress directions based on the fracture orientation data using T-Tecto software (Figure 2). We combined stereographic projection analyses and borehole gamma logs to investigate how the orientations of different types of fractures might have changed with depth, mainly focusing on the orientations of open and filled fractures as those might provide the best insights within past and present fluid flow patterns. We have determined the locations where the fractures might intersect with each other below surface by using apparent dip data of the fracture planes on vertical planes through any two of the three studied boreholes (Figures 3 and 4). Eventually we aim to create a three-dimensional model of the fracture network using three-dimensional visualization tools within the ArcScene® software package. Preliminary data shows that dominant fracture orientation patterns change with depth in each of the three bore holes, and the fracture orientation patterns in each of the three boreholes have little or no similarity with each other (Figure 5). Some of the fractures show evidence of past fluid flow in the form of filled veins. Since some of the fractures are not filled, this might indicate multiple episodes of fracture formation which would be consistent with the tectonic history of the area. This is also indicated by the T-Tecto plots, which show multiple stress directions (Figure 2). Also two dimensional models of fracture plane intersections have shown possible intersection points among different fracture types that may be conduits for groundwater flow. Here we will present our data, and discuss the potential implications of our analyses on understanding past and present groundwater flow in the studied region. References Hart, David J., (2011): Comparison of Groundwater Flows into Three Closely Spaced Crystalline Bedrock Wells. Geological Society of America Abstracts with Programs, Vol. 43, No. 5, p. 287 Schulz, Klaus, and Cannon, William (2007). The Penokean orogeny in the Lake Superior region. Precambrian Research, Volume 157, Issues 1-4, pages 4-25 70 �Figure 2: T Tecto Plot of all fractures in well 9 Figure 1: Map of Study Area Pittsville Wisconsin Figure 3: Calculations used to determine depth at which different fracture planes intersected the three boreholes Figure 4: 2D graph showing possible fluid flow connection. Figure 5: Stereographic Projections (left to right)Well 7 data filled fractures depth between 90-120 Well 8 data filled fractures depth between 80-110 Well 9 data filled fractures depth between 80-110 71 �The Parent Lake Volcanics: Product of a phreatomagmatic eruption of basalt during deposition of the Michigamme Formation? SCHULZ, K.J.1, CANNON, W.F.1, and WOODRUFF, L.G.2 1 U.S. Geological Survey, 954 National Center, Reston, VA 20192, kschulz@usgs.gov, wcannon@usgs.gov 2 U.S. Geological Survey, 2280 Woodale Ave., Mounds View, MN 55112, woodruff@usgs.gov. An unusual mafic volcanic layer occurs within an otherwise monotonous sequence of turbidites of the Michigamme Formation in a set of outcrops about 40 km south of L’Anse (NW ¼ Sec. 9 and NE ¼ Sec. 8, T. 48 N., R. 33 W), at the extreme west end of the Marquette trough in Michigan’s Upper Peninsula. The volcanic rocks are about 150 m thick and dip steeply (80-85 degrees) to the south. However, they are overturned and face north as indicated by both graded beds in adjacent graywacke and bedding-cleavage relations within the volcanic rocks. Typical Michigamme graywacke both underlies and overlies the mafic volcanic rocks, although the upper contact is intensely sheared and may not be a depositional contact. The mafic volcanic rocks are divided into two units. The lower unit is a fine-grained, mafic pyroclastic rock having fragments of highly vesicular basalt typically less than a few centimeters in long dimension. It is massive to crudely layered and shows greenschist metamorphic assemblages, including a colorless amphibole, chlorite, and clinozoisite as the most common minerals; secondary carbonate is abundant. The upper unit is a coarse breccia containing fragments of both vesicular basalt and exotic fragments of crystalline rocks, presumably derived from the underlying Archean basement and older Paleoproterozoic sedimentary rocks. These fragments are as much as 1 m diameter and commonly are well rounded. Lithologies include a variety of granitic rocks and quartzose sedimentary rocks. The breccia is crudely bedded in meter-scale layers; the matrix is intensely sheared and contains abundant secondary carbonate. A distinctive feature of some exotic fragments is a rind or shell of fine vesicular material that is more resistant to weathering than the matrix of the breccia (Fig. 1). These clasts look similar to cored bombs that formed during phreatomagmatic eruptions in some recent marr/diatreme complexes (Hanson and Elliot, 1996; Rosseel and others, 2006; Sottili and others, 2010). Cored bombs display a chilled shell of juvenile material, generally basalt, surrounding a lithic core, reflecting the thermal interaction of magma with fragments of wall rock in the vents of marr/diatreme complexes (Sottili and others, 2010). The highly vesicular, fragmental, and crudely bedded and graded nature of the Parent Lake mafic volcanic units, as well as the presence within them of angular to rounded clasts of basement rocks—some with the appearance of cored bombs—are all features compatible with formation by highly explosive basalt eruptions in a marr/diatreme complex (Hanson and Elliot, 1996). However, the location of the source vent for these deposits is unknown. Although not common, other largely fragmental mafic volcanic units are present in the Michigamme Formation, including the Clarksburg Volcanics to the east (Cannon, 1975) and unnamed mafic volcanics north of Iron River to the south (Cannon and Klasner, 1980). References Cannon, W.F., 1975, Bedrock geologic map of the Republic Quadrangle, Marquette County, Michigan: U.S. Geological Survey, Map I-862, scale 1:24,000. Cannon, W.F., and Klasner, J.S., 1980, Bedrock geologic map of the Kenton-Perch lake area, Northern Michigan: U.S. Geological Survey, Map I-1290, scale 1:62,500. Hanson, R.E., and Elliot, D.H., 1996, Rift-related Jurassic basaltic phreatomagmatic volcanism in the central Transantarctic Mountains: Precursory stage to flood-basalt effusion: Bulletin of Volcanology, v. 58, p. 327−347. 72 �Rosseel, J.-B., White, J.D.L., and Houghton, B.F., 2006, Complex bombs of phreatomagmatic eruptions: Role of agglomeration and welding in vents of the 1886 Rotomahana eruption, Tarawera, New Zealand: Journal of Geophysical Research, v. 111, B12205, doi:10.1029/2005JB004073. Sottili, G., Taddeucci, J., and Palladino, D.M., 2010, Constraints on magma-wall rock thermal interaction during explosive eruptions from textural analysis of cored bombs: Journal of Volcanology and Geothermal Research, v. 192, p. 27−34. Figure 1. Chert clast with a rind of fine vesicular basalt in the upper unit of the Parent Lake volcanics. Quarter for scale. 73 �Geochemical and petrographic study of a Mesoarchean felsic metavolcanic unit near Musselwhite Mine, North Caribou greenstone belt, northwestern Ontario SMYK, Emily1, HOLLINGS, Pete1, and BICZOK, John2 1. Geology Department, Lakehead University, 955 Oliver Rd, Thunder Bay, Ontario, P7B 5E1, Canada 2. Goldcorp Inc. Musselwhite Mine, P.O. Box 7500, Thunder Bay, Ontario, P7B 6S8, Canada The ~3 Ga North Caribou greenstone belt, North Caribou Terrane, Superior Province, comprises greenschist- to upper amphibolites-facies, ultramafic to felsic metavolcanic and metasedimentary rocks, intruded by 3000 to 2700 Ma felsic plutonic rocks. The study area is centered on a thin (~100m wide), north-trending, felsic metavolcanic unit on the western side of Opapimiskan Lake, 5 km northwest of Musselwhite Mine, in the South Rim Volcanic Unit (SRV). This rhyolitic unit has yielded a preliminary U-Pb age of 3053 Ma (McNicoll, unpublished data). The unit was mapped in detail over a distance of 2 km. Samples of the felsic rocks as well as the associated metabasalts and metapelites were collected. The metavolcanic and metasedimentary lithologies were classified by their petrography and whole rock geochemistry. The metarhyolitic units are characterised by a mineral assemblage of quartz + plagioclase + Kfeldspar with accessory muscovite + biotite + chlorite ± titanite ± clinozoisite ± zircon. Field observations identified some possible tuffaceous and pyroclastic felsic units as well as possible flows. The calculated anhydrous SiO2 values for the felsic metavolcanic rocks in the SRV range from 75 to 81 wt%. These highsilica, calc-alkaline rhyolites have been classified as FII-type rhyolites (based on a classification by Lesher et al. [1986]). The unaltered felsic metavolcanic rocks are characterised by LREE-enrichment and εNd values of -2.79 and -0.89 (Fig. 1). The tholeiitic and calc-alkaline basaltic and komatiitic-basaltic units have been metamorphosed to amphibolites consisting of plagioclase and hornblende with accessory titanite + chlorite + opaques ± rutile ± biotite ± zircon. The komatiitic-basalt has an MgO value of 14 wt% (compared to the range of 3 to 9 wt% MgO for the basalts) and has higher Ni and Cr contents. The basalts can be subdivided into two groups based their REE contents (Fig. 2). The εNd values are +0.78 and +0.58 for the flat REE basalts and -1.58, -1.61 and -3.35 for the LREE-enriched basalts. The three outcrops of metasedimentary rock found in this area have been metamorphosed into schists and a quartzite. The protolith of the schists was likely pelitic sedimentary rocks based on the mineral assemblage of strongly foliated quartz + biotite + muscovite + chlorite ± garnet ± zircon ± clinozoisite. These schists are associated with the felsic metavolcanic rocks. The quartzite is similar to the felsic metavolcanic units but does not contain feldspars. Its mineral assemblage contains quartz + muscovite + biotite + garnet. The rhyolites are calc-alkaline rocks with consistent LREE enrichment and negative Nb and Ti anomalies consistent with a suprasubduction zone environment. However, the negative εNd values indicate that these rocks have been contaminated by older continental crust suggesting emplacement in a continental arc. The tholeiitic mafic volcanic rocks with the flat REE is consistent with an oceanic island plateau environment generated by a mantle plume (Hollings and Kerrich, 1999). The positive εNd values support this conclusion as a positive value implies that the melt was derived from a depleted mantle source. 74 �1000.00 100.00 10.00 1.00 0.10 0.01 Th  Nb  La  Ce  Pr  Nd  Zr  Hf  Sm  Eu  Ti  Gd  Tb  Dy  Y  Ho  Er  Tm  Yb  Lu  Al V  Sc  Figure 1: Trace elements of the felsic metavolcanic rocks plotted on a primitive mantle‐normalized plot. Figure 2: Trace elements of the mafic metavolcanic rocks plotted on a primitive mantle‐normalized plot. References Biczok, J., Hollings, P., Klipfel, P., Heaman, L., Maas, R., Hamilton, M., Kamo, S., and Friedman, R. 2012. Geochronology of the North Caribou greenstone belt, Superior Province Canada: Implications for tectonic history and gold mineralization at the Musselwhite Mine; Precambrian Research, 192-195, 209-230. Hollings, P., Kerrich, R., 1999. Trace element systematics of ultramafic and mafic volcanic rocks from the 3 Ga North Caribou greenstone belt, northwestern Superior Province. Precambrian Research 93, 257–279. Lesher, C.M., Goodwin, A.M., Campbell, I.H., Gorton, M.P., 1986. Trace-element geochemistry of ore-associated and barren, felsic metavolcanic rocks in the Superior Province, Canada. Canadian Journal of Earth Sciences 23, 222–237. unpublished data, McNicoll, V., Geological Society of Canada, 2012. 75 �Petrology, mineralization, and alteration of the Thunder mafic to ultramafic intrusion, Midcontinent Rift, Thunder Bay TREVISAN, Brent1, HOLLINGS, Pete1, and AMES, Doreen2 1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1 Canada 2 Geological Survey of Canada, Central Canada Division, 750-601 Booth St., Ottawa, ON K1A 0E8 Canada The northern Lake Superior region is host to a large segment of the North American Mesoproterozoic Midcontinent Rift (MCR). Since the discovery of high grade Ni-Cu-PGE mineralization hosted by mafic to ultramafic intrusions at the Current Lake deposit in Ontario, and the Eagle deposit in Minnesota, considerable exploration activity has been focused within the region. However, the small size of these buried deposits makes them difficult to locate both on the ground and on regional magnetic survey maps. The Thunder prospect is a mineralized mafic to ultramafic intrusion located on the outskirts of the City of Thunder Bay with mineral claims currently held by Rio Tinto (formerly Kennecott Canada Exploration Inc.). The intrusive body has been interpreted to be associated with the early magmatic stages of the MCR based on geochemical similarities with rocks of the Nipigon Embayment (Hollings et al., 2007). However, unlike other mineralized MCR intrusions, the Thunder intrusion has relatively low Ni grades (< 0.08%) and a large range in PGE tenors (combined platinum-palladium values > 0.5 g/t; Bidwell and Marino, 2007). The Thunder intrusion is also the only mineralised MCR intrusion hosted within the Archean Shebandowan greenstone belt as others, including Current Lake, intrude the Mesoproterozoic Sibley Group and/or the Archean Quetico metasedimentary subprovince (Williams et al., 1991; Hart and McDonald, 2007). This MSc study will characterise the petrology, mineralization, and alteration footprint of the Thunder intrusion and place it within the context of the MCR as a whole, in order to identify criteria for vectoring towards mineralization. The ~800m x ~800m Thunder intrusion consists of a mafic-ultramafic basal section that is overlain by a mafic sill-like body termed the “gabbroic cap”. The contact is xenolith-rich and lacks a well-developed chill margin. The mafic-ultramafic basal section of the intrusion consists of three transitional cumulate igneous phases: olivine websterite, olivine melagabbro, and olivine gabbro. Olivine websterite hosts up to 30% disseminated pyrrhotite, chalcopyrite, pyrite, rare bornite, and unknown platinum group minerals along its intrusive contact with significant drill intercepts including 20m at 0.22% Cu, 0.06% Ni, 0.25 g/t Pt, 0.29 g/t Pd, and 0.04 g/t Au (Bidwell and Marino, 2007). The Thunder gabbroic cap consists of three igneous phases: gabbro, pegmatitic gabbro, and leucogabbro all of which display a subophitic texture. The gabbro and leucograbbro are transitional and host up to 15% vein and 76 �disseminated chalcopyrite, pyrite and rare bornite however, no significant mineralisation was intercepted (Bidwell and Marino, 2007). The pegmatitc gabbro occurs along the contact between the two intrusive components of the Thunder and locally cross-cuts the olivine websterite. This unit has been interpreted to be a late magmatic phase of the Thunder. Marginal country rocks include metavolcanic and metasedimentary assemblages of the Shebandowan Greenstone Belt that have been structurally deformed and overprinted by regional greenschist facies metamorphism (Williams et al., 1991). The intrusive contact between the Thunder intrusion and the marginal country rock is sharp, xenolith-rich, lacks a well-developed chill margin, and contains blebs of granophyric material which suggests partial melting of the wall rock and/or wall rock assimilation. Within 100m of the Thunder intrusion a contact metamorphic aureole overprints the country rock consisting mostly of pervasive hornfels alteration. During Kennecott’s 2005 drill program a drill hole located ~400m southeast of the Thunder intrusion intercepted a mineralized unit 58m down depth. This peculiar unit is 4m thick, runs 1.7 g/t Au and 0.53% Cu, and was initially interpreted to be a massive sulphide magnetite garnet skarn associated with the emplacement of the Thunder intrusion (Marino and Bidwell, 2007). However, recent findings and discussions have challenged classifying this mineralized unit as skarn alteration and it is currently being investigated. Additional petrography, whole rock geochemistry, and SEM and S-isotopic analyses are currently being applied in order to further characterise the Thunder intrusion, mineralization and alteration assemblages both within the Thunder and the marginal country rock. In addition, this study will date the intrusion using U/Pb of zircon and/or baddeleyite. References Bidwell, G. E., and Marino, F., 2007., Geoinformatics Exploration Canada Limited, Thunder Project 2007 Field Program, Diamond Drilling on the 1245457 claim, Thunder Bay Mining Division, Ontario. Thunder Bay Resident Geologist’s Office, assessment files 20000002091-2.34638. 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. Hollings, P., Hart, T., Richardson, A., MacDonald, C.A., 2007. Geochemistry of the Mesoproterozoic intrusive rocks of the Nipigon Embayment, northwestern Ontario: evaluating the earliest phases of rift development. Canadian Journal of Earth Sciences 44: 1087–1110. 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 I: 485-539. 77 �Glacial Lake Ontonagon and the Development of Large Scale Landslides Vitton, Stanley J., Michigan Technological University, Houghton, MI, 49931 A massive landslide occurred in 2005 along the East Branch of the Ontonagon River in northern Michigan adjacent to US-45 (Figure 1A). The landslide initially blocked the river causing it to redevelop a new flow channel. While other massive landslides occur along this section of the river, they tend to be infrequent with respect to the general form of mass wasting such as slope regression due to river under cutting and surface erosion. An investigation of the landslide indicated two very distinct soil units that appear to correspond to the two phases of glacial Lake Ontonagon (Figure 1B). The two soil units have a relatively distinct boundary as seen in Figure 1C. The lower unit consists of a red till, which forms the floor of the valley, grading upward into alluvial sand (Figure 1D), while the upper unit is a distinct lacustrine soil deposit (Figure 1E). The massive landslide failure zone developed in the lower soil unit. It is unclear at this point as to whether the failure was due to softening of the lower red till or liquefaction induced failure caused by increased pore pressure development during the spring runoff in the alluvial sand. Due to the extensive development of soil liquefaction features (Figure 1F), however, it is believed that failure was induced via liquefaction in the transitional zone between the red till and the clean sand in the lower soil unit where the percent of fines in the sand prevent adequate drainage. Additional analysis of the soil’s strength and dynamic properties are needed, however, to make a more definitive determination (Smith, 2012). The origins of glacial Lake Ontonagon was first addressed by Leverette (1929) and later by Hack (1965), Farrand and Drexler (1985) and Attig, Clayton and Mickelson (1985). The formation Lake Ontonagon soils are believed to have developed in the post-Twocreekan time, around 11,800 Before Present (BP). The postTwocreekan glacier advance completely filled the Lake Superior basin with two ice lobes that were split by the Keweenaw Peninsula. The Superior lobe reached the position of the Nickerson moraine southwest of Duluth, while the Lake Michigan-Green Bay lobe moved southward across the northern peninsula of Michigan, ultimately reaching the Two Rivers moraine at Manitowoc, Wisconsin, about 11,800 BP. Following the Two Creek advance, de-glaciation formed lakes and drainage channels in front of the glacier lobes in which glacial lakes Duluth and Ontonagon formed. Lake Ontonagon drained westward into Lake Ashland and eventually to the St. Croix River, which drained southward to the Mississippi River at about 11,000 BP. Between 11,000 and 10,700 BP the glacier retreated into the Lake Superior Basin forming a much larger Lake Duluth and eventually as the ice retreated and the glacial rebound occurred lowering Lake Duluth to form Lake Algonquin. It is believed that the lower soil unit formed during this period of time. At about 10,000 BP, however, the last glacial re-advance, known as the Marquette Phase, advanced back into the Lake Superior Basin covering most of the northern portion of the Upper Peninsula. At about 9,900 BP the ice retreated again forming a series of lakes along the front of the ice sheet. Lake Ontonagon reformed at this time along with Lake’s Ashland and Nemadjic. Eventually the lakes became confluent and drained westward to the St. Croix outlet. At that time the lake levels for Ashland and Nemadjic dropped about 20 feet. Lake Ontonagon, on the other hand, dropped about 200 feet, (Leverett, 1929) leaving much of its lake bed dry land surface. It is believed that the upper lacustrine soil unit formed during this period of time. References Attig, W.J., Clayton, L. and D.M. Mickelson, 1985. Correlation of late Wisconsin glacial phases in the western Great Lakes area, Geological Society of America Bulletin vol. 96, no. 12; pp 1585-1593. 78 �Farrand, W.R. and Drexler, C.W. 1985. Late Wisconsin and Holocene History of the Lake Superior Basin, Quaternary Evolution of the Great Lakes, P.F Karrow and P.E. Calkin, editors, Geological Assoc. of Canada Special Paper 30. Hack , John, 1965. Postglacial drainage evolution and stream geometry in the Ontonagon area, Michigan, Geological Survey Professional Paper 504-B, Washington, D.C., 45 p. Leverett, Frank, 1929. Moraines and shorelines of the Lake Superior basin: U.S. Geological Survey Professional Paper 154-A, 72 p. Smith, J. 2012. Large Scale Landslide on the Ontonagon River, Michigan, Masters of Science Report, Michigan Technological University, Houghton, Michigan, 17 p. A B US-45 Upper Unit Lower Unit East Branch Ontonagon River C D Soil Unit Interface Lower LowerUnit Unit––Alluvial AlluvialSand Sand E F Upper Unit – Lacustrine Lateral spreading Figure 1. A) Large scale landslide adjacent US-45 on the East Branch of the Ontonagon River. B) Two soil units. C) Interface between soil units. D) Lower soil unit - alluvial sand. E) Upper soil unit – lacustrine soil unit. F) Liquefaction induces lateral spreading. 79 �A PRELIMINARY SURVEY OF THE GEOLOGY OF THE PRE-MICHIGAN BASIN ROCKS OF THE SOUTHERN PENINSULA VOICE, Peter, HARRISON, William, and THAKURTA, Joyashish, Department of Geosciences and the Michigan Geological Repository for Research and Education, Western Michigan University, 1903 W. Michigan Ave, Kalamazoo, MI 49008 The Michigan Geological Repository for Research and Education (MGRRE) is the premier core repository for Michigan Basin sedimentary rock materials. The collection holds roughly half a million linear feet of core. Archived with the MGRRE collection are multiple wells with cores (9 total) and a larger number of wells with drill cuttings (35 total) from preBasin rocks (Fig. 1). The majority of wells identified with pre-Basin rocks are from southern and southeastern Lower Peninsula. A handful of age dates are available from these wells, though most of the analyses for geochronological results were determined in the 1960s. Cuttings rich in biotite were age dated with both Rb-Sr and K-Ar from the McClure #2 State Beaver Island well and obtained ages of 1,040 and 1,090 Ma respectively (Lidiak et al. 1966). These dates suggest a relationship with the Mid Continent Rift System. The St. Blair 2-24 from Grand Traverse County was age dated with U-Pb from zircons from granite and yielded an age of 1,472 Ma (Hoppe et al. 1983). The St. Blair 2-24 well does have core though it is not archived with MGRRE. A series of wells from the southeastern Lower Peninsula were sampled for granite and granite-gneiss cuttings. These samples were dated with both the Rb-Sr and K-Ar systems and yield ages between 840 and 970 Ma (Lidiak et al. 1966, Summerson, 1962). Hinze et al. (1975) interpreted these rock units as being part of the Grenville province. Initial investigation of the collection of available cores has shown a complex set of lithologies preserved in the basement of the Michigan Basin in Branch and St. Joseph counties. Four closely spaced wells alternate between porphyritic granite and biotite-rich gneisses. A deep stratigraphic test drilled in Gratiot Co. cored through fine-grained turbiditic sediments interpreted to being part of the Mid Continent Rift System (Fowler and Kuenzi, 1978). The southeastern Lower Peninsula (Arenac, Huron and St. Clair counties) wells with core exhibit a diverse assemblage of metasediments, granites, and granite gneisses. Hinze et al. (1975) published a basement provinces map for the Lower Peninsula interpreted on the basis of limited well data, mostly from descriptions on driller’s reports and the geochronological results published by Lidiak et al (1966) and Summerson (1962). The addition of more than twice as many wells into the Lower Peninsula basement rock data set provides the opportunity to update Hinze et al.’s (1975) province map and produce the first basement geologic map of Michigan’s Lower Peninsula. To aid this effort, we are attempting to generate detailed lithological descriptions and a new geochronology of events. 80 �Figure 1. Distribution of wells in the Lower Peninsula of Michigan that drilled into the basement beneath the Michigan Basin. References Fowler, J. H., and Kuenzi, W. D. 1978. Keweenawan Turbidites in Michigan (Deep Borehole Red Beds): A Foundered Basin Sequence Developed During Evolution of a Proterozoic Rift System. Journal of Geophysical Research, 83: 5833-5843. Hinze, W. J., Kellog, R. L., and O’Hara, N. W. 1975. Geophysical Studies of Basement Geology of Southern Peninsula of Michigan. American Association of Petroleum Geologists Bulletin, 59: 1562-1584. Hoppe, W. J., Montgomery, C. W., and Van Schmus, W. R. 1983. Age and signficance of Precambrian Basement Samples from Northern Illinois and Adjacent States. Journal of Geophysical Research, 88:7276-7286. Lidiak, E. G., Marvin, R. F., Thomas, H. H.; and Bass, M. N.1966. Geochronology of the Miccontinent Region, United States. 4. Eastern Area. Journal of Geophysical Research, 71: 5427-5438. Summerson, C. H. 1962. Precambrian in Ohio and Adjoining Areas; State of Ohio, Department of Natural Resources, Division of Geological Survey. Report of Investigation. 44, 16 pp. 81 �Sedimentology and Geochemistry of a Regressive Surface in the Chemical Sediments of the Paleoproterozoic Gunflint Formation YIP, Christopher and FRALICK, Philip, Department of Geology, Lakehead University, Thunder Bay, ON, Canada, P7B 5E1, philip.fralick@lakeheadu.ca The 1878 ±1Ma Gunflint Formation is a chemical-sedimentary unit deposited in the Animike Basin; it shows a sequence of transgressive-regressive cycles. Wolff (1917) and Broderick (1923) divided the Gunflint into several individual members; lower cherty, lower slaty, upper cherty and upper slaty. These members were then grouped into two different sequences; the upper and lower sequences. The first and most extensive transgressive- regressive cycle is made up of the lower cherty member, while overlying transgressive-regressive cycles are made up of the lower slaty member, the lower cherty member and the upper slaty member. An outcrop present near Mink Mountain UTM: 329,520 E/5,338,163N, shows a complete section through the peak lower regressive-transgressive sequence. A detailed description of the section was logged through this sequence, which was divided into three main units; 1) a grainstone unit is the bottom unit and lies directly below 2) stromatolites which is capped off by a 3) oncolithic unit. The grainstone directly below the stromatolites is brecciated and shows injections of jasper and hematite throughout (Fig 1A). Microscopically the grainstone unit is composed of angular to rounded grains of chert. The cement is predominantly chert with some blocky quartz found forming at grain boundaries. The stromatolite unit is present above the pre-lithified grainstone unit and contains distinct stratiform and columnar stromatolites. The top unit is an oncolith-rich grainstone. The grains have a nucleus composed of either microquartzrich chert or blocky quartz. The cement of the unit is composed of a combination of a chalcedony-rich chert and blocky quartz. Several samples were taken up through this section and sent to the OGS lab in Sudbury for ICP-MS analysis for rare earth elements. The results were normalized to Taylor and McLennan (1985) Post Archean Australian Shale values and plotted (Fig 2A,B). All the samples taken from the grainstone layer and three samples taken from the stromatolite show a characteristic europium anomaly and a distinct positive cerium anomaly. The Ce anomaly is indicative of an oxidized environment where Ce (IV) was being precipitated and scavenged by the sediments (Peters, 2003). This requires oxygen production in the near-shore, and precipitation of Ce from sea-water that had not been previously exposed to significant oxygen. A B Figure 1. A) The lithified surface directly beneath the stromatolite section at Mink Mountain. B) A close up photograph of stromatolite lamiae and oncoliths in the depression between stromatolites from a polished hand sample. 82 �A B Figure 2. A) Rare earth element spider diagram for the grainstone unit at Mink Mountain. B) Rare earth element spider diagram for the stromatolites from Mink Mountain. References Broderick, T.M., 1920, Economic geology and stratigrsphy in the Gunflint iron district, Minnesota: Economic Geology 15: 422-452. Peters, J.M., 2003. Ancient iron formation: their genesis and use in the exploration of strataform base metal sulphide deposits, with examples from the Bathurst Mining Camp, in Lentz, D.R., ed., Geochemistry of Sediments and Sedimentary Rocks: Evolutionary Consideration to Mineral Deposits-Forming Environments: Geological Association of Canada. GeoText 4. P. 145-176 Taylor , S.R., and Mclennan, S.M., 1985. The continental crust; its composition and evolution; an examination of the geochemical record preserved in sedimentary rocks. Blockwell, Oxford, p. 312 Wolff, J.F., 1917, Recent geologic developments on the Mesabi Iron Range, Minnesota: Am. Institute of Mining and Metallurgical Engineers, Transactions 56: 229-257 83 �Sponsors The following organizations made general contributions to the 59th Annual Meeting. We thank the for their commitment to the Institute on Lake Superior Geology. For the past 59 years this organization has thrived as a result of the interest of individuals, corporations, universities and government agencies. The dedication to an exchange of scientific ideas and a passion for field trips has enabled the Institute to provide one of its primary objectives - to promote better understanding of the geology of the Lake Superior region. Eagle Mine � https://digitalcollections.lakeheadu.ca/files/original/74297d1a8a15b17000b67423e21aa2ba.pdf f54f6098f3af779b3b9135dc2d2d400e PDF Text Text Institute on Lake Superior Geology 59th Annual Meeting Houghton, Michigan May 8 - 11, 2013 Proceedings Volume 59 Part 2 - Field Trip Guidebook Editors: Theodore J. Bornhorst and Robert J. Barron www.lakesuperiorgeology.org ��Institute on Lake Superior Geology 59TH ANNUAL MEETING MAY 8-11, 2013 HOUGHTON, MICHIGAN SPONSORED BY: A. E. Seaman Mineral Museum Michigan Technological University THEODORE J. BORNHORST AND ALLAN R. BLASKE Co-Chairs Proceedings Volume 59 Part 2 – Field Trip Guidebook EDITED BY THEODORE J. BORNHORST AND ROBERT J. BARRON Cover Photo: Native copper from the Central Mine, Keweenaw Peninsula, Michigan. Collection of the A.E. Seaman Mineral Museum. Photograph by George Robinson. ��59TH INSTITUTE ON LAKE SUPERIOR GEOLOGY PROCEEDINGS VOLUME 59 CONSISTS OF: PART 1: PROGRAM AND ABSTRACTS PART 2: FIELD TRIP GUIDEBOOK TRIP 1: GEOLOGIC OVERVIEW OF THE KEWEENAW PENINSULA, MICHIGAN TRIP 2: CALEDONIA MINE, KEWEENAW PENINSULA NATIVE COPPER DISTRICT, ONTONAGON COUNTY, MICHIGAN TRIP 3: GEOLOGY OF SILVER MOUNTAIN, HOUGHTON COUNTY, MICHIGAN TRIP 5: GEOLOGY OF THE KEWEENAWAN SUPERGROUP, PORCUPINE MOUNTAINS, ONTONAGON AND GOGEBIC COUNTIES, MICHIGAN TRIP 6: GEOLOGY AND ENVIRONMENTAL SITE CONDITIONS OF THE COPPERWOOD DEPOSIT, GOGEBIC COUNTY, MICHIGAN Reference to material should follow the example below: Cannon, W. F. Woodruff, L. G., and Schulz, K.. J., 2013, The Hiawatha Graywacke of the Iron River-Crystal Falls district, Michigan: a megaturbidite triggered by seismicity related to the 1850 Ma Sudbury impact [abstract]: Institute on Lake Superior Geology Proceedings, 59th Annual Meeting, Houghton, MI, v. 59, part 1, p. 14-15. Published by the 59th Institute on Lake Superior Geology and distributed by the ILSG Secretary: Peter Hollings Department of Geology Lakehead University Thunder Bay, ON P7B 5E1 CANADA peter.hollings@lakeheadu.ca ILSG website: http://www.lakesuperiorgeology.org ISSN 1042-9964 i ��Table of Contents TRIP 1 – GEOLOGIC OVERVIEW OF THE KEWEENAW PENINSULA, MICHIGAN 1 TRIP 2 – CALEDONIA MINE, KEWEENAW PENINSULA NATIVE COPPER DISTRICT, ONTONAGON COUNTY, MICHIGAN 43 TRIP 3 – GEOLOGY OF SILVER MOUNTAIN, HOUGHTON COUNTY, MICHIGAN 59 TRIP 4 – A.E SEAMAN MINERAL MUSEUM – NO GUIDE TRIP 5 – GEOLOGY OF THE KEWEENAWAN SUPERGROUP, PORCUPINE MOUNTAINS, ONTONAGON AND GOGEBIC COUNTIES, MICHIGAN 69 TRIP 6 – GEOLOGY AND ENVIRONMENTAL SITE CONDITIONS OF THE COPPERWOOD DEPOSIT, GOGEBIC COUNTY, MICHIGAN 97 ii ��Field Trip 1 Geologic Overview of the Keweenaw Peninsula, Michigan Theodore J. Bornhorst A.E. Seaman Mineral Museum, Michigan Technological University, 1404 E. Sharon Avenue, Houghton, MI 49931 Robert J. Barron Department of Geological and Mining Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931 Introduction The geology of the far western Upper Peninsula of Michigan consists of three temporally distinct episodes. During the Mesoproterozoic, between about 1.15 and 1.03 Ga, up to 30 km of Keweenaw Supergroup volcanics and clastic sediments filled an intracratonic rift, the Midcontinent Rift (MCR) (Figs. 1 and 2) (Heaman et al., 2007; Davis and Paces, 1990; Cannon et al., 1989). After a 500 million year period of erosion, the MCR rocks were buried by Phanerozoic sedimentary rocks from about 500 Ma to 175 Ma (Catacosinos et al., 2001). Pleistocene continental glaciations, beginning about 2 million years ago, removed the Phanerozoic rocks from the Keweenaw Peninsula leaving only a few outliers. About 10,000 years ago, as the last remaining glaciers retreated, and they left behind a variety of unconsolidated clastic sediments. The geologic evolution of the far western Upper Peninsula is illustrated in cartoon form in Figure 3. 75o 50o Midcontinent Rift 100o o 50 Precambrian bedrock below unconsolidated glacial sediments Canada Canada Minnesota Canada Wisconsin Grenville Tectonic Zone Iowa Nebraska Kansas 0 Shaded area = Phanerozoic bedrock belowunconsolidated glacial sediments o 35 100o North Slate Shoreline Islands Superior Shoal Bg/Og Bg/Og Ks 5 Plv Paleoproterozoic? Manitou Island Js Manitou structural zone 35o 75o South Shoreline Js Plv/Ks Ks A A 400 kilometers Fault blocks/ intrusives 10 A 15 Ks = Keweenawan Supergroup older than Portage Lake Volcanics Js = Jacobsville Sandstone Og = Oronto Group Bg = Bayfield Group Plv = Portage Lake Volanics A = Archean rock Stratigraphic column given in Fig. 4 Figure 1: Generalized bedrock geologic map of the Midcontinent Rift. Grenville tectonic zone after Cannon (1994) and interpretative cross-section across the Lake Superior segment of the Midcontinent rift after Cannon et al. (1989). 1 �Mesoproterozoic Midcontinent Rift Around Lake Superior Native Copper occurrences Sedimentary Rocks Igneous Rocks Several Major Faults Abundant Ontario Isle Royale Minnesota Lake Superior Keweenaw Peninsula native copper district Ontario Upper Peninsula of Michigan Wisconsin Lake Michigan 0 100 200 kilometers Archeanmetamorphosed sedimentaryandigneous rocks N Paleoproterozoic metamorphosed sedimentary and igneous rocks Phanerozoic sedimentary rocks Figure 2: Generalized bedrock map showing the exposed rocks of the Midcontinent Rift around Lake Superior and the bedrock of the Upper Peninsula of Michigan. Locations of concentrations of native copper are shown around Lake Superior. Modified from Bornhorst and Barron (2011). In the strictest sense, the geographic area of the Keweenaw Peninsula proper extends northeast of a NW-SE line drawn through L’Anse (Fig. 4), however, the term Keweenaw Peninsula has also been applied to the area containing MCR rocks farther to the south. Bornhorst and Barron (2011) used the name Keweenaw Peninsula native copper district to describe native copper deposits hosted by MCR rocks as far south as the White Pine Mine. The geologic description in this field trip guide is restricted to the Keweenaw Peninsula proper and does not include the southern bedrock such as at Silver Mountain described in Field Trip 3 (this volume) or Cambrian to Devonian rocks at Limestone Mountain described by Milstein (1987). The descriptions of the geology of the Keweenaw Peninsula provided here were modified from a combination of Bornhorst and Barron (2011), Bornhorst and Lankton (2009), and Bornhorst and Rose (1994), and Bornhorst et al. (1983). Specific citation or quotation is not given in all instances. 2 �D. Late compression and rift-flanking basin ~ 1.06 to 1.03 Ga H. Continental Glaciation Rift-flanking basin < 2 Ma Isle Royale Lake Superior Keweenaw Peninsula Reverse inversion of normal faults C. Sedimentary infillingduring sagging G. Burial by Phanerozoic sedimentary rocks ~ 1.092 to 1.05 Ga ~ 500 to 175 Ma Clastic sedimentary rocks B. End of Magmatic Phase ~ 1.098 to 1.092 Ga Sheeted dikes F. Extended period of erosion Basalt lava flows ~ 1.15 to 1.098 Ga Downward percolating groundwater Diabase dike Gabbro A. EarlyStage of Rifting ~ 1.03 Ga to 500 Ma (0.5 Ga) Basalt lava flows E. Formation of native copper deposits during late compression ~ 1.06 to 1.04 Ga Extension Crust Deep Magma Chamber Native copper precipitation Mantle Ore mineralizing fluids Figure 3: Cartoon NW to SE cross sections from Minnesota (left) to the Upper Peninsula (right) illustrating the progressive geologic evolution. Modified from Bornhorst and Lankton (2009). Midcontinent Rift Lithologic Units The Keweenaw Peninsula is located on the southern margin of the Lake Superior segment of the MCR (Figs. 1and 2). The rock units that are associated with the MCR have been termed the Keweenawan Supergroup (Fig. 5). These rocks were deposited from about 1.15 and 1.03 Ga (Heaman et al., 2007; Davis and Paces, 1990; Cannon et al., 1989). The MCR beneath Lake Superior is filled with up to about 30 km of volcanic rocks (Figs. 1 and 3) (Hinze et al., 1990; Cannon et al., 1989; Cannon, 1992). The MCR geology of the Keweenaw Peninsula can be divided into northwest-dipping, rift-filling volcanic and clastic sedimentary rocks located on the northwest side of the Keweenaw Peninsula and flat to low-dipping, rift-flanking clastic sedimentary rocks located on the southeast side (Fig. 4). These two contrasting lithologic settings are separated by the Keweenaw Fault which was originally a graben-bounding fault, but today is a high-angle reverse fault (Fig. 3). 3 �4 �Figure 5: Lithostratigraphic bedrock units of the Mesoproterozoic Midcontinent rift system of Michigan. Portage Lake Volcanics The Portage Lake Volcanics (Figs. 4 and 5) is a 2,500 to 5,200 m thick formation dominantly composed of subaerial basalt lava flows with less than 1 % by volume intermediate to felsic volcanic and subvolcanic rocks located stratigraphically near the base of the exposed formation. Less than 5 % by volume is stratigraphically scattered interflow reddish-colored conglomerate and sandstone units that are greater in abundance towards the top of the formation (Butler and Burbank, 1929; White, 1968). The base of the formation is truncated by the Keweenaw Fault. The lavas flowed from fissure vents that tended to be located nearer the axis of the rift zone which produced a layered succession of flood basalts comparable to the rift zones of East Africa and Iceland (e.g., Nicholson, 1992 and reference therein). The Portage Lake Volcanics erupted over 2 to 3 million years from 1,096.2+/-1.8 (Copper City flow, Fig. 6) to 1,094.0+/-1.5 (Greenstone flow, Fig. 6) (Paces and Miller, 1993; Davis and Paces, 1990). There are more than 200 individual basaltic lava flows in the exposed Portage Lake Volcanics which are typically aphyric, Mg-rich, high-Al olivine tholeiites (Paces, 1988). The most abundant type of basalt flows are olivine tholeiites, followed by primitive olivine tholeiites and quartz tholeiites. Iron-rich olivine tholeiites are generally lesser in abundance (Table 1). The thicker lava flows are compositionally stratified due to magmatic differentiation after eruption, especially the Greenstone flow, which is the thickest individual flow in the formation (Cornwall, 1951a and b; Broderick, 1935; Broderick and Hohl, 1935). The composition of the basalts is cyclical with minor and major cycles superimposed on an overall trend toward more primitive compositions towards the top of the formation. The basalt magmas were derived by partial melting of sub-continental upper 5 �mantle with an overall stratigraphically upwards trend towards younger, to more primitive basalt compositions as a result of less contamination by crustal rocks (Paces, 1988; Paces and Bell, 1989). The repeated magmatism at the rift axis and progressive crustal thinning provided pathways for transport of magma to the surface creating less extended contact with crustal rocks and hence, less contamination. The youngest rocks of the Portage Lake Volcanics in the Keweenaw Peninsula have compositions similar to MORB suggesting the MCR nearly formed an ocean basin. The major geochemical cycles are due to fractional crystallization and replenishment in large magma chambers near the crust/mantle interface whereas the minor cycles are due to closed system fractional crystallization in small magma chambers within the crust (Paces, 1988). The Portage Lake Volcanics were likely derived by partial melting of trace element enriched plume-related mantle (Nicholson et al., 1997; Nicholson and Shirey, 1990; Paces and Bell, 1989). Table1: Average and representative geochemical data for least altered lavas of the Portage Lake Volcanics (from Paces, 1988). Tholeiites were grouped by Ni content. Primitive Intermediate Olivine Olivine olivine olivine tholeiite tholeiite tholeiite tholeiite Ni (ppm) Wt.% SiO₂ Al₂O₃ FeOt MgO CaO Na₂O K₂O TiO₂ P₂O₅ MnO PPM Ni Cu Zr Iron-rich olivine and Andesite Dacite Rhyolite quartz tholeiites 400-300 n=5 300250 n=9 250200 n=14 200-100 n=8 100-15 n=6 n=1 n=1 n=1 47.82 15.89 9.77 12.44 10.58 2.04 0.19 0.98 0.16 0.14 47.34 15.27 11.82 11.69 10.24 2.10 0.22 1.13 0.19 0.16 48.03 15.32 12.32 9.85 10.16 2.25 0.33 1.35 0.22 0.16 48.55 15.12 12.86 9.06 9.65 2.31 0.42 1.60 0.25 0.18 49.94 13.28 14.91 7.78 6.64 2.91 1.43 2.34 0.36 0.24 56.39 13.78 9.87 5.52 5.10 3.94 2.27 1.83 1.00 0.30 68.44 15.17 4.46 1.14 1.40 4.74 3.86 0.51 0.19 0.08 77.89 12.77 1.11 0.17 0.04 3.67 4.28 0.08 0.01 0.01 231 73 101 172 86 126 54 126 212 10 5 430 7 13 573 5 61 145 326 279 37 51 78 85 FeOt=total Fe as FeO 6 �All observed basalt lava flows in the Portage Lake Volcanics were erupted subaerially and consist of a massive (vesicle-free) interior capped by a vesicular and/or brecciated flow top. The only evidence for involvement of water during eruption is a single thin hyaloclastic unit in the upper part of the formation (locally termed the ashbed). Subaerial eruption resulted in degassing of volatiles, notably SO2 (Cornwall, 1951c). The lava flows range in thickness from 1 to 450 m with most of them between 10 to 20 m thick (Paces, 1988; White, 1960). Most of the lava flows cannot be traced along strike with confidence although a few such as the Scales Creek, Kearsarge, and Greenstone flows have well documented lateral continuity flows (Fig. 6). The Greenstone flow has been correlated down dip across the Lake Superior syncline to Isle Royale (Longo, 1982; Huber, 1975). The uppermost 5 to 20% of the tops of most (89 %) individual lava flows are vesicular with between 5 and 50% vesicles (White, 1986). The tops of 21 % of the flows are brecciated with clasts of vesicular basalt. The vesicles in most lava flows within the Portage Lake Volcanics, except for the stratigraphically uppermost, are filled with secondary minerals and are amygdules. Thus, amygdaloids are lava flows with vesicle-only tops and fragmental amygdaloids are those with vesicular and brecciated tops. There are the minor amounts of andesite, dacite, and rhyolite lava flows and subvolcanic plutons that interfinger with and cross cut the basalts of the Portage Lake Volcanics (Table 1). Most of these occur in the stratigraphically lowermost portion of the Portage Lake Volcanics. A few dikes of intermediate composition and a diorite stock at Mt. Bohemia intrude the exposed Portage Lake Volcanics. The rhyolitic volcanic setting is analogous to the shield-type central volcanoes of Iceland (Nicholson, 1991). Interflow sedimentary units are important stratigraphic markers in an otherwise monotonous succession of basalt lava flows that can be traced up to 90 km along strike and so many of them are given informal names (see Fig. 6). They consists of red-colored, well-lithified, pebble-to-boulder conglomerates with lesser amounts of interbedded sandstone and occasional significant amounts of siltstone and shale ranging in thickness from a few cm up to about 40 m (Merk and Jirsa, 1982; White, 1968; Butler and Burbank, 1927). The typical conglomerate has an exposed interflow lithology characterized by sub-rounded to angular pebbles in a sandy matrix. Clast size varies from pebbles to boulders and clast lithologies are predominantly felsic, although there is considerable variation within and between specific beds reflecting diversity in source terrane. Within the interflow Calumet and Hecla Conglomerate, Kalliokoski and Welch (1985) interpreted a subunit as a caliche soil profile. The interflow clastic sedimentary beds were deposited during intervals of volcanic quiescence most likely in terrestrial alluvial fans in an arid to sub-arid climate. Deposition was on top of the shallow-dipping to flat-lying lava flows by streams flowing from the topographic high on the margin of the MCR toward the center of the rift basin (now under Lake Superior) (White, 1968). 7 �NE M eters 300 600 0 Top of the Portage Lake Volcanics Hancock Conglomerate Ashbed Flow Pewabic West Conglomerate Ashbed Flow Greenstone Flow Pewabic Flow Allouez Conglomerate 300 600 900 Evergreen Flows Kearsarge Flow Winona Flow 1200 Isle Royale Flow Wolverine Sandstone 1500 Scales Creek Flow 1800 Baltic Flow Gratiot Flow Copper City Flow Upper Limit of Epidote in Flows Upper Limit of Quartz in Flows Lower Limit of Prehnite in Flows 0 3 6 km Exceptionally Thick Lava Flows Location of Mine within Stratigraphic Strike Parallel Section Figure 6: Generalized stratigraphic position parallel to strike of informal units within the Portage Lake Volcanics. Modified from Stoiber and Davidson (1959). Figure 5 shows location of GreenlandMass subdistrict (Michigan, Caledonia, Mass, Adventure Mines) and Copper Harbor. Copper Harbor Formation The Copper Harbor Formation is the oldest formation in the Oronto Group and conformably overlies and interfingers with the Portage Lake Volcanics (Figs. 4 and 5). It consists of red-brown clastic sedimentary rocks with a maximum exposed thickness 2,000 m. The Copper Harbor Formation in the Keweenaw Peninsula includes a succession of subearially deposited lava flows informally named the Lake Shore Traps. The depositional environment of the Copper Harbor Formation was deposited in a prograding coalescing alluvial fan complex with proximal-to-distal braided stream and sheet flood facies on the alluvial fans to distal sand flats and flood plain facies (Elmore, 1984). The climate was probably arid with flashy seasonal streams. The highlands from which the Copper Harbor Formation were derived to the southeast, now buried under the Jacobsville Sandstone. 8 �Conglomerates and sandstones are the dominant lithologies in the Copper Harbor Formation. The formation fines distally and up section, reflecting a waning sediment supply due to progressive erosion of the source area (Elmore, 1984). The poorly-sorted clasts in the conglomerates range in size from granules to boulders that are subrounded to rounded and are mostly volcanic in origin and have a ratio of mafic-to-intermediate + silicic composition of about 2:1 (Daniels, 1982). The conglomerates include clast-supported and matrix- supported varieties; some of the latter are diamictites. The conglomerates are interpreted as high-energy channel deposits on a coalescing alluvial fan (Elmore, 1984). The diamictites are debris flow in origin. Sandstones are predominantly red-brown, subangular-to-angular lithic graywackes with volcanic lithic fragments. These exhibit current-ripples, trough-cross beds, current and parting lineations, and reduction spots. Sandstone interbeds are more common in the upper 2/3 of the formation. The abundant calcite cement in the conglomerate and coarse sandstone was probably deposited as vadose carbonate or caliche (Kalliokoski, 1986). Thin red-colored siltstone and shale interbeds have desiccation cracks and are interpreted as filling abandoned channels on the alluvial fan surface. In the Copper Harbor area, there are also laminated cryptoalgal carbonate beds and ooid lenses occurring within the same general stratigraphic position. These are laterally-linked contorted layers in shale-siltstone that are draped over cobbles and are found as poorly developed mats in coarse sandstone (Elmore, 1983). The laminated carbonate beds are algal stromatolite (genus Colleria). The stromatolites formed in shallow, medial fan lakes and possibly abandoned channels on the alluvial fan surface (Elmore, 1983). The Lake Shore Traps (Lane, 1911), an informal member of the Copper Harbor Formation (Fig. 5), are well exposed near the tip of the Keweenaw Peninsula where the unit is composed of 31 lava flows and one interflow conglomerate about 600 m thick (Paces and Bornhorst, 1985). The composition of the Lake Shore Traps is different than the underlying Portage Lake Volcanics reflecting the change from active rift-filling magmatism to active rift-filling clastic sedimentation with little to no magmatism. The rocks range from Fe-rich olivine tholeiitic basalt at the base to Ferich olivine-bearing tholeiitic basaltic andesites to tholeiitic andesites. Strato-geochemical relationships can be explained by a combination of fractional crystallization, parental magma replenishment, and wall rock assimilation (Paces and Bornhorst, 1985). Davis and Paces (1990) report a U-Pb age on zircon of 1087.2 +/- 1.6 Ma for the Lake Shore Traps. Nonesuch Formation The Nonesuch Formation conformably overlies and locally interfingers with the Copper Harbor Formation (Figs. 4 and 5). It consists of dominantly black-to-gray-to-green fine clastic sedimentary rocks with a maximum exposed thickness 240 m. Exposures in the Keweenaw Peninsula are limited with the best exposure at the Hancock Campground on M-203. This formation will not be visited for this field trip. The Nonesuch Formation was deposited in a generally anoxic lacustrine environment ranging from marginal lacustrine (sandflat-mudflat) to lacustrine to lacustrine-to-fluvial subenvironments (Elmore et al., 1989). Siltstone and shale are the dominant lithologies with lesser very-fine sandstone and minor carbonate laminates. While gray (reduced) color characterizes most of this formation, the stratigraphic upper beds have more red-brown colors (Bornhorst and Williams, in press). Well-laminated to massive 9 �black to dark-gray siltstone and shale were deposited in the lacustrine subenvironment. The lacustrine lithologies at the base of the Nonesuch Formation host economic quantities of chalcocite and native copper at the now closed White Pine Mine (Mauk et al., 1992) and chalcocite at the Copperwood project (Bornhorst and Williams, in press; and Field Trip 5 this guidebook). A thin carbonate laminate yielded a Pb-Pb isochron age of 1,081 ± 9 Ma (Ohr,1993) Freda Formation The Freda Formation is the youngest formation of the Oronto Groupand overlies the Nonesuch Formation with no explosed top (Figs. 4 and 5). The contact between the Freda and Nonesuch Formations is gradational. The exposed thickness is greater than 3,700 m, however, it is poorly exposed except along the Lake Superior shoreline. This formation will not be visited for this field trip. The Freda Formation is presumably overlain by the Jacobsville Formation. The Freda Formation was deposited in an environment characterized by shallow meandering streams (Daniels, 1982). Red-brown fine to very-fine sandstone, siltstone, and mudstone are the dominant lithologies in the Freda Formation. Fining-upward sequences occur on the scale of a few meters. Based on regional correlations the age of the Freda Formation is likely 1,060 to 1,040 Ma (Cannon, 1992). Jacobsville Sandstone The Jacobsville Sandstone is the youngest Mesoproterozoic bedrock formation in the Keweenaw Peninsula (Figs. 4 and 5). Its stratigraphic relationship with other units is uncertain. It occurs in a contiguous geographic region bound on the northwest by the Keweenaw Fault and to the southeast, it angularly unconformably overlies Paleoproterozoic and Archean rocks (Fig 3). The Jacobsville Sandstone is estimated to be more than 2,900 m thick and the top is not exposed (Kalliokoski, 1982). The Jacobsville Sandstone was deposited in an environment characterized by shallow meandering streams (Kalliokoski, 1988). The formation occurs in a rift-flanking basin and at least part of it was deposited during active reverse movement along the Keweenaw Fault. Red to red-brown sandstone is the dominant lithology with lesser amounts of red-brown conglomerate, siltstone, and shale. The sandstone varies from subarkose to quartz sublithic arenite although there are some beds of arkose and quartz arenite (Kalliokoski, 1982). Rounded-tosubrounded, very-fine to coarse sand grains of quartz, feldspar, and lithic fragments occur in massive to cross-bedded, fining-upward sequences. Quartz grains show evidence of volcanic and metamorphic origin. Ripple marked bedding surfaces and cross-bedding are common in some localities. The sandstone varies in color from red to a cream-white or purplish-red color; creamwhite color occurs as spherical reduction spots and layers that tend to follow bedding or fractures. Conglomerate is more common in localities near the Keweenaw Fault or near the unconformable contact. Near the Keweenaw Fault, pebble to boulder sized clasts in the conglomerates are composed of felsic and mafic volcanic rocks, similar to Keweenaw Supergroup lithologies. Near the unconformable contact, clast lithologies are of locally derived chemically resistant debris such as quartz and iron formation. There are no interbedded volcanic rocks or cross-cutting igneous dikes within the Jacobsville Formation and while the older age is constrained the upper age is not. The 10 �Jacobsville Formation is approximately 1.06-1.04 Ga to 1.03? Ga (Cannon, 1992). Midcontinent Rift Structure The last episode of the MCR was characterized by a compression of the continent. This compression transformed original graben-bounding normal faults into reverse faults, reactivated other extensional rift-related faults/fractures, and produced new compression-only faults/fractures and folds. Cannon et al. (1993) have determined that compression occurred at about 1,060+/-20 Ma. The probable cause of this event was continental collision along the Grenville front (Fig. 1) beginning as early as 1.08 Ga and ending by 1.04 Ga (Cannon, 1994; Cannon and Hinze, 1992; Hoffman, 1989). The rift-filling Keweenawan Supergroup strata dips moderately northwesterly toward the center of the rift (Lake Superior) (Figs. 3 and 7). Their dip angles increase toward the exposed stratigraphic base which is truncated by the Keweenaw Fault. The present day dip of the strata within the MCR is a combination of syn-depositional downwarpage and tilting in response to reverse movement along Eagle Harbor 28 3 area of fissure deposits Strike and dip of bedding Major copper deposits 24 Eagle River 30 1 Fault or fissure 28 U=up thrown side D=down thrown side U D 2 50 3 4 26 5 6 3 7 Calumet 83 8 19 9 5 Hancock Houghton 11 N 10 0 12 10 20 58 Kilometers 1 Name of Deposit Given in Table 2 Figure 7: Simplified geologic map showing the location of the major deposits within the Keweenaw Peninsula native copper district, Michigan. Table 2 provides the names and production for the numbered deposits. The areas shown on the map are the mined out down-dip portion projected to the surface. All of the native copper mines are hosted by the Portage Lake Volcanics. Modified from Bornhorst and Barron (2011). Table 2: Production from 1845 to 1968 of refined copper from native copper deposits (after Weege and Pollock, 1971). 11 �Million lbs Produced Refined Copper Location Number Shown on Figure 7 Calumet & Hecla Conglomerate 4,229 7 Kearsarge Flow Top 2,263 3 Baltic Flow Top 1,845 12 Pewabic Flow Top 1,077 9 Osceola Flow Top 578 8 Isle Royale Flow Top 341 10 Atlantic Ashbed 143 11 Allouez Conglomerate 73 6 Houghton Conglomerate 38 4 Kingston Conglomerate 20 5 Greenland-Mass Subdistrict 72 See Figure 3 Other Flow Top and Conglomerate Deposits 137 Cliff Fissure 38 1 Central Fissure 53 2 Other Fissure Deposits 123 Name of Deposit District Total 11,030 the Keweenaw Fault produced by continental compression. Bedding in the rift-flanking Jacobsville Sandstone dips less than 5O in most areas, except near the Keweenaw Fault, where dips steepen in response to drag along the fault. Compression-related deposition produced the Jacobsville Sandstone. There are many faults/fractures in the Keweenaw Peninsula. Some of these were exclusively formed during extension of the MCR when graben-bounding normal faulting was prominent along the margin (Fig. 3). However, most faults/fractures were likely reactivated by or related to the compressional event that inverted the major graben-bounding fault, the Keweenaw Fault, into an overall high-angle reverse fault (Cannon et al., 1989; White, 1968). The Keweenaw Fault strikes and dips more or less parallel to the bedding of the truncated Portage Lake Volcanics (Fig. 7) and is not necessarily one fault, as it is a zone with branches up to 0.8 km from the main fault (Butler and Burbank, 1929). Although the Keweenaw Fault would make an ideal conduit for movement of hydrothermal fluids, there are no native copper deposits along it similar to other ore bearing districts where the main faults are not well mineralized. The rocks within and adjacent are altered especially by paragenetically late hydrothermal fluids. Several reverse faults occur oblique to the strike of bedding. In the Eagle River area, high-angle faults with displacement from 0 to 200 m, faultcontrolled native copper veins are common (Butler and Burbank, 1929). The Allouez Gap fault bisects the largest lava flow top hosted native copper deposit in the district and was likely a significant conduit for native copper mineralizing hydrothermal fluids (Bornhorst, 1997). The Allouez Gap fault may have been a reactivated original rift fault. Faults were the principal pathway for the upward movement and focusing of ore fluids into the stratabound lava flow tops in the Baltic and Isle Royale deposits (Broderick, 1931) as well as those in the Greenland-Mass subdistrict (Field Trip 2, this guidebook). Faulting occurred before, during and after deposition of native copper along 12 �with associated alteration minerals based on fault brecciation and re-cementation of alteration minerals. There is a close relationship between faulting/fracturing produced by or reactivated by compression. These compressional structures acted as pathways for native copper mineralizing hydrothermal fluids. Broad open synclines and anticlines, with wavelengths of around 10 km and various orientations, are superimposed on the regional dip. Faults with displacement and mineralized tension breaks are common near the crests of anticlines (Butler and Burbank, 1929). These post-depositional folds are likely related to the Keweenaw Fault (White, 1968). Keweenaw Peninsula Native Copper District Active copper mining occurred from 1845 to 1968 in the Keweenaw Peninsula native copper district. The estimated pre-mining geologic resource for the district is 19.7 billion lbs of copper and small quantities of temporally and spatially associated native silver (Bornhorst and Barron, 2011). The major ore producing horizons are located in a 45 km-long belt in the Keweenaw Peninsula (Figs. 5 and 7) and in a subdistrict to the southwest (Field Trip 2, this guidebook). Accompanying native copper and silver, the only economic metallic minerals, were a suite of nonmetallic alteration minerals (Fig. 8). Sulfide minerals are uncommon in the native copper deposits; chalcocite only occurs in trace amounts. Pyrite, an acid-producer when exposed to oxygenated waters, is absent. Several chalcocite deposits of unknown connection to the native copper deposits are hosted by the stratigraphically older Portage Lake Volcanics; the largest of these contains roughly 230 million lbs of copper (Maki and Bornhorst, 1999). These will not be discussed here. Native Copper Ore Bodies Ore bodies in the Keweenaw Peninsula are tabular, stratabound concentrations of native copper in Portage Lake Volcanics host rocks with sufficient original porosity including brecciated and amygdaloidal flow tops (58.5% of production) and interflow conglomerate beds (39.5% of production). Secondary porosity occurs along fractures/faults host veins (about 2% of production). Since the deposits represent important stratigraphic horizons, the host rocks were given informal member names (Butler and Burbank, 1929). Several mines with different names often worked the same deposit or same lithostratigraphic unit. About 85% of the total district production came from four deposits: Calumet and Hecla Conglomerate, top of the Kearsarge lava flow, top of the Baltic lava flow, and the top of the Pewabic lava flow. The most common host rocks for native copper deposits are brecciated flow tops (fragmental amygdaloid) as their original porosity was typically much greater than vesicular (amygdaloidal) flow tops (White, 1968). The stratabound flow top deposits are “sandwiched” between a footwall consisting of the same flow as the mineralized flow top and hanging wall of barren massive basalt interior in the succeeding lava flow. Native copper is often more abundant near the top and bottom of the brecciated/fragmental amygdaloid interval of the flow top, however, in rich ore shoots, the entire brecciated/fragmental amygdaloid flow top contains significant amount of copper. As brecciated/fragmental amygdaloidal grades downward into massive basalt, it becomes deficient in native copper. In some cases, ore shoots are located in tongues of brecciated flow tops within massive basalt (Weege and Schillinger, 1962). The lateral and vertical distribution of 13 �brecciated/fragmental amygdaloid within the top of the lava flow is irregular and hence, so is the grade of copper. In general, mined stope heights are from 3 to 5 m. Ore shoots are elongate, but also occur in a wide variety of shapes, with widths of 30 to 150 m and down dip lengths from 50 m to 600 m (White, 1968). The strike length for major ore bodies ranges from 1.5 to 11 km with down dip mineralization extending from 1.5 to 2.6 km below the surface on the inclined deposit. (Butler and Burbank, 1929; White, 1968). Although interflow conglomerate beds make up only a small volume of the Portage Lake Volcanics, about 40 % of the district productions were hosted by them. These deposits were also tabular and stratabound, just like the flow top deposits. They are “sandwiched” between a footwall consisting of the top of the underlying lava flow and hanging wall of barren massive basalt interior in the underlying lava flow. The porosity of underlying brecciated/fragmental amygdaloid lava flow top is greatly decreased by silt and sand filling the origin open space between fragments. Native copper tends to be concentrated along specific stratigraphic bands that are 0.5 to 5 m thick (Weege et al., 1972). Interflow conglomerates overall host a significant fraction of the districts production. Specifically, the Calumet and Hecla Conglomerate was by far the largest single native copper deposit producing 4.2 billion lbs, as compared to the next largest deposit hosted by the Kearsarge flow top which produced 2.3 billion lbs (Fig. 7 and Table 2). The Calumet and Hecla Conglomerate was mined along a strike length of 4.9 km, and down-dip 2.8 km. The productive area corresponds to a thickening of the conglomerate from less than 1 m up to 6 m (Butler and Burbank, 1929; Weege et al., 1972). Ore grades decrease with depth where the width of the conglomerate is greater where essentially the same amount of copper is distributed throughout a greater volume. (Butler and Burbank, 1929). The highest grades correspond to beds where there is relatively little fine interstitial material or where interstitial spaces are filled with coarse sand or small pebbles (Weege et al., 1972). Thus, localization of native copper ore is dependent on sedimentary environmental factors. The first mines in the district were developed on tabular steeply dipping deposits that cross cut bedding at high angles. The veins have widths of up to 3 m or more (Butler and Burbank, 1929). Veins are not single tabular bodies, but rather a series of parallel of anastomosing filled open spaces. While brecciation within the deposit is common, gouge is not present (Butler and Burbank, 1929). These adjacent lava flow tops and conglomerates are mineralized. The distribution of native copper in veins is more erratic than in either lava flow top or conglomerate deposits. The richest ore veins tend to be spatially associated with the intersections of the vein and well-oxidized lava flow tops (Butler and Burbank, 1929). Native copper occur as both finely disseminated with associated alteration minerals and as masses weighing many tons. These vein deposits are of slight economic importance in the district. Several small vein deposits are localized just beneath the thickest basalt flow in the district. A good example is the Greenstone flow in which hydrothermal fluids moved up along the cross fractures until blocked by the very thick impermeable massive interior of the Greenstone Flow. 14 �There are veins spatially and genetically associated with the stratabound lava flow top or conglomerate deposits; these veins occur along faults that intersect major deposits such as the Baltic and Isle Royale (Broderick, 1931). This suggests that ore fluids moved upward along faults and outward into the permeable flow tops. The intersection of subsidiary faults with locally thick permeable horizons is a key factor in concentrating ore such as the Kearsage deposit (see Stop 4 and Fig. 12). White (1968) suggested that for the movement of ore fluids to occur, permeability due to fracturing was more important than primary permeability. Faults and small fractures cutting massive interior of lava flows were likely important for upward transport of ore fluids also. Overlapping of successive lava flows and minor unconformities suggests that simple up-dip movement of ore fluids was not likely without a network of fractures. Secondary Hydrothermal Minerals The rocks within the Keweenaw Peninsula native copper district were pervasively altered by lowtemperature, low-pressure hydrothermal/burial metamorphic fluids. Alteration was most intensely associated with the native copper deposits, although to some degree, secondary hydrothermal minerals occur in all rocks of the Portage Lake Volcanics. Areas in the Keweenaw Peninsula more distal to the native copper deposits were less altered. The intensity and degree of alteration also varies as a function of position within lava flows; the massive interiors of lava flows being much less altered whereas the lava flow tops are relatively more altered. Lava flows in close proximity to cross cutting features tend to be more altered. The minerals occur as amygdule and vein fillings, and as whole rock replacements. Within the Portage Lake Volcanics, some original igneous minerals are present in the massive interiors of flows, but secondary minerals exist in the massive interiors of all flows regardless of their thickness. While the massive interiors of lava flows contain secondary minerals, their original igneous geochemical composition is often only slightly modified by secondary hydrothermal processes. There are over 100 different secondary alteration minerals in the Keweenaw Peninsula; most of them are related to hydrothermal process and some are related to supergene processes. Only about 24 alteration minerals are common. Native copper with small quantities of native silver represents over 99% of the metallic minerals in the mined ore bodies of the district. Most of the native copper carries a small amount of arsenic in solid solution (typically less than 0.2 % arsenic in total copper + silver + arsenic; Broderick, 1929). Copper-nickel arsenides occur in veins that are paragenetically late (Moore, 1971; Stoiber and Davidson, 1959; Butler and Burbank, 1929). Within the native copper deposits, paragenetically late chalcocite occurs as small veins cutting lava flow top deposits, and as coatings on joints containing calcite in conglomerate deposits (White, 1968). 15 �Flow Top Deposits and Veins Microcline Chlorite Epidote Pumpellyite Prehnite Native Copper Datolite Silver Ankerite Quartz Sericite Calcite Arsenides Sulphides Albite Adularia Chlorite Laumontite Analcite Sulphates Conglomerate Deposits Little Little (barite, anhydrite, gypsum) Relative Age Abundant Relative Age Not abundant Figure 8: Paragenesis and relative abundance of secondary hydrothermal alteration minerals in the Keweenaw Peninsula native copper district. Modified from Butler and Burbank (1929). District-wide there is a well-defined mineral paragenesis (Fig. 8), although individual deposits may not exactly follow the district-wide timing of precipitation. There is a general spatial variation of hydrothermal minerals in the Calumet area of the district (Fig. 9). Epidote and the appearance of quartz are spatially associated with major native copper deposits (Stoiber and Davidson, 1959). A detailed study by Stoiber and Davidson (1959) of the Kearsarge deposit shows that native copper is much more irregularly distributed than secondary mineral zones, but there is a general correlation with the abundance of native copper associated with the variation of quartz and microcline (see Stop 4 and Fig. 14). On the tip of the Keweenaw Peninsula, the suite of hydrothermal alteration minerals consists of low temperature zeolite minerals except within about 750 m of the Keweenaw Fault where there is epidote (Cornwall, 1955; Cornwall and White, 1955). Amygdule-filling minerals are equivalent to zeolite and prehnite-pumpellyite metamorphic facies. The hydrothermal /metamorphic mineral zones dip more gently towards Lake Superior than the strata, implying that the strata were tilted prior to hydrothermal alteration (Livnat, 1983; Broderick, 1929). The paragenetic succession of alteration minerals at the Kearsarge deposit begins with low-temperature (80 to 100oC) agates followed by higher temperature native copper and temporally associated minerals (about 225OC) and the final stage is superimposition of lower temperature late-stage barren laumontite and calcite. This progression represents a waxing and waning of the hydrothermal system. Much later, the native copper has been altered by oxidized groundwaters generating copper oxide minerals. Native copper mineralization is younger than the Copper Harbor Conglomerate, which hosts rare veins of calcite and native copper (see Stop 6). White (1968) interpreted the age of native copper mineralization as after the deposition of parts or all of the Freda Sandstone. Minor amounts of 16 �native copper occur within the lower beds of the Jacobsville Sandstone near Rice Lake. Based on field relations, hydrothermal alteration is younger than deposition of rift-filling strata and at least some of the rift-flanking Jacobsville Sandstone. The absolute age of hydrothermal alteration is between 1060 and 1047 Ma (+/- ~ 20 Ma) (Bornhorst et al., 1988). This age is consistent with the approximate age of 1060 Ma for regional continental compression that caused reverse faulting along the Keweenaw Fault (Cannon et al., 1993). Thus, the age of hydrothermal alteration is about 1070 to 1040 Ma contemporaneous with regional continental compression and some 10 to 30 million years after eruption of the Portage Lake Volcanics. Calumet Cross Section Geographic Position Top of Portage Lake Volcanics ~ 200 oC Stratigraphic Position of Native Copper Mines %of District Production Epidote & quartz present 10%Quincy 38%C&H 500 ~200 to 250 oC 5%Osceola 21 %Kearsarge 0 250 to 300 oC Disappearance of ferrian prehnite ~ 3%Isle Royale 17%Baltic No actinolite oC < 300 to 325 ~ KeweenawFault Figure 9: Distribution of prominent secondary hydrothermal alteration minerals in the Portage Lake Volcanics in a cross-section in vicinity of Calumet at the center of the major deposits of the Keweenaw Peninsula native copper district. Data compiled from Livant (1983) and Stoiber and Davidson (1959) and modified from Bornhorst and Rose (1994). Genesis of the Native Copper Deposits Native copper occurs throughout the MCR in Wisconsin, Minnesota, and Ontario (Fig. 2) which suggests a regional distribution of mineralizing hydrothermal waters. The regional Cu-bearing hydrothermal fluids can be best explained by their generation during burial metamorphism of rift-filling basalts with temperatures reaching a thermal maximum 10 to 30 million years after the end of widespread rift magmatism (Fig. 3). The coincidence of regional continental compression with the thermal maximum provided an integrated paleohydrologic system through reactivated and new faults and fractures. This allowed the upward movement of hydrothermal fluids to 17 �focus in sites of future copper deposits at the very time of greatest fluid availability (Bornhorst 1997). During generation of the regional hydrothermal ore fluids, a few ppm of copper was leached from the rift-filling basalt strata (Jolly, 1974; White, 1968; Stoiber and Davidson, 1959). Simple calculations demonstrate that only a 3 km down dip volume of the 10 km thick rift-filling basalts along the 45 km strike length of the major copper deposits need be leached to generate sufficient copper for deposition, or in other words, the rift-filling basalts are a viable source rock for the copper. The hydrothermal fluids were low in sulfur since most of the sulfur in the buried rift-filling source rocks was degassed during eruption so subsequent deposition up dip in the same sulfurpoor rocks preferentially lead to deposition of native copper rather than copper sulfides. Precipitation of native copper was caused by mixing of ore fluids with cooler, more dilute shallower fluids, ore fluid-rock reactions, and cooling of ore fluids. The localization of large native copper deposits within the Keweenaw Peninsula rather than elsewhere in the MCR may be controlled by favorable geometric orientation within the regional continental compression stress field. Since much of the MCR strata is buried, perhaps another area of native copper deposits remains hidden. Phanerozoic The last events in the geologic development of the MCR in the Keweenaw Peninsula were the formation of the native copper deposits and deposition of the Jacobsville Sandstone during regional continental compression at 1.06 to 1.04 Ga; the Jacobsville deposition may have continued after compression until 1.03? Ga (Fig. 3). The Keweenaw Peninsula was subsequently subjected to a 500 million year period of erosion, from about 1.03 Ga to 0.5 Ga, 500 Ma and multiple kilometers of rock were eroded exposing the native copper deposits at the surface (Fig. 3) (Bornhorst and Robinson, 2004). Downward percolating groundwaters supergene altered native copper and produced a suite of including cuprite, tenorite, malachite, and chrysocolla. The rocks of the Keweenaw Peninsula were subsequently buried by Paleozoic sedimentary rocks associated with the Michigan basin beginning about 500 Ma (Fig. 3). Over the past two million years, the Keweenaw Peninsula was subjected to several continental glacial periods which removed all of the overlying Paleozoic sedimentary rocks with the exception of a Paleozoic outlier slightly south (Fig. 3). After the last glacial episode, the native copper deposits were exposed at roughly the same erosional level as at 500 Ma or the end of the Precambrian. The continental glaciers sculpted the bedrock of the Keweenaw Peninsula and when the last glacier retreated about 10,000 years ago, it left behind a variety of unconsolidated glacial-related sediments that included entrained boulders of native copper. The glaciers carved out the topographic low the Lake Superior basin corresponding to the less competent clastic sedimentary rocks under the center of the MCR. After the glaciers retreated, very large volumes of water filled this topographic low and initially all but the highest land elevations were underwater of a large glacial lake. The glacial lake levels successively dropped over time to the current level of Lake Superior (Farrand 1960). As the lake levels receded humans populated the area. Objectives of Field Trip 18 �This field trip is designed to provide a geologic overview of the Keweenaw Peninsula and the Keweenaw Peninsula native copper district. The Mesoproterozoic MCR bedrock and hosted native copper deposits are unconformably overlain by unconsolidated Pleistocene glacial sediments. The descriptions and stops of this field trip provide glimpses into both of these distinct geologic events. The MCR rocks and native copper deposits are the focus of this field guide. Figure 10 provides the regional geologic setting of the Stops. Figure 10: Geologic map of the far western part of the Upper Peninsula of Michigan showing field trip stops. 19 �Stop 1: Razorback Center Directions: Drive west through downtown Houghton on US-41to south M26. Drive 0.9 miles (1.4km) to Sharon Ave. at first stoplight. Turn left on Sharon Ave. for 0.1 miles(0.2km) to Razorback Dr. Turn right on Razorback Dr. for 0.1 miles (0.2km) to strip mall on left built on top of a small hill. Turn left and outcrop exposure is behind the strip mall. [UTM 5218745N 160379747E (NAD27 CONUS)] Figure 11: Photograph of the rock cut at Razorback Center looking northwest. The rock cut at the edge of the parking lot in the rear (south side) of Razorback Center, Houghton provides an excellent example of the characteristics of subaerial basalt lava flows that comprise the Portage Lake Volcanics, the host rock unit for native copper deposits of the Keweenaw Peninsula native copper district and the rock unit that holds up the spine of the Keweenaw Peninsula (Fig. 10). This of exposure of subaerial lava flows is located stratigraphically between the Calumet and Hecla and Kingston Conglomerates (Fig. 11). It is also located between the stratigraphic level of the Isle Royale and the Quincy Mines. The lava flows strike about N30oE and dip about 55o to the northwest (towards Lake Superior). The rock cut is at an oblique angle to the strike of the lava flows. When facing the exposure, the stratigraphic top is towards the northwest or toward the right. At the far eastern end, or far left, only the amygdaloidal top of the oldest basalt lava flow in this 20 �rock cut is exposed. Stratigraphically upwards, towards the west/right, this flow top is overlain by a thick section of dark-gray to black massive basalt representing the interior of a prominent lava flow. Progressively, the abundance of amygdules increases upwards and the color of the basalt changes to greenish tones reflecting an increased degree of alteration. This zone represents the amygdaloidal top of the prominent lava flow in this rock cut. The amygdules tend to be concentrated along layers and near the upper contact; they coalesce into a continuous now filled open space. The contact between the amygdaloidal top of this prominent lava flow and the massive basalt of the overlying flow is well exposed (Fig. 11). Along most of the exposed contact, amygdaloidal basalt lies directly below the massive basalt indicating the lava flow had a smooth top (pahoehoe lava flow), however, at the level of the parking lot, the planar contact bends and there is a small zone of brecciated flow top. The entire cross section of the prominent lava flow is exposed in the Razorback Center rock cut. A typical lava flow in the Portage Lake Volcanics is between 10 to 20 m thick, the prominent lava flow at Razorback Center. Stratigraphically further upwards, towards the west, the prominent lava flow is overlain by a thick section of dark-gray to black massive basalt representing the interior of the overlying flow (Fig. 11). On the far western end, there is amygdaloidal basalt representing the top of this overlying flow; an almost complete cross section of this flow is exposed here. Volcanic textures and structures at Razorback Center are typical of subaerial lava flows within the Portage Lake Volcanics. The basalts are mainly olivine tholeiites erupted as thick, ponded subaerial lava sheets. The very top and bottom of such lava flows typically consist of aphanitic chilled basalt. The contact between the underlying and overlying lava flows occurs where amygdules disappear abruptly and the overlying flow consists of massive basalt. The upper surface of the main flow was brecciated slightly by movement of lava after the formation of an upper crust, but rapidly grades downward to a non brecciated, highly vesicular flow top. The layered nature of amygdules in the prominent flow here at Razorback Center is likely a result of preferential accumulation of vesicles along laminar flow planes. The flow top breccia is laterally discontinuous for this flow. Slow cooling of the lava flow caused solidification toward the flow interior at a rate which allowed development of subophitic to ophitic textures (large oikocrysts of clinopyroxene enclosing a felted framework of An-rich plagioclase and intergranular olivine). The resulting massive, non-vesicular flow interior constitutes about two-thirds of the flow. The effects of regional hydrothermal alteration can be observed within the amygdaloidal flow tops. The massive interiors are much less altered except along fractures. The original plagioclase in the massive basalt has been replaced by albite and the mafic minerals by chlorite, pumpellyite, and iron oxides. The massive interior of the flow is much less altered than the flow top which represents a relatively impermeable horizon in the paleohydrologic system except in the vicinity of selected fractures. The pseudomorphic alteration minerals in the massive interior of the basalt are similar to those which fill the amygdules. The amygdules here are filled with a variety of secondary minerals including: calcite, chlorite, epidote, prehnite, pumpellyite, quartz (not in order of abundance), and traces of native copper. Late stage laumontite abundantly fills some amygdules. Stop 2: Float Copper US-41 Calumet 21 �Directions: Get back onto M26 at the traffic light and turn right. Stay on north M26/US41 and cross the bridge over to Hancock. Drive through downtown Hancock and continue 10 miles (16km) north to Calumet. Float copper is located on the left side of US41/M26 past the first traffic light just before Red Jacket Rd. [UTM 5233057N 160390423E (NAD27 CONUS)] A float copper boulder weighting 4,263 kg (9,392 lbs) is on display at Stop 2 (Fig. 10). This mass of glacially transported native copper was found in 1970 about 4.5 miles SW of Calumet in less than three feet of surficial sediments. Native copper deposits of the Keweenaw Peninsula were exposed at the bedrock surface at the time of the last period of Pleistocene glaciations. The glacial ice plucked masses of malleable native copper from the tabular lodes and fissures which were subsequently smoothed and flattened by abrasion from other rocks carried by the glacial ice. When the glaciers retreated about 10,000 years ago, unconsolidated rock debris (rounded boulders to clay sized material) were left behind by the melting ice including masses of native copper such as this one “floating” among the unconsolidated rock debris. While some of the rocks in the glacial deposits are from far north of the Keweenaw Peninsula, most of them are recognizable as from local MCR strata exposed in the Keweenaw Peninsula. The large float copper masses could not have moved far from their source, but smaller masses have been transported quite far and have been found in Lower Michigan and Wisconsin. The largest known float copper was discovered in the early 2000s and weighed about 25 tons (50,000 lbs) near the Houghton County airport; it was cut into smaller masses and sold to be smelted and refined. Most pieces of float copper are small, ranging from a few to 50 cm across. The famous example of float copper was the Ontonagon boulder, a 3,700 pound specimen visited by numerous explorers and finally removed from the Keweenaw to the nation’s capital in 1843. The Ontonagon boulder is part of the Smithsonian’s collection. This and other float copper masses have been surface altered by oxygenated groundwater and shallow precipitation since the glaciers retreated. This surface alteration consists of forms of copper including cuprite (copper oxide; Cu2O), tenorite (copper oxide; CuO), malachite (hydrated copper carbonate; (Cu2(CO3)(OH)2) and rarely azurite (hydrated copper carbonate, (Cu3(CO3)2(OH)2). Even when small cm sized masses of float copper are cut, the typical surface alteration is less than one mm thick; native copper is highly resistant to surface weathering. Float copper makes an attractive decorator specimen when a part of the surface is polished and buffed exposing shiny copper color. The basalt mine rock buildings are part of the Keweenaw National Historical Park. The park was established on October 27, 1992, by U. S. Congress Public Law 102-543. The enabling legislation ascertained that the Keweenaw was nationally significant because of: its unique geology; the prehistoric use of its copper by Native Americans; the importance of the region as a leading copper producer and developer of new technologies; its long history of corporate paternalism; and because it became home to so many European ethnic groups that migrated to the United States. Older mining districts typically had only single-industry economies and when the mines shut down, the communities suffered major contraction. In 1910, nearly 40,000 people resided within a few miles of Stop 2 whereas now, fewer people live in all of Houghton County. 22 �The idea that maybe the future of Calumet resided in its past was generated in the late 1980s; history could be “sold” to revitalize the community by increasing tourism. The national park itself only owns a few structures in Calumet, including these, and instead relies on public and private partners termed Keweenaw Heritage Sites. The heritage sites contain and interpret significant cultural and/or natural resources that together with park assets help tell the story of copper mining in the Keweenaw Peninsula. The Quincy Mine property on the edge of Hancock and the A.E. Seaman Mineral Museum on the campus of Michigan Tech are two among multiple Keweenaw Heritage Sites. Stop 3: Bumbletown Hill THE ROCK PILES DESCRIBED FOR THIS STOP ARE ON PRIVATE PROPERTY AND PERMISSION IS REQUIRED ACCESS THEM. Directions: Continue on US-41 3.6 miles (5.8 km) past headquarters of the Keweenaw National Historical Park denoted by park sign and large specimen of glacial float native copper to Bumbletown Rd. and turn left (west). Drive about 0.4 miles (0.6 km) to rock pile. [UTM 5237960N 160393345E (NAD27 CONUS)]. To access the overlook leave the rock pile and continue on Bumbletown Road west about 0.5 miles (0.8 km) to overlook at the top of the hill near towers. [UTM 5238215N 160392860E (NAD27 CONUS)] The description of this stop is reproduced from Bornhorst and Barron (2011). The Allouez conglomerate (informal member) is one of a small number of interflow clastic sedimentary horizons within the Portage Lake Volcanics visible in the rock pile at this stop (Fig. 10). This particular conglomerate bed can be traced along strike from the tip of the Keweenaw Peninsula, to at least the Mass area, a strike length of more than 120 km (Fig. 6). The Allouez conglomerate is stratigraphically just below the Greenstone flow, arguably the largest basalt flow in the world, within the Portage Lake Volcanics. The rock piles at the base of Bumbletown Hill are from the Allouez Mine. The Allouez conglomerate consists of mostly red-colored conglomerate with lesser amounts of sandstone and siltstone. The largest contained boulders at this locality are about 65 cm in diameter and the median size is about 8 cm. A pebble count of boulders more than 20 cm across gave the following results: mafic rock, mostly amygdaloidal basalt, 16%; quartz porphyritic rhyolite, 36%; feldspar porphyritic rhyolite, 11%; and granophyre, 37% (White, 1971). The mines on the Allouez conglomerate yielded only about 75 million pounds of refined copper (Table 2). Some evidence of native copper mineralization can be seen in rocks at this stop. Occasionally, one can find a specimen with native copper filling the void space between clasts and grains. Calcite and chlorite are the dominant pore-filling secondary minerals visible on this rock pile. Thin black veinlets cutting the Allouez conglomerate consist of calcite with chalcocite “dust.” While supergene alteration resulting from the downward percolation of groundwater is not common in most the native copper deposits, at this stop, supergene alteration minerals are common including chrysocolla, malachite, and cuprite. From the overlook on a clear day, Isle Royale may be seen 80 km to the northwest and the Huron 23 �Mountains may be seen beyond Keweenaw Bay, 60 km to the southeast. The land slopes very gradually to the northwest toward Lake Superior, as it does throughout most of the length of the Keweenaw Peninsula. The area is underlain mainly by conglomerates and sandstones of the Copper Harbor Formation dipping at about 20o. The southeastern flank of the Keweenaw Peninsula has a steeper slope at the skyline, following approximately the line of the Keweenaw fault. The low-lying plain between the fault and Keweenaw Bay is underlain by flat-lying Jacobsville Sandstone. Bumbletown Hill is located on the southwest side of the Allouez Gap, a NW- to SE-trending valley. The valley follows the Allouez Gap fault, a zone of faults and fractures, along which the Portage Lake Volcanics and Keweenaw fault, are offset. At this gap, the strike of the Portage Lake Volcanics swings from about N35oE to N50oE (Figs. 5 and 7). Almost every permeable horizon near the Allouez Gap fault contains above average amounts of native copper; nowhere else in the district are there so many mineralized beds (Fig. 7). About 60% of the district production can be linked to the fault as a primary pathway for ore fluids. The fault bisects the Kearsarge deposit (see Fig. 12), which was the second largest copper producer in the native copper district. The line of rock piles demarking the many mines along the Kearsarge deposit is a little more than 1,500 m southeast of Bumbletown Hill. The Kingston Mine, a small deposit that produced 20 million pounds of copper (1963 to 1968; one of the most recent native copper mines to open and last to close), is bisected by the Allouez Gap fault. About 1,200 m N65oE of the hilltop, the Houghton conglomerate and the Iroquois flow produced 33 million pounds of copper. Looking northeast along the strike of the Portage Lake Volcanics, one can see the cuesta form of the ridge upheld by the Greenstone flow. To the right of the ridge, the more distant hills are formed by lava flows lower in the Portage Lake Volcanics sequence. At Bumbletown Hill, the Greenstone flow is only 85 m thick, but the flow thickens abruptly to more than 400 m near end of the cuesta ridge. It dips northward at about 25o toward the center of the Lake Superior. The Greenstone flow can be traced along much of the Keweenaw Peninsula and has been stratigraphically and geochemically correlated with a similar unit on Isle Royale, 90 km away, on the opposite side of the rift. Thus, the areal extent of this great flow exceeds 5,000 km2, and its volume is on the order of 800 to1,500 km3 (Longo, 1983). The geochemical composition of the Greenstone flow magma is more evolved than typical olivine tholeiites of the Portage Lake Volcanics. 24 �Stop 4: Seneca Mine Rock Pile THE ROCK PILES DESCRIBED FOR THIS STOP ARE ON PRIVATE PROPERTY AND PERMISSION IS REQUIRED ACCESS THEM. Directions: Drive back to US41/M26 on Bumbletown Rd. and turn left. Continue northeast on US41/M26 0.6 miles(0.9km) to B St. Turn Left on B St. Drive about 0.3 miles(0.5km) to rock pile. [UTM 5238775N 160394119E (NAD27 CONUS)] The Kearsarge lode was worked by the Seneca Mine, one of multiple mines which produced native copper from the top of the Kearsarge basalt lava flow over a strike length of more than 12 km and down-dip as much as 2,500 m (Figs. 10 and 12). About 1,026 million kg of refined copper were produced at an average grade of 1.05% Cu, making the Kearsarge deposit the largest flow top hosted deposit and the second largest producer in the district behind the C&H Conglomerate mines (Table 2). Production of copper from the Kearsarge lode began in 1887 and stopped in 1967. The Kearsarge lava flow has been recognized for a distance of about 55 km along strike and dips between 35 and 40o NW (Fig. 12). It lies directly above the Wolverine Sandstone (Fig. 6). The amygdaloidal and/or brecciated top of the Kearsarge flow ranges from near zero up to 10 m in thickness. The productive top has an average thickness of around 2 m and consists of brecciated basalt (individual fragments of amygdaloidal basalt are generally less than 15 cm in greatest dimension). The brecciated basalt grades downward into amygdaloidal basalt with amygdules concentrated in layers. Further downward, the top grades into a zone of fewer and larger amygdules, and then into aphyric massive basalt in the interior of the flow. Just below the brecciated and/or amygdaloidal top of the flow, there is distinct plagioclase porphyritic basalt. The abundance and size of the plagioclase phenocrysts in this zone is variable, but they can make up a large percentage of the rock, with phenocrysts up to 2.5 cm in length. This zone is probably the result of plagioclase in situ floating during surface crystallization of the flow. Specimens with abundant plagioclase phenocrysts can be found on this rock pile. The basalt itself in the Kearsarge flow is well oxidized. Albitized and pumpellyitized basalt consists of pseudomorphically replaced plagioclase set in a fine-grained to cryptocrystalline groundmass. Original igneous minerals were replaced in areas where alteration was intense. Olivine is almost invariably completely replaced while other igneous mineral are replaced by alteration minerals to varying degree. The amygdule and interfragmental space-filling gangue minerals in the Kearsarge lode are generally (in order of most to least abundant): calcite, epidote, K-feldspar, quartz, and lesser amounts of chlorite, prehnite, pumpellyite, laumontite, and sericite. Native copper is closely associated in time and space with the secondary amygdule minerals (Stoiber and Davidson, 1959). Paragenetically, chlorite; epidote; microcline; and prehnite are early-formed minerals, and the latest-formed minerals are quartz; native copper; calcite; and chlorite (Fig. 14). A zonal stratabound arrangement of amygdule minerals in the Kearsarge deposit is seen in the Ahmeek Shaft No. 3 (Fig. 15). The zoning may be explained by deposition of secondary minerals from a hydrothermal solution moving along a permeable channel. 25 �SW NE Productive Area 90 Thickness 60 meters 30 Top of Wolverine Sandstone Location along strike South Centennial Kearsarge North 1 2 2 Wolverine Kearsarge 1 4 3 2 11 2 3 Allouez Gap Fault Seneca 2 3 Mohawk Ahmeek 2 1 5 3 4 2 1 4 6 Limit mining of Very high grade copper ore 0 Upper limit of quartz 600 1200 meters Lower limit of microcline Figure 12: Thickness of the Kearsarge lava flow from showing the location of the productive area where the top of the flow is thickest. Modified from Butler and Burbank (1929). The most productive area corresponds to the thickest part of the flow which is bisected by the Allouez Gap fault. Bottom diagram is a down-dip strike parallel section project to vertical showing distribution of higher grade native copper ore and occurrence of important alteration minerals. Modified from Stoiber and Davidson (1959). Abundance of quartz in amygdules is greater than 10 % on the down-dip side (lower) of the line shown and K-feldspar is absent on the down-dip side (lower) of the line shown. The Kearsage flow dips about 35 to 40o NW and all data are projected. 26 �Chlorite Microcline Prehnite Hematite Epidote Pumpellyite Quartz Sericite Native Copper Calcite Relative Age Less abundant More Abundant Figure 13: Paragenesis of secondary hydrothermal alteration minerals in the Kearsarge deposit at the Wolverine No. 2 Mine. North South Hanging Wall chlorite Hanging Wall microcline-calcite calcite-epidote calcite-epidote quartz-epidote chlorite Contact between Kearsarge flow top and overlying massive flow bottom 5 0 meters Abundant native copper Mineral Assemblage Band Volume Percent Amygdule Filling chlorite Chlorite Microcline Epidote Calcite Quartz Pumpellyite 100 0 0 trace 0 0 chloritemicroclinecalcite microclinecalcite quartzepidote calciteepidote 69-74 15-25 0-1 0-5 0-5 0-6 0-3 45-82 5-10 0-47 0-8 0-trace 0 0 90-96 0-1 4-9 0 0 0 12 87 1 trace Figure 14: Cross section of the top of the Kearsarge lava flow (amygdaloid) deposit showing the distribution of secondary hydrothermal amygdule-filling alteration minerals at the Ahmeek Mine, 35th level, 400 to 500 ft south of the shaft. Modified from Stoiber and Davidson (1959). Data from the back and walls are projected to a horizontal plane. There is a barren laumontitequartz-calcite zone not shown here. 27 �Chlorite and microcline would have been deposited first, along the outer limits of the solution channel; followed by quartz and epidote in the center of the channel; and finally, deposition of calcite in the remaining openings. This observation is consistent with the paragenetic relationships seen in individual samples. No strict correlation exists between the stratabound zoning and the grade of native-copper mineralization (Stoiber and Davidson, 1959). The amygdule minerals and grade of copper mineralization vary with depth. Within the upper limit of quartz (Fig. 12), the quartz content is typically about 15 % of open space fillings although it is considerably less than 10% at shallower depths. The lower limit of microcline may also mark the limit of significant copper mineralization. The amount of native copper present is much more irregular than variation of the mineralized zones. The Allouez Gap Fault bisects the thickest segment of the Kearsarge Flow along its 55 km strike length (Fig. 12). Higher grades and production occur northeast of the fault where fractures with orientations that parallel the fault are more abundant. Within the Allouez Gap Fault zone, early epidote and quartz were brecciated and recemented by calcite, quartz, and native copper. After another episode of brecciation, the fault zone was recemented again with calcite; quartz; and lesser laumontite (Butler and Burbank, 1929). Movement along the fault occurred before, during, and after deposition of native copper. The fault apparently was a conduit for transport of ore fluids to the permeable flow top. The coincidence of this fault with the relatively thick flow top resulted in the second largest deposit in the district. The Seneca Mine is an excellent locality to study the character of a representative basaltic flow top hosted native copper deposit. Specimens of massive basalt, massive basalt abundant plagioclase phenocrysts,and amygdaloidal basalt can be found on this rock pile. Masses of native copper are readily collectable especially when using a metal detector. Open-space filling minerals (amydgules and between breccia fragments) that occur in the lode can be found on the rock pile. Stoiber and Davidson (unpublished data) made a quantitative analysis of open-space filling minerals for the Seneca Mine rock pile and found open-space filling minerals consisted of: calcite, 57%; red feldspar 8%; pink feldspar 15%; epidote, 17%; prehnite, trace; pumpellyite, trace and quartz, trace. Many specimens contain multiple minerals and illustrate paragenetic relationships. Stop 5: Eagle River Falls Directions: Continue northeast on US-41/M26 9.5 miles (15km) to Phoenix and turn left on M26. Continue 2.3 miles (3.7km) to Eagle River and park by the bridge. [UTM 5251824N 160402193E (NAD27 CONUS)] The water falls of Eagle River is near the contact between the top of the Portage Lake Volcanics and the base of the Copper Harbor Formation (Fig. 10). The contact dips about 30o NNW. The beds strike roughly parallel to the shoreline of Lake Superior; the orientation of the Keweenaw Peninsula changes from NE in vicinity of Houghton to ENE at Eagle River to E-W near the tip. The tholeiitic basalt subaerial lava flows just below the contact are pahoehoe with ropy upper surface. The orientation of the ropes indicates that the flow was erupted from a vent to the north geographically under Lake Superior. That the ropy flow top is preserved suggests that little erosion occurred between deposition of the last of the lava flows of the Portage Lake Volcanics and the Copper 28 �Harbor Formation. The Copper Harbor Formation consists of red-brown rhyolite-pebble conglomerate, but includes many sandstone and even some shale beds. Under the bridge, one can get a good view of the lithology of the lower part of the Copper Harbor Formation. The Copper Harbor Formation was deposited in an alluvial fan shed off of a highland area to the SE (opposite of Lake Superior) likely buried under the rift-flanking Jacobsville Sandstone (Elmore, 1984). This contact marks an abrupt change in the geologic evolution of the Midcontinent rift. Below this contact there is a thick succession of basalt subaerial lava flows with more than 200 individual flows and a cumulative thickness of about 5,000 m, thus magmatic activity dominated the Midcontinent rift at that time. Abruptly above the contact lava flows are strikingly absent and clastic sedimentation dominated the Midcontinent rift. While generally absent, a last gasp of magmatic activity will be seen at Stop 10 where a thin package of mafic to intermediate volcanic rocks, the Lake Shore Traps, are interfingered within the Copper Harbor Formation. Stop 6: Great Sand Bay Directions: Continue driving northeast on M26 through Eagle River for 4.8 miles(7.7km) until the Great Sand Bay overlook. [UTM 5253974N 160406985E (NAD27 CONUS)] The description of this stop is reproduced with minor modifications from Bornhorst and Barron (2011). The Great Sand Bay overlook provides a beautiful view of Lake Superior (Fig. 10). Very large volumes of water filled the Lake Superior basin as a result of melting of the glaciers, turning it into a glacial lake. The levels of the glacial lakes depended on the position of the ice front, outlets, and crustal rebound (a result of removing the weight of the ice). There are 15 lake stages recognized in the Lake Superior basin (Farrand 1960). As the lake levels receded to the current level of Lake Superior, more and more of the Keweenaw Peninsula emerged. At the road level, the sand dunes are remains of the Lake Nipissing Stage (4,000 to 5,000 years ago) when the lake level was about 9 m (30 feet) higher than today. After lake stages at about 3,200, 2,000, and 1,000 years ago, the waters receded toward the present level termed Lake Superior. The underlying bedrock is the Copper Harbor Formation. In the Keweenaw Peninsula there is a succession of basalt lava flows interbedded near the middle of the formation (see Stop 8). The massive interiors of these lava flows are more resistant to erosion than the underlying and overlying conglomerates and sandstones of the Copper Harbor Formation. As a result, harbors such those at Eagle Harbor and Copper Harbor are maintained by lava flows visible at their mouths. While not visible, lava flows occur at the mouth of Great Sand Bay too. There are many extensive underwater fissure vein deposits which cross cut the Eagle River shoals located about 0.5 to 1 km offshore. Many of them are often quite rich in native copper and can contain long continuous stringers protruding up to 1.5 m in height and extending almost 6 meters in length. Most of veins are less than 50 cm in width and are primarily composed of quartz or calcite with minor amounts of laumontite , datolite, prehnite, and traces of silver. Veins will locally contain 29 �clay pockets which can produce well defined copper crystal specimens. The largest copper specimen ever recovered underwater was a massive 17 ton unattached copper boulder in July of 2001. It was recovered from one of these vein deposits north of Jacobs Creek in about 9 m of water. To date, there have been 36 underwater copper veins discovered from the eastern tip of Great Sand Bay to Eagle River, about 3.2 km west. Stop 7: Hebard Park Directions: Continue driving 10.5 miles (16.9km) east on M-26 until arriving at Hebard Park conglomerate exposure on left. [UTM 5258659N 160428890E (NAD27 CONUS)] The description of this stop is reproduced from Bornhorst and Barron (2011). The Copper Harbor Formation is exposed along the Lake Superior shoreline at Hebard Park (Fig. 10) and is stratigraphically above the Lake Shore Traps (Fig. 5). The lithologies at at this stop consist of interbedded conglomerates and sandstones that characterize the Copper Harbor Formation. Clast-supported conglomerate beds consist of rounded, cobble- to boulder-sized clasts with a matrix of coarse sand-sized subangular grains cemented with carbonate and iron oxide. Clasts are predominantly of silicic volcanic rocks, with subordinate basalt, pyroclastic, plutonic, and metamorphic rocks. Several finer grained interbeds higher in the exposed section exhibit crossbeds, current lineations, current ripples, parting lineation, and reduction spots. In particular, one should note the calcite-rich cemented zones that may represent vadose carbonate or paleocaliche (Kalliokoski, 1986). There is a thin continuous zone of laminated cryptoalgal carbonate, laterallylinked stromatolite, that is draped over cobbles and contorted layers in mudstone-siltstone. 30 �Stop 8: Hunter’s Point Park Directions: Continue driving 2.4 miles (3.8km) east on M-26 to North Coast Rd. and then turn left. Drive 0.3 miles (0.4km) to Harbor Coast Lane and turn right. Drive 0.3miles (0.4km), park at the end of the road and walk down to shoreline. [UTM 5258263N 160432253E (NAD27 CONUS)] Lake Superior Copper Harbor Formation Hunter’s Point Park Lake Shore Traps Copper Porters Island N 0 500 feet Copper Harbor Ft. Wilkins State Park Garden Copper Harbor Formation Figure 15: Geologic map of the Copper Harbor area taken directly from Cornwall (1955) showing the location of Hunter’s Point (Stop 8), Brockway Nose (part of Stop 9), and Fort Wilkins Historic State Park (Stop 10). Hunter’s Point Park was established in 2005 when funding provided by the Michigan Natural Resources Trust Fund and many generous private donors (www.hunters-point.org) allowed the land to be purchased. Prior to becoming an official park the point was a popular hiking destination for visitors (Fig. 10 and 15). The land owners subdivided the area for residential housing which would have restricted public access without its conversion into a park. The name of Hunter’s Point is uncertain but it could have been named after A.W. Hunter, an early resident in the town of Copper Harbor who purchased the point from the U.S. Government. The Copper Harbor Formation is overall composed of volcanogenic clastic sedimentary rocks, dominantly conglomerates with lesser sandstone, siltstone, and shale such as observed at Stop 7. These rocks were deposited in a fining upward prograding alluvial fan complex (Elmore, 1984). Typically conglomerates are composed of clasts with a ratio of mafic-to-intermediate+felsic composition of about 2:1 (Daniels, 1982). Towards the tip of the Keweenaw Peninsula, the Copper Harbor Formation is informally subdivided into an inner (land side) “member” and an outer (lake side) “member.” Between these two “members” there is a thin succession of interbedded lava flows collectively known as the Lake Shore Traps. The Lake Shore Traps consist of Fe-rich olivine tholeiite, basaltic andesite, and andesite lava that were erupted during the waning stage of volcanism within the MCR; the youngest flows tend to be more intermediate in 31 �composition. At 1087.2 +/- 1.6 Ma (Davis and Paces, 1990), the Lake Shore Traps are among the youngest magmatism within the MCR. The thickest section of the Lake Shore Traps is about 15 km to the east at the tip of the peninsula. Volcanologically, the lower lava flows are interpreted as erupted as ponded sheets while the upper lava flows erupted on a low positive slope such as a shield volcano. The Lake Shore Traps were subaerially erupted pahoehoe lava flows. At Hunter’s Point, the top of the andesitic lava flows of Lake Shore Traps are conformably overlain by contact conglomerates of the Copper Harbor Formation (Fig. 15) simple geo map of Copper Harbor and Hunter’s point). The strike of bedding is about E-W and dip is about 35o to the north (towards the lake). The orientation of the contact is roughly parallel to the orientation of Hunter’s Point. From the Hunter’s Point parking lot, follow the walkway to beach towards the west side of the point. As the walkway ends, you will be on outcrops of lava flows of the Lake Shore Traps (Fig. 15). Walking to the east, the beach gives way to a rocky shoreline. In erosional coves, you can see contacts between lava flows, represented by vesicular to amygdalodoidal andesitic lava (top of the lava flow) overlain by massive andesitic lava (massive interior of the overlying lava flow). The massive lava flow interiors within the Lake Shore Traps often retain relict olivine and interstitial glass due to the overall low degree of alteration (weathering and hydrothermal). Highly visible red hematitic bands form circular patterns within the massive interior; this banding is interpreted to be the result of alteration. Secondary minerals filling amygdules include agate, chalcedony, quartz, laumontite, analcite, calcite, and smectite in amygdules; this suite of minerals is equivalent to zeolite facies metamorphism. In contrast, in massive lava flow interiors within the Portage Lake Volcanics the olivine and interstitial glass are completely replaced by Mg-Fe phyllosilicates and amygdule filling minerals are equivalent to higher degree of metamorphism, greenschist facies. The Lake Shore Traps are geographically more distal to the thermal high and increased hydrothermal activity that resulted in the native copper deposits, hence, lower degree and grade of burial metamorphic/hydrothermal alteration. To the west from the walkway, you can see a rocky point extending towards Lake Superior, the rocks in this point are conglomerates of the Copper Harbor Formation. The sharp contact between the uppermost lava flow of the Lake Shore Traps and the conglomerates can be viewed on the eastern edge of this rocky point. The conglomerate above the contact is dominated by rounded to sub-rounded boulders that are matrix-supported. There are proportionately more basaltic and andesitic clasts in this conglomerate bed than stratigraphically higher elsewhere along the Lake Superior shoreline such as at Stop 7 as these clasts are derived from erosion of the Lake Shore Traps updip towards the highlands on the edge of the rift (the updip rocks are now missing having been removed by erosion). The very poor sorting and fine matrix-supporting the clasts suggest this conglomerate could have been deposited as a debris flow. Sedimentary debris flows are common in alluvial fan depositional environments. The Copper Harbor Formation was deposited in an alluvial fan derived from highlands to the south in the vicinity of Keweenaw Bay. Additional outcrops of the Copper Harbor Formation can be seen on the far western end of the cobble beach. These outcrops consist of interbedded conglomerates and sandstone that are typical of the formation as a whole. The conglomerates are described at Stop 7. There are several prominent 32 �white-colored calcite -filled fractures (calcite veins) within these outcrops. The calcite veins are northerly oriented consistent with the orientation of faults cutting the Portage Lake Volcanics about 5 km to the south. Calcite veins are a common occurrence in the Copper Harbor Formation and some of them contain native copper such as those described at Stop 6, Great Sand Bay, and at Stop 10, Fort Wilkins. Stop 9: Brockway Nose and Brockway Mountain Viewpoints Directions: Continue driving 2.7 miles (3.8km) east on M-26 to Brockway Mtn. Drive. Turn right and drive 0.6 miles (0.4km) to Brockway Nose turnoff. [UTM 5257463N 160432304E (NAD27 CONUS)] Brockway Mountain Drive intersects M-26 just west of Copper Harbor. After a steep climb upwards there is a pullover at the second hairpin curve which is Brockway Nose viewpoint (Figs. 10 and 15). Brockway Nose provides an excellent view of Copper Harbor and Lake Fanny Hooe (Figure for Hunter’s Point). The top of Brockway Mountain is accessed by continuing upwards from Brockway Nose. Brockway Mountain is a conglomerate ridge that reaches and elevation of over 400 m, with excellent views of the ridge and valley topography of the northern shore of the Keweenaw Peninsula. From Brockway Nose viewpoint, the town of Copper Harbor is the prominent visible feature (Fig. 15). The town of Copper Harbor began as a boom town in 1843, following the discovery of copper in the vicinity. Porter's Island, at the mouth of Copper Harbor on the west side (left) was the site of the first government land office. Hunter’s Point is west of Porter’s Island. On the east side of the mouth of Copper Harbor, the Copper Harbor Lighthouse, built in 1866, is visible. Lake Fanny Hooe is located south of Copper Harbor. Fort Wilkins is located on the north shore of Lake Fanny Hooe on the thin strip of land between the lake and harbor. It was built in 1844, with the intent to protect the miners from potentially hostile Indians. Fort Wilkins is now a Historic State Park and is discussed more at Stop 10. Nearby, the Estivant Pine is a 2.06 km2 nature sanctuary established in 1973, containing one the last stands of virgin white pines in the Midwest and the last stand in the Upper Peninsula. Some of the trees are up to 600 years old (www.michigannature.org). In 1955, the white pine was designated the state tree of Michigan. Copper Harbor and several other harbors between here and Eagle River are located within the Lake Shore Traps. Dipping massive interior of the basaltic to andesitic lava flows of the Lake Shore Traps occur at the head of the harbors. From the Brockway Mountain viewpoint there are an excellent 360o views. Underfoot, the Copper Harbor Conglomerate dips about 20o to the north. Near the base of the ridge on the south side, opposite Lake Superior, there is an exposure of a single basaltic lava flow erupted as part of the Lake Shore Traps. With care, southwest of the gift shop at the high point, one can view the dipping conglomerates of the Copper Harbor Formation and see the lava flow near the base of the ridge. To the west, the Lake Shore Traps form island chains on a prominent ridge in the vicinity of Agate Harbor and Esrey Park. The rocks of the Copper Harbor Formation are found in the drowned 33 �valleys and along the outer ridge jutting into Agate Harbor and associated island chain. The ridges of the Lake Shore Traps and Copper Harbor Formation along the Keweenaw Peninsula’s north shore are also the site of numerous shipwrecks. Lake Bailey (with the small island) and Lake Upsom occupy a topographically low valley on a finer-grained clastic horizon (sandstone and siltstone) within the Copper Harbor Formation which is overall composed of conglomerates. Just to the south of Lake Bailey, is the ridge of Mt. Lookout, marking the contact between the basal conglomerates of the Copper Harbor formation and the uppermost basalt lava flows of the Portage Lake Volcanics. The inland lake almost directly south, is Lake Medora, and just before the lake is a prominent ridge which marks the stratigraphic position of the Greenstone flow (see Stop 4). In the distance, farther to the south across Lake Medora, is Mount Bohemia, a dioritic stock-sized intrusion within the lower section of the Portage Lake Volcanics. To the southwest, a distant ridge is Gratiot Mountain, which is a small shallow rhyolite intrusive body that cuts the Portage Lake Volcanics. To the east are the communities of Copper Harbor and Lake Fanny Hooe (better viewed from Brockway Nose), both of which occupy the same stratigraphic horizon as Lake Bailey. Just south of Copper Harbor is a golf course that is part of Brockway Mountain lodge. Brockway Mountain lodge was built during the Great Depression in the 1930’s by the WPA. To the north, Lake Superior is the prominent feature. On the skyline 65 km away, is Isle Royale National Park, which can be visible on a clear day. The skyline of Isle Royale is formed by the Greenstone Flow, as it is on the Peninsula. The beds on Isle Royale dip towards the Keweenaw Peninsula forming the Lake Superior “syncline.” Viewed from here, the Midcontinent Rift proper extends from the Keweenaw Fault, originally a graben bounding fault on the edge of the rift, just south of Mt. Bohemia to the Isle Royale Fault, also originally a graben bounding fault on the edge of the rift, just northwest of Isle Royale. Glacial erosion exposed Keweenawan and pre-Keweenawan relatively hard and competent bedrock on the edges of the Midcontinent rift system. Dipping well-cemented conglomerates of the Copper Harbor Formation are exposed at Brockway Mountain and basaltic lava flows of the Portage Lake Volcanics are exposed when viewing south. Both are relatively resistant to glacial erosion. On Isle Royale, on the southeast (Keweenaw side) are exposed the same conglomerates of the Copper Harbor Formation and on the northwest side, there are exposed basaltic lava flows of the Portage Lake Volcanics. In the center of what is now Lake Superior, much less competent, nearly flat lying, very fine sandstone and siltstone of the Freda Formation was at the bedrock surface. The latest glacial advance(s) preferentially eroded out the less competent rocks in the center of the rift, resulting in present day Lake Superior following the horseshoe shape of the MCR. Very large volumes of water filled the basin as a result of melting of the glaciers, turning it into a glacial lake. The Duluth Glacial Lake was the largest of these glacial lakes and only elevations above roughly 400 m (1,300 ft) were emergent. Brockway Mt. and Mt. Bohemia. 34 �Stop 10: Fort Wilkins Historic State Park Directions: Continue driving east on M-26 4.6 miles (7.4 km) until arriving at entrance to Fort Wilkins State Park on right. [UTM 5257338N 160434763E (NAD27 CONUS)] The description of this stop is reproduced with modifications from Bornhorst and Barron (2011). Fort Wilkins was built in 1844 by the U.S. Army to provide order on the Keweenaw frontier and to protect the copper resources during the Civil War (Figs. 10 and 15). The army built 27 structures to house two full strength infantry divisions. After the soldiers were needed in the Mexican War in 1846, the fort was abandoned. Fort Wilkins became a State Park in 1923. During the 1930s under the Work Project Administration, the fort underwent extensive restoration. Many of these structures still survive today and have been either been restored or rebuilt after archeological excavations. Today, the restored buildings are a museum and contain exhibits on the mining history of the area. Fort Wilkins is a popular destination in the summer for recreation and camping. Considerable exploration activity took place in the immediate vicinity of the fort, and there are shafts and exploration pits between Lake Fanny Hooe and the harbor, mostly from exploration during the period from1843 to 1846. Just north of the park store, several pits provide evidence of early mining activity by European settlers. The Pittsburgh and Boston Mining Company operated here in the 1840's on a vein of native copper within the Copper Harbor Formation; the vein was reported to be up to 0.3 m wide. This venture was not profitable. In 1853 and for several decades thereafter mining activity took place about 4.4 km south of the fort in a series of workings called the Clark Mine. The mineralization at the Clark Mine is hosted in both fissures and basalt flow tops. It consists of prehnite, epidote, analcite, quartz, laumontite, adularia, microcline, chlorite, datolite, calcite and several copper minerals including native copper, chalcocite, cuprite and tenorite. Agates are conspicuous as vesicle fillings in the Copper Harbor area especially in the Lake Shore Traps. Opposite Fort Wilkins, on the harbor shoreline is a view of the Copper Harbor Lighthouse, one of the first on Lake Superior built in 1866. Near the lighthouse on the Lake Superior shoreline is the famous "green rock". The "green rock" is a vein that was described by Douglass Houghton. Houghton himself may have never really understood the uniqueness of the district. Conventional wisdom at the time led him to the interpretation that the “green rock” was the surficial alteration of a sulfide ore (Krause, 1992). Nevertheless, Houghton had a profound impact in promoting the district. His report to the Michigan legislature started the first major mining rush in North America to the Keweenaw Peninsula where the first economic discovery in 1845 at the Cliff Mine (Stop 11) was followed by many more until mining ceased in 1968. Douglas Houghton drowned in 1845 near Eagle River, MI while leading a geological expedition. 35 �Stop 11: Cliff Mine Rock Pile Directions: Get back onto M26 and continue driving east to stoplight in Copper Harbor. Turn right and drive 21.8 miles(35km) southwest on US41 past Phoenix until Cliff Dr. Turn right on Cliff Dr. and drive 0.4 miles(0.6km) to mine site.[UTM 52247173N 160400875E (NAD27 CONUS)] The description of this stop is reproduced with minor modifications from Bornhorst and Barron (2011). Fissure (vein) deposits were of little importance to the overall copper production from the Keweenaw Peninsula native copper district (Figs 10 and 7). Only a few fissure mines, including the Cliff Mine, were profitable. The Cliff Mine worked the Cliff fissure (vein) from 1845 to 1887 and produced a total of about 38 million lbs of refined copper (Table 2). The Cliff fissure is nearly at right angles to the attitude of bedding and dips steeply to the east. The productive portion of the fissure is under the Greenstone flow. While most of the mineralization was confined to the fissure, some lava flow tops (amygdaloids) cut by the fissure contained native copper. Multiple large masses of native copper, some up to 100 tons, were taken out of the Cliff Mine. Among the fissure deposits, the Cliff Mine produced the most native silver. Minerals other than native copper and native silver include adularia, apophyllite, calcite, chlorastrolite, chlorite, datolite, epidote, laumontite, and prehnite (alphabetical). Many specimens contain multiple minerals and illustrate paragenetic relationships. Fissures ranges in size from tight cracks to more than 3 m wide. In this part of the native copper district, fissures strike across the lava flows and dip steeply. Fissures formed as tension cracks related to bending of the lava beds, transverse to the axis of the MCR (Butler and Burbank, 1929). The steep ridge near the Cliff rock pile is the Greenstone flow (see also Stop 9). Here it makes up the entire high ridge from bottom to top and with a northward dip of about 25o. The very thick massive relatively impermeable interior of the Greenstone flow likely played an important role in the localization of native copper. The fissures acted as efficient pathways for fluid movement. On a local scale, fluids migrating upward through these open fractures and were impeded beneath the massive interior of the Greenstone flow and were forced to move laterally into adjacent permeable horizons. In general, flows beneath the thicker section of the Greenstone flow in this area contain more dispersed native copper than elsewhere, but economic deposits are not common. 36 �Stop 12: Jacobsville Formation M-26 Tamarack Directions: Drive 6 miles (9.6km) south west on Cliff Dr. until it intersects with US41/M26. Turn right and continue 5.2 miles (8.3km) to Calumet and turn left at the second stoplight onto Lake Linden Ave/M26 south. Drive 3.9 miles(6.3km) downhill to Lake Linden(blinking light) and turn right on M26. Drive 5.8 miles (9.3km) through Tamarack City to sandstone roadside outcrops. [UTM 5222120N 16038915E (NAD27 CONUS)] The description of this stop is reproduced from Bornhorst and Barron (2011). The Jacobsville Sandstone is a red-bed succession consisting of feldspathic and quartzose sandstones, conglomerates, siltstones, and shales up to 1,000 m thick that were deposited by fluvial processes in a rift-flanking basin (Fig. 10). Overall, there are neither interbedded lava flows nor cross-cutting dikes and, thus, the age of the Jacobsville Sandstone is inferred to be ca. 1,060 to 1,020 Ma. Jacobsville sedimentation was the last Precambrian event associated with the development of the MCR. The Jacobsville Sandstone at this stop displays features characteristic of the unit as a whole. At the northeastern end of the outcrop, reddish shale and red-brown siltstone are exposed at the highway level. They are overlain by two fining-upward sequences of conglomerate and red, red-brown, and white cross-bedded sandstone. The lower conglomeratic bed is planar and can be traced 30 m to the southwest, along with the directly underlying shale and siltstone. Farther to the southwest, the section is almost entirely cross-bedded red sandstone; some beds are contorted and mottled. The sandstone consists of almost equal parts of rounded-to-sub-rounded quartz, feldspars, and lithic fragments. Clasts in the lower conglomerate are predominately sub-angular, and rhyolitic in composition, with subordinate mafic volcanic rocks. Mining history will be viewed along M-26 from Lake Linden to Mason as an extension of the actual Stop described above, history summarized here from Molloy (2007). Just before leaving Lake Linden on the left is the C&H Mill. The C&H Mill was first built in 1867 with several expansions as milling practices changed and closed in 1956. Like Quincy, C&H also reclaimed copper from the sand tailings. The Houghton County Historical Museum is located on the edge of Lake Linden and exhibits mining and local history. Just outside of Lake Linden on the left are the remains of the Calumet and Hecla (C&H) smelter. C&H was the largest native copper producer in the district. The large building at the north end of the site next to the highway was the C&H mineral storage building where crushed and concentrated copper ore was smelted. It is now occupied by Peninsula Copper Industries which primarily recovers copper from scrap copper such as printed circuit boards to make copper sulfate as a fungicide for the wood preservative industry and to make other specialty copper compounds. About 2 miles (3.2 km) southwest of Lake Linden, is the only remaining steam stamp that was part of the Ahmeek Mill. The steam-driven stamp hammers could deliver about 104 blows per minute and process 7,000 tons of ore a day. About 1.5 miles southwest of the Ahmeek Mill, the Quincy Mining Company built a reclamation plant in 1942 to 1943 to reprocess the stamp sand tailings along Torch Lake, and from 1943 to 1967 recovered approximately 50,000 tons of copper. One of the mining dredges used in the recovery process sank in a storm in 1956 and is located just offshore. The foundations between the road and the dredge are part of the Quincy Mills for processing native copper ore. 37 �Acknowledgements We thank Allan Blaske for valuable comment that improved this field guide. References Cited Bornhorst, T.J., 1997, Tectonic context of native copper deposits of the North American Midcontinent Rift system: Geological Society of America Special Paper 312, p. 127-136. Bornhorst, T.J., and Barron, R.J., 2011, Copper deposits of the western Upper Peninsula of Michigan: Geological Society of America Field Guide, v. 24, p. 83-99. Bornhorst, T. J., and Lankton, L. D., 2009, Copper mining: A billion years of geologic and human history: in Schaetzl, R., Darden, J., and Brandt, D. (eds.), Michigan Geography and Geology, Pearson Custom Publishing, New York, p. 69-90. Bornhorst, T.J., Paces, J.B., Grant, N.K., Obradovich, J.D., and Huber, N.K., 1988, Age of native copper mineralization, Keweenaw Peninsula, Michigan: Economic Geology, v. 83, p. 619-625. Bornhorst, T. J., and Robinson, G.W., 2004, Precambrian aged supergene alteration of native copper deposits in the Keweenaw Peninsula: Michigan: Institute on Lake Superior Geology Proceedings, v. 50, part 1, p. 40-41. Bornhorst, T.J., and Rose, W.I., Jr., 1994, Self-guided geological field trip to the Keweenaw Peninsula, Michigan: Institute on Lake Superior Geology Proceedings, v. 40, part 2, 185 p. Bornhorst, T.J., Rose, W.I., Jr., and Paces, J.B., 1983, Field guide to the geology of the Keweenaw Peninsula, Michigan: Institute on Lake Superior Geology, v. 29, part 2, 116p. Bornhorst, T.J., and Williams, W.C., in press, The Mesoproterozoic Copperwood sedimentary rockhosted stratiform copper deposit, Upper Peninsula, Michigan: Economic Geology. Broderick, T.M., 1929, Zoning in Michigan copper deposits and its significance: Econ. Geol., v. 24, p. 149-162, 311-326. Broderick, T.M., 1931, Fissure vein and lode relations in Michigan copper deposits: Economic Geology, v. 26, p. 840-856. Broderick, T.M., 1935, Differentiation in lavas of the Michigan Keweenaw: Geological Society of America Bulletin, v. 46, p. 503-558. Broderick, T.M., and Hohl, C.D., 1935, Differentiation in traps and ore deposition: Economic Geology, v. 64, p. 342-346. Butler, B.S., and Burbank, W.S., 1929, The copper deposits of Michigan: U.S. Geological Survey Professional Paper 144, 238 p. 38 �Cannon, W.F., 1992, The Midcontinent Rift in the Lake Superior region with emphasis on its geodynamic evolution: Tectonophysics, v. 213. p. 41-48. Cannon, W. F., 1994, Closing of the Midcontinent Rift - A far field effect of Grenvillian contraction: Geology, v. 22, p. 155-158. Cannon, W.F., and Hinze, W.J., 1992, Speculations on the origin of the North American Midcontinent rift: Tectonophysics, v. 213, p. 49-55. Cannon, W. F., Green, A. G., Hutchinson, D. R., Lee, M.W., Milkereit, B., Behrendt, J.C., Halls, H.C., Green, J.C., Dickas, A.B., Morey, G.B., Sutcliffe, R., and Spencer, C., 1989, The North American mid-continent rift beneath Lake Superior from Glimpse seismic reflection profiling: Tectonics, v. 8, p. 305-332. Cannon, W. F., Peterman, Z.E., and Sims, P.K. 1993, Crustal-scale thrusting and origin of the Montreal River monocline - A 35-km-thick cross section of the Midcontinent Rift in northern Michigan and Wisconsin: Tectonics, v. 12, p. 728-744. Catacossinos, P.A., Harrison, W.B., Reynolds, R.F., Westjohn, D.B., and Wollensak, M.S., 2001, Stratigraphic lexicon for Michigan: Michigan Department of Environmental Quality, Geologic Survey Division Bulletin 8. Cornwall, H.R., 1951a, Differentiation in lavas of the Keweenawan series and the origin of the copper deposits of Michigan: Geological Society of America Bulletin, v. 62, p. 159-201. Cornwall, H.R., 1951b, Differentiation in magmas of the Keweenaw series: Journal of Geology, v. 59, p. 151-172. Cornwall, H.R., 1951c, Ilmentite, magnetite, hematite, and copper in lavas of the Keweenawan series: Economic Geology, v. 46, p. 51-67. Cornwall, H.R., 1955, Geologic map of the Fort Wilkins quadrangle, Michigan: U.S. Geol. Survey Geologic Quadrangle Maps of the United States Map GQ-74. Cornwall, H.R. and White, W.S., 1955, Bedrock geology of the Manitou Island quadrangle, Michigan: U.S. Geol. Survey Geologic Quadrangle Maps of the United States Map GQ-73. Daniels, P. A., 1982, Upper Precambrian sedimentary rocks: Oronto Group: Geological Society of America Memoir 156, p. 107-134. 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. Elmore, R.D., 1983, Precambrian non-marine stromatolites in alluvial fan deposits, the Copper Harbor Conglomerate, upper Michigan: Sedimentology, v. 30, p. 829-842. 39 �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. Elmore, R.D., Milavec, G.J., Imbus, S.W., and Engel, M.H., 1989, The Precambrian Nonesuch Formation of the North American Mid-Continent Rift, sedimentology and organic geochemical aspects of lacustrine deposition: Precambrian Research, v. 43, p. 191-213. Farrand, W.R., 1960, Former shorelines in western and northern Lake Superior basin: unpublished Ph.D. dissertation No. 5366, University of Michigan, Ann Arbor, 226p. Heaman, L.M., Easton, R.M., Hart, T.M., MacDonald, C.A., Hollings, P., and Smyk, M., 2007, Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon region, Ontario: Canadian Journal of Earth Sciences, v. 44, p. 1055-1086. Hinze, W.J., Braile, L.W., and Chandler, V.W., 1990, A geophysical profile of the southern margin of the Midcontinent rift system in western Lake Superior: Tectonics, v. 9, p. 303-310. Hoffman, P. F., 1989, Precambrian geology and tectonic history of North America: in Bally, A.W., and Palmer, A.R., eds., The Geology of North America-An overview, Boulder, Colorado, Geol. Soc. America, The Geology of North America, v. A, p. 447-512. Huber, N.K., 1975, The geologic story of Isle Royale National Park: U. S. Geological Survey Bulletin 1309, 66p. Jolly, W.T., 1974, Behavior of Cu, Zn, and Ni during prehnite-pumpellyite rank metamorphism of the Keweenawan basalts, northern Michigan: Economic Geology, v. 69, p. 1118-1125. Kalliokoski, J., 1982, Jacobsville Sandstone: Geological Society of America Memoir 156, p. 147-155. Kalliokoski, J., 1986, Calcium carbonate cement (caliche) in Keweenawan sedimentary rocks (~1.1 Ga), Upper Peninsula of Michigan: Precambrian Research, v. 32, p. 243-259. Kalliokoski, J., 1988, Jacobsville Sandstone: An up-date: in Upper Keweenawan rift-fill sequence Midcontinent rift system, Michigan, Michigan Basin Geological Society 1988 Fall Guidebook, p. 127136. Kalliokoski, J., and Welch, E.J., 1985, Keweenawan-age caliche paleosol in the lower part of the Calumet and Hecla Conglomerate, Calumet, Michigan: Geological Society of America Bulletin, v. 96, p. 1188-1193. Krause, David., 1992, The making of a mining district: Keweenaw native copper 1500-1870: Wayne State University Press, Detroit, MI, 305 p. Lane, A.C., 1911, The Keweenawan series of Michigan: Michigan Geological and Biological Survey Publication 6 (Geology series 4), 297p. Livnat, A., 1983, Metamorphism and copper mineralization of the Portage Lake Lava Series, northern Michigan: Ph.D. Dissertation, University of Michigan, Ann Arbor, 292p. 40 �Longo, A.A., 1982, A geochemical correlation, with correlative inferences from petrographic and paleomagnetic data, of the Greenstone flow, Keweenaw Peninsula and Isle Royale, Michigan: Institute on Lake Superior Geology Proceedings, v. 28, part 1, p. 22-23. Maki, J.C., and Bornhorst, T.J., 1999, The Gratiot chalcocite deposit, Keweenaw Peninsula, Michigan: Institute on Lake Superior Geology Proceedings, v. 44, p. 33-34. Mauk, J.L., Brown, A.C., Seasor, R.W., and Eldridge, C.S., 1992, Geology and stable isotope and organic geochemistry of the White Pine sediment-hosted stratiform copper deposit: Society of Economic Geologists Guidebook Series, v. 13, p. 63-98. Merk, G.P., and Jirsa, M.A., 1982, Provenance and tectonic significance of the Keweenawan interflow sedimentary rocks: Geological Society of America Memoir 156, p. 97-105. Milstein, R.L., 1987, Anomalous Paleozoic outliers near Limestone Mountain, Michigan: Geological Society of America Centennial Field Guide, v. 3, p. 263-268. Molloy, L.J., 2007, A Visitor’s Guide to the Historic Quincy Mine: published by Great Lakes Geoscience LLC, 61 p. Moore, P.B., 1971, Copper-nickel arsenides of the Mohawk No. 2 mine, Mohawk, Keweenaw Co., Michigan: American Mineralogist, v. 56, 1319-1331. Nicholson, S.W., 1991, Geochemistry, petrography, and volcanology of rhyolites of the Portage Lake Volanics, Keweenaw Peninsula, Michigan, U. S. Geological Survey Bulletin, 1970B, p. B1-B57. Nicholson, S.W., and Shirey, S.B., 1990, Evidence for a Precambrian mantle plume: a Sr, Nd, and Pb isotopic study of the Midcontinent Rift System in the Lake Superior region: Journal of Geophysical Research, v. 95, p. 10851-10868. Nicholson, S.W., Shirey, S.B., Schulz, K.J., and Green, J.C., 1997, Rift-wide correlation of 1.1 Ga Midcontinent rift system basalts: implications for multiple mantle sources during rift development: Canadian Journal of Earth Sciences, v. 34, p. 504-520. Ohr, M., 1993, Geochronology of diagenesis and low-grade metamorphism in pelites: Ph.D. dissertation, The University of Michigan, Ann Arbor, MI.,161 p. Paces, J.B., 1988, Magmatic processes, evolution and mantle source characteristics contributing to the petrogenesis of Midcontinent rift basalts: Portage Lake Volcanics, Keweenaw Peninsula, Michigan: Ph.D. Dissertation, Michigan Technological University, Houghton, 413p. Paces, J.B., and Bell, K., 1989, Non-depleted sub-continental mantle beneath the Superior Province of the Canadian Shield: Nd-Sr isotopic and trace element evidence from Midcontinent rift basalts: Geochimica Cosmochima Acta, v. 53, p. 2023-2035. 41 �Paces, J.B., and Bornhorst, T.J., 1985, Geology and geochemistry of lava flows within the Copper Harbor Conglomerate, Keweenaw Peninsula, Michigan: 31st Annual Institute on Lake Superior Geology Proceedings (Kenora, Ontario), p. 71-72. Paces, J.B., and Miller, J.D., Jr., 1993, Precise U-Pb ages of the Duluth Complex and related mafic intrusions, northeastern Minnesota: Geochronological insights to physical, petrogenetic, paleomagmatic, and tectnomagmatic processes associated with the 1.1 Ga Midcontinent Rift system: Journals of Geophysical Research, v. 98, p. 13,997-14,013. Stoiber, R.E., and Davidson, E.S., 1959, Amygdule mineral zoning in the Portage Lake Lava Series, Michigan copper district: Economic Geology, v. 54, p. 1250-1277, p. 1444-1460. White, W.S., 1960, The Keweenawan lavas of Lake Superior, an example of flood basalts: American Journal of Science, v. 258A, p. 367-374. White, W.S., 1968, The native-copper deposits of northern Michigan: in Ridge, J.D., ed., Ore Deposits of the United States, 1933-1967 (the Graton Sales volume), American Institute of Mining, Metallurgical, and Petroleum Engineering, New York, p. 303-325. White, W.S., 1971, Field Trip A-2 – Houghton to Calumet via South Range quarry and Eagle River: Society of Economic Geologists, Guidebook for field conference, Michigan copper district, Sept. 30-Oct. 2, 1971, p. 68-75. Weege, R.J., and Pollack, J.P., 1971, Recent developments in native-copper district of Michigan: Society of Economic Geologists Field Conference, Michigan Copper District, September 30 - October 2, 1971, p. 18-43. Weege, R.J., Pollock, J.P., and the Calumet Division Geological Staff, 1972, The geology of two new mines in the native copper district: Economic Geology, v. 67, p. 622-633. Weege, R.J., and Schillinger, A.W., 1962, Footwall mineralization in Osceola amygdaloid, Michigan native copper district: A.I.M.E. Transactions, v. 223, p. 344-350. 42 �Field Trip 2 Caledonia Mine, Keweenaw Peninsula Native Copper District, Ontonagon County, Michigan Theodore J. Bornhorst A.E. Seaman Mineral Museum, Michigan Technological University, 1404 E. Sharon Avenue, Houghton, MI 49931 Robert J. Barron Department of Geological and Mining Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931 Richard C. Whiteman Red Metal Minerals, 202 Ontonagon Street, Ontonagon, MI 49953 Directions: Leave downtown Houghton and head south on M-26 towards South Range. Stay on M-26(highway turns into M-38 past the Mass City turnoff) for 39 miles past Greenland(Pat’s Auto & Sports Center) to Ridge Road. Turn left (south) on Ridge Road approximately 1 mile to Caledonia Rd. Drive southwest 1.7 miles (2.7 km) to the Caledonia Mine. The Caledonia mine is privately owned and permission is required to enter this property [UTM 5179684N 160338022E (NAD27 CONUS)] The Caledonia Mine, surface and underground, is strictly private property. Permission is required to enter the property. Introduction The Caledonia Mine is part of the Keweenaw Peninsula native copper district of the western Upper Peninsula of Michigan (Fig. 1). The Caledonia Mine is located in the Greenland-Mass subdistrict about 40 km southwest of the Baltic Mine, the southernmost major native copper mine (Fig. 2). In total the mines on the Evergreen succession produced about 73 million lbs of copper at grades ranging from 0.5 to 1.25 % (Weege and Pollock, 1971) and, as compared to total district production of 11,000 million lbs of copper, this subdistrict was a minor producer. The largest producer among the Greenland-Mass subdistrict mines was the Mass Mine which produced about 51 million lb of refined copper from 1851 to 1923 (Butler and Burbank, 1929). The Adventure Mine produced about 11 million lb of copper. Despite its low production of copper, the geologic characteristics of the Greenland-Mass subdistrict deposits are typical of the native copper deposits elsewhere in the Keweenaw Peninsula. Since native copper mining in the Keweenaw Peninsula ceased in 1968, underground access to observe or study the native copper deposits is limited today to four mines, two of which are in the Greenland-Mass subdistrict (Caledonia and Adventure Mines). The Caledonia Mine is one of the few remaining localities where a typical native copper ore body hosted by the top of a basalt lava flow can be observed up close underground. 43 �This field trip guide relies on existing publications by Bornhorst and Whiteman (1992 and 1995) and Bornhorst and Barron (2011). The geologic and human history overview is summarized from Bornhorst and Lankton (2009). Since professional and collector oriented underground field trips to the Caledonia Mine are common, the introductory explanations have been expanded to provide a more readily stand alone field trip guide. Underground, the Caledonia Mine is relatively dry and regular field boots are usually sufficient; hard hats and lights are required. The field trip involves an easy walk underground to observe the character of native copper mineralization in an adit and a drift that parallels the strike of the tabular native copper ore body (lode) that is hosted by the top of the Knowlton lava flow. The cross-cutting adit provides opportunity to observe the Knowlton lode in cross-section as well as to observe underlying lava flows of the Evergreen succession. An optional more difficult segment of the field trip involves climbing up into an underground stope on the Knowlton lode to observe the character of mineralization and to collect specimens. Specimens of native copper and associated minerals from hosted by rocks of the Caledonia Mine can be collected on the rock pile adjacent to the adit. The Caledonia Mine is owned and operated by Red Metal Minerals as an educational facility and to recover specimens for resale in the general public and mineral collector markets. Mesoproterozoic Midcontinent Rift SystemAround Lake Superior Native Copper occurrences Sedimentary Rocks Igneous Rocks Several Major Faults Abundant Ontario Isle Royale Minnesota Lake Superior Keweenaw Peninsula native copper district Ontario 46.00 Wisconsin Lake Michigan 0 100 200 kilometers Archean metamorphosed sedimentaryandigneous rocks N Paleoproterozoic metamorphosed sedimentary and igneous rocks Phanerozoic sedimentary rocks Figure 1: Generalized bedrock geologic map of showing the Keweenaw Peninsula Native Copper District. 44 �Regional Geologic and Human History Overview The Keweenaw Peninsula is home to the largest known accumulation of native copper on the planet termed the Keweenaw Peninsula native copper district. The district is unique in comparison to copper mining districts elsewhere in that native copper comprises nearly all of the metallic minerals in the mined ore bodies. Approximately 11 billion pounds of refined copper from 380 million tons of ore were produced from native copper mines from 1845 to 1968 (Weege and Pollock, 1971). Small quantities of native silver occur with the native copper. Copper sulfides are uncommon in the Keweenaw Peninsula native copper district although chalcocite occurs in veinlets cutting the deposits (White, 1968). Near the tip of the Keweenaw Peninsula, there are several small unmined chalcocite-dominated deposits but their connection with the native copper deposits is uncertain (see Field Trip 3 this volume, Maki and Bornhorst, 1999). The native copper deposits of the Keweenaw Peninsula are hosted by rocks of the Mesoproterozoic Midcontinent Rift (MCR) (Fig. 1and 2). The MCR is filled with more than 25 km of volcanic rocks and 8 km of clastic sedimentary rocks (Cannon et al., 1989 and 1993). This thick succession of rocks was emplaced between about 1.15 to 1.03 Ga (Cannon et al., 1989; Davis and Paces, 1990; Heaman et al., 2007). Volcanic rocks were erupted on land surface initially over a broad area above a mantle plume and later, were erupted from fissure volcanoes within the normal fault-bounded rift graben. The volcanic rocks erupted during this syn-rift phase of the MCR were predominantly subaerial tholeiitic flood basalt lava flows. The subaerial basalt lava flows have a top which is vesicular (amygdaloid) and/or brecciated (fragmental amygdaloid) underlain by a massive (vesicle-free) interior. The typical flow is 10 to 20 m thick. Minor gravels and sands were deposited on top of the lava flows during hiatuses in volcanic activity and today occur as red conglomerate and sandstone that are interbedded with the lava flows. After active rifting and volcanic activity ended, the rift basin continued to sag. Rivers carried gravels, sands, silts, and muds to fill this sagging rift basin, and with subsequent burial they were lithified into clastic sedimentary rocks which occupy the center portion of the rift today (Merk and Jirsa, 1982). Volcanic rocks crop out around the margin of the rift (Fig. 1). The last and final phase of the MCR resulted from a regional compressional event due to collision of continental land mass along the eastern edge of North America at that time (Grenville Orogeny, Cannon, 1994). Compression inverted the rift-bounding normal faults into reverse faults as well as folding, faulting, and fracturing rift-filling volcanic and clastic sedimentary rocks. Native copper and related minerals were emplaced during this regional compressional event about 1.06 to 1.04 Ga (Bornhorst, 1997). For roughly 500 million years, from about 1.0 Ga to 500 Ma, there were no geologic events recorded by rocks of the Upper Peninsula. During this time interval, erosion exposed the native copper deposits to the surface and downward percolating oxidizing groundwaters had access to alter the native copper (Bornhorst and Robinson, 2004). After being buried beneath Phanerozoic sedimentary rocks (500 Ma to 175 Ma) (Catacosinos and others 2001), Pleistocene continental glaciations removed all but a few outliers of these rocks from the Keweenaw Peninsula and exposed the native copper deposits at the surface. 45 �Native people began exploiting native copper by ca. 7,000 years ago as the land surface of the Keweenaw Peninsula emerged above the retreating glacial lake levels. At first these prehistoric ancient miners likely found boulders of native copper (locally termed float copper) with their distinctive green weathered crust of malachite among the brown, gray, red, and white rocks. As they discovered the usefulness of native copper, they moved on to mining of bedrock. Because of the scars these early exploits left on the landscape, most mines of the Keweenaw Peninsula were rediscovered later including the Caledonia and those nearby. The first major mining rush in North America was started by Douglass Houghton through his report to the Michigan legislature in 1841. The Cliff Mine became the first profitable mine in the district in 1849. The Minesota Mine, in the southwest cluster near Caledonia Mine (Fig. 3) became profitable soon after the Cliff. In 1880, copper production from native copper mines of the Keweenaw Peninsula accounted for up to 80 % of the nation’s copper production. The peak copper production occurred in 1916 at 267 million pounds with mining ending in 1968. The Keweenaw National Historical Park was created in 1992 to preserve and interpret the historical importance of native copper mining to the history of the U.S. Native Copper Deposits of the Keweenaw Peninsula The pre-mining geologic resource of the Mesoproterozoic Keweenaw Peninsula native copper district totaled about 20 billion lbs of Cu (Bornhorst and Barron, 2011). Most of the native copper in the district is hosted in the permeable and porous brecciated and amygdaloidal lava flow tops (~58.5% of production) and interflow conglomerate-sandstone horizons (~39.5% of production). The ore is "sandwiched" above and below between barren massive basalt that lacks permeability and porosity and is geometrically found in tabular bodies between 3 and 5 m thick that have the same orientation as surrounding host rocks, i.e., stratiform lode. The typical lode has a lateral extent of 1.5 to 11 km and extends down-dip 1.5 to 2.6 km (Butler and Burbank, 1929; White, 1968). Native copper fills open spaces from a few cm across (e.g., vesicle-fillings) to small-to-moderately sized openings (e.g., space between lava flow top breccia fragments or between clasts in conglomerate) that contain native copper masses weighing up to several pounds and rarely weighing tons. A minor amount of native copper was produced, ~ 2%, from high-angle tabular veins that cut across the volcanic-dominated strata. Native copper is closely associated with over 100 different minerals in the Keweenaw Peninsula although only about 25 of them are common. These minerals fill the same open spaces along with and instead of native copper (Butler and Burbank, 1929; Stoiber and Davidson, 1959; White, 1968). The suite of minerals is similar to those found where rocks have undergone very low to low grade burial metamorphism, < 300OC. Overall, higher temperature assemblages are spatially associated with the area of native copper deposits where the thermal anomaly was greatest because of focused hydrothermal fluid flow. In areas more distal to the deposits, the open spaces are filled with lower temperature assemblages. At any one location, there is a recognizable sequence in the precipitation of minerals from hydrothermal fluids due to changing hydrothermal fluid temperature and composition. The absolute age of the hydrothermal activity is coincident with the age of the regional compressional event at about 1.06 to 1.04 Ga (Bornhorst et al., 1988). The compressional event provided the plumbing system, faults/fractures, that facilitated movement of the hydrothermal fluids into the sites of future mineable deposits of native copper (Bornhorst, 1997). 46 �47 �The native copper mineralizing event was widespread throughout the exposed MCR (Fig. 1). Burial metamorphism at depth of rocks down-dip from the deposits was the likely source of the mineralizing hydrothermal fluids; small amounts of copper and other constituents were leached from the rift-filling basalt-dominated rocks. Subaerial eruption of the basalt lava flows likely resulted in the degassing of most of the contained sulfur leaving them sulfur poor. Thus, the hydrothermal fluids generated from them were low in sulfur and the movement of these fluids through the same sulfur poor rocks resulted in sites of ore deposition where host rocks were also sulfur poor. This low sulfur environment favored the deposition of native copper rather than copper sulfide. The heating of the volcanic rocks during burial probably reached a maximum millions of years after they were erupted and it was most likely the coincidence of increased fluids generated at this temperature maximum with the regional compressional event which played a critical role in providing the plumbing system necessary for producing the deposits (Bornhorst, 1997). Geology of the Evergreen Series The native copper mines in the Greenland-Mass subdistrict produced native copper from the tops of rift-filling lava flows that comprise the Evergreen succession within the Portage Lake Volcanics (Fig. 3 and 4). The Evergreen succession of basalt lava flows have a total thickness of about 210 m. The individual copper-rich lava flows within the succession were each informally named. From bottom to top the Evergreen succession consists of: the Evergreen flow: a 3 to 15 m thick plagioclase porphyritic otherwise aphanitic basalt lava flow; the Ogima flow, a 30 to 43 m thick slightly plagioclase glomerophyritic basalt lava flow; the Butler flow, a 15 to 27 m thick plagioclase glomerporphyritic basalt lava flow; and a horizon of thin plagioclase glomeroporphryitic flows 75 to 90 m thick. This latter stratigraphic horizon of multiple flows includes the Mass flow. The South Knowlton flow overlies this horizon and is a plagioclase glomeroporphyritic lava flow up to 15 m thick and at the top of the Evergreen succession is the Knowlton flow, a 9 to 21 m thick plagioclase glomeroporphyritic lava flow (Calumet and Hecla, 1958). The volcanic rocks nearby underlying the Evergreen succession are ophitic and aphanitic basalt lava flows. The nearby overlying volcanic rocks are ophitic basalt lava flows. The Evergreen succession is stratigraphically at the level of the Isle Royale flow near Houghton. The tops of these flows were productive over a strike length of about 5 km. Of the lava flows in the Evergreen succession, the Butler flow top yielded the most copper followed by the Evergreen and Knowlton flow tops which also yielded significant amounts of copper. Most of the Evergreen succession basalt lava flow tops are brecciated (fragmental amygdaloid) with considerable lateral (along strike) variation in the degree of brecciation and thickness. In some areas, thin lava flows lack brecciated tops and are simply vesicular (amygdaloid). In general, the best grades of copper occur where the brecciated flow top thickens. Secondary minerals in all of the flow tops are quite similar. Quartz, feldspar, pumpellyite, chlorite, calcite, and epidote are abundant minerals filling amygdules and spaces between breccia fragments. There is less abundant native silver, prehnite, datolite, and laumontite. 48 �Greenland-Mass Subdistrict Bedrock Geology City Nonesuch Formation Native Copper Mine 45 Strike and Dip of Bedding Freda Sandstone Fault 12 Anticline 20 28 38 Syncline 35 43 44 45 North Lake Toltec Greenland A Adventure Belt 45 South Lake Lake Mass 45 Mass Old Mass Knowlton Caledonia 48 45 Rockland 45 Nebraska Nassau Superior Rockland Flintsteel Bumblebee National Minesota Michigan 55 60 Algomah 46o45' N 0 55 1 mile 0 60 5 Jacobsville Sandstone T51N T50N 2 kilometers A’ 20 NW 6 SE NonesuchFormation A' A FASL 0 Freda Sandstone Jacobsville Sandstone -2000 -4000 1 0 0 mile 1 kilometer Figure 3: Generalized bedrock geologic map of the Greenland-Mass subdistrict of the Keweenaw Peninsula Native Copper District showing the location of native copper mines. Geologic cross section shows the Evergreen Succession within the Portage Lake Volcanics; a lithostratigraphic column is given in Figure 4. Subdistrict bedrock geology and cross section modified from Whitlow (1974). 49 �The Evergreen succession in the Greenland-Mass subdistrict dips about 45o NW towards Lake Superior and forms a local broad open anticlinal structural bend (Fig. 3). The largest mine, the Mass Mine, occurs near the maximum bend in this anticline. Faults with significant vertical displacement are uncommon as most have displacement of < 1 m. There are multiple veins in tension fractures that cut perpendicular across the lava flows in association with the anticline (Butler and Burbank, 1929). However, some veins are parallel to strike of the lava flows but dip in the opposite direction. In the stratigraphically equivalent Isle Royale lode, Broderick (1931) describes similar strike parallel veins which he interpreted to be feeders of ore fluid into the top of the lava flow. Figure 4: Stratigraphic position of the Evergreen succession that hosts native copper deposits of the Greenland-Mass subdistrict as compared to lithostratigraphic units of the Keweenaw Peninsula, Michigan. Units from the Powder Mill Group to the Jacobsville are all Mesoproterozoic in age and related to the Midcontinent rift. Geology of the Caledonia Mine In the context of mines in the Keweenaw Peninsula native copper district, the Caledonia Mine was very small, producing only about 6.8 million lb of refined copper from the top of the Knowlton basalt lava flow (Knowlton lode), the youngest basalt lava flow of the Evergreen Series (Fig. 4). The near horizontal adit of the mine follows approximately along strike of the Knowlton lava flow where it connects with the Knowlton Mine (Fig. 5) and to where it connects to the stopes of the Mass Mine “C” shaft (not shown). At the Mass Mine, the stratigraphically lower Bulter lava flow top was the principal focus of native copper mining. In the GreenlandMass subdistrict the Butler lava flow top was developed for about 2000 m along strike and to a maximum depth of 300 m along dip. The most abundant secondary minerals in the Butler are quartz and calcite with slightly lesser amounts of K-feldspar and epidote. Prehnite and pumpellyite are usually much less abundant and chlorite is present in amounts < 1 %. The Butler contains a high number of veins, usually they strike subparallel to the strike of the Butler lava flow top and have dips both similar to the dip of bedding and at a high angle to bedding (Butler and Burbank, 1929). 50 �51 �In the Greenland-Mass subdistrict, the Knowlton flow top was developed for about 3000 m along strike and to a maximum depth of about 375 m. The Knowlton was the focus of native copper mining at the Caledonia Mine. The Knowlton lava flow top is a brecciated flow top or fragmental amygdaloid. Stopes at Caledonia were raised on the Knowlton lode upwards from the drift toward the topographic high of the Caledonia bluff (Fig. 6). The floor (footwall) of the stopes reflects the original depositional irregularities of the top of the underlying lava flow such as gentle flexures. The average thickness of the Knowlton lava flow top is about 2.5 m but locally it can thicken to around 6 m (Calumet and Hecla, 1958). In general, a thicker flow top results in better ore. While most of the ore occurs in the top of the Knowlton lava flow top, there are pockets of ore that extend into the underlying Knowlton massive flow interior footwall and are closely associated with strike-parallel fractures and veins. The grade of the ore in the flow top may correlate with these footwall pockets of ore; there is an approximate strike parallel (drift parallel) orientation of the grade of the ore (Fig. 5). The workings of the Caledonia Mine provide excellent access to observe the 3-D geometry of the Knowlton lode. An elongated volume of highly epidotized basalt characterizes a footwall ore pocket that was mined out in the 1990s by Red Metal Minerals and will be visited on this field trip. Veins are of two types. Veins within the Knowlton flow top that extend into underlying Knowlton massive flow interior and contain the same basic minerals as those found in the lode itself (including native copper) are considered synchronous with the native copper deposit at the Caledonia Mine. Native copper occurs as small to large masses in the fractures and seems to be more abundant in the overlying adjacent tabular ore body within the lava flow top. The fractures typically have little or no displacement. Adjacent to the fractures even massive basalt can be highly altered and host native copper. One main-stage vein that has been studied in more detail by Bornhorst strikes subparallel with the strike of the flow top but dips more steeply, about 80oNW dip of the vein as compared to 45oNW of the lava flow top. This vein has been traced along strike for over 100 m. It extends into the footwall, but is hard to identify as it enters the lode. Within this vein the intensity of alteration varies from slight to very high. Original basalt can be completely converted to a green soft epidote and lesser chlorite rock, or a hard epidote and lesser quartz rock. Overall mineralogy and paragenesis in the vein is similar to the lode. The vein contains pockets of a soft blue-green mineral identified as corrensite by XRD (mixed layered clay mineral with 50/50 chlorite and smectite unit cells stacked in perfect alternation). NW SW 1300 1200 Caledonia Mine adit 1100 1000 Stope 900 Drift 800 0 500 Horizontal Scale feet 5X vertical exxageration Figure 6: Cross-section sketch of the topography of Caledonia bluff showing the positions of the Caledonia Mine adit and the top of the Knowlton lava flow. 52 �This vein has yielded outstanding museum quality specimens of crystalline native silver (now part of the A.E. Seaman Mineral Museum collection). These native silver specimens were encased in white calcite; the calcite was removed by acid cleaning. This vein also yielded clusters of colorless calcite crystals internally laced with native copper from open vugs. Several masses of native copper weighing over 100 kg and small groups of copper crystals originally encased in white calcite (removed by acid cleaning) have also been recovered during exploration. Some of the copper crystals were coated with very small cubic native silver crystals. There are multiple other veins at Caledonia synchronous with native copper precipitation. One of these veins along the drift that will be visited is notable for hosting datolite; the datolite commonly contains very-fine inclusions of native copper. The occurrence of veins at the Caledonia Mine is quite similar to veins described by Broderick (1931) occurring in the Baltic and Isle Royale Mines near Houghton. At Caledonia, they are interpreted as pathways for ascending hydrothermal fluids and thus, played an important role in the deposition of native copper and associated minerals. At Caledonia, there are veins that crosscut the native copper mineralized lode. These post-copper mineralization veins contain calcite and laumontite and are barren of copper. Several of these post-mineral veins are readily visible along the down-dip side of the Caledonia adit. At the Caledonia Mine, the most abundant mineral filling amygdules and spaces between fragments in the Knowlton lode is calcite which is closely followed by subequal amounts of quartz, epidote and red K-feldspar. There are lesser amounts of prehnite, pumpellyite and chlorite. Native copper is present in small amounts with an average grade of about 1.2 % Cu. Native silver and datolite are present in much lesser amounts. Least abundant are laumontite, adularia, and corrensite (clay mineral). No major differences exist in the abundance of secondary minerals averaged over the scale of 100's of meters. In contrast, over the scale of a few meters the distribution of secondary minerals is variable to highly variable. While a secondary mineral may be completely absent in one zone and extremely abundant in another, the meter scale variation does display a degree of regularity. For example, within the Knowlton flow top secondary minerals may occur in overlapping bands; the bands are consistent with the progressive filling of open spaces indicated by amygdule paragenesis. The intensity of alteration is highest near both the hanging wall and footwall of the brecciated flow top lode where apparently there was preferential flow of hydrothermal fluids. Distribution of secondary minerals also has a poorly defined correlation with the occurrence of synchronous veins. In general, native copper tends to be more commonly associated with epidote, calcite, and quartz. Rarely is native copper abundant in areas with abundant K-feldspar. Paragenetically, K-feldspar is an early formed mineral followed by epidote and then datolite, prehnite, pumpellyite, chlorite, calcite and quartz. Native copper is found as inclusions in epidote, calcite, quartz, and datolite. Much of the calcite that is overall synchronous with native copper precipitation does not contain obvious inclusions of native copper. Post-native copper mineralization hydrothermal minerals in veins and open space fillings as coatings on earlier formed minerals include calcite, laumontite, adularia, and corrensite. These likely formed from superposition of later lower temperature hydrothermal fluids on earlier higher temperature formed minerals associated with native copper during collapse of the hydrothermal system. This relationship is found elsewhere in the broader district. 53 �Post-emplacement alteration of hydrothermal minerals is most obvious for native copper. At Caledonia, tenorite and cuprite (Cu oxide) often but not always occurs as a thin coating on native copper that is found in open space fillings. The tenorite and cuprite could have its origin during Precambrian weathering and downward-percolating ancient groundwater leading to supergene alteration of the native copper prior to Phanerozoic marine submergence or during more recently since Pleistocene glaciations eroded and exposed the Caledonia deposits at the surface. Today, only a few fractures within the stopes of Knowlton lode are damp with meteoric groundwater despite the shallow depth, an argument in favor of Precambrian tenorite and cuprite. However, the presence of modern groundwater flow into the mine, such as near the intersection of the cross-cut and the Knowlton drift, suggests Pleistocene age for the tenorite and cuprite cannot be precluded. In addition to tenorite and cuprite, there are occasional copper carbonate minerals (such as malachite), brochantite (hydrated Cu sulfate), atacamite (hydrated Cu chloride) and unknown green to blue-green minerals on native copper surfaces. These may also be supergene in origin. However, there is at least one mineral, gerhardtite (hydrated Cu nitrate) that is the result of postmining chemical reactions. An extension of the Caledonia adit cuts across the lava flows towards the Nebraska Mine; the cross-cut was a component of Calumet and Hecla’s exploration program (Fig. 5). This cross-cut intersects the South Knowlton lava flow top directly below the base of the Knowlton lava flow. An NSF-sponsored Teachers Earth Science Institute (educating middle and high school teachers about mining) drilled, blasted, and mucked out the South Knowlton lava flow top to its present expanded opening in the early 2000s. Below the South Knowlton, there are several thin basalt lava flows that can be identified before the stratigraphic level of the Butler lava flow. While the Butler lava flow top is readily identified in the cross-cut by abundant amygdules filled with Kfeldspar and calcite, it lacks significant native copper here. A small fault can also be seen along the cross-cut. This cross-cut and adit connects the Caledonia Mine to the Nebraska Mine where the Butler lava flow top was a principal target of mining. Mining History of the Caledonia Mine The Caledonia Mining Company began its operations in 1863 after acquisition of the mining rights of the former Nebraska Company and acquisition of the adjacent Kansas Properties. The workings at that time consisted of a horizontal adit driven about 90 m to the Butler deposit on the west end of the Caledonia bluff and two shafts about 60 m deep (Nebraska Mine) (Fig. 5). A vein showing mineralization was explored on the north side of the bluff by four adits and while the vein proved to contain too little copper, the adits intersected the Knowlton, South Knowlton, Mass, and Butler lava flow tops. About 900,000 lbs of refined was produced from 1863 to 1870 by the mining of native copper at a grade of about 1.25 % Cu from the Knowlton and Butler lava flow tops. Production halted in 1870 when a fire destroyed the processing facility. Subsequently, the Caledonia Mining Company acquired the Flintsteel properties in 1870 and despite investing in a new processing facility, the Caledonia operations closed before significant ore was processed. Captain Martin leased the Caledonia properties and from 1873 to 1881 produced more than 330,000 lbs of copper, including a single mass weighing 80,000 lbs (40 short tons). After the lease expired, mining ceased. In 1901 there was a failed proposal to merge the Caledonia properties with other mineral rights in the Greenland-Mass subdistrict and to construct a processing facility on Lake Superior some distance away. There was no reported mining from the Caledonia properties for 56 years from 1881 to 1937. 54 �The Calumet and Hecla Consolidated Mining Company was the major producer of copper from the mines north of Houghton. Calumet and Hecla did exploration core drilling and reopened the Caledonia adit. They drifted some 600 m along the strike of the Knowlton flow top and estimated the grade of native copper ore to be 1.45 % Cu. The workings from the nearby Nebraska Mine were connected to the Caledonia with a cross-cut and adit. Exploration of the Caledonia Mine by Calumet and Hecla ended in c.a. 1941 as a result of World War II. After World War II, Calumet and Hecla resumed exploration of Caledonia and removed a 200 ton bulk sample from the Knowlton lode in 1950. The sample had a very promising grade of 1.84 % Cu. From 1951 to 1958, Calumet and Hecla produced 5.55 million lbs of refined copper with an average grade of 1.24 %. This program included the stoping on the Knowlton lode visible above the drift today. Subsequently, Copper Range Company acquired the mineral rights at the Caledonia Mine. The Caledonia Mine was a candidate for in-situ leaching of Cu, hence a limited evaluation of the mine was completed by Copper Range and the U.S. Bureau of Mines from 1971 to 1972. The in-situ leaching option was abandoned due to potential problems with groundwater pollution. In 1985, Red Metal Minerals acquired the mineral rights for the Caledonia Mine and other properties in the Greenland-Mass subdistrict from the Copper Range Company. The Caledonia adit was reopened and reconditioned to undertake a limited program of exploration. Red Metal Minerals mucked out broken rock as well as drilled and blasted new areas to recover native copper and other minerals. Over 28 years from 1985 to present, Red Metal has removed a single mass weighing 3000 lbs. Masses of recovered native copper are sold to the general public and mineral collectors. Red Metal distributes native copper on a wholesale basis to retail outlets that distribute Caledonia native copper around the world. You may find native copper from the Caledonia Mine for sale in unexpected places, even visitor gift shops at other copper mines! A large mass of native copper is on exhibit at the A.E. Seaman Mineral Museum, on the campus of Michigan Tech in Houghton. When the native copper is dispersed through the rock, Red Metal uses this material to prepare decorative bookends and cut slabs. The Caledonia Mine has yielded minerals sought after by mineral collectors such as native copper crystals, native silver crystals, datolite nodules, copper in calcite crystals as well as adularia and epidote. At this time, Red Metal does not undertake in underground drilling and blasting to recover specimens from the mine. Instead, the Caledonia Mine serves as an educational facility, used by Michigan Tech and others, and to recover specimens from already broken rock for resale in the general public and mineral collector markets. Since most of the native copper mines of the Keweenaw Peninsula are closed and flooded, the Caledonia Mine is significant today because it provides rare access to observe the character of native copper mineralization underground in three dimensions. Acknowledgements We thank Allan Blaske for helpful reviews which improved the quality of this field trip guide. References Cited Bornhorst, T.J., 1997, Tectonic context of native copper deposits of the North American Midcontinent Rift System: Geological Society of America Special Paper 312, p. 127-136. 55 �Bornhorst, T.J., and Barron, R.J., 2011, Copper deposits of the western Upper Peninsula of Michigan: Geological Society of America Field Guide, v. 24, p. 83-99. Bornhorst, T.J., and Lankton, L.D., 2009, Copper mining: A billion years of geologic and human history: in Schaetzl, R., Darden, J., and Brandt, D., eds, Michigan Geography and Geology, Pearson Custom Publishing, New York, p. 150-173. Bornhorst, T.J., Paces, J.B., Grant, N.K., Obradovich, J.D., and Huber, N.K. 1988. Age of native copper mineralization, Keweenaw Peninsula, Michigan: Economic Geology, v. 83, p. 619625. Bornhorst, T.J., and Robinson, G.W., 2004, Precambrian aged supergene alteration of native copper deposits in the Keweenaw Peninsula: Michigan; Institute on Lake Superior Geology Proceedings and Abstracts, v. 50, part 1, p. 40-41. Bornhorst, T.J., and Whiteman, R.C., 1995, Native copper and associated minerals in basalts at the Caledonia Mine, western Upper Michigan: Institute on Lake Superior Geology Proceedings, v. 41, part 1, p. 3-4. Bornhorst, T.J., and Whiteman, R.C., 1992, The Caledonia native copper mine, Michigan: Society of Economic Geologists Guidebook Series, v. 13, p. 139-144. Broderick, T.M., 1931, Fissure vein and lode relations in Michigan copper deposits: Economic Geology, v. 26, p. 840-856. Butler, B.S., and Burbank, W.S., 1929, The copper deposits of Michigan: U.S. Geological Survey Professional Paper 144, 238 p. Calumet and Hecla, 1958, Unpublished report for Defense Minerals Exploration Administration, 29p. Cannon, W. F., 1994, Closing of the Midcontinent Rift - A far field effect of Grenvillian contraction: Geology, v. 22, p. 155-158. Cannon, W. F., Green, A. G., Hutchinson, D. R., Lee, M.W., Milkereit, B., Behrendt, J.C., Halls, H.C., Green, J.C., Dickas, A.B., Morey, G.B., Sutcliffe, R., and Spencer, C., 1989, The North American mid-continent rift beneath Lake Superior from Glimpse seismic reflection profiling: Tectonics, v. 8, p. 305-332. Cannon, W. F., Peterman, Z.E., and Sims, P.K. 1993, Crustal-scale thrusting and origin of the Montreal River monocline - A 35-km-thick cross section of the Midcontinent Rift in northern Michigan and Wisconsin: Tectonics, v. 12, p. 728-744. Catacossinos, P.A., Harrison, W.B., Reynolds, R.F., Westjohn, D.B., and Wollensak, M.S., 2001, Stratigraphic lexicon for Michigan: Michigan Department of Environmental Quality, Geologic Survey Division Bulletin 8. Lansing, MI. 56 �Davis, D.W., and Paces, J.B., 1990, Time resolution of geologic events on the Keweenaw Peninsula and implications for development of the Midcontinent Rift system: Earth and Planetary Science Letters, v. 97, p. 54-64. Heaman, L.M., Easton, R.M., Hart, T.M., MacDonald, C.A., Hollings, P., and Smyk, M., 2007, Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon region, Ontario: Canadian Journal of Earth Sciences, v. 44, p. 1055-1086. Maki, J.C., and Bornhorst, T.J., 1999, The Gratiot chalcocite deposit, Keweenaw Peninsula, Michigan: Institute on Lake Superior Geology Proceedings and Abstracts, v. 45, part 1, p. 33-34. Merk, G.P., and Jirsa, M.A., 1982, Provenance and tectonic significance of the Keweenawan interflow sedimentary rocks: Geological Society of America Memoir 156, p. 97-105. Stoiber, R.E., and Davidson, E.S., 1959, Amygdule mineral zoning in the Portage Lake Lava Series, Michigan copper district: Economic Geology, v. 54, p. 1250-1277, p. 1444-1460. Weege, R.J., and Pollack, J.P., 1971, Recent developments in native-copper district of Michigan: Society of Economic Geologists Field Conference, Michigan Copper District, September 30 October 2, 1971, p. 18-43. White, W.S. 1968, The native-copper deposits of northern Michigan: in Ridge, J.D., ed., Ore Deposits of the United States, 1933-1967 (the Graton Sales volume). American Institute of Mining, Metallurgical, and Petroleum Engineering, New York: p. 303-325. Whitlow, 1974, Geologic map of the Greenland and Rockland quadrangles, Ontonagon County, Michigan: U.S. Geological Survey Miscellaneous Field Studies Map MF-596. 57 ��Field Trip 3 Geology of Silver Mountain, Houghton County, Michigan Evgeniy Kulakov Department of Geological and Mining Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931. Directions: Leave downtown Houghton and head east on Montezuma avenue and College Avenue. Continue on US 41south for 27 miles. In Baraga MI turn right and continue to follow M38 west for 13.5 miles. Make left turn on S Laird road and follow it for 5.7 miles. Take a left turn onto Forest Rd 2276 and continue 1.8 miles. Turn right onto Forest Rd 193/NF-2270. In 0.9 miles make a right turn onto Forest Rd 922 road. Arrive in 0.4 miles. [UTM 5171550N 367079 E (NAD27 CONUS)]. Introduction Figure 1: View from the top of Silver Mountain looking east. 59 �Silver Mountain is located at the south-eastern part of Keweenaw Peninsula, near Sturgeon River Falls, Houghton County, Michigan (sections 1 and 12 of T 49 N, R 36 W, and section 6 of T 49N, R 35 W). A former US Forest Service lookout tower, Silver Mountain is now designated as a scenic view. Silver Mountain is a 100 meter high glacially polished dome-shaped hill with a 360° view (Fig.1) of the surroundings. The low lying areas around Silver Mountain consist of unconsolidated glacial deposits unconformably overlying Mesoproterozoic Jacobsville Sandstone. Fourteen shallow-dipping tholeiitic lava flows are exposed on the top and sides of the mountain. Silver Mountain was first geologically mentioned by Burt (1849) who reported its location and small amounts of copper sulfides associated with calcite. Foster and Whitney (1851) described Silver Mountain as basaltic flows made up of labradorite and hornblende with scattered nodules of quartz and chalcedony. They interpreted the mountain as igneous rocks that intruded the surrounding sedimentary strata. In the mid 1850s, the National Company drove an approximately 30 m long adit. However, metallic mineralization was insufficient to justify additional mining. Lane (1909) concluded that the rocks of Silver Mountain were typical Keweenawan lava flows. Regional Geology The Midcontinent Rift (MCR) extends from NE Kansas northward to Lake Superior and through Michigan (Cannon et al., 1989). Rifting began ~ 1.1 Ga during an interval of reversed polarity of geomagnetic field with the oldest erupted material being reversely magnetized. These oldest lava flows include the Siemens Creek Formation of the Powder Mill Group, the lowermost part of North Shore Volcanics, Osler Volcanics, and the lower part of Mamainse Point Formation, located around Lake Superior (Paces and Miller, 1993; Davis and Green, 1997) (Fig.2 and 3). This early stage of magmatism occurred from 1109 to approximately 1105 Ma (Heaman et al., 2007; Paces and Miller, 1993) and was followed by a quiescence period when the geomagnetic field reversed to normal polarity (Davis and Green, 1997). Magmatism resumed by 1102 Ma (Paces and Miller, 1993) during the normal polarity interval. During this interval, the main stage of the rift-related magmatism was represented by a sequence of approximately 200 lava flows of the Portage Lake Volcanics that erupted within a short interval around 1095 Ma (Davis and Paces, 1990). Magmatism of the Portage Lake Volcanics ended and the rift basin was filled with clastic sedimentary rocks during continued sagging (Bornhorst and Lankton, 2009). The final phase of the MCR was continental compression at about 1060 Ma related to the Grenville Orogeny which inverted original rift-bounding graben normal faults into high angle reverse faults (Cannon, 1994). During late rift compression, rift-wide burial metamorphic/hydrothermal fluids altered rift-filling rocks and formed the native copper deposits of the Keweenaw Peninsula native copper district (Bornhorst, 1997; Bornhorst and Barron, 2011). 60 �Figure 2: a. Generalized geologic map of Keweenaw Peninsula. The inset shows the geology of the Lake Superior segment of the Midcontinent Rift. The inset from Ojakangas et al, (2001); b, Section of Keweenaw Peninsula along the Line A-A’ (B- B’part of the section is present on the map). Section is taken and modified from Cannon and Nicholson (2001). 61 �Fourteen tholeiitic lava flows have been recognized at Silver Mountain and that dip NE at about 15o. The lava flows of Silver Mountain are characterized by moderate to high magnetic anomalies and overall high positive gravity anomaly (Campbell, 1952). The magnetic and gravity anomalies are similar to those attributable to the Siemens Creek Formation. Based on this geophysical data, the lava flows at Silver Mountains were interpreted as being Keweenawan in age and part of the South Range Traps (termed Siemens Creek Formation today) (Campbell, 1952). However, Silver Mountain is an isolated knob and there is no direct geological contact with the rest of the Powder Mill Group. The mountain is located in the immediate vicinity of the Marenisco fault (Fig.2). Reverse and thrust faulting occurred during the latest compressional stage of the MCR evolution and resulted in uplifting the deeply buried strata to the surface. At Silver Mountain, the Marenisco fault has uplifted the lava flows stratigraphically older than the Portage Lake Volcanics which are exposed in the Keweenaw Peninsula because of uplifting along the Keweenaw fault. The Jacobsville Sandstone surrounds the uplifted basalt flows at Silver Mountain and vicinity. The Jacobsville Sandstone was deposited in a rift-flanking basin that while initiated by, and contemporaneous with compression, its deposition continued after compression. About 10 km west-southwest of Silver Mountain, there are gravity and magnetic anomalies resulting from the Echo Lake Gabbro which was also uplifted during compression along another fault. This geophysical anomaly was drilled in 1994 and confirmed as a layered intrusion (Waggoner, 1994). Additional drilling by Bitterroot Resources identified a 5.5 m thick interval containing 0.5 to1 ppm total Pt + Pd + Au (Cannon and Nicholson, 2001). There is at least two faults cross-cutting Silver Mountain (Roberts, 1940). One fault can be observed at the adit at the base of Silver Mountain. It strikes N85°E and dips approximately 60° N (Roberts, 1940). The other fault is exposed near the NE side of Silver Mountain. This fault strikes N45°W and dips at about 80° N. Figure 3: Stratigraphic column of Midcontinent Rift rocks in the western Upper Peninsula of Michigan. The available radiometric ages shown are from Davis and Paces, (1990). ”R” and “N” indicate reversed and normal polarity of remnant magnetization, respectively. 62 �The massive interiors of the lava flows are fine-grained with intergranular texture. The predominant rock forming mineral is plagioclase with laths up to about 2 mm in length with an aspect ratio of around 10:1 . Typically, more equant altered mafic minerals, less than 1 mm across, and opaques, less than 0.2 mm across, fit between the plagioclase laths. In most massive interiors, the mafic minerals are completely pseudomorphically replaced by chlorite, however, in some interiors; patches of original pyroxene have survived a combination of burial metamorphic and hydrothermal alteration. Some plagioclase has altered to sericite. The plagioclase laths and space between them contain irregular patches of calcite. The abundance of calcite is near zero in some flow interiors and much greater in others, but always less than a few percent. Amygdules are filled with quartz, calcite, chlorite, adularia, sericite, hematite, bornite, and chalcopyrite. Small amounts of copper sulfides are particularly noted in flow tops cropping out at the top of the mountain. The nonmetallic minerals are similar to those found throughout the Keweenaw Peninsula (Butler and Burbank, 1929). However, the occurrence of the copper sulfides chalcopyrite and bornite in amygdules is very uncommon in the Keweenaw Peninsula. Copper sulfides are reported by Robertson (1975) in the tops of lava flows at Mount Bohemia near the tip of the peninsula. New Paleomagnetism Data Rocks of the MCR are probably among the worlds most extensively studied by paleomagnetic methods (Halls and Pesonen, 1982). Reversed polarity of natural remanence was recently reported in the flows of Silver Mountain (Kulakov et al, 2012). Well-defined characteristic remanent magnetization in samples from 13 flows revealed a paleomagnetic mean direction typical for reversely magnetized Keweenawan rocks (Fig. 4) (Kulakov et al., 2012). The paleomagnetic direction was similar to that found in the Lower North Shore Volcanics that have been dated at 1107.9±0.8 Ma (Davis and Green, 1997) and the Powder Mill Group (Halls and Pesonen, 1982) dated at 1107.3±1.6 Ma (Davis and Green, 1997). Thus, the likely age of the Silver Mountain basalts is 1107 to 1108 Ma and the same as the Siemens Creek Formation, Powder Mill Group. Figure 4: Equal area projection showing the mean paleomagnetic directions for selected reversely magnetized rocks from the MCR. Open square - Silver Mountain (N=13) (Kulakov et al, 2012.); open triangle – Powder Mill Group (N=9) (Palmer and Halls, 1986); open circle – North Shore volcanics (N=21) (Halls and Pesonen, 1982). 63 �Geochemistry Major and trace element geochemical analysis were conducted on samples from seven flows (Fig 5.) These samples were characterized by very uniform composition for both major and trace elements. The major and trace element composition of the Silver Mountain flows are very similar to that reported for the Upper Siemens Creek formation and equivalent rocks of the lowermost part of the North Shore Volcanics and Osler Volcanics. These youngest rift-related flows belong to basalt type II of Nicholson et al, (1997). The compositional similarity of the flows of Silver Mountain to this group of basalts, and in particular to the Upper Siemens Creek rocks further confirms the close relationship of the Silver Mountain lava flows to the Powder Mill Group. Nicholson et al. (1997) concluded that these basalts were derived from a mantle plume, but were contaminated by continental lithospheric crust. Figure 5: Primitive mantle normalized plot comparing the average trace element composition of rocks of Silver Mountain (N=7) (open squares) and Basalt type II from the Upper Siemens Creek Formation (N=18) (open circle). Siemens Creek Formation data from Nicholson et al. (1997) and Silver Mountain data from Kulakov et al. (2012) Field Trip Stop This field trip provides the opportunity to observe the lava flows at Silver Mountain and to enjoy a scenic view from the top. The trip begins from the parking area at the bottom of Silver Mountain with a short hike up a trail consisting of a combination of rocky terrain and built in stairs to the top. The Silver Mountain adit is located near the parking area with the 1850s poor rock pile scattered near the entrance. It mainly consists of amygdaloidal basalt from the oldest exposed flow at Silver Mountain. The adit exists because the top flow here was more mineralized adjacent to a fault striking parallel to the adit. Fragments of fault breccia can be observed. 64 �After viewing the mineralized rocks from the adit, the trip will proceed laterally to view the relatively thick lava flows at the base of Silver Mountain. The trip will then continue with a climb to the top of the mountain and involves strenuous physical exertion. Along the way there will be an opportunity to observe the character of the massive interiors and amygdaloidal flow tops of several flows with different thicknesses. The thickest ( ~ 6 m) lava flow at Silver Mountain crops out approximately half way to the top. Towards the top, the thickness of the flows tends to decrease. At the top, there is an excellent scenic view of the surrounding area. The shallow-dipping lava flows roughly parallel the gentle sloping topography of the top of Silver Mountain making it more difficult to observe contacts between the cross sectional views of the lava flows. The highest point is on the updip side (southwestern side). Proceeding northeast from the top and slightly down the steep side one can observe the amygdaloidal top and underlying massive interior of a flow. The flow top is particularly notable because the amygdules are filled with copper sulfides (chalcopyrite) and calcite. The trip to the top is not recommended in stormy or wet weather. The glacially polished rocks surfaces at the top can be quite slippery when wet. REFERENCES CITED Bornhorst, T.J., 1997, Tectonic context of native copper deposits of the North American Midcontinent Rift System: Geological Society of America Special Paper 312, p. 127-136. Bornhorst, T. J., and Lankton, L. D., 2009, Copper mining: A billion years of geologic and human history: in Schaetzl, R., Darden, J., and Brandt, D. (eds.), Michigan Geography and Geology, Pearson Custom Publishing, New York, p. 69-90. Burt, A.W., 1849, Message from the President of the United States to the two Houses of Congress at the Commencement of the First Session of the 31st Congress, December 24, 1849: Pt.111, Geological Report of W.A. Burt, On Survey of Township Lines in 1846, p. 849. Butler, B.S., and Burbank, W.S., 1929, The copper deposits of Michigan: U.S. Geological Survey Professional Paper 144, 238 p. Campbell, R.E., 1952, Geophysical investigation of the Silver Mountain Area – Houghton County, Michigan: In Partial fulfillment of the Requirement for the Degree of Master of Science. Michigan College of Mining and Technology, 64 p. Cannon, W. F., Green, A. G., Hutchinson, D. R., Lee, M.W., Milkereit, B., Behrendt, J.C., Halls, H.C., Green, J.C., Dickas, A.B., Morey, G.B., Sutcliffe, R., and Spencer, C., 1989, The North American mid-continent rift beneath Lake Superior from Glimpse seismic reflection profiling: Tectonics, v. 8, p. 305-332. 65 �Cannon, W. F., 1994, Closing of the Midcontinent Rift - A far field effect of Grenvillian contraction: Geology, v. 22, p. 155-158. Cannon, W.F., and Nicholson, S.W., 2001. Geologic map of the Keweenaw Peninsula and adjacent area, Michigan. United States Geological Survey, Geologic Investigations Series, Map I-2996. 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. 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. Foster, K.W. and Whitney, J.D., 1851, Report on Geology of the Lake Superior Land District: Pt.11, 32nd Congress, Special Sessions Supt. Executive Documents, v. 41, p. 68-69. Halls, H.C. and Pesonen, L.J., 1982, Paleomagnetism of Keweenawan rocks: Geological Society of America, Memoir, 156, 173-201. Heaman, L.M., Easton, R.M., Hart, T.M., MacDonald, C.A., Hollings, P., and Smyk, M., 2007, Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon region, Ontario: Canadian Journal of Earth Sciences, v. 44, p. 1055-1086. Kulakov, E.V, Smirnov, A.V., Bornhorst, T.J., Cundari, R., and Hollings, P. N., 2012. Paleomagnetism and geochemistry of the Geordie Lake and Silver Mountain basalts: Implications for the Midcontinent Rift evolution. American Geophysical Union 2012 Fall meeting abstract. GP21A-1130 Lane, A.C., 1909, The Keweenawan Series of Michigan: Michigan Geological Survey, Publication, 6. Geological Series 4, p. 628-629. 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: implication for multiple mantle sources during rift development: Canadian Journal of Earth Sciences, v. 34, p. 504-520. Paces, J.B., and Miller, J.D., Jr., 1993, Precise U-Pb ages of the Duluth Complex and related mafic intrusions, northeastern Minnesota: Geochronological insights to physical, petrogenetic, paleomagmatic, and tectnomagmatic processes associated with the 1.1 Ga Midcontinent Rift system: Journals of Geophysical Research, v. 98, p. 13,997-14,013. Palmer, H.C. and Halls H.C., 1986, Paleomagnetism of the Powder Mill Group, Michigan and Wisconsin: A Reassessment of the Logan Loop. Journal of Geophysical Research, 91, 11, 11571-11580. 66 �Robertson, J.M., 1975, Geology and mineralogy of some copper sulfide deposits near Mount Bohemia,Keweenaw County, Michigan: Economic Geology, v. 70, p. 1202-1224. Roberts, E., 1940, Geology of the Alston District Houghton and Baraga counties, Michigan: In Partial fulfillment of the Requirement for the Degree of Master of Science, California Institute of Technology, Pasadena, California. 47p. Waggoner, T.D., 1994. Echo Lake Gabbro, Houghton County, Michigan. Abstract, Institute on Lake Superior Geology, 40th Annual Meeting, Houghton, Mich, 1994, Programs and Abstracts, pt. 1, p 70. 67 ��Field Trip 6 Geology and Environmental Site Conditions of the Copperwood Deposit, Gogebic County, Michigan Theodore J. Bornhorst A.E. Seaman Mineral Museum, Michigan Technological University, 1404 E. Sharon Avenue, Houghton, MI 49931 Allan Blaske AECOM, 401 S. Washington Square, Suite 100, Lansing, Michigan, 48933 Dave Anderson and Thomas D. Repaal Orvana Resources US Corp., 10199 Lake Road, Ironwood, Michigan 49938 97 �Introduction The Copperwood deposit is a stratiform copper deposit in Gogebic County, Upper Peninsula, Michigan that is hosted by gray to black shales and siltstones filling the Midontinent rift (MCR) (Fig. 1). Copperwood is a reduced-facies or Kuperschiefer-type sedimentary rock-hosted stratiform copper deposit (Bornhorst and Williams (in press)). Mineralization at Copperwood contains 33.2 million short tons of Canadian National Instrument 43-101 compliant Measured and Indicated resources with an average grade of 1.65 % Cu and 4.34 ppm Ag, There are 3.0 million short tons of inferred resources with an average grade of 1.07 % Cu and 2.01 ppm Ag (Ward 2011). The resources are based on a cutoff composite grade of 0.8 % Cu and thickness of 5 ft (1.5 m). The Porcupine Mountains sedimentary rock-hosted copper district (Bornhorst and Barron, 2011) encompasses the White Pine Mine and the Copperwood deposit (Fig. 1). The White Pine Mine produced approximately 4.5 billion lbs of Cu and 50 million ounces of Ag from 1953 to 1996 with a few interruptions in production. Copperwood was discovered in 1956 (a subsidiary of AMAX) after a USGS publication in Economic Geology (White and Wright, 1954) indicated potential for copper mineralization in the Western Syncline. In addition to Copperwood, the U. S. Metals and Refining Company exploration program discovered 3 lower grade mineralized areas within the Western Syncline with indicated and inferred resources of 1,348 million short tons with an average grade of 1.34 % Cu (Kulla and Thomas, 2011). During1957 to 1958, a 71 m vertical shaft, 635 m of drifting, and 3 small stopes were completed in the higher grade Copperwood deposit (Bornhorst and Williams, in press). A mine was not developed because of presumed ground stability issues that would force excess dilution during mining. Advances in mining technology combined with higher Cu prices make Copperwood an economic deposit today. Orvana Minerals Corp began exploration and environmental baseline studies at Copperwood in 2008. This was quickly followed by the first Canadian National Instrument 43101 compliant mineral resource reported in 2010 (Kulla and Parker, 2010), prefeasibility in 2011, and feasibility in 2012 (Keane et al., 2012). Copperwood was granted a mining permit by the State of Michigan in 2012 and in February 2013, the Michigan Department of Environmental Quality granted the wetlands permit which is the last major permit needed before construction and production can proceed. Production is projected to begin in the near future. The descriptions in this field guide are based on Orvana (2011) and Bornhorst and Williams (in press). This field guide is published with permission granted by Orvana Minerals Corporation; however, the content is the sole responsibility of the authors. Regional Geologic Setting The broad 300 km wide MCR in Michigan consists of more than 25 km thick succession of riftfilling tholeiitic flood basalts with minor interbedded red conglomerates and sandstones overlain by 8 km thick succession of rift-filling clastic sedimentary rocks (Cannon, 1993). A rift-flanking basin is filled with 3 km of sandstone. These rocks are make up the Keweenawan Supergroup in Michigan and were deposited from 1.15 to about 1.03 Ga (Fig. 2) (Heaman et al., 2007; Davis and Paces, 1990; Cannon et al., 1989). 98 �75 o 50o o 100 50 o Precambrian bedrock at surface Canada Minnesota Canada Lake Huron Midcontinent Rift Porcupine Mountains sediment-hosted copper district Wisconsin Canada Grenville Front Tectonic Zone Iowa Nebraska Lake Nipigon Calumet Copperwood project White Pine Mine Houghton Kansas 0 35o 100o Major native copper deposits 400 35o 75o kilometers Stipled area - Phanerozoic bedrock at surface Ontario Ontario Lake Superior Minnesota Wisconsin 46.00 Phanerozoic Sedimentary rocks Mesoproterozoic Midcontinent Rift Sedimentary rocks Igneous rocks Paleoproterozoic Metamorphosed sedimentary and igneous rocks Archean Metamorphosed sedimentary and igneous rocks Lake Michigan N 0 100 kilometers Major Faults Figure 1: Generalized bedrock geologic map of the Midcontinent Rift. Modified from Bornhorst and Barron (2011). The bedrock at Copperwood is part of the rift-filling clastic sedimentary rocks. Continental compression occurred at 1.06 Ga in response to the Grenvillian collision along the eastern edge of the North American continent inverting the original graben-bounding faults into reverse thrust faults (Fig. 2) (Cannon, 1994). A syncline at Copperwood is a result of this compressional event. Erosion followed continental compression from about 1.03 Ga to 0.5 Ga (500 Ma) during which time multiple km of bedrock was removed and likely exposing the Copperwood orebody at the bedrock. This would have allowed a period of downward percolating groundwater into the Copperwood deposit (Bornhorst and Robinson, 2004). Marine submergence beginning 500 Ma buried the Precambrian bedrock under multiple km of Phanerozoic sedimentary rocks; evidence of Phanerozoic rocks is missing at Copperwood. 99 �Pleistocene glaciation over the last 2 million years removed the Phanerozoic rocks overlying Copperwood and once again exposed the orebody at the surface. The retreat of the last glaciers about 10,000 years ago left behind unconsolidated glacial deposits that today overlie the Precambrian bedrock at Copperwood. Peterson (1985, 1986) determined that four distinct advances of glacial ice occurred in the western Upper Peninsula during the late Wisconsinan time (14,500 to 10,200 years B. P.). Two of these advances and retreats produced the current surficial features at Copperwood. The first advance moved out of the Lake Superior basin south to the Wisconsin-Michigan border and upon retreat, left behind the lower till. The youngest advance occurred approximately 10,200 years B.P., when the glacier overrode previous till deposits at Copperwood. The approximate southern limit of this last advance was approximately 3 km south of Copperwood. Following this final retreat, glacial Lake Duluth covered the Copperwood area. Several Lake Duluth shorelines are evident along the topography at Copperwood. o o 88o 89 90 Copper Harbor N Calumet Yj Yn Hancock Houghton 47o Copperwood Deposit 2- 6 Ontonagon L'Anse White Pine mine 1 Ironwood Wakefield 3 Stop Location 0 fault 10 20 30 kilometer Figure 2: Bedrock geologic map and lithostratigraphic column for the western part of the Upper Peninsula of Michigan. The location of the field trip stops are shown. 100 �Mesoproterozoic Bedrock Geology The Copperwood deposit is on the southwest limb of the open shallow-plunging Western Syncline (Fig. 3). The bedrock at Copperwood consists of clastic sedimentary rocks of the Mesoproterozoic Oronto Group (Fig. 2) that strike approximately east-west and dip approximately 10 degrees to the north. The MCR bedrock at Copperwood is overlain by unconsolidated Pleistocene glacial sediments. The lowermost lithostratigraphic layer at Copperwood is the Copper Harbor Formation that consists of primarily of reddish-sandstone. The Nonesuch Formation overlies and interfingers with the Copper Harbor Formation. It consists of gray-black shale and siltstone to gray-white siltstone to brownish-red siltstone that are subdivided into multiple informal subunits. The Freda Formation gradationally overlies the Nonesuch Formation and consists of brown siltstone at Copperwood. Lithostratigraphy Copper Harbor Formation. Overall, the Copper Harbor Formation is composed of red-brown conglomerates and sandstones with lesser siltstones in an upward- and basinward-fining sequence. In Michigan, there is a maximum exposed thickness of about 2,000 m (Elmore 1984). The Copper Harbor Formation sedimentary rocks are fluvial and deposited a coalescing alluvial fan environment. At Copperwood, the Copper Harbor Formation is the oldest lithostratigraphic bedrock formation. It is lithologically dominated by red-brown to white and gray fine- to coarse-grained, arkosic sandstone. The Copper Harbor Formation in one drill hole consisted of 140 m of sandstone, a 1 m thick red, matrix-supported conglomerate, and more sandstone. Outcrops of the Copper Harbor Formation along the southern portions of Namebinag Creek and an unnamed creek indicate an increase in conglomerate facies in the lower portions of the formation. Outcrops along these streams are predominantly conglomerates with lesser amounts of sandstones. The uppermost few feet of the Copper Harbor Formation intersects in all of the Orvana and legacy exploration drilling at Copperwood. The uppermost Copper Harbor Formation consists of interlaminated red-brown siltstones and shales with occasional beds of very fine-grained sandstones. Uncommonly, there are interbedded, thin beds of dark-gray shales and siltstones less than 1.5 cm below the upper contact indicative of interfingering overlying Nonesuch lithologies. Absent in some holes, is the red-brown siltstone at the top of the Copper Harbor Formation. It can be up to 1 m thick, but is typically less than 30 cm in thickness. Assay data has shown, the uppermost 1 m of the Copper Harbor Formation (red-brown siltstone and sandstone) does not carry significant amounts of copper. There is a dramatic and abrupt change from the reddish Copper Harbor Formation to dark-gray to black shales and siltstones of the overlying Nonesuch Formation. At Copperwood, this abrupt transition defines the change from the Copper Harbor Formation to the Nonesuch Formation. 101 �11 A’ Porcupine Mountains 22 21 20 11 6 29 30 28 27 33 34 7 32 31 36 4 6 8 2 1 5 6 4 3 20 A N Copper Harbor Formation 1 0 kilometers 11 12 25 8 7 10 9 Presque Isle River GeologyfromunpublishedmapsbyForbes(1959), Copper RangeCompany, andOrvanaexploration 0 M117 0 M57 Meter 500 1000 Feet 2000 N Orvana Drill Hole (C) M27 Western Sector of deposit M24 C68 M21 M13 C41 C42 C60 C62 C37 C111 1 Section Number M25 M63 C52 C39 C40 Section Line C100 36 M70 C16 M62 C44 M54 M23 C99 C58 C54 C101 P2 P5 C114 P6 P7 C31 P1 C73 C70 C55 C119 C116 C35A C36 C56 C66 M64 C32 C17 M48 M59 C79 M46 C78 C75 C71 C33 C50 P4 M69 C86 C81 P3 M28 C110 C29 P9 M22 C142 P8 C74 C49 C98 C13 C25 C30 C45 C28 C140 C83 C85 C80 C122 P10 C65 C136 C87 C104 C26 C106 C115 C82 C84 C138 C141 M53 C24 M31 C27 C47 C143 C95 C107 C97 C67 M72 C61 M11 C96 M52 M75 C108 M159 C88 M58 C102 C20 C63 C59 C77 P12 C92 C137 C139 C94 C121 C123 P11 M18 C21 C91 M9 P17 C133 C109 C93 C90 C43 C103 C69 C48 C105 P13 P18 M108 M12A C128 C23 C127 C126 M109 M80 M19 C38 M114 C132 M49 C130 M116 M4A P22 C89 C9 C22 C125 C129 C118 C11 C131 C113 C117 P16 P14 P19 P15 Nonesuch Formation at Bedrock Surface M103 C57 USMR Legacy Hole (M) Bear Creek Legacy Hole (P) M26 C34 C46 C112 M32 Freda Formation at Bedrock Surface C53 C72 C64 C51 C76 Eastern Sector of deposit Copper Harbor Formation at Bedrock Surface 2 Tabular CBSprojected to surface 11 Measured and Indicated Resources Inferred Resources Fault 1 1 6 12 12 7 T49N R46W P20 P21 T49N R45W Canadian National Instrument 43-101 Compliant Resource Classification Figure 3: Geologic map of the Western Syncline and Copperwood deposit. Modified from Bornhorst and Williams (in press). 102 �Nonesuch Formation. Overall, the Nonesuch Formation is composed of characteristically blackto-gray -green siltstones, shales, carbonate laminates, and minor sandstones with a maximum thickness of 215 m. Elmore et al. (1989) and Suszek (1997) interpreted the depositional environment of the Nonesuch Formation to be dominantly anoxic lacustrine ranging from marginal lacustrine (sandflat-mudflat) to lacustrine to lacustrine-to-fluvial subenvironments. At Copperwood, a completed stratigraphic section is exposed in the northeast part of the property, at a thickness of 200 to 215 m. The upper contact is missing due to erosion throughout most of Copperwood property. The formation has been subdivided into multiple informal members on the basis of lithologic variations (Fig. 4). All of these members have remarkable lateral continuity throughout the Copperwood area (Fig. 5). The Parting Shale member is at the base of the Nonesuch Formation and is further subdivided into units (Fig. 4). The three lower units of the Parting Shale are the host to copper mineralization at Copperwood and together are termed the Copper-Bearing Sequence (CBS). The lowermost unit of the Parting Shale and CBS is termed Domino after terminology used at the White Pine Mine. Domino averages 1.6 m thick in the western sector and thins to about 60 cm thick in the eastern sector. The overall average thickness is 90 cm with a range from 9 cm to 2.3 m. Domino is characterized by laminated dark-gray to black shales and siltstones. Domino hosts the highest-grade copper at Copperwood. The contact between Domino and the overlying Red Massive unit is sharp and easily recognized in drill core as an abrupt change from the dark gray/black (Domino) to red-brown (Red Massive). The Red Massive unit of the Parting Shale and medial unit of CBS averages 35 cm thick, ranges in thickness from near zero to 1.2 m, but is usually less than 50 cm thick. It is somewhat thicker in the eastern sector than in the western sector. Red Massive is characterized by massive dark red-brown siltstones with interbedded red-brown, fine-grained sandstones. The contact between Red Massive and the overlying Gray Laminated unit is gradational and is placed where the color changes from reddish gray to gray. The Gray Laminated unit of the Parting Shale and upper unit of CBS averages 1.1 m thick and ranges in thickness from 50 cm to 4 m. Gray Laminated is characterized by of light-to-medium, gray-to-reddish-gray laminated siltstones; some intervals are massive. The contact between Gray Laminated and the overlying Red Laminated is gradational and is placed where the color changes from gray-dominated to mixed maroon and gray. The Red Laminated unit of the Parting Shale and hanging wall of the CBS ranges in thickness from 10 cm to 3.4 m and is more typically 1.2 to 1.8 m thick. Red Laminated is characterized by laminated siltstones with bimodal color distribution of maroon/red-brown and gray. Typical Red Laminated has mottled or wavy maroon intervals interspersed with medium-gray to reddish-gray siltstones. The contact between Red Laminated and the overlying Gray Siltstone is gradational. 103 �0 Copperwood Terminology 0 10 Feet 100 Surface Meters 20 30 Glacial clay-rich till Alpha Code for Sections Freda Formation L M Brown-red and greyish-red siltstone K Reddish-black siltstone J Greyish-red Siltstone I H Cross-stratified grey siltstone Copperwood Terminology Upper Sandstone Grey and red-brown interbedded siltstone 9 8 Nonesuch Formation 7 G Black laminated siltstone 6 F Grey concretion siltstone E Dark grey laminated siltstone 5 4 Red Laminated 3 D C B A Gray Laminated Galaxy Stripey 2 Red Massive Upper Shale 1 Upper sandstone Domino Parting Shale 0 Copper Harbor Formation RedSiltstone Sandstone Red and white sandstone meters Figure 4: Lithostratigraphic column of Copperwood bedrock units with detail of Parting Shale member. Modified from Bornhorst and Williams (in press). 104 �Freda Formation. The top of the Nonesuch Formation gradually transitions into the Freda Formation over an interval of about 10 m where beds of coarse, light-brown siltstones and massive to cross-bedded, dark reddish-brown siltstones are intercalated with grayish-red siltstones. The contact is placed at the base of a brown to white, cross-bedded siltstone. At Copperwood, the Freda Formation is up to 120 m thick above and consists of brown siltstone. Only the base of the formation occurs at Copperwood as the rest has been removed by erosion. Structure The structure at Copperwood is simple and consists of bedrock units that dip gently to the north on the southwest limb of the Western Syncline (Fig. 5). Under the unconsolidated glacial sediments, dips for all bedrock units vary from 12° in the south near the subcrop to 8° in the north nearer the synclinal axis. The bottom surface of the CBS approximates a gently curved, dipping plane lacking significant undulations. One fault has been identified at Copperwood (Fig. 3). This fault is interpreted to be a shallow, north-dipping reverse fault with 3 to 7 m of vertical displacement. Minor fractures with less than 1 inch of displacement were observed in multiple holes and these fractures are typically healed by calcite. South A 229 Subcrop Base of Nonesuch Formation 168 1,300 meters or 4,263 feet C89 C87 C55 292ft East 265ft West C/D B North A’ C99 C41 211ft West 183ft East 162ft West 750 H K J G M J L I I E H C/D B 107 550 K H J G G I E H 350 E 46 -15 MASL 6 5 4 3 2 1 0 No Vertical Exaggeration C/D B C/D B 150 G E C/D B -50 FASL C89 C87 C55 Red Laminated Red Laminated Upper CBS Upper CBS Domino Domino C99 C41 RL Red Laminated Upper CBS D Domino 20 15 10 5 0 CBS- Datum- topof Copper Harbor Formation Vertically Exaggerated M117 M57 LakeSuperior M27 South-North Cross SectionA- A’ M24 M26 C34 M32 C57 C46 C41 C111 C112 B’ C37 M63 M25 C40 C42 C39 C52 C60 C62 C100 M62 C44 M70 M54 M23 C64 C16 C101 C99 C58 C54 C76 C114 P7 C31 C55 C70 C116 C35A C36 C56 C66 M48 M46 M59 C32 C17 C79 M64 C75 C78 C71 C33 C50 C29 C86 C81 M69 C83 M28 C110 P8 M22 C49 C74 C98 C28 C13 C25 C30 C45 C80 C65 C122 C140 C85 C87 C104 C26 C115 C106 C82 C84 C138 C24 M53 C95 M31 C107 C97 C27 C96 M11 C47 C67 M72 C61 C77 M52 M75 C102 M159 C88 M58 C108 C121 C20 C63 C59 C94 C 92 C123 C21 M18 C109 M9 C91 C93 C90 C43 C105 C103 C48 C69 P18 M108 C126 C128 C127 C23 M12A M109 M19 M80 M114 C38 M49 P22 M116 M4A C9 C22 C89 C118 C11 C113 C117 C68 36 C53 C119 M103 M21 M13 C72 C73 C51 B P9 P2 P5 P6 P1 P4 C142 P3 C143 P10 C136 C141 P17 C133 C137 C139 P11 P12 P13 C130 P19 C132 C125 C129 P16 C131 P14 P15 NoMineralizedHorizon 2 1 Copper 11 12 P20 P21 Harbor Formation1 6 12 7 HoleusedinCrossSection T49NR46W T49NR45W Figure 5: Geologic cross section of the Copperwood deposit. Modified from Bornhorst and Williams (in press). 105 �Copperwood Deposit At Copperwood, copper mineralization is hosted by gray to black shales and siltstones of the CBS (Fig. 4). The footwall consists of red-bed sandstones and minor siltstones of the Copper Harbor Formation and the hanging wall consists of maroon/red-brown to gray siltstones of the Red Laminated unit. Geologic cross sections using the base of the CBS as the horizontal datum demonstrate the high degree of lithologic continuity of the CBS. The copper orebody is a conformable and tabular with an average thickness of 2.5 m. In the western sector the CBS averages 2.9 m thick whereas in the eastern sector it averages 2.1 m thick. The Copperwood deposit contains a total (all categories) undiluted geologic resource of about 1.16 billion lbs of Cu and 4.4 million ounces of Ag. The deposit (CBS) is characterized by copper in the form of chalcocite with minor amounts of silver. Other copper minerals such as chalcopyrite and bornite occur above the CBS (Bornhorst and Williams, in press). Pyrite is virtually nonexistent in the CBS, but above the Parting Shale, pyrite does occur in low abundance. The CBS is dominantly siltstones that are composed of over 90% silicate minerals (quartz, clinochlore, muscovite, illite, K-feldspar, plagioclase), about 2% calcite, and 3% hematite. Overall, pyrite and other minerals with the potential to generate acids are lacking whereas calcite, which is an acid-neutralizing mineral species, is abundant. The White Pine Mine produced copper from almost the entire Parting Shale and from the overlying Upper Shale (Fig. 4) (Mauk, 1992; Ensign et al., 1968). Whereas the three layers that compose the CBS at Copperwood are lithologically similar to those at the White Pine Mine, their thicknesses and proportions are not. The total Parting Shale is much thicker at Copperwood and e.g., the Domino within the western sector is typically 2.5 times thicker than at White Pine. At the White Pine Mine, the ore minerals are chalcocite and native copper whereas Copperwood is devoid of native copper. At White Pine, the chalcocite and minor native copper is stratiform ore interpreted as being related to diagenesis (Brown, 1971; Ensign et al., 1968). The native copper represents a second stage of mineralization (Mauk et al., 1992) hosted in faults and fractures and is interpreted as being related to the native copper deposits of the Keweenaw Peninsula (Bornhorst, 1997; see Field Trip 1 this volume). Copperwood lacks the complexity of the White Pine deposit (Bornhorst and Williams, in press). The White Pine copper deposit straddles a right-lateral strike-slip fault and an anticline (Ensign et al., 1968; Johnson et al., 1995). Thrust faults, strike-slip faults, and normal faults are encountered throughout White Pine and these faults and folds are mostly related to late rift compression. Some clearly compression-related thrust faults host sheets of native copper. The second-stage mineralizing fluids were likely of the same origin as those related to native copper in the Keweenaw Peninsula (Bornhorst, 1997). The Copperwood deposit is an example of a reduced facies or Kupferschiefer-type sedimentary rock-hosted copper deposit (Bornhorst and Williams, in press). Bornhorst and Williams (in press) proposed that at Copperwood “chalcocite replacement of pyrite in unlithified sediments during diagenesis is a result of emanating upward-focused, compaction-driven, Cu-bearing saline basinal waters whose Cu was leached from the underlying red-bed paleoaquifer.” In comparison to most examples of reduced facies or Kupferschiefer-type (Hitzman et al., 2010, 2005; Cox et al., 2003), Copperwood is notable for its simplicity (Bornhorst and Williams, in press). 106 �Pleistocene Unconsolidated Glacial Deposits The unconsolidated deposits at Copperwood consist primarily of a reddish-brown glacial till. This glacial till unconformably overlies the bedrock and ranges in thickness from 0 (at outcrops along streambeds south of the subcrop of the Copperwood deposit) to 43 m; average thickness is approximately 25 m (Fig. 6). The top of the bedrock surface is generally smooth. Boulders or otherwise weathered bedrock were encountered in some borings at the bedrock surface, but at most locations the glacial till sits on a scoured bedrock surface. The surface of the bedrock is more or less parallel to the ground surface topography, and slopes toward the north-northwest. Figure 6: Schematic cross section through the Copperwood project with generalized geology. The glacial till at Copperwood is a mud-matrix supported diamictite. This diamictite is massive with no stratification, lamination or fining upward or downward and the matrix is a uniform mix of sand, silt, and clay. Grain-size distribution curves are typical of those associated with subglacial tills (Fig. 7). The characteristics lead to the interpretation of the glacial till at Copperwood as a subglacial diamictite (Kemmis, 2008), meaning that it was deposited beneath the glacial ice. The diamictite is dense and overconsolidated, characteristic of subglacial till, but unlike normally consolidated to slightly overconsolidated lacustrine deposits. The glacial till was described during the soil boring program as variations of a silty clay based on slight variations in the observed portions of silt, clay, and sand. Field classification ranged from silt, clay, silty clay, clayey silt, silty sand, and sandy silt. The till was found to contain trace (1 to 9%) to little (10 to 19%) to some (20 to 34%) amounts of sand and gravel. Soil samples tested for particle size distribution indicated that the average composition of the cohesive (silt/clay) was approximately 50% silt, 20% clay, and 30% sand (Fig. 8). Sample analysis of the till fine fraction (<200 sieve size) by X-ray diffraction indicate that quartz was the major component, 107 �Figure 7: Grain size distribution of glacial till samples from the Copperwood site. with very little clay minerals (kaolinite, illite, montmorillonite, etc.) present. Analyses also indicated the presence of small amounts of feldspar, micas, calcite, and hematite. Testing using dilute hydrochloric acid indicated the presence of very finely-ground calcite, volumetrically too low to be detected by X-ray diffraction. The gravel/cobble portion was difficult to estimate from the laboratory analyses, due to the mass of the samples subjected to sieve analysis. Some samples contained no gravel-sized fraction, while others contain up to 45% (by dry weight) of gravel (coarse and fine). Inspection of Rotosonic soil cores revealed as much as 10 to 20% coarse fraction (% gravel by volume) in the recovered soil core. Gravel, cobbles and boulders larger than 9 cm (diameter of drill sampling device) were encountered during the drilling as well as during site reconnaissance activities. Large boulders (up to 1 m in diameter) were present within the bluffs along the Lake Superior shoreline. The gravel portion of the till consisted of dark reddish-brown sandstone (between 60% and 70%), diabase (between 8% and 18%), granite/gneiss (between 7% and 13%), basalt/amygdaloid (between 1% and 5%), and other types of rocks (between 2% and 8%). The sandstone and basalt are likely to have been locally derived from the Keweenawan Supergroup bedrock. Beach stones on the Lake Superior shoreline represent a material washed from the entire thickness of the till outcrops along the lakeshore. The predominance of red sandstone (likely Freda Formation) in the till accounts for the overall red-brown color of the glacial material. 108 �Figure 8: Soil texture plot of glacial deposits from the Copperwood site. Figure 9: Stratigraphic section of the glacial overburden deposits at the Copperwood site. 109 �The glacial till can be divided into two units (upper till and lower till), based on matrix grain size, amount and size of gravel, and vertical distribution (Fig. 9). In addition, there are thin (< 3 m with an average thickness of < 1 m) and isolated layers of coarser (non-cohesive) sediments throughout Copperwood in various borings that are located predominantly between the upper and lower till units. These generally consisted of fine to medium sand with varying amounts of silt and clay. These granular deposits, when encountered, are not laterally extensive, and for the most part cannot be correlated between adjacent borings. They are interpreted as lacustrine, intra-till sands or subglacial melt water deposits. Additional granular deposits were found in three borings at the base of the overburden, on the top of the bedrock surface. Peterson (1985) described the glacial deposits of the area as thin (< 10 m) drift over bedrock. Hack (1965) described the Ontonagon Plain (located on the east side of the Porcupine Mountains) to be underlain by reddish-brown glacial lake sediments and till. He described three units within the glacial deposits – the lower, intermediate, and upper units. The lower unit is described as a stony till containing locally derived subangular boulders and fragments. The intermediate unit is described as till and laminated silt and clay in distinct layers that are believed to be lacustrine sediments. This unit is less stony than the lower unit. The upper unit is described as a clayey till that is much less stony than either of the lower units. Thin lacustrine deposits related to glacial Lake Duluth are described as a patchy thin (< 60 cm) overlying veneer of strongly-laminated clay, silt and sand of variable thickness. The three primary glacial units described by Hack (1965) are present at Copperwood: the lower till which is slightly coarser grained with more (and larger) clasts than the upper till, the intermediate unit, less well defined at Copperwood, but composed of the laterally inconsistent layers of silty and sandy sediments, and the upper till. A thin (< 1 m thick) of lacustrine deposits related to Lake Duluth exists at Copperwood. The glacial deposits at Copperwood are composed predominantly of subglacially deposited till that was streamlined by glacial movement into elongated drumlins and flutes. While the flutes or ridges are not obvious within at Copperwood, such features may have been the cause of the distinct modern Copperwood drainage pattern. 110 �Objectives of Field Trip This field trip is designed to provide a geologic overview of Copperwood deposit hosted by the Mesoproterozoic MCR bedrock through descriptions and observations of drill core. The environmental site conditions at Copperwood will be observed and discussed onsite, especially those associated with surface and groundwater. The unconsolidated Pleistocene glacial deposits that unconformably overly the deposit will be observed as they play an important role in the environmental site conditions. The environmental site conditions are a critical aspect of permitting a modern mine. The location of Field Trip stops depend on site activities and accessibility. Those described below are based on full access and moderately dry conditions. Participants will be provided a map at the time of the field trip. Access to the Copperwood site is strictly forbidden without express permission from Orvana Minerals US Corp. No samples of any kind are allowed to be removed from the Copperwood site during this field trip. STOP 1: Orvana Offices, Ironwood, Michigan The first stop of the field trip will be to the offices of Orvana Resources U. S. Corp. in Ironwood. An overview of the Copperwood geology and project will be provided. Core of the bedrock and unconsolidated glacial deposits will be available for inspection and discussion. Core drilling of the Copperwood deposit will be available for inspection. Since there is little lateral variation within the Copperwood orebody, core from only a few drill holes are necessary to observe the character of the orebody as a whole. The unconsolidated glacial deposits were sampled using Rotosonic drilling techniques. Core samples of the glacial till will be available for inspection. Slight differences in the matrix composition and gravel content can be observed in the various core samples. STOP 2: Copperwood Historic Test Mine Rock Pile and Weather Station The Copperwood historic mine rock pile is a result of testing mining in 1957 to 1958. This rock pile was once much larger, as prior to Orvana’s activities at Copperwood, this rock was used as fill in wet areas of roadways and along stream crossings. The rock pile was used by Orvana as a staging area for exploration and pre-development activities and for an environmentally focused rock pile study. The rock pile will be removed upon creation of the tailings disposal facility. The rocks in this 1957-58 rock pile are dominated by unprocessed ore (CBS), but include hanging wall and footwall rocks as well. None of the ore was processed during this 1957-58 test mining, except for bulk samples delivered to Michigan Tech (then known as Michigan College of Mining and Technology) for process testing. The rock pile has been subjected to slightly more than 50 years of weathering which continues today. There is no evidence of acid drainage from the rock pile. Blocks of black shale, likely Domino, which contains the most copper within the CBS, are present on the surface of the rock pile and can be identified by notable green surface coloration. The green mineral has been identified by X-ray diffraction as malachite (copper carbonate). These blocks readily separate into smaller fragments along bedding planes 111 �and, upon separation, the malachite is only visible for less than 2 cm from the edge of the block even when the bedding planes are visibly moist. An environmentally focused study was initiated by Orvana to validate bench scale laboratory determined rates of release for chemical constituents and evaluate the long-term environmental impact of weathering of Copperwood unprocessed ore-bearing rock. After 50 years of well aerated, well-drained, and high-infiltration leaching, the rock pile has been and continues to be acid-neutralizing since the precipitation today is acidic. The study of the rock pile will be discussed at this stop. Immediately to the south of the rock pile a weather station is located in a clearing on the east side of the entrance road. This station was installed in 2008 and used to collect site-specific temperature, wind, precipitation, ground temperature, and air quality information for the Environmental Impact Assessment. STOP 3: Monitoring Well Sites Monitoring well nests were installed across the site to determine groundwater and aquifer characteristics (flow direction, flow rate, vertical gradients, groundwater chemistry, etc.). As we travel towards the Lake Superior shoreline, several sets of monitoring wells can be observed, and we will stop briefly to discuss the data collected from them. At each monitoring well site, a well was installed into the glacial deposits, and a second well installed into the underlying bedrock. Groundwater elevation data collected from the well network indicates that the groundwater in the glacial deposits, upper portions of the Copper Harbor formation, and the Nonesuch formation flow to the north-northwest, toward Lake Superior, and generally follows the slope of ground surface topography. The indicated groundwater velocity is from 0.63 to nearly 2.8 feet per year (fpy) within the bedrock units and from 0.7 fpy to nearly 1.1 fpy in the glacial deposits. The highest concentrations of total dissolved solids (TDS) and chloride in the groundwater are located in the top of the Copper Harbor Formation and the bottom of the Nonesuch Formation; concentrations decrease upward through the Nonesuch (Fig. 10). The groundwater in the glacial deposits generally contains much lower TDS concentrations. There is very little connection between groundwater within the various units, except for the uppermost unit in the glacial deposits which illustrates characteristics of recharge from surface water. 112 �113 �Groundwater in the uppermost portions (upper 30 feet) of the glacial deposits has relatively low TDS (average of 477 mg/L). It is depleted in sodium, chloride, and sulfate, and is mainly calcium bicarbonate type water. This water type is typical of groundwater near a precipitationfed recharge zone that has had relatively short contact time with the geologic materials. Groundwater within the remainder of glacial deposits has an average TDS of 878 mg/L (ranging between 110 mg/L to 7,300 mg/L) and it is depleted in magnesium and sulfate. The ion composition varies from sodium chloride, calcium chloride, sodium bicarbonate/carbonate, to calcium bicarbonate/carbonate water types. These variations in water type and TDS concentrations indicate the groundwater is not in connection with a precipitation-fed recharge zone and lacks connection with or flow path between the overlying uppermost zone. Sodiumand/or chloride-type water typically indicates that groundwater is equilibrating with the surrounding geologic matrix. For wells screened in the Nonesuch Formation, TDS ranges between 115 mg/L to 34,000 mg/L, with an average of 6,800 mg/L, which is about eight times that for groundwater residing in the overlying glacial deposits. Calcium chloride is the dominant water type, particularly when TDS is elevated. Sodium is also a dominant cation in groundwater at some wells. This groundwater is not near a precipitation-fed recharge zone and is not connected with the overlying glacial deposits. Wells screened in the Copper Harbor Conglomerate have a range of TDS between 140 mg/L to 66,000 mg/L, with an average of 10,545 mg/L, which is about 1.5 times that for groundwater in the overlying Nonesuch Formation. Groundwater in one well has TDS at 66,000 mg/L, which is greater than the TDS of sea water (35,000 mg/L). The groundwater is calcium-chloride type. In addition to calcium and chloride, sodium and bicarbonate/carbonate ions are dominant in groundwater from wells with lower TDS. This groundwater lacks connection with other groundwater zones beneath the site. STOP 4: Surface Water Monitoring Points Monitoring of the flow characteristics of the surface water in streams was performed at several locations at Copperwood. As we travel towards the Lake Superior shoreline, streams can be observed, and we will stop briefly to discuss the data collected from them. The surficial drainage system at Copperwood is part of the Lake Superior watershed and is composed entirely of small streams, roughly parallel to one another, flowing to the northwest from higher ground towards the south directly into Lake Superior. There are no lakes at Copperwood. Water flow within the streams is flashy and significantly controlled by timing and duration of precipitation. No groundwater contribution has been observed in these streams. High flow in streams occurs during spring when the snow melts and after significant rain events. Flow increases and decreases quickly during rain events. All of the streams have periods of zero measurable flow either due to dry conditions or freezing in the winter. During the summer between rain events, a slight “trickle” of water can be observed flowing between cobbles in the stream bed which enters and exits multiple isolated pools. The many isolated pools found on all of the streams are also maintained by water flow just beneath the stream-bottom substrate. The 114 �upper reaches of the streams at Copperwood can be classified as ephemeral (flow only during or immediately after periods of precipitation) and the lower reaches as intermittent (flows only during certain times of the year). Perennial streams, which have continuous flow, are not present at the site. Ephemeral streams have no base flow and the stream beds are above the water table. Intermittent streams have base flow for at least some periods of the year. At the Copperwood site, this base flow is extremely small. No springs, seeps, or areas of wetland vegetation were observed that would indicate groundwater discharge to the surface water environment. Surface water at Copperwood has a neutral to slightly alkaline pH with most values are between 6.5 and 8.0. Lake Superior water is slightly alkaline (average and median pH of about 8). The average and median dissolved oxygen values for surface water (8.3 mg/L and 7.9 mg/L) and Lake Superior water (8.9 mg/L and 8.0 mg/L) indicate the waters are oxidized as is typical for surface water in contact with the atmosphere. The surface water is calcium-bicarbonate type, which is associated with precipitation and little or no contact with soil. The surface water contains TDS at levels of approximately 20% of that in the groundwater in the uppermost glacial till, which is consistent with very little groundwater contribution to surface water. STOP 5: Incised Stream Channels The streams at the site are deeply incised into the glacial overburden. Along the entrance road, the stream channels are only a few feet deep, yet at the north end near Lake Superior, the channels are as much as 12 m deep. As we travel towards the Lake Superior shoreline, streams can be observed, and we will stop briefly to discuss the characteristic of these channels. The incised stream channels are contained within steep-walled valleys. Active erosion is present within the steep-walled portions of the stream valleys. The floors of the valleys are generally flat and range in width between 15 and 60 m. The streams meander within the bottoms of the valleys with significant portions of the streams dammed by beavers creating tiered meadows within the valleys. The upper portions of the stream valleys are generally narrower and shallower than those further downstream. The overall gradient of the streams at Copperwood is approximately 20 m per km. STOP 6: Glacial Exposures along Lake Superior Shoreline At this stop we will examine the exposure of unconsolidated glacial material in the eroding bluff along the Lake Superior shoreline. The current shoreline of Lake Superior is dominated by a steep bluff which rises as much as 15 m above the lake surface. The entire face of the bluff is composed of slumped blocks of silt/clay-rich till. The majority of the bluff is not vegetated, but slumped blocks of soil often contain trees and surface vegetation that were brought down from the top of the bluff; the bluff is experiencing significant erosion. A beach of mixed sand and cobbles is present at the base of the bluff. The maximum width of this beach is 10 m and in many places is much narrower. The beach does little to protect the base of the bluff from wave action and in many places, wave action reaches across the narrow beach to the base of the bluff. 115 �The upper till unit of the glacial diamictite is exposed on the face of the bluff. The diamictite has a massive structure with no observed stratification or laminations. It is composed of a silty clay matrix with scattered gravel. The gravel is mostly cobbles and pebbles less than 8 cm in diameter, but there are boulders up to one meter in diameter. The cobbles and pebbles on the beach are predominantly red sandstone which is likely from the Freda Formation. The exposed diamictite is interpreted as a subglacial till. References Bornhorst, T.J., 1997, Tectonic context of native copper deposits of the North American Midcontinent Rift System: Geological Society of America Special Paper 312: pp. 127-136. Bornhorst, T.J., and Barron, R.J., 2011, Copper deposits of the western Upper Peninsula of Michigan: Geological Society of America Field Guide, v. 24, p. 83-99. Bornhorst, T. J., and Robinson, G.W., 2004, Precambrian Aged Supergene Alteration of Native Copper Deposits in the Keweenaw Peninsula, Michigan [abstract]: Institute on Lake Superior Geology Proceedings, v. 50, Part 1: pp. 40-41. Bornhorst, T.J., and Williams, W.C., in press, The Mesoproterozoic Copperwood sedimentary rock-hosted stratiform copper deposit, Upper Peninsula, Michigan: Economic Geology. Brown, A.C., 1971, Zoning in the White Pine Copper deposit, Ontonagon County, Michigan: Economic Geology , v. 66, p. 543-573. Cannon, W. F., 1994, Closing of the Midcontinent Rift - A far field effect of Grenvillian contraction: Geology 22, p. 155-158. Cannon, W.F., Green, A.G., Hutchinson, D.R., Lee, M., 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. Cannon, W. F., Peterman, Z.E., and Sims, P.K., 1993, Crustal-scale thrusting and origin of the Montreal River monocline - A 35-km-thick cross section of the Midcontinent Rift in northern Michigan and Wisconsin: Tectonics, v. 12, p. 728-744. Cox, D.P., Lindsey, D.A., Singer, D.A., and Diggles, M.F., 2003, Sediment-hosted copper deposits of the world: Deposit models and database: U.S. Geological Survey Open-File Report 03-107. 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 97, p. 54-64. 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. 116 �Elmore, R.D., Milavec, G.J., Imbus, S.W., Engel, M.H., 1989, The Precambrian Nonesuch Formation of the North American Mid-Continent Rift: Sedimentology and Organic Geochemical Aspects of Lacustrine Deposition: Precambrian Research, v. 43, p. 191-213. Ensign, C.O., Jr., White, W.S., Wright, J.C., Partick, J.L., Leone, R.J., Hathaway, D.J., Trammell, J.W., Fritts, J.J., and Wright, T.L., 1968, Copper deposits in the Nonesuch shale, White Pine, Michigan: SME Graton Sales Ore Deposits of the United States 1933-1967, v. 1, p. 460-488. Hack, J. T., 1965, Postglacial Drainage Evolution and Stream Geometry in the Ontonagon Area, Michigan, U.S. Geological Survey Professional Paper 504-B. Heaman, L.M., Easton, R.M., Hart, T.M., MacDonald, C.A., Hollings, P., and Smyk, M., 2007, Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon region, Ontario: Canadian Journal of Earth Sciences, v. 44, p. 1055-1086. Hitzman, M., Selley, D., and Bull, S., 2010, Formation of sedimentary rock-hosted stratiform copper deposits through Earth history: Economic Geology, v. 105, p. 627-640. Hitzman, M., Kirkham, R., Broughton, D., Thorson, J., and Selley, D., 2005, The sedimenthosted stratiform copper ore system: Economic Geology 100th Anniversary Volume, p. 609642. Johnson, R.C., Andrews, R.A., Nelson, W.S., Suszek, T., and Sikkila, K., 1995, Geology and mineralization of the White Pine copper deposits: unpublished Copper Range Company Report. Keane, J. M., Milne, S., and List, D., 2012, Feasibility Study on the Copperwood Project, Michigan, USA NI 43-101 technical report: KD Engineering, SEDAR published report. Kemmis, T., 2008, Unraveling the Complexity of Glacial Successions: in Course Workbook for Improving the Description and Characterization of Glacial Successions for Environmental and Engineering Projects, Midwest Geosciences Group, Western Michigan University, June 2008. Kulla, G., and Parker, H., 2010, Copperwood project, Michigan, USA NI 43-101 technical report: AMEC, SEDAR published report. Kulla, G., and Thomas, D., 2011, Copperwood S6 and satellite project NI 43-101 technical report, Michigan, USA: AMEC, SEDAR published report. Mauk, J.L., 1992, Geology and stable isotope and organic geochemistry of the White Pine sediment-hosted stratiform copper deposit: Society of Economic Geologists Guidebook Series, v. 13, p. 63-98. 117 �Mauk, J.L., Kelly, W.C., van der Pluijm, B.A., Seasor, R.W., 1992, Relations between deformation and sediment-hosted stratiform copper mineralization: evidence from the White Pine part of the Midcontinent rift system: Geology, v. 20, p. 427–430. Orvana, 2011, Copperwood Mine Orvana Resources US Corp. Part 632 Mine permit application: submitted September 26, 2011, published online by Michigan Department of Environmental Quality. Peterson, W. L., 1985, Surficial Geologic Map of the Iron River 1o x 2o Quadrangle, Michigan and Wisconsin: USGS Miscellaneous Investigation Series, Map I-1390-C. Peterson, W. L., 1986, Late Wisconsinan Glacial History of Northeastern Wisconsin and Western Upper Michigan: U. S. Geological Survey Bulletin 1652. Suszek, T., 1997, Petrography and sedimentation of the Middle Proterozoic (Keweenawan) Nonesuch Formation, western Lake Superior region, Midcontinent Rift system: Geological Society of America Special Paper 312, p. 127-136. Ward, M.B., 2011, Resource estimate and NI 43-101 technical report for Copperwood project, Ironwood, Michigan for Orvana US Corp: Marston & Marston Inc., SEDAR published report. White, W.S. and Wright, J.C., 1954, The White Pine copper deposit, Ontonagon County, Michigan: Economic Geology, v. 49, p. 675-716. 118 �Geology of the Keweenawan Supergroup, Porcupine Mountains, Ontonagon and Gogebic Counties, Michigan Laurel G. Woodruff1, William F. Cannon2, Suzanne W. Nicholson2, Klaus J. Schulz2, and Robert Wild3 1 U.S. Geological Survey, St. Paul, Minnesota; 2U.S. Geological Survey, Reston, Virginia; 3 Porcupine Mountain Wilderness State Park, Ontonagon, Michigan Introduction This field trip examines the geology of the rocks of the Keweenawan Supergroup (1.1 Ga) and related intrusive rocks of the Midcontinent rift system (MRS) exposed in and around the Porcupine Mountains. Most stops on this trip were visited in a previous Institute on Lake Superior Geology field trip guidebook (Cannon and others, 1992). The stop descriptions here are taken largely from that field guide with minor updates, new location maps and photographs. Because of uncertainties of weather, road conditions, and remaining snow pack in early May, the specific stops that we will visit will not be known until the date of the trip. Latitudes and longitudes are from GPS readings using WGS 84 datum. General Geology The 1.1 Ga Midcontinent rift system (MRS) is a prominent 2500 km linear feature on gravity and magnetic maps that extends from Kansas north to Lake Superior and then southeast beneath the Michigan basin to where it is cut off by the Grenville Front near Detroit, Michigan (Fig. 1). The MRS cuts across Early Proterozoic and Archean terranes and is attributed to crustal extension resulting from upwelling and decompression melting of an anomalously hot mantle plume at the base of the continental lithosphere (Nicholson and others, 1997). In the Lake Superior region, nearly complete crustal separation accompanied emplacement of as much as 2 million km3 of extrusive basalt and possibly an equal volume of intrusive rocks from about 1108 Ma to about 1087 Ma (Hutchinson and others, 1990; Nicholson and Shirey, 1990; Allen and others, 1995). In the Lake Nipigon area, intrusive rocks that have been attributed to the Midcontinent Rift event suggest that magmatism may have started as early as 1115 Ma (Easton and others, 2007). The deepest part of the rift subsided along large normal growth faults to a depth of nearly 30 km, accommodating at least 20 km of rift-related volcanic rocks. Following the volcanic phase of rifting, thermal subsidence accommodated deposition of up to 10 km of overlying sedimentary rocks in central rift grabens and flanking basins. Rocks of the MRS in western Lake Superior are known as the Keweenawan Supergroup and contain a remarkably complete record of igneous intrusion, flood basalt volcanism, and clastic sedimentation (Fig. 1). Along the south shore of western Lake Superior, initial subsidence of the MRS is recorded by deposition of the Bessemer Quartzite (Fig. 2), a blanket of relatively pure, fluvial sandstone as much as 100 m thick (Ojakangas and Morey, 1982). The Bessemer Quartzite is overlain by a great thickness of subaerial flood basalt flows and lesser intermediate and rhyolitic rocks (Powder Mill Group and Portage Lake Volcanics). Conformably overlying the volcanic rocks are fluvial sedimentary rocks (Copper �Harbor Conglomerate and Freda Formation) and lesser lacustrine sedimentary rocks (Nonesuch Formation) which are as much as 8 km thick beneath Lake Superior (Cannon and others, 1993) and at least 5 km thick on shore in the field trip area. Along the south shore of western Lake Superior, post-rift movement along the rift-bounding Keweenaw fault has caused large block rotation so that much of the Keweenawan Supergroup section is steeply to vertically dipping. Figure 1. Regional bedrock geologic map of western Lake Superior showing the distribution of rocks related to the Midcontinent rift system (N is magnetically normal; R is magnetically reversed) and area of the geologic map in Fig. 3 (modified after Miller and Chandler, 1997). �Figure 2. Age in Ma, magnetic polarity (N is normal and R is reversed), and stratigraphic section for rocks of the Keweenawan Supergroup in western Michigan and northeastern Wisconsin. Field trip stop numbers are placed in their relative stratigraphic positions (modified from Zartman and others, 1996). �Volcanic rocks of the Keweenawan Supergroup (Fig. 2) range in composition from olivine tholeiite to rhyolite. By far the dominant rock type is high-Al olivine tholeiite (Al203 = 15 to 19 wt. %) followed by lesser high-Fe tholeiite and rocks of intermediate and felsic composition (Green, 1982; Paces, 1988; Nicholson and others, 1997). The basalts commonly are ophitic in texture and the dominant phenocryst is plagioclase. The most primitive Keweenawan basalts are geochemically similar to ocean island basalts and have incompatible trace elements ranging from slightly to strongly enriched compared to depleted or primitive mantle. Radiogenic isotope analyses (Sr, Nd, and Pb) of the main stage high-Al olivine tholeiites suggest that a likely source of the voluminous basalts was a trace element-enriched mantle plume (Paces and Bell, 1989; Nicholson and Shirey, 1990; Nicholson and others, 1997). Flows near the base of the exposed Portage Lake Volcanics north of the Keweenaw fault were erupted at about 1096 Ma and those near the top of the formation at about 1094 Ma (Davis and Paces, 1990). Thus, the great thickness of Portage Lake Volcanics, at least 8 km in this area, was erupted in only a few million years. Because synchronous volcanism occurred along the entire trend of the rift, the rift system as a whole was producing basalt at a rate unrivalled by any modern analog (Cannon, 1992). The Porcupine Volcanics (Fig. 2) create much of the topography of the Porcupine Mountains. The Porcupine Volcanics overlie the Portage Lake Volcanics, and represent a volcanic center that became active late in the volcanic history of the region, at about 1093 Ma. As much as 5 km of andesite, rhyolite, and basalt were erupted in a large shield volcano and deposited on top of a flat-lying lava plain composed of Portage Lake Volcanics. The unit has a lateral extent of about 35 km on the present erosion surface. The present arcuate shape of the Porcupine Mountains and the unusual hook-shaped map pattern of the Porcupine Volcanics are partly a reflection of the original shape of the volcanic shield (Fig. 3). Rhyolite near the top of the section has an age of 1093±1.4 Ma (Zartman and others, 1997). The Porcupine Volcanics consist of a sequence of subaerially deposited (in order of abundance) andesite, basalt, felsite, and quartz-porphyry lava flows, and minor interbedded volcaniclastic lithic sandstone, siltstone, and conglomerate (Hubbard, 1975). The abundance of felsic rocks, most common near the top of the formation, where they occur as both lava flows and domes (stops 3 and 8), and the predominance of andesite over basalt clearly distinguish the Porcupine Volcanics from underlying Portage Lake Volcanics. An abundance of intermediate and felsic volcanic rocks, such as the Porcupine Volcanics, is atypical of the MRS as a whole and is limited to only a few felsic volcanic centers associated with shield volcanoes. The major element compositions of the basalt, basaltic andesite, and andesite of the Porcupine Volcanics and the Portage Lake Volcanics are similar, but the Porcupine Volcanics are distinctly enriched in light rare earth elements (LREE) and Th compared to the Portage Lake Volcanics (Fig. 4). The two formations differ more significantly in their rhyolite chemistry and mineralogy. The rhyolite that occurs most commonly in the Portage Lake Volcanics is aphyric or may contain sparse quartz phenocrysts. In contrast, numerous rhyolite bodies in the Porcupine Volcanics range from rhyolites that are aphyric to those with abundant quartz and/or feldspar phenocrysts. Rhyolites of the Portage Lake Volcanics on the Keweenaw Peninsula typically have lower abundances of incompatible trace elements �Figure 3. Generalized geologic map of the Porcupine Mountains area, modified from Cannon and others (1995). Field trip stops are shown as red diamonds. �(such as LREE, Zr, Y, Hf, and Th) than rhyolite of the Porcupine Volcanics (Fig. 4A). Radiogenic isotope analyses suggest that most Portage Lake rhyolites were derived by partial melting of already erupted Keweenawan basalt with a minor contribution, if any, from older basement, whereas rhyolites of the Porcupine Volcanics have a much larger contribution from older basement. Figure 4. A. Spidergram illustrating the average compositions of Portage Lake Volcanics Type 1 rhyolites, Porcupine Volcanics average rhyolite, and a sample of rhyolite from the quarry at stop 8. B. Spidergram illustrating the average compositions of Portage Lake Volcanics average basalts, Porcupine Volcanics average basalts, and samples of the Lake Shore traps from the area of stop 7. �Abrupt changes in thickness of the Porcupine Volcanics are inferred along prominent structural breaks that are especially evident on the aeromagnetic map of the area (King, 1987). These breaks are believed to be synvolcanic normal faults, which outlined a central caldera. A major gravity low, centered just south of the Porcupine Mountains, was interpreted by Klasner (1989) as a large, shallow felsic intrusion. This intrusion may have been the subvolcanic felsic magma chamber that erupted much of the Porcupine Volcanics. Eruption of the Porcupine Volcanics marked an end of major volcanism in the Midcontinent Rift, with further events dominated by fluvial and lesser lacustrine sedimentation. The Copper Harbor Conglomerate is a sequence of red to brown arkosic conglomerates and sandstones, interpreted as a northward prograding alluvial fan complex (Daniels, 1982). The Copper Harbor crops out along a discontinuous belt from the east end of the Keweenaw Peninsula westward into Wisconsin. There is an inverse relationship between the thickness of the Copper Harbor and the Porcupine Volcanics such that the Copper Harbor thins as the Porcupines Volcanics become thicker (White, 1972; Cannon and Nicholson, 1992). This suggests that the broad shield volcano that formed the Porcupine Mountains was a persistent topographic high during the time of Copper Harbor deposition. In the vicinity of the Porcupine Mountains, true conglomerate is rare; the Copper Harbor typically is a platy, reddish, fine- to medium-grained sandstone/siltstone (stop 6). Within the Copper Harbor, there are up to 31 basaltic andesite to andesite lava flows interspersed with sediment, generally in the upper part of the formation (Fig. 2). These flows, informally known as the Lake Shore traps, have an age of about1087±1.6 Ma (Davis and Paces, 1990) and form the prominent north flank of the Porcupine Mountains (stop 7). A unit of dark gray shale and siltstone, the Nonesuch Formation (stops 1, 4 and 5), conformably overlies and interfingers with the upper Copper Harbor Conglomerate. The Nonesuch is generally interpreted to have been deposited in a perennial lake located at the toe of a transgressing-regressing alluvial fan complex (Daniels, 1982, Elmore and others, 1988; Suszek, 1997). Basal beds of the Nonesuch and locally the top of the Copper Harbor contain regional-scale low-grade stratiform copper mineralization (White and Wright, 1954). Economic–grade ore bodies are located on the western (Presque Isle) and eastern (White Pine) flanks of the Porcupine Mountains. Copper typically occurs as fine-grained disseminated chalcocite; at White Pine chalcocite is accompanied by minor native copper. The White Pine mine, a large underground mine just east of the Porcupine Mountains, produced more than 1.8 million metric tons of copper from 1955 until the mine closed in 1995. The Copperwood deposit, as the Presque Isle deposit is now known, has just completed the pre-mining permitting process. The historical Nonesuch Mine (stop 4) is an example of the early mining history in the region. Deposition of the Nonesuch Formation was succeeded by a return to fluvial redbed deposition during which at least several kilometers of red to brown sandstone and siltstone of the Freda Sandstone (stops 1 and 2) were deposited. The Freda, although still rich in volcanic detritus, is compositionally more mature than older sandstones, which probably indicates that Archean and Early Proterozoic basement in the source area was exposed by erosion of the overlying Keweenawan basalts. Structures along much of the MRS are generally simple, consisting of thick monoclinical sections of rock titled toward the rift axis by a combination of rift subsidence, and later compression andrift inversion. Near the Porcupine Mountains, the structure is somewhat more complicated. The �major structure along the south shore of western Lake Superior is the Keweenaw fault, a reverse fault which partly inverted the central graben of the rift. A seismic section just east of the area shows the Keweenaw fault to be a north-dipping listric thrust (Hinze and others, 1990). Thus, in the area of the field trip, all sedimentary and volcanic units are on the upper thrust plate. Several cross faults that probably developed during eruption of the Porcupine Volcanics complicate the geology of the upper plate as do the gentle folds in the Porcupine Mountains, which produce a repetition of stratigraphy. Thrust faulting and folding can be indirectly dated in the interval from approximately 1060 to 1040 Ma (Ruiz and others, 1984; Bornhorst and others, 1988; Cannon and others, 1993). The regional compression that reversed the sense of movement along rift-bounding faults is likely the result of the Grenville orogeny (Cannon, 1994). The bustling community of Nonesuch, drawn in 1884 by Agnes Hathaway, a miner’s daughter. The town grew up around the Nonesuch copper mine that operated sporadically from 1866 until 1912. Now a ghost town, Nonesuch in its heyday once had a post office, school house, lumber mill, boarding house, and baseball team. The site (stop 4) became part of Porcupine Wilderness State Park in 1988. Image provided by Robert Wild. �Stop 1: Nonesuch Formation and Freda Sandstone at the mouth of the Presque Isle River: 46.7087ºN -89.9734ºW The upper portion of the Nonesuch Formation and the base of the Freda Sandstone are well exposed in the gorge of the Presque Isle River near its mouth and along the shore of Lake Superior west of the river (Fig. 5). Continuous exposures along the picturesque gorge of the river extend from just upstream of Nawadaha Falls to the lakeshore. Exposures continue in bluffs along the lakeshore for about half a mile west of the river mouth. An examination of exposures near the river mouth and a short distance to the west along the shore require a round trip hike of nearly a mile, mostly on wellmaintained trails and stairways. Figure 5. Location and geologic setting for stops 1 and 2. Geology is generalized from Cannon and others (1995). Structures are dotted and contacts are thin lines. �The rocks exposed here are on the northeast limb of the Presque Isle syncline, a gentle northwest-plunging fold. Dips range from nearly flat to about 10º SW. The Nonesuch Formation is distinguished from other sedimentary units of the Keweenawan Supergroup by a predominance of gray, green, or black fine-grained sediments. Lower Keweenawan felsic, intermediate, and mafic volcanic rocks were the major contributor of detritus to the Nonesuch, with contributions from Early Proterozoic and Archean crystalline rocks increasing up section (Suszek, 1997). Many of the units here show trough cross-bedding, symmetrical and asymmetrical ripples, rib and furrow structures, and parting lineations (Fig. 6A-C). A good example of ball-and-pillow structure, probably indicative of seismically-generated slumping, is seen in a one meter thick bed best exposed on the west bank of the river just upstream from the lower gorge (Fig. 6B). Finer-grained rocks include well-laminated shales, which are most abundant lower in the section. The Nonesuch displays coarsening-upward sequences at scales ranging from a few meters to the entire thickness of the unit. On a smaller scale, fining upward sequences are common in units from a few centimeters to a few meters thick. The Nonesuch grades upward to the Freda Sandstone through a zone of dark gray laminated and small-scale cross-bedded siltstone and sandy mudstone are interbedded with medium to coarse-grained reddish brown sandstone. There is a gradual change in oxidation state, and grain-size and bedding thickness both increase, reflecting increased environmental energy (Daniels, 1982). Although not exposed in this area, the lower part of the Nonesuch section is strongly mineralized with very fine-grained chalcocite. Orvana Mineral Corporation is in the process of developing the Copperwood deposit, with an estimate of total proven and probable reserves of 27.42 at 1.41% Cu and 3.6 ppm Ag for contained metal of 852 million pounds of Cu and 3.2 million ounces of Ag (Bornhorst and Williams, 2011). While the lower, mineralized fine-scale stratigraphy at Copperwood is directly correlative with the stratigraphy at the White Pine mine, the upper stratigraphic sequence of the Nonesuch in the Presque Isle syncline does not correlate as well with the Nonesuch stratigraphy at White Pine. This indicates that at least during early Nonesuch deposition, sedimentary conditions were very uniform over the entire region surrounding the Porcupine Mountains. Copperwood also lacks the structural complexity and hydrothermal overprint characteristic of White Pine. The Copperwood deposit occurs along a single dipping plane and only one fault has been recognized; White Pine is cut by a major strike-slip fault and numerous smaller thrust faults. The widespread chalcocite mineralizing event is hydrothermally overprinted at White Pine by a second stage influx of Cu-bearing fluids that deposited native copper along faults and adjacent parting planes (Mauk and others, 1992). Stop 2: Freda Sandstone along the Presque Isle River: 46.6962ºW -89.9744ºN At this stop, near the axis of the Presque Isle syncline, reddish cross-bedded sandstone typical of the lower part of the Freda Sandstone is exposed (Fig. 5). The gently southwest dipping beds are probably 100 to 200 m above the base of the formation and slightly higher stratigraphically than those at stop 1. These are dominantly lithic somewhat micaceous sandstones. The Freda marks a return to fluvial redbed deposition following the lacustrine deposition of the underlying Nonesuch Formation. The Freda Sandstone is a very thick unit in much of the rift in the western Lake Superior region and volumetrically is the dominant unit of the post-rift sedimentary fill. Thirty kilometers to the west, at the mouth of the �A. Potholes along the lower gorge of the Presque Isle River eroded into nearly horizontal shale of the Nonesuch Formation. B. Ball-and-pillow structures. Approximately 1-meter-thick bed of disrupted sediments between laminated siltstone. C. Ripple marks in laminated siltstone. Figure 6. Sedimentary structures in the Nonesuch Formation exposed in the Presque Isle River during low flow. Photographs by Bill Cannon. �Montreal River, about 3500 m of the upper part of the Freda is exposed in lakeshore bluffs. Seismic sections indicate the Freda is even thicker under the lake. The Freda generally becomes finer-grained and more mature upwards. Daniels (1982) interprets the cyclic sandstone-mudstone sedimentation of the Freda as an alluvial channel-fill sequence. Stop 3: Porcupine Volcanics - Rhyolite at Summit Peak and Beaver Creek: 46.7433ºN -89.7711ºW Summit Peak is the highest point in Porcupine Mountains Park and one of the highest points in the state. The observation tower at the summit provides a panoramic view of the field trip area. To the south, the highlands are underlain by the Portage Lake Volcanics and Porcupine Volcanics along the main monocline of volcanic rocks of the Keweenawan Supergroup. The lowlands immediately to the southeast are underlain by rocks of the Oronto Group in the east-plunging Iron River syncline. Looking east along strike, the stack from the former smelter at White Pine can be seen in the distance. To the north, the interior of the park extends over the rugged topography in the foreground to Lake Superior in the distance. The interior of the park is maintained as a wilderness area with access by hiking only. The park also contains some of the largest stands of virgin timber in Michigan. Most of the park interior is underlain by a thick unit of rhyolite composed of a series of lava flows and domes typified by the rocks seen at this stop (Fig. 7). There are good exposures of coarse rhyolite breccia present along the trail leading to Summit Peak and at the overlook platform west of the summit. This breccia is probably the carapace of a rhyolite dome (stop 3A). Excellent exposures of typical intermediate and felsic units of the Porcupine Volcanics occur on the north side of the hill (549 m elevation, stop 3B) along the Beaver Creek Trail about 0.5 mi from the Summit Peak parking lot. The units dip to the south and include, in stratigraphic order, sparse outcrops of intermediate to mafic rocks as well as massive aphyric rhyolite in the creek bed. Moving up the slope, these rocks are overlain by a coarse rhyolite breccia or debris flow. The breccia contains clasts ranging in size from nearly a meter to less than 1 cm. The breccia is clast-supported and some clasts are subrounded, whereas others are flowbanded. Overlying the breccia is a medium-grained, vesicular basalt flow. Capping the hill, and overlying the basalt, is an aphanitic massive rhyolite that is microspherulitic with some crackle breccia on the easternmost end. The following latitude and longitude readings are given as a guide to locating the different units in the Beaver Creek section. 1) 46.7409ºN -89.77678ºW: massive aphyric rhyolite in stream bed 2) 46.74067ºN -89.77821ºW: rhyolite breccia; some fragments flow-banded 3) 46.74037ºN -89.77795ºW: vesicular basalt flow 4) 46.73981ºN -89.77702ºW: massive to flow-banded microspherulitic rhyolite; eastern end is crackle breccia 5) 46.74067ºN -89.77821ºW: rhyolite breccia; some fragments flow-banded �Figure 7. Location and geologic setting for stops 3A and B. Geology generalized from Cannon and others (1995). Structures are dotted and contacts are thin solid lines. Box indicates the area of multiple units along the Beaver Creek Trail for stop 3B. Stop 4: Historic Nonesuch Mine Site: 46.760ºN -89.620ºW The Nonesuch Mine first opened in 1866, extracting finely disseminated native copper from sandstone and shale near the contact between the Copper Harbor Conglomerate and the Nonesuch Formation (Fig. 8). The mine went through a long history of openings and closings. In the 1880’s the prospects looked quite encouraging, with an operating stamp mill that eventually produced 110 tons of copper, transported by tram to a dock 5 miles away at Union Bay. Four separate shafts on either side of the river extended to a depth of about 460 feet (Butler and Burbank, 1929). A town site with around 300 people sprang up around the operation. However, the very fine-grained nature of the copper made �Figure 8. Location and geologic setting for stop 4. Geology generalized from Cannon and others (1995). Contacts are thin lines and faults are thick lines. extraction difficult. A final attempt with chemical leaching that was successful in small pilot trials proved to be unsuccessful on a large-scale, and the mine closed again late in 1884, with most of the machinery subsequently stripped from the site. Several unsuccessful efforts were made to reopen the mine, one in 1906 and another in 1912, after which the mine closed for good. Total production for the Nonesuch Mine is estimated at 389,000 pounds of copper from 1868-1885 (Butler and Burbank, 1929). The former town site is now listed as a ghost town, with old foundations and lilac and apple trees in a grassy field as the only evidence of the town today. The contact between the Copper Harbor Conglomerate and base of the Nonesuch Formation is well exposed in small rapids on the Little Iron River where beds dip about 30o to the east (Fig. 9A). The upper Copper Harbor is a fluvial sandstone, well cross-bedded, and contains lenses of conglomerate as much as 0.5 m thick with clasts as large as 15 cm. (Fig. 9B). Above a transition zone less than a meter thick, is thinly laminated shale and siltstone of the basal Nonesuch Formation. This is the horizon from which the ore was mined, although mineralization is not evident in this exposure. �A. Laminated gray shale at the base of the Nonesuch Formation a few meters above the contact with the Copper Harbor Conglomerate. B. Conglomerate lens in the uppermost Copper Harbor Conglomerate. The base of the Nonesuch is exposed at the base of the falls (far left). Figure 9. Nonesuch Formation and Copper Harbor Conglomerate exposed during low flow in the Little Iron River near at the Nonesuch mine site. Photographs by Bill Cannon. �Stop 5: Nonesuch Formation at Bonanza Falls: 46.8177ºN -89.5701ºW The most complete exposure of the Nonesuch Formation in the region is along the Big Iron River near Bonanza Falls, although access can be difficult and dangerous at times of high water (Fig. 10). The Nonesuch is exposed nearly continuously in a gently southeast-dipping section from just upstream of Bonanza Falls to the sharp bend in the river near the northeast corner of section 13 (Fig. 11A). A detailed measured section is presented by Suszek (1991). The exposed rocks total 226 m of section, which includes nearly the entire Nonesuch, although neither the upper nor lower contact is directly exposed. Figure 50. Location and geologic setting for stop 5. Geology generalized from Cannon and others (1995). Structures are dotted lines, contacts are thin lines, and faults are thick lines. �A. Exposure of the Nonesuch Formation in the Big Iron River at Bonanza Falls, looking upstream. Beds dip gently upstream. B. Contorted bedding in the Nonesuch at Bonanza Falls, probably generated by seismic liquefaction. Scale card is 85 cm wide. Figure 6. Nonesuch Formation exposed at Bonanza Falls during low flow in the Big Iron River. Photographs by Bill Cannon. �The Nonesuch Formation in the Big Iron River section is dominantly siltstone and fine-grained sandstone with minor shale. Many rocks have trough cross-bedding, symmetrical and asymmetrical ripples, rib and furrow structures, parting lineations, and soft sediment deformational features (Fig. 11B). The finer-grained rocks include well-laminated shale, which is most abundant lower in the section. The shaley units commonly have ball-and-pillow structures and calcareous concretions. The Nonesuch here displays coarsening-upward sequences at scales ranging from a few meters to the entire thickness of the unit. On a smaller scale, upwardly fining sequences are common in units from a few centimeters to a few meters thick. In the lower 10 m of the section, copper mineralization occurs as concentrations of chalcocite, bornite and malachite along bedding planes. The mineralization is cogenetic with the major copper mineralization in the White Pine mine, where the downdip extension of this unit was mined just to the south and east. A good exposure of the mineralized base of the Nonesuch Formation and the top of the Copper Harbor Conglomerate occurs along the Little Iron River near the center of the SW1/4, Section 13, but requires a walk of about 1 mi south from Highway 107. It is an easy walk along an unmaintained trail on the east bank for those who can spend more time in the area. At this location, remains of early mining efforts for native silver occurs there, as well as ‘ore’ specimens from old dumps. Copper was initially introduced to the lower Nonesuch Formation in both the White Pine and Copperwood districts during early diagenesis, probably by upward circulating connate waters which dissolved copper from the underlying redbeds of the Copper Harbor Formation (Swenson and Person, 2000). Chalcocite, largely of microscopic size, formed by the replacement of diagenetic pyrite. A later phase of copper mineralization documented by Mauk and others (1992) in the White Pine mine, introduced native copper mostly along fault zones and adjacent strata. This second stage of native copper mineralization commonly occurs as large thin plates of native copper developed along bedding planes in the Nonesuch. It is locally known as sheet copper and is probably cogenetic with the classic native copper mineralization of the Portage Lake basalts along the Keweenaw Peninsula. Stop 6: Upper part of Copper Harbor Conglomerate at Union Bay Campground: 46.8253ºN -89.6418ºW Good exposures of reddish sandstone containing thin conglomerate beds are abundant along the shore of Lake Superior in the park at the Union Bay Campground (Fig. 12). The Copper Harbor Conglomerate is typically characterized by coarse volcanogenic conglomerate, which forms most of the lower part of the section throughout much of its outcrop belt and which grades up into finer grained sandstone (Elmore, 1984). However, south of the village of Ontonagon, there is a different facies relationship. The lower part of the formation here is mostly sandstone, siltstone, and basalt or andesite lava flows; conglomerate is very subordinate. These rocks underlie the high hills immediately south of Highway 107. A coarse conglomerate facies occurs higher in the section, but forms less than 10 percent of the thickness. The exposures here at Union Bay are near the base of the upper unit and are probably about 1,000 m above the base of the formation. Sandstone layers at Union Bay dip 10-20° to the north. Sandstone is volcanogenic and quartz-poor. There are excellent examples of trough cross-bedding, generally indicating a northeastward current vector. A variety of other sedimentary features including �desiccation cracks, rip-up clasts, oscillation and current ripples, and swash marks are also present (Fig. 13A-D). Figure 7. Location and geologic setting for stop 6. Geology generalized from Cannon and others (1995). Structures are dotted and contacts are thin lines. The exposures of Copper Harbor Conglomerate north of the Porcupine Mountains are the farthest from the source highlands to the south. The relative scarcity of thick coarse conglomerate compared to exposures farther south probably reflects the distal nature of these rocks. These northernmost outcrops are a good representative of much of the Copper Harbor beneath Lake Superior. The rocks at Union Bay show a an irregular coarsening-upward trend as opposed to the fining-upward trend typical of the more proximal parts of the unit. This relationship is consistent with northward prograding alluvial plain deposition. Several large boulders are distributed along the beach and are composed of conglomerate typical of the lower part of the Copper Harbor elsewhere. In the boulders the conglomerate contains, almost exclusively, clasts of Keweenawan Supergroup volcanic rock types common in the region. An �A. Ripple marks B. Cross-bedding C. Mudcracks D. Lineations on bedding surface, possibly indicating current directions. Note variations in direction between various bedding planes. Figure 13. Sedimentary features in red siltstone and sandstone of the Copper Harbor Conglomerate along the Lake Superior shoreline at Union Bay Campground. Scale card is 85 cm in width. Photographs by Bill Cannon. �interesting question is the source of these boulders. Although the Copper Harbor does contain some conglomerate beds nearby (for example, Fig. 9B), none of the streams entering Lake Superior in this area seem capable of transporting such large boulders. Most streams, especially near the lakeshore, have low gradients, and streambeds do not contain such large boulders. These boulders are very likely glacial eratics that have been transported from some distance away. Ice movement was from the northeast, roughly parallel to the present shoreline. The boulders probably came from the Copper Harbor Conglomerate farther east toward the Keweenaw Peninsula where thick conglomerate is common. Stop 7: Basalt flows within the Copper Harbor Conglomerate (Lake of the Clouds overlook): 46.8031ºN -89.7641ºW Along Highway 107 leading to the Lake of the Clouds overlook are several exposures of conglomerate within the Copper Harbor Conglomerate. To the north is a good view of Lake Superior and the lowlands underlain by sedimentary rocks of the Oronto Group. From the overlook parking lot, a short hike leads to the overlook and a spectacular view of Lake of the Clouds and the Porcupine Mountains Wilderness State Park (Figs. 14 and 15). Figure 8. Location and geologic setting for stop 7. Geology generalized from Cannon and others (1995). Contacts are thin lines. �Figure 15. Panoramic view of Lake of the Clouds, Porcupine Mountains Wilderness State Park. Photograph by Bill Cannon. �The overlook is along the south escarpment of a high ridge supported by a series of northdipping lava flows within the Copper Harbor Conglomerate (Fig. 16). These flows are known as the Lake Shore traps. Comparable flows within the Copper Harbor Conglomerate at the tip of the Keweenaw Peninsula have an age of 1087.2 ± 1.6 Ma (Davis and Paces, 1990). The low area south of the ridge, including Lake of the Clouds, is underlain by sandstone and siltstone and a few basalt flows which constitute the lower part of the Copper Harbor Conglomerate. The higher regions farther south are underlain by volcanic rocks, mostly rhyolite of the Porcupine Volcanics. Toward the east end of the overlook area, a large glaciated surface shows a series of thin basalt flows, which average a few meters thick. Individual flows can be readily identified by chilled vesicular bases, in places containing inclusions of older flows, and by rubbly or vesicular tops. Abundant epidote alteration and vesicle fillings impart a distinctive greenish cast to flow margins (Fig. 16). Hubbard (1975) described the flows in the Copper Harbor as mostly andesite with minor basalt, but chemical analyses of two samples from this locality indicate that they are basalt, similar in composition to average basalt from the Porcupine Volcanics. An interesting feature of some of these flows is the incorporation of very fine-grained red sediments both in vesicles near their base and in thin fractures extending for a meter or more up into flows. Thin vestiges of these sediments are also along flow contacts. We interpret these sediments as wind-blown dust that accumulated on flow surfaces shortly after eruption. It was still unconsolidated when the next flow was erupted. The soft sediment was then injected upward by the weight of the overlying flow. Compared to Portage Lake Volcanics, these basalts are enriched in incompatible trace elements and show a distinct negative Nb-Ta anomaly in primitive mantle normalized incompatible trace element patterns similar to the basalts from the Porcupine Volcanics (Fig. 4B). This negative anomaly is a likey indication of crustal contribution to the parent magmas. Figure 16. Contact between two basalt flows at the Lake of the Clouds overlook. The vesicular top of one flow (bottom of photo) has amygdules filled with greenish epidote. The base of the overlying flow has pipe vesicles near the lower contact, grading up to a more massive interior (towards the top of the photo). Reddish fine-grained clastic material occurs in cracks in the upper flow. Scale card is 85 cm wide. Photograph by Bill Cannon. �Stop 8: Porcupine Volcanics: Rhyolite quarry near White Pine: 46.7155ºN -89.4435ºW The rocks exposed here are part of a small rhyolite body near the top of the Porcupine Volcanics (Fig. 17). The body probably does not extend much beyond the hill into which the quarry is cut. The quarry provides a cross section through part of a subaerial agglutinate deposit. Agglutinate deposits form at vents by spatter of erupting magma and buildup of mounds of hot, viscous material. The mound of erupted material eventually flows outward under its own weight, resulting in large flow folds such as seen in this quarry. The near-vent nature of this deposit is deduced from the presence of lithic fragments within the rhyolite, large and small contorted flow folds, and stratification of rhyolite (light and dark units) (Fig. 18A-C). At the edges of agglutinate deposits, flowage typically has homogenized the magma, and, as such, stratification and folding are generally not preserved. Zartman and others (1997) reported an age of 1093.6 +/- 1.8 Ma for rhyolite from this quarry. Figure 17. Location and geologic setting for stop 8. Geology generalized from Cannon and others (1995). Contacts are thin lines and faults are thick lines. �This rhyolite contains feldspar phenocrysts, which are typically aligned parallel to the stratification and foliation. Rhyolite of the Porcupine Volcanics is enriched substantially in such incompatible trace elements as Th, Ba, Zr, Hf, and LREE compared to rhyolite in the Portage Lake Volcanics on the Keweenaw Peninsula. Isotopically, rhyolite at this stop has an initial Nd isotopic signature (εNd(1100Ma) about -14) that reflects a substantial contribution from older crustal sources. The Portage Lake rhyolite from the Keweenawan Peninsula (εNd(1100Ma) about 0) is thought to be derived from partially melted early Keweenawan basalts (Nicholson and others, 1997). A. Lithic fragment showing flow contact between light and dark rhyolite. B. Flattened and drawn-out lithic fragments in rhyolite matrix. C. Contoured flow pattern showing contrasting rhyolite melts. Figure 18. Agglutinate textures in Porcupine Volcanics rhyolite, exposed in quarry blocks. Photographs by Bill Cannon. �References Allen, D.J., Braile, L.W., Hinze, W.J., and Mariano, J., 1995, The Midcontinent rift system, U.S.A.: a major Proterozoic continental rift, in Olsen, K.H. (ed.), Continental rifts: evolution, structure, tectonics: Elsevier, New York, pp. 375-407. 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Cannon, W.F., Nicholson, S.W., Woodruff, L.G., Hedgman, C.A., and Schulz, K.J., 1995, Geologic map of the Ontonagon and part of the Wakefiled 30′ x 60′ quadrangles, Michigan: U.S. Geological Survey Miscellaneous Investigations Series Map I-2499, scale 1:100,000. Daniels, P.A., Jr., 1982, Upper Precambrian sedimentary rocks: Oronto Group, Michigan-Wisconsin, in Wold, R.J. and Hinze, W.J. (eds.), Geology and tectonics of the Lake Superior basin: Geological Society of America Memoir 156, p. 107-133. 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. Elmore, R.D., 1984, The Copper Harbor Conglomerate: a late Precambrian fining-upward alluvial fan sequence in northern Michigan: Geological Society of America Bulletin 95, p. 610-617. Elmore, R.D., Milavec, G., Imbus, S., Engel, M.H., and Daniels, P., 1988, The Precambrian Nonesuch Formation of the North American Midcontinent Rift: Sedimentary and organic geochemical aspects of lacustrine deposition: Precambrian Research, v. 43, p. 191-213. �Easton, M., Hart, T.R., Hollings, P., Heamon, L.M., MacDonald, C.A., and Smyk, M., 2007, Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon Region, Ontario: Canadian Journal of Earth Sciences, v. 44, p. 1055-1086. Green, J.C., 1982, Geology of Keweenawan extrusive rocks, in Wold, R.J., and Hinze, W.J. (eds.), Geology and tectonics of the Lake Superior basin: Geological Society of America Memoir 156, p. 4755. Hinze, W.J., Braile, L.W., and Chandler, V.W., 1990, A geophysical profile of the southern margin of the Midcontinent Rift system in western Lake Superior: Tectonics, v. 9, p. 303-310. Hubbard, H.A., 1975, Geology of the Porcupine Mountains in Carp River and White Pine quadrangles, Michigan: U.S. Geological Survey Journal of Research, v. 3, p. 519-528. Hutchinson, D.R., White, R.W., Cannon, W.F., and Schulz, K.J., 1990, Keweenaw hot spot: geophysical evidence for a 1.1 Ga mantle plume beneath the Midcontinent Rift System: Journal of Geophysical Research, v. 95, p. 10869-10884. King, E.R., 1987, Aeromagnetic map of the Iron River 1º x 2º quadrangle, Michigan and Wisconsin: U.S. Geological Survey Miscellaneous Investigations Map I-1306F, scale 1:250,000. Klasner, J.S., 1989, Bouger gravity anomaly map and geologic interpretation of the Iron River 1º x 2º quadrangle, Michigan and Wisconsin: U.S. Geological Survey Miscellaneous Investigations Map I1306E, scale 1:250,000. Mauk J.L, Kelly, W.C., van der Pluijm, B.A., and Seasor, R.W., 1992, Relations between deformation and sediment-hosted copper mineralization: Evidence from the White Pine part of the Midcontinent rift system: Geology, v. 20, p. 427-430. Miller, J.D., and Chandler, V.W., 1997, Geology, petrology, and tectonic significance of the Beaver Bay Complex, northeastern Minnesota: Geological Society of America Special Papers 312, p. 73-96. Nicholson, S.W., and Shirey, S.B., 1990, Evidence for a Precambrian mantle plume: a Sr, Nd, and Pb isotopic study of the Midcontinent Rift System in the Lake Superior region: Journal of Geophysical Research, v. 95, p. 10851-10868. 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. Ojakangas, R.W., and Morey, G.B., 1982, Keweenawan pre-volcanic quartz sandstones and related rocks of the Lake Superior region, in Wold, R.J., and Hinze, W.J. (eds.), Geology and tectonics of the Lake Superior basin: Geological Society of America Memoir 156, p. 85-96. Paces, J.B., 1988, Magmatic processes, evolution and mantle source characteristics contributing to the petrogenesis of Midcontinent rift basalts: Portage Lake basalts, Keweenaw Peninsula, Michigan: unpublished Ph.D. dissertation, Michigan Technological University, Houghton, Michigan. Paces, J.B., and Bell, K., 1989, Non-depleted sub-continental mantle beneath the Superior Province of the Canadian Shield: Nd-Sr isotopic and trace element evidence from Midcontinent rift basalts: Geochimica et Cosmochimica Acta, v. 53, p. 2023-2035. Ruiz, J., Jones, L.M., and Kelly, W.C., 1984, Rubidium-strontium dating of ore deposits hosted by Rbrich rocks using calcite and other common Sr-bearing minerals: Geology, v. 12, p. 259-262. �Suszek, T.J., 1991, Petrography and sedimentation of the Middle Proterozoic (Keweenawan) Nonesuch Formation, western Lake Superior region, Midcontinent Rift System: unpublished M.S. thesis, University of Minnesota, Duluth, Minnesota. Suszek, T.J., 1997, Petrography and sedimentation of the middle Proterozoic (Keweenawan) Nonesuch Formation, western Lake Superior region, Midcontinent Rift System: Geological Society of America Special Paper 312, p. 195- 210. Swenson, J.B., and Person, M., 2000, The role of basin-scale transgression and sediment compaction in stratiform copper mineralization: implications from White Pine, Michigan, USA: Journal of Geochemical Exploration, v. 69-70, p. 239-243. White, W.S., 1972, The base of the upper Keweenawan, Michigan and Wisconsin: U.S. Geological Survey Bulletin 1354-F, p. F1-F23. White, W.S., and Wright, J.C., 1954, The White Pine copper deposit, Ontonagon County, Michigan: Economic Geology, v. 49, p. 675-716. Zartman, R.E., Nicholson, S.W., Cannon, W.F., and Morey, G.B., 1997, U-Th-Pb zircon ages of some Keweenawan Supergroup rocks from the south shore of Lake Superior: Canadian Journal of Earth Sciences, v. 34, p. 549-161. � 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, 2013 Description An account of the resource Institute on Lake Superior Geology. Houghton, Michigan. May 8-11, 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/3bcf982a44576e47069c02b68e895c56.pdf 618d1ae16e9ef5c5a6bfbd10589516cd PDF Text Text �Institute on Lake Superior Geology 60TH ANNUAL MEETING May 14-17, 2014 Hibbing, Minnesota Sponsored by PRECAMBRIAN RESEARCH CENTER, UNIVERSITY OF MINNESOTA DULUTH and MINNESOTA GEOLOGICAL SURVEY James D. Miller and Mark A. Jirsa Co-Chairs Proceedings Volume 60 Part 1 – Program and Abstracts Edited by Jim Miller, University of Minnesota Duluth Cover Photo Credit View of mines and the city of Hibbing looking south. Gray area in foreground is the footprint of Hibbing Taconite’s mining; partially flooded, dark red areas in the mid-ground are remnants of historic natural (hematite) ore mines, including the Hull-Rust, Mahoning, Susquehanna, and Scranton; City of Hibbing in background, showing location of meeting hotel (oval). Modified from image provided by Dave Witt—Aero-Environmental Consulting, LLC, Cook, MN i �Table of Contents Institutes on Lake Superior Geology, 1955-2014 iii Sam Goldich and the Goldich Medal vi Goldich Medal Guidelines viii Goldich Medalists and Goldich Medal Committee x Citation for Goldich Medal Award to Laurel Woodruff xi Memorial to Ernest Lehmann xiii Memorial to Jack Everett xiv Eisenbrey Student Travel Awards xv Joe Mancuso Student Research Awards xvi Doug Duskin Student Paper Awards and Award Committee xvii Board of Directors, Local Committee, and Banquet Speaker xviii Session Chairs and Field Trip Leaders xix Corporate and Individual Sponsors of Student Travel Scholarships xxi Report of the Chair of the 59th Annual Meeting xxii Program xxiv Poster Presentations xxix Abstracts 1-130 Reference to material in Part 1 should follow the example below: Field trip authors, date, title: Institute on Lake Superior Geology Proceedings v. 60, Part 1, p. XX. Proceedings Volume 60, Part 1—Program and Abstracts, and Part 2—Field Trip Guidebook are published by the 60th Institute on Lake Superior Geology and distributed by the Institute Secretary: Peter Hollings Department of Geology Lakehead University Thunder Bay, ON P7B 5E1 CANADA peter.hollings@lakeheadu.ca Some figures in this volume were submitted by authors in color, but are printed grayscale to conserve printing costs. Full color imagery will appear in the digital version of the volume when it is available on-line at http://www.lakesuperiorgeology.org. ISSN 1042-99 ii �Institutes on Lake Superior Geology, 1955-2014 95 o o 85 o Wabigoon subprovince90 o 80 48 o Wawa-Abitibi subprovince 48o Wawa-Abitibi subprovince o 45 45o Minnesota River Valley subprovince MEETING LOCATIONS Phanerozoic Mesoproterozoic Map by Mark Jirsa Paleoproterozoic o 90 95o 85o Archean Superior Province # Date Place Chairs 1 1955 Minneapolis, Minnesota C.E. Dutton 2 1956 Houghton, Michigan A.K. Snelgrove 3 1957 East Lansing, Michigan B.T. Sandefur 4 1958 Duluth, Minnesota R.W. Marsden 5 1959 Minneapolis, Minnesota G.M. Schwartz & C. Craddock 6 1960 Madison, Wisconsin E.N. Cameron 7 1961 Port Arthur, Ontario E.G. Pye 8 1962 Houghton, Michigan A.K. Snelgrove 9 1963 Duluth, Minnesota H. Lepp 10 1964 Ishpeming, Michigan A.T. Broderick 11 1965 St. Paul, Minnesota P.K. Sims & R.K. Hogberg 12 1966 Sault Ste. Marie, Michigan R.W. White 13 1967 East Lansing, Michigan W.J. Hinze 14 1968 Superior, Wisconsin A.B. Dickas 15 1969 Oshkosh, Wisconsin G.L. LaBerge 16 1970 Thunder Bay, Ontario M.W. Bartley & E. Mercy iii �# Date Place Chairs 17 1971 Duluth, Minnesota D.M. Davidson 18 1972 Houghton, Michigan J. Kalliokoski 19 1973 Madison, Wisconsin M.E. Ostrom 20 1974 Sault Ste. Marie, Ontario P.E. Giblin 21 1975 Marquette, Michigan J.D. Hughes 22 1976 St. Paul, Minnesota M. Walton 23 1977 Thunder Bay, Ontario M.M. Kehlenbeck 24 1978 Milwaukee, Wisconsin G. Mursky 25 1979 Duluth, Minnesota D.M. Davidson 26 1980 Eau Claire, Wisconsin P.E. Myers 27 1981 East Lansing, Michigan W.C. Cambray 28 1982 International Falls, Minnesota D.L. Southwick 29 1983 Houghton, Michigan T.J. Bornhorst 30 1984 Wausau, Wisconsin G.L. LaBerge 31 1985 Kenora, Ontario C.E. Blackburn 32 1986 Wisconsin Rapids, Wisconsin J.K. Greenberg 33 1987 Wawa, Ontario E.D. Frey & R.P. Sage 34 1988 Marquette, Michigan J. S. Klasner 35 1989 Duluth, Minnesota J.C. Green 36 1990 Thunder Bay, Ontario M.M. Kehlenbeck 37 1991 Eau Claire, Wisconsin P.E. Myers 38 1992 Hurley, Wisconsin A.B. Dickas 39 1993 Eveleth, Minnesota D.L. Southwick 40 1994 Houghton, Michigan T.J. Bornhorst 41 1995 Marathon, Ontario M.C. Smyk 42 1996 Cable, Wisconsin L.G. Woodruff 43 1997 Sudbury, Ontario R.P. Sage & W. Meyer 44 1998 Minneapolis, Minnesota J.D. Miller & M.A. Jirsa 45 1999 Marquette, Michigan T.J. Bornhorst & R.S. Regis 46 2000 Thunder Bay, Ontario S.A. Kissin & P. Fralick 47 2001 Madison, Wisconsin M.G. Mudrey & Jr., B.A. Brown 48 2002 Kenora, Ontario P. Hinz & R.C. Beard 49 2003 Iron Mountain, Michigan L. Woodruff & W.F. Cannon iv �# Date Place Chairs 50 2004 Duluth, Minnesota S. Hauck & M. Severson 51 2005 Nipigon, Ontario M. Smyk & P. Hollings 52 2006 Sault Ste. Marie, Ontario A. Wilson & R. Sage 53 2007 Lutsen, Minnesota L. Woodruff & J. Miller 54 2008 Marquette, Michigan T. Bornhorst & J. Klasner 55 2009 Ely, Minnesota J. Miller, G. Hudak, & D. Peterson 56 2010 International Falls, Minnesota M. Jirsa, P. Hollings, & T. Boerboom, P. Hinz & M.Smyk 57 2011 Ashland, Wisconsin T. Fitz 58 2012 Thunder Bay, Ontario P. Hollings 59 2013 Houghton, Michigan T. Bornhorst & A. Blaske 60 2014 Hibbing, Minnesota J. Miller & M. Jirsa v �Sam Goldich and the Goldich Medal Sam Goldich received an A.B. from the University of Minnesota in 1929, a M.A. from Syracuse University in 1930, and a Ph.D. from the University of Minnesota in 1936. During World War II Sam worked for the U.S. Geological Survey in mineral exploration. In 1948, Sam returned to the University of Minnesota, and became Professor and Director of the Rock Analysis Laboratory the following year. He rejoined the U.S. Geological Survey in 1959 and was appointed as the first Branch Chief of the Branch of Isotope Geology. Sam returned to academia in 1964 when he went to Pennsylvania State University. He left PSU in 1965 and moved to the State University of New York at Stony Brook, where he stayed for 3 years. Restless yet again, he moved to Northern Illinois University in 1968 where he was a professor until his retirement in 1977. Sam’s final move was to Denver where he became an emeritus at the Colorado School of Mines. Sam died in 2000, less than a month before his 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 vi �INSTITUTE ON LAKE SUPERIOR GEOLOGY GOLDICH MEDAL vii �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. viii �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. ix �Goldich Medalists 1979 Samuel S. Goldich 1997 Ronald P. Sage 1980 not awarded 1998 Zell Peterman 1981 Carl E. Dutton, Jr. 1999 Tsu-Ming Han 1982 Ralph W. Marsden 2000 John C. Green 1983 Burton Boyum 2001 John S. Klasner 1984 Richard W. Ojakangas 2002 Ernest K. Lehmann 1985 Paul K. Sims 2003 Klaus J. Schulz 1986 G.B. Morey 2004 Paul Weiblen 1987 Henry H. Halls 2005 Mark Smyk 1988 Walter S. White 2006 Michael G. Mudrey 1989 Jorma Kalliokoski 2007 Joseph Mancuso 1990 Kenneth C. Card 2008 Theodore J. Bornhorst 1991 William Hinze 2009 L. Gordon Medaris, Jr 1992 William F. Cannon 2010 William D. Addison & 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 GOLDICH MEDAL RECIPIENT Laurel Woodruff Goldich Medal Committee Serving through the meeting year shown in parentheses. Graham Wilson (2014) Turnstone Consulting Bernhardt Saini-Eidukat (2015) North Dakota State University Mark Smyk (2016) Ontario Geological Survey x �Citation for the Goldich Medal Award to Laurel G. Woodruff It is my pleasure and honor to present the 2014 Goldich Medal to Laurel G. Woodruff. Laurel has been one of the most active and involved members of the Institute for more than 20 years. During that time she has chaired or co-chaired three annual meetings (47th, 49th, 53rd) and served corresponding terms on the board of directors. She was chair of the board of directors in 1995-1996, 2002-2003, and 2006-2007. She has served twice on the student paper award committee, and most recently, from 2010-2013, was a member of the Goldich Award committee, and chaired the committee in 2012-2013. In Laurel in the Brooks Range, Alaska in addition, she has been co-leader of three 2007 during the Alaska Soil Institute fieldtrips and has made numerous Geochemistry Transect. technical presentations at Institute meetings. In case no one has yet noted, this is the first Goldich citation in which the pronoun “she” has been used. Most of Laurel’s career, spanning more than thirty years, has been with the USGS mineral resources research program, with more than 25 of those years in the Lake Superior region. Prior to that Laurel received her formal education at University of Michigan (BS in Geology 1973), Michigan Technological University (MS in Geology 1977), and the University of Chicago (PhD in geology 1989). After completing her MS degree and beginning a PhD at Chicago, Laurel was hired to run the light stable isotope laboratory at the University of Wisconsin-Madison and she participated in a broad variety of stable isotope research. The highlight of this part of her career was research on modern seafloor hydrothermal deposits, which culminated in publication of her Journal of Geophysical Research paper on the stable isotope geochemistry of seafloor hydrothermal vent systems. Laurel joined the USGS in 1983. Her initial assignment was establishing the light stable isotope laboratory in the Branch of Eastern Minerals Resources. When the laboratory became operational, she was responsible for light stable isotope analyses (S, O, and C) of rocks, ores, and mineral samples from a number of locations throughout the world in support of research on seafloor sulfide formation, and precious metal mineralization. In 1986-87 Laurel returned to the University of Chicago to complete her PhD and conducted her dissertation research on diabases of the eastern U.S. Triassic basins as part of a large USGS project on the mineral potential of xi �those basins. Laurel’s scientific contributions to the geology of the Lake Superior region began in the late 1980’s when she became a member of a USGS project team studying the geology and mineral potential of the Midcontinent Rift in Michigan and Wisconsin. Laurel’s contributions included: 1) field work to collect bedrock and mineral deposit samples, 2) preparation of geologic maps and reports, 3) stable isotope analyses to constrain metal sources and characterize regional alteration patterns, and 4) geochemical and 2-D thermal modeling to better understand the origin and distribution of copper mineralization in the rift In the past decade, Laurel has become increasingly involved in environmental research and has been a leader in fostering the incorporation of geology and geochemistry into multidisciplinary studies of the behavior of elements of concern such as arsenic and mercury. Her studies of the effect of forest fires on the mercury content of soils in the Lake Superior region and the cycling of mercury in aquatic ecosystems, conducted in cooperation with colleagues in soil science, hydrogeochemistry, and aquatic biology have provided fundamental new understanding of the mercury cycle. Laurel also has been a key figure in establishing procedures for and conducting geochemical baseline studies from local to national scale. The recently completed soil geochemical survey of the conterminous U.S. has produced a new database and a national geochemical atlas based on 15,000 samples from about 5,000 sites across the country. Laurel was a key member of that project from the earliest planning phase, through pilot studies, and the full survey, to the current activities of producing interpretive research papers. Laurel also continues her research on the Precambrian geology and resources of the Lake Superior region and is the coordinator of a new USGS multidisciplinary project on metallogeny and mineral potential of the St. Croix horst in Wisconsin and Minnesota. On a personal note, Laurel has been a great friend and colleague for more than 25 years as we have wended our way through a kaleidoscope of research from hard rocks, through glacial deposits, and soils, to lake-bottom muck; wanderings across Alaska, the “death marches” on Isle Royale, and hard days of canoeing and portaging through the Boundary Waters and Voyageurs Park. Many of the most vivid and pleasant (at least in hindsight) memories of my career are from those days. Our research has commonly been guided by Laurel’s often expressed philosophy of “Let’s do something even if it’s wrong!” She’s shown over and over through her proclivity for action and her eagerness to plunge into new work, that it is so much easier to make mid-course corrections of something in progress than it is to overcome the inertia of over planning, indecision, and inaction; an attribute that has been unfailingly valuable in so much of what she has done during her career. So, in recognition of her decades of accomplishments and of her dedication to the geology of the Lake Superior region and to the Institute on Lake Superior Geology it is my pleasure to present the 2014 Goldich Medal to Laurel. Bill Cannon, Geologist Emeritus U.S. Geological Survey xii �In Memoriam Ernest K. Lehmann (1929-2013) On December 13, 2013, the Institute on Lake Superior Geology lost one of its industry giants with the passing Ernest (Ernie) K. Lehmann. Ernie was an exploration geologist whose lifelong work in the mining industry took him around the globe. He was awarded the ILSG’s Goldich Medal in 2002 for his pioneering contributions to base and precious metal exploration in the Lake Superior region, especially in Minnesota and Wisconsin. He tirelessly contributed his time and talents to professional organizations such as SME and AIPG, mining advocacy groups such as Mining Minnesota, and minerals outreach programs such as the Minnesota Minerals Education Workshops. Ernie was admired by family, friends and colleagues for his honesty, integrity and perseverance and his ability to tell a tale. He was a quietly generous and caring man who will be greatly missed. Ernie was born in Heidelberg, Germany and emigrated with his parents to the United States in 1935. He was educated in the public primary and secondary schools of New Rochelle, N.Y. and graduated from Williams College, Mass. in 1951 with highest honors in geology. In 1951-52, he did graduate study in geology at Brown University and in 1984 completed the Owners and Presidents Management Program at the Harvard Business School. His career in the mining industry began in 1950 when he worked as a miner and then geologist at a gold mine in Bannack, Montana and then joined Kennecott Copper’s exploration subsidiary, where he was head of a team that discovered the south end of the “New Lead Belt” in Missouri in the 1950s. With the consulting firm he founded in 1958, he undertook a variety of successful exploration projects, including industrial limestone; gold in Montana, the Northwest Territories and Argentina and copper-nickel-platinum group metals in Minnesota. He managed a small fluorspar mine in southern France and undertook valuation of various mining projects in Africa, Indonesia, South and Central America and North America for IFC (International Finance Corporation), the World Bank, major mining companies and metal trading companies. He served as president of North Central Mineral Ventures (NCMV) since its incorporation in February 1986. NCMV has served as Manager of Vermillion Gold since December 2007. Ernie served as a member of the Advisory Board to the Natural Resources Research Institute of the University of Minnesota and the MGS (Minnesota Geological Survey) State Mapping Advisory Committee. He served as a member of the Governor’s Committee of Minnesota’s Mining Future from 2004 to 2007 and as a member of the Minnesota Legislature’s Mineral Coordinating Committee for over 10 years. He was an officer or director of several other private companies, including a Director of Silverthorn Exploration Inc. He was a charter and honorary member of the AIPG, of which he served as national President in 1985; a life member and fellow of the Society of Economic Geologists; a Legion of Honor member of the Society of Mining Engineers; and a member of several other professional and technical societies. Ernie was president of Mining Minnesota, a trade association representing Minnesota’s non-ferrous and precious metals mining industry. xiii �In Memoriam Jack V. Everett (1921-2013) The ILSG lost another long-time supporter with the passing of Jack V. Everett, who slipped away peacefully on August 12, 2013 at his summer home on Ottertail Lake, MN. Jack will be most remembered for his sitting in the front row of ILSG meetings and snapping pictures of slide presentations. Jack lived in Duluth, MN for most of his professional career working as a consulting mining geologist. Jack was born in Roseburg, Oregon, but spent most of his childhood in lower Michigan. He enrolled at Michigan State in the class of 1944 in wildlife management, conservation and zoology, but later chose to major in geology. World War II interrupted his studies and he enlisted on June 6, 1942 in ROTC in field artillery with basic training at Fort Bragg, NC, and was called for active duty on April 16, 1943. He married Eleanor Brown Everett, class of ’44, from Onaway, Michigan at that time. The Army needed infantry officers and on May 6, 1944 sent his entire class to be retained as infantry officers at Fort Benning, GA. On July 18, 1944 he was assigned as a cadre training officer at Fort Meade, MD. After the Japanese surrendered, on September 3, 1945 he received orders to be transferred to Japan and was assigned to serve in the occupation forces of the 77th Division in Hakodate on the northern island of Hokkaido. He was discharged out of the service on September 5, 1946. He went back to MSU and graduated in 1947 with a B.S. degree, cum laude, in Geology. Honors included Phi Kappa Phi for scholastic, Sigma Gamma Epsilon for geologic, and Tau Sigma for scientific. Jack later served in various U. S. Army Reserve and Minnesota National Guard units in Brainerd and Duluth, and retired as a Major in June of 1972. Jack’s professional career started when he was hired as a District Geologist for Pickands Mather & Co. on the Minnesota Cuyuna Iron Range where he discovered four iron – manganese deposits near Emily, MN. These deposits are currently being developed for their manganese ore potential. In 1951 he took a position with W.S. Moore Company as Chief Geologist & Exploration Manager and moved to Duluth, MN. During the 50s and 60s he conducted exploration programs for iron ore deposits in various locations across the United States and Canada, and also Parana, Brazil. In the 1960s he conducted major prospecting programs in unmapped areas of the Northwest Territories exploring for gold deposits from Yellowknife to the Arctic coast and was quoted as saying that for 20 years, he spent 50% of his life living in tents. He also conducted exploration programs in Northern MN and discovered one major copper nickel deposit. Jack started a career as a Certified Professional Geologist in 1971 and worked for more than 100 US and Canadian mining companies as an independent consulting geologist. He conducted exploration programs for copper and gold deposits in Wisconsin. Jack was an avid hunter and fisherman. Although he supported mining, he was a conservationist and supported preservation of unique natural resources and was on the Governor Elmer L. Andersen committee as a consulting geologist and first chair of the Duluth Chapter of the Citizens Committee for the establishment of Voyageurs National Park. In the 1980s he explored and developed placer gold deposits in Alaska. In later years he worked on a variety of geology, geotechnical and hydrology projects, including the tunnel projects on the North Shore of Lake Superior for the MnDOT. In 1995 he became Vice PresidentExploration & Director of Leadville Mining and Milling Corp. where he was involved with the development, geology and exploration of their underground gold mine near Leadville, CO. More recently he worked on the geology and development of El Chanate Gold Project in Sonora, Mexico as a Director for Capital Gold Corporation. And most recently he has been working on the geology and development of Lake Victoria Gold deposit in Tanzania, Africa. He always joked that he planned to retire soon. 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 onehalf of any additional proceeds from each annual meeting, after all other commitments and expenses are covered. • The ILSG Board of Directors will be responsible for selecting a minimum of two awards each year. The ILSG Treasurer will issue the awards. • The ILSG Student Research Fund is available for undergraduate or graduate students working on geology in the Lake Superior region. • The applications are due to the ILSG Secretary by August 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 2013, the ILSG Board of Governors awarded three $500 awards from the Student Research Fund. The winners were: Michael Doyle University of Minnesota-Duluth, Department of Geological Sciences Current degree program: MS Candidate (Advisor: Jim Miller) Geologic and Geochemical Attributes of the Beaver River Diabase and Greenstone Flow: Testing a Possible Intrusive-Volcanic Correlation in the 1.1 Ga Midcontinent Rift Sarah Sauer University of Minnesota-Duluth, Department of Geological Sciences Current degree program: MS Candidate (Advisor: Jim Miller) The Petrology of the DLS “Chill” – Evidence of Venting of Hydrous Magma from the Layered Series at Duluth? Nicholas Fedorchuk University of Wisconsin-Milwaukee, Department of Geosciences Current degree program: MS Candidate (Advisor: John Isbell) Biogenicity of Mesoproterozoic Lacustrine Stromatolites from the Copper Harbor Conglomerate 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 long-time friend of the Institute. The 2012 ILSG Board of Directors approved adding Doug’s name to the award to acknowledge his contributions, and distribute those donations in a manner that would have pleased him. The Duskin Student Paper Committee is appointed by the Meeting Chair. Criteria for best student paper—last modified by the Board in 2001—follow: 1) The contribution must be demonstrably the work of the student. 2) The student must present the contribution in-person. 3) The Student Paper Committee shall decide how many awards to grant, and whether or not to give separate awards for poster vs. oral presentations. 4) In cases of multiple student authors, the award will be made to the senior author, or the award will be shared equally by all authors of the contribution. 5) The total amount of the awards is left to the discretion of the meeting Chair in conjunction with the Secretary, but typically is in the amount of about $500 US (increase approved by Board, 10/01). 6) The Secretary maintains, and will supply to the Committee, a form for the numerical ranking of presentations. This form was created and modified by Student Paper Committees over several years in an effort to reduce the difficulties that may arise from selection by raters of diverse background. The use of the form is not required, but is left to the discretion of the Committee. 7) The names of award recipients shall be included as part of the annual Chair’s report that appears in the next volume of the Institute. Student papers will be noted on the Program. Student Paper Awards Committee Andrew Ware – PolyMet Mining Prajukti Bhattacharyya – University of Wisconsin-Whitewater Robert Cundari – Ontario Geological Survey xvii �Board of Directors Board appointment continues through the close of the meeting year shown in parentheses, or until a successor is selected Jim Miller (2014-2017) – University of Minnesota Duluth Allan Blaske (2013-2016) – AECOM Peter Hinz (2012-2015) – Ontario Geological Survey Tom Fitz (2011-2014) – Northland College Pete Hollings - Secretary (2013-2016) – Lakehead University Mark Jirsa – Treasurer (2011-2014) – Minnesota Geological Survey Local Committee Chairs Jim Miller – Program Chair Department of Geological Sciences and Precambrian Research Center University of Minnesota Duluth Mark Jirsa – Field Trip Chair Minnesota Geological Survey University of Minnesota Volume Editors Jim Miller – Proceedings Volume Department of Geological Sciences and Precambrian Research Center University of Minnesota Duluth Mark Jirsa and Terry Boerboom – Field Trip Guidebook Minnesota Geological Survey, University of Minnesota Special Projects Amy Radakovich – Minnesota Geological Survey, University of Minnesota Banquet Speaker Dr. Francis M. Carroll University of Manitoba - Winnipeg and St. Johns University "A Line in the Trees: History of the US-Canadian Boundary from Lake Superior to Lake of the Woods" xviii �Session Chairs Al MacTavish – Panoramic Resources, Thunder Bay, ON Joyashish Thakurta – Western Michigan University, Kalamazoo, MI Geoff Pignotta – University of Wisconsin – Eau Claire Marcia Bjornerud – Lawrence University, Appleton, WI Mary Louise Hill – Lakehead University, Thunder Bay, ON Bernie Saini-Eidukat – North Dakota State University, Fargo, ND Field Trip Leaders Field trips have been the mainstay of the ILSG since its inception 60 years ago. We want to give a special thanks to the field trip leaders who volunteered their time and talent in carrying that tradition forward. 1) STRATIGRAPHY, SEDIMENTOLOGY, STRUCTURE AND MINERALIZATION OF THE BIWABIK IRON FORMATION, CENTRAL MESABI IRON RANGE Phil Larson - Duluth Metals Ltd. Marsha Patelke - Natural Resources Research Institute, UMD Jakob Wartman - United Taconite, Cliffs Natural Resources Michael Totenhagen - Arcelor Mittal Mark Jirsa - Minnesota Geological Survey Steven Losh - Minnesota State University – Mankato Peter K. Jongewaard - Cliffs Natural Resources (retired) 2) A WALK IN THE PARK—NEOARCHEAN GEOLOGY OF LAKE VERMILION STATE PARK George J. Hudak - Natural Resources Research Institute – UMD Amy Radakovich - Minnesota Geological Survey Geoff Pignotta - University of Wisconsin - Eau Claire Kelly Schwierske - University of Wisconsin - Eau Claire 3) WESTERN MESABI RANGE MINING OPERATIONS Douglas Halverson - Cliffs Natural Resources, Duluth Daniel Cervin - Cliffs Natural Resources, Hibbing Taconite William Everett – Essar Steel Kevin Kangas - Essar Steel Joseph Nielsen - Magnetation xix �5) VISIONS OF MATURI: THE GEOLOGY OF THE SOUTH KAWISHIWI INTRUSION Dean Peterson - Duluth Metals Ltd. 6) THE ST. LOUIS SUBLOBE AND GLACIAL LAKE UPHAM Phil Larson - Duluth Metals Ltd. Alan Knaeble - Minnesota Geological Survey Howard Mooers - University of Minnesota Duluth Lisa Marlo - Halcon Resources Corporation 7) GEOLOGY AND GOLD MINERALIZATION OF THE VIRGINIA HORN AREA Mark Jirsa - Minnesota Geological Survey William Rowell - Vermillion Gold LLC Richard Sandri - Vermillion Gold LLC Jason Richter - Minnesota Department of Transportation A) STATE DRILL CORE LIBRARY—HIBBING MINNESOTA Minnesota Department of Natural Resources—Division of Lands and Minerals Dave Dahl – Minnesota Department of Natural Resources, Div. of Lands and Minerals Dean Rossell - Kennecott Exploration, Rio Tinto B) HIBBING’S IRON MINING AND CULTURAL HISTORY Henry Djerlev - Superior GEO-Services (retired) Bob Kearney – Hibbing High School (retired) Erica Larson and other Hibbing Historical Society staff C) MINNESOTA DISCOVERY CENTER, CHISHOLM, MN Discovery Center Staff D) COLERAINE MINERALS RESEARCH LABORATORY Natural Resources Research Institute, University of Minnesota-Duluth Dick Kiesel - CMRL Director Dave Hendrickson - Director Strategic Planning Matt Mlinar - Program Coordinator Mineral Processing Basak Anameric - Program Coordinator High Temperature Process) E) MINEVIEW FROM A CANOE : Mark Jirsa - Minnesota Geological Survey Daniel Jordan - Iron Range Resources and Rehabilitation Board Dale Cartwright - Minnesota Dept. of Natural Resources, Div. of Lands and Minerals xx �Sponsors The following organizations and individuals made general contributions to the 60th Annual Meeting. We thank them for their commitment to the Institute on Lake Superior Geology. All of the funds contributed this year go toward travel awards for student registrants. Midwest Institute of Geosciences and Engineering INDIVIDUAL CONTRIBUTORS TO STUDENT TRAVEL SCHOLARSHIPS MARY ARTHUR JOHN BERKLEY KARL EVERETT JOHN GREEN GEORGE HUDAK PETER JONGEWAARD STEVEN LOSH ALLAN MACTAVISH GORDON MEDARIS, JR. MICHAEL MUDREY JILL PETERMAN With an especially generous donation provided by RON SEAVOY xxi �Report of the Chairs of the 59th Annual Meeting Theodore J. Bornhorst and Allan R. Blaske Houghton, Michigan The 59th annual meeting of the Institute on Lake Superior Geology (ILSG) was held May 8 to 11, 2013 in Houghton, Michigan, at the Franklin Square Inn. The meeting was hosted by the A. E. Seaman Mineral Museum of Michigan Technological University and was chaired and organized by Ted Bornhorst (A. E. Seaman Mineral Museum) and Allan Blaske (AECOM). The meeting was attended by a total of 228 delegates from 14 U.S. states (Arizona, Colorado, Illinois, Indiana, Iowa, Massachusetts Michigan, Minnesota, New York, North Dakota, Ohio, Texas, Virginia, Wisconsin) and 4 Canadian provinces (British Columbia, Ontario, Manitoba, Quebec). There were 58 student attendees. The two-day technical session began on Thursday morning with oral presentations on Archean topics and continued on Friday with presentations on Keweenawan and Quaternary geology. There were a total of 25 oral presentations, 10 of which were presented by students. The technical session included a total of 18 poster presentations, 10 of which were presented by students. The meeting offered 5 field trips that highlighted the Keweenawan geology of the western Upper Peninsula of Michigan. Two pre-meeting trips were held on Wednesday: Geologic Overview of the Keweenaw Peninsula, Michigan, led by Ted Bornhorst (A. E Seaman Mineral Museum) and Caledonia Mine, Keweenaw Peninsula Native Copper District, Ontonagon County, MI, led by Bob Barron (Michigan Tech) and Richard Whiteman (Red Metal Minerals). A third scheduled pre-meeting field trip was cancelled because of the unusual lingering snowpack which prevented access to Silver Mountain. Friday afternoon featured a “field trip” open house at the A. E. Seaman Mineral Museum, with guided tours led by Museum Director, Ted Bornhorst. Two post-meeting trips were held on Saturday: Geology of the Keweenawan Supergroup, Porcupine Mountains, Ontonagon and Gogebic Counties, MI, led Laurel Woodruff (USGS), Bill Cannon (USGS), and Robert Wild (Porcupine Mountains Wilderness State Park) and Geology and Environmental Site Conditions of the Copperwood Deposit, Gogebic County, MI, led by Ted Bornhorst (A. E. Seaman Mineral Museum), Allan Blaske (AECOM), Dave Anderson (Orvana Resources US Corp) and Tom Repaal (Orvana Resources US Corp.). The field trips were well-attended with each being at maximum capacity (sold out). The unusual snow cover in the Keweenaw and cold weather on Saturday made some shuffling of field trip logistics necessary. Four Doug Duskin Best Student Paper Awards were given for student paper presentations. Awards were presented for oral and poster presentations, with an award within each category for undergraduate students and for graduate students. The student awardees were Breanne Beh (Lakehead University, graduate student) and Emily Smyk (Lakehead University, undergraduate student) for their oral presentation and Jonathan Dyess (University of Minnesota – Duluth, graduate student) and Brynley Nadziejka (Lawrence University, undergraduate student) for their poster presentation. Eisenbray Student Travel Awards are funded by ILSG and forty students received a travel award. Thanks to very generous support from ILSG corporate sponsors (AECOM, Coleman Engineering Company, Rio Tinto–Eagle Mine, and Superior Copper Corporation) we were able to award ILSG Corporate Student Registration Awards to all students who attended the meeting. The ILSG Corporate Student Registration Award consisted of the meeting registration fee. In addition, students who were presenting papers received additional monetary support in the form of an ILSG Corporate Presenter Award. We are pleased to report that $5,325 were awarded to students. Through the support of corporate sponsors, the ILSG can better promote geologic studies of the Lake Superior region to the next generation of professional geoscientists. The ILSG social and banquet were held at the Franklin Square Inn, Houghton. There were 140 people at the banquet. Jim Ashley of the Lunar Reconnaissance Orbiter Camera Science Operations xxii �Center (LROC) at Arizona State University delivered the banquet address, entitled “Rusty Metal at the Martian Equator: The Search for Life on the Red Planet,” which discussed the occurrence and weathering of iron meteorites on the surface of Mars, and the implications these samples have to the history of water on Mars. The highlight of the banquet was the awarding of the 2013 Goldich Medal to Tom Waggoner (retired chief geologist and lands manager for Cleveland-Cliffs and currently a consulting geologist). Ron Seavoy provided a brief summary of Tom’s contributions to the geology of the Lake Superior region and the ILSG. Tom was greeted with warm applause upon receiving the prestigious ILSG award. The Institute’s Board of Directors met on May 9 to discuss the business of ILSG. The meeting was attended by Ted Bornhorst (Board of Directors meeting Chair), Allan Blaske, Al MacTavish, Tom Fitz, Peter Hinz, Jim Miller, Mark Jirsa and Pete Hollings. ILSG Secretary Hollings took the minutes of the meeting, which are as follows: 1. Accepted report of the Chairs for the 58th ILSG, Thunder Bay, Ontario; as printed in the Proceeding Volume (Hinz), and minutes of last Board meeting, May 17, 2012 (Hollings). 2. Received, discussed, and accepted the 2012-2013 ILSG Financial Summary (Jirsa). 3. Received, discussed, and accepted the 2012-2013 report of the Secretary (Hollings). 4. Approved Allan Blaske as ILSG Board member representing the 2013 meeting. 5. Approved Hibbing as the site for the 60th annual ILSG meeting. The meeting will be hosted by Jim Miller and Mark Jirsa. 6. Discussed and approved renewal of Peter Hollings as Institute Secretary (end of term 2013). This was later approved by a vote of the membership during the technical session. 7. Discussed and approved replacing Laurel Woodruff as the “member from government” on Goldich Committee (end of term 2013) with Mark Smyk. 8. Discussed the possibility of having a short themed session with invited speakers at future meetings. 9. Requested that Secretary Hollings contact past chairs in order to compile statistics on the number of responses to the first circular in relation to attendance at the meeting. 10. Discussed the topic of the level of student participation required to be eligible for an Eisenbrey Student Travel Award. While there was no formal vote, the Board agreed that meeting Chairs should include a statement on the application indicating that full participation in the meeting was required to receive the award. In other words, a student cannot come for ½ day of the technical session or only attend a field trip to pick up the award. The meeting co-Chairs would like to thank all those who assisted with this year’s meeting either by chairing sessions, judging student papers, leading field trips, driving vehicles for field trips, helping with the registration desk, operating the meeting and banquet projectors, and more. These volunteers made the quality of the meeting better and of course, made the job of the meeting chairs far easier than it may otherwise have been. A special thank you goes to Darlene Comfort, who tirelessly and patiently managed the registration and logistics for the meeting. We are gratified by all of the positive comments by participants. While chairing and organizing an ILSG meeting involves time and a bit of stress on occasion, we are happy to have had the opportunity to serve the geological community of the Lake Superior region. We look forward to the 2014 meeting when we can be much more relaxed! Respectfully submitted, Ted Bornhorst and Allan Blaske Co-chairs, 59th Institute on Lake Superior Geology xxiii �PROGRAM WEDNESDAY MAY 14, 2014 All trips leave from the northeast entrance of the Hibbing Park Hotel 8:00am - 5:30pm PRE-MEETING FIELD TRIPS FIELD TRIP 1: STRATIGRAPHY, SEDIMENTOLOGY, STRUCTURE, AND MINERALIZATION OF THE BIWABIK IRON FORMATION, CENTRAL MESABI IRON RANGE Phil Larson, Duluth Metals Ltd. Marsha Patelke, Natural Resources Research Institute, UMD Jakob Wartman, United Taconite, Cliffs Natural Resources Michael Totenhagen, Arcelor Mittal Mark Jirsa, Minnesota Geological Survey Steven Losh, Minnesota State University – Mankato Peter K. Jongewaard, Cliffs Natural Resources (retired) FIELD TRIP 2: A WALK IN THE PARK - NEOARCHEAN GEOLOGY OF LAKE VERMILION STATE PARK George J. Hudak, Natural Resources Research Institute – UMD Amy Radakovich, Minnesota Geological Survey Geoff Pignotta and Kelly Schwierske, University of Wisconsin - Eau Claire FIELD TRIP 3: WESTERN MESABI RANGE MINING OPERATIONS Douglas Halverson, Cliffs Natural Resources—Duluth Daniel Cervin, Cliffs Natural Resources—Hibbing Taconite William Everett and Kevin Kangas, Essar Steel Joseph Nielsen, Magnetation 4:00 pm - 10:00 pm Registration at Hibbing Park Hotel (hallway outside Arrowhead Ballroom) 7:00 pm - 10:00 pm Ice Breaker Social (Arrowhead Ballroom) and Poster Session (Whispering Pines Room) xxiv �THURSDAY MAY 15, 2014 Asterisk * denotes a student eligible for Best Student Paper Award 7:30 am - noon REGISTRATION 8:00 am OPENING REMARKS Jim Miller and Mark Jirsa, Co-Chairs, 2014 ILSG TECHNICAL SESSION I Session Chairs: Al MacTavish – Panoramic Resources Joyashish Thakurta – Western Michigan University 8:10 Peter Hollings and Geoff Heggie Rethinking the Midcontinent Rift – Puncturing the “Plume Paradigm” 8:30 Paul Bedrosian Electrical resistivity structure of the Midwestern United States from EarthScope magnetotelluric data 8:50 Elisa Piispa*, Aleksey Smirnov and Lauri Pesonen Mesoproterozoic Midcontinent Rift intrusives in Thunder Bay area, Ontario, Canada: a paleomagnetic review 9:10 Adam Leu* and Jim Miller Geology and petrology of the Wilder Lake Intrusion, Duluth Complex, northeastern Minnesota 9:30 Klaus Schulz, Laurel Woodruff and Suzanne Nicholson Midcontinent Rift-related satellite mafic-ultramafic intrusions hosting Fe-Ti-V oxide deposits 9:50 COFFEE BREAK AND POSTER SESSION 10:20 Gabe Sweet, Dean Peterson, Phil Larson, Molly Finnegan, Evan Finnes, Charlie Parent, Bob Nowak, Tyler Boley Sulfide highway revisited: New ideas on internal structure and sulfide mineralization of the Nickel Lake Macrodike 10:40 Molly Finnegan and Phil Larson Geochemistry of basalt xenoliths entrained in mineralized troctolitic and anorthositic intrusions, northeastern Minnesota 11:00 Alex Steiner* and Jim Miller Genesis of sulfide mineralization within the footwall granite of the Maturi Cu-NiPGE Deposit of the South Kawishiwi Intrusion, Duluth Complex, NE Minnesota 11:20 Brent Trevisan*, Peter Hollings and Doreen Ames The Thunder mafic to ultramafic intrusion: a PGE and precious metal-bearing early rift conduit system in the Midcontinent Rift 11:40 Jeff Mauk, Laurel Woodruff and Ester Stewart Variable copper mineralization in the lower Nonesuch Formation of the Midcontinent Rift System: Constraints on regional controls xxv �Noon LUNCH BUFFET (free to all registrants) ILSG BOARD MEETING TECHNICAL SESSION II Session Chairs: Geoff Pignotta – University of Wisconsin – Eau Claire Marcia Bjornerud – Lawrence University 1:30 Bill Cannon, Laurel Woodruff, Stacy Saari, and Molly Hagstrom A new occurrence of the Sudbury impact layer in the Gogebic Iron Range of Wisconsin 1:50 Monica Karman*and Philip Fralick Sedimentology and paleogeographic reconstruction of the layers in and adjacent to the Subury Impact Layer in the Lake Superior Basin 2:10 Leif Johnson and Brad Dunn An exploration update and mineralogical study of the Emily District Maganese Deposit, Cuyuna Iron Range, Minnesota 2:30 Adrian Arts* and Philip Fralick Nanoscale features within freshwater lacustrine ferromanganese nodules: Nanospheres, nanotubes and nanowires 2:50 James Walsh Strontium isotope study of Mesabi Iron Range groundwater 3:10 COFFEE BREAK AND POSTER SESSION 3:40 Robert Seal The danger of “Sulfide Mining” in the Lake Superior Region 4:00 Tim McIntyre* and Philip W. Fralick Sedimentology and geochemistry of the Mesoarchean chemical sediments of Wallace Lake and Red Lake 4:20 Rob Cundari, Mark Smyk and Peter Hollings Geology and geochemistry of the Mesoproterozoic Badwater Intrusive Complex, Ontario: Implications for GEON 15 magmatism 4:40 Terry Boerboom, Karl Wirth and Joseph Evers Five newly acquired high-precision U-Pb ages in Minnesota, and their geologic implications 6:00 RECEPTION – CASH BAR 7:00 ANNUAL BANQUET (Arrowhead Ballroom) − Announcement of 61st Annual Meeting Location − 2014 Goldich Award Presentation to Laurel Woodruff − Banquet Presentation by Dr. Francis M. Carroll, Univ. of Manitoba A Line in the Trees: History of the US-Canadian Boundary from Lake Superior to Lake of the Woods xxvi �FRIDAY MAY 16, 2014 Asterisk * denotes a student eligible for Best Student Paper Award 8:00 OPENING REMARKS, UPDATES Jim Miller and Mark Jirsa, Co-Chairs, 2014 ILSG TECHNICAL SESSION III Session Chairs: Mary Louise Hill – Lakehead University Bernie Saini-Eidukat – North Dakota State University 8:10 Jack Berkley Deer Lake Complex redux: Memories and reflections, 1970 - 1972 8:30. Ben Kuzmich*, Peter Hollings and Michel Houle Geochemistry and mineralogy of the Fe-Ti-V-P mineralized ferrogabbroic intrusions of the McFaulds greenstone belt, Superior Province, northern Ontario, Canada 8:50 Jordan Quinn*, Peter Hollings and John Biczok Geochemistry and petrography of a mafic metavolcanic sequence south of Musselwhite Mine 9:10 Lionnel Djon*, Gema Olivo, Jim Miller and Rob Stewart Petrology of the layered North Lac des Iles Intrusion, Ontario: Part I. Stratigraphy and mineral-chemical evidence for multiple magma injection 9:30 Skylar Schmidt* and Mary Louise Hill Structural control of mineralization at Lac des Iles Mine 9:50 COFFEE BREAK AND POSTER SESSION 10:20 Amanda Van Lankvelt*, M. Williams, D. Schneider, S. Seaman, Garnet in the deep crust: The key to linking Archean TTG generation and vertical block motions? 10:40 Jonathan Dyess* and Vicki Hansen Structural and kinematic analysis of the Shagawa Lake Shear Zone: Implications for Archean tectonic processes in the Southern Superior Province 11:00 Simon Dolega* and Mary Louise Hill Strain analysis on the Max Lake polymictic conglomerates in the Wabigoon Subprovince, Ontario 11:15 Leah Clapp* and Mary Louise Hill Evidence of simultaneous brittle and ductile deformation in the Main Break Fault System, Kirkland Lake, Ontario 11:30 Jared Liimu* and Mary Louise Hill The role of brittle-ductile deformation and competency contrast in gold mineralization in the C-zone at Hemlo 11:45 Daniel LaFontaine* and Mary Louise Hill Structural control on the Borden Gold Deposit in Chapleau, Ontario xxvii �Noon LUNCH BUFFET (free to all registrants) 1:20 BEST STUDENT PAPER AWARDS STUDENT TRAVEL AWARDS 2:00-6:00 FRIDAY AFTERNOON FIELD TRIPS FIELD TRIP A: STATE DRILL CORE LIBRARY – HIBBING, MINNESOTA (MINNESOTA DEPARTMENT OF NATURAL RESOURCES –DIVISION O F LANDS AND MINERALS Dave Dahl, Minnesota Department of Natural Resources Dean Rossel, Kennecott Exploration, Rio Tinto FIELD TRIP B: HIBBING’S IRON MINING AND CULTURAL HISTORY Henry Djerlev and Staff from the Hibbing Historical Society FIELD TRIP C: MINNESOTA DISCOVERY CENTER Discovery Center Staff FIELD TRIP D: COLERAINE MINERALS RESEARCH LABORATORY, NATURAL RESOURCES RESEARCH INSTITUTE, UNIVERSITY OF MINNESOTA DULUTH Dick Kiesel, Director CMRL Dave Hendrickson, Director Strategic Planning Matt Mlinar, Program Coordinator Mineral Processing Basak Anameric, Program Coordinator High Temperature Process FIELD TRIP E: MINEVIEW FROM A CANOE Mark Jirsa, Minnesota Geological Survey Dan Jordan, Iron Range Resources and Rehabilitation Board Dale Cartwright, Minnesota Department of Natural Resources SATURDAY MAY 17, 2014 8:00am – 5:00pm POST-MEETING FIELD TRIPS FIELD TRIP 5: VISIONS OF MATURI: THE GEOLOGY OF THE SOUTH KAWISHIWI INTRUSION Dean Peterson, Duluth Metals Ltd. FIELD TRIP 6: THE ST. LOUIS SUBLOBE AND GLACIAL LAKE UPHAM Phil Larson, Duluth Metals Ltd. Alan Knaeble, Minnesota Geological Survey Howard Mooers, University of Minnesota Duluth Lisa Marlow, Halcon Resources Corp. FIELD TRIP 7: GEOLOGY & GOLD MINERALIZATION OF THE VIRGINIA HORN AREA Mark Jirsa, Minnesota Geological Survey Bill Rowell, Vermilion Gold LLC Rick Sandri, Vermilion Gold LLC Jason Richter, Minnesota Department of Transportation xxviii �POSTER PRESENTATIONS Asterisk * denotes a student eligible for Best Student Paper Award Steven D. J. Baumann, Alex B. Cory and David Wilson Fault offsetting in the Proterozoic Lorraine Formations along Government Road, south of Echo Bay, Ontario, Canada Mark Baumgardner, Nathan Brown, Matt Grotte, Alan Jacobson, Jamie Kendall, Claire Ostwald, Nathan Schriner, Justin White, and Dean Peterson Bedrock geologic map of the Gafvert Lake area, St. Louis County, northeastern Minnesota Patrick Belshaw* and Mary Louise Hill Relationship between Microstructure and Rock Mechanics in Shear-Zone-HostedGold Deposits Terry Boerboom and John Green Bedrock geologic map of the Marr Island and Hovland 7.5'quadrangles, North Shore of Lake Superior,Minnesota Anthony Boxleiter*, Joyashish Thakurta, and Thomas Quigley Geochemical investigation of the origin of the Back Forty volcanogenic massive sulfide deposit, Menominee County, Michigan Tom Buchholz, A. Falster, and W. Simmons Zirconium/Hafnium fractionation in some pegmatites of the upper Midwest, USA Val Chandler and Richard Lively Continued work on using the horizontal-to-vertical spectral ratio (HVSR) passive seismic method for determining Quaternary sediment thickness in Minnesota Ben Drenth, Ray Anderson, Klause Schulz, Val Chandler, Bill Cannon, Ben Bloss, Paul Bedrosian, Josh Feinberg, Rob McKay Preliminary interpretation of Precambrian lithology and structure from high-resolution, multi-method geophysics, northeast Iowa and southeast Minnesota Jonathan Dyess* and Vicki Hansen Determination of Vorticity in Archean Tectonites Nicholas Fedorchuk*, Stephen Dornbos, John Isbell, Julie Bowles, Frank Corsetti, Dylan Wilmeth, Victoria Petryshyn Bedrock Geologic Map of the Putnam Lake Area, St. Louis County, NE Minnesota – Precambrian Research Center Capstone Project Kiel Finn* and Julie Bowles Magnetic Mineralogy of Reversely Magnetized Chengwatana Lava Flows of St. Croix Falls, Wisconsin Sidney Firmin* and Julie Bartley An Unusual Mesoproterozoic Carbonate Unit: Relic of a Saline Lake? Paul Fix, Stephen Ginley, Lauren Schraeder, Aaron Summers, Michael Doyle,Terry Boerboom Geology of the Brule River area of the Pine Mtn quadrangle, Minnesota: Capstone mapping project for the Precambrian Research Center’s 2013 field camp xxix �Marine Foucher*, Renee Curganus, Elisa Piispa, Aleksey Smirnov, and Lauri Pesonen Evolution of the Midcontinent Rift system: Paleomagnetic, rock magnetic and anisotropy of magnetic susceptibility study of the Mesoproterozoic Baraga - Marquette dike swarm, MI, USA VJ Grauch, Val Chandler, and Rich Lively Compilation of existing geophysical models in preparation for 3D modeling of the Midcontinent Rift System in the western Lake Superior region, Minnesota, Wisconsin, and Michigan Matt Grotte* and George Hudak, A field and petrographic study of Neoarchean variolitic pillow lavas, Newton Belt, Vermilion District, NE Minnesota. Ivan Guzman* Stratigraphic framework and landsystem correlation for deposits of the Saginaw Lobe, Michigan, USA George Hudak, Stephen Monson Geerts, Larry Zanko, Sara Post, and Bryan Bandli The Minnesota Taconite Workers Health Study: Environmental Study of Airborne Particulate Matter - 2014 Update Darcy Jacobson*, Elisa Piispa, Aleksey Smirnov, and Lauri Pesonen Silica Remobilization in the Biwabik Iron Formation, Minnesota USA Monica Karman* and Phil Fralick Impact ejecta features in the Lake Superior basin from the 1850Ma Sudbury Impact Event Stephen Kissin and Gregory Brumpton PDFs in Sudbury Ejecta in the Gunflint Formation, Ontario: A Comparison of Methods Matthew Lamb* and Prajukit Bhattacharyya Ru-Rh-Pd Mobilization in Flambeau Massive Sulfide Deposit Gordon Medaris, Jr., Tim Flood, Brian Jicha and Bradley Singer Composition and 40Ar/39Ar Age of Pegmatitic Amphibole in the Wausau Syenite Complex, Marathon County, Wisconsin Jim Miller, Sarah Sauer, Jordan Benningfield, Jackson Graham, Sara Kozmor, and Ann Marie Prue Geology of the Lake Three Troctolite, Duluth Complex - 2013 Precambrian Field Camp Capstone Connor Mulcahy, Dan Romanelli, Roger Schulz, Steve Moorhead, Mitchell May, and Mark Jirsa Geologic mapping of Neoarchean and Paleoproterozoic rocks near Hanson Lake, NE Minnesota, by students of the Precambrian Research Center’s 2013 field camp Brynley Nadziejka* and Marcia Bjornerud Petrographic characterization of the Penokean Twelvefoot Falls Shear Zone, Marinette County, Wisconsin: Evidence for coeval ductile and seismic behavior xxx �Ainslee Nolan* and Mary Louise Hill Metamorphism and Deformation at the Wabioon-Quetico subprovince boundary in the Decourcey Lake area Dean Peterson Bedrock geologic map of the Twin Metals Minnesota Project, Northern South Kawishiwi Intrusion and adjacent areas Nadine Piatak, Robert Seal, Perry Jones, and Laurel Woodruff Potential for copper toxicity caused by surface water and stream sediments in unmined mineralized watersheds of the Duluth Complex Patrick Quillen* and Jim Miller Documenting the first lava flows of the Midcontinent Rift by digital mapping and petrographic analysis Amy Radakovich and Howard Hobbs The Arrowhead Pilot Project: Mapping of Precambrian and Quaternary geology in two diverse geologic areas of northeastern Minnesota Bill Rose and Erica Vye Tools for interpreting Keweenaw geoheritage to a broad public Kelly Schwierske*, Geoff Pignotta, and George Hudak The 2.7 billion year old Mt. St. Helens of northern Minnesota: Petrography, geochemistry and economic significance of the Neoarchean Gafvert Lake sequence Laurel Woodruff, Bill Cannon, Federico Solano, and David Smith Geochemistry and Mineralogy of Glacial Soils in the Upper Midwest Chris Yip* and Phil Fralick The evolution of the atmosphere-hydrosphere: A geochemical comparison of two Paleoproterozic Gunflint weathering profiles xxxi �Abstracts ��Nanoscale features within freshwater lacustrine ferromanganese nodules: Nanospheres, nanotubes and nanowires ARTS, Adrian and FRALICK, Philip. Department of Geology, Lakehead University, Oliver Rd. Thunder Bay, ON, P7B 5E1, Canada Iron-hydroxide and manganese-oxide precipitates, often referred to as ferromanganese nodules (FMN), are common occurrences on lake bottoms worldwide (Sozanski and Cronan, 1978). The nodules form at the sediment-water interface, generally on a sandy substrate, at neutral pH (Kindle, 1932). Detailed studies documenting their morphology and geochemistry have been conducted by several authors (Sommers et al., 2002). FMNs can take varying morphological forms. However, they most typically accrete as disk-shaped precipitates with a concentric growth pattern of alternating Fe- and Mnrich bands, around a central pebble or small cobble nucleus (Sozanski and Cronan, 1976) (Fig. 1A, 1B). Bacterially mediated precipitation and changes in redox conditions are believed to be a significant factor in their growth (Dean and Greeson, 1979; Boudreau, 1988). These features are environmentally important as the iron hydroxides composing them adsorb arsenic with concentrations up to 4900 ppm. Despite the considerable amount of work conducted on the nodules, no research has been undertaken to explore FMNs at the micro- and nanoscale to investigate the extreme arsenic uptake abilities. This study was conducted to provide new insights on the micro- and nanoscale features within FMNs and to determine the geochemical composition of these features. The nodules examined were collected from Shebandowan Lake (Ontario), Sowden Lake (Ontario), and Lake Charlotte, (Nova Scotia). The utilization of high resolution field emission scanning electron microscopy (SEM) revealed an intriguing range of nanoscale forms previously undocumented in FMNs. Coccus bacterial forms were commonly found implanted in extracellular polymeric substance (EPS). Figure 1C illustrates the high concentrations of ovoid to round nanospheres (100-200nm diameter) which are embedded within the EPS. Semi-quantitative analysis (SEM-EDX) indicates the areas containing nanospheres are enriched in iron. It has been postulated that similar structures in carbonates provide nucleation sites for biologically induced mineralization (Aloisi et al., 2006) preventing the cellular membrane from being entombed by the precipitates (Bontognali et al., 2008). Nanotubes were also documented and appear to be ubiquitous in the FMN samples. The nanotubes appear as a tangled mass of worm like structures, which range in length, from 2-40 µm, with a diameter range of 50-400 nm. Uwins et al. (1998) reported similar structures in Triassic and Jurassic sandstones. Utilizing three different RNA staining techniques they deduced that the structures are biogenic, contain RNA, and have thus referred to them as nanobes. Finally, nanowires appear to be a common formational constituent of samples from each of the three lakes (Fig. 1E). These wires build together into large sheet-like- masses. SEM-EDX analysis show the wires to be composed of manganese oxides. This study provides new insight as to how FMNs accrete, and how they are able to accumulate high concentrations of toxic metals. Similar to the environmental goals of artificially produced metal nanotubes, the biogenic iron hydroxide nanotubes greatly increase the reactive area allowing far greater arsenic adsorption. 1 �(A) (B) (C) 2.00μm (E) (D) 1.00μm 5.00μm Figure 1. Images of Ferromanganese nodules at different scales. Picture of dorsal (A) and ventral (B) side of a FMN forming around a cobble nucleus, with distinct concentric laminations. (C) SEM image of nanospheres embedded into a smooth layer of extracellular polymeric substance. Mineralization can be seen increasing from the top to bottom of the image. (D) Intertwined mass of nanotubes coated in an iron precipitate. The varying diameters and lengths are evident in the image. (E) Wispy mass of manganese oxide nanowires. They can be seen growing together to form sheet-like layers. References Aloisi, G., Gloter, A., Krüger, M., Wallmann, K., Guyot, F., and Zuddas, P. 2006. Nucleation of calcium carbonate on bacterial nanoglobules. Geology. 34, 1017-1020. Bontognali, T.R., Vasconcelos, C., Warthmann, R.J., Dupraz, C., Bernasconi, S.M., and McKenzie, J.A. 2008. Microbes produce nanobacteria-like structures avoiding cell entombment. Geology. 36, 663-666. Boudreau, B. 1988. Mass transport constraints on the growth of discoidal ferromanganese nodules. Journal of American Science. 288, 777-797. Dean, W.E., and Greeson, P.E. 1979. Influences of algae on the formation of freshwater ferromanganese nodules Oneida Lake, New York. Archiv fur Hydrobiologie. 86, 181-192. Folk, R.L. 1993. SEM imaging of bacteria and nannobacteria in carbonate sediments and rocks. Journal of Sedimentary Petrology. 63, 990-999. Gorham, E, and Swaine, D. J. 1965. The influence of oxidizing and reducing conditions upon the distribution of some elements in lake sediments. Limnology and Oceanography. 10, 268-279. Harriss, R.C., and Troup, A.G. 1969. Freshwater ferromanganese concretions: chemistry and internal structure. Science. 166, 604-606. Kindle, E.M. 1932. Lacustrine concretions of manganese. American Journal of Science. 5(24), 496-504. Sommers, M., Dollhopf, M., and Douglas, S. 2002. Freshwater ferromanganese stromatolites from Lake Vermilion, Minnesota: Microbial culturing and scanning electron microscopy investigations. Geomicrobiology Journal. 19, 207-227. Sozanski, A.G., and Cronan, D.S. 1976. Environmental differentiation of morphology of ferromanganese oxide concretion in Shebandowan Lakes, Ontario. Limnology and Oceanography. 21, 894-898. Sozanski, A.G., and Cronan, D.S. 1978. Ferromanganese concretions in Shebandowan lakes, Ontario. Canadian Journal of Earth Science. 16, 126-140. 2 �FAULT OFFSETTING IN THE PROTEROZOIC LORRAINE AND JACOBSVILLE FORMATIONS, ALONG GOVERNMENT ROAD, SOUTH OF ECHO BAY, ONTARIO, CANADA BAUMANN, Steven D.J.1, DYLKA, Sandra K.1 1 Geology Section, Midwest Institute of Geosciences and Engineering, 2328 W. Touhy Ave. Chicago, IL 60645 Along the east side of Government road (a small road that runs parallel to Trans Canada 17) exists an outcrop about 850 feet long at GPS: 46.46141o -84.05810o. The outcrop is mostly of the Jacobsville Formation, with an approximately 300 foot long outcrop of the Lorraine Formation near the center (see Figure 1). The Lorraine is much more indurated than the Jacobsville. The Lorraine was quarried at this location during sometime in the past. The Lorraine is a nearly white, thinly bedded, crystalline, fine to medium grained, quartz arenite, with minor beds of red jasper and white quartz conglomerate that has been metamorphosed. The Lorraine exposed at the outcrop is probably near the top of the formation. The Jacobsville is dominantly a reddish purple mottled pale yellow brown, cross bedded, fine to medium grained, non-metamorphosed, quartz arenite. Lenses of dark red sandy siltstone with red and green shale breccia are common in the Jacobsville (see Figure 2). The exact stratigraphic position of the Jacobsville is not known at this location. The outcrop displays a section of the Lorraine “poking up” through the Jacobsville (see Figure 2). There are several ways this relationship could have formed. 1) The Lorraine existed as a paleo-high and the Jacobsville was deposited around it, making the contact depositional in nature. A similar situation exists in the Baraboo Range at the Upper Narrows in Wisconsin, where the Precambrian Baraboo Quartzite is in contact with the Cambrian sandstones and conglomerates. 2) The Jacobsville could have been deposited with some initial dip, and as more sediments accumulated the weight created growth faulting along the Lorraine-Jacobsville contact. 3) The outcrop is a horst structure, where the Jacobsville was originally deposited with little to no initial dip, and later extensional tectonic forces lowered the Jacobsville relative to the Lorraine. Due to the field relationships of the outcrop, we believe the exposure to be a horst structure (see Figures 1 and 2) for the following reasons. 1) Perhaps the most compelling line of evidence is that no recognizable clasts of Lorraine exist within the Jacobsville at this outcrop, unlike what is seen at the Upper Narrows in Wisconsin, where clasts of Baraboo Quartzite are commonly seen in the local Cambrian sandstones. 2) The brecciated nature of the green and red shale cobbles within the siltstone facies of the Jacobsville appear jumbled. They were probably deposited as clay in stream beds and later brecciated during faulting. This makes sense that the shale and siltstone would have been more susceptible to deformation than the surrounding quartz arenites. 3) The variation in strike and dip between the Jacobsville surrounding the Lorraine. The Lorraine was not deformed during faulting. However, the Jacobsville north of the north fault has a different strike than it does on the south side of the south fault (the south fault may also have some lateral strike-slip movement). 4) There is complex breccia exposed at the north fault (see Figure 4). 5) Slickensides are present on the faces of the Lorraine at both the north and south faults. References: Baumann, S.D.J., 2013. Contact of the Precambrian, Lorraine and Jacobsville Formations, along Government Road, South of Echo Bay, in an Abandoned Quarry, Ontario. Midwest Institute of Geosciences and Engineering, M-122013-2A Jackson, S.L., 2001. On the Structural Geology of the Southern Province between Sault Ste. Marie and Espanola, Ontario. Ontario Geological Survey, Open File Report 5995 Johns, G.W., Mcllraith S., Muir, T.L., 2003. Precambrian Geology Compilation Series, Sault Ste. Marie-Blind River Area, Ontario Ministry of Northern Development and Mines, MAP 2670 3 �Figure 3: Photo of the Stratigraphic Relationships at the North Fault, U.S. Dollar coin for scale Figure 1: Diagram of the Outcrop and Location Map Figure 2: Conceptual Diagram of the Horst Structure along Government Road 4 �BEDROCK GEOLOGIC MAP OF THE GAFVERT LAKE AREA, ST. LOUIS COUNTY, NORTHEASTERN MINNESOTA Mark Baumgardner1, Nathan Brown2, Matt Grotte3, Alan Jacobson4, Jamie Kendall5, Claire Ostwald6, Nathan Schriner7, Justin White8, and Dean Peterson9 1 Wayne State University, 2Virginia Tech University, 3University of Minnesota Duluth, 4University of Wisconsin Milwaukee. 5Swarthmore College, 6Boston University, 7University of Cincinnati, 8Northwest Missouri State University, 9Duluth Metals Limited and UMD Natural Resources Research Center Each year, students from the Precambrian Research Center (PRC) geology field camp complete “capstone” projects that encompass approximately one week of detailed field mapping followed by one week of mapmaking and map publishing. During the fifth and sixth weeks of the 2013 field camp, eight PRC field camp students, under the direction of PRC Assistant Director Dean Peterson, mapped Neoarchean rocks of the informally named Gafvert Lake Sequence (Peterson and Jirsa, 1999, Peterson, 2001) between eastern Lake Vermilion’s Mud Creek Bay and Armstrong Lake, 6 miles to the east-southeast (Baumgardner et al., 2013). This capstone mapping project sought to: 1) identify the lithologies and determine the detailed stratigraphy within the Neoarchean supracrustal strata in this area; 2) define and characterize the nature of the contacts between various units of the Neoarchean supracrustal strata and intrusive rocks; 3) obtain a better understanding of geological structures and their orientations within the area; 4) produce a detailed geological map of the entire Gafvert Lake sequence stratovolcano. Mapping was carried out over five days by eight students (Figure 1) of PRC field camp and 617 new outcrops were mapped by hiking and lakeshore canoe mapping in the field area. The final map incorporated over 1,500 outcrops from historical work in the area. Emphasis was placed on defining the structure of an Archean stratovolcano within the Vermilion Greenstone Belt. Figure 1. Students of the Gafvert Lake Capstone. Mark Severson, while mapping in the area for US Steel in the early 1980's, first recognized that the volcanic rocks in the area around Gafvert Lake represent a proximal facies dacitic to andesitic volcanic edifice. The morphology of the volcanic complex is best seen at map-scale, 5 �which provides an almost perfect cross section through an Archean stratovolcano of dacitic to andesitic composition. The sequence overlies the Soudan Iron Formation, is overlain by the Upper member of the Ely Greenstone to the east and north, and interfingers with reworked tuff and greywacke of the Lake Vermilion Formation on the west. In simple terms, the complex consists of a core of dacite lava flows that are overlain by coarse fragmental volcanic rocks of dacitic to andesitic composition. The fragmental rocks are in turn overlain by thin- to medium-bedded dacitic lapilli and ash tuffs. The whole complex is cut by multiple intrusions of coarse-grained quartz-feldspar porphyry (with rounded quartz phenocrysts up to 1.5 cm across), which occur as a central plug and thick sills to the east and west. The presence of pumice and scoriaceous clasts in the fragmental rocks indicates that much of the sequence was erupted in extremely shallow water or subaerially. Two large bodies of quartz-feldspar porphyry intrude pillow basalts of the overlying Upper member of the Ely Greenstone and probably represent the last episode of igneous activity associated with the sequence. Tuffaceous greywacke of the Lake Vermilion Formation is inferred to be derived largely from this dacitic complex, and possibly other felsic complexes developed along this stratigraphic horizon. Capping the Gafvert Lake sequence to the east and north is a distinct horizon of multiple-facies iron-formation. References Baumgardner, M., Brown, N, Grotte, M., Jacobson, A., Kendall, J., Ostwald, C., Schriner, N., White, J., and Peterson, D., 2013, Bedrock Geologic Map of the Gafvert Lake Area, St. Louis County, Northeastern Minnesota; Precambrian Research Center, PRC/MAP 2013-04. Peterson, D.M., 2001, Development of Archean Lode-Gold and Massive Sulfide Deposit Exploration Models using Geographic Information System Applications: Targeting Mineral Exploration in Northeastern Minnesota from Analysis of Analog Canadian Mining Camps; University of Minnesota Ph.D. thesis, 503 pages, 12 plates, 1 CD-Rom. Peterson, D. M., and Jirsa, M.A., 1999, Bedrock geologic map and mineral exploration data, western Vermilion district, St. Louis and Lake Counties, northeastern Minnesota: Minnesota Geological Survey, Miscellaneous Map M-98, scale 1:48,000. 6 �ELECTRICAL RESISTIVITY STRUCTURE OF THE MIDWESTERN UNITED STATES FROM EARTHSCOPE MAGNETOTELLURIC DATA BEDROSIAN, Paul, U.S. Geological Survey, PO Box 25046, MS 964, Denver, CO, 80225 As part of the EarthScope USArray program, long-period magnetotelluric (MT) data have been collected within the Midwestern United States. From 2011-2013, 237 MT stations were collected with 70 km nominal station spacing over all of Minnesota, Wisconsin, Michigan and Iowa as well as parts of Illinois, Indiana, Missouri, Kansas, Nebraska, and Ohio. This data set is unique in its ability to constrain subsurface electrical resistivity at crustal and lithospheric scales over this broad region. Coupled with advances in three-dimensional (3D) MT inversion, the EarthScope MT data can be used to create 3D resistivity models with coverage and resolution comparable to seismic tomography models. I will present a preliminary 3D resistivity model derived from these data. The discretized resistivity inverse model has a horizontal cell size of 10 km, with cell thickness starting at 100 m and increasing logarithmically with depth. The full MT impedance tensor was inverted at 11 periods ranging from 10 – 20,000 sec; vertical magnetic-field transfer functions at these same periods were also inverted. The resulting 3D resistivity model reflects the complex structural collage from the Archean to present. Several zones of high conductivity from the surface to ~5 km depth mimic the distribution of Phanerozoic sediments within the Michigan, Illinois, and Forest City basins. In contrast, structural highs such as the Transcontinental Arch and the Wisconsin and Ozark domes are electrically resistive at these same depths. At upper- to mid-crustal depths, the resistivity model illuminates the first-order structure of the 1.1 Ga Mid-Continental Rift (MCR) system. Highly resistive rocks coincide spatially with high magneticfield anomalies, and are attributed to Keweenawan volcanic rocks along the length of the southwest rift arm. In addition, flanking conductive anomalies trace out thick packages of Keweenawan clastic rocks, some of which appear to extend to ~15 km depth. The structure of the MCR system as imaged within the 3D resistivity model is most striking within the Iowa Horst, but can be traced throughout the known extent of the MCR system, including beneath the Michigan Basin within the southeastern rift arm. Structures predating the MCR system are also reflected in the resistivity model, particularly in the northern half of the model area. One example, the Flambeau anomaly, is imaged as a 300-km long, eastwest trending structure through northern Wisconsin and upper Michigan at 46°N. The southern boundary of this high conductivity zone appears coincident with the Niagara Fault. 7 �Figure 1: Preliminary resistivity structure at 9 km depth as constrained by 3D inversion of EarthScope magnetotelluric data. Background hillshade shows the total magnetic-field anomaly for comparison. 8 �RELATIONSHIP BETWEEN MICROSTRUCTURE AND ROCK MECHANICS IN SHEAR-ZONE-HOSTED-GOLD DEPOSITS Belshaw, Patrick and Hill, Mary-Louise Department of Geology, Lakehead University, 955 Oliver Rd, Thunder Bay, ON P7B 5E1, Canada. Orogenic gold deposits of northern Ontario are often hosted in steeply dipping, mylonitic shear zones. The common challenge of identifying, and mitigating the effects of penetrative planar fabrics with respect to rock mass properties is important in determining the safety of any mining operation. In addition to the relative effect on rock mass behaviour associated with fault planes and joint surfaces, highly ordered flaws in the material can have a significant effect on the orientation, initiation, and propagation of tensile Griffith fractures. In these deposits, where inherent flaws parallel to foliation dip steeply, spalling conditions are often observed in rocks that appear to be adequately competent. Most rocks in these deposits have also undergone grain size reduction during progressive deformation, making the interpretation of penetrative fabrics much more difficult on the outcrop scale. On the microscopic scale however, the morphology and density of grain boundaries and other flaws can be evaluated to develop relationships between the microscopic texture, and macroscopic material properties of the rock mass. 9 �10 �DEER LAKE COMPLEX, REDUX: MEMORIES AND REFLECTIONS, 1970 - 1972 BERKLEY, Jack, Department of Geosciences, Houghton Hall, SUNY Fredonia, Fredonia, NY 14063 USA The Deer Lake Complex (DLC), located between the town of Big Fork and southern tip of Deer Lake in Itasca County, MN, has been the target of precious and industrial metals exploration for over forty years. It was a major Cu-Ni prospect in the early 1970s, but has recently received renewed scrutiny as a gold, PGM, and Cu-Ni prospect. Although written off during the late 60s – early 70s exploration boom as a “dry hole”, application of modern technology and imaginative exploration strategies could conceivably bear positive results in the near future. The DLC consists of at least two gravity-stratified mafic-ultramafic sills, each roughly 250 meters thick, intruded into an Archean greenstone-meta-sedimentary terrain within the Wawa subprovince of the southern Superior province, Northern Minnesota (Berkley, 1972). Each sill differentiated into a lower wherlite / hornblende peridotite, overlain by distinct layers of clinopyroxenite, poikilitic and non-poikilitic gabbro, topped off by a layer of quartz-hornblende diorite (Berkley and Himmelberg; Ripley, 1978). Upper-most diorite units are commonly transected by thin, randomly dispersed veins of granitic assemblages (mostly microcline + quartz), and also display sheaf-like assemblages of acicular augite (Fig. 2a) plus skeletal plagioclase, the result of extreme undercooling at emplacement contacts (Ripley, 1972). Exposed lower contacts of sills (below peridotite) have chilled margins consisting of fine-grained basalt that – along with quench textures noted above – suggest shallow emplacement. Pyroxene reaction rims on peridotite olivine grains (Fig. 2b) indicate emplacement of less than about 6 km depth (e.g., Longhi and Pan, 1987). (a) (b) Figure 1: (a) Location map for DLC exposures. Textured squares represent areas mapped 1970-1973 (Berkley & Himmelberg, Ripley, 1978). (b) DLC geologic map from Ripley, 1978. Initial interest in the area was prompted by 1970 USS Corp. aeromagnetic plots that portrayed pronounced NE-trending, parallel linear patterns. Recent UMD grads, Jack Berkley and David Witt, were dispatched by Sid Iverson to the area to determine the cause of the magnetic anomalies. Upon entering the area using a logging road winding south from highway MN-1, they were eventually rewarded by the discovery of highly sheared black, blue, and blue- 11 �green – magnetite-rich serpentine, instantly accounting for the mag anomalies. What followed was a program using techniques and equipment that might seem archaic by today’s standards, but that nevertheless demonstrate the value of systematic field work leading to discovery. Subsequent mapping during August, 1970 required using a sun compass to compensate for the Brunton compass’ tendency to confuse peridotite exposures for the north magnetic pole. Topographic maps of the area were non-existent (the USGS team arrived the next year), thus geospatial positioning required finding, and using county-installed PLS section posts. Section lines festooned with colorful plastic flagging tape served as base lines for traverses that inevitably crossed insect-infested, soggy high-grass wetlands, waist-high blackberry thickets, and black spruce groves good only for obscuring views of whatever outcrops loomed ahead. By the end of August 1970 our team had completed a crude map and hand-written report, likely representing the first geological report on the DLC ever produced. It reported the existence of possible layered, mafic-ultramafic igneous intrusions, consisting of – at the very least -peridotite, pyroxenite, and gabbroic or diorite components. Berkley returned the next summer (1971) to study and complete a map of the DLC to fulfill the requirements for a master’s degree in Geology at the University of Missouri, Columbia under the tutelage of Dr. Glen R. Himmelberg (PhD, 1965 UM, Twin Cities). As he had in 1970, Berkley resided that summer in the cabin on Deer Lake (Fig. 2c) owned by Dave Witt’s parents, where they were visited at times by various UMD Geology alums and other friends. One very important visitor was UMD’s Dr. Richard Ojakangas, our indefatigable undergraduate instructor who was eager to see the work of his students. We duly escorted him into the depths of the DLC so he could plot the complex on the revised Hibbing Sheet of the Minnesota State Geologic Map. It remains there to this very day! Figure 2. a) Super-cooled pyroxene sheaves, from upper sill contact zone (hand specimen). b) Olivine with augite reaction rim, both enclosed by hornblende (photomicrograph). c) DLC field partners, Dave Witt and Jack Berkley (with “field dogs”) outside the Deer Lake cabin, 1971. References Berkley, J. and Himmelberg, G., 1978, Cumulus mineralogy of the Deer Lake Complex, Itasca County, Minnesota, Report of Investigations 20-A, Minnesota Geological Survey, 18pp. Longhi, J. and Pan, V., 1987. Olivine / low-Ca pyroxene liquidus relations and their bearing on eucrite petrogenesis. Lunar and Planetary Sci. XVIII: 570-571. Ripley, Edward, 1978, Sulfide Minerals in the Layered Sills of the Deer Lake Complex, Report of Investigations 20-B, Minnesota Geological Survey, 32pp. 12 �FIVE NEWLY ACQUIRED HIGH-PRECISION U-PB AGES IN MINNESOTA, AND THEIR GEOLOGIC IMPLICATIONS BOERBOOM, Terrence J., Minnesota Geological Survey, boerb001@umn.edu WIRTH, Karl R., and EVERS, Joseph, F., Macalester College, wirth@macalester.edu In the past year, with much appreciated cooperation from Dr. Mark Schmitz, director of the Boise State University isotope geology laboratory, four new high-precision U-Pb ages (three zircon and one baddeleyite) have been acquired for rocks from various geologic terranes throughout Minnesota. We summarize these results and also include the results from a suite of detrital zircon ages obtained as part of a Macalester College student research project. Funding provided by the USGS Statemap program and the MGS County Atlas program. UTM coordinates given below are in NAD ’83, Zone 15. Midcontinent Rift System – Two samples - a porphyritic rhyolite flow (DG073-AD; Grand Marais rhyolite) and a ferromonzonite intrusion (MH047-AD; Hovland sill) were dated as a followup to detailed 1:24,000 scale bedrock mapping (Boerboom and Green, 2010; Boerboom and Green, 2013. Sample DG073-AD – Six zircon crystals were selected for CA-TIMS (Chemical Abrasion Thermal Ionization Mass Spectrometry) analysis, from which five grains produced concordant isotopic ratios, with a weighted mean 206Pb/238U date of 1095.00±0.33 (MSWD (Mean Square Weighted Deviation) = 0.07) and a weighted mean 207Pb/206Pb age of 1097.26±0.67 (n=5; MSWD 1.47). Using the 207 Pb/206Pb weighted mean date, this age is only slightly younger than the Devil’s Kettle rhyolite (1097.7±1.7; Davis and Green, 1997), which lies roughly 8,000 feet stratigraphically below and is separated by several thick mafic to felsic volcanic units. The nearly identical ages for these two units indicates rapid and voluminous volcanic activity in the upper part of the northeast limb of the North Shore Volcanic Group. Sample from roadcut at the west edge of Grand Marais. (UTM 698364E, 5291847N) Sample MH047-AD – Six baddeleyite crystals selected for dissolution were all variably discordant, but gave equivalent 207Pb/206Pb dates with a weighted mean of 1095.94±0.62 (n=6; MSWD 0.37). This age falls within the range of published ages for various other units of the Beaver Bay Complex, including the Wilson Lake ferrogabbro (1095.75±0.92; Hoaglund, 2010), Sonju Lake intrusion (1096.1±0.8; Paces and Miller, 1993), Silver Bay ferrogabbro (1095.8±1.2; Paces and Miller, 1993), Pine Mountain granophyre (1095.3±3.8; Vervoort and others, 2007), as well as others. The Hovland sill occurs near the base of the upper northeast limb of the NSVG. The sample, a coarse prismatic olivine-pyroxene ferromonzonite that forms the upper differentiated cap to the Hovland sill, was collected from a roadcut on Highway 6, 1.3 miles northeast of the Brule River near Hovland. (UTM 722714E, 5301024N) Yavapai-interval intrusion – Sample 11-BUC-1-459.5. Seven zircon grains were selected for CA-TIMS analysis, and five of these seven analyses were concordant and equivalent, with a weighted mean 206Pb/238U date of 1779.93±0.56 (MSWD = 0.71), and a weighted mean 207Pb/206Pb date of 1782.31±0.64 Ma (MSWD 1.12). This drill core sample is from the heart of the Minnesota River Valley (MRV) subprovince in southern Minnesota, within a prominent north-south elongate magnetic low about 9 x 1.3 miles in dimension. The age of this granite places it in the Yavapai interval, along with the granites that make up the east-central Minnesota Batholith (1772 – 1800 Ma) and other mafic-intermediate intrusions along the southern Minnesota border emplaced into the MRV subprovince, which have been dated at ca. 1792 Ma (Southwick, 1994) and ca. 1760 Ma (Van Schmus, 2006). (UTM 352525E, 4890299N) 13 �Penokean Orogen – Sample HB5716-AD. Three abraded zircon fragments produced concordant isotopic ratios, with a weighted mean 206Pb/238U date of 1882.32±1.21 (MSWD = 0.23), and a weighted mean 207Pb/206Pb date of 1882.96±0.67 Ma (MSWD 1.12). This sample is from a metagabbroic sill encountered in the 7,440’ (2,268.3m) – long ‘Hattenberger’ core (HB-1) located within the Moose Lake-Glen Township panel, in the heart of the Penokean fold-and-thrust belt in eastcentral Minnesota. The drill hole intersected interlayered mafic volcanic and sedimentary rocks metamorphosed under lower amphibolite-grade conditions, as well as lesser proportions of mafic sills thought to be petrologically related to the mafic volcanic rocks (Southwick and others, 2005). The dated sample is from a coarse-grained, feldspathic zone in the interior of a 500-foot thick mafic sill that has chilled upper and lower margins. This sill mostly retains its primary igneous fabric despite the mafic mineral assemblage being composed almost entirely of blue-green metamorphic hornblende. UTM 503780E, 5149030N. The ca. 1,882 Ma age of this sill clearly predates the geon 17 Yavapai-interval ages of the voluminous east-central Minnesota batholith and the ca. 1858-1877 Ma Bradbury Creek granodiorite (Holm and others, 2005); but it postdates the 2,009 Ma Mille Lacs granite (Holm and others, 2005). It is barely older than the 1878.3±1.3 Ma lapilli tuff in the upper Gunflint Iron Formation (Fralick and others, 2002), and the 1,874±9 Ma Hemlock Volcanics interlayered with the Negaunee Iron Formation (Schneider and others, 2002). It overlaps with ages from the Pembine-Wausau portion of the Wisconsin Magmatic Terrane (1,860-1,889; Sims and others, 1989 as reported in Schulz and Cannon, 2007). The sill intruded a carbonate-rich unit tentatively correlated with the Denham Formation, which has a maximum depositional age of 2,072.7±17.9 Ma based on detrital zircons, (Vorhies, 2006). Sample 05BWS001 - Little Falls Formation detrital zircon – A drill core sample of staurolite-garnet schist (Little Falls Formation) yielded abundant detrital zircon grains with U-Pb ages from 1,833 to 2,784 Ma. Most grains have ages from 1,833 - 1,898 Ma; smaller numbers of grains yield clusters (>3 grains each) at 2,420, 2,668, 2,695, and 2,704 Ma. Analysis of the main cluster of ages suggests a separate group with an age of 1,844 Ma, considered to be the maximum depositional age (MDA). The preponderance of ca. 1844 Ma ages (46 of 90 grains analyzed) offers new insights into the long-standing debate about age of the Little Falls Formation. The Little Falls Formation covers a large area of east-central Minnesota, and based on geophysical data is thought to form the upper plate of a thrust sheet that has been ramped west-northwest over rocks correlative with the Penokean Mille Lacs Group. Although it is well known that the Little Falls Formation was affected by the widespread ca. 1760 Ma regional metamorphic event throughout east-central Minnesota (e.g. Holm and others, 2007), the actual depositional age has never gone beyond speculation. If the 1844 Ma maximum depositional age is verified by further studies, this would imply that the Little Falls Formation was deposited synchronous with the much lower grade Animikie basin and that the basin covered a much broader area than previously recognized, necessitating a rethinking of models of the geologic evolution of the Penokean and Yavapai orogens in central Minnesota. Drill hole 05BWS001 UTM 391454E, 5071884N. References: Boerboom, T.J., and Green, J.C., 2010, Minnesota Geological Survey Miscellaneous Map Series Map M-189, scale 1:24,000. Boerboom, T.J., and Green, J.C., 2013, Minnesota Geological Survey Miscellaneous Map Series Map M-195, scale 1:24,000. Fralick, P., Davis, D.W., and Kissin, S.A., 2002, CJES v. 39 no. 7, p. 1085-1091. Hoaglund, S., Miller, J.D., Crowley, J.L., and Schmitz, M.D., 2010, ILSG 56th Annual Meeting; Program and abstracts, p. 25-26. Holm, D.K., D.A. Schneider, D.A., Rose, S., Mancuso, C., McKenzie, M., Foland, K.A., and Hodges, K.V., 2007, Precambrian Research, V. 157, nos. 1-4, p. 106-206. Holm D.K., Anderson, R., Boerboom, T.J., Cannon, W.F., Chandler, V., Jirsa, M., Miller, J., Schneider, D.A., Schulz, K.J., and Van Schmus, W.R., 2007, Precambrian Research V. 157 p.71–79 Schneider, D., Bickford, M., Cannon, W., Shulz, K., and Hamilton, M., 2002, C.J.E.S., v. 39, p. 999–1012. Schulz, K.J., and Cannon, W.F., 2007, Precambrian Research, V. 157, p. 4-25. Sims, P.K., Van Schmus, W.R., Schulz, K.J., Peterman, Z.E., 1989, C.J.E.S., v. 26, 2145–2158. Sims, P.K., Schulz, K.J., Peterman, Z.E., 1992, US Geol. Surv. Prof. Pap. 1517, 65 pp. Southwick, D.L., 1994, Minnesota Geological Survey Report of Investigations 43, p. 1-19. Southwick, D.L., Morey, G.B., Christopher, J.M., McSwiggen, P.L., and Boerboom, T.J., 2005, MN. Geol. Survey R.I. 63, 63 p. Van Schmus, W.R., 2006, University of Kansas, Lawrence; Reported in Jirsa, M.A., Miller, J.D., Jr., Severson, M.J., and Chandler, V.W., 2006, Minnesota Geological Survey Open-File Report OF-06-03, 49 p. Vorhies, S., 2006, B.A. Honors Paper, Smith College, John Brady, Faculty advisor. 14 �BEDROCK GEOLOGIC MAP OF THE MARR ISLAND AND HOVLAND QUADRANGLES, NORTH SHORE OF LAKE SUPERIOR, MINNESOTA BOERBOOM, Terrence J., Minnesota Geological Survey, boerb001@umn.edu GREEN, John C., University of Minnesota-Duluth, jgreen@d.umn.edu The Minnesota Geological Survey has continued ongoing mapping of the bedrock geology of 7.5’ quadrangles adjacent to Lake Superior as part of the USGS STATEMAP program, resulting to date in twenty published 1:24,000 scale maps from Duluth northeast to beyond Hovland, in addition to 10 quadrangles already published under the former USGS COGEOMAP program. The Marr Island and Hovland quadrangles, (Boerboom and Green, 2013), are the most recent of these maps (Fig. 1). These maps are available at the MGS website (www.mngs.umn.edu). Outcrop mapping was augmented by nearly 60 sets of high-quality water well cutting samples, collected at 10 foot intervals by McKeever Well Drilling of Little Marais, Minnesota. This mapping has refined the volcanic stratigraphy of the North Shore Volcanic Group (NSVG) in this area, as well as details and extents of intrusions. In keeping with prior work, the NSVG is subdivided into informal lithostratigraphic packages (listed below), which on this map follow closely those units identified by Green, 2002. Most of the rocks in this quadrangle contain typical zeolite mineral assemblages; however in proximity to mafic intrusions the volcanic rocks contain minor garnet and/or epidote. Volcanic rocks—The volcanic rocks in this map area cross the boundary between the upper (normally-polarized) and lower (reversely-polarized) portions of the northeast limb of the NSVG. The volcanic rocks are intruded by mafic to felsic intrusions inferred to be mostly related to the Beaver Bay Complex in timing. The major lithostratigraphic units are listed below from lowest to highest in the volcanic stratigraphy. Hovland lavas—Predominantly strongly porphyritic trachyandesite, famous for its abundant and large tabular plagioclase phenocrysts. Also includes a poorly-mapped rhyolite known only from water well cuttings. Only the uppermost of the Hovland lavas are present here, most of the unit being to the northeast. Brule River lavas—Predominantly rhyolite, interlayered with variably porphyritic intergranular basalts, pigeonitic ferroandesite, ophitic basalt, and minor sandstone. The upper half of this sequence of lavas is made up of the quartz-and feldspar-phyric Devil’s Kettle rhyolite (1,097.7 ±1.7 Ma: Davis and Green, 1997); other rhyolites contain only feldspar phenocrysts, including the Big Bay rhyolite near the base of the upper normally-polarized lavas, which has a U-Pb zircon age of 1100.2±2 Ma (Davis and Green, 1997). Marr Island lavas—An approximately 1,000 meter thick sequence of dominantly mafic to intermediate lava flows (Green, 2002) that range from ophitic Fe-tholeiite to andesite with minor proportions of icelandite, rhyolite, and sandstone. Pigeonite is present in the basalts and andesites, and some andesites contain fresh glass in the mesostasis. Kimball Creek felsites—Icelandite and rhyolite; the bulk of this unit occurs to the west in the Kadunce River quadrangle (Boerboom and Green, 2011), and only the very base of the section reaches this map area. Intrusive rocks—Multiple intrusions range from coarse-grained gabbroic anorthosite, cumulate- differentiated sills, felsic to intermediate intrusions, and small diabase to ferrodiorite dikes and sills. The major intrusions are summarized below from earliest to latest in timing. Carlson Creek gabbro complex—Anorthositic gabbro with pods of gabbroic pegmatite, and felsic-intermediate rocks that may be related, which locally contain xenoliths of pure anorthosite. Intrusions tentatively assigned to the Brule-Hovland complex—Mainly ophitic gabbro and diabase but including erromonzodiorite hybrids. Ophitic olivine gabbro contains inclusions of volcanic rocks and interflow sandstone. Reservation River diabase—A gently dipping sheet-like body of ophitic olivine diabase that is present mainly to the north and east of the map area; intrudes reversely-polarized flows but is normally-polarized. Horseshoe Bay diabaseOphitic diabase (normally polarized), troctolitic diabase, and ferromonzonite. Troctolitic phase contains Fo62-72 olivine and augite of Mg#70; ophitic diabase contains Fo50-58 olivine and augite of MG#70. Orientations of olivine streaks and sheet joints indicate troctolite is a gently south-dipping sill; this possibly grades into the ophitic diabase. Ophitic diabase contains large inclusions of amygdaloidal basalt, and there are local narrow hybrid melt zones where ophitic diabase intruded rhyolite. Small plug-like bodies of prismatic pyroxene-quartz ferromonzonite may have formed as partial melt segregations from the underlying rhyolite. Chicago Bay ophitic diabasePresumably a sill (normally-polarized) beneath the Hovland sill; may be a marginal phase to it. Augite compositions of Mg#70 and partially olivine average Fo60. 15 �Brule River sillVariably granophyric mafic sill; the lowest part is a cumulate with locally abundant ilmenite and minor poikilitic olivine. Higher parts have layers bearing clots of poikilitic olivine alternating with more coarsegrained granophyric layers. The upper portion may grade into overlying Pine Mountain granodiorite. Lookout sillA south-dipping sill, like the Hovland sill in that it contains cumulate plagioclase, augite, olivine (Fo23 near base, Fo15 higher up), apatite, and abundant ilmenite. The upper part is a miariolitic prismatic ferromonzonite that contains fayalitic olivine (Fo10). Hovland sillAn approximately 15-degree south dipping, subcordant, 300 m-thick sill composed of a basal noncumulate ferrogabbro, a middle cumulate-foliated granophyric ferrogabbro, and an upper coarse-grained felsic cap. Not physically continuous with the Lookout sill but very similar and may be related in timing and paragenesis. Monazite U-Pb age of 1095.94±0.62 (Boerboom and others, this volume). The lower ferrogabbro contains distinguishing, evenly distributed 3-4mm altered olivine clots (5%), intergranular augite (Mg# 59 to 54), and up to 2% pigeonite (Mg# 41). The middle section is strongly cumulatefoliated and typically coarse-grained; the lowest part of this contains abundant cumulate ilmenite plates but higher in the stratigraphy magnetite becomes dominant over ilmenite; also contains cumulate plagioclase, augite, Fe-Ti oxides, olivine (mostly altered), and minor apatite. Olivine content is generally around 2%, but near the top increases to as much as 13%. Pigeonite (Mg# 49) rims augite, for which average Fe/Mg ratios increase from Mg# 57 at the base to 35 at the top (ferroaugite). Mg numbers for olivine range from Fo29 near the base to Fo17 near the top. The coarse-grained felsic cap contains 8-15% prismatic ferroaugite (Mg#35) and 10-15% fayalitic olivine (Fo10) that is mostly altered and varies in form from irregular coarse clots and prismatic grains to acicular trellises up to 30cm in length. Pine Mountain GranophyrePart of a larger body of granophyre and granodiorite located mainly to the west of this map area (e.g. Boerboom and Green, 2011); with a reported U-Pb age of 1,095.3 ± 3.8 Ma (Vervoort and others, 2007). Compositions are gradational from leucogranite into gabbro of the Brule River sill, implying that the Brule River Sill, and by extension, the Lookout sill, may all be close to 1,095 Ma in age. Miscellaneous intrusionsA wide variety of small intrusions are present throughout the map area. These include small hybrid/contaminated ferromonzonitic dikes with intermingled partially melted rhyolite, fine-grained ferrodiorite dikes, ophitic to intergranular olivine diabase, medium-grained pyroxene granodiorite, and ferromonzodiorite. Most of these are dikes, but some appear to be sills. References Boerboom, T.J., and Green, J.C., 2010, Minnesota Geological Survey Miscellaneous Map Series Map M-189, scale 1:24,000. Boerboom, T.J., and Green, J.C., 2013, Minnesota Geological Survey Miscellaneous Map Series Map M-195, scale 1:24,000. Davis, D.W., and Green, J.C., 1997, Canadian Journal of Earth Science, Volume 34, No. 4, April 1997, p. 476-488. Green, J.C., 2002, Minnesota Geological Survey Report of Investigations 58, p. 94-102. Vervoort, J.D., Wirth, K., and Kennedy, B., 2007, Precambrian Research, vol. 157, no. 1-4, p. 235-268. Figure 1. Simplified geologic map of the Marr IslandHovland quadrangles, showing the major lithostratigraphic units discussed in the text. Inset location map shows the extent of the Keweenawan Midcontinent Rift System in Minnesota, and the locations of the Marr IslandHovland quadrangles (dark gray box). 16 �GEOCHEMICAL INVESTIGATION OF THE ORIGIN OF THE BACK FORTY VOLCANOGENIC MASSIVE SULFIDE DEPOSIT IN MENOMINEE COUNTY, MI Anthony Boxleiter1, Joyashish Thakurta1, and Thomas O. Quigley2 1. Department of Geosciences, Western Michigan University, Kalamazoo, MI 49008, anthony.r.boxleiter@wmich.edu; 2 Aquila Resources Inc., Menominee, MI 49858 The Back Forty Volcanogenic Massive Sulfide deposit is located in Menominee County, Michigan. Several Volcanogenic Massive Sulfide (VMS) deposits can be found trending along the Penokean Volcanic Belt in northern Wisconsin and the Michigan Upper Peninsula (Fig. 1). The Back Forty deposit is a Paleoproterozoic ore deposit which formed during the Penokean Orogeny (1874 ± 4 Ma; Schulz et. al., 2007). This deposit is unique because it contains low amounts of copper and high amounts of zinc and gold when compared to other VMS deposits associated with the Penokean Volcanic Belt, such as Crandon and Flambeau. Mineralization of the Back Forty deposit consists of massive, semi-massive, stringer sulfide zones, and sulfide-poor Au and Ag enriched zones (Thakurta and Quigley, 2013). Three chemically distinct varieties of host rhyolite have been identified based upon trace element characteristics, two of which are found to host sulfide mineralization (Quigley et al., 2008). The relationship between rhyolite geochemistry and VMS mineralization has been proposed by Thurston (1981) and Campbell et al. (1982) as an exploration tool for discerning prospective VMS deposits, based on Archean VMS deposits in the Canadian Shield. From this groundwork, Lesher et al. (1986) and Barrie et al. (1993) developed a formal classification scheme for felsic volcanic rocks based on trace element concentrations and they suggested that certain types of rhyolites are more prospective for sulfide mineralization than others. They classified rhyolites associated with VMS deposits into three types: FI, FII, FIIIa, and FIIIb. Conclusions drawn from Lesher et al. (1986), Lentz (1998), and Hart et al. (2004) are that Archean VMS deposits are hosted mainly by FIII rhyolites, whereas most post-Archean VMS deposits are hosted predominantly by FII rhyolites. Under the classification scheme developed by Lesher et al. (1986) and Barrie et al. (1993), FI and FII type rhyolites are least favorable for VMS mineralization while FIIIa/FIIIb types have been proposed as the most prospective. FI type rhyolites appear to be particularly associated with gold-rich VMS deposits, such as the world-class Laronde deposit. The FI rhyolites are alkaline to calc-alkaline, with strongly fractionated REE patterns and strongly negative Ta and Nb anomalies. The FII rhyolites are calc-alkaline to transitional, with moderately fractionated REE patterns and moderate Ta and Nb anomalies and considered more favorable than FI rhyolites. The FIIIa and FIIIb rhyolites are tholeiitic and considered to have the greatest potential for hosting VMS deposits. The FIIIa rhyolites show weakly fractionated REE patterns and weak to nonexistent Nb and Ta anomalies. The FIIIb rhyolites are high-temperature rhyolites with flat REE patterns that lack Ta and Nb anomalies. (Gaboury and Pearson, 2008) Trace element analysis plotted as [La/Yb]CN versus YbCN for the two rhyolites hosting sulfide mineralization in the Back Forty deposit classifies these rhyolites as FI type under the scheme proposed by Lesher et al. (1986) and Barrie et al. (1993). Trace element analysis plotted as Zr/Y versus Y places these two rhyolites under the FII/FIIIa classification. The elements Zr, Y, La, and Yb are most useful for trace element analysis because they are generally immobile during hydrothermal alteration and are representatives of petrogenetic processes (Gaboury and Pearson, 2008). The FI classification of these rhyolites based upon [La/Yb]CN versus YbCN and the high amount of gold associated with the Back Forty is consistent with FI type association under this classification scheme. However, the trace element plot of Zr/Y versus Y places these rhyolites under the FII/FIIIa classification scheme. While this classification scheme has demonstrated the usefulness of rhyolite geochemistry for exploration in some areas, more work is required to characterize each type based on actual mineral deposits. For this reason, Gaboury and Person (2008) suggest that a combination of rhyolite geochemistry, volcanic facies, and the style of sulfide mineralization may be more meaningfully applied in exploration than rhyolite type alone. This is particularly important in the case of FI and FII rhyolites associated with VMS deposits of post-Archean age, such as the Back Forty deposit. 17 �This research will explore the use of not only rhyolite classification, but sulfur isotope analysis and petrographic techniques to characterize the Back Forty VMS deposit. This research will investigate the relationship between the pattern of distribution of sulfur isotopes in sulfide minerals of the Back Forty deposit, the mode of occurrence of the ore body, and textural characteristics of the sulfide ore minerals. Sulfur isotope values will help to characterize the distribution of sulfide minerals in the Back Forty deposit and to model the origin of sources. Sulfur isotope analysis may reveal episodic pulses of hydrothermal fluids as well as the source of sulfur (i.e., magmatic sulfur with δ34S values of 0 ± 2 per mil, biogenic sulfur with negative δ34S values, and surface-derived sulfur with positive δ34S). Sulfur isotope values measured in VMS deposits in other parts of the world, notably the Archean Kidd Creek VMS deposit in Ontario, Canada, indicate isotopic disequilibrium. In the study conducted on Kid Creek by Hannington et al. (2006), δ33S values in conjunction with δ34S values were used to model sulfur isotope systematics in Archean ore deposits. A similar study was conducted by the U.S. Geological Survey and U.S. Department of Interior (Taylor et al., 2010) on the Greens Creek VMS deposit located in Admiralty Island, Southeastern Alaska. Sulfur isotope analysis on the Greens Creek VMS produced δ34S values of 11 to -16‰ and has been interpreted by Taylor et al. (2010) as resulting locally from the organic reduction of seawater sulfate to H2S. Sulfur isotope analysis has never been used on the Back Forty deposit. The relationship of the mode of occurrence of sulfide mineral deposits at Back Forty with sulfur isotope signatures will provide important geochemical constraints on the origin of the deposit. This geochemical dataset will also be useful to model the origins of other VMS deposits in the Penokean Volcanic Belt and to explore for new economic sulfide deposits associated with rhyolitic host rocks. Fig. 1: Locations of VMS deposits along the E-W trend of the Penokean Volcanic Belt in northern Wisconsin. The Back Forty is the easternmost deposit of this trend and is the only VMS deposit found in the Michigan Upper Peninsula. References Barrie, C.T., Ludden, J.N., and Green, T.H., 1993. Geochemistry of volcanic rocks associated with Cu-Zn and Ni-Cu deposits in the Abitibi subprovince: Economic Geology, v. 88, p. 1341-1358. Campbell, I.II., Coad, P., Franklin, J.M., Gorton, M.P., Scott, S.D., Sowa, J., and Thurston, P.C., 1982. Rare earth elements in volcanic rocks associated with Cu-Zn massive sulfide mineralization. A preliminary report: Canadian Journal of Earth Sciences, v. 19, p. 619-623 Gaboury, D. and Pearson, V., 2008, Rhyolite geochemical signatures and association with volcanogenic massive sulfide deposits: Examples from the Abitibi Belt, Canada, Economic Geology, 103, 1531-1562 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, 1003-1013 Hannington, M., Jamieson, J., Wing, B. and Farquhar J., 2006, Evaluating isotopic equilibrium among sulfide mineral pairs in Archean ore deposits: Case study from the Kidd Creek VMS deposit, Ontario. Economic Geology, 101. p. 1055-1061. Lentz, D.R., 1998. Petrogenetic evolution of felsic volcanic sequences associated with Phanerozoic volcanic-hosted massive sulphide systems: the role of extensional geodynamics: Ore Geology Reviews v. 12 p. 289-327. Lesher, C.M. Goodwin, A.M., Campbell, I.II., and Gorton, M.P., 1986. Trace-element geochemistry of ore-associated and barren, felsic metavolcanic rocks in the Superior province. Canada: Canadian Journal of Earth Sciences, v. 23, p. 222-237. Quigley, T., Mahin, B., and Aquila Field Office Geologic Staff, 2008, Back Forty Geology and Mineralization: 54th Annual Institute on Lake Superior Geology, Field Trip #3 Ross, C., Hudak, G., Morton, R., Quigley, T. and Mahin, R., 2011, Preliminary stratigraphy and physical volcanology associated with the Paleoproterozoic Back Forty VMS deposit, Menominee County, Michigan, Institute of Lake Superior Geology Schulz, K.J. and Cannon, W.F., 2007, The Penokean Orogeny in the Lake Superior region. Precambrian Research, 157, 4-25 Taylor, C.D. and Johnson, C.A., 2010, Editors. U.S. Geological Survey Professional Paper 1763, 429 p. Thakurta, J. and Quigley, T.O., 2013. Geochemical characterization of the Back Forty volcanogenic massive sulfide deposit in Menominee County, MI. Western Michigan University; Kalamazoo, MI. Thurston, P.C., 1981. Economic evaluation of Archean felsic volcanic rocks using REE geochemistry: Geological Society of Australia Special Publication 7, p. 439-450. 18 �ZIRCONIUM/HAFNIUM FRACTIONATION IN SOME PEGMATITES OF THE UPPER MIDWEST, USA Buchholz, T. W.1, Falster, A. U.2, and Simmons, W. B. 2 1 1140 12th Street North, Wisconsin Rapids, Wisconsin 54494, 2Department of Earth and Environmental Sciences, University of New Orleans, New Orleans, Louisiana 70148 Zirconium and hafnium form a coherent pair, being of similar radius and chemistry, and mutually substitute in various Zr minerals. Zircon, nominally ZrSiO4, incorporates Hf levels reflecting the relative Hf contents of the crystalizing melt, which in turn reflects the degree of fractionation or evolution of the melt, rarely culminating (in highly evolved pegmatites) in the very rare Hf dominant analog hafnon. In the course of our studies of pegmatites in Wisconsin, Michigan and Minnesota, considerable data have been developed regarding chemistry of zircons from these pegmatites, in particular regarding Hf, which will be discussed below. HfO2 contents in zircons from source granites are generally low, in the range of 1-2 wt% (Fleischer, 1955, Wang et al, 2000), though higher levels have been reported in evolved granites (e.g. Wang, et al 2000). For local comparisons, a zircon from typical Nine Mile granite, Marathon Co, WI was analyzed, and found to contain 1.0-1.2wt% HfO2, zircons from the Zunker property (site of a former zircon mining attempt) in the Stettin Complex, Marathon Co, WI contained 1.8 to 2.2wt% HfO2, and zircons from the Bell Creek Granite (Marquette Co, MI) contained 3.8 to 4.3wt% HfO2. Results from Midwest pegmatite zircons are summarized below. The lists of associated minerals are not intended to be complete, but rather to suggest the degree of fractionation achieved, with emphasis on Mn/Fe, Nb/Ta, W, Sn and F. Waterloo Quarry, Jefferson Co, WI: (Small fractionated pegmatites emplaced in quartzites/metapelites, associated minerals: columbite-(Mn), tantalite-(Mn), microlite, gahnite, Bi): 8.1 to 10.9 wt% HfO2. Wimmer Pit #3, Marathon Co, WI: (Small fractionated pegmatite emplaced in Nine Mile Granite, associated minerals: cassiterite, tantalian cassiterite, columbite-group minerals, U-rich pyrochlore, and a U-niobate phase): 1.6 to 4.9 wt% HfO2, Hf-rich rims on zircon. Maguire Pit, Marathon Co, WI: (Greisenized pegmatites and aplites emplaced in Nine Mile Granite, associated minerals: huebnerite/ferberite, cassiterite, topaz, W-rich columbite-group minerals, zinnwaldite, fluorite, prosopite, cryolite): 1.8 to 10.4wt% HfO2, Hf enriched cores in and rims on zircon. Pegmatite #22, Koss Pit, Marathon Co, WI: (Small fractionated pegmatite emplaced in aplite body in Nine Mine Granite, associated minerals: columbite-group minerals, tapiolite-(Fe), cassiterite, microlite, monazite, xenotime-(Y), xenotime-(Yb), fluorite): 7.4 to 14.6 wt% HfO2, Hf-rich rims on Urich zircon. Woodland Drive Pegmatite, Marathon Co, WI: (Small unusually fractionated silica-saturated REE & Th-poor pegmatite emplaced in tabular syenite phase of the Stettin Complex, associated minerals: Ta-rich columbite-Fe, Ta-rich pyrochlore, cassiterite, ilmenite, unidentified phases): 3.9 to 8.5 wt% HfO2. Hoskin Lake pegmatite field (Florence Co, WI): (Moderate sized highly fractionated LCT pegmatites, associated minerals: elbaite tourmaline, tantalite-Mn, stibiotantalite, tantite, pegmatite phosphates, pollucite, rynersonite, behierite): 6.1 to 19.8wt% HfO2. Groveland Pegmatite, Dickinson Co, MI: (Small fractionated pegmatite emplaced in metasediments adjacent to the old Groveland Mine, associated minerals: columbite-(Fe), tantalite-(Fe), tapiolite-(Fe), microlite, gahnite, beryl): 1.7 to 6.5wt% HfO2. 19 �Black River Pegmatite, Marquette Co, MI: (Small pegmatite emplaced in Archean Bell Creek Gneiss, associated minerals: columbite-(Fe), late microlite, monazite-(Ce), synchysite-(Ce), synchysite(Y), topaz, fluorite): 3.9 to 5.3wt% HfO2. Orr Pegmatite, St Louis Co, MN: (Moderate sized pegmatite emplaced in biotite schist-rich migmatite, associated minerals: Mn-rich almandine garnet, magnetite, Th-rich monazite, possible chevkinite-(Ce), columbite-Fe, allanite-(Ce), titanite): 2.1 to 5.3wt% HfO2. Substantial enrichment in Hf is evident in many of the above pegmatites and is in accordance with observations made by Fleischer (1955) and Linnen & Kepler (2002). The enrichment present in the Woodland Drive pegmatite, emplaced in the alkalic Stettin Pluton, is particularly interesting as in all other Stettin zircon samples Hf contents are quite low. The highest HfO2 levels, from pegmatites in the Proterozoic Nine Mile Granite, are closely associated with highly fractionated Nb-Ta minerals, as well as high to very high F levels (as evidenced by late fluorite and other F-rich minerals). It is likely that high Hf levels in the evolved zircons are related to the relatively greater stability of Hf-F vs Zr-F complexes (Linnen & Kepler, 2002). Break down of Hf-F complexes triggered by the formation of F-rich minerals released Hf which was then incorporated into the outer zones of growing zircon crystals. The high levels of enrichment in HfO2 observed in the Florence County pegmatites can be attributed to the extreme level of fractionation achieved in these LCT pegmatites. F is present in various phases, but does not approach the high levels observed in the Nine Mile Granite and the Stettin Complex, and is unlikely to be the driving factor in this fractionation. References Fleischer, M. 1955. Hafnium Content and Hafnium/Zirconium Ratio in Minerals and Rocks. US Geological Survey Bulletin 1021-A Linnen, R. L. and Keppler, H. 2002. Melt composition control of Zr/Hf fractionation in magmatic processes. Geochimica et Cosmochimica Acta, 86, no. 18: 3293-3301. Wang, R.C., Zhao, G.T., Lu, J.J., Chien, X.M. and Wang, D.Z. 2000. Chemistry of Hf-rich zircons from the Laoshan I- and A-type granites, Eastern China. Mineralogical Magazine, 64/5: 867-877. 20 �A NEW OCCURRENCE OF THE SUDBURY IMPACT LAYER IN THE GOGEBIC IRON RANGE OF WISCONSIN CANNON, William F.1, WOODRUFF, Laurel G2., SAARI, Stacy M.3, and HAGSTROM, Molly C.3 1 U.S. Geological Survey, MS 954, Reston, VA 20192 wcannon@usgs.gov U.S. Geological Survey, 2280 Woodale Drive, Mounds View, MN 55112 3 Gogebic Taconite, LLC, 402 Silver Street, Hurley, WI 54534 2 Exploration drilling in 2014 by Gogebic Taconite, LLC in the western Gogebic Iron Range in northern Wisconsin provides seven new intersections of the Sudbury impact layer (SIL) (Fig. 1). Together with a hole drilled previously, they reveal features of the SIL along 6 km of strike length. Data presented here are observations of drill core and preliminary microscopic examination. The SIL lies at the expected stratigraphic position- at or very near the contact of the underlying Ironwood Iron-formation and Tyler Formation. The Ironwood-Tyler contact appears transitional as evenly bedded magnetic iron-formation of the Pence Member of the Ironwood grades upward into nonmagnetic laminated argillite of the lower Tyler Formation. The precise location of the contact is generally arbitrary; we have seen no indications of a hiatus in sedimentation between the Ironwood and Tyler. Although the Pence Member is predominantly evenly- and thinly-bedded at this locality, it contains a few interbeds of wavy bedded granular iron-formation suggesting that at the time of deposition of the SIL the area was submerged to depths only slightly below the maximum depth of surface wave agitation. The SIL here consists of ejecta having similarities to many other SIL localities reported previously in the Lake Superior region. The most definitive features are accretionary lapilli (Fig. 2), altered glass spherules and fragments of diverse character (Fig. 3), and a very sparse suite of quartz grains showing relict planar deformation features (Fig. 4). The rocks are somewhat metamorphosed so that biotite, chlorite, and sericite are common in the matrix. Secondary carbonate is widespread and obscures much of the original texture. Fragments of argillite are also common. Together, these lithologies vary in total thickness from about 20 m to only 0.1 m along the 6 km strike length studied to date. In some drill cores, a zone of seismically disrupted sediments underlies the ejecta. Several features suggest that the SIL is largely, or entirely, reworked ejecta mixed with more local sediments. One drill hole contains several clasts of lapillistone at least 5 cm diameter about 15 cm above the base of a 1.2 m thick ejecta layer (Fig. 2). The clasts appear to be original lapilli-bearing material that was lithified (or frozen?), fragmented, and redeposited in its present position. Many clasts of Tyler-like laminated argillite are included within ejecta and vary from thin wisps of apparently soft sediments to much thicker intervals. The thickest of the SIL drilled sections contains four intervals of distorted laminated argillite from 3 m to less than 1 m thick. A second drill core contains a 4 m interval composed of lapilli-bearing ejecta interlayered with five intervals of laminated argillite as much as 1.5 m thick. We interpret the argillite to be clasts of semiconsolidated Tyler Formation that were incorporated into debris flows composed originally of ejecta. This implies that a nearby elevated area existed onto which ejecta originally was deposited and subsequently slumped into the deeper water of this area. Such an elevated area probably existed only a few tens of kilometers to the east. The classic five-fold stratigraphy of the Ironwood, defined in the central and eastern Gogebic Range, includes the Anvil Member, a 21 �shallow water granular iron-formation that overlies the Pence Member in that area. Deposition of the Anvil was probably followed by uplift that raised the Anvil above sea level. The base of the Tyler in that area is a basal conglomerate (Pabst Member of the Tyler Formation) that marks a short-lived erosion interval between the Tyler and Ironwood (see Cannon and others, 2008 for a summary of previous publications on stratigraphic details). Our preliminary model, therefore, begins with deposition of the Sudbury ejecta, in part in a terrestrial setting, to the east of our study area. The ejecta deposit was partly lithified before being remobilized and traveling as debris flows that incorporated newly deposited argillite to the current depositional site. Thus, at least part of the SIL in the western Gogebic Range may not record the instant of impact, but rather is a younger deposit whose deposition was delayed sufficiently to allow partial lithification of the ejecta and deposition of at least a thin layer of Tyler argillite. Cannon, William F., LaBerge, Gene L., Klasner, John S., and Schulz, Klaus, J., 2008, The Gogebic Iron Range-a sample of the northern margin of the Penokean fold and thrust belt: U.S. Geological Survey Professional Paper 1730, 44 p. 22 �CONTINUED WORK ON USING THE HORIZONTAL-TO-VERTICAL SPECTRAL RATIO (HVSR) PASSIVE SEISMIC METHOD FOR DETERMINING QUATERNARY SEDIMENT THICKNESS IN MINNESOTA Val W. Chandler and Richard S. Lively Minnesota Geological Survey, 2642 University Ave., St. Paul, MN 55114 chand004@umn.edu Work has continued on using the horizontal-to-vertical-spectral ratio (HVSR or sometimes H/V) passive seismic method for determining the thickness of Quaternary sediments in Minnesota, which consist chiefly of Pleistocene glacial deposits. The HVSR method is used to estimate the primary resonant frequency (shear wave) of unconsolidated sediments. If the acoustic impedance (density*seismic velocity) at the sediment-bedrock contact differs by a factor of at least 2, and if this surface is reasonably flat, the thickness (z) of the unconsolidated materials can be estimated by the relationship: z=af0b Where f0 is the estimated primary resonant frequency, and a and b are parameters that are calibrated empirically for a given area from control points where a wide range of bedrock depths are known. Once calibrated, the relationship can be used to estimate depths at points lacking control. Due to the pronounced variations in shear-wave velocities for glacial deposits, calibrated parameters are reliable only within fairly localized areas, and multiple calibrations may have to be conducted for larger regions. At control points where z is known, the average shear-wave velocity (Vs) of the unconsolidated sediments can also be estimated. During the spring and summer of 2013 more than 425 passive seismic stations were acquired, bringing the total to over 1100 passive seismic stations in Minnesota and adjacent parts of Wisconsin (Figure 1). The most recent work has been focused in the “Arrowhead” area in northeastern Minnesota, in Kanabec County in east-central Minnesota, and along profiles that traversed parts of Becker, Clay, Hubbard, Todd and Wadena Counties in northwestern Minnesota (Figure 1) Considerable scatter is observed in the f0 vs z relationships at control points in the new study areas, implying that Vs values vary significantly, both laterally and vertically, and more than one depth calibration may be needed for each area. In addition, HVSR results were commonly poor in parts of the Arrowhead area where unconsolidated sediment was thin (<15 meters), most likely reflecting an irregular bedrock surface. In spite of these limitations, the HVSR method was nonetheless useful in mapping the thickness of Quaternary sediments in both the Arrowhead and Kanabec County areas. In Kanabec County the HVSR-results were combined with drillhole and gravity data to produce residual gravity data that further helped in mapping the thickness of Quaternary sediments. Preliminary analysis of HVSR data in northwestern Minnesota indicates good results in Clay County, and in western Becker, northern Hubbard, northern Todd, and southern Wadena Counties, whereas generally poor results were observed elsewhere. Further work is being planned for the northwestern part of the state this summer. 23 �In summary the HVSR passive seismic method continues to be a very useful tool for estimating the thickness of Quaternary sediments in Minnesota and adjacent areas, provided the appropriate cautions are heeded. In some situations the HVSR methods will provide a suitable and much cheaper alternative to conventional seismic sounding, and when not, it will at least help in prioritizing and targeting areas where conventional seismic sounding is necessary. Figure 1. Map showing all passive seismic stations that have been acquired in Minnesota and adjacent parts of Wisconsin through the summer of 2013 (red circles). Stations highlighted in white represent control points where bedrock depth is known through either drillholes or seismic soundings. 24 �EVIDENCE OF SIMULTANEOUS BRITTLE AND DUCTILE DEFORMATION IN THE MAIN BREAK FAULT SYSTEM IN KIRKLAND LAKE, ON L. B. Clapp and M. L. Hill Department of Geology, Lakehead University, 955 Oliver Rd., Thunder Bay, Ontario, P7B 5E1, lbclapp@lakeheadu.ca Microstructural analysis provides evidence of significant ductile deformation concentrated along the main break, in Kirkland Lake, ON. The Main Break is an east-west striking mineralized fault system that has sustained multiple gold mines since its discovery in 1911. It is found in the southern Abitibi gold belt on Kirkland Lake Gold Inc.’s Lakeshore Mine property, which is a structurally controlled orogenic gold deposit. Oriented samples for microstrucural analysis were collected from two new 1-meter long channels across the main break spaced eight meters apart (Fig.1). Samples were examined in transmitted and reflected light microscopy. Evidence of ductile deformation by dislocation creep to produce mylonite includes porphyroclasts of potassium feldspar in an extremely fine-grained matrix, mineral alignment, undulatory extinction and subgrains in potassium feldspar, as well as lenticular aggregates of feldspar and muscovite showing dextral shear sense (Fig.2). Dislocation creep occurs in feldspar during ductile deformation at temperatures in the amphibolite facies of metamorphism or higher which puts deformation of the Main Break at temperatures above 400°C. Sericite aggregates replacing potassium feldspar likely enhanced grain softening during this ductile deformation process. Sericite as well as evidence of pressure solution along grain boundaries indicate the presence of a hydrous fluid during deformation. The most significant evidence of simultaneous brittle-ductile deformation is a potassium feldspar porphyroclast with strong undulatory extinction and subgrains in one half of the grain and microfractures in the other half (Fig.3). We conclude that the main break is a narrow ductile shear zone with minor brittle deformation. Figure 1. Main Break outcrop with channel samples shown in black lines 25 �Figure 2. Lenticular aggregates of feldspar and muscovite showing dextral shear sense Figure 3. K-spar porphyroclast showing mutually overprinting brittle and ductile deformation 26 �GEOLOGY AND GEOCHEMISTRY OF THE MESOPROTEROZOIC BADWATER INTRUSIVE COMPLEX, ONTARIO: IMPLICATIONS FOR GEON 15 MAGMATISM CUNDARI, Robert1, SMYK, Mark1 and HOLLINGS, Peter2 1 Resident Geologist Program, Ontario Geological Survey, Ministry of Northern Development and Mines, 435 James St. S., Suite B002, Thunder Bay, ON, P7E 6S7 Canada 2 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1 Canada The Mesoproterozoic Badwater Intrusive Complex (a.k.a. Waweig Troctolite Complex; cf. Borradaile and Middleton, 2006) intrudes Archean Wabigoon Subprovince country rocks, 13 km southwest of Armstrong, Ontario. This proposed complex comprises the Badwater Gabbro (BG) and the Badwater Syenite (BS). It is believed to form a multi-phase, intrusive complex which is expressed by a circular magnetic anomaly 12 km in diameter (Borradaile and Bennett, 2008). Initial mapping by MacDonald (2004) identified a variety of intrusive rocks, ranging from gabbro to quartz monzonite and syenite. They are unconformably overlain and largely obscured by Pillar Lake volcanic rocks which were possibly erupted at 1129 ± 4.6 Ma (U-Pb age from titanite) which would then likely represent early Midcontinent Rift magmatism (Heaman et al., 2007; Smyk et al., 2011). The poorly exposed BG was first recognized in 2000 by East West Resources Corp. (Middleton 2004) and was tested for PGE mineralization in drilling campaigns carried out in 2004 and 2008. The BG is described as a layered troctolite-gabbro complex consisting of olivine gabbro, anorthosite, troctolite, glomeroporphyritic rocks and layers of magnetite with sulphides (Middleton and Bennett, 2008). Magmatic layering dips ~45° to ~55° southeast. Modal mineralogy for typical olivine gabbro is listed as: plagioclase (labradorite/bytownite) 55%; clinopyroxene (augite?) 25%; biotite 10%; olivine (partly relict) 3%; talc/sericite (after olivine) 2%; amphibole (secondary actinolite) 2%; opaque (magnetite? pyrrhotite?) 2%; clay?/sericite (after plagioclase) trace (Middleton and Bennett, 2008). The BG is undeformed and displays generally fresh plagioclase and relatively unaltered olivine. The Badwater Syenite likely represents a multi-phase intrusion, characterized by intrusive breccias and hybrid rocks resulting from assimilation and contamination. Syenite dykes crosscut BG and BG xenoliths occur in syenite. High-precision U-Pb dating of baddeleyite yielded an emplacement age of 1598.7 ± 1.1 Ma for the BG and a U-Pb zircon age of 1590.1 ± 0.8 Ma for the BS, which supports observed cross-cutting relationships (Heaman et al., 2007). A possible genetic relationship between the BG and the BS can be tested using geochemistry. The BG is geochemically distinct from the BS on primitive mantle-normalized diagrams. The BG shows a flatter REE pattern with slight LREE enrichment, moderate HREE fractionation (Gd/Ybn = 2.4 to 3.9) and pronounced negative Zr and Hf anomalies (Fig. 1A). The BS shows a steeper REE pattern characterized by strong LREE enrichment, weak HREE fractionation (Gd/Ybn = 1.3 to 2.2) and pronounced negative Eu and Ti anomalies (Fig. 1B). It should be noted that two BG samples (03CAM115 and 03CAM305) were taken from mafic phases within the BS on the shore of Pillar Lake (not from within the main body of the BG) and display lower Gd/Ybn ratios than those taken from the two BG outcrops north of Pillar Lake (BW-01 and BW-02) which display Gd/Ybn ratios of 3.88 and 3.90, respectively. The trace element patterns for the BS show distinct similarities to those for the nearby 1546.5 ± 3.9 Ma (Heaman et. al., 2007) English Bay granite-rhyolite complex (EBC) (Fig. 1C). 27 �Based on similar trace element geochemistry, it would appear that the BS was derived from a similar source to the EBC, despite the 50 m.y. gap between the two units, whereas the BG appears to be sourced from a deeper source region. Hollings et al. (2004) suggested that the anorogenic EBC was derived from a mantle plume and it recorded the northern portion of a Mesoproterozoic plume track which produced anorogenic granites throughout North America. If the BS and the EBC are genetically related, the plume would have been attached to the base of the lithosphere for ~50 m.y. before detaching to create the anorogenic granites to the south in the United States. The BG could represent an early expression of the plume emplaced through a lithospheric-scale structure allowing for the tapping of a deeper-seated source. Alternatively, it may represent an earlier, unrelated plume that exploited the same structures as the EBC. Further work will elucidate intrusive relationships and possible regional associations. References Borradaile, G.J. and Middleton, R.S. 2006. Proterozoic paleomagnetism in the Nipigon Embayment of northern Ontario: Pillar Lake Lava, Waweig Troctolite and Gunflint Formation tuffs. Precambrian Research 144: 6991. Heaman, L.M., Easton, M., Hart, T.R., Hollings, P., MacDonald, C.A. and Smyk, M., 2007. Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon region, Ontario; Canadian Journal of Earth Sciences 44: 1055-1086. Hollings, P., Fralick, P. and Kissin, S. 2004. Geochemistry and geodynamic implications of Mesoproterozoic English Bay granite-rhyolite complex, northwestern Ontario; Canadian Journal of Earth Sciences 41: 13291338. MacDonald, C.A. 2004. Precambrian geology of the south Armstrong-Gull Bay area, Nipigon Embayment, northwestern Ontario; Ontario Geological Survey, Open File Report 6136, 42p. Middleton, R.S. 2004. Diamond drilling on Red Granite property, Pillar Lake sheet, Armstrong, Ontario; unpublished assessment file, Thunder Bay North District, Thunder Bay, 58p. Middleton, R.S. and Bennett, 2008. Drill Report, Armstrong, (Red Granite) property, Pillar Lake area, Thunder Bay Mining Division, Ontario; unpublished assessment file, Thunder Bay North District, Thunder Bay, 125p. Smyk, M., Hollings, P., and Cundari, R. 2011. The Pillar Lake Volcanics: new insights into an enigmatic Mesoproterozoic suite near Armstrong, Ontario. 58th Institute on Lake Superior Geology, Annual Meeting, Ashland, WI, May 18-21, 2011, Proceedings Volume 57, Part 1, p.75-76. 28 �PETROLOGY OF THE LAYERED NORTH LAC DES ILES INTRUSION, ONTARIO; PART I. STRATIGRAPHY AND MINERAL-CHEMICAL EVIDENCE FOR MULTIPLE MAGMA INJECTION Djon, M. L.1, Olivo, G.R.1, Miller, J.D.2 and Stewart, R. D3. 1 Queen's University, Department of Geological Sci. and Geological Engineering, Kingston, Ontario K7L 3N6 Department of Geological Sciences, University of Minnesota Duluth, Duluth, MN 55812 3 North American Palladium Ltd, 10th Avenue, Thunder Bay, Ontario, P7B 2R2. 2 The North Lac Des Iles Intrusion (NLDI-I) is a large, multi-phase, layered ultramafic intrusive component of the Lac des Iles Complex located in the northwest Ontario, which also includes the mafic Mine Block intrusion that has been extensively studied because of the palladium mining over two decades. The NLDII is an extensive (~25km2) well-layered tadpole-shaped complex, characterized by a series of nested bodies emplaced into the Archean tonalitic basement rocks of the Central Wabigoon Subprovince (McCracken et al., 2014). Two major intrusive centres, the Northern Ultramafic Centre (NUC) and the Southern Ultramafic Centre (SUC), are distinguishable based on dominant lithologies and attitudes of layering (Sutcliffe and Sweeney, 1986; Brugmann et al., 1989; Gupta et al., 1990; Brugmann et al., 1997). The focus of this study is the eastern limb of the NUC, which is a nearly circular funnel-shaped body with a mean diameter of approximately 4 km (Stone et al., 2003). Gravity modelling indicates that the NUC body thickens southward from a thickness of 1 km at the north end of NLDI to 3 km at the contact with the SUC (Gupta et al., 1990). North American Palladium’s integration of historical geological data with recent geophysical surveys shows that the magmatic layering is concentric and is preserved in the eastern part of northern centre but, is disrupted in the west by several smaller bodies of irregular to semielliptical shape. Bedrock mapping and reconstruction of 1.5 km long stratigraphic traverse of the eastern flank of the NUC show that it consists of a shallow, west- to northwest-dipping, layered sequences of ultramafic cumulate assemblages composed of olivine, chromite, clinopyroxene (augite) and orthopyroxene (bronzite). These sequences are characterized by regular repetitions of cumulate assemblage that define cyclic units that range from 50 to 250 meters in thickness. The upward progression of a typical cyclic unit is a basal olivine-chromite cumulate (dunite) grading into an olivine-bronzite-augite ± chromite cumulate (olivine websterite) and capped by a locally feldspathic augite-bronzite cumulate (websterite). In some cyclic units, however, the olivine websterite cumulate is absent. Contacts between cumulate assemblages within cyclical units are gradational over 0.5 to 3m to less commonly sharp. Minor plagioclase commonly occurs as an intercumulus phase in the websterite, but locally is abundant (up to 70 modal %) and can display cumulus texture in thin lenticular intervals. Although cyclic units appear throughout the NLDI-I, cyclic units in the lower part of layered sequence are dominated by websterite assemblages and are thus designated as the Pyroxenite Zone. This zone is composed of three, thick (200-250m) cyclic units that have websterite/dunite unit thickness ratios averaging 3:1. The upper cyclic units, in contrast, tend to be thinner (20-175m) and are dominated by olivine-bearing intervals (websterite:dunite unit thickness ratios average 2:3. This sequence is designated as the Peridotite Zone. Electron-microprobe analyses of cumulus olivine, chromite, and pyroxene compositions from drill core NL12-100, which profiles the two zones, are shown in Figure 1. The total ranges of compositions are not significantly different between the two zones, implying that the composition of the parent magma was likely the same for both sequences. Although a cyclical cryptic variation is evident throughout the core and consistent among the different cumulus phases, the breaks in mineral composition (dashed lines in Fig. 1) do not consistently correlate with cyclical boundaries. If this cryptic variation was due to repeated magma recharge pulses followed by fractional crystallization, the mineralogically most primitive dunite cumulate units would be expected to have the highest mg# (=MgO/(MgO+FeO), mole%) and the upper websterites to be the most evolved. However, this type of cryptic variation is clearly evident only in the 29 �upper (third) cycle of the Pyroxenite Zone. The cryptic break at the boundaries of cyclic units 1-2 and 3-4 is displaced upward from the lithologic contact. The density of data in the Peridotite Zone cycle is not sufficient to evaluate a correlation between cumulus phase layering and cryptic layering. The cause of the deviations from expected correlations between cumulus phase layering and cryptic layering and other petrologic aspect of the NLDI-I stratigraphy are still under investigation. Some possible processes being evaluated include: 1) variations in trapped liquid shift wherein primitive cumulus compositions are reset to lower mg#s by reequilibration with intercumulus liquid; 2) differences in the partitioning of MgO and FeO between cumulus olivine, clinopyroxene, and orthopyroxene (MgO is more compatible in pyroxenes than olivine); 3) changes from eutectic to peritectic relations between olivine and orthopyroxene, which could explain why olivine websterite cumulates are not always present;; and 4) contrasting densities of hot, primitive recharging magma and cooler, evolved resident magmas. If the recharging magma is denser than the resident magma, it should intrude beneath the resident magma and produce a sharp phase and cryptic change. If the recharging magma density is lower than the resident magma, it will plume into the chamber and would result an abrupt phase change and a more gradual cryptic shift to more primitive compositions. The relative volumes of recharging and resident magmas will also control the phase and mineral chemical effects. Figure 1: Cryptic variation shown by olivine, chromite, clinopyroxene, and orthopyroxene in the Peridotite and Pyroxenite zones of the Northern Ultramafic Center of the North Lac des Iles Complex. References Brügmann, G.E., Naldrett, A.J., Macdonald, A.J., 1989, Magma Mixing and Constitutional Zone-Refining in the Lac-Des-Iles Complex, Ontario - Genesis of Platinum-Group Element Mineralization: Economic Geology, v. 84, p. 1557-1573. Brugmann, G.E., Reischmann, T., Naldrett, A.J. and Sutcliffe, R.H. 1997. Roots of an Archean volcanic arc complex: The Lac des Iles area in Ontario, Canada; Precambrian Research 81, p. 223-239. Gupta, V.K. and Sutcliffe, R.H. 1990. Mafic–ultramafic intrusives and their gravity field: Lac des Iles area, northern Ontario; Geological Society of America Bulletin, v.102, p.1471-1483. McCracken, T., Kanhai, T., Bridson, P., McBride, W. R., Small, K., Penna, D., Technical Report Lac Des Iles Mine, Ontario, 2014. Stone, D., Lavigne, M.J., Schnieders, B., Scott, J., Wagner, D., 2003, Regional geology of the Lac des Iles area: Ontario Geolgical Survey Open File Rep 6120:15-1, p. 15-25. Sutcliffe, R.H. and Sweeny, J.M., 1986. Precambrian Geology of the Lac des Iles Complex, District of Thunder Bay, Ontario. Ontario Geological Survey, Map 3047, Geological Series-Preliminary Map, scale 1:15840. 30 �STRAIN ANALYSIS ON THE MAX LAKE POLYMICTIC CONGLOMERATES IN THE WABIGOON SUBPROVINCE, ONTARIO, CANADA Simon Dolega and Mary Louise Hill Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario P7B 5E1 Canada (sdolega@lakeheadu.ca) The Max Lake polymictic conglomerates are exposed near Highway 527, about 88 km north of the intersection with Highway 11-17. The conglomerates are part of the Beardmore-Geraldton belt in the Wabigoon Subprovince, Superior Province of Ontario, Canada. The Rf/Phi method for initially elliptical objects was used to estimate the amount strain on the conglomerates. In the Max Lake conglomerates, chlorite-actinolite clasts are more deformed than amphibolite clasts, which are more deformed than granitoid clasts. Heterogeneous strain also occurs among different outcrops. The overall amount of strain is lower where larger, more abundant and more competent clasts are found. Petrographic and microstructural analyses were used to determine the peak metamorphic grade preserved by each clast in the polymictic conglomerate. The matrix of the conglomerate and the chlorite-actinolite clasts preserve a peak metamorphic mineral assemblage stable in the greenschist facies. The amphibolite clasts preserve a peak metamorphic mineral assemblage stable in the amphibolite facies. The preservation of the amphibolite facies metamorphic mineral assemblage in the amphibolite clasts indicates that these clasts were derived from a metamorphic terrane. 31 �32 �PRELIMINARY INTERPRETATION OF PRECAMBRIAN LITHOLOGY AND STRUCTURE FROM HIGH-RESOLUTION, MULTI-METHOD GEOPHYSICS, NORTHEAST IOWA AND SOUTHEAST MINNESOTA DRENTH, Benjamin1, ANDERSON, Raymond2, SCHULZ, Klaus3, CHANDLER, Val4, CANNON, William3, BLOSS, Benjamin1, BEDROSIAN, Paul1, FEINBERG, Joshua M.5, and McKAY, Robert6 1 U.S. Geological Survey, PO Box 25046, MS 964, Denver, CO, 80225 2 Dept. Earth and Environmental Sciences, Univ. Iowa, Iowa City, IA, 52242 3 U.S. Geological Survey, 12201 Sunrise Valley Dr., MS 954, Reston, VA, 20192-6320 4 Minnesota Geological Survey, 2642 University Avenue W., St. Paul, MN, 55114-1032 5 Dept. Earth Sciences, Univ. Minnesota, 310 Pillsbury Dr. SE, Minneapolis, MN, 55455-0219 6 Iowa Dept. Natural Resources—Iowa Geological and Water Survey, Iowa City, IA, 52242 Large amplitude gravity and magnetic highs over northeast Iowa are interpreted to reflect a buried intrusive complex composed of mafic/ultramafic rocks, the northeast Iowa Intrusive Complex (NEIIC), intruding Yavapai Province (1.8-1.72 Ga) rocks. The age of the complex is unproven, although it has been considered to be Keweenawan (~1.1 Ga). Because only four boreholes reach the complex, which is thought to be covered by 200-700 m of Paleozoic sedimentary rocks, geophysical methods are critical to developing a better understanding of the nature and mineral resource potential of the NEIIC. An initial airborne data collection campaign in the region of Decorah, Iowa, included high-resolution magnetic, gravity gradient (AGG), and time-domain electromagnetic (TDEM) data. Geophysical interpretations are presented in the form of a preliminary geologic map of the basement Precambrian rocks (Fig. 1), largely constructed by interpreting lithologies and cross cutting relationships expressed in magnetic and AGG anomalies. Numerous magnetic anomalies are coincident with AGG highs, indicating widespread strongly magnetized and dense rocks of likely mafic/ultramafic composition. A Yavapai age (Van Schmus et al., 2007) metagabbro unit (Ymg) is interpreted to be part of a layered intrusion with subvertical dip, and is thought to be among the oldest rocks present in the survey area. Another presumed Yavapai-age unit (Ysp) has low density and weak magnetization, observations consistent with granitic plutons. Northeast-trending, linear magnetic lows are interpreted to reflect reversely magnetized diabase dikes, and modeling shows that the anomalies are consistent with Keweenawan magnetization. The interpreted dikes are cut in places by normally magnetized mafic/ultramafic rocks (mi), suggesting that the latter represent younger Keweenawan rocks. Large magnetic highs without coincident AGG highs are interpreted to reflect intermediate or silicic intrusive rocks (ii). Distinctive horseshoe-shaped magnetic and AGG highs correspond with a known gabbro (dg, undated), and surround rocks with weaker magnetization and lower density (dwm). Here called the Decorah Complex, the source body has notable geophysical similarities to Keweenawan alkaline ring complexes, such as the Coldwell and Killala Lake Complexes, and Mesoproterozoic anorogenic complexes, such as the Kiglapait, Hettasch, and Voisey’s Bay intrusions in Labrador. Most units are cut by suspected northwest-trending faults imaged as magnetic lineaments, and one produces apparent sinistral fault separation of a dike in the eastern part of the survey area. The location, trend, and apparent sinistral sense of motion are consistent with the suspected faults being part of the Belle Plaine fault system, a complex transform fault zone within the Midcontinent Rift System. The TDEM data fail to directly image Precambrian rocks, due to their large burial depth. However, most of the overlying sedimentary section is well imaged, and depths to Precambrian rocks are estimated at 400-600 m based on preliminary TDEM interpretations and downward extrapolation of known stratigraphy. This interpretation is consistent with the limited borehole data. Reference Van Schmus, W.R., Schneider, D.A., Holm, D.K., Dodson, S., and Nelson, B.K., 2007, New insights into the southern margin of the Archean-Proterozoic boundary in the north-central United States based on U-Pb, SmNd, and Ar-Ar geochronology: Precambrian Research, v. 157, p. 80-105. 33 �Figure 1: Preliminary geologic map of Precambrian crystalline rocks, interpreted from limited boreholes and high-resolution airborne gravity gradient and magnetic data. 34 �Structural and Kinematic Analysis of the Shagawa Lake Shear Zone: Implications for Archean Tectonic Processes in the Southern Superior Province (Part 2 of 2) DYESS, Jonathan and HANSEN, Vicki, Department of Geological Sciences, University of Minnesota Duluth, 1114 Kirby Drive, Duluth MN 55812 The Archean (3.85-2.5 Ga) Superior Province, to a first approximation, consists of a series of east-west trending subprovinces of supracrustal rocks (greenstone belts) and granitoid rocks (e.g., Percival et al., 2007, and references therein). The Wawa Subprovince, southern Superior Province, is widely interpreted as a transpressional margin with shear zones recording unidirectional dextral strike-slip along the subprovince boundary (Hudleston et al., 1988; Bauer and Bidwell, 1990; Schultz-Ela and Hudleston, 1991), an interpretation held up as fundamental evidence for Archean plate-tectonic processes (Sleep, 1992). Others interpret these shear zones as recording dominantly oblique- to dip-slip shear possibly formed during greenstone sagduction between rising granitoid diapirs (Erickson, 2008, 2010; Wolf, 2006; Goodman, 2008; Karberg, 2009). Differing interpretations invoke different assumptions about non-coaxial shear direction. Due to the proximity of the Shagawa Lake shear zone to the Wawa subprovince boundary, structural and kinematics fabrics recorded within the Shagawa Lake shear zone have direct implications for crustal assembly of the southern Superior Province. If the Shagawa Lake shear zone records significant unidirectional strike-slip, then supported plate-tectonic models for Wawa Subprovince formation will be further constrained. If the Shagawa Lake shear zone does not record significant unidirectional strike-slip, then existing plate-tectonic and structural models of terrane amalgamation along the Southern Superior Province require reevaluation. We conducted a structural and kinematic analysis of the Shagawa Lake shear zone in three phases: 1) analysis of regional tectonic fabrics through Light Detection and Ranging altimetry data; 2) structural analysis of outcrop-scale structures through detailed field mapping; and 3) thin-section kinematic analysis. The Shagawa Lake shear zone contains a regional subvertical metamorphic foliation with an average strike of 065 but varies locally from 065 to 100. Two types of elongation lineation occur within the Shagawa Lake shear zone. These include ridge-in-groove striations on C-foliation surfaces (Lc) and stretching lineations on Ssurfaces (Ls) (Lin and Williams, 1992; Lin et al., 2007). Lc and Ls plunge steeply to obliquely, with local zones of shallow plunge, and non-coaxial shear direction is sub-parallel to elongation lineation (Dyess et al., 2014). Thus non-coaxial shear was dominantly dip- to oblique-slip with localized strike-slip. Microstructures, within foliation-normal, lineation-parallel sections, record both north-side-up and south-side-up shear in different samples. Samples with oblique lineation commonly record an apparent dextral strike-slip shear-sense despite varying lineation orientation. Our data indicate the Shagawa Lake shear zone experienced both N-side-up and Sside-up dip- to oblique-slip with relatively minor apparent dextral strike-slip and does not record significant unidirectional strike-slip as required by accepted plate tectonic models. 35 �References Bauer, R. L and Bidwell, M. E., 1990. Contrasts in the response to dextral transpression across the Quetico-Wawa subprovince boundary in northeastern Minnesota. Canadian Journal of Earth Sciences, 27, 1521-1535. Dyess, J.E., Hansen, V.L., Goscinak, C., 2014. Determination of vorticity in Neoarchean tectonites (Part 1 of 2). Institute on Lake Superior Geology annual meeting, Hibbing, MN. Erickson, E., 2008. Structural and kinematic analysis of the Shagawa Lake shear zone, Superior Province, northeastern Minnesota. M.S. Thesis, University of Minnesota Duluth, MN. Erickson, E., 2010. Structural and kinematic analysis of the Shagawa Lake shear zone, Superior Province, northern Minnesota: implications for the role of vertical versus horizontal tectonics in the Archean. Canadian Journal of Earth Sciences, 47, 1463-1479. Goodman, S., 2008. Structural and Kinematic Analysis of the Kawishiwi Shear Zone, Superior Province. M.S. Thesis, University of Minnesota Duluth, MN. Hudleston, P.J., Schultz-Ela, D., Southwick, D. L., 1988. Transpression in an Archean greenstone belt, northern Minnesota. Canadian Journal of Earth Sciences, vol 25, 10601068. Karberg, S M., 2009. Structural and Kinematic Analysis of the Mud Creek Shear Zone, Northeastern Minnesota. M.S. Thesis, University of Minnesota Duluth, MN. Lin, S., Williams, P.F., 1992. The origin of ridge-in-groove slickenside striae and associated steps in an S-C mylonite. Journal of Structural Geology 14, 315e321. Lin, S., Jiang, D., Williams, P., 2007. Importance of differentiating ductile slickenside striations from stretching lineations and variation of shear direction across a high-strain zone. Journal of Structural Geology, 29, 850-862. Percival, J.A., 2007, Geology and metallogeny of the Superior Province, Canada, in Goodfellow,W.D., ed.,Mineral Deposits of Canada:ASynthesis ofMajor Deposit-Types, District Metallogeny, the Evolution of Geological Provinces, and Exploration Methods: Geological Association of Canada, Mineral Deposits Division, Special Publication No. 5, p. 903-928. Schultz-Ela, D.D., Hudelston, P.J., 1991. Strain in an Archean greenstone belt of Minnesota. Tectonophysics, 190, 223-268. Sleep, N., 1992. Archean plate tectonics: what can be learned from continental geology?. Canadian Journal of Earth Sciences, 29, 2066-2071. Wolf, D. E., 2006. The Burntside Lake and Shagawa/Knife Lake shear zones: Deformation kinematics, geochemistry and geochronology; Wawa Subprovince, Ontario, Canada. Masters Thesis, Washington State University. 36 �Determination of Vorticity in Archean Tectonites (Part 1 of 2) DYESS, Jonathan, HANSEN, Vicki, and GOSCINAK, Christopher, Department of Geological Sciences, University of Minnesota Duluth, 1114 Kirby Drive, Duluth MN 55812 There is no consensus about the processes responsible for the formation of Archean crust. The Superior Province of North America is widely interpreted as a series of accreted terranes with subprovinces representing individual terranes (Talbot, 1973; Goodwin and Ridler,1970; Langford and Morin, 1976; Dimroth et al., 1983a, b; Ludden et al., 1986; Sylvester et al., 1987). The Neoarchean (2.7-2.5 Ga) Wawa Subprovince (Fig. Location) forms a NE-trending belt of sub-greenschist to greenschist facies supracrustal rocks cut by multiple shear zones marked by a well-developed metamorphic foliation (Fm) and elongation lineation (Le). The Wawa is interpreted as a transpressional plate-margin with significant dextral strike-slip displacement (Hudleston et al., 1988; Schultz-Ela and Hudleston, 1991). This interpretation is held up as evidence for Archean plate-tectonic processes based on plate-tectonic models that require strike-slip shear zones more than 1200 km of long (Sleep, 1992). Despite apparent broad acceptance of this interpretation, the nature of shear zone deformation within the Wawa remains poorly constrained. Displacement direction and magnitude is genetically linked to vorticity, marked by the vorticity-normal-section (VNS) and vorticity axis (pole to VNS), within L-S tectonites (Passchier, 1998; Xypolias, 2010 and references therein). However, geometric relationships between displacement direction and macroscopic structures, such as Le, can vary depending on shear zone kinematics (Passchier, 1998). Le can form parallel, perpendicular, or oblique to displacement direction during L-S tectonite formation. Therefore determination of L-S tectonite vorticity requires careful consideration before interpretation of displacement direction. A determination of the vorticity axis and observation of the geometric relationships between the VNS and Le can allow for the use of Le as a reference to noncoaxial shear direction. In this contribution, we determine the vorticity for seven samples from two Neoarchean shear zones from the Wawa subprovince. We use a combination of thin-section, X-Ray Computed Tomography, and quartz petrofabric data. We demonstrate that the vorticity axis lies approximately within the Fm and is normal to Le and that the VNS lies approximately within Fm-normal, Le-parallel planes for all seven samples. Thus non-coaxial displacement direction is sub-parallel to Le. Regional Le orientation varies within the two shear zones ranging from steeply to obliquely plunging, with local zones of shallow plunge (Hudleston, 1976; Hudleston et al, 1988; Bauer and Bidwell, 1990; Jirsa, et al., 1992; Goodman, 2008; Erickson, 2010; Johnson, 2009; Karberg, 2009). Data indicate that noncoaxial shear direction is sub-parallel to Le regardless of Le geographic orientation. 37 �References Bauer, R. L and Bidwell, M. E., 1990. Contrasts in the response to dextral transpression across the Quetico-Wawa subprovince boundary in northeastern Minnesota. Canadian Journal of Earth Sciences 27, 1521-1535. Dimroth, E., Imreh, L., Goulet, N., Rocheleau, M., 1983a. Evolution of the south-central segment of the Archean Abitibi Belt, Quebec Part II tectonic evolution and geomechanical model. Can J Earth Sci 20, 1355-1373. Dlmroth, E., Imreh, L., Goulet, N., Rocheleau, M., 1983b. Evolution of the south-central segment of the Archean Abitibi Belt, Quebec Part III plutonic and metamorphic evolution and geotectonic model. Can J Earth Sci 20 1374-l 388. Goodman, S., 2008. Structural and Kinematic Analysis of the Kawishiwi Shear Zone, Superior Province. M.S. Thesis, University of Minnesota Duluth, MN. Goodwln, A.M. and Ridler, R.H., 1970. The Abitibi orogenic belt In Symposium on Basins and Geosynclines of the Canadian Shield. Geol Surv Can Pap 70-40, 1-24. Erickson, E., 2010. Structural and kinematic analysis of the Shagawa Lake shear zone, Superior Province, northern Minnesota: implications for the role of vertical versus horizontal tectonics in the Archean. Canadian Journal of Earth Sciences, 47, 14631479. Hudleston, P.J., 1976. Early deformational history of Archean rocks in the Vermillion district, Northeastern Minnesota. Canadian Journal of Earth Sciences 13, 579-592. Hudleston, P.J., Schultz-Ela, D., Southwick, D. L., 1988. Transpression in an Archean greenstone belt, northern Minnesota. Canadian Journal of Earth Sciences 25, 10601068. Johnson, T., 2009. Structural, Kinematic, and Hydrothermal Fluid Investigation of the GoldBearing Murray Shear Zone, northeastern Minnesota. M.S. Thesis, University of Minnesota Duluth, MN. Karberg, S.M., 2009. Structural and kinematic analysis of the Mud Creek shear zone, northeastern Minnesota; implications for Archean (2.7 Ga) tectonics. M.S. Thesis, University of Minnesota Duluth, MN. Langford, F.F. and Morin, M.A. 1976. The development of the Superior Province of Northwestern Ontario by merging island arcs. Am J Sci 276, 1023-1034. Ludden, J.N., Hubert, C., Gariepy, C., 1986. The tectonic evolution of the Abitibi greenstone belt of Canada. Geol Mag 123, 153-166. Passchier, C.W., 1998. Monoclinic model shear zones. Journal of Structural Geology. 20 (8): 1121-1137. Schultz-Ela, D.D. and Hudleston, P.J., 1991. Strain in an Archean greenstone belt of Minnesota. Tectonophysics 190, 233-268. Sleep, N., 1992. Archean plate tectonics: what can be learned from continental geology?. Canadian Journal of Earth Sciences, 29, 2066-2071. Sylvester, P.J., Attoh, K and Schulz, K.J., 1987. Tectonic setting of late Archean bimodal volcanism in the M1- chipicoten (Wawa) greenstone belt, Ontario. Can J Earth Sci 24, 1120-1134. Talbot, C.J., 1973. A plate tectonic model for the Archean crust. Philos Trans Soc London 273, 413- 427. Xypolias, P., 2010. Vorticity analysis in shear zones: A review of methods and applications. Journal of Structural Geology 32, 2072-2092. 38 �EVALUATING THE BIOGENICITY OF FLUVIAL-LACUSTRINE STROMATOLITES FROM THE MESOPROTEROZOIC COPPER HARBOR CONGLOMERATE, UPPER PENINSULA OF MICHIGAN, USA Nicholas D. Fedorchuka, Stephen Q. Dornbosa,b, John L. Isbella, Julie A. Bowlesa, Frank A. Corsettic, Dylan T. Wilmethc, Victoria A. Petryshynd a Department of Geosciences, University of Wisconsin-Milwaukee, Milwaukee, WI 53211, USA Geology Department, Milwaukee Public Museum, Milwaukee, WI 53232, USA c Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089, USA d Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, Los Angeles, CA 90095, USA b The Mesoproterozoic (1.09 Ga) Copper Harbor Conglomerate represents alluvial fan, fluvial and lacustrine deposition into the Midcontinent Rift System. The formation outcrops in the Upper Peninsula of Michigan where it contains carbonate stromatolites preserved within both siltstone and conglomerate facies. The purpose of this study is to evaluate the biogenicity of these stromatolites, which lack direct microfossil evidence. The stromatolites were placed into their depositional context, their macro-scale features and thin section microfabrics were analyzed, and growth angles were measured of cobble-draping samples to determine if a phototrophic response existed. A methodology that uses magnetic susceptibility as a biosignature was also performed on these stromatolites. The result of these analyses reveals two distinct types of stromatolites. Stromatolites from the siltstone facies are interpreted as biogenic. They contain detrital laminae, hematite-rich micritic laminae, and fenestral fabrics. The stromatolites formed as microbial mats grew over a mudflat or sandflat with carbonate filled dessication cracks and an eroded topography. Stromatolites from the conglomerate facies are interpreted to have formed by a mix of chemical and biological processes. They are microdigitate and have abiogenic features such as isopachous laminae with radial fibrous calcite fans and botryoids. They also lack a phototrophic response, suggesting that growth was not controlled by cyanobacteria. These stromatolites also have some biogenic signatures such as conical wavy laminae that have been separated by gas build-ups. These stromatolites are interpreted as having formed in a flooded braidplain setting with restricted circulation. Magnetic susceptibility tests yielded inconclusive results in this case because the stromatolites in question contain secondary hematite. This study supports previous studies of these stromatolites, as well as microbial structures and organic-rich paleosols that have suggested freshwater microbial communities were abundant in the Midcontinent Rift during the Mesoproterozoic. It also highlights how variable environmental factors can influence stromatolite growth, even in similar depositional settings and with a consistent microbial presence. 39 �40 �GEOCHEMISTRY OF BASALT XENOLITHS ENTRAINED IN MINERALIZED TROCTOLITIC AND ANORTHOSITIC INTRUSIONS, NORTHEASTERN MINNESOTA FINNEGAN, Molly L1 and LARSON, Phillip C1, 1 Duluth Metals Limited, 306 W. Superior Street, Suite 610, Duluth, MN 55802 Troctolites of the Nickel Lake Macrodike (NLM), South Kawishiwi (SKI) and Partridge River (PRI) Intrusions, as well as the varying lithologies of the Anorthositic Series (An-series), within the 1.1 Ga Duluth Complex, host numerous basaltic xenoliths. These xenoliths provide evidence that these intrusive bodies originally emplaced within the basalts and other accompanying lithologies of the North Shore Volcanic Group (NSVG; Miller and Weiblen, 1990). These xenolith packages, which include Biwabik Iron Formation, Virginia Formation and Colvin Creek metasediment, and An-series material, in addition to both magnetic and non-magnetic basalts, are often associated with sulfide-mineralized troctolitic and anorthositic lithologies. Lithogeochemical data suggests these basalt xenoliths can be grouped into three distinct categories, correlating with different mineralization styles in the host rock lithologies. This study aims to determine the nature of the apparent correlation between the presence of basalt xenoliths and CuNi-PGE mineralization as well as attempt to correlate the basalt compositions with possible sources. Samples were collected from drill holes and outcrop from the NLM, eastern margin of the SKI (East Shore), and the northeast corner of the PRI (Rook)(Fig. 1). Three groups of basalt are distinguished by relative abundances of MgO, Al2O3, Fe2O3, TiO2, Zr, and Mg# (calculated as Mg/(Mg+Fe3+)*100), using factor analysis (which identifies trends between multiple variables) to provide a more robust classification. Type 1 basalt xenoliths are characterized by Mg#s which range from 42-52 accompanied by elevated MgO and Al2O3, and low TiO2 and Zr. Type 2 has a range of Mg#s from 19-33, along with elevated Al2O3, TiO2, and Zr, and low MgO. Type 3 has a mid-range Mg# with respect to the other two types (27-36), accompanied by low MgO and Al2O3, and high TiO2, Zr, and Fe2O3*. An Mg# vs. Zr plot clearly discriminates the three different basalt xenoliths types (Fig. 2). Comparing these basalt xenoliths to basalt compositions from the NSVG using an MgO vs. TiO2 plot demonstrates Type 2 and Type 3 basalts correlate well with compositions of basalts within the NSVG, whereas the composition of the Type 1 basalt is particularly anomalous (Fig. 3). The nearest compositional correlation to Type 1 comes from sample KEW-6, collected from the Larsmont basalts of the NSVG near Knife River, MN (Boerboom et al., 2002). A closer match appears to be the P-Magma of Miller and Weiblen (1990) (Mg# 48), their representative primitive high-Al olivine tholeiitic basalt composition, which plots within the range of the Type 1 basalt xenoliths (Fig. 3). Type 1 basalts are spatially associated with the lower contact of a Cu-Ni mineralized zone at the southwest end of the NLM. Other xenolith lithologies occurring with Type 1 basalts include Virginia Formation and Biwabik Iron Formation. Type 2 basalts are spatially associated with high-grade PGM mineralization along the SKI-An-series contact, occurring with Colvin Creek and An-series xenoliths. Type 3 basalts are spatially associated with high-grade Cu-Ni-PGE mineralization in the PRI. They are the most variable in composition, as well as geographically widespread, occurring along the eastern margin of the SKI and within the NLM as well. Xenolith lithologies associated with Type 3 basalt include Colvin Creek, Virginia Formation, and An-series. 41 �Basalt xenolith populations can potentially be used as an indicator of the prospectivity of heterolithic mafic rocks. Correlating these xenoliths with their source areas potentially allows reconstruction of the sources and pathways of mineralized troctolitic and anorthositic magmas in the Duluth Complex. Figure 2: Plot of Mg# (=Mg/(Mg+Fe3+)) versus Zr (ppm). Type 1 has clustered around higher Mg# values, while Type 2 and 3 have lower and more variable Mg# ranges in conjunction with higher Zr content. Figure 1: Map indicating spatial relationships between the intrusions and showing the xenolith rich areas of the NLM, SKI, and PRI. Figure 3: Plot of TiO2 versus MgO for the three differentiated types of basalt as well as known compositions within the NSVG. Sample KEW-6 (Boerboom et al, 2002) is the sample plotted closest to the Type 1 basalt xenolith trend. The composition of P-Magma (Miller and Weiblen, 1990) is also shown. References Cited Boerboom, T.J., Green, J.C., and Jirsa, M.A., 2002, Bedrock geologic map of the Knife River quadrangle, St. Louis and Lake Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-129, scale 1:24,000. Miller, J. and Weiblen, P., 1990. Anorthositic rocks of the Duluth Complex: Examples of rocks formed from plagioclase crystal mush. Journal of Petrology, 31, 295-339. 42 �MAGNETIC MINERALOGY OF REVERSELY MAGNETIZED CHENGWATANA LAVA FLOWS OF ST. CROIX FALLS, WISCONSIN FINN, Kiel and BOWLES, Julie Department of Geosciences, University of Wisconsin-Milwaukee, Lapham Hall 366, Milwaukee, WI 53201 Previous paleomagnetic studies on the 1.1 billion year old Chengwatana Volcanic lava flows, located near St. Croix Falls, Wisconsin, found a period of reversed polarity within a predominantly normal sequence that was very similar to that found at another location within the Keeweenawan Rift at Mamainse Point in Ontario. Based on this observation, the two sequences were correlated with each other. However, new research at Mamainse Point (Swanson-Hysell et al., 2011) has discovered that some of the lava flows there carry a self-reversed magnetization. This means that the magnetization is opposite in direction to the Earth’s field in which the rocks originally cooled. Further, data from some of the Chengwatana samples reported by Kean et al. (1997) also showed two components of magnetization approximately antiparallel to each other. The goal of this study was to further investigate the magnetic mineralogy of the Chengwatana Volcanic lava flows in order to test that reversely-magnetized samples from Chengwatana indeed reflect a period of reversed field polarity and not a self-reversed magnetization. Thirty samples were collected in two different locations at Interstate State Park in St. Croix Falls, Wisconsin. The natural remanent magnetization was demagnetized both by alternating field (AF) and thermal demagnetization at the University of Wisconsin – Milwaukee (UWM). Also at UWM, Curie temperature was measured via temperature-dependent susceptibility using an MFK1 Kappabridge susceptibility bridge with furnace insert. Hysteresis measurements to determine grain size were carried out on a vibrating sample magnetometer at the Institute for Rock Magnetism at University of Minnesota. Results indicate that the magnetic remanence held by both normal and reverse polarity flows, as defined previously by Kean et al. (1997), is held by multi-domain to pseudo-single-domain magnetite. Individual samples that carry two anti-parallel components of magnetization are also dominated by magnetite, and the high-coercivity or high-temperature component is similar in direction and polarity to single-component samples from the same flow. It is likely that the antiparallel component was acquired by partial remagnetization during a reheating event during a later period of opposite polarity. It does not completely overprint the primary magnetization, and the polarity sequence identified Kean et al. (1997) remains unchanged. This is in contrast to the self-reversed magnetization found in some of the Mamainse Point basalts by Swanson-Hysell et.al (2011). In those samples, the antiparallel component was controlled by fine grained hematite that acquired its magnetization during the formation of martite within the rocks. There is no indication that the Chengwatana flows share this mineralogy and the conclusions of Kean et al. remain valid. References Kean, W.F., Williams, I., Chan, L., Feeney, J., 1997 Magnetism of the Keweenawan age Chengwatana lava flows, northeast Wisconsin. Geophysical Research Letters, vol. 24, no.12, 1523-1526 Swanson-Hysell, N. L., Feinberg, J. M., Berquo, T.S., Maloof, A.C. 2011 Self-reversed magnetization held by martite in basalt flows from the 1.1-billion-year-old Keweenawan rift, Canada. Earth and Planetary Science Letters, 305, pp.171-184 43 �44 �AN UNUSUAL MESOPROTEROZOIC CARBONATE UNIT: RELIC OF A SALINE LAKE? FIRMIN, Sydney and Bartley, Julie K. Department of Geology, Gustavus Adolphus College, St. Peter, Minnesota, 56082 The Mesoproterozoic Rossport Formation of Ontario, Canada is primarily made up of sandstone and shale. The Rossport Formation is approximately 1.4 billion years old (Franklin et al., 1980) and is generally interpreted to have been deposited in an Figure 1 intracratonic basin, most likely a rift-related lake (Rogala et al., 2005). The Middlebrun Bay Member, in the middle of the formation, consists of cherty limestone containing stromatolites. While examining outcrops of the Middlebrun Bay Member on the Channel Islands of Lake Superior, we discovered an unusual limestone bed on Copper Island. This calcite does not contain stromatolites; it has an unusual bright white color and lacks internal structure (Fig. 1). Previous work on the Rossport Formation suggests that the stromatolites formed when lake levels were low and not much sand was making it to the basin (Rogala et al., 2007). In this model, stromatolites would have formed in a hypersaline lake environment during intervals of low clastic influx. If this interpretation is correct, the non-stromatolitic “white-bed” could have formed as an evaporite bed, now replaced by calcite. In this study, we investigate the hypothesis that the massive white limestone unit is calcitized evaporite. At the outcrop level, the massive white carbonate is approximately 1.1 m thick and occurs between layers of sandstone. The carbonate unit contains sandstone clasts (Fig. 2). This outcrop relationship is consistent with either collapse of sandstone during evaporite dissolution, or upward growth of evaporite rock, causing sandstone to wedge, split, and form clasts surrounded by evaporite. Figure 2 Figure 3 The macroscopic texture of the carbonate unit is both massive and coarsely crystalline, with a texture reminiscent of chicken-wire evaporite (Fig. 3). Chicken-wire texture forms when nodules of gypsum crystals grow and push other material to their edges, forming a coarsely crystalline structure with a network of residuum outlining large crystal domains. Thin-sections of the massive white carbonate were compared to those from Middlebrun Bay Member Stromatolites from Channel Island. The stromatolitic thin sections show relatively small crystals and fine lamination, consistent with their macroscopic texture. In contrast, the “whitebed” rock had large subhedral to euhedral crystals, with zonation apparent by 45 �cathodoluminescence. “White-bed” samples also had a large number of stylolites, indicating substantial dissolution along crystal boundaries. Both stylolites and large crystal edges contain accumulations of insoluble residue (Fig. 4), indicating that dissolution and reprecipitation processes were important in generating the final texture of the white bed. Figure 4 Chemical evidence is consistent with an evaporite origin of the “white-bed” carbonate. Trace element concentrations, measured by ICP-MS, were generally highly elevated in stromatolite samples and moderately elevated in the massive carbonate unit, compared to average Proterozoic carbonate compositions. Taken together, geochemistry suggests that both the stromatolites and the white bed were deposited in a hypersaline lake environment. Trace elements were concentrated in carbonate during precipitation of stromatolites. Primary evaporite phases would also have been highly concentrated in trace elements, but these concentrations would have decreased during dissolution of evaporites and precipitation of secondary calcite. Similar patterns of trace element enrichment are observed in hypothesized calcitized evaporites from the Mesoproterozoic Atar Group, Mauritania (Manning-Berg and Kah, 2013). Based on the evidence collected both in the field and lab, it seems likely that the “white bed” carbonate possesses a unique texture because it was originally precipitated as gypsum. The massive, coarsely crystalline texture indicated pervasive recrystallization, consistent with a primary evaporite miner, like gypsum, which was secondarily replaced by calcite, resulting in coarse, featureless carbonate and collapse of overlying sandstone layers. Geochemical results are consistent with deposition under hypersaline conditions. In further research we will look at sulfate concentrations, both as total S and as carbonate associated sulfate (CAS). Other calcitized evaporites have shown elevated CAS concentrations (Manning-Berg and Kah, 2013). A depositional environment where gypsum formed would indicate a saline lake, consistent with previously proposed environmental conditions for the Rossport Formation. References 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. Rogala, B., and Fralick, P.W., 2005, Stratigraphy and sedimentology of the Mesoproterozoic Sibley Group and related igneous intrusions, northwestern Ontario: Ontario Geological Survey Open File Report 6174, 128 pp. Franklin, J. M., McIlwaine, W.H., Poulsen, K.H., and Wanless, R.K., 1980, Stratigraphy and depositional setting of the Sibley Group, Thunder Bay district, Ontario, Canada: Canadian Journal of Earth Sciences, v. 17, p. 633-651. Manning-Berg, A.R., and Kah, L.C., 2013, Calcitized Evaporites and the Evolution of Earth’s Early Biosphere: Geological Society of America Abstracts with Programs, v. 45(7), p. 628. 46 �GEOLOGY OF THE BRULE RIVER AREA OF THE PINE MOUNTAIN QUADRANGLE, MINNESOTA: CAPSTONE MAPPING PROJECT FOR THE PRECAMBRIAN RESEARCH CENTER’S 2013 FIELD CAMP Paul J. Fix1, Stephen J. Ginley1, Lauren A. Schraeder1, Aaron J. Summers1, Michael S. Doyle2,Terrence J. Boerboom3 1 2013 Field Camp Participants, Precambrian Research Center, Natural Resources Research Institute, University of Minnesota Duluth, 5013 Miller Trunk Hwy., Duluth, MN 55811 2 Dept. of Geological Sciences, University of Minnesota Duluth, 229 Heller Hall, 1114 Kirby Drive, Duluth, MN 55812 3 Minnesota Geological Survey (MGS), University of Minnesota, 2642 University Ave. West, St. Paul, MN 55114 The Precambrian Research Center at the University of Minnesota Duluth conducted its seventh annual Precambrian Field Camp during the summer of 2013.This presentation is one of a series that detail the results of the 2013 Capstone Mapping Projects which represent the culmination of activities at the field camp. The Capstone projects, conducted during the final two weeks of the field camp, are meant to test the skills obtained by the camp participants by conducting field studies and creating geological maps of areas of poorly understood geology. This Capstone Project involved mapping near Brule River area, in the Pine Mountain 7.5’ Quadrangle, approximately 25 miles north-northwest of Grand Marais, MN. Figure 1. Geology of northeastern Minnesota showing the area of the Pine Mountain capstone mapping project. From Miller and Green, 2002 Prior to this work, the area had only been mapped in ay reconnaissance fashion, and consequently the geologic detail was poorly understood and interpretations were largely derived from only geophysical data. The map area is in a geologically complex Mesoproterozoic terrane comprised largely of mafic to 47 �felsic lavas of the Keweenawan North Shore Volcanic Group (NSVG), and later mafic to felsic intrusions related largely to the Beaver Bay Complex. The map area lies along the western extension of the reversely-polarized Hovland lavas (1107.7 ± 1.9 Ma; Davis and Green, 1997), shown only as a single unit of undivided volcanic rocks on published maps (e.g. Miller and others, 2001). The area is bordered on the south by the Brule-Hovland gabbro complex, and is cut by roughly east-west trending diabase dikes inferred to also be related to the BruleHovland gabbro. Geophysical evidence implies that the latter dikes form a forked dike set which cuts the middle of the mapping area. Our mapping has shown that multiple east-west striking, southeast-dipping, lava flows composed of alternating rhyolite and andesite are present within the map area. The rhyolites are generally quartzand plagioclase-phyric but local aphyric varieties with large (5-10 cm) pumice inclusions were noted. Contorted flow banding and flow-aligned plagioclase laths in the rhyolites give evidence of viscous flow. The andesites are generally plagioclase phyric to glomeroporphyritic, with plagioclase phenocrysts as large as 1-5 cm. Some of the flows contain abundant pyroxene and ilmenite, and may be basaltic in composition, but no thin sections or geochemical analyses were obtained to verify this. The orientations of the lava flows in the andesites were determined by measuring the orientations of oxidation-lamination. Flow contacts were noted via the presence of amygdaloidal flow tops, and amygdules in both the rhyolites and andesites contain quartz, epidote, and chlorite, indicating that they experienced low grade contact metamorphism due to emplacement of the surrounding mafic intrusions. The volcanic rocks are cut by a series of mafic intrusions related in timing to the Beaver Bay Complex (~1096 Ma; Miller and Chandler, 1997). Based on field relationships the oldest intrusive unit is a medium-grained, variably porphyritic, slightly granophyric, poorly- to moderately-foliated anorthositic gabbro. This is intruded by medium-grained ophitic olivine gabbro, which is much more extensive throughout the map area than was recognized prior to this work. Hybrid ferrodioritic rocks are common along the margins of this ophitic gabbro, especially where in contact with felsic volcanic rocks. This hybrid unit contains abundant rhyolite inclusions and felsic stringers mixed with light to dark gray, finegrained chilled mafic phases. The shape and textures of these felsic stringers implies that they were formed from partial melting of the rhyolite inclusions, and that these melts commingled with mafic magma. Sparse inclusions of meter to outcrop scale hornfels basalt are also found in this unit. Two narrow and parallel, high-amplitude linear aeromagnetic anomalies, formerly interpreted as two parallel east-west trending diabase dikes, instead may be caused by the magnetic margins of a single, thick dike of ophitic olivine gabbro. However the ophitic gabbro also covers a large area that is characterized by lower amplitude magnetic anomalies; in order to resolve this further petrographic and rock property studies would need to be completed. In summary, although not all outcrops in the capstone field map area were examined due to time constraints, we have shown that there is much more geologic complexity than had been documented by previous reconnaissance mapping. This small map window should lay the groundwork for and encourage future mapping endeavors in this poorly mapped area. This and other capstone maps produced by the Precambrian Research Center can be viewed at www.d.umn.edu/prc. References: Miller, J.D. Jr., and Chandler, V.W., 1997 , in Ojakangas, R.W., Dickas, A.B., and Green, J.C., eds., Middle Proterozoic to Cambrian rifting, central North America: GSA Special Paper 312, p. 73-96. Miller, J.D., Jr., and Green, J.C. 2002, in Miller,, J.D., and others, 2002, MGS Rept. of Investigations 58, p. 144-163 Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.M., 2001, Minnesota Geological Survey Miscellaneous Map Series Map M-119, scale 1:200,000. Davidson, D.M., Jr., and Burnell, J.R., Jr., 1977, Minnesota Geological Survey Miscellaneous Map M-29, scale 1:24,000 Davis, D.W., and Green, J.C., Canadian Journal of Earth Sciences, v. 34, no.4, p.476-488. 48 �EVOLUTION OF THE MIDCONTINENT RIFT SYSTEM: PALEOMAGNETIC, ROCK MAGNETIC AND ANISOTROPY OF MAGNETIC SUSCEPTIBILITY INVESTIGATION OF THE MESOPROTEROZOIC BARAGA - MARQUETTE DIKE SWARM (MICHIGAN, USA) FOUCHER, Marine, CURGANUS, Renee, PIISPA Elisa J., SMIRNOV, Aleksey V. Department of Geological and Mining Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive, 630 DOW ESE Building, Houghton, MI 49931-1295, USA and PESONEN, Lauri J. P. Department of Physics, Division of Geophysics and Astronomy, University of Helsinki, Helsinki, Finland The Midcontinent Rift System (MRS) is characterized by multiple diabase dike swarms parallel to subparallel to the rift axis (e.g. Green et al. 1987). The dikes are generally considered to be feeders to now eroded lava flows once deposited on the flanks of the rift. We report the results of a detailed investigation of rock magnetism, paleomagnetism, and anisotropy of magnetic susceptibility (AMS) from 24 dikes of the east-west trending Baraga-Marquette (BM) dike swarm exposed in the Upper Peninsula of Michigan. In addition, preliminary rock magnetic and paleomagnetic data from five dikes of the Central Wisconsin (CW) dike swarm are presented. Thermomagnetic and magnetic hysteresis analyses indicate that the principal magnetic carrier in the studied dikes is single- to pseudo-single domain low-titanium titanomagnetite. Approximately a third of the dikes contain minor amounts of hematite. In addition, several dikes from highly mineralized areas exhibit an additional magnetic phase likely pyrrhotite or maghemite. Twelve of the investigated BM dikes yielded well-defined characteristic remanent magnetization (ChRM) directions similar to the typical directions observed from other reversely magnetized MRS rocks and the directions observed in a prior study of the BM swarm by Pesonen and Halls (1979). The new data from reversely magnetized dikes are combined with the prior study data and the combined dataset (20 dikes) is subjected to paleosecular variation analysis. The results are also compared with the paleomagnetic data obtained from other nearly coeval dike swarms of MRS. Three BM dikes yielded normal ChRM directions with steep inclinations, significantly different from the direction exhibited by other normally magnetized MRS sequences. The normal and reversed polarity dikes are also distinguishable with respect to their magnetic grain size. While the statistical significance of these observations requires further investigation, taken at face value it suggests that the BM swarm may represent at least two emplacement episodes with normally magnetized dikes being older. Three CW dikes yielded stable ChRM directions (two normal and one reversed) typical for the MRS time. The anisotropy of magnetic susceptibility analyses yielded well-constrained magma flow directions in most of the studied dikes. The magma flow directions of the BM dike swarm are discussed in the context of the tectonic evolution of MRS. REFERENCES Green, J.C., Bornhorst, T.J., Chandler, V.W., Mudrey, M.G. Jr., Myers, P.E., Pesonen, L.J., Wilband, J.T., 1987. Keweenawan dikes of the Lake Superior region: evidence for evolution of the middle Proterozoic Midcontinent Rift of North America. In: Halls, H.C., Fahrig, W.F. (Eds.), Geological Association of Canada, Special Paper 34, 289–302. Pesonen, L.J. and Halls, H.C., 1979. The paleomagnetism of Keweenawan dikes from Baraga and Marquette Counties, northern Michigan. Canadian Journal of Earth Sciences 16: 2,136-2,149. 49 �50 �Compilation of existing geophysical models in preparation for 3D modeling of the Midcontinent Rift System in the western Lake Superior region, Minnesota, Wisconsin, and Michigan GRAUCH, V.J.S.1, CHANDLER, Val2 and LIVELY, Richard S.2 1 U.S. Geological Survey, PO Box 25046, MS 964, Denver, CO, 80225 2 Minnesota Geological Survey, 2642 University Avenue W., St. Paul, MN, 55114-1032 Over the past several decades, both 2D and 3D geophysical models have played a large part in developing our understanding of the subsurface structure and composition of the Midcontinent Rift System (MRS). The overall configuration of the MRS is prominently expressed in regional gravity and magnetic maps. Details of the subsurface configuration of volcanic layers and structures are evident in high-resolution aeromagnetic data and published seismic-reflection sections. Improvements in the resolution and coverage of gravity and magnetic data and technical advances in modeling capabilities in the last decade provide the motivation for new attempts at 3D modeling of the MRS. In particular, the MRS has complex structure in the western Lake Superior region that is appropriate for 3D modeling. In this area, bounding faults of the NNE-trending St. Croix horst abruptly turn more easterly at White’s Ridge and transition into the ENE-striking Lake Owen and Keweenaw faults (Fig. 1). Gravity data west of White’s Ridge suggest that relations between the St. Croix Horst and the Duluth Complex are also 3D in nature (Fig. 2). Thus, we are developing a new 3D model of the MRS for the area surrounding White’s Ridge (Figs. 1 and 2). We start with the 3D gravity models of Allen (1994), who constrained his modeling using seismic-reflection lines (Fig. 1). Refinements are made by incorporating new 2D geophysical models and constraints from analysis of more recent geophysical and geologic data sets. We also plan to re-evaluate the gravity effects of the lower crust and mantle, as interpretations of new deep-looking geophysical data become publically available. As the model develops, the generalized rock units can be subdivided and additional structure added. The first step in the modeling process is to compile existing models and information into the 3D model space so that discrepancies or other issues can be easily recognized. Images captured from published 2D geophysical models, interpreted seismic-reflection sections, and geologic cross-sections (Fig. 1) were input as displays along section lines. The two 3D gravity models of the MRS constructed by Allen (1994) for western Lake Superior and the Minnesota-Wisconsin section (Figure 2) were recently combined by Chandler and Lively (2011) into a 3D visualization of surfaces, where each surface represents the base of a generalized rock package. Grid points from these surfaces were input into the 3D model space, then projected onto the section displays to quickly discover discrepancies between models. The discrepancies found are mostly explained by how rock units are generalized and what rock properties are assigned to which rock units. After consideration of updated rock property information, we chose the following model units and associated densities (in kg/m3) for the 3D modeling, which generally follows those of Allen (1994): Bayfield Group—2,450; Oronto Group—2,650; volcanic rocks—2,950; Duluth Complex—3,000; pre-rift upper crust (<20 km depth)—2,750; and lower crust (>20 km depth)—2,900. References Allen, D. A., 1994, An integrated geophysical investigation of the Midcontinent Rift System: western Lake Superior, Minnesota and Wisconsin [PhD]: Purdue University, 267 p. Cannon, W. F., Woodruff, L. G., Nicholson, S. W., and Hedgman, C. A., 1996, Bedrock geologic map of the Ashland and the northern part of the Ironwood 30' X 60' quadrangles, Wisconsin and Michigan: U.S. Geological Survey Miscellaneous Investigations Map I-2566, scale 1:100,000. Chandler, V. W., and Lively, R. S., 2011, Compilation of Minnesota and western Wisconsin geoscience for the USGS national geologic carbon dioxide sequestration assessment: Enhanced geophysical model for extent and thickness of deep sedimentary rocks: Minnesota Geological Survey Open-File Report 11-03, 37 p. Ferderer, R. J., 1982, Gravity and magnetic modeling of the southern half of the Duluth Complex, northeastern Minnesota [MA]: Indiana University, 86 p. 51 �Figure 1: Rock units of the Midcontinent rift in the western Lake Superior region, area covered by the new 3D model, and locations of previous 2D geophysical models and seismic lines. Geographic boundaries are shown by dashed lines. Figure 2: Color shaded-relief image of Bouguer gravity, showing the new 3D model area (bold black outline). Sections are as in Fig. 1. The white dashed lines outline the two 3D model areas of Allen (1994). 52 �A Field and Petrographic Study of Neoarchean Variolitic Pillow Lavas, Newton Belt, Vermilion District, Northeastern Minnesota GROTTE, M. J.1,2 and HUDAK, G. J.2,3     1 Department of Geological Sciences, University of Minnesota Duluth, grot0133@d.umn.edu Precambrian Research Center, Natural Resources Research Institute, University of Minnesota Duluth, 3 Minerals Division, Natural Resources Research Institute, University of Minnesota Duluth, Duluth, MN   2 The Newton Lake Formation, located northeast of Ely, Minnesota, comprises a series of tholeiitic to komatiitic lava flows, mafic to ultramafic intrusions, and associated clastic sedimentary rocks. Approximately one-half mile north of CR-88 on the Echo Trail, a sequence of exceptionally well-preserved, steeply dipping, south-topping, lower greenschist-facies metamorphosed variolitic pillow lavas are exposed on the northwest side of the road. This outcrop was recently visited during the 55th Annual Institute of Lake Superior Geology Field Trip 7 and was described as “spherulitic pillow basalt” (Peterson et al., 2009). Although mapped on a regional scale (Peterson et al., 2005), to date no detailed geological sketches or petrographic analyses of these variolitic pillowed flows have been completed for the purpose of understanding the genesis of these variolites. As varioles may be the result of blotchy alteration, magma mingling, or quench crystallization (Arndt and Fowler, 2004; Fowler et al., 2002), a detailed petrographic study was conducted to evaluate the genesis of the variolites that exist at this location. Scanning electron microscopy will be utilized to better understand the compositional characteristics of the variolites. A series of hand samples from this exposure of Newton Lake Formation pillow lavas was collected during fall, 2013. Samples were chosen at different distances from the crusts of individual pillows, as well as from areas where both pillow selveges and associated pillow hyaloclastite occurred. All sample locations were documented using a hand-held GPS unit in the UTM NAD 83 Zone 15 North coordinate system. A series of photographs was taken, and a panorama of these photos was constructed to assist in the mapping of this exceptional outcrop. Samples were prepared into standard and polished thin sections for analysis on a Leica DM EP polarizing microscope and by energy dispersive spectroscopy using a JEOL JSM-6490LV scanning electron microscope, both at the University of Minnesota Duluth. In outcrop, the pillow lavas vary from bun- to mattress-shaped (Dimroth et al., 1978) and range from <1 m to >2.5 m in diameter. A typical cross-section through these pillow lavas is shown in Figure 1. Pillow cores tend to be dark green to pale yellow-green in color depending on the degree of secondary epidote alteration present. Locally, quartz-filled vacuoles are present near the stratigraphic tops of individual pillows. Variolitic textures are most common in the border zone of individual pillows within 0.5 m of the crust of individual pillows, shown in Figure 2. Here, 5-15% individual rounded to spherical variolites up to 1 cm in diameter, as well as nearly massive, globular, coalescing variolites oriented sub-parallel to pillow margins are present. Petrographic studies indicate exceptional preservation of hyaloclastite adjacent to the crusts of individual pillows (Figure 3). The hyaloclastite comprises shard-shaped, jigsaw-puzzle fit lapilli composed of light brown to light green altered glass similar to that comprising the pillow crusts. Small (<5 mm) spherical plagioclase variolites are commonly present in the hyaloclastite shards. Variolites in the border region are zoned moving inward toward the pillow core, with an outer zone comprising of individual, small (2-5 mm), spherical to oval plagioclase spherulites with minor (<20%) fan-shaped to bow-tie shaped inclusions of altered mafic minerals. A secondary zone comprising larger (5-10 mm), round plagioclase spherulites in a matrix composed of fine-grained, chaotically-oriented skeletal amphibole that may be pseudomorphs of original pyroxene. A third zone composed of semi-massive to massive, globular, coalescing spherical plagioclase spherulites that contain 30-55% fan-shaped to bow-tie inclusions of mafic minerals, and a fourth zone containing <10% rounded plagioclase spherulites up to 15 mm in 53 �diameter in a matrix of coarser grained, chaotically oriented skeletal amphibole. The cores of the pillow lavas are composed of fine-grained tabular to acicular plagioclase intergrown with finegrained tabular to prismatic amphibole pseudomorphs of pyroxene. Scanning electron microscopy studies are currently in progress to evaluate compositional differences between the variolites and groundmass minerals in the various variolite zones and pillow cores. Based on the results of this study, variolites in this exceptional exposure of Newton Lake Formation pillow lavas are dominantly composed of rounded to oval, radiating plagioclase spherulites with rare, axiolitic plagioclase spherulites locally present. The presence of needle-like to acicular skeletal plagioclase crystals and absence of phenocrysts suggest that lavas responsible for the pillow lava flows at this location were erupted at temperatures above the liquidus and experienced relatively large degrees of undercooling before undergoing rapid crystallization on the Neoarchean seafloor. Figure 1. Typical pillow lava cross-section. Figure 3. Thin section of pillow margin containing small varioles and well preserved hyaloclastite. Figure 2. Outcrop photo of pillow margin, varioles transitioning into hyaloclastite (from right to left). References Arndt, N. & Fowler, A. D., 2004, Textures in Komatiites and Variolitic Basalts. The Precambrian Earth - Tempos and Events: Elsevier, p. 298-311. Dimroth, E., Cousineau, P., Leduc, M., and Sanschagrin, Y., 1978, Structure and organization of Archean subaqueous basalt flows, Rouyn-Noranda area, Quebec, Canada: Canadian Journal of Earth Sciences, v. 15, p. 902-918. Fowler, A. D., Berger, B., Shore, M., Jones, M. I., and Ropchan, J., 2002, Supercooled rocks: development and significance of varioles, spherulites, dendrites, and spinifex in Archean volcanic rocks, Abitibi Greenstone Belt, Canada: Precambrian Research, v. 115, p. 311-328. Peterson, D. M., Jirsa M. A., and Hudak, G. J., 2009, Field Trip 7 - Architecture of an Archean Greenstone Belt: Stratigraphy, Structure, and Mineralization: Institute on Lake Superior Geology, Proceedings Volume 55 Part 2 – Field Trip Guidebook, p. 178-215. 54 �STRATIGRAPHIC FRAMEWORK AND LANDSYSTEM CORRELATION FOR DEPOSITS OF THE SAGINAW LOBE, MICHIGAN, USA GUZMAN, Ivan R., Department of Geosciences, Western Michigan University, Kalamazoo, MI 49008 Since the time of the Last Glacial Maximum (LGM) the south-central portion of the Lower Michigan Peninsula has been subject to several glacial advances and retreats by the Saginaw lobe. As part of the U.S Geological Survey Great Lakes Geological Mapping Coalition projects, several rotasonic borings were drilled between 2006 and 2013 in Barry, Kalamazoo and Calhoun Counties. Gamma ray logs and textural analyses were completed for each core. Five of these borings were selected according to their diamicton (till) content and correlated using water well logs and surficial geology maps. Glacial deposits such as diamicton serve as evidence of glacial advance/retreat, and are usually present as nearly continuous layers of sediments. Analysis of these layers affords the ability to accurately correlate these types of sediments across an area. Three cores, BA-10-02 and BA-09-02, KA-12-02 were drilled along the Kalamazoo moraine, each one containing 1 to 3 diamicton units separated by lacustrine sediments. The last two cores, CA-11-01 and KA-13-01 were drilled on a drumlinized till plain; both contain 2 to 4 diamicton units separated by outwash sediments. These diamicton units indicate the presence of at least one major and two minor advances/retreats of the Saginaw Lobe. 55 �56 �RETHINKING THE MIDCONTINENT RIFT – PUNCTURING THE “PLUME PARADIGM” HOLLINGS, Peter1, and HEGGIE, Geoff2 1 Department of Geology, Lakehead University, 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada. peter.hollings@lakeheadu.ca 2 Panoramic Resources Ltd., 1004 Alloy Drive, Thunder Bay, ON P7B 6A5 Canada The mantle plume origin for the Midcontinent Rift (MCR) is widely accepted in the literature (e.g., Hutchinson et al., 1990; Nicholson and Shirey, 1990; Nicholson et al., 1997). However, recent geochronological, geochemical and mineralogical data suggest that the simple plume model should no longer be applied and it is necessary to evaluate alternate models. So, why do we need to rethink the rift? Geochronology – The majority of plume-related Large Igneous Provinces (LIPs) are characterized by a short-duration magmatic pulse or pulses (less than 1–5 My; Ernst et al., 2013a). Recent geochronology has shown that MCR magmatism spans at least 20 million years (Heaman et al., 2007, Hollings et al., 2010 and Dunlop, 2013) and possibly as much as 60 million years. Ultramafic rocks – One argument often put forward in favour of a plume origin for the rift is the presence of ultramafic rocks. Although there are, indeed, ultramafic rocks in the MCR, all of which are hosted in intrusions, mineral chemistry analyses from these intrusions shows that they have maximum olivine forsterite compositions not in equilibrium with the mantle: Seagull (Fo85), Thunder Bay North (Fo82), Eagle (Fo85; Ding et al., 2010) and Tamarack (Fo88; Goldner, 2011). Numerical models of these olivine compositions suggest a parental magma with 8-10 wt% MgO. Even the most primitive Tamarack intrusion suggests a primary magma with a composition of 12wt% MgO and 11wt% FeO (Goldner, 2011). Dyke swarms – The majority of plume-related LIPs are associated with, or even recognized by, the presence of giant, radiating dike swarms up to 3000 km long which project for long distances into cratonic hinterlands and provide evidence for paleo-stress regimes consistent with a central piercing point, likely a plume (Ernst et al., 2013a). To date, no radiating dike swarm has been recognized nor associated with the MCR. Rather, the majority of MCR-related dikes occupy extensional, rift arm-parallel structures (e.g. Hollings et al., 2010). Hanson et al. (1998, 2004, 2006) recognized ~1100 Ma magmatism in the Kalahari Craton, termed the Umkondo event. Ernst et al. (2013b) have recently proposed that this magmatic event may be considerably more widespread, with the recognition of ~1100 Ma magmatism in the Congo, the Amazon and India. They proposed that paleogeographic reconstructions are permissive of these events representing a single LIP that, based on paleomagnetic reconstructions, was distinct from Keweenawan magmatism (Ernst et al., 2013b). The presence of multiple, broadly contemporaneous LIP events suggests that the Mesoproterozoic may have been a period of significant mantle overturn (Stein and Hofmann, 1994) and atypically increased magmatic activity. 57 �The long duration of MCR magmatism, absence of primary ultramafic magmas and lack of a radiating dike swarm all suggest that a passive rifting model may be more appropriate for the rift. According to this model, rifting of the Superior Craton, possibly a response to the Umkondo LIP event, allowed for upwelling of material underplated by earlier plume events thought to have been centered in the vicinity of the present-day Lake Superior (e.g. the Marathon LIP, Halls et al., 2008). References Ding, X., Li, C., Ripley, E.M., 2010. The Eagle and East Eagle sulfide ore-bearing mafic-ultramafic intrusions in the Midcontinent Rift System, upper Michigan: Geochronology and petrologic evolution. G3 Geochemistry Geophysics Geosystems, 11, 1-22. Dunlop, M., 2013. The Eagle Ni-Cu-PGE Magmatic Sulfide Deposit and Surrounding Mafic Dikes and Intrusions in the Baraga Basin, Upper Michigan: Relationships, Petrogenesis, and Implications for Magmatic Sulfide Exploration. Unpublished MSc thesis, Indiana University, 105p. Ernst, R., Bleeker, W, Soderlund, U. and Kerr, A., 2013a. Large Igneous Provinces and supercontinents: Toward completing the plate tectonic revolution. Lithos, 174, 1-14. Ernst, R., Pereirac, E., Hamilton, M., Pisarevsky, S., Rodriques, J., Tassinari, C., Teixeirah, W., and VanDunemi, V., 2013b. Mesoproterozoic intraplate magmatic ‘barcode’ record of the Angola portion of the Congo Craton: Newly dated magmatic events at 1505 and 1110 Ma and implications for Nuna (Columbia) supercontinent reconstructions. Precambrian Research, 230, 103-118. Goldner, B.D., 2011. Igneous Petrology of the Ni-Cu-PGE-Mineralized Tamarack Intrusion, Aikin and Carlton Counties, Minnesota. Unpublished MSc thesis, University of Minnesota, 155p. Halls, H.C., Davis, D.W., Stott, G.M., Ernst, R.E., 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 margin of the North American Superior Province. Precambrian Research 162, 327–353. Hanson, R.E., 2003. Proterozoic geochronology and tectonic evolution of southern Africa. In: Yoshida, M., Windley, B., Dasgupta, S. (Eds.), Proterozoic East Gondwana: Supercontinent Assembly and Breakup, vol. 206. Geological Society of London, Spec. Publ, pp. 428–463. Hanson, R.E., Crowley, J.L., Bowring, S.A., Ramezani, J., Gose, W.A., Dalziel, I.W.D., Pancake, J.A., Seidel, E.K., Blenkinsop, T.G., Mukwakwami, J., 2004. Coeval large-scale magmatism in the Kalahari and Laurentian cratons during Rodinia assembly. Science 304, 1126–1129. Hanson, R.E., Harmer, R.E., Blenkinsop, T.G., Buller, D.S., Dalziel, I.W.D., Gose, W.A., Hall, R.P., Kampunzu, A.B., Key, R.M., Mukwakwami, J., Munyanyiwa, H., Pancake, J.A., Seidel, E.K., Ward, E.K., 2006. Mesoproterozoic intraplate magmatism in the Kalahari craton: a review. Journal of African Earth Sciences 46, 141–167. 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. Hutchinson, D.R., White, R.W., Cannon, W.F., Schulz, K.J., 1990. Keweenaw hot spot: geophysical evidence for a 1.1 Ga mantle plume beneath the Midcontinent Rift System. Journal of Geophysical Research 95, 10,869–10,884. Nicholson, S.W., 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 95 (10), 10851– 10868. Nicholson, S.W., Shirey, S., Schulz, K., Green, J., 1997. Rift-wide correlation of 1.1 Ga Midcontinent rift system basalts: implications for multiple mantle sources during rift development. Canadian Journal of Earth Sciences 34, 504–520. Stein, M. and Hofmann, A.W., 1994. Mantle plumes and episodic crustal growth. Nature. 372. 63–68. 58 �THE MINNESOTA TACONITE WORKERS HEALTH STUDY: ENVIRONMENTAL STUDY OF AIRBORNE PARTICULATE MATTER 2014 UPDATE HUDAK, George1, MONSON GEERTS, Stephen1, ZANKO, Larry1, POST, Sara1, BANDLI, Bryan2 1 2 Natural Resources Research Institute, 5013 Miller Trunk Highway, Duluth, MN, 55811 Department of Geological Sciences, University of Minnesota Duluth, 229 Heller Hall, 1114 Kirby Drive, Duluth, MN 55812 The Natural Resources Research Institute (NRRI) continues to conduct a detailed characterization of mineral dust in northeastern Minnesota. The purpose of this research is to evaluate the effects of present emissions from taconite mining and processing on air quality throughout the Mesabi Iron Range (MIR) (Figure 1) by characterizing airborne mineral particulate matter within currently operating taconite processing plants, in MIR communities surrounding taconite mining/processing operations, and in population centers in Minnesota not associated with taconite mining. Characterization studies of agedated lake sediments are also being conducted to determine the composition of past particulate matter deposition. NRRI’s sampling and characterization work represents the community/environmental component of the Minnesota Taconite Workers Health Study, a broad University of Minnesota (UM) research effort involving both the NRRI and the School of Public Health. Figure 1. Locations of taconite processing plants on the Mesabi Iron Range being sampled during this study (after Oreskovich and Patelke, 2006) Air sampling was performed within taconite operations, MIR communities, and non-MIR communities by NRRI scientists during both winter and summer seasons from 2009-2012. Sampling was conducted at four process locations within taconite operations, including: 1) secondary crushers; 2) magnetic separators/concentrators; 3) agglomerators/ball drums; and 4) kiln/pellet discharge areas. Sampling within the MIR communities took place on centrally-located rooftops of public buildings, whereas sampling in non-MIR communities occurred on either rooftops, or in remote sampling locations, 59 �so that background air quality away from the MIR could be evaluated. Airborne particles were collected using: 1) a micro orifice uniform deposit impactor (MOUDI) (Marple et al., 1991, 2014), which enables size-fractionated particulate matter collection; and 2) a Total Filter Sampler (TFS). Particulate matter was evaluated via gravimetric analysis and was subsequently subjected to comprehensive particulate matter characterization that included: 1) scanning electron microscopy (SEM) imaging; 2) energy dispersive xray spectroscopy (EDS); 3) electron backscattered diffraction (EBSD); 4) proton induced x-ray emission (PIXE); 5) the Minnesota Department of Health’s 852 Method Transmission Electron Microscopy (TEM) Analysis for Mineral Fibers in Air; and 6) the International Standardization Organization’s Indirect Method 13794 for Ambient air – Determination of Asbestos Fibers. NRRI’s research methods do not produce exposure data, and are not meant to provide data for regulatory purposes. During the past year, the NRRI has been evaluating the physical (gravimetric, morphology, concentration), mineralogical, and chemical characteristics of the particulate matter obtained from sampling at the taconite operations and MIR/non-MIR communities. This includes analysis of samples obtained during 14 sampling events at taconite operations and 79 sampling events at locations within communities and sites in northeastern Minnesota (73) and Minneapolis (6). Lake sediment analysis continues, and will provide important historical data regarding potential mineralogical inputs from iron mining and processing from ~1840 (which pre-dates iron mining on the MIR) to the present, which includes the period where the transition from natural ore mining to taconite mining took place. Community results to date are as follows: • measured particulate matter concentrations for PM2.5 in all MIR communities have been below 12 µg/m3, and for total PM have been below 16µg/m3; • particulate matter concentrations on the MIR are similar to those in the two NE Minnesota background sites (Duluth NRRI, Ely Fernberg site), and are lower than those obtained in Minneapolis (UM Mechanical Engineering Building rooftop); • mineral particulate matter in community air samples reflects the mineralogy of the Biwabik Iron Formation and other Minnesota rock types and geological materials; • elongate mineral particles (EMP) are present in MIR community ambient air samples; however, asbestiform amphiboles were rarely observed (1 asbestiform amphibole EMP in ~22,800m3 of air). In-plant results to date are as follows: • plant environments can be very dusty, with the most dusty environments associated with the agglomerator and kiln discharge areas; • particulate levels (PM1, PM2.5, PM10, and total PM) show a slight increase in the five MIR communities during plant/mine activity, but this increase is not statistically significant compared to when the plants were not operating. The NRRI plans to complete this work in 2014. References ISO 13794 (1999), Ambient air — Determination of asbestos fibres — Indirect-transfer transmission electron microscopy method. Marple, V. A., Rubow, K. L., and Behm, S. M., 1991, A micro orifice uniform deposit impactor (MOUDI): description, calibration, and use: Aerosol Science and Technology, v. 14, p. 434-446. Marple, V., Olson, B., Romay, F., Hudak, G., Monson Geerts, S., and Lundgren, D., 2014, Second Generation Micro-Orifice Uniform Deposit Impactor, 120 MOUDI-II: Design, Evaluation, and Application to LongTerm Ambient Sampling: Aerosol Science and Technology, v. 48-4, p. 427-433. MDH. Method 852 (1999) T.E.M. analysis for mineral fibers in air – 852. Minnesota Department of Health, Microparticulate Unit, St. Paul, MN. 42 pp. Oreskovich, J. A., and Patelke, M. M., 2006, Historical use of taconite byproducts as construction aggregate materials in Minnesota: A Progress Report: Natural Resources Research Institute Report of Investigation NRRI-RI-2006-02, 10 p. 60 �ROCK MAGNETISM AND PALEOMAGNETISM OF THE ~1144 MA LAMPROPHYRE DYKES, THE EASTERN LAKE SUPERIOR REGION, ONTARIO, CANADA JACOBSON, Darcy, M. Department of Physics, Michigan Technological University, Houghton, MI, PIISPA, Elisa J., SMIRNOV, Aleksey V. Department of Geological and Mining Engineering and Sciences, Michigan Technological University, Houghton, MI, and PESONEN, Lauri J. P., Department of Physics, Division of Geophysics and Astronomy, University of Helsinki, Helsinki, Finland Despite several decades of intensive research, the origin of the formation of the ~1.1 Ga Midcontinent Rift system (MRS) remains an open question. The proposed hypotheses vary from active rifting due to a mantle plume or plumes (e.g. Nicholson et al., 1997; Hollings et al., 2010), to passive rifting related to the Grenville Orogeny (Gordon and Hempton, 1986), or separation of the Amazon craton from Laurentia (Stein et al., 2014). Abundant ~1144 Ma lamprophyre dykes in the Eastern Lake Superior region (Ontario, Canada) (Queen et al., 1996) are coeval with the ~1141 Ma Abitibi diabase dyke swarm (Krogh et al. 1987) in the same area. In addition, both dyke suites share similar alcalic composition and appear to fan out from a locus in the present-day Lake Superior. These observations hint that the lamprophyre dykes and Abitibi dykes may form a single radiating dyke swarm representing the earliest magmatic stage of MRS. The existence of such a swarm is consistent with the arrival of a mantle plume, hence supporting the active rifting hypothesis. In order to test this hypothesis, we sampled 173 independently oriented samples from 22 lamprophyre dykes. In addition, at three sites, samples for the baked contact test were collected from the presumably baked and unbaked host rocks. The dependence of low-field magnetic susceptibility versus temperature indicates low titanium titanomagnetite as the dominant magnetic mineral. Subordinate hematite was observed in several samples. Magnetic hysteresis measurements reveal single to pseudo-single domain behavior in most dykes except for four dykes that show multidomain behavior. Both alternating field (AF) and thermal demagnetization were used to determine the paleomagnetic directions. In general, the AF demagnetization technique proved to be more effective in revealing the characteristic paleomagnetic directions. For several dykes, temperature treatment resulted in unstable demagnetization behavior due to alteration. Preliminary measurements of the anisotropy of magnetic susceptibility were also conducted on selected dykes in order to test whether the flow directions are consistent with the potential plume center. The paleomagnetic results of this study will be compared with the results obtained from the Abitibi dykes by Ernst and Buchan (1993) and the possible implications related to the formation of the MRS will be discussed. References Ernst, R.E., and Buchan, K.L. 1993. Paleomagnetism of the Abitibi dyke swarm, southern Superior Province, and implications for the Logan Loop. Canadian Journal of Earth Sciences, 30: 1886- 1897. Gordon, M. B., and M. R. Hempton (1986), Collision-induced rifting: The Grenville Orogeny and the Keweenawan Rift of North America, Tectonophysics, 127(1–2), 1–25, doi:10.1016/00401951(86)90076-4. Hollings, P., Smyk, M., Heaman, L.M., and Halls, H. 2010. The geochemistry, geochronology, and paleomagnetism of dikes and sills associated with the Mesoproterozoic Midcontinent Rift near Thunder Bay, Ontario, Canada. Precambrian Research. Precambrian Research, v. 183, iss. 3, p.553-571. 61 �Krogh, T.E., Corfu, F., Davis, D.W., Dunning, G.R., Heaman, L.M., Kamo, S.L., Machado, N., Greenough, J.D., and Nakamura, N.1987. Precise U–Pb isotopic ages of diabase dykes and mafic to ultramafic rocks using trace amounts of baddeleyite and zircon; in Mafic Dyke Swarms, (ed.) H.C. Halls and W.F. Fahrig; Geological Association of Canada, Special Paper 34, p. 147–152. Nicholson, S. W., S. B. Shirey, K. J. Schulz, and J. C. Green (1997), Rift-wide correlation of 1.1 Ga Midcontinent rift system basalts: Implications for multiple mantle sources during rift development, Can. J. Earth Sci., 34(4), 504–520. Queen, M., Heaman, L.M., Hanes, J.A., Archibald, D.A., and Farrar, E. 1996. 40Ar/39Ar phlogopite and U–Pb perovskite dating of lamprophyre dikes from the eastern Lake Superior region: evidence for a 1.14 Ga magmatic precursor to Midcontinent Rift volcanism. Canadian Journal of Earth Sciences 33, 958–965. Stein, C. A., S. Stein, M. Merino, G. Randy Keller, L. M. Flesch, and D. M. Jurdy (2014), Was the Midcontinent Rift part of a successful seafloor-spreading episode?, Geophys. Res. Lett., 41, 1465–1470, doi:10.1002/2013GL059176. 62 �AN EXPLORATION UPDATE AND MINERALOGICAL STUDY OF THE EMILY-DISTRICT MANGANESE DEPOSIT, CUYUNA IRON RANGE, MINNESOTA 1 JOHNSON, Leif A. and 2DUNN, Brad M. Barr Engineering Company, 4700 W 77th St. Minneapolis, MN 55435 1 ljohnson@barr.com, 2bdunn@barr.com The Cuyuna Range in east-central Minnesota produced in excess of 100 million tons of manganiferous iron ore from initial discovery in 1904 to the final mine closing in 1984. The presence of higher percentages of manganese (greater than 10 percent) is the main component that distinguishes the Cuyuna Range from other Early Proterozoic iron mining districts in the Lake Superior Region. Similar to the Mesabi Iron Range, The Cuyuna Range has been documented as part of the Animikie Group, containing the Virginia Formation, "Unit A" iron formation (similar to the Biwabik Iron-Formation), and the Pokegma Quartzite (Morey and Southwick, 1993) Numerous works have studied the manganese deposits of the Emily-District. Summaries of the regional structural setting by Southwick et al. (1988) and Morey et al. (1981) show that deformation within the Emily-district is associated the Penokean Orogeny. Regionally, rocks within the Emily district form a broad synclinorium that plunges to the east. Morey and Southwick (1993) presented an in-depth summary of the stratigraphic and sedimentology characteristics that show possible geologic controls of manganese distribution. Dahl et. al. (1994) characterized the mineralogy for the purposes of utilizing insitu mining techniques using two holes drilled in 1990. Other studies have utilized historic drill core obtained from the Minnesota Department of Natural Resources core library in Hibbing. Cooperative Mineral Resources (CMR) controls 80 acres within the Emily-District. Historic resource estimates for this district showed one to two million tons of manganese resource. On their controlled property, thirteen historic drill holes were drilled, which indicated a sizeable manganese resource. In 2011 and 2012, CMR commissioned an expanded exploration drilling program with seven additional holes. This drilling confirmed the historic resource, which is divided into upper and lower manganese-rich zones. Assays from these modern drill holes showed stratigraphically continuous manganese grades of 15 to 20 percent. Further work has continued on the CMR Emily deposit. In 2013, metallurgical studies included a mineral liberation analyses (MLA). The MLA report analyzed fourteen core samples from the 2011 and 2012 drill holes using an automated scanning electron microscope equipped with energy dispersive detectors (SEM-EDS) and MLA software. The MLA technique analyzes X-ray spectrometry (XRF) from individual grains, assigns an elemental composition based on the geometric center of each grain, and assigns the most likely mineralogy based on a database of XRF. The results of the MLA confirmed the mineralogy described by past studies on the Emily-District. Predominant iron mineralogy was assumed to be hematite. Manganese mineralogy occurred in several manganese oxide (MnO) phases. Most notably, MnO was assumed to be the composition of manganite. Several manganese-iron (FeO-Mn) phases were found, which were not assigned a specific mineralogy. Crpytomelane (K12Mn8O16), hollandite (BaMn8O16), stilpnomelane (K(Fe,Mg)8(Si,Al)12(O,OH)27), and calcite_Mn ((Ca,Mn)CO3) were secondary manganese minerals. Quartz is the principal gangue mineral. Past metallurgical work on the manganese resource has shown that primary and secondary grinding, associated with flotation is insufficient in separating individual manganese-oxide from iron-oxide grains 63 �and quartz. The MLA report showed the mineral liberation at various grind meshes. These results will be further refined to develop an alternative processing technique for the manganese resource. References Dahl, L.J., Brink, S.E., Blake, R.L, Tuzinksi, P.A. and Adamson, N.R., 1994. Site characterization of Minnesota Manganese deposits for determining in situ mining potential: Society For Mining, Metallurgy, and Exploration Inc.: Transactions Volume 294, p. 1892-1905. Morey, G. B., Olsen, B. M, and Southwick, D. L., 1981, Geologic Map of Minnesota, east-central Minnesota, bedrock geology: Minneapolis, Minnesota Geological Survey, scale 1:250,000. Morey, G. B and Southwick, D. L., 1993. Stratigraphic and Sedimentological Factors Controlling the Distribution of Epigenetic Manganese Deposits in Iron-Formation of the Emily District, Cuyuna Iron Range, East-Central Minnesota: Economic Geology, v. 88, p. 104-122. Southwick, D. L., Morey, G. B., and McSwiggen, P. L., 1988. Geologic Map (scale 1:250,000) of the Penokean orogeny, central and eastern Minnesota, and accompanying text: Minnesota Geological Survey, Report, Investigation 37, 25p. 64 �SEDIMENTOLOGY AND PALEOGEOGRAPHIC RECONSTRUCTION OF THE LAYERS IN AND ADJACENT TO THE SUDBURY IMPACT LAYER IN THE LAKE SUPERIOR BASIN KARMAN, Monica M.1 and FRALICK, Philip W.1 1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1, mmkarman@lakeheadu.ca, pfralick@lakeheadu.ca Various locations around the area of the Lake Superior Basin reveal stratified layers of the 1878.3±1.3 Ma (Fralick et al., 2002) Gunflint Formation, the 1850 Ma (Krogh et al., 1984) Sudbury Impact Layer, and the overlying 1832±3 Ma (Addison et al., 2005) Rove Formation. Samples were collected and tested, and stratigraphic logs were drawn from these locations to determine the sedimentology of the lithologic units in an attempt to reconstruct the paleogeographic setting at the time of deposition, along with diagenesis. Focus is given to two units, one above and one below the Sudbury Impact Layer (S.I.L), which display time gaps in the stratigraphic record. The units below and above the S.I.L. display a ~28 Ma, and an ~18 Ma year time gap respectively (Addison et al., 2010), indicating a period of subaerial exposure and ultimately, erosion; during this time the Rove Sea had regressed back to the southern edge of the continent. Samples collected below the S.I.L. near the Terry Fox monument and the Harbour Expressway in Thunder Bay, ON, indicate that subaerial exposure was present in this unit. The Terry Fox site displays stalactite-like structures (Figure 1A) composed of silica, indicating growth above the water table. The Harbour Expressway site reveals the same stalactite-like features, except that only a very small, eroded outcrop was found, showing plan-view stalactites termed by the author as silica flowerettes (Figures 1B, 1C). In addition, the Harbour Expressway site reveals botryoidal gypsum-like rosettes (Figure 2), indicative of an arid environment. Although these rosettes were most likely comprised of gypsum at one point in time, XRD and geochemical analysis show that this feature has been overprinted by calcite. Samples collected from the S.I.L. indicate that subaerial exposure affected this ejecta unit, made evident by silica invasion, carbonate crystal and cement growth or replacement. Ejecta features such as sphere-in-sphere structures (Figure 3A), and vesicular glass bubbles (Figure 3B) have been infilled, overprinted/replaced, and/or broken because of subaerial exposure. Samples collected above the S.I.L. display formation in subaerial conditions exemplified by a unit resembling gypsum laths sampled from drill core BDQ (Figure 4), collected near Hwy 588, Thunder Bay, ON. As with the gypsum-like rosettes found near the Harbour Expressway site, these gypsum laths had to have precipitated in an arid, subaerial environment. Another sample taken from drill core BDQ displays a chicken-wire texture of zoned carbonate crystals (Figure 5). SEM-EDX analysis of the crystals displays Mg and Fe zonation, indicating that formation occurred in a sabhka environment with access to meteoric water, rather than being covered by the Rove Sea. Although these zoned crystals were formed through the percolation of meteoric waters, the Rove Sea seems to have abruptly transgressed into the Animikie Basin, as seen by the silt and shale unit situated directly on top of the carbonate unit, seemingly choking it out. 65 �4 1A 1B 1C 2 3A 3B 5 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, n 3, p.193-196. Addison, W.D., Brumpton, G.R., Davis, D.W., Fralick, P.W., and Kissin, S.A., 2010, Debrisites from the Sudbury impact event in Ontario, north of Lake Superior, and a new age constraint: Are they base-surge deposits or tsunami deposits?, in Gibson, R.L., and Reimold, W.U., eds., Large Meteor Impacts and Planetary Evolution IV: Geological Society of America Special Paper 465, p. 245-268. Fralick, P.W., Davis, D.W., and Kissin, S.A., 2002. The age of the Gunflint Formation Ontario, Canada: single zircon U-Pb age determinations from reworked volcanic ash. Canadian Journal of Earth Sciences, v. 39, p. 1085-1091. Krogh, T.E., Davis, D.W., Corfu, F., 1984. Precise U-Pb zircon and baddeleyite ages for the Sudbury area. In, E.G. Pye ed., The Geology and Ore Deposits of Sudbury Structure. Ontario Geological Survey, Special Volume 1, p. 431-446. 66 �IMPACT EJECTA FEATURES IN THE LAKE SUPERIOR BASIN FROM THE 1850 MA SUDBURY IMPACT EVENT KARMAN, Monica M.1 and FRALICK, Philip W.1 1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1, Canada mmkarman@lakeheadu.ca, pfralick@lakeheadu.ca Between the 1878.3±1.3 Ma (Fralick et al., 2002) Gunflint Formation, and the overlying 1832±3 Ma (Addison et al., 2005) Rove Formation, unconformably lies the 1850 Ma (Krogh et al., 1984) impact ejecta unit from the Sudbury Impact Event. These distal ejecta sites that have been discovered in the Lake Superior basin, extend approximately 600-800 kilometers, or ~5-7 crater radii (Spray et al. 2004), from the Sudbury impact crater. The Sudbury Impact Layer (S.I.L.) is composed of two constituents: 1) A chaotic debrisite portion that includes clasts and rip-ups of carbonate grainstone, along with blocks of chert and stromatolite that have been sheared from the underlying Gunflint Formation; 2) The ejected material from the impact event (Addison et al., 2010). Ejecta features of the S.I.L. unit include devitrified glass (Figures 1A, 1B), spherules (Figures 2A, 2B, 2C), planar features (Figure 3), and lapilli (Figure 4). During deposition of the 1850 Ma Sudbury Impact Layer, it is assumed that deposition took place in a subaerial environment (Fralick and Burton, 2008). Samples taken from two locations in the field directly below and above the S.I.L. unit display what seems to be gypsum rosettes (Figures 5A, 5B) indicating an arid environment (Karman and Fralick, 2014). Because of subaerial exposure, many ejecta features have been affected by carbonate replacement/alteration and silicification, either overprinting, or destroying them. Figures: Devitrified vesicular impact glass (DVIG); 1A: Oval DVIG with carbonate-filled vesicles (plane polarized light HEW site), 1B: Spherical DVIG with carbonate-filled void (plane polarized light BC site); 2 - Spherules replaced by chalcedony; 2A: Spherule cluster with irregular intricate banding (crossed polarized light HCP site), 2B: “LP record” spherules (plane polarized light BM site), 2C: Fibrous radial rimmed spherules with carbonate-filled previously hollow centers (crossed polarized light HEW site); 3 - Planar Deformation Feature (PDF) (crossed polarized light JN34 slide); Two sets of PDFs in quartz grain decorated with inclusions; 4 - Lapilli; Accretionary lapilli ~2.5cm width (588 site); 5 Replaced gypsum rosettes; 5A: Cross section of radially banded gypsum rosette replaced by carbonate directly under ejecta (HCP site), 5B: Cluster of botryoidal gypsum rosettes replaced by calcite growing in mat-like form overlying the S.I.L. (HEW site). 1A 1B 67 �2A 2B 2C 3 5B 5A 4 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, n 3, p.193-196. Addison, W.D., Brumpton, G.R., Davis, D.W., Fralick, P.W., and Kissin, S.A., 2010, Debrisites from the Sudbury impact event in Ontario, north of Lake Superior, and a new age constraint: Are they base-surge deposits or tsunami deposits?, in Gibson, R.L., and Reimold, W.U., eds., Large Meteor Impacts and Planetary Evolution IV: Geological Society of America Special Paper 465, p. 245-268. Fralick, P.W., Davis, D.W., and Kissin, S.A., 2002. The age of the Gunflint Formation Ontario, Canada: single zircon U-Pb age determinations from reworked volcanic ash. Canadian Journal of Earth Sciences, v. 39, p. 1085-1091. Fralick, P. W., and Burton, J., 2008, Geochemistry of the Paleoproterozoic Gunflint Formation carbonate: Implications for early hydrosphere-atmosphere evolution: Geochimica et Cosmochimica Acta, special supplement, v. 72, no. 125, p. A280. Karman, M. M., and Fralick, P. W., 2014, Sedimentology and paleogeographic reconstruction of the layers in and adjacent to the Sudbury Impact Layer in the Lake Superior Basin, M.Sc. Thesis (in progress), Lakehead University. Krogh, T.E., Davis, D.W., Corfu, F., 1984. Precise U-Pb zircon and baddeleyite ages for the Sudbury area. In, E.G. Pye ed., The Geology and Ore Deposits of Sudbury Structure. Ontario Geological Survey, Special Volume 1, p. 431-446. 68 �PDFS IN SUDBURY EJECTA IN THE GUNFLINT FORMATION, ONTARIO: A COMPARISON OF METHODS KISSIN, Stephen A. and BRUMPTON, Gregory R. Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1 Canada Planar deformation features (PDFs) in quartz are considered to be definitive evidence, along with other features, of large terrestrial impact (French 1998). The ejecta from the 1850 Ma Sudbury impact structure has been described in the Animike Group and correlative units in the western Lake Superior area in Michigan, Minnesota and Ontario (Cannon and Addison 2007). PDFs are known to develop in response to shock pressures greater than those developed by terrestrial events (e.g. volcanic eruptions) and form on rational planes in the quartz crystal (von Engelhardt and Bertsch 1969). Indexed planes of PDFs from the Baraga Group and correlative units from Michigan have been reported by Pufahl et al. (2007) and Cannon et al. (2010); however, although PDFs in Gunflint Formation of Ontario have been described (Addison et al. 2005), they have not been indexed. In this study, we have indexed 22 PDFs in 11 thin sections of ejecta from drill holes BP99-2 and MC95-1, which have been described by Addison et al. (2005). The conventional method for indexing of PDFs, as described by von Engelhardt and Bertsch (1969), utilizes the universal stage to determine the pole of a PDF and the orientation of the c-axis of the quartz grain. Based on the polar angle between the c-axis and the pole of the PDF, the results are then compared to a template of the projection of known indices of PDFs in a stereographic projection. The standard template devised by von Engelhardt and Bertsch (1969) allows a margin of error of 5º in the stereographic projection. The conventional method described above suffers from some imprecision in the plotting and manipulations of measurements on a Wulff net, which must be done manually. In order to overcome errors of this sort, Huber et al. (2011) created the Automated Numerical Index Executor program (ANIE), which is based on angular calculations from spherical trigonometry. They demonstrated the apparent superiority of use of the program over manual methods in indexing of PDFs. In our study, we have compared the application of the conventional method with that of the ANIE program, as well as presenting results from the Gunflint Formation, which lies approximately 200 km west of the Baraga Group and hence, more distally from the Sudbury structure. Quartz grains examined in this study in most cases contained only one orientation of PDF in the thin section examined. An example of an exceptional grain with a high density of PDFs in two orientations is shown in the figure. The Miller-Bravais indices of PDFs examined, according to the ANIE program, with numbers determined in parentheses are as follows in order of increasing polar angle: {1014} (1set); {1013} (1 set); {1012} (1 set); {1122} (1 set); {1011} (1 set); {1121} (4 sets); {2131} (2 sets); {2241} (1 set); {3141} (2 sets); {5161} (6 sets); {5160} (1 set); unindexed (1 set). There is good agreement with the planes recorded by Cannon et al. (2010); however, they found most planes to be of low index, whereas the high index planes, e.g. {5161}, of this study are indicative of high shock intensity. Pufahl et al. (2007) found most of the planes seen in this study with a fairly uniform number of occurrences. Using the conventional method with the Wulff net, a number of planes were "near misses" on the template and thus would appear to be unindexed. This a result similar to that found by Huber et al. (2011), which indicates that the ANIE program is superior to the conventional method. As well, the format for input of measurements from the universal stage accepts an error of ± 1º, providing for a more realistic accounting of error of measurement. The program also can provide output in tabular form for ease of recording of data. 69 �Figure 1. A "toasted" quartz grain from section JN34 with two indexed directions of PDFs. References Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Davis, D.W., Kissin, S.A., Fralick, P.W. and Hammond, A.L. 2005. Discovery of distal ejecta from the 1850 Ma Sudbury impact event, Geology 33: 193196. Cannon, W.F., and Addison, W.D. 2007. The Sudbury impact layer in the Lake Superior iron ranges: A time-line from the heavens: Institute on Lake Superior Geology, Proceedings 53, Part 1: 20-21. 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, Geological Society of America Bulletin 122: 50-75. French, B.M. 1998. Traces of Catastrophe: A Handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures, Lunar and Planetary Institute , Contribution 954. Huber, M./S., Ferriére, L., Losiak, A. and Koeberl, C. 2011. ANIE: A mathematical algorithm for automated indexing of planar deformation features in quartz grains, Meteoritics and Planetary Science 46: 1418-1424. Pufahl, P.K., Hiatt, E.E., Stanley, C.R., Morrow, J.R., Nelson, G.J. and Edwards, C.T. 2007. Physical and chemical evidence of the 1850 Ma Sudbury impact event in the Baraga Group, Michigan, Geology 35: 827-830. von Engelhardt, W. and Bertsch, W. 1969. Shock induced planar deformation structures in quartz from the Ries crater, Germany, Contributions to Mineralogy and Petrology, 20: 203-23. 70 �GEOCHEMISTRY AND MINERALOGY OF FE-TI-V-P MINERALIZED FERROGABBROIC INTRUSIONS OF THE MCFAULDS GREENSTONE BELT, SUPERIOR PROVINCE, NORTHERN ONTARIO, CANADA KUZMICH, Ben1, HOLLINGS, Pete1, HOULÉ, Michel G.2 1 Department of Geology, Lakehead University, 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada Geological Survey of Canada, GSC-Quebec, 490 rue de la Couronne, Québec, Quebec G1K 9A9 2 The McFaulds Lake area (i.e., Ring of Fire) located in northern Ontario (Canada) has been the site of recent exploration leading to the discovery of several mineralization types including chromite and nickel sulfide deposits. Although the majority of exploration has been focused on chromium, the area also contains significant Fe-Ti-V-P mineralization associated with gabbroic intrusions, of which the Thunderbird and Butler occurrences are the best defined. The study has been focused on the geochemical and petrographic characterization of the intrusions to investigate their petrogenesis. The intrusions are widely distributed throughout the McFaulds Lake area and can be grouped into two main types: (1) large mafic-dominated intrusions and (2) subconcordant to slightly discordant mafic-dominated sills/dikes characteristic of the Thunderbird and the Butler intrusions respectively. Both types are composed of an evolved mafic suite termed the ‘Ferrogabbro’ characterized by the presence of Fe-Ti oxides. Detailed core logging has shown that both intrusions are largely composed of very similar lithologies including iron-rich gabbros, leucogabbros, and anorthosites. Two types of Fe-Ti oxide mineralization occur within these intrusions: (1) Fe-Ti-V and (2) Fe-Ti-P mineralization. Fe-Ti-V mineralization has been intersected within both intrusions, whereas the Fe-Ti-P mineralization has only been identified within the Thunderbird intrusion. The mineralization occurs dominantly as disseminated magnetite and ilmenite (1-10%), but is also present as semi-massive (50-80%), to massive layers (>80%: Fig. 1). These layers typically contain distinct sharp, stratigraphically lower contacts and gradational upper contacts typical of primary igneous layering (Fig. 2). The ilmenite and magnetite occurs as anhedral to subhedral crystals and to a lesser extent, as very fine-grained exsolutions within anhedral magnetite grains. 71 �Figure 1. Massive magnetitite from Butler East intrusion (BP11-V01). Figure 2. Magmatic layering within the Thunderbird intrusion (NOT09-2G25). This research project has also addressed the use of TiO2/V2O5 ratio as a potential vector towards vanadium and/or phosphorous mineralized horizons within the ferrogabbroic intrusions of the McFaulds Lake area. Preliminary data in the Butler intrusions, strongly suggest that the ratio TiO2/V2O5 ratio values are independent of rock type, abundance of magnetite-ilmenite, and/or alteration and could be useful in determining favorable horizons for further vanadium mineralization. Furthermore, the samples from this intrusion that exhibit significant vanadium contents (>0.50 weight % V2O5) are restricted to a narrow range of TiO2/V2O5 values (between 8 and 12). This ratio has the potential to be a significant exploration tool to target magmatic Fe-Ti-V-P mineralization and it has also been an instrumental tool interpretation of stratigraphy of the Butler and Thunderbird intrusions. The ferrogabbroic intrusions may be petrogenetically related to the abundant ultramafic rocks within the McFaulds Lake area, and could possibly represent the late stage end member of a magmatic sequence as has been suggested for the Bushveld complex. However, the rare or absent ultramafic components spatially associated with these ferrogabbroic intrusions, combined with some ultramafic units crosscutting the ferrogabbroic units within the Butler intrusion, may suggest that they could represent two distinct magmatic events rather than a dismembered layered intrusion, as proposed by previous workers. 72 �STRUCTURAL CONTROL ON THE BORDEN GOLD DEPOSIT IN CHAPLEAU, ONTARIO D.J. LaFontaine1 and M.L. Hill1 1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, P7B 5E1, djlafont@lakeheadu.ca The Borden gold deposit is located 20 km east of Chapleau, 180 km southwest of Timmins, within the Kapuskasing Structural Zone and Wawa subprovince of the Superior Province. The deposit is a low grade, bulk tonnage style hosting 4.3 million ounces at 1.03 g/t Au in upper amphibolite to granulite facies metamorphic rocks. The metamorphic minerals at Borden include biotite, muscovite, hornblende, sillimanite, garnet, kyanite, cordierite and pyroxene. Based on the abundance of aluminous metamorphic minerals, the protolith is inferred to be pelitic. At such high metamorphic temperatures, deformation is dominantly by ductile mechanisms although microfracturing of competent minerals is also possible. On the macroscopic scale, gold mineralization seems to be controlled by strain heterogeneity related to metamorphic grade and competency. Competent lithons (boudins?) of granulite facies rock appear to be surrounded by more ductile amphibolite facies gneisses and schists, suggesting polymetamorphism with retrograde amphibolite facies metamorphism after granulite facies metamorphism. Gold is typically observed in low strain rocks with weakly developed foliation and also in low strain rocks that are bordered by strongly foliated units. Gold mineralization has been observed at grain boundaries of quartz, within cleavage planes of biotite and associated with euhedral pyrite and anhedral pyrrhotite. This project will provide specific structural and microstructural parameters to guide further exploration and development of the mineralized zone at the Borden gold deposit. 73 �74 �RU-RH-PD MOBILIZATION IN FLAMBEAU MASSIVE SULFIDE DEPOSIT LAMB, Matthew T., and BHATTACHARYYA, Prajukti Department of Geography and Geology, UW-Whitewater, 120 Upham Hall, 800 Main St., Whitewater, WI, 53190 The Flambeau deposit located in Rusk county Wisconsin is a Volcanogenic Massive Sulfide (VMS) deposit that plays host to large amounts of copper (chalcopyrite) and zinc (sphalerite). The Flambeau deposit is located in the Ladysmith-Rhinelander volcanic complex and has a felsic center which is a steeply dipping section of interlayered fragmental quartz-sericite, andalusitebiotite, quartz-eye, chlorite-garnet, actinolite, and chlorite schist (DeMatties, 1994). The deposit has gone through major alteration due to preliminary sulfide mineralization, regional metamorphism, and supergene alteration near the surface (May and Dinkowitz 1996). However, the characteristics of hydrothermal alterations associated with sulfide mineralization at deeper levels of Flambeau deposit have not been well studied. The goal of this research project is to study how hydrothermal fluids at Flambeau deposit concentrated Cu, Zn, and Fe sulfides, and mobilized other metals such as Ru, Rh, Pd, Ti, and Cr in the process. In order to accomplish this goal I am analyzing a core sample from the Flambeau deposit using a Bruker handheld X-Ray Fluorescence (XRF) analyzer. I am collecting data from primary bedrock layers (sericite-rich layers with little or no sulfide), discrete, thin (1-2 cm think) sulfide bands within the host rock, bedrock with visible amounts of sulfide minerals (mixed layers), sharp boundary layers between bedrock and massive sulfide layers, and massive sulfide layers with no visible bedrock. Preliminary data collected from the depths of 620 feet to 640 feet (which is within the hypogene massive sulfide deposit) show that Ru and Pd concentrations are lower within the massive sulfide layers, and progressively increase towards the sericite-rich bedrock layers, while Rh concentration progressively decreases going from massive sulfide layers towards bedrock (Figure 1). Besides Ru, Rh, and Pd, no other PGE, or common “pathfinder” elements like Ba, Co, Ni, etc. are present in the samples in detectable quantities. Ti concentrations are higher in bedrock layers compared to the sulfide layers, but Cr concentrations stay relatively constant in all the layers. The distribution patterns of Ru, Rh, and Pd associated with Flambeau VMS deposit might provide important insights regarding how sulfide mineralization may affect the distribution of trace elements already present in host rocks. Results from this research can potentially help in future explorations for other, similarly formed VMS deposits around the world. References DeMatties, Theodore (1994). Early Proterozoic Volcanogenic Massive Sulfide Deposits in Wisconsin: An Overview. Economic Geology. 1994: 1122-1151 May, Edwarde, and Dinkowitz, Stephen (1996). An Overview of the Flambeau Supergene Enriched Massive Sulfide Deposit: Geology and Mineralogy, Rusk County, Wisconsin. Volcanogenic Massive Sulfide Deposits of Northern Wisconsin: A commemorative volume: (LaBerge, G. L., Ed), Institute on Lake Superior Geology Proceedings, 42nd Annual Meeting, Cable, WI, v. 42, part 2, 67-93 75 �Massive sulfide Ru Sulfide bands in bedrock Boundary Bedrock and mixture Rh Pd Figure 1. Change in Ru, Rh, and Pd within Massive Sulfide, Boundary Layers, and Sericite-Rich Bedrock Layers (620-640 Ft). 76 �GEOLOGY AND PETROLOGY OF THE WILDER LAKE INTRUSION, DULUTH COMPLEX, NORTHEASTERN MINNESOTA LEU, Adam, and MILLER, Jim Department of Geological Sciences, University of Minnesota Duluth, Duluth, Minnesota 55812. In a massive forest fire in the autumn of 2011, 160 square miles of dense forest in the Boundary Waters Canoe Area Wilderness (BWCAW) was intensely burned. Underlying what was termed Pagami Creek burn area are mafic intrusive rocks of the 1.1 Ga Duluth Complex - a large, arcuate-shaped, multiple intrusive igneous complex that underlies most of northeastern Minnesota and that constitutes the largest exposed plutonic component of the 1.1 Ga Midcontinent Rift. Situated in the center of the burn area is the Wilder Lake Intrusion (WLI), an incompletely studied, northward-dipping sheet-like mafic layered intrusion known only from reconnaissance mapping of its lake-accessible western extent. The intense burn created a time-sensitive opportunity to access freshly exposed outcrops along the entirety of its 10 km strike length. The WLI is part of the Layered Series – a collection of mafic layered intrusions within the Duluth Complex emplaced near the base of a comagmatic volcanic edifice (North Shore Volcanic Group). While most Layered Series intrusions were emplaced beneath an earlier intrusive suite of anorthositic rocks (Anorthositic Series; Fig. 1), the WLI was intruded entirely within Anorthositic Series rocks (Miller et al., 2002), The overall objective of this study is to document the igneous stratigraphy along its entire strike-length with the goal of better understanding the composition and emplacement and crystallization history of its parental magma(s) that produced its unique petrologic attributes. These attributes, noted by others and confirmed here, include 1) its unique cumulate stratigraphy where Fe-Ti oxide becomes a cumulus phase before augite; 2) the cumulus reversal indicated by the change from a four-phase cumulate of Pl+Cpx+Ox+Ol abruptly giving way up section to a troctolitic (Pl+Ol) cumulate; and 3) its reversed cryptic variation of Fo in olivine and En’ in clinopyroxene (Miller and Ripley, 1996). Detailed field mapping (1:12,000), petrographic observations, and geochemical analyses were conducted to accomplish these goals and objectives. WLI was first discovered by reconnaissance mapping by Phinney (1972) who documented exposures of well-foliated and layered gabbros and troctolites that extend from North Wilder Lake to the west and Arrow Lake to the east, a strike-length of about 10 kilometers. Phinney noted that internal layering and foliation dips to the north to northeast between 15° and 35°, which contrasts with the southerly to easterly (riftward) dip of most layered series intrusions of the Duluth Complex. Reconnaissance mapping and follow-up petrologic studies were conducted by Jim Miller who noted a distinct cumulate reversal (Pl+Cpx+Ox+Ol  Pl + Ol) in the upper section of the western portion of the intrusion, as well as identifying a reversed cryptic variation of upwardly increasing En’ and Fo content of pyroxene and olivine, respectively (Miller, 1986; Miller and Ripley, 1997). Unpublished field, petrographic and geochemical data also collected in the western WLI by Joy Turnbull in 2004 verified the reversed cryptic variation and phase layering in the western part of the WLI. Detailed mapping conducted in 2012 and 2013 for this study shows that most cumulate units of the WLI can be followed along the entire 10km strike length of the WLI, but with some notable exceptions. Remapping in the western part of the WLI has confirmed that the 2km-thick igneous stratigraphy exposed here starts with a basal unit of heterogeneous, intergranular olivine oxide gabbro that is in sharp contact with Anorthositic Series rocks. This marginal gabbro is overlain by a troctolitic unit of Pl+Ol cumulates, which can be subdivided into a heterogeneous subunit, a layered subunit and an anorthosite inclusion subunit. The troctolite unit is overlain by a thin (20-100m thick) oxide troctolite unit of Pl+Ol+ Ox cumulates which abruptly gives way to an olivine oxide gabbro unit of Pl+Cpx+Ox+Ol cumulates. Above the gabbro, an upper troctolitic unit occurs marking a cumulus reversal back to Pl+Ol cumulates. Mapping of the excellent exposures created by the burn reveal that the upper troctolite unit cross-cuts and locally scours out the four-phase gabbro. Thus it is interpreted as a recharge of more primitive magma into the upper part of the WLI chamber rather than a downward crystallizing roof zone unit as proposed by Miller (1986). Detailed mapping by overland traverses in the central and eastern extents of the WLI show it to thin from 2 km in the west to 1 km in the east. Moreover, several units pinch out in the eastern section of the intrusion. The oxide troctolite pinches out just east of center, but swells back to about 20 meters in thickness before pinching out again with the upper gabbro farther east. The lower gabbro also pinches out around the same place and is replaced by a taxitic unit that dominates at the eastern margin; a similar heterogeneous unit also can be found at the western margin. 77 �Petrographic studies of 223 thin sections collected along three profiles across the intrusion at its western, east central and eastern extents helped to confirm and refine the mineralogy and textures described from field observations. In addition, olivine and pyroxenes from many of the thin section samples were analyzed by UMD’s SEM-EDS to document cryptic variation of Mg/Fe ratios. This mineral chemical data was acquired to verify the reversed cryptic variation previously documented in the west and to determine if this variation persists along strike to the east. Reversed cryptic variation of upwardly increasing magnesium number (mg#=MgO/(MgO+FeO), mole%) in olivine (Fo) and pyroxene (En’) was confirmed in the west and in the eastern profiles. However, the data also reveal that the mg# tends to decrease at a particular stratigraphic horizon from west to east. We interpret the reversed cryptic variation up section to be due to a reduced trapped liquid shift within the oxide troctolite and olivine oxide gabbro units. Trapped liquid shift occurs where high mg# cumulus olivine re-equilibrates with low mg# intercumulus liquid. As evidenced by their strong foliation, these rocks are adcumulates with very little intercumulus minerals (i.e., trapped liquid component) and thus retain their high-mg# cumulus compositions. The lateral decrease in mg# to the east as well as the disappearance of the oxide troctolite unit is thought to be caused by the thinning of the intrusion causing the eastern portion of the intrusion to cool more rapidly than the west. This more rapid cooling would have caused more trapping of intercumulus liquid (and thus a stronger trapped liquid shift) and would have promoted oxide and pyroxene to crystallize more synchronously since their liquidus temperatures are not very different. We are currently evaluating whole rock analyses of the basal intergranular gabbro samples to determine if they may be representative of a parental liquid composition. We are using a MELTS-based modeling program, Pele (Boudreau, 2006), to evaluate the phase equilibrium of these compositions and to see if fractional crystallization of these compositions under different conditions of oxygen fugacity and cooling rate can replicate the cumulate stratigraphy observed in the WLI. REFERENCES Boudreau, A. , 2006, Pele. (7.07). Computer modeling program. Duke University. www.nicholas.duke.edu/eos/ Miller, J.D., Jr., 1986, The geology and petrology of anorthositic rocks of the Duluth Complex, Snowbank Lake quadrangle, northeastern Minnesota (PhD thesis) University of Minnesota Miller, J.D. Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.E., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota. Minnesota Geological Survey Report of Investigations 58, 207p. w/ CD-ROM Miller, J.D., Jr., and Ripley, E.M., 1996, Layered Intrusions of the Duluth Complex, Minnesota, USA. In: Cawthorne, R.G., Layered Intrusions: Amsterdam, Elsevier Science, p. 257-301 Phinney, W.C., 1972. Northwestern part of Duluth Complex. In: Sims, P.K. & Morey, G.B. (eds.) Geology of Minnesota -A centennial volume. Minnesota Geological Survey, p. 335-345 Wilder Lake Intrusion Figure 1. Generalized geology of NE Minnesota showing the location of the Wilder Lake Intrusion and the Pagami Creek Burn Area. Pagami Creek Burn Area 78 �THE ROLE OF BRITTLE-DUCTILE DEFORMATION AND COMPETENCY CONTRAST IN GOLD MINERALIZATION IN THE C-ZONE AT HEMLO LIIMU, Jared and HILL, Mary Louise Department of Geology, Lakehead University, 955 Oliver Rd., Thunder Bay, ON, P7B 5E1, Canada The Hemlo gold mine is located along the Hemlo shear zone within the Hemlo-Schreiber greenstone belt (Muir, 2002). This greenstone belt lies within the Archean Wawa subprovince of the Superior province (Muir, 2002). The characteristics of an active mineralized stope in the Czone were studied. The area was interpreted to be under amphibolite facies metamorphism, based on ductile deformation of feldspar, and the presence of sillimanite. This is consistent with findings by Powell et al. (1999), who also noted a peak kyanite phase followed by a decrease in pressure but a relatively minor decrease in temperature. Lingering high temperatures are supported by our observation of annealed grain boundaries. Brittle-ductile deformation is present throughout the C-zone, on all scales of measure. Figure 1 shows evidence of this on a microstructural scale, where quartz shows evidence of ductile deformation (undulose extinction and subgrains), as well as fracturing. Different minerals show evidence for different behaviours; for example, clinozoisite deformed in a purely brittle manner. Mutually overprinting brittle and ductile deformation, as well as competency contrasts are also evident on outcrop scale. Competency contrast within the area of study is most obviously seen between the more competent metavolcanics and more ductile biotite schist. Quartz veins within the metavolcanics tend to have relatively straight contacts with the host rock, whereas quartz veins in the biotiteschist tend to exhibit boudinage or pinch and swell textures. Quartz veins in the metavolcanics are localized within this unit and do not extend into the surrounding biotite-schist (Fig. 2). Greenschist facies pressure and temperature are considered by many to be the perfect conditions for simultaneous brittle-ductile deformation due to competency contrasts (Weinberg et al., 2012), which in turn provides an ideal Figure 1: Photomicrograph of a microshear zone with evidence for brittle and ductile deformation within quartz. Figure 2: More competent metavolcanic unit (1) and the less competent biotite-schist (2). 79 �setting for gold mineralization. Results from this project show that similar mineralization can occur under amphibolite facies conditions where ductile deformation dominates. There are three modes of mineralization observed within the C-zone. The first two include mineralization of gold along competence contrast boundaries. The third involves mineralization of gold within metavolcanic hosted fractures. The role competency plays in gold mineralization is two-fold. The first is that competent bodies tend to fail via brittle deformation. This allows pore space for gold-hosting fluids to infiltrate. This may be the basis for mineralization within the metavolcanic hosted fractures. The second role competency plays involves high-temperature diffusion of gold-hosting fluids along boundaries between competent and ductile lithologies. This causes mineralization, a) within the biotite-schist along the boundary of quartz boudins, and b) within the metavolcanics along the boundary with the biotite-schist. References Muir, T., 2002. The Hemlo gold deposit, Ontario, Canada: principal deposit characteristics and constraints on mineralization, Ore Geology Reviews, v. 21, p. 1-66 Powell, W., Pattison, D., Johnston P., 1999. Metamorphic history of the Hemlo gold deposit from Al2SiO5 mineral assemblages, with implications for the timing of mineralization, Canadian Journal of Earth Sciences, v. 36, p. 33-46 Weinberg, R., Groves, D., Hodkiewicz, P., van der Borgh, P., 2012. Controls on gold endowment: Shear Zone Comparison, Hydrothermal Systems, v. 3, p.101-108 80 �VARIABLE COPPER MINERALIZATION IN THE LOWER NONESUCH FORMATION OF THE MIDCONTINENT RIFT SYSTEM: CONSTRAINTS ON REGIONAL CONTROLS MAUK, Jeffrey L.1, WOODRUFF, Laurel G.2, and STEWART, Esther3 1 U.S. Geological Survey, PO Box 25046, MS 964, Denver, CO, 80225 U.S. Geological Survey, 2280 Woodale Ave, St. Paul, MN 55112 3 Wisconsin Geologic and Natural History Survey, 3817 Mineral Point Road, Madison, Wisconsin 537055100 2 The lower Nonesuch Formation in the Lake Superior portion of the Midcontinent Rift System is a host of major sediment-hosted stratiform Cu mineralization, including the White Pine mine, which produced more than 2 Mt Cu, and the Copperwood deposit, which has measured, indicated, and inferred resources of 0.5 Mt Cu. However, the lower Nonesuch Formation is not uniformly mineralized; instead some areas are relatively well-endowed with Cu mineralization, whereas other areas, such as the Ashland syncline in northern Wisconsin, contain only trace quantities of Cu. These variations in Cu content lead to questions on the possible first-order controls of regional to local mineralization, and we critically evaluate new and preexisting data to help identify possible controls on Cu mineralization. The western Lake Superior portion of the Midcontinent Rift contains clastic sedimentary rocks of the Oronto Group. The basal unit is the conglomerate, sandstone, and siltstone that form red beds of the Copper Harbor Conglomerate. This is overlain by gray siltstone, shale, and fine-grained sandstone of the Nonesuch Formation, which is overlain by reddish brown sandstone of the Freda Formation. Sediment-hosted stratiform Cu deposits such as White Pine and Copperwood occur in the lowest gray beds, which contain organic matter and diagenetic pyrite that can serve as a reductant for cupriferous brines that are introduced from red beds of the underlying stratigraphy. Previous work on the lower Nonesuch has documented that the total organic carbon and sulfur contents are similar in the Ashland syncline and the White Pine-Copperwood area, suggesting that the sedimentary rocks in both places had similar reduction potential for trapping metallic minerals. The hydrothermal fluid that transported metals to the reduction site is widely accepted to be oxidized cupriferous brines. Neither the Ashland syncline nor the White Pine-Copperwood area have abundant evaporite minerals, but both contain evidence for local to minor evaporite minerals in the lower Nonesuch Formation, and the red beds of the Copper Harbor Conglomerate underlie the Nonesuch Formation in both areas, so we infer that the carrying capacity of the diagenetic basinal fluids was similar in both areas. The ultimate source of the Cu is typically interpreted to be the basalt that underlies the Oronto Group, and because both areas are underlain by extensive basalt, we infer that a source of Cu was not the greatest limitation. The sedimentary facies in each of the main formations of the Oronto Group are similar in both areas; this suggests that first-order control of fluid flow by major facies variations was not a likely control on different Cu endowments. However, the detailed stratigraphy of the lower Nonesuch Formation, which shows remarkable continuity in the White Pine-Copperwood area, differs in the Ashland syncline: most of the marker beds in the White Pine-Copperwood area are poorly developed and rarely occur in the Ashland syncline. This observation is consistent with previous interpretations that the Ashland syncline lies within a different subbasin that was either partially or completely separated from the large rift basin that hosts the White Pine and Copperwood deposits. 81 �Taken together, these results suggest that one major constraint on the Cu endowment of favorable strata in the Nonesuch may have been the size of the available mineralizing diagenetic fluid source. The White Pine-Copperwood deposits are on the margin of a large rift basin that would have been able to contribute significant quantities of mineralizing fluid. In contrast, the presumably smaller size and restricted connectivity of the rift sub-basin in the Ashland syncline area would have had a smaller fluid source thereby limiting the potential metal endowment in that area. 82 �SEDIMENTOLOGY AND GEOCHEMISTRY OF THE MESOARCHEAN CHEMICAL SEDIMENTS OF WALLACE LAKE AND RED LAKE MCINTYRE, Tim and FRALICK, Philip, Department of Geology, Lakehead University, Thunder Bay, ON, Canada, P7B 5E1, philip.fralick@lakeheadu.ca In the western Superior Province, Mesoarchean (~2.93Ga) carbonates and iron formation of the Uchi Subprovince represent a large carbonate platform that extended between the Wallace Lake and Red Lake greenstone belts. The aerial extent, sedimentary structures, and geochemistry of the platform indicate that significant changes in oceanic processes were occurring in the Mesoarchean. These changes include the precipitation of aerial extensive platform carbonates and evidence for the addition of free oxygen to limited areas of the hydrosphere by its initial production on semi-restricted platforms. Paleoarchean marine carbonates consisted of thin bedded units precipitating from anoxic water basins (ex. Strelley Pool Chert (Allwood et al., 2010)). Geographically scattered large Neoarchean carbonate platforms show evidence for the gradual build-up of oxygen during this time and leading to a relatively oxygenated atmosphere by 2.4Ga (ex. Steep Rock platform (Fralick and Riding, in press) and the CampbellrandMalmani platform (Kendall et al., 2010)). The occurrence of this large carbonate platform in the earliest transitional period of this change and its similarities to younger Neoarchean platforms is significant in that the information gathered has significant import to processes responsible for the change in carbonate deposition through the Paleoarchean to Paleoproterozoic. The lithofacies of the Wallace Lake and Red Lake carbonate platform represent deposition from peritidal to basinal environments, with many of the structures being present in younger Neoarchean and Proterozoic carbonate platforms. The peritidal lithofacies assemblage consists of herringbone calcite, pseudomorph fans, and tufa. The sub-tidal environment is characterized by large pseudomorph fans (Figure 1A) and laterally linked domal stromatolites (Figure 1B). Upper slope environments consisted of slumps, ribbon rock, and carbonate associated oxide-facies iron formation. Chert-oxide facies iron formation defines the basinal environment. These lithofacies typify younger Neoarchean carbonate platforms contributing to the gradual oxidation of the atmosphere (cf. Sumner and Grotzinger, 2004; Kendall et al., 2010; Fralick and Riding, in press). The rare earth element (REE) geochemistry of the Wallace Lake and Red Lake chemical sediments (Figure 2) can significantly contribute to our understanding the changing early oceans. The PAAS normalised REE patterns (REE(PAAS)) of the basin lithofacies (oxide-facies iron formation) is characterized by LREE depletion and positive Eu anomalies (Figure 2). This pattern mirrors REE geochemistry of Paleoarchean oceans (cf. Allwood et al., 2010). However, the carbonates show very little Figure 1. A) Pseudomorph fans after aragonite. B) Laterally linked domal LREE/HREE fractionation, positive La, Eu, and Y anomalies, stromatolites. and negative Ce anomalies. This implies a significant change A B 83 �REE(PAAS) in ocean chemistry from basin to the shallow carbonate platform. The transitional facies between basin and peritidal platform (upper slope) is characterized by a REE(PAAS) pattern similar to that of the platform carbonates. The significant difference in basin and platform REE(PAAS) patterns and the REE(PAAS) pattern of the upper slope suggests that the platform was semi-restricted and evaporitic. This would lead to density contrasts between the shallow platform and basin waters allowing for seeping and down-welling of platform waters to the basin and imparting the shallow carbonate REE(PAAS) pattern to the upper slope environment. The presence of Ce anomalies in the 1 Average Carbonate (n=28) carbonates and lack thereof in the basin Average Oxide-Facies Iron Formation (n=6) iron formation is indicative of a redox boundary separating the basin and peritidal Carbonate Associated Iron Formation (n=2) environments. The negative Ce anomalies 0.1 are the largest and most consistent in the pseudomorph fan facies. These pseudomorph fans are common structures found in Precambrian carbonate platforms, 0.01 range in depositional environments from La Ce Pr Nd Sm Eu Gd Tb Dy Y Ho Er Yb Lu peritidal to sub-tidal, and are thought to be pseudomorphs after aragonite (Sumner and Grotzinger, 2004). The fans of the Wallace Lake and Red Lake carbonate platform are also characterized by increased Ba, Sr, P, and K concentrations. The presence of the negative Ce anomalies in the fans and lack thereof in most of the rest of the platform indicate that the fans precipitated from relatively oxygenated water. It is suggested here that the fans are not indicative of a particular environment of deposition, but limited to periods of relative oxygenation of the platform and, when occur, are a platform wide occurrence. To summarize: the Wallace Lake and Red Lake carbonate platform is the oldest known carbonate platform in a series of geographically scattered platforms contributing to a gradual change in oceanic processes in the Mesoarchean and Neoarchean that led to an extensively oxygenated atmosphere by 2.4Ga. Figure 2. REE plot normalized to Taylor and McLennen’s (1989) Post Archean Australian Shale (PAAS) (n refers to the number of samples analyzed). References Allwood, A., Kamber, B. S., Walter, M. R., Burch, I. W., and Kanik, I. (2010). Trace elements record depositional history of an Early Archean stromatolitic carbonate platform. Chemical Geology, 270(1), 148-163. Fralick, P. and Riding, R. (in press). Anatomy and geochemistry of an Archean carbonate platform. Kamber, B. (2001). The geochemistry of late Archaean microbial carbonate: Implications for ocean chemistry and continental erosion history. Geochimica Et Cosmochimica Acta, 65(15), 2509-2525. Kendall, B., Reinhard, C.T., Lyons, T.W., Kaufman, A. J., Poulton, S.W., and Anbar, A.D., (2010). Pervasive oxygenation along late Archaean ocean margins. Nature Geoscience (3), 647- 652. Sumner, D. and Grotzinger, J., (2004). Implications for Neoarchaean ocean chemistry from primary carbonate mineralogy of the Campbellrand-Malmani Platform, South Africa. Sedimentology, 51(6), 1273-1299. 84 �COMPOSITION AND 40AR/39AR AGE OF PEGMATITIC AMPHIBOLE IN THE WAUSAU SYENITE COMPLEX, MARATHON COUNTY, WISCONSIN MEDARIS1, Gordon Jr., FLOOD2, Tim, JICHA1, Brian and SINGER1, Bradley 1 Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706 St. Norbert College, De Pere, WI 54115 medaris@geology.wisc.edu, tim.flood@snc.edu, bjicha@geology.wisc.edu, bsinger@geology.wisc.edu 2 Granitic pegmatites are of great interest for their unusual and exotic minerals. Because they are highly differentiated chemically, they may also contain silicate minerals of endmember compositions, such as arfvedsonite, aegirine, and albite. End-member amphibole crystals ≤ 30 cm in length occur in an NYF (niobium-yttrium-fluorine) granitic pegmatite that intrudes quartz syenite of the Wausau Complex (Fig. 1). The Wausau Complex is one of four, alkaline syenitic to granitic concentric complexes proximate to Wausau in Marathon County. The igneous complexes decrease in age from 1565 Ma in the north (Stettin) to 1506 Ma in the south (Nine Mile) and precede the Wolf River batholith in intrusive age by 30 to 90 m.y. (Van Wyck et al., 1984; Dewayne and Van Schmus, 2007). The granitic pegmatite in the Wausau Complex is ~6m long × ~1m wide and consists of 35% euhedral amphibole, 40% subhedral microcline, 25% anhedral quartz, and accessory pyroxene, albite (Ab 99.7), fluorite, ilmenite, and magnetite. The amphibole is mainly arfvedsonite (Table 1; Fig. 2), with a mean atomic composition of: (Na0.76K0.10)A(Na1.09K0.15Ca0.35Fe2+0.41)B(AlVI0.20Ti0.14Fe3+1.14Fe2+3.22Mn0.13Mg0.17)C (Si7.91AlIV0.09)TO22(OH1.49F0.51) A-site occupancy varies from 0.73 to 0.97, and F contents vary from 0.51 to 1.26 wt.%. In contrast, the rims of amphibole and fine-grained amphibole associated with aegerine are intermediate arfvedsonite-riebeckite, with a mean atomic composition of: (Na0.42K0.02)A(Na1.60K0.06Ca0.01Fe2+0.32)B(AlVI0.10Ti0.03Fe3+1.56Fe2+3.09Mn0.02Mg0.20)C (Si8.27)TO22(OH1.92F0.08) A-site occupancy varies from 0.35 to 0.48, and F contents vary from 0.13 to 0.43 wt.%. The intermediate arfvedsonite-riebeckite contains less TiO2, Al2O3, MnO, CaO, and K20 than the predominant arfvedsonite and is closer to a Na-Fe-Si end-member composition. However, the Si content of arfvedsonite-riebeckite exceeds 8.0 apfu, which may indicate the presence of a pyribole component, which is under further investigation. The pyroxene is very close to end-member aegirine, with a mean atomic composition of: (Na0.956Ca0.004)(AlVI0.038Ti0.007Fe3+0.896Fe2+0.093Mn0.001Mg0.002)(Si2.004)O6 85 �Aegirine occurs interstially and intergrown with arfvedsonite-riebeckite at the rims of arfvedsonite crystals. Such textural and compositional relations are likely the result of increasing oxygen fugacity during crystallization, which stabilizes riebeckite at the expense of arfvedsonite and stabilizes aegirine at the expense of alkali amphibole (Ernst, 1962; Scaillet & MacDonald, 2001). The presence of ilmenite inclusions in arfvedsonite and an association of magnetite with interstitial aegerine are consistent with this interpretation. Ar analysis of a single arfvedsonite crystal was performed by step heating using a CO2 laser, which yielded a well-defined plateau at 1513 ± 5 Ma (Fig. 3), representing ~75% of the 39Ar released. This age is slightly younger than, but within error of, the 1522 ± 6 Ma U-Pb zircon age for syenite in the Wausau Complex (Dewayne and Van Schmus, 2007). The geon 15 igneous intrusions near Wausau are classic examples of alkaline granitic to syenitic concentric complexes, the differentiated parts of which, such as the NYF pegmatite in the Wausau Complex, contain end-member silicate mineral compositions. These igneous intrusions represent an important magmatic event in the Precambrian evolution of the southern Lake Superior region, appearing somewhat earlier than the more voluminous and slightly less alkaline geon 14 Wolf River batholith. References Ernst WG (1962) Journal of Geology, v. 70, 689-736. Myers PE et al. (1984) Institute on Lake Superior Geology, v. 30, Field Trip #3, 58 pp. Scaillet B & MacDonald R (2001) Journal of Petrology, v. 42, 825-845. Van Wyck N et al. (1984) Institute on Lake Superior Geology, v. 30, Part 1 Program and Abstracts, 81-82. 86 �GEOLOGY OF THE LAKE THREE TROCTOLITE, DULUTH COMPLEX - 2013 PRECAMBRIAN FIELD CAMP CAPSTONE MAPPING Jim MILLER, Sarah Sauer, Jordan Benningfield, Jackson Graham, Sara Kozmor, and Ann Marie Prue Precambrian Research Center, University of Minnesota Duluth, Duluth, MN 55812 As a capstone mapping project for the 2013 Precambrian field camp, a crew of four students, a teaching assistant (Sauer) and an instructor (Miller) conducted five days of field mapping bedrock geology in the southern area of Lake Three in the Boundary Waters Canoe Area. The field area was affected by the Fall 2011 Pagami Creek Fire, which burned an area of approximately 93,000 acres. The area immediately south of Lake Three experienced moderate fire intensity such that significant amounts of deadfall still existed on the forest floor. Moreover, in the second summer after the burn, a thick cover of schrubs and vines covered the forest floor creating difficult conditions for inland traverses. Despite these less than ideal conditions, three crews of field partners conducted multiple inland traverses that ultimately extended detailed mapping over a mile south of the Lake Three shoreline. The Lake Three Troctolite (L3T) is a sub-circular body of troctolitic cumulates exposed along the southern shore of Lake Three and intruded into Anorthositic Series rocks. Both the troctolitic and anorthositic rocks are parts of the extensive Duluth Complex, which is the largest exposed intrusive component of the 1.1Ga Midcontinent Rift. Shoreline exposures of the L3T were first mapped by Phinney (1972) and Miller (1986). Additional reconnaissance mapping to south of Lake Three by Miller (1986) and more recent post-fire mapping by Jirsa (2013) suggests that the L3T may extend to as far as 3 miles south of the Lake Three shoreline. This study sought to extend detailed mapping of the L3T south of the shoreline into the area burned by the Pagami Creek Fire. This detailed mapping shows the L3T to have broad asymmetrical synformal structure that trends NNE-SSW with a thinner eastern limb dipping NW and thicker western limb dipping SE. Moreover, the capstone mapping distinguished two distinct map units – an outer (lower) zone of varitextured troctolitic cumulates rich in anorthositic inclusions, and an interior (upper) zone of homogeneous ophitic augite troctolite. More specifically, the outer troctolite unit is composed of light to dark gray, commonly vari-textured (medium- to coarse-grained), locally layered, poorly to well foliated, subophitic to ophitic, Pl-phyric troctolitic rock types that are locally rich in irregular masses of anorthositic lithologies. Modal variants include melatroctolite, leucotroctolite and augite troctolite. The unit typically contains 2%-15% plagioclase phenocrysts typically about 1cm diameter. Modal layering is typically defined by variable olivine:plagioclase concentrations. Contacts with anorthositic series rocks are not exposed, but near inferred contacts, the troctolite becomes vari-textured, generally finer grained, and contains numerous anorthosite inclusions from cm to several meters. Contacts with the interior ophitic augite troctolite are broadly gradational over several to tens of meters. Rocks of the inner augite troctolite unit are typically light gray to dark gray, medium-grained, poor to moderately foliated, homogeneous, ophitic, Pl-phyric augite troctolite. Plagioclase phenocrysts are typically 1-2 cm in diameter, but some are up to 4 cm. Augite oikocryst range from 2-5 cm in diameter and iron oxide commonly occur as subpoikilitic clots less than 1 cm across. Contacts with the troctolite unit are gradational over several to tens of meters. 87 �Anorthositic Series rocks are composed of various anorthositic to leucogabbroic rock types including anorthosite, troctolitic anorthosite, olivine gabbroic anorthosite, leucotroctolite, and augite leucotroctolite.. All lithologies have greater than 75% plagioclase with less than 25% olivine, augite, and Fe-Ti oxides. Augite and oxide always occur as poikilitic to subpoikilitic crystal relative to lathy plagioclase, whereas olivine ranges in habit from subhedral granular to poikililtic. Oikocrysts of olivine up to 20 cm diameter have been observed and commonly recessively weather to create a pocked surface. Most anorthositic lithologies have a well- developed igneous foliation defined by plagioclase alignment, but modal and textural layering are rare. Locally anorthositic series rocks occur as inclusions into both the troctolite and augite troctolite units of the L3T. A previously unrecognized lithology was found along the eastern margin of the L3T and as a large xenolith at the western troctolite-augite troctolite contact during the capstone mapping. It is typically a fine- to medium fine-grained, equigranular (granoblastic) olivine gabbro to augite troctolite. It is typically homogenous, but locally contains abundant coarse-grained clots and stringers rich in oxides and olivine. In some areas, a swirly mixture of medium fine-grained olivine oxide gabbro, fine-grained augite leucotroctolite, and fine grained leuctroctolite with 1cm oikocrysts of olivine is observed. Subtle modal layering with hints of crossbedding have been observed in a few locations. These masses are similar to other areas of the Duluth Complex that have been interpreted to be intensely metamorphosed inclusions of mafic volcanics of the North Shore Volcanic Group. Abundant unburned blowdown trees and thick forest floor vegetation prevented traverses to reach south much more than a mile. Confirmation of the southern extent of the L3T will require additional mapping north of the Isabella River. References Miller, J.D., Jr., 1986, The geology and petrology of anorthositic rocks of the Duluth Complex, Snowbank Lake quadrangle, northeastern Minnesota. Unpublished Ph.D. thesis, University of Minnesota, Minneapolis, MN, 525 p. Phinney, W.C., 1972, Northwestern part of Duluth Complex. In Sims, P.K. & Morey, G.B. (eds.) Geology of Minnesota - A Centennial Volume. Minnesota Geological Survey, p. 335-345 88 �GEOLOGIC MAPPING OF NEOARCHEAN AND PALEOPROTEROZOIC ROCKS NEAR HANSON LAKE, NE MINNESOTA, BY STUDENTS OF THE PRECAMBRIAN RESEARCH CENTER’S 2013 FIELD CAMP MULCAHY, Connor1, ROMANELLI, Dan1, SCHULZ, Roger1, MOORHEAD, Steve1, MAY, Mitchell1, and JIRSA, Mark2 1 2013 Field Camp Students, Precambrian Research Center, Natural Resources Research Institute, University of Minnesota Duluth, Duluth, Minnesota 55811 2 Minnesota Geological Survey (MGS), University of Minnesota, 2642 University Avenue W., St. Paul, Minnesota 55114 (jirsa001@umn.edu) The University of Minnesota-Duluth’s Precambrian Research Center conducted its seventh annual field camp in 2013, and this presentation is one of a series that show some of the results. During the fifth and sixth weeks of camp, teams of students participate in “capstone projects” that test student skills by creating new geologic maps in areas of poorly known geology. This capstone project involved mapping an area of the Boundary Waters Canoe Area Wilderness (Fig. 1) accessed by 20 lakes, with Hanson Lake at its center. The map provides details about the complex depositional and deformation history of a Neoarchean, largely metasedimentary terrane that is part of the Wawa subprovince of Superior Province. Figure 1. Generalized bedrock geologic map of northeastern Minnesota showing the Hanson Lake capstone area. The unit labeled “Knife Lake Group” also encloses older volcanic sequences that are not delineated separately at this scale. Dashed line is the border of the Boundary Waters Canoe Area Wilderness. 89 �The Hanson Lake map area lies west of the boundary between the Saganaga Tonalite (ca. 2690 Ma), and superjacent sedimentary strata of the Knife Lake Group that are inferred to have been derived in part from it. Both rock units were variably tilted, folded, faulted, and metamorphosed to low greenschist facies during a regional deformation event at about 2680 Ma, which brackets deposition of the Knife Lake Group between ca. 2690-2680 Ma. Our mapping demonstrated that strata of the Knife Lake Group in this area form a broad, northeast-trending synclinorium, bounded by the Saganaga Tonalite on the east, and an apparently uplifted fault-block of metabasalt on the west. The limbs of this large structure are marked by smaller sympathetic folds, and are dissected by faults and shear zones. Several major faults enclose what appear to be discrete blocks that were uplifted, tilted, and eroded to expose different crustal levels of the stratigraphic succession. As a result, some of the blocks contain older metavolcanic and meta-intrusive rocks that are unconformably overlain by Knife Lake strata. The dominant rock types are interbedded graywacke and slate, which were subdivided into eastern and western sections having somewhat different attributes. The eastern section contains sandstone, graywacke, and mudstone that is locally interlayered with several types of conglomerate and one thin unit of banded iron-formation, and intruded by rare peperite. A conglomerate near Nawaska Lake contains rounded, cobble-to-boulder sized clasts of tonalite, metabasalt, and metagabbro, implying fluvial deposition from streams draining a lithologically diverse terrane. Conglomeratic strata near Gift Lake contain amoeboid and diffuse-edged clasts of dacitic rock, inferred to have been weathered to saprolite before incorporation. Shreds of mafic peperite occur in chaotically bedded gritstone that contains angular grains of feldspar and mafic minerals identical with those in the peperite, indicating synchronous magmatism and sediment deposition. The western section consists of graywacke and slate, together with localized occurrences of more enigmatic strata. At Lake of the Clouds, a trachybasaltic crystal-lapilli tuff and breccias, containing clasts as large as 25cm occurs within an overall package of gray wacke. This implies episodic explosive calc-alkalic volcanism was synchronous with or just predated deposition of graywacke. At least macroscopically, this rock and the peperite appear magmatically-related. In the South Arm of Knife Lake, a unit of metabasalt is capped by basaltic conglomerate having both angular and amoeboid clasts. Presence of the latter implies the basaltic substrate was weathered prior to erosion, which is consistent with the inference that the sedimentary sequence lies unconformably on older basaltic substrate within the fault-bounded block. Taken together, these attributes portray deposition in a largely fault- and unconformitybounded, Timiskaming-type extensional basin. We infer that after the Saganaga Tonalite intruded older basaltic strata, the terrane was uplifted, weathered, and eroded to contribute detritus from both source rocks into the developing Knife Lake basin. The abundant graywacke and slate are interpreted to represent deposition in a lacustrine or marine setting, and the interlayered coarser, polymictic clastic strata may represent braided stream, alluvial fan, or subaqueous fan deposition of sediment shed off the uplifted flanks of the basin. The layered strata exhibit chaotic softsediment deformation features, local growth faults, and abrupt facies changes, suggesting that deposition was synchronous with episodic basin subsidence. Thin layers and lenses of ironformation that are associated with graywacke and slate are inferred to represent chemical precipitation into what may have been a shallow marine environment during periods of relative quiescence. These attributes, together with the association of syn-sedimentation magmatism, are consistent with the model of a Timiskaming-type basin assemblage. Several N20°W-trending, vertically dipping diabasic dikes were also encountered in the area. They are locally as thick as 30m, coarse-grained, subophitic, and have chilled margins. The dikes are inferred to be part of the Paleoproterozoic Kenora-Kabetogama dike swarm on the basis of similarities in trend, thickness, and macromineralogy. This and other capstone mapping projects can be viewed at www.d.umn.edu/prc. 90 �PETROGRAPHIC CHARACTERIZATION OF THE PENOKEAN TWELVEFOOT FALLS SHEAR ZONE, MARINETTE COUNTY, WI: EVIDENCE FOR COEVAL DUCTILE AND SEISMIC BEHAVIOR NADZIEJKA, Brynley and BJØRNERUD, Marcia Geology Department, Lawrence University, Appleton, Wisconsin, 54911 USA The Twelvefoot Falls Shear Zone in northeastern-most Wisconsin lies 25 km south of, and approximately parallel to, the NW-SE-striking Niagara Fault, which is thought to represent the ca. 1.88 Ga Penokean suture between the Archean-Paleoproterozoic Superior Craton and Paleoproterozoic island arc rocks of the Pembine-Wausau terrane (Schulz & Cannon, 2006). The shear zone is well exposed along the Pike River at Twelvefoot and Eightfoot Falls, where it cuts through quartz diorite with a U-Pb zircon crystallization age of 1889 +/- 6 Ma (Schulz & Schneider, 2005). This intrusive body was emplaced into the Quinnesec Formation, a metavolcanic unit interpreted to be part of a precollisional suprasubduction zone ophiolite-island arc complex (LaBerge et al., 2003). The Twelvefoot Falls Quartz Diorite lies on the southern flank of the Dunbar Gneiss Dome, a younger, post-collisional composite intrusion dated at 1862 +/- 5 Ma (Sims et al., 1985; Sims, 1990). Detailed petrographic study of samples from outcrops at Twelvefoot and Eightfoot Falls reveals a complex, multistage deformational history. The rocks have a pervasive, though heterogeneously developed, subvertical NW-striking foliation defined by the preferred orientation of relatively large (2-5 mm) hornblende porphyroclasts and planar quartz-rich domains. The quartz in these bands is typically fine grained (<0.1 mm), with undulose extinction, irregular grain boundaries, and in places ‘core and mantle’ structure. These textures record ductile deformation, dynamic recrystallization and subsequent partial annealing. The hornblende grains have ragged grain boundaries and are commonly dismembered or boudinaged. In many cases, once-joined grain fragments can be recognized through their optical continuity (simultaneous extinction). Alteration of hornblende to chlorite is common. Narrow (<1 cm wide) mylonite zones transect the foliation at oblique angles. Within these bands, both the hornblende and quartz grains are finer than they are in the rest of the rock, but their microstructural characteristics are similar: the hornblende is fragmented in a quasi-brittle manner, while the quartz forms narrow, high-strain bands. This suggests that both the foliation and mylonite zones developed under similar temperature conditions, namely between ca. 300°C (threshold for quartz ductility) and 600°C (onset of hornblende ductility under hydrous conditions; Hacker & Christie, 1999). This is consistent with the upper greenschist- to amphibolite-facies metamorphic conditions in the area surrounding the Dunbar Gneiss Dome (Sims et al., 1985) and also with temperature estimates for rocks with similar compositions and textures from mylonite zones in the Grenville Province (Babaie & LaTour, 1994). At Eightfoot Falls, dark, branching discordant veins 0.3-0.5 cm wide and 10-15 cm long cut across the foliation in the rocks. In thin section, these are found to contain a mesh of fine hornblende crystals with high aspect ratio, arranged with no preferred orientation in a non-crystalline matrix that is dark in plane light. These macro- and micro-scale characteristics suggest that the veins represent devitrified pseudotachylyte – frictional melt glass generated on a fault plane during seismic slip, and injected as ‘hydro’-fractures into the surrounding rock. Significantly, the pseudotachylyte material can be seen in thin section to have been cut by, and in places incorporated into, the mylonitic bands, indicating that brittle seismic failure occurred at least once while the rocks were still at depths and temperatures where crystal plastic deformation was predominant. Such mutually cross-cutting relationships between mylonites and pseudotachylytes have been reported from a small number of sites around the world (e.g., Sibson, 1975; Hobbs et al., 1986) and are interpreted as records of large earthquake ruptures, 91 �usually in convergent tectonic settings, that propagated downward from the fully brittle upper crust into the upper part of the ductile regime. We interpret the fabrics in the rocks from the Twelvefoot Falls Shear Zone to be of mid-late Penokean age, based on the proximity and parallelism of the Zone with the Niagara Fault and on the regional evidence that intrusion, deformation and metamorphism of the Dunbar Gneiss Dome coincided with later Penokean (ca. 1.85 Ga) crustal shortening at the time of the collision of the Marshfield terrane with the Pembine-Wausau terrane (Schulz & Cannon, 2006; Sims, et al., 1985). If so, our results provide insight in the rheology of the middle crust in the heart of a growing mountain belt. References Babaie, H. and LaTour, T., 1994. Semibrittle and cataclastic deformation of hornblende-quartz rocks in a ductile shear zone. Tectonophysics, v. 229, p. 19–30. Hacker, B. and Christie, J., 1990. Brittle/ductile and plastic/cataclastic transitions in experimentally deformed and metamorphosed amphibolite. The Brittle-Ductile Transition in Rocks. American Geophysical Union Geophysical Monograph 56, p. 127-147. Hobbs, B., Ord, A. and Teyssier, C., 1986. Earthquakes in the ductile regime? Pageoph, v. 124, p. 309-336. LaBerge, G., Cannon, W.F., Schulz, K., Klasner, J. and Ojakangas, R., 2003. Paleoproterozoic stratigraphy and tectonics along the Niagara suture zone, Michigan and Wisconsin. In: Cannon, W.F. (ed.), Institute on Lake Superior Geology Field trip Guidebook, v. 49, p. 1-32. Schulz, K. and Cannon, W.F., 2006. The Penokean orogeny in the Lake Superior region. Precambrian Research, v. 157, p. 4-25. Schulz, K. and Schneider, D. 2005. Age constraints on the Paleoproterozoic Pembine ophiolite-island arc complex and implications for the evolution of the Penokean orogen. Geological Society of America Abstracts with Programs, v. 37, no. 5, p. 4 Sibson, R., 1975. Generation of pseudotachylyte by ancient seismic faulting. Geophysical Journal of the Royal Astronomical Society, v. 43, p. 775-794. Sims, P. K., Peterman, Z., and Schulz, K., 1985. The Dunbar Gneiss-granitoid dome: Implications for early Proterozoic tectonic evolution of northern Wisconsin. Geological Society of America Bulletin, v. 96, p. 1101-1112. Sims, P.K. 1990. Geologic Map of Precambrian Rocks of Iron Mountain and Escanaba 1° × 2° Quadrangles, Northeastern Wisconsin and Northwestern Michigan. U.S. Geologic Survey Miscellaneous Investigations Series Map I-2056. 92 �METAMORPHISM AND DEFORMATION AT THE WABIOONQUETICO SUBPROVINCE BOUNDARY IN THE DECOURCEY LAKE AREA A.E., Nolan and M.L., Hill Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, Canada, P7B 5E1 The Wabigoon and Quetico subprovinces are east-west trending belts of metasedimentary and metavolcanic rocks located within the Superior province, the world’s largest preserved Archean craton. The Wabigoon-Quetico subprovince boundary in the Decourcey Lake area is complex and several kilometers wide. The 130m-long roadcut on Hwy 527 near Decourcey Lake is composed of a garnet-biotite to biotite schist with three intrusions, a pegmatitic intrusion, a felsic intrusion and a mafic dyke. There are also several quartz-carbonate veins that have been folded and boudinaged; the folded veins crosscut the boudinaged veins that are parallel to foliation. In the southern portion of the outcrop, mats of fibrolite in the schist indicate peak metamorphism at amphibolite facies or higher. However, some parts of the outcrop show evidence of overprinting retrograde metamorphism. The amount of retrograde metamorphism increases from south to north in the outcrop. In the southern part of the outcrop there are minimal amounts of chlorite (~1%) and the amount increases to about 30% in the north. The chlorite replaces biotite, the main mica in the southern portion of the roadcut. Chlorite replacing biotite is indicative of retrograde metamorphism. Other evidence for this retrograde metamorphism is the presence of stable garnet in the southern portion of the outcrop and metastable garnet in the northern portion, indicated by euhedral unfractured crystals in the south and anhedral, fractured and separated grains in the north (fig. 1). a b c Figure 1. a) Garnet from southern portion of the outcrop (stable), b) garnet from middle portion of the outcrop (metastable), c) garnet from northern portion of the outcrop (metastable) Throughout the schist, quartz with undulose extinction, irregular grain boundaries and subgrains provides evidence for deformation by dislocation creep. In the southern portion of the outcrop, undulose extinction in feldspar indicates deformation at amphibolite facies temperatures or higher. This higher temperature deformation of feldspar is not evident in the northern portion of the outcrop where retrograde metamorphism is more pervasive. The pegmatite and felsic 93 �intrusions preserve evidence of deformation at the amphibolite facies or higher, including undulose extinction in feldspar and bent mica grains (Fig. 2). Figure 2. Photomicrograph of the felsic intrusion showing evidence for dislocation creep The preservation of evidence for amphibolite facies metamorphism and high temperature deformation in schist in the southern portion of the outcrop as well as in the pegmatite and felsic intrusions suggests that the Decourcey Lake roadcut is composed of metamorphosed Quetico lithologies. Therefore, the Quetico-Wabigoon subprovince boundary lies to the north of this roadcut. The increase in retrograde metamorphism toward the northern portion of the outcrop (in the direction of the subprovince boundary) suggests that this lower temperature metamorphism may be associated with the boundary. 94 �BEDROCK GEOLOGIC MAP OF THE TWIN METALS MINNESOTA PROJECT, NORTHERN SOUTH KAWISHIWI INTRUSION AND ADJACENT AREAS Dean M. Peterson, Senior Vice President, Exploration, Duluth Metals Limited, 306 West Superior Street, Suite 407, Duluth, MN 55802 Twin Metals Minnesota LLC (TMM), is the joint venture company between Duluth Metals Limited (60% ownership interest) and Antofagasta plc (40% ownership interest). TMM is currently in the process of completing a prefeasibility study of the Maturi Cu-Ni-PGE deposit in the northern South Kawishiwi intrusion (SKI) of the Duluth Complex, northeastern Minnesota. Assuming a favorable outcome of the prefeasibility study, TMM will embark on a bankable feasibility study that will include extensive study of groundwater within and around the facilities (the proposed underground mine at Maturi, a concentrator facility, and a tailings storage facility). All of these studies would be incorporated into an environmental review of the project and would be open for comment and review by the general public. As the joint venture looks to the immediate future, Duluth Metals Limited believes that it is imperative that this process be as transparent as can be possible, especially on the publication of confidential geological data that could possibly be linked to water issues in an environmental review process. The bedrock geological map presented in this poster is the result of nearly two decades of geological work by the author integrated with data from seemingly innumerable government (Minnesota Geological Survey, U.S. Geological Survey, Natural Resources Research Institute, Minnesota Department of Natural Resources), academic (Minnesota-Duluth, Indiana, Minnesota, Wisconsin), and industry (Duluth Metals, Twin Metals, Franconia, INCO, Hanna, Bear Creek, Kennecott, Newmont, Duval, Encampment, etc…). 95 �96 �POTENTIAL FOR COPPER TOXICITY CAUSED BY SURFACE WATER AND STREAM SEDIMENTS IN UNMINED MINERALIZED WATERSHEDS OF THE DULUTH COMPLEX. PIATAK, Nadine M.1, SEAL, Robert R. II1, JONES, Perry M.2, WOODRUFF, Laurel G.2, 1 U.S. Geological Survey, Reston, VA 20192, npiatak@usgs.gov, rseal@usgs.gov U.S. Geological Survey, Mounds View, MN 55112, pmjones@usgs.gov, woodruff@usgs.gov 2 The characterization of baseline conditions in unmined mineralized watersheds of the Mesoproterozoic Duluth Complex, northeastern Minnesota, is essential to understanding how to responsibly extract minerals in one of the most prospective mining areas in the United States. Mining could release metals into watersheds that already contain ecologically-significant naturally-occurring amounts of some elements such as Cu and Ni. The potential for metals to be toxic to aquatic organisms is influenced by the amount of organic carbon in the aquatic environment, the cumulative effects of multiple metals, cation competition for biologic binding sites, and speciation of metals. We estimated toxicity in mineralized watersheds using approaches that incorporate these water and sediment quality parameters. Surface-water and streambed-sediment samples were collected from sites along three geologically distinct watersheds in the Duluth Complex: 1. Filson Creek where Cu-Ni-PGM mineralization occurs at the bedrock surface along the basal Duluth Complex; 2. Keeley Creek where Cu-Ni-PGM mineralization occurs only at great depth; and 3. the St. Louis River in the vicinity of Fe-Ti oxide ultramafic intrusions, which occur at the subcrop beneath glacial cover. Samples were collected in September 2012 near base-flow conditions in watersheds dominated by lakes, wetlands, and streams. The geochemistry of the surface waters and stream sediments reflects underlying rock types, glacially transported unconsolidated materials, mineralization style within each watershed, and geochemical processes occurring in the streams. The surface water is oxic, near neutral to slightly acidic (pH 5.9 to 7.6), has low total dissolved solids (41 – 94 mg/L), and is characterized by moderate hardness (18 – 50 mg/L CaCO3), moderate carbonate species concentrations (11 – 38 mg/L CaCO3 as bicarbonate), low sulfate (<0.8 – 3 mg/L), and high dissolved organic carbon (DOC) concentrations (18 – 47 mg/L). The dominant dissolved trace elements are Fe (472 – 3,950 µg/L), Al (54 – 228 µg/L), Cu (0.8 – 8 µg/L), Ni (1 – 5 µg/L), and Co (0.4 – 3 µg/L). Stream sediments contain significant Al (7 – 11 wt. %), Ca (1.5 – 6 wt. %), Fe (1 – 7 wt. %), and Na (2 – 4 wt. %). Sulfur is very low (<0.05 wt. %). Organic carbon reaches 4.7 wt. % in one sample but is ≤1.5 wt. % in all the other samples. Trace metals are dominated by Cr (14 – 346 mg/kg), Cu (10 – 179 mg/kg), Ni (13 – 127 mg/kg), and Zn (23 – 95 mg/kg). On average, Cu and Ni are highest in Filson Creek surface waters and sediments where Ni-Cu-PGM mineralization occurs at the surface. Samples collected from the St. Louis River watershed, where Fe-Ti oxide-bearing ultramafic rocks and a Paleoproterozoic shale/greywacke unit (Virginia Formation) occur, contain the highest average concentrations of As, Fe, and Pb in both surface water and sediments, Cr and Zn in sediment, and sulfate in waters. In water, the toxicity of most metals is assessed on the basis of hardness-based criteria that adjust for the protective effects of Ca and Mg ions, which compete with metal ions for binding sites on organisms. For sediment, consensus-based total-metal guidelines are routinely used and rely on laboratory toxicity tests that document increased toxicity caused by increased metal concentrations (McDonald and others, 2000). However, new guidelines that rely on the Biotic Ligand Model (BLM) utilize a more sophisticated approach incorporating more water and sediment quality parameters including the cumulative effects of multiple metals in sediment, metal speciation in water, and organic carbon complexes in both water and sediment (Di Toro and others, 2005; Paquin and others, 2001). The baseline surface-water and sediment metal concentrations can be compared to aquatic guidelines using the hazard quotient (HQ), which is the ratio of the concentration of a metal in the sample to the guideline. Values above 1 imply toxic conditions, whereas those below do not. In water, HQs for Cu are greater than 1 for several sites in the Filson Creek watershed when calculated using hardness-based criteria (Figure 1). However, as shown in Figure 1, HQs for Cu in water calculated based on the BLM model are significantly less than 1, suggesting a lack of toxicity from all samples. The radically different results from the hardness-based and BLM-based approaches suggest that the former may be inadequate to describe metal toxicity in these watersheds because it is based on a more limited set of parameters (i.e., only hardness). The complexation of Cu with DOC likely significantly affects the bioavailability of dissolved Cu, helping mitigate its toxicity. 97 �The sediment BLM approach also suggests a different level of predicted toxicity from sediments than predicted from consensus-based guidelines. Several HQs for Ni and one HQ for Cu are greater than 1 when calculated using the consensus-based guidelines, which suggests toxic conditions (Figure 2A). In comparison, no toxicity to uncertain toxicity is predicted based on Equilibrium Partitioning Sediment Benchmark (ESB) (USEPA, 2005) (Figure 2B). Specific considerations of this approach include: 1. determining extractable (a proxy for bioaccessible) metal concentrations (i.e., combined simultaneously extracted metals, ƩSEM); 2. adjusting them for potential incorporation into less bioaccessible sulfides (i.e., acid volatile sulfide, AVS); 3. and adjusting for complexation with organic carbon (i.e., fraction of organic carbon, foc). The high organic carbon in some of the sediments could sequester significant amounts of trace elements; however, the low AVS suggest trace elements bound to sulfides are not significant components in these sediments. Applying these more sophisticated and holistic approaches enhances our capability to predict metal toxicity. This improved understanding will be advantageous when developing successful strategies to help minimize future mining impacts and develop appropriate restoration goals. References Di Toro, D.M., McGrath, J.M., Hansen, D.J., Berry, W.J., Paquin, P.R., Mathew, R., Wu, K.B., and Santore, R.C., 2005, Predicting sediment metal toxicity using a sediment biotic ligand model: Methodology and initial application: Environmental Toxicology and Chemistry, v. 24, no. 10, p. 2410-2427. MacDonald, D.D., Ingersoll, C.G., and Berger, T.A., 2000, Development and evaluation of consensus-based sediment quality guidelines for freshwater ecosystems: Archives of Environmental Contamination and Toxicology, v. 39, no.1, p. 20-31. Paquin, P.R., Gorsuch, J.W., Apte, Simon, Batley, G.E., Bowles, K.C., Campbell, P.G.C., Delos, C.G., Di Toro, D.M., Dwyer, R.L., Galvez, Fernando, Gensemer, R.W., Goss, G.G., Hogstrand, Christer, Janssen, C.R., McGeer, J.C., Naddy, R.B., Playle, R.C., Santore, R.C., Schneider, Uwe, Stubblefield, W.A., Wood, C.M., and Wu, K.B., 2002, The biotic ligand model: A historical overview: Comparative Biochemistry and Physiology, v. 133, no. 1-2, p. 3–35. U.S. Environmental Protection Agency, 2005, Procedures for the derivation of equilibrium partitioning sediment benchmarks (ESBs) for the protection of benthic organisms: Metal mixtures (cadmium copper, lead nickel, silver, and zinc): U.S. Environmental Protection Agency 600-R-O2-011, variously paginated. 98 �MESOPROTEROZOIC MIDCONTINENT RIFT INTRUSIVES IN THE THUNDER BAY AREA (ONTARIO, CANADA): A PALEOMAGNETIC REVIEW PIISPA, Elisa J., SMIRNOV, Aleksey V. Department of Geological and Mining Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive, 630 DOW ESE Building, Houghton, MI 49931-1295, USA and PESONEN, Lauri J. P. Department of Physics, Division of Geophysics and Astronomy, University of Helsinki, Helsinki, Finland Mafic sills and dykes extend over 300 km from south of Thunder Bay to northeast of Lake Nipigon, representing the northern expression of the ~1.1 Ga Midcontinent Rift System (MRS). Recent geochemical and geochronological studies have significantly improved our understanding of the area geology. The Logan sills, south of Thunder Bay, are geochemically similar but not identical to the sills exposed in the vicinity of Lake Nipigon, which in turn can be divided into three separate groups based on their geochemical signatures (e.g. Hollings et al., 2010, 2012). In addition, several discrete mafic/ultramafic intrusions and dyke swarms that represent both the earliest and the latest stages of the Midcontinent Rift magmatism are exposed along the north shore of Lake Superior (Heaman et al. 2007; Hollings et al., 2010). The dykes within the Thunder Bay area are currently grouped into four lithological units based on their orientation, petrology and geochemical differences: the Mount Mollie dyke and the Sibley, Pigeon River and Cloud River dykes (Hollings et al. 2010, 2012; Cundari et al., 2012). In general, the magnetostratigraphy of the MRS can be summarized as follows: a) The earliest rock sequences (~1115-1105 Ma) are reversely magnetized; b) A polarity reversal (or reversals) occurred between 1105-1102 Ma; c) The rocks emplaced after ~1102 Ma are normally magnetized. Withstanding the age uncertainty, this geomagnetic polarity sequence allows for an approximate correlation within and between the MRS rock sequences. When combined with high quality geochronological and petrographical observations as well as detailed geochemical and isotope data, paleomagnetic data can provide valuable information of the development of the MRS. We present the new results of our rock magnetic and paleomagnetic investigation of several MRS intrusives exposed in the vicinity of Thunder Bay (Ontario, Canada). We also re-evaluate the previously published paleomagnetic data based on the newly published geochronological and geochemical data from the sills and dykes of the Thunder Bay area. Finally, we will critically address the observed inconsistencies between the field observations 99 �and paleomagnetic and geochronological data. This study contributes to better understanding of the MCR magnetostratigraphy and further improvement of the late Mesoproterozoic apparent polar wander path for North America. References: Cundari, R.M., Hollings, P., Smyk, M.C., Scott, J.F. and Campbell, D.A. 2012. Whole rock and isotope data from the Midcontinent Rift: implications for crustal contamination history; in Summary of Field Work and Other Activities 2012, Ontario Geological Survey, Open File Report 6280, p.18-1 to 18-10. 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.289303. Hollings, P., Smyk, M., Heaman, L.M., and Halls, H. 2010. The geochemistry, geochronology, and paleomagnetism of dikes and sills associated with the Mesoproterozoic Midcontinent Rift near Thunder Bay, Ontario, Canada. Precambrian Research. Precambrian Research, v. 183, iss. 3, p.553-571. Hollings, P., Smyk, M.C. and Cousens, B. 2012. The radiogenic isotope characteristics of dikes and sills associated with the Mesoproterozoic Midcontinent Rift near Thunder Bay, Ontario, Canada; Precambrian Research, v.214-215, p.269-279. 100 �DOCUMENTING THE FIRST LAVA FLOWS OF THE MIDCONTINENT RIFT BY DIGITAL MAPPING AND PETROGRAPHIC ANALYSIS QUILLEN, Patrick, and Miller, Jim Department of Geological Sciences, University of Minnesota Duluth, Duluth, MN 55812 The Ely’s Peak basalts (EPB) located west of Duluth represent the first lava flows to be erupted from the Midcontinent Rift. The EPB lie conformably on siltstones, sandstones, and conglomerates of the Nopeming sandstone, which in turn unconformably overlies the Paleoproterzoic Thomson Formation. This classic outcrop area (Jirsa and Morey, 1979; Green, 1999; Jirsa and Green, 2011) has been a popular field trip location for many years, and today it’s used as a field mapping exercise for a Geologic Maps course at UMD (GEOL 3000) and the Precambrian Research Center’s field camp (GEOL 4500). The area was initially mapped by J.A. Kilburg as part of his MS thesis at UMD (Kilburg, 1972) with a reconnaissance map later produced by Kilburg and Morey (1977). Kilburg (1972) noted that the Duluth Complex thermally metamorphosed the EPB flows and made them difficult to distinguish. The focus of this project is exposure of the EPB north of Interstate 35 (Fig. 1). The research questions for this project were to determine the number of lava flows represented in the area, their petrographic characteristics and their geographical distribution of exposure. The data collected from several years of mapping and new mapping conducted for this study were also compiled and digitized with ArcMAP 10. For this study, 17 new were collected and made into thin sections and combined with 17 samples collected previously. A petrographic analysis of this suite of sections were made, noting textures and mineral assemblages comprising the groundmass, phenocrysts, and amygdules that might be useful for distinguishing individual lava flows and revealing the nature and intensity of thermal metamorphism. Figure 1. Bedrock geology of the field area based on previous mapping by Kilburg (1972). Significantly more outcrops have been discovered by mapping conducted for the UMD geology courses. 101 Prior mapping by Kilburg (1972) and UMD classes concluded that there were two main types of flows. The first is a lower sequence of variably amydaloidal, dense, pyroxene-phyric basalts overlying the Nopeming sandstone. Pyroxene phenocryts up to 1 cm in diameter appear to compose between 10 and 40 vol.% of these flows, but up to 90% pyroxene phenocrysts have been locally observed. Amygdaloidal zones composed of chlorite, quartz and epidote amygdules are locally observed, but flow contacts are difficult to recognize, presumably due to anealling by thermal metamorphism. In the eastern third of the EPB exposure area (Fig. 1), a distinctly aphyric basalt is recognized. It is also distinguishable from the pyroxene-phyric flow by being moderately magnetic. The contact �with the Duluth Complex on the east side of the field area is somewhat ambiguous by being marked by a fine-grained, massive gabbro that has been variably interpreted to be intensely metamorphosed basalt or chilled gabbro. Petrographic observations from this study reveal several interesting and unexpected results. Most notable is that the previous interpretation of pyroxene-phyric basalt overlain by aphyric basalt is a significant oversimplification of the area. Several samples collected from the porphyritic basalt area are actually aphyric. Also, what have been assumed to be pyroxene phenocrysts are actually clusters of pyroxene that may be glomerphenocrysts or perhaps xenoliths of pyroxenite. Curiously, they commonly have amphibolitic reaction rims, suggesting that they may be out of equilibrium with the enclosing basaltic groundmass and thus be xenoliths. The few exposures that have been mapped as comprising an area of aphyric basalt instead appear to be more related to the Duluth Complex. They are subophitic, olivine gabbro and could potentially be the chilled contact zone of the Duluth gabbro. Another surprising observation is that given the density of these basalts, granoblastic recrystallization textures are rarely noted. Instead, original intergranular igneous textures dominate the sections observed. This poster presentation will show a revised geological map and of the area and display photomicrographs of the textures and mineral assemblages observed. References Green, J.C., 1999, Proposal to designate the Grandview Area as a state Scientific and Natural Area for its geological importance. Unpublished report submitted to the MN Dept. of Natural Resources, 6p. Jirsa, M.A., and Morey, G.B., 1979, Jay Cooke State Park and Grandview area: Evidence for a major Early Proterozoic-Middle Proterozoic unconconformity in Minnesota. Geological Society of America Centennial Field Guide – North Central Section, p. 67-72. Jirsa, M.A. and Green, J.C, 2011, Classic Precambrian geology of northeast Minnesota . In Miller, J.D., Hudak, G.H., Wittkop, C., and McLaughlin, P.I., eds., Archean to Anthropocene: Field Guides to the Geology of the Mid-Continent of North America: Geological Society of America Field Guide 24, p. 25-45. Kilburg, J.A, 1972, Petrology, structure, and correlation of the upper Precambrian Ely’s Peak basalt. Unpublished MS thesis, University of Minnesota Duluth, 97p. Kilburg, J.A., and Morey, G.B., 1977, Reconnaissance geologic map of the Esko quadrangle, St. Louis and Carlton Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-25, scale 1:24,000. 102 �GEOCHEMISTRY AND PETROGRAPHY OF A MAFIC METAVOLCANIC SEQUENCE SOUTH OF MUSSELWHITE MINE QUINN, Jordan1, HOLLINGS, Pete1, BICZOK, John2 1 2 Department of Geology, Lakehead University, 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada Goldcorp Canada Ltd., Musselwhite Mine, P.O. Box 7500, Thunder Bay, Ont., P7B 6S8, Canada The North Caribou Greenstone Belt (NCGB), within the North Caribou Terrane of the Archean Superior Province, is host to multiple ~3.0Ga metavolcanic and metasedimentary assemblages. The assemblages have been metamorphosed from greenschist to upper-amphibolite grade and are bounded by ~2.7-3.0 Ga granitoids and gneisses (Biczok et al., 2012). The study area is located approximately 5km south of Musselwhite Mine within what was previously thought to be the Opapimiskan-Markop metavolcanic assemblage. There were three lithologies identified within the study area (basaltic, komatiitic, and felsic volcanic flows) which have been subdivided into four separate volcanic suites: Volcanic Suite A, Volcanic Suite B, Volcanic Suite C, and Felsic Volcanic Suite. Volcanic Suite A is comprised of a succession of massive and pillowed basaltic flows that were metamorphosed to amphibolites. These flows exhibit a mineral assemblage of amphibole, chlorite, and plagioclase with minor quartz, muscovite, titanite and epidote. Major element geochemistry reveals that this suite is compositionally similar to that of a high-Mg tholeiitic basalt. Primitive mantle normalized plots for this volcanic suite are characterized by a flat rare-earth element (REE) pattern comparable to tholeiites from the South Rim Unit (SRU) (Fig. 1), which have been interpreted to represent oceanic island plateau basalts formed from a mantle plume (Hollings et al., 1999). Figure 1: Comparison of primitive mantle plots from Volcanic Suite A versus tholeiites from the South Rim Unit (Blue trace). Volcanic Suite B was comprised of pillowed and massive basaltic flows that have been metamorphosed to amphibolites. The main mineral assemblage observed in this suite was amphibole and chlorite with minor plagioclase, clinozoisite, quartz, titanite and dolomite. Major element geochemistry indicates that this suite is comprised of high-Mg tholeiitic basalts, komatiitic basalts, and a komatiite. Primitive mantle normalized plots display a relatively flat REE pattern but with a negative Nb anomaly (Fig. 2). The similar 103 �trace element geochemistry of Volcanic Suites A and B suggests that they are both derived from a plume source, however, the negative Nb anomaly in Volcanic Suite B indicates that it has undergone crustal contamination during emplacement. Figure 2: Primitive mantle normalized plot of the samples from Volcanic Suite B. Volcanic Suite C was also comprised of massive and pillowed basaltic flows with a main mineral assemblage of amphibole and chlorite. A single sample was taken from this suite and it was determined to be a high-Fe tholeiitic basalt based on major element geochemistry. Primitive mantle plots of this suite are light rare-earth element (LREE) enriched with a negative Nb anomaly and positive Zr and Hf anomalies. A similar REE pattern was observed in the tholeiitic basalts from the Opapimiskan-Markop Unit (OMU) (Hollings and Kerrich, 1999). The felsic volcanic suite overlies the mafic volcanic suites and is comprised of rhyolitic flows. This suite was LREE enriched with a relatively flat heavy rare-earth element (HREE) pattern and negative Nb and Ti anomalies in conjunction with positive Zr and Hf anomalies. Similar REE patterns were observed in the SRU and interpreted to be derived from a subduction tectonic setting (Hollings et al., 1999). The results of this study are consistent with previous work in the region and suggest that the early history of the area preserved the interaction of a mantle plume with preexisting continental crust. In addition this study has refined the boundaries of the various assemblages within the NCGB. References Biczok, J., Hollings, P., Klipfel, P., Heaman, L., Maas, R., Hamilton, M., Kamo, S., Friedman, R., 2012. Geochronology of the North Caribou greenstone belt, Superior Province Canada; implications for tectonic history and gold mineralization at the Musselwhite Mine. Precambrian research 192:209230. Hollings, P., Wyman, D., Kerrich, R., 1999. Komatiite-basalt-rhyolite volcanic associations in northern Superior Province greenstone belts; significance of plume-arc interaction in the generation of the proto continental Superior Province. Lithos 46.1:137-161. Hollings, P., Kerrich, R., 1999. Trace element systematics of ultramafic and mafic volcanic rocks from the 3Ga North Caribou greenstone belt, northwestern Superior Province. Precambrian research 93.4:257-279. 104 �THE ARROWHEAD PILOT PROJECT: MAPPING OF PRECAMBRIAN AND QUATERNARY GEOLOGY IN TWO DIVERSE GEOLOGIC AREAS OF NORTHEASTERN MINNESOTA RADAKOVICH, A.R.1, and HOBBS, H.C.1 1 Minnesota Geological Survey, St. Paul, MN 55114, rada0042@d.umn.edu, hobbs001@umn.edu The Arrowhead Pilot Project was undertaken by the Minnesota Geological Survey (MGS) to explore the feasibility of County Geologic Atlas-style mapping in NE Minnesota. It integrates new field mapping by the authors with archived data to provide both Precambrian and Quaternary geologic interpretations of two areas of interest in northeastern Minnesota (Fig. 1). Two distinct areas were mapped: the western one contains significant exposure of Archean, Paleoproterozoic and Mesoproterozoic bedrock, and relatively thin and patchy Quaternary deposits; the eastern area has limited bedrock exposure of Mesoproterozoic rocks, and thick Quaternary glacial cover. Together these areas cover part or all of fifteen 7.5’ quadrangles. The intent of this study was multi-faceted: (1) to compile previous maps (both Precambrian and Quaternary) of these areas into single, coherent maps; (2) to augment gaps in data with new mapping; (3) to assess the usefulness of LiDAR (Light Detection And Ranging altimetry) imagery in identifying bedrock outcrop and surficial features; and (4) to assess the costs and feasibility of a mapping project at a similar level of detail over a much larger area, such as that typical of a County Geologic Atlas produced by the MGS. The motivation to produce large scale, comprehensive regional map products is in part driven by increased interest in extracting mineral resources, as well as management of ground and surface water resources. Further, the area’s complex bedrock geology provides insight into more than 1.5 billion years of earth’s history in three distinct terranes: the Archean Giants Range batholith, the Paleoproterozoic Animikie Group, and the Mesoproterozoic Duluth Complex. This study area is also key to understanding the glacial history of the region, as it includes the interlobate junction between the Rainy and Superior lobes, and spans the transition in the provenance of Rainy lobe clasts from the Duluth Complex-dominated eastern portion to the granitedominated western part. Figure 1. Location of study areas showing 1-meter LiDAR (Light Detection and Ranging Altimetry) imagery. 105 �The bedrock portion of the study resulted in creation of modified bedrock geologic and bedrock topographic maps, and a detailed fault and lineament analysis. In the western area, new mapping north of the Mesabi Iron Range helped provide a more complete characterization of the Archean Giants Range batholith. Mapping south of the Iron Range also helped better define layers of the Mesoproterozoic South Kawishiwi Intrusion in the Duluth Complex. In the eastern area, new outcrop data combined with improved aeromagnetic and gravity data helped modify and extend unit contacts within Mesoproterozoic volcanic sequences. LiDAR analysis combined with field mapping and structural analysis identified significant bedrock-controlled linear topographic features which were classified as faults, lineaments, or igneous foliations. Such features are known to potentially play significant roles in the hydrogeologic system. Finally, mapping of the bedrock surface incorporated data from County Well Index (CWI) records, passive seismic data, outcrop elevations, and LiDAR; It revealed that depth to bedrock varies considerably across the area: from zero to as much as 250 feet (~75m). The Quaternary portion of the study revealed the Highland moraine of the Superior lobe to be highly collapsed and strewn with ice-walled lake plains, indicating widespread stagnant ice. The overall texture of the unsorted material of the Highland moraine is rocky sandy loam. The lake plains are composed of sorted material, typically sand and gravel on the raised rims, grading vertically down to fine sand and silt. The center of the plains are presumably composed of silt and clay, but were not investigated. Subglacial meltwater from two large esker systems coalesced and spewed large volumes of meltwater, which deposited outwash that followed the edge of the retreating Rainy lobe. The meltwater was ponded for a time in glacial Lake Dunka, and ultimately flowed through a gap in the Giant’s Range into a lake indirectly connected to glacial Lake Agassiz. The Rainy lobe built several distinct recessional moraines in the mapping area; the Vermilion moraine is the last Rainy lobe moraine south of the International Border. The ice deposited a relatively thin, extremely coarse till between the moraines, and did not discharge copious amounts of meltwater. Rogen moraine ridges are common north of the Vermilion moraine; these ridges formed under the ice, and do not represent ice margins. The Arrowhead Pilot Project demonstrates that successful products can result from regional mapping in areas of northeast Minnesota given appropriate time, funding, and creativity. However, areas of especially sparse data and remote settings will necessarily result in diminished mapping detail, particularly in the depiction of the subsurface distribution of Quaternary materials, compared to other parts of the state where the MGS has produced County Geologic Atlas maps. Nevertheless, even with that limitation, County Geologic Atlas map products for the region would provide a markedly improved geologic framework that would facilitate resource management decisions. Selected References Boerboom, T.J., and Miller, J.D., Jr., 1994., Bedrock geologic map of the Silver Island Lake, Wilson Lake, and western Toohey Lake quadrangles, Lake and Cook Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map M-81, scale 1:24,000. Jirsa, M.A., Chandler, V.W., and Lively, R.S., 2005, Bedrock geology of the Mesabi Iron Range, Minnesota: Minnesota Geological Survey Miscellaneous Map M-163, scale: 1:100,000. Jirsa, M.A., and Miller, James D., Jr., 2004, Bedrock geology of the Ely and Basswood Lake 30' X 60' quadrangles, northeast Minnesota: Minnesota Geological Survey Miscellaneous Map M-148, scale: 1:100,000. Foose, M.P., and Cooper, R.W., 1978, Preliminary geologic report on the Harris Lake area, northeastern Minnesota: U.S. Geological Survey Open-File Report 78-385, 24 p., 1 pl., scale 1:12,000. Friedman, Albert L., 1981, Surficial geology of the Isabella quadrangle, northeastern Minnesota: Unpublished Master's thesis, University of Minnesota. Miller, James D., Jr., and Severson, M.J., 2005, Bedrock geology of the Babbitt quadrangle, St. Louis and Lake Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map M-159, scale 1:24,000. Miller, James D., Jr., and Severson, M.J., 2005, Bedrock geology of the Babbitt Northeast quadrangle, St. Louis and Lake Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map M-160, scale 1:24,000. Miller, James D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.M., 2001, M-119 Geologic map of the Duluth Complex and related rocks, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map M-119, scale: 1:200,000 and 1:500,000. Miller, J.D., Jr., Boerboom, T.J., and Jerde, E.A., 1994, Geologic map of the Cabin Lake and Cramer 7.5-minute quadrangles, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-82, scale 1:24,000. Severson, M.J., 1994, Igneous stratigraphy of the South Kawishiwi intrusion, Duluth Complex, northeastern Minnesota: Duluth, University of Minnesota, Natural Resources Research Institute Technical Report NRRI/TR-93/34, 210 p. + plates. Stark, James R., 1977, Surficial geology and ground-water geology of the Babbitt-Kawishiwi area, northeastern Minnesota with planning implications: Unpublished Master's thesis, University of Wisconsin-Madison. 106 �TOOLS FOR INTERPRETING KEWEENAW GEOHERITAGE TO A BROAD PUBLIC ROSE, William I and VYE, Erika, Department of Geological and Mining Engineering and Sciences. Michigan Technological University, Houghton, MI 49931 In the United States, the public is seemingly isolated from geoheritage, perhaps due to a disconnect between the geoscience academic community and how we communicate what we know. Recently retired, and with nearly 45 years in Houghton, a place with a strong geoheritage, the first author has begun to focus on communicating Earth Science to the a broader public with vital help from his co-author. This has generated new interpretive activities and tools paramount for engaging the public in learning about how and why their place has come to be the way it is. Boulder Gardens. Around Lake Superior we have an abundant teaching resource of glacial erratics. We moved some of the most exemplary ones in our region (some with difficulty!) to a public square in the center of campus to serve as an educational and cultural focus (Rose, 2011). The boulders have fresh, glacially polished surfaces and are an assemblage of dozens of outcrops representing all the lithologies of the Keweenaw Rift in one succinct location. The site has drawn educational attention, and is especially useful as an introduction to field trips of the area. GPS, smart phones, QR codes, EarthCaches. We have embraced treasure hunting and technology-based tools to engage people in learning about geolocations, fundamental to 3D visualization and “reading the landscape”. We have identified more than 150 field sites in the Keweenaw and linked them to .kmz file information, Google Maps, and QR codes to be accessed via smartphones. We have also very successfully contributed to GSA’s EarthCache efforts and database (Gochis et al. 2013). Community geotours - bike/walk and trolley. We fashioned a geotour of our local town (Houghton) which can be done on foot or bike. The tour identifies and interprets a variety of features such as mines, large lava flows, faults, veins, aa and pahoehoe flows, amygdaloids, glacial features, river deltas, kame terraces and Anthropocene features in the town (Rose et al. 2013). The Geotour is integrated with other heritage tours and has been successfully used as a fundraiser that uses the local trolley to access a wider range of sites. Grassroots Partnership. We work with local groups who share common goals of conservation and public access to field sites. Partners include the local Chamber of Commerce, Copper Country Trail 107 �National Byway, the Keweenaw Land Trust, the Michigan Nature Association, the Nature Conservancy, national and state parks, local towns and villages, historical societies, museums, and local businesses all wishing to disseminate geoheritage information. A demonstration of geotours with a broad audience (ex. “elderhostal” format) is being done this summer with five two-day geotours which use combined van and boat transport to visit many remote sites of the Keweenaw. The goal is to facilitate public geotours of greater depth which last for several days. We have built our network to engage Earth science teachers, who can replicate geointerpretation efforts and transfer them to their students. This, in turn, helps us find more geosites as they are in everyone’s own yards! International Geoheritage links. We have a strong partnership with colleagues at European Geoheritage sites who are teaching us successful strategies for international geoheritage status; common in Europe, China and other parts of the world, but so far unknown in the US (VanWyk deVries, 2013). All of these efforts are found on a single Keweenaw Geoheritage website (http://www.geo.mtu.edu/ ~raman/SilverI/KeweenawGeoheritage), which broadly links shared technical, geographic and heritage information. We invite public input about this process; how can we improve geoheritage outreach? References: ROSE, WI 2011, KEWEENAW BOULDER GARDEN—A REVITALIZED KAME TERRACE ON CAMPUS, USED AS A TEACHING LABORATORY GSA Abst w Programs 43 (5), p 25 (https://gsa.confex.com/gsa/2011AM/ finalprogram/abstract_195146.htm) GOCHIS, Emily E.1, ROSE, William I.1, VYE, Erika C.1, HUNGWE, Kedmon2, MATTOX, Stephen R.3, and PETCOVIC, Heather4, 2013, INCREASING AWARENESS OF GEOHERITAGE SITES & EARTH SCIENCE LITERACY THROUGH TEACHER-DEVELOPED EARTHCACHES GSA Annual Meeting in Denver: (27-30 October 2013) Paper No. 349-6 (https://gsa.confex.com/gsa/2013AM/finalprogram/abstract_233117.htm) VYE, Erika C.1, ROSE, William I.1, KLAWITER, Mark F.2, and GOCHIS, Emily E. 2013, THE IMPORTANCE OF PARTNERSHIPS FOR IMPROVED EARTH SCIENCE LITERACY AND THE COMMUNICATION OF GEOHERITAGE GSA Annual Meeting in Denver: (27-30 October 2013) Paper No. 349-5 (https://gsa.confex.com/ gsa/2013AM/finalprogram/abstract_232797.htm) ROSE, William I.1, VYE, Erika C.2, KLAWITER, Mark F.2, and GOCHIS, Emily E.2 2013, GEO/BIKE WALK COMMUNICATES GEOHERITAGE IN HOUGHTON, MICHIGAN GSA Annual Meeting in Denver: (27-30 October 2013) Paper No. 318-6 (https://gsa.confex.com/gsa/2013AM/finalprogram/ abstract_226444.htm) VAN WYK DE VRIES, Benjamin, 2013 GEOHERITAGE AND SENSE OF PLACE OF THE CHAîNE DES PUYS AND LIMAGNE FAULT: HOW PEOPLE UNDERSTAND GEOSCIENCE THOUGH BELONGING TO THEIR LANDSCAPE, GSA Annual Meeting in Denver: (27-30 October 2013) Paper No. 318-10 (https://gsa.confex.com/ gsa/2013AM/finalprogram/abstract_223880.htm) 108 �STRUCTURAL CONTROL OF MINERALIZATION AT LAC DES ILES MINE S. SCHMIDT and M.L. HILL Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, P7B 5E1 SSchmid1@lakeheadu.ca North American Palladium Ltd.’s Lac des Iles mine is located approximately 90km north of Thunder Bay, ON and is the only mine in Canada that is a primary producer of palladium. The mafic Mine Block intrusion that hosts the ore is located in the Wabigoon subprovince north of the regional boundary with the Quetico subprovince, within the Superior province. The property yields evidence for high-temperature deformation in the solid state, indicating that the intrusion is pre- and/or syntectonic, rather than post-tectonic as commonly presumed. The purpose of this MSc thesis project is to discover and examine evidence for high-temperature deformation and assess any controls this deformation may have on the mineralization. Evidence for hightemperature deformation is documented in the North Varitextured rim, Baker zone, Sheriff zone, and Creek zone. From the analysis of structural measurements and field relationships in these areas, two populations of narrow ductile shear zones have been recognized in the North Varitextured rim (NVT), Baker zone, Sheriff zone, and Creek zone. One population has an average orientation of 319/85 (n=30) with a dextral sense of shear, the other population has an average orientation of 058/83 (n=44) with a sinistral shear sense. The NW-striking dextral shear zones are commonly parallel to intrusive features and are variably foliated. The NE-striking sinistral shear zones are discordant to intrusive features. The presence of quartz-carbonate veins within the NE-striking sinistral shear zones may indicate reactivation and/or a local tensile component to stress. The orientation of the intersection lineation between the two populations of ductile shear zones is 81/108. A mutually cross-cutting relationship has been found indicating conjugate formation under the same stress field. 109 �110 �MIDCONTINENT RIFT-RELATED SATELLITE MAFIC-ULTRAMAFIC INTRUSIONS HOSTING FE-TI-V OXIDE DEPOSITS Schulz, K.J., U.S. Geological Survey, 954 National Center, Reston, VA 20192, kschulz@usgs.gov, Woodruff, L.G., U.S. Geological Survey, 2280 Woodale Ave., Mounds View, MN 55112, woodruff@usgs.gov, and Nicholson, S.W., Geological Survey, 954 National Center, Reston, VA 20192, swnich@usgs.gov. The best known Fe-Ti-V oxide deposits in the Midcontinent Rift are in the Duluth Complex, northeastern Minnesota, in two types of deposits: 1) titanomagnetite/ilmenite-rich layers in the early (1107 Ma) Poplar Lake intrusion (formerly Nathan’s Layered Series), and 2) late discordant Oxide-bearing Ultramafic Intrusions (OUI) such as the Longnose and Water Hen intrusions. Less well known are Fe-Ti-V oxide deposits that occur in relatively small (<10 km) mafic-ultramafic intrusions emplaced in country rocks surrounding the Midcontinent Rift. These intrusions are currently known to extend from northwestern Wisconsin into southeastern Minnesota and northeastern Iowa based on geophysics and limited drill core (Fig. 1). The Round Lake intrusion in northwestern Wisconsin is characterized by a large amplitude negative aeromagnetic anomaly; the other intrusions (Clam Lake, WI; Fillmore B1, MN; and Osborne, IA) are characterized by large positive amplitude aeromagnetic anomalies. The Clam Lake intrusion appears to be a plug-like body (Mudrey and others, 2003); the other intrusions appear as linear, northeast-trending dikelike bodies. All the intrusions show variable modal silicate-oxide mineral layering at scales ranging from centimeters to meters; oxide mineral content (Ti-magnetite with variable ilmenite) varies from a few percent to locally massive layers and from intercumulus to cumulus in texture. Strong to moderate igneous flow foliation, defined by aligned plagioclase crystals, is common in all the intrusions. The Clam Lake intrusion is composed of oxide gabbro (plag+cpx+oxide) with some clinopyroxenite layers. Round Lake is dominantly composed of oxide troctolite and melatroctolite (plag+ol+oxide). Both the Clam Lake and Round Lake intrusions are cut by diabase dikes. The Fillmore B1 and Osborne intrusions show greater variability particularly with respect to olivine content and contain oxide dunite and peridotite (ol+oxide) as well as oxide troctolite and melatroctolite (plag+ol+oxide). The Osborne intrusion is oxideand olivine-rich in the upper portion and becomes more plagioclase-rich with depth; it also contains oxide-rich noritic anorthosite layers (plag+opx+oxide). The major element compositions of these intrusions largely reflect their cumulate mineralogy but are dominated by their oxide mineral content. Phosphorous contents are uniformly low (<0.5 wt.%) in all samples and not correlated with TiO2 content. Overall trace element abundances are mostly low as would be expected of dominantly cumulate rocks with low interstitial melt contents. Cobalt, Ni, Sc, and V generally show positive correlations with TiO2 content suggesting that their concentrations are controlled by oxide mineral content. Samples from Round Lake have V contents considerably higher than samples from the other intrusions (up to ~4,500 ppm); the differences in V content may reflect differences in oxygen fugacity between intrusions as V partitioning is strongly dependent on oxygen fugacity. High field strength element (HFSE) covariations within and between intrusions are variable. Samples with high TiO2 mostly show positive Nb-Ta anomalies on primitive mantle-normalized (PMN) trace element plots. However, positive Nb-Ta anomalies are highest in samples from the Clam Lake and Osborne intrusions and weak to absent in samples from Round Lake. In contrast, samples show varying Zr-Hf anomalies on PMN trace element plots ranging from positive anomalies in samples from the Iowa intrusion to no or negative anomalies in samples from Round Lake and Clam Lake. Given the general correlation between HFSE and TiO2 content, it is likely that the HFSE variations are controlled by oxide minerals with different partition coefficients controlled by changing oxygen fugacity. 111 �The REE data show that the intrusions are related to more than one magma type. The Round Lake intrusion has relatively steep REE patterns with enriched light REE and depleted heavy REE. The REE patterns match those of the basal, magnetically reversed basaltic lavas from Pigeon Point and Ely’s Peak. The Fillmore B1 and Osborne intrusions have similar slightly enriched light REE and flat heavy REE patterns. They are likely related to Portage Lake-Chengwatana-equivalent high-TiO2 basalt. The REE patterns for the Clam Lake intrusion have flat light REE and depleted heavy REE; they overlap REE patterns from the BIC intrusion and Eagle Deep dikes in the Baraga basin of Michigan. The Midcontinent Rift-related satellite mafic-ultramafic intrusions and their Fe-Ti-V oxide deposits are very similar to the intrusions hosting Fe-Ti-V oxide deposits in the Permian Emeishan large igneous province of southwest China (Pang and others, 2010). Like the China examples, the Midcontinent Rift intrusions likely formed as conduits experiencing frequent replenishment of fractionated, crystal-rich high-Ti mafic magmas. References cited Mudrey, M.G., Jr., Ervin, C.P., and Olmstead, J.F., 2003, Middle Keweenawan basin evolution inferred from geophysical analysis of a strongly magnetic intrusion, Clam Lake, Wisconsin: Wisconsin Geological and Natural History Survey, Open-File Report 2003-04, 17 p. Pang, K-N, Zhou, M-F, Qi, Liang, Shellnutt, Gregory, Wang, C.Y., and Zhao, Donggao, 2010, Flood basalt-related Fe-Ti oxide deposits in the Emeishan large igneous province, SW China: Lithos, v. 119, p. 123–136. Figure 1. Location of Midcontinent Rift-related satellite mafic-ultramafic intrusions hosting Fe-Ti-V oxide deposits. 112 �THE 2.7 BILLION YEAR OLD MT. ST. HELENS OF NORTHERN MINNESOTA: PETROGRAPHY, GEOCHEMISTRY AND ECONOMIC SIGNIFICANCE OF THE NEOARCHEAN GAFVERT LAKE SEQUENCE SCHWIERSKE, Kelly L.1, PIGNOTTA, Geoffrey S.1 and HUDAK, George J.2 1 University of Wisconsin-Eau Claire, Department of Geology, 105 Garfield Ave., Eau Claire, WI 54701 Precambrian Research Center, Minnesota Natural Resources Research Institute, University of Minnesota-Duluth, 5013 Miller Trunk Hwy, Duluth, MN 55811 2 The Neoachean Gafvert Lake sequence comprises part of the Vermilion District in the Wawa-Abitibi Terrane in northeastern Minnesota and is located in Minnesota’s newest state park, Lake Vermilion State Park (Fig. 1). The Wawa-Abitibi Terrane is the most economically important granite-greenstone belt in the Superior Province, and hosts a wide variety of mineral deposits (including but not limited to shear zone hosted gold deposits, volcanogenic massive sulfide deposits, komatiite-hosted copper-nickelplatinum group element deposits, rare earth element deposits, diamond deposits) in its extents from Minnesota to northeastern Quebec. There has been minimal historic economic mineral exploration in this region despite the striking similarities between the Vermilion District and prolific metal (e.g., Au, Cu, Zn) producing regions across the border in Ontario, Canada. The Gafvert Lake sequence was initially recognized by Peterson and Jirsa (1999) and appeared to represent a stratovolcano complex located immediately up-section from the Soudan Iron Formation, an Algoma-type iron formation unit that hosted Minnesota’s first iron mine, the Soudan Mine. Recent mapping in the Vermilion District, northeast of Ely, MN has documented the regional distribution of rocks associated with the Gafvert Lake sequence which consists of intermediate to felsic volcanic and volcaniclastic rocks intruded by intermediate plutons that are likely age equivalent (Hudak et al., 2004). A dacitic tuff breccia from the Gafvert Lake sequence yielded a 2689.7 ± 0.8 Ma U-Pb age indicating that these deposits lie unconformably on Lower Ely and Soudan members of the Soudan belt (Fig. 1; Lodge et al., 2013). This project examines the petrographic, geochemical and structural characteristics of the Gafvert Lake sequence. In the field, this package of volcanics and associated plutons is strikingly similar to other arc volcano-plutonic complexes found in more recent, Mesozoic and Cenozoic subduction zone related arc systems, like those exposed along the western margin of North America. Field, petrologic, and structural relationships suggest that the Gafvert Lake sequence volcanic rocks are dominantly intermediate in composition and comprised of a series of flows, welded tuffs, and volcaniclastic breccias. Petrographic analyses also show that primary textures are generally well preserved in the volcanics. Preliminary geochemistry indicates that the sequence is dominantly rhyodacite to dacite. Trace element chemistry suggest that the sequence formed in a volcanic arc setting. The volcanics are intruded by a very coarse crystalline to porphyritic tonalite to granite complex called the Gafvert Lake intrusive complex that is geochemically identical to the volcanic package. The volcano-plutonic complex is cut by several steeply dipping, east-west trending, dextral shear zones with stretching lineations that are shallowly east plunging. The tectonic and structural setting of the Gafvert Lake sequence suggests that there is economic potential in this package of rocks due to its strikingly similar characteristics to other economically viable volcano-plutonic systems in the Wawa-Abitibi Terrane. 113 �Figure 1. The Gafvert Lake sequence is exposed in the Vermilion District between Soudan Mine and the Mud Creek shear zone (modified from Lodge et al., 2013). References Hudak, G.J., Heine, J., Jirsa, M. and Peterson, D.M., 2004, Volcanic stratigraphy, hydrothermal alteration, and VMS potential of the lower Ely Greenstone, Fivemile Lake to Sixmile Lake area. 50th Annual Meeting, Institute on Lake Superior Geology, Field Trip Guidebook volume 50. Lodge, R.W.D., Gibson, H.L., Stott, G.M., Hudak, G.J., Jirsa, M.A. and Hamilton, M.A., 2013, New U–Pb geochronology from 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), 264-277. Peterson, D.M. and Jirsa, M.A., 1999, Bedrock geological map and mineral exploration data,western Vermilion District, St. Louis and Lake counties, northeastern Minnesota: St. Paul, Minnesota Geological Survey Miscellaneous Map Series M-98. Peterson, D.M., Jirsa, M.A., and Hudak, G.J., 2009, Architecture of an Archean Greenstone Belt: Stratigraphy, structure and mineralization. 55th Annual Meeting, Institute on Lake Superior Geology, Field Trip Guidebook volume 55. 114 �THE DANGER OF “SULFIDE MINING” IN THE LAKE SUPERIOR REGION SEAL, Robert R. II1, PIATAK, Nadine M.1, and WOODRUFF, Laurel G.2 1 2 U.S. Geological Survey, Reston, VA 20192, rseal@usgs.gov, npiatak@usgs.gov U.S. Geological Survey, Mounds View, MN 55112, woodruff@usgs.gov The danger of “sulfide mining” is in the term itself. The term “sulfide mining” undermines meaningful evaluation of environmental risks associated with metal mining because it fails to recognize important influences that geology, hydrology, climate, mining methods, ore-processing methods, and continued evolution of environmental management practices have on environmental risks specific to prospective deposit types. It also places an overemphasis on a single environmental risk – acid generation from the weathering of sulfide minerals – when other risks, such as trace elements in water and solids warrant thorough consideration as well. The mineral deposits in the Lake Superior region, especially the Cu-Ni-PGM ores, highlight the importance of a geologically-based context for assessing environmental risk and designing sound environmental management practices for mining. Acid-mine drainage results from the oxidative weathering of sulfide minerals, principally pyrite or pyrrhotite, in the presence of oxygen or other oxidant, such as dissolved ferric iron. The risk of acid generation from waste rock or mill tailings is directly proportional to the amount of these sulfide minerals present, and inversely proportional to the amount of acid-neutralizing potential available. In the case of the Cu-Ni-PGM ores in the Lake Superior region, the deposits anchor both ends of a wide spectrum in terms of sulfide abundance and therefore acid-generating potential. At one end of the spectrum are the magmatic disseminated sulfide deposits of the Duluth Complex, northern Minnesota; at the other end is the Eagle magmatic massive sulfide deposit in Upper Peninsula of Michigan. Drill core from along the strike of the basal mineralized zone in the Partridge River and South Kawishiwi intrusions of the Duluth Complex, sampled from the Minnesota Department of Natural Resources Core Library, average 0.4 ± 0.7 weight percent S (range <0.05 – 2.6 wt. %), whereas semi-massive to massive sulfide ore at the Eagle mine has sulfur contents ranging from 13 to 36 weight percent (Kennecott Eagle Minerals, 2006). Further, the hydrometallurgical technique likely to be used on the disseminated Duluth Complex ores uses a bulk sulfide concentrate, effectively removing most of the sulfide from the solid waste, whereas the Eagle mine plans to use traditional froth flotation to produce separate copper and nickel concentrates with the pyrrhotite left in the solid waste. These mineralogical, geochemical, and ore-processing features of the proposed ores all affect the acidgenerating potential of solid waste and associated waste management approaches. The acid-generating potential of mine waste is commonly evaluated through a technique known as “acid-base accounting” that compares the acidgenerating potential (AP) of the material to its acid-neutralizing potential (NP). The AP is inferred from the sulfide content of the material and the NP is typically inferred from its carbonate content. The acid-generating potentials of the disseminated and massive sulfide ores vary dramatically, although neither has significant carbonate acidneutralizing potential (Figure 1). From a mine waste management perspective, NP:AP ratios below 1 are considered to be “probably acid-generating waste”, those above 2 are considered to be “non-probably acid-generating waste”, and those between 1 and 2 are considered to have uncertain potential. Silicate minerals are generally not considered because of their limited NP and slow reaction rates; however, two silicate minerals that have some of the highest NP values are olivine and calcic plagioclase (Jambor et al., 2002), which are important constituents of the host rocks of the Lake Superior Cu-Ni-PGM ores, especially the troctolites of the mineralized basal portion of the Duluth Complex. Trace metals, especially Cu and Ni, in mine waste warrant consideration relative to human health and aquatic ecosystem risks. Both Cu and Ni show strong correlations with sulfur concentrations in drill core from the basal mineralized zone (Figure 2), similar to variations found by Ripley (2014). Metallurgical testing on ore from the NorthMet deposit resulted in 88 to 91 percent recovery of Cu and 67 to 73 percent recovery of Ni; this yielded tailings material with Cu concentrations ranging between 320 and 390 mg/kg and Ni concentrations between 280 and 350 mg/kg (Dreisinger, 2009). The difference in recovery between Cu and Ni suggests that approximately 20 percent of the Ni is hosted by a non-sulfide phase such as olivine. Ripley (2014) found Ni concentrations up to 1,800 mg/kg in olivine from the Partridge River intrusion, which is consistent with this interpretation. Nickel in olivine in mill tailings is less likely to be labile and bioavailable than Ni in residual sulfides. The concentrations of Cu and Ni in the experimental tailings are well below both residential and industrial soil screening levels for human health protection (US Environmental Protection Agency, 2013). However, these ranges 115 �are above probable effects concentrations for sediments relative to aquatic ecosystem protection (MacDonald et al., 2000), indicating that waste management practices must be designed to guard against accidental release of tailings to nearby waterways. The potential release of Cu and Ni from mine waste to surface water and groundwater will depend upon hydrologic setting and chemical setting (factors such as the availability of oxygen or other oxidants) of mine waste in the context of waste management practices. Figure 1. Plot of acid-generating potential (AP) and acid-neutralizing potential (NP) of various magmatic NiCu-PGM deposits, modified from Schulz et al. (2010). Figure 2. Plot of bulk sulfur, copper, and nickel concentrations of drill core from the basal mineralized zone of the Duluth Complex. Consideration of the geologic, mineralogical, and ore-processing characteristics of magmatic mineral deposits in the Lake Superior region provides greater insights into environmental challenges associated with mining and mineral extraction than those from the oversimplified perspective of “sulfide mining”. These insights extend beyond acid-generating potential and include the assessment of potential risks to human health and aquatic ecosystems from trace metals. The identification of environmental risks enables effective mine planning for environmental protection. References Dreisinger, D., 2009, Keynote address: hydrometallurgical process development for complex ores and concentrates. In Proceedings of Hydrometallurgy Conference, v. 2009, p. 187-212. Jambor, J.L., Dutrizac, J.E., Groat, L.A., and Raudsepp, M., 2002, Static tests of neutralization potentials of silicate and aluminosilicate minerals: Environmental Geology, v. 43, p. 1-17. Kennecott Eagle Minerals, 2006, Eagle Project Mining Permit Application, Volume I, 126 p. MacDonald, D.D., Ingersoll, C.G., and Berger, T.A., 2000, Development and evaluation of consensus-based sediment quality guidelines for freshwater ecosystems: Archives of Environmental Contamination and Toxicology, v. 39, no. 1, p. 20–31. Ripley, E.M., 2014, Ni-Cu-PGE Mineralization in the Partridge River, South Kawishiwi, and Eagle Intrusions: A Review of Contrasting Styles of Sulfide-Rich Occurrences in the Midcontinent Rift System: Economic Geology, v. 109, p. 309-324. Schulz, K.J., Chandler, V.W., Nicholson, S.W., Piatak, Nadine, Seal, II, R.R., Woodruff, L.G., and Zientek, M.L., 2010, Magmatic sulfide-rich nickel-copper deposits related to picrite and (or) tholeiitic basalt dike-sill complexes—A preliminary deposit model: U.S. Geological Survey Open-File Report 2010–1179, 25 p. (Available at http://pubs.usgs.gov/of/2010/1179/). U.S. Environmental Protection Agency, 2013, Regional Screening Level (RSL) Summary Table (TR=1E-6, HQ=1) November 2013: available only online at http://www.epa.gov/reg3hwmd/risk/human/rbconcentration_table/Generic_Tables/docs/master_sl_table_run_NOV2013.pdf. (Accessed March 26, 2014.) 116 �GENESIS OF SULFIDE MINERALIZATION WITHIN THE FOOTWALL GRANITE OF THE MATURI CU-NI-PGE DEPOSIT OF THE SOUTH KAWISHIWI INTRUSION, DULUTH COMPLEX, NE MINNESOTA STEINER, Ronald Alex and MILLER, Jim Department of Geological Sciences, University of Minnesota Duluth, Duluth MN 55812 The development of the 1.1 Ga Midcontinent Rift (MCR) generated voluminous magmatism resulting in the extensive flood basalts and sub-volcanic intrusions exposed along the flanks of Lake Superior (Miller et al., 2002). In northeastern Minnesota, two intrusions of the Layered Series, the Partridge River Intrusion (PRI) and South Kawishiwi Intrusion (SKI), are known to hosts significant Cu-Ni-PGE sulfide mineralization (Miller et al., 2002). The Maturi Cu-Ni-PGE deposit occurs along the basal zone of the SKI where it is in contact with granitic rocks of the Archean Giants Range Batholith (GRB). Generally Cu-Ni-PGE-enriched sulfides are disseminated throughout a 50-150m-thick basal mineralized zone (BMZ) and locally may be semimassive to massive sulfide (Bonnichsen, 1974). Several researchers (Severson, 1993; Peterson, 1997; Sawyer, 2002; Hovis, 2003) have noted significant sulfide mineralization in the dominantly granitic footwall. Extensive drilling by Twin Metals Minnesota since 2006 has shown that the mineralization within the footwall is typically disseminated sulfide, to locally massive sulfide veins, that is dominantly composed of chalcopyrite, pyrrhotite, pentlandite, as found in the BMZ. The mineralization extends as deep as 100 meters below the basal contact with the SKI (Kevin Boerst, 2013, personal comm.). While sulfur isotope data show that the sulfide in the mineralized granite originated from the same source as that in the overlying gabbro (Ripley, 1986; Molnar, 2009), the mechanism by which footwall mineralization occurred is unconfirmed. The purpose of this study is to evaluate evidence for possible mechanisms by which the Giants Range Batholith may have become mineralized. Two hypotheses will be evaluated: 1. Partial melting of the GRB resulting in buoyant exchange of dense magmatic sulfide fluid and less dense anatectic melts rising from the GRB. 2. Hydrothermal fluids mobilizing sulfide from the mineralized gabbro into the granitoid rocks. These two hypotheses are being tested by acquiring petrographic and geochemical data from four drill cores from the Maturi deposit that penetrate the gabbro-footwall contact and reach below the mineralized zone in the granite. Three of the drill cores represent variations in different styles of mineralization in the gabbro and the granite recognized in recent exploration drilling (Peterson, 2012). A fourth core was selected for the extensive occurrence of mineralization into a large biotite schist enclave within the batholith. Preliminary results suggest a relationship between the degree of partial melting of the GRB and sulfide mineralization. Core logging and subsequent petrographic observations indicate that the footwall experienced pyroxene hornfels grade metamorphism producing orthopyroxene and clinopyroxene at the expense of biotite and amphibole that extends in excess of 30 meters from of the SKI-GRB contact. The volatiles produced by recrystallization of biotite and amphibole likely played a role in promoting anatectic melting of the granite as well. Petrographic evidence of partial melting of the GRB recognized in this and previous studies (Sawyer, 1999 2002; Hovis, 2003) included mylonitic textures, pockets of polygonal quartz-orthoclase-plagioclase aggregates, and lattice-dislocation textures in plagioclase. Leucosome patches have been observed to contain massive to semi-massive sulfide suggesting a relationship between escaping partial melts and sulfide liquid. A retrograde alteration of metamorphic pyroxene to biotite, cummingtonite/actinolite, and chlorite is evidence of post-metamorphic hydrothermal alteration. This late hydrothermal alteration assemblage, which is recognized throughout the granite, typically does not contain significant sulfide. Additionally, where sulfides are present they appear largely unaffected by hydrothermal alteration indicating that this event did not cause significant sulfide remobilization or recrystallization. The presence of rare, late gypsum may indicate that the hydrothermal fluids were strongly oxidized and that any remobilized sulfur was crystallized as sulfate. Petrographic observations implying exchange of anatectic melts and sulfide liquid are also supported by geochemical analyses. All REE became increasingly depleted with increased proximity to the gabbro 117 �contact except for Eu with appears as a peak on the diagrams. During partial melting, Eu is likely being retained in plagioclase whereas other REE will be partioned into partial melts which are able to escape the system. Plotting S concentration against Eu/Ce (Fig. 1A) shows a positive correlation indicating that the amount of anatectic melt escaped generally correlates with an increase in sulfide mineralization. Another proxy of increased escape of anatectic melt is an increase in plagioclase relative to quartz and alkali feldspar. A plot of CIPW norm values of Ab/(Or+Qtz) vs. wt% S (Fig. 1B) shows a similar, though broader, positive correlation. Research is ongoing to further test these hypotheses by evaluating isocon plots (Grant, 1986) of whole rock geochemistry and by acquiring SEM-EDS analyses of pyroxene and amphibole compositions. The isocon method will be applied to determine element mobility through the system in order to better identify the mechanism of mineralization. Partial melting or hydrothermal alteration have distinct elemental signatures that can be identified in isocon modeling. Mineral chemistry acquired by SEM-EDS analyses will be used to trace changes in mineral composition relative to distance from the SKI-GRB contact. References Grant, James A. 1986 "The Isocon Diagram-A Simple solution to Gresens' Equation for Metasomatic Alteration."Economic Geology. Vol. 81, 1976-1982 Hovis, Steven T., 2003,.”Observations on Cu-Ni Mineralization in the Giants Range Batholith Footwall of the South Kawishiwi Intrusion, Duluth Complex, Northeastern Minnesota”., Natural Resources Research Institute; University of Minnesota, NRRI/TR-2003/24 Miller, J.D. Jr., Green, J.C., Severson, M. J., Chandler, V. W., Hauck, S. A., Peterson, D. M., and Wahl, T. E., 2002, “Geology and Mineral potential of the Duluth Complex and related rocks of Northeastern Minnesota”, Minnesota Geological Survey, Report of Investigations 58 Molnar, F., Peterson, D. M., Arehart, G. B., Hauck, S. A., 2009, “Sulfur isotope constraints for a dynamic magmatic sulfide ore deposition model in the sill-like South Kawishiwi Intrusion of the Duluth Complex, Minnesota, USA”, Geological Society of America, Abstract. Peterson D.M. 2012, “Maturi Geological Model” Duluth Metals ltd Presentation to Twin Metals Minnesota LLC Sawyer, E. W., 1999, Criteria for the Recognition of Partial Melting, Physical Chemistry Earth, Vol. 24, No. 2, pp. 269-279 Sawyer, E. W., 2002, “Report on Thin Sections From DDH WM-1, Spruce Road Cu-Ni Deposit, South Kawishiwi Intrusion, Duluth Complex”, Natural Resources Research Institute; University of Minnesota, NRRI/RI-2002/13 Severson, M. J., 1994, “Igneous stratigraphy of the South Kawishiwi Intrusion, Duluth Complex, Northeastern Minnesota”, Natural Resources Research Institute, University of Minnesota, Duluth, NRRI/TR-93/34 Ripley, E. M. & Alawi, J. A., 1986, “Sulfide mineralogy and chemical evolution of the Babbitt Cu—Ni deposit, Duluth Complex, Minnesota”, Canadian Mineralogist Vol. 24, 347-368 118 �SULFIDE HIGHWAY REVISITED: NEW IDEAS ON INTERNAL STRUCTURE AND SULFIDE MINERALIZATION OF THE NICKEL LAKE MACRODIKE SWEET, Gabriel J., PETERSON, Dean M., LARSON, Philip C., FINNEGAN, Molly L., FINNES, Evan, PARENT, Charles, NOWAK, Robert, BOLEY, Tyler D. Duluth Metals, 306 W. Superior St., Suite 610, Duluth, MN 55802 Recent exploratory drilling within the Nickel Lake Macrodike (NLM) by Duluth Metals has facilitated the first subsurface investigations of the magma conduit into the prolifically mineralized South Kawishiwi Intrusion (SKI). Seventeen holes were drilled in late 2012 and early 2013 along 6500’ feet of strike length of the NLM, with 4 holes reaching depths of over 4000’. Intercepts of the primary surface lithologies suggests that the youngest units of the NLM (the variably pegmatoidal oxide gabbro (N-xG) and layered troctolite (N-Tl)) are dipping irregularly (~30⁰) towards the northwest anorthositic sidewall of the NLM. Based on limited pierce points through this internal stratigraphy, the oxide gabbro appears to extend into the anorthosite series wall rocks beyond the surface-mapped northwestern sidewall contact of the NLM. At depth, the top of the N-xG truncates the xenolithbearing (dominantly hornfelsed North Shore Volcanic Basalt and Biwabik Iron Formation) heterogeneous troctolite (N-Th) and sulfide-bearing troctolite (N-Ts) packages emplaced along the southeast-dipping (60⁰) northwest margin of the NLM. Below the N-xG and N-Tl units is a second package of heterogeneous troctolite (Th). Unlike the heavily xenolith-bearing N-Th unit present at and near surface, this troctolite is sparsely populated by small (~10’) hornfelsed basalt xenoliths. This same heterogeneous troctolite hosts a series of large (100’ thick) rafts of Virginia Formation argillite and greywacke at depth (~3000’) in the south-central portion of the NLM. Sulfide mineralization was encountered along the northwestern margin of the NLM both as the down-dip extension of outcropping and subcropping mineralization, as well as at greater depths. Three distinct types of mineralization were defined with respect to overall sample grades, interval thicknesses and lithological associations: Type 1 - long intervals (~50’-200’+) of moderate grade disseminated to blebby chalcopyrite and pyrrhotite (broadly 0.4%Cu, 0.1%Ni and 0.15g/t Pt+Pd+Au) associated with variably hornfelsed basalt xenolith-bearing Th, Type 2 - short intervals (~5’-35’) of higher grade disseminated to blebby chalcopyrite and pyrrhotite (upwards of 0.55%Cu, 0.11%Ni and from 0.25g/t to over 2.0g/t Pt+Pd+Au) generally intercepted deeper than the larger, moderate grade intervals, and Type 3 - variable-length intervals (~25’-400’) of low grade, disseminated to coarse-grained pyrrhotite and minor chalcopyrite (generally < 0.25%Cu, <0.10%Ni and <0.10g/t Pt+Pd+Au) hosted by a variably oxide-rich pyroxenite. Mineralization Type 1 and Type 2 tend to occur at shallow depths in the majority of drill holes along the western margin of the NLM. However, the Type 3 mineralization is confined to the shallow southwestern NLM, where it is found in close proximity to a large iron-formation xenolith (~1300’ strike length as mapped at surface). Comparison of the geochemical signature of NLM sulfide mineralization types to basal SKI mineralization suggests a distinctly different fractional history for the NLM sulfide populations. Copper-nickel ratios for Types 1 and 2 tend to fall between 4:1 to 5:1, and 5:1 to 6:1, respectively. Coupled with low to moderate TPM tenors, NLM mineralization is distinct from the broadly 3:1 Cu:Ni ratio and propensity towards elevated precious metal tenors of the basal SKI mineralization. At this time, no direct analogue to basal SKI mineralization has been identified in the NLM. The deviations in precious metal tenor between Type1 and Type 2 mineralization, and the SKI may speak more directly to processes operating “downstream” of the NLM. With Cu:Ni ratios of 1:1 to 3:1, Type 3 mineralization is distinctly more Ni-rich than Types 1 and 2, and the basal SKI mineralization. Type 3 mineralization is further distinguished by its unique pyroxenite host rock, and highly elevated P (up to 1%) and Zn (generally >175ppm), respectively up 119 �to two orders of magnitude and double that of the vast majority of basal SKI mineralization. However, the Type 3 lithological association and distinct geochemical signature shows affinity with reported rock types and whole rock compositions of apatite-bearing, mineralized oxide-rich ultramafic intrusions (OUIs; Ripley et al., 1998). The spatial association with a large xenolith of metamorphosed iron formation is also in line with the observations of Severson (1995), who noted that OUIs in the basal central Duluth Complex occur in close proximity to metamorphosed Biwabik Iron Formation. The lithological relationships noted in the drilling confirm the intrusive sequence of the NLM as suggested by Peterson et al. (2006), but imply a slightly different geometry for the youngest intrusive phases internal to the NLM. The difference between the xenolith populations in the near surface N-Th unit and the Th at depth may indicate origination from different pulses of troctolitic magma through the conduit. The long intervals of xenolith-poor Th (up to 2500’+) at depth within the central NLM may represent areas of high magma flow through the conduit. The existence of multipe types of mineralization along the margins of the NLM magma conduit indicates sulfide mineralized magmas passed through the conduit. Variation of the NLM mineralization grade and tenor from that of basal SKI mineralization may be the result of fractionation processes that occurred down-stream of the NLM conduit (e.g., sulfide dissolution upgrading; Kerr and Leitch, 2005), or it may ultimately correlate with undiscovered mineralization with the SKI-NLM system. Mineralization Type Type 1 Type 2 Type 3 Boulder Lake Figure 1 – Geological map of the southern Nickel Lake Macrodike with major lithological units and Duluth Metals’ drill pads (black dots). Figure 2 – Zn vs P2O5 comparison of NLM mineralization types with oxideapatite- rich samples from DDH IV-2 from the Boulder Lake Intrusion in the southwestern Duluth Complex, from Ripley et al., 1998. REFERENCES Kerr, A. and Leitch, A., 2005, Self-Destructive Sulfide Segregation Systems and the Formation of High-Grade Magmatic Ore Deposits: Economic Geology, Vol. 100, p. 311-332. Peterson, D.M, Albers, P.B., and White, C.R., 2006, Bedrock Geology of the Nickel Lake Macrodike and Adjacent Areas, Lake County, northeastern Minnesota: University of Minnesota Duluth, Natural Resources Research Institute, Map Series NRRI/MAP-2006-04, scale 1:10,000. Ripley, E.M., Severson, M.J. and Hauck, S.A., 1998, Evidence for Sulfide and Fe-Ti-P-Rich Liquid Immiscibility in the Duluth Complex, Minnesota: Economic Geology, Vol. 93, p. 1052-1062. Severson, M.J., 1995, Geology of the Southern Portion of the Duluth Complex: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report NRRI/TR-95/26, 185p. 120 �THE THUNDER MAFIC TO ULTRAMAFIC INTRUSION: A PGE AND PRECIOUS METAL BEARING EARLY-RIFT CONDUIT SYSTEM IN THE MIDCONTINENT RIFT TREVISAN, Brent1, HOLLINGS, Pete1, and AMES, DOREEN2 1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, P7B 5E1 Geological Survey of Canada, Central Canada Division, 750-601 Booth St., Ottawa, Ontario, K1A 0E8 2 In 2002, high grade massive Ni-Cu-PGE sulphide mineralization at the Eagle deposit near Marquette, Michigan was discovered stimulating exploration programs in search of small mafic to ultramafic intrusions hosting “conduit-type” magmatic sulphide mineralization associated with the early stages of the Mesoproterozoic Midcontinent Rift (MCR; Miller and Nicholson, 2013; Ripley, 2014). Since the Eagle discovery over a half-dozen poorly exposed mineralized early-rift mafic to ultramafic intrusions have been discovered within the Lake Superior region, prompting active petrological research (e.g., Ding et al., 2010) and re-evaluation of the current MCR tectono-magmatic model (e.g., Miller and Nicholson, 2013). However, from an exploration stand point the small size of these buried mineralized mafic to ultramafic intrusions makes them difficult to locate both on the ground and on regional magnetic survey maps (Ames et al., 2012). This study is a collaborative project between the Geological Survey of Canada, the Ontario Geological Survey, and Lakehead University as part of the Ni-Cu-PGE-Cr project, Targeted Geoscience Initiative-4 (TGI-4; Ames et al., 2012). The objective is to characterise the petrology, mineralization, and alteration footprint of the Thunder intrusion within the context of the MCR as a whole, in order to identify criteria for targeting buried mineralization. The Thunder intrusion is a small, layered mafic to ultramafic intrusion located on the outskirts of Thunder Bay, ON, which has been explored by Rio Tinto (formerly Kennecott Canada Exploration Inc.) in 2005 and 2007 (Bidwell and Marino, 2007). The intrusion is interpreted to be associated with the early magmatic stages of the MCR based on geochemical similarities to mafic and ultramafic rocks of the Nipigon Embayment (Hollings et al., 2007) and an unpublished 207Pb/206Pb baddeleyite age of 1110.33±0.92 Ma (Ames, pers. comm., 2014). This intrusion is distinct from the other known mineralized early-rift intrusions as it is the only known occurrence hosted by the Archean Shebandowan greenstone belt. The intrusion is approximately 800 by 1000m by < 500 m thick and dips steeply to the south. Major textural and geochemical differences can be used to divide the lithostratigraphy into a lower mafic to ultramafic basal unit and an upper gabbroic unit, however, similar trace and rare earth element ratios of the two units suggests they formed from a single magmatic pulse that has undergone subsequent fractionation. Ni-Cu-PGE mineralization is hosted by clinopyroxenite in the lower mafic to ultramafic unit adjacent to the basal wall rock, including 20 m of 0.22% Cu, 0.06% Ni, 0.25ppm Pt, 0.29ppm Pd (Bidwell and Marino, 2007). Sulphides rarely comprise up to 30% by volume but more typically 1-5%, with textures ranging from medium- to fine-grained disseminated, globular and rarely net-textured. Pyrrhotite, chalcopyrite and rare pentlandite with common secondary marcasite-pyrite replacement are present along with trace kotultskite, naldrettite, merenskyite, sperrylite, electrum and native silver. The δ34S values of sulphide minerals from the Thunder intrusion are similar to the adjacent wall rock forming a tight range between +3.8 and -3.1‰. Although δ34S values are broadly consistent with a 121 �mantle origin (0 ± 2‰) the involvement of crustal sulphur during the mineralization process remains a possibility. Radiogenic isotopes were measured from select samples to investigate possible contamination of the Thunder intrusion. The εNd values from the intrusion range between -0.74 and +0.99, with no trends towards wall rock compositions, whereas the 87Sr/86Sr values range from 0.7031 and 0.7061 and trend towards wall rock values of 0.7071 and 0.7087. The decoupling of the two radiogenic isotope signatures is consistent with crustal contamination at depth and local contamination during the emplacement of the Thunder intrusion. References Ames, D.E. et al. 2012. Update on Research Activities in the Targeted Geoscience Initiative 4 MagmaticHydrothermal Nickel-Copper-Platinum Group Elements Ore System Subproject: System Fertility and Ore Vectors. Summary of Field Work and Other Activities 2012. Ontario Geological Survey, Open File Report 6280. Bidwell, G. E., and Marino, F. 2007. 2007 drilling assessment report for the Geoinformatics Exploration Canada Ltd Thunder Project; Thunder Bay South District, Assessment Files, AFRO report number 2.34638, 112p. Ding, X., Li, C., Ripley, E.M., Rossell, D., and Kamo, S. 2010. The Eagle and East Eagle sulfide ore‐bearing maficultramafic intrusions in the Midcontinent Rift System, upper Michigan: Geochronology and petrologic evolution. Geochemisty, Geophysics, Geosystems, v. 11, p. 1-22. Hollings, P., Hart, T., Richardson, A., and MacDonald, C.A. 2007. Geochemistry of the Mesoproterozoic intrusive rocks of the Nipigon Embayment, northwestern Ontario: evaluating the earliest phases of rift development. Canadian Journal of Earth Sciences, v. 44, p. 1087–1110. Miller, J., and Nicholson, S. 2013. Geology and Mineral Deposits of the1.1Ga Midcontinent Rift in the Lake Superior Region: An Overview, in Field Guide to Copper-Nickel-Platinum Group Element Deposits of the Lake Superior Region. Precambrian Research Center Guidebook, v. 13-01, p. 1-50. Ripley, E. M. 2014. Ni-Cu-PGE Mineralization in the Partridge River, South Kawishiwi, and Eagle Intrusions: A Review of Contrasting Styles of Sulfide-Rich Occurrences in the Midcontinent Rift System. Economic Geology, v. 100, p. 309-324. 122 �GARNET IN THE DEEP CRUST: THE KEY TO LINKING ARCHEAN TTG GENERATION AND VERTICAL BLOCK MOTIONS? VAN LANKVELT, A1, WILLIAMS, ML1, SCHNEIDER, DA2, SEAMAN, SJ1 1 Department of Geosciences, University of Massachusetts-Amherst, 611 North Pleasant St, Amherst, MA 01003 USA 2 Department of Earth Sciences, University of Ottawa, 140 Louis-Pasteur Pvt. Ottawa, ON K1N 6N5 Canada There is substantial debate about the differences between geologic processes that operated during the Archean and those operating today (e.g. Percival et al., 2006; Van Kranendonk, 2010). Two notable differences between modern terranes and Archean cratons are the presence of very large volumes of TTGs (tonalite-trondhjemite-granodiorite) and tectonic structures suggesting that locally, TTG blocks moved up relative to adjacent mafic greenstone belts. Explanations for the structural differences between modern and early Earth vary between two endmember models: density-driven (vertical) and modern-style (horizontal) tectonics. Many field-based structural studies of Archean rocks contain evidence for both vertical and horizontal tectonics (e.g. Lin, 2006). When the spatial scales of vertical and horizontal structures are considered, horizontal structures seem to dominate the large-scale province framework and are more prevalent in high-grade rocks, whereas low-grade and local (greenstone belt-scale) structures exhibit more evidence for vertical processes (Van Kranendonk, 2010). Current interpretations for the origins of TTGs favor partial melting of a mafic, hydrous, garnetbearing source rock. TTGs are chemically similar to modern arc rocks, so the suprasubduction-zone setting of modern arcs is commonly invoked for the generation of TTGs as well (see Moyen & Martin, 2012, and references therein). Other suggestions invoke plume-related process for generating larger volumes of TTGs than are found in modern arc settings (e.g. Moyen & Martin, 2012). Below, we attempt to integrate geodynamic and petrogenetic models for Archean tectonics. One approach to better understand geodynamic and petrogenetic processes operating in the Archean is to compare rocks from different crustal levels. The North Caribou Terrane is located in the central Superior Province, and it is dominated by Meso- to Neoarchean TTGs. Lin (2006) studied the structures within and adjacent to greenstone belts in the North Caribou and concluded that these rocks preserve structures consistent with synchronous vertical and horizontal tectonism. Several studies of the TTGs in the North Caribou show that their compositions are consistent with typical Archean TTGs (e.g. Wyman et al., 2011). The thermobarometric and geochemical data we present from TTGs in the North Caribou are consistent with Wyman et al.’s (2013) data and indicate that the TTGs were emplaced in the mid-crust and later metamorphosed at shallower levels. The Athabasca granulite triangle is an exposure of deep-crustal rocks that straddles the Snowbird Tectonic Zone north of Lake Athabasca. The granulite-facies (0.9-1.9 GPa, 700-950 ºC; Baldwin et al., 2003) terrane contains several generations of mafic rocks, two of which, Neoarchean gabbros and Paleoproterozoic mafic dikes, are possible analogues to TTG source rocks, as both preserve primary hornblende and contain garnet. Although the source for the older gabbros is not fully understood, the dikes are not associated with arc magmatic rocks (Flowers et al., 2006). Some of the mafic dikes have undergone anatexis due to dehydration melting of hornblende, resulting in tonalitic melt and peritectic garnet (Williams et al., 1995). Implications The rocks in the Athabasca provide an interesting option for the generation of TTGs. The mafic dikes, which are not related to subduction, indicate the potential for long-term storage of water in the 123 �lithospheric mantle, so concurrent subduction and melting are not necessarily required. Their anatectic textures also suggest the possibility for melting through underplating or mantle upwelling (Williams et al., 1995). Regardless of the melting mechanism, extraction of significant volumes of TTG parent magma from mafic rocks would leave a garnet-rich restite, similar to what has been postulated by Saleeby et al. (2003) to exist below the Sierra Nevada batholith. Foundering of this gravitationally unstable lithospheric root may be the cause of the uplift of the Sierra Nevada (Saleeby et al., 2003), and a similar delamination scenario in the Archean may explain relative uplift of TTGs compared to adjacent greenstone belts. This is observed in the North Caribou, and a higher frequency of delamination events in the Archean due to widespread extraction of melt could explain the evidence for vertical tectonics. Delamination of dense restite and subsequent mantle upwelling could also trigger additional melting, creating a positive feedback mechanism that could produce significant amounts of tonalitic magma, like the large batholiths that are common in the North Caribou. This scenario would require a wet mantle, either through the release of primordial water or an earlier introduction of volatiles. Structural evidence for horizontal tectonics preserved in large-scale structures suggests that subduction may have operated during the Archean, but not all TTG-type magmas need to have been derived at subduction zones. Instead, TTGs could be generated in several non-unique tectonic settings, and garnet-driven delamination of the lower crust can explain both evidence for vertical tectonics and large volumes of TTGs. References Baldwin, JA, Bowring, SA, Williams, ML. 2003. Petrological and geochronological constraints on high pressure, high temperature metamorphism in the Snowbird tectonic zone, Canada. J Metamorphic Geol 21, 81-98. Flowers, RM, Bowring, SA, Williams ML. 2006. Timescales and significance of high-pressure, hightempperature metamorphism and mafic dike anatexis, Snowbird tectonic zone, Canada. Contrib Min Petrol 151, 558-581. Lin, S. 2006. Synchronous vertical and horizontal tectonism in the Neoarchean: Kinematic evidence from a synclinal keel in the northwestern Superior Craton, Canada. Precam Res 139, 181-194. Moyen, J-F, Martin, H. 2012. Forty years of TTG research. Lithos 148, 312-336. Percival, JA, Sanborn-Barrie, M, Skulski, T, Stott, GM, Helmstaedt, H, White, DJ. 2006. Tectonic evolution of the western Superior Province from NATMAP and Lithoprobe. Can J Earth Sci 43, 1085-1117. Saleeby, J, Ducea, M, Clemens-Knott, D. 2003. Production and loss of high-density batholithic root, southern Sierra Nevada, California. Tectonic 22, 1064-1087. Van Kranendonk, MJ. 2010. Two types of Archean continental crust: plume and plate tectonics on early Earth. Am J Sci 310, 1187-1209. Williams, ML, Hanmer, S, Kopf, C, Darrach, M. 1995. Syntectonic generation and segregation of tonalitic melts from amphibolite dikes in the lower crust, Striding-Athabasca mylonite zone, northern Saskatchewan. J Geophys Res 100, 15717-15734. Wyman, DA, Hollings, P, Biczok, J. 2011. Crustal evolution in a cratonic nucleus: Granitoids and felsic volcanic rocks of the North Caribou Terrane, Superior Province, Canada. Lithos 123, 37-49. 124 �STRONTIUM ISOTOPE STUDY OF MESABI IRON RANGE GROUNDWATER Walsh, James F. Minnesota Department of Health, St. Paul, MN 55164 On the Mesabi Iron Range, significant differences in 87Sr/86Sr exist between long residence time groundwater from wells completed in the Biwabik Iron Formation, especially where covered by the Virginia Formation, and short residence time groundwater from wells completed in overlying glacial aquifers and in surface waters. The relatively low 87Sr/86Sr observed at the iron formation wells falls within the range commonly observed for weathering of Phanerozoic marine carbonates, whereas the higher values observed at the drift wells and surface waters are more characteristic of weathering of Archean silicate minerals. Short residence time water from Biwabik Iron Formation wells situated in the subcrop of the formation span a wide range of 87Sr/86Sr and in some cases is more radiogenic than that observed at the glacial drift wells and surface water bodies. These results likely reflect the impact of glacial provenance on the distribution of strontium-bearing minerals within their groundwater flow pathways. The glacial deposits on the Mesabi Range are dominated by northeast-sourced glaciers of the Rainy Lobe, whose sediments are characterized by an abundance of Archean granitic material and scarcity of Phanerozoic marine sediments. However, northwest-sourced glacial sediments are recognized locally and have contributed sediments relatively rich in Phanerozoic marine carbonate and shale, especially along the west-central Mesabi Range. It is likely that water samples with high 87Sr/86Sr and low strontium concentrations are predominantly influenced by recharge through Rainy Lobe glacial sediments. In contrast, those that are relatively low in 87Sr/86Sr but high in strontium concentration are predominantly reflecting dissolution of carbonate minerals from northwest-sourced glacial deposits or from the iron formation itself. 125 �126 �GEOCHEMISTRY AND MINERALOGY OF GLACIAL SOILS IN THE UPPER MIDWEST WOODRUFF, Laurel G., U.S. Geological Survey, St. Paul, MN 55112 (woodruff@usgs.gov) CANNON, William F. and SOLANO, Federico, U.S. Geological Survey, Reston, VA 20192 SMITH, David B., U.S. Geological Survey, Denver, CO 80225 The U.S. Geological Survey has recently completed a low-density (1 site per 1,600 square kilometers, 4,857 sites) geochemical and mineralogical survey of soils of the conterminous United States (Smith et al., 2013). Three samples were collected, if possible, from each site; (1) a sample from a depth of 0 to 5 centimeters, (2) a composite of the soil A horizon, and (3) a deeper sample from the soil C horizon or, if the top of the C horizon was at a depth greater than 1 meter, from a depth of approximately 80–100 centimeters. The <2-millimeter fraction of each sample was analyzed by a combined inductively coupled plasma-atomic emission spectrometry/mass spectrometry method for a suite of 45 major and trace elements following near-total multiacid digestion. The major mineralogical components in samples from the soil A and C horizons were determined by a quantitative X-ray diffraction method. Regional- and national-scale element and mineral patterns can be related to (1) soil parent materials, (2) climate factors, (3) soil age, and (4) possible anthropogenic loading to surface soils. This presentation will describe the influence of source provenance and soil age factors on the geochemistry and mineralogy of the soil A and C horizons in the upper Midwest. In the upper Midwest, melting of glacial ice left the region mantled with a blanket of mixed, immature sediments from which present day soils developed. Individual ice lobes of the late Wisconsinan glaciation created distinct patterns in soil geochemistry and mineralogy because of varying provenance and transport paths. Carbonate- and shale-rich ‘gray’ tills in Minnesota, North Dakota, South Dakota, and Iowa, deposited by the Des Moines and James lobes were derived from Cretaceous sedimentary rocks (dolostone, limestone, shale); glaciolacustrine sediments of Glacial Lake Agassiz along the North Dakota/Minnesota border have a similar provenance (Wright, 1972). Gray tills were transported significant distances to the south and southeast from their source and deposited on Precambrian bedrock that is largely devoid of carbonate minerals. ‘Red’ tills were deposited in northeastern Minnesota and northern Michigan and Wisconsin by the Rainy and Superior lobes. The Rainy lobe provenance is mainly Precambrian crystalline rocks of the Canadian Shield and the Superior lobe provenance is mainly basalts and sediments of the Precambrian Keweenawan Supergroup (Wright, 1972). In the lower Great Lakes region, carbonate- and shale-bearing tills sourced from the Cambrian to Devonian sedimentary bedrock units that rim the Michigan basin were deposited by the Green Bay and Lake Michigan lobes in western Wisconsin and northern Illinois, by the Saginaw lobe in central Michigan, and by the Huron-Erie lobe in eastern Indiana and western Ohio (Johnson, 1986; Hofer and Szabo, 1993). Soils developed on glacial sediments are relatively young and often retain easily weathered minerals and mobile elements, such as carbonates and related elements (e.g., Ca and Mg), typically leached from older, more mature soils beyond the southern extent of the last glaciation. As expected from their differing provenance, soils developed on red tills have much lower clay contents and much higher quartz and feldspar contents compared to soils developed on gray tills. This divergent mineralogy creates striking contrasts in element concentrations. Soils on gray tills have higher Ca contents from carbonate as well as higher As, Cd, Mo, Sb, and U concentrations, likely contributed by the shale component, compared to soils on red tills, which have higher Na and K contents from the higher feldspar content. Soils developed on the James lobe have somewhat higher Mn contents than soils developed on the Des Moines lobe, perhaps related to local redox conditions. Soils developed on Lake Agassiz clays have relatively higher Li, Sc, Ti, V, and Zn contents compared to soils developed on surrounding gray tills. One of the more dramatic characteristics of some glacial soils in the upper Midwest is a high concentration of primary dolomite. Tills of the Green Bay and Lake Michigan lobes are characterized by an especially high content of dolomite relative to calcite, and as a consequence, these soils have some of the 127 �highest soil Mg concentrations in the conterminous United States. The Green Bay and adjacent Lake Michigan lobes, as well as the Saginaw and adjacent Huron-Erie lobes are all largely sourced from similar rocks (dominantly dolostone, limestone, and black shale). However, a strong contrast in Mo contents in the soil C horizon between the Green Bay and Lake Michigan lobes and between the Saginaw and Huron-Erie lobes is an indication of the proportion of black shale incorporated into their respective glacial sediments by the individual lobes (Figure 1). The higher the percentage of black shale, the higher Mo content of soils, as well as a number of other elements such as As, Cd, Co, K, Sb, Tl, U, and Zn, all of which may be enriched in black shale. Because of this shale influence, large areas of northern Ohio and Indiana have some of the higher soil Mo concentrations in the conterminous United States. Thus, glacial dispersal of materials sourced from different bedrock sources, especially relatively thin shale units, had a widespread effect on soil geochemistry and mineralogy throughout the glaciated upper Midwest. Figure 1. Interpolated concentration map depicting molybdenum (Mo) in the soil C horizon in the lower Great Lakes region. The hachured black line is the maximum southern extent of Wisconsinian glaciation; dotted lines are the approximate margins of named individual ice lobes, with arrows indicating major ice flow direction (after Grimley, 2000; Hofer and Szabo, 1993). References Grimley, D.A., 2000. Glacial and nonglacial sediment contributions to Wisconsin Episode loess in the central United States. Geological Society of America Bulletin 112, 1475-1495. Hofer, J.W., and Szabo, J.P., 1993. Port Bruce ice-flow directions based on heavy-mineral assemblages in tills from the south shore of Lake Erie in Ohio. Canadian Journal of Earth Sciences 30, 1236-1241. Johnson, W.H., 1986. Stratigraphy and correlation of the glacial deposits of the Lake Michigan lobe prior to 14 ka BP. Quaternary Science Reviews 5, 17-22. Smith, D.B., Cannon, W.F., Woodruff, L.G., Solano, Federico, Kilburn, James E., and Fey, David L., 2013. Geochemical and mineralogical data for soils of the conterminous United States. U.S. Geological Survey Data Series 801, 19 p. Wright, H.E., 1972. Quaternary history of Minnesota, in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota: a centennial volume. Minnesota Geological Survey, 515-547. 128 �THE EVOLUTION OF THE ATMOSPHERE-HYDROSPHERE: A GEOCHEMICAL COMPARISON OF TWO PALEOPROTEROZIC GUNFLINT WEATHERING PROFILES YIP, Christopher and FRALICK, Philip, Department of Geology, Lakehead University, Thunder Bay, ON, P7B 5E1, cyip@lakeheadu.ca, philip.fralick@lakeheadu.ca The 1878±1 Ma year old Gunflint Iron Formation is a chemical sedimentary unit that forms one of the members of the Animike Group. It is well known that the Gunflint Formation is made up of a transgressive-regressive-transgressive sequence, which represent the advance and retreat of the ancient sea that filled the Animike basin. This sequence traps a unique point in the evolution of the atmospherehydrosphere during the Precambrian. This interaction with the atmosphere should be seen in the rock record at the point where the water depth was at its shallowest, such as the initial transgression or the regressive surfaces. If oxygen was present, the rocks underlying the transgression, as well as the initial transgressive strata that were precipitated in the shallow ocean should contain geochemical markers such as Ce anomalies. The newly created two-lane Highway 11/17 outside of Thunder Bay, shows a clean example of the basal section of the Gunflint Formation (Figure 1a). This section of the Gunflint overlies an Archean granodiorite unit. The overlying Gunflint carbonate grainstones show no unique features. What is important is the underlying granodiorite unit. The granodiorite shows a very clear example of spheroidal weathering that should occur if joints in the bedrock were the site of intense chemical weathering. Samples were collected starting from the base of the granodiorite where it is the freshest and least weathered up through the increasingly altered portions of the weathering profile into the Gunflint grainstone. Samples were prepared and analysed for major oxides as well as trace and rare earth elements. Another prime example of the basal section of the Gunflint Formation can be seen at Schreiber Beach outside of Schreiber ON, which was studied by Polat et al. (2012) (Figure 1b). This area differs from the 11/17 site in that the Gunflint strata sits above a Neoarchean pillow basalt sequence that shows the well preserved basaltic pillows overlain by hyaloclastites made up of shattered pillow breccias and flows. Directly below the contact with the Gunflint, the pillow sequence has been weathered and exfoliated creating red to brown highly fractured pillows topped off by brown to green pillow basalt soils. A geochemical comparison of these two sample locations was performed. When plotted on Nesbitt (2003) A-CN-K feldspar diagrams the 11/17 outcrop shows an enrichment in Al2O3 (Figure 2a), whereas the ACN-K diagram plotted by Polat et al. (2012) shows that the weathered layer is enriched in K2O and Al2O3 (Figure 2b). The difference in the parent material of the two weathered profiles and possibly potassic metasomatism in the basaltic material is controlling these weathering trends. The spheroidal weathering granodiorite also has an intense enrichment in Fe, Mn and Mg, probably the result of interactions with Gunflint derived fluids, which overprinted the effects of weathering. This period of alteration by Gunflint fluids also resulted in intense leaching of rare earth elements. 129 �A B Figure 1.a) the outcrop on the side of Highway 11/17, showing the weathering profile starting with the granodiorite and working up through the weathered section eventually capped by Gunflint grainstone. B) The stratigraphic sequence of the Schreiber beach outcrop modified from Polat et al., (2012). B A Figure 2. a) A-CN-K diagram for the data collected from the Highway 11/17 outcrop outside of Thunder Bay, ON. The fresh granodiorite samples plot in the middle and the weathered samples plot at the top showing CaO, Na2O and K2O depletion. B) The A-CN-K diagram from Polat et al.(2012) showing the enrichment of K from the unweathered pillows to the weathered brown to green basalts. References: Nesbitt, H.W., 2003, Petrogenesis of silicalstic sediments and sedimentary rocks, in Lentz, D.R., ed., Geochemistry of sediments and sedimentary rocks: Evolutionary Considerations to Mineral Deposit-Forming Environments: Geological association of Canada, GeoText4, p. 39-51 Polat, A., 2012, Extreme element mobility during transformation of Neoarchean (ca. 2.7 Ga) pillow basalts to a Paleoproterozic (ca. 1.9 Ga) paleosol, Schreiber Beach, Ontario, Canada. Chemical Geology, 326-327, 145173. 130 � https://digitalcollections.lakeheadu.ca/files/original/cfaebe60172260fedcb902ba9425270e.pdf a55faff9c466cceda748bca918036e0e PDF Text Text �INSTITUTE ON LAKE SUPERIOR GEOLOGY 60TH ANNUAL MEETING May 14-17, 2014 Hibbing, Minnesota Sponsored by PRECAMBRIAN RESEARCH CENTER, UNIVERSITY OF MINNESOTA DULUTH, and MINNESOTA GEOLOGICAL SURVEY James D. Miller and Mark A. Jirsa Co-Chairs Proceedings Volume 60 Part 2 – Field Trip Guidebook Edited by Mark Jirsa, Terrence Boerboom, and Amy Radakovich, Minnesota Geological Survey Cover Photos Historic photo of steam shovel was acquired from the archives of Minnesota Discovery Center Research Library; all other photos were taken by Jirsa. Photos vaguely represent the subject matter of some field trips described in this guidebook (relevant trip numbers are shown in parentheses). Photos in large “6” include conjugate faults (1) in oxidized Biwabik Iron Formation at Susquehanna Mine; glacial till in Albany Mine (6); taconite pellets (3); folded Soudan Iron Formation (2); gray-green stromatolites photographed from polished sample in collection of Dan England, Eveleth Fee Office (1); and jointed, oxidized Biwabik Iron Formation from Glenn Mine (E). Photos in large “0” include taconite-bearing drill core (A); red stromatolites from Dan England collection (1); pillowed metabasalt from near Gilbert (7); and historic photo of steam shovel, location and date unknown (B, C). i �Generalized geologic map showing locations of field trips in this guidebook Note that no Trip 4 is shown, as it was cancelled Geology simplified from Minnesota Geological Survey Map S-21 statewide bedrock geology ii �TABLE OF CONTENTS PROCEEDINGS VOLUME 60 PART 2— FIELD TRIP GUIDEBOOK Pre-meeting field trips, Wednesday, May 14 TRIP 1: STRATIGRAPHY, SEDIMENTOLOGY, STRUCTURE, AND MINERALIZATION OF THE BIWABIK IRON FORMATION, CENTRAL MESABI IRON RANGE ............................................ 1 TRIP 2: A WALK IN THE PARK – NEOARCHEAN GEOLOGY OF LAKE VERMILION STATE PARK .............................. 37 TRIP 3: WESTERN MESABI RANGE MINING OPERATIONS .................................................. 76 TRIP 4: LAURENTIAN VISION RECLAMATION Cancelled Post-meeting field trips, Saturday, May 17 TRIP 5: VISIONS OF MATURI: THE GEOLOGY OF THE SOUTH KAWISHIWI INTRUSION 86 TRIP 6: THE ST. LOUIS SUBLOBE AND GLACIAL LAKE UPHAM 102 TRIP 7: GEOLOGY AND GOLD MINERALIZATION OF THE VIRGINIA HORN AREA ………….119 Syn-meeting field trips, Friday afternoon, May 16 TRIP A: STATE DRILL CORE LIBRARY – HIBBING, MINNESOTA ...................................... 137 TRIP B: TRIP C: TRIP D: TRIP E: HIBBING’S IRON MINING AND CULTURAL HISTORY ........................................... 140 MINNESOTA DISCOVERY CENTER ..................................................................... 146 COLERAINE MINERALS RESEARCH LABORATORY .............................................. 147 MINEVIEW FROM A CANOE................................................................................ 148 The editors extend sincere thanks to all who contributed to this field trip guidebook. The time and effort expended to prepare field trip descriptions are greatly appreciated. Special thanks to Minnesota Coaches Voyageur Bus Company in Duluth for substantially discounting transport costs, and to Greyhound Bus Museum in Hibbing for providing a vintage bus for field trip B. Reference to material in Part 2 should follow the example below: Field trip authors, date, title: Institute on Lake Superior Geology Proceedings v. 60, Part 2, p. XX. Proceedings Volume 60, Part 1—Program and Abstracts, and Part 2—Field Trip Guidebook are published by the 60th Institute on Lake Superior Geology and distributed by the Institute Secretary: Peter Hollings Department of Geology Lakehead University Thunder Bay, ON P7B 5E1 CANADA peter.hollings@lakeheadu.ca Some figures in this field trip guidebook were submitted by authors in color, but are printed grayscale to conserve printing costs. Full color imagery will appear in the digital version of the guide when it is available on-line at http://www.lakesuperiorgeology.org. ISSN 1042-9964 iii ��FIELD TRIP 1 Wednesday, May 14, 2014 STRATIGRAPHY, SEDIMENTOLOGY, STRUCTURE AND MINERALIZATION OF THE BIWABIK IRON FORMATION, CENTRAL MESABI IRON RANGE LEADERS: Phil Larson (Duluth Metals) Marsha Patelke (Natural Resources Research Institute – Duluth) Jakob Wartman (United Taconite, Cliffs Natural Resources) Michael Totenhagen (Arcelor Mittal) Mark Jirsa (Minnesota Geological Survey) Steven Losh (Minnesota State University – Mankato) Peter K. Jongewaard (Cliffs Natural Resources, recently retired) Figure 1. Bedrock geologic map of the central Mesabi Iron Range showing 3 main field trip localities. Stops 1-4 are located on United Taconite’s Thunderbird North and South Mines near Eveleth; stop 5 is the Mary Ellen Mine near Biwabik. Archean metavolcanic, metasedimentary, and granitic rocks are shades of green, blue, and pink, respectively; Paleoproterozoic Pokegama Quartzite is yellow; Biwabik Iron Formation is red; and Virginia Formation is gray (modified from Jirsa and others, 1998). 1 �INTRODUCTION This field trip explores the geology of the Paleoproterozoic Biwabik Iron Formation (BIF) in the central Mesabi Iron Range of northeastern Minnesota. The formation hosts iron-ore deposits that have been mined continuously for nearly 125 years, constituting the most economically significant mining district in the United States. This trip will visit exposures in 3 localities (Fig. 1): the Cliffs Natural Resources - United Taconite LLC Thunderbird North Mine in Eveleth, an active magnetite taconite mine; the inactive satellite Thunderbird South Mine; and the Mary Ellen Mine near Biwabik, a closed directshipping (hematite) ore mine. Because this field trip guide was compiled by a large number of co-leaders having different experiences and perspectives, it may contain some content that is repetitive or has minor inconsistencies. The iron-bearing strata of the Biwabik Iron Formation were first noted in 1866 by Henry Eames, on what was to become the eastern end of the Mesabi Iron Range. Sporadic exploration for iron ore deposits began soon after, notably by Peter Mitchell and the Ontonagon Syndicate. Lack of infrastructure hampered exploration and development until 1884, when the Duluth and Iron Range Railroad to the Soudan Mine on the Vermilion Range was completed. The resulting exploration boom initially focused on the well-exposed, metamorphosed iron-formation near the contact with the Duluth Complex. It was not until 1890 that the Merritt Brothers re-directed their exploration focus to an area farther SE along the Mesabi range, correctly surmising that iron ore float located south of the Giants Range had been transported some distance by glaciation. Their discovery of soft, friable, high-grade iron ore at the future Mountain Iron Mine on November 16, 1890 revolutionized the global iron ore and steel industries. The first shipment of 4,245 tons in 1892 came at the crest of a wave of exploration that was to quickly discover and develop billions of tons of iron ore along the 175 km strike-length of the Biwabik Iron Formation. The Mesabi Iron Range rapidly developed into the largest iron mining district in the United States, a status it continues to hold. For decades after its discovery, it was the largest iron ore district on earth, accounting for nearly half of global production in the late 1940s. The Biwabik Iron Formation ranges from 0.5 to 5.0 km width (0.25 to 3 miles) along a strike length of 175 km (100 miles) (Fig. 2). The formation, as much as 220 meters (750 feet) thick, generally dips gently to the southeast at angles of about 7° to 15°. Unweathered, unaltered iron-formation, colloquially known as “taconite,” contains about 30 percent iron and 45 percent silica, with the balance (2-10%) composed principally of MgO, CaO, and MnO. In numerous places along the length of the range, typically along joints, fractures, folds, and other structurally prepared zones, silica and other elements were leached under tropical weathering conditions, locally enriching the iron content to as much as 64 percent. So-called “natural” or direct-shipping ores dominated production through the Second World War. Depletion of higher grade reserves, combined with increasingly stringent quality requirements, led to a rapid conversion of the industry during the 1950s. Perfection of fine grinding, magnetic separation, and pelletizing technology (the “taconite process”) allowed for economic exploitation of pristine ironformation. Between 1956 and 1977, eight taconite facilities with a combined capacity in excess of 54 million tons per year (mtpy) were brought to production. Production of beneficiated iron ores (gravity and taconite concentrates) exceeded that of direct-shipping ores in 1958, and by 1967, taconite concentrates accounted for over half of production. At this writing, six taconite (40 mtpy capacity), and three tailings recovery (3 mtpy capacity) facilities are in production (Fig. 3). 2 �Figure 2. Location map of the Mesabi Iron Range (maroon). Note the Duluth Complex (Keweenawan, 1.1 Ga) on the east side). Figure 3. A) Aerial distribution of taconite pits and cities. B) A longitudinal section of the Biwabik Iron Formation showing: average thickness of the iron-formation at each taconite operation (along with the thickness of the various members at each operation), and mined taconite intervals (as black columns adjacent to the sections). From Severson and others (2009). 3 �REGIONAL GEOLOGY Basement rocks in the central Mesabi area consist of 2.7 Ga Archean granitic and greenstone terrains of the Superior craton. These were intruded by the 2.1 Ga Kenora-Kabetogama dike swarm, but were apparently eroded to a peneplained surface by about 1.9 Ga. The peneplained Superior craton formed the platform upon which a Paleoproterozoic continental shelf and margin assemblage was deposited. 1.88 Ga iron-formation and associated clastic sediments are preserved along 3000 km of strike length near the margin of the craton, and were once much more extensive. Available evidence indicates iron-formation accumulation occurred at all margins of the craton simultaneously, and is a reflection of global ocean chemical conditions rather than local basin geometry. Most of the iron-formation is preserved in mobile belts at the craton margin, and are thus variably deformed and metamorphosed (Trommald Iron Formation of east-central Minnesota, Gogebic Iron Formation of Wisconsin-Michigan, Negaunee Iron Formation of Michigan, Sokoman Iron Formation of Labrador and Quebec, Cape Smith belt of Quebec, Kipalu Iron Formation of Hudson Bay, Sutton Inlier iron-formation of Ontario). Relatively flat-lying, undeformed, and un- or weakly-metamorphosed iron-formation is locally preserved inboard of the craton margin (Temiscamie Iron Formation of Quebec, the Gunflint Iron Formation of Minnesota-Ontario, and the Biwabik Iron Formation). These were likely deposited on a clastic-starved platform, and indicate that an epeiric sea may have nearly or completely inundated the peneplained craton during the peak of ironformation accumulation. Paleoproterozoic sedimentation and iron-formation accumulation followed a general sequence of depositional events throughout the Superior craton. Nearshore, tidally influenced clastic sedimentation was succeeded by chemically-precipitated iron-formation. The transition from clastic to iron-formation is typically abrupt; however the presence of iron-rich, chert-cemented epiclastic strata indicate that iron and silica precipitation was occurring prior to significant accumulation of the epiclastic-poor iron-formation. Significant iron-formation accumulation was apparently triggered by a lack of epiclastic input, rather than abrupt onset of favorable iron-precipitating geochemical conditions. Iron-formation accumulation proceeded at very low rates until resumption of clastic deposition in the basin. Available evidence indicate that the Biwabik Iron Formation records accumulation over as many as 15 million years within a significantly larger time span (Larson, 2013). Iron-formation accumulation across the craton was likely diachronous within this time span, as significant internal disconformities are evident within individual iron-formations, and correlations between individual iron-formations are problematic. Iron-formation accumulation proceeded until resumption of clastic deposition in the basin; similar to the basal epiclastic strata, significant amounts strata containing iron-rich precipitates are found in the overlying epiclastic units. In the Animikie Basin, the basal, near-shore, epiclastic sequence is represented by the Pokegama Formation, which was succeeded by deposition of the Biwabik Iron Formation. In excess of 200m of iron-formation accumulation was abruptly terminated at the contact with the overlying argillites, siltstones, and greywackes of the Virginia Formation. Within the regional Paleoproterozoic depositional system, this contact is also marked by chaotic (paleoseismic) deformation and deposition of ejecta related to the 1.85 Ga Sudbury meteorite impact event. (Jirsa and others, 2011). In the central Mesabi area, a conglomeratic bed containing angular argillite, chert, and carbonate clasts within the Upper Slaty member Dolomite/Limestone unit (submember US-2; see Stratigraphy section below) (Severson and others, 2009) has been correlated with the Sudbury meteorite impact event (Addison et al., 2005). Turbiditic sediment of the overlying Virginia Formation was ultimately sourced from 1.87-1.83 Ga volcanic rocks, and represents the collision of an island arc with the southern margin of the Superior craton during the Penokean orogen. Collision and sedimentation at the continental margin led to development of a foreland basin and the thick turbidite sequence. Paleoproterozoic sedimentary rocks (including the Animikie Group) along the southern margin of the Superior craton were bisected by the 1.1 Ga Midcontinent Rift, a 2000km (1200 mile)-long rift system extending in an arcuate fashion from northeastern Kansas to southeastern Michigan. The rift consists predominantly of mafic flows and intrusions overlain by rift-fill sedimentary strata. In addition, numerous 4 �mafic dikes, chonoliths, and other small intrusions were emplaced locally into rocks of the Animikie Group rocks at significant distances from the rift axis. These include mafic dikes cross-cutting the Biwabik Iron Formation near Keewatin, and a series of sills emplaced into iron-formation in the vicinity of Aurora. However, no such intrusions are known from the Central Mesabi area. Significant thermal metamorphism of iron-formation is limited to the area generally east of Aurora, within the aureole of the Duluth Complex. By the end of the Cretaceous, peneplaination produced topography similar to that of modern day. The central Mesabi area lay close to the eastern extent of the Cretaceous Interior Seaway, and the Biwabik Iron Formation locally is overlain by near-shore shale and sandstone. A basal iron-ore-bearing conglomerate is locally present, indicating extensive formation of supergene-enriched, direct-shipping ores predated Cretaceous sedimentation. The roughly coeval formation of secondary iron oxides and hydroxides implies that supergene enrichment may have occurred under a tropical climate during the Cretaceous (Symons, 1966; Purucker, 1973). During this and subsequent weathering, it is likely that the iron-formation served as a geochemically resistant “cap” that protected the underlying Giants Range Batholith, and is indirectly responsible for formation of the Missabe Wachu (“Big Man Hills” from the classic David D. Owen’s 1852 Report of a Geological Survey of Wisconsin, Iowa, and Minnesota). The area is now known physiographically as the Giants Range. Ice sheets advanced across the Biwabik Iron Formation multiple times during the course of the Pleistocene, primarily during the last 2 million years. Unconsolidated saprolite, including the supergene enriched direct-shipping ore (DSO), was preferentially eroded, leaving only remnants in deep, structurehosted, trough-shaped bodies and stratiform layers variably protected by resistant cap rocks. Locally, large blocks of weathered iron-formation and even DSO were eroded by glaciotectonic activity. A variable thickness of till, outwash, and glaciofluvial sediment was deposited over the iron-formation during the final glacial cycle (See related Field Trip in this guidebook). Archean Rocks The Neoarchean bedrock of the central Mesabi Iron Range lies near the southern edge of the Wawa subprovince of the Superior Province, and constitutes the southwestern-most exposures of the terrane. The Archean supracrustal rocks on the Mesabi Range are separated from the well-known Vermilion district to the north by the Giants Range batholith, a large, composite body consisting of granitoid rocks of several generations and compositions. The Archean rocks are covered to the south, east, and west by Paleoproterozoic strata, including iron-formation of the Mesabi Iron Range. The Archean supracrustal rocks are subdivided into northern and southern panels on the basis of contrasting metamorphic grade and deformation style (Fig. 4). The northern panel, adjacent to the Giants Range batholith, contains intensely lineated, amphibolite-grade schist of volcanic, intrusive, and clastic protolith. The southern panel contains a similar stratigraphic sequence, but has minerals that indicate it underwent metamorphism to much lower grades, ranging from prehnite-pumpellyite to low greenschist facies. The two panels are separated by the east-trending, post-metamorphic, Laurentian fault. Amphibolite-grade rocks of the northern panel (north of the Laurentian fault) comprise the Minntac sequence that contains locally strongly layered schists having geochemical and outcrop-scale characteristics of volcanic, intrusive, and turbiditic protoliths. The lower grade strata within the southern panel are subdivided into the Mud Lake and Midway sequences. The Mud Lake sequence forms a broad, southwest plunging syncline (the Mud Lake syncline) defined by outer limbs of calc-alkalic and tholeiitic strata, and cored by graywacke, slate, and minor felsic tuff. The Mud Lake strata are unconformably overlain by, and locally lie in fault contact with, fluvial and alluvial conglomerate, subaerially deposited trachyandesitic flows, and pyroclastic rocks that comprise the Midway sequence. 5 �Figure 4. Geologic map of the central Mesabi range area, illustrating features of the Archean bedrock (From Jirsa and Boerboom, 2003). The Z-shaped fold/fault structure apparent in the strike of iron-formation is known locally as the “Virginia horn.” 6 �Paleoproterozoic Animikie Group The Paleoproterozoic Animikie Group unconformably overlies the Mille Lacs Group to the south in central Minnesota, and the Archean basement on the Mesabi Range to the north (Southwick and Morey, 1991). The Animikie Group consists of three major formations on both the Mesabi and equivalent Gunflint Iron Ranges. The respective units are the Pokegama Formation and the Kakabeka Quartzite (the lowest units), the Biwabik and Gunflint Iron Formations (the middle units) and the Virginia and Rove Formations (the upper units, composed of graywacke and shale). The Thomson Formation in the northern part of east-central Minnesota is inferred to be correlative with the Virginia and Rove Formations. The Biwabik and Gunflint Iron Formations are on strike with each other and were probably continuous prior to intrusion of the Duluth Complex at about 1.1 Ga. Age The age of the Animikie Group is relatively poorly constrained in the central Mesabi area due to a paucity of datable cross-cutting or intercalated units. A minimum age of deposition for the Pokegama Formation is 1,930 ± 25 Ma (Pb/Pb), which was obtained from quartz veins that cut the Pokegama Formation (Hemming and others, 1990). An age of 1,878 ± 2 Ma (U/Pb on euhedral zircons) has been obtained from an ash layer in the upper Gunflint Iron Formation (Fralick and others, 2002); this horizon may correlate with the Intermediate Slate of the Lower Slaty Member of the Biwabik Iron Formation (LS-1 submember). The ejecta layer correlated with the 1,850 Ma Sudbury impact event dates the stratigraphic top of iron-formation (Addison and others, 2005; Jirsa 2010). Zircon ages from an ash layer at the very base of the overlying Virginia Formation are dated at 1,832 ± 3 Ma (Addison and others, 2005). The latter sample was collected a few inches above the base of the Virginia Formation in drill hole VHD-00-1, located immediately to the west of the Thunderbird Mine. Vallini and others (2007) dated a metamorphic xenotime overgrowth on detrital zircon from the Pokegama Formation at 1763 ± 14 Ma, attributing this to a ~1786 Ma regional basin-wide, subtle thermal pulse. Stratigraphy In the central Mesabi area, the Animikie Group is composed of three formally defined formations: the Pokegama Formation, Biwabik Iron Formation, and Virginia Formation; and an informally named unit of breccia and ejecta related to the Sudbury meteorite impact event. Pokegama Formation The Pokegama Formation consists of up to 300’+ of shale, siltstone, and chert-cemented quartz arenite (quartzite). The formation consists mostly of siltstone and shale. Silica-cemented quartz arenite is confined to an interval a few meters thick beneath the overlying Biwabik Iron Formation. Ojakangas (1983) used gross stratigraphic relationships to subdivide the Pokegama Quartzite into three informal members: a basal member consisting dominantly of thinly bedded to laminated shale and lesser amounts of siltstone; a middle member consisting of shale and siltstone and scattered thin beds of quartz arenite; and an upper member consisting mostly of quartz arenite. The formation was deposited on an irregular Archean bedrock surface. A basal conglomerate contains a poorly sorted array of clasts derived from the underlying bedrock set in a matrix of fine-grained sandstone to siltstone. Basal strata are marked by a second conglomerate that has angular to sub-rounded, pebble- and granule-size clasts of chert, jasper, algal fragments, and vein quartz. This implies that Pokegama-like clastic sedimentation and Biwabik-like chemical precipitation were, for a time, contemporaneous (Jirsa and Morey, 2003). At most localities, the contact between the Pokegama Quartzite and the overlying Biwabik Iron Formation is conformable and gradational; the presence of beds of chert 6 to 12 meters beneath the Biwabik–Pokegama contact has been cited as evidence for a gradational sedimentary regime between the two formations (Ojakangas, 1983). Biwabik Iron Formation The Biwabik Iron Formation ranges from about 175-300 feet thick at the extreme eastern end of the Mesabi Iron Range (Dunka Pit) (Bonnichsen, 1968), to 730-780 feet thick in the Central Mesabi area near 7 �Eveleth, decreasing to around 500 feet thick on the western Mesabi Iron Range near Coleraine, and eventually exhibits a “nebulous ending about 15 miles southwest of Grand Rapids” (Marsden and others, 1968). The formation is subdivided into four informal members (from bottom to top): Lower Cherty, Lower Slaty, Upper Cherty, and Upper Slaty (Wolff, 1917). Individual beds can be described as either sand-textured granular iron-formation (gif), composed predominantly of rounded oolitic grains and intraclasts, or mud-textured and laminated banded iron-formation (bif). Although interlayering of these two lithotypes occurs on all scales, the “cherty” members are composed predominantly of medium- to thick-bedded gif; the ‘slaty’ members are composed predominantly of thin-bedded bif. The terms “slaty” and “cherty” were originally used by miners, and are not indicative of metamorphism or slaty cleavage, or a predominance of silica as chert. The cherty gif members are largely composed of iron oxides and chertcemented granules of iron silicates and carbonates. The slaty members are generally composed of laminated iron silicates and iron carbonates. The slaty bif members are envisioned to have been deposited below storm wave base. Granules comprising the cherty gif members formed in high-energy environments. Two models have been applied to explain formation of granules in the Paleoproterozoic iron-formations: direct precipitation, and reworking of intraclasts shoreward during storm events where they are reworked into granules in shallower water. Some granules appear to be the product of reworking of laminated bif material, however the self-similarity of granule sizes, lack of apparent intraclast source material, and geochemical dissimilarity between gif and bif material suggest the gif formed due to direct precipitation as oolites in shallow water. A few diagnostic marker units within the formation allow basin-scale correlation. Two stromatolitebearing intervals several feet thick are present, one at the base of the Lower Cherty member and the other in the middle of the Upper Cherty member (UC-6 submember). For submember terminology, see Detailed Stratigraphy of Biwabik Iron Formation discussion below. The black Intermediate Slate (LS-1 submember) at the base of the Lower Slaty member reportedly contains ash-fall tuff, with up to 5.5% Al2O3 (Morey, 1992). The top of the Upper Slaty member (US-2 submember) contains several feet of limestone and dolomite. Most of these marker units, which are prominent in the eastern and central parts of the range, pinch out in the vicinity of Hibbing, about 60 miles from the west end of the range (Severson and others, 2009). The Lower Slaty member is not present at the far western end of the range. Sudbury Impact Layer The contact between iron-formation and overlying slate of the Virginia Formation is marked by a thin layer of deformed substrate and overlying ejecta formed during the 1850 Ma Sudbury meteorite impact event. The horizon can only be seen in drill core on the Mesabi Iron Range; however, it is well exposed in the equivalent Gunflint Formation to the northeast. There, the deformed layer consists of ≤ 7m of chaotically folded and locally brecciated substrate iron-formation. At least some of the iron-formation beds were ductily deformed, implying they were not yet fully lithified at the time of deformation. The rheologic behavior of siliceous layers was more brittle, and they were dislodged and shattered. In the context of a major meteorite impact event, the deformation is inferred to have occurred by impact-induced seismicity (Jirsa and others, 2011). The deformed strata are draped by ejecta (≤ 1m-thick) containing abundant petrographic evidence of impact origin, including the presence of zoned spherules and quartz fragments displaying multiple planar deformation features. The impact-related horizon, known as the Sudbury Impact Layer, is well exposed in several locations in the Lake Superior region (Fig. 5). It occurs in the Gunflint Lake area of northeast Minnesota (op. cit.), in the Thunder Bay area of adjacent Ontario (Addison and others, 2005), and in Michigan (Cannon and others, 2010; Pufahl and other, 2007). At exposures near Thunder Bay and Gunflint Lake, the contact between the impact layer and the overlying Rove Formation (Virginia equivalent) is inferred to be a disconformity that may represent a significant depositional or erosional hiatus, perhaps as long as 15-40 million years (Jirsa and others, 2011). Whether this is also true for the contact on the Mesabi range is currently unclear. It is noteworthy that the depositional environment in which the Virginia and Rove Formations were deposited is nearly identical with that of the underlying iron-formation—the primary difference being a paucity of chemical precipitates (iron and silica) in the former. 8 �Figure 5. Generalized correlation diagram of Paleoproterozoic strata in the Lake Superior region. The position of the Sudbury Impact Layer is from Cannon and Addison (2007). Virginia Formation The Virginia Formation overlies the Biwabik Iron Formation and Sudbury Impact Layer on the south side of the Mesabi Iron Range, and is inferred to extend southward beneath glacial cover for an unknown distance. Presumably, it reappears in east-central Minnesota as a folded and metamorphosed sequence called the Thomson Formation. The Virginia Formation is a thick turbidite sequence composed of interbedded argillite, graywacke, and volcaniclastic rocks (Fig. 5). The 450 meters of Virginia Formation strata penetrated at a site south of Biwabik have been described in considerable detail in Lucente and Morey (1983). The lower part of the formation is composed almost entirely of alternating beds of darkgray, silty mudstone and black carbonaceous shale. Quartz-rich siltstone and very fine- to fine-grained lithic graywacke become more abundant stratigraphically higher. The basal part of the Virginia Formation contains several beds of coarse-grained feldspathic graywacke and volcaniclastic rocks, as well as many lenses and irregular beds of limestone and dolomite. Dolomite-rich concretions of various sizes and shapes, also characterize the lower several hundred meters of the formation. Sandstone beds become coarser-grained and more abundant up-section, and are composed of angular quartz and feldspar grains in a matrix of muscovite and chlorite. Much of the matrix consists of diagenetically altered lithic fragments. Hemming and others (1995) documented that shales of the Virginia Formation have Nd-depleted, mantle model ages of 2.35 to 2.14 Ga. Interbedded volcaniclastic sediments have younger model ages of 1.99 to 1.86 Ga. Craddock and others (2013) showed a dominant spectrum of Penokean orogeny ages (1.85-1.8 Ga) for detrital zircon populations from the correlative Thompson and Rove Formations. These data indicate Virginia Formation sediment was sourced from a comparatively young, differentiated volcanic arc, most likely from the Wisconsin Magmatic Terrane and equivalent rocks in Minnesota to the south. Depositional Environments The Animikie Group records a sedimentalogical transition from nearshore, tidally influenced, allochthonous clastic deposition, through shallow, autochthonous, chemical platform deposition, interrupted by impact-dominated deformation and deposition, and followed by deep water basinal turbidite sedimentation (Fig. 6). The Pokegama Formation is interpreted to have been deposited in a tidally influenced, shallow marine setting near the shoreline, having received clastic detritus from the Archean basement to the north (Ojakangas, 1983; Craddock and others, 2013). In the central Mesabi area, the Pokegama Formation consists of a lower (argillaceous) member, a middle member of intercalated argillaceous and silty sedimentary strata, and an upper member of quartz sandstone. This succession is interpreted to represent a 9 �transition from upper tidal flat to lower tidal flat/subtidal depositional environments (Ojakangas, 1983). Elsewhere along its strike length, the Pokegama contains pebble conglomerate that may represent a transgressive lag. The transition from near-shore, clastic-dominated sedimentation to autochthonous chemical sedimentation is recorded by an abrupt gradation into iron-formation. The abrupt decrease in clastic input is consistent with non-accretionary transgression across the peneplained craton, whereby relatively small rises in eustatic sea level translate into dramatic shifts in shoreline position. In general, iron-formation can be geochemically divided into two components: an autochthonous chemically precipitated component, and an allochthonous clastic component. The elements comprising the autochthonous component (Fe, Si, Mg, Ca, Mn, P) are precipitated directly from seawater, while the allochthonous component (Al, Ti, K, Na) is derived from terrestrial sources (dust or suspended sediment). In general, the allochthonous component of the BIF averages 2.5%. The close correlation between lithofacies and mineralogy indicates that iron-formation mineralogy is a sensitive recorder of redox conditions at the sediment-water interface and, by extension, the depositional environment. The Lower Cherty Member (LC) of the Biwabik Iron Formation is interpreted to have been deposited on a shallow marine shelf. The LC is dominated by granular iron-formation (gif), characterized by rounded oolitic grains, cross-bedding, and other features indicative of a high-energy environment. Accumulation was driven nearly entirely by autochthonous chemical precipitation, with very little epiclastic input. Ferric hydroxide apparently co-precipitated with carbonates, evidenced by an abundance of ankerite. The common presence of stylolites indicates significant volume loss of both iron- and carbonate minerals by chemical dissolution during compaction. The presence of herringbone crossstratification in the lowermost LC indicates deposition in a tidally-influenced environment. The presence of thin laminae of slaty banded iron-formation interbedded with medium-bedded gif farther up-section within the LC suggests a transition to a deeper shelf environment. There, steady deposition of mudtextured banded iron-formation (bif) below the fair-weather wave base was periodically punctuated by rapid accumulation of gif, perhaps by shelf-to-basin transport during storm events. The upper portion of the LC contains a thick-bedded, coarse-textured (with intraformational conglomerate), pink (oxidized) gif overlain by a medium-bedded, sand-textured, green (reduced) gif (LC6/LC-7 contact) with abundant interbedded bif layers. This major lithostratigraphic break represents a significant disconformity within the overall BIF sequence. Trace element data show an abrupt transition from a high Al:Ti clastic source for LC strata below the break, to a low Al:Ti clastic source for all BIF strata above the break, indicating a change in sediment provenance. The LC sequence above the LC6-LC-7 disconformity records a transition to deep water pelagic bif sedimentation, culminating in the Lower Slaty (LS) member Intermediate Slate (LS-1) submember. The Intermediate Slate/LS-1 is composed of iron silicates and sulfides in a laminated bif. The relatively large clastic content indicates a decrease in the rate of autochthonous chemical sediment accumulation, driven in part by redissolution of ferric hydroxide precipitates in anoxic bottom water. The LS to Upper Cherty (UC) (LS-2 to UC-4) sequence is dominated by laminated bif, punctuated by lenticular or channel-shaped bodies of gif (Interbedded Cherts or IBCs). The bifs record deepwater sedimentation; the gifs consist of sand-textured granules that were exported from a high-energy environment and deposited in bypass channels on the shelf (channels) or deepwater fans (lenses). The upper portion of the UC (submembers UC-5 to UC-7) is characterized by gifs deposited in a high-energy environment, as evidenced by abundant rounded oolitic grains, stromatolites, and cross bedding. Mineralogically, these submembers are characterized by the presence of significant amounts of primary hematite relative to the Biwabik Iron Formation as a whole, and the Algal Chert Horizon/UC-6 submember is significantly enriched in Mn, indicating deposition in oxic conditions. Combined, these features suggest deposition in shallow, tidally-influenced or subtidal environment. The high-energy environment, combined with a paucity of clastic sediment input, suggest this may have been analogous to a modern carbonate platform environment. Chauvel and Dimroth (1974), noted similar features (chiefly oolitic and intraclastic sand) in the Sokoman Iron Formation, and their corresponding textural similarity to sediments in modern carbonate environments. They applied a carbonate facies model to the iron-formation, and attributed gif deposition 10 �to a lagoonal platform depositional environment, characterized by rapidly shifting banks of oolitic and intraclastic sand, mud banks, and small, lagoonal basins ringed by oolite shoals. Sommers and others (2000) concluded that ooids and stromatolites from the Lower Algal Chert member of the Gunflint Formation (stratigraphic equivalent of the UC-6) were originally deposited as carbonates and later replaced by silica. Rankey and Reeders (2011) described how the interaction of wind, waves, and tides with channel morphology on platforms to create “Spin Cycle” currents which control formation, transport, grain size, sorting, and deposition of chemically precipitated oolites. Such currents are a viable mechanism for keeping precipitating granules frequently in suspension until they reached a critical size; this in turn provides an explanation for the remarkable self-similarity in granule size observed in gifs within the BIF (Patelke, in progress). The sequence from UC to Upper Slaty (UC-8 to US-2) is dominated by laminated bifs, indicating a return to a deeper-water shelf environment. The US-2 is a limestone of enigmatic origin. The BIF is overlain by argillite and graywacke of the Virginia Formation, deposited by northwardadvancing basinal turbidite sedimentation. Some volcanic ash evidently settled into the basin, locally forming graded beds with a totally volcanic composition. The dominance of black, fissile shale suggests the "raining out" of clay and deposition in deep, anoxic water below the wave base. Minor, thin, sandstone lenses were deposited by bottom currents (Lucente and Morey, 1983) . Figure 6: Possible siliciclastic environments for deposition of portions of the Biwabik Iron Formation in cross-sectional view (facies division in a shallow sea). Compiled, and drawn, by Marsha Patelke (in Severson and others, 2009) from sources including Boggs (2001), Leeder (1999), Nichols (1999), and Pratt and others (1992). 11 �Mesoproterozoic Despite being regionally extensive, no known Mesoproterozoic intrusive rocks are known in the Central Mesabi area. Similarly, the entire area of this field trip lies beyond the extent of thermal metamorphism of the Biwabik Iron Formation by the Duluth Complex. Cretaceous Cretaceous rocks are thought to be more or less continuous beneath glacial drift throughout the western half of the State, and form numerous outliers in the eastern half. On the Mesabi Range, Cretaceous rocks have been exposed by mining, and recognized in drill core and well cuttings (Fig. 7). The Cretaceous Coleraine Formation in the Mesabi district comprises iron-ore conglomerate, shale, and sandstone that form a thin irregular mantle over an uneven surface on the underlying bedrock (Sloan, 1964). The rocks are dominantly of marine origin in the west, but grade eastward in the central Mesabi to continental (fluvial and deltaic) sediments. In the western part of the district, fine conglomerate grades upward into a ferruginous grit and sandstone, including bluish-green shale in the vicinity of the Hill Annex mine near Calumet. The rocks grade eastward into continental sediments, including a widespread basal conglomerate composed of fragments of iron ore. The Cretaceous strata are virtually horizontal except locally, where they are interpreted to have slumped or to have compacted differentially over depressions in the underlying bedrock (Owens, 1956). Depressions and slumps were common topographic expressions of supergene-enriched natural iron ores, enabling the preservation of Cretaceous remnants post-glaciation. In at least two notable central Mesabi locations, the Coleraine Formation conglomerates were mined as ore. At the Judson Mine near Buhl, eroded iron ore was transported short distances southward, and conglomerates were deposited in northwestward-trending channels cut in the underlying bedrock (Everett, 1956). Owens (1956) describes a 60-ft section of iron ore conglomerate, red and white shales, and lignite in the Enterprise Mine northeast of Virginia. The occurrence of Cretaceous iron-ore conglomerates have been confirmed just east of Gilbert (Sloan, 1964). Figure 7. Panoramic view of the Enterprise Mine near Virginia; slump area in foreground and light-colored unit in background are Cretaceous sandstone (Owens, 1956). 12 �Quaternary The central Mesabi area was glaciated repeatedly during the course of the Pleistocene. Unconsolidated saprolite, including a significant amount of supergene enriched direct-shipping ore, was preferentially eroded, leaving only remnants in deep, structure-hosted trough-shaped bodies and in stratiform layers variably protected by resistant cap rock. Final ice retreat occurred about 13 C 14 thousand years before present, when the margin of the Rainy Lobe retreated north of the Giant’s Range, depositing a sandy-textured, boulder till in its wake. The St. Louis Sublobe advanced in a surge across the glacial lake to the south, reaching the toe of the Giant’s Range, depositing a silty, boulder-poor till. A later glacial lake capped the low lying areas with silty glaciolacustrine sediment. Glaciotectonism played a significant role in glacial erosion of direct-shipping ores. Large blocks of loose, porous oxidized and weathered iron-formation were frozen en masse onto the toe of the glacier, and thrust into the debris load. In the vicinity of the Fayal Mine in Eveleth, stripping operations in advance of a ‘milling’ mine encountered a block of direct-shipping ore entirely encased by glacial drift. A similar occurrence can be seen in the pit wall of the Glen Mine near Chisholm (Field Trip E, this guide book). The so-called Moose Track Mine produced in excess of 30,000 tons of ore, indicating the block contained over 10,000 m3 of material (Leith, 1903). BIWABIK IRON FORMATION: STRATIGRAPHY, STRUCTURE, MINERALOGY, AND ORE DEPOSITS Stratigraphy of the Biwabik Iron Formation The four-fold stratigraphy of Lower Cherty, Lower Slaty, Upper Cherty, and Upper Slaty members (Wolff, 1917) is still used at each of the currently operating (and inactive) iron mines on the Mesabi Iron Range. However, each of the mining companies further subdivides the Biwabik Iron Formation into several submembers based on lithostratigraphic units and mineralogical assemblages observed in drill core and mine exposures (Fig. 8). District-wide correlation of individual mine stratigraphy is problematic because lithostratigraphic or mineralogically defined units important at the mine scale are not necessarily extensive at the district scale. Severson and others (2009) defined and correlated 25 laterally extensive submembers within the BIF based on examination of more than 380 drill holes along 75 miles strike length. Units were named for their characteristic bedding types. Definition of these 25 “Rosetta” units serve as a starting point for more detailed sequence stratigraphic studies and basin analysis (Fig. 9). There clearly are variations in some of the units and the four main iron-formation members along strike that are related to facies changes. Figure 8. Textural characteristics of the Biwabik Iron Formation (from Severson and others, 2009 – modified from Pfleider and others, 1968). Dark bands represent magnetite-rich rock. 13 �Figure 9. Graphic summary of the 25 major “Rosetta” units in the Biwabik Iron Formation that were identified and described in Severson and others (2009). Most of these units have corresponding submember designations at each of the taconite mines. 14 �In addition to the member subdivisions recognized in the BIF throughout the Mesabi Iron Range, Thunderbird Mine geological staff further subdivide the iron-formation into 23 submembers based on lithologic, metallurgical, and mineralogical characteristics. These submembers form the basis for both resource estimation and grade control at the mine, and are described in detail below. For reference, the corresponding “Rosetta” unit name of Severson and others (2009) is listed in parentheses after the Thunderbird Mine submember name. The drilled thickness of the Biwabik Iron Formation in the vicinity of the Thunderbird North Deposit is approximately 686’ (Table 1). Member Thickness Upper Slaty 51 feet (15.5 meters) Upper Cherty 347 feet (121 meters) Lower Slaty 52 feet (16 meters) Lower Cherty 236 feet (72 meters) Table 1. Average thicknesses of the four members, as recognized at Thunderbird North Lower Cherty The Lower Cherty member is approximately 236 feet thick in the Thunderbird North deposit. It has been subdivided into the following eight subunits: LC-1 (Basal Red Unit) The submember is a pink-green-gray heterogeneous unit comprised of interbedded thin- bedded slaty and thin-bedded cherty carbonate-silicate(minnesotaite-talc-stilpnomelane) iron-formation. LC-1 comprises the basal 64 feet of the iron-formation. It is defined as the footwall thickness of the iron formation, the magnetite grade of which is subeconomic. It is in general poorly described since the majority of exploration and development drilling terminates in the upper few feet of this unit. LC-2 (Regular-Bedded Unit) The submember is a gray thin-bedded cherty carbonate silicate(minnesotaite-talc) iron-formation. Magnetite occurs as disseminated and diffuse idiomorphic granules and as replacement of thin slaty laminae and early burial stylolites. Magnetite (slaty) laminae often have thin stringers of white talc. LC-2 averages 16 feet in thickness, but varies across the extent of the Thunderbird North Deposit, being thinner in the southwest extent of the deposit and thicker in the northeast extent. In the northeast portion of the deposit, the unit is of sufficient thickness and grade to warrant mining despite dilution by the overlying LC-3 waste unit. LC-3 (Regular-Bedded Unit) Rocks of the submember are characterized by interbedded greenish-gray thin-bedded cherty and green medium-laminated slaty iron-formation. The unit is weakly magnetic, with the cherty beds conspicuously low in magnetite. The unit averages 13 feet thick, but varies across the deposit. In the southwestern extent of the deposit, the unit is up to 30 feet thick and predominantly composed of slaty iron formation. In the northeastern extent of the deposit, the unit is consistently 10 feet thick and composed predominantly of thin bedded non-magnetic granular chert. LC-4 (Wavy-Bedded Unit) The submember is composed of gray medium-bedded cherty oxide-carbonate(ankerite)silicate(minnesotaite-talc) iron-formation with minor thin irregular thin beds of slaty (magnetite) iron formation. Magnetite occurs as disseminated idiomorphic granules, patchy haloes cored by coarse slaty intraclasts, and replacement of thin slaty laminae. LC-4 varies from 40-50 feet thick at Thunderbird North, thickening to the southwest. Notable features of LC-4 are magnetite haloes or reaction rims around 15 �small intraclasts, and wispy laminae of magnetite, likely a later diagenetic overprint of early burial stylolites. The LC- 4 and its equivalents are widespread across the Mesabi District, and perhaps the most economically significant subunit, having a high weight recovery (36%) and being capable of producing a low silica concentrate (~2.0%). LC-5 (Wavy-Bedded Unit) The submember is composed of pink-gray medium- to thick-bedded cherty oxide-chertcarbonate(ankerite/kutnahorite) iron-formation. Magnetite occurs as disseminated grains and in mottles. The unit is notable for its high carbonate content, containing up to 3.0% CaO in ankerite. LC-5 varies from 40-50 feet thick at Thunderbird North, thickening to the southwest. LC-5 contains a small but variable amount of ‘primary’ (e.g. pre-supergene oxidation) hematite. LC-5 has appreciably more matrix chert than the underlying LC-4, and produces a significantly higher silica concentrate (~6.0%). LC-6 (Variably-Bedded and/or Mottled Unit) The submember averages seven feet thick, and consists of a pink massive- to thick-bedded cherty oxide-chert-carbonate(kutnahorite) iron-formation with conspicuous pink carbonate mottles. The unit is composed principally of coarse grained intraclasts, reflecting a relatively high energy depositional environment. The unit also contains an appreciable content of “primary” hematite, and has relatively low magnetite recovery. The unit is remarkably tough, and poses a challenge to mining in that it resists fragmentation during blasting and tends to produce large chunks. LC-7 (Bold Striped Unit) The submember is composed of interbedded thick irregular magnetite-carbonate-silicate slaty and green thin- to medium-bedded cherty carbonate(siderite)-silicate(greenalite) iron-formation. The unit averages 13 feet thick. The unit is remarkable in that magnetite occurs predominantly in the thick slaty laminae, resulting in a boldly-striped appearance in drill core. Green LC-7 sharply overlies the pink LC-6, and the contact is a highly visible stratigraphic marker throughout the Virginia Horn area. The transition upward from thick-bedded coarse-grained to thin-bedded fine-grained iron-formation, as well as the contrasting mineralogical assemblages at the LC-6/LC-7 contact, suggests an abrupt transition in the depositional environment. Very fine-grained magnetite (25-45 µm) and intimate association with very fine-grained chert and siderite contribute to grinding and processing difficulties with the LC-7. LC-8 (Mesabi Select Unit) The submember is visually similar to LC-7, consisting of a interbedded green medium- to thicklaminar massive slaty and greenish-gray thin-bedded granular cherty carbonate(siderite)silicate(greenalite) iron-formation. However, the unit contains little or no magnetite and is a waste product that makes excellent aggregate material. The LC-8 averages 21 feet in thickness. LC-8 from the Thunderbird North Mine is the type material for “Mesabi Select” crushed aggregate currently being marketed regionally for road construction, noted for its high specific gravity and angular fragmentation. Lower Slaty The Lower Slaty member, as defined at Thunderbird North, averages 52 feet thick and is characterized by non-magnetic and thin-bedded waste rock between the Lower Cherty and Upper Cherty member ore horizons. Other interpretations (Severson and others, 2009) of the Lower Slaty in the Virginia Horn area extend it to up to the top of the UC-4 submember, and up to 309 feet thick. LS-1 (Intermediate Slate) The submember is composed of predominantly black massive- to thinly-laminated slaty carbonate(siderite)- silicate (stilpnomelane-minnesotaite)-sulfide iron formation. The LS-1 averages 17 feet in total thickness, and is divisible into a lower half composed of a composed of thick bedded massive intraformational debris flow breccias and an upper half composed of thinly-laminated planar- bedded slaty iron-formation. Locally, thin to medium bedded black flinty cherts are present in the lower portion; these flinty cherts typically occur in pod-like bodies extending a few 100s of feet along strike. The upper portion of LS-1 has undergone extensive bedding-parallel deformation; essentially the entire horizon served as a low-angle fault plane. Small-scale folds are common, as are bedding-parallel syntectonic 16 �quartz-carbonate(ankerite-siderite) veins. The thinly-laminated planar-bedded slaty iron-formation in the upper portion is the so-called Intermediate Slate, a district-scale marker horizon. LS-1 is notable in that it contains a very high percentage of Al2O3 (1.86%) and other elements indicative of clastic input, suggesting the basin was experiencing either an influx of clastic detritus, or a sharp reduction in the rate of iron-formation deposition. LS-2 (Lowermost Thin-Bedded Unit) The submember is composed of a green to greenish-gray well-cemented very thinly-laminated slaty carbonate-silicate(minnesotaite) iron-formation. The unit averages 35 feet thick. The top of the LS-2 is defined by the appearance of significant magnetic slaty iron-formation. Commonly, this corresponds to the first appearance of thin-bedded intraclast breccias. These breccias commonly have a magnetite-rich matrix. Upper Cherty The Upper Cherty member at Thunderbird North contains all potential ore horizons situated above the Lower Slaty waste horizons. This comprises 347 feet in thickness and the remainder of the thickness of the iron-formation exposed in the present workings. The lowermost 257 feet of the Upper Cherty, as defined at Thunderbird North, consists of alternating horizons of dominantly slaty- and cherty-ironformation; these horizons have been included in the Lower Slaty by other workers (Severson and others, 2009) including US Steel. The Upper Cherty has been subdivided into eleven subunits at Thunderbird North. LUC-1 (Ore Zone of Lowermost Thin-Bedded Unit) The submember is composed of gray laminar thin-bedded slaty chert-silicate(stilpnomelane) magnetite iron- formation. The unit averages 18 feet in thickness and is notable for producing a very high silica concentrate (up to ~10% SiO2). This unit, in common with the other slaty iron-formation horizons in the Upper Cherty member, has a relatively high Al 2O3 content (~0.56%). LUC-2 (Lower IBC Unit) The submember is a heterogeneous unit, composed variously of green-gray thin-bedded slaty ironformation, interbedded green-gray thin-bedded slaty iron-formation, thin-bedded cherty iron- formation, and gray thick-bedded cherty iron-formation. The unit as a whole averages 46 feet thick. For the unit as a whole, thin-bedded granular cherty horizons predominate over thin- to medium-laminated shales. The abundance and frequency of cherty horizons generally increases up-section within the unit. Locally, pink to green-grey massive- to thick-bedded, coarse-grained, magnetite- bearing granular cherts, upwards of 20 feet thick, are present within the unit. These beds are characterized by significantly higher weight recovery, and significantly lower concentrate silica grades than the unit as a whole. LUC-3 ( Middle Thin-Bedded Unit) The submember is composed of dark reddish-brown thin/planar-bedded slaty chert-silicate ironformation. The unit averages 19 feet thick. Nodules and beds of chert are increasingly abundant upsection, culminating in the presence of a 1-2 foot thick horizon containing thin bedded flinty chert, an important marker horizon in the mine. UC-1 (Middle IBC Unit) The submember is composed of pinkish-gray thick-bedded cherty oxide-chert-silicate iron-formation. The unit averages 29 feet thick; however, it is not extensive through the deposit. The overall aspect of UC-1 is of a lenticular body on the order of several km in extent. UC-1 is notable in that it contains an appreciable content of ‘primary’ hematite; this hematite is intimately intergrown with magnetite, and thus, is recovered in the Fairlane concentrator circuit. UC-2 ( Middle Thin-Bedded Unit) The submember is a dark reddish-brown thin-bedded slaty chert-silicate iron-formation, averaging 46 feet in thickness. UC-2 is generally characterized by low weight recovery and a high concentrate silica, and thus, is marginal ore at best. 17 �UC-3 and UC-3A (Upper IBC Unit) The submembers are gray thick-bedded cherty iron-formation. Combined, they average 91 feet in thickness. Similar to UC-1, these units are not laterally extensive, and have the overall aspect of lenticular bodies on the order of several km in extent. The two units comprise a single depositional package; however, the lower half (UC-3) is generally characterized by low weight recovery, while the upper half (UC-3A) is characterized by higher carbonate and magnetite content. UC-3A was historically one of the primary ore units at the Thunderbird North Mine; however, it is not now being mined and is only poorly exposed in the pit. UC-3A is notable for an abundance of coarse-grained jasper intraclasts; the vivid colors of these intraclasts have resulted in the name ‘confetti ore’ being attached to UC-3A. UC-4 (Uppermost Thin-Bedded Unit) The submember is a dark reddish-brown thin-bedded slaty silicate iron-formation, averaging 18 feet in thickness. In areas where UC-1, UC-3 and UC-3A are absent, UC-4 is the upward continuation of UC2. The unit typically has a very low weight recovery. The uppermost 1-5 feet of the subunit is commonly a black, thin-bedded non-magnetic slaty silicate iron-formation. This has been recognized as an important marker horizon, and for some workers (Fig. 9), marks the top of the Lower Slaty member. UC-5 (Alternating-Bedded Unit) The submember consists of interbedded reddish-brown thin-bedded slaty silicate iron-formation and thin-bedded cherty iron-formation. The unit averages 15 feet thick. The thin cherty beds commonly contain abundant coarse-grained jasper intraclasts. UC-6 (Algal/Conglomerate Unit) The submember is very distinct in that it is composed of red medium- to thick-bedded coarse-grained intraclast conglomerates. Clasts in the conglomerate are composed predominantly of resedimented cherty algal stromatolites (oncolites). The conglomeratic matrix is commonly composed predominantly of manganiferous carbonates., including rhodochrosite (Zeilinski and others, 1994). UC-6 is notable in having the highest manganese content in the Biwabik Iron Formation, averaging ~6.0% Mn (7.8% MnO). UC-7 (Regular/Medium-Bedded Unit) The submember is composed of gray to red thick-bedded oolitic cherty oxide-chert-carbonate ironformation. The unit averages 37 feet thick in the Thunderbird North Mine area, and is known mostly from oxidized drill hole intercepts. The unit appears to consist of a lower red hematitic oolitic cherty ironformation and an upper gray magnetite-bearing oolitic chert- carbonate (ankerite) cherty iron-formation. The upper gray horizon contains abundant coarse poikiloblasts of ankerite; commonly these are weathered away, leaving vesicle-like vugs in the oolitic cherts. UC-8 (Thin-bedded unit) Similar to UC-5, the UC-8 consists of interbedded green-red thin-bedded slaty silicate iron- formation and thin-bedded cherty iron-formation. The unit averages 28 feet thick. UC-8 is known only from (commonly) oxidized drill hole intercepts. The thin cherty beds commonly contain abundant coarsegrained jasper intraclasts. The contact between UC-8 and the overlying US-1 is poorly defined. Upper Slaty The Upper Slaty member in the vicinity of the Thunderbird North Mine is only known from oxidized intercepts in a few drill holes, and is not exposed in outcrop. US-1 The submember is comprised predominantly of reddish-brown thin-bedded slaty iron-formation, and is about 50 feet thick. US-2 The submember is not exposed in outcrop or mine workings in the Thunderbird Mine, nor has it been intercepted in drilling. Regional drilling data indicates the unit is composed of grey thin-bedded micritic calcareous carbonate, and is about 19 feet thick. 18 �Structure of the Biwabik Iron Formation Recent studies of bedrock structure along the Mesabi Iron Range (Jirsa and others, 1998; 2002; 2005a; 2005b; Jirsa, 2006) reveal that a protracted history of deformation affected the Biwabik Iron Formation. Much of the formation forms a south-dipping homocline that contains little evidence of tectonic disruption, with the exception of locally well-developed deformation structures. A general sequence of deformation events can be inferred from those localized structures. The precise ages of events on the Mesabi range are unknown; however, a relative chronology for various structural elements can be established from cross-cutting relationships. Assigning deformation events to specific structures is speculative; nevertheless, the "D 0, D1, D2…" nomenclature is applied here to refer to suites of apparently related structures. The oldest are those presumably related to soft-sediment deformation (D0), including slumps, sedimentary breccias, and structures that appear to be the result of differential compaction and localized faulting synchronous with deposition. The earliest "regional" deformation (D 1) is manifest in localized, small-scale rotational structures, bedding-parallel slickensides, and larger nappe and sheath folds. The structures commonly lie along boundaries between units having strong rheologic contrast, such as the contacts between packages dominated by mudstone vs. those composed of siliceous, intraclastic grainstone. Nearly all of these structures display a sense of asymmetry that indicates southover-north tectonism. This northward vergence, and the apparent timing relative to later structures, is consistent with compressional deformation—potentially related to the Penokean orogen. One of the longstanding controversies in iron-ore genesis is the question of whether oxidation and leaching of ironformation to form the high-grade hematite ores occurred by supergene or hypogene processes. Although not conclusive, the observation of several early-formed, south-dipping thrust faults with folded, mineralized wall rocks, and bedding-parallel slickensides that host abundant secondary iron and silica implies that at least some of the mineralization was coincident with compressional deformation, perhaps during Penokean orogenesis. This is consistent with the hypogene model proposed by Morey (1999) that attributes oxidation and leaching to groundwater flow driven northward from uplift in the Penokean fold and thrust belt. A second regional suite of structures (D2) is largely extensional. These structures are monoclines and normal faults that are mutually transgressive; that is, faults that have sympathetically folded wall rocks, and folds that pass gradationally into faults along the trend of axial planes. These are some of the major structures along which oxidation and leaching has occurred, and the focus of most direct-shipping (hematite) ore mining. Veins, vugs, and other secondary mineralization features are abundant. D2 structures likely formed as localized responses to regional tilting. The most recent deformation effects (D3) are trough-like collapse structures, presumably related to post-leaching subsidence. The collapse, and associated oxidation and weathering, are best developed in the uppermost subcrop of iron-formation, implying supergene alteration played a significant role in their development. Thus, the answer to the supergene vs. hypogene debate appears to be that both processes were significant, perhaps at different times. Lacking finite ages, the structures can only be inferred to record components of Penokean (Geon 18), Yavapai (Geon 17), Mazatzal (Geon 16), and/or Keweenawan (Geon 11) deformation events. The overall shallowly-dipping, northeast-striking homoclinal trend of Paleoproterozoic strata along the Mesabi Iron Range is interrupted near the city of Virginia, where strata are warped around an apparent anticline-syncline pair to form the structure known locally as the Virginia horn. In this area, dips as steep as 25° occur, and strikes trend N, NE, and NW. The origin of this structure has been variously ascribed to faulting and folding associated with uplift of Archean bedrock that now cores the anticlinal portion of the Z-shaped horn structure. Intuitively, the granitoid basement rocks were too competent to accommodate ductile compression, and it is therefore unlikely that the horn formed by simple flexural folding. The Alpena fault and several others are marked by differences in the thickness of internal units across them, indicating some deformation was synchronous with deposition (i.e., growth faults). Several faults are essentially continuous along strike with those in the Archean basement. Where displacement sense or magnitude differs significantly between these faults in the two ages of bedrock, the portions affecting Paleoproterozoic strata are inferred to have been reactivated along faults of Archean parentage. The conceptual model shown below (Fig. 10) depicts an interpretation of the structural development in the 19 �horn that invokes some combination of faulting and folding, and addresses the reactivation of what were likely Archean faults reactivated during the Paleoproterozoic. The development of direct shipping (hematite-goethite) ores appears to have been localized to varying degrees by fault and fold structures within iron-formation. Regionally, natural ore bodies extend from the bedrock surface to depths as great as 120 m. These ores formed by oxidation, hydration, and subsequent Figure 10. Schematic model showing structural evolution of the Virginia horn structure (from Morey, 2003. 20 �leaching by through-flowing solutions after lithification. Bedding plane fractures, folds, and faults presumably acted as hydraulic conduits and traps for descending and/or ascending solutions that selectively altered certain lithologic units. The direction of fluid movement and the possibility of multiple episodes of alteration are currently unclear, but work to more fully understand these questions is underway (e.g., Losh and Rague, 2013; see Diagenesis, Alteration, and Fluid Flow discussion below). Zones of oxidation along structures are apparent in derivative aeromagnetic imagery (Fig. 11). Linear zones of less magnetic, presumably oxidized iron-formation typically cross the strike of iron-formation at varied angles. Where it can be verified on the ground, many of these zones are coincident with mapped faults, axial planes of minor folds, and major joint networks. Some are also coincident with mined natural ore bodies; though to be clear, most magnetic lows depicted do not represent mineable deposits of natural ore. However, they do represent local zones having variably decreased overall magnetite content. Figure 11. Derivative aeromagnetic map of the central Mesabi Range. Image was created from total field magnetic data, which was regridded from flight lines and band-pass filtered to remove broad wave-length (low frequency) anomalies and reveal contrasts in the short wave-length (near-bedrock surface) anomalies. Magnetic highs are light; lows are dark. In the north-trending limb of the Virginia horn structure, most of the linear magnetic lows that cross the strike of the overall high correspond with folds and faults that have been mapped within iron-formation (White, 1954; and field work by the authors). North-south striping is an artifact of gridding flight line data. 21 �Origin of Iron-Formation Iron-formation formed by chemical precipitation of dissolved ferrous (Fe2+) iron as a solid phase, most likely a ferric (Fe3+) bearing species. A reduced or low-oxygen atmosphere relative to modern conditions was necessary to allow accumulation of high concentrations of dissolved ferrous iron in seawater. Mineralogical and geochemical evidence indicates co-precipitation of variable amounts of Mg, Ca, Mn, P, Si, and CO2 in addition to Fe. Silica precipitation may have occurred by adsorption onto ferric iron species settling through the water column (Fischer and Knoll, 2009), by diagenetic reaction with ferric iron precipitates at the sediment-water interface, or direct precipitation on the seafloor (Maliva and others, 2005). It is likely that all of these mechanisms may have played a role. Geochemistry of Iron Deposition A number of mechanisms have been proposed to explain iron precipitation and deposition, including direct oxidation as a byproduct of oxygenic photosynthesis, anoxygenic photosynthesis utilizing iron as an electron acceptor, and abiotic photochemical oxidation. Regardless of mechanism, the reactions can be generalized as: Fe2+ + O2 + H2O → Fe(OH)3 + H+ (1), or 2+ 4Fe + 11H2O + CO2 → 4Fe(OH)3 + CH2O + 8H+ (2) In each of these models, iron oxidation is placed close to the surface within the photic zone. The photosynthetic models intimately associate iron precipitation with biological activity, and presume that iron precipitates are raining out of the water column along with organic material. Each of these models also presumes a reservoir of dissolved iron in anoxic waters lying beneath a chemocline. In the case of shallow water iron precipitation, this implies a current- or tidal-driven flux of anoxic bottom waters into shallower water environs. Fixation of Ferric Iron into Ferrous Iron species Ferric iron hydroxide (Fe(OH)3) precipitates formed at or near the top of the water column and settled to the bottom. Hydrous ferric iron oxides have not been recognized as primary minerals in iron-formation. All iron-bearing minerals in iron-formation may have been produced by diagenetic reactions at or near the sediment-water interface. Basic iron-fixing reactions include: 2Fe(OH)3 → Fe2O3 + 3H2O (Hematite) (3) Fe(OH)3 +CO2 + H+ → FeCO3 + H2O (Siderite) (4) 3Fe(OH)3 + 2SiO2(aq) + 3H+ → Fe3Si2O5(OH)4 + 4H2O (Greenalite) (5) Fe(OH)3 + 2HS- → FeS2 + 3H2O (Pyrite) (6) With the exception of hematite, all these iron-bearing minerals contain ferrous (Fe2+) iron, indicating diagenetic iron-fixation was accompanied by iron reduction. Iron reduction was driven by a combination of settling of precipitates through the chemocline into anoxic bottom water, or respiration of organic material at the sediment-water interface, or a combination of both processes. Absent carbonate, silica, or sulfur with which to react and form a stable mineral species, the transformation of insoluble ferric iron precipitate to highly soluble ferrous iron would return dissolved iron back into the water column. Formation of geologically stable iron-formation is not a function of deposition of ferric iron precipitates, rather that of fixation of ferric iron into stable, dominantly ferrous iron mineral species. It is more appropriate to speak in terms of iron-formation accumulation and accumulation rates than ironformation deposition and deposition rates. Viewed in this context, apparent decreases in “deposition” rate are actually decreases in accumulation rates, and may reflect a lack of suitable fixative at the sedimentwater interface rather than a decrease in iron precipitation rates at the top of the water column. Iron-Formation Mineralogy The iron-bearing minerals in iron-formation consist of oxides, carbonates, silicates, and sulfides. James (1954) recognized that the iron mineralogy varied systematically, and reflected distinct lithostratigraphic facies, at least in part. His iron-formation facies concept (oxide, silicate, carbonate, and sulfide facies) continues to provide a compelling framework within which to interpret iron-formation sedimentology and mineralogy. The diversity of iron minerals found in iron-formation (Table 2) is a 22 �direct reflection of the diversity of the sedimentological and geochemical environments in which the ironformation formed. The likelihood of extensive replacement of primary iron precipitates has resulted in significant controversy regarding the precise nature of the primary precipitate, and the precise reaction pathways responsible for formation of the observed mineral assemblages (Simonson, 2003). Eh and pH are major controls on the stability of the iron minerals in both the depositional and diagenetic environments (Ojakangas and others, 2005). Klein (2005) has suggested that the original precipitate materials were probably hydrous Fe-silicate gels of a greenalite-type composition; Na-, K- and Al-containing gels approximating stilpnomelane compositions; SiO2 gels; Fe(OH)2 and Fe(OH)3 precipitates; and very finegrained carbonate oozes. A variety of other primary chemical precipitates for iron-formation in general have also been postulated by an assortment of authors and include siderite, iron hydroxides, iron silicates (Konhauser and others, 2002; Rajan and others, 1996), and colloidal iron silicates (Lascelles, 2007). “Clastic” components such as Al2O3, TiO2, K2O, and Na2O were likely deposited as eolian dust, and reflect a far-travel clay mineral component eroded from exposed cratons. Nevertheless, within the ironformation these elements are typically found in the iron silicate mineral stilpnomelane, suggesting that even the clastic component participated in diagenetic reactions. Mineral Oxides Magnetite Hematite Goethite Silicates Chert Chalcedony Microcrystalline Quartz Stilpnomelane Minnesotaite Talc Greenalite Chamosite (Al-rich Fe-chlorite) Carbonates Siderite Ankerite Kutnohorite - Ferroan Dolomite Kutnohorite Calcite Sulfides Pyrite Pyrrhotite Formula Fe3O4 Fe2O3 FeO(OH) SiO2 SiO2 SiO2 K(Mg, Fe+2, Fe+3)8(Si,Al)12(O,OH)27 Fe3Si4O10(OH)2 Mg3Si4O10(OH)2 Fe3Si2O5(OH)4 Fe3(Al,Si)2O5(OH)4 FeCO3 Ca(Fe,Mg)(CO3)2 (Ca,Mn)(CO3)2 - Ca(Mn,Mg,Fe)(CO3)2 CaMg(CO3)2 CaCO3 FeS2 Fe(1-x)S Table 2. Common mineral names and formulas associated with the Biwabik Iron Formation (excluding the more highly metamorphosed eastern Mesabi Iron Range in proximity to the Duluth Complex). 23 �Oxides Hematite is the iron-bearing mineral most commonly associated with iron-formation. However, primary hematite is a relatively rare component of the BIF, occurring most prominently in oxide facies iron-formation of the UC member. Magnetite is common throughout the BIF sequence. The relatively coarse-grained idiomorphic magnetite characteristic of gifs are late diagenetic in origin (LaBerge, 1964; LaBerge and others, 1987; Zanko and others, 2003) and form by the replacement of pre-existing iron silicates and iron carbonates (French, 1973). Fine-grained magnetite in ‘slaty’ bif layers likewise formed by diagenetic reaction of iron silicates and carbonates. Earlier work on the oxidized taconites of the western Mesabi Iron Range was accomplished by Bleifuss (1964). He showed that late hematite was developed by the oxidation and pseudomorphic replacement of magnetite octahedra, that layers of goethite were precipitated from solutions likely derived from the oxidation of siderite, and that some goethite formed by the oxidation of acicular iron silicate minerals. Silicates Greenalite is considered to most closely reflect the composition of an initial ferric hydroxide/silica gel precipitate in that it exhibits no detectable replacement of any pre-existing phase (French, 1973; LaBerge and others, 1987; Simonson, 1987; Klein, 2005). Within gifs, greenalite most often occurs as roundshaped granules that are <1 mm in diameter. Stilpnomelane is a secondary mineral that commonly replaces early iron silicates (greenalite) (French, 1973). The presence of alumina and potassium suggests reaction with the detrital dust component found in the iron-formation. French (1973) suggests that stilpnomelane formed under conditions ranging from diagenesis to low-grade metamorphism. Minnesotaite is a common component of throughout the BIF sequence, and the type locality for this mineral was located in the north end of the Thunderbird North mine. Stoichiometrically analogous to magnesian talc, but structurally dissimilar, it generally occurs as sheaves or needles replacing greenalite granules (French, 1973). True talc, including ferroan talc, locally comprises a significant amount of the silicate fraction within gifs. McSwiggen and Morey (2008) show that both chamosite and talc are common throughout portions of the Biwabik Iron Formation. Carbonates Ankerite and siderite are common early diagenetic minerals. Siderite commonly occurs within laminated bifs, while ankerite commonly occurs as idiomorphic replacement of primary granules within gifs. Locally, coarse-grained poikiloblastic aggregates of ankerite (mottles) are found in gifs. These mottles are clearly late diagenetic replacements. McSwiggen and Morey (2008) report manganese substitution for iron in the dolomite-ankerite series, leading to kutnohorite and ferroan kutnohorite composition in the Lower Cherty. Both Mg and Mn substitute for Fe in siderite in the Lower Cherty, with some samples containing as much as 20 to 25 mole percent MnCO3. The average composition of siderite from the Lower Cherty [(Fe.60-.78Mg.15-.19Ca.02-.03Mn.04-22)CO3] differs considerably from that of the remainder of the iron-formation, where siderite compositions averages [(Fe.88-.84Mg.03-.11Ca.02-.03 Mn.02.07)CO3] (McSwiggen and Morey, 2008). Sulfides Sulfide minerals are ubiquitous throughout the BIF sequence. Pyrite and pyrrhotite are the most common, with minor amounts of arsenopyrite, cobaltite and galena (Theriault, 2011). Pyrite occurs ubiquitously as in trace amounts as idiomorphic cubic or dodecahedral crystals, framboids, and spheroids. Less commonly, sulfide occurs in larger blebs. The Intermediate Slate (LS-1) contains the greatest abundance of sulfide, apparently associated with elevated mercury and arsenic concentrations (Morey and Lively, 1999). 24 �Alteration and Regional Fluid Flow Common features within the Biwabik Iron Formation are quartz-carbonate±iron silicate veins that occupy vertical fractures and bedding-parallel slip planes. These veins postdate magnetite formation and diagenesis of the iron-formation, but display textures indicative of syntectonic growth, suggesting they may be related to far-field deformation, perhaps associated with the Penokean orogen. In the vicinity of direct-shipping ore deposits, these veins are commonly overprinted by: 1. complete or partial dissolution of carbonate minerals; 2. brecciation of the quartz, perhaps associated with volume loss collapse of the iron formation; and 3. recementation by secondary iron oxides and silica. Recent fieldwork in the Hibbing Taconite, Thunderbird North, and Thunderbird South/Fayal Mines, combined with petrographic, SEM, fluid inclusion, and geochemical techniques, have elucidated oxidation by deep, saline, hydrothermal-diagenetic waters at relatively low water/rock ratios (e.g., Losh and Rague 2013). Fluid inclusions in fault breccia and low-angle and high-angle veins containing secondary minerals (quartz, calcite, minnesotaite, stilpnomelane, hematite) have average homogenization temperature of 155°C ± 17°C (n=278), and salinity of 9.5 ± 5.3 wt% NaCl equivalent (n=160). Temperature correction due to pressure is on the order of 50°C. There is no significant difference in fluid inclusion homogenization temperature or salinity between fault breccias (including quartz cement) and veins of diagenetic affinity. These results agree well with oxygen isotopic temperatures of 150°–200°C for diagenesis determined by Perry and others (1973). The oxidizing fluids, a mixture of diagenetic and meteoric fluids, infiltrated along high-angle faults that contain vein quartz cemented by quartz ± iron oxides (typically goethite), and brought about oxidation of magnetite to hematite and Fe-silicates to goethite (the latter reaction also yielding silica as a reaction product), accompanied by quartz recrystallization. Silica liberated from this oxidation filled microfractures, typically only a few microns in width, and pits within altered magnetite grains. This contributes to the general observation of high silica in magnetite concentrates from oxidized ores. As silica was not dissolved but rather only remobilized during this oxidation event, ore was not significantly upgraded; in fact, the introduction of quartz into microfractures in magnetite locally diminished magnetic taconite ore quality. Quartz-filled microfractures in magnetite are also observed in unweathered ‘slaty’ iron formation near bedding-parallel faults, as in the Thunderbird North mine. Ore Deposits The Biwabik Iron Formation contains about 30% iron throughout its thickness, irrespective of lithofacies, mineralogy, and grain size. However, only a fraction of the formation hosts recoverable iron mineralization. Historically, two deposits types have proven economically feasible to mine: directshipping (natural) ores (DSO) and magnetic taconite. Direct-shipping ores are composed of hematite and goethite enriched by supergene leaching of silica from the pristine iron-formation. Early in the development of the Mesabi Range, DSO were shipped directly from the mine. In later years, gravity concentration was used to upgrade the iron content of the ores. DSOs formed from leaching of any lithofacies ranging from thin-bedded slaty band iron-formation to thick-bedded, coarse-grained granular iron-formation. Because of this, DSO quality was quite variable in terms of deleterious elements, including phosphorus, alumina, manganese, and structural water (from goethite). This necessitated an elaborate and extensive system of ore grading and blending of railcar size shipments at rail yards and ore docks at the shipping ports to maintain consistent quality blast furnace feed. Magnetic taconite ores are composed predominantly of coarser grained magnetite found in granular iron-formation. These rocks are capable of producing a high-grade magnetite concentrate after fine grinding by magnetic separation. The magnetic concentrates are agglomerated and fired into the 3/8”-1/2” taconite pellets familiar to would-be slingshot assassins throughout the Great Lakes region. The resulting pellets have a significantly higher iron grade, significantly lower deleterious element content, and superior smelting properties relative to the DSO production they have replaced. Furthermore, the product is easily transportable. 25 �Direct-shipping Ores The direct-shipping ores of the Mesabi Iron Range fall under the Soft Iron Ore category of Marsden and others (1968). They are generally porous masses of hematite, goethite, and minor magnetite and manganese oxides. Gangue minerals consist of quartz, clay, and minor carbonate. Fundamentally, soft ore formation is the product of preferential leaching of silicate and carbonate components from the ironformation, and alteration of the primary iron oxide, silicate, and carbonate minerals to secondary hematite and hydrous iron oxides (Marsden and others, 1968). The iron content of the iron-formation is increased by loss of gangue material (primarily silica and carbonate), rather than enrichment or replacement by supergene iron minerals. On the Mesabi Iron Range, soft ore bodies occur in trough, fissure, and irregular ore bodies, reflecting variable degrees of ore formation along faults, folds, or zones of fracturing. They also occur as stratiform ore bodies, reflecting ore formation along favorable horizons. Generally, soft ore bodies extend from the bedrock surface to depths of 400 or 500 feet (Marsden and others, 1968). Ore formation was evidently a multi-stage process. Early desilicification of the iron-formation was accompanied by alteration of primary magnetite to hematite, and alteration of primary iron silicates and iron carbonates to goethite. Much direct-shipping ore exhibits textures indicative of a second stage of enrichment. Secondary porosity induced during desilicification is commonly filled by paragenetically late iron hydroxides and hydrous iron oxides. Dripstone textures indicate that at least some of these secondary iron hydroxides were precipitated in the vadose zone. Leith (1903) noted that hydrous minerals were more abundant in the shallower portions of the deposits, suggesting the presence of a supergene enrichment zone, perhaps coincident with a paleo-water table. Ore formation and desilicification were accompanied by mass loss (as much as 50% by weight) and, to a variable extent, volume loss. Unaltered iron-formation has a specific gravity of 3.3-3.4; Leith (1903) reported typical direct-shipping ore specific gravities in the range of 2.6-3.1, with some ores as low as 2.0-2.1. Mass loss was typically accompanied by structural collapse and formation of a synclinal structure in the ore body (D 3 deformation; see discussion above in Structure of the Biwabik Iron Formation). Commonly, the ore retained bedding and geochemical traits inherited from the precursor iron-formation, with the steepest dips adjacent to the margins of the deposits. The nature and timing of ore formation has been the subject of much debate. The clear association of many deposits with fault and fracture zones, as well as the sharp wall contacts, has been cited as evidence in favor of a hydrothermal origin (Morey, 1999). In contrast, the clear association of the stratiform bodies with the paleosurface argues strongly in favor of a supergene origin. The complex paragenesis of the ores suggests that multiple events may ultimately have been responsible for development of the ores. Oxidation of iron-formation in the Mesabi Range has long been thought to have been solely the result of near-surface interaction with meteoric water, most intensely during saprolite formation during the Cretaceous (Leith, 1903; Sloan, 1964; Marsden and others, 1968), or during other time periods with a tropical climate. Gruner (1956) and Morey (1999) proposed that at least some if not all of the intense oxidation was of hydrothermal origin but did not characterize the effects, nature, or ultimate source of the fluids responsible for this alteration. Hydrothermal oxidation, accompanied by dissolution of non-oxide minerals, has been implicated in the upgrading of iron ores in Australia (Thorne and others, 2008) and Brazil (Rosiere and others, 2008). On the Mesabi Iron Range, oxidized magnetic taconite ore has been locally characterized by high Davis Tube concentrate silica values, particularly adjacent to faults. Hydrothermal oxidation may have taken place during the Paleoproterozoic, when the currently-exposed Biwabik Iron Formation was the most deeply buried and was undergoing diagenesis. Later lateritic weathering, perhaps during the Cretaceous, dissolved silica and all other non-iron oxides, resulting in the natural ores as found in the Fayal Mine. Geochemically, these ores are characterized by pronounced cerium anomalies, which can result from intense oxidation near the surface, consistent with a lateritic interpretation for these ores. The older, fault-related hydrothermal oxidation did not produce cerium anomalies, consistent with its deep-seated setting. 26 �Magnetic Taconite Ores The taconite reserves of the Mesabi Range are comprised of magnetite-rich horizons in the Biwabik Iron Formation. Although the iron content of the Biwabik Iron Formation is relatively uniform, the proportion contained in magnetically recoverable magnetite is highly variable, ranging from less than 10% (typically in slaty banded iron-formation) to greater than 25% (typically in cherty granular ironformation). Mineable horizons exist throughout the entire iron-formation thickness in the central Mesabi district. Principal ore units include the middle 100’ of the Lower Cherty (UTAC LC-4, LC-5), and in variable-thickness Upper Cherty submembers (e.g. LUC-2, UC-3 & 3A). Ores are classified on their concentrate weight recovery (typically >25%), crude magnetic iron (~1725%), and concentrate Fe and SiO 2 grades (product averages 66% Fe, 4.5-5% SiO2). Certain cherty ores (LC-4) can produce concentrate silica grades as low as 2% with standard grinding and separating techniques (75-80% -325 mesh, or P80 45 µm), with most cherty ores averaging ~5-6% concentrate SiO2. Slaty magnetite taconite ores produce concentrates of higher concentrate SiO 2, reflecting finer magnetite grain size and textural intergrowth with gangue minerals. Minor contaminants (CaO, MgO, MnO from carbonates, K2O and Al2O3 from silicates) are related to very specific stratigraphic horizons, allowing accurate mine-to-mill blending reconciliations. Three-position blending and maximizing the use of a single high-silica ore source are required for stable processing operations. The magnetite in magnetite taconite ores formed as a result of low-temperature diagenetic recrystallization, likely from reaction of oxide and/or carbonate precursors: Fe2O3 + FeCO3 → Fe3O4 + CO2 (7) (Burt, 1972) Textural relationships also suggest formation directly from a carbonate or silicate precursor: 3FeCO3 + 3H+ → Fe3O4 + CO2+ 3 H2O (8) Fe3Si2O5(OH)4 → Fe3O4 + 2SiO2 + 3H2O + H+ (9) Magnetite-rich iron-formation is typically enriched in iron relative to non-magnetite rich iron-formation (Fig. 12), suggesting reactions with dissolved ferrous iron may play a role in magnetite formation: Fe2O3 + Fe2+ + H2O → Fe3O4 + 2H+ (10) (Ohmoto, 2003) 2Fe(OH)3 + Fe2+ → Fe3O4 + 2H2O + H+ (11) 2FeCO3 + Fe2+ → Fe3O4 + CO2 (12) Reaction of precursor ferric oxide with organic material has also been proposed as a mechanism: 3Fe2O3 + CH2O → 2Fe3O4 + CO2 + 2H+ (13) (modified from Perry and others, 1973) Overall, magnetite formation shows a clear affinity for horizons with sedimentological, mineralogical, and geochemical evidence for a significant carbonate component in the primary precipitate. The association of carbonate with subsequent magnetite formation suggests that a buffered pH was as significant a control as Eh. Textural relationships indicate that magnetite formation occurred after the onset of burial stylolitization and significant chemical compaction. However, it was apparently complete prior to post-Penokean deformation and fluid flow, as it is cross cut by hydrothermal quartzcarbonate veins associated with this event. Figure 12. Total iron versus percent of iron in magnetic fraction of unweathered, unmetamorphosed Biwabik Iron Formation. Note that the highest total iron contents (>35%) are associated with the highest magnetic iron fraction; this suggests magnetite formation was accompanied by iron enrichment. 27 �Production Annual production of direct-shipped ore and taconite pellets produced on the Mesabi Iron Range are shown in Figure 13. Production of direct-shipping ore started in 1892 and rose steadily until 1942, when a record 54 million tons were produced. Gravity concentrate production rose steadily thereafter, until a record 77 million tons of direct-shipping and gravity concentrate ore was produced in 1953. Reserve Mining Company initiated the first large scale taconite operation in 1955, and by 1967 taconite production from six taconite facilities accounted for more than half of iron ore production. The Mesabi Range iron ore industry weathered the global resource recession of the 1980s largely intact, accounting for over 75% of US iron ore production by the end of the decade. The industry continues to evolve, with six taconite facilities (40 mtpy capacity), three tailings recovery facilities (3 mtpy capacity), and a valueadded direct reduced iron facility (0.5 mtpy capacity) in production. Figure 13. Annual production of direct-shipping ore, gravity concentrates, and taconite concentrates from the Mesabi Iron Range for the period 1892-2012. 28 �DESCRIPTION OF FIELD STOPS STOP 1—Stratigraphic section of the Biwabik Iron Formation, Thunderbird North Mine 535000E/5257910N (UTM Zone 15 coordinates, NAD83 datum) Eveleth 7.5’ USGS Quadrangle; SWSW, Section 29, T58N, R17W *NOTE: THIS SITE IS LOCATED ON AN ACTIVE MINE SITE. DO NOT ATTEMPT TO ENTER WITHOUT FIRST OBTAINING PERMISSION. Directions: Beginning in Hibbing, proceed east on US Highway 169 to the interchange with Highway 53 in Virginia (22 miles). Turn south (right) to merge onto US Highway 53, and drive 4.1 miles to the stoplight intersection with Grant Avenue in Eveleth. Turn west (right) onto Grant Avenue, and drive south 0.5 miles to the Cliffs Natural Resources Thunderbird Mine entrance. Historical Overview: Material mined at the Thunderbird North Mine consists of taconite ore horizons from the Lower Cherty, Lower Slaty, and Upper Cherty members. Direct-shipping ore, also referred to a natural ore, was originally mined in the immediate vicinity from the Auburn (1894-2002), Virginia (1910-1953), and Gross-Nelson (1944-1977) deposits. Exploration for magnetic taconite at this site began in earnest in 1960, after the opening of pioneering taconite operations at the Reserve Mining Company (now Northshore Mining) and Erie Mining Company (now Cliffs Erie site) in the mid-1950s. Drilling by Oglebay Norton Company identified a substantial magnetic taconite deposit in the area and the property was jointly developed with the Ford Motor Company – groundbreaking occurred in June, 1964. The Thunderbird North mine and Fairlane plant began producing in November, 1965, with an initial rated capacity of 1.6 million tons of iron ore pellets per year. In 1977, addition of a second concentrating and pelletizing line, and the opening of the adjacent Thunderbird South mine, increased rated capacity to 6.0 million tons of pellets. The Thunderbird South mine closed in 1992, and in 1996, ownership of the operation was transferred to Eveleth Mines LLC. Eveleth Mines closed the concentrating and pelletizing Line 1 in May, 1999, reducing capacity to 4.2 million tons of pellets. The remaining operation was idled in May, 2003. The idled facility was purchased and re-opened by United Taconite LLC in December, 2003 (now owned 100% by Cliffs Natural Resources). They subsequently refurbished and reactivated Line 1 in December, 2004, which increased the annual rated capacity to 5.2 million tons of pellets. Description: Depending on access, one or more sites with mine exposures of the Lower Cherty, Lower Slaty, and Upper Cherty members will be visited. Refer to the detailed stratigraphy section for more information regarding the submembers visited. STOP 2—Fault and Associated Quartz Veining and Alteration/Oxidation, Thunderbird South Mine 534100E/5255520N (UTM Zone 15 coordinates, NAD83 datum) Eveleth 7.5’ USGS Quadrangle SWSW, Section 5, T57N, R17W *NOTE: THIS SITE IS LOCATED ON AN ACTIVE MINE SITE. DO NOT ATTEMPT TO ENTER WITHOUT FIRST OBTAINING PERMISSION. Directions: Proceed southward on company roads through the Thunderbird North Mine. Cross County Highway 101 (through two remote operated gates on either side of the highway) and continue southeast to the north side of Thunderbird South. 29 �Description: The site contains multiple exposures of Lower Slaty units on several benches, with the conspicuous fault/quartz vein area trending SW into the flooded pit. The complete Lower Cherty section remains as reserve in Thunderbird South The exposure in the Thunderbird South pit contains a N45E-trending high-angle fault in the LS2/LUC1 units. The fault is approximately 30 cm wide, and contains quartz veins that have been brecciated and cemented by very fine-grained quartz intergrown with goethite. Fluid inclusions in the quartz cement, which is inferred to have precipitated during the hydrothermal oxidation event (hence its intergrowth with goethite), have average homogenization temperatures of 155° C (n=22) and salinity of 7.3 wt% NaCl equivalent, clearly indicating the involvement of saline hydrothermal fluids in goethiteforming oxidation, and furthermore implicating more widely-distributed diagenetic fluid in that alteration. Breccia clasts of quartz vein from this fault zone have essentially the same homogenization temperatures and salinities as the quartz cement. In terms of trace element geochemistry, the fault breccia is remarkably similar to iron-formation, implying it was largely buffered by iron -formation in a rockdominated system. Notably, the fault breccia displays a distinctive positive Europium anomaly, as does the iron-formation (see also Planavsky and others, 2009). Similar oxidized high-angle faults are known throughout the Iron Range. Adjacent to these faults, iron-formation is oxidized, with iron silicates altered to goethite + quartz, chert textures are overprinted by recrystallized quartz, and magnetite is oxidized to martite. The hydrothermal oxidation is commonly overprinted by late red hematite (+/- goethite) that formed near Earth’s surface: it coats fractures and is associated with silica dissolution. STOP 3—Security Reserve/Fayal Complex Direct-shipping Ore 535070E/5255050N (UTM Zone 15 coordinates, NAD83 datum) Eveleth 7.5’ USGS Quadrangle NWSE, Section 6, T57N, R17W *NOTE: THIS SITE IS LOCATED ON AN ACTIVE MINE SITE. DO NOT ATTEMPT TO ENTER WITHOUT FIRST OBTAINING PERMISSION. Directions: Drive around the western and southern sides of the Thunderbird South pit, past the crusher, and east to ramp into the Fayal Pit. The site is an approximately 200 meters (650 feet) long exposure of ironformation and direct-shipping ore exposed along a northeast-trending access ramp into the flooded Fayal Mine complex. Historical Background: The Fayal Mine (1895-1965; total production 44.5 million tons) was discovered in November, 1893 by David T Adams of Duluth. The mine site was initially explored by the McInnis Mining Company and was sold to the Minnesota Iron Company (a component of the 1901 United States Steel merger), after which the mine was operated by the Oliver Iron Mining Company. Production of direct-shipping ore began in 1895 and was initially extracted by shaft from underground operations. Open pit operations facilitated a rapid increase in production, reaching1.9 million tons in 1902. Through the end of 1919, the complex had yielded an aggregate of 29.9 million tons – more than a million tons per year since 1895 (and two-thirds the ultimate production). The Fayal complex was closed in 1933, but was reopened on a smaller scale, as an open-pit truck operation, to recover lower grades of ore between 1944 and 1965. Final development plans included recovering approximately 794,000 tons of Lower Slaty- and lower Upper Cherty-hosted ores along the south side of the deposit (Security Reserve). However, by the time final mining was contemplated by Auburn Minerals LLC (ca. 2000), the sulfur content of the reserve was deemed unacceptably high. 30 �Description: Included in the Security Reserve is an access ramp to the flooded Fayal Mine. Along this ramp, direct-shipping ore is exposed in both the floor and wall of the ramp. This site is one of the few remaining locations on the Mesabi Iron Range to view in situ direct-shipping ore. All direct-shipping ore in the Fayal deposit falls under the Soft Iron Ore Classification of Marsden (1968). The Fayal ore consists predominantly of hematite and goethite, with minor magnetite and manganese oxides, as is common with the other soft ore deposits of the Mesabi Iron Range. Silica and clay minerals are the predominant gangue minerals. In 1901, Fayal direct-shipping ore was reported to averaged 63.8% iron, 0.037% phosphorus, and 2.95% silica (dry basis; Leith, 1903). The direct-shipping ore visible in the Fayal ramp occurs along the margin of the deposit and likely has lower iron and higher silica content than the typical higher grade ore shipped from the Fayal deposit for most of its life. Iron-formation exposed along the east side of the Fayal ramp parallels the contact of the directshipping ore deposit. The ore is formed from predominantly slaty proto-ore, and displays varying degrees of desilicification (leaching) and oxidation. Bedding in direct-shipping ore on the north end of the ramp clearly displays a relatively steep dip to the west and lies near the center of the trough. The west side of the ramp parallels the northeast-trending Fayal fault, a high-angle, west-dipping normal fault, and one of the larger structures cross-cutting the Biwabik Iron Formation. The fault is occupied by a thick, brecciated, and re-cemented quartz ± carbonate vein. Visible immediately adjacent to the large vein is drag folding in the footwall iron-formation, indicating a hangingwall- (westside-) down sense of motion. STOP 4—Drill Core Display 535000E, 5257910N (UTM Zone 15 coordinates, NAD83 datum) Eveleth 7.5’ USGS Quadrangle SWSW, Section 29, T58N, R17W *NOTE: THIS SITE IS LOCATED ON AN ACTIVE MINE SITE. DO NOT ATTEMPT TO ENTER WITHOUT FIRST OBTAINING PERMISSION. Directions: Proceed back north along the same route to the core shack within the Thunderbird North Mine. Description: A drill hole cored through most of the Biwabik Iron Formation, and a portion of the upper Pokegama Formation, will be on display inside the core shack or, weather permitting, will be laid outside. Ironformation submembers will be labeled according to the Thunderbird North classification scheme. STOP 5—Algal/Conglomerate unit of the Upper Cherty member, Mary Ellen Mine 548260E, 5264380N (UTM Zone 15 coordinates, NAD83 datum) Biwabik 7.5’ USGS Quadrangle NENW, Section 10, T58N, R16W *NOTE: THIS SITE IS LOCATED ON AN ACTIVE MINE SITE. DO NOT ATTEMPT TO ENTER WITHOUT FIRST OBTAINING PERMISSION. Directions: From the Thunderbird Mine entrance, turn left (north) on Grant Avenue. Proceed to the .intersection with Highway 53 (0.5 miles), and turn left (north). Proceed 1.5 miles to the intersection with Highway 135, and turn right (east) to merge onto MN Highway 135. Drive east 10 miles to the intersection with County Road 715, located just outside the western outskirts of Biwabik. Turn left (north) on 715, and proceed 0.2 miles. The entrance to the Mary Ellen Mine will be on the south (left) side of the road. 31 �Historical Background: The Mary Ellen Mine was first opened in 1924 by Pioneer Mining (Stanley Mining, operator), and saw regular production of what was termed ‘hard, bluish-red siliceous hematite’ through 1928. Stanley Mining reopened the Mary Ellen in 1948, and it experienced sporadic production through to final topography in 1962 (last operated by Pittsburgh-Pacific). Total shipments in the period 1924-1962 were 4.6 million tons of gravity concentrates. Description: The Mary Ellen mine is perhaps most notable for its exposures of the algal submember of the Upper Cherty (equivalent to the UC-6 submember at the Thunderbird Mine, and the I submember of Gundersen and Schwartz (1962)). Here, stromatolites occur as mounds of fossilized algal colonies separated by intraformational jasper conglomerates. The algal and conglomeratic units exhibit a combined thickness of 2-20 feet. This horizon occurs only in the eastern half of the Mesabi Iron Range, pinching out in the vicinity of Hibbing. Planavsky and others (2009) attribute the stromatolites to Fe-oxidizing bacteria present in the Animikie Basin and in similar settings world-wide, where microbial communities proliferated at specific shallow-water redox boundaries in late Paleoproterozoic oceans (see Fig. 14, below). The Mary Ellen mine is noted for the abundance of colonies of finely-laminated, small (~1cm diameter) digitate, columnar stromatolites. They occur in mound-like aggregations that appear to have been buried in-situ on the seafloor, in contrast to the largely resedimented oncoliths comprising the algal chert submember farther to the west. Discussion: The presence of stromatolites and intraformational conglomerate at this stratigraphic horizon within the iron-formation is consistent with extremely shallow water, and perhaps even emergent (subaerial) conditions. This likely represents maximum marine regression during the transgressive-regressivetransgressive cycle that characterizes deposition of the Biwabik Iron Formation. The carbonate rocks that comprise the uppermost Upper Slaty member of the iron-formation (submember US-2), though enigmatic, may relate to development of a second regression. Figure 14. Model of stromatolite depositional environment (Planavsky and others, 2009). 32 �REFERENCES Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Davis, D.W., Kissin, S.A., Fralick, P.W., and Hammond, A.L., 2005, Discovery of distal ejecta from the 1840 Ma Sudbury impact event: Geology 33, p. 193-196. 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Vallini, D.A., Cannon, W.F., Schulz, K.J., and McNaughton, N.J., 2007, Thermal history of low metamorphic grade Paleoproterozoic sedimentary rocks of the Penokean orogeny, Lake Superior region: Evidence for a 1786 Ma overprint: Precambrian Research, v. 157, p. 169-187. White, D.A., 1954, Stratigraphy and structure of the Mesabi range, Minnesota: unpublished PhD Dissertation, University of Minnesota, 146 p., 4 plates. Wolff, J.F., 1917, Recent geologic developments on the Mesabi Iron Range, Minnesota: American Institute of Mining and Metallurgical Engineers, Transactions, v. 56, p. 229-257. Zanko, L.M., Severson, M.J., Oreskovich, J.A., Heine, J.H., Hauck, S.A., and Ojakangas, R.W., 2003, Oxidized taconite geological resources for a portion of the western Mesabi Range (west half of the Arcturus Mine to the east half of the Canisteo Mine), Itasca County, Minnesota—a GIS-based resource analysis for land-use planning: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report NRRI/TR-2001/40, 85 p. Zielinski, A.M., Mancuso, J.J., Frizado, J.P., and Waidler, R.J., 1994, Manganese-rich oncolites in the Biwabik Iron Formation, Eveleth Mine, Mesabi Iron Range, Minnesota: in D.L. Southwick, ed., Short Contributions to the Geology of Minnesota 1994, Minnesota Geological Survey Report of Investigations 43, pp. 48-59. 36 �FIELD TRIP 2 Wednesday, May 14, 2014 A WALK IN THE PARK—NEOARCHEAN GEOLOGY OF LAKE VERMILION STATE PARK LEADERS: George J. Hudak (Natural Resources Research Institute – Duluth) Amy Radakovich (Minnesota Geological Survey) Geoff Pignotta and Kelly Schwierske (Geology Dept., University of Wisconsin - Eau Claire) INTRODUCTION The Vermilion District of northeastern Minnesota contains one of the classic greenstone belts in the United States. The district comprises the southwestern part of the Wawa Abitibi Terrane (Stott et al., 2007; Stott and Mueller, 2009) which encompasses Neoarchean metavolcanic, metasedimentary, and meta-intrusive rocks that extend northeastward through northwestern Ontario and Quebec. In Canada, this terrane hosts numerous volcanogenic massive sulfide deposits (e.g. Winston Lake, Geco, Noranda), goldrich volcanogenic massive sulfide deposits (Horne (Noranda camp), Bousquet 2 – LaRonde 1, LaRondePenna; Mercier-Langevin et al., 2010), as well as a large number of lode (orogenic) gold deposits (for example, in the Hemlo, Timmins, and Kirkland Lake camps). The Vermilion District is known for its numerous, previously mined massive hematitic iron ore deposits (including the Pioneer Mine in Ely and the Soudan Mine in Soudan) which locally occur within regional extensive Algoma-type banded iron formations. To date, no volcanogenic massive sulfide, gold-rich volcanogenic massive sulfide, or lode gold deposits have been discovered in the Vermilion District, although several studies (Peterson and Jirsa, 1999; Peterson, 2001; Hudak et al., 2002a; Peterson and Patelke, 2003; Hoffman, 2007; Hudak et al., 2007; Hudak et al., 2012; Lodge et al., 2013) have indicated that evidence for volcanic, hydrothermal, and structural processes associated with these types of mineral deposits is present throughout the Vermilion District. Since the late 1990’s considerable geological research has been conducted in the region between Tower, MN (in the west) to Ely, MN (in the east) within the Vermilion District. Much of this research has been conducted to better understand the stratigraphy, structural geology, and economic geology of the belt, and the results of these studies have provided a solid foundation for the geological research that has, and currently is, taking place in Lake Vermilion State Park. Several recent Institute on Lake Superior Geology (ILSG) field trips (Hudak et al., 2004; Jirsa et al., 2004; Peterson and Patelke, 2004; Larson and Mooers, 2009; Peterson et al., 2009a; Jirsa and Hillman, 2009; Peterson et al., 2009b) describe these finding for specific areas in and around the Vermilion District. The Vermilion District’s iron ore mining heritage is currently preserved at two state parks located near Soudan, Minnesota. With the donation of land and infrastructure associated with the former Oliver Iron Mining Division’s Soudan Mine by United States Steel to the State of Minnesota in 1965, Soudan Underground Mine State Park was established. This state park currently preserves the historical surface and underground workings from, as well as the wilderness adjacent to, Minnesota’s oldest iron ore mine, the Soudan Mine. This mine operated from 1882 until December, 1962 and produced approximately 15.5 tons of hematic iron ore. This popular tourist site continues to be the focus of a wide variety of research spanning geology, geochemistry, hydrogeology, biology, biochemistry and physics. Lake Vermilion State Park is Minnesota’s newest state park, and comprises over 3,000 acres of land, including over five miles of undeveloped shoreline on Lake Vermilion (Bakst, 2013). In 2008, Minnesota State Legislature set aside $20 million in bonding authority to buy, plan, and develop the park, which is located immediately east of Soudan Underground Mine State Park. The park was established in June 2010 after land was 37 �purchased from U. S. Steel Corporation. At the present time, the park is undergoing considerable development, including establishment of trails, roads, and campsites. The park boasts a rich natural and human history, including a wide variety of ~2.7 billion year old rocks that were formed by a wide variety of genetic process, abundant wildlife, as well as archaeological evidence for human habitation dating back over 6,000 years. Additionally, considerable evidence for recent (within the past 140 years) mineral exploration efforts can be readily identified in the park. In 2010 and 2011, students and faculty from the Precambrian Research Center at the University of Minnesota Duluth had the opportunity to conduct detailed (1:5000 scale) geological mapping in both Soudan Underground Mine State Park (Vallowe et al., 2010) and Lake Vermilion State Park (Radakovich et al., 2010; Heim et al., 2011). Twelve students (Nick Heim, Robert Kilduff, Chris Mahr, Charlie Parent, Molly Partridge, Rita Pierce, Amy Radakovich Andrew Ritts, Christine Rahtz, Heather Scott, Andrew Vial, Spencer Young) and instructor George Hudak performed geological mapping in the northwestern (2010) and northeastern (2011) parts of Lake Vermilion State Park. Recently, Geoff Pignotta and Kelly Schwierske of the Geology Department at the University of Wisconsin Eau Claire compiled these geological maps and conducted lithogeochemical evaluations of several lithological units in the park (Schwierske et al., in press). This trip builds on these findings, and will be the first formal geology field trip in Lake Vermilion State Park. It will include a walk up-section through the stratigraphic sequence exposed along a single two-track trail that traverses the park. As well, two outcrops outside the state park boundary will be investigated, as they comprise classic outcrops that will add context to the geological story developed through observing rocks in the park. REGIONAL GEOLOGIC SETTING Supracrustal rocks in the Vermilion district consist of volcanic-dominated stratigraphic sequences of the Wawa Abitibi Terrane within the Superior Province of the Canadian Shield. Rocks of the Wawa Abitibi Terrane in northern Minnesota are divided on the basis of stratigraphic and structural setting into: (1) the Soudan belt, to the south, and (2) the Newton belt, to the north (Jirsa et al., 1992; Southwick et al., 1998). The boundary between these contrasting structural panels can be traced geophysically across the width of Minnesota, and was informally designated the Leech Lake structural discontinuity (Jirsa et al., 1992). In the region west and north of the Lake Vermilion State Park, the Leech Lake structural discontinuity occurs along the Mud Creek shear zone (Hudleston et al., 1988), small segments of the Vermilion and Wolf Lake faults (Sims and Southwick, 1985), and the Bear River fault (Jirsa et al., 1992). A simplified regional geological map of the Neo-Archean terranes of northeastern Minnesota and adjacent Ontario is presented in Figure 1. The Soudan belt (Figure 2) contains large, broad folds involving calc-alkalic and tholeiitic volcanic strata overlain by, and locally interdigitated with, turbiditic rocks. In contrast, the Newton belt consists of elongate, northeast-trending, and mostly northward-younging volcanic and volcaniclastic sequences. Volcanic rocks of the Newton belt differ from those of the Soudan belt in containing locally abundant komatiitic flows and peridotitic sills. The two belts are fault- bounded, and the relationships between stratigraphic units within each belt are largely conformable (although faults obscure contacts locally). In its eastern extension, the Soudan belt is continuous with the Saganagons assemblage in Ontario and terminates against the Saganaga pluton and Northern Light Gneiss. The Newton belt extends discontinuously eastward into the Shebandowan District of Ontario to form the Greenwater and Burchell assemblages. Intrusive rocks in both belts vary from gabbroic and felsic porphyries demonstrably related to volcanism, to large plutons emplaced post-tectonically. Both districts contain unconformable, Timiskaming-type sequences composed of calc-alkalic volcanic rocks, conglomerates, and finer grained sedimentary rocks. Lithostratigraphic units in the western Vermilion district (Table 1) include: (1) the Lower member, Soudan Iron-Formation member, and Upper member (Upper Ely) of the Ely Greenstone Formation, the Lake Vermilion Formation (including the informally named Britt and Gafvert Lake sequences), and the Knife Lake Group of the Soudan belt; (2) the Bass Lake sequence (Peterson and Jirsa, 1999) and the 38 �Newton Lake Formation of the Newton belt; and, (3) syn- to post-tectonic granitoid intrusions of the Giants Range batholith, and a suite of post-tectonic alkalic stocks and plutons. Contacts between the different units are typically conformable, although considerable overlap in time and space is documented between volcanic and sedimentary sequences (Southwick, 1993). Geochronological information for supracrustal and intrusive lithologies in the Vermilion District is relatively sparse (Figure 2). Peterson et al. (2001) obtained a U-Pb zircon age date of 2722 ± 0.9 Ma from a quartz-phyric rhyolite dome in the Fivemile Lake Sequence of the Lower Member of the Ely Greenstone Formation. Lodge et al. (2013) obtained a U-Pb zircon date of 2689.7 ± 0.8 Ma for a Gafvert Lake Sequence dacitic tuff breccia that occurs approximately 2m north of the contact with the Soudan Iron-Formation member of the Ely Greenstone Formation. As well, Lodge et al. (2013) obtained detrital zircon dates ranging from 2680-2690 Ma from greywackes that comprise the Lake Vermilion formation. This date confirms the source of the detritus in the Lake Vermilion Formation was derived locally from the volcaniclastic rocks comprising the Gafvert Lake Sequence. Jirsa et al. (2012) obtained a U-Pb age of 2690.7 ± 0.6 Ma for synvolcanic intrusions that cross-cut volcaniclastic rocks that comprise the Knife Lake Group. The upper part of the Knife Lake Group includes conglomerates which contain clasts derived from the Saganaga Tonalite, which has been dated by Driese et al. (2011) at 2690.83 ± 0.26 Ma. Peterson et al. (2001) also dated a non-foliated feldspar porphyry intruded into Newton Belt strata at 2683.1 +1/-4 Ma. This date provides a minimum age for the regional D2 deformation event that is described below. Wawa-Abitibi Terrane Figure 1. Simplified correlation map of Neoarchean assemblages in Minnesota and northwestern Ontario (after Peterson et al., 2001). Inset map illustrates location of the Wawa-Abitibi Terrane in Minnesota and northwestern Ontario (Stott et al., 2007). The Leach Lake structural discontinuity is illustrated in red. The red star symbols indicate location of Lake Vermilion State Park. 39 �Figure 2. Generalized geology of the Vermilion District in the vicinity of the Tower-Soudan anticline (modified after Peterson, 2001). Locations, ages, and sources of U-Pb ages dates within the district are noted in the callout boxes. Generalized lithologies for each of the groups, formations or sequences are also noted. 40 �Intrusive Rocks Late Intrusions Vermilion Granitic Complex Giants Range Batholith Supracrustal Rocks Newton Belt Newton Lake Formation Bass Lake Sequence Soudan Belt Knife Lake Group Lake Vermilion Formation Gafvert Lake Sequence Britt Sequence Upper Member – Ely Greenstone Soudan Member – Ely Greenstone Lower Member – Ely Greenstone Central Basalt Sequence Fivemile Lake Sequence Plutons and stocks of syenite, monzonite, diorite, and lamprophyre. A U-Pb zircon age date of a non-foliated feldspar porphyry intrusion in the Newton belt is 2683 ± 1.4 Ma (Peterson et al., 2001). Granite, schist, amphibolite, and schist-rich migmatite Granite, granodiorite, monzodiorite, and schist-rich migmatite Tholeiitic and komatiitic basalt lava flows, intrusions, and clastic strata Tholeiitic basalt lava flows, iron-formation, and felsic porphyries Graywacke, slate, conglomerate, and sheared equivalents Graywacke, slate, dacitic tuff, minor conglomerate. Detrital zircons from planar bedded, normal-graded resedimented volcaniclastic rocks have U-Pb age dates of 2680-2690 Ma (Lodge et al., 2013) Dacitic to rhyodacitic tuff, lapilli-tuff, tuff-breccia, and ironformation. Basal dacite tuff-breccia deposits in Lake Vermilion State Park have U-Pb age date of 2689.7 ± 0.8 Ma (Lodge et al., 2013) Tholeiitic basalt lava flows Tholeiitic basalt lava flows and iron-formation Oxide-facies iron formation with intercollated basalt lava flows and felsic volcaniclastic rocks Calc-alkaline and tholeiitic basalt-rhyolite lava flows, tuffs, epiclastic rocks, and minor iron-formation Calc-alkaline to tholeiitic sparsely amygdaloidal basalt and minor basaltic andesite lava flows with MORB-like or back arc basin-like chemical affinities within 100-200 meters of the overlying Soudan Member iron-formation; FII- and FIIIa-type felsic volcanic and volcaniclastic rocks Calc-alkaline to transitional moderately to highly vesicular basalt and andesite lava flows and volcaniclastic rocks with arc-like chemical affinities: FI-, FII-, and FIV-type felsic volcanic and volcaniclastic rocks. Rhyolite dome at near Fivemile Lake has U-Pb age date of 2722.6 ± 0.9 Ma (Peterson et al., 2001). Epithermal-like zinc stringer mineralization is present near Fivemile Lake (Hudak et al., 2002a). Table 1. Lithostratigraphic units within the western Vermilion District (modified after Peterson and Jirsa, 1999; Peterson et al., 2009; Hudak et al., 2012). 41 �STRUCTURAL GEOLOGY The structural geology of the Vermilion District has been well described by Peterson et al. (2009), and is reproduced below. Periods of generally N-S directed compression resulted in three major regional deformation events in the Neoarchean terranes of northern Minnesota. The earliest deformation event (D1) produced broad, locally recumbent folds within the Soudan belt and major fault zones throughout the region. In the Newton belt, D1 was accommodated by thrust imbrication of large crustal blocks, resulting in mainly northward stratigraphic facing. Field relationships indicate that uplift, faulting, and the deposition of Timiskaming-type clastic sedimentary sequences in local fault-bounded basins occurred late in D1 deformation (Jirsa, 2000). A large, map-scale structure related to D1 deformation in the western Vermilion District is the Tower-Soudan Anticline, which is a west-plunging anticline within which the axis and plunge changes orientation along strike from nearly vertical in basalts to shallow NE plunging in the western sedimentary rocks. Axial-planar cleavage associated with this early fold typically is lacking, although Bauer (1985), Hooper and Ojakangas (1971), Hudleston (1976), and Jirsa et al. (1992) have described early cleavage (S1) locally. A second deformation event (D2) associated with synchronous regional metamorphism resulted in foliation development and structures having largely dextral asymmetry. D2 is constrained in the Vermilion District to the time period 2674 to 2685 Ma (Boerboom and Zartman, 1993), and between about 2680 and 2685 Ma in the Shebandowan (Corfu and Stott, 1998). Because D2 deformation affected all of the supracrustal rocks in the area and is reasonably constrained by geochronology, the regional foliation (S2) can be used in the field to temporally relate other structural, intrusive, and deformation events. The relationship between S2 fabric and shear structures indicates that most shearing occurred relatively late in the D2 event. Major shearing that produced the Mud Creek and related shear zones is attributed to the late stages of D2 dextral transpression. Structures related to the third deformation event (D3), which led to juxtaposition of the Wawa Abitibi and Quetico terranes (Peterson and Patelke, 2003), include abundant NE- and NW-trending faults that dissect the stratigraphic assemblages. Named structures related to D3 include the NE-trending Waasa and Camp Rivard faults east of the Soudan Mine area, and the WNW-trending, crustal-scale Vermilion and related faults that form the Wawa-Quetico Subprovince boundary. 42 �Figure 3. Geologic map of Lake Vermilion State Park (after Peterson and Patelke, 2003; Radakovich et al., 2010; Heim et al., 2011; Schwierske et al., in press). 43 �GEOLOGY OF LAKE VERMILION STATE PARK Lake Vermilion State Park contains a variety of supracrustal and intrusive lithological units. Supracrustal rocks that can be observed in the park (Figure 3) include the Lower Member of the Ely Greenstone Formation (both the Fivemile Lake and Central Basalt Sequences), the Soudan Member of the Ely Greenstone Formation, and the Gafvert Lake Sequence of the Lake Vermilion Formation. As well, a wide variety of syn- and post-volcanic intrusive rocks crop out within the park, including diabase, gabbro, diorite, granodiorite, various types of quartz-feldspar porphyries, feldspar-porphyries, and lamprophyre (Peterson and Patelke, 2003; Radakovich et al., 2010; Heim et al., 2011; Schwierske et al., in press). Two northwest-trending faults (which based on detailed mapping (Peterson and Patelke, 2003; Radakovich et al., 2010; Heim et al., 2011) possess higher concentrations of synvolcanic hydrothermal alteration mineral assemblages proximal to the structures) appear to be reactivated synvolcanic structures that offset stratigraphic units in the central and northwestern part of the park. As well, Peterson and Patelke (2003) have identified four major, more-or-less east-west trending shear zones that displace stratigraphy in the southern one-third of the park. The northernmost two shear zones represent the northern and southern limits of the Mine Trend Shear Zone, which extends westward into Soudan Underground Mine State Park, and appears to have played a key role in the development of hematite-rich iron orebodies that were historically mined there. The Mine Trend Shear Zone displaces lithological units higher in the stratigraphic sequence to the east. The southern two shear zones represent the northern and southern edges of the Murray Shear Zone (Peterson and Patelke, 2003). This fault system also displaces rocks higher in the stratigraphic sequence to the east. Rocks sandwiched between the southern edge of the Mine Trend Shear Zone and the northern edge of the Murray Shear Zone are in a structural domain known as the Linking Zone (Peterson and Patelke, 2003). According to Peterson and Patelke (2003), the net slip along the Mine Trend Shear Zone may have been as much as 7 km, whereas the net slip along the Murray Shear Zone may have been as much as 13.8 km (Table 2). Table 2. Calculated displacements among the Mine Trend and Murray Shear zones (Peterson and Patelke, 2003). Ranges of values were calculated geometrically by using the average plunges of lineations associated with the shear zones, and two measured lines of possible correlative stratigraphy offset by the bounding shear zones. See Peterson and Patelke (2003) for further details. 44 �45 Figure 4. Regional stratigraphic correlations across the Vermilion District (after Hudak et al., 2012). �During our field trip, we will be observing exposures of various lithologic units that occur within the Vermilion District. Overall, lithological units observed in Lake Vermilion State Park correlate well with lithological units mapped regionally in the Vermilion District. A diagram (Hudak et al., 2012) illustrating stratigraphic columns from the Soudan Mine Area (in the west), the Fivemile Lake area, and the Twin Lakes area (in the east) is provided in Figure 4. Stratigraphic and intrusive units that occur within Lake Vermilion State Park are described below in order from oldest to youngest units. The Fivemile Lake Sequence, Lower Member of the Ely Greenstone Formation The Fivemile Lake Sequence comprises the lowermost mafic to intermediate and felsic volcanic and volcaniclastic lithologies associated with the Lower Member of the Ely Greenstone Formation. This generally east-west striking, north-topping sequence is dominated by well-vesiculated basaltic to andesitic pillow lavas (Hudak et al., 2007; Peterson et al., 2009; Hudak et al., 2012) that display bun, mattress, and lobe morphologies using the nomenclature of Dimroth et al. (1978). Locally, these pillow lavas display exceptional multiple selveges (Hudak et al., 2002b). Multiple pillow selvedge morphologies have been interpreted as an indication of eruption in shallow water active volcanic settings (Kawachi and Pringle, 1988). Within Lake Vermilion State Park, horizons of Fivemile Lake pillow lavas are up to 1100m thick. Subordinate massive sheet lava flows associated with the Fivemile Sequence have been identified by Hoffman (2007) in one locale near Soudan. As well, numerous relatively thin horizons of massive to bedded basalt tuff, lapilli-tuff, and lapillistone deposits are present in the southwest ¼ of Section 25 in the south-central part of Lake Vermilion State Park. According to Hoffman (2007), these deposits vary from 50-150 meters thick, have a strike length of up to 350 meters, and comprise poorly-sorted and poorlygraded, matrix-supported, thickly to very-thickly bedded mafic pyroclastic deposits containing up to 65% lapilli- (64mm) to block-sized (>64mm) scoria fragments. Hudak et al (2007, 2012) have evaluated the lithogeochemical characteristics of Fivemile Lake Sequence mafic to intermediate lava flows and have found them to be dominantly calc-alkaline to transitional basalt and andesite/basalt using the classification schemes of Ross and Bedard (2009) and Pearce (1996). These rocks also are characterized by significant negative niobium (Nb) anomalies when plotted on primitive mantle-normalized spider diagrams. This suggests derivation of the magmas associated with the Fivemile Lake sequence in an arc-like volcanic terrane. Felsic volcanic and volcaniclastic rocks also occur within the Fivemile Lake Sequence, and crop out within the southern one-third of Lake Vermilion State Park based on mapping completed by Peterson and Patelke (2003). These include coherent and volcaniclastic facies dacitic to rhyolitic lithologies including lava flows, monolithic and heterolithic breccia deposits, and tuff and lapilli- tuff deposits. Lithogeochemically, Hudak (2007) and Hudak et al. (2012) have shown felsic rocks in the Fivemile Lake Sequence to be calc-alkaline to transitional andesites to rhyolites using the classification schemes of Pearce (1996) and Ross and Bedard (2009), respectively. As well, these felsic rocks have trace element characteristics of FI, FII, and FIV rhyolites based on classifications from Hart et al. (2004). One relatively thin horizon (<20 meters thick) of oxide-facies iron-formation identified as being within the Fivemile Lake Sequence has been observed proximal to the northern margin of the Murray Shear Zone approximately 600 meters east of the Lake Vermilion State Park boundary. This iron formation typically occurs as localized, thin (<3m thick) horizons interbedded with Fivemile Lake Sequence pillowed lava flows (Peterson and Patelke, 2003). The various lithofacies comprising the Fivemile Lake Sequence of the Lower Member of the Ely Greenstone Formation are summarized in Table 3. 46 �Lithofacies Associated with the Fivemile Lake Sequence, Lower Member, Ely Greenstone Formation Unit Symbol (Figure 3) Lithofacies FM1a Massive Basalt - Andesite FM1b Pillow Basalt - Andesite FM1h Scoriaceous Basalt –Andesite Tuff and Lapilli-Tuff FM1i Foliated Basalt-Andesite FM2a Coherent Dacite – Rhyolite (lava flows and lava domes) FM2c Felsic Polymict Breccia FM2d Felsic Monolithic Breccia FM2e Dacite-Rhyolite Tuff FM4a Oxide-facies Banded Iron-Formation Table 3. Lithofacies and map symbols associated with lithologies comprising the Fivemile Lake Sequence of the Lower Member of the Ely Greenstone Formation. The Central Basalt Sequence, Lower Member of the Ely Greenstone Formation The Central Basalt Sequence crops out in the east-central part of Lake Vermilion State Park, and is composed of massive and pillowed basalt lava flows, structurally deformed foliated basalt, and local thin (up to 3 meters thick) horizons of Algoma-type banded iron-formation. Within Lake Vermilion State Park, the Central Basalt Sequence varies from approximately 300-1000 meters in stratigraphic thickness. The Central Basalt Sequence mafic lava flows appear to be regionally correlative with basaltic lava flows comprising the Armstrong Lake Sequence in the northernmost parts of the Eagles Nest Quadrangle mapped by Jirsa et al., 2001 (Peterson and Patelke, 2003). The Central Basalt Sequence mafic lava flows can be distinguished from the Fivemile Lake Sequence mafic-intermediate lava flows using the following criteria: 1) the Central Basalt Sequence mafic lava flows commonly comprise exceptionally well-preserved primary volcanic textures - such textures are generally not present in the Fivemile Lake Sequence mafic flows due to destruction of these textures from a combination of synvolcanic hydrothermal alteration combined with recrystallization during greenschistfacies regional metamorphism; 2) the Central Basalt Sequence mafic volcanic rocks tend to be dark green to green colored, whereas the Fivemile Lake Sequence mafic-intermediate volcanic rocks typically vary from gray green to blueish green in color; 3) the Central Basalt sequence mafic volcanic rocks tend to lack amygdules or be sparsely amygdaloidal, whereas the Fivemile Lake Sequence mafic-intermediate volcanic rocks tend to contain abundant amygdules; and 4) to date, multiple selvege pillow lavas have not been identified in the Central Basalt Sequence, whereas they are locally abundant within the Fivemile Lake Sequence. Within Lake Vermilion State Park, the Central Basalt Sequence is composed primarily of pillowed basalt lava flows. These mafic volcanic rocks are medium green to dark green in color and tend to be sparsely amygdaloidal (<5% 2mm-1cm rounded to oval gray quartz-filled amygdules). Dark green, locally exceptionally well-preserved interpillow hyaloclastite deposits, ranging from 1-5cm wide, separate individual pillow structures. Locally these rocks are moderately- to strongly quartz- and epidote-altered. As well, in the southeastern part of the park, hydrothermally altered interpillow hyaloclastite deposits containing abundant andradite garnets have been identified. Massive basalt lava flows (interpreted as sheet flow facies lava flows) are also quite common, and comprise green to dark green, aphyric to sparsely-plagioclase phyric basalt. Foliated basalts locally occur in close proximity to shear zones present in the park. Hudak et al (2007, 2012) have evaluated the lithogeochemical characteristics of mafic lava flows in the Central Basalt Sequence in the vicinities of Needleboy and Sixmile Lakes, which are located 47 �approximately 4-5 kilometers east of the eastern boundary of Lake Vermilion State Park. These researchers have found them to range from calc-alkaline to tholeiitic basalt and andesite/basalt using the classification schemes of Ross and Bedard (2009) and Pearce (1996), respectively. Hudak et al. (2007) first observed that Central Basalt Sequence mafic flows could be divided into two types based on rareearth element characteristics. The first of these types is characterized by calc-alkaline to transitional compositions with arc-like chondrite- and primitive-mantle-normalized rare earth element spider diagrams. The second type comprises tholeiitic basalt characterized by flat chondrite-normalized and primitive mantle-normalized rare earth spider diagrams that are characteristic of mafic volcanic rocks erupted within mid-ocean ridge (MORB) or back-arc basin (BABB) extensional tectonic environments. Detailed mapping indicates that these tholeiitic, MORB/BABB compositions only occur within 200 meters of the lower contact with the overlying Soudan Member Iron Formation. Hudak et al. (2007, 2012) have used both these lithogeochemical results, and results from detailed regional mapping at Lake Vermilion State Park, the Needleboy Lake-Sixmile Lake area, the Twin Lakes area (located approximately 14km east of the eastern boundary of Lake Vermilion State Park), and the Purvis Lake area (on the southern limb of the Tower-Soudan anticline approximately 5km south-southeast of the southern boundary of Lake Vermilion State Park) to indicate that the major hydrothermal event that led to the formation of the Soudan Member Algoma-type iron-formation occurred immediately following the opening of a nascent rift or back-arc basin environment during the youngest part of Central Basalt Sequence mafic volcanism. Hudak et al. (2002b) and Hoffman (2007) have identified several localized occurrences of felsic coherent and volcaniclastic strata within the Central Basalt Sequence to the south and east of Lake Vermilion State Park. Hudak et al. (2007, 2012) have evaluated the lithogeochemical characteristics of these rocks, and have found them to be calc-alkaline andesite to rhyolite/dacite using the classification schemes of Ross and Bedard (2009) and Pearce (1996), respectively. As well, these felsic rocks have trace element characteristics of FII and FIIIa rhyolites based on classifications from Hart et al. (2004). The various lithofacies comprising the Central Basalt Sequence of the Lower Member of the Ely Greenstone Formation are summarized in Table 4. Lithofacies Associated with the Central Basalt Sequence, Lower Member, Ely Greenstone Formation Unit Symbol (Figure 3) Lithofacies Cb1a Massive Basalt Cb1b Pillow Basalt Cb1i Foliated Basalt Cb1u Undivided Basalt Cb2eh Polymict Dacite-Rhyodacite Tuff and Lapilli-Tuff Cb2e Dacitic-Rhyodacite Tuff and Lapilli-Tuff Cb2f Felsic Epiclastic Deposits Cb4a Interbedded Oxide-facies Banded Iron-Formation and Basalt Table 4. Lithofacies and map symbols associated with lithologies comprising the Central Basalt Sequence of the Lower Member of the Ely Greenstone Formation. The Soudan Member of the Ely Greenstone Formation The Soudan Member of the Ely Greenstone formation is dominantly composed of laminated to thinly bedded Algoma-type oxide facies banded iron-formation, with subordinate, locally interstratified, sparsely amygdaloidal massive to pillowed basalt lava flows and resedimented felsic tuff deposits. Regionally, the stratigraphic thickness of the Soudan Member of the Ely Greenstone Formation varies 48 �from 50-3,000 meters, with an average stratigraphic thickness of approximately 700 meters (Peterson et al., 2009). Within Lake Vermilion State Park, the Soudan Member ranges in stratigraphic thickness from approximately 300 – 680 meters in thickness. Individual horizons of oxide-facies iron formation range from approximately 70-345 meters thick, whereas the Soudan basalt lava flow units range from approximately 60-300 meters in thickness. A gradational contact over several tens of meters to two hundred meters occurs between the underlying Central Basalt Sequence rocks and the overlying oxide facies iron-formations of the Soudan Member. This transitional zone is characterized by a decrease in abundance of basalt lava flows and associated volcaniclastic rocks, and an increase in the abundance and thickness of oxide-facies ironformation horizons, as one moves toward the basal contact of the Soudan Member (Hudak et al., 2002b; Peterson and Patelke, 2003; Hudak et al., 2007; Hoffman, 2007; Hudak et al., 2012). Several characteristics suggest that the Soudan Member was deposited in relatively quiet water in a relative deep subaqueous environment (>200m, probably greater than 1400 m). This evidence includes: 1) a lack of primary mafic or felsic pyroclastic deposits within the stratigraphic sequence; 2) a lack of multipleselvege pillow lavas in the stratigraphic sequence; 3) planar laminations and bedding combined with an absence of any wave-associated sedimentary bedforms within both the chemical and clastic sedimentary rocks within the sequence; and 4) lithological and geochemical evidence for the development of an extensional tectonic environment that resulted in deepening of the depositional environment in the uppermost sections of the stratigraphically underlying Central Basalt Sequence. Within Lake Vermilion State Park, the Soudan Member oxide-facies banded iron-formation is planar laminated to medium-bedded, with black magnetite-rich horizons, light gray to black chert horizons, red to blueish-black hematite-rich horizons, and red jasper horizons defining the bedding. Locally, very tight, chaotically oriented folds, resulting from syn-depositional soft sediment deformation and subsequent tectonic deformation, are present. These iron formation deposits can be intimately interbedded with basalt lava flows such that mapping individual iron-formation and basalt horizons is impossible at 1:5000 scale. Where this occurs, these rocks have been mapped as a stratigraphic unit called “Basalt and IronFormation” by Peterson and Patelke (2003). Basalt lava flows associated with the Soudan Member of the Lower Ely Greenstone are characterized by a medium green to dark green color. They are aphyric to sparsely plagioclase ± pyroxene (now actinolite)-phyric. Plagioclase phenocrysts are present in abundances up to 3%, are typically less than or equal to 1mm in length, and vary from subhedral to euhedral tabular in morphology. Locally, 5-7% dark green actinolite pseudomorphs of pyroxene phenocrysts may be present. Where amygdaloidal, the unit contains up to 7% oval to round, light gray quartz-filled amygdules ranging from <1-4mm in diameter. The various lithofacies comprising the Soudan Member of the Ely Greenstone Formation are summarized in Table 5. Lithofacies Associated with the Soudan Member, Ely Greenstone Formation Unit Symbol (Figure 3) Lithofacies S1a Massive Basalt S2eq Aphyric- to Quartz-phyric Rhyodacite Tuff S4a Oxide Facies Banded Iron-Formation Table 5. Lithofacies and map symbols associated with lithologies comprising the Soudan Member of the Ely Greenstone Formation. The Gafvert Lake Sequence, Lake Vermilion Formation The Gafvert Lake Sequence (mapped as the “Upper Sequence” by Peterson and Patelke, 2003; Radakovich et al., 2010: and Heim et al., 2011) comprises dacitic to rhyodacitic volcaniclastic and epiclastic rocks that are locally interbedded with Algoma-type banded iron-formation and chert deposits. 49 �This sequence, which is part of the Lake Vermilion Formation, has been found to unconformably overlie the Soudan Member of the Ely Greenstone Formation in the north-central and northwestern parts of Lake Vermilion State Park based on recent mapping and geochronological evidence reported by Lodge et al. (2013). Within Lake Vermilion State Park, the overall stratigraphic thickness of the Gafvert Lake Sequence is up to approximately 1300 meters thick, with individual felsic volcaniclastic deposits having stratigraphic thicknesses ranging from approximately 75 – 400 meters thick, and individual Algoma-type oxide facies banded iron formations and associated massive- to bedded chert deposits ranging from 25250 meters and up to 175 meters in stratigraphic thickness, respectively. To the wes,t in Soudan Underground Mine State Park, the Gafvert Lake Sequence is locally interlayered with, and overlain by, greywacke deposits associated with the Lake Vermilion Formation. Within Lake Vermilion State Park, several lithofacies comprise the Gafvert Lake Sequence. The basal member of this sequence comprises massive, very-thickly bedded, quartz- and plagioclase-phyric polymict dacitic to rhyodacitic tuff, lapilli-tuff, and tuff-breccia deposits. These light gray, non-sorted, non-graded, matrix-supported deposits contain 3-8% 1-2mm (rare 3mm) pale gray anhedral to subhedral quartz phenocrysts, 10-15% <1-2mm subhedral to euhedral tabular plagioclase phenocrysts, and a wide variety of lapilli- to block-sized clasts including: 1) 10-20% 1-10 cm quartz- and plagioclase-phyric coherent dacite to rhyodacite lapilli and blocks; 2) 5-7% <3cm diameter pale gray-green lens-shaped, locally quartz- and plagioclase-phyric pumice lapilli; 3) up to 1% dark gray to light gray angular chert lapilli ranging from 0.5-3cm in diameter; and 4) 1-3% 0.5-5cm dark gray to black to red magnetite-rich, hematite-rich, or jasper-rich banded iron formation lapilli. These deposits are overlain by, and interbedded with, light gray, matrix-supported, non-sorted and non-graded quartz- and plagioclase-phyric dacitic to rhyodacitic tuff deposits which contain 10-25% 1-3mm subhedral to euhedral tabular plagioclase phenocrysts, 1-3% 1-3mm subhedral to anhedral, commonly broken, quartz phenocrysts, as well as 10-15% subangular quartz- and plagioclase-phyric coherent dacite to rhyodacite lapilli and up to 5% locally quartz- and plagioclase-phyric pumice lapilli. Spectacular felsic epiclastic deposits comprising polymict volcaniclastic conglomerates and lithic sandstones are also present in the Gafvert Lake Sequence and crop out west of Lake Vermilion State Park in Stunz Bay (Radakovich et al., 2010). In addition to felsic volcaniclastic and epiclastic rocks, two types of chemical sedimentary rocks have also been identified in the Gafvert Lake Sequence. These include laminated to medium-bedded Algomatype banded iron formation that varies from red (hematite- and jasper-rich) to dark gray (magnetite-rich) to light gray (chert-rich) in color. Immediately west of the Lake Vermilion State Park boundary, light gray to black laminated to very thickly bedded black chert deposits are present. These chert deposits may represent the distal facies equivalent of Algoma-type banded iron-formation horizons that are present in the northeast part of Lake Vermilion State Park south of Cobble Bay. A limited number of Gafvert Lake Succession felsic volcaniclastic rocks have been studied by lithogeochemical means by Geoff Pignotta and Kelly Schwierske at the University of Wisconsin Eau Claire (Figure 5). These researchers (Schwierske et al., in press) have found that the volcaniclastic and epiclastic deposits associated with the Gafvert Lake Sequence consistently plot as rhyodacite/dacite in composition when using the immobile element lithological classification scheme of Winchester and Floyd (1977). These compositions are very similar to the composition of a quartz- ± plagioclase-phyric rhyodacite sill that crops out in the northeastern part of Lake Vermilion State Park (see Field Trip Stop 9 below), although the sill has consistently higher Nb/Y ratios than the Gafvert Lake volcaniclastic and epiclastic rocks. This sill may represent a synvolcanic intrusion that is genetically related to the evolution of the Gafvert Lake sequence based on this lithogeochemical evidence, as well as field evidence from Peterson and Jirsa (1999), which indicates that this intrusion is most commonly observed within the Gafvert Lake Sequence where it is thickest, in proximity to Gafvert Lake west of the Mud Creek Road. Peterson (2001) has hypothesized that this intrusion represents feeder intrusions to a Gafvert Lake Sequence stratovolcano located in this area. The various lithofacies comprising the Gafvert Lake Member of the Lake Vermilion Formation are summarized in Table 6. 50 �Figure 5. Chemical classification of various lithologies within Lake Vermilion State Park (Schwierske et al., in press) using the classification scheme of Winchester and Floyd (1977). Open triangles represent samples from a quartz- ± plagioclase-phyric rhyodacite/dacite sill in the northeastern part of Lake Vermilion State Park. The black squares, large black diamonds, and small black diamonds represent various Gafvert Lake Succession volcaniclastic and epiclastic rock units. Lithofacies Associated with the Gafvert Lake Sequence, Lake Vermilion Formation Unit Symbol (Figure 2-3) Lithofacies US1,4 Interbedded Basalt and Oxide-facies Banded Iron-Formation US2b Dacite-Rhyodacite Tuff-breccia US2cf/US2f Dacite-Rhyodacite Epiclastic Deposits US2e Quartz- + Plagioclase-phyric Dacite-Rhyodacite Tuff/Lapilli-tuff US2eh Polymict Dacite-Rhyodacite Tuff/Lapilli-tuff US4a Oxide-facies Banded Iron-Formation Table 6. Lithofacies and map symbols associated with lithologies comprising the Gafvert Lake Sequence of the Lake Vermilion Formation. 51 �Intrusive Rocks Eight types of intrusive bodies have been mapped within the boundaries of Lake Vermilion State Park (Peterson and Patelke, 2003; Hoffman, 2007; Radakovich et al., 2010; Heim et al., 2011; Figure 3). From oldest to youngest, these intrusions include:  Gabbro (mapped by Peterson and Patelke, 2003; Hoffman, 2007; Radakovich et al., 2010; Heim et al., 2011, unit Gb) – Identified as sills throughout the central one-third of Lake Vermilion State Park, this unit is characterized by grayish-green to black, medium-grained equigranular gabbro that is locally highly magnetic and displays ophitic texture.  Diabase (mapped by Peterson and Patelke, 2003; Hoffman, 2007, unit Db) – Identified in the southern one-third of Lake Vermilion State Park, this unit comprises black to dark green, finegrained plagioclase-phyric diabase dikes and sills that have been interpreted to represent feeder dikes to mafic volcanic rocks located stratigraphically up-section.  Coarsely porphyritic Quartz-Feldspar Porphyry (mapped by Heim et al., 2011, unit GLIC) – Identified in the northeastern part of Lake Vermilion State Park, this intrusion comprises light gray, massive, quartz ± plagioclase-phyric coherent rhyodacite. The light gray aphanitic groundmass contains 3-7% gray to light blue subhedral rounded to euhedral square quartz phenocrysts that range from 3-10mm in diameter, and 10% pale gray to tan, subhedral to euhedral tabular plagioclase phenocrysts ranging from 1-4mm in length. Similar intrusive rocks have been mapped in the vicinity of Needleboy and Sixmile Lakes by Hudak et al. (2002b), and near Gafvert Lake by Peterson and Jirsa (1999) and Peterson (2001).  Diorite (mapped by Peterson and Patelke, 2003; Hoffman, 2007, unit D) – Occurs as a generally east-west striking sill in the southern one-third of Lake Vermilion State Park. Composed of gray to gray-green, fine- to medium-grained, equigranular diorite. This unit was informally named the “Sugar Mountain Diorite” by Peterson and Patelke (2003), and is notable for its massive, indurated nature and lack of prominent joints, veins, and alteration.  Granodiorite (mapped by Peterson and Patelke, 2003; Hoffman, 2007, unit Gd) – Identified in the central part of Lake Vermilion State Park, and composed of whitish-pink to gray-green, fine- to medium-grained, commonly xenolith-rich granodiorite and locally hornblende granodiorite.  Quartz-Feldspar Porphyry (mapped by Peterson and Patelke, 2003; Hoffman, 2007; Radakovich et al., 2010; Heim et al., 2011, unit Qfp) - Found locally throughout Lake Vermilion State Park, this intrusion comprises a light gray to pale green-gray groundmass that contains 20-25% 1-3mm (locally up to 5mm) subhedral to euhedral tabular plagioclase phenocrysts and 7-12% 1-3mm (locally up to 5mm) subhedral to euhedral gray-blue quartz phenocrysts  Feldspar Porphry (mapped by Peterson and Patelke, 2003; Hoffman, 2007; Radakovich et al., 2010; Heim et al., 2011, unit Fp) – Identified in the south and central parts of Lake Vermilion State Park, this intrusion is white to pink in color, and contains subhedral rounded to euhedral tabular 4mm feldspar phenocrysts and locally, subhedral to euhedral prismatic to tabular actinolite pseudomorphs of hornblende phenocrysts.  Lamprophyre (mapped by Peterson and Patelke, 2003, unit L) – Located in the southwestern part of Lake Vermilion State Park, this intrusion is characterized by black, fine-grained, massive hornblende-feldspar rock that contains 10-15% fine hornblende needles in a gray-black to red matrix, as well as large (>25cm) rounded granite and supracrustal rock xenoliths. 52 �TERMINOLOGY OF VOLCANICLASTIC ROCKS It is important to note the terminology utilized in this field trip guide for: 1) volcaniclastic rocks; and 2) bedding characteristics. Use of consistent terminology is required in order to accurately describe these geological features. Volcaniclastic rocks contain abundant volcanic material irrespective of their origin or depositional environment. Such rocks can be formed directly from volcanic eruptions (whether subaerial or subaqueous), result from resedimentation of non-lithified volcanic deposits (for example, resedimentation of pyroclasts prior to lithification), or result from weathering and resedimentation of pre-existing lithified volcanic rocks. Primary (juvenile) volcaniclastic particles result directly from eruptive processes, and are of three types:  Pyroclasts, which form by explosive fragmentation of magma into particles (including ash, highly vesiculated glass (pumice, scoria), crystals and crystal fragments, and lithic fragments);  Hydroclasts, which form by explosive interaction with external water (via phreatic (steam only) and/or phreatomagmatic (steam and magma) explosions) or by non-explosive quenching and granulation of lava (for example, the formation of hyaloclastite fragments on the margins of submarine lava flows or intrusions into wet sediments); and  Autoclasts, which form by frictional breakage of moving viscous lava flows (for example, to form carapace breccias on the margins of subaerial lava flows). Based on these different types of fragmentation, four types of primary volcaniclastic deposits have been identified by White and Houghton (2006):  Pyroclastic deposits, which are generated from volcanic plumes and jets or pyroclastic density currents as particles first come to rest. Deposition mechanisms associated with these processes include suspension settling, traction, or en masse freezing;  Autoclastic deposits, which are generated during effusive volcanism when lava cools and fragments as a result of thermal processes, or recently cooled lava breaks during flow. Deposition for these types of rocks is under the influence of continued lava flowage;  Hyaloclastite deposits, which are generated during effusive volcanism when magma or flowing lava is chilled and fragmented as a result of contact with water. Deposition of such deposits is under the influence of the continued emplacement of the lava in the presence of water; and  Peperite deposits, which are generated when magma intrudes into unconsolidated clastic material and mingles with (generally wet) debris to form a volcaniclastic deposit. Deposition of peperite deposits takes place essentially in-situ. Secondary volcaniclastic particles are known as epiclasts:  Epiclasts are lithic clasts and/or crystals derived from physical weathering and erosion of preexisting rocks. Epiclasts are volcaniclasts when the pre-existing rocks are volcanic. In recent years, the terminology for volcaniclastic rocks has become increasingly confusing because different classification schemes (for example Fisher, 1961; Fisher 1966; Schmid, 1981; Cas and Wright, 1987; McPhie et al., 1993; White and Houghton, 2006) are preferentially used in different parts of the world, and terminology relating to volcaniclastic rocks is commonly misused. Four classification schemes have been used most commonly in the recent geological literature:  Fisher (1961, 1966) – Classification based on particle size, particle formation, or particle fragmentation mechanism;  Schmid (1981) – Particle type within the deposit;  Cas and Wright (1987) – Mode of fragmentation and deposition; and  McPhie et al. (1993) – Transport and deposition mechanisms. According to R. V. Fisher (1998), the difficulties with volcaniclastic rock classification can be understood because “volcaniclastic rocks are essentially igneous on the way up and sedimentary on the way down”. 53 �In fact, Fisher’s thesis advisor, when observing the volcaniclastic rocks that were the focus of his thesis studies, indicated that they were “the ugliest and most undistinguished rocks I’ve seen in my 30 years of petrology!” As well, classification is especially difficult in ancient volcaniclastic rocks because key aspects of classification can be obscured by subsequent metamorphism and/or structural deformation (e.g. particle type, particle size) or because genetic processes cannot be ascertained unambiguously (e.g. transport and deposition mechanism, fragmentation mechanisms). Figure 6. Volcaniclastic rock classification schemes of Fisher (1966) and White and Houghton (2006). This field trip guidebook will classify volcaniclastic rocks using Fisher’s (1966) classification scheme. For this field trip guidebook, we will utilize Fisher’s (1966) classification (Figure 6) for volcaniclastic rocks. This classification scheme is based on the relative proportions of ash-sized material (< 2mm), lapilli-sized material (64mm), and blocks/bomb sized material (>64mm) in the rock. Both Gibson et al. (1999) and Mueller and White (2004) suggest that this classification be used for field-based rock classification (mapping, diamond drill core logging, petrography) of ancient volcaniclastic deposits for the following reasons:  The classification scheme is “field-user friendly” because it accommodates both the historically important pyroclastic rock names and enables comparison at both the hand sample and thin section scale (Mueller and White, 2004);  It is a Wentworth-based scale, and thus enables comparison of volcaniclastic deposits to sedimentary deposits; and  Rock classification does not require knowledge of the specific transport mechanism or depositional processes involved with the genesis of the deposit. More recently, White and Houghton have developed a modified version of Fisher’s (1966) volcaniclastic classification scheme (Figure 6). The scheme is essentially equivalent to the Fisher (1966) scheme, with the exception that the lapill-tuff field in the White and Houghton (2006) classification comprises the lapilli-tuff and lapillistone fields of Fisher’s (1966). 54 �Specific terms for bedding thicknesses are also used in this guidebook The terms used, and their bedding thickness characteristics, have been adopted from McPhie et al. (1993) and include:  Laminated <1 centimeters thick  Very thinly bedded 1-3 centimeters thick  Thinly bedded 3-10 centimeters thick  Medium bedded 10-30 centimeters thick  Thickly bedded 30-100 centimeters thick  Very thickly bedded >100 centimeters thick ROAD LOG AND FIELD TRIP STOPS All stop locations for this field trip are given in Universal Transverse Mercator (UTM) coordinates, Zone 15N, using the North American Datum of 1983 (NAD83). Section subdivisions read from smallest to largest quarter; e.g., “NW, SE” should be read “NW quarter of the SE quarter.” The small topographic map insets illustrating field trip stop locations have been taken from the Tower and Soudan USGS 7.5minute quadrangle maps. A selected number of field trip stops will take place outside Lake Vermilion State Park, with the majority of the stops taking place along a 4.5 mile traverse through the state park. More detailed geological maps with stop locations are given in Figures 7 and 8 later in this guidebook. From Hibbing, our field trip route will proceed north and east from the Hibbing Park Hotel along Minnesota Highway 169 North. We will make one stop at a spectacular outcrop displaying various flow facies of Central Basalt flows that are located south of Lake Vermilion State Park on the south side of Highway 169 North (Figure 7). We will then proceed into Lake Vermilion State Park (Figure 8) where we will observe an outcrop of Neoarchean gabbro just southeast of the Old Ely Road. After a coffee break, we will strap on our hiking boots and make several field trip stops at outcrops along an approximately 4.5 mile traverse through Lake Vermilion State Park. All major stratigraphic units in the park will be observed during this traverse. We will then leave Lake Vermilion State Park, and start our return to Hibbing via Highway 169 South. We will make one final field trip stop on the south side of Highway 169 South just west of Tower to investigate a recent road cut comprising Gafvert Lake Sequence rhyodacite tuffs, lapilli tuffs, and tuff breccias (Figure 7) prior to returning to the Hibbing Park Hotel via Highway 169 South. Mileage for this roadlog starts at the intersection of East Howard Street and Highway 73/169 North. Roadlog (vehicle) mileage will be denoted in bold italic text. Mileage for the traverse through the park will be denoted in italic text. Bus Log 0.0 miles 22.0 miles 26.6 miles 54.1 miles Turn north on Highways 73/169 North and proceed to Virginia, Minnesota. Turn left on to Highways 53/169 North. Veer right at the exit for Ely, Minnesota on Highways 1/169 North. You will pass the “Y” store at approximately 44.2 miles, and the intersection for Highway 135 on the west side of Tower at approximately 48.3 miles. Continue on Highway 1/169 North through Tower. At approximately 50.2 miles you will see the intersection of Main Street, Soudan, Minnesota, and a sign for Soudan Underground Mine State Park. Continue on Highway 1/169 past Soudan. At approximately 53.4 miles you encounter the intersection of Highway 1/169 North and the Murray Forestry Road (on the south side of Highway 1/169). Get prepared to turn south off of Highway 1/169 on to a dirt road in approximately 0.6 - 0.7 miles. Turn right (south) on to the dirt road and immediately park in the open area at the base of the hill. Hike 0.15 miles (approximately 240 meters) up the hill on the dirt road to Field Trip Stop 1. 55 �Figure 7. Map illustrating regional geology in the vicinity of Lake Vermilion State Park. Field trip stops 1 and 2 are outside the state park boundaries and are illustrated. The location of Figure 8, a more detailed map illustrating field trip stops in the state park, is illustrated by the bold black box. 56 �Figure 8. Detailed geologic map of Lake Vermilion State Park (after Peterson and Patelke (2003), Radakovich et al. (2010) and Heim et al. (2011)). Field trip stops within the park are labeled. 57 �Stop 1: Central Basalt Sequence Sheet Flows, Pillow Lavas, and Perlitic Hyaloclastite Location: T. 62N, R.14W, sec. 19, SE, SW, Soudan 7.5-minute quadrangle UTM: 562,000E / 5,297,805N This classic outcrop has been visited during field trips associated with both the 2004 and 2009 ILSG conferences (Hudak et al., 2004; Peterson et al., 2009). This is a no-hammer outcrop, as the preservation of the delicate textures here rivals those observed in other classic Neoarchean camps in the Superior Province containing well-preserved volcanic textures such as Noranda, Quebec and Timmins, Ontario. The description and figure below has been taken from Peterson et al. (2009). The Central Basalt sequence (Peterson and Patelke, 2003) comprises a steeply north-dipping (75°vertical), north-facing sequence of sparsely amygdaloidal pillowed and massive lava flows of basalt andesite to basalt composition that are believed to be correlative with the tholeiitic Armstrong Lake volcanic sequence mapped in the Eagles Nest quadrangle (Jirsa et al., 2001), approximately 11km to the east. Hudak et al. (2007), Jansen et al. (2007), and Hudak et al. (2012) have shown that the lowermost sections of the Central Basalt Sequence are composed of submarine basaltic andesite to basalt lava flows that have rare earth element lithogeochemical patterns similar to mafic rocks in oceanic volcanic arcs. However, locally, submarine basalt lava flows that occur within 50-200m stratigraphically below the contact between the Central Basalt Sequence and the overlying Soudan Member of the Ely Greenstone Formation illustrate MORB-like or back-arc basin-like lithogeochemical patterns. This change in rare earth element characteristics may be interpreted to indicate a change from an oceanic arc to back-arc environment immediately prior to the deposition of the Soudan Member. Relative to massive and pillowed basalt and andesite flows in the Fivemile Lake sequence, Central Basalt sequence lava flows are notably less amygdaloidal, and lack multiple pillow rind structures. In addition, the Central Basalt sequence lacks the thick sequences of scoriaceous basalt-andesite lapilli tuffs that are commonly interstratified with lava flows in the Fivemile Lake sequence. These characteristics of the Central Basalt sequence indicate eruption and deposition in a deeper submarine environment than the stratigraphically older Fivemile Lake sequence, and suggest overall increasing water depth during the temporal development of the Lower Ely. Deepening of the water column could be accommodated by extensional tectonics and normal faulting associated with the development of the proposed back-arc environment. The outcrop comprises two east-southeast striking massive basalt flows, ranging from at least five to nine meters in thickness, that are separated by a ten meter thick flow unit comprising pillows and pillow lobes (Fig. 9). All three lava flows at this vicinity illustrate tholeiitic, MORB-like lithogeochemistries (Hudak et al., 2007). 58 �Figure 9. Detailed geological map of sheet flows, pillow lavas, and associated hyaloclastite deposits at field trip Stop 1. Flow 1, at the southern part of the outcrop, is composed of a pale- to dark green, faintly feldspar-phyric (~10% 0.5-1 mm laths), sparsely amygdaloidal, basalt sheet flow that locally exhibits tortoise-shell jointing formed in response to contraction during cooling. The uppermost 10-40 cm of the coherent part of Flow 1 is generally silicified and epidotized. Petrographic observations indicate that this section of the flow also contains up to 70% <0.1 cm round spherulites. An irregular contact occurs between the coherent basalt flow and an overlying one- to two meter thick unit of dark green, exceptionally well-preserved perlitic in-situ hyaloclastite and associated self-peperite (c.f. Batiza and White, 2000). The hyaloclastite formed from non-explosive fracturing of the basalt glass developed on the flow top due to quenching by 59 �water, whereas the perlite formed following deposition by hydration of volcanic glass. An irregular contact occurs between the hyaloclastite and Flow 2, which is composed of north-facing mattress- to bunshaped pillow lavas and pillow lobes with numerous “neck and knob” structures. Individual Pillow structures have well developed perlitic hyaloclastite margins that range from 1-4 cm in width. Pillow buds indicate propagation from east to west, suggesting the volcanic vent was located east of this location. The coherent pillows and lobes are overlain by up to 2.5 meters of hyaloclastite breccia that contains 20-40% subround to subangular pale gray green basalt lapilli in a jigsaw puzzle-fit dark green perlitic hyaloclastite matrix. The upper contact of Flow 2 and the overlying basalt sheet flow (Flow 3) is irregular, and is marked by thin (1-8 cm thick), sheet- like basalt fragments that are up to 1.6 meters in length. These fragments locally appear to be isoclinally folded about an east-west-trending fold hinge. Although the genesis of this structure is currently not well understood, it may be due to syneruptive deformation of either thin slabs of hot, basal flow margin crust from the overlying flow, or thin injections of basalt magma into the hyaloclastite from either the underlying pillows or the overlying sheet flow. Flow 3 comprises an at least ten-meter thick pale green-gray, slightly feldspar-phyric, sparsely amygdaloidal sheet flow. Steep, NNE-trending west dipping D3 joints are well developed in this unit, as are lens-shaped pseudo-pillows that are up to 50 cm in diameter. Return to the bus by walking back down the hill. 54.1 miles 54.7 miles 54.8 miles 56.1 miles 56.4 miles 56.9 miles Turn left and follow Highway 1/169 South approximately 0.6-0.7 miles to the west. You will see the Murray Forestry Road on your left (south side of road) Turn right (north) on the dirt road immediately north of the intersection with Highway 1/169 South and the Murray Forestry Road. Proceed north approximately 0.1 miles. Park the vehicle on the Old Ely Road immediately outside the gate to Lake Vermilion State Park. Walk approximately 0.3 miles northeast on the Old Ely Road, then approximately 0.04 miles southwest along the trail to Field Trip Stop 2. Drop off field trip participants on Old Ely Road immediately east of the gate to Lake Vermilion State Park. After dropping off field trip participants, the bus will drive northeast up the Old Ely Road for 1.32 miles. Make sharp left turn on dirt road farthest to the south. Continue west on dirt road approximately 0.28 miles to the gate at Lake Vermilion State Park. Enter Lake Vermilion State Park through gate. Proceed 0.48 miles west, where you will see a two-track trail on the south of the road immediately before the well-maintained road turns right sharply to the north. Park the bus in the grass on the south side of the road at the intersection of the well-maintained road and the two-track trail. Park Bus. 60 �Stop 2: Neoarchean Gabbro Sill Location: T. 62N, R. 14W, sec. 19, SW, SW, Soudan 7.5-minute quadrangle UTM: 561,385E / 5,297,735N Detailed geologic mapping in the Vermilion District (Peterson, 2001; Hudak et al., 2002a, Hudak et al., 2002b; Hudak et al., 2006) has indicated the presence of several gabbro/diabase dikes and sills within both the Fivemile Lake and Central Basalt sequences of the Lower Member of the Ely Greenstone Formation. These intrusive rocks vary from fine-grained diabase with well-developed trachytic textures, to medium- to coarse-grained gabbro and quartz gabbro that locally display well developed ophitic and sub-ophitic textures. Petrographic observations indicate the presence of relatively unaltered (minor sericite ± carbonate alteration) euhedral to subhedral plagioclase and subhedral to anhedral actinolite pseudomorphs of original clinopyroxene. At this location, we will observe dark green to dark greenish-black medium- to coarse-grained gabbro that locally displays exceptional sub-ophitic and ophitic textures. Return to the bus for our morning coffee break and a brief explanation of our traverse through the park We will now begin our traverse through Lake Vermilion State Park. Make sure to have proper field gear (hat, rain gear, etc), your lunch, and drinks along with you, as we will be out on the trails in the park for nearly the remainder of the trip. Per state park rules, no hammering on the outcrops, or taking of samples, will be allowed while we are on the traverse. Traverse Log 0.0 miles 0.35 miles Leave the bus and go through the gate at the entrance to Lake Vermilion State Park for 0.26 miles. Turn south along the dirt road/trail and proceed approximately 0.9 miles along the trail to Stop 3. Stop at outcrop on ridge of hillside. Stop 3: Garnet-altered Central Basalt Sequence Pillow Lavas Location: T. 62N, R. 15W, sec. 25, SW, NE, Soudan 7.5-minute quadrangle UTM: 560,670E / 5,297,210N In several locations in the vicinities of Sixmile Lake (Hudak et al., 2006) and Twin Lakes (Moosavi et al., 2007), mafic volcanic and volcaniclastic rocks in the Central Basalt Sequence have been intensely altered to form mineral assemblages comprising quartz, epidote (both pistacite and zoisite/clinozoisite), actinolite, sericite, and/or chlorite (both Mg-rich and Fe-rich compositions). Locally, these altered rocks also 61 �contain minor to moderate abundances of subhedral to euhedral, dark reddish-brown garnets that have been identified as andradite via x-ray diffraction analysis (andradite chemical formula is Ca 3Fe2Si3O12, a member of the ugrandite garnet series (Phillips and Griffen, 1981, p. 117)). At this location, you will observe hydrothermally altered Central Basalt Sequence pillow lavas that contain an abundance of andradite garnet in hydrothermally-altered interpillow hyaloclastite deposits. Recognition of mineral phases that are not consistent with greenschist-facies metamorphism of original basalt composition protoliths is essential to identifying hydrothermal alteration zones. Detailed mapping of such alteration mineral assemblages provides important data regarding the processes and timing of hydrothermal alteration, which in turn, provide essential clues to economic geologists in their quests to find mineralization. Return to Old Ely Road along the same path used to access Stop 3. 0.45 miles 0.65 miles 1.10 miles At the intersection of the trail leading to Stop 3 and Old Ely Road, turn left (west) and walk approximately 0.2 miles to the intersection of a two-track road that leads to the north. Turn north on the two-track road and proceed north-northeast. Walk approximately 0.27 miles to the first major curve in the road. It is likely to be a bit wet here, so plan to continue walking north-northeast along the edge of the trail. Continue for another approximately 0.27 miles northeast along the two-track road. You will encounter a series of five outcrops along the two-track road that will extend for a distance of approximately 0.1 miles. This sequence of outcrops is Stop 4. Stop 4a: Central Basalt Sequence Pillow Lavas Location: T. 62N, R. 15W, sec. 24, SE, SW, Soudan 7.5-minute quadrangle UTM: 560,440E / 5,297,700N At this location, a series of six small outcrops occurs as one traverses approximately 125 meters up a small hill from southwest to northeast. Here you will observe well-preserved, commonly muffin-shaped, sparsely vesicular variably altered Central Basalt Sequence pillow lavas. In several locations, dark reddish-brown sulfide burn can be observed where sulfide minerals (pyrite, locally minor chalcopyrite) have been oxidized. Near the central part of the outcrop exposure (outcrop number four as one procedes from south to north), locally strong silicification and actinolite alteration may be observed. 1.2 miles 1.3 miles From the northernmost outcrop associated with Stop 4, proceed north approximately 0.1 miles. You will see a series of small outcrops that extend for approximately 0.1 miles along the east side of the two-track road. 62 �Stop 4b: Hydrothermally Altered Central Basalt Sequence Pillow Lava Location: T. 62N, R. 15W, sec. 24, SE, SW, Soudan 7.5-minute quadrangle UTM: 560,525E / 5,297,860N This stop comprises a series of east-northeast striking, north-topping, locally silicified and actinolite-altered Central Basalt Sequence basalt to basaltic-andesite pillow lavas that extend for approximately 0.1 miles as one traverses toward the north. Note the brown stains within both the interpillow hyaloclastite deposits and the cores of the pillow lavas that result from weathering of minor amounts of pyrite in the rock. 1.4 miles 1.75 miles 1.77 miles From the northernmost outcrop exposure, continue walking northwest along the twotrack road. Gather on the two-track road, and follow the field trip leader approximately 0.02 miles (~30 meters) through the bush. Gather on the north slope of the north-south trending outcrop. This is Stop 5. Stop 5: Contact Between Soudan Member Banded Iron Formation and Soudan Basalt Location: T. 62N, R. 15W, sec. 24, NW, SW, Soudan7.5-minute quadrangle UTM: 560,165E / 5,298,240N Mapping by Jirsa et al. (2001), Peterson and Patelke (2003), Radakovich et al. (2010) and Heim et al. (2011) has shown that the Soudan Member of the Ely Greenstone Formation is composed of Algoma-type oxide-facies banded iron formation horizons interbedded with massive and pillowed basalt lava flows, aphyric- to quartz-phyric rhyolite tuffs, and locally, polymict quartz- and plagioclase-phyric dacitic to rhyodacitic lapilli tuff deposits. To the south and east, as well as within the 2700 drift of the Soudan Underground Mine, shearing of the interbedded iron formation and basalt horizons has resulted in a rock comprising chaotically intermingled chlorite schist and banded iron formation which Peterson and Patelke (2003) most appropriately termed “Schist ‘n’ BIF”. The Algoma-type iron formations of the Soudan Member comprise laminated- to medium-bedded iron formation containing dark gray to black magnetite-rich bands, bluish-gray to red hematite-rich bands, red jasper bands, and light gray to black chert bands. Planar bedding is most common, with tight, commonly chaotic folds present that have been, in part, interpreted to be the result of soft sediment deformation. The unit is typically strongly magnetic, but is locally moderately to weakly magnetic where dominated by hematite-rich horizons or chert horizons. The Soudan Basalts comprise medium-green to dark green, aphyric to sparsely plagioclase- ± pyroxenephyric massive to amygdaloidal basalt. Typically, the recrystallized matrix (now chlorite-epidoteactinolite) contains up to 3% <1mm subhedral to euhedral tabular plagioclase phenocrysts and locally, 5- 63 �7% <1mm dark green actinolite pseudomorphs of pyroxene phenocrysts. Locally, amygdaloidal basalt flows contain 5-7% oval to round, light gray to white, quartz- ± epidote- ± chlorite-filled amygdules ranging from <1-4mm in diameter. Locally, brownish-tan colored ankerite alteration and dark green chlorite alteration are present. Figure 10. Detailed (1:5000 scale) map illustrating the complex contact relationships between Soudan Member oxide facies iron formation and basalt units in the vicinity of Stop 5. Figure 10 is a reproduction of a detailed field map (originally mapped at 1:5000 scale) in the general vicinity of Stop 5. The map illustrates the complex contact relationships between Soudan Member banded iron formation and basalt units at this location. Here we will see the nature of the contact between one of the banded iron formation units and an adjacent massive basalt lava flow. 1.77 miles 1.79 Miles 2.14 miles Walk north-northwest through the bush back to the two-track road. Proceed west-southwest along the two-track road. At the fork in the road, proceed to the northwest along the two-track road. Continue walking along the two-track road for approximately 0.35 miles. At this point you will encounter a series of outcrops within, and along the south and north edges, of the two-track road. This will be Field Trip Stop 6a. 64 �Stop 6a: Folded Soudan Iron-Formation Member Banded Iron Formation Location: T. 62N, R. 15W, sec. 23, NE, SE, Soudan 7.5-minute quadrangle UTM: 559,725E / 5,298,155N As is well exemplified at the “classic” outcrop of Soudan Member banded iron formation located in the NE ¼ NE ¼ Sec. 27, T.62N, R. 15W (see stop 7-10 in Peterson et al., 2009), the Soudan Member banded iron formation commonly displays multiple generations of tight folds which can result in complex interference patterns (Figure 11). At this location, and at several other small outcrops along the north side of the twotrack trail, we can observe highly folded, moderately- to strongly magnetic, magnetite-rich Soudan Member oxide facies iron formation. 2.14 miles 2.16 miles 2.17 miles Continue walking southwestward along two-track road for approximately 0.02 miles. You will see several outcrops between 0.01 and 0.02 miles into the bush on the northwest side of the two-track road. Proceed to these outcrops. This will be Stop 6b. Figure 11. Tightly folded Soudan Member Algoma-type banded iron formation 65 �Stop 6b: Contact between Folded Soudan IronFormation Member Banded Iron Formation and Diabase/Gabbro Intrusion Location: T. 62N, R. 15W, sec. 23, SE, SE, Soudan 7.5-minute quadrangle UTM: 559,560E / 5,298,035N Note: Be extremely careful on this outcrop, especially if it is wet. The glacially polished surface combined with wet moss makes for very slippery conditions. This outcrop is once again composed primarily of laminated to thinly-bedded Soudan Member oxidefacies banded iron formation. On the far western side of the outcrop, as well as in several small outcrops to the northeast, we can observe a massive, fine- to medium-grained, dark green to grayish-green rock which has been interpreted to represent a diabase sill. This sill appears to intrude the contact between Soudan Member oxide-facies iron formation (to the south) and Soudan Member massive basalt lava flows (to the north). Based on the outcrop distribution in the park, this unit appears to get coarser grained to the east, where it represents dark green to blackish-green medium-grained gabbro. 2.17 miles 2.19 miles 2.30 miles 2.49 miles 2.60 miles Return to the two-track road. Walk southwest, then west, down the hill along the two-track road. Cross bridge over small creek between unnamed pond (to the south) and creek/swamp (to the north). Be extremely careful crossing this bridge as it may be wet and slippery! Continue west, then southwest along the two-track road and begin to climb a moderately steep hill. Continue walking up the hill. The outcrop ridge on both sides of the two-track road comprise poorly exposed interbedded Soudan Member banded iron formation and Soudan Member basalt lava flows exhibiting both sheet flow facies and associated flow breccia facies. Continue walking another 0.11 miles up to the top of the hill. We are now at the top of the hill. We will reassemble here before moving on to the remainder of the outcrops along our traverse. To the west, the prominent hill and low lying outcrops along the trail comprise magnetite-rich Soudan Member banded iron formation. After reassembling the group, we will proceed north, then northeast, along the two-track trail for 0.09 miles. 2.69 miles 2.80 miles 2.81 miles Take the left fork and proceed to the northwest along the two-track road. We will once again reassemble the group at this location. Once reassembled, we will walk north-northeast approximately 0.01 miles up a hill through the bush to Field Trip Stop 2.7. Field Trip Stop 2.7 66 �Stop 7: “Contact” Between Soudan Member Banded Iron Formation and Rhyodacite Polymict Lapilli Tuff/Tuff Breccia of the Gafvert Lake Sequence Location: T. 62N, R. 15W, sec. 23, NW, SE, Soudan 7.5-minute quadrangle UTM: 558,995E / 5,298,230N Here we will see one of the few places where the nature of the contact between the Soudan IronFormation Member oxide facies iron-formation and the overlying dacitic to rhyodacitic volcaniclastic rocks associated with the informally named Gafvert Lake Sequence (which is part of the Lake Vermilion Formation) can be observed. Based on regional mapping, Sims and Southwick (1980), Southwick (1993), and Southwick et al. (1998) have indicated that the contact between the underlying Soudan Iron-Formation Member of the Ely Greenstone Formation and the overlying Lake Vermilion Formation is locally an unconformity. Geochronological work in the Vermilion District (Peterson et al., 2001; Lodge et al., 2013), combined with detailed field mapping in the limited number of locations where the contact between the Soudan Iron-Formation Member and the Lake Vermilion Formation occurs, bears out this interpretation. Peterson et al. (2001) obtained a U-Pb zircon age of 2722 ± 0.9 Ma from a quartz-phyric rhyolite dome within the Fivemile Lake Sequence at the Fivemile Lake prospect, located approximately 850 meters stratigraphically below the base of the overlying Soudan Member Iron-Formation unit. Regionally extensive detailed mapping in the stratigraphic units that occur between the Fivemile Lake Sequence rhyolite dome and the base of the Soudan Member Iron-Formation has been completed by a number of researchers (Peterson and Jirsa, 1999; Peterson, 2001; Hudak et al., 2002a; Hudak et al., 2002b; Peterson and Patelke, 2003; Hoffman, 2007; Radakovich et al., 2010; Heim et al., 2011). Based on this detailed mapping, there are no indications of any unconformities within the Fivemile Lake Sequence or the Central Basalt Sequence that comprise the footwall to the Soudan Iron-Formation Member. As well, unconformities at the contacts between the Fivemile Lake Sequence and the Central Basalt Sequence, and the Central Basalt Sequence and the overlying Soudan Iron-Formation do not appear to be present. Hudak et al. (2007; 2012) have noted that the contact between the Central Basalt Sequence and the overlying Soudan Iron-Formation Member is transitional over several hundred meters, with the presence of ironformation horizons near the top of the Central Basalt Sequence increasing in abundance, and the abundance of basalt lava flows and associated volcaniclastic rocks decreasing in abundance, as one approaches the base of the Soudan Iron-Formation Member. Therefore, it appears that the Fivemile Lake Sequence is stratigraphically overlain by the Central Basalt Sequence, which in turn is stratigraphically overlain by the Soudan Iron-Formation Member. Furthermore, there appears to be a major period of volcanism and associated hydrothermal activity between 2722 and 2718 Ma in the western part of the Wawa Abitibi Terrane in Ontario that produced both the volcanic rocks and volcanogenic massive sulfide orebodies that occur at the Winston Lake and Geco deposits (Lodge et al., 2013). Based on both the geochronological work and detailed mapping in the Vermilion District, as well as the regional volcanic and hydrothermal events in the western Wawa-Abitibi belt, we currently believe that the Soudan IronFormation Member was deposited between 2722 and 2718 Ma. Further geochronological studies within the stratigraphic package that comprises the Soudan Iron-Formation Member of the Ely Greenstone Formation will need to be completed in order to verify our current interpretation. Based on field relationships recognized by Radakovich et al. (2010), Lodge et al. (2013) collected a sample of the basal part of the Gafvert Lake polymict dacite- to rhyodacite lapilli-tuff / tuff-breccia deposits that occur at this outcrop in order to determine the age of volcanism of the Gafvert Lake 67 �Sequence relative to the ages of the Lower and Soudan Iron-Formation members of the Ely Greenstone Formation. Zircons from the sample of polymict rhyodacite tuff-breccia from this outcrop approximately 2m north of the contact with the Soudan Iron-Formation Member produced a high precision U-Pb age of 2689.7 ± 0.8 Ma using thermal ionization mass spectrometry (Lodge et al., 2013). Given that the basal Gafvert Lake Sequence deposits contain angular intraclasts of chert and banded iron formation, and that there appears to be no intense structural fabric in either the Soudan Iron-Formation Member or the Gafvert Lake volcaniclastic rocks, Lodge et al. (2013) interpreted the contact here to represent a disconformity, a type of unconformity characterized by strata that are essentially parallel on either side of the erosional or non-depositional surface. Several outcrops occur at this location, but at 1:5000 scale mapping they have been combined into a single east-northeast trending outcrop (Figure 12). The majority of the outcrop, which extends east up the hill, is composed of laminated to medium bedded Soudan Iron-Formation Member. Alternating magnetite-rich horizons, chert horizons, and jasper horizons display planar bedding and are locally folded. Moving toward the northwest part of the outcrop, we observe a small break in the outcrop exposure. This break occurs directly above the contact between the Soudan Member Iron-Formation and the Gafvert Lake volcaniclastic rocks. In this area, note the lack of deformation in both lithological units. The lack of structural deformation at this contact, as well as geochronological data obtained from the Gafvert Lake volcaniclastic rocks near this contact (Lodge et al., 2013), supports the interpretation of a disconformity. Figure 12. Detailed (1:5000 scale) map illustrating the disconformable contact between the Soudan Member Algoma-type banded iron-formation (unit S4a) and the Gafvert Lake Sequence quartz- and plagioclase-phyric polymict dacite-rhyodacite tuff-breccia / lapilli tuff deposits (unit US2eh). We will 68 �start our investigation where Stop 7 is indicated, and traverse along the path indicated by the red dashed line over a series of outcrops. We will assemble on the two-track trail where indicated by the star symbol before proceeding to Stop 8. Moving to the northwest, we observe the basal several meters of the Gafvert Lake Succession volcaniclastic rocks. Here, the rock is composed of a very thickly bedded quartz- and plagioclase-phyric polymict dacite-rhyodacite tuff-breccia / lapilli tuff. The rock is characterized by up to 5% 1-3mm diameter subhedral to euhedral gray to blue-gray quartz phenocrysts and locally, 5-10% subhedral to euhedral light gray to tan tabular plagioclase phenocrysts set in a fine-grained quartzo-felspathic matrix that is locally sericite altered. Accidental fragments comprising laplli-sized light gray to grayish black angular to subangular chert, gray to dark gray subangular to angular banded iron formation (Figure 13), and rare angular to subangular reddish brown jasper fragments are present. As well, juvenile fragments comprising lapilli- to locally block-sized pumice are present. Lapilli- to block-sized accessory fragments of quartz- and plagioclase-phyric coherent dacite and rhyodacite are also present, in abundances up to 5%. B A A Figure 13. Gafvert Lake Sequence quartz- and plagioclase-phyric polymict dacite-rhyodacite tuff-breccia / lapilli tuff from the Gafvert Lake Sequence. A. Typical appearance of very thickly bedded quartz- and plagioclase-phyric polymict dacite-rhyodacite lapilli tuff. B. Closeup of unit illustrating tannish-white subhedral to euhedral tabular plagioclase phenocrysts, gray to gray-blue anhedral quartz phenocrysts, and 1cm diameter angular accidental fragment composed of jasper-rich banded iron formation. 2.81 miles 2.90 miles 3.32 miles 3.41 miles 3.48 miles 3.50 miles We will proceed north down the north-sloping hillside for about 0.09 miles over a series of outcrops comprising Gafvert Lake Succession rhyodacite tuffs, lapilli tuffs, and tuff breccias. Observe the subtle changes in crystal content and fragment compositions and abundances while moving down the hill toward the two-track trail. We will reassemble the group on the two-track trail. We will proceed to walk eastnortheast along the two-track road for approximately 0.42 miles. Cross bridge – there will be a large unnamed pond to the east. Continue 0.09 miles up hill to intersection and wait where the two-track road makes a turn from north-trending to east-trending. This is where we parked our truck each day during our 2010 capstone mapping project for the PRC Field Camp. We will now walk northeast along the road for 0.07 miles. Follow the field trip leader approximately 0.01-0.02 miles southeast into the bush to Stop 8. Stop 8. 69 �Stop 8: Gafvert Lake Sequence Tuffs and Lapilli Tuffs Location: T. 62N, R. 15W, sec. 23, SE, NE, Soudan 7.5-minute quadrangle UTM: 559,675E / 5,298,700N We will stop here to observe several small outcrops of the Gafvert Lake Sequence tuffs and lapilli tuffs. These deposits comprise very thickly bedded, light gray, quartzand plagioclase-phyric dacitic to rhyodacitic tuffs and lapilli tuffs. The light gray recrystallized matrix generally contains 10-15% <1-2mm subhedral to euhedral tabular plagioclase phenocrysts which locally appear to be broken, as well as 3-8% <1-2mm pale gray anhedral, locally broken, anhedral to subhedral quartz phenocrysts. Various types of lapilli may be observed, including: 1) 10-20% 1-3cm diameter quartz- and plagioclase-phyric coherent dacite to rhyodacite lapilli; 2) 5-7% <3cm diameter pale gray green, lens-shaped, locally quartzand plagioclase-phyric pumice lapilli; 3) <1mm dark gray to light gray angular chert lapilli ranging from 0.5-3cm in diameter; and 4) 1-3% 0.5-5cm dark gray to black magnetite-rich banded iron formation lapilli. 3.50 miles 3.51 miles 4.00 miles Traverse approximately 0.01 miles north through the bush back on to the two-track road. Proceed northeast, then northwest, then northeast along the two-track trail for approximately 0.48 miles. We will assemble the group at this location before the group follows the leader on a 0.01mile traverse north-northwest into the bush to Stop 9. Stop 9. Stop 9: Quartz ± plagioclase-phyric Rhyodacite Sill (informally named the Gafvert Lake Intrusive Complex) Location: T. 62N, R. 15W, sec. 24, NW, NW, Soudan 7.5-minute quadrangle UTM: 560,015E / 5,299,175N At this location we will observe a spectacular light gray, massive, quartz- ± plagioclase-phyric coherent rhyodacite which, based on regional mapping (Peterson and Jirsa, 1999; Peterson, 2001; Hudak et al., 2002b; Heim et al., 2011) comprises a sill-dike complex that extends from the northern extents of Lake Vermilion State Park over 20km eastward to Mitchell Lake. This intrusion is most prevalent in the vicinity of Gafvert Lake, where it comprises several sills and dikes that intrude into the thickest section of Gafvert Lake Sequence volcaniclastic rocks. Based on the distribution of sills and dikes, coherent-facies Gafvert Lake Sequence deposits, and an abundance of coarse polymict breccias in this region, Peterson (2001) has interpreted this area to be the remnants of a stratovolcano that produced the Gafvert Lake Sequence dacitic to rhyodacitic volcaniclastic rocks. For this reason, this unique quartz-feldspar porphyry intrusion has been informally named the Gafvert Lake Intrusive Complex (GLIC). Lithogeochemical work recently completed at the University of Wisconsin Eau Claire by Geoff Pignotta and Kelly Schwierske indicates that the GLIC and Gafvert Lake volcaniclastic rocks have very similar major, trace and rare earth element characteristics suggesting that they may be genetically related. However, geochronological studies will need to be 70 �performed to determine unambiguously if the GLIC and Gafvert Lake volcaniclastic rocks are genetically related. The GLIC comprises light gray, massive, quartz ± plagioclase-phyric coherent rhyodacite. The light gray aphanitic groundmass contains 3-7% gray to light blue subhedral rounded to euhedral square quartz phenocrysts that range from 3-10mm in diameter, and 10% pale gray to tan, subhedral to euhedral tabular plagioclase phenocrysts ranging from 1-4mm in length. A variety of xenoliths may be found in this intrusion, including: 1) brown mudstone lapilli; 2) green to gray-green massive and/or amygdaloidal basalt lapilli; and 3) light gray aphyric coherent rhyodacite lapilli. In the field, the presence of large 5mm-10mm diameter gray to blue gray quartz phenocrysts distinguishes the GLIC from other quartzfeldspar-porphyry intrusions in the Vermilion District. 4.00 miles 4.01 miles 4.59 miles Traverse south through the bush 0.01 miles back to the two-track trail. Once you return to the two-track trail, walk 0.58 miles east-northeast. At the end of the two-track trail you will intersect a well-maintained dirt road. The bus will be parked at this location. Obtain refreshments and get on the bus. End of traverse log Bus Log (Continued) 56.9 miles Pick up field trip participants after their traverse through the park. Drive east on wellmaintained dirt road 0.48 miles back to the intersection with Old Ely Road. 57.7 miles Turn right (southwest) and follow the Old Ely Road approximately 1.3 miles to the intersection with the dirt road immediately before the gate to Lake Vermilion State Park. 59.0 miles Turn south on dirt road and return to Highway 1/169. 59.1 miles Turn west on Highway 1/169 South. Proceed approximately 5.2 miles past Soudan and through Tower to the intersection between Highway 1/169 and Highway 135. 64.3 miles Pull bus off on to shoulder of Highway 1/169 South just west of the intersection of Highways 1/169 and Highway 135. The outcrop on the south side of the road is Field Trip Stop 10. Stop 10: Recently Exposed Outcrop of Gafvert Lake Sequence Lapilli-tuffs and Tuff-Breccias Location: T. 62N, R. 15W, sec. 32, SW, SW, Tower 7.5-minute quadrangle UTM: 553,500E / 5,294,510N NOTE: Take extreme care when crossing the highway at this location! The final stop on our field trip is to a recently exposed (~2012) outcrop comprising polymict Gafvert Lake Sequence tuff, lapilli-tuff and tuff-breccia deposits. Although never mapped in detail by any of the coauthors in this guidebook, this exposure appears to contain several individual volcaniclastic units that may be distinguished by the size and abundance of the phenocrysts and fragments present. The pale gray to gray aphanitic matrix contains variable percentages and sizes of quartz and plagioclase phenocrysts. Locally, abundant (up to 10%) <1mm euhedral pyrite cubes are disseminated in the matrix. Fragment 71 �composition is also variable, with gray chert, light gray quartz- and plagioclase-phyric coherent rhyodacite, light gray to tan pumice, and rare massive pyrrhotite fragments present. The rock also possesses a moderately- to well-defined schistosity with lineations that plunge moderately to steeply to the northeast (Sims, 1973). 64.3 miles 112.5 miles Field trip participants should return to bus. Take extreme care when crossing Highway 1/169. Follow Highway 1/169 south and retrace route to Vermilion District back to the Hibbing Park Hotel in Hibbing, Minnesota. End of field trip at parking lot in Hibbing Park Hotel. Acknowledgements Characterizing and evaluating the detailed geology of Lake Vermilion State Park involved a team effort between Minnesota Department of Natural Resources (MDNR) personnel, the Minnesota Geological Survey, NRRI geologists, and students and faculty from the Precambrian Research Center Field Camp as well as the Univesity of Wisconsin Eau Claire. The lead author would like to thank Jim Essig (Manager, Soudan Underground Mine State Park and Lake Vermilion State Park) and James Pointer (Interpretive Supervisor, Soudan Underground Mine State Park and Lake Vermilion State Park) from the MDNR for their support, assistance, and guidance while planning and conducting detailed geological mapping by PRC students and faculty in Lake Vermilion State Park in 2010 and 2011. Also, Minnesota Geological Survey geologists Amy Radakovich, Mark Jirsa, and Terry Booerboom are thanked for their assistance (and patience!) during the development of this field trip guide. As well, Dean Peterson, the late Richard Patelke, Mark Severson, John Heine, Peter Jongewaard, Steve Hovis and Adam Hoffman are thanked for their excellent mapping in the southern part of what was to become Lake Vermilion State Park. This work by former and current NRRI colleagues became the foundation upon which new mapping in the park was based. Additionally, Geoff Pignotta, Kelly Schwierske, and the Department of Geology at the University of Wisconsin, Eau Claire are thanked for intellectual efforts and financial support to further evaluate the geology and geochemistry of Lake Vermilion State Park. Finally, PRC students Chris Heim, Rob Kilduff, Chris Mahr, Charlie Parent, Molly Partidge, Rita Pierce, Amy Radakovich, Christine Rahtz, Andrew Ritts, Heather Scott and Andrew Vial are thanked for their outstanding field mapping, compiling, computer map generation, and companionship during the four weeks in 2010 and 2011 that it took to produce the recent detailed geologic maps in Lake Vermilion State Park. Without these exceptional students, our knowledge of the fascinating geology of Lake Vermilion State Park would not be nearly what it is today. REFERENCES Bakst, B., 2013, Minnesota’s Lake Vermilion State Park evolving at a cautious pace: TwinCities.com, Pioneer Press, http://www.twincities.com/localnews/ci_23939683/lake-vermilion-state-parkminnesotas-newest-evolving-at. Batiza, R., and White, J. D. L., 2000, Submarine lavas and hyaloclastite: in Sigurdsson, H., Encyclopedia of Volcanoes: Academic Press, San Diego, CA, p. 361-381. Cas, R. A. F., and Wright, J. V., 1987, Volcanic Successions – Modern and Ancient: George Allen and Unwin, London, 528 p. Dimroth, E., Cousineau, P., Leduc, M., and Sanschagrin, Y., 1978, Structure and organization of Archean subaqueous basalt flows, Rouyn-Noranda area, Quebec: Canadian Journal of Earth Sciences, v. 15, p. 90918. Driese, S. G., Jirsa, M. A, Ren, M., Brantley, S. L., Sheldon, N. D., Parker, D., and Schmitz, M. 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M., 2004, Trace element geochemistry and petrogenesis of felsic volcanic rocks associated with volcanogenic Cu-Zn-Pb sulfide deposits: Economic Geology, v. 99, p. 1003-1013. Heim, N., Scott, H., Kilduff, R., Rahtz, C., Vial, A., Young, S., Mahr, C., and Hudak, G., 2011, Preliminary bedrock geology map of the eastern part of Lake Vermilion State Park, St. Louis County, NE Minnesota: Precambrian Research Center Map Series Map PRC/Map – 2010-01, 1:5000 scale. Hoffman, A. T., 2007. Lithostratigraphy, Hydrothermal Alteration, and Lithogeochemistry of Neoarchean Rocks in the Lower and Soudan Members of the Ely Greenstone Formation, Vermilion District, NE Minnesota: Implications for Volcanogenic Massive Sulfide Deposits: unpublished M. S. thesis, University of Minnesota – Duluth, 295 p. Hudak, G. J., Heine, J., Hocker, S. M., and Hauck, S., 2002b, Geological mapping of the Needleboy Lake – Sixmile Lake area, northeastern Minnesota: a summary of volcanogenic massive sulfide potential: Natural Resources Research Institute Report of Investigation NRRI/RI-2002/14, 15 p. Hudak, G.J., Heine, J., Jirsa, M.A., and Peterson, D.M., 2004, Field Trip 1 - Volcanic stratigraphy, hydrothermal alteration, and VMS potential of the Lower Ely Greenstone, Fivemile Lake to Sixmile Lake area: 50th Annual Meeting, Institute on Lake Superior Geology, Proceedings Volume 50, Part 2 – Field Trip Guidebook, p. 1-45. Hudak, G. J., Heine, J., Lodge, R. W. D., and Jansen, A., 2012, Recent developments understanding the volcanic, magmatic, tectonic, and metallogenic evolution of the Ely Greenstone Formation, Vermilion District, NE Minnesota: Geological Association of Canada – Mineralogical Association of Canada, Abstracts and Program, v. 35, p. 59. Hudak, G. J., Heine, J., Newkirk, T., Odette, J., and Hauck, S., 2002a, Comparative geology, stratigraphy, and lithogeochemistry of the Fivemile Lake, Quartz Hill, and Skeleton Lake VMS occurrences, Vermilion district, NE Minnesota: A report to the Minerals Coordinating Committee, DNR, Minerals Division, State of Minnesota: Natural Resources Research Institute Technical Report NRRI/TR2002/03, 390 pages. Hudak, G. J., Hocker-Finamore, S. M., and Heine, J., 2006, Field distribution, petrography, and lithogeochemistry of epidosites in the vicinities of Fivemile, Needleboy, and Sixmile Lakes, Vermilion District, NE Minnesota: 52nd Annual Meeting, Institute on Lake Superior Geology, Proceedings Volume 57, Part 1 – Programs and Abstracts, p. 30-31. Hudak, G. J., Hoffman, A. T., Peterson, D. M., and Heine, J., 2007, Recent developments understanding the volcanic, magmatic, tectonic, and metallogenic evolution of the Ely Greenstone Formation, Vermilion District, NE Minnesota: 53rd Annual Meeting, Institute on Lake Superior Geology, Proceedings Volume 53, Part 1 – Program and Abstracts, p. 443. Jansen, A. C., Hudak, G. J., Heine, J. J., and Peterson, D. M., 1999, Lithogeochemical evaluation of Neoarchean mafic volcanic rocks comprising the footwall to the Soudan Member of the Ely Greenstone Formation, northeastern Minnesota: 55th Annual Meeting, Institute on Lake Superior Geology, Proceedings Volume 55, Part 1 – Program and Abstracts, p. 46-47. Jirsa, M. A., Boerboom, T. J., and Peterson, D. M., 2001, Bedrock geological map of the Eagles Nest Quadrangle, St. Louis County, Minnesota: Minnesota Geological Survey, Miscellaneous Map M-114, scale 1:24,000. Jirsa, M. A., Boerboom, T. J., Green, J. C., Miller, J. D., Morey, G. B., Ojakangas, R. W., and Peterson, D. 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Larson, P., and Mooers, H., 2009, Field Trip 2 – Glacial geology of the Vermilion Moraine: 55th Annual Meeting, Institute on Lake Superior Geology, Proceedings Volume 55, Part 2 – Field Trip Guidebook, p. 81-99. Lodge, R. W. D., Gibson, H. L., Stott, G. M., Hudak, G. J., Jirsa, M. A., and Hamilton, M. A., 2013, New U-Pb geochronology from the Timiskaming-type assemblages in the Shebandowan and Vermilion greenstone belts, Wawa Subprovince, Superior Craton: Implications for the Neoarchean development of the southwestern Superior Province: Precambrian Research, v. 235, p. 264-277. McPhie, J., Doyle, M., and Allen, R., 1993, Volcanic Textures: A Guide to the Interpretation of Textures in Volcanic Rocks: CODES Key Centre, University of Tasmania, Hobart, Tasmania, 198 p. Mercier-Langevin, P., Hannington, M. D., Dubé, B., and Bécu, V., 2010, The gold content of volcanogenic massive sulfide deposits: Mineralium Deposita, v. 46, p. 509-539. Moosavi, S., Johnson, T., Wendland, C., Anderson, A., and Hudak, G., 2007, Bedrock geology map of the footwall of the Soudan Iron Formation south of Twin Lakes, St. Louis County, northeastern Minnesota: Precambrian Research Center Map Series Map PRC/Map – 2007-04, 1:5000 scale. Mueller, W. U., and White, J. D. L., 2004, 4.2 – Terminology of Volcanic and Volcaniclastic Rocks: in Eriksson, P. G., Altermann, W., Nelson, D. R., Mueller, W. U., and Catuneanu, O., (eds.), The Precambrian Earth: Tempos and Events: Developments in Precambrian Geology, v. 12, Elsevier, Amsterdam, p. 273-277. Peterson, D. M., 2001, Development of Archean lode-gold and massive sulfide deposit exploration models using geographic information system applications: targeting mineral exploration in northeastern Minnesota from analysis of analog Canadian mining camps: unpublished Ph. D. dissertation, University of Minnesota, Duluth, Minnesota, 503 p. Peterson, D. M., and Jirsa, M.A., 1999, Bedrock geologic map and mineral exploration data, western Vermilion district, St. Louis and Lake Counties, northeastern Minnesota: MGS Miscellaneous Map M-98, scale 1:48,000. Peterson, D., Jirsa, M., and Hudak, G., 2009a, Field Trip 7 – Architecture of an Archean Greenstone Belt: Stratigraphy, Structure, Mineralization: 55th Annual Meeting, Institute on Lake Superior Geology, Proceedings Volume 55, Part 2 – Field Trip Guidebook, p. 178-215. Peterson, D. M., and Patelke, R. L., 2003, National Underground Science and Engineering Laboratory (NUSEL): Geological site investigation for the Soudan Mine, northeastern Minnesota: Natural Resources Research Institute Technical Report NRRI/TR-2003/29, 88 p.a Peterson, D. M., and Patelke, R. L., 2004, Field Trip 7 – Economic geology of Archean gold occurrences in the Vermilion District, northeast of Soudan, Minnesota: 50th Annual Meeting, Institute on Lake Superior Geology, Proceedings Volume 50, Part 2 – Field Trip Guidebook, p. 200-226. Peterson, D. M., Pointer, J., and Marshak, M., 2009b, Field Trip 3 – Soudan Iron Mine and Physics Lab Tour: 55th Annual Meeting, Institute on Lake Superior Geology, Proceedings Volume 55, Part 2 – Field Trip Guidebook, p. 100-109. Pearce, J. A., 1996, A user’s guide to basalt discrimination diagrams: in Wyman, D. A., ed., Trace Element Geochemistry of Volcanic Rocks: Applications for Massive Sulphide Exploration: Geological Association of Canada, Short Course Notes, v. 12, p. 79-113. 74 �Philips, W. R., and Griffen, D. T., 1981, Optical Mineralogy – The Nonopaque Minerals: W. H. Freeman and Company, San Francisco, 677 p. Radakovich, A. L., Parent, C. T., Partridge, M. E., Ritts, A. D., Pierce, R., and Hudak, G. J., 2010, Reconnaissance bedrock geological map of the northern part of Soudan Underground Mine State Park and the northwestern part of Lake Vermilion State Park, St. Louis County, Minnesota: Precambrian Research Center Map Series Map PRC/Map – 2010-04, 1:5000 scale. Ross, P.-S., and Bédard, J. H., 2009, Magmatic affinity of modern and ancient subalkaline volcanic rocks determined from trace-element discrimination diagrams: Canadian Journal of Earth Sciences, v. 46, no. 11, p. 823-839. Schmid, R., 1981, Descriptive nomenclature and classification of pyroclastic deposits and fragments; recommendations of the IUGS subcommission on the systematics of igneous rocks: Geology, v. 9, p. 41-43. Schwierske, K.L., Pignotta, G. S., and Hudak, G. J., in press, The 2.7 billion year old Mt. St. Helens of northern Minnesota: Petrography, geochemistry, and economic significance of the Neoarchean Gafvert Lake Sequence: 60th Annual Meeting, Institute on Lake Superior Geology, Proceedings Volume 60, Part 1 – Programs and Abstracts. Sims, P. K., 1973, Geologic map of the western part of the Vermilion District, Northeastern Minnesota: Minnesota Geological Survey, Miscellaneous Map M-13, scale 1:48,000. Sims, P. K., and Southwick, D. L., 1985, Geologic map of Archean rocks, western Vermilion district, northern Minnesota: U. S. Geological Survey, Miscellaneous Investigations Map I-1527, scale 1:48,000. Southwick, D. L., (compiler), 1993, Bedrock geologic map of the Soudan-Bigfork area, northern Minnesota: Minnesota Geological Survey, Miscellaneous Map M-79, scale 1:100,000. Southwick, D. L., Boerboom, T. J., and Jirsa, M. A., 1998, Geologic setting and descriptive geochemistry of Archean supracrustal and hypabyssal rocks, Soudan-Bigfork area, northern Minnesota: implications for metallic mineral exploration: Minnesota Geological Survey, Report of Investigations 51, 69 p. Stott, G., Corkery, T., Leclair, A., Boily, M., and Percival, J., 2007, A revised terrane map for the Superior Province as interpreted from Aeromagnetic Data: 53rd Annual Meeting, Institute on Lake Superior Geology, Proceedings Volume 53, Part 1 – Program and Abstracts, p. 74-76. Stott, G. and Mueller, W., 2009, Superior Province: The nature and evolution of the Archean continental lithosphere: Precambrian Research, v. 168, p. 1-3. Vallowe, A. M., Thalhamer, E. J., Rhoades, D. L., and Peterson, D. M., 2010, Surface and subsurface geologic maps of the Soudan Underground Mine State Park, St. Louis County, northeastern, Minnesota: Precambrian Research Center Map Series Map PRC/Map – 2010-01, 1:2500 and 1:5000 scale. White, J. D. L., and Houghton, B. F., 2006, Primary volcaniclastic rocks: Geology, v. 34, no. 8, p. 677680. Winchester, J. A., and Floyd, P. A., 1977, Geochemical discrimination of different magma series and the differentiation products using immobile elements: Chemical Geology, v. 20, p. 325-343. 75 ��FIELD TRIP 3 Wednesday, May 14, 2014 WESTERN MESABI RANGE MINING OPERATIONS LEADERS: Douglas Halverson (Cliffs Natural Resources—Duluth) Daniel Cervin (Cliffs Natural Resources—Hibbing Taconite), William Everett and Kevin Kangas (Essar Steel); and Joseph Nielsen (Magnetation). INTRODUCTION This field trip will visit three distinct iron mining operations along the western part of the Mesabi Iron Range (Fig. 1). The morning will be spent at the Hibbing Taconite operation managed by Cliffs Natural Resources. There, participants will get a pit tour of the mining and reclamation operations, followed by a tour of the processing facility. The afternoon will include a tour through the new construction of Essar Steel’s taconite processing facility near Nashwauk (near the old Butler Taconite site), and may view some core that intersects strata including what is inferred to be the 1850 Ma Sudbury Impact Layer. This will be followed by a tour of Magnetation’s two-year old, 1.2 million ton per-year facility located near Bovey, where iron concentrate is being extracted from the tailings of historic mining of natural (hematite) ores. Figure 1. Bedrock geologic map of the western Mesabi Iron Range showing the 3 operations that will be visited during this trip. Map is clipped from MGS Miscellaneous Map M-163 (Jirsa and others, 2005; published scale=1:100,000). Reddish unit represents subsurface extent of Biwabik Iron Formation. 76 �HIBBING TACONITE PIT AND PLANT Hibbing Taconite Company, managed by Cliffs Natural Resources Doug Halverson and Dan Cervin (Cliffs Natural Resources) Field Trip participants will get a pit tour of the mining and reclamation operations. There may be an opportunity to view a blast, dependent upon blasting schedule and blast location. Hibbing Taconite Company (HTC) is managed by Cliffs Natural Resources, an international mining and natural resources company. Cliffs is the largest producer of iron ore pellets in North America, a major supplier of directshipping lump and fines iron ore out of Australia and a significant producer of metallurgical coal. The annual production capacity of Hibbing Taconite Company is 8.0 million tons of taconite pellets, operating 24 hours per day, year-round, employing 770 people. Through 2013, HTC has produced 260 million gross tons of pellets. HTC is jointly owned by Arcelor Mittal (62.3%), Cliffs Natural Resources (23%), and US Steel Canada (14.7%). The HTC plant is located approximately four miles northwest of Hibbing, Minnesota (Fig.2), just north of the Laurentian Divide. The initial taconite pit was developed in 1975. Since inception, this pit has expanded east, west, and south along the northern crest of the historic Hull-Rust Mahoning natural ore mine. The Hull-Rust Mahoning Mine, actually a combination of 30 separate mines, was developed along an east/west-trending fault structure and operated from 1895 to 1979. Material movement from this "largest open pit iron mine in the world" totaled more than 1.1 billion tons. Figure 2. Airphoto image of Hibbing Taconite’s plant, tailings facility, and mines that extend nearly 7 miles along the strike of iron-formation. The four main subdivisions of the Biwabik Iron Formation are present in the vicinity of HTC. From bottom to top they are Lower Cherty, Lower Slaty, Upper Cherty, Upper Slaty. Erosion has removed the Upper Slaty and most of the Upper Cherty members within the area of the current pit. Where present, the 77 �Lower Slaty member and the upper 30 feet of the Lower Cherty member are stripped as rock waste. Approximately 150 feet of cherty and slaty taconite is mined from the central portion of the Lower Cherty. The formation strikes northeast and dips 6°-8° to the southeast. The Pokegama Quartzite forms the footwall of the iron-formation and outcrops to the north, along the south edge of the divide. The only significant structural features are common, but minor, northwest-trending normal faults. Numerous natural ore mines were located along these oxidized structures. The taconite mined by HTC averages 20 percent magnetic iron, with the general mineralogy consisting of quartz, magnetite, siderite, ankerite, minnesotaite, stilpnomelane and hematite. Ore units in the Lower Cherty (120 feet thick) are predominantly "cherty" taconite, with 6- to 12-inch-thick massive silicate-chert zones separated by 1/8- to 2-inch-thick slaty bands. The lower two units (30 feet thick) are predominantly "slaty" taconite with inter-bedded argillite, magnetite, and minor hematite forming slaty bands from 2-10 inches in thickness separated by 2- to 4-inch massive cherty zones. Stripping materials include glacial overburden, waste rock, lean oxidized taconite, and old stockpiles, all varying widely in thickness from area to area. Standard rotary drilling, blasting, electric shovel loading, and 240-ton truck haulage are the mining methods utilized. The processing flowsheet differs significantly from the standard Mesabi Range taconite plant in the area that involves crushing and grinding. By contrast, the HTC plant utilizes autogenous mills, which do not contain grinding media. A single stage of gyratory crushing in one of two 60-inch crushers reduces the crude to 10 inches. This is followed by autogenous grinding in one of nine 36-foot¬diameter mills. Water is added and as the ore tumbles, it reduces itself to powder fineness. Field Trip Stops at Hibbing Taconite: Stop 1 – The first stop in the field trip will be in Hibbing Taconite’s Group 4 to view the stratigraphy of the Biwabik Iron Formation at the mine site. In this currently inactive portion of the pit, overlying stockpiles from natural ore mines, glacial till, Lower Slaty rock stripping and Lower Cherty ore horizons are exposed in the high wall of the mine pit. Opportunity for the collection of typical cherty taconite ore will be available at this site. Stop 2 – The tour will travel east to Group 2 and view the “footwall” of the mine. Opportunity to collect samples of more slaty, jasper rich ore horizons will be available while discussing the in-pit enrichment processing that is used to improve these horizons prior to plant processing. Stop 3 – The tour will continue east to view active mining activities in Group 1. Typical mine production activities such as production drilling, loading and hauling will be observed from this location. Stop 4 – View a production blast or plant tour. Depending on the production blasting schedule and the visibility of the blast from a safe location the tour may have the opportunity to view a production blast, typically involving 500,000 to 1,000,000 tons of taconite ore or overlying rock. Alternate - If scheduling or location does not allow the viewing of a blast, the tour will view the Hibbing Taconite processing plant. Portions of the process that include crushing, grinding, concentration, balling and induration will be toured to show the flow of material from crude ore to finished product. Stop 5 – Lunch at mine view in old North Hibbing. The scale of the Hibbing Taconite mining operation will be seen from this scenic overview . 78 �ESSAR STEEL’S NEW TACONITE PIT AND PLANT Essar Steel Minnesota, LLC, Nashwauk, MN William Everett (Area Manager of Mining) Essar Steel Minnesota, LLC (ESML) is an iron ore mining company engaged in the development of a fully integrated iron ore mine and pellet plant located on the western end of the Mesabi Range. The Project is adjacent to the City of Nashwauk located in Itasca County, approximately 15 miles (24 km) west of Hibbing and 20 miles (32 km) east of Grand Rapids. This mining project is probably the last major taconite facility to be built on the Mesabi Range. The facility construction is nearly 65% completed and will cost $1.8 billion dollars when finished, with an expected design capacity of 7.0 million tons per year of fluxed, standard and DR-grade pellets. All of the required permits required for construction and operation are in place for the designed capacity. The Crushing & Concentration Facility is separated from the Pelletizing Facility, as shown in the Figure 3. When completed, we believe this mining operation will be one of the lowest cost iron ore pellet producers in North America. Figure 3 – Essar Steel Minnesota site layout. The taconite resource at this project site was originally mined by Butler Taconite Mining Company, a company managed by Hanna Mining Company. Butler Taconite was a jointly owned mining operation and was in production from 1967 to 1985. When one of the owners declared bankruptcy, the other two owners closed the operation, and the facility underwent demolition. In 2007, Essar Steel Holdings acquired Minnesota Steel Industries (MSI), a development company owned by some of the mineral owners on the property. In 2008, Essar renamed the MSI to Essar Steel Minnesota, LLC (ESML) The ESML deposit is a low grade iron-formation with magnetite as the predominate resource. In 2011, ESML conducted a diamond drilling program to bring the deposit into compliance with Canadian 79 �NI43-101 ore reserve standards. A total of 63 diamond drill holes were drilled across the length of the deposit totaling 41,720 feet. The new drilling information combined with the historic drilling, defined the ore zone over the strike length and down dip of the deposit. The drilling program identified 1.7 billion tons of proven and probable ore, having an average stripping ratio of 1.69 and an average weight recovery of 29.1%. Within the project area, the Biwabik Iron Formation is underlain by the Pokegama Quartzite and is overlain by the Virginia Formation. The Biwabik Iron Formation subcrop and the Virginia Formation are overlain by scattered Cretaceous marine deposits, and all these formations are covered by glacial drift. The Biwabik Formation strikes generally E-NE (065°) with a 5° S-SE dip. The stratigraphy in the ESML Project area was characterized in detail by Hanna Mining Company geologists. The Biwabik Iron Formation has four distinct members: Upper Slaty Member – Slaty non-magnetic taconite; Upper Cherty Member – Cherty weakly magnetic taconite; Lower Slaty Member – Slaty non-magnetic taconite; and Lower Cherty Member – Cherty magnetic taconite. The Lower Cherty Member has been divided into ten distinct subunits. The ore zone lies in the LC4A, LC4B, LC4C, and LC5A subunits of the Lower Cherty Member, and averages about 200 feet thick. The La Rue fault system runs along the strike length of the formation, bisecting the historic pits within the project area. The Patrick Shear Zone crosscuts the deposit between Pits 2 and 5 of the original Butler Taconite pits. The taconite has been locally oxidized along these fault zones. Figure 4 is a map depicting the geology and historic drilling across the project site. A basic geologic column at the ESML Project site is shown in Figure 5. Figure 4. Map of bedrock geology and historic drilling (yellow dots) within the project site. Geologic contacts are black; faults are red. 80 �Figure 5. Geologic Column at the Essar property. During the 2011 diamond drilling campaign, down-dip drill holes intersected a well-defined contact between the Virginia Formation and the Upper Slaty Horizon of the Biwabik Iron Formation. This very distinctive contact zone is characterized by a chaotic mix of broken and deformed strata, uncharacteristic for the uniformly bedded strata above and below this zone. The zone of turbulence matches contacts found in the Gunflint Iron Formation which are attributed to the Sudbury Meteorite Impact event. Above the zone, little evidence of magnetic beds would seem to suggest a disruption in the deposition of ferrous iron minerals by this catastrophic event. Figure 6 provides photographic documentation of the contact in two separate diamond drill holes. 81 �Figure 6. Images of core that intersected the Upper Slaty – Virginia Contact. 82 �Taconite mining will start with a development cut adjacent to the new crushing complex and progress down dip into the old Butler Pit 5 taconite pit. Pumping is presently in progress to remove water from the historic mine pit. To date, the water level in Pit 5 has been lowered approximately 60 feet, with another 80 feet needed to reach pit bottom. The mine will be developed using a level bench configuration, hydraulic shovels, and a small fleet of 240-ton haul trucks. The initial mining area has been pre-stripped of glacial overburden north of the Butler Pit 5, and the first blast will be directly in the taconite production zone, as shown in Figure 7 (labeled “Mine Development”). Figure 7. Map showing facilities and planned location for initial mine development adjacent to the historic Butler Taconite pit. 83 �MAGNETATION Joe Nielsen (Magnetation Inc.). This trip will visit a unique new iron ore venture currently operating in the Bovey area. Founded in 2006, the privately-held Magnetation Inc. was created with the intention of utilizing magnetic separation technology to capture iron ore particles left over from previous mining operations that existed on the iron range dating back to the 1890s. Owners Al Fritz and Rod Hunt focused the company’s early efforts on research and development of a beneficiation process centered on the Ferrous Wheel®, a technology Al Fritz invented in the 1970s that uses permanent magnets to separate iron ore from waste materials and produce an upgraded iron ore concentrate. The location near Bovey is the second of three plants operating on the Mesabi Iron Range; the others are near Keewatin and Chisholm. The Bovey plant commenced operations in May 2012, and produces about 1.2 million tons per-year of iron ore concentrate. The operations consist of excavation and transport of iron-bearing tailings to the concentrator facility, where the iron-rich portions are reclaimed. The resulting iron ore concentrate is then trucked to the Jessie LoadOut, where it is shipped by rail to Magnetation customers. A new pelletizing plant under construction in Reynolds, Indiana, will begin processing ore concentrate late 2014. The Ore Magnetation mines the iron ore tailings from mining days long ago. We dig up all the discarded tailings in the Pit and bring them to the Plant one truck load at a time. In the plant the tailings go through various stations to become Iron Concentrate. The trucks then take the Concentrate to our train loading station to ship to our customers. The Pit Unlike a conventional open mine pit our material requires no drilling and blasting. We dig up what was left behind as waste, remove the iron, and return the material back to the basin to be eventually replanted with vegetation. The Plant In the plant, we are using Magnetation’s unique processes and equipment to remove the iron from our feed material. We are constantly adjusting processes to accommodate the variations in feed from the pit and continually improving to further the iron yield from the feed material. www.magnetation.com 84 ��FIELD TRIP 5 Saturday, May 17, 2014 VISIONS OF MATURI: THE GEOLOGY OF THE SOUTH KAWISHIWI INTRUSION LEADER: Dean M. Peterson (Duluth Metals, Ltd.) INTRODUCTION Twin Metals Minnesota (TMM), a private joint venture company owned by Duluth Metals Ltd (60%) and Antofagasta plc (40%), is currently finishing a robust prefeasibility study (due mid 2014) to develop the Maturi Cu-Ni-PGE deposit in the northern part of the South Kawishiwi Intrusion (SKI). The mineralization at Maturi is confined to the basal 500’ of the intrusion and has been the focus of numerous ILSG-based field trips in recent years. In October 2013, UMD’s Precambrian Research Center hosted a workshop on Cu-Ni-PGE deposits in the Lake Superior area and the Duluth Complex fieldtrip guidebook of that workshop (Severson et al., 2013) is perhaps the most up-to-date geologic description of Duluth Complex Cu-Ni-PGE deposits. As such, this field trip is focused mainly on the “rest of the rocks” of the SKI as a way to put the Cu-Ni-PGE ores in better context to the vast majority of the rocks of the intrusion. The importance of understanding these rocks and the overlying glacial deposits will be ever increasing as the TMM Project goes into bankable feasibility, environmental review and permitting, since virtually all of the water in the region (surficial and deep groundwater) interacts mostly with the “other rocks” of the SKI. Duluth Metals’ understanding of the SKI has been facilitated by extensive bedrock geologic mapping in the local region (22,200 outcrops mapped), drilling (2,300,000 feet in 1,556 holes), geochemistry (4,500 tills, 1,800 rocks, and ~110,000 drill core assays), and geophysics (3 recent airborne VTEM surveys covering 178,600 acres, borehole EM surveys, Titan-24 survey, and RIM cross-hole imaging). REGIONAL GEOLOGIC SETTING, DULUTH COMPLEX The Duluth Complex and associated intrusions of Keweenawan age (~1.1 billion years) in northeastern Minnesota constitute one of the largest mafic intrusive complexes in the world, second only to the Bushveld Complex of South Africa (Miller et al., 2002). These rocks cover a 2,200 square mile (5,700 square km) arcuate area associated with the two strongest gravity anomalies (+50 and +70 milligals) in North America, implying intrusive roots over 8 miles (13 km) deep (Allen and others, 1997). The comagmatic flood basalts and intrusive rocks underlying much of northeastern Minnesota were emplaced during development of the Mesoproterozoic Midcontinent rift, which can be traced geophysically from exposures in the Lake Superior region along a 1250 mile (2,000 km) long, segmented, arcuate path to Kansas and Lower Michigan. The Duluth Complex is defined as the more or less continuous mass of mafic to felsic plutonic rocks that extends for >170 miles (275 km) in an arcuate fashion from Duluth nearly to Grand Portage (Fig. 1). It is bounded by a footwall of Paleoproterozoic sedimentary rocks and Archean granite-greenstone terranes (Peterson and Severson, 2002), and a hanging wall largely of comagmatic, rift-related flood basalts and hypabyssal intrusions of the Beaver Bay Complex. In genetic terms, the Duluth Complex is composed of multiple discrete intrusions of mafic to felsic tholeiitic magmas that were episodically emplaced into the base of a volcanic edifice between 1108 and 1098 Ma. The geology of the Duluth Complex and adjacent areas has recently been described in two major publications by the Minnesota Geological Survey (MGS). These include a 1:200,000 scale regional bedrock geological map of northeastern Minnesota (Miller et al., 2001), and a comprehensive written description of the geology depicted on this map (Miller et al., 2002), commonly referred to as the “bible” by geologists working on Duluth Complex geology. Readers’ interested in more detailed descriptions of 86 �the geologic setting of the Duluth Complex should begin their quest for knowledge by downloading these publications from the MGS website (ftp://mgssun6.mngs.umn.edu/pub2/). Within the nearly continuous mass of intrusive igneous rock forming the Duluth Complex, four general rock series are distinguished on the basis of age, dominant lithology, internal structure, and structural position within the complex. Felsic series—Massive granophyric granite and smaller amounts of intermediate rock that occur as a semi-continuous mass of intrusions strung along the eastern and central roof zone of the complex, that were emplaced during early stage magmatism (~1108 Ma). Early gabbro series—Layered sequences of dominantly gabbroic rocks that occur along the northeastern contact of the Duluth Complex, emplaced during early stage magmatism (~1108 Ma). Anorthositic series—Structurally complex suite of foliated, but rarely layered, plagioclase-rich gabbroic anorthosite emplaced throughout the complex during main stage magmatism (~1099 Ma). Layered series—Suite of stratiform troctolitic intrusions that comprises at least 11 variably differentiated mafic layered intrusions that occur mostly along the base of the Duluth Complex. These intrusions were emplaced shortly after the Anorthositic series (~1099 Ma). Figure 1. Generalized geologic map of northeastern Minnesota (modified from Miller et al., 2002). 87 �LOCAL GEOLOGIC SETTING, THE SOUTH KAWISHIWI INTRUSION The SKI consists almost entirely of troctolitic rocks that generally dip gently to the southeast. However, it is not well known that shallow southwesterly dipping troctolite of the upper SKI in the northeastern portion of the intrusion defines an asymmetric funnel-shaped body that emerged from the Nickel Lake macrodike. The basal mineralization of the SKI is exposed in an arc-shaped area that extends from the Serpentine deposit, in the southwest, to the Spruce Road deposit, in the northeast (Fig. 2). Footwall rocks include the Paleoproterozoic Virginia Formation, Biwabik Iron Formation and Archean Giants Range Batholith, the latter is the dominant footwall rock type. Figure 2. Simplified geological and ore deposit map of the northwestern Duluth Complex. GEOLOGIC MAPPING Detailed geological mapping, generally at a scale of 1:5,000 or greater, has been the most important component of Duluth Metals’ understanding of the geology of the SKI. Geologists associated with the company have mapped over 17,000 outcrops (~1,400 total acres of outcrop covering >77,000 acres of ground) within the SKI and adjacent rock units. True understanding of the SKI rocks in their natural environment, In The Field, has led to much improved interpretation of the rocks observed in drill core, the geochemistry of glacial tills, and the interpretation of geophysical studies. Geologists working on the mineral deposits within the Duluth Complex that have not spent a considerable amount of time in the field mapping the rocks will have a difficult time interpreting what they see and log in drill core. Such in the field knowledge is especially important as projects advance and true 3D geological models have to be 88 �constructed for mine planning in feasibility studies. All geological interpretations will be scrutinized and audited once the banks get involved in project financing. A historical account of geological mapping programs within the SKI is presented in Figure 3, and the aerial extent of mapped rock types within the bounds of the SKI are given in Table 1. It is important to note that there are nearly 4,600 acres of sulfide-bearing bedrock exposed on the Earth’s surface within the SKI (1,230 gossanous outcrops mapped). Figure 3. History of geological mapping in the SKI. Table 1. Distribution of rock types exposed on the surface in the SKI. Rock Type Acres % Area 1 6 14 14 206 482 3,112 0.00% 0.01% 0.02% 0.02% 0.31% 0.72% 4.67% Sulfide-bearing Troctolite Augite Troctolite Anorthositic Troctolite 4,594 5,554 20,364 6.89% 8.34% 30.56% Troctolite Grand 32,286 66,633 48.45% 100.00% Diabase dike Iron Formation xenoliths Sandstone xenoliths Ultramafic rocks Gabbroic xenoliths Basalt xenoliths Anorthosite xenoliths 89 �IGNEOUS STRATIGRAPHY & LITHOGEOCHEMISTRY Integration of geological, geochemical, and geophysical data over the last few years by Duluth Metals has resulted, via integration, in a new interpretation of the bulk igneous stratigraphy of the SKI. The proposed new stratigraphy of the intrusion is presented in Figure 4 and consists of five regionally extensive units. From the top down these units include: Upper SKI – Medium to coarsegrained, locally layered troctolite and anorthositic troctolite. Well layered as defined by olivine-rich horizons. SKI Break – Chromium oxide-rich, heterogeneous dunite and melatroctolite. Interpreted to be a magmatic unconformity within the SKI. Middle SKI – Medium to coarsegrained, locally layered troctolite and anorthositic troctolite. Layering defined by olivine. Main AGT – Coarse-grained, homogeneous, augite troctolite with highdensity ophitic Augite grains. This unit is never layered and is interpreted to be the solidified basaltic liquid that carried the phenocrysts and immiscible sulfide droplets of the BMZ. BMZ – Heterogeneous, sulfide-bearing troctolitic rocks. Interpreted to have formed from a sulfide-rich, crystalladen magmatic slurry. Figure 4 Interpreted igneous stratigraphy of the SKI. A simplified composite lithogeochemical compilation profile through the SKI is presented in Figure 5. This geochemical compilation clearly displays the strong correlation of economically important base (Cu, Ni) and precious (Pt, Pd, Au) metals into the basal mineralized zone (BMZ) and adjacent footwall granitoids. In addition, the common 3:1 Cu:Ni ratio of the BMZ is clearly completely different than the vast majority of the intrusion, where in fact Cu averages about 100 ppm and Ni averages about 200 ppm. Strong olivine layering in the Middle and Upper SKI can easily be seen in the Mg % profile and the break between the Middle and Upper SKI is clearly displayed in the Cr (ppm) profile. A geologic cross section roughly along Minnesota State Highway #1 is presented in Figure 6, and integrates hundreds of thousands of feet of drilling into this new regional context. Note the extremely large xenolith of Anorthositic Series rocks and North Shore Volcanic Group lavas in the center of the intrusion. This xenolith of older rocks played an important role in the development of higher grades of Cu, Ni, and PGEs in the Maturi deposit compared to other deposits in the district (Peterson and Boerst, 2013). 90 �Figure 5. Generalized lithogeochemical compilation profile through the SKI. Data from continuous sampling of Duluth Metals drill holes within the western SKI in the Maturi Deposit in 2007 – 2009 (drill holes MEX-072, Mex-109, and MEX-155), and along the northeastern margin of the intrusion in 20122013 (drill holes 12-DM-14, 12-DM-15, and 13-DM-45). Figure 6. Geologic cross section through the northern South Kawishiwi Intrusion. 91 �DRILLING Exploration for Cu-Ni deposits at the base of the Duluth Complex began in 1948, about 12 miles southeast of Ely, MN, when strongly mineralized rocks were uncovered in an excavation used to source road material for Spruce Road. Local prospector Fred S. Childers of Ely noted copper stains in the material and he, along with Roger V. Whiteside of Duluth, began searching along the basal contact in the vicinity of the Kawishiwi River. In 1951, they diamond drilled a 188 foot deep hole and intersected mineralized troctolite that averaged 0.36% Cu and 0.13% Ni. In 1952, the International Nickel Company (INCO) began intensive exploration efforts along the zone that coincided with the basal contact and eventually picked up the Childers-Whiteside properties (Spruce Road and Maturi deposits). Since this initial hole, an additional 1,555 holes (Fig. 7) have been drilled in the intrusion (2,300,000 feet of total drilling) in eight prospective areas (Maturi, Spruce Road, Dunka Pit, Maturi SW, Serpentine, Filson Creek, Birch Lake, and East Shore). Figure 7. Historic perspective of the amount of drilling completed in the SKI since 1951. 92 �ORE DEPOSITS, TWIN METALS MINNESOTA A detailed description of Maturi deposit has recently been published (Peterson and Boerst, 2013) and field trip participants that wish to delve deeper into the geology and geochemistry of the basal mineralized zone (BMZ) should acquire that guidebook. However, since publication of that deposit description, Duluth Metals has received an updated independent NI 43-101 Technical Report completed by AMEC E&C Services Inc. (AMEC) on the Maturi and Maturi SW deposits. The extent of the resource categories for the Maturi, Maturi SW, Birch Lake, and Spruce Road deposits are presented in Figure 8. The updated study utilizes 922 drill holes and 312 wedge offsets, and reports a significant portion of the Maturi deposit upgraded to the Measured Resource category. The mineral resources have been estimated using CIM Definition Standards for Mineral Resources and Reserves dated November 2010. Figure 8. Map of the NI 43-101 qualified resources of Twin Metals Minnesota within the SKI. The majority of the increase in total contained metals in the 2014 resource estimates reflects the addition of the Maturi Southwest Deposit. The updated mineral resources estimate has 295 million tons in the Measured category at a 0.3% copper cut-off in the Maturi Deposit, which may potentially provide an early start-up area for future mining. The change in category for a significant portion of the Indicated Resource to Measured Resource reflects the excellent continuity of the resource demonstrated by the close-spaced fence drilling completed at Maturi. Base case qualified resources for these deposits are given in Table 2, and the combined metal contents for the measured, indicated, and inferred mineral resources are provided in Table 3. The enormity of the metal resource in these deposits is certainly quite staggering and clearly shows why so much time, effort, and money has been spent on these deposits in recent years. 93 �Table 2. Twin Metals Minnesota’s NI 43-101 base case Qualified Mineral Resources Resource Cut-off Tons Cu Deposit Ni Pt Pd (Mst) (%) Name Class Cu (%) (%) (ppm) (ppm) Au (ppm) Maturi Maturi Maturi Measured Indicated Inferred 0.3 0.3 0.3 295 774 562 0.63 0.58 0.51 0.20 0.19 0.17 0.148 0.160 0.138 0.345 0.360 0.317 0.084 0.085 0.071 Maturi Southwest Maturi Southwest Indicated Inferred 0.3 0.3 103 32 0.48 0.43 0.17 0.15 0.080 0.065 0.185 0.157 0.048 0.041 Birch Lake Birch Lake Indicated Inferred 0.3 0.3 100 239 0.52 0.46 0.16 0.15 0.233 0.180 0.511 0.370 0.114 0.087 Spruce Road Inferred 0.3 480 0.43 0.16 Table 3. Contained Metals in the TMM Resource (effective date October 8, 2013) * Metal Copper Nickel Platinum Palladium Gold Measured Resource 3.7 billion lbs. 1.2 billion lbs. 1.3 million ozs. 3.0 million ozs. 0.7 million ozs. Indicated Resource 11.0 billion lbs. 3.5 billion lbs. 4.5 million ozs. 10.2 million ozs. 2.5 million ozs. Measured + Indicated 14.7 billion lbs. 4.7 billion lbs. 5.8 million ozs. 13.2 million ozs. 3.2 million ozs. Inferred Resource 12.3 billion lbs. 4.2 billion lbs. 3.6 million ozs.** 8.0 million ozs.** 1.8 million ozs.** * Based on mineral resources estimated at base case 0.3% Cu cut-off grade; for tons and grade see Tables 2 further below. ** Contained ounces of platinum, palladium, and gold in the Inferred category do not include the Spruce Road deposit. Additional exploration potential highlighted by AMEC outside of the four mineral resources (Maturi, Maturi Southwest, Birch Lake and Spruce Road deposits) and in addition to the TMM defined mineral resource are considered targets for further exploration. An estimate of the exploration potential is between 1.3 to 2.1 billion tons contiguous to the boundaries of the four deposits (Fig. 8). For Maturi and Maturi Southwest, AMEC Assumed that mining, processing and G+A costs would be approximately $15/t, $8/t and $2.50/t respectively for a total of $25.50/t. At Birch Lake and Spruce Road, AMEC assumed that mining, processing and G+A costs would be approximately $16/t, $12/t and $2/t respectively for a total of $30/t. This indicates a breakeven NSR of approximately $30 per ton. Resources meeting an NSR cutoff of $30/t approximately equate to a copper cutoff of 0.3%. The geological subunit within basal mineralized zone (BMZ) in the Maturi Deposit, known as the Stage 3 (Peterson and Boerst, 2013), hosts higher grades ore (172 million short tons at 0.72% Cu, 0.23% Ni, 0.188 ppm Pt, 0.438 ppm Pd and 0.104 ppm Au in the Measured category) that is a subset of the base case mineral resource estimate that may have potential as an early start-up area. The AMEC 2014 Technical Report update on the Measured, Indicated and Inferred categories is presented below for Maturi (Table 4), Maturi SW (Table 5), Birch Lake (Table 6), and Spruce Road (Table 7) deposits. 94 �Table 4. Maturi Deposit Mineral Resources by Copper Cutoff Grade (base case is highlighted) Measured Mineral Resources Cut-off Cu (%) Tons (Mst) Cu (%) Ni (%) Pt (ppm) Pd (ppm) Au (ppm) 0.2 0.3 0.4 0.5 0.6 312.5 295.3 262.2 224.9 174 0.61 0.63 0.66 0.70 0.74 0.20 0.20 0.21 0.22 0.24 0.143 0.148 0.157 0.168 0.179 0.334 0.345 0.366 0.392 0.419 0.081 0.084 0.089 0.094 0.101 Indicated Mineral Resources 0.2 829.4 0.3 774.2 0.4 678.2 0.5 518.2 0.6 366.8 0.56 0.58 0.61 0.66 0.71 0.18 0.19 0.20 0.21 0.22 0.154 0.160 0.171 0.192 0.209 0.345 0.360 0.384 0.431 0.470 0.082 0.085 0.091 0.101 0.109 Inferred Mineral Resource 0.2 804 0.3 562 0.4 399 0.5 266 0.6 147 0.43 0.51 0.57 0.63 0.70 0.14 0.17 0.19 0.20 0.22 0.117 0.138 0.162 0.194 0.233 0.265 0.317 0.370 0.437 0.523 0.060 0.071 0.083 0.097 0.114 *Effective Date is 8 October 2013. *Dr. Harry Parker, RM SME, is the QP for the estimate and is a Professional Geologist licensed in Minnesota. *The resources are based on a US$30/t NSR that assumes a mining cost of $15.00/t, a process cost of $8.00/t and G&A charges of $2.50/t; global metallurgical recoveries of 94.3% (Cu), 60.8% (Ni), 82.3% (Au), 36.1% (Pd), and 42.5% (Pt); and long-term consensus metal prices of $3.00/lb Cu, $9.50/lb Ni, $1,200/troy oz Au, $700/troy oz Pd and $1,650/troy oz Pt. *The NSR equates to a 0.3% Cu cut-off grade. *Figures have been rounded and may not sum. * Mst = million short tons. Table 5. Maturi Southwest Deposit Mineral Resources by Copper Cutoff Grade (base case is highlighted) Indicated Mineral Resources Cut-off Cu (%) Tons (Mst) Cu (%) Ni (%) Pt (ppm) Pd (ppm) Au (ppm) 0.2 0.3 0.4 0.5 0.6 131 103 71 40 16 0.43 0.48 0.53 0.59 0.67 0.15 0.17 0.18 0.20 0.22 0.071 0.080 0.093 0.108 0.124 0.164 0.185 0.217 0.256 0.294 0.042 0.048 0.055 0.064 0.071 Inferred Mineral Resource 0.2 57 0.3 32 0.4 16 0.5 7.2 0.6 3.2 0.35 0.43 0.51 0.60 0.66 0.13 0.15 0.17 0.20 0.22 0.052 0.065 0.082 0.102 0.115 0.126 0.157 0.197 0.251 0.279 0.033 0.041 0.050 0.063 0.069 *Footnotes the same as in Table 4. 95 �Table 6. Birch Lake Deposit Mineral Resources by Copper Cutoff Grade (base case is highlighted) Indicated Mineral Resources Cut-off Cu (%) Tons (Mst) Cu (%) Ni (%) Pt (ppm) Pd (ppm) Au (ppm) 0.2 0.3 0.4 0.5 111.9 99.7 85.4 54.9 0.49 0.52 0.55 0.60 0.15 0.16 0.17 0.18 0.217 0.233 0.247 0.269 0.474 0.511 0.543 0.591 0.106 0.114 0.120 0.130 0.41 0.46 0.51 0.58 0.13 0.15 0.16 0.18 0.156 0.180 0.203 0.228 0.320 0.370 0.423 0.480 0.076 0.087 0.098 0.111 Inferred Mineral Resource 0.2 313.1 0.3 239.2 0.4 158.4 0.5 76.8 * Effective Date is 15 September 2012. * Dr. Harry Parker, RM SME, is the QP for the estimate and is a Professional Geologist licensed in Minnesota. * The resources are based on a US$30/t NSR that assumes a mining cost of $16.00/t, a process cost of $12.00/t and G&A charges of $2.00/t; global metallurgical recoveries of 90.8% (Cu), 57.4% (Ni), 863.3% (Au), 63.6% (Pd), and 55.2% (Pt); and long-term consensus metal prices of $3.00/lb Cu, $9.38/lb Ni, $1,050/troy oz Au, $850/troy oz Pd and $1,840/troy oz Pt. * The NSR equates to a 0.3% Cu cut-off grade. * Figures have been rounded and may not sum. * Mst = million short tons. Table 7. Spruce Road Deposit Mineral Resources by Copper Cutoff Grade (base case is highlighted) Inferred Mineral Resource Cut-off Cu (%) Tons (Mst) Cu (%) Ni (%) 0.2 0.3 0.4 0.5 674 480 254 101 0.38 0.43 0.50 0.57 0.14 0.16 0.18 0.21 * Effective Date is 15 September 2012. * Dr. Harry Parker, RM SME, is the QP for the estimate and is a Professional Geologist licensed in Minnesota. * The resources are based on a US$30/t NSR that assumes a mining cost of $16.00/t, a process cost of $12.00/t and G&A charges of $2.00/t; global metallurgical recoveries of 90.8% (Cu), 57.4% (Ni); and long-term consensus metal prices of $3.00/lb Cu, $9.38/lb Ni. * The NSR equates to a 0.3% Cu cut-off grade. * Figures have been rounded and may not sum. * Mst = million short tons. 96 �DESCRIPTION OF FIELD TRIP STOPS The location of the South Kawishiwi Intrusion field trip stops is presented in Figure 9, and short descriptions of the geology and important take-away knowledge (bedrock geology, glacial geology and dynamics, exploration geochemistry, environmental review, etc.) from each stop is given in the following descriptions. It is important to note that the whole northern SKI is located in the scoured bedrock terrain of the Wisconsinan cycle of the Pleistocene Laurentide ice sheet (herein the Rainy Lobe about 12,000 years ago). The local end moraine of the Rainy Lobe is located immediately south of Stop #1 along the new Tomahawk Road. Work by Duluth Metals has positively shown that the mean transport length (the distance where ½ of material in the glacial deposits is sourced from) in the field trip area is <0.5 miles. Figure 9. Location map of the field trip stops in the South Kawishiwi Intrusion. 97 �STOP 1: Upper SKI Troctolite UTM NAD83 Coordinates: 598664E, 5287937N. PLS: T61N, R11W, S26 Glacially scoured outcrops of weakly layered anorthositic troctolite perfectly exposed at the bottom of a State of Minnesota gravel pit. As is typical in much of the Upper and Middle SKI, the rocks dip shallowly to the southeast with olivine layers striking 35° and dipping 8° to the southeast. Exposures such as this are extremely important in environmental reviews of proposed mining operations as they display the natural occurrence of bedrock surfaces beneath the thin veneer of glacial sediments. As we at Duluth Metals have become aware during the course of prefeasibility studies of TMM’s Maturi deposit, many hydrogeologists (consulting, governmental, academic) believe that there has to be several hundred feet of fractured bedrock immediately below the Quaternary-Mesoproterozoic contact. If these envisioned fractured bedrock zones exist, they would clearly be permeable (even to the Precambrian geologist author) to near-surface groundwater flow, thus requiring identification and study any environmental review of a mining operation. Please note the glacial scours and examine the glacial till bank on the north edge of the gravel pit. PGEenriched sulfide-bearing boulders (with >0.5% Cu) in these glacial deposits have been obtained from this till bank. The nearest identified and exposed up-ice Cu-Ni-PGE occurrence is the Spruce Road deposit, approximately 7 miles to the NNE, which is over 15 times the mean glacial transport distance. STOP 2: Mesabi Black Quarry, Coldspring Granite Company UTM NAD83 Coordinates: 599131E, 5289159N. PLS: T61N, R11W, S24 Coldspring’s Mesabi Black® quarry opened in 2000 and furnishes the dimension stone industry with poikilitic gabbroic anorthosite. The company utilizes a mix of mining techniques at the quarry to harvest the blocks but the technique that will most interest field trip participants is the diamond wire cuts. Get ready, this is perhaps the best locality in existence where one can examine Anorthositic Series rocks of the Duluth Complex in 3D and wrap your mind around the viscosity of a plagioclase crystal mush. The quarry is in the heart of the USGS mapping of the Harris Lake area (Foose and Cooper, 1978). Troctolitic rocks in the area dip (as defined by olivine layering and plagioclase crystal foliation) moderately to the southeast at approximately 25°. The operation ships blocks of rocks from a large Anorthositic Series xenolith from within the Upper SKI. For the ILSG regulars, an anecdote herein is required… Duluth Metals geologists invited Dr. Paul Weiblen to visit the quarry in the spring of 2013. After about an hour looking at Anorthositic Series rocks in perfect 3D cuts, Paul stated to the author (and I quote), “I learned by far more today about the Anorthosite Series than I have over the last 50 years mapping and studying these rocks”. STOP 3: The SKI Break - Middle to Upper SKI Troctolite Contact UTM NAD83 Coordinates: 600417E, 5292338N. PLS: T61N, R10W, S18 Duluth Metals geologists first took interest in the rocks of this field trip stop in 2011 during follow up field work around several highly anomalous till geochemistry (Cu, Ni, Pt and Pd) samples. Numerous angular boulders of melatroctolite and peridotite were discovered containing highly anomalous Cu-NiPGE geochemistry. A detailed mapping and sampling program ensued and the locally sulfide-bearing, chromium oxide-rich ultramafic SKI Break was discovered in outcrops along the valley of Keeley Creek. This stop will involve a moderately long walk (½ mile) walk through large exposures of anorthosite xenolith-rich, shallow dipping (here to the west-southwest), foliated troctolite of the basal portion of the Upper SKI into the recessive weathering SKI Break. As we walk along U.S. Forest Service road 1468 please note the rugged topography of the glacially scoured outcrops of the Upper SKI. The rugged nature of the topography in the Upper SKI becomes more subdued in glacially polished outcrops of the Middle SKI. Subtle differences in geochemistry/mineralogy/texture of these SKI units must have resulted in differential weathering and saprolite development of these similar troctolite units of the SKI. 98 �STOP 4: Middle SKI Troctolite UTM NAD83 Coordinates: 597134E, 5293919N. PLS: T61N, R11W, S11 In the summer of 2013, geologists from Duluth Metals and Twin Metals Minnesota completed detailed structural mapping of nearly 100 sites around the Maturi deposit in the initial field phase of an upcoming geohydrology program for the project. Nearly 800 structural elements were measured and 1,321 new outcrops were mapped in detail. The goal of this work was to attempt to define the dip direction and angle of numerous topographic lineaments that cross the SKI (refer back to Figures 6, 8, and 9). Many workers and NGO activists believe that each and every one of these topographic features represent deepseated faults that directly transport groundwater throughout the entire mass of the intrusion. We at Duluth Metals would beg to differ and it is hoped that a conversation on this topic originates on the outcrop. A simplified single example of this type of geological mapping is presented in Figure 10, and the original field sheet will be available to examine during the field trip. At this stop, massive, moderately layered and foliated troctolite (strike 45°, dip 22°) of the Middle SKI is exposed in numerous outcrops. This stop epitomizes the “Sea of Troctolite” that occurs throughout the vast majority of the SKI. We will take a short walk to the south onto large massive exposures of glacially scoured troctolite to investigate weak jointing developed along the eastern margin of the bedrock exposures and discuss the interpretation of the NNE trending topographic lineaments. Figure 10. Example of detailed structural mapping. STOP 5: Main Augite-Troctolite, the “Main AGT” UTM NAD83 Coordinates: 594743E, 5296267N. PLS: T62N, R11W, S33 Recent road cut along the south side of Minnesota Highway #1 of massive, extremely homogeneous augite troctolite of the Main AGT unit of Severson (1994). Troctolite of the Main AGT unit differs from the Middle and Upper SKI troctolite in two distinctive ways: 1) ophitic augite crystals are black, distinctly associated with Fe-Ti oxides + apatite, and occur as high-density ophitic crystals from 1 to 3 inches in diameter. In the Middle and Upper SKI, ophitic augite crystals are brown, not associated with Fe-Ti oxides, and occur as large (up to 15 inches) low-density grains; and 2) The Main AGT is never layered. Geologists at Duluth Metals interpret the units’ homogeneity and lack of layering as evidence that the Main AGT magma lacked phenocrysts of olivine and plagioclase and represents the end product of topdown and bottom-up solidification of a basaltic liquid. We currently interpret the Main AGT as the solidification of much of the “carrier liquid” of the underlying sulfide-bearing BMZ magmatic slurry. STOP 6: Basalt Xenolith in the BMZ, the Spruce Road Deposit UTM NAD83 Coordinates: 599404E, 5298990N. PLS: T62N, R11W, S24 A short field trip stop up onto a small knob of basalt hornfels within the center of the Spruce Road Cu-Ni deposit. Sulfide-bearing troctolitic rocks of the Spruce Road Cu-Ni deposit are distinctly different in several ways to similar rocks within the Maturi Cu-Ni-PGE deposit. First and foremost is the fact that the precious metal content (Pt-Pd-Au) of Spruce Road ores are much less than within Maturi. The second 99 �fact is that the Spruce Road deposit contains a large amount of sulfide-barren xenoliths. In fact, the mapped proportion of barren xenoliths at Spruce Road approaches 15% of the total rocks within the heart of the deposit (Table 8). At Maturi, such accessory and exotic xenoliths account for <<1% of the Cu-NiPGE mineralized zone. Table 5-8. Extent of mapped rock types in the Spruce Road deposit. SKI Rock Type Acres Extent 187.3 82.5% 6.8 3.0% 14.7 6.5% Barren troctolite (early chill margin?) 9.9 4.4% Biwabik Iron Formation 5.1 2.3% Anorthosite 3.0 1.3% Virginia Formation 0.2 0.1% Sulfide-bearing, heterogeneous troctolite Sulfide-bearing melatroctolite to dunite Xenoliths Basalt The difference between these deposits is interpreted to be the result of the timing of magma injection. The Spruce Road deposit is believed to have formed prior to Maturi and the lithology and amount of xenoliths are the end product of the system cleaning out the pathways that the magma traveled upwards from depth (Peterson and Boerst, 2013). STOP 7: U.S. Forest Service Borrow Pit, BMZ in the Spruce Road Deposit UTM NAD83 Coordinates: 598826E, 5298384N. PLS: T62N, R11W, S25 Beginning in the late 1940s, the U.S. Forest Service utilized locally derived glacial tills and weathered bedrock gossans as road building materials during the construction of the Spruce Road. As we take a short hike into one of these borrow pits, we will walk by the 1973 INCO bulk sample site in the Spruce Road deposit. This short stop will examine the bottom of an old borrow pit where participants can walk on and sample sulfide-bearing troctolite gossans. Please note the friable nature of the rocks in the weakly saprolitic exposure and look for rounded core-stones where weathering over the eons was less intense. STOP 8: Sulfide-bearing Troctolite & layered Melatroctolite, Maturi SW Deposit UTM NAD83 Coordinates: 590572E, 5293036N. PLS: T61N, R11W, S7 Classic roadside exposures of heterogeneous sulfide-bearing troctolite and layered melatroctolite of Severson’s (1994) Basal Heterogeneous (BH) and Ultramafic 3 (U3) units of the SKI. A large core-stone is well exposed in the weakly saprolitic heterogeneous troctolite outcrop. Several small xenoliths of finegrained troctolite can be observed on top of the outcrop and are interpreted as Stage 1 chilled margin autoliths (Peterson and Boerst, 2013). Within the exposure of the overlying U3 layered melatroctolite, olivine layers strike 17° and dip steeply 51° to the ESE. The steep dip is apparently associated with two defined north-south trending faults east of these exposures. Recent drilling by Twin Metals Minnesota in this area has led to the definition of the Maturi SW deposit (see Fig. 8 and Tables 2 and 5). STOP 9: Basal Heterogeneous, Sulfide-poor Troctolite UTM NAD83 Coordinates: 590313E, 5292211N. PLS: T61N, R11W, S18 At this stop, we’ll examine perhaps the best exposure of Severson’s (1994) BH unit in the whole SKI. The heterogeneous troctolitic rocks at this stop are generally poorly mineralized and thus lack a gossanous saprolitic weathering profile which lets one see the true nature of the heterogeneity within the troctolite. I believe that all geologists who ever will log drill core within the Cu-Ni deposits of the Duluth Complex (or who attempt to model such deposits for mine planning purposes) should be required to spend several days examining the rocks within the area around both Stops 8 and 9. All participants should imagine a drill core cutting this exposure and how they would interpret the geology of that core without first examining this outcrop. Such thoughts are why the Precambrian Research Center’s field camp has for many years had its students complete a 1:5,000 scale bedrock geology map of this area. 100 �STOP 10: Giants Range Batholith, the Footwall UTM NAD83 Coordinates: 590518E, 5296544N. PLS: T62N, R11W, S31 The footwall rocks of the whole northern SKI consist of the Neoarchean Giants Range batholith (GRB), such as is exposed along Highway 1 at this field trip stop. The rocks here consist of porphyritic hornblende quartz monzonite with distinctive 1-2 cm potassium feldspar phenocrysts. The massive nature of this unit creates an excellent footwall for the intrusions Cu-Ni-PGE deposits as it lacks bedding and thus rarely (if ever) gets incorporated into the mineralized zone as barren xenoliths. In addition, the melting of the GRB beneath long-lived magma channels (Peterson and Boerst, 2013) at the base of the Maturi deposit has contaminated the SKI and induced additional sulfide immiscibility and the formation of Ni- and Co-rich massive sulfide bodies (see the bottom of Fig. 5). REFERENCES Allen, D.J., Hinze, W.J., Dickas, A.B., and Mudrey, M.G., Jr., 1997, Integrated geophysical modeling of the North American Midcontinent rift system: New interpretations for western Lake Superior, northwestern Wisconsin, and eastern Minnesota, in Ojakangas, R.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. Foose, M.P., and Cooper, R.W., 1978, Preliminary geologic report on the Harris Lake area, northeastern Minnesota: U.S. Geological Survey, Open-File Report 78-385, 24 p., 1 pl., scale 1:12,000. Miller, J.D., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.M., 2001, Geological map of the Duluth Complex and related rocks, Northeastern Minnesota; Minnesota Geological Survey, Miscellaneous Map M119, scale 1:200,000. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations RI-58, 207 p. Peterson, D.M. and Severson, M.J., 2002, Chapter 4, Archean and Paleoproterozoic rocks forming the footwall of the Duluth Complex, in Geology and mineral potential of the Duluth Complex and related intrusions of northeastern Minnesota, Minnesota Geological Survey, Report of Investigations 58, pp. 76-93. Peterson, D.M. and Boerst, K., 2013, Twin Metals Minnesota’s Maturi Deposit, in Severson, M.J., Peterson, D.M., Ware, A., and Boerst, K., 2013, Cu-Ni-PGE Deposits of the Duluth Complex, Geology and Development: Precambrian Research Center, Workshop on the Copper, Nickel, Platinum Group Element Deposits of the Lake Superior Region, October 6-13, 2013, Field Trip Guidebook, pp. 45- 57. Severson, M.J., 1994, Igneous stratigraphy of the South Kawishiwi intrusion, Duluth Complex, northeastern Minnesota: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report NRRI/TR 93/34, 210 p. (with plates). Severson, M.J., Peterson, D.M., Ware, A., and Boerst, K., 2013, Cu-Ni-PGE Deposits of the Duluth Complex, Geology and Development: Precambrian Research Center, Workshop on the Copper, Nickel, Platinum Group Element Deposits of the Lake Superior Region, October 6-13, 2013, Field Trip Guidebook, 60 p. 101 �FIELD TRIP 6 Saturday, May 17, 2014 THE ST. LOUIS SUBLOBE AND GLACIAL LAKE UPHAM LEADERS: Phil Larson (Duluth Metals Limited) Alan Knaeble (Minnesota Geological Survey) Howard Mooers (University of Minnesota Duluth) with contributions by Lisa Marlo (Halcon Resources Corporation) INTRODUCTION The flat plain lying to the south of the Mesabi Iron Range at first glance stands in stark contrast to the varied topography and geology of northeastern Minnesota. Largely covered by peatland, early travelers on the St. Louis and Savanna Rivers no doubt appreciated their quick passage through the mosquitoinfested terrain north to Lake Vermilion and west to Sandy Lake. After iron ore was discovered on the Mesabi, mine developers at first appreciated the gentle grades and straight lines of the railroads vital to transporting iron ore to market, but learned to respect the difficulties inherent in maintaining lines across water-logged ground prone to sink beneath the heavy traffic. Agricultural settlement was almost an afterthought, memorable more for its heroic efforts than lasting success. Even so, on a long drive south from the Range, a traveler cannot help to wonder, “Why so flat, and why here?” As it turns out, the Glacial Lake Upham Plain (as it is called) is the location of one of the more interesting episodes in the long history of glaciation in Minnesota and the Upper Midwest (Fig. 1). As the last Laurentide Ice Sheet retreated to the north, the long-standing pattern of ice flowing from Hudson Bay in the north to central Minnesota and beyond to the south was dramatically disrupted by a surge of ice from the Red River Valley to the west. As the ice entered the flat Upham Plain, flow took right-turns to either flank, advancing to the southwest as far as Aitkin, and more remarkably, advancing from south to north as far as the Giant’s Range. Despite this dramatic entry, almost as rapidly, it melted away and was gone. The glacial advance by the St. Louis sublobe was rapid, short-lived, and barely left a mark on the landscape. Nevetheless, the signs of this light touch are there for the careful observer to see. This field trip explores sites illustrating the landforms and sediments associated with the St. Louis sublobe and its associated glacial lakes. HISTORICAL BACKGROUND Leverett (1932) first noted the presence of a distinct clayey, calcareous grey till in the region south of the Mesabi Iron Range. He correlated the till with similar fine-textured, calcareous till associated with the Des Moines lobe to the west, and first applied the name St. Louis sublobe to the corresponding glacial advance. He also noted the presence of similar reddish clayey till in the immediate vicinity of the Mesabi, attributing the red color to local incorporation of red hematite iron-ore. He also mapped the extent of the lobe, as well as the associated overlying glacial lake basins, named Glacial Lakes Aitkin and Upham. Although Wright (1955) initially attributed the red color in the drift to reddish drift from a Superior lobe advance from the southeast, later work by Baker (1964) and Wright and Watts (1969) returned to Leverett’s conclusion that the St. Louis sublobe advanced from the northwest. Baker (1964) recognized that two tills were associated with the St. Louis sublobe advance (named the Alborn Phase by him): a reddish clayey till, and an overlying grey (brown when oxidized) silty till, named the Alborn till and Prairie Lake till, respectively. Wright and Watts (1969) attributed the red color of the Alborn till to 102 �incorporation and mixing of reddish lake clay from Glacial Lake Upham I into a glacial debris load composed of grey (-brown) silty till. The red lake clay was believed due to reddish sediment ultimately sourced from the northern margin of the Superior lobe. Farnham and others (1964) published two radiocarbon dates attributing relatively young dates to the Alborn phase. Wright and Watts (1969) recognized that meltwater from the St. Louis sublobe and its successor lakes flowed around the northern margin of the Thomson-Nickerson moraine during the Nickerson phase of the Superior lobe. Winter (1971) and Winter and others (1973) investigated St. Louis sublobe deposits along its northern extent in detail. They questioned that the grey(-brown) Prairie Lake till and reddish Alborn till could be sourced from the same provenance, noting the difference in texture and clast composition. Hobbs (1983) described the history of meltwater flow associated with the St. Louis sublobe and its successor lakes, including the flow of meltwater from Lake Koochiching (potentially a Glacial Lake Agassiz precursor) through the Upham and Aitkin basins. Ballantine (1991) conducted additional stratigraphic investigations in the southern Upham basin. Meyer (1993) defined the St. Louis sublobe as referring only to ice flowing east of the Giant’s Range, distinguishing it from the much more extensive northwestern provenance ice advance to the west. More recently, Knaeble and others (2004) and Knaeble and Hobbs (2009) conducted the first detailed modern investigations of the southern margin of the St. Louis sublobe in Crow Wing and Carlton Counties. Marlow (2004) investigated the widespread occurrence of eolian deposits on the beds of Glacial Lakes Aitkin and Upham II, and Jennings and Reynolds (2005) re-mapped glacial stratigraphy along the axis of the Mesabi Iron Range. GEOLOGY Regional Background Bedrock in the area glaciated by the St. Louis sublobe is underlain by shales, mudstone, and greywackes of the Paleoproterozoic Virginia Formation. This formation has proven relatively less resistant to weathering and erosion than the iron-formation and Archean granite-greenstone terrane to the north, the Mesoproterozoic Duluth Complex to the east, and the fold-and-thrust belt of the Paleoproterozoic Penokean orogen to the south. Consequently, a natural basin existed in the area prior to glaciation. The area was repeatedly glaciated over the course of the Pleistocene. Most recently, during the Wisconsinan glaciation, ice of the Rainy lobe advanced from the north-northeast from the Labradoran sector of the Laurentide ice sheet (bearing approximately 215°), carrying relatively coarse-grained, sandy textured sediment dominated by crystalline igneous and metamorphic rocks of the Canadian Shield. At about the same time, to the south, the Labradoran ice funneled into the Lake Superior basin as the Superior lobe advanced roughly parallel to the Rainy lobe. The Superior lobe carried an abundance of reddish sediment eroded from rift-filling sedimentary rocks of the Mesoproterozoic Midcontinent Rift. During the waning stages of the last glaciation, the Rainy lobe margin retreated back to the northnortheast across the area. Retreat was characterized by a relatively steady rate of margin retreat, punctuated by the occasional minor re-advance and moraine building event. When the line of retreat reached the region underlain by the Virginia Formation (Animikie Basin), proglacial lakes developed in the natural topographic low, Glacial Lake Aitkin to the southwest and Glacial Lake Upham to the northeast (Fig. 1). These lakes received meltwater-borne sediment from both the Rainy lobe to the northeast and the Superior lobe to the south. Glacial Lake Upham I Lake Upham I formed as a proglacial lake as the Rainy lobe ice margin retreated to the northeast. Little is known about the extent or duration of the lake, as its existence is largely inferred from the red clayey lacustrine sediment incorporated into the debris load of the later St. Louis sublobe advance. In situ Upham I sediment is rarely observed. 103 �The distinctive red color of Upham I sediment is due to the presence of hematite in the clay size fraction. Leverett (1932) believed this red color reflected erosion and incorporation of soft hematite iron ore from the Mesabi Iron Range to the north. Leith (1903) recognized that significant glacial erosion of soft iron ores had taken place, and suggested that much of this eroded material would be resident in finegrained glacial sediments. Wright (1955) correlated Upham I sediments with the Superior lobe, believing the red color came from incorporation of red shale from the Mesoproterozoic Midcontinent Rift System. Both are viable hypotheses, and it is not unlikely that both sources contributed to the red color of Upham I sediments. The lake basin was bounded by stagnant ice-cored topography to the north and west, and glacial ice of the Rainy lobe to the northeast and the Superior lobe to the south. Large patches of ice-cored topography likely persisted in the lake basin, particularly along northwest-southeast oriented moraines formed by the retreating Rainy lobe. The outlet to the lake was likely to the southwest through Lake Aitkin I and ultimately the Mississippi River. The age of Upham I is constrained by two bracketing events: retreat of the Rainy lobe from the St. Croix moraine in central Minnesota, tentatively dated at no later than ~15.1 14C kyr BP (Birks, 1976; Mooers and Lehr, 1997), and the Alborn phase advance of the St. Louis sublobe. The lake was likely contemporaneous with the Automba phase of the Superior lobe. St. Louis Sublobe Gradual retreat of the Rainy lobe and deglaciation of the Upham basin was abruptly interrupted by advance of the St. Louis sublobe from the northwest (Fig. 1). In contrast to the Rainy lobe, formed by ice flowing from the northeast and the Labradoran accumulation center of the Laurentide Ice Sheet, the St. Louis sublobe originated from ice flow southward from the Lake Winnipeg basin into the Red River Valley. For most of the glacial cycle, a relatively high flux of ice over the Canadian Shield of northwestern Ontario and northeastern Minnesota blocked eastward expansion of ice streaming south from Lake Winnipeg, funneling this ice into the Minnesota River Valley. Retreat of the Rainy lobe ice margin from the Itasca-St. Croix moraines opened a low elevation, ice-free corridor to the north of the Itasca moraine. Continued high ice flux in the Red River Valley rapidly surged into this gap, expanding south of the Mesabi Range to the northeast and southwest across the fine-grained lacustrine sediments of Lakes Aitkin I and Upham I. This advance and its associated glacial deposits are grouped as the Alborn phase. Figure 1. Maximum extent of St. Louis sublobe advances (white) and inferred location of Lakes Aitkin I and Upham I (stippled pattern). Arrows indicate inferred ice flow directions. Western limit of Alborn member/’red clayey’ till along the Mesabi Range (red line) from Winter and others 104 �(1973). Proglacial Lake North of Nashwauk is the high-level (elev. >1500’) proglacial lake dammed by the initial phase of the St. Louis sublobe. Chronology The absolute timing of the Alborn phase is poorly constrained. As mentioned above, the advance could not have occurred prior to ~15.1 14C kyr BP. Two radiocarbon dates have been attributed directly to St. Louis sublobe associated deposits (Farnham and others, 1964). A buried soil within Lake Aitkin II sediment near Aitkin, MN was dated at 11.6 14C kyr BP. Wood recovered from a red clayey till at the Mariska Mine in Gilbert, MN, near the northeastern limit of the Alborn phase, was dated at 11.2 14C kyr BP. However, the Lake Aitkin II soil was developed on lacustrine sediment, and therefore significantly post-dates the Alborn phase and formation of Lake Aitkin II. The Mariska Mine wood is unlikely to have been incorporated during advance of the St. Louis sublobe into a forested environment. Rather, it perhaps represents wood incorporated into a flow till developed on ice-cored topography, and also significantly post-dates the Alborn phase. These young dates therefore are minimum bracketing dates for the Alborn phase. Relative age relationships provide additional insight. During the Split Rock phase, meltwater from the Superior lobe apparently flowed northwest from the Cloquet moraine into the ice-free Upham I basin (Wright and Watts, 1969). In the later Nickerson phase, meltwater from the eastern margin of the St. Louis sublobe and from post-glacial Lake Upham II flowed down the proto-St. Louis River and around the Superior lobe margin (Thomson moraine) into the St. Croix River drainage. Significant meltwater flow down the St. Louis River persisted after retreat of the Superior lobe from the Thomson moraine and formation of Lake Duluth. However, meltwater inflow from Lake Upham II into Lake Duluth ceased prior to readvance of the Superior lobe to the western end of the Lake Duluth basin during the Marquette phase (Mooers and others, 2005), dated at 10.0 14C kyr BP (Lowell and others, 1999). Glacial Dynamics The St. Louis sublobe was a thin, temperate glacier. It was on the order of perhaps 100-200 m thick at its maximum extent (Knaeble and others, 2005). The apparent confinement of the lobe by topography during its advance is a direct consequence of this relative thinness; flow was apparent confined by stagnant ice in the Itasca and Outing moraines to the south and west, and by stagnant Rainy lobe ice to the north. Notwithstanding the relatively thin ice, expansion into the Lakes Aitkin I and Upham I basins was facilitated by the low shear strength lacustrine sediment substrate. The base of the glacier was apparently at the pressure melting point throughout the Alborn phase. Preexisting proglacial lakes precluded development of permafrost in the Aitkin I and Upham I basins, so much of the glacial advance was over a ‘warm’ bed. Little evidence for freeze-on and entrainment of sediment into glacial ice exists, either in the form of thick Alborn phase glacial sediment accumulations or stagnation topography. Alborn phase tills tend to be compositionally homogeneous, and relatively uniform in thickness. Local incorporation of basal sediment (lacustrine clays and silts) is evident in the basal Alborn member tills, however erosion and entrainment occurred at very low rates relative to the polythermal Rainy lobe just to the north. Smeared pods of Alborn till are often incorporated into the overlying Prairie Lake member till at the contact. However, the overall composition of Prairie Lake till is homogeneous and distinct from Alborn till, suggesting erosion, entrainment, and mixing of substrate into the debris load was occurred at very low rates. These features suggest sediment transport by the St. Louis sublobe is best explained as having occurred as a subglacial deforming layer. Other than Lake Upham I lacustrine sediments, Alborn phase deposits show little evidence of erosion, entrainment, or other modification of the older Rainy lobe glacial deposits over which the St. Louis sublobe advanced. Rainy lobe landforms such as eskers, drumlins, and moraines are clearly visible beneath a veneer of Alborn phase drift, and the hummocky ‘moraines’ found at the margins of the St. Louis sublobe are composed primarily of older, relatively coarse-grained Rainy and Superior lobe drift. 105 �The inability of the St. Louis sublobe to form its own landforms, or modify older landforms, is a consequence of a general lack of englacial sediment, and an inability to transmit shear stress into the substrate. The relative thinness of the ice and the low shear strength substrate suggest that the St. Louis sublobe advance was as one, or possibly two, surge(s) from the main Red River lobe ice mass. By analogy with modern ice streams in Antarctica and Greenland, ice may have streamed into the Upham and Aitkin basins at flow rates up to 10 km/yr, and may have taken only a few decades for the St. Louis sublobe to reach its maximum extent. It is unclear how many individual surges, or advances, from the main Red River lobe ice mass may have occurred over the history of the St. Louis sublobe. The two distinct tills associated with the Alborn phase likely record two distinct phases of ice flow through the St. Louis sublobe. The Goodland esker (Knaeble and others, 2005) formed in a large englacial meltwater conduit developed in response to the initial west-to-east St. Louis sublobe advance blocking southward meltwater flow from the Rainy lobe to the north. A reconstruction indicates around 75 m of ice was necessary to block this flow. However, the Goodland esker and the surrounding ice-cored Rainy lobe deposits are mantled by upper (later) Prairie Lake member tills, suggesting this area was later overrun by thicker ice in a later phase of the St. Louis sublobe advance. Similar to other surging glaciers, the lobe may have stagnated en masse, with an abrupt retreat of the ‘active’ ice margin by as much as 150 km. Ice down-flow of the ‘active’ margin ceased flowing and melted without forming recessional moraines. The lack of significant englacial debris transport precluded formation of significant supraglacial sediment accumulation, even in marginal areas. Consequently, during stagnation and wastage of the glacier an insulating debris layer did not form to retard melting. Perhaps 5 m of surface melting of the clean ice may have occurred each season, meaning even a 200 m thick glacier would have persisted only 40 years after cessation of ice flow. Alborn Phase Deposits Glacial deposits associated with the Alborn phase are placed in the Aitkin formation lithostratigraphic unit (Johnson and others, in press). Two members have been formally defined: the Alborn member, and the Prairie Lake member (Baker, 1964). The Alborn member is the lower member, and is interpreted to have been derived to a great degree from erosion of underlying Lake Upham I lacustrine sediment. The overlying Prairie Lake member contains a significant component of Paleozoic carbonate and Cretaceous shale lithologies, derived from the Red River Valley and Winnipeg basin. The distribution of the two members is poorly understood within and adjacent to the Upham basin, however the Prairie Lake member is less extensive than the underlying Alborn member. Both members are recognized primarily as till lithofacies, however minor glaciofluvial and glaciolacustrine elements locally occur. Alborn Member The Alborn member consists predominantly of reddish-brown to dark reddish-grey, clay to clay loam till (Fig. 2a). The pebble, cobble, and boulder clast content is distinctly lower than underlying Rainy lobe deposits (Knaeble and Hobbs, 2009) (Fig. 2b). Alborn member tills along the northern margin of the Upham basin, adjacent to the Mesabi Range, are distinctly more clay rich than tills along the southern margin of the Upham basin (Winter and others, 1973) (Fig. 2a). This may reflect incorporation of a greater amount of clayey lacustrine sediment by the glacier along a longer flow path across the bed of Upham I. Alborn till is patchily distributed near the margins of the St. Louis sublobe. The once continuous till sheet deposited on ice-cored topography has been disrupted by subsequent meltout and collapse of the underlying substrate. 106 �Figure 2. Matrix texture (a. left triangle) and lithologic composition (b. right triangle) of the 1-2 mm sand fraction of Alborn member tills. Circles are from QDI (Quaternary Data Index) database (Minnesota Geological Survey), triangles are ‘red clayey till’ of Winter and others (1973), and large circle is mean in Carlton County (Knaeble and Hobbs, 2009). The ‘red clayey till’ from the northern end of the Upham basin is distinctly more clay-rich than Alborn member tills from the southern end of the basin. The tills contain only a minor amount of carbonate, and no gray shale, reflecting the composition of recycled underlying northeastern provenance (Rainy lobe) drift. Figure 3. Matrix texture (a. left triangle)and lithologic composition (b. right triangle) of the 1-2 mm sand fraction of Prairie Lake member tills. Circles are from QDI (Quaternary Data Index) database (Minnesota Geological Survey), triangles are ‘brown silty till’ of Winter and others (1973), large circle is mean in Carlton County (Knaeble and Hobbs, 2009). The tills are variably enriched in carbonate and gray shale, reflecting a northwest (Red River Valley) provenance. 107 �Prairie Lake Member The Prairie Lake member consists predominantly of yellow-brown to brown to dark grey, loam to clay loam till (Fig. 3a). The pebble, cobble, and boulder clast content is similar to Alborn member till, and likewise distinctly lower than underlying Rainy lobe deposits. Unleached (grey) Prairie Lake tills average ~10% carbonate and ~17% grey shale clasts in the very coarse sand fraction, indicative of a northwestern provenance (Fig. 3b). Carbonate leaching averages about 1 m depth (Knaeble and Hobbs, 2009). In contrast to Alborn till, Prairie Lake till shows no apparent systematic textural variation across the Upham basin. Glacial Lake Upham II Advance and stagnation of the St. Louis sublobe was followed by a rapid melting of the thin, warm ice, as discussed above. Melting of the ice was accompanied by formation of a series of proglacial lakes, which ultimately coalesced to form Lakes Aitkin II and Upham II (Fig. 4). Initially, Aitkin II and Upham II were contiguous. Opening of successively lower elevation outlets led to drawdown and drainage of both lakes into the proto-St. Louis River. Continued isostatic rebound raised the sill between Aitkin II and Upham II, leading to re-inundation of Aitkin II. Aitkin II ultimately drained with the opening of an outlet into the Mississippi River on the southwestern end of the basin. Figure 4. Location of major meltwater inflow and discharge points for Lakes Aitkin II and Upham II, St. Louis sublobe areal footprint (olive), and active Rainy and Superior lobe ice (white) during main phase of Upham II (>ca. 11.6 kyr BP). As much as 67 m (220’) of differential isostatic rebound occurred over the 160 km extending from the southwestern extent of Aitkin II to the northeastern extent of Upham II, based on strandline correlations (Marlow, 2004). Adjusting for isostatic rebound, the absolute elevation difference between the uppermost Upham II outlet and the final outlet is about 38 m (125’), a number also corresponding to the maximum depth of Upham II. 108 �Meltwater Drainage In addition to their immediate catchments, Aitkin II and Upham II received meltwater from up to 500 km of the margin of the Laurentide Ice Sheet (Fig. 5). Advance of the St. Louis sublobe into the area recently vacated by the retreating Rainy lobe blocked the flow of meltwater south from the Rainy lobe ice margin. Meltwater apparently pooled in the interlobate area forming proglacial lakes, however discharge necessitated flow either through subaerial channels around the northeastern margin of the glacier, or through sub-, en-, or supra-glacial meltwater conduits in the glacier itself. Morphology of the Goodland esker suggests that a significant amount of meltwater flowed in a supraglacial channel southward across the surface of the glacier, at least during the earlier phase of the advance. Later meltwater flow may have been accommodated at least in part by subaerial channels along the northeastern margin of the glacier. Following stagnation and melting of the glacier, and formation of Lakes Aitkin II and Upham II, meltwater entered the Aitkin II and Upham II basins at three major inlets. Retreat of the Rainy lobe north of the Laurentian divide led to formation of proglacial Lake Norwood (Winchell, 1901). Discharge from Norwood entered northeastern portion of the Upham II basin through the Embarrass Gap (Fig. 5). Norwood’s initial outlet had an elevation of 1490’, with successive outlets developed at 451 m (1480’) and 450 m (1476’) elevation (Lehr and Hobbs, 1992). A lower outlet at 443 m (1454’) elevation is correlated with the Mizpah phase of Lake Koochiching (Hobbs, 1983; Lehr and Hobbs, 1992). The final outlet through the Gap has an elevation of around 428 m (1405’), however meltwater drainage ceased when the sill between the Pike and Embarrass Rivers (435 m (1427’) elevation) was exposed. Meltwater also flowed down the proto-Prairie River southward into Lake Aitkin II. This outlet has been nominated as a potential discharge point for the lower elevation Gemmell phase of Lake Koochiching, and even as a potential outlet for Lake Climax, an early, high level of Lake Agassiz (Hobbs, 1983). However, the sill over the Laurentian divide into the Prairie River drainage stands at 418 m (1373’), approximately the same as the lower Koochiching outlets in the Embarrass Gap (corrected for rebound), and channel morphology in the vicinity of the sill shows no evidence of high meltwater discharge. The lower reach of the Prairie River does show evidence for significant meltwater discharge. This was likely limited to a short interval after stagnation and melting of the St. Louis sublobe, and before retreat of the Rainy lobe ice margin from the Laurentian divide and expansion of Lake Norwood. West of Grand Rapids, MN, Lake Sucre formed in the area covered by the ‘upstream’ expanse of the stagnant St. Louis sublobe (Larson and others, 2004). A considerable amount of meltwater from the Red River lobe flowed into the western end of this lake, eventually reaching the western end of Aitkin II. At the time of maximum meltwater discharge through Upham II, flow was ultimately channeled around the northern margin of the Nickerson phase Superior lobe into the Kettle River meltwater channel (Wright and Watts, 1969). Assuming an average melt rate of 2.5 m/yr over an ablation zone extending 200 km from the ice margin, average annual discharge through this channel was roughly 8000 m3/s. Discharge was seasonably variable, so maximum discharge rates were considerably higher. Carney (1996) evaluated peak discharge through this channel examining a variety of parameters, concluding that maximum peak discharges of 12000 to 17000 m3/s were reasonable. This number suggests that meltwater discharge through Upham II and the Kettle River channel solely reflected ablation from the ice margins immediately adjacent to the lakes, and does not contain a Lake Climax/Agassiz discharge component. Significant meltwater discharge through Lakes Aitkin II and Upham II was ultimately restricted to the period between stagnation and melting of the St. Louis sublobe and opening of the Macintosh channel into Lake Climax in the Red River Valley, at the end of the Cass phase (c. 11.6 kyr BP) (Fenton and others, 1983). High Level Outlets A series of outlets to high level proto-Upham II proglacial lakes are present along the eastern margin of the Upham II basin, including include the Chicken Creek (three outlets; 1500’ to 1465’) and Us-Kab- 109 �Wan-Ka River (428 m to 422 m; 1404’ to 1386’) outlets. Initially, these channels drained the eastern margin of the St. Louis sublobe, and a portion of the southern margin of the Rainy lobe. Formation of Lake Norwood may have occurred about the time the Us-Kab-Wan-Ka River outlets formed. As Norwood expanded to the west, an increasing discharge of meltwater was channeled along the eastern margin of the St. Louis sublobe. Meltwater flowed over an unstable, ice-cored landscape, resulting in frequent shifts in outlet location, and drops in outlet elevation. The Us-Kab-Wan-Ka outlets were succeeded by a series of outlets formed in the vicinity of Hellwig Creek ranging from 422 m (1386’) to 418 m (1370’) elevation. Main Lake Stage The highest well-developed strandlines in the Upham II basin correlate to a series of outlets between 413 m (1354’) and 407 m (1336’) elevation at Hellwig Creek. The Hellwig Creek outlets were abandoned as ice-cored topography in the Culver moraine continued to melt and collapse, in favor of lower level outlets in the Artichoke River 404 m (1324’) and Spider Creek 396 m (1300’) channels. The highest strandlines in Upham II are commonly obscured by collapsed topography, indicating these shorelines formed on topography still underlain by stagnant Rainy lobe ice. The final outlet to Upham II was established with opening of an outlet down the proto-St. Louis River channel. Terraces correlating with the uppermost outlet had an elevation of about 392 m (1285’). The broad, wide floodplain and terraces corresponding to the upper St. Louis River outlet indicates a significant amount of meltwater continued to flow through Aitkin II and Upham II at the time this channel was established. At least four subsequent stable lower outlets are present, at 390 m (1278’), 386 m (1266’), 381 m (1250’), and 377 m (1238’) elevation. In contrast to the higher elevation outlets abruptly abandoned with opening of lower elevation outlets, the St. Louis River outlet records stepwise drops within a single channel to an ultimate elevation of 375 m (1230’). Figure 5. Location of outlets for early ice marginal drainage channels (Chicken, Us-Kab-Wan-Ka, Hellwig) and main stage Lake Upham II outlet channels (Hellwig, Artichoke, Spider, and St. Louis). 110 �Eolian Activity in the Upham II basin Upham II was apparently stable at the higher Hellwig Creek and Artichoke River outlet levels for a considerable length of time, long enough to allow deposition of well-sorted lacustrine sediments in the lake, ranging from very fine to fine sand in the nearshore areas to clay in the deepest portion of the basin. Later lowering of lake level and exposure of these nearshore sands triggered eolian activity, leading to extensive dune field development (Marlow, 2004). Final Drainage of Lake Upham II Separation of Upham II from Aitkin II occurred when the lake level in Upham II dropped below the elevation of the Swan River sill (382 m; 1252’). Outflow occurred through a broad, shallow channel, suggesting meltwater inflow into the Aitkin II basin had ceased. Downcutting of the St. Louis River outlet may have been triggered by cessation of meltwater influx into the lakes. Discharge over the Swan River sill eroded dune fields developed on Upham II’s bed, indicating significant eolian activity does not post-date final cessation of drainage from Aitkin II into Upham II. This relationship further indicates that significant eolian activity in the Upham II basin was largely confined to the interval between opening of the lower Hellwig Creek outlets and the 381 m (1250’) elevation St. Louis River outlet. Final drainage of Upham II occurred as the St. Louis River down-cut to its modern level of about 375 m (1230’) at the final outlet to Upham II. Final Drainage of Lake Aitkin II Lake Aitkin II substantially drained as the outlet to Upham II dropped to below 382 m (1252’); this drawdown in lake level may have triggered development of the peat dated by Farnham and others (1964) (11.6 14C kyr BP). Continued differential rebound between the Swan River sill on the northeastern side, and the southwestern side of the basin led to re-inundation of the lake (and deposition of the upper lacustrine sequence reported by Farnham and others (1964)). Hobbs (1983) reported a radiocarbon date of 9.1 14C kyr BP from a snail shell recovered from a marl deposited in Aitkin II; this date indicates Aitkin II persisted for at least 2500 years after separation from Upham II. Aitkin II ultimately drained with opening of a new outlet into the Mississippi River in the southwest corner of the basin at about 366 m (1200’) elevation. 111 �DESCRIPTION OF FIELD TRIP STOPS Figure 6. Location of field trip stops relative to major features of the Upham basin. 1. Glacial Lake Upham II Beach 499660E/5236650N (UTM Zone 15, NAD83 datum) Silica 7.5’ USGS Quadrangle NENE, Section 4, T55N, R21W This site is located on the uppermost relatively well-developed beach associated with Lake Upham II. It likely formed in response to establishment of a relatively stable outlet in the vicinity of Hellwig Creek, on the opposite time of the basin. Subsequent to the time of upper beach formation, Lake Upham II experienced a series of relatively gradual drops in water level at this site. Downward stepping clinoforms visible in a ground penetrating radar profile are interpreted to represent a forced regressive shoreface (Fig. 6). Progradation of the bedforms occurs as a result of shoreline regression, and indicates a constructional shoreline. At this site, regression was triggered by a combination of gradual relative lake level drop due to differential isostatic rebound relative to the more southerly outlets, and relatively abrupt lake level drops triggered by development of new, lower elevation outlets. 112 �Figure 6. West to east ground penetrating radar profile of Site 1. Profile length 120 m, vertical scale ~9 m. From Knaeble and others (2005). 2. Toivola Esker 514200E/5226890N (UTM Zone 15, NAD83 datum) Toivola 7.5’ USGS Quadrangle NENE, Section 1, T54N, R20W Most of the landforms within the Glacial Lake Upham basin predate development of the lakes. This exposure is an example of an esker deposited during retreat of the Rainy lobe that was later overrun by the St. Louis sublobe and subsequently modified by wave action. This esker and others like it became wave-washed "islands" once Glacial Lakes Aitkin and Upham I and II formed. Exposed at the base of the sequence are coarse-grained gravels containing northeast provenance clasts, including granites, ironformation and locally derived shale and greywacke of the Paleoproterozoic Virginia Formation. This esker segment, and numerous similar examples in the Upham basin, was deposited by a beaded esker system during retreat of the Rainy lobe. Overlying the gravels is a yellow-brown fine-grained till (Prairie Lake Member). Overlying the till is a sequence of nearshore sands and gravels. These presumably eroded from that portion of the esker rising above the level of glacial Lake Upham II; the strandline formed at about 397 meters (1,300 feet) elevation. The uppermost portion of the sequence is a blanket of eolian sand and silt derived from the surrounding lake plain to the upland after final drainage of Lake Upham II. 3. Prairie Lake Member Till 507820E/5182550N (UTM Zone 15, NAD83 datum) Prairie Lake 7.5’ USGS Quadrangle SWSE, Section 20, T50N, R20W There are two distinct tills at this road cut on the east side of State Highway 73 on the northeast side of Prairie Lake (Figure 1). Both units are deposits of the St. Louis sublobe. The elevation at the top of the exposure is about 1330 and the upper till, the Prairie Lake Member (Baker, 1964) of the Aitkin Formation (Johnson and others, in press), 113 �is approximately 15 feet thick, yellow-brown (2.5Y5/4) to brown (10YR5/4), calcareous, and loam textured. Three samples have textures averaging 37-35-28 (sand-silt-clay percentages, respectively) and lithologic percentages of the 1-2 mm coarse-grained sand fraction, averaging 46-12-42 (crystallinecarbonate-gray shale, respectively). There are trace amounts (<1%) of red Superior-source sand grains. Five feet of the lower till, the Alborn Member (Baker, 1964) of the Aitkin Formation, is exposed at the base of the ditch. Above the contact between the two tills there are, in places, streaks of red-brown till incorporated into the base of the yellow-brown till. An auger boring in the ditch at the base of the outcrop penetrated another 30 feet. The upper 29 feet detected calcareous red-brown clay loam till. Six samples of this till have textures averaging 23-37-40 (sand-silt-clay percentages, respectively) with coarse-grained sand fraction amounts averaging 3% carbonate, no gray shale, and 11% red Superior-source. There are some, but not many pebbles in the till, which tends to become finer with depth, possibly due to incorporation of underlying lake sediment. The last foot was gray clayey silty lake sediment (possibly Glacial Lake Aitkin I). Previous interpretations suggest that the tills of these two members were deposited by one ice advance (Baker, 1964; Wright, 1972). The Prairie Lake till represents the original yellow-brown and brown characteristics of the sediment in the ice as it advanced into glacial Lakes Aitkin I and Upham I, and the Alborn till depicts the incorporation and mixing of red lake sediments of glacial Lakes Aitkin I and Upham I into the basal portion of the ice as the glacier advanced across the basin. This produced ice deposits with brownish sediment overlying and/or intermixed with red sediments. In contrast, subsequent interpretations suggest that each member was a separate ice advance (Knaeble and Hobbs, 2009). 4. Alborn Member Till 521450E/5190450N (UTM Zone 15, NAD83 datum) Martin Lake 7.5’ USGS Quadrangle NWNW, Section 35, T51N, R19W At this private pit located just south of St. Louis CR 856 there are three tills exposed along the west wall (Figure 2). The elevation at the top of the exposure is about 1350. The upper 8 feet is composed of redbrown (5YR4/3 to 7.5YR4/3), non-calcareous, clay loam Alborn Member till with some pebbles. A sample at 6 foot has a textural analysis result of 22-44-34 (sand-silt-clay percentage, respectively), with no carbonate (leached) or gray shale, and ~10% red Superiorsource in the coarse-grained sand fraction. Below a sharp contact there is 3 feet of red-brown (5YR4/3) sandy loam Cromwell Formation (Wright, 1972; Johnson and others, in press) till of the Automba phase of the Superior lobe. Textural and lithologic analysis results for two samples of this till average 39-47-14 (sand-silt-clay percentages, respectively) with 1% carbonate (one sample was leached), no gray shale, and 21% red Superior-source in the 1-2 mm coarse-grained sand fraction. Below another sharp contact is 2 feet of brown (10YR6/3) cobbly, sandy Independence Formation (Johnson and others, in press) till, a deposit of the Rainy lobe. Two samples of this till averaged 53-40-7 (sand-silt-clay percentage, respectively) with trace amounts of carbonate, no gray shale, and 17% red Superior source in the coarse-grained sand fraction. The pebble concentration basically doubled in each underlying till unit. Underlying the 3 till units at the base of the exposure there is pebbly, cobbly sand and gravel. 114 �This site is about a mile or two north of the southern extent of St. Louis sublobe ice deposits. Here Alborn Member till thinly covers older Superior and Rainy lobe deposits. This same stratigraphic sequence is evident in other pit exposures as far east as Brookston. The Toimi drumlins (Wright and Ruhe, 1965) east of Brookston and the St. Louis River are surface exposures of the Rainy lobe deposits that at this site were covered, first by Automba phase deposits of the Superior lobe and later by the Alborn Member deposits of the St. Louis sublobe. 5. St. Louis River Outlet Channel 530800E/5191070N (UTM Zone 15, NAD83 datum) Brookston 7.5’ USGS Quadrangle SWSW, Section 26, T51N, R18W This stop is located at the intersection of the Artichoke and St. Louis Rivers, where there is a prominent terrace at 375 meters (1,230’) elevation that extends 1 kilometer (0.6 mile) across. The terrace is predominantly composed of a thick deposit of relatively coarse-grained gravel, ultimately derived from erosion of Rainy lobe drift in the Culver moraine. The terrace gravels are overlain and partially infilled by loess. The loess likely originated from lacustrine sediment from the bed of the Lake Upham II, eroded by wind as the littoral zone was episodically exposed by rapid drawdown associated with establishment of new, lower elevation outlets. 6. Spider Creek Outlet Channel 531600E/5201600N (UTM Zone 15, NAD83 datum) Alborn 7.5’ USGS Quadrangle NWNE, Section 26, T52N, R18W Spider Creek occupies one of a series of around 10 successive outlets that drained Lakes Upham II, and by extension Aitkin II. The broad (800 m wide), flat-bottomed channel formed when collapse of underlying ice-cored Rainy lobe drift opened an outlet some 24’ in elevation below the Artichoke River outlet. The channel morphology indicates a significant amount of meltwater was still discharging out of Upham II. Baker (1965) reported a bulk radiocarbon date of 13,000 ± 400 14C yr bp from a sequence of lacustrine marl (sample W-1234) within the Spider Creek outlet. The marl must post-date the cessation of drainage through the channel because marl formation requires shallow, still water. Baker (1965) expressed concern that this date was too old due to possible contamination by lignite. However, this date is consistent with the other evidence presented in this field trip description. The Spider Creek date places the minimum age of Glacial Lakes Aitkin and Upham II, and therefore the maximum limit of the St. Louis sublobe, prior to 13.0 14C kyr B.P. 115 �7. Birch Esker 530580E/5208810N (UTM Zone 15, NAD83 datum) Payne 7.5’ USGS Quadrangle NESE, Section 34, T53N, R18W There are multiple exposures in this large county pit (Figure 3). The eastern most exposure is a 25 foot cut adjacent to the railroad crossing revealing 3 separate tills. At a surface elevation of approximately 1350 the soil has been stripped from a 1 foot thick layer of leached mixed till and sand lenses. Underlying the leached layer is 5 feet of yellow-brown (2.5Y6/3 to 10YR6/4), calcareous, silt loam textured Prairie Lake Member till. Near the base of the unit there are shear bands (streaks, pods, and lenses) of incorporated material from the underlying red-brown till. A sample at a depth of 5 feet had a texture of 28-56-16 (sand-silt-clay percentages, respectively), and 8% carbonate, 2 % gray shale, and 1% red Superior-source in the 1-2 mm coarse-grained sand fraction. The underlying red-brown (7.5YR5/4 to 5YR4/4), slightly calcareous, loam textured Alborn Member till is about 3 feet thick with more pebbles and cobbles than the overlying till. There is a cobble-boulder stone line or lag at the base of the unit. A sample at the depth of 8 feet had a texture of 40-40-20 (sand-silt-clay percentages, respectively), and 5% carbonate, no shale, and 9% red Superior-source in the 1-2 mm coarse-grained sand fraction. The lowest unit is about 9 feet thick and exposes brown (10YR6/3) to gray-brown (10YR6/2) non-calcareous, sandy Independence Formation till with abundant pebbles and cobbles. A sample at the depth of 15 feet had a texture of 55-37-8 (sand-siltclay percentages, respectively), and 1% carbonate, no gray shale, and 15% red Superior-source in the 1-2 mm coarse-grained sand fraction. There is about 10 feet of slump to the pit floor below these units. At this site deposits of both members of the St. Louis sublobe are present in typical stratigraphic position. At Stop 2 the Alborn Member was above both the Cromwell Formation Automba phase till and the Independence Formation till. Here at Stop 3 we are further northeast beyond the depositional extent of the Automba phase deposits and therefore have only the Independence Formation till at the base. 8. Alborn Member Till on Collapsed Rainy Lobe Ice-cored Topography 520160E/5251950N (UTM Zone 15, NAD83 datum) Kirk 7.5’ USGS Quadrangle NESE, Section 15, T57N, R19W The Aitkin and Upham basins were occupied by glacial lakes on two separate occasions during the Late Wisconsin glaciation. Retreat of the Rainy lobe from the Mille Lacs and Outing moraines (Mooers 1988) led to the formation of Lakes Aitkin I and Upham I. The extent and timing of these lakes is poorly understood, as their presence is largely inferred from incorporation of lacustrine sediment into the overlying St. Louis sublobe till. Exposed at the base of the sequence are steeply south-dipping foreset beds of a subaqueously deposited fan. These sediments are Rainy lobe provenance, deposited along the southern margin of stagnant ice lying on the southern slope of the Giant’s Range, an area now characterized by collapsed ice-cored topography. The upper portion of the sequence is a St. Louis sublobe till. Between the fan sediments and till are a number of elongate slabs of fine-grained lacustrine sediment derived from Glacial Lake Upham I. This lacustrine sediment was eroded from deeper water and thrust onto the fan during the advance of the St. Louis sublobe (Figs. 11 and 12). The relationships visible in this exposure indicate that the St. Louis sublobe advance occurred while a substantial amount of debrismantled, stagnant Rainy lobe ice was still present south of the Giant’s Range. 116 �REFERENCES Baker, R.G., 1964, Late-Wisconsin glacial geology and vegetation history of the Alborn area, St. Louis County, Minnesota: University of Minnesota Master’s Thesis, 44 pp. Baker, R.G., 1965, Late-glacial pollen and plant macrofossils from Spider Creek, So. St. Louis County, MN: Geological Society of America Bulletin v. 45, p. 645-665. Ballantine, J.W., 1991, Late-Wisconsin Stratigraphy and Glacial History of Southwestern St. Louis County, Minnesota: Unpublished Master’s Thesis, University of Minnesota Duluth, 154 pp. Birks, H.J.B., 1976, Late Wisconsinan vegetational history at Wolf Creek, central Minnesota: Ecological Monographs v. 46, p. 395-429. Carney, S.J., 1996, Paleohydrology of the Western Outlets of Glacial Lake Duluth: Unpublished Master’s Thesis, University of Minnesota Duluth, 129 pp. Farnham, R.S., McAndrews, J.H., and Wright, H.E., 1964, A Late-Wisconsin Buried Soil Near Aitkin, Minnesota, and its Paleobotanical Setting: American Journal of Science, v. 262, p. 393–412. Fenton, M.M., Moran, S.R., Teller, J.T., Clayton, L., 1983. Quaternary stratigraphy and history in the southern part of the Lake Agassiz Basin. In Teller, J.T., Clayton, L., eds., Glacial Lake Agassiz, Geological Association of Canada Special Paper v. 26, p. 49-74. Hobbs, H.C., 1983, Drainage relationships of Glacial Lakes Aitkin and Upham and early Lake Agassiz in northeastern Minnesota. In Teller, J.T. and Clayton, L., eds., Glacial Lake Agassiz. Geological Association of Canada Special Paper v. 26, 245-259. Jennings, C.E., and Reynolds, W.K., 2005, Surficial Geology of the Mesabi Iron Range, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, v. 164. Johnson, M.D., Adams, R.S., Gowan, A.S., Harris, K.L., Hobbs, H.C., Jennings, C.E., Knaeble, A.R., Lusardi, B.A., and Meyer, G.N., in press, Quaternary lithostratigraphic units of Minnesota: Minnesota Geological Survey Report of Investigations RI-68. Knaeble, A.R., and Hobbs, H.C., 2009, Surficial geology, pl. 3 of Boerboom, T.J., project manager, Geologic atlas of Carlton County, Minnesota: Minnesota Geological Survey County Atlas, C-19, pt. A, 6 pls., scale 1:100,000. Knaeble, A.R., Meyer, G.N., and Hobbs, H.C., 2004, Surficial geology, pl. 3 of Setterholm, D.R., project manager, Geologic atlas of Crow Wing County, Minnesota: Minnesota Geological Survey County Atlas C-16, pt. A, 6 pls., scale 1:100,000. Knaeble, A.R., Meyer, G.N., Marlow, L.M., Larson, P.C., and Mooers, H.D., 2005, Deposits and Landforms in the Region Glaciated by the St. Louis Sublobe, in Robinson, L. ed., Field Trip Guidebook for Selected Geology in Minnesota and Wisconsin, Minnesota Geological Survey, Minneapolis, p. 40–79. Larson, P.C., Mooers, H.D., and Marlow, L.M., 2004, Early advance of the St. Louis sublobe: A revised chronology of the deglaciation of northeastern Minnesota [abstract]; Institute on Lake Superior Geology Proceedings, 50th Annual Meeting, Duluth, MN, v. 50, part 1, p.100-101. Lehr, J.D., and Hobbs, H., 1992, Field Trip Guidebook for the Glacial Geology of the Laurentian Divide Area, St. Louis and Lake Counties, Minnesota: Minnesota Geological Survey Guidebook Series 18. Leith, C.K., 1903, The Mesabi Iron-Bearing District of Minnesota: USGS Monograph, v. 43, pp. 345. Leverett, F., 1932, Quaternary Geology of Minnesota and Parts of Adjacent States: USGS Professional Paper, v. 161, 149 pp. Lowell, T. V., Larson, G.J., Hughes, J.D., and Denton, G.H., 1999, Age verification of the Lake Gribben forest bed and the Younger Dryas Advance of the Laurentide Ice Sheet: Canadian Journal of Earth Sciences, v. 36, p. 383– 393. Marlow, L.M., 2004, Late Glacial and Early Holocene history of the Glacial Lakes Aitkin and Upham basin, NorthCentral Minnesota: Implications for the timing of post-glacial eolian activity. Unpublished Master’s Thesis, University of Minnesota Duluth, 82 pp. Meyer, G.N., 1993, Surficial geologic map of parts of Koochinging, Itasca, and Beltrami Counties, north-central Minnesota: Minnesota Geological Survey Miscellaneous Map M-76, scale 1:250,000. Mooers, H.D., and Lehr, J.D., 1997, Terrestrial record of Laurentide ice sheet reorganization during Heinrich events: Geology v. 25, p. 987-990. Mooers, H.D., 1988, Quaternary history and ice dynamics of the late Wisconsin Rainy and Superior lobes, central Minnesota. Unpublished Doctoral Thesis, University of Minnesota, 200 pp. Winchell, N.W., 1901, Glacial Lakes of Minnesota. Geological Society of America Bulletin v. 12, p. 109-128. Winter, T.C., 1971, Sequence of Glaciation in the Mesabi-Vermilion Iron Range Area, Northeastern Minnesota: USGS Professional Paper, v. 750-C, p. C82–C88. 117 �Winter, T.C., Cotter, R.D., and Young, H.L., 1973, Petrography and Stratigraphy of Glacial Drift, Iron Range Area, Northeastern Minnesota: USGS Bulletin, v. 1331-C, p. 50. Wright, H.E., Jr., 1955, Valders drift in Minnesota: Journal of Geology, v. 63, p. 403-411. Wright, H.E., and Watts, W.A., 1969, Glacial and Vegetational History of Northeastern Minnesota: Minnesota Geological Survey Special Publication, v. 11, 59 pp. 118 ��FIELD TRIP 7 Saturday, May 17, 2014 GEOLOGY AND GOLD MINERALIZATION OF THE VIRGINIA HORN AREA LEADERS: Mark Jirsa (Minnesota Geological Survey), William Rowell and Richard Sandri (Vermillion Gold LLC), and Jason Richter (Minnesota Department of Transportation) INTRODUCTION The local term “Virginia horn” applies to an area near the town of Virginia, where the generally easttrending, Paleoproterozoic, Biwabik Iron Formation makes an abrupt bend to the southwest, creating a marked anomaly in the map pattern (Fig. 1). The iron-formation unconformably overlies well-exposed Neoarchean bedrock within an uplifted, wedge-shaped block. This trip visits exposures that provide an overview of the Archean metavolcanic, meta-igneous, and metasedimentary rocks—including a Timiskaming-type successor-basin sequence, and Paleoproterozoic iron-formation and associated strata. Archean quartz-feldspar porphyry intrusions were the locus of deformation, alteration, and associated gold mineralization. The area has a long history of intermittent gold prospecting, and the potential for mineable quantities is currently under investigation by Vermillion Gold, LLC. Channel sampling and drill core from that exploration work will be displayed. Some of the most recently acquired core in the area is a product of highway relocation work underway to accommodate proposed new iron mining. The following field guide is modified from Jirsa and Green (2011). The stops are ordered for expeditious travel, rather than stratigraphy or geochronology. All UTM coordinates are given using NAD 83, Zone 15N. It is likely that more stops are described here than can reasonably be covered in a single day. Stops missed during this excursion can be visited by individuals using the guide, with the caveat that some stops may require permission from land owners. Figure 1. Generalized geologic map of northeastern Minnesota showing location of the Virginia Horn area (black outline). 119 �Geology of Archean Rocks (STOPS 1-5, 9, 10) The Archean rocks in the Virginia horn area (Fig. 2) are part of the Wawa subprovince of Superior Province, and are similar in most respects to other greenstone-granite terranes of the subprovince. The supracrustal rocks in the horn are separated from the well-known Vermilion district to the north by the Giants Range Batholith—a large, composite body consisting of several intrusive generations and compositions. The mafic volcanic and hypabyssal intrusive components in the Virginia horn may be equivalent to the Ely Greenstone in the Vermilion district, which has a 207Pb/206Pb zircon age of associated felsic strata of 2722.6±0.9 Ma (Peterson and others, 2001). The supracrustal rocks in the horn are subdivided into northern and southern panels on the basis of metamorphic grade and deformation style. The northern panel, immediately south of the Giants Range batholith, contains intensely lineated, amphibolite-grade schist having volcanic, intrusive, and clastic protoliths (Minntac sequence). The southern panel contains lithologically and stratigraphically similar rocks that were metamorphosed to much lower grades, ranging from prehnite-pumpellyite to low greenschist (Mud Lake sequence). The two panels are separated by the east-trending, post-metamorphic, Laurentian fault. The two sequences are stratigraphically, lithologically, and geochemically identical; suggesting that they may represent different crustal exposure levels of the same stratigraphic package. The metamorphic cleavage-forming event in both panels was the second (D2) of three regional scale deformation events—no metamorphic effects are recognized from the other two deformations. The first (D1) involved upright folding, soft-sediment deformation, and complex faulting. Strata of the southern panel form the broad, southwest-plunging, Mud Lake syncline (Fig. 2.B) and many smaller sympathetic folds—all inferred to be D1 structures. The syncline is cored by graywacke, slate, and minor felsic tuff, and has outer limbs of calc-alkalic and tholeiitic volcanic strata. The Mud Lake strata and D1 structures (F1 fold axes) developed in it were cut by felsic quartz- and feldspar-phyric (QFP) intrusions. However, the QFP intrusions are lithologically similar to some layers within graywacke, suggesting the possibility of temporal overlap between dacitic magmatism and sedimentation. Strata in the Mud Lake syncline, and the quartzofeldspathic dikes that intrude them, are bisected by a fault- and unconformitybounded, alluvial fan-fluvial-volcanic succession known as the Midway sequence (see discussion below). All three sequences described above were metamorphosed and deformed during D 2, which has been bracketed locally between about 2674 Ma and 2682 Ma (Boerboom and Zartman, 1993). The third deformation event (D3) produced localized semi-brittle crenulation of D1 and D2 structures, and selective reactivation of earlier-formed faults. Based on seismic and geochronologic work in adjacent Ontario (e.g., Percival and Helmstaedt, 2006), D1 may be equated with the Shebandowanian orogeny at about 2695 Ma. It may represent collision of the Wawa subprovince with the composite Superior superterrane to the north. The second deformation (D 2) occurred at about 2680 Ma during the Minnesotan orogeny. It can be attributed to oblique collision of the Minnesota River Valley subprovince with the Superior superterrane along a north-dipping suture known as the Great Lakes Tectonic Zone in south-central MN (Percival and others, 2006). The three major deformation events are more or less coaxial, reflecting a continuum of roughly NW-SE-directed compression. The D3 event appears to have been a late manifestation of this, perhaps at a time when rocks were at sufficiently high crustal levels to elicit mainly partitioned brittle responses. The Midway sequence—a Timiskaming-type assemblage (STOPS 2, 5) The Midway sequence forms a wedge of strata <500 meters thick that youngs consistently southward. The basal (NW) contact is not exposed, but intersections in 10 drill holes indicate the contact is a fault in some localities, and an unconformity in others. The latter is indicated in drill core by the presence of sand-filled fractures in the subjacent Viking porphyry intrusion, and the abundance of QFP clasts in the “overlying” Midways sequence. The Midway sequence contains attributes of Timiskaming-type successions, including temporal and geographic association with large fault systems, alkalic and calcalkalic volcanic and intrusive rocks (hornblende trachyandesite), and conglomerate containing clasts of trachyandesite, together with those derived from older plutonic and strongly foliated substrate bedrock 120 �(Jirsa and Boerboom, 2003). The origin of such Timiskaming-type assemblages has been variously ascribed to localized extension in regional transpressional regimes (Jirsa, 2000; Corcoran and Mueller, 2007), or regional extension in response to imbrication and crustal loading during terrane accretion (Bleeker, 2012). Isolated occurrences of sequences similar to the Midway are exposed in other parts of Minnesota—near International Falls (Seine Group), in the Vermilion district (Gafvert Lake sequence), and in the Knife Lake area (Ogishkemuncie conglomerate). The ages for deposition of the Seine Group are 2693±1 to 2692±1 Ma (Fralick and Davis, 1999); the Gafvert Lake sequence is 2689.7± 0.8 Ma (Lodge and others, 2013); and the Ogishkemuncie conglomerate contains clasts of 2690.83 Ma Saganaga Tonalite (Driese and others, 2011). The broad equivalence of these sequences, and the fact that most young southward, indicates more or less synchronous development, which is consistent with an origin involving a single event of regional extension. Filler—taken from an old GAC GEOLOG publication 121 �Fig ure 2. Geologic map (A.) and schematic cross-section (B.) of the Virginia Horn area (modified from Jirsa and others, 1998) showing details of field trip STOPS 1 to 10. Block arrows on B indicate directions of stratigraphic facing. 122 �Gold Mineralization (STOP 3) In the 1930s visible gold was discovered by Minnesota Geological Survey geologist J.W. Gruner (Grout, 1937) in Archean rocks adjacent to a railroad grade cutting through the Virginia Horn (Fig. 2A and STOP 3). Perhaps because of the regional emphasis on iron ore mining, there was no systematic exploration for gold in the Virginia Horn until the 1980s. During the 1980s Newmont Mining, Rhude and Fryberger, Resources Limited, and American Shield conducted exploration programs that included 43 drill holes totaling 20,000 feet, geologic mapping, soil and outcrop geochemical surveys, and ground geophysical surveys. Most of the 1980s exploration focused on well exposed knobs of what is known locally as the Viking quartz-feldspar porphyry (QFP), which strikes west-southwest from the western side of the Pike River fault (Fig. 2B). Within the QFP, gold is concentrated in zones of variable brittle-ductile deformation (likely both D2 and D3), with associated carbonate-sericite alteration and abundant quartz veins. Proximal to the Pike River fault, higher grade gold mineralization occurs in one to three cm-thick quartz veins with pyrite, arsenopyrite and free gold, and in greyish quartz flooded zones with acicular arsenopyrite but no visible gold. One kilometer to the west of the Pike River Fault, most of the known gold mineralization is associated with the Viking QFP and variably sericitized porphyritic dacite that rarely outcrops. Gold is predominantly concentrated in one to three cm anastomosing quartz veins with pyrite and arsenopyrite concentrated along vein margins. Arsenopyrite occurs in irregular masses and not in the acicular habit associated with gold mineralization one km to the east. Recent studies of the geology of the Virginia Horn area by the Minnesota Geological Survey have shown that the Virginia Horn prospect is located within a Timiskaming-type geologic setting with good potential for gold mineralization beyond the porphyry and into adjacent metasedimentary and metavolcanic rocks (Jirsa and Boerboom, 2003; Bleeker, 2012). Subsequent analyses of samples from outcrop and drill core have confirmed that these rock types are also gold-enriched. Geologic Setting of Paleoproterozoic rocks (STOPS 6-8) The Paleoproterozoic strata exposed in the Virginia Horn are part of the Animikie Group, a sequence of sedimentary rocks, including basal quartzite and siltstone (Pokegama Quartzite), medial iron-bearing strata (Biwabik Iron Formation), and upper graywacke and shale of turbidite origin (Virginia Formation). Current models indicate deposition in a back-arc basin that evolved into a northward-migrating fore-deep during the compressional phase of the Penokean orogen—largely complete by about 1850 Ma (Schulz and Cannon, 2007; Pufahl and others, 2010). A depositional age for iron-formation can be inferred from interbedded volcanic tuff in the equivalent Gunflint Iron Formation to the northeast, which produced a UPb zircon date of approximately 1878 Ma (Fralick and others, 2002). The contact between ironformation and overlying slate of the Virginia Formation is marked by the presence of breccia and ejecta formed during the 1850 Ma Sudbury meteorite impact event. The ejecta contain abundant petrographic evidence of impact origin, including the presence of zoned spherules and quartz fragments displaying multiple planar deformation features. The impact-related horizon, known as the Sudbury Impact Layer, is well exposed in the Gunflint Lake area of northeast Minnesota (Jirsa and others, 2011), in the Thunder Bay area of adjacent Ontario (Addison and others, 2005), and in Michigan (Cannon and others, 2010; Pufahl and other, 2007); however, it can be seen only in drill core on the Mesabi range. Tuffaceous layers in basal strata of Virginia Formation were sampled a few meters above the Sudbury Impact Layer and produced an age of 1832±3 Ma (Addison and others, 2005). The belt of exposure forming the Mesabi Iron Range defines a regional monocline striking ENE and dipping shallowly (0-12 degrees) southward. The exception to this trend is in the Virginia Horn, where strike varies from N-S to NE, and dips as great as 25 degrees are recorded. This paired syncline-anticline is inferred to be related to uplift of a horst, now manifest in the Archean core of the structure, which formed by a combination of folding and faulting (Morey, 2003). Offset along the Laurentian fault, which was south-side down after the D2 Archean metamorphic and deformation event, was subsequently reactivated during the Paleoproterozoic to produce north-side down movement during and after deposition of the Animikie Group. 123 �DESCRIPTION OF FIELD TRIP STOPS STOP 1 Archean pillowed and massive greenstone—Old Gilbert school Location: UTM (NAD 83, Zone 15): 539,820E/5,259,750N; north edge of athletic fields for former Gilbert Junior High School off Wisconsin Avenue, 4 blocks northwest of State Highway 37. Description: This outcrop of pillowed and massive basalt is part of the Archean Mud Lake sequence, metamorphosed to low greenschist-grade. Pillow shapes indicate stratigraphic facing is to the northwest, consistent with the location of this outcrop on the south side of a major D 1 structure known as the Mud Lake syncline. Note also the presence locally of fractures and shallow depressions on the outcrop surface that are filled with reddish jasper, presumably deposited by overstepping of Paleoproterozoic seas onto the eroded surface of Archean bedrock during deposition of the Biwabik Iron Formation. Discussion: Detailed structural study by Jirsa and others (1998) and Jirsa and Boerboom (2003b) demonstrate that the tholeiitic and calc-alkalic volcanic rocks and tholeiitic intrusions are conformably overlain by graywacke and slate of the Mud Lake sequence (STOPS 3 and 4). In detail, the Mud Lake sequence forms a broad, twice-deformed, west-plunging syncline that has been segmented by faults of several generations. STOP 2 Archean volcanogenic conglomerate of the Midway sequence Location: UTM: 539120E/5261040N; Old Railroad trail Description: This former railroad cut exposes parts of the volcanic and lower conglomerate facies of the Timiskamingtype Midway sequence. Figure 3 shows the position of this exposure within a composite stratigraphic section. The rocks here are characterized by disorganized beds of poorly sorted conglomerate and volcanic breccia. Dark red, green, and purple clasts of hornblende- and plagioclase-phyric trachyandesitic composition are most abundant. Both normal and reversed grading are preserved locally. Clasts are as large as 25cm, and diamictites containing outsized clasts are common in this unit. Flattening of clasts in the plane of D2 is apparent, and a matrix of varied grain size locally displays anastomosing S 2 cleavage. Overall, the conglomerate contains clasts representing all lithologic components of the Mud Lake sequence and the porphyry dikes that intruded it. However, significant variations in clast content and internal organization of bedding characterize these units, and these attributes vary gradationally both laterally and with stratigraphic height. Figure 4 shows the map and cross-section distribution of the Midway sequence based on both drill core and surface exposures. The Upper conglomerate facies will be visited at STOP 5. Discussion: Remarkably, the polychromatic trachyandesite clasts are identical with those in parts of the Shebandowan assemblage exposed some 240 km, more or less along strike to the NE in Ontario (e.g., Aubut and Campbell, 2012), despite the intervening Giants Range batholith and other terranes. Figure 3. Composite cross section of the Midway sequence. Dark polygons represent clasts of trachyandesitic to trachybasaltic composition; white polygons represent clasts of quartzofeldspathic porphyry identical with the Viking QFP (STOP 3); which is represented diagrammatically by the dot pattern. (From Jirsa and Boerboom, 2003, Fig. 2.4). 124 �STOP 3 STOP 2 Figure 4. Surface and drill core-based geology of the “Viking Prospect area (STOPS 2 and 3). A) shows geologic map view; B) shows drill holes that intersected the northeastern (basal) contact of Midway sequence with adjacent rocks. From Jirsa and Boerboom, 2003, Fig. 2.6. 125 �STOP 3 Archean Graywacke, argillite, and quartzofeldspathic porphyry with Au mineralization Location: UTM: 537715E/5261240; Old Railroad trail Description: This former railroad cut and outcrops nearby expose interlayered graywacke and argillite of the Mud Lake sequence, intruded locally by quartzofeldspathic porphyry. One of the earliest reports of gold in Minnesota (1930) was made at this locality, and visible gold can still be found associated with small quartz veins. The porphyry intrusions here are identical with and presumably apophosial offshoots of the larger Viking QFP exposed to the east along and south of the trail (Fig. 4A). Regionally, the porphyry contains ornately embayed phenocrysts of quartz as large as 2cm, smaller albite phenocrysts, and minor mica in an aphanitic quartzofeldspathic groundmass. Groundmass is commonly crossed by anastomosing shear planes and altered to combinations of quartz, sericite, dolomite, iron-carbonate (ankerite and ferroan dolomite), pyrite, and locally arsenopyrite. Channel samples on this outcrop (Fig. 5) and core from several nearby exploration drill holes will be displayed and discussed. Discussion: During 2009 and 2010, Vermillion Gold, LLC (http://vermilliongold.com) completed nine drill holes designed to reevaluate areas where gold mineralization was intersected by the 1980s drill holes, and test new targets outside of the Viking QFP. Five of the nine drill holes have focused on gold mineralization in altered porphyritic dacite that outcrops adjacent to a railroad grade on the western side of the property (STOP 3). A channel sample of intercalated porphyritic dacite and metasediments taken from the outcrop by Newmont in the 1980s averaged 1.2 gpt over a sample length of 77.5 ft. Vermillion Gold’s 2009 drill hole (Fig. 6) beneath the railroad grade outcrop intersected 1.1 gpt gold over an interval of 195.7 feet and includes intersections of 16.1 gpt/3.6 ft and 11.4 gpt/4.2 ft. A 2010 drill hole, located approximately 150 m south of the railroad outcrop, intersected a thick unit of porphyritic dacite locally cut by thin fingers of Viking QFP. A 224.9 ft section of the drill hole averages 1.0 gpt gold and includes a 77.9 gpt/2.3 ft sample with visible gold in a quartz vein. Within the thick, gold-enriched intersections, values of 100 to 300 ppb are associated with disseminated pyrite and arsenopyrite. Samples with multi-gram gold values include quartz veins with free gold. There is a strong correlation between gold and arsenic values, however, samples with the highest grade gold values reflect free gold and not increased arsenopyrite content. 126 �Figure 5. Location (upper image) and analytical results (lower image) of channel samples on outcrops at STOP 3 (imagery from Vermillion Gold, LLC) 127 �Figure 6. Partial log of core from drill hole VH-09-4, showing lithologic and analytical results (from records of Vermillion Gold, LLC). 128 �STOP 4 Archean graywacke and slate, intruded by quartzofeldspathic porphyry—Bourgin Road Location: UTM: 536,311E/5,260,659N; road cut on east side of Bourgin Road. Description: Outcrops along this side of the road expose quartzofeldspathic porphyry (QFP) intruded into variably deformed graywacke, siltstone, and slate of the Mud Lake sequence. The sedimentary rocks here are moderately deformed, but much of that deformation is inferred to predate the main cleavage-forming event D2, and some may have occurred prior to lithification. The QFP is large and continuous to the east, but at this locality, appears to be segmented into a zone of multiple anastomosing dikes. Both graywacke and QFP are intensely altered to some combination of iron-carbonate minerals (ankerite, ferroan dolomite) and sericite. Regionally, this style of alteration is commonly, though not always associated with the QFP intrusions—presumably because they remained more structurally rigid than the enclosing sedimentary rocks during the shear-related deformation event that accompanied alteration late in D2. Most gold mineralization in the area is closely allied to this alteration, yet this outcrop is apparently barren. STOP 5 Archean conglomerate—Midway sequence Location: UTM: 535,713E/5,259,459N; driveway at No. 7 Mesabi Lane, village of Midway. NOTE: This is private property! Permission must be obtained before entering. Description: The Archean conglomerate and lithic sandstone that form this driveway surface represents the Upper conglomerate facies of the Timiskaming-type Midway sequence. It differs from the Lower conglomerate facies at STOP 2 in containing more diverse clast content, more rounded clasts, graded bedding, and more abundant sandy beds. These attributes imply submarine deposition. Taken in the context of the sequence stratigraphy (Fig. 3), this may indicate basin deepening over time, which is consistent with observations of other Timiskaming-type successions (Bleeker, 2012). The conglomerate contains clasts of basalt, graywacke, quartzofeldspathic porphyry (QFP), and porphyritic trachyandesite. This provenance indicates that the older Archean rocks of the Mud Lake sequence were intruded by QFP, deformed, uplifted and eroded to provide detritus to what was probably a “pull-apart” or extensional basin developed along a major structure now occupied by the Pike River fault zone. The southward-younging basin is bounded by both faults and an inferred unconformity (Fig. 7). Discussion: Note also the presence of red jasper in depressions and joints as at STOP 1, indicating that this outcrop surface represents paleo-seafloor during deposition of the Paleoproterozoic Biwabik Iron Formation. Figure 7. Schematic illustration of Midway basin geometry prior to steepening by D2 compressional deformation (from Jirsa and Boerboom, 2003). STOP 6 Paleoproterozoic Biwabik Iron Formation-Highway 53 Location: UTM: 536,263E/5,256,200N; Outcrops along north-bound exit ramp from Hwy 53 to Hwy 37, Eveleth. Description: This exposure of gently south-dipping strata is part of the Lower Cherty member of the Biwabik Iron Formation. It lies nearly at the crest-line of the anticline that forms half of the Horn structure. The ironformation forms a transitional contact with the underlying Pokegama Quartzite exposed at Stop 7. Both formations have fine- to coarse-grained sandy textures and cross-bedding, consistent with a high-energy, 129 �near-shore depositional environment. Bimodal-bipolar cross-strata in the iron-formation indicate that tidal currents may have played an active role in deposition (Ojakangas, 1993), though tidal bundles have not been documented. The most significant difference between these two formations is the abrupt change in sediment source from the extrabasinal quartz grains in the Pokegama, to recycled, chemically precipitated chert nearly devoid of detrital grains in the Biwabik. Discussions: 1) One possible explanation for the abrupt change in sediment source in the transition from Pokegama Quartzite to basal Biwabik Iron Formation may be related to topography of the watershed. If the terrane was a relatively flat peneplain, the continued rise of sea level may have essentially drowned the detrital source region. 2) The Biwabik Iron Formation is generally divided into 4 members; termed lower Cherty, Lower Slaty, Upper Cherty, and Upper Slaty (Fig. 8). These are convenient field names based on readily apparent bedding attributes; however, they are somewhat misleading. The cherty units are beds of recycled granular chemical precipitates including chert, iron oxides, iron carbonates, and iron silicates. They are interbedded on all scales with “slaty” units of fine-grained, laminated iron silicates and iron carbonates. In most of the Mesabi range, the iron-formation and associated strata were not significantly metamorphosed, and much of the textural and mineralogic attributes are products of diagenesis and subsequent fluid movement. As a result, no slaty cleavage exists, and thus the term “slate” is applied only as a field identifier. Most, though not all of the iron ore mined on the Mesabi range is extracted from cherty members (Fig. 8B). Figure 8. Simplified geologic map (A) of the Mesabi Iron Range showing locations of taconite mines (black) and drill holes (numbered), and cross-section (B) showing subdivision of the Biwabik Iron Formation based on mined sections and drill core, and approximate intervals mined for taconite at each locality (modified from Jirsa and others, 2008). 130 �STOP 7 Paleoproterozoic Pokegama Quartzite-Highway 53 Location: UTM: 535,956E/5,256,913N; Outcrops along north-bound entrance ramp onto Hwy 53 from Hwy 37, Eveleth. Description: In the area of the Virginia horn, the Pokegama consists largely of siltstone and shale. This exposure represents the sandy, upper member of the Pokegama Quartzite, which is only of fraction of the units’ total thickness of 26-51 m. It is quartz arenite characterized by coarse grain size, intraclasts of shale and siltstone, and massive beds as thick as 1.5 m, separated by thin beds of shale and siltstone. Ojakangas (1993) interpreted that the deposition of this facies occurred within a high-energy, lower tidal or subtidal environment. Because stratigraphic dip is southward, the outcrop at this location is inferred to be several meters stratigraphically beneath the iron-formation at STOP 6. Discussions: 1) The basal strata of the Pokegama Quartzite is marked locally by conglomerate composed of a poorly sorted array of clasts derived from underlying Archean and Paleoproterozoic (diabase dikes) bedrock. The patchy distribution of conglomerate, and the presence of red jasper in fractures on some Archean exposures (as at STOPS 1 and 5), implies that chemical sedimentation abruptly overstepped clastic deposition during early evolution of the Animikie basin. 2) The contact between the Pokegama Quartzite and overlying Biwabik Iron Formation is conformable and gradational. In the transition zone, both units contain similar sedimentary structures and grain size, implying continuity of depositional process. The primary difference between them is grain composition—the Pokegama grains are epiclastic vs. those in the Biwabik are reworked from poorly lithified or unlithified chemical precipitates. The absence of epiclastic grains in the nearly 200 m-thick stratigraphic section of the Biwabik begs the question: How was this detritus abruptly shut off from the watershed? STOP 8 Abandoned and flooded Rouchleau “natural ore” mine Location: UTM: 535,710E/5,261,650N; Mineview in the Sky overlook near Virginia. Description: North from this overlook is a 3-mile-long complex of abandoned mining properties, known collectively as the Rouchleau mine, all developed within the Paleoproterozoic Biwabik Iron Formation. Actually, within this view there were some 15 separately named mines that collectively shipped ore during the period 1893-1986. All of them, and nearly 400 more along the 150-mile long Mesabi Iron Range, extracted oxidized (hematite- or goethite-rich) and leached (silica-depleted) iron-formation referred to locally as “natural ore.” Iron-formation at this point lies on the north-trending limb separating the syncline to our west and the anticline to the east. The natural ore deposits here are localized along a set of faults (Fig. 2A) that presumably provided the plumbing system for fluids that first oxidized the formation and produced permeability, then leached silica from the porous zones, thus increasing ore tenor. Natural ores typically contained as much as 50 percent iron and less than 10 percent silica. Since about the 1950’s, the principal “ore of choice” has shifted from hematite- to magnetite-rich deposits. The mammoth open-pit mine in the distance to the northwest, and another just southwest of the highway, are developed in unoxidized magnetite ore containing about 30 percent iron, and 50 percent silica. The ore mined at these locations, and several others along the range is the source of the iron concentrate known as taconite. The name taconite has also been applied generally to magnetite-bearing iron-formation where it contains sufficient iron content to be mined for a profit using today’s technology. 131 �Discussions: 1) Origin of “natural ore” Nearly 70 percent of the 3.6 billion metric tons of iron ore produced on the Mesabi range between years 1892 and about 2000 was extracted as natural ores. Although it is generally accepted that these ores formed by oxidation and leaching along folds, faults, and bedding planes, the source and composition of altering solutions and the timing of alteration have been subjects of considerable debate among economic geologists for nearly 80 years. Much of the literature and geologic observations on the issue are reviewed in Morey (2003). Many writers support the concept of descending meteoric waters to account for the dissolution of silica and oxidation of iron minerals. Others, including Gruner (1930) believed the geologic features were better explained by ascending hydrothermal solutions. Gruner’s theory failed to gain common acceptance, in part because no driving mechanism for such a hydrothermal system could be envisioned. The integration of Animikie Group strata into the tectonic context of the Penokean orogen in east-central Minnesota revived the theory of hydrothermal fluid flow within the Pokegama Quartzite and ultimately the iron-formation, as part of a continent-scale, gravity-driven ground-water system (Morey, 1999). The debate continues—fueled in part by the observation that most of the alteration occurs near the present land surface. Field trip #1 in this guide explores these issues more fully. 2) Highway relocation This overlook will soon be gone, as taconite mining to the southwest is slated to expand into the Rouchleau pit area. As a result, U.S. Highway 53 will be rerouted to skirt the new mining. Currently the Minnesota Department of Transportation (MnDOT), the Department of Natural Resources, and the mining company are engaged in geotechnical work and discussions to evaluate the various potential new routes for the highway. Exploratory drilling (for both ferrous and non-ferrous metallic mineral potential), geotechnical drilling, and geophysical surveying have been conducted by MnDOT to assess the financial and engineering risks of two relocation alternatives proposed through the Rouchleau Pit (Fig. 9), as well as a third alternative through the mine site to the southwest. This type of apparent conflict between surface infrastructure and mining has characterized development along the Mesabi Iron Range for more than 120 years. It was particularly acute during the shift in the “ore of choice” in the 1950’s and 1960’s from natural ores that typically occur in narrow and steep, structurally-controlled deposits; to taconite that occurs in more widely distributed layers. Entire cities have been moved and removed, as at Hibbing (See Field Trip B in this guide book for example). 3) Wind turbines on northern horizon Minnesota Power’s Taconite Ridge Energy Center is visible to the north, located on U.S. Steel property in Mt. Iron. It consists of 10 wind turbines that generate 25-megawatts, capable of powering the equivalent of 8,000 homes annually. (Reference=http://www.hometownfocus.us/news/2013-09-06/Mining_Features; accessed 2/2014) 132 �Figure 9—Map showing potential new routes for a portion of Highway 53 (blue and green lines through Rouchleau mine complex), and associated test drilling (magenta dots=holes to iron-formation; green dots=holes to Archean) to evaluate both the engineering and resource implications of proposed realignments. Colored overlay on air photo imagery shows geologic units and faults from Jirsa and others (2012). Image created using data from MnDOT. STOP 9 Archean Giants Range batholith at “Confusion Hill” Location: UTM: 534,337E/5,269,458N; outcrops at Laurentian Wayside, near Highway 169 south of its split with Highway 53. Description: Exposures at Confusion Hill are a small part of the Giants Range batholith, which forms the core bedrock of the Laurentian (drainage) divide. The batholith is a belt of intrusions that can be traced on geophysical maps and outcrop east to the Mesoproterozoic Duluth Complex, and west beyond the western border of Minnesota. It separates Archean supracrustal sequences in the Virginia Horn from those of the Vermilion District to the north—making stratigraphic correlation between the two districts speculative in the near absence of high-precision geochronologic data. Exposed near this wayside and in road cuts on both sides of the highway is an array of variably layered intrusions having both tonalitic (white) and dioritic (black) compositions. A cursory look shows intrusive relationships that conclusively demonstrate that diorite was emplaced into tonalite at one locality; and at another, tonalite was emplaced into diorite. In detail, all compositions intermediate between the two end members are also present locally. Although the dioritic component is abundant here, the bulk of the mapped unit is tonalitic. Emplacement of this unit, now known as the Lookout Mountain tonalite, probably involved some degree of magma mingling. It may be equivalent to tonalitic gneiss exposed along strike to the east and having a somewhat imprecise U-Pb zircon age of 2718±67 Ma (Southwick, 1994). Dikes of tonalite that cut the adjacent high-grade supracrustal rocks of the highgrade Minntac sequence contain metamorphic fabrics, yet little evidence of metamorphic origin can be 133 �seen in the interior of the body, implying it is syntectonic to pre-tectonic with respect to D2 deformation. U-Pb zircon dates (Boerboom and Zartman, 1993) of two components of the batholith exposed to the north bracket the age of D2 deformation between about 2674 and 2682 Ma. STOP 10 Archean diorite in Giants Range batholith Location: UTM: 534,337E/5,269,458N; outcrops at Laurentian Wayside, near Highway 169 south of its split with Highway 53. Description: Rock type exposed in this now partially reclaimed quarry is a massive hornblende-pyroxene-biotite diorite. Currently, little is known about the intrusion, as no petrologic study, geochronologic analysis, or mapping of its contacts has been conducted by the authors. Nevertheless, it is similar to other small alkalic plutons in and adjacent to the Giants Range Batholith. These vary in composition—in some cases within a single intrusion—from syenite to monzodiorite to lamprophyre and pyroxenite (Boerboom, 1994). It is interesting to speculate that this intrusion may fall into the category of the late alkalic to calcalkalic intrusions that are temporally, and likely geochemically, related to Timiskaming-type assemblages such as the Midway sequence. 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. Aubut, A., and Campbell, D., 2012, Field Trip 4—Shebandowan Mine area: in Hollings, P., MacTavish, A., and Addison, W., Institute on Lake Superior Geology Proceedings, Part 2, p. 67-73. Bleeker, W., 2012, Targeted Geoscience Initiative 4. Lode Gold deposits in ancient deformed and metamorphosed terranes: The role of extension in the formation of Timiskaming basins and large gold deposits, Abitibi Greenstone Belt—A discussion: in Summary of Field Work and other activities 2012, Ontario Geological Survey Open File Report 6280, p. 47-1 to 47-12. Boerboom, T.J., 1994, Alkalic plutons of northeastern Minnesota: in Southwick, D.L., ed., Short contributions to the geology of Minnesota, Minnesota Geological Survey Report of Investigations 43, p. 20-38. Boerboom, T.J., and Zartman, R.E., 1993, Geology, geochemistry, and geochronology of the central Giants Range batholith, northeastern Minnesota: Canadian Journal of Earth Sciences, v. 30, p. 2510-2522. Cannon, W.F., Schultz, K.J., Horton, W. Jr., and Kring, D.A., 2010, The Sudbury impact layer in the Paleoproterozoic iron ranges of northern Michigan, USA: Geological Society of America Bulletin, v. 122, p. 50-75. Corcoran, P.L., and Mueller, W.U., 2007. Time-transgressive Archean unconformities underlying molasse basin-fill successions of dissected oceanic arcs, Superior Province,Canada. Journal of Geology 115, 655–674. Driese, S.G., Jirsa, M.A., Ren, M., Sheldon, N.D., Brantley, S.L., Parker, D., and Schmitz, M., 2011, Neoarchean paleoweathering of tonalite and metabasalt: Implications for reconstructions of 2.69 Ga early terrestrial ecosystems and paleoatmospheric chemistry: Precambrian Research, v. 189, p. 1-17. Fralick, P., and Davis, D.W., 1999, The Seine-Coutchiching problem revisited: Sedimentology, geochronology and geochemistry of sedimentary units in the Rainy Lake and Sioux Lookout areas: in Harrap, R.M., and Helmstaedt, H., eds., 1999 Western Superior Transect Fifth Annual Workshop 70, Lithoprobe Secretariat, University of British Columbia, p. 66-75. Fralick, P., Davis, D.W., and Kissin, S.A., 2002, The age of the Gunflint Formation, Ontario, Canada: single zircon U-Pb age determinations from reworked volcanic ash: Canadian Journal of Earth Sciences, v. 39, p. 1085-1091. Grout, F.F., 1937, Petrographic study of gold prospects of Minnesota: Economic Geology, v. 37, p. 56-68. Gruner, J.W., 1930, Hydrothermal oxidation and leaching experiments; their bearing on the origin of Lake Superior hematite-limonite ores: Economic Geology, v. 21 pp. 697-719, 837-867. Jirsa, M.A., 2000, The Midway sequence: A Timiskaming-type, pull-apart basin deposit in the western Wawa subprovince, Minnesota: Canadian Journal of Earth Sciences, v. 37, p. 1-15. 134 �Jirsa, M.A., and Boerboom, T.J., 2003, Geology and mineralization of Archean bedrock in the Virginia Horn: in Jirsa, M.A., and Morey, G.B., eds., Contributions to the geology of the Virginia Horn area, St. Louis County, Minnesota: Minnesota Geological Survey Report of Investigations 53, p. 10-73. Jirsa, M.A., Boerboom, T.J., Chandler, V.W., 2012, Geologic map of Minnesota—Precambrian bedrock geology: Minnesota Geological Survey State Map Series S-22, scale 1:500,000. Jirsa, M.A., Boerboom, T.J., and Morey, G.B., 1998, Bedrock geologic map of the Virginia Horn, Mesabi Iron Range, St. Louis County, Minnesota: Minnesota Geological Survey, Miscellaneous Map Series M-85, scale 1:48 000. Jirsa, M.A., Fralick, P.W., Weiblen, P.W, and Anderson, J.L.B., 2011, The Sudbury impact layer in the western Lake Superior region: in Miller, J.D., Hudak, G.J., Wittkop, C., and McLaughlin, P.I., eds., Archean to Anthropocene: Field Guides to the Geology of the Mid-Continent of North America: Geological Society of America Field Guide 24, p. 147-169. Jirsa, M.A., and Green, J.C., 2011, Classic Precambrian geology of northeast Minnesota: in Miller, J.D., Hudak, G.J., Wittkop, C., and McLaughlin, P.I., eds., Archean to Anthropocene: Field Guides to the Geology of the Mid-Continent of North America: Geological Society of America Field Guide 24, p. 25-45. Jirsa, M.A., Miller, J.D. Jr., and Morey, G.B., 2008, Geology of the Biwabik Iron Formation and Duluth Complex: Regulatory Toxicology and Pharmacology, v. 52, p. S5-S10. Lodge, R.W.D., Gibson, H.L., Stott, G.M., Hudak, G.J., Jirsa, M.A., and Hamilton, M.A., 2013, New U-Pb geochronology from Timiskaming-type assemblages in the Shebandowan and Vermilion greenstone belts, Wawa subprovince, Superior Craton: Implications for the Neoarchean development of the southwestern Superior Province: Precambrian Research v. 235, p. 264-277. Morey, G.B., 1999, High-grade iron ore deposits of the Mesabi range, Minnesota—Product of a continental-scale Proterozoic ground-water flow system: Economic Geology, v. 94, p. 133-142. Morey, G.B., 2003, Paleoproterozoic Animikie Group, related rocks, and associated iron-ore deposits in the Virginia Horn: in Jirsa, M.A., and Morey, G.B., eds., Contributions to the geology of the Virginia Horn area, St. Louis County, Minnesota: Minnesota Geological Survey Report of Investigations 53, p. 74-102. Ojakangas, R.W., 1993, Pokegama Quartzite: in Institute on Lake Superior Geology Proceedings, 39th Annual Meeting, Eveleth Minnesota, v. 39, Part 2, p.19-21 and 46-48. Percival, J.A., and Helmstaedt, H., 2006, The Western Superior Lithoprobe and NATMAP transects: Introduction and summary: Canadian Journal of Earth Science, v. 43, p. 743-747 and articles therein. Percival, J.A., Sandborn-Barrie, M., Skulski, T., Stott, G.M., Helmstaedt, H., and White, D.J., 2006, Tectonic evolution of the western Superior Province from NATMAP and Lithoprobe studies: Canadian Journal of Earth Science, v. 43, p. 1085-1117. Peterson, D.M., Gallup, C., Jirsa, M.A., and Davis, D.W., 2001, Correlation of Archean assemblages across the U.S.-Canadian border: Phase I geochronology (abs): Institute on Lake Superior Geology, 47 th Annual Meeting, Madison, Wisconsin, Proceedings v. 47, Part 1, p. 77-78. Pufahl, P.K., Hiatt, E.E., and Kyser, T.K., Does the Paleoproterozoic Animikie Basin record the sulfidic ocean transition?: Geology, v.38, p. 659-662. Pufahl, P.K., Hiatt, E.E., Stanley, C.R., Morrow, J.R., Nelson, G.J., and Edwards, C.T., 2007, Physical and chemical evidence of the 1850 Ma Sudbury impact event in the Baraga Group, Michigan: Geology, v. 35, p. 827-830. Southwick, D.L., 1994, Assorted geochronologic studies of Precambrian terranes in Minnesota: A potpourri of timely information: in Southwick, D.L., ed., Shorter Contributions to the Geology of Minnesota, Minnesota Geological Survey Report of Investigations 43, p. 1-19. Schulz, K.J., and Cannon, W.F., 2007, The Penokean orogeny in the Lake Superior region: Precambrian Research, v. 157, p. 4-25. 135 �FRIDAY AFTERNOON FIELD TRIPS MAY 16, 2014 136 �FIELD TRIP A Friday, May 16, 2014 STATE DRILL CORE LIBRARY—HIBBING MINNESOTA Minnesota Department of Natural Resources—Division of Lands and Minerals LEADERS: Dave Dahl, (MnDNR), and Dean Rossell (Kennecott) INTRODUCTION The Minnesota Department of Natural Resources maintains a Drill Core Library in Hibbing, Minnesota. It serves as the single State of Minnesota repository for archiving bedrock and earthen material core samples collected during minerals exploration, engineering, and geoscience research programs across the state. The library attracts a worldwide audience of scientists who use core samples to develop new ideas about the capacity of the state’s bedrock and glacial materials to host mineral resources, and to model the geologic forces and features that have shaped the state’s foundation. This trip will provide opportunities for visitors to tour the facility and view some very old exploration data collections, century-old historical cores, and recent scientific and exploratory cores. The latter include cores taken from iron-formation, Midcontinent rift peridotite and gabbro, greenstone belt prospects, and sedimentary and glacial settings. Figure 1. Interior of Building #3 showing stacked core boxes. 137 �Figure 2. Map of Minnesota showing locations of drill holes from which cores stored at DNR were extracted. 138 �The three buildings that comprise the core library facility house more than 3 million lineal feet of drilled core samples archived from approximately 9,000 exploratory and scientific borings. Some archived samples are well cuttings (depending on the drilling method employed during sampling). The archive collection contains approximately 7,000 mineral exploration cores, 1,500 roadway and bridge foundation cores, and 500 cores collected during scientific, governmental and academic research investigations (Fig. 2). Building #1, built in 1972 has a storage capacity of 400,000 lineal feet of core. Building #2, constructed in 1979 has a 600,000 lineal foot storage capacity. Building #3 (Fig. 1), originally constructed in 1989 with a capacity of 800,000 lineal feet, has been expanded twice. In 1995 the building was doubled in size through an addition, and in 2009 the building was nearly doubled in size again through addition of a wing. In present configuration, the three buildings have capacity to store approximately 4 million lineal feet of NQ-sized core samples. Core samples are normally transferred to the facility in two-foot long boxes, 5 core segments per box, or 10 feet of core per box. Box storage capacities range from 7 segments for small diameter (A- and E-size) core to 2 segments for large diameter (PQ—size) core. Boxes are generally designed to hold 50 lbs weight or less. Today, most exploratory boring samples are delivered to the library in fulfilment of statutory requirements (M.S. 103 I.601 and 103 I.605) which have been in effect since 1980. The library archives are augmented by substantial collections of historical (pre-1980) core samples that have been received via donation from mineral exploration companies or through consolidation of agency core collections. The earliest known collar date in the core collection is 1905, for exploratory cores taken along the Gunflint trail. Mineral exploration archive documents housed in Hibbing indicate that core samples were obtained in the Vermilion district as early as the 1870’s. Core samples are expensive to obtain and maintain, but that money is well spent. Samples acquired to meet one investigative objective commonly are “recycled and reused” several times in subsequent programs. They can be used to test new working models of geologic processes, and to provide new insights on the disposition and location of mineral resources. Analytical techniques (geochemical, geochronologic, and geophysical) are constantly evolving, and these cores provide ready materials for testing within well constrained geologic contexts. Archived cores have provided the basis for advancement in the evaluation of several copper-nickel, gold, titanium, and iron deposits and prospects (Tamarack, Birch Lake, Maturi, Spruce Road, Serpentine, Mud Creek, Lost Lake, Virginia Horn, Longnose, TiTac, Buckeye, Emily and others). New private investments to advance these properties, some of which include School Trust or other state-owned mineral lands, are on the order of $200 million over the past decade. The DNR recently convened a working group to increase efficient delivery of core library services and to attract research and investment in the evaluation and understanding of Minnesota resources. 139 ��FIELD TRIP B Friday, May 16, 2014 HIBBING’S IRON MINING AND CULTURAL HISTORY LEADERS: Henry Djerlev, Bob Kearney, Erica Larson, and Hibbing Historical Society Staff INTRODUCTION The nearly 120 years of iron ore mining in the Hibbing area has certainly played a large part in shaping the history and culture of the towns and residents and the rest of the Mesabi Iron Range. Within the outline of what is now called the Hull-Rust-Mahoning open pit, more than 30 separate mines operated from 1895 to the present. The early miners emigrated from dozens of countries, and all of their various languages were spoken in the mines. The mix of cultures made for awkward communications in the mines, and often created ethnic neighborhoods in the mining locations and larger towns. Initial underground mining quickly changed to open pit due to the nature of the ore body and the introduction of large steam operated drills, shovels and trains. Very quickly, with eastern monies invested, some of our larger corporations emerged, such as United States Steel. Very small "mining locations" grew into formal towns like Hibbing founded by Frank Hibbing and A. J. Trimble. Open pit mining quickly progressed from small individual pits, into one large open pit that was nicknamed "The Grand Canyon of the North". This expansion made it necessary to physically move, from 1919 to 1921, what was called North Hibbing to the south in order to make room for increased mining. This gave Hibbing another nickname: "The Town that Moved". During this short tour of Hibbing, the National Historic Landmark of the Hull-RustMahoning Mine will be visited and several of the historic buildings that were made possible by iron mining dollars. These include the spectacular Hibbing High School and the Hibbing Historical Society Museum. Stop locations given in latitude/longitude. *Departure Point- Hibbing Park Hotel (47o 25' 38.71'' N/92o 35' 26.22''W) On the short trip from the Hibbing Park Hotel to Stop 1 the bus will pass by several historic points in Hibbing such as the one time home of Andrew "Bus Andy" Anderson, built in 1920. Andy with his partner Carl Wickman initiated what was to become The Greyhound Bus Company. Carl, after losing his job at the Alice mine started as a salesman for the Hupmobile company. In 1914, after seeing the failing sales of the seven passenger Hupmobile, he tried to show Figure 1: Early photo of one of Mesabi Transportations Hupmobile Buses. his clients what a great product the 140 �Hupmobile was by taking people for short rides. Wickman was soon giving miners rides to and from work for a cheap fifteen cents a ride. When they found out that giving the miners transportation was more profitable, Wickman and Anderson created the Mesaba Transportation Company. Three years after starting the company, they were running 18 buses and were making $40,000 a year. In 1922, he sold the company for $60,000. In 1933, the company was formally named The Greyhound Corporation and was running nationally. Other historic points along this leg of the tour will be the Sons of Italy Hall (1923), Mesabi Railway Company(1921)—now Zimmy’s Restaurant, the Androy Hotel (1923), Bennett Park, and the Greyhound Bus Museum. The small city got its start by a German immigrant named Frank Hibbing who founded the town in 1893. Originally named Frans Dietrich Von Ahlen, Frank took his mother's last name of "Hibbing" which comes from English descent. Frank decided to do this out of honor for his mother who passed away in his infancy and thought that this would be a good move for his exploration of the "New World." Frank Hibbing originally settled in Beaver Dam, Wisconsin where he worked at a farm and shingle mill. Originally he had hopes of becoming a lawyer but after finding out the extreme differences between the German and English language, he decided to forego that dream and then became interested in the Figure 2 - Frank Hibbing area's most abundant resource: timber. In 1887, Frank Hibbing moved to Duluth and became a real estate salesman which eventually lead him further north into the Vermillion Range. It was not until 1892 when he and thirty men set out to cut a road from Mountain Iron to what was then called Section 22. While cutting this road, Frank Hibbing found iron ore on the ground and realized what that meant to the area's economy. Little did Frank Hibbing know at the time, this ore deposit would be one of the largest in the world! In 1893, the city of Hibbing was laid out and named in honor of Frank Hibbing. The city even has a statue to the German who had the sense to notice the reddish soil and the value that it had. Artist Robert Mitchell, born in Alice location, created the statute which was dedicated on October 21, 1941. Robert was the son of a Hanna Company mining captain for whom Mitchell location was named. Frank took so much pride in his new town that he used personal means to finance the first water plant, electrical plant, hotel, saw mill, and bank building. Frank Hibbing made Duluth his home for the last ten years of his life until his death from appendicitis on July 30, 1897. He did retain close communication with "his" town during that time. Frank Hibbing was only 40 years old. Figure 3- Frank Hibbing Park 141 �Stop 1- HULL-RUST MAHONING MINE (47o 26' 50.53'' N/92o 56' 45.86''W) "The Largest Open Pit Iron Ore Mine in the World" is more than three miles long, two miles wide and 600 feet deep. This man-made "Grand Canyon of the North" was the one of the first open pit mines on the Mesabi Iron Range. This amazing view continues to grow as the Hibbing Taconite Company mine expands its mining operations. Rotary drills, 33-cubic-yard shovels and 240-ton production trucks can been seen in action at this National Historic Site. Occasionally, you may witness a production mining blast of nearly 1 million tons used to clear bedrock away and break the taconite ore for processing in the plant. Figure 4: Recent photo of the Hull-Rust-Mahoning Pit Since 1895 more than 1.4 billion tons of earth have been removed on its 2,000 acres of land, and more than 800 million gross tons of iron ore have been shipped from the mine. At peak production in the 1940's, as much as one quarter of the ore mined in the United States came from the Hull Rust Mine. Currently Hibbing Taconite Company (Cliffs) produces approximately 8 Million tons of taconite pellets annually. Over 520 million tons of waste material and 690 million tons of iron ore have been removed. A slide presentation in the observation building explains the colorful history of the mine and early mining. An observation building, mine exhibits, mine shovel bucket, mining truck, interpretive graphics and a walking trail complete the trip to the Hull Rust Mine View. A miner poses near the edge of the pit. This area of the Mesabi Range was first explored in 1893, shortly after the Mountain Iron Mine was established in 1892. The early development was as an underground mine, but open pit mining soon proved to be a better choice because of the shallow nature of the ore deposits. The many smaller open pit mines developed in the area soon merged into one large mine. The growth of the mine even resulted in the town of Hibbing being relocated to accommodate expansion. The move started in 1919 and took two years to complete at a cost of $16,000,000. A total of 185 houses and 20 businesses were moved, and some of the larger buildings had to be cut in half for the move. Only a portion of the network of city streets and foundations from old North Hibbing remain in the vicinity of the Mine Observation Figure 5- Early Miner along the crest Overlook. of the Hull-Rust Pit Figure 6- Street sign in Other historic points along the second leg of the tour will be Frank Hibbing North Hibbing Park (1941), the Godfrey House (1920's) and the Mitchell-Tappan House (1897). 142 �Stop 2- The Hibbing High School - 1922 (47o 25' 33.30'' N/92o 55' 57.08''W) One of the buildings that was built during the Oliver Mining Company's relocation of North Hibbing was the Hibbing High School. Built in 1921 by the Oliver Mining Company, the school building originally cost almost $4 million dollars. In today's dollars that would be close to $45 million dollars! Figure 7- Hibbing High School under construction. Figure 8- Present Day Hibbing High School Why did it cost so much? Well it is simple, in order to lure prospective workers and miners to work in dangerous situations such as tunnels and around explosions, they had to provide a first class environment for their families and especially their children. So they went all out for the education. The auditorium of the high school was modeled after the famous Capitol Theatre in New York City. The auditorium has cut glass chandeliers which were imported from Belgium which light the 1800 velvet seated venue. The chandeliers were originally priced at $15,000 when built and today are insured for over $250,000 each. The auditorium has a rarity in it as well, which is a Barton pipe organ. Only two are in existence in North America. With 1800 pipes, it can synthesize any instrument excluding the violin. Figure 9- Hibbing High School Auditorium A few celebrities have attended this high school including basketball star and musician Robert Zimmerman aka "Bob Dylan". Robert Zimmerman may have been born in Duluth, Minnesota on May 24, 1941 but it is Hibbing, Minnesota where he grew up. Robert grew up in Duluth, MN until he was six years old. It was when his father was sick with polio that his family moved back to his mother's hometown of Hibbing, MN. This is where he fell in love with music and formed many bands throughout his high school career including such Figure 10- Dylan Family Home names as "The Shadow Blaster" and "The Golden Chords." 143 �During the high school talent show, Danny and the Juniors, played so loud that the principal cut off the microphone. Robert Zimmerman left Hibbing in 1959 to move to Minneapolis so he could enroll at the University of Minnesota. This is where Robert Zimmerman did two things; he fell in love with folk music and changed his name to Bob Dylan. The reason for the change was that Bob was very familiar with the poetry of Dylan Thomas. In a 2004 interview, Bob stated, "You're born, you know, the wrong names, wrong parents. I mean, that happens. You call yourself what you want to call yourself. This is the Figure 11- Bob Dylan, 1963 land of the free." Stop 3- Hibbing Historical Society Museum (47o 25' 25.92'' N/92o 56' 13.36''W) The current focus of the Hibbing Historical Society is the documentation and presentation of the early "Mining Locations” that grew up in the vicinity of the surrounding mines. There were three types of mining locations, but all had the common factor that they placed on company-owned land. There were over 175 locations between 1892 and the 1920's on the Mesabi Range. Initially miners and sometimes their families just found an open piece of land near the mine they worked at where they built a very small home from whatever building materials they could locate. These were "squatter's locations" and often called 'chicken towns' as there were often farm animals in the yards or even in the living quarters themselves, during the winter months. Figure 12- Mahoning Location and Rail Shops As time progressed the companies decided that if they organized a mining location and built modest homes for its miners, they had better success in keeping a stable and highly trained workforce. These were called "company locations" and were actually townsites that were surveyed in and could included paved streets with water and sewer utilities. Mahoning location is a fine local example of a company location in the vicinity of Hibbing. The homes in these Locations were built by the mining firms and then leased or rented to the employees. In some cases the residents were permitted to purchase their house, but not the land. Common monthly rental fees might be $1.00 per $100 invested by the mining company. Very rarely a mining company, such as United States Steel, would design and build “Model Locations". These communities would consist of more elaborate homes with full services plus community buildings such as recreation halls, hospitals and fire departments. Stellar examples of a Model Location would be Morgan Park in Duluth and the community of Coleraine on the western end of the Mesabi Range. The company plan here was for a more attractive community with higher quality constructed homes that would show them in a better light. 144 �More than 30 squatter's locations and company locations were located just to the north, east and west of Hibbing to service a like number of small early mines. In 1915, the town of Hibbing had 20,000 people who all had to uproot their homes and families and move them south to the small village of Alice. Many of the buildings were actually lifted and rolled down to Alice. The Oliver Mining Company (later to become US Steel) was the brainchild behind this move and agreed that if the town relocated 2 miles south to Alice, they would develop the downtown buildings with low interest loans for the retailers. At the Hibbing Historical Society Museum there is a large scale model of the Hibbing with excellent exhibits that describe the physical moving of that "North Hibbing" portion of the town. The move started in 1919 after four years of careful planning and was completed in 1921. The buildings were all moved down what was called at the time, "the First Avenue Highway" which is still in existence today. In total, about 200 structures were moved to the new town while new structures were also built including the Hibbing High School, the Androy Hotel, the Rood Hospital, and the Village Hall. These buildings were created with mining company money to Figure 13- North Hibbing at crest of Mining. help ease the settlers' mood about having to move the entire town. Only one structure did not make it to the new town during the move. A hotel tumbled off the rollers and crashed into a million pieces. One eyewitness referred to it as "an enormous pile of kindling." The city of Alice was then renamed to Hibbing and annexed. The land size of the city of Hibbing is the largest in Minnesota, even surpassing both Minneapolis and St. Paul's city limits! A children's book chronicles the amazing story of the "Town that Moved". On the very short final leg of the tour we'll pass-by the family home of Bob Dylan (Robert Zimmerman). End of tour and return to the Hibbing Park Hotel: Sources:  City of Hibbing website- (http://www.hibbing.mn.us/index.asp?Type=B_LIST&SEC={0BA3C1786F53-4831-B739-0315936323C6})  Hibbing Historical Society  Hibbing High School website- (http://www.hibbing.k12.mn.us/)  "Hibbing Historical Walking Tour" pamphlet - Hibbing Daily Tribune & Roger Saccoman Architecture  Hibbing Chamber of Commerce website- (http://www.hibbing.org/pages/History/)  Hibbing: The Town That Moved website(http://minnesotaghosts.com/index.php/library/mnhistory/81-hibbing-the-town-that-moved)  "The "Locations" - Company Communities on the Minnesota's Iron Ranges",1982, by Arnold R. Alanen (Minnesota Historical Society). 145 �FIELD TRIP C Friday, May 16, 2014 MINNESOTA DISCOVERY CENTER LEADERS: Discovery Center Staff Figure 1. Miners statue near MDC entrance The Minnesota Discovery Center museum and research library in Chisholm (a few miles north of Hibbing) houses artifacts, examines mining methods, explores regional geology, and hosts traveling exhibits that highlight the story of the predominantly European immigrants who migrated to this region at the turn of the 20th century to find work in the burgeoning iron ore industry. Their stories document the development of the Mesabi Iron Range, a region that became the nation’s largest producer of iron ore. The museum, formerly known as “Ironworld,” is perched at the edge of a lakefilled gorge that represents the collective footprint of many open-pit and underground mine properties. This field trip includes a guided tour of the museum, and a trolly ride across a portion of the mine. 146 �FIELD TRIP D Friday, May 16, 2014 COLERAINE MINERALS RESEARCH LABORATORY Natural Resources Research Institute University of Minnesota-Duluth LEADERS: Dick Kiesel (Director CMRL) Dave Hendrickson (Director Strategic Planning) Matt Mlinar (Program Coordinator Mineral Processing) Basak Anameric (Program Coordinator High Temperature Process) This trip will tour the Coleraine Minerals Research Laboratory (CMRL) in Coleraine, about 25 miles SW of Hibbing. The CMRL conducts applied research that supports technology-based economic development for iron ore mining, non-ferrous minerals, industrial minerals, environmental remediation, alternative iron making, and the use of taconite mining products for various value-added aggregate applications. The facility consists of an analytical laboratory, mineral processing and pyrometallurgical processing capabilities from bench to pilot scale for applied research and development projects. Geographic proximity to the nation’s largest iron mining district (the Mesabi Iron Range) has meant that the CMRL has historically conducted minerals development research, and contributed to the training and development of a substantial number of iron mining and minerals industry professionals. Demand varies from solving short term problems, identifying unique market niches, to providing medium to long range technical innovation and developing products and processes for the future. 147 �FIELD TRIP E Friday, May 16, 2014 MINEVIEW FROM A CANOE LEADER: Mark Jirsa (Minnesota Geological Survey); with assistance from Daniel Jordan (Iron Range Resources and Rehabilitation Board), and Dale Cartwright (Minnesota Department of Natural Resources, Division of Lands and Minerals) Figure 1. Airphoto image of the collection of inactive natural (hematite-goethite) ore mines that form what is referred to here as “Ironworld Pit Lake,” just south of Chisholm. Image shows general locations of 2 main geologic features (ovals) that will be viewed from the gunnels, and other local landmarks. Width of photo ~ 2.5 miles. This trip offers a unique duck’s-eye view of the geology at “Ironworld Pit Lake” in Chisholm, a few miles north of Hibbing. The lake occupies the abandoned footprints of as many as 15 separate natural (hematite-goethite) ore mine properties. These include the Pillsbury (operating years1898-1969), Glen (1902-1957), Leonard-Burt (1909-1974), Leonard (1903-1974), Clark (1900-1925), Monroe-Tener (19051981), South Tener (1928-1981), Bruce Annex (1929-1937), Dunwoody (1917-1977), Douglas (19421977), Neville R (1947-78), Duncan (1914-1970), Pillsbury-Brown (1951-1978), Chisholm (1901-1967), and Godfrey (1926-1963) mines. One wall of the pit exposes a 30-foot thick slab of what is inferred to be Cretaceous iron-rich conglomerate that was glaciotectonically dislodged and thrust over till (Fig. 1). In the early days of mining (1892-1950’s), these hematite-pebble conglomerates were prized as extremely high-grade ore. 148 �Another wall portrays fold and fault structures that likely were genetically related to the formation of natural ores by oxidation and leaching of various layers of Biwabik Iron Formation. The central part of mine pit appears to follow major NW-trending fault/fold structures and subparallel joints (Fig.2), and thus obliquely crosses the strike of iron-formation. As a result, a comparatively thick section of strata is exposed along pit walls—perhaps including parts of the Lower slaty, Upper cherty, and Upper slaty members, depending on water level (SEE Field Trip 1, this guidebook for vernacular). Intuitively, most of the natural ore was exhausted from this site; however, exposures of oxidized (near-ore) can be seen locally on pit walls, depending on water level. The various types of natural ore can generally be color-correlated with the inferred protore (via Gruner, 1946). For example, the precursor of “blue ore,” composed of semi-massive martite (magnetite pseudomorphed by hematite), may have been cherty magnetite-rich layers. Yellow-colored ores that contain primarily goethite and limonite (a generic term for undifferentiated, hydrated iron oxides; typically hydrated goethite) formed from layers of thinly bedded to laminated (slaty) iron-silicates. Brown-colored ores consist of mixtures of goethite, limonite, hematite, and martite, and likely were derived from intimately interbedded cherty and slaty layers. The processes of oxidation and localized leaching of silica and iron-carbonate results in considerable volume loss (as much as 50%), and some of the slump structures visible on pit walls are a product of collapse related to this alteration. Some are also undoubtedly related to collapse into historic underground workings. In general, it is difficult, and in some cases impossible, to assign specific episodes of deformation to individual structures (SEE discussion of iron-formation structures in Field Trip 1, this guidebook). Figure 2. Bedrock geologic map of the Chisholm area showing mine pit lakes (pale blue), faults (solid and dashed thick black lines with letters denoting inferred fault movement; Up, Down), fold axes (thick blue lines), and subsurface extent of shallowly southeast-dipping Biwabik Iron Formation (reddish). Brown line in iron-formation approximates the surface trajectory of the Intermediate slate unit, which marks the stratigraphic top of the Lower cherty member. Dashed green lines represent known extent of Cretaceous strata. Width of photo ~ 6 miles (gray section lines). Map is clipped from 1:100,000-scale (Jirsa and others, 2005). Historically, mining geologists and engineers on the Mesabi Iron Range classified natural ore bodies into 3 main types: trough, fissure, and flat-lying (Wolff, 1917). Trough ore bodies are as large 3000 feet long, 1000 feet wide, and 200-400 feet deep. They formed typically along permeable faults or joint sets 149 �by selective leaching and oxidation. Consequent collapse into linear zones of reduced volume produced the synclinal or trough shapes. Fissure ore bodies are similar, but smaller (≤ 200’X2-5’X50’)—having formed along lesser joint structures and typically involving only minor collapse. Flat-lying ore bodies represent oxidation and leaching along select stratigraphic intervals. They are irregular in shape, commonly follow bedding planes outboard of vertical faults or joints, and therefore persist over considerable distances. The Godfrey Mine that lies just south of Ironworld Pit Lake was developed in such an ore body. Ore was mined there from a 20-30 foot-thick silicate horizon that lies at the stratigraphic top of the Lower cherty member, just beneath what’s known as the Intermediate Slate or Paint Rock horizon. The Godfrey Mine’s underground workings extend for nearly a mile, and produced more than 12 million tons of ore. The ores were extracted from this area utilizing first underground mining, followed by open-pit methods. The underground workings were recently digitized by the Lands and Minerals Division of the Department of Natural Resources (DNR) using historic paper records. The resulting 3D imagery reveals an extensive network of underground workings at various depths (Fig. 3). The deepest of the mines shown here was the Monroe-Tener, at ~ 220 feet below surface. Much of the current lake basin was created by subsequent open-pit mining. Figure 3. Airphoto image of the Chisholm area showing extensive underground workings digitized in 3D. Color coding for drifts and shafts differs for each mine, but represents various depths of workings. From Cartwright and others, 2011. Width of photo ~ 1.5 miles. See website http://www.dnr.state.mn.us/lands_minerals/underground/index.html for more details. The work by DNR and associated geologic mapping by the Minnesota Geological Survey (Jirsa and Meyer, 2007; Jirsa and others, 2002, 2005; Jennings and Reynolds, 2005) was undertaken in large part to evaluate connectivity of ground and surface waters between historic and active mines on the Mesabi Iron Range. Obviously, the underground workings have significant influence on water movement—at least in 150 �the upper several hundred feet—and they present engineering challenges for potential mining of taconite in the future. On-going surface subsidence into these historic underground workings (via sink holes) continues to damage local infrastructure. Despite extensive underground operations on parts of the Mesabi Iron Range between 1892 and 1961, the only remaining head frame is that for the Bruce Mine just north of “Ironworld Pit Lake” (Fig. 4). Figure 4. Head-frame from underground mining at the Bruce Mine; the last of its kind on the Mesabi Iron Range. REFERENCES Cartwright, D.F., Oreskovich, J.A., and Oberhelman, M.W. 2011. Central Iron Range Underground Mine Mapping: Minnesota Department of Natural Resources, Division of Lands and Minerals, DVD Disk #1. Gruner, J.W., 1946, Mineralogy and geology of the Mesabi Range: publications of the Office of the Commissioner of the Iron Range Resources and Rehabilitation, 127 p. Jennings, C.E., and Reynolds, W.K., 2005, Surficial geology of the Mesabi Iron Range, Minnesota: Minnesota Geological Survey Miscellaneous Map M-164, scale 1:100,000. Jirsa, M.A., Chandler, V.W., and Lively, R.S., 2005, Bedrock geology of the Mesabi Iron Range, Minnesota: Minnesota Geological Survey Miscellaneous Map M-163, scale 1:100,000 Jirsa, M.A. and Meyer, G.N., 2007, Bedrock and Quaternary geology of the Central Mesabi Iron Range, northeastern Minnesota: Minnesota Geological Survey Open-File Report OFR-07-03. Jirsa, M.A., Setterholm, D.R., Bloomgren, B.A., and Lively, R.S., 2002 , Bedrock topographic and depth to bedrock maps of the western half of the Mesabi Iron Range, northern Minnesota: Minnesota Geological Survey Miscellaneous Map M126, scale 1:100,000. Wolff, J.F., 1915, Ore bodies of the Mesabi range: Engineering and Mining Journal, v. 100, p. 219-224. 151 � 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, 2014 Description An account of the resource Institute on Lake Superior Geology. Hibbing, Minnesota. May 14-17, 2014. 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 2014 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/15972bb6d69bb343e7dfe2bf006624d3.pdf 80e63708d307845e80958b75b276229d PDF Text Text 61st ANNUAL MEETING InstItute on Lake superIor GeoLoGy Dryden, Ontario - May 20-24, 2015 Part 1 – Proceedings and Abstracts �Sponsors The following organizations made generous contributions to the 61st Annual Meeting. We thank them for their commitment to the Institute on Lake Superior Geology. All of the funds contributed this year go toward travel awards for student registrants. For the past 60 years this organization has thrived as a result of the interest of individuals, corporations, universities and government agencies. The dedication to an exchange of scientific ideas and a passion for field trips has enabled the Institute to provide one of its primary objectives – to promote better understanding of the geology of the Lake Superior Region. Mary Arthur Steve Baumann Leonard Espinosa Gordon Medaris Jr. Allan MacTavish Jim Miller Paul Weiblen Canadian Institute of Mining and Metallurgy Thunder Bay Branch �61st annuaL MeetInG InstItute on Lake superIor GeoLoGy Supported by ONTARIO MINISTRY OF NORTHERN DEVELOPMENT AND MINES May 20-24, 2015 Dryden, Ontario HOSTED BY: Rob Cundari & Peter Hinz Co-Chairs Ontario Geological Survey Proceedings - Volume 61 Part 1 – Proceedings and Abstracts Edited by Mark Smyk Cover photos: Top - Max the Moose (courtesy of Peter Hinz), Middle - Pickle Crow, No. 3 Headframe, Pickle Lake, ca.1989 (courtesy John Scott), Bottom - Field Trip, Thierry Mine, Pickle Lake (courtesy Mark Smyk) �61st InstItute on Lake superIor GeoLoGy VoLuMe 61 consIsts of: part 1: proGraM and abstracts part 2: fIeLd trIp GuIdebook trIp 1: The CenTral red lake Gold BelT trIp 2: WesTern WaBiGoon suBprovinCe TranseCT, dryden To MeGGisi lake trIp 3: CanCelled trIp 4: Thunder lake (GoliaTh) projeCT trIp 5: ClassiC ouTCrops of The dryden area trIp 6: GOLD OCCURRENCES OF VAN HORNE TOWNSHIP, VAN HORNE GOLD PROPERTY FLAMBEAU EXPLOSURES trIp 7: unique MineralizinG evenT aT The pidGeon MolyBdenuM deposiT sTripped surfaCe exposure trIp 8: GeoloGy and Mineral deposiTs of The piCkle lake GreensTone BelT trIp 9: The GhosT lake BaTholiTh and relaTed peGMaTiTes trIp 10: MaTTaBi/sTurGeon lake hisToriC vMs CaMp Reference to material in Part 1 should follow the example below: Arts, A., and Fralick, P., 2015.Iron-rich siliceous stromatolites from the upper algal unit of the Gunflint and Biwabik iron formations. In; Smyk, M., (Ed.), Institute on Lake Superior Geology Proceedings, 61st Annual Meeting, Dryden, Ontario, Part 1 - Program and Abstract, v.61, part 1, 7-8. Published by the 61st 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 61st ILSG Annual Meeting - Part 1 Table of Contents Institutes on Lake Superior Geology, 1955-2015 ii Sam Goldich and the Goldich Medal v Goldich Medal Guidelines vii Goldich Medalists and Goldich Medal Committee ix Citation for Goldich Medal Award to Rodney Ikola x Eisenbrey Student Travel Awards xiii Joe Mancuso Student Research Awards xiv Doug Duskin Student Paper Awards and Award Committee xv Board of Directors, Local Committee, and Banquet Speaker xvi Session Chairs and Field Trip Leaders xvii Corporate and Individual Sponsors of Student Travel Scholarships xviii Report of the Chairs of the 60th Annual Meeting xix Program xxii Poster Presentations xxviii Abstracts 1-90 Some figures in this volume were submitted by authors in color, but are printed grayscale to conserve printing costs. Full color imagery will appear in the digital version of the volume when it is available on-line at http://www.lakesuperiorgeology.org. i �Proceedings of the 61st ILSG Annual Meeting - Part 1 Institutes on Lake Superior Geology, 1955-2015 # Date Place Chairs 1 1955 Minneapolis, Minnesota C.E. Dutton 2 1956 Houghton, Michigan A.K. Snelgrove 3 1957 East Lansing, Michigan B.T. Sandefur 4 1958 Duluth, Minnesota R.W. Marsden 5 1959 Minneapolis, Minnesota G.M. Schwartz & C. Craddock 6 1960 Madison, Wisconsin E.N. Cameron 7 1961 Port Arthur, Ontario E.G. Pye 8 1962 Houghton, Michigan A.K. Snelgrove 9 1963 Duluth, Minnesota H. Lepp 10 1964 Ishpeming, Michigan A.T. Broderick 11 1965 St. Paul, Minnesota P.K. Sims & R.K. Hogberg 12 1966 Sault Ste. Marie, Michigan R.W. White 13 1967 East Lansing, Michigan W.J. Hinze ii �Proceedings of the 61st ILSG Annual Meeting - Part 1 14 1968 Superior, Wisconsin A.B. Dickas 15 1969 Oshkosh, Wisconsin G.L. LaBerge 16 1970 Thunder Bay, Ontario M.W. Bartley & E. Mercy 17 1971 Duluth, Minnesota D.M. Davidson 18 1972 Houghton, Michigan J. Kalliokoski 19 1973 Madison, Wisconsin M.E. Ostrom 20 1974 Sault Ste. Marie, Ontario P.E. Giblin 21 1975 Marquette, Michigan J.D. Hughes 22 1976 St. Paul, Minnesota M. Walton 23 1977 Thunder Bay, Ontario M.M. Kehlenbeck 24 1978 Milwaukee, Wisconsin G. Mursky 25 1979 Duluth, Minnesota D.M. Davidson 26 1980 Eau Claire, Wisconsin P.E. Myers 27 1981 East Lansing, Michigan W.C. Cambray 28 1982 International Falls, Minnesota D.L. Southwick 29 1983 Houghton, Michigan T.J. Bornhorst 30 1984 Wausau, Wisconsin G.L. LaBerge 31 1985 Kenora, Ontario C.E. Blackburn 32 1986 Wisconsin Rapids, Wisconsin J.K. Greenberg 33 1987 Wawa, Ontario E.D. Frey & R.P. Sage 34 1988 Marquette, Michigan J. S. Klasner 35 1989 Duluth, Minnesota J.C. Green 36 1990 Thunder Bay, Ontario M.M. Kehlenbeck 37 1991 Eau Claire, Wisconsin P.E. Myers 38 1992 Hurley, Wisconsin A.B. Dickas 39 1993 Eveleth, Minnesota D.L. Southwick 40 1994 Houghton, Michigan T.J. Bornhorst 41 1995 Marathon, Ontario M.C. Smyk 42 1996 Cable, Wisconsin L.G. Woodruff 43 1997 Sudbury, Ontario R.P. Sage & W. Meyer 44 1998 Minneapolis, Minnesota J.D. Miller & M.A. Jirsa 45 1999 Marquette, Michigan T.J. Bornhorst & R.S. Regis 46 2000 Thunder Bay, Ontario S.A. Kissin & P. Fralick 47 2001 Madison, Wisconsin M.G. Mudrey & Jr., B.A. Brown iii �Proceedings of the 61st ILSG Annual Meeting - Part 1 48 2002 Kenora, Ontario P. Hinz & R.C. Beard 49 2003 Iron Mountain, Michigan L. Woodruff & W.F. Cannon 50 2004 Duluth, Minnesota S. Hauck & M. Severson 51 2005 Nipigon, Ontario M. Smyk & P. Hollings 52 2006 Sault Ste. Marie, Ontario A. Wilson & R. Sage 53 2007 Lutsen, Minnesota L. Woodruff & J. Miller 54 2008 Marquette, Michigan T. Bornhorst & J. Klasner 55 2009 Ely, Minnesota J. Miller, G. Hudak, & D. Peterson 56 2010 International Falls, Minnesota M. Jirsa, P. Hollings, & T. Boerboom, P. Hinz & M.Smyk 57 2011 Ashland, Wisconsin T. Fitz 58 2012 Thunder Bay, Ontario P. Hollings 59 2013 Houghton, Michigan T. Bornhorst & A. Blaske 60 2014 Hibbing, Minnesota J. Miller & M. Jirsa 61 2015 Dryden, Ontario P. Hinz & R. Cundari iv �Proceedings of the 61st ILSG Annual Meeting - Part 1 Sam Goldich and the Goldich Medal Sam Goldich received an A.B. from the University of Minnesota in 1929, a M.A. from Syracuse University in 1930, and a Ph.D. from the University of Minnesota in 1936. During World War II Sam worked for the U.S. Geological Survey in mineral exploration. In 1948, Sam returned to the University of Minnesota, and became Professor and Director of the Rock Analysis Laboratory the following year. He rejoined the U.S. Geological Survey in 1959 and was appointed as the first Branch Chief of the Branch of Isotope Geology. Sam returned to academia in 1964 when he went to Pennsylvania State University. He left PSU in 1965 and moved to the State University of New York at Stony Brook, where he stayed for 3 years. Restless yet again, he moved to Northern Illinois University in 1968 where he was a professor until his retirement in 1977. Sam’s final move was to Denver where he became an emeritus at the Colorado School of Mines. Sam died in 2000, less than a month before his 92nd birthday. In the late 1970’s, Geological Society of America Special Paper 182, which included seminal geochronological studies by Sam Goldich and coworkers on the Archean rocks of the Minnesota River Valley, was nearing completion. At this time various ILSG regulars began discussing the possibility of recognizing Sam for his pioneering work on the resolution of age relationships and thus the geology of Precambrian rocks in the Lake Superior region. Three members, R.W. Ojakangas, J.O. Kalliokoski and G.B. Morey, presented the idea to the ILSG Board of Directors in 1978. The Board approved the creation of an award, provided funding could be obtained. It was suggested that collecting one or two dollars at registration for a dedicated account would provide resources for striking the medal. A general request was made to the ILSG membership for donations and Sam himself offered a challenge grant to match the contributions. In total $4,000 was collected and thus began the work of creating the Goldich Medal. The initial Goldich Award was presented to Sam by G.B. Morey in 1979 and consisted of a large paper proclamation. For the actual medal, G.B. Morey consulted with the foundry on production details, while Dick Ojakangas and Jorma Kalliokoski worked on the design of the award, suggesting that it be given for “outstanding contributions to the geology of the Lake Superior region.” Simultaneously, a committee of J.O. Kalliokosi, W.F. Cannon, M.M Kehlenbeck, G.B. Morey, and G. Mursky developed the Award Guidelines that were approved by the ILSG Board. By 1981 all the elements of the Goldich Award had come together, and the second recipient, Carl E. Dutton, Jr., received the Goldich Medal for 50 years of significant contributions to the understanding of the geology of the Lake Superior region. Since the beginning, the Awards Committee has consisted of individuals representing industry, government and academia, with each member of the Committee serving for three years. The medal is now awarded every year at the annual ILSG meeting. Reference: Morey, G.B. and Hanson, G.N. (editors). 1980. Selected studies of Archean gneisses and Lower Proterozoic rocks, southern Canadian Shield. Geological Society of America, Special Paper 182, 175 p. Prepared by various Goldich Medal Awardees, 2007 v �Proceedings of the 61st ILSG Annual Meeting - Part 1 INSTITUTE ON LAKE SUPERIOR GEOLOGY GOLDICH MEDAL vi �Proceedings of the 61st ILSG Annual Meeting - Part 1 Goldich Medal Guidelines (Adopted by the Board of Directors, 1981; amended 1999) Preamble The Institute on Lake Superior Geology was born in 1955, as documented by the fact that the 27th annual meeting was held in 1981. The Institute’s continuing objectives are to deal with those aspects of geology that are related geographically to Lake Superior; to encourage the discussion of subjects and sponsoring field trips that will bring together geologists from academia, government surveys, and industry; and to maintain an informal but highly effective mode of operation. During the course of its existence, the membership of the Institute (that is, those geologists who indicate an interest in the objectives of the ILSG by attending) has become aware of the fact that certain of their colleagues have made particularly noteworthy and meritorious contributions to the understanding of Lake Superior geology and mineral deposits. The first award was made by ILSG to Sam Goldich in 1979 for his many contributions to the geology of the region extending over about 50 years. Subsequent medallists and this year’s recipient are listed in the table below. Award Guidelines 1) The medal shall be awarded annually by the ILSG Board of Directors to a geologist whose name is associated with a substantial interest in, and contribution to, the geology of the Lake Superior region. 2) The Board of Directors shall appoint the Goldich Medal Committee. The initial appointment will be of three members, one to serve for three years, one for two years, and one for one year. The member with the briefest incumbency shall be chair of the Nominating Committee. After the first year, the Board of Directors shall appoint at each spring meeting one new member who will serve for three years. In his/her third year this member shall be the chair. The Committee membership should reflect the main fields of interest and geographic distribution of ILSG membership. The out-going, senior member of the Board of Directors shall act as liaison between the Board and the Committee for a period of one year. 3) By the end of November, the Goldich Medal Committee shall make its recommendation to the Chair of the Board of Directors, who will then inform the Board of the nominee. 4) The Board of Directors normally will accept the nominee of the Committee, inform the medallist, and have one medal engraved appropriately for presentation at the next meeting of the Institute. 5) It is recommended that the Institute set aside annually from whatever sources, such funds as will be required to support the continuing costs of this award. vii �Proceedings of the 61st ILSG Annual Meeting - Part 1 Nominating Procedures 1) The deadline for nominations is November 1. Nominations shall be taken at any time by the Goldich Medal Committee. Committee members may themselves nominate candidates; however, Board members may not solicit for or support individual nominees. 2) Nominations must be in writing and supported by appropriate documentation such as letters of recommendation, lists of publications, curriculum vita’s, and evidence of contributions to Lake Superior geology and to the Institute. 3) Nominations are not restricted to Institute attendees, but are open to anyone who has worked on and contributed to the understanding of Lake Superior geology. Selection Guidelines 1) Nominees are to be evaluated on the basis of their contributions to Lake Superior geology (sensu lato) including: a) importance of relevant publications; b) promotion of discovery and utilization of natural resources; c) contributions to understanding of the natural history and environment of the region; d) generation of new ideas and concepts; and e) contributions to the training and education of geoscientists and the public. 2) Nominees are to be evaluated on their contributions to the Institute as demonstrated by attendance at Institute meetings, presentation of talks and posters, and service on Institute boards, committees, and field trips. 3) The relative weights given to each of the foregoing criteria must remain flexible and at the discretion of the Committee members. 4) There are several points to be considered by the Goldich Medal Committee: a) An attempt should be made to maintain a balance of medal recipients from each of the three estates—industry, academia, and government. b) It must be noted that industry geoscientists are at a disadvantage in that much of their work in not published. 5) Lake Superior has two sides, one the U.S., and the other Canada. This is undoubtedly one of the Institute’s great strengths and should be nurtured by equitable recognition of excellence in both countries. viii �Proceedings of the 61st ILSG Annual Meeting - Part 1 Goldich Medalists 1979 Samuel S. Goldich 1997 Ronald P. Sage 1980 not awarded 1998 Zell Peterman 1981 Carl E. Dutton, Jr. 1999 Tsu-Ming Han 1982 Ralph W. Marsden 2000 John C. Green 1983 Burton Boyum 2001 John S. Klasner 1984 Richard W. Ojakangas 2002 Ernest K. Lehmann 1985 Paul K. Sims 2003 Klaus J. Schulz 1986 G.B. Morey 2004 Paul Weiblen 1987 Henry H. Halls 2005 Mark C. Smyk 1988 Walter S. White 2006 Michael G. Mudrey 1989 Jorma Kalliokoski 2007 Joseph Mancuso 1990 Kenneth C. Card 2008 Theodore J. Bornhorst 1991 William Hinze 2009 L. Gordon Medaris, Jr. 1992 William F. Cannon 2010 William D. Addison & 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 2015 GOLDICH MEDAL RECIPIENT RODNEY J. IKOLA Goldich Medal Committee Serving through the meeting year shown in parentheses. Bernhardt Saini-Eidukat (2015) North Dakota State University Mark Smyk (2016) Ontario Geological Survey Hélène Lukey (2017) Cliffs Natural Resources ix �Proceedings of the 61st ILSG Annual Meeting - Part 1 Citation for the Goldich Medal Award to Rodney J. Ikola Rodney J. (Rod) Ikola was born and raised in the Finnish community of Esko, Minnesota. As he puts it, he had learned his first “foreign language” (ENGLISH) by the end of his first year in school. He has continued to be active in Finnish organizations throughout his life, such as Festival Finlandia at Ironworld in Chisholm, MN, and FinnfestUSA in Duluth in 2008. He has dual citizenship in the U.S. and Finland and served as a member of the Expatriate Parliament of the Republic of Finland for a number of years. Rod has a list of accomplishments in the field of geophysics that would make any Finnish Mother proud, and we suggest that they are worthy of adding Rodney Ikola’s name to the list of Goldich Medal recipients. He did his undergraduate studies at the University of Minnesota Duluth, with a major in geology and a minor in mathematics. He continued his education at the University of Utah on a scholarship in geophysics from the Continental Oil Company, during which time he obtained sufficient math credits to fulfill the requirements for a degree in mathematics. After one year he transferred to the University of Minnesota to complete his Masters degree. During his time at the University of Minnesota, Professor Hal Mooney knew that Rod had been doing gravity work around the southern end of the Duluth Complex and west into Carlton County, simply out of personal curiosity, using a gravimeter made available to him compliments of U. S. Steel. Mooney suggested that Rod should write up the gravity work he had already done and he would accept that as a Master’s thesis. This gravity survey showed several gravity anomalies at the western edge of Carlton County, which became known as the Tamarack Intrusion; this is currently being drilled for copper, nickel and PGE by Rio Tinto-Kennecott. During his days at the U of M, Rod worked on several geophysical projects. In 1959 he conducted his first geophysical survey; a magnetic study of the Barden’s Peak Intrusive of the Duluth Complex. During the summers of 1960 and 1962 he worked on field geophysical exploration for U.S. Steel under the supervision of their geophysicist, George Durfee. The 1960 project consisted of running magnetometer lines across every dip needle anomaly in northwestern Wisconsin (most of which were located in swamps). In 1962 he conducted an extensive gravity survey of Jackson County in central Wisconsin, for the Jackson County Iron Company, a subsidiary of Inland Steel; a taconite mine was developed a decade later in the Archean rocks in Jackson County. In 1965 Director Paul Sims obtained funding to significantly expand the activities of the Minnesota Geological Survey and asked Rod if he would join the Survey to start a systematic gravity survey of the State. Rod accepted the offer and he and G. B. Morey started working for the Survey on the same day. Most of Rod’s time with the Survey was consumed with the gravity survey of the state. This resulted in the publication of several Bouguer gravity maps at a scale of x �Proceedings of the 61st ILSG Annual Meeting - Part 1 1:250,000. During this time he produced the first gravity map of the entire Duluth Complex. He was also periodically on loan to the U.S. Army Topographic Command to establish geodetic control over the central U.S. At the AIME meeting in Duluth in 1970, Fred Chase, chief geologist of the Hanna Mining Co., asked Rod if he would be interested in joining Hanna. They had started a large exploration program in the greenstone belts of Minnesota and needed a geophysicist for the project. Rod accepted the offer, eventually was appointed chief geophysicist for Hanna in 1974, and became responsible for world- wide exploration, a position he held until 1982. Initially his work with Hanna was mainly involved with geophysical exploration in support of Hanna’s geological activities in Minnesota’s greenstone belts and the Duluth Complex. However, he soon began working on all of Hanna’s projects around the world. The list of projects is too long to list here, but demonstrates a wide range of geophysical techniques that Rod mastered. With the downturn of the iron ore market in 1982, Hanna eliminated their entire exploration program as the first step in the eventual demise of the entire company. With this event, Rod decided to go on his own as a consulting geophysicist, which he has been for the last thirty years. Almost immediately he became heavily involved in geophysical consulting in the Lake Superior region for many of the world’s major mining companies. Some of these include Newmont, INCO, American Copper and Nickel, Cominco American, Noranda, Phelps Dodge, DeBeers, (through their regional affiliate), plus numerous junior companies. Most of this work remains proprietary but one project in particular can be mentioned. He did all the geophysical work for Noranda that led to the discovery of the large Lynne (Cu-Zn) Deposit in northern Wisconsin. Only environmental issues prevented the development of the project into a commercial venture. He also became extensively involved in Freeport-McMoRan for many years and acted as de facto geophysicist on many of their worldwide exploration efforts. He worked extensively in the Iberian Pyrite Belt of Spain and Portugal. One of these efforts led to the discovery of the Agua Blanca nickel deposit, which is Europe’s largest nickel producer. He also spent considerable time doing geophysics in the Grasberg area of Indonesia. And he worked on porphyry exploration for Freeport in Baja California. Consulting work has taken Rod to numerous mining camps around the world. He has spent time at Noril’sk in Russia studying their geophysical exploration techniques. He also did some work for a consortium of companies exploring for gold in the “reefs” south of Lake Victoria in Africa, diamond exploration in Brazil, nickel exploration in Western Australia, uranium exploration in the Athabasca area of Canada, and deeply buried porphyry deposits in the southwest U.S.. In recent years, with the upsurge of mineral exploration in the Lake Superior region, he has spent more time close to home. He has been involved with Polymet Mining and Duluth Metals in the Duluth Complex, and Keweenaw Copper Co. and Bitteroot Exploration in Michigan. However, the results of this work remain proprietary. In recent years, Rod has been involved in numerous geophysical projects pertaining to environmental and groundwater problems. On a project with Barr Engineering, he helped develop a groundwater resource at the Sherco Power Plant in Becker, MN. For this work the Minnesota Society of Professional Engineers awarded the group a Distinguished Achievement Award. On another project the group used a unique application of the SP geophysical method to delineate karst features in the Keweenawan sandstones near Askov, MN, to help prevent pollution from their sewage treatment plant. xi �Proceedings of the 61st ILSG Annual Meeting - Part 1 Rod has always been interested in the use of geophysics in archaeological investigations, and he has participated in many studies. He spent two summers in Greece (one on top of Mt. Olympus with the Gods!!) looking at sites from the Homeric Age. On another occasion he used geophysics to look for buried Mayan tombs in Belize. Rod has been affiliated with many professional organizations during his career. He is an emeritus member of the Society of Exploration Geophysicists, belongs to the Australian Society of Exploration Geophysicists, a member of the American Institute of Professional Geologists and has belonged to the Society of Mining Engineers for many years (and served on seven national committees for them). He is also a founding member, and was on the board of directors of the Minnesota Exploration Association (now Mining Minnesota). He is a Registered Earth Scientist in Minnesota and a Professional Geophysicist in California. He has been involved with the Institute on Lake Superior Geology for over fifty years. The first Institute meeting he attended was the fourth one, in Duluth in 1958, and has subsequently attended approximately 45 meetings. During his career as a geophysicist, Rod Ikola has made many contributions to our understanding of the geology of the Lake Superior Region, particularly in the areas of government surveys and industry. These accomplishments, as well as his geophysical studies at so many other places around the world, make him highly qualified for the Goldich Medal. Respectfully submitted, Gene L. LaBerge Richard W. Ojakangas xii �Proceedings of the 61st ILSG Annual Meeting - Part 1 Eisenbrey Student Travel Awards The 1986 Board of Directors established the ILSG Student Travel Awards to support student participation at the annual meeting of the Institute. The name “Eisenbrey” was added to the award in 1998 to honor Edward H. Eisenbrey (1926-1985) and utilize substantial contributions made to the 1996 Institute meeting in his name. “Ned” Eisenbrey is credited with discovery of significant volcanogenic massive sulfide deposits in Wisconsin, but his scope was much broader—he has been described as having unique talents as an ore finder, geologist, and teacher. These awards are intended to help defray some of the direct travel costs of attending Institute meetings, and include a waiver of registration fees, but exclude expenses for meals, lodging, and field trip registration. The number of awards and value are determined by the annual Chair in consultation with the Secretary and Treasurer. Recipients will be announced at the annual banquet. The following general criteria will be considered by the annual Chair, who is responsible for the selection: 1) The applicants must have active resident (undergraduate or graduate) student status at the time of the annual meeting of the Institute, certified by the department head. 2) Students who are the senior author on either an oral or poster paper will be given favored consideration. 3) It is desirable for two or more students to jointly request travel assistance. 4) In general, priority will be given to those in the Institute region who are farthest away from the meeting location. 5) Each travel award request shall be made in writing to the annual Chair, and should explain need, student and author status, and other significant details. Successful applicants will receive their awards during the meeting. xiii �Proceedings of the 61st ILSG Annual Meeting - Part 1 Joe Mancuso Student Research Awards The 2005 Board of Directors established the ILSG Student Research Fund with $10,000 US from the Institute’s general fund to encourage student research on the geology of the Lake Superior region. A minimum of two awards of $500 US each for research expenses (but not travel expenses) will be made each year. Students are expected to present their research orally or during a poster session at an ILSG meeting. The award winners will also be automatically eligible for the Eisenbrey Travel Awards. To allow the fund to grow, the Fund will receive onehalf of any additional proceeds from each annual meeting, after all other commitments and expenses are covered. • The ILSG Board of Directors will be responsible for selecting a minimum of two awards each year. The ILSG Treasurer will issue the awards. • The ILSG Student Research Fund is available for undergraduate or graduate students working on geology in the Lake Superior region. • The applications are due to the ILSG Secretary by August 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 2014, the ILSG Board of Governors awarded a $1000 award from the Student Research Fund to Justin Beermaert. It should be noted that an especially generous donation was once again provided by Ron Seavoy. xiv �Proceedings of the 61st ILSG Annual Meeting - Part 1 Doug Duskin Student Paper Awards Each year, the Institute selects the best of student presentations and honors the presenters with a monetary award. Funding for the award is generated from registrations of the annual meeting, and from generous donations to the fund in honor of Doug Duskin—an exploration geologist and long-time friend of the Institute. The 2012 ILSG Board of Directors approved adding Doug’s name to the award to acknowledge his contributions, and distribute those donations in a manner that would have pleased him. The Duskin Student Paper Committee is appointed by the Meeting Chair. Criteria for best student paper—last modified by the Board in 2001—follow: 1) The contribution must be demonstrably the work of the student. 2) The student must present the contribution in-person. 3) The Student Paper Committee shall decide how many awards to grant, and whether or not to give separate awards for poster vs. oral presentations. 4) In cases of multiple student authors, the award will be made to the senior author, or the award will be shared equally by all authors of the contribution. 5) The total amount of the awards is left to the discretion of the meeting Chair in conjunction with the Secretary, but typically is in the amount of about $500 US (increase approved by Board, 10/01). 6) The Secretary maintains, and will supply to the Committee, a form for the numerical ranking of presentations. This form was created and modified by Student Paper Committees over several years in an effort to reduce the difficulties that may arise from selection by raters of diverse background. The use of the form is not required, but is left to the discretion of the Committee. 7) The names of award recipients shall be included as part of the annual Chair’s report that appears in the next volume of the Institute. Student papers will be noted on the Program. Student Paper Awards Committee Amy Radakovich– Minnesota Geological Survey Mark Severson – Teck American Dave Good – Western University xv �Proceedings of the 61st ILSG Annual Meeting - Part 1 Board of Directors Board appointment continues through the close of the meeting year shown in parentheses, or until a successor is selected Robert Cundari (2015-2018) – Ontario Geological Survey Jim Miller (2014-2017) – University of Minnesota Duluth Allan Blaske (2013-2016) – AECOM Peter Hinz (2012-2015) – Ontario Geological Survey Pete Hollings – Secretary (2013-2016) – Lakehead University Mark Jirsa – Treasurer (2014-2017) – Minnesota Geological Survey Local Committee Chairs Peter Hinz – Co-Chair Ring of Fire Secretariat, Ministry of Northern Development and Mines Robert Cundari – Co-Chair Resident Geologist Program, Ontario Geological Survey Volume Editors Mark Smyk – Proceedings Volume Resident Geologist Program, Ontario Geological Survey Allan MacTavish – Field Trip Guidebook Panoramic PGMs (Canada) Limited Banquet Speaker Steve Beneteau (Senior Diamond Advisor / Chief Gemmologist for the Province of Ontario and the Manager of the Diamond Sector Unit, Ministry of Northern Development and Mines) “Ontario’s Diamonds: A Journey from Mine to Market” xvi �Proceedings of the 61st ILSG Annual Meeting - Part 1 Session Chairs Jim Miller – University of Minnesota Duluth Anthony Pace - Ontario Geological Survey Dean Peterson – Peterson Geoscience LLC Mark Smyk - Ontario Geological Survey Ann Wilson – Ontario Geological Survey Laurel Woodruff – United States Geological Survey Field Trip Leaders Field trips have been the mainstay of the ILSG since its inception 60 years ago. We want to give a special thanks to the field trip leaders who volunteered their time and talent in carrying that tradition forward. Pre-Meeting: 1. Red Lake Geology (2-day) Tuesday May 19th and Wednesday May 20th, 2015 Leaders: Andreas Lichtblau (OGS) and Carmen Storey (OGS) 2. Western Wabigoon Subprovince Transect (Dryden to Meggisi Lake) Wednesday May 20th, 2015 Leaders: Mark Puumala (OGS) and Dorothy Campbell (OGS) 3. Mine Reclamation and Legacy Issues at the South Bay Mine Wednesday May 20th, 2015 Leader: Rob Purdon (MNDM) [CANCELLED DUE TO UNFORESEEN CIRCUMSTANCES] 4. Geological Setting of the Thunder Lake Gold Deposit Wednesday May 20th, 2015 Leaders: Treasury Metals Inc. Half-Day: 5. Classic outcrops of the Dryden Area Friday May 22nd, 2015 Leader: Peter Hinz (MNDM) 6. Gold Occurrences of Van Horne Township Friday May 22nd, 2015 Leader: Steve Meade (OGS) 7. Unique mineralizing event at the Pidgeon Molybdenum Occurrence Friday May 22nd, 2015 Leaders: Craig Ravnaas (OGS) Post-Meeting: 8. Historic Pickle Lake Camp (1.5-day) Friday May 22nd and Saturday May 23rd, 2015 xvii �Proceedings of the 61st ILSG Annual Meeting - Part 1 Leaders: Mark Smyk (OGS), Pete Hollings (Lakehead University) and Neil Pettigrew (PC Gold Inc.) 9. Ghost Lake Batholith and Related Pegmatites Saturday May 23rd, 2015 Leader: Shannon Zurevinski (Lakehead University) 10. Mattabi/Sturgeon Lake Historic VMS Camp (2-day) Friday May 22nd and Saturday May 23rd, 2015 Leader: George Hudak (Natural Resources Research Institute – University of Minnesota Duluth) Sponsors The following organizations made generous contributions to the 61st Annual Meeting. We thank them for their commitment to the Institute on Lake Superior Geology. All of the funds contributed this year go toward travel awards for student registrants. For the past 60 years this organization has thrived as a result of the interest of individuals, corporations, universities and government agencies. The dedication to an exchange of scientific ideas and a passion for field trips has enabled the Institute to provide one of its primary objectives – to promote better understanding of the geology of the Lake Superior Region. Mary Arthur Steve Baumann Leonard Espinosa Gordon Medaris Jr. Allan MacTavish Jim Miller Paul Weiblen Canadian Institute of Mining and Metallurgy Thunder Bay Branch xviii �Proceedings of the 61st ILSG Annual Meeting - Part 1 REPORT OF THE CHAIRS OF THE 60TH ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY HIBBING, MINNESOTA The Precambrian Research Center (PRC) of the University of Minnesota Duluth (UMD) and the Minnesota Geological Survey (MGS) of the University of Minnesota – Twin Cities hosted the 60th Annual Institute on Lake Superior Geology on May 14– 17, 2014 at the Hibbing Park Hotel in Hibbing, Minnesota in the heart of the Mesabi Range. This was the first time that the ILSG has been held in Hibbing. We are pleased to report a very vigorous attendance of 225 registrants, including 57 students. The organizing committee for the meeting was comprised of Jim Miller of UMD-PRC (technical program, meeting and field trip logistics, and registration) and Mark Jirsa of the MGS (field trip coordinator and guidebook editor, student travel awards, and sponsorships). Amy Radakovich of the MGS helped with the procurement and design of T-shirts and beer glasses. Terry Boerboom, also of the MGS, assisted with the organization of the field guidebook. Julie Ann Heinz, an executive office administrator at the UMD Natural Resources Research Institute, provided assistance with meeting registration and creating name tags. During the meeting, a number of UMD students assisted with on-site registration and other institute business. The two-day technical session held at the Hibbing Park Hotel on Thursday and Friday (5/15 and 5/16) included 30 talks and 35 posters presentations, including 17 oral and 20 poster presentations by students. The number of student presenters and attendees are both ILSG records. The meeting opened with a remembrance of Jack Everett and Ernie Lehmann who passed away in August, 2013 and December, 2013, respectively. Both Jack and Ernie were exceptional exploration geologists who devoted much of their careers to minerals exploration in the Lake Superior region. Both were long-time supporters of the ILSG, with Ernie receiving the Goldich medal in 2002. This year’s Goldich medal recipient was Laurel Woodruff of the US Geological Survey. Laurel was recognized for her long and productive 30 year career with the USGS’s mineral resources research program, mostly in the Lake Superior region and for her scientific contributions and service to the ILSG, especially her serving as meeting chair in 1996 (Cable, WI), 2003 (Iron Mtn, MI), and 2007 (Lutsen, MN). Laurel was presented the medal at the annual banquet by Bill Cannon, her colleague and mentor at the USGS who received the Goldich medal in 1992. The evening banquet talk was presented by Dr. Francis M. Carroll of the University of Manitoba - Winnipeg and St. Johns University. The title of his talk was: "A Line in the Trees: History of the US-Canadian Boundary from Lake Superior to Lake of the Woods". The meeting offered six full-day and three Friday afternoon field trips that highlighted various aspects of the geology, ore deposits, and culture of the central Mesabi Range. Most trips were filled to capacity with a cumulative total of 255 field trip attendees. Three pre-meeting field trips run on Wednesday, May 14 included: 1) Stratigraphy, Sedimentology, Structure, and Mineralization of the Biwabik Iron Formation, Central Mesabi Iron Range led by Phil Larson (Duluth Metals), Marsha Patelke (UMD-NRRI), Jakob Wartman (Cliffs NR), Michael Totenhagen (Arcelor xix �Proceedings of the 61st ILSG Annual Meeting - Part 1 Mittal), Mark Jirsa (MGS), Steven Losh (Minnesota State University-Mankato), and Peter Jongewaard (Cliffs NR); 2) A Walk in the Park – Neoarchean Geology of Lake Vermilion State Park led by George Hudak (UMD-NRRI), Amy Radakovich (MGS), Geoff Pignotta (UW - Eau Claire), and Kelly Schwierske (UW - Eau Claire); and 3) Western Mesabi Range Mining Operations led by Doug Halverson (Cliffs NR), Dan Cervin (Cliffs NR), William Everett (Essar Steel), Kevin Kangas (Essar Steel), and Joey Nielsen (Magnetation). Three Friday afternoon trips included: 1) State Drill Core Library – Hibbing, Minnesota led by Dave Dahl, Barry Frey, and other MNDNR staff and Dean Rossell (Kennecott Exploration, Rio Tinto); 2) Hibbing’s Iron Mining and Cultural History led by Henry Djerlev, Bob Kearney, Erica Larson and other Hibbing Historical Society staff; and 3) Mineview from a Canoe led by Mark Jirsa (MGS), Dan Jordan (IRRRB), and Dale Cartwright (MNDNR). Three post-meeting trips were run on Saturday, May 17 and included: 1) Visions of Maturi: The Geology of the South Kawishiwi Intrusion led by Dean Peterson (Duluth Metals Ltd.); 2) The St. Louis Sublobe and Glacial Lake Upham led by Phil Larson (Duluth Metals Ltd.), Alan Knaeble (MGS), Howard Mooers (UMD), and Lisa Marlo (Halcon Resources Corp.); and 3) Geology and Gold Mineralization of the Virginia Horn Area led by Mark Jirsa (MGS), Bill Rowell (Vermillion Gold), Rick Sandri - (Vermillion Gold), and Jason Richter (MN DOT). The student paper committee comprised of Andrew Ware (PolyMet Mining), Prajukti Bhattacharyya (University of Wisconsin-Whitewater), and Rob Cundari (Ontario Geological Survey had the onerous task of judging 17 oral and 20 poster presentations by students. The committee awarded 4 Doug Duskin Student Paper Awards with a cash prize of $500 each to Amanda Van Lankfelt (U Mass.), Adrian Arts (Lakehead), Monica Karman (Lakehead), and Darcy Jacobson (Michigan Tech.). To defray student’s expenses for travel and registration, a total of $6400 was distributed to 32 students representing 8 different schools. This generous aid was provided by the Eisenbrey Student Travel Awards. Additional student travel support was provided by funds contributed by 11 meeting registrants (Mary Arthur, Jack Berkley, Karl Everett, John Green, George Hudak, Peter Jongewaard, Steven Losh, Al MacTavish, Gordon Medaris, Michael Mudrey, and Jill Peterman),and several corporations and organizations (Eagle Mine, Teck American, Midwest Institute of Geosciences and Engineering, and GEI Consultants). Ron Seavoy provided a particularly generous donation to establish and maintain the Joe Mancuso Student Research Grant program. Three $500 Mancuso research grants were awarded in 2014 to Michael Doyle (UMD), Michael Fedorchuk (UW-Milwaukee), and Sarah Sauer (UMD). The Institute’s Board of Directors met on May 15, 2014. The meeting was attended by meeting co-chair Jim Miller, Treasurer Mark Jirsa, Secretary Peter Hollings, and board members Allan Blaske (2013 chair) and Al MacTavish (2012 chair). Incoming chair for the 2015 ILSG, Pete Hinz, also attended. Secretary Hollings took the minutes of the Board meeting that are as follows: 1. Accepted report of the Chairs for the 59th ILSG, Houghton, Michigan; as printed in the Proceeding Volume (Blaske), and minutes of last Board meeting, May 9, 2013 (Hollings) xx �Proceedings of the 61st ILSG Annual Meeting - Part 1 2. 3. 4. 5. Received, discussed, and accepted 2013-2014 ILSG Financial Summary (Jirsa). Received, discussed, and accepted 2013-2014 report of the Secretary (Hollings). Approved Jim Miller as on-going ILSG Board member Approved Dryden as the site for the 61st annual ILSG meeting. The meeting will be hosted by the Ontario Geological Survey with Peter Hinz and Rob Cundari serving as co-chairs. 6. Discussed and approved renewal of Mark Jirsa as Institute Secretary (end of term 2017). This was later approved by a vote of the membership 7. Discussed and approved replacing Graham Wilson as the “member from industry” on Goldich Committee (end of term 2017) with Helene Lukey 8. Discussed student attendance and presentations at future meetings. We would like to thank the participants, especially the students, for supporting the Institute by their attendance and enthusiasm, the field trip leaders for their hard work, the presenters for their high quality and informative talks and posters, the session chairs and subcommittee members for their important contributions, and the meeting sponsors for their generosity in helping students participate in the Institute. Respectfully submitted, Jim Miller and Mark Jirsa Co-Chairs, 60th Institute on Lake Superior Geology xxi �Proceedings of the 61st ILSG Annual Meeting - Part 1 PROGRAM SCHEDULED ACTIVITIES AND FIELD TRIPS Tuesday, May 19th, 2015 8:00 a.m. – 6:00 p.m. Field Trip 1 Red Lake Geology (Day One) Wednesday, May 20th, 2015 8:00 a.m. – 6:00 p.m. Field Trip 1 Field Trip 2 Field Trip 3 Field Trip 4 7:00 p.m. – 10:00 p.m. Red Lake Geology (Day Two) Western Wabigoon Subprovince Transect Mine Reclamation and Legacy Issues at the South Bay Mine [cancelled] Geological Setting of the Thunder Lake Gold Deposit / Gold occurrences of Van Horne Township Registration at Best Western Poster Session (Gemini Room) / Ice Breaker Social (Centennial Room) Thursday, May 21st, 2015 8:00 a.m. – 12:00 p.m. Registration continues 9:00 a.m. – 12:00 p.m. Technical Session I 12:00 p.m. – 1:30 p.m. Lunch (provided) 1:30 p.m. – 4:00 p.m. Technical Session II 6:00 p.m. – 7:00 p.m. Mixer / Cash Bar 7:00 p.m. – 10:00 p.m. Annual Banquet, Keynote Speaker and Awards Presentation (Sunset Ballroom) Friday, May 22nd, 2015 9:00 a.m. – 11:10 a.m. 11:10 a.m. – 12:00 p.m. 12:00 p.m. – 1:30 p.m. 1:30 p.m. – 6:00 p.m. 6:00 p.m. – 7:00 p.m. Saturday, May 23rd, 2015 8:00 a.m. – 6:00 p.m. Sunday, May 24th, 2015 8:00 a.m. – 6:00 p.m. Technical Session III Student Awards presentations Lunch (provided) Field Trip 5 Classic outcrops of the Dryden Area Field Trip 6 Gold Occurrences of Van Horne Township Field Trip 7 Unique mineralizing event at the Pidgeon Molybdenum Occurrence Field Trip 8 (departs for Pickle Lake) Field Trip 10 (departs for Ignace) Field Trip 8 Field Trip 9 Historic Pickle Lake Camp (return to Ignace / Dryden) Ghost Lake Batholith and Related Pegmatites (return to Dryden) Field Trip 10 Mattabi / Sturgeon Lake Historic VMS Camp (Day One; return to Ignace) Field Trip 10 Mattabi / Sturgeon Lake Historic VMS Camp (Day Two; return to Dryden) TUESDAY, MAY 19TH 8:00 am – 6:00 pm Pre-Meeting Field Trip: 1. Red Lake Geology (Day One; overnight in Red Lake) Leaders: Andreas Lichtblau (OGS) and Carmen Storey (OGS) xxii �Proceedings of the 61st ILSG Annual Meeting - Part 1 WEDNESDAY, MAY 20TH 8:00 am – 6:00 pm Pre-Meeting Field Trips: 1. Red Lake Geology (Day Two; return to Dryden) Leaders: Andreas Lichtblau (OGS) and Carmen Storey (OGS) 2. Western Wabigoon Subprovince Transect Leaders: Mark Puumala (OGS) and Dorothy Campbell (OGS) 3. Mine Reclamation and Legacy Issues at the South Bay Mine Leader: Rob Purdon (MNDM)[cancelled due to unforeseen circumstances] 4. Geological Setting of the Thunder Lake Gold Deposit Leaders: Treasury Metals Inc. 7:00 p.m. – 11:00 p.m. Registration at Best Western Poster Session (Gemini Room) / Ice Breaker Social (Centennial Room) THURSDAY, MAY 21ST 8:00 – 12:00 pm Registration continues 8:50 - 9:00 am OPENING REMARKS, UPDATES Peter Hinz and Robert Cundari, Co-Chairs, 2015 ILSG 9:00 – 9:10 am Welcoming Remarks TECHNICAL SESSION I (*denotes a student eligible for Best Student Paper Award) Session Chairs: Laurel Woodruff – United States Geological Survey Anthony Pace - Ontario Geological Survey 9:10 – 9:50 am Bill Cannon, Bill Addison, Greg Brumpton and Mark Jirsa The Sudbury Impact Event in the Lake Superior region: Ten years of research on ten minutes of geologic time 9:50 – 10:10 am Daniel Lafontaine* and Mary Louise Hill Structural control on the Borden Gold deposit, Chapleau, ON 10:10 – 10:40 am COFFEE BREAK AND POSTER VIEWING xxiii �Proceedings of the 61st ILSG Annual Meeting - Part 1 10:40 – 11:00 am Jim Miller Role of Felsic and Feldspathic Rocks in Triggering Subvolcanic Emplacement of Mafic Intrusions: Evidence from the Midcontinent Rift in Northeastern Minnesota 11:00 – 11:20 am David Good, Peter Hollings, Robert Cundari and Doreen Ames Significance of LREE-enriched mantle source to genesis of basalt in the Coldwell Alkaline Complex, Midcontinent Rift, Ontario 11:20 – 11:40 am Michael Doyle* and Jim Miller Geologic and geochemical attributes of the Beaver River Diabase and Greenstone Flow: Testing a possible intrusive-volcanic correlation in the 1.1 Ga Midcontinent Rift 11:40– 12:00 pm Sarah Sauer* and Jim Miller Petrologic study of the "Chill" zone of the Layered Series at Duluth: Testing a possible plutonic-volcanic correlation within the Midcontinent Rift 12:00 – 1:30 pm LUNCH (provided) / MEETING OF THE BOARD OF DIRECTORS TECHNICAL SESSION II (*denotes a student eligible for Best Student Paper Award) Session Chairs: Dean Peterson – Peterson Geoscience LLC Ann Wilson – Ontario Geological Survey 1:30 – 1:50 pm Robert Mahin The Eagle Mine in Production: U.S.A.’s Only Primary Nickel Producer 1:50 – 2:10 pm Kristofer Asp*, Christian Schardt, and Lev Spivak-Birndorf Evidence of high temperature Ni isotopic fractionation during the formation of Cu-Ni-PGE sulfide deposits in the Duluth Complex xxiv �Proceedings of the 61st ILSG Annual Meeting - Part 1 2:10 – 2:30 pm David Good, Louis Cabri and Doreen Ames Comparison of PGM assemblages for the Marathon, Geordie Lake and Area 41 deposits, Coldwell Alkaline Complex, Ontario 2:30 – 2:50 pm Jeffrey Mauk, Poul Emsbo and Peter Theodorakos Evaporated seawater formed sediment-hosted stratiform copper orebodies and second-stage copper mineralization in the Mesoproterozoic Nonesuch Formation of the Midcontinent Rift 2:50 – 3:20 pm COFFEE BREAK AND POSTER VIEWING 3:20 – 3:40 pm Amos Albert*, Jessica Eagle-Bluestone* and Bernie Saini-Eidukat Chemistry and Mineralogy of Nopeming metasiltstone at the Grandview Site, Duluth, Minnesota 3:40 – 4:00 pm Adrian Arts* and Phil Fralick Iron-rich siliceous stromatolites from the upper algal unit of the Gunflint and Biwabik Iron Formations 4:00 – 4:20 pm Christopher Yip* and Phil Fralick Exposure Surfaces of the Gunflint Iron Formation, Northwestern Ontario 4:20 – 4:40 pm Riku Metsaranta and Phil Fralick Sedimentology and Geochemistry of a 1.4 Ga Continental Playa System, the Lower Sibley Group, Northwestern Ontario: Implications for the Mesoproterozoic Hydrosphere and Atmosphere 4:40 –5:00 pm Paul Fix* and Tamara Diedrich Characterization of secondary minerals formed on weathered Duluth Complex Cu-Ni-PGE deposit rock and implications for controls on metal mobility 6:00 pm RECEPTION – CASH BAR 7:00 pm ANNUAL BANQUET (Sunset Ballroom) − Announcement of 62nd Annual Meeting Location xxv �Proceedings of the 61st ILSG Annual Meeting - Part 1 − 2015 Goldich Award Presentation to Rodney Ikola − Banquet Presentation by Steve Beneteau (MNDM) “Ontario’s Diamonds: A Journey from Mine to Market” FRIDAY, MAY 22ND 8:50 – 9:00 am OPENING REMARKS, UPDATES Peter Hinz and Rob Cundari, Co-Chairs, 2015 ILSG TECHNICAL SESSION III Session Chairs: Mark Smyk - Ontario Geological Survey Jim Miller – University of Minnesota Duluth 9:00 – 9:20 am Brent Trevisan, Pete Hollings, Doreen Ames and Nicole Rayner The petrology, mineralization and regional context of the Thunder mafic to ultramafic intrusion, Midcontinent Rift, Thunder Bay, Ontario 9:20 – 9:40 am Seamus Magnus Geology and geochemistry of the Lang Lake greenstone belt, Uchi Domain, Superior Province 9:40 – 10:00 am Phil Fralick Lateral Geochemical Gradients and Physical Processes Associated with the Genesis of Iron Formations: Examples from the Paleoproterozoic to Mesoarchean of Superior Province 10:00 – 10:30 am COFFEE BREAK AND POSTER VIEWING 10:30 – 10:50 am Steve Kissin Rainy River, northwestern Ontario's first meteorite 10:50 – 11:10 am Dennis Smyk, William Ross and Mark Smyk Images on stone: Pictographs of the Ignace area, northwestern Ontario xxvi �Proceedings of the 61st ILSG Annual Meeting - Part 1 11:10 – 11:30 am Dean Peterson So, an Environmental Impact Statement is required: Some Lake Superior area Geologic Parameters for Geologists, Consultants, Companies, and Regulators 11:30 – 12:00 pm BEST STUDENT PAPER AWARDS AND STUDENT TRAVEL AWARDS 12:00 – 1:30 pm LUNCH (provided) 1:30 – 6:00 pm FRIDAY AFTERNOON FIELD TRIPS 5. Classic outcrops of the Dryden Area Leader: Peter Hinz (MNDM) 6. Gold Occurrences of Van Horne Township Leader: Steve Meade (OGS) 7. Unique mineralizing event at the Pidgeon Molybdenum Occurrence Leader: Craig Ravnaas (OGS) 8. Historic Pickle Lake Camp (departs Dryden for Pickle Lake) Leaders: Mark Smyk (OGS), Peter Hollings (Lakehead University) and Neil Pettigrew (PC Gold Inc.) 6:00 – 7:00 pm 10. Mattabi / Sturgeon Lake Historic VMS Camp (departs Dryden for Ignace) Leader: George Hudak (NRRI – University of Minnesota Duluth) SATURDAY, MAY 23RD 8:00 am – 6:00 pm POST-MEETING FIELD TRIPS 8. Historic Pickle Lake Camp (returns to Ignace / Dryden in evening) 9. Ghost Lake Batholith and Related Pegmatites Leader: Shannon Zurevinski (Lakehead University) 10. Mattabi/Sturgeon Lake Historic VMS Camp (Day One) SUNDAY, MAY 24TH 8:00 am – 6:00 pm POST-MEETING FIELD TRIPS 10. Mattabi/Sturgeon Lake Historic VMS Camp (Day Two; returns to Ignace / Dryden in evening) xxvii �Proceedings of the 61st ILSG Annual Meeting - Part 1 POSTER PRESENTATIONS (*denotes a student eligible for Best Student Paper Award) Eric Anderson, Ashley Quigley, Patrick Quigley and Thomas Monecke Geophysical imaging of the bedrock geology of the Pembine-Wausau terrane, Wisconsin: Constraints on the setting of volcanogenic massive sulfide deposits Eric Anderson, V. Grauch, Michael Powers and Bill Cannon Seismic, gravimetric, and magnetic modeling over the Bayfield Peninsula, Wisconsin: Testing hypotheses on the source of a gravity low Jordan Baird* and Mary Louise Hill Fold analyses in the Gunflint Formation: working towards a characterization of regional deformation in the Animikie Group near Thunder Bay, Ontario Steven Baumann, Alexandra Cory and Sandra Dylka Interpretation of the St. Amour Deep Stratigraphic Test Well, Alger County, Michigan Greg Brumpton and Steve Kissin Large hypervelocity impacts on Earth: Empirical observations and validation of computational model predictions for Sudbury and Chicxulub Tom Buchholz, Alexander Falster and W. B. Simmons Tainiolite from the Stettin Intrusion, Wausau Complex, Marathon County, Wisconsin Benjamin Drenth, Chad Ailes and Eric Anderson Re-digitized public aeromagnetic data for the Baraga basin and surrounding region, Upper Peninsula, Michigan Espree Essig*, George Hudak, Geoff Pignotta and Robert Lodge Petrographic Analysis of Felsic Tuffs within the Neoarchean Soudan Member of the Ely Greenstone Formation, Northeastern Minnesota V.J.S. Grauch, Michael Powers, Eric Anderson and Bill Cannon Preliminary 3D model of the Midcontinent Rift System in western Lake Superior region Steve Hauck, John Heine, Mark Severson, Sara Post, Sarah Chlebecek, Stephen Monson Geerts, Julie Oreskovich, Sarah Gordee and George Hudak Geological and Geochemical Reconnaissance for Rare Earth Element (REE) Mineralization in Minnesota Jonathan Haynes*, Joyashish Thakurta and Tom Quigley Petrological and geochemical evaluation of the Sturgeon Falls Igneous Body and its relationship with the Penokean Orogenic Belt Benjamin Hinks*, Joyashish Thakurta and Bob Mahin xxviii �Proceedings of the 61st ILSG Annual Meeting - Part 1 Geochemical and petrological studies on the origin of Ni-Cu sulfide mineralization at the Eagle Intrusion in Marquette County, Michigan George Hudak, Stephen Monson Geerts, Larry Zanko, Sara Post and Bryan Bandli The Minnesota Taconite Workers Health Study: Environmental Study of Airborne Particulate Matter - 2015 Update Mark Jirsa, Terry Boerboom, Val Chandler and Mark Schmitz Geology and geochronology of Archean rocks in the International Falls and Littlefork 30X60’ quadrangles, north-central Minnesota Steve Kissin and Greg Brumpton Studies on PDFs in shocked quartz from distal Sudbury ejecta in the Thunder Bay area compared with Chicxulub Benjamin Krogmeier, Dylan McKevitt, Elizabeth Roepke, Michael Sara, Paul Szkilnyk and Mark Jirsa Geologic mapping of Neoarchean and Proterozoic rocks near Knife Lake, Northeastern Minnesota, by students of the Precambrian Research Center’s 2014 field camp Nathan Lentsch* and Jim Miller Incorporation of Duluth Complex maps into GIS platform Jim Miller, Christopher Beaver, Timothy Hahn, Nikolas Miller, Joseph Puliese and Erick Wright Geology of the North and South Temperance Lakes Area of the Boundary Waters Canoe Area, Cook County, Minnesota - 2014 Precambrian Field Camp Capstone Mapping Doug Nikkila* and Shannon Zurevinski The mineralogy and petrology of a newly discovered REE occurrence within the Coldwell Complex near Marathon, Ontario Sean O’Brien*, Pete Hollings and Jim Miller Petrology, geochemistry and mineral chemistry of the Crystal Lake and Mount Mollie mafic intrusions, Northwestern Ontario Mark Puumala, Rob Cundari, Dorothy Campbell, Desmond Rainsford and Riku Metsaranta New airborne geophysical data for the Lake Superior Region of northwestern Ontario: A new tool for the identification of Neoarchean to Mesoproterozoic structures and associated maficultramafic intrusions Patrick Quigley* and Thomas Monecke Spectrum of Volcanogenic Massive Sulfide Deposits in the Penokean Volcanic Belt, Great Lakes Region, USA Andrew Sasso* and Joyashish Thakurta xxix �Proceedings of the 61st ILSG Annual Meeting - Part 1 Geochemical and Petrologic Characterizations of Peridotite, Marquette County, Michigan Laurel Woodruff and Carrie Jennings Bedrock and Soil Chemistry in Paired Watersheds in Northeastern Minnesota xxx �Proceedings of the 61st ILSG Annual Meeting - Part 1 Chemistry and Mineralogy of Nopeming metasiltstone at the Grandview Site, Duluth, Minnesota ALBERT, Amos, EAGLE-BLUESTONE, Jessica, and SAINI-EIDUKAT, Bernhardt Department of Geosciences, North Dakota State University, Fargo, ND 58102 USA The series of outcrops in the Grandview area of Duluth, Minnesota, is considered a classic geologic exposure (Mattis, 1972; Jirsa and Morey, 1987). Here, we examined the contact between the Middle Proterozoic Nopeming Sandstone and the lower magnetically reversed Ely’s Peak Basalts of the North Shore Volcanic Group. We investigated whether prominent light/dark banding in the uppermost siltstone portion of the Nopeming formation resulted from sedimentary deposition or metamorphism, and whether any chemical and mineralogical differences exist. The sample was taken from the metasiltstone layer directly beneath the basalt (Fig. 1). After carrying out petrographic analysis, polished samples were examined by SEM-EDS (NDSU). In preparation for XRD (NDSU) and ICP analysis (Activation Laboratories), we crushed the sample and hand-sorted the light and dark grains. Grain size varies from fine silt in the light bands to clay size in the dark colored bands. Although there are areas of disrupted banding where light and dark materials are intimately mixed, the bands counted (n= 30 total) have average widths of 2.8 mm and 2.25 mm respectively (Fig. 2). Light bands contain coarser grains compared to dark bands. Fining from light to dark bands represents a graded bedded sequence of sedimentation. A few thin bands, less than ~40 µm in width, of opaque grains with reaction rims surrounding each grain were also observed. XRD and petrography show the light bands are richer in quartz and albite, plus some augite. Dark bands consist mainly of quartz, albite, hornblende, and actinolite. SEM-EDS indicates a reaction rim assemblage of ~ 25 µm grains consisting of ilmenite cores with titanite rims (Fig. 3), in a matrix of actinolite and albite. Zircon, diopside, potassium feldspar and quartz were also observed. Whole rock chemistry shows both light and dark bands are silica rich, but the dark bands contain less silica (61.59 wt. %) than the light bands (69.22 wt.%) (Table 1). Higher amounts of Al2O3, Fe2O3, MnO, MgO, CaO, K2O and TiO2 were found in the dark bands. Dark bands also contain more total REE (147 ppm) vs. light (131 ppm), consistent with the concept of higher original clay content in the dark bands. Both light and dark bands show marked CN LREE enrichment with a small negative Eu anomaly and flat HREEs. The overall pattern is similar to average sedimentary and crustal REE patterns (McLennan, 1989) (Fig. 4). On a PAAS normalized diagram, both light and dark bands plot near unity, although the LREES have ratios slightly <1 while the HREE's are slightly >1 (Fig. 4). It is unclear why a disproportional amount of barium (1467 ppm) was found in the dark bands, vs. 273 ppm in the light ones. Ilmenite-titanite reaction rims appear as sharp boundaries between the two minerals. Ilmenite has high FeTiO3 content suggesting that it crystallized in conditions of higher T and/or lower fO2. Titanite often occurs as product of late stage oxidation. Textural evidence suggests that titanite + ferroactinolite assemblage is due to a hydration reaction, following such as below from Harlov et al. (2006) (Fig. 5, Reaction 4). 6 Hedenbergite + 3 Ilmenite + 5 Quartz + 2 H2O = 2 Fe-actinolite + 3 Titanite We conclude the banding resulted from a process producing fining upward graded bedding, perhaps by small turbidities in a shallow aqueous environment. The presence of hornblende and actinolite indicates the emplacement of the Ely’s Peak Basalt altered the original mineralogy to a 1 �Proceedings of the 61st ILSG Annual Meeting - Part 1 metamorphic grade between hornblende-hornfels and amphibolite facies. This event also produced the observed ilmenite-titanite assemblage. Fig. 1. Metasiltstone in outcrop. UTM 15T 555,616E/5,174,699N Fig. 4. Chondrite and PAAS normalized REE spiderplots for the light and dark bands. Fig. 2. Cut sample showing banding Fig. 5. Schematic phase relationships in the system CaOFe2O3-TiO2-SiO2-H2O-O2 involving titanite (Ttn), ilmenite (Ilm), magnetite (Mt), hedenbergite (Hed), Fe-actinolite (Act), and quartz (Qtz) as a function of logfO2 and logfH2O at constant temperature and pressure (Harlov et al., 2006) Fig. 3. SEM image of ilmenite w titanite rim. 1: Ilmenite; 2: Titanite; 3,4: Actinolite; 5: Albite wt. % Light Dark SiO2 69.22 61.59 Al2O3 7.98 8.94 Fe2O3t 3.34 5.3 MnO 0.179 0.203 MgO 6.02 9.28 CaO 9.37 9.74 Na2O 1.95 2.03 K 2O 0.98 1.3 TiO2 0.678 0.834 P 2O 5 0.09 0.1 LOI n.a. 0.89 Total 99.81 100.21 Table 1. Whole rock major element chemistry for the light and dark bands of the metasiltstone. n.a.: not analyzed Funding from the Three Affiliated Tribes of North Dakota to J.E.-B.is gratefully acknowledged. REFERENCES Harlov, D., Tropper, P., Seifert, W., Nijland, T., Förster, H.-J., 2006. Formation of Al-rich titanite (CaTiSiO4OCaAlSiO4OH) reaction rims on ilmenite in metamorphic rocks as a function of fH2O and fO2, Lithos 88, 72– 84. Jirsa, M.A. and Morey, 1987, Jay Cooke State Park and Grandview areas: evidence for a major early Proterozoic - middle Proterozoic unconformity in Minnesota, in Biggs, D.L., ed., Centennial Field Guide 3: Boulder, CO, Geological Society of America, p. 67-72. Mattis, A.F., 1972, The petrology and sedimentation of the basal Keweenawan sandstones of the north and south shores of Lake Superior. Unpubl. M.S. Thesis, Univ. Minn. Duluth, 123 p. McLennan, S.M., 1989. Rare earth elements in sedimentary rocks; influence of provenance and sedimentary processes. Reviews in Mineralogy and Geochemistry, v. 21, p. 169-200. 2 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Seismic, gravimetric, and magnetic modeling over the Bayfield Peninsula, Wisconsin: Testing hypotheses on the source of a gravity low ANDERSON, Eric D.1, GRAUCH, V.J.S.1, POWERS, Michael H.1, and CANNON, William F.2 1 US Geological Survey, MS 964, PO Box 25046, Denver, CO 80225 USA 2 US Geological Survey, 12201 Sunrise Valley Dr., MS 954, Reston, VA 20192 USA A prominent gravity low lies over the Bayfield Peninsula in northern Wisconsin (Figure 1). The mapped bedrock geology includes sedimentary rocks of the Oronto and Bayfield Groups that overlie Midcontinent rift-related volcanic and intrusive rocks. The nearly 100 mGal amplitude anomaly has been interpreted to reflect low density Archean granite that is surrounded by higher density basalt (White, 1966; Allen and others, 1997). Two-dimensional (2D) gravity and magnetic models, constrained by limited seismic reflection data, are being developed to test possible sources for the gravity anomaly. Seismic reflection data acquired in 1984 were licensed from Seismic Exchange International for portions of several lines on the Bayfield Peninsula. Plots of two-way travel times across the western gradient of the gravity low (Figure 1) show relatively flat and continuous reflections within the Bayfield and Oronto Group rocks. Calculated depths from two-way travels times for the base of the sedimentary rocks vary slightly from 2.7 km in the west to 3.6 km in the east, indicating that the rocks have an apparent dip to the east. On the western side of the seismic profile are reflections that dip moderately to the west which can be observed to estimated depths of 7.6 km. These westerly dipping reflections pinch-out at around 3.5 km depth near the eastcentral part of the profile. The contrasting dip direction indicates that the source rocks are in unconformable contact with the overlying, gently dipping reflections attributed to the sedimentary rocks. This angular unconformity has been interpreted to represent the contact between the Oronto Group and the Midcontinent rift-related volcanic rocks (Allen and others, 1997). Seismic reflections are not apparent beneath the volcanic rocks where the geology is inferred to be Archean granite. Publically available gravity and magnetic data map contrasting physical properties that are related to changes in subsurface geology. The gravity data with stations spaced approximately 2 km on-shore and 5 km off-shore indicate that low-density crustal material underlies the Bayfield Peninsula. Magnetic anomaly data show that a moderate amplitude, long wavelength anomaly high occurs over much of the gravity low which likely reflects a relatively deep magnetic source. Forward models of gravity and magnetic data along a 160 km east-west profile that spans the Bouguer gravity low (Figure 1; line A-A`) were constructed using reported physical property values (Chandler and Lively, 2011). The results confirm that the gravity anomaly can be explained by a ridge of low density material, possibly Archean granite, flanked by west-dipping high density rocks, both of which are overlain by low density sedimentary rocks. Magnetic depth estimates and model sensitivity to changes in source magnetization at depths ranging 3 to 12 km suggest that the west-dipping rocks, constrained by the seismic data, are magnetic. However, reported basalt magnetizations did not produce acceptable model response, indicating that true magnetizations are much lower and possibly indistinguishable from the adjacent Archean rocks. These results suggest basalt is present, but may have reversed-polarity or reduced magnetization compared to elsewhere. 3 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Figure 1: Generalized geology map of western Lake Superior showing location of prominent gravity low over the Bayfield Peninsula. Seismic lines provide detailed subsurface imaging that helps constrain 2D gravity and magnetic models to test possible sources for the gravity low. Forward models of gravity and magnetic data were constructed along line A-A`. REFERENCES Allen, D.J., Hinze, W.J., Dickas, A.B., and Mudrey, M.G., 1997. Integrated geophysical modeling of the North American Midcontinent rift system: new interpretations for western Lake Superior, northwestern Wisconsin, and eastern Minnesota. In Ojakangas, R.W., Dickas, A.B., and Green, J.C. (Eds.), Middle Proterozoic to Cambrian Rifting, central North America: Geological Society of America Special Paper 312: 47-72. Chandler, V.W., and Lively, R.S., 2011. Compilation of Minnesota and western Wisconsin geoscience for the USGS National Geologic Carbon Dioxide Sequestration Assessment: Enhanced geophysical model for extent and thickness of deep sedimentary rocks. Minnesota Geological Survey Open-File Report 2011-03: 37 pages. White, W.S., 1966. Tectonics of the Keweenawan basin, western Lake Superior region. U.S. Geological Survey Professional Paper 524-E: E1-E23. 4 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Geophysical imaging of the bedrock geology of the Pembine-Wausau terrane, Wisconsin: Constraints on the setting of volcanogenic massive sulfide deposits ANDERSON, Eric1, QUIGLEY, Ashley2, QUIGLEY, Patrick2, and MONECKE, Thomas2 1 US Geological Survey, MS 964, PO Box 25046, Denver, CO 80225 USA 2 Department of Geology and Geological Engineering, Colorado School of Mines, 1516 Illinois St., Golden, CO 80401 The Pembine-Wausau terrane in Wisconsin (Figure 1) represents a major Paleoproterozoic belt of metavolcanic rocks that have formed in an island-arc setting at the southern limit of the Superior Craton (Schulz and Cannon, 2007). The terrane is known to host a number of significant volcanogenic massive sulfide deposits, including the world-class Crandon deposit which comprises a total resource of 65.8 million metric tons of massive sulfides. Despite its potential future economic significance, little is known about the bedrock geology of this terrane. A thick cover of glacial deposits makes field observations difficult. Existing geologic reconnaissance map compilations indicate that many of the known VMS prospects occur within a succession dominated by bimodal metavolcanic rocks (Nicholson and others, 2004). Existing regional-scale potential field data provide a continuous set of observations across the entire terrane. This study reinterprets these data and applies filters to highlight changes in rock properties that may in part reflect magmatic controls on the location of the VMS deposits. The interpretations are being integrated with on-going geochemical and geochronological studies to better understand the observed geophysical anomalies over an accreted island-arc setting. The gravity compilation contains stations spaced approximately 1.6 km where access was not limited (Snyder and others, 2004). The data were gridded to a 400 m cell size from which filtered data sets were generated. The complete Bouguer anomaly map highlights dense mafic volcano-plutonic rocks that were intruded by less dense, syn- and post-tectonic granite-tonalite rocks. Large northeast-southwest and east-west trending gradients coincide with mapped and inferred buried faults that indicate offset crustal blocks of varying densities within, or beneath, the bimodal metavolcanic rocks. Such structures may reflect bounding faults enclosing grabens within which the VMS deposits may have formed. Aeromagnetic data were collected along north-south flight lines spaced 800 m at a nominal height of 150 m (Karl, 1986). These data were contoured using a cell size of 250 m from which filtered data sets were generated. The reduced-to-pole (RTP) transformation shows that the mafic volcano-plutonic rocks produce strong magnetic anomaly highs. Moderate amplitude RTP anomaly highs are observed over the younger granite-tonalite plutons. Bimodal volcanic rocks produce magnetic lows; however, within these lows are circular and linear magnetic highs that trend northeast-southwest and east-west, some of which are associated with gabbro rocks. The analytic signal (AS) transformation shows high gradients over the mafic volcano-plutonic rocks. The AS highlights isolated magnetic anomalies within the bimodal volcanic rocks, some of which may be imaging synvolcanic plutons that may have acted as heat sources for VMS hydrothermal systems. Several circular and linear AS anomalies occur along the gravity gradients and mapped faults. The tilt derivative (TDR) transform highlights a northeast-trending magnetic fabric within both the mafic volcano-plutonic rocks and the bimodal volcanic rocks. TDR lineaments occur in higher concentrations proximal to mapped faults. These lineaments are parallel to bedding orientations and major structures and, are therefore, interpreted to reflect the strike of mafic flows within the volcanic rock package. 5 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Together, the gravity and magnetic data are able to identify the structural framework and the buried extents of the bimodal metavolcanic rocks that are the targets for VMS exploration. In addition, these data help map the location of plutons that are critical to understanding the thermal evolution of the region. Figure 1: Generalized geology map showing major rock types within the Pembine-Wausau terrane between the Niagara fault and Eau Pleine shear zone (Nicholson and others, 2004). Red triangles and circles depict VMS deposits and prospects, respectively. REFERENCES Karl, J.H., 1986. Total magnetic intensity map of northern Wisconsin: Wisconsin Geological and Natural History Survey Map 86-7: scale 1:250,000. Nicholson, S.W., Dicken, C.L., Foose, M.P., and Mueller, J.A.L., 2004. Preliminary integrated geologic map databases of the United States: Minnesota, Wisconsin, Michigan, Illinois, and Indiana. U.S. Geological Survey Open-File Report 2004-1355. Schulz, K.J. and Cannon, W.F., 2007. The Penokean orogeny in the Lake Superior region. Precambrian Research, 157: 4-25. Snyder, S.L., Geister, D.W., Daniels, D.L., and Ervin, C.P., 2004. Principal facts for gravity data collected in Wisconsin: A website and CD-ROM for distribution of data. U.S. Geological Survey Open-File Report 03157. 6 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Iron-rich siliceous stromatolites from the upper algal unit of the Gunflint and Biwabik iron formations ARTS, Adrian and FRALICK, Philip Department of Geology, Lakehead University, Oliver Rd. Thunder Bay, ON, P7B 5E1, Canada The Gunflint and Biwabik formations comprise the middle units of the Proterozoic Animikie Group that crop out along the north shore of Lake Superior. Two stromatolitic units are present within these formations; a basal unit that the stromatolites grow directly on the peneplained Archean basement or on the conglomerate which forms the base of the Animikie, and a second upper unit roughly 45 meters above the base. The Animikie stromatolites are unique in that they are mainly composed of finely laminated, fine grained, bands of iron-rich silica. This is unusual as most stromatolites, modern and ancient are composed primarily of carbonate. Since their initial discovery, speculation as to their original mineralogy have been raised (Barghoorn & Tyler, 1965; Cloud 1965; Lougheed 1983; Sommers et al, 2000). This study was conducted on the two stromatolitic horizons to determine whether these iron-silica-rich stromatolites represent a primary mineralogy or if there is evidence for silicification of an earlier carbonate phase. The utilization of high resolution, field emission scanning electron microscopy (SEM), x-ray diffraction (XRD), whole rock geochemistry, and transmitted light microscopy revealed several pieces of compelling evidence within the upper algal unit. Hand samples cut horizontally, (Fig. 1A) show the fine grained, hematite-rich laminae located within the columns (white arrows). In thin section, erosive scouring of the siliceous stromatolite column tops is common, with new siliceous bacterial mat truncating the old (Fig. 1B). The sharp contact between the scour-truncation suggest lithification prior to the development of the younger laminae. The microquartz which the stromatolitic laminae are composed of, is in sharp contrast to the mega quartz cement found within the interspace between the columns (Fig. 1C). Note the thin (≤10µm) microquartz wisps overlaying the coated grains bridging the columns, suggesting a fossilized bacterial mat (white arrow). Intraformational clasts containing pieces of lithified stromatolite are common, especially as nucleation sites of ooids (Fig. 1D, 1E). Finally, energy dispersive x-ray (EDX) mapping show distinct alternation of silica-iron-manganese in ooid coatings and stromatolite laminae (Figs. 1F-1I). The above strongly indicates the Gunflint and Biwabik stromatolites were originally siliceous and formed by a different precipitation mechanism than Proterozoic carbonate stromatolites or modern agglutinated forms did. REFERENCES Barghoorn, E.S. and Tyler, S.A. 1965. Microorganisms from the Gunflint chert, Science, 147, 563-577. Cloud, P, 1965. Significance of the Gunflint (Precambrian) microflora. Science, 148(3666), 27-35. Lougheed, M.S. 1983. Origin of Precambrian iron-formations in the Lake Superior region. Geological Society of America Bulletin, 94, 325-340. Sommers, M.G., Awramik, S.M., Woo, K.S., 2000. Evidence for initial calcite-aragonite composition of lower algal chert member ooids and stromatolites, Paleoproterozoic Gunflint Formation, Ontario, Canada. Canadian Journal of Earth Sciences, 37(9), 1229-1243. 7 �Proceedings of the 61st ILSG Annual Meeting - Part 1 (A ) (B) (C ) (D ) (E) (F) (G ) (H ) (I) Figure. 1. Features of the silica-iron-rich stromatolites within the Gunflint and Biwabik Formations. (A) Horizontal section through stromatolite columns showing fine grained hematite laminae within the column (white arrows). (B) Photomicrograph illustrating the truncation and subsequent overgrowth of stromatolite columns by fine grained, thin laminae composed of iron-rich jasper (red/brown) and quartz (clear). (C) XPL photomicrograph highlighting difference between microquartz stromatolitic laminae, and blocky quartz cement in interspace. The bridging by a thin siliceous algal mat between two columns (white arrow), suggests a rapid growth of mat over the grainstone. (D) Clast containing coated grains and piece of stromatolite column. (E) SEM-BSE image of an ooid containing a broken piece of stromatolite as its nucleation point. This suggests the stromatolite was lithified prior to the development of the ooid lamination. (F-I) EDX false colour images of alternating silica (G), iron (H), manganese (I) stromatolitic laminae. 8 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Evidence of high-temperature Ni isotopic fractionation during the formation of Cu-Ni-PGE sulfide deposits in the Duluth Complex ASP, Kristofer1, SCHARDT, Christian1, and SPIVAK-BIRNDORF, Lev2 1 Department of Earth and Environmental Sciences, University of Minnesota-Duluth, 1114 Kirby Dr., Duluth, MN 55812 USA 2 Department of Geological Sciences, University of Indiana, 1001 East 10th St., Bloomington, IN 47405 USA The Duluth Complex in northeastern Minnesota is an extensive, largely gabbroic body that formed during the 1.1 Ga Midcontinent Rift event. The basal zones of the complex, the South Kawishiwi (SKI) and Partridge River intrusions (PRI), host extensive Cu-Ni-PGE mineralization in a number of recognized deposits that are actively being explored. Previous research, using sulfur isotopes, indicates the underlying Virginia formation as a source of sulfur during the formation of these deposits [1]. Recent studies have shown significant Ni isotopic variations of up to 1.1‰ in high-temperature magmatic rocks associated with magmatic sulfide mineralization [24]. The heterogeneous mineralogy and mafic lithologies in the basal Duluth Complex indicate a range of different processes active during crystallization. The primary goal of this study is to examine mineralized and unmineralized Duluth Complex material to assess the potential of Ni isotopic fractionation between early cumulates, subsequent Cu-Ni-PGE mineralization, and weathering products. Of particular interest is the exploration potential of Ni isotopes for magmatic sulfides recorded in weathered products at the surface. Sample material collected includes till, outcrop material, and in-situ mineralization as well as control samples unconnected to the SKI or PRI. Massive sulfides, disseminated sulfides, and non-mineralized gabbro were selected to obtain Ni isotopic signatures from both a sulfide and silicate source. Till and surface samples were collected in the vicinity of known Cu-Ni-PGE deposits, including Spruce Road, Maturi, Mesaba, Serpentine, and NorthMet. In-situ mineralized material from drill core was provided by local mining companies (Duluth Metals/Twin Metals, PolyMet, Teck, Encampment Minerals) that hold mineral rights to individual deposits. Material from other deposits, including Birch Lake, Wetlegs, and Wyman Creek was sampled from drill core available from the Minnesota DNR. Samples were processed to produce thin sections and polished thick to observe representative textures and mineral compositions for the Cu-Ni-PGE mineralization and host rock gabbro. Material from each deposit was analyzed using XRD, whole rock, and trace element geochemistry. Electric pulse disaggregation (EPD) was used to separate 1 cm3 samples into individual mineral grains and olivine was separated for isotopic analysis. EPD olivine crystals, along with till, massive sulfide material, and weathered surface samples were ground to < 70 µm using a shatterbox and send for nickel isotope analysis to the University of Indiana using the double-spiking method outlined in [2]. Ni isotope ratios are reported relative to the NIST SRM 968 standard with conventional delta notation and a general 2σ error of 0.06‰. Isotopic results show a spread of δ60/58Ni values from -0.97‰ to +0.21‰, within the range of Ni isotopic values reported previously [2,3]. The least fractionated values come from unmineralized mafic intrusives (-0.07‰), while Ni isotopic ratios become progressively lighter with increasing sulfide content, ranging from -0.16 ‰ to -0.97‰ (Fig. 1). Till samples record intermediate values (-0.02 ‰ to -0.77‰) and weathered surface samples can span the entire range. 9 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Figure 1: δ60/58 Ni values for sulfide, olivine, and till compared to background values and data from previous studies. The isotopic data indicate an isotopic fractionation trend in the Duluth Complex from unfractionated values around zero, assumed to be bulk silicate earth, for early crystallizing phases to notably fractionated sulfide mineralization, accumulated at some later stage. This fractionation of up to 0.81‰ suggests that Ni was fractionated during the Ni sulfide formation by incorporating preferentially lighter Ni into the accumulating sulfide melt and resulting Ni sulfides. Ni isotopic values for till and mineralized surface samples, and their correlation with known deposits, may be useful in distinguishing regions overlying Cu-Ni-PGE mineralization from barren areas. Data may also help to identify the entry point of the mineralizing magma based on the location and isotopic signature of individual sulfide deposits. This will require a more detailed sampling of selected locations and materials. REFERENCES Gueguen B., Rouxel O., Ponzevera E., Bekker A., Fouquet Y. (2013) Ni isotope variations in terrestrial silicate rocks and geological reference materials measured by MC-ICP-MS. Geostandards and Geoanalytical Research 3: 297-317 Hiebert RS., Rouxel, O., Houlé, MG., Bekker, A. (2014) Ni isotope fractionation between komatiite and sulfide mineralization at the Neoarchean Hart deposit, Abitibi greenstone belt, Canada. Geological Society of America Abstracts 46: 467 Ripley, E. (2006) Sulfur isotopic studies of the Dunka Road Cu-Ni deposit, Duluth Complex, Minnesota. Economic Geology 76: 610-620 Wasylenki, L.E, Howe, Haleigh D., Spivak-Birndorf, L.J., Bish, DL. (2015) Ni isotope fractionation during sorption to ferrihydrite: implications for Ni in banded iron formations. Chemical Geology 400: 56-64 10 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Fold analyses in the Gunflint Formation: working towards a characterization of regional deformation in the Animikie Group near Thunder Bay, Ontario BAIRD, Jordan, and HILL, Mary Louise Department of Geology, Lakehead University, 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada Deformation in the Animikie Group near Thunder Bay is characterized by intense, localized fold-and-thrust belt deformation within units that are otherwise relatively undeformed and flat-lying. This deformation is interpreted to be the result regional stress during Proterozoic tectonism. Stereographic analyses of structural measurements and outcrop observations are used to determine beta-axes (mean fold axes) for fold populations observed, as well as to determine structural relationships within the region. Two main fold populations are observable in the data. The primary population exhibits a north-south trending beta-axis, indicating east-west compressive stress. The secondary population exhibits an east-west trending beta-axis, indicating either north-south compressive stress or the presence of lateral ramps. Slickenlines are also observed to trend north-south and east-west, depending on outcrop location. East-west trending slickenlines tend to be in areas of more intense folding, indicating that they may be older than the north-south trending slickenlines. Older slickenlines may have been destroyed when fault surfaces were reactivated in areas of less intense folding, replaced by the slickenlines associated with the most recent deformation. Additional observations include the presence of fold-hinge breccia associated with non-cylindrical folding, which may indicate lateral ramp formation, as well as the presence of a possible cleavage duplex structure, which may indicate repetitive east-verging thrusting. A specific tectonic history for the Animikie Group has been suggested based on these observations. It has been proposed here that primary east-verging thrusting was associated with the Trans-Hudson orogeny in the Paleoproterozoic. Following this, there may or may not have been a secondary compressional phase due to the Yavapai-Mazatzal orogenies during the early to middle Proterozoic; the effects of these orogenies remain unclear. Deformation likely culminated in the late Proterozoic with north-south extension related to the Midcontinent Rift.   11 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Interpretation of the St. Amour Deep Stratigraphic Test Well, Alger County, Michigan BAUMANN, Steven D.J.1, CORY, Alexandra B.1, and DYLKA, Sandra K.1 1 Geology Section, Midwest Institute of Geosciences and Engineering, 1321 W. Touhy Ave. 2S, Chicago, IL 60626 The Amoco Production St. Amour (1-29R) was a petroleum deep test well, drilled in 1988 to a depth of 7238 feet below ground surface (bgs). Surface elevation of the well was approximately 910 feet above mean sea level. The GPS coordinates for the well are 46.354591o 86.584370o. The well passed through 110 feet of glacial material at the surface. Below the glacial material is 411 feet of Paleozoic sediments. Under the Paleozoic is 2132 feet of the Precambrian Jacobsville Formation (this differs from Ojakangas’ 2002 interpretation). The Jacobsville can be subdivided into three distinct units that are very similar to the formations of the Bayfield Group in Wisconsin. Chequamegon type lithology is encountered at 521-1470 feet bgs, Devils Island type lithology is encountered at 1470-1939 feet bgs, and Orienta type lithology is encountered at 19392653 feet bgs. Beneath the Jacobsville lies 3197 feet of the Freda Formation, which can also be divided into three units. There is an upper red to gray and light brown, fine to medium grained arkose (2653-2830 feet bgs). The middle unit is composed of mostly red, silty, very fine to medium grained arkose, with beds of red siltstone and shale (2830-3260 feet below the surface). The basal Freda is a fining upwards sequence of red to gray mottled pale brown (occasionally green), fine to coarse grained arkose with beds of deep red shale, and pebbles are common. Below the Freda is a unit not seen elsewhere in the Oronto Group. There is 675 feet of a red, hematitic quartz arenite with thick beds of quartz conglomerate (unit Y(x), Figure 1). This unit is very mature for its stratigraphic position and appears unique from above and below units. From a depth of 625-6933 was described as a “heterogeneous unit” (Ojakangas 2002). It consists of 203 feet of faulted basalt interbedded with sandstone, siltstone, and conglomerate (unit Y(xx), Figure 1). This unit appears to be unconformable at its base, although this is difficult to determine for sure in the core. Below this unit (6728-6783 feet bgs) is what has been interpreted as 145 feet of the Nonesuch Formation. We agree with this interpretation since the stratigraphy matches up well with the lower three units in the Big Iron River at Bonanza Falls (Susek 1997). Below the Nonesuch lies 50 feet of a basalt flow (unit B1, Figure 1) over a gabbro diabase (unit B2, Figure 1). The gabbro diabase appears to be a later intrusion into units Y(bb) and Y(xxx) and may have been emplaced during the Grenville Orogeny. Under the gabbro is 10 feet of red and gray siltstone and shale underlain by conglomerate, unit Y(xxx). The deepest unit penetrated is 350 feet of rhyolite and ignimbrite, which has been dated at 1.083 + 0.003 Gya old. The only known igneous rocks younger than this within the Mid-continental Rift belong to the Bear Lake Rhyolite Stock in the Freda Formation (1.054 + 0.034 Gya). There are two faults present within the St. Amour core. There is a 32 foot long stretch highly sheered red and green sandstone from 6573-6605 feet bgs. A smaller second fault, which contains 12 feet of gray and red sandstone and shale is present at 6691-6703 feet bgs. Figure 1 shows the interpretive relationship between the geologic units from 6400-7000 feet bgs. In our interpretation both faults are high angle reverse faults that dip about 73o from the horizontal (the strike of the core is not known so the dip direction could not be obtained). The smaller of the two faults has been modeled parallel to the larger fault. However, in reality, it likely branches off from it and represents a faulted shatter zone. Total fault displacement is about 121 feet. There are two basalt units above the Nonesuch within the core. The upper one is 48 feet thick and lies 12 �Proceedings of the 61st ILSG Annual Meeting - Part 1 above the faults. The second is about 43 feet thick and lies between the two faults. Both basalts have mostly sandstone and siltstone with some conglomerate below them. Due to their similar thickness and lithology, we interpret them to be the same unit offset by the faults (units Y(ba), Figure 1). The basalt has not been dated but since it overlies the rhyolite, it has to be younger. If it were to be dated we could be able to bracket the time of deposition for the Nonesuch Formation. Based on our analysis of the core we partially agree with Ojakangas and Dickas’ 2002 interpretation that late Mid-continent Rift volcanism occurred later than in other areas because of the core’s proximity to the mantle plume. We also postulate that the area around the core may represent a buried stratovolcano similar to the one at Porcupine Mountains (located about 150 miles west-northwest of the St. Amour core location). We propose that a high altitude composite volcano existed in the area until about 1.080 Gya, until the complete deposition of the Nonesuch Formation. At that time main volcanism ceased and the area began to rapidly subside allowing for the deposition of thick quartz sandstone (unit Y(x), Figure 1) over the youngest basalt (unit Y(ba), Figure 1), thus burying the volcano. The main fault in the core may have originated as a normal growth fault that was later reactivated as a reverse fault during the Grenville Orogeny, creating smaller offshoot faults. Until more deep cores are obtained from the area of the St. Amour test well, the presence of a subsurface volcano cannot be verified. REFERENCES Bornhorst, T.J., Rose, W.I., 1994. Self-Guided Geologic Field Trip to the Keweenaw Peninsula, Michigan. Institute on Lake Superior Geology, volume 40, pp. 161-164 Dickas, A.B., Mudrey Jr., M.G., 1992. Keweenaw Sedimentary Rock of the South Shore, Lake Superior. Institute on Lake Superior Geology, volume 38, pp. 43-102 Friedhoff-Miller, Diana, 1988. Record of Well Drilling or Deepening, St. Amour 1-29R. State of Michigan Department of Natural Resources, Geological Survey Division Ojakangas, R.W., Dickas, A.B., 2002. The 1.1-Ga Midcontinent Rift System, central North America: sedimentology of two deep boreholes, Lake Superior Region. Journal of Sedimentary Geology 147 (2002) pp. 13-36 Suszek, T. J., 1997. Petrography and sedimentation of the Middle Proterozoic (Keweenaw) Nonesuch Formation, western Lake Superior region, Midcontinent rift system, Geological Society of America, Special Paper 312 13 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Figure 1: Structural Interpretation of the St. Amour Core from 6400-7000 feet Below the Surface 14 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Linking the Ordovician L-Chondrite Event to the Terrestrial Cratering Record: a NorthAmerican Perspective BLEEKER, Wouter Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario, Canada, K1A 0E8 Based on abundant evidence of shock metamorphism and partially or wholly reset Ar ages of a class of meteorites (low metal or “L-chondrites”), meteorite researchers have hypothesized that a catastrophic impact and breakup event took place in the asteroid belt about 500 million years ago (Keil et al., 1994), spawning a large population of meteoritic fragments some of which were perturbed into Earth crossing orbits. This hypothesis received a major boost with the discovery of numerous L-chondrite meteorite fragments in limestone quarries in southern Sweden (Thorslund et al., 1984; Nyström et al., 1988; Schmitz et al., 1996), preserved in-situ across a section of Middle Ordovician stratigraphy (ca. 470 Ma). The Lchondrite-bearing, condensed, shallow marine limestones are also characterized by a sharply increased heavy mineral count of extraterrestrial Ni spinels, also of L-chondrite affinity (Schmitz et al., 2003). This Ni spinel “rain out” has now been documented not only in Sweden but also in China (Heck, et al., 2010), and by all expectations should constitute a global signal. Since then, more detailed Ar-Ar dating of the partially degassed meteorites has refined the likely age of the breakup event to 470±6 Ma (Korochantseva et al., 2007). A number of small meteorite impact craters in Scandinavia has been linked to this event, e.g. the 458 Ma Lockne crater in central Sweden (Grahn et al., 1996; Alwmark and Schmitz, 2007). With Earth moving through a dynamically evolving swarm of asteroid debris (e.g., Nesvorný et al., 2009), the effects of this event should have been global. Numerous, possibly large, impact craters should be linked to this event, particularly in North America with its large cratonic target area and a robust population of ~60 confirmed impact structures. We have previously linked the well-known Brent impact crater to this event (Bleeker, 2011), with a stratigraphically constrained age of ca. 460-450 Ma (Lozej and Beales, 1975) and with a melt/breccia sheet that is known to contain geochemical traces of an L-chondrite bolide (Palme et al., 1981). Among the ~32 confirmed and possible impact craters in Canada alone (e.g., see Grieve, 2006), there could be as many as 5-10 structures that are linked to the same broad event: Brent, Holleford, Skeleton Lake, Nicholson, Pilot Lake, Presqu’ile, Couture, La Moinerie, and the large Slate Island structure in Lake Superior. Scattered and non-definitive K-Ar and Ar-Ar ages could extend this list to the large (~50-55 km-diameter) Carswell structure, and perhaps the buried but unconfirmed Can-Am crater. We have recently redated this structure, using Ar-Ar stepwise heating on pristine adularia crystals, to 481±1 Ma (Bleeker et al., 2015), confirming it as part of the Ordovician impact spike. Even if only the most likely subgroup of this list is indeed related to a ca. 470-440 Ma impact spike of L-chondrites, the proportion of impact craters linked to this event is very large (1 in 5?), as similarly suggested by the more limited sample of just the Swedish crater record alone. The same conclusion is also reached for craters in the remainder of North America (mainly the USA), where among ~30 confirmed craters the following could be linked to the same event: Ames, Calvin, Glasford, Glover Bluff, Newporte, Rock Elm and Versailles (again, a proportionally similar and very large subpopulation, as in Canada). Several of these structures have stratigraphically constrained ages in the 470440 Ma interval (e.g., see Koeberl et al., 2001, for Ames; Millstein, 1994, for Calvin; McHone et al., 1986 for Glasford). It is concluded that the Ordovician L-chondrite event left a major imprint in the North American and by inference global cratering record and, as recognized by Birger Schmitz and coworkers (Schmitz et al., 2008), must have jarred the Earth system throughout much of the Middle and Late Ordovician. During the 470-440 Ma interval, the flux of large impactors appears to have been an order of magnitude higher than during the remainder of the Phanerozoic. To gauge the full scope of this event, an integrated effort to produce better and more precise ages for all major impact structures is needed, with equal emphasis on stratigraphic and isotopic constraints. Important rewards could be an improved understanding of the 15 �Proceedings of the 61st ILSG Annual Meeting - Part 1 dynamical evolution of an asteroidal breakup swarm, a quantification of the overall impact flux during this interval, and a better appreciation of how the Earth system and biota responded to this major event. REFERENCES Awlmark, C., and Schmitz, B., 2007. Extraterrestrial chromite in the resurge deposit of the early Late Ordovician Lockne crater, central Sweden. Earth and Planetary Science Letters, vol. 253, p. 291-303. Bleeker, W., 2011. Linking the Ordovician L-chondrite event to the terrestrial cratering record: A North American perspective. Ottawa 2011, GAC-MAC Joint Annual Meeting, University of Ottawa, May 25-27, Abstracts, vol. 34, p. 19. Bleeker W., LeCheminant, A.N., Alwmark, C., Page, L., Scherstén, A., and Söderlund, U., 2015. The age of the Carswell impact structure. AGU-GAC-MAC-CGU Joint Assemblee, 3-7 May 2015, Montreal. Grahn, Y., Nolvak, J., and Paris, F., 1996. Precise chitinozoan dating of Ordovician impact events in Baltoscandia. Journal of Micropaleontology, vol. 15, p. 21-35. Grieve, R.A.F., 2006. Impact structures in Canada. Geological Association of Canada, Geotext 5. Heck, P.R., Ushikubo, T., Schmitz, B., Kita, N.T., Spicuzza, M.J., and Valley, J.W., 2010. A single asteroidal source for extraterrestrial Ordovician chromite grains from Sweden and China: High Precision oxygen three-isotope SIMS analysis. Geochimica et Cosmochimica Acta, vol. 74, p. 497-509. Keil, K., Haack, H., and Scott, E.R.D., 1994. Catastrophic fragmentation of asteroids: evidence from meteorites. Planetary and Space Science, vol. 42 (12), p. 1109-1122. Korochantseva, E.V., Trieloff, M., Lorenz, C.A., Buykin, A.I., Ivanova, M.A., Schwarz, W.H., Hopp, J., and Jessberger, E.K., 2007. L-chondrite asteroid breakup tied to Ordovician meteorite shower by multiple isochron 40Ar-39Ar dating. Meteroritics & Planetary Science, vol. 42 (1), p. 113-130. Lozej, G.P., and Beales, F.W., 1975. The unmetamorphosed sedimentary fill of the Brent meteorite crater, southeastern Ontario. Canadian Journal of Earth Sciences, vol. 12, p. 606-628. Koeberl, C., Reimold, W.U., and Kelly, S.P., 2001. Petrography, geochemistry, and argon-40/argon-39 ages of impact-melt rocks and breccias from the Ames impact structure, Oklahoma: The Nicor Chestnut 18-4 drill core. Meteoritics & Planetary Science, vol. 36, p. 651-669. McHone, J.F., Sargent, M.L., and Nelson, W.J., 1986. Shatter cones in Illinois: evidence for meteoritic impacts at Glasford and Des Plaines. Meteoritics, vol. 21, p. 446. Millstein, R.L., 1994. The Calvin impact crater, Cass County, Michigan: identification and analysis of a subsurface Ordovician astrobleme. Ph.D. thesis, unpublished, Oregon State University, 114 p. Nesvorný, D., Vokroulicky, D., Morbidelli, and Bottke, W., 2009. Asteroidal source of L chondrite meteorites. Icarus, vol. 2009, p. 698-701. Nyström, J.O., Lindström, M., and Wickman, F.E., 1988. Discovery of a second Ordovician meteorite using chromite as a tracer. Nature, vol., 336, p. 572-574. Palme, H., Grieve, A.F., and Wolf, R., 1981. Identification of the projectile at the Brent crater, and further considerations of projectile types at terrestrial craters. Geochimica et Cosmochimica Acta, vol. 45, p. 24172424. Schmitz, B., Lindström, Asaro, F., and Tassinari, M., 1996. Geochemistry of meteorite-rich marine limestone strata and fossil meteorites from the Lower Ordovician and Kinnekulle, Sweden. Earth and Planetary Science Letters, vol. 145, p. 31-48. Schmitz, B., Häggström, T., and Tassinari, M., 2003. Sediment-dispersed extraterrestrial chromite traces a major asteroid disruption event. Science, vol. 300, p. 961-964. Schmitz, B., Harper, D.A.T., Puecker-Ehrenbrink, B., Stouge, S., Alwmark, C., Cronholm, A., Bergström, Tassinari, M., and Xiaofeng, W., 2008. Asteroid breakup linked to the Great Ordovician Biodiversification event. Nature Geoscience, vol. 1, p. 49-53. Thorslund, P., Wickman, F.E., Nyström, J.O., 1984. The Ordovician chondrite from Brunflo, central Sweden, I. General description and primary minerals. Lithos, vol. 17, p. 87-100. 16 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Large hypervelocity impacts on Earth: Empirical observations and validation of computational model predictions for Sudbury and Chicxulub BRUMPTON, Gregory R. and KISSIN, Stephen A. Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1 Canada Johnson and Melosh (2014) reported on a high-resolution, two-dimensional computational model of effects during a hypervelocity 10km impactor collision with Earth at 20km/s. This early version of the model focused on three impact products, melt droplets, melt fragments and accretionary impact lapilli. They estimated the size of the ejecta products using simple analytical expressions and information determined from their hydrocode models. Prediction of the size of the ejecta products depends on the impactor size, impact velocity and ejection velocity from the forming crater. Johnson and Melosh sought to find a consensus between model predictions describing the formation of the ejecta products and actual geological observations. Modeled estimates of the sizes of melt droplets and accretionary impact lapilli are generally within one order of magnitude of limited empirical measurements at Chicxulub and Sudbury. This agreement acts as a validation of their model and illustrates a process whereby geologic observations can be applied so as to improve the model. Our studies on Sudbury ejecta from the Thunder Bay area (Addison et al. 2005; 2010) have considered the predictions of the Johnson and Melosh model with the results indicated below: Prediction: The model predicts order of magnitude estimates of the size [mm-scale] of melt droplets and melt fragments. - Our studies of Sudbury ejecta and comparison with literature information on Chicxulub (Yancey and Guillemette 2008;Yancey and Liu 2013) verify the predictions of the model. Prediction: Millimeter-sized melt droplets should be found together with accretionary impact lapilli and rarer melt fragments (tektites). Figure 1. Accretionary lapillus (left), tektite (horizontal arrow) and melt droplet (vertical arrow). MC18A SEM, XPL. Prediction: A wide range of sizes of melt droplets should be found at any given site. Figure 2. Melt droplets illustrating a range of sizes. JN23, XPL. Prediction Accretionary impact lapilli form during the ejection process in the turbulent ejecta curtain from fine-grained, solid fragments and molten silicate acts as a binding agent. Lapilli range from larger than 1cm to less than 1mm diameter. 17 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Figure 3. Accretionary lapillus with multiple rings of opaque silicate, cored with crystal fragment. JN29A-2 SEM, XPL Prediction: There is a tendency for the small solid fragments to follow vapor streamlines so that the small fragments may be swept around the growing lapilli and as a result accrete in rows. Figure 4. Detail of an accretionary lapillus showing streamlined, rows of fragments apparently deposited by turbulent vapors during growth of the lapillus. JN29A-2 SEM, XPL. Prediction: The largest melt fragments (tektites) will come from more lightly shocked, near surface, target material. The composition of melt fragments in Sudbury ejecta is consistent with that of surficial sedimentary rocks and granitic gneiss of the target area. They show little if any isotropic silicate melt on the exterior. Prediction: "A more detailed comparison of our models to known ejecta layers will allow us to test the predicted dependence of ejecta product size on impactor size and may even allow us to empirically constrain some additional products ..." (Johnson and Melosh 2014). 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. Addison, W.D., Brumpton, G.R., Davis, D.W., Fralick, P.W., and Kissin, S.A. 2010. Debrisites from the Sudbury impact event in Ontario, north of Lake Superior, and a new age constraint: Are they base-surge deposits or tsunami deposits? in Gibson, R.L. and Reihold, U.W., eds. Large Meteorite Impacts and Planetary Evoution IV: Geological Society of America Special Paper, 465: 245-268. Johnson, B.C. and Melosh, H.J. 2014. Formation of melt droplets, melt fragments, and accretionary impact lapilli during a hypervelocity impact. Icarus, 228: 347-363. Yancey, T.E. and Guillemette, R.N. 2008. Carbonate accretionary lapilli in distal deposits of the Chicxulub impact event. Geological Society of America Bulletin, 120: 1105-1118. Yancey, T.E. and Liu, C. 2013. Impact-induced sediment deposition on an offshore, mud-substrate continental shelf, Cretaceous-Paleogene boundary, Brazos River, Texas, U.S.A. Journal of Sedimentary Research, 83: 354-367. 18 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Tainiolite from the Stettin intrusion, Wausau Complex, Marathon County, WI. BUCHHOLZ, THOMAS W.1, FALSTER, Alexander U.2, and SIMMONS2, W. B. 1 1140 12th Street North, Wisconsin Rapids, Wisconsin 54494 2 Maine Mineral and Gem Museum, PO Box 500, 99 Main Street, Bethel, Maine 04217. The Wausau Syenite Complex (WSC) is composed of four plutons, of which the Stettin Pluton (1565 Ma +3-5) (Van Wyck, 1994) is the oldest and most alkalic. Tainiolite is a relatively uncommon Li-Mg mica; KLiMg2(Si4O10)F2 and occurs in alkalic igneous rocks such as syenites and associated pegmatites. Recently tainiolite has been identified from two sites located in the Stettin Pluton, and probable tainiolite has been found at one other site within the pluton. The first occurrence is in the Ravine Pegmatite, a small irregular evolved pegmatite located on the west side of the long-dormant Dehnel quarry, near the western edge of the Stettin Pluton. Here, tainiolite occurs as colorless crystals of composition (K0.921Na0.080)Σ1.001Li1.000(Mg1.201Fe0.705Mn0.049Al0.033Ti0.012Ca0.009)Σ2.009(Si3.910Al0.090)Σ4.000O22 [(F1.811OH0.189)]Σ2.000 in the miarolitic core zone of the pegmatite and in adjacent intermediate zones, associated with abundant zircon, pyrochlore, bastnaesite-(Ce), bastnaesite-(La), bastnaesite-(Nd), columbite-(Fe), fersmite, aegerine, riebeckite, microcline, albite, and rare baddeleyite. The given analytical results, determined using EMP, XRD and DCPS, are in good agreement with recent published data for tainiolite (e.g. Armbruster et al, 2007). An additional occurrence was noted in summer 2014 in a cobble of syenite pegmatite recovered from a rock pile located in the western portion of the Stettin intrusion on the north side of Evergreen Drive (southern portion of S. 10, T 29N, R 6 E), near the contact between amphibole syenite and the discontinuous nepheline syenite outer rim of the pluton. Here the tainiolite, of composition (K0.969Na0.032)Σ1.001Li1.000(Mg1.109 Fe0.762Al0.041Mn0.030Ti0.010Ca0.011)Σ1.936(Si3.9100Al0.090)Σ4.000O22[(F1.851OH0.149)]Σ2.000, determined using EMP and DCPS, is found as colorless crystals associated with riebeckite, aegerine and Kspar. Interestingly, despite the likely simultaneous crystallization of riebeckite and probable tainiolite, the riebeckite is virtually Mg-free, suggesting that, given a limited supply of Mg vs Na, Mg may be preferentially taken up by tainiolite as opposed to riebeckite. The final (probable) occurrence was identified in material from the old Summit prospect (a failed attempt to mine U from a Th-rich pegmatite) obtained from a mineral collection assembled in the early 1960’s. The prospect worked a pegmatite located in the south-central portion of the Stettin Intrusion. Here the probable tainiolite occurs as sparse light yellow-brown to colorless flakes in intergrown aegerine, fluorite, zircon, bastnaesite-(Ce) and minor quartz. REFERENCES Armbruster, T., Richards, R. P., Gnos,E., Pettke, T., Herwegh, M. (2007): Unusual fibrous sodian tainiolite epitactic on phlogopite from marble xenoliths of Mont Saint-Hilaire, Quebec, Canada. The Canadian Mineralogist, Vol. 45, pp. 541-549. 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. 19 �Proceedings of the 61st ILSG Annual Meeting - Part 1 The Sudbury impact event in the Lake Superior region: Ten years of research on ten minutes of geologic time CANNON, W.F.1, ADDISON, W.D.2, BRUMPTON, Gregory R.3, and JIRSA, M.J.4 1 U.S. Geological Survey, MS 954, Reston, VA 20192 2 371 Crossbow Court, Thunder Bay, ON, P7G 1H5, Canada 3 Lakehead University, Department of Geology, Thunder Bay, ON, P7B 5E2, Canada 4 Minnesota Geological Survey, 2609 W. Territorial Rd., St. Paul, MN 55114 The ancient meteor impact near Sudbury, Ontario is the second largest impact event preserved in the geologic record. The impactor was most likely an ordinary or enstatite chondrite based on siderophile element concentrations in melt rocks (Huber et al., 2014; Petrus et al., 2015). It struck Earth at 1850 Ma, the age of the Sudbury Igneous Complex (SIC) (Davis, 2008), a remnant of the melt sheet generated by the impact. The impact formed a crater, now deeply eroded, with a diameter variously estimated from 130 to 250 km. The impactor, if chondritic, must have been 10 to 15 km in diameter to provide adequate energy to account for the crater size and melt volume. An impact of that magnitude undoubtedly spread a layer of debris hundreds of kilometers beyond the crater. Sudbury impact debris (ejecta) was first reported in the Lake Superior region in 2005 (Addison et al., 2005). Many other sites have been identified since in the western Lake Superior region (Pufahl et al., 2007; Cannon et al., 2010; Jirsa et al., 2011). The impact produced a variety of sedimentary and seismic features; the rocks in which these have been found are known collectively as the Sudbury Impact Layer (SIL). Compelling evidence of meteor impact is relict planar deformation features in quartz grains (Fig. 1A), because they are uniquely produced by impact-generated shock. Other common features are millimeter-scale spherules of devitrified impact glass (Fig. 1B), angular glass particles (Fig 1C), and accretionary lapilli (Fig. 1D). During deposition and reworking, ejecta became mixed with local material to form a hybrid rock. Seismic effects include fracturing and dislocation of pre-impact rocks and seismically triggered submarine debris flows. The effects of the impact across the Lake Superior region can be simulated using a computational model (Collins et al., 2005). Although there is no unique solution, input parameters of: 1) a chondritic impactor with a density of 3.4 g/cm3, 2) an impact velocity of 25 km/sec, 3) an impactor diameter of 15 km, 4) an impact angle of 45o, and 5) crystalline target rocks, predict: 1) a crater diameter of 193 km, well within the range of estimates, 2) a melt volume of about 12,000 km3, similar to previous estimates (Deutsch et al., 1995), and 3) a melt sheet thickness of about 1.2 km, less than the observed thickness of the preserved SIC. Using those same parameters, the model predicts effects of the impact at various distances from Sudbury, which provides a framework in which to judge observed features (Figure 2). The currently known extent of the SIL stretches from the Dead River Basin in Michigan, 500 km west of the impact site, to Coleraine in the western Mesabi Iron Range in Minnesota, about 1000 km west of the impact site. It spans the transition from proximal to distal deposits and its character changes markedly over that distance. The SIL also spans a north-south distance of about 150 kilometers, across which it was deposited in conditions ranging from dry land in the Thunder Bay area, across a continental shelf to the south, to deep water in the Iron River and Crystal Falls area. This combination of factors makes the Lake Superior region a unique natural laboratory in which to study the range of effects of the Sudbury impact beyond the crater margin. 20 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Seismicity: The Sudbury impact generated an earthquake of Richter magnitude 10 or greater, larger than any possible tectonic-related earthquake. The severity of shaking was as great as Mercalli intensity VIII at the more proximal sites and gradually waned to the west. Because the velocity of seismic waves is greater than the velocity of ejecta, severe shaking began minutes before the arrival of ejecta. This shaking caused liquefaction of pre-impact sediments and brecciation of basement rocks, probably aiding in their incorporation into ground surges of ejecta that followed within minutes. At Gunflint Lake the upper 5-7 m of the Gunflint Iron Formation were liquefied to formed coarse breccias of chert in an iron silicate matrix. This is the most distal locality at which strong seismicity has been documented. In the eastern Gogebic Range, upper parts of the Ironwood Iron Formation were mobilized into submarine slump deposits. In the Iron River-Crystal Falls area, in deep-water, massive debris flows were triggered from both the shelf edge to the north and an island arc to the south, resulting in slump deposits as much as 150 m thick. They remained active long enough for ejecta to reach the deep seabed and be incorporated into the slump deposits. In the Dead River Basin, the most proximal SIL localities, Archean basement rocks were intensely fractured and overlying sediments were injected downward into these fractures, probably a few tens of meters below the sea floor. Ejecta: The Sudbury impact blasted material (ejecta) away from the crater as an “ejecta curtain”, which swept across the entire western Lake Superior region over a span of about 3 minutes with likely velocities of 1.5-2 km/sec. The ejecta included fragments of the pre-impact target rocks of varying sizes, with a predicted average fragment size of 2 cm at the more proximal sites, masses of melted rock, which eventually solidified into spherules and glass fragments (Figure 1B, C), and part of the impacting body itself, largely as high temperature vapor. On landing, the huge mass of material was propelled across the landscape by its forward momentum as a turbulent groundhugging density current (base surge) for hundreds of kilometers. The energy within the base surge allowed entrainment of underlying rock and unconsolidated sediments resulting in hybrid deposits of true ejecta (material from the crater itself) mixed with more local rock ranging from fine particles to meter-scale boulders. As the energy and velocity of the base surges waned, a discontinuous layer of debris was deposited as a variable thickness of bedded material whose internal structure records the chaotic nature of the transport and deposition of the impact debris 21 �Proceedings of the 61st ILSG Annual Meeting - Part 1 (Fralick et al., 2012). The most distal base surge deposits are at Gunflint Lake, about 700 km from the impact site, where a layer of breccia and lapillistone about a meter thick overlies the seismically disturbed upper beds of the Gunflint Iron Formation (Jirsa et al., 2011). At more proximal sites near Thunder Bay, base surge deposits are as thick as 4+ m and contain meter-scale and finer clasts of the underlying Gunflint Formation, ejecta rock fragments, devitrified glass, and accretionary lapilli (Addison et al., 2010). In Michigan, deposits are discontinuous. The SIL is absent in many exposures and drill core, but is as thick as 26 m in the Baraga Basin and 40 m in the Dead River Basin (Cannon et al., 2010). All of these deposits contain variable amounts of relatively local material, so the thickness of ejecta is significantly less than the total thickness of the surge deposits. At sites along the Mesabi Iron Range in Minnesota, beyond the outer fringes of ground surges, the SIL is a discontinuous layer, only tens of centimeters thick, composed mostly of glass spherules and minor clasts of quartz and feldspar, all of which are probably ejecta. These were deposited from a cloud of suspended ejecta material that spread over the region in the minutes to hours after the impact and settled onto the sea floor. The most distal known occurrence of the SIL, near Coleraine, Minnesota, is nearly 1000 km from the impact site, the greatest distance at which rocks of suitable age to record the Sudbury event are exposed in the Lake Superior region. Figure 3. Schematic cross section showing the stratigraphic position and depositional setting of the Sudbury Impact Layer across the Penokean foreland. The SIL was deposited within an active tectonic belt of the Penokean orogeny that varied from a low lying land area on the north, across a marine continental shelf southward, into a foredeep at the southernmost localities in the Iron River-Crystal Falls area (Fig. 3). The nature of its deposition varied accordingly. Ground surge deposition on the northern land area is the most straight-forward to comprehend, lacking the complexities of interaction of ejecta with seawater. To the south, in the Gunflint Lake, Mesabi, Gogebic, Baraga Basin, and Dead River Basin areas, ejecta appears to have been deposited in a relatively shallow sea in which iron-formations and ferruginous cherts were being deposited. The manner in which the extremely energetic ejecta and ground surges interacted with the ambient shallow ocean remains largely speculative. Tsunamis: The last impact-generated event that is recorded in the SIL is the inferred reworking of ejecta and underlying breccia by tsunami waves that were generated by the impact. Because these 22 �Proceedings of the 61st ILSG Annual Meeting - Part 1 waves move slowly relative to seismic waves and ejecta, they swept the area some hours after the ejecta was deposited. The magnitude of the tsunamis is poorly understood in theory and depends in part on whether or not the crater was developed in an eastward extension of the ocean that existed at that time in the Lake Superior region. In any case, tsunami waves must have been substantial and possibly enormous. Features formed by those waves have not been reliably differentiated from waning phases of ground surges in many deposits. Perhaps the best indication of wave reworking of ejecta is at Gunflint Lake, where upper beds of the SIL consist of ejecta intermixed with large clasts that are more rounded than typical of lower parts of the layer, indicating energetic reworking. The day after: An impact of the magnitude of Sudbury surely had global consequences, just as the Chicxulub impact did at the end of the Cretaceous period. A global layer of fine impact material was probably deposited, but has not yet been identified outside of the Lake Superior region. More locally, within the Lake Superior region, land areas were mantled with ejecta that probably dominated both the chemistry of water and physical nature of sediment being carried to the adjacent shallow ocean for a considerable time after the impact. Model studies suggest that the impact also resulted in a global-scale “nuclear winter”, a period of cold and dark conditions, as fine particles in the upper atmosphere blocked sunlight for months or years. This may have severely affected photosynthesizing microorganisms, whose short life cycles coupled with the prolonged lack of sunlight may have led to major population declines if not extinctions. Further study of the immediate post-SIL strata may yield critical information on such effects. REFERENCES Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Davis, D.W.,Kissin, S.A., Fralick, P.W., and Hammon, A.L., 2005, Discovery of distal ejecta from the 1850 Ma Sudbury impact event. Geology, 33: 193196. Addison, W.D., Brumpton, G.R., Davis, Don W., Fralick, P.W., and Kissin, S.A., 2010, Debrisites from the Sudbury impact event in Ontario, north of Lake superior, and a new age constraint: are they base surge deposits or tsunami deposits?, Geological Society of America Special Paper 465: 245-268. Cannon, W.F., Schulz, K.J., Horton, J.W. Jr., and Kring, D.A., 2010, The Sudbury impact layer in the Proterozoic iron ranges of northern Michigan, USA: Geological society of America Bulletin, 122, : 50-75. Collins, G.S., Melosh, H.J., Marcus, R.A., 2005, Earth Impact Effects Program: A web-based computer program for calculating the regional environmental consequences of a meteoroid impact on Earth. Meteoriteics and Planetary Science, 40:817-840. http://www.purdue.edu/impactearth/ Davis, D.E., 2008, Sub-million year age resolution of Precambrian igneous events by thermal extraction-thermal ionization mass spectrometer Pb dating of zircon: application to crystallization of the Sudbury impact melt sheet. Geology, 36: 383-386 Deutsch, A., Grieve, R.A.F., Avermann, M., Bischoff, L., Brockmeyer, P., Buhl, D., Lakomy, R., Muller-Mohr, V., Ostermann, M., and Stoffler, D., 1995, The Sudbury structure (Ontario, Canada: a tectonically deformed multi-ring impact basin. Geoligische Rundschau, 84: 697-709. Fralick, P., Grotzinger, J., and Edgar. L., 2012, Potential recognition of accretionary lapilli in distal impact deposits on mars: a facies analog provided by the 1.85 Ga Sudbury impact deposit, in Sedimentary Geology of Mars. SEPM Special Publication, 102: 211-227. Jirsa, M.A., Fralick, P.W, Weiblen, P.W., and Anderson, J.L.B., 2011, Sudbury impact layer in the western Lake Superior region. Geological Society of America Field Guides, 24: 147-169. Huber, M.S., McDonald, and Koeberl, C., 2014, Petrography and geochemistry of ejecta from the Sudbury impact event. Meteoritics and Planetary Science, 49: 1749-1768. Petrus, J.S., Ames, D.E., and Kamber, B.S., 2015, On the track of the elusive Sudbury impact: geochemical evidence for a chondritie or comet bolide. Terra Nova, 27: 9-20. Pufahl, P.K., Hiatt, E.E., Stanley, C.R., Morrow, J.R., Nelson, H.J., and Edwards, C.T., 2007, Physical and chemical evidence of the 1850 Ma Sudbury impact event in the Baraga Group, Michigan. Geology, 35: 827830. 23 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Geologic and geochemical attributes of the Beaver River Diabase and Greenstone Flow: Testing a possible intrusive-volcanic correlation in the 1.1 Ga Midcontinent Rift DOYLE, Michael S.1 and MILLER, James D. Jr.1 1 Department of Earth and Environmental Sciences, University of Minnesota Duluth, 229 Heller Hall, 1114 Kirby Drive, Duluth, MN 55812 Over the last century, numerous geological studies have fairly well constrained the overall tectonomagmatic evolution of the Midcontinent Rift (MCR). However, until now, the correlation of the numerous flood basalts with their intrusive feeder systems has not been attempted. This study proposes one such correlation between two of the largest igneous bodies in the MCR: the Beaver River Diabase (BRD) intrusive complex in northeastern Minnesota and the Greenstone Flow (GSF) lava sheet in northern Michigan. The objective of this research is to test the validity of this link through a detailed analysis of the field relationships, petrographic characteristics, and geochemical attributes of these two units. The GSF is an enormous (at least 1,650 km3) lava sheet exposed over a ~5,000 km2 area on Isle Royale and the Keweenaw Peninsula in northern Michigan (White, 1960; Longo, 1984). The GSF forms the prominent ridge that runs the length of the Keweenaw Peninsula (~90 km) where it reaches a maximum thickness of 460 m (Cornwall, 1951). The GSF has been correlated across Isle Royale (Lane, 1893; Longo, 1984) where it reaches a maximum thickness of 260 m (Huber, 1973). The BRD is an extensive, composite dike and sill complex exposed over a ~600 km2 area in northeastern Minnesota. Perhaps the most intriguing feature of the BRD is the occurrence of numerous large (≤ 500m in diameter), lower crustal anorthosite xenoliths in the BRD (Miller and Green, 2002). That these diabase feeder dikes were at one time wide enough to accommodate such large blocks within several kilometers of the Earth’s surface implies that such conduits would most certainly have reached the surface and resulted in enormous outpourings of lava such as those that would have created the GSF. Field mapping and previous studies have shown that both the BRD and GSF are composite systems formed by multiple pulses of successively fractionated magma. BRD dikes and sills occur as ophitic olivine diabase, with 0.5 – 10 cm augite oikocrysts, which grades into coarser and more subophitic to intergranular olivine oxide gabbro in the medial portions of larger dikes and sills. A distinctive textural attribute of the ophitic diabase if the occurrence of clustered, often radiating, plagioclase laths. Within these dikes and sills are numerous, smaller composite intrusions of more highly fractionated lithologies (ferrodiorite to quartz ferromonzonite) that locally display modal layering and strong foliation. Contacts between dioritic rocks and the enclosing gabbros are sharp, but unchilled. In addition, ophitic olivine diabase locally occurs as altered xenoliths in the intermediate composite intrusions. These composite bodies are especially prevalent in the southern extent of the BRD where they are termed the Silver Bay Intrusions (SBI) (Miller and Green, 2002). Paces and Miller (1993) reported a U-Pb age of 1095.8 ± 1.2 Ma which is within analytical error of the date reported by Davis and Paces (1990) for the GSF (1094.0 ± 1.5 Ma). Similarly, the GSF can be divided into distinct lithological zones (from bottom to top): the lower ophite, heterolithic, upper ophite, and entablature. The upper and lower ophite zones are composed of ophitic olivine basalt with 0.1-4 cm augite oikocrysts and displaying clustered, often radiating, plagioclase laths. Occupying the central 1/4 to 1/2 of the lava sheet is the heterolithic zone, which is composed of coarser, subophitic to intergranular olivine oxide gabbro to subprismatic, locally foliated, ferrodiorite. Within the gabbroic/dioritic rocks of the heterolithic 24 �Proceedings of the 61st ILSG Annual Meeting - Part 1 zone occur numerous en echelon bodies of granophyre-rich lithologies (ferromonzodiorite to quartz ferromonzonite). As in the BRD-SBI intrusions, contacts between these rocks and the host gabbros/diorites are sharp and display no discernable chilled margins. And, as in the BRD intrusions, ophitic olivine basalt commonly occurs as inclusions in the heterolithic zone gabbros and diorites. Whole-rock geochemical analysis shows significant similarities in major and trace element compositions between BRD and GSF lithologies. REE patterns on primitive mantle-normalized diagrams are nearly identical for comparable rocks types of each unit. Gabbroic rocks are characterized by moderate LREE enrichment (La/Smn 1.54-2.49) and weak HREE fractionation (Gd/Ybn = 1.45-1.79). The more highly fractionated rocks of the SBI and GSF heterolithic zone show higher LREE enrichment (La/Smn = 1.86-2.54) and HREE fractionation (Gd/Ybn = 1.552.74), as well as strongly negative Sr anomalies, moderate Zr-Hf anomalies, and weak negative Eu anomalies. SEM-EDS analysis showed a similar range in pyroxene compositions between comparable rocks of the BRD and GSF. Pyroxenes within the gabbroic rocks of each unit were predominantly augite with lesser pigeonite and minor enstatite while the more highly differentiated lithologies of the SBI intrusions and GSF heterolithic zone were generally more Fe-rich (ferroaugite to ferrosilite). Olivine compositions tended to be more Fe-rich in BRD samples than those in the GSF ophites Fo28-69 and Fo50-66, respectively. Within the GSF, fresh olivine was only found in the upper and lower ophites so rocks of the heterolithic zone could not be compared with comparable rocks in the BRD-SBI. SEM-EDS analysis was also used to measure the An content in plagioclase phenocrysts in GSF samples. Three phenocrysts from GSF ophites of Isle Royale were found to have anomalously high anorthite contents (An71-81) with respect to the groundmass plagioclase (An30-60). The An content of these calcic phenocrysts (xenocrysts?) is consistent with those reported by Morrison (1983) for the anorthosites xenoliths in the BRD (An54-78) and could be indicative of a similar source. Based on the evidence obtained during this research, we propose the first ever intrusivevolcanic link between an MCR flood basalt and its intrusive feeder system. Based on the 1) overlap in U-Pb ages; 2) similar composite lithologies and contact relationships; 3) similar mineralogical and textural attributes, especially the occurrence of clustered plagioclase laths; 4) similar major and trace element compositions; 5) similar primary mineral chemistries; 6) similar An contents between anorthosite xenoliths in the BRD and plagioclase megacrysts in the GSF; and, 7) enormous volumes represented by each unit. Collectively, this data points to the conclusion that the BRD acted as the feeder conduit for the GSF. If this is the case, it more than doubles the total volume of the GSF making it perhaps the largest single lava flow on Earth. REFERENCES Cornwall, H. R. (1951). Differentiation in the lavas of the Keweenawan series and the origin of the copper districts of Michigan. Geological Society of America Bulletin, 62, 159–202. 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, 97(1-2), 54–64. Huber, N. K. (1973a). The Portage Lake Volcanics (Middle Keweenawan) on Isle Royale, Michigan. United State Geological Survey Professional Paper 754-C, C1–C32 Lane, A. C. (1893). Geological report on Isle Royale, Michigan. Geological Survey of Michigan, 6, 1–265. Longo, A. (1984). A correlation for a Middle Keweenawan flood basalt: The Greenstone Flow, Isle Royale and the Keweenaw Peninsula, Michigan. Michigan Technological University. 25 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Miller, J.D., and Green, J.C., 2002a, Geology of the Beaver Bay Complex and related hypabyssal intrusions. In Miller, J.D. Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.E., and Wahl, T.E., Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota. Minnesota Geological Survey Report of Investigations 58, p. 144-163. Morrison, D. A., Ashwal, L. D., Phinney, W. C., Shih, C., and Wooden, J. L. (1983). Pre-Keweenawan anorthosite inclusions in the Keweenawan Beaver Bay and Duluth Complexes, northeastern Minnesota. Geological Society of America Bulletin, 94, 206–221. Paces, J. B., & Miller, J. D. Jr. (1993). Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: Geochronological insights to physical, petrogenetic, paleomagnetic, and tectonomagmatic processes associated with the 1.1 Ga Midcontinent Rift System. Journal of Geophysical Research, 98, 13997–14013. White, W. (1960). The Keweenawan Lavas of Lake Superior, an example of flood basalts. American Journal of Science, 258-A, 367–374. 26 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Re-digitized public aeromagnetic data for the Baraga basin and surrounding region, Upper Peninsula, Michigan DRENTH, Benjamin, AILES, Chad, and ANDERSON, Eric 1 Crustal Geophysics and Geochemistry Science Center, U.S. Geological Survey, PO Box 25046 MS 964, Denver, CO, 80225 USA The public aeromagnetic database (Daniels et al., 2009) for Michigan’s western Upper Peninsula (UP) is widely regarded as unsuitable for intermediate- and detailed-scale geologic mapping and mineral exploration applications. There are several limitations of the data, including being available only in a native analog format, being acquired with too wide of a line spacing and too high of a terrain clearance, and being digitized at a lower level of detail than shown on original contour maps. This abstract describes a recent experimental effort to re-digitize sample aeromagnetic data from original contour maps in the greatest detail possible. The area chosen is the Proterozoic Baraga basin, containing the Eagle Ni-Cu deposit, and surrounding Archean rocks (Sheet 2 of Case and Gair, 1965). A fixed-wing total-field aeromagnetic survey was flown in the region in 1950, along north-south lines spaced 400 metres at a nominal terrain clearance of 150 metres (Case and Gair, 1965). After removal of an unspecified base level, the acquired data were interpolated onto contour maps with a minimum contour interval of 50 nT (see Case and Gair, 1965). A subsequent digitization effort from the contour maps followed at an unknown time, and the resulting digitized data are those publically available today from the USGS (e.g., Daniels et al., 2009). However, that digitization effort sampled the contour maps along only every other flightline, effectively simulating a survey with 800 metre line spacing. This resulted in very poor geologic resolution. The same problem plagues several other vintage aeromagnetic datasets acquired in the western UP. As an experiment we re-digitized a portion of this aeromagnetic dataset, sampling each contour from the original contour map and effectively capturing all of the available detail. The experiment is considered a success, as the resulting map is a far more effective representation of the region’s geology. As originally interpreted by Case and Gair (1965), many Keweenawan diabase dikes are imaged against a background of generally weakly magnetized Archean rocks and older Proterozoic metasedimentary rocks. The magnetic anomaly over the Eagle-hosting intrusion is difficult to pick out in the published data due to the wide data spacing, yet is readily apparent in the re-digitized data. In spite of this successful experiment, the recovered data still have several major and minor limitations that must be considered by interpreters. First, the survey was flown at too wide a line spacing (400 meters) and too far above the ground (~150 meters) for detailed geologic mapping and mineral exploration. Second, the minimum contour interval of 50 nT shown on the original contour maps means that more subtle anomalies and geologic details undoubtedly present in the flightline data will never be recoverable. Third, the exact terrain clearance of the magnetometer was not recorded, and in several localities may have varied significantly from the nominal 150 meter clearance. Finally, the base level removed from the magnetic data wasn’t recorded, meaning that the total field intensity and formal total field anomalies cannot be calculated. 27 �Proceedings of the 61st ILSG Annual Meeting - Part 1 REFERENCES Case, J.E., and Gair, J.E., 1965. Aeromagnetic map of parts of Marquette, Dickinson, Baraga, Alger and Schoolcraft Counties, Michigan, and its geologic interpretation: U.S. Geological Survey Geophysical Investigations Map GP-467. Daniels, D.L., Kucks, R.P., Hill, P.L., and Snyder, S.L., 2009, Michigan magnetic and gravity maps and data: a website for the distribution of data: U.S. Geological Survey Data Series 411 available only online at http://pubs.usgs.gov.ds/ds411. 28 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Petrographic Analysis of Felsic Tuffs within the Neoarchean Soudan Member of the Ely Greenstone Formation, NE Minnesota ESSIG, Espree1, HUDAK, George 2, PIGNOTTA, Geoff3, and LODGE, Robert3 1 Department of Geological Sciences, University of Minnesota Duluth, Duluth, MN 2 Precambrian Research Center, Natural Resources Research Institute, University of Minnesota Duluth, Duluth, MN 3 Department of Geology, University of Wisconsin Eau Claire, Eau Claire, WI The Vermilion District of northeastern Minnesota contains one of the classic greenstone belts in the United States, and is composed of a wide variety of greenschist facies metamorphosed Neoarchean volcanic, sedimentary, and intrusive rocks that comprise the southwestern part of the Wawa Abitibi Terrane (Stott et al., 2007). The Ely Greenstone Formation occurs within the Western Vermilion District and is composed of calc-alkaline to tholeiitic massive to pillowed basalt, andesite, dacite, and rhyolite lava flows and volcaniclastic rocks (Lower Member); Algoma-type banded iron formations with interbedded tholeiitic massive to pillowed basalt lava flows, rhyodacitic to dacitic tuffs, and polymict volcaniclastic rocks (Soudan Member); and pillowed to massive tholeiitic basalt lava flows and interbedded Algoma-type banded formation horizons (Upper Member; Peterson and Jirsa, 1999). The purpose of this study is to identify and characterize potential felsic tuff horizons that are interbedded with Algoma-type banded iron formations within the Soudan Member of the Ely Greenstone Formation. This has been accomplished by detailed field mapping, petrographic studies, scanning electron microscopy (SEM) studies, and lithogeochemical studies. In addition to texturally, mineralogically, and chemically characterizing potential felsic tuff units, our research seeks to determine if the felsic tuff units could potentially yield age dates by means of future U/Pb geochronological studies in a manner similar to that which has been done in the Abitibi Belt in northeastern Ontario (Thurston et al., 2008). Despite the long history of geological studies in the Western Vermilion District, relatively few absolute age dates are present (Fig. 1A; Peterson et al., 2001; Lodge et al., 2013), and no absolute age dates exist for the Soudan Member of the Ely Greenstone Formation. Detailed mapping has identified a light gray, 30-50cm thick, laminated to thinly-bedded, possibly resedimented felsic tuff horizon that is interlayered with laminated to very thinly-bedded magnetite-chert Algoma-type banded iron formation within the uppermost 25 meters of the Soudan Member in the central part of Lake Vermilion State Park. In thin section, the tuff is sparsely quartz-phyric, and comprises a matrix of fine-grained, recrystallized polygonal quartz with up to 1%, up to 1mm in diameter subhedral, recrystallized quartz phenocrysts. Hydrothermal alteration of the tuffs varies from moderate to intense (up to 75% alteration minerals), with the greenschist-facies metamorphosed synvolcanic hydrothermal alteration assemblage now composed of variable amounts of iron carbonate (siderite, ankerite), actinolite, chlorite, and various epidote-group minerals (pistacite, clinozoisite/zoisite). Due to the extremely fine-grained texture of the tuff, and the locally pervasive hydrothermal alteration within the tuff, searching for zircons using standard petrographic analysis has proven to be difficult. On-going SEM analysis will be used to constrain the mineralogy of the existing alteration and to seek out zircons in a more systematic manner. Lithogeochemical analysis and lithogeochemical classification by means of immobile trace elements (e.g., Pearce, 1996) is ongoing. 29 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Figure 1: A: Generalized section of the Ely Greenstone Formation in Lake Vermilion State Park (modified from Hudak and Peterson, 2014); B: Field appearance of felsic tuff unit within the uppermost 25 meters of the Soudan Member; C and D: Cross-polarized and plane polarized appearance of hydrothermally altered felsic tuff. REFERENCES Hudak, G.J., and Peterson, D. ., 2014, Non-Ferrous Mineralization Associated with the Wawa-Abitibi Terrane and Duluth Complex Cu-Ni-PGM Deposits, NE Minnesota: Society of Economic Geologists, SEG Guidebook Series Guidebook 47, 150 p. Lodge, R. W. D., Gibson, H. L., Stott, G. M., Hudak, G. J., Jirsa, M. A., and Hamilton, M. A., 2013, New U-Pb geochronology from the Timiskaming-type assemblages in the Shebandowan and Vermilion greenstone belts, Wawa Subprovince, Superior Craton: Implications for the Neoarchean development of the southwestern Superior Province: Precambrian Research, v. 235, p. 264-277. Pearce, J. A., 1996, A user’s guide to basalt discrimination diagrams: in Wyman, D. A., ed., Trace Element Geochemistry of Volcanic Rocks: Applications for Massive Sulphide Exploration: Geological Association of Canada, Short Course Notes, v. 12, p. 79-113. Peterson, D. M., Gallup, C., Jirsa, M. A., and Davis, D. W., 2001, Correlation of Archean assemblages across the U.S.- Canadian border: Phase I geochronology: 47th Annual Meeting, Institute on Lake Superior Geology, Proceedings Volume 47, Part 1 – Programs and Abstracts, p. 77-78. Peterson, D. M., and Jirsa, M.A., 1999, Bedrock geologic map and mineral exploration data, western Vermilion district, St. Louis and Lake Counties, northeastern Minnesota: MGS Miscellaneous Map M-98, scale 1:48,000. Stott, G., Corkery, T., Leclair, A., Boily, M., and Percival, J., 2007, A revised terrane map for the Superior Province as interpreted from Aeromagnetic Data: 53rd Annual Meeting, Institute on Lake Superior Geology, Proceedings Volume 53, Part 1 – Program and Abstracts, p. 74-76. Thurston, P. C., Ayer, J. A., Goutier, J., and Hamilton, M. A., 2008, Depositional Gaps in Abitibi Greenstone Belt Stratigraphy: A Key to Exploration for Syngenetic Mineralization: Economic Geology, v. 103, p. 1097-1134. 30 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Characterization of secondary minerals formed on weathered Duluth Complex Cu-Ni-PGE deposit rock and implications for controls on metal mobility FIX, Paul M.1, and DIEDRICH, Tamara R.2 1 Department of Earth and Environmental Sciences, University of Minnesota, Duluth, MN, USA 2 Barr Engineering Company, Duluth, MN, USA Secondary minerals and other weathering products can serve as an important control on the concentration of trace metals in mine-impacted waters (for example, Jambor, 2003). Ultimately, aqueous metal concentrations will reflect combined effect of constituent release from primary minerals (e.g. sulfides), as well as, attenuation during weathering from mechanisms such as precipitation of secondary phases, sorption onto solid phase surfaces, and co-precipitation. We characterized weathering products formed on weathered exposures of mineralized Duluth Complex to specifically investigate possible solubility controls for Cu and Ni. Combining these solid phase characterization results with both standard-method mine waste studies (see Lapakko, 2012) and numerical modeling should yield an improved understanding of the mobility of trace metals that would be released during weathering of potential future waste rock. Opportunistic sampling was conducted at five Duluth Complex exposures at the Mesaba deposit (currently held by Teck American and formerly known as the Babbitt deposit). Exposures included both natural outcrops and a railroad cut. Six samples were collected with the intent of capturing the variety of visual alteration. Powder X-ray diffraction (XRD) was conducted on weathering products isolated by scraping the outer surfaces of hand sample specimens. XRD of iron-rich material (rusty coatings) generally did not produce diffraction patterns that allowed phase identification, suggesting the material structure is very poorly crystalline to amorphous. However, in one case, poorly crystalline goethite (FeOOH) was identified and appeared to be a product of sulfide mineral replacement. In addition, the secondary minerals malachite (Cu2(CO3)(OH)2), rozenite (FeSO4·4H2O), alunogen (Al2(SO4)3·17H2O), and epsomite (MgSO4·7H2O) were identified. The morphology and semi-quantitative chemistry of weathering products was characterized using a JEOL JSM-6590LV scanning electron microscope, combined with an INCA X-ACT energy dispersive spectroscopy system at University of Minnesota- Duluth. SEM observations of rusty coatings reveal micro-scale banded (alternating Fe and Si rich) features (Figure 1). The coatings commonly contain Fe and Si as major components and variable amounts of Al, S, and Cu as minor elements. It should be noted that Ni was not found to be commonly associated with these features. This could be due to sub-detection limit (< 0.1 wt. %) quantities or absence of Ni. Sorption of trace metals by hydrous ferric oxides in mine-water systems is common. SEM-EDS analyses indicate Ni and Cu can be associated with sheet silicate minerals, in concentrations that can exceed several percent by weight. Research on similar materials found Nirich metallic particles along laths of serpentine and chlorite (Suárez, 2011) and sorption of Ni(OH)2 onto primary mineral grains (Plante, 2010). It may be possible that Cu and Ni were incorporated during late stage deuteric alteration and not surface weathering. Planned analyses include SEM-EDS of non-weathered rock from the same lithology to determine if trace metal enrichment of sheet silicates is unique to weathered samples as well as TEM analyses to determine nano-scale mineral properties. Collectively, the mineral characterization techniques employed provide evidence for attenuation of constituents released during weathering of Duluth Complex rock by means of: (1) 31 �Proceedings of the 61st ILSG Annual Meeting - Part 1 secondary mineral precipitation (malachite and sulfate salts); (2) incorporation of Cu and Ni by sheet silicates (likely, but not necessarily occurring during sub-aerial weathering); and (3) formation of iron-oxide rich coatings which may retain trace metals (especially Cu) though adsorption. The degree to which trace metals are attenuated will be function of drainage pH, total iron content, trace metal content, and reactive surface area among other variables. +@A&)!"#$#%& '&()* +, -.(/ 4" 2(5 0# 2(6 :; /(< =, 3(9 78 /(2 7? 9(1 7" 9(3 > .9(3 +@A&)6 !"#$#%& '&()* 0# 12(3 +, 1(1 78 3(9 + -(2 4" -(1 > 19(2 Figure 1: SEM-Backscatter electron image of a weathered Duluth Complex sample. Lower left region shows banding typical of iron rich surface rinds. Upper right phase appears to be a weathered sheet silicate with significant Cu enrichment. Similar phases in our samples have been observed to contain up to 4 wt. % Ni. Iron rich phases (light strips) can also be seen where sheets have parted. Spots indicate locations of semi-quantitative SEM-EDS compositional data (right) and are sized to the approximate analytical volume at these working conditions (15 kV). REFERENCES Jambor, J.L. 2003. Mine-waste mineralogy and mineralogical perspectives of acid-base accounting, In: Jambor, J., Blowes, D., Ritchie, A. (Eds) Environmental Aspects of Mine Wastes. Mineralogical Association of Canada. Short Course Series 31, pp. 117-145. Lapakko, K., Antonson, D. A. 2012 Duluth Complex Rock Dissolution and Mitigation Techniques: A summary of 35 years of DNR research, Minnesota Department of Natural Resources, (p. 56). Plante, B., Benzaazoua, M., Bussière, B. 2010. Study of Ni sorption onto Tio mine waste rock surfaces. Applied Geochemistry 25, 1830–1844. Suárez, S., Nieto, F., Velasco, F., Martín, F.J. 2011. Serpentine and chlorite as effective Ni-Cu sinks during weathering of the Aguablanca sulphide deposit (SW Spain). TEM evidence for metal-retention mechanisms in sheet silicates. European Journal of Mineralogy 23, 179–196. 32 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Lateral geochemical gradients and physical processes associated with the genesis of iron formations: Examples from the Paleoproterozoic to Mesoarchean of Superior Province FRALICK, Philip Department of Geology, Lakehead University, Thunder Bay, ON, Canada, P7B 5E1, The presence of Fe, Si, Mn and P in mineral phases with precursors that were quasi-stable precipitates from the overlying water column necessitates that disequilibrium conditions existed during their formation. Commonly a two box model is used to explain iron precipitation in the Precambrian with an oxic to sub-oxic phytoplankton-rich or CO2-rich surface layer driving iron precipitation in the underlying anoxic, Fe+2-rich ocean. But what is the evidence for this model? 1) On the wide, Paleoproterozoic Gunflint-Mesabi shelf carbonate iron formation (IF) dominated the shallow areas, whereas oxide IF accumulated in more offshore locations. Storm induced geostrophic flows delivered oxygenated inner shelf waters to the offshore instigating formation of the discrete iron to iron+manganese+silica+mud laminae that compose the fine-grained IF. Here frequent storm mixing would have destroyed vertical stratification and Fe+2 transport across the 150km wide shallow shelf indicates anoxic conditions across the shelf during fair-weather times. 2) The Neoarchean delta-top IFs in the Lake St Joseph area record extremely rapid accumulation of iron hydroxides during limited sediment flux to portions of distributary mouth bar complexes. IF accumulated on short-lived reactivation surfaces of gravel bars to ripples in water depths of a few meters. No IF accumulated further offshore. Its chemistry is virtually identical to deep-water IF deposits. The IF was probably formed by high nutrient flux to the shallows promoting cyanobacteria. 3) Carbonate was deposited on the Mesoarchean, stromatolitic, oxygenated Steep Rock platform, while the generation of free oxygen caused Fe and Mn precipitation offshore. Onshore water movement, probably driven by storm events, deposited iron-rich layers in the limestone and shifted the offshore from Fe to chert or mud accumulation. In these examples IF deposition was dependent on lateral, not vertical, geochemical differences augmented by storm induced mixing. 33 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Comparison of PGM assemblages for the Marathon, Geordie Lake and Area 41 deposits, Coldwell Alkaline Complex, Ontario GOOD, David1, CABRI, Louis 2, and AMES, Doreen 3 1 Department of Earth Sciences, University of Western Ontario, London, ON N6A 5B7 2 Cabri Consulting Inc., 700-702 Bank Street, PO Box 14087, Ottawa, ON, K1S 5P5 3 Geological Survey of Canada, 750-601 Booth St., Ottawa, ON, K1A 0E8 Numerous Cu-PGE deposits in the Coldwell Alkaline Complex exhibit widely varying mineralization styles and Cu/Pd and Pd/Pt ratios (Ruthart, 2012; Meghji et al., 2013; Good et al., 2015). However, the host gabbro and ultramafic bodies are believed to be co-genetic and differences between the deposits may be explained by variations in mineralizing processes and the respective magma conduit setting. This study presents results for heavy mineral separates from 5 mineralized zones within three deposits: W horizon and Main zone in the Marathon deposit, Main zone at the Geordie Lake deposit, and the Main and PGE-enriched zones at the Area 41 occurrence. A total of over 9000 PGM comprising 46 PGM species, numerous unknown PGM, and 7 Au/Ag minerals were identified. The mineral separates were prepared by two different methods on two different groups of samples. The first group was prepared by hydroseparation (HS) of screened size-by-size composites and the second group by electric-pulse disaggregation (EPD) of drill core (e.g., Cabri et al. 2008). All mineral separates were mounted on polished sections and analysed by SEM-EDS techniques to characterize the platinum group minerals (PGM). The sum of measured surface areas for each mineral are collated to provide estimates of mineral abundances. Large representative sample sets were prepared from each location. In group one, mineral separates for three composite samples with similar Cu and Pd abundances (Fig. 1a), one from each intrusion, a total of 513 precious metal grains were found and characterized in 15 sized monolayer polished sections. In group two, mineral separates for samples with relatively high PGE abundances were prepared from 12 pieces of drill core, 4 from each of the Main zone, W Horizon, and Area 41 intrusion (Fig. 1b). The results from group 1 show that three mineralized zones (Main zone, W horizon and Geordie Lake) have distinct precious mineral signatures (Fig. 2), as expected based on the variation of Cu/Pd and Pd/Pt values, and differences between local intrusive settings and 34 �Proceedings of the 61st ILSG Annual Meeting - Part 1 mineralization styles. The Pd PGM in the Main zone sample are dominated (mass%) by arsenian PGM (80%) with less bismuthides (12%); in the Geordie Lake sample, arsenides dominate (51%), followed by arsenian-antimonian (33%), and arsenian-nickeloan PGM (16%); and in the Area 41 sample, plumbian PGM dominate (47%), followed by arsenian PGM (17%), arsenian-antimonian PGM (14%), bismuthides (13%), stannides (6%), and tellurides (3%). The Pt PGM for the Main zone and Geordie Lake are the same, where sperrylite predominates, but are very different for the Area 41 sample where Pt alloys are abundant: sperrylite (~53%), isoferroplatinum (~31%), and tetraferroplatinum (~16%). Results are summarized in Figure 2. The PGM assemblage at Area 41 resembles that for the W Horizon and is consistent with the very wide range and notably low Cu/Pd values present at both locations. Further, the host gabbros exhibit similar petrographic features and trace element abundances that suggest they formed by similar processes. REFERENCES Cabri, L.J., Rudashevsky, N.S., Rudashevsky, V.N., and Oberthür, T., 2008, Electric-Pulse Disaggregation (EPD), Hydroseparation (HS) and their use in combination for mineral processing and advanced characterization of ores. Canadian Mineral Processors 40th Annual Meeting, Proceedings, Paper 14, 211-235. Good, D.J., Epstein, R., McLean, K., Linnen, R.L. & Samson, I.M., 2015, Evolution of the Main Zone at the Marathon Cu-PGE sulfide deposit, Midcontinent Rift, Canada: spatial relationships in a magma conduit setting. Economic Geology (in press). Meghji I., Linnen R.L., Samson I.M., Ames D.E., Good D.J., 2013, The character and distribution of Cu-PGE mineralization at the Geordie Lake Deposit within the Coldwell Complex, Ontario, GAC-MAC, Poster presentation. Ruthart R., 2012, Characterization of High-PGE, Low-Sulphur Mineralization at the Marathon PGE-Cu Deposit, Ontario, M.Sc. thesis, University of Waterloo, 145 p. 35 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Significance of LREE-enriched mantle source to genesis of basalt in the Coldwell Alkaline Complex, Midcontinent Rift, Ontario GOOD, David1, HOLLINGS, Peter 2, CUNDARI, Robert 2, 3, and AMES, Doreen 4 1 Department of Earth Sciences, University of Western Ontario, London, ON N6A 5B7 2 Department of Geology, Lakehead University, 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada 3 Ontario Geological Survey, Ministry of Northern Development, Mines and Forestry, Suite B002, 435 James St. South Thunder Bay, ON P7E 6S7 Canada 4 Geological Survey of Canada, 750-601 Booth St., Ottawa, Ontario, K1A 0E8 At least three distinct basaltic packages occur within the Coldwell Alkaline Complex: Lower and Upper basalt units located within the Eastern Gabbro Suite, and the Coubran basalt. Geochemical evidence suggests the Coubran basalt is co-genetic with the Two Duck Lake gabbro, a late phase of the Eastern Gabbro with an age of 1108±1 Ma (Heaman et al., 2007) and thus formed as part of the Early Magmatic stage of the rifting event. The Upper and Lower basalt were re-crystallized during pyroxene hornfels grade metamorphism during intrusion of the Eastern Gabbro suite, and are older than 1108 Ma and possibly represent magma formed during the Initiation stage of the rifting event. Major element abundances for the Coldwell basalt units are comparable to other Early Stage basaltic units of the Midcontinent rift. Based on Mg number and Ni abundance, the three groups, listed in order from most primitive to most evolved, are Lower basalt, Upper basalt and Coubran basalt. Spider diagrams were prepared following the method of Pearce (2008) whereby data are normalized in two steps: first by N-MORB and then by Tb (Fig. 1). Note that we normalize to Tb and not Ti. All Coldwell units show depleted HFSE relative to LREE, similar to that demonstrated by Groups 1 and 2 at Mamainse Point. The units listed, from most to least depleted, are Coubran basalt, Upper basalt and Lower basalt. Although LREE abundances in the Coldwell basalts are consistent with OIB source, the regular pattern of LREE enrichment relative to HFSE indicates a more complicated origin and cannot be explained by crustal contamination. Crustal contamination, if present, would have resulted in elevated SiO2 and Zr abundances as well as higher Th/La, but these affects were not observed. Further, La enrichment relative to Zr in the Coubran basalt (Fig. 2) cannot easily be explained by crustal contamination. Therefore, it seems more likely that the mantle source was LREE enriched and HFSE abundances are a better indicator of either MORB or OIB source. The HFSE suggest the Coubran basalt magma was intermediate between E-MORB and OIB. Similarly, the Lower and Upper basalt groups show HFSE signatures that are intermediate between E-MORB and N-MORB. Coldwell units exhibit Gd/Yb that are intermediate between E-MORB and OIB. These units, in order of decreasing Gd/Yb, are Lower basalt, Coubran basalt and Upper basalt, whereby Lower basalt resembles that of OIB and Upper basalt resembles MORB. These results suggest a progressive change in the mantle source for the Coldwell basaltic magmas. The mantle source areas have varying degrees of LREE enrichment, possibly introduced by an earlier subduction event as discussed by Shirey et al. (1994) for the MPVG. Our observations for the Coldwell basalts may be explained by partial melting of sources that, from oldest to youngest show, deep depleted mantle having moderate enriched LREE signature (Lower basalt); shallow depleted mantle with significant LREE enrichment (Upper basalt); and finally, 36 �Proceedings of the 61st ILSG Annual Meeting - Part 1 enriched mantle from intermediate depth with significant but heterogeneous LREE enrichment (Coubran basalt). A model to test origin of Eastern gabbros from heterogeneous LREE enriched mantle source similar to that which produced Coubran basalt is proposed in Figure 2. Figure 1: Average data for Coubran basalt and Lower and Upper basalts compared to averages for MPVG 1, 2 and 3a (after Shirey et al. 1994). MORB, OIB and E-MORB after Sun and McDonough (1989).   Figure 2: Model for derivation of Eastern gabbros based on contamination trend of Coubran basalt REFERENCES Cundari R., 2012, Geology and geochemistry of Midcontinent rift-related igneous rocks: M.Sc. thesis, Thunder Bay, ON, Lakehead University, 122 p. Good, D.J., Epstein, R., McLean, K., Linnen, R.L. & Samson, I.M., 2015, Evolution of the Main Zone at the Marathon Cu-PGE sulfide deposit, Midcontinent Rift, Canada: spatial relationships in a magma conduit setting. Economic Geology (in press). Heaman, L.M., Easton, M., Hart, T.R., Hollings, P., Macdonald, C.A., and Smyk, M., 2007, Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon region, Ontario: Canadian Journal of Earth Sciences v. 44, p. 1055–1086. Pearce, J.A., 2008, Geochemical fingerprinting of oceanic basalts with applications to ophiolite classification and the search for Archean oceanic crust. Lithos, v. 100, p. 14-48. Shirey, S.B., Klewin K.W., Berg, J.H. and Carlson, R.W., 1994, Temporal changes in the sources of flood basalts: Isotopic and trace element evidence from the 1100 Ma old Keweenawan Mamainse Point Formation, Ontario, Canada, Geochimica et Cosmochimica Acta, V. 58, P 4475-4490. 37 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Preliminary 3D model of the Midcontinent Rift System in western Lake Superior region GRAUCH, V.J.S., POWERS, Michael H., ANDERSON, Eric D., and CANNON, William F., U.S. Geological Survey, PO Box 25046, MS 964, Denver, CO, 80225 Over the past several decades, geophysical models have played a large part in developing our understanding of the Midcontinent Rift System (MRS). Technical advances in modeling capabilities, expanded and improved data coverage, and renewed interest in the mineral resources of the MRS provide the motivation for new attempts at 3D digital modeling of the MRS. Efforts are focused on modeling the structure and configuration of sedimentary and volcanic basins in the western Lake Superior region (Fig. 1). Previous workers used geophysical data to identify extremely thick volcanic and sedimentary sections that commonly have unconformable contacts against intervening structural highs. An improved 3D model of the area helps visualize the relations, provides mechanisms to test hypotheses about tectonic history and the spatial distribution of mineralization, and helps identify areas where more detailed analysis is required. The 3D model is regional and intended to show broad variations in geology. Only three generalized, MRS-related geologic packages are represented, following previous 3D models of Allen (1994; Allen et al., 1997). The packages, from oldest to youngest, are 1) undivided Keweenawan plutonic and volcanic rocks, 2) Oronto Group sedimentary rocks, and 3) Bayfield Group and equivalent sedimentary rocks. A fourth package represents undivided pre-rift rocks (basement). In addition, three major fault systems are modeled: the Douglas, Lake Owen, and Keweenaw fault systems (Fig. 1). The modeling strategy involves digitizing the bottoms of the rift-related geologic packages, fault locations, and general orientation data along 2D sections and from a geologic map. The 3D modeling software then connects the digitized points into surfaces and volumes in 3D space, using simple geologic rules for stratigraphic and onlap relations and for the lateral extents of the influences of faults. Steps that have been accomplished for the new 3D model are the following. 1. Images were captured from published 2D geophysical models, interpreted seismic-reflection sections, and geologic cross-sections. They were geo-registered and hung in 3D model space by projecting them onto 20 different sections crossing the model area. Digital and analog geologic maps from several sources provided information in plan view. 2. Contacts between geologic packages were determined for available, industry seismicreflection time sections using surfaces derived from previous 3D models (Allen and others, 1997) as guides. In addition, selected, proprietary industry seismic-reflection data from the Bayfield Peninsula, which only a few workers have previously seen, were licensed to the U.S. Geological Survey. Time was converted to depth for the sections using root-meansquare velocities derived from industry data processing. 3. Selected points representing fault surfaces and the bases of the geologic packages were digitized, then rendered into 3D volumes and surfaces by the modeling software. A 3D perspective view of the modeled volume of the Oronto Group is shown in Figure 2. Although this and the previous model (Allen and others, 1997) are intentionally very similar, there are significant differences. First, the newly acquired seismic data provided corrections for several significant errors in the current digital rendition of the previous 3D model that would not have been known other-wise. Second, due to advances in software capabilities, faults are now properly represented rather than handled by gridded surfaces. Finally, even though all apparent constraints from 2D sections were met, the Oronto Group in the Bayfield Basin does not extend as far south underneath the Bayfield Group in the new model as it does in the old one. This discrepancy 38 �Proceedings of the 61st ILSG Annual Meeting - Part 1 brings up the difficult problem of determining the contact between the Bayfield and Oronto Groups from geophysical data, and is yet to be resolved. Future work involves more extensive analysis of existing seismic data in conjunction with gravity and magnetic modeling; division of intrusive from extrusive Keweenawan igneous rocks; and addition of layers to the volumes of the generalized geologic packages to better represent dips, stratigraphic variations, and unconformities. 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., ed., Middle Proterozoic to Cambrian Rifting, Central North America: Geological Society of America Special Paper 312, p. 47-72. Figure 1: Generalized rock units of the Midcontinent rift system in the western Lake Superior region and area covered by the 3D model. Geographic boundaries are shown by dashed lines. Figure 2: 3D perspective view of the modeled base of the Oronto Group (green) and surface representing the Douglas fault (dark purple) for the model area located on Fig. 1. View is to the west-northwest. 39 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Geological and geochemical reconnaissance for rare earth element (REE) mineralization in Minnesota HAUCK, Steven, HEINE, John, SEVERSON, Mark, POST, Sara, CHLEBECEK, Sarah, MONSON GEERTS, Steven, ORESKOVICH, Julie, GORDEE, Sarah, and HUDAK, George Natural Resources Research Institute, University of Minnesota Duluth, 5013 Miller Trunk Highway, Duluth, MN 55811 The purpose of this study was to: 1) collect rock samples from across Minnesota and assay them for Rare Earth Elements (REEs); 2) evaluate the assay results; and 3) via a combination of acquisition and evaluation of both new data and historical geochemical data, identify locations or regions within Minnesota that possess anomalous REE concentrations that may warrant further characterization for potentially economic REE mineral deposits (Hauck et al., 2014). Currently, approximately 90-95% of the world’s REE processing and sales are controlled by China. REEs are vital to the U.S. economy, particularly in components used by the U.S. military, in windmills and other equipment that use REE-containing magnets, and in green economy products such as hybrid vehicles. This study is multi-dimensional, and includes: 1) collection of new REE geochemical data through detailed geological mapping, sampling, and analysis; 2) compilation of previously published REE data from a variety of scientific publications; and 3) re-analysis of one historical sample containing anomalous REE contents to confirm the validity of the historic analysis. Resulting from this study are: 1) a new, detailed 1:5,000 scale geologic map in NE Minnesota showing anomalous REE contents; 2) a new lithogeochemical dataset containing 287 samples compiling both new and historic REE data to provide the exploration industry up-to-date lithogeochemical resource data for designing future REE exploration programs in the State; 3) new maps illustrating both historic and new geochemical sampling location and 4), a new interpretation of areas that may host anomalous REE mineralization. Based upon currently mined REE ore deposits, igneous rocks or ionic clays are most likely to contain anomalous REE contents, and, therefore, sample collection concentrated primarily on silica-rich igneous rocks (rocks containing >67% SiO2). Two hundred twenty-two rock samples were collected. Of these, 147 rock samples were analyzed. Based upon the availability of outcrops and diamond drill core samples, the majority of the samples were from St. Louis, Lake of the Woods, Koochiching, and Lake Counties. The rock samples were analyzed at Acme Labs, Vancouver, B.C., for multi-element chemical analyses, including a complete suite of REEs+Y+Sc. The geochemical data received was combined with lithogeochemistry from a previous study (Klenner et al., 2012) to provide as complete a dataset of REE values of Minnesota rocks for interpretation as possible. This combined data set contains over 280 REE analyses, with sample analyses from diamond drill holes comprising ~33% of the combined geochemical assays and ~67% of the data were from outcrop samples from all regions of the state. This compiled data set indicated that 25 samples from around the State had TREEs > 425 ppm. The highest seven REE analyses occurred in samples from NE MN in Koochiching and St. Louis Counties. Lac Qui Parle County, in SW MN, contained the only anomalous sample outside of the NE section of the State, and this sample contained 760.36 ppm TREE. The most promising sample in this study was identified from a review of publications on Minnesota geology. Morey and McDonald (1989) reported a highly anomalous sample, GSP-47, 40 �Proceedings of the 61st ILSG Annual Meeting - Part 1 from near Ray, MN that had a TREE analysis of 11,313.50 ppm and 1,100 ppm Thorium (a common component of REE mineralization). Although this original TREE analysis was incomplete (some heavy rare earth elements (HREEs) were missing), the location of the sample, as well as a powdered sample split from the original analysis were both available, and afforded the NRRI the opportunity to re-evaluate the original sample, which provided an almost duplicate REE and Th analysis, and added the missing HREEs. The sample location is west of Ray, MN. The Minnesota Geological Survey provided a split of the original sample. Reanalysis confirmed its anomalous nature, i.e., 11,139.46 ppm TREE and 1,162.3 ppm Th. Upon confirmation of the original anomalous REE contents of GSP-47, detailed geological mapping (1:5000 scale) and additional geochemical sampling was done west of Ray, MN. The sample site was mapped and sampled (8 samples), and two samples assayed had > 4,000 ppm TREE and > 425 ppm Th, confirming the anomalous nature of this geologic area. Further work is recommended to more fully characterize the nature of the geological areas associated with anomalous TREE and Th contents identified in this study. Such work recommended for future exploration and characterization includes detailed airborne and ground radiometric surveys of exposed rock outcroppings of favorable REE host rocks, followup/confirmation assaying, detailed geologic mapping, and, if warranted, diamond drilling. As well, future geologic and geophysical studies should be conducted to further characterize, and if possible, confirm the existence of carbonatites in the southwest part of the state as suggested by Southwick (2014). Such carbonatites are currently the major source of REEs in China and the U.S. (Molycorp) and may afford Minnesota an excellent target for future REE exploration and characterization. REFERENCES Hauck, S., Heine, J., Severson, M., Post, S. Chlebecek, S. Monson Geerts, S., Oreskovich, J., Gordee, S., and Hudak, G., 2014, Geological and geochemical reconnaissance for rare earth element mineralization in Minnesota: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report NRRI/TR-2014/39, 572 p. Klenner, R., Gosnold, W., Heine, J.J., Severson, M.J., Hauck, S.A., Hudak, G., and Fosnacht, D.R., 2012, New Heat Flow Map of Minnesota Corrected for the Effects of Climate Change and an Assessment of Enhanced Geothermal System Resources: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report NRRI/TR-2012/01, 109 p. Morey, G.B., and MacDonald, L.L., 1989, Analytical Results of the Public Geologic Sample Program, 1987-1989 Biennium: Minnesota Geological Survey, Information Circular 29, 66 p. Southwick, D.L., 2014, Reexamination of the Minnesota River Valley Subprovince with Emphasis on Neoarchean and Paleoproterozoic Events: Minnesota Geological Survey Report of Investigations 69, 52 p. 41 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Petrological and geochemical evaluation of the Sturgeon Falls Igneous Body and its relationship with the Penokean Orogenic Belt HAYNES, Jonathan1, THAKURTA, Joyashish1, and QUIGLEY, Tom2 1 Department of Geosciences, Western Michigan University, 1903 W. Michigan Ave. Kalamazoo, MI 49048. 2 Aquila Resources Inc., 414 10th Avenue, Suite 1,Menominee MI 49858 The Sturgeon Falls Igneous Body (SFIB) is a mafic to ultramafic intrusion located along the Michigan-Wisconsin border just south of the town of Norway, MI. The SFIB is bounded to the north by the Niagara Shear Zone and the Michigamme Formation (Schulz and Cannon, 2006) and to the south by an unnamed thrust fault zone and the Quinnesec Formation (Sims and Schulz, 1993). Field mapping has shown that the SFIB is composed almost entirely of metagabbro, with isolated outcrops peridotite and serpentinite. The metagabbro reached greenschist facies (Prinz, 1959), and is mostly composed of plagioclase, clinopyroxene, and hornblende. Hornblende is present as both a primary and secondary alteration mineral. The level of alteration within this rock is spatially varied within the SFIB. The portions of the SFIB near the fault zones have been metamorphosed to the point where their original fabric and mineral composition has been lost. This area has been named the Heterogeneous Altered zone. It is rich with secondary amphibole, with weak foliation often present. Prinz (1959) observed that the metagabbro cut the ultramafic rocks indicating that they were formed as part of an earlier magmatic event. Schulz and LeBerge (2003) proposed that the SFIB is part of a larger suprasubduction ophiolite sequence that formed along with the Pembine-Wausau Terrane. Major and trace element geochemistry of approximately 35 samples was compared to other known suprasubduction zone ophiolites, as well as island arcs suites. The results show that the major element geochemistry of the SFIB matches well with both suprasubduction zone ophiolites and island arc terranes. Trace element geochemical signatures such as enrichment in light REE and LILE elements are often used to distinguish suprasubduction zone ophiolites (Shervais, 2001), however in this case, these methods prove problematic for several reasons. First, light REE are highly mobile during metamorphism (Yumal, 1996). Secondly, these characteristics are also common to Island Arcs, so they are not in themselves sufficient to distinguish an ophiolite sequence. In order to identify an ophiolite sequence, a comprehensive study of the lithology, structure, and geochemistry must be used, one method by itself is insufficient. While the geochemistry of the SFIB is roughly in agreement with the ophiolite theory, the evidence available for consideration is not sufficient to distinguish the SFIB as an ophiolite sequence as opposed to an arc related intrusion. 42 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Representative geochemistry from each rock unit within the SFIB Sample   Lithology   SiO2   Al2O3   Fe2O3   MnO   MgO   CaO   Na2O   K2O   P2O5   Cr2O3   TiO2   BaO   LOI   Total                                 SF-­‐149   SF-­‐097   CY-­‐05     MG   HET   UM     46.75   52.49   39.03     17.83   14.61   0.49     11   10.42   11.29     0.166   0.18   0.17     8.14   8.94   36.2     10.802   3.956   0.1     2.41   4.39   0.05     0.27   1.25   <0.01     0.028   0.066   <0.01     0.08   0.02   0.75     0.49   0.67   0.01     <0.004   0.03   <0.01     2.82   3.63   11.85     100.8   100.66   99.94                 Table 1 is a representative geochemistry from each rock unit within the SFIB shown as oxides. MG= metagabbro, HET= Heterogenous Altered Zone, UM= Ultramafic. Concentrations expressed in weight %. REFERENCES Prinz, W.C., Geology of the southern part of the Menominee district, Michigan and Wisconsin: US Geological Survey open-file report. April 17, 1959, 221p. Schulz, K. J., & Cannon, W. F. (2006). The Penokean orogeny in the Lake Superior region. Precambrian Research, 157, 4-25. Schulz, K., and LaBerge, G. (2003). Pembine-Wausau Magmatic Terrane. Institute on Lake Superior Geology 49th Annual Meeting Proceedings, 49, 33-47. Sims, P.K., Schulz, P.K., Geologic Map of Precambrian Rocks of Parts of Iron Mountain 30’ x 60’ Quadrangles, Northeastern Wisconsin and Adjacent Michigan. US geological Survey Miscellaneous Investigation Series Map _2356. Scale 1:100,000 Shervais, J. (2001). Birth, death, and resurrection: The life cycle of suprasubduction zone ophiolites. Geochemistry Geophysics Geosystems, 2. 43 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Geochemical and petrological studies on the origin of Ni-Cu sulfide mineralization at the Eagle Intrusion in Marquette County, Michigan HINKS, Benjamin1, THAKURTA, Joyashish1 and MAHIN, Robert2 1 Department of Geosciences, Western Michigan University, 1903 W. Michigan Ave. Kalamazoo, MI 49008 2 Eagle Mine, Lundin Mining Corporation, 4547 County Road, Champion, MI 49814 The ~1.1 Ga Eagle deposit is a small mafic to ultramafic sulfide-bearing intrusion located in the north central portion of Michigan’s Upper Peninsula within Michigamme Township, Marquette County (Figure 1). The Eagle and East Eagle intrusions penetrate Paleoproterozoic rocks of the Marquette Supergroup that were deposited in the 400 km2 Baraga Basin during the ~1.85 Ga Penokean orogeny (Ding et al. 2010). The Eagle and East Eagle intrusions are a part of the east-west trending Marquette-Baraga dike swarm that is associated with ~1.1 Ga Midcontinent Rift System (MRS) magmatism (Ding et al. 2010). The age of the Eagle intrusions were determined using uranium-lead dating to be 1107.3 ± 3.7 Ma, which constrains their formation to the early stages of Midcontinent Rift System formation (Ding et al. 2010). The current proven and probable reserves for Eagle are 5.2 million tonnes with an average grade of 3.11% Ni, 2.55% Cu, 0.08% Co, 0.69 gpt Pt, 0.47 gpt Pd, and 0.28 gpt Au (R. Mahin, pers. comm.). This study will attempt to constrain the mechanisms responsible for the formation of sulfide minerals at the Eagle deposit and the source(s) of external sulfur required to produce the sulfide ores. Principle objectives include identifying the source(s) of external sulfur required to form sulfide minerals and the geochemical/ petrological relationships between the intrusions, country rocks and sulfide deposits. It is well known that the Eagle intrusion hosts high-grade Ni-Cu sulfide ores, while the East Eagle intrusion is weakly mineralized (Ding et al. 2010). As of now a relationship between the two intrusions has not been well established. Further analysis will attempt to constrain the connection between the Eagle and East Eagle intrusions. Geochemical studies done through previous research have suggested multiple sources of external sulfur from Archean and Paleoproterozoic country rocks, even though sulfur isotope signatures are indicative of mantle values (Ding et al. 2012). Further analysis of sulfur isotope data from the Eagle and East Eagle deposits will help to address the question of external sulfur in the magmatic system. Petrographic analysis of rocks from the intrusions and the country rocks will be used to study textural characteristics. From the results generated in this study we hope to identify a set of characteristics that can be used for the identification of other sulfide deposits in the Upper Peninsula of Michigan. Preliminary hand sample analyses of previously collected core samples have yielded three main rock types in decreasing olivine contents: feldspathic peridotite, melatroctolite and olivine melagabbro (Ding et al. 2010). Primary sulfur textures are disseminated, semi-massive and massive sulfide mineralization with major sulfide minerals of pyrrhotite, chalcopyrite, pentlandite and traces of bornite. Disseminated sulfide mineralization tends to occur as scattered blebs with 315% sulfide minerals. Semi-massive sulfide mineralization occurs as a net-textured matrix containing 30-50% sulfide minerals. Massive sulfide mineralization is characterized by a leopardtextured matrix with stringers of sulfide minerals. Massive sulfides contain >50% sulfide minerals. 44 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Figure 1: Eagle area geology. Mafic dykes determined from magnetics are shown in red. Purple zones are the locations of the Eagle and East Eagle intrusions. Fault zones determined from magnetics are shown as black lines. Blue lines represent gravity lineaments from Resolve electromagnetics. Black dots are representative of borehole locations. From: Rossell et al. 2005 REFERENCES Ding, X., C. Li, E. M. Ripley, D. Rossell, and S. Kamo (2010), The Eagle and East Eagle sulfide ore-bearing maficultramafic intrusions in the Midcontinent Rift System, upper Michigan: Geochronology and petrologic evolution, Geochem. Geophys. Geosyst., 11, Q03003, doi:10.1029/2009GC002546. Ding, X., E. M. Ripley, C. Li (2010), PGE geochemistry of the Eagle Ni-Cu-(PGE) deposit, Upper Michigan: constraints on ore genesis in a dynamic magma conduit: Miner Deposita, doi: 10.1007/s00126-0350-y. Ding, X., E. M. Ripley, S. B. Shirey, C. Li (2012), Os, Nd, O and S isotope constraints on country rock contamination in the conduit-related Eagle Cu-Ni-(PGE) deposit, Midcontinent Rift System, Upper Michigan: Geochemica et Cosmochimica Acta, 89, pp. 10-30. Owen, M. L. and Meyer L. H. I. (2013), NI 43-101 Technical Report on the Eagle Mine, Upper Peninsula of Michigan, USA. Report for Lundin Mining Corporation, dated July 26, 2013, pp. 1-241. Rossell, D and Coombes, S, (2005), The Geology of the Eagle Nickel-Copper Deposit Michigan, USA. Report for Kennecott Exploration, dated April 29, 2005, pp. 1-35. Schneider, D.A., Bickford, M.E., Cannon, W.F., Schultz, K.J., and Hamilton, M.A., (2002). Age of volcanic rocks and syndepositional iron formations, Marquette Range Supergroup: implications for the tectonic setting of Paleoproterozoic iron formations of the Lake Superior region. Can. J. Earth Sci., vol. 39, pp. 999-1012. 45 �Proceedings of the 61st ILSG Annual Meeting - Part 1 The Minnesota Taconite Workers Health Study: Environmental Study of Airborne Particulate Matter - 2015 Update HUDAK, George1, MONSON GEERTS, Stephen1, ZANKO, Larry1, POST, Sara1, and BANDLI, Bryan2 1 Natural Resources Research Institute, 5013 Miller Trunk Highway, Duluth, MN, 55811 2 Department of Geological Sciences, University of Minnesota Duluth, 229 Heller Hall, 1114 Kirby Drive, Duluth, MN 55812 The Natural Resources Research Institute (NRRI) continues to conduct a detailed characterization of mineral dust in northeastern Minnesota. The purpose of this research is to evaluate the effects of present emissions from taconite mining and processing on air quality throughout the Mesabi Iron Range (MIR) (Figure 1) by characterizing airborne mineral particulate matter (PM) within currently operating taconite processing plants, in MIR communities surrounding taconite mining/processing operations, and in population centers in Minnesota not associated with taconite mining. Characterization studies of age-dated lake sediments are also being conducted to determine the composition of past PM deposition. NRRI’s sampling and characterization work represents the community/environmental component of the Minnesota Taconite Workers Health Study, a broad University of Minnesota (UM) research effort involving both the NRRI and the School of Public Health. Figure 1. Locations of taconite processing plants on the Mesabi Iron Range being sampled during this study (after Oreskovich and Patelke, 2006) Air sampling was performed within taconite operations, MIR communities, and non-MIR communities by NRRI scientists during both winter and summer seasons from 2009-2012. Sampling was conducted at four process locations within taconite operations, including: 1) secondary crushers; 2) magnetic separators/concentrators; 3) agglomerators/ball drums; and 4) kiln/pellet discharge areas. Community sampling took place on centrally-located rooftops of public buildings, or in the case of the northern most background site, in a remote sampling location to evaluate the air quality away from the MIR. Airborne particles were collected using: 1) a micro orifice uniform deposit impactor (MOUDI) (Marple et al., 1991, 2014), which enables size46 �Proceedings of the 61st ILSG Annual Meeting - Part 1 fractionated PM collection; and 2) a Total Filter Sampler (TFS). Particulate matter was evaluated via gravimetric analysis and was subsequently subjected to comprehensive characterization that included: 1) scanning electron microscopy (SEM) imaging; 2) energy dispersive x-ray spectroscopy (EDS); 3) electron backscattered diffraction (EBSD); 4) proton induced x-ray emission (PIXE); 5) the Minnesota Department of Health’s 852 Method Transmission Electron Microscopy (TEM) Analysis for Mineral Fibers in Air; and 6) the International Standardization Organization’s Indirect Method 13794 for Ambient air – Determination of Asbestos Fibers. NRRI’s research methods do not produce exposure data, and are not meant to provide data for regulatory purposes. During the past year, the NRRI has been evaluating the physical (gravimetric, morphology, concentration), mineralogical, and chemical characteristics of the PM obtained from sampling at the taconite operations and MIR/non-MIR communities. This includes analysis of 55 taconite plants; 73 northeastern Minnesota community and 6 Minneapolis samples. Lake sediment analysis has been completed, and will provide important historical data regarding potential mineralogical inputs from iron mining and processing from ~1840 (which pre-dates iron mining on the MIR) to the present, which includes the period where the transition from natural ore mining to taconite mining took place. Community results to date are as follows: • measured particulate matter concentrations for PM2.5 in all MIR communities have been below 12 µg/m3, and for total PM have been below 16µg/m3; • particulate matter concentrations on the MIR are similar to those in the two NE Minnesota background sites (Duluth NRRI, Ely Fernberg site), and are lower than those obtained in Minneapolis (UM Mechanical Engineering Building rooftop); • mineral particulate matter in community air samples reflects the mineralogy of the Biwabik Iron Formation and other Minnesota rock types and geological materials; • elongate mineral particles (EMP) are present in MIR community ambient air samples; however, asbestiform amphiboles were rarely observed (1 asbestiform amphibole EMP in ~22,800m3 of air). Taconite plant results to date are as follows: • plant environments can be dusty, with the most dusty environments associated with the agglomerator and kiln discharge areas; • particulate matter levels (PM1, PM2.5, PM10, and total PM) show a slight increase in the five MIR communities during plant/mine activity, but this increase is not statistically significant compared to when the plants were not operating. • significantly higher concentrations of EMPs, including amphiboles, were detected in the eastern most plant compared with the other five plants, but the morphology of these structures more closely resembles cleavage fragments rather than asbestiform morphologies. REFERENCES ISO 13794 (1999), Ambient air — Determination of asbestos fibres — Indirect-transfer transmission electron microscopy method. Marple, V. A., Rubow, K. L., and Behm, S. M., 1991, A micro orifice uniform deposit impactor (MOUDI): description, calibration, and use: Aerosol Science and Technology, v. 14, p. 434-446. Marple, V., Olson, B., Romay, F., Hudak, G., Monson Geerts, S., and Lundgren, D., 2014, Second Generation MicroOrifice Uniform Deposit Impactor, 120 MOUDI-II: Design, Evaluation, and Application to Long-Term Ambient Sampling: Aerosol Science and Technology, v. 48-4, p. 427-433. 47 �Proceedings of the 61st ILSG Annual Meeting - Part 1 MDH. Method 852 (1999) T.E.M. analysis for mineral fibers in air – 852. Minnesota Department of Health, Microparticulate Unit, St. Paul, MN. 42 pp. Oreskovich, J. A., and Patelke, M. M., 2006, Historical use of taconite byproducts as construction aggregate materials in Minnesota: A Progress Report: Natural Resources Research Institute Report of Investigation NRRI-RI-200602, 10 p. 48 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Geology and geochronology of Archean rocks in the International Falls and Littlefork 30X60’ quadrangles, north-central Minnesota JIRSA1, Mark A., BOERBOOM1, Terrence J., CHANDLER1, V., and SCHMITZ2, Mark D., 1 Minnesota Geological Survey (MGS); 2Department of Geosciences, Boise State University The International Falls and Littlefork quadrangles provide a transect across parts of 3 subprovinces of the Archean Superior Province—Wabigoon, Quetico, and Wawa (Fig. 1). The interpretation of bedrock geology described here was recently published as MGS Miscellaneous Map M-197. The map incorporates field work and geophysical modeling by the authors, augmented by lidar, air photography, reprocessed aeromagnetic data, and unpublished field notes of former MGS geologists. In addition, 4 samples were submitted for high-precision U-Pb geochronologic analysis of zircons (Fig. 2) to quantify ages for some units and temporally constrain some events. The map depicts a complex history of volcanism, sedimentation, intrusion, multiple episodes of migmatization involving partial melting and melt dispersion, and several periods of deformation and metamorphism. Figure 1. Generalized geologic setting of subject map area showing the approximate locations from which samples were taken for geochronologic analysis (solid circles #1-4). See Fig. 2 for sample details. The structural grain of the subprovinces is largely a product of three major orogenic events— each involving a component of NW-SE-directed compressional and transpressional deformation— referred to here as D1, D2, D3. The Wabigoon and Wawa subprovinces are greenstone-granite terranes inferred to represent oceanic and island arc settings. The intervening Quetico subprovince consists of sedimentary rocks deposited in continental margin and oceanic environs that were subsequently deformed, metamorphosed, partially melted, and multiply intruded. Various estimates bracket volcanism in the Wabigoon subprovince in Ontario between ~ 2728-2725 Ma. Geochronologic analyses conducted for this mapping project indicate that a younger sequence of felsic volcanic strata (2702.9±0.6 Ma) marks the southernmost part of the subprovince (Figs.1 and 2, sample 1.). Similar rock types occur in the northern Wawa subprovince, but ages there are less well constrained, as only a single age of ~2722 Ma exists from a sample acquired 85 miles east of the map area. A new age of 2715.8±0.5 Ma (Figs. 1 and 2, sample 2) acquired from felsic volcanic rocks in this map area establishes broad equivalence across the northern part of the Wawa subprovince. Rocks of the Quetico subprovince consist of biotite-plagioclase schist, granitoid and minor mafic intrusions, and complex migmatite containing multiple paleosomatic and neosomatic components. Based on detrital zircons in Canadian analogs, the schist was derived from graywacke and pelitic sediments deposited ~2698-2692 Ma in an accretionary prism during early 49 �Proceedings of the 61st ILSG Annual Meeting - Part 1 stages of collision between the Wawa subprovince island arc to the south, and the Superior craton (superterrane) to the north. Later stages of this D1 collisional event produced tilting, folding that included broad nappe structures locally, and thrust imbrication of volcanoplutonic rocks in subprovinces north and south of the Quetico, and recumbent folding within the Quetico. This D1 deformation was followed by what may have been a regional extensional event that produced localized calc-alkalic volcanism, and sediments containing clasts derived from all precursor rock types deposited in isolated, unconformity- and fault-bounded basins. The remnants of one such basin in the International Falls area is known as the Seine Group. A new age of 2693.9±0.6 Ma was acquired from a clast of trachyandesite in Seine conglomerate (Figs. 1 and 2, sample 3). A second deformation event (D2) occurred at about 2680 Ma during the Minnesotan orogeny. It produced regional penetrative fabrics, folds, and prograde metamorphism in all 3 subprovinces, and partial melting of Quetico schist to form an early suite of leucogranite, granodiorite, trondhjemite, and tonalite that is interlayered on all scales with biotite schist, forming complex migmatites. A third deformation event (D3) is manifest in shear and fault zones in the volcanoplutonic subprovinces; and broad, east- and west-plunging folds (synforms, antiforms) of D2 fabrics in the Quetico subprovince. At least part of this deformation was synchronous with or just preceded migmatization and emplacement of 2-mica leucogranite and slightly younger, typically red, variably magnetic biotite granite known as the Lac La Croix. An age of 2661.3±0.3 Ma was acquired in this map area from a late granitic intrusion lithologically similar to the Lac La Croix (Figs. 1 and 2, sample 4). The grade of metamorphism is more or less symmetrical along the northeast-trending axis of the Quetico subprovince, having greenschist facies at the margins, and middle to upper amphibolite facies near the axis. Metamorphism was generally syntectonic with D2 and D3 deformations; and a contact metamorphic overprint occurs locally in rocks adjacent to the Lac La Croix Granite and similar late intrusions. The axis of the Quetico is coincident with a large post-metamorphic anticlinorium, increased neosome abundance, and moderately high magnetic and gravity expression, despite the presence of less dense rocks at surface. Collectively, these attributes indicate exposure of more deeply buried crust. The presence of dense, magnetic rocks at depth that may represent the uplifted floor on which sedimentary strata were deposited. Figure 2. Concordia plots showing results of LAICPMS geochronologic analysis of zircons. Ages reported here are taken from calculations of 207 Pb/206Pb. Details of this work are available from published MGS digital files associated with Miscellaneous Map M-197. Sample locations in UTM NAD83 coordinates: 1. 436644E/5382588N 2. 454692E/5316937N 3. 485860E/5383472N 4. 476489E/5354520N Mapping and geochronologic analyses were funded in part by 2013 USGS STATEMAP element of the National Geologic Mapping Program 50 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Rainy River, northwestern Ontario's first meteorite KISSIN, Stephen A. Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1 Canada The only meteorite previously known from northern Ontario is the Osseo iron meteorite, found in 1931 in the easternmost part of Ontario in the vicinity of the Cobalt silver camp (Buchwald, 1975). In 2000, Robert Weaver found a 3.26 kg iron object in a field on his farm, near Rainy River, Ontario. It is now in the possession of Howard Williams of Winnipeg, who presented the specimen for identification in 2013. A polished slab of 47.26 g has been deposited as the type specimen in the Royal Ontario Museum. The meteorite is oxidized on its exterior and is of a roughly ellipsoidal shape with dimensions of 17.4 cm x 13.2 cm x 9.5 cm (Figure 1). The interior is unoxidized except minor patches near the thin exterior iron oxide crust and along kamacite grain boundaries. The meteorite is an octahedrite, with coarse kamacite lamellae of irregular width and a stubby aspect of L: W= 3:1 to 4:1. The residual taenite lamellae are very narrow where preserved and only one area of net plessite was observed (Figure 2). The other minerals present are troilite, in the form of small, intergranular veinlets, and schreibersite, as small, globular forms and as small, euhedral, crystals known as rhabdites. The rhabdites are very abundant and display strong preferred orientation within kamacite lamellae. The meteorite has experienced moderate cosmic shock as seen in the abundant Neumann lines, some of which have been bent in places. The kamacite lamellae have been polygonalized due to brittle fracture. Hardness of kamacite (VHN=254±13) and taenite (VHN= 492±28) is also evidence of work hardening due to shock. Analysis by neutron activation yielded the following minor and trace element composition: Ni= 7.23 wt%, Co=0.463 wt%, Sb= 410 ppb, all in ppm Cr=19, Cu=114, Ga=91, Ge=170, As 13.9, W=<10, Re=0.64, Ir=3.87, Pt=7.8, Au=1.47. Together with the details of the structure, the meteorite is a member of the group IAB complex, as defined by Wasson and Kallemeyn (2003). However, the Au-content is at low extreme of the group, thus suggesting that the meteorite may belong to the Algarrabo duo of Wasson and Kallemeyn. Moreover, other trace element contents, especially Ge and Ga, are similar to those of one of the duo, Livingston (Tennessee). As well, the structure of Livingston (Tennessee) is similar to that of Rainy River in that Ni-content does not agree with the normally expected kamacite bandwidth, which should be in the range of 2.5-3 mm in well-defined Widmanstaetten structure (Buchwald, 1975). Buchwald further proposed an usual thermal history for Livingston (Tennessee), which may also have applied to Rainy River. 51 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Figure 1: Main mass of the Rainy River meteorite, less off-cut for type specimen. Scale in cm. Figure 2: Polished surface of the type specimen. Some kamacite lamellae outlined by weathering. Scale in cm. REFERENCES Buchwald, V.F. 1975. Handbook of Iron Meteorites. University of California Press. Wasson, J.T. and Kallemeyn, G.W. 2003. The IAB iron-meteorite complex: A group, five subgroups, numerous grouplets, closely related, mainly formed by crystal segregation in rapidly cooling melts. Geochimica et Cosmochimica Acta 66: 2445-2473. 52 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Studies on PDFs in shocked quartz from distal Sudbury ejecta in the Thunder Bay area compared with Chicxulub KISSIN, Stephen A. and BRUMPTON, Gregory R Department of Geology, Lakehead University, 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada Planar deformation features (PDFs) in Sudbury ejecta in the Marquette, Michigan area have been studied briefly by Cannon et al. (2010) and more extensively by Pufahl et al. (2007) and briefly in the Thunder Bay area by Kissin & Brumpton (2014). We have made an extensive study of PDFs in quartz from ejecta from various localities in the Thunder Bay area. A total of 153 PDFs from 104 quartz grains were measured on the universal stage (Table 1). Of these, 77 PDFs were measured in 55 grains within accretionary lapilli, and 76 PDFs were measured in 49 grains in the matrix of the ejecta. Indexing was carried out by both the use of stereographic projection and use of the ANIE program of Huber et al. (2011). Agreement between the two methods was generally good, although the ANIE program eliminated drawing errors that may be introduced in the stereographic projection method. Comparison of the distribution of indexed PDFs in grains in lapilli vs. those of grains in the matrix revealed no significant difference at 95% confidence using the Wilcoxon matched-pairs signed-ranks test, although the source of the grains may have differed. Differences were noted in that grains within lapilli are smaller (50-100 µm) as opposed to those in the matrix (100-500 µm). As well, grains in the matrix were frequently rounded, whereas those in the lapilli were almost always angular. These observations suggest that the grains in lapilli were predominantly from crystalline basement rocks of the target area, whereas those in the matrix had a considerable contribution from sedimentary surficial rocks. The distribution of the PDFs in this study was compared with the results of Nakano et al. (2008) who studied PDFs in distal ejecta at various distances from the Chicxulub impact crater (Table 2). The PDF sets are grouped in five-degree bins containing most of the major PDFs. Note that Nakano et al. did not include the {1014} set, which is abundant in both their and our data sets. Analysis of all the data sets in Table 2 using the Friedman test yields a Friedman Statistic of 0.9592. Thus, the probability is > 95% that the sum of the ranks (rows) is the same in each column. As well, the PDF sets from each of the Chicxulub sites was compared with the Sudbury ejecta set using the Willcoxon matched-pairs signed ranks test. In all cases except for the DSDP 536 set, the nonparametric Spearman correlation coefficient and the one-tailed P value indicated pairing of the ranks. The DSDP 536 data were slightly less than statistically significantly paired. These tests indicate the similarity of distribution of PDF sets in Sudbury and Chicxulub distal ejecta. The formation of PDFs in quartz has been experimentally calibrated in response to shock pressure, as reviewed by Stöffler and Langenhorst (1994). The PDFs in Sudbury and Chicxulub ejecta correspond to a wide range of shock pressures extending as high as 35 GPa. Even higher shock pressures are indicated by the occurrence of diaplectic quartz glass and incipient melting of quartz grains. This range of shock pressures is indicative of the impact process and the location of the quartz grains to it, as has been modeled by Nakano et al. (2008). 53 �Proceedings of the 61st ILSG Annual Meeting - Part 1 !"""#$% CB,! C-B*! !!"!! !$% *CB.! !!"!! !!! .BE! !!"!! !N! *BC! !!!!! !!! @BA! !!"!! !N! 1B,! !!"!! !N! 1B@! !!!!! !}! 1B,! !!"!! !N! @B-! !!!!! !N! *BC! !!"!! !!! -! !!"!! !!! .BE! !!"!! !}! *BC! !!"!! !}! -B.! !!!!! !}! @BA! !!"!! !N! OF3FP'Q'P! .BE! Table 1. Absolute frequency of PDF sets (153 sets) Table 2 REFERENCES 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, Geological Society of America Bulletin 122: 50-75. Huber, M./S., Ferriére, L., Losiak, A. and Koeberl, C. 2011. ANIE: A mathematical algorithm for automated indexing of planar deformation features in quartz grains, Meteoritics & Planetary Science 46: 1418-1424. Kissin S.A. and Brumpton, G.R. 2014. PDFs in Sudbury ejecta in the Gunflint Formation, Ontario: A comparison of methods., Institute on Lake Superior Geology Proceedings 60, Part 1, 69-70. Nakano, Y., Goto, K., Matsui, T., Tada, R. and Tajika, E. 2008. PDF orientations in shocked quartz grains around the Chicxulub crater, Meteoritics & Planetary Science 43: 745-760. Pufahl, P.K., Hiatt, E.E., Stanley, C.R., Morrow, J.R., Nelson, G.J. and Edwards, C.T. 2007. Physical and chemical evidence of the 1850 Ma Sudbury impact event in the Baraga Group, Michigan, Geology 35: 827-830. Stöffler, D. and Langenhorst, F. 1994. Shock metamorphism of quartz in nature and experiment: I. Basic observation and theory. Meteoritics, 29: 155-181. 54 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Geologic mapping of Neoarchean and Proterozoic rocks near Knife Lake, northeastern Minnesota, by students of the Precambrian Research Center’s 2014 field camp KROGMEIER, Benjamin1, McKEVITT, Dylan1, ROEPKE, Elizabeth1, SARA, Michael1, SZKILNYK, Paul1, and JIRSA, Mark2 1 2014 Field Camp Students, Precambrian Research Center, Natural Resources Research Institute, University of Minnesota Duluth, 5013 Miller Trunk Highway, Duluth, Minnesota 55811 2 Minnesota Geological Survey, University of Minnesota, 2609 W. Territorial Rd., St. Paul, Minnesota 55114 The University of Minnesota-Duluth’s Precambrian Research Center conducted its eighth annual field camp in 2014, and this presentation is one of a series that show results of “capstone” mapping projects. The projects test student skills by creating new geologic maps in areas of poorly known geology, which benefits both students and mentor organizations. This capstone project involved mapping an area of ~12 mi2 in the Boundary Waters Canoe Area Wilderness (BWCAW), centered on the south arm of Knife Lake (Fig. 1). The resulting map provides details about the complex depositional and tectonic history of a Neoarchean metavolcanic and metasedimentary terrane that is part of the Wawa subprovince of Superior Province, and rare diabasic dikes that intruded it. Figure 1. Generalized bedrock geologic map of NE Minnesota showing the Knife Lake capstone area (solid black polygon). The Neoarchean unit labeled “Supracrustal Rocks” encloses both older volcanic sequences and younger, largely sedimentary ones. Outline of Boundary Waters Canoe Area Wilderness is dashed. The Neoarchean rocks in the central BWCAW comprise a Timiskaming-type extensional basin and its apparent wall- and floor-rocks. The geologic units are parceled into structural lozenges separated by anastomosing shear and fault zones. Although rock types are comparatively pristine within each lozenge, correlation of units from one fault-bounded block to another is challenging. Nevertheless, this project and the several that preceded it in prior years of mapping attempt to “unstrain” the rocks within each parcel to reveal stratigraphic variations that may reflect fluctuations in basin geometry and progressive erosional dissection of basin wall rocks. Understanding the lithologic details and the apparent post-depositional tilt of individual lozenges of rock are essential to this objective. The Knife Lake map area provides a window into this complex terrane. It consists of 2 sequences of broadly folded metasedimentary rocks that are part of the Knife Lake Group, separated by an east-trending block of vertically dipping, southward-facing, variolitic metabasalt as thick as 0.5 km. The sedimentary strata include tuffaceous graywacke and mudstone, locally containing conglomeratic and gritstone layers having clasts of the metabasalt, Saganaga Tonalite (~2.69 Ga), and other calc-alkalic igneous rocks. All these rocks were deformed and metamorphosed to very low greenschist facies during the Minnesotan Orogeny (~2.68 Ga)—thus constraining deposition of the sediments to the approximate interval 2.69-2.68 Ga. The primary objective of this mapping was to delineate and interpret the nature of contacts between the 55 �Proceedings of the 61st ILSG Annual Meeting - Part 1 apparently older metabasalt and superjacent sedimentary strata derived in part from it. Highlights of our mapping include the following: 1) The abundance of graded sequences of sand- to mud-sized detritus implies deposition of much of the strata was submarine (or lacustrine). 2) Local sequences of polymictic and oligomictic conglomerate, and arkose containing matrixsupported, subrounded metabasalt fragments as large as 20 cm implies occasional subaerial deposition of more chaotic debris flow, alluvial fan, and fluvial sediments. This is consistent with episodic uplift of wall rocks adjacent to the developing fault- and unconformity-bounded basin. 3) The abundance of white-weathered tuff and tuffaceous siltstone and mudstone implies calc-alkalic volcanism may have been contemporaneous with deposition, or volcanic strata were not fully lithified at the time of deposition. 4) Although both northern and southern sedimentary sequences are broadly folded; stratigraphic facing near contacts with the medial basalt block is consistently away from the basalt. 5) The northern contact of basalt with sedimentary strata is poorly exposed, but appears to be a fault. It lies along the down-section part of the basalt, and thus is stratigraphically discordant. 6) The southern contact of basalt with sedimentary strata is visible in several areas where it varies from an angular unconformity developed on relatively fresh basalt, to one developed on paleosaprolitic basalt. A spectacular paleosaprolite of basaltic protolith is exposed at one locality. 7) Major fold axes in the northern sedimentary sequence plunge shallowly to the northeast; those in the southern sequence plunge to the southwest. 8) From these observations it appears that the block containing basalt unconformably overlain by the southern sedimentary sequence was uplifted on its north side (Fig. 2). In addition, the divergence of major fold plunges in sedimentary sequences north and south of the fault implies a scissor motion that tilted the southern block down on the west. Judging from inferred structural position and lithologic similarities, it is also likely that some portions of the two sedimentary sequences may have been fault-duplicated. Although the precise age of rare diabase dikes is unknown, most trend northwest and dip nearly vertically, similar to the Paleoproterozoic Kenora-Kabetogama dike swarm. However, a Mesoproterozoic age may be indicated by the trend of one dike that is nearly horizontal, which is quite anomalous for Paleoproterozoic dikes in the Superior Province. We speculate that it may represent emplacement at very high levels in the crust, where comparatively less lithostatic load permitted delamination along horizontal exfoliation structures in host rocks. This and other capstone mapping projects can be viewed at www.d.umn.edu/prc. Figure 2. Schematic cross-sectional model to explain the distribution of map units and partial repetition of stratigraphic sequences by faulting. 56 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Structural control on the Borden Gold deposit, Chapleau, Ontario LAFONTAINE, Daniel and HILL, Mary Louise Department of Geology, Lakehead University, 955 Oliver Rd. Thunder Bay, ON P7B 5E1 Canada The multi-million-ounce Borden Gold deposit is located 20 km east of Chapleau, within the Wawa Subprovince of the Superior Province. Interestingly, it is hosted in upper amphibolite to granulite facies metamorphic rocks at the southern margin of the Kapuskasing Structural Zone. Competent lithons of granulite facies rock appear to be surrounded by more ductile amphibolite facies gneisses and schists, suggesting polymetamorphism with retrograde amphibolite facies metamorphism after granulite facies metamorphism. Competency contrasts between the granulite and retrograde amphibolite facies lithologies created heterogeneous strain, ideal for gold mineralization, during ductile deformation at amphibolite facies metamorphic temperatures. Gold is typically observed in competent rocks with weakly developed foliation and also in competent rocks that are bordered by strongly foliated units. Garnet-biotite geothermometry on unzoned almandine garnets yields temperatures ranging from 579°C to 690°C ±50°C for metamorphism of the garnet-biotite schist. Temperatures increase from the garnet core towards the rim, indicating that garnets equilibrated rapidly during prograde metamorphism from the upper amphibolite to granulite facies. Fieldwork and microstructural analysis have identified a variety of competent lithologies and minerals, which provide low-strain environments for gold mineralization. On the macroscopic scale, the relict granulite facies rock behaves more competently than the retrograde amphibolite facies rock. Competent minerals that provide a low-strain site for fluid transport and gold mineralization include relict orthopyroxene, garnet, pyrite and coarse sillimanite. Preliminary results indicate an important relationship between gold mineralization, metamorphism and deformation, and understanding this relationship will benefit exploration and development of the Borden Gold deposit. 57 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Incorporation of Duluth Complex maps into GIS platform LENTSCH, Nathan1 and MILLER, Jim1 1 Department of Geological Sciences, University of Minnesota Duluth, Duluth, MN 55812 In 2001, Jim Miller and colleagues compiled much of the mapping existing at the time into a digital geologic map and database for the Duluth Complex that was published by the Minnesota Geological Survey (MGS) as Miscellaneous Map M-119 (Miller et al., 2001). Because this compilation was focused on areas of the complex that were open to minerals exploration, a substantial amount of historical field data collected within the Boundary Water Canoe Area Wilderness (BWCAW) was largely excluded from this digital compilation, this includes data collected by Phinney (1972) and Miller (1986). In the 1960’s, William Phinney, an igneous petrology professor at the University of Minnesota-Twin Cities, conducted bedrock mapping for the MGS in the northwestern part of the Duluth Complex. In the summers of 1966 to 1969 Phinney conducted extensive reconnaissance mapping of lakeshore exposures by canoe and floatplane support in areas of the Duluth Complex now contained entirely within the BWCAW (established in 1976) and only accessible by canoe or by foot. Although he did not publish any maps from this field work, Phinney summarized his studies in the MGS’s Centenial Volume on the Geology of Minnesota publication (Phinney, 1972). In 1970, he left the University to take a job with NASA. In 1979, Dr. Phinney passed along all field journals, maps, and thin sections of his Duluth Complex mapping to Dr. Miller, who at the time was a PhD candidate planning his own detailed mapping study in part of an area of the complex that Phinney had previous reconnaissance mapped. In 1981, Dr. Miller conducted four months of detailed mapping in an area focused on the Lake One - Lake Four chain in the Snowbank 7.5’ quadrangle. Miller’s outcrop mapping is preserved only as a blueprint map in his PhD thesis (Miller, 1986), field maps on airphoto bases, and field notebooks. Only his geological linework and structural measurements were digitized for the M-119 map. None of Phinney’s original outcrop data has been digitized. The goal of my research project, which was funded by the University of Minnesota Duluth’s UROP program (Undergraduate Research Opportunity Program), was to compile outcrop-based field data into a digital database using ArcMAP 10 for the area of the Duluth Complex mapped by Phinney (unpublished data) and Miller (1986) in the Snowbank Lake 7.5’ quadrangle. Digitally compiling these field observations, measurements, and sample locations is important for several reasons: 1) it will preserve an important database of geologic information, some of which previously only existed as fragile paper copies; 2) whereas only one copy of Phinney’s and Miller’s field data had existed, digitally archiving their field maps and observations will allow open access to future geologists and researchers; 3) with most of their mapping covering areas deep into the BWCAW, it is likely that in many areas, their mapping will be all that exists for the foreseeable future; and 4) this project gave me the opportunity to learn the powerful geospatial tool that is ArcMap. To efficiently and accurately trace the locations and shapes of the numerous outcrops mapped by Dr. Miller, scans of field maps on aerial photo bases were inserted as a layer under a partially transparent topographic map layer in ArcMAP. Figure 1 shows an example of what this process looked like. As of this writing, over 1,000 outcrops have been digitized, each linked to a table of attributes. Recorded attributes include the outcrop station, the date visited, the major and minor lithologies observed, and a short description for each. An example of this table is shown in Figure 2. 58 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Figure 1: GIS map showing areas of the BWCAW. Aerial photo as a base layer in bottom left. Light gray outcrops around Lake Two were digitized from Dr. Miller’s 1981 mapping data (Miller, 1986). Figure 2: Table of attributes associated with outcrops in GIS. Each outcrop can be referenced by major/minor lithology and physical description. Given the time constraints of this project, only Dr. Miller’s field data was able to be digitized thus far. This leaves the door open for future work done by another eager undergraduate who would like to become familiar with GIS and the Duluth Complex mapped by Phinney. REFERENCES Miller, J.D., Jr., 1986, The geology and petrology of anorthositic rocks in the Duluth Complex, Snowbank Lake quadrangle, northeastern Minnesota. unpublished Ph.D. dissertation, University of Minnesota, Minneapolis, 280 p. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.E., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota. Miscellaneous Map Series, M-119, scale 1:200,000, 2 sheets. Phinney, W.C., 1972. Northwestern part of Duluth Complex. In: Sims, P.K. & Morey, G.B. (eds.) Geology of Minnesota -A centennial volume. Minnesota Geological Survey, p. 335-345 59 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Geology and geochemistry of the Lang Lake greenstone belt, Uchi Domain, Superior Province MAGNUS, Seamus Earth Resources and Geoscience Mapping Section, Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, ON P3E 6B5 Canada The Lang Lake greenstone belt, located 70 km WNW of Pickle Lake, Ontario, lies within the central Uchi domain of the greater North Caribou terrane. The belt is composed of ca. 2750 Ma dominantly greenschist facies volcanic, volcaniclastic and sedimentary rocks, and is intruded by various syn-volcanic and syn-to-post-tectonic plutonic rocks. Following the initial discovery of gold in the Shonia Lake area in 1928 and subsequent mapping of the surrounding greenstone by Laird (1930), the Lang Lake greenstone belt received little attention until mapping by the Ontario Geological Survey in the 1970s (Fenwick 1970, 1971; Fenwick & Srivastava 1972). These 3 maps were compiled and ground-checked as part of “Operation Pickle Lake” by Sage and Breaks (1973), who later released an Ontario Geological Survey Open File Report (Sage and Breaks 1982) which included a brief lithologic description of the greenstone belt. Unlike the neighbouring greenstone belts of the central Uchi domain, which have been subject of modern geological studies (government and academic) throughout the 1990s and 2000s, the Lang Lake greenstone belt represented a significant gap in our regional geoscience knowledge. To fill this gap and to supplement the Cat Lake First Nations land use plan, the Lang Lake greenstone belt was mapped during the 2014 field season at a scale of 1:20,000. 278 hand samples were collected for whole rock major and trace element geochemical analysis, 6 samples were submitted for U-Pb zircon geochronological analysis, and 16 samples were submitted for whole rock Sm-Nd isotopic analysis as part of an HBSc thesis conducted by Matthew Hanewich at Carleton University. Preliminary geochemical analysis of the coherent facies metavolcanic rocks and synvolcanic dikes indicates the presence of primitive (i.e. derived from “Primitive Mantle”) Fetholeiitic basalt, calc-alkaline basaltic andesite and calc-alkaline FII rhyolite. Volcaniclastic rocks contain whole rock major and trace element compositions equivalent to calc-alkalic basalt, andesite, dacite and rhyolite, suggesting that they were proximally sourced, and may represent a mixture of material from the aforementioned tholeiitic and calc-alkalic flows. Interflow volcaniclastic mudstones and wackes are intercalated with bands of magnetitechert iron formation which extend across the entire strike-length of the greenstone belt. Similar bands of iron formation are interbedded with clastic metasedimentary rocks which dominate the eastern half of the greenstone belt. Facies include mudstone, wacke, arenite and several lenses of granitoid-clast-bearing conglomeratic rocks similar to those found in the Billet Assemblage of the nearby Meen-Dempster greenstone belt (Stott and Corfu 1993). Whole rock geochemical trends and concentrations overlap those of the metavolcanic and metavolcaniclastic rocks, thus making them difficult to distinguish geochemically. Whole rock geochemical analysis of the mafic intrusive rocks at McVicar Lake and the tonalitic stock which cuts them suggests that these intrusive rocks represent a high level magma chamber which acted as a feeder conduit for the overlying metavolcanic rocks. Magma mingling, assimilation, hybridization and fractional crystallization textures are visible at an outcrop scale throughout the mineralogically and geochemically diverse pluton, supporting the proposed genetic relationship between the local intrusive and extrusive igneous rocks. 60 �Proceedings of the 61st ILSG Annual Meeting - Part 1 The supracrustal rocks are surrounded and intruded by calc-alkalic, magnesian, peraluminous to metaluminous biotite tonalite to granodiorite. A pluton of alkali-calcic syenodiorite to quartz syenite of unknown age is hosted by volcaniclastic rocks between Lang Lake and McVicar Lake, and contains trace element concentrations indicative of a metasomatised mantle source. In contrast the late alkalic Otoskwin pluton (granodiorite to gabbronorite), just northeast of the greenstone belt, and several intermediate alkalic dikes, appear to have been derived from an unaltered mantle source. Four phases of deformation have been identified within the greenstone belt, including i) one purely compressional event that occurred during emplacement of the surrounding granodioritic intrusions, ii) a sinistral transpressional event which produces the S-asymmetry observed throughout the belt and within the surrounding intrusive rocks, iii) a dextral transpressional event associated with movement along the northwest trending Bear Head Shear Zone at the west end of the belt, and finally another iv) compressional event which formed a series of discrete NNEtrending shear zones that offset all of the previous structural features. This mapping project has provided some broad insights into the petrogenesis and structural history of the Lang Lake greenstone belt. The belt contains abundant prospects for further academic study which would help relate this belt to others within the Uchi province, provide new insights into Archean igneous processes, and aid in the pursuit of economic gold mineralization. REFERENCES Fenwick, K.G. 1970. Lang–Cannon lakes area (west half); Ontario Geological Survey, Preliminary Map P.581, scale 1:31 680. Fenwick, K.G. 1971. Lang–Cannon lakes area (central part); Ontario Geological Survey, Preliminary Map P.665, scale 1:31 680. Fenwick, K.G. and Srivastava, P. 1972. Lang–Cannon lakes area (eastern part); Ontario Geological Survey, Preliminary Map P.738, scale 1:31 680. Laird, H.C. 1930. Shonia Lake area, District of Kenora (Patricia Portion), Ontario; Ontario Geological Survey, Map 39d, scale 1:63 360. Sage, R.P. and Breaks, F.W. 1982. Geology of the Cat Lake–Pickle Lake area, districts of Kenora and Thunder Bay; Ontario Geological Survey, Report 207, 238p. Stott, G.M. and Corfu, F. 1991. Uchi Subprovince; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 1, p.145-238. 61 �Proceedings of the 61st ILSG Annual Meeting - Part 1 The Eagle Mine in Production: U.S.A.’s Only Primary Nickel Producer MAHIN, Robert Eagle Mine LLC, Exploration Department, 200 Echelon Drive, Negaunee, MI 49866 Lundin Mining Corporation’s Eagle Mine is located in Marquette County in the Upper Peninsula of Michigan. The Eagle deposit is an ultramafic-intrusive-hosted high grade Ni-Cu deposit, with associated cobalt, platinum, palladium, silver and gold. Published (2014) P&P reserves for Eagle are 5.2MT @ 3.11% NI and 2.55% Cu. Lundin acquired the partially developed project from Rio Tinto in July, 2013. Capital expenditure, including purchase and construction completion are $770M. As of Q1 2015, full commercial production was achieved significantly ahead of schedule. Annual production over the first three full years (2015 - 2017) is expected to average approximately 23,000 tonnes of nickel and 20,000 tonnes of copper per annum, with additional byproduct credits of precious metals and cobalt. The orebody is accessed from surface by a 1 mile long, 13% grade decline. The mine employs longhole open stoping. The majority of the stopes will be mined as transverse bench and fill stopes, with some thinner zones mined as longitudinal retreat stopes. Stope drilling is accomplished by top hammer vertical drilling from the top sill cut with a breakthrough to the bottom sill cut. Stope dimensions are 10 meters wide by 18 to 29 meters high sill to sill. The stope lengths vary with the thickness of the orebody (15 to 85 meters). The sill will be cut the full width of the stopes at 10 meters wide and 5 meters high. Level spacing varies between 18-29 meters and there are nine mining horizons. Stopes will be mined from the bottom up in an alternating sequence of primary and secondary stopes with cemented rock fill in the primaries and rock fill in the secondary stopes. After mining the uppermost stopes, backfill will be placed tight to the stope backs with a jammer to prevent subsidence. Approximately forty-five truckloads per day deliver ore 65 miles to the Humboldt Mill. The mill is a renovated pellet processing plant with a capacity of 2000 tonne per day. Conventional flotation produces separate nickel and copper concentrates with approximate recoveries of 82% Ni and 93% Cu. Flotation tailings are thickened and deposited subaqueously into the flooded Humboldt open pit. Concentrates are railed directly to either Canadian smelters or to port for overseas shipping. The average life of mine (8 years) production of 17 ktpa Ni and 17ktpa Cu is expected at cash costs (excl. royalties) of approximately $2.50/lb Ni. Since inception the company has striven for open and transparent communication with the community. Eagle has committed to a 75% local hire goal, created programs to support small business development, and hosts semiannual town hall meetings. In addition, the company helped develop a precedent-setting third party community environmental monitoring program (CEMP). The CEMP provides independent verification monitoring for the Eagle Mine, Humboldt Mill, and transportation route. Full-time employment, including long-term primary contractors will be approximately 330. Over the 13 year life of the mine, including construction and closure, the economic impact for Marquette County is predicted to be on the order of $4 billion. Eagle is actively exploring for additional mineralization. Exploration is largely geology driven and based on an open-system magma conduit (chonolith) model. Efforts have focused on identifying and tracing the feeder dikes to Eagle and a sister intrusion, Eagle East (a.k.a. the Yellowdog Peridotite). In 2014, exploration successfully intersected significant mineralization in what is interpreted as the feeder dike to Eagle East. 62 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Evaporated seawater formed sediment-hosted stratiform copper orebodies and second-stage copper mineralization in the Mesoproterozoic Nonesuch Formation of the Midcontinent Rift MAUK, Jeffrey L., EMSBO, Poul, and THEODORAKOS, Peter U.S. Geological Survey, MS-973 Denver Federal Center, P O Box 25046, Denver, CO 80225-0046 The Mesoproterozoic North American Midcontinent Rift System contains sediment hosted stratiform Cu deposits at White Pine and Copperwood, which combined host at least 4 Mt Cu and 75 Moz Ag (Nicholson et al., 1992; Bornhorst and Williams, 2013). Main stage sediment-hosted Cu mineralization formed at both deposits during diagenesis at temperatures near or cooler than 100°C. White Pine also contains structurally controlled second-stage mineralization that was likely synchronous with Keweenaw Peninsula native Cu mineralization (Mauk et al., 1992). Here, we report chemical data from fluid inclusions from main- and second-stage mineralization at White Pine to constrain the origin of these mineralizing fluids. We measured the solute compositions of ore-forming brines from fluid inclusions in mainstage chalcocite, and second-stage calcite and chalcocite from the White Pine deposit. Fluid inclusions were extracted from 100-600 mg of calcite and chalcocite, and analyzed for Na+1, NH4+1, Ca+2, Mg+2, K+1, Rb+1, Sr+2, Ba+2, Cl-1, Br-1, F-1, S2O3-2, SO4-2, and acetate using the ion chromatography methods described by Viets et al. (1996). The Cl-Br-Na data from main- and second-stage minerals from White Pine plot in a relatively small compositional field, with Cl/Br molar ratios that are less than 300, and Na/Cl molar ratios that are less than 0.3. Main- and second-stage fluids may occupy slightly different fields, but this minor difference may only be apparent due to the relatively few analyses of mainstage chalcocite. The Cl-Br-Na data plot close to or along the seawater evaporation curve with no evidence for the dissolution of salt or input from non-marine brines, which would have much greater Cl/Br ratios (Fig. 1). The Na/Cl molar ratios are distinctly depleted compared to those of most basinal fluids worldwide, suggesting that the brines evolved significantly beyond halite precipitation, and approached Mg- and K-salt saturation. Terrestrial fluvial environments played a key role in the sediment deposition that followed the volcanic phase of the Midcontinent Rift, but debate continues on whether the fine-grained clastic sedimentary rocks of the Nonesuch Formation, which host the White Pine and Copperwood deposits, formed in a marine or lacustrine environment (e.g., Cumming et al., 2013, and references therein). Our data require seawater that evaporated to the point of salt deposition, which supports a marine depositional environment for the Nonesuch Formation. Furthermore, the evaporation of seawater beyond halite saturation required by our data, plus the enormous volume of brine required to form the Cu deposits, would require a significant evaporite basin filled with gypsum, halite, and potentially even bittern salts occurred somewhere in the rift basin. If preserved, the most likely location of this thick evaporite sequence was in the thickest and deepest axial portion of the rift, which lies under present-day Lake Superior. The similar composition of main- and second-stage brines raises the intriguing possibility that these two stages of mineralization, despite apparently being separated by nearly 60 m.y. (Ohr, 1993), formed from the same brine. If so, a large basin was required to store the large volume of brine necessary to form second-stage Cu at White Pine. 63 �Proceedings of the 61st ILSG Annual Meeting - Part 1 $#!!" ,+"-& !"#$%& .&/01000& *+,-./0+12" /234-5./0+12" -4-.462"3+674-+02" 82+9+026" !"#$%&'()& $!!!" #!!" 4-" 46+= :;+< !" !%!" !%&" !%'" !%(" *+#!"&'()& !%)" $%!" Figure 1: Cl/Br versus Na/Cl molar ratios for main-stage chalcocite and second-stage calcite and chalcocite from the White Pine deposit. Integrating these results with current understanding of basin architecture and location of deposits may provide new insights into why some areas of the rift produced world-class deposits and other segments are barren. Furthermore, analyses of minerals from the Keweenaw native Cu deposits, which presumably formed synchronously with White Pine second-stage mineralization (Mauk et al., 1992; Bornhorst, 1997), could test whether these large Cu endowments formed from the same fluid, or whether different brines produced mineralization in different portions of the rift. REFERENCES Bornhorst, T. J., 1997, Tectonic context of native copper deposits of the North American Midcontinent Rift System: Geological Society of America Special Papers, v. 312, p. 127-136. Bornhorst, T. J., and Williams, W. C., 2013, The Mesoproterozoic Copperwood sedimentary rock-hosted stratiform copper deposit, Upper Peninsula, Michigan: Economic Geology, v. 108, p. 1325-1346. Cumming, V. M., Poulton, S. W., Rooney, A. D., and Selby, D., 2013, Anoxia in the terrestrial environment during the late Mesoproterozoic: Geology, v. 41, p. 583-586. Mauk, J. L., Kelly, W. C., van der Pluijm, B. A., and Seasor, R. W., 1992, Relationships between deformation and sediment-hosted copper mineralization: Evidence from the White Pine portion of the Midcontinent rift system: Geology, v. 20, p. 427-430. 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. Ohr, M., 1993, Geochronology of diagenesis and low-grade metamorphism in pelites: Unpub. PhD thesis, University of Michigan., 147 p. Viets, J., Hofstra, A. H., and Emsbo, P., 1996, Solute composition of fluid inclusions in sphalerite from North America and European Mississippi Valley-type ore deposits: Ore fluid derived from evaporated seawater, in Sangster, D. F., ed., Carbonate-Hosted Lead-Zinc Deposits, Society of Economic Geologists Special Publication 4 p. 465483. 64 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Sedimentology and Geochemistry of a 1.4 Ga Continental Playa System, the Lower Sibley Group, Northwestern Ontario: Implications for the Mesoproterozoic Hydrosphere and Atmosphere METSARANTA, Riku T.1 and FRALICK, Philip2 1 Ontario Ministry of Northern Development and Mines, Sudbury, Ontario, Canada, 2 Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada, P7B 5E1 The 900 m thick Sibley Group consists of playa to deltaic to aeolian deposits outcropping north of Lake Superior and east of Thunder Bay. The lowermost 100 m thick succession of highly oxidized siliciclastic rocks and dolostone was deposited in a north-south trending half graben. The sediments can be divided into 15 lithofacies associations representing distinct depositional environments. The lower siliciclastic unit contains: boulder conglomerate-sandstone-dolocrete (proximal ephemeral braided stream), pebble to cobble conglomerate (ephemeral braided stream), trough cross-stratified sandstone (braided stream), green sandstone-siltstone (delta), massive cobble conglomerate (transgressive shoreline lag), planar cross-stratified sandstones (nearshore lacustrine sandwaves), and thinning-upward sandstones (lacustrine storm sand sheets). The overlying mixed siliciclastic-carbonate unit contains: red siltstone (non-saline lake), red siltstonedolostone or dolomitic sandstone (saline lake), and halite-mudstone (ephemeral salt pans). Next is the upper siliciclastic unit with: sheet sandstones (lake infilling) and stromatolitic dolostone-chert (shoreline). After final desiccation of the lake terra rosa soils, collapse breccias and intraformational conglomerates developed. Paleocurrents, detrital zircon geochronology and sulfur isotopes indicate a change in drainage directions resulting in sand sheets infilling the saline lake. Sr isotopes reflect shallow groundwater circulation and lacustrine dolostone containing significant radiogenic Sr. Carbon and O isotopes are heavier upward in the saline lake deposits, probably due to evaporation and residence time effects. Most interestingly, REE patterns for dolomite in the dolocrete, stromatolitic shoreline deposits and overlying intraformational conglomerates have patterns similar to modern oxygenated groundwater, whereas the saline lake dolomites have hatshaped patterns resembling modern groundwater draining waterlogged, organic-rich areas. 65 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Role of felsic and feldpathic rocks in triggering subvolcanic emplacement of mafic intrusions: evidence from the Midcontinent Rift in northeastern Minnesota MILLER, James D. Dept. of Earth and Environmental Sciences and Precambrian Research Center, University of Minnesota Duluth, Duluth, MN 55812 Subvolcanic mafic intrusions make up over 40% of the igneous rocks of NE Minnesota associated with the 1.1Ga Midcontinent Rift. The greatest concentration of mafic intrusions comprise the enormous (~20,000 km3) Duluth Complex, which was emplaced into the basal section of a 5-10 km thick edifice of comagmatic volcanics. Most Duluth Complex intrusions, and mafic intrusions emplaced higher in the volcanic pile, occur as sheet-like bodies that commonly underlie either felsic rocks (rhyolite flows or granophyre intrusions) or feldspathic rocks (gabbroic to troctolitic anorthosite). In all cases, field relationships indicate that the felsic/feldspathic rocks are consistently older than the mafic rocks that underlie them. Three styles of mafic underplating are recognized in the Midcontinent Rift of northeastern Minnesota (Fig. 1): 1) Mafic layered intrusions beneath large granophyre bodies. Examples of this style of underplating are the Poplar Lake Intrusion beneath the Misquah Hill Granophyre in the Gunflint Trail area; the Sawbill Lake intrusion beneath the Eagle Mountain Granophyre, and the Sonju Lake Intusion beneath the Finland Granite. In all three cases, the mafic intrusion is well differentiated and in gradational contact with the overlying granophyre. The gradational contact is best explained by partial melting of the base of the granophyre and subsequent assimilation. 2) Mafic layered intrusions beneath anorthositic rocks. This style of mafic underplating is evident in almost all Layered Series intrusions of the Duluth Complex which are intruded beneath Anorthsotic Series rocks (Fig. 1). The Layered Series intrusions that will be highlighted in this talk include the Layered Series at Duluth, the Partridge River Intrusion, and the Tuscarora Intrusion. 3) Diabase sheets beneath thick rhyolite flows. Examples of this style of mafic underplating include the Endion Sill beneath the Tischer Creek Rhyolite and the Lester River Sill beneath the Lakewood Rhyolite, both in the Duluth area, the Beaver River Diabase beneath the Palisade Head Rhyolite in the Beaver Bay Complex, and the Mink Mountain diabase emplaced beneath and into the Grand Marais Felsites. Empirical evidence and density/viscosity/thermal considerations suggest that the felsic/feldspathic rocks served as density barriers to mafic magmas, which had risen into the upper crust to the point of neutral bouyancy. The felsic/feldspathic rocks not only triggered underplating of the mafic magmas, but also commonly served as thermal insulators to the underplated mafic bodies, thus resulting in their slow cooling and crystallization differentiation. 66 �Proceedings of the 61st ILSG Annual Meeting - Part 1 67 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Geology of the North and South Temperance Lakes area of the Boundary Waters Canoe Area, Cook County, Minnesota - 2014 Precambrian field camp capstone mapping MILLER, Jim, BEAVER, Christopher, HAHN Timothy, MILLER Nikolas, PULIESE Joseph, and WRIGHT, Erick Precambrian Research Center, University of Minnesota Duluth, Duluth, MN 55812 As a capstone mapping project for the 2014 Precambrian field camp, a crew of five students under the supervision of Jim Miller conducted five days of field mapping bedrock geology in the South and North Temperance Lakes area. This area is located in the Boundary Waters Canoe Area west of Brule Lake in Cook County, Minnesota. The area is accessible from the Caribou Trail, which heads north from Tofte to a canoe landing on Brule Lake, and then via a 5-mile paddle to the west end of Brule Lake and a portage into South Temperance Lake. The main objective of this project was to conduct bedrock geologic mapping of rocks that previous capstone mapping has shown to comprise part of the footwall to the Sawbill Lake intrusion (Brooker and Miller, 2013). Previous studies of the Temperance Lakes area include reconnaissance mapping by Grout et al. (1959) and Davidson (1977). Grout’s (~1:100,000-scale) township maps of the area (Figures XXIII and XXIV, Grout et al., 1959) show it to contain granophyric granite and mafic volcanic rocks intruded by gabbro. Davidson’s (1:24,000-scale) reconnaissance map of the Cherokee Lake 7.5’ quadrangle show a similar mix of rock types in the Temperance Lakes area, but he subdivides the gabbroic rocks into an olivine gabbro unit and an anorthositic gabbro unit. The latter unit, Davidson correlates to the anorthositic series of the Duluth Complex. Capstone mapping conducted for UMD’s Precambrian field camp in the summers of 2007 (Frost et al., 2007), 2009 (Blakely et al., 2009), 2010 (Brooker et al., 2010), and 2011 (Asp et al., 2011) and field mapping conducted by Ben Brooker as part of his MS thesis at UMD in 2011 (MS, in preparation) revealed the existence of well differentiated, mafic layered intrusion which has been named the Sawbill Lake intrusion (SbLI; Brooker and Miller, 2012, 2013). Mapping of the lower contact of the SbLI showed is lower troctolitic cumulates were in contact with a footwall of evolved ferrodioritic cumulates that locally contain hornfels basalt inclusions. This observation and aeromagnetic data (Chandler, 1983) showing curvilinear anomalies in the footwall rocks that are conformable to similar anomalies internal to the SbLI suggest that another well differentiated mafic layered intrusion might exist beneath the SbLI. The 2014 capstone mapping project focused mapping shoreline exposures in the South and North Temperance Lakes which covers a 3 km wide by 4 km tall area extending north from the basal contact of the SbLI. The result of this mapping clearly shows that a well differentiated tholeitic mafic layered intrusion indeed is situated conformably beneath the SbLI. This as yet unnamed intrusion can be subdivided into seven distinct units based on dominant lithology, internal structure and position within the intrusion. Internal structure within the intrusion (layering and igneous foliation dips 20-40°) to the south. The base of the intrusion is exposed in the northern part of North Temperance Lake where a fine- to medium-grained, locally plagioclase-phyric, subophitic to ophitic olivine diabase is found in intrusive contact with granophryric leucogranite of the Misquah Hills granophyre – part of Duluth Complex Felsic Series. The diabase cuts the granophyre in an orthogonal pattern of N-S/EW dikes and shows chilled contacts. In the southern part of North Temperance Lake, the olivine diabase grades into an ophitic augite troctolite that locally displays moderate foliation and layering defined by augite oikocryst concentrations. This Pl+Ol cumulate persists upsection into the northern part of South Temperance Lake. In the mid-section of South Temperance Lake, a 68 �Proceedings of the 61st ILSG Annual Meeting - Part 1 troctolite-olivine gabbro transitional unit is defined by the intermittent (cyclical?) occurrence of cumulus augite and Fe-Ti oxide. The southern part of South Temperance Lake is dominated by a medium-grained, locally layered, moderately to well foliated, intergranular olivine oxide gabbro – a Pl+Cpx+Ox+Ol cumulate. Along the portage trail following the Temperance River south of South Temperance Lake, the olivine oxide gabbro transitions into an apatite ferrodiorite (Pl+Cpx+Ox+Ol+Ap cumulate), which in turn grades into a ferromonzodiorite with abundant basaltic hornfels inclusions. At the south end of the portage trail, the ferromonzodiorite abruptly transitions into a fine-grained, subophitic olivine diabase, which is the base of the overlying Sawbill Lake Intrusion. This olivine diabase is also observed to cross cut the upper three units of the Temperance Lakes sequence, clearly indicating that the Sawbill Lake was emplaced later by overplating the Temperance Lake sequence. Plans for the 2015 capstone mapping project are to follow the igneous stratigraphy defined in the Temperance Lakes area to the west and north into the Cherokee Lake area. REFERENCES Asp, K., Leu, A., Parisi, A., Sletten, D., Brooker, B., Miller, J., 2011, Bedrock geology of the Sawbill Lake area: University of Minnesota Duluth, Precambrian Research Center, PRC/MAP-2011-04, 1: 12,000. Blakely, S., Brown, A., Foley, D., Rowland, A., Stifter, E., and Miller, J., 2009, Bedrock geology map of Homer Lake and adjacent areas; Cook County, Northeastern Minnesota: University of Minnesota Duluth, Precambrian Research Center, PRC/MAP-2009-01, 1: 12,000. Brooker, B.P., and Miller, J.D., 2013, Bedrock geologic map of the Sawbill Lake Intrusion, Cook County, MN. Precambrian Research Center Map Series PRC/Map-2013-01, scale 1:24,000. Brooker, B.P., and Miller, J.D., 2012, Geology and petrology of a Mesoproterozoic layered mafic intrusion in portions of the Brule Lake and Cherokee Lake 7.5’ Quadrangles, northeastern Minnesota. Institute on Lake Superior Geology Proceedings, 58th Annual Meeting, Thunder Bay, Ontario, Part 1 - Proceedings and Abstracts, v. 58, part 1, 15-16. Brooker, B.P.,Hadley, M.L., Markwood, L.W., Olson, J., Tomlinson, A.P., and Miller, J.D.,2010, Bedrock geologic map of the Jack Lake and Weird Lake areas, Cook County, northeastern Minnesota: University of Minnesota Duluth, Precambrian Research Center, PRC/Map-2010-05, 1: 12,000. Chandler, Val W, 1983, Aeromagnetic map of Minnesota, Cook and Lake counties: Minnesota Geological Survey, Aeromagnetic Map Series, Map A-1, scale 1:250,000 Davidson, D.M., 1977, Reconnaissance geologic map of the Cherokee Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-30, scale 1:24,000 Frost, S.J., Juda, N.A., and Miller, J., 2007, Bedrock Geology Map of Homer Lake and Adjacent Areas; Cook County, Northeastern Minnesota: University of Minnesota Duluth, Precambrian Research Center, PRC/MAP-2007-02, 1: 12,000 Grout, F.F., Sharp, R.P., and Schwartz, G.M., 1959, The geology of Cook County, Minnesota: Minnesota Geological Survey Bulletin 39, 163 p. 69 �Proceedings of the 61st ILSG Annual Meeting - Part 1 The mineralogy and petrology of a newly-discovered REE occurrence within the Coldwell Complex near Marathon, Ontario NIKKILA, D. and Zurevinski, S. Dept. of Geology, Lakehead University, Thunder Bay, ON P7B 5E1 The Coldwell Complex is situated within the Archean Schreiber-White River metavolcanicmetasediment of the Superior Province. Spanning over 25 km in diameter, it is the largest alkaline intrusion in North America (Figure 1). The 1108 +/- 1 Ma age of the Coldwell complex and close spatial proximity supports a strong relationship to the magmatism of the Keweenawan Midcontinent Rift (Heaman and Machado 1992). Early studies define three magmatic centers of the Coldwell Complex, which in order of intrusion are Center I, Center II and Center III (Mitchell and Platt 1982). Center I consists of an oldest phase gabbro, which borders a ferroaugite syenite to the east and north. Center II includes a nepheline-bearing biotite-gabbro and several intrusions of nepheline syenites, and Center III is composed of four syenites which in order of intrusion are: magnesiohornblende syenite, contaminated ferroedenite syenite, ferroedenite syenite, and quartz syenite. .000000 510000 .000000 520000 .000000 530000 .000000 540000 .000000 550000 .000000 .000000 5423000 .000000 $ 5414000 5414000 .000000 5423000 .000000 5432000 500000 .000000 .000000 5432000 .000000 490000 Radio Hiill .000000 Terrace Bay ! . ! . . !! . 5396000 .000000 Mink Creek ! P 490000 0 2.5 5 .000000 10 500000 15 .000000 20 510000 25 .000000 520000 .000000 530000 .000000 540000 .000000 Marathon 550000 .000000 Frog ! P 5396000 5405000 ! P 5405000 .000000 ! . .000000 30 Km Figure 1: Regional geology of the Coldwell Complex (purple). As part of an undergraduate thesis project, the focus of this study was to classify syenite rocks related to the three intrusive centers, and identify any REE-bearing minerals present. Fieldwork was completed along HW-17 roadcuts, and North of the highway on Canada Rare Earth claim blocks, termed the ‘Radio Hill’ occurrence (Figure 1). The syenites of the Radio Hill occurrence had not previously been identified due to limited access to the area. Through petrography, Highway-17 samples were classified as Center II Nepheline syenites, where the 70 �Proceedings of the 61st ILSG Annual Meeting - Part 1 textures of amphibole, biotite, pyroxene and natrolite compared well to the nephelene syenites previously described by Mitchell and Platt (1982). Syenites of the Radio Hill occurrence were classified as Center III, specifically, ferroedenite syenites. The Radio Hill syenites show an increase in the modal abundance of quartz, and a decrease in natrolite. Compositions of the amphiboles from the Radio Hill syenites compare well with the silicic ferroedenite and hastingsitic hornblende compositions, with a trend to Na, Si, and Fe enrichment with Ca and Al depletion. Radio Hill mica has been identified as annite end-member compositions, with Mg # ranging from 0.082 to 0.294. Radio Hill plagioclase feldspar compositions show An % from 0 to 12.04 %, representing albite to oligioclase end-members. Rare earth element minerals were described and identified from the Radio Hill occurrence using qualitative identification methods with the scanning electron microscope (SEM-EDX) at Lakehead University. Minerals found occurring in the Radio Hill syenites, in order of abundance, include apatite (elevated La, Ce, Nd, and Th), plumbopyrochlore ((Pb, Y, U, Ca)2-xNb2O6(OH)), ceriopyrochlore ((Ce, Ca, Y)2(Nb, Ta)2O6(OH, F)), monazite ((La, Ce, Nd)PO4), and fluorite. REFERENCES 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. Mitchell, R.H., Platt, R.G., Lukosius-Sanders, J., Artist-Downey, M. and Moogk-Pickard, S. 1993. Petrology of syenites from centre III of the Coldwell alkaline complex, northwestern Ontario, Canada; Canadian Journal of Earth Sciences, v. 30, p. 145-158. Mitchell, R.H. and Platt, R.G. 1982. Mineralogy and Petrology of Nepheline Syenites from the Coldwell Alkaline Complex, Ontario, Canada; Journal of Petrology, v. 23, p. 186-214. 71 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Petrology, geochemistry and mineral chemistry of the Crystal Lake and Mount Mollie mafic intrusions, northwestern Ontario O’BRIEN, Sean1, HOLLINGS, Peter1, and MILLER, Jim2 1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1 Canada 2 Department of Geological Sciences, University of Minnesota Duluth, 1049 University Drive, Duluth, MN 55812, United States The Midcontinent Rift (MCR) extends ~2500 km through Canada and the United States, and comprises ~1,500,000 km3 of volcanic and intrusive rocks spanning four distinct stages of activity ranging from 1150-1087 Ma (Heaman et al., 2007). The 1108-1105 and 1100-1094 Ma periods has been interpreted by Heaman et al. (2007), to represent the main formation and maturation of the rift system and is associated with the majority of the igneous activity, producing mafic to ultramafic intrusions, basaltic sills, dikes and flows as well as alkaline rocks. The extent and volume of magmatic activity has led previous researchers to conclude that a plume was most likely the cause of the MCR (Miller and Nicholson, 2013). In this study, two mafic intrusions related to the MCR will be investigated using detailed petrography, geochemistry and mineral chemistry. The two intrusions, Crystal Lake and Mount Mollie, are located ~40 km south of Thunder Bay, Ontario and are located within a few km of each other. Crystal Lake is a Y-shaped layered intrusion with a north limb striking W-NW for 5 km and a south limb striking E-NE for 2.75 km. Mount Mollie varies from 60 to 350 m wide and extends for ~35 km, and is located just east of the Crystal Lake intrusion (Fig. 1). The intrusions have been targets for exploration for the past few decades as they both contain disseminated sulphides and are host to Ni-Cu-PGE mineralization (Smith and Sutcliffe, 1987; Lightfoot and Lavigne, 1995). The two units are very similar, consisting mainly of gabbro, troctolite and olivine gabbro with some disseminated sulphides, chromite and other spinels. The close spatial relationship led to the belief that the intrusions were related and contemporaneous (Lightfoot and Lavigne 1995), however, recent geochronology has shown that the Mount Mollie intrusion has an age of 1109.3 ± 6.3 Ma whereas Crystal Lake has been dated at 1099.6 ± 1.2 Ma (Hollings et al., 2010). Figure 1. Generalized map of Crystal Lake gabbro, Mount Mollie dike and surrounding rocks, adapted from Cundari (2013). 72 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Preliminary results suggest the Mount Mollie dike may in fact be contemporaneous with Crystal Lake. Core logging revealed that ~10 m of intrusive mafic rocks occur at the top of the drill core, overlaying ~20 m of sandstone, followed by gabbroic rocks. It is unclear whether the gabbros above and below the sandstone layer are directly related. With two generations of igneous activity a possibility, the temporal relationship between the two intrusions needs to be investigated further. Full descriptions of the mineralogy and textures of both intrusions, particularly the layering styles (i.e., modal, graded, and phase layering), will be completed through core logging and detailed petrography of thin sections. These results will be combined with whole-rock geochemical data to investigate fractionation trends, mixing and crustal contamination signatures to understand the evolution of the intrusions and the genesis of the mineralized horizons. Mineral chemistry of olivine and spinels will be used to determine cryptic layering. Olivine mineral chemistry is especially important as it can further constrain the evolution of the magmas, with an emphasis on the forsterite-fayalite and nickel contents, and to give insight into parental melt compositions. Spinel mineral chemistry will be used to help understand parental melt compositions as chromite is one of the first minerals to crystalize from the melt and is refractory. SEM analysis of the platinum group minerals (PGMs) will be conducted to determine textural and mineralogical associations as well as what compositional varieties are present, (i.e. alloys, sulphides, arsenides, etc.). This detailed study will allow us to build a model for how these two intrusions formed, how they fit into the Midcontinent Rift, and what the style of mineralization is. REFERENCES Cundari, R.S., Smyk, M., Campbell, D. and Puumala, M., 2013. Geology, Geochemistry and Cu-Ni-PGE Mineralization of the Crystal Lake Gabbro, 6th Annual PRC Professional Workshop Cu-Ni-PGE Deposits of the Lake Superior Region, Duluth, Minnesota. Heaman, L., Easton, R., Hart, T., Hollings, P., MacDonald, C. and Smyk, M., 2007. Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon region, Ontario. Canadian Journal of Earth Sciences, 44: 10551086. Hollings, P., Smyk, M., Heaman, L.M. and Halls, H., 2010. The geochemistry, geochronology and paleomagnetism of dikes and sills associated with the Mesoproterozoic Midcontinent Rift near Thunder Bay, Ontario, Canada. Precambrian Research, 183: 553-571. Lightfoot, P.C. and Lavigne, Jr., M.J., 1995. Nickel, copper, and platinum group element mineralization in Keweenawan intrusive rocks: new targets in the Keweenawan of the Thunder Bay region, northwestern Ontario; Ontario Geological Survey, Open File Report 5928, 32p. Miller, J., and Nicholson, S., 2013, Geology and mineral deposits of the 1.1Ga Midcontinent Rift in the Lake Superior region: an overview, in field guide to copper-nickel-platinum group element deposits of the Lake Superior region. Precambrian Research Center, Guidebook, 13: 1-50. Smith, A.R., and Sutcliffe, R.H., 1987. Keweenawan intrusive rocks of the Thunder Bay area. in Summary of Field Work, 1987. Ontario Geological Survey, Miscellaneous Paper 137: 248-255. 73 �Proceedings of the 61st ILSG Annual Meeting - Part 1 So, an Environmental Impact Statement is required: Some Lake Superior area Geologic Parameters for Geologists, Consultants, Companies, and Regulators PETERSON, Dean1 1 Peterson Geoscience LLC, 306 West Superior Street, Suite 410, Duluth, Minnesota, 55802. Since the birth of the environmental movement in the early 1970s, modern society has evolved into an “environmentally concerned world”, where the public demands that industry (herein the mining industry) reduce its physical, social and environmental footprint have translated into specific national legislation. Under United States environmental law, an environmental impact statement (EIS) is a document required by the National Environmental Policy Act (NEPA) for certain actions "significantly affecting the quality of the human environment". Similar Canadian environmental law documents are required by the Canadian Environmental Assessment Act. An EIS type document is a project specific tool for decision making, and typically includes four sections: (1) an introductory statement of the Purpose and Need of the Proposed Action, (2) a description of the Affected Environment, (3) a Range of Alternatives to the proposed action, and (4) an Analysis of the environmental of each of the possible alternatives. In the Lake Superior area, certain environmental groups vehemently oppose all activities related to the mining industry and actively coordinate their opposition to the general public through meetings, web sites, social media, and the press. Most coordinated opposition ties directly into published EIS documents in general, and specifically on the Analysis of water quality for the proposed actions. To the author, environmental misinformation, half-truths, and outright lies constitute much of the anti-mining opposition. Have you read about “Sulfide Mining”, acid mine drainage, degraded water quality, highly fractured bedrock, canoeing to the white house, the 500 years, etc…..? So, what are field geologists to do? Geologists must first recognize and defend the fact that the basis for every type of geologic study related to an EIS is fundamentally rooted in observations made of rocks in their natural habitat, “in the field”. An intimate understanding of the projects geology gives company geologists an appreciation of the coherent and compelling field-based scientific arguments from which all other geology-based EIS interpretations grow. Company geologists working on mining related projects have to interact with a diverse group of people during the EIS process, and must vigorously defend the fundamental observations of the projects “in the field” geology. Geologists have to ask themselves this question: Do the lawyers, PR folks, environmental consulting firms, regulators, NGO’s, and the general public know the details of the projects geology as much as I do? As geologists, we must always remember Francis Pettijohn’s famous quote, “The rocks are the final court of appeal”. This talk will highlight some fundamental geologic parameters of the Lake Superior area that geologists, consultants, companies, and regulators need to understand to better design and complete mining related EIS documents. 74 �Proceedings of the 61st ILSG Annual Meeting - Part 1 New airborne geophysical data for the Lake Superior Region of northwestern Ontario: A new tool for the identification of Neoarchean to Mesoproterozoic structures and associated mafic-ultramafic intrusions PUUMALA, Mark1, CUNDARI, Rob1, CAMPBELL, Dorothy1, RAINSFORD, Desmond2, and METSARANTA, Riku2 1 Ontario Geological Survey, Ministry of Northern Development and Mines, Resident Geologist Program, Suite B002, 435 James St. South Thunder Bay, ON, P7E 6S7, Canada 2 Ontario Geological Survey, Ministry of Northern Development and Mines, Earth Resources and Geoscience Mapping Section, 933 Ramsay Lake Road, Sudbury, ON, P3E 6B5, Canada. Recent discoveries of mafic-ultramafic intrusion hosted Ni-Cu-PGE mineralization throughout the Midcontinent Rift (MCR) region (e.g. in Canada: Current Lake, Sunday Lake, Thunder, Steepledge, Marathon; in the U.S.A. Tamarack, Eagle) have revitalized exploration interest in the area surrounding Lake Superior. Airborne magnetic and radiometric surveys that were flown during 2014 (Figure 1; Ontario Geological Survey, 2015a, b) have provided new highresolution public domain airborne geophysical coverage that will assist in these exploration efforts. The new surveys cover a large portion of the northwest Lake Superior region in Ontario including areas underlain by MCR-related rocks, Paleoproterozoic and Mesoproterozoic sedimentary rocks of the Animikie and Sibley Groups, as well as parts of the Archean Quetico, Wawa and Wabigoon subprovinces. In the Lake Superior region, Ni-Cu-PGE mineralized mafic-to-ultramafic rocks were emplaced in a variety of settings during several tectonic events that occurred over a time period extending from the Mesoarchean to the Mesoproterozoic (Smyk et al. 2002, Smyk and Franklin 2007). As a result, these intrusions display a wide range of geochemical affinities and morphologies. In spite of these geochemical and physical differences, most of these mafic-toultramafic intrusions have a close spatial association with major crustal-scale structures (Rogers et al. 1995, Hart and MacDonald 2007), and many can be recognized by their distinctive magnetic signatures (i.e., positive or negative anomalies). When the new airborne survey magnetic data are combined with data from previous magnetic surveys (Ontario Geological Survey 2003, 2004), they highlight numerous structures that could have controlled the emplacement of mineralized MCR-related intrusions into Archean country rocks along the northwest margins of the rift north of Thunder Bay. One such structure is marked by several magnetic discontinuities and anomalies that can be traced for at least 145 km along an east-northeast (063°) trending line from the southeast end of Northern Light Lake (near the Ontario-Minnesota border), through to Greenwich Lake (50 km northeast of Thunder Bay). This structure is approximately parallel to Midcontinent Rift-related faults and dikes that are located farther to the southeast (Sutcliffe 1991) and Neoarchean faults that have been mapped to the northwest (Hart and MacDonald 2007). A second parallel structure is also evident in the magnetic data approximately 10 km further to the south-southeast. Ni-Cu-PGE mineralized mafic-ultramafic rocks of the Sunday Lake, Steepledge Lake and Current Lake intrusive complexes occur in a linear array that closely follows the Northern LightGreenwich Lakes structure (NLGLS), suggesting that it may have played a role in their emplacement. The NLGLS, which has been mapped as a fault over a portion of its length (Lodge et al. 2014), is also located in close proximity to Neoarchean gold (Tower Mountain) and komatiite-hosted Ni-Cu-PGE (Bateman Lake) mineralization in Conmee Township. This observation, together with its proximity to Mesoproterozoic and Neoarchean faults of similar orientations, suggests that the NLGLS may have been tectonically active both during the accretion 75 �Proceedings of the 61st ILSG Annual Meeting - Part 1 of the Superior Craton, and during the Midcontinent Rift event. As a result, it presents an attractive target for both Ni-Cu-PGE and gold exploration. Preliminary observations indicate that the NLGLS may continue further to the northeast, where it eventually merges with the Gravel River fault (GRF) approximately 60 km north of Terrace Bay). The GRF extends northeast along approximately the same trend to the James Bay Basin (Williams 1991), and it is also interesting to note its close spatial association with the Albany Graphite deposit. Figure 1. Location of new airborne geophysical surveys carried out over the Thunder Bay region during 2014 (geology from Ontario Geological Survey 2011). The data were released during the winter and spring of 2015 as Geophysical Data Sets 1077 (Mahon & Flatrock Lakes) and 1078 (Lac des Mille Lacs – Nagagami). REFERENCES Hart, T.R. and MacDonald, C.A. 2007. Proterozoic and Archean geology of the Nipigon Embayment: Implications for emplacement of the Mesoproterozoic Nipigon diabase sills and mafic to ultramafic intrusions; Canadian Journal of Earth Sciences v.44, p.1021-1040. Lodge, R.W.D., Ratcliffe, L.M. and Walker, J.A. 2014.Geology and mineral potential of Sackville and Conmee Townships, Wawa Subprovince; in Summary of Field Work and Other Activities 2014, Ontario Geological Survey, Open File Report 6300, p.9-1 to 9-17. Ontario Geological Survey 2003. Ontario airborne geophysical surveys, magnetic data, Shebandowan area; Ontario Geological Survey, Geophysical Data Set 1021 - Revised. Ontario Geological Survey 2004. Ontario airborne geophysical surveys, magnetic and gamma-ray spectrometer data, Lake Nipigon Embayment Area; Geophysical Data Set 1047. Ontario Geological Survey 2011. 1:250 000 scale bedrock geology of Ontario; Ontario Geological Survey, Miscellaneous Release-Data 126- Revision 1. Ontario Geological Survey 2015a. Ontario airborne geophysical surveys, magnetic and gamma-ray spectrometric data, grid and profile data (ASCII and Geosoft® formats) and vector data, Mahon Lake and Flatrock Lake areas; Ontario Geological Survey, Geophysical Data Set 1077. Ontario Geological Survey 2015b. Ontario airborne geophysical surveys, magnetic and gamma-ray spectrometric data, grid and profile data (Geosoft® format) and vector data, Lac des Mille Lacs–Nagagami Lake area; Ontario Geological Survey, Geophysical Data Set 1078b. 76 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Rogers, P.C, Thurston, P.C, Fyon, J.I, Kelly, R.I. and Breaks, F.W. 1995. Descriptive mineral deposit models of metallic and industrial deposit types and related mineral potential assessment criteria; Ontario Geological Survey, Open File Report 5916, 241p. Smyk, M.C. and Franklin, J.M. 2007. A synopsis of mineral deposits in the Archean and Proterozoic rocks of the Lake Nipigon region, Thunder Bay District, Ontario; Canadian Journal of Earth Sciences v.44, p.1041-1053 Smyk, M.C., Mason, J.K., Schnieders, B.R. and Stott, G.M. 2002. A synopsis of Archean and Proterozoic platinum group element mineralization in the Thunder Bay District, Ontario; in 9th International Platinum Symposium, Billings, Montana, July 25, 2002, Extended Abstract Vol., p.433-434. Stone, D. 2010. Precambrian geology of the central Wabigoon Subprovince, northwestern Ontario; Ontario Geological Survey, Open File Report 5422, 130p. Sutcliffe, R.H. 1991. Proterozoic geology of the Lake Superior region; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 1, p.627-658. Williams, H.R. 1991. Quetico Subprovince; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 1, p.383-403. 77 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Spectrum of volcanogenic massive sulfide deposits in the Penokean Volcanic Belt, Great Lakes Region, USA QUIGLEY, Patrick* and MONECKE, Thomas Department of Geology and Geological Engineering, Colorado School of Mines, 1516 Illinois Street, Golden, Colorado 80401 The Paleoproterozoic (ca. 1880Ma) Penokean volcanic belt extends for over 250 kilometers across northern Wisconsin and western Michigan. The dominantly submarine volcanic rocks comprising the belt were formed in an island arc-related setting at the southern edge of the Superior craton. Despite relatively minor exploration, the majority of which occurred intermittently from 1960 to 1990, approximately 100 million metric tons of polymetallic massive sulfide ores have been delineated. Only the supergene enrichment zone of the Flambeau deposit has reached commercial production. The mined resource accounts for less than 2% of known mineral reserves, which makes the Penokean volcanic belt one of the most accessible, undeveloped, and underexplored volcanic terranes worldwide. The present study aims to characterize the volcanic setting, deposit characteristics, and alteration signature of significant deposits within the Penokean volcanic belt to provide a comprehensive metallogenetic model. To accomplish this goal, detailed core logging has been conducted at seven deposits across the belt, namely Back Forty, Bend, Flambeau, Horseshoe, Lynne, Reef, and Ritchie Creek. Representative sampling has been conducted at all deposits for detailed petrographic and geochemical investigation. Ongoing research has revealed a wide spectrum of volcanic environments and alteration styles across the Penokean volcanic belt. All major deposits occur within felsic-dominated volcanic successions and are hosted by vent-proximal volcanic facies associations. For example, the Back Forty deposit is hosted within a felsic succession (apparent stratigraphic thickness of 1,200 m) comprising coherent rhyolite units and associated volcanic breccias. Felsic volcanism was broadly contemporaneous with the deposition of mass-flow-derived volcaniclastic debris presumably generated by an explosive eruption of a rhyolite source. Mafic-dominated host rock successions are less common in the Penokean volcanic belt and appear to host some of the smaller tonnage deposits, including Horseshoe and Ritchie Creek. The styles of hydrothermal alteration vary between deposits, with sericite-chlorite-quartz alteration occurring at Back Forty, Bend, and Horseshoe. Acid-style alteration represented by andalusite-biotite-sericite schists has been noted at Flambeau and calc-silicate mineral assemblages are present at Lynne, Ritchie Creek, and Reef. Calc-silicate mineral associations have also been observed at the Pelican River and Spirit deposits, possibly suggesting that the volcanic host rocks were originally interbedded with limestone. Regional metamorphism varies from lower greenschist to amphibolite grade and has obscured relationships in some deposits. Most notable, primary volcanic textures are difficult to recognize at the Reef deposit, which is an unusual disseminated to quartz-sulfide vein confined Au-Cu deposit hosted by strongly deformed and recrystallized rocks. Recognition of significant variations in setting and deposit characteristics across the Penokean volcanic belt likely reflects first-order tectonostratigraphic controls during the development of the Penokean orogeny. 78 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Geochemical and petrologic characterizations of peridotite, Marquette County, Michigan SASSO, Andrew, and THAKURTA, Joyashish Department of Geosciences Western Michigan University1903 W Michigan Ave Kalamazoo MI 49008-5241 USA The discovery of the Eagle magmatic sulfide deposit, in 2002, sparked a renewed interest in exploration for magmatic sulfide mineral deposits associated with peridotite in Michigan’s Upper Peninsula. This study is a preliminary attempt to determine if other sulfide mineral deposits could potentially exist in association with the peridotites of Marquette County, Michigan. In order to achieve its goal, this study is attempting to determine if any petrologic or geochemical relationship exists between the peridotites of Marquette County, Michigan. As shown by Figure 1, peridotite has been mapped at four locations across the county: the Yellowdog Peridotite, located at the site of the Eagle Mine (Rossell and Coombes, 2005), the Presque Isle Peridotite, located in Marquette (Gair and Thaden, 1968), the Deer Lake Peridotite, located just north of Ishpeming (Clark, Cannon, and Klassner, 1975), and Black Rock Point, located north of Big Bay (Case and Gair, 1965). Field work was conducted in May, 2014 and was followed by petrographic analyses of collected samples. A peridotite rock unit could not be located at Black Rock Point by initial field work and petrographic studies. The area is composed of three major rock units. The southernmost unit is gabbro, the largest of the three units present. To the north there is an abrupt change to a heavily veined granite which is cut by at least two mafic dikes. The northern most unit of the area is a gneissic rock which has a much smaller outcrop than the other units. Thin section analysis of samples collected from Presque Isle revealed that the rock is a serpentenized lherzolite. Primary minerals include olivine, clinopyroxene, and orthopyroxene. A large portion of the rock has been altered to serpentine. Other secondary minerals such as chlorite and calcite are also present. Thin sections from samples of the Deer Lake peridotite show a fine-grained rock with a texture suggestive of hypabyssal origin, which has been heavily serpentenized. Serpentine is by far the most abundant mineral in all samples analyzed thus far. Primary minerals including olivine, clinopyroxene and orthopyroxene are present in small quantities. Other secondary minerals include chlorite, and calcite. New geochemical, petrologic, and structural data collected by this study will be included in detailed geologic maps of each site. The investigation will address the relationships of the peridotite units with respect to one another as well as their relationships with the surrounding rocks. The final results generated from this study will be useful in the creation of a new set of criteria to assess the mineralization potentials of peridotite units in Marquette County. 79 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Figure 1. Modified map of locations of the four research areas and their surrounding geology. Compiled by Simms, 1992. REFERENCES Case, James and Gair, Jacob., 1965Aeromagnetic Map of Parts of Marquette, Dickinson, Baraga, Alger, and Schoolcraft Counties, Michigan, and Its Geologic Interpretation. Clark, Lorin D., William F. Cannon, and J. S. Klasner., 1975, Bedrock Geologic Map of the Negaunee SW Quadrangle, Marquette County, Michigan. Reston, VA: Survey. Gair, Jacob Eugene, and Robert E. Thaden., 1968, Geology of the Marquette and Sands Quadrangles, Marquette County, Michigan. Rossell, Dean and Coombes, Steven., 2005, The geology of the Eagle Nickel-Copper Deposit Michigan, USA. Kennecott Minerals Co. Sims, P. K.., 1992, Geologic Map of Precambrian Rocks, Southern Lake Superior Region, Wisconsin and Northern Michigan. U.S. Geological Survey. 80 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Petrologic study of the “Chill” zone of the Layered Series at Duluth: Testing a possible plutonic-volcanic correlation within the Midcontinent Rift SAUER, Sarah and MILLER, Jim Department of Earth and Environmental Sciences, University of Minnesota Duluth, Duluth, Minnesota 55812 The Duluth Complex is a multiple intrusive mafic complex that represents the largest exposed plutonic component of the 1.1 Ga Midcontinent rift. Results from extensive field mapping and petrologic studies (Miller and Green, 2008a, 2008b; Green and Miller, 2008) of the mafic cumulates comprising the type locality of the Duluth Complex at Duluth have confirmed that it is composed of two fundamentally distinct rock series, and provided further detail regarding the igneous stratigraphy, internal structure of each series and thus the petrogenetic relationship between them. The Anorthositic Series (DAS), which forms a ~ 1-km thick cap to the Duluth Complex at Duluth, is a suite of structurally complex plagioclase-rich gabbroic rocks which are interpreted to have formed by multiple intrusions of a plagioclase crystal mush from a lower crustal magma chamber (Miller and Weiblen, 1990). Underlying the DAS cap is a 3-4.5 km thick, well differentiated, stratiform sequence of troctolitic to gabbroic cumulates forming the layered series at Duluth (DLS). The DLS is thought to have formed by open system crystallization differentiation (Miller and Ripley, 1997). Based on lithology and stratigraphic position, the DLS can be subdivided into five major zones: basal contact zone, troctolite zone, cyclic zone, gabbro zone and upper contact zone. The DAS had long been interpreted to be significantly older than the DLS based on the abundance of DAS inclusions in the DLS and, especially, on the occurrence of a fine-grained mafic rock that occurs at the sharp upper contact of the DLS with the overlying DAS, referred to as the DLS “chill”. However, high precision U-Pb ages from DAS and DLS samples (Paces and Miller, 1993) has shown that these two rock series are essentially identical in age at 1099±0.5 Ma relative to the 30m.y. window of MCR magmatism. This revelation warrants a reinterpretation of the relationship between the two series, along with reevaluating the origin of the DLS “chill.” Since the similar ages of the DLS and DAS preclude the DLS “chill” being a thermal quench of DLS parental magma against the DAS, Miller (Miller and Ripley, 1997; Miller, 2011) has suggested that quenching of DLS magma was caused by the decompression of a volatile-saturated magma accompanying volcanic venting from the subvolcanic DLS chamber. Several features lend evidence in support of a decompression quenching of hydrous magma interpretation including: 1) the evolved composition of the DLS “chill”, 2) the presence of biotite phenocrysts in the “chill”, and 3) the extensive hydrothermal alteration of overlying DAS rocks. Miller has further suggested that periodic venting of hydrous magma may have played an important role in the formation of the cyclic zone in the medial part of the DLS, particularly the occurrence of microgabbro cumulates in the upper parts of phase-layered macrocycles. This study seeks to test whether the DLS “chill” composition could be formed by decompression quenching of a volatile saturated magma and whether that magma is in equilibrium with the microgabbros of the Cyclic Zone and possible volcanic products represented in the NSVG overlying the Duluth Complex. To accomplish this, the lithological, petrographic and geochemical attributes of the DLS “chill”, microgabbros and flows from the NSVG were evaluated. Taking advantage of the fact that Brannon (1984) analyzed the chemistry of all mafic lavas occurring between 3-5 kilometers above the top of the Duluth Complex, the chemostratigraphy of the overlying NSVG were searched for compositions matching the DLS “chill”. If a correlative lava can be found, it would provide valuable constraints on the depth and pressure of the DLS magma 81 �Proceedings of the 61st ILSG Annual Meeting - Part 1 chamber. Once a pressure of crystallization is established, the DLS “chill” composition can be applied to PELE, a MELTS-based phase equilibrium modeling program developed by Boudreau (2006), to simulate whether pressure fluctuations caused by devolatilization and venting could effectively quench a volatile-saturated magma during decompression. PELE can also be used to evaluated the comagmatic relationship between the DLS “chill” and Cyclic Zone microgabbros by determining the mineral phases in equilibrium with the “chill” composition and comparing them to the phase compositions observed in the microgabbros. Preliminary results indicate two sequences of flows (Brannon’s (1984) flows 28-38 and 65) have the best match to the “chill” composition. Both series had the best fit for most major and trace elements, but most notable, however, was flow 65 which corresponds to the upper-most flow of an eight-flow sequence that defines an obvious differentiation trend. Flow 65 shows a good fit to the “chill” composition and parallel patterns of depletion of incompatible trace elements evident in the successively lower flows of the differentiation sequence. Treating these two sequences of flows as the potential volcanic products would indicate that the top of the DLS was emplaced within the volcanic edifice at a depth of ~4-5km. This depth is consistent with estimates for the formation of shallow reservoirs of magma (2-4km) beneath mafic volcanic centers like Kilauea in Hawaii (Ryan, 1987). Modeling with the PELE program is just underway. We hope to have results to present at the time of the meeting. REFERENCES Boudreau, A., 2006, Pele. (7.07). Computer modeling program. Duke University. (www.nicholas.duke.edu/eos/) Brannon, J.C. 1984. Geochemistry of successive lava flows of the Keweenawan North Shore Volcanic Group. Ph.D. thesis, Washington University, St. Louis, MO, . Green, J.C., and Miller, J.D., Jr., 2008, Bedrock geology of the Duluth quadrangle, St. Louis County, Minnesota. Minnesota Geological Survey Miscellaneous Map M-182, scale 1:24,000 Miller, J.D., 2011, Igneous stratigraphy of the Layered Series at Duluth – Type intrusion of the Duluth Complex. Institute on Lake Superior Geology, Proceedings Vol. 57, Part 2 - Field Trip Guidebook, p. 3-29. Miller, J.D., Jr.,and Ripley, E.M., 1996. Layered intrusions of the Duluth Complex, Minnesota, USA, in Cawthorn, R.G., ed., Layered intrusions: Amsterdam, Elsevier Science, p.257-301. Miller, J.D., Jr., and Green, J.C., 2008a, Bedrock geology of the Duluth Heights and eastern portion of the Adolph quadrangles, St. Louis County, Minnesota. Minnesota Geological Survey Miscellaneous Map M-181, scale 1:24,000 Miller, J.D., Jr., and Green, J.C., 2008, Bedrock geology of the West Duluth and eastern portion of the Esko quadrangles, St. Louis County, Minnesota. Minnesota Geological Survey Miscellaneous Map M-183, scale 1:24,000. Miller, J.D., Jr., and Weiblen, P.W., 1990, Anorthositic rocks of the Duluth Complex: Examples of rocks formed from plagioclase crystal mush: Journal of Petrology, v. 31, p. 295–339. Paces, J.B., and Miller, J.D., Jr. 1993. Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: Geochronological insights to physical Petrogenetic, paleomagnetic and tectonomagmatic processes associated with the 1.1 Ga Midcontinent rift system: Journal of Geophysical Research 98: 13,997-14,013. Ryan, M.P. 1987. Neutral buoyancy and the mechanical evolution of magmatic systems. In Magmatic processes physiochemical principles. B.O. Mysen (ed.) The Geochemical Society Special Publication 1, p. 259-287. 82 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Images on stone: Pictographs of the Ignace area, northwestern Ontario SMYK, Dennis W.1, ROSS, William.2 and SMYK, Mark C.3 1 P.O. Box 989,153 Balsam St., Ignace, ON P0T 1T0 2 William Ross Archaeological Research Associates, 189 Peter Street, Thunder Bay ON P7A 5H8 3 Resident Geologist Program, Ministry of Northern Development and Mines, Ontario Geological Survey, Suite B002, 435 James Street South, Thunder Bay, ON P7E 6S7 More than 400 rock paintings (pictographs) had been documented on outcrops of the Canadian Shield from Quebec, across Ontario and as far west as Saskatchewan (Rajnovich 1994). The senior author, an avocational archaeologist, has found and documented an additional 150 pictograph sites over the past 50 years. Most of them are situated within 160 km of Ignace, midway between Thunder Bay and Kenora in northwestern Ontario. Pictographs are the legacy of the Algonquian-speaking, early Cree and Ojibway peoples, whose roots may extend to the beginnings of post-glacial human occupancy in the area almost 10,000 years ago. This region of northwestern Ontario is underlain by Archean rocks of the Superior Province that are overlain by thin, discontinuous, unconsolidated glacial, lacustrine and organic deposits. The vast majority of pictograph sites are located on shoreline bedrock exposures of lakes and interconnecting rivers. Sites with relatively homogeneous and leucocratic bedrock faces (e.g., granitoids) are preferred, although more mafic rocks also host pictographs. Many sites consist of single elements (e.g., a canoe), while others are more complex with many figures and motifs. In many cases, there may only be one site on a lake, whereas in other examples, several sites may be scattered along a lakeshore. The largest concentration of pictographs found to date has 28 sites on cliffs on both sides of a 8 km-long, narrow stretch of lake. In almost all cases, the paintings are red, but one site with a gold handprint was found and documented by the senior author on Eagle Lake, southwest of Dryden. Discovered in the mid1960s, the unique Smyk Site (Figure 1), northeast of Ignace, is the only known local site with the three distinct colours of red, gold and dark purple. Although there is some scattered and anecdotal evidence of ochre quarrying, there has been only limited research into possible sources of natural pigmenting agents and the ochres themselves. Unlike sites in the Thunder Bay area, the geoarchaeology of the area west of the Lake Superior basin remains largely cursory. Much work remains in identifying and documenting pictograph sites and relating them in the larger context of spiritual places, habitation sites, transport and trading routes, local geologic materials and deglaciation history. REFERENCE Rajnovich, G. 1994. Reading Rock Art: Interpreting the Indian Rock Paintings of the Canadian Shield; Natural Heritage / Natural History Inc., Toronto, ON. 83 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Figure 1: Pictographs of three distinct colours (red, gold and dark purple) on Archean granitoid rocks at the Smyk Site, northeast of Ignace. Exposed panel is approximately 1 m across. 84 �Proceedings of the 61st ILSG Annual Meeting - Part 1 The petrology, mineralization and regional context of the Thunder mafic to ultramafic intrusion, Midcontinent Rift, Thunder Bay, Ontario TREVISAN1, Brent, HOLLINGS1, Pete, AMES2, Doreen and RAYNER2, Nicole 1. Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada. 2. Geological Survey of Canada, Ottawa, Ontario, K1A 0E8, Canada The 1108 Ma Thunder mafic to ultramafic intrusion is a small, 800 x 100 x 500 m, Cu-PGE (platinum group element) mineralized body, located on the outskirts of Thunder Bay, Ontario. The intrusion was explored by Rio Tinto (formerly Kennecott Canada Exploration Inc.) in 2005 and 2007 (Fig. 1; Bidwell and Marino, 2007). It is associated with the early magmatic stages of the Midcontinent Rift (MCR) based on geochemical similarities to mafic and ultramafic rocks of the Nipigon Embayment and a 207Pb/206Pb zircon age of 1108.0 ± 1.0 Ma (Trevisan, 2014; Trevisan et al., 2015). Figure 1: Geological map of the greater Thunder Bay area including major road networks and outline of the current mineral claims that enclose the Thunder intrusion. The Thunder intrusion is located on the outskirts of the City of Thunder Bay, within Gorham Township and situated within the eastern limb of the Archean Shebandowan greenstone belt. Geospatial data from OGS (2011). The Thunder intrusion is similar to the other known mineralized early-rift MCR intrusions; however, it is the only known mafic/ultramafic intrusion of the MCR hosted in an Archean greenstone belt (Shebandowan). Major textural and geochemical differences can be used to subdivide the intrusion into a lower mafic to ultramafic unit and an upper gabbroic unit; the similar trace and rare earth element ratios of the two units suggest a single magmatic pulse that has undergone subsequent fractional crystallization and related cumulate phase layering. The estimated parental composition of the Thunder intrusion has a mg# (MgO/(MgO+FeOTot), mole %) of 57 which represents a more evolved magma than other early-rift mafic to ultramafic intrusions. This may indicate the involvement of multiple staging chambers during the ascent of the parent magma. 85 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Trace and rare earth element geochemical patterns are consistent with a mantle plume ocean island basalt-like source but, with high Th concentrations and the presence of a negative Nb anomaly, indicative of contamination . ԐNdt values from the intrusion range between -0.7 and +1.0, but lack trends indicative of progressive wall rock contamination, whereas the 87Sr/86Sri ratios range from 0.70288 to 0.70611 and trend towards wall rock values of 0.70712 and 0.70873. The weak correlation at Thunder between ԐNdt and 87Sr/86Sri is also a feature of the Nipigon Sills where it has been interpreted to be due to shallow-level crustal contamination whereas plots of MgO and SiO2 versus ԐNdt indicate contamination at depth by an older crustal source. Ni-Cu-PGE sulphide mineralization (20 m of 0.22 wt. % Cu, 0.06 wt. % Ni, 0.25 ppm Pt and 0.29 ppm Pd) is hosted by feldspathic peridotite in the lower mafic to ultramafic unit adjacent to the footwall of the Thunder intrusion. Sulphides typically occur from 1 - 5 modal %, rarely up to 30 modal %, with textures ranging from medium- to fine-grained disseminated, globular and rarely net-textured. Pyrrhotite, chalcopyrite and rare pentlandite with common secondary marcasite pyrite replacement occur along with the trace Pd, Ag, Au and rare Pt minerals, michenerite, kotulskite, merenskyite, sperrylite, hessite, electrum and argentian pentlandite. Whole-rock geochemical data display fractionated Ni-Cu-PGE patterns with depletion of iridium subgroup relative to the platinum subgroup of the platinum group elements. Sulphide δ34S values from the Thunder intrusion range from -2.0 to +3.8 ‰ and are similar to values for the metavolcanic host rock that range from -3.1 to +2.3 ‰. Two samples from the basal mineralization zone sulphides yield Δ33S values of 0.066 and 0.122 ‰ and one sample from the metavolcanic wall rock yields 0.149 ‰. The δ34S and Δ33S values for the Thunder intrusion fall within range of typical upper mantle compositions. The sulphur source appears to be of mantle origin; however, assimilation of crustal sulphur is a possibility but hard to resolve as the wall rock S isotope and S/SeTot signature is similar to that of upper mantle. REFERENCES Bidwell, G. E., and Marino, F., 2007, Thunder Project: 2007 Field program diamond drilling on the 1245457 claim: Thunder Bay Regional Geologist office, Assessment Files 2.34638. Trevisan, B.E., 2014, The petrology, mineralization and regional context of the Thunder mafic to ultramafic intrusion, Midcontinent Rift, Thunder Bay, Ontario: Unpublished M,Sc. Thesis, Thunder Bay, ON, Lakehead University, 299 p. Trevisan, B.E., Hollings, P., Ames, D.E., and Rayner, N., 2015. The petrology, mineralization, and regional context of the Thunder mafic to ultramafic intrusion, Midcontinent Rift, Thunder Bay, Ontario, In: Targeted Geoscience Initiative 4: Canadian Nickel-Copper-Platinum Group Elements-Chromium Ore Systems — Fertility, Pathfinders, New and Revised Models, (eds) D.E. Ames and M.G. Houlé; Geological Survey of Canada, Open File 7856. 86 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Bedrock and soil chemistry in paired watersheds in northeastern Minnesota WOODRUFF, Laurel G.1 and JENNINGS, Carrie E.2 1 U.S. Geological Survey, 2280 Woodale Drive, St. Paul, MN 55112 2 Minnesota Department of Natural Resources, 500 Lafayette Road, St. Paul, MN 55155 Bedrock and soil have been collected and analyzed in two adjacent watersheds (Filson and Keeley) in northeastern Minnesota (Fig. 1). This sample collection effort is part of a three-year study to determine baseline geochemistry of solid media and water quality in this part of Minnesota. The Filson watershed, which includes Filson and South Filson Creeks, drains an area of about 26.3 km2 (~10.2 mi2) and discharges into the South Fork of the Kawishiwi River. The geology of the Filson watershed is complex. Bedrock includes the Archean Giants Range Batholith and the Duluth Complex. The Duluth Complex in the Filson watershed is mainly represented by the South Kawishiwi Intrusion, a thick sequence of troctolite and augite troctolite (with a heterogeneous sulfide-bearing, basal zone in contact with Archean quartz monzonite), the Anorthosite Series, and the Nickel Lake Macrodike, composed of oxide-rich gabbro and foliated troctolite (Fig.1). Two major mineral deposits in the Filson watershed are the Spruce Road deposit and the South Filson deposit (Fig.1). At the Spruce Road deposit, discontinuous sulfide-bearing heterogeneous troctolite is exposed at the surface across the northern part of the watershed. The South Filson deposit is a combination of primary disseminated Cu-Ni sulfide mineralization and secondary hydrothermal mineralization along fracture zones. Primary mineralization at South Filson occurs both at depth and sporadically in troctolite outcrop within a limited area; secondary mineralization occurs in fine-grained veinlets proximal to a northeast-southwest trending, highly altered fault zone. The adjacent Keeley Creek watershed, south and west of the Filson watershed, drains an area of about 28 km2 (~10.9 mi2) and discharges into Birch Lake. The geology of the Keeley watershed is fairly simple, with bedrock dominated by relatively homogeneous, typically unmineralized anorthositic troctolite to troctolite of the South Kawishiwi Intrusion (Fig.1). A thin zone of sulfidebearing melatroctolite has recently been mapped within the trace of the creek (D. Peterson, personal communication, 2014). In both the Filson and Keeley watersheds topography is largely controlled by the resistance to chemical weathering of underlying bedrock and subsequent removal of the saprolith by glacial erosion. Glacial cover is very thin. The landscape within the watersheds mainly consists of bedrock highlands surrounded by wetlands. In the Filson watershed soil was collected from 16 upland sites along two broad transects that cut across the general trend of the major bedrock types. In the Keeley watershed, the monotonous sea of anorthosite troctolite resulted in selection of 14 upland soil sites based on the rather problematic road access. At all soil sites, up to 3 samples were collected using hand tools, including the soil O horizon (where present – this area has abundant invasive earthworms that typically covert organic soil to mineral soil), the soil A horizon, and a deeper soil. Final sample depths were typically constrained by the stony substrate. Bedrock samples were collected from the abundant outcrop within each watershed. Bedrock sample sites were selected to be proximal to soil sample sites, if possible, or to capture the diversity of rock types. Bedrock was collected at 14 sites in the Filson watershed and 9 sites in the Keeley watershed. Soil and bedrock were analyzed for 44 major and trace elements following a near-total 4-acid digestion. 87 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Copper and Ni concentrations in mineralized bedrock in the vicinity of the exposed Spruce Road deposit are 4,600 ppm Cu and 1,130 ppm Ni. Non-mineralized South Kawishiwi Intrusion anorthosite contained Cu concentrations that range from 50 to 203 ppm and Ni concentrations that range from 119 ppm to 296 ppm; rocks of the Anorthosite Series in the Filson watershed have Cu concentrations from 67 to 235 ppm and Ni concentrations from 56 to 70 ppm. Soils collected over the exposed footprint of the Spruce Road deposit and in the down-ice direction from the Spruce Road have high Cu and Ni concentrations; a single soil sample collected in the vicinity of the South Filson deposit has relatively high Cu and Ni (Fig. 2A). The distribution of Ni and Cu in Fig. 2A is consistent with variable contributions to soil parent materials from sulfide (for example, high Cu and Ni) versus ferromagnesian silicate Figure 1. Location map showing the distribution of minerals (for example, high Ni but low Cu); soil and bedrock sample sites in the Keeley and Filson plots of other elements, such as Co, Cr, Fe, Mg, watersheds. and Mn provide similar evidence. Surface soils typically have metal values consistent with deeper soils. Because glacial transport distances are short and glacial cover thin, soil chemistry, for the most part, can be related back to bedrock contributions to soil parent material (Fig. 2B). Although these data are rather sparse, they describe the natural distribution of many elements within these two watersheds that can be attributed to geologic processes. The combination of these data with the on-going collection of water quality data in this three-year study will provide valuable information on the geochemical landscape in this region of potential mineral resource development. Figure 2. A) Box plot of Ni vs. Cu (in ppm) in soil; B) Ternary plot of Cu-Ni-Co in soil; bedrock data within solid similarly colored fields. 88 �Proceedings of the 61st ILSG Annual Meeting - Part 1 Exposure Surfaces of the Gunflint iron formation, northwestern Ontario YIP, Christopher, and FRALICK, Philip Department of Geology, Lakehead University, 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada Exposure surfaces present in Precambrian rocks can be used as an environmental record of the conditions prior to their being covered by depositing sediments. The surfaces can show alteration of the preexisting rocks, which were exposed to the Precambrian atmosphere. The 1.88Ga Gunflint Formation in northern Ontario has two identifiable exposure surfaces found within its stratigraphy. The first one makes up the basal contact of the iron formation and is comprised of the basement rocks in places overlain by the thin Kakabeka Conglomerate all capped off by microbialite and/or grainstone of the Gunflint Formation marking the initial transgression of the sea. The second exposure surface is found approximately 45m above the basal contact. It records the regression of the ancient sea and is underlain by Gunflint grainstone and overlain by stromatolitic growth marking the shallowing of the sea. In the basal contact, the Archean rocks forming the exposed surface can show high levels of alteration. There are three outcrops present near Thunder Bay, Ontario, which contain complete sections of the basal exposure surface. The sample sites selected are two outcrops on the shoulder of Highway 11/17 and one on the shoulder of Highway 590. These three outcrops exhibit various alteration patterns within rocks near the paleosurface (Figure 1). Figure 1: Three examples of the Gunflint Iron Formation’s basal contact exposure surface showing the complete section through the exposure surface. The alteration horizons (1) are demarked. A) The formation of large core stones during alteration of the outcrop on the shoulder of Highway 11/17. B) The alteration of the KOA Hill outcrop showing the change in foliation from the vertical schistosity to the flaky altered horizon. C) The outcrop on Kakabeka Falls showing an indistinct difference between the unaltered and darker altered portion of the granodiorite unit. 89 �Proceedings of the 61st ILSG Annual Meeting - Part 1 The first outcrop on Highway 11/17 shows high levels of alteration through the formation of the large core stones as well as replacement of the original mineralogy of the granodiorite by mostly iron-rich chlorite. The outcrop at KOA hill on the shoulder of Highway 11/17 exhibits a change in the foliation in the Archean metasedimentary basement from a vertical schistosity to a flaky layer with no discernable pattern. The appearance of the Kakabeka Falls outcrop exhibits minor amounts of dark discolouration, but extensive replacement of the original mineralogy. The alteration history of these three outcrops and in particular the former, can be related to an earlier phase of surface weathering overprinted by massive diagenetic addition of Fe and Mn and extensive leaching of the initial constituents of the rocks. The exposure surface that is approximately 45m above the base of the Gunflint Formation consist of lithified grainstone blocks, some up to boulder size, that were in places rotated by current activity (Figure 2). This rubble zone and fractured basement below it contains small, wispy hematite dykes. Stromatolites developed on this lithified brecciated surface. Figure 2: Two representations of the exposure surface present approximately 45m above the base of the outcrop. A) The outcrop to the immediate north of Mink Mountain exhibits brecciation of the lithified grainstones with hematite dykes filling the fractures. B) A boulder removed from an outcrop present on Old School Road showing stromatolites forming above a brecciated grainstone boulder as well as hematite dykes filling the fractured boulder. 90 � https://digitalcollections.lakeheadu.ca/files/original/df52e8901482197925f050c5a95285c8.pdf 6927d891e486e9981bf1513e5ed0a06b PDF Text Text 61st ANNUAL MEETING InstItute on Lake superIor GeoLoGy Dryden, Ontario - May 20-24, 2015 Part 2 – Field Trip Guidebook �Sponsors The following organizations made generous contributions to the 61st Annual Meeting. We thank them for their commitment to the Institute on Lake Superior Geology. All of the funds contributed this year go toward travel awards for student registrants. For the past 60 years this organization has thrived as a result of the interest of individuals, corporations, universities and government agencies. The dedication to an exchange of scientific ideas and a passion for field trips has enabled the Institute to provide one of its primary objectives – to promote better understanding of the geology of the Lake Superior Region. Mary Arthur Steve Baumann Leonard Espinosa Gordon Medaris Jr. Allan MacTavish Jim Miller Paul Weiblen Canadian Institute of Mining and Metallurgy Thunder Bay Branch �61st annuaL MeetInG InstItute on Lake superIor GeoLoGy Supported by ONTARIO MINISTRY OF NORTHERN DEVELOPMENT AND MINES May 20-24, 2015 Dryden, Ontario HOSTED BY: Rob Cundari & Peter Hinz Co-Chairs Ontario Geological Survey Proceedings - Volume 61 Part 2 – Field Trip Guidebook Edited by Al MacTavish & Pete Hollings Cover photos: Top - Sakoose Mine, circa 1937 (from Humphrey and Tymura: The New Klondike to the Manitou), Middle - Central Patricia Headframe, Pickle Lake (courtesy Mark Smyk), Bottom - Pickle Crow Au mine (courtesy of Rob Cundari) �61st InstItute on Lake superIor GeoLoGy VoLuMe 61 consIsts of: part 1: proGraM and abstracts part 2: fIeLd trIp GuIdebook trIp 1: The CenTral red lake Gold BelT trIp 2: WesTern WaBiGoon suBprovinCe TranseCT, dryden To MeGGisi lake trIp 3: CanCelled trIp 4: Thunder lake (GoliaTh) projeCT trIp 5: ClassiC ouTCrops of The dryden area trIp 6: GOLD OCCURRENCES OF VAN HORNE TOWNSHIP, VAN HORNE GOLD PROPERTY FLAMBEAU EXPLOSURES trIp 7: unique MineralizinG evenT aT The pidGeon MolyBdenuM deposiT sTripped surfaCe exposure trIp 8: GeoloGy and Mineral deposiTs of The piCkle lake GreensTone BelT trIp 9: The GhosT lake BaTholiTh and relaTed peGMaTiTes trIp 10: MaTTaBi/sTurGeon lake hisToriC vMs CaMp Reference to material in Part 2 should follow the example below: Lichtblau, A., and Storey, C., 2015. Field trip 1 - The Central Red Lake Gold Belt. In; MacTavish, A. and Hollings, P., (Eds.), Institute on Lake Superior Geology Proceedings, 61st Annual Meeting, Dryden, Ontario, Part 2 - Field trip guidebook, v.61, part 2, 2-23. Published by the 61st 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 61st ILSG Annual Meeting - Part 2 Table of Contents Introduction, safety considerations and acknowledgements ...............................................1 Field Trip 1 - The Central Red Lake Gold Belt ..................................................................2 Field Trip 2 - Western Wabigoon Subprovince Transect, Dryden to Meggisi Lake .........24 Field Trip 3............................................................................................................ cancelled Field Trip 4 - Thunder Lake (Goliath) Project ..................................................................40 Field Trip 5 - Classic Outcrops of the Dryden area ..........................................................46 Field Trip 6 - Gold Occurrences of Van Horne Township, Van Horne Gold property Flambeau explosures.................................................................................................51 Field Trip 7 - Unique Mineralizing Event at the Pidgeon Molybdenum Deposit Stripped Surface Exposure ......................................................................................................60 Field Trip 8 - Geology and Mineral Deposits of the Pickle Lake Greenstone Belt ..........67 Field Trip 9 - The Ghost Lake Batholith and Related Pegmatites ..................................112 Field Trip 10 - Mattabi/Sturgeon Lake Historic VMS Camp ........................................116 -i- �Proceedings of the 61st ILSG Annual Meeting - Part 2 Introduction, safety considerations and acknowledgements Rob Cundari and Peter Hinz Ontario Geological Survey, Thunder Bay, Ontario, Canada This volume is intended to serve not only as a guide for 61st 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 Zone 15 datum for the majority of stops, as well as instructions on how to reach them. For some of the stops on private land we have withheld the UTM coordinates to respect the privacy of the property owner. As some of the stops are on private and staked land, please be sure to obtain the land owners’ permission before entering their land. For up-to-date information on land ownership please contact the Kenora Resident Geologists’ Office at (807) 468-2819 or (for the Pickle Lake Area) the Thunder Bay Resident Geologist’s Office at (807) 4751331. Sample collection is prohibited at some stops on private land or within Provincial Parks. Many of the fieldtrips will be visiting stops along major and secondary highways or busy logging roads. Please take care when crossing or parking along these roads. For those field trips that are visiting active mine sites personal protective equipment will be required. For those field trips that are visiting past-producing mine sites please be very careful around visible open holes such as shafts, pits, or trenches and keep a wary eye out for hidden holes which may be overgrown with vegetation and therefore are very difficult to see. Please notify the field trip leaders if you have any medical conditions that may be of concern during the trip. Each trip leader is equipped with a first aid kit and satellite/ cell phone, so please notify them of any incident. We would like to thank all authors who contributed to this field guide and also all those who provided comments and assisted with the running of the field trips themselves. We appreciate the assistance and cooperation of the exploration and mining companies in providing us access and information concerning their properties. We are particularly grateful to Goldcorp Inc., Glencore plc, Cadillac Ventures Inc., PC Gold Inc. and Treasury Metals Inc. for running field trips on their properties. Figure 1. Map showing the general locations of field trips for the 2015 meeting. -1- �Proceedings of the 61st ILSG Annual Meeting - Part 2 Field Trip 1 - The Central Red Lake Gold Belt Andreas Lichtblau and Carmen Storey Ontario Geological Survey, Red Lake, Ontario, Canada A Brief History of The Beginnings Of The Red Lake Mining Camp This excerpt from Horwood (1940) details the early history of the camp and ends with the first mine that went into production, the Howey, in 1930: A trading post, known as Red Lake House, was established by the North West Company some time prior to 1786 at Post narrows near the northeast end of the lake. Red Lake itself is shown on Arrowsmith’s map of 1801, which outlines the route travelled by Alexander Mackenzie in 1789. The post was taken over by the Hudson’s Bay Company in 1821 and has been in continuous operation ever since. In 1926 the site was changed to the west side of Howey bay opposite the Howey mine. company staked a group of claims near Slate Bay in McDonough township but abandoned the property after doing some development work and sinking a small shaft. In 1912, when the Provincial Geologist issued a report that the Keewatin rocks of the Patricia District should contain deposits of gold, some prospecting was done; but the work did not meet with success. In 1922, a party of prospectors became interested in the area. A press report of their discovery of a vein containing quartz and argentiferous galena attracted a number of other prospectors and resulted in some activity. Gus McManus, one of the prospectors, found some small gold-bearing quartz stringers near the outlet of Red Lake and staked several claims. Bruce, who was doing geological work to the south for the Ontario Department of Mines, heard of the finds and came in to make an examination. The area proved of such interest that the Department sent him back to complete his preliminary investigation. The map and report, published late in 1924, aroused considerable interest, and early in the summer of 1925 Lorne Howey and George McNeely, representing Haileybury interests, and Ray Howey and W. F. Morgan, representing McIntyre- Porcupine Mines, Limited, came in to prospect. Late in July, Lorne Howey and his partner found quartz stringers containing native gold and staked a group of claims on what is now the property of Howey Gold Mines, Limited. Ray Howey and his partner discovered gold mineralization to the southwest and; staked an adjoining group of claims, now part of the Hasaga property, for the McIntyre. News of the finds quickly reached the outside, and the Red Lake country had its first gold rush. The backers of Lorne Howey and George McNeely interested J. E. Hammell in their property, and he formed the Howey Red Lake Syndicate to develop the showing. In order to get men and supplies into the country before freeze-up seven Forestry planes, which were stationed at Sioux Lookout, were chartered. The success of this freighting venture was directly responsible for the development of commercial flying as an adjunct to mining From 1812 to 1872 no mention is made of Red Lake by the various exploring parties that penetrated the west. Its position near the heightof-land and away from the main canoe routes discouraged exploration, and it remained an outof-the-way but active outpost of the Hudson’s Bay Company. In 1872 A. R. C. Selwyn, who was making an exploratory trip down the English River from Lac Seul to Lake Winnipeg, heard of Onimini Sagaigan or Red Paint Lake from a group of Indians. They told him of the occurrence of slaty rocks, which, to him, suggested the presence of a belt of sedimentary and volcanic rocks to the north. He did not have time, however, to visit the area. In 1883, Bell made a track survey of the lake and confirmed Selwyn’s opinion of the presence of a wide belt of what he termed Huronian rocks. In 1893, Dowling examined the area and made a map, which till 1924 was the only source of geological information. Some prospecting was done between 1898 and 1924, but no mineral deposits of commercial importance were located. The first recorded discovery of gold was made in 1897 by the Northwestern Ontario Exploration Company, a prospecting venture headed by R. J. Gilbert. This -2- �Proceedings of the 61st ILSG Annual Meeting - Part 2 development in the more inaccessible parts of the north. By the spring of 1926 a fairly regular airservice had been established. In the fall of that year H. A. Oaks, one of the pioneer fliers, and James Richardson, Winnipeg financier, founded Western Canada Airways. This enterprise was known as Canadian Airways when taken over by Canadian Pacific Airlines in April, 1942. Red Lake District: 1. Uchi Sub-province rocks in the Red Lake District comprise the Red Lake and Birch-Confederation Lake greenstone belts in which the bulk of exploration and mining activity has taken place. The supracrustal rocks of the Red Lake greenstone belt can be subdivided into several assemblages with ages ranging from circa (ca.) 2990Ma to ca. 2700Ma (Table 1). Major granitoid intrusions show a range from ca. 2734Ma to 2699Ma (Table 2). Late in 1925 Mr. Hammell succeeded in interesting Dome Mines, Limited, in the Howey Syndicate. They took an option on the property and, by the summer of 1926, had completed an extensive diamond-drilling campaign. They decided, however, that the grade of the ore was too low to warrant the expenditure of more funds and in August of that year dropped their option. Many people became discouraged over the possibilities of the camp and left the area. Mr. Hammell, however, still had faith in the Howey property and continued work. With the financial assistance of W. S. Cherry a mill was built and the mine came into production on April 2, 1930. The undertaking proved to be a profitable venture and continued operations till November 3, 1941. 2. English River Sub-province rocks, south of the Uchi Sub-province, are predominantly metasedimentary and host minor intrusive rocks similar to those in the Quetico Sub-province. 3. To the north, the Berens River Sub-province formed the core of a microcontinent. This area is underlain by ca. 2750-2690Ma felsic plutonic rocks interpreted as a magmatic arc formed at an Andean-style margin that culminated in the Kenoran Orogeny. These plutonic rocks intruded an older substratum (North Caribou terrane) on which Mesoarchean volcanic rocks of the Red Lake belt are also interpreted to have formed. We will drive by an open stope at the Howey mine site during the tour. Regional Geology The Red Lake District (Fig. 1) is underlain by Archean rocks of the Superior Province of the Canadian Shield. Rocks of four sub-provinces are found in the Figure 1. Western Uchi Subprovince (modified from Percival et al., 2000). 4. The Sachigo Sub-province comprises crustal blocks ranging from Paleoarchean (>3.4Ga) to Neoarchean (ca. 2.7Ga) in age. Geology of the Red Lake Greenstone Belt (adapted from Sanborn-Barrie et al. 2001) The geology of the Red Lake greenstone belt (Fig. 2) is dominated by the (ca. 2990Ma) mafic-ultramafic Balmer assemblage, an oceanic plain sequence; minor calc-alkalic volcanic rocks of arc-like affinity terminate the assemblage. The majority of lode gold deposits in the camp are hosted by the basal mafic-ultramafic sequence. A later diverse lithologic association, the Ball assemblage, appears to represent a shallow marine, volcanic edifice built upon the Balmer substrate. Widespread circa (ca.) 2894Ma calc-alkaline volcanism is represented in Red Lake by the Bruce Channel assemblage. Overlying this is the ca. 2850Ma Trout Bay assemblage which includes substantial basaltic and gabbroic rocks in western Red Lake which are prospective for PGE mineralization, and which includes minor intermediate pyroclastic rocks throughout central Red Lake. The Trout Bay assemblage may correlate with Woman assemblage rocks of the Confederation Lake belt. -3- A regional angular unconformity is interpreted �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 2. General geology of the Red Lake greenstone belt (after Parker, 2000). Tour stops as indicated. to separate the Mesoarchean assemblages from the Neoarchean Confederation assemblages. Volcanogenic massive sulphide mineralization is associated with the younger sequence. A significant number of felsic units are classed as FII- and FIII-type rhyolites, considered highly prospective for large (Kidd Creek/Noranda type) massive sulphide deposits (Parker, 1999). A newly recognized component of the Neoarchean supracrustal package is the Huston sedimentary assemblage that includes surface exposures of polymictic cobble- to pebble-conglomerate and argillite; clasts include jasperoidal chert iron formation, massive sulfide pebbles, mafic flow rocks as well as well-bedded, graded turbiditic wacke and argillite. The Huston assemblage conformably to unconformably overlie the McNeely sequence (Confederation assemblage) and underlies the Graves assemblage (Sanborn-Barrie et al., 2004). The U-Pb age of detrital zircons give single age peaks of 2743 and 2746Ma at the cemetery and Madsen sites, respectively (Skulski et al., 2001), indicating single source derivation from erosion of pre-existing Confederation age rocks, and deposition after ca. 2743Ma. In the Campbell-Red Lake Deposit area the conglomerate defines the interface between the Balmer or Bruce Channel assemblages and the Confederation assemblage. Exposures on 16 Level of the Red Lake Mine reveal that (Dubé et al., 2004): “... the [Huston] conglomerate is a polymictic proximal conglomerate or breccia (debris flow) dominated by subangular to subrounded laminated cherty clasts with local jasper-rich and green mica fragments; it is similar to the Temiskaming conglomerate… it also contains clasts of andalusite-rich altered basalt and a few local clasts of layered (possibly sheeted) carbonate veins with small crustiform banding and cockade texture.” Detrital zircon analyses from the conglomerate on 16 Level of the Red Lake Mine define a single population with a mean age of 2747±4Ma (Dubé et al., 2004). This implies that the former two assemblages were exposed at surface by 2747Ma. The presence of local andalusite-rich clasts and clasts of carbonate vein material in an unaltered matrix demonstrates that there was a period of at least some -4- �Proceedings of the 61st ILSG Annual Meeting - Part 2 Table 1. Summary of supracrustal lithologies and radiometric ages in the Red Lake greenstone belt (modified from Parker 2000; with new ages from Sanborn-Barrie et al. 2001 and Skulski et al. 2001; final error estimates are not cited for the new unpublished ages of T. Skulski). Supracrustal assemblage English River? U-Pb Age (Ma) <2700±6 ConfederationGraves (north Red Lake) 2733±1.5 Huston (Cemetery) ≤2743 Huston (16 Level, Lake Mine) Rock types and descriptions References Polymictic pebble conglomerate. Thought to correlate with the Austin tuff, host to the Madsen gold deposit. Calc-alkaline andesitic to dacitic pyroclastic rocks Sanborn-Barrie et al. 2001 Well-bedded argillite and polymictic conglomerate Skulski et al. 2001 turbiditic wacke; Corfu and Andrews 1987 Dubé et al. 2004 Red ≤2747±4 Polymictic conglomerate; laminated cherty and jasper clasts; green mica fragments; aluminousaltered basalt and iron-carbonate vein clasts ConfederationHeyson (southeast Red Lake) 2739±3 Basal sequence is commonly tholeiitic to calcalkaline with lobe-hyaloclastite rhyolite flows; intermediate pyroclastic rocks; basalt; and feldspar-phyric andesite. Calc-alkaline rocks are more abundant at higher stratigraphic levels. Corfu and Wallace 1986 2742 2748+10/-5 Dominated by calc-alkaline, intermediate lapillituff breccia and lapilli tuff Sanborn-Barrie et al. 2001 2853 Lower tholeiitic basalt sequence with associated gabbroic rocks overlain by fine-grained clastic metasedimentary rocks (wacke, argillite) interlayered with subordinate intermediate pyroclastic rocks and chert-magnetite iron formation. Overlain by tholeiitic, pillowed basalts. Strongly calc-alkaline intermediate pyroclastic rocks overlain by pebble conglomerate, thinly bedded wacke and capped by chert-magnetite iron formation Interlayered, feldspathic wacke, lithic wacke and argillite; lenses of pebble and cobble conglomerates and quartz-rich pebble conglomerate and quartz arenite. Typically calc-alkaline intermediate pyroclastic rocks and rhyolite flows; komatiitic to tholeiitic basalt; overlain by chert-magnetite iron formation and dolomitic marble which contains stromatolites. Tholeiitic basalt, basaltic komatiite and komatiite interlayered with subordinate chert-magnetite iron formation; minor clastic metasedimentary rocks; minor intermediate to felsic pyroclastic rocks; and rhyolite. Sanborn-Barrie et al. 2001 ConfederationMcNeely (central and SE Red Lake) Trout Bay Bruce Channel Slate Bay Ball Balmer 2894±1.5; 2894±2 ≤2916 2940±2; 2925±3 2992+20/-9; 2989±3; 2964+5/-1 -5- Corfu and Wallace 1986; Corfu and Andrews 1987 Corfu et al. 1998 Corfu and Wallace 1986 Corfu and Andrews 1987 �Proceedings of the 61st ILSG Annual Meeting - Part 2 Table 2. Summary of lithologies and radiometric ages for major granitoid intrusions in the Red Lake greenstone belt (modified from Parker, 2000; new ages cited in Sanborn-Barrie et al., 2001 and elsewhere do not have final error estimates assigned, as this U-Pb data is not yet published). Granitoid intrusion Cat Island pluton Walsh Lake pluton Killala-Baird batholith Hammel batholith Dome stock U-Pb Age (Ma) 2699 2699 2704±1.5 Lake 2717±2 2718±1 McKenzie stock 2720±2 Red Crest stock 2729±1.5 Little Vermilion batholith Douglas Lake pluton Rock types and descriptions Potassium feldspar granodiorite Potassium feldsparand quartz-phyric monzogranite; xenolith-rich, diorite or granodiorite; possible oxidized phase at Ranger Lake with broad magnetic anomaly Potassium feldsparand quartz-phyric monzogranite; xenolith-rich, diorite or granodiorite, diorite or granodiorite; oxidized, magnetite-bearing marginal phase. Potassium feldspar and quartz porphyritic monzogranite; associated anorthositic intrusion. Granodiorite and augite porphyritic diorite/gabbro. Augite porphyritic diorite-gabbro; some ultramafic rocks; granodiorite Augite porphyritic diorite-gabbro 2731±3 Hornblende tonalite-granodiorite 2734±2 Biotite tonalite aluminous and carbonate alteration prior to deposition of the Huston conglomerate (Dubé et al. 2004). The position of colloform-crustiform iron-carbonate±quartz veins in the Campbell-Red Lake Deposit underneath the interpreted subaerial unconformity also leads Dubé et al. (2004) to interpret the veins as near-surface, epithermal-epizonal products, part of a protracted hydrothermal alteration event spanning pre- to postHuston assemblage time, a period of more than 35m.y. Recent age dating (Skulski et al. 2001) has also yielded multiple ages of detrital zircons from a fragmental unit thought to correlate with the Austin “tuff” ore zone at the former Madsen mine. Most of the Meso- and Neoarchean assemblages exposed in Red Lake are represented in this unit. Maximum age of deposition is consequently ≤2700±6Ma. Deformation (adapted from Sanborn-Barrie et al., 2001) The Red Lake greenstone belt has undergone at least three phases of deformation: 1. D0, a non-penetrative, early (pre-2748Ma) event involving overturning of the Balmer assemblage; References Sanborn-Barrie et al. 2001 Noble 1989 Corfu and Andrews 1987 McMaster 1987 Corfu 1987 Corfu 1987 Corfu 1987 Corfu 1987 Corfu 1998 and Andrews and Andrews and Andrews and Andrews and Stone 2. D1, (bracketed between 2733-2742Ma) resulted in a north-trending foliation that is axial planar to F1 folds and involved east-west shortening; and 3. D2, (ca. 2720-2700Ma) resulted in a dominantly east- to northeast-striking foliation that refolds F1 folds. A local ‘deflection’ of S2 around the McKenzie stock created an east-southeast-striking corridor of heterogeneous strain forming the “Mine Trend”, from Cochenour through the Balmertown area, hosting the major gold deposits of the camp. Hydrothermal Alteration Parker, 2000) (adapted from The Red Lake greenstone belt has been affected by a large-scale (10’s of kilometres) hydrothermal alteration system, resulting in approximately contemporaneous a) strong to intense, distal calcite carbonatization that affects rocks of all ages, and b) less extensive (kilometre), proximal, strong to intense ferroandolomite and potassic alteration, found in almost all areas hosting gold mineralization. Carbonate alteration affects both the Dome (2718±1Ma) and McKenzie (2720±2Ma) stocks and is overprinted by calc-silicate, skarn-like alteration formed during the intrusion of the -6- �Proceedings of the 61st ILSG Annual Meeting - Part 2 Killala–Baird batholith (2704±1.5Ma) and the Walsh Lake pluton (2699Ma). The significant carbonate alteration event is therefore bracketed between 2718 and 2704Ma, during D2. The main macroscopic features of carbonate alteration are pervasive replacement of rock matrix, open-space filling/replacement of primary porosity (vesicles, pillow selvages, hyaloclastite matrix), filling of extension veins with massive, colloform, crustiform and cockade breccia textures, networks of variably oriented veins, and “jigsaw puzzle” breccia veins. Multiple stages of carbonate alteration and veining have been recognized, indicating continuous carbonatization during D2 deformation. Potassic metasomatism takes the form of sericite/ muscovite alteration in greenschist-facies rocks; in ferroan-dolomite altered ultramafic rocks fuchsite occurs instead of sericite. Potassic alteration in amphibolite-facies mafic and ultramafic rocks takes the form of pervasive biotite±muscovite. Centimetreto m-wide, strong to intense, biotite±calcite±ferroandolomite±disseminated pyrite alteration halos often enclose ferroan-dolomite veins in amphibolite-facies mafic rocks. Biotite altered zones in amphibolite-facies rocks are characterized by a diverse assemblage of aluminosilicate minerals such as andalusite, staurolite and cordierite, with garnet, chloritoid, cummingtonite, and anthophylite. Barren, pervasive silicification within proximal alteration zones may be due to release and remobilization of silica during periods of pervasive carbonatization. The majority of gold deposits in the Red Lake belt are quartz and arsenopyrite rich selective replacement zones of colloform-crustiform ferroan-dolomite veins and breccia. Geology of the Campbell-Red Lake Gold Deposit (adapted from Dubé et al., 2002) Gold has been continuously produced from the Red Lake (formerly known as the Campbell-Dickenson) deposit since 1948. Current production levels and reserves are given in Table 3. Historic production figures for the Red Lake greenstone belt are shown in Table 4. Alteration facies in the High Grade Zone at Goldcorp Inc.’s Red Lake Mine have been described by Dubé et al. (2002) as: 1. An outer, metre-wide, garnet-chlorite-magnetite alteration with chlorite-amphibole-andalusite and locally associated centimetre- to metre-wide ‘bleached zone’ containing andalusite-muscovitequartz-ilmenite; 2. A proximal, centimetre- to metre-wide, massive to laminated, reddish-brown, biotite-carbonate alteration with disseminated pyrite (3-5%) and carbonate veinlets in well foliated basalt; and 3. A gold-rich, strongly foliated, silicified zone with abundant fine-grained arsenopyrite, sericite, and rutile, and lesser amounts of pyrite, pyrrhotite, magnetite, and stibnite (≤15%). This third alteration facies is adjacent to the silicified auriferous carbonate veins and replaces the biotite-carbonaterich alteration. The chronology of gold-rich replacement textures suggests a syn-D2 mineralizing event, dominated by silicification of carbonate veins, contemporaneous with boudinage of the veins. The silicified carbonate veins are hosted mainly by basalt; areas of high-grade gold mineralization are controlled by F2 fold hinges deforming the basalt-ultramafic contact. Multiple periods of silicification and gold deposition overprint and replace the carbonatization in these lower pressure hinge zones. The extremely high grade ore (>2.0oz/t Au) currently mined at Goldcorp Inc.’s Red Lake Mine, is possibly due to a combination of factors, including the presence of a low-permeability ultramafic cap, allowing the build-up of very high fluid pressure in the footwall basalt; the high iron content of the tholeiitic basalt, creating a chemical, as well as structural, trap for the Table 3. Current gold production and reserves, Goldcorp Inc., Red Lake Gold Mines Production in 2013 Mine Goldcorp Inc. Red Lake Gold Mines (1) Tonnage @ Grade 786 900 tonnes @ 20.33 g/t Au Total Commodity 493 000oz Au Production in 2014 Tonnage @ Grade 684 100 tonnes @ 19.47 g/t Au (1) Goldcorp Inc., news release MD&A, February 19, 2015. -7- Total Commodity 414 400 ounces Au Reserves (Proven and Probable) at end of 2014 Contained Ounces Gold 2 060 000 �Proceedings of the 61st ILSG Annual Meeting - Part 2 Table 4. Historical gold production, Red Lake greenstone belt GOLD PRODUCTION IN THE RED LAKE GREENSTONE BELT to December 31, 2014 GOLD PRODUCED RED LAKE BELT PRODUCER RED LAKE GOLD MINES CAMPBELL RED LAKE GOLDCORP (DICKENSON) MADSEN COCHENOURWILLANS MCKENZIE RED LAKE HOWEY HASAGA STARRATT OLSEN H.G. YOUNG MCMARMAC GOLD EAGLE RED LAKE GOLD SHORE BUFFALO ABINO LAKE ROWAN RED SUMMIT MOUNT JAMIE TOTAL YEARS OF PRODUCTION 2006 to present (1) ORE MILLED (SHORT TONS) TROY OUNCES OUNCES PER TON GRAMS PER METRIC TONNE 6,906,322 5,285,590 0.694 23.80 19,944,241 11,216,443 0.510 17.49 1948 - 2005 9,606,894 5,962,948 0.621 (4) 21.28 1938 - 1976, 1997 – (5) 1999 1939 - 1971 8,678,143 2,452,388 0.283 (6) 9.69 2,311,165 1,244,279 0.538 (7) 18.46 1935 - 1966 2,353,833 651,156 0.277 4,630,779 1,515,282 907,813 288,179 152,978 180,095 86,333 421,592 218,213 163,990 55,244 45,246 40,204 21,100 0.091 0.144 0.181 0.192 0.296 0.223 0.244 31,986 2,733 13,023 591 552 57,061,260 1,656 1,397 1,298 277 265 27,369,086 0.052 0.511 0.100 0.469 0.480 0.335 1949 – 2005 (2) 1930 - 1941, 1957 1938 - 1952 1948 - 1956 1960 - 1963 1940 - 1948 1937 - 1941 1936 - 1938 1981 - 1982 1985 - 1986 1986 - 1988 1935 - 1936 1976 (8) 9.48 (9) 1. Includes total production from the Red Lake complex from January 1, 2006, and production from the Campbell complex subsequent to May 12, 2006, the date of acquisition. 2. Includes production figures under Placer Dome (CLA) Ltd., to May 12, 2006. 3. For 1997, 1998 and 1999, no production due to strike by unionized employees. 4. From 1970, includes production from Robin Red Lake. 5. Includes clean up of ore and materials from the mine site. 6. Historic grade, actual grade for 1999 was 0.14 ounce per ton gold. 7. Includes production from Annco and Wilmar properties. 8. Continuous production 1930 to 1941; includes 268 ounces recovered from clean up in 1957. 9. The ore mined at Howey, before sorting totalled 5 158 376 tons. The average production from run-ofmine ore was therefore 0.0817 ounce per ton gold. -8- 3.12 4.94 6.19 6.57 10.14 7.65 8.38 1.78 17.53 3.42 16.07 13.30 17.4 �Proceedings of the 61st ILSG Annual Meeting - Part 2 auriferous fluids; multiple D2 strain events; repeated episodes of gold deposition and remobilization into a low pressure F2 fold hinge hosting the High Grade Zone. garnet-biotite-staurolite-amphibole; metre-wide stockwork amphibole veins and veinlets alternate with the pervasive alteration. Timing of this alteration is pre- to syn-D1, but its relationship to gold mineralization is not yet known; indeed, it could be classified as the amphibolite-facies equivalent of volcanogenic massive sulphide (VMS) type alteration, related to a Confederation age synvolcanic hydrothermal alteration system; Field Trip Stops The Field Trip starts in the west, at the Suffel Lake (Flat Lake) Road turn-off from Highway.618 and continues east and north, to end at the Redcon Prospect, approximately 17.2km north on Nungesser Road. Stop 1: Suffel Lake Road and Highway 618. Contact between Confederation and Balmer Assemblages (Fig. 3, Table 5) UTM Coordinates: NAD83; 15U 0434844E / 5645498N Exposures on the south side of the highway are part of the lowermost units of the Neoarchean Confederation assemblage, the dominantly calc-alkaline McNeely sequence. The outcrops here are amphibolite-facies, quartz-feldspar-porphyritic lapilli-crystal tuff, with thin, dark grey, collapsed pumice fragments; occasional lapilli sized lithic clasts are also observed. Strike of the rocks is generally northeast, facing and dipping steeply southeast. A sample from this unit, 800m northeast of the intersection, gave an age of 2744±1Ma (Corfu and Andrews, 1987). 2. An inner zone comprising a banded-laminated texture, characterized by bands of actinolitehornblende-microcline-calcite-tourmaline, alternating with biotite-rich bands. The amphibole is commonly randomly oriented. Diopside locally forms disseminated crystals up to 7-8cm long, or veinlets. Ore zones occur within the inner alteration zone, and comprise finely layered, sulphide-rich lenses up to a few metres wide. Sulphides (8-10%) comprise pyrrhotite, pyrite and/or arsenopyrite with trace chalcopyrite, and are found as disseminations or veinlets parallel to lamination/foliation. Gold occurs in the native state as inclusions in silicate minerals and locally as coatings on sulphide minerals. The highest grade is found in areas of most intense alteration, The north side of the road exposes highly altered tholeiitic, mafic volcaniclastic rocks of the Balmer assemblage. Abundant garnet and biotite rims clasts; minor andalusite is present. This outcrop, barren at this locality, forms part of the Austin “tuff” ore zone, described further below. Stop 2: Power Line Outcrops, Madsen Deposit UTM Coordinates: NAD83; 15U 0435235E / 5646040N Time limitations of the tour do not permit a complete visit of the Madsen deposit; a brief description of the deposit follows: Geology of the Madsen Deposit (adapted from Dubé et al., 2000) Madsen is a stratabound, replacement-style, disseminated gold deposit (Fig. 3), exhibiting two alteration facies, the mineralogy of which is now represented by two amphibolite-facies zones: 1. A pervasive aluminous, metre- to tens-of-metreswide, low-strain, outer zone, containing andalusite- Figure 3. Geology of the Madsen mine area (modified from Dubé et al., 2000) -9- �Proceedings of the 61st ILSG Annual Meeting - Part 2 Table 5. Major and trace element analyses Stop 1 lithologies (major oxides in %, trace elements in ppm). Sample Number UTM-Easting UTM-Northing Rock Type Sample No. SiO2 TiO2 Al2O3 Fe2O3 K2O MgO MnO CaO Na2O P2O5 LOI Total 2009-AL-02 434891 5645522 QFP 2009AL-02 76.19 0.1 12.06 1.58 4.86 0.36 0.03 0.94 2.53 0.01 0.82 99.48 2009-AL-03 434849 5645513 Basalt 2009AL-03 48.61 1.55 14.59 19.21 0.91 4.37 0.51 9.57 0.41 0.21 0.73 100.67 Au (opt) Ag (opt) <0.01 <0.1 0.01 <0.1 Rb Ba Sr Sc La Ce Nd Sm Eu Gd Tb 88.23 1044.1 76 3.5 47.57 86.49 27.27 3.78 0.47 2.46 0.303 19.66 84.2 44 33.9 7.84 18.69 12.85 3.85 1.26 5.23 0.936 represented by a quartz-biotite-muscovite-microcline assemblage in mm-cm bands or layers. Crenulation of alteration bands, sulphides and calcite veinlets by S2 as well as the large-scale deformation and folding of Austin ore lenses by F2 folds are consistent with pre- to early-D2 timing of gold mineralization. A minimum age on the deposit is 2699±4Ma (Corfu and Andrews, 1987), the age of a cross-cutting post-ore granodiorite dyke. Proximal alteration and style of mineralization may indicate the Madsen Deposit to be related to higher temperature (400º-600ºC) gold deposits and gold-skarn deposits hosted by mafic volcanic rocks (Parker, 2000). Sample Number Yb Lu Y Zr Th U Hf Nb Ta Cs Dy Er Ho Pr Tm Be Cd Ga Li Mo Sb Sn Tl V W Zn Pb Cu Cr Ni Co Bi Ti 2009AL-02 0.838 0.13 8.53 108 15.16 2.98 3.21 4.33 0.4 2.5 1.68 0.86 0.31 8.54 0.125 0.73 0.11 13.47 12.9 3 1.47 1.03 0.46 <10 1.32 30.56 12.6 22 36 5 1.7 0.094 605.94 2009AL-03 4.389 0.68 41.59 67 0.99 0.24 1.9 3.82 0.2 1.83 6.55 4.36 1.43 2.66 0.665 0.57 0.14 19.02 24.1 0.95 1.11 0.79 0.1 358.45 0.75 146.21 1.8 86 109 99 47.8 0.041 8772.63 South Austin Zone – Powerline Section The integrated mapping, geochronological, and lithogeochemical projects completed during the Federal–Provincial NATMAP program in the Red Lake greenstone belt from 1999 to 2004 significantly advanced the understanding of the geological history of the belt. Key outcrops elucidating the 300Ma history of the belt include the surface expression of the main ore zone (“South Austin Zone”) of Claude Resources Inc.’s past-producing Madsen Mine (produced 2.5 million ounces Au between 1938 and 1999; see Table 4), which is interpreted to occupy the position of the unconformity between the Mesoarchean Balmer Assemblage and Neoarchean Confederation - 10 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 4. Location of the South Austin Zone (adapted from Dubé et al., 2000). Assemblage (Sanborn-Barrie et al., 2001). Geological field tours of the Red Lake mining camp have been given since the start of the Red Lake office of the Resident Geologist Program in 1968. The South Austin Zone outcrop has become an important field trip stop since NATMAP re-interpretation of local geology, illustrating the complex interplay of lithology, alteration, deformation, and economic gold mineralization. The series of poorly exposed hillside outcrops are located approximately 600m southwest of the Madsen shaft (Fig. 4). They were mechanically stripped in the fall of 2009 and power-washed and channel-sampled by S. McDonald in the summer of 2010. The exposure is now complete from the Austin Zone footwall, through the Confederation Assemblage quartz-feldspar rhyolite, to the overlying Huston conglomerate (Fig. 5). Following is a preliminary description of the lithology and geochemistry of the exposure. The base of the exposure is a well banded/layered, locally contorted, example of Austin “tuff”, from which the bulk of the 2.5 million ounces Au of the Madsen deposit were mined. At this locality, the Austin Zone is a strongly altered (biotite, amphibole, garnet) mafic volcaniclastic/epiclastic rock, with up to 20% wacke/tuff (?) clasts, occupying the position of the unconformity between Balmer and Confederation assemblages. Major oxide chemistry (Table 8), uncorrected for alteration, indicates that 11 of 12 samples collected from this unit are tholeiitic (Irvine and Baragar, 1971, Fig. 2). A minimum age on the deposit is 2699±4Ma (Corfu and Andrews, 1987), the age of a crosscutting post-ore granodiorite dike (not exposed at this locality). Weakly to moderately foliated quartz-feldspar rhyolite porphyry (“QFP”) forms the structural and stratigraphic hanging wall of the deposit and marks the beginning of Confederation Assemblage time (McNeely sequence of Sanborn-Barrie et al., 2001). Contact with the underlying Austin Zone is sharp, with the Austin Zone characterized by an 80cm-wide transition zone of contorted and brecciated rock. Whole rock analyses of two samples of porphyry plot in the calc-alkaline rhyolite field of Jensen (1976; Fig. 1). Overlying the quartz-feldspar rhyolite is a conglomerate unit, part of the Huston Assemblage, which yielded a single peak in detrital U-Pb zircon ages of ≤2746Ma (Sanborn-Barrie et al., 2001). Fragments are generally intermediate to mafic, with a mafic biotite-garnet-andalusite altered matrix. Major - 11 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 5. Detail of stripping and sampling, South Austin Zone. Sample series 2010SM-01 through 2010SM-27. oxide chemistry (Table 8), uncorrected for alteration, indicates that the 4 samples collected from this unit are of tholeiitic affinity (Irvine and Baragar, 1971, Fig. 2). Tables 6 and 7 show the trace element analyses for samples collected during this study. Heyson felsic volcanic rocks, including spherulitic and lobe-hyaloclastite flows are seen in outcrops on both sides of Highway 618 on the way to Stop 3. Samples from all three rock units (Tables 6, 7, 8, and 9) were submitted to the Pacific Centre for Isotopic and Geochemical Research (“PCIGR”), Department of Earth and Ocean Sciences, University of British Columbia for geochronological analysis. The almost complete sequence across the interpreted unconformity is exhibited in these outcrops. A cross-section of age data was deemed important to understanding the relationships here, and in the >100km of unconformity interpreted throughout the Red Lake belt. TIMS dating employing the single grain chemical abrasion (CA-TIMS) technique (Plot 1). Five single grains analysed give results that are concordant and overlapping at 2741Ma. The best age estimate for the rock, 2741.0±0.8Ma, is based on the weighted average of all five 207Pb/206Pb dates. Sample 2010SM-031: The Huston Conglomerate was dated by the laser ablation (LA-U-Pb) technique, with 59 grains analysed (Plots 2 and 3). All analysed grains are less then ±5% discordant and 207Pb/206Pb A 20-litre sample was collected from each unit; sample preparation by PCIGR can be found at http://pcigr.eos.ubc.ca/services/samplepreparation. php#Geochronology. Description of results from PCIGR includes: Sample 2010SM-029: South Austin Zone-No zircons were found Sample 2010SM-030: QFP lapilli tuff; U-Pb Plot 1: Uranium-lead concordia diagram of QFP lapilli tuff. - 12 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Table 6. Trace element analyses (ppm) of South Austin Tuff samples (sample number prefix is 2010SM, i.e., 2010SM-01). Sample No. Rock Type -01 Austin -02 Austin -03 -04 Austin -05 Austin -06 Austin Austin -07 -08 Austin Austin -09 -10 Austin -11 Austin -12 Austin Austin Rb 100.79 64.46 73.7 108.19 115.67 104.61 70.52 122.2 109.1 96.68 82.53 142.82 Ba 259.6 199.3 357.5 282.5 264.8 275.1 226.6 249.2 245.9 125.7 166.2 190.8 Sr 31 80 126 98 77 65 41 44 41 6 9 9 Sc 43.3 34.7 42.2 40.1 53.7 51.8 42.9 50.9 53.2 47.9 32.3 31.1 La 5.64 5.4 7.39 6.8 7.66 7.31 6.26 6.58 7.24 11.35 7.01 9.96 Ce 14 13.14 18 16.56 18.96 17.77 16.23 17.16 17.23 24.24 15.08 20.37 Nd 9.5 8.75 12.05 11.61 12.64 11.81 11.37 11.16 11.77 15.18 9.49 12.27 Sm 2.9 2.82 3.8 3.72 3.98 3.58 3.72 3.44 3.55 4.14 2.57 3.11 Eu 1 1.19 1.29 1.25 1.4 1.33 1.45 1.24 1.18 1.37 0.81 0.79 Gd 3.71 3.73 5.03 4.96 5 4.67 4.99 4.1 4.46 4.95 2.89 3.7 Tb 0.686 0.655 0.889 0.896 0.895 0.821 0.925 0.738 0.839 0.863 0.482 0.629 Yb 3.412 2.886 4.092 4.013 3.822 3.41 3.959 3.702 4.158 4.356 2.054 2.415 Lu 0.52 0.43 0.61 0.59 0.57 0.5 0.6 0.57 0.63 0.69 0.31 0.36 Y 28.23 26.63 36.82 36.72 34.9 31.92 38.04 29.44 34.11 34.81 18.05 24.04 Zr 103 54 38 47 99 105 74 106 99 77 67 81 Th 0.99 0.71 0.99 0.96 1.01 1 0.94 1.03 0.95 1.01 0.7 0.97 U 0.25 0.22 0.26 0.25 0.22 0.24 0.24 0.28 0.23 0.25 0.25 0.37 Hf 2.97 1.61 1.3 1.41 2.8 3.05 2.13 3 2.78 2.19 1.99 2.36 Nb 4.13 2.91 4.16 4 4.25 4.1 3.78 4.24 3.92 3.88 2.86 4.28 Ta 0.3 0.2 0.3 0.2 0.3 0.3 0.2 0.3 0.3 0.3 0.2 0.3 Cs 6.25 2.95 2.16 4.1 6.12 5.41 2.99 6.22 5.39 6.27 3.7 6.08 Dy 4.74 4.53 6.15 6.14 6.04 5.58 6.34 5.17 5.87 5.94 3.14 4.15 Er 3.18 2.97 4.15 4.04 3.85 3.47 4.15 3.57 3.99 4.08 2.04 2.6 Ho 1.04 0.98 1.34 1.33 1.28 1.18 1.37 1.14 1.29 1.32 0.67 0.88 Pr 1.98 1.91 2.6 2.4 2.68 2.54 2.39 2.39 2.51 3.39 2.11 2.79 Tm 0.489 0.438 0.61 0.598 0.573 0.513 0.596 0.543 0.604 0.63 0.3 0.371 Be 0.69 3.06 2.23 1.94 1.24 0.87 0.62 0.52 0.49 0.23 0.28 0.75 Cd 0.11 0.1 0.13 0.07 0.07 0.08 0.21 0.12 0.16 0.28 0.07 1.86 Ga 20.34 18.27 20.93 20.51 22.75 22.26 19.38 22.42 20.46 18.14 14.51 18.55 Li 46 20.7 20.6 29.2 40.2 37.4 19.7 38.2 30 26.2 21.1 26.2 Mo 1.25 1.47 1.07 1.21 0.63 0.48 0.51 0.49 0.47 0.66 0.77 1.22 Sb 0.76 2.44 3.46 2.03 0.82 0.8 1.95 1.09 0.97 0.71 0.78 1.14 Sn 0.87 0.83 0.73 0.87 0.75 0.91 0.84 0.81 0.93 0.73 0.67 0.58 Tl 0.52 0.33 0.33 0.51 0.55 0.5 0.37 0.59 0.54 0.52 0.41 0.86 V 403.74 251.4 410.39 398.18 453.66 429 399.06 431.52 420.2 433.06 227.59 267.23 W 9.61 67.03 63.65 45.25 18.88 9.16 19.03 3.21 2.3 3.4 9.59 11.86 Zn 105.7 66.58 81.62 77.45 91.71 69.39 173.52 121.95 108.11 107.27 34.15 388.81 Pb 3.5 3.5 3.7 2.8 2.9 3.1 3.4 2.2 2.7 6.2 2 21.4 Cu 45 74 64 75 114 86 135 107 69 86 37 182 Cr 113 90 111 116 118 116 104 117 111 106 154 162 Ni 87 67 89 100 94 94 70 84 78 75 118 127 Co 44.7 34.9 50.9 52.7 52.7 57.1 43.7 51.9 44.5 39.5 33.1 38.8 Bi 0.031 0.065 0.105 0.086 0.032 0.033 0.083 0.051 0.077 0.066 0.036 0.128 Ti 9630.2 6115.15 9527.75 9381.44 10239.1 10082.4 8816.34 10119.2 9356.36 9089.29 5094.75 6837.98 Analyses by Geoscience Laboratories, Ministry of Northern Development, Mines and Forestry, Sudbury, Ontario. - 13 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Table 7. Trace element analyses (ppm) of QFP and Huston conglomerate samples (sample number prefix is 2010SM, i.e., 2010SM-13). Analyses for samples 2010SM-25 and -26 are pending. Sample No. Rock Type -13 QFP -14 QFP -15 Huston -16 Huston -17 Huston -18 Huston -19 Huston -20 Huston -21 Huston -22 Huston -23 Huston -24 Huston Rb 85.22 74.47 107.78 68.11 64.93 63.87 77.81 77.86 61.36 63.04 53.71 66.62 Ba 722 569.1 495.9 283.4 271.4 584.6 334.3 293.1 250.6 276.9 273.8 261.1 Sr 73 54 217 40 38 77 80 66 56 44 40 41 Sc 5 4 28.5 41.4 37.7 21 28.2 35.7 39.3 45.1 39.9 42.9 La 27.9 20.46 23.3 6.55 7.12 20.45 14.54 7.78 6.57 9.42 7.56 7.78 Ce 52.26 38.29 45.04 14.46 16.56 39.34 27.97 15.81 14.82 20.41 16.52 17.22 Nd 15.14 11.48 17.35 8.39 10.47 15.7 12.19 7.91 8.24 10.52 9.11 9.99 Sm 2.26 1.63 3.29 2.28 3.17 3.09 2.72 2 2.25 2.68 2.42 2.57 Eu 0.58 0.44 1.09 0.82 0.97 0.91 0.76 0.95 0.86 0.87 0.81 0.91 Gd 1.75 1.25 3.04 2.84 4.61 2.69 2.72 2.38 2.77 3.08 2.95 3.12 Tb 0.277 0.199 0.494 0.494 0.818 0.429 0.45 0.426 0.495 0.554 0.505 0.552 Yb 1.037 1.012 1.888 2.086 3.384 1.512 1.624 1.902 2.127 2.398 2.163 2.297 Lu 0.16 0.15 0.29 0.32 0.51 0.23 0.25 0.29 0.33 0.36 0.33 0.35 Y 10.83 8.66 18.68 18.81 35.54 14.91 16.06 16.41 19.34 21.67 19.85 21.14 Zr 125 122 109 72 73 132 87 86 81 98 75 74 Th 12.36 12.32 6.33 1.06 1 8.18 3.38 2.67 1.46 2.37 1.26 1 U 2.36 2.2 1.52 0.38 0.32 2.98 1.06 0.79 0.51 0.73 0.34 0.33 Hf 3.72 3.64 3.19 2.13 2.11 3.74 2.42 2.48 2.39 2.71 2.12 2.17 Nb 4.69 4.46 4.26 3.39 3.25 5.71 4.1 3.9 3.72 4.59 3.48 3.63 Ta 0.4 0.4 0.4 0.2 0.2 0.6 0.3 0.3 0.3 0.3 0.2 0.2 Cs 1.09 1.28 4.78 2.33 2.25 1.56 2.39 1.85 1.82 2.1 1.8 2.38 Dy 1.76 1.35 3.2 3.31 5.53 2.7 2.81 2.89 3.32 3.7 3.43 3.69 Er 1.08 0.93 1.97 2.1 3.66 1.57 1.69 1.88 2.13 2.4 2.21 2.33 Ho 0.36 0.3 0.67 0.7 1.22 0.55 0.59 0.61 0.72 0.8 0.73 0.78 Pr 4.68 3.59 4.84 1.93 2.29 4.43 3.22 1.94 1.93 2.57 2.13 2.28 Tm 0.157 0.144 0.285 0.31 0.518 0.229 0.248 0.288 0.324 0.354 0.326 0.349 Be 1.04 1.11 1.4 0.82 0.65 1.14 0.61 0.61 0.74 0.76 0.46 0.57 Cd 0.03 0.03 0.37 0.69 1.17 0.19 0.27 0.37 0.8 0.39 0.28 0.79 Ga 16.27 15.14 16.66 13.94 14.33 21.19 16.76 16.31 15.24 14.44 9.8 11.78 Li 17.9 18.6 26.7 24.9 26.7 33.5 30.1 25.5 27.9 44.6 25.6 24.3 Mo 1.52 1.05 1.51 1.14 0.93 1.33 1.66 1.81 1.43 1.11 1.06 0.96 Sb 0.34 0.22 0.93 1.25 1.15 0.62 0.61 1.13 1.42 1.49 1.02 0.74 Sn 0.73 0.73 0.8 0.58 0.54 0.87 0.8 0.7 0.56 0.62 0.69 0.59 Tl 0.31 0.29 0.55 0.36 0.32 0.28 0.39 0.35 0.3 0.33 0.29 0.37 V 18.54 12.18 189.3 284.59 263.86 145.66 202.73 249.17 270.06 307.75 270.75 288.7 W 1.76 3.9 2.3 2.43 1.98 3.05 1.39 1.62 1.57 1.46 1.34 2.16 Zn 21.37 26.71 139.65 105.4 131.72 51.19 77.43 99.6 101.71 50.85 30.67 72.94 Pb 6.5 4.8 8.1 9.7 8.8 5.6 5.1 6.6 8.2 4.7 5.7 6 Cu 4 3 382 235 199 44 172 191 191 225 145 197 Cr 32 <24 190 466 326 122 343 428 397 502 388 334 Ni 14 19 119 193 162 76 131 136 162 232 154 170 Co 3.7 5 38.6 62.7 55.8 25.5 39.4 37 53 62.7 47.3 58 Bi 0.024 0.061 0.024 0.088 0.104 0.028 0.088 0.106 0.123 0.086 0.102 0.05 Ti 1274.8 1039.0 4366.4 5646.3 5878.0 3920.6 4674.2 5442.6 6041.0 7120.2 5825.7 6060.0 Analyses by Geoscience Laboratories, Ministry of Northern Development, Mines and Forestry, Sudbury, Ontario. - 14 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Table 8. Major element chemistry of all lithologies, South Austin Tuff exposure. Sample No. Rock Type SiO2 (%) TiO2 (%) Al2O3 (%) Fe2O3 (%) K2O (%) MgO (%) MnO (%) CaO (%) Na2O (%) P2O5 (%) LOI (%) Total (%) 2010SM-01 Austin 54.33 1.67 15.68 15.38 4.66 3.55 0.37 1.72 0.55 0.15 1.65 99.71 2010SM-02 Austin 52.45 1.07 14.35 14.45 2.34 4.37 0.32 6.79 0.38 0.12 2.26 98.90 2010SM-03 Austin 46.23 1.64 15.22 14.31 3.34 4.97 0.40 10.25 0.20 0.18 2.51 99.23 2010SM-04 Austin 52.95 1.61 15.44 12.90 3.65 4.14 0.28 6.61 0.17 0.18 2.51 100.43 2010SM-05 Austin 53.84 1.68 16.44 11.79 4.37 3.93 0.21 4.47 0.29 0.20 2.39 99.60 2010SM-06 Austin 54.55 1.69 16.71 10.66 4.35 4.03 0.21 4.40 0.34 0.20 2.33 99.48 2010SM-07 Austin 47.39 1.53 14.45 16.10 2.61 5.92 0.45 8.57 0.36 0.16 2.12 99.67 2010SM-08 Austin 49.95 1.69 16.48 15.00 4.74 4.29 0.27 3.86 0.25 0.20 2.79 99.52 2010SM-09 Austin 53.44 1.59 15.49 14.73 4.36 3.74 0.27 3.33 0.21 0.19 2.37 99.71 2010SM-10 Austin 50.62 1.55 14.95 23.23 3.62 3.11 0.48 0.45 0.04 0.17 1.23 99.46 2010SM-11 Austin 64.42 0.86 14.08 12.20 3.57 1.89 0.27 0.24 0.05 0.10 1.76 99.44 2010SM-12 Austin 58.16 1.13 14.19 15.78 4.65 2.30 0.39 0.36 0.04 0.14 2.01 99.15 2010SM-13 QFP 74.89 0.21 14.08 1.64 3.79 0.74 0.04 1.53 1.01 0.03 1.83 99.80 2010SM-14 QFP 76.14 0.17 13.28 2.27 3.27 0.82 0.04 1.17 1.08 0.03 1.78 100.05 2010SM-15 Huston 62.92 0.72 14.84 8.94 3.45 2.79 0.15 2.65 1.88 0.05 1.42 99.83 2010SM-16 Huston 60.37 0.97 14.21 15.04 2.55 3.33 0.29 1.46 0.20 0.07 1.49 99.99 2010SM-17 Huston 59.94 0.99 13.16 16.81 2.34 2.98 0.42 1.58 0.21 0.06 1.04 99.54 2010SM-18 Huston 64.49 0.67 16.82 7.53 2.93 2.24 0.14 2.11 0.84 0.08 1.99 99.84 2010SM-19 Huston 64.50 0.80 13.64 9.40 2.80 2.85 0.17 2.62 0.90 0.07 1.26 99.01 2010SM-20 Huston 62.65 0.94 13.46 12.23 2.73 3.01 0.20 1.85 0.46 0.06 2.23 99.82 2010SM-21 Huston 60.95 1.05 13.82 14.09 2.22 3.10 0.22 1.65 0.33 0.06 1.40 98.89 2010SM-22 Huston 60.45 1.21 14.48 14.76 2.22 3.38 0.19 1.11 0.29 0.06 1.26 99.40 2010SM-23 Huston 63.56 1.04 13.74 14.01 2.05 2.93 0.28 0.98 0.28 0.06 0.99 99.92 2010SM-24 Huston 58.64 1.04 14.61 16.14 2.53 3.08 0.33 1.52 0.27 0.08 0.68 98.92 Analyses by Geoscience Laboratories, Ministry of Northern Development, Mines and Forestry, Sudbury, Ontario. than 2700Ma. Data from this sample suggest that the maximum age of the breccia is 2700Ma, and including the very youngest result the maximum age could be as young as 2655±9Ma. The 2741±0.8Ma age of the QFP lapilli tuff correlates Plot 2: Uranium-lead concordia diagram of Huston conglomerate. dates for nearly all are likely to record primary crystallization ages. Reversely discordant grains 25 and 33 are the only ones that give results younger Plot 3: Uranium-lead probability plot, Huston conglomerate. - 15 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Table 9. Precious metal analyses of Austin Tuff exposure samples Units Detect Limit 2010SM-01 2010SM-02 2010SM-03 2010SM-04 2010SM-05 2010SM-06 2010SM-07 2010SM-08 2010SM-09 2010SM-10 2010SM-11 2010SM-12 2010SM-13 2010SM-14 2010SM-15 2010SM-16 2010SM-17 2010SM-18 2010SM-19 2010SM-20 2010SM-21 2010SM-22 2010SM-23 2010SM-24 2010SM-25 2010SM-26 2010SM-15 SP Au ppb 6 37 431 73 35 30 9 <6 8 <6 29 9 36 <6 <6 22 11 16 6 12 15 9 10 8 <6 6 13 19 Pd ppb 1.3 <1.3 <1.3 <1.3 <1.3 <1.3 <1.3 <1.3 <1.3 <1.3 <1.3 3.9 7 <1.3 <1.3 3.4 7.3 5.8 2.2 4.3 5.6 6.1 6.4 6.1 8 6.2 6.1 3.2 Pt ppb 0.4 1 0.9 0.8 0.9 1 0.9 0.9 1 1 1 4.1 4.7 <0.4 <0.4 4.3 8.5 7.2 3 5.2 6.8 8.1 7.9 8 9.1 8.2 7.7 4.7 detrital ages. A similar unit (called the “Madsen Conglomerate”) was sampled by Sanborn-Barrie et al. (2004), approximately 1.2km northeast of the South Austin zone locality. A maximum depositional age of this unit was interpreted to be 2700±6Ma, based on a diverse detrital zircon profile (n=56), with ages that correspond to volcanic assemblage as old as ca. 2.9Ga and younger post-volcanic plutons ca. 2.7Ga. Its relationship to gold mineralization remains unknown. The questions arising from different age profiles obtained from the same outcrop of conglomerate, and the very young depositional ages of detrital zircons (English River assemblage) can be addressed by additional geochronology, detailed mapping of the units in the immediate area and inspection of core from diamond drilling of the Austin ore zone by Claude Resources Inc. and Pure Gold Mining Inc. Stop 3: Buffalo Deposit – Dome Stock mineralization UTM Coordinates: NAD83; 15U 0439890E / 5650800N The approximately 7km diameter hornblendebiotite granodiorite Dome Stock (Table 2) has been dated at 2718±1Ma (Corfu and Andrews 1987) and is interpreted to have been emplaced during D2 (SanbornBarrie et al. 2001). The stock is variably iron-carbonate, sericite, and chlorite altered and deformed. Exposures to be visited are at its southern contact; here it intrudes, and contains xenoliths of, foliated Balmer assemblage mafic volcanic rocks (Fig. 6). well with previous dates of 2744±1Ma obtained by Corfu and Andrews (1987) from the same lapilli tuff or immediately adjacent unit 150m to the southwest, and a felsic spherulitic flow approximately 120m in the hangingwall, dated at 2746+36/-17Ma (Corfu and Andrews, 1987). The same outcrop of interpreted Huston conglomerate at the present locality and one 15km to the northeast (referred to as the “Cemetery conglomerate”) were previously dated by T. Skulski at less than ca. 2746Ma, based on prominent (n=5060) single age peaks (cited in Sanborn-Barrie et al., 2004). They concluded a single-source derivation from Confederation assemblage volcanic rocks. The current data from Madsen suggests that this conglomerate unit may have sampled a very diverse substrate, and therefore displays a wider range of The Dome Stock hosts several gold occurrences and two small, past-producing mines 1) the Red Lake Gold Shore produced 21,100 ounces gold, and 2) the Buffalo Mine produced 1656 ounces gold. The Buffalo Mine was discovered in 1925 and explored several times since then. Note: the adit was reopened by Claude Resources Ltd. in October 1998 to further explore the Buffalo deposit. Rehabilitation of the adit started in 2004. Gold is hosted within two sets of quartz-tourmalinepyrite-calcite veins in conjugate orientation (cm-wide northeast-trending veins at 239°/73°N, and decimetrewide northwest-trending veins at 119°/76°S; Pettigrew, 1999). Their orientation may be a result of the intersection of two previously interpreted (Durocher and Hugon, 1983) deformation zones (St. Paul BayMartin Bay and Flat Lake-Howey Bay Deformation Zones). The dominant vein set strikes northwest and primary quartz vein fill was replaced by tourmaline, concomitant with bleached pink metasomatic halos - 16 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 6. Detailed geology of south side of Buffalo Pit (from Lavigne et al., 1986). developing around tourmaline-rich portions of the veins. Gold is concentrated in the calcite-albitesulphide halos, in particular at its outer fringe, where chalcopyrite and tellurides were deposited. A second stage of gold mineralization is associated with Bitellurides in fractures and cavity fillings in quartz and late fracture-filling pyrite, hosted within the quartztourmaline-pyrite-calcite veins. Stop 4: Flat Lake–Howey Deformation Zone Bay–Flat (similar in appearance to the Dome stock) containing mafic xenoliths and quartz-tourmaline veinlets. Table 10. Major and trace element analyses of mylonite, Stop 4 (major oxides in %, trace elements in ppm) Lake UTM Coordinates: NAD83; 15U 0441670E / 5651602N The Howey Bay–Flat Lake deformation zone was defined by Durocher and Hugon (1983), and was interpreted to be part of a belt-wide system of transcurrent shear zones hosting most of the major gold deposits. Recent detailed work has led to a reevaluation of this concept (Sanborn-Barrie et al., 2000). This stop is approximately 750m southwest of the Howey mine. Intense deformation at this stop has destroyed most primary textures that might be used to identify the rocks. They have been recently re-interpreted by Sanborn-Barrie et al. (2004) as mylonitized dioritic rocks (Table 10), representing a mafic border phase to the Dome stock. Pink felsic dikes that cut the mafic rocks are also mylonitized. Ironcarbonate veins are boudinaged and transposed into the sinistral shear direction. The far western extremity of the outcrops exposes a deformed quartz-feldspar dyke - 17 - Sample # UTM-Easting UTM-Northing Rock Type Sample No. SiO2 TiO2 Al2O3 Fe2O3 K2O MgO MnO CaO Na2O P2O5 LOI Total 2009AL-07 441670 5651602 Mylonite 2009AL-7 61.11 0.62 15.69 5.84 2.04 2.19 0.07 3.11 4.63 0.12 5.1 100.52 Au (opt) Ag (opt) <0.01 <0.1 Rb Ba Sr Sc La Ce Nd Sm Eu Gd Tb 51.6 333.5 518 14.1 20.47 37 19 3.57 1 2.95 0.41 Sample No. Yb Lu Y Zr Th U Hf Nb Ta Cs Dy Er Ho Pr Tm Be Cd Ga Li Mo Sb Sn Tl V W Zn Pb Cu Cr Ni Co Bi Ti 2009AL-7 1.154 0.18 11.25 116 3.12 0.88 3.03 1.74 <0.2 1.34 2.27 1.12 0.42 5.01 0.163 1.07 0.05 18.87 22.1 1.22 2.27 0.79 0.31 96.62 1.36 71.04 7.8 30 71 60 15 0.073 1730.68 �Proceedings of the 61st ILSG Annual Meeting - Part 2 Stop 5: Howey Mine (Fenced In Pit) – Drive-By by calcite. This represents the distal, outer halo of carbonate alteration, as discussed by Parker (2000). UTM Coordinates: NAD83; 15U 0442328E / 5652067N On the north side of Hammell Road a cement foundation marks the site the former Howey mine. Behind the fenced-off area is the site of the crown pillar mined out in the final stages of the mine. The Howey Mine was the first Au producer (1930-1941) in the Red Lake camp and remains the lowest grade profitable gold mine in Canadian mining history (final average grade 0.08oz/t Au, having produced 422,000oz gold). The Howey (and adjacent Hasaga) ore bodies occur in a boudinaged, variably sericitized and silicified quartz-feldspar porphyry dyke trending approximately 065°/80°S. Cm-wide, auriferous quartz veinlets trend 080°, making an angle of 15° with the contacts of the dyke and dip at 80°S. Gold-bearing quartz veinlets formed as the last of three episodes of quartz veining. Gold is associated with pyrite-sphalerite-galenatourmaline±tellurides. Small flat outcrops between the highway and the fence are highly deformed intermediate rocks of the Howey Mine hanging wall. This site lies within the northeast-trending Howey Bay–Flat Lake deformation zone and comprises Confederation age rocks. Stop 6: Chicken Chef Outcrop; Huston conglomerate UTM Coordinates: NAD83; 15U 0442396E / 5652067N This small outcrop is a clast-supported conglomerate, exhibiting cobble-sized, rounded granitoid clasts and stretched lapilli-sized intermediate to mafic clasts in a feldspar-phyric, foliated groundmass. Some of the granitoid clasts are moderately iron-carbonatized; a highly iron-carbonate-altered lense of detritus appears to be partially wrapped around a granitoid cobble. Similar clast lithologies, but also including rounded cherty fragments, are found in an outcrop 200m south along Highway 105. Stop 7: Outcrops on west side of Highway 125 and Sandy Bay Road; Calcite carbonatized pillowed flows (distal carbonate alteration facies). UTM Coordinates: NAD83; 15U 0446915E / 5654682N Slightly deformed pillows of the Confederation assemblage show pervasive calcite carbonatization, calcite veins, and pods (Table 11). Amygdules are also filled (replaced?) with calcite. Jig-saw puzzle breccias (created by fluid overpressure at depth) are cemented Stop 8: Woodland Cemetery Road and Highway 125; Meso–Neoarchean Contact UTM Coordinates: NAD83; 15U 0447235E / 5655572N These outcrops show altered, relatively low-strain, pillowed basaltic komatiite flows of the Balmer Assemblage (Table 12) unconformably overlain by polymictic conglomerate of the Huston assemblage. The exposures are in the transition from calcite carbonatization (distal alteration) to ferroan-dolomite (proximal) alteration. The pillowed and minor massive flows show extensive iron carbonate alteration as well as iron carbonate and quartz veins. Fuchsite is present in the central part of the outcrops on the west side of the highway (cemetery side). The Campbell Mine is approximately 1.5km to the north. While the mafic flows have not been directly dated at this locality, they are typically variolitic, and show a geochemical similarity with known Balmer-age rocks elsewhere; the massive and pillowed flows here can be traced to Balmertown, where an intercalated rhyolite at the Campbell mine was dated at 2989±3Ma (Corfu and Andrews, 1987). Variolitic flows occur in the northern part of the outcrops on the east side of the highway. However, they are unconformably overlain by Huston polymictic conglomerate further south along the outcrop. The conglomerate contains a large proportion of rounded cherty, jasperoidal and pyritic fragments. It represents an apron of Confederation assemblage (McNeely-age 2743Ma) detritus deposited at the break in paleoslope between the Confederation volcanic centre and its Balmer-age substrate. Stop 9: Cochenour Outcrop; Proximal ferroancarbonate alteration; carbonate veining; silicification; and gold UTM Coordinates: NAD83; 15U 0443689E / 5658640N This outcrop was mapped in detail by Williamson and Dubé (2003) at a scale of 1:150. It is situated directly above past-producing workings of the Cochenour Mine. The outcrop is underlain by maficultramafic Balmer assemblage volcanic rocks; the Meso- Neoarchean contact is exposed approximately 900m to the north. - 18 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Table 11. Major and trace element analyses, Stop 7 lithologies (major oxides in %, trace elements in ppm) Sample # UTM-Easting UTM-Northing 2009-AL-08 446914 5654682 Rock Type Andesite SiO2 TiO2 Al2O3 Fe2O3 K2O MgO MnO CaO Na2O P2O5 LOI Total 43.93 0.55 14.98 9.05 0.18 3.01 0.22 12.88 3.91 0.08 10.81 99.59 2009-AL-09 446914 5654682 Intermediate Dyke 56.29 0.84 16.27 6.81 0.41 4.28 0.09 5.64 4.74 0.42 4.21 100.02 Au (opt) Ag (opt) <0.01 <0.1 <0.01 <0.1 Rb Ba Sr Sc La Ce Nd Sm Eu Gd Tb Yb Lu 4.3 117.8 355 25.2 8.72 18.54 9.58 2.15 0.63 2.36 0.378 1.304 0.19 10.13 100.5 776 14.7 59.04 128.13 64.69 11.54 2.71 7.7 0.892 1.483 0.21 The outcrop contains exposures of iron-carbonate veins and breccia with spectacular colloformcrustiform and cockade textures, as well as a highgrade, auriferous, silicified zone (Williamson and Dubé, 2003). The latter is characterized by strong silicification, with varied proportions of sericite/green mica and carbonate. The presence of green mica hints at a mafic-ultramafic protolith; however, the presence of local subparallel layering in a feldspar- and quartzrich rock argues for a flow-banded felsic protolith. The silicified zone has been offset 50m to the west by latestage black line faults. Extensional iron-carbonate veins are generally barren of gold, except when cut by mm-scale, fine, Sample # Y Zr 2009-AL-08 13.43 47 2009-AL-09 21.16 177 Th 1.83 9.93 U Hf Nb Ta Cs Dy Er Ho Pr Tm Be Cd Ga Li Mo Sb Sn Tl V W Zn Pb Cu Cr Ni Co Bi Ti 0.47 1.19 2.98 0.2 0.62 2.46 1.45 0.51 2.35 0.208 0.45 0.14 14.54 24 1.01 1.58 0.54 0.03 148.81 <0.5 85.32 3.5 78 122 169 48.9 0.022 3160.87 2.27 4.53 8.15 0.5 0.63 4.42 1.9 0.75 16.21 0.246 1.46 0.05 21.74 33 0.79 1.72 1.26 0.06 126.67 <0.5 89.72 8.2 19 43 70 25 0.14 5021.85 auriferous arsenopyrite veinlets. Stop 10. Redcon Carbonate Zone; West and East sides of Nungesser Road; Proximal ferroancarbonate alteration; carbonate veining UTM Coordinates: NAD83; 15U 0448918E / 5661396N This area is approximately 4km north of the Campbell–Red Lake Deposit, still within the proximal, ferroan-carbonate alteration facies. The outcrops are weakly foliated (145°), dominantly massive to pillowed Balmer assemblage basalts (Table 13), occurring within the amphibolite-facies metamorphic aureole of the Walsh Lake Pluton. - 19 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Table 12. Major and trace element analyses, Stop 8 basalt (major oxides in %, trace elements in ppm). Sample # UTM-Easting UTM-Northing Rock Type Sample No. SiO2 TiO2 Al2O3 Fe2O3 K2O MgO MnO CaO Na2O P2O5 LOI Total 2009-AL-10 447235 5655572 Basalt 2009AL-10 61.58 0.5 6.7 18.09 1.28 3.85 0.36 3.13 <0.01 0.04 3.42 98.86 2009-AL-11 447235 5655572 Basalt 2009AL-11 49.76 0.81 10.58 10.92 1.89 4.05 0.33 7.9 0.29 0.05 13.02 99.6 Au (opt) Ag (opt) <0.01 <0.1 <0.01 <0.1 Rb Ba Sr Sc La Ce Nd Sm Eu Gd Tb Yb Lu Y 86.5 73.6 20 65.4 2.11 5.73 4.44 1.44 0.39 1.67 0.279 1.041 0.18 6.81 62.09 122.7 91 40.3 2.4 6.57 5.4 1.8 0.68 1.97 0.305 0.749 0.11 6.86 The stripped area on the east side of the Nungesser road was mapped in detail (Fig. 7) by Redcon Gold Mines in 1981 (assessment files) and now forms part of Goldcorp Inc.’s holdings. Here, a 1-2m wide carbonate vein is exposed near its southeastern termination. The vein can be traced in outcrop and drilling for approximately 750m to the west-northwest and will be seen at the next stop on the west side of the road. Gold occurs in north-northwest-trending, irregular, cm-thick quartz-actinolite stringers (tension gashes?) within the carbonate vein. After initial, pervasive potassic (biotite) Sample No. Zr Th U Hf Nb Ta Cs Dy Er Ho Pr Tm Be Cd Ga Li Mo Sb Sn Tl V W Zn Pb Cu Cr Ni Co Bi Ti 2009AL-10 32 0.19 0.06 0.75 1.3 <0.2 19.34 1.6 0.85 0.3 0.92 0.139 0.39 0.03 8.59 52.4 0.47 4.1 0.43 0.76 294.17 5.38 40.48 1 210 >600 655 100.5 0.037 2827.8 2009AL-11 30 0.28 0.07 0.83 1.76 <0.2 2.89 1.69 0.78 0.3 1.07 0.114 0.43 0.09 13.4 58.5 0.29 4.95 0.58 0.17 286.05 3.22 38.28 1 139 >600 493 83.5 0.06 4595.4 metasomatism of the basalt, the intrusion of the Walsh Lake Pluton produced a decimetre-scale, meshlike texture of amphibole-quartz veinlets that are prominently displayed due to their positive weathering features. Cross-cutting relationships suggest the following sequence of formation (from Lavigne et al. 1986): 1. quartz-calcite veins 2. ferroan-dolomite veins 3. mafic dyke 4. auriferous quartz veins - 20 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Silicification evident in the pillowed flow on the northern half of the outcrop is barren and apparently not related to the gold-rich silicification event; rather, it may be due to local silica dumping following pervasive carbonate metasomatism. Table 13. Major and trace element analyses, Stop 10 lithologies (major oxides in %, trace elements in ppm). Sample # UTM-Easting UTM-Northing 2009-AL-15 448918 5661396 2009-AL-16 448918 5661396 Basalt-Si+ Basalt Sample No. SiO2 TiO2 Al2O3 Fe2O3 K2O MgO MnO CaO Na2O P2O5 LOI Total 2009-AL-14 448918 5661396 Fe-Carb vein 2009AL-14 3.05 0.1 0.74 10.34 0.14 13.84 0.64 28.6 <0.01 0.02 41.85 99.18 2009AL-15 56.09 1.88 17.14 9.73 0.97 4.71 0.14 5.54 1.58 0.22 2.1 100.08 2009AL-16 50.04 1.69 14.65 15.95 0.86 7.5 0.15 5.74 0.77 0.17 1.89 99.42 Au (opt) Ag (opt) <0.01 <0.1 <0.01 <0.1 <0.01 <0.1 Rb Ba Sr Sc La Ce Nd Sm Eu Gd Tb Yb Lu Y 3 13.1 46 6.8 2.2 4.18 3.23 1.05 0.53 1.57 0.256 0.816 0.14 11.67 30.74 89.9 52 44.2 12.76 33.18 21.28 5.72 1.72 5.17 0.741 2.311 0.34 21.2 22.98 145.1 42 46.2 7.53 19.28 13.99 4.08 1.32 5.09 0.897 3.516 0.51 34.71 Sample No. Zr Th U Hf Nb Ta Cs Dy Er Ho Pr Tm Be Cd Ga Li Mo Sb Sn Tl V W Zn Pb Cu Cr Ni Co Bi Ti 2009AL-14 11 <0.09 0.03 0.26 0.21 <0.2 0.17 1.59 0.89 0.32 0.64 0.124 0.39 0.23 1.19 6.2 0.14 0.91 0.13 0.03 35.32 8.7 124.59 1.5 4 <24 44 9.4 0.015 441.71 2009AL-15 106 0.9 0.23 2.89 5.29 0.3 3.78 4.43 2.55 0.89 4.66 0.368 0.39 0.09 20.89 54.4 0.35 3.28 1.21 0.23 476.84 13.91 124.21 2.8 193 66 70 35.9 0.02 11217.71 2009AL-16 86 0.73 0.18 2.29 5.33 0.3 2.9 5.95 3.7 1.27 2.92 0.54 0.45 0.13 20.82 49.7 0.41 4.83 1.13 0.19 443.67 4.77 162.95 1.7 88 133 63 45.2 0.038 10004.7 Rock Type A “black line” fault occurs in the northern wallrocks of the main carbonate vein. A mafic (or lamprophyre) dyke (unit 4, above) cuts the vein, but is itself cut by late quartz-actinolite-gold stringers. The western outcrops are approximately 300m westnorthwest of the previous exposures. Things to note on the western series of outcrops: • differing colours of cross-cutting carbonate veins • colloform/crustiform textures in carbonate veins • andalusite-garnet-biotite alteration of pillows cut by calc-silicate veins (diopside±calcite, quartz, tourmaline; retrograding to epidote, tremolite, actinolite/hornblende, magnetite) • calc-silicate veins cross-cut by carbonate veins • folding of carbonate veins by the “Mine trend” S2 References (Used in Guide) Corfu, F. and Andrews, A.J. 1987. Geochronological constraints on the timing of magmatism, deformation and gold mineralization in the Red Lake greenstone belt, northwestern Ontario; Canadian Journal of Earth Sciences, v.24, p.1302-1320. Corfu, F. and Stone, D. 1998. Age, structure and orogenic significance of the Berens River composite Batholiths, western Superior Province; Canadian Journal of Earth Sciences, v.35, p.1089-1109. Corfu, F. and Wallace, H. 1986. U-Pb zircon ages for magmatism in the Red Lake greenstone belt, northwestern Ontario; Canadian Journal of Earth Sciences, v.23, p.27-42. Corfu, F., Davis, D.W., Stone, D., and Moore, M. 1998. Chronostratigraphic constraints on the genesis of Archean greenstone belts, northwestern Superior Province, Ontario, Canada; Precambrian Research, v.92, p.277-295. Dubé, B., Balmer, W., Sanborn-Barrie, M., Skulski, T., and Parker, J. 2000. A preliminary report on amphibolite-facies, disseminated-replacement-style mineralization at the Madsen gold mine, Red Lake, Ontario; in Current Research 2000-C17, Geological Survey of Canada, 12p. Dubé, B., Williamson, K., and Malo, M. 2002. Geology of the Goldcorp Inc. High Grade zone, Red Lake mine, Ontario: an update, in Current Research 2002-C26, Geological Survey of Canada, 15p. - 21 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 7. Detailed geology of the Redcon prospect (modified from Lavigne et al. 1986). Dubé, B., Williamson, K., McNicoll, V., Malo, M., Skulski, T., Twomey, T., and Sandborn-Barrie, M. 2004. Timing of gold mineralization at Red Lake, Northwestern Ontario, Canada: New constraints from U-Pb Geochronology at the Goldcorp HighGrade Zone, Red Lake mine, and the Madsen mine; Economic Geology, v.99, No.8, p.1611 to 1641. Durocher, M.E. and Hugon, H. 1983. Structural geology and hydrothermal alteration in the Flat Lake-Howey Bay deformation zone, Red Lake area, in Summary of Field Work, 1983, Ontario Geological Survey, Miscellaneous Paper 116, p.216 to 219. Horwood, H.C. 1940. Geology and mineral deposits of the Red Lake area, in Forty-ninth Annual Report of the Ontario Dept. of Mines, vol. XLIX, Pt. II, 231p. Irvine, T.N. and Baragar, W.R. 1971. A guide to the chemical classification of the common volcanic rocks; Canadian Journal of Earth Sciences, v.8, p.523-548. Jensen, L.S. 1976. A new cation plot for classifying subalkaline rocks; Ontario Geological Survey, Miscellaneous Paper 66, 22p. Lavigne Jr., M.J., Hugon, H., Andrews, A.J., and Durocher, M.E. 1986. Gold deposits of the Red Lake District, Relationships of gold mineralization to regional deformation and alteration in the Red Lake greenstone belt, Ontario, in Gold ‘86, Excursion Guidebook, ed. Pirie, J. and Downes, M.J., p.167 to 211. McMaster, N.D. 1987. A preliminary 40Ar/39Ar study of the thermal history and age of gold in the Red Lake greenstone belt; unpublished M.Sc. thesis, University of Toronto, Toronto, Ontario, 107p. Noble, S.R. 1989. Geology, geochemistry and isotope geology of the Trout Lake Batholith and the UchiConfederation lakes greenstone belt, northwestern Ontario, Canada; unpublished Ph.D. thesis, University of Toronto, Toronto, Ontario, 288p. Parker, J.R. 1999. Exploration potential for volcanogenic massive sulphide (VMS) mineralization in the Red Lake greenstone belt; in Summary of Field Work and Other Activities 1999, Ontario Geological Survey, - 22 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Canada, 14p. Open File Report 6000, p.19-1 to 22-26. Parker, J.R. 2000. Gold mineralization and wall rock alteration in the Red Lake greenstone belt: a regional perspective; in Summary of Field Work and Other Activities 2000, Ontario Geological Survey, Open File Report 6032, p.22-1 to 22-27. Chi, G., Dubé, B., and Williamson, K. 2003. Fluid evolution and pressure regimes in the Campbell-Red Lake gold deposit, Red Lake mine trend, Red Lake, Ontario; in Current Research 2003-C28, Geological Survey of Canada, 18p. Percival, J.A., Bailes, A.H., Corkery, M.T., Dubé, B., Harris, J.R., McNicoll, V., Panagapko, D., Parker, J.R., Rogers, N., Sanborn-Barrie, M., Skulski, T., Stone, D., Stott, G.M., Thurston, P.C., Tomlinson, K.Y., Whalen, J.B., and Young, M.D. 2000. An integrated view of Western Superior crustal evolution: highlights of 2000 NATMAP studies, in Summary of Field Work and Other Activities 2000, Ontario Geological Survey, Open File Report 6032, p.13-1 to 13-17. Dubé, B., Williamson, K., and Malo, M. 2001. Preliminary Report on the Geology and Controlling Parameters of the Goldcorp Inc. High Grade Zone, Red Lake Mine, Ontario; Geological Survey of Canada, Current Research 2001-C18, 13p. Pettigrew, N. 1999. Structural and alteration history of the Buffalo Gold Deposit, Red Lake, Ontario; unpublished B.Sc. Thesis, University of New Brunswick, 154p. Sanborn-Barrie, M., Skulski, T., and Parker, J. 2001. Three hundred million years of tectonic history recorded by the Red Lake greenstone belt, Ontario, in Current Research 2001-C19, Geological Survey of Canada, 32p. Sanborn-Barrie, M., Skulski, T., and Parker, J. 2004. Geology, Red Lake greenstone belt, Western Superior Province, Ontario; Geological Survey of Canada, Open File 4594, scale 1:50,000. Sanborn-Barrie, M., Skulski, T., Parker, J., and Dubé, B. 2000. Integrated regional analysis of the Red Lake greenstone belt and its mineral deposits, western Superior Province, Ontario, in Current Research 2000-C18, Geological Survey of Canada, 16p. Skulski, T., Sanborn-Barrie, M. and Sanborn, N. 2001 New U-Pb geochronology in the Red Lake greenstone belt, Western Superior NATMAP, unpublished poster. Williamson, P.K. and Dubé, B. 2003. Detailed geology, hydrothermal alteration and gold mineralisation of the Cochenour stripped outcrop, Red Lake gold district, Ontario; Geological Survey of Canada, Open File 1673. Bibliography of Recent Research (not used in Guide) Chi, G., Dubé, B., and Williamson, K. 2002. Preliminary fluid-inclusion microthermometry study of fluid evolution and temperature-pressure conditions in the Goldcorp High-Grade zone, Red Lake Mine, Ontario, in Current Research 2002-C27, Geological Survey of 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; in Current Research 2003-C21, Geological Survey of Canada, 15p. Gulson, B.L., Mizon, K.J., and Atkinson, B.T. 1993. Source and timing of gold and other mineralization in the Red Lake area, northwestern Ontario, based on leadisotope investigations, Canadian Journal of Earth Sciences, v.30, p.2366 to 2379. Parker, J.R. 2001. Intermediate to Felsic Plutons in the Red Lake Greenstone Belt: Relationship to Deformation and Gold Mineralization; in Summary of Field Work and Other Activities 2001, Ontario Geological Survey, Open File Report 6070, p.19-1 to 19-10. Penczak, R.S. and Mason, R. 1997. Metamorphosed Archean epithermal Au-As-Sb-Zn-(Hg) vein mineralization at the Campbell Mine, Northwestern Ontario, Economic Geology, v.92, p.696 to 719. Penczak, R.S. and Mason, R. 1999. Characteristics and origin of Archean pre-metamorphic hydrothermal alteration at the Campbell Gold Mine, Northwestern Ontario, Canada, Economic Geology, v.94. p.507-528. Pirie, J. and Downes, M.J., eds. 1986. Gold ‘86 Excursion Guidebook. Stone, D. and Hallé, J. 2000. Geology of the Blackbear, Yelling and Stull Lake areas, Northern Superior Province, Ontario, in Summary of Field Work and Other Activities 2000, Ontario Geological Survey, Open File Report 6032, p.15-1 to 15-9 Thompson, P.H. 2003. Toward a new metamorphic framework for gold exploration in the Red Lake greenstone belt; Ontario Geological Survey, Open File Report 6122, 52p. Twomey, T. and McGibbon, S. 2002. The geological setting and estimation of gold grade of the High-Grade zone, Red Lake mine, Goldcorp Inc.; Exploration and Mining Geology, Nos. 1 and 2, p.19 to 34. - 23 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Field Trip 2 - Western Wabigoon Subprovince Transect, Dryden to Meggisi Lake Mark Puumala and Dorothy Campbell Resident Geologist Program, Ontario Geological Survey, Thunder Bay, Ontario, Canada Craig Ravnaas Resident Geologist Program, Ontario Geological Survey, Kenora, Ontario, Canada Introduction This field trip will provide participants with an overview of the geology of a portion of the western Wabigoon Sub-province along an approximately 52km long transect that follows the Highway 502 corridor between the City of Dryden and Meggisi Lake (see Fig. 1, A-B; Fig. 3). The trip will begin in metasedimentary rocks near the southern margin of the Sioux Lookout domain, crossing over the Wabigoon fault into the Atikwa-Manitou volcano-plutonic domain. The majority of the trip will be spent observing supracrustal rocks of the Eagle-Wabigoon-Manitou Lakes (EWM) greenstone belt and plutonic rocks of the Atikwa batholith and Taylor Lake stock. The area has long been known for its gold exploration potential, with modest levels of production (approximately 13,000oz. Au) occurring between 1895 and 1948. As a result, the trip will also include a stop to observe a gold occurrence located near Flambeau Lake. Regional Geology Blackburn et al. (1991) describe the Wabigoon Subprovince as a 900km long by 150km wide granitegreenstone sub-province in the northwestern Superior Province. It consists of metavolcanic and subordinate metasedimentary rocks that are surrounded and cut by granitoid batholiths. The Wabigoon Sub-province is bounded on the north by the Winnipeg River and English River Sub-provinces and on the south by the Quetico Sub-province (Fig. 1). The Wabigoon Sub- Figure 1. General geology of the western Wabigoon Sub-province, showing its extent, major supracrustal belts and structural features (Blackburn et al., 1991). The transect is defined by the A-B line. - 24 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 province has been divided into three regions, referred to as: i) the western region; ii) the central region; and iii) the eastern region. As noted above, this field trip will transect a portion of the western region. The following summary of the regional geology of the western Wabigoon Sub-province is excerpted from Beakhouse et al. (1995). “The western Wabigoon region is characterized by interconnected, arcuate, metavolcanic dominated ‘greenstone belts’ surrounding large elliptical batholiths. The metavolcanic component of greenstone belts includes minor ultramafic (komatiitic), through abundant mafic (tholeiitic, calc-alkalic and minor alkalic) to felsic (mostly calc-alkalic) varieties. Except locally, metasedimentary rocks are volumetrically minor but diverse including turbiditic, volcaniclastic deposits, alluvial fan-fluvial deposits and chemical (magnetite ironstone and chert) deposits. Stratigraphic sequences generally comprise basal, laterally extensive, mafic metavolcanic sequences overlain by laterally limited, diverse mafic to felsic sequences. Minor clastic metasedimentary deposits are associated with some of the intermediate to felsic volcanism. Very locally, coarse clastic-dominated metasedimentary sequences with subordinate chemically distinct metavolcanic rocks unconformably overlie the diverse volcanic sequences. The principal exception to the generalized lithologic proportions outlined above occurs in an area north of the Wabigoon fault in the Dryden-Sioux Lookout area where metasedimentary rocks predominate. A more detailed review of the stratigraphy and geochemistry of western Wabigoon greenstone belts is presented in the guide portion of this volume. generalization include the Dryberry and Ghost Lake batholiths which are younger than and compositionally distinct from metavolcanic rocks in this area. The smaller stocks are predominantly late- to post-tectonic and range compositionally from diorite to granite and syenite. Minor, late alkalic intrusions (e.g., Sturgeon Narrows) occur in the Sturgeon Lake area. The deformational style of much of the western Wabigoon region, and particularly that portion lying to the south of the Wabigoon fault, is dominated by structural domes cored by large batholithic masses giving rise to apparent synclinal keels of greenstone belts surrounding the batholiths. In detail, it is not possible to correlate units on either side of the apparent ‘synclinal axes’ and these zones of opposing stratigraphic facing correspond, in part, to faults that have juxtaposed segments of volcanic rock of contrasting ages. Laterally continuous deformation zones exhibiting complex kinematics typically occur along the central axis of the greenstone belts where greenstone sequences face one another and may be related to this faulting. The northern portion (north of the Wabigoon fault) of the sub-province has a distinct structural style reflected in linear, fault bounded panels trending parallel to the sub-provincial boundary that contrasts with that of the remainder of the western Wabigoon region. Here there is evidence for early recumbent folding and thrust faulting as well as a later phase of dextral, transcurrent shear. Greenschist-grade regional metamorphic mineral assemblages characterize much of the greenstone belts. The principle exceptions to this generalization are narrow amphibolite-grade zones that occur at the contact with granitoid batholiths and at sub-province boundaries. A particularly noteworthy exception occurs in the Dryden area where there is widespread evidence for in situ partial melting of pelitic metasedimentary rocks. Granitoid rocks within the western Wabigoon region include large elliptical to multi-lobate batholiths that define the architecture of the greenstone belts as well as smaller stocks. Most of the large batholiths (Aulneau, Atikwa, Sabaskong) range compositionally from ultramafic to granitic but are predominantly tonalitic to granodioritic. These are closely associated petrogenetically and temporally with the metavolcanic rocks of the greenstone belts and are interpreted to represent sub-volcanic chambers that have risen into their own volcanic ejecta. Exceptions to this U-Pb geochronological constraints indicate that metavolcanic rocks were deposited between 2775 and 2711Ma, and much of this in the narrow interval of time between 2740 and 2720Ma. Large granitoid batholiths occurring to the south of the Wabigoon fault were emplaced synchronously with adjacent volcanic rocks, whereas those to - 25 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 the north of the fault tend to be younger than 2710Ma. Small, post-tectonic plutons were emplaced over a 15Ma interval commencing at ~2699Ma.” different interpretations of their relative age of emplacement. Presence of layering in the Gabbro Lake body suggests that they are horizontally-emplaced sills, and regional crosscutting relationships suggest their emplacement subsequent to major folding. This argument led to the suggestion (Blackburn, 1980) that they were emplaced subsequent to overthrusting of the Boyer Lake volcanics. However, an age of 2722Ma later obtained (Davis et al., 1982) from the same sill is not consistent with a postthrusting emplacement. These problems remain unresolved.” Manitou-Stormy Lakes Greenstone Belt The first half of the field trip will be spent viewing rocks located in the Manitou-Stormy Lake portion of the EWM Lakes greenstone belt. Beakhouse et al. (1995) describe the Manitou-Stormy Lakes greenstone belt as follows. “Stratigraphic and structural relationships in this greenstone belt, supported by age dating (Davis et al., 1982; Davis, 1990; Parker et al., 1989: summarised in Blackburn et al., 1991), suggest a history in which two early mafic tholeiitic sequences, the Wapageisi Lake group (not dated directly, but in the range 2745 to 2730Ma) and the Boyer Lake group (not dated directly, but >2722Ma) have been juxtaposed against an intervening sequence of later calcalkalic pyroclastics and overlying clastic metasedimentary rocks (Manitou-Stormy groups: <2706Ma and >2696Ma). To the northwest of the Manitou Straits fault, supracrustal units (Upper Manitou Lake, Pincher Lake, Lower Wabigoon volcanics: >2732Ma) that extend northward to the Wabigoon fault are examples of volcanic ejecta that are invaded by their own magma chamber (Atikwa batholith, Dore Lake lobe: 2732Ma). The early mafic suites are distinctly different, and may represent the ensimatic mafic plane within which the emergent pyroclastic edifices rose in island arc-related environments. The mafic plane analogue is supported by the presence within Wapageisi volcanics of two 200m thick, laterally extensive (>10km strike length) plagioclase-phyric flow units, 1500m apart. Differing lines of evidence from two tabular gabbro bodies (Gabbro Lake, Mountdew Lake) within the Boyer Lake group lead to Economic Geology Exploration and Production History The Wabigoon Lake and Upper Manitou Lake areas have long been recognized as having significant gold exploration potential. During the time period between 1895 and 1943, several small gold deposits were developed and modest amounts of gold were produced in these areas. The majority of the historic gold production occurred in the Gold Rock mining camp near Upper Manitou Lake. Past-producers at Gold Rock included the Laurentian, Big Master (aka Kenwest), Elora (aka Jubilee) and Gold Rock mines (Parker, 1989). Total production from these four mines was 12,113.21oz Au. Production statistics for the individual mines are provided in Table 1. Gold production from the Wabigoon Lake area during the same time period was much less significant, with a total of 613.02oz Au mined from four deposits (Parker, 1989). The past producing mines and their production statistics are listed below (Table 2). Since 1948, the Wabigoon Lake and Upper Manitou Lake areas have both seen sporadic exploration activity, including a period of base metal exploration during the 1960s and 1970s (Parker, 1989). However, most exploration activity that has been completed in these areas since 1980 has continued to focus on gold. Table 1. Gold production statistics for the Gold Rock Mining Camp (from Parker, 1989). Mine Laurentian Big Master (Kenwest) Elora (Jubilee) Gold Rock Total Production (oz Au) 8143 2565.52 1369.69 35 - 26 - Years of Operation 1906-09 1902-05, 1942-43 1936-37, 1939 1929 �Proceedings of the 61st ILSG Annual Meeting - Part 2 Table 2. Gold production statistics for the Wabigoon Lake area (from Parker, 1989). Mine Redeemer Bonanza Rognon Van Houten Total Production (oz Au) 343.96 243.85 22.21 3 The most notable recent exploration activity in the Wabigoon Lake area has occurred on the Laurentian Goldfields Ltd. Van Horne Gold Property. This property will be visited during this field trip, and is described in detail as field trip Stop 11. In the Upper Manitou Lake area, Manitou Gold Inc. has been active recently on a number of claim groups, including their Elora and Kenwest properties. These properties include the past-producing Laurentian, Big Master, and Elora Mine sites. There are 61 known gold showings, occurrences, prospects, and deposits in the Upper Manitou Lake area (Maunula and Wilson, 2010). These showings include the Starr Gold Occurrence that is described below as an optional self-guided field trip stop (Stop 8). Years of Operation 1904-06, 1918 1920, 1923 1916-18 1940 Lake Volcanics, and underlies a pillowed mafic flow sequence occurring at the top of the volcanic succession, the Upper Wabigoon Volcanics.” The stratigraphic and structural controls on gold mineralization in the Upper Manitou Lake area are described as follows by Parker (1989). “It is apparent that stratigraphy plays an important role in the concentration of gold at Upper Manitou Lake. Gold deposits are situated near the contacts between the Blanchard Lake, Upper Manitou Lake, and Pincher Lake Volcanic Groups. The mines at Goldrock, and the significant The location of the Van Horne gold property and the historic Gold Rock mining camp are shown on Figure 2. This map also illustrates mining claims that were in good standing as of April 20, 2015 and the location of mineral exploration projects that were reported by Lichtblau et al. (2015) to have been active during 2014. Gold Mineralization Gold occurrences in the Eagle-Wabigoon-Manitou Lakes greenstone belt show strong spatial and genetic correlations with major deformation zones such as the Wabigoon and Pipestone Manitou Straits faults (Parker, 1989). Parker (1989) noted the following structural and stratigraphic relationships to gold mineralization in the Wabigoon Lake area. “Gold-bearing quartz veins west of Wabigoon Lake are controlled by northwest-trending tension fractures and east- and east-northeast-trending shear zones. The majority of known gold occurrences at Eagle and Wabigoon Lakes are situated within a mixed sequence of mafic to felsic metavolcanics grouped together as the Lower Wabigoon Volcanics: it overlies a thick sequence of massive and pillowed mafic flows, the Eagle Figure 2. Map illustrating locations of Van Horne gold property and Gold Rock historic mining camp relative to field trip stops. Mining claims and exploration properties active during 2014 are also shown. - 27 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 gold occurrences and prospects in the area, are situated within the Pincher Lake Group, near its contact with the underlying Upper Manitou Lake Group. The Pincher Lake Group may be identical to the Lower Wabigoon Volcanics, which host numerous gold deposits and past producing mines at Eagle and Wabigoon Lakes. The Pincher Lake Group and Lower Wabigoon Volcanics are mixed sequences of calc-alkaline to tholeiitic, mafic and felsic metavolcanic rocks. exposure of folded Thunder Lake metasedimentary rocks in outcrops located adjacent to the Dryden Walmart parking lot. As noted above, there are significant differences in structural style on the north and south sides of the Wabigoon fault, suggesting that these rocks represent domains that evolved separately and have been juxtaposed against each other along the fault. Rocks located to the north of the Wabigoon fault, including the Thunder Lake sediments, are collectively referred to as the Sioux Lookout domain (Beakhouse et al., 1995). The fact that gold deposits are concentrated northwest of the Manitou Straits Fault is partly due to the fact that geological successions on either side of the fault are substantially different. The rocks southeast and east of the fault consist of the dominantly mafic Boyer Lake Group, and the Manitou Lake Group, both of which host a few scattered gold occurrences. Rocks northwest of the fault are dominantly calcalkaline to tholeiitic and are more typical of gold-bearing environments elsewhere in the Dryden-Ignace area. Structural disruption is also more significant northwest of the fault, providing numerous dilatant zones for the emplacement of gold-bearing quartz veins. Extensive and intense northeast shearing controls the majority of goldbearing quartz veins. At Peak Lake, northeast of Upper Manitou Lake, gold-bearing quartz veins are controlled by northwest-trending fractures which crosscut east and northeast-trending shear zones. This style of deformation is similar to gold deposits at Dinorwic Lake, and differs from the dominant northeast controls at Goldrock. Blackburn et al. (1991) suggest that the Wabigoon fault displays an early history of thrusting (south side up), followed by dextral strike-slip movement. Evidence for the former is most evident to the south of the fault, where tight folds with sub-horizontal axes occur in the Wabigoon volcanic rocks. Evidence for strike-slip motion is most notable to the north of the fault, where the Thunder Lake metasedimentary rocks are folded about sub-vertical axes. The Thunder Lake metasedimentary rocks are described as follows by Beakhouse (2000). “The Thunder Lake sediments include two separate panels of rock separated along a portion of their strike-length by the Thunder Lake volcanics. Thin- to medium-bedded wackesiltstone characterized by even, continuous bedding is the predominant component in both panels. Thin magnetite ironstone layers are a conspicuous minor component within the Thunder Lake sediments north of the Thunder Lake volcanics but are rare within the southern panel. Minor, thin garnet-rich (<70% garnet) and calc-silicate layers may represent original more pelitic and marly layers, respectively. In one location, the calc-silicate material forms discordant veins, and suggests that some of this material is remobilized or originated by secondary alteration processes. The garnet-rich layers are often closely spatially associated with the ironstone layers. Competency and susceptibility to fracturing of the host rock is the controlling influence on the concentration of the quartz veins: ductility contrasts between felsic dikes and mafic metavolcanics have focussed zones of extension and compression, commonly resulting in fracturing of the more rigid felsic dikes.” Field Trip Stops Stop 1: Thunder Lake metasedimentary rocks UTM Coordinates: NAD83; 15U 0513498E / 5514739N The Walmart store property located on TransCanada Highway 17 (Government Street) at the east end of Dryden. This stop provides an opportunity to view an - 28 - A limited number of determinations indicate that tops, although locally reversed by tight to isoclinal minor folding, are generally to the south in both the north and south panels. Contact relations with the Brownridge volcanics have not been observed but the data are permissive of a conformable stratigraphic relationship. The contacts with the Thunder Lake volcanics �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 3. General geology with field trip stops (geology from Ontario Geological Survey 2011). - 29 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Lake pluton (~2698Ma). A laser ablation inductively coupled mass spectrometric (LAICP–MS) U/Pb age of 2727±5Ma for a gabbro occurring to the southeast of Stormy Lake (Stone et al., 2010) may approximate the age of the unit; however, the sampled unit occurs at the contact between the Wapageisi and Bending Lake volcanics that both contain gabbro intrusions. The basal contact of the Wapageisi volcanics is in part intrusive where younger phases (Islet Lake Stock, Stone, Hellebrandt, and Lange, 2011b; Meggisi pluton, Blackburn, 1982) are in contact with the basal volcanic units. However, a well-developed, commonly shallowly dipping, penetrative deformation fabric is restricted to the basal 500m of the Wapageisi volcanics with the intensity of fabric development increasing towards the contact suggesting that the local intrusive contacts may be superimposed on a tectonic contact. The character of the northern contact of the Wapageisi volcanics differs across the area.”x appear to also be conformable although these are commonly moderately highly strained and a loci of abundant quartz veining.” This outcrop exposure illustrates the effects of two phases of deformation that resulted in the development of refolded folds in the Thunder Lake metasedimentary rocks. The first phase of deformation resulted in the formation of isoclinal folds about steeply-plunging axes. These folds were subsequently refolded into open z-folds indicative of a late phase of dextral transcurrent shear along the Wabigoon fault (Beakhouse et al., 1995). The axial planes of the late folds strike approximately east-northeast. Parker (1989) also noted the presence of structures (e.g., northwest-trending tension fractures) on the south side of the Wabigoon fault that are consistent with a late phase of dextral transcurrent motion. We will now travel for approximately one hour along Highways 594 and 502 until we reach the southern end of the transect at Meggisi Lake. Stop 2: Wapageisi metavolcanic rocks UTM Coordinates: NAD 83; 15U 0524073E / 5463090N to 0524055E / 5463690N Turn left onto Highway 17 and travel 650m to the traffic lights at the intersection with Highway 594 (Duke Street). Turn left onto Highway 594 and travel 8km to the intersection with Highway 502. Turn left onto Highway 502 and travel 64.5km south to Meggisi Road. This field trip stop consists of a series of road cuts extending northward from Meggisi Road for approximately 600m along the west side of Highway 502. Uphill Road is intersected at a distance of 350m along the section. These bedrock exposures provide excellent examples of relatively undeformed pillowed and massive flows of the Wapagesi metavolcanic rocks. The Meggisi-Uphill Roads section through the Wapagesi metavolcanic rocks commences in plagioclase-phyric pillowed basalt (Fig. 4), typical of the two laterally extensive, plagioclase-phyric flow units that provide stratigraphic markers near the base of the Wapagesi metavolcanic rocks (Beakhouse et al., 1995). The flows at this location are upward-facing with a shallow dip (approximately 25°) toward the north. Moving north, the plagioclase-phyric unit is overlain by aphyric massive basalt that is overlain by pillowed flows and pillow breccia. A 10cm wide mafic dike can be observed cross-cutting the pillow breccia. The The Wapageisi metavolcanic rocks are described as follows by Beakhouse (2011). “The Wapageisi volcanics comprise a homoclinal, generally northward-facing panel of submarine mafic volcanic and related gabbroic rocks. Plagioclase glomeroporphyritic units are laterally continuous and make up approximately 10 to 15% of the unit. The age of the Wapageisi volcanics is constrained to being older than both the Manitou–Stormy supracrustal association (~2703Ma) and the late- to post-tectonic Taylor Figure 4. Plagioclase-phyric pillowed flows. - 30 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 5. Quartz-carbonate filled “eyebrow” structures. northeast-striking dike parallels shear fractures and dips approximately 70° toward the northwest. This unit is overlain by a second massive flow that passes into another sequence of pillowed material. The massive flow contains large quartz-carbonate filled “eyebrow” structure gas cavities (Fig. 5) whose long axes are oriented approximately parallel to stratigraphy. Figure 6. Roadside exposure of pillowed Wapageisi metavolcanic rocks. are scattered throughout. Narrow selvages and lack of vesicles suggest deep-water emplacement but the presence of carbonate-filled gas cavities appears to contradict this. Size of pillows ranges from 20cm or less to about 1m. On the east side of the highway, excavation for the road cut, followed by winter frost heave, has exposed exceptionally well preserved three-dimensional pillow shapes. In one specimen, the budding neck of the pillow can clearly be seen in three dimensions.” More pillowed flows and flow-breccia are present to the north of the Uphill Road turnoff. Cavities in the breccia have been in-filled with quartz, carbonate and epidote. Stop 3: Pillowed lavas in Wapageisi metavolcanic rocks UTM Coordinates: NAD83; 15U 0524129E / 5464528N Travel 1.8km north of Meggisi Road along Highway 502. Because these roadside outcrop exposures are located on a corner, park at a roadside stop located on the west side of the highway at UTM Co-ordinates: NAD83; 15U 0524385E / 5464742N. The outcrops are located approximately 350m to the southwest of the parking spot. This stop provides a good opportunity for participants to obtain photographs of excellent examples of pillowed mafic metavolcanic flows in the lower, tholeiitic mafic plane sequence of the Wapageisi metavolcanic rocks (Fig. 6). The following description of the outcrop exposures is excerpted from Beakhouse et al. (1995). “In the west face, pillow shape and packing indicates tops to the northwest. Good examples of bun-shaped to mattress-shaped to budding forms can be seen. Sparse feldspar phenocrysts Stop 4: Manitou Group conglomerate and quartzfeldspar porphyry UTM Coordinates: NAD83; 15U 0521390E / 5469239N Travel 5.4km north from Stop 3 along Highway 502 to the intersection with Mosher Road. Turn west on Mosher Road and travel 6.6km This stop provides an opportunity to view polymictic conglomerate of the Manitou group that has been intruded by a quartz-feldspar porphyry dike. Beakhouse et al. (1995) describe the Manitou group as follows. The correlatable Manitou and Stormy Lake groups are typical of upper, emergent, chemically and texturally diverse, predominantly calc-alkaline sequences that provided fill for marginal sedimentary basins. In other greenstone belts these have been termed “Temiskaming” type. The base of the sequence unconformably (in places with high angle) rests on Wapageisi Lake group tholeiites. Calc-alkaline pyroclastics are typically coarse, and intruded by comagmatic, dacitic, subvolcanic porphyry stocks, one of which has been dated at 2699Ma (Don Davis, personal - 31 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 communication, 1989). At the top of the pyroclastic sequence lies a mafic alkaline (trachybasalt) flow unit (Sunshine Lake formation) that is unique in the western portion of the Wabigoon Subprovince. Rubble at the top of the unit marks the passage into the epiclastic suite, in which there is vertical and lateral facies variation from alluvial fan and fluvial into resedimented conglomerates, sandstones and mudstones. The resedimented facies show all the characteristics of turbidite sequences: coarse heterolithic conglomerates, graded wacke to mudstone beds, and loading textures (e.g. flame structure) (Teal and Walker, 1977; Blackburn, 1981). The polymictic conglomerate seen here forms part of the Mosher Bay metasedimentary rock sequence. The Mosher Bay metasedimentary rocks are an approximately 2000m thick, north-northwest facing, steep northerly- to vertically-dipping sequence comprised of conglomerate, sandstone, mudstone and minor iron formation. The lower two thirds of the sequence are dominated by conglomerate, while the upper third is a mixture of conglomerate, sandstone and mudstone. The conglomerates include both volcanic clast-dominated and polymictic horizons (Blackburn, 1981). access road located just to the south of Rattlesnake Creek. The outcrops on both sides of the highway provide a good exposure of the late- to post-tectonic 2695Ma (Davis et al., 1982) Taylor Lake Stock. The stock is a relatively small intrusion (approximately 11km by 6km) that is completely enclosed by supracrustal rocks. The intrusion is inhomogeneous and ranges in composition from granodiorite to monzodiorite. However, the highway transects the relatively homogeneous, granodioritic, western portion of the stock (Blackburn, 1981). Interesting features that are visible in these outcrops include exfoliation fractures (Fig. 7), shear fractures, a pegmatite dike that parallels a shear, and numerous mafic xenoliths. Stop 6: Manitou Group alkaline metavolcanic rocks UTM Coordinates: NAD83; 15U 0526602E / 5471785N Travel 1.4km north from Stop 5 to outcrop exposures located on both sides of the highway, north of Rattlesnake Creek. The following description of this stop is excerpted from Beakhouse et al. (1995). The polymictic conglomerates have variable clast sizes (pebbles, cobbles and boulders) and include a mixture of lithologies including granitoid rocks, volcanic rocks, chert, jasper, and magnetite iron formation. The proportion of volcanic clasts found in the polymictic conglomerates decreases upward in the sequence (Blackburn, 1981). “The river valley follows the east-striking Mosher Bay - Washeibemaga (MBW) fault, along which the Boyer Lake volcanics may have been thrust over the Manitou and Stormy Lake groups. In the outcrop on the east side of the highway alkaline volcanics of the Manitou group are intruded by granitic rocks of the late tectonic Taylor Lake stock. Sigmoidal shears in the alkaline volcanics may be related to movement on the MBW fault. The Mosher Bay metasedimentary rocks are intruded by a concordant quartz feldspar porphyry intrusion that covers a strike length of more than 3 km and ranges in width from tens of metres to 300m. The Mosher Bay porphyry is petrographically identical to nearby porphyries at Sunshine and Thundercloud lakes and contains abundant 2 to 4mm phenocrysts of quartz and feldspar throughout. Several similar, much smaller porphyry intrusions occur in proximity to the Mosher Bay porphyry and are likely to be genetically related (Blackburn, 1981). Stop 5: Taylor Lake Stock UTM Coordinates: NAD83; 15U 0526461E / 5470489N Return to Highway 502 from Stop 4 and turn left back onto Highway 502. Travel 1.3km north along Highway 502 to the intersection with a Domtar forest Figure 7. Exfoliation fractures in the Taylor Lake stock. - 32 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 To the north across the valley is a narrow gabbro unit, similar to those at the last stop, emplaced into the Boyer Lake volcanics. Although not evident at this stop, the MBW fault is cut out for a portion of its length by the Taylor Lake stock, and has in turn been sinistrally offset along the north-northeast-striking Taylor Lake fault.” by Asamera Inc. failed to identify any significant mineralization at depth or along strike. The location of the original surface showing can be recognized as a notch in the exposure where shallow blasting was carried out following the initial discovery. Stop 7 (Optional Self-Guided Tour Stop): Mountdew Lake gabbro UTM Coordinates: NAD83; 15U 0525623E / 5474074N Travel 2.4km north from Stop 6 along Highway 502 to the intersection with a logging road. Turn right and park on the side of the logging road. Outcrops of the Mountdew Lake gabbro sill are visible here near Highway 502 and along a logging road on the east side of the highway. This gabbro body discordantly intrudes mafic metavolcanic rocks of the Boyer Lake group (Blackburn, 1981). The intrusion post-dates formation of the Kamanatogama syncline which has affected the surrounding metavolcanic rocks (Blackburn, 1982). Although the intrusion has not been mapped in detail, a number of intrusive phases were identified by Blackburn (1981). The dominant phase of the intrusion (visible at this stop) is a medium-grained, dark green gabbro exhibiting an ophitic to subophitic texture in which amphibole derived from pyroxene is intimately intergrown with saussuritized plagioclase feldspar (Blackburn, 1981). Other intrusive phases include coarse-grained to pegmatitic gabbro containing considerable quantities of magnetite or ilmenite, altered pegmatitic gabbro with colourless “eyes” of quartz and/or potassic feldspar, and granophyre comprised dominantly of pink potassic feldspar with minor quartz (Blackburn, 1981). Stop 8 (Optional Self-Guided Tour Stop): Starr Gold Occurrence UTM Coordinates: NAD83; 15U 0524624E / 5483569N Travel 10.6km north from Stop 7 along Highway 502. Located immediately north of a paved pull-over area on the east side of the highway. Visible gold was discovered in a narrow quartz vein located near the south end of the outcrop on the east side of the highway in 1982 by local area prospector E. Starr. Follow-up prospecting identified another gold showing approximately 15m east of the highway. However, follow-up drilling of the occurrence in 1983 Rocks at this location are deformed and altered pillowed mafic metavolcanic flows of the Pincher Lake group. The deformation, alteration and gold mineralization observed here are associated with a structure known as the Gold Rock splay (Beakhouse et al., 1995). It is a subsidiary structure to the Manitou Straits fault, which crosses the highway approximately 4km to the south. As noted previously, the most significant gold deposits in the Manitou-Stormy Lakes greenstone belt occur nearby. They are also located to the northwest of the Manitou Straits fault in rocks of the Pincher Lake group, near its southern contact with the Upper Manitou group. Alteration includes silicification and carbonatization with numerous quartz-carbonate pods, veinlets and stringers. In many locations, the alteration is accompanied by disseminated pyrite mineralization. Stop 9: Giant pillows in Pincher Lake metavolcanic rocks UTM Coordinates: NAD83; 15U 0521772E / 5485196N Travel 3.5km north from Stop 8 along Highway 502. Park vehicles at the entrance to the Ministry of Transportation gravel pit located on the east side of the highway. The outcrop is located opposite the Fort Frances 140km sign. Mafic metavolcanic rocks of the Pincher Lake group are exposed on this road cut located on the east side of the highway. These rocks are pillowed flows that show wide shape and size variations. This exposure is notable for the remarkable size of many of the pillows, some of which span the entire height of the road cut (a distance of at least 5m) and are up to 2m thick (Fig. 8). Pillow selvages are narrow, with a few much smaller pillows visible between the giant ones. In a well-exposed road cut such as this, these rocks can easily be classified as pillowed mafic metavolcanic flows. However, this type of exposure illustrates the potential pitfalls of trying to classify flow morphology in areas where only small surface exposures are available (i.e., giant pillows could easily be mistaken for massive flows). - 33 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 LILE-enriched mantle may be present in these plutons remains a question for further study. These rocks, chemically high-Mg andesites and originally referred to as sanukitoid, have many geochemically overlapping characteristics with basalt-derived tonalite but differ importantly in having high-Mg as well as Ni and Cr (e.g., Stern et. al., 1989; Sutcliffe et. al., 1990). An important difficulty in distinguishing these two origins, however, arises from the probability that even limited fractionation will obscure some of the diagnostic characteristics of primitive sanukitoid magmas.” Figure 8. Giant pillows of the Pincher Lake metavolcanic rocks. Stop 10: Atikwa Batholith UTM Coordinates: NAD83; 15U 0516387E / 5498799N Travel 15.7km north from Stop 9 along Highway 502. The Atikwa Batholith is one of a number of intrusions in the western Wabigoon Sub-province that may represent subvolcanic magma chambers that have risen into their own volcanic ejecta. The following description of these intrusions is excerpted from Beakhouse et al. (1995). “The Aulneau, Sabaskong and Atikwa batholiths, that are interpreted to represent the plutonic root to much of the volcanism in the western Wabigoon Sub-province, are dominated by rocks within the compositional spectrum tonalite-quartz diorite-granodiorite. The absence of inherited zircons (Davis et. al, 1988) and equivocal nature of field evidence for an unconformable relationship with potential underlying sialic crust, suggest development in an ensimatic regime. These rocks have distinctive geochemical characteristics including low K and Rb and high Ca, Sr and Na/K, moderately fractionated REE with HREE at 1-4x chondrite, LREE at 30-60x chondrite and negligible Eu anomalies and mantle-type radiogenic (Sr and Nd) isotopic signatures. These characteristics have led to the interpretation that many of these rocks were derived from the partial melting of tholeiitic basalt at mantle or lower crustal depths (Davis and Edwards, 1985; Beakhouse and McNutt, 1991). The extent to which a component derived from direct melting of The outcrop at this location consists largely of medium-grained, massive to weakly foliated tonalite of the Dore Lake lobe of the Atikwa Batholith. The tonalite contains numerous relatively small, partiallyassimilated mafic xenoliths. A prominent feature visible in the exposure on the eastern side of the highway is a 20cm wide mafic dike that strikes 165° and dips 70° to the west (Fig. 9). Such dikes are common near the eastern margin of the Dore lobe and may be indicative of the cooled carapace to an active magma chamber (Beakhouse et al., 1995). The eastern exposure also exhibits a prominent set of joints that strike approximately 135° and dip steeply (approximately 70°) toward the southwest. Stop 11: Van Horne Gold Property – Pritchard exposure trench UTM Coordinates: NAD83; 15U 0504890N / 5507870N Travel 17.2km north from Stop 10 along Highway 502 to the intersection with Flambeau Road. Note that this field trip stop is accessed via a private, gated road, Figure 9. Mafic dike cross-cutting tonalite of the Atikwa batholith. - 34 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 and that permission must be obtained from the property owner. The Pritchard trench is located approximately 850m southeast of the intersection with Highway 502. A summary of the exploration history and geology of the Van Horne gold property is provided below. (Fig. 10). The metavolcanic rocks occur together with synvolcanic diorite to quartz diorite intrusions and contain discontinuous horizons of interflow sedimentary rocks. Mafic and felsic dikes cut all of these rock types. The mafic dikes are mainly gabbro to diorite, while the felsic dikes include both feldspar porphyry and quartz-feldspar porphyry. All of these rock types have been cut by quartz veins. The youngest rock type on the property is diabase that occurs as a prominent west-trending dike that appears to have been emplaced after the formation of the quartz veins (Lengyel and Rennie, 2009). Exploration History Laurentian Goldfields Ltd. initiated an exploration program in 2008 on the Van Horne Gold Property, approximately 8km southwest of Dryden. There are16 historical shafts, 53 test pits, and 73 trenches located on the property. Some have underground workings and gold production has occurred at 4 of these excavations. There are approximately 24 historical gold occurrences on the property (Lengyel, 2007). The majority of this historical work occurred during a period of intense exploration and development activity that occurred between 1897 and the 1940’s. Structural Geology Rock Types All of the rock types on the property exhibit variable strain. Shearing and veining observed on the outcropscale in exploration trenches provide the basis for property-scale structural interpretations. With the exception of minor northeast-trending veins (D1 axial planar), most shear structures and veins lie along four dominant orientations associated with D2 dextral transpression. These structures are listed below: The Van Horne Property is situated in a mixed sequence of mafic to felsic metavolcanic rocks grouped together as the Lower Wabigoon volcanics • East-trending (90°) structures parallel to D2 regional Figure 10. Regional stratigraphy and location of Laurentian Goldfields Ltd.’s Van Horne gold property (from Lengyel and Rennie, 2009). - 35 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Disseminated euhedral magnetite occurs throughout the property in most rock types. However, greater magnetite concentrations occur on the western part of the property. Rennie et al. (2012) have proposed that the “magnetic anomaly underlying the Flambeau zone is associated with hydrothermal magnetite disseminated through the diorite and quartz-diorite dykes.” foliation; • West-northwest-trending (280° to 290°) D2 Riedel (R) shears; • North-northwest-trending (325° to 345°) D2 Riedel (R’) shears; and • Northwest-trending (300° to 310°) tension features. Most historical exploration and development on the property has targeted the gold potential of quartz veins. These quartz veins are structurally controlled and hosted in a variety of rock types (Lengyel and Rennie, 2009). Based on the results of a channel sampling program completed on the Van Horne Property exposures, Rennie and Chiang (2012) were able to identify the quartz vein systems that host gold mineralization and described them as follows: Sulphides “The vein array consists of west-trending shear veins and northwest-trending tension veins as well as a lesser array of sub-horizontal ladder veinlets. Shear veins tend to show dramatic pinch-and-swell but are locally continuous over up to about tens metres. Tension veins locally form left-stepping, echelon patterns and the individual veins tend to be narrow (up to 20 cm) with openly anastomosing planar forms. The shear veins and tension veins intersect without cross-cutting relation, suggesting that they are coeval and are interpreted to have formed during a single progressive strain event.” Sulphide minerals are found in all rock types, including the quartz veins. Samples collected during a second phase of sampling tested the sulphide-bearing rocks adjacent to gold-bearing quartz veins that were identified during the initial sampling. Some of the channel-cut samples collected from this sampling program returned anomalous gold values (Rennie and Chiang, 2012). Exposures – Trenching Alteration In 2009, mechanical removal of overburden and pressure washing was conducted at seven areas on the property (Fig. 11). The objective of the trenching was to provide a north-south cut across stratigraphy in areas of high outcrop density. Some of these areas are adjacent to historical excavations, whereas others were testing mineral potential sites identified from Mobile Metal IonTM (MMI) survey results and responses from a helicopter-supported airborne magnetometer geophysical survey. These exposures were geologically mapped and the mineral potential of selected parts of outcrops was tested by two phases of channel cutting and sampling. All rock types on the property exhibit variable hydrothermal alteration. The intensity of alteration in the exposures often increases where quartz veining is more abundant. Some parts of the exposures that lack quartz veins also display hydrothermal alteration (Lengyel, 2007). Several alteration features were identified during examination of the boundary between the differentiated phases within the mafic intrusive rock by Rennie et al. (2012). “... in many instances the primary mineralogy and textures in the rock are completely replaced by alteration minerals. Multiple phases of alteration are identified in drill core; these include an early phase of sericite and chlorite replacement, overprinted by carbonate and finally pervasively silicified. The main alteration minerals are carbonate, ankerite, chlorite, magnetite and silica with lesser amounts of albite and tourmaline.” Gold Mineralization Based on the assay results of samples collected by Laurentian Goldfields Ltd. since the initiation of activity on the property, Rennie and Chiang (2012) and Rennie et al. (2012) have proposed the following: • Both shear veins and tension veins contain anomalous concentrations of gold. However, the gold concentrations are highly variable; • Gold occurs in both barren and pyrite-bearing quartz veins; • Altered wall rocks with secondary pyrite locally contain high concentrations (up to 8g/t) of gold, but gold values are highly localized; • Lithological contacts and mechanically competent rock units are important host rocks for mineralization; - 36 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 11. Location of stripped exposures and historical excavation on part of the Laurentian Goldfields Ltd. Van Horne gold property (modified from Lengyel and Rennie, 2009). • The gold-mineralized zone is enriched in pathfinder elements, such as arsenic, silver and particularly tungsten; and • Hydrothermal magnetite is disseminated throughout the diorite and quartz diorite dikes. Future exploration efforts on the property could identify additional quartz-carbonate vein-hosted gold mineralization, and the sulphide-bearing rocks adjacent to known gold-bearing quartz veins may host additional gold-mineralized zones. Quartz veins and hydrothermal alteration zones localized along lithological boundaries between differentiated phases within the interiors of mafic intrusions should also be examined for gold mineralization. Pritchard Exposure Trench The Pritchard Exposure trench (Fig. 12) will be visited during this field trip. The following description of this gold occurrence is excerpted from Lengyel and Rennie (2009): “The Pritchard trench is underlain by felsic tuff and tuff-breccia, massive rhyolite, mafic intrusive, and one outcrop of [an] altered and brecciated mafic unit of unknown origin at the north end. Both mafic units and the felsic tuff adjacent to them at the south end of the trench contain strong pervasive ankeritization. Strong Figure 12. Pritchard trench geology map showing areas where anomalous gold assays have been obtained (modified from Lengyel and Rennie 2009) - 37 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 pervasive silica is ubiquitous in the trench. Sericitization is moderate to strong in the felsic units, and is also strong in the altered northern mafic outcrop. Beakhouse, G.P. and McNutt, R.H., 1991. Contrasting types of Late Archean plutonic rocks in northwestern Ontario: Implications for crustal evolution in the Superior Province. Precambrian Research, 49, p.141165. Structures observed in the trench were limited to faulting and strong fracturing in the felsic units. A north- to northeast-trending fault (D1 axial planar) in the felsic volcanics immediately south of the access road is defined by a 1-2m deep cleft in the outcrop. The fault extends across the entire unit, and associated tensional faults and quartz veins define a sinistral sense of movement. The rhyolite unit is more strongly faulted, with a 0.5m east-trending (D2) wide fault zone at its northern end and a 5m wide east-trending D2 fault zone at its southern end. A minor northwesttrending normal fault was also observed at the southern end of the rhyolite. Beakhouse, G.P., Blackburn, C.E., Breaks, F.W., Ayer, J., Stone, D., and Stott, G.M. 1995. Precambrian ’95 western Superior Province field trip guidebook; Ontario Geological Survey, Open File Report 5924, 94p. Blackburn, C.E. 1980. Towards a mobilistic tectonic model for part of the Archean of northwestern Ontario; Geoscience Canada, v.7, p.64-72. Blackburn, C.E. 1981. Geology of the Boyer Lake-Meggisi Lake area, District of Kenora; Ontario Geological Survey Report 202, 107 p. Accompanied by Maps 2437 and 2438, scale 1:31,680. Blackburn, C.E. 1982. Geology of the Manitou Lakes area, District of Kenora, stratigraphy and petrochemistry; Ontario Geological Survey, Report 223, 61p. Accompanied by Map 2476, scale 1:50,000. Despite the strongly-fractured, brittle nature of the felsic units, the bulk of the veining observed in the trench exists within the mafic units. The brecciated and strongly altered northern mafic outcrop contains four zones of <1cm wide tensional quartz-ankerite±tourmaline veins over 4m. The larger mafic intrusive unit at the south end of the trench is cut by numerous east-trending (D2) and northwest-trending (tensional), 2-26cm wide quartz-ankerite-tourmaline veins. The east-trending veins typically cut and offset the northwest-trending veins. Veining within the rhyolite is mainly associated with a zone of faulting at its south end, although the easttrending quartz-ankerite-tourmaline-muscovite vein is observed cross-cutting the faulting and local mafic dykes. Blackburn, C.E., Johns, G.W., Ayer, J., and Davis, D.W., 1991. Wabigoon Sub-province; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 1, p.303-381. Davis, D.W. 1990. The Seine-Couchiching problem reconsidered: U-Pb geochronological data concerning the source and timing of Archean sedimentation in the Western Superior Province; in Proceedings, 36th annual Meeting, Institute on Lake Superior Geology, pt.1, Abstracts, p.19-21. Davis, D.W. and Edwards, G.R. 1985. The petrogenesis and metallogenesis of the AtikwaLawrence volcanicplutonic terrain; in Geoscience Research Grant Program, Summary of Research 1984-85, Ontario Geological Survey, Miscellaneous Paper 127, p.101111. Davis, D.W., Blackburn, C.E., and Krogh, T.E. 1982. Zircon U-Pb ages from the WabigoonManitou Lakes region, Wabigoon Sub-province, northwest Ontario. Canadian Journal of Earth Sciences, 19, p.254-266. Significant assays (>1g/t Au) from the Pritchard trench include grab samples of 7.12g/t Au, 3.58g/t Au, 2.46g/t Au, 2.18g/t Au, and 1.55g/t Au. Significant channel samples include 2.21g/t Au over 0.6 m and 1.52g/t Au over 0.6m. Davis, D.W., Sutcliffe, R.H., and Trowell, N.F., 1988. Geochronological constraints on the tectonic evolution of a Late Archean greenstone belt, Wabigoon Sub-province, northwest Ontario, Canada. Precambrian Research, 39, p.171-191. References Beakhouse, G.P. 2000. Precambrian geology of the Wabigoon area; in Summary of Field Work and Other Activities 2000, Ontario Geological Survey, Open File Report 6032, p.20-1 to 20-8. Beakhouse, G.P. 2011. Western Wabigoon Sub-province Synthesis Project; in Summary of Field Work and Other Activities 2011, Ontario Geological Survey, Open File Report 6270, p.9-1 to 9-8. Lengyel, P. 2007. Compilation report on the Van Horne Area of Interest gold property; Kenora District Geologist’s office, assessment file 52E10NW BBB-1, City of Dryden. Lengyel, P., and Rennie, C. 2009. Assessment report on the Van Horne gold property; AFRO number 2.46195, Kenora District Geologist’s office, assessment file 52E10NW DDD-7, Laurentian Goldfields Limited. Lichtblau, A.F., Ravnaas, C., Storey, C.C., Tims, A., Debicki, R.L., Pettigrew, T.K., Wilson, A.C., and Wetendorf, J. - 38 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 2015. Report of Activities 2014, Resident Geologist Program, Red Lake Regional Resident Geologist Report: Red Lake and Kenora Districts; Ontario Geological Survey, Open File Report 6301, 83p. Maunula, T. and Wilson, J. 2010. Manitou Gold Inc., Kenwest property, national instrument 43-101compliant technical report, 55p. http://www.manitougold.com/_ resources/kenwest/43-101_KW.pdf Ontario Geological Survey 2011. 1:250 000 scale bedrock geology of Ontario; Ontario Geological Survey, Miscellaneous Release Data 126—Revision 1. Parker, J.R. 1989. Geology, gold mineralization and property visits in the area investigated by the Dryden-Ignace Economic Geologist, 1984-1987; Ontario Geological Survey, Open File Report 5723, 306p. Parker, J.R., Blackburn, C.E. and Davis, D.W. 1989. Constraints on timing and placement of gold mineralization in the Wabigoon Sub-province near Dryden, Ontario: evidence for synvolcanic through late tectonic emplacement; Program with Abstracts, Geological Association of Canada-Mineralogical Association of Canada, Annual Meeting, v.l4, p.A92. Rennie, C. and Chiang, M. 2012. Assessment report on the Van Horne gold property; AFRO number 2.52196, Kenora District Geologist’s office, assessment file 52E10NW DDD-9, Laurentian Goldfields Limited. Rennie, C., Chiang, M., and Meade, S. 2012. Assessment report on the Van Horne gold property; AFRO number 2.52620, Kenora District Geologist’s office, assessment file 52E10NW DDD-8, Laurentian Goldfields Limited. Stern, R.A., Hanson, G.N. and Shirey, S.B., 1989. Petrogenesis of mantle-derived, LILEenriched Archean monzodiorites and trachyandesites (sanukitoids) in southwestern Superior Province. Canadian Journal of Earth Sciences, 26, p.1688-1712. Stone, D., Davis, D.W., Hamilton, M.A., and Falcon, A. 2010. Interpretation of 2009 geochronology in the central Wabigoon Sub-province and Bending Lake areas, northwestern Ontario; Ontario Geological Survey, Open File Report 6260, p.14-1 to 14-13. Stone, D., Hellebrandt, B., and Lange, M. 2011. Precambrian geology of the Bending Lake area (south sheet); Ontario Geological Survey, Preliminary Map P.3624, scale 1:20 000. Sutcliffe, R.H., Smith, A.R., Doherty, W. and Barnett, R.L., 1990. Mantle derivation of Archean amphibolebearing granitoid and associated mafic rocks: evidence from the southern Superior Province, Canada. Contributions to Mineralogy and Petrology, 105, p.255-274. Teal, P.R. and Walker, R.G. 1977. Stratigraphy and sedimentology of the Archean Manitou group, northwestern Ontario; in Report of Activities, Part A, Geological Survey of Canada, Paper 77-1A, p.181184. - 39 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Field Trip 4 - Thunder Lake (Goliath) Project Treasury Metals Incorporated Staff Wabigoon, Ontario Please Note: All information presented within this document was taken from the website of Treasury Metals Incorporated and all data is within the public domain. The project site is not open to the public and special permission must be obtained from Treasury Metals before access of any kind is granted. speculative geologically to have the economic considerations applied to them that would enable them to be categorized as mineral reserves, and there is no certainty that the preliminary economic assessment will be realized. Mineral resources that are not mineral reserves do not have demonstrated economic viability. Project Highlights/Introduction Location Treasury Metals Incorporated is a Canadian gold exploration and development company focused on its 100% owned high-grade Goliath Gold Project (Thunder Lake), which currently consists of an Indicated and Inferred resource of 1.7 Moz. The Project, which is near Dryden, Ontario and is slated for near-term Canadian gold production. Treasury Metals Inc.’s 50-km2 Goliath Property land package (Fig. 1) lies adjacent to the Trans-Canada Highway 17 and is located midway between Thunder Bay and Winnipeg. The closest city is Dryden, Ontario located 15km to the west. The community of Wabigoon is located six kilometres to the east. Treasury Metals Inc.’s Goliath Gold Project offices are located in former Ontario government industrial offices, workshops, and storage facilities within the community of Wabigoon, Ontario. Treasury Metals is advancing through the Canadian permitting process to begin production at its open-pit gold mine and 2,500tpd processing facility in the near future. Subsequent underground operations will be developed in the latter years of mine life and will be funded from the project’s initial cash flow. Beneficial features of the Project location are: • Year-round fieldwork can be comfortable working conditions • Excellent year-round road access • On-site power supply • Trans-Canada Natural Gas Pipeline • Neighbouring major industrial services in Dryden Low initial start-up CAPEX of C$92M with cashflows from initial 3 years of open-pit production funding underground development; • Nearby workforce in surrounding communities • In August 2012, Treasury Metals completed an updated preliminary economic assessment (PEA) on the project. Key highlights include: • After-tax NPV 5% of $144M and 32.4% IRR The project demonstrates numerous highly prospective targets that show potential to host gold mineralization including about six kilometres of prospective strike along trend from the Goliath deposit. • Producing 80,000 -- 100,000 oz annually over 10+ years of mine life • Payback Period of 2.8 years • Low initial start-up CAPEX of $92M • LOM average feed grade of 2.87 g/t Au and 9.30 g/t Ag 100%-owned high-grade, demonstrating indicated and inferred resource of 1.7 Moz; • Results of 2013 drilling program have defined high-grade near-surface intersections, indicating significant upside potential for both resource and project economics; • • an The PEA is preliminary in nature and includes inferred mineral resources that are considered too completed in Project Geology and Mineralization The Goliath Project is within the Wabigoon Subprovince of the Archean Superior Province, north of the Wabigoon Fault (Fig. 2). An amphibolite metamorphic grade volcanogenic-sedimentary complex topped by an upper layer of greenschist characterizes much of the project area. The assemblage itself comprises quartzporphyritic felsic to intermediate metavolcanic rocks represented by biotite gneiss, mica schist, quartzporphyritic mica schist, a variety of metasedimentary rocks, and minor amphibolites. Compositional layering - 40 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 1: Location of the Goliath Gold Project area. in metasedimentary rocks strikes ~70 to 90° and dips from 70 to 80· south-southeast. The Thunder River Mafic Metavolcanics underlie the southern part of the property. All rocks have been subjected to folding and moderate to intense shearing with local hydrothermal alteration, quartz veining and sulphide mineralization. muscovite schist, and metasedimentary rocks), with the footwall comprising metasedimentary rocks with minor porphyries, felsic gneiss, and schist. Gold within the central unit is concentrated in a pyritic alteration zone, consisting of quartz-sericite schist (MSS), quartz-eye gneiss, and quartz-feldspar gneiss. The main zones of mineralization within the Goliath Deposit consist of the Main Zone, Footwall Zone (B, C, and D subzones), and Hangingwall Zone (H and H1 subzones). Mineralized zones strike approximately east-west and dip 70° to 80° to the south-southeast. High-grade gold mineralization (>3g/t) is concentrated in several steeply dipping, west-plunging shoots with strike lengths (>50m) and significant down-plunge continuity. The high-grade shoots are interpreted to be the result of the intersection between tight F1 isoclinal folding of the mineralized horizon and regional F2 folding with gold often concentrated in fold noses. These high-grade “shoots”, which appear to occur in regular intervals, are separated by lower grade Au/Ag mineralized rocks. Goliath Mineralization The mineralized zones are tabular composite units characterized by anomalous to strongly elevated gold concentrations, increased lead and zinc sulphide content, and distinctive altered rock units. Stratigraphically, gold mineralization is contained in an approximately 100 to 150m wide central zone composed of intensely altered felsic metavolcanic rocks (quartz-sericite and biotite-muscovite schist) with minor metasedimentary rocks. Overlying hanging wall rocks consist of altered felsic metavolcanic rocks (sericite schist, biotite- Exploration Focus Treasury Metals has completed more than 100,000m of diamond drilling since 2008 (in addition to historic drilling by Teck Resources). The Company’s current exploration and drilling program has focused on targets located in the northeast and east of the Goliath - 41 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 2: Geology Map of the Goliath Gold Project. Gold Deposit within the >49 km2 property block. Significant gold values intercepted in previous drilling campaigns, as well as re-interpreted airborne EM and aeromagnetic geophysical surveys, are being used to guide the current drilling program. Diamond drilling exploration was completed during 2012 and 2013 to test additional targets projected over 11 km of potential ore zone strike extension. Recent exploration focus has been principally to the northeast and east of the Goliath Gold deposit, in the Project’s land package. Significant gold values intercepted in previous drilling campaigns, as well as re-interpreted airborne EM and magnetometer geophysical surveys were used to guide the drilling program. The program focused on pursuing strike extensions of previously identified mineralization as well as following potential new ore shoots down dip within the currently defined resource area. Most recently the program concentrated on delineating the C Zone mineralization, now largely in the Inferred category, both within and to the east of the proposed open pit boundary. The C Zone in-fill program focused primarily on defining near surface resources, converting inferred ounces to indicated ounces, and defining a recently discovered high-grade C zone ore shoot. A previously sparsely drilled area was delineated measuring approximately 1.2km in strike-length and infill drilling of select targets was also completed. The C Zone has the potential to represent an increase in the current open pit mineable resource and a reduction of the overall waste to ore stripping ratios. With the completion of the first phase of the C Zone in-fill drilling program, the company completed a gap analysis in 2014 to determine the location of holes required to upgrade all ounces within the current open pit design from inferred to the indicated category including both the Main and C Zones. The scope and size of the in-fill and expansion drill program was determined from the gap analysis. - 42 - Refer to Figure 3 below for the location of the �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 3: Goliath Deposit mineralized zones and 2103 diamond drill collars. Project History various Goliath mineralized zones and the locations of drill holes completed in 2013. Resource Estimate The Goliath Gold Project is an amalgamation of two historic properties: the Thunder Lake Property purchased from Teck Resources Limited and Corona Gold Corporation, and the Laramide Property, transferred to Treasury from Laramide Resources Ltd. upon the Company’s spin-out in 2008. The detail of the most recent resource estimate (August 2012) for the Project is presented in Table 1, below. At present the deposit consists of an NI-43-101compliant indicated open pit (OP) and underground (UG) resource of 9.14 million tonnes (Mt) grading 2.6 grams per tonne (g/t) Au and 10.4g/t Ag for 760,000 contained gold ounces and 3,070,000 contained silver ounces. Also included within the estimate is an inferred open pit and underground resource of 15.9Mt grading 1.7g/t Au and 3.9g/t Ag for 870,000 contained gold ounces and 1,990,000 contained silver ounces (AuEq refers to Au equivalent grade; i.e. the value of all payable metals calculated as Au grade). Treasury Metals has since brought the Project to a level sufficient to begin the company’s Feasibility Study. Successfully completed Goliath Gold Project programs include: • More than 160,000 metres of diamond drilling. • Two NI43-101 compliant Resource Estimates since 2008. • Full Environmental Baseline Study Program completed. • Robust PEA released in August 2012. - 43 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 • Figure 4: Aerial view of the conceptualized open pit and the main mineralized zones. • Feasibility level Metallurgical testing. The majority of the pre-2008 exploration work was completed by Teck Resources beginning in 1990 with Teck’s first discovery hole and continuing through to a 250m underground drift and 2,375 tonne bulk sample taken in 1998 when the project was put on hold before being acquired by Laramide in 2007. There is only limited documentation of the prospecting and early exploration activity conducted on the Project properties prior to 1989. Material exploration activity on the property completed by Teck, which ultimately defined the Thunder Lake mineralization now known as the Goliath deposit, began in 1989 after reconnaissance work. The discovery hole on the Main Zone of the Goliath Deposit was drilled in 1990 and intersected multiple horizons of gold mineralization. In 1994, Teck and Corona entered into an option agreement for the development of their property, pursuant to which they formed a Joint Venture partnership in 1996 and drilled together until 1998 at which time Teck collected the bulk sample. Table 1: Treasury Metals 2012 Resource Estimation for the Goliath Gold Deposit. Resource Category Cut-off Grade Average Tonnes (g/t Au) Grade (g/t Au) Contained Au (oz) Average Ag Grade (g/t) Contained Ag (oz) Ag Total AuEq AuEq oz oz (Au+Ag) Indicated Resource Estimate Surface 0.3 6,002,000 1.8 326,000 7.1 1,257,000 22,000 348,000 Underground 1.5 3,136,000 4.3 433,000 18.0 1,812,000 32,000 465,000 9,140,000 2.6 760,000 10.4 3,070,000 54,000 810,000 Total Inferred Resource Estimate Surface 0.3 11,093,000 1.0 352,000 3.3 1,184,000 21,000 374,000 Underground 1.5 4,789,000 3.3 514,000 5.2 807,000 14,000 528,000 Total 15,900,000 1.7 870,000 3.9 1,990,000 35,000 900,000   - 44 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 By 1999, Teck and Corona’s exploration led to the outlining of the Goliath deposit and the reporting of a historic (non-NI43-101-compliant) inferred mineral resource estimate of 2.974 million tonnes at 6.47g/t Au, using a 3.0g/t Au cut-off and a minimum thickness of 3.0 metres. the company’s latest metallurgy results, is scheduled for 2015, a major milestone in the mine permitting process. Present metallurgy by G&T Metallurgical Services demonstrates 95+% gold recovery, 60-70% of which is recoverable by gravity, with 8-12 hours leach time, and a medium hardness rock. Strategic land continued to be held by Laramide, an exploration and development company involved in advancing U.S. and Australian uranium projects to development and permitting. The Laramide Property, historically referred to as the Goliath Gold Project, was located immediately south of the western portion of the Thunder Lake Property (Teck and Corona). Laramide completed preliminary exploration in 1994 and 1996 that included 1,622m of diamond drilling in eight holes. In 2008, Treasury Metals acquired all properties surrounding the deposit. New discoveries at targets outside the Goliath Gold Project’s current resource area could add significant resources to the Goliath Gold Project. Engineering activities are progressing to support advancement of this full feasibility study. Metallurgical Studies Treasury has completed several detailed metallurgical tests including a 2375 tonne bulk sample and a 400kg sample. The studies were completed by ALS Metallurgy (G&T Metallurgical Systems) of Kamloops to Feasibility level standards. The project consistently returns extremely high recoveries that are easily scalable with low leach times and low reagent consumptions. • 2375 tonne sample previously completed with 97% recovery. 70% recovery from gravity alone. • G&T Metallurgical Services obtained 95+% Au recovery, 60-70% recovery by gravity, 8-12 hours leach time, medium hardness rock. • Gekko Systems Australia are currently testing their Python Process to confirm amenability of Goliath gold ore to treatment using VSI and continuous gravity concentration CGR. This would significantly reduce treatment and permitting requirements. Environmental Baseline Study Program Baseline studies are completed to gain an understanding of the current natural environment of the site, support mine development decisions and management plans, and to provide support to rigorous on-going monitoring and mine closure plans. Treasury Metals is completing baseline studies to provide the necessary data to support the Goliath Gold Project. Treasury Metals began a complete program of baseline studies in 2010 in support of the Goliath Gold Project with an operational team of professionals based locally and nationally. Baseline studies have continued to present day supporting current physical, biological, and socio-economic decisions. Baseline study results will be provided and reported as part of the federal and provincial permitting regulations. Permitting and Mine Development The permitting process has been under way since 2012 with the acceptance of Treasury Metals Inc.’s Goliath Project Description by the Canadian Environmental Assessment Agency (CEAA). The CEAA subsequently issued its Environmental Impact Statement (EIS) guidelines to the company in February 2013. A full bankable feasibility study (BFS), including - 45 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Field Trip 5 - Classic Outcrops of the Dryden area Peter Hinz Ring of Fire Secretariat, Ontario Ministry of Northern Development and Mines, Thunder Bay, Ontario, Canada Foreword This trip will examine three stops of an eight-stop field trip which was developed for the Dryden High School Geography Department by the author in 2000. The three stops (Fig. 1) represent the most photoworthy of the eight stops and display some of the more significant depositional, structural, and metamorphic features in the Dryden Area. The field trip area lies within the Western Wabigoon Terrane formerly known as the Wabigoon Sub-province. The geological discussions related to tectonic setting, lithologies, and structure has been compiled from work completed by Beakhouse between 1999 and 2010. The field trip stop discussions include observations by the author and interpretations of host lithologies noted by Beakhouse. The photograph below of Dryden area pillowed mafic basalts is from Satterly (1943). with minimal superimposed strain. This area has a complex deformational history with an early, generally bedding-parallel fabric (D1) deformed into a series of megascopic to regional-scale, southwest-plunging, Z-asymmetric folds with the development of a second fabric (D2) parallel to the axial surface of these folds. Metamorphic grade varies regionally from upper greenschist to upper amphibolite with the lowest grade generally occurring nearest to the Wabigoon fault. The Atikwa domain occurs to the south of the Wabigoon fault and is characterized by dominantly volcanic sequences that face away from large, coeval batholiths (e.g., Atikwa Batholith, Aulneau Batholith). Within the map area, the Wabigoon Metavolcanics are typical of these sequences with a thick basal portion consisting almost entirely of mafic metavolcanic rocks overlain by a more heterolithic portion, which, although still dominantly mafic metavolcanic, includes minor intermediate to felsic metavolcanic rocks and rare metasedimentary rocks. Mineral assemblages indicative of regional middle to upper greenschist facies predominate with narrow amphibolitic contact metamorphic aureoles adjacent to some of the plutons. Lithologies (Beakhouse, 2000 & 2002) Wabigoon Metavolcanic rocks Introduction and (Beakhouse, 2005) Tectonic Setting The Wabigoon-Dinorwic area is transected by the Wabigoon fault, which is a major regional structure that separates two geologically distinct domains within the Wabigoon Sub-province. Distinct mineral deposit types and styles also characterize these domains. The Sioux Lookout domain, lying to the north of the Wabigoon fault, is characterized by a series of alternating sedimentary-dominated and volcanicdominated panels that consistently face to the south. Many of these panels are regionally interpreted to have fault contacts; however, some of the contacts appear to be conformable depositional contacts The Dinorwic area occurs within the Atikwa domain and, with the exception of several small, late intrusions, is underlain by the Wabigoon Metavolcanics. Mafic metavolcanic rocks dominate the Wabigoon Metavolcanics in the Dinorwic area. Massive and pillowed flows are approximately subequally abundant with minor, widely distributed flow breccia and hyaloclastite. Massive flows range from fineto medium-grained. Many of the pillowed flows and some of the massive flows are moderately vesicular. Equigranular flows are most abundant with conspicuous moderate to coarse plagioclase porphyritic flows occurring locally. Massive flows include both magnetite-poor (magnetic susceptibility ~0.5 to 1.0) and magnetite-rich (magnetic susceptibility commonly greater than 50) varieties, whereas the pillowed flows consistently have magnetic susceptibilities comparable - 46 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 1. Field trip stop locations (from Blackburn et al., 1981). to the magnetite-poor massive flows. The colour of both weathered and fresh surfaces is highly varied due to a range in intensity of a variety of types of alteration including carbonatization (both calcitic and ferroan carbonate), silicification, and epidotization. Thunder Lake Sediments The Thunder Lake Sediments include two separate panels of rock separated along a portion of their strike-length by the Thunder Lake Volcanics. Thinto medium-bedded wacke-siltstone characterized by even, continuous bedding is the predominant component in both panels. Thin magnetite ironstone layers are a conspicuous minor component within the Thunder Lake sediments north of the Thunder Lake Volcanics but are rare within the southern panel. Minor, thin garnet-rich (<70% garnet) and calc-silicate layers may represent original more pelitic and marly layers, respectively. In one location, the calc-silicate material forms discordant veins, and suggests that some of this material is remobilized or originated by secondary alteration processes. The garnet-rich layers are often closely spatially associated with the ironstone layers. A limited number of determinations indicate that tops, although locally reversed by tight to isoclinal minor folding, are generally to the south in both the north and south panels. Contact relations with the Brownridge Volcanics have not been observed but the data are permissive of a conformable stratigraphic relationship. The contacts with the Thunder Lake Volcanics appear to also be conformable although these are commonly moderately- to highly-strained and a loci of abundant quartz veining. Structural Geology (Beakhouse, 2000) The areas north and south of the Wabigoon fault have markedly different deformational styles. South of the Wabigoon fault, the dominantly volcanic sequence is a northward-younging homocline, but penetrative fabrics are sporadically and weakly developed. Primary form of pillows and other primary structures is well-preserved in horizontal sections, although there is currently little constraint on the extent of possible vertical stretching. - 47 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 The area north of the Wabigoon fault is characterized by alternating volcanic- or sedimentary-dominated units that are generally southward-facing and has heterogeneous, weakly- to strongly-developed penetrative fabrics. An earlier (D1), ubiquitous, weaklyto strongly-developed fabric is parallel to bedding with the exception of one outcrop where it is interpreted to be parallel to the axial surface of rootless, tight to isoclinal folds. The D1 fabric development heterogeneity is, in part, controlled by primary lithological characteristics (e.g., fabric in mafic volcanic sequences is better developed in interflow hyaloclastite/breccia units than massive, fine- to medium-grained units). The D1 fabric, along with bedding, is deformed on a variety of scales into southwesterly plunging (D2) folds having Z-asymmetry. These regional-scale folds, as well as the contrasting characteristics to the south of the Wabigoon fault, are readily apparent on regional aeromagnetic maps. On a detailed scale, most outcrops having suitable markers or well developed D1 fabrics display Z-asymmetric minor folds that commonly have a D2 fabric (spaced cleavage to pervasive penetrative schistosity) developed parallel to the axial surface of the minor folds. Alteration (Beakhouse, 2001) Noteworthy abundances of garnet, that may be indicative of alteration, occur in two settings. In the Thunder Lake Sediments, extremely garnet-rich (6080% garnet) rocks form layers that are commonly, though not exclusively, closely associated with magnetite layers and calc-silicate mineral assemblages. It is not clear if these layers are indicative of hydrothermal alteration or whether they may represent isochemical metamorphism of unusually aluminous sedimentary rocks. Field Trip Stops Figure 2. Pillows with younging direction to the upper right. pillowed mafic metavolcanic rocks which are part of the Boyer Lake group. U-Pb age determinations of 2719±3Ma and 2722 ±5Ma are reported for the Boyer Lake group by Davis (1990). The pillows display distinct thick selvages and provide excellent younging direction (Fig. 2). Small, well-developed vesicles are observed which suggest a high confining pressure and emplacement in deep water. Small sections of pillow breccia are also seen amongst the well developed pillows. Further north along the outcrops sections of interflow sediments may also be observed, suggesting a hiatus in eruptive activity. Moving northward a 2 to 3m thick unit of interflow sedimentary rocks is observed, suggesting a hiatus in extrusive activity. The sediments display bedding as well as cross-bedding. Overlying the interflow sedimentary rocks is a massive flow unit, the lower portion of which does not contain vesicles. Oriented vesicles appear in the upper portion of the flow unit (Fig. 3). From the MTO rest stop turn right and travel for 23.8km to Elm Bay Road, turn left and after 75m turn left again. Continue for approximately 350m and turn Depart Dryden from the traffic lights by McDonalds Restaurant on Trans-Canada Highway 17. Travel 39.7km east to the Snake Bay Road. Turn around and return 0.8km to the west. Park the vehicles at the MTO rest stop. Walk approximately 200m to the north to the low outcrops on the east side of the highway. Stop 1: Pillowed mafic metavolcanic and interflow sedimentary rocks of the Wabigoon Volcanics. UTM Coordinates: NAD83; 15U 0540211E / 5496631N The southern portion of this outcrop contains Figure 3. Oriented vesicles within the massive flow unit. - 48 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 have been sheared through, producing isolated fold noses. The form of these folds is a tight Z, in which axial planes are parallel to the local east-southeast formational strike. Differentially weathered beds in the metapelites and wackes serve to outline these folds, portions of some having been completely detached by shearing. A second phase of open folding refolds the first phase.” left. Drive an additional 100m and park. Walk 45m eastward along a trail to the large smooth outcrop. Stop 2 – Isoclinal folding in Thunder Lake Sediments UTM Coordinates: NAD83; 15U 0522545E / 5512553N The description below (Beakhouse et al., 1995) is for an outcrop located approximately 300m to the east along-strike and on Highway 17. The outcrop we are visiting was identified subsequent to Beakhouse et al. (1995) and was exposed as part of an aggregate operation. “Road-cut outcrops on either side of Highway 17 show style of folding and order of superposition of folding in this portion of Warclub group metasedimentary rocks. On the south side of the highway, on a smooth sloping glaciated surface, two phases of folding can be seen. Transposition of bedding into the plane of schistosity is seen where the limbs of first phase isoclinal folds The outcrop displays distinctive z-fold asymmetrical folding as well as parasitic folds. Detailed descriptions of the structural components that are visible at this stop are provided above in the Structural Geology section. Return to Highway 17, turn left and continue for approximately 10km, turn left into the Walmart parking lot and proceed to the low outcrop located on the west side of the building. Stop 3 – Folding and metamorphism/alteration of Thunder Lake Sediments UTM Coordinates: NAD83; 15U 0513496E / 5514716N At this stop pelitic sedimentary rocks and iron formation of the Thunder Lake sediments display a similar deformation history as observed at Stop 2. Thin beds of iron formation display striking Z-asymmetrical folding as well as parasitic folds on fold noses and limbs across the outcrop (Fig. 4). Garnet-rich beds are also observed throughout the outcrop. As noted in the discussion it is unclear whether the garnets are a result of hydrothermal alteration or isochemical metamorphism (Fig. 5). For detailed descriptions of the structural and alteration features at this stop, refer to the Structural Geology and Alteration sections above. Figure 4. Folding in iron formation and pelitic sedimentary rocks of the Thunder Lake sediments. - 49 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 References Blackburn, C.E., Beard, R.C., and Rivett, A.S. 1981, KenoraFort Frances, geological compilation series, Kenora and Rainy River Districts, Ontario Geological Survey, Geological Map 2443, scale 1:253,440. Beakhouse, G.P. 2000, Precambrian geology of the Wabigoon area; in Summary of Field Work and Other Activities 2000, Ontario Geological Survey, Open File Report 6032, p.20-1 to 20-8. Beakhouse, G.P. 2001. Precambrian geology of the Thunder Lake Segment, Wabigoon Area; in Summary of Field Work and Other Activities 2001, Ontario Geological Survey, Open File Report 6070, p.15-1 to 15-6. Beakhouse, G.P. 2002. Precambrian Geology of the Dinorwic Area, Wabigoon Subprovince; in Summary of Field Work and Other Activities 2002, Ontario Geological Survey, Open File Report 6100, p.10-1 to 10-6. Beakhouse, G.P. 2005. Precambrian Geology of the Dinorwic-Butler Lakes Area, Wabigoon Subprovince; in Summary of Field Work and Other Activities 2005, Ontario Geological Survey, Open File Report 6172, p.9-1 to 9-6. Beakhouse, G.P., Blackburn, C.E., Breaks, F.W., Ayer, J., Stone, D., and Stott, G.M. 1995. Precambrian ’95 western Superior Province field trip guidebook; Ontario Geological Survey, Open File Report 5924, 94p. Figure 5. Abundant garnet in pelitic sediments. Davis, D.W. 1990. The Seine-Coutchiching problem reconsidered: U-Pb geochronological data concerning the source and timing of Archean sedimentation in the western Superior Province; Institute on Lake Superior Geology, v.36, pt.1, p.19-21. Satterly, J. 1943. Geology of the Dryden-Wabigoon area, Kenora District; Ontario Department of Mines, Annual Report, 1941, v.50, pt.2, 67p. - 50 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Field Trip 6 - Gold Occurrences of Van Horne Township, Van Horne Gold property - Flambeau explosures Steve Meade Ontario Geological Survey, Sudbury, Ontario, Canada Craig Ravnaas Resident Geologist Program, Ontario Geological Survey, Kenora, Ontario, Canada Location and Access The Van Horne Gold Property and the Flambeau Exposures are located 8km southwest of Dryden. The exposures are located on private lands and can be accessed from Highway 502 (Fig. 1). Exploration Activity Laurentian Goldfields Ltd. initiated an exploration program in 2008 on the Van Horne Gold Property (Hogg, 2009). There are 16 historical shafts, 53 test pits, and 73 trenches located on the property. Some have underground workings and gold production has occurred at 4 of these excavations. There are approximately 24 historical gold occurrences on the property (Lengyel, 2007). A majority of this historical work is the result of the intense exploration and development activity occurring in the area between 1897 and the 1940’s. Rock Types The Van Horne property is situated in a mixed sequence of mafic to felsic metavolcanics rocks grouped together as the Lower Wabigoon Volcanics (Fig. 2). These rocks have been intruded by synvolcanic diorite to quartz diorite intrusions. Discontinuous interflow sedimentary rocks occur throughout the property. Mafic and felsic dikes cut all of these rocks types. The mafic dikes are mainly gabbro to diorite. The felsic dikes include both feldspar porphyry and quartzfeldspar porphyry. All of these rock types have been cut by quartz veins. One of the youngest rock types on the property is the prominent west-trending diabase dike that appears to been emplaced after the formation Figure 1. Location and access to Van Horne Property and the Flambeau Exposures. - 51 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 2. Regional stratigraphy and location of the Van Horne gold property (from Lengyel and Rennie, 2009). of quartz veins (Lengyel and Rennie, 2009). Quartz Veins Strain A majority of the historical exploration efforts and development has targeted the gold potential of quartz veins on the property. These quartz veins are structurally controlled and hosted in a variety of rock types (Lengyel and Rennie, 2009). Based on the results from the Phase II channel sampling program completed on the Van Horne property exposures, Rennie and Chiang (2012, p.29) identified the quartz veins that hosted gold mineralization: All of the rock types on the property exhibit varying amounts of strain. Shearing and veining within the trenches are consistent with structural interpretations for the property. With the exception of minor northeasttrending veins (D1 axial planar), structures and veins lie along four dominant orientations associated with D2 dextral transpression. Orientation of structures and quartz veins: • east-trending (Az 090), parallel to D2 regional foliation; • west-northwest-trending (Az 280 to 290) D2 Riedel (R); • north-northwest-trending (Az 325 to 345) D2 Riedel (R’); and • northwest-trending (Az 300 to 310) tensional features. - 52 - “The vein array consists of west-trending shear veins and northwest-trending tension veins as well as a lesser array of sub-horizontal ladder veinlets. Shear veins tend to show dramatic pinch-and-swell but are locally continuous over up to about tens metres. Tension veins locally form left-stepping, echelon patterns and the individual veins tend to be narrow (up to 20cm) with openly anastomosing planar forms. The shear veins and tension veins intersect without cross-cutting relation, suggesting that they are coeval and are interpreted to have formed during a single progressive strain event.” �Proceedings of the 61st ILSG Annual Meeting - Part 2 Alteration All rock types on the property exhibit varying amounts of hydrothermal alteration. The intensity of alteration in the exposures often increases where quartz veining is more abundant. Some parts of the exposures which lack quartz veins also display hydrothermal alteration (Lengyel, 2007). Several alteration features were identified during examination of the boundary between the differentiated phases within the mafic intrusive rock by Rennie et al. (2012, p.31): “... in many instances the primary mineralogy and textures in the rock are completely replaced by alteration minerals. Multiple phases of alteration are identified in drill core; these include an early phase of sericite and chlorite replacement, overprinted by carbonate and finally pervasively silicified. The main alteration minerals are carbonate, ankerite, chlorite, magnetite, and silica with lesser amounts of albite and tourmaline.” higher magnetite content in the rocks underlying the western part of the property. Rennie et al. (2012, p.31) have proposed that the “magnetic anomaly underlying the Flambeau zone is interpreted to be associated with hydrothermal magnetite disseminated through the diorite and quartz-diorite dykes”. Sulphides Sulphides are found in all rock types, including the quartz veins. Samples collected during the Phase II sampling program tested the sulphide-bearing rocks adjacent to gold-bearing quartz veins identified from the Phase I program. Some of the channel-cut samples collected from this Phase II program returned anomalous gold values (Rennie and Chiang, 2012). Exposures - Trenching Disseminated euhedral magnetite occurs in a majority of the rock types on the property. There is a In 2009, mechanical removal of overburden and pressure washing was conducted at seven areas on the property (Fig. 3). The objective of the trenching was to provide a north-south cut across stratigraphy in areas of high outcrop density. Some of these areas are Figure 3. Location of stripped exposures and historical excavation on part of the Van Horne Gold Property (modified from Lengyel and Rennie, 2009). - 53 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 adjacent to historical excavations, whereas others were testing mineral potential sites identified from Mobile Metal IonTM (MMI) survey results and responses from a helicopter-supported airborne magnetometer geophysical survey. These exposures were geologically mapped and the mineral potential of selected parts of the exposed outcrops were tested by two phases of channel cutting and sampling. Gold Mineralization Based on the assay results of samples collected by Laurentian Goldfields Ltd. since the initiation of activity on the property, Rennie and Chiang (2012) and Rennie et al. (2012) have proposed: • Both shear veins and tension veins contain anomalous concentrations of gold; however, the gold concentrations are highly variable; • Gold occurs in both barren and pyrite-bearing quartz veins; • Altered wallrocks with secondary pyrite locally contain high concentrations (up to 8g/t) of gold, but gold values are highly localized; • Lithological contacts and mechanically competent rock units are important host rocks for mineralization; • The gold-mineralized zone is enriched in pathfinder elements, such as arsenic, silver, and particularly tungsten; and • Hydrothermal magnetite is disseminated throughout the diorite and quartz diorite dikes. Exploration efforts could identify additional quartz-carbonate vein-hosted gold mineralization. The sulphide-bearing rocks adjacent to known goldbearing quartz veins could represent additional goldmineralized zones. Quartz veins and hydrothermal alteration zones localized along lithological boundaries between differentiated phases within the interiors of mafic intrusions should also be examined for gold mineralization. Field Trip Stops The following tour stop exposures descriptions and figures are gleaned from Rennie and Chiang (2012). The locations of the exposures are shown in Figure 3. The geological legend for Figures 4 through 7 is shown Figure 4. Pritchard exposure – North part highlighting sample sites which returned assay results >1.0g/t Au. - 54 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 5. Pritchard exposure – Central part highlighting sample sites which returned assay results >1.0g/t Au. faulted, with a 0.5m wide east-trending (02) fault zone at its northern end and a 5m wide east-trending (02) fault zone at its southern end. A minor northwesttrending normal fault was also observed at the southern end of the rhyolite. in Figure 8. Stop 1: Pritchard Exposure (Figs. 4 and 5) UTM Coordinates: NAD 83; 15U 0504905E, 5507910N The Pritchard trench exposure is underlain by felsic tuff and tuff-breccia, massive rhyolite, mafic intrusive rocks, and one outcrop of an altered and brecciated mafic unit of unknown origin located at the north end of the trench. Both mafic units and the felsic tuff adjacent to them at the south end of the trench contain strong pervasive ankeritization. Strong pervasive silica is ubiquitous in the trench. Sericitization is moderate to strong in the felsic units and is also strong in the altered northern mafic outcrop. Structures observed in the trench were limited to faulting and strong fracturing in the felsic units. A north- to northeast-trending fault (01 axial planar) in the felsic volcanics immediately south of the access road is defined by a 1-2m deep cleft in the outcrop. The fault extends across the entire unit, and associated tensional faults and quartz veins define a sinistral sense of movement. The rhyolite unit is more strongly Despite the strongly-fractured, brittle nature of the felsic units, the bulk of the veining observed in the trench exists within the mafic units. The brecciated and strongly altered northern mafic outcrop contains four zones of <1cm wide tensional quartzankerite±tourmaline veins over 4m. The larger mafic intrusive unit at the south end of the trench is cut by Pritchard Exposure (assay results >1.0g/t Au) Channel Cut Samples (H 463_ _ _ sample series) Samples mainly from quartz veins and adjacent wall rock Cut # PR-02 PR-04 Sample # 140 147 g/t Au 1.52 2.21 Rock-types quartz vein and felsic massive flow quartz vein and mafic intrusive Grab Samples (H E44_ _ _ sample series) Samples mainly from quartz veins and adjacent wallrock Sample # 569 571 572 - 55 - g/t Au 3.58 2.00 2.46 Rock-types quartz vein and felsic volcanic lapilli tuff-breccia felsic volcanic lapilli tuff-breccia felsic volcanic lapilli tuff-breccia �Proceedings of the 61st ILSG Annual Meeting - Part 2 numerous east-trending (02) and northwest-trending (tensional), 2-26cm wide quartz ankerite-tourmaline veins. The east-trending veins typically cut and offset the northwest-trending veins. Veining within the rhyolite is mainly associated with a zone of faulting at its south end, although the east-trending quartzankerite-tourmaline-muscovite vein is observed crosscutting the faulting and local mafic dykes. Widow Showing Exposure assay results > 1.0 g/t Au Channel Cut Sample (H 463_ _ _ sample series) Samples mainly from quartz veins and adjacent wall rock Cut # WSB-01 WSB-02 WSB-03 WSB-04 g/t Au 1.77 2.08 2.50 1.12 Rock-type mafic volcanic quartz vein & mafic volcanic quartz vein & mafic volcanic quartz vein & mafic volcanic Grab Samples (G011_ _ _ sample series) Samples mainly from quartz veins and adjacent wall rock Sample # 822 823 824 827 828 830 Stop 2a: Widow’s Showing Exposure (Fig. 6) UTM Coordinates: NAD83; 15U 0504870E, 5507610N The Widow’s Showing exposure is entirely underlain by massive mafic volcanic flows with 4% 0.1-0.5mm, euhedral magnetite and local strong ankerite associated with veins and fault zones. The outcrop is cut by two main structures, a main east-trending 02 fault adjacent to the east-trending D2 vein that displays dextral Sample # 120 125 128 131 g/t Au 1.54 2.68 8.96 5.07 5.61 1.25 Rock-type quartz vein and mafic volcanic quartz vein and mafic volcanic quartz vein and mafic volcanic quartz vein and mafic volcanic quartz vein and mafic volcanic mafic volcanic wrench movement with later normal dip-slip, and an associated northwest-trending dilatational fault. Sense of motion on the east-trending structures is evidenced by northwest-trending tensional shears and drag-folded Figure 6. Widow’s Showing exposure –Highlighting sample sites which returned assay results >1.0g/t Au. - 56 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 tension veins. Widow Trench Exposure (assay results >1.0 g/t Au) Channel Cut Samples (H 463_ _ _ sample series) Samples mainly from quartz veins and adjacent wall rock The outcrop contains numerous veins, which are dominantly northwest- and north-northwest-trending, with one main east-trending vein. The northwesttrending tensional veins and north-northwest-trending R’ veins dip 60° to the northeast, and are observed being dragged into the east-trending fault with dextral sense of motion. Veins are 1-30cm wide and composed of quartz, ankerite, and tourmaline with 10% pinkishwhite feldspar and minor light brown feldspar. Cut # WS-02 Sample # 088 g/t Au 1.54 Rock-types quartz vein and mafic volcanic Grab Samples (G011_ _ _ sample series) Samples mainly from quartz veins & adjacent wall rock Sample # 805 808 809 810 811 812 815 818 g/t Au 9.37 1.29 4.22 1.59 2.50 3.06 1.18 1.29 Rock-types quartz vein and mafic feldspar porphyry quartz vein and mafic feldspar porphyry quartz vein and mafic feldspar porphyry quartz vein and mafic feldspar porphyry quartz vein and mafic feldspar porphyry quartz vein and mafic feldspar porphyry quartz vein and mafic feldspar porphyry quartz vein and mafic feldspar porphyry Stop 2b: Widow’s Trench Exposure (Fig. 7) diameter pyrite and trace chalcopyrite is observed The Widow’s Trench is underlain by three main lithologies, quartz diorite, quartz-K-feldspar crystal tuff, and diorite. The quartz diorite is cut by plagioclase-phyric mafic dykes. Alteration in the quartz diorite comprises strong pervasive ankeritization, and approximately 4% euhedral disseminated 1-2mm throughout. Alteration in the quartz-K-feldspar crystal tuff is dominated by strong pervasive silicification and sericitization, with local mo derate pervasive ankerite. The diorite is very weakly deformed and altered, and contains strong pervasive magnetite. Figure 7. Widow’s Trench exposure – North part highlighting sample sites which returned assay results >1.0g/t Au. - 57 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 The main structural features in the Widow’s Trench are northwest-trending faults and quartz veins that are restricted to the quartz diorite and quartz-K-feldspar crystal tuff. The two most dominant structures are a northwest-trending high-angle reverse fault in the northeast corner of the trench and a northwest-trending shear zone that crosses the baseline at 47m (5507635 mN). Several indicators of displacement indicate sinistral movement on the shear zone. The sense of motion and orientation of the shear zone are consistent with the interpretation that it is an R’ Riedel shear related to the main east-trending dextral shear observed in the Widow’s Showing. In addition to the dominant northwest-trending quartz veins, minor north-trending quartz veins occur and are observed being cross-cut by the northwesttrending veins. Tourmaline is observed locally in the northwest-trending vein set, but is absent in the northtrending set. northward to determine structural relationships between the large northeast-trending, faulted cliff face and any newly-exposed bedrock. Approximately 3ft (1m) of overburden was removed from the northern half of the trench exposing mafic flows, mafic lapilli tuffs, and a mafic dyke. The area quickly filled in with water, and was subsequently backfilled for safety reasons, therefore only the southern exposure is described in detail below. The trench is underlain by three lithologies, with sharp east-northeast-trending contacts except where they are in northwest-trending faulted contact. The units comprise felsic quartz crystal tuff at the northern and southern ends of the exposed trench, with finegrained mafic flows and medium-to coarse-grained mafic lapilli tuffs in the middle of the exposure. The The number of northwest-trending veins displayed on Map 7 is under-represented. Many 0.5-2cm quartz and quartz-feldspar veins exist within the northwesttrending exposure north of 40m and west of 68m. Stop 3: Flambeau Exposure (Fig. 8) UTM Coordinates: NAD83; 15U 0505100E, 5507620N The Flambeau trench totals 95m in length; however, the bulk of the exposed bedrock exists within the southernmost 45m. The trench was continued Flambeau Exposure (assay results >1.0g/t Au) Channel Cut Samples (H 463_ _ _ sample series) Samples mainly from quartz veins and adjacent wall rock Cut # FL-03 FL-05 FL-06 tuff FL-08 tuff FL-09 FL-10 tuff FL-11 tuff Sample # 181 187 192 g/t Au 1.20 1.92 1.72 Rock-types quartz vein and mafic lapilli tuff quartz vein and mafic lapilli tuff quartz vein and felsic quartz crystal 200 1.49 quartz vein and felsic quartz crystal 205 209 24.80 ** 1.49 quartz vein and mafic volcanic quartz vein and felsic quartz crystal 212 2.84 quartz vein and felsic quartz crystal Grab Samples (G011_ _ _ sample series) Samples mainly from quartz veins & adjacent wall rock Sample # 833 836 837 838 842 844 902 905 907 g/t Au 5.01 1.87 3.85 2.47 1.99 2.14 1.50 2.44 47.80 ** Rock-types mafic volcanic mafic lapilli tuff mafic lapilli tuff mafic lapilli tuff quartz vein and mafic volcanic quartz vein and mafic volcanic quartz vein and mafic volcanic quartz vein and felsic quartz crystal tuff quartz vein and felsic quartz crystal tuff Figure 8. Flambeau exposure – Highlighting sample sites which returned assay results >1.0g/t Au. - 58 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 9. Legend for all Flambeau exposure figures. fine-grained mafic dykes are observed cross-cutting the mafic lapilli tuff and the felsic quartz crystal tuff with sharp contacts. Structurally, the Flambeau trench has two distinct features. Firstly, a northwest-trending high-angle reverse fault that crosses the baseline at 34.5m has resulted in blocky/broken ground at the topographic apex of the trench. A conjugate fracture set measured around 29m on the baseline indicates pure dip-slip movement (reverse). Secondly, an east-northeasttrending (D2) well-developed 7m wide shear zone centered at 20m on the baseline (5507604 mN) has deformed the mafic and felsic host rocks into fissile schists. Strong chloritization and ankeritization are pervasive. The felsic tuffs are strongly sericitized and locally silicified, and the mafic units are moderately silicified toward the central fault zone. The large finegrained mafic unit and the adjacent felsic tuff unit to the north contain 10% fine-grained, euhedral magnetite. The Flambeau trench contains three distinct orientations of 1-10 cm wide quartz-ankerite veins: 1. West-northwest-trending, closely-spaced (1030cm) Riedel veins in the southern half of the exposed bedrock; 2. Northwest-trending tensional veins in the northern half of the exposed bedrock; and 3. Minor northeast-trending 01 axial-planar veins within the central shear zone. References Lengyel, P. 2007. Compilation report on the Van Horne Area of Interest gold property; Kenora District Geologist’s office, assessment file 52F10NW BBB-1, City of Dryden. Lengyel, P. and Rennie, C. 2009. Assessment report on the Van Horne gold property; AFRO number 2.46195, Kenora District Geologist’s office, assessment file 52F10NW DDD-7, Laurentian Goldfields Limited. Hogg, S. 2009. Assessment report on the Van Horne gold property; AFRO number 2.44181, Kenora District Geologist’s office, assessment file 52F10NW DDD4, Laurentian Goldfields Limited. Rennie, C. and Chiang, M. 2012. Assessment report on the Van Horne gold property; AFRO number 2.52196, Kenora District Geologist’s office, assessment file 52F10NW DDD-9, Laurentian Goldfields Limited. Rennie, C., Chiang, M., and Meade, S. 2012. Assessment report on the Van Horne gold property; AFRO number 2.52620, Kenora District Geologist’s office, assessment file 52F10NW DDD-8, Laurentian Goldfields Limited. - 59 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Field Trip 7 - Unique Mineralizing Event at the Pidgeon Molybdenum Deposit Stripped Surface Exposure Craig Ravnaas Ontario Geological Survey, Kenora, Ontario, Canada Introduction The Pidgeon Molybdenum Deposit (Pidgeon Mo) mineralization occurs within a granodiorite body referred to as the Lateral Lake Stock (Figure 1). The known molybdenum mineralization is situated near the east-south-eastern margin of the stock and consists of molybdenite within quartz-pegmatite veins and as clots and disseminations within the rocks flanking the veins. Molybdenite is also present within unaltered and nonquartz vein parts of the stripped exposure. MPH Ventures Corp., who completed the last phase of exploration work on the property, have proposed that a 30m wide envelope containing molybdenite occurs within the eastern portion of the Lateral Lake Stock and that envelope appears to follow the boundary between the stock and adjacent supracrustal rocks (Figure 3). Mineralization has been traced for 2km in a northeasterly direction (30°) and has a dip of approximately 45° to the southeast. The field trip will examine the rocks, structural features, alteration, and mineralization exposed by a recently stripped exposure located above the historic Pidgeon Mo adit. Exploration History The exploration history for the Pidgeon Molybdenum Deposit described below was summarized from Colvine and McCarter (1977), Busch (2008), and Duke (2012). 1906: Molybdenum was discovered in a pegmatite south of Gullwing Lake by C.D. Coates. 1946: Molybdenite occurrences are reported by the Ontario Department of Mines at the east end of Lateral Lake. 1950: Initial claims are staked by G.L. Pidgeon. 1954: Claims are optioned to Detta Minerals Ltd., who drilled two sub-horizontal holes, totalling 107m. A 35m adit, located at the stripped exposure, was driven to collect a 115kg bulk sample for metallurgical testing. 1957: Pidgeon Molybdenum Mines Ltd. (PPLM) was incorporated. 1957 and 1958: Rio Tinto Canadian Exploration Ltd. optioned the property from PMML and drilled 21 holes, totalling 2348 metres. A possible, historic and non-NI43-101 compliant, resource of 568,000 tons grading 0.57% Mo was estimated. 1958: DeCoursey Brewis Minerals Ltd. completed 612m in 5 diamond drill holes along the south contact of the Lateral Lack Stock, north of Moly Lake. 1963: Denison Mines Ltd: completed 858 m in eight diamond drill holes. 1965 to 1966: Rio Algom completed a magnetometer survey and 29 diamond drill holes, totalling 3474m. The company released an internal, historic and nonNI43-101 compliant, mineral resource of 416,000 tons grading 0.57% Mo. 1977: The Lateral Lake stock was mapped by A.C. Colvine and P. McCarter of the Ontario Geological Survey and released in Miscellaneous Paper MP55. 1979 to 1980: Rio Algom completed 27 diamond holes, totalling 3710 metres. A capital and operating cost analysis was completed by Strathcona Mineral Services Ltd. who outlined an historic and nonNI43-101 compliant mineral resource of 9.0 million tonnes (Mt) grading 0.096% Mo. 1981: Rio Algom completed 3 diamond drill holes, totaling 352 metres. 2006 to 2011: MPH Ventures Corp. optioned the property in 2006. In 2007 the company completed 7 diamond drill holes, totalling 1210 m, and calculated an NI43-101 compliant revised Mineral Resources Estimate containing an inferred resource of 8.5 Mt grading 0.099% Mo. The next year (2008) MPH completed 31 diamond drill holes, totalling 2644 m, removed overburden to expose the mineralized zone above the historic adit, and completed a detailed mapping and sampling program on the stripped zone. 2012: NI43-101 Mineral Resource Estimate. During 2012 MPH Ventures completed another NI43-101 Technical Report and revised Mineral Resource - 60 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Estimate on the Mo mineralization (MPH Ventures Ltd. September 13 2012 Press Release; and Duke, 2012). The resource estimate using a cut-off grade of 0.04% Mo was calculated at: • 2.66 million tonnes (Mt) indicated @ 0.117% Mo; and • 12.39 Mt inferred @ 0.084% Mo. Location and Access The Pidgeon Molybdenum Deposit is located between the towns of Dryden and Sioux Lookout (Fig. 1). The stripped exposure to be examined is accessed by following Trans-Canada Highway 17 east of Dryden for a distance of 28km to the town of Dinorwic. Turn left (north) onto Highway 72 and drive for approximately 32km until you reach the junction with the Kathyn Logging road. Turn left onto the Kathyn Road and travel northwest for a distance of 8.8km to a trail that leads 250m west into the Field Trip Stop at the Pidgeon Mo stripped exposure. Regional Geology The Pidgeon Molybdenum Deposit is located within the eastern part of the Lateral Lake Stock (Figures 1 and 2). The Lateral Lake Stock is an elongate body extending approximately 12km from Gullwing Lake easterly to Lateral Lake and it is up to 2.8km in width. It consists predominantly of granodiorite with gradational contacts into a marginal quartz monzonite and larger country rock inclusions. Biotite is the principal mafic mineral and contains 1 to 2% epidote intergrowths. Foliation within the Lateral Stock varies from indistinct in the central part of stock to strongly foliated near the margins. The contact of the stock is concordant with the dip of the surrounding supracrustal rocks, varying from 45° on the southern side tod 30° on the northern side (Colvine and McCarter, 1977). The axial plane of a regional anticline structure trends along the central part of the Lateral Lake Stock. This regional anticline also extends into the supracrustal rocks (Fig. 2). The flanks of the anticline dip 40 to 60°, while the crestal zone plunges 25 to 30° in the east, 25 to 30° in the west. The fold is probably Figure 1. Kenora District geology map with location of Pidgeon Molybdenum Deposit (geology from OGS 2011). - 61 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 2. Regional geology and location of the Pidgeon Molybdenum Deposit and the stripped surface exposure (Blackburn, 1978). the combined result of gentle doming associated with the emplacement of the Lateral Lake Stock (Colvine and McCarter, 1977). exploration activity by MPH Ventures Ltd. provides a good exposure to view rock types, structure, alteration, and mineralization associated with the occurrence. The property and stripped exposure is situated at the eastern end of the Lateral Lake Stock, near the axial plane of the regional anticline (Fig. 2). Exposed Rock-types The metavolcanic rocks surrounding the stock consist of fine-grained chloritic units interbedded with medium- to coarse-grained amphibolitic units. Metaconglomerate units overlie the metavolcanic rocks both north and south of the stock; they contain well rounded trondhjemite (leucotonalite), chert, aplite, and mafic volcanic clasts in a quartz-feldspar-biotite matrix (Busch, 2008). The felsic intrusive rocks underlying the stripped exposure (Figs. 3 and 4) consist of coarse-grained granodiorite and quartz monzonite that are composed of quartz, albite plagioclase, orthoclase, microcline, and biotite with minor chlorite, muscovite, carbonate, sericite, sphene (titanite), epidote, and apatite (Busch, 2008). Structural Features Field Trip Stop All observed structural features are illustrated on Figures 4 and 5. UTM Coordinates: NAD 83; 15U 0545830E / 5533428N At the stripped exposure the contact of the Lateral Lake Stock with the supracrustal rocks is 70° and dips 45° SE (Fig. 3). Previous exploration has mainly focused on the mineral potential near the eastern boundary of Lateral Lake Intrusion (Fig. 3). The Pidgeon Molybdenum Deposit, exposed by stripping during the last phase of Colvine and McCarter (1977) mention that “foliation varies from indistinct in the centre of the Lateral Lake - 62 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 3. Rock-types and location of exposed rocks and mineralization and MPH Ventures Corp. diamond drill holes (Busch, 2008). Figure 4. Structural features, compositional banding, adit, and the large quartz vein in Pidgeon Mo stripped exposure (modified from Busch, 2008). - 63 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Stock to very strong on the margins, producing a finely banded to gneissic texture through separation of biotite, quartz and feldspar bands”. This compositional banding trends 42° and dips 80°E. The intrusion of several magmatic phases probably resulted in the formation of pink and grey-white banding (Busch, 2008). The pink banding is dominantly quartz and feldspar; whereas the gray bands are composed almost entirely of feldspar. The trend of this banding (42°) is not parallel to the contact of exposed supracrustal rocks (70°). The banding was overprinted by all of the other structural features present. There also appears to be increase in number of these 40°N-dipping fractures as distance increases from the contact and they overprint the 110°-trending fractures. The sloping edges of the outcrop, located in the western part of exposure near the adit entrance, appear to follow 30 to 80°-trending, 40° N dipping fractures. A similar feature comprising the slope of stripped exposure situated south of adit also appears to be associated with a 40° N-dipping fracture (Fig. 4). These slopes could comprise the footwall of a fracture. Prominent fractures observed within the stripped exposure have a strike of 110° and dip to the northwest. The dip of these fractures near the boundary with the supracrustal rocks is 45°NW, whereas that dip increases to 60°NW as the distance from the contact increases. The spacing between these fracture sets varies from 1 to 5 m. The 45-60° N-dipping and the 40° N-dipping fractures could be classified as joints and appear to have formed before the hydrothermal quartz veining event. There are also other randomly trending fractures such as a weak set of north-northeast-trending; vertically dipping fractures that overprint the quartz veins, the 45 to 60° N-, and the 40° N-dipping fracture/joint sets. A second prominent fracture set within the stripped exposure trends north-easterly between 30° and 80° and exhibits a consistent dip of 40°N irrespective of distance from the intrusion/supracrustal rock contact. It is interesting to note that the contact of the Lateral Lake stock and the supracrustal rocks dips at 45°SE, which is opposite to the dip direction of the all fracture sets observed within the stripped exposure. Figure 5. Fractures, quartz veining, alteration, and sample sites with high Mo values in the Pidgeon Mo stripped exposure (modified from Busch, 2008). - 64 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Quartz Veins present) in wallrocks; Quartz has filled some of the fractures present within the stripped exposure (Fig. 5). Quartz is not observed in the 110° trending fracture/joints but is often present within the northeast-trending 40°N dipping fracture/ joints. Quartz is not found in the northerly trending vertical dipping fractures. Quartz is also not observed along the contact of the intrusion nor extending from the intrusive rocks into the adjacent the supracrustal rocks. A majority of the quartz veins observed within the stripped exposure occur within the randomly trending fractures. These fractures could be related to strain associated with the regional anticlinal flexure. A large easterly-trending quartz vein, present in the central part of the stripped exposure, cuts the prominent 110°-trending fracture/joints (Figs. 4 and 5). Alteration Based on regional mapping and diamond drill intersections Busch (2008) proposed that “aplitic sills and potassic feldspar-bearing pegmatite are concentrated in the mineralized zone”. Busch (2008) also suggested that “based on mapping of the exposure these pegmatite and aplitic sills are [a] succession of coarse-grained alkali feldspar immediately adjacent to and within the quartz vein flanked by a uniform finegrained zone of alkali feldspar”. This alteration halo is up to 1.5m thick flanking the large easterly-trending quartz veins and is considerably narrower adjacent to other, smaller quartz veins (Figure 5). The compositional banding is commonly (but not always) destroyed in the vicinity of the alteration flanking the quartz veins. Biotite is irregularly distributed in the felsic intrusive rocks, sometimes as large patches, and is not abundant in the quartz-pegmatite veins. Mineralization Colvine and McCarter (1977) presented a summary of the molybdenite mineralization they observed at the Pidgeon Mo Deposit as follows: • Isolated grains in quartz veins and pegmatites; • Comminuted along vein margins; • Isolated grains and rosettes in wall rocks adjacent to pegmatite veins and quartz vein stockworks; • Narrow bands and lenses parallel to foliation (where • Comminuted along fractures and slippage planes in wallrocks not parallel to foliation (where present). Exploration activity, especially by MPH Ventures Corp, has outlined a 30m wide envelope containing molybdenite, extending for at least 1200 m, which occurs within the eastern part of the Lateral Lake Stock and appears to follow the boundary with the supracrustal rocks (Fig. 3). A majority of these exploration programs targeted the mineral potential of the Lateral Lake Stock along the boundary zone and associated quartz veins. The following summary of mineralization is based on examination of the exposures by Busch (2008), Duke (2012), and field visits by Kenora District OGS geological staff. Figure 6 shows the location of sample sites that returned anomalous Mo values. Molybdenite mineralization occurs in the quartz and pegmatite veins and is disseminated within the felsic intrusive rocks adjacent to veining. Molybdenite is very clotty in the quartz veins and pegmatites with a preference for the boundary between quartz and pegmatite and the boundary between the pegmatite and aplite. These clots comprise near solid molybdenite aggregates up to 3cm wide by 30cm long. Molybdenite also occurs as finer, more evenly distributed disseminations within the wallrock adjacent to the veins (Busch, 2008). Disseminated fakes of molybdenite are found in a majority of the 30 to 80°-trending, 40°N-dipping fracture/joints (Fig. 4). Molybdenite can also be found within micro-fractures cutting the felsic intrusive rocks adjacent to the 40°N-dipping fracture/joints. The mineralization present within the quartz veins can be disseminated, occur within fractures, or can concentrate along vein boundaries. Not all quartz veins present contain molybdenite (Fig. 5). Portions of the stripped exposure contain concentrations of disseminated molybdenite. These zones can contain up to 15% molybdenite, with rosettes ranging up to 1cm diameter, but do not occur within or adjacent to quartz veins, pegmatites, fractures, or fracture/joints sets (Fig. 4). Fluorite, amethyst, epidote, and tourmaline have been reported in drill logs, but were not directly related to molybdenum mineralization. Weak correlations associated with anomalous Mo mineralization are evident with B, Cd, S, SE, Th, and U values (Busch, 2008). - 65 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 6. Channel samples cuts, quartz veining, alteration, and sample sites with high Mo values in the Pidgeon Mo stripped exposure (modified from Busch, 2008) References Blackburn, C.E. 1978. Geological compilation, Kenora–Fort Frances; Ontario Geological Survey, Map 2243, scale 1:253,440. Ontario Geological Survey, 2011. 1:250 000 scale bedrock geology of Ontario; Ontario Geological Survey, Miscellaneous Release-Data 126 – Revision 1. Busch, D.J. 2008. Pidgeon Moly Assessment Report; Kenora District Geologist’s office, assessment file 52F16NW 028, AFRO# 2.41635, MPH Ventures Corp. Colvine, A.C. and McCarter, P. 1977. Geology and Mineralization of the Lateral Lake Stock; in Summary of Field Work 1977, Ontario Geological Survey, Miscellaneous Paper 75 No 47, p.205 to 208. Duke, C. 2012. Technical Report, Pidgeon Molybdenum project, mineral resource summary, prepared for MPH Ventures Corp, by Riverbend Geological Services Inc.; NI 43-101 Technical Report, filed September 13, 2012 with SEDAR®, see SEDAR Home Page. - 66 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Field Trip 8 - Geology and Mineral Deposits of the Pickle Lake Greenstone Belt Mark Smyk Resident Geologist Program, Ontario Geological Survey, Thunder Bay, Ontario, Canada Pete Hollings Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada Neil Pettigrew PC Gold/Fladgate Exploration Consuting Corporation, Thunder Bay, Ontario Introduction Exploration and Mining History Despite the fact that the Pickle Lake area has been the subject of geological mapping, mineral exploration, and mining for over a hundred years, it still remains a relatively unknown and poorly understood greenstone belt in northwestern Ontario. Renewed academic and exploration interest over the last two decades has provided new data and insights into its tectonic evolution and metallogeny in a modern context. The Uchi domain has a long history of economic mineral production, including several current and pastproducing gold mines in the Red Lake and Pickle Lake areas, and two past-producing base metal mines at Confederation Lake (South Bay volcanogenic massive sulphide (VMS) copper-zinc deposit) and Pickle Lake (Thierry copper-nickel-platinum group elements (PGE) deposit). The past-producing mines (Map 1) in the Pickle Lake belt are listed in Table 1 along with their production statistics and most recent published reserve figures. More detailed statistics are provided in the individual stop descriptions. This field trip will provide an overview of this greenstone belt, focusing on gold and base metal deposits in the vicinity of Pickle Lake (Pickle Lake Greenstone Belt Map below). To the authors’ knowledge, this is the first “formal” field trip that has been conducted in the Pickle Lake area. Many of these field trip stops are on mine properties which require access permission from the property owners. Resident Geologist Program staff (Ontario Geological Survey, Ministry of Northern Development and Mines) can provide current ownership and contact information for these properties. Field trip participants should adhere to all safety protocols and exercise caution around highways, mine workings and other potential hazards. Acknowledgments The authors have benefited from the guidance and support of a number of individuals and company personnel in developing this field guide and gaining access to mine properties: • Brian Newton (Billiken Management Services Inc.) • Norman Brewster (Cadillac Ventures Inc.) • PC Gold Inc. • Mike Aziz (Goldcorp Canada Ltd.) • Jim Hickey (Sigfusson Northern Ltd.) • Gerry White, Robert Cundari, Mark Puumala, Stuart Dunlop, Doug Lowman (Resident Geologist Program, MNDM). This synopsis of the exploration and mining history of the Pickle Lake camp has been modified from those of Thomson (1939) and Hennessey et al. (2011). A reconnaissance survey was made along the Crow River between 1903 and 1905 by McInnes (1906) of the Geological Survey of Canada. Prospecting in the Pickle Lake area commenced in 1926. In 1927, Louis Cohen of Haileybury formed a prospecting group and sent Alex and Murdock Mosher in to stake the first claims in December, 1927 on what ultimately became the Central Patricia Gold Mines Property. These claims were optioned by F. M. Connell & Associates in August 1928, and Central Patricia Gold Mines Limited was incorporated on February 19, 1929. Diamond drilling commenced at Central Patricia in February 1929 and production in March, 1930. In the spring of 1930 a mining plant was assembled at the Central Patricia and underground work was done during that summer. An ore body was outlined and a 50-ton mill recommended, but, because of financial conditions, the mill was ordered only in 1933 and was completely installed by May, 1934. In 1928, gold was discovered by Albany River Mines Ltd. at the No. 16 Vein on the Albany River claims to the east of the then Pickle Crow Property. The Crow River area had attracted little attention until the summer - 67 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Map 1: Pickle Lake greenstone belt geology and past-producing mine locations. of 1928, when these promising gold discoveries were announced and a gold rush ensued. During the following winter months an area extending 20km east of Pickle Lake was largely staked and subsequently prospected in the summer of 1929. Following the gold rush of 1928, M. E. Hurst (1931) made a preliminary geological examination of the Pickle Lake-Crow River area. Articles on the Central Patricia mine were written by J. M. Connie and J. A. Reid. Up to that time the most promising discoveries were at the Central Patricia Mine, the showings of Northern Aerial Minerals Exploration (NAME), a company set up in 1928 by J. E. (Jack) Hammell (later becoming the Pickle Crow Mine), and a vein on the Springer claims, which were under option to F. M. Connell and associates. Development on other groups of claims did not provide much encouragement, although on many claims very little rock was exposed and no real exploration could be done. The Central Patricia discovery paved the way for exploration in the region which led to the discovery and initial drilling (1929) of the first Pickle Crow ore body, the No. 1 (Howell) Vein, by NAME. The original discoveries on Central Patricia and NAME ground were drilled in 1929. Pickle Crow Gold Mines, Limited was organized and took over the property of Northern Aerial Canada Golds, Limited, successors to NAME. Underground work had commenced at the - 68 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Table 1: Past-producing mines in the Pickle Lake area (modified after Puumala, 2009). Mine Name UTM Zone Central 15 Patricia #1 15 Central Patricia #2 Easting (m) 697509 Northing (m) 5707914 Years of Production 1934–1951 703404 5708271 1938–1940 15 702034 5699957 1989–1994 Golden 15 Patricia Pickle Crow 15 629877 5692270 1988–1997 704366 5709757 1935–1966 684148 5708433 1976–1982 Dona Lake Thierry 15 Total Production Reserves 621 806oz (17,628kg) Au 13,158oz (373kg) Au None published 246 500oz Au 5080t @ 8.05g/t Au open pit reserves (1993)1 40,000t @ 5.95g/t Au (1994)1 None published 619,796oz (17,572kg) Au 2 10.150Mt @ 3.9g/t Au (2011) (~1.26M oz) 1,446,214oz (41,000 kg) Au 51,528,960kg Cu Measured and Indicated Resources: 8.131Mt @ 1.46% Cu, 0.18% Ni, 3.7g/t Ag; Inferred Resource: 11.507Mt @ 1.46% Cu, 2 0.15% Ni, 6.1g/t Ag (2012) K1-1 Deposit: Inferred Resource: 51.044 2 Mt @ 0.31% Ni, 0.08% Cu, 1.5g/t Ag (2012)   reserve estimates that cannot be confirmed to comply with the reporting standards of National Instrument 43Historical 101. 1 Resource estimate determined in compliance with the reporting standards of National Instrument 43-101. 2 original discovery (Howell vein) in the fall of 1933 and soon indicated an ore body. A 125-ton mill was ordered and production commenced on April 17, 1935. The immediate success attained at both properties led to a resumption of activity in the area. In the summer of 1936, 14 companies were at work. Owing to the large areas of overburden, detailed examination of the country was carried on by means of geological mapping, geophysical surveys, surface trenching, and diamond-drilling. Albany River Mines Ltd. sank the Albany Shaft to a depth of 190m (625ft) between 1933 and 1938 and completed extensive underground development. Winoga Patricia Gold Mines was created in 1936 and drilled 73 surface diamond drill holes on a pie-shaped property located between PCGM’s holdings and the Albany River Mines ground to the east. A shaft was subsequently sunk on the property in 1938. That same year, PCGM took over ownership of both Albany River Mines and Winoga Patricia Gold Mines through a new company called Albany River Gold Mines Ltd. It is believed that the Winoga Patricia Gold Mines shaft later became the No. 3 Shaft of the Pickle Crow operation. The Cohen-MacArthur zone, located 2km to the north of the developing Pickle Crow Mine, was discovered in 1933. A total of 14 surface diamond drill holes were drilled at Cohen-MacArthur in the winter of 1936. This property also was optioned by PCGM in 1938. With the acquisition of the CohenMacArthur claims, PCGM became one of the largest land holders in the Pickle Lake area. The Geological Survey of Canada completed a regional synthesis of the Pickle Crow greenstone belt during this period as well. Ground and airborne geophysical surveys have been completed over all or parts of the Pickle Crow property at various times during its early history. A dip-needle survey completed in 1936 on the Pickle Crow property was useful in tracing out the bands of iron formation. A detailed magnetic survey was carried out over the property by Teck (or its predecessor companies) around 1960. With the outbreak of World War II and the shortage of labour, mine operations slackened off considerably. Prospecting virtually came to a standstill, although both Pickle Crow and Central Patricia Mines continued production throughout the war. Pickle Crow was the last of the original gold mines to close, ending operations in 1966. Central Patricia Gold Mines began copper-nickel exploration in the Kapkichi Lake area in the mid1940s. Kapkichi Nickel Mines Limited followed, carrying out surveys and drilling from 1956 to 1966. Mining claims covering the Thierry copper-nickel deposit were optioned by Union Minière Explorations and Mining Corporation (UMEX) from Kapkichi Nickel Mines in 1969. UMEX’s follow-up work led to a decision to proceed with development of the deposit in 1974. The Thierry Mine was in production from 1976 to 1982 (Puritch et al., 2012). The property is currently held and is being explored by Cadillac Ventures Inc. In 2012, Cadillac revised the Resource Estimate for the underground portion of their Thierry Project, based upon the operating costs of conceptually combining the operations of the Thierry underground - 69 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 and the K1-1 open pit deposits. In early 1984, exploration by Dome Exploration (Canada) led to the discovery of the Dona Lake gold deposit, southeast of the main Pickle Lake camp (Cohoon 1986). Jointly owned by Dome Mines and Campbell Red Lake Mines, the Dona Lake Mine began production in 1988. After producing approximately 250,000oz of gold, it suspended operations in 1997; the property is now maintained by Goldcorp Canada Inc. The Golden Patricia Mine, 65km west of Pickle Lake, was discovered by St. Joe Gold Corp. Operated by Bond International Gold/LAC Minerals, the mine produced over 600,000oz of gold from 1988 to 1997. There has been ongoing exploration in the Pickle Lake camp since the suspension of mining operations. The most notable of the gold exploration projects has been that of PC Gold Inc. on the Pickle Crow Mine property. Since assuming ownership in 2008, the company’s exploration efforts have generated a 1.26 million-ounce, NI 43-101-compliant gold resource (10,150,000 tonnes averaging 3.9g/t gold). Mapping of the Pickle Lake area has been undertaken by the Ontario Department of Mines and its successor, the Ontario Geological Survey. The reader is referred to a number of maps and reports, including: Hurst (1931); Harding (1936); Thomson (1939); Evans (1941); Pye (1956; 1975; 1976); Ferguson (1966); Sage and Breaks (1982), Stott et al. (1989a, b) and Stott (1996). Regional Geology of the Pickle Lake Greenstone Belt The regional geology of the Pickle Lake belt has recently been elucidated by modern lithostratigraphic, geochemical, geochronologic and structural studies. These include research and mapping conducted by Stott (1996), Hollings (1998, 2002), Hollings and Kerrich (2004), Young (2003), Young and Helmstaedt (2001) and Young et al. (2006), among others. The Pickle Lake greenstone belt is located within the central Uchi Sub-province (Fig. 1). Stott and Corfu (1991) divided the belt into four volcanic assemblages based on stratigraphic relationships, isotopic age data, and aeromagnetic data (Fig. 1); in order of decreasing age these are the Northern Pickle (~2900Ma), Pickle Figure 1. Regional geology of the Pickle Lake Greenstone Belt (from Young et al., 2006). - 70 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Crow (2870Ma), Woman (2820Ma), and Confederation (2750Ma) assemblages (Fig. 1; cf. Sage and Breaks, 1982; Corfu and Stott, 1989). More recently, a revised stratigraphy has been developed by Young et al. (2006), from which the following synopsis is taken: “The Archean Uchi subprovince of the western Superior Province contains an imperfectly preserved record of over 300 million years of crustal evolution, characterized by episodic volcanism and plutonism initiated at ~3.0 Ga and culminating with subprovince-scale orogenesis at ~2.7Ga (Stott and Corfu 1991; Corfu and Stott 1993a, 1993b, 1996; Sanborn-Barrie et al. 2001; Thurston et al. 1991). The east–westtrending Uchi subprovince consists of narrow supracrustal belts surrounded by broad plutonic domains that occur at the southern margin of the North Caribou terrane (NCT, Fig. 1; Thurston et al. 1991). time of amalgamation of the Winnipeg River and Wabigoon subprovinces, respectively, against the NCT (White et al., 2003). These regional tectonic fabric-forming events are associated with gold mineralization throughout the Uchi subprovince (Stott and Corfu 1991). In particular, gold mineralization at the world-class Red Lake mining district is well studied and is most likely associated with ~2.718Ga regionally penetrative structures (Sanborn-Barrie et al. 2001; Dubé et al. 2003). In contrast, structures hosting gold mineralization in the Pickle Lake belt were thought to have formed prior to ~2.744Ga (Stott and Corfu 1991; Stott 1996); however, we can show that these structures are younger, approximately correlative with the timing of mineralization in Red Lake. The Pickle Lake greenstone belt comprises a 25km wide and ~70km long belt of Archean supracrustal rocks and internal granitoid plutons surrounded by large granitoid batholiths (Fig. 2). The supracrustal rocks have been metamorphosed to greenschist facies with amphibolite facies Neoarchean regional scale deformation and metamorphism is widespread along the southern margin of the NCT (Stott and Corfu 1991) at ~2.72 and ~2.70Ga, which probably identifies the Figure 2. Geology of the Pickle Lake Greenstone Belt (from Young et al., 2006). - 71 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 to 2740Ma trondhjemitic to granodioritic Ochig Lake pluton and Pickle Lake stock, as well as the ~2697 to 2716Ma Hooker–Burkoski stock. occurring within the narrow (<1km wide) thermal aureoles of younger plutonic bodies (Stott 1996). Although all of these rocks have been metamorphosed, in this paper the prefix “meta-“ is dropped for convenience. Up to three phases of deformation, variably developed, are recognized in the Pickle Lake belt and are described in detail later. The earliest recognized deformation (D1) is recorded by a local bedding-parallel foliation in the Pickle Crow assemblage. This foliation is truncated by the ~2735Ma Albany quartz– feldspar porphyry dyke and is not recognized in the volcanic rocks of the Confederation assemblage. The early deformation event is attributed to overturning of the Pickle Crow assemblage prior to deposition of the ~2744 to 2729Ma Confederation assemblage. Subsequent deformation and development of a regionally penetrative planar fabric (S2) postdates ~2729Ma volcanism, pre-dates the intrusion of the ca. <2716Ma Hooker–Burkoski stock and is host to gold mineralization. Corfu and Stott (1993a, 1993b) and Stott (1996) described a tectono-stratigraphy in the Pickle Lake belt. New field work, U–Pb ages, geochemical data, and Sm–Nd isotopic analyses have established the timing and determined the nature of volcanism, deformation, and tectonic assembly of the Pickle Lake greenstone belt [Young and Helmstaedt 2001, Young 2003, Young et al. 2006]. The >2860Ma Pickle Crow assemblage has been redefined to include the former Northern Pickle assemblage on the basis of stratigraphic continuity and similar volcanic geochemistry between the two units across a previously inferred fault contact. The Pickle Crow assemblage consists of tholeiitic basalt with thin, but laterally extensive, oxide-facies iron formation overlain by alkalic basalts and minor calcalkaline andesites to dacites with primitive Nd isotopic compositions (εNd 2.89 Ga = +2.1 to +2.4) suggestive of deposition in a sediment-starved oceanic basin. Integration of new and previously published field, geochronological, and geochemical data has permitted revision of the distribution and contact relationships of supracrustal assemblages in the Pickle Lake greenstone belt. The revisions can be summarized as follows: 1. Based on the lateral continuity of lithologic units and Nd isotopic compositions, the rocks formerly attributed to the inferred ~3Ga Northern Pickle assemblage have been assigned to the ~2.89Ga Pickle Crow assemblage. An earlier assumed accretionary boundary between these two assemblages is not supported by the present work. The ~2km thick 2836Ma Kaminiskag assemblage (former Woman assemblage) consists of tholeiitic basalt interbedded with intermediate and rare felsic pyroclastic flows with primitive Nd isotopic compositions (εNd 2.836 Ga = +2.4). Two samples of intermediate volcanic rocks interbedded with southeast-younging pillowed basalt, previously inferred to be part of the Pickle Crow assemblage, yielded U–Pb zircon ages of 2744+3/-2Ma and 2729±3Ma. These rocks are thus part of the younger Confederation assemblage, which consists of intercalated basalt and dacite (εNd2.74 Ga = +0.1 to +0.8) exhibiting diverse compositions probably reflecting eruption in a continental margin arc to back-arc setting. The contact between the Confederation and Kaminiskag assemblages is assumed to be a fault. 2. Based on new geochronology, the limits of the ~2.74–2.73Ga Confederation assemblage are known to extend further north and east in the belt and include rocks formerly assigned to the Pickle Crow assemblage. Opposite younging directions and enriched isotopic compositions suggest that the Confederation assemblage unconformably overlies the Pickle Crow assemblage. 3. The ~2836Ma Kaminiskag assemblage (formerly Woman assemblage), previously assumed to have developed autochthonously on the Pickle Crow assemblage, is thought to be in fault contact with the younger Confederation assemblage, having developed outboard of the North Caribou terrane. The greenstone belt is intruded by late synto post-tectonic plutons including the composite quartz dioritic to gabbroic July Falls stock with a U–Pb zircon age of 2749+4/-2Ma, and the ~2741 The proposed tectonic evolution incorporates the revised supracrustal assemblages and can be - 72 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 assemblages of the Uchi Sub-province are dominated by plume-related magmatism (c.f. Hollings et al., 1999; Tomlinson et al., 1998) whereas younger assemblages (<2.8 Ga) are characterized by arc related magmatism (c.f. Hollings et al., 2000; Hollings and Kerrich, 1999). The Pickle Lake belt (Fig. 2) offers the opportunity to investigate the transition between these regimes as he ~2.8Ga Pickle Crow assemblage offers a valuable opportunity to evaluate the tectonic processes occurring on the margins of the ancestral Superior Province during this significant transition. Hollings (2002) reported data on the Northern Pickle assemblage and distinguished three distinct volcanic suites (Fig. 3). Suite I, from the north of the assemblage, comprises tholeiitic basalts with unfractionated rare earth elements (REE). Suite II basalts are light REE (LREE) enriched with negative mantle normalised Nb anomalies relative to Th and La, similar to the CA suite of Sajona et al. (1996). Basalts from Suite III are also LREE enriched but lack negative Nb anomalies. Normalised Nb abundances are either similar to, or higher than Th and La, comparable to Nb-enriched basalts (NEB) from Phanerozoic arcs. Variable high field strength elements and heavy REE (HREE) systematics support the subdivision of Suite III basalts into two spatially distinct suites. Suite IIIa is interpreted to have melted at relatively shallow depths in the presence of Nb-Ti-bearing silicates whereas Suite IIIb melted in the presence of garnet. Variations in the geochemistry of the three suites can be accounted for by interaction between primitive mantle, adakite melts, and subduction modified mantle. The association of the three suites is interpreted to be the result of rifting of Suite I tholeiites in a backarc environment, characterized by Suite I tholeiites, permitting asthenospheric upwelling of subduction modified NEB and associated arc-like volcanic rocks (Suites II and III). The association of arc like basalts (Suite II) with NEB (Suite III) in the Northern Pickle assemblage extends the known occurrence of Archean NEB beyond the ~2.7 Ga age of previously recognised examples. summarized as follows: 1. Deposition of the Pickle Crow assemblage in a back-arc to emergent-arc setting prior to 2.86Ga. The isotopically enriched tholeiitic lower sequence may represent deposition on or near a thinned or juvenile continental margin; whereas, compositionally diverse and more evolved rocks of the upper sequence may have formed in a transitional arc to back-arc setting. 2. North- to northwest-vergent shortening (D1), widely bracketed between 2892 and 2744Ma, may have been responsible for overturning the Pickle Crow assemblage and producing a major recumbent fold. Prior to the deposition of the Confederation assemblage, the upper limb of this recumbent fold was eroded, preserving the downward-younging lower limb. This episode of deformation is one of the few documentations of pre-2.75Ga tectonism and, by implication, metamorphism in the western Superior Province other than in the North Caribou greenstone belt (Thurston et al. 1991) and cryptic evidence in the Red Lake belt (Sanborn-Barrie et al. 2001). 3. The isotopically juvenile ~2836Ma Kaminiskag assemblage developed parautochthonously with respect to the North Caribou terrane in an arc to back-arc setting followed by intrusion of isotopically enriched ~2821Ma tonalite (Quarrier tonalite gneiss). 4. The ~2744 to 2729Ma Confederation assemblage was deposited unconformably on the overturned Pickle Crow assemblage. Arc-derived plutonic rocks (the Ochig Lake pluton and Pickle Lake stock) intruded the Pickle Crow and Confederation assemblages during this interval of Neoarchean volcanism. 5. Subsequent to ~2739Ma Confederation assemblage volcanism, and prior to the intrusion of the ~2697–2716Ma Hooker–Burkoski stock, regionally penetrative deformation resulted in steep foliations and telescoping of the Kaminiskag assemblage against, and possibly tectonically interleaved with, the Confederation assemblage. Gold mineralization in the Pickle Lake belt is associated with these tectonic fabrics and is therefore contemporaneous with mineralization in the Red Lake camp, possibly reflecting a NCTwide mineralization event.” Geochemical evidence shows that the older (>2.9Ga) Hollings and Kerrich (2004) showed that the ~2.89Ga Pickle Crow assemblage comprises a basal unit of tholeiitic basalts with uniform SiO2 contents, and MgO spanning 12.4 to 3.1 wt.%. The basalts plot on a hyperbolic mixing array from a high Nb (4-8ppm), La/Ybn, but low Zr/Nb member to a low Nb (2-4 ppm), La/Ybn, but high Zr/Nb counterpart, indicative of a heterogeneous sub-arc mantle source (Fig. 3). The tholeiites were interpreted to have formed in a back-arc basin to the later calc-alkaline rocks found - 73 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 3. Chondrite- and primitive mantle-normalised plots for representative samples from the Pickle Crow Assemblage. Normalizing values are those of Sun and McDonough (1989). stratigraphically above them (Stott and Corfu, 1991; Young and Helmstaedt, 2001). The presence of NEB within the Northern Pickle assemblage, interpreted by Young and Helmstaedt (2001) to stratigraphically underlie the Pickle Crow assemblage is consistent with that model and overall the geochemical data are consistent with a paired back-arc and arc, where the high-Nb basalts are generated from back-arc mantle, and the lower-Nb basalts from sub-arc mantle. Field Trip Stops The field trip stops are shown in Figure 4 and a road log for the field trip is presented in Table 2. Stop 1: Pillowed Basalt, Pickle Lake Turn-Off UTM Coordinates: NAD83, 15U, 0696981E / 5706785N Two large, glacially polished outcrops just south of the junction of Highway 599 and Pickle Lake Road expose pillowed basalt (Fig. 5) of the tholeiitic Lower Sequence of the Pickle Lake Assemblage (Young, 2003). The tholeiitic rocks of the lower sequence display SiO2 contents of 44–51 wt. %, MgO of 4–11 wt. % and Fe2O3 of 9–15 wt. %. The majority of samples are characterized by unfractionated or weakly depleted light REE (LREE) and mildly fractionated heavy REE (HREE) (La/Smpm = 0.75–1.10; Gd/Ybpm = 0.97–1.35). The Th/La and Zr/Y ratios are generally less than the primitive mantle values of 0.11 and 2.44, reflecting the depleted character of the tholeiites. In addition, some samples show negative Zr and Ti anomalies relative to neighbouring elements on a primitive mantlenormalized plot (Young, 2003). Unfortunately, the westernmost of the two large outcrops has fallen victim to geo-vandals, who have applied white paint and localized spray paint to much of the outcrop. Despite this, recessively weathered sections and unpainted surfaces suggest that it is, indeed, pillowed basalt. The easternmost outcrop has escaped the fate of its neighbour and does offer a great exposure of the same pillowed basalt. Pockmarked by recessively weathered selvages, possible drainage cavities and fractures, these grey-weathering bun - 74 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Table 2: Road Log for Field Trip; trip stops are shown in Figure 4. All UTM co-ordinates on road log and field trip stops are stated as NAD83, UTM Zone 15. STOP NAME STOP NO. Starting Point Pillowed Basalt DISTANCE (km) EASTING NORTHING Junction Highway 599 and Pickle Lake Road 0.0 679055 5706910 Turn south on Hwy. 599; to Pull-off / parking area on right 0.05 696981 5706785 687610 5709480 1 THIERRY MINE AREA Sheared basalt LANDMARK Junction Highway 599 and Pickle Lake Road (reset) 0.0 Central Patricia Mine 0.9 Bridge across Crow (Kawinogans) River 1.0 Junction, Highway 599 and Thierry Mine Road 3.2 Reset, turn west onto Thierry Mine Road 0.0 Road to K1-1 Zone pit 10.3 2a 10.8 Mine Gate 14.4 Pillowed basalt, south of West Pit 2b 684010 5708540 Sheared basalt and gabbro, north of East Pit 2c 684107 5708717 Mafic dyke in sheared basalt, north of East Pit 2d 684140 5708686 “Ribboned” basalt, north of East Pit 2e 684122 5708670 “Breccia ore”, northwestern end of East Pit 2f 684086 5708638 PICKLE CROW MINE AREA Junction Highway 599 and Pickle Lake Road (reset) 0.0 Junction, Highway 599 and Pickle Crow Road 0.5 Reset, turn east onto Pickle Crow Mine Road 0.0 Mine Gate 7.1 703600 5709080 Field office and core yard 3a Trench C (No. 5 and No. 11 veins) 3b 7.5 703890 5709300 No. Vein stockpile and Shaft Pit 3d 8.2 704355 5709860 - 75 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Table 2: Continued Trench A (Conduit Zone) 3c DONA LAKE AREA 10.3 Junction Highway 599 and Pickle Lake Road (reset) 0.0 Junction Highway 599 and Dona Lake Mine Road 11.0 Reset, turn onto Dona Lake Mine Road 0.0 705820 5711200 Dona Lake Mine Portal 4a 702099 5699732 Sulphidic Banded Iron Formation 4b 701767 5699990 Pillowed Basalt 4c 701686 5700056 Banded Iron Formation 4d 701708 5699991 BIF, southern end of pit 4e 701764 5699994 693228 5693898 Junction Highway 599 and Dona Lake Road (reset) Ochig Lake Pluton 5 0.0 3.2   pillows are outlined by thin, dark selvages and locally developed pillow breccia. Pillow cusps and packing indicate “tops” to the southwest. A foliation, striking at ca. 125° may parallel flow contacts. Localized flexures and crenulations in this fabric were also noted. Stop 2: Thierry Copper-Nickel Mine (Sub-stops are described below) (N.B. Permission to enter the site must be granted by Cadillac Ventures Inc.) The exploration and development history of the Thierry Mine property has been taken and modified from Puritch et al. (2006). Despite Pickle Lake’s reputation as a gold camp, base metal occurrences were discovered in the late 1940s and became a significant focus of exploration and mining activity in the late 1970s. Central Patricia Gold Mines Limited completed diamond drilling from 1946 to 1950 on several gabbro-hosted copper-nickel prospects in the Kapkichi Lake area, west of Pickle Lake. Kapkichi Nickel Mines Limited conducted geophysical surveys and diamond drilling in this area between 1956 and 1958. The actual claims covering the mine site were optioned by Union Miniere Explorations and Mining Corporation (UMEX) from Kapkichi Nickel Mines in 1969. UMEX completed further exploration and drilling that eventually outlined four high-priority areas of copper-nickel mineralization: the K1-1, K2-1, G, and J anomalies, respectively. The K2-1 anomaly eventually became the Thierry Mine Deposit. In 1969, following a joint-venture agreement with Kapkichi, UMEX crews started ground electromagnetic, magnetometer and geologic surveys on the Kapkichi property. As a follow-up of the McPhar and UMEX geophysical surveys, drilling intersected low-grade (0.40% Cu, 0.11% Ni) mineralization in mafic and ultramafic rocks underlying Kapkichi Lake. This discovery was later named the “J” and “G” deposits. In September 1970, the first hole drilled outside the area covered by the joint-venture agreement with Kapkichi Nickel Mines, intersected 6m (20ft) of sulphides in biotite-chlorite schist containing 1.24% Cu and 0.14% Ni. This drill hole marked the actual discovery of the Thierry deposit, which was completely covered by overburden and thus had escaped discovery by earlier prospectors. Immediately following the discovery drill hole, the Thierry Deposit was drilled off, on a grid of cross sections 200ft apart, by 77 holes, totaling 45,000ft. In December 1971, at the end of the surface drilling campaign, the undiluted in-situ, drill-indicated reserves were estimated at 11,500,000 tons averaging 1.68% Cu and 0.18% Ni. In view of these results, UMEX decided to proceed with an underground development and - 76 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 4. Field trip stop locations, Pickle Lake Greenstone Belt geology (geology from Young et al., 2006). exploration program. Kilborn Engineering was awarded a contract to prepare a preliminary feasibility study of the deposit and to assume the project engineering. Figure 5. Pillowed basalt, Pickle Lake turn-off (Stop 1). After studying Kilborn Engineering’s feasibility report, it was decided to proceed with the development of the deposit. The official decision to place the property into production was made in 1974. Shaft Sinking started on June 27, 1972 to a planned depth of 2200ft. On December 8, 1973, after installing the cage and skip, cross-cut stations were started at the 600-, 1200-, and 1600-foot levels. At the end of 1974, the crosscuts on these levels were completed. Work progressed to complete the drifts in the hanging wall at the 600- 77 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 and 1600-foot levels and the drift in the footwall at the 1200-foot level. Horizontal and inclined holes were drilled from stations at 100-foot intervals in the drifts. Underground development, mill construction (started late 1974), and infrastructure development (town site, upgrading of access road, upgrading of the power line, etc.) were accelerated as much as possible to be ready for start-up by July 1, 1976. On July 1, 1976 the 4000 short tons per day mill was completed and production started at the Thierry Deposit in two separate small open pits (West and East) from the surface down to the 170-foot level. Dilution was very high (approximately 30%) due to the transverse mining method applied by the contractor. Ore was separated in two stock piles: high-grade (above 0.5% Cu), and low-grade (below 0.5% Cu). Underground production, utilizing the sub-level (blast hole) stoping method started in 1977. Underground work went on continuously but the milling rate was reduced from 4000 short tons per day to 2000 short tons per day in July 1977 given the low copper prices. At the same time, underground primary development was halted until October 1978. Production continued until April 1982 when the mine and mill were decommissioned. Production from the Thierry Deposit was accomplished by two open pits followed by underground operations. Underground development of the deposit included the development of a three-compartment shaft to a depth of 543m (1778ft) and 2890m of excavations at the 180, 360 and 850m levels. A total of 15,850m of underground diamond drilling was completed. Approximately 5.8 million tons of ore were mined and processed, with an average grade of 1.13% Cu and 0.14% Ni, between October 1976 and April 1982. This resulted in the production of approximately 217,750 short tons of concentrate that contained 113.6 million pounds of copper and 2.8 million pounds of nickel. Initially, only a copper concentrate was produced; however, by 1981 UMEX recognized the value of the nickel and a limited amount of nickel concentrate Table 3: Thierry Mine Estimated Historical Production (Novak and Mlot, 2004).   Year Production (tons) Cu (%) Ni (%) Pt+Pd+Au (ounces) Concentrate (tons) 1976 1977 1978 1979 1980 1981 1982 TOTAL 215,017 956,428 913,103 1,021,572 1,160,558 1,309,298 255,556 5,831,532 1.17 1.26 1.29 1.15 1.08 0.90 1.32 1.13 0.10 0.13 0.11 0.11 0.11 0.09 0.10 0.13 0.043 0.037 0.041 0.04 0.033 0.018 0.043 0.04 9,072.44 40,071.89 38,365.67 39,140.70 43,304.39 35,871.18 11,924.59 217,749 was produced which slightly enhanced the net smelter value of the ore. In addition, minor payable amounts of precious metals and PGE were also reported: 17,500 troy oz platinum; 47,000 troy oz palladium; 17,000 troy oz gold, and 900,000 troy oz silver. Mine production data were estimated from available UMEX production records and was tabulated by Novak and Mlot (2004). The estimate is summarized in Table 3 below. The mine was shutdown in 1982 due to low metal prices, and lower than anticipated ore grades. In June 1987, the mine was allowed to flood. After the mine closure in 1982, very little additional exploration was carried out; however, between 1987 and 1989 analyses were carried out for platinum and palladium. Between 1974 and 1975, 221 drill core samples from the Thierry Deposit were assayed by UMEX for Pt and Pd. In 1987, UMEX staff geologist D. Unger, implemented re-sampling and assaying of selected diamond drill holes. In 1987, R. Dahl was retained by UMEX to undertake a complete reevaluation of the PGE potential of the Thierry Mine and vicinity. An airborne geophysical survey (EM/ Resistivity/Magnetometer/VLF-EM) was flown by DIGHEM in 1988 over the Kibler Lake Stock. In 1989, UMEX Inc. re-evaluated the economic potential of the deposit with a view to reopening of the Thierry Cu-Ni Mine. UMEX, as a result of the PGE studies undertaken between 1987 and 1988, was aware that the mine contained nickel-copper zones that were coincident with anomalous PGE concentrations. UMEX also reported a large, low-grade zone of disseminated copper-nickel mineralization at the K1-1 anomaly. The K1-1 zone is tabular, 1500m long and up to several hundred metres thick. Test mining of this zone was undertaken in 1981. The average grade of the zone is 0.31% Cu and 0.1% Ni, at a cut-off of 0.2% Cu. Etruscan Resources Inc. purchased the property in 1990 with a view to placing the property into production. In 1991, Watts, Griffis and McOuat Limited (WGM) were engaged by Etruscan to prepare an economic analysis for the reactivation of the Thierry operation. WGM reported non-NI43-101-compliant diluted underground “mineral reserves” of 2.7 million short tons with average grades of 1.78% copper and 0.25% nickel. Etruscan completed reclamation of the mill and shaft facilities from 1993 to 1995. During this period, the entire drill core storage facility, including the contained core, was destroyed. PGM Ventures Corporation acquired the property from Etruscan under an asset purchase agreement - 78 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 in late 2000. PGM Ventures reviewed and initiated development of a digital database from available UMEX data. In 2002, PGM Ventures undertook a drilling campaign of 25 drill holes, totaling 8952m, to test mineralization at the Thierry Deposit and at other targets on the property. In total, PGM Ventures drilled 11 holes to confirm the presence of economically interesting mineralization at the Thierry Deposit. Drilling confirmed mineralization on the eastern and western portions of the deposit. Five drill holes intersected mineralization from depths of 1200ft to 1800ft below surface. PGM Ventures also took samples of ore representative of that previously mined from surface stockpiles to confirm the presence of consistently attractive PGE values in the well-developed breccias and massive sulphide ores. PGM engaged JVX Ltd. to complete a Time-Domain electromagnetic and magnetometer survey. The work was designed to identify geophysical targets along the main westtrending structure, which hosts both the Thierry and the Ros Zone (K1-1 zone). The survey results outlined coincident magnetic and EM anomalies, similar in intensity and extent to those at the Thierry Mine itself. Richview Resources Inc. acquired the property and commenced a diamond drilling program to explore the Thierry deposit and other areas of the property from October, 2004 through March 2005. Apart from the drilling program no other exploration work was complete by Richview. Puritch et al. (2012) outlined the subsequent work at Thierry. An NI43-101-compliant resource estimate with an effective date of February 1, 2006 consisted of 4,623,000 tonnes of Measured and Indicated material at a grade of 1.81% Cu, 0.20% Ni, along with 4,366,000 tonnes of Inferred material at a grade of 1.71% Cu and 0.18% Ni. Richview commenced its summer validation and exploration program on May 9, 2007 and completed a 14,000m drilling program. Surface drilling around the K1-1 open pit area to confirm and validate the historic drilling was also undertaken. A compilation of all mine data was conducted. A 3km corridor of unexplored ground between the Thierry Mine and the K1-1 deposit was cleared of overburden. A summer work program including excavation, geological mapping, prospecting, and geochemical sampling was completed by October 2008. A Mobile Metal Ion (MMI) geochemical survey of the Thierry Project was also conducted. The amalgamation of Cadillac Ventures Inc. and Richview Resources Inc., pursuant to a three-cornered agreement, became effective on Jan 15, 2010. Cadillac assumed 100% control of the Thierry Project. The following synopsis of the post-amalgamation period is taken from Cadillac’s Management’s Discussion and Analysis, January 26, 2015. Following the amalgamation, Cadillac obtained access to all the detailed geological and technical information on Thierry that was available. This data was then evaluated with the objective of developing a comprehensive work program. Cadillac revised the resource estimate for Thierry to include the additional data generated by the drilling of 20 holes in 2007 and 2008 by the former operators of the property as well as the drilling used in previous NI43-101-compliant resource estimates. Cadillac reported (press release, June 9, 2010) that the resource estimate update for the former Thierry Mine then consisted of a Measured and Indicated resource of 6,228,000 tonnes containing 1.92% Cu and 0.2% Ni and an Inferred resource of 8,379,000 tonnes containing 1.79% Cu and 0.16% Ni, using an NSR cut off of $46/tonne. This report enabled Cadillac to identify target areas for a surface diamond drilling campaign in conjunction with a dewatering initiative at Thierry. The objective of the planned drilling program was to infill drill an area at depth where there had been a lack of information in the Thierry Deposit model. Cadillac commenced the drilling program in May 2010 and in May 2011 reported the successful completion of all six holes, totalling 6800m. Assay results from each of the six holes drilled indicated a successful intersection of the Thierry mineralized zone at depth. Additionally, a diamond drilling program designed to test for shallower extensions of mineralization from the known and modeled deposit, commenced in January 2011. Cadillac subsequently reported that the assay results from these drill holes confirmed the extension of mineralization both to the west and to the east of the known deposit with both directions remaining open. The results of this drilling program, together with the extensions of the strike length to the west and east, formed the basis of an updated resource estimate (press release, September 1, 2011). This updated resource consisted of a Measured and Indicated resource of 8,281,000 tonnes containing 1.73% Cu and 0.2% Ni and an Inferred resource of 14,639,000 tonnes containing 1.70% Cu and 0.16% Ni, using an NSR cut off of $46/tonne. Cadillac subsequently completed an NI43-101-compliant Technical Report and Resource Estimate in December 2011. - 79 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Following a successful drilling program on the nearby K1-1 deposit in November and December 2011, Cadillac further revised the Thierry underground resource based upon conceptually combining the operations of the Thierry underground and the K1-1 open pit deposit (which resulted in a $5/tonne decrease in the NSR cut-off to $41/tonne). The further updated resource consisted of a Measured and Indicated resource of 8,815,000 tonnes containing 1.66% Cu and 0.19% Ni and an Inferred resource of 14,922,000 tonnes containing 1.64% Cu and 0.16% Ni, using the revised NSR cut-off of $41/tonne (press release, February 23, 2012). Cadillac subsequently completed an NI43-101compliant Technical Report and Resource Estimate in March 2012. Outside of the Thierry underground mine area, the eastern component of the property encompasses a project referred to as K1-1. This area, which lies 3 km east of the Thierry Mine, was drill-tested by UMEX who defined the mineralization in 1973 and 1981. UMEX had identified an estimated, non-compliant 75,000,000 tonne resource containing 0.38% Cu and 0.11% Ni. During February and March 2011, Cadillac completed three shallow drill holes on the K1-1 occurrence in order to test previous UMEX results and intersected wide zones of mineralization. During the first quarter of fiscal year 2012 Cadillac completed an additional 13-hole drilling program on the K1-1 deposit designed to confirm the historic data calculation and facilitate an NI 43-101-compliant resource. Cadillac subsequently reported the results of the 13 holes drilled, with each of the holes intersecting widespread shallow mineralization with localized higher-grade occurrences. The results of this drilling program enabled Cadillac to calculate the size of this open pit deposit and in October 2011, Cadillac reported the completion of a mineral resource estimate and exploration target for the K1-1 open pit project. This estimate was based on a combination of historic drilling by previous project operators and more recent drilling by Cadillac. The Inferred mineral resource estimate for K1-1 within a whittle pit shell consists of 19,897,000 tonnes containing 0.42% Cu and 0.10% Ni, using an NSR cut off rate of $ 15/tonne. The exploration target for K1-1 located outside and below the resource pit shell was also estimated to contain 45,000,000 to 55,000,000 tonnes grading 0.32% to 0.36% Cu and 0.08% to 0.12% Ni. Cadillac subsequently completed a NI 43-101-compliant Technical Report and Resource Estimate in December 2011. Cadillac then completed a further exploration program on the K1-1 deposit during November and December 2011 utilizing two diamond drill rigs and completing a total of 26 holes. The purpose of this program was to upgrade and expand the mineralization and models at K1-1 by infill drilling within the area of the pits and adjacent to the modeled pits, as well as targeting areas under the pits and along strike in the exploration program. Cadillac reported that the assay results of samples from the 26 holes enabled Cadillac to further update the initial K1-1 resource estimate. The updated Inferred mineral resource at K1-1 had been estimated within an economically optimized Whittle pit shell consisting of 53,614,000 tonnes containing 0.38% Cu and 0.10% Ni, using an NSR cut off rate of $11/tonne (press release, February 14, 2012). Cadillac subsequently completed an NI43-101compliant Technical Report and Resource Estimate in March 2012. The K1-1 deposit consists of a Global Mineralized inventory including the optimized pit and adjacent mineralization of 75,857,000 tonnes containing 0.38% Cu and 0.10% Ni, using an NSR cut off rate of $11/tonne (press release, February 14, 2012). This updated K1-1 resource estimate will impact on a future production decision at Thierry as the close proximity of both the K1-1 deposit and the Thierry underground mine to each other will allow cost efficiencies based on the sharing of the infrastructure and the processing plant capacity which should enable Cadillac to realize lower production costs and therefore process material of a lower grade than would be envisioned if either deposit were operating independently of each other. In May 2012, Cadillac received a positive Preliminary Economic Assessment (PEA) which demonstrated the technical and potential economic viability of reopening the Thierry Mine and processing material from both the underground Thierry Mine deposit and the K1-1 open pit deposit on a combined basis (press release, May 12, 2012). Cadillac had reported that it had commissioned a dewatering plan for the Thierry mine site. During fiscal year 2013, Cadillac completed more drilling at K1-1 (8 DDH, totalling 2200 m) to potentially increase the mineralized resource. Cadillac subsequently reported that each hole had successfully encountered mineralization outside of the current Whittle Pit-defined compliant resource and returned assay grades comparable, or better than, those contained within the current Whittle Pit-defined compliant resource (press release, September 13, - 80 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 2012). Cadillac also completed a field prospecting/ mapping program designed to provide field testing and sampling of correlated geophysical and coincident magnetic anomalies on the property outside of the area of the Thierry Mine and K1-1 during the year. Cadillac continues to evaluate the results of the aforementioned drilling, prospecting and mapping programs with a view to determining the next phase of the exploration program at the Thierry Mine and K1-1. Geology of the Thierry Mine The description of the geology of the Thierry Mine, below is taken from Anderson (2007). A vertical crosssection through the deposit showing the pit and some of the underground workings is present in Figure 6. Regional setting The Thierry deposit is situated in the northwest portion of the Archean Pickle Lake greenstone belt in the Uchi volcano-plutonic subprovince of the western Superior Province. Supracrustal rocks in this portion of the Pickle Lake belt are included in the Mesoarchean (>2860Ma) Pickle Crow assemblage, which consists of massive and pillowed tholeiitic basalt flows, with minor intercalated sedimentary rocks, iron formations, and calcalkaline andesite and dacite (Young et al., 2006). Along the northwest margin of the Pickle Lake belt, the Pickle Crow assemblage is intruded by stocks, plugs and sills of mafic to ultramafic composition. The largest of these intrusions, the July Falls stock, is composed of gabbro, diorite and quartz diorite, the latter of which has yielded a U-Pb zircon age of 2749Ma (Young et al., 2006). South of the Thierry deposit, these rocks are intruded by the 2740Ma Pickle Lake stock, which is interpreted to be broadly syntectonic Figure 6. Cross section of the Thierry deposit (Patterson and Watkinson, 1984). - 81 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 edge of the open pit, most outcrops consist of strongly foliated mafic tectonites, which appear to be derived from pillowed mafic flows and flow-breccia, with subordinate intercalations of sedimentary rocks. Individual pillows are typically strongly flattened and attenuated, although convincing, bun-shaped examples of pillows are observed locally. Intercalated mafic breccia layers tend to be more strongly tectonized than the pillowed flows, and locally exhibit moderate to strong epidote- calc-silicate alteration. Layering in these rocks dips moderately to steeply northwest. Mafic sills and dikes are abundant and consist mainly of medium-grained equigranular leucocratic and mesocratic gabbro. (Young et al., 2006). To the north, the Pickle Crow assemblage is intruded by the post-tectonic Bow Lake batholith. Metamorphic mineral assemblages indicate middle greenschist-facies regional metamorphism, although amphibolite-facies assemblages are developed in the thermal aureoles of the Neoarchean plutons. Overprinting relationships described by Young et al. (2006) indicate a relatively simple deformation history. A regionally pervasive and penetrative planar shape fabric, which is oriented subparallel to bedding and the general trend of the supracrustal belt, is assigned to the D2 deformation phase (Young et al., 2006). In the northern portion of the belt, the S2 fabric dips steeply northwest and is axial planar to tight to isoclinal, moderately to steeply-plunging, F2 folds. This fabric also contains a variably developed L2 stretching lineation that plunges steeply to the northeast or north-northeast. On the northwest flank of the East pit, the mafic volcanic and intrusive rocks are structurally overlain to the north by a 10–20m thick unit of well-layered chloritic tectonite, which appears to have been derived from stratified mafic to intermediate volcaniclastic rocks. These rocks are weathered dark green to light grey and are fine-grained, chloritic and feldspathic. Local geology Bedrock exposures in the footwall of the Thierry deposit consist mainly of pillowed mafic flows and intercalated flow-breccia, with subordinate mafic intrusions. The mafic flows weather greenish-grey to black and are finegrained, aphyric and non-amygdaloidal. Wellpreserved, bun-shaped to amoeboid pillows range up to 1.5m in maximum dimension, with 1–2cm thick selvages. Interpillow material consists mainly of strongly recrystallized and altered hyaloclastite. On the southwest margin of the East pit, individual pillowed flows range from 1 to 5m thick and are intercalated with similarlythick units of monolithic mafic breccia and pillow-fragment breccia. The subvertical flow contacts trend roughly north and a pillow cusp in one location indicates tops to the east. Mafic tectonites in the hanging wall of the Thierry deposit are intruded by fine-grained, lateto post-tectonite felsic dikes that weather pale brown and contain sparse quartz and feldspar phenocrysts. Structural geology Overprinting relationships in a single outcrop of pillowed basalt in the area roughly midway between the fenced shaft and West pit indicate at least three generations of ductile deformation fabric. In this outcrop, an early penetrative planar fabric that trends generally north is defined by foliated chlorite and amphibole, and a variably developed tectonite layering. This early fabric is overprinted by open to tight, symmetric to Z-asymmetric folds that trend northeast and plunge steeply to the north. These folds are, in turn, transected by a finely-spaced crenulation cleavage that dips subvertically and trends eastnortheast. In this location, the mafic volcanic rocks are intruded by minor dikes and sills of medium to coarse-grained gabbro/pyroxenite. In the largest of these intrusions, leucogabbro, melagabbro and pyroxenite exhibit highly contorted and locally gradational contact relationships that are suggestive of magmatic-mixing. Although these relationships clearly indicate three generations of ductile deformation, the available information is insufficient to determine the relationship (if any) of these structures to those observed in the East and West pits, or their significance with respect to the regional deformation history described by Young et al. In the hanging wall of the Thierry deposit, on the northwest margin of the East pit, bedrock exposures exhibit significantly higher finite strain than those in the footwall. Along the northwest - 82 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 common feature of the generally east-trending greenstone belts in the Superior Province. Such features can be interpreted in terms of two distinct generations of structure (e.g., early vertical stretching overprinted by transcurrent shearing), or a single generation of structure formed during progressive deformation within a transpressional shear zone (i.e., a shear zone with a significant component of zone-normal shortening). In the present case, the available data is insufficient to differentiate between these, or other, possibilities. (2006). It is noteworthy however, that the overall style, orientation and sequence of deformation structures observed in these outcrops correspond closely to structures assigned to the regional D1, D2 and D3 deformations by Young et al. (2006). As described above, finite strain appears to be considerably lower in the footwall of the Thierry deposit, as compared to the hanging wall. The stratification observed in the outcrops of pillowed mafic flows and breccia on the south margin of the East pit trends generally north, at a high angle to the strike of the mineralized zone. Open to tight, west-trending folds, such as those observed south of the West pit, could account for this marked obliquity. In this regard, it is possible that the localization and orientation of the shear zone that hosts the Thierry deposit was controlled by the axial surfaces or limbs of pre-existing, west-trending fold structures. Alternatively, the obliquity could simply result from progressive folding along the margins of the mineralized zone during shearing, in which case the strata further down in the footwall would be expected to gradationally change orientation into the more regional easterly trend. The tectonite fabric is overprinted by at least two generations of folds, which are particularly well-developed in the layered mafic to intermediate tectonite on the northwest margin of the East pit. The earlier folds are open to tight, strongly asymmetrical, similar-style structures. The fold hinges plunge moderately to steeply toward the northwest or north. Z-asymmetrical folds predominate over S-folds, although both are observed in single outcrops. This opposing vergence is consistent with the local occurrence of sheath-like fold closures. A sheath-fold interpretation for these closures is supported by the fact that the fold hinges are oriented subparallel to a locally observed quartz-ribbon lineation and the more extensively-developed L-fabric in the underlying mafic tectonites. As described previously, rocks in the hanging wall of the deposit consist mainly of variably tectonized mafic volcanic rocks. The tectonite fabric in these rocks consists of both planar and linear fabric elements. The planar fabric element dips steeply to the north-northwest or northwest and is defined by flattened primary features (e.g., pillows, clasts), transposed veins and primary layering, and foliated chlorite and amphibole. The presence of steeply plunging sheath folds and stretching lineations in the mafic to intermediate tectonite indicates a significant component of dip-slip shear, in apparent conflict with the abundance of dextral kinematic indicators on the horizontal outcrop surfaces. Although such features are not inconsistent with transpressional shear zones, they may be more readily interpreted in terms of a two-stage deformation history involving early dip-slip shear, overprinted by later strike-slip (dextral) shear. Such complex deformation paths would have important implications for the localization and geometry of Cu-Ni ore bodies in the Thierry deposit (see below). The linear fabric element is defined by aligned hornblende porphyroblasts, rare quartz ribbons and, in the mafic intrusions, aligned aggregates of amphibole and chlorite that likely represent pseudomorphic replacements after primary pyroxene phenocrysts. Some of these rocks approach pure L-tectonites. On the northwest margin of the East pit, the L-fabric plunges 65o towards the north. On the east flank of the East pit, the L-fabric plunges 40o toward the northwest. The tectonite fabric and early asymmetrical folds are overprinted by gentle to open, upright folds with steeply plunging hinges and northwesttrending axial planes. Asymmetric boudins and shear bands are abundant on horizontal outcrop surfaces and consistently indicate dextral shear. Tectonite zones exhibiting well-developed kinematic indicators on horizontal outcrop surfaces and steeply plunging stretching lineations are a In the mafic tectonite, the mineral assemblage appears to comprise chlorite, hornblende, feldspar, epidote, biotite and quartz, consistent - 83 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 have obscured or destroyed primary textures. Patterson and Watkinson (1984) describe the host-rocks as consisting of 75% metagabbro, 20% mafic metagabbro and 5% talc-carbonate schist, and provide convincing textural evidence of a peridotitic to dunitic protolith for the talccarbonate schist. Novak and Mlot (2004) cite previous drill-log descriptions of chemical sedimentary rocks in the mine sequence, which were not described by Patterson and Watkinson (1984). with upper greenschist- to amphibolite-facies metamorphism. Hornblende porphyroblasts are strongly aligned in the planar shape-fabric, and locally define a penetrative mineral lineation that plunges parallel to the stretching lineation. In one location, hornblende porphyroblasts are also randomly oriented within and across the planar shape-fabric, suggesting that hornblende growth outlasted penetrative ductile deformation. In the western portion of the large stripped outcrop on the northwest flank of the East pit, the mafic tectonites are cross-cut by a series of widely-spaced, discrete, brittle-ductile faults. These faults dip steeply toward the northnortheast and are defined by very planar slipsurfaces that sharply truncate the tectonite fabrics in the wall-rocks. The faults contain narrow (<20cm), laminated fault-fill quartz veins which locally widen into more irregular quartz breccia veins. Oblique internal fabrics in the laminated portions of the veins indicate dextral strike-slip shear. The most significant of these faults contains narrow, discontinuous veins and irregular blebs of remobilized chalcopyrite with minor pyrrhotite. The rock-types tentatively identified by this author in the mineralized drill cores from the Thierry deposit include possible chemical or siliciclastic sedimentary rocks, ultramafic intrusive rocks, and mafic intrusive/extrusive rocks. The possible ultramafic rocks are dark green to black, fine- to medium-grained and equigranular, and composed almost exclusively of chlorite (±carbonate, talc, serpentine and amphibole). The possible sedimentary rocks are light to dark grey or reddish-brown, fine-grained, siliceous and biotitic, and typically exhibit a finely laminated to layered structure. Evidence of intense transposition is provided by the common occurrence of tight to isoclinal, strongly asymmetric and rootless folds. From the descriptions in Patterson and Watkinson (1984, p.4), Novak and Mlot (2004), Keller (2005) and Puritch et al. (2006), it seems likely that most of these rocks were previously described as ‘mylonite’ or ‘chlorite-biotite schist’, with the implication that they were derived through intense deformation and hydrothermal alteration of mafic or ultramafic intrusive precursors. However, this interpretation appears inconsistent with the observation that these rocks commonly lie in very sharp contact with ultramafic and mafic intrusive rocks that lack mesoscopic deformation fabrics, which would require very abrupt strain-gradients from the mylonite into the precursor rocks. Discrete shear-fractures and faults locally form complex, diffuse networks within, and along the margins of, the zone of mafic-intermediate tectonite, and are particularly well-developed in the massive, equigranular gabbro sills. Asymmetric fabrics and minor offsets of the main tectonite fabric consistently indicate dextral strike-slip shear. The shear fractures dip steeply to the north and northeast, and appear to define a Riedel-type shear-fracture system to which the brittle-ductile faults described above may be related. Riedel-type systems comprise multiple orientations of synthetic and antithetic shear factures that, in a strike-slip system, will exhibit a common, sub-vertically plunging line of intersection. Orebodies overprinted by these types of shear-fracture systems would be expected to exhibit significant structural complexities. The gabbro is fine- to medium-grained and typically massive and equigranular, with a weak to moderate foliation defined mainly by chlorite. Leucogabbro grades continuously into melanogabbro, consistent with the descriptions of Patterson and Watkinson (1984), and the occurrence of possible magma-mixing textures in gabbro exposed on the south margin of the East Host-rocks to mineralization Identification of primary rock-types in the examined intervals of mineralized drill core from the Thierry deposit is significantly complicated by the combined effects of intense metamorphic recrystallization, penetrative deformation and local hydrothermal alteration, which - 84 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 irregular-shaped fragments or grains of silicate gangue. This style of mineralization typically contains 5 to 10% total sulphide. pit. Blocky, relatively undeformed inclusions of melanocratic gabbro in the layers of possible sedimentary origin are tentatively interpreted as gabbro dikes that underwent boudinage and segmentation during intense transposition of the host-rocks. Unless the host-rocks were significantly strained prior to emplacement of the gabbro dikes, the relatively undeformed aspect of the boudins would tend to support a sedimentary, rather than tectonic, origin for the fine-grained and laminated aspects of biotitic layers. Net-textured mineralization consists of irregular, inter-connected veinlets of [chalcopyrite] and [pyrrhotite] which surround fragments or grains of silicate gangue. Typically, this style of mineralization contains 10 to 25% total sulphide and, with increasing sulphide content, grades into breccia mineralization. Breccia mineralization consists of near-solid (50-80% total sulphide) to solid (>80% total sulphide) [chalcopyrite] and [pyrrhotite] that contains angular to sub-rounded inclusions of wall-rocks or other silicate gangue. Breccia mineralization tends to form discrete, relatively sharp-walled veins that are apparently controlled by irregular fracture arrays in relatively undeformed (i.e., weakly to non-foliated) wall rocks. These veins may result from ductile flow of sulphides during deformation (i.e., piercement veins) or fluid-state remobilization during, or subsequent to, deformation (e.g., Gilligan and Marshall, 1987). The breccia veins typically do not exceed 50cm in maximum thickness, and are composed mainly of [chalcopyrite], with subordinate to subequal [pyrrhotite]. Some veins exhibit a marked segregation of [pyrrhotite and chalcopyrite]. Styles of mineralization Patterson and Watkinson (1984) describe four types of sulphide ore in the Thierry deposit: breccia ore, mylonite ore, bornite ore and disseminated sulphides. Novak and Mlot (2004) propose a similar classification, but instead refer to the mylonite ore as ‘chlorite-biotite schist’ ore and include an additional, poorly defined, ‘oxidized’ ore-type. “… sulphide mineralization in the Thierry deposit is subdivided on the basis of texture into disseminated, matrix-textured, net-textured and breccia styles. It is recommended that the terms ‘mylonite’ or ‘chlorite-biotite schist’ should be avoided, as these describe only the hostrocks to the sulphide mineralization, not the mineralization proper…” Late-tectonic emplacement of the breccia veins is indicated by their relatively undeformed state, and the local presence of multiphase deformation structures in wall-rock inclusions. In addition, breccia mineralization is observed to discordantly cut tight to isoclinal folds in the laminated rocks of possible sedimentary origin, indicating emplacement subsequent to development of the intense transposition fabrics. These aspects, coupled with the relatively finegrained, non-annealed nature of the breccia mineralization indicate emplacement late in the tectono-thermal evolution of the host-rocks. Disseminated mineralization consists of isolated, monomineralic to polymineralic grains of pyrrhotite (po), chalcopyrite (cp) and/or pentlandite that typically do not exceed 2mm across and are evenly distributed throughout the host-rock. Typically, this style of mineralization contains less than 5% total sulphide. Disseminated mineralization is observed in ultramafic and mafic rocks of inferred intrusive origin, and has been interpreted to represent recrystallized primary magmatic sulphide (e.g., Patterson and Watkinson, 1984). It should be noted however, that disseminated sulphides are also locally observed in the finely laminated rocks of possible sedimentary origin, indicating that alterative mechanisms may be required to explain at least some of this mineralization. Nevertheless, some of the breccia veins do preserve evidence of a more protracted, multiphase emplacement history. One of the breccia veins intersected in DDH PGM 05-60, for example contains two generations of breccia mineralization. In this vein the early generation of breccia is inclusion-poor and distinctly layered and is sharply cross-cut by a later generation of Matrix-textured mineralization consists of blebs and irregular veinlets of [chalcopyrite] and [pyrrhotite] that form the matrix to equant to - 85 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 relatively massive and inclusion-rich breccia. chalcopyrite, pyrrhotite, pentlandite and pyrite. Although most examples of breccia mineralization are relatively undeformed, some exhibit evidence of ductile and brittleductile deformation, in the form of a moderate to strong planar fabric defined by aligned inclusions of silicate gangue. This mineralization is appropriately referred to as ‘durchbewegt’ (Marshall and Gilligan, 1989). In [one] example…the ductile fabric is cross-cut by enechelon arrays of [chalcopyrite]-filled extension veins (i.e., tension-gashes). Four principal types of sulphide mineralization are recognized at the Thierry Deposit (Patterson and Watkinson, 1984) with Patterson (1980) noting a fifth: Puritch et al. (2012) provided another synopsis of copper-nickel-PGE mineralization at the Thierry Deposit: • Chlorite-Biotite Schist Mineralization (mylonitic mineralization): 56% of all mineralized rock (CBS ore), containing 5-20% sulphide as stringers of chalcopyrite, pyrrhotite, pentlandite and pyrite; the stringers parallel foliation and where gradational with breccia mineralized rock, the breccia fragments are flattened and elongated. “Mineralization at the main Thierry and adjacent K1-1 deposits, is more or less coincident with what is best characterized as a chloritebiotite-hornblende altered mylonitic shear zone (the “CBS shear zone”). The shear zone extends across the ultramafic intrusive along a strike length of about one kilometre and a width up to 50m. Within the shear zone mineralization is hosted by highly schistose rocks containing stringer sulphides to less schistose ultramafic rocks containing massive stringers or veins and disseminated sulphides. Primary sulphides, listed in approximate order of decreasing abundance are pyrrhotite, chalcopyrite, pyrite and pentlandite. Cubanite, bornite, magnetite and minor ilmenite have also been identified. Violarite and mackinawite have developed from alteration of pentlandite. • Breccia Mineralization: 40% of all mineralized rock and composed of 20-30% sulphide, consisting of rounded to angular fragments of gangue in a matrix of chalcopyrite, pyrrhotite, pyrite and pentlandite. Breccia mineralized rock grades into CBS mineralized rock. • Bornite Mineralization: 2% of all mineralized rock, containing 1-5% sulphide as stringers and disseminations of chalcopyrite and bornite in carbonate veins associated with blocks of amphibolite schist in the main shear zone. • Primary Disseminated Sulphide Mineralization: 1% of all mineralized rock, occurring as blocks of chalcopyrite (with exsolution of bornite or cubanite) plus pyrrhotite and pentlandite between remnants of olivine. • Oxidized Mineralization: 1% of all ore comprised of several varieties of ore, characterized by violarite, millerite, bornite etc. Outside of the main mineralized zone, chalcopyrite and bornite occur as stringers as well as finely dissemination sulphides. Bornite is commonly associated with carbonate and quartz veins. Oxidized mineralizations are reported to contain violarite, millerite and bornite. The mylonite and breccia mineralization has a copper-to-nickel ratio of 8:1, compared to a 2:1 ratio in the disseminated sulphides. In addition, the chalcopyrite:pyrrhotite ratio is approximately 1:1 in the mylonite mineralization and 1:10 in the disseminated sulphides (Patterson, 1980). Copper-nickel-PGE mineralization at the Thierry and K1-1 deposits is hosted within a highly deformed and altered ultramafic sequence. Copper-nickel-PGE mineralization consists of: Precious metal minerals have been found in the Thierry Deposit in two distinct associations: • • In the breccia mineralization, the precious metal minerals merenskyite, moncheite, stutzite and an unnamed mineral Ag3BiTe3 occur with chalcopyrite, pyrrhotite. pentlandite, pyrite and violarite. Sulphide matrix breccia; • Blebs and small stringers, occasionally net textured sulphides; and • • In the bornite mineralized rock, the precious metal minerals, native silver, acanthite, stutzite and merenskyite are associated with chalcopyrite, Disseminated sulphides. The sulphide mineral assemblage consists of - 86 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 bornite and copper bismuth sulfosalt (wittichenite and emplectite). rocks along the Thierry Mine access road (STOP 2a) and at the mine site (Stops 2b to 2f) to the tholeiitic Suite I Upper Sequence of the Pickle Lake assemblage, correlative with Suite I of Hollings (2002). These basalts are characterized by unfractionated or weakly depleted light REE and mildly fractionated heavy REE (La/Smpm= 0.70–1.5; Gd/Ybpm = 1–1.2). The strongest positive correlation of metals is between silver and copper. There is a corresponding negative correlation between silver and nickel at values of nickel greater than 0.5%. A plot of Pt/(Pt+Pd) versus Cu/(Cu+Ni) shows average head grades of the Thierry Deposit to be enriched in copper and somewhat in platinum relative to other similar deposits (Naldrett and Cabri, 1976). From a PGE perspective the Thierry mineralization falls into two groups, both of which fall well off a characteristic trend line defined by Naldrett and Cabri (1976) for typical PGE ores. The first group is pyrrhotite-rich and correspondingly has a high Ni content. This group is platinum poor compared to the second Cu-rich, chalcopyrite rich fraction which has a high platinum content. Stop 2a: Mafic Metavolcanic Rocks, south of K1-1 Zone, south side of Thierry Mine access road UTM Coordinates: NAD83; 15U 0687610 E / 5709480N This stripped area exposes strongly sheared amphibolitized and epidotized mafic metavolcanic rocks south of the K1-1 mineralized zone (Fig. 8). Metrescale M- and S-folds are visible, as are boudinaged or ptygmatically folded felsic dykelets and quartz veins. A pervasive, sub-vertical foliation strikes 090° and is also locally folded. Millimetre-scale shear bands and spaced cleavages are ubiquitous. Disseminated pyrite generates localized gossan. No primary features are noted in these metavolcanic rocks. Ores at the Thierry Deposit underwent intense modification after their initial deposition as magmatic sulphides. Dynamic metamorphism has mobilized much of the breccia and mylonite mineralization. Stop 2b: Thierry Mine, open pits; pillowed basalt, south of the West Pit Sub-Stop Descriptions, Thierry Mine (Fig. 7) Geochemical sampling and analyses conducted by Young (2003) ascribed all the mafic metavolcanic UTM Coordinates: NAD83; 15U 0684010E / 5708540N Figure 7. Field trip stop locations, Thierry Mine Property. - 87 - Outcrops north of the exploration office and near �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 8. Sheared and folded mafic metavolcanic rocks, Stop 2a, Thierry Mine access road. the capped shaft and vent raises expose sheared and locally pillowed mafic metavolcanic rocks with pervasive epidote veins and patches and ptygmatically folded quartz-feldspar-epidote veins. Flattening and stretching have produced pillow aspect ratios of up to 10:1, molar-shaped pillows and preclude unequivocal “tops” direction determination. Pillows have dark, chlorite-, biotite- and amphibole-rich selvages and variably bleached cores (Fig. 9). Foliations in this area are more-or-less east-striking and have variable dips. Felsic dykes may be discordant to foliation or may be sheared and Z-folded, suggesting a protracted intrusion history. Recognizable pillowed basalt gives way to strongly foliated, phyllitic, amphibolitic equivalents closer to the southern margin of the west pit. The access ramp at THE southeastern end of the West Pit provides a crosssection through these phyllitic rocks, dipping circa (ca.) 50° north. Local malachite staining was noted in some of the feldspathic neosome of some of the banded amphibolite. An ~100m wide deformation zone (aka CBS Shear Zone) extends through the middle of the West Pit. In the West Pit, the main mineralized zone is situated on the southern margin of the deformation zone. Around the margins of the pit, contacts are exposed between phyllitic mafic metavolcanic rocks and weakly foliated, synvolcanic (?) gabbroic rocks. These gabbroic rocks are thought to host the bulk of the Thierry mineralized zone at depth, whereas chloritebiotite schist hosts the mineralization at surface to shallow levels of the mine (A. Carlson, Richview Resources Inc., pers. comm., 2008). Minor fold axes are generally northeast-plunging, as are ore shoots within the main mineralized zone. Steeply plunging stretching lineations are evident on foliation planes. On the eastern margin of the West Pit, massive mafic metavolcanic rocks locally host a chloritic, 110°-striking cataclasite unit which may offset the mineralized zone between the West and East pits. Biotite schist is locally developed along the contacts with gabbroic units. Stop 2c: Thierry Mine, open pits; sheared Basalt and Gabbro, north of East Pit UTM Coordinates: NAD83; 15U 0684107E / 5708717N Sharp contacts between sheared, locally laminated basalt and medium- to coarse-grained gabbro are wellexposed on these glacially polished exposures north of the western end of the East Pit and just east of the road that extends between the two pits. Stop 2d: Thierry Mine, open pits; mafic dyke in sheared basalt, north of East Pit UTM Coordinates: NAD83; 15U 0684140E / 5708686N Stop 2e: Thierry Mine, open pits; “ribboned” basalt, north of East Pit UTM Coordinates: NAD83; 15U 0684122 / 5708670N Figure 9. Deformed pillowed basalt flow, south of West Pit, Thierry Mine (Stop 2b). Sheared amphibolite/pillowed basalts predominate north of the East Pit and in some places are highly stretched and ribboned. Felsic dykes are folded and millimetre-scale shear bands are developed in sheared intermediate dykes. A distinctive, 25cm-wide dyke, hosted in amphibolite, is characterized by numerous, rounded granitoid xenoliths (Fig. 10). - 88 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 10. Mafic dyke with rounded granitoid clasts in deformed metabasalt, north of West Pit, Thierry Mine (Stop 2d). Stop 2f: Thierry Mine, open pits; “breccia ore”, northwestern end of East Pit UTM Coordinates: NAD83; 15U 0684086E / 5708638N At the northwestern end of the East Pit a rusty, malachite-coated exposure displays chalcopyrite- and pyrrhotite-rich, “breccia ore”-style-mineralized zones that crosscut folded metavolcanic rocks. Host rocks are well-foliated and resemble the main chlorite-biotite schist host to sulphide mineralization. Small, lenticular “islands” of more competent or xenolithic (?) material are enveloped by schistose rocks. Felsic dykes are dismembered. Stop 3: Pickle Crow Mine (N.B. permission to access the site must be granted by PC Gold Inc.) The Pickle Crow Mine was discovered in the early 1930’s by Northern Aerial Mineral Exploration which began sinking the No. 1 Shaft in 1933. Northern Aerial was acquired by Pickle Crow Gold Mines (PCGM) in 1934 and commercial production at the mine began in 1935. The Pickle Crow mine operated until 1966, during which time it produced 1,446,214 troy oz gold and 168,757 troy oz silver from 3,070,475 short tons of ore milled (at an average grade of 0.47oz/t or 16.14 g/t Au). After the mine closed in 1966 there was little work done until Highland Crow/Noramco Mining Corporation acquired the property in the 1980’s. Noramco completed over 46,000m of surface drilling, dewatered the mine workings to the 750 foot level, and completed an additional 9,000m of underground drilling. With the end of the flow-through era the property sat dormant until Wolfden Resources Inc. acquired the property in 1999. Wolfden subsequently entered into a surface mining agreement (top 100m of the deposit) in June, 2000 with privately held Cantera Mining Limited, resulting in fragmentation of the property ownership. Cantera constructed a 225 ton per day (tpd) extreme gravity mill on the site, submitted a partially completed production closure plan with MNDM, and began constructing a tailings management facility within the historic Pickle Crow tailings area. Cantera also commenced stockpiling of material mined from the historic No. 1 Vein shaft and crown pillar area. Cantera ceased work in 2002 and was placed into receivership in 2004. On November 5, 2007, Premier Gold Mines (successor to Wolfden) and Don Ross (successor to Cantera) announced the signing of a Letter of Intent (LOI) to sell their interests in the property to PC Gold, at that time a private company. A definitive agreement was signed on December 21, 2007. On May 13, 2008 PC Gold satisfied the terms of the definitive agreement and completed the acquisition by completing an initial public offering and listing on the TSX. PC Gold Inc. has been actively exploring the Pickle Crow property since 2008. The mineral concessions of the Pickle Crow project consist of 106 patented mining claims and 119 unpatented claims for a total area of approximately 21,690ha. Since acquiring the property PC Gold has completed a massive program of digitizing historical data and building a detailed 3D model of the mine and surrounding property. PC Gold has to date has completed 98,000m of drilling, as well as several trenching, mapping, and geophysical programs. Most recently, PC Gold has been pursuing financing for small-scale production using the onsite 225tpd mill with ramp access on the near surface, highgrade No. 22 and 23 veins. The Pickle Crow deposit currently hosts an NI43101-compliant inferred resource of 1.3 million ounces, including 637,000 high-grade vein-hosted ounces at an average grade of 9.1g/t Au. Local Geology Many geological investigations have been completed on the Pickle Crow Property (Fig. 11). Detailed rock descriptions on a property scale have been completed by Thomson (1939), Pye (1956, 1975), Ferguson (1966), and MacQueen (1987). Other workers, such as former PCGM employees R. J. - 89 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 11. General geology of the Pickle Lake Mine area, showing Field Trip Stop locations. Graham and L. D. S. Winter, wrote unpublished reports that contain much valuable information. The following descriptions of the geological units of the Pickle Crow Property are derived from the detailed, property-scale work referenced above, and placed into the tectonicstratigraphic framework of Young et al. (2006). The Pickle Crow Property in the immediate vicinity of the mine is underlain by rocks of both the Pickle Crow and Confederation assemblages; rocks of the Kaminiskag assemblage occur to the east. On the property, the Pickle Crow assemblage is dominated by tholeiitic basalts with intercalated sedimentary rocks, primarily banded iron-formation (BIF), and rare calc-alkaline volcanic and volcaniclastic units. The assemblage is interpreted to be unconformably overlain by the Confederation assemblage. An unnamed, Temiskaming-like sedimentary assemblage was identified and dated (<2752.2±2Ma) by PC Gold in 2009 and comprises polymictic conglomerate, sandstone, siltstone, argillite, and argillaceous iron formation. The assemblage occupies a small, fault-bound basin near the contact between the Pickle Crow and Confederation assemblages, and likely represents the erosional unconformity between the two assemblages. The assemblage appears to represent a similar event to that which produced the Houston Lake assemblage of the nearby Red Lake greenstone belt. Pickle Crow Assemblage Rare ultramafic rocks (>20 wt% MgO) were noted in drill core, particularly in deep drilling in holes PC08-014A and PC-10-085 at Shaft 1 and 3, respectively. These units are typically 5 to 20m thick and are intensely talc-altered, sometimes presenting a severe obstacle to drilling. Due to their alteration and deformation, the protolith cannot be accurately determined. No relict spinifex texture has been observed. These ultramafic units are always found intercalated with tholeiitic basalt and are currently interpreted to represent subvolcanic sills. The tholeiitic lavas are compositionally consistent; no flows of intermediate composition have been noted. - 90 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Greenish-grey-weathering, massive basalt is generally fine-grained. Although pillowed units are ubiquitous, it has not been possible to subdivide or map individual flows. Some flow contacts are marked by beds of iron formation. Pillows range in size from 0.5 to 1m and have narrow selvages ~1cm thick. Generally, there are no amygdules in the pillows. They display a variety of shapes and in only a few places on surface has it been possible to make top determinations. Flow-top breccias locally contain light-coloured, angular fragments up to 30cm in diameter. Parts of the flows are medium-grained with individual crystals up to 1.5mm in diameter. These rocks are of medium-grey colour with small light grey feldspars. All these medium-grained rocks have been included with the volcanic rocks and no dykes or sills have been mapped separately on surface. The mediumgrained metabasalts consist essentially of fibrous amphibole, chlorite, and highly altered plagioclase with small amounts of carbonate, epidote, saussurite, and quartz, and subordinate leucoxene, apatite, sphene, and sometimes pyrrhotite (Pye, 1956). One main band of banded iron formation (BIF) is known to be interbedded with the basalts adjacent to the workings of the Pickle Crow Mine but, in places, there are additional local bands. This BIF has been traced in the Shaft 1 area by surface mapping and by drilling for 2700m and ranges in thickness from ~1m up to 25m or, where it has been thickened by folding, to ~45m. In the No. 1 Shaft workings the BIF is known to extend down-dip for 1200m and is thought likely to persist to much greater depths. Magnetite-carbonate BIF is prominently banded with alternating layers, varying in thickness from thin laminae up to 5cm. The more siliceous layers may be light or dark grey, laminated chert. No jasper bands are known to be present. Some of the darker layers contain a high proportion of magnetite but the magnetite content varies along strike; consequently, magnetic surveys have been only partially successful in tracing these beds. The weathering of some BIF produces a rusty iron oxide alteration. This variety of BIF consists of bands of cryptocrystalline quartz and siderite, varying amounts of magnetite and pyrrhotite, and occasional streaks of chlorite (Hurst, 1931). Some of the bands are composed almost wholly of magnetite in small, angular grains. Pyrrhotite occurs as patches, streaks, or grains replacing iron carbonate and chlorite or as veinlets traversing the various bands. Streaks of chlorite, which probably represent inclusions of schistose greenstone, are often associated with the carbonate bands. Carbonate (ankerite) BIF is exposed in outcrops and old trenches on Pickle Crow claims PA774 and 777 in the Cohen-MacArthur area. This BIF is ~550m in length, with thicknesses up to 10m and has sharp contacts. Limonite from the weathering of this BIF has stained adjacent host rocks. At the northeastern end of this zone some outcrops are typically BIF but elsewhere, the more siliceous bands do not weather in relief and the weathered surface of the rock has a uniform surface. The fresh surface of the rock varies in colour from light to dark grey and some specimens of this BIF are very hard and siliceous. Sulphide-chert-rich argillaceous BIF is abundant in the Central Pat East Zone where it typically occurs as interbedded, magnetite-poor, chert-rich BIF and sulphide-rich (pyrite) argillite with minor intermediate tuff. Thin section work (Kolb, 2011) indicated that this BIF is also very carbonate-rich. Although extensive, this BIF displays great local variation in thickness and type (e.g. oxide- versus sulphide-facies). It is not wellexposed at surface and is known almost entirely from diamond drilling. The Pickle Crow assemblage contains significant amounts of calc-alkaline rocks. Although rare in the core mine trend, dacites are the most common rock type outside of it. Most units are lenticular and in cannot be easily correlated between adjacent outcrops. The presence of beds containing breccia fragments is a widespread and characteristic feature. There is no known interbedding between the basic metavolcanic rocks and the acid to intermediate metavolcanic rocks. In places, the contact is marked by interbedded sedimentary rocks. Feldspar-phyric dacite occurs in the northwestern part of the property, often in tuff-breccia units. Such rocks outcrop, or have been recorded in drill core, in the area between the No. 1 Shaft and Powderhouse Lake. Quartz–phyric rocks are very rare, but have been reported in the Central Pat East Zone. Although portions of the local Temiskaming-like sedimentary assemblage, notably the argillite and conglomerate, had been reported historically (Graham, 1965) it was not recognized as a separate unit from the Pickle Crow assemblage until 2009 (Lynch, 2010). The sedimentary basin is quite restricted in its dimensions extending from just northeast of Shaft 3 (hole PC-09050) to just north of the Springer Shaft. The unit appears to be a fault-bound basin that contains polymictic conglomerate at its northeastern end, grading into finegrained wacke, siltstone, argillite, and argillaceous - 91 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 BIF to the southwest. The assemblage is particularly pyrite-rich, specifically the conglomerate (interstitial pyrite) and argillite (bedded nodular pyrite). Locally, this pyrite has been remobilized and converted to pyrrhotite, especially in the conglomerates. The Confederation assemblage unconformably overlies the Pickle Crow assemblage on the property and includes: • The lowermost basaltic unit. • A thick succession of intermediate to felsic metavolcanic rocks in the southeastern part of the property. A thin, discontinuous unit of pillowed basalt occurs at the base of the Confederation assemblage and is distinguished from the lower sequence of the Pickle Crow assemblage on the basis of trace element geochemistry. This lower basalt unit, or geochemical Suite I, is characterized by elevated FeO contents (13 to 16 wt%) and displays weak LREE enrichment and HREE fractionation with minor negative niobium anomalies (Young et al., 2006). The southeastern part of the property is underlain by calc-alkaline felsic to intermediate volcanic and volcaniclastic material that is similar in most respects to the calc-alkaline rocks in the Pickle Crow assemblage. Feldspar-phyric dacite and related breccias predominate and are both widely distributed in this part of the Pickle Crow Property. The dacites contain feldspar, quartz, and sericite together with minor amounts of chlorite, epidote and leucoxene (Pye, 1956; Ferguson, 1966). The dacite breccias contain scattered, light-coloured angular felsic fragments, usually less than 5cm in maximum dimension. Near the eastern boundary of the property the fragments in the volcaniclastic rocks are rounded and were termed “agglomerate” or “volcanic conglomerate” by Thomson (1939). There are two porphyry stocks and several porphyry dykes within the property boundaries. The Pickle Crow porphyry stock occurs northwest of No. 3 Shaft and the Albany River porphyry stock outcrops near the Albany shaft. Dikes have been mapped on claims PA729, 1139 and 2011 (Ferguson, 1966). The Pickle Crow porphyry (2909+15Ma; Young et al., 2006) is elliptical in plan, 1.8km long and 200m wide. The major axis strikes 055° and appears to be generally conformable in strike and dip with the enclosing rocks. But, on the 229m (750ft) level plan it is shown to crosscut the trend of the volcanic units at a small angle. The complete outline of the stock has not been established on the 869m (2850ft) level, but over this vertical distance the porphyry appears to maintain its shape, becomes slightly wider, dips at 77° to the northwest, and does not appear to plunge. A few porphyry dykes or sills are present near the stock but the outline is regular, without apophyses extending outward from the main intrusion. On the 229m (750ft) level the southern contact of the intrusion with the adjacent country rocks is sharp. The Pickle Crow porphyry is distinguished by large (2 to 10mm) quartz phenocrysts which are rounded to oval in cross-section, but a few are rectangular with rounded corners. On the light grey weathered surface, quartz phenocrysts are enclosed in a matrix of kaolinized feldspar. Microscopically, the rock contains distinct, well-formed, but smaller, fractured crystals of albite (Ferguson, 1966). The matrix of the rock is an aggregate of tiny anhedral grains of quartz and altered plagioclase with accessory amounts of magnetiteilmenite, leucoxene, apatite, sphene, and rutile. The Albany porphyry (2735+10Ma; Young et al., 2006) is 670m long and 120m wide, striking 060°. This stock is somewhat irregular at the ends, with lobes and dyke-like apophyses. Some of the associated dikes are parallel with the trend of the enclosing rocks but others are crosscutting. From surface to the 625ft (190m) level, the stock dips 65° northwest. On surface, the major axis of the stock makes a small angle with the strike of the enclosing rocks. The stock appears to maintain a similar shape and extend down the dip with no known plunge. This faintly pink porphyry contains abundant feldspar phenocrysts, scattered quartz phenocrysts, and a few biotite crystals, all of which are about 2mm in size. In its most unaltered state it appears granodioritic in composition. In thin section the feldspar is considerably altered to white mica and in some local areas to saussurite. Some of the quartz phenocrysts are individual crystals, but other phenocrysts consist of clusters of crystals. Quartz and carbonate occur as trains of interstitial material between the larger feldspar phenocrysts. Small crystals of apatite are enclosed within the large biotite crystals. Miscellaneous feldspar porphyry dykes also occur on the mine property. A prominent northwest-trending diabase dyke cuts across the western portion of the property. Narrow, fine-grained diabase dykes have also been mapped in the workings and encountered in underground drilling at the Pickle Crow Mine. One dike occurs in the - 92 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 hanging wall of the No. 1 Vein and appears to be earlier than the vein, but another dike of similar appearance cuts this vein on the 411 m (1,350 ft) level (Pye, 1956). Diabase dykes also occur in the Pickle Crow porphyry. Although some of these dykes are parallel to the margins of the stock, others are at angles of from 20°to 40° to the contact. All of these dykes within the porphyry are cut by the mineralized veins. A biotite lamprophyre dyke outcrops along the southern side of the rock exposures on claim PA760, in the northwestern part of the Pickle Crow property and, a dyke of similar composition occurs in the Pickle Crow porphyry and cuts a mineralized vein. The lamprophyres are massive, dark grey to black, mediumgrained rocks with large (locally >5mm) biotite phenocrysts. Two varieties have been recognized. One, which cuts the Howell Vein, is composed chiefly of biotite, orthoclase, chlorite, and carbonate, and may be an altered minette. A second, post-ore dyke, which cuts the No. 2 Vein system, is made up of biotite, andesite, quartz, subordinate clinopyroxene, and accessory apatite and zircon (Pye, 1956). Lamprophyre dykes, similar to those exposed at Shaft 1, are also very common and are intimately associated with gold mineralization at the Central Pat East Zone. The Hooker-Burkowski quartz-phyric trondhjemite stock, southwest of the Pickle Crow property, is undeformed and intrudes all Pickle Lake greenstone belt assemblages. Structural Geology The structural history of the Pickle Crow Property, although extensively studied, is by no means definitely understood. The focus has been mainly on the Pickle Crow assemblage and is largely based on the descriptions of Ferguson (1966) and the deformation history of MacQueen (1987). Stratigraphic younging determinations and structural-facing directions have been based on rare, unequivocal pillow tops and graded argillaceous beds. One persistent magnetite-carbonate BIF forms an important marker unit within the Pickle Crow tholeiites. The general strike of rocks on the property is northeast and the dip is 75° to 80° northwest. The plunge of folds in the BIF near No. 1 Shaft is due north at 75° to 80°. The rake of the three productive veins in the No. 1 Shaft area is 70° at 020°. The Pickle Crow porphyry stock and the Albany porphyry stock both extend down the dip and do not appear to plunge. Several of the anticlines narrow and plunge beneath the younger rocks in a pattern that would be consistent with a plunge to the northeast. Some other anticlines maintain a constant width for considerable distances and some anticlines have a shape, in plan, which suggests a plunge to the southwest. Along some fold axes the stratigraphic sequence is repeated in reverse order which indicates plunge reversals. The major anticlines on the property are: the Pickle Crow; Albany Shaft; Pumphouse Lake; Sawmill; Pumphouse Creek; Powderhouse Lake south; Powderhouse Lake central; and Powderhouse Lake north. The major adjacent synclines are: the Albany Shaft; Township Line; No. 3 Shaft; Pickle Crow No. L; Pickle Crow No. 2; and Pumphouse Lake (Ferguson, 1966). It is important to note that this complex folding is essentially confined to the Pickle Crow mine area, and that the upper (northerly) part of the Pickle Crow assemblage outside of the property is a simple, north-facing, homoclinal sequence (Young et al., 2006). The general trend of the fold axes is northeast, but the Pickle Crow No. 2 syncline, the Pumphouse Creek anticline, and the Township Line syncline have fold axes which curve across the major fold axes. The folds strike northeast and dip steeply northwest, forming isoclinal folds with overturned southeastern limbs. A well-developed schistosity is present in the metavolcanic rocks in the mine area and on the limbs of folds. This schistosity conforms with the dip of the bedding and with the axial planar cleavage of the latest period of folding. The porphyries are not strongly sheared, but the platy minerals developed in the matrix are aligned in conformity with the schistosity of the adjacent volcanic rocks. Lithologic units in the Pickle Crow area have been metamorphosed to greenschist facies. The greenschistamphibolite facies isograd was defined below the mine workings in Shaft 1 (at approximately 1600m depth) and is identified by the appearance of hornblende and abundant garnet. Greenschist mineral assemblages in basaltic rocks were petrographically determined to be: chlorite+actinolite+epidote+quartz+albite; and chlor ite+sericite+quartz+albite. The chlorite and actinolite have a preferred orientation parallel to the second deformation foliation, suggesting that this was the peak metamorphic episode. MacQueen (1987) described a complex polyphase deformation history that includes four tectonometamorphic episodes summarized below (Figure 12): - 93 - “The earliest generation of structures (D1) is present as rare 1- to 3m isoclinal fold closures or �Proceedings of the 61st ILSG Annual Meeting - Part 2 hinge zones (F1) within BIF units that have been subsequently refolded inside second generation (F2) fold closures. Due to refolding, the F1 folds have axial planes with a mean strike direction between east-northeast to east and a steep dip to the north with hinge lines (L1) plunging steeply (60° to 70°) to the northeast (Pye, 1956). striking sets of shear fractures have, in some instances, developed into discrete 2 to 5m wide Type 2 shear zones that are strongly foliated and run between substantial Type 1 structures. These Type 2 shears are an important structural control on gold mineralization. D2 deformation was contemporaneous with gold-bearing vein emplacement and deformation definitely continued sometime after vein emplacement, as evidenced by Z-folding of the No. 1 Vein in Shaft 1 and even more intense folding and boudinaging of veins in Shaft 3. D2 structures are the most prominent in terms of metamorphic overprint and the current distribution of rock units and mineralization on the property. In the mine area, D2 is characterized by a penetrative axial planar schistosity (S2), parallel, or at a small angle, to bedding/S1, striking northeast and dipping 75° to 87° to the northwest. Stretching lineations (L2) in the S2 plane, defined by chert and magnetite in BIF, quartz phenocrysts in quartz feldspar porphyry and varioles in tholeiites, are steeply plunging (70° to 85°) to the north-northeast. The effect of this stretching lineation cannot be overstated, as pillows have been measured with stretching ratios in excess of 30 to 1. The lineation is typically the strongest and most consistent fabric, with the S2 foliation often a distant second. As a result, the most continuous direction of lithological continuity (and mineralization) on the property is vertical. D2 fold closures (F2) have axial surfaces that strike northeast and dip steeply (75° to 85°) to the northwest, and hinge lines (L2) that plunge 60° to 80° to the northeast. D2 closures are characterized by 1 to 200m wide, tight, to isoclinal, similar folds, (best developed in BIF). They have thickened hinge zones, attenuated limbs and wavelengths of 300m. The large D2 folds outlined by the BIF include the Pickle Crow Anticline, Pickle Crow Syncline, Sawmill Anticline, and Powder House Anticline (Ferguson, 1966). D2 shear zones occur throughout the Pickle Crow property as zones parallel to S2 surfaces (Type 1) and as discrete shear zones (S2’) that splay off the Type 1 shear zones in a eastnortheast direction, connecting Type l shear zones. Type 1 shear zones are strongly foliated zones greater than 30m wide which dip steeply (75° to 85°) to the northwest. They include the Pickle Crow Fault, Highland Crow Shear Zone, Pumphouse Shear Zone and Powderhouse Shear Zone. Shear fractures in outcrop surfaces (Stott, 1996) trend east-northeast at low to moderate angles (20° to 40°) to S2. These east-northeast- The third generation of structures on the Pickle Crow property (D3) consists of northwest- and north-striking conjugate faults, steeply dipping to the northeast, and crosscutting and displacing earlier structural fabrics. Late undeformed veins, felsic dikes and lamprophyre dikes have been emplaced along fractures parallel to northwestand north-striking conjugate sets of fractures which crosscut D2 fabrics. The final (D4) deformation generation is represented by the development of late, continuous, northwest-striking, en-echelon extensional fractures that crosscut earlier fabrics (Sage and Breaks, 1982). These fractures are up to 20m wide and have considerable strike lengths, up to hundreds of metres. The Pickle Crow diabase dike, and other diabase dikes hosted in these structures, crosscut all strata, including the Hooker-Burkowski stock and the granitic terrane to the north and south of the Pickle Lake greenstone belt. The Pickle Crow diabase dike has been displaced sinistrally along a northeast-trending fault, the only record of post-D4 deformation within the Pickle Crow area.” Gold Deposits PC Gold considers the gold occurrences in the Pickle Lake mining camp to be classical examples of deposits grouped under the descriptive model of Archean low-sulphide, gold-quartz veins. The major gold ore bodies at Pickle Lake are hosted by the Pickle Crow assemblage, which locally hosts at least two such large structural discontinuities or “breaks”: the core mine-trend break, which preserves Temiskaminglike sedimentary rocks; and the Cohen-MacArthur deformation zone, which is up to 100m wide. The gold-bearing veins at Pickle Crow fill pre- or syn-ore - 94 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 12. Structural elements of the Pickle Crow Mine area. faults, shears, and fractures in the various host rocks. Auriferous sulphide zones that are stratabound and contained within BIF occur adjacent to shear zones in some areas. The historically mined ore at Pickle Crow was contained in quartz veins that are generally banded (crack-seal) with tiny streaks of tourmaline, chlorite or sericite, and in fracture fillings. Quartz is by far the main vein mineral, with lesser carbonates, including siderite, ferruginous dolomite, and calcite. Minor albite, chlorite, sericite, and local traces of tourmaline, magnetite, and scheelite have been noted. Native gold was the main ore mineral. The main sulphide minerals are pyrrhotite and pyrite, which combined are usually less than 2% of the vein material, along with trace arsenopyrite, chalcopyrite, galena, and sphalerite. Some gold is closely associated with the sulphides and traces are found in the altered wall rock but, in general, the gold is free and occurs along sericitechlorite-fuchsite-lined fractures and seams in-filling minute fractures in the quartz. Spectacular samples of visible gold have been observed in a number of places in the mine. As a general rule, however, the gold is very finely divided and practically invisible to the naked eye (Pye, 1956). At Pickle Crow, alteration minerals include silica, sericite, chlorite, carbonate, and pyrite. Host rocks for the gold mineralization at Pickle Crow include tholeiitic basalts, BIF, intermediate volcanic/volcaniclastic rocks, and quartz feldspar porphyry. Gold occurrences are associated with four styles of mineralization: • Narrow, high-grade, gold-scheelite-bearing quartz veins, which were the main source of gold produced at the Pickle Crow Mine from 1935 to 1966. • BIF-hosted gold mineralization adjacent to vein structures. BIF contains stringers and discontinuous lenses of quartz and the iron-bearing minerals have been replaced by sulphides. Both quartz and sulphides are gold-mineralized. Only a limited amount of this type of material was processed at the Pickle Crow Mine. However, BIF-hosted gold was - 95 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 the main ore type at the adjacent Central Patricia Mine to the southwest. • Shear zone-hosted gold mineralization, consisting of complex, wide zones of intense shearing and alteration which are intimately associated with the intrusion of the Albany porphyry and characterized by disseminated pyrite, discontinuous quartz veining, and sulphidation of interflow BIF. • Arsenopyrite-associated gold mineralization which typically occurs as disseminated to semi-massive arsenopyrite and quartz-arsenopyrite stockworks hosted by BIF, but can be also found, to a lesser extent, in shear zones and/or quartz veins in volcanic rocks. Similar arsenopyrite-rich BIF-hosted gold was the main ore type at the adjacent Central Patricia Mine. A substantial number of auriferous quartz veins have been located on the property, along with several occurrences of BIF-hosted mineralization. The following quartz vein descriptions are mainly from the work of Thomson (1939), Pye (1956), and Ferguson (1966), while the subsequent BIF mineralization descriptions also include information from MacQueen (1987) and Winter (1988). Gold was produced from the No.1, No. 2, No. 5, No. 6, No. 7, No. 8, and No. 9 Veins during the life of the Pickle Crow Mine. The only additional mineralization of this type that was processed at the Pickle Crow mill came from exploration drifts at the Albany Shaft area. The most productive of the quartz vein ore bodies was the No. 1 Vein (Fig. 13). This vein has been traced on surface over a strike length of 900m and extends below the lowest level of the mine, or beyond a depth of 1500m (almost 700m below the lowest historically mined level of the deposit). The average thickness of this vein is 0.9m. The eastern part of the vein is highly contorted with an overall strike of 083°, cuts across the lithologic units at an angle of 30° to 40°, and has an average dip of 73° north. The western part of the vein has an overall strike of 058°, cuts the formations at about 10°, and has an average dip of 75° northwest. The No. 1 ore body is a ‘shoot’ within the No. 1 Vein, with the eastern boundary determined by gold content and the western boundary by diminishing vein thickness (Pye, 1956). Although the No. 1 Vein is typical of the veinstyle mineralization at the Pickle Crow Mine there are variations throughout the mine property. Veins in the Shaft 1 area (Fig. 14) are relatively undeformed, and more laminated, with more fine-grained gold. Figure 13. Cross-section through the No. 1 Shaft area, Pickle Crow Mine. They have very little shearing or wall rock alteration and have significant down-dip continuity. Veins in the Shaft 3 area (e.g., No. 2), are more deformed, have few laminations with more coarse-grained gold, and possess wider zones of shearing and alteration in the wall rock. They are more en echelon in nature and have less down-dip continuity. Veins in the Albany Shaft area (e.g., No. 16) are even more deformed, with few laminations and generally rare visible gold, possess wide zones of shearing and intense alteration of the wall rocks, and generally poor continuity. The No. 1 Vein consists largely of white or greyish, coarse- to fine-grained, almost sugary quartz, a little ferro-dolomite, tourmaline, scheelite, and subordinate amounts of metallic sulphides. The ankerite, tourmaline, and scheelite, although locally occurring as patches completely enclosed by the quartz, generally occur in thin seams replacing chloritized basalt in ‘book and ribbon’ structures or in the walls, and so - 96 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 14. Structural relationship between mineralized zones, No. 1 Shaft area (Coates and Anderson 2008). help accentuate the banded character of the vein. Two distinct generations of quartz are recognizable; one gold-bearing and making up the body of the No. 1 Vein, the other barren and occurring, along with calcite, in narrow transverse veinlets that cut sharply across the earlier type. Most of the other mineralized veins at the Pickle Crow property have similar characteristics to the No. 1 Vein. Quartz is by far the most abundant mineral of all the veins and the two generations of quartz are found in many of the veins. The quartz in other veins in the metabasalt is a light grey colour and is banded due to the presence of inclusions of schist and dark coloured minerals. Similarly, the second generation of quartz consists of veinlets, generally less than a centimetre in thickness, which are approximately at right angles to the veins and extend completely across the veins but rarely extend into the wall rock. These veins consist of quartz with abundant white or pink calcite (Ferguson, 1966; Pye, 1956). Pyrite and pyrrhotite are the most abundant sulphides and both are about equally common in the veins in the metabasalt, but pyrite is more abundant in the No.2 Vein, and only pyrite occurs in the No.16 Vein. In the metabasalt and iron formation wall rocks, pyrrhotite occurs as irregular masses, disseminated grains, and narrow seams. It occurs in fractures in the veins, or the wall rock, or both, and as grains healing broken crystals of arsenopyrite and pyrite. The pyrrhotite is later than the quartz and the late variety of pyrite, but is - 97 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 replaced by sphalerite, chalcopyrite and galena. The second style of mineralization at Pickle Crow is the gold-bearing BIF type. Considerable mineralization of this type was identified by PCGM on the property during its exploration and development work, mainly in the No. 1 Shaft area. These were locations adjacent to the No. 1 ore body (the No. 1 Iron Formation Zone or Eastern Iron Formation), the No. 5 ore body (the No. 5 Iron Formation Zone), and an area approximately midway, at depth, between the No. 1 and No. 5 Veins known as the Central Iron Formation Zone. After running iron formation material from test stopes in all three zones, PCGM found that it was unable to satisfactorily process both vein and iron formation mineralization styles due to their different metallurgical characteristics (Winter, 1987). It has also been reported that the average auriferous iron formation grade, believed to be about 6.85g/t Au (0.20oz/ton Au), was below the then cut-off grade of 8.57g/t Au (0.25oz/ton Au) (MacGregor, 1989). The Eastern, Central, and No. 5 iron formations or quartz-sulphide zones comprise stringers and discontinuous lenses of quartz within BIF. The iron-bearing minerals of the BIF have been replaced by sulphides adjacent to the veins and gold is present in the veins and the associated sulphides. Approximately 15,000t of this type of mineralization, taken from test stopes in each of the above occurrences, were processed during the historic mining operations. The BIF-hosted mineralization consists of bleached and altered iron formation with variable amounts of pyrite, pyrrhotite, and occasionally arsenopyrite, often with heavy secondary magnetite and quartz, and carbonate veins and veinlets. The gold mineralization at Pickle Crow (both quartz vein- and iron formation-hosted) is localized along, or adjacent to, Type 2 shear zones, i.e., shear zones that are developed oblique to greenstone belt lithological trends and cross between adjacent Type 1, lithologically concordant shear zones, or faults. The shear zone-hosted type of mineralization is restricted to the Albany Shaft area, and is referred to as Conduit style mineralization after Conduit Zones 1, 2, and 3 (formerly the A, B, and C zones). The mineralization is characterized by wide, highly complex zones (both lithologically and structurally) of shearing with discontinuous quartz veining, sulphidized interflow BIF, and disseminated pyrite. All rock types can be mineralized, with a preference for the interflow BIF where the highest grades occur, often in association with a pronounced crenulation fabric and abundant Z-folds. Lithological complexity is a key component, providing abundant small-scale competency contrasts. Conduit-style mineralization, when present in homogeneous rocks such as massive basalt, is much less intense and lower-grade. Alteration mineralogy includes widespread carbonate (some calcite, but primarily ankerite), strong sericitization, chlorite, silicification, quartz veining, and abundant disseminated pyrite. Visible gold was not observed although grades greater than 1oz/ton have been recorded. Minor alteration minerals include tourmaline, hematite, and fuchsite. The geometry of the mineralization is poorly understood. In the case of Conduit Zone 1 it has been defined by drilling to be an approximately 40m wide, northerly-plunging (~55°), pipe-shaped body; however Conduit Zones 2 and 3 do not mimic this geometry. There is also strong evidence that the Conduit style of mineralization is simply a much stronger manifestation of the shearing that surrounds the high-grade quartz veins at the Pickle Crow Mine. For instance there is evidence that Conduit Zone 2 is the southwest extension of the No. 16 Vein. The mineralization is often moderate- to low-grade and possibly amenable to open pit or bulk underground mining methods. Arsenopyrite-bearing gold mineralization was described early on in the history of the Pickle Crow property when it was discovered at the CohenMacArthur Zone and MacArthur Vein in the 1930s. These are located north of the core mine trend and just south of the Kawinogans (Crow) River. While historically it was not a significant style of mineralization at Pickle Crow, it was the principal ore at the nearby Central Patricia Mine. Subsequent work by PC Gold has found this style of mineralization to be much more widespread on the property than previously thought. The Cohen-MacArthur deformation zone is a wide (up to 100m) zone of intense shearing and carbonate (ankerite)-sericite alteration that roughly parallels the core mine trend and runs the entire length of the property. It was identified through geophysics and drilling in 2010. The Cohen-MacArthur structure is the strongest structure present on the property and forms a dividing line between mineralization styles. All mineralization north of this structure is associated with arsenopyrite whereas the mineralization south of it (with the rare exceptions of the Arsenide Vein and No. 21 Vein) are associated with minor scheelite and low arsenopyrite contents. Arsenopyrite mineralization can occur in several forms such as localized shear zones, quartz veins, quartz stockworks, and disseminations in - 98 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 both volcanic and chemical sedimentary rocks. The most widespread mineralization and highest grades occur within iron formation such as the Central Pat East Zone, where the mineralization is also spatially associated with several late unmineralized lamprophyre dikes which presumably used the same structures as the gold mineralization originally exploited. The tenor of the gold mineralization is very closely tied to arsenopyrite content, as well as the degree of silicification, quartz flooding and/or veining. Alteration minerals associated with arsenopyrite mineralization are widespread and include often intense carbonate (ankerite) alteration and strong silicification, quartz flooding and/or veining, and moderate sericite alteration. Minor alteration minerals include tourmaline, pyrrhotite, and chalcopyrite. Petrographic studies by Kolb (2011) on the Central Pat East indicate that the gold is free, located within fractures or next to arsenopyrite crystals. An important structural component of the arsenopyrite-associated gold mineralization is the presence of fractures and/or veinlets at high angles to bedding. At the Central Patricia Mine, the host BIF possessed an easterly strike; however, the mineralization was present in sulphide-rich fractures oriented perpendicular to the strike of the BIF. On the 750 level, mineralized fracture patterns occur at a high angle to the strike of the BIF, generating ore bodies (Tigert, 1949). At Central Patricia miners exercised care to drill holes at approximately right angles to the locally prevailing fracture pattern (i.e. parallel to bedding). Normal drilling, at right angles to the drift (i.e. perpendicular to bedding), would produce very false results (Tigert, 1949). These fractures plunge approximately 55° to the east and formed very consistent high-grade shoots within the BIF. Similar structures have been observed at the Central Pat East Zone, where arsenopyrite-filled fractures and arsenopyrite-quartz veinlets cut bedding at high angles. Similar high-angle quartz veinlets, plunging to the northeast and perpendicular to the strike of the host iron formation, have been observed in the arsenopyritepoor iron formation mineralization at the Sawmill Vein and may play an important role in controlling the geometry of this style of gold mineralization as well. Mineral Resources The NI43-101-compliant inferred mineral resource estimate was compiled by Fladgate Exploration Consulting Corporation and audited by Micon Internation Limited in April 2011. It was amended by Fladgate in August 2014 with the inaugural estimates on the No. 22 and 23 veins. The Pickle Crow project resource estimate is divided into three distinct areas within the core mine trend: the Shaft 1 area; the Shaft 3 area; and the Albany Shaft area. The drill hole database used for the resource estimation comprised drill holes, underground chip samples, and surface trench channel samples. In total, 1597 drill holes, totalling ~138,000m, were used, of which 167 drill holes (~50,000m) belonged to PC Gold drilling campaigns. A total of 27,826 chip samples taken by PCGM and 45 surface trench channel samples, taken by PC Gold, were used for estimation purposes. Tables 4 and 5 detail the resource estimate and Figure 15 is a longitudinal section showing resource areas for the Pickle Crow property. Table 4: Updated Pickle Crow Inferred Mineral Resources – August, 2014 Category Cut-off Grade (g/t Au) Tonnes Grade (g/t Au) Ounces Percentage of Total Ounces High-Grade Vein Underground 2.8 2,165,000 9.1 637,000 49% Bulk Underground* 2.0 4,510,000 3.7 536,000 41% Total Underground 2.25** 6,680,000 5.5 1,173,000 90% 0.35 3,628,000 1.1 126,000 10% 10,303,000 3.9 1,299,000 100% Total Open Pit Grand Total *Bulk Underground resources comprises primarily Banded Iron Formation (BIF) hosted mineralization.   **Represents a combination of potentially bulk mineable underground resources (2.0 g/t cut-off) and cut-and-fill underground resources (2.8 g/t cut-off, with vein intersections diluted to a minimum of 1 metre). - 99 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Table 5: Pickle Crow Updated Inferred Mineral Resources – August, 2014; Detailed Breakout by Shaft and Zone Area Shaft 1 Shaft 3 Albany Shaft Total Pickle Crow Zone Host Mining Technique Grade Au g/t Metric Tonnes Contained Ounces Au Cut-off Grade g/t BIF BIF & Vein Open Pit 1.1 3,628,000 126,000 0.35 BIF BIF & Vein Bulk Underground 3.7 4,320,000 508,000 2.0 No. 1 Vein Vein Underground 10.1 718,000 233,000 2.8 No. 5 Vein Vein Underground 5.2 141,000 24,000 2.8 No. 9 Vein Vein Underground 5.4 203,000 35,000 2.8 No. 11 Vein Vein Underground 6.5 18,000 4,000 2.8 No. 19 Vein Vein Underground 14.0 381,000 171,000 2.8 Shaft 1 Total Ounces 3.6 9,409,000 1,100,000 No. 2 Vein Vein Underground 9.1 96,000 28,000 2.8 No. 6 Vein Vein Underground 8.2 156,000 41,000 2.8 No. 7 Vein Vein Underground 5.8 49,000 9,000 2.8 No. 8 Vein Vein Underground 7.9 64,000 16,000 2.8 No. 12 Vein Vein Underground 11.9 14,000 5,000 2.8 No. 13 Vein Vein Underground 6.5 103,000 22,000 2.8 No.22 Vein* Vein Underground 5.9 28,000 5,000 2.8 No.23 Vein* Vein Underground 7.9 125,000 32,000 2.8 Shaft 3 Total Ounces 7.7 635,000 158,000 CZ1 Conduit-Style Bulk Underground 4.9 168,000 27,000 2.0 CZ3 Conduit-Style Bulk Underground 2.7 22,000 2,000 2.0 No. 15 Vein Vein Underground 4.7 42,000 6,000 2.8 No. 16 Vein Vein Underground 6.3 28,000 6,000 2.8 Albany Shaft Total Ounces 4.9 260,000 41,000 Total Open Pit 1.1 3,628,000 126,000 0.35 Total Bulk Underground 3.7 4,510,000 536,000 2.0 Total Underground 9.1 2,165,000 637,000 2.8 Grand Total 3.9 10,303,000 1,299,000   *2014 Resource Estimates Notes: 1. The mineral resource estimate is entirely classified as inferred mineral resources. 2. CIM Definition Standards were followed for mineral resources. 3. The cut-and-fill (highgrade vein) underground component of the mineral resource has been estimated at a cut-off grade of 2.8 g/t Au over a minimum width of 1 metre. Vein widths less than 1 metre were diluted to 1 metre prior to application of the 2.8 g/t Au cut-off grade. Grade and tonnes for the cut-and-fill component of the mineral resource are reported as diluted grade and tonnes. 4. The long-hole bulk underground (moderate-grade) component of the mineral resource has been estimated at a cut-off grade of 2.0 g/t Au. 5. The open pit (lowgrade) component of the mineral resource has been estimated at a pit discard cut-off grade of 0.35 g/t Au, using a preliminary Whittle pit shell to constrain the resource estimate and other assumed pit parameters. 6. The open pittable mineral resource extends to a depth of approximately 150 metres below surface. Only mineralization located within the pit shell has been reported at open pit cut-off grades. 7. The mineral resource has been estimated using a gold price of US$1,100 per ounce. 8. High-grade assays have been capped. Each domain was capped with respect to their unique geology and statistics. Caps for cut and fill (high-grade vein) underground resources range from 35 g/t to 145g/t. 9. Bulk density of 3.14 t/m3 was used for BIF and 2.70 t/m3 was used for veins. 10. The mineral resource was calculated via block model. Three dimensional wireframes were generated using geological information. A combination of Kriging and inverse distance estimation methods were used to interpolate grades into blocks of varying dimensions depending on geology and spatial distribution of sampling. 11. Mineral resources that are not mineral reserves do not have demonstrated economic viability. 12. Mineral resources have been adjusted for mined out areas. Small rib and sill pillars around old stopes have not been considered. 13. Numbers may not add due to rounding. - 100 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 15. Longitudinal section showing resource areas, Pickle Crow property Sub-stop Descriptions, Property (Fig. 16) Pickle Crow Deposit Stop 3c: Trench A (Figure 18) UTM Coordinates: NAD83; 15U 0705820E / 5711200N Stop 3a: PC Gold Inc. field office and core yard UTM Coordinates: NAD83; 15U 0703600E / 5709080N The geological setting of the Pickle Crow deposit along with various styles of mineralization will be discussed in detail at this stop. The property’s core library, which covers all rock types, alteration, and mineralization styles found on the property will be laid out for display and discussion. Stop 3b: Trench C UTM Coordinates: NAD83; 15U 0703890E / 5709300N This trench is an excellent example of the Core Mine Trend BIF-style mineralization (Fig. 17). The trench exposed the No. 5 vein in the northeastern corner and the No. 5 BIF zone in the centre of the trench, as well as extensive low-grade BIF mineralization. The highgrade, but narrow No. 11 vein is also exposed in the southeastern corner of the trench. It should be noted that although these structures appear to be relatively narrow and have a short strike-length, they have incredible down-plunge continuity, having been traced to more than 1150m at depth. This trench is the best exposure of what is referred to as the conduit-style mineralization. This mineralization is part of the high-grade vein Au-W style, but with some notable differences. In the Albany shaft area, the deformation of the vein is much more intense and hence continuity is diminished. In addition to this, the degree of alteration is also much more intense, with wide (tens of metres) alteration halos surrounding the veins. In the Albany area, the veins have been deformed to the point that they often form boudinaged fold hinges. These hinges have limited strike extent and complex internal structure, but with strong down-plunge continuity. Stop 3d: High-grade No. 1 Vein stockpile and Shaft Pit (Fig. 16) UTM Coordinates: NAD83; 15U 704355E / 5709860N This high-grade stockpile is composed of vein material from the No. 1 Vein crown pillar (Fig. 19). The larger stockpile behind it is composed of BIF-style mineralization which surrounds the No. 1 Vein, also from the crown Pillar. Abundant fine free gold can be observed along the chlorite-sericite-filled seams in the high-grade veins. Sheared, isoclinally folded (F2) BIF - 101 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 16. General geology of the southern half of the Pickle Crow property, showing veins, trenches and Field Trip Stop locations. is well-exposed in the nearby No. 1 Shaft Pit (Figure 20). Carbonate-altered lamprophyre dykes are also present in the No. 1 Vein surface stockpile and exposed in the No. 1 Shaft Pit. The 225tpd extreme gravity mill build by Cantera in 2002 lies a short distance away. STOP 4: Dona Lake Mine UTM Coordinates: NAD 83; 15 U listed with sub-stop descriptions (N.B. permission to enter the site must be granted by Goldcorp Canada Inc.) Figure 17. Stripped area at Trench C (Stop 3b), Pickle Crow property, showing contact between No. 5 Zone BIF and metavolcanic rocks. The past-producing Dona Lake Mine deposit, approximately 9km southeast of Pickle Lake, was discovered by Dome Exploration Limited in 1980 (Cohoon, 1986). Between 1980 and 1985 geophysical and geological surveys and diamond drilling were carried out by Dome Exploration. Advanced exploration, including sinking of a 176m exploration shaft, was conducted by Dome Mines and Campbell Red Lake Mines between 1985 and 1987. Developed jointly by Dome Mines and Campbell Red Lake Mines, it was put into production by Placer Dome Canada Limited - 102 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 18. General geology of the southern half of the Pickle Crow property, showing veins, trenches and Field Trip Stop locations. Figure 19. Broken chunks of No. 1 vein on stockpile at Stop 3d, showing typical laminated (“crack-seal”) structure (gloves for scale). Mill is in background. Figure 20. F2 fold in BIF, No. 1 Shaft pit, Stop 3d. Field of view is 2 metres. - 103 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 in February 1989 at a rated concentrator capacity of 550tpd, with Proven and Probable reserves totalling 754,000 tons (684,000 tonnes), averaging 0.24oz/t Au (8.23g/t Au; Coates and Anderson 2008). Placer Dome Canada operated the site until July 1993, at which time ownership was transferred to Ross-Finlay who operated the mine until closure in August 1994. The mine closed in 1994 due to exhaustion of viable reserves. During this time period, it is estimated that a total of 939,237 tonnes of ore was milled to produce approximately 6988kg (246,500 oz) of gold (Puumala 2009). This estimate is based on grade and tonnage estimates obtained from The Northern Miner (article, May 30, 1994), Janes et al. (1994) and Jen and McCutcheon (1995). The first source summarizes production under the ownership of Placer Dome from 1989 to 1993, and the second and third sources summarize production under the ownership of Ross Finlay from 1993 to 1994. At the time of mine closure, it is estimated that reserves of approximately 40,000 tonnes grading 5.95g/t Au remained at depth (Chronicle-Journal, article, June 24, 1994). Responsibility for the tailings reclamation remained with Placer Dome Canada which became Goldcorp Canada on May 12, 2006. Ochig Lake Pluton. The entire assemblage has been metamorphosed to amphibolite grade, as indicated by the presence of garnet porphyroblasts, biotite, local amphibolites with blue-green hornblende, and the relative lack of chlorite. The Dona Lake deposit is located in iron formation near the east-central portion of the property. The general geology in the immediate vicinity of the deposit, as derived from scattered outcrops, drilling and ground magnetic surveys, consists of tholeiitic basalt separated by several major units of iron formation, and intruded by felsic dykes and albite porphyry. The volcanics and sediments strike north-south to locally northwest-southeast and dip to the east and northeast at 60°. Tops, as determined from wellpreserved pillows, are also to the east. Several stages of deformation are evident in the area of the deposit: • The basalts, which are normally pillowed and massive, are very schistose and foliated in the vicinity of the iron formations. Some, but not all, of the felsic dykes are also affected by this foliation event. The Dona Lake Deposit is predominantly BIFhosted, geologically similar to the auriferous iron formation-hosted zones at the Central Patricia and Pickle Crow mines. The most comprehensive description was provided by Cohoon (1986): “The Dona Lake property is south of the previous producers [i.e. Pickle Crow, Central Patricia, etc.] in a separate greenstone sequence that trends south and merges with the OsnaburghPickle Lake belt [sic]. The main trend on the property is described by the nearly circular, 11kmlong arc of high magnetics which wraps around the tongue of the Ochig Lake Pluton. The high magnetics are caused by a major, semi-continuous unit and numerous minor discontinuous units of oxide iron formation. • Virtually all of the iron formation has been isoclinally folded. The fold planes are parallel to overall stratigraphy and the fold axes plunge east down the dip of the iron formation. These folds have wavelengths of about 1m and amplitudes of up to 10m. • Superimposed on the isoclinal folds with schistosity are low-amplitude cross folds with wavelengths of about 200m and amplitudes of 20m. • A stratigraphically controlled shear zone has been identified over a distance of 1km in the footwall of the iron formation. It is composed almost entirely of chlorite and presumably postdates the metamorphic event. The iron formations occur within a package of tholeiitic, usually pillowed basalt and amphibolite with local tuffs and minor felsic volcanics and clastic sediments. These units dip away from the pluton at a very consistent 60° and also young away from the pluton, suggesting a pre-erosion domal structure over the intrusive. All of the volcanic and sedimentary units have been intruded by sodium-rich felsic dykes and albite porphyry with a composition similar to the It is within the fold axis of one of these broadwavelength cross folds, in iron formation, that the Dona Lake gold deposit occurs. The spatial relationship is illustrated in [the Figure referred is not included in this guide; refer to Cohoon, 1986] which depicts the surface projection of the mineralization and the host iron formation. The other structural feature apparent in the diagram is a horizontal overlap of the iron formation of - 104 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 approximately 150m, caused by a north-south sinistral fault at a low angle to the stratigraphy. Vertical movement on this fault is unknown but is likely to exceed 300m. This feature may be related to the chloritic shear mentioned above. altered to pyrrhotite and/or grunerite. • Visible gold is common on the polished outer surface of core but has rarely been seen on broken surfaces. Visible gold occurrences usually correlate well with assay results. Gold mineralization occurs in both of the iron formations illustrated but the majority of the established reserves are in the one to the east. Mineralization has been defined over a strike length of 200m, but more than 80% of the reserves are in the central 100m in the core of the crossfold axis. With width exceeding 25m in the core, the overall geometry is that of a flattened pod, which plunges east down the axis of the crossfold, entirely within iron formation. • Gold mineralization is accompanied by traces of microscopic chalcopyrite and sphalerite. However, neither mineral was noted microscopically in drill core. Several features are notable by their absence: • There are virtually no quartz veins and the few which do occur seldom contain gold. There is also no visible or geochemically evident siliceous alteration, at least not within the basalts. Within the iron formation, variable quartz content and the possibility of remobilization of original chert makes identification of siliceous alteration difficult in drill core. ... The iron formations occur within the tholeiitic pillow basalt and amphibolite, which is strongly foliated in proximity to the mineralization. Intruding these units are numerous irregular albite-rich felsic dykes, albite porphyries and lamprophyre dykes. • There is no arsenopyrite and no evidence of geochemically anomalous arsenic. This situation should be contrasted with the other gold deposits in the Pickle Lake camp where vein quartz was usually the immediate host and arsenopyrite was often the main sulphide. The iron formations are usually classic oxidefacies iron formation, composed of finely bedded magnetite, chert and hornblende, with local grunerite, garnet, calcite and sulphides. A finely bedded chert-sulphide unit in the hanging wall has been genetically grouped with the iron-rich minerals other than sulphides and might be more properly termed a chert, since the sulphides may be secondary. Significantly, no carbonate iron formation, nor iron-rich carbonates, have been located on the property. Detailed studies concerning the genesis of the deposit are just beginning but, in general, it is assumed that the important factors relating to localization of mineralization are: • The iron formation which provided a competency contrast with the adjacent basalts and a chemically attractive site for the replacement of iron-rich minerals by iron sulphide. Limited polished section work on mineralized iron formation indicates that the magnetite beds are finely fractured and brecciated, leaving what would presumably be a very porous network for the movement of fluids. All of the gold occurs in oxide-facies iron formation. Mineralized sections display the following characteristics: • Between 5% and 15% pyrrhotite virtually always accompanies gold. Notably, this relationship does not apply to pyrite; even when pyrrhotite is abundant, if the pyrite content exceeds 3%-4%, gold values are usually low. The pyrrhotite is fine-grained and wispy, cross cutting bedding and apparently replacing or displacing other minerals. • Cross-folds which induced the fracturing in the iron formation and provided a locus for the entry and deposition of mineralized solutions. • In most cases, evidence of original bedding in drill core has been partially or completely destroyed. • The sodium-rich felsic dykes and albite porphyry, whose exact role is uncertain, but whose association with most gold deposits in the Canadian Shield empirically suggests that they are a necessary factor. • There is some evidence to suggest that the magnetite content is considerably reduced in mineralized sections, perhaps having been • Amphibolite-grade metamorphism, which - 105 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 may have resulted in ductile, as opposed to brittle, deformation. It is assumed that this is the reason for the amorphous zone of sulphide replacement rather than a distinct break or shear zone with quartz filling. All of the above hypotheses assume that the deposit is epigenetic, that the pyrrhotite mineralization is secondary and partially or completely replaced original iron-rich minerals, and that the gold mineralization was penecontemporaneous with the pyrrhotite. However, the sequence and mechanisms of deformation, and even the degree and nature of sulphide replacement, are open to further study. Outcrop is very sparse in the vicinity of the deposit, except for some of the hanging wall basalts and one small exposure of the unmineralized chert. [Exposures were subsequently created during mining and development activities.] Consequently, considerable reliance was placed on geophysics and geochemistry in the early stages of exploration, prior to detailed diamond drilling. Not surprisingly, magnetics were the most useful tool in defining the distribution and gross structural characteristics of the iron formation. However, very closely spaced readings on a 50m box grid were required to define the subtle variations in structure caused by the crossfold hosting the gold mineralization. Horizontal loop EM surveys with a 100m coil separation gave a weak but discernable response over the deposit. It is assumed that the response is weak due to the wispy and patchy nature of the sulphide mineralization, despite sulphide concentrations in excess of 15%. Surface soil geochemistry, involving both Cand A-horizon soils, failed to reveal the deposit. This failure is attributed to the uniform blanket of till which rests directly on [bed]rock. Rock geochemistry was somewhat more useful during initial stages of exploration prior to diamond drilling. Samples of basalt in the hanging wall returned up to 82ppb gold and more than half of the samples, in a radius of 200m, contain more than 10ppb gold. Background elsewhere on the property, including in iron formation, is in the order of 1ppb gold. The Dona Lake gold deposit is intimately associated with a folded, oxide-facies iron formation. It is the iron formation which played a crucial role in providing a structural and chemical trap for mineralizing solutions.” Structural setting for gold mineralization East of the Ochig Lake pluton, in the vicinity of the Dona Lake Mine, Stott and Corfu (1991) described shear zones with normal sense movement that are parallel and dip away from the margin of the pluton and interpreted these shear zones as related to pluton emplacement. East of the northeastern lobe of the Ochig Lake pluton, within a mapped contact strain domain, in the vicinity of the Dona Lake area, Stott (1996) documented margin-parallel shear zones dipping away from the intrusion, showing pluton-side-up kinematics. A discussion of gold in high-strain zones associated with pluton emplacement was given in Smyk et al. (2011). Stott and Biczok (2010) had related the structural geology of Musselwhite Mine (125km north of Pickle Lake) to the emplacement of the crescentshaped North Caribou pluton and revisited the idea (Stott et al., 1989) that gold mineralization there may also be related to the intrusion of this pluton. In this model, lateral compression imparted by the intrusion of the pluton produces shallowly plunging, isoclinal and locally transposed folds, and high-strain or shear zones in relatively incompetent lithologic layers that fold more tightly. These narrow high-strain/shear zones reflect coaxial strain and would not be related to any through-going, transcurrent, regional shear zone. Any non-coaxial shear features on folded limbs can be accounted for by subjecting rocks to the limit of flattening and by accommodating the extreme shortening by rotation and lateral shear. These zones, however, may focus hydrothermal fluid flow and host fault-fill veins (cf. Dubé and Gosselin, 2007). Many such zones are characterized by replacement and/or alteration zones, rather than discrete vein systems. The mineralized zones that result from such focussed fluid flow are localized and may only represent a very small proportion of a favourable host lithology (e.g. banded iron formation). Gold mineralization at Dona Lake is manifested as a flattened, plunging pod, hosted by a folded, sulphidized banded iron formation, largely in the core of a crossfold axis (Cohoon, 1986). There is a notable lack of brittle features, quartz veining and a mineralized shear zone or ‘break”, ascribed in part to the relatively high grade (amphibolite facies) of metamorphism. The deposit is situated in the amphibolite-facies contact - 106 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 metamorphic and strain aureole surrounding the Ochig Lake pluton (Stott, 1996). This pluton (2741Ma) has post-dated and overprinted the regional penetrative deformation fabrics with its own contact strain fabric (cf. Young et al. 2006). Sub-stop Descriptions, Dona Lake Gold Mine (Fig. 21): Stott (1996) and Stott and Smith (1988) listed several different settings for gold deposits directly attributed to felsic pluton emplacement: UTM Coordinates: NAD83; 15U 0702099E / 5699732N • Type A: A set of shear or high-strain zones bordering an uplifted and rotated wedge of supracrustal rock; • Type B: Shear or high-strain zone defining the outer margin of a pluton-induced strain aureole; • Type C: Shear or high-strain zone focussed along a contrast in rock ductility (e.g., BIF-basalt; Dona Lake Mine, Musselwhite Mine); • Type D: Shear or high-strain zones in conjugate sets at small angles to the schistosity; this strain environment may correspond to regional orogenic shortening or to a pluton-induced strain aureole; (e.g., Pickle Crow Mine); and • Type E: Shear or high-strain zones, tangential to a strain aureole, which may have been initiated during regional orogenic shortening and locally reactivated. Stop 4a: Dona Lake Mine Portal area The rehabilitated site of the former Dona Lake Mine is now marked by a grassy field along the sides of the access road, south of Sika Pond. The mine portal, now sealed, lies just south of the road. Foliated and folded mafic metavolcanic and gabbroic rocks outcrop in the vicinity of the site and along shoreline exposures on Sika Pond. The exposures near the reclaimed mine site are dominated by gabbro and pillowed basalt of the lower sequence of the Pickle Crow Assemblage with local banded iron formation (Young 2003). Confederation assemblage mafic volcanic rocks occur east of the Dona Lake Mine area. Well-preserved pillowed flows east of the mine area indicate east-southeast younging (Young 2003). Young (2003) has noted that in the Dona Lake area, gabbro is common and has a weathered surface characterized by a ‘knobby’ texture caused by coarse-grained hornblende and anastomosing fractures superficially resembling pillowed basalt (Fig. 22). Figure 21. Dona Lake Mine area with Field Trip Stop locations. - 107 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 basalt (Fig. 24) is situated on the west side of the mine road, across from where a pit had been developed in BIF. Bun pillows and brecciated pillow fragments have thin, dark, amphibole-rich selvages. Figure 22. Coarse-grained, amphibolitized gabbro, south of Sika Pond (Stop 4a). Stop 4b: Banded Iron Formation UTM Coordinates: NAD83; 15U 0701767E / 5699990N; 0701708E / 5699991N, and 0701764E / 5699994N Banded iron formation outcrops (Fig. 23) in a number of locations (see three sets of UTM coordinates above) along a mine access road that skirts the western shore of Sika Pond and accesses a flooded mine pit. A southeasterly trending foliation and M- and Z-folds are developed in these BIF units. A rusty BIF sample collected by OGS staff returned <0.01 z/ton Au and 0.3 oz/ton Ag (unpublished data, Resident Geologist’s Files, Thunder Bay) Figure 24. Pillowed basalt, west side of road, near pit, west of Sika Pond (Stop 4c). Stop 5: Ochig Lake Pluton Stop 4c: Pillowed Basalt UTM Coordinates: NAD83; 15U 0693228E / 5693898N. UTM Coordinates: NAD83; 15U 0701686E / 5700056N A small outcrop of relatively undeformed pillowed Figure 23. Rusty, folded BIF near pit, west of Sika Pond (Stop 4b). One of the few exposures of the Ochig Lake Pluton occurs on the eastern side of Highway 599, south of Fault Lake. These low-lying outcrops expose grey, medium-grained, equigranular biotite granodiorite with local, lenticular quartz-filled gashes (Fig. 25). One of the internal granitoids in the Pickle Lake belt, the Ochig Lake Pluton is semi-circular in plan, with a domical internal structure defined by outward-dipping foliation (Stott 1996). A northeasterly trending, steeply southeast-dipping foliation was noted by Stott et al. (1989a) just northeast of this location. Most of the pluton consists of homogeneous, medium- to finegrained granodiorite to trondhjemite. The Ochig Lake pluton yielded a U-Pb zircon age of 2741±2Ma (Corfu and Stott 1993a). - 108 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 mineralization within 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. Evans, J.E.L. 1941. Geology of the Eastern Extension of Crow River area; Ontario Department of Mines, Annual Report, v.48, pt.7, 9p. Ferguson, S. A., 1966. Geology of Pickle Crow Gold Mines Limited and Central Patricia Gold Mines Limited, No. 2 Operation”; Ontario Department of Mines, Miscellaneous Paper 4. Figure 25. Quartz veins and lenses in equigranular granodiorite of the Ochig Lake Pluton, Highway 599 (Stop 5). References Anderson, S.D. 2007. Thierry Mine project, Pickle Lake belt, Ontario: Observations pertaining to the structural geology of the Thierry Cu-Ni-(PGE) deposit; unpublished report, Richview Resources Inc., 39p. Coates, H. and Anderson, W., 2008. Technical Report on the Pickle Crow Gold Property, Patricia Mining Division, Ontario, Canada; unpublished report, PC Gold Inc. Cohoon, G.A. 1986. Gold in an iron formation, the Dona Lake deposit; The Northern Miner Magazine, v.1, no.8, p.16-20. Corfu, F. and Stott, G.M. 1989. U-Pb geochronology of the central Uchi Subprovince, northwestern Ontario; p.A55 in Geological Association of Canada, Program with Abstracts, v.14, 140 p. Corfu, F., and Stott, G.M. 1993a. Age and petrogenesis of two late Archean magmatic suites, northwestern Superior Province, Canada: zircon U–Pb and Lu–Hf isotopic relations; Journal of Petrology, v.30, p.1179– 1196. Corfu, F., and Stott, G.M. 1993b. U–Pb geochronology of the central Uchi subprovince, Superior Province. Canadian Journal of Earth Sciences, v.78, p.53–63. Corfu, F., and Stott, G.M. 1996. Hf isotopic composition and age constraints on the evolution of the Archean central Uchi subprovince, Ontario, Canada. Precambrian Research, v.78, p53–63. Dubé, B. and Gosselin, P. 2007. Greenstone-hosted quartzcarbonate vein deposits; in Mineral deposits of Canada: A synthesis of major deposit-types, district metallogeny, the evolution of geological provinces, and exploration methods, Geological Association of Canada, Mineral Deposits Division, Special Publication No. 5, p.49-73. Dubé, B., Williamson, K., and Malo, M. 2003. Gold Gilligan, L.B. and Marshall, B. 1987. Textural evidence for remobilization in metamorphic environments; Ore Geology Reviews, v.2, p.205–229. Graham R. J., 1965. Unpublished letter to Dr. S. A. Ferguson, Ontario Department of Mines, Pickle Crow Gold Mines internal report. Harding, W.D. 1936. Geology of the Cat River–Kawinogans Lake area; Ontario Department of Mines, Annual Report, 1935, v.44, pt.6, p.53-75. Hennessey, B.T., San Martin, A.J. and Shoemaker, S.J. 2011. A Mineral Resource Estimate for the Pickle Crow property, Patricia Mining Division, northwestern Ontario, Canada; unpublished report, PC Gold Inc., 209p. Hollings, P. 1998. Geochemistry of the Uchi subprovince, northern Superior Province: an evaluation of the geodynamic evolution of the northern margin of the Superior Province ocean basin. Unpublished Ph.D. thesis, University of Saskatchewan, Saskatoon, Saskatchewan. Hollings, P., Stott, G. and Wyman, D., 2000. Trace element geochemistry of the Meen-Dempster greenstone belt, Uchi subprovince. Canadian Journal of Earth Sciences, 37, 1021-1038. Hollings, P. 2002. Archean Nb-enriched basalts in the northern Superior Province; Lithos, v.64, p.1–14. Hollings, P. and Kerrich, R. 1999. Trace element systematics of ultramafic and mafic volcanic rocks from the 3Ga North Caribou greenstone belt, northwestern Superior Province. Precambrian Research, v.93, p.257–279. Hollings, P. and Kerrich, R. 2004. Geochemical systematics of tholeiites from the 2.86Ga Pickle Crow assemblage, northwestern Ontario: arc basalts with positive and negative Hf–Nb anomalies; Precambrian Research, v.134: p.1–20. Hollings, P., Wyman, D.A., and Kerrich, R. 1999. Komatiitebasalt-rhyolite volcanic associations in northern Superior Province greenstone belts: Significance of plume-arc interaction in the generation of the protocontinental Superior Province; Lithos, v.46: p.137– 161. Hurst, M.E. 1931. Pickle Lake-Crow River area; Ontario - 109 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Department of Mines, Annual Report, 1930, v.39, pt.2, p.1-35. Janes, D.A., Seim, G.W., Hinz, P. and Storey, C.C. 1994. Sioux Lookout Resident Geologist’s District—1993; in Report of Activities 1993, Resident Geologists, Ontario Geological Survey, Open File Report 5892, p.106-128. Jen, L. and McCutcheon, B. 1995. Principal Canadian nonferrous and precious metal mine production in 1994; Canadian Minerals Yearbook, 1995, p.68.1 to 68.7. Keller, G.D. 2005: Technical Report, Thierry Deposit, Pickle Lake, Ontario; SRK Consulting (Canada) Inc., 150 p. Kolb, M. J., 2011. PC Gold thin section report, Central Patricia East, unpublished, internal company report, PC Gold Inc. Lynch, T. 2010. Technical Report for MNDM Assessment Purposes: PC Gold - Pickle Lake Property. Ontario Ministry of Northern Development, Mines and Forestry, unpublished Assessment Report, Resident Geologist’s Files, Thunder Bay. MacGregor, H. 1989. Report on Mineral Reserves at the Pickle Crow Property dated January 20, 1989; unpublished company report for Noramco Mining Corporation, by Watts Griffis and McOuat Limited. MacQueen, J. K. 1987. Stratigraphy, structure and gold mineralization of the No.5 Vein/Iron Formation Zone, Pickle Crow Gold Mines, Pickle Lake, Ontario. Unpublished M.Sc. thesis, Carleton University, Ottawa, Ontario. Marshall, B. and Gilligan, L.B. 1989. Durchbewegung structure, piercement cusps and piercement veins in massive sulfide deposits: formation and interpretation; Economic Geology, v.84, p.2311–2319. McInnes, W. 1906. The headwaters of the Winisk and Attawapiskat rivers; in Summary report of the Geological Department of Canada for the calendar year 1905, Geological Survey of Canada; Geological Survey of Canada, Summary Report (1905), 1906; p.76-80. Naldrett, A.J. and Cabri, L.J. 1976. Ultramafic and related mafic rocks; their classification and genesis with special reference to the concentration of nickel sulfides and platinum-group elements. Economic Geology, v.71, p.1131-1158. Novak, N. and Mlot, S. 2004. Technical report on the geology and mineral resources of the Thierry Copper-Nickel project, Pickle Lake, Ontario; Internal Consultants Report, 38 p. Patterson, G.C. 1980. The Geology of the Kapkichi Lake ultramafic-mafic bodies and related Cu-Ni mineralization, Pickle Lake, Ontario; unpublished Ph.D. thesis, Carleton University, Ottawa, Ontario. Patterson, G.C. and Watkinson, D.H. 1984. The geology of the Thierry Cu-Ni mine, northwestern Ontario; Canadian Mineralogist, v.22, p.3–11. Puritch, E., Armstrong, T., Burga, D., Routledge, R., Pearson, J.L., Hayden, A., Orava, D. and Rodgers, K. 2012. Technical report and Preliminary Economic Assessment of the Thierry and K1-1 Cu-Ni-PGE deposits, Thierry Project, Pickle Lake area, Patricia Mining District, northwestern Ontario, Canada; unpublished company report, Cadillac Ventures Inc., 230p. Puritch, E., Ewert, W.D., and Armstrong, T. 2006. Technical report and resource estimate on the Thierry Cu-NiPGE mine property, Pickle Lake area, Patricia mining district, northwestern Ontario, Canada; P& E Mining Consultants Inc., 161 p. Puumala, M.A. 2009. Mineral occurrences of the central and eastern Uchi domain; Ontario Geological Survey, Open File Report 6228, 294p. Pye, E.G. 1956. “Geology and Mineral Deposits of the Crow River Area”, Ontario Department of Mines, p.1-239. Pye, E.G. 1975. Crow River area, District of Kenora (Patricia Portion); Ontario Geological Survey, Preliminary Map P.1009, scale 1:12,000. Pye, E.G. 1976. Geology of the Crow River area, District of Kenora (Patricia Portion); Ontario Geological Survey, Open File Report 5152, 264p. Sage R.P. and Breaks, F.W. 1982. Geology of the Cat Lake -Pickle Lake area, Districts of Kenora and Thunder Bay; Ontario Geological Survey, Report 207. Sajona, F. G., Maury, R. C., Bellon, H., Cotten, J. and Defant, M. J. 1996. High field strength element enrichment of Pliocene-Pleistocene island arc basalts, Zamboanga Peninsula, western Mindanao (Philippines). Journal of Petrology, v.37, p.693–726. Sanborn-Barrie, M., Skulski, T., and Parker, J. 2001. Three hundred million years of tectonic history recorded by the Red Lake greenstone belt, Ontario. Geological Survey of Canada, Current Research 2001-C19. Smyk, M.C., White, G.D., Lockwood, H.C., and Bennett, N.A. 2011. Report of Activities 2010, Resident Geologist Program, Thunder Bay North Regional Resident Geologist Report: Thunder Bay North District; Ontario Geological Survey, Open File Report 6262, 47p. Stott, G.M. 1996. The geology and tectonic history of the central Uchi subprovince; Ontario Geological Survey Report 5952, 178p. Stott, G.M. and Biczok, J. 2010. North Caribou greenstone belt: gold and its possible relation to the North Caribou pluton emplacement—a belt-wide contactstrain aureole?; in Summary of Field Work and Other Activities 2010, Ontario Geological Survey, Open File Report 6260, p.22-1 to 22-12. Stott, G.M., Corfu, F., Breaks, F.W., and Thurston, P.C. 1989. Multiple orogenesis in northwestern Superior Province; Geological Association of Canada– - 110 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Mineralogical Association of Canada, Joint Annual Meeting, Program with Abstracts, v.14, p.A56. Stott, G.M., Brown, G.H., Coleman, V.J., Green, G.M., and Reilly, B.A. 1989a. Precambrian geology of the Pickle Lake area, western part; Ontario Geological Survey, Preliminary Map P.3056, scale 1:50,000. Stott, G.M., Brown, G.H., Coleman, V.J., Green, G.M., and Reilly, B.A. 1989b. Precambrian geology of the Pickle Lake area, eastern part; Ontario Geological Survey, Preliminary Map P.3057—Revised, scale 1:50,000. Stott, G.M., and Corfu, F. 1991. Uchi subprovince. Geology of Ontario, Ontario Geological Survey Special Vol. 4, Part 1. Ontario Geological Survey, Ontario, p.144– 236. Stott, G.M. and Smith, P.M. 1988. Development of goldbearing structures in the Archaean: the role of granitic plutonism; in Bicentennial Gold 88, extended abstracts, poster programme, v.1, Geological Society of Australia, Abstracts Series no.23, p.48-50. Sun, S.S. and McDonough, W.F. 1989. Chemical and isotopic systematics of oceanic basalts; implications for mantle composition and processes; in Magmatism in the ocean basins. Saunders, A.D. and Norry, M.J. (Editors), Geological Society of London, London; v.42, p.313-345. Thomson, J.E. 1939. The Crow River area; Ontario Department of Mines, Annual Report, 1938, v.47, pt.3, p.1-65. Thurston, P.C., Osmani, I.A., and Stone, Northwestern Superior Province: review analysis. Geology of Ontario, Ontario Survey, Special Vol. 4, Part 1. Ontario Survey, Ontario, p.80–142. D. 1991. and terrane Geological Geological Tomlinson, K.Y., Stevenson, R.K., Hughes, D.J., Hall, R.P., Thurston, P.C., and Henry, P. 1998. The Red Lake greenstone belt, Superior Province: evidence of plume-related magmatism at 3Ga and evidence of an older enriched source; Precambrian Research, v.89, p.59–76. White, D.J., Musacchio, G., Helmstaedt, H., Harrap, R.M., Thurston, P.C., van der Velden, A., and Hall, K. 2003. Images of a lower crustal slab: Direct evidence for tectonic accretion in the Archean western Superior Province; Geology, v.31, p.997-1000. Winter, L.D.S. 1987. Geological Reserve (Mineral Inventory) Estimate, Pickle Crow Property, Pickle Lake, Ontario; unpublished company report for Highland Crow Resources. Winter, L.D.S. 1988. Geological Reserve (Mineral Inventory) Estimate, Pickle Crow Property, Pickle Lake, Ontario; unpublished company report for Noramco Mining Corporation. Young, M. 2003. New structural, geochronological and geochemical constraints on the tectonic assembly of the Archean Pickle Lake greenstone belt, Uchi subprovince, western Superior Province; Unpublished M.Sc. thesis, Queen’s University, Kingston, Ont. Young, M. and Helmstaedt, H. 2001. Tectonic evolution of the northern Pickle Lake greenstone belt, northwestern Superior Province, Ontario. Geological Survey of Canada, Current Research 2001-C20. Young, M.D., McNicoll, V., Helmstaedt, H., Skulski, T., and Percival, J.A. 2006. Pickle Lake revisited: new structural, geochronological and geochemical constraints on greenstone belt assembly, western Superior Province, Canada; Canadian Journal of Earth Sciences, v.43, p.821–847. Tigert, T. T. 1949. Geology of the Central Patricia Mine; Canadian Mining Journal, p.72-75. - 111 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Field Trip 9 - The Ghost Lake Batholith and Related Pegmatites Shannon E. Zurevinski Dept. of Geology, Lakehead University, Thunder Bay, Ontario, Canada Introduction The Late Archean (2685 Ma) Ghost Lake Batholith (GLB) is a roughly 280km2 elongated intrusion which trends east to northeast, following regional structures (Fig. 1). The GLB is exposed from Eagle River, northeast to Ghost Lake, and is classified as a two mica granitoid (biotite and muscovite). Mineralogy of the peraluminous fertile S-type granitoid includes cordierite, sillimanite, Mg-garnet, tourmaline, beryl, and rare dumortierite (a fibrous borosilicate). The batholith shows trends of increasing geochemical fractionation, and as a result, is host to exotic rare element pegmatite occurrences, such as the Mavis Lake pegmatite group. pegmatites can be difficult when pegmatites are not found in association with the fertile primary parent granite. This is further hindered when the melts separate and mix with the crustal material during ascent and emplacement, producing hybrid geochemical and mineralogical signatures. In contrast, the Ghost Lake Batholith represents an intrusive complex where the rare element pegmatite facies exists within its consanguineous, primitive peraluminous granite parent (Breaks and Moore, 1992). This relationship shown in outcrop presents an opportunity to assess mechanisms that concentrate rare elements in pegmatites. For this reason, the Ghost Lake Batholith has been the subject of much past academic research, and likely will continue to be for years to come. Identifying the initial sources and processes that concentrate lithophile metal enrichment in rare element This ½ day field trip will examine granite and pegmatitic outcrop from the Ghost Lake Batholith, Figure 1. Generalized map showing the Sioux Lookout Terrane boundary and the location of the Ghost Lake Batholith and the Mavis Lake pegmatite group (from Brand et al., 2009). - 112 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 and two associated pegmatites, both of the Mavis Lake Pegmatite Group. This field guide builds upon those previously written and compiled by Breaks (1982), Breaks (1985), Breaks and Janes (1991) and Beakhouse et al. (1995). One stop is a road cut, so caution must be exercised. Never stand on the paved portions of the road, and be aware of the traffic at all times. Road intersection. Regional Geology This road cut is located in the western part of the batholith and features different plutonic phases. Most of the outcrop at this location was termed GLB-1 by Breaks and Janes (1991). GLB-1 is one of the most common granitic units of the batholith. It is an inequigranular, coarse grained, massive to weakly foliated biotite and cordierite-biotite granite. Zones of inhomogenous biotite granite with inclusions of metasedimentary rocks and mafic segregations occur throughout GLB-1. Biotite and cordierite-biotite pegmatitic leucogranite segregations occur sporadically throughout the unit. Breaks and Janes (1991) document the pegmatitic masses as consisting of blocky, graphic K-feldspar-quartz masses up to 60cm wide, grading into smoky quartz segregations in association with blocky K-feldspar, biotite, and apatite. Cordierite can be seen occurring as square megacrysts, but it appears more often as dark masses that are up to 1.5cm in diameter. It can be replaced by chlorite, muscovite, and andalusite. Accessory minerals include garnet, apatite, zircon, monazite, and sillimanite. Secondary muscovite occurs after K-feldspar, plagioclase (An10-30), biotite and cordierite. Other plutonic phases at this location are dikes of fine-grained, grey, muscovite-biotite granite and coarse-grained, white biotite-muscovite leucogranite and pegmatite segregations. The GLB and related Mavis Lake Pegmatite group occur within the Winnipeg River–Wabigoon Subprovince boundary zone, referred to as the Sioux Lookout Terrane (SLT) (Beakhouse, 1989). The SLT is characterized by metasedimentary and metavolcanic assemblages exhibiting a wide range of metamorphic grade, including zones of migmatized metasedimentary rocks (Breaks and Moore, 1992). This terrane is host to about 150km of peraluminous granite plutons, with the GLB being the largest of the group. Geochemically, the plutons of the SLT exhibit enrichment in lithophile elements and rare-earth elements (REE’s), including Cs, Be, Li, Rb, Nb, Ta, Ga, U, Th, Mo, W, and Sn. The GLB is broadly concordant to the eaststriking foliations and is weakly foliated. Evidence of the foliations is shown by preferred orientations of phyllosilicates, and biotite-cordierite-sillimanite segregations. Foliation diminishes towards the eastern segment of the batholith. Multiple events of ductile deformation in the GLB are described by Breaks and Moore (1982), and will be discussed at each field stop. The GLB is subdivided into two portions comprising the Western GLB and the Eastern GLB. The Western GLB is also known as the lower intrusion, where the granitoids are less homogenous, and form transitional contacts with the migmatized sedimentary host. In the Eastern GLB, also known as the upper intrusion, the contacts with the metasedimentary rocks are abrupt and contain fewer sedimentary inclusions. Traversing from the lower intrusion through to the upper intrusion, there are marked trends of geochemical fractionation involving enrichment of B, Be, Ga, Li, Nb, and Rb; and depletion of Ba, Sr, Zr, and total REE. Ghost Lake Batholith stops Starting at the parking lot of the Best Western Hotel and Conference Center, Dryden, drive 13km west along the Trans-Canada Highway 17 to the village of Oxdrift. Park vehicles in the old Oxdrift School parking lot and walk to the road cut located just west of the Corner Stop 1: GLB-1, Oxdrift School Stop, Muscovitebiotite granite, biotite-muscovite granite, rare granodiorite (± cordierite). UTM Coordiantes: NAD 83 15U 0499697E / 5518078N Mavis Lake Group Pegmatites stops Drive east from Stop 1 on Highway 17 back through Dryden for approximately 16.4km until you reach Airport Road. Turn left onto Airport Road and travel north for 4.8km to Ghost Lake Road. Turn right on Ghost Lake Road and follow the main road to the junction with a logging road at UTM coordinates 527090E, 5522510N, a distance of approximately 11.6km from Airport Road. Turn south and drive along this main road to an intersection with a secondary logging road at UTM coordinates 526970E, 5521410N, a distance of about 1.4km. Veer west along this secondary road and up the hill where several vehicles can park. Most of the better outcrops at this stop are located south of the logging road. - 113 - Beakhouse et al. (1995) describe the Mavis Lake �Proceedings of the 61st ILSG Annual Meeting - Part 2 Pegmatite Group as an east-striking, 8km by 1.5km, concentration of rare-element pegmatites and related metasomatic zones. With increasing distance from the parent Ghost Lake Batholith, the pegmatite mineral assemblages exhibit well-defined zonations. More than 12 distinctive spodumene pegmatites are described from the Mavis Lake area and each ranges in size from 3 by 15m to 15 by 280m. Occurring parallel to the foliation of the host metavolcanic rocks, the pegmatites intrude as roughly lensoidal bodies. The rare-element pegmatites are classified as the albite-spodumene type (after Černy, 1982). Mavis Lake Group Pegmatites are structurally confined to the northern limb of the westplunging Thunder Lake Syncline and show localized effects of late-tectonic deformation, for example: strained contacts; healed fractures involving tourmaline and spodumene buckling; and boudinage of pegmatitic granitic dykes near the contact zones. STOP 2: Dryden Airport Pegmatite UTM Coordinates: NAD 83 15U 0526720E / 5521360N This stop shows pegmatitic-aplite dykes intruding metasedimentary rocks (Fig. 2). This dyke is described as a barren potassic pegmatite with a characteristic mineral assemblage including garnet, muscovite, biotite, abundant coarse tourmaline, albite, quartz, blocky microcline (Or70-77Ab23-30), and rare, large (up to 10cm in length) lime-green beryl. Remnant rock saw cuts represent poached rare beryl. Road. Turn down Mavis Forest Road and travel 2.6km and park to your right at a small BMX trail entrance (just after the powerline). Follow the trail in and veer to your first left. Follow the trail until you reach a fork in the trail and then veer right and follow the intersection of the rutabaga trail and out onto a large ridge. Follow this around to the pegmatite clearing. STOP 3: Fairservice Spodumene-Beryl-Tantalite Pegmatites UTM Coordinates: NAD 83 15U 0523789E / 5518050N The Fairservice pegmatite intrudes foliated and gneissic mafic metavolcanic rocks and fine-grained laminated metagreywackes. There is some vague internal zonation - in which this particular occurrence is representative of a quartz-rich core and spodumenerich, albite-quartz pegmatite. This pegmatite hosts a randomly oriented, green primary spodumene, large coarse quartz pod-like segregations, thick yellowgreen muscovite, albite, apatite, lesser white and blue beryl, large black tourmaline (Fig. 3), blue apatite, and orange garnet in a matrix of light grey massive quartz. A sharp contact with laminated metagreywackes is noted. Healed fractures of tourmaline are shown at this occurrence. References Return to the vehicles and drive back to Highway 17. Turn right (east) onto the highway and drive 3.7km until you reach Thunder Lake Road. Turn left onto Thunder Lake Road and travel 2.7km to Mavis Forest Beakhouse, G.P., Blackburn, C.E., Breaks, F.W., Ayer, J., Stone, D., and Stott, G.M. 1995. Western Superior Province Fieldtrip Guidebook. Precambrian 1995 Meeting. Geological Survey of Canada Open File 3138/Ontario Geological Survey Open File Report 5924. Figure 2. Dryden Airport Pegmatite in contact with metasedimentary rocks. Figure 3. Fractured tourmaline from the Fairservice Occurrence, Mavis Lake Pegmatite Group - 114 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Brand, A.A., Groat, L.A., Linnen, R.L., Garland, M.I., Breaks, F.W. and Giuliani, G. 2009. Emerald mineralization associated with the Mavis Lake pegmatite group, near Dryden, Ontario. Canadian Mineralogist, 47, p.315-336. Breaks, F.W. and Moore, J.M, Jr. 1992. The Ghost Lake Batholith, Superior Province of Northwestern Ontario: A fertile, S-type, peraluminous graniterare-element pegmatite system: The Canadian Mineralogist, v.30, p.835-875. Breaks, F.W. 1989. Origin and evolution of peraluminous granite and rare-element pegmatite in the Dryden area of Northwestern Ontario; unpublished PhD thesis, Carleton University, Ottawa, Ontario, 594p. Breaks, F.W., Selway, J.B., and Tindle, A.G. 2001. Fertile peraluminous granites and related rare-element mineralization in pegmatites, Superior Province, Northwest and Northeast Ontario in Summary of Field Work and Other Activities, 2001, Ontario Geological Survey, Open File Report 6070, p.39-1 to 39-9. Breaks, F.W. and Janes, D.A. 1991. Granite-related mineralization of the Dryden area, Superior Province of Northwestern Ontario. GAC-MAC-SEG Joint Annual Meeting, 1991, Field Trip B7 Guidebook, 71p. Breaks, F.W. and Kuehner, S. 1984. Precambrian geology of the Eagle River-Ghost Lake area, Kenora District; Ontario Geological Survey, Map P.2623, 1:31,680. Černy, P. 1982. Petrogenesis of granitic pegmatites. In Granitic Pegmatites in Science and Industry (P. Černy editor) Mineralogical Association of Canada, Short Course Handbook 8, p.405-461. - 115 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Field Trip 10 - Mattabi/Sturgeon Lake Historic VMS Camp George J. Hudak Minerals Division, Natural Resources Research Institute, University of Minnesota Duluth, Duluth, MN Introduction showings. Detailed field, petrographic, and lithogeochemical studies performed since the mid-1980’s have indicated that the south Sturgeon Lake region of northwestern Ontario is underlain by an extremely well-preserved, though partially eroded, Neoarchean subaerial to submarine volcanic caldera complex (Fig. 1: Morton et al., 1991; Hudak et al., 2003). This caldera complex, the Sturgeon Lake Caldera Complex (SLCC), hosted six massive sulphide orebodies which produced nearly 20 million tons of polymetallic volcanogenic massive sulfide (VMS) ore during mining operations between 1972 and 1991 (Table 1), as well as numerous sub-economic massive and semi-massive sulphide The combination of well-preserved volcanic and hydrothermal alteration textures, the variable 55° to 90° dip of a north-facing, essentially homoclinal volcanic sequence, and more than 600,000m of diamond drilling over an apparent 4,500 meter stratigraphic interval has enabled geologists the opportunity to examine the lithological, lithogeochemical, and metallogenic evolution of the SLCC. This includes the synvolcanic subvolcanic intrusions (Biedelman Bay Intrusive Complex), initial subaerial to shallow subaqueous precaldera volcanism (Pre-caldera Sequence, PCS), early subaerial to submarine explosive silicic volcanism (Early Caldera Sequence, ECS) that is associated Figure 1. Location map (inset) and regional geological map of the south Sturgeon Lake region (modified after Trowell, 1983; Morton et al., 1991; Morton et al., 1999; Galley et al., 2000). - 116 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Table 1. Grade and tonnage statistics from the VMS ore deposits of the south Sturgeon Lake region (after Franklin, 1996). Caldera sequences include the Early Caldera Sequence (ECS) and the Late Caldera Sequence (LCS; Hudak, 1996; Morton et al., 1999; Hudak et al., 2003). F-Group Caldera Sequence ECS 106 Tons 0.34 Cu % 0.64 Zn % 9.51 Pb % 0.64 Ag (g/t) 60.4 Ag (g/t) 2.13 Mattabi ECS/LCS 12.55 0.74 8.28 0.85 104.0 3.67 Sturgeon Lake LCS 2.07 2.55 9.17 1.21 164.2 5.79 Creek Zone/Sub-Creek Zone* Lyon Lake* LCS 0.91 1.66 8.80 0.76 141.5 4.99 LCS 3.95 1.24 6.53 0.63 141.5 4.99 19.82 1.06 8.50 0.91 119.7 3.85 Deposit Total/Average * VMS deposits interpreted to have originally formed as part of Sturgeon Lake deposit with the development a multi-cyclic, piecemeal caldera complex up to 25km in strike length, and later intracaldera submarine mafic to felsic, dominantly effusive submarine volcanism and clastic and chemical sedimentation (Late Caldera Sequence, LCS). Postvolcanic structural deformation has not only led to the apparent transposition of non-caldera associated strata (Lyon Lake Fault Sequence, LLFS) up-section from the caldera-associated volcanic sequence, but has locally led to remobilization of massive sulphide mineralization along a major northwest-southeasttrending fault zone to form smaller, but locally economic, massive sulphide deposits (e.g., the Lyon Lake, Creek Zone, and Sub-Creek Zone deposits). The stratigraphic evolution in the SLCC represents one of the few examples of Neoarchean calderas which illustrate the modern “caldera cycle” (Smith and Bailey, 1968; Hudak et al., 2003; Mueller et al., 2004; Mueller et al., 2008). This field trip has several purposes: 1) to illustrate the textures, lithologies, and geological structures resulting from various volcanological and sedimentological processes associated with various stages of caldera complex development; 2) to illustrate the hydrothermal alteration mineral assemblages within the caldera complex, and their spatial relationships to volcanogenic massive sulphide (VMS) mineralization; and 3) to illustrate methods by which exploration geologists can use physical volcanology, hydrothermal alteration mineral assemblages, and structural geology, along with other lithogeochemical and geophysical data, to effectively explore for VMS deposits in the Sturgeon Lake camp and elsewhere. This guidebook represents an updated version of previous field trip guidebooks for the Sturgeon Lake region that were associated with the Geological Association of Canada – Mineralogical Association of Canada meeting during May, 1996 (Morton et al., 1996), as well as the Precambrian Research Center Professional Workshop on VMS and lode gold deposits in Archean greenstone belts (Hudak et al., 2008). In addition to this guidebook, the reader is referred to two papers which describe the physical volcanology and mineralization present within the SLCC. These papers provide detailed overviews of the regional geology as well as the literature related to subaerial and submarine volcanic processes and the genesis of VMS mineralization. Morton et al. (1991) presents an early interpretation of the stratigraphic sequence within the SLCC, and discusses the interrelationships between volcanological and mineralizing processes that formed the Mattabi VMS orebody. This classic paper is the first to discuss the development of a submarine caldera complex in the south Sturgeon Lake region, and how processes associated with caldera development led to conditions favourable for massive sulphide mineralization. Hudak et al. (2003) is a detailed discussion of submarine explosive silicic volcanological processes within the SLCC, and how these processes dictated the stratigraphic intervals, as well as the intracaldera locations, where economic VMS mineralization occurs within the SLCC. In addition, Hudak et al. (2003) includes the most upto-date stratigraphic nomenclature and interpretations of the volcanological and ore-forming processes associated with the genesis of this exceptionally wellpreserved Neoarchean subaerial to submarine caldera complex. In essence, Hudak et al. (2003) indicate that - 117 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 larger VMS deposits correlate with larger explosive eruption events within the SLCC. Regional Geologic Setting The SLCC is located within the South Sturgeon Sequence of the Savant Lake-Sturgeon Lake Greenstone belt that occurs in the Wabigoon Greenstone Belt of the Superior Province (Fig. 1; Sanborn-Barrie and Skulski, 1999). The SLCC is up to 25 km in strike length, and contains an approximately 3000 meter stratigraphic thickness of north-facing, vertical to moderately northdipping (55°) intracaldera fill (Morton et al., 1991; Morton et al., 1999; Hudak et al., in press) composed of greenschist to locally amphibolite facies (Trowell, 1974, 1983; Groves et al., 1988) metavolcanic, metasedimentary, and meta-intrusive rocks of Neoarchean age (Table 2). The eastern margin of the SLCC has been interpreted by Morton et al. (1991, 1999) to be the Lac David Fault. The western margin of the SLCC is poorly constrained by limited outcrop and diamond drilling, but based on the western limit of intracaldera strata, numerous dikes, and extensive hydrothermal alteration, is believed to be located beneath Biedelman Bay at the West Sturgeon Fault (Fig. 1). The caldera complex and associated ore deposits formed within an evolved, continental margin oceanic arc (Sanborn-Barrie et al., 2001; Galley, 2002) which contained magmas derived from back-arc basalts (Galley, 2003) that were not contaminated to any large extent by older 3.0 Ga Wabigoon Province continental crust (Bernier et al., 1999). Two coeval intrusive complexes intrude into the pre-caldera supracrustal volcanic strata in the Sturgeon Lake region, and have been extensively studied by Galley et al. (2000), Galley (2002), and Galley (2003). The Pike Lake Layered Complex (PLLC) consists of massive to crudely layered ferrogabbro, melanogabbro, and gabbro that, combined, are up to 10km in strike length and up to 2500m thick. The PLLC is intruded along its eastern margin by the Beidelman Bay Intrusive Complex (BBIC), which is approximately 20km in strike length and up to 2000m thick (Trowell, 1983; Galley et al., 2000). The BBIC is composed of at least six separate intrusive phases, from oldest to youngest including xenolithic tonalite, quartz-porphyritic pre-main phase dikes, main phase leucotonalite, post-main-phase leucotonalite dikes, quartz-plagioclase-phyric porphyritic dikes, and late phase quartz monzonite and granodiorite stocks (Galley, 2002; Galley, 2003). Wide variations in geochemical signatures and age dates for these intrusive rocks indicate a complex magmatic history encompassing at least three separate magma sources over a time span of up to 20 my (Table 2; Galley, 2002). The pre-main phase dikes, main phase leucotonalite, and post-main phase leucotonalite dikes are interpreted to be syncaldera magmatic phases based on similar U/Pb zircon Table 2. Summary of geochronology in the south Sturgeon Lake region (modified from King et al., 2000). Sample No. Source Rock Unit King et al., 2000 Rock Unit (this study) Date (Ma) PN76-13 Davis & Trowell, 1982 Galley et al., 2000: Galley, 2002 Post-caldera felsic volcaniclastic N/A 2717.9 JH82-2 Davis & Trowell, 1982 Davis et al., 1985 Swamp Lake rhyolite 2735.2 JH82-1 Davis et al., 1985 Swamp Lake rhyolite 2734.8 JH82-5 Davis et al., 1985 Lyon Lake Andesite and Rhyolite Lyon Lake Andesite and Rhyolite Lyon Lake Andesite and Rhyolite Not Reported Post-caldera felsic volcaniclastic Biedelman Bay quartz-plagioclase porphyritic dike Swamp Lake rhyolite 2735.0 JH82-4 Davis et al., 1985 Lyon Creek dacite lava Middle L tuff PN76-15 JH82-3 Davis et al., 1985 Davis et al., 1985 Middle L tuff Mattabi (?) tuff 2734.7* 2736.3 DD78-17 Davis & Trowell, 1982 Davis & Trowell, 1982 Galley, 2002 Pike Lake Complex 2732.7 Beidelman Bay biotite trondhjemite Beidelman Bay biotite leucotonalite 2733.8 96-GIA-328 PN76-14 DD78-18 Not Reported Lyon Creek Lava Dome Breccias Mattabi Ash Flow Lyon Creek Lava Dome Breccias Pike Lake Pluton Gabbro Beidelman Bay biotite trondhjemite N/A * Davis et al. (1985) note this is a minimum age. - 118 - 2720 2736.0 2735.5 2734.0 Analytical Error (Ma) + 2.7 - 1.5 +3.5 - 3.0 +1.8 -1.8 +6.9 -3.2 +2.8 -2.5 +1.7 -1.7 +3.0 -1.9 ±1.6 +9.3 -3.9 +3.6 -2.0 +1.4 -1.3 +3 -3 �Proceedings of the 61st ILSG Annual Meeting - Part 2 dates (2738.5-2732.5 Ma; Davis and Trowell, 1982; Davis et al., 1985; Galley, 2002) and trace element geochemical characteristics (Campbell et al., 1981; Galley et al., 2000; Galley, 2002) that correspond with SLCC intracaldera tuffs. These intrusive phases are believed, in part, to have provided thermal energy to drive regionally extensive hydrothermal systems which formed semi-conformable zones of sericite ± dolomite alteration that were subsequently cross-cut by diffuse, semi-conformable to disconformable alteration zones comprising iron carbonate (ferruginous dolomite, ankerite or siderite), iron-chlorite, chloritoid, and aluminum silicate (pyrophyllite, andalusite and/or kyanite) (Franklin et al., 1975; Campbell et al., 1981; Hudak, 1989; Jongewaard, 1989; Morton et al., 1991; Galley et al., 2000; Galley, 2002; Holk et al., 2002, Galley, 2003) that are genetically associated with VMS formation. Intrusive and hydrothermal breccias, and quartz-plagioclase-phyric dikes associated with porphyry-style Cu-Mo mineralization described by Poulsen and Franklin (1981) are approximately 15 my younger than the synvolcanic intrusive phases, indicating that porphyry mineralization in the south Sturgeon Lake area post-dates caldera-associated VMS mineralization (Galley et al., 2000; Galley, 2002). Three distinctive types of faults have been recognized in the south Sturgeon Lake area, as well as at numerous other VMS deposits (Gibson et al., 1999). These include: a) synvolcanic faults; b) post-volcanic highangle faults; and c) post-volcanic shear zones which may occur at a relatively low angles to the stratigraphy and may represent thrust faults. Synvolcanic faults are recognized using the following criteria: a) apparent offset of a layered unit, with subsequent units not offset; b) abrupt thickening or thinning of a volcanic, volcaniclastic, or sedimentary unit; c) rapid changes in alteration intensity or abrupt changes in alteration mineral assemblages; and d) the presence of apophyses or dikes associated with synvolcanic intrusions (Gibson et al., 1999; Hudak et al., 2003). Post-volcanic high angle faults have the following characteristics: a) offset of all stratigraphic units; b) abundance of brittle fracturing and/or zones of lost core; and c) abundance of quartz ± carbonate ± tourmaline veins filling fractures. Post-volcanic shear zones (thrust faults?) can be identified using a combination of the following criteria: a) intense shearing within adjacent units; b) displacement of older stratigraphic units into positions up-section from younger stratigraphic units; c) presence of “lamprophyre” and/or massive diorite dikes; and d) locally, the presence of graphite-rich breccia zones containing angular quartz-rich fragments (Hudak, 1996). Several episodes of faulting and folding have taken place in the south Sturgeon Lake region. Numerous synvolcanic fault zones were identified by Morton et al. (1991, 1999) based on abrupt thickness changes in volcanic strata, terminations of fault zones by overlying volcanic strata, and proximity to intense hydrothermal alteration and/or VMS mineralization. Two episodes of post-volcanic faulting are preserved in the south Sturgeon Lake area. The earliest event is a low angle shear zone (possibly a thrust fault) which juxtaposed caldera-associated strata and non-caldera associated strata in the eastern and northeastern parts of the south Sturgeon Lake area (Hudak, 1996: Morton et al., 1999). The fault zone interpretation is supported by a) the presence of highly strained rocks along the contact between the two sequences (Dube et al., 1989; Koopman, 1993; Hudak, 1996); b) the lack of consistent stratigraphy along a 20-50 m thick zone adjacent to the contact in the vicinities of the Sub-Creek Zone, Creek Zone, and Lyon Lake VMS orebodies (Hudak, 1996); c) abrupt terminations of alteration mineral assemblages across the contact (Hudak, 1996); and d) sharp oxygen isotopic gradients across the contact (Moss, 1992; Holk et al., 2002). This fault zone has been successfully geophysically imaged by Nedmović and West (2002). A subsequent faulting event formed a series of NNE-trending faults that offset the intracaldera supracrustal strata and the early low angle fault (Figure 1: Koopman, 1993; Hudak, 1996; Morton et al., 1999). The major fold event in the region resulted from a ca 2.7 Ga north-south compression event associated with the collision of Neoarchean rocks in the south Sturgeon Lake region and a Mesoarchean volcanic rift sequence to the north (Sanborn-Barrie and Skulski, 1999). The north-facing and north-dipping stratigraphic sequence in the south Sturgeon Lake assemblage represents the southern limb of an E-SE-trending, shallow eastplunging F1 fold axis located in the Post-Lake – Barge Lake region (Sanborn-Barrie et al., 1998). A major 116°-trending, 16°E-SE plunging fold documented by Dube et al. (1989) and Koopman (1993) near the Lyon Lake and Creek Zone orebodies is consistent with the orientation of this F1 syncline. Generalized Stratigraphy and Physical Volcanology of the SLCC Thirteen supracrustal stratigraphic successions have been grouped into four stratigraphic sequences based on their temporal and genetic relationships to caldera - 119 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 development, as well as their stratigraphic position relative to the Mattabi VMS orebody (after Hudak, 1996). Progressing stratigraphically up-section, these sequences are: a) the Pre-caldera Sequence (PCS); b) the Early Caldera Sequence (ECS); c) the Late Caldera Sequence (LCS); and d) the Lyon Lake Fault Sequence (LLFS). A major zone of structural deformation interpreted as either a shear zone (Koopman, 1993) or a thrust fault (Hudak, 1996) marks the contact between the PCS, ECS, and LCS in the south, and the LLFS to the north. Figures 2, 3, and 4 illustrate the geology in the western, central, and eastern parts of the SLCC, respectively. Figure 5 contains stratigraphic columns in various parts of the caldera complex. The PCS (Figs. 6 and 7) comprises a 200-2100m thick succession of subaerial and shallow subaqueous basalt lava flows, scoria-rich volcaniclastic deposits, and minor associated rhyolite lava flows (Groves et al., 1988; Morton et al., 1991; Morton et al., 1999). Pillow lavas and hyaloclastite are conspicuously absent in this sequence, except locally in the easternmost regions of the south Sturgeon Lake area (Jongewaard, 1989). Groves et al. (1988) and Morton et al. (1999) have interpreted the pre-caldera volcanic environment as a subaerial to shallow subaqueous shield volcano with local fields of scoria cones and tuff cones. The ECS (Figs. 6, 8, 9, and 10) contains a 650-1300m thick succession of volcanic and volcaniclastic strata. Up-section, these include: a) subaerially deposited ash fall tuff deposits (Jackpot Lake Succession); b) interstratified polymict breccias and subaerially and subaqueously deposited quartz-phyric lapilli tuff and tuff deposits (High Level Lake Succession); c) subaerial felsic lava flows, mafic-intermediate lapilli tuffs and volcaniclastic deposits (Bell River Succession); d) interstratified subaqueously deposited polymict breccias, volcanic sandstones and mudstones, and dacitic to andesitic lava flows and tuffs (Tailings Lake Succession); and e) subaqueous, massive to locally well-bedded, quartz-phyric lapilli tuff and tuff deposits (Mattabi Succession). Hudak (1996) interpreted this stratigraphic sequence as indicative of the early stages of caldera development (c.f. Smith and Bailey, 1968; Busby-Spera, 1984; Lipman, 1976; Lipman, 1997). The distribution of voluminous felsic volcaniclastic units and associated polymict breccias suggest that Figure 2. Geological map of the western one-third of the Sturgeon Lake Caldera Complex (modified after Morton et al., 1999). Lines A – A’ and B – B’ correlate to stratigraphic sections in Figure 5. - 120 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 3. Geological map of the central one-third of the Sturgeon Lake Caldera Complex (modified after Morton et al., 1999). Line C – C’ correlates to stratigraphic section in Figure 5. Figure 4. Geological map of the eastern one-third of the Sturgeon Lake Caldera Complex (modified after Morton et al., 1999). Lines D – D” and E-E” correlate to stratigraphic sections in Figure 5. - 121 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 5. Stratigraphic sections across the Sturgeon Lake Caldera Complex (Hudak et al., in prep.). Section is hung on top of High Level Lake Rhyodacite-Rhyolite Tuff/Lapilli-tuff unit. - 122 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 6. Immobile trace element Geochemical classification (after Winchester and Floyd, 1977) of least-altered volcanic and volcaniclastic rocks associated with the Sturgeon Lake Caldera Complex. Pre-caldera strata are illustrated in diagram A, early caldera strata are illustrated in diagrams B and C, Late-Caldera strata are illustrated in diagrams D and E, and Lyon Lake Fault Sequence strata are illustrated in diagram F. Data from Groves (1984), Hudak (1989, 1996), Jongewaard (1989), Walker (1993) and Franklin (unpublished data). - 123 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 7. Pre-caldera (Darkwater Succession) strata associated with the SLCC. A) rare pillow lavas near Lac David; B) massive basalt-andesite tuff with carbonate-altered scoria lapilli; C) photomicrograph of scoria in basalt -andesite tuff (field of view 8mm); D) fusiform- and spindle-shaped bombs in basalt-andesite tuff; E) cored bomb in basalt-andesite tuff; and F) flow-banding in rhyodacite – rhyolite lava flow. - 124 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 8. Strata from the Jackpot Lake and High Level Lake Successions. A) Jackpot Lake Succession lapilli tuff; B) photomicrograph of recrystallized ash matrix and pumice from Jackpot Lake Succession tuff (field of view 8mm); C) High Level Lake Succession polymict breccia (mesobreccia); D) High Level Lake Succession polymict breccia (megabreccia); E) pumice lapilli in massive High Level Lake Succession tuff south of the Mattabi orebody; F) interbedded High Level Lake Succession lapilli-tuffs and tuffs near the F-Group VMS deposit. - 125 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 9. Strata from the Bell River and Tailings Lake Successions. A) photomicrograph of amygdaloidal Bell River Succession rhyodacite-rhyolite lava flow; B) massive Bell River basalt lapilli tuff with carbonate- and quartz-altered scoria; C) polished drill core sample of Bell River basalt lapilli-tuff (scale bar = 2cm); D) Tailings Lake Succession polymict breccia from south of the Mattabi orebody. initial caldera development occurred simultaneously with the deposition of the High Level Lake Succession. A synvolcanic basin, bounded to the west and east by the Darkwater and Lac David Faults respectively (Fig. 1), appears to represent a nested caldera produced simultaneously with the eruption of the Mattabi tuffs (Hudak et al., 2003; Hudak et al., in prep). The LCS (Figs. 6, 11, 12, 13, and 14) comprises a 500-1500m thick succession of quartz- and quartz + feldspar-phyric tuff and lapilli tuff deposits, volcaniclastic sedimentary rocks, andesitic to dacitic lava flows, domes, and cryptodomes, and Algomatype banded iron formations (Koopman, 1993; Hudak, 1996). Hudak (1996) interpreted this sequence as characteristic of a maturing, late-stage caldera complex (c.f. Smith and Bailey, 1968; Lipman, 1997), and Hudak et al (in prep.) have utilized GIS analysis of the Sturgeon Lake geology that reflects relative percentages (by area and volume) of caldera collapse breccias, eruption-fed tuffs, lava flows, and sedimentary rocks that is consistent with this interpretation (Table 3). The most voluminous explosive eruptions in the LCS produced the Middle L lapilli tuffs and tuffs which are the host rocks for all the VMS orebodies in the LCS. Several intra-caldera intrusive rocks have been identified and include: a) the Beidelman Bay Intrusive Complex (BBIC), a multiphase, dominantly trondhjemitic intrusive complex that petrochemical and lithogeochemical data indicate was, in part, the subvolcanic intrusion associated with the SLCC; b) massive to amygdaloidal rhyolite which occurs as feeder dikes to the Bell River Succession rhyolite lava flows; c) coarse-grained massive feldspar-phyric diorite which occurs as feeder dikes to the Lyon Creek Succession lava domes and cryptodomes; and d) finegrained massive to amygdaloidal diorite and quartz - 126 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 10. Mattabi Succession strata: A) massive lapilli tuff (compass is 55cm wide); B) pumice block in massive Mattabi Succession lapilli tuff from immediately southwest of the Mattabi VMS orebody; C) laminated to thinly-bedded tuff and subeconomic VMS horizon north of the F-Group VMS orebody (scale bar divided into centimeters); D) drill core appearance of bedding replaced by sulphides (mainly pyrite; scale bar is 2cm). Table 3. Geographic information system (GIS) analysis of Pre-Caldera, Early Caldera, and Late Caldera sequences in the Sturgeon Lake Caldera Complex (Hudak et al., in prep.) Distribution of Lithology Types in the Sturgeon Lake Caldera Complex by Area % Lithology Type Pre-Caldera % Early Caldera % Late Caldera % Caldera Collapse Breccia 0.0 49.6 0.0 Lava Flows 94.1 8.6 41.0 Eruption-Fed Tuffs 4.8 41.0 12.3 Sedimentary Rocks 1.1 0.8 46.7 Distribution of Lithology Types in the Sturgeon Lake Caldera Complex by Volume %* Lithology Type Pre-Caldera % Early Caldera % Late Caldera % Caldera Collapse Breccia 0.0 57.6 0.0 Lava Flows 98.4 3.5 42.1 Eruption-Fed Tuffs 1.6 38.7 6.3 Sedimentary Rocks 0.1 0.2 51.6 *Volume estimates of intracaldera strata completed by assuming that the Sturgeon Lake Caldera Complex was a rcular caldera structure similar to Cenozoic subaerial ash flow calderas such as Valles (Smith and Bailey, 1968), Santorini (Druitt and Francaviglia, 1992), and Crater Lake (Bacon and Druitt, 1988), and submarine ash flow calderas such as Healy (Wright et al., 2003) and Myojin Knoll (Fiske et al., 2001). Similar assumptions have been used to estimate eruption volumes in other ancient volcanic sequences (Busby-Spera, 1984; Kokelaar and Busby, 1992). - 127 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 11. Lower L and Middle L strata within the SLCC: A) bedded Lower L rhyodacite-rhyolite tuff and lapilli tuff (lens cap is 55mm); B) Middle L Succession laminated to thinly bedded rhyo-dacite-rhyolite tuffs overlain by very thickly bedded lapilli tuff (scale bar divided into centimeters); C) massive Middle L tuff with 1-3 mm subhedral to euhedral quartz phenocrysts (scale bar is 2cm); D) normal graded Middle L tuff (up is to right of photo, scale bar is 2cm); E) outcrop appearance of Middle L rhyodacite-rhyolite tuff breccia (scale bar divided into centimeters); F) close-up view of Middle L rhyodacite-rhyolite tuff-breccia (note jigsaw puzzle-fit lapilli and bombs, scale bar divided into centimeters). - 128 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 12. Upper L and Bell River Lake succession strata: A) Upper L Succession rhyodacite-rhyolite tuff and lapilli tuff (scale bar divided into centimeters); B) Upper L Succession crystal-rich reworked tuff (large divisions on scale bar are centimeters); C) massive, crudely graded, reworked Upper L tuff (scale bar divided into centimeters); D) Bell River Lake Succession quartz- and plagioclase-phyric rhyodacite – rhyolite lava flow (compass at left for scale). - 129 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 13. No Name Lake succession strata: A) pillow lava with concentric cooling joints; B) pillow lava with multiple selvedges (large divisions on scale bar are centimeters); C) laminated tuff (resedimented hyaloclastite; D) amoeboid basalt andesite dike with surrounding peperite and resedimented hyaloclastite; E) close-up of relationship between basalt-andesite dike and surrounding peperite; F) close-up of close-packed blocky to irregular peperite. Compass in all photos is 55 mm wide. - 130 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 14. Lyon Creek Succession strata: A) vertical facies from margin (left) to center (right) of andesite-dacite cryptodome; B) andesite-dacite lithic tuff-breccia that occurs along margin of cryptodome; C) andesite-dacite lithic tuff breccia containing both fine- and coarse-grained feldspar phyric lava clasts; D) resedimented crystal-lithic tuff; E) laminated to very thinly bedded graphitic mudstone; F) folded magnetite-chert banded iron formation interbedded with chlorite-rich mudstones. Scale bar in photos 14A-14E is 2 centimeters. Scale bar in photo 14F is divided into centimeters. - 131 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 diorite which dilate the stratigraphy across the entire caldera complex and may have been a subvolcanic sill complex which fed the No Name Lake Sequence basaltic and andesitic lava flows (Morton et al., 1999). Post-caldera intrusive rocks also occur throughout the SLCC. These include: a) fine-grained, massive, locally feldspar-phyric diorite; b) coarse-grained amphibole-phyric diorite; and c) thin (<1-13m wide) “lamprophyre” dikes that occur within high strain zones associated with the structural deformation between caldera-associated strata and the non-caldera associated Lyon Lake Fault Sequence. Hydrothermal Alteration Assemblages in the SLCC Mineral Hydrothermal solutions, both ore-forming and nonore forming, have altered the volcanic and intrusive rocks associated with the Sturgeon Lake Caldera Complex. Following alteration, rocks in the region were metamorphosed to greenschist and locally amphibolite facies. Therefore, the alteration minerals now present are, in part, metamorphosed equivalents of alteration minerals formed during the synvolcanic hydrothermal alteration events. Hydrothermal alteration is widespread within the complex and in the uppermost parts of the Beidelman Bay Intrusive Complex (Franklin et al., 1975; Jongewaard, 1989; Hudak, 1989; Walker, 1993; Hudak, 1996; Galley et al., 2000; Galley, 2002; Galley, 2003). Discrete assemblages of metamorphosed alteration minerals form zones that are a) widespread and largely conformable to the volcanic stratigraphy (semiconformable alteration); and b) locally lens-shaped or pod-like beneath massive sulphide occurrences and deposits (semi-conformable alteration); and c) narrow and elongate that cross-cut stratigraphy and are proximal to synvolcanic structures or synvolcanic faults (disconformable alteration; Franklin et al., 1975; Groves, 1984; Morton and Franklin, 1987; Jongewaard, 1989; Hudak, 1989; Walker, 1993; Hudak, 1996; Gibson et al., 1999). Hydrothermal fluids formed five distinct, mapable massive sulphide ore-associated alteration mineral assemblages in the area (Fig. 15). From least- to most-altered, these assemblages are: a) widespread, semi-conformable carbonatization and silicification; b) widespread, semi-conformable iron carbonate ± iron-rich chlorite; c) widespread, semi-conformable chloritoid ± iron-carbonate and/or iron-rich chlorite; d) localized lens-shaped to pod-like, locally linear zones of aluminum silicate (pyrophyllite, andalusite, and/or kyanite) + chloritoid; and e) localized linear disconformable and semi-conformable, generally stratiform zones of aluminum silicate. Algoma-type iron formations associated with the LCS are associated with semi-conformable to disconformable veins, patches, and lenses of iron-carbonate + iron- rich chlorite + magnetite ± Mn-rich almandine garnet ± grunerite. Late sericite and/or magnesium-rich chlorite alteration locally overprints these five alteration mineral assemblages. Iron-carbonate ± iron-rich chlorite assemblage rocks contain at least 10% iron-carbonate + iron-rich chlorite with less than 5% chloritoid or aluminum silicate minerals. Outcrops containing this alteration assemblage can generally be easily identified by their orange to orange-brown stained, commonly pitted surfaces. Staining varies from irregular patches up to 15cm in diameter, to veins and veinlets 1-15mm in width that are aligned parallel to the foliation. Pumice/ scoria that has been replaced by iron-carbonate can be recognized as rounded to oval, orange-brown stained pits up to 5cm in diameter which commonly contain rounded- to lens-shaped quartz amygdules (10-60%). In thin section, this assemblage contains iron-carbonate ± iron-rich chlorite (10-60%), sericite (up to 30%), magnesium-rich chlorite (up to 50%), and locally, traces of chloritoid and/or aluminum silicate minerals. Chloritoid ± iron carbonate and/or iron-rich chlorite assemblage rocks contain greater than 5% chloritoid, and are characterized by the presence of 1-3mm dark green to greenish-black chloritoid prisms and rosettes. The presence of chloritoid commonly gives the rocks a “salt and pepper” appearance. Locally, chloritoid porphyroblasts occur with sericite (10-55%) in 1-5mm wide grey green veins that vary from semi-conformable to disconformable in orientation. Other minerals present include iron-carbonate (up to 60%), ironrich chlorite (2-20%, locally as a retrograde product of chloritoid), magnesium-rich chlorite (1-20%) and pyrite (up to 12%). Aluminum silicate + chloritoid bearing rocks typically contain 1-3mm chloritoid porphyroblasts (up to 33%) in a grey to grey-pink matrix composed of massive pyrophyllite (5-20%), 1-3mm blocky pink andalusite (up to 10%), and/or blue tabular to bladed kyanite porphyroblasts (up to 8%). Chloritoid occurs as 1-3mm prisms or rosettes disseminated throughout the rock or in chloritoid + andalusite veins up to 1cm in width. In addition, iron-rich chlorite (up to 7%), - 132 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 15. Field and drill core hydrothermal alteration mineral assemblages in the SLCC. A) Iron-carbonate assemblage in the F-Group area; B) chloritoid + chlorite alteration assemblage from Area 17; C) aluminum silicate alteration (kyanite + sulfides) from footwall to the Mattabi VMS deposit; D) patchy aluminum silicate (andalusite) alteration proximal to VMS mineralization in Area 17; E) iron-formation associated alteration (iron-chlorite + iron carbonate + chloritoid + magnetite) in Area 23; F) vein of iron formation-associated alteration (iron carbonate + magnetite + iron chlorite) in Area 23. - 133 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 magnesium-rich chlorite (up to 35%), iron carbonate (3-35%), pyrite (up to 30%) and sphalerite (up to 5%) are also associated with this alteration assemblage. Figure 16 illustrates the distribution of the VMS ore-associated alteration mineral assemblages in the vicinity of the F-Group orebody. Iron carbonate ± iron-rich chlorite assemblage rocks form a large, semiconformable zone along the southern, eastern, and northern boundaries of the area. Locally, pod-like regions (up to 50m in diameter) of this assemblage occur within the chloritoid assemblage zone, which exists as a large, semi-conformable zone up to 100m thick in the hanging wall to the F-Group orebody, and is present as pipe-like alteration in the F-Group deposits footwall. More proximal to mineralization, semi-conformable and locally linear pipe-like zones containing the aluminum silicate ± chloritoid alteration assemblage are present. Aluminum silicate assemblage rocks are most closely associated with the mineralization at the F-Group deposit. These rocks are distributed in two distinct patterns: a) in disconconformable, pipe-like linear, 5-30m wide zones that trend NNE in proximity to synvolcanic fault zones; and b) in a broad semiconformable zone (700m by 500m at surface) located both in the footwall and the hanging wall rocks to the F-Group orebody. Figure 17 illustrates the distribution of the ore- associated alteration at the Mattabi deposit. Semiconformable iron-carbonate (ankerite- and/or sideritebearing) and silicified rocks form a broad zone in the lower footwall rocks. This zone is cross-cut by several westward-dipping tabular zones which contain an aluminum silicate-rich core and aluminum silicate ± chloritoid margin. These zones occur in close proximity to synvolcanic fault zones which lead upward to, and cross-cut, a broad semi-conformable zone of aluminum silicate + chloritoid altered rocks that crudely surrounds the Mattabi orebodies. The aluminum silicate-rich tabular zones, associated with the synvolcanic fault zones, spread out into a semiconformable alteration zone directly stratigraphically below and lateral to the deposit. Stratigraphically overlying the deposit is a semi-conformable zone of chloritoid ± iron-rich chlorite ± iron-carbonate up to 150m thick (Walker, 1993). Figure 18 illustrates the distribution of iron formation-associated alteration mineral assemblages that occur in Areas 17 and 23 southwest of the Lyon Lake and Creek Zone VMS ore deposits. It is important to keep in mind that based on stratigraphic and structural data, the iron formation-associated alteration assemblage has no genetic relationships to the Lyon Lake and Creek Zone deposits, as these deposits are believed to have been structurally remobilized into Figure 16. Distribution of hydrothermal alteration mineral assemblages associated with the F-Group VMS orebody. - 134 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Figure 17. Distribution of hydrothermal alteration mineral assemblages associated with the Mattabi VMS orebody. Figure 18. Distribution of hydrothermal alteration mineral assemblages associated with the Algoma-type iron formation and VMS mineralization in Areas 17 and 23. - 135 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 erupt simultaneously with caldera collapse that generates the High Level Lake Succession polymict breccia deposits (megabreccia and mesobreccia). Facies analyses suggest initial trap-door like caldera collapse where the floor of the caldera complex collapses into a subaqueous environment west of a synvolcanic fault located approximately at the boundary between the F-Group claims and Area 15 (refer back to Fig. 5). The eastern two-thirds of the caldera complex, from the Darkwater Fault to the Lac David Fault, remains in a subaerial depositional environment. their present locations (Koopman, 1993; Hudak, 1996; Morton et al., 1999). Summary of the Geologic History of the SLCC The following summary represents the temporal sequence of geological, volcanological, and oreforming processes that occurred during the genesis of the Sturgeon Lake Caldera Complex. Pre-Caldera Geological History 1. Subaerial basalt shield volcanism (Darkwater Succession basalt-andesite lava flows) with minor effusive felsic volcanic activity (Darkwater Succession rhyodacite-rhyolite lava flows and associated flow breccias). 2. Intrusion of spherulitic rhyolite dikes in southern Area 16 - Area 17 which feed the eruption of Bell River Succession dacite-rhyolite lava dome in a subaerial environment in the eastern two-thirds 2. Subaerial-shallow subaqueous scoria cone and tuff cone genesis (Darkwater Succession basalt-andesite tuffs, lapilli tuffs, and tuff-breccias) as the end stage in the development of a shield volcano – tuff-cone – scoria-cone complex. 3. Initial intrusion of the ancestral Beidelman Bay Intrusive Complex and the Pike Lake Layered Complex as a high-level magma chamber. This causes regional tumescence which results in the formation of pre-caldera structures that evolve into ring fractures. 4. Subaerial explosive felsic volcanism, probably erupting through vents proximal to pre-caldera structures caused by extension related to regional tumescence, deposits the Jackpot Lake Succession rhyodacite and rhyolite tuffs. This eruption can be interpreted as minor, pre-caldera explosive activity which commonly precedes caldera formation in the Smith and Bailey (1968) caldera cycle model. Further development of pre-caldera faults continues during this time. Early Caldera Geological History 1. Formation of the ~25km strike length Sturgeon Lake Caldera Complex occurs as a result of voluminous subaerial explosive volcanism (at least 16km3, possibly on the order of ~500-900km3 based on observed caldera diameter and eruption volume observation (Cas and Wright, 1987; Hudak et al., 2003; Fig. 19) coupled with simultaneous caldera collapse. Explosive volcanism produces the High Level Lake rhyodacite-rhyolite lapilli tuffs which Figure 19. Comparison of estimated eruption volumes and caldera diameters for modern subaerial and subaqueous caldera eruptions, and the caldera-forming eruptions associated with the SLCC (Hudak et al., 2003, modified from Cas and Wright, 1987). Point A represents relationships for the initial SLCC event which resulted from the combined eruptions of the Jackpot Lake and High Level Lake succession rhyodacite-rhyolite lapilli tuffs and tuffs. Point B represents relationships for the Mattabi eruptive event, which formed the Mattabi Succession rhyodacite-rhyolite lapilli tuffs and tuffs, and a nested caldera between the Darkwater and Lac David faults. Point C represents the Middle L eruptive event, which formed the Middle L rhyodacite-rhyolite breccias, tuff-breccias, lapilli tuffs and tuffs, and possible two nested calderas of 1.5km and 3.1km in diameter (equivalent to a single caldera of ~3.4km diameter) in the eastern and western parts of the SLCC in the vicinities of the F-Group deposit and Areas 17 and 23. - 136 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 of the SLCC. Post caldera-collapse submarine hydrothermal activity is centered in the F-Group and Darkwater regions of the SLCC, and forms the F-Group VMS orebodies. Based on observations of modern VMS mineralization in terms of tectonic setting, metal contents, physical volcanology and hydrothermal alteration mineral assemblages (Gibson et al., 1999; Morgan and Schulz, 2012; Monecke et al., 2014), it is likely that these VMS orebodies were formed as replacement-style VMS deposits (c.f. Doyle and Allen, 2003) at water depths less than 1500 meters (Hudak, 1996); Subaerial hydrothermal activity occurs in the eastern twothirds of the caldera complex, and forms subaerial epithermal-like stringer mineralization in the High Level Lake lapilli tuffs and tuffs, as well as the Bell River Succession lava dome. 3. Eruption and deposition of the Bell River Succession basalt lapillistones and lapill tuffs from Area 16Area 17 in the vicinity of the Bell River lava dome. These deposits pond to the west along the Darkwater fault, which marks the western limit of a paleo-basin that extends eastward to the Lac David Fault. 4. Intracaldera clastic sedimentation forms the Bell River Succession laminated to thickly bedded polymict tuffs and tuffs. 5. Continued subsidence of the basin between the Darkwater and Lac David faults changes the depositional environment from a subaerial to a shallow subaqueous setting. Scalloping of the basin margins leads to deposition of the Tailings Lake Succession massive to stratified polymict tuffs and lapilli tuffs. Periodic effusive eruption of andesitic (Tailings Lake andesite lava flows) and small volume explosive eruptions of dacite-rhyolite (Tailings Lake dacite-rhyolite tuff) also occurs. 6. High temperature hydrothermal activity forms the Mattabi “E” ore lens within the Tailings Lake dacite-rhyolite tuff. 7. Pulsating, voluminous (~30km3: Hudak et al., 2003) subaqueous explosive eruptions form hot (?) highand low-concentration eruption fed density currents within the basin between the Darkwater and Lac David faults, and cool high- and low-concentration, eruption-fed density currents as outflow sheets in the western one-third of the SLCC (Mattabi bedded quartz-phyric rhyodacite-rhyolite lapilli tuffs and tuffs). A nested caldera is formed during these eruptions between the Darkwater and Lac David faults, enabling the submarine environment to subside to depths in which VMS may form. 8. Regional low temperature submarine hydrothermal activity alters the volcaniclastic strata to regional sericite and carbonate alteration assemblages. This regional alteration is responsible for semiconformable zone of Na-depletion that extends for 25km along strike within the Sturgeon Lake camp. The regional carbonate present is generally dolomite. Post-eruptive, high temperature synvolcanic hydrothermal activity in the vicinity of synvolcanic structures forms the Mattabi “B”, “C”, and “D” lenses, primarily as sheet-like replacement VMS deposits (c.f. Doyle and Allen, 2003) at water depths <1500 meters. Hydrothermal alteration associated with this mineralization leaches alkali elements and locally adds iron and manganese to form the premetamorphic precursors to the iron carbonate + iron-rich chlorite assemblage, the chloritoid ± ironrich chlorite assemblage, the aluminum silicate + chloritoid assemblage, and the aluminum silicate assemblage. Regional dolomite reacts with the hydrothermal solutions to form ferrodolomite, ankerite, and siderite as one approaches VMS mineralization (Franklin et al., 1975). Downwelling of cool seawater deposits magnesium and potassium to form late magnesium-rich chlorite and sericite veins. 9. Subaqueous deposition and post hydrothermal reworking of Mattabi Succession massive rhyodaciterhyolite tuffs. Continued regional submarine hydrothermal alteration produces regional semiconformable alteration zones dominated by sericite. Late Caldera Geological History 1. Subaqueous eruption, deposition, and post-eruptive reworking of the Lower L Succession rhyodaciterhyolite tuffs and lapilli tuffs. 2. Continued erosion of caldera walls, subaqueous reworking of intracaldera deposits, and low temperature hydrothermal activity generates the Lower L Succession interbedded graphitic mudstones, tuffs, and lapilli tuffs). 3. Initial eruptions of No Name Lake Succession basalt-andesite lava flows east of the Bell River, and Bell River Lake basalt-andesite lava flows adjacent to the Darkwater fault. 4. Eruption of rhyodacite-rhyolite lava dome, and formation of VMS deposits on the dome, approximately 2km west of the present location of - 137 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 the Sturgeon Lake VMS deposit (indirect evidence from rhyodacitic to rhyolitic lava lapilli and blocks in the Middle L breccia and tuff-breccia deposits). 5. Subaqueous disintegration of rhyodaciterhyolite lava dome and associated VMS deposits, simultaneously with subaqueous rhyodaciticrhyolitic plinean (phreato-plinean?) volcanism form high- and low-concentration eruption fed mass flows which deposit Middle L Succession rhyodacite breccia, tuff-breccia, lapilli tuffs, and tuffs within a nested caldera that has a well-defined western margin near the Bell River, and a poorly defined eastern margin (the location of the eastern margin is no longer able to be recognized at surface due to the structural deformation associated with the Lyon Lake Fault Zone). Simultaneous submarine plinean eruptions appear to have occurred north of the F-Group VMS orebody, and deposited relatively thick sequences of quartz-phyric rhyodacite-rhyolite lapilli tuffs and tuffs in a localized synvolcanic basin (Middle L Succession rhyodacite-rhyolite breccia, tuff-breccia, lapilli tuff, and tuff). 6. Post-eruptive high temperature hydrothermal activity forms the Mattabi “A” ore lens, the Sturgeon Lake VMS deposit, and subeconomic VMS deposits hosted in the Middle L Succession lapilli tuffs and tuffs in Area 17 and near the border between the F-Group claims and Area 15. Regional low temperature submarine alteration of the rocks yields semi-conformable sericite-rich and carbonate-rich alteration zones. 7. Intracaldera low temperature hydrothermal activity, sedimentation and reworking of intracaldera volcanic and volcaniclastic rocks (Upper L Succession interbedded graphitic mudstones, tuffs, lapilli tuffs). 8. Eruption, subaqueous deposition, and subaqueous reworking of plagioclase- + quartz-phyric volcaniclastic rocks (Upper L rhyodacite-rhyolite tuffs and lapilli tuffs). 9. Post-eruptive synvolcanic hydrothermal activity forms subeconomic VMS deposits in the Upper L Succession lapilli tuffs and tuffs in the eastern onethird of the SLCC. 10. Continued intracaldera sedimentation and submarine reworking of intracaldera deposits (Upper L Succession interbedded graphitic mudstones, tuffs, and lapilli tuffs). 11. Intermittent subaqueous mafic-intermediate and felsic volcanism (No Name Lake Succession and Bell River Lake Succession basalt-andesite lava flows, and Bell River Lake Succession rhyodaciterhyolite lava flows). 12.Continued intracaldera sedimentation and reworking of intracaldera deposits (Upper L Succession interbedded graphitic mudstones, tuffs, lapilli tuffs). 13. Intrusion of plagioclase-phyric diorite-dacite dikes and sills, and the formation of the Lyon Creek Succession andesite-dacite lava flows, lava domes and cryptodome. Subsequent erosion of flows and domes yields sediment which lithifies into Lyon Creek Succession feldspathic tuffs, lapilli tuffs, and tuff breccias. 14. Development of a localized basin within the eastern part of the Lyon Creek Succession dome/cryptodome complex, and the deposition of volcaniclastic sediments with the basin (Lyon Creek Succession feldspathic tuffs, lapilli tuffs, and tuff breccias). 15. Low temperature hydrothermal activity within the Lyon Creek basin forms Lyon Creek Succession Algoma-type iron formation, chert, and graphiterich mudstones. 16. Continued intracaldera sedimentation (Lyon Creek Succession feldspathic tuffs, lapilli tuffs, and tuff breccias). Post-caldera Geological History 1. Regional compression, and formation of the Lyon Lake Fault Zone, possibly as a low-angle thrust fault; shearing of the Sturgeon Lake VMS orebody, remobilization of sulphides, and emplacement of Lyon Lake, Creek Zone, Sub-Creek Zone VMS orebodies within the high strain zone associated with the Lyon Lake Fault 2. 2.7Ga regional north-south compression yields north-facing homoclinal sequence, regional greenschist and lower amphibolite facies regional metamorphism. Field Trip Stops This field trip is designed to be completed in one to two days, depending upon the time spent, and the depth of discussions, at each field trip stop location. The field trip stops have been numbered so that the participants observe rocks from the base toward the top of stratigraphic sequence (older rocks to younger rocks) throughout the excursion. In addition to moving - 138 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 more or less up-section, the field trip will successively investigate volcanic rocks, hydrothermal alteration, and mineral deposits in the western, central, and eastern parts of the SLCC. N.B. All field trip stops are located on GlencoreCanada Property; therefore, permission must be obtained before attempting access to the field trip stops. The first day of the field trip will concentrate on rocks that comprise the subvolcanic Beidelman Bay Intrusive Complex, the Darkwater Succession (pre-caldera rocks upon which the Sturgeon Lake Caldera Complex was developed), and the High Level Lake, Tailings Lake, and Mattabi Successions which comprise the Early Caldera Sequence, as well as host the majority of the VMS mineralization with the Sturgeon Lake camp. The second day of the field trip will focus on rocks that comprise the Late Caldera Sequence, including variably (and locally intensely) altered Middle L Succession tuffs and tuff-breccias (which host the VMS deposits at the Sturgeon Lake Mine about a kilometer east-southeast of our field trip stops), coherent dacites that comprise the Lyon Creek Succession cryptodome, and exceptionally well-preserved Neoarchean pillow lavas, volcaniclastic sediments, and locally, peperite deposits that make up the No Name Lake Succession. Please note, there are no co-ordinates available for the stops described within this field trip guide. UTM Co-ordinates; however, will be supplied to participants at the beginning of the field trip. Day 1 – Synvolcanic Intrusion, Precaldera Rocks, and Early Caldera Rocks Associated with the Sturgeon Lake Caldera Complex The Beidelman Bay Intrusive Complex Stop 1: Beidelman Bay Intrusive Complex Two coeval intrusive complexes intrude the precaldera supracrustal strata in the Sturgeon Lake region (refer to Fig. 1), and have been extensively studied by Galley et al. (2000), Galley (2002) and Galley (2003). The Pike Lake Layered Complex (PLLC) is composed of massive to crudely layered ferrogabbro, melanogabbro, and gabbro, which combined have a strike length of approximately 10km and a composite thickness of approximately 2500m. The PLLC is intruded along its eastern margin by the Beidelman Bay Intrusive Complex (BBIC), which has a strike length of approximately 20km and a composite thickness of approximately 2000m (Trowell, 1983; Galley et al., 2000). The BBIC is composed of at least six separate intrusive phases. From oldest to youngest, these include xenolithic tonalite, quartz-porphyritic premain phase dikes, main phase leucotonalite, post-main phase leucotonalite dikes, quartz- and plagioclasephyric dikes, and late-phase quartz monzonite and granodiorite stocks (Galley, 2002, 2003).Wide variations in geochemical signatures and age dates for these intrusive phases indicate a complex magmatic history encompassing at least three separate magma sources over a time span of up to 20 million years (refer to Table 2). At this field trip stop, we will investigate the product of the middle intrusive event, the main phase biotite leucotonalite of the BBIC. According to Galley (2002), the main phase biotite leucotonalite makes up the bulk of the BBIC. It comprises a massive, mediumgrained hypidiomorphic to seriate-textured rock that displays few internal variations in texture or grain size. Galley (2002) notes that this phase of the BBIC has lithogeochemical characteristics that matches favourably with those of the High Level Lake, Mattabi, and Middle L pyroclastic rocks within the SLCC, in agreement with earlier interpretations (Davis and Trowell, 1982; Trowell, 1983) that the BBIC was the subvolcanic magma chamber that erupted to form the intracaldera felsic pyroclastic rocks within the SLCC. Precaldera Strata – The Darkwater Basalts and Darkwater Rhyolites Stop 2: Darkwater Basalts and Darkwater Rhyolites The Precaldera Sequence extends along strike across the entire south Sturgeon Lake region, and is composed of a 200-2100m thick succession of subaerial and shallow submarine basalt and andesite lava flows (Darkwater basalt-andesite lava flows), volcaniclastic deposits (Darkwater basalt-andesite tuffs and lapillituffs), and minor rhyodacite-rhyolite lava flows (Darkwater rhyodacite-rhyolite lavas; Figs. 2, 3 and 4). At this field trip location, we will have the opportunity to investigate Darkwater basalt-andesite lava flows as well as Darkwater rhyodacite-rhyolite lavas. The Darkwater basalt-andesite lava flows have been studied in detail by Groves (1984) and Groves et al. (1988). These lava flows comprise a 200-1800m thick sequence of aphyric to plagioclase-phyric, massive to amygdaloidal (2-25% amygdules) basalt – andesite lava flows. Groves (1984) and Groves et al. (1988) indicate that individual lava flows vary in - 139 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 stratigraphic thickness from 3-40m, and are commonly characterized by billowy flow tops containing 45-70% amygdules and locally flow marginal autobreccias that contain 40-70% poorly sorted, massive to scoriaceous basalt-andesite lapilli and blocks. It is only on the far eastern end of the south Sturgeon Lake region that pillow lavas, sheet flows, and associated hyaloclastite deposits have been recognized (Jongewaard, 1989), suggesting that these pre-caldera strata were formed largely in a subaerial environment. At this location we will investigate the physical characteristics of Darkwater Succession basalt-andesite lava flows. Moving to the north, we will also investigate aphyric to quartz-plagioclase-phyric, typically massive but locally spherulitic and/or flow banded rhyodaciterhyolite lava flows that occur within the Darkwater Succession. Individual lava flows are up to 80m thick, and are separated by clast-supported volcanic breccias that have been interpreted by Groves (1984) and Groves et al. (1988) to be autobreccias. Similar felsic lava flows with stratigraphic thicknesses up to 10m locally overlie thin basalt-andesite lava flows in the eastern part of the south Sturgeon Lake region near Lac David (Jongewaard, 1989). The F-Group VMS deposit was discovered from airborne and ground geophysical surveys combined with exploration diamond drilling conducted by Mattagami Lake Mines in 1969. The orebody was mined by open pit methods from 1981-1984, and produced approximately 377,564 short tons of ore which contained 0.64% Cu, 9.51% Zn, 0.64% Pb, and 60.4g/ton Ag (M. Patterson, personal communication, 1990; Franklin, 1996). Stop 3: High Level Lake Tuffs and Polymict Breccias (Mesobreccia) F-Group Region Intracaldera Strata At this location (Fig. 20), approximately 75m southeast of the F-Group pit, one can observe the contact between massive to sparsely graded polymict breccia deposits (the High Level Lake Succession polymict breccias) and overlying rhyolitic massive lapilli tuffs and tuffs (the High Level Lake Succession Tuffs). The green to green grey polymict breccia contain a finegrained matrix compose of chlorite, sericite, and locally iron-rich carbonate (ferrodolomite and/or ankerite). Lapilli- to small block-sized clasts vary in abundance from 30-70%, and include: a) easily recognizable, light grey to pale white angular to subangular, locally flowbanded rhyolite lava flow fragments which have been In the F-Group and Darkwater regions, participants will observe Early- and Late Caldera Succession strata which illustrate the three stratigraphic horizons which host VMS orebodies in the SLCC. These stops will include: a) interbedded polymict breccias (High Level Lake Succession polymict breccias) and rhyolite tuffs and lapilli tuffs (High Level Lake Succession rhyodacite-rhyolite lapilli tuffs and tuffs) that formed during simultaneous explosive felsic eruptions and foundering of the region above the shallow ancestral Beidelman Bay intrusion during formation of the Sturgeon Lake Caldera Complex; b) extreme aluminum silicate alteration within the High Level Lake Succession breccias and tuffs that occur down-section and along strike from the F-Group VMS deposit; and c) well-bedded quartz-phyric lapilli tuffs and tuffs that comprise the Mattabi VMS ore horizon (including a small replacement-type sphalerite-rich VMS deposit) in the western part of the caldera complex, and the hanging wall rocks to the Mattabi VMS deposit in this part of the caldera complex, the Middle L Succession rhyodacite-rhyolite lapilli tuffs and tuffs. The Middle L Succession lapilli tuffs and tuffs are the host rocks for the Sturgeon Lake VMS deposit in the eastern part of the SLCC. Figure 20. Geological sketch map of the F-Group area, with field trip stop locations (from Morton et al., 1996). - 140 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 derived from the underlying Darkwater Succession rhyodacite to rhyolite lava flows; b) light grey to pale white, angular to subangular, locally silicified rhyolite tuff lapilli (presumably derived from the underlying Jackpot Lake Succession tuffs); c) green, commonly difficult to recognize, angular to rounded amygdaloidal basalt lapilli (derived from the underlying Darkwater Succession basalt-andesite lava flows); and d) green, commonly difficult to recognize, generally rounded to subrounded, locally pale-brown iron carbonate altered scoria lapilli (derived from the underlying Darkwater Succession andesite tuffs, lapilli tuffs, and tuff breccias). As one moves from the southwestern part of the outcrop toward the northeast, the abundance of lapilli and blocks within the breccia deposits decreases, and the abundance of <1mm quartz phenocrysts increases to 1-2% over a zone which varies from 1-5m in width. This zone represents the gradational contact between the High Level Lake mesobreccia deposits and the overlying and intercalated High Level Lake quartzphyric lapilli tuffs and tuffs. This gradational contact formed during mixing of the two units as they were deposited simultaneously (Hudak, 1989). Similar intercalations of felsic pyroclastic deposits and polymict breccia deposits have been documented in calderas within the San Juan Mountains in Colorado by Lipman (1976), and have been interpreted to represent deposits which result from simultaneous explosive volcanism and caldera collapse. Continuing to move to the north on the outcrop, the gradational contact zone gives way to massive deposits of the High Level Lake Succession lapilli tuffs and tuffs. The light grey quartz- and sericite-rich, recrystallized ash matrix of these deposits contains 5-25% euhedral to subhedral, locally resorbed 1mm quartz phenocrysts and up to several percent, typically difficult to see, subangular to subrounded pumice lapilli. Irregular patches and lenses of red-brown iron carbonate alteration vary from 1-10cm in length, and locally comprise 10-15% of the outcrop. In thin section, many of the altered 1-3cm carbonate patches appear to be altered pumice lapilli. The polymict breccia deposits at this outcrop, and numerous other outcrops in the Sturgeon Lake camp, are interpreted to be mesobreccias and megabreccias. Mesobreccias and megabreccias form from material that slumps off oversteepened walls of a caldera during and after caldera collapse. By definition (Lipman, 1976; Lipman, 1997; Lipman, 2000), megabreccia deposits contain blocks which are dominantly greater than 1m in diameter, whereas mesobreccia deposits contain lapilli and blocks which are dominantly less than 1m in diameter. Megabreccia deposits are generally formed proximal to caldera walls. Mesobreccia and megabreccia deposits which occur in the footwalls to the Mattabi and Sturgeon Lake Mine orebodies are stratigraphically equivalent to the polymict breccia deposits which are observed at this location. Stop 4: Altered High Level Lake Polymict Breccias, Lapilli Tuffs, and Tuffs (optional) Intensely aluminum silicate- and aluminum silicate + chloritoid-altered, intercalated High Level Lake Succession breccias and rhyolite lapilli tuffs / tuffs occur at this location, approximately 300m southwest of the F-Group pit (Fig. 20). Although difficult to recognize, these rocks comprise the same stratigraphic units exposed at the first field trip stop. Here, the High Level Lake Succession polymict breccia deposits vary from green to grey-green to pale pinkish-grey, and contain up to 50% lapilli and blocks. The fragments are of three principal types: 1) 5% rounded, 3-10mm in diameter, intensely amygdaloidal quartz- and sericite-rich fragments which petrographic observations indicate are altered pumice; b) 5-10% light grey lapilli- to small block-sized felsic lava fragments (Darkwater Succession rhyodacite/ rhyolite lava flows); and c) up to 30% subround to oval, 3-10mm diameter chlorite-rich amygdaloidal fragments which petrographic observations suggest are scoria and amygdaloidal basalt. The matrix of this unit is generally composed of magnesium-rich chlorite, quartz, and sericite, but locally, where pale pinkish grey, andalusite and/or pyrophyllite also occur. High Level Lake Succession rhyolite lapilli tuffs and tuffs overlie, and are locally intercalated with, the High Level Lake polymict breccia deposits in this area. The felsic lapilli tuffs and tuffs contain 1-20% 1mm diameter euhedral to subhedral quartz phenocrysts in a recrystallized ash matrix, and vary in colour from greygreen to greyish-pink depending upon the alteration mineral assemblage present. Grey-green regions contain an alteration assemblage of quartz, sericite, and locally magnesium- and/or iron-rich chlorite. Pinkish-grey regions are composed of an alteration mineral assemblage composed dominantly of quartz and up to 1mm diameter ragged anhedral to blocky subhedral andalusite. Light grey, subrounded to oval pumice lapilli vary from 3-20mm in diameter, and - 141 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 are composed of recrystallized quartz. Petrographic observations indicate that the pumice lapilli contain 30-50% <1mm round quartz-filled vesicles. Pervasive aluminum silicate alteration, greenschist facies metamorphism, and subsequent retrograde processes have led to the development of three different aluminum silicate phases within this group of outcrops. Andalusite is the most common aluminum silicate phase present (5-50%), and occurs as 1-6mm ragged anhedral to equant subhedral pink porphyroblasts. Thin section examinations indicate that the ragged andalusite porphyroblasts have locally undergone retrograde reactions and now contain inclusions and rims composed of sericite and/or pyrophyllite. Kyanite is present in two distinctive forms: a) as ragged tabular porphyroblasts up to 5mm in length within the altered matrix of the tuffs; and b) as pale blue blades ranging from 3-20mm in length within white to reddish-brown quartz-iron carbonate veins that are up to several centimeters in width. Pyrophyllite can commonly be found along the margins of both the andalusite and kyanite porphyroblasts, and may also occur in veins up to several centimeters in width as soft, pale greenishwhite radiating micaceous aggregates. Pyrophyllite is commonly present where quartz veins intersect kyaniterich veins. Minor amounts of chloritoid (generally <5% but locally up to 10%) are locally associated with these aluminum silicate minerals. This series of outcrops is interpreted to be proximal to, and in part, within, a synvolcanic fault zone. These faults provided cross-stratal channel ways in which high temperature, acidic metalliferous hydrothermal fluids traveled upward through the subseafloor to the seafloor. The aluminum silicate alteration has been shown via mass balance analyses (Jongewaard, 1989; Hudak, 1989; Hudak, 1996) to have developed as the acidic fluids leached cations from the rocks (for example, during the alteration of feldspar or volcanic glass), leaving them rich in aluminum and silica (and presumable with a clay-rich pre-metamorphic alteration mineral assemblage). Andalusite and kyanite certainly formed during the greenschist facies metamorphism of the strata; however, studies of both modern and ancient hydrothermal alteration assemblages (White and Hedenquist, 1990; White and Hedenquist, 1995) associated with epithermal mineral deposits indicate that andalusite may also form as a primary, high temperature alteration mineral phase. Pyrophyllite may have formed as a primary mineral as well, but based on textural evidence, appears primarily to be due to retrograde metamorphism of kyanite and andalusite. Stop 5: The F-Group Trench Excavated in 1989, the F-Group reclamation trench was designed to channel runoff waters from the F-Group waste dump into the F-Group pit (Fig. 20). Four different lithologies are exposed in this trench: 1) a dark grey to green, locally amygdaloidal gabbro to quartz-diorite sill-like synvolcanic intrusion; 2) light grey to pink, aphyric to locally quartz-phyric, bedded to massive Mattabi Succession lapilli tuffs and tuffs; 3) semi-massive to massive, replacement-type lenses of pyrite ± sphalerite which occur at the equivalent stratigraphic horizon to the “B”-lens of the Mattabi VMS orebody; and 4) grey to grey-green, bedded to massive, quartz- ± feldspar-phyric lapilli tuff and tuff deposits of the Middle L Succession. Several alteration mineral assemblages can also be recognized at this locality. These include: a) iron carbonate ± iron chlorite assemblage; b) the chloritoid assemblage; c) the chloritoid + aluminum silicate ± sericite assemblage; d) the aluminum silicate ± sericite assemblage; and e) locally, silicification. Note the cross-cutting relationships of the various alteration assemblages which can be observed by close examination of the trench wall rocks. Structurally, two different generations of faults have been identified by Walker and Hudak (1989) within the F-Group trench. Synvolcanic structures, which locally led to minor differences in the thicknesses of the volcanic units within the trench, produced cross-stratal permeability that focused upwelling metalliferous fluids in pathways to the paleoseafloor. Metasomatism resulting from interactions between the volcaniclastic strata and these synvolcanic metalliferous fluids produced the extensive hydrothermal alteration in the area. Cooling and neutralization of these fluids by cooler seawater or lower temperature hydrothermal fluids within the tuffs within the shallow seafloor led to the development of replacement-style (Doyle and Allen, 2003) massive sulphide occurrence within the Mattabi tuffs. Locally, strongly sheared, commonly sericite-rich, east-northeast trending high strain zones represent post-volcanic structural deformation which is believed to be related to splays off the northeasttrending Sturgeon Narrows Shear Zone (Figure 1) that is located to the north and west of this location beneath Sturgeon Lake. Stop 6: Bedded and Graded High Level Lake Mesobreccia Deposits (optional) - 142 - As indicated previously, it appears that the initial �Proceedings of the 61st ILSG Annual Meeting - Part 2 development of the Sturgeon Lake Caldera Complex involved trap-door like caldera collapse in which the most significant displacement occurred in the western part of the caldera. Here, in the western one-third of the caldera complex (refer to Fig. 1), we can observe planar thin- to thick-bedded, matrix-supported, normal and locally reverse-graded polymict lapilli-tuffs and tuffs. The finer-grained deposits on this group of outcrops display Ta, Tb, and Te-type bedding units (c.f. Bouma, 1962), whereas coarser deposits display R2R3 and S1-S3 bedding units (c.f. Lowe, 1982). Mattabi Region (Area 16) Intracaldera Strata In the vicinity of the Mattabi VMS deposit, field trip participants will observe: a) the High Level Lake Succession polymict breccias and rhyolite lapilli tuffs and tuffs in the central part of the SLCS: b) thick, massive, pumice-rich, quartz-phyric lapilli tuffs and tuffs that comprise the immediate footwall and host strata to the Mattabi VMS deposit (Mattabi Succession lapilli tuffs and tuffs); and c) hanging wall basalt to andesite pillow lavas, pillow breccias, and locally, bedded volcaniclastic sedimentary strata and peperite (the No Name Lake Succession lava flows and associated interflow sedimentary strata). Previous field trips to the Sturgeon Lake area also included a stop at an exceptional exposure of well-bedded polymict breccia deposits and associated normal graded volcaniclastic sandstones and mudstones which comprise the Tailings Lake Succession sedimentary strata; however, during reclamation, this exposure, which occurs immediately east of the former Mattabi headframe, was buried by approximately 3-5m of fill. Due to the importance of this exposure in terms of understanding the development of the SLCC, we have chosen to include its field trip description although we will not be able to observe the outcrop. The Mattabi VMS deposit was discovered by Mattagami Lake Mines in 1969 from follow-up drilling of airborne geophysical anomalies. The orebodies were mined by open pit and underground mining methods, and comprised five stratiform lenses of massive sulphide ore separated by stringer base-metal mineralization or barren host rock which occur in three distinct stratigraphic successions (Tailings Lake, Mattabi, and Middle L successions, respectively). Massive sulphide lenses which cropped out and extended to approximately 250m below the surface were mined via open pit methods between 1972 and 1980. The deeper VMS deposits were mine using underground mining methods until reserves were depleted in 1988. Combined, the five lenses comprising the Mattabi VMS deposit produced approximately 12.55 million tons of VMS ore grading 8.28% Zn, 0.74% Cu, 0.85% Pb, and 104g/ton Ag (M. Patterson, personal communication, 1990; Franklin, 1996). Stop 7: Mattabi Footwall - High Level Lake Polymict Breccias and Rhyolite Lapilli Tuffs/Tuffs This series of outcrops is located approximately 500m stratigraphically below the lowermost lens of the Mattabi VMS orebody. The southern portion of these outcrops comprises coarse polymict breccias which are interpreted to be mesobreccia deposits formed during caldera collapse. The far northeastern outcrops comprise quartz-phyric rhyolite tuffs which overlie, and are intercalated with, the polymict breccias (the field relationship seen previously in the vicinity of the F-Group orebody). Figures 21 and 22 illustrate the locations of the outcrops described below. Stop 7a: Polymict Breccia Deposits (High Level Lake Succession Mesobreccia) These outcrops comprise coarse, polymict, breccia that contains up to 50% 1-25cm light-coloured subangular felsic lithic fragments (Darkwater Succession rhyolite lava flow lapilli), up to 10% lapillito block-sized pumice fragments, which commonly have silicified rims, and <5% amygdaloidal mafic lapilli, which are up to 5cm in diameter (Darkwater Succession amygdaloidal basalt lapilli). Petrographic observations indicate that the matrix is composed of a mixture of fine-grained recrystallized quartz (40%), chloritoid (12%), magnesium-rich chlorite (30%), white mica (sericite ± pyrophyllite, 15%), and opaque minerals (pyrite and/or magnetite, 3%). The northeastern section of the outcrop contains a bussized felsic block (>10m in diameter) with similar composition to the smaller felsic lapilli which occur throughout the breccia deposits (Walker, 1993). Stop 7b: High Level Lake Succession Tuffs High Level Lake Succession tuffs at this location are light tan to grey in colour and contain 2-5% 0.5-1.5mm diameter subhedral to euhedral, locally resorbed quartz phenocrysts. In thin section, only “ghosts” of 2-10mm diameter pumice lapilli can be observed within a matrix composed of recrystallized, inequigranular polygonal quartz (up to 65%), fine-grained sericite (25%), magnesium-rich chlorite (6%), 3% pyrite, and 1% magnetite and/or ilmenite. Further to the - 143 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 east, the unit contains a coarse, fragmental texture. Here, the rock contains 60-70% lapilli- to blocksized subangular to angular felsic lithic fragments in a felsic matrix. The texture has been interpreted by Walker (1993) to represent deposits formed by postdepositional slumpage of partially consolidated High Level Lake Succession tuffs. Only minor variations in the alteration mineralogy can be found between the matrix and the fragments present at this location. Stop 7c: Chloritoid–Aluminum Silicate-Chlorite Altered High Level Lake Mesobreccia This exposure is composed of High Level Lake polymict breccia deposits which contain 20-30% rounded, 2-30cm, highly amygdaloidal scoria lapilli and blocks, as well as up to 10% 1-15cm subangular felsic lapilli and blocks which are similar in composition to those observed in outcrop M-1a. In thin section, the matrix comprises 30% quartz, 20% magnesium-rich chlorite, 10% iron carbonate, 15% sericite ± pyrophyllite, 10% chloritoid, up to 10% ragged andalusite, and 5% opaque minerals. Chlorite alteration increases toward the center of the outcrop where massive chlorite veining up to 50cm in width occurs. Figure 21. Geological plan map of the Mattabi area, with field trip stop locations (after Walker, 1993; Morton et al., 1996). The chloritic veining at this location is interpreted to have resulted from recharge of cool, fresh seawater into a hot hydrothermal system. This appears to have resulted in magnesium-dumping and subsequent chlorite alteration (Walker, 1993; also see Seyfried et al., 1999). The quartz-filled tension fractures which Figure 22. Geological sketch map of the High Level Lake Succession outcrops south of the Mattabi VMS orebody (after Walker, 1993; Morton et al., 1996). - 144 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 occur at this location may be the result of volume changes due to hydrothermal alteration. Kyanite can locally be found in these fracture-filling veins on the southern part of the outcrop. A 10-20cm wide band of silicified rock, striking North-South, bisects the outcrop. Samples from this silicified rock contain 50% quartz, 15% sericite ± pyrophyllite, 8% iron-rich carbonate, 7% magnesium-rich chlorite, 2% andalusite, and 2% opaque minerals. This band of more intense alteration may represent a conduit responsible for upward, hightemperature, acidic metalliferous fluid movement which was later overprinted by lower temperature, more neutral hydrothermal fluids that produced sericite and chlorite alteration. Stop 7d: Chloritoid – Aluminum Silicate - Fecarbonate Altered Mesobreccia This series of outcrops illustrates a definable, confined and symmetrically zoned increase in alteration intensity within a synvolcanic fault zone in the High Level Lake polymict breccia unit. Original volcanic textures within the breccia unit within this synvolcanic structure are strongly overprinted, but still recognizable when carefully inspected. In particular, one can still relatively easily recognize the abundance of relatively unaltered felsic lithic lapilli. Alteration in the small outcrops on the road and immediately north of the road consist of 1-4cm clots of iron-carbonaterich material surrounded by anastomosing veinlets of quartz, chloritoid, and andalusite. The dominant change in rock mineralogy from the previous outcrop (Stop 7c) is an increase in the amount of Fe-carbonate and chloritoid. Stop 8 (No Longer Available): Mattabi Footwall Tailings Lake Succession Bedded Sediments Note: Reclamation in the vicinity of the former Mattabi Mine headframe has unfortunately resulted in the burying of the classic outcrop of the Tailings Lake Succession. Although this outcrop can no longer be observed, I have included its description below, as the field relationships and textures observed in this former outcrop were extremely important for the development of the Sturgeon Lake Caldera Complex volcanological model described by Morton et al. (2001), Morton et al. (1999), Walker (1993) and Hudak et al. (2003). The Tailings Lake Succession consists primarily of highly variable polymict breccias, sandstones, and mudstones, with minor intercalated felsic tuff horizons and intermediate to mafic lava flows. The polymict breccias vary in their clast composition, clast abundance, and bedding characteristics. Three clast types are most common: 1) mafic lithic clasts, which are commonly replaced by chlorite and/or ironcarbonate (up to 50%); 2) fined-grained cherty felsic lithic lapilli (up to 30%); and 3) rounded pumice lapilli (up to 20%). Bedding is uncommon, but where present, is usually defined by sorting of the clasts, as well as changes in the compositions of the clasts. The Tailings Lake polymict breccia deposits are lithogeochemically indistinguishable from the High Level Lake polymict breccia deposits, and suggests their provenance is similar (e.g., from infilling of a basin by clastic material derived primarily from Pre-caldera strata). Many of the characteristics of the Tailings Lake polymict breccias are conspicuous in the exposure located beside the Mattabi Mine ventilation shaft. This exposure is atypical in its cross-sectional view, and because of the presence of well-defined bedding within the breccia unit. Here, the unit contains 5-30% 2-30mm diameter mafic lapilli (which contain 5-30% oval to rounded iron-carbonate- and quartz-filled amygdules or weathering pits), as well as 2-30mm felsic lithic or pumice lapilli (5-30%). In thin section, the matrix comprises quartz (35%), iron-carbonate(20%), and andalusite (10%), as well as late patches and veins of magnesium-rich chlorite (20%) and sericite/ pyrophyllite (10%). Bedding in the unit is defined by various abundances of mafic and felsic fragments. It appears mafic fragments are generally normal graded and felsic pumice lapilli are reverse graded. Such reverse grading of pumice clasts is commonly attributed to the slow settling of cold, highly vesicular fragments as water infiltrates vesicles in an aqueous environment (Whitham and Sparks, 1986). Bedding trends approximately 100° and dips 60-70° to the north. Stop 9: Mattabi Footwall – Aluminum SilicateAltered Mattabi Lapilli Tuffs and Tuffs This large outcrop on the southwestern side of the Mattabi open pit consists of moderately- to stronglyhydrothermally altered, massive Mattabi Succession quartz-phyric rhyolite lapilli tuffs and tuffs (Fig. 21). This exposure contains 10-50% subrounded to rounded juvenile felsic lapilli, 5-15% 5-30cm (although locally, up to 70-80cm diameter) delicate amoeboid to subrounded pumice, and 2-7% 0.5-1.5mm diameter subhedral to euhedral quartz phenocrysts. The recrystallized altered ash matrix consists of finegrained polygonal quartz (40-50%), chloritoid (1030%), sericite ± pyrophyllite (15-35%, after andalusite), - 145 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 andalusite (up to 5%), and opaque minerals (generally pyrite, 1-5%). Lithic fragments consist of quartz ± sericite ± pyrophyllite and the pumice are replaced, and locally contain amygdules with, quartz ± sulphide minerals. Bounding facies, as well as the presence of massive sulphide mineralization, indicate that these strata were deposited in a subaqueous environment. To the north (up-section), this unit grades into a series of bedded and locally normal graded aphyric tuffs and quartz-phyric tuffs. Petrographic studies indicate the presence of heat retention structures (lithophysae) and spherulites (indicative of the original glassy nature of the deposits) within the massive lapilli tuffs at this location (Walker, 1993: Hudak et al., 2003). White (2000) has shown that massive lapilli tuff deposits, which contain delicate, amoeboid-shaped pumice as well as heat retention textures, which are overlain by bedded tuffs of similar composition represent primary, hot, eruption-fed high concentration mass flows which are essentially true submarine pyroclastic flow deposits (e.g. originally deposited from collapse of hot, gas charged flows of juvenile volcanic material). Hudak et al. (2003) have, based on textures at this outcrop as well as in several diamond drill core intersections, shown that the Mattabi tuffs were, at least locally in the vicinity of the Mattabi VMS orebody, deposited as submarine pyroclastic flows from voluminous submarine explosive rhyolitic volcanism. End of Day 1 of Sturgeon Lake Field Trip Day 2 – The Late Caldera Sequence Lyon Lake and Sturgeon Lake Mine Regions (Areas 17 and 23) In this part of the caldera complex, we will observe two important units within the Late Caldera Sequence: 1) the Middle L Succession tuff breccias, lapilli tuffs, and tuffs (which are the host rocks to the Sturgeon Lake VMS deposit approximately 1.5km to the east of the field trip stop location); and 2) the Lyon Creek Succession dacite cryptodome–lava dome complex, which are interpreted to comprise the final igneous products associated with the genesis of the Sturgeon Lake Caldera Complex. Four VMS orebodies occur in the eastern part of the SLCC: 1) the Sturgeon Lake Mine, a 2.07 million ton ore deposit mined by open pit methods which contained 2.95% Cu, 9.17% Zn, 1.21% Pb, and 164g/ ton Ag; 2) the Lyon Lake deposit, a 3.95 million ton ore deposit mined by underground methods which contained 1.24% Cu, 6.53% Zn, 0.63% Pb, and 142g/ ton Ag; 3) the Creek Zone deposit; mined via open pit methods; and 4) the Sub-Creek Zone deposit, mined by underground methods. Combine, the Creek Zone and Sub-Creek Zone deposits contained 0.91 million tons of ore which graded 1.66% Cu, 8.80% Zn, 0.76% Pb, and 141g/ton Ag (Franklin, 1996). The genesis of these four VMS deposits has historically been controversial in the Sturgeon Lake Camp. The Lyon Lake, Creek Zone, and Sub-Creek Zone deposits were originally known as the NBU ore deposits. Harvey and Hinzer (1981) interpreted abrupt facies changes, coarser volcaniclastic rocks, increased alteration intensity, and a greater MnO/FeO ratio in the Sturgeon Lake, Creek Zone, and Lyon Lake deposits to indicate formation along the same stratigraphic horizon at various distances from high temperature hydrothermal vents. Harvey and Hinzer (1981) suggested that these VMS orebodies occurred stratigraphically up-section from the Mattabi and F-Group VMS deposits. Severin (1981) postulated that the Sturgeon Lake deposit occurred on the same stratigraphic horizon as the Mattabi orebody, and that the Lyon Lake, Creek Zone, and Sub-Creek Zone deposits formed in topographic lows from hydrothermal activity which post-dated the formation of the Sturgeon Lake deposit. More recent detailed mapping (Dube et al., 1989: Koopman, 1993: Hudak, 1996; Morton et al., 1999), petrographic studies, and lithogeochemical evaluations now indicate that the Sturgeon Lake deposit is cut by a major postvolcanic fault zone, and that the Lyon Lake, Creek Zone, and Sub-Creek Zone deposits represent parts of the Sturgeon Lake deposit which were moved into their present locations by this structural deformation. Stop 10. Aluminum Silicate Altered Quartz-Phyric Middle L Tuffs The Middle L Succession comprises a sequence up to 150m thick composed of quartz-phyric rhyolite tuff breccias, lapilli tuffs, and tuffs which can be followed along strike for at least 15km across the SLCC. VMS orebodies occur within this sequence of rocks at both the Mattabi (Mattabi ore lens “A”) and Sturgeon Lake Mine deposits. In addition, anomalous Cu and Zn concentrations occur in the Middle L tuffs under Sturgeon Lake near the F-Group – Area 15 property boundary. - 146 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 This small outcrop (Fig. 23), located immediately south of a drill road which runs approximately eastwest through Area 17, is one of the few exposures of the Middle L Succession in the eastern part of the caldera complex. At this location, the matrix varies from pale grey to pinkish-grey in colour and contains 1-2% <12mm subhedral to euhedral quartz phenocrysts. Pumice and felsic lithic lapilli are not obvious on this exposure, but occur in minor amounts (1-2%) in thin sections and from diamond drill core intersections of the unit. The colour of this outcrop is largely due to the presence of 5-15% 1-3mm blocky subhedral andalusite and finegrained quartz. Immediately down-dip and up-section from this exposure, a thin (1m thick) massive sulphide horizon occurs within bedded ash deposits in diamond drill hole 17-64. Due to a lack of exposure, it is difficult to determine whether the aluminum silicate alteration at this location represents disconformable alteration within a synvolcanic structure, or semi-conformable aluminum silicate alteration along the stratigraphic horizon which hosts the Sturgeon Lake deposit to the east. Stop 11: Middle L Succession Tuff Breccia This stop is located immediately north of the Area 17 drill road approximately 30m north of the previous stop (Fig. 23). The spectacular volcanic breccia (tuffbreccia) is composed of a chlorite-, sericite-, iron carbonate-, and locally biotite-altered matrix which contains 30-60% subangular to angular, 1-35cm light grey felsic lava lapilli and blocks which contain 1-5% 1mm quartz phenocrysts, as well as rare massive sulphide lapilli and iron-carbonate-altered pumice lapilli. The felsic lava flow fragments are composed of spherulitic quartz- and K-spar-phyric rhyolite lava, are vaguely reverse graded, commonly have fine-grained (apparently chilled) rims, and often exhibit jigsaw puzzle-fit with adjacent fragments. The tuff-breccia deposits can be followed several hundred meters down dip in numerous diamond drill holes within Area 17. Bounding facies indicate that these deposits were formed in a submarine environment. Hudak (1996) and Hudak et al. (2003) believe that these tuff-breccia deposits resulted from the collapse of a Middle L Succession submarine lava dome. This lava dome was likely located near a synvolcanic fault within a few hundred meters of this location. This tuffbreccia essentially represents deposits from block and ash flows that occurred in a submarine environment (Gibson et al., 1999). Massive sulphide lapilli within the tuff-breccias suggest that massive sulphides were being deposited on or within the lava dome prior to its collapse. Stop 12: Lyon Creek Succession Cryptodome The Lyon Creek Succession is composed of the youngest strata clearly associated with the development of the Sturgeon Lake Caldera Complex. As indicated above, these andesitic to dacitic lava flows, lava domes, cryptodomes and associated clastic and chemical sedimentary strata have been interpreted by Hudak (1996), Morton et al. (1999), and Hudak et al. (2003) to represent lava dome building and associated intracaldera clastic and hydrothermal sedimentation associated with the terminal stages of caldera development within a Valles-like caldera cycle (Smith and Bailey, 1968). Figure 23. Geological plan map in Areas 17 and 23 (after Walker, 1993; Morton et al., 1996). This stop (Fig. 23), located near the top of a small hill immediately west of the Lyon Lake mine road, is one of a handful of small exposures of the Lyon Creek dacite cryptodome. At this location we observe light grey massive plagioclase-phyric dacite lava which comprises the central part (and most - 147 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 coarsely porphyritic) of the cryptodome. Hudak et al. (in prep.) have shown a systematic increase in the maximum size of plagioclase phenocrysts from the margins toward the center of the cryptodome. Tan to orange-brown, locally carbonate-altered tabular plagioclase phenocrysts (10-15%) generally vary from 1-3mm in diameter. Locally, one may observe plagioclase phenocrysts as large as 6mm in diameter. Hydrothermal alteration at this location comprises anastomosing 2-10mm wide veins containing ironcarbonate, chlorite, magnetite, and locally, burgundyred Mn-rich almandine garnets. Locally, 1-2mm tabular deep green chloritoid porphyroblasts occur in the cryptodome immediately adjacent to the veins, and suggest hydrothermal processes which included the chemical breakdown of feldspar combined with iron metasomatism to form chloritoid. Alteration in this outcrop is genetically associated with the formation of Algoma-type banded iron formation which occurs upsection on the northeastern margin of the cryptodome– dome complex. matrix consist of 0.5-2cm angular andesite lapilli within a blocky, fine- to coarse-ash-sized matrix composed of delicately preserved hyaloclastite with convex fragment edges. Interflow sedimentary strata are finely bedded, and are locally intruded by amoeboid to relatively straight, locally discontinuous amygdaloidal andesite dikes which are up to 50cm in width. Look for blocky peperite where the wet sediments interacted with magma along the edges of the dikes. Stop 13: No Name Lake Andesite and Interflow Sediments and Peperite At this, the final stop of our field trip (Fig. 21), we will observe “classic” pillow lavas associated with the No Name Lake Succession. At this location, the extremely well preserved, moderately- to highly amygdaloidal “bun-” and “mattress-”shaped pillows (nomenclature of Dimroth et al., 1978) illustrate exception concentric cooling cracks and locally, what may be multiple pillow selvedges. Stratigraphic topping directions for these pillow lavas are consistently to the north. The tannishgreen color of these submarine lava flows is indicative of moderate intensity iron carbonate ± iron chlorite alteration that has been observed petrographically. The No Name Lake Succession comprises basaltic to andesitic sheet flows, pillow lavas, pillow breccias, interflow sedimentary rocks, and locally, peperites. Only the uppermost section of this succession is exposed at this location (Fig. 21); the lower sections can be observed only in diamond drill core and appear to consist primarily of thick, amygdaloidal sheet flows and pillow lavas. The first outcrop (behind the core racks) consists of thin (30-70cm thick) sheet flows with 5-25% oval-shaped carbonate-filled amygdules (2-30mm in diameter) that are generally aligned parallel to strike and the dominant east-west-trending rock foliation. In thin section, these rocks are composed of 20-30% fine laths of plagioclase and 15% quartz in a secondary groundmass composed of chlorite (40%), biotite (10%), and epidote (2%). Pillow breccia, hyaloclastite, interflow sedimentary strata, and peperite are exposed in the outcrop immediately east of the water tower. These rocks consist of approximately 25% amygdaloidal pillow lapilli and blocks (5-50cm) in a matrix comprising hyaloclastite. The pillow breccia fragments contain 10-20%, 1-4cm diameter carbonate-filled amygdules (these are commonly weathered-out to form small pits on the outcrop surface). The hyaloclastite portion of the Pillowed andesite flows are exposed in the outcrop north of the water tower. This rock consists of wellformed 1-4m long amygdaloidal pillows with 25-30% 1-2mm carbonate-filled amygdules. The amygdules illustrate a bimodal size distribution; most amygdules are 1-4mm, with another distinct group being larger and up to 30mm in diameter. Massive pillow selvedges vary from 5-15cm thick. Such thick selvedges may indicate proximity to an eruptive vent (Kennish and Lutz, 1998; Hudak et al., 2002). Stop 14: No Name Lake Andesite Pillow Lavas End of Day 2 of Sturgeon Lake Field Trip Acknowledgements The author would like to thank the many people and organizations that have made his research in the Sturgeon Lake region possible, and have made this field trip a reality. First and foremost, my mentors Ron Morton and Jim Franklin gave me numerous opportunities during my graduate and post-doctoral research to map, analyze, interpret, reinterpret, and publish many of our findings during our 20+ years of research in the Sturgeon Lake area. As well, Ron and Jim, along with the University of Minnesota Duluth and the Geological Survey of Canada, provided me not only research funding, but found ways to fund my travels to many key geological locations and - 148 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 important geological conferences that enhanced my ability to understand the geological environment and mineralization associated with the Sturgeon Lake region. I benefitted immensely from my years of research with my Sturgeon Lake research colleagues Jamie Walker and Peter Jongewaard. As well, Dean Peterson played a major role in developing the GIS studies that helped to further understand the evolution of the SLCC. Having the ability to toss around and develop ideas with these three exceptional field-oriented economic geologists was instrumental in developing many of the key concepts needed to understand the volcanological, hydrothermal, and mineralization processes that took place within the Sturgeon Lake Caldera Complex. As well, funding from Noranda Exploration, Mattabi Mines Ltd., Rio Algom, and Minnova during the late 1980’s and early 1990’s was instrumental in being able to conduct the extensive field mapping programs, petrographic research, and geochemical studies necessary to evaluate this spectacular volcanic feature and its associated mineral deposits. Key industry personnel that contributed greatly to further understanding the SLCC included Wally Gibb (Mattabi Mines, Ltd.), Mike Patterson (Mattabi Mines, Ltd.), Al Smith (Noranda Exploration) and Ron Kennedy (Mattabi Mines, Ltd.). Lucy Potter and Aaron MacDonell (both with Glencore Canada Corporation) are thanked for their key roles in allowing access to the Sturgeon Lake VMS camp for this field trip. Finally, I am humbled that the Institute on Lake Superior Geology requested that I lead this field trip to this exceptional geological area. References Bacon, C.R. and Druitt, T.H. 1988. Compositional evaluation of the zone calc-alkaline magma chamber of Mount Mazama, Crater Lake, Oregon: Contributions to Mineralogy and Petrology, v.98, p.224-256. Bernier, F., Stevenson, R.K., Gariepy, C., and Franklin, J.M. 1999. Nd isotopic studies in the south Sturgeon Lake Greenstone Belt, northwestern Ontario: a progress report: Fifth Annual Workshop, Lithoprobe Western Superior Transect, Canada, p.117-121. Bouma, A.H.. 1962. Sedimentology of some Flysch Deposits: A Graphic Approach to Facies Interpretation: Elsevier, Amsterdam, 198p. Busby-Spera, C.J. 1984. Large volume rhyolitic ash flow eruptions and submarine caldera collapse in the lower Mesozoic Sierra Nevada, California. J. Geophysical Research, v.89, p.8417-8427. Campbell, I.H., Franklin, J.M., Gorton, M.P., Hart, T.R., and Scott, S.D. 1981. The role of subvolcanic sills in the generation of massive sulphide deposits: Economic Geology, v.76, p.2248-2253. Cas, R.A.F. and Wright, J.V. 1987. Volcanic Successions: Modern and Ancient. Allen and Unwin, London. Davis, D.W. and Trowell, N.F. 1982. U-Pb zircon ages from the eastern Savant Lake-Crow Lake metavolcanicmetasedimentary belt, northwestern Ontario. Canadian Journal of Earth Sciences, v.19, p.868-877. Davis, D.W., Krogh, T.E., Hinzer, J., and Nakamura, E. 1985. Zircon dating of polycyclic volcanic at Sturgeon Lake and implications for base metal mineralization. Economic Geology, v.80, p.1942-1952. Dimroth, E., Cousineau, P., Leduc, M., and Sanschagrin, Y. 1978. Structure and organization of Archean subaqueous lava flows, Rouyn-Noranda area, Quebec, Canada: Canadian Journal of Earth Sciences, v.15, p.902-918. Doyle, M.G. and Allen, R.L. 2003. Subsea-floor replacement in volcanic-hosted massive sulphide deposits: Ore Geology Reviews, v.23, p.183-222. Druitt, T.H. and Francaviglia, V. 1991. Caldera formation on Santorini and the physiography of the islands in the late Bronze Age: Bulletin of Volcanology, v.54, p.484-493. Dube, B., Koopman, E.R., Franklin, J.M., Poulsen, K.H., and Patterson, M.R. 1989. Preliminary study of the stratigraphic and structural controls of the Lyon Lake massive sulphide deposit, Wabigoon Subprovince, northwest Ontario: in Current Research, Part C, Geological Survey of Canada, Paper 89-1C, p.275284. Fiske, R.S., Naka, J., Iizasa, K., Yuasa, M., and Klaus, A. 2001. Submarine silicic caldera at the front of the Izu-Bonin Arc, Japan: voluminous seafloor eruptions of rhyolite pumice: Geological Society of America Bulletin, v.113, p.813-824. Franklin, J.M. 1996. Volcanic-associated massive sulphide base metals; in Eckstrand, O.R., Sinclair, W.D., Thorpe, R.I. (eds.), Geological Survey Canada, Geology of Canada 8, p.158-183. Franklin, J.M., Kasarda, J., and Poulsen, K.H. 1975. Petrology and chemistry of the alteration zone of the Mattabi massive sulphide deposit: Economic Geology, v.70, p.63-79. Galley, A. 2002. Characteristics of composite subvolcanic intrusive complexes associated with Precambrian VMS districts: in Balley, A., Bailes, A., Hannington, M., Holk, G., Katsube, J., Paquette, F., Paradis, S., Santaguida, F, and Taylor, B., Database for Camiro Project 94E07: Interrelationships between subvolcanic intrusions, large-scale alteration zones, and VMS deposits: Geological Survey of Canada Open File Report 4431, p.1-40. Galley, A. 2003. Composite synvolcanic intrusions associated with Precambrian VMS-related hydrothermal systems: Mineralium Deposita, v.38, p.443-473. - 149 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Galley, A., van Breemen, O., and Franklin, J. M. 2000. The relationship between intrusion-hosted Cu-Mo mineralization and the VMS deposits of the Archean Sturgeon Lake mining camp, northwestern Ontario; Economic Geology, v.95, p.1543-1550. Gibson, H.L., Morton, R.L., and Hudak, G.J. 1999. Submarine volcanic processes, deposits and environments favourable for the location of volcanicassociated massive sulphide deposits: Rev. Economic Geology, v.8, p.13-51. Groves, D.A. 1984. Stratigraphy and alteration of the footwall volcanic rocks beneath the Archean Mattabi massive sulphide deposit, Sturgeon Lake, northwestern Ontario: unpublished M. Sc. Thesis, University of Minnesota – Duluth, 115 p. Groves, D.A., Morton, R.L., and Franklin, J.M. 1988. Physical volcanology of the footwall rocks near the Mattabi massive sulphide deposit, Sturgeon Lake, Ontario; Canadian Journal of Earth Sciences, v.25, p.280-291. Harvey, J.D. and Hinzer, J.B. 1981. Geology of the Lyon Lake ore deposits, Noranda Mines Limited, Sturgeon Lake area, Ontario: Canadian Institute of Mining and Metallurgy Bulletin 74, p.77-84. Holk, G., Taylor, B., Galley, A., Hannington, M., and Timbal, A. 2002. Geochemical and alteration studies of the Sturgeon Lake Caldera Complex: in Balley, A., Bailes, A., Hannington, M., Holk, G., Katsube, J., Paquette, F., Paradis, S., Santaguida, F, and Taylor, B., Database for Camiro Project 94E07: Interrelationships between subvolcanic intrusions, large-scale alteration zones, and VMS deposits; Geological Survey of Canada Open File Report 4431, p.333-369. Hudak, G.J. 1989. The physical volcanology and hydrothermal alteration associated with the F-Group Archean massive sulphide deposit, Sturgeon Lake, northwestern Ontario: unpublished M. Sc. Thesis, University of Minnesota – Duluth, Duluth, Minnesota, 172p. Hudak, G.J. 1996. The physical volcanology and hydrothermal alteration associated with late caldera volcanic and volcaniclastic rocks and volcanogenic massive sulphide deposits in the Sturgeon Lake region of northwestern Ontario: unpublished Ph. D. Dissertation, University of Minnesota, Minneapolis, Minnesota, 463p. Hudak, G.J., Heine, J.J., Newkirk, T.T., Odette, J., and Hauck, S. 2002. Comparative geology, stratigraphy, and lithogeochemistry of the Five Mile Lake, Quartz Hill, and Skeleton Lake VMS occurrences, Vermilion District, NE Minnesota: Natural Resources Research Institute Open File Report NRRI/TR-2002/03, 350p. Hudak, G.J., Morton, R.L., Franklin, J.M., and Peterson, D.M. 2003. Morphology, distribution, and estimated eruption volumes for intracaldera tuffs associated with volcanic-hosted massive sulphide deposits in the Archean Sturgeon Lake Caldera Complex, northwestern Ontario: American Geophysical Union Geophysical Monograph 140 (Explosive Subaqueous Volcanism), p.345-360. Hudak, G.J., Morton, R.L., Franklin, J.M., Peterson, D.M., Walker, J.S., Jongewaard, P.K., and Murphy, C. in prep. Lithostratigraphy and volcanic evolution of the Archean Sturgeon Lake Caldera Complex. Hudak, G., Morton, R., and Peterson, D. 2008. Field guide to the volcanology, structure, alteration, and mineralization of Archean greenstone belts in the vicinities of Sturgeon Lake and Rainy River, Ontario, and Lake Vermilion, Minnesota: Short Course and Field Investigation of Physical Volcanology, Structure, and Hydrothermal Alteration associated with VMS and Lode Gold Deposits in Archean Greenstone Belts: Precambrian Research Center Guidebook 08-01, 209p. Jongewaard, P.K. 1989. Physical volcanology and hydrothermal alteration of the footwall rocks to the Archean Sturgeon Lake massive sulphide deposit: unpublished M. Sc. Thesis, University of Minnesota – Duluth, Duluth, Minnesota, 141p. Kennish, M.J. and Lutz, R.A. 1998. Morphology and distribution of lava flows on mid-ocean ridges: a review: Earth-Science Reviews, v.43, p.63-90. Kokelaar, P. and Busby, C. 1992. Subaqueous explosive eruptions and welding of pyroclastic deposits: Science, v.257, p.196-201. King, E.M., Valley, J.W., and Davis, D.W. 2000. Oxygen isotope evolution of volcanic rocks at the Sturgeon Lake volcanic complex, Ontario: Canadian Journal of Earth Sciences, v.37, p.39-50. Koopman, E.R. 1993. Stratigraphy, structural geology, and stratigraphic controls of ore distribution of the Lyon Lake massive sulphide deposit, Sturgeon Lake, Ontario: unpublished M. Sc. Thesis, Carleton University, Ottawa, Ontario, 170 p. Lipman, P.W. 1976. Caldera collapse breccias in the western San Juan Mountains, Colorado. Geol. Soc. Am. Bull. 87, p.1397-1410. Lipman, P.W. 1997. Subsidence of ash-flow calderas: relation to caldera size and magma chamber geometry. Bull. Volcanol. 59, p.198-218. Lipman, P.W. 2000. Calderas. In Sigurdsson, H. (ed.), Encyclopedia of Volcanoes, Academic Press, p.643662. Lowe, D.R. 1982. Sediment gravity flows II: Depositional models with special reference to the deposits of high density turbidity currents: Journal of Sedimentary Petrology, v.52, p.279-297. Monecke, T., Petersen, S., and Hannington, M. 2014. Constraints on water depth of massive sulfide formation: evidence from modern seafloor hydrothermal systems in arc-related settings: Economic Geology, v.109, p.2079-2101. - 150 - �Proceedings of the 61st ILSG Annual Meeting - Part 2 Morgan, L.A. and Schulz, K.J. 2012. Physical volcanology of volcanogenic massive sulfide deposit in volcanogenic massive sulfide occurrence model: U. S. Geological Survey Scientific Investigations Report 2010-5070C, chap. 5, 36p. Morton, R.L. and Franklin, J. M. 1987. Twofold classification of Archean volcanic-associated massive sulphide deposits: Econ. Geol. 82, p.1057-1063. Morton, R.L., Hudak, G.J., and Franklin, J.M. 1999. Geology, south Sturgeon Lake area, Ontario. Geol. Surv. Canada Open File Rpt. 3642. Morton, R.L., Hudak, G.J., and Koopman, E. 1996. Physical volcanology, hydrothermal alteration, and massive sulphide deposits of the Sturgeon Lake Caldera. GAC-MAC Field Trip Guidebook B3, 37p. Morton, R.L., Walker, J.S., Hudak, G.J., and Franklin, J.M. 1991. The early development of an Archean submarine caldera complex with emphasis on the Mattabi ash flow tuff and its relationship to the Mattabi massive sulphide deposit: Econ. Geol. 86, p.1002-1011. Moss, R. 1992. An oxygen isotope study of the Lyon Lake area, northwestern Ontario: Potential application to exploration for volcanogenic massive sulphide deposits in medium grade metamorphic terrains: unpublished B. Sc. Thesis, University of Toronto, Toronto, Ontario, 57p. Mueller, W.U., Stix, J., White, J.D.L., Corcoran, P.L., Lafrance, B., and Daigneault, R. 2008. Characterization of Archean subaqueous calderas in Canada: physical volcanology, carbonate-rich hydrothermal alteration, and a new exploration model: in Gottsman, J., and Marti, J., (editors), Developments in Volcanology, Volume 10, Caldera Volcanism: Analysis, Modeling and Response: Elsevier, Amsterdam, p.181-232. Mueller, W.U., Stix, J., White, J.D.L., and Hudak, G.J. 2004. Chapter 4.6: Archean Calderas: in Eriksson, P. G., Altermann, W., Nelson, D. R., Mueller, W. U., and Catuneanu, O., eds., The Precambrian Earth: Elsevier, p.345-357. Nedimović, M.R. and West, G.F. 2002. Shallow threedimensional structure from two-dimensional crooked line seismic reflection data over the Sturgeon Lake Volcanic Complex: Economic Geology, v.97, p.17791794. Poulsen, K.H. and Franklin, J.M. 1981. Copper and gold mineralization in an Archean trondhjemite intrusion, Sturgeon Lake, Ontario: Geol. Surv. Canada Paper 81-1A, p.9-14. Sanborn-Barrie, M. and Skulski, T. 1999. Tectonic assembly of continental margin and oceanic terranes at 2.7 Ga in the Savant Lake-Sturgeon Lake greenstone belt, Ontario: in Current Research 1999-C, Geological Survey of Canada, p.209-220. Sanborn-Barrie, M., Skulski, T., and Parker, J. 2001. 300 m.y. of tectonic history recorded by the Red Lake greenstone belt, Ontario: in Current Research 2001C, Geological Survey of Canada, p.15-22. Sanborn-Barrie, M., Skulski, T., and Whalen, J.B. 1998. Tectono-stratigraphy of central Sturgeon Lake, Ontario: deposition and deformation of submarine tholeiites and emergent calc-alkaline volcanosedimentary sequences: in Current Research 1998-C, Geological Survey of Canada, p.115-126. Seyfried, W.E., Jr., Ding, K., Berndt, M. E., and Chen, X. 1999. Experimental and theoretical controls on the composition of mid-ocean ridge hydrothermal fluids: Reviews in Economic Geology, 8, 181-200. Severin, P. W. A., 1981, Geology of the Sturgeon Lake Cu0Zn-Pb-Ag-Au deposit: C.I.M. Bulletin, v. 75, no. 846, p. 107-123. Smith, R. L. and Bailey, R. A. 1968. Resurgent Cauldrons: Geol. Soc. America Mem. 116, p.613-662. Trowell, N.F. 1974. Geology of the Bell Lake – Sturgeon Lake area, Districts of Kenora and Thunder Bay. Ontario Div. Mines Rep. 114, 67p. Trowell, N.F. 1983. Geology of the Sturgeon Lake area, Districts of Thunder Bay and Kenora: Ontario Geol. Surv. Rept. 221, 97p. Walker, J.S. 1993. Physical volcanology and hydrothermal alteration of the footwall rocks to the Archean Mattabi massive sulphide deposit, northwestern Ontario: unpublished M. Sc. Thesis, University of Minnesota – Duluth, 174p. Walker, J.S. and Hudak, G.J. 1990. Detailed geological mapping, F-Group reclamation trench “A”: Mattabi Mines Ltd. Unpublished map. White, J.D.L. 2000. Subaqueous eruption-fed density currents and their deposits; Precambrian Res. 101 (24), p.87-109. White, N.C. and Hedenquist, J.W. 1990. Epithermal environments and styles of mineralization: variations and their causes, and guidelines for exploration: Journal of Geochemical Exploration, v.36, p.445474. White, N.C. and Hedenquist, J.W. 1995. Epithermal gold deposits: styles, characteristics, and exploration: Society of Economic Geology Newsletter, v.23, p.113. Whitham, A.G. and Sparks, R.S.J. 1986. Pumice: Bulletin of Volcanology, v.48, p.209-223. Winchester, J.A. and Floyd, P.A. 1977. Geochemical discrimination of different magma series and their differentiation products using immobile elements: Chem. Geol. 20, p.325-343. Wright, I C., Gamble, J.A., and Shane, P.A.R. 2003. Submarine silicic volcanism at the Healy Caldera, southern Kermadec Arc (SW Pacific): I – volcanology and eruption mechanisms: Bulletin of Volcanology, v.65, p.15-29. - 151 - � 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, 2015 Description An account of the resource Institute on Lake Superior Geology. Dryden, Ontario. May 20-24, 2015. 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 2015 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/88b14dde3e7a16aeb5b6b67dc352d17c.pdf f477a80c8302e8a3243aa0d3acca505a PDF Text Text Institute on Lake Superior Geology 62ND ANNUAL MEETING May 4-8, 2016 Duluth, Minnesota Sponsored by PRECAMBRIAN RESEARCH CENTER AND DEPARTMENT OF EARTH AND ENVIRONMENTAL SCIENCES AT THE UNIVERSITY OF MINNESOTA DULUTH Meeting Co-Chairs James Miller, Christian Schardt, and Dean Peterson Proceedings Volume 62 Part 1 – Program and Abstracts Edited by Christian Schardt and Jim Miller \ i �ii �Table of Contents Institutes on Lake Superior Geology, 1955-2016 iv Sam Goldich and the Goldich Medal vi Goldich Medal Guidelines viii Goldich Medalists and Goldich Medal Committee x Citation for Goldich Medal Award to Mark Jirsa xi Memorial to Leon Gladen xiii Eisenbrey Student Travel Awards xiv Joe Mancuso Student Research Awards xv Doug Duskin Student Paper Awards and Award Committee xvi Board of Directors, Local Committee, and Session Chairs xvii Field Trip Leaders xviii Corporate and Individual Sponsors of Student Travel Scholarships xix Report of the Chair of the 61st Annual Meeting xx Duluth Entertainment and Conventions Center Floor Plan xxii Technical Program xxiii Poster Presentations xxx Abstracts 1-160 Reference to abstracts in Part 1 should follow the example below: Authors, 2016, abstract title. 62nd Institute on Lake Superior Geology Proceedings v. 62, Part 1-Program and Abstracts, p. XX. Proceedings Volume 62, Part 1—Program and Abstracts, and Part 2—Field Trip Guidebook are published by the 62nd Institute on Lake Superior Geology and distributed by the Institute Secretary: Peter Hollings Department of Geology Lakehead University Thunder Bay, ON P7B 5E1 CANADA peter.hollings@lakeheadu.ca Some figures in this volume were submitted by authors in color, but are printed grayscale to conserve printing costs. Full color imagery will appear in the digital version of the volume when it is available on-line at http://www.lakesuperiorgeology.org. ISSN 1042-99 iii �Institutes on Lake Superior Geology, 1955-2016 95 o o 85 o Wabigoon subprovince90 o 80 48 o Wawa-Abitibi subprovince 48 o Wawa-Abitibi subprovince o 45 45o Minnesota River Valley subprovince MEETING LOCATIONS Phanerozoic Mesoproterozoic Map by Mark Jirsa 95o Paleoproterozoic o 90 85o Archean Superior Province # 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 1955 1956 1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 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 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 iv �# Date 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 Place 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 Chairs Thunder Bay, Ontario Milwaukee, Wisconsin Duluth, Minnesota Eau Claire, Wisconsin East Lansing, Michigan International Falls, Minnesota Houghton, Michigan Wausau, Wisconsin Kenora, Ontario Wisconsin Rapids, Wisconsin Wawa, Ontario Marquette, Michigan Duluth, Minnesota Thunder Bay, Ontario Eau Claire, Wisconsin Hurley, Wisconsin Eveleth, Minnesota Houghton, Michigan Marathon, Ontario Cable, Wisconsin Sudbury, Ontario Minneapolis, Minnesota Marquette, Michigan Thunder Bay, Ontario Madison, Wisconsin Kenora, Ontario Iron Mountain, Michigan Duluth, Minnesota Nipigon, Ontario Sault Ste. Marie, Ontario Lutsen, Minnesota Marquette, Michigan Ely, Minnesota 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 v M.M. Kehlenbeck G. Mursky D.M. Davidson P.E. Myers W.C. Cambray D.L. Southwick T.J. Bornhorst G.L. LaBerge C.E. Blackburn J.K. Greenberg E.D. Frey & R.P. Sage J. S. Klasner J.C. Green M.M. Kehlenbeck P.E. Myers A.B. Dickas D.L. Southwick T.J. Bornhorst M.C. Smyk L.G. Woodruff R.P. Sage & W. Meyer J.D. Miller & M.A. Jirsa T.J. Bornhorst & R.S. Regis S.A. Kissin & P. Fralick M.G. Mudrey & Jr., B.A. Brown P. Hinz & R.C. Beard L. Woodruff & W.F. Cannon S. Hauck & M. Severson M. Smyk & P. Hollings A. Wilson & R. Sage L. Woodruff & J. Miller T. Bornhorst & J. Klasner J. Miller, G. Hudak, & D. Peterson M. Jirsa, P. Hollings, & T. Boerboom, P. Hinz & M.Smyk T. Fitz P. Hollings T. Bornhorst & A. Blaske J. Miller & M. Jirsa R. Cundari & P. Hinz J. Miller, C. Schardt, & D. Peterson �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 vi �INSTITUTE ON LAKE SUPERIOR GEOLOGY GOLDICH MEDAL vii �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. viii �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. ix �Goldich Medalists 1979 Samuel S. Goldich 1997 Ronald P. Sage 1980 not awarded 1998 Zell Peterman 1981 Carl E. Dutton, Jr. 1999 Tsu-Ming Han 1982 Ralph W. Marsden 2000 John C. Green 1983 Burton Boyum 2001 John S. Klasner 1984 Richard W. Ojakangas 2002 Ernest K. Lehmann 1985 Paul K. Sims 2003 Klaus J. Schulz 1986 G.B. Morey 2004 Paul Weiblen 1987 Henry H. Halls 2005 Mark Smyk 1988 Walter S. White 2006 Michael G. Mudrey 1989 Jorma Kalliokoski 2007 Joseph Mancuso 1990 Kenneth C. Card 2008 Theodore J. Bornhorst 1991 William Hinze 2009 L. Gordon Medaris, Jr 1992 William F. Cannon 2010 William D. Addison & 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 2015 Rodney J. Ikola 2016 GOLDICH MEDAL RECIPIENT Mark Jirsa Goldich Medal Committee Serving through the meeting year shown in parentheses. Mark Smyk (2016) Ontario Geological Survey Hélène Lukey (2017) Cliffs Natural Resources Shannon Zurevinski Lakehead University x �Citation for the Goldich Medal Award to Mark A. Jirsa I am honored to be able to present the 2016 Goldich Medal to Mark Jirsa. I first met Mark in 1983, when I was hired for a six month stint at the Minnesota Geological Survey. The first time I walked into the MGS office I was immediately introduced to both Mark Jirsa and Jim Miller, and the three of us were quickly dispatched to northern Minnesota to conduct field mapping for an unusual project involving the Minnesota Waste Management Board. I soon realized that I was working with a mapping Zen-master, and was very fortunate to have started my career with not one, but two people who are now both at the forefront of their respective areas of expertise. It was also the beginning of a long friendship and comradery. Mark’s career has never strayed far from Lake Superior – he earned a B.S. degree from the University of Wisconsin – Eau Claire in 1976, and an M.S. from the University of Minnesota – Duluth 1980. His Master’s thesis was on the petrology and tectonic significance of the interflow sediments in the Keweenawan North Shore Volcanic Group of Northeastern Minnesota. He worked a short time in northwestern Minnesota for Exxon Minerals, and then joined the Minnesota Geological Survey. As if the ‘normal’ work load of annual stints of field mapping, compilation, and publication of geologic maps and reports isn’t enough, Mark is also the Technical Editor for every map and report that is published at the MGS. The wide range of topics for the things he reviews – including Precambrian, Phanerozoic, and Quaternary geology, as well as hydrostratigraphy, testifies to the breadth of Mark’s geologic knowledge. Mark has been a mainstay of the ILSG for many years – his first paper was in 1978 in Milwaukee, on the topic of his Master’s thesis. He has probably attended every meeting since then. As most of you know, Mark has been one of the most active and involved members of the Institute on Lake Superior Geology for many years. He has co-chaired meetings in Minneapolis, International Falls, and Hibbing. He has led or co-lead 13 field trips starting with Eveleth in 1993 through the most recent 2014 meeting in Hibbing, and after this meeting we can bump that total up to 15. He has submitted over 30 abstracts for oral talks and posters, and has been session chair multiple times. He was the Secretary-Treasurer from 1994-2002, returned as Treasurer in 2005 and still is today. He has also led field trips for GSA meetings. If you have ever been on an ILSG trip that Mark is taking part in, you also know he has his nose right on the outcrop talking to the leaders and other participants about the rocks, not only for that stop, but for how they tie into the rest of the world. If you are a new young member of ILSG you should stick close to Mark and ask any question you want because he will fully engage you, and to him there is no such thing as a ‘stupid question’. However, the Goldich award is for more than just involvement with the ILSG. It is also about one’s contribution to the geology of the Lake Superior region. In the latter, Mark has contributed a great amount. His work has focused mainly on deciphering the complex xi �geology of the Archean rocks in both northern Minnesota and in the Minnesota River Valley, but he has also contributed a great deal to understanding the Paleoproterozoic terranes of Minnesota, including the east-central Minnesota batholith and environs, the Sioux Quartzite, the Biwabik and Gunflint Iron Formations, and the Sudbury ejecta deposits in Minnesota. In all of these cases his work has benefited not only Minnesota, but has applications elsewhere in the Lake Superior region. He has authored or co-authored more than 60 maps and reports published by the MGS, has authored or co-authored numerous publications in refereed journals, and selflessly agrees to give presentations to the public on a wide variety of topics pertaining to his work Mark’s latest focus is on unraveling the Timiskaming-type assemblages in northern Minnesota, which has given him cause to lead nine Precambrian Research Center capstone projects aimed at tracing and deciphering these assemblages. These capstone projects have given dozens of aspiring geologists the opportunity to map with a great mentor. He always found a way to sandwich these capstone projects in between all the other contractual mapping obligations of the MGS. Mark first started at the Minnesota Geological Survey in 1979, as a Junior Geologist, and one of his first projects was making a geologic map of Paleozoic strata of the Twin Cities basin. Fortunately for us hard rock types, he quickly moved on to what he loves, Precambrian rocks. Mark has an uncanny knack for field mapping (especially picking out graded beds!) and accompanying drill core logging. His ability to unravel the structural attributes of everything from a single outcrop to an entire greenstone belt never ceases to amaze me. Equally as amazing and inspirational to me is his tireless work ethic, be it long days in the field or dark winter days in the office. I’ve never known him to knock off a day of field work because of any type of weather conditions – more than once I’ve been back indoors, warm and dry, for several hours due to atrocious field conditions, but I won’t see Mark until after dark when he comes in stomping mud off his boots telling me about some great thing he discovered that day. Subsequent to my being hired full-time at MGS in 1987, I have worked almost continuously with Mark on a wide variety of mapping and drilling projects throughout all of the different Precambrian terranes of Minnesota. Early on I had the job of being his field assistant, which was great fun since my main task was helping peel outcrops – and boy did we peel! This led to other larger mapping projects where we divided up map areas, drilling projects that went through entire winters, and independent mapping projects. During every one of these, and continuing to this day, Mark has always set the bar when it comes to initiating projects, field work, and interpreting the rocks – from the outcrop through map compilation and publication. More than once Mark would have some idea for a grand mapping project for which my initial reaction was “Really? You think we can do that?”, then a couple of years later there it was - a finished project. The year 1983 was 33 years ago. Since I am currently 56 years old, that means I have known Mark for well over half my life. We have spent months living out of the same motel room, driving to and from field work, to ILSG meetings, etc. I don’t know if the feeling is mutual, but I had a great time through all of it. And my conclusion after having lived half my life with the guy is that he is most deserving of this medal. Terry Boerboom Precambrian Geologist Minnesota Geological Survey xii �In Memoriam Leon Wayne Gladen, 82, of Hibbing, died April 23rd, 2016. He was born in Bemidji, MN on April 25th, 1933, to Leonard and Hattie (Holmberg) Gladen. Leon grew up on the family farm in Bemidji and he served in the U.S. Army during the Korean War. He was a graduate of UMD and he held a Master’s Degree in Geology. He worked as a geologist for the Minnesota Department of Natural Resources for ten years and later for Lehman & Associates out of Minneapolis. Leon and Bernice (Grzybowski) were united in marriage on July 1, 2001 in Kenai, Alaska. Leon and Bernice enjoyed traveling the world together for his work assignments. He enjoyed reading, hunting, collecting fly fishing rods, and gardening. xiii �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. xiv �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 2015, the ILSG Board of Governors awarded three awards from the Joe Mancuso Student Research Fund. The winners were: Amanda Van Lankvelt University of Massachusetts-Amherst, Department of Geosciences Current degree program: PhD candidate (Advisor: M.L. Williams) Determining the Deformation Age of the Baraboo Syncline Award: $500 Detaya Johnson University of Wisconsin-Milwaukee, Department of Geosciences Current degree program: Bachelors of Science (Advisor: Dyanna Czeck) Geochemical analysis of deformed metaconglomerates Award: $500 Laura Cuccio Utah State University, Department of Geology Current degree program: MS Candidate (Advisor: James Evans) Evaluating the nature of sedimentary rock-crystalline basement interface and its control on hydrologic processes. Award: $500 xv �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. 2016 Student Paper Awards Committee Karl Everett – KEA Associates Dyanna Czeck - University of Wisconsin-Milwaukee Tim Kroeger – Bemidji State University (MN) Michael Zieg – Slippery Rock University (PA) Dorothy Campbell – Ontario Geological Survey xvi �Board of Directors Board appointment continues through the close of the meeting year shown in parentheses, or until a successor is selected Christian Schardt (2016-2019) – University of Minnesota Duluth Rob Cundari (2015-2018) – Ontario Geological Survey Jim Miller (2014-2017) – University of Minnesota Duluth Allan Blaske (2013-2016) – AECOM Pete Hollings - Secretary (2013-2016) – Lakehead University Mark Jirsa – Treasurer (2011-2014) – Minnesota Geological Survey Local Committee Jim Miller – General Meeting Chair Department of Earth and Environmental Sciences University of Minnesota Duluth Christian Schardt – Technical Program Chair Department of Earth and Environmental Sciences University of Minnesota Duluth Dean Peterson – Field Trip Chair Peterson Geoscience LLC and Precambrian Research Center University of Minnesota Duluth Session Chairs Robert Lodge – University of Wisconsin – Eau Claire Gerry White – Ontario Geological Survey, Thunder Bay, ON Ben Drenth – U.S. Geological Survey, Denver, CO Jeff Lynott – Foth, Green Bay, WI Latisha Brengman – University of Minnesota Duluth Esther Kingsbury Stewart – Wisconsin Geological and Natural History Survey xvii �Field Trip Leaders Field trips have been the mainstay of the ILSG since its inception 62 years ago. We want to give a special thanks to the field trip leaders who volunteered their time and talent in carrying that tradition forward. 1) GLACIAL GEOLOGY OF THE LAURENTIAN UPLANDS Phil Larson – Vesterheim Geoscience PLC Howard Mooers – Dept. of Earth and Environmental Sciences, UMD 2) NEOARCHEAN GEOLOGY OF THE WESTERN VERMILION DISTRICT Mark Jirsa, Amy Radakovichm Terry Boerboom - Minnesota Geological Survey 3) Cu-Ni-PGE DEPOSITS OF THE DULUTH COMPLEX Mark Severson – Tech American Andrew Ware – PolyMet Mining Kevin Boerst – Twin Metals Minnesota Steve Monson-Geerts – Natural Resources Research Institute, UMD 5) GEOLOGY OF THE ENDION SILL ALONG THE DULUTH LAKEWALK Jim Miller - Dept. of Earth and Environmental Sciences, UMD 6) GEOLOGY AND TROUT FISHING ALONG AMITY CREEK, DULUTH Dean Peterson – Peterson Geoscience LLC George Hudak - Natural Resources Research Institute, UMD 7) ARCHEAN AND PROTEROZOIC GEOLOGY OF THE GUNFLINT TRAIL Mark Jirsa - Minnesota Geological Survey 8) KEWEENAWAN GEOLOGY OF THE HOVLAND AREA Terry Boerboom – Minnesota Geological Survey John Green - Dept. of Earth and Environmental Sciences, UMD 9) DULUTH HARBOR GEOLOGIC HISTORY BOAT CRUISE: QUATERNARY TO ANTHROPOCENE Irv Mossberger, Mehgan Blair, Eric Dott – Barr Engineering Andy Breckenridge – University of Wisconsin - Superior Todd Kremmin - Dept. of Earth and Environmental Sciences, UMD xviii �Sponsors The following organizations and individuals made general contributions to the 62nd Annual Meeting. We thank them for their commitment to the Institute on Lake Superior Geology. All of the funds contributed this year go toward travel awards for student registrants. INDIVIDUAL CONTRIBUTORS TO STUDENT TRAVEL SCHOLARSHIPS WILLIAM EVERETT JOHN BERKLEY HENRY DJERLEV STEVE HOAGLUND ALLAN MACTAVISH RYAN DAYTON MARY ARTHUR DAN COSTELLO HARVEY THORLIEFSON DANIEL ROMANELLI GORDON MEDARIS, JR. ERIC DOTT With an especially generous donation provided by RON SEAVOY xix �Report of the Chair of the 61st Annual Meeting Dryden, Ontario The 61st ILSG was held in Dryden, Ontario on May 19-24, 2015. The meeting was chaired by Robert Cundari (Ontario Geological Survey) and Peter Hinz (Ministry of Northern Development and Mines) with considerable assistance from the organizing committee (Mark Smyk, Al MacTavish and Pete Hollings). The meeting was attended by a total of 123 delegates including 31 students. Special thanks to individuals who provided financial support for the meeting (Mary Arthur, Steve Baumann, Leonard Espinosa, Gordon Medaris Jr., Allan MacTavish, Jim Miller and Paul Weiblen) as well as the Thunder Bay Branch of the Canadian Institute of Mining and Metallurgy (CIM) for its generous donation. The two-day technical session began on Thursday May 21st which focused on Midcontinent Riftrelated geology and special topics including an extended oral presentation summarizing research to date on the Sudbury Impact Event in the Lake Superior Region. Technical talks continued through Friday morning with talks focusing largely on Archean geology. A total of 21 talks were given, 8 of which were presented by students. A total of 24 posters were displayed, 9 of which were presented by student authors. The 2015 Goldich Medal was awarded to Rodney J. Ikola from Esko, Minnesota. Thomas Waggoner presented the award during the annual banquet citing Rodney’s many contributions to the geoscience and mining community of Minnesota. The evening banquet speaker was Steve Beneteau – Senior Diamond Advisor / Chief Gemmologist for the Province of Ontario and the Manager of the Diamond Sector Unit for the Ontario Ministry of Northern Development and Mines. The title of his talk was: “Ontario’s Diamonds: A Journey from Mine to Market”. The meeting offered three multi-day field trips, three one-day field trips and three half-day field trips covering the Archean Geology of northwestern Ontario. Three pre-meeting field trips were offered on Tuesday May 19th and Wednesday May 20th, including Red Lake Geology (2-day) led by Andreas Lichtblau and Carmen Storey (Ontario Geological Survey), a Western Wabigoon Subprovince Transect (Dryden to Meggisi Lake) led by Mark Puumala and Dorothy Campbell (Ontario Geological Survey) and the Geological Setting of the Thunder Lake Gold Deposit led by Treasury Metals Inc. personnel. Three half-day trips were offered on Friday May 22nd, including Classic outcrops of the Dryden Area led by Peter Hinz (Ministry of Northern Development and Mines), Gold Occurrences of Van Horne Township led by Steve Meade (Ontario Geological Survey) and the Unique mineralizing event at the Pidgeon Molybdenum Occurrence led by Craig Ravnaas (Ontario Geological Survey). Three post-meeting field trips were offered starting Friday afternoon running through Sunday May 24th, including the Historic Pickle Lake Camp (1.5-day) led by Mark Smyk (Ontario Geological Survey), Pete Hollings (Lakehead University) and Neil Pettigrew (Fladgate Exploration Consulting Corp.), the Ghost Lake Batholith and Related Pegmatites led by Shannon Zurevinski (Lakehead University) and the Mattabi/Sturgeon Lake Historic VMS Camp (2-day) led by George Hudak (University of Minnesota Duluth). xx �The Institute’s Board of Directors met on Thursday May 21st to discuss the business of the Institute. The meeting was attended by meeting co-chair Robert Cundari, Treasurer Mark Jirsa, Secretary Peter Hollings and board members Jim Miller (2014 chair), Theodore Bornhorst (2013 chair) and Al MacTavish (2012 chair). Secretary Hollings took the minutes of the Board meeting that are as follows: 1. Accepted report of the Chairs for the 60th ILSG, Hibbing, Minnesota; as printed in the Proceeding Volume (Miller), and minutes of last Board meeting, May 15, 2014 (Hollings) 2. Received, discussed, and accepted 2014-2015 ILSG Financial Summary (Jirsa). 3. Received, discussed, and accepted 2014-2015 report of the Secretary (Hollings). 4. Approved Rob Cundari as on-going ILSG Board member 5. Approved Duluth as the site for the 62nd annual ILSG meeting. The meeting will be hosted by Jim Miller and Christian Schardt. 6. Discussed and approved replacing Bernhardt Saini-Eidukat as the “member from academia” on Goldich Committee (end of term 2015) with Shannon Zurevinski 7. Discussed student research grants. It was agreed that the application deadline will be switched to April 1 with results announced by April 30 each year. The next competition will be held in 2016. Jirsa to provide a list of all recipients of these awards so their attendance at the Annual Meeting can be tracked. 8. Jirsa to investigate the possibility of obtaining Director’s insurance including the cost and what would be covered. Also to investigate the implication of turning the Institute into a LLC. 9. Jirsa to develop a budget template for Meeting Chairs to help with the reporting of expenses and revenue when submitting the financial summaries to the Treasure The chairs would like to thank all those who assisted with the meeting including the organizing committee, the field trip leaders, the session chairs, service providers and those who provided support behind the scenes. The chairs would also like to thank those who participated in the meeting including the field trip attendees and the oral and poster presenters for their enthusiastic involvement with the Institute. Respectfully submitted, Robert Cundari and Peter Hinz Co-chairs, 61st Institute on Lake Superior Geology xxi �xxii �TECHNICAL PROGRAM WEDNESDAY MAY 4, 2016 All trips leave from the Harbor Side entrance (G) of the Duluth Entertainment and Convention Center 8:00am - 5:30pm PRE-MEETING FIELD TRIPS 1) GLACIAL GEOLOGY OF THE LAURENTIAN UPLANDS Phil Larson – Vesterheim Geoscience PLC Howard Mooers – Dept. of Earth and Environmental Sciences, UMD 2) NEOARCHEAN GEOLOGY OF THE WESTERN VERMILION DISTRICT Mark Jirsa, Amy Radakovichm Terry Boerboom - Minnesota Geological Survey 3) Cu-Ni-PGE DEPOSITS OF THE DULUTH COMPLEX Mark Severson – Tech American Andrew Ware – PolyMet Mining Kevin Boerst – Twin Metals Minnesota Steve Monson-Geerts – Natural Resources Research Institute, UMD 1:00pm - 5:30pm HALF-DAY PRE-MEETING FIELD TRIP 5) GEOLOGY OF THE ENDION SILL ALONG THE DULUTH LAKEWALK Jim Miller - Dept. of Earth and Environmental Sciences, UMD 4:00 pm - 10:00 pm Registration (Horizon Foyer) 7:00 pm - 10:00 pm Welcoming Reception (Horizon Foyer) Poster Session (Horizon Foyer and Room 202) xxiii �THURSDAY MAY 5, 2016 Asterisk * denotes a student eligible for Best Student Paper Award 7:30 am - noon REGISTRATION 8:00 am OPENING REMARKS Jim Miller and Christian Schardt, Co-Chairs, 2016 ILSG TECHNICAL SESSION I Session Chairs: Robert Lodge – University of Wisconsin – Eau Claire Ann Wilson – Ontario Geological Survey 1A) PETROLOGY AND METALLOGENESIS OF ARCHEAN AND PALEOPROTEROZOIC IGNEOUS RX 8:10 Dave Peck, Lionnel Djon, Cameron McLean, Gary DeSchutter, Jill Maxwell, Kelsey Privett, Denis Decharte, Chris Roney, Michelle Huminicki, & Bob Stewart The Lac Des Iles PGE-Cu-Ni deposit, Canada: an organized mega-breccia unit? 8:25 M. L. Djon*, G.R. Olivo, J.D. Miller, D.C. Peck, and B. Joy PGE Mineralization in the Northern Ultramafic Center of the Lac des Iles Complex, Ontario: Evidence of Magmatic and Hydrothermal processes 8:40 Erik Haroldson*, Brian Beard, Aaron Satkoski, Clark Johnson, and Philip Brown U-Th-Pb isotopes of the Reef Deposit; a Au-Cu occurrence in central Wisconsin 8:55 Ashley Quigley*, Thomas Monecke, Eric Anderson, Nigel Kelly, and Patrick Quigley Setting of volcanogenic massive sulfide deposits of the Paleoproterozoic Penokean volcanic belt 9:10 Robert Lodge, Geoffrey Pignotta, Brigitte Gélinas, Kelly Schwierske, and George Hudak Volcanological, Geochemical, and Geochronological Comparisons of the Gafvert Lake Sequence in Minnesota and Shebandowan Assemblage in Ontario 9:25 9:40 G. Gamelin*, V. Stinson, Yuanming Pan, and M. Nadeau A comparative study of mafic and felsic lithologies from the Borden Belt and adjacent greenstone belts in the Wawa-Abitibi Terrane Brigitte Gélinas* and Peter Hollings The Geology and Geochemistry of the Laird Lake Property, Red Lake Greenstone Belt, Ontario 9:55 COFFEE BREAK AND POSTER SESSION xxiv �1B) PRECAMBRIAN GEOCHRONOLOGY / ARCHEAN SEDIMENTATION AND STRUCTURE 10:15 Michael Mudrey Continued Evaluation of the Dilatancy Model for Discordant Uranium-Lead Age Determination of Zircon 10:30 Ben Frieman*, Yvette Kuiper, Nigel Kelly, and Thomas Monecke Provenance and tectonic evolution recorded by successor basins in the Abitibi-Wawa terrane: Insights from new U-Pb LA-ICP-MS analyses of detrital zircon 10:45 Sophie Kurucz* and Philip Fralick Giant Domes of the Mosher Carbonate, Steep Rock, Ontario 11:00 Matthew Svensson* and Philip Fralick The Badwater gabbro as an analogue for the weathering of Martian basalts 11:15 Victoria Stinson*, Yuanming Pan, Gleceria Gamelin, and Matthew Nadeau A re-examination of the Kapukasing structural zone 11:30 Tracy Carson*, Brittany Deley, and Mary Louise Hill Microstructural comparison of the Hardrock Project at Geraldton, Ontario and the Coffee Gold Project, Yukon 11:45 LUNCH BREAK ILSG BOARD MEETING TECHNICAL SESSION II Session Chairs: Ben Drenth – U.S. Geological Survey, Denver, CO Jeff Lynott – Foth, Green Bay, WI 2A) MIDCONTINENT RIFT GEOLOGY AND MINERALIZATION 1:15 Klaus Schulz and Suzanne Nicholson The Geochemistry of the Siemens Creek Formation and the Nature of Early Midcontinent Rift Basaltic Magmatism in the Western Lake Superior Region 1:30 Mark Smyk, Peter Hollings, and Philip Fralick A Preliminary Investigation of Enigmatic Igneous Rocks on Big Powder Island, Northern Lake Superior: A Possible Mesoproterozoic Magmatic Event 1:45 Sean O’Brien*, Peter Hollings, and Jim Miller Petrology, geochemistry and sulphur isotopes of the Crystal Lake gabbro and Mount Mollie dyke, Northwestern Ontario 2:00 Robert Cundari, Peter Hollings, David Good, and Sarah Davis Geochemistry and petrogenesis of volcanic rocks in the Coldwell Alkaline Complex; new insights from the Wolfcamp Lake volcanic rocks 2:15 David Good, Robert Linnen, and Iain Samson The Cu/Pd diagram and metal/sulfur variation as an exploration tool: Examples from the Coldwell Alkaline Complex, Ontario xxv �2:30 Robert Mahin and Steven Beach The Eagle East Magmatic Nickel-Copper Discovery 2:45 Connor Mulcahy*, Jim Miller, Robert Mahin, Steven Beach, and Bob Nowack Emplacement and Crystallization History of Ni-Cu-(PGE) Sulfide-mineralized Peridotites in the Eagle Intrusion, Upper Michigan 3:00 Christian Schardt Metal isotopic signatures in the Duluth Complex associated with magmatic Cu-NiPGE mineralization 3:15 COFFEE BREAK AND POSTER SESSION 2B) ENVIRONMENTAL GEOLOGY RELATED TO MINING AND EXPLORATION 3:35 Alex Brown Unique characteristics of sediment-hosted stratiform copper mineralization resulting from exceptional latent volcanic heat at White Pine, northern Michigan 3:50 Robert Seal, Perry Jones, Nadine Piatak, and Laurel Woodruff Potential value of pre-mining baseline oxygen, hydrogen, and sulfur isotopic data from surface waters for proposed large mining projects in northern Minnesota 4:05 Andrew Manning, Richard Wanty, and Jean Morrison Preliminary groundwater age and chemistry data from cover overlying Duluth Complex Ni-Cu-PGE deposits, NE Minnesota 4:20 Nadine Piatak, Robert Seal, Perry Jones, and Laurel Woodruff Copper toxicity and dissolved organic matter: Resiliency of mineralized watersheds in northern Minnesota and Michigan 4:35 Andrea Reed, Barry Frey, and Kevin Hanson Expanding the historical exploration document collection at the Minnesota Department of Natural Resources: the Polaris Joint Venture exploration program 4:50 George Hudak, Monson Geerts Stephen, Larry Zanko, Sara Post, and Euan Reavie The Minnesota Taconite Workers Health Study: Environmental Study of Airborne Particulate Matter ‐ 2015 Update 6:00 RECEPTION/POSTER SESSION – CASH BAR (Harbor Side Foyer) 7:00 ANNUAL BANQUET (Harbor Side Ballroom)  Announcement of 63rd Annual Meeting Location  2016 Goldich Award Presentation to Mark Jirsa  Banquet Presentation - Peter Clevenstine, Asst. Director of Minerals, MN DNR “Managing Minnesota's Mineral Resources and the DNR’s Conservation Agenda” xxvi �FRIDAY MAY 6, 2016 Asterisk * denotes a student eligible for Best Student Paper Award 8:00 OPENING REMARKS, UPDATES Jim Miller and Christian Schardt, Co-Chairs, 2016 ILSG TECHNICAL SESSION III Session Chairs: Latisha Brengman –University of Minnesota Duluth Esther Kingsbury Stewart– Wisconsin Geological and Natural History Survey 3A) PROTEROZOIC TECTONICS AND SEDIMENTATION 8:10 Paul Bedrosian Making it and breaking it in the upper Midwest: Constraints on continental assembly and rifting from EarthScope2 8:25. Esther Kingsbury Stewart and Jeffrey Mauk Sequence stratigraphy and basin evolution of the Mesoproterozoic Nonesuch Formation, Ashland syncline, northern Wisconsin 8:40 V.J.S. Grauch, Michael Powers, and Eric Anderson Progress on 3D modeling of the Midcontinent Rift System in the western Lake Superior region and an isopach map of the Oronto Group 8:55 Robyn Jones* and Philip Fralick Sedimentology of a pre-vegetation prograding deltaic assemblage: the Mesoproterozoic Kama Hill and Outan Island Formations, Ontario 9:10 Philip Fralick and Kamil Zaniewski Sedimentology of a Pre-Vegetation Floodplain Assemblage: the Mesoproterozoic Hele Member of the Sibley Group, Ontario 9:25 Julie Bartley, John Berger, Tanner Eischen, Sydney Firmin, and Lindsey Reiners Hypersaline conditions for stromatolite growth in the Rossport Formation (Mesoproterozoic, Ontario) 9:40 Richard Ojakangas What Happened in Northern Minnesota Between 2700 Ma and 1900 Ma? The Answer Is in the Pokegama Formation: A Multicycle Sedimentary History! 9:55 COFFEE BREAK AND POSTER SESSION 3B) GUNFLINT IRON FORMATION AND BARABOO QUARTZITE 10:25 William Cannon, Laurel Woodruff, and Stacy Saari Traces of the Sudbury meteor impact in the western Gogebic Iron Range, northern Wisconsin 10:40 Ruby Reid-Sharp* and Mary Louise Hill Characterizing deformation of Gunflint Formation in contact with Archean basement rocks east of Thunder Bay, Ontario xxvii �10:55 Carli Nap* and Philip Fralick Mesoproterozoic Alteration of the Paleoproterozoic Gunflint Formation: Analogies with Martian Blueberries 11:10 Esther Kingsbury Stewart, Eric Stewart, and Matthew Lamb Discovering hidden folds and faults in the Precambrian: new insights into Baraboointerval stratigraphy and deformation in southern Wisconsin 11:25 Gordon Medaris, Jr. Quantifying Mass Fluxes of Potassium in Weathering and Metasomatism of Paleosols 11:40 LUNCH BREAK TECHNICAL SESSION IV 10 YEAR ANNIVERSARY OF THE PRECAMBRIAN RESEARCH CENTER AT UMD 1:15 Jim Miller, Dean Peterson, and George Hudak Ten Years of Educating the Next Generation of Precambrian Field Geologists 1:30 Dean Peterson, Jim Miller, and George Hudak The PRC's Precambrian Field Camp ‐ A Decade of Training Students Geologic Mapping of the Canadian Shield 1:45 George Hudak and Dean Peterson Future Directions for the Precambrian Research Center 2:00 Mark Jirsa Nine years of capstones: A summary of PRC field camp capstone projects in the Neoarchean Knife Lake Group and associated rocks, central BWCAW, Minnesota 2:15 UPCOMING FIELD OPPORTUNITIES BEST STUDENT PAPER AWARDS STUDENT TRAVEL AWARDS 3:00 END OF TECHNICAL SESSION 4:00 pm POST-MEETING FIELD TRIPS 6) GEOLOGY AND TROUT FISHING ALONG AMITY CREEK, DULUTH Dean Peterson – Peterson Geoscience LLC George Hudak - Natural Resources Research Institute, UMD 7) ARCHEAN AND PROTEROZOIC GEOLOGY OF THE GUNFLINT TRAIL Mark Jirsa - Minnesota Geological Survey 8) KEWEENAWAN GEOLOGY OF THE HOVLAND AREA Terry Boerboom – Minnesota Geological Survey John Green - Dept. of Earth and Environmental Sciences, UMD xxviii �SATURDAY MAY 7, 2016 8:00am – 5:00pm POST-MEETING FIELD TRIP 9) DULUTH HARBOR GEOLOGIC HISTORY BOAT CRUISE: QUATERNARY TO ANTHROPOCENE Irv Mossberger, Mehgan Blair, Eric Dott – Barr Engineering Andy Breckenridge – University of Wisconsin - Superior Todd Kremmin - Dept. of Earth and Environmental Sciences, UMD xxix �POSTER PRESENTATIONS Asterisk * denotes a student eligible for Best Student Paper Award Eric Anderson, V.J.S Grauch, and Michael Powers Reprocessed seismic data image geology and structure near the Douglas fault on the Bayfield Peninsula, Wisconsin Kira Arnold* and S.E. Zurevinski An Investigation of the Ney’s Lookout Lamprophyric Dyke, Marathon, ON Kristofer Asp* and Christian Schardt An Investigation of Ni and Cu Isotopic Fractionation in Basal Duluth Complex Cu-Ni-PGE Mineralization, Northeastern Minnesota Steven Baumann, Alexandra Cory, and Sandra Dylka Lithological sedimentary divisions of the Copper Harbor Formation in Gogebic and Ontonagon Counties, Michigan Thomas Buchholz, Falster, Alexander, and Wm. B. Simmons The occurrence of Li, B, Sn, and W in the Nine Mile Pluton, Wausau Syenite Complex, Marathon County, Wisconsin William Cannon Mobilization of silica by flash heating of silica gel beneath the Sudbury Impact Layer, Baraga Basin, Michigan Val Chandler and Amy Radakovich Utility of the horizontal-to-vertical spectral ratio (HVSR) passive seismic method for determining Quaternary sediment thickness and bedrock elevation in north-central Minnesota: Fun with little control and generally poor data Jonathan Clark, Kristen Eshler, Patrick Groff, Taylor McClendon, Alexander Rode, Emily Salings, Kristen Spinelli, Kyle Vander Wyst, Aiden Walsh, Kristofer Asp, and Phillip Larson Bedrock Geology of the Devilfish Lake Area, Cook County, Minnesota Sarah Davis*, Peter Hollings, and Rob Cundari Mineralogy and Geochemistry of the Wolfcamp Lake Basalts Brittany Deley* and Mary Louise Hill Origin of the gold-hosting porphyry at Geraldton, Ontario Benjamin Drenth and Chad Ailes Re-digitized public aeromagnetic data for parts of the west-central Upper Peninsula, Michigan Benjamin Drenth, Raymond Anderson, Klaus Schulz, Joshua Feinberg, Val Chandler, and William Cannon Progress on Geophysical Mapping of the Northeast Iowa Intrusive Complex Simon Dolega* and Philip Fralick Geochemistry of deep and shallow water Archean banded iron formations, and their postdepositional implications in the western Superior Province, Canada xxx �Espree Essig*, Howard Mooers, and Karen Gran 3D Geological Mapping Using Terrestrial LiDAR at Soudan Underground Mine F. Glass Evidence in the Eastern Canadian Shield of regular fault patterns of crustal origin for the loci of some mineral deposits and late-stage intrusive events. Michael Felzan * and Marcia Bjørnerud Multi-stage development of breccias in the Baraboo Quartzite, Rock Springs, Wisconsin Carol Finn, Michael Zientek, Paul Bedrosian, Benjamin Bloss, Bethany Burton, Dean Petersen, and Heather Parks Geophysical Imaging of Layered Mafic Complexes and Relation to Platinum Group Element Exploration Carlin Green*, Robert Seal, William Cannon, and Nadine Piatak Quantitative abundance and preliminary morphological characterization of amphiboles in the Ironwood Iron-Formation, Gogebic Iron Range, Wisconsin Gregory Guenther* and Esther Kingsbury Stewart Paleocurrent interpretation of the Cambrian Elk Mound group using geophysical optical borehole image (OBI) logs from two new boreholes, Dodge county, Southern Wisconsin Samuel Helmuth* and Robert Lodge A New Rusk County: Producing an new Precambrian geological map from new field observations and compilations of historic geological/geophysical datasets Benjamin Hinks*, Joyashish Thakurta, Robert Mahin, and Steve Beach Geochemical and petrological studies on the origin of Ni-Cu sulfide mineralization at the Eagle and Eagle East intrusions in Marquette County, Michigan Sheree Hinz* and Peter Hollings Preliminary Observations of the Ultramafic Metavolcanic Rocks in the Eastern Portion of the Shebandowan Greenstone Belt, northwestern Ontario Nathaniel Jackson*, Bruno De Moura Merss, and Robert Lodge Lithostratigraphy and Ore Petrology of the Eisenbrey Zn-Cu-Pb Deposit, Rusk County, Wisconsin Shiyun Jin and Huifang Xu Incommensurately modulated structure of plagioclase as an indicator of cooling history of igneous rock Detaya Johnson* and Dyanna Czeck Geochemistry of Seine River metaconglomerates from Mine Centre, Ontario: interpreting fluid flow and volume changes during deformation with implications for strain analysis Alexandra Kozlowski* and S.E. Zurevinski Mineralogy and petrology of the diamondiferous Madonna Dyke, Marathon, ON Timothy Kroeger Preliminary Report on the Palynology of the Gervais Formation (Pleistocene), Red Lake County, Minnesota xxxi �Matthew Lamb* and Esther Kingsbury Stewart A comparison of Baraboo-Interval (Late Paleoproterozoic) Iron-Formation, Southern Wisconsin Crystal Lambert* and John Swenson Millennial-scale shoreline bluff retreat rates in the western arm of Lake Superior Matthew Matko* and Christian Schardt Small scale microanalysis of rock and mineral textures and its relationship to mineral separation S. Metteer* and S.E. Zurevinski Mineralogy and Petrology of the Rabbit Foot Dyke, White River, ON Jim Miller, Aaron Balles, Ellie Brown, Ryan Helms, Greta Penzel, and Luke Smith Geology of the Cherokee Lake area of the Boundary Waters Canoe Area, Cook County, MN - 2015 Precambrian field camp capstone mapping D. Nikkila*, R.H..Mitchell, and S.E. Zurevinski Investigations of the Layered Series Nepheline Syenite within Center II of the Coldwell Complex, Marathon, ON Maile Olson* and Robert Lodge Ore Petrography and Precious Metals of the Primary Flambeau Massive Sulfide Ore Mark Puumala and Seamus Magnus A preliminary evaluation of the structural controls on gold mineralization in the Jackfish Lake area, northwestern Ontario Lindsey Reiners*, Tanner Eischen*, and Julie Bartley The building blocks of stromatolites: Comparisons across time and environment Tyler Sager* and Nigel Wattrus Evaluating H/V analysis of passive seismic data as a means to map sediment thickness in the Duluth-Superior harbor Andrew Sasso* and Joyashish Thakurta Geochemical and Petrological Comparisons of Peridotite Units in Marquette County, Michigan Ruth Schulte, Nadine Piatak, Robert Seal, and Laurel Woodruff Acid-Generating and Acid-Neutralizing Potential of Silicate Rocks from the Basal Mineralized Zone of the Duluth Complex, Minnesota V. Smith* and S. Zurevinski The Mineralogy, Petrography and Geochemistry of the Anderson Lake Pegmatite Occurrence Matthew Svensson* and Stephen Kissin Source of Native Iron in Canadian Arctic Artifacts Margaret Upton, Ryan Puzel, Jaron Christenson, Morgan Kent, Steven Spreitzer, and Mark Jirsa Geologic mapping of Neoarchean and Proterozoic rocks near Kekekabic Lake, northeastern Minnesota, by students of the Precambrian Research Center’s 2015 field camp xxxii �Gerrit VanderWaal* and Christian Schardt Influence of mineral liberation on metal leaching and dissolution rates in ore material and associated host rock Blake Wallrich* and Michael Zieg Small-Scale Petrographic Variations in a Nipigon Diabase Sill Zacharie Zens* and Robert Lodge Geochemistry and Petrography of the Volcanic Strata Hosting the Flambeau Cu-Zn-Au Deposit in Rusk County, WI: A Re-examination of Wisconsin’s Only Past-Producing Volcanogenic Massive Sulfide Mine. Michael Zieg and Blake Wallrich Evidence for Episodic Emplacement History of a Nipigon Diabase Sill Michael Zientek, Klaus Schulz, Laurel Woodruff, William Cannon, Suzanne Nicholson, Lukas Zürcher, Heather Parks, and Connie Dicken Assessment of Undiscovered Nickel-Copper-Platinum Group Element (Ni-Cu-PGE) Resources Related to Conduit-Type Mineralization in the Midcontinent Rift System, Michigan, Minnesota, Ontario, and Wisconsin xxxiii �ABSTRACTS 1 �Reprocessed seismic data image geology and structure near the Douglas fault on the Bayfield Peninsula, Wisconsin ANDERSON, Eric D., GRAUCH, V.J.S., POWERS, Michael H., US Geological Survey, MS 964, PO Box 25046, Denver, CO 80225 USA A prominent gravity low lies over the Bayfield Peninsula in northern Wisconsin. The mapped bedrock geology includes sedimentary rocks of the Oronto and Bayfield Groups that overlie Midcontinent rift-related volcanic and intrusive rocks. The nearly 100 mGal amplitude anomaly has been interpreted to reflect low density Archean granite that is surrounded by higher density basalt (White, 1966; Allen and others, 1997), informally called White’s Ridge (Figure 1). Reprocessed seismic reflection data are helping to understand the geology and structures that are encompassed by the gravity low. Seismic reflection data acquired in 1984 were obtained for portions of several lines on the Bayfield Peninsula (Figure 1). Initially, only stacked time-series sections were available. The raw shot records were obtained for two lines within the western gradient of the gravity low and reprocessed using modern seismic data processing techniques. New stacking velocities were determined and used to create a new migrated time section and convert it to depth. The results show significantly more detail than was evident in the original time sections. These reprocessed data provide new insights about the geology to depths as great as 12 km. The reprocessed seismic lines run north-south and east-west for a total of about 65 line km. The north-south line is perpendicular to the mapped Douglas fault and confirms that it is south-dipping, thrusting Keweenawan volcanic rocks over as much as 3.7 km of younger, southdipping Bayfield and Oronto Group rocks to the north. At approximately 1 km depth, an anticlinal structure is evident south of the mapped Douglas fault. The velocity model indicates that the anticlinal structure is within the sedimentary rock package that overlies the volcanic rocks. North of the Douglas fault, both seismic sections indicate the Oronto Group increases in thickness towards the fault. The reprocessing of the east-west line has revealed several significant reflections in what was previously considered as reflection-poor Archean gneiss. These events are at depths from 9 to 12 km and may indicate the presence of layered strata of unknown origin at the easternmost end of the seismic line. The reprocessed data show in both lines a wavy texture in parts of the volcanic rocks. This texture is not evident in the vintage time sections. In the east-west line, the wavy texture is more pronounced where the volcanic rocks are in unconformable contact with the overlying Oronto Group. In the north-south line, the wavy texture only appears at depth in the volcanic rocks north of the Douglas fault. This texture may be due to heterogeneity in the physical properties and/or layering of the volcanic rocks or be the expression of local intrusive activity. The new seismic observations are being integrated into broader scale gravity and magnetic models that span the gravity low over the Bayfield Peninsula. Preliminary models indicate that relatively low density pre-rift rocks can explain the gravity low. The seismic data add considerable detail to the geologic model and are helping to define stratigraphic and structural relationships between the lithologies at depth. 2 �Figure 1: Generalized geology map of western Lake Superior showing location of reprocessed seismic data within the western gradient of the gravity low that overlies the Bayfield Peninsula. Anomalous gravity lows occur at both White’s and Grand Marais Ridges. The seismic lines provide detailed subsurface images that are being integrated with 2D gravity and magnetic models to help characterize the source of the gravity lows. REFERENCES Allen, D.J., Hinze, W.J., Dickas, A.B., and Mudrey, M.G., 1997. Integrated geophysical modeling of the North American Midcontinent rift system: new interpretations for western Lake Superior, northwestern Wisconsin, and eastern Minnesota. In Ojakangas, R.W., Dickas, A.B., and Green, J.C. (Eds.), Middle Proterozoic to Cambrian Rifting, central North America: Geological Society of America Special Paper 312: 47-72. White, W.S., 1966. Tectonics of the Keweenawan basin, western Lake Superior region. U.S. Geological Survey Professional Paper 524-E: E1-E23. 3 �An Investigation of the Ney’s Lookout Lamprophyric Dyke, Marathon, ON ARNOLD, Kira1, and ZUREVINSKI, S.E.1 1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1, kaarnold@lakeheadu.ca The Coldwell Complex of the Superior Province is known to host multiple lamprophyric dykes varying from ultramafic to alkaline affinities (Heaman and Machado 1992). The 1108 +/1 Ma age of the Coldwell complex and close spatial proximity supports a strong relationship to the magmatism of the Keweenawan Midcontinent Rift. The Ney’s Lookout dyke is located within the Center II portion of the Coldwell Complex, crosscutting assimilated syenites. This area of Center II is highly brecciated and assimilated, which is interpreted to be representative as evidence of the cauldron collapse (Mitchell and Platt 1978). The lamprophyre is devoid of diamonds, and has not been thoroughly analyzed other than brief field assessments. The objective of this research is to complete a mineralogical assessment of the lamprophyre dyke in order to characterize and classify the lamprophyre according to the IUGS classifications of lamprophyre-clan rocks. The main minerals that comprise the lamprophyre are euhedral zoned pyroxene phenocrysts and flow-aligned anorthoclase laths in a fine-grained groundmass composed of biotite, pyroxene, amphibole and feldspars. The pyroxene phenocrysts are poikilitic with inclusions of biotite, amphibole and relic olivine similar to the groundmass. Relic chloritized olivine phenocrysts are present in the groundmass and as inclusions in pyroxenes. The pyroxenes present in the sample are classified as diopside with minor augite. Mineralogical compositions of the Ney’s Lookout amphibole classify as Ferroan Paragasitic Hornblende. When compared to other lamprophyres within the Coldwell Complex, there were mineralogical and textural similarities between Ney’s Lookout Lamprophyre and local sannaites, such as porphyritic texture and mantling of amphiboles on pyroxene (Mitchell et al. 1991). Aside from minor variances in modal mineralogy, the lamprophyre best resembles a sannaite under the alkaline lamprophyre classification. The chill margin along the dyke is a fine-grained rim with no contact metamorphism present, interpreted as representing a moderate emplacement temperature. Laths of anorthoclase are directionally aligned in the lamprophyre and are interpreted as flow textures. Zonation in the pyroxene is distinct in the phenocrysts, representing increasing Cr and Ti outward toward the rim with increased Fe in the core and decreasing amounts in the rim. Uralitization is the altertion of pyroxene phenocrysts to amphibole around the outter rim. The zonation and uralitization often occur from magma mixing events. The Ney’s Lookout Sannaite Lamprophyre, has intruded into an area of highly brecciated and assimilated country rock. As well, the Little Pic River fault, a North-South trending fault, is located to just to the West of the lamprophyre. It is likely that the Ney’s Lookout lamprophyre was emplaced along planes of weakness similar to other lamprophyres in the area. 4 �Figure 1. A. Pyroxene phenocrysts present in the lamprophyre dyke are poikilitic and euhedral with simple twinning. B. The distinct zonation of pyroxene phenocrysts with a Fe rich core. Cr and Ti wt. %’s increase towards the rim. Al wt. % varies between zones, with no distinctive trend. References: 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, 289-303. Mitchell, R. H., and Platt, R. G., 1978. Mafic mineralogy of ferroaugite syenite from the Coldwell alkaline complex, Ontario, Canada. Journal of Petrology, v. 19, 627-651. Mitchell, R. H., Platt, R. G., Downey, M., and Laderoute, D. G. 1991. Petrology of alkaline Lamprophyres from the Coldwell alkaline complex, northwestern Ontario. Canadian Journal of Earth Sciences, v. 28, 1653-1663. 5 �An Investigation of Ni and Cu Isotopic Fractionation in Basal Duluth Complex Cu-Ni-PGE Mineralization, Northeastern Minnesota Asp, Kristofer1, Schardt, Christian 1 1 Department of Earth and Environmental Sciences, University of Minnesota-Duluth, 1114 Kirby Dr. Duluth, MN 55812 USA Cu-Ni-PGE magmatic sulfide-style mineralization occurs along the western margin of the Duluth Complex in northeastern Minnesota. Previous studies have demonstrated a notable fractionation of 60Ni and 58Ni in terrestrial materials, including both primary and secondary phases, with a total range of 2.1 ‰ [1 – 5]. Additional work has indicated a fractionation of 65Cu and 63Cu, with pronounced differences between primary copper sulfides and secondary copper phases in a variety of magmatic deposit types [6, 7]. Prior to this research, no δ60/58Ni or δ65/63Cu values have been measured in Duluth Complex rocks. The primary goal of this study is to measure Ni and Cu isotope values in a variety of Duluth Complex samples, and develop a possible model for the δ60/58Ni and δ65/63Cu isotopic systems in this geologic terrane. A secondary goal is to determine whether or not Ni and Cu isotope values in surface material could be used as an exploration tool for identifying Cu-Ni-PGE mineralization at depth. Based on the findings of previous studies, samples were collected to determine the isotopic differences between sulfide-bearing and sulfide-barren material. The potential effects of weathering were also taken into account by collecting samples at the surface in addition to unweathered, primary mineralized drill core. Samples were collected from a variety of locations in the basal Duluth Complex, including glacial till beds and surface outcrops in the vicinity of the Spruce Road, Maturi, Serpentine, Mesaba, and NorthMet deposits. Drill core from the Birch Lake, Wyman Creek, and Wetlegs deposits, along with those listed previously, was provided by several companies, including Duluth Metals/Twin Metals, Teck, PolyMet, and Encampment Minerals. Several pieces of drill core were also obtained from the MN DNR core facility in Hibbing, MN. A detailed characterization of till, weathered surface, and primary drill core samples revealed three main sources of nickel in Duluth Complex material: silicate, sulfide, and secondary oxide. The 24 δ60/58Ni values have an overall range from -0.97 to 0.22 ‰, but are correspondingly distinct in each type of material: silicate (-0.03 ‰ average), sulfide (-0.36 ‰ average), secondary oxide (-0.50 ‰ average). Further geochemical and microprobe work, along with the isotopic values, indicate two main stages of Ni fractionation in basal Duluth Complex rocks: a high temperature stage during crystallization, and a low temperature stage during surficial weathering. High-T fractionation is defined by a preferential incorporation of 58Ni into sulfide, while silicates, especially olivine, are reflective of the Bulk Silicate Earth value [3]. Low-T fractionation results in a preferential incorporation of 58Ni into secondary oxide, while 60 Ni possibly enters solution and leaves the system [1; 5]. The 22 measured Duluth Complex δ65/63Cu values have an overall range from -1.28 ‰ to 0.36 ‰, with an overall average of -0.35 ‰. This range is roughly similar to the magmatic sulfide values measured by [6], but are a significant departure from values measured in other deposit types. Specifically, deposits containing abundant secondary copper phases, including Cucarbonates and hydroxides, are significantly more enriched in 65Cu, with values up to 2.41 ‰ reported [7]. Due to the lack of observed secondary Cu minerals in project samples, it is possible that the low-T fractionation of 65Cu into secondary phases may not occur as readily in the basal Duluth Complex. Accordingly, the only indicated fractionation in the Duluth Complex is a high 6 �temperature fractionation, where 63Cu is preferentially incorporated into Cu sulfides during crystallization. Based on the measured isotopic values, there are distinct differences between sulfidebearing and sulfide-barren material at the surface that may indicate the presence of mineralization at depth. There is potential for these isotopic systems to be used as an exploration tool, but further research is required to fully evaluate the processes associated with these systems. Figure 1: Developed model for Ni isotopic fractionation in the basal Duluth Complex. Primary fractionation occurs at high temperatures during the crystallization of olivine and sulfide, while secondary fractionation leads to a preferential incorporation of 58Ni into secondary oxides. Also during secondary fractionation, 60Ni enters solution, and may be incorporated into other secondary phases including ferromanganese crusts and secondary Ni silicates (garnierite). References 1. Cameron, V. and Vance, D. (2014). Heavy nickel isotopic compositions in rivers and the ocean. Geochemica et Cosmochimica Acta 128: 195-211 2. Gall, L., Williams, H.M., Siebert, C., Halliday, A.N., Herrington, R.J., Hein, J.R. (2013). Nickel isotope compositions of ferromanganese crusts and the constancy of deep ocean inputs and continental weathering effects over the Cenozoic. Earth and Planetary Science Letters 375: 148-155 3. Gueguen B., Rouxel O., Ponzevera E., Bekker A., Fouquet Y. (2013) Ni isotope variations in terrestrial silicate rocks and geological reference materials measured by MC-ICP-MS. Geostandards and Geoanalytical Research 3: 297-317 4. Hiebert RS., Rouxel, O., Houlé, MG., Bekker, A. (2014) Ni isotope fractionation between komatiite and sulfide mineralization at the Neoarchean Hart deposit, Abitibi greenstone belt, Canada. Geological Society of America Abstracts 46: 467 5. Wasylenski, L.E, Howe, Haleigh D., Spivak-Birndorf, L.J., Bish, DL. (2015) Ni isotope fractionation during sorption to ferrihydrite: implications for Ni in banded iron formations. Chemical Geology 400: 56-64 6. Larson, P.B., Maher, K., Ramos, F.C., Chang, Z., Gaspar, M., Meinert, L.D. (2003). Copper isotope ratios in magmatic and hydrothermal ore-forming environments. Chemical Geology 201: 337-350 7. Markl, G., Lahaye, Y., Schwinn, G. (2006) Copper isotopes as monitors of redox processes in hydrothermal mineralization. Geochemica et Cosmochimica Acta 70: 4215-4228 7 �Hypersaline conditions for stromatolite growth in the Rossport Formation (Mesoproterozoic, Ontario) BARTLEY, Julie K., 1BERGER, John, EISCHEN, Tanner, 2FIRMIN, Sydney, and REINERS, Lindsey Department of Geology, Gustavus Adolphus College, St. Peter, Minnesota 56082 1 Present Address: Bay West LLC, St. Paul, Minnesota 55103 2 Present Address: Department of Geography and the Environment, University of Denver, Denver, CO 80208 The Mesoproterozoic Rossport Formation of Ontario, Canada is approximately 1.4 billion years old (Franklin et al., 1980) and is generally interpreted to have been deposited in an intracratonic basin, most likely a rift-related lake Figure 1 (Rogala et al., 2005). While the Rossport consists dominantly of sandstone and shale, the Middlebrun Bay Member, in in the middle of the formation, is a carbonate unit. The Middlebrun Bay Member, in exposures on the Channel Islands and along the North Shore of Lake Superior, consists of massive limestone and cherty, stromatolitic carbonate. Key features of these carbonates suggest an interval of low lake level, low clastic influx, high salinity, and local conditions suitable for microbialite development. They provide an important Figure 2 environmental constraint on conditions in this lake, and additionally suggest environments in which complex, carbonate-precipitating microbial communities could thrive. The presence of a massive, recrystallized limestone on Copper Island suggests of hypersaline, evaporite-producing conditions. This unit is devoid of stromatolites or microbial laminae and has an unusual bright white color lacking internal structure (Fig. 1) with a coarsely recrystallized texture. These features suggest dissolution and replacement of a primary, soluble phase such as an evaporite mineral. The presence of large sandstone clasts let down from the overlying bed is reminiscent of collapse breccia associated with dissolution of that primary mineralogy (Fig. 2). Additionally, geochemical data suggest broad similarity with other Proterozoic carbonates interpreted as calcitized evaporites (Manning-Berg and Kah, 2013) Based on these data, we interpret the massive carbonate exposed on Copper Island as a calcitized evaporite, probably deposited originally as gypsum and replaced by calcite during diagenesis (Firmin and Bartley, 2014). Previous work on the Rossport Formation suggests that the Middlebrun Bay Member formed when lake levels and clastic influx were low (Rogala et al., 2007). Combined with an evaporite interpretation of the massive carbonate unit, we suggest the Middlebrun Bay interval may have been deposited during a period of increased aridity in the region. In this model, stromatolites would have formed in a hypersaline lake environment during intervals of low clastic influx. 8 �Figure 3 Middlebrun Bay stromatolites have low-relief stratiform to columnar morphology (Figure 3). Early diagenetic chert is locally abundant, and the meso- to micro-scale texture suggests in situ precipitation of microbial laminae and rapid cementation of stromatolite form. Stromatolites occur in lacustrine environments throughout geologic history, commonly under conditions of high alkalinity or high salinity (e.g., Gomez et al., 2013). During the Mesoproterozoic, though, stromatolites are more often associated with marine environmental conditions (Kah et al., 2009) and only occasionally occur in association with indicators of elevated salinity (e.g., Neudert and Russell, 1981). The Middlebrun Bay stromatolites, therefore, provide an important datum in space and time, allowing comparison of their morphology, texture, and fabric in relation to the features of younger, more commonly hypersaline stromatolites and contemporaneous, normal-marine forms. REFERENCES Firmin, S., and Bartley, J.K., 2014, An unusual Mesoproterozoic carbonate unit: Relic of a saline lake? Institute on Lake Superior Geology, v. 60, p. 45-46. Franklin, J. M., McIlwaine, W.H., Poulsen, K.H., and Wanless, R.K., 1980, Stratigraphy and depositional setting of the Sibley Group, Thunder Bay district, Ontario, Canada: Canadian Journal of Earth Sciences, v. 17, p. 633-651. Gomez, F.J., Kah, L.C., Bartley, J.K., and Astini, R.A., 2014, Microbialites in a high-altitude Andean lake: Multiple controls on carbonate precipitation and lamina accretion: PALAIOS, v. 29, p. 233-249. Kah, L.C., Bartley, J.K., and Stagner, A.F., 2009, Reinterpreting a Proterozoic enigma: ConophytonJacutophyton stromatolites of the Mesoproterozoic Atar Group, Mauritania: International Association of Sedimentologists Special Publication 41, p. 277-295. Manning-Berg, A.R., and Kah, L.C., 2013, Calcitized Evaporites and the Evolution of Earth’s Early Biosphere: Geological Society of America Abstracts with Programs, v. 45(7), p. 628. Neudert, M.K., and Russell, R.E., 1981, Shallow water and hypersaline features from the Middle Proterozoic Mt. Isa Sequence: Nature, v. 293, p. 284-286. Rogala, B., and Fralick, P.W., 2005, Stratigraphy and sedimentology of the Mesoproterozoic Sibley Group and related igneous intrusions, northwestern Ontario: Ontario Geological Survey Open File Report 6174, 128 pp. Rogala, B., Fralick, P.W., Heaman, L.M., and Metsaranta, R., 2007, Lithostratigraphy and chemostratigraphy of the Mesoproterozoic Sibley Group, northwestern Ontario, Canada: Canadian Journal of Earth Sciences, v. 44, p. 1131-1149. 9 �Lithological sedimentary divisions of the Copper Harbor Formation in Gogebic and Ontonagon Counties, Michigan BAUMANN, Steven D.J.1, CORY, Alexandra B.1, DYLKA, Sandra K.1 1 Geology Section, Midwest Institute of Geosciences and Engineering, 1321 W. Touhy Ave. 2S, Chicago, IL 60626 The sedimentary assemblage of the Copper Harbor Conglomerate shows four to five distinct sedimentary units (with interbedded volcanics) exposed for a length of approximately 35 miles along Lake Superior in Gogebic and Ontonagon Counties. 7.5’ quadrangle scale mapping in 2014 to 2015 led to the recognition of four distinct sedimentary facies extending from the North Ironwood east along Lake Superior to at least the White Pines 7.5’ quadrangles (see figure 1). A fifth sedimentary unit at the top of the formation exists in the Black River Harbor 7.5’ quadrangle east to the Carp River East 7.5 minute quadrangle. The five sedimentary units can be lumped into two main facies. The lower two units are conglomerate dominated (Units 1 and 2), while the upper three units are dominantly sandstones (Units 3, 4, and 5). These two basic facies appear to be separated by an unconformity, where the contact is exposed along the Black River. The basal unit of the lower facies (Unit 1) is reddish brown to brown, massive, clast supported, polyometic orthoconglomerate about 1700 to 3000 feet thick. The clasts are well rounded and range in size from pebbles to boulders (<14” in diameter). Lithic arkose makes up the majority of the matrix at <15% of the rock. Along the Black River this unit contains about 480 feet of volcanics at about the halfway point within the unit, informally called the “Black River Flows”. These flows are dominantly andesite with some beds of sedimentary rock. The flows are stratigraphically lower in section than the base of the Lake Shore Traps. Unit 2 overlies Unit 1 in a gradational to intertonguing relationship over about 200 feet, except where the Lake Shore Traps are present. The Lake Shore Traps separate the two units with sharp contacts. Unit 2 is a deep red to reddish brown, thick bedded, clast supported, polyometic sandy orthoconglomerate. The clasts are all well rounded and generally larger than in Unit 1 (<22” in diameter). Lenses of deep red, medium to coarse grained, lithic arkosic arenite exist within the unit and are <4’ thick by <50 feet wide. Unit 2 is about 700 to 2800 feet thick. Unit 2 is separated from the upper facies (Unit 3) by a sharp contact, which is a probable unconformity. This unconformity separates the two main facies. Unit 2 thickens at the expense of Unit 3 in the Carp River West and East quadrangles. Unit 3 is deep purplish gray to brown, thin to medium cross bedded, fine to coarse grained, arkosic arenite. It contains beds of polyometic sandy diconglomerate with well rounded clasts <8” in diameter. Ripple marks are very common on bedding planes. Unit 3 is 800 to 3000 feet thick. Unit 3 grades up into Unit 4 over about 30 feet. The lithology of Unit 4 closely resembles the Freda Formation. It is deep red mottled pale yellow brown becoming purplish near the top, medium bedded to cross bedded, fine to coarse grained, arkosic arenite. The unite fines up section. It contains isolated thick beds of polyometic sandy paraconglomerate, with clasts <6” in diameter. Ripple marks are common. Unit 4 has the most consistent thickness at 1000 to 1700 feet. Unit 5 shows an intertonguing and gradational contact with Unit 4. Gradation is over about 20 feet. Unlike the lower four units, it is not continuous throughout the area. It exists as a large lens extending about 21 miles from the North Ironwood to Carp River East quadrangle. It is a deep purple to dark gray, laminated to massive, sandy shale, with some polyometic sandy paraconglomerate beds. Clasts are <3” in diameter. The sandy parts of the unit are mostly coarse lithic arenites. It is the thinnest of the units at 0 to 400 feet thick. The top of the unit is locally covered, except in the Carp River East quadrangle, where the top is gradational with the overlying Nonesuch Formation. The top of the Copper Harbor Formation at Nonesuch Falls along the Little Iron River (White Pine quadrangle), resembles Unit 5 and may be equivalent. However, complex local folding and faulting makes a definite correlation difficult. Units 1 and 2 thin dramatically on the east side of the Porcupine Mountains and pinch out altogether just south of the mountains (at about 46.76o by -89.62o). However, lithologies resembling Units 1 and 2 appear to be traceable on the north shore of the Keweenaw Peninsula, where Unit 1 can be traced along with a facies change in Unit 2. At Horseshoe Harbor (47.473o by -87.809o) Unit 2 includes more continuous beds of sandstone with shale, along with 10 �stromatolites. The thinning of Units 1 and 2 observed around the Porcupine Mountains may just be a local phenomenon caused by the extremely thick Porcupine Volcanics. The four to five fold division of the Copper Harbor Formation is persistent and traceable from about -90.19o (North Ironwood quadrangle) to -89.57o longitude (White Pine quadrangle) along Lake Superior. The four to five fold division west of longitude -90.19o, begins to break down. West of longitude -90.28o (in the Little Girls Point quadrangle), the Copper Harbor Formation appears to be undivided. The Copper Harbor pinches out entirely in Copper Falls State Park, north of Mellen, Wisconsin. References: Bornhorst, T.J., Rose, W.I., 1994. Self Guided Geologic Field Trip to the Keweenaw Peninsula, Michigan. Institute on Lake Superior Geology, volume 40, pp. 161-164 Cannon, W.F., 1995, Geologic Map of the Ontonagon and Part of the Wakefield 30’ x 60’ Quadrangles, Michigan, United States Geological Survey, Miscellaneous Investigation Series, Map I-2499 Dickas, A.B., Mudrey Jr., M.G., 1992. Keweenaw Sedimentary Rock of the South Shore, Lake Superior. Institute on Lake Superior Geology, volume 38, pp. 43-102 White, W.S., Wright, J.C., Lithofacies of the Copper Harbor Conglomerate, Northern Michigan, United States Geological Survey, Professional Paper 400-B, pp. B5-B8 Figure 1: Mapped Quadrangles in Gogebic and Ontonagon Counties 11 �Making it and breaking it in the upper Midwest: Constraints on continental assembly and rifting from EarthScope Paul A. Bedrosian U.S. Geological Survey, Denver Federal Center, MS 964, Denver, CO, 80225 The North American mid-continent presents a window into craton growth and stabilization as well as the 1.1 Ga rifting event that nearly tore Laurentia apart. Unique to this region is the preservation of this tectonic collage, largely unmodified by subsequent tectonic events, which permits examination of if and how such events are preserved in the continental lithosphere. Focusing on the upper Midwest, I will discuss the implications of a three-dimensional resistivity model derived from EarthScope magnetotelluric data [Bedrosian, 2016]. The resistivity model reveals the distribution of highly conductive Penokean age metasedimentary rocks in Minnesota, Michigan, and Wisconsin. These rocks are correlated with metagraywackes of the Michigamme Formation in MI and WI, and with graywackes of the Virginia and Rove Formations in MN. The electrical signature of these rocks is unique throughout the entire midcontinent region. Their high conductivity is attributed to metallic sulfides and in some cases graphite. The former is considered a potential source of sulfur for certain types of mineral deposits found in the region. A more detailed magnetotelluric survey, in addition to a reconnaissance airborne electromagnetic survey, is being carried out to map these rocks in greater detail. An isolated sliver of similarly conductive rocks is mapped in the subsurface in northwest Iowa (sub-parallel to the Spirit Lake Tectonic Zone); I speculate that Penokean-age rocks may be preserved further west than currently assumed. The Paleoproterozoic structural collage was interrupted by the 1.1 Ga Mid-continent Rift System (MRS). The type electrical signature of the MRS is found in Iowa, where a resistive medial horst is imaged, flanked by deep sedimentary basins filled with Bayfield and Oronto Group equivalent sediments. A distinction is observed between the moderately conductive Oronto Group and the highly conductive Bayfield Group. Translating this basic picture to the Lake Superior graben, a pronounced west to east asymmetry is seen, with the Thiel fault being the most obvious division. The asymmetry is taken to reflect the different geometric response of each half of the basin to compression during the Grenville orogeny [Cannon, 1994]. The orientation of the rift-bounding faults in relation to the stress field at the time is speculated to have resulted in a greater degree of compression in the western half of the Superior graben than in the eastern half. Additionally, there appears to be a horst and graben geometry across each half of the Superior graben. At a much larger scale, the resistivity of the mantle lithospheric beneath the region is surprisingly heterogeneous. The spatial pattern of these variations bears little resemblance to the crustal imprint of past tectonic events or to the direction of North American absolute plate motion. I argue that these resistivity variations reflect differing degrees of hydration (metasomatism) preserved within the lithosphere. Lithospheric hydration in the upper Midwest is speculated to have occurred during MRS magmatism. This interpretation is consistent with geochemical and isotopic analyses of MRS basalts and their inferred mantle sources [Nicholson et al., 1997]. REFERENCES Bedrosian, P.A., 2016. Making it and breaking it in the Midwest: Continental assembly and rifting from modeling of EarthScope magnetotelluric data, Precambrian Research, doi:10.1016/j.precamres.2016.03.009 Cannon, W.F., 1994. Closing of the Midcontinent rift-A far-field effect of Grenvillian compression. Geology 22, 155–158. Nicholson, S.W., Schulz, K.J., Shirey, S.B., Green, J.C., 1997. Rift-wide correlation of 1.1 Ga Midcontinent rift system basalts: implications for multiple mantle sources during rift development. Canadian Journal of Earth Sciences 34, 504–520. 12 �Unique characteristics of sediment-hosted stratiform copper mineralization resulting from exceptional latent volcanic heat at White Pine, northern Michigan BROWN, Alex C., 13250 rue Acadie, Pierrefonds, QC, H9A 1K9, acbrown@polymtl.ca Most studies of copper mineralization hosted by basal greybeds of the Nonesuch Formation at White Pine, Michigan, have concluded that the main-stage sediment-hosted stratiform copper (SSC) mineralization resulted from an upward influx of cupriferous brine from a coarse-grained footwall aquifer, the Copper Harbor Conglomerate (White and Wright, 1966; Brown, 1971). An early diagenetic timing is interpreted mainly from textural evidence that copper deposited largely by replacement of in situ fine-grained syndiagenetic pyrite (Fig. a), that infiltrations of oreforming brine would have been relatively rapid before advanced compaction and lithification of the basal fine-grained Nonesuch strata, and that main-stage mineralization pre-dated later structurally controlled mineralization (Mauk, 1993). Curiously, the replacement of fine-grained disseminations of euhedral pyrite by cupriferous sulfides at White Pine is clearly visible in a transition zone at the top of the cupriferous zone (cms in width), but not within the cupriferous zone proper (meters in width) where disseminated chalcocite (Fig. b) occurs with a grain-size considerably greater than that of disseminated pre-ore pyrite found above the cupriferous zone. The only strong visual evidence for chalcocite replacement of pyrite within the cupriferous zone is the occurrence of chalcocite nodules which presumably were initially syndiagenetic pyrite nodules; commonly, the chalcocite nodules are surrounded by halos of hematite which probably represent iron released during pyrite replacement. Curiously too, the White Pine deposit is unique in that most other SSC deposits world-wide exhibit fine-grained euhedral pyrite replacements within their cupriferous zones. Also, the White Pine copper mineralization is composed virtually only of disseminated chalcocite with a remarkably narrow transition (Py→Cp→Bn→Cc) into overlying pyritic strata (Fig. c), whereas most SSCs world-wide exhibit gradual copper-iron sulfide transitions over the breadth of their cupriferous zones. The recent proposal (Brown, 2013, 2014) that the upwardly infiltrating cupriferous brine at White Pine was warmed to ~100oC by latent heat from the underlying buried Porcupine Volcanics dome (Fig. d) could offer explanations for the unique character of the White Pine SSC deposit: a) the anomalously warm ore-stage environment could have resulted in a more rapid and complete reaction through the Py→Cp→Bn→Cc sequence to form uniquely chalcocitic mineralization rapidly within the cupriferous zone proper while the copper front advanced upward through the basal Nonesuch graybeds; b) under warm conditions, fine-grained chalcocite could have recrystallized to coarser-grained chalcocite; and c) the preservation of fine-grained zoned replacement textures and pseudomorphs after pyrite euhedral along the top of the cupriferous zone could represent replacements of pyrite at low temperatures prevailing toward the end of the main-stage copper mineralization event. References Brown, A.C., 1971, Zoning in the White Pine Copper Deposit, Ontonagon County, Michigan: Economic Geology, v. 66, p. 543-573. Brown, A.C., 2013, Brine viscosity vs. temperature: A key to explaining copper mineralization in the finest-grained basal Nonesuch Formation in the White Pine-Presque Isle district, northern Michigan: Institute on Lake Superior Geology, Proc. of Annual Meeting, Houghton, Michigan, v. 59, p. 9-10. Brown, A.C., 2014. Latent thermal effects from Porcupine Volcanics calderas underlying the White PinePresque Isle stratiform copper mineralization, northern Michigan: Economic. Geology, v. 109, p. 2035-2050. 13 �Mauk, J.L., 1993, Geological and geochemical investigations of the White Pine sediment-hosted stratiform copper deposit, Ontonagon County, Michigan: Ph.D. thesis, Univ. of Mich., Ann Arbor, 194 p. White, W.S. and Wright, J.C., 1966, Sulfide-mineral zoning in the basal Nonesuch Shale, northern Michigan: Economic Geology, v. 61, p. 1171-1190. a) Disseminated very fine-grained pyrite (some framboidal) in Nonesuch graybeds above the cupriferous zone, White Pine deposit. (Relatively coarse grain of pre-ore stage chalcopyrite (Cp) in center of view). c) Narrow pyrite to chalcocite transition (Py→Cp→Bn→Cc) at the top of the cupriferous zone, White PinePresque Isle district. b) Fine-grained chalcocite (white) of the cupriferous zone, basal Nonesuch graybeds. Chalcocite grain-size decidedly greater than that of pyrite in a). Note also the absence of pseudomorphs after euhedral pyrite. d) Schematic illustration of Porcupine Volcanics dome contributing latent volcanic heat during early diagenetic infiltrations of Cu-bearing brine into basal Nonesuch graybeds, White Pine-. Presque Isle district. See Brown (2014) for further explanations. 14 �The occurrence of Li, B, Sn, and W in the Nine Mile Pluton, 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. The Nine Mile Pluton is the youngest (≈1505 Ma, Dewane & Van Schmus, 2007) and most silicic of the four intrusions comprising the Wausau Syenite Complex, and is primarily composed of granite and quartz monzonite. Over the last several decades we have developed considerable data on the behavior of some minor elements during the formation of this complex, notably Li, B, Sn and W, which are addressed here. Identification was via electron microprobe (EMP), X-ray diffraction (XRD), energy dispersive spectromentry (EDS) and direct coupled plasma spectroscopy (DCPS) as indicated. Although overall sparse, Li mineralization has been identified in several areas of the pluton, associated with pegmatites and greisens exposed in quarrying operations exploiting easily excavated “rotten granite” or grus. In the former Wimmer pits and adjacent operations, the Limica zinnwaldite, KLiFe2+2Al(Al2Si3O10 (an intermediate composition between siderophyllite and polylithionite), has been identified (EMP, DCPS) from several fractionated pegmatites, associated with columbite- pyrochlore- and euxenite-group minerals. Additionally, approximately 2 gms of elbaite tourmaline (XRD, EMP), Na(Li1.5Al1.5)Al6(Si6O18)(BO3)3 (OH)3(OH), were found in a large (2.4x2.2x0.4 m) miarole in a pegmatite located in the former Thurber pit (later Wimmer #3). Zinnwaldite (EMP, DCPS) has also been found in several pegmatites in the former Koss pit in the central portion of the pluton (associated with cassiterite, columbite-, euxenite- and pyrochlore-group minerals); in greisenized pegmatites and aplites in the former Maguire Pit in the south-central portion of the pluton (associated with cassiterite, ferberite/huebnerite, topaz, cryolite, prosopite and other minerals), and in a greisen-related episyenite and vein assemblage in the Ladick operation (associated with cassiterite, pseudobrookite, monazite, molybdenite and other minerals) in the southwest portion of the pluton. Overall, Li contents in Nine Mile Pluton pegmatite wall zones reach 10-20 ppm, while in the granite itself Li contents range from below detection limit to 4 ppm (DCPS). Boron minerals are extremely rare in the Wausau Complex, having only been found as the aforementioned elbaite tourmaline, and a few grams of schorl tourmaline (EMP) found in a small miarole (5 cm diameter) in the former Wimmer pit #2. This scarcity is probably a consequence of very low boron levels in the Complex. In general, DCPS analysis of Nine Mile Pluton pegmatite wall zones indicate B contents ranging from below detection limit to 3 ppm, and of the granite itself from below detection limit to 1 ppm, although DCPS analysis of the wall zone of the elbaite-bearing pegmatite indicated anomalous B contents of 12-15 ppm. Sn mineralization is not abundant in the Nine Mile Pluton, but is relatively widespread. Cassiterite, SnO2, has been found as apparent epitaxial overgrowths on ilmenite in a small miarolitic granite body on the western side of the former Wimmer Pit #3 in the NE portion of the pluton (EDS). Tantalian cassiterite (EMP) was found in a small fractionated pegmatite nearby where it was closely associated with columbite-group minerals, U-rich pyrochlore, a U-niobate mineral and hafnian zircon, and was also identified (EDS, EMP) from a fractionated pegmatite exposed by workings in the former Koss Pit in the central portion of the pluton Sparse cassiterite has also been found as red-brown grains in chloritized biotite from an euxenite and fluorite-rich pegmatite exposed in the Kafka operation in the western portion of the pluton (EDS), and in a 15 �thin quartz-pyrite vein briefly exposed in the floor of the Red Rock Southwest Pit in the southwest portion of the pluton (EDS). Cassiterite was abundant in greisenized pegmatites and aplites exposed by workings in the former Maguire Pit in the southern portion of the Nine Mile Granite (EDS, EMP), and tantalian cassiterite (EMP) was common in the greisen-related episyenite and vein assemblage in the Ladick quarry briefly described above. It is interesting that several analyzed cassiterites have elevated tantalum contents of 0.3 to 0.4 wt% Ta2O5, while their Nb2O5 contents are much lower, suggesting that Ta is more compatible in cassiterite than Nb. Tungsten mineralization is very limited within the Nine Mile Pluton, and has only been found in the greisenized pegmatites and aplites that were exposed in the former Maguire Pit briefly described above. “Wolframite” (sensu lato) was common as crystals to about 1.5 mm, and varied from Fe-rich ferberite, FeWO4, to Mn-rich huebnerite, MnWO4, often within the same crystal (EMP). Small grains of a Pb-tungstate mineral, probably either raspite or stolzite (EDS, EMP, both Pb(WO4)), were present as inclusions in ferberite. Closely associated columbite-(Fe) in some cases contained up to 12 wt.% WO3 (EMP), resulting in tungstenian columbite-(Fe). Such high-W columbites are likely disordered, and may be “wolframoixiolite”, (Nb,W,Ta,Fe,Mn)2O4 (MINDAT, accessed 03/2016), but due to limited amounts of material this hypothesis has not been confirmed. Finally, scheelite (EMP), CaWO4, was noted as small grains in heavy mineral separates from several dikes, and ferberite pseudomorphs of octahedral to cubic morphology were found in one dike (No. 5), and are likely ferberite replacements of scheelite. Reference: Dewane, T. J., Van Schmus, W. R. (2007): U-Pb geochronology of the Wolf River batholith, north-central Wisconsin: Evidence for successive magmatism between 1484 Ma and 1468 Ma. Precambrian Research, V. 157, pp. 215-234. 16 �Mobilization of silica by flash heating of silica gel beneath the Sudbury Impact Layer, Baraga Basin, Michigan CANNON, W.F. U.S. Geological Survey, Reston, VA 20192 The Baraga Basin in northern Michigan is a structural depression in which sedimentary rocks of the Paleoproterozoic Baraga Group are preserved. The section includes ejecta-bearing rocks of the Sudbury Impact Layer (SIL) deposited 1,850 million years ago (Cannon and others, 2010). At the time of the Sudbury impact, the area of the Baraga Basin was covered by a shallow, perhaps intertidal, sea in which chemical sediments, now interlayered chert, carbonate minerals, and minor iron minerals were accumulating. The ejecta layer in this area, about 500 kilometers west of the impact site, is typically a few meters thick and consists mostly of impact glass in the form of spherules and shards and some interlayers rich in accretionary lapilli. The SIL was deposited across the area from a fast-moving curtain of ballistic ejecta that, in some of the area, was transformed into a base surge that incorporated parts of the underlying rock units into the ejecta-bearing layer. A distinguishing feature of the SIL in the Baraga Basin is the prevalence of a very siliceous matrix surrounding ejecta particles. The SIL also contains many particles that are themselves very siliceous, many of which are highly amygdular. Both features suggest an unusual involvement of silica in the deposition of the ejecta layer. Some of the uppermost beds of the sedimentary sequence at the time of the impact may have been silica gel. Gels can contain more than 90 wt. % water. When these gels came into contact with newly deposited high temperature ejecta, or were fragmented and incorporated into it, flash heating by the hot ejecta resulted in explosive release of steam and generation of silica-rich fluids that infused much of the ejecta layer. Most ejecta particles appear to have been solid during deposition, but flattened and stretched forms suggest that they were hot enough to have been soft and easily deformed. The presence of soft gels is shown microtexturally by siliceous (now cherty) inclusions in the ejecta that are distorted as indicated by both their external shape and internal features (Fig. 1). Especially compelling textures are shown by slivers of hardened silica that sharply penetrated into gelatinous clasts (Fig. 2). Many chert clasts have an unusual internal structure consisting of closely spaced amygdules whose outlines are shown by a film of sericite grains, but are otherwise entirely a recrystallized mosaic of fine quartz grains (Fig. 3). The term “popcorn chert” seems appropriate for them in that they appear to have formed by flash vaporization of their original high water content to produce these once highly vesicular silica masses. Commonly the amygdules are moderately to strongly flattened parallel to bedding (Fig. 4). Some samples show arcuate films of sericite grains that may be fragments of broken bubble walls (Fig. 5 ) suggesting that boiling also produced independent hollow structures that were later collapsed and broken. A well preserved contact of the base of the ejecta with underlying chert, seen in one drill hole, appears to have preserved this flash boiling in progress (Fig 6). The cuspate and discordant upper contact of chert containing thin magnetite laminae is overlain by a selvage of popcorn chert that may have formed in situ as chert gel was converted to a silicasteam mixture upon heating from the overlying ejecta. These features, together, suggest that a substrate of hydrous silica played an important role in determining the character of the SIL in the Baraga Basin. Cannon, W.F, Schulz, K.J., Horton, J. Wright, Jr., and. Kring, David A., (2010) The Sudbury impact layer in the Paleoproterozoic iron ranges of northern Michigan, USA: Geological Society of America Bulletin, v. 122, p. 50-75. 17 �1. Irregular mass of highly vesicular chert surrounded by glassy ejecta. 2. Sliver of chert that penetrated and deformed a clast of chert-magnetite gel. 3. Fragment of popcorn chert in glassy ejecta. Amygdules are outlined by a coasting of sericite. 4. Fragment of popcorn chert whose upper half is compressed and highly flattened. 5. Films composed mostly of sericite in a siliceous matrix. These curved films may be the walls of broken bubbles. 6. Upper contact of chert with magnetite lamellae and overlying glassy ejecta. Popcorn chert at contact. 18 �Traces of the Sudbury meteor impact in the western Gogebic Iron Range, northern Wisconsin CANNON, W.F.1, WOODRUFF, Laurel G.2, and SAARI, Stacy3 1 U.S. Geological Survey, MS 954, Reston, VA 20192 2 U.S. Geological Survey, 2280 Woodale Drive, Mounds View, MN 55112 3 Global Minerals Engineering, LLC, Hibbing MN 55746 Rock layers containing ejecta particles and other indications of the 1850 Ma Sudbury meteor impact have been observed in core from 12 drill holes along 40 km of the Gogebic Iron Range in northern Wisconsin. This ejecta-bearing material, found about 650-700 km west of the impact site near Sudbury, Ontario, is distinct from other occurrences of the Sudbury Impact Layer around the Lake Superior region. It occurs in multiple beds of reworked ejecta that commonly have ultra-potassic compositions. Ejecta-bearing material is found in two types of deposits: 1thick (up to 20 m) debris flows of coarse breccia, and 2- multiple thinner (0.02- 2 m) turbidite beds throughout the lower 25 m of the Tyler Formation (Fig.1). Figure 1. Stratigraphic relationships of ejecta-bearing beds to the regional stratigraphy of the Gogebic Iron Range. The Pence Member of the Ironwood Iron Formation is an even-bedded, generally moderately magnetic, iron formation containing minor units of granular ironformation. The Tyler Formation, in its lower part, is laminated argillite and shale containing variable amounts of carbon and pyrite. It contains thin units of chert and magnetic chert in some drill holes. Debris flows- Three drill cores contain coarse breccia from 10-20 m thick at the contact of the Pence Member of the Ironwood Iron Formation and Tyler Formation. They appear to be three separate units because breccias are absent in intervening drill holes. Where breccia is present, the thickness of the underlying Pence Member is less than in drill holes without breccia, suggesting that the debris flows incised their own channels into the Pence. The flows consist of a mixture of apparently local rocks and ejecta, including accretionary lapilli, and altered impact glass. They are massive to crudely bedded. A sparse set of quartz grains with shock metamorphic features (Fig. 2A) is a definitive indication of a relationship to the Sudbury impact. Turbidite beds- Ejecta-bearing beds that appear to be turbidites are interbedded with laminated shale and siltstone of the lower 25 m of the Tyler Formation. The number of beds varies from 0 to 8 in various drill holes and the beds vary in thickness from a few cm to 2 m. Many beds have prominent normal size grading. These beds cannot be correlated either in number or thickness between closely spaced drill holes showing that they are individual tongues of material with limited lateral extent, at least along the current strike of the iron range. The occurrence of multiple ejecta-bearing beds leads to the obvious conclusion that they are 19 �reworked material derived from the original ejecta blanket. These beds were commonly identified as tuff on drill logs, but a suite of features characteristic of ejecta, including a sparse but well developed suite of impact shocked quartz grains (Fig. 2B) indicate a link to the Sudbury event. Figure 2. Examples of quartz grains with relict planar deformation features indicative of intense shock. A-from a debris flow; B-from a turbidite bed. Figure 3. A-microcline phenocrysts in a vesicular glass fragment, B-microcline phenocrysts in a quench-textured microcline-rich fragment, C-amygdular glass fragment composed almost entirely of fine-grained mosaic-textured microcline. Both types of beds have ultra-potassic compositions. K2O ranges from 8.4 to 13.0 wt. % in five samples analyzed. The K content results both from abundant small phenocrysts and fragments of microcline, and from a finer mosaic of microcline that overprints original textures of clasts. The phenocrysts are a very early component of the ejecta, being enclosed both in accretionary lapilli and glass fragments (Fig. 3A, B). The finer microcline may be secondary-a result of seafloor K-metasomatism akin to formation of K-bentonites in younger volcanic ash. Individual clasts of K-rich ejecta indicate that they were altered before being incorporated into the present beds. The ejecta-bearing beds record a set of processes including: 1) deposition of an original ejecta blanket in a shallow marine setting probably not far from the current study area, 2) alteration of the ejecta to K-rich partly consolidated sediments, and 3) remobilization of the metasomatized ejecta to produce the debris flows and multiple ejecta-bearing turbidite beds. 20 �Microstructural comparison of the Hardrock Project at Geraldton, Ontario and the Coffee Gold Project, Yukon CARSON, Tracy, DELEY, Brittany, and HILL, Mary Louise Department of Geology, Lakehead University, 955 Oliver Road Thunder Bay, ON, P7B 5E1, Canada The Superior Province of the Canadian Shield and the North American Cordillera both host extensive gold mineralization. These regions are distant from each other and very different in age, but the similarities between microstructures within gold-hosting units are striking. Separate microstructural studies were done on gold-hosting units in the Coffee Gold Project south of Dawson, Yukon and the Hardrock gold project at Geraldton, Ontario and the results are compared. The focus in each project is to examine the microstructures within gold-hosting rock units to determine the protolith of the host and assess structural control on gold mineralization. Based on mineralogy, each study concludes that a felsic igneous rock is the likely protolith to the gold-hosting lithology. Each study finds similar styles of deformation resulting in microstructures typical of a mylonite. The microstructures include a foliation defined by parallel alignment of muscovite and rigid porphyroclasts of feldspar in a fine-grained groundmass. Together, these observations lead to the conclusion that the host for gold mineralization in each study is a deformed felsic plutonic rock. Both gold-hosting lithologies have similar mineralogy consisting dominantly of feldspar, quartz and muscovite. Rigid porphyroclasts of plagioclase are situated within a groundmass composed of <0.1mm quartz, feldspar and muscovite. Plagioclase is the dominant feldspar in both studies. Feldspar is present both in the groundmass and as rigid porphyroclasts. The plagioclase porphyroclasts commonly have asymmetrical tails, indicating non-coaxial strain, and commonly contain deformation twins. Sericite alteration on porphyroclasts is evidence for the presence of hydrous metamorphic fluids. In the Geraldton study these rigid porphyroclasts are fractured. Quartz is common within the groundmass in both studies, and late quartz veins are abundant within the Yukon study. Undulose extinction and subgrain boundaries indicate dislocation creep in quartz. Dimensional preferred orientation of muscovite defines the foliation and muscovite bends around the rigid porphyroclasts. The Coffee Gold Project is hosted within the Yukon–Tanana Terrane bounded between the Tintina and Denali Faults and along strike of the Teslin Fault, this area is located within the Tintina Gold Province. The study examines three zones, Kona, Supremo and Latte. In each of the zones a felsic plutonic rock with varying degrees of deformation was interpreted to be the protolith. The Kona zone is mapped as granite and is the least deformed of the three zones. The Kona zone’s mineralogy consists of feldspar, quartz, sericite and biotite with an average grainsize of approximately 2.5mm. Deformation in the Kona zone includes undulose extinction, subgrain boundaries and serrated grain boundaries in the quartz as well as sericite alteration of feldspars. Supremo and Latte zones show increased deformation with the development of a foliation bending around rigid porphyroclasts and deformation features in quartz and feldspar. Quartz microstructures such as subgrain boundaries, serrated grain boundaries and undulose extinction are common and indicate deformation by dislocation creep. Rigid porphyroclasts of feldspar are common providing a minimal original grainsize of ~3mm. Deformation twins observed within feldspars provide further evidence of deformation. The Latte zone is mapped as biotite schist, however biotite is present in only small amounts. The mineralogy in the Latte zone 21 �consists dominantly of quartz and feldspar suggesting a felsic igneous protolith. A relationship between deformation and gold mineralization is observed; as deformation increases the gold mineralization increases. Gold mineralization based on published assay values was lowest in the Kona zone and increased in the more deformed Latte and Supremo zones. The protolith of all three zones is likely a deformed felsic plutonic rock with varying degrees of metamorphism and deformation. The Hardrock Project is located within the Beardmore-Geraldton belt. The BeardmoreGeraldton belt lies between the Wabigoon and Quetico subprovinces and it is comprised of multiple fault-bounded metavolcanic and metasedimentary belts. The Hardrock Project is one of several past-producing gold mines situated within the Beardmore-Geraldton gold camp. The Beardmore-Geraldton gold camp is bounded by major steeply dipping structures that are comparable to structures in the Tintina Gold Province. Mineralogy and microstructure of the “porphyry” within the Hardrock Project are consistent with a deformed felsic plutonic rock similar to the Latte and Supremo zones in the Coffee Gold Project. The Latte and Supremo zones in the Coffee Gold Project have mylonitic features that are comparable to the “porphyry” at the Hardrock Project. The “porphyry” is known to have significantly more mineralization than its protolith, Croll Lake stock, suggesting that gold mineralization is related to deformation. This is similar to deposits within the Tintina Gold Province such as the Coffee Gold Project. The Beardmore-Geraldton belt may be an older Cordilleran-type gold province with structural controls on gold mineralization similar to the Tintina Gold Province. This comparison gives insight into the similarities of shear-zone-hosted gold deposits. In both examples, mylonitized felsic plutonic rock hosts gold mineralization. 22 �Utility of the horizontal-to-vertical spectral ratio (HVSR) passive seismic method for determining Quaternary sediment thickness and bedrock elevation in north-central Minnesota: Fun with little control and generally poor data Val W. Chandler and Amy L. Radakovich Minnesota Geological Survey, 2609 Territorial R., St. Paul, MN 55114 Work continues in Minnesota on using the horizontal-to-vertical-spectral ratio (HVSR) passive seismic method to help map the thickness of Quaternary glacial sediments and the topography of the bedrock surface. A total of 570 passive seismic stations have been acquired in and around Becker, Cass, Hubbard, and Wadena Counties in north-central Minnesota as part of the County Geologic Atlas (CGA) program of the Minnesota Geological Survey (MGS). This area is characterized by thick and complex glacial sequences, widely scattered drillhole control, and, unfortunately, generally poor HVSR data. Ideally a HVSR spectrum yields a largeamplitude, singular peak that represents the bedrock surface (Chandler and Lively, 2014; Lane and others, 2008). In north-central Minnesota, the observed HVSR spectra instead commonly consist of low-amplitude and multiple peaks, where recognition of bedrock signatures is difficult when observing spectra individually. The cause of the poor signal quality is unknown, but could be related to a highly irregular bedrock surface or to the complexity of the overlying glacial sequences. In areas with these types of issues we have made increased use of multi-station spectral cross-sections that have been converted to approximate elevation sections (Fig. 1). This conversion uses a power-law calibration that is based on observed HVSR results at control points, usually consisting of water wells or seismic refraction soundings. Due to the lack of a sufficient number of control points in the study area, a generalized calibration is used, which is based on 303 widely distributed control points in Minnesota. More refined sections may eventually be possible with locally derived calibrations, but our generalized approach produces useful sections that readily allow lateral correlation of HVSR features within a framework of available geologic control. Figure 1. Color HVSR section across central Becker County. West end of profile located at UTM 252686E/5203087N; East end of profile located at UTM 335129E/5210337N) White X’s designate HVSR-based estimates of the bedrock surface. White diamonds indicate bedrock horizons at control points, either drill-holes with well number (6 digit label), or seismic refraction sites (4 digit label). Bedrock horizons are: K, the top of Cretaceous sediments; Sap the top of saprolite; and Fbr the top of fresh bedrock. Individual stations are labeled in black. AM approximates the western margin of the Alexandria Moraine. Vertical scale is elevation and horizontal scale is cumulative distance between successive stations. 50X vertical exaggeration. 23 �Using the cross-sectional approach as a guide, viable maps have been produced that estimate the elevation and depth of the Precambrian bedrock surface that underlies the study area. Trough- like depressions in the Precambrian surface are indicated beneath northeastern and central Becker County and beneath northeastern Wadena and eastern Hubbard Counties, where total depths are 500-1000 ft. (150-300 m). These depressions are known to locally contain Cretaceous strata, but the HVSR method cannot discriminate the soft Cretaceous rocks from the glacial sediments. Broad rises in the Precambrian surface are indicated near the junction of Becker, Hubbard, and Wadena Counties, and much of southern and eastern Cass Counties. These preliminary interpretations have been largely corroborated by follow-up seismic refraction profiling and test drilling. The results of this study indicate that when used with the proper precautions, the HVSR method can be useful for bedrock depth and elevation studies, even under less-than-ideal conditions. REFERENCES Chandler, V. W., and Lively, R. S., 2014, Evaluation of the horizontal-to-vertical spectral ratio (HVSR) passive seismic method for estimating the thickness of Quaternary deposits in Minnesota and adjacent parts of Wisconsin: Minnesota Geological Survey Open File Report 14-01, 52 p. Lane, J. W., Jr., White, E. A., Steele, G. V., and Cannia, J. C., 2008, Estimation of bedrock depth using the horizontal-to-vertical (H/V) ambient-noise seismic method: in Symposium on the Application of Geophysics to Engineering and Environmental Problems, Proceedings of the Environmental and Engineering Geophysical Society, 13 p. 24 �Bedrock Geology of the Devilfish Lake Area, Cook County, Minnesota CLARK, Jonathan1, ESHLER, Kristen2, GROFF, Patrick3, MCCLENDON, Taylor4, RODE, Alexander5, SALINGS, Emily6, SPINELLI, Kristen7, VANDER WYST, Kyle8, WALSH, Aiden9, ASP, Kristofer10, and LARSON, Phillip11 1 Northwest Missouri State University, Maryville, MO, 2Temple University, Philadelphia, PA,3California State University, East Bay, Hayward, CA, 4Huffington Department of Earth Science, Southern Methodist University, Dallas, TX, 5Wayne State University, Detroit, MI, 6Missouri State University, Springfield, MO, 7Department of Geological Studies, Binghamton University, Binghamton, NY, 8University of Wisconsin-Milwaukee, Milwaukee, WI, 9 School of the Environment, Washington State University, Pullman, WA, 10Department of Earth and Environmental Sciences, University of Minnesota Duluth, 1114 Kirby Drive, Duluth, MN 55812, 11Vesterheim Geoscience PLC, 120 Greenwood Ln, Duluth, MN 55803 and Department of Earth and Environmental Sciences, University of Minnesota Duluth, 1114 Kirby Drive, Duluth, MN 55812   Grout and others (1959) has heretofore been the primary published work investigating the contact between the reversed polarity lower northeast sequence of the North Shore Volcanic Group (NSVG) and the early magmatic stage of the Duluth Complex (DC) in the vicinity of Devilfish Lake, and by extension the eastern limit of the Mesoproterozoic Duluth Complex, Minnesota, USA. Geologic mapping and gravity surveying by the Precambrian Research Center Field Camp in the summers of 2014 and 2015 provides additional detail to better contextualize Grout’s pioneering work with more recent understanding of the geological and geophysical nature of the DC, NSVG, and Midcontinent Rift as a whole (e.g. Miller et al., 2012). Targeted mapping addressed 1) understanding the contact between the DC and hangingwall lower northeast sequence NSVG volcanic rocks, 2) identifying small-scale mafic intrusive dikes and sills cross-cutting the NSVG and/or DC, and 3) infilling state-wide gravity station coverage along roads and trails constructed since prior gravity surveying in the late 1960s. In this area, volcanic rocks of the ~1108 Ma lower northeast series of the NSVG form an ~9 km thick, shallow, southwest-dipping sequence overlying the Paleoproterozoic Rove Formation. The base of the sequence is composed of relatively primitive tholeiitic basalts of the Grand Portage lavas, and is capped by the relatively evolved basaltic andesites, andesites, icelandites, and rhyolites of the Hovland lavas. The Hovland lavas are characterized by abundant feldspar phenocrysts. This study recognizes a sequence of thick-bedded, relatively evolved, aphyric basalts and basaltic andesites intermediate to the Grand Portage and Hovland lavas, herein named the Esther lavas. The Grand Portage and Esther lavas successively truncate against the DC toward the west. The base of lower northeast sequence was subsequently intruded by the Crocodile Lake gabbro (CLG) and Cucumber Lake granophyre (CL); these intrusions have been correlated with the ~1109 Ma Poplar Lake intrusion. A general lack of volcanic or sedimentary xenoliths in the CLG and CL suggest they intruded as sheet-like bodies along the basal contact of the NVSG with the underlying Rove Formation, with little incorporation or removal of wall rock, suggesting the pinch-out of the Grand Portage and Esther lavas may be a primary characteristic reflecting original volcanic basin geometry. The contact between the DC (CL) and overlying NSVG is much more irregular than indicated by previous mapping, consistent with a relatively shallow, southward-dipping contact. Hornfelsed NSVG volcanic rocks immediately overlying this contact form a prominent linear trend of hills immediately south of the contact, and apophyses of granophyre are common in the hornfels. Overall, the contact between the DC and NSVG appears to mirror the dip of flows within the NSVG. 25 �Mapping identifies at least four mafic intrusive phases with distinct mineralogy and geochemistry cross-cutting the NSVG and DC. Sill-like(?) gabbroic intrusions cross-cutting the Esther lavas are the only pre- or syn-DC intrusions recognized by this mapping. Multiple mafic intrusive phases cross-cutting the DC occur as dikes in both NW-SE and SW-NE trending structures. E-W oriented, normal polarity dikes (Chester dikes) contains abundant anorthosite xenoliths, suggesting correlation with the ~1096 Ma Beaver River diabase. Mafic dikes cross-cutting the CLG display both coarse-grained and chilled, aphanitic textures, suggesting multiple intrusive events during the cooling of the CLG. Previous to this study, state-wide gravity data indicated an ~45 mgal gradient in the Bouguer anomaly over ~5 km between points collected over the Rove Formation (north) and the contact between the CLG and CL intrusions and hangingwall NSVG volcanics (south). Infill surveying of this gap suggests that the CLG and CL intrusions are southward-dipping sheet-like bodies. Truncation of this E-W oriented gradient coincident with the eastern extent of the DC suggests intrusion of the CLG/CL was controlled in part by subsidence along a rift-perpendicular tear fault. Mapping results support the hypothesis that the early, eastern extent of the DC was emplaced as sheet-like intrusions at the base of the lower northeast sequence NSVG volcanic pile. The NSVG pile may preserve pinch-out of individual volcanic units reflecting original basin geometry. Multiple geochemically, texturally, and mineralogically distinct mafic dikes cross-cutting both the early DC and NSVG attest to a heretofore unrecognized long and complex history of post-early magmatic stage igneous activity in this sector of the Midcontinent Rift. References Grout, F.F., Sharp, R.P., and Schwartz, G.M., 1959, The geology of Cook County Minnesota: Minnesota Geological Survey Bulletin 39, 163 p., 16 pls. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, 207 p. 26 �Geochemistry and petrogenesis of volcanic rocks in the Coldwell Alkaline Complex; new insights from the Wolfcamp Lake volcanic rocks CUNDARI, Robert1, HOLLINGS, Peter2, GOOD, David3, and DAVIS, Sarah2 1 Resident Geologist Program, Ontario Geological Survey, Ministry of Northern Development and Mines, 435 James St. S., Suite B002, Thunder Bay, ON, P7E 6S7 Canada 2 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1 Canada 3 Department of Earth Sciences, University of Western Ontario, London, ON N6A 5B7 Recent work carried out on volcanic rocks present within the Coldwell Alkaline Complex has unveiled new insights into the genesis of Midcontinent Rift-related volcanic rocks along the North Shore of Lake Superior, Ontario. Coldwell Complex volcanic rocks display broadly similar trace element patterns to basalt type I and II (cf. Nicholson et al. 1993) suggesting eruption in the early magmatic stage of the Midcontinent Rift (Good et al. 2015; Cundari et al. 2012). This is supported by the intrusion of the Eastern Gabbro suite into Upper and Lower metabasalt units at 1108±1 Ma and the intrusion of centre II syenites into the Coubran Lake and Wolfcamp Lake basalts at 1109+8-4 Ma (Heaman and Machado, 1992). Good et al. (2015) proposed three distinct basaltic packages present within the Coldwell Complex, listed here as progressively more evolved units based on Mg number and Ni abundance (Figure 1a); the Lower meta-basalts (LMB), the Upper meta-basalts (UMB) and the Coubran Lake basalts (CLB). The Wolfcamp Lake basalts (WLB) represent a fourth unit in this sequence more evolved than previously recognized volcanic units in the Coldwell Complex. WLB display an average TiO2 content of 1.86 compared to 0.86 for the CLB, 1.15 for the UMB, and 1 for the LMB suggesting the Wolfcamp rocks are distinctly different and more evolved than those suites (Figure 1b). The WLB also exhibit the highest total alkali abundance of all four volcanic units present within the Coldwell complex. WLB display greater LREE abundances compared to the CLB but lack a negative niobium anomaly suggesting the LREEenrichment is not due to crustal contamination. Additionally, SiO2 content for the WLB is too low to be explained by contamination of continental crust. The CLB were interpreted to be the volcanic equivalent of the Two Duck Lake gabbro based on trace element abundances by Cundari (2012). Trace element abundances for the Geordie Lake gabbro trend towards the WLB suggesting a co-genetic relationship whereby the WLB may represent the volcanic equivalent of the Geordie Lake gabbro. Gd/Ybn ratios for the WLB are broadly similar to those for the CLB suggesting a similar depth of partial melting. The extreme LREE enrichment exhibited by the WLB compared to the CLB may be due to lower degrees of partial melting although it should be noted that field evidence such as a reddish staining of alkali material suggests that WLB have been altered by syenite intrusions which may have contributed to the LREE content of these rocks. A number of mechanisms may be responsible for the LREE enrichment characteristics observed in the Coldwell Complex volcanic rocks, notably the CLB and the WLB. Extreme LREE enrichment may be controlled by apatite accumulation, suggested by a positive correlation between Th and Gd with P2O5 although LREE abundances may be too high to be accounted for entirely by apatite. An alternative mechanism for generation of LREE enriched volcanic rocks may be partial melting of an enriched mantle source suggesting a possible relationship with Coldwell syenitic intrusions. Further work is required to develop the relationships between the volcanic units present within the Coldwell Complex, most notably, the relationship between the Coubran Lake and Wolfcamp Lake volcanic units. Modelling of fractional crystallization of average Coubran basalt composition trending toward Geordie Lake gabbro and Wolfcamp basalt may explain the petrogenesis of these units and establish the magmatic relationship between them. 27 �a b c d Figure 1: Major and trace element abundances of Coldwell Complex volcanic units compared to Coldwell Complex intrusive units. Figure 2: La/Sm vs. Gd/Yb plot of Coldwell Complex volcanic units compared to Coldwell Complex intrusive units. References Cundari R., 2012, Geology and geochemistry of Midcontinent rift-related igneous rocks: M.Sc. thesis, Thunder Bay, ON, Lakehead University, 122 p. Good, D.J., Hollings, P., Cundari, R. and Ames, D. Significance of LREE-enriched mantle source to genesis of basalt in the Coldwell Alkaline Complex, Midcontinent Rift, Ontario. 61st Institute on Lake Superior Geology, Dryden, ON, May 20-24, 2015, Proceedings Volume 61, Part 1, p.70-71. Heaman, L.M. and Machado, N., 1992. Timing and origin of midcontinent rift alkaline magmatism, North America: evidence from the Coldwell Complex; Contr. to Mineralogy and Petrology, 110, p.289-303. Nicholson, S.W., Shirey, S., Schulz, K., and Green, J., 1997. Rift-wide correlation of 1.1 Ga Midcontinent rift system basalts: implications for multiple mantle sources during rift development. Canadian Journal of Earth Sciences 34: 504-520. 28 �Mineralogy and Geochemistry of the Wolfcamp Lake Basalts DAVIS1, Sarah, HOLLINGS1, Pete and CUNDARI2, Rob 1. Department of Geology, Lakehead University, Thunder Bay, ON, Canada 2. Ontario Geological Survey, Ministry of Northern Development, Mines and Forestry, Suite B002, 435 James St. South Thunder Bay, ON P7E 6S7 Canada The Wolfcamp Lake Basalts are Midcontinent Rift-related volcanic rocks (Walker et al., 1993) that have been linked with other MCR volcanic rocks such as the Coubran Lake, Penn Lake, Bamoos Lake and Foster Island volcanic units along the north shore of Lake Superior which have all been interpreted to be roof pendants within the Coldwell Complex (Puskas, 1967; Walker et al., 1993). Basalts of the Wolfcamp Lake volcanic unit are exposed in two main areas bisected by the TransCanada highway northwest of Marathon, Ontario. The entire unit is approximately 2 km east-west by 4 km north-south with one large exposure northeast of the highway east of Wolfcamp Lake and a second exposure south of the TransCanada highway cropping out as large cliffs, railway cuts and outcrops on the north shore of Lake Superior near Port Munroe. Flow thicknesses are generally 2-4 m with variations present locally. The main lithology comprises basalt flows which are dominantly ophitic or sub-ophitic. The mineralogy is dominated by feldspars (primarily plagioclase), olivine and pyroxenes. Alteration minerals including hornblende, sillimanite, chlorite, biotite, opaques (primarily magnetite) are present in all samples at concentrations from 3% up to 40%. All of the samples of the Wolfcamp Lake basalts show very consistent trace element geochemistry with OIB-like characteristics on primitive mantle normalised plots. They are characterised by negative zirconium, hafnium and titanium anomalies with no negative niobium anomalies observed. The Wolfcamp Lake basalts were compared to the closest volcanic rocks which are the Coubran Lake Basalts, but differences were observed when compared to other volcanic rocks in the area. When compared to the three types of basalts identified in the Coubran Lake unit the Wolfcamp Lake basalts lack the strong negative niobium anomaly displayed by the Coubran Types A and B which was interpreted to be the result of contamination (Fig. 1). The Wolfcamp Lake Basalts also show lower Mg numbers than the Coubran Lake Basalts indicating that the former are more evolved. This suggests that the Wolfcamp basalts are uncontaminated MCR magmas despite their more evolved compositions. The Wolfcamp Lake basalts are broadly similar to those of the Lower Osler volcanic rocks, with similar LREE patterns and no negative niobium anomaly (Fig. 2). However, the Osler volcanics display a lower La/Smn ratio on a Gd/Ybn vs La/Smn diagram (Fig. 3) indicating a more primitive composition compared to the Wolfcamp Lake Basalts. 29 �Figure 1: Spider plots comparing geochemistry of Average Wolfcamp Lake Basalts to Coubran Lake Basalt Types A, B and C. Comparative data from Cundari (2012). Figure 2: Spider Plot comparing Wolfcamp Lake Basalts to the Lower Osler Volcanics. Comparative data from Hollings et al. (2007). Figure 3: Plot of La/Sm vs Gd/Yb showing Midcontinent Rift volcanics. Normalizing values from Sun and McDonough (1989). Comparative data from Cundari (2012). REFERENCES Cundari, R. 2012. Geology and Geochemistry of Midcontinent Rift-related igneous rocks. Masters Thesis, Lakehead University, Thunder Bay, Ontario. Good, D.J. 1993. Genesis of Copper-Precious Metal Sulfide Deposits in the Port Coldwell Alkalic Complex, Ontario. Ph.D. Thesis, McMaster University, Hamilton. Ontario Geological Survey, Open File Report 5839. Hollings, P., Fralick., P. and Cousens, B., 2007c. Early history of the Midcontinent Rift inferred from geochemistry and sedimentology of the Mesoproterozoic Osler Group, northwestern Ontario. Canadian Journal of Earth Sciences 44: 389-412. Puskas, F.P. 1967. Geology of Port Coldwell Area, District of Thunder Bay, Ontario. Ontario Department of Mines. Open File Report 5014. Sun, S.S. and McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes, In: Summary of field work. Ontario Geological Survey, Miscellaneous Paper 100, P. 26-29. Walker, E.C., Sutcliffe, R.H., Shaw, C.S.J., Shore, G.T., Penczak, R.S., 1993. Precambrian Geology of the Coldwell Alkalic Complex. Ontario Geological Survey. Open File Report 5868. 30 �Origin of the gold-hosting porphyry at Geraldton, Ontario DELEY, Brittany and HILL, Mary Louise Department of Geology, Lakehead University, 955 Oliver Road Thunder Bay, ON, P7B 5E1, Canada The lithologic unit known as the “porphyry” at Geraldton, Ontario is host to widespread gold mineralization, but its origin is poorly understood. The porphyry occurs within the BeardmoreGeraldton greenstone belt in the Superior Province of the Canadian Shield. Gold was first discovered in the Beardmore-Geraldton belt in 1917, and production occurred between 1930 and 1970 at 20 mines throughout the belt, producing 4.1 million ounces of gold (www.premiergoldmines.com, 2016). Exploration is currently active again in the BeardmoreGeraldton belt. Microstructural analysis was done on samples from the “porphyry” to better understand its origin. Samples from the porphyry contain approximately 1mm porphyroclasts of plagioclase within a groundmass composed of approximately 0.1mm grains of quartz, plagioclase, and muscovite. The fine-grained groundmass is formed as a result of grainsize reduction by dislocation creep. The plagioclase porphyroclasts commonly have asymmetrical tails, and muscovite crystals tend to bend around the porphyroclasts. Asymmetrical tails are an indication of non-coaxial strain. Plagioclase porphyroclasts are commonly fractured with calcite infilling the fractures. The presence of calcite indicates a CO2-bearing fluid phase. Sericite alteration occurs on porphyroclasts indicates the presence of H2O-bearing metamorphic fluids. Plagioclase commonly contains deformation twins. Undulose extinction and subgrain boundaries indicate dislocation creep in quartz. The porphyry also has a well-developed foliation defined by parallel alignment of muscovite. Based on microstructural analysis, it appears that the most likely protolith for the porphyry is a felsic plutonic rock. The microstructures are typical of a mylonite. Characteristic features of mylonitic rocks include a strong foliation produced by the parallel alignment of minerals, the presence of a fine grained matrix produced by grainsize reduction mechanisms with porphyroclasts, and the presence of a certain asymmetry. Approximately 30km east of Geraldton, near Longlac, Ontario, is a 150 km2 elliptical, granodiorite intrusion, the Croll Lake stock. This intrusion is the nearest felsic and plutonic rock to the porphyry at Geraldton. The Croll Lake stock is also deformed. Deformational features in quartz include undulose extinction, subgrains and serrated grain boundaries. Plagioclase commonly contains deformation twins. Evidence for deformation of the stock increases westward toward its margin where it resembles the porphyry. Microstructural analysis of the porphyry and the Croll Lake stock suggest that the “porphyry” is a mylonitized fragment of the Croll Lake stock. 31 �PGE Mineralization in the Northern Ultramafic Center of the Lac des Iles Complex, Ontario: Evidence of Magmatic and Hydrothermal processes Djon, M. L.1,3, Olivo, G.R.1 Miller, J.D.2, Peck, D.C.3and Joy, B1 1 Queen's University, Department of Geological Sci. and Geological Engineering, Kingston, Ontario K7L 3N6 Department of Geological Sciences, University of Minnesota Duluth, Duluth, MN 55812 3 North American Palladium Ltd, 10th Avenue, Thunder Bay, Ontario, P7B 2R2. 2 The Archean Lac des Iles Complex in northwest Ontario consists of a mafic complex (South LDI complex) and a predominantly ultramafic complex (North LDI complex). The South LDI complex is ~6 km long and 1.5-2.5km wide and comprises the Mine block intrusion, South LDI intrusion, and Camp Lake Intrusion. The Mine Block Intrusion is the best known of the individual intrusions within the LDIC due to the presence of significant Pd-rich platinum group element (PGE) mineralization that has been mined since 1993 (Lavigne and Michaud 2001). The North LDI complex is a composite, tadpole-shaped, intrusive body with dimensions of 6 km by 4.5 km and featuring a preponderance of ultramafic over mafic cumulate rocks that are distributed in two centres: the Southern Ultramafic Centre and the Northern Ultramafic Centre (Sutcliffe and Sweeney 1985). Various exploration programs conducted over a 50 year span have identified PGE occurrences with >1 g/t Pd+Pt. The best documented of these ultramafic-hosted PGE occurrences are in the Sutcliffe Zone, which consists of four subparallel, stratiform PGE-enriched horizons exposed within the Northern Ultramafic Centre (NUC). Detailed field mapping, petrographic studies, and lithogeochemistry of recently acquired drill cores indicate that the 4 km diameter, >1900m-thick, lopolith-shaped NUC can be subdivided into an Eastern Marginal Zone and a Layered Series. Furthermore, the Layered Series can be divided into fourteen cyclic units defined by cumulus mineral paragenesis of two general types. Cyclic unit type A (CUA) displays a cumulate stratigraphy of Ol+Sp (dunite)  Ol+Opx+Cpx±Sp (olivine-websterite) Cpx+Opx (websterite)  Pl+Opx±Cpx (gabbronorite). Cyclic unit type B (CUB) displays a simpler cumulate sequence of Ol+ Sp (dunite)  Ol+Cpx (olivine clinopyroxenite). Cyclic unit boundaries are sharp between similar types but also where CUA overlies CUB. In contrast, where CUB overlies CUA, the contact is gradational and is marked by the complex hybrid unit. Within individual cyclic units, cyclical cryptic variation in mg# of olivine (Fo) and pyroxenes (En) shows a progressive upward decrease followed by an abrupt to gradual increase at or just below cyclic unit boundaries. Olivines of CUB rocks have lower Ni concentrations and higher Fo contents than olivines in CUA rocks. CUA and CUB samples are further distinguished by having different abundance ranges in incompatible elements, and Zr/Y and Ce/Yb ratios. Djon et al. (submitted) interpreted CUA and CUB type cyclic units to have formed from two compositionally distinct parental magmas. CUA parental magmas would have been enriched in Si, Ni and incompatible trace elements and depleted in Ce/Yb ratio relative to CUB parental magmas. The cyclical cryptic variation within the NUC cyclic units is suggested to have formed by crystal fractionation within individual magma inputs randomly interrupted by alternating recharge of CUA or CUB magmas. The PGE-enriched horizons occur exclusively in four of the CUA-type cyclic units and show very different enrichment patterns depending on whether it is sharply overlain by another CUA unit or is capped by hybrid zone below a CUB unit (Fig. 1). The upper three PGE- enriched zones occur in CUA6, -8 and -11 (which are overlain by the hybrid units and CUB) and are hosted in websterite and/or gabbronorite units. They form 15-20m-thick disseminated zones, where the PGE grades gradually increase toward the top of the cycle and rapidly decrease into the hybrid unit. The lowermost PGEenriched zone occurs halfway through the CUA-5, which is overlain by CUA-6. It is hosted in olivine websterite and websterite cumulates, and forms 10-15m-thick zone in which PGE grades slowly decreases toward the top of the cycle (Fig. 1). 32 �The host rocks show weak to moderate hydrothermal alteration with variable proportion of mainly actinolite-tremolite, serpentinite, and chlorite. The primary PGE mineralization is commonly found in association with disseminated and blebby sulfide (0.2-2 vol%; mainly pyrrhotite, chalcopyrite, and pentlandite with minor cubanite, troilite, and cobaltite) and locally with primary hydrous phase (fluorine- bearing magnesiohornblende). In altered rocks, the primary sulfides have been partially replaced by chalcopyrite, pentlandite, sphalerite, heazlewoodite, and chalcocite. Palladium occurs either in solid solution with primary pentlandite and cobaltite or as Pd-telluride; however, Pd-bearing minerals containing mainly Pb and to lesser extent Ge, Sn, Bi, Rh, and Ag (e.g. zvyagintsevite; Pbbearing palladium telluride) occur at the margin of secondary sulfides (e.g. heazlewodite, pentlandite) and altered chrome-spinel, along pyroxene fractures, or included in serpentine and amphibole. Ptbismuthotellurides and sperrylite commonly occur associated with primary sulfides at sulfide–silicate contacts and, minor laurite is found enclosed in chrome-spinel. In general, PGE enrichments are related to increases in total S, Cu and Zr contents and a decrease in Mg:Fe ratios of pyroxenes. The mineralized zones averaging 0.358 ppm Pd+Pt ( 643 ppm Pd+Pt in 100% sulfide), Pd/Pt and Pd/Ir ratios ranging from 0.9 to 3.5 and 35 to 537, respectively, and a wide range of S/Se ratios (500-6000). The highest PGE (Pd+Pt) grades up to 11 ppm (4377 ppm in 100% sulfide) are found in serpentinized olivine websterite, which yield an average Pd/Pt ratio of 3.5 and a S/Se ratio of approximately 2.000. Our data suggest that magmatic crystal fractionation leading to early sulfur saturation and late concentration of base and precious metals in a volatile-rich fluid and possibly magma mixing can be accounted as dominant process for the different PGE occurrences throughout the CUA-type cyclic units. The PGE contents could have been enhanced during a post-magmatic stage by hydrothermal fluids which have interacted with early-formed assemblages resulting in dissolution and redistribution of PGE. Figure 1: (A) Detailed crosssection of the Sutcliffe Zone within the Layered Series showing the distribution of the PGE enriched horizons with their stratabound Pd and Pt concentrations. (B) illustrates the stratigraphic location of the PGE-rich horizons throughout the cyclic unit types from drill hole NL12101, in relation with the variation of modal rock type, the whole rock concentrations of selected PGE and trace elements (from North American Palladium Ltd database) and the average enstatite (En') content of clinopyroxene (Cpx) from (Djon et al., submitted) REFERENCES Lavigne, M.J., Michaud, M.J. 2001 Geology of North American Palladium Ltd.'s Roby Zone Deposit, Lac des Iles. Exploration and Mining Geology 10: 1-17 Djon, M. L., Miller, J.D., Olivo, G.R., and Peck, D.C. (submitted) Petrogenesis of Cyclic Units in the Northern Ultramafic Centre of the Lac des Iles Complex, Ontario, Canada: Evidence of Two Distinct Parental Magmas. Cont to Min & Petro. Sutcliffe, R.H. and Sweeny, J.M., 1986. Precambrian Geology of the Lac des Iles Complex, District of Thunder Bay, Ontario. Ontario Geological Survey, Map 3047, Geological Series-Preliminary Map, scale 1:15840. 33 �GEOCHEMISTRY OF DEEP AND SHALLOW WATER ARCHEAN BANDED IRON FORMATIONS, AND THEIR POST DEPOSITIONAL IMPLICATIONS IN THE WESTERN SUPERIOR PROVINCE, CANADA Dolega, S., and Fralick, P. Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario P7B 5E1 Canada (sdolega@lakeheadu.ca) Geochemical studies have been conducted on banded iron formations for over 40 years to attempt to better understand their synthesis, depositional environment and ocean chemistry. Previous studies have made assumptions that the geochemistry of the iron formation reflects the exact geochemistry of seawater during time of formation (Konhauser et al. 2011; Planavsky et al. 2014). These studies have not taken to account the possibilities of a post depositional mechanism, such as diagenesis and metamorphism, which can affect the mobility of elements and can alter interpretations inferred from geochemical analyses. Several locations containing Archean banded iron formations have been chosen for this study. They include the banded iron formations of the Beardmore-Geraldton Belt, Wabigoon Subprovince, Lac-St Joseph of the Uchi Subprovince, Shebandowan Greenstone Belt of the Wawa Subprovince, Weagamow – North Caribou Greenstone Belt of the Sachigo Subprovince, Red Lake Greenstone Belt of the Uchi Subprovince and the Steep Rock Group of the Wabigoon Subprovince. These locations were chosen based on their differences in age, depositional environment and differences in metamorphic grade. Geochemical analyses were conducted using an Inductively Coupled Plasma Optical Emission Spectrometer and Inductively Coupled Plasma Mass Spectrometer at Lakehead University. Preliminary results show that the Na/K ratio is greater in the magnetite-rich samples and lower in the hematite-rich samples (Figure 1). This is consistent for all the iron formations chosen for this study. The differences in the Na/K ratio cannot reflect the original geochemistry of seawater because the magnetite and hematite layers form by the same mechanism, the precipitation of ferric hydroxides. During diagenesis, magnetite will form where organic carbon reduces some of the iron (Drever, 1974). Changes in the Na/K ratio within the iron formation indicate that there has been element mobility after deposition either by diagenesis or regional metamorphism. Therefore, any geochemical conclusions inferred from banded iron formations without taking into account element mobility must be questioned. 34 �Na vs K ratios in Banded Iron Formations 140 Beardmore, Gerladton and Lac St. Joseph Magnetite 120 Fe2O3/FeO 100 Beardmore, Geraldton and Lac St. Joseph Hematite 80 60 40 Musselwhite Magnetite 20 0 0 20 40 60 80 100 120 Na2O/K2O Figure 1: Ratio scatter plot showing how Na and K concentrations vary with the mineralogy of the iron bearing phase in the iron formations studied. References Drever, J. I. (1974). Geochemical Model for the Origin of Precambrian Banded Iron Formations. Geological Society of America Bulletin, 85, pp. 1099-1106. Konhauser, K. O., Lalonde, S. V., Planavsky, N. J., Pecoits, E., Lyons, T. W., Mojzsis, S. J., Rouxel, O. J., Barley, M. E., Rosìere, C., Fralick, P. W., Kump, L. R., Bekker, A. (2011). Aerobic bacterial pyrite oxidation and acid rock drainage during the Great Oxidation Event. Nature, 478(7369), 369-373. doi:10.1038/nature10511 Planavsky, N. J., Asael, D., Hofmann, A., Reinhard, C. T., Lalonde, S. V., Knudsen, A., Wang, X., Wang, X., Ossa Ossa, F., Pecoits, E., Smith, A. J., Beukes, N. J., Bekker, A., Johnson, T. M., Konhauser, K. O., Lyons, Rouxel, O. J. (2014). Evidence for oxygenic photosynthesis half a billion years before the Great Oxidation Event. Nature Geoscience Nature Geosci, 7(4), 283286. doi:10.1038/ngeo2122 35 �Progress on Geophysical Mapping of the Northeast Iowa Intrusive Complex DRENTH, Benjamin1, ANDERSON, Raymond2, SCHULZ, Klaus3, FEINBERG, Joshua M.4, CHANDLER, Val5, and CANNON, William3 1 U.S. Geological Survey, PO Box 25046, MS 964, Denver, CO, 80225 2 Dept. Earth and Environmental Sciences, Univ. Iowa, Iowa City, IA, 52242 3 U.S. Geological Survey, 12201 Sunrise Valley Dr., MS 954, Reston, VA, 20192-6320 4 Dept. Earth Sciences, Univ. Minnesota, 310 Pillsbury Dr. SE, Minneapolis, MN, 55455-0219 5 Minnesota Geological Survey, 2642 University Avenue W., St. Paul, MN, 55114-1032 Large amplitude gravity and magnetic highs over northeast Iowa are interpreted to reflect a buried intrusive complex composed of mafic/ultramafic rocks, the northeast Iowa Intrusive Complex (NEIIC), intruding Yavapai Province (1.8-1.72 Ga) rocks. The age of the complex is unproven, although it has been considered to be Keweenawan (~1.1 Ga). Because only four boreholes reach the complex, which is thought to be covered by 200-700 m of Paleozoic sedimentary rocks, geophysical methods are critical to developing a better understanding of the nature and mineral resource potential of the NEIIC. An initial airborne data collection campaign in the region of Decorah, IA, included high-resolution magnetic and gravity gradient data. Geologic interpretation of those data (Drenth et al., 2015) highlighted circular magnetic and gravity gradient highs thought to represent an alkaline ring complex or a mafic anorogenic intrusive complex, concepts that may be applicable to other similar anomalies over the broader NEIIC. Interpreted northeast trending dikes, thought to be Keweenawan, are also critical to understanding the age of the NEIIC and country rocks. A large (~22,700 line km) follow-up aeromagnetic-only survey was flown during the fall of 2015 over the southern half of the NEIIC, in the region of Cedar Rapids, Waterloo, and Manchester, IA. Those data are here presented publicly for the first time, in the context of other older geophysical datasets in the region. No rigorous interpretive effort has yet been made, and much ground gravity data remains to be acquired in the region, but the aeromagnetic data show many interesting features that are similar to those observed in and interpreted from the Decorah geophysical data: Large circular aeromagnetic anomalies are consistent with significant individual intrusive complexes, and numerous northeast trending linear anomalies are consistent with dikes of possible Keweenawan age. Follow-up geochronologic and paleomagnetic work may also help to clarify the geologic relations. Reference 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 Iowa-Minnesota border: Canadian Journal of Earth Sciences, v. 52, p. 279-293. doi: 10.1139/cjes2014-0178. 36 �Re-digitized public aeromagnetic data for parts of the west-central Upper Peninsula, Michigan DRENTH, Benjamin1, and AILES, Chad 1 1 Crustal Geophysics and Geochemistry Science Center, U.S. Geological Survey, PO Box 25046 MS 964, Denver, CO, 80225 USA The public aeromagnetic database (Daniels et al., 2009) for Michigan’s west-central Upper Peninsula (UP) is widely regarded as unsuitable for intermediate- and detailed-scale geologic mapping and mineral exploration applications. There are several limitations of the data, including being available only in a native analog format, being acquired with too wide of a line spacing and too high of a terrain clearance, and being digitized at a much lower level of detail than shown on original contour maps. This abstract describes an ongoing effort to re-digitize aeromagnetic data from original contour maps in the greatest detail possible. To date, the Michigan F (Case and Gair, 1965), E (U.S. Geological Survey 1967a, 1967b, and 1967d), and C (U.S. Geological Survey 1967c and 1967d) surveys in the west-central UP have been redigitized. Each survey was a fixed-wing total-field aeromagnetic survey flown at a nominal terrain clearance of 150 meters (Daniels et al., 2009). The Michigan C survey was flown in 1948 with 533 m line spacing along N-S lines, the F survey was flown in 1950 with 400 m line spacing along N-S lines, and the E survey was flown in 1949 with 400 m line spacing along E-W lines. After removal of an unspecified base level, the acquired data were interpolated onto contour maps with a minimum contour interval of 50 nT. A subsequent digitization effort from the contour maps followed at an unknown time, and the resulting digitized data are those publically available today from the USGS (e.g., Daniels et al., 2009). However, that digitization effort inexplicably sampled the contour maps along only every other flightline, effectively simulating surveys with double the actual line spacing. This resulted in poor geologic resolution. The same problem plagues several other vintage aeromagnetic datasets acquired in the central and western UP. We have re-digitized each of these aeromagnetic datasets in their entirety, sampling each contour from the original contour map and effectively capturing all of the available detail. The results are a much more effective representation of the region’s geology. Several known geologic features in the region are well represented, including the Marquette and Menominee iron ranges, dike swarms, blocks of Archean rocks, and the plutons hosting the Eagle and Eagle East deposits. However, the recovered data still have several major and minor limitations that must be considered by interpreters. First, the surveys were flown at too wide a line spacing and too far above the ground for detailed-scale geologic mapping and mineral exploration. Second, the minimum contour interval of 50 nT shown on the original contour maps means that more subtle anomalies and geologic details undoubtedly present in the original flightline data will never be recovered. Third, the exact terrain clearance of the magnetometer was not recorded, and in several localities may have varied significantly from the nominal 150 meter clearance. Finally, the base level removed from the magnetic data wasn’t recorded, meaning that the total-field intensity and formal total-field anomalies cannot be calculated. 37 �REFERENCES Case, J.E., and Gair, J.E., 1965. Aeromagnetic map of parts of Marquette, Dickinson, Baraga, Alger and Schoolcraft Counties, Michigan, and its geologic interpretation: U.S. Geological Survey Geophysical Investigations Map GP-467. Daniels, D.L., Kucks, R.P., Hill, P.L., and Snyder, S.L., 2009, Michigan magnetic and gravity maps and data: a website for the distribution of data: U.S. Geological Survey Data Series 411 available online at http://pubs.usgs.gov.ds/ds411. U.S. Geological Survey, 1967a, Aeromagnetic map of the Crystal Falls quadrangle and part of the Florence quadrangle, Iron County, Michigan, U.S. Geological Survey Geophysical Investigations Map GP-607. U.S. Geological Survey, 1967b, Aeromagnetic map of the Ned Lake quadrangle and part of the Witch Lake quadrangle, Iron, Baraga, and Marquette Counties, Michigan, U.S. Geological Survey Geophysical Investigations Map GP-609. U.S. Geological Survey, 1967c, Aeromagnetic map of parts of the Ralph and Norway quadrangles, Dickinson County, Michigan, U.S. Geological Survey Geophysical Investigations Map GP-610. U.S. Geological Survey, 1967d, Aeromagnetic map of Sagola quadrangle and part of the Iron Mountain quadrangle, Dickinson, Iron, and Marquette Counties, Michigan, U.S. Geological Survey Geophysical Investigations Map GP-611. 38 �3D Geological Mapping Using Terrestrial LiDAR at Soudan Underground Mine ESSIG, Espree, MOOERS, Howard, and GRAN, Karen Department of Earth and Environmental Sciences, University of Minnesota, Duluth, MN essig008@d.umn.edu Light Detection and Ranging, or LiDAR, data technology, has the potential to revolutionize the way we visualize and interpret 3 dimensional relationships in space. Conceptualization of underground geology and mine workings can be particularly challenging. This study evaluates the use of terrestrial LiDAR to visualize the physical setting and geology at Soudan Underground Mine, which today serves as a historical State Park, physics laboratory and point of significant geological interest. Using a Faro Focus 3D laser scanner, courtesy of Karen Gran and the University of Minnesota Duluth’s Department of Earth and Environmental Sciences, 3D point clouds and associated photographs have been collected on at the 27th level at Soudan. These point clouds provide submillimeter resolution and spatially accurate representations of anything in range of the laser beam. These individual scans are then stitched together in the Faro Scene® software using a combination of spherical target, plane, and point recognition approaches with an error of less than 2 mm. Data collection, stitching and rendering of these point clouds is ongoing. The goal of this project is to create a working 3 dimensional model of the underground drift workings at Soudan using LiDAR that can serve as a platform for various applications. Geology of the Soudan Mine mapped by Thompson (2016), Peterson and Patelke (2003), and Vallowe, Thalhamer, Rhoades, and Peterson (2010) are overlain on the 3D imagery. The scope of this project uses modelling as a tool to clearly visualize the spatial relationships among mapped geologic units in 3D space. The Soudan Iron Formation is an Algoma-type banded iron formation (BIF) on the southern end of the Ely Greenstone Belt. Originally chemically deposited as magnetite with chert in association with mafic volcanics, the Soudan Iron Formation was later hydrothermally altered to rich hematite ore through metasomatism and syntectonic deformation. Interpretations and details of its genesis have caused ongoing geological debates, many of which depend on structural and resulting geochemical controls. We evaluate the use of 3D mapping as a geological interpretive tool at Soudan Mine. Pictured to the left are 3D rendered point cloud representations of the 27th level at Soudan Mine 39 �References Oliver Iron Mining Company., 1953-1954. Soudan Mine Area Worksheet. Peterson, D. M. and Patelke, R.L., 2003. Bedrock Geologic Map of the Soudan Mine Area, St. Louis County, Northeast ern Minnesota. Natural Resources Research Institute Technical Report NRRI/TR-2003/29. Scale 1:5,000. Peterson, D.M. and Patelke, R.L., 2003. Geologic Map of the 27th Level East Drift, Soudan Mine. Peterson, D.M. and Patelke, R.L., 2003. National Underground Science and Engineering Laboratory (NUSEL) Geological Site Investigation for the Soudan Mine, Northeastern Minnesota. Economic Geology Group, Natural Resources Research Institute, University of Minnesota Duluth. Peterson D.M., Rhoades, D.L., Thalhamer, E.J. and Vallowe, A.M., 2010. Surface and Subsurface Geologic Maps of the Soudan Underground Mine State Park, St. Louis County, Northeastern Minnesota. Precambrian Research Center Map Series. PRC/MAP2010-01. Scale 1:2,500. Thompson, Adam, 2015, M.S. Thesis (Advisor: Mooers). A hydrothermal model for metasomatism of neoarchean Algoma-Type banded iron formation to massive hematite ore at the Soudan Mine, NE Minnesota. 40 �Multi-stage development of breccias in the Baraboo Quartzite, Rock Springs, Wisconsin FELZAN, Michael and BJØRNERUD, Marcia Geology Department, Lawrence University, 711 E Boldt Way, Appleton, Wisconsin 54911 USA. The ca. 1650 Ma Baraboo Quartzite contains localized zones of breccia in which angular fragments of the red to purple quartzite are engulfed in a stockwork of coarse white vein quartz and subsidiary kaolinite. The time of formation of these breccias relative to the formation of the Baraboo Syncline is not well constrained because the highly competent quartzite experienced little internal deformation, and thus there are no clear cross-cutting relationships between the breccias and folding-related mesoscale structures. At two breccia localities, muscovite that appears to be in textural equilibrium with the vein quartz and kaolinite has yielded 40Ar/39Ar plateau ages of 1405 + 5 Ma and 1459 + 3 Ma (Medaris et al., 2002), broadly consistent with 40Ar/39Ar ages of 1460 to 1484 Ma obtained from muscovite that lies parallel to cleavage planes in the Seeley Slate, which overlies the Baraboo Quartzite but is only known from drill core samples (Medaris et al., 2009). Fluid inclusion analyses of the white vein quartz, combined with the equilibrium coexistence of quartz + kaolin + muscovite, constrain the conditions of breccia formation to 200-280° C and 0.5-2 kbar, or about 2-8 km depth (Medaris et al., 2011). The most extensive brecciated zone in the Baraboo Range occurs on the north limb of the syncline near the town of Rock Springs, WI, extending from the west side of Ablemans Gorge to the Rock Springs Quarry east of the Baraboo River. At this location, the brecciated zone is at least 50 m thick, >3 km long, and approximately parallel with the vertical, ENE-striking bedding. We recently obtained permission to collect samples at the Rock Springs Quarry, where the brecciated zone is well exposed on the south wall of the main pit. Here it is clear that the white vein quartz breccia in fact overprints two other distinctive types of broken and veined rock. The older of these are cm-scale bands of fragmented host quartzite and greyish vein quartz, the latter recognizable in thin section because of its significantly larger grain size and elongate grain shapes. Grain boundary textures suggest that the host rock had already been metamorphosed to quartzite at the time of this early fragmentation. Broken zircon grains occur in these early breccias, and in one thin section, two halves of what seems to have been a single original grain can be seen on opposite walls of the brecciated zone. This suggests that the initial failure along these zones occurred under transient stresses high enough to fracture zircon, a famously tough mineral. The vein quartz in these oldest breccia bands exhibits kink-like deformation lamellae, indicative of quasi-ductile deformation and temperatures just below the onset of quartz plasticity (ca. 350°C). This is close to the peak metamorphic temperature inferred from the absence of biotite in the Seeley Slate (Medaris et al. 2011). These deformed early breccias are in turn cut by distinctive black, hematite-rich veins that occur in three different geometries. The most common form is a mesh-like framework around cm-size fragments of host quartzite. In these occurrences, the hematite-rich zones coincide with areas of very fine, apparently cataclasized, quartz grains much smaller (0.1 mm) than the grain size typical of the host quartzite (>1 mm). The hematite occurs as tiny (<0.1 mm) needle-like crystals that ‘decorate’ the edges of the quartz grains. In many instances, white (late-stage) vein quartz clearly cuts across this black, hematitic cataclasite. The second mode of occurrence of the hematite-rich material is as planar zones – apparently bounded by tensile fractures – that show no contrast in grain size with the adjacent, unmineralized host quartzite. As in the cataclastic zones, hematite occurs as very fine fibrous crystals that fill interstitial spaces around and between quartz grains. The third geometric mode of the hematitic material is as selvages around quartzite fragments that are enclosed in a matrix of white (late) vein quartz. Here the black material seems to have adhered to the edges of clasts when they were separated by the emplacement of the coarse vein quartz. The textural similarities of the hematite in these three types of occurrence suggest that it formed under the same conditions, involving a fluid of distinct composition, over a limited period of time. Finally, the white vein quartz was emplaced in complex three-dimensional networks that cut through not only the two earlier types of broken/mineralized rock but also previously unfractured host quartzite. This stage of brecciation and fluid flow affected a much larger mass of rock than the earlier two and involved large dilatant strains. In representative hand specimens, the white vein quartz constitutes as much as 40% of the rock volume. The fragments of host quartzite have a wide range of sizes (mm to m scales) and morphologies, ranging from tabular to equant to highly irregular three-dimensional shapes. Crescentic and splinter-like clasts suggest energetic spallation from the walls of the veins. Although the separated clasts have a ‘jigsaw’-like appearance, it is difficult in most cases to fit the pieces back together by matching their shapes or truncated internal features. 41 �This could indicate significant translation and/or rotation of the fragments within the zone prior to the formation of the vein quartz. Fluid inclusions are very abundant in the vein quartz and typically occur in parallel planar arrays within a given crystal, pointing to episodic growth over many cycles of fluid infiltration. In some cases, the vein quartz occurs as zoned, phantom, and terminated prismatic crystals up to 5 cm long, growing into open space or into masses of kaolin. In thin section, the vein quartz shows sweeping undulose extinction, indicating that it too experienced significant deviatoric stress after its crystallization (van Lankvelt and Bjørnerud, 2010). Collectively, these observations suggest that brecciation and veining at Rock Springs occurred in several distinct stages, involving different modes of deformation and fluids of varying composition, over a protracted period of time. First, an early period of brittle failure, apparently at very high – possibly co-seismic -- stresses, fractured the host quartzite and even cracked detrital zircon grains. An early generation of quartz veins were precipitated into the fractures, and these veins then became the locus of quasi-ductile deformation at temperatures close to the inferred metamorphic peak of 350°. Next, a period a cataclasis along anastomosing shear fractures created cm-scale zones of finely comminuted quartz grains. These cataclasized zones acted as conduits for an iron-rich fluid that infiltrated the rock and was overpressured enough to create additional pathways for itself through hydrofracturing. Next, a large volume of rock, including but extending beyond the previously fractured area, was pervasively broken into fragments whose varied and complex shapes suggest violent, implosive failure. Silicic fluids at temperatures of 200-280°C then flowed repeatedly through this brecciated rock, eventually enclosing the clasts in a matrix of white, fluid inclusion-rich quartz and lesser kaolinite. Finally, continuing stresses caused slight deformation of the vein quartz. The temporal relationships between these inferred events and the regional folding is not clear. The parallelism of the Rock Springs breccia zone with bedding in the Baraboo Quartzite could indicate that flexural slip during the main folding event played a role in the formation of the breccia. The Rock Springs breccia occurs within an especially massive stratigraphic interval of the quartzite, with little interbedded phyllite (Van Hise Rock being the notable exception). Perhaps at some point in the folding process, bending stresses became so great within the stiff quartzite beam that it failed along a bedding-parallel zone. Failure then allowed overpressured fluids to rush into the fragmented rock, leading to further fracturing and successive stages of veining. The source of the fluids, and in particular the iron-rich fluid that left the unusual hematite veins, is not known. The Baraboo Quartzite as a whole – one of Earth’s earliest ‘red bed’ sequences -- owes its color to oxidized iron, so the iron could have been scavenged from the host rock itself. Alternatively, a little-known iron formation, the Freedom Formation, lies stratigraphically above the Baraboo Quartzite and Seeley Slate and occurs (in the subsurface) in the core of the Baraboo syncline (Roe and Bjørnerud, 2012). If the formation of the hematite-rich areas post-dated the folding and rotation of bedding in the north limb to its present vertical orientation, fluid flow from the Freedom Formation might have been lateral rather than downward. Another fundamental question is whether all the breccias in the Baraboo district (and other Baraboo-interval quartzites) are of the same age and origin or whether they reflect only local stresses and fluid flow. The 14001460 Ma ages of muscovite from other breccia localities as well as the Seeley Slate suggest that deformation and metamorphism at Baraboo were linked to Wolf River Batholith magmatism. In any case, it seems the Baraboo breccias have a more complex story to tell than previously thought and may provide a new window into the processes that contributed to the formation of the still-enigmatic Baraboo Mountains. References cited Medaris L.G., Jr., Singer B., Dott, R.H., Jr., Naymark, A., Johnson, C., and Schott, R., and 2003. Late Paleoproterozoic climate, tectonics, and metamorphism in the southern Lake Superior region and Proto-North America: Evidence from Baraboo-Interval quartzites. Journal of Geology 111, p. 243-257. Medaris, L.G., Jr., Jicha, B., Dott, R.H., Jr. and Singer B., 2009. A 1465 Ma 40Ar/39Ar age for the Seeley Slate: Implications for metamorphism in the Baraboo Range, Wisconsin. Institute on Lake Superior Geology Program with Abstracts 55, p. 59-60. Medaris, L.G., Jr., Dott, R.H., Jr., Craddock, J., and Marshak, S., 2011. The Baraboo District: A North American Classic. Geological Society of America Field Guide 24, p. 63-82. Roe, C. and Bjørnerud, M., 2012. The ca. 1650 Ma Freedom Formation: A late iron formation in the Lake Superior region. Institute on Lake Superior Geology Program with Abstracts 58, p. 73-74. Trepmann, C.A., Stöckhert, B., 2002. Cataclastic deformation of garnet: a record of synseismic loading and postseismic creep. Journal of Structural Geology 24, p. 1845–1856. van Lankvelt, A. and Bjørnerud, M., 2010. Revisiting the Baraboo breccias, Institute on Lake Superior Geology Program with Abstracts 56, p. 67-68. 42 �Geophysical Imaging of Layered Mafic Complexes and Relation to Platinum Group Element Exploration FINN, Carol A., ZIENTEK, Michael, BEDROSIAN, Paul, BLOSS, Benjamin, BURTON, Bethany, PETERSEN, Dean and PARKS, Heather Layered ultramafic to mafic intrusions such as the Duluth Complex are economically important because they can host magmatic ore deposits containing economic concentrations of nickel, copper, titanium, vanadium and platinum-group elements (PGE’s). Because most new discoveries lie under cover, modern exploration for PGE’s relies heavily on understanding the geophysical signature of the entire magmatic system in which buried deposits form. Geophysical surveys and methods of analysis provide higher resolution views of the environments in which PGE’s develop than were previously available. Combined analysis of potential field and electromagnetic data can provide constraints on the volume of the intrusion, its extent under cover, possible locations of sulfide mineralization. These data can also effectively map smallscale (~3 meters) structures within layered intrusions that host PGE-bearing magmatic ore deposits. Previous interpretations of aeromagnetic and gravity data helped map the geology of the Duluth Complex under cover and confirmed that the complex consists of multiple intrusions (Chandler, 1990; Chandler et al., 1998; Miller et al., 2001). The advent of new filtering, imaging, and modeling techniques will allow us to refine these earlier interpretations, and new high resolution magnetic and electromagnetic (EM) data provide detailed views of the Kawishiwi intrusion. For example, the tilt derivative of the aeromagnetic data significantly enhances small scale features, such that linear aeromagnetic anomalies in the Poplar Lake and Misquah Hills intrusions, Duluth Complex, possibly related to layering, can be observed. In other intrusions, like the Stillwater Complex, the linear aeromagnetic anomalies primarily relate to boundaries between major stratigraphic units and olivine-bearing rock layers altered to a mixture of serpentine and magnetite. Gravity highs characterize the exposed and interpreted buried extent of several intrusive components of the Duluth Complex (Chandler, 1990; Chandler and Ferderer, 1989; Chandler and Lively, 1998). Two dimensional modeling of the Duluth Complex indicate that the Complex thickens to the southeast (Chandler and Ferderer, 1989; Chandler and Lively, 1998). Similar to the Bushveld (Cole et al., 2013; Finn et al., 2015a) and Stillwater Complex layered mafic intrusions (Finn et al., 2015b), modeling of the gravity data constrained by other geophysical data, may help constrain the 3D extent of the Duluth Complex. Existing gravity data over the Kawishiwi intrusion indicate that the Cu-Ni-PGE deposits along its base align along a gravity gradient indicating a density contrast between basement and mafic intrusive rocks (Condor Consulting Company, 2011, internal report). Stochastic inversion of the EM data aids in enhancing low resistivity features that could indicate undiscovered deposits. Existing MT data images the configuration of low resistivity sulfides within the Animikie basin adjacent to the Duluth Complex (Bedrosian, 2016), important because the degree of reaction and assimilation of the Animikie rocks with the mafic magmas resulted in a variety of mineralization zones. In addition, the Animikie basin may have influenced the extent of the Duluth Complex as the Transvaal basin did for the Bushveld intrusion (Finn et al., 2015a). 43 �REFERENCES Bedrosian, P.A., 2016., Making it and breaking it in the Midwest: Continental assembly and rifting from modeling of EarthScope magnetotelluric data. Precambrian Research, doi:10.1016/j.precamres.2016.03.009. Chandler, V.W. and Ferderer, R.J., 1989. Copper-nickel mineralization of the Duluth Complex, Minnesota; a gravity and magnetic perspective. Economic Geology, 84(6), pp.1690-1696. Chandler, V.W., 1990. Geologic interpretation of gravity and magnetic data over the central part of the Duluth Complex, northeastern Minnesota. Economic Geology, 85(4), pp.816-829. Chandler, V. W. and R. S. Lively, 1998, Gravity and magnetic modeling of the Duluth Complex in the Allen 7.5minute quadrangle, St. Louis County, Minnesota, University of Minnesota, Minnesota Geological Survey Miscellaneous Map Series, Map M-90. Cole, Janine, Susan J. Webb and Carol A. Finn, 2014, Reassessing geophysical models of the Bushveld Complex - have we come full circle?: Journal of South African Earth Sciences, v. 92, p. 97-118 http://dx.doi.org/10.1016/j.jafrearsci.2014.01.012 1464-343X. Finn, Carol A., Paul Bedrosian, Janine Cole, Tshepo David Khoza and Susan J. Webb, 2015a, Mapping the extent of the Northern Lobe of the Bushveld layered mafic intrusion from geophysical data: Precambrian Research, v. 268, 279-294, doi:10.1016/j.precamres.2015.07.003. Finn, CA, Michael Zientek, Paul Bedrosian, Janine Cole, Susan Webb and Heather Parks, 2015b, Geophysical exploration for PGE deposits in layered mafic intrusions, Abstract NS42A-01 presented at 2015 Fall Meeting, AGU, San Francisco, CA. 44 �Sedimentology of a Pre-Vegetation Floodplain Assemblage: the Mesoproterozoic Hele Member of the Sibley Group, Ontario FRALICK, Philip and ZANIEWSKI1, Kamil Department of Geology, Lakehead University, Thunder Bay, ON philip.fralick@lakeheadu.ca Descriptions of fluvial systems operating prior to significant terrestrial macrophyte vegetation have concentrated on assessing the impact of plant evolution on channel style; this is probably in part due to the scarcity of well developed floodplain successions in fluvial assemblages of these ages. This study describes wet, pre-vegetation floodplain deposits and processes observed and inferred form a continuous succession of drill core through an extensive, 1.4 Ga, delta-top channelfloodplain assemblage forming a portion of the Sibley Group, Ontario, Canada. Sub-aerial deposits are dominated by flaser, wavy and lenticular bedded, find-grained sandstones, siltstones, and mudstones, with abundant small mudstone rip-up clasts. Soft-sediment deformation of these units is ubiquitous, with loading and injection features being the most prominent. Thicker medium-grained sandstone beds, representing crevasse splays, commonly have poorly developed protosols in their upper portions. Well-laminated sediments with wave ripples, and only rarely containing rip-up clasts and soft sediment deformation, were deposited in floodplain ponds. These deposits differ from post-vegetation floodplain sediments in having: (i) better preservation of layering without rootlet bioturbation; (ii) dominance of rippled sand on the floodplain, probably due to lack of vegetation-induced baffling and thus higher velocities of overbank flows away from the levees; (iii) large-scale generation of small, intraformational clasts caused by intense drying of the thin upper layer of sediment due to the lack of shade; (iv) desiccation crack fills consisting of peds and locally derived intraclasts, probably transported by overland flow during rainfall events that did not result in sediment delivery from the main channels; (v) ubiquitous soft-sediment deformation features in sub-aerial deposits denoting a groundwater table very close to the surface; and (vi) well-laminated, commonly oxidized sediment that accumulated in floodplain ponds attributable to low levels of organic loading, though some green pond sediments indicate that microbial microflora, and probably microfauna, did exist in these ponds. These attributes are the direct result of the lack of macrophyte vegetation, and produce floodplain assemblages that are distinctly different from those currently forming in similar climatic settings. 45 �Provenance and tectonic evolution recorded by successor basins in the Abitibi-Wawa terrane: Insights from new U-Pb LA-ICP-MS analyses of detrital zircon FRIEMAN, Ben M.1, KUIPER, Yvette D. 1, KELLY, Nigel M.2, MONECKE, Thomas1 Dept. of Geology & Geological Engineering, Colorado School of Mines, 1516 Illinois St., Golden, CO, 80401; 2Dept. of Geological Sciences, University of Colorado at Boulder, 2200 Colorado Ave., Boulder, CO, 80309 Understanding the detrital zircon provenance of syntectonic successor basins can provide insight into the processes related to amalgamation of the Superior Province. The aim of this study was to investigate the provenance of successor basins in the southern Abitibi subprovince (SAS), which comprises the southeastern most extent of the Superior Province. To establish the provenance of sedimentary rocks in successor basins of the SAS we have utilized statistically robust U-Pb dates of detrital zircon grains obtained by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The results were compared to detrital zircon data from successor basins in the Wawa subprovince from Lodge et al. (2013) to establish the characteristics of provenance throughout the Abitibi-Wawa terrane. The Abitibi-Wawa terrane (Stott et al., 2010) is one of the largest, best-exposed, well-studied, and economically well-endowed Archean greenstone belts in the world. The Abitibi and Wawa subprovinces occur to the east and west, respectively, of the Paleoproterozoic Kapuskasing uplift in Ontario. In recent years, extensive high-resolution, U-Pb zircon geochronology coupled with new and existing mapping has revealed a characteristic lithotectonic progression for the development of the Abitibi-Wawa terrane (e.g., Ayer et al., 2002). The research, primarily focused on the SAS, indicates that construction of the terrane was dominated by bimodal volcanism in a subaqueous environment from ~2750 to ~2695 Ma. The termination of submarine volcanism at ~2695 Ma was marked by the onset of regional tectonism, which progressively localized along major regional deformation zones and resulted in the formation of orogenic gold deposits. Synchronous with tectonism was the development of thick sedimentary successions in successor basins. In the SAS, two distinct types of successor basins are recognized based on sedimentological, structural, and geochronological constraints: the ~2695-2685 Ma, submarine, turbidite-dominated Porcupine assemblage and the ~2680-2670 Ma, largely subaerial, coarse clastic-dominated Timiskaming assemblage. The Timiskaming assemblage is typically fault- and/or unconformitybounded, represents a critical temporal and structural marker unit, and commonly hosts orogenic gold deposits. To establish the provenance of successor basins in the SAS a representative set of sixteen samples was collected from across the subprovince. Sampling targeted well-constrained localities with a variety of stratigraphic relationships, spanning many of the major structural zones and mining camps in the SAS. To evaluate the detrital zircon date distributions of samples from these successor basins, probability density (PDF) and cumulative distribution (CDF) functions were calculated. These data provide a robust characterization of the provenance of Porcupine and Timiskaming assemblage sedimentary rocks. The PDFs for all of the samples display similar principal populations of detrital zircon grain dates that are dominated by ~28002650 Ma results. However, primary differences between Porcupine and Timiskaming assemblage detrital zircon populations were indicated by differing proportions of older, pre-2800 Ma dates. The CDF calculations indicate that zircon grains with pre-2800 Ma dates comprise ≤5% of the Porcupine assemblage data and ~10-15% of the Timiskaming assemblage data. Similar to SAS 1 46 �samples, Wawa detrital zircon samples (Lodge et al., 2013) are predominately composed of grains with 2800-2650 Ma dates. Furthermore, the Wawa data displays a similar proportion of pre-2800 Ma dates to Porcupine assemblage samples (i.e., <5%), but also contain a relatively high proportion of ~2650 Ma and younger zircon grains. Lodge et al. (2013) interpreted the young dates as a result of metamorphism, and therefore they may not reflect primary depositional differences in provenance between the southern Abitibi and Wawa subprovinces. The Wawa successor basins display sedimentological characteristics that are ‘Timiskaming like’ (i.e., dominated by coarse clastic sedimentary rocks), although the maximum age of deposition is locally well-constrained by detrital zircon geochronology to be ~2690 Ma (Lodge et al., 2013). Consequently, despite broad similarities to the Timiskaming assemblage in the SAS, the Wawa successor basins appear to be the same age as, and contain detrital zircon provenance that is most comparable to, the Porcupine assemblage in the SAS. In general, the provenance of SAS detrital zircon samples indicates that the successor basins are dominantly composed of local detritus, consistent with geochemical and sedimentological studies (e.g., Feng and Kerrich, 1990; Cocoran and Mueller, 2007). However, the occurrence of pre-2800 Ma detrital zircon grains may be indicative of provenance from adjacent subprovinces, as the SAS does not contain rocks in this age range (e.g., Ayer et al., 2002). Therefore, we interpret these data as an indicator of exhumation and erosion of an emergent hinterland to the north (i.e., the Winnipeg River, Marmion, and Opatica subprovinces). Potential sources for pre2800 Ma grains also occur to the south in the Minnesota River Valley terrane. However, these rocks also contain zircon grains with ~3500-3300 Ma dates (Bickford et al., 2006), which are not observed in the SAS data. In the SAS, it is well-established that the Porcupine and Timiskaming assemblages were deposited at different times based, in part, on the ages of the youngest detrital zircon grains they contain (e.g., Ayer et al., 2002). Our data additionally indicates that Timiskaming assemblage rocks contain a higher abundance of zircon grains with pre-2800 Ma dates relative to Porcupine assemblage rocks. This suggests that detritus from the hinterland was more prevalent during the later stages of tectonism at ~2680-2670 Ma, supporting models that invoke north to south propagation of the tectonic front. REFERENCES Ayer, J., Amelin, Y, Corfu, F., Kamo, S., Ketchum, J., Kwok, K., and Trowell, N., 2002. Evolution of the southern Abitibi greenstone belt based on U-Pb geochronology: Autochthonous volcanic construction followed by plutonism, regional deformation and sedimentation. Precambrian Research, v. 115, p. 63–95. Bickford, M.E., Wooden, J.L., and Bauer, R.L., 2006. SHRIMP study of zircons from Early Archean rocks in the Minnesota River Valley: Implications for the tectonic history of the Superior Province. GSA Bulletin, v. 118, p. 94–108. Corcoran, P.L., and Mueller, W.U., 2007. Time-transgressive Archean unconformities underlying molasse basin-fill successions of dissected oceanic arcs, Superior Province, Canada. Journal of Geology, v. 115, p. 655–674. Feng, R., and Kerrich, R., 1990. Geochemistry of fine-grained clastic sediments in the Archean Abitibi greenstone belt, Canada: Implications for provenance and tectonic setting. Geochimica et Cosmochimica Acta, v. 54, p. 1061–1081. Lodge, R.W.D., Gibson, H.L., Stott, G.M., Hudak, G.J., Jirsa, M.A., and Hamilton, M.A., 2013. New U-Pb geochronology from the Timiskaming-type assemblages in the Shebandowan and Vermilion greenstone belts, Wawa Subprovince, Superior Craton: Implications for the Neoarchean development of the southwestern Superior Province. Precambrian Research, v. 235, p. 264–277. Stott, G.M., Corkery, M.T., Percival, J.A., Simard, M., and Goutier, J., 2010. A revised terrane subdivision of the Superior Province. Ontario Geological Survey Open File Report 6260, p. 20-21–20-10. 47 �A comparative study of mafic and felsic lithologies from the Borden Belt and adjacent greenstone belts in the Wawa-Abitibi Terrane GAMELIN, G., Stinson, V.R., Pan, Yuanming, and Nadeau, M. Department of Geology, University of Saskatchewan, 114 Science Place, Saskatoon, SK, Canada S7N 5E2, glecy.gamelin@usask.ca The Borden Belt in the Kapuskasing Structural Zone of the Superior Province of Canada contains schists and gneisses that reached upper amphibolite to granulite facies of metamorphism. Whole rock geochemistry and trace element analyses indicate igneous protoliths of sub-alkaline affinity, ranging from ultramafic to felsic compositions. Igneous tectonic geochemical signatures are preserved which indicate shallow mantle melting environment and crustal input, characteristic to arc-subduction zone magmatism. The Wawa-Abitibi Terrane can be defined as a tectonostratigraphic terrane, by comparing trace element geochemistry and field relationships of the Borden belt, Michipicoten Greenstone Belt, and the southern Abitibi Greenstone Belt in Ontario. All three belts share similar geochemical signatures which indicate lithologies associated with the formation and progression of a mature volcanic arc in a shallow marine setting within the Wawa-Abitibi Terrane. 48 �The Geology and Geochemistry of the Laird Lake Property, Red Lake Greenstone Belt, Ontario GÉLINAS, Brigitte1 and HOLLINGS, Pete1 1 Department of Geology, Lakehead University, Thunder Bay, Ontario The Laird Lake property is situated on what has been previously interpreted as the angular unconformity 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 (Corfu and Wallace, 1986; Corfu and Andrews, 1987; Stott and Corfu, 1991). The Balmer assemblage is characterized by mafic tholeiites, ultramafic flows, local banded-iron formations and minor felsic volcanic rocks, whereas the Confederation comprises mafic to felsic volcanic rocks with a calc-alkalic affinity (Parker, 2000; Sanborn-Barrie et al., 2001, 2004). Contrary to previous interpretations, the Laird Lake property displays metamorphic assemblages up to amphibolite grade facies which are attributed to regional metamorphism, rather than being related to the metamorphic aureole of the Killala-Baird batholith (2704 ± 1.5 Ga) north of the field area (Sanborn-Barrie et al., 2004; Dubé et al. 2000). Multiple gold occurrences are found on the property and generally occur within 200 m of the interpreted unconformity. The occurrences are hosted by multiple rock types with some of the highest recorded values from grab samples of quartz veins (101.90 g/t), banded-iron formation (35.34 g/t), altered mafic volcanic rocks (38.57 g/t) and quartz-feldspar porphyritic crystal-tuff (>0.1 g/t; LeBlanc, 2015). This study will develop an integrated model for the geology and gold mineralization of the Laird Lake area by 1) characterizing the primary host rocks and 2) investigating the nature, timing and origin of the Au-mineralization. Mapping and sampling of the various rock types in the field area at surface and within five drill cores that intersect the gold bearing zone will provide the geological constraints for this study. Petrographic and SEM analysis will characterize the textures and mineralogy of the rocks, whereas whole rock geochemistry will be used to characterize their composition in order to differentiate the Balmer and Confederation assemblages. Quartz vein samples (barren and mineralized) will be analyzed for oxygen isotopes and fluid inclusion work will be conducted if possible. Additionally, Sm-Nd isotope analyses will be conducted on carefully selected samples in order to acquire information on the source of the host rocks to help establish the tectonic history of the area, and last, a geochronology sample of a felsic volcanic unit within the Balmer assemblage will be dated by U-Pb zircon analysis. Previous models suggest that the Balmer and Confederation assemblages cannot be distinguished in the field, however, observations made over the field season indicate that the two assemblages show clear differences. The Balmer comprises a fine-grained, aphyric mafic metavolcanic, locally pillowed, with various amounts of biotite and carbonate alteration and banded-iron formation. The Confederation was observed to have phenocrystic (feldspar and/or amphiboles) and rare aphyric mafic metavolcanic rocks intercalated with intermediate and felsic metavolcanic rocks. The mafic to felsic metavolcanic rocks were not observed to have pillows and display much weaker alteration than the Balmer. Both Balmer and Confederation rocks show a dominant east-trending fabric with increasing foliation observed when approaching the unconformity. Detailed mapping at the “Gold Bearing Zone” trench and whole-rock geochemistry indicates that the outcrop is part of the Balmer assemblage with aphyric mafic metavolcanic tholeiites and banded iron formations. Known gold occurrences at the trench are localized within a banded iron formation cut by a gold-bearing shear. This relationship would suggest a 49 �sulphidation reaction in order to precipitate the gold, samples of which have yielded values up to 35.34 g/t over 2m (LeBlanc, 2015). Primitive mantle-normalized trace element profiles for Balmer and Confederation volcanic units show distinct trends that support the subdivision of assemblages within the field. As expected, the Balmer mafic volcanic rocks show a tholeiitic signature, with two distinct trends; trend 1 has a low Th/Nb ratio and a weak negative Ti anomaly whereas trend 2 has a more LREE depleted profile with a distinctly higher Th/Nb ratio and a stronger negative Ti anomaly. The Balmer ultramafic volcanic units equally show two trends; trend 1 is enriched in LREE while trend 2 is depleted in LREE. Both show high Th/Nb ratios with flat HREE. The Confederation assemblage volcanic rocks show calc-alkaline signatures with enriched LREE, flat HREE, high Th/Nb ratios and strong negative Ti anomalies, all indicative of rocks formed above a subduction zone. Figure 1: Primitive mantle-normalized trace element profiles for Balmer and Confederation assemblage metavolcanic rocks (normalization factors are after McDonough et al. 1992). (A) Balmer assemblage. (B) Confederation assemblage. REFERENCES Corfu, F. and Andrews, A.J. 1987. Geochronological constraints on the timing of magmatism, deformation, and gold mineralization in the Red Lake greenstone belt, northwestern Ontario; Canadian Journal of Earth Sciences, v.24, p.13021320. Corfu, F. and Wallace, H. 1986. U–Pb zircon ages for magmatism in the Red Lake greenstone belt, northwestern Ontario; Canadian Journal of Earth Sciences, v.23, p.27-42. Dubé, B., Balmer, W., Sanborn-Barrie, M., Skulski, T. and 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, 12p. LeBlanc, J. 2015. Project Summary Report for the Laird Lake Gold Project, Red Lake, Ontario, Canada. Unpublished Company Report for Bounty Gold Corp. 50p. McDonough, W. F., S. Sun, A. E. Ringwood, E. Jagoutz, and A. W. Hofmann (1992), K, Rb, and Cs in the earth and moon and the evolution of the Earth mantle, Geochim. Cosmochim Acta, S. R. Taylor Symposium volume, 1001-1012 Parker, J.R. 2000. Gold mineralization and wall rock in the Red Lake greenstone belt: A regional perspective; in Summary of Field Work and Other Activities, 2000, Ontario Geological Survey, Open File Report 6032, p.22-1 to 22-28. Sanborn-Barrie, M., Rogers, N., Skulski, T., Parker, J., McNicoll, V. and 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. Sanborn-Barrie, M., Skulski, T. and 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, 30p. Stott, G.M. and Corfu, F. 1991. Uchi Subprovince; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 1, p.145-238. 50 �Evidence in the Eastern Canadian Shield of regular fault patterns of crustal origin for the loci of some mineral deposits and late-stage intrusive events Glass, F., Quebec Qualitative analysis of aeromagnetic data and examination of other data sets (geomorphology, mapped outcrops ) have been used to determine the presence of interpreted lineaments at known mineral occurrences or late-stage intrusive events ( mapped or aeromagnetic observation ). These lineaments are thought to represent ductile or brittle fault traces and to have origin within the crust. The prevalence of particular fault-trace orientations is consistently regular over a large area of the Canadian Shield. The age of the dated mineral deposits or intrusive events associated with particular fault traces or fault loci vary from 2,7 Ga to 1,1 Ga. Within a 1 000 km map block area or greater, the inter-relationship of certain mineral deposits and late-stage intrusive events appears to consistently correlate to specific geographic orientations. Within the boundary of a smaller block ( 100 km or smaller in size ) anywhere inside the larger block, the inter-relationship between local deposits or nearby late-stage intrusives, coincides with some of those lineaments evident at the regional scale. In addition, there are also different orientations, not necessarily observed with extensive continuity, unequivocally present. These features of local extent are, however, oddly present, with near exact correlation to specific regional orientations, over a similarly large area of the Canadian Shield. In order to explain the origin of the above observations requires likely two different depths for the far and local stress fields or, more plausibly, a single cause for the stress field possibly acting on two crustal layers, the near-surface one delaminated from the deeper crust below. Prior to at least 1,1 Ga, the Canadian Shield appears to have behaved as a single rigid crustal block. ( The Mount Polley alkaline complex has a N27W faulted boundary to the west; the Labrador Trough to the M7 earthquake ( 1929 ) in the St-Lawrence ( Quebec Embayment Structure ) is at N27W ). 10 slides have been prepared showing repetitions of a few common lineament orientations ( all flat-map representations ) with a final slide illustrating the necessity to use polar projection ( spherical maps ) over large regional distances. All 11 slides are presented on one poster. 51 �The Cu/Pd diagram and metal/sulfur variation as an exploration tool: Examples from the Coldwell Alkaline Complex, Ontario GOOD, David J.1, LINNEN, Robert L.1 and SAMSON, Iain M.2 1 2 Department of Earth Sciences, Western University, London, ON, Canada, N5A 5B7, dgood3@uwo.ca Department of Earth & Environmental Sciences, University of Windsor, Windsor, ON, Canada N9B 3P4 Mapping out variations in Cu/Pd, Cu/S and Pd/S through a Ni-Cu-PGE sulphide deposit provides important insight to the mechanisms acting during deposit formation. For instance, the Cu/Pd diagram has been used: to illustrate PGE depletion by previous sulphide segregation (Barnes et al., 1993); to estimate R-factors required to form deposits in the Duluth Complex (Thériault and Barnes, 1998); and as an indicator of extreme PGE enrichment (Barnes and Ripley, 2016). We propose here that patterns and features shown by a large data set on the Cu/Pd diagram can provide insight into the cumulative history of the deposit, particularly in a magmaconduit setting where intrusive bodies formed by build up of multiple sills. This study compares spatial and compositional relationships for 15,000 assays from 11 mineralized zones within the Marathon, Geordie Lake, and Area 41 deposits and the Four Dams and Redstone occurrences located within the 1.1 Ga Midcontinent Rift related Coldwell Alkaline Complex. The deposits are proposed to be co-genetic and, except for Geordie Lake, exhibit common petrologic features indicating formation in a magma conduit setting at or just above the contact between mafic meta-volcanic rocks and the Archean basement (Good et al., 2015). The Geordie Lake deposit is located closer to the middle of the complex and was cut by syenitic rocks during Cycle I of the Coldwell intrusive event. The host rocks for the deposits consist of some combination of ophitic gabbro and pegmatite; apatitic clinopyroxenite; or augite troctolite. Mineralization consists predominantly of disseminated assemblages of chalcopyrite ± pyrrhotite ± bornite. Trends for metal abundances across mineralized intervals in all zones, except for the W Horizon at Marathon, show a correlation between Cu, Pd, Pt and S. But metal tenor (metal/S) and/or Cu/Pd in these sections commonly exhibit saw-tooth patterns that change gradually up through intrusions with sharp steps occurring within and between individual zones. Mineralized intervals that exhibit increasing Cu, S and Pd with decreasing Cu/Pd are consistent with models for accumulation of sulphides from magma with similar R-factor. Step-like changes in Cu/Pd or Cu/S within a zone are interpreted to represent contacts between individual pulses of sulphidebearing magma with an inherent R-factor attribute. Taken together, the range of Cu/Pd for all samples in a mineralized zone represents the cumulative history of individual magma pulses. Contouring the data for each zone by point density defines a characteristic shape and trend line for the zone, the slope of which is dependent on the total range of R-factors for the accumulated magma pulses. Shallow-dipping trend lines, such as that observed for the Geordie Lake deposit and the Marathon footwall zone, indicate a simple intrusive history with magma pulses having a limited range of R-factors. Steeply negative trend lines extending from the Pd-depleted to the Pd-enriched fields represent magma pulses with a very broad range of R-factors and indicate a very dynamic intrusive history, as is the case for data from the W horizon and the top unit at Area 41. The latter relationship shows there is a spatial, and possibly a temporal, relationship between the most copper-rich and Pd-rich zones at Marathon and Area 41. 52 �REFERENCES Barnes, S-J, Couture, J-F, Sawyer, EW, & Bouchaib, C., 1993. Nickel-copper sulfide occurrences in BelleterreAngliers belt of the Pontiac sub-province and the use of Cu/Pd in interpreting platinum-group element distributions. Econ Geol 88: 1402–1418. Barnes, S.-J. & Ripley, E.M., 2016. Highly Siderophile and Strongly Chalcophile Elements in Magmatic Ore Deposits, Reviews in Mineralogy & Geochemistry 81: 725-774. Good, D.J., Epstein, R., McLean, K., Linnen, R.L. & Samson, I.M., 2015, Evolution of the Main Zone at the Marathon Cu-PGE sulfide deposit, Midcontinent Rift, Canada: spatial relationships in a magma conduit setting. Economic Geology 110, p. 983-1008. Thériault, R. D., Barnes S.-J., and Severson M. J., 1997, The influence of country-rock assimilation and silicate to sulfide ratios (R factor) on the genesis of the Dunka Road Cu - Ni - platinum-group element deposit, Duluth Complex, Minnesota, Can. J. Earth Sci. V. 34, p. 375-389. 53 �Progress on 3D modeling of the Midcontinent Rift System in the western Lake Superior region and an isopach map of the Oronto Group GRAUCH, V.J.S., POWERS, Michael H., and ANDERSON, Eric D. U.S. Geological Survey, PO Box 25046, MS 964, Denver, CO, 80225 Efforts are underway to develop a three-dimensional (3D) model of the structure and configuration of sedimentary and volcanic basins related to the Midcontinent Rift System (MRS) in the western Lake Superior region (Fig. 1). The current efforts build on a 3D model previously developed in the 1990s (Allen et al., 1997) and concepts developed by many other previous workers. Technical advances in modeling capabilities, expanded and improved geophysical data coverage, and renewed interest in the mineral resources of the MRS provide the motivation for new attempts at 3D digital modeling of the MRS. An improved 3D model of the area helps visualize geologic relations, provides mechanisms to test hypotheses about tectonic history and the spatial distribution of mineralization, and helps identify areas where more detailed analysis is required. Our 3D model is regional and intended to show broad variations in geology. Only three generalized, MRS-related geologic packages are represented, following Allen et al. (1997). The packages, from oldest to youngest, are 1) undivided Keweenawan plutonic and volcanic rocks, 2) Oronto Group sedimentary rocks, and 3) Bayfield Group and equivalent sedimentary rocks. A fourth package represents undivided pre-rift rocks (basement). In addition, three major fault systems are modeled: the Douglas, Lake Owen, and Keweenaw fault systems (Fig. 1). The modeling strategy involves digitizing the bases of the geologic packages, fault locations, and general orientation data along 2D sections and from geologic maps. The 3D modeling software then connects the digitized points into surfaces and volumes in 3D space, using simple geologic rules for stratigraphic and onlap relations and for the lateral extents of the influences of faults. Digitized points from the previous 3D model (Allen et al., 1997) serve as a guide, but we are re-evaluating geophysical data and making sure that updated geologic concepts have been incorporated. In particular, we are revisiting the analyses of available seismic-reflection sections with modern software and developing new gravity and magnetic models to substantiate the interpretations. For example, we have applied modern reprocessing techniques to proprietary seismic-reflection data in the Bayfield Peninsula to document previously unpublished results and provide a better image of the geology. We are also developing rootmean-square velocity maps for analog seismic-reflection sections to establish depths to strong reflections. Some of these new depth interpretations are revealing discrepancies between interpreted seismic sections where they cross each other, which merit further work. An isopach map of Oronto Group thickness constructed from the 3D model (Fig. 2) illustrates the tremendous thickness of sediments that were deposited within present-day, western Lake Superior during post-magmatic subsidence and before Bayfield basin formation. The thickest parts of the basin are generally located in between White’s and Grand Marais ridges, respectively. A significant observation is the broad asymmetry of the Oronto basin in western Lake Superior, with the thickest part of the basin (>12 km) near the south shore adjacent to the Porcupine Mountains. The asymmetry is present in similar, previous maps (e.g., Allen et al., 1997), but is more pronounced in our map. The broad, southeastdeepening asymmetry next to the Porcupine Mountains contrasts with isopach thicknesses and previous workers' interpretations of GLIMPCE seismic line C (Fig. 2). Along line C, the thickest Oronto Group is concentrated within a narrow (~25-km) zone adjacent to the south shore. The local thickening next to the Douglas fault at the southeast side of White's Ridge is present in both our and Allen's model, derived independently from informal observations of proprietary seismic data. 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., ed., Middle Proterozoic to Cambrian Rifting, Central North America: Geological Society of America Special Paper 312, p. 47-72. 54 �Figure 1: Generalized rock units of the Midcontinent rift system in the western Lake Superior region and area covered by the 3D model. Geographic boundaries are shown by dashed lines. Figure 2: Isopach map of the Oronto Group from the 3D model, shown as a color-shaded relief image with illumination from the west. Gravity lows interpreted as basement highs (Allen et al., 1997): White's Ridge (WR) and Grand Marais Ridge (GM). Location of GLIMPCE seismic line C is indicated by red and white line. 55 �Quantitative abundance and preliminary morphological characterization of amphiboles in the Ironwood Iron-Formation, Gogebic Iron Range, Wisconsin GREEN, Carlin J., SEAL, Robert, R., II, CANNON, William F., and PIATAK, Nadine U.S. Geological Survey, MS 954, Reston, VA 20192 The western portion of the Gogebic iron range in northern Wisconsin constitutes one of the largest undeveloped iron resources of the Lake Superior region (Cannon et al., 2007). Knowledge of mineralogical changes related to metamorphism in the Gogebic iron range will be essential to planning for potential future resource development, especially solid mine waste management practices. The purpose of this study is to document the distribution and morphological character of amphibole minerals in the Ironwood Iron-Formation related to the thermal impact of the Mellen Intrusive Complex (MIC). The Paleoproterozoic Ironwood Iron-Formation is the principal iron-bearing unit in the Gogebic iron range. The Ironwood has five named members (from the base upward): Plymouth, Yale, Norrie, Pence, and Anvil (Anvil Member is absent in the western Gogebic iron range) (Cannon et al., 2007). The Ironwood contains ferruginous chert, Fe-oxides, Fe-silicates, and Fecarbonates in granular or oolitic wavy and finely laminated beds. Contact metamorphism related to the Mesoproterozoic MIC had the combined effects of increasing the magnetite content and progressively changing the silicate mineralogy of the Ironwood Iron-Formation (Cannon et al., 2007). Three drill holes, selected for study based on the completeness of their stratigraphic section and location relative to the MIC contact, were examined to determine variability in mineral assemblages with changes in metamorphic grade. The drill holes are oriented east-west, increasing in distance from the MIC eastward. Drill hole A is approximately 2 km from the contact, B is approximately 3.2 km from the contact, and C is approximately 4.7 km from the contact. Samples were selected from lithologic sub-units within the 4 different members to create a representative sample suite for each member. A separate topical set of samples was also chosen from areas of particular interest to elucidate amphibole paragenesis. A few samples were also collected from the underlying Palms Formation. All samples were examined by optical microscopy and scanning electron microscopy (SEM), and analyzed by powder X-ray diffraction (XRD) using Co Kα radiation. Quantitative estimates of mineral abundances were calculated from XRD data using the Rietveld method. The three drill cores show a well-defined progression from low-grade metamorphic conditions in the easternmost drill hole (A) to high-grade metamorphic conditions in the westernmost drill hole (C). Figure 1 displays the mineral distribution in the representative suite of samples from each drill hole. Samples are shown at the drill hole depth from which they were taken. Drill hole A primarily exhibits low-grade metamorphism characteristics (as described in Klein, 1983). Quartz is commonly present in major amounts. Magnetite typically ranges from approximately 10 to 40 weight percent (wt. %) but is absent in a small number of samples. Siderite and members of the dolomite-ankerite series are common and make up significant portions of the total rock volume. Chlorite-group minerals are commonly present in trace to minor amounts. The presence of Fe- and Mg-carbonates and trace amounts of Fe-silicates such as stilpnomelane are indicative of the low-grade metamorphic conditions. Amphiboles were not detected within the representative samples, but trace to minor amounts were found in several of the topical samples from the Norrie and Plymouth members. 56 �Central drill hole B commonly contains amphiboles of the grunerite-cummingtonite series, diagnostic of medium-grade metamorphic conditions (see Klein, 1983). These amphiboles are present in each member of the Ironwood, ranging in quantities from 3 to 65 wt. %. Magnetite and quartz remain the most common minerals. The near-complete absence of Fe- and Mg-carbonates is due to metamorphic reactions of these minerals with quartz, producing members of the grunerite-cummingtonite and actinolite-tremolite series along with calcite. Chlorite-group minerals remain common but at diminished amounts. Drill hole C has indications of high-grade metamorphism, such as the appearance of pyroxene, fayalite, and garnet. Quartz and magnetite continue to be common. Members of the gruneritecummingtonite and actinolite-tremolite series remain common and are present from trace amounts up to approximately 40 wt. %. Chlorite-group minerals become rare at these conditions. Research on amphibole morphology and mineral chemistry is ongoing. Examination of thin sections using petrographic microscopy and SEM reveals a range of amphibole crystal habits and grain sizes. Distinctions in primary morphological classifications of individual amphibole crystals, based on degree of elongation of one or more dimensions and aspect ratio, is underway. Very fined-grained crystalline aggregates composed of grains less than 10 µm in size, as well as large single crystals up to 5.5 mm also have been observed. The distribution of a variety of amphiboles in the Ironwood Iron-Formation has been documented by powder XRD and SEM-EDS (energy dispersive spectroscopy). Quantitative mineral abundance estimates show that the presence of amphiboles is dependent on the proximity to the MIC. Preliminary microscopic investigation reveals a range of crystal habits and geometric variability. These will be characterized in more detail in continuing studies, which will include electron probe micro-analysis to determine quantitative mineral chemistry. Figure 1. Mineral distribution in weight percent in the Ironwood Iron-Formation. REFERENCES Cannon, W.F., LaBerge, G.L., Klasner, J.S., and Schulz, K.J. 2007, The Gogebic iron range—a sample of the northern margin of the Penokean fold and thrust belt: U.S. Geological Survey Professional Paper 1730, 44 p. Klein, C., 1983, Diagenesis and metamorphism of Precambrian banded iron-formations: In, Trendall, A.F. and Morris, R.C. (Eds.), Iron-formation: Facts and Problems, Elsevier, Amsterdam, p. 417-465. 57 �PALEOCURRENT INTERPRETATION OF THE CAMBRIAN ELK MOUND GROUP USING GEOPHYSICAL OPTICAL BOREHOLE IMAGE (OBI) LOGS FROM TWO NEW BOREHOLES, DODGE COUNTY, SOUTHERN WISCONSIN GUENTHER, Gregory1, KINGSBURY STEWART, Esther2 1Department 2Wisconsin of Geology and Geography, University of Wisconsin - Whitewater, 800 W Main St, Whitewater, WI 53190 USA Geological and Natural History Survey, 3817 Mineral Point Rd, Madison, WI 53705 Paleocurrents measured from cross-beds in sandstone indicate sediment transport direction or the flow direction of water at the time of deposition. Characterizing variation in paleocurrent direction through a stratigraphic succession is one tool for investigating evolution of depositional environment through time. In southern Wisconsin the Cambrian Elk Mound Group is comprised of, from oldest to youngest, the Mount Simon Formation sandstone, Eau Claire Formation shale, and Wonewoc Formation sandstone. In south-central Wisconsin the Eau Claire Formation undergoes a facies change from shale to sandstone. Here the Elk Mound Group is present mostly in the subsurface so it is understood primarily through observation of drill core, drill cuttings, and downhole geophysical logs. The purpose of this study is (1) to test the validity of using measurements from optical borehole image (OBI) logs to measure paleocurrents in sandstones present in the subsurface and (2) investigate how paleocurrent direction varies through two thick, sandstone-dominated intervals of the Elk Mound Group in Dodge County, Wisconsin. OBI logs are oriented, down-hole images taken from drill site boreholes. The images return as 360 degree unwrapped digital representations of the borehole, and these images record sedimentary structures like cross-beds, laminations, and bedding planes. We use Wellcad software to measure the strike and dip of cross-beds and laminations from the Elk Mound Group that were imaged with OBI logs from two boreholes that the Wisconsin Geological and Natural History Survey drilled in Dodge County in 2015. The Slinger site encountered 243 feet (74 m) of Elk Mound Group sandstone before hitting Precambrian iron-formation. The Westphal 2 site encountered 422 feet (129 m) of Elk Mound Group sandstone and did not hit Precambrian basement. We compare OBI logs to drill core collected at each site. We compare our paleocurrent measurements to existing published data. Finally, we use elemental data collected using a handheld x-ray fluorescence (XRF) instrument at one foot intervals along each core to subdivide the Elk Mound Group. We observe how paleocurrent measurements vary between each of these preliminary subdivisions of the Elk Mound Group. The paleocurrent directions we measured from OBI logs are consistent with published data for the Elk Mound Group (Michelson et al., 1973; Driese et al., 1981; Hagadorn et al., 2002). Published data report primarily south-directed paleocurrent directions with a secondary bimodal direction for the Elk Mound Group. At the Slinger and Westphal 2 study locations, we subdivide the Elk Mound Group into four informal units based on changes in elemental concentrations of Al, Ti, Zr, and K. The lower two units are characterized by southwest-directed paleocurrents. Paleocurrents become bimodal in the upper two units. Based on the similarity with published paleocurrent data, we conclude that paleocurrents may be measured accurately from OBI logs. Furthermore, the change in paleocurrent direction we observe from southwest in the lower Elk Mound Group to bi-modal in the upper Elk Mound Group likely reflects a change in environmental conditions that control sediment transport and deposition. Measuring paleocurrent direction from OBI logs is therefore a useful tool to aid subdivision of seemingly monotonous, thick packages of sandstone that are present in the subsurface. 58 �We acknowledge Dr. Prajukti Bhattacharyya, University of Wisconsin-Whitewater, for her assistance with this project. Driese, S. G., Byers, C. W. (1981). Tidal Deposition in the Basal Upper Cambrian Mt. Simon Formation in Wisconsin. Journal of Sedimentary Research, vol. 51, no. 2, p. 367-381. Hagadorn, J. W., Dott, R. H. Jr., Damrow, D. (2002). Stranded on a Late Cambrian shoreline: Medusae from central Wisconsin. Geology, vol. 30, no. 2, p. 147-150. Michelson, P. C., Dott, R. H. Jr. (1973). Orientation Analysis of Trough Cross Stratification in Upper Cambrian Sandstones of Western Wisconsin. Journal of Sedimentary Research, vol. 43, no. 3, p.784-794. NOTE: Journal of Sedimentary Research formerly known as Journal of Sedimentary Petrology. 59 �U-Th-Pb isotopes of the Reef Deposit; a Au-Cu occurrence in central Wisconsin HAROLDSON, Erik1 BEARD, Brian1 SATKOSKI, Aaron1 JOHNSON, Clark1 BROWN, Philip1 1 Department of Geoscience, University of Wisconsin-Madison, 1215 W. Dayton, Madison, WI, 53706 USA The Reef deposit, located within the Wausau Volcanic complex (WVC), is a vein hosted Au-Cu occurrence historically calculated to contain ~454,600 tons grading 0.262 opt gold, 0.25 opt silver and ~0.28% copper located approximately 15 miles east of Wausau, Wisconsin. The Ladysmith-Rhinelander volcanic complex (LRVC), adjacent to the North of the WVC within the Pembine-Wausau volcanic subterrane, is known to contain many volcanogenic massive sulfide (VMS) deposits, several of which are considered to be economically viable to mining (DeMatties 1996). The Reef deposit has been described in the past as a lode gold or shear-zone hosted gold deposit (DeMatties 1996) separating it genetically from the VMS deposits to the north, along with a physical separation of the hosting volcanic complex (WVC vs. LRVC). The Reef deposit has also been described as the root zone of a Cu-Zn or Zn-Cu VMS deposit (Scott 1988) and although there are no known VMS deposits of economic interest, the WVC is known to host minor VMS mineralization (DeMatties 1996). An orogenic lode gold deposit could potentially be associated with various tectonic events in the history of central North America including; ~1700 Ma Yavapai, ~1600 Mazatzal or even anorogenic magmatic activity of the ~1510 Ma Stettin syenite complex or the ~1470 Ma Wolf River Batholith. The Reef deposit consists of seven mineralized zones which consist of Au-Cu bearing quartz-sulfide veins hosted in primarily basaltic material (mafic metavolcanics and gabbroic intrusions) (Kennedy and Harding, 1990). Whilst ore zones of quartz-sulfide veins are also crosscut by similar gabbroic intrusives it is unclear when the Au mineralization was deposited. The zones trend northeast, dip to the northwest and are closely associated with felsic intrusions (Kennedy and Harding 1990). Felsic dikes and sills are intermingled with the host gabbro intrusives as a swarm of granophyric to porphyritic, locally aplitic units. The deposit area is flanked to the west and northwest by dominantly basalt of massive and pillowed flows and mafic tuff. Felsic intrusions flanking the deposit are concentrated in the western and southeastern areas adjacent to the deposit. U-Th-Pb isotopes analyses were made of sulfide phases which were micro-drilled from drill core selections and aliquots of coarse reject from split drill core assay material, previously split and crushed by Aquila Resources. The coarse reject samples are referred to as ‘whole rock’ (WR) samples. WR and sulfide (chalcopyrite, pyrite and pyrrhotite) 206Pb/204Pb, 207Pb/204Pb and 208 Pb/204Pb values range from 15.732 to 22.897, 15.222 to 16.058 and 35.139 to 41.794 respectively (Fig 1). Pb isotope values indicate a source region for sulfides at Reef similar to that of the Flambeau VMS deposit. The age of volcanogenic massive sulfide deposits in the LRVC have been estimated at ~1860 Ma (DeMatties 1996) and a Pb-Pb isochron for Reef of 1925 ± 92 Ma does overlap the estimate. Measured 238U/204Pb (μ) values range from 0.185 to 4.45 for sulfides and 1.016 to 14.202 for WR samples. 232Th/204Pb (ω) values range from 0.0436 to 3.084 for sulfides and 0.623 to 19.051 for WR samples. Only some μ and ω values support highly radiogenic Pb values to have been derived from in-situ decay (Fig 2). Samples are either subject to later coincident uranium and thorium loss, or lead addition from a high μ source. 60 �A genetic model for the Reef deposit remains elusive; but a primary association with the Penokean orogen has been established. Figure 1 – 207Pb/204Pb vs 206Pb/204Pb plot of results from the Reef deposit and the Flambeau and Lynne VMS deposits. Smaller plot is inset with same data plotted. WR = Whole Rock; WRHM = Whole Rock Hand Magnetic fraction. Dark filled squares are previously published values for VMS deposits (Afifi et al 1984). Curve plotted is lead evolution from Stacey and Kramers (1975). Figure 2 - plots investigating the in-situ decay of U-Th in Reef deposit samples. Symbols are same as figure 1. Reference isochron of 1925 Ma is shown in each plot. REFERENCES Afifi, A., Doe, B.R., Sims, P.K., Delevaux, M.H., 1984, U-Th-Pb isotope chronology of sulfide ores and rocks in the early Proterozoic metavolcanic belt of northern Wisconsin, Economic Geology, v. 79, pp. 338-353 Dematties, T.A., 1996, A geologic framework for early Proterozoic volcanogenic massive sulfide deposits in Wisconsin: an exploration model, Institute on Lake Superior Geology, Volcanogenic massive sulfide deposits of northern Wisconsin: a commemorative volume, pp. 31-65. Kennedy, L.P., and Harding, T.A.,1990, Summary report of the Reef joint venture Marathon County, Wisconsin, Noranda Exploration Inc., 63 pp. Scott, W.P., 1988, A volcanic hosted gold occurrence in Marathon County, Wisconsin, A thesis submitted in partial fulfillment of the requirements for the degree of master of science geology, University of WisconsinMadison, 99 pp Stacey, J.S., and Kramers, J.D., 1975, Approximation of terrestrial lead isotope evolution by a two-stage model, Earth and Planetary Science Letters, vol. 26, pp 207-221. 61 �A New Rusk County: Producing an new Precambrian geological map from new field observations and compilations of historic geological/geophysical datasets HELMUTH, Samuel L.1, LODGE, Robert W.D.1 1 Department of Geology, University of Wisconsin-Eau Claire, Eau Claire, WI 54701 USA Precambrian rocks of the Penokean Orogeny within the Wisconsin Magmatic Terrane host world-class volcanogenic massive sulfide deposits (VMS) near the southern limit of the Canadian Shield (DeMatties, 1994). This is a Paleoproterozoic juvenile and continental arc sequence consisting of mafic to felsic volcanic assemblages, sedimentary sequences, and associated plutonic rocks that are about 1.8-1.9 billion years old (Schulz & Cannon, 2007). These deposits are significant sources of metals such as zinc, copper, lead, silver and gold in the form of sulfide minerals. The size and concentration of VMS deposits in Wisconsin gives the region potential to become a world-class mining district. However, a thick cover of glacial deposits and Paleozoic sedimentary strata limit the Precambrian exposure in the region and make regional tectonic and metallogenic interpretations of the Precambrian geology difficult. The primary objective of this project was to create a new geologic map of the Precambrian geology in Rusk County, Wisconsin, describing the distribution and petrologic characteristics of the Precambrian bedrock beneath this cover by compiling historic geological and geophysical datasets with new field observations and combining these observations with modern geochemical data and interpretations. There were two phases of mapping related to this project. The first phase included sampling rocks from drill core, which was collected during mineral exploration and mining activities, in regions in the county where no bedrock exposure exists. These uppermost parts of these drill cores were described and sampled. The second phase involved field mapping and sampling of Precambrian rock outcrops in the rivers, road cuts, and quarries throughout Rusk County. All samples were processed and analyzed via XRF to determine their major and minor element geochemistry and thin sections from select samples were used for petrographic analysis. The volcanic and tectonic insights gained through petrographic and geochemical interpretations were compiled and geospatially integrated with historic geologic maps published through the Wisconsin Geological and Natural History Survey (Mudrey et al. 1987) and aeromagnetic geophysical surveys available through the USGS (Daniels & Snyder, 2002) to better constrain the Precambrian lithostratigraphic units beneath Cambrian and Quaternary cover. This integration of field mapping, drill core sampling, and data compilation of geological and geophysical datasets in ArcGIS created a new and improved geological map of the Precambrian rocks of Rusk County (Figure 1). In addition, petrographic and geochemical interpretations of these rocks will add a new layer of understanding that will significantly improve our tectonic, volcanological, and metallogenic framework of the Precambrian bedrock in Wisconsin. Based on aeromagnetic geophysical patterns, distribution of various volcanic, plutonic, and sedimentary rocks, and geochemical domains throughout Rusk County, we subdivided the Precambrian geology into four supracrustal units: (1) Blue Hills Metasedimentary Unit, (2) Thornapple Metavolcanic Unit, (3) Flambeau-Jump River Metavolcanic Unit, and (4) Weyerhaeuser-Forks Metavolcanic Unit. The Blue Hills Metasedimentary Unit underlies the northwestern portion of the county and has abundant outcropping quartzite and phyllite. The Thornapple Metavolcanic Unit is located throughout the north-central portion of the map forming a moderately magnetic geophysical domain and hosts the Zn-Cu-Pb Eisenbrey VMS 62 �deposit. Bedrock exposures and core samples observed included primarily intermediate volcanic assemblages with lesser basalt, rhyolites, and iron formation forming. The lower magnetic portion of the southeastern part of the county is predominantly mafic to intermediate volcanic assemblages that hosts the Flambeau Cu-Au VMS deposit. Lastly, the Weyerhaeuser-Forks Unit underlies the highly-magnetic areas in the southwest and northeast parts of the map. Here, alkali basalts and associated intrusions have been discovered. The aeromagnetic geophysical surveys also allowed us to better identify felsic plutonic bodies in the mapping area. Figure 1: Preliminary Precambrian geology of Rusk County, Wisconsin, showing major faults (bolded lines) and contacts (thin lines) shown in relation to the airborne magnetic geophysical map for the state (Daniels & Snyder, 2002). Circles in mapping area show where core and field samples have been collected and analyzed. Numbers 1 through 4 correlate to approximate location of supracrustal units described in text. REFERENCES Daniels, D.L. & Snyder, S.L., 2002. Wisconsin Aeromagnetic and Gravity Maps and Data: A Web Site for Distribution of Data. USGS Open File Report 02-493. DeMatties, T.A., 1994. Early Proterozoic volcanogenic massive sulfide deposits in Wisconsin: An overview. Economic Geology, 89: 1122-1151. Mudrey, M.G., LaBerge, G.L., Myers, P.E., and Cordua, W.S., 1987. Bedrock geology of Wisconsin: Northwest sheet. Wisconsin Geological and Natural History Survey Regional Map Series. Scale: 1:250,000. Schulz, K.J. & Cannon, W.F., 2007. The Penokean orogeny in the Lake Superior region. Precambrian Research, 157: 4-25. 63 �Geochemical and petrological studies on the origin of Ni-Cu sulfide mineralization at the Eagle and Eagle East intrusions in Marquette County, Michigan HINKS, Benjamin1, THAKURTA, Joyashish1, MAHIN, Robert2, and BEACH, Steve2 1 Department of Geosciences, Western Michigan University, 1903 W. Michigan Ave. Kalamazoo, MI 49008, benjamin.d.hinks@wmich.edu 2 Eagle Mine, Lundin Mining Corporation, 4547 County Road, Champion, MI 49814 The Eagle deposit is a small, high-grade Ni-Cu-bearing sulfide deposit located in the north central portion of Michigan’s Upper Peninsula in Marquette County. The Eagle and the Eagle East intrusions, separated by 600m, are parallel to the east-west trending Marquette-Baraga dike swarm that is associated with ~1.1 Ga Midcontinent Rift System magmatism (Ding et al. 2010). Recently, in 2014 a discovery was made within the Eagle East intrusion. Drilling by Lundin Mining Corporation intersected a small section of high grade sulfide minerals near the base of the intrusion. The purpose of this study is to characterize the magmatic sulfide deposit at Eagle and its relationship to the surrounding country rocks. The relationship between Eagle sulfide ores and the newly discovered Eagle East sulfide ores have also been analyzed. Petrographic analysis has shown that Eagle and Eagle East sulfide mineralization consists of three minerals: pyrrhotite, pentlandite and chalcopyrite. Olivine crystals tend to host sulfide minerals, indicating that an immiscible sulfide liquid must have formed prior to olivine formation. Rare earth element (REE) analysis from peridotite samples at Eagle and Eagle East confirms that the intrusions are very similar. The REE diagram depicts similar concentrations and patterns of REEs with shallow slopes for the two intrusions, indicating low degrees of crystal fractionation and high degrees of partial melting that formed the magmas at Eagle and Eagle East. Sulfur isotope analysis was also performed on sulfide ores from Eagle, Eagle East, Michigamme Formation rocks and Archean basement rocks. The three intrusive sulfide zones (disseminated, semi-massive, and massive) for both intrusions are within the range of 0‰ to 5‰. Michigamme Formation rocks display δ34S values from 6‰ to 20‰. Archean basement rocks display a wide range of sulfur isotope values from -11‰ to 7‰. The δ34S values reported from the main sulfide ore bodies of Eagle and Eagle East are indicative of slight enrichment of 34S, most likely caused by interaction and mixing of mantle-sourced sulfur (0‰) with surrounding crustal-sourced sulfur. Country rock contamination δ34S signatures found within the immiscible sulfide liquid may have been averaged or totally erased through interaction of mantle magmas with the crustal sulfide liquid. δ34S signatures could have additionally been altered by the mixing of sulfur from the two crustal sources, Archean and Proterozoic rocks. Archean rocks typically have a range of high and low δ34S values from -11‰ to 7‰, while Proterozoic rocks have high δ34S values of 6‰ to 20‰. Mixing of sulfur sourced from Archean rocks with sulfur sourced from Proterozoic Michigamme Formation rocks could have averaged the δ34S signatures seen in the ore bodies to ~3.5‰. Previous researchers Ding et al. (2012) determined δ34S values at the Eagle deposit for disseminated and massive sulfides ranging from 0.3‰ to 4.6‰, while semi-massive sulfides were characterized by δ34S values ranging from 2.2‰ to 5.3‰. The δ34S values of sulfide ores hosted in the intrusion are much lower than would be expected if assimilation of crustal-sourced sulfur had occurred. Ding et al. (2012) attributed the low δ34S values within the Eagle intrusion 64 �to mantle-sourced sulfur (0‰) coming into contact with sulfide liquid that had higher δ34S values attributed to crustal rock assimilation and depleting the δ34S values closer to 0‰. However, since the δ34S values within intrusive sulfides at both Eagle and Eagle East in this study are so similar, it is proposed that mixing of crustal-sourced sulfur form the Michigamme Formation rocks and Archean basement rocks could have additionally averaged the δ34S values seen within the intrusions to a range between 0‰ to 5‰. Mixing of sulfur derived from crustal rocks and the interaction of mantle-sourced sulfur could have averaged the δ34S values seen within the ore bodies from that of crustal contamination signatures. Figure 1: Hypothetical diagram illustrating a model for the mixing of sulfur from Archean and Proterozoic sources, resulting in δ34S values between 0‰ to 5‰ for sulfides hosted within the Eagle and Eagle East intrusions. REFERENCES Ding, X., C. Li, E. M. Ripley, D. Rossell, and S. Kamo (2010), The Eagle and East Eagle sulfide ore‐bearing mafic‐ ultramafic intrusions in the Midcontinent Rift System, upper Michigan: Geochronology and petrologic evolution, Geochem. Geophys. Geosyst., 11, Q03003, doi:10.1029/2009GC002546. Ding, X., E. M. Ripley, S. B. Shirey, C. Li (2012), Os, Nd, O and S isotope constraints on country rock contamination in the conduit-related Eagle Cu-Ni-(PGE) deposit, Midcontinent Rift System, Upper Michigan: Geochemica et Cosmochimica Acta, 89, pp. 10-30. 65 �Preliminary Observations of the Ultramafic Metavolcanic Rocks in the Eastern Portion of the Shebandowan Greenstone Belt, northwestern Ontario HINZ, Sheree and HOLLINGS, Pete Department of Geology, Lakehead University, Thunder Bay, Ontario P7B 5E1 The ultramafic metavolcanic rocks in the eastern portion of the Shebandowan greenstone belt are located within Conmee Township (Fig. 3), which is part of the larger Wawa-Abitibi terrane (Stott et al., 2010). The Shebandowan greenstone belt has been divided into three main assemblages based on geochronological studies by Stott and Corfu (1998) and Lodge (2012), namely; the Greenwater assemblage (circa 2720 Ma), the Kashabowie assemblage (circa 2695 Ma) and the Shebandowan assemblage (circa 2690 to 2680 Ma). This study focuses on the ultramafic metavolcanic rocks of the Greenwater assemblage that are best exposed in the Bateman trenches dug by Linear Metals in 2008 and the Freewest and Dawson trenches (Fig. 3). These trenches display near continuous stratigraphic sections including spinifex-textured flows, massive flows, flow breccia and quenched autobreccia (Lodge, 2014). Other rock types in this assemblage include mafic, intermediate and felsic metavolcanic rocks, as well as metasedimentary rocks. Preliminary field observations include the identification of preserved macroscopic primary volcanic textures, despite the greenstone belt being metamorphosed to greenschist facies. The macroscopic textures and associations are important because on a microscopic scale, all primary mineralogy has been altered and metamorphosed. Spinifex texture is widely observed throughout the map area; this texture is formed through undercooling of a very hot magma and is a critical feature of komatiites (Fig. 2). Associated with the spinifex-textured komatiite in the Linear Metals trenches is a unit with polyhedral jointing analogous to columnar jointing in basaltic flows (Arndt 2008). In several localities an ultramafic volcanic breccia with spinifex-textured angular fragments in a glassy groundmass is present (Fig. 1). These macroscopic textures are difficult to put into context with the surrounding units because of the gaps in the trenches where contacts should exist. Preliminary geochemical analyses show that these rocks have lower than normal MgO for a true komatiitic rock, with an average of 11.2 wt% but some samples showing up to 27wt% MgO. The SiO2 content is also higher than normal for a typical komatiite, with an average of 50%, with some samples as low as 40wt%. The Ni content fits well with the typical komatiite composition with an average of 607ppm and average Cr content of 1133ppm. This suggests that the ultramafic rocks in the study area are predominantly komatiitic basalts with rare komatiites. Both komatiites and komatiitic basalt display spinifex texture. Further mapping will be completed in the summer of 2016 in order to correlate the units over a wider area. Petrographic studies are underway and geochemical data will be analyzed once obtained. By combining detailed petrographic studies with whole-rock geochemical data this study will investigate fractionation trends, mixing and crustal contamination signatures to understand the evolution of the komatiites. This study will produce a detailed map of the komatiites and develop a model for the formation of these rocks. 66 �Figures 1) Spinifex texture in ultramafic rock, from Linear Metals trench 2) Ultramafic breccia from Freewest trench Figure 3) Regional geology map showing the location of study areas within Conmee Township in relation to the Shebandowan greenstone belt. Bedrock geology modified from Santaguida (2001a, 2001b) and Lodge et al. (2015). REFERENCES Arndt, N.T., Lesher, C.M, Barnes, S.J. 2008. Komatiite. Cambridge University Press. 467p. 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. Lodge, R.W.D., Gibson, H.L., Stott, G.M., Franklin, J.M. and Hudak, G.J. 2015. Geodynamic setting, crustal architecture, and VMS metallogeny of ca. 2720 Ma greenstone belt assemblages of the northern Wawa subprovince, Superior Province; Canadian Journal of Earth Sciences, v.52, p.196-214. Lodge, R.W.D. 2012. Preliminary results of uranium–lead geochronology from the Shebandowan greenstone belt, Wawa Subprovince; in Summary of Field Work and Other Activities 2012, Open File Report 6280, p.10-1 to 10-10. Lodge, R.W.D., Ratcliffe, L.M., and Walker, J.A. 2014. Geology and Mineral Potential of Sackville and Conmee Townships, Wawa Subprovince; in Summary of Field Work and Other Activities 2014, Open File Rpt 6300, p 9-1 to 9-17. Santaguida, F. 2001a. Precambrian geology compilation series—Quetico sheet; Ontario Geological Survey, Map 2663, scale 1:250 000. ——— 2001b. Precambrian geology compilation series—Thunder Bay sheet; Ontario Geological Survey, Map 2664, scale 1:250 000. Stott, G.M., Corkery, M.T., Percival, J.A., Simard, M and Goutier, J. 2010. A revised terrane subdivision of the Superior Province; in Summary of Field Work and Other Activities 2010, Ontario Geological Survey, Open File Report 6260, p.20-1 to 20-10. 67 �The Minnesota Taconite Workers Health Study: Environmental Study of Airborne Particulate Matter - 2015 Update HUDAK, George1, MONSON GEERTS, Stephen1, ZANKO, Larry1, POST, Sara1, and REAVIE, Euan1, 1 Natural Resources Research Institute, 5013 Miller Trunk Highway, Duluth, MN, 55811 The Natural Resources Research Institute (NRRI) conducted a detailed characterization of mineral dust in northeastern Minnesota. The purpose of this research was to evaluate the effects of present emissions from taconite mining and processing on air quality throughout the Mesabi Iron Range (MIR) (Figure 1) by characterizing airborne mineral particulate matter (PM) within currently operating taconite processing plants, in MIR communities surrounding taconite mining/processing operations, and in population centers in Minnesota not associated with taconite mining. Characterization studies of age-dated lake sediments were also conducted to determine the composition of past PM deposition. NRRI’s sampling and characterization work represents the community/environmental component of the Minnesota Taconite Workers Health Study, a broad University of Minnesota (UM) research effort involving both the NRRI and the School of Public Health. Figure 1. Locations of taconite processing plants on the Mesabi Iron Range being sampled during this study (after Oreskovich and Patelke, 2006) Air sampling was performed within taconite operations, MIR communities, and non-MIR communities by NRRI scientists during both winter and summer seasons from 2009-2012. Sampling was conducted at four process locations within taconite operations, including: 1) secondary crushers; 2) magnetic separators/concentrators; 3) agglomerators/ball drums; and 4) kiln/pellet discharge areas. Community sampling took place on centrally-located rooftops of public buildings, or in the case of the northern most background site, in a remote sampling location to evaluate the air quality away from the MIR. Sampling and analytical techniques for all project work are described in detail in several reports that are in preparation. NRRI’s research methods do not produce exposure data, and are not meant to provide data for regulatory purposes. 68 �NRRI has evaluated the physical (gravimetric, morphology, concentration), mineralogical, and chemical characteristics of the PM obtained from sampling at the taconite operations and MIR/non-MIR communities. This included analysis of 55 taconite plants; 73 northeastern Minnesota community and 6 Minneapolis samples. Age-dated lake sediment cores were collected from Silver Lake in Virginia, MN, on the central MIR, and “North-of-Snort” Lake on MIR’s eastern end, near Babbitt, MN, and 36 analyses were completed. The results provide historical data regarding potential mineralogical inputs from iron mining and processing from ~1840 (which pre-dates iron mining on the MIR) to the present, including the natural ore mining to taconite mining transition period. Community results are as follows:  measured particulate matter concentrations for PM2.5 in all MIR communities have been below 12 µg/m3, and for total PM have been below 16µg/m3;  particulate matter concentrations on the MIR are similar to those in the two NE Minnesota background sites (Duluth NRRI, Ely Fernberg site), and are lower than those obtained in Minneapolis (UM Mechanical Engineering Building rooftop);  mineral particulate matter in community air samples reflects the mineralogy of the Biwabik Iron Formation and other Minnesota rock types and geological materials;  elongate mineral particles (EMP) are present in MIR community ambient air samples; however, asbestiform amphiboles were rarely observed (1 asbestiform amphibole EMP in ~22,800m3 of air). Taconite plant results are as follows:  plant environments can be dusty, with the most dusty environments associated with the agglomerator and kiln discharge areas;  particulate matter levels (PM1, PM2.5, PM10, and total PM) show a slight increase in the five MIR communities during plant/mine activity, but this increase is not statistically significant compared to when the plants were not operating.  significantly higher concentrations of EMPs, including amphiboles, were detected in the eastern most plant compared with the other five plants, but the morphology of these structures more closely resembles cleavage fragments rather than asbestiform morphologies. Lake sediment results are as follows:  a water elutriation method developed by Webber et al. (2008) was effective for isolating PM2.5 particles from age-dated sediment intervals.  the mineralogy of isolated PM2.5 EMPs reflects bedrock and glacial geology in the vicinity of both lakes.  a portion of the insoluble PM within the sediment of the two MIR lakes is most likely attributable to atmospheric inputs (fugitive dust) generated by historic iron ore/taconite mining activity References MDH. Method 852 (1999) T.E.M. analysis for mineral fibers in air – 852. Minnesota Department of Health, Microparticulate Unit, St. Paul, MN. 42 pp. Oreskovich, J. A., and Patelke, M. M., 2006, Historical use of taconite byproducts as construction aggregate materials in Minnesota: A Progress Report: Natural Resources Research Institute Report of Investigation NRRI-RI-200602, 10 p. Webber, J. A., Blake, D. J., Ward, T. J., and Pfau, J. C., 2008, Separation and characterization of respirable amphibole fibers from Libby, Montana, Inhalation Toxicology, 20:8, p 733-740. 69 �Lithostratigraphy and Ore Petrology of the Eisenbrey Zn-Cu-Pb Deposit, Rusk County, Wisconsin JACKSON, Nathaniel, DE MOURA MERSS, Bruno, and LODGE, Robert W.D. Department of Earth Science, University of Wisconsin-Eau Claire, Eau Claire, WI The primary objective of this research is to study the geological characteristics of the poorly understood Eisenbrey Zn-Cu-Pb deposit in Rusk County, Northwestern Wisconsin. Volcanogenic massive sulfide (VMS) deposits are significant sources of metals such as zinc, copper, lead, silver and gold in the form of sulfide minerals (Franklin et al. 2005). Despite the proximity of the Eisenbrey deposit to the better known, past-producing Flambeau Cu-Au VMS deposit, there has been essentially no research completed on the rocks hosting the Eisenbrey nor has there been any volcanic and tectonic linkages made to the strata hosting the Flambeau. Understanding the tectonic and metallogenic framework of the Eisenbrey and any potential genetic relationship to the Flambeau deposit will significantly improve our understanding of the Precambrian geology of northwestern Wisconsin (e.g. DeMatties 1996). The samples of the metalliferous ores and their host rocks are currently being analyzed to determine their petrographic and geochemical characteristics and to re-interpret the economic geology of this region. Initial phases of research included visiting the outcrop exposures of the Paleoproterozoic rocks that host the deposit that are present along the Thornapple River approximately 4 miles northwest of the Flambeau deposit. The ore horizon is present as interlayered sulfide-bearing magnetite-chert iron formation and strongly foliated chloritic mafic units exposed with a total stratigraphic thickness of ~10 m. The ore horizon is hosted in variably silicified intermediate to felsic volcaniclastic rocks. These Paleoproterozoic rocks are intruded to the north and west by gabbro and pyroxenite dykes associated with the midcontinent rift. The second phase of this research involved re-logging and sampling of historic drill core collected during mineral exploration (Figure 1). The ores and altered rocks are hosted within a thick pile of moderately foliated dacitic volcaniclastic unit consisting of tuffs and lapilli tuffs that have feldspar phenocrysts up to 3mm in size and composes 1-2% of the rock. These rocks are altered to an anthophyllite-magnetite±sericite schist that is generally fine grained and has a moderate foliation. This altered unit is variably mineralized and contains thin pyrite stringers and disseminations. Based on lithostratigraphic associations, the Eisenbrey deposit is a bimodal felsic-type deposit. In addition, the presence of an ore horizon dominated by sphalerite, chalcopyrite, pyrite and pyrrhotite supports this classification (Franklin et al., 2005). However, massive to semimassive ore are generally low grade and are composed of mostly pyrrhotite and pyrite with local abundances of chalcopyrite and sphalerite composing a combined 10% over 1-2 meters. Reflected light and scanning electron microscopy are currently being carried out on these ores. In addition to confirming the major ore minerals, trace amounts of silver telluride have been found. 70 �Figure 1: Representative cross section of the Eisenbrey VMS deposit showing location and stratigraphic context of holes logged for this study (bold lines and labels). Figure modified from LeBerge (1996). REFERENCES DeMatties, T.A., 1994. Early Proterozoic volcanogenic massive sulfide deposits in Wisconsin: An overview. Economic Geology, 89: 1122-1151. Franklin, J.M., Gibson, H.L., Jonasson, I.R., Galley, A.G., 2005. Volcanogenic massive sulphide deposits, in: Hedenquist, J.F.H., Goldfarb, R.J., Richards, J.P. (Eds.), Economic Geology, 100th Anniversary Volume, pp. 523-560. LeBerge, G.L. (ed), 1996. Volcanogenic massive sulfide deposits of northern Wisconsin: A commemorative volume. Institute on Lake Superior Geology, Proceedings, 42nd Annual Meeting, Cable, WI, vol. 42, part 2, 179 p. 71 �Incommensurately modulated structure of plagioclase as an indicator of cooling history of igneous rock JIN, Shiyun, and XU, Huifang Department of Geoscience, University of Wisconsin-Madison, 1215 W. Dayton Street, Madison, WI 53706 Plagioclase feldspar is the most abundant group of mineral in the earth’s crust. Intermediate plagioclase formed at low temperature display a highly ordered aperiodic structure, resulting in satellite diffractions with irrational indices (e-reflections), which has been an enigma for more than half a century since its first discovery in 1940(Chao and Taylor 1940). Three plagioclase crystals of similar chemical composition from Duluth igneous complex were analyzed with single crystal X-Ray diffraction. The incommensurately modulated structure were solved in (3+1)D hyperspace. Detailed differences of the crystal structures were revealed, while the structures solved with only main reflections are basically the same as volcanic labradorite. The plagioclase in troctolite (An57) and gabbro (An54) from Duluth Igneous complex has a modulation period of 42Å and 45Å respectively, which are significantly larger than the modulation period of a metamorphic plagioclase with similar composition (~30Å). The structures display density modulation with an amplitude of less than 10 mole%, which is hard to characterize accurately due to the limited second order satellite reflections (f-reflections). The plagioclase in the anorthosite with composition of about An60, on the other hand, shows a modulation period of 40Å. The structure is obviously more ordered than the other two crystals, displaying a density modulation of 18 mole% in amplitude. The number of f-reflections collected is about triple of the troctolite sample and more than 10 times of the gabbro sample. The fitness of the refinement result and the experimental data is significantly better. The degree of orderliness characterized by the XRay diffraction data indicates a different origin of the anorthosite from the Duluth Layered Serious. Thus the modulation period and amplitude of e-plagioclase may be used as an indicator of the cooling history of the host rock. Chao, S.H. and Taylor, W.H. (1940) Isomorphous replacement and superlattice structures in the plagioclase feldspars, pp. 76-87. 72 �Nine years of capstones: A summary of Precambrian Research Center field camp capstone projects in the Neoarchean Knife Lake Group and associated rocks, central Boundary Waters Canoe Area Wilderness, Minnesota JIRSA, Mark A. Minnesota Geological Survey, University of Minnesota; jirsa001@umn.edu An important mission of the University of Minnesota-Duluth’s Precambrian Research Center (PRC) is training students to map the geology of Precambrian terranes. The arrowhead region of Minnesota generally—and the Boundary Waters Canoe Area Wilderness specifically—contains some of the best localities for such mapping. Since its inception in 2006, the PRC has conducted 9 seasons of geologic field camps in the region (2007-2015), with an emphasis on the rock types, alterations, and structures that host much of the world’s metallic mineral endowment. The training involves several weeks of mapping exercises, followed by a “capstone” project conducted in small groups with practicing geologists. These projects test student skills by creating new geologic maps in areas of poorly known geology, which benefits both students and mentor organizations. Students conduct field work for 7-8 days in remote locations, then remarkably turn their observations into geologic maps during 4 days and present their results before an audience of academic, government, and industry professionals. This presentation describes one set of capstone projects in the central part of the Boundary Waters Canoe Area Wilderness (BWCAW). The geology was mapped to varied levels of detail by the author and 41 students during 9 individual capstone projects. Although these capstones focused on lithologic and structural complexities of the Neoarchean Knife Lake Group (Fig. 1), the resulting maps provide details about other parts of the Wawa subprovince of the Archean Superior Province, rare diabasic dikes, unconformitybounded Paleoproterozoic iron-formation, and the basal Mesoproterozoic Duluth Complex. Figure 1. Generalized bedrock geologic map of NE Minnesota showing the area of capstone mapping projects (outline labeled “Knife Lake Capstones;” ~ area of Fig. 2). The Neoarchean unit labeled “Supracrustal Rocks” encloses both older volcanic sequences and younger, largely sedimentary ones. Outline of BWCAW is dashed. Volcanic, sedimentary, and intrusive rocks of the Knife Lake Group comprise a Timiskaming-type extensional basin and its apparent wall- and floor-rocks. The geologic units are parceled into structural lozenges separated by anastomosing shear and fault zones (Fig. 2). Although rock types are comparatively pristine within each lozenge, correlation of units from one fault-bounded block to another is challenging. Nevertheless, these projects attempt to “unstrain” the rocks within each lozenge to reveal stratigraphic variations that may reflect fluctuations in original basin geometry and progressive erosional dissection of basin wall rocks. Understanding the lithologic details and the apparent post-depositional tilt of individual lozenges of rock is essential to this objective. An integrated depositional and tectonic model of the basin is evolving using Gruner’s (1941) work, unpublished theses, and capstone mapping. Basin evolution appears to have involved early subaerial calc-alkalic volcanism synchronous with sediment deposition, uplift, erosion, and surface weathering that contributed alluvium to localized fault basins. This was followed by development of braided streams, and transitioned to subaqueous deltaic and turbiditic deposition. Strata containing clasts of the ca. 2690 Ma Saganaga Tonalite were deformed and metamorphosed at ca. 2680 Ma, which brackets basin development within a 10 Ma period. 73 �Figure 2 Historic geologic map of central BWCAW from Gruner (1941) showing fault zones that bound lozenges or segments of internally coherent strata. Bounding faults are modified from Gruner’s work based on subsequent mapping. Mapping funded in part by the Precambrian Research Center contributed to the following publications: Abstracts—Institute on Lake Superior Geology, 2008-2016. Lead authors/meeting years: Jirsa 2008, 2009, 2012; Fahrenkrog 2010; Birkmeier 2011; Korman 2013; Mulcahy 2014; Krogmeier 2015; Christenson 2016. Driese, S.G., Jirsa, M.A., Ren, M., Sheldon, N.D., Brantley, S.L., Parker, D., and Schmitz, M., 2011, Neoarchean paleoweathering of tonalite and metabasalt: Implications for reconstructions of 2.69 Ga early terrestrial ecosystems and paleoatmospheric chemistry: Precambrian Research, v. 189, p. 1-17. Jirsa, M.A., 2011, Bedrock geology of the western Gunflint Trail area, northern Minnesota: Minnesota Geological Survey Miscellaneous Map M-191, scale 1:24,000. Jirsa, M.A., Boerboom, T.J., Chandler, V.W., Mossler, J.H., Runkel, A.C., and Setterholm, D.R., 2011, Geologic map of Minnesota-Bedrock geology: Minnesota Geological Survey State Map Series S-21, scale 1:500,000. Jirsa, M.A., Leu, A., and Miller, J.D., Jr., 2013, Preliminary bedrock geologic map of the Pagami Creek fire area, northeastern Minnesota: Minnesota Geological Survey Open-File Report OFR-13-01, scale 1:24,000. Jirsa, M.A., Starns, E.C., and Schmitz, M.D., in press, Bedrock geologic map of the 2006 Cavity Lake forest fire area, Boundary Waters Canoe Area Wilderness, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map M-193, scale 1:24,000 (currently MGS Open-File Report OFR-2008-05). Lodge, R.W.D., Gibson, H.L., Stott, G.M., Hudak, G.J., Jirsa, M.A., and Hamilton, M.A., 2013, New U-Pb geochronology from Timiskaming-type assemblages in the Shebandowan and Vermilion greenstone belts, Wawa subprovince, Superior Craton: Implications for the Neoarchean development of the southwestern Superior Province: Precambrian Research v. 235, p. 264-277. REFERENCE CITED Gruner, J.W., 1941, Structural geology of the Knife Lake area, northeastern Minnesota: Geological Society of America Bulletin 52:1577-1642, and map at scale 1:42,240. 74 �Geochemistry of Seine River metaconglomerates from Mine Centre, Ontario: interpreting fluid flow and volume changes during deformation with implications for strain analysis JOHNSON, Detaya1, and CZECK, Dyanna M.1 1 Department of Geosciences, University of Wisconsin-Milwaukee, P.O. Box 413, Milwaukee, WI 53201 USA Ductile shear zones are formed by tectonic plate interactions and are often thought to be deep-seated equivalents to faults. They can act as barriers or conduits for fluid flow, with many acting as both conduits for zone-parallel flow and barriers to cross-zone flow (e.g. Selverstone et al., 1991; Yonkee et al., 2013). Fluids have major impacts on deformation; they can alter deformation mechanisms, metamorphic reactions, and strain accumulation. Even though fluid is such an important factor in deformation, it is difficult to study due to its transient nature. Geochemical analyses may be used to interpret fluid conditions during deformation. In particular, major element geochemical analyses may be conducted to determine whether fluids assisted depletion of soluble elements resulting in volume changes (e.g. Newman and Mitra, 1993; Srivastava et al., 1995; Yonkee et al., 2003). This method has been used to demonstrate significant volume loss in some shear zones (e.g. up to 70% reported in Newman and Mitra, 1993) and none in others (Srivastava et al., 1995). The Seine River metaconglomerates in the Rainy Lake region of northwestern Ontario, Canada are located in a wedge of deformed rocks between the Quetico Fault and the Seine RiverRainy Lake Fault. The metaconglomerates underwent ductile deformation and greenschist facies metamorphism. They have clasts with varied lithology including felsic to ultramafic volcanic and granitoid clasts. Selected samples were previously analyzed to determine strain magnitudes by Czeck et al. (2009); they demonstrated that different clast types had differing rheologies that resulted in a range of strain magnitudes. Strain magnitude also varied across the region (Fig. 1). In the most highly strained outcrops, significant carbonate alteration suggests that fluid flow was an important factor during deformation, and there may have been a symbiotic relationship between fluid localization and enhanced deformation. Geochemical analysis and evaluation for fluid-rock interaction had not previously been performed on the Seine River metaconglomerates and was the focus of this study. Compositions of several clasts types (granitoids, mafic volcanics, and felsic volcanics) were determined for a range of strain magnitudes using major and minor element X-Ray Fluorescence (XRF). Results from 36 analyses were reviewed to see if there were significant changes in composition from low to high strain sites. These preliminary results show a high degree of variability amongst major constituents within the same clast type at individual sites. For a given clast type, the mean concentrations of all major elements are indistinguishable at low and high strains (Table 1). For example, in granitoid clasts, the mean concentration of SiO2 is 70.43% (st. dev. of 4.17%) at low strain and 69.13% (st. dev. of 6.06%) at high strain. The high variability and overlap of data for all major elements indicates that either A) there was no significant clast volume change between low and high strain regions or B) the geochemical signal of any volume change is masked by a large variability in initial compositions. So while there is clear evidence from alteration patterns that fluid flow differed between low and high strain outcrops, this did not result in measurable volume change with increasing strain. The inherent heterogeneity in metaconglomerates allows them to be extremely useful for strain analysis, but complicates geochemical characterization. In order to capture the variability within the population of clasts and determine the extent of fluid-assisted alteration in 75 �deformation, we may require a larger sample size similar to the study by Yonkee et al. (2013) in deformed diamictites. Figure 1. Seine metaconglomerates at a variety of strain magnitudes. A) Low strain ($1 CAD for scale). B) Moderate strain (lens cap for scale). C) High strain with carbonate alteration ($1 CAD for scale) Table 1. Results of XRF analysis. The compositional signatures of each clast type are highly variable. In all cases, the mean concentrations of elements are indistinguishable between low and high strain when the standard deviation is taken into account. References Czeck, D. M., Fissler, D. A., Horsman, E., and Tikoff, B., 2009. Strain analysis and rheology contrasts in polymictic conglomerates: an example from the Seine metaconglomerates, Superior Province, Canada. Journal of Structural Geology 31, 1365-1376. Newman, J., Mitra, G., 1993. Lateral variations in mylonite zone thickness as influenced by fluid-rock interactions, Linville Falls fault, North Carolina. Journal of Structural Geology 7, 849-863. Selverstone, J., Morteani, G., Staude, J.M., 1991. Fluid channelling during ductile shearing; transformation of granodiorite into aluminous schist in the Tauern Window, Eastern Alps. Journal of Metamorphic Geology 9, 419-431. Yonkee, W. A., Czeck, D. M., Nachbor, A., Barszewski, C. B., Pantone, S., Balgord, E., and Johnson, K. R., 2013. Strain accumulation and fluid-rock interaction in a naturally deformed diamictite, Willard thrust system, Utah (USA): Implications for crustal rheology and strain softening. Journal of Structural Geology 50, 91-118. Yonkee, W.A., Parry, W.T., Bruhn, R.L., 2003. Relations between progressive deformation and fluid-rock interaction during shear-zone growth in a basement cored thrust sheet, Sevier Orogenic Belt, Utah. American Journal of Science 303, 1-59. 76 �SEDIMENTOLOGY OF A PRE-VEGATATION PROGRADING DELTAIC ASSEMBLAGE: THE MESOPROTEROZOIC KAMA HILL AND OUTAN ISLAND FORMATIONS, ONTARIO JONES, Robyn and FRALICK, Philip Water Resource Science, Department of Geology, Lakehead University, ON, Canada, rjones2@lakeheadu.ca This study focused on the Kama Hill and Outan Island Formations of the Sibley Group located approximately 30 to 130 km east and northeast of Thunder Bay, Ontario. A lack of associated extrusive igneous rocks has resulted in an imprecise age for the unit of between approximately 1460 to 1339 Ma (Rogala, 2003; Franklin, 1978). A paleomagnetic pole position for the Sibley plots at 1400 Ma on the apparent polar wander path (Robertson, 1973) and further defines its age. The principle objective of this study was to more precisely understand the depositional environment of this section of the Sibley Group. Cheadle (1986) interpreted the Kama Hill Formation as a mud-flat environment, but Rogala (2003) and Rogala et al. (2007) believed it to be a prograding deltaic assemblage. Previous studies have found evidence of desiccation cracks in outcrop, as reported by Cheadle (1986) in the lower Kama Hill Formation. While other studies have interpreted the depositional environments as deltaic to fluvial; with fining upwards sequences as channel-fill deposits and coarsening upwards sequences as sub-aqueous distributary mouth bars (Rogala, 2003; and similar to Haszzeldaine, 1984). The lithofacies associations in this study were constructed based on four cored drill-holes within the Nipigon Plate and currently reside in the MNDM Conmee core library. Along with drill-core logging of the 340m thick interval, multiple thin sections were prepared to analyze the microstructures and grain arrangements. The drill cores were cross-correlated and divided into different lithofacies associations (LA) based on grain-size and primary sedimentary structures: fine-grained, silt-rich, rippled sandstone LA; cross-stratified LA; massive medium-grained sandstone LA; fine-grained sandstones LA; silt-rich, very fine-grained sandstone LA, and upper fine-grained LA. The lower two thirds of the succession studied consists of two coarsening- and thickening-upward sequences, with the lower one terminating in distributary mouth bar sediments and the upper one overlain by a fluvial floodplain-channel assemblage. Finally, there are desiccation cracks within the interval studied; however, they are located within areas in the upper third of the core interpreted as subaerial floodplain, where a sediment moisture deficit could easily occur. Also, the lower contact with the Rossport Formation was difficult to determine in some core and as the Rossport has mud cracks it may have led to confusion in the past. The most striking difference between this delta and modern examples is that the upper distributary mouth bar is mostly composed of massive, thick sandstones that appear to have been deposited by slurry flows down the distributary channels, and may be related to the lack of vegetation. This study gives unique insight into the primary structures of deltas, the role that flora and fauna play within deltaic systems, and has determined that the Kama Hill and Outan Island Formations were formed in deltaic environments. 77 �Figure 3: He-02-02 depth; 166.3 to 170.8 m; nice ripple lamination with numerous mud drapes present, part of the fine grained lithofacies association. Figure 1: NB-97-04 depth: 489.75-495.64 m; shows possible mud crack located within the floodplain assemblage. Figure 2: NB-97-04: 692.00 to 697.87m; mud-chip conglomerate between rippled sandstones located in the silt-rich lithofacies association. References Cheadle B. A. 1986. Stratigraphic and sedimentation of the Middle Proterozoic Sibley Group, Thunder Bay District, Ontario. Unpublished PhD. thesis, the University of Western Ontario, London, 434p. Franklin, J.M., 1978. The Sibley Group, Ontario. In, Ed. By R.K. Wanless and W.D. Loveridge. Geological Survey of Canada, Paper 77-14, 31-34. Haszeldine, R.S. 1984. Muddy deltas in freshwater lakes, and tectonism in the Upper Carboniferous Coalfield of NE England. Sedimentology, 31, 811-822. Robertson, W.A., 1973. Pole position from thermally cleaned Sibley Group sediments and its relevance to Proterozoic magnetic stratigraphy. Canadian Journal of Earth Sciences. 10, 180-193. Rogala, B. 2003. The Sibley Group: A lithostratigraphic, Geochemical and Paleomagnetic Study. Master’s Thesis, Geology Department, Lakehead University, Thunder Bay. 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, 44, 1131-1149. 78 �Mineralogy and petrology of the diamondiferous Madonna Dyke, Marathon, ON KOZLOWSKI, Alexandra1, and ZUREVINSKI, S.E.1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1, akozlows@lakeheadu.ca 1 Diamondiferous intrusive lamprophyric dykes have been identified occurring just North of Lake Superior, near Marathon, Ontario. The Madonna Dyke is a diamondiferous occurrence that lies within the Superior Province within a region host to multiple alkalic and carbonatitic complexes, many of which are linked to the intrusive activity resulting from the Midcontinent rifting event at 1.1 Ga. The Madonna Dyke is a hypabyssal rock with medium- to fine-grained phenocrysts of pseudomorphed olivine, spinel, pyroxene, and amphibole set in a dark green to black altered groundmass of mainly calcite after melilite, REE-poor apatite, phlogopite and spinel. 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. Clinopyroxenes are aluminous diopside with Al2O3 ranging from 3.11 to 14.47 wt.%. Groundmass mica show kinoshitalite – phlogopite compositions with up to 4 wt.% BaO and 20.9 wt.% Al2O3. Spinel-group mineral compositions follow Magnetic Trend #2 – the Titanomagnetite Trend, where spinels range in composition from aluminous magnesian chromites to titanian magnesian chromites to titanian chromites to members of the ulvöspinel-magnetite series. 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 spinels are 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 1: Regional map of the area north of Lake Superior between Terrace Bay and Marathon showing the alkaline complexes and structures of the area (after Smyk et al., 1993). The location of the Madonna Dyke is denoted by the red star. 79 �Figure 2: Microphotographs of the Madonna Dyke. (A) SEM-BSE image with red arrow pointing to an atoll spinel with a resorbed spinel core, calcite lagoon and magnetite rim. (B) Transmitted light PPL image of a pseudomorphed olivine macrocryst and reddish-brown chromium spinels. 80 �Preliminary Report on the Palynology of the Gervais Formation (Pleistocene), Red Lake County, Minnesota Timothy J. Kroeger Center for Environmental, Economic, Earth and Space Studies, Bemidji State University, 1500 Birchmont Dr. NE, Bemidji MN 56601. tkroeger@bemidjistate.edu The Gervais Formation (Harris and others, 1974) is the oldest glacial till stratigraphic unit exposed in northwestern Minnesota. The Gervais is exposed just above river level in a cutbank of the Red Lake River about 2 km northwest of the city of Red Lake Falls. The Gervais is described as very dark gray, unbedded, silty, clay loam that contains few pebbles and cobbles. The outcrop sampled is unusual because there are abundant wood fragments, small logs, insects, and mollusk fragments—concentrated in the lower portion of the formation. Reported radiocarbon dates for the organic materials are: >39,000, >39,900 (wood), >46,900 (coleopterous material) (Shotten and others, 1975). An Early Wisconsinan or pre-Wisconsinan age is suggested for the formation (Harris and others, 1974). Most of the pebbles within the Gervais Formation are carbonate rock although a few pebbles of coal were found. Harris and others (1974) suggested that the till of the Gervais Formation was derived largely from glacially modified lacustrine and fluvial sediment and the silts and clays were probably locally derived. Five samples for palynological analysis were collected from the Gervais Formation. The samples were macerated using standard palynological extraction techniques (Doher, 1980). Palynomorph bearing residues were mounted on microscope slides using glycerine jelly as a mounting medium. All five of the samples from the Gervais Formation proved to be palyniferous. Palynomorph preservation ranges from excellent to moderately good. Algal palynomorphs are common, including Pediastrum, multiple forms of peridinioid dinoflagellate cysts, chorate dinoflagellate cysts, and several additional palynomorph morphologies that are most likely produced by algae. Some of the algal forms are more typical of marine or marginal marine environments. Very few spores of ferns and mosses are present; they include spores that were probably produced by Sphagnum. Conifer grains are relatively common and include grains morphologically similar to Pinus, Picea, Abies, and Thuja. Angiosperm pollen is also common. In addition to pollen forms that would be typical of Pleistocene environments, angiosperm pollen forms that are stratigraphically restricted to the Upper Cretaceous and Paleogene are also present. Included are the form taxa, Aquilapollenites sp., Tricolpites microreticulatus, and Retitrescolpites anguloluminosus. Overall, there is a mixture of palynomorphs that were most likely locally produced and palynomorphs that are reworked from Upper Cretaceous and Paleogene rocks. There is little obvious difference in preservation between grains that may represent local palynomorph production versus those that are reworked. The presence of pollen indicative of Upper Cretaceous and Paleogene rocks and the abundance of peridinioid and chorate dinoflagellate cysts strongly suggests that Upper Cretaceous and Paleogene rock materials were entrained by the glacier; such rocks are common to the west and northwest of the Red Lake Falls area in North Dakota and Saskatchewan where Upper Cretaceous and Paleocene aged marine and terrestrial rock units are exposed or subcrop beneath glacial materials. The clay-rich lithology of the Gervais Formation is also consistent with glacial transport of the shales and mudrocks from a northwesterly source area. 81 �References Cited Doher, L.I., 1980, Palynomorph preparation procedures currently used in the paleontology and stratigraphy laboratories, U.S. Geological Survey: U.S. Geological Survey Circular 830, 29 p. Harris, K.L., Moran, S.R., and Clayton, L., 1974, Late Quaternary stratigraphic nomenclature Red River Valley, North Dakota and Minnesota: Miscellaneous Series 52, North Dakota Geological Survey, 47 p. Shotton, F.W., Williams, R.E.G., and Johnson, A.S., 1975, Radiocarbon 1975, Birmingham University Radiocarbon Dates IX: Radiocarbon, v. 17, no. 3, p. 255-275. 82 �Giant Domes of the Mosher Carbonate, Steep Rock, Ontario Kurucz, Sophie and Fralick, Philip Department of Geology, Lakehead University, Thunder Bay, ON, skurucz@lakeheadu.ca The Giant Dome Lithofacies of the Mosher Carbonate is located within the Steep Rock Group, 5 km north of Atikokan in northwestern Ontario. The Mosher carbonate has been studied for over a century and is one of the most well preserved Archean carbonate sequences in the world (Grotzinger, 1989). The giant domes that constitute the uppermost 70m of the 500m thick carbonate sequence are referred to as the Elbow Point Member. The giant domes are meter-sized, elongate in shape, and are internally composed of alternating crystal fan fabric and cuspate and net-like microbialite fabric. Crystal fan fabric consists of centimeter to decimeter tall radiating fans that have been argued to be originally aragonite that precipitated directly on the seafloor. Microbialites have been described by Sumner (1997) as being composed of draping, mat-like laminae, vertically oriented microbial support structures, and cement-filled voids. While both microbialite and crystal fan fabrics are common in the late Neoarchean to early Proterozoic, their occurrence with one another, and even as alternating lithologies, may be isolated to the Neoarchean. Therefore, the environmental factors that controlled the development of the giant domes, and their unusual internal composition, is not well understood. The image on the left shows the giant domes of the Mosher Carbonate with a hat for  scale. The image on the right shows the lithologies seen within the giant domes and their  interbedded nature. Polished slab is 10cm wide.  The giant domes have been interpreted to have formed in a rimmed platform environment, where a fluctuating redox boundary resulted in the alternation in precipitation of aragonite and calcite (Fralick and Riding, 2015). Major element geochemical data suggests that the crystal fan fabric contains lower concentrations of Mn and Fe than adjacent fenestrate microbialite fabric, while Mg concentrations do not show any change in concentration. This trend may be a good indication that the primary mineralogy of the crystal fan fabric and fenestrate microbialite fabric is different. Similarly, cements that are interstitial and mantle the crystal fans contain relatively higher concentrations of Mg than the crystal fans themselves, indicating that the deposition of the fans and void-filling cements was not synchronous. 83 �Mn Sample S1 1.2 Mn net‐like fabric Mn crystal fan fabric Fe Sample S1 0.8 Fe net‐like fabric Fe crystal fan fabric 0.7 1 0.6 0.5 Fe % Mn % 0.8 0.6 0.4 0.3 0.4 0.2 0.2 B.D. 0 47.5 48 48.5 49 49.5 50 0.1 B.D. 0 50.5 47.5 Ca % 48 48.5 49 Ca % 49.5 50 50.5 Atomic % of Mn and Fe vs. Ca for adjacent net‐like microbialite and crystal fans. The net‐like  fabric displays higher concentrations of both Fe and Mn relative to the associated crystal fan  fabric. B.D.=below detection. There are also features that suggest periodic subaerial exposure of the giant domes. A wellpreserved desiccation surface with a typical polygonal crack pattern can be seen on a sample that in cross-section displays cuspate fenestrate fabric. The association of the support structures with the cracks that are expressed on the surface, leads to consideration of a desiccation related process in their formation. While there is an otherwise complete lack of preserved desiccation surfaces, Fe-rich red-brown surfaces that intervene between the interbedded layers of the giant domes at periodic intervals may represent desiccation or subaerial exposure surfaces that were later destroyed. Lastly, Fe- and carbon-rich dissolution surfaces separate some adjacent lithologies. These surfaces are continuous and can be seen to mark the boundary between crystal fan fabric and fenestrate microbialite fabric within certain samples. They are composed of microcrystalline quartz and zoned dolomite. The dissolution surfaces may represent hardgrounds that formed as a result of a hiatus in sedimentation and/or as result of early cementation and provided preferential fluid pathways for later silicification. References Fralick, P., and Riding, R., 2015. Steep Rock Lake: Sedimentology and geochemistry of an Archean carbonate platform. Earth Science Reviews, v. 151, p. 132-175. Grotzinger, John P., 1989. Facies and evolution of Precambrian carbonate depositional systems: Emergence of the modern platform archetype. SEPM Special Publication No. 44, p. 79106. Sumner, Dawn Y., 1997. Late-Archean calcite-microbe interactions: Two morphologically distinct microbial communities that affected calcite nucleation differently. PALAIOS, vol. 12, no. 4, p. 302-318. 84 �A COMPARISON OF BARABOO-INTERVAL (LATE PALEOPROTEROZOIC) IRON-FORMATION, SOUTHERN WISCONSIN LAMB, Matthew T1. and KINGSBURY STEWART, Esther2 1 Department of Geography, Geology, and Environmental Science, UW-Whitewater, 120 Upham Hall, 800 Main St., Whitewater, WI, 53190. 2 Wisconsin Geologic and Natural History Survey, 3817 Mineral Point Road, Madison, Wi 53705 The Freedom Formation is part of the Baraboo-interval sediments that were deposited after ca. 1.71 Ga and later deformed by the 1.6 Ga Mazatzal Orogeny (Holm et al., 1998; Medaris et al., 2003). In the Baraboo Range of Sauk County, Wisconsin, the Freedom Formation conformably overlies the Seeley Slate, which conformably overlies the Baraboo Quartzite (Weidman, 1904). The Freedom Formation is only present in the subsurface and is known from 100-year-old drill cores that were recovered along the hinge of the Baraboo syncline (Sauk County). In Sauk County, the Freedom Formation is divided into two members: a lower iron-rich unit made up of finely-interbedded to interlaminated, hematite-rich micrite, chert, and silt- to clay-sized siliciclastic material and an upper member comprised of dolomitic micrite. Drill cuttings from three wells between 5 and 50 miles (8 to 80 km) east of the Baraboo Range recovered hematite- and magnetite-rich fine-grained sediments and quartzite, suggesting that the Freedom Formation is present, along with Baraboo-interval quartzite, in the subsurface across a broad area in southern Wisconsin. Additionally, a drill core recovered in 2015 by the Wisconsin Geologic and Natural History Survey encountered iron-formation beneath the Cambrian Elk Mound Group in the subsurface of Dodge County, some 50 miles (80 km) east of Baraboo. The purpose of this research is to compare the Freedom Formation recovered from the Baraboo Range, Sauk County to the iron-formation recovered from drill core in Dodge County in order to better constrain correlation of Baraboo interval sediments. To accomplish this, we identify and log primary lithofacies from the Sauk and Dodge County drill cores. We also analyze thin sections from each lithofacies for mineralogical similarities. Lastly we collect elemental concentrations using a handheld Thermo Fisher Niton XL3t GOLDD+ X-ray florescence (XRF) analyzer on each core at 1 foot intervals to compare geochemistry with lithofacies observed through core logging. We identify three main lithofacies in core from both location: 1) gray to white, apparently massive carbonate, 2) laminated carbonate and gray siltstone, 3) laminated red (hematite-rich) siltstone, gray siltstone, and white to gray carbonate. In thin sections we observe a primary mineral assemblage of fine-grained carbonate, quartz, hematite, chlorite, and magnetite. The relative abundance of these minerals varies by lithofacies. Recrystallization is evident in thin section and core from both locations, but is more pronounced in the Dodge County location. Elemental XRF analyses broadly correlate with lithofacies for the Sauk County location, but the eight feet of iron-formation recovered from Dodge County did not provide enough material to meaningfully compare XRF analyses from both locations. From observation of core and thin section, we show that the Precambrian iron-formation recovered from drill core in Dodge County has similar lithofacies and mineralogy to the Freedom Formation of Sauk County. Based on these similarities, we conclude that the iron-formation from Dodge County is correlative with the Freedom Formation of Sauk County. Therefore, the Baraboo-interval sediments include an ironformation that is laterally continuous over a more than 1,000 km2 area in southern Wisconsin. 85 �Figure 1. A. Location map. Dots show location of cores that recovered iron-formation in Sauk and Dodge counties. B. Core photographs showing examples of iron-formation recovered from each location. Holm, D., Schneider, D.A., Coath, C., 1998. Age and deformation of Early Proterozoic quartzites in the southern Lake Superior region: implications for extent of foreland deformation during final assembly of Laurentia. Geology 26, 907–910. Medaris, L.G., Singer, B.S., Dott, R.H., Naymark, A., Johnson, C.M., and Schott, R.C., 2003. Late Paleoproterozoic Climate, Tectonics, and Metamorphism in the Southern Lake Superior Region and Proto-North America: Evidence from Baraboo Interval Quartzites. The Journal of Geology. 111, 243-257. Weidman, 1904. The Baraboo Iron-Bearing District of Wisconsin. Wisconsin Geological and Natural History Survey Bulleting No. 13, Economic Series No. 8, 190 pp. 86 �Millennial-scale shoreline bluff retreat rates in the western arm of Lake Superior LAMBERT, Crystal A., and SWENSON, John B. Department of Earth and Environmental Sciences, University of Minnesota Duluth, Duluth, MN 55812 In the far western arm of Lake Superior, shoreline bluff retreat is a significant problem for landuse planners and policymakers, who require knowledge of the bluff retreat rate. Previous studies of shoreline bluff retreat employ careful differencing of aerial photos, in combination with direct field observations (e.g., Johnson and Johnston, 1995; Swenson et al., 2006; NRPC; 2012). The large variability in retreat rate reported in these decadal-scale studies reflects the intrinsically stochastic nature of hillslope and coastal processes. By extension, these short-term studies downplay the importance of the well-documented, long-term rise in relative lake level (RLL) driven by differential uplift of the Lake Superior basin (e.g., Mainville and Craymer, 2005). On longer (geologic) timescales, this RLL rise is the fundamental driver for bluff retreat. We developed a simple geometric-kinematic model of bluff retreat rate that addresses explicitly the role of RLL rise. Our model is motivated by the observation that the nearshore lake bottom is a surface of non-deposition and erosion (Kemp et al., 1978; Thomas and Dell, 1978). In the sequence-stratigraphic nomenclature, the lake bottom is a wave-cut transgressive ravinement surface, sensu Posamentier and Allen (1999). On the north shore, where the bluff material is Precambrian bedrock, this transgressive surface shows a very sparse cover lag of cobbles and boulders atop a clean (eroded) bedrock surface; on the south shore, where the bluffs are composed of unconsolidated glacial till, the scoured surface is located lakeward of a zone of littoral drift. With knowledge of the rate of RLL rise (Vz), the dip () of this transgressive surface constrains the long-term bluff retreat rate (Vx) (Fig. 1). Model predictions of bluff retreat rate hold for multi-century to millennial timescales and, as such, remove much of the variability that plagues short-term measurements. One limitation of our model is that it is tied explicitly to knowledge of the long-term rate of RLL rise, which is only moderately well constrained in the western arm of Lake Superior (Mainville and Craymer, 2005; Yu et al., 2013). We applied our model to the extreme western arm of Lake Superior. With Duluth as the western boundary, our study area extended eastward to the mouths of the Knife and Brule rivers on the north and south shores, respectively. Our study area is well suited to analyze the importance of lithology in determining long-term bluff retreat rate: Bluffs on the north shore are composed of Keweenawan-aged igneous rocks of various lithologies, whereas those on the south shore are composed of relatively homogeneous Holocene-aged glacial till. Common to both coastlines was the history of RLL rise. Recent LiDAR data (NOAA, 2010; Fig. 1) provide highresolution bathymetry for the north shore; on the south shore, we digitized the low-resolution NOAA navigation charts. Preliminary results of our modeling efforts suggest that millennial-scale bluff retreat rates on the south shore are as much as an order of magnitude greater than those of the north shore; this result is not surprising, given the significant difference in bulk lithology between the coastlines. Using a widely accepted RLL rise rate of 2.5 mm·a-1 in the Duluth area (Mainville and Craymer, 2005), bluff retreat rates on the north shore average approximately 5 cm·a-1, with considerable variability that correlates broadly with lithology. In map view (Fig 1), this variability manifests itself as resistant headlands and erodible bays with tall escarpments. Thick felsic volcanic units show the greatest bluff retreat rates (>7 cm·a-1), whereas mafic intrusives, e.g. the Endion Sill, are most resistant to erosion, e.g. the Endion sill. The correlation between retreat rate and 87 �lithology is reduced by ‘buttressing’ effects, i.e. a strong mafic intrusive unit protecting or supporting an adjacent felsic unit. Figure 1. (Left) Cartoon illustrating relationship between transgressive surface, rate of RLL rise, and bluff retreat rate. (Right) Map of eastern Duluth shoreline, showing nearshore LiDAR-derived bathymetry (NOAA, 2010) and mapped bedrock units (Green and Miller, 2008). Relatively low quality bathymetric data hinder somewhat the application of our model to the south shore, which consists of unconsolidated till. The relatively smooth coastline—lacking bays and headlands—and the generally planar offshore bathymetry together suggest little variability in bluff retreat rate with position along the coast. The primary difficulty in applying our model here is identifying the closure depth of the littoral zone—the region of active longshore sand transport—and, by extension, the nearshore extent of the transgressive surface. Our preliminary best estimate for the slope of the transgressive surface yields a millennial-scale bluff retreat rate of between 50 and 100 cm·a-1. Our model predictions for long-term bluff retreat rates on both coastlines are within the ranges of values reported from decadal-scale studies (Johnson and Johnston, 1995; Swenson et al., 2006). REFERENCES Green, J.C., and Miller, J.D., Jr. (2008) Bedrock geology of the Duluth quadrangle, St. Louis County, Minnesota. Minnesota Geological Survey Miscellaneous Map M-182, scale 1:24,000. Johnson, B. L., and Johnston, C. A. (1995) Relationship of lithology and geomorphology to erosion of the western Lake Superior coast. Journal of Great Lakes Research, 21(1), 3-16. Kemp, A.L.W., Dell, C.I., and Harper, N.S. (1978) Sedimentation rates and a sediment budget for Lake Superior. Journal of Great Lakes Research, 4, 276-287. Mainville, A., and Craymer, M. R. (2005) Present-day tilting of the Great Lakes region based on water level gauges. Geological Society of America Bulletin, 117(7-8), 1070-1080. National Ocean and Atmospheric Administration (NOAA) (2010) Data verification report Lake Superior bathymetric LiDAR. Northwest Regional Planning Commission (NRPC) (2012) Lake Superior South Shore Bluff Recession Rate Study. Posamentier, H.W., Allen, G.P. (1999) Siliciclastic sequence stratigraphy: concepts and applications. SEPM Concepts in Sedimentology and Paleontology, no. 7, 210 p. Swenson, M. J., Wu, C. H., Edil, T. B., & Mickelson, D. M. (2006) Bluff recession rates and wave impact along the Wisconsin coast of Lake Superior. Journal of Great Lakes Research, 32(3), 512-530. Thomas, R.L., Dell, C.I. (1978) Sediments of Lake Superior. Journal of Great Lakes Research, 4(3-4), 264-275. Yu, SY., Colman, S.M., and Milne, G.A. (2013) Radiocarbon Dating of Basal Peats Supports Separation of Lake Superior from Lakes Michigan-Huron about 1250 years ago. Earth and Planetary Science Letters, 375, 319-325. 88 �Volcanological, Geochemical, and Geochronological Comparisons of the Gafvert Lake Sequence in Minnesota and Shebandowan Assemblage in Ontario LODGE, Robert W.D. 1, PIGNOTTA, Geoffrey S.1, GÉLINAS, Brigitte, R.2, SCHWIERSKE, Kelly L.1, and HUDAK, George J.3 1 Department of Geology, University of Wisconsin-Eau Claire, Eau Claire, WI 2 Department of Geology, Lakehead University, Thunder Bay, ON 3 Precambrian Research Center, Minnesota Natural Resources Research Institute, University of Minnesota-Duluth The magmatic and tectonic evolution of Archean greenstone belts is complex and rocks are formed and modified by episodes of pre-deformation volcanic and plutonic events, syndeformation orogenic magmatism and sedimentation, and post-orogenic plutonism. In addition to contributing to the understanding Archean magmatic and tectonic processes, unravelling the different episodes of the formation of these greenstone belts have important metallogenic implications. Many of the greenstone belts in the Superior province are host to a variety of base and precious metal deposits in a variety of deposit types. The well exposed Neoarchean greenstone belts in the westernmost part of the Wawa-Abitibi terrane present an excellent opportunity to study each of these phases of formation. This study examines and compares the volcanic and plutonic rocks that are part of the syndeformational Timiskaming-type assemblages in the Vermilion and Shebandowan greenstone belts in Minnesota and Ontario (Figure 1). The Gafvert Lake sequence of the Vermilion greenstone belt and the Shebandowan assemblage of the Shebandowan greenstone belt contain coeval volcanic assemblages that were deposited in D2 transtensional basins formed during the accretion of the Wawa-Abitibi terrane to the rest of the Superior Province around 2690 Ma (Lodge et al, 2013). The interpreted geodynamic setting for these assemblages is similar to that of the Timiskaming group in the Kirkland Lake area that hosts several world-class lode gold deposits. The parallel settings between these areas have metallogenic implications and therefore it is critical to fully understand the characteristics of the Timiskaming-like assemblages in the western Wawa-Abitibi terrane to understand their potential precious metal endowments. Newly acquired geochemical and geochronological data from the Gafvert Lake sequence and published geochemical data from the volcanic and plutonic assemblages in the Shebandowan greenstone belt (Gélinas et al. 2016) have allowed a more detailed comparison of the two assemblages. The most notable difference is that these two volcanic-plutonic deposits are compositionally distinct despite being coeval in formed in similar geodynamic settings. The Gafvert Lake sequence is notably more felsic with calc-alkalic, quartz-phyric volcanic and plutonic rocks dominating the strata. In contrast, the volcanic and plutonic rock in the Shebandowan assemblages are calc-alkalic to alkalic in composition with predominately andesitic volcanic compositions and monzonite to syenite plutonic suites. Geochemically, the Shebandowan assemblage are notably more enriched in their LREE and other incompatible elements relative to the Gafvert Lake Sequence, but are still significantly less alkalic than the volcanic and plutonic rocks that are found in the Kirkland Lake region. These compositional variations in the magmatic rocks are likely contributing to the relative differences in gold prospectivity in these greenstone belts. 89 �Figure 1: Regional geologic map of the Vermilion and Shebandowan greenstone belts in Minnesota and Ontario. Inset map shows location of these greenstone belts in relation to the western part of the Wawa-Abitibi terrane if the Superior Province. VGB, SGB, WGB, and MGB are referring to the Vermilion, Shebandowan, Winston Lake, and Manitouwadge greenstone belts, respectively. Figure is modified from Lodge et al. (2013). References Gélinas, B.R., Lodge, R.W.D., Gibson, H.L., 2016. Characterization of the Mineralization and Alteration at Tower Mountain, Conmee Township, Shebandowan Greenstone Belt, Ontario Ontario Geological Survey, Miscellaneous Release - Data 330. Lodge, R.W.D., Gibson, H.L., Stott, G.M., Hudak, G.J., Jirsa, M.A. and Hamilton, M.A., 2013, New U–Pb geochronology from 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), 264-277. 90 �The Eagle East Magmatic Nickel-Copper Discovery MAHIN, Robert A. and BEACH, Steven T. Eagle Mine LLC/Lundin Mining Corp., 200 Echelon Dr., Negaunee, MI 49866 In June 2015, Lundin Mining Corporation announced the discovery of high grade magmatic nickel-copper mineralization similar in style to its 100%-owned Eagle deposit (5.2 MT reserves @ 3.12% Ni, 2.56% Cu), located in Michigan’s Upper Peninsula. The new zone, Eagle East, is approximately two kilometers east of the Eagle mine and 950 meters deep. Assay highlights include:  30.85 meters of massive (MSU) and semi-massive sulphide (SMSU) with 5.23% Ni, 8.74% Cu, and 9.49 g/T combined Pt-Pd-Au. Included in this interval was 16.38 meters of MSU with 6.70% Ni, 13.59% Cu, and 16.43 g/T combined Pt-Pd-Au (DDH 14EA331I; 1,139.85m to 1,170.70m).  23.85 meters of MSU and SMSU with 5.34% Ni, 4.41% Cu, and 4.37 g/T combined PtPd-Au, including 9.83 meters of MSU with 8.16% Ni, 7.10% Cu, and 5.14 g/T combined Pt-Pd-Au (DDH 14EA331H; 1,142.18m to 1,166.03m). Eagle and the new Eagle East zone are hosted by separate 1Ga Midcontinent Rift-related ultramafic intrusions that intrude the Baraga Basin, a gently dipping syncline of Paleoproterozoic slates, shales, and greywackes. The intrusions are interpreted to be part of the plumbing system for the Keweenaw flood basalts that host the renowned native copper deposits. Massive sulphides at the Eagle East discovery consist of generally greater than 90% pentlandite, chalcopyrite, and pyrrhotite. Semi-massive sulphides consist of net-textured pentlandite, chalcopyrite, pyrrhotite, and silicates. Also present are magmatic breccias containing inclusions of basal cherts/quartzites and granitic Archean basement. The mineralisation is filling a subhorizontal chonolith or conduit. Similar to the Eagle deposit, the new zone also contains horizontal MSU sills that intrude from the conduit laterally out into the surrounding metasediments. The discovery was made by applying a dynamic conduit model of magmatic sulfide mineralisation in which multiple deposits should exist along a main feeder dike. The nickel and copper tenors of the uneconomic mineralisation of Eagle East (aka Yellow Dog Peridotite) were known to be higher than that of Eagle. If mineralisation was found, the tenors predicted it could be higher grade than Eagle. The drilling program involved attempting to trace the feeder dyke of from the lower portion of Eagle East into undrilled territory using directional drilling. Critical to the success of the program was the ability to use Devico directional drilling to steer each hole to a specific target and using of a single parent drill hole as a platform for multiple directional kick-off daughter holes. Semi-massive sulphides and magmatic breccias occupying the core of a 15 to 40 meter wide peridotite dyke were intersected early in the program and spurred further drilling along strike. Semi-massive sulphides continued to be intersected for over a 250 meter strike length to where thicker SMSU and high-grade MSU were encountered. The discovery drilling was made by using just two parent holes and a total of 15 kick-offs. The high grade zone has been traced for over a 60 meter strike length and is open in all directions. The project is still in an exploration and drill-out phase and the full extent of Eagle East has not been determined. 91 �Preliminary groundwater age and chemistry data from cover overlying Duluth Complex Ni-Cu-PGE deposits, NE Minnesota MANNING, Andrew H.1, WANTY, Richard B.2, and MORRISON, Jean M.2 1 Central Mineral and Environmental Resources Science Center, U.S. Geological Survey, Denver, CO 2 Crustal Geophysics and Geochemistry Science Center, U.S. Geological Survey, Denver, CO The U.S. Geological Survey initiated a project in 2015 aimed at evaluating geochemical exploration methods for covered deposits in the northern Midcontintent Rift. A first round of soil and groundwater samples were collected in September from unconsolidated material overlying the Spruce Road, Wyman Creek, and Skibo deposits with the objective of determining effective sampling methodologies and characterizing the general geochemical signature of these deposits within the shallow cover. Twenty-seven water samples were collected, including 21 groundwater samples and 6 surface water samples. Groundwater samples were collected from minipiezometers (plus one spring) having depths of mainly 0.5 to 1.5 m installed mostly in areas of suspected local groundwater discharge to wetlands. Five piezometers were installed along a flow-line-parallel transect ending at Filson Creek overlying the Spruce Road deposit. All samples were analyzed for major and trace element chemistry and stable isotopes of water (2H and 18O). Ten samples were also analyzed for groundwater age tracers, including dissolved noble gases (He, Ne, Ar, Kr, and Xe), 3He/4He ratio, tritium, and/or chlorofluorocarbons (CFC11, -12, and -113). Age data were collected along with chemistry to survey the range of groundwater residence times in the cover and to investigate chemical evolution and metal transport processes along flow paths. Groundwater conditions in the site area presented several challenges to obtaining wellconstrained age determinations. For example, high DOC concentrations led to oxygen depletion in CFC sample bottles prior to analysis, resulting in CFC degradation and erroneously old computed CFC ages. However, difficulties with individual dating methods were overcome by utilizing multiple age tracers together, allowing samples to be sorted into the following age categories with reasonable confidence: <0.5 yr old; 0.5 to 2 yr old; 2 to 10 yr old, and 15 to 30 yr old. Water <0.5 yr old was distinguishable mainly based on heavy 18O values (-8 to -10.5‰ compared to -11 to -12‰; Levy et al., 2014) and warm noble gas recharge temperatures (9 to 15°C compared to 2 to 5°C), along with apparent 3H/3He ages of 0 yr, indicating 2015 spring or summer recharge. Water >2 yr old was distinguishable mainly based on apparent 3H/3He ages, with 15-30 yr old water containing substantial terrigenic 4He (produced from U-Th decay) and resulting low 3He/4He ratios 30 to 60% below atmospheric values. Tritium concentrations range from 7 to 10 TU, suggesting samples contained little pre-modern water >60 yr old. Groundwater Cu concentrations range from <0.5 to 150 g/L (median of 5.2 g/L) and Ni from <1 to 348 g/L (median of 5.5 g/L), similar to concentrations reported for the site area in previous studies (Siegel and Ericson, 1980; Miller et al., 1992). Concentrations are generally greater at Spruce Road, where Cu and Ni medians are 21 and 19 g/L, respectively. Groundwater Cu and Ni are roughly correlated with soil Cu and Ni concentrations at the piezometer location/depth, as expected. However, a relatively well defined negative correlation is apparent between Cu concentration and pH, as well as Ni and pH, suggesting that pH is another important control on Cu and Ni mobility in the groundwater system (Figs. 1A, B). Measured pH ranges from 5.7 to 8.6 (median of 7.2), and dissolved Cu and Ni concentrations are commonly observed to decrease as pH increases above the acidic range in mineralized settings 92 �due to adsorption on negatively charged mineral surfaces. Figure 1C suggests that pH generally increases with age along flow paths, probably due to weathering of abundant mafic minerals (Fig. 1D). This increasing pH is the likely cause of an apparent decrease in Cu and Ni concentrations with greater age (Figs. 1E, F). The observed negative correlation between Cu/Ni concentrations and age is significant because it may provide a limit on Cu/Ni concentrations and mobility within the deeper groundwater system in the site area, and must be taken into account in geochemical exploration approaches. It also corroborates the finding of Walton-Day et al. (1990) that discharging deeper, older groundwater in the cover may not be a primary contributor to enriched Cu and Ni concentrations in surface water in streams and wetlands in the Spruce Road area. Figure 1. (A and B) Dissolved Cu and Ni concentration vs. pH. WC = Wyman Creek, SK = Skibo, SR = Spruce Road. (C) pH vs. interpreted groundwater age. Interpreted age is approximate midpoint of assigned age range referred to in text. Power law trend line is for Spruce Road samples. (D) Dissolved Ca (solid symbols) and Mg (open symbols) concentration vs. interpreted groundwater age. (E and F) Dissolved Cu and Ni concentration vs. interpreted groundwater age. Power law trend line is for Spruce Road samples. REFERENCES Levy, Z.F., Siegel, D.I., Dasgupta, S.S., Glaser, P.H., and Welker, J.M., 2014. Stable isotopes of water show deep seasonal recharge in northern bogs and fens. Hydrological Processes, v. 228, p. 4938–4952. Miller, W.R., Ficklin, W.H., and McHugh, J.B., 1992. Geochemical exploration for copper-nickel deposits in the cool-humid climate of northeastern Minnesota. Journal of Geochemical Exploration, v. 42, p. 327-344. Siegel, D.I., and Ericson, D.W., 1980. Hydrology and water quality of the copper-nickel study region, northeastern Minnesota. U.S. Geological Survey Water-Resources Investigations Open-File Report 80-739, 87 p. Walton-Day, K., Filipek, L.H., and Papp, C.S.E., 1990. Mechanisms controlling Cu, Fe, Mn, and Co profiles in peat of the Filson Creek Fen, northeastern Minnesota. Geochimica et Cosmochimica Acta, v. 54, p. 2933-2946. 93 �Small scale microanalysis of rock and mineral textures and its relationship to mineral separation MATKO, Matthew W.1 and SCHARDT, Christian1 1 Department of Earth and Environmental Sciences, University of Minnesota Duluth, 229 Heller Hall, 1114 Kirby Drive, Duluth, MN 55812 Ore deposits form in a wide variety of geologic settings, and the processes involved in the formation of these deposits are well understood. Previous research along these lines has typically focused on more macro-scale processes such as fluid migration, chemical and metal transfer, and the associated physical and chemical changes that result (Cathles, 1981; Zientek, 2012). The ideas behind these processes have since been applied to large-scale features based on field observations, laboratory experiments, and theoretical assumptions to create models for the formation of these ore deposits. However, our understanding of how small-scale properties can influence the mineralization and alteration processes in these deposits is currently insufficient. Small-scale in-situ properties such as mineral grain size and shape variation, fracture dispersal, and the interconnectivity of pore space, along with the spatial distribution of these properties, may well play an important role in how ore deposits develop. Because mineralization styles such as disseminated, net-texture, or vein-style ores are all influenced by these properties it is important that they are investigated to see what influence they have on larger-scale processes (Prince et al., 1995; Wennberg et al., 2009; Holzheid, et al., 2000; and Liu, et al., 2014). The primary goal of this project is to examine selected small-scale physical rock properties such as pore space, pore connectivity, fracture dispersal, as well as mineral grain size and orientation. Data collected from the examination of these properties will be used to create preliminary models for the physico-chemical formation of different mineralization and alteration styles. The investigation of these small-scale features will result in improved ore deposit models that take into account a greater set of formation variables. Materials were selected from a range of ore deposit types exhibiting different mineralization styles and/or alteration. Samples will be studied using a combination of analytical techniques such as X-ray Computed Tomography (xCT), Electro Pulse Disaggregation (EPD), and Mineral Liberation Analysis (MLA). xCT will be used to examine small cores taken from sample material to examine in-situ properties such as pore-space distribution, mineral grain morphology or fracture dispersal. EPD is being utilized as an alternative to traditional methods of material separation as this method is a touch-free way to liberate individual mineral grains. Electric pulses traveling through the material along zones of weakness, such as mineral grain boundaries or fractures, allow material disaggregation preferentially along these boundaries and preserve the original mineral grain morphology (Cabri et al., 2008). MLA will be performed using a scanning electron microscope (SEM) on portions of the samples disaggregated by the electro pulse technique. The MLA will be employed to characterize the degree of ore mineral separation from gangue material and other ore minerals, grain shape patterns, and mineral grain size variation. Preliminary qualitative analysis of disaggregated material under a binocular microscope indicates that the EPD technology excels at separating ore minerals from silicate material and gangue, while preserving the original crystal morphology. The separation of individual ore minerals from each other in the case of massive ore will need to be assessed using the MLA technique because visual observation through the binocular microscope is not adequate. Samples of the disaggregated material will be scanned using an optical particle analyzer to assess the preserved crystal morphology. 94 �Figure 1) A sample of chromite ore disaggregated using EPD technology (top left). Sample image of a porosity analysis from a sample run through an xCT (bottom left) A sample grain mount after an MLA has been performed (right). References Cabri, Louis J., et al. "Electric-pulse disaggregation (Epd), hydroseparation (Hs) and their use in combination for mineral processing and advanced characterization of ores." Canadian Mineral Processors, 40th Annual Meeting, Ottawa, Proceedings. Vol. 211. 2008. Cathles, L.M. (1981) Fluid flow and ore genesis of hydrothermal ore deposits: Economic Geology 75th Anniversary Volume, p. 424 – 457 Zientek, M.L. (2012) Magmatic Ore Deposits in Layered Intrusions - Descriptive Model for Reef-Type PGE and Contact-Type Cu-Ni-PGE Deposits: U.S. Geological Survey Open File Report 2012-1010, p. 48 Prince, C. M., Ehrlich, R., Anguy, Y. (1995) Analysis of spatial order in sandstones; II, Grain clusters, packing flaws, and the small-scale structure of sandstones: Journal of Sedimentary Research 65, p. 13 - 28 Wennberg, O.P., Rennan, L., Basquet, R. (2009) Computed tomography scan imaging of natural open fractures in a porous rock; geometry and fluid flow: Geophysical Prospecting 57, p. 239 – 249 Holzheid, A., Schmitz, M.D., and Timothy L. Grove, T.L. (2000) Textural equilibria of iron sulfide liquids in partly molten silicate aggregates and their relevance to core formation scenarios: Journal of Geophysical Research, 105, p. 13,555 - 13,567 Liu, P.P., Zhou, M.F., Chen, W.T., Boone, M., and Cnudde, V. (2014) Using Multiphase Solid Inclusions to Constrain the Origin of the Baima Fe–Ti–(V) Oxide Deposit, SW China: Journal of Petrology, v. 55, p. 951 - 976 95 �Quantifying Mass Fluxes of Potassium in Weathering and Metasomatism of Paleosols MEDARIS, L. Gordon Jr. Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706 medaris@geology.wisc.edu Paleosols provide significant insights into ancient climates, and weathering mass fluxes can be used to evaluate various climofunctions such as mean annual precipitation (MAP), mean annual temperature (MAT), and atmospheric pCO2. However, the chemical compositions of many paleosols have been modified by potassium metasomatism, and such modification must be taken into account when calculating mass fluxes associated with weathering. Method - The Chemical Index of Alteration (CIA) is a good measure of degree of weathering, attaining a value of 100 upon complete removal of labile oxides (Nesbitt & Young, 1982; CIA = 100 × molar Al2O3/[Al2O3+CaO*+Na2O+K2O], where CaO* is CaO in silicate minerals). It has been demonstrated that modern weathering produces chemical trends subparallel to the A-C*N side of an A-C*N-K plot (Nesbitt & Young, 1984; Fig. 1). The compositions of many paleosols are displaced from weathering trends towards the K apex, as illustrated by the saprolite in Fig. 1, which is a result of potassium metasomatism subsequent to weathering. The original CIA value (CIACALC) for the metasomatized saprolite can be determined by constructing a line from the K apex through the measured saprolite composition to intersect the weathering trend. A second line is then constructed horizontally from the point of intersection to the vertical CIA scale on the left, giving the original CIA value (Fedo et al., 1995). In an analogous manner, the pre-metasomatic K2O content of the saprolite is determined by constructing a line from the C*N apex through the CIA intersection point on the weathering trend to the A-K side of the figure, where the molar K2O:Al2O3 ratio is given (Fig. 1). The pre-metasomatic K2O content of the saprolite is then calculated from this ratio and the measured Al2O3 content of the saprolite, converting molar values to wt. % (Medaris et al., 2015). However, K2O values calculated in this manner are only minima, because after complete removal of plagioclase, further weathering with removal of K feldspar may have proceeded along the A-K side of the figure toward the A apex. Note that the difference between minimum calculated K2O contents and possible actual contents is a function of lithology (Fig. 2), where the difference between the two decreases with a decrease in original K feldspar content, e.g. progressing from granite to granodiorite to tonalite. 96 �Application - The Baraboo paleosol is a mature, 800 cm-thick weathering profile that developed by intense weathering of Baxter Hollow granite at ~1700 Ma, which resulted in complete removal of both plagioclase and K feldspar from the paleosol and corresponding absence of detrital K feldspar in the overlying Baraboo Quartzite (Driese & Medaris, 2008). With one exception, all saprolite and regolith samples are displaced from the weathering trend due to ~1460 Ma potassium metasomatism (Fig. 3). Relative to the Al2O3 content of mean granite protolith, ~30% K2O was added to the paleosol (Fig. 4), which integrated over the entire profile, amounts to 0.13 mols/cm2 K2O. The calculated amount of K2O removed by weathering, following the method described above, was ~25%, corresponding to 0.16 mols/cm2. Because both plagioclase and K feldspar were completely removed by weathering, the actual % change of K2O may be taken as equal to that of Na2O, yielding an estimate of 0.59 mols/cm2 K2O removal over the profile (Fig. 4). Atmospheric pCO2 - Calculated atmospheric pCO2 is a function, among other things, of the total mass flux of weathering. Despite the large relative difference between calculated and estimated weathering fluxes of K2O, this difference has little effect on calculated pCO2, because the flux of K2O is small compared to the combined flux of SiO2, MgO, CaO, and Na2O ( = -5.03 mols/cm2). Thus, pCO2 for calculated and estimated K2O fluxes is 18.1 vs. 19.0 × PIAL, respectively. Calculated pCO2 is much more sensitive to duration of weathering and paleosol removal by erosion than to the uncertainty in K2O flux, e.g. an increase in weathering duration from 105 to 106 years corresponds to a reduction in pCO2 from 19.0 to 1.9 × PIAL, and pCO2 prior to 10% erosion was 26.4 × PIAL, rather than a post-erosion value of 19.0 × PIAL. References Driese SG & Medaris LG Jr (2008) Journal of Sedimentary Research, v. 78, 443-457 Fedo CM et al. (1995) Geology, v. 23, 921-924 Medaris LG Jr et al. (2015) Precambrian Research, v. 257, 83-93 Nesbitt HW & Young GM (1982) Nature, v. 299, 715-717 Nesbitt HW & Young GM (1984) Geochimica et Cosmochimica Acta, v. 48, 1523-1534. 97 �Mineralogy and Petrology of the Rabbit Foot Dyke, White River, ON Metteer, S1, and Zurevinski, S.E.1 1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1 Diamond-bearing macrocrystic, and xenolithic rocks have been identified near the town of White River, Northwestern Ontario. Initial assessment of these rocks has led to their being classified “melnoite”, a term used to describe potentially diamondiferous ultramafic lamprophyres. Olivine occurs as two distinct phases within the rocks of the Rabbit Foot Dyke; macrocrystal olivine and groundmass olivine. Macrocrystal olivine ranges in Mg/(Mg+Fe) from Fo to Fo , while phenocrystal and groundmass olivine ranges from Fo to Fo . Phlogopite compositions range from Ba-phlogopite to tetraferriphlogopite. Spinel-group minerals commonly have chromite cores with titanomagnetite rims. Spinel compositions follow the “Magmatic trend 2”, the titanomagnetite trend. Spinel-group minerals can be observed displaying atoll textures. Spinel and perovskite are commonly spatially related, and are observed surrounding larger macrocrysts such as olivine in a necklace texture. Perovskite compositions are relatively pure CaTiO , lacking any REE concentrations. In at least one outcrop of xenolithic rock, spherical magma clasts occur up to 10cm in diameter, with cores of fragmented olivine macrocrysts. The rocks of the Rabbit Foot Dyke are in many ways analogous to kimberlite in texture and mineralogy, however, significant petrogenetic overlap with melnoites, or ultramafic lamprophyres, is evident. The macrocrystic and xenolithic rocks of the Rabbit Foot Dike are characterized here as kimberlite with melnoitic (ultramafic lamprophyre) affinity. 98 �GEOLOGY OF THE CHEROKEE LAKE AREA OF THE BOUNDARY WATERS CANOE AREA, COOK COUNTY, MN - 2015 PRECAMBRIAN FIELD CAMP CAPSTONE MAPPING Jim MILLER, Aaron Balles, Ellie Brown, Ryan Helms, Greta Penzel, Luke Smith Precambrian Research Center, University of Minnesota Duluth, Duluth, MN 55812 As a capstone mapping project for the 2015 Precambrian field camp, a crew of five students under the supervision of instructor Jim Miller conducted four days of field mapping bedrock geology in the Cherokee Lake area. This area is located in the Boundary Waters Canoe Area west of Brule Lake in Cook County, Minnesota. The area is accessible from the Caribou Trail, which heads north from Tofte to a canoe landing on Brule Lake. The 8-mile paddle and 4 portages totaling over 300 rods between the landing and Cherokee Lake started out with attempting a 5-mile paddle across the western half of Brule Lake into a strong wind and high waves. After a harrowing episode of one of our canoes being blown off course and “temporarily separated from the group - long story”, we took refugee on an island and resumed our trip to Cherokee Lake under much calmer conditions the next day. The main objective of this capstone project was to conduct bedrock geologic mapping of intrusive igneous rocks to the northwest of the 2014 capstone projects on North and South Temperance Lakes (Beaver et al., 2014; Miller et al., 2015). That capstone mapping revealed that the footwall to the well differentiated Sawbill Lake intrusion, previously defined by capstone mapping in 2007 (Frost et al., 2007), 2009 (Blakely et al., 2009), 2010 (Brooker et al., 2010), and 2011 (Asp et al., 2011) and additional mapping by Ben Brooker for his MS thesis (Brooker and Miller, 2013), is composed of yet another as yet unnamed, well differentiated mafic layered intrusion. It was hoped that this footwall layered mafic intrusion defined in the Temperance Lakes area would project into the southern part of Cherokee Lake. This turned out not to be the case, as mostly Anorthositic Series and Felsic Series rocks of the Duluth Complex were found in the Cherokee Lake area. Previous studies of the Cherokee Lake area include reconnaissance mapping by Grout et al. (1959) and Davidson (1977). Grout’s 1:100,000-scale map of Township 63 North, Range 4 West (Fig. XXIII, Grout et al., 1959) shows the area around Cherokee Lake to contain mostly gabbro with minor granophyric granite, intermediate rocks, and anorthositic gabbro. Davidson’s (1:24,000-scale) reconnaissance map of the Cherokee Lake 7.5’ quadrangle (Davidson, 1977) shows the dominant rock type to be anorthositic gabbro with minor areas of granophyric granite, intergranular granite, intermediate intrusives, metavolcanics, and olivine gabbro. The discovery that the gabbroic anorthosite mapped by Davidson in North and South Temperance Lakes (Davidson, 1977) and in Homer Lake (Davidson and Burnell, 1977) to the southeast was incorrect led to the suspicion that Davidson’s identification of gabbroic anorthosite in the Cherokee Lake area was also erroneous. Turns out Davidson got it right. The 2015 capstone mapping project focused mapping shoreline exposures around the perimeter and along the many islands in Cherokee Lake. Overall, about 350 outcrops were mapped. Four general rock types were encountered in the area. In order of abundance, these were gabbroic anorthosite (90% of exposure), granophyric granite, diabase dikes, and mixed diabase/granophyre dikes. As is typical of Anorthositic Series rocks found elsewhere in the Duluth Complex, anorthositic rocks in the Cherokee Lake area are comprised of various modal types including gabbroic anorthosite, olivine leucogabbro, olivine gabbroic anorthosite and anorthosite. Most exposures show subophitic to ophtic textures with Cpx oikocrysts up to 5cm 99 �diameter. Olivine, when present, ranges from small anhedral grains to poikilitic oikocrysts up to 3cm diameter. Fe-Ti oxides are typically subpoikilitic clots up to 0.5 cm diameter. Most rocks show some degree of foliation that is variably oriented on an outcrop scale. Granophyric granite is salmon pink, fine- to medium fine-grained, intergranular to micrographic leucogranite, quartz monzonite, and quartz ferromonzodiorite. It contains 3-10% mafics (amphibole, oxide, and Fe-pyroxene). The large mass of granophyre east of Cherokee Lake is the western extension of the Misquah Hill granophyre of the Felsic Series of the Duluth Complex, which predates the Anorthositic Series (1108 vs. 1099Ma, Vervoort et al., 2007). Although smaller granite bodies north and west of Cherokee Lake were initially interpreted as Felsic Series equivalents (Miller et al., 2001), contact relationship observed in this study indicate that these granites cut the Anorthositic Series and are thus not part of the Felsic Series. Small-scale intrusions cutting the Anorthositic Series rocks throughout the area include 1) thin (5-30 cm) granophyre dikes, 2) narrow (5cm-5m), very fine-grained, columnar-jointed diabase dikes, and 3) hybrid dikes of mixed diabase and granophyre. The hybrid dikes contain a mixture of medium fine-grained granophyre and aphanitic to fine-grained massive diabase, as well as intermediate hybrid lithologies. The felsic and mafic components commonly occur in sharp lobate contacts suggesting two-magma mixing. Locally, the diabase is cut by more angular apophyses of granophyre. This rock type is similar to late hybrid intrusions cutting the Sawbill Lake intrusion to the south (Brooker and Miller, 2013). Plans for a future capstone mapping project in the area would be to map between Sawbill Lake and Cherokee Lake in order to better define the Sawbill Lake “footwall” mafic layered intrusion exposed in the Temperance Lakes. References Asp, K., Leu, A., Parisi, A., Sletten, D., Brooker, B., Miller, J., 2011, Bedrock geology of the Sawbill Lake area: University of Minnesota Duluth, Precambrian Research Center, PRC/MAP-2011-04, 1: 12,000. Blakely, S., Brown, A., Foley, D., Rowland, A., Stifter, E., and Miller, J., 2009, Bedrock geology map of Homer Lake and adjacent areas; Cook County, Northeastern Minnesota: University of Minnesota Duluth, Precambrian Research Center, PRC/MAP-2009-01, 1: 12,000. Brooker, B.P., and Miller, J.D., 2013, Bedrock geologic map of the Sawbill Lake Intrusion, Cook County, MN. Precambrian Research Center Map Series PRC/Map-2013-01, scale 1:24,000. Brooker, B.P.,Hadley, M.L., Markwood, L.W., Olson, J., Tomlinson, A.P., and Miller, J.D.,2010, Bedrock geologic map of the Jack Lake and Weird Lake areas, Cook County, northeastern Minnesota: University of Minnesota Duluth, Precambrian Research Center, PRC/Map-2010-05, 1: 12,000. Chandler, Val W, 1983, Aeromagnetic map of Minnesota, Cook and Lake counties: Minnesota Geological Survey,Aeromagnetic Map Series, Map A-1, scale 1:250,000 Davidson, D.M., 1977, Reconnaissance geologic map of the Cherokee Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-30, scale 1:24,000 Davidson, D.M, Jr., and Burnell, J.R., Jr., 1977, Reconnaissance geologic map of the Brule Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-29, scale 1:24,000 Frost, S.J., Juda, N.A., and Miller, J., 2007, Bedrock Geology Map of Homer Lake and Adjacent Areas; Cook County, Northeastern Minnesota: University of Minnesota Duluth, Precambrian Research Center, PRC/MAP2007-02, 1: 12,000 Grout, F.F., Sharp, R.P., and Schwartz, G.M., 1959, The geology of Cook County, Minnesota: Minnesota Geological Survey Bulletin 39, 163 p. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.E., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota. Miscellaneous Map Series, M-119, scale 1:200,000 Vervoort, J.D., Wirth K., Kennedy, B., Sandland , T., Harpp, K.S., 2007, The magmatic evolution of the Midcontinent rift: New geochronologic and geochemical evidence from felsic magmatism: Precambrian Research 157, p. 235–268. 100 �Continued Evaluation of the Dilatancy Model for Discordant Uranium-Lead Age Determination of Zircon MUDREY, M.G., Jr. 106 Ravine Road, Mount Horeb, WI 53572, USA - mgmudrey@mhtc.net The dilatancy model for lead loss in zircon explains the apparent linearity of data for suites of zircon plotted on a concordia diagram (Goldich and Mudrey, 1972; 1975). The lower intercept age of the discordia dates the approximate time of loss of lead through the escape of fluids which entered the metamict zircon. Time is required for the cumulative effects of U and Th decay to produce radiation damage . Uplift and erosion bring the zircon-bearing rock close to the surface and the resultant relief of pressure causes micro-fracturing and a concomitant increase in the volume or dilation of the rock. This, in turn, permits the entry of water and reactions between the water and the solid phase Under these conditions some of the solution carrying radiogenic lead escapes. The zircon U- Pb ages become discordant. The decay of uranium to lead yields decay equations which can be coupled by the construction of a concordia diagram, permitting the calculation of 206Pb/238U, 207Pb/235 U, and 207 Pb/206Pb ages. Substitution of uranium for zirconium leads to radiation damage in zircon that produces a complex of mixture of crystallites, and amorphous compounds. The extent of the damage depends on the original uranium and thorium content and the age of the mineral. Upon subsequent events, zircon can lose or gain both uranium and lead leading to the plot of data not in concordance, but commonly along a line from the original age to a lower intercept that has been variously interpreted. The figure 1 is a concordia diagram for the Rainy Lake district, Ontario and includes discordant heritage data (Hart and Davis, 1969), and modern single crystal concordant isotopic data (Lodge and others, 2013). The discordia line with an apparent Paleozoic lower intercept was rejected by Hart and Davis on the basis that its interpretation in the episodic lead-loss model requires an event at approximately 500 Ma for which there is no evidence. These same data, however, are subject to different interpretation in terms of the dilatancy model and may represent the exhumation of paleoplanes. Goldich and Mudrey submit that the excellent fit of the data to the 2,720-60 Ma chord clearly favors the dilatancy model in explaining the age discordance in the Rainy Lake district. The non-analytical, or 'geologic,' reasons for discordance are (1) mixing of two or more populations of zircon, (2) Pb loss or U gain, (3) intermediate progeny product disequilibrium, and (4) initial Pb isotopic composition. Other factors such as the U isotopic composition of the sample may also be germane. Peterman and others (1986) illustrate the difficulty in interpreting of poly-episodic events in the Lake Superior region. Episodic lead loss may be by subsequent high temperature metamorphic or thermal events, or by low-temperature dilatancy and uplift. Water, either as H2O or as hydroxyl (OH)- occurs both in the primary crystal structure, and in micro capillary channels and pores which result from radiation damage and stress release. The release of pressure during uplift and erosion results in expansion of the rock mass from micro fractures, rifting and jointing, particularly in quartz-rich rocks. This dilatancy permits 101 �some of the water with dissolved radiogenic lead to escape. This low temperature lead loss model, occurs relatively late in the history of the zircon, and, therefore, the 207Pb/206Pb age commonly approaches the true age, provided the area is not reburied under a significant stratigraphic sequence. Time since crystallization and uranium concen- tration determine the extent of dilantancy. As shown in figure 2, U/Pb analyses on abraded and HF leached single zircons of AS3 of the Duluth Complex containing ap- proximately 100 ppm U (Min and others, 2000) yield a 207Pb/206Pb age of 1097 Ma. Also is plotted an unabraded zircon from a Logan Sill near Lake Nipigon (Davis and Sutcliffe, 1985) with approximately 3000 ppm U and an 207Pb/206Pb age of 1107 Ma. The unabraded zircon has not had sufficient time for dilatancy, although the uranium content is high. The Illinois-Deep-Hole-Project, drilled three deep core holes along the Illinois Wisconsin state line (Coates and others, 1983). The zircon discordia upper intercept and the whole rock Rb/Sr are in agreement, 1,430 Ma (Peterman and others, 1986). The lower intercept of the discordia gives an apparent age of 180 Ma. Zimmerman (1986) reports an apatite fission track age of 140 Ma at the Precambrian surface at 614 m, suggesting very strongly that the lower discordia intercept is a geologically interpretable event of uplift and exposure of the Mesoproterozoic granite. I estimate that based on in situ stress and strain measurements (Engelder, 1993, p.267), the depth at which dilantancy is occurs is greater than 200 m but less than 1000 m. SELECTED REFERENCES Coates, M.S. and others, 1983, Introduction to the Illinois Deep Hole Project: Journal of Geophysical Research. v. 88, p. 7267-7285. Davis, D.W. and Sutcliffe, R.H., 1985, U-Pb ages from the Nipigon plate and northern Lake Superior Geological Society of America Bulletin, v. 96, p. 1572-1579. Engelder, T., 1993, Stress Regimes in the Lithosphere: Princeton Press, Princeton, New Jersey, 451 p Goldich, S.S., and Mudrey, Jr., 1972, Dilatancy model for discordant U-Pb zircon ages, in Contributions to recent geochemistry and Analytical chemistry (Vinogradov Volume), A.I. Tugarinov, ed., pp. 415-18 Moscow: Nauka Publ. Office 1972 (in Russian). Goldich, S.S., and Mudrey, M.G., Jr., 1975, Dilatancy model for discordant U-Pb zircon ages, in Tugarinov, A.I., ed., Recent contributions to geochemistry and analytical chemistry: New York, John Wiley & Sons, p. 466470 (English version of 1972 paper). Hart, S.R. and Davis, G.L., 1969, Zircon U-Pb and Whole-Rock Rb-Sr Ages and Early Crustal Development near Rainy Lake, Ontario: Geological Society of America Bulletin, v. 80, p. 595-616. Lodge, R.W.D. and others, 2013, New U–Pb geochronology from Timiskaming-type assemblages in the Shebandowan and Vermilion greenstone belts, Wawa subprovince,Superior Craton: Implications for the Neoarchean development of the southwestern Superior Province: Precambrian Research, v 235, p. 264-277. Min, A. and others, 2000, , A test for systematic errors in 40Ar/39Ar geochronology through comparison with U/Pb analysis of a 1.1-Ga rhyolite: Geochimica et Cosmochimica Acta, v. 64, p. 73–98, 2000 Peterman, Z.E., and others, 1986, Geochronology of Basement Granite, Stephenson County, Illinois: US Geological Survey Bulletin 1622, p. 41-50. Peterman, Z.E., and others, 1986 , A protracted Archean history in the Watersmeet gneiss dome, northern Michigan: US Geological Survey Bulletin 1622, p. 51-64 Zimmermann, R.A. 1986, Fission-track dating of the Illinois drill-hole core: US Geological Survey Bulletin 1622, p. 99-108. 102 �Emplacement and Crystallization History of Ni-Cu-(PGE) Sulfide-mineralized Peridotites in Eagle Intrusion, Upper Michigan MULCAHY, Connor1, MILLER, Jim1, MAHIN, Robert2, BEACH, Steven2, and NOWACK, Robert2 1 Department of Earth and Environmental Sciences, University of Minnesota Duluth, Duluth, MN 55812; 2 Eagle Mine, 4547 County Road 601, Champion, MI 49814. The Eagle deposit and associated Eagle East prospect near Marquette, MI, are small, high-grade orthomagmatic Ni-Cu-(PGE) sulfide deposits hosted in ultramafic intrusive rocks associated with the Midcontinent Rift (Ding et al., 2010). The Eagle deposit is currently being developed by Lundin Mining Corporation. This study, which is the focus of the lead author’s MS thesis at UMD, has four main goals: 1) to determine the number of magmas involved in Eagle’s genesis, 2) to test an emplacement model for Eagle proposed by Lundin geologists, 3) to determine the petrogenesis of enigmatic “pyroxenite” inclusions (actually melagabbronorite) in parts of the Eagle intrusion, and 4) to determine the emplacement and crystallization history of the Eagle East intrusion. This talk will report on data that address the first and second goals. With the exception of pyroxenite inclusions, the host rock of the Eagle intrusion is generally characterized by Lundin geologists as peridotite, which they subdivide into four grades of sulfide mineralization (mineralization unit types): < 5 vol% poorly mineralized (Per); 5-25% - disseminated (MPer), 25-75% - semimassive (SMSU), and > 75% - massive sulfide (MSU). Petrographic work on silicate mineralogy conducted by Ding et al. (2010) defined four main modal rock types in Eagle and Eagle East – dunite, feldspathic peridotite, melatroctolite and melagabbro. However, they used a non-traditional classification scheme that did not discriminate between pyroxene types and did not factor in textures. In this study, petrographic observations of 121 samples from three Eagle drill cores and three Eagle East drill cores reveal that most samples representing all four mineralization types modally classify as a feldspathic lherzolite to feldspathic olivine websterite. Interpreting cumulate nomenclature from mineral habits and modes, the samples uniformly classify as olivineclinopyroxene-orthopyroxene cumulates with variable amounts of intercumulus plagioclase, hornblende, Fe-Ti oxide, and apatite (OCpH to OCpH [α, f, a] in the cumulate code nomenclature of Miller et al., 2002). In evaluating the number of magmas involved in the formation of the Eagle intrusion, Ding et al. (2010) interpreted variations in incompatible trace element ratios to indicate the involvement of at least two to as many as four compositionally distinct parental magmas. However, reevaluating the lithogeochemistry of the peridotitic rocks in terms of mixtures of cumulus minerals and postcumulus minerals (=trapped liquid), the range of incompatible trace element ratios can be attributed more simply to a single parental magma type. Orthocumulates, which contain elevated amounts of trapped liquid (now represented by intercumulus plagioclase, amphibole, Fe-Ti oxide, and apatite), should have incompatible trace element ratios closer to their parental magmas. In contrast, adcumulates, with little to no trapped liquid component, would be expected to have incompatible trace element ratios more reflective of the partition coefficients of their cumulate mineralogy. As shown in Figure 1, the variation of incompatible trace element ratios such as Zr/Y correlates well with the amount of Eu, Ti, and P, which are all elements associated with intercumulus minerals. Using the mineral-liquid partition coefficients reported by Bedard (1993) and taking the Zr/Y and La/Yb ratios of 6.5 and 5.0, respectively, for orthocumulates as approximating parental magma ratios (green ovals, Fig. 1), Zr/Y and La/Yb were calculated for adcumulates ranging in Ol:Cpx:Opx mode from 80:15:5 to 10:60:30 (black ovals, Fig. 1). It is noteworthy that samples with more abundant sulfide are more likely to be hosted in adcumulates. This may indicate that low density intercumulus silicate melt was displaced by the infiltration of high density sulfide liquid. Lundin geologists have developed a two-stage emplacement/mineralization model based on spatial relationships of the three styles of mineralization. They speculate that initial pulse was intruded into a narrow Y-shaped conduit as a sulfide-oversaturated ultramafic magma that then experienced density driven settling of sulfide liquid to create the massive sulfide unit (MSU) in the neck of the intrusion and the disseminated mineralization in the upper parts of the intrusion. This was followed by emplacement of an olivine porphyritic magma along the margin of the still molten massive sulfide resulting in the creation of the semi-massive body adjacent to the massive sulfide and projecting upward into the disseminated lherzolite. While the trace element data suggest that a single parental magma composition was likely involved in the creation of the Eagle intrusion, it is still possible that this magma was emplaced more than once. 103 �Figure 1) Zr/Y and La/Yb vs. TiO2 + P2O5 (A & B) and vs. Eu (C & D) for Eagle samples. Ti, P and Eu are taken as proxies of intercumulus minerals of Fe-Ti oxide, apatite, and plagioclase, respectively, and give a qualitative measure for the amount of trapped liquid in Ol-Cpx-Opx cumulates. The green ovals represent estimated trace element ratios for the parent magmas approximated by orthocumulates. The black ovals represent the compositional range of trace element ratios of adcumulates in equilibrium with such parental magmas based on experimentally determined mineral-liquid partitions coefficients (Bedard, 1994). Pink areas indicate the range of trace element ratios that might be expected in orthocumulates to adcumulates generated from a single magma composition. Another possible test off whether two major magma pulses were involved in the creation of the Per-MPerMSU sequence and the SMSU is to evaluate whether the Ni content of olivine is different between the two mineralized suites. While the presence of euhedral olivine phenocrysts in the SMSU suggest that olivine crystallized prior to sulfide liquation in the second pulse, the timing of olivine crystallization and sulfide liquation in the initial pulse is not clear. If sulfide liquation occurs before olivine begins to crystallize, Ni should be strongly depleted in olivine. Electron microprobe analyses of olivine from all four mineralization types show undepleted Ni abundances (Fig. 2). This implies that, if there were two pulses of magma, olivine crystallized prior to sulfide liquation in both. As shown in Figure 2, almost all analyses of olivine within a particular sample show a positive correlation between Ni abundance and Fo content, which is consistent with Ni depletion due to fractional crystallization of olivine. With the exception of one sample, the positive correlation Ni and Fo, especially from SMSU and MSU samples, indicates that olivine did not re-equilibrate with enclosing sulfides. This contrasts with Li et al’s (2007) analyses of olivine in mineralized rocks from Voisey’s Bay which show evidence of re-equilibration by their negative Ni-Fo trends. Figure 2) Ni-Fo trendlines by sample for all samples analyzed in the study. Orange points denote semi-massive sulfide, blue points denote disseminated sulfide. Red trendline shows the only negative Ni-Fo trend indicative of olivine-sulfide reequilibration. References Bedard, Jean H. (1994). A procedure for calculating the equilibrium distribution of trace elements among the minerals of cumulate rocks, and the concentration of trace elements in the coexisting liquids. Chemical Geology 118, 143-153. Ding X., Li C., Ripley E. M., Rossell D. and Kamo S. (2010). The Eagle and Eagle East sulfide ore-bearing mafic–ultramafic intrusions in the Midcontinent Rift System, Upper Michigan: geochronology and petrologic evolution. Geochemistry Geophysics Geosystems. Li, C, Naldrett, A.J., and Ripley E. (2007). Controls on the Fo and Ni Contents of Olivine in Sulfide-bearing Mafic/Ultramafic Intrusions: Principles, Modeling, and Examples from Voisey’s Bay. Earth Science Frontiers Vol. 14, Issue 5. Miller, J. D., J. C. Green, M. J. Severson, V. W. Chandler, S. A. Hauck, D. M. Peterson, and T. E. Wahl (2002), Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota, Minn. Geol. Surv. Rep. Invest., 58, 207 pp., Minn. Geol. Surv., Saint Paul.  104 �Mesoproterozoic Alteration of the Paleoproterozoic Gunflint Formation: Analogies with Martian Blueberries NAP, Carli1, FRALICK, Philip2 1 Department of Geology, Lakehead University, Thunder Bay, ON, cnap@lakeheadu.ca 2 Department of Geology, Lakehead University, Thunder Bay, ON, pfralick@lakeheadu.ca NASA’s Opportunity rover landed on Meridiani Planum in summer of 2004 with the intention of studying a rich concentration of hematite in much finer detail than what the preliminary images from the orbiting Mars Global Surveyor could possibly allow. Small, diagenetic, 4mm spherules composed primarily of hematite were discovered embedded in sand blasted bedrock, arming the scientific community with further evidence for a past presence of water on Mars (NASA, 2012). This thesis is an attempt at providing a terrestrial analogue for the formation of Martian spherules by using hematite-rich concretions observed in the minimally metamorphosed, 1.8Ga Gunflint formation as proxy. Approximately 40 minutes eastbound of Thunder Bay at intersections with Mirror Lake (ML) and West Loon (WL) road lie two recently exposed, iron oxidized grainstone outcrops standing ~2-3m vertically. Original deposition occurred in the Paleoproterozoic at 1,876Ma in a storm-dominated shallow shelf. The grainstone has a typical grey-green ankeritic to white chert colour with spherical to rhombic hematite-rich concretions averaging <2mm to 2cm in diameter present in thin layers oriented parallel to the shallowly dipping, lensy bedding or randomly distributed within lenses as independent concretions or hematitic masses. Three fundamental questions occur in response to these outcroppings: Why are these rocks so iron stained when compared with the upper cherty and shaley Gunflint at outcrops of nearby Pass Lake (PL) road?; How do these concretions form?; Is the mechanism responsible for the formation of these concretions a reasonable analogue for those observed on Mars? A scaling down approach to these queries consisted of site mapping and stratigraphic sectioning, qualitative and quantitative petrographic and SEM analysis, and quantitative geochemical analysis using data acquired from ICP-MS, ICP-AES and XRD techniques. It is well known that the Gunflint is high in iron that precipitated out of seawater solution as an insoluble chemical precipitate. In the studied upper cherty member, individual grainstone grains have fine, iron-rich laminae that had precipitated onto the grain surface itself or were accumulated by rolling over an Fe-rich substrate. An implicit, unconformable upper contact with the Sibley group, present above and just tens of meters away from the Gunflint outcrop, which would have allowed for iron rich and potentially oxidative fluid migration into the underlying Gunflint. Large, centimeter scale, iron-rich fracture sets as well as very fine, micrometer scale capillary networks provide evidence for fluid migration. Variability in the red colouration, from blood red to maroon, can be differentiated on concentric layers of individual concretions, as well as overprinting masses, and is suggestive of multiple phases of redox fluid front migrations. It is by some combination of intrinsic and extrinsic iron combined with oxidation that gave these rocks their ultimate red colouration. Hematite concentrations within the ML and WL outcrops are always associated with carbonates that are at varying stages of decay. These hematite-bearing carbonates have been identified through geochemical, XRD and SEM analysis to be of ferroan dolomite to ankerite in composition. They are often found nucleating on or within siliceous and hematite altered grains as rhombs, and commonly mimic the entire grain. Spherical concretions occur when several carbonate altered grains are enclosed by the growth of successive poikilitic carbonate and rarely 105 �display a distinguishable nuclei: the appearance of framboidal pyrite central to a select group of concretions and scattered within the groundmass strongly suggests the influence of bacterial sulfate reduction (BSR) on carbonate growth. The variation in size, morphology and distribution of iron-bearing carbonates within individual grainstone lenses as concretionary spheres or masses, as well as the presence of framboidal pyrites, suggests that primary, hematite-poor carbonate concretion formation occurred in a shallow phreatic, anaerobic environment at some time after cementation and prior to compaction as the grainstone layers accumulated. Gunflint hematite concretions differ in several ways to those observed on Mars. Negative weathering, original iron-carbonate composition, the promotion of growth by BSR and a lesser random to common bedding parallel stratigraphic distribution define the concretions observed at the terrestrial site whereas positive weathering, jarosite-hematite-alunite composition, spherulitic growth by supersaturation of Fe-rich fluids, and non-conformable growth over all stratigraphic units define the concretions at the Martian sites (Morris et al, 2010). Concretions preserve fluid chemistry and are blueprints for flow regimes and are as such important to the evolution of water on Mars and early Earth. References Morris, R.V., Golden, D.C., Ming, D.W., 2010, Spherulitic Growth of Hematite Under Hydrothermal Conditions: Insights into the Growth Mechanism of Hematite Spherules at Meridiani Planum Mars [abstract]: NASA, 41st Lunar and Planetary Science Conference (2010), PDF 2541. NASA, 2012, Opportunity on Mars - Eight Years and Counting - January 24, 2012 Animation: [http://science.nasa.gov/missions/mars-rovers/]. 106 �Investigations of the Layered Series Nepheline Syenite within Center II of the Coldwell Complex, Marathon, ON Nikkila, D1., Mitchell, R.H1., and Zurevinski, S.E1. 1 Department of Geology, Lakehead University, ON The Coldwell Alkaline Complex is the largest alkaline intrusion in North America, and was emplaced during initial magmatism of the Keweenawan Midcontinent Rift (MCR) at 1.1 Ga. Located on the north shore of Lake Superior, the Coldwell Complex was emplaced along the Thiel fault (the northern component of the Trans-Superior Tectonic Zone), and is host to rare earth elements (REE), Cu, Ni, PGE, and other high field element mineralization. Emplaced from east to west, the oldest – termed “Center I” is host to gabbro and Fe-rich augite syenite; “Center II” hosts biotite-gabbro and nepheline syenite; and “Center III” is host to a variety of syenite. The focus of this study is to understand the magma systematics involved in the emplacement and crystallization of the different intrusive centers, specifically Center II. This involves field mapping, extensive sampling and mineralogical study of the complex syenites. There is an emphasis on identifying the REE minerals occurring in the nepheline syenites. This detailed assessment will help to understand the complex systematics of alkaline rocks in the Superior Province, and will assist with mineral exploration within the complex, which is currently underexplored for REEs. Layered series nepheline syenite rocks display a cumulus texture perthitic feldspar, with post cumulus amphibole and associated pyroxene, biotite, and zeolites. The feldspar exhibits secondary albitization of an earlier alkali feldspar which has produced lamellae of albite (An% 04.43). Amphiboles dominantly plot in the hastingsite range; with zoning compositions displaying a trend to Fe and Mn enrichment with Mg, Ca, Ti depletion. Biotite are classified as annite with rare siderophyllite, and occur as alteration products associated with amphibole. REE analysis of the layered series has displayed elevated levels of LREE in apatite (La, Ce, Nd) along with Th and Y, and minor abundances of britholite-(Ce, La) and wohlerite. Massive syenites associated with the layered series at higher structural levels contain abundant wohlerite and britholite-(Ce, La) with apatite containing minor amounts of LREE. Pegmatites representing altered massive syenite; contain the most diverse suite of accessory minerals including: wohlerite, britholite, NbAl-Fe bearing titanite, U-bearing pyrochlore, zoned apatite, zircon, and minor sulphides. Apatite has proven to be a viable mineral for future tracer isotope analysis; dominantly forming 20100µm (rarely up to 300µm) subhedral elongate to hexagonal crystals. Commonly displaying irregular to concentric zoning with an increased LREE rim composition, apatite is found disseminated in the feldspar groundmass or along fractures and grain boundaries of amphibole. 107 �Figure 1: Geological Map of the Coldwell Complex (Note: Star represents sample area). 108 �Petrology, geochemistry and sulphur isotopes of the Crystal Lake gabbro and Mount Mollie dyke, Northwestern Ontario O’BRIEN, Sean1, HOLLINGS, Peter1, and MILLER, Jim2 Department of Geology, Lakehead University, Thunder Bay, ON, P7B 5E1 Canada 2 Department of Geological Sciences, University of Minnesota Duluth, Duluth, MN 55812, United States 1 The Midcontinent Rift (MCR) formed from ~1150 to 1087 Ma, with the majority of the igneous activity occurring in two pulses between ~1108-1105 Ma and ~1100-1094 Ma (Heaman et al., 2007). A magnetic polarity reversal is recognized by Davis and Green (1997) in MCR related volcanic and intrusive rocks during the gap in time between the main pulses. In this study two mineralized MCR-related intrusions, the Crystal Lake gabbro (CLG) and the Mount Mollie dyke (MMD), have been investigated using petrography, geochemistry and sulphur isotopes. The CLG is a “Y” shaped intrusion with a 5 km long north arm and a 3.75 km long southern arm (Fig. 1). The ~ 35 km long MMD extends east from the CLG (Fig. 1). The spatial relationship and similar rock types led to the belief that the two were co-genetic and/or or contemporaneous, however, recent age dating has revealed that the CLG formed at 1099.6 ± 1.2 Ma and the MMD formed at 1109 ± 6.3 Ma (Heaman et al., 2007; Hollings et al., 2010). There are still unresolved issues with this age gap, most notably that both have the same paleomagnetic N-polarity, where one would expect a R-polarity for MCR related rocks that are older than 1105 Ma. This study will use petrography, whole rock geochemistry and sulphur isotopes in an attempt to resolve the conflicting evidence regarding the relationship between the CLG and MMD. Samples of the CLG were collected from an 828 m diamond drill core from the relatively little known about southern arm, whereas MMD samples were collected from a 1087 m diamond drill core. Figure 1. Generalized map and magnetic polarity of the Crystal Lake gabbro, Mount Mollie dyke and surrounding rocks within the Logan Basin (Cundari, 2013). Petrographic studies have revealed that the CLG and MMD are mineralogically and texturally similar, with troctolite and sub-ophitic to ophitic olivine gabbro being the most abundant rock types. The bottom of the MMD hole begins with a sequence of troctolite to olivine gabbro with variable amounts of sub-ophitic to ophitic clinopyroxene. Around -600 m a ~15 m sequence of alternating very coarse- and medium-grained olivine gabbro. The next unit is a clinopyroxene bearing troctolite to gabbro sequence, with sub-ophitic to ophitic clinopyroxene at the bottom and intergranular clinopyroxene in the top 100 m of the sequence. The top 150 m of the drill core consists of granophyre, hornfels, and diabase units. The bottom of the CLG drill core consists of 109 �shale of the Rove Formation and a thin 20 m fine-grained diabase unit. Next is a 100 m unit of medium- to coarse-grained troctolite and olivine gabbro with thin (mm) layers of fine-grained chrome spinel chadacrysts present 10 m above the base of this unit. A 75 m unit of fine-grained and plagioclase phyric diabase, petrographically similar to the 20 m diabase unit, separates two CLG units. The CLG units have chilled contacts with the diabase unit above and below, suggesting that it is significantly younger. The main CLG consists of a sequence of troctolite to gabbro with ophitic to sub-ophitic clinopyroxene with thin layers of chrome spinel chadacrysts near the base. The top 40 m of the hole consists of strongly altered sulphidic olivine gabbro. The MMD and main gabbro unit in the CLG drill core show relatively smooth fractionation trends, with increasing concentrations of Fe2O3, SiO2, Na2O, TiO2, Ba and V and decreasing concentrations of Al2O3, CaO, MgO and Ni from the bottom to the top of the hole; however, CaO values are constant in the CLG. In and around the very coarse-grained unit of the MMD, there are deviations from the smooth trends observed in the other units. This unit is currently being investigated using mineral chemistry to help explain these variations. The 20 m and 75 m thick diabases in the CLG drill hole are geochemically distinct from the CLG. This suggests that these units are likely from a separate source, consistent with the petrographic observation of chilled margins. To determine the role of crustal assimilation in the CLG, δ34S was analyzed from the visible sulphides in various horizons of the drill core. The CLG had visible sulphides occurring at the top and bottom 10 to 50 m of both units. Overall the δ34S ranged from 4.1 to 21.0 ‰, with higher values generally found in the lower unit. Due to the high variability of the δ34S ~ -1 to 33‰ (Johnston et al., 2006) of the surrounding Rove Formation, which the CLG is assumed to have assimilated, it is difficult to determine the degree of assimilation. This is because a high degree of assimilation of a moderate δ34S would yield similar results to a lower degree of a high δ34S value material. Regardless, there is enough data to suggest that crustal assimilation played a role in the sulphur saturation history of the CLG. References Cundari, R.S., Campbell, D., and Puumala, M., 2013. Geology, Geochemistry and Cu-Ni-PGE Mineralization of the Crystal Lake Gabbro, 6th Annual PRC Professional Workshop Cu-Ni-PGE Deposits of the Lake Superior Region, Duluth, Minnesota. 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(4): 476-488. Heaman, L., Easton, R., Hart, T., Hollings, P., MacDonald, C. and Smyk, M., 2007. Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon region, Ontario. Canadian Journal of Earth Sciences, 44(8): 1055-1086. Hollings, P., Smyk, M., Heaman, L.M. and Halls, H., 2010. The geochemistry, geochronology and paleomagnetism of dikes and sills associated with the Mesoproterozoic Midcontinent Rift near Thunder Bay, Ontario, Canada. Precambrian Research, 183(3): 553-571. Johnston, D. T., Poulton, S. W., Fralick, P. W., Wing, B. A., Canfield, D. E., & Farquhar, J., 2006. Evolution of the oceanic sulfur cycle at the end of the Paleoproterozoic. Geochimica et Cosmochimica Acta, 70(23): 5723-5739. 110 �What Happened in Northern Minnesota Between 2700 Ma and 1900 Ma? The Answer Is in the Pokegama Formation: A Multicycle Sedimentary History! OJAKANGAS, Richard W. Department of Earth and Environmental Sciences, University of Minnesota Duluth, Duluth, Minnesota 55812 rojakang@d.umn.edu The Giants Range Batholith is the core of the Algoman Mountains in the area just to the north of the Mesabi Iron Range. This composite batholith, the Vermilion Complex, and the Saganaga Batholith were emplaced beneath volcanic arcs at about 2700 Ma by subduction processes. Except for 2100 Ma mafic dikes, there is no preserved geologic record in northern Minnesota until the sedimentary rocks of the Paleoproterozoic Animikie Basin, a foreland basin, at ~ 1850 to 1900 Ma. Certainly erosion was going on, for the Algoman Mountains were essentially eroded away as topographic prominences by the time the Animikie Basin developed to the south of the mountains. This is a time span of about 800 million years. If we can assume that the Algoman Mountains were indeed respectable mountains, uplift and erosion must have generated thousands of feet of sediment. We also can reasonably assume that a significant portion of the detritus consisted of resistant quartz eroded from the granitic bodies that intruded into what was likely a volcanic roof, as documented by amphibolitic inclusions in the granitic outcrops. Based on regional relationships and cross bedding, we can assume that the paleoslope was to the south, toward and into the Animikie Basin. The lowest stratigraphic unit in the basin, the Pokegama Formation, less than 50 m thick in total, contains abundant detrital quartz only in its uppermost member, which was a quartz sandstone and is now a quartzite. Therefore the question is: “Where is the expected large quantity of quartz, the likely resistant detritus of 800 million years of erosion?” At Blueberry Hill within the Hibbing Taconite complex of open pits, there is a conglomerate at the base of the Pokegama Formation, which overlies the Archean tonalite to granodiorite basement and underlies the Biwabik Iron Formation. The conglomerate is especially interesting in that it contains numerous rounded pebbles of quartz arenite, which consist of well-sorted and spherical unit quartz grains. At least a partial answer to the above question is that older quartz sands (pre-Pokegama) were transported, abraded, deposited, lithified, re-eroded, and redeposited during those 800 million years. The pebbles of quartz sandstone are therefore proof of a multicycle history of at least two cycles, and three counting the Pokegama. How many other undetected quartz sandstones may have been present but have been totally removed by erosion? A cross bedding plot for the Biwabik Iron Formation is bimodal-bipolar, indicating a tidally influenced environment of deposition that is probably applicable to the Pokagama as well. Although the excellent rounding of resistant quartz grains also fits a tidal model, the total lack of vegetation on land during both Late Archean time and Paleoproterozoic time would have resulted in constant wind abrasion of sand-sized quartz grains by wind, the “best rounder” of sand grains. Therefore, the quartz grains (plus rare feldspar grains) were very likely wellrounded before they reached their final depositional site in the Pokegama Formation. 111 �Rounded pebbles of quartz arenite in a basal conglomerate of the Pokegama Formation 112 �Ore Petrography and Precious Metals of the Primary Flambeau Massive Sulfide Ore OLSON, Maile J. and LODGE, Robert W.D. Department of Geology, University of Wisconsin-Eau Claire, Eau Claire, WI The Flambeau volcanogenic massive sulfide (VMS) deposit, near Ladysmith, WI, was the only one of several potentially economic deposits to be extracted in the Penokean Orogeny of Wisconsin. Northern Wisconsin is home to at least thirteen VMS deposits hosted within the Wisconsin Magmatic Terrane (DeMatties, 1994), a Precambrian juvenile arc sequence consisting of volcanic rocks, sedimentary sequences, and associated plutonic rocks that are about 1.8-1.9 billion years old (Schulz & Cannon, 2007). The Flambeau mine was considered as a copper-gold deposit because of the supergene-enriched zone that was the only portion of the ore body to be extracted. Interestingly, the enriched zone of the Flambeau deposit that was extracted accounted for ¼ of the total volume of ore and the primary ore was left in the ground. Following an initial surge of academic publications following the discovery and exploration of the Flambeau (e.g. DeMatties 1994), research largely ceased along with the mining and exploration activities. What research was done was predominantly carried out on the secondary enriched zone (e.g. Ross, 1997). Preliminary research indicates the primary ore body being more enriched in zinc and lead (Zens et al. 2015). This project is designed to examine the mineralogy, texture, and composition of the primary ore body, determine the base and precious metal phases that are present in the ore. Petrographic and geochemical analyses of ore-forming minerals have helped constrain the nature and evolution of economic mineralization. The current phase of this research project has involved mineral analyses of polished thin sections of Flambeau ore samples. Using a Scanning Electron Microscope (SEM) in the Materials Science Center at UW – Eau Claire, the habit of gold, silver, empressite, electrum, and Sb-bearing alloy grains have been documented (Fig. 1). As expected with metamorphosed VMS ores, the main ore phases were euhedral pyrite, sphalerite, and lesser galena and chalcopyrite. Most of the thin section samples there are abundant sub-microscopic inclusions of galena, frequently found in pyrite, but also found in sphalerite and chalcopyrite. Bi-Ag-alloy has been observed with these galena inclusions. Various trace minerals found in the primary ore include monazite, barite, cinnabar, and cassiterite within the host mineral pyrite and to lesser extent sphalerite. Within sphalerite and pyrite, native silver, native gold, electrum, empressite, and Bi-Te-Sb alloys were identified. The primary gangue mineral phases are quartz and anthophyllite. Research utilizing reflected light petrography and the SEM have improved the identification of precious metal ore minerals, their chemistry, and characterization of the precious metal mineral-hosts. The data gathered here is contributing to a larger study that is seeking a complete a geochemical and petrographic understanding of Wisconsin’s VMS deposits and to fully characterize the Precambrian and economic geology of rocks hosting the Flambeau mine. 113 �Figure 1: Various images showing mineral hosts of precious metals in the primary Flambeau ore body. Photos A-C are backscatter images from Scanning Electron Microscope. Photo D is photomicrograph from petrographic microscope. Mineral abbreviations: Sp – Sphalerite; Py – Pyrite; Ga – Galena; Au – Native Gold; El – Electrum ± Antimony alloy; Ar – Arsenopyrite; An – Anthophyllite; Em – Empressite (AgTe). References DeMatties, T.A., 1994. Early Proterozoic volcanogenic massive sulfide deposits in Wisconsin: An overview. Economic Geology, 89: 1122-1151. LeBerge, G.L. (ed), 1996. Volcanogenic massive sulfide deposits of northern Wisconsin: A commemorative volume. Institute on Lake Superior Geology, Proceedings, 42nd Annual Meeting, Cable, WI, vol. 42, part 2, 179 p. Schulz, K.J. & Cannon, W.F., 2007. The Penokean orogeny in the Lake Superior region. Precambrian Research, 157: 4-25. Zens, Z.A., Helmuth, S.L., & Lodge, R.W.D. (2015). Geochemistry and petrography of the strata hosting the Flambeau Cu-Zn-Au Deposit: Revisiting Wisconsin’s only past-producing volcanogenic massive sulfide mine. Geological Society of America, Abstracts with Programs, 47(5). Ross, Andrew M. Supergene Gold Enrichment of the Precambrian Aged Flambeau Gossan, Flambeau Mine, Rusk County, Wisconsin. 1997. Print 114 �THE LAC DES ILES PGE-CU-NI DEPOSIT, CANADA: AN ORGANIZED MEGA-BRECCIA UNIT? Dave PECK1, Lionnel DJON1, Cameron McLEAN1, Gary DeSCHUTTER1, Jill MAXWELL1, Kelsey PRIVETT1, Denis DECHARTE1, Chris RONEY1, Michelle HUMINICKI2 and Robert STEWART3 1 North American Palladium Ltd., Exploration Department, 556 Tenth Avenue, Thunder Bay, ON, Canada P7B 2R2 Micro Analytical Facility, Brandon University, 270 18th Street Brandon, Manitoba Canada R7A 6A9 3 330 Ridgevale Drive, Bedford, Nova Scotia, Canada B4A 3M1 2 The Lac des Iles Intrusive Complex (LDI-IC), located in the Thunder Bay Mining District of northwestern Ontario, is one of several ~2.68 Ga mafic +/- ultramafic intrusive bodies that were emplaced into both the Eastern Wabigoon Terrane and the eastern part of the older Marmion Terrane of the western Superior Province. The LDI-IC consists of one mafic complex (South LDI complex) and one predominantly ultramafic complex (North LDI complex). All known mineral resources in the LDI-IC are hosted within the South LDI complex, with the vast majority occurring in the western part of the ~3 km long x 1.5km wide Mine Block intrusion (MBI). Historical PGE-Cu-Ni resources in the western MBI, including mined out and existing resources, now exceed 200 million tonnes at an average grade of >2 g/t Pd. This historical global resource includes approximately 50 million tonnes of higher-grade resources featuring average Pd grades in excess of 4 g/t over true widths of <5 to ~80m that commonly includes narrow widths (1-10m) of ‘bonanza-grade’ Pd mineralization (e.g., >10 g/t to 2-3 oz/t). Most of these resources occur within a small surface area having approximate, known dimensions of ~1 km N-S by 500m E-W by >2 km vertically. Recent insights stemming from 3D assay, wholerock geochemistry, applied mineralogy, geophysical and structural logging data imply a dynamic, multi-stage mineralizing process. Previous research addressing LDI ore forming processes have typically cited one or both of the following: 1) magmatic sulfide collection of PGE and base metals; and, 2) magmatic volatile-related PGE localization and upgrading. Over 50 years of exploration and twenty years of mining at LDI have generated a tremendous geoscientific database including over 500,000 assays and approximately 750,000 metres of exploration and definition drill core and logging. This database underpins several immutable facts relating to the geology, mineralogy, geochemistry and morphology of the PGE-Cu-Ni resources at LDI. These include: 1. Palladium tenors at LDI are amongst the highest documented from magmatic PGE-Cu-Ni deposits, typically exceeding several hundred ppm Pd and locally exceeding several thousand ppm Pd in 100% sulfide; 2. Pd:Pt and Pd:Au ratios increase with increasing Pd grade that, although documented in a small number of other intrusive bodies (e.g. Skaergaard intrusion, Keays and Tegner, 2016) remains an atypical feature of magmatic PGE-Cu-Ni deposits; 3. The highest Pd grades, although principally located along an inferred pre-magmatic regional feeder fault, locally follow other major faults as well as lithological contacts - especially those between vari-textured noritic rocks and earlier-formed intrusive units; 4. Semi-continuous brecciation of pre-existing rock units, including both early phases of the MBI and basement orthogneiss, occurred during the emplacement of fluid-rich noritic magmas and is interpreted as having been coeval with the formation of most of the highest grade PGE-Cu-Ni mineralization; 5. Palladium mineralization is commonly, but not universally, associated with disseminated Fe-CuNi sulfide mineralization comprising a characteristic assemblage of pyrite + pyrrhotite + pentlandite + chalcopyrite +/- millerite with total sulfide mineral abundances generally falling in 115 �6. 7. 8. 9. the range of 0.5 to 2% but with local, narrow occurrences of net-textured and semi-massive sulfide mineralization; Palladium principally occurs in Te-, Bi- and S-rich ore minerals displaying a wide range in texture, locking characteristics and mineral associations but having strong similarities to Pdbearing minerals documented from other PGE deposits; Recent studies of remanent magnetism in the LDI area indicate that the bulk of the PGE-Cu-Ni mineralization was formed after a reversal of the Earth’s magnetic field such that both normallypolarized and reversely-polarized rocks are present in the South LDI-IC; The Offset Fault separates the Roby and Offset structural blocks and is currently interpreted to be an oblique thrust fault – however, the timing of oblique slip movement along the Offset fault remains equivocal as does the relative timing of the currently defined major faults on the mine property and their relative importance in focusing and displacing mineralization; and, Hydrous alteration associated with magmatic volatiles derived from fluid-rich noritic magma produced chlorite-amphibole +/- talc +/- epidote alteration along the contact between a preexisting, largely unmineralized gabbroic unit (EGAB) and mineralized vari-textured noritic rocks. Although many altered structures and contacts are present in the MBI, only a few of these appear to have acted as “sinks” for PGE. Taken together, these observations support an interpretation involving multiple injections of noritic to gabbronoritic magma into the proto South LDI-IC, with different pulses having distinctive geochemical and mineralogical characteristics. The earliest period of magmatism produced massive to layered, iron-enriched and locally oxide-saturated leuco- to ferrogabbronorite and gabbronorite. The second magmatic episode produced massive to weakly layered norite and melanocratic norite featuring local PGE- and base metal sulfide enrichment. The final and principal ore-generating magmatic episode involved the emplacement of water-rich and sulfur-saturated noritic magma that produced a deeply-rooted and laterally- and verticallyextensive magmatic mega-breccia unit. The latter comprises a varitextured leuconorite to melanorite matrix and both cognate and basement-derived fragments of variable size and exhibiting a wide range in alteration intensity and degrees of resorption and partial melting. Although the mega-breccia unit is focused along the main north-south feeder structure it locally tracks along other major faults and intersects most of the pre-existing major rock units in the MBI. With deeper drilling currently confined to the western MBI, the extent (and mineral potential) of the mega-breccia in the central and eastern parts of the intrusion remains unclear. Although having a chaotic appearance on the local scale, recent exploration findings suggest that the mega-breccia is well-organized on the deposit scale. Accordingly, vectoring to higher-grade subzones is becoming possible using routine exploration data including logging information, geochemistry, 3D structural interpretations and geophysical properties. The favourable 3D continuity, thickness, PGE grades and sub-vertical orientation of the LDI PGE-Cu-Ni deposit provides strong motivation for future exploration in the region. The general category of contact-type PGE-Cu-Ni sulfide deposits should now be extended to include deeplyrooted, structurally-controlled magmatic mega-breccia systems like LDI. The higher-grade parts of these breccia systems are expected to occur in areas of maximum magma throughput, such as primary feeder conduits and structural intersections along these feeders. Narrow zones of high-grade PGE mineralization having low total sulfide content represent difficult exploration targets but should carry most of the value of a given mega-breccia system. Reference KEAYS, R.R. and TEGNER, C., 2016: Magma chamber processes in the formation of the low-sulphide magmatic Au–PGE mineralization of the Platinova reef in the Skaergaard intrusion, East Greenland. J. Petrology, in press. 116 �Copper toxicity and dissolved organic matter: Resiliency of mineralized watersheds in northern Minnesota and Michigan. PIATAK, Nadine M.1, SEAL, Robert R. II1, JONES, Perry M.2, WOODRUFF, Laurel G.2, 1 2 U.S. Geological Survey, Reston, VA 20192, npiatak@usgs.gov, rseal@usgs.gov U.S. Geological Survey, Mounds View, MN 55112, pmjones@usgs.gov, woodruff@usgs.gov Copper (Cu) toxicity in surface waters was estimated in watersheds containing contrasting mineral-deposit types in the Duluth Complex (northern Minnesota) and in the Porcupine Mountains Cu district (western Upper Peninsula of Michigan); these waters contain high dissolved organic carbon (DOC) and low to moderate hardness, both of which mitigate metal toxicity. Mineral deposits in these areas are related to rocks of the Midcontinent Rift System. In the Duluth Complex, deposits include magmatic Cu-Ni (nickel)-PGM (platinum group metal) sulfide deposits in mafic rocks and iron (Fe) and titanium (Ti) oxide ultramafic intrusions. Mineral deposits in the Porcupine Mountains district are stratiform Cu deposits hosted by the Nonesuch Formation, which consists of gray to black shales and siltstones. This study compared aquatic life water-quality criteria for Cu calculated on hardness alone, which had been the standard regulatory procedure for the past few decades, and compared it to criteria based on the Biotic Ligand Model (BLM), a newer approach that has supplanted the hardness-based approach for Cu and incorporates major element chemistry, metal speciation, and organic carbon complexation. Surface-water samples were collected along streams in three geologically distinct watersheds in the Duluth Complex: 1. Filson Creek where Cu-Ni-PGM mineralization occurs at the bedrock surface along the basal Duluth Complex; 2. Keeley Creek where Cu-Ni-PGM mineralization occurs only at depth; and 3. the upper St. Louis River in the vicinity of Fe-Ti oxide ultramafic intrusions, which occur at the subcrop beneath glacial cover. Samples were collected during low- and high-flow conditions between September 2012 and April 2014. In addition, surface-water samples were collected in September 2014 from several watersheds in the Porcupine Mountains; a short temporal variation in flow was examined due to a significant storm that occurred between sampling events. Sample locations included 1. upstream of, or not influenced by, the Nonesuch Formation, or 2. downstream of the Nonesuch (influenced) or downstream of mining impacted areas (impacted). The geochemistry of the surface waters reflects underlying rock types, glacially transported unconsolidated materials, mineralization style within each watershed, and geochemical processes occurring in the streams. Waters from the Duluth Complex and the Porcupine Mountains are similar in composition in that they are generally oxic, near neutral to slightly acidic (pH 5.0 to 7.6), and are characterized by moderate carbonate species concentrations (4 – 65 mg/L CaCO3 as bicarbonate) and low sulfate concentrations (< 0.8 – 8 mg/L). Calcium (Ca), sodium (Na), magnesium (Mg), and silica (SiO2) are the main dissolved major cations and the predominant dissolved trace elements include aluminium (Al), Cu, and Fe. Total dissolved solids, Ca, and Cu are generally lower in waters collected from the Duluth Complex compared to the Porcupine Mountains, whereas DOC, Fe, Al, and Ni are generally higher. Despite some variations in their chemistries, these watersheds display an atypical chemical signature when compared to most surface waters in the United States; the surface waters are rich in DOC (18 - 47 mg/L for Duluth; 2 - 22 mg/L for Porcupine Mountains) and have generally either low (10-53 mg/L CaCO3 for Duluth Complex) or low-moderate hardness (21-135 mg/L CaCO3 for Porcupine Mountains). The concentrations of major and trace elements vary seasonally and after large storm events with lower concentrations generally being found during higher flow conditions, consistent with dilution by rain or snowmelt. However, in the Duluth Complex watersheds, dissolved loads for major and trace elements are greater during higher flow conditions, likely from elements accumulating in wetlands and groundwater during dry and winter conditions and then being flushed downstream during higher flow. The aquatic toxicity of most metals (i.e., Ag, Cd, Cr, Cu, Ni, Pb, Zn) is routinely assessed on the basis of hardness-based criteria that adjust for the protective effects of Ca and Mg ions. In 2007, a new guideline was adopted by United States Environmental Protection Agency for determining aquatic life criteria for Cu that relies on the BLM. The BLM evaluates the biological availability of metals in aquatic systems for several organisms (i.e., fishes, water fleas) and incorporates major element chemistry as well as additional water-quality characteristics including metal speciation and organic carbon complexation (Paquin et al., 2002); BLMs for metals other than Cu, including Ag, Cd, Ni, Pb, and Zn, are also being developed. Concentrations of metals in surface waters can be evaluated with regard to the hardness-based and BLM-based criteria using a hazard quotient (HQ), which is the ratio of the dissolved concentration of the metal in the sample to the criterion. Values above 1 imply toxic conditions, whereas those below imply non-toxic conditions. Hazard Quotients based on the hardness-based criteria for Cu are greater than 1 for most sites in Filson Creek and in most waters collected after the storm and, in a few, before the storm in the Porcupine Mountains (Figure 1A), which is a reflection of the low hardness of these waters. However, as shown in Figure 1A, HQs for Cu calculated based on the BLM model are significantly less than 1 for some of 117 �these same sites, in particular for the Filson Creek watershed during low flow; for these sites, the hardness-based approach would predict toxic conditions, whereas the BLM model does not predict toxic conditions. The low HQs based on the BLM reflect the ability of DOC to complex Cu, rendering it unavailable biologically. In contrast, hardness-based HQs for a few sites in the Porcupine Mountains are lower than 1, whereas BLM-based HQs are greater than 1. The different results from the hardness-based and BLM-based approaches suggest that the former may be inadequate to describe metal toxicity especially in watersheds with high DOC and low to moderate hardness. The formation of Cu-DOC complexes significantly reduces the amount of dissolved Cu available to interact with the biotic ligand (the gill) of aquatic organisms. The protective effects of cations, such as Ca and Mg, competing with Cu to complex with the biotic ligand are likely not as important as DOC in many of these waters. The composition of DOC also influences its ability to mitigate metal toxicity and is an input parameter for the BLM model. The humic acid (HA) fraction of DOC is assumed to be the reactive fraction available to complex with dissolved metals. As shown in Figure 1B for a site in the Porcupine Mountains, varying the DOC concentrations and HA fraction in the BLM model significantly changes the predicted chronic water-quality criteria for Cu; the fraction of HA has a greater influence on the criteria as total DOC concentrations are increased (i.e., steeper slope). Based on the measured DOC (16 mg/L) and Cu concentration (38 µg/L) at this site, if the HA fraction of the DOC is less than approximately 40%, the Cu concentration in the water exceeds the criterion, predicting toxic conditions, whereas if HA is greater than approximately 40%, the Cu concentration is less than the criterion. It is also noteworthy that there is a limited range of DOC and HA values for which the BLM is calibrated (Figure 1B); waters collected in the Porcupine Mountains fall within the DOC calibrated range (0.05- 29.6 mg/L), whereas numerous samples from the Duluth Complex exceed the upper limit. Naturally-occurring concentrations of Cu at some sites currently exceed criteria prior to mining. The BLM approach to predicting aquatic water-quality criteria for Cu is likely needed for these waters in order to evaluate the effects of the high DOC and better predict resiliency to increased dissolved Cu concentrations. Additional investigations are needed to examine the composition of the DOC, which influences its ability to mitigate toxicity, as well as some of the limitations of the calibrated range for DOC concentrations in the BLM model. REFERENCES Paquin, P.R., Gorsuch, J.W., Apte, Simon, Batley, G.E., Bowles, K.C., Campbell, P.G.C., Delos, C.G., Di Toro, D.M., Dwyer, R.L., Galvez, Fernando, Gensemer, R.W., Goss, G.G., Hogstrand, Christer, Janssen, C.R., McGeer, J.C., Naddy, R.B., Playle, R.C., Santore, R.C., Schneider, Uwe, Stubblefield, W.A., Wood, C.M., and Wu, K.B., 2002, The biotic ligand model: A historical overview: Comparative Biochemistry and Physiology, v. 133, no. 1-2, p. 3–35. 118 �A preliminary evaluation of the structural controls on gold mineralization in the Jackfish Lake area, northwestern Ontario PUUMALA, Mark1, MAGNUS, Seamus2 1 Ontario Geological Survey, Ministry of Northern Development and Mines, Resident Geologist Program, Suite B002, 435 James St. South Thunder Bay, ON, P7E 6S7, Canada 2 Ontario Geological Survey, Ministry of Northern Development and Mines, Earth Resources and Geoscience Mapping Section, 933 Ramsey Lake Road, Sudbury, ON, P3E 6B5, Canada. Jackfish Lake lies within the Wawa Subprovince, in the western portion of the Schreiber-Hemlo greenstone belt. The supracrustal rocks in this area have been assigned to the Schreiber assemblage (Williams et al. 1991), and include approximately equal proportions of metavolcanic and metasedimentary rocks (Walker 1967). These supracrustal rocks have been intruded by a number of lateto post-tectonic plutons, including the Terrace Bay batholith and the Santoy Lake pluton. No age determinations are available for the supracrustal or intrusive rocks in this portion of the greenstone belt. However, they are presumed to be Neoarchean, based on their stratigraphic relationship to rocks that have been dated elsewhere in the belt (Magnus and Walker 2015). Several late- to post-tectonic faults and shear zones occur in the Jackfish Lake area (Walker 1967). These structures include a major northweststriking shear zone that was recently described by Magnus and Walker (2015) and occurs in the supracrustal rocks between the Terrace Bay batholith and Santoy Lake pluton (see Figure 1). Magnus and Walker (2015) also noted evidence of a late north-south-directed deformation event. As discussed below, this event may have played a role in the localization of gold mineralization at Jackfish Lake. The Jackfish Lake area has a long history of gold exploration that dates back to 1873, when Donald McKellar reported the discovery of gold at Victoria Cape and Jackfish (Schnieders et al. 1996). Since that time, numerous gold occurrences have been found in the Terrace Bay batholith, and in the supracrustal rocks that surround the intrusion. These occurrences include the Empress Mine, which produced 112 ounces of gold during 1896-97 (Schnieders et al. 1996). During 2015, work began on a study to examine the structural controls on gold mineralization in the vicinity of the Terrace Bay batholith. This paper presents the preliminary results of this work, which will be refined and expanded upon during a two year Ontario Geological Survey (OGS) – Lakehead University study set to commence in the summer of 2016. To date, field data have been collected from 11 gold occurrences in the Jackfish Lake area, near the eastern end of the Terrace Bay batholith. This field work has been complemented by the review of information that is on file for 20 other nearby gold and sulphide occurrences at the OGS Thunder Bay South Regional Resident Geologist’s office. Based on a review of the information collected to date, gold has been found to occur in the following structural settings.  Mineralized shear/fault zones that occur in supracrustal rocks near the margins of the Terrace Bay batholith.  Quartz-carbonate vein systems that parallel the batholith-supracrustal rock contact and are located at or near the contact.  Quartz-carbonate veins that occupy late brittle fracture systems within the batholith. The most notable gold mineralized shear zone in the supracrustal rocks is known as the Empress structure (shown on Figure 1). It is a 15 to 25 m wide shear zone that strikes 70˚ with a moderate to steep dip toward the south. This structure hosts a number of gold occurrences, including the Empress Mine. The most significant gold values are obtained where the shear zone is at its widest and exhibits intense folding and quartz vein development. Fold axes in this structure plunge moderately toward the east. The gold-mineralized contact- and batholith-hosted vein systems tend to have the following general characteristics.  One grouping consists of narrow (generally less than 0.2 m wide), shallow-dipping (often northstriking) to near-horizontal veins. Many of these veins occur in en-echelon sets. 119 � The second grouping of veins is approximately north-striking with near-vertical dip. Vein widths are highly variable, ranging from a few millimetres to more than a metre. All of the observations listed above suggest that gold mineralization is most likely to have been associated with a late north-south-directed compression event, such as that proposed by Magnus and Walker (2015). Gold generally appears to be associated with structural traps, both along the Empress structure (i.e., in zones of folding where dilatant structures have developed), and in the Terrace Bay batholith (brittle fractures). In the Empress structure, structural traps are most likely to occur at the nose of significant folds, where quartz veining has occurred in dilatant zones. In the Terrace Bay batholith, the mineralized fracture-fill veins are most likely to have been “dead-end” fractures that were connected on one end to structures that acted as the primary gold bearing fluid conduits. Figure 1. Gold and sulphide occurrences of the Jackfish Lake area (geology from Walker 1967 and Magnus and Walker 2015). All co-ordinates provided in NAD83 Zone 16. REFERENCES Magnus, S.J. and Walker, J. 2015. Geology and mineral potential of Walsh, Tuuri and Syine Townships, SchreiberHemlo greenstone belt; in Summary of Field Work and Other Activities 2015, Ontario Geological Survey, Open File Report 6313, p.14-1 to 14-12. Schnieders, B.R., Smyk, M.C., Speed, A.A. and McKay, D.B. 1996. Mineral occurrences in the Nipigon–Marathon area; Ontario Geological Survey, Open File Report 5951, 912p. Walker, J.W.R. 1967. Geology of the Jackfish-Middleton area, District of Thunder Bay; Ontario Geological Survey, Report 50, 41p. 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.485-539. 120 �Setting of volcanogenic massive sulfide deposits of the Paleoproterozoic Penokean volcanic belt QUIGLEY, Ashley1, MONECKE, Thomas1, ANDERSON, Eric2, KELLY, Nigel3, and QUIGLEY, Patrick1 1 Department of Geology and Geological Engineering, Colorado School of Mines, 1516 Illinois Street, Golden, CO 80401 2 U.S. Geological Survey, MS 964, PO Box 25046, Denver, CO 80225 3 Department of Geological Sciences, University of Colorado-Boulder, UCB 399, Boulder, CO 80309 The Paleoproterozoic (ca. 1875 Ma) Penokean volcanic belt represents one of the world’s principal orogens hosting volcanogenic massive sulfide (VMS) deposits (Franklin et al., 2005). Sporadic exploration from 1970-1995 has identified a large number of VMS deposits and prospects throughout the belt, including the world-class Crandon deposit that comprises an estimated 61 million tonnes of polymetallic massive sulfide ore (Lambe and Rowe, 1987). Despite successful exploration and the significant economic potential in the Penokean volcanic belt, only limited academic research has been conducted focusing on constraining the tectonic, structural, and volcanic setting of the VMS deposits. Many key aspects of the regional geology are not well understood, which is mostly due to extensive glacial cover of the Paleoproterozoic bedrock. The present study aimed to use whole rock major and trace element geochemistry and U-Pb geochronology to investigate the geologic framework and tectonic setting of the belt and to constrain the timing of massive sulfide formation. Regional geochemical samples were taken from geophysically defined geologic domains. The high-precision chemical abrasion ID-TIMS U-Pb dating technique was used on zircon grains from seven felsic volcanic samples from the host rock successions of some of the major VMS deposits within the Penokean volcanic belt. The results of the geochemical analyses reveal subtle differences between the geophysical domains. The majority of the volcanic rocks sampled have a tholeiitic affinity with fewer calc alkaline and transitional rocks. Geochemical evidence points to an island arc subduction environment with either intra-arc or back-arc extension. Results of the ID-TIMS U-Pb geochronological analyses revealed that four of the deposits, namely Bend, Horseshoe, Lynne, and Pelican River were all formed at about 1874 Ma. This is evidence of a major period of volcanism and related VMS deposition that likely occurred in an extensional setting at this time. The Back Forty massive sulfide deposit occurs on the east end of the Penokean volcanic belt. High-precision ID-TIMS U-Pb zircon geochronology of the host rhyolite yielded an age of about 1833 Ma, which is approximately 50 million years younger than the host rock successions of the other deposits of the Penokean volcanic belt. There are two possible explanations for this apparent age. The first explanation is that this represents a crystallization age and the host rocks to the Back Forty are part of a distinctly younger volcanic succession. Alternatively, a thermal event at 1833 Ma may have reset the U-Pb age. The rhyolite sampled from the host rock successions of the Lynne and Back Forty deposits contain rare Archean-aged zircon grains. These zircon grains yielded U-Pb ages of approximately 2700 Ma and are presumably inherited from an Archean basement. These data 121 �support the model that the Superior craton extends beneath the Pembine-Wausau terrane south of the Niagara Fault. REFERENCES Franklin, J.M., Gibson, H.L., Jonasson, I.R., and Galley, A.G., 2005, Volcanogenic massive sulfide deposits: Economic Geology 100th Anniversary Volume, p. 523-560. Lambe, R.N., and Rowe, R.G., 1987, Volcanic history, mineralization, and alteration of the Crandon massive sulfide deposit, Wisconsin: Economic Geology, v. 82, p. 1204-1238. 122 �Expanding the historical exploration document collection at the Minnesota Department of Natural Resources: the Polaris Joint Venture exploration program REED, Andrea, FREY, Barry, and HANSON, Kevin Mineral Potential Evaluation Section, Lands and Minerals, Minnesota Department of Natural Resources The Polaris Joint Venture was a large-scale five year-long exploration program targeting copper-zinc prospects in the greenstone belts of northern Minnesota during the early 1980s. Ernest K. Lehmann & Associates, in partnership with Getty and Billiton, completed airborne and ground geophysical surveys, geologic mapping, geochemical sampling, and 50 drill holes within a 13,000 square mile study area that stretched roughly from Bemidji to Ely. In December 2014, the Lehmann Family Foundation donated the Venture’s results and $10,000 to the Minnesota Department of Natural Resources (DNR). Due to the large areal extent of the donation and quantity of work conducted in previously unexplored areas, the DNR decided to make the data digitally available to the public as soon as possible. Due to the manageable size of this donation, it also gave the DNR the perfect opportunity to experiment with new ways of managing its historical exploration document collection. The full historical exploration document collection can be found online at http://minarchive.dnr.state.mn.us/information.html (Fig. 1). Resulting from the experimentation process with the Polaris Joint Venture data is the website: http://www.dnr.state.mn.us/lands_minerals/polaris/index.html (Fig. 2). From this site, the user should be able to smoothly navigate through and find the wanted data using the DNR’s downloadable Microsoft Excel catalog, downloadable GIS shapefile and PDF links to the data, as well as being able to spatially search online through the web map application. Updates to the Polaris Joint Venture collection are planned and the DNR will be accepting input from outside users. Once the Polaris Joint Venture collection portal is finalized, the DNR’s next steps will be to adapt the new historical document management system to accommodate the other collections. Fig. 1. The old historical document collection system. Fig. 2. A new historical document collection and organization system. 123 �Characterizing deformation of Gunflint Formation in contact with Archean basement rocks east of Thunder Bay, Ontario REID-SHARP, Ruby and HILL, Mary Louise Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1 Canada New expansion along Highway 11/17 east of Thunder Bay, Ontario exposes faults and damage zones that cut through and displace the Gunflint Formation of the Animikie Group, the underlying Archean basement rock, and the basal unconformity between them. Observations and stereographic projections were used to characterize outcrop-scale deformation along a curved road-cut on a hill approximately 1km long and covering an increase in elevation from west to east (where the unconformity is exposed) of about 65m. The Highway 11/17 road cut exposes a fault zone network that has accommodated a minimum of 60m of vertical displacement and unknown horizontal displacement. Two sets of well-preserved slickenlines record normal oblique sense movement on the fault zone. A void-filling calcite vein system is exposed in conjunction with the fault systems and is associated with brecciation. Calcite crystals reach up to 10cm in diameter where larger voids are filled. At least two major fault zones with damage zones at least 1m wide are exposed at the study site and are seen at the far east and far west ends of the road-cut. The Eastern Fault Zone (EFZ) has an average strike and dip of 071°/72° cutting through both Archean basement rock and Gunflint Formation. The Western Fault Zone (WFZ) average strike and dip is 067°/65° and cuts through Gunflint Formation rocks. Two relatively minor reverse faults striking 345° and dipping 48° to 58° cut through the Gunflint Formation about 80m southwest of where the WFZ first appears. The relationship between the EFZ and WFZ is obscured by an oblique fault cutting through Archean rock between them with an average strike and dip of 040° and 60°. The site is a potentially important study location for understanding fault/fluid interactions and how extensional faulting can accommodate fluid transportation. All major extensional features in the study area have a roughly northeast strike which aligns well with the orientation of the Mid-Continent Rift to the southeast suggesting that this fault zone is associated with extension ca. 1Ga. 124 �The building blocks of stromatolites: Comparisons across time and environment REINERS, Lindsey, EISCHEN, Tanner, and BARTLEY, Julie K. Geology Department, Gustavus Adolphus College, St. Peter, MN 56082 Approximately one billion years separate the stromatolites of the Shakopee (Ordovician, Minnesota) and Rossport (Mesoproterozoic Sibley Group, Ontario) formations, yet they may share similar carbonate building blocks, despite stark differences in age, large-scale morphology, and depositional environment. The Shakopee Formation (Ordovician) was deposited in a shallow epeiric sea that covered much of the upper Midwest. Stromatolites in the Shakopee are diagnostic of the carbonate-rich, nearshore facies of the Shakopee (Davis, 1966; Johnson and Simo, 2002), perhaps marking the presence of restricted, warm, shallow marine environments. Stromatolites range in macroscopic form from lowrelief, nearly stratiform morphotypes through low-relief columnar forms to internally complex domal structures. In contrast, the Rossport Formation (Mesoproterozoic) accumulated in an inland lacustrine environment that was at least intermittently hypersaline (Rogala et al., 2007). Robust stromatolite development is restricted to the Middlebrun Bay Member of the Rossport and represents an interval dominated by carbonates within an otherwise mainly siliciclastic succession. In macroscopic form, the Rossport stromatolites are stratiform to columnar, with low inheritance and low synoptic relief. Despite these differences in environment and macroscopic outcrop expression, the two stromatolite occurrences share similar mesoscale to microscale textures – the structural building blocks of stromatolites. Although post-depositional processes, including recrystallization, silicification, and dolomitization, have affected the present-day preservation of these stromatolites, primary textures can be inferred in both units, permitting interpretation of the original carbonate building blocks of these ancient structures. At the mesoscale (visible in hand sample) level, stromatolites show isopachous lamination, oversteepened lamina, and complex internal textures that are characteristic of typical Proterozoic stromatolites formed by in situ precipitation of carbonate during stromatolite growth. At the microscale (visible under a microscope), stromatolites from both localities preserve (1) evidence of a primary micritic texture, with peloidal or clotted form, commonly associated with microbially-influenced carbonate precipitation, and (2) syndepositional cement growth coating stromatolites and forming the mesoscale isopachous laminae. Both stromatolite occurrences lack evidence of trapping-and-binding as a mechanism for stromatolite growth. when. Similarities in stromatolite structural components across time and space suggest that the mechanisms of microbialite construction are broadly comparable in many settings, including ancient marine as well as both ancient and modern lacustrine environments. Modern marine stromatolites, such as those in the Bahamas and building blocks and are likely the outliers in microbialite construction across time and space. REFERENCES Davis, R.A. Jr. 1966. Willow River Dolomite: Ordovician analogue of modern algal stromatolite environments: Journal of Geology v. 74, p. 908-923. Johnson, C.L., and Simo, J.A. 2002. Sedimentology and sequence stratigraphy of a lower Ordovician mixed siliciclastic-carbonate system, Shakopee Formation, Fox River Valley of east-central Wisconsin: Geoscience Wisconsin v. 17, p. 21-33. Rogala, B., Fralick, P.W., Heaman, L.M., Metsaranta, R. 2007. Lithostratigraphy and chemostratigraphy of the Mesoproterozoic Sibley Group, northwestern Ontario, Canada: Canadian Journal of Earth Sciences v. 44, p. 1131-1149. 125 �Evaluating H/V analysis of passive seismic data as a means to map sediment thickness in the Duluth-Superior harbor SAGER, Tyler A. and WATTRUS, Nigel. of Earth and Environmental Sciences, University of Minnesota, Duluth, MN Drilling during the construction of the Interstate High Bridge between Duluth, MN and Superior, WI documented: 1) significant variations in depth to bedrock, showing that the thickness of the sediment cover increases towards Duluth; 2) that there are significant amounts of anthropogenic material (dredge spoil, waste material linked to historical sawmill activity in the harbor) in the areas surrounding the modern day harbor. The seismic velocity structure of near-surface sediments can be determined from H/V analysis of passive seismic data collected with a 3-component geophone (ie the instrument measures ground motion in three orthogonal directions). This technique utilizes the ambient background seismic noise generated by nearby human activities (such as traffic or heavy machinery), winds and other atmospheric phenomena, and ocean waves. The response of the near-surface to excitation by ambient seismic noise varies with its’ seismic velocity structure. Specifically, it exhibits unique seismic resonance frequencies that are linked to the structure of the near-surface. The H/V spectral ratio method was originally introduced by Nogoshi and Igarshi (1971) with further development by Nakamura (1989). At each recording site, amplitude spectra are calculated for each of the three-component records of the ambient seismic noise. The ratio between the amplitude spectra of the horizontal (H) to vertical (V) frequency components identifies the resonance frequencies within the near-surface sediment package (SESAME, 2005). The lowest (fundamental) resonance frequency can be used to determine the thickness of the surface sediment layer. H/V analyses of passive seismic observations made at multiple sites in the Duluth-Superior harbor area, are used to create a map depicting the spatial variation in seismic resonance frequency in the area. A map of sediment thickness in the harbor area is derived from this map by applying an empirical calibration function that links seismic resonance frequency to sediment thickness. This relationship is derived by making resonance measurements at sites with well control. A more detailed interpretation of the near-surface shear-wave velocity structure is undertaken at selected sites. At these sites, the recorded data is used to construct a model of the near-surface velocity structure by iteratively perturbing a trial model, so that its calculated response matches the recorded data. This profile can yield valuable geotechnical information (specifically stiffness) about the near surface sediments (Lai and Rix, 1998; Xia et al., 1999). REFERENCES Lai, C. G. and Rix, G. J.: Simultaneous inversion of Rayleigh phase velocity and attenuation for near-surface site characterization, Georgia Institute of Technology, 1998 Nogoshi M. and Igarashi T. (1971) On the amplitude characteristics of microtremor (part 2) (in Japanese with English abstract). Journal of Seismological Society of Japan, 24, 26-40. Nakamura, Y. (1989) A method for dynamic characteristics estimation of subsurface using microtremor on the ground surface. Quarterly Report of the Railway Technical Research Institue 30 (1), 25-30. SESAME, 2005. Guidelines for the implementation of the H/V spectral ratio technique on ambient vibrations measurements, processing and interpretation. SESAME European Research Project, Deliverable D23.12, 62p. Xia, J., Miller, R. D., and Park, C. B.: Estimation of near-surface shear-wave velocity by inversion of Rayleigh waves, Geophysics, 64, 691–700, 1999 126 �Geochemical and Petrological Comparisons of Peridotite Units in Marquette County, Michigan SASSO, Andrew, and THAKURTA, Joyashish Department of Geosciences Western Michigan University1903 W Michigan Ave Kalamazoo MI 49008-5241 USA This study characterizes the following rock units in Marquette County, Michigan in terms of geochemistry and petrology: (1) Presque Isle Peridotite, (2) Deer Lake Peridotite, and (3) Yellowdog Peridotite. Analyses were conducted to determine if any petrological or geochemical relationships exist between these units, and to assess the potential of these units to host magmatic sulfide deposits. Based on the findings, these units have been separated into the two groups presented below. Deer Lake Peridotite Mineralogical compositions and textural characteristics of Deer Lake Peridotite are similar to those observed in the Presque Isle Peridotite and Yellowdog Peridotite. Prior to alteration, the unit’s mineralogical composition was dominated by olivine and pyroxene. This rock type displays a cumulate texture. In some cases a poikilitic texture in which pyroxene oikocrysts enclose olivine chadacrysts is also present. However, these similarities are not sufficient to conclude that this unit shares a common origin with either of the other peridotite units. Also, it is noteworthy that the degree of serpentinization and hydrothermal alteration observed in the Deer Lake Peridotite is different, being far greater than that observed in either of Marquette County’s other peridotite units. Geochemical comparison of Deer Lake Peridotite with the other three units addressed in this study reveals obvious differences in chemical composition. Geochemical analysis also reveals that the Deer Lake Peridotite crystallized from a magma which formed as a result of shallow melting; whereas, the parent magmas of Presque Isle Peridotite and Yellowdog Peridotite formed as a result of deep melting. Truncation of the Deer Lake Peridotite along its south-western margin by the Great Lakes Tectonic Zone suggests that the unit must have been formed either during, or prior to the formation of the GLTZ (2.7-1.85 Ga). This window of time for this formational event proves that the Deer Lake Peridotite predates the Yellowdog Peridotite’s age (1.1 Ga) by no less than 750 Ma. Trace element, and petrographic and geochemical analysis also suggest that Deer Lake Peridotite may actually represent two separate peridotite units emplaced during two separate events. Type 2 Deer Lake Peridotite displays foliation which is not present in Type 1 Deer Lake Peridotite. Possibly, Type 2 Deer Lake Peridotite was emplaced early in the formation of the GLTZ, and later deformed during the Penokean Orogeny (1.86-1.83 Ga), with Type 1 crystallizing during this later compressive phase. Bornhorst et al. (1993) postulated that the Deer Lake Peridotite may represent the subvolcanic base of the Mona Formation. Deer Lake Peridotite also appears to be associated with the metavolcanics of the Kitchi Formation. It is possible that these successive metavolcanic units may correspond to the two ultramafic units of Deer Lake Peridotite. Based on geochemical and petrographic analysis, in conjunction with the geologic setting of the Deer Lake Peridotite, it is reasonable to conclude that the Deer Lake Peridotite was formed independently, and substantially earlier, than Presque Isle Peridotite and Yellowdog Peridotite. It is likely that this unit crystallized from a parent magma which resulted from shallow melting during the formation of the Great Lakes Tectonic Zone. Presque Isle Peridotite and Yellowdog Peridotite Presque Isle Peridotite and Yellowdog Peridotite, upon first examination, appear to be very different units. Presque Isle Peridotite is more finely grained, and lacks visible plagioclase feldspar crystals, such as those observed in the Yellowdog Peridotite. Presque Isle Peridotite has also undergone a notably higher degree of serpentinization, as is evidenced by a greater density of hydrothermal veins. 127 �Additional examination also reveals that primary mineral assemblages in both units include a large fraction of olivine. Pyroxene, mostly in the form of augite, with a much smaller fraction of enstatite, also constitutes a substantial fraction of both units. They both display a cumulate texture. Additionally, a poikilitic texture in which rounded olivine chadacrysts are partially, or fully enclosed by pyroxene oikocrysts can be observed in both the Presque Isle Peridotite and Yellowdog Peridotite. Petrographic analysis of both units also reveals olivine hosted sulfide inclusions. Such inclusions may indicate the presence of an immiscible sulfide liquid at the time of crystallization. Geochemical comparison of the Presque Isle Peridotite and Yellowdog Peridotite indicates that both formed from a parent melt of mantle origin, and became contaminated by crustal material prior to crystallization. Minor trace element analysis of the two units, reveals that they share a very similar geochemical composition. Limited outcrop exposure of the Yellowdog Peridotite, and the lack of accessibility to host rock contacts at the Presque Isle Peridotite, make it difficult to draw conclusions based strictly on geologic setting relationships at both sites. However, Yellowdog Peridotite has intruded through the Late-Archean granite basement of the “Northern Complex”, and Paleoproterozoic sediments of the Baraga Basin (as confirmed by the exploration teams of both Kennecott and Lundin Mining). Ding et al. (2010), by the use of U-Pb baddeleyite dating, has confirmed that this unit crystallized at 1107.2 ± 5.7 Ma. This date allows for the reasonable conclusion that the Yellowdog Peridotite formed during the Midcontinent Rift event. The only unit which has been observed in direct contact with the Presque Isle Peridotite is the uppermost member of the Keweenawan series, the Jacobsville Sandstone. Here, it is clear that Presque Isle Peridotite is nonconformably overlain. Radiometric dating of zircons from Jacobsville Sandstone confirms that the unit is no younger than 960 Ma (Malone et al., 2015). Observation of the peridotite’s contacts with other units is not possible because they are concealed beneath Lake Superior. However, it can be safely assumed that the Presque Isle Peridotite has also intruded through the Archean granite basement of the “Northern Complex”. No geochronological dates have ever been obtained for the Presque Isle Peridotite. The findings suggest the possibility that the Presque Isle Peridotite may also date to the Mesoproterozoic, at which time, it may have been formed contemporaneously with the Yellowdog Peridotite, during the early stages of the Midcontinent Rift event (1.1 Ga). Geochemical similarities between these units also suggest the likelihood that the plume induced, parent melts of both units were very similar, and may have been directly related. Presque Isle Peridotite is shown to display the following: (1) High nickel content, comparable to that of the Yellowdog Peridotite (as shown by XRF analysis). (2) Sulfide inclusions within olivine. (3) Primary magmatic sulfide assemblages of chalcopyrite and pentlandite. (4) Incompatible element enrichment. Comparison of Ni content with Fo molar percentage, also yields results similar to those observed in both the “Eagle” and “Eagle East” intrusions of Yellowdog Peridotite. All these factors make Presque Isle Peridotite a prime target for future exploration, as they suggest the possibility that the unit has the potential to host magmatic sulfide deposit similar to those hosted by both intrusions of the Yellowdog Peridotite. It can be concluded from the available data, that peridotite units of Keweenawan age, located in the Lake Superior region should be considered high priority targets for magmatic sulfide exploration. References Bornhorst, T. J. and R. C. Johnson, 1993, Geology of Volcanic Rocks in the South Half of the Ishpeming Greenstone Belt, Michigan. United States Geological Survey. USGS Publication Warehouse. Ding, Xin, Chusi Li, Edward M. Ripley, Dean Rossell, and Sandra Kamo, 2010, "The Eagle and East Eagle Sulfide Ore-bearing Mafic-ultramafic Intrusions in the Midcontinent Rift System, Upper Michigan: Geochronology and Petrologic Evolution." Geochemistry Geophysics Geosystems 11.3. Malone, David H., Carol A. Stein, John P. Craddock, Jonas Kley, Seth Stein, and John Malone, 2015, "Maximum Depositional Age of the Neoproterozoic Jacobsville Sandstone, Michigan: Implications for the Evolution of the Midcontinent Rift." Jacobsville/MCR. Illinois State University. 128 �Metal isotopic signatures in the Duluth Complex associated with magmatic Cu-Ni-PGE mineralization SCHARDT, Christian Department of Earth and Environmental Sciences, University of Minnesota-Duluth, 1114 Kirby Dr. Duluth, MN 55812 USA Magmatic Cu-Ni-PGE sulfide mineralization occurs in a series of deposits along the western margin of the Duluth Complex in northeastern Minnesota. While the mineralogy, geochemistry, and formation of these deposits has been investigated in some detail, there is little information available on the metal isotopic signatures (Ni, Cu) of these deposits and if this information allows to I) gain more insight into the genesis and evolution of these deposits and II) holds potential as an exploration tool. Recent studies have demonstrated that Ni isotopic fractionation exists in both high and low-temperature terrestrial material with a range of 2.1 ‰ [1-3]. Fractionation of Cu isotopes in magmatic systems has been established and significant differences between primary and secondary sulfides are noted [4,5]. For the Duluth Complex, recently reported Cu isotopic values range from -0.85 to 0.45 ‰ [6]. Sulfide-bearing drill core samples (massive, disseminated) and sulfide-barren material from all major deposits in the Duluth Complex were collected and analyzed, along with weathered surface material from mineralized outcrops and till samples. δ60/58Ni values in olivine (ave: -0.03 ‰), sulfides (ave: 0.36 ‰), and secondary oxides (ave: -0.50 ‰) are comparable to previous work [1-2] and indicate i) high temperature fractionation of Ni into olivine with bulk silicate earth signature (BSE) during initial crystallization, iii) increasing lighter Ni isotope incorporation as a function of sulfide content, and iii) preferred 58Ni incorporation into secondary oxides and silicates due to weathering processes. 60Ni is presumed to enter solution during weathering, supported by isotopic signatures of garnierite (1.5 ‰) from a magmatic sulfide deposit in Germany. δ65/63Cu values for the Duluth Complex deposits range from -1.28 ‰ to 0.36 ‰ (Figure 1), comparable to published data [6]. Other magmatic Cu-Ni-PGE deposits in the area (Eagle, Tamarack; massive sulfides) show a distinctly heavier isotopic signature (> 0.69 ‰) while disseminated ore material from Eagle is much lighter (-0.16). While both massive and disseminated sulfides from all deposits in the Duluth Complex show similar Cu isotopic values (ave: -0.32 to -0.35 ‰), individual deposits differ in their isotopic signature and are quite distinct from each other. Surface material generally shows variable negative enrichment of 63Cu, attributed to the weathering process and the preferential enrichment of lighter Cu isotopes into the weathering products. Due to the difference in Ni isotopic values between unmineralized material (ave: -0.05) and sulfide-bearing samples (up to -0.97 with increasing sulfide content), Ni isotopes may be useful to distinguish barren magmatic systems (BSE signature) from mineralized systems because of the preferential incorporation of isotopically light Ni into sulfide-bearing rocks. The significance and use of Cu isotopes in the Duluth Complex is less clear since a variety of processes (Cu source, sulfide fractionation) likely influenced the overall Cu isotopic signature. However, if a mantle signature of ~ 0 ‰ is assumed, the relative contribution of sedimentary Cu in the Duluth Complex (ave: 0.97 ‰) and magmatic Cu may be determined and differences in the formation of individual deposits in the Duluth Complex assessed. 129 �Figure 1: Cu isotopic values of primary and secondary sulfide material from Duluth Complex Cu-Ni-PGE ore deposits compared to other magmatic sulfide deposits and selected VMS deposits. Data for orange bars are taken from [6]. References 1. 2. 3. 4. 5. 6. Gueguen B., Rouxel O., Ponzevera E., Bekker A., Fouquet Y. (2013) Ni isotope variations in terrestrial silicate rocks and geological reference materials measured by MC-ICP-MS. Geostandards and Geoanalytical Research, v. 3, p. 297-317 Hiebert RS., Rouxel, O., Houlé, MG., Bekker, A. (2014) Ni isotope fractionation between komatiite and sulfide mineralization at the Neoarchean Hart deposit, Abitibi greenstone belt, Canada. Geological Society of America Abstracts, v. 46: p. 467 Wasylenski, L.E, Howe, Haleigh D., Spivak-Birndorf, L.J., Bish, DL. (2015) Ni isotope fractionation during sorption to ferrihydrite: implications for Ni in banded iron formations. Chemical Geology, v. 400, p. 56-64 Larson, P.B., Maher, K., Ramos, F.C., Chang, Z., Gaspar, M., Meinert, L.D. (2003). Copper isotope ratios in magmatic and hydrothermal ore-forming environments. Chemical Geology 201: 337-350 Markl, G., Lahaye, Y., Schwinn, G. (2006) Copper isotopes as monitors of redox processes in hydrothermal mineralization. Geochemica et Cosmochimica Acta 70: 4215-4228 Ripley, E.M., Dong, S., Li, C., Wasylenski, L.E. (2015) Cu isotope variations between conduit and sheet-style Ni–Cu–PGE sulfide mineralization in the Midcontinent Rift System, North America. Chemical Geology, v. 414, p. 59–68 130 �Acid-Generating and Acid-Neutralizing Potential of Silicate Rocks from the Basal Mineralized Zone of the Duluth Complex, Minnesota SCHULTE, Ruth F.1, PIATAK, Nadine M.1, SEAL, Robert R., II1, and WOODRUFF, Laurel G.2 1 2 U.S. Geological Survey, MS 954, Reston, VA 20192 U.S. Geological Survey, 2280 Woodale Drive, St. Paul, MN 55112 The acid-generating potential (AP) and acid-neutralizing potential (NP) of the basal zone of the Mesoproterozoic Duluth Complex are atypical of most mafic hosted copper-nickel-platinum-group-metal (Cu-Ni-PGM) ore deposits because: 1) the sulfide ores are typically disseminated, and 2) the gangue assemblages generally lack carbonate minerals; thus, any acid-neutralizing potential must be derived from more slowly reacting silicate minerals. Recent technological advances in mineral processing have improved the viability of developing these low grade, large tonnage Cu-Ni-PGM, which represent some of the largest undeveloped deposits of their type on earth (Eckstrand and Hulbert, 2007; Miller and Nicholson, 2013). The potential development of these resources necessitates an understanding of their environmental behavior including assessment of the AP and NP of future mine waste. The 1.1 Ga Duluth Complex is a large (> 5,000 km2) composite body of troctolitic, gabbroic and anorthositic intrusions located in northeastern Minnesota. The complex is part of the Midcontinent Rift system that occurs in the Lake Superior region. The intrusions were emplaced into footwall rocks of Archean granite and greenstone terranes, Proterozoic metasedimentary rocks (Biwabik Iron Formation and Virginia Formation) and penecontemporaneous North Shore Volcanic Group basalts. In order to evaluate the AP and NP of potential mine waste from the Duluth Complex, five drill cores were sampled at the Minnesota Department of Natural Resources Core Library in Hibbing, Minnesota. Drill core in the vicinity of the NorthMet (26075), Local Boy (10089 and 10107), Mesaba (B1-95), and Maturi (DU-14) Cu-Ni-PGM deposits was included in this study. Mineralogical, petrographic, and geochemical analyses were undertaken to evaluate NP contained in carbonate and silicate minerals and AP contained in sulfides. For most of the drill cores, the main analytical focus was on samples in the basal zone hosting disseminated sulfide mineralization, although representative samples from above and below the mineralized zones were also assessed to evaluate variations in AP and NP based on rock type and stratigraphic position. If the Cu-Ni-PGM deposits are developed, mine waste will consist of processed rock from the mineralized zones as well as material adjacent to those zones. The ability of mine waste to generate or neutralize acid is commonly estimated using AP and NP values. In general, if the acid consumption or neutralization value (NP) exceeds the acid producing value (AP), the material will likely not be a source of acidity; whereas, if AP exceeds NP, the material may generate acid. The AP is based on the sulfide content and is typically estimated from total sulfur (S) content and calculated assuming all S occurs as the acidgenerating mineral pyrite (Sobek et al., 1978). The NP of mine waste is typically based on direct or, more commonly, indirect measurement of its carbonate content. Carbonate minerals are considered the most effective neutralizing agents to mitigate the acid generated by sulfide oxidation in solid mine waste. However, this is problematic in deposits without significant amounts of carbonate minerals, such as those in the Duluth Complex. For these cores, we examined the potential for other minerals, particularly silicates, to contribute to the NP; if these deposits are developed, silicate minerals would constitute the bulk majority of the mine and milling waste. The main silicate minerals identified in drill cores from the Duluth Complex include plagioclase feldspar, olivine, pyroxene, mica, amphibole, and secondary minerals, such as serpentine 131 �and chlorite. The NP contribution from the silicate minerals was calculated based on NP values of monomineralic samples from Jambor et al. (2002; 2007) and weighted by their abundances in the drill core samples from powder X-ray diffraction (XRD) analyses. Olivine contributes the most NP of the silicate minerals, at 38 kilograms of calcium carbonate equivalent per ton (kg CaCO3/t) (Jambor et al., 2007). The NP of plagioclase feldspar depends on the anorthite content and ranges from less than 1 up to about 14 kg CaCO3/t (Jambor et al. 2007). The NP of pyroxene is 4.6 kg CaCO3/t (Jambor et al. 2007). As shown in Figure 1, calculated NP values from these cores range from 6.7 to 19 kg CaCO3/t. Because the carbonate content is low in the Duluth Complex core samples, the NP contribution from the silicate minerals is significantly greater than NP from carbonate (Fig. 1). It is important to note that not all of the drill cores are predicted to be acid generating (Fig. 2). Specifically, all drill core samples from 26075 (NorthMet deposit) and most from DU-14 (Maturi deposit) have AP values that are less than their NP values. As a result, the NP in these cores may be sufficient to neutralize acid generated by sulfides. Furthermore, if we assume that 90 percent of sulfides will be recovered during mineral processing, the AP values of mill tailings decrease dramatically; thus, bulk samples previously predicted to generate acid no longer fall into the acid-generating category because most of the sulfides have been removed (Fig. 2). This is particularly important in drill core samples with significant sulfide contents, such as the Local Boy ore zone (drill cores 10089 and 10107). NP and AP also can be discussed in terms of a neutralization potential ratio (NPR), where NPR = NP/AP. For these cores, NPR values vary from 0.1 to 14.2 assuming no sulfide has been recovered during mineral processing. Generally, NPR ratios less than 1 are considered acid producing and ratios greater than 2 are non-acid producing (Price 2009). NPR ratios between 1 and 2 are capable of producing acid rock drainage. Based on these benchmark values, some samples from the Duluth Complex Cu-Ni-PGM deposits could contribute to the production of acid mine drainage (Fig. 2). However, at 10 percent of the AP (assuming 90% sulfide recovery), the neutralization potential ratios are all above 1, with an average NPR value of 49, suggesting the mine waste generated after recovering sulfides from the ores may not be a source of acidity. Although silicates weather more slowly than carbonates, the NP of the silicates can facilitate the neutralization of acids generated in mine waste. Neutralization from silicate minerals is likely to have the greatest impact in tailings storage facilities where groundwater should have longer residence times due to the significantly finer grain size of the material compared to that of typical waste rock. REFERENCES Eckstrand, O.R., and Hulbert, L.J., 2007, Magmatic nickel/copper/platinum-group element deposits, in Goodfellow, W.D., ed., Mineral deposits of CanadaA synthesis of major deposit types, district metallogeny, the evolution of geological provinces, and exploration methods: Geological Association of Canada, Mineral Deposits Division, Special Publication 5, p. 205-222. Jambor, J.L., Dutrizac, J.E., Groat, L.A., and Raudsepp, M., 2002, Static tests of neutralization potentials of silicate and aluminosilicate minerals: Environmental Geology, v. 42, p. 1-17. Jambor, J. L., Dutrizac, J.E., and Raudsepp, M., 2007, Measured and computed neutralization potentials from static tests of diverse rock types: Environmental Geology, v. 52, p. 1019-1031. Miller, J. and Nicholson, S.W., 2013, Geology and mineral deposits of the 1.1 Ga Midcontinent Rift in the Lake Superior region– an overview, in Miller, J., ed., Field guide to the copper-nickel-platinum group element deposits of the Lake Superior Region: Journal of Precambrian Research Center Guidebook, 13-01. Price, W.A., 2009, Prediction manual for drainage chemistry from sulphidic geologic materials: Mine Environment Neutral Drainage (MEND) Report 1.20.1, 579 p. Sobek, A.A., Schuller, W.A., Freeman, J.R. and Smith, R.M., 1978, Field and laboratory methods applicable to overburden and minesoils: Environmental Protection Agency, 600/2-78-054, 203 pp. 132 �The Geochemistry of the Siemens Creek Formation and the Nature of Early Midcontinent Rift Basaltic Magmatism in the Western Lake Superior Region SCHULZ, Klaus J., and NICHOLSON, Suzanne W. U.S. Geological Survey, 12201 Sunrise Valley Drive, MS 954, Reston, VA 20192 The Siemens Creek Formation is the lower member of the Powder Mill Group and represents the earliest (>1108 Ma) exposed lava flows of the Midcontinent Rift in northern Michigan and Wisconsin (Hubbard, 1975). It extends from Sturgeon Falls in Michigan to as far west as at least Atkins Lake in Wisconsin, although it is disrupted by the intrusion of the Mellen Complex near Mellen, Wisconsin (Cannon and others, 2006). It is best exposed in a prominent east-west belt of knobby hills north of U.S. Route 2 between Bessemer, Michigan and Upson, Wisconsin. Near Ironwood, Michigan the Siemens Creek Formation is about 1,340 m thick (Hubbard, 1975). The Siemens Creek Formation lies conformably upon the Bessemer Quartzite; rare quartzite interbeds also occur in the lower 30 m. The basal flows, 35 to 50 m thick, are typically pillowed, strongly chloritized, and at least locally contain hyaloclastite breccia. Basal flows west of a point about midway between Upson and Hurley, Wisconsin typically have olivine and pyroxene phenocrysts and range from picrite to basalt. In contrast, to the east of that point the basal flows mostly lack olivine-pyroxene phenocrysts and range from basalt to andesite. The western basal pillowed flows are locally overlain by a stromatolitic carbonate layer that divides the Siemens Creek into lower and upper members (basalt Types I and II, respectively of Nicholson and others, 1997). The upper lava flows are generally thinner (3 to 14 m) than the lower flows and were subaerially erupted with pahoehoe and sparsely vesicular flow tops. The upper flows range from basalt to andesite with more evolved andesites containing small plagioclase phenocrysts. The western basal flows (Type I olivine-pyroxene-phyric picrites and related basalts) are geochemically distinct from the upper Siemens Creek Formation (Type II basalts). The basal Type I flows are characterized by high MgO (~8 to 17%), FeOt (~13 to 16%), and TiO2 (~2-3%), and low Al2O3 (~8 to 10%). Those samples that are chloritized have high water contents (2 to 3%), and also appear to have lost Na2O (have high K2O/(K2O+Na2O)). Based on the major element characteristics, the Type I flows would be classed as ferropicrites and ferrobasalts (Gibson and others, 2000). Their trace element characteristics also are distinctive with steep, strongly light REE enriched REE patterns, high high-field-strength element and V contents, and primitive mantle normalized patterns that peak at Ta, have a saddle at Zr-Hf, and positive V anomalies (Fig. 1A). Isotopically these rocks have an initial ɛNd of ~0. The trace element characteristics are similar to some ocean island basalts (OIB), particularly alkaline HIMU OIB, and alkaline meymechites (low Al, high Mg rocks) in the Siberian Traps of Russia (Fig. 1A). They are also similar to the basal Keweenawan lava flows at Ely’s Peak near Duluth, in the Grand Portage area, and at the base of the Osler Group in Ontario (Fig. 1B). The eastern basal flows, although also pillowed, are Upper Siemens Creek Type II basalts in composition. Type II basalts are broadly similar in overall composition, although in detail three compositional types can be distinguished that appear to have distinct stratigraphic and lateral distribution (Type IIA, B, C in Fig 1C). The basal flows (Type IIA) extend from about the midpoint between Upson and Hurley eastward to Sturgeon Falls. They have high SiO2 ~50 to 58%, MgO ~5 to 10% (one picrite with 17%), FeOt ~10 to 13%, Al2O3 ~10 to 15%, and TiO2 ~1.5 to 2.5% with low Al2O3/TiO2 ratios (5-8). They show light REE enrichment with positive Th and mostly large negative Nb-Ta anomalies when normalized to primitive mantle (IIA in Fig. 1C). The Type IIB flows are directly above the basal flows and extend from east of Mellen to just east of Lake Gogebic. They show a smaller compositional range with SiO2 ~50 to 55%, MgO ~5 to 8%, FeOt ~10 to 12%, Al2O3 ~14 to 16%, and TiO2 ~ 1.4 to 2% with Al2O3/TiO2 ratios of 8 to 10. They are more enriched in light REE than the Type IIA basalts and most do not show negative Nb-Ta anomalies when normalized to primitive mantle (IIB in Fig. 1C). The last type (Type IIC) occurs as the uppermost Upper Siemens Creek flows in the area between just west of Hurley to Silver Mountain. They overlap in composition with the basalts below but mostly have lower TiO2 contents (~1 to 1.8%) and higher Al2O3/TiO2 ratios (9 to14). Their trace elements are similar to those of 133 �Type IIA basalts but they extend to more enriched Th and also have negative Nb-Ta anomalies when normalized to primitive mantle (IIC in Fig. 1C). The high SiO2 contents, enriched Th, negative Nb-Ta anomalies, and negative initial ɛNd values (-2 to -7) suggest significant, probably lower crustal contamination of the Type II Upper Siemens Creek basalts. The lava flows of the Siemens Creek Formation and correlative units around western Lake Superior appear to record the initial decompressive melting of an enriched mantle plume (Fig. 1D). The first picritic to basaltic flows represent low degree partial melts (~1-3%) derived from considerable depth well in the garnet stability field (>120km) (Fig. 1D). Progressive lithospheric extension, however, appears to have relatively quickly given rise to basaltic melts derived at higher degrees of melting at intermediate to relatively shallow depths (spinel stability field), and that ponded and interacted with the lower crust (Fig. 1D). Figure 1. A. Primitive mantle normalized trace element plot for the western basal Type I Siemens Creek lava flows and field for Siberian meymechites; B. Primitive mantle normalized trace element plot for basal Keweenawan lava flows in western Lake Superior; C. Primitive mantle normalized trace element plot for lower Type I and upper Type II Siemens Creek basalts; D. Ce/Yb vs Sm/Yb plot comparing Siemens Creek basalts with model melts generated by progressive lithospheric extension. Tick marks on model melt curve indicate depth of final melt segregation in km (after Ellam, 1992). Cannon, W.F., Woodruff, L.G., Nicholson, S.W., and Hedgman, C.A., 1996, Bedrock geologic map of the Ashland and the northern part of the Ironwood 30’x60’ quadrangles, Wisconsin and Michigan: U.S. Geological Survey Miscellaneous Investigation Series Map I-2566, scale 1:100,000. Ellam, R.M., 1992, Lithospheric thickness as a control on basalt geochemistry: Geology, v. 20, p. 153–156. Gibson, S.A., Thompson, R.N., and Dickin, A.P., 2000, Ferropicrites–geochemical evidence for Fe-rich streaks in upwelling mantle plumes: Earth and Planetary Letters, v. 174, p. 355–374. Hubbard, H.A., 1975, Lower Keweenawan volcanic rocks of Michigan and Wisconsin: U.S. Geological Survey Journal of Research, v. 3, no. 5, p. 529–541. 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. 134 �Potential value of pre-mining baseline oxygen, hydrogen, and sulfur isotopic data from surface waters for proposed large mining projects in northern Minnesota SEAL, Robert R. II1, JONES, Perry M.2, PIATAK, Nadine M.1, & WOODRUFF, Laurel G.2 1 U.S. Geological Survey, Reston, VA 20192, rseal@usgs.gov, npiatak@usgs.gov 2 U.S. Geological Survey, Mounds View, MN 55112, pmjones@usgs.gov, woodruff@usgs.gov Large mining projects, such as those proposed for the disseminated Cu-Ni-platinum group metal ores of the basal zone of the Duluth Complex, face a number of long-term challenges regarding environmental management due to their large size and the duration of mine lives. Leachates from mine waste may reach groundwater and eventually surface water from sources difficult to identify because they may be concealed by extensive piles of waste material. Acid drainage, even from low-sulfide mine waste, is one type of leachate that can be especially problematic to the environment by contributing acid, sulfate, and trace metals to the surrounding watershed. Large mines can have mine lives that span multiple decades. Under such long time spans, climate change, particularly drought, may have unanticipated effects on water quality and quantity in mine settings presently dominated by wetlands, such as those in northern Minnesota. Bacterial sulfate reduction is a common process acting in wetlands that consumes sulfate and organic matter to produce sulfide and alkalinity. The resulting sulfide can combine with iron or other metals to form authigenic sulfide minerals that sequester sulfur under the water-saturated anoxic conditions within wetlands. If these wetlands become unsaturated due to drought conditions, these authigenic sulfide minerals may oxidize and release sulfate to the watershed as surfacewater levels decline, which can confound attempts to identify sources of dissolved sulfate and trace elements in regional watersheds. Pre-mining baseline characterization of the oxygen and hydrogen isotopic composition of water and the sulfur and oxygen isotopic composition of dissolved sulfate provides a potential means of identifying the future influx of both acid drainage leached from mine-waste piles and sulfate derived from oxidation of naturally occurring authigenic sulfide minerals from drying wetlands. Three watersheds, Filson Creek, Keeley Creek, and the Saint Louis River, all of which transect the basal zone of the Duluth Complex, were sampled for baseline water-quality and stable isotope geochemistry over the course of a year focusing on peak-flow (June 2013) and base-flow (September 2012, 2013) conditions. The oxygen and hydrogen isotopic compositions of surface waters reflect the varying amount of evaporation that these waters have experienced. Isotopic analyses from samples from base-flow conditions reflect the influence of significant evaporation (δ18O = -9.4 – -4.1 ‰; δD = -65.5 – -38.8), whereas samples taken during peak flow show minimal deviation from the meteoric water line and imply little or no evaporation. Comparison of the oxygen isotope composition of water and dissolved sulfate reflects the relative importance two pyrite oxidation reactions that primarily differ in terms of their oxidants: FeS2 + 7/2 O2 + H2O  Fe2+ + 2 SO42- + 2 H+ (Reaction 1) FeS2 + 14 Fe3+ + 8 H2O  15 Fe2+ + 2 SO42- + 16 H+ (Reaction 2) Reaction 1 with molecular oxygen as the oxidant is most important at pH values above 4 – conditions of incipient sulfide oxidation. In contrast, Reaction 2 with ferric iron as the oxidant is most important below pH 4 – classic acid-mine drainage conditions. The oxygen isotopic composition of the water and dissolved sulfate can be used to estimate the relative importance of these two reactions in the generation of dissolved sulfate. The oxygen isotopic composition of 135 �dissolved sulfate in surface waters around the basal zone of the Duluth Complex (δ18O = -0.2 – 4.3 ‰) is consistent with greater than 70 percent of its derivation from Reaction 1 (Figure 1). Any future input of acid-mine drainage would be expected to produce sulfate with a distinctly lower oxygen isotopic composition. 6.0 4.0 2.0 δ18O SO4 0.0 ‐2.0 ‐4.0 ‐6.0 Filson Creek ‐8.0 Keeley Creek ‐10.0 ‐12.0 ‐12.0 ‐11.0 ‐10.0 St. Louis River ‐9.0 ‐8.0 ‐7.0 ‐6.0 ‐5.0 ‐4.0 δ18O H2O Figure 1. Oxygen isotope composition of dissolved sulfate and water from surface waters samples in the Filson Creek, Keeley Creek, and Saint Louis River watersheds near the basal zone of the Duluth Complex. The dashed lines correspond to the relative proportions of Reaction 1 and Reaction 2 contributing to the dissolved sulfate through sulfide oxidation. The sulfur isotopic composition of dissolved sulfate has potential for being a sensitive indicator of sulfate derived from future emergence and drying of wetlands due to drought conditions. The sulfate-sulfur isotopic composition of dissolved sulfate (δ34S = 5.6 – 8.6 ‰) overlaps known variations for ore and country rocks sulfides, suggesting that the current dissolved sulfate is derived from near surface oxidation of sulfide minerals from the basal zone of the Duluth Complex and the Virginia Formation. In contrast, the sulfur isotope fractionation caused by bacterial sulfate reduction can produce sulfide with isotopic compositions that can be between 15 and 70 ‰ lower than the corresponding sulfate. The sulfur isotope composition of sulfate derived from emergent wetland sulfide minerals in the future may be markedly different from the current sulfate in the watersheds. Baseline isotopic data for waters and dissolved sulfate can potentially be the basis for sensitive indicators of the future onset of acid-mine drainage, or the release of sulfate from wetlands that emerge due to prolonged droughts. The recognition of either of these processes near their earliest inception will facilitate appropriate responses for environmental protection during mining and after closure. 136 �The Mineralogy, Petrography and Geochemistry of the Anderson Lake Pegmatite Occurrence V. Smith, S. Zurevinski Department of Geology, Lakehead University, Thunder Bay, Ontario Anderson Lake Pegmatite is a S-type granitic pegmatite derived from the Hilma Lake Granite within the Quetico Terrane of the Southern Superior Province. The Anderson Lake pegmatite mineralogy consists of potassium feldspar, muscovite, quartz, beryl, and molybdenite. The pegmatite is crosscut by later quartz veins which occasionally host amethyst. The molybdenite within the pegmatite is syngenetic and occurs directly within quartz rich areas of the pegmatite, as well as within late stage dark gray quartz veins which crosscut the pegmatite. The molybdenite is occurring as coarse-grained euhedral florets, as well as pod-like aggregates. Of the three main trenches within the property, the molybdenite is more abundant in trench A, with minor occurrences in B and C. Within trench C, ferrimolybdenite is present within fractures alongside the molybdenite. Re-Os dating of the molybdenite within the pegmatite produced an age of 2689 +/- 12 Ma, which predates much of the plutonism, metamorphism and subsequent pegmatite injections within the Quetico Terrane (Percival and Sullivan, 1986). The pegmatite is roughly N-S trending along the contact between the host metasedimentary rocks and the Hilma Lake Granite. The associated Hilma Lake granite is classified within this study as a equigranular, coarse-grained monzogranite to syenogranite. The Hilma Lake granite contains plagioclase – potassium feldspar – quartz – biotite – muscovite – titanite – garnet – apatite and the biotite is heavily altered to chlorite with magnetite inclusions between the sheets of the biotite. When plotted on a tectonic discrimination diagram (Rb vs. Y + Nb) (Pearce et al., 1984), the Hilma Lake Granite and the Anderson Lake Pegmatite plot as volcanic arc granites. This would be expected had they formed during the subduction and transpression of the Wabigoon Terrane to the north and the Wawa Terrane to the south, when the Quetico was an accretionary complex composed of sediments derived from the volcanic arcs to the north and south (Percival and Williams, 1989). Figure 2: Tectonic discrimination diagram of the Hilma Lake Granite and the Anderson Lake Pegmatite based on (Pearce et al., 1984). 137 �Figure 3: A) Pegmatitic Beryl crystal found in trench A. B) Amethyst within a quartz vein in trench C. C) Hand sample from trench A showing a large molybdenite crystal. REFERENCES: Pearce, J. A., Harris, N. B., & Tindle, A. G., 1984. Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. Journal of petrology, 25(4), 956-983. Percival, J. A., & Williams, H. R., 1989. Late Archean Quetico accretionary complex, Superior province, Canada. Geology, 17(1), 23-25. Percival, J. A., & Sullivan, R. W., 1986. Age constraints on the evolution of the Quetico belt, Superior Province Ontario. Geological Survey of Canada: Radiogenic Age and Isotopic Studies: Report 2, 97-108. 138 �A Preliminary Investigation of Enigmatic Igneous Rocks on Big Powder Island, Northern Lake Superior: A Possible Mesoproterozoic Magmatic Event SMYK, Mark C.1, HOLLINGS, Peter 2 and FRALICK, Philip2 1 Resident Geologist Program, Ontario Geological Survey, Ministry of Northern Development and Mines, Suite B002, 435 James Street South, Thunder Bay, ON P7E 6S7 2 Department of Geology, Lakehead University, 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada A previously undescribed package of igneous rocks was investigated in 2015 on the southern shore of Big Powder Island (aka Anguros Island) in northern Lake Superior near Rossport. These relatively flat-lying rocks apparently overlie Paleoproterozoic Animikie Group sedimentary rocks of the Rove Formation (ca. 1835 Ma) and appear to be overlain by Mesoproterozoic clastic sedimentary rocks of the Pass Lake Formation, the lowermost formation of the ca. 1.4 Ga Sibley Group. All local rocks are intruded by ca. 1.1 Ga Nipigon diabase sills and dykes of the Midcontinent Rift (MCR). This package of igneous rocks, originally ascribed to the MCR by Giguere (1975) comprise a variety of fragmental units with aphanitic to phaneritic groundmasses. Locally displaying columnar joints, these rocks contain features that suggest that they may have been alternatively emplaced as shallow intrusions, flows and/or pyroclastic units. Three broad varieties of fragmental rocks have been identified:    Polymictic rocks with < 50% fragments; Monomictic rocks with < 30% black, fine-grained fragments; and Fragment-poor, aphanitic rocks with possible vesicles/amygdules. The majority of fragments are sub-angular to sub-rounded and resemble chemical and clastic Animikie Group sedimentary rocks. Baked margins are visible on some fragments, while others show evidence of partial resorption, flow alignment and/or flow-induced folding. The entire recovered zircon population appears to consist of detrital grains that exhibit metamorphic rounding but apparently lack metamorphic overgrowths. U-Pb data are concordant, yield typical Archean greenstone belt (2718-2720 Ma) and granitoid (2678 Ma) ages, and have normal igneous Th/U. These xenocrysts may have come from a xenolith that had been assimilated and heated sufficiently to cause resorption on the zircon surfaces, but not enough to produce melted rims (S. Kamo, personal communication, 2016). Or, alternatively, they represent a detrital zircon population that was abraded during transport and incorporated into a magma prior to overgrowth formation. Unlike nearby Nipigon sills, the primary mineralogy of these rocks has been extensively altered. Plagioclase and microcline have been sausseritized and sericitized, respectively. Primary ferromagnesian minerals have been altered to amphibole and chlorite. Small, irregular patches of quartz are suggestive of partial melting or resorption. Whole rock geochemistry of relatively fragment-free rocks is consistent with andesites to dacites (SiO2 = 57-70 wt%), with generally elevated K2O contents of 1.8 to 6.1 wt%, placing them in the calc-alkaline field on an AFM diagram. Very low Na2O/K2O ratios for most of the samples strongly suggests removal of Na during near-surface weathering, though the significant amount of K in the samples indicates the weathering was not extensive. The Big Powder Island (BPI) rocks are characterized by enriched LREE (La/Smn = 2.3-3.9), weakly fractionated HREE (Gd/Ybn = 1.2-1.7) and strong negative Nb anomalies. 139 �When compared to MCR-related magmatic suites, the BPI rocks have similar La/Smn and Gd/Ybn characteristics to both the Nipigon sills and the Osler volcanic rocks. Two thin dikes that cut a nearby Nipigon sill are geochemically similar to the BPI rocks rather than the sills, suggesting that they could be co-magmatic. However, the BPI fragmental rocks have lower TiO2 contents at a given Mg# than the majority of MCR rocks, suggesting a distinct source region. Interestingly, the only other MCR suite that shows similar TiO2 systematics is from the Moss Lake Intrusion. On a regional scale, the ~1590 Ma Badwater intrusive rocks show similar trends to the BPI with variable Mg#s at broadly constant TiO2 contents (Hinz 2015) whereas the ~1540 Ma English Bay granites (Hollings et al. 2007) do not. Lacking unequivocal primary zircon age data and based on apparent field relationships with other local Proterozoic rocks, it is suggested that the BPI rocks may represent magmatism that occurred between 1.4 and 1.8 Ga. Detrital zircon data from the overlying Sibley Group rocks contain populations that cluster between 1.9 to 1.8 Ga and 1.6 to 1.4 Ga (Rogala et al. 2007). Very few igneous rocks of those ages occur in this part of northwestern Ontario, suggesting that perhaps the BPI rocks could be a possible source of some of these zircons. Further work is required to establish and constrain field relationships and determine the nature and age of these enigmatic igneous rocks. REFERENCES Giguere, J.F. 1975. Geology of St. Ignace Island and adjacent islands, District of Thunder Bay; Ontario Division of Mines, Geological Report 118, 35p. Hinz, S., 2015. Geochemistry of the Badwater Gabbro south of Armstrong, Ontario. Unpublished HBSc thesis, Lakehead University, 98p. 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. Rogala, B., Fralick, P., 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, pp. 1131-1149. 140 �SEQUENCE STRATIGRAPHY AND BASIN EVOLUTION OF THE MESOPROTEROZOIC NONESUCH FORMATION, ASHLAND SYNCLINE, NORTHERN WISCONSIN KINGSBURY STEWART, Esther1 and MAUK, Jeffrey L.2, 1 Wisconsin Geologic and Natural History Survey, 3817 Mineral Point Road, Madison, Wisconsin 53705- 5100 2 U.S. Geological Survey, PO Box 25046, MS 964, Denver, CO, 80225 The late Mesoproterozoic Oronto Group consists of the Copper Harbor Conglomerate, successively overlain by the Nonesuch Formation and the Freda Sandstone. These sediments were deposited within the Midcontinent Rift above about 20 km of mostly volcanic rocks. The Nonesuch Formation is a significant host for copper and silver, a potential unconventional hydrocarbon resource, and a potential seal for carbon sequestration (Thorleifson, 2011, Bornhorst and Williams, 2013). In addition, the formation is the focus of fundamental research into environmental controls on the expansion of terrestrial life (Cumming et al., 2013). Despite its significance to applied and fundamental geologic questions, the Nonesuch Formation’s detailed sedimentology and stratigraphy remain relatively unstudied, and the formation has never been interpreted within a genetic, sequence stratigraphic framework. We present a sequence stratigraphic framework for the Nonesuch Formation within the Ashland Syncline, northern Wisconsin, based on core description and geochemistry. Because the Midcontinent Rift developed as a series of relatively enclosed basins, we use lacustrine facies associations to interpret a sequence stratigraphic framework (Carroll and Bohacs, 1999). This does not exclude the probability of episodic marine incursions, and lacustrine environments described and discussed herein may be partly to fully marine (Hieshima and Pratt, 1991). This framework ties geologic variability to geologic processes, thus improving predictability of the distribution of physical, geochemical, and biologic characteristics of the Formation. We observe ten lithofacies which we group into fluvial-alluvial, fluctuating-profundal, and fluvial-lacustrine facies associations. Handheld XRF analyses provide geochemical data that help classify these lithofacies. We observe three regional stratigraphic surfaces: two flooding surfaces, and one progradational surface that is a potential sequence boundary. The lithofacies succession we observe records a progression from a fluvial and alluvial depositional environment recorded by the upper Copper Harbor Conglomerate, to a balancefilled and then overfilled lacustrine environment within the Nonesuch Formation, and finally returning to a fluvial and alluvial environment within the uppermost Nonesuch Formation and lower Freda Sandstone. This interpreted evolution of depositional environments suggests that the primary control on sediment deposition and preservation was tectonic rather than climatic. The fluvial-alluvial facies association of the upper Copper Harbor Conglomerate was deposited when tectonic subsidence was relatively low. The basal Nonesuch contact records renewed tectonic subsidence, and the facies associations and stacking within the overlying Nonesuch Formation and lower Freda Sandstone record an evolution of sedimentary environments that developed in response to waning tectonic subsidence. The relative thicknesses and distribution of proximal and distal depositional environments, interpreted from lithofacies, show interpreted isopachs with arcuate shapes that thicken to the east. This indicates greater subsidence to the east. From this, we interpret deposition within an asymmetric half-graben bounded by one or more westdipping normal faults to the east. Growth faults like these resulted in a series of relatively isolated sub-basins within the Midcontinent Rift basin structure. Distinct stratigraphic architecture and basin-bounding faults of these sub-basins may have acted as barriers to 141 �mineralizing fluids and likely explain the difference between the unmineralized Nonesuch Formation in the Ashland Syncline and the ore-grade Cu mineralization in the Western Syncline and White Pine areas. We demonstrate that a modern, sequence stratigraphic approach may be applied to finegrained Precambrian sediments by using traditional rock description techniques and supporting lithogeochemistry. Our identification of a characteristic succession of lithofacies in Mesoproterozoic sediments demonstrates fundamental controls that are commonly interpreted for Phanerozoic lake systems may be extended into the Precambrian; these fundamental controls result in a predictable association of lithofacies, with distinct physical, biological, and geochemical properties. Figure 1: A. Fence diagram showing cores described for this study. Grain size increases to the right in each column. B. Nonesuch Fm. isopach map showing location of inferred basinbounding fault(s). Bornhorst, T.J., Williams, W.C., 2013. The Mesoproterozoic Copperwood sedimentary rock-hosted stratiform copper deposit, Upper Peninsula, Michigan. Economic Geology 108, 1325-1346. Carroll, A.R., Bohacs, K.M., 1999. Stratigraphic classification of ancient lakes: Balancing tectonic and climatic controls. Geology 27, 99-102. Cumming, V.M., Poulton, S.W., Rooney, A.D., Selby, D., 2013. Anoxia in the terrestrial environment during the late Mesoproterozoic. Geology 41, 583-586. Hieshima, G.B., Pratt, L.M., 1991. Sulfur/carbon ratios and extractable organic matter of the Middle Proterozoic Nonesuch Formation, North American Midcontinent Rift. Precambrian Research 54, 65-79. Thorleifson, L.H., 2011. Potential for implementation of mineral carbonation as a carbon sequestration method in Minnesota. Minnesota Geological Survey Open-File Report 11-2. 142 �DISCOVERING HIDDEN FOLDS AND FAULTS IN THE PRECAMBRIAN: NEW INSIGHTS INTO BARABOO-INTERVAL STRATIGRAPHY AND DEFORMATION IN SOUTHERN WISCONSIN KINGSBURY STEWART, Esther1, STEWART, Eric D.1, LAMB, Matthew2 1 Wisconsin Geologic and Natural History Survey, 3817 Mineral Point Road, Madison, WI 53705 2 Department of Geology and Geography, University of Wisconsin - Whitewater, 800 W Main St, Whitewater, WI 53190 Laurentia records a complex history of continental collision and construction, accretion, stabilization, and reactivation of continental lithosphere. In the Southern Lake Superior Region, the Baraboo-interval sediments, including the Baraboo Quartzite, Seeley Slate, the iron-rich Freedom Formation (Sauk County) and the Waterloo Quartzite (Dodge and Jefferson Counties), were deposited after ca. 1.71 Ga and then deformed during ca. 1.65- 1.63 Ga Mazatzal accretion (Holm et al., 1998; Medaris et al., 2003). Ca. 1.4 Ga intrusion of the Wolf River Batholith and 1.0 Ga failed Midcontinent rifting subsequently affected the region. Since then, the Southern Lake Superior region has remained tectonically stable for the past billion years and thus uniquely preserves the complex history of Laurentian assembly but also results in subdued topography and a thick cover of Phanerozoic sediments that obscures direct observation of these rocks. We present preliminary results from geologic mapping of the buried Precambrian basement for a ~9000 km2 area in southern Wisconsin that is based on integration of data from new bedrock drill core, sparse outcrops, and existing geophysical and subsurface data sets. Our mapping revises the stratigraphy of the Baraboo interval quartzites and allows us to interpret the deformation history of the area. We describe a two-stage deformation history, beginning with regional folding and ending with thick-skinned thrusting. We integrate regional aeromagnetic data (Snyder and Daniels, 2002) with data from outcrops, existing bedrock geologic maps (Dalziel and Dott, 1970), drill cuttings, and new and existing bedrock drill cores (e.g. Weidman, 1904; Schmidt, 1951) and associated geophysical logs in a manner analogous to the NICE Working Group (2007). Aeromagnetic anomalies reflect the distribution of magnetic minerals in the upper crust and can be used to locate buried faults and characterize the distribution and structure of magnetic rocks. In the Lake Superior Region, Pleistocene glacial deposits and Paleozoic sedimentary rocks are magnetically transparent, so the aeromagnetic map reflects changes in the composition and geometry of the buried Precambrian crust. Importantly, a new drill core collected by the Wisconsin Geological and Natural History Survey in Dodge County recovered Precambrian iron-formation over a strata-bound aeromagnetic high present in the aeromagnetic anomaly map of Snyder and Daniels (2002). We correlate this iron formation to the Freedom Formation (Lamb and Stewart, this volume), allowing the strata-bound aeromagnetic high to be used as a marker unit in the aeromagnetic anomaly map, even in areas of southern Wisconsin with no surface exposure of Precambrian rocks. Patterns observed in the aeromagnetic data suggest the development of broad-scale doubly plunging folds within Baraboo-interval sediments and underlying post-Penokean granites and rhyolites. These folds occur across >9000 km2 of southern Wisconsin. The Baraboo syncline in Sauk County represents the western edge of regional folding. In Dodge County, the Waterloo Quartzite occurs in a broad basin stratigraphically above the aeromagnetic high marker unit interpreted as the Freedom Formation, indicating the Waterloo quartzite is a distinct quartzite unit stratigraphically above the Baraboo quartzite, the Seeley Slate and the Freedom Formation. 143 �Aeromagnetic map patterns further suggest that regional folds were truncated by at least two basement-involved thrust faults. The well-known Baraboo Syncline lies in the hanging wall of one of these thrusts, which placed folded post-Penokean granites and rhyolites, as well as the lower portion of the Baraboo-interval stratigraphy, over stratigraphically higher folded Baraboointerval sediments. Mazatzal-age accretion is interpreted to have caused this two-stage (i.e. folding followed by faulting) deformation history. Figure 1. Preliminary Precambrian basement map. Dalziel, I.W.D. and Dott, R.H., 1970. Geology of the Baraboo District, Wisconsin: A description and field guide incorporating structural analysis of the Precambrian rocks and sedimentologic studies of the Paleozoic strata. Wisconsin Geological and Natural History Survey Information Circular 14. Holm, D., Schneider, D.A., Coath, C., 1998b. Age and deformation of Early Proterozoic quartzites in the southern Lake Superior region: implications for extent of foreland deformation during final assembly of Laurentia. Geology 26, 907–910. Medaris, L.G., Singer, B.S., Dott, R.H., Naymark, A., Johnson, C.M., and Schott, R.C., 2003. Late Paleoproterozoic Climate, Tectonics, and Metamorphism in the Southern Lake Superior Region and Proto-North America: Evidence from Baraboo Interval Quartzites. The Journal of Geology. 111, 243-257. NICE working group, 2007. Reinterpretation of Paleoproterozoic accretionary boundaries of the north-central United States based on a new aeromagnetic-geologic compilation. Precambrian Res. 157, 71–79. Schmidt, 1951. The Subsurface Geology of Freedom Township ni the Baraboo Iron-Bearing District of Wisconsin. UW Msc. Thesis, 40 pp. Snyder, S.L. and Daniels, D.L., 2002. Wisconsin Aeromagnetic and Gravity Maps and Data: A web site for distribution of data. USGS Open File Report 02-493. 144 �A RE-EXAMINATION OF THE KAPUKASING STRUCTURAL ZONE STINSON, Victoria R., PAN, Yuanming, GAMELIN, Gleceria, and NADEAU, Matthew 114 Science Place, Saskatoon, Saskatchewan, S7N 5E2; vis211@mail.usask.ca The Kapukasking Structural Zone in the Wawa-Abitibi terrane crosscuts the Superior province and exposes amphibolites, granulites, and migmatites of the middle and lower crust. The Wawa Gneiss Domain west of the Kapukasing Structural Zone is composed of various types of felsic intrusive bodies that crosscut diatexite migmatites. The diatexite migmatites transition to metatexites into the Kapukasing Structural Zone and are migmatized amphibolites, granulites, and metaconglomerate. The transition from the Wawa Gneiss Domain into the western Kapukasing Structural Zone is gradational and is defined by a systematic pattern of regional to microscopic pinch and swell and boudinage structures. The boudin necks are defined by penetrative, tightly-spaced schistosity and gneissosity, dominantly east-west to northeast-southwest striking, steeply to moderately dipping south to southeast or north to northwest, steeply to moderately plunging dominantly east or west mylonites in dextral ductile to brittle-ductile shear zones. The rheologically competent boudins, acting as regional to microscopic lithons, display varying types of boudinage structures, and have wider-spaced gneissosity, strike east-west to northeastsouthwest, are horizontal to gently dipping north or south, and horizontal to gently plunging east or west. The formation of the Kapukasing Structural Zone can be explained by continental collision to dextral transpression between the Minnesota River Valley and the Wawa-Abitibi terranes in the Neoarchean to Paleoproterozoic. The oblique continental collision produced lithospheric-scale boudinage, creating and exposing the Kapukasing Structural Zone and metamorphic core complex of the Wawa Gneiss Domain, which were subsequently brittlely deformed throughout the Proterozoic. 145 �Source of Native Iron in Canadian Arctic Artifacts SVENSSON, Matthew, and KISSIN, Stephen Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1 Canada A collection of fourteen samples of metal made from iron artifacts recovered from the Canadian Arctic Archipelago was received from the Canadian Conservation Institute, Department of Canadian Heritage in order to examine the origin of the materials. In Greenland, three sources of iron in artifacts have been determined: terrestrial native iron, manmade iron of European origin and meteoritic iron. The present study was undertaken in order to determine the origin of the metal in the Canadian artifacts. Observation of polished surfaces made from the samples by reflected light microscopy revealed the presence of the Widmanstätten structure deformed in all cases by cold working, clearly indicating the meteoritic origin of all of the artifacts. The Widmanstätten structure is an oriented intergrowth of kamacite (α-Fe) lamellae in taenite (γ‐Fe).  This structure forms as the result of the exsolution of kamacite during the diffusion of the Ni into the taenite phase in octahedrites. The Ni distribution and concentration of the Ni in the kamacite phase can be used as an indicator of the equilibrium state of the specimen and in the classification of the meteorite. The results of a step scan across the entire bandwidth of the kamacite lamellae can yield an Mshaped profile indicating the diffusion of Ni. Although kamacite and taenite can be found in terrestrial iron sources, the Widmanstätten texture is unique to meteoritic iron and is commonly found in octahedrites such as the Cape York meteorite. The Cape York meteorite landed close to Thule and Dorset territory in southern Greenland. It is reasonable to hypothesize that the Cape York meteorite is the source of iron for these artifacts because of its close proximity to Thule and Dorset territory. A comparison was done of the Ni-content and the bandwidth of the kamacite lamellae in the artifacts with that of the Cape York meteorite in order to precisely test this hypothesis. Here, we report the results of the mineral chemistry and compare these results to those reported for the Cape York meteorite in order to draw conclusions. Only 0.5 to 2.0mm sized samples were taken for use in SEM-EDX analysis in order to preserve as much of the artifacts as possible. A step-scan and line scan were performed roughly perpendicular to the kamacite taenite interface to acquire a representative sample of the artifact’s whole composition. The standard used for this analysis was the Filomena meteorite. Filomena is comprised almost entirely of kamacite, making it an excellent standard for the analysis of the kamacite lamellae. Both the deformation of the kamacite lamellae caused by the cold forging of the artifacts, and the small sample size prevent direct observation of the full bandwidth of the kamacite lamellae. The bandwidth of the kamacite lamellae may be calculated through the use of Goldstein’s (1965) plot of average half-width of kamacite vs the average Ni-content in the kamacite, which also related mean kamacite bandwidth to Ni-content in the meteorite The M-profiles in this study are nearly flat, reflecting the approach to equilibrium in diffusion of Ni from kamacite lamellae. The kamacite bandwidth as measured in SEM images and by determination from Goldstein’s (1965) figure 9 yield a range of widths of 0.042 to 0.0196mm . These bandwidths are significantly smaller than Buchwald (1975) reported for Cape York at 1.20±0.2mm. However, Buchwald’s measurements were made on the Savik I mass of Cape York, which has the lowest reported Ni-content of the Cape York masses as reported by Esbensen et al. (1982). The Thule mass (table 1) has a much higher Ni-content and therefore a 146 �narrower kamacite bandwidth. However, the bandwidths for other than Savik I have not been reported, and kamacite bandwidths can vary widely in a given meteorite. We conclude that this study reveals that the Canadian Arctic metal artifacts are made from meteoritic iron and further, that the likely source is the Cape York meteorite. Table 1: Ni-content of Cape York Meteorite Masses Fragment Ni (wt %) Savik I 7.46 Savik II 7.54 Ahnighito East 7.46 Ahnighito West 7.63 Woman 7.65 Dog 7.89 Agpalilik 8.25 Thule 8.52 Data from Esbensen et al. (1982) REFERENCES Buchwald V.F., 1975. Handbook of Iron Meteorites. University of California Press Buchwald V.F., Mosdal G., 1985. Meteoritic iron, telluric iron and wrought iron in Greenland. Monographs on Greenland, Man and Society 242: 3-49 Esbensen K.H., Buchwald V.F., Malvin D.J., Wasson J.T., 1982. Systematic compositional variations in the Cape York iron meteorite. Geochimica et Cosmochimica 46: 1913-1920 Goldstein J.I., 1965. The formation of the kamacite phase in metallic meteorites. Journal of Geophysical Research 70: 6223- 6232 147 �The Badwater gabbro as an analogue for the weathering of Martian basalts SVENSSON, Matthew1, FRALICK, Philip1 1 Department of Geology, Lakehead University, 955 Oliver Rd. Thunder Bay, ON P7B 5E1 Canada The best way to study the surficial processes of other planets apart from indirect observation through rovers and orbiters is through the use of analogues. Studying the surface of Mars has become a popular topic as more sophisticated direct measurements are undertaken using instruments such as those on board the Curiosity rover. It is known from the study of Martian meteorites that the crust is entirely basaltic with no evidence of felsic igneous rocks. Through weathering, the basaltic crust was broken down and now comprises the material analyzed by Curiosity. Therefore, investigating the possible effects of weathering of basalts in a low oxygen atmosphere is important in order to better understand data retrieved from rovers such as Curiosity. Here, we analyze the petrography, mineral chemistry and whole-rock geochemistry of the Badwater gabbro with a focus on the effects of weathering. The results were interpreted in order to assess the viability of the uppermost weathered zone of the Badwater gabbro as an analogue for the planet Mars. The Badwater gabbro is a 1598 ± 1.1Ma, coarse-grained, intrusive unit that is disconformably overlain by the pillowed Pillar Lake volcanics. The gabbro has a striking paleoweathering profile developed in its upper five meters. The age of this weathering profile is poorly constrained, but it is Mesoproterozoic, constrained between the age of the Badwater gabbro and an 1100 Ma Midcontinent Rift related sill, which cuts the Pillar Lake volcanics. Sandstones present interbedded with the volcanics are similar to those of the ~1400 Ma Sibley group, which outcrops to the south. Data was collected from drill core samples obtained by East West Resources Corporation in 2004 during their search for PGE mineralization, near Armstrong, Ontario. Trends in the whole rock geochemistry are some of the most telling features of the Badwater gabbro. Potassium was found to increase from 0.482%, 1612cm below the contact with the Pillar Lake volcanics to 2.508%-3.573% towards the Pillar Lake volcanics contact (Figure 1a). Similarly, the magnesium was found to increase from 7.319% to 14.602% towards the contact (Figure 1b). These are thought to be the influence of weathering due to the presence of a saline lake such as those for which the neighboring Sibley group is known. Aluminum and sodium were found to show little variance near the contact. An average aluminum composition of 13.341±1.889% (Figure 2a), and an average sodium concentration of 4.325±1.502% (Figure 2b) were found. The aluminum data reflects the lack of overall loss or gain in the major elements. Therefore it can be concluded that the trends in magnesium and potassium do not reflect mass loss in the system. Similarly, sodium follows the same trend as Al, but is a much more mobile element and is therefore usually lost during weathering in subaerial or freshwater environments. This suggests that weathering took place in a saline lake environment thus restricting the mobility of Na. The upward increase in K and Mg is unusual as these elements, in particular K, are commonly depleted during weathering. The upward enrichment without similar enrichment in Al means they were being added from above during weathering. This implies the presence of a saline water mass or groundwater system near a saline waterbody. The overlying pillowed basalts reinforce the probability of an overlying saline lacustrine system, which had abundant K and Mg that was incorporated into the loose sediment of the weathering profile. The saline 148 �lacustrine sediments of the Sibley group attest to the development of the appropriate climatic conditions in the Mesoproterozoic for the formation of saline systems. b) MgO vs Depth 4 20 3 15 MgO K2O a) K2O vs Depth 2 10 1 5 0 0 0 500 1000 1500 2000 0 500 Depth (cm) 1000 1500 2000 Depth (cm) Figure 1: 1a) K2O vs depth. 1b) MgO vs depth. Both these oxides increase significantly in concentration through the weathered zone towards the contact. b) Na2O vs Depth 20 8 15 6 Na2O Al2O3 a) Al2O3 vs Depth 10 5 4 2 0 0 0 500 1000 1500 0 2000 Depth (cm) 500 1000 1500 2000 Depth (cm) Figure 2: 2a) Al2O3 vs depth. 2b) Na2O vs depth. Neither of these oxides show any significant increase or decrease in concentration towards the contact. Saline lakes would have low amounts of sulfur thus invoking less of a restriction on dolomite production. The magnesium derived from the dolomite is the likely source of the anomalous magnesium values. These conclusions are consistent with the widely accepted magnesium concentrations and dolomitic evaporites in the Sibley lakes. Given that weathering is dominated by the presence of saline water in a low oxygen environment it has these in common with weathering on the red planet as its water mass decreased. However, during this period on Mars the water became acidic resulting in iron being transported and iron sulfates commonly forming, whereas the non-acidic waters studied were capable of forming potassic clays and magnesium evaporites. It is reasonable to suggest that the Badwater gabbro may be as close to an analogue for Mars that can be studied on Earth, but would not make a totally appropriate analogue. REFERENCES Hinz, 2015. Geochemistry of the Badwater Gabbro south of Armstrong, Ontario. Lakehead University Mezger, K., Debaille, V., Kleine, T. 2012. Core formation and mantle differentiation on Mars. Space Science Reviews, 174: 27-48 Nesbitt, 2003. Petrogenesis of siliciclastic sediments and sedimentary rocks. Geochemistry of Sediments and Sedimentary Rocks: Evolutionary Considerations to Mineral Deposit-Forming Environments, 4: 39-51 149 �Geologic mapping of Neoarchean and Proterozoic rocks near Kekekabic Lake, northeastern Minnesota, by students of the Precambrian Research Center’s 2015 field camp Abstract refers to poster entitled: “Bedrock geologic map of the Knife Lake Group and related intrusions near Kekekabic Lake, Lake County, Minnesota.” First author: “Christenson” UPTON, Margaret1, PUZEL, Ryan1, CHRISTENSON, Jaron1, KENT, Morgan1, SPREITZER, Steven1, and JIRSA, Mark2 1 2015 Field Camp Students, Precambrian Research Center, University of Minnesota-Duluth, 5013 Miller Trunk Highway, Duluth, Minnesota 55811 2 Minnesota Geological Survey, University of Minnesota, 2609 W. Territorial Rd., St. Paul, Minnesota 55114 (jirsa001@umn.edu) The University of Minnesota-Duluth’s Precambrian Research Center conducted its ninth annual field camp in 2015, and this presentation shows results of one of several “capstone” mapping projects. The projects test student skills by creating new geologic maps in areas of poorly known geology. This benefits both students and mentor organizations, and contributes to our collective understanding of Minnesota geology. The capstone project described here involved mapping an area of ~12 mi2 in the Boundary Waters Canoe Area Wilderness (BWCAW), centered on the western part of Kekekabic Lake, but included all or parts of Spoon, Dix, Skoota, Missionary, Pickle, Kek, Strup, and Wisini Lakes (Fig. 1). The resulting map provides details about the complex depositional, magmatic, and tectonic history of a Neoarchean metavolcanic and metasedimentary terrane that is part of the Wawa subprovince of Superior Province, the basal Mesoproterozoic Duluth Complex and diabase dikes. Compared with the other 8 capstones mentored by Jirsa (2007-2014), this one presented the greatest lithologic diversity and logistical challenges. Figure 1. Generalized bedrock geologic map of NE Minnesota showing the Kekekabic Lake capstone area (solid black polygon). The Neoarchean unit labeled “Supracrustal Rocks” encloses both older volcanic sequences and younger, largely sedimentary ones. Outline of Boundary Waters Canoe Area Wilderness is dashed. The Neoarchean rocks in the central BWCAW comprise a Timiskaming-type extensional basin deposit that consists of a broad array of sedimentary (terrestrial and shallow marine), volcanic, and intrusive rocks in close proximity. The Kekekabic Lake map area (Fig. 2) provides a window into this complex terrane. Highlights of the mapping include the following:  Tightly folded graywacke and mudstone (Fig. 2, unit Aks), locally containing thin lenses and layers of iron-formation. The latter implies deposition in marine or restricted basin settings.  A unique sequence of unsorted oligomictic conglomerate composed of hornblende-, pyroxene-, and plagioclase-phyric trachyandesite clasts that grades up stratigraphic section to sandstone and gritstone containing abundant mafic minerals and chlorite presumably derived from them (unit Akg). Ripple marks and trough cross-bedding in sandy portions indicate fluvial transport (Fig.3).  Both units are cut by a large, lithologically diverse intrusion (unit Akp) composed of porphyritic rocks that vary from monzonite to hornblende-, pyroxene- and biotite-bearing lamprophyre. 150 � Heterogeneous gabbro and troctolite of the basal Duluth Complex intruded the Knife Lake rocks, creating a Hornfels contact aureole that displays metamorphic recrystallization, thermally augmented deformation features, and magmatic brecciation of the host Neoarchean rocks. Figure 2. Gray-scale version of the 14-map unit Kekekabic Lake geologic map highlighting pertinent units discussed above. NAD83, Zone 15 UTM coordinates. Figure 3. Field photographs of unit Akg. Left: unsorted volcanic conglomerate; Right: ripple-marked volcanic sandstone. See discussion above. North shore of Kekekabic Lake. The results of this and other capstone mapping projects can be viewed at www.d.umn.edu/prc. 151 �Influence of mineral liberation on metal leaching and dissolution rates in ore material and associated host rock VANDERWAAL, Gerrit and SCHARDT, Christian Department of Earth and Environmental Sciences, University of Minnesota Duluth, 1049 University Drive, Duluth, MN 55812, United States Current methods of sulfide ore comminution produce harmful waste in the form of leached metals and acidic runoff. Conventional ore processing techniques leave some ore minerals with the tailings, the efficiency of which depends on many factors. In the case of sulfide mining, these ore minerals oxidize to form sulfate and free H+ ions, lowering the pH of the system and accelerating sulfide oxidation and the release of metals, a phenomenon known as “acid rock drainage” (Nicholson et al., 1990; Lapakko et. al, 2013). A technology that has recently become commercially available, electric pulse disaggregation (EPD), breaks apart rock at mineral grain boundaries, void spaces, and other cavities within the rock. A given sample is submerged in water and variable amounts of electricity (200 kV in this experiment) are pulsed through the water, generating plasma streamers which break apart the rock by shockwaves formed by rapid heating and expansion (Cabri et al., 2008). By breaking the rock apart into individual mineral grains, the surface area of both sulfides and silicates is drastically increased, improving the recovery of ore minerals and increasing the buffering capacity of silicates, thereby limiting the potential production of acidic runoff. EPD should increase ore recovery efficiency while simultaneously decreasing the amount of waste released into the environment by separating ore minerals from the gangue at grain boundaries. An ongoing experiment utilizing Cu-Ni-PGE sulfide ore material from Minnesota’s Duluth Complex is currently testing this hypothesis. Six different experiments, three with material processed via EPD (material stored in water at room temperature, at 50°C, in a solution with a starting pH of 4) and three with conventionally crushed material (same conditions as above) were set up and run over the course of 8+ weeks. Temperature, pH, conductivity, and mass measurements are being taken on a weekly basis. Samples of the aqueous solution will be taken twice over the course of the experiment and will be subjected to geochemical analysis. The reaction product will eventually be analyzed via x-ray diffraction to determine the final composition of the rock. Preliminary data indicate a slight increase in free ions in the material processed via EPD compared to the mechanically treated material but other parameters are similar. All experiments exhibit a decrease in mass to the current date and the pH of 4 experiment quickly reached neutral conditions (within five weeks) as expected, while the other experiments showed no signs of significant change. REFERENCES Cabri, L. J., Rudashevsky, N. S., Rudashevsky, V. N. and Oberthür, T. 2008. Electric-pulse disaggregation (EPD), hydroseparation (HS) and their use in combination for mineral processing and advanced characterization of ores. In Proceedings of the 40th Annual Canadian Mineral Processors Conference. p. 213. Ontario, Canada: Canadian Mineral Processors. Lapakko, K. A., Olson, M. C., and Antonson, D. A. 2013. Duluth Complex tailings dissolution: Ten-year laboratory experiment. Minnesota Department of Natural Resources. p. v. Nicholson, R., Gillham, R., and Reardon, E. 1990. Pyrite oxidation in carbonate-buffered solution: 2. Rate control by oxide coatings. Geochimica Et Cosmochimica Acta, 54(2), p. 395-402. 152 �Small-Scale Petrographic Variations in a Nipigon Diabase Sill WALLRICH, Blake M. and ZIEG, Michael J. Department of Geography, Geology, and the Environment, Slippery Rock University, Slippery Rock, PA, 16057 In this study, we report ongoing results describing the petrographic characteristics of a Nipigon diabase sill, part of the 1.1 Ga Midcontinent rift magmatic suite of ultramafic to mafic sills located in the Lake Superior region (Hollings et al., 2007). A ~250 m continuous core (BSE-07-01) drilled by RPT Uranium Inc. (2007) in the vicinity of Black Sturgeon Lake, southwest of Lake Nipigon has been archived in the geology department at Slippery Rock University (SRU). Previous research by SRU students and faculty has identified several reinjection horizons where new magma has intruded older, partially solidified magma within a crystal mush zone (e.g., Zieg, 2014). This investigation focused on a detailed petrographic and textural analysis of a 20-meter section (65-85 m above lower contact) to provide a better understanding of petrographic signatures of the reinjection process within this interval. The primary data for this study were variations in modal mineralogy (obtained by point counting) and plagioclase mean length (determined by manually tracing crystals in photomicrographs). Using this data, we have identified a zone where plagioclase sizes deviate from a normal coarsening-inward trend. This deviation is accompanied by variations in modal olivine: anomalous decreases in plagioclase mean length are associated with sudden increases in olivine abundance. The decreases in mean plagioclase size are thought to represent soft internal chills. The olivine accumulations are interpreted as suspended crystals that settled out of the reinjected magma soon after emplacement, collecting on top of the more viscous crystal mush at the base of the freshly injected magma (Hayes et al. 2015; Zieg, 2014). In addition to grain size we also examined plagioclase orientation. In the section with anomalous textures, we also found plagioclase grains with a preferred orientation, possibly induced by shear stresses related to the inflowing magma, parallel to reinjection margins (perpendicular to the core axis). The combination of plagioclase alignment, finer-grained textures, and the sudden increase in olivine abundance supports the hypothesis that magma was emplaced into the existing mush at this location as an olivine-rich slurry (Fig. 1, Fig. 2; Zieg, 2014). The results of this project will contribute to a larger study (Zieg, this volume) that evaluates the processes of emplacement and identifies diagnostic criteria for recognizing reinjection horizons within sills. This data will then be used to refine model parameters for magma chamber evolution within the Earth’s crust. Future work will focus on further defining the petrologic characteristics of reinjection horizons using textures and modal mineralogy, and eventually geochemical analysis, to further constrain the physical and chemical consequences of reinjection in sills. References Hayes, B., Bédard, J. H., and Lissenberg, C. J., 2015, Olivine slurry replenishment and the development of igneous layering in a Franklin sill, Victoria Island, Arctic Canada: Journal of Petrology, v. 56, p. 83-112. Hollings, P., Hart, T., Richardson, A., and MacDonald, C. A., 2007, Geochemistry of the Mesoproterozoic intrusive rocks of the Nipigon Embayment, Northwestern Ontario: Evaluating the earliest phases of rift development: Canadian Journal of Earth Sciences, v. 44, p. 1087-1110. Zieg, M. J., 2014, Petrologic evolution of a Nipigon diabase sill, Ontario, Canada: Insights from compositional and textural profiles: Economic Geology, v. 109, p. 1383-1401. 153 �Figure 1(left to right): Variations in modal olivine, mean plagioclase length, and plagioclase alignment factor ( ) 85 m 75 m 72 m 71 m Figure 2: Variations in olivine abundance, mean plagioclase length, and alignment factor with height. FOV in micrographs 4.1 x 3.3 mm 154 65 m �Assessment of Undiscovered Nickel-Copper-Platinum Group Element (Ni-CuPGE) Resources Related to Conduit-Type Mineralization in the Midcontinent Rift System, Michigan, Minnesota, Ontario, and Wisconsin ZIENTEK, Michael L.1, SCHULZ, Klaus J. 2, WOODRUFF, Laurel G.3, CANNON, William, F. 2, NICHOLSON, Suzanne W. 2, ZÜRCHER, Lukas4, PARKS, Heather L. 1, and DICKEN, Connie2 1 U.S. Geological Survey, 904 West Riverside Avenue, Spokane, WA 99201 2 U.S. Geological Survey, 12201 Sunrise Valley Drive, MS 954, Reston, VA 20192 3 U.S. Geological Survey, 2280 Woodale Drive, St. Paul, MN 55112 4 U.S. Geological Survey, 520 North Park Avenue, Tucson, AZ 85719 The U.S. Geological Survey (USGS) uses a geology-based three-part mineral resource assessment approach. The three parts for a specific deposit type are: 1) delineation of areas permissive for undiscovered mineral resources permitted by the geology; 2) estimation of the number of undiscovered deposits within each delineated area; and 3) estimation of the amount of resources contained in the undiscovered deposits, using appropriate ore characteristics and metal contents defined by worldwide grade and tonnage models. The USGS is completing such an assessment for conduit-type Ni-Cu- PGE sulfide deposits (defined as magmatic sulfide mineralization restricted to small- to medium-sized mafic and ultramafic dikes and sills derived from picritic and tholeiitic basaltic magma; Schulz and others, 2014) in rocks related to the Midcontinent Rift System (MRS) in Michigan, Minnesota, Ontario, and Wisconsin. The name of this deposit type, conduit-type, emphasizes the relation of these Ni-Cu-PGE sulfide-rich deposits to small- to medium-sized mafic and ultramafic dikes and sills that served as pathways for the flow-through of picritic and tholeiitic basaltic magmas. These intrusions are orders of magnitude smaller than many layered intrusions that host contact-type Ni-Cu±PGE sulfide (e.g., Duluth Complex) or reef-type PGE deposits (e.g., Stillwater Complex, Montana). Critical processes of conduit-type deposit ore formation are used to identify essential criteria appropriate for a regional-scale assessment and to define proxies for these system components. For this assessment, we emphasized a metal source (MRS-related mafic and ultramafic magmas), magma pathways (mafic and ultramafic dike swarms and small intrusions emplaced during development of the MRS), and a source of sulfur in country rocks that, if assimilated, could result in sulfur saturation of MRS magmas (in this setting, sulfide-bearing Paleoproterozoic metasedimentary and metavolcanic rocks). It is difficult to predict, however, if a particular mafic or ultramafic intrusion emplaced into favorable country rock served as a high-flux magma pathway and thus, could contain a magmatic conduit-type sulfide deposit. Also, at the regional scale of an assessment, there are no proxies for predicting if and where sulfide minerals might accumulate in sills and dikes. As a result, all areas where MRS-related mafic and ultramafic dikes or sills are inferred to be present in sulfur-rich Paleoproterozoic rocks are considered to be permissive for the occurrence of conduit-type magmatic Ni-Cu-PGE sulfide mineralization. Thus, for this assessment, five permissive tracts are identified: 155 �1) Animikie tract – strata of the Proterozoic Animikie Group in northern Minnesota (one known deposit (Tamarack) but no other known occurrences) 2) Little Falls tract – metamorphosed and deformed metasedimentary and volcanic rocks that are part of the Penokean-age fold and thrust belt in Minnesota (no known deposits or occurrences) 3) Michigamme tract – the known and inferred distribution of the Michigamme Formation, Michigan and Wisconsin (one known deposit (Eagle mine) and multiple occurrences (e.g., BIC, Roland Lake, Eagle East)) 4) Pigeon Point tract – strata of the Animikie Group in northeast Minnesota and southern Ontario (one known deposit (Great Lakes Nickel deposit) and several occurrences) 5) Pembine-Wausau tract – volcanic and sedimentary rocks of the Pembine-Wausau magmatic terrane, Wisconsin (no known deposits or occurrences) After a discussion of conduit-type deposit requirements and the favorable (or unfavorable) geology of the each tract, assessment team members made separate estimates of the numbers of undiscovered deposits. Estimators were asked for the least number of deposits of a given type that they believed could be present at three specified levels of certainty (90 percent, 50 percent, and 10 percent). Each person made initial estimates without sharing their results until everyone was finished; and then the results were compiled and discussed. Following the discussion, individual scores were adjusted and a single mean estimate of the number undiscovered deposits in each tract down to 2 km depth was determined. This final estimate reflects both uncertainties in what could exist and in the favorability of a tract. As might be expected from the number of known deposits and occurrences, the Michigamme tract was assessed to have the highest number of possible undiscovered deposits, with a mean estimate of five undiscovered deposits. The Animikie, Pigeon Point, and PembineWausau tracts each had mean estimates of two undiscovered deposits, whereas the Little Falls tract had a mean estimate of one undiscovered deposit. New grade and tonnage models for conduit-type sulfide deposits were used in Monte Carlo simulations to obtain estimated probability distributions of undiscovered metals in each tract. The current known and, here estimated, undiscovered resources represented by MRS-related conduit-type deposits are significantly less than the known Ni-Cu±PGE resources in the undeveloped contact-type deposits along the western margin of the Duluth Complex; conduittype deposits, however, are typically much higher grade, and therefore remain very attractive exploration targets. Reference Schulz, K.J., Woodruff, L.G., Nicholson, S.W., Seal, R.R., II, Piatak, N.M., Chandler, V.W., and Mars, J.L., 2014, Occurrence model for magmatic sulfide-rich nickel-copper-(platinum-group element) deposits related to mafic and ultramafic dike-sill complexes: U.S. Geological Survey Scientific Investigations Report 2010–5070–I, 80 p. http://dx.doi.org/10.3133/sir20105070I 156 �Geochemistry and Petrography of the Volcanic Strata Hosting the Flambeau Cu-Zn-Au Deposit in Rusk County, WI: A Re-examination of Wisconsin’s Only Past-Producing Volcanogenic Massive Sulfide Mine. ZENS, Zacharie A., and LODGE, Robert W.D. Department of Geology, University of Wisconsin-Eau Claire, Eau Claire, WI Volcanogenic massive sulfide (VMS) deposits are polymetallic mineral deposits and are the source of many important base (e.g., Cu, Zn, Pb) and precious metals (e.g., Au, Ag, Ga) (LaBerge, 1996; DeMatties, 1994). The Flambeau Cu-Zn-Au Mine and other VMS deposits are hosted within the Paleoproterozoic juvenile and continental volcanic arc sequences of the Wisconsin Magmatic Terrane of the Penokean Orogeny (Schulz and Cannon, 2007). Despite the Flambeau Mine being the only partially extracted VMS deposit in the Penokean orogeny (only the supergene-enriched zone was mined), the volcanic and tectonic setting of the rocks hosting the deposit are poorly constrained. Research on the deposit essentially ceased after the mine closure in 1997. The mine site has since been successfully reclaimed and all bedrock exposures were covered. This study revisits the volcanic strata hosting the Flambeau VMS deposit through examination of historic drill cores to describe the geologic, geochemical and alteration characteristics of the deposit in light of almost 20 years of advances in the fields of geochemistry, economic geology, and the tectonic evolution of the region. Due to the absence of outcrop in the area surrounding the past-producing mine, Flambeau Mine drill cores were obtained and re-logged at the Wisconsin Geological and Natural History Survey core repository, in Mount Horeb, Wisconsin. Core samples were analyzed using X-ray Fluorescence and Inductively Coupled Plasma Mass Spectrometry at the Materials Science Center at UW-Eau Claire. This new geochemical data was compiled with historic mine maps and cross sections to develop a coherent scientific model describing the nature and evolution of the volcanic and hydrothermal system that hosts and formed the deposit. The metamorphosed and recrystallized altered strata hinder the interpretation of the intensity of hydrothermal alteration. Presently, the altered rocks of the Flambeau consist of biotiteandalusite±sericite schists and quartz-sericite-pyrite stringer zones. There is little visual correlation between porphyroblast abundance/size and the degree of alteration. However, major element mobility and geochemical alteration indices emphasize the variable intensities that are present throughout the stratigraphy and potentially highlight several fluid pathways and new ore horizons (Figure 1). Trace elements reveal that the protoliths of the hanging wall strata consist of primarily dacites with interlayered mafic units. The protoliths of the foot wall consist of thick dacite and rhyolite units with local thin mafic units increasing up section. Based on these preliminary geochemical characteristics of the stratigraphy, the Flambeau deposit is likely a series of stacked ore lenses in a rifting arc geodynamic setting where a submarine volcanic arc was undergoing extension and likely developing a back-arc rift (Zens et al., 2015). This is evidenced by the dacite-dominated volcanic pile with increasing abundance of mafic protoliths toward the ore horizon and stratigraphic hanging wall. This data provides invaluable constraints on the petrogenesis of the volcanic assemblages in this part of the Penokean Orogeny. 157 �Figure 1: Downhole profiles of several hydrothermal alteration indices for the volcanic strata hosting the Flambeau Mine compiled using representative holes of the hanging wall (hole 22-2) and foot wall (hole 22-60). AI – Ishikawa Alteration Index (Ishikawa et al., 1976); CCPI – Chlorite-carbonate-pyrite index (Large et al., 2001). REFERENCES DeMatties, T.A., 1994, Early Proterozoic volcanogenic massive sulfide deposits in Wisconsin: An overview. Economic Geology, 89: 1122-1151. Ishikawa, Y., Sawaguchi, T., Iwaya, S. and Horiochi, M. 1976. Delineation of prospecting targets for Kuroko deposits based on models of volcanism of underlying dacite and alteration halos; Mining Geology, v. 26, p.105-117. Large, R.R., Gemmell, J.B., Paulick, H. and Huston, D.L. 2001. The alteration box plot—A simple approach to understanding the relationship between alteration mineralogy and lithogeochemistry associated with volcanic-hosted massive sulfide deposits; Economic Geology, v. 96, p.957–971. LeBerge, G.L. (ed), 1996, Volcanogenic massive sulfide deposits of northern Wisconsin: A commemorative volume. Institute on Lake Superior Geology, Proceedings, 42nd Annual Meeting, Cable, WI, v. 42, part 2, 179 p. Schulz, K.J. and Cannon, W.F., 2007, The Penokean orogeny in the Lake Superior region. Precambrian Research, 157: 4-25. Zens, Z.A., Helmuth, S.L., and Lodge, R.W.D., 2015, Geochemistry and petrography of the strata hosting the Flambeau Cu-Zn-Au Deposit: Revisiting Wisconsin’s only past-producing volcanogenic massive sulfide mine. Geological Society of America, Abstracts with Programs, 47: 5. 158 �Evidence for Episodic Emplacement History of a Nipigon Diabase Sill ZIEG, Michael J. and WALLRICH, Blake M. Department of Geography, Geology, and the Environment, Slippery Rock University, Slippery Rock, PA 16057 The emplacement of magma into a magma chamber is one of the least-understood parts of the igneous cycle. Melting, crystallization, and differentiation can all be simulated experimentally as well as modeled theoretically. Emplacement, however, is an inherently large-scale, dynamic, and complex process that doesn't easily lend itself to experimental simulation or straightforward modeling. By the time an intrusive body is exposed at the surface, emplacement effects are commonly masked by subsequent processes including differentiation and textural reequilibration, making it more difficult yet to recognize and reconstruct emplacement conditions. Despite this difficulty, research on the timing and consequences of magma reinjections into an existing chamber is a dynamic topic of ongoing research. It is now well-accepted that magma chambers grow through incremental inflation (e.g., Menand et al., 2015, and references therein). By supplying new material, including more primitive liquids with or without suspended crystals, they can reset differentiation trends, and potentially deposit cumulate layers (Brandriss et al., 2014). Through a combination of thermal and compositional effects, reinjections into existing magma chambers have also been shown to be crucial in establishing conditions for the formation of magmatic ore bodies (e.g., Charlier et al., 2010; Maier et al., 2013). We are attempting to develop a better understanding of the nature and consequences of these reinjection events using a continuous drill core profile through a ~250 m thick diabase sill from Nipigon, Ontario. A moderately thick sill was chosen for this study because it is large enough to have experienced a complex emplacement history, while also cooling quickly enough to avoid the pervasive textural re-equilibration effects associated with larger intrusions. Using petrographic (plagioclase grain sizes, alignment factors, modal mineralogy, Fig. 1) and compositional (modal mineralogy and bulk-rock geochemistry, Fig. 2) trends in this sill, we have identified multiple horizons that can be tentatively identified as reinjection sites. The intervals interpreted as reinjection horizons are marked by an association between a sudden increase in olivine content and a decrease in plagioclase grain size. Additional characteristics include reversals in fractionation trends and changes in the composition of olivine populations. These horizons are believed to represent the influx of a new pulse of magma that arrives bearing a suspended load of olivine phenocrysts. These phenocrysts settle toward the base of the newly-injected liquid layer, producing an olivine accumulation. Meanwhile, the older, partiallycrystalline mush undergoes textural coarsening due to the heat from the fresh magma, enhancing the textural contrast between rocks crystallized from the old and new magma pulses. Future work will focus on three key questions: 1) what exactly are the diagnostic petrographic and geochemical characteristics of a reinjection horizon? 2) what processes control the development of these signatures? and 3) what can these reinjection horizons tell us about the thermal evolution of mafic sills in general, and this sill in particular? 159 �Figure 1. Petrographic data. Dashed lines represent the locations of inferred reinjection horizons. Figure 2. Compositional data. Dashed lines are the same proposed reinjection horizons, which were identified based on petrographic (primarily textural) variations. REFERENCES Brandriss, M.E., Mason, S., and Winsor, K., 2014, Rhythmic layering formed by deposition of plagioclase phenocrysts from influxes of porphyritic magma in the Cuillin Centre, Isle of Skye: Journal of Petrology, v. 55, p. 1479–1510. Charlier, B., Namur, O., Malpas, S., de Marneffe, C., Duchesne, J.C., Vander Auwera, J. and Bolle, O., 2010, Origin of the giant Allard Lake ilmenite ore deposit (Canada) by fractional crystallization, multiple magma pulses and mixing: Lithos, v. 117, p. 119–134. Maier, W.D., Barnes, S.J. and Groves, D.I., 2013, The Bushveld Complex, South Africa: Formation of plRatinum– palladium, chrome- and vanadium-rich layers via hydrodynamic sorting of a mobilized cumulate slurry in a large, relatively slowly cooling, subsiding magma chamber: Mineralium Deposita, v. 48: p. 1–56. Menand, T., Annen, C., and de-Saint Blanquat, M., 2015, Rates of magma transfer in the crust: Insights into magma reservoir recharge and pluton growth: Geology, v. 43, p. 199–202. 160 � https://digitalcollections.lakeheadu.ca/files/original/363792e794c7e2a9a4b835f47d25c32d.pdf 4e56613284f2d80676e46ab8fcfdb9aa PDF Text Text INSTITUTE ON LAKE SUPERIOR GEOLOGY DULUTH, MINNESOTA MAY 4-8, 2016 HOSTED BY: JAMES D. MILLER JR., UNIV. OF MINN. DULUTH MEETING CHAIRPERSON CHRISTIAN SCHARDT, UNIV. OF MINN. DULUTH DEAN M. PETERSON, PETERSON GEOSCIENCE LLC PROCEEDINGS VOLUME 62 Part 2 Field Trip Guidebook Compiled by Dean M. Peterson (Peterson Geoscience LLC) i �TABLE OF CONTENTS Proceedings Volume 62 Part 2 – Field Trips PRE-MEETING FULL DAY FIELD TRIPS, WEDNESDAY, MAY 4 1) Glacial Geology of the Laurentian Uplands 1 Trip Leaders: Phil Larsen (Vesterheim Geoscience PLC) and Howard Mooers (UMD) 2) Neoarchean Geology of the Western Vermilion District 14 Trip Leaders: Mark Jirsa, Terry Boerboom, and Amy Radakovich (MGS) 3) Cu-Ni-PGE Deposits of the Duluth Complex 27 Trip Leaders: Mark Severson (Teck American), Andrew Ware (PolyMet Mining), and Kevin Boerst (Twin Metals Minnesota), Steve Geerts (UMD-NRRI) PRE-MEETING AFTERNOON FIELD TRIPS, WEDNESDAY, MAY 4 4) Duluth Stream Geomorphology and the June 2012 Flood – CANCELLED 79 Trip Leader: Karen Gran (UMD) 5) Geology of the Endion Sill, Duluth 80 Trip Leader: Jim Miller (UMD) POST-MEETING EVENING FIELD TRIP, FRIDAY, MAY 6 6) Geology and Trout Fishing along Amity Creek, Duluth 102 Trip Leaders: Dean Peterson (Peterson Geoscience) and George Hudak (UMD-NRRI) POST-MEETING FULL-DAY TRIPS, SATURDAY, MAY 7 (and SUNDAY, MAY 8 for TRIP 7) 7) Archean and Proterozoic Geology of the Western Gunflint Trail (Two-Day Trip) 110 Trip Leader: Mark Jirsa (MGS) 8) Keweenawan Geology of the Hovland Area 137 Trip Leaders: Terry Boerboom (MGS) and John Green (UMD) 9) Duluth Harbor Geologic History Boat Cruise: Quaternary to Anthropocene Trip Leaders: Irv Mossberger, Mehgan Blair, Eric Dott (Barr Engineering), Andy Breckenridge (UW-Superior), Todd Kremmin (UMD) ii 160 �FIELD TRIP 1 Wednesday, May 4, 2016 GLACIAL GEOLOGY OF THE LAURENTIAN UPLANDS Phil Larson, Vesterheim Geoscience PLC Howard Mooers, University of Minnesota Duluth with contributions by Margretta Meyer, University of Minnesota Duluth INTRODUCTION The glacial geology of NE Minnesota has been the subject of study for over 100 years and our knowledge of glacial history and chronology have steadily evolved. Within the area that is the focus of this field trip there has been relatively little study. Wright (1956) was first to describe in detail the glacial sediments and landforms. The only detailed investigations since then are the MS theses of Friedman (1981), Fenalon (1986), and Meyer (2009); the map compilation Hobbs et al. (1988); a study of the Rögen moraine by Kryzer (2013). However, the recent availability of high-resolution digital elevation LiDAR has changed the way we map and interpret glacial geology. This field trip will highlight heretofore unrecognized landforms that significantly change our understanding of glacial landform genesis, the history glacial recession, and the nature of subglacial erosional processes. Widespread occurrence of ice-walled lake plains and other subtle ice-stagnation landforms reveal a complex history of ice recession punctuated by numerous ice-marginal stabilizations or minor readvances. These features suggest that the retreating ice must have been relatively debris poor and thin (2-3 meters) sheets of stagnant ice existed over large areas. Recognition of large tracts of Rögen moraine within the Toimi drumlin field the suggest an evolving subglacial erosional landscape that is interpreted as an indication that the subglacial system switched from depositional to erosional at or near the Last Glacial Maximum. The orientation of individual ridges within tracts of Rögen moraine and their association with eskers suggest that these features formed late during deglaciation in area of the glacier bed that were well drained. Lastly, we will highlight aspects of the scoured bedrock surface that allow interpretation of the nature of the depth of glacial scour of bedrock. Saprolite of varying thickness is exposed sporadically throughout the region. These occurrences, often in fractures or other protected settings, indicate that in large part scour of the Precambrian shield was limited by the depth of preglacial weathering. GLACIAL HISTORY Northeastern Minnesota was continuously covered by ice from the earliest Late Wisconsin ice advance approximately 27-29ka (Clayton and Moran, 1982; Mooers and Lehr, 1997) until about 11ka by the Rainy and Superior lobes of the LIS (Fig. 1.). The earliest formal studies of glacial deposits in northeastern Minnesota were conducted by Upham (1894) who identified a series of moraines across Minnesota. He identified the Vermillion moraine, as the 12th moraine, although he did not define its entire length. Winchell (1900), as the first Minnesota state geologist, organized systematic mapping of the glacial geology of Minnesota. Along with Upham and others, Winchell (1899) was one of the first to map large portions of northeastern Minnesota and describe the surficial deposits. Todd (1898) postulated two lobes 1 �of ice, the Lake Superior lobe, which flowed along the axis of Lake Superior, and the Red River lobe, which advanced from the west. Elftman (1898) suggested two lobes for the northeastern portion of Minnesota because of observed till differences and provenances; he named these the Superior and Rainy lobes; the Rainy lobe referring to the ice flowing from the Rainy River area. Figure 1. a-c, General sequence of glaciation modified from Mooers and Lehr (1996). d, landforms of NE Minnesota modified from Lehr and Hobbs (1992). Leverett (1932), based mostly on the work of his predecessors, proposed that northeastern Minnesota was glaciated by three separate lobes of ice. He recognized that the earliest drift in the area was the result of ice flowing from the Patrician [Labradoran] ice center located in the Hudson Bay Lowlands between the Keewatin and Labradorean ice accumulation centers. In terms of the overall glacial history of northeastern Minnesota, the modern understanding began with Wright (1956) who was the first to conduct systematic fieldwork in the area between the border lakes and Lake Superior. Wright (1964) recounted the glacial history of Minnesota as phases of the different ice lobes and was the first to identify and interpret tunnel valleys, drumlins, and eskers (Wright, 1972). In addition to laying out the general glacial geologic framework, Wright (1964, 1972) established the first regional chronology. The late Wisconsin maximum limit of the Rainy lobes was placed at the St. Croix moraine in central Minnesota (Wright 1964) ca. 20,500 BP. Wright (1972) then suggested that the Rainy lobe retreated and readvanced to the Vermilion moraine by about 18,000 BP, however, no evidence of ice recession between these two phases was presented. Little additional work was done in this area until the MS theses of Friedman (1981) and Fenelon (1986) followed by the compilation of the surficial geologic map of the Isabella area by Hobbs et al. (1988). Lehr and Hobbs (1992) outline the glacial history and landforms of the area and 2 �describe the stratigraphy of the Independence Till from three rotasonic cores in the Toimi drumlin field. Since the work of Lehr and Hobbs (1992) the only significant investigations of the glacial geology are those of Meyer (2009) and Kryzer et al. (2013), which focused on the distribution and genesis of Rögen moraine. CHRONOLOGY The chronology of Wright (1972) mentioned above has long been in question. The date of 20,500 for the St. Croix phase is based on the basal radiocarbon date at Wolf Creek, an interdrumlin swale behind the St. Croix moraine (Wright, 1972; Birks, 1976). The Vermilion moraine was correlated with the Mille Lacs/Highland moraine system which was dated ca. 18,000 BP; a date inferred as intermediate between the 20,500 date at Wolf Creek (Wright, 1972; Birks, 1976) and a date of 16,150 BP at Kotiranta Lake associated with the Split Rock phase of the Superior lobe (Wright and Watts, 1969). There are, however, basal radiocarbon dates from lakes in the Toimi drumlin field. Florin and Wright (1969) and Banerjee et al. (1979) report basal dates on aquatic mosses of 14,690 BP at Weber Lake and 16,500 Kylen Lake, respectively. Lowell et al. (2009) got a similar date of 14,050 BP on aquatic moss from the base of a core of nearby Salo Lake. Despite the general agreement of radiocarbon ages from the Toimi drumlin field, the possibility of significant carbonate error exists as the Independence Till is calcareous at depth. DESCRIPTION OF FIELD TRIP STOPS Figure 2. Location of field trip stops. 3 �STOP 1 – Ice-walled Lake Plain in Highland Moraine 564355E/5201830N (UTM Zone 15, NAD83 datum) Fredenberg 7.5’ USGS Quadrangle This site is located at a prominent ice-walled lake plain situated on the crest of the Highland Moraine (Fig. 3). Although composed predominantly of sand and gravel, this sediment is technically glaciolacustrine, deposited in an ice-dammed basin located in an icecored end moraine. The Highland Moraine is composed of hundreds of similar ice-walled lake plains, coalesced to form a belt over 100 km long and as much as 109 km wide; the massive volume of the moraine is a factor of the considerable length of time the Superior Lobe margin stood at this margin, the high sediment flux of the Superior Lobe, or both. Figure 3. Ice-walled lake plain in Highland moraine north of Duluth. 4 �STOP 2 - Superior Lobe Outwash Mantled Over Fluted Rainy Lobe Drift 567180E/5210690N (UTM Zone 15, NAD83 datum) Thompson Lake 7.5’ USGS Quadrangle This site is located on an outwash plain extending westward from the junction of the Rainy and Superior Lobes (Fig. 4). The intersection of two ice lobes forms a trough on the ice surface that focuses both surface and subglacial meltwater and sediment discharge. Sedimentladen meltwater discharge deposited a relatively flat, westward sloping outwash surface. In the vicinity of Stop 2, the outwash plain is partially collapsed, revealing fluted subglacial topography associated with northeast- to southwest-flowing ice of an older Rainy Lobe phase. These relationships indicate that retreat of the active Rainy Lobe ice margin in this area was accompanied by stagnation of a large area of the marginal zone, rather than gradual retreat of the ice margin. Sediment-poor clear stagnant ice rapidly melted until incipient melting of the sediment-rich basal debris layer formed an insulating blanket of supraglacial sediment. The outwash plain was deposited over this relatively thin layer of stagnant ice, later collapsing as the last of the buried ice melted. Figure 4. Collapsed outwash overlying Rainy Lobe stagnant ice topography. Ice margins indicated by dashed line, meltwater flow direction by blue arrows, and Rainy Lobe ice flow direction by black arrow. 5 �The relative elevation difference between the intact surface of the outwash plain and the lowest parts of the collapsed area places an important constraint on the relative thickness of the basal debris layer of the Rainy Lobe ice. The apparent thickness – about 2 m – is consistent with basal debris layer thickness observed in the modern Greenland and Antarctic ice sheets. STOP 3 - Rögen Moraine Superimposed on Drumlins 568050E/522170N (UTM Zone 15, NAD83 datum) Boulder Lake Reservoir NE 7.5’ USGS Quadrangle Stop 3 is at a road cut through one of a series of ice flow-perpendicular sediment ridges known as Rögen moraine (Fig. 5). Rögen are a common occurrence in the Toimi Drumlin field and adjacent up-ice Rainy Lobe terrane. They appear to be the product of remobilization of older subglacial sediment by sliding glacial ice, in this case the underlying drumlins. The ridges themselves are characterized by about 5 m of relief, and are spaced at a characteristic 100 m along flow lines. Here and elsewhere in the Toimi Drumlin field, Rögen moraine shows a close spatial association with subglacial meltwater discharge (the esker). Ice flow-parallel elongate corridors of Rögen moraine commonly flank tunnel valleys and eskers. This suggests that Rögen formed under warmed-bedded conditions near the ice margin, a conclusion distinctly at odds with other models for their formation. It further suggests that periodic fluctuations in basal shear stress associated with annual variations in subglacial meltwater discharge may play a role in Rögen formation, in particular cyclic stick-slip coupling of basal ice to subglacial sediment. Figure 5. Ice-marginal Rögen moraine superimposed on drumlins. Esker is sinuous feature in southeast quadrant; ice flow direction indicated by arrow. 6 �STOP 4 - Toimi Drumlin Field 595550E/5250390N (UTM Zone 15, NAD83 datum) Mount Weber 7.5’ USGS Quadrangle Stop 4 is located at a road cut through one of the hundreds of drumlins that collectively form the Toimi Drumlin field (Fig. 6). This particular drumlin is located near the center of the field. Toward the east and northeast, in the up-ice flow direction, the drumlins show increasing frequency of bedrock cores and are perhaps better described as cragand-tail features. Toward the southwest, in the down-ice flow direction, the drumlins display the characteristic streamlined form. In the near vicinity, borehole records indicate drift thicknesses of 30 to 65 m, predominantly composed of till (Lehr and Hobbs, 1992). Relief on the drumlins is about 30 m, suggesting the drumlin field is composed of a somewhat discontinuous layer of drift. Significantly, deeper tills encounrtered in borehole are weakly to moderately calcareous. Figure 6. Typical Toimi drumlins, crag and tail features in the southeast quadrant. 7 �STOP 5 - Thermokarst in Sediment-poor Stagnation Moraine 602510E/5250800N (UTM Zone 15, NAD83 datum) Mount Weber 7.5’ USGS Quadrangle This stop is located in an ice flow-perpendicular belt characterized by thin ice-walled lake plains (pannukaku) (Fig. 7). Pannukaku typically show as little as 1 m of relief with their surroundings, and are commonly draped over subglacial topography. Belts of pannukaku in the Toimi Drumlin field define minor surgestagnation moraines deposited by the retreating Rainy Lobe. The relatively small volume of sediment contained in these moraines is a reflection of relatively low sediment flux on the part of the warmedbedded Rainy Lobe. Figure 7. Rainy Lobe surge-stagnation moraine, defined by belt of pannukaku. Ice margin defined by dashed line, pannukaku by thin dashed outlines. 8 �STOP 6 - Superior Lobe Outwash 603980E/5280650N (UTM Zone 15, NAD83 datum) Slate Lake East 7.5’ USGS Quadrangle Stop 6 is located at a road cut through Superior Lobe sand and gravel, deposited by meltwater flowing from southeast to northwest (Fig. 8). Following retreat of the Rainy Lobe ice margin north of the Laurentian Upland, meltwater from the Superior Lobe was able to flow northwest into Glacial Lake Dunka and ultimately Glacial Lake Norwood. This meltwater deposited a distinct tongue of Superior Lobe-provenance sand and gravel cross-cutting continuous Rainy Lobe drift. Figure 8. Superior Lobe meltwater channel cross-cutting older Rainy Lobe drift. 9 �STOP 7 - Relict Pre-glacial Saprolite 601510E/5289820N (UTM Zone 15, NAD83 datum) Bogberry Lake 7.5’ USGS Quadrangle Much of what is commonly thought of as typical ‘glacially sculpted’ rugged shield topography is better explained as the morphology of the base of a pre-glacial saprolite (Feininger, 1971). This stop highlights relict pre-glacial saprolite exposed in a road cut during recent (2013) highway reconstruction. STOP 8 - Ice-contact Outwash Fan 615910E/5284460N (UTM Zone 15, NAD83 datum) Mitawan Lake 7.5’ USGS Quadrangle Stop 8 is located on an ice-contact outwash fan formed on a stagnant Rainy Lobe ice margin (Fig. 9). The fan itself is composed largely of sand and gravel, with some gravels at the proximal head of the fan approaching >1 m in mean grain size. Esker-like segments in the fan suggest deposition on stagnant ice. The volume and coarse-grained nature of the sediment suggest a highly energetic, high discharge meltwater system. A number of such fans are evident in the area, suggesting frequent reorganization of a broad zone of stagnant ice at the Rainy Lobe margin. Figure 9. Ice-contact Rainy Lobe outwash fan. 10 �STOP 9 - Rögen Moraine 627670E/5284350N (UTM Zone 15, NAD83 datum) Sawbill Landing 7.5’ USGS Quadrangle Stop 9 is located in a road cut through a single Rögen moraine, described by Meyer (2009) (Fig. 10). The Rögen is composed of exceedingly dense till, and is part of a system of Rögen moraine characterized by 5-10 m of relief spaced 300-400 m apart along flowlines. In this respect, these Rögen, situated at the very up-ice limit of the Toimi Drumlin field, are significantly different from the lower amplitude, shorter wavelength Rögen characteristic of the southwestern portion. Ground-probing radar profiles through this and other Rögen ridges in the vicinity display structures evocative of northeast-dipping foresets, onlapping to the south (Fig. 11). These features suggest that Rögen may form by erosion of lee-side sediment and re-deposition on the stoss side of down-ice ridges, akin to migrating dune forms in fluvial systems. In this respect, Rögen may ‘migrate’ up-ice under flowing ice as the overall subglacial surface is lowered by net erosion. Figure 10. Rögen moraine near Sawbill Landing. 11 �Figure 11. Ground penetrating radar (GPR) profile of a Rögen ridge near Sawbill Landing. Vertical scale is approximate based on radar two-way travel time. Ice flow direction is from left to right. REFERENCES Banerjee, S.K., and Lund, S.P., 1979, Geomagnetic record in Minnesota lake sediments - Absence of the Gothenburg and Erieau excursions: Geology, v. 7, p. 588–591. Birks, H.J.B., 1976, Late Wisconsinan vegetational history at Wolf Creek, central Minnesota: Ecological Monographs v. 46, p. 395-429. Birks, H.J.B., 1981, Late Wisconsin vegetational and climatic history at Kylen Lake, northeastern Minnesota: Quaternary Research, v. 16, p. 222–355. Björck, S., 1990, Late Wisconsin History North of the Giants Range, Northern Minnesota, Inferred from Complex Stratigraphy: Quaternary Research, v. 33, p. 18–36. Brown, T.R., 1988, Eskers and heavy mineral prospecting, northeastern Minnesota: M.S. Thesis, p. 103 p. Buchheit, R.L., Malmquist, K.L., and Niebuhr, J.R., 1989, Glacial Drift Geochemistry for Strategic Minerals; Duluth Complex, Lake County, Minnesota: Minnesota Department of Natural Resource Division of Lands and Minerals Project, v. 262, no. part I, p. 95 p. Clayton, L, and Moran, S.R., 1982, Chronology of late Wisconsin glaciation in middle North America. Quaternary Science Reviews, v.1, pp55-82. Elftman, A.H., 1898, The geology of the Keweenawan Area in northeastern Minnesota, Part I.: The American Geologist, v. 21, p. 90–109. Eyles, N., Putkinen, N., Sookhan, S., and Arbelaez-Moreno, L., 2016, Erosional origin of drumlins and megaridges: Sedimentary Geology, doi: 10.1016/j.sedgeo.2016.01.006. Fenelon, J.M., 1986, Glacial geology of the Cramer quadrangle, northeastern Minnesota. [M.S. Thesis]: Milwaukee, University of Wisconsin, 76pp. Florin, M.B. and Wright, H.E., 1969. Diatom evidence for the persistence of stagnant glacial ice in Minnesota, Geological Society of America Bulletin, 80(4), 695-704. Friedman, A.L, 1981, Surficial geology of the Isabella quadrangle, northeastern Minnesota [M.S. thesis]: Minneapolis, University of Minnesota, 66pp. 12 �Friedrich, H.G., 2011, Assessment of Sand and Gravel and Clay Deposits in Parts of Northern St. Louis and Lake Counties: Minnesota Department of Natural Resource Division of Lands and Minerals Project, v. 380, p. 1– 47.Fries, M., 1962, Pollen Profiles of Late Pleistocene and Recent Sediments from Weber Lake, Northeastern Minnesota: Ecology, v. 43, no. 2, p. 295–308. Hobbs, H.C., Friedman, A.L., Fenelon, J.M., and Stark, J.R., 1988, Surficial Geologic Map of the Greenwood Lake, Isabella, and Cramer Quadrangles, Minnesota: Minnesota Geological Survey Open-File Report, v. 88-02. Johnson, M.D., Adams, R.S., Gowan, A.S., Harris, K.L., Hobbs, H.C., Jennings, C.E., Knaeble, A.R., Lusardi, B.A., and Meyer, G.N., 2016, Quaternary lithostratigraphic units of Minnesota: Minnesota Geological Survey Report of Investigations RI-68. Kryzer, R., Mooers, H.D., and Larson, 2013, Rögen moraine as a transitional bedform in an erosional subglacial system: Geological Society of American Abstracts with Programs, v. 45, no. 7, p. 119. Lehr, J.D., and Hobbs, H., 1992, Field Trip Guidebook for the Glacial Geology of the Laurentian Divide Area, St. Louis and Lake Counties, Minnesota: Minnesota Geological Survey Guidebook Series 18. Leverett, F., 1932, Quaternary Geology of Minnesota and Parts of Adjacent States: USGS Professional Paper, v. 161, 149 pp. Lowell, T. V., Fisher, T.G., Hajdas, I., Glover, K., Loope, H.M., and Henry, T., 2009, Radiocarbon deglaciation chronology of the Thunder Bay, Ontario area and implications for ice sheet retreat patterns: Quaternary Science Reviews, v. 28, no. 17-18, p. 1597–1607, doi: 10.1016/j.quascirev.2009.02.025. Lund, S.P., and Banerjee, S.K., 1985, Late Quaternary paleomagnetic field secular variation from two Minnesota Lakes: Journal of Geophysical Research: Solid Earth, v. 90, no. B1, p. 803–825, doi: 10.1029/JB090iB01p00803. Martin, D., and Eng, M., 1985, Esker Prospecting Over the Duluth Complex in Northeastern Minnesota: Minnesota Department of Natural Resource Division of Lands and Minerals Project, v. 246, p. 27. Meyer, M.S., 2009, Paleoglaciological context of Rögen moraine, northeastern Minnesota: University of Minnesota Duluth, 71 p. Mooers, H.D., and Lehr, J.D., 1997, Terrestrial record of Laurentide ice sheet reorganization during Heinrich events: Geology v. 25, p. 987-990. Todd, J.E., 1898, A revision of the moraines of Minnesota: American Journal of Science (ser. 4), v. 6, p. 469-477. Upham, W., 1894, Preliminary report of the field work during 1893 in northeastern Minnesota, chiefly relating to the glacial drift, in Winchell, N. H., Geological and Natural History Survey of Minnesota, 22nd Annual Report, for the year 1893, pp.18-86. Winchell, N.H., 1899, The geology of the north part of St. Louis County, in Winchell, N.H., The geology of Minnesota, Volume IV of the final report: Geological and Natural History Survey of Minnesota, p. 222265. Winchell, N.H., 1900, Glacial Lakes of Minnesota. Geological Society of America Bulletin v. 12, p. 109-128. Winter, T.C., 1971, Sequence of Glaciation in the Mesabi-Vermilion Iron Range Area, Northeastern Minnesota: USGS Professional Paper, v. 750-C, p. C82–C88. Wright, H.E., and Watts, W.A., 1969, Glacial and Vegetational History of Northeastern Minnesota: Minnesota Geological Survey Special Publication, v. 11, 59 pp. Wright, H. E. Jr., 1964, The classification of the Wisconsin Glacial Stage. Journal of Geology 72, 628-637. Wright, H.E., 1972, Quaternary history of Minnesota, in Sims, P.K. and Morey, G.B. eds., Geology of Minnesota: A Centennial Volume, St. Paul, Minnesota, p. 515–547. Wright, H.E., 1956, Sequence of glaciation in eastern Minnesota: Geological Society of America Guidebook for Field Trips, v. 3, p. 1–24. 13 �FIELD TRIP 2 Wednesday, May 4, 2016 NEOARCHEAN GEOLOGY OF THE WESTERN VERMILION DISTRICT Mark Jirsa, Terry Boerboom, and Amy Radakovich Minnesota Geological Survey Figure 1. Regional bedrock geologic map showing locations of field trip stops (squares numbered 1-10), published geochronologic sample sites (stars), and pertinent geologic features. Modified from Jirsa and others, 2012. INTRODUCTION This field trip takes a preliminary look at stratigraphic relationships between older largely volcanic rocks inferred to be equivalent to the Ely Greenstone (~2720 Ma) and the apparently 30 Ma younger, volcanic, volcaniclastic, and epiclastic sedimentary strata of the Lake Vermilion Formation. New mapping supports recently acquired geochronologic data that indicates sediments of the Lake Vermilion Formation were deposited unconformably on the variably weathered, “deep-water” volcanic rocks. The significant hiatus, contrasting depositional environments, evidence for magmatism synchronous with sedimentation, and local unconformable contacts between the two units implies that the Lake Vermilion Formation formed in a late-tectonic extensional basin. 14 �The field guide is brief, primarily for expediency, but also reflecting the tentative nature of newly acquired outcrop information on which it is based. The field work was the first phase of a multi-year effort by the Minnesota Geological Survey to create geologic atlases of St. Louis and Lake Counties— two of the largest counties in Minnesota. It was supported by a grant from the U.S. Geological Survey STATEMAP element of the National Geologic Mapping program, and by the Minnesota Environmental and Natural Resources Trust Fund. Although geochronologic analyses were conducted as part of this mapping, the new data are not yet ready for publication. GEOLOGIC SETTING The traditional definition of the Neoarchean Lake Vermilion Formation describes the unit as complexly interbedded strata that vary from felsic volcaniclastic rocks, to rocks having evidence of reworking, to mixed-source graywacke-siltstone. The informally named Gafvert Lake sequence (Fig. 1) consists of quartz- and plagioclase-phyric, dacitic to rhyodacitic breccia and tuff that yielded a 207Pb/206Pb age of 2689.7±0.8 Ma from magmatic zircons (Lodge and others, 2013). The sequence has been inferred to lie disconformably atop the Soudan Iron Formation member of the Ely Greenstone. Previous mapping (Jirsa and others, 2001) demonstrated that the iron-formation is transitional with metabasaltic rocks of the Lower Ely Greenstone, and a felsic unit within the greenstone yielded an age of 2722±0.9 Ma (Peterson and others, 2001). Thus, the Gafvert Lake sequence is approximately 30 Ma younger than the subjacent metabasalt- and iron-formation-bearing rocks. Quartzofeldspathic sediments apparently derived from the Gafvert Lake sequence make up a variable, but locally large proportion of the detritus in the Lake Vermilion Formation. Regionally, a series of outcrops from Gafvert Lake westward shows an irregular transition from proximal, perhaps subaerial deposition on the east, to distal submarine turbiditic fan deposition to the west. On this basis, the Gafvert is now considered part of the Lake Vermilion Formation, which by extension is also 30 Ma younger than greenstone. Recent field work was conducted in part to explore lithologic attributes of the Lake Vermilion Formation and ascertain the nature of contacts between these strata and the older greenstone. This field trip examines outcrop evidence that we believe documents the unconformable nature of these strata outboard of the Ely Greenstone. The evidence acquired to date is consistent with the inference that the Lake Vermilion Formation represents deposition in a Timiskaming-type successor basin, much like the equivalent Knife Lake Group to the northeast (Driese and others, 2011; Jirsa and others, in prep.) and the Midway sequence to the southwest (Jirsa, 2000). The Ely Greenstone and Lake Vermilion Formation—as defined here—are part of the Wawa subprovince of the Superior Province. Temporal distinctions between various geologic components of this terrane are evolving with new geochronologic analyses. Nevertheless, they remain largely based on fabrics and structures that resulted from three major phases of deformation, denoted D1, D2, and D3. The D1 event involved generally pre-lithification deformation of graywacke sequences (in some localities forming large nappe structures), and tilting, broad folding, and thrust imbrication of the thick, more rigid volcanic strata. D2 deformation accompanied regional metamorphism to greenschist to amphibolite facies, and produced pervasive metamorphic foliation and lineation, folding, and strike-slip faulting. U-Pb dates of intrusions bracket the D2 event between about 2,674 and 2,685 Ma (Boerboom and Zartman, 1993). The TowerSoudan anticline (Fig. 1) is considered a D1 structure because both limbs of the complex fold are transected by D2 cleavage that trends more northeasterly than bedding. Deposition of the Lake Vermilion Formation is bracketed to an approximately 10 million year period between volcanism of the Gafvert Lake sequence at ca. 2690 Ma, and its deformation during D2 at ca. 2680 Ma. D3 is assigned to partitioned deformation that produced crenulation and faults within rocks affected by D2. All three deformation events can be attributed to variably north-northwest—south-southeast-directed transpression. 15 �FIELD TRIP STOP DESCRIPTIONS NOTE: all UTM coordinates are given in NAD 83, Zone 15 STOP 1 – Felsic pyroclastic breccia and tuff—Gafvert Lake sequence; Lake Vermilion Formation Location: UTM: 0553467E/5294482N, Highways 1 and 169, west edge of village of Tower. Description: The Gafvert Lake sequence consists of dacitic to rhyodacitic lava flows and pyroclastic rocks to the northeast that are more or less transitional with volcaniclastic strata to the west. This outcrop lies in the transition zone. It is poorly sorted and contains angular to subrounded clasts of dacitic composition (plagioclase and quartz-phyric) that range in size from several millimeters to 20 cm. Rare angular to ornate clasts of pyrrhotite and pyrite imply a pyroclastic origin. However, the presence of siltstone clasts locally, the subrounded nature of some dacitic fragments, and the rare appearance of bedding imply reworking has occurred. Figure 2. Poorly sorted fragmental rock containing clasts of quartz- and plagioclase-phyric dacite, rusty sulfides, and rare black mudstone. [Field station LS046] DIRECTIONS: Continue west ~4 mi. along highways 1 and 169 to highway 77, turn north and proceed ~ 0.5 miles and cross Pike River to pull-off on west (left) side of highway. STOP 2 – Feldspathic greywacke-mudstone of Lake Vermilion Formation—the “classic” outcrop Location: UTM: 0547272E/5293368N; west side of Hwy. 77, north side of Pike River near dam. Description: This glacially scoured outcrop exposes a nearly perfect cross-section of tabular-bedded, variably graded, feldspathic graywacke and dark gray slate. The feldspar-rich, dacitic composition of the sandy textured beds is presumed to represent derivation from the Gafvert Lake sequence exposed to the east (Stop 1). The beds contain numerous “soft-sediment” deformation features including load structures, flames, intrafolial slump folds, and growth-faults. Bedding is nearly vertical, and graded beds indicate younging to the south. This topping direction is consistent with a position on the south limb of a large, south-overturned regional D1 fold structure—inferred to be the western extension of the Tower-Soudan Anticline (Jirsa and Boerboom, 2003). Northeast-trending kink bands, fault zones, and quartz veins traversing the outcrop are Figure 3. Road cut exposing white tonalite assigned to the latest, D3 deformation event. dike with large inclusions of adjacent black siltstone. Note 40 cm-long hammer in leftcenter of photo for scale. [Field station LS051] DIRECTIONS: Walk/drive north ~700 feet to road cut on east (right) side of Highway 77 16 �STOP 3 – Tonalitic dikes cutting sedimentary strata of the Lake Vermilion Formation Location: UTM: 0547193E/5293565N; road cut on east side of County Rd 77 Description: On first glance, this outcrop appears to represent chaotic dacitic (arkosic) sedimentary strata interbedded with gray sandstone and black siltstone, compositionally similar to that at Stops1 and 2. Graded sandstone-siltstone couplets indicate stratigraphic facing to the south, as at stop 2. Closer inspection reveals the “dacitic” rocks are tonalitic dikes emplaced more or less along bedding planes of the enclosing sandstone-siltstone. Discordance is only evident locally. Tonalitic dikes are fine- to mediumgrained, equigranular, and contain abundant quartz. The intrusion suspended numerous angular to subangular xenolithic inclusions of mudstone and siltstone as large as 1m in diameter. Many of the inclusions have dark, presumably contact metamorphic rinds. A smaller tonalitic intrusion exposed in the northern part of the outcrop is cut by a 2-meter wide lamprophyric dike. Figure 4. Graded beds of white arkosic sandstone and feldspathic graywacke, black siltstone and mudstone. Stratigraphic facing is up in the photo (southward). DIRECTIONS: Return to vehicle near Pike River; drive north on Hwy 77 approximately 1.3 mi. to County Road 104 (Bois Forte or Vermilion Reservation Rd.); drive east ~0.7 mi. to Waters if Vermilion Rd. on the south; drive south ~0.2 mi. to STOP 4. STOP 4 – Arkosic sandstone and siltstone of Lake Vermilion Formation Location: UTM: 0548130E/5294880N; PRIVATE PROPERTY! Description: This apparently blasted and cleaned outcrop consists of white, coarse to fine grained arkose, with rare gray-black siltstone layers. The arkose is inferred to have been sourced from the Gafvert Lake sequence. The exposure provides a 3-dimensional view of some inferred depositional, dewatering, and compaction features. Where it can be determined, stratigraphic facing is southward as at previous stops 2 and 3. The remarkable similarity between the arkose here and the tonalitic dikes at stop 3 invite correlation and a preliminary interpretation that magmatism was synchronous with sedimentation. A. B. Figure 5. A. White arkose with rare lenses, layers, and fragments of black siltstone (Amy Radakovich for scale). B. Ball and pillow structures inferred to have formed during settling and dewatering of inferred mass flow deposit. [Field station LS056, “Majestic Rocks” development] 17 �DIRECTIONS: Return to Highway 77, turn north (right) and proceed 0.6 miles to road cut on east side of highway. STOP 5 – Mixed-source graywacke of Lake Vermilion Formation cut by quartz-plagioclase (tonalitic) dikes Location: UTM: 0547145E/5296230N; Long road cut on east side of Highway 77. Description: Complex exposure of mixed-source graywacke cut by several fine- to mediumgrained quartzofeldspathic (tonalitic) dikes. The southern third of the roadcut consists entirely of tonalite. Normal faulting is apparent locally. Stratigraphic facing is northward (Fig. 6A)—the inverse of that at prior stops 2-4—which reflects a geographic position north of an inferred D1 antiformal nappe structure that may be the western extension of the Tower-Soudan anticline (see Jirsa and Boerboom, 2003). A B. Figure 6. A. Graded bedding in mixed-source feldspathic graywacke-siltstone; stratigraphic facing is to the left in photo (north in outcrop). B. Mixed source graywacke-siltstone cut by one of several tonalitic dike dipping to right (south in outcrop). [Field station LS058] DIRECTIONS: Continue north on Highway 77 for ~0.5 miles to road cut on east side of highway. STOP 6 – Large D2 folds and shear zones in mixed-source graywacke of Lake Vermilion Formation Location: UTM: 0546970E/5297030N; Road cut on east side of Highway 77. Description: Long road cut that exposes complex shearing and folding in mixed-source graywackesiltstone beds. Shearing is manifest as ankeritesericite-chlorite phyllite near the south end of the cut. Folds exposed farther north along the road cut are tight to isoclinal synforms and antiforms having D2 fold axes that are steeply dipping. Considerable rodding lineation of more competent sandy beds indicates shallow plunge (~47º) to the east (away from the viewer). Figure 7. D2 folds (dashed white lines) of thinly bedded graywacke-siltstone [Field station LS059] 18 �DIRECTIONS: To STOP 7A—U-turn to head south on Highway 77 for ~1.4 mi. to Lost Lake Road; turn west (right) and proceed 0.8 miles to gated trail entry. To STOP 7B—drive west on Lost Lake Road ~0.2 mi. and turn south (left) on Holter Road and proceed 0.2 mi. to shallow gravel pit on east side of road. NOTE: BOTH STOPS ON PRIVATE PROPERTY. STOPS 7A, 7B –Conglomerate with abundant volcanic and sulfidic clasts similar to Stop 1 Locations: A. UTM: 0545650E/5294580N; B. 0545365E/5294515N; both exposures on floors of shallow gravel pits—PRIVATE PROPERTY! Description: These two stops (7A and B) expose slightly different versions of conglomerate in an area essentially surrounded by arkosic and mixed-source sedimentary strata similar to stops 2-6. Both stops are glacially polished outcrops of clast-supported conglomerate. The conglomerate contains abundant felsic to intermediate volcanic fragments, together with clasts of layered siliceous rock, dacitic porphyry, and sulfide-rich rock. Clasts vary from rounded to subangular. The overall composition of fragments and the presence of sulfide clasts indicates a potential correlation with volcanic strata of the Gafvert Lake sequence as seen at stop 1, which we interpret to lie stratigraphically beneath the exposures at stops 2-6. On this basis, we infer that the conglomerate represents a localized uplift of the basin floor on which other sediments of the Lake Vermilion Formation were deposited. This is consistent with a structural position near the axis of an antiformal D1 nappe structure inferred to be the western extension of the Tower-Soudan anticline (as shown on Jirsa and Boerboom, 2003). Figure 8. Lithologically diverse conglomerate (including abundant sulfide clasts on right side) at STOP 7A. [Field station LS141] DIRECTIONS: Continue south on Holter road ~1 mi. to Highway 1; turn east on Hwy 1 and proceed 1.5 miles to junction with Highway 169; turn southwest (right) on 169 and travel 0.8 miles to Peyla Road; turn east and travel to STOP 8 described below. Access will depend on road conditions, and several scattered exposures off Peyla Road will be examined. STOP 8 – Peyla sequence – basalt, conglomerate, and sandstone Location: UTM: 0549822E/5292233N and environs, Tower quadrangle, Peyla Road east of Highway 169. Description: Near the end of Peyla Road is a series of outcrops of pillow basalt overlain by mafic conglomerate interbedded with sandstone (Figure 9). Published geologic maps (Ojakangas and others, 1978, Sims and Southwick, 1985, Southwick, 1993) show the basalts but do not distinguish the conglomerate and sandstone from the typical graywacke of the Lake Vermilion Formation. The Peyla conglomerate and interbedded sandstone overlie variably variolitic pillow basalt (Figure 9). Clasts in the conglomerate range from less than 1 cm to as much as 40 cm in size and are very angular; in fact the term ‘sedimentary breccia’ might be more appropriate in most cases. Topping indicators in both the basalts and sediments are difficult to find. 19 �The conglomerate (Figure 10A) is dominated by fine-grained basalt clasts that include weakly porphyritic, variolitic, and amygdaloidal phases. Medium-grained clasts of metagabbro/lamprophyre are common and in a few places predominant; other less common clast types include fine-grained possibly tuffaceous felsic rocks, and rare clasts of sulfides (pyrite) and hornblende-phyric andesitic hypabyssal intrusive rocks (Figure 10B). The matrix is similar to the adjacent sandstone. The varied types of basalt clasts (amygdaloidal, massive, variolitic, and porphyritic) imply reworking of the basalt substrate, and the polymictic nature of the conglomerate leads to the inference that this is a “Timiskaming-type” sedimentary package. Figure 9. Preliminary geologic map of the Peyla sequence on a 1 m lidar base. Most of fringing area has not been re-examined as part of recent field work, and the full extent of Timiskaming-type sandstone and conglomerate has not been established. The units shown here as green sandstone and conglomerate had not been distinguished on prior published maps, thus the fringing area is subject to revision by future mapping endeavors. 20 �B A Figure 10. Photograph (A; scale in cm) and photomicrograph (B) of typical Peyla conglomerate. Photomicrograph shows the edge of a basalt clast (B), a clast of hornblende-plagioclase porphyry (HAP), and hornblende (H) and plagioclase crystals (P) in the sandy matrix, which also contains minor quartz (Q). Lithic clasts outlined by white dashed line. Scale bar= 1 mm. The photomicrograph is from a different sample than that shown in the left photo. The ‘green sandstone’ (field term) interbedded with the conglomerate contains detrital plagioclase which commonly exhibits blocky and broken shapes, minor quartz derived from a volcanic source, blocky to euhedral detrital hornblende, small mafic to felsic volcanic rocks fragments, and rare detrital sphene and apatite along with metamorphic epidote, hornblende, biotite. The sandstone is typically quite massive and poorly bedded. The Peyla basalts are commonly variolitic, with irregular pillow shapes that commonly don’t yield reliable topping indicators. Locally the interiors of the pillows exhibit an incipient state of brecciation (Figure 11), outlining fragments similar in size and shape to those in the conglomerate. The reason for this brecciation is not known, but speculatively may be due to weathering and paleosol development prior to or during deposition of the overlying conglomerate and sandstone. Further development of this fracturing/brecciation process may have produced disaggregated fragments of angular basalt that were then shed into the adjacent sediments. Figure 11. Incipient brecciation in basalt. The sizes and shapes of these fragments are similar to the angular basalt clasts in the overlying mafic conglomerate. The white veining between the fragments is composed of clinozoisite, quartz, and carbonate. 21 �Mafic lamprophyric dikes, typically less than 2 meters wide and with sharp straight edges, intrude the sedimentary rocks, but none have been noted in the basalts so far. Multiple generations of lamprophyric dikes are visible on some outcrops, as well as rare instances of apparent lamprophyric peperite (Figure 12). The presence of mafic peperite and the clasts of hornblende-rich mafic intrusive rocks in the conglomerate imply that the lamprophyres may be related to subalkalic igneous activity contemporaneous with deposition of the sedimentary rocks. In contrast, very few if any mafic/lamprophyric dikes were noted in the Peyla basalts, based on the mapping completed thus far. The reason for this is not clear, but could be that the basalts were relatively impermeable to the dikes compared to the overlying sedimentary rocks. Figure 12. Thin dark green lamprophyric intrusion; on one side is in sharp, straight contact with the conglomerate and on the other side is diffuse and peperitic. DIRECTIONS: Return to Highway 169 and continue south. Drive ~2 miles from Peyla road to Flaim road, which is at the south end of a long road cut. Turn right (west) on Flaim road and park (STOP 9). STOP 9 – Sandstone, pillowed basalt, and lamprophyric dikes. Location: UTM: 0546116E/5287910N (Intersection of Flaim Road and Hwy. 169; Park on Flaim Road). Note: the road shown on the published Biwabik NW quadrangle is the old road which has been straightened and now cuts further west. Description: This is a new road cut that runs north-south parallel to Highway 169, and along the north side of Flaim Road west of Highway 169. It exposes metasedimentary strata on the south, and metavolcanic strata on the north, and mafic dikes emplaced into both rock types. Start on the westernmost outcrops along the north side of Flaim road and work east toward the highway. This glacially polished flat outcrop consists primarily of light tan-colored, fine-grained sandstone, having weakly graded beds that indicate southward stratigraphic facing. The sedimentary strata were intruded by 2 lamprophyric/dioritic dikes. Both the dikes and the bedding in the sedimentary strata dip steeply. One of the dikes contains xenoliths of varied rock types that range from 5 to 60 cm in size, and include layered 22 �tonalitic gneiss, porphyritic dacitic volcanic rocks, granodioritic to tonalitic intrusive rocks, and mafic schist. Adjacent to (south of) the inclusion-rich lamprophyre is a >30 cm thick dike of hornblende- and plagioclase-phyric diorite. This dike contains abundant phenocrysts of equant hornblende and blocky plagioclase as large as 3mm (Figure 13A), and small apatite crystals visible only in thin section. A B Figure 13. Plane light photographs of thin sections of samples from this stop (sections are approximately 2.5 cm wide). A – Hornblende- and plagioclase-phyric lamprophyric dike that intruded sedimentary strata. B – Lamprophyric dike that intruded pillow basalt showing trachytoid alignment of euhedral hornblende crystals. Continue walking east to Highway 169 along the sandstone outcrop, then go north along the freshly blasted roadcut. It is difficult to distinguish between the sandstone and subjacent basalt on the fresh rock faces; however, the transition is marked by a rusty interval that is visible from a distance (look to the outcrops across the highway). Two cherty horizons occur within the basalt– a layer that is orange in color, and further north a black chert horizon that contains thin layers of pyrite. Numerous dark green 0.5 – 10 m thick lamprophyric/mafic dikes are present within the basalt. Continue to north end of roadcut, climb up to top, and walk back south. Here on the glacially polished exposure one can see pillowed metabasalt and the mafic dikes emplaced into them. The dikes are weakly hornblende-phyric (Figure 13B), have chilled margins, and appear to be less deformed than the basaltic host. One highly unusual feature in the pillow basalt is the presence of two 7 cm rounded xenoliths of pink, medium-grained granite (Figure 14). The nearly vertical contact between the pillowed metabasalt to the north and adjacent metasedimentary strata to the south is fairly straight, abrupt, and lacks evidence for shearing that might indicate a fault origin. Although the contact zone is quite rusty (presumably pyritic), the contact is unremarkable. Nevertheless, the contrast in apparent depositional environments, and the abrupt termination of pillowed basalt (i.e., no gradation of pillows to flow-top breccia) implies that the contact is an unconformity. 23 �Figure 14. Rounded xenolith of pink, medium-grained granite in pillow basalt, on upper flat outcrop surface. UTM coordinates notes as 546129, 5288055 (Nad83 zone 15). STOP 10 – Conglomerate and metagraywacke, mafic dikes. Location: UTM: 0546327E/5286624N (Hwy. 26 0.6 mi east of Hwy. 169). Description: Highly flattened/lineated multi-lithic conglomerate interbedded with tightly folded graywacke, which is cut by multiple north-dipping, weakly foliated mafic/lamprophyric dikes. The conglomerate contains 1-10cm (longest dimension in end view) clasts that are also lineated (foliation N85°E, 70°N; lineation plunge 45° to N70°E). The clasts show weak size grading, and vary from felsic (light grayish-tan to pink and fine-grained) to mafic (dark green plagioclase-phyric, hornblende-rich; Figure 15). The matrix contains little if any quartz, and is generally dark green and amphibolitic; however it is commonly difficult to distinguish between matrix and pseudomatrix (i.e., flattened, less competent clasts). The surface of the conglomerate outcrops has a patchy pink staining, which is likely due to abundant microcline. In thin section the fine-grained light-colored clasts are composed predominantly of finely granoblastic microcline, with some larger (but less than 1mm) grains of irregularly-shaped plagioclase that may have been phenocrysts, and little if any quartz. Other clasts cannot be distinguished from matrix/pseudomatrix. Some of the clasts are more coarse-grained and microcline-rich, and may in part be granoblasticrecrystallized syenite. The matrix is composed of granoblastic plagioclase and metamorphic amphibole, lesser K-feldspar, and minor carbonate, and has small blocky, saussuritized plagioclase crystals. The metagraywacke in the western portion of this outcrop exhibits some weakly graded beds which mostly indicate south topping, thus are slightly overturned. Fine-grained amphibolitic lenses are present in the graywacke, and overall it has a slight orange stain due to finely disseminated pyrite. The mafic dikes here are likely of the same timing as those which cut the pillow basalts at stop 9. 24 �Figure 15. Strongly flattened and lineated dark green mafic-intermediate plagioclase-phyric clasts and smaller fine-grained, light-colored clasts in conglomerate. REFERENCES Boerboom, T.J., and Zartman, R.E., 1993, Geology, geochemistry, and geochronology of the central Giants Range batholith, northeastern Minnesota: Canadian Journal of Earth Science, v.30, p. 2510-2522. Driese, S.G., Jirsa, M.A., Ren, M., Sheldon, N.D., Brantley, S.L., Parker, D., and Schmitz, M., 2011, Neoarchean paleoweathering of tonalite and metabasalt: Implications for reconstructions of 2.69 Ga early terrestrial ecosystems and paleoatmospheric chemistry: Precambrian Research, v. 189, p. 1-1. Jirsa, M.A., 2000, The Midway sequence: a Timiskaming-type, pull-apart basin deposit in the western Wawa subprovince, Minnesota: Canadian Journal of Earth Sciences, 37:1-15. Jirsa, M.A., and Boerboom, T.J., 2003, Bedrock geology of the Vermilion Lake 30’ X 60’ quadrangle, northeast Minnesota: Minnesota Geological Survey Miscellaneous Map M-141; scale 1:100,000. Jirsa, M.A., Boerboom, T.J., and Chandler, V.W., 2012, Geologic map of Minnesota: Precambrian bedrock: Minnesota Geological Survey State Map Series S-22, scale 1:500,000. Jirsa, M.A., and Boerboom, T.J., and Peterson, D.M., 2001, Bedrock geologic map of the Eagles Nest quadrangle, St Louis County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-114; scale 1:24,000. Jirsa, M.A., Starns, E., and Schmitz, M.D., in prep., Bedrock geologic map of the Cavity Lake fire area, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map M-193, scale 1:24,000 [in preparation—in the interim, refer to MGS Open-File Report OF-08-05, or contact lead author for details]. 25 �Lodge, R.W.D., Gibson, H.L., Stott, G.M., Hudak, G.J., and Jirsa, M.A., and Hamilton, M.A., 2013, New U-Pb geochronology from Timiskaming-type assemblages in the Shebandowan and Vermilion greenstone belts, Wawa subprovince, Superior Craton: Implications for the Neoarchean development of the southwestern Superior Province: Precambrian Research v. 235, p. 264-277. Ojakangas, R.W., Sims, P.K., and Hooper, P.R., 1978, Geologic map of the Tower quadrangle, St. Louis County, Minnesota: U.S. Geological Survey geologic quadrangle map GQ-1457, scale 1:24,000. Peterson, D.M., Gallup, C., Jirsa, M.A., and Davis, D.W., 2001, Correlation of Archean assemblages across the U.S.-Canadian border: Phase geochronology: Institute on Lake Superior Geology, 47th Annual Meeting, Madison, Wisc., Proceedings v. 47, Part 1., p.77-78. Sims, P.K, and Southwick, D.L, 1985, Geologic maps of Archean rocks, western Vermilion District, northern Minnesota: U.S. Geological Survey Miscellaneous Investigation Series Map I-1527, scale 1:48,000. Southwick, D.L., 1993, Geologic map of Archean bedrock, Soudan-Bigfork area, northern Minnesota: Minnesota Geological Survey Miscellaneous Map Series map M-79, scale 1:100,000. Southwick, D.L., 1994, Assorted geochronologic studies of Precambrian terranes in Minnesota: A potpourri of timely information: in Southwick, D.L., (Ed.), Short contributions to the geology of Minnesota: Minnesota Geological Survey Report of Investigations 43, p. 1-19. [age reference in Figure 1] 26 �CU-NI-PGE DEPOSITS OF THE DULUTH COMPLEXGEOLOGY AND DEVELOPMENT Figure from Dean Peterson. (Note that resources are exclusive of reserves, and that various mineral resources were added to various mineral reserves just to allow comparisons in orders of magnitude. Note also that some of the categories only consider mineral in the ground and do not take into consideration the costs, recoveries, and other relevant factors associated with extraction and recovery of the metal or mineral) Mark Severson (Teck American Incorporated) Andrew Ware (PolyMet Mining) Kevin Boerst (Twin Metals) Stephen Monson Geerts (Natural Resources Research Institute) Portions previously written by: Richard L. Patelke (formerly with PolyMet Mining) Tim Jefferson (formerly with Teck-Cominco Inc.) Dean M. Peterson (formerly with Duluth Metals Limited) 27 �EXPLORATION AND DEVELOPMENT BACKGROUND By the late Richard Patelke (PolyMet Mining) with modifications by Mark Severson Large resources of low-grade copper-nickel sulfide ore that locally contain PGE concentrations are well documented by drilling in the basal zones of the Partridge River, Bathtub, and South Kawishiwi intrusions. At least eleven occurrences of significant mineralization have been delineated in the basal 300 to 1000 feet of these intrusions. Of these eleven occurrences, two projects have undergone fairly recent definition drilling, including the Mesaba deposit (Teck American) and Maturi deposit (Twin Metals). Definition drilling at the Birch lake deposit (Twin Metals) took place up to four years ago. A fourth project, the NorthMet deposit (PolyMet Mining) is currently undergoing environmental review and mine permitting. Recent exploration drilling has taken place at the Serpentine deposit (Encampment Resources). Overall, the copper-nickel mineralization consists predominantly of disseminated sulfides that historically are estimated to contain over 4.4 billion tons of material averaging 0.66% Cu and 0.20% Ni at a 0.5% Cu cut-off, according to an earlier study (Listerud and Meineke, 1977): note that this estimate is historic and does not follow reporting guidelines as established by CIM Definition Standards). As outlined in Miller et al. (2002a), serious exploration for Cu-Ni deposits at the base of the Duluth Complex began in 1948, about 8 miles to the southeast of Ely, MN, when strongly mineralized rocks were uncovered in an excavation used to source road material for Spruce Road. Local prospector Fred S. Childers of Ely noted copper stains in the material and he, along with Roger V. Whiteside of Duluth, began searching along the basal contact in the vicinity of the Kawishiwi River. In 1951, they diamond drilled a 188 foot deep hole and intersected mineralized gabbro that averaged 0.36% Cu and 0.13% Ni. In 1952, both Bear Creek Mining Company (BMC) and the International Nickel Company (INCO) began intensive exploration efforts along a 38 mile-long zone that coincided with the basal contact. INCO eventually picked up the Childers-Whiteside properties (Spruce Road and Maturi deposits); whereas, BMC concentrated most of their effort near the town of Babbitt which resulted in the discoveries of the Babbitt (formerly called Minnamax and now known as Mesaba) and Serpentine deposits. By 1960, these exploration efforts indicated that very large tonnages of disseminated Cu-Ni mineralization were present; however, the low-grade nature of the deposits and the unavailability of state-owned mineral lands at the time led to suspension of activities. In 1966, state mineral leases were offered by the Minnesota Department of Natural Resources (DNR) and were awarded to successful bidders, resulting in renewed exploration activity (including the return of BMC and INCO). Since 1966, over 20 companies have been actively involved in exploration for Cu-Ni and Fe-Ti-V deposits along the basal contact of the Complex. Over 2,600 holes, totaling over 4.5 million feet of core, have been drilled. Exploration efforts during this period also defined several more deposits including: Dunka Road (now NorthMet) and Wyman Creek (United States Steel Corp.), Birch Lake (Duval Corporation and Newmont Mining), South Filson Creek (Hanna Mining), Dunka Pit (Erie Mining, BMC, and Exxon), and Wetlegs (BMC and Exxon). AMAX Exploration Inc. leased the Babbitt deposit from BMC and renamed it the Minnamax deposit in 1973. During mid to late 1970s, the Spruce Road and Minnamax deposits came closest to development. Mining plans were submitted, test shafts were sunk (one each at the Maturi and Minnamax/Mesaba deposits), surface bulk samples were collected from three sites, and various land-use and water-use permits were requested from State and Federal agencies. In 1974, the Minnesota Environmental Quality Board required that a regional Environmental Impact Statement (EIS) be conducted prior to acceptance of any site-specific EIS mining-related proposals. The DNR discontinued lease sales of State lands (1974-1982) until completion of the regional EIS. However, by the time the regional EIS was submitted in 1979, development of the Cu-Ni deposits was put on hold by the most of the mining companies involved due to weakened copper and nickel markets and the inability to make marketable (i.e., "smeltable") separate copper and nickel concentrates. Amax abandoned their plans to develop an underground high grade ore zone within the Minnamax/Mesaba deposit (known as the Local Boy ore body) in late 1982. 28 �Then starts the “PGE” era. During the early period of drilling (prior to 1980), all of the exploration companies recognized that the Cu-Ni deposits had some potential for hosting PGEs. Based on very limited sampling, the companies assumed that the typical Cu-Ni ore contained no more than a few hundred parts per billion (ppb) combined platinum and palladium. In 1985, the DNR and Minerals Resource Research Center (MRRC of the U of M) conducted a geochemical evaluation of portions of a Duval drill hole (DU-15), from the Birch Lake area, and found significant values of up to 9 parts per million (ppm) combined Pt and Pd (Sabelin and Iwasaki, 1985, 1986). This was at a time when demand for these elements was increasing due to their use in automotive catalysts. A short time later, Morton and Hauck (1987) compiled all of the known PGE data for the Complex and reported the presence of anomalous PGE values, often associated with high Cu values, at several other Cu-Ni deposits. These discoveries sparked renewed interest in the Cu-Ni deposits as potential polymetallic deposits (Miller et al., 2002; and references therein). E.K. Lehman and Associates of Minnesota obtained mineral leases from the state of Minnesota and began drilling wedges off the discovery hole (DU-15W) in the Birch Lake area. These Lehmann leases were later incorporated into Franconia Minerals holdings. Additional drill holes were sampled and analyzed for PGEs by several other companies throughout the Duluth Complex, and as a result, significant PGEs were found at many deposits. The occurrences of PGE mineralization for each deposit will be more thoroughly discussed later in this guidebook. Enter the “Hydromet” era. Early development of the deposits was hampered both by state leasing issues, complex metallurgy that resulted in an inability at the time to make marketable separate Cu and Ni concentrates, and by general environmental concerns regarding sulfide mining and conventional pyrometallurgical processes. In the mid to late 1990s, the potential of developing the Cu-Ni deposits using hydrometallurgical techniques once again sparked renewed activity in the Duluth Complex. PolyMet plans to use the PlatSol technique, developed and patented by SGS Lakefield on NorthMet ores, to recover Cu, Ni, Co, and PGE at the NorthMet deposit. REGIONAL GEOLOGIC SETTING, DULUTH COMPLEX The Duluth Complex and associated intrusions of Keweenawan age (~1.1 billion years) in northeastern Minnesota constitute one of the largest mafic intrusive complexes in the world, second only to the Bushveld Complex of South Africa (Miller et al., 2002). These rocks cover a 2,200 square mile (5,700 square km) arcuate area associated with the two strongest gravity anomalies (+50 and +70 milligals) in North America, that imply intrusive roots more than 8 miles (13 km) deep (Allen and others, 1997). The co-magmatic flood basalts and intrusive rocks underlying much of northeastern Minnesota were emplaced during development of the Mesoproterozoic Midcontinent rift, which can be traced geophysically from exposures in the Lake Superior region along a 1250 mile (2,000 km) long, segmented, arcuate path to Kansas and Lower Michigan. The Duluth Complex is defined as the more or less continuous mass of mafic to felsic plutonic rocks that extends for >170 miles (275 km) in an arcuate fashion from Duluth nearly to Grand Portage (Fig. 3-1). It is bounded by a footwall of Paleoproterozoic sedimentary rocks and Archean granite-greenstone terranes (Peterson and Severson, 2002), and a hanging wall largely of comagmatic, rift related flood basalts and hypabyssal intrusions of the Beaver Bay Complex (Fig. 3-1). In genetic terms, the Duluth Complex is composed of multiple discrete intrusions of mafic to felsic tholeiitic magmas that were episodically emplaced into the base of a volcanic edifice between 1108 and 1098 Ma. The geology of the Duluth Complex and adjacent areas has recently been described in two major publications by the Minnesota Geological Survey (MGS). These include a 1:200,000 scale regional bedrock geological map of northeastern Minnesota (Miller et al., 2001), and a comprehensive written description of the geology depicted on this map (Miller et al., 2002), commonly referred to as the “bible” by geologists working on Duluth Complex geology. Readers’ interested in more detailed descriptions of 29 �the geologic setting of the Duluth Complex should begin their quest for knowledge by downloading these publications from the MGS website (ftp://mgssun6.mngs.umn.edu/pub2/). Within the nearly continuous mass of intrusive igneous rock forming the Duluth Complex, four general rock series are distinguished on the basis of age, dominant lithology, internal structure, and structural position within the complex. Felsic series—Massive granophyric granite and smaller amounts of intermediate rock that occur as a semi-continuous mass of intrusions strung along the eastern and central roof zone of the complex, that were emplaced during an early stage magmatism (~1108 Ma). Early gabbro series—Layered sequences of dominantly gabbroic cumulates that occur along the northeastern contact of the Duluth Complex, emplaced during early stage magmatism (~1108 Ma). Anorthositic series—Structurally complex suite of foliated, but rarely layered, plagioclase-rich gabbroic cumulates emplaced throughout the complex during main stage magmatism (~1099 Ma). Layered series—Suite of stratiform troctolitic intrusions that comprises at least 11 variably differentiated mafic layered intrusions that occur mostly along the base of the Duluth Complex. These intrusions were emplaced shortly after the Anorthositic series (~1099 Ma). Figure 3-1. Generalized geologic map of northeastern Minnesota (modified from Miller et al., 2002). Rock Type and Unit Classification Igneous rock types in the Duluth Complex are classified at each of the deposits by visually estimating the modal percentages of plagioclase, olivine, and pyroxene, and using a rock classification scheme (Figure 3-2) modified from Phinney (1972). Using this classification, the majority of rocks at the various deposits consist of troctolite, augite troctolite, anorthositic troctolite, and norite (near the basal contact) with local 30 �ultramafic layers consisting of melatroctolite to dunite. Due to subtle changes in the percentages of the estimated minerals, there can be subsequent variations in the defined rock types within a specific igneous stratigraphic rock unit on a hole by hole basis, on an interval by interval basis, and even on a geologist by geologist basis. Figure 3-2. Modified Phinney (1972) diagram for rock type classification. Overall, stratigraphic unit definitions are based on: dominant rock type; textural relationships; mineralogy; sulfide content; and context with respect to bounding surfaces (i.e., ultramafic horizons, oxide-rich horizons). Unit definitions are not always immediately clear in logging, but are usually clarified when drill holes are plotted on cross-sections. In other words, to correctly identify a particular stratigraphic unit, the context of the units directly above and below should also be considered. LOCAL GEOLOGIC SETTING-PARTRIDGE RIVER, SOUTH KAWISHIWI, AND BATHTUB INTRUSIONS By: Mark Severson The three deposits under review for this trip are located in three of the oldest intrusions in the Duluth Complex. The NorthMet deposit and parts of the Mesaba deposit are in the Partridge River intrusion, the majority of the Mesaba deposit in the newly defined Bathtub intrusion, and Maturi deposit in the South Kawishiwi intrusion (Fig. 3-3). Partridge River intrusion The Partridge River intrusion (PRI) consists mainly of troctolitic cumulates, dips gently to the southeast, and is exposed in an arc-shaped area that extends from the Water Hen deposit, on the southwest, to the southern edge of the Mesaba/Babbitt deposit, on the northeast (Fig. 3-3). Footwall rocks include the Paleoproterozoic Virginia Formation and locally the Biwabik Iron Formation. The basal 3000 ft. (900 meters) are known in great detail from studies of abundant drill core (Severson and Hauck, 1990) and are subdivided into seven or more units that can be traced over a strike-length of 15 miles (24 kilometers). 31 �Figure 3-3. Location of Cu-Ni±PGE sulfide deposits, Fe-Ti±V oxide deposits (Oxide-bearing Ultramafic Intrusion - OUI), and other exploration areas along the western edge/base of the Duluth Complex. Note that the NorthMet deposit was referred to as the Dunka Road deposit and the Mesaba deposit was referred to as the Babbitt deposit; the most recent names for these two deposits are used in this guidebook. The Birch Lake deposit will not be discussed in this guidebook. The units of the Partridge River intrusion (PRI) are recently described in Miller and Severson (2002) and are depicted in Figure 3-4. At the base of the PRI is Unit I which consists of a suite of heterogeneous- 32 �textured troctolitic rocks that contain the vast majority of disseminated sulfide-mineralized zones. The top of Unit I is marked by a fairly persistent ultramafic horizon, which in actuality is at the base of Unit II. Within Unit I are several laterally-discontinuous ultramafic horizons and abundant footwall sedimentary inclusions of the Virginia Formation. Noritic rocks are common at the basal contact and adjacent to the inclusions due to silica contamination from assimilated footwall rocks. Unit II consists of more homogenous-textured troctolitic rocks with minor sulfide-bearing zones. However, at the Wetlegs deposit, both Units I and II contain abundant laterally-discontinuous ultramafic horizons, interbedded with troctolitic rocks that are collectively referred to as the Wetlegs Layered Interval (Fig. 3-4). Figure 3-4. Generalized stratigraphy of the basal zone of the Partridge River intrusion (modified from Severson, 1994). Roman numerals (I through VIII) denote igneous units in the Partridge River intrusion; BT1 and BT4 denote igneous units in the Bathtub intrusion; and OUI denotes Oxide-bearing Ultramafic Intrusions. Overlying Unit III in the PRI are units IV through VIII. Unit IV varies from a troctolite to augite troctolite, often contains an ultramafic base, and commonly grades upward into Unit V which is coarsergrained and varies from a troctolite to troctolitic anorthosite. Units VI and VII, and additional units above VII, are generally homogenous-textured troctolitic to anorthositic troctolitic rocks; each with a persistent ultramafic base that record magma injection events. Unit III is a major marker bed throughout much of the PRI (Wetlegs to Mesaba deposits - Figs. 3-3 and 34) in that it is characterized by a poikilitic leucotroctolite with olivine oikocyrsts that are randomly dispersed throughout the rock giving it a mottled appearance. This mottled-appearance, and the relatively fine-grained nature of Unit III, give it a distinct appearance in drill core and it is easily identified. Unit III pinches out to the west of the Wetlegs deposit and is present on only the southern fringe of the Mesaba deposit. The rapid pinch-out of Unit III to the north within the Mesaba deposit appears to be related to emplacement of a distinctly different sub-intrusion herein referred to as the Bathtub intrusion (see discussion below). Bathtub intrusion The Bathtub intrusion (BTI) is wholly contained in the central portion of the Mesaba (Babbitt) deposit. It has recently been singled out as a separate intrusion to explain the abrupt change from typical Partridge 33 �River intrusion stratigraphy in the extreme southern part of the deposit to a completely different stratigraphy, to the north in the remainder of the deposit (Severson and Hauck, 2008). There are three structural features that are pertinent to understanding the intrusive history of the BTI that include (Fig. 35): 1. an east-west trending paired syncline and anticline in the footwall rocks referred to as the Bathtub Syncline and Local Boy Anticline; 2. a zone that is closely associated with the Local Boy Anticline, referred to as the “Hidden Rise,” that separates the PRI and BTI; and 3. a north-trending fault zone, referred to as the Grano Fault, that is situated on the extreme eastern portion of the Mesaba deposit – the fault has been postulated to have been the feeder zone for the BTI and footwall-injected massive sulfides of the Local Boy ore zone. The “Hidden Rise” is a loosely-defined zone wherein scattered hornfels inclusions, and associated noritic rocks, are fairly common. When viewed collectively, the inclusions in “The Hidden Rise” define an eastwest trending “ridge” that is coincident with the Local Boy Anticline and roughly positioned at the contact between the PRI and BTI. Thus, “the “Hidden Rise” is used to both define this hornfels-bearing “ridge” and to artistically, and conveniently, divide the BTI from the PRI (Fig. 3-5). The morphology of this feature suggests that it may have originally served as the southern edge of an earlier intruded BTI and later served as a wall along the floor and north edge of the PRI as its upper units were emplaced. The BTI has been subdivided into two main units, BT1 and BT4, each of which contain several internal subunits (Fig. 3-5). In the vicinity of the Bathtub Syncline, ultramafic layers and modally-bedded rocks are extremely common within the BT4 Unit and have been collectively referred to as the Bathtub Layered Interval (BTLI). Figure 3-5. Schematic “type-section” looking east through the Mesaba deposit that crudely displays the spatial distribution of most of the igneous units in the Bathtub intrusion and pertinent structural features. Note that not all of the PRI units are shown on the right side of the figure. Cu-rich massive sulfides are locally present at the Mesaba deposit in a small zone referred to as the Local Boy ore zone. Local Boy is positioned along the crest of the Local Boy Anticline, in close to proximity to the “Hidden Rise,” and just west of the Grano Fault. Most of the massive sulfides are associated with either hornfelsed sedimentary inclusions above the basal contact or with footwall rocks below the contact while the interfingering intrusive rocks (mostly norite) are relatively barren of massive sulfides (Severson and Barnes, 1991). This suggests that the massive sulfide ores were not formed by the gravitational settling of sulfides, but rather, the ores formed by injection of an immiscible sulfide melt into structurally prepared areas within the footwall rocks along the Local Boy anticline in a vein-like setting. A possible 34 �feeder vent for the sulfide injection event may have been the Grano Fault, which was repeatedly reactivated during emplacement of the Complex. West-directed increases in Cu and PGE, associated with the massive sulfides at Local Boy, suggest that the immiscible sulfide melt fractionally crystallized and became progressively enriched in Cu and PGE as it was deposited in an east-to-west direction. Partridge River and Bathtub intrusion footwall rocks Because the footwall at NorthMet and Mesaba is so similar, the following is a generic description appropriate to both deposits. The drilled footwall rock types at Mesaba and NorthMet consist mainly of the Virginia Formation and Biwabik Iron Formation. Both are Paleoproterozoic in age (approximately 1.9-1.8 Ga) and are the two upper units of the Animikie Group. Any discussion on these two formations must include a description of their type-section on the Mesabi Range, as well as, a description of them as related to the metamorphism and partial melting that was produced during emplacement of the Complex. Lying beneath the Biwabik Iron Formation, but encountered in only a few drill holes are the Paleoproterozoic Pokegama quartzite (also of the Animikie Group), along with granitic rocks of the Archean Giant’s Range Batholith. Biwabik Iron Formation The Biwabik Iron Formation (BIF) exposed on the nearby Mesabi Range has typically been subdivided into four informal lithostratigraphic members (Wolff, 1917) that are, from the bottom up: Lower Cherty, Lower Slaty, Upper Cherty, and Upper Slaty. Diamond drill holes at Mesaba and NorthMet generally pierce the top submembers of the Upper Slaty, and end in submember C or D. Submember A is comprised of chert and marble, submember B is characterized by alternating bands of green diopside and chert with very coarse-grained hedenbergite, and submember C is a thin-bedded, green rock consisting of chert-fayalite-ferrohypersthene with black magnetite-rich bands. Virginia Formation below the PRI and BTI The Virginia Formation is a thick sequence of argillite, siltstone, and graywacke at the top of the Animikie Group. In close proximity to the Complex the effects of partial melting are profound and portions of the hornfelsed Virginia Formation no longer even remotely resemble a sedimentary rock. Severson et al. (1994a) subdivided the hornfelsed Virginia Formation, in both the footwall and in inclusions within the Duluth Complex, into at least five informal units based largely on metamorphic attributes, which are each related to varying degrees of partial melting. These members, and a pre-Duluth Complex sill, are described below and are schematically portrayed in Figure 3-6 - although in real occurrence, this idealized metamorphic progression is more erratic, often with rapid lateral and vertical changes between the four metamorphic units discussed below. 35 �Figure 3-6. Schematic cross-section showing the general relationships of the metamorphosed footwall rocks beneath the Duluth Complex at the Mesaba, NorthMet, Wetlegs, and Serpentine deposits. Cordieritic hornfels Directly beneath the basal contact of the Duluth Complex, the adjacent Virginia Formation typically consists of massive/non-foliated, cordierite-rich hornfels that display a bluish-gray color in drill core. The rock is generally fine-grained, granoblastic, and biotite-poor (due to loss of water into the Complex) and locally may contain porphyroblastic and/or poikiloblastic cordierite. Original bedding planes are preserved in some localities, but mostly the bedding planes have been obliterated by contact metamorphism. Recrystallized unit (RXTAL) Beneath the cordieritic “capping” the next metamorphic variant of the Virginia Formation nearest to the Duluth Complex is a rock that is referred to as the RXTAL unit. The RXTAL unit is properly classed as a diatexite and is characterized by fine- to medium-grained cordierite, plagioclase, biotite, quartz, and Kspar with lesser amounts of Opx and opaques. Bedding planes of the original argillaceous rocks are obliterated and what remains is a massive recrystallized rock with decussate biotite that contains enclaves (blocks and folded boudins) of more structurally competent calc-silicate hornfels and thin-bedded siltstone Disrupted unit (DISRUPT) With increased distance from the Complex, the RXTAL unit progressively grades into the DISRUPT unit which is a thin-bedded rock that is visibly deformed and underwent less degrees of partial melting. Textures that characterize the DISRUPT unit are bedding planes that are extremely chaotic and random in orientation due to pervasive small-scale folding, faulting, and brecciation. Superimposed on this chaotic pattern are abundant zones of leucocratic partial melts that are also chaotic and folded. The rock consists of varying amounts of quartz, cordierite, K-spar, biotite, plagioclase, and muscovite with leucosome veins and patches containing quartz, K-spar (microperthite), plagioclase, and muscovite (Duchesne, 2004). The DISRUPT unit is properly classed as a metatexite. 36 �Graphitic argillite and Bedded Pyrrhotite (BDD PO) units Carbonaceous argillite of the lower portion of the Virginia Formation is commonly preserved as either the BDD PO unit, or graphitic argillite, in close proximity to the Duluth Complex. This rock commonly contains over 5% disseminated pyrrhotite and/or extremely thin-bedded pyrrhotite laminae (hairlinethick), and variable amounts of graphite, staurolite(?) and sillimanite. Wherever the unit contains conspicuous and regularly-spaced laminae of pyrrhotite (0.5-3.0 mm thick at 1-20 mm spacings) it is informally referred to as the bedded pyrrhotite unit (BDD PO unit). In some areas, the BDD PO served as a local sulfur source to both disseminated and massive sulfide occurrences at the base of the Duluth Complex. VirgSill The VirgSill is generally present in the bottom 0.5-130 feet of the Virginia Formation, and as local apophyses into the top of the Biwabik Iron Formation. The VirgSill was intruded along the contact between the Virginia Formation and Biwabik Iron Formation and exhibits a granoblastic texture indicating that it was metamorphosed by the Duluth Complex (and thus the VirgSill is pre-Duluth Complex in age). On this basis, the VirgSill is inferred to be equivalent to the Logan sills (circa 1,109 Ma); as is another sill, the BIFSill, in the C submember of the Biwabik Iron Formation (Hauck et al., 1997). However, the VirgSill and BIFSill are different chemical entities (the VirgSill is much more Crenriched), and thus, these two sills may be related to at least two different intrusive events. Identification of the VirgSill in drill core is hampered by the fine-grained granoblastic texture that makes it difficult to distinguish from the enclosing hornfelsed Virginia Formation rocks. The VirgSill is subdivided into two textural varieties (Severson et al., 1994a; Park et al., 1999) referred to as: 1. the Massive Gray unit (MG unit); and 2. a coarser-grained interior with obvious hornblende and/or olivine. South Kawishiwi intrusion The South Kawishiwi intrusion (SKI) consists mainly of troctolitic cumulates and dips gently to the southeast. The SKI is exposed in an arc-shaped area that extends from the Serpentine deposit, on the southwest, to the Spruce Road deposit, on the northeast (Fig. 3-3). Footwall rocks include the Paleoproterozoic Virginia Formation, Biwabik Iron Formation and Archean Giants Range Batholith, the latter is the dominant footwall rock type. The presence of Biwabik Iron Formation as inclusions, from the Birch Lake deposit to as far north as the Spruce Road deposit, indicates that the majority of Paleoproterozoic units were assimilated and removed from the footwall during emplacement of the South Kawishiwi intrusion (Severson et al., 2002). The basal stratigraphic section of the SKI is known in great detail from studies of abundant drill core and is subdivided into 17 different units (Fig. 3-7) that are present over a strike-length of 19 miles (31 kilometers). The lowermost units are unevenly distributed along the strike length of the intrusion in a “compartmentalized” fashion, suggesting a complicated intrusive history (Miller and Severson, 2002). A few salient features to keep in mind regarding the igneous stratigraphy of the SKI include: • The vast majority of sulfide mineralization is confined to the BH (Basal Heterogeneous Unit), BAN (Basal Augite Troctolite and Norite Unit), UW (Updip Wedge Unit), and U3 (Ultramafic 3 Unit) – the latter three of these units are combined and referred to as the BMZ (Basal Mineralized Zone) by Twin Metals at their Maturi deposit; • Major marker beds, at specific areas in the SKI, include three horizons that contain abundant cyclic ultramafic layers (U1, U2, and U3 Units) and a pegmatite-bearing unit (PEG Unit originally recognized by Foose, 1984). The U1, U2 and U3 Units represent periods of rapid and 37 �continuous magma replenishment that crystallized more primitive ultramafic layers before mixing with the resident magma (Severson et al., 2002); • The U3 Unit is unique in that it contains several massive oxide pods (titanomagnetite-rich), as well as, recognizable inclusions of bedded Biwabik Iron Formation; especially at the Birch Lake deposit. The spatial correspondence between the U3 Unit and footwall iron-formation suggests that most of the massive oxide pods are iron-rich “restite” produced by assimilation and partial melting of the iron-formation (Muhich, 1993; Severson, 1994; Severson et al., 2002); • The U3 Unit contains the vast majority of high PGE values, especially within the Birch Lake area and possibly at the Maturi deposit. However, high PGE values are also present in the PEG Unit (Birch Lake area and Maturi deposit), the top of the BH Unit (Maturi deposit), and very locally in troctolitic rocks situated well above the basal contact (South Filson Creek deposit); and • A large inclusion/pillar of anorthosite is present at the Maturi deposit. This pillar, and possible proximity to a vent area and magma flow paths (see discussion for Maturi deposit) are some of the inferred reasons for high PGE values at the Maturi deposit. Figure 3-7. Generalized stratigraphy of the basal zone of the South Kawishiwi intrusion (modified from Severson, 1994; and included in Miller and Severson, 2002). The lowermost igneous units are: BAN = Basal Augite Troctolite and Norite; BH = Basal Heterogeneous; U3 = Ultramafic 3; PEG = Pegmatitic unit of Foose (1984); U2 = Ultramafic 2; U1 = Ultramafic 2; AT-T = Anorthositic Troctolite to Troctolite; UW = Updip Wedge; Main AGT = Main Augite Troctolite. 38 �PART 3A: POLYMET NORTHMET DEPOSIT By: the late Richard Patelke with modifications by Andrew Ware NORTHMET PROJECT SUMMARY NorthMet, located in the Partridge River intrusion of the Duluth Complex, is a large, disseminated sulfide deposit in heterogeneous troctolitic rocks associated with the 1,100 million year old Mid-Continent rift. Metals of interest are copper, nickel, cobalt, platinum, palladium, and gold. The majority of the metals are concentrated in four sulfide minerals: chalcopyrite, cubanite, pentlandite, and pyrrhotite, with platinum, palladium and gold also found in bismuthides, tellurides, and alloys. NorthMet is one of eleven coppernickel-PGE deposits along the northern margin of the Complex (PGE: platinum, palladium, gold). All of these share grossly similar geologic settings–disseminated sulfides with minor local massive sulfides in heterogeneous rocks forming the basal unit of the Duluth Complex along the contact with older rocks. The deposit is on the southern flank of the Mesabi Iron Range, which is host to six large operating taconite mines, the closest of which is less than two miles (3.2 km) north of the planned NorthMet pits (Figure 5A-1). Ore from NorthMet will be processed at a rate of 32,000 short tons per day through the former LTV Steel Mining Company iron ore concentration plant (“Erie Plant) with new facilities for processing of the NorthMet copper-nickel-PGE concentrates through a hydrometallurgical method to produce copper metal and various hydroxide and concentrate products of nickel-cobalt-PGE (Figure 5A2). PERMITTING and THE FINAL ENVIRONMENTAL IMPACT STATEMENT After 10 years of environmental review, the Final Environmental Impact Statement (FEIS) for the NorthMet Project was released in November 2015. The Minnesota DNR deemed the FEIS adequate in March 2016, and this DNR decision initiated the permitting process. The two other regulatory decision documents on the FEIS (Records of Decision from the US Forest Service and the US Army Corp of Engineers) are expected in 2016. The timelines for submitting and obtaining permits will be different for each permit. EXPLORATION and DEVELOPMENT There have been four major drilling programs since 1969, re-sampling for PGE began in 1989, three PolyMet joint ventures were pursued and dissolved in the 1990's, processing technology was developed in the late 1990's, the former LTV Steel Mining Company concentrator and other property was optioned in 2003, and the metallurgical process was refined in 2005-2008. Drilling programs have been conducted by United States Steel (USS, 1969-1974) and PolyMet Mining Inc. (Reverse Circulation or “RC” drilling and core drilling in 1998-2000 & two phases of core drilling in 2005 and 2007), plus two (actually two pairs of twins) holes by NERCO Minerals Company in 1991. This drilling encompasses 285,756 feet over 371 holes as of May 2008. Over 35,973 acceptable assays have been taken from this drilling (216,344 feet assayed). Table 3A-1 gives a breakdown of years, footages, and number of assays for all project drilling. United States Steel (USS) began core drilling at NorthMet (as the Dunka Road project) in 1969. Drilling targeted a conductor that turned out to be in the footwall metasedimentary rocks, but the first drill hole hit massive sulfide in the Duluth Complex. Drilling continued over five years for 112 holes with 133,716 feet of intercept. The working assumption was to mine the deposit from underground, sampling was limited to the most continuous zones with strong visible copper-nickel mineralization, and only about 2,200 samples representing about 22,000 feet were taken. USS assayed only for copper, nickel, sulfur, and iron. PGE 39 �presence was known from sampling on concentrates, but the economics of PGE recovery were apparently not pursued. Project work stopped while apparently incomplete and was not restarted. USS did not do much follow-up, but kept their land ownership, core, pulps, coarse rejects, and records for the project. In the mid 1980's the Minnesota Department of Natural Resources (MDNR) began sampling various historic drill core intervals in the Duluth Complex for PGE and got some good, but localized, results. In 1989 Fleck Resources (Fleck) leased the Dunka Road property from USS and began a program of re-assaying USS pulps and coarse rejects with a much more extensive multi-element suite, as well as adding in some new samples from existing core through cooperative work with the Natural Resources Research Institute (NRRI). The results were very positive in showing elevated PGE values in the deposit and confirming the previous copper-nickel assays. Fleck partnered with NERCO in 1991 for some bulk sample work, mine plans, environmental reviews etc., done through Fluor Daniel Wright engineers, but the partnership was eventually dissolved. In 1995 Fleck joined with Argosy Mining Corp. to do more work on the project, again with no major progress towards production. In June 1998, Fleck became PolyMet and focused their resources on Dunka Road, which was renamed NorthMet. Without partners, except for a brief venture with North Mining (North), PolyMet drilled and sampled 87 holes in 1998-2001, and sent two large bulk metallurgical samples to Lakefield Laboratories (now SGS) in Lakefield, Ontario for development and refinement of the PlatSol hydrometallurgical process and began some environmental background work. In the summer of 2000, North was taken over by Rio Tinto. The joint venture agreement was terminated upon consideration that NorthMet appeared to be a low priority to Rio Tinto. However, much of the North funding was already in place and was used to partially finance the 2001 pre-feasibility study. After release of the pre-feasibility study (2001), a brief hiatus, and a major re-evaluation of how the project should proceed, PolyMet became active again in 2003 with new management and a new development plan. This plan involves integrating the former LTV Steel Mining Company iron ore concentration plant (“Erie Plant) with new facilities for processing of the NorthMet copper-nickel-PGE concentrates through a hydrometallurgical method at rate of 32,000 short tons of ore per day to produce copper metal and various hydroxide and concentrate products of nickel-cobalt-PGE. Geologic work towards this end began in 2004 and first focused on a careful and total re-compilation of the historic NorthMet project drill hole related data. This effort organized and verified all drilling metadata, location, downhole survey, lithology, and assay data, and cataloged all paper (and digital) records for the project. Of note is that this resulted in an increase in the number of acceptable assays from 12,000 to around 17,200 and an improved geologic picture from careful consolidation of existing records. This work was used as background for a revised resource estimate in January 2005 and planning of a drill program for 2005. The 2005 program entailed drilling and sampling 109 holes (77,000 feet), collection of a forty ton metallurgical bulk sample for pilot scale testwork, geotechnical (oriented core) drilling, in-fill sampling of previously drilled core, and extensive collection of waste characterization data. The 2005 drilling program added 13,450 multi-element assay records to the existing database. A PolyMet report covers the details of historic drilling and assaying (Patelke & Geerts, 2006). 40 �Figure 3A-1. PolyMet NorthMet project site. 41 �Figure 3A-2. Detail of Erie Plant site showing existing facility and new construction. 42 �Drilling in 2007 for 24.530 feet with 3,546 assays concentrated on defining mineralization in the upper units in the west part of the deposit (the “Magenta Zone”). This drilling and the subsequent re-modeling of the deposit turned about 50 million tons of material previously classed as waste to ore. There is also over 34,000 feet of hydrogeology drilling and “stratigraphic holes” (drilling by other companies not done as part of the NorthMet project). No assays are in use from these 44 holes which are used for geologic control. Approximately 89.5% of Unit 1 and about 57% of the upper units have been sampled across the deposit. The sampled percentages are higher in the anticipated area of mining. Table 3A-1. Total drilling and assaying for NorthMet project. No. of drill holes Total footage for group No. of assay intervals used in “accepted values” tables Assayed footage used in final database Assay Laboratories 1969-1974 1969-1974, 19891991, 19992001, 20052006, 2008 112 133,716 11,259 73,303 USS, ACME, ALS-Chemex 1991 1991 2 (4) 842 165 822 ACME 1998-2000 1998-2000 52 24,650 4,765 23,767 ACME PolyMet core drilling 1999-2000 2000-2001, few in 2005 32 22,156 4,058 20,727 ALS-Chemex PolyMet RC drilling deepened with AQ core tail 2000 2000 3 2,696 524 2,610 ALS-Chemex PolyMet core drilling 2005 2005-2006 109 77,166 11,656 71,896 ALS-Chemex PolyMet core drilling 2007 2007 61 24,530 3,456 23,310 ALS-Chemex 371 285,756 35,973 216,344 Company US Steel NERCO PolyMet reverse circulation drilling Drilling years Assaying years Totals for Exploration Drilling:    US Steel stratigraphic holes 1970's? none 6 9,647 none none 1956 none 3 2,015 none none Humble Oil / Exxon 1968-1969 none 3 9,912 none none Bear Creek / AMAX 1967-1977 none 11 8,893 none none PolyMet / Barr Engineering (hydrologic testing) 2005-2007 none 21+ 3,459+ none none INCO Sampling in Unit 1 (the main mineralized zone) is now mostly continuous through the zone for all generations of drilling. The PolyMet RC and core holes have continuous sample through the upper waste zones (which do have some intercepts of economic mineralization). Work in 2005 through 2008 43 �essentially completed the sampling of historic USS core within the area likely to be mined. This broad sampling limits the possibility of location bias in the sample set. The entire USS core footage has not been sampled, however there is no known un-sampled mineralized intervals. Table 3A-2. Large metallurgical samples collected at NorthMet. Bulk Sample Year Tons Location of sample USS Bulk sample pit No. 1 1971 Unknown, but small Pit in center of property USS Bulk sample pit No. 2 1971 300 Pit at east end of property USS Bulk sample pit No. 3 1971 20 Pit at east end of property NERCO PQ drill core 1991 Argosy Mining 1995 Unknown, but small Composited from USS coarse rejects PolyMet RC drill cuttings 1998 26 One composite, mostly from what is now considered east part of 10 year pits PolyMet RC drill cuttings 2000 33 One composite, mostly from what is now considered east part of 10 year pits PolyMet 4 inch and PQ core and coarse reject 2005 10.5, 21.5, and 10.7 Three composites from within ten year pits across property PolyMet coarse reject 2006 4.2 and 4.94 One composite from 10 year east pit, one from 20 year pit across property PolyMet ¼ core from 2005 and 2007 Drilling 2007 500  kg One composite,  from east and west pit areas PolyMet ¼ core from 2005 and 2007 Drilling 2008 4.44 One composite,  from east and west pit areas PolyMet ¼ core from 2005 and 2007 Drilling 2008 4.48 One composite,  from east and west pit areas Estimated at 4.5 One PQ drill hole from each tons or less by drill end of property core size There have been numerous bulk samples taken at NorthMet. Samples have been representative by Unit and rock type. Agreement between calculated grades (based on core sampling) and analyzed grades of final sample has been excellent. Earlier bulk samples represented the first ten years of production, more 44 �recent samples used material from across the deposit. Each bulk sample has built upon the previous, and work has progressed to the point where PolyMet has confirmed the ability to make separate, saleable, copper and nickel concentrates. This will allow the company to develop cash flow from sales much earlier in production while completing construction of the hydrometallurgical facility. The planned hydrometallurgical process (PlatSol) was developed on NorthMet ores. The process uses pressure oxidation (225°C, over 30 atmospheres) in the presence of chloride to capture all base and precious metals in the concentrate. Hydromet process recoveries are all over 98%. Other geologic data collected includes: recovery and RQD measurements on all core, over 7,000 specific gravity measurements, over 900 whole rock analyses, over 300 Rare Earth Element packages and a large amount of microprobe data collected for waste characterization purposes. GEOLOGY OF THE NORTHMET DEPOSIT NorthMet consists of seven igneous units that dip southeast, with most economic sulfide mineralization in the top parts of the lowermost unit (Unit 1). The following is a summarized description of the geology of the deposit, based on observations from drill core and limited outcrop mapping. Quaternary Geology In general the Quaternary geology of the region is a thin (0-30 feet or 0-10 meters, but locally thicker) blanket of glacial deposits including till, lacustrine materials, and outwash. Low spots are usually peat bog or open wetland. Topography is subdued and drainage is poor. Site specific geologic studies of the drift have not been done, though a series of geophysical soundings were carried out in 2006 to better define drift thickness outside the area to be mined (Ikola, 2006). Structural Geology The general structure of the NorthMet deposit, as defined by igneous contact dips, foliation in serpentinized zones, bedding trends in the Biwabik Iron Formation (BIF) and in the Virginia Formation, is dominated by an overall dip ranging from 15-25° to the southeast, striking about N56°E. Dips in the seven igneous units are grossly similar, but dips of the mineralized zone are up to 60° in the east pit area. Dips in both the Animikie and the Duluth Complex rocks can be attributed to crustal loading, associated with the input of large volumes of magma originating from the Mid-continent Rift System (Sims and Morey, 1972). Numerous faults have been proposed across the NorthMet Deposit, based largely on reconciling dips in the footwall rocks. There is insufficient evidence, based on drilling to indicate with certainty the exact location of offsets or faulting within the igneous rock units or the footwall rocks on a hole-to-hole basis. Clearly however, pre-intrusion offset or faulting probably exists within the footwall rocks, due to substantial offsets in the BIF (assuming an average 20° dip) as evidenced between drill holes portrayed in cross-sections. Many of these same offsets can be correlated in adjacent cross-sections. Fault zones are apparent in drill core and show up as brecciated intervals (up to several feet thick), including gouge mineralization (clay, calcite, quartz, etc.), slickensides on serpentinized fracture faces, and/or severely broken (rubble) core. Extensive angle drilling in 2005 and 2007 (142 of 170 holes) brought no great clarity to this issue (virtually all previous drilling was vertical). The current geological model and working cross-sections are therefore constructed with minimal faulting influence, especially within the igneous rock units of the Partridge River intrusion. 45 �Logging and Mapping Units A summary of the general stratigraphy of the NorthMet Deposit is outlined below. Rock units and formations are listed in descending order, as would be observed from top to bottom in drill hole. NorthMet units are labeled as Units 1 through 7 (Units I through VII in Severson’s terminology), bottom to top. Unit 3 is probably the oldest, the intrusion sequence of the other units is not clear. The broad picture is of a regular stratigraphy of troctolitic to anorthositic rock units, dipping southeast at 20° to 25°, with basal ultramafic units defining the boundaries of some of these units. The basal ultramafic zones tend to have diffuse tops, sharp bases, and are commonly serpentinized and foliated. Geologists have generally picked the unit boundaries at the base of these ultramafics though there are local exceptions. Economic sulfide mineralization is ubiquitous in the basal igneous unit (Unit 1) and is locally present, but restricted, in the upper units. Unit Definitions and Descriptions Descriptions of the general igneous Stratigraphy for the NorthMet deposit is described below and presented in a stratigraphic column in Figure 3A-3. Unit 7 Unit 7 is the uppermost unit intersected in drill holes at the NorthMet Deposit. It consists predominantly of homogeneous, coarse-grained anorthositic troctolite and troctolitic anorthosite, characterized by a continuous basal ultramafic subunit that averages 20 ft. thick. The ultramafic consists of fine- to mediumgrained melatroctolite to peridotite and minor dunite. The average thickness of Unit 7 is unknown due to erosion removing the upper parts. Unit 7 is generally not mineralized. Unit 6 Very similar to Unit 7, Unit 6 is composed of homogeneous, fine- to coarse-grained, troctolitic anorthosite to troctolite. It averages 400 ft. thick and has a continuous basal ultramafic subunit that averages 15 ft. thick. Overall, sulfide mineralization is minimal, although a number of drill holes in the southwestern portion of the NorthMet Deposit contain significant sulfides and associated elevated PGEs (Geerts 1991, 1994). Sulfides within Unit 6 generally occur as disseminated chalcopyrite/cubanite with minimal pyrrhotite. This mineralized occurrence, the “Magenta Zone”, transitions into Units 3, 4, and 5, and is discussed in greater detail below. Unit 5 Unit 5 exhibits an average thickness of 250 ft. and is composed primarily of homogeneous, equigranulartextured, coarse-grained anorthositic troctolite. Anorthositic troctolite is the predominant rock type, but can locally grade into troctolite and augite troctolite towards the base of the unit. The lower contact of Unit 5 is gradational and lacks any ultramafic subunit; therefore the transition into Unit 4 is a somewhat arbitrary pick. Due to the ambiguity of this contact, thicknesses of both units vary dramatically. However, when Units 5 and 4 are combined, the thickness is fairly consistent deposit-wide. Aside from Magenta Zone mineralization in the west, Unit 5 is not mineralized. Unit 4 Being somewhat more mafic than Unit 5, Unit 4 is characterized by homogeneous, coarse-grained, ophitic augite troctolite with some anorthosite troctolite. Unit 4 averages about 250 ft. thick. At its base, Unit 4 may contain a local thin (usually no more than 6 inch) ultramafic layer or oxide-rich zone. The lower contact with Unit 3 is generally sharp. Unit 4 is rarely mineralized outside the Magenta Zone. 46 �Figure 3A-3. Generalized stratigraphic column for NorthMet units (modified after Geerts, 1994) Unit 3 Unit 3 is used as the major “marker bed” in determining stratigraphic position in the PRI. It is composed of fine- to medium-grained, poikilitic and/or ophitic, troctolitic anorthosite to anorthositic troctolite. Characteristic poikilitic olivine gives the rock an overall mottled appearance. On average Unit 3 is 300 ft. thick. As with Units 4 and 5, the thickness of Units 2 and 3 tend to be highly variable, whereas if combined into one unit, it is more consistent deposit-wide (though not as consistent as Units 4 & 5). Unit 2 Unit 2 is characterized by homogeneous, medium- to coarse-grained troctolite and augite troctolite with a consistent basal ultramafic subunit. The continuity of the basal ultramafic subunit, in addition to the relatively uniform grain size and homogeneity of the troctolite, makes this unit distinguishable from Units 1 and 3. Unit 2 has an average thickness of 100 ft. The ultramafic subunit at the base of Unit 2 is the 47 �lowermost continuous basal ultramafic horizon at the NorthMet Deposit, averages 25 ft. thick, and is composed of melatroctolite to peridotite and minor dunite. Unit 1 Of the seven igneous rock units represented within the NorthMet Deposit, Unit 1 is the only unit that contains significant deposit-wide sulfide mineralization. Sulfides occur primarily as disseminated interstitial grains between a dominant silicate framework and are chalcopyrite > pyrrhotite > cubanite >pentlandite. Unit 1 is also the most complex unit, with internal ultramafic subunits, increasing and decreasing quantities of mineralization, complex textural relations and varying grain sizes, and abundant sedimentary inclusions. It averages 450 ft. thick, but is locally 1,000 feet thick and is characterized lithologically by fine- to coarse-grained heterogeneous rock ranging from anorthositic troctolite (more abundant in the upper half of Unit 1) to augite troctolite with lesser amounts of gabbro-norite and norite (becoming increasingly more abundant towards the basal contact) and numerous sedimentary inclusions. By far the dominant rock type in Unit 1 is medium-grained ophitic augite troctolite, but the textures can vary wildly. Two internal ultramafic subunits occur in drill holes in the southwest, and have an average thickness of 10 ft. Footwall rocks are covered in the Partridge River intrusion description. Inclusions Two broad populations of inclusions occur at NorthMet: hanging wall metabasalts (Keweenawan) and footwall metasedimentary rocks. Basalts are fine-grained, generally gabbroic, with no apparent relation to any mineralization. Footwall inclusions may carry substantial sulfide (pyrrhotite) and often appear to contribute to the local sulfur content. Footwall inclusions are all Virginia Formation, no iron-formation, Pokegama Quartzite, or older granitic rock has been recognized as an inclusion at NorthMet. Sedimentary inclusions make up about 4% of the logged rock types, and basalt inclusions sum to less than 1% of the drilling footage. Other Igneous Units Quadrangle scale outcrop mapping indicates that other igneous stratigraphic units are present above Unit 7. These units are similar to Units 6 and 7 in that they consist of homogeneous-textured troctolitic rocks with basal ultramafic members. There are minor, mineralized, pre-Complex sills in both the Virginia Formation and Biwabik Iron Formation at NorthMet (VirgSill and BIF Sill in footwall descriptions above). In neither case is there any apparent relation to Duluth Complex mineralization. Alteration The vast majority of rock within the NorthMet Deposit would be considered fresh and is unaltered or only weakly altered. Types of alteration most commonly observed in NorthMet rocks are serpentinization / chloritization of olivine, sericitization and saussuritization of plagioclase, and uralitization of pyroxenes. Most alteration is related to close proximity of fractures and/or joints that cross-cut the troctolitic rocks. Likewise, on a microscopic level the center of alteration is focused around microfractures. This pattern suggests that both fracturing and accompanying alteration of the rock occur as a result of the migration of late-stage deuteric fluids during the cooling phase. The vast majority of sulfide mineralization is independent of alteration. Nickel in Silicates (Lab Assay Nickel vs. Recoverable Nickel) It has been characteristic of NorthMet and other Duluth Complex deposits to show lower nickel recoveries in process test work than would be expected from laboratory assays on drill core. Generally there is a loss of about 25-35% of the nickel compared to drill core assays when concentrating sulfides. From previous work, it is known that small amounts of unrecoverable nickel occur as a magnesium-iron- 48 �nickel silicate [(Mg,Fe,Ni) SiO ] that is tied up in the mineral olivine, which is one of three significant gangue minerals that occur across the NorthMet deposit.. 2 4 Figure 3A-4. Geologic map of NorthMet Deposit, all units dip southeast, Magenta Zone is projected upward, does not actually subcrop. 49 �Figure 3A-5. Cross section 35700 at west end of property and 45600 at east end. Purple shading indicated ore zones, bar graphs along holes indicate grades expressed as dollar values, where red = $7.42 cut-off to average grade (~$14.39), and purple shows above average grade, blue are zones of potential lean ore should metals prices rise. 50 �ECONOMIC MINERALIZATION The majority of economic mineralization (copper, nickel, cobalt, platinum, palladium, and gold) at NorthMet occurs in the upper parts of basal Unit 1, with copper and nickel in chalcopyrite, cubanite, and pentlandite, all in the presence of pyrrhotite. Cobalt is contained in sulfides. Platinum, palladium, and gold, while showing good correlation with sulfur and the other metals, are also in a variety of tellurides, bismuthides, and alloys, as well as associated with the major and minor sulfides. Table 3A-3 shows correlation of metals values in drill core data. Table 3A-3. Simple correlation ® table for economic metals and sulfur Cu % Ni % S % Pt ppb Pd ppb Au ppb Pt+Pd+Au Co ppm Zn ppm Cu % 1.000 Ni % 0.860 1.000 S% 0.541 0.572 1.000 Pt ppb 0.568 0.508 0.195 1.000 Pd ppb 0.750 0.635 0.292 0.673 1.000 Au ppb 0.591 0.472 0.250 0.482 0.699 1.000 Pt+Pd+Au 0.760 0.645 0.292 0.778 0.983 0.755 Co ppm 0.544 0.704 0.621 0.217 0.281 0.241 0.288 1.000 Zn ppm -0.021 -0.004 0.286 -0.041 -0.037 -0.017 -0.039 0.093 1.000 1 The simple correlation table above (number of samples=19,516) shows the strong relation of copper, nickel, and palladium, and a somewhat surprising relation of cobalt to sulfur. Zinc’s low factor is probably related to its multiple origins as either magmatic or derived from assimilation of footwall rock, hence representing two populations of data. The sulfur vs. metal correlation is probably greatly affected by iron, the presence of which is not shown here, but is in excess in all rocks. Grades are highest at the top of Unit 1 and fade going down hole. Grades appear to be higher down-dip though this may be an artifact of less dense sampling. There is a smaller zone of economic mineralization (about 50 million tons) at the western end of the property in the upper units, known as the “Magenta Zone.” This zone is generally copper and PGE-rich (sulfur-poor relative to metals) and of “average” reserve grade. The minerals of interest from a waste characterization perspective are the same as above, but pyrrhotite is expected to be the main mineral affecting water quality in regards to waste rock, though the traces of chalcopyrite, cubanite and pentlandite are studied for waste rock storage. Trace pyrite and pyrrhotite are the main sulfide minerals found in the tailings. Most sulfide mineralization at NorthMet is thought to be of a distant source (magmatic?), some is locally modified by sulfur derived from footwall metasedimentary rocks (Virginia Formation). Minor veins and other cross-cutting relations indicate some movement of sulfides within the deposit, but there is no evidence recognized for large scale relocation of sulfides, nor any macroscopic evidence for any hydrothermal event that may have remobilized PGE’s or sulfides. Element distributions, on a single section through the west pit in figure 5A-6 are, located on a single page at the end of the PolyMet Section. The Magenta Zone mineralization cutting across upper intrusive units, is illustrated in this section. 51 �RESOURCE The PolyMet resource and reserve (Table 3A-4) models have been done in cooperation with several consultants, most recently PEG Mining of Toronto. PolyMet supplies the geologic solids model, database, and block model geometry. Geostatistics and population of the block model, and hence the resource estimate, are done in consultation, with finalized resource block models then sent forward to engineers for reserve calculation and mine planning. Table 3A-4. The NorthMet resource and reserve values work was done by Wardrop Engineering 2007. Cut-off based on “Net Metals Value” per ton, accounting for grade, average flotation and Hydromet recovery, realization costs, metal prices, and other factors. See Desaultels and Patelke, 2008 for resource calculation details. Enough reserve has been shown for 20 years of permit constrained production. RESERVES-2007 Proven Probable Proven and Probable Cut-off value $7.42 $7.42 $7.42 Million Tons 118.1 156.5 274.6 Cu % 0.30 0.27 0.28 Cut-off value $7.42 $7.42 $7.42 $7.42 Million Tons 202.5 491.7 694.2 229.7 Cu % 0.285 0.256 0.265 0.273 Ni % 0.09 0.08 0.08 Co Ppm 75 72 73 Pt ppb 75 75 75 Pd ppb 275 248 260 Au ppb 38 37 37 RESOURCES-2007 Measured Indicated Measured & Indicated Inferred Ni % 0.083 0.075 0.077 0.079 Co ppm 74 70 71 56 Pt ppb 71 66 68 73 Pd ppb 258 231 239 263 Au ppb 36 34 35 37 ASSUMPTIONS Metal and Units Assumed Metal Price Average % recovery, as used in DFS Cu % $1.25 lb. 92.33 Ni % $5.60 lb. 70.34 Co Ppm $15.25 lb. 40.75 Pt ppb $800 oz. 75.74 Pd ppb $210 oz. 72.69 Au ppb $400 oz. 67.04 In the center of the deposit the highest, near surface, Unit 1 grades transition into the middle of the unit, while in the east, mineralization is strong and vertically persistent throughout the unit. The top of the merged Unit 1 and Unit 2 mineralized domain (domain 1) forms a hard boundary that, combined with the bedrock ledge (depth to bedrock) surface, forms the bottom and top estimation boundaries for the upper units (exclusive of the “Magenta Zone”, which is internal to this domain). There is no conclusive relation between specific Unit 1 specific rock type and presence or grade of mineralization except that noritic rocks are generally of lower grade. Units 2 and 3: These units are treated as one unit in the geologic model, with PolyMet geologists considering them as a single package grading from an ultramafic base to an anorthositic top for modelling purposes. The thickness of the package stays relatively constant, though the thickness of the two individual units varies, primarily due to Unit 2 locally thinning. While generally barren, Unit 2 has mineralization at its base in the western half of the deposit. These zones may not be strictly equivalent to Unit 1 type mineralization. Copper and nickel values are lower, as is pyrrhotite, but behavior of other metals is inconsistent, with PGE (Pt + Pd +Au) content varying locally relative to nearby grades at the top of Unit 1. Above the basal zone of Unit 2 it is usually barren, medium-grained, and homogenous in texture. Average PGE in Unit 2 is slightly above that of Unit 1. 52 �Unit 3 shows mineralization in the west, in the middle of the unit and near the top. This occurrence is merged into the Magenta Zone. Units 4 and 5 are also modeled as a geologic package. There is no compelling geologic reason to fully separate these units, the boundary between them being an arbitrary pick based on overall changes in texture from homogenous to heterogeneous, grain size, and plagioclase content, but without a welldefined bounding horizon. The top boundary of Unit 5 is the basal ultramafic of Unit 6, which is an unused hard boundary in grade modelling. The bottom boundary of Unit 4 is a discontinuous ultramafic horizon. There are also discontinuous oxide-rich zones along the contact between Units 3 and 4. Metals and sulfur grades in Unit 4 are proportional to Unit 1, but consistently lower. Unit 4 has few high copper or sulfur assay intervals. There is some near surface mineralization, modelled as a part of the Magenta Zone, described below. Otherwise there is only low grade, discontinuous material at the base. Unit 6 and Unit 7: These units are very similar in nature. Both are homogenous anorthositic troctolite with well-defined ultramafic bases. No top for Unit 7 has been seen in drill hole. Units 3, 4, 5 and 6 host a zone of mineralization, modeled as the Magenta Zone. Unit 6 mineralization was described by Geerts (1994) as the “Magenta Horizon” when originally found in six drill holes. Further drilling has extended these copper rich, sulfur poor zones (of moderate overall grade) into more than fifty drill holes in Units 3, 4, 5, and 6. The zone transitions across the ultramafic base of Unit 6 and into Units 3, 4 and 5, (i.e., does cross the igneous stratigraphy) which is problematic if the emplacement model of these units representing individual pulses of magma is correct. There is no gross evidence for this mineralization being hydrothermal, which could cross boundaries, but would presumably alter large masses of rock. Unit 7 has a few good assay intercepts, but no apparent continuity for sulfides. Copper, nickel, and sulfur values in Table 3A-5 are calculated after removing samples with less than 0.05% copper. 53 �Table 3A-5. Average values for assays by unit after removal of the less than 0.05% copper intervals (drill core samples). Unsampled zones not accounted for here. Data complete through 2006. Cu Ni % % S Co Cu+Ni Total % of unit Pt+Pd+Au ppb Cu/Ni Cu/S % ppm % sampled Average sample length-feet Unit 1 0.3 0.09 0.83 349 76 0.39 3.35 0.43 90 5.3 Unit 2 0.2 0.07 0.39 365 73 0.27 2.74 0.61 80 5.6 Unit 3 0.19 0.05 0.5 286 62 0.25 3.19 0.53 71 7.2 Unit 4 0.21 0.06 0.58 269 66 0.28 3.40 0.44 51 7.6 Unit 5 0.27 0.07 0.54 398 65 0.35 3.64 0.54 41 7.8 Unit 6 0.33 0.08 0.48 532 69 0.41 3.74 0.69 27 7.2 Unit 7 0.2 0.06 0.32 330 83 0.26 3.60 0.72 11 8.4 • Gatehouse (North Mining) did report some geochemical cyclicity in unit 1, but this has not been revisited with the larger data set; • Poor assay grades in the noritic rocks are related to footwall assimilation and contamination, otherwise there is little connection between grades and specific rock type. About 83% of the igneous rocks at NorthMet are troctolites, 6% anorthositic rocks, 4% ultramafic rocks, and 4% footwall inclusions. The remainder are norite, gabbro and others; • Within Unit 1 copper:sulfur ratio tends to be highest at top, then diminishes with depth, following the pattern of PGE’s; • The upper units have higher copper:sulfur ratios than Unit 1 (i.e., more chalcopyrite rich), but lower overall copper values; • Ratio of PGE to copper is lowest in Unit 1, but Unit 1 has greatest quantities of both; • Chalcopyrite is the dominant sulfide in the upper units regardless of total sulfur content; Sulfide (Ore) Mineral Proportions Various metallurgical test programs have been conducted on NorthMet ores since the 1970's. Reported sulfide mineral proportions have not been entirely consistent between these tests. Sulfide mineralogy within the NorthMet Deposit has been described in detail through petrographic observations and microprobe analysis. Approximately 95-98% of all sulfide mineralization consists of 4 predominant species, in decreasing order of abundance: chalcopyrite (cp) > pyrrhotite (po) > cubanite (cb) > pentlandite (pn). In general, Po:Cp+Cb ratios increase towards the basal contact or in proximity to sedimentary inclusions. Likewise, Cp:Cb ratios increase with increased distance away from the footwall rocks. In core logging and other work, chalcopyrite is often not distinguished from cubanite. Mining Mining at NorthMet will begin with contractor clearing and overburden stripping of the pit and stockpile areas. Engineered stockpile bases and liner systems must be in place before mining begins, as does the overall water collection system for treatment and pumping to the tailings basin. Ore and waste production will start in the east pit, with production from the west pit ramping up soon afterwards. Up through about year 11 or 12 production from both pits will be equal until the east pit is mined out. At that point, backfilling of the east pit will begin, with the ultimate goal of constructing a wetland in that pit. The central pit area will be mined last. Ore will be moved at a rate of 32,000 tons per day. Waste to ore strip ratio will be about 1.46:1. Ore will be moved by truck to the “superpocket” and loaded to 100 ton capacity side dump rail cars by pan feeder. There will be twenty trains per day of 16 cars each. 54 �Ore and waste categorization (“ore control”) will be by assay of core and / or blast holes and careful pit mapping. Waste material will be sorted to stockpiles, and stockpile liners will be built, according to the sulfur and metals content of the waste rock. Fig. 3A-6. Economic element distribution, sulphur and NSR in the west pit. Section is orthoganal to the NE-SW strike of the Virginia Fm - Duluth Complex contact. 55 �PART 3B: TECK AMERICAN MESABA DEPOSIT By: Mark Severson (portions originally by Tim Jefferson) BACKGROUND Previous exploration and development work The Mesaba deposit was first discovered along the base of the Duluth Complex in 1958 by Bear Creek Mining Company (BMC). Between 1958 and 1960, BMC completed 55 shallow drill holes for 43,000 feet (13,952 meters). BMC renewed drilling activities in 1967-1971 completing 149 additional holes. Drill hole B1-105 intersected substantial amounts of semi massive to massive sulfide mineralization between 1,400 and 1,800 feet (425 and 550 meters) below surface in footwall rock. Subsequent drilling defined a high grade zone appropriately named the Local Boy ore zone after BCM geologist Stuart Behling, the “local boy,” who encouraged BMC to continue drilling this site. In late 1973, AMAX Exploration, Inc. agreed to take over BMC’s state and private leases. During the next four years (1974-1978) AMAX continued drilling the deposit (completing 228 drill holes). In particular their focus was drawn to the Local Boy ore zone (Watowich and others, 1981), and following successful permitting, they sank a shaft in 1976-1977. Four drifts totaling 3,800 feet (1,160 meters) were developed and 218 underground holes were completed. Based on this work, AMAX reported an overall underground estimate of 364 million tons (330.2 million tonnes) averaging 0.84% Cu and 0.19% Ni, with a Local Boy-only resource of 5 million tons (4.54 million tonnes) grading 1.89% Cu, 0.36% Ni (Watowich, 1978). Both underground resources were estimated based on a 0.60% Cu cut-off (Watowich, 1978). Due to weakening copper and nickel markets and the inability to produce separate high grade Cu and Ni concentrates, AMAX abandoned their plans to develop the deposit in late 1981. Rhude and Fryberger obtained leases and, along with the NRRI, evaluated the PGE potential of the Local Boy ore zone circa 1990. Arimetco Inc., picked up the Babbitt deposit leases, renamed it the Mesaba deposit, and evaluated the property circa 1994-1996. They did not complete any drilling but collected two bulk samples for metallurgical test work. Arimetco reported a resource estimate to 3,300 million tons (2,993.7 million tonnes) grading 0.46% Cu, 0.12% Ni, cut-off 0.38% Cu (Miller et al, 2002). Arimetco Inc. declared bankruptcy in late 1996. Present exploration and development work Teck American Incorporated acquired a package of state and private leases covering the Mesaba deposit in 1998. Teck drilling began in 2007-2008 for a total of 67,430 feet (20,560 meters) in 64 drill holes (Fig. 3B-1). This drilling was concentrated on the western portion of the deposit to complete a 400 foot (120 meter) grid infill program. In 2012-2013, Teck conducted three additional drilling campaigns (Fig. 3B-1). 56 �Figure 3B-1. Drill hole location map for Mesaba Deposit. Grid north is about 33° west of north. Re-logging of historic holes at the Mesaba deposit over an 18 year period, in addition to information gained from logging of holes completed in 2007-2008 and 2012-2013, indicates that the deposit is primarily hosted by a previously unrecognized intrusion within the Duluth Complex (Severson and Hauck, 2008). It is believed that this intrusion, informally named the Bathtub intrusion (BTI) lies between the Partridge River intrusion (PRI) to the south, and the South Kawishiwi intrusion (SKI) to the north and east. This intrusion is believed to have been fed by a vent in the Grano Fault area on the east side of the Mesaba deposit. The BTI is believed to pre-date the SKI and to be coeval to slightly older than the PRI. It is further believed, based on drill hole evidence, that the upper igneous units of the PRI overlap specific BTI units. Supporting evidence for this new interpretation is based on igneous units that are unique to either the PRI or BTI, and different styles of sulfide mineralization between the two. The following geologic discussion is largely based on the work of Severson and Hauck (2008) but is condensed and summarized. 57 �Figure 3B-2. Preliminary geologic map of the Mesaba deposit showing major geologic units of the Bathtub, Partridge River, and South Kawishiwi intrusions. Major structural features associated with the deposit are also shown. 58 �GEOLOGIC SETTING Footwall Rocks As there is great commonality between the footwall rocks at the NorthMet Deposit and the Mesaba Deposit, they are discussed in the regional geology section. Structure As discussed earlier in this guidebook, there are three structural features that are pertinent to understanding the intrusive history of the BTI that include (Fig. 3B-2): 1. an east-west trending paired syncline and anticline in the footwall rocks referred to as the Bathtub Syncline and Local Boy Anticline; 2. a zone referred to as the “Hidden Rise” that separates the PRI and BTI; and 3. a north-trending fault zone, referred to as the Grano Fault, that has been postulated to have been the feeder zone for the BTI and footwall-injected massive sulfides of the Local Boy ore zone. The “Hidden Rise,” as discussed earlier, is a loosely-defined zone wherein scattered hornfels inclusions of footwall Virginia Formation are fairly common. When viewed collectively, the “Hidden Rise” defines an east-west trending “ridge” that is roughly positioned at the contact between the PRI and BTI. Along the far eastern edge of the Mesaba deposit is the north-trending Grano Fault, so named for the abundant and sometimes voluminous amounts of late granitoid and oxide rich pyroxenitic lenses (OUIs) associated with the fault zone (Severson, 1994). The late intrusive lenses are interpreted to have vertical configurations and were injected along subsidiary fault zones parallel to, and immediately west of, the Grano Fault. The late OUI and granitoid bodies cut the troctolitic rocks and thus demonstrate that the fault was active during and after emplacement of the BTI and PRI. BATHTUB INTRUSION The newly named Bathtub intrusion (BTI) is wholly contained in the central portion of the Mesaba (Babbitt) deposit. The BTI has recently been singled out as a separate intrusion to explain the abrupt change from typical Partridge River intrusion (PRI) stratigraphy, in the southern part of the deposit, to a completely different stratigraphy to the north in the remainder of the deposit. The BTI has been divided into two major units, BT1 and BT4, each of which contain several subunits. These units, in addition to footwall rocks, and structural features, are portrayed in Fig. 3-5. BT1 Unit The lowermost unit of the BTI is referred to as the BT1 Unit. It is very similar to Unit I of the nearby PRI in that it is heterogeneous-textured at all scales, contains abundant hornfels inclusions near the basal contact, and is the main sulfide-bearing unit at Mesaba. However, there are some important differences between Units I and BT1 that include: • Massive sulfide occurrences are more common near the basal contact in the BT1 than in Unit I (excluding the unique Local Boy ore zone) indicating that sulfide settling may have been a more important mineralization mechanism in the BTI; • Coarse- to very coarse-grained disseminated sulfides (up to several centimeters across) are exceedingly common in the lowermost portions of BT1; whereas, this same relationship is not so obvious in Unit I – this again implies the importance of a sulfide settling origin, and; • Ultramafic horizons and patches are very common in portions of the BT1; whereas, similar ultramafic horizons are not as common in Unit I of the PRI. 59 �The BT1 Unit has been further subdivided into several internal subunits that are discussed below. BT1-a This subunit of the BT1 is a heterogeneous-textured augite troctolite grading to olivine gabbro. The BT1a subunit is more common in the bottom half of the BT1 Unit and increases up dip (to the north) at the expense of most other subunits of the BT1. BT1-c At the base of the BT1 there is significant silica contamination of the magma, due to assimilation of the footwall rocks, and noritic rocks (norite to gabbro norite), with common hornfels inclusions, are the dominant rock types. Graphite occurrences are also commonly found in various rock types of BT1-c. The BT1-c subunit spatially occurs as a rind or coating along the basal contact. BT1-uz Wherever olivine-rich ultramafic rocks are common over appreciable intervals in the BT1 Unit, this subunit is used to designate ultramafic zones. The ultramafic rocks in these zones range from welldefined layers to zones where irregular ultramafic patches are presumably peppered throughout a troctolitic host rock. BT1-at This subunit of the BT1 is used to denote areas where anorthositic troctolite is the dominant rock type. BT-sli A few holes in the western end of the BTI exhibit well-defined modally-bedded rocks consisting of alternating troctolitic and ultramafic rocks. These intervals are designated as BT-sli for the Bathtub Side Layered Interval. The BT-sli subunit occurs about in the center of BT1 unit in close proximity to the “Hidden Rise.” BT4 Unit The uppermost unit of the BTI is referred to as the BT4 Unit. It was originally correlated with Unit IV of the PRI. However, the BT4 Unit is distinctly different from Unit IV in that the BT4 Unit at Mesaba is: • Often more heterogeneous-textured at all scales and composed of many alternating rock types; • commonly contains local sulfide-bearing zones; whereas, Unit IV is mostly sulfide-barren – the sulfides in BT4 are generally chalcopyrite-rich in comparison to chalcopyrite/cubanite ores in the underlying BT1 Unit; • floored by a semi-persistent ultramafic layer termed the "± Picrite" (see discussion below) in the central portion of the Bathtub ore zone; and • ultramafic layers and modally-bedded zones, termed the Bathtub Layered Interval (BTLI), are common in the central portion of the Bathtub ore zone. The BT4 Unit has been further subdivided into several internal subunits based on the presence of a dominant rock type. The various subdivisions of the BT4 Unit are briefly discussed below. BT4-a This subunit of the BT4 on the cross-sections denotes areas where heterogeneous-textured augite troctolite is the dominant rock type. BT4-at 60 �This subunit of the BT4 is used to denote areas where anorthositic troctolite is the dominant rock type. "± Picrite" At the base of BT4 is a semi-persistent olivine-enriched ultramafic horizon referred to as the "± Picrite." It is present in about 70% of the drill holes in the BTI-portion of the Mesaba deposit. The "± Picrite" is generally absent in the up dip direction (to the north) and is variably present to the south in the contact zone between the PRI and BTI. Where present, the "± Picrite" is about 1-15 feet thick, but exceptions are locally present. In some areas, the "± Picrite" consists of several stacked ultramafic horizons, or modal beds, that are interlayered with troctolitic rocks, and thus, the zone represents a collection of several cyclic layers. In other areas of the Mesaba deposit, the "± Picrite" is not always easily singled out as it occurs in close proximity to a downward thickening BTLI with similar ultramafic layers and modal beds. Therefore, in some instances it is difficult to pick the "± Picrite" out of a myriad of ultramafic horizons associated with either the BTLI or BT-sli. Bathtub Layered Interval (BTLI) In the vicinity of the Bathtub Syncline and the “Hidden Rise,” ultramafic layers and modal-bedded zones are extremely common within the BT4 Unit. In the eastern half of the Mesaba deposit the BTLI appears to be present in a subhorizontal saucer-shaped morphology. Conversely, in the western half of the deposit, the BTLI is confined to one or two cylinder-shaped zones, albeit with irregular edges, that is positioned in close proximity to the “Hidden Rise.” Overall, the ultramafic rock types of the BTLI are characterized by alternating assemblages of either/or: melatroctolite (picrite), feldspathic peridotite, peridotite, dunite (minor), olivine-rich troctolite, and troctolite with modal beds of olivine-rich layers. One or more of these rock types may be stacked above the other in no particular order, and the thickness of this assortment may be highly variable between drill holes. The number of individual ultramafic layers present within the BTLI for any particular drill hole varies drastically. The range in thickness for each of the individual ultramafic beds also shows considerable variation, ranging from a few inches to over tens of feet thick. Although the BTLI can be correlated as a package of alternating troctolitic and ultramafic layers, each of the individual ultramafic layers cannot be correlated on a hole by hole basis. This situation indicates that the ultramafic layers either: 1) bifurcate/divide into many thin ultramafic layers; 2) pinch out or have very limited spatial extent; 3) some may actually represent dike-like features (filter pressed crescumulates?); or 4) combinations of the above. Further complicating the picture, the inclination of contacts and modal bedding associated with the ultramafic layers are highly variable, ranging from 5°-80° (with localized overturned beds). This variation in inclinations can even be present in a single drill hole. For the most part, the bedding and contact inclinations in the BTLI are steeper higher up in the drill hole and gradually shallow with depth. The shallow to steep angles exhibited by the BTLI may reflect that the ultramafic layers originated via a variety of mechanisms that include: 1) crystal settling to form subhorizontal layers (dominant in the eastern half of the deposit); 2) filter-pressing to form localized dike-like morphologies; 3) slumpage and folding of the beds took place before they were fully crystallized to form highly irregular and overturned beds; 4) compaction differences took place during lithostatic loading of the crystal pile to form steep and irregular beds; 5) cooling and crystallization, or size/density sorting, took place along, and parallel to, the southern wall of the BTI (up against “The Hidden Rise”); or 6) combinations of all of these mechanisms. Whatever their origin, the steep beds displayed by the BTLI in the western half of the Mesaba deposit are inordinately associated with the “Hidden Rise” and the southernmost edge of the BTI. PARTRIDGE RIVER INTRUSIVE (PRI) AT MESABA Many of the igneous rock units that are present at the nearby NorthMet deposit are also present along the southern edge of the Mesaba deposit and are believed to represent units of the Partridge River Intrusion (see previous geologic setting discussion). Additionally, Units IV through VI of the PRI appear to extend 61 �northward and overlie the heterogeneous-textured BT4 Unit. This relationship, also depicted in Figure 3B-2, suggests that the BTI was eventually over-ridden/overlain by the upper units of the PRI. The overall timing of emplacement for the lower PRI units versus the BTI is unknown but correlations in cross-sections crudely suggest the following: • Units I through III were intruded first along the southern edge of the Mesaba deposit with a vent area located somewhere to the southwest. The “Hidden Rise” generally marks the northern extent of this intrusive activity and originally formed as part of the floor to these units. Unit III may have been intruded as thin lenses across and north of the “Hidden Rise” – this may explain the local presence of Unit III-like inclusions in the BTI. • Concurrent with or after the above activity, the BT1 Unit was intruded from a vent area located somewhere to the east, possibly from the Grano Fault area. The “Hidden Rise” formed the southern wall of this particular magma chamber. • The BT4 Unit was intruded into the same magma chamber but was emplaced above the BT1 Unit. • Concurrent with or after the above activity, Units IV through VII+ of the PRI were intruded from a vent area located somewhere to the southeast. These upper units were emplaced over the BT4 Unit. MINERALIZATION AT MESABA The Mesaba deposit is characterized by disseminated sulfide mineralization, which occurs most commonly as accumulations of chalcopyrite, cubanite, pyrrhotite, and pentlandite. Additionally, common occurrences of talnakhite have been noted in close proximity to the “Hidden Rise.” Short intercepts of semi-massive to massive sulfide mineralization are locally encountered in the BT1-c. Sulfur isotope analyses have indicated that the source of the sulfur used in the formation of the sulfides at Mesaba is the Virginia Formation (Ripley, 1986). The model of sulfide deposition entails turbulent injection of units of the BTI wherein immiscible sulfide droplets coalesce within the silicate melts and attract the chalcophile elements (chiefly copper and nickel) through magma mixing. Thus, the most contaminated magma (from assimilation of footwall Virginia formation) hosts basal sulfides that contain excess sulfur and iron relative to intrusive units higher above the footwall. The sulfide content of the rock increases, often dramatically, as the footwall is approached. This sulfide content increase with depth is accompanied by the increasing presence of pyrrhotite and a subsequent change in the copper bearing sulfides (cubanite is dominant over chalcopyrite with depth). The disseminated mineralization is generally composed of 1-4% sulfides, but can reach upwards of 8-12 % sulfides as the footwall is approached. The most important mineralized zone at Mesaba is the basal zone, starting at the footwall Virginia Formation contact, and ranges between 200 and 400 feet thick (60 and 125 meters thick). Higher up in the intrusive package, often overlapping the BT1-BT4 unit boundary, is a second zone of disseminated sulfide mineralization that is more erratic and discontinuous in nature but contains markedly lower amounts of cubanite and pyrrhotite. The mineralized footprint of the Mesaba deposit is oblong to arcuate in shape, 3,000 by 13,000 feet (925 by 4,000 meters) in approximate dimensions, crops out at the surface on the northern/up-dip side and extends to approximately 1,650 feet (500 meters) below surface in the southern/downdip direction. The strongest basal mineralization is often localized within the Bathtub Syncline. Here, concentration of sulfides by gravitational settling into the footwall depression has likely occurred. 62 �PGE Mineralization at Mesaba Platinum group element (PGE) mineralization in the BTI at Mesaba occurs to a lesser degree than the other deposits and intrusions. Generally, the highest PGE values at Mesaba are associated with Cu-rich massive to semi-massive sulfides in the Local Boy ore zone. Analyses from sampled intervals (5-15 feet thick) record values as high as 11.1 ppm Pd, 8.3 ppm Pt, 13.1 ppm Au, and 62 ppm Ag in the sulfide-rich ores (Severson and Barnes, 1991; Hauck and Severson, 2000). The majority of the anomalous PGE values are spatially distributed along the axis of the Local Boy anticline with the highest Cu and PGE values occurring in the west half of Local Boy. The Grano Fault may have served as a feeder zone to the massive sulfides that were injected into the footwall rocks along the Local Boy Anticline as an immiscible sulfide melt. This melt fractionally crystallized in an east-to-west direction and progressively became enriched in PGE towards the west (see discussion below). MASSIVE SULFIDES AT THE LOCAL BOY ORE ZONE OF THE MESABA DEPOSIT Cu-rich massive sulfides near the basal contact of the Complex are locally present at the Mesaba deposit in a small zone referred to as the Local Boy ore zone. In 1976, AMAX Inc. completed a 1,700-foot-deep exploratory shaft (Minnamax shaft), and in 1977, completed four drifts (A, B, C, and D; Figures 3B-3 and 3B-4). Underground Fan drilling (217 holes) was completed in 1978 to further define the massive sulfide distribution. Sulfide minerals include pyrrhotite, pentlandite, chalcopyrite, talnakhite, cubanite, maucherite (nickel arsenide), sphalerite, bornite and late mackinawite, chalcocite, covellite, godlevskite, and native silver (Severson and Barnes, 1991). Footwall Structures in the Local Boy Ore Zone Several investigators have recognized that pre-existing structural conditions in the footwall rocks strongly influenced the basal contact of the Duluth Complex (Mancuso and Dolence, 1970; Watowich, 1978; Holst et al., 1986; Martineau, 1989; Severson and Barnes, 1991). Major irregularities in the basal contact are generally related to folds in the underlying country rock indicating that intrusion proceeded more or less along bedding planes in the footwall rocks (Holst et al., 1986). This is readily expressed by a major eastwest -trending trough and ridge in the basal contact at Mesaba that coincides exactly with a synclineanticline that is defined by the top of the Biwabik Iron Formation (BIF). The thickness of preserved Virginia Formation between the Complex and the BIF is variable due to the amount of material assimilated by the Complex. The Local Boy ore zone is also situated over this anticlinal ridge. The majority of massive sulfide ore zones, hosted mainly by the Virginia Formation (Severson and Barnes, 1991), are broadly coincident with the axis of the anticline. The contoured top of the BIF in the Local Boy area is shown in Figure 3B-3 (left). Similar anticline geometries are also present for the basal contact as shown in Figure 3B-3 (right). All the data indicate that an EW-trending anticline is the major structural feature present within the footwall rocks of the Local Boy area. 63 �Figure 3B-3. Contoured top of the Biwabik Iron Formation at Local Boy (left), and the contoured top of the basal contact between the footwall Virginia Formation and the Partridge River intrusion at Local Boy (right). Diagrams from Severson and Hauck, 2008. Mineralization Trends in the Massive Sulfide at the Local Boy Ore Zone The vast majority of massive sulfides at Local Boy are contained within the Paleoproterozoic Virginia Formation. Even though the massive sulfides straddle the basal contact, most of the massive sulfides are associated with either hornfelsed sedimentary inclusions above the contact or with footwall rocks below the contact while the interfingering intrusive rocks are relatively barren of massive sulfides (Severson and Barnes, 1991). This suggests that the massive sulfide ores were not formed in this area by the gravitational settling of sulfides, but rather, the ores formed by injection of an immiscible sulfide melt into structurally prepared areas within the footwall rocks along the Local Boy anticline in a vein-like setting. A similar mechanism is proposed for the Norilsk-Talnakh deposits in Russia. Even though the basal contact of the Complex with the Virginia Formation is highly undulatory, the massive sulfides exhibit a definite top and bottom. The ore is distributed such that most of it is contained within a zone between 20 feet and 300 feet above the top of the Biwabik Iron Formation. The geologic constraint for the bottom of the ore zone generally corresponds to the top of the VirgSill (a structurally competent unit). The constraints for the upper portion of the ore zone are unknown and may have been obliterated during emplacement of the Complex. Figure 5B-7 is an attempt to show, in a plan view, where massive sulfide zones are present. Also shown in the figure are the different massive sulfide types (ranging from pyrrhotite-dominant to Cu-rich) relative to structural features. The relationships shown in Figure 3B-4 indicate that the massive sulfides show a progressive change in an east-to-west direction from Cu-poor massive sulfides to Cu-rich massive sulfides in the vicinity of the Local Boy anticline. These relationships suggest that the injected immiscible sulfide melt underwent fractional crystallization 64 �and progressively became more Cu and PGE enriched as it moved through the footwall rocks in an eastto-west direction. Figure 3B-4. Potential distribution of semi-massive to massive sulfide types (Cu-poor versus Cu-rich) at the Local Boy ore zone (left); and an isopach map of the cumulative thickness of the massive sulfide zones at the Local Boy ore zone (right). Note that the massive sulfides are not present as a continuous blanket, but rather, as one or more stacked disjointed/separated multiple horizons near the basal contact. Diagrams from Severson and Hauck, 2008. A possible feeder vent for the sulfide injection event may have been the Grano Fault, which was repeatedly reactivated during emplacement of the Complex. Other data that indicates that the Grano Fault was a potential feeder vent include: 1) the massive sulfides are more common, and thicker (Figure 3B-4), close to the Grano Fault (feeder) and along the axis of the Local Boy anticline (structurally-prepared site); 2) the VirgSill rarely contains significant amounts of disseminated sulfides – except near the Grano Fault; and 3) the Biwabik Iron Formation rarely contains sulfides – except near the Grano Fault. In summary, the massive sulfides at the Local Boy ore zone are interpreted to be structurally controlled in that they are situated along the axis of the Local Boy anticline. The massive sulfides are locally Cu-rich (5-25% Cu; Severson and Hauck (2008) – based on historical assay data on file at MDNR) and are almost exclusively hosted by the Virginia Formation. Sulfide textures suggest that the massive sulfides were injected as an immiscible sulfide melt into the footwall rocks. The overall pattern of sulfide types and PGE contents suggest that the sulfides formed via a process of fractional crystallization of an immiscible sulfide melt as it migrated into the footwall rocks. The Grano Fault is inferred to represent the potential feeder zone in this scenario. 65 �PART 3C: TWIN METALS MINNESOTA’S MATURI DEPOSIT Kevin D. Boerst – Twin Metals Minnesota (Portions previously written by Dean M. Peterson – formerly with Duluth Metals Limited) INTRODUCTION Twin Metals Minnesota’s Maturi deposit is the largest and highest-grade classified Cu-Ni-PGE deposit in the 1.1 Ga. Duluth Complex of northeastern Minnesota. The deposit is located near the north end of the South Kawishiwi intrusion (SKI) west-southwest of the junction of the Nickel Lake Macrodike (NLM) and the SKI (Peterson et al., 2006; Peterson and Albers, 2007; Tharlason et al., 2007; Peterson, 2008; Gal, 2008; and White, 2010). The deposit was discovered utilizing a genetic ore deposit model that identified channelized magma flow within the SKI under a large xenolith/pillar of anorthosite. The model led to exploratory drilling in 2006; deposit discovery and resource estimations in 2007 & 2008; a joint venture with Antofagasta plc in 2010; and significant resource expansion and classification upgrades in 2012 and 2014 (Fig. 3C-1). The company is currently optimizing its business case with the aim of completing a Mine Plan of Operation (MPO) in the future. RESOURCES The Mineral Resource estimate for the Maturi deposit (completed by AMEC) incorporate assay data from 564 drill holes totaling 1,466,641 feet (excluding wedges) drilled on the Maturi deposit that includes 75 legacy holes also in the geologic data base. The April 2014 Resource Estimates for the Maturi (as well as the Birch Lake and Spruce Road deposits) is based on a 0.3% copper cut-off grade to define the resource model. Based on AMEC`s review of metal prices, process recoveries, refining costs and underground mine operating costs likely to apply at the Twin Metals site, the 0.3% copper cut-off grade (highlighted) is considered the base case for the statement of Indicated and Inferred Mineral Resources at this time. The estimates at the cut-off grades higher and lower than the base case are provided to show sensitivity of the cut-off grade (Table 3C-1). Inferred Resources Indicated Resources Measured Resources Table 3C-1. April 2014 resource estimate for TMM’s Maturi Deposit. Cu% Million Cu Ni Pt Cut-off Tons % % ppm 0.2 327 0.61 0.20 0.141 0.3 308 0.63 0.20 0.146 0.4 275 0.66 0.21 0.5 237 0.70 0.22 0.6 183 0.74 0.2 881 0.3 0.4 Au Ag Co ppm ppm ppm ppm 0.328 0.080 2.2 105 0.339 0.083 2.3 107 0.155 0.359 0.088 2.4 110 0.165 0.383 0.093 2.5 113 0.24 0.177 0.411 0.100 2.7 116 0.56 0.18 0.148 0.336 0.080 2.0 102 822 0.58 0.19 0.155 0.350 0.083 2.1 104 716 0.61 0.20 0.166 0.375 0.089 2.2 106 0.5 547 0.66 0.21 0.186 0.420 0.099 2.4 109 0.6 379 0.71 0.22 0.205 0.461 0.108 2.6 111 0.2 768 0.42 0.13 0.116 0.262 0.059 1.6 81 0.3 531 0.49 0.16 0.138 0.314 0.070 1.8 98 0.4 358 0.57 0.19 0.167 0.376 0.083 2.0 110 0.5 235 0.63 0.20 0.202 0.449 0.099 2.3 112 0.6 127 0.69 0.21 0.246 0.545 0.118 2.5 111 66 Pd �Figure 3C-1. Map of Twin Metals Minnesota’s deposits and resource classification. 67 �GEOLOGY OF THE MATURI DEPOSIT The Maturi Deposit is located within the South Kawishiwi Intrusion (SKI), a shallow dipping (~24º eastsoutheast) sill-like troctolitic intrusion exposed in an 8- x 32-kilometer arcuate band along the northwestern margin of the Duluth Complex. Lithologic units within the Maturi deposit include Mesoproterozoic rocks of the SKI and Anorthositic Series of the Duluth Complex as well as basalt xenoliths of the North Shore Volcanic Group. At Maturi, SKI magmas intruded between hanging wall anorthositic rocks and footwall granitic rocks of the Neoarchean Giants Range batholith (Fig. 5C1). Brief descriptions of the lithostratigraphic units within the Maturi Deposit are given in Table 3C-2. Table 3C-2. Lithostratigraphic units within the Maturi deposit. Duluth Complex and related rocks (1.1 Ga.) Anorthositic troctolite to troctolite (ATA Series) - Medium to coarse-grained, homogeneous, wellfoliated and locally layered anorthositic troctolite, troctolite, and ophitic troctolitic rocks. In the field, this unit is commonly referred to as the “sea of troctolite”. SKI Augite-bearing troctolite (Main AGT) - Homogenous, coarse-grained, subophitic to ophitic, poorly foliated augite troctolite characterized by scattered augite-rich pegmatitic clots and patches. Commonly capped by hanging wall inclusions (HB & Ai) and interpreted to be the solidified basaltic liquid that carried the BMZ crystals and sulfides. Sulfide-bearing troctolite (BMZ) - Heterogeneous, sulfide-bearing, varitextured troctolite, augite troctolite, anorthositic troctolite, and olivine gabbro with 0.5 - 5% disseminated chalcopyrite, cubanite, talnakhite, pentlandite and pyrrhotite. Anorthosite (An-Series & Ai) - Undifferentiated Anorthositic Series inclusions. Includes wellfoliated very coarse-grained anorthosite, troctolitic-anorthosite, poikilitic troctolitic anorthosite, gabbroic anorthosite, gabbro, and locally troctolite. Inclusions range from a few cm’s to elongate bodies measured in km’s. Xenoliths in the SKI Anorthositic gabbro to gabbro (Upper Gabbro) - Mixed group of Anorthositic Series rocks that occur in the central portion of the map area. Includes well-foliated anorthositic gabbro, gabbro, oxide gabbro, anorthosite, and augite troctolite. Basaltic hornfels (Upper Basalt, HB) - Fine-grained, granoblastic to poikiloblastic basaltic hornfels; consists of variable amounts of plagioclase, augite, olivine, hypersthene, and inverted pigeonite. Commonly associated with Anorthosite xenoliths (unit Ai). Giants Range Batholith (2.68 Ga.) Footwall Porphyritic quartz monzonite (GRB) - Pink, coarse-grained, hornblende-phyric, porphyritic quartz monzonite with large (1-2 cm) orthoclase phenocrysts. Also contains irregular zones of aplite, lamprophyre, and supracrustal xenoliths. Strongly recrystallized and partially melted locally adjacent to the contact with the SKI. Early modeling of the Maturi Deposit was limited due to wider-spaced drill holes across the deposit area, though Severson (1994) did a remarkable job in defining the igneous stratigraphy of the SKI along a 19 mile strike length. Severson recognized the fact that the rocks below the lower pegmatite (PEG) unit typically contained sulfides and that rocks above the PEG unit were a monotonous sequence of sulfidebarren anorthositic troctolite to troctolite (AT/T), and augite-troctolite (Main AGT). Two detailed geologic cross sections through the Maturi Deposit are presented in Figure 3C-2. These sections display the continuity of the basal mineralization as well as the differences in the hanging wall 68 �stratigraphy from west to east through the deposit. In the east, the deposit is located under an extremely thick (>1,000m) megaxenolith of Anorthosite Series rocks, and in essence the basal SKI can be viewed as a thin sill-like body. To the west, the anorthosite xenolith ends and the immediate hanging wall rocks to the deposit are sulfide-barren troctolites of the Main AGT unit. We interpret that the Main AGT as the solidified troctolite melt that carried the crystals and sulfide droplets of the magmatic slurry. Figure 3C-2. Geologic cross sections through the Maturi Deposit. 69 �In 2008, geologists from Duluth Metals came to the realization that the initial basaltic composition SKI magmas that ultimately solidified to create the Maturi deposit intruded as sulfide-bearing, crystal-laden (olivine and plagioclase crystals), magmatic slurries. Based on this new interpretation, the company reinterpreted Severson’s (1994) regional basal stratigraphy (units U3, BH, BAN) of the SKI (Fig. 3C-3) at Maturi into the Basal Mineralized Zone, or BMZ. The company believes that the geometry of the system (sill-like sub-horizontal intrusion) and the inherent crystallinity of the basaltic melts (phenocrysts of plagioclase and olivine) led to crystal sorting and melting of the footwall granitic rocks to create the heterogeneous lithologies and textures of the BMZ. Figure 5C-3. Simplified crystal-liquid slurry model for the SKI in the Maturi area. MATURI ORE DEPOSIT MODEL In 2012, the geology of the mineralized portions (the BMZ) of the Maturi deposit were reevaluated by the geologic staffs of Duluth Metals, Twin Metals, and geologists from AMEC utilizing a significant volume of new, high-quality geochemical and geological data during the completion of an updated mineral resource classification by the consulting firm AMEC. Mineralization in both the BMZ and footwall at Maturi were reclassified based on patterns in the physical distribution of mineralization as projected on down-hole plots. Sulfide mineralization is characterized by several distinct patterns, including (1) very low grade mineralized intervals showing low variability (Stage 1), (2) moderate grade mineralized intervals showing low variability (Stage 2), and 3) higher grade mineralized intervals showing higher variability and commonly bounded by low grade selvages (Stage 3) (Fig. 3C-5). Significantly, the contacts between different mineralized intervals are typically quite abrupt. A single hole might contain one or several distinct mineralized intervals within the BMZ, including higher grade intervals with the highest grade occurring at the top, middle, or bottom of the section. Based on these criteria, four intrusive subunits, characterized by common grade profiles, were defined in the BMZ. In addition, two distinct suites of mineralization were identified in the footwall 70 �rocks, including Ni-Co enriched semi-massive to massive sulfide zones and disseminated Cu-PGE enriched zones deep in the footwall granitoids. The classifications derived from this exercise were validated by multivariate statistical analysis of multielement geochemical data, including principal component analysis (Fig. 3C-6) and factor analysis. This investigation revealed a significant correlation of multi-element geochemistry to mineralization within the BMZ as well as several possible subdivisions of the BMZ based on both the physical distribution patterns of mineralization and the geochemistry of the host rocks. The Maturi subunits so defined and validated were determined to occur in a consistent stratigraphic order, and are correlative across the deposit. Figure 3C-5. Revised igneous stratigraphy of the BMZ and adjacent rocks within the Maturi deposit. Figure 3C-6. Multi-element principal component analysis plot of MEX-Series drill hole geochemical data. 71 �Typical geochemical plots of Maturi drill holes are presented in Figure 3C-7 and display several of the patterns that were originally identified in the development of the revised geological model of the Maturi deposit. As well, an idealized intrusive sequence model for the SKI in the Maturi deposit area is given in Figure 3C-8 and a NW to SE cross section of the modeled units of the BMZ is presented in Figure 3C-9. Figure 3C-7. Downhole geochemical and principal component plots of typical drill holes within the Maturi deposit. 72 �Figure 3C-8. Idealized intrusive sequence model of the SKI in the Maturi deposit area. Figure 3C-9. Northwest (left) to Southeast (right) cross section through the Maturi deposit depicting the recently modeled geological units of the BMZ. Detailed descriptions of the seven units modeled for the Maturi deposit is well beyond the scope of this field trip guidebook. However, brief descriptions are provided in Table 3C-3 below and geochemical plots of copper, nickel, and precious metals are given in Figure 3C-10. One of the most important outcomes of the reinterpretation of the geology and mineralization within the Maturi deposit has been the identification of the higher-grade Stage 3 (S3) intrusive unit of the SKI (Table 3C-4). S3 has the highest grade and is the most widely distributed of the four BMZ units (Fig. 3C11). Cu, Ni, and PGEs and are all significantly elevated in S3 relative to the other BMZ units and the 73 �mineralized GRB. Stage 2 (S2) mineralization is overall much lower grade than S3, but locally S2 is well mineralized, and will likely contribute significantly to the deposit economics. Mineralization in the GRB is overall low grade and discontinuous. However, local zones of the unit G-N (where Cu and especially Ni locally occur as massive and semi-massive sulfides) are very high grade and may contribute to the resource. Table 3C-3. Interpreted lithostratigraphic-chemostratigraphic units within and adjacent to the BMZ within the Maturi deposit. Unit UH Description Stage 3 Continuous, higher-grade, PGE-enriched, heterolithic troctolite and melatroctolite Stage 2 Continuous, moderate-grade, heterolithic, oxide-bearing, augite-troctolite to troctolite Stage 1 Discontinuous, barren to very low-grade, homogeneous troctolite, gabbro, anorthosite and/or norite G-N Irregular, locally high-grade, Ni- and Co-enriched semi-massive to massive sulfide pods and veins at or immediately below the basal SKI contact. G-M Discontinuous, low to moderate-grade, disseminated Cu and PGE enriched mineralization. G-B Continuous, barren granitoid footwall rocks Discontinuous, barren to low-grade, highly variable troctolite Figure 3C-10. Geochemical boxplots of composited drill hole geochemistry for units within and adjacent to the BMZ within the Maturi deposit. 74 �The current lithostratigraphic model of the Maturi deposit effectively discriminates between higher- and lower-grade mineralization and provides a realistic geological model. The new data allowed correlation of units from hole-to-hole and section-to-section resulting in a very robust geologic model upon which to build mine plans and further our understanding of the magmatic processes that occurred to generate TMM’s Maturi Deposit. Figure 3C-11. Plan views of Stage 3 Cu, Ni, and TPM grades from the TMM Maturi deposit block model. 75 �REFERENCES Desaultels, P., and Patelke, R., 2008, Updated Technical Report on the NorthMet Deposit, Minnesota, USA, by Wardrop Engineering, (Toronto) for PolyMet, 109 p. 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Hauck, S.A., Severson, M.J., Zanko, L.M., Barnes, S.-J., Morton, P., Aliminas, H.V., Foord, E.E., and Dahlberg, E.H., 1997, An overview of the geology and oxide, sulfide, and platinum-group element mineralization along the western and northern contacts of the Duluth Complex, in Ojakangas, R.W., Dickas, A.B., and Green, J.C., eds., Middle Proterozoic to Cambrian rifting, central North America: Geological Society of America Special Paper 312, p. 137-185. Hauck, S.A., and Severson, M.J., 2000, Platinum-group element-gold-silver-copper-nickel-cobalt assays primarily from the Local Boy ore zone, Babbitt deposit, Duluth Complex, northeastern Minnesota: An update: Natural Resources Research Institute, University of Minnesota, Duluth, Report of Investigations NRRI/RI-2000/01, 48 p. Holst, T.B., Mullenmeister, E.E., Chandler, V.W., Green, J.C., and Weiblen, P.W., 1986, Relationship of structural geology to the Duluth Complex to economic mineralization: Minnesota Department of Natural Resources, Division of Minerals Report 241-2. Ikola, R.J., 2006, Vertical Electrical Soundings - NorthMet Project - St. Louis County, Minnesota, Prepared for PolyMet Mining Corp.: R.J. Ikola and Associates, Inc., Hibbing, Minnesota, 115 p. Jennings, C.E., and Reynolds, W.K., 2005, Surficial Geology of the Mesabi Iron Range, Minnesota: Minnesota Geological Survey, Miscellaneous Map M-164, scale 1:100,000. Lehr, J.D., and Hobbs, H.C., 1992, Glacial geology of the Laurentian divide area, St. Louis and Lake Counties, Minnesota: Minnesota Geological Survey Field Trip guidebook series, 18, Prepared for the 39th Midwest Friends of the Pleistocene Field Trip, Biwabik, Minnesota, 1992, 73 p., map, 1:250,000. Listerud, W.H., and Meineke, D.G.., 1977, Mineral resources of a portion of the Duluth Complex and adjacent rocks in St. Louis and Lake Counties, northeastern Minnesota: Minnesota Department of Natural Resources, Division of Minerals, Report 93, 74p. Mancuso, J.D., and Dolence, J.D., 1970, Structure of the Duluth Gabbro Complex in the Babbitt area, Minnesota (abs.): 16th Annual Institute on Lake Superior Geology, Thunder Bay, Ontario, p. 27. Martineau, M. P., 1989, Empirically derived controls on Cu-Ni mineralization: A comparison between fertile and barren gabbros in the Duluth Complex, Minnesota, U.S.A.: in Prendergast, M. D., and Jones, M. J., eds., Magmatic Sulphides - The Zimbabwe Volume: Inst. Min. Metall., London, p. 117-137. Miller, J.D., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.M., 2001, Geological map of the Duluth Complex and related rocks, Northeastern Minnesota; Minnesota Geological Survey, Miscellaneous Map M119, scale 1:200,000. Miller, J.D., Jr. and Severson, M.J., 2002, Geology of the Duluth Complex in Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E., 2002a, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations RI-58, p. 106-143. 76 �Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E., 2002a, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations RI-58, 207 p. Morton, P.M., and Hauck, S.A., 1987, PGE, Au and Ag contents of Cu-Ni Sulfides Found at the Base of the Duluth Complex, Northeastern Minnesota: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report NRRI/GMIN-TR-87-4, 94 p. Muhich, T.G., 1993, Movement of titanium across the Duluth Complex–Biwabik Iron Formation contact at Dunka Pit, Mesabi Iron Range, northeastern Minnesota: Duluth, Minn., University of Minnesota Duluth, M.S. thesis, 154 p. Park, Y.R., Ripley, E.M., Severson, M.J., and Hauck, S.A., 1999, Stable isotopic studies of mafic sills and Proterozoic metasedimentary rocks located beneath the Duluth Complex, Minnesota: Geochimica et Cosmochimica Acta, v. 63, no. 5, p. 657-674. Patelke, R.L., 2003, Exploration drill hole lithology, geologic unit, copper-nickel assay, and location database for the Keweenawan Duluth Complex, northeastern Minnesota: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report, NRRI/TR-2003/21, 96 p. Patelke, R.L., and Geerts, S.D.M., 2006, PolyMet NorthMet Drill Hole / Geological Database Recompilation: Location, Downhole Survey, Assay, Lithology, Geotechnical Data, and Related Information, 2004 to 2006, PolyMet Mining Inc. Internal report, 86 p. Peterson, D.M. and Albers, P.B., 2007, South Kawishiwi Intrusion Cu-Ni-PGE mineralization in association with the Nickel Lake Macrodike, Institute on Lake Superior Geology, 53rd Annual Meeting, Field Trip Guidebook, Lutsen, Minnesota, Volume 53. Peterson, D.M., 2001a, Development of a conceptual model of Cu-Ni-PGE mineralization in a portion of the South Kawishiwi Intrusion, Duluth Complex, Minnesota: Laurentian University – Society of Economic Geologists, Second Annual PGE Workshop, Sudbury, Ontario. Peterson, D.M. and Severson, M.J., 2002, Chapter 4, Archean and Paleoproterozoic rocks forming the footwall of the Duluth Complex, in Geology and mineral potential of the Duluth Complex and related intrusions of northeastern Minnesota, Minnesota Geological Survey, Report of Investigations 58, pp. 76-93. Peterson, D.M., 2008, Bedrock geologic map of the Duluth Complex in the northern South Kawishiwi intrusion and surrounding area, Lake and St. Louis Counties, Minnesota: Natural Resources Research Institute, Map Series NRRI/MAP-2008-01, scale 1:20,000. Phinney, W.C., 1972, Duluth Complex, history and nomenclature, in Sims, P. K., and Morey, G. B., eds., Geology of Minnesota: A Centennial Volume: Minn. Geol. Survey, pp. 333-334. Ripley, E.M., 1986, Origin and concentration mechanisms of copper and nickel in Duluth Complex sulfide zones – a dilemma: Economic Geology, v. 81, p. 974-978. Sabelin, T., and Iwasaki, I., 1985, Metallurgical evaluation of chromium-bearing drill core samples from the Duluth Complex (Contract report of Minnesota Department of Natural Resources, Division of Minerals): Minerals Resources Research Center, University of Minnesota, Minneapolis, MN, 58 p. Sabelin, T., and Iwasaki, I., 1986, Evaluation of platinum group metal occurrence in Duval 15 drill core from the Duluth Complex: Internal report, Minerals Resources Research Center, University of Minnesota, Minneapolis, MN, 23 p. Severson, M.J., 1994, Igneous stratigraphy of the South Kawishiwi intrusion, Duluth Complex, northeastern Minnesota: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report NRRI/TR 93/34, 210 p. (with plates) Severson, M.J., and Barnes, R.J., 1991, Geology, mineralization and geostatistics of the Minnamax/Babbitt Cu-Ni deposit (Local Boy area), Minnesota, Part II: Mineralization and geostatistics: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report, NRRI/TR-91/13b, 216 p. Severson, M.J., and Hauck, S.A., 1990, Geology, geochemistry, and stratigraphy of a portion of the Partridge River intrusion: Natural Resources Research Institute, University of Minnesota-Duluth, Technical Report, NRRI/GMINTR-89-11, 236p. (with plates). 77 �Severson, M.J., and Hauck, S.A., 2003, Platinum group elements (PGEs) and platinum group minerals (PGMs) in the Duluth Complex: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report NRRI/TR-2003/37, 296 p. Severson, M.J., and Hauck, S.A., 2008, Finish logging of Duluth Complex drill core (And a reinterpretation of the geology at the Mesaba (Babbitt) deposit): University of Minnesota Duluth, Natural Resources Research Institute, Technical Report NRRI/TR-2008/17, 68 p. Severson, M.J., Miller, J.D., Jr., Peterson, D.M., Green, J.C., and Hauck, S.A., 2002, Mineral potential of the Duluth Complex and related intrusions in Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E., 2002a, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations RI-58, p. 164-200. Severson, M.J., Patelke, R.L., Hauck, S.A., and Zanko, L.M., 1994a, The Babbitt copper-nickel deposit, Part B: Structural datums: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report NRRI/TR-94/21b, 48 p. Severson, M.J., Patelke, R.L., Hauck, S.A., and Zanko, L.M., 1994b, The Babbitt copper-nickel deposit, Part C: Igneous geology, footwall lithologies, and cross-sections: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report NRRI/TR-94/21c, 79 p. Sims, P.K., and Morey, G.B., eds. 1972, Geology of Minnesota: A Centennial Volume, Minnesota Geological Survey. Tharalson, E., Sweet, G., Boisjoli, T., Lentz, B., Fellows, T., and Peterson, D., 2007, Geological Map of the Nickel Lake Macrodike and Northern Bald Eagle Intrusion, Lake County, Northeastern Minnesota: Precambrian Research Center Map Series PRC/MAP-2007-01, scale 1:10,000. Watowich, S.N., Malcomb, J.B., and Parker, P.D., 1981, A review of the Duluth Gabbro Complex as a domestic source of critical and strategic metals: Society of Mining Engineers of AIME, preprint 81-351, 9 p. White, C., 2010, The Nokomis Deposit, a Masters of Geology thesis: University of Minnesota, Duluth. Wolff, J.F., 1917, Recent geologic developments on the Mesabi range, Minnesota: American Institute of Mining and Metallurgical Engineers Transactions, v. 56, p. 142-169. Zanko, L.M., Severson, M.J., and Ripley, E.M., 1994, Geology and mineralization of the Serpentine copper-nickel deposit, Duluth Complex, Minnesota: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report, NRRI/GMIN-TR-93-52, 90p. 78 �FIELD TRIP 4 May 4, 2016 DULUTH STREAM GEOMORPHOLOGY AND THE JUNE 2012 FLOOD Karen Gran Department of Earth and Environmental Sciences University of Minnesota Duluth FIELD TRIP CANCELLED 79 �FIELD TRIP 5 May 4, 2016 GEOLOGY OF THE ENDION SILL EXPOSED ALONG DULUTH’S LAKEWALK James D. Miller Department of Earth and Environmental Sciences and the Precambrian Research Center University of Minnesota Duluth Introduction The Endion Sill is a mafic to felsic, hypabyssal intrusion that was emplaced into the North Shore Volcanic Group lavas during the 1.1 Ga Midcontinent Rift (Miller and Green, 2002). The sill is the lowest hypabyssal intrusion in the volcanic rocks overlying the Duluth Complex at Duluth. This gently east-dipping, approximately 425m-thick intrusion is semi-continuously exposed along a half-mile stretch of Lake Superior shoreline in wave-washed outcrops adjacent to Duluth’s Lakewalk path between 16th and 28th avenues east. This field trip will highlight the mineralogic and textural attributes of the diverse lithologies exposed in these shoreline outcrops and consider the various ideas for its emplacement and crystallization history. All who have studied the Endion Sill (Schwartz and Sandberg, 1940; Ernst, 1955, 1960; Oestrike, 1983; Gardner, 1987; and Jerde, 1991) have noted its being generally composed of a lower gabbroic zone and an upper intermediate to felsic zone. The sill was emplaced between basaltic to basaltic andesite flows comprising the upper part of the Leif Erickson Park Lavas (unit Mnb, Fig. 1) and the 300m-thick, Congdon Park rhyolite (unit Mnc, Fig. 1) which forms the lowermost flow unit of the Lakeside Lavas of the NSVG (Green and Miller, 2008). The emplacement of mafic intrusions beneath felsic lavas and intrusions is a commonly observed phenomenon in igneous rocks of the Midcontinent Rift and likely is triggered by the felsic rocks serving as a density barrier to mafic magma (Miller, 2015). The gradational contacts typically observed between of felsic rock types and underlying mafic intrusions further suggests partial melting and assimilation of the felsic hanging wall is involved in the evolution of the mafic intrusion. This contamination process appears to have occurred in the Endion Sill as well. Previous Studies Schwartz and Sandberg (1940) were the first to report on the petrology of mafic and felsic components of the Endion Sill and of related sills in the Duluth area, the Northland and Lester River sills. From their field observations and petrographic studies and major and minor element wet chemical analysis of seven samples, they considered five different hypotheses to explain the bimodal distribution of rock types, and particularly the occurrence of felsic rock types (commonly referred to as granophyre or “red rock”): 1) as a composite felsic intrusion, 2) by metamorphism of the rhyolite, 3) by hydrothermal alteration, 4) by a mix of assimilation and differentiation, and 5) by differentiation alone. They concluded “that many factors had some effect, but that differentiation within each sill accounts for the major part of the segregation of rock types into two distinct facies.” Nevertheless, they acknowledge that simple fractional crystallization of the diabase could not produce the large amount of felsic material in the sills. 80 �Figure 1. Geologic setting of the Endion Sill (brown unit Mne) in the Duluth 7.5’ quadrangle (from Green and Miller, 2008). Footwall rocks (to the west) comprise the basaltic Leif Erickson Park Lavas and hanging wall rocks are composed of the Congdon Park Rhyolite (Unit Mnc) and overlying mixed lavas of the Lakeside lavas. The Mep unit is a granophyre that occurs at the diabase-rhyolite contact. Geochemical analyses of Esamples of Jerde (1991) are given in Table 1. For his MS thesis at the University of Minnesota under the advisement of SS Goldich, Ernst (1955, 1960) conducted a detailed petrographic study of 27 samples collected from lakeshore exposures and streambed exposures up Tischer Creek (Fig. 2). Ernst (1955) defined several stratiform map units within the sill. The lower half of the sill is composed of medium-grained, subophitic to ophitic diabase. The diabase is overlain over a narrowly gradational contact by a medium-grained, intermediate “mottled” granodiorite which gradually transitions upward into granophyre/”red rock” which he calls a “sodalite-adamellite”. This “medial granophyre” then abruptly grades upward into a fine-grained diabase. The upper 10 meters of the sill is capped by a fine-grained “upper granophyre” which is in sharp irregular contact with the diabase. The upper granophyre-Congdon Park rhyolite contact is observed to be abrupt in sea cliff exposures and in streambed outcrops along Tischer Creek (where it crosses East 2nd Street, Fig. 2). The transition into rhyolite is marked by the abrupt decrease in grain size. Ernst (1955, 1960) concluded that much of the mafic and intermediate rock types in the lower part of the sill (about 40% of the total thickness) were likely related by fractional crystallization. However, like Schwartz and Sandberg (1940), he recognized that there was too much felsic material to have been produced by in situ differentiation. He offered two possible explanations for the large volume of granophryre in the upper part of the sill - 1) additional granophyre generated by differentiation from a downdip, thicker section of the sill had injected itself into an up-dip section of the sill, where it is currently exposed or 2) the granophyre represents a separate intrusion of felsic magma unrelated to the mafic component. 81 �Figure 2. Geology of the Endion Sill from Ernst (1955) showing his sample locations. For his University of Illinois MS thesis, Oestrike (1983) conducted a more thorough petrographic and mineral chemical study of the Endion Sill along the shoreline and Tischer Creek sections (Figs. 3 & 4). He subdivided the sill into 3 main units that he termed the Gabbroic Zone, the Acidic Zone and the Intermediate Zone. Grading abruptly up from a basal chill zone, the Gabbroic Zone is largely composed medium-grained, subophitic to ophitic, oxide gabbro to olivine gabbro with variable concentrations of “red spots” (interstitial concentrations of granophyre) and felsic dikelets. About 100 meters above the basal contact, Oestrike noted that the gabbro is in sharp contact with a medium-grained, subprimatic ferromonzodiorite – a rock type commonly found in the Acidic Zone. The next exposure upsection of the ferromonzodiorite is more subophitic gabbro, but the contact is not exposed. Oestrike calls this interval the Gabbroic Zone Acidic Layer (Figs. 3&4). The contact between the Gabbroic and Acidic Zone is about 220 meters above the basal contact and is gradational over several 10’s of meters. It is marked by the transition from a subophitic to subprismatic pyroxene texture and an increase in interstitial granophyre to above 10%. Within the Acidic Zone this trend continue to where the rock becomes a subprismatic to prismatic quartz ferromonzonite and develops a very deep salmon red color. Oestrike notes that these rocks have tridymite paramorphs of quartz. The upper 50 meters of the sill, an interval Oestrike calls the Intermediate Zone, is largely composed of a gray, fine-grained ferrodiorite that locally contains granophyric clot and dikes. Its lower contact with the Acidic Zone s gradational over several meters and its upper contact is characterize by an irregular mixture of ferrodiorite and massive red granophyre over a 3 meter interval. This Intermediate Zone is not observed in the Tischer Creek section. The granophyre-rhyolite contact is inaccessible along the shore due to sea cliffs. 82 �| Int Zn | Acidic Zone | Gabbroic Zone Figure 3. Geology and lithostratigraphy of the Endion Sill from Oestrike (1983) showing his sample locations. Figure 4. Cryptic variation of An content in plagioclase and En% (relative to Fs and Wo) in pyroxene through shoreline exposure of the Endion Sill. Mineral compositions were determined by electron microprobe (from Oestrike, 1983). Citing constant and distinct mineral compositions within the Gabbroic and Acidic Zones (Fig. 4), Oestrike (1983) disagreed with the conclusions of Schwartz and Sandberg (1940) and Ernst (1955, 1960) that the compositional variations in the Endion Sill represent mostly the effects of magmatic differentiation. Instead, he concluded that the Endion Sill was formed by the composite emplacement of mafic and intermediate-felsic magmas in rapid succession. He speculated that the two magmas did not readily mix due to the effects of double-diffusive convection. He did not have a satisfactory explanation for the origin of the Intermediate Zone ferrodiorite. 83 �A lithogeochemical study of the Endion Sill, including Sm-Nd isotopes, was conducted by Gardner (1987) for his MS thesis at Washington University. (I have not been able to track down a copy of his 350 page thesis, so I will paraphrase the conclusions of his study from a Lunar and Planetary Science Conference Abstract (Gardner et al., 1987) and from a summary by Jerde (1991)). Although Jerde reports that Gardner (1987) conducted 9 analyses of Sm-Nd isotopes for his thesis study, Gardner et al. (1987) reported Sm-Nd analyses of only one Gabbroic Zone sample and one Acidic Zone sample. Gardner’s (1987) INAA analyses of 26 samples through the sill show incompatible trace element concentrations increasing uniformily through the sill. He notes that REE patterns and concentrations of Gabbroic Zone samples resemble andesitic NSVG compositions. He further notes that the 30m-thick granophyre interval, which he calls the altered rhyolite zone and which occurs between the top of the sill and the base of the unaltered rhyolite, is depleted in REE and other incompatible trace elements (Ta, Rb, Th) and enriched in more compatible elements such as Sr, Ba, Co, and Sc relative to unaltered rhyolite. He suggests that devolution of fluids from the rhyolite may have mobilized trace element into the underlying sill. Gardner et al. (1987) reports ƐNd (1100Ma) values for the Gabbroic Zone sample of -0.6 and -2.0 for the Acidic Zone sample (Fig. 5). Gardner concludes that if the Congdon Park Rhyolite has an ƐNd value of <8, its partial melting and assimilation along with fractional crystallization of the Endion sill may explain its Nd isotopic compositions. Gardner did not analyze the Sm-Nd isotopic compositions of the Congdon Park Rhyolite, but Vervoort and Green (1997) report a ƐNd (1100Ma) value of -4.1 (Fig. 5). Figure 5. ƐNd (1100Ma) and 1/Nd values for samples from the Gabbroic and Acidic zones of the Endion Sill compared to primitive NSVG basalt, the Congdon Park Rhyolite, and 5% and 10% model partial melts from Superior Province crust. The values show for the Congdon Park Rhyolite are from Vervoort and Green (1997). All other data are from Gardner et al. (1987). Gardner (1987) conclude from his geochemical data that the Gabbroic and Acidic zones represent two separate intrusions into the sill and are not related by in-situ fractional crystallization. Both show contamination which may have been contributed by the Congdon Park rhyolite or by partial melts of Archean crust during the intrusion of mantle-derived magmas (Fig. 5). He interpreted the more contaminated Acidic Zone to have been intruded later and above the semi-molten gabbroic zone. Gardner (1987) further interpreted the upper Intermediate Zone to be a more contaminated upper contact of the Gabbroic Zone. 84 �Jerde (1991) studied the lithogeochemistry of the Endion Sill as a part of a larger study of hypabyssal mafic intrusions into the NSVG for his PhD dissertation at UCLA. Locations of the 17 samples he analyzed by INAA and microprobe analyses of fused glass are shown in Figure 1 and the data are presented in Table 1. By his own admission, he conducted only reconnaissance field investigations and instead relied on the previous studies of Oestrike (1983) and Gardner (1987). Table 1. Geochemical analyses of Endion Sill samples from Jerde (1991). Locations shown in Figure 1. Table 1 (cont.) 85 �The chemostratigraphic variations of major and trace elements of Jerde’s (1991) data are shown in Figures 6 and 7, respectively, relative to the lithostratigraphy of Oestrike (1983). The major element data (SiO2, TiO2, mg# and Na2O+K2O) clearly show abrupt compositional changes between the gabbroic and acidic zones across 50 meter-thick interval that Jerde (1991) termed the transitional zone (AZ/GZ, Fig., 6). Oestrike (1983) recognized this hybrid transitional zone petrographically and interpreted this as a mixing zone between Acidic Zone magma that was compositely emplaced above the partially solidified Gabbroic Zone. Interestingly, the Intermediate Zone (IZ, Fig. 6) has major element compositions that are comparable with the transitional zone compositions suggesting that it is the upper hybrid chill zone of the Acidic Zone rather than a remnant chill of the Gabbroic Zone as interpreted by Gardner (1987). The transitional major element compositions of the recrystallized rhyolite (xRhy, Fig. 6) between the unaffected rhyolite and the Intermediate Zone rocks suggests that the recrystallized rhyolite has been contaminated by and provided contaminants to the Endion Sill as suggested by Gardner (1987). 86 �Figure 6. Chemostratigraphic variation of major elements through the Endion Sill. Lithogeochemical data from Jerde (1991), see Table 1. Lithostratigraphic units modified from Oestrike (1983); see Fig. 3 – GZGabbroic Zone; GZa- acidic layer in the Gabbroic Zone; AZ/GZ – transitional zone between the Acidic and Gabbroic Zones; AZ – Acidic Zone; IZ – Intermediate Zone; xRhy – recrystallized rhyolite; Rhy – unaffected rhyolite. Darker shades indicate felsic compositions. Jerde’s (1991) trace element data (Zr, Ba, Ni, and Cr, shown in Figure 7) are more consistent with the Acidic and Gabbroic Zones being formed by composite intrusions, as opposed to in situ fractional crystallization. Moreover, the similarity of trace element abundances between the transitional zone (AZ/GZ, Fig. 7) with the Intermediate Zone is consist with the IZ being an upper hybrid chill of the Acidic Zone magma. However, the very uniform incompatible trace element ratios of Ce/Yb and Zr/Hf indicate that both zones were formed from a common parent magma which presumably differentiated at depth. The subtle increase in the Th/Hf ratio in the Acidic zone is consistent with Gardener’s (1987) Sm-Nd isotopic data that implies contamination of the acidic zone magma by the overlying Congdon Park rhyolite. Most Midcontinent Rift rhyolites are interpreted from negative ƐNd isotopic compostions to have formed by partial melting of Archean to Paleoproterzoic crust, which should also be enriched in Th (Vervoort and Green, 1997). 87 �Figure 7. Chemostratigraphic variation of trace elements through the Endion Sill. Lithogeochemical data from Jerde (1991), see Table 1. Lithostratigraphic units modified from Oestrike (1983) – GZ-Gabbroic Zone; GZa- acidic layer in the Gabbroic Zone; AZ/GZ – transitional zone between the Acidic and Gabbroic Zones; AZ – Acidic Zone; IZ – Intermediate Zone; xRhy – recrystallized rhyolite; Rhy – unaffected rhyolite. Darker shades indicate more felsic compositions. The REE data reported by Jerde (1991), and plotted in normalized spidergrams in Figure 8, also provide some insight into the petrogenetic relationship between the Gabbroic and Acidic Zones. That the general slopes of the REE curves from the Endion Sill samples are general coparallel are again consistent with the Gabbroic and Acidic Zones being evolved from a common magma. However, the GZ and AZ samples clearly cluster in two distinct REE abundance groups. The basal chill sample (black dot in Fig. 8) shows enriched REE abundances relative Gabbroic Zone samples, but show a similar trend and lack of Eu anomaly. This suggests that the gabbroic zone samples may have cumulate tendencies (i.e. concentrations of primocrysts over parental magma) as suggested by Jerde (1991) and supported by his noting higher than normal concentrations of olivine in his sample E6 from the Gabbroic Zone (Table 1). The Acidic Zone samples have distinctively higher REE concentration with a similar slope to the GZ samples, but a moderate negative Eu anomaly. Interestingly, two sample from the transitional (AZ/GZ) zone have distinctive compositions with one lining up with the Gabbroic Zone samples and the other similar to the Acidic Zone samples, though with a less pronounced negative Eu anomaly. Interestingly, the REE-enriched transitional zone sample is most similar to the REE compositions of the upper 88 �Rock/Primitive Mantle Intermediate Zone. This lends additional evidence to the suggestions of Jerde (1991), Gardner (1987) and Oestrike (1983) that the acidic zone is a later composite intrusion above the semi-crystallized Gabbroic Zone and that the Intermediate Zone and transitional zone are upper and lower margins of that later impulse of intermediate magma. The REE patterns of the overlying Congdon Park Rhyolite and its recrystallized lower interval in contact with the Intermediate Zone are somewhat similar, but have steeper LREE slopes and a much more pronounced negative Eu anomaly. That Eu values for Acidic Zone straddle the range between Intermediate Zone and transition zone samples and those of the rhyolite and recrystallized rhyolite are also consistent with Gardner (1987) Nd isotope data that suggest that the Acidic Zone assimilated a modest amount of anatectic melt from the rhyolite. Figure 8. REE normalization plot of samples from the Endion Sill analyzed by Jerde (1991); see Table 1. Samples are color coded to the lithostratigraphic column modified from Oestrike (1983). REE abundance normalized to primitive mantle composition of Sun and McDonough (1989). 89 �Recent Studies The shoreline exposures of the Endion Sill, which will be the focus of this field excursion, were mapped in detail in September 2007 by the author as the final element of bedrock mapping integrated into the geologic map of the Duluth 7.5’ quadrangle (Green and Miller, 2008); See Figure 1. A total of 27 sample were collected during the mapping and thin sections were made for 25 of these. These sections were only recently investigated and photographed earlier this year (nothing like leading a field trip to finally getting around to it). Field stop descriptions given in the next section will incorporate the field and petrographic observations and will be illustrated with field photos and photomicrographs from these recent studies. Based on these field and petrographic studies, a revised lithostratigraphy for the Endion Sill is proposed and summarized in Figure 9. This figure highlights the textures, mineralogy and reddish hemitization of the various lithologies with thumbnail scans of thin sections and summary petrographic descriptions. The main differences with the stratigraphic column proposed by Oestrike (1983) is to expand the base of the AZ/GZ transition zone from 200 meters down to 100 meters. This corresponds to the sharp intrusive contact between gabbroic rocks and ferromonzodiorite of his “acidic layer in the Gabbroic Zone” (Fig. 3). Oestrike (1983) resumes the Gabbroic Zone over the acidic layer based on interpreting his sample E-24 as being gabbro. However, sample MD769 is observed to be varitextured oxide gabbro with about 10% granophyre and no evidence of olivine. Indeed olivine is only observed in the lower 100 meters of the sill. Field description of this outcrop notes that this sample was taken from the least granophyric sample. As such, it bears greater resemblance to the variably granophyric olivine-barren gabbros ranging in pyroxene texture from subophitic to intergranular to poikiloprismatic that are common in the transition zone. Unfortunately, Jerde (1991) did not sample this exposure to see if it maintains a Gabbroic Zone chemistry or starts to show evidence of mixing with the Acidic Zone magma. (Don’t know if Gardner (1987) sampled here). Other elements of the lithostratigraphic column are as portrayed by Oestrike (1983). Questions to Consider As we progress up section through the excellent shoreline exposures of the Endion Sill, some questions to consider include: • What is the petrogenetic relationship between the Gabbroic Zone and the Acidic Zone and particularly theAZ/GZ transition zone between them? • Do the field relations (and geochemical data) give any indication that fractional crystallization is involved? • What is the evidence for composite emplacement of the GZ and AZ, and what is the relative timing of their emplacement? • What does the ferrodioritic Intermediate Zone represent? - an upper chill of the Acidic Zone? a remnant of the upper chill of the Gabbroic Zone? or something unrelated to either? • What is the source of granophyric material in the sill? – partial melting of the overlying rhyolite or fractional crystallization in situ or in a remote staging chamber? 90 �Figure 9. Revised lithostratigraphy of the Endion Sill based on recent field and petrographic studies. 91 �Field Stop Descriptions The field trip will start at a large glacially polished and striated outcrop (location 2) just several meters east of the Lakewalk where 17th Ave. East projects to the shore and an elevated walkway crosses the interstate highway. Proceed to the SW edge of the outcrop to observe the basal contact (Location 1). Area A – Gabbroic Zone of the Endion Sill Location 1: Basal Chill of the Endion Sill UTM (NAD83): 570490_5183340 Description: Exposed here is a sharp contact between an intermediate volcanic (icelandite?) and an aphanitic gabbroic rock with sparse small phenocrysts of plagioclase and an altered mafic (olivine?). The volcanic occurs in an 5 4 outcrop just SW of the fine gabbro and as a 0.5 m thick 3 slab/septum of similar intermediate volcanic about 1 meter above the (unexposed) basal contact (Fig. 11). Fragments 2 of the volcanic also occur as blocks in the fine gabbro. The 1 contact between the volcanic slab and the very chilled hackly fractured surface of the gabbro has a strike and dip of N30°W/18°NE. A small (unexposed) fault is evidenced by an apparent left lateral offset of the volcanic slab across a N15E trending gap in exposure. Given the NE dip of the contact, this offset may also indicate east-side up displacement, which would imply it formed during late rift compression. IV F BC Figure 11. Panoramic photo of the basal chill (BC) of the Endion Sill with a slab/septum of intermediate volcanic (IV) offset by a reversed? fault (F). Location 2: Glacially polished whale-back outcrop granophryic gabbro. UTM (NAD83): 570620_5183305 Description: Progressing northeasterly across this glacially smoothed, polished, and striated outcrop, it is easy to observe the progressive coarseing in texture and the increase in granophyre clots, which stand out in relief, from about 5% to over 30% (Fig. 12A) Samples MD765D and MD765E (Fig. 10) show a transition from a medium fine-grained, felty subophitic slightly granophryic oxide gabbro to a mediumgrained, ophitic apatitic, granophryric olivine gabbro with 1-2 cm clots of micrographic Ksp+Qtz and free quartz (Fig. 13). All igneous phase show moderate degrees of alteration to bowlingite (Ol), uralite (Cpx), and sericite (Pl). Small inclusion of fine-grained, locally amygaloidal basalt occur in the gabbro and tend to concentrate granophyre at their margins (Fig. 12B). Also, the outcrop is cut by alteration veins trending N15E (parallel to the fault) along which liesegang redox bands are developed. 92 �A B Figure 12. A) Glacially polished and striated subophitic gabbro with irregular granophyre clots standing out in relief; B) Basaltic inclusion with concentrations of granophyre in surrounding gabbro. B A Cp Ol gp B’ A’ Figure 13. Photomicrographs of felty subophitic gabbro texture of sample MD765D (A & A’) and subophitic granophyric olivine gabbro texture of MD765E (B & B’). Poikilitic augite (Cp), bowlingite-altered olivine (Ol) and granophyre-rich (gp) area noted in B. All photos at 1.25x, scale bar = 3mm Location 3: Ophitic olivine gabbro UTM (NAD83): 570665_5183350 Description: Medium fine-grained, poorly foliated, ophitic olivine gabbro. Augite oikocrysts are about 0.5cm diameter (Fig. 14A). Granular olivine tends to cluster in the inter-ophite areas (Fig. 14B) and preferentially weather out on the bedrock surface to form pits. 93 �B A Figure 14. Photomicrographs from Sample 766. A) Ophitic augite with plagioclase chadacryts. B) Altered (bowlingite) olivine clusters in interophite areas. Both photos at 4x, scale bar = 0.5mm Location 4: Columnar jointed, ophitic olivine diabase UTM (NAD83): 570725_5183402 Description: The best developed columnar jointing in the Endion Sill are exposed here (Fig. 15A) as is obvious ophitic texture (Fig. 15B). Cpx oikocryts range from 0.5 to 2 cm diameter over the outcrop. Thin sections show this gabbro to contain less than 5% granophyric mesostasis. B A Figure 15. Field photos of Location 4 outcrop of ophitic olivine diabase displaying columnar jointing (A) and <1cm oikocrysts of pyroxene on a weathered surface (B). Location 5: Contact between Gabbroic Zone and AZ/GZ Transitional Zone UTM (NAD83): 570851_5183420 Description: Exposed in the outcrop at this point is a sharp contact between medium-grained, intergranular oxide gabbro and intergranular to subprismatic ferromonzodiorite (Fig. 16B). This contact has been recognize by all workers and marks the lower contact of Oestrike’s acidic layer in the Gabbroic Zone (Fig. 3) . The gabbro near the contact is intergranular, moderately granophyric (5-10%) and devoid of olivine (Fig. 16C), but the westernmost exposures grade into a subophitic to ophitic, mildly granophyric (<5%) olivine gabbro typical of the Gabbroic Zone (Fig. 16D) . Given the textural zoning of the gabbro against the rather homogeneous ferromonzodiorite, one could argue that the gabbro is intrusive 94 �into the ferromonzodiorite. This of course, is counter to interpretations of Oestrike (1983), Gardner (1987) and Jerde (1991) that the intermediate rocks intruded the gabbro. A B D C Figure 16. Progression of rock types at Location 5. A) subprismatic ferromonzodiorite, B) sharp contact between ferrodiorite and intergranular granophyric gabbro, C) intergranular oxide gabbro 1 meter form contact, subophitic olivine gabbro 5 meters from the contact. Area B – Transitional AZ/GZ Zone of the Endion Sill Location 6: Heterogeneous granophyric gabbro UTM(NAD83): 570995_5183600 Description: Although Oestrike (1983) shows this exposure as belonging to the Gabbroic Zone, this medium-grained, variably granophyric (15-25%) and varitextured (poikiloprismatic to subophitic to intergranular) oxide gabbro seems better grouped in the transitional AZ/GZ zone (Fig. 17). B’ B gp A Cp Cp Figure 17. Field and petrographic photos of sample MD769 from Location 6. A) Field photo of the variably granophyric composition of this exposure. B) Plane and cross-polar images of intergranular to subophitic augite (Cp) and interstitial areas rich in K-feldspar and quartz (granophyre). Photomicrographs at 1.25x, scale bar = 3mm. 95 �Location 7: Gradation from Granophyric Gabbro to Ophitic Gabbro UTM(NAD83): 571125_5183608 Description: Across this point, a very gradational transition in rock type is observed. At the SW end of the exposure, the rock is a subprismatic to subophitic granophyric (5-10%) oxide gabbro (Fig. 18A) . Crossing the point to the southeast, the granophyre abundance drops to below 5% and an ophitic texture is clearly developed with Cpx oikocrysts up to 3 cm across evident (Fig. 18B). At one location, a softballsized anorthosite inclusion is present. Plagioclase phenocrysts are present throughout the ophtic gabbro. A B gp gp A’ B’ Figure 18. Photomicrographs from samples at Location 7. A) Subophitic granophyric oxide gabbro with granophyric mesostasis (gp) (Sample MD770A). B) Ophitic oxide gabbro (Sample MD770B). All photos at 1.25X; scale bar = 3mm. Location 8: Foliated, Subprismatic, Apatitic Granophyric, Oxide Gabbro. UTM (NAD83): 571185_5183635 Description: Across a 5 meter gap in exposure from the ophitic gabbro at the east end of Location 7, the rock type abruptly changes to a medium fine-grained, subprismatic to intergranular, apatitic oxide gabbro with up to 20% granophyre. Locally, the rock displays a crude foliation of plagioclase and pyroxene (Fig. 19). This is the first significant occurrence of more than 1% apatite as slender needles, which persists upsection throughout the Acidic Zone. 96 �B B’ A Figure 19. Textures of foliated intergranular granophric gabbro in outcrop (A) and thin section (B) at Location 8. Location 9: Subprismatic Apatitic Quartz Ferromonzodiorite UTM(NAD83): 571245_5183675 Description: The stretch of Endion ledges marks the consistent onset of subprismatic to prismatic texture of mafic phases, the increase in granophyric mesostasis to greater than 15%, the presence of quartz paramorphs after tridymite, and the occurrence of 2-3% apatite (Fig. 20). This interval marks the upper part of the AZ/GZ transitional zone. The rock has a pinkish hue, but its does not develop the deep red color characteristic of the Acidic Zone until the next outcrop at Location 10. A B C Figure 20. Photomicrographs of subprimatic, apatitic quartz ferromonzodiorite sample MD770D(A) and MD 770E(B & C). Quartz paramorphs of tridymite are commonly associated with granophyre patches. Scale bar = 3 mm. 97 �Area C – Acidic Zone of the Endion Sill Locations 10 - 13: Hematized Prismatic Quartz Ferromonzonite UTM (NAD83): 571320_5183720 to 571607_5183960 Description: Homogeneous exposures along this stretch of shore display the strongly hematized rock that characterizes the Acidic Zone (or Red Rock of Schwartz and Sandberg, 1940). Granophyre content through this section is consistently between 30 and 50%, 20-35% plagioclase laths are commonly bleached white, and altered pyroxene becomes is consistently subprismatic to prismatic (Fig. 21). In thin section, tridymite paramorphs are ubiquitous. Apatite ranges from 0.5 to 2 %. B A C Figure 21. Prismatic to subprismatic textures of ferromonzonite exposures at Locations 10, 11 and 12. A B Figure 22. Strongly hematized and granophyre+tridymite mineralogy of samples MD771B (A) and MD772A (B) from locations 10 and 12, respectively. Scale bars = 3mm. 98 �Area D – Intermediate Zone and Upper Contact of the Endion Sill Location 14: Heterogeneous mix of ferromonzodiorite and granophyre UTM (NAD83): 571615_5183990 to 571660_5184125 Description: Along this continuous stretch of shoreline outcrop, the prismatic quartz ferromonzonite grades into a less granophyric composition of ferrodiorite to ferromonzodiorite that becomes mixed with irregularshaped masses of granophyre (Fig. 23). The host ferromonzodiorite still locally displays a very prismatic texture (Fig. 23A). Progressing north across this mixed zone, the granophyric and dioritic components become more strongly contrasted (Fig. 23B  Fig. 23C) with the intermediate component becoming dominant over granophyre and more ferrodioritic in composition (less granophyric mesostasis). A B C Figure 23. Exposures of complexly mixed granophyre and ferromonzodiorite observed at Location 14 progressing north. A) irregular masses of granophyre mixed with prismatic ferromonzodiorite. B) Broader scale view of complexly mixed granophyre with ferromonzodiorite. C) Granophyre occurring as irregular dikes and dikelets in more homogeneous ferrodiorite. Location 15: Ferrodiorite of the Intermediate Zone UTM(NAD83): ~ 571685_5184160 Description: At this point along the shore, the dominant rock is a dark ferrodiorite with only local occurrences of irregular granophyre masses. This is the main rock type of what other workers (Oestrike, 1983; Gardner, 1987; Jerde, 1991) have termed the intermediate zone. Petrographically, it is a mediumgrained, subprismatic to poikiloprismatic quartz ferrodiorite to ferromonzodiorite with significant 99 �amounts of apatite (4%), tridymite paramorphs (15%), and primary brown amphibole as rims on pyroxene prisms (Fig. 24). Strongly zoned pyroxene and plagioclase display preferentially altered cores and fresh rims (Fig. 24B). A B A’ B’ Figure 24. Photomicrographs of Samples MD773E (A) and MD773F (B) displaying the subprismatic to poikiloprismatic texture of the quartz ferromonzodiorite/ferrodiorite that forms the Intermediate Zone of the Endion Sill. Scale Bar = 3mm. The ferrodiorite is cut here by a 0.5-1.5 meter wide diabase dike trending to the NE and dipping steeply to the NW. The diabase shows well-developed columnar joints that are curved indicating fault motion before the dike completely cooled (Fig. 25). Figure 25. Columnar-jointed diabase dike cutting ferromonzodiorte at location 15. Location 16: Contact of Intermediate Zone and Recrystallized Rhyolite UTM(NAD83): ~ 571710_5184240 100 �Figure 26. Panoramic view of the contact between ferrrodiorite and granophyre (recrystallized rhyolite) at Location 16. Description: Exposed in the sloping ledge at this location is the subhorizontal (slightly NE-dipping) contact between the medium fine-grained ferrodiorite and medium fine-grained granophyre (Fig. 26). In detail, the contact is very irregular and interfingering with granophyre occurring as irregular masses to dike-like bodies in the ferrodiorite. In the overlying granophyre, some lobate masses of fine-grained ferrodiorite suggestive of two magma mixing textures. The granophyre transitions into flow banded rhyolite in the seacliffs that line this bay (inaccessible except by boat). This transition is observed in Tischer Creek, where it is observed to occur over several meters (Ernst, 1955; Oestrike, 1983). References Ernst, W.G., Jr., 1955, Petrology of the Endion Sill. M.S. thesis, University of Minnesota-Minneapolis, 31p. Ernst, W.G., Jr., 1960, Diabase-granophyre relations in the Endion Sill, Duluth, Minnesota. Journal of Petrology, v. 1, p. 286-303. Gardner, J.E., 1987, Origin of the Endion Sill. M.A. thesis, Washington University, St. Louis, 359p. Gardner, J.E., Haskin, L.A., Brannon, J.C., 1987, Possible assimilation by a mafic magma: the Endion Sill, Duluth, Minnesota. Lunar Planetary Science Conference Abstracts, v. 18, p. 312-313. Green, J.C. and Miller, J.D., 2008, Bedrock geology of the Duluth quadrangle, St. Louis County, Minnesota. Minnesota Geological Survey Miscellaneous Map M-182, scale 1:24,000 Jerde, E. A., 1991, Geochemistry and petrology of hypabyssal rocks associated with the Midcontinent Rift, northeastern Minnesota; Appendix D – The Endion Sill. Ph.D. dissertation, University of California - Los Angeles, p. 278-288. Oestrike, R.W., 1983, The Endion Sill, Duluth, Minnesota: mineralogy and petrology of a composite intrusion. M.S. thesis, University of Illinois, Urbana-Champaign, 141 p. Schwartz, G.M. and Sandberg, A.E., 1940, Rock series in diabase sills at Duluth, Minnesota. Geological Society of America Bulletin, v. 51, p. 1135-1172. Sun, S, -S. and McDonough, W.F., 1989, Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In: A.D. Saunders and M.J. Norry (eds.), Magmatism in Ocean Basins. Geol. Soc. London, Special Publ., v. 42, p. 313-345. 101 �FIELD TRIP 6 May 6, 2016 GEOLOGY AND TROUT FISHING ALONG AMITY CREEK, DULUTH Dean Peterson Peterson Geoscience LLC George Hudak UMD-NRRI Figure 1. Bedrock geology and field trip stop map of Amity Creek, Duluth, Minnesota. 102 �Introduction: Circa 1985…. was the time, as seemingly all of the University of Minnesota Duluth (UMD) geology undergraduate and graduate students were aware, of monthly handouts of the infamous 5-pound bricks of government “eutectic” cheese as well as bags of rice and other edible things that we cannot remember. As a poor geology undergraduate student living on these government handouts and potatoes at the UMD, the first author spent many many days on Amity Creek fishing for meat (brook trout) for dinner after class. Most trips were successful back then and still are today. This short field trip will include the geology of the Seven Bridges Road portion of Amity Creek as well as trout fishing tips and perhaps some stories and seemingly tall tales from two proud products of the geology department of UMD. We will end our trip with a BBQ where additional tales may be told. History of Seven Bridges Road: This field trip largely follows the path of Duluth pioneer and former mayor Samuel Snively, who was instrumental in the development (late 1890s) and construction of our roadway, Seven Bridges Road. Seven Bridges Road is one of many parkways in Duluth, a city renowned for its parks and outdoor amenities. We will travel up Seven Bridges Road to Hawk Ridge and onto Duluth's famous hilltop boulevard, Skyline Parkway. Winding our way up Amity Creek through a mixed forest of pine, fir, maple, and birch you will see how the road intertwines and crosses with the creek over seven stone-arch bridges, and the reason for the road’s current name. Each bridge is faced with local 1.1 billion year old basalt and diabase collected from the creek bed or blasted from nearby outcrops and the pink cap rocks consist of granite quarried in St. Cloud, Minnesota. The following historical description of the development of Seven Bridges Road is modified from Ryan (1999). Work on the original section of the road was begun in 1899, and opened for use in the next year, though it took over three decades before the upper connection across Hawk Ridge was finally completed. The drive was built by Samuel Snively, a Duluth pioneer who owned a large 400 acre farm in the back country above Duluth suburbs of Lester Park and Lakeside. His hilltop farm was well-known for its thoroughbred stock and the beauty of its layout, which included a glimpse of Lake Superior through the nearby Amity Creek valley. Snively apparently often hiked the valley and explored the woods on both sides of the ridge line overlooking the east end of Duluth. During these strolls, Snively began to envision a park drive that would rival any other in Duluth. Donating sixty acres of his own property, Snively set to work contacting all the other landowners in the area, successfully garnering donations of the necessary rights-of-way for his road, as well as some of the necessary monies to build it. A crew of workers from the surrounding countryside was hired and construction on Snively's road began in the late fall of 1899 and continued into the following summer. The road crew started its work at the junction of Oriental and Occidental boulevards near Lake Superior, two carriage paths that ran through and alongside the boundaries of Lester Park, a city park in east Duluth. Despite its popularity as a scenic parkway, the city of Duluth neglected to maintain Snively's road, and within a decade all the wooden bridges had fallen to ruin, making the road impassable to vehicle traffic. In 1910, the road's destiny changed for the better when it was handed over to the Duluth's park commission, and a new plan for its rejuvenation was developed. The park board hired an architectural landscaping firm to design a new series of bridges for the road. In the fall of that year, the firm of Morell & Nichols of Minneapolis presented the park commissioners with sketches and blueprints for nearly a dozen new stone-arch bridges to replace the wooden ones Snively had built. During 1911, the roadway was regraded and graveled, and several first class stonemasons from the Duluth area were hired to build the bridges simultaneously. When completed, the newly refurbished road would become an official part of the Duluth's boulevard system. News of the park board's intentions delighted Snively, for the plans were exactly what he had in mind when he first built the road. 103 �When Snively's road reopened on July 6th, 1912, it was renamed Amity Parkway and added nearly 6 miles to the city's growing boulevard system. The new Amity Parkway became a popular destination for tourists and locals alike. Winding its way up into the eastern hills, the route presented many scenic sights of the landscape and rushing creek. Flowers lined many of the drive's turns and curves, and in the autumn, the poplar and birch forests presented spectacular colors for the tourist. "When the park board decided to take over and improve this roadway, it greatly pleased me, for it assured the consummation of the very purpose I had in view, the appropriation by the city for park and boulevard purposes of some of the scenic and natural park property in and about the city...Our possible park system rightly developed will be the city's greatest asset and advertisement." Samuel Snively Satisfied for the time being, Snively moved on again to other things in his life, but would return some two and a half decades later, (this time as the mayor of Duluth) to build the final leg to his road, the segment of the road leading to Hawk Ridge. Two of the bridges (#8 & #9) fell into disuse (by automobiles) when the eastern extension Winter Bridge to and along Hawk Ridge was completed in the thirties. Remnants of these two closely-built bridges still stand along a pedestrian pathway that shoots off from the main road near the last bridge crossed before the road ascends toward Hawk Ridge Nature Reserve. The remaining seven bridges are still used by vehicle traffic, but years of weather, combined with vandalism, and vehicle accidents had taken their toll. In the mid-1990s the city of Duluth, realizing the historic significance of the bridges, initiated a program to repair and restore the structures. Bridge #2, just south of the Lakeview hockey rinks, being the most damaged of the lot, was the first to be restored. The original blueprints were consulted with the work beginning in late 1996, and completed the following summer. The bridge was restored to its original condition, and the project was hailed a success. In the spring of 1998, the Duluth Preservation Alliance awarded the restoration with a plaque at its annual awards ceremony. Bridge #6 restoration work was begun the following year, a century after Samuel Snively began construction on the original road. Repairs to the remaining five bridges are slated to take place over the next few years. Although in fairly rough condition and its paved section in need of resurfacing, Seven Bridges Road remains one of Duluth's more idyllic drives. Traveling the road, you'll often meet hikers and bicyclists, equestrians, and automobiles. Fisherman can be spotted angling for trout along the creek bank, and during the warmer days of summer, swimmers are often seen cooling themselves in pools such as the one situated just beneath the falls near Historic Bridge #6. During the winter months, snowmobilers share the route with hikers, cross-country skiers, and snowshoe enthusiasts. 104 �One day, back in early November of 1934, Sam Snively stood along Hawk Ridge overlooking eastern Duluth and the blue expanse of Lake Superior. Much of his celebrated farm, less than a mile away, had been destroyed sixteen years before in a devastating forest fire that had swept through the area. He sold the property soon after. Now, as he stood there, fast approaching his 75th birthday, and well into in his last term as mayor, he contemplated his long life in Duluth. "Sometimes, when I become discouraged, I say to myself, I should have gone to another city to seek my fortune. But when I look over these hills and see the great natural beauties of our community, I console myself and wonder--where in all this wide world could I find such a view as this?" Samuel Snively Geologic Setting: As Amity Creek decends the steep hillside of Duluth to Lake Superior, it flows over mafic and felsic lava flow sequences and hypabyssal diabase dikes and sills of the 1.1 Ga. North Shore Volcanic Group as well as locally over red glacial rift. The variability in resistance to weathering and erosion of these rocks has lead to the varied character of Amity Creek and its most famous inhabitants, the brook trout. We will spend a late afternoon exploring the rocks, the character of the creek, and the possibility of hooking some trout. The authors wish to let all of the field trip participants know that neither of us have ever formally mapped the geology we will be looking at during this short Duluth-centric field trip. The experts on the geology of this area are University of Minnesota Duluth geology professors Dr. John Green and Dr. James Miller, and thus we will be critiqueing their geology (Fig. 1) as shown on the Minnesota Geological Survey Miscellaneous Map M-182 (Green and Miller, 2008) during this field trip. However, as two experienced NE Minnesota geologists we can perhaps answer most questions and will explore the geologic details during the field trip simply along with all of the participants. Angling Setting: Unlike the trout fishery of southeastern Minnesota’s driftless area, the streams of Minnesota’s north shore of Lake Superior generally are only fair trout streams. These waterways depend on unstable runoff for their flow and surge after large rain events and during spring snowmelt which can dwindle to a trickle during drought and the winter season. In the summer some stretches (especially the upper portions of the stream systems) can get warmer than is best for trout. The 1.1 Ga. volcanic bedrock over which the North Shore streams flow contain few of the water-soluble minerals that help keep the water alkaline and the aquatic invertebrate population large. Consequently, these streams tend to be soft, slightly acidic to neutral, and not particularly productive. Because they generally lack spring water, the streams get very cold in winter and can form "anchor ice" on the bedrock of the streambed, destroying aquatic life and habitat. However, North Shore streams have two things in their favor for the development of a trout fishery. The first is the cool Lake Superior-moderated climate and the second is the deep shade provided by the ubiquitous forest bank cover, which generally keeps these streams just cool enough to support trout. Trout are not native to the upper reaches of the North Shore streams. Coaster brook trout occupied Lake Superior and ascended the rivers as far as the first barrier falls-usually less than a mile from the lake. Only during the last century have brook trout, rainbow trout, and locally brown trout been stocked above the barrier falls of North Shore streams. 105 �FIELD TRIP STOP DESCRIPTIONS NOTE: all UTM coordinates are given in NAD 83, Zone 15 and keyed to Figure 1. STOP 1—Lakeside lavas and the confluence of Amity Creek and the Lester River at Lester Park Location: UTM NAD83 coordinates 575795E, 5187900N General description: The confluence of Amity Creek and the Lester River occurs at Duluth’s Lester Park, where these two stream systems flow on undifferentiated mafic flows of the Lakeside Lavas (Green and Miller, 2008). The lavas consist of dark gray to brown, aphyric to sparsely porphyritic basalt and basaltic andesite lava flows. Individual flows are generally 5 to 30 meters thick, with an amygdaloidal upper zone and smooth (rarely rubbly) upper surface. Phenocrysts, where present, are predominantly plagioclase, with minor altered olivine, magnetite, and augite. Angling tip: Lester Park provides numerous angling opportunities (young steelhead and occasional brown or brook trout) with the best chances of success in pools immediately downstream of the massive interiors of lava flows. The park’s streams also seasonally (spring and fall) receive runs of large adult salmon, steelhead, and kamloops rainbow from Lake Superior. Anglers should be aware that only lures with single hooks are allowed at Lester Park. Figure 2. The lower Lester River at Lester Park. STOP 2—Lakeside lavas waterfalls of Amity Creek at “The Deeps” Location: UTM NAD83 coordinates 575470E, 5188350N General description: A very short walk downstream from our parking spot leads us to numerous waterfalls and deep pools associated with the erosion of the Amity Creek diabasic basalt. This huge (100 meters thick) lava flow is a dark gray to brown, fine- to medium-grained, locally seriate to porphyritic, heterogeneously textured basalt flow. The flow contains up to 10 percent phenocrysts of thin plagioclase tablets and minor phenocrysts of ilmenite, magnetite, and oxidized olivine. Groundmass is intergranular/intersertal to ophitic to felty and locally diktytaxitic, and contains plagioclase, augite, oxidized olivine, tabular ilmenite, and magnetite, and a mesostasis including K-feldspar, quartz, apatite, and chlorite. Diktytaxitic cavities contain chlorite, quartz, calcite, and laumontite. The massive nature of the Amity Creek diabasic basalt has lead to the formation of numerous waterfalls and deep pools, which are favorite swimming/jumping/diving spots for local Lakeside area kids. 106 �Angling tip: Within these pools prowl some of the largest stream-bound rainbow trout of the whole North Shore of Lake Superior in Minnesota. The summer angler must get up early in the morning (and bring a net) to fish these pools as swimmers always abound later in the day. The bridge we will cross and immediately park after is the Posted Boundary of Amity Creek. Below this bridge anglers are only allowed to fish lures with single hooks, while above the bridge treble hooks are allowed. Figure 3. Waterfall of Amity Creek into “The Deeps”. STOP 3—Resistant diabase dike in icelandite lavas forming the “Rainbow Trout Pool” Location: UTM NAD83 coordinates 575410E, 5188000N General description: A steep hike down from Seven Bridges Road is needed to observe first hand the diabasic Lakeside intrusion. The intrusion here is a black, fine- to medium-grained, plagioclase-phyric, intergranular olivine diabase with abundant apatite, interstitial quartz, and K-feldspar. This exposure shows it to be a vertical dike cutting the Lester Park icelandite lava flow. On lakeshore exposures, the western contact is crosscutting the Lester Park icelandite. These relationships are interpreted to indicate that the diabase came in along a high-angle reverse fault. The resistant diabase dike forms a beautiful waterfall and large pool in Amity Creek immediately downstream from the dike. Angling tip: Small to moderate size (8-13 inches) rainbow trout are abundant in the pool and several casts of a 1/32 oz panther martin spinner into this pool should always be tried by anglers. Anglers should always remember to lower your rod tip when the hooked rainbow trout jumps out of the water. There have been days when the first author hooked rainbow trout on his first 12 casts. Figure 4. Subvertical diabase dike and the “Rainbow Pool”. 107 �STOP 4—Flaggy weathering of icelandite lava and the meandering of Amity Creek Location: UTM NAD83 coordinates 575300E, 5189465N General description: A stop to observe the large Lester Park icelandite, a pink, red, or tan, porphyritic icelandite lava flow, about 180 meters (590 feet) thick. Mostly massive, but the upper part is strongly flow-laminated and folded, with local large, round vugs containing calcite, quartz, barite, and fluorite. Phenocrysts (5 to 10 percent) are of plagioclase, oxidized Fe-silicate, magnetite, zircon, and apatite. The groundmass contains poikilitic quartz (paramorphs after tridymite), K-feldspar, oxidized Fe-silicate, and magnetite, calcite, and chlorite. Angling tip: Perhaps we as a group we can locate the best brook trout pool since the first author has never had much luck in the past angling in this portion of Amity Creek. STOP 5—Waterfalls and pools off of the Northland Sill and the excellent brook trout fishing trail along the upper portions of Amity Creek Location: UTM NAD83 coordinates 574970E, 5190135N General description: Brook Trout Central!! This portion of Amity Creek is, in the opinion of the first author, the best place to first start angling on Amity Creek. The geology is dominated by the contact of the highly resistant Northland Sheet intrusion and the Amity Creek Icelandite lava flow, which culminates for the angler in a series of waterfalls and excellent brook trout pools. The Northland sheet is a brown, fine- to mediumgrained, diktytaxitic intergranular/intersertal diabase grading to augite quartz ferromonzonite. The diabasic intrusion contains minor traces of low-Ca pyroxene, olivine, and primary hornblende and abundant magnetite, ilmenite, and apatite. The sheet-like intrusion is variable in thickness and cuts across more than 500 meters (1,640 feet) of volcanic flows of the Lakeside lavas. Angling tip: At this stop, we’ll investigate waterfalls and excellent brook trout fishing pools situated at the uppermost portions of the Amity Creek icelandite lava flow as well as those within the basal portions of the diabasic Northland Sheet. An excellent horse/hiking trail heads upstream from our parking spot and anglers should be aware that numerous large hook-jawed brook trout swim in these waters. If one is a believer in “take a kid fishing”, then perhaps this is one place to begin (Fig. 5). Figure 5. His smile will last forever, Nathan’s first trout, 2015 (nephew of Dean M. Peterson). 108 �REFERENCES Green, J.C. and Miller, J.D., 2008, Bedrock geology of the Duluth quadrangle, St. Louis County, Minnesota: Minnesota Geological Survey, Miscellaneous Map M-182, scale 1:24,000. Ryan, Mark, 1999, The history of Duluth, Minnesota’s Seven Bridges Road, Samuel Snively and the building of a Northern Minnesota parkway: http://www.amitycreek.com/sevenbridges/index.html. 109 �FIELD TRIP 7 May 7-8, 2016 ARCHEAN AND PROTEROZOIC GEOLOGY OF THE WESTERN GUNFLINT TRAIL Mark Jirsa Minnesota Geological Survey Figure 1. Regional geologic map of northeastern Minnesota showing the location of western Gunflint Trail area (modified from Jirsa and Miller, 2005). Inset box outlines Figure 2. INTRODUCTION This field trip along the western end of the Gunflint Trail explores Neoarchean, Paleoproterozoic, and Mesoproterozoic rocks, and a diversity of well displayed unconformable and intrusive contact relationships. The trip is modified from the ill-fated one attempted for the 2007 ILSG meeting in Lutsen (Jirsa and Weiblen, 2007) that was canceled due to outbreak of forest fire. For expediency, some of that field guide is repeated here. In addition, this guide borrows heavily from two other field trips: a 110 �workshop on iron-formation hosted by the Precambrian Research Center (Jirsa and Fralick, 2010), and a Geological Society of America field guide that focused on the Sudbury Impact Layer (Jirsa and others, 2011). Readers should consult those publications for additional information and references. The order and number of stops that will be visited during this trip will be determined by time, weather, and access issues. The bedrock geology of the field stops is portrayed on a recent map of the Western Gunflint Trail area published as Minnesota Geological Survey Miscellaneous Map M-191 (Jirsa, 2011; Fig. 2). In this area, the Neoarchean greenstone-granite terrane of the Wawa subprovince of Superior Province is represented by a succession of metavolcanic rocks (~2720 Ma) known informally as the Paulson Lake volcanic sequence, intruded by the Saganaga Tonalite (~2690 Ma) and Paleoproterozoic diabasic dikes. The Neoarchean and diabasic rocks are unconformably overlain by Paleoproterozoic sedimentary strata of the Animikie Group (~1870-1830 Ma), which includes the Gunflint Iron Formation. The stratigraphic top of the iron-formation is marked by seismically deformed and brecciated strata and ejecta—known collectively as the Sudbury Impact Layer—that resulted from a meteorite impact near Sudbury Ontario (~1850 Ma). A disconformity separates the impact layer from overlying siltstone and graywacke of the Paleoproterozoic Rove Formation (~1835 Ma). Mesoproterozoic rifting is manifest in hypabyssal dikes and sills of the Logan intrusions (~1115 Ma), and several phases of the Duluth Complex (~1100 Ma), emplaced into both the Archean and Paleoproterozoic rocks. Figure 2. Geologic map (Jirsa, 2011) of the western Gunflint Trail (Highway 12 dashed), showing pertinent features of geology and field trip stops. Note that stops 2 and 3 lie just off the northwest corner of the map. Sudbury Impact Layer is reddish; Proterozoic dikes are shown as thin red lines; Logan intrusions are shades of purple. Image reduced from 1:24,000 map scale. 111 �GEOLOGIC SETTING Neoarchean The oldest rocks exposed in the region are Neoarchean metavolcanic and metasedimentary strata that are part of the Wawa subprovince of the Superior Province. They are probably equivalent to, but not demonstrably continuous with, the Ely Greenstone and Newton Lake Formation. Although the supracrustal successions are dissected by faults and intrusions, some correlation can also be made with adjacent terranes in Ontario. Most recent regional mapping in this terrane was conducted by Jirsa and Miller (2004) in the 1:100,000-scale, Ely-Basswood Lake map sheet that lies just west of the Gunflint Trail. A more detailed map of the Cavity Lake forest fire area that lies just west of the Gunflint is still in review and production (Jirsa and others, in prep.). Rocks of the terrane are divided into a number of faultbounded segments, each having distinct geologic characteristics that cannot be easily correlated from place to place. The Gunflint Trail area exposes the eastern edge of what Gruner (1941) referred to as the Gabimichigami segment. Supracrustal rocks in this segment include an older suite of variably pillowed, mafic to ultramafic flows and hypabyssal intrusions of the Paulson Lake volcanic sequence (stop 1), and a younger suite of hornblende-bearing andesitic to dacitic pyroclastic and volcaniclastic rocks that comprise the Knife Lake Group that are exposed just east of Fig. 2. Based on stratigraphic facing directions established from pillowed metabasalt flows, the Paulson Lake sequence forms an east-trending and steeply south dipping and younging homocline. The Saganaga Tonalite (stops 2-6) was emplaced into metabasaltic rocks and defines the northern edge of the supracrustal succession. The metamorphic grade of greenstone along this contact is locally increased from greenschist facies that is typical of much of the belt, to amphibolite grade, and foliation in both tonalitic and volcanic rocks is well-developed near the contact. The western edge of the Gabimichigami segment is terminated by a north-northeast-trending fault that juxtaposed greenstone against sedimentary and volcanic rocks of the Knife Lake Group. The Knife Lake Group includes the informally named “Ogishkemunce conglomerate” that contains detrital clasts of the Saganaga Tonalite, along with clasts of iron-formation, metabasalt, metagabbro, and gneiss. This distinctive sequence of conglomerate, sandstone, and alkalic rocks is interpreted to have been deposited in a complex array of successor basins developed along early-formed faults at some time after emplacement of the Saganaga Tonalite at ca. 2690 Ma (Driese and others, 2011; Jirsa, 2016). Lacking detailed geochronologic data for this immediate area, much of the temporal distinction between various geological elements of the Neoarchean bedrock is based on the correlation of U-Pb zircon dates acquired elsewhere with regionally developed fabrics and structures that resulted from three major phases of deformation, denoted D1, D2, and D3. All three deformation events are the result of N-S- to NW-SEdirected compression. The timing of D1 deformation is bracketed between deposition of the metabasaltic and associated rocks of the Wawa subprovince at ca. 2722 Ma (Peterson and others, 2001), and emplacement of the Saganaga Tonalite at ca. 2690 Ma. Folds attributed to D1 deformation in the Ely Greenstone and related rocks are truncated by faults associated with Knife Lake strata, indicating that the latter is synchronous with or post-dates deposition and early deformation of the Ely. As such, the Knife Lake Group is inferred to be a Timiskaming-type extensional basin sequence temporally equivalent to the Shebandowan assemblage exposed in adjacent parts of Ontario (Lodge and others, 2013; Jirsa and others, 2016). D2 deformation and metamorphism affected all of the Archean supracrustal units and can be crudely bracketed by U-Pb dates of intrusions in the Giants Range batholith to the southwest that place the regional deformation and metamorphic event between about 2674 Ma and 2685 Ma (Boerboom and Zartman, 1993). Thus, the age of sedimentary and volcaniclastic rocks is confined to a 10 million year duration between the emplacement of tonalite at ca. 2690 Ma and D2 metamorphism and deformation at ca. 2680 Ma. In this region, D3 deformation is manifest as crenulations and faults in rocks affected by D2. 112 �Paleoproterozoic Mafic dikes Mafic dikes emplaced into the Saganaga Tonalite are prominent on aeromagnetic maps as positive linear anomalies trending northward and eastward. Exposures mapped along the magnetic trajectories (thin red lines on Fig. 2) indicate that the dikes vary from diabasic to lamprophyric, and from a meter or less in width, to more than 30 meters. The roadcut at stop 4 exposes diabase inferred to lie along one of the anomalies. The precise age of the dikes is unclear, and there may be suites representing more than one age. At least one of the northwest-trending dikes is unconformably overlain by, and shed fragments into conglomeratic strata of the basal Paleoproterozoic Animikie Group, indicating a syn- to prePaleoproterozoic age. Animikie Group Sedimentary rocks of the Paleoproterozoic Animikie Group are exposed in an east-trending belt that extends from Thunder Bay on Lake Superior to a point 12 miles (19km) west of the Gunflint Trail, where the belt is truncated by the Mesoproterozoic Duluth Complex (Fig. 1). The Animikie Group in this area consists of locally developed basal conglomerate and sandstone, and iron-bearing strata of the Gunflint Iron Formation. The stratigraphic top of the iron-formation is marked by a major unconformity and ejecta that resulted from a meteorite impact near Sudbury Ontario ca. 1850 Ma (Krogh, 1984; Davis, 2008). Of the 188 known and scientifically verified terrestrial impacts, the Sudbury event is the third largest (based on crater size) and forth oldest (TUwww.unb.ca/passc/ImpactDatabaseUT). The resulting ejecta blanket has been identified in Ontario (Addison and others, 2005), Michigan (Pufahl and others, 2007; Cannon and others, 2010), and here in Minnesota near Gunflint Lake (Jirsa and others, 2011), and in drill core along the Mesabi Iron Range (Addison and others, 2005). The overlying Rove Formation—a mudstone and turbiditic sandstone unit—was deposited directly on the ejecta. The sequence is broadly correlative with Animikie strata exposed to the southwest along the Mesabi Iron Range. The rocks form a homocline that dips gently southward, except where deformed by folding, faulting, and emplacement of Mesoproterozoic intrusions. For example, local folding, inferred to be associated with emplacement of the Logan intrusions, may be the product of magmatic delamination and shouldering of the sedimentary country rocks. Sequence "inflation" by the Logan sills may explain the observation that the dip of Paleoproterozoic rocks increases from 10º away from the contact with Duluth Complex, to 60º in some places near it. Faults are present locally, but few have displacements greater than 50 feet. A notable exception is the Lookout fault (Fig. 2) that crosses the Gunflint trail. As much as 200 feet of uplift on the west and south is speculated (Morey and others, 1981; Jirsa, 2011). In addition, the dip of Animikie strata on the west side of the fault is much steeper than that on the east, which explains in part the difference in the widths of map units apparent from the geologic map. Much of the complex-looking fold pattern east of the fault is an artifact of moderately high topographic relief and shallowly dipping units. The Lookout fault displaces Animikie strata; however the relationship between faulting and the emplacement of Logan and Duluth Complex intrusions is unclear. Gunflint Iron Formation (field trip stops 7-10) As on the Mesabi Iron Range, the Gunflint Iron Formation has historically been subdivided into so-called cherty (granular) and slaty (argillaceous) informal subunits or members. The members are denoted lower cherty, lower slaty, upper cherty, and upper slaty (Wolff, 1917). Although this terminology has some descriptive utility in the field, the subdivision employed here is based instead on a sedimentalogical model (after Pufahl and Fralick, 2004). In this model, depicted in Figure 3, the lower cherty and parts of the lower slaty members represent deposition during a single marine transgression. This was followed by a regression that deposited the lower part of the upper cherty member—the resulting sedimentary strata are collectively and informally termed lower sequence here. The upper part of the upper cherty represents the onset of a second transgression that continued through deposition of the thick upper slaty member, 113 �and is collectively termed the upper sequence here. The contact between the two sequences is a diastem inferred to represent a period of maximum regression. The initial stages of the second transgression is marked by intraformational conglomerate containing oncoliths, fragments of what appear to have been semi-lithified grainstone derived from the lower sequence, and both in-place and dislodged stromatolites. The uppermost strata of iron-formation are variably brecciated and/or chaotically folded, carbonatebearing, and capped by granular ejecta from the ca. 1850 Ma Sudbury meteorite impact event, collectively termed Sudbury Impact Layer here. More detailed descriptions are given below: Lower sequence—Irregularly graded sequence recording marine transgression, followed by regression. It grades from conglomerate and sandstone at the base, unconformably overlying Neoarchean bedrock; to locally stromatolitic, siliceous grainstone; to interlayered, laminated to massive chert, to iron-rich mudstone, and finally to siliceous grainstone. Total thickness is approximately 50 m. The basal part of the sequence is marked by discontinuous conglomerate and minor fine- to medium-grained quartzofeldspathic sandstone that is typically thinner than 1 m. Conglomerate contains pebbles to small cobbles of quartz, Saganaga Tonalite, metabasalt, and diabase. Thicker sections of this facies exposed in Canada are known as the Kakabeka Conglomerate. The uppermost siliceous grainstone forms prominent ridges. It appears to have been partially lithified prior to deposition of, and contributed grainstone fragments to, the basal part of upper sequence. Upper sequence—Siliceous grainstone and laminated chert; locally contains stromatolitic and intraclastic conglomerate at base of the sequence; which grades irregularly up-section to increasingly mudstone-rich; and typically parallel-laminated to wavy-bedded. Total thickness is approximately 45-55 m. Reworked volcaniclastic zircons from the upper sequence exposed in Ontario yielded a U-Pb age of 1878±1 (Fralick and others, 2002). Sudbury impact layer (SEE Discussion below)—Brecciated and complexly deformed iron-formation as much as 10 m thick, overlain locally by less than 1 m of mesobreccia and granular ejecta. Both deformed (seismically shattered and chaotically folded) iron-formation and ejecta are inferred to be related to the Sudbury meteorite impact event (Jirsa, and others, 2011). The macroscopically most apparent feature of ejecta is the presence of 0.1-1.0 cm, concentrically zoned spheres inferred to be accretionary lapilli. Microscopic evidence that this material has an impact origin includes rare occurrence of quartz fragments marked by planar deformation features. Metamorphism here in the contact aureole of the superjacent Duluth Complex presumably has obscured or obliterated other diagnostic attributes (e.g., French and Koeberl, 2010). 114 �Figure 3. Schematic stratigraphic section of Gunflint Iron Formation and adjacent rocks comparing older stratigraphic nomenclature in left column with that used informally here on right. Siliceous grainstone (cherty) units in the iron-formation are represented by pale color; iron-rich mudstone is darker. Approximate stratigraphic positions of field trip stops are shown in boxes. DISCUSSION OF SUDBURY IMPACT LAYER (Field Trip stops 11-15) The Gunflint Lake exposures lie some 480 miles (770 km) west of Sudbury, making this one of the most distant sites known to contain what is considered “proximal ejecta” from the ca. 1850 Ma Sudbury meteorite impact event. Similar deposits have been discovered in Thunder Bay, Ontario, and Michigan that are well documented by Addison and others (2005), Cannon and others (2010), and Pufahl and others, (2007). Deposits near Gunflint Lake appear to be consistently thicker than in other areas, even though these sites are more distal than those in Michigan and Ontario. The impact deposits at sites further away from the crater than Gunflint Lake are much thinner and lapilli are only rarely present. This has led Addison and others (2010) to hypothesize that the Gunflint Lake deposits may represent thick ramparts, 115 �as described for end-of-flow Martian base-surge deposits (Kenkmann and Schonian, 2006; Osinski, 2006; Mouginis-Mark and Garbeil, 2007; Fralick and others, 2012). It should be noted at the onset, that only a small portion of the material described here can be considered true ejecta; i.e., air-borne detritus derived from the impact site. The great majority of the 7 meter-thick deposit is breccia that consists of thoroughly disheveled fragments that appear to have been derived from subjacent iron-formation. Like the deposits near Thunder Bay, the breccia is sandwiched between Gunflint Iron Formation and sedimentary strata of the Rove Formation. Unlike deposits near Thunder Bay, the breccia lies within the metamorphic aureole of the Mesoproterozoic Tuscarora and Poplar Lake Intrusions of the Duluth Complex (ca. 1100 Ma), and is intruded by diabasic sills of the Logan Intrusions (ca. 1115 Ma; Heaman and Easton, 2005). Pervasive carbonate mineralization and metamorphism has overprinted and obscured much of the original, delicate mineralogic features, but macroscopic textures and geochemical content that convey information about protolith and depositional mechanisms are preserved. In the following discussion, the term Sudbury Impact Layer (SIL) is applied to all facies of sedimentary rocks inferred to have been formed or deformed in response to the ca. 1850 Ma Sudbury meteorite impact. By that definition, it includes autochthonous material interpreted to be seismically folded and shattered iron-formation (ejecta-absent), and overlying strata composed largely of allochthonous material derived from the impact site (ejecta-bearing). In no single outcrop are all facies present; however, an approximation of temporal relationships can be inferred from the juxtaposition of two or more facies in individual outcrops (Fig.4). Figure 4. Stratigraphic framework derived from 8 exposures along a 2 mile strike-length near Gunflint Lake; hung from the contact (bold dashed line) between ejecta-bearing (upper) and ejecta-absent (lower) facies of the Sudbury Impact Layer. Facies below are described in apparent stratigraphic order from oldest to youngest: Ejecta-Absent UContorted iron-formation facies:U The uppermost layers of iron-formation are chaotically folded and exhibit both ductile and brittle behavior in close proximity at the scale of individual outcrops. The rheologic response depends on the apparent rigidity of material at the time of deformation. Silica-rich layers display brittle, shattered to semi-ductile, boudinage-like textures. By contrast, much of the iron-silicate mudstone layers behaved in a ductile fashion, locally showing evidence of 116 �fluidization and injection into superjacent strata. Folds are non-systematic in trend and style, and multiple hinge detachments occur. These attributes counter-indicate a regional tectonic origin, and instead are best viewed in the context of impact-generated seismicity imposed on semi-lithified substrate. UParautochthonous breccia facies:U At several locations, straight-bedded iron-formation passes laterally along strike into irregular zones in which the silica-rich layers have been broken and disheveled, while still retaining some semblance of jigsaw-puzzle fit. UMegabreccia faciesU: This term is used for breccia composed of unsorted slabs (as large as 5 m), blocks, and smaller fragments of iron-formation. The fragments are angular and in most places have random orientations. Fragments of green, iron-silicate mudstone typically show some evidence of semi-ductile behavior, and locally this material was fluidized to form irregular matrix and clastic (muddy) dikes. Ejecta-Bearing Mesobreccia facies: This is fragmental rock containing angular, subrounded, and amoeboid clasts (up to 5 cm long) of dark green material and scattered accretionary lapilli. Petrography shows that much of the original structure of the clasts has been metamorphically recrystallized and annealed; however, relicts of amygdaloidal and fluid-looking textures remain—implying a glassy protolith. Lapillistone-gritstone facies: Accretionary lapilli as large as 1.5 cm occur as irregular masses and layers interbedded with sandy to silty gritstone. In areas least affected by metamorphism, several grains of shocked quartz with planar deformation features have been identified in thin section. Spherule, pellet, small lapilli facies: The upper parts of ejecta horizons locally contain layers, lenses, and interbeds of accretionary or relict glass grains that are smaller than typical lapilli and generally lack concentric zonation. These apparently accreted particles may represent waning ejecta plume deposition. Ejecta-bearing conglomerate facies: In a few localities, the uppermost part of impactite deposits consist of conglomerate containing subrounded fragments of iron-formation (in contrast to anglular fragments typical in breccias described above), and matrices containing variably abraded lapilli. U U U U U U U U The apparent contrast in rheologic response to seismicity between siliceous and interbedded “muddy” strata indicates that the relative competancy of the two sediment types was significantly different at the time of impact. One explanation is that the silicification process occurred very early, perhaps just beneath the sediment/water interface, and produced the more cohesive (but not yet fully lithified) siliceous layers. Seismic deformation brecciated those layers selectively, while folding and liquefacting the interbedded muds. This interpretation is relevant to the understanding of depositional environment. It implies that upper layers of the iron-formation were either in a shallow submarine setting or only recently emergent at the time of impact. The arrangement of facies described above and depicted on Figure 4 can be interpreted in the context of experimental evidence and observations from lunar and smaller terrestrial impacts. Using calculations from Collins and others (2005), based primarily on estimated crater dimensions, one can predict arrival times for various effects of the impact here, some 480 miles west of the impact site as follows: EVENT 1) Fireball 2) Earthquake 3) Ejecta Ground Surge 4) Air blast 5) Tsunami * APPROXIMATE ARRIVAL TIME 13 seconds (the modern equivalent of 3rd degree burns) 2-3 minutes (>10.9 at epicenter) 5-10 minutes (predicts ejecta 1-3 meters thick, grain sizes ~1cm) 40 minutes (sonic boom) 1-3 hours P 117 P �[*The latter is speculation, as the arrival time and effects of tsunami are dependent on pre-impact position relative to strand line, and basin bathymetry which is nearly impossible to establish.] Intuitively, only three of these events are likely to have produced a record in the rocks: earthquake, ejecta surge, and tsunami. Nearly all contacts between individual facies are gradational, with one very important exception—in all exposures, the boundary between ejecta-absent and ejecta-bearing facies is extremely sharp. This is inferred to reflect a fundamental shift in geologic processes from intense seismic perturbation of uppermost iron-formation represented by the ejecta-absent facies, to deposition by the passing ejecta plume. The uppermost conglomerate facies is inferred to represent mixing of local and exotic detritus, presumably by tsunamis or other post-impact fluvial or marine processes. Rove Formation The Rove Formation consists of carbonaceous, thinly bedded argillite to slate, and fine- to mediumgrained graywacke (stops 15 and 16). Primary sedimentary structures indicate turbidity current flow was dominantly to the south. The basal several meters of the formation are irregularly bedded, carbonate-rich, and locally conglomeratic (stop 15). Detrital zircons taken from lower parts of the formation in Ontario yielded ages as young as1827±8 (Addison and others, 2005), indicating some considerable hiatus separated deposition of the Rove from that of the underlying 1850 Ma Sudbury Impact Layer. Contact metamorphism The iron-formation, SIL, and overlying argillaceous strata of the Rove Formation were variably replaced by carbonate and metamorphosed by the Duluth Complex to amphibole and pyroxene hornfels. Floran and Papike (1978) delineated irregularly northwest-striking metamorphic zones recognized on the basis of the dominant iron-silicate mineral present in iron-formation. From least metamorphosed on the northeast, to most metamorphosed on the southwest, these indicator minerals are greenalite+minnesotaite, grunerite, hedenbergite, fayalite, and ferrohypersthene. Despite this metamorphism, macroscopic sedimentary textures are well preserved in most outcrops. Metamorphic effects adjacent to the Logan Intrusions are minor. Mesoproterozoic Mesoproterozoic mafic intrusions comprise the remaining exposures in the Gunflint Trail area. The rocks represent early magmatic stages of the Midcontinent Rift. The apparently earliest of these are diabasic sills and dikes emplaced into the Animikie strata and collectively referred to as the Logan intrusions. A baddeleyite age of 1115 ± 1 Ma is reported from a sample of a Logan sill near Thunder Bay (Heaman and Easton, 2005). The sills are intruded, with slight angular discordance, by medium to coarse-grained gabbro and troctolite of the Poplar Lake and Tuscarora intrusions of the Duluth Complex. The Poplar Lake is part of the early gabbroic series of the complex. A basal gabbroic unit of the intrusion yielded a date of 1106.9 ±.8 (Miller and Severson, 2002). Field relationships indicate that the Poplar Lake is intruded by the Tuscarora intrusion, which is considered to be part of the layered series of Duluth Complex. This is consistent with a U-Pb age of 1098.81±0.32 from a sample just west of the Gunflint Trail area (Hoaglund and others, 2010). Logan Intrusions The Logan intrusions (stops 11, 15, 16) are exposed along a series of prominent, east-trending ridges formed by the differential erosion of diabase sills and sedimentary rocks, particularly the Rove Formation. Individual sills are as much as 1100 ft (33m) thick, and can be traced along strike for several kilometers. Branching and merging of individual sills is common, and many sills thicken and thin down-dip. Some 118 �sills terminate against joints and inferred faults. Locally, fractures in the Rove Formation are occupied by thin dikes, which give a box-work configuration to the hypabyssal intrusions. Rock types include aphyric basalt, fine- to medium-grained diabase with ophitic clinopyroxene enclosing plagioclase, plagioclase cumulates, and granophyre (Jones, 1984). Outcrops in the field trip area commonly have plagioclasephyric phases (stop 11). Chilled margins form sharp contacts with, and locally contain inclusions of, the country rocks. Diabase coarsens to medium-grained near the center of individual sills, and clinopyroxene is ophitic throughout. Minor differentiation is manifest in accumulations of plagioclase or granophyric intergrowths (quartz, sodic plagioclase, and orthoclase) in upper parts of sills. Duluth Complex The Duluth Complex (Fig. 5) is a sequence of generally discordant plutonic rocks consisting of many separate intrusions. Two of these intrusions, the Poplar Lake and Tuscarora, are exposed along the Gunflint Trail. The Poplar Lake intrusion, formerly referred to as Nathan's layered series, consists of interlayered gabbroic cumulates, with minor amounts of troctolitic and anorthositic cumulates. Rocks of the Poplar Lake have reversed magnetic polarity, and thus are broadly correlative with lower lavas of the North Shore Volcanic Group and with the Logan intrusions. The Poplar Lake intrusion is composed of at least 27 sheet-like units of mafic cumulates and intermediate to felsic rocks (the so-called Nathan’s layered series). The Tuscarora intrusion irregularly cuts across the layered gabbro of the Poplar Lake intrusion (Morey and Nathan, 1978). The basal part of the Tuscarora intrusion consists of a fine-grained, augite-poikilitic, olivine gabbro (stops 17 and 18). Within 0.3 mi (0.5km) of the basal contact, finegrained troctolite coarsens to medium-grained. The troctolite units consist of 65-70 percent plagioclase and 10-15 percent olivine. Relative amounts of poikilitic augite and iron-titanium oxides vary locally. Orthopyroxene mantles olivine and occurs in symplectic intergrowth with plagioclase. Biotite is locally present in association with iron-titanium oxides. Modal layering is well developed and generally concordant with unit boundaries that dip gently to the south—typically more steeply dipping than the subjacent Animikie Group strata. The basal part commonly contains chalcopyrite, pyrrhotite, and minor pentlandite interstitial to plagioclase and olivine. The sulfide concentrations are subeconomic, but locally form mappable zones (stop 18). 119 �FIELD TRIP STOP DESCRIPTIONS NOTES: 1) Many of these outcrops are scientifically important—the subjects of on-going research, and all are on private or National Forest lands. For these reasons, we ask that you refrain from hammering and sampling without first checking with the leader. 2) This group is unlikely to visit all of the stops described below during this trip. A number of stop descriptions are included in this guide to provide context and for future visits to the region. 3) The stops are presented in general geochronologic order from oldest to youngest. 4) At the time of this writing, many of these stops had not been visited for several years. Regrowth after forest fires and other factors may preclude visiting some stops, and alternative locations may be substituted. 5) Some descriptions below show locations on images extracted from published 7.5-minute U.S.G.S. quadrangles. 6) All locations are given in NAD 83, Zone 15N UTM coordinates. Figure 5. Geologic map and schematic cross-section (A-A') showing the approximate geographic and stratigraphic positions of field trip stops (geology modified from Jirsa, 2011). Note that the horizontal distances on the cross section are much greater than those on the map, and the section is vertically exaggerated by 3.5 X, resulting in apparent dips of contacts steeper and units thicker than true. 120 �STOP 1—Neoarchean pillowed basalt and basal Gunflint Iron Formation Location: UTM locations of several substops along the Kekekabic Hiking Trail west of Hwy 12— Gunflint Trail (Fig. 5) are given below: 1a = Outcrops along the irregular unconformable contact between metabasalt and iron-formation in the vicinity of UTM 661970E/5328460N 1b = Outcrops near the junction of Kekekabic and Lookout trails at UTM 661230E/5328410N 1c= "Paulson Mine" at UTM 660980E/5328320N; about 900 feet west of junction of Kekekabic and Lookout trails General description: Follow the Kekekabic Hiking Trail from the parking lot, westward to several stops listed above, and perhaps others enroute. The Kekekabic trail parallels the base of the Gunflint Iron Formation where it rests unconformably on Neoarchean metabasalt. Several small outliers of the lower sequence (lower cherty member) of the iron-formation containing iron-silicates and magnetite can be found along the route, implying that this gently south-dipping surface is very near the unconformable contact between Neoarchean and Paleoproterozoic rocks. Exposures of metabasalt vary from massive to pillowed, autobrecciated, and locally variolitic. Analyses of a fine-grained hypabyssal intrusion associated with the metabasalt indicate that it has a komatiitic composition (Jirsa and Weiblen, 2007). Intrusions of this composition are also found in the Newton Lake Formation, some 30 miles to the southwest, and high-Mg tholeiitic basalt flows and pyroxenitic to peridotitic sills were described by Vervoort (1987) in the JAP Lake area two miles along strike to the west. Pillow shapes indicate moderate flattening by regional D2 deformation. Bedding trends to the east-northeast, and is steeply southward dipping and facing. The steep north-facing slope immediately south of the trail contains exposures of the lower sequence (lower slaty member). The iron-formation has been strongly metamorphosed in this area and now consists of various assemblages of quartz-grunerite-fayalite-magnetite and quartz-cummingtonitegrunerite-pyroxene-magnetite. Several test pits and shafts can be seen along more than a mile of the trail, including one that is fenced and labeled "Paulson Mine 1893" (stop 1c). In reality, this and other scattered shafts collectively made up the Paulson “mines.” They are developed in the lower sequence of iron-formation, and waste piles contain abundant pyrrhotite, other sulfide minerals, and magnetite. Despite construction of a rail line to Port Arthur (now Thunder Bay), only one train car of “ore” was ever shipped. Presumably the low iron and large sulfide content precluded further work, though the 1893 “financial panic” may also have played a role. STOP 2—Felsic phase of the Neoarchean Saganaga Tonalite cut by a small mafic dike. Location: UTM: 656333E/5335730N; End of the Trail Campground; Campsite #18 Description: Most of the outcrops in this area consist of gray, massive to trachytoid-foliated, medium to coarse-grained tonalite, having plagioclase in much greater abundance than microcline. Large quartz phenocrysts, or eyes, as much as 1 cm in diameter are characteristic of this phase, which is typical of 90% of the batholith (Fig. 6). Quartz eyes are polycrystalline aggregates, in which each crystal has a different optical orientation. Quartz also occurs as an interstitial mineral to subhedral plagioclase (An20-28). Small amounts of microcline occur as antiperthitic exsolution in plagioclase, as rims on plagioclase, and as small interstitial grains. Hornblende is the dominant ferromagnesian mineral, together with minor amounts of augite, biotite, epidote, and chlorite. 121 �Figure 6. Texture typical of the Saganaga Tonalite, including lineated quartz "eyes" (medium gray, trachytoid-aligned). The small dike of aphanitic mafic rock in this exposure has not been analyzed, but is inferred to be related to larger north-trending diabasic and lamprophyric dikes that form prominent north-trending anomalies on aeromagnetic maps. The pronounced foliation in the Saganaga Tonalite was inferred by Grout (1933) as a primary flow fabric (trachytoid). The tonalite is inferred to have been emplaced into Archean metavolcanic rocks shortly after early (D1) deformation, based a U-Pb date of 2689±1 Ma in Canadian exposures (Corfu and Stott, 1998), and 2690.83±0.26 Ma (Driese and others, 2011) just west of the Gunflint Trail. As such, it experienced major regional metamorphism and transpression associated with D2 deformation at ca. 2680 Ma. As with many large plutons in such terranes, the debate remains unresolved about whether the fabrics are wholly magmatic, wholly tectonic, or some hybrid of the two. STOP 3—Neoarchean Saganaga Tonalite with rounded dioritic to granodioritic inclusions Location: UTM: 656219E/5336079N; Campsite #13. Description: Equigranular tonalite with characteristic quartz eyes, containing a wide variety of inclusions. The term "inclusion" has a tortured usage—we prefer to use the term to apply to material that has a contrasting composition or appearance from its host, regardless of origin, size, shape, degree of assimilation, or extent of equilibration with the enclosing host magma. Inclusions may represent blocks of county rock (xenoliths) incorporated into the Saganaga Tonalite, or cognate phases of the intrusion (autoliths). Note that each inclusion contains the same mineralogic components (hornblende, plagioclase, quartz), but the components occur in varied proportions. Although these inclusions have not been studied in detail, field work in the region has identified multiple phases of the intrusion that are compositionally identical with the inclusions, and they are therefore considered autholiths. 122 �STOP 4—Granodioritic phase of Neoarchean Saganaga Tonalite with inclusions; cut by diabase dike Location: UTM 658575E/5335837N; Roadcuts on both sides of Gunflint Trail (Fig. 2) Description: Exposures on the west side of the road consist of pinkish to gray hornblende granodiorite to granite inferred to be a border phase of the Saganaga Tonalite and containing inclusions that are both more felsic and more mafic than the enclosing rock (Fig. 7). For example, the large angular block shown in the photo consists of quartz-eyebearing tonalite, much like rock that is typical of the main phase of the intrusion. Foliation is poorly developed and likely magmatic in origin. This exposure demonstrates that the Saganaga is a composite intrusion that, despite its apparent homogeneity, consists of quite varied magmatic phases, particularly near its border. Tonalite on the east side of the road is cut by a fine-grained diabasic dike several meters in width. Although this location lies some distance east of the prominent north-trending aeromagnetic trends associated with dikes that are described in the text above, the dikes presumably are related. Figure 7. Granodioritic phase of Saganaga Tonalite, containing many and varied inclusions. Large, lighter colored block in center of exposure is inferred to be an autolith of quartz-eye-bearing tonalite similar to the major phase of the batholith. 123 �STOP 5—Border phase Neoarchean Saganaga Tonalite with flattened inclusions and well-developed foliation in contact zone with Neoarchean metabasalt Location: UTM 661834E/5329257N; Roadcut on east side of Gunflint Trail (Fig. 5) Description: This location exposes the border phase of the Saganaga batholith, characterized by a granodioritic composition, general lack of quartz eyes, and an abundance of dioritic inclusions consisting of varied proportions of hornblende, pyroxene, biotite, plagioclase, and minor quartz (Fig. 8). The irregular ovoid and discoid shape of inclusions is oriented subparallel to well developed, steeply dipping and east-trending foliation. Hornblende crystals and aggregates define a prominent lineation plunging shallowly to the east. Petrology indicates that much of this fabric appears magmatic, yet foliation may be a hybrid of approximately coaxial magmatic flow and regional tectonic deformation (D2). This is typical of the border zone of the intrusion against Archean metabasaltic country rocks, which presumably lie in the low ground just to the south. A preliminary comparative geochemical study, summarized in Jirsa and Weiblen (2007), indicates that the mafic inclusions are not partially assimilated, recrystallized, and tectonically deformed country rock volcanic xenoliths as implied by some earlier workers. They have the common chemical characteristics of the sanukitoid suite of Archean granitoid rocks. Thus, we infer that the inclusions studied are autoliths derived from a separate, primitive sanukitoid-magma. Figure 8. Granodioritic border phase of Saganaga Tonalite containing mafic inclusions. 124 �STOP 6—Contact zone of Neoarchean Saganaga Tonalite and metabasalt; unconformably overlain by Paleoproterozoic Kakabeka conglomerate and lower sequence of Gunflint Iron Formation Location: 3 exposures along bush path off Gunflint Trail [individual UTM coordinates given below] (Fig. 5) Description: Stop 6a [UTM 661925E/5329065N] Archean metavolcanic rocks containing abundant granitic sheets and dikes, presumably related to border phases of Saganaga Tonalite (Fig. 9). The boundary between tonalite and metabasalt has been mapped in many places as a fault (Weiblen and others, 1971). These exposures do not preclude that possibility, but they imply that passive emplacement of the intrusion has also occurred, at least locally. Figure 9. Outcrop of border zone of Saganaga Tonalite. Dark gray material probably represents various phases of the intrusion; lighter gray, wedge-shaped area in the foreground is inferred to be metavolcanic country rock. Stop 6b [UTM 661965E/5329062N] Conglomerate developed at the gently southward dipping unconformity between Neoarchean intrusive and metavolcanic rocks and the overlying basal part of the Paleoproterozoic Animikie Group. The unit, regionally known as the Kakabeka Conglomerate, is present only locally on the western end of the Gunflint. In most places, the Lower cherty member of ironformation lies directly on eroded Archean surfaces. This small outcrop is one of the few places where the conglomerate is exposed and accessible along the contact. The conglomerate is greenish gray, poorly bedded, and contains subangular to subrounded fragments of Saganaga Tonalite and related granitoid rocks, metabasalt, and quartz, in a granular siliceous matrix. Stop 6c [UTM 661965E/5329037N] Walking southward from the basal conglomerate is a low "step-up" onto southward dipping strata of the Lower cherty member of Gunflint Iron Formation. 125 �Figure 10. Outcrop photograph of Kakabeka conglomerate lying unconformably on vertically foliated and eroded Archean Saganaga Tonalite (uniform light gray area on right side of photo). Note diversity of fragment types, including Archean metabasalt (dark) and tonalite (light), quartz pebbles (light, subrounded), and Paleoproterozoic diabase (dark gray). STOP 7—Lower Sequence of Gunflint Iron Formation. Location: UTM 661844E/5328896N; Road cut on Gunflint Trail (Highway 12) just north of parking lot for west end of Magnetic Rock Hiking Trail. Description: Gently southward-dipping, thinly interbedded granular and argillaceous iron-formation typical of the lower sequence. In earlier parlance, this stratigraphic position is the lower part of the Upper Cherty member of the Gunflint Iron Formation (Fig. 3). STOP 8—Stromatolitic grainstone at the diastem separating lower and upper sequences of Gunflint Iron Formation. Location: 3 exposures—specific coordinates given below; all adjacent to Magnetic Rock Hiking Trail (Fig. 5). CAUTION and ADVICE: This is a fairly long hike, approximately 1 mile round-trip; please be prepared with water and other field needs. Although this is not in the BWCAW, it does lie within Superior National Forest and is frequented by hikers. For this reason, and to preserve scientific value, please be respectful in matters of hammering and sampling. Description: Stop 8a [UTM 662034E/5328885N] Thin-bedded, fine-grained, chert-amphibole-magnetite-bearing strata assigned to the upper part of the lower sequence of Gunflint Iron Formation (“Lower slaty member”). Beds strike ENE and dip generally less than 8 degrees southward. Stop 8b [UTM 662401E/5329093N] Stromatolites lie within and just above a major regressiontransgression boundary (diastem) that is marked by intraformational conglomerate containing fragments of the underlying granular chert that appear to have been cohesive (though likely not lithified) at the time of incorporation, and in-situ and dislodged stromatolites. Irregular domal and laminar stromatolite forms are present. Note the presence of granules, intraclasts, and oncoliths—the latter consist of intraclasts coated with what likely was biogenic material, now composed largely of silica. Stop 8c [UTM 662583E/5329257N] Crest of ridge exposes the same boundary described above, here with abundant 3-dimensional views of stromatolites, intraformational conglomerate, and stromatolite "hash," all in a peloidal to ooidal, siliceous grainstone matrix. Given the apparent mineralogic replacement and moderate metamorphic grade, little of the original carbon-based material is likely present. Despite this, examples of nearly all morphological forms of stromatolites can be found, including columnar-digitate, domal, and laminar (Fig. 11). 126 �Figure 11. Cherty, stromatolitic Gunflint Iron Formation. A=Oblique surface of small columnar-digital stromatolites; B=Horizontal surface of irregular domal or laminar stromatolites; C=Vertical section of laminar stromatolites; D="Stromatolite hash." STOP 9—The “Magnetic Rock” Location: UTM 0663740E/5329670N; approximately 1 mile walk east of stop 8 on the Magnetic Rock Hiking Trail (Fig. 5) Description: Although most of the trail has magnetic iron-formation underfoot, the trail’s actual namesake lies about a mile walk to the east. The “Magnetic Rock” is a slab of iron-formation in which bedding is essentially vertical and standing nearly 30 feet above the surrounding land surface (Fig. 12). The appearance of this tombstone-shaped block raises the question of how glaciers could have up-ended it, but left the delicately balanced slab intact during ablation. My answer invokes glacial rotation of a “cube” of rock, followed by spalling along bedding planes during repeated cycles of freeze/thaw (frost-heaving). Figure 12. Slab of iron-formation. (Blue-handled hammer against lower 1/3rd of the rock is 40 cm long). 127 �STOP 10—Upper-most, largely argillaceous, Gunflint Iron Formation Location: UTM 663754E/5328212N; Gravel pit north of Gunflint Trail on U.S. Forest Service road #1347 (Fig. 5). Description: This dip-slope exposure consists of interbedded granular (cherty) and laminated (slaty) strata of the uppermost Gunflint Iron Formation. The slope defines the southern limb of a large, shallowly east-plunging anticline. The gentle dip of this limb illustrates the observation that open folding and moderate-relief topography are responsible for the complex map pattern. Note that large ridge visible to the south represents the basal Mesoproterozoic Duluth Complex. The bedding surface is marked by what have been referred to in earlier literature as “syneresis cracks”. The cracks, now filled with quartz, occur both concentrically and radially around a central, apparently raised core within a single granular layer of siliceous iron-formation (Fig. 13). Syneresis cracks are defined generally as shrinkage cracks formed by dewatering in a gel or colloidal suspension. They differ from septarian cracks that may develop in a similar way, in that the latter typically occur in concretions. Surprisingly diverse interpretations can be found in the literature about syneresis cracks (summarized in Pratt, 2001). There is, however, general agreement that they represent localized tensional failure during sediment dewatering. The explanation for localized semi-brittle response to what likely were formationwide stresses—caused by compaction or vibration due to syn-sedimentary earthquakes—is more contentious. It has been ascribed variously to the localization of cements, locally increased pore pressure, or zones of granular sediment made coherent by "microbial glue." It is interesting to note that syneresis structures are more prevalent in Precambrian and Cambrian rocks than younger ones. This may be due in part to more uniform organic bonding of clays in younger strata, which reduced the occurrence of stresslocalization. Recent mapping by the author (Jirsa, 2011) indicates that these quartz-filled cracks occur only in some granular siliceous layers that lie near the stratigraphic top of the iron-formation. This stratigraphic position, and their enigmatic structural attributes, may indicate an origin by impact-induced seismic wave passage through cohesive, semi-rigid chert during the Sudbury meteorite impact event. Figure 13. Polygonal quartz veining on eroded bedding surface of thinly bedded Gunflint Iron Formation. STOP 11—Paleoproterozoic ejecta and breccia from the Sudbury meteorite impact, intruded by sill and dikes of the Mesoproterozoic Logan Intrusions. Location: UTM 664785E/5329200N Description: This traverse provides a cross-section through diabase of the Logan Intrusions and underlying deposits of iron-formation, breccia, and ejecta. The diabase is medium- to coarse-grained in its core to the south, and grades to finer grained and more porphyritic near its base to the north. The northernmost outcrops lie along a steep cliff that exposes the upper Gunflint Iron Formation overlain by a 128 �thick sequence of iron-formation breccia that represents the ejecta-absent facies (Fig. 14.A) of the Sudbury Impact Layer. This is overlain by irregular lenses of bedded lapillistone, mesobreccia (Fig. 14.B), and reworked breccia containing rounded fragments of iron-formation in a matrix composed largely of accretionary lapilli that collectively represent the ejecta-bearing facies. The precise stratigraphic position of the latter two rock types is not entirely clear, though the strata containing accretionary lapilli (true ejecta) appear to lie near the top of the deposit. B. A. Figure 14. A. Large-fragment breccia (ejecta-absent); B. Bedded lapillistone and mesobreccia (ejecta-bearing). STOP 12—Sudbury Impact Layer—folded iron-formation overlain by ejecta. Location: UTM 663700E/5328967N Off Magnetic Rock Hiking Trail Description: Folded siliceous and argillaceous iron-formation overlain by a thin, discontinuous layer of mesobreccia containing scant accretionary lapilli. Note the structural detachment at the base of the outcrop that separates gently dipping, planar-bedded iron-formation layers from the overlying meter or so of folded strata. The chaotic fold style (Fig. 15A) indicates soft-sediment deformation prior to deposition of ejecta, which lends credence to the inference that iron-formation was not yet fully lithified at the time of impact. The walk from here to Stop 11 crosses several exposures of variably deformed iron-formation, all considered part of the ejecta-absent facies of SIL. These outcrops demonstrate the rheologic contrasts of substrate during deformation, and highlight the interpretation that at least some components of ironformation were unlithified at the time of impact deformation (Fig. 15B and 15C). 129 �Figure 15. Outcrop photographs of soft-sediment deformation in the ejecta-absent facies of SIL, locally overlain by ejecta, and demonstrating that deformation occurred during and after silicification of mudstones, but prior to complete lithification. A. Folded siliceous (light-colored) and iron-silicate (darker) iron-formation overlain by thin skin of ejecta containing accretionary lapilli (Bill Addison and Bevan French for scale). B. Irregularly layered siliceous and iron-silicate mudstone cut by a mudstone “clastic” dike (darkest narrow feature running up-down in center of photo). C. Folded and brecciated iron-formation, in which the siliceous layer (light gray) is attenuated and shattered in contrast with the enclosing iron-silicate mudstone that is ductily folded. 130 �STOP 13—Sudbury Impact Layer—deformed substrate, mesobreccia, gritstone and lapillistone. Location: UTM 663535E/5329100N Off Magnetic Rock Hiking Trail Description: This small outcrop provides a complete cross section of the SIL, and some unique sedimentalogical features not seen elsewhere. The stratigraphic sequence is shown in Fig. 16A. Of particular importance are the scoured (channelized) appearance at the base of lapillistone, and the presence of larger fragments of gritstone in lapillistone (Fig. 16B). Both indicate moderately high energy delivery of detritus—presumably by the passing ejecta plume or ground surge. Figure 16A. Photograph and graphic sedimentological analysis of stop 13. Black angular polygons represent fragments of iron-formation; black circles represent lapilli. White box shows approximate location of photo Fig. 16B. In detail, the basal part of this deposit consists of disorganized-bedded boulder “megabreccia”, with clasts composed of rock types characteristic of the underlying Gunflint Iron Formation. The megabreccia is overlain by a decimeters-thick, matrix-supported, pebble “mesobreccia” and massive, pebbly sandstone— here termed gritstone due to its content of moderately sorted, but primarily angular grains. Scattered accretionary lapilli occur in this unit locally, implying that it may be a mixture of ejecta and locally derived detritus. The mesobreccia and gritstone are overlain by lapillistone, composed of tightly packed accretionary lapilli. These fill shallow scours in the top of the mesobreccia and gritstone, or deeper scours that remove strata all the way down to megabreccia locally. The bases of the scours are commonly overlain by a one-centimeter-thick wisp of coarse-grained gritstone, followed vertically by the accretionary lapilli. The scours give a paleocurrent direction of 260 degrees—the bearing from Sudbury 131 �to Gunflint Lake is 280 degrees. At other locations, where individual smaller scours at the base of the lapillistone are not present, the basal, clast-supported lapillistone bed drapes shallow erosive scours. The lowermost accretionary lapillistone is massive-textured, as are overlying accretionary lapilli-rich beds, except where rare, small-scale, low-angle cross-stratification dipping towards 060 degrees is visible. The diameter of accretionary lapilli in the bed at the base of the lapilli-rich interval average 0.7 to 0.8 cm, and those higher in the section and interbedded with sandstone range from 0.2 to 0.4 cm. Gritstone beds become more dominant in the upper few decimeters. Here they are medium- to finegrained with stringers and patches of small accretionary lapilli. Some beds are massive with abundant, isolated lapilli. Parallel lamination to undulating parallel lamination is common in the non-massive beds. Approximately 10 cm of thinly laminated siltstone caps the impact deposit. Figure 16B. Close-up view of lapillistone containing entrained fragment of layered gritstone. STOP 14—Sudbury impact layer—deformed Gunflint Iron Formation overlain by thin ejecta layer that includes small spherules. Location: UTM 663628E/5329186N Off Magnetic Rock Hiking Trail Description: This cliff and ridge-top exposure includes a 7m-thick breccia, abruptly overlain by mesobreccia (Fig. 17A), and capped by strata composed of small (2-5mm) accretionary pellets and slightly larger, concentrically zoned lapilli (Fig. 17B). Some of these small particles may be relict glass spherules; however, metamorphism precludes definitive identification. A. B. Figure 17. A. Megabreccia sharply overlain by mesobreccia and other ejecta. B. Layers composed of accretionary pellets, small lapilli, and inferred relict spherules. 132 �STOP 15—Paleoproterozoic Sudbury impact layer, basal Rove Formation, and Mesoproterozoic Logan Intrusion. Location: UTM 665200E/5329300N Description: This outcrop affords a great number and variety of views of the ejecta and breccia (Fig. 18) because the exposed surface is nearly parallel with strike and dip of formations. The stratigraphic sequence is similar to that at Stop 11; however, this site lies along the top of the deposit, showing the relationship between ejecta and breccia more clearly. Just to the south is the eastern extension of the Logan sill traversed at Stop 11. In the intervening 0.25 mi., the basal contact of the sill cross-cut stratigraphic units to here overlie about 10 feet of slate and graywacke inferred to be the basal section of the Rove Formation. Figure 18. Breccia irregularly overlain by a “skin” of lapillistone. STOP 16—Mesoproterozoic Logan sill and Paleoproterozoic slate of the Rove Formation at Cross River Location: UTM 0665120E/5328890N Description: Outcrops on the north shore of Cross River and in it lie at the top of the same Logan sill that capped basal Rove Formation at stop 15. The cliff on the south shore exposes thinly bedded graywacke and mudstone of the Rove. Crossing the river may not be possible at this time. 133 �STOP 17—Mesoproterozoic Tuscarora intrusion of Duluth Complex—atypical border phase Location: UTM 662074E/5327457N; Roadcut on Cross Lake road (CR#47) south of Hwy 12 Description: A confusing exposure of the lower units of the Tuscarora intrusion. The outcrop consists of intergranular to ophitic gabbro and augite troctolite, with pods and veinlets of coarse mafic pegmatite and shear bands containing sulfide mineralization. Spheroidal weathering produced "core stones" locally. STOP 18—Mesoproterozoic Tuscarora intrusion of Duluth Complex Location: UTM 666638E/5327433N; roadcut on Gunflint Trail east of CR#50. (Fig. 5) Description: Just south of the parking pull-off is the rather poorly exposed intrusive contact between Paleoproterozoic Rove Formation and the Mesoproterozoic Tuscorara Intrusion (Fig. 5 explanation; Morey and others, 1981). The basal unit is a typical example of Cu-sulfide mineralized augite troctolite that is found at the base of the Duluth Complex here and in the Hoyt Lakes-Kawishiwi area to the southwest. It contains disseminated pyrite, pyrrhotite, and chalcopyrite. In the 1970's, International Nickel Company (INCO) drilled 7 holes in the basal Duluth Complex (Tuscarora and western Poplar Lake intrusions) to evaluate potential for Cu-Ni mining. All of these holes lie along the basal part of the intrusion within a few miles east and west of this stop. The archived drill cores were studied by Mogessie (1976) and Mogessie and others (1976). REFERENCES Addison, W.D., Brumpton, G.R., Davis, D.W., Fralick, Philip W., and Kissin, S.A., 2010, Debrisites from the Sudbury impact event in Ontario, north of Lake Superior, and a new age constraint: Are they base-surge deposits or tsunami deposits? Geol. Soc. Am., Special Paper 465. Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N. J., Davis, D.W., Kissin, S.A., Fralick, P.W., and Hammond, A.L., 2005, Discovery of distal ejecta from the 1850 Ma Sudbury impact event: Geology: 33:193196. Boerboom, T.J., and Zartman, R.E., 1993, Geology, geochemistry, and geochronology of the central Giants Range batholith, northeastern Minnesota: Canadian Journal of Earth Sciences, 30:2510-2522 Cannon, W.F., Schulz, K.J., Horton, J.W. Jr., Kring, D.A., 2010,The Sudbury impact layer in the Paleoproterozoic iron ranges of northern Michigan, USA: Geological Society of America Bulletin v. 122, p. 50-75. Corfu, F., and Stott, G.M., 1998, Shebandowan greenstone belt, western Superior Province: U-Pb ages, tectonic implications, and correlations: Geological Society of America Bulletin 110:1467-1484 Davis, D.W., 2008, Sub-million-year age resolution of Precambrian igneous events by thermal extraction-thermal ionization mass spectrometer Pb dating of zircon: Application to crystallization of the Sudbury impact melt sheet: Geology, v. 36, p. 383-396. Driese, S.G., Jirsa, M.A., Ren, M., Sheldon, N.D., Brantley, S.L., Parker, D., and Schmitz, M., 2011, Neoarchean paleoweathering of tonalite and metabasalt: Implications for reconstructions of 2.69 Ga early terrestrial ecosystems and paleoatmospheric chemistry: Precambrian Research, v. 189, p. 1-17. Floran, R.J., and Papike, J.J., 1978, Mineralogy and petrology of the Gunflint Iron Formation, Minnesota-Ontario: Journ. Petrology, 19:215-288. Fralick, P.W., Davis, D.W., and Kissin, S.A., 2002, The age of the Gunflint Formation, Ontario, Canada: single zircon U-Pb age determinations from reworked volcanic ask: Can. J. Earth Sci., 39:1085-1091. Fralick, P., Grotzinger, J., and Edgar, L., 2012, Potential recognition of accretionary lapilli in distal impact deposits on Mars: A facies analog provided by the 1.85 Ga Sudbury impact deposit: Society for Sedimentary Geology Special Publication 102, p.211-227. French, B.M., and Koeberl, Christian, 2010, The convincing identification of terrestrial meteorite impact structures: what works, what doesn’t, and why: Earth Science Reviews, v. 98, p. 123-170. Grout, F.F., 1933, Structural features of the Saganaga granite of Minnesota-Ontario: Report of XVI International Geological Congress, Washington, p. 255-270. Gruner, J.W., 1941, Structural geology of the Knife Lake area of northeastern Minnesota: Geological Society of America Bulletin 52:1577-1642. 134 �Heaman, L.M., and Easton, R.M., 2005, Proterozoic history of the Lake Nipigon area, Ontario: Constraints from UPb zircon and baddeleyite dating: in Easton, M., and Hollings, P., eds., Institute on Lake Superior Geology Proceedings, 51st Annual Meeting, Nipigon, Ontario, Proceedings and Abstracts, v. 51, part 1, p. 24-25. Hoaglund, S.A., Miller, J.D., Crowley, J.L., and Schmitz, M.D., 2010, U-Pb zircon geochronology of the Duluth Complex and related hypabyssal intrusions: Investigating the emplacement history of a large multiphase intrusive complex related to the 1.1 Ga Midcontinent rift: Institute on Lake Superior Geology Proceedings, v. 56, part 1, p.25-26. Jirsa, M.A., 2011, Bedrock geology of the western Gunflint Trail area, northern Minnesota: Minnesota Geological Survey Miscellaneous Map M-191, scale 1:24,000. Jirsa, M.A., 2016, Nine years of capstones: A summary of Precambrian Researcch Center field camp capstone mapping projects in the Newoarchean Knife Lake Group and associated rocks, central Boundary Waters Canoe Area Wilderness, Minnesota: (abs.) in Institute on Lake Superior Geology, Part 1—Abstracts and Program; abs., this volume. Jirsa, M.A., Boerboom, T.J., and Radakovich, A.L., 2016, Neoarchean geology of the western Vermilion District: Field Trip 2 in Institute on Lake Superior Geology, Part 2—Field trip guidebook; this volume. Jirsa, M.A., and Fralick, P.W., 2010, Field Trip 4: Geology of the Gunflint Iron Formation and Sudbury Impact Layer, in Field Guide to the Geology of Precambrian Iron Formations in the western Lake Superior region, Minnesota and Michigan: Precambrian Research Center Guidebook 10-01, p.77-92. Jirsa, M.A., Fralick, P.W., Weiblen, P.W., and Anderson, J.L.B., 2011, Sudbury impact layer in the western Lake Superior region, in Miller, J.D., Hudak, G.J., Wittkop, C., and McLaughlin, P.I., eds., Archean to Anthropocene: Field Guides to the Geology of the Mid-Continent of North America: Geological Society of America Field Guide 24, p. 147–169. Jirsa, M.A., and Miller, J.D., Jr., 2004, Bedrock geology of the Ely and Basswood Lake (U.S. portion) 30’ x 60’ quadrangles, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-148, scale 1:100,000. Jirsa, M.A., Starns, E., and Schmitz, M.D., in prep., Bedrock geology of the 2006 Cavity Lake fire area: Minnesota Geological Survey Miscellaneous Map M-193, scale 1:24,000. [in preparation; in the interim, refer to OpenFile Report OF-08-05] Jirsa, M.A., and Weiblen, P.W., 2007, Geology along the Gunflint Trail: Field Trip 6 in Miller, J.D., and Peterson, D.M., compilers; Institute on Lake Superior Geology, Part 2-Field Trip Guidebook; p. 143-168. Jones, N.W., 1984, Petrology of some Logan sills, Cook County, Minnesota: Minnesota Geological Survey Report of Investigations 29, 40p. Kenkmann, T., and Schonian, F., 2006, Ries and Chicxulub: Impact craters on Earth provide insights for Martian ejecta blankets: Meteoritics and Planetary Science, v. 41, p. 1587-1603. Krogh, T.E., Davis, T.W., and Corfu, F., 1984, Precise U-Pb zircon and baddeleyite ages for the Sudbury area: in The geology and ore deposits of the Sudbury Structure, E.G. Pye, A.J. Naldrett, and P.E. Giblin, eds, Ontario Geological Survey Special Volume 1, p. 431-446. Lodge, R.W.D., Gibson, H.L., Stott, G.M., Hudak, G.J., and Jirsa, M.A., and Hamilton, M.A., 2013, New U-Pb geochronology from Timiskaming-type assemblages in the Shebandowan and Vermilion greenstone belts, Wawa subprovince, Superior Craton: Implications for the Neoarchean development of the southwestern Superior Province: Precambrian Research v. 235, p. 264-277. MillerJ.D., Jr., and Severson, M.J., 2002, Geology of the Duluth Complex: in Miller, J.D., Jr., Green, J.C., Severson, J.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E., Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, p. 106-143. Mogessie, A., 1976 Petrologic study of copper-nickel mineralization in the Tuscarora intrusion Duluth Complex, northeastern Minnesota: M.S. thesis, University of Minnesota, Twin Cities, 137p. Mogessie, A., Stumpfl, E.F., and Weiblen, P.W., 1976, The role of fluids in the formation of platinum-group minerals, Duluth Complex, Minnesota: Mineralogic, textural, and chemical evidence: Econ. Geol. 86:15061518. Morey, G.B., and Nathan, H.D., 1978, Geologic map of the Gunflint Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Misc. Map M-42, scale 1:24,000. Morey, G.B., Weiblen, P.W., Papike, J.J., and Anderson, D.H., 1981, Geologic map of the Long Island Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Misc. Map M-46, scale 1:24,000. Mouginis-Mark, P.J., and Garbeil, H., 2007, Crater geometry and ejecta thickness of the Martian impact crater Tooting: Meteoritics and Planetary Science, v. 42, p. 1615-1625. 135 �Osinski, G., 2006, Effect of volatiles and target lithology on the generation and emplacement of impact crater fill and ejecta deposits on Mars: Meteoritics and Planetary Science, v. 41, p. 1571-1586. Peterson, D.P., Gallup, C., Jirsa, M.A., and Davis, D.W., 2001, Correlation of Archean assemblages across the U.S.Canadian border: Phase I geochronology, (abstract): Institute on Lake Superior Geology Proceedings, 47th Annual Meeting, Madison, WI, 2001, Part 1, p.77-78. Pratt, B.R., 2001, Syneresis cracks: subaqueous shrinkage in argillaceous sediments caused by earthquake-induced dewatering: Sedimentary Geol., 117:1-10. Pufahl, P.K., and Fralick, P.W., 2000, Depositional environments of the Paleoproterozoic Gunflint Formation: in Institute on Lake Superior Geology Proceedings, 46th Annual Meeting, Thunder Bay, Ontario, Part 2 Field Trip Guidebook, v. 51. Pufahl, P.K., and Fralick, P.W., 2004, Depositional controls on Paleoproterozoic iron formation accumulation, Gogebic Range, Lake Superior region, USA: Sedimentology 51:791-808. Pufahl, P.K., Hiatt, E.E., Stanley, C.R., Morrow, J.R., Nelson, G.J., and Edwards, C.T., 2007, Physical and chemical evidence of the 1850 Ma Sudbury impact event in the Baraga Group, Michigan: Geology 35:827-830. Vervoort, J.D., 1987, Petrology and geochemistry of the Archean of the JAP Lake area, northeastern Minnesota: M.S. Thesis, University of Minnesota-Duluth, 193 p. Weiblen, P.W., Morey, G.B., and Mudrey, M.G., 1971, Guide to the Precambrian rocks of northwestern Cook County as exposed along the Gunflint Trail: in Davidson, D.M., Darby, D.G., Green, J.C., and Grant, J.A., eds., Technical Sessions, Abstracts and Field Guides, 17th Annual Institute on Lake Superior Geology, Annual meeting, Duluth, Minnesota, p.97-127. Wolff, J.F., 1917, Recent geologic developments on the Mesabi Iron Range, Minnesota: American Institute of Mining and Metallurgical Engineers Transactions, v. 56 p. 229-257. 136 �FIELD TRIP 8 Saturday, May 7, 2016 KEWEENAWAN GEOLOGY OF THE HOVLAND AREA Terry Boerboom (Minnesota Geological Survey John Green (University of Minnesota-Duluth Emeritus) INTRODUCTION This field trip will show several examples of the varied types of volcanic rocks within the Northeast sequence of the North Shore Volcanic Group (NSVG), beginning in the upper part of the lower, reversely-polarized sequence and working up the stratigraphic section into the upper, normally-polarized sequence (table 1). It also includes some stops in the Hovland sill, a layered intrusion that is subconformable to the host volcanic rocks, and some small hybrid intrusions. The trip will start NE of Hovland, and work back southwest toward and past Grand Marais (Figure 1). Although brief driving directions are given, the UTM coordinates should provide the most accurate locations. Some of the easternmost UTM coordinates (for stops 1-7) are given using NAD 83, Zone 16N, the rest (stops 7-21) use NAD 83 Zone 15N. It is likely that more stops are described here than can reasonably be covered in a single day. Stops missed during this excursion can be visited by individuals using the guide, with the caveat that some stops may require permission from land owners. Figure 1. General stop locations with respect to the towns of Grand Marais and Hovland. Almost all the stops are along Highway 61, which parallels the shore. The North Shore Volcanic Group (Figure 2), part of the Mesoproterozoic Midcontinent Rift System, is well described in a multitude of publications. Some of these include the Geological Society of America Special Paper 312 (Ojakangas, Dickas, and Green, editors, 1997); Minnesota Geological Survey Report of Investigations 58 (Green, 2002); and field trip number 7 in the Geological Society of America Field Guide 24 (Green and others, 2011). These are just a few examples, and within those publications numerous references to other publications on the topic are listed. Given the widespread background descriptions already available, the interested user is referred to those publications, and to the references therein. A series of detailed geologic maps, based on 1:24,000-scale quadrangles, are available for almost all of the quadrangles that intersect the shoreline of Lake Superior. These maps as well as all Minnesota 137 �Geological Survey publications are available for free download at the Minnesota Geological Survey website (www.mngs.umn.edu) under the ‘search or browse’ link. The geologic maps pertinent to the stops for this field trip are listed below. Published Minnesota Geological Survey bedrock geology maps (all 1:24,000 scale) pertaining to this field trip: Stops 1-10, MGS Map M-195, Marr Island and Hovland quadrangles (Boerboom and Green, 2013) Stops 11-15, MGS Map M-190, Kadunce River (Boerboom and Green, 2011) Stop 16, MGS Map M-189, Grand Marais (Boerboom and Green, 2010) Stops 17-21, MGS Map M-179, Deer Yard Lake – Good Harbor Bay (Boerboom and Green, 2008) Figure 2. Generalized geology of the Mesoproterozoic rocks of northeastern Minnesota showing the major subdivisions of the North Shore Volcanic Group. The black bar denotes the general traverse of this field trip. 138 �Table1. Generalized stratigraphy of the northeast limb of the North Shore Volcanic Group showing U/Pb ages (Davis and Green, 1997; Green and others, 2001; Boerboom and others, 2014). Positions of intrusions denote approximate stratigraphic level affected and not age of emplacement, except rocks of the Beaver Bay Complex affect multiple stratigraphic levels. Thickness(m) Lithostratigraphic units Lithologic character U/Pb ages 7359 325 Total section Schroeder–Lutsen sequence (normal polarity) Lutsen basalts 3998 olivine tholeiite; includes Indian Camp sandstone and thin conglomerate. angular unconformity Upper northeast sequence (normal polarity) 130 100 131 Terrace Point basalt (within Good Harbor Bay andesites) (Stop 21) Cut Face Creek sandstone (Stop 21) Good Harbor Bay andesites (Stop 20) ophitic, olivine tholeiite basalt red, laminated, ripple-marked sandstone brown, porphyritic basaltic andesites Beaver Bay Complex – Beaver River diabase, Leveaux ferrodiorite, etc. (Stops 2, 3, 5 & 6) 122 Breakwater basalt (Stop 17, 19) 348 Grand Marais felsites (Stop 18) 335 Croftville basalts (Stop 16) 250 70 Devil Track rhyolite (Stop 15) Maple Hill rhyolite (Stop 14) 274 Red Cliff basalts (Stop 13) 366 Kimball Creek felsites (rhyolite and icelandite) (Stops 11 & 12) 539 Marr Island lavas (Stops 9 & 10) 198 235 900 Naniboujou basalts Devil’s Kettle rhyolite Brule River lavas (Stop 4) brown, columnar-jointed basalt flow pink to gray porphyritic rhyolite and felsite intergranular basalt and andesite flows, thick interflow sandstone aphyric, intergranular rhyolite flow porphyritic rhyolite flow ophitic olivine tholeiite flows, some plagioclase-phyric Porphyritic Kimball Creek rhyolite flow; Kadunce icelandite. mixed basalt, tholeiitic andesite, and icelandite flows intergranular basalt flows porphyritic ash-flow tuff interbedded basalt and rhyolite flows Brule Lake gabbro, Hovland sill, (of the Beaver Bay Complex) (Stops 7 & 8) 3036 1097.7±1.7 1100.2±1.9 1095.94±0.62 (Hovland sill) Lower northeast sequence (reversed polarity) 1932 Hovland lavas (Stop 1) 67 92 945 Red Rock rhyolite Deronda Bay andesite Grand Portage lavas 60 Puckwunge Sandstone mostly plagioclase-phyric basalt flows, some rhyolite and andesite red/tan, porphyritic rhyolite tan/brown, porphyritic andesite basalt lava flows, pillowed at base BASE 60 1097.26±0.67 Cross-bedded quartz sandstone slight angular unconformity Rove Formation (Paleoproterozoic) 139 1107.7±1.9 1107.9±1.8 �FIELD TRIP STOPS STOP 1 – Porphyritic andesitic lavas – lower reverse sequence Location: UTM (NAD 83, Zone 16T): 0284488E/5305983N, Hovland quadrangle, Milepost 133.2. Park along Highway 61 just beyond the Axtell mailbox. Note: This is private property except for the roadside outcrops. Index map: rrd – Reservation River diabase; hyf – hybrid dike (stop 2); pqm; ferromonzonitic dike (stop 3); nnu – NSVG undivided, normally-polarized; nrh – Hovland lavas rhyolite; nhp – Hovland lavas porphyritic andesite (stop 1). Relationship between hyf and pqm modified from M-195. The gray areas on this and subsequent index maps denote mapped outcrops. Description: Strongly porphyritic basaltic andesite lava flow with as much as 35% large plagioclase phenocrysts (An46) up to 5 cm in length which show a subparallel flow alignment. There are also a few small phenocrysts of magnetite and altered olivine. Upper amygdaloidal portions of the flows contain amygdules filled with epidote and chlorite. The lava flows at this stop are part of the Hovland lavas (mainly basalts and basaltic andesites), which are near the top of the reversely-polarized Lower Northeast sequence of the North Shore Volcanic Group. UPb ages for rhyolites within the Lower Northeast sequence include the 1107.7 ±1.9 Ma Tom Lake rhyolite (located inland to the northwest), and the 1107.9 ±1.8 Ma Red Rock rhyolite, which is located further northeast up the shore (Davis and Green, 1997). --------------------------------------------DIRECTIONS: Cross the highway and proceed to the lake shore in the small cove by the boat house. Note: This is private property and permission from the house across the road is needed to access. ---------------------------------------------STOP 2 –Hybrid ferromonzonite to ferrodiorite dike Location: UTM (NAD 83, Zone 16T): 0284647E/5306009N, Hovland quadrangle, Milepost 133.3 (see inset map under stop 1 for location). This is just across the highway and slightly east of the last stop, outcrops in a small cove on the lake shore. Description: Pink to gray, variably porphyritic, intermingled rhyolitic (finely granophyric) to basaltic rock types that exhibit mutually cross-cutting relationships. Contacts between the phases vary from sharp to gradational. Plagioclase phenocrysts vary from few to abundant; also present are small phenocrysts of apatite, pyroxene, oxides, and possible altered olivine. Viewed from the proper perspective this dike appears to be curvilinear in form. This dike can be traced to the west-southwest, subparallel to the shore, for nearly two miles. Other similar small hybrid dikes and intrusions have been mapped to the southwest in the Marr Island, Kadunce River, and Devil Track Lake quadrangles. These dikes commonly contain resorbed quartz and feldspar xenocrysts, implying that they may be the product of mafic magmas having melted porphyritic rhyolite flows at depth, and comingling with the felsic melts as they were emplaced. The hybrid dike at this stop was shown to be reversely-polarized (K.G. Books, unpub. Data); it is considered to be part of the reversely-polarized Grand Portage dike swarm (Green and others, 1987). On the published bedrock geologic map (M-195) this information was overlooked, and this hybrid dike was mistakenly portrayed as cross-cutting the larger ferromonzodiorite dike that we will visit at the next stop. --------------------------------------------- 140 �DIRECTIONS: From the driveway near Stop 1, proceed back southwest approximately 0.5 mile to an outcrop along the north side of the highway and park on the edge of the road --------------------------------------------STOP 3 –Ferromonzodiorite dike Location: UTM (NAD 83, Zone 16T): 0283964E/5305764N, Hovland quadrangle (see inset map under stop 1 for location). Description: Medium-coarse grained pyroxene-quartz ferromonzodiorite dike – gray, but weathers to a pinkish color, due to granophyre in the mesostasis, and cm-sized clots of granophyre are commonly visible on weathered faces. Contains an average of 49% plagioclase, 20% micrographic felsic mesostasis, 22% blocky to prismatic augite, 6% Fe-Ti oxides, up to 3% altered pigeonite, and trace amounts of apatite and possible altered olivine. This north–south vertical dike averages 328 feet (100 meters) in width and forms a prominent topographic ridge that makes a point out in the lake and extends inland about one mile from the shore. Its full extent, or how it relates to other hypabyssal diabase intrusions to the north, is not known as that area is incompletely mapped. ---------------------------------------------DIRECTIONS: Go back to Highway 61 and drive southwest approximately 2.5 miles to small road leading to a gravel pit, directly across from Big Bay Point road. Veer left at the first intersection on this road and proceed into an old gravel pit which has outcrops in the pit floor. ---------------------------------------------- STOP 4 –Porphyritic rhyolite (Big Bay rhyolite; 1,100.2± 2 Ma) Location: UTM (NAD 83, Zone 16T): 0280325E/5304581N, Hovland quadrangle. Index map: hfd – Hybrid ferromonzonite (stop 5); hfm – Hybrid ferromonzonite and remobilized rhyolite; hod– ophitic diabase; cgf – augite ferromonzodiorite; hba – coarse-grained amygdaloidal basalt; nbf – porphyritic rhyolite (stop 4). Description: Maroonish-pink, feldspar-phyric rhyolite that is quite vesicular (drusy) at the western-most outcrops, but going east passes through a less vesiculated, spherulitic zone and farthest east into a dense, grayish-pink more massive zone; speculatively passing from the upper to lower part of a flow. The western-most outcrops contain possible relict fiamme features, and the eastern/speculatively lower part of the flow may be a welded tuff. Small garnets are locally present, presumably due to contact metamorphism by the surrounding diabases. A sample from here gave a U-Pb zircon age of 1,100.2 ± 2 Ma (the Big Bay rhyolite of Davis and Green, 1997). This rhyolite was determined by Val Chandler (Minnesota Geological Survey, pers. comm.) to have normal magnetic polarity, and thus must be very near the base of the Upper Northeast sequence. Based on the phenocryst assemblage and flow characteristics, this rhyolite is grouped with other disparate occurrences of similar rhyolite in the area, but which are separated by one of the many intrusions. ---------------------------------------------DIRECTIONS: Go back to Highway 61 and drive southwest approximately 0.4 miles to roadcut on both sides of the highway, just past a right bend in the road. NOTE: There is a small pull-off on the north side of the highway just past/west of the outcrop roadcut where one could park a car. The highway is narrow and dangerous – please be careful! ---------------------------------------------- 141 �STOP 5 – Hybrid ferromonzonite phase within ophitic diabase Location: UTM (NAD 83, Zone 16T): 02799459E/5304014N, Hovland quadrangle (see inset map for stop 4 for location). Description: This outcrop shows a gradation from ophitic olivine diabase (the Horseshoe Bay ophitic diabase – Beaver Bay Complex) into a plug-like body of prismatic pyroxene-quartz ferromonzonite. The western-most outcrop is spheroidally-weathered diabase with normal cm-sized pyroxene oikocrysts; going east the oikocrysts transition into bronzy clotted ophites, then to prismatic pyroxene grains; concurrently the diabase texture grades from ophitic, to intergranular and weakly granophyric, into increasingly coarse-grained, granophyric, and prismatic ferromonzodiorite, and ultimately into very coarse-grained ferromonzonite with large curved-prismatic clinopyroxene and plagioclase laths greater than 1cm in size. At the east end within the monzonite the pyroxene prisms and plagioclase laths are aligned into a vertical to steeply east-dipping flow structure. One petrographic sample of the ferromonzonite contains 40 percent strongly zoned plagioclase, 20 percent variably uralitized prismatic augite, 8 percent Fe-Ti oxide minerals, 20 percent felsic mesostasis that is dominated by micrographic quartz/alkali feldspar but also includes independent quartz and sanidine, 10 percent red-brown secondary clay-type minerals, 1 percent hornblende, and nearly 1 percent apatite. This ferromonzonite body is one of several similar bodies that occur within or marginal to the Horseshoe Bay ophitic diabase (unit hod). Some of the marginal bodies may have formed as partial melt segregations from the underlying rhyolite. ---------------------------------------------DIRECTIONS: Continue southwest on Highway 61 for approximately 1.1 miles, to the Flute Reed River in the town of Hovland. Park near the river at a safe place along the highway or on the street parallel to the river and find your way down to the river. The water level must be sufficiently low to access the outcrops. ---------------------------------------------- STOP 6 – Chicago Bay ophitic olivine diabase Location: UTM (NAD 83, Zone 16T): 0278355E/5303179N, Hovland quadrangle. Index map: cbd – ophitic diabase (stop 6); htd – troctolitic ophitic diabase; ndk– Devil’s Kettle rhyolite; nbo – ophitic basalt; nbf – feldspar-phyric rhyolite; nb – sparsely porphyritic basalt. Description: This ophitic olivine diabase (part of the Beaver Bay Complex) exhibits strong sheet joints that dip more or less 10 degrees toward Lake Superior. It locally verges on augite troctolite; and in general contains 60 to 65% plagioclase (dominantly labradorite but includes andesine and bytownite; average An66Ab33Or1), 11 to 20% ophitic augite (Wo38En43Fs19, Mg# 70), 2 to 3% Fe-Ti oxides, 7 to 20% partially iddingsite-altered olivine (average Fo60), trace to 5% felsic mesostasis and quartz, up to 2% hypersthene, up to 1.5% fine-grained chlorite and/or clay mesostasis, and trace amounts of pigeonite, apatite, hornblende, and biotite. This typical ophitic olivine diabase is thought to be a sill-like body that underlies the Hovland sill, but it is not clear whether or not they are related. The extent of this unit is well established in the Hovland quadrangle by outcrops, water well cuttings, and topography; but the extension to the west in the Marr Island quadrangle below the Hovland sill is based on only one set of water well cuttings. This unit is of normal polarity (H.C. Palmer, unpub. data, 1972). 142 �---------------------------------------------DIRECTIONS: From the Flute Reed River continue southwest on Highway 61approximately 1.3 miles to a set of outcrops on the uphill side of the road next to a small pull-off. ---------------------------------------------- Overview of the Hovland Sill, Stops 7 and 8 (actual stops below) The Hovland sill, previously mapped in part by Jones (1963), is a gently-dipping (approximately 15° SSE), subcordant body composed of a basal massive ferrogabbro (stop 7), a middle zone of cumulatefoliated granophyric ferrogabbro to ferromonzodiorite (stop 7), and an upper coarse-grained felsic cap (stop 8). Overall the sill is estimated to be at least 984 feet (300 meters) thick. This sill is very similar to a less well exposed unit a few miles north that has informally been named the Lookout sill, which dips approximately 15° S-SE, and like the Hovland sill, has a cumulate portion with abundant coarse ilmenite plates and an upper felsic cap that contains fayalitic olivine. Both of these evolved intrusions are broadly similar to the ‘Silver Bay ferrogabbro’ type of zoned intrusions that are late intrusions associated with the Beaver Bay Complex (e.g. Miller and Green, 2002). A sample (MH047A-AD) of coarse prismatic olivine-pyroxene ferromonzonite from the upper felsic phase (Stop 8) was submitted for age dating to Dr. Mark Schmitz at the Boise State Isotope Laboratory. No zircon was separated from the sample, however relatively abundant, although small (approximately 100 microns in long dimension), flattened, light brown baddeleyite crystals were recovered. Six baddeleyite crystals selected for dissolution were all variably discordant, but gave equivalent 207 Pb/206Pb dates with a weighted mean of 1095.94±0.62 (n=6; MSWD 0.37; Figure 3; Boerboom and others, 2014). This age falls within the range of published ages for various Figure 3. Isochron diagram for sample MH047A-AD. other units of the Beaver Bay Complex, including the Wilson Lake ferrogabbro (1095.75±0.92; Hoaglund and others, 2010), Sonju Lake intrusion (1096.1±0.8; Paces and Miller, 1993), Silver Bay ferrogabbro (1095.8±1.2; Paces and Miller, 1993), Pine Mountain granophyre (1095.3±3.8; Vervoort and others, 2007), as well as others. The sample was collected from a roadcut on Highway 6, 1.3 miles northeast of the Brule River near Hovland. (UTM zone 15T 722714E, 5301024N). Figures 4 and 5 demonstrate various aspects of chemical differentiation trends within the Hovland sill, and Figure 6 shows examples of the varied textures between the phases. 143 �Figure 4. Variation in Mg# of olivine (A) and An content of plagioclase (B) within the Hovland sill. The olivine Mg plot shows the number of points analyzed for each individual sample in parentheses. The An diagram is from many samples which are not differentiated. Semi-quantitative SEM analyses were provided by Jeff Thole, Macalester College. Figure 5. Whole-rock compositional variations through the Hovland sill. Note the increase in TiO2 near the transition from the lower to middle zone, which is reflected by abundant cumulate ilmenite plates near the bottom of the cumulate zone. Analyses provided by Karl Wirth, Macalester College. 144 �A B C Figure 6. Photographs of thin sections (plane-polarized on left, cross-polarized on right) of phases of the Hovland sill. A. Upper felsic cap (Stop 8) B. Middle cumulate phase (stop 7) C. Lower massive phase STOP 7 – Strongly foliated cumulate ferromonzodiorite of the Hovland sill Location: UTM (NAD 83, Zone 16T): 0276907E/5301831N, Hovland quadrangle. Index map: fdd – small ferrodiorite dikes; hcg – Hovland sill cumulate ferromonzodiorite (stop 7); hgc– Hovland sill ferrogabbro); cbd – ophitic diabase. Description: This stop is within the middle cumulate zone of the Hovland sill. The cumulate ferromonzodiorite is strongly foliated, and granophyric, with abundant magnetite and plates of ilmenite plates (Figure 6B). The cumulate phases consist of plagioclase, augite, Fe-Ti oxides, olivine, and minor apatite. Prismatic augite crystals up to 2 centimeters in length are randomly oriented within the foliation plane. Olivine content (mostly altered) is generally low, around 2-4% in most of the intrusion. Pigeonite (Wo13En49Fs38, Mg# 49) occurs as thin discontinuous rims on augite and as small post-cumulate grains within the felsic mesostasis. Samples from this unit examined petrographically contain 45 to 55% strongly zoned plagioclase, 16 to 33% augite, 0 to 3% pigeonite, 5 to 11% Fe-Ti oxides, 2 to 10% mostly 145 �altered olivine, 8 to 15% felsic mesostasis, minor apatite, and near the top, rare hornblende. The felsic mesostasis is composed of a mixture of quartz paramorphs of tridymite, euhedral sanidine, micrographic to granophyric quartz and alkali feldspar, and abundant secondary iddingsite and clay minerals. Based on limited SEM semiquantitative analyses, average Fe/Mg ratios of augite increase from the base (augite; Wo35En38Fs27, average Mg# 57) to the top (ferroaugite; Wo37En22Fs41, average Mg# 35). Mg numbers for olivine (Figure 4A) range from Fo29 near the base, to Fo17 near the top; olivine is typically altered to reddish-brown iddingsite and/or green bowlingite (saponite). Limited feldspar analyses (Figure 4B) indicate that plagioclase becomes increasingly sodic, ranging from labradorite to mainly andesine at the base (average An51Ab47Or2), and andesine to oligoclase near the top (average An41Ab56Or3). Sanidine is common within the felsic mesostasis (average An2Ab45Or53). The basal ferrogabbro (Figure 6C) is poorly exposed along the highway to the east and will not be visited by this trip. It is dark greenish-gray with a rusty-weathered surface, medium- to coarse-grained, non- to weakly-foliated, and typically contains evenly distributed 3- to 4-millimeter, reddish-brown altered olivine spots. Based on several point counts this lower unit contains 48 to 55% plagioclase (labradorite), 25 to 35% granular augite (Wo37En41Fs22, Mg# 59 at the base and Wo32En37Fs32, Mg# 54 at the top), up to 2% pigeonite (Wo10En37Fs53, Mg# 41), up to 5% altered olivine clots, 5 to 8% Fe- Ti oxides, 6 to 8% felsic mesostasis, 1% reddish-brown clay-like material, and traces of apatite. ---------------------------------------------DIRECTIONS: Continue towards Grand Marais for about 2.2 miles; pull over near the auto repair shop and go to outcrop on lake side of highway under powerline, just east of driveway marked by 4300. Be careful if you walk across highway! NOTE: Entering UTM zone 15 (NAD 83). ---------------------------------------------STOP 8 – Upper felsic phase of the Hovland sill Location: UTM (NAD 83, Zone 15T): 722453E, 5300915N; Marr Island quadrangle. Index map: hfg – upper felsic cap of Hovland sill (stop 8); hcg – Hovland sill cumulate ferromonzodiorite (stop 7) Description: Very coarse-grained, prismatic ferromonzonitic upper felsic portion of the Hovland sill; location of age date sample MH047A.AD (Figure 3). Note prismatic to trellis-shaped pyroxene, incomplete fayalitic olivine trellises, and brownish granophyric matrix. Overall description of this unit taken from published geologic bedrock map (M-195) below: Figure 6A shows the typical texture of this unit. Prismatic olivine-pyroxene ferromonzodiorite to ferrogranite— Rusty reddish-brown where weathered, dark brownish- to greenish-gray where fresh, coarse-to very coarse-grained, granophyre-rich, prismatic. Contains 30 to 50 percent strongly zoned andesine (average An39Ab58Or3) to oligoclase, 8 to 15 percent variably prismatic ferroaugite (Wo22En41Fs37, Mg# 35), 10 to 15 percent fayalitic olivine (Fo10) that is mostly altered to reddish-brown iddingsite and varies from irregular coarse prismatic grains and clots to acicular trellises up to30 centimeters in length, 2 to 8 percent Fe-Ti oxides, 30 to 40 percent felsic mesostasis (combinations of micrographic quartz and alkali feldspar, crystalline quartz, and sanidine crystals), 1 to 2 percent apatite, trace amounts of rutile within quartz, and rare fine-grained bornite. The felsic mesostasis also contains abundant reddish-brown iddingsite-like needles interpreted to be former fine-grained masses and prisms of Fe-olivine. Outcrops at the border zone along the Brule River are generally darker in color, slightly more fine-grained, and locally contain small round chlorite amygdules. This unit is of normal polarity (K.G. Books, unpub. data, 1972). 146 �---------------------------------------------DIRECTIONS: Continue towards Grand Marais for about 2.5 miles to where the beach nearly touches the highway. Pull over and park along the small pull off near the beach. Walk back east along the beach to low outcrops, or park farther east along the edge of the highway. ---------------------------------------------STOP 9 – Icelandite of the Marr Island lavas Location: UTM (NAD 83, Zone 15T): 719901E, 5299658N; Marr Island quadrangle. Index map: hfg – upper felsic cap of Hovland sill (stop 8); nic – icelandite (stop 9) na – pigeonitic andesite; nob – ophitic basalt; nmr – aphyric rhyolite; nmb – strongly amygdaloidal basalt; ndk –Devil’s Kettle rhyolite. All volcanic units are part of the Marr Island lavas. The volcanic units shown here are also known as the Naniboujou basalts; this unit has been incorporated into the larger Marr Island lava package on M-195 (Boerboom and Green, 2013). Description: Low outcrops along the beach are fine-grained, sparsely porphyritic icelandite, with phenocrysts mainly of plagioclase but also some phenocrysts of magnetite, pyroxene, and apatite in a matrix of fine felty plagioclase and brownishweathered alkali feldspar mesostasis. As you work east along beach there are zones that are variably amygdaloidal, but it is difficult to demarcate flow contacts. Icelandite is a felsic rock characterized by 62-66% SiO2, high FeO (~7%), and Na2O + K2O (6.5-9%) (Carmichael, 1964, as summarized in Green and Fitz, 1993). Icelandites characteristically contain a few percent of small rectangular plagioclase phenocrysts that bleach white on an otherwise pinkish-brown weathered surface. Icelandite can be hard to differentiate from plagioclase-phyric rhyolite; however icelandite typically is brownish in color, has a fine felty texture, is weakly magnetic, and has small apatite phenocrysts compared to rhyolite which is more pink, saccharoidal in texture, non-magnetic, and lacking in apatite phenocrysts. Icelandite in some exposures has a strong flaggy parting which forms slabs about 6-10 cm thick (which would make ideal paving stones!). ---------------------------------------------DIRECTIONS: Continue towards Grand Marais for about 2 miles to County road 14, and directly across from it, turn left towards the lake onto an old section of the highway (Fire number 3500) and park. Walk west on the old highway and cut down to the beach to outcrops of ophitic basalt. ---------------------------------------------STOP 10 – Ophitic basalt of the Marr Island lavas, and another small hybrid intrusion. Index map: nhd – hybrid ferromonzonite (stop 10); na – pigeonitic andesite; nmo – ophitic basalt (stop 10);nba –basaltic andesite; npa – strongly porphyritic andesite. All volcanic units are part of the Marr Island lavas. Dashed red line is path of newer Highway 61. Location: UTM (NAD 83, Zone 15T): 716788E, 5297938N; Marr Island quadrangle. Description: Typical ophitic basalt, not only of the Marr Island lavas but of the North Shore Volcanic Group in general. Examine old road cuts and beach outcrops. Note scattered plagioclase phenocrysts. 147 �---------------------------------------------DIRECTIONS: Continue towards Grand Marais for about 1 mile to the intersection with Kelly’s Hill Road. Outcrop at the northwest corner of the intersection. ---------------------------------------------STOP 11 – Rangeline icelandite of the Kadunce icelandites. Location: UTM (NAD 83, Zone 15T): 715255E, 5297698N; Kadunce River quadrangle. Index map: nhd – hybrid ferromonzonite; nki – Kadunce icelandite (stop 11); nkq – porphyritic rhyolite; na – pigeonitic andesite; nmo – ophitic basalt. Description: The roadcuts along the north side of the highway expose the Rangeline icelandite, which is typical of the icelandites of the NSVG. It is brownish, with ~15% phenocrysts of mainly plagioclase but also altered Fe-olivine, Fe-augite (En15Wo42Fs43), magnetite, and apatite in a fine-grained groundmass of mainly plagioclase, alkali feldspar, and quartz (Figure 7). Figure 7. Photomicrographs (plane-polarized light) of the Rangeline icelandite showing phenocryst assemblage. The glomerophenocryst in the center of photo on left includes pale green-altered pyroxene; the brownish matrix is alkali feldspar. Pf – plagioclase; Ex-Ol – altered olivine; Px – ferroaugite; Ap – apatite. ---------------------------------------------DIRECTIONS: Continue towards Grand Marais for about 2.3 miles and park just beyond mile marker 118 at base of trail going up the hill on north side of the highway. Note: this is private property so please use discretion and stay near the road. The loose pieces of rhyolite at the base of the cliff are identical to those on the cliff face, which is dangerous. ---------------------------------------------- 148 �STOP 12 – Kimball Creek Rhyolite Rheoignimbrite Location: UTM (NAD 83, Zone 15T): 711750E, 5296730N; Kadunce River quadrangle. Index map: mld – Monker Lake diabase; nkr – Kimball Creek rhyolite (stop 12). Description: The Kimball Creek rhyolite is thought to be the second largest felsic flow in the NSVG. It is ~350 m thick and extends at least 20 miles/32 km to the west (Green and Fitz, 1993). The composition of this rhyolite is midway between typical rhyolites and typical icelandites. It contains 5-10% small phenocrysts, mostly plagioclase but also magnetite, zircon, apatite, quartz, and altered Feaugite. The fine-grained groundmass is composed mainly of alkali feldspar and variably poikilitic quartz, much of which occurs as paramorphs after tridymite tablets. As in the Devil Track rhyolite, the size of these ‘ex-tridymite’ tablets increases toward the flow center from the top and the base, implying emplacement as a single cooling unit. At both the top and bottom of this flow, outcrops show pyroclastic texture, with flattened fiamme and shards in a dense, probably originally vitric-ash groundmass. Near the base, these stretched fiamme are involved in the flow-folds (Green an Fitz, 1993). These observations imply that this flow was emplaced as a high-temperature ignimbrite that consolidated and underwent bulk flow. ---------------------------------------------DIRECTIONS: Continue toward Grand Marais for about 1.4 miles to mailbox #2524. Walk down the driveway toward the lake to outcrop on the shore below house. NOTE: This is private property – please obtain permission of the owner before proceeding to the shore! NO HAMMERS! Alternate outcrops of this unit can be viewed at roadcuts along the highway, but they are not as strongly porphyritic. ---------------------------------------------STOP 13 – Porphyritic basalt flow of Red Cliff basalts Location: UTM (NAD 83, Zone 15T): 710132E, 5295236N; Kadunce River quadrangle. Index map: nrb – Red Cliff basalts (stop 13); nkr – Kimball Creek rhyolite (stop 12). Description: The Red Cliff basalts are a series of olivine tholeiite flows sandwiched between large rhyolite flows in this section of the Upper Northeast sequence. This group of basalt flows is approximately 300 m thick, and can be traced inland to the west for at least 18 mi / 30 km. The thick ophitic to subophitic flow at this stop is remarkable for its concentration of large plagioclase phenocrysts (~An70) near its top; the phenocrysts apparently floated in the lava after eruption. Locally it also contains dm-sized, angular inclusions of coarse-grained anorthosite. It appears as though some of the phenocrysts may have floated away from disaggregating anorthosite inclusions (Figure 8) 149 �Figure 8. Photograph of Red Cliff basalt flow with coarse-grained anorthosite inclusion and plagioclase phenocrysts that appear to have disaggregated and floated away from it. Hammer is 40 cm long. ---------------------------------------------DIRECTIONS: Continue towards Grand Marais for about 1.9 miles and park along the edge of the highway near outcrops on the uphill side of the road. ---------------------------------------------STOP 14 – Maple Hill rhyolite Location: UTM (NAD 83, Zone 15T): 707109E, 5295074N; Kadunce River quadrangle. Index map: nhd – hybrid ferromonzonite; ndr – Devil Track rhyolite (stop 15); nwo – Woods Creek basalt; nhr – Maple Hill rhyolite (stop 14); nrb – Red Cliff basalts. Description: The Maple Hill rhyolite varies from 260 to nearly 400 feet in thickness, and extends west for at least 12 mi / 20 km and possibly as far as 25 mi / 40 km. Structures such as lineated vesicles and folded vesicle trains, coupled with the lack of pyroclastic textures, indicate that this rhyolite erupted as a lava flow rather than a rheoignimbrite (Green and Fitz, 1993). The upper part of the flow has quartz-lined stretched vesicles and amygdules of quartz and calcite, and local veinlets of purple fluorite. Below the upper vesiculated zone the rhyolite commonly shows tightly folded flow layering, and locally contains abundant spherulites and lithophysae as large as 3 centimeters. In general, this unit contains 4 to 6 percent alkali feldspar phenocrysts, 2 to 4 percent quartz phenocrysts, and rare microphenocrysts of zircon in a groundmass of fine-grained quartz and feldspar with minor fluorite (Fitz, 1988), as well as phenocrysts of plagioclase, altered Fe-olivine and Fe-augite, and Fe oxides (Green and Fitz, 1993). This rhyolite, in contrast to the aphyric Devil Track rhyolite (stop 15), contains abundant phenocrysts. 150 �An enigmatic thin flow of ophitic basalt (Woods Creek basalt; now on inset map), which is exposed a couple miles inland to the west and also intersected in water wells, appears to have erupted at the same time as the Maple Hill rhyolite, or more likely between the Maple Hill and Devil Track rhyolites. Remarkably, the same stratigraphic relationships are noted over 40 km to the west in the Lutsen quadrangle (Boerboom and others, 2007), where a thin basalt flow overlies the western extension of the Maple Hill Rhyolite and in turn is overlain by the Devil Track rhyolite. ---------------------------------------------DIRECTIONS: Continue towards Grand Marais for about 0.25 miles and park along the edge of the highway near outcrops at the abandoned wave-cut cliff on the uphill side of the road. ---------------------------------------------STOP 15 – Devil Track rhyolite Location: UTM (NAD 83, Zone 15T): 706718E, 5294928N; Kadunce River quadrangle (see inset map for stop 14 above for location). Description: The Devil Track rhyolite is the largest known flow of felsic volcanic rocks in the North Shore Volcanic Group and is inferred to have been either a hot superliquidus lava flow or possibly a thick, hot rheoignimbrite that flowed and underwent complete crystallization after deposition (Green and Fitz, 1993). Basal outcrops just northeast of here show strong lamination containing a marked flow lineation. This rhyolite varies from 750 to over 950 feet / 230-300 meters) in thickness, and extends west for at least 25 mi / 40 km from the mouth of the Devil Track River (and an unknown distance to the east, beneath Lake Superior). This rhyolite is light pink to grayish-pink, fine-grained and saccharoidal textured, essentially aphyric, and contains abundant small, tabular paramorphs of quartz after primary tridymite. Grain size increases toward the center of the flow (Green and Fitz, 1993). Flaggy parting is typical, as well as a planar flow layering that is gently warped and is generally not parallel to the flaggy parting; neither parting nor flow layering provide consistent measured structural orientations. A well that penetrates the top of this rhyolite to the southwest of here shows that the upper part of the flow is perlitic, and is overlain by a thin, discontinuous sandstone. ---------------------------------------------DIRECTIONS: Drive to Grand Marais, and at the east edge of town, follow signs to go up the Gunflint Trail (Cook County Highway 12). Drive up the Gunflint Trail for approximately 2.5 miles, and turn right on the road to Pincushion Mountain ski trails. Drive this road to the parking lot. ---------------------------------------------STOP 16 – Andesitic Croftville lavas – Pincushion Mountain overlook Location: UTM (NAD 83, Zone 15T): 701012E, 5294305N; Grand Marais quadrangle. Index map: nco – ophitic to intergranular pigeonitic basalt; nca – andesite (stop 16). Both are part of the Croftville lavas. Description: Artists’ point and the breakwater that forms the enclosure around the Grand Marais Harbor visible below are formed by the Breakwater basalt flow, which will be the next stop. This landform, with an island tied to the mainland by a gravel bar, is a classical tombolo. The rubbly outcrops here below this overlook are fine-grained, sparsely porphyritic andesite which is part of the 151 �Croftville lava sequence. Exposures on the dipslope and in creek valleys below the overlook show the andesite flows contain thick rubbly Aa-type flow tops, typical of lava flows of this composition. ---------------------------------------------DIRECTIONS: Drive back down the Gunflint Trail to Grand Marais, turn right (west) on Highway 61, and proceed to Broadway Avenue. Turn left on Broadway and drive to the parking lot next to the harbor just before the Coast Guard station, and continue walking toward the lake to the breakwater. ---------------------------------------------STOP 17 – Breakwater basalt – Artist’s Point Location: UTM (NAD 83, Zone 15T): 699945E, 5291458N; Good Harbor Bay quadrangle. Index map: mmd – Murphy Mountain diabase; nbb – Breakwater basalt (stop 17); nba – Amygdaloidal porphyritic basalt; ngp –Grand Marais porphyritic rhyolite (stop 18); ngr – Grand Marais aphyric rhyolite. Description: The broad ledges here that help form the Grand Marais harbor are made of a thick (>100 m) flow of transitional basalt called the Breakwater basalt. It has distinctive texture and columnar jointing, and forms some of the ridges of the ‘Sawtooth Range’ visible to the west from the Breakwater. The western end of the breakwater has wellpreserved glacial striations and well-developed joint-plucking. This is the tombolo seen from the last stop. The basalt is massive, gray to maroon, and fine- to medium- grained, with abundant small clustered plagioclase phenocrysts and minor augite, altered olivine, and magnetite in a felty-intergranular groundmass. The Breakwater basalt can be traced at least 7.5 mi/12 km to the west, where it apparently pinches out. It is not present in the Cascade River, which is approximately 10.5 mi / 16 km to the west. ---------------------------------------------DIRECTIONS: Drive back to Highway 61, turn left (west) and go 1 mile up the hill to small roadcut on the right (north) side of the road opposite the lake shore. It might be best to pull into the Harbor Light parking lot and walk back to outcrop. ---------------------------------------------STOP 18 – Porphyritic Grand Marais rhyolite (1097.26±0.67 Ma) Location: UTM (NAD 83, Zone 15T): 698364E, 5291847N; Good Harbor Bay quadrangle. Index map units same as stop 17. Description: This outcrop (underneath large billboard) is the location of age date sample DG073-AD, porphyritic rhyolite (207Pb/206Pb age of 1097.26±0.67). This strongly porphyritic rhyolite contains in general 2-4% each of quartz and feldspar phenocrysts, as well as minor magnetite and altered mafic silicate phenocrysts. The rhyolite varies from massive to flow-banded, and is commonly strongly blocky-fragmental, a texture well-exhibited in the small creek just east of this outcrop where angular, flow-banded rhyolite blocks up to 1.5 m in size are evident; mapping in the vicinity indicates that loose blocks of rhyolite were overrun by the Breakwater basalt (see next stop for more description). 152 �Age dating on rhyolite from this outcrop was performed by Dr. Mark Schmitz at the Boise State Isochron lab. Six zircon crystals were selected for CA-TIMS (Chemical Abrasion Thermal Ionization Mass Spectrometry) analysis, from which five grains produced concordant isotopic ratios, with a weighted mean 206Pb/238U date of 1095.00±0.33 (MSWD (Mean Square Weighted Deviation) = 0.07) and a weighted mean 207Pb/206Pb age of 1097.26±0.67 (n=5; MSWD 1.47). Using the 207Pb/206Pb weighted mean date, this age is only slightly younger than the Devil’s Kettle rhyolite (1097.7±1.7; Davis and Green, 1997), which lies roughly 8,000 feet stratigraphically below and is separated by several thick mafic to felsic volcanic units. The nearly identical ages for these two units indicates rapid and voluminous volcanic activity in the upper part of the northeast limb of the North Shore Volcanic Group. ---------------------------------------------DIRECTIONS: Continue southwest on Highway 61 approximately 1.7 miles to the Fall River. Park on wide spot at edge of Highway near river. First cross highway and follow trail along east side of river to the shore, then cross the river (if possible) to outcrops on the west. ---------------------------------------------- STOP 19 – Breakwater basalt and Grand Marais rhyolite, Fall River Location: UTM (NAD 83, Zone 15T): 695809E, 5290906N; Good Harbor Bay quadrangle. Index map: nga – Good Harbor Bay andesites; nbb – Breakwater basalt; nba – amygdaloidal Breakwater Basalt; ngp – Grand Marais porphyritic rhyolite; nbd – inclusion-rich basalt sill or dike; nmi – icelandite (outcrop not shown). Description: Outcrops of porphyritic rhyolite cross-cut and/or overrun by the Breakwater basalt. Dikelets of basalt intruded into fractures in the rhyolite contain abundant small chips and slivers of rhyolite, and larger inclusions of rhyolite or possibly more andesitic rocks with stretched vesicles are contained in the basalt. Although the rhyolite here may be as inclusions in the basalt, farther up Fall Creek and also along the shoreline there are several ‘windows’ through the Breakwater basalt where the underlying rhyolite is exposed. In all cases, it appears as though the rhyolite was a ‘loose breccia’ that was overrun by the Breakwater basalt. Figure 9 is a photograph taken along the shore towards Grand Marais, which shows a block of rhyolite that is draped by the Breakwater basalt; evidence for a loose, blocky rhyolite surface having been overrun by a basalt flow. 153 �Figure 9. Block of porphyritic rhyolite overrun by the base of the Breakwater basalt flow, lakeshore west of Grand Marais (see map for stop 18, small unit labeled ‘ngp’ along shoreline due south of stop 18. Chilled, finely amygdaloidal margin of the basalt drapes over the block of rhyolite. ----Hike back up to the highway and climb down into the river just upstream of the highway----Exposed here (on the east side of the river) is another contact between the Breakwater basalt and rhyolite, which in this case has only feldspar phenocrysts. The basalt in general becomes increasingly amygdaloidal near the rhyolite, but the relationships are somewhat ambiguous because there are also inclusions of identical-appearing amygdaloidal basalt within the rhyolite. Another example of this can be found farther upstream, where the rhyolite is extremely brecciated, with fragments from 1 m or more to as small as 1 cm, and the fragments are intruded by the Breakwater basalt, or a possibly a feeder dike to the basalt. ---------------------------------------------DIRECTIONS: Continue southwest on Highway 61 for about 2.3 miles and park along the road edge adjacent to a long roadcut. 154 �STOP 20 – Good Harbor Bay andesites Location: UTM (NAD 83, Zone 15T): 692225E, 5289747N); Good Harbor Bay quadrangle. Index map: ngs – Cut Face Creek sandstone (stop 21); nga – Good Harbor Bay andesites (stop 20); nbb – Breakwater basalt. Description: The Good Harbor Bay andesites extend west from this location for nearly 20 mi\32 km, to beyond the Onion River, where they are terminated by the Leveaux Porphyry. The unit is at least 200 ft / 60 m thick. It overlies the Breakwater basalt and is overlain by the Cut Face Creek Sandstone, which in turn is overlain by the Terrace Point basalt flow (Stop 21). However, approximately 22 km to the west of here the sandstone pinches out, and the Good Harbor Bay andesites are in direct contact with the Terrace Point basalt. This locality is typical of the Good Harbor Bay andesites – fine-grained, fresh, sparsely porphyritic, and moderately to strongly magnetic. This roadcut exposes a flow contact where a lower rubbly amygdaloidal Aa flow-top is overlain by a massive flow base. A discontinuous, meter thick bed of sandstone locally overlies the rubbly flow-top breccia. The flow-top breccia can be recognized by blocks of strongly amygdaloidal/vesicular andesite infilled by sandstone, and the base of the overlying flow contains abundant amygdules that are highly stretched parallel to the flow contact. The sandstone between the two flows can be identified by its red-spotted appearance (oxidation spots). Just east of this outcrop is a small creek that crosses the highway. If the water is low enough to traverse up the creek, there are excellent fresh exposures of the Good Harbor Bay andesites. Approximately 200 m upstream is a small waterfall formed by an approximately N20°E, 20° north dipping, 30 cm wide sharply bounded brittle fault that has pink zeolite minerals infilled around the fault breccia clasts. Other small, flat faults, some with slickensides, may be visible in the stream bed. ---------------------------------------------DIRECTIONS: Continue southwest on Highway 61 about ¾ of a mile to the scenic overlook across from the high road cut. ---------------------------------------------- STOP 21A – Terrace Point basalt flow and Cut Face Creek sandstone (Cut Face Creek Road cut) Location: UTM (NAD 83, Zone 15T): 691780E, 5289170N Good Harbor Bay quadrangle. Index map: ngt – Terrace Point basalt flow; ngx – basaltic breccia; ngs – Cut Face Creek sandstone; nga – Good Harbor Bay andesites. Description: Thick interflow sandstone with ripple marks, deformation features in sandstone at base of flow, shale rip-up chips, and desiccation cracks. At the west end is a fragmental/scoriaceous phase of the Terrace Point basalt intruded and overrun by the main basalt flow. In high roadcut on northwest side of highway is an obvious contact between the Cut Face Creek Sandstone and the overlying Terrace Point basalt flow. The sandstone overlies the Good Harbor Bay andesites (stop 20). 155 �Basalt – The Terrace Point basalt, which overlies the sandstone, is a major ridge-forming unit from here to the southwest, forming ‘sawtooth mountains’. It is a distinctive flow characterized by a dark green color, white thomsonite amygdules, uniform 3-4mm ophitic texture, and scattered small glassy plagioclase phenocrysts. In general the contact with the sandstone is sharp and straight, but in places the sandstone has been slightly deformed, though not appreciably metamorphosed, by the basalt flow. Near the south end of the road cut is a unit of scoriaceous, fragmental basalt that is intruded and overrun by the Terrace Point flow. Similar rock types and relationships have been observed to the southwest, also near the flow base. The breccia contains 1-100 cm angular basalt fragments that are both massive and amygdaloidal, and scattered large blocks of massive basalt. At this locality and others, sub-volcanic dikes of Terrace Point basalt that intrude the fragmental basalt are slightly chilled, and contain small amygdules stretched parallel to, and columnar joints perpendicular to, curvilinear dike margins. The fragmental unit is interpreted as a cinder cone or lahar-type deposit that may have formed by interaction between a basalt feeder and water-saturated sediment. Sandstone –Jirsa (1984) measured approximately 73 meters of sandstone and 3 meters of shale in this section of the Cut Face Creek sandstone, but reported overall that nearly 30 percent of it is composed of thinly bedded, graded layers of fine-grained sand, silt, and clay. He reports both symmetrical and asymmetrical ripple marks, and bimodal paleocurrent distribution, and concluded that the Cut Face Creek sandstone was deposited in a fluvial-lacustrine environment. Unlike nearly all the other exposed interflow sandstones within the NSVG, planar cross-bedding is predominant, but some trough cross beds are present near the top of the section. Other features that may be visible in the sandstone are desiccation cracks filled with sandstone or coarse pink zeolite minerals, and rip-up textures. Compositionally it is predominantly a lithic arkose composed mostly of plagioclase feldspar and mafic rock fragments, with lesser amounts of quartz, altered clinopyroxene, and opaque grains; cemented by calcite and zeolite. The base of the sandstone is exposed to the north in the Cut Face Creek valley, where it overlies the Good Harbor Bay andesite (see stop 21B). The Cut Face Creek Sandstone was earlier considered to represent clastic deposition during a significant hiatus in volcanism prior to eruption of the Terrace Point/Schroeder-Lutsen basalts. It was also considered to be somewhat unique in that it was one of the few thick sandstone units in the northeast limb of the North Shore Volcanic Group, along with the 68 meter thick Indian Camp sandstone (which is within the Schroeder-Lutsen basalts). However, recent remapping has shown that there are likely at least three more substantially thick sandstone units that occur within the lower series of lava flows to the north of the Good Harbor Bay lavas. Thus, it is now recognized that the Cut Face Creek and Indian Camp sandstones, exposed because of more active erosion near the Lake Superior coast, are only one part of a larger set of thick sandstone units (Figure 10), some of which are located over 5 miles inland and at least 500 and possibly more than 1,000 meters downsection from here. The recognition of thick interflow sandstones throughout several different series of lava flows at different stratigraphic levels implies active sedimentation during a prolonged period of volcanism. Fragmental flow tops in the Good Harbor Bay Lavas contain abundant sandstone infillings, and thin, discontinuous, layers and crack fillings (clastic dikes) of sandstone are common in the Schroeder-Lutsen basalts. Elevated levels of clastic deposition during active volcanism may account for the relative abundance of sand at the tops of the lava flows, and the thick interflow sandstones may have formed during periods of relative volcanic quiescence, but continued basin subsidence. Available data indicate that the thicker sandstones may vary in thickness along strike, consistent with deposition onto an irregular lava surface. 156 �Figure 10. Simplified geologic map showing the distribution of sandstone units in the northeast limb of the NSVG. The black arrows at the lower left indicate thin sandstone units (one of which is the southwestern extension of the Cut Face Creek Sandstone at stop 21). ---------------------------------------------DIRECTIONS: Drive back towards Grand Marais a short distance to the wayside rest parking area on the lake side of the highway. ---------------------------------------------STOP 21B – Traverse up Cut Face Creek; Good Harbor Bay andesites, Cut Face Creek sandstone Location: UTM (NAD 83, Zone 15T): 691953E, 5289526N Good Harbor Bay quadrangle. See index map for stop 21A. Description: Small outcrop directly across highway from parking area and to northeast, and outcrops at the mouth of Cut Face Creek are fine-grained, brownish-gray, sparsely porphyritic Good Harbor Bay andesite. The andesite here is overlain by the Cut Face Creek sandstone, and the contact is exposed in numerous locations in the meandering Cut Face Creek valley. The basal contact appears to be conformable with the flat-surfaced, uneroded, amygdaloidal andesite. Small pebbles of massive to amygdaloidal andesite are common in the lower 3 meters of the sandstone, typically in 3-25 cm thick planar cross-bedded pebbly beds. The upper part of the andesite typically contains stretched amygdules, and has cracks filled with sandstone (Figure 11). 157 �Figure 11. Photograph of base of Cut Face Creek sandstone (red arrow and above), showing clastic dike (black arrow) filling a crack in underlying andesite. Hammer in ellipse is 45 cm long. REFERENCES Boerboom, T.J., Wirth, K., 2014, and Evers, J.F., 2014, Five newly acquired high-precision U-Pb ages in Minnesota, and their geological implications [abs]: Institute on Lake Superior Geology, May 14-17, 2014, Proceedings Volume 60, Part 1 – Programs and Abstracts, p.13-14. Boerboom, T.J., and Green, J.C., 2013, Bedrock geology of the Marr Island and Hovland quadrangles, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series Map M-195, scale 1:24,000. Boerboom, T.J., and Green, J.C., 2011, Bedrock geology of the Kadunce River quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series Map M-190, scale 1:24,000. Boerboom, T.J., and Green, J.C., 2010, Bedrock geology of the Grand Marais quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series Map M-189, scale 1:24,000. Boerboom, T.J., and Green, J.C., 2008, Bedrock geologic map of the Deer Yard Lake and Good Harbor Bay quadrangles, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series Map M-179, scale 1:24,000. Boerboom, T.J., Green, J.C., and Albers, P.B., 2007, Bedrock geology of the Lutsen quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous map M-174, scale 1:24,000. Carmichael, I.S.E, 1964, The petrology of Thingmuli, a Tertiary volcano in eastern Iceland: Journal of Petrology 5, p. 435-460. Davis, D.W., and Green, J.C., 1997, Geochronology of the North American Midcontinent Rift in western Lake Superior and implications for its geodynamic evolution: Canadian Journal of Earth Sciences, v. 34, no. 4, p. 476488 Fitz, T.J., III, 1988, Large felsic flows in the Keweenawan North Shore Volcanic Group in Cook County, Minnesota: unpubl. M.S. thesis, University of Minnesota-Duluth, Duluth, MN; 165 p. 158 �Green, J.C., Boerboom, T.J., Schmidt, S.Th., and Fitz, T.J., 2011, The North Shore Volcanic Group: Mesoproterozoic plateau volcanic rocks of the Midcontinent Rift System in northeastern Minnesota: Geological Society of America Field Guide 24, Trip number 7, p. 121-146; Miller, J.D. Jr. Houdak, G.J., Wittop, C, and McLughlin, P.I, eds. Green, J.C., 2002, Volcanic and sedimentary rocks of the Keweenawan Supergroup in northeastern Minnesota, in Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., and Wahl, T.E., Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, p. 94-102. Green, J. C., Davis, D.W., and Schmitz, M.D., 2001, Three new zircon dates for the Midcontinent Rift, North Shore, Minnesota: More data, more questions: Institute on Lake Superior Geology, 47th Annual Meeting, Madison, Wis., Proceedings, pt. 1, Programs and Abstracts, p. 29. Green, J.C., Bornhorst, T.J., Chandler, V.W., Mudrey, M.G., Jr., Myers, P.E., Pesonen, L.J., and Wilband, J.T., 1987, Keweenawan dykes of the Lake Superior region: Evidence for evolution of the Middle Proterozoic Midcontinent Rift of North America, in Halls, H.C., and Fahrig, W.F., eds., Geological Association of Canada special paper 34, Mafic Dyke Swarms, Publication No. 0120 of the International Lithosphere Program, p. 289 – 302. Green, J.C., and Fitz, T.J. III, 1993, Extensive felsic lavas and rheoignimbrites in the Keweenawan Midcontinent Rift plateau volcanics, Minnesota: petrographic and field recognition: Journal of Volcanology and Geothermal Research, v. 54, p. 177-196. Hoaglund, S.A., Miller, J.D., Jr., Crowley, J.L., and Schmitz, M.D., U-Pb zircon geochron9llgy of the Duluth Complex and related hypabyssal intrusions: Investigating the emplacement history of a large multiphase intrusive complex related to the 1.1 Ga Midcontinent rift. Jirsa, M.A., 1984, Interflow Sedimentary Rocks in the Keweenawan North Shore Volcanic Group, Northeastern Minnesota; Minnesota Geological Survey Report of Investigation 30, 20 p. Jones, N.W., 1963, The relationship between the Duluth Gabbro and the dikes and sills in the vicinity of Hovland: Minneapolis, University of Minnesota, M.S. thesis, 90 p. Miller, J.D., Jr., and Green, J.C., 2002, Geology of the Beaver Bay Complex and related hypabyssal intrusions, in Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, p. 144-163. Ojakangas, RW., Dickas, A.B., and Green, J.C., eds., 1997, Middle Proterozoic to Cambrian rifting, central North America: Geological of Society Special Paper 312, 322 p. Paces J.B., and Miller, J.D., Jr., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: Geochronological insights to physical, petrogenetic, paleomagnetic, and tectonomagmatic processes associated with the 1.1 Ga Midcontinent Rift System: Journal of Geophysical Research, V. 98, No. B8, p. 13,997-14,013. Vervoort, J.D., Wirth, K., and Kennedy, B., 2007, The magmatic evolution of the Midcontinent rift: New geochronologic and geochemical evidence from felsic magmatism: Precambrian Research, vol. 157, no. 1-4, p. 235-268. 159 �FIELD TRIP 9 Saturday, May 7, 2016 DULUTH HARBOR GEOLOGIC HISTORY BOAT CRUISE - PLEISTOCENE TO ANTHROPOCENE Dr. Andy Breckenridge (University of Wisconsin-Superior; Natural Sciences Department) Todd Kremmin (University of Minnesota-Duluth; Dept. of Earth & Environmental Sciences) Eric Dott, P.G. and Irvin Mossberger, P.G. (Barr Engineering, Duluth, MN) “there is nothing -- absolutely nothing -- half so much worth doing as simply messing about in boats” -Wind in the Willows by Kenneth Grahame Introduction The St. Louis River flows into western Lake Superior to create one of the most remarkable coastal systems on the Great Lakes - the St. Louis estuary and harbor (Figure 1). The system is a ria, a coastal inlet formed by submergence of a former river valley. Post-glacial rebound drives the modern submergence of the St. Louis River valley, which probably began around 1000 years ago, but this is only the most recent of at least three periods of submergence, the first two of which were succeeded by lake level lowering and downcutting of the former lake plain. Morning stops highlight a few recent efforts to improve our understanding of the coastal morphology and geologic history of the western basin. These include Minnesota Point, the longest freshwater baymouth bar in the United States, and a tour of the RV Blue Heron, the only UNOLS research vessel on the Great Lakes. In the afternoon we will board the Vista Queen, a commercial tourist vessel, to draw attention to modern environmental challenges within the estuary. The harbor is the largest and busiest on the Great Lakes, and the long history of industry led to the St. Louis River being one of many federally designated “Great Lakes Areas of Concern,” which are targeted for remediation and restoration due to severe environmental degradation. At the forefront of these restoration efforts are geoscientists. We hope that a cruise along the river will reveal that despite development, the St. Louis estuary is an urban wilderness waterway, with exceptional recreational opportunities and a natural beauty that rivals the Great Lake. Background The Pleistocene The Superior Lobe of the Laurentide Ice Sheet filled the western Lake Superior basin during the last glacial maximum. Superior Lobe sediments are distinguished by a striking red color created by erosion of redbed sandstones and mafic igneous rock that underlie the lake. Ice retreat took thousands of years, and was punctuated by multiple readvances. Determining the extent of retreat prior to each readvance is challenging because readvances bury and often erode older landforms and sediments. Multiple Superior Lobe till sheets have been mapped. Younger tills are enriched in silt and clay upsection, suggesting that successive readvances overrode clay-rich glaciolacustrine sediments from pro-glacial lakes that formed after the ice front retreated to positions within the Lake Superior drainage basin. 160 �Figure 1. Field Trip 9 Overview: St. Louis River, Park Point, and Field Trip Stops. In the western Lake Superior basin, most of the glaciogenic sediment has been mapped as the Barnum Formation (Hobbs, 2003; Knaeble and Hobbs, 2009; Johnson et al., 2016). Within the Barnum Formation are three texturally distinct tills (i.e. members). The youngest tills are the Moose Lake and Knife River members. Between these two tills are deltaic and lacustrine sediments mapped as the Wrenshall Member, which were deposited in glacial Lake Wrenshall (Figure 2A; Wright et al., 1970; Johnson et al., 2016). Glacial Lake Wrenshall (also referred to as an early phase of glacial Lake Duluth) was a pro-glacial lake fronted by Superior Lobe ice within the Lake Superior basin (Breckenridge, 2013). Overflow of glacial Lake Wrenshall routed initially through the Moose Lake (or Portage) outlet into the Kettle River, but ice margin retreat opened the slightly lower Brule outlet into the St. Croix River (Figures 2A, 3). At this time, lake levels within the St. Louis estuary were at around 325 m (~1070 ft) above sea level, probably at the Epi-Duluth level (Figure 4). The ancestral St. Louis River entered the lake just downstream of the area which is now Jay Cooke State Park and deposited deltaic sediment that has not been mapped in detail. Because the ancestral St. Louis River drained meltwater and glacial lakes associated with the Rainy Lobe to the north (Figure 2A), the discharge must have been far greater than for the modern river. Finer-grained lacustrine sediments were deposited distal to the river mouth, including glacial varves (Wright et al., 1970; Knaeble and Hobbs, 2009; Hobbs and Breckenridge, 2010). Clay associated with these lacustrine sediments was quarried for bricks, near the town of Wrenshall during the late 19th and early 20th centuries (Grout, 1919). The timing of glacial Lake Wrenshall is poorly constrained. Basal radiocarbon dates from kettle lakes on the Nickerson moraine, (which precedes Lake Wrenshall), suggest the lake is at least as old as 12,800 cal 161 �yr BP (~10.8 14C ka), but these are minimum ages and likely lag ice retreat (Florin and Wright, 1969). Spruce logs have been dated to as old as 11,700 cal yr BP (~10.1 14C ka) on the former Lake Figure 2. Selected paleogeographic reconstructions of the Great Lakes region through time (adapted from Breckenridge et al., 2010). Outlets for glacial lakes are provided by white arrows. Lake Superior outlets varied through time, and include the Brule (BR), Au train-Whitefish (AWH), St. Mary’s River (SMR), and North Bay (NB). 162 �Wrenshall lake plain in Wisconsin and Michigan, which necessitates lake levels lowered below the Brule outlet by this time (Black, 1976). This requires that the Superior Lobe retreated far enough to the northeast to open lower elevation routes into the Lake Michigan basin. This would have resulted in the first period of downcutting of the former lake plain by the St. Louis River within the reach that is now the St. Louis estuary. Note that a disconformity from this period of low lake levels has not been found within the western basin, but the area lacks detailed stratigraphic research. The radiocarbon dated logs on the former Lake Wrenshall plain were buried by red clayey till, therefore the Superior Lobe re-advanced, blocked routes into the Lake Michigan basin, and raised lake levels back to the Brule outlet. This advance, known as the Marquette Advance in the eastern Lake Superior basin, created glacial Lake Duluth. The name Lake Duluth originates from Leverett (1928) after a series of prominent gravel beaches in the city of Duluth described by early geologists (Taylor, 1894). The Duluth level follows in a general way Skyline Parkway, but development and construction within the city of Duluth have obscured evidence of the strandline. Notable locations along Skyline Parkway that are at the lake Duluth level include a prominent terrace along the Spirit Mountain ski slope, the 1st United Methodist Coppertop church, and Heller Hall on the University of Minnesota Duluth campus where the Department of Geological Sciences is housed (Hobbs and Breckenridge, 2013). At the Lake Duluth level, overflow routed through the Brule outlet into the St. Croix River for a second time. Eventually ice retreat once again opened eastern outlets to the Lake Michigan basin causing lake levels to fall. Unlike the prior phase of lower levels, which is only inferred by wood on the former lake plain, the geomorphologic record of these lowered levels is preserved by a series of strandlines that are clearly visible on high resolution lidar DEMs (Figure 4; Breckenridge, 2013). Every one of these lower lake levels are named, typically after a town in which a strandline associated with the lake level is found (e.g. Highbridge, Washburn, and Beaver Bay) (Farrand, 1960; Farrand and Drexler, 1985). These strandlines are particularly useful for understanding the nature of glacial isostatic adjustment (GIA) because they can be traced around the basin. Glacial isostatic adjustment (or post-glacial rebound) is the rise of the lithosphere following deglaciation. Rebound rates are highest where the ice was thickest. In the Lake Superior basin, geomorphologic features provide a record of former lake levels (waterplanes) that rise in elevation to the northeast, where ice was thickest (Figure 3). Older waterplanes have undergone a longer period of GIA, and therefore rise more steeply. The end of glacial Lake Duluth occurred when ice retreat allowed a union with glacial Lake Minong, a lake that initially was limited in extent to Whitefish Bay in southeastern Lake Superior and fronted to the northwest by ice from the Marquette advance (Figure 2B). The merger between glacial lakes Duluth and Minong created a lake that retains the name glacial Lake Minong (Farrand, 1960) (Figure 2C). Glacial Lake Minong drained over a drift-covered bedrock sill, called the Nodaway barrier, into the St. Mary’s River (SMR). Progressive downcutting of the drift-covered sill at the St. Mary’s River caused falling lake levels in Lake Minong, which include the Minong through post-Minong levels (Figure 2C; Breckenridge, 2013). 163 �Figure 3. Map of Lake Superior with inlets and outlets (arrows), and contours for glacial isostatic adjustment used in Figure 4. Figure 4. Former waterplanes of Lake Superior (adapted from Breckenridge, 2013). 164 �The Holocene Not until ~2500 years after the onset of the Holocene ~9100 cal yr BP (8100 14C yr BP), did glacial meltwater cease to discharge into the glacial Lake Minong (Breckenridge et al., 2004; Hyodo and Longstaffe, 2011). The earliest post-glacial (i.e. post-Minong) lake level is called the Houghton, which was established when the Nadoway barrier was cut down to bedrock at the St. Mary’s River (SMR) (Figure 2D). Glacial isostatic adjustment (GIA) depressed the bedrock sill relative to the western basin, which resulted in the lowest lake levels ever in the Twin Ports region, probably 60-m below the modern level. In the Michigan and Huron basins, in situ stumps have been found that establish the immediate, post-glacial lake levels, and they appear to be ~20-m lower that the outlets for each lake (Lewis et al., 2008). This suggests a drier climate created closed-basins. No data has been found from the Superior basin to determine whether or not Lake Superior was a closed-basin lake at this time. By 8300 cal yrs BP, with the onset of a wetter climate, lake levels appear to have risen enough to overflow from Lake Huron into the North Bay outlet, which discharged to the Ottawa River (Figure 2D). During the early to mid-Holocene, differential GIA caused the outlet for Lake Huron (North Bay) to rise above the elevation of Lake Superior’s St. Mary’s River (SMR) outlet. Rising levels in Lake Huron drowned the SMR and created a shared waterplane between the Superior, Michigan, and Huron basins known as Lake Nipissing (Figure 2E; Larsen, 1985; Baedke and Thompson, 2000). Lake Nipissing levels peaked in the Huron and Michigan basins at around 4500 cal yr BP (Thompson et al., 2011). At this time, rising lake levels likely breached the basin drainage divide between the Huron and Erie basins, or perhaps the Michigan basin and Mississippi River via the Chicago River (Thompson et al., 2011; Johnston et al., 2012). This resulted in a major shift in the drainage pathway for the upper Great Lakes, from a route into the Ottawa River via North Bay (Figure 2E), to a southern outlet (Figure 2F). Subsequent lake levels dropped due to a combination of sill incision of the new outlet and perhaps climate change (Thompson et al., 2011). The Lake Nipissing level is commonly referred to as the Nipissing highstand, and strandlines formed at this level are readily apparent across much of the upper Great Lakes. At Sault Ste Marie (SSM), the Nipissing strandline is at 198 meters asl, 16-m above the modern level (Cowan, 1985), but the Nipissing elevation decreases to the west, converging towards modern lake levels due to GIA (Figure 4). Prior studies have suggested that Connor’s and Rice’s Point were a former baymouth bar formed by lake level rise to the Nipissing level (Loy, 1963; Barlaz, 1983). Longshore drift of sand eroded primarily from the southern shore, combined with sediment sourced from the Nemadji and St. Louis Rivers, likely built a spit across the head of the lake, but there have been no sediment or geomorphic studies to test this hypothesis. Former lake levels since the Nipissing highstand have been established for Lake Superior by coring and dating foreshore sand deposits from multiple strandplains in the eastern Lake Superior basin (Figure 5; Johnston et al., 2012). The work is an impressive undertaking, and is currently the most detailed paleohydrograph anywhere on the Great Lakes. The data indicate a lake-level drop of almost 4-m shortly after the peak Nipissing, followed by steady lake level lowering until around 1000 cal yr BP; thereafter lake levels have been stable near the Lake Superior outlet. The steady drop in lake levels from 4000 to 1000 cal yr BP was probably the result of GIA; an outlet on the southern side of a basin would have caused lake levels to fall everywhere north of the outlet. Stabilization of lake levels at 1000 cal yr BP is attributed to the separation of Lake Superior from Lake Huron (Johnston et al., 2012). Lake levels have been constant at SSM since this time, but this paleohydrograph must be corrected for GIA to understand former lake levels within the estuary and Twin Ports region. GIA is causing SSM to rise relative to the St. Louis River estuary. For example, gauge data suggest that lake levels in the estuary have risen around 25cm over the last 100 years due to differential GIA (Mainville and Craymer, 2005). 165 �For this field guide, an empirically derived model of GIA by Lewis et al (2005) has been adapted to correct the Sault Ste Marie paleohydrograph using the isobases of Breckenridge (2013) (Figure 6). The model applies an exponential decay function to estimate the rate of uplift necessary to result in warped strandlines in the Great Lakes of known age (Figure 6B). The modeled GIA correction is poorly constrained and could be in error, but the underlying processes that affected lake levels in the Twin Ports are generally understood. The resultant hydrograph (Figure 6C) suggests rapid drawdown from the Nipissing highstand around 4000 cal yr BP was too fast to be countered by relatively slow rates of GIA, resulting in a rapid lake level drop. Steadily falling lake levels at Sault Ste Marie from 4000 to 1000 cal yr BP were likely countered by GIA in the Twin Ports, which may have resulted in relatively stable lake levels. When lake levels stabilized at Sault Ste Marie at 1000 cal yr BP, lake levels would have risen in the Twin Ports. This abrupt change to rising lake levels is most likely responsible for drowning the St. Louis River to create the estuary. In addition, the rising lake levels probably initiated formation of Minnesota and Wisconsin Points. Ground penetrating radar surveys of Minnesota and Wisconsin Points suggest that the baymouth bar system is prograding lakeward (Morrison et al., 2015), presumably in response to lake level rise and increased sediment supply. One possibility is that the baymouth bar system is accreting vertically and lakeward on a former beach ridge that is just one of many that comprise a drowned strandplain now buried in the St. Louis Harbor. Evidence for this strandplain exists on the incredibly detailed bathymetric survey of the harbor and estuary completed by William Hearding in 1861 (Figure 7). Testing this model will necessitate detailed sediment and stratigraphic analysis of the harbor and baymouth bar sediments, combined with robust age dating. Figure 5. Lake levels at Sault Ste Marie since the Nipissing highstand, adapted from Johnston et al. (2012). 166 �Figure 6. Relative lake level curves for the Twin Ports (unpublished). A) Lake levels since 12,000 cal yr BP include a lowstand that exposes the glacial Lake Wrenshall plain, which is reflooded, presumably due to ice re-advance around 11,500 cal yr BP. Subsequent ice retreat lowers lake level in abrupt drop as new outlets open. The lowest level is the Houghton, which is also the end of glacial Lake Minong. Lake levels rise to the Nipissing due to GIA of the North Bay and St. Mary’s River outlets. B) Lake levels since the Nipissing highstand have been constrained Johnston et al., 2012 (see also Figure 5). Differential GIA caused levels to rise relative to Sault Ste Marie (SSM). C) Modeled GIA has been subtracted from the SSM hydrograph to estimate relative lake levels in the Twin Ports. Figure 7. Bathymetric data from Hearding (1861) converted to raster data and overlayed with modern LIDAR DEM. Water depths in the harbor were generally between 6 and 9 feet, except for the deep river channel. 167 �The Anthropocene Various Native American peoples inhabited the western Lake Superior area throughout the Holocene. After the glaciers receded, Paleo-Indian cultures were the first to inhabit the land, succeeded by Eastern Archaic peoples, until about 1,000 B.C (Dierckins, 2006). The Eastern Archaic peoples gave way to the Woodland cultures, until roughly 1600 A.D. Next were Dakota tribes, who were soon pushed west by the Ojibwe, near the same time as the arrival of the first explorers and gun/fur traders, including Daniel Greysolon, Sieur du Lhut. The 1842 and 1854 Treaties of La Pointe ceded rights of ownership near Lake Superior in areas of Wisconsin and Minnesota respectively, to European settlers in the region, ushering in development of the Industrial Age. The first iteration of modern locks at Sault Ste. Marie was completed in May 1855. In the Duluth/Superior Harbor, breakwaters were built, a shipbuilding industry began, and commercial fishing was established. Jay Cooke brought the Lake Superior & Mississippi Railroad to Duluth from St. Paul in the 1860’s, spurring logging throughout the region, with lumber mills appearing from Rice’s Point (Figure 1) to West Duluth (Dierckins, 2006). Grain elevators and railroad docks soon followed, connecting the waterfront to the railway. Other railroads began working their way to Duluth. In 1870 Duluth incorporated as a city. At this time the Superior Entry was the only waterway connecting Lake Superior to the Duluth/Superior Harbor. Soon after, the Duluth Shipping Canal was built from 1870 to 1877. This new connection to the lake changed the currents and hydrodynamics of the harbor. In the 1880’s, with the development of the Mesabi, Cuyuna, and Vermillion Iron Ranges, iron ore shipping began, along with the subsequent construction of docks to handle the ore. In 1907 Duluth surpassed New York City in shipping tonnage (Dierckins, 2006). The United States Steel Corporation began construction of a steel plant at Spirit Lake around this time. Shipbuilding operations were founded in Superior and later Riverside around the time of the two World Wars. The uppermost reach of the St. Louis River estuary became constrained by the Fond du Lac Dam in Jay Cooke State Park, when its construction was completed in 1924. In 1959 the St. Lawrence Seaway was created, allowing large ocean vessels into the Port. To accommodate ever-larger ships, navigational dredging deepened the harbor in shipping channels, and the dredge spoils were used variously as fill for the port, to create man-made islands (e.g., Barker’s Island, Figure 1), and the 90-acre Erie Pier dredge disposal facility. Recently, dredge spoils have begun to be beneficially reused to restore habitat at the 21st Ave. W. (Stop 3), 40th Ave. W., and Grassy Point sites (Figure 1). The Western Lake Superior Sanitary District (WLSSD) treatment plant began operating in September 1978, consolidating 17 inadequately treated wastewater discharges. Water quality in the St. Louis River rapidly improved from essentially a sewer condition to becoming suitable again for fishing and recreation. The St. Louis River Great Lakes Area of Concern (AOC) was established in 1987 by the EPA. Work continues today to remove several beneficial use impairments from the AOC. For example, river sediment cleanup projects have been initiated at Stryker Bay (Stop 4) and the Former US Steel plant upstream at Spirit Lake. A revitalization plan for West Duluth neighborhoods was begun in 2012 to capitalize on the neighborhood’s unique location along the St. Louis River corridor. Today, the harbor still handles many commodities ranging from coal, iron ore, grain, and limestone to cement, salt, wood pulp, wind turbine components and other heavy equipment. 168 �Field Trip Stops Vehicle Tour (meet at Stop 1 at 9:00am) Stop 1: Blue Heron Research Vessel – Lakehead Boat Basin (Breckenridge) Location: UTM Zone 15, 569377E 5180363N The RV Blue Heron is part of the fleet of University National Oceanographic System (UNOLS) vessels and is operated by the Large Lakes Observatory at the University of Minnesota Duluth as a charter vessel for research scientists. The vessel was built in 1985 for fishing the Grand Banks, but was sold to the University of Minnesota and converted for research in 1998. Over the last 18 years many researchers have used a wide array of equipment aboard the Blue Heron to extend our knowledge about the geologic and modern history of Lake Superior. Notable equipment includes acoustic seismic profilers that operate on a range of frequencies for imaging both shallow and deep sediments, side-scan sonar for imaging the sea floor surface and composition, and a piston corer capable of recovering sediment cores up to 9 meters in length. At this stop, we can tour the vessel and examine geophysical data collected aboard the Blue Heron from the harbor and greater lake. Examples include side scan sonar images of the harbor and the Thomas Wilson, a whaleback freighter that sank just outside the Duluth entrance in 1902. Selected sections from Lake Superior sediment cores BH02-5P, taken aboard the Blue Heron, will also be available for inspection. The core sections are on loan from the National Lacustrine Core Repository in Minneapolis where they are permanently archived in cold storage. Core BH02-5P (Figure 8) is from the deep Caribou sub-basin of Lake Superior, east of the Keeweenaw peninsula (Figure 3). BH02-5P has been correlated with photographs from S62-8, a long gravity core that penetrated to bedrock. By combining these records, 1406 varves (annual couplets) have been measured that overlie a red till and the Cambrian Jacobsville sandstone. The entire record dates to between 9.3 and 8.1 14C ka BP (11,500-9,100 cal yr BP). The most intriguing aspect of this record is a series of ~36 anomalously thick varves that correlate with those from the northern Lake Superior sub-basins. These varves are at the very top of the glaciolacustrine record, when the ice margin was most distal, and must have resulted from anomalously high sediment and water fluxes into the Superior basin (Breckenridge et al., 2004; Breckenridge, 2007). In the Isle Royale trough, these individual varves can be up to 14-cm thick. Presumably this 36-year event of anomalously high sediment flux was caused by abnormally high discharge by the ice sheet, or by the influx of anomalously great overflow from glacial Lake Agassiz which spilled into the basin (Figure 2C). These great floods of water may have caused short term rises in lake levels in the Michigan, Huron, and Superior basins; presumably the flux of water was too great to be accommodated by outlet channels. Evidence for rapid, short term increases in water level at around this time, perhaps 12-m or more, is found in small lakes that appear to be flooded by sediments from Lake Superior, including Fenton Lake (Breckenridge et al., 2010) and Beaver Lake (Fisher and Whitman, 1999). 169 Figure 8. Glacial varves (annual couplets) from BH02-5P (see Fig.3 for core location). Winter (W) and summer (S) sediment laminae are noted. �Stop 2: – Formation of Minnesota/Wisconsin Point and the St. Louis River Estuary (Kremmin/Breckenridge) Location: UTM Zone 15, 572438.5E 5175762.3N. PLS: T.49N., R.14W., S.13, NE1/4. Figure 9. View of Lake Superior, Duluth Harbor, Superior Bay, Minnesota Point, and Wisconsin Point from Enger Tower, Duluth, Minnesota (October 3rd, 2015). Photographer – Todd Kremmin. Introduction – Situated at the southwestern tip of Lake Superior, Minnesota Point and Wisconsin Point form a baymouth bar stretching northwest-southeast between Duluth, Minnesota and Superior, Wisconsin establishing the outer breakwater for the largest and farthest inland freshwater seaport in North America (Figure 9 and 10) (Duluth Seaway Port Authority, 2015). In combination, the baymouth bar of Minnesota and Wisconsin Points is approximately 16 kilometers in length and considered one of the longest freshwater bars in the world (Loy, 1962 & 1963; Bernard and Davidson, 1969). The material necessary for the formation of this sandy baymouth bar is attributed largely to littoral drift of sediments from the south shore of Lake Superior in Wisconsin, as well as secondary sources of sediment derived from outflows of the St. Louis and Nemadji Rivers. Research pertaining to the geomorphic development of the system has remained relatively surficial in nature: examining present day geomorphic structures, surveying bathymetry and topography, collecting surface sediment samples, extrapolating offset boreholes, and comparing analogous marine system bar developments (Merrill, 1939; Loy, 1962 & 1963; Kemp et al., 1978; Thomas and Dell, 1978; Sydor et al., 1979; Barlaz, 1983; Rasid and Hufferd, 1989; Rasid, et al., 1992; Albrecht, 2005; Hobbs, 2009). Moreover, research regarding the development of this baymouth bar in the context of post glacial isostatic rebound of the Lake Superior basin along with lake level variation has not been investigated. 170 �DULUTH Lake Superior Duluth Harbor Minnesota Point St. Louis River Park Point SUPERIOR Superior Bay Wisconsin Point Nemadji River Allouez Bay 0 1 Miles Figure 10. Map view of Lake Superior, Duluth Harbor, Superior Bay, Allouez Bay, Minnesota Point, Park Point, Wisconsin Point, City of Duluth, City of Superior, St. Louis River, and Nemadji River (Google Earth, 2016). The objective of this thesis research is to enhance previous observations of geomorphic development and continue further investigation into the subsurface of the baymouth bar system using Ground-Penetrating Radar (GPR). 40 kilometers of GPR data using both 250MHz and 100MHz antennae were obtained on Minnesota Point near the Park Point Recreational Area (Figure 11) for this project. To supplement and ground truth the GPR data, 8 vibracores (11.24 meters total) were drilled within the expanse of the acquired geophysical data (Figure 11). All sediment cores underwent Loss On Ignition (LOI) analysis and select portions of the cores were radiocarbon dated to provide chronology to the subsurface system and ultimately extrapolate findings across the entire baymouth bar. Background – Ground-Penetrating Radar GPR is a geophysical method (similar to seismic reflection) which enables indirect insight into the subsurface using radar energy. This geophysical method allows researchers to view large expanses of the subsurface without altering the landscape, uncovering the architectural radar stratigraphy of shallow well sorted fluvial, aeolian, or coastal sediments (Stoker et al., 1997; Jol and Bristow, 2003). Radar signals are “pinged” into the ground and reflected back to a receiver, recording travel times (a proxy for depth) and the variable signatures of properties and materials encountered. Within the region of Minnesota Point and 171 �specifically the Park Point Recreation Area, water saturation and organic content of the subsurface play an important role in the variations of reflections received. A variety of antennas can be used in GPR surveys, higher frequency antennas provide high resolution records, but limit subsurface penetration depths, lower frequency antennas provide deeper penetration depths, but limit resolution. When collected in tight grid spacing (≤1m), a spatially large (3D) volume of GPR data can be observed in planar dimensional slices with depth, permitting views on how systems have geomorphically changed over time at larger scales than traditional core observations. Lake Superior Superior Bay 100 0 Meters Figure 11. Map view of Park Point Recreation Area on Minnesota Point. GPR grids – (Grid_Orig, Grid_01, Grid_02, Grid_03) and lines are shown along with core locations - (1A, 1B, 1C, 2A, 2B, 4A, 5A, 6A). (Google Earth, 2016). Historical Context and Anthropogenic Influences The baymouth bar made up of Minnesota and Wisconsin Points’ remained in a natural state up until the 1860’s (Figure 12) (Hearding, 1863; Demeter, 1993; Zenith City, 2016). During this period Duluth had recently paved a railroad under the direction of Jay Cooke. This railroad provided a shaky commercial industry, which began to rival the neighboring city of Superior, Wisconsin. The City of Superior had been commercially well off prior to the increase in population within Duluth simply due to the natural opening 172 �of the baymouth bar. This natural opening, known as the Superior entry, allowed ships to enter from Lake Superior and easily harbor at the shores near the City of Superior. Around 1869 Duluth leaders discussed the idea of developing their own entry, but instead the Superior entry was securely modified with concrete N 0 1 Miles Figure 12. Historical Survey of the Northern and North Western Lakes made in obedience to acts of Congress and under the direction of The Bureau of Topographical Engineers of the War Department. Insets highlight the Duluth Canal prior to excavation (upper inset), along with the Park Point Recreation Area prior to dredge dumping (lower inset). (Hearding, 1863). breakwaters in 1869 by the U.S. Army Corps of Engineers (Demeter, 1993). Outraged, Minnesota leaders lobbied for their own ship canal, and, in 1870, began dredging. Businessmen of Superior filed a federal suit against these actions, fearing a new canal would diminish the City of Superior’s commercial industry. Shortly before the courier could arrive with the U.S. Supreme Courts orders to desist from excavating the canal, legend has it that all available citizens of Duluth had picked up their shovels and hand dug the remainder of the canal. This legend may be exaggerated, as other accounts claim the dredging operations handled most of the excavation. Regardless, a new unnatural entry had been built and marked the beginnings of the anthropogenic influences of the baymouth bar system (Zenith City, 2016). Following the establishment of the “Twin Ports,” dredging operations have been ongoing and extensive to keep channels open for shipping vessels. In the mid 1930’s Duluth Parks Superintendent Rodney Paine submitted a proposal to the federal government for $338,000 to develop a new recreation center on Minnesota Point (Figure 13). Unfortunately, this proposal was submitted in the midst of a financial crisis during the depression era, 173 �ending the idea before it even gained attention. President Franklin D. Roosevelt’s Works Progress Administration (WPA) program revived the Park Point Recreation Area project, and in 1936 workers brought in approximately 150,000 yards³ of fill to create new ‘land’ just south of 43rd street (Paine, 1936). A comparison of natural and man-made land is available to view in Figure 14. This historical context has relevance to this thesis project because a considerable amount of GPR data was collected on this new land made up of dredge material. These dredge spoils consist mainly of silty-sand and pebbles and can be readily differentiated between the natural underlying materials in both GPR and core observations. 100 0 Meters Figure 13. Preliminary drawings of proposed Park Point Recreation Area Project (Zenith City Online, 2016). N Natural System 1863 0 100 Meters Figure 14. LiDAR map showing existing Park Point Recreation area and Minnesota Point system (grayscale). Natural Minnesota Point system (yellow) (Hearding, 1863; DNR LiDAR Data, 2016). 174 �Results The most prominent grid - (Grid_02) (Figure 15) is 85m x 85m, with 1.0m spacing between lines, 0.05m step size, 0.25m antennae separation, and was collected in a half day using the 250 MHz Noggin Smartcart™ from Sensors & Software, Inc. Select planar slices were chosen showing amplitude variation of the evolving geomorphic system in 0.1m intervals (Figures 16 and 17). The lower right corner of each grid slice is North. Cross sections of GPR data are presented in Figures 18 and 19. Lake Superior A’ GRID_02 B’ B A Superior Bay 0 100 Meters Figure 15. Map view of Park Point Recreation Area on Minnesota Point. GPR Grid_02 is shown with cross section lines A-A’ and B-B’ - please reference Figures 18 and 19 to view cross sections (Google Earth, 2016). 175 �B’ A’ A A B C D B Figure 16. Grid_02 planar depth slices showing amplitude variation (red-high amplitude, blue-low amplitude). Variations in amplitude are believed to be derived from water and organic content variability in the subsurface (i.e. red = changing water/organic content with depth, blue = no change in water/organic content with depth). The lower right corner of each grid slice is North. Crosshatched region contains no data. Depths slices are indicated as the following intervals: A) 0.7-0.8m, B) 0.8-0.9m, C) 0.9-1.0m, D) 1.0-1.1m. A-A’ and B-B’ cross sections – refer to Figure 18 and 19. (Please refer to original color version for precise amplitude variation analysis in upcoming master’s thesis – Todd Kremmin UMD ‘16). High amplitude coherent variations of radar energy are visible (red) with depth beginning from 0.7m1.5m. These coherent packages seem to migrate North-Northwest over time and extend from Superior Bay towards Lake Superior/Duluth (Figures 16 and 17). Medium to lower amplitude variations of radar energy (yellow-blue) do not have as coherent of patterns in these depth ranges. Figure 16 (C) and (D) and Figure 17 (A) and (B) show blotchy medium-low amplitude variation in the right half of the depth slices. 176 �B’ A A’ A B C D B Figure 17. Grid_02 planar depth slices showing amplitude variation (red-high amplitude, blue-low amplitude). Variations in amplitude are believed to be derived from water and organic content variability in the subsurface (i.e. red = changing water/organic content with depth, blue = no change in water/organic content with depth). The lower right corner of each grid slice is North. Crosshatched region contains no data. Depths slices are indicated as the following intervals: A) 1.1-1.2m, B) 1.2-1.3m, C) 1.3-1.4m, D) 1.4-1.5m. A-A’ and B-B’ cross sections – refer to Figure 18 and 19. (Please refer to original color version for precise amplitude variation analysis in upcoming masters thesis – Todd Kremmin UMD ‘16). 177 �A A’ Figure 18. Cross Section GPR line A-A’ (Grid_02 Line X65). Depth in meters on left, time in nanoseconds on right, and position lines in meters at top. Red window shows planar depth slice from 1.3-1.4m. 0.0-0.7m is believed to be dredge material. 0.7-3.0m is believed to be dune clinoforms. 3.0-5.0m is thought to be glacio-lacustrine material. B B’ Figure 19. Cross Section GPR line B-B’ (Grid_02 Line Y15). Depth in meters on left, time in nanoseconds on right, and position lines in meters at top. Red window shows planar depth slice from 1.3-1.4m. 0.0-0.7m is believed to be dredge material. 0.7-3.0m is believed to be dune clinoforms. 3.0-5.0m is thought to be glacio-lacustrine material Conclusions Subsurface (0.7-3.0m) natural baymouth bar system development in a back bar setting indicates progradational/aggradational migration towards Lake Superior/Duluth. Above this interval (0.7-3.0m), incoherent radar amplitude reflectors differentiate the natural system with dredge material dumped here around 1936 (0.0-0.7m) (Paine, 1936). Core observations and radiocarbon ages support this hypothesis*. GPR cross section lines A-A’ show clinoformal migration towards Lake Superior/Duluth at depths (0.73.0m). On occasion, GPR cross sections happen to cut clinoformal structures along-strike or obliquely (AA’ position lines 20-35m), (B-B’ position lines 54-70m), negating the true stratigraphic architecture and dip of the reflectors. Loss of signal with depth indicates radar energy is attenuated (which happens when radar energy interacts with fine-grained materials). It is believed the baymouth bar system has developed over glacial/lacustrine material. The western Lake Superior Basin has seen increases in lake levels throughout the last ~2000 years due to differential isostatic rebound and changing outlets (Farrand and Drexler, 1985; Mainville and Craymer, 2005). This baymouth bar has responded to the increasing lake 178 �level in a progradational and aggradational fashion. Smaller climatic driven lake level variations are observed in detail from core observations. Although this baymouth bar is a young, non-marine system, reconstructing its geomorphic evolution in response to lake level change may become a useful analogue for similar, larger systems involved with base level change. In addition, stratigraphic findings of how such a system’s architecture is configured may yield insightful clues towards vintage conventional exploration reservoirs. Finally, a stronger understanding of how such a system geomorphically evolved in the context of the Lake Superior region post glaciation may aid in reshaping knowledge of how other geomorphic features and processes have developed throughout the region, perhaps providing tangible framework for future engineering and environmental management undertakings. *Sediment cores, radiocarbon ages, and additional GPR data will be available to view at Stop 2 during the field trip. Lunch at Barr Engineering 5th floor lunchroom (see handout map) Location: UTM Zone 15, 568970E 5181519N Vista Queen Tour (boarding at 1:00pm, departing 1:30pm) Stop 3: 21st Ave. West/Miller Creek restoration (Dott/Mossberger) Location: UTM Zone 15, 567314E 5178430N The St. Louis estuary is the largest estuary on Lake Superior, and is an important source of biological productivity, and wetland and aquatic habitat types for a wide variety of fish and wildlife communities. The 21st Avenue West Habitat Complex (Figure 20) is one of several habitat restoration projects that are incorporating beneficial reuse of navigational dredge material (Host, et al., 2013). The site is located near the WLSSD treatment plant, which discharges an average of 43 million gallons per day of treated wastewater. The intent of the restoration is to improve habitat by implementing remedial activities to address sediment contamination while complementing the desired ecological vision of stakeholder teams. The US Army Corps of Engineers started a pilot project for placement of material in the 21st Ave. W. Complex in 2013. Barr Engineering performed turbidity monitoring to help evaluate how suspended sediment generated during material placement would be transported, and provided preliminary cut and fill estimates for material quantities needed to implement the habitat plan. The MPCA received an approved Work in Public Waters permit in spring 2015 from the MNDNR for the design shown in Figure 20, so the final stages of the 21st Ave. W. Project may begin. It is anticipated that the base material and features will be constructed by 2017. 179 �Figure 20. 21st Ave. W. Placement Plans – Islands (modified from USACE 2016). Stop 4: SLIDRT, aka Stryker Bay (Dott/Mossberger) Location: UTM Zone 15, 563072E 5174671N The 255-acre St. Louis River/Interlake/Duluth Tar (SLRIDT) Superfund site is the largest sediment remediation project in Minnesota’s history. Seven decades of industrial use had left polycyclic aromatic hydrocarbons (PAHs) and other contaminants in the St. Louis River estuary. Barr’s work at the site included development of one of the nation’s first “hybrid” sediment remedies, which combined dredging and capping at the site; surcharge capping to maintain wetland habitat; use of an activated carbon mat to prevent re-contamination – the first commercial application of this technology; and integration of remediation, mitigation and restoration. Costing $90 million less than an all-dredging approach, the project restored 106 acres of aquatic and riparian habitat for fish, wildlife, and the Duluth community. Part of the investigation phase involved researching the history of the development of Stryker Bay and associated docks and slips. Figure 21 (SERVICE, 2002) shows a schematic of development of the site throughout the 98 years from 1903-2001. Fill (stippled areas in Figure 21) was brought into the protoStryker Bay in various phases, extending the land southward to accommodate industrial uses. Interestingly, the dock walls became confining barriers to shallow groundwater flow from the uplands to the river, causing artesian conditions to exist on the docks. An industrial water supply well was drilled near the southern tip on Dock 7, which was an artesian flowing well. After the site was investigated with numerous soil borings and sediment cores, the remedial project began with capping of impacted material in Slip 7 (Figure 22). In subsequent years, construction of a dike across Slip 6 created a Confined Aquatic Disposal facility (CAD). Impacted material was dredged from Stryker Bay and the shipping channel, hydraulically pumped to the CAD, and evenly spread throughout 180 �the CAD using a spreader barge. Part of Stryker Bay was separated from the rest with a sheet pile wall and was covered with an activated carbon mat and a surcharge to compress impacted material and cap it in place. The impacted material in the CAD was covered with an activated carbon mat, clean cap sand, and “environmental media” – material dredged and hydraulically pumped from a restoration project at Tallas Island approximately 2 miles upstream. The dike across the CAD was later removed, reconnecting it with the estuary. The site is currently in a long-term monitoring and maintenance phase to confirm that the constructed caps are properly containing the contaminants of concern and that aquatic plant and benthic communities are recovering consistently with other areas of the St. Louis River estuary. Figure 21. Industrial development of the SLRIDT site since 1903 (Modified from SERVICE, 2002). 181 �Figure 22. SLRIDT Caps and Covers Map (Modified from Hard Hat Services, 2013). Stop 5: St. Louis River and Glacial Lake Duluth Geomorphology: Pokegama Bay, Strandlines (Breckenridge/Mossberger) Location: UTM Zone 15, 562881E 5173280N The Pokegama River is a tributary of the St. Louis River, originating near Jay Cooke State Park on the Minnesota/Wisconsin border (Figure 23). It flows through the Superior Municipal Forest, where it empties into Pokegama Bay and the St. Louis River. The Pokegama-Carnegie wetlands are identified by the WDNR Bureau of Endangered Resources as a Lake Superior Basin Priority Site due to high quality wetland and occurrence of rare plant populations. The river is an important spawning area for walleye, northern pike, and other fish species. The Pokegama and its tributaries are deeply incised into red clay of the former lake bed, often forming steep stream banks with exposed clay. The exposed clay is susceptible to slumping and accelerated erosion (Mossberger, 2010). Slumping of stream banks, such as those in the Pokegama River, increases erosion and downstream sedimentation by supplying freshly exposed sediment to the stream. Slumping and erosion can also threaten the stability of nearby structures. If a slump intersects a confined aquifer, water under high hydraulic head in the confined aquifer can then seep to the surface, contribute baseflow, and increase the sediment load to the streams. Sedimentation from the St. Louis River deposits about half of the sediment yielded to the Duluth-Superior harbor (NRCS, 1996), and causes various economic and environmental problems. For example, a large plume of suspended sediment from the Pokegama River is often visible in the St. Louis River after rainstorms (Figure 24). 182 �Figure 23. LiDar image of Clough Island and Dwight’s Point at Pokegama Bay (DNR LiDAR Data, (2016). Figure 24. Aerial image of Clough Island and Dwight’s Point at Pokegama Bay (Google Earth, 2016). 183 �One economic impact of excessive turbidity is the cost of dredging. Dredging is necessary to maintain adequate draft for ships that use the 17 miles of harbor shipping channels. Insufficient draft requires ships to reduce their cargo load, leading to increased transportation costs. The United States Army Corps of Engineers (USACE) has estimated that approximately 33,000 tons of sediment each year is dredged from the Duluth-Superior harbor (NRCS, 1998). The St. Louis River’s contribution to the dredged sediment is approximately 14,000 tons, or 1,000 dump-truck loads, of sediment to the harbor annually, resulting in a burden to taxpayers for dredging and disposal of the river’s sedimentation. The USACE‘s current dredge disposal facility for the harbor, Erie Pier needed to be expanded and reengineered because it was expected to run out of capacity in 2017 due to the accumulation of excess fine-grained material. In addition to economic impacts, sediments can cause environmental problems. Suspended sediments are considered non-point pollution when they occur in high enough concentrations in designated surface waters. Industrial pollutants such as mercury, dioxins, and PCBs can attach to sediment particles, trapping toxins or oxygen-demanding materials in the harbor (Bridges, 2008). Dredging agitates and re-suspends settled pollutants, elevating toxin levels in the water and biota (MPCA, 1992). The harbor became one of the 43 Areas of Concern (AOC) under the Great Lakes Water Quality Agreement (WQA) in 1972. In 1987, Remedial Action Plans (RAPs) were developed to improve the health of the Nemadji and St. Louis Rivers. The harbor was designated as impaired for five uses, including fish consumption advisories, degradation of benthos, restrictions on dredging, degradation of aesthetics, and loss of fish and wildlife habitat (NRCS, 1998). In Pokegama Bay, there is a seasonal anoxic zone. The NOAA Sentinel Site Program is designed to create a national network of long-term research sites that measure the effects of climate change on our estuaries. The Lake Superior National Estuarine Research Reserve (LSNERR) has set up a Sentinel Site in Pokegama Bay. The goal will be to measure changes in climate and the associated effects on water quality, erosion, decomposition, marsh morphology, vegetation, and wildlife. A weather station, a water quality station, and 12 surface elevation tables (SETs) used to measure sediment accretion and subsidence were installed in 2011-2014. Acknowledgements Thanks to the following people and sources of information that helped shape this guidebook: Mehgan Blair and Pete Kero, Barr Engineering. Dan Breneman, Minnesota Pollution Control Agency. http://zenithcity.com/ http://wlssd.com/about-us/history/ http://www.lre.usace.army.mil/Missions/Recreation/SooLocksVisitorCenter/SooLocksHistory.aspx http://www.duluthport.com/port-history.php 184 �REFERENCES Albrecht, D.R. (2005). Geochemical Fingerprinting and the Role of the Nemadji River in Sediment Transport within Western Lake Superior. [Master’s Thesis]: University of Minnesota Duluth, Duluth, Minnesota. Barlaz, D.B. (1983). Sedimentation in the Duluth-Superior Harbor, Lake Superior. [M.S. Thesis]: University of Minnesota Duluth, Duluth, Minnesota. Bernard, J.M., and Davidson, D.W. (1969). A Floristic Resurvey of a Landfill Area 32 Years after Deposition: The Oatka Beach Addition, Minnesota Point, Minnesota. American Midland Naturalist, 82(2), 559-563. Black, R.F., 1976. 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Prepared March 14, 20013 for XIK Corp. 185 �Hearding, W.H. (1863). Survey of the Northern and North Western Lakes: West End of Fond Du Lac of Lake Superior Map. The Bureau of Topographical Engineers of the War Department. Hobbs, H.C., 2003, Surficial geology of the Knife River quadrangle, St. Louis and Lake Counties, Minnesota; Minnesota Geological Survey Miscellaneous Map M-137, scale 1:24,000. Hobbs, H.C. (2009). Surficial Geology of the Duluth Quadrangle, St. Louis County, Minnesota. Minnesota Geological Survey. Hobbs, H.C., Breckenridge, A., 2011. Ice advances and retreats, inlets and outlets, sediments and strandlines of the western Lake Superior basin. In: Miller, J.D., Hudak, G.J., C. Wittkop, C., McLaughlin, P.I. (Eds.), Archean to Anthropocene: Field Guides to the Geology of the Mid-Continent of North America, GSA Field Guide 24, 299-315. Hyodo, A., Longstaffe, F.J., 2011. The chronostratigraphy of Holocene sediments from four Lake Superior subbasins. Canadian Journal of Earth Sciences 48, 1581–1599 . Host, James, Buttermore, et al., An Ecological Design for the 21st Avenue West Remediation-to-Restoration Project. Natural Resources Research Institute, University of Minnesota Duluth Large Lakes Observatory, U.S. Fish and Wildlife Service. USFWS Cooperative Agreement F11AC00517June 2013 NRRI/TR2013/24. Johnson, M. D., Adams, R. S., Gowan, A. S., Harris, K. L.; Hobbs, H. C., Jennings, C. E., Knaeble, A.R.; Lusardi, B. A.; Meyer, G. N.. (2016). RI-68 Quaternary Lithostratigraphic Units of Minnesota. Minnesota Geological Survey. Retrieved from the University of Minnesota Digital Conservancy, http://hdl.handle.net/11299/177675. Johnston, J.W., Argyilan, E.P., Thompson, T.A., Baedke, S.J., Lepper, K., Forman, S.L., Wilcox, D.A., 2012. A Sault-outlet-referenced mid- to late-Holocene paleohydrograph for Lake Superior constructed from strandplains of beach ridges. Canadian Journal of Earth Sciences 49, 1364-1371. Jol, H.M., & Bristow, C.S. (2003). GPR in Sediments: Advice on Data Collection, Basic Processing and Interpretation, a Good Practice Guide. Geological Society London, Special Publications, 211, 9-27. Kemp et al., (1978). Sedimentation rates and a sediment budget for Lake Superior. Journal of Great Lakes Research, 4, 276-287. Knaeble, A.R., and Hobbs, H.C., 2009, surficial geology, Plate 3, and Quaternary stratigraphy, Plate 4 of Boerboom, T.J., project manager, geologic atlas of Carlton County, Minnesota: Minnesota Geological Survey County Atlas Series C-19, Part A, 6 plates, scale 1:100,000. Larsen, C.E., 1985. Lake Level, uplift, and outlet incision, the Nipissing and Algoma Great Lakes. In: Karrow, P.K., Calkin, P.E. (Eds), Quaternary Evolution of the Great Lakes: Geological Association of Canada Special Paper 30, pp. 63-77. Leverett, F. 1928. Moraines and shorelines of the Lake Superior basin. USGS paper 154-A, p. 1-72. Lewis, CFM, et al., 2008. Dry climate disconnected the Laurentian Great Lakes, EOS 89(52), 541-542. Loy, W.G. (1962). The Coastal Geomorphology of Western Lake Superior. [Master’s Thesis]: University of Chicago, Chicago, Illinois. Loy, W.G. (1963). Formation of the Duluth-Superior Harbor. The Minnesota Academy of Science Proceedings, 31, 28-35. Loy, W.G. (1963). The Evolution of Bay-Head Bars in Western Lake Superior. Journal of Great Lakes Research, 10, 150-157. Mainville, A., & Craymer, M.R. (2005). Present-Day Tilting of the Great Lakes Region Based on Water Level Gauges. Geological Society of America Bulletin, 117(7), 1070. Minnesota Pollution Control Agency and Wisconsin Department of Natural Resources, 1992. St. Louis River System Remedial Action Plan, Stage One. Merrill, J.A. (1939). The Wonderland of Lake Superior. Minneapolis, Minnesota: Burgess Publishing Company. 186 �Morrison, S. M., Jol, H. M, Alger, R. Using radar stratigraphic analysis to identify erosion and deposition in the Duluth bay barrier, Lake Superior, NC-GSA poster presentation, Madison, WI. Mossberger, I. G. (2010). Potential For Slumps, Sediment Volcanoes, And Excess Turbidity In The Nemadji River Basin (Masters Thesis, University of Minnesota-Duluth). Natural Resources Conservation Service, 1996. Memorandum. Natural Resources Conservation Service and U.S. Forest Service, 1998. Executive summary report: Erosion and sedimentation in the Nemadji River Basin: Nemadji River Basin Project, 16 p. Paine, R.F (1936). City of Duluth Park Department – Annual Report 1936. Duluth Public Library, Duluth, Minnesota. Call 352.7. No. D88a. Vol.1936. Rasid, H., & Hufferd, J. (1989). Hazards of Living on the Edge of Water: The Case of Minnesota Point, Duluth, Minnesota. Human Ecology, 17, 85-100. Rasid et al., (1992). Coping with Great Lakes Flood and Erosion Hazards; Long Point, Lake Erie, vs. Minnesota Point, Lake Superior. Journal of Great Lakes Research, 18(1), 29-42. SERVICE Engineering Group, 2002. Geology-Hydrogeology (of the) St. Louis River/Interlake/Duluth Tar Site, Duluth, MN. Appendix GH of the Data Gap Report. Stoker, M.S., Pheasant, J.B., and Josenhans, H., 1997, Seismic Methods and Interpretations, in Davies, T.A., et al., eds., 1997, Glaciated Continental Margins: An Atlas of Acoustic Images; London, Chapman & Hall, 315 p. Sydor et al., (1979). Red Clay Turbidity and its Transport in Lake Superior. Environmental Protection Agency Document. Grant No. R005175-01. Taylor, F. B., 1894. A reconnaissance of the abandoned shore lines of the south coast of Lake Superior. American Geologist, 365-383. Thomas, R.L. & Dell, C.I. (1978). Sediments of Lake Superior. Journal of Great Lakes Research, 4(3-4), 264-275. Thompson, T.A., Lepper, K., Endres, A.L., Johnston, J.W., Baedke, S.J., Argyilan, E.P., Booth, R.K., Wilcox, D.A. 2011. Mid Holocene lake level and shoreline behavior during the Nipissing phase of the upper Great Lakes at Alpena, Michigan, USA. Journal of Great Lakes Research 37, 567–576. U. S. Army Corps of Engineers, 2016. 21st Ave. West Restoration Placement Plan – Islands. Sheet GI001. Unpublished. Wright, H.E., Jr., Mattson, L.A., Thomas, J.A., 1970. Geology of the Cloquet quadrangle, Carlton County, Minnesota: Minnesota Geological Survey Geologic Map Series GM-3. Zenith City Historical Archive, (2016). Retrieved from: http://zenithcity.com/zenith-city-history-archiv 187 ��1 � 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, 2016 Description An account of the resource Institute on Lake Superior Geology. Duluth, Minnesota. May 4-8, 2016. 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 2016 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/8cc74af26712cd59ea3d647b21961546.pdf 465d7b12c828300cfef32450ada060e7 https://digitalcollections.lakeheadu.ca/files/original/9d120470c1c05757ed646d2dee949394.pdf 28759b6a6e8f43b02466eb45a44d0338 PDF Text Text 63rd Annual Meeting Institute on Lake Superior Geology Wawa, Ontario May 8 - 12, 2017 Wawa Wild Goose or Land of the Big Goose in Ojibway Proceedings Volume 63 Part 2: Field Trip Guidebook �Institute on Lake Superior Geology 63rd ANNUAL MEETING May 8-12, 2017 Wawa, Ontario SPONSORED BY: ONTARIO GEOLOGICAL SURVEY MINISTRY OF NORTHERN DEVELOPMENT AND MINES AND A. E. SEAMAN MINERAL MUSEUM MICHIGAN TECHOLOGICAL UNIVERSITY Meeting Co-Chairs Anthony Pace, Ann Wilson, and Theodore J. Bornhorst Proceedings Volume 63 Part 2: Field Trip Guidebook Compiled by Theodore J. Bornhorst and Margaret J. Hanson Cover Photos: Top left, Calcite crystals on pyrite from George W. MacLeod Mine, Wawa, collection of the A. E. Seaman Mineral Museum donate, photograph by George Robinson; iconic Wawa goose at junction of Trans-Canada Highway and Highway 101, image from http://www.northernontario.travel/algoma-country/fun-facts-about-the-famous-wawa-goose; Sir James Dunn open pit iron mine on the Eleanor Iron Range, photograph by Anthony Pace. �63rd INSTITUTE ON LAKE SUPERIOR GEOLOGY VOLUME 63 CONSISTS OF: PART 1: PROGRAM AND ABSTRACTS PART 2: FIELD TRIP GUIDEBOOK TRIP 1: ARCHEAN AND PROTEROZOIC GEOLOGY OF THE MARATHON-HEMLO AREA Day 1: Geology of the Coldwell Alkaline Complex Part 1: Transect through the Coldwell alkaline complex Part 2: Marathon Cu-PGM deposit Day 2: Geology of the eastern Schreiber-Hemlo Greenstone Belt in the vicinity of Heron Bay and Hemlo TRIP 2: MORE UNUSUAL DIAMOND-BEARING ROCKS OF THE WAWA AREA TRIP 3: GEOLOGY OF THE WAWA GOLD PROJECT TRIP 4: GEOLOGY OF THE ISLAND GOLD MINE TRIP 5: GEOLOGY OF THE RENABIE AREA TRIP 6: KAPUSKASING STRUCTURAL ZONE AND BORDEN LAKE GOLD DEPOSIT Reference to material in Part 1 should follow the example below: Authors, 2017, abstract title, 63rd Institute on Lake Superior Geology Proceedings, v. 63, Part 1, Field Trip Guidebook, p. xx. Proceedings Volume 63, Part 1: Program and Abstracts, and Part 2: Field Trip Guidebook are published by the 63rd Institute on Lake Superior Geology and distributed by the Institute Secretary: Peter Hollings Department of Geology Lakehead University Thunder Bay, ON P7B 5E1 CANADA peter.hollings@lakeheadu.ca Some figures in this volume were submitted by authors in color, but are printed grayscale to conserve printing costs. Full color imagery will appear in the digital version of the volume when it is available on-line at: http://www.lakesuperiorgeology.org ISSN 1042-9964 i �Part 1: Field Trip Guidebook Table of Contents Trip 1: Archean and Proterozoic geology of the Marathon-Hemlo area 1 Trip 1, Day 1: Geology of the Coldwell Alkaline Complex 1 Trip 1, Day 2: Geology of the eastern Schreiber-Hemlo Greenstone Belt 44 Trip 2: More unusual diamond-bearing rocks of the Wawa area 79 Trip 3: Geology of the Wawa gold Pproject 109 Trip 4: Geology of the Island Gold Mine 148 Trip 5: Geology of the Renabie area 164 Trip 6: Kapuskasing structural zone and Borden Lake gold deposit 187 ii �Field Trip 1 Archean and Proterozoic geology of the Marathon-Hemlo area Day 1 Geology of the Coldwell Alkaline Complex The Coldwell Alkaline Complex portion (Day 1) of Field Trip 1 consists of two parts: 1) a west to east transect through southern part of the complex and 2) a visit to the Marathon Cu-PGE Deposit. 1 �Part 1: Transect Through the Coldwell Alkaline Complex Allan MacTavish1 and Mark Smyk2 Panoramic PGMs (Canada) Limited, Thunder Bay, ON; 2 Resident Geologist Program, Ontario Geological Survey, Thunder Bay, ON 1 A variety of Mesoproterozoic, Midcontinent Rift-related alkalic and carbonatitic rocks occur within several intrusive complexes on or near the northern shore of Lake Superior (see Figures 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) (see Figure 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, as is shown in Table 1 below: Table 1 Lithologic Unit/Complex Age(s) (Method) Reference Coldwell Alkaline Complex 1108 ± 1 Ma (U/Pb) Heaman and Michado (1987) Be-Zr Zone crosscutting Dead Horse Creek diatreme Prairie Lake Carbonatite Lamprophyre Dyke, McKellar Harbour Gabbro (biotite), Killala Lake Complex 1112.7 ± 4 Ma (U/Pb)1 1128.7 ± 6 Ma (U/Pb)2 1130 ± 10Ma (Rb/Sr) 1145 + 15/10 Ma (U/Pb) 1185 ± 90 Ma (K/Ar) Krogh and Wilkinson (M. Smyk pers. Comm. 1995) Pollock (1987) Queen et al. (1996) Coats (1970) Syenite, Killala Lake Complex 1050 ± 35 Ma (Rb/Sr) Bell and Blenkinsop (1980) (1-2.49% discordant; 2 1.82% discordant) It has been postulated that the TSTZ may represent 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 cross-faulting (Sage 1991). The Coldwell and Killala Lake 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. 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 Precambrian apparent polar wander path (Lewchuk and Symons 1990). The localization of the alkalic magmatism off-axis, 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 2 �tholeiitic and alkaline, have a uniform, nearly chondritic isotopic composition (ibid). Samariumneodymium 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. 2. 3. 4. 5. Magmatic Cu-Ni-PGE (± Au, Ag) in gabbros of the Killala Lake and Coldwell complexes; Magmatic Ti-V±apatite deposits in the Eastern Border Gabbros of the Coldwell Complex; Magmatic U, Nb (+ wollastonite, apatite) in the Prairie Lake carbonatite (Sage 1987); Late-stage magmatic Nb-Y-F-family rare earth elements in syenite pegmatites (Alexander 2007); 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 (see Figure 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 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 3, superimposed ring sub-complexes or magmatic centers (Mitchell and Platt 1978) that young progressively (Centers 1 to 3) to the southwest (see Figure 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: 1. 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); 2. 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 3. Center 3: Silica-oversaturated alkalic rocks with oversaturated residue; consisting of magnesio-hornblende syenites, quartz syenites, and minor granites (Mitchell and Platt, 1994; Lukosius-Sanders, 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): 1. 2. 3. 4. 5. 6. Mafic ocellar lamprophyre (camptonitic variety) Quartz-bearing, mafic lamprophyres (camptonitic variety) Sannaite-type lamprophyres Monchiquitic-type lamprophyres Feldspar glomeroporphyritic and alkali basalt dikes Analcime tinguaite (heronite) 3 �Figure 1. Mid-continent Rift geology and the locations of mafic/ultramafic intrusions (After Miller et al. 1995). 4 �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. 5 �Figure 3. Generalized geology of the Coldwell Alkaline Complex (after Walker et al. 1993) with field trip stops. 6 �Figure 4. Coldwell Alkaline Complex magmatic centres: CI (Centre 1), CII (Centre 2), and CIII (Centre 3). Generalized geology after Mitchell and Platt, 1994. 7 �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 sub-horizontal 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 (fine-grained gabbros). The mafic intrusive rocks occurring within Centers 1 and 2 are tabulated, with their associated mineralized zones, below: Table 2 Intrusion (Centre) Eastern Gabbro (1) Lithologic Units Reference(s) Shaw (1994, 1997); Lum (1973); Barrie et al. (2002) Malpas Lake (1) 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 Geordie Lake (2?) Troctolite, olivine gabbro Alkalic gabbro (2) Biotite gabbro Biotite- and olivine-gabbro Mulja (1989); MacTavish et al. (1987) Mitchell and Platt (1982b) Walker et al. (1993a) Western Gabbro (1) Two Duck Lake (1) Penczak (1992); Wilkinson (1983) Shaw (1994, 1997) Dahl et al. (1987) Shaw (1994, 1997) 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 (see Table 3). 8 �Table 3 Mineralized Zone Grade / Significant Assays Marathon Measured and Indicated In-Pit 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) Chalcopyrite Marathon PGM > cubanite  pyrite ; s Corporation hollingworthite, atokite-zvyaginstevite, Ohnenstetter et al. sperrylite, Bi-kotulskite, michenerite, (1991); Watkinson merenskyite, monceite, stibiopalladinite, and Ohnenstetter paolovite, merteite II, palladoarsenide, (1992); Good and unnamed (Pd5As2), nickeline, majakite, Crocket (1994a, argentian gold 1994b) 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, bornite, pyrite, millerite, siegenite, pentlandite, galena, chalcocite, melonite, hessite, unnamed (Ag3Te2), altaite, kotulskite, merenskyite, michenerite, sopcheite, Pd-bismuthotelluride, paolovite, Pd-arsenide, guanglinite, Pd-antimonide, sperrylite, electrum, Pd1.6As1.5Ni, AgSb4 Chalcopyrite, pyrrhotite, pentlandite, sphalerite, pyrite Chalcopyrite, bornite, pentlandite, cobaltite, galena, chalcocite; telargpalite, polarite, kotulskite, taimyrite, merteite, zvyagintsevite, plumbopalladinite, majakite, tetraferroplatinum n/a Geordie Lake Middleton Skipper Lake Area 41 average grade of 1.05 g/t Pd+Pt+Au over 12 m 0.48 g/t Pt+Pd+Au over 202 m, incl. 1.23 g/t Pt+Pd+Au over 61 m Ore Mineralogy Reference(s) news release, Marathon PGM Corporation, May 04, 2010 Mulja (1989); Mulja and Mitchell (1990, 1991) Penczak (1992) MacTavish (2000) Benton Resources Corp. 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, silicate-sulphide associations, trace-element trends and 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 mineralization processes. Watkinson and Ohnenstetter (1992) presented field, petrographic and mineral-chemical 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. 9 �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). 10 �Coldwell Alkaline Complex Transect - Field Trip Stops STOP C1: Natrolite-Bearing Syenite and Massive Fe-Ti-oxides Location: Lat: 48o47'45" N, Long: 86o39'15"W; UTM 525528E, 5405511N 29.4 to 29.9km west of the Highway 626 and Highway 17 junction Description: This exposure displays natrolite-bearing, pegmatitic syenite (see 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 (see 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 (see 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 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 1. Pink, natrolite-bearing, pegmatitic augite syenite. Photo credit D. Campbell. 11 �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. 12 �STOP C2: Little Pic River Breccia Zone Location: Lat: 48o47'45"N, Long: 86o37'30"W; UTM 527478E, 5405531N 27.3 km west of the Highway 626 and Highway 17 junction Description: 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, see Photo 4) that consist of angular to rounded blocks of fine- to medium-grained, equigranular, mafic (gabbroic?) rocks within a groundmass of pink, medium-grained, 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 2 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 2 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 3 different types of xenoliths: the most abundant are dark grey to black, very fine-grained 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 (see Photo 6; what do you think?)? Although isolated xenoliths are 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 granites and have been interpreted to be the result of fractional crystallization of mantle-derived, basaltic magma (Lukosius-Sanders 1988; Mitchell et al. 1993). 13 �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. 14 �Photo 6. Rounded dark grey, amphibole-phyric xenoliths/inclusions within a fine- to medium-grained, pinkish mafic groundmass (breccia dyke?). Photo credit M. Puumala. Stops C3: Prisoners Cove, Neys Provincial Park (Sample Collecting Prohibited!). The descriptions (unpublished mapping/field descriptions, MacTavish 1992) herein are for 14 sub-stops along the shoreline south of Prisoner’s Cove; however, due to time constraints only the northernmost stops will be visited. Location: Lat: 48o46'30"N, Long: 86o37'10"W; UTM 527984E, 5402537N; 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 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 enclave-bearing, 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. 15 �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 post-intrusion 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 flat-bottomed boats, mainly consists of foliated, wispy, hybridized amphibole-nepheline syenite with diffuse discontinuous “layers” (see 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 amphibole-nepheline syenite. At the northwestern tip of the outcrop is an elongate, diffuse zone of apparently non-hybridized, 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-northwest-striking, 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 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 body 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 (see Photo 8). 16 �Photo 7. Wispy, relatively mafic in appearance, hybrid amphibole-nepheline syenite exhibiting diffuse discontinuous layers. Photo credit A. MacTavish. Photo 8. Large biotite gabbro xenolith/inclusion (?) crosscut by veins and dykes of amphibole-nepheline syenite. Photo credit A. MacTavish. 17 �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 (see 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 coarse-grained, 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) (see Photo 11). There are numerous leucocratic clots (oikocrysts?) of plagioclase (see Photo 12) throughout. 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. 18 �Photo 10. Disaggregating biotite gabbro xenoliths exhibiting subparallel reaction haloes. Photo credit M. Puumala. Photo 11. Biotite gabbro crosscut by fine-grained to pegmatitic syenite dyke (centre) and thinner syenite veins (centre left). Photo credit A. MacTavish. 19 �Photo 12. Leucocratic clots of plagioclase (oikocrysts) within biotite gabbro. Photo credit A. MacTavish. Sub-Stop C3f (527982E, 5402324N): This sub-stop (~115m west-southwest of Sub-stop C3e) consists of an irregular, variably assimilated zone of mafic (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 C3g (527978E, 5402292N): This location (~30m south of Sub-stop C3f) consists of locally well-developed modal layering within medium- to locally coarse-grained amphibole-nepheline syenite. The magmatic layering dips shallowly to the west and west-southwest at between 20 and 26o 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. Sub-Stop C3h (527971E, 5402265N): At this location (~27m south of Sub-stop C3g) are 2, sub-parallel, aphanitic to fine-grained, ocellar lamprophyre dykes (see Photo 13) occupying a narrow southeast-striking fracture zone. The dykes dip to the northeast between 54o 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 apple-green and greyish minerals (zeolites?), and possibly white calcite. 20 �Photo 13. Ocellar lamprophyre dyke in narrow fracture zone. Photo credit A. MacTavish 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-size-layered (?) amphibole nepheline syenite. 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. Finer-grained 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 are composed 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. Sub-Stop C3k (528000E, 5402039N): Located 77m south of Sub-stop C3j. This sub-stop consists of a well-layered block of amphibole-nepheline syenite (~6.5m by 3.5m in dimensions) 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. This isolated block is composed of a sequence of 3 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. Sub-Stop C3l (528004E, 5402016N): A further 23m south of Sub-stop C3k is a zone characterized by well-developed syenite layering (see Photo 14), 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 (18oW dip). 21 �Photo 14. Well-developed, relatively flat-lying magmatic layering within amphibole-nepheline syenite. Photo credit A. MacTavish Sub-Stop C3m (528011E, 5402006N): This sub-stop 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 (see Photo 15) composed of mobile “flow-banded” material. The “vein” margins 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 composed of medium- to very coarse-grained, modally- and normally grain-size graded syenite layers (see Photo 16). This was interpreted as slump folding due to the presence of at least 3 axial planar directions present within 3 separate folds all in close proximity to each other. Unfortunately since mapping was completed in 1992 this exposure has become partially obscured by the growth of lichen. 22 �Photo 15. Crosscutting, vein-like body possibly resulting from the movement of volatile-rich magmatic fluids. Photo credit A. MacTavish. Photo 16. Lichen-obscured slump fold within layered amphibole-nepheline syenite. Photo credit A. MacTavish. 23 �STOP C4: Hornfelsed Basaltic Roof Pendants, Wolf Camp Lake Location: Lat: 48o47'40"N, Long: 86o26'05"W; UTM 541775E, 5404189N 8.6km 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 tholeiitic lineage, contemporaneous with the Coldwell Complex. Fresh, metasomatized and hornfelsed, andesine-oligoclase basalt flows are estimated to attain a thickness of 5km (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 1km 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 3 main units: 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 (see Photo 17). Well-defined, amygdule-bearing zones dip 8o 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. Amygdules within the basaltic roof pendant located near Wolf Camp Lake. Photo credit D. Campbell. 24 �STOP C5: Layered Fe-rich Augite Syenite (Alternate stop if time allows) Location: Lat: 48o44'20"N, Long: 86o23'25"W; UTM 544782E, 5398443N 680m 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, 0.3km past CPR tracks (park at 544864E, 5398750N) 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 15o 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. 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 070o and dips 60o north. The layering is unusual in that it is defined by an intercumulus mineral (augite) rather that by cumulus phases (feldspar). STOP C6: Layered Eastern (Border) Gabbro Location: Lat: 48o44'00"N, Long: 86o20'00"W; UTM 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 20o to 60o 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 at ~45o. 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 (see Photo 18). Here, the macrorhythmic layering (see Photo 19) produces relatively thin (1 to 5cm) to medium thick (5 to 100cm) layers that can be traced for over 35m along 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. 25 �Photo 18. Macrorhythmic layering within the Eastern Border Gabbro. Photo credit M. Smyk. Photo 19. Macrorhythmic modal layering within the Eastern Border Gabbro. Photo credit A. MacTavish. 26 �STOP C7 (Alternate Stop): Eastern Contact of the Coldwell Complex Location: Lat: 48o43'10"N, Long: 86o19'30"W; UTM 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 quartzfeldspar-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). 27 �COLDWELL ALKALINE COMPLEX REFERENCES Alexander, M. 2007. The mineralogy of NYF pegmatites from the Coldwell Alkaline Complex, northwestern Ontario; unpublished MSc thesis, Lakehead University, Thunder Bay, Ontario. Barrie, C. Tucker, MacTavish, A.D., Walford, P.C., Chataway, R., and Middaugh, R., 2002. Contact-type and magnetite reef-type Pd-Cu mineralization 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. 87 86 Bell, K. and Blenkinsop, J. 1980. Grant 42: Ages and initial Sr- Sr 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 and geochemical study; unpublished MSc thesis, McMaster University, Hamilton, Ontario, 135p. Coates, M.E. 1970. Geology of the Killala–Vein lakes area, Ontario; Ontario Department of Mines, Geological Report 81, 35p. 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. 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. ———. 1994b. Origin of albite pods in the Geordie Lake gabbro, Port Coldwell alkaline complex, northwestern Ontario: Evidence for late-stage hydrothermal Cu-Pd mineralization; The Canadian Mineralogist, v.32, p.681701. 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. ——— 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 and deposits in northwestern Ontario; Ontario Geological Survey, Open File Report 5889, 145p. 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.146-156. 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. 28 �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 property), St. Joe Canada Inc.; unpublished report, Resident Geologist’s Files, Thunder Bay, 7p. 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. ——— 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. ——— 1982b. Mineralogy and petrology of nepheline syenites from the Coldwell alkaline complex, Ontario, Canada; Journal of Petrology, v.23, p.186-214. ——— 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., Artist-Downey, 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. ——— 1991. The Geordie Lake intrusion, Coldwell Complex, Ontario: Palladium- and 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 Deadhorse Creek diatreme, northwestern Ontario; unpublished MSc thesis, Lakehead University, Thunder Bay, Ontario. 29 �Puskas, F.W. 1967. Port Coldwell area; Ontario Department of Mines, Preliminary Map P.114, scale 1:31 680. th ——— 1970. The Port Coldwell alkali complex; in Proceedings, 16 Institute on Lake Superior Geology, Thunder Bay, Ontario, p.87-100. 40 39 Queen, M., Heaman, L.M., Hanes, J.A., Archibald, D.A. and Farrar, E. 1996. Ar/ Ar 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. Sage, R.P. 1982. Mineralization in diatreme structures north of Lake Superior; Ontario Geological Survey, Study 27, 79p. ——— 1985. Geology of carbonatite-alkaline rock complexes in Ontario: Chipman Lake area; Ontario Geological Survey, Study 44, 40p. ——— 1986. Alkalic rock complexes – carbonatites of northern Ontario and their economic potential; unpublished PhD thesis, Carleton University, Ottawa, Ontario, 335p. ——— 1987. Geology of carbonatite-alkaline rock complexes in Ontario: Prairie Lake carbonatite complex, District of Thunder Bay; Ontario Geological Survey, Study 46, 91p. ——— 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. 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. Shaw, C.S.J. 1994. Petrogenesis of the eastern gabbro, Coldwell alkaline complex, Ontario; unpublished PhD thesis, University of Western Ontario, London, Ontario, 292p. ——— 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. Tuominen, H.V. 1967. Port Coldwell area; Ontario Department of Mines, Map P.114, scale 1:15 840. 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. ——— 1993a. Precambrian geology of the Coldwell Alkaline Complex; Ontario Geological Survey, Open File Report 5868, 30p. ——— 1993b. Precambrian geology, Port Coldwell complex, west half; Ontario Geological Survey, Preliminary Map P.3232, scale 1:20 000. ——— 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.183-191. 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. 30 �PART 2: MARATHON CU-PGM DEPOSIT 3 David Good1 and John McBride2 Earth Sciences Dept., Western University, London, ON 2 Stillwater Canada Inc., Marathon, ON 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 (Figure 1). 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) 31 �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 (Figure 2). The dip at the east contact is variable from nearly flat (at the south end) to vertical and locally over turned 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 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. 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 the 6300 feeder but are not shown. 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 units are summarized in Figure 3. The metabasalt is interpreted to correlate with Mamainse Point Volcanic Group 1 (Figure 4) which was emplaced at approximately 1108 Ma (Keays and Lightfoot, 2015). 32 �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. 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. 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). 33 �Mineralization: Disseminated sulfide mineralization is hosted by the Two Duck Lake gabbro and associated breccia (Figure 5) and occurs within several thick and continuous shallow-dipping lenses that parallel the footwall contact. 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 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. 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. 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 chalcopyrite and bornite with minor pyrrhotite and trace amounts of pentlandite, cobaltite, and pyrite (Ruthart, 2012). 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). 34 �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. Deposit Model: Figure 7: Step 3 of schematic illustration for magmatic plumbing system (after Barnes et al, 2016) Evidence for magma conduit setting at Marathon include: • • • • • • • • 35 association with volcanic rocks fault control brecciation and assimilation accumulation in trough setting flow through PGE upgrading tube shaped intrusion gravity driven back flow high heat flux �Marathon Cu-PGM Deposit - Field Trip Stops Figure 8: Map at south end of Marathon deposit showing location of stops 1 and 2. 36 �Stop 1a: South end of Troctolite Sill (Figures 8 and 9b): 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 (Figure 1). The sill dips moderately west at the north end, but flattens out in the south to sub-horizontal (Figure 10b). Note layering is approximately east west at stop 1a, consistent with rotation of sill from west dipping in the north to sub horizontal or north dipping in the south. Figure 9: 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). 37 �Stop 1b: South end of Marathon deposit (Figures 9, 11 and 12): Trench outcrop exposure of Two Duck Lake gabbro shows shallow dipping W Horizon and Main zone type mineralization. Figure 10: Plan map of trench at the south end of the Marathon deposit. 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. Note textural evidence for cross cutting intrusions. East-west channel samples 44 to 56 (just north of red dot at stop 1b). Interval average: 3.37 ppm Pd+Pt+Au, and 0.35% Cu over 18.6 m. 38 �Stop 2a: Splat trench and the W Horizon Figure 11: Figure 12: 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. 39 �Stop 2b: Splat trench and the W Horizon Figure 12: Stop 2b: Splat Trench - NW branch 40 �Stop 3: Ultramafic magnetite-olivine-clinopyroxene-apatite intrusions Numerous pod shaped ultramafic apatite intrusions occur within the metavolcanic pile to the west of and stratigraphically above the Two Duck Lake intrusion. Similar intrusions occur within the TDL intrusion. These units were previously referred to as reef type accumulations, but trenched exposures show they are discontinuous intrusive bodies that cut the metavolcanic and Layered Gabbro Series. The Four Dams and Willie Lake Cu-Pd occurrences are larger examples of this type of mineralization. At the Marathon deposit, the ultramafic units occur as discontinuous pods that range in size from about 20 to 100 m along strike and less than about 10-20 m in thickness. The best intersections in surface outcrops include 9.3m at 1.6 g/t PGM+Au and 0.13% Cu and 4.94 m at 2.15 g/t PGM+Au and 0.29% Cu. The ultramafic pods are concentrated in the vicinity above trough structures in the footwall and are potential indicators of underlying magma conduits. These intrusions are proposed to have formed by backflow of dense minerals in a conduit setting. 41 �Stop 4: Main Zone at Two Duck Lake 42 �MARATHON Cu-PGE DEPOSIT REFERENCES Barnes SJ, Cruden AR, Arndt N, Saumur BM, 2016. The mineral system approach applied to magmatic Ni–Cu–PGE sulphide deposits, Ore Geology Reviews 76, 296-316. Good DJ, 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 DJ and Crocket JH, 1994. Genesis of the Marathon Cu-Platinum-Group Element Deposit, Port Coldwell Alkalic Complex, Ontario: A Midcontinent Rift-Related Magmatic Sulfide Deposit, Economic Geology, v. 89, p. 131-149. Good, DJ, Epstein, R, McLean, K, Linnen, RL & Samson, IM, 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. Keays RR, and Lightfoot PC, 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. 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. Samson, IM, Fryer, BJ, and Gagnon, JE, 2008. The Marathon Cu-PGE deposit, Ontario: Insights from sulphide chemistry and textures, in Goldschmidt conference, p. 820. 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. Walker EC, Sutcliff RH, Shaw CSJ, Shore GT and Penczak RS, 1993. Precambrian geology of the Coldwell Alkaline Complex, Ontario Geological Survey, Open File Report, v. 5868, 30p. Watkinson DH 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. 43 �Field Trip 1 Archean and Proterozoic geology of the Marathon-Hemlo area Day 2 Geology of the eastern Schreiber-Hemlo Greenstone Belt in the vicinity of Heron Bay and Hemlo Mark Puumala and Mark Smyk Resident Geologist Program, Ontario Geological Survey, Thunder Bay, Ontario, Canada and Tom Muir Ontario Geological Survey (retired) INTRODUCTION Day 2 of the Marathon-Hemlo field trip will begin in the Heron Bay area, just to the east of the contact between the Mesoproterozoic Coldwell alkaline complex and the eastern half of the Neoarchean Schreiber-Hemlo greenstone belt. The first three stops will highlight the lowermetamorphic grade (predominantly greenschist facies) supracrustal rocks and variable deformation that are characteristic of this portion of the belt. The trip will then proceed eastward to the world-renowned Hemlo gold camp (production to date of close to 22 Moz Au) where the bulk of the trip will be spent examining the strongly deformed and higher-metamorphic grade (amphibolite facies) supracrustal rocks that are exposed along Highway 17 adjacent to Barrick Gold Corporation’s Williams Mine property. These roadside exposures provide an overview of the bedrock types, geological structures, mineralization and alteration that are present in the vicinity of the Hemlo gold deposit, and include outcrops of the up-dip projections of two goldmineralized zones. REGIONAL GEOLOGY The Schreiber-Hemlo greenstone belt is located in the Neoarchean Wawa subprovince. The belt extends from Schreiber in the west, almost to White River in the east, and is bisected by the Mesoproterozoic Coldwell alkaline complex between the Little Pic River and Marathon. Consequently, the two halves of the greenstone belt are sometimes referred to separately as the Schreiber greenstone belt and the Hemlo greenstone belt (see Figure 1). 44 �Figure 1. Regional geological setting of the Schreiber-Hemlo greenstone belt (from Jackson et al. 1998). The western half of the Schreiber-Hemlo greenstone belt consists of a mixture of tholeiitic mafic metavolcanic rocks, calc-alkalic mafic to felsic volcanic rocks and sedimentary rocks whose stratigraphic and structural relationships have not previously been resolved (Williams et al. 1991). An Ontario Geological Survey (OGS) mapping project that is currently being undertaken in the western portion of the belt (Magnus and Walker 2015; Magnus and Arnold 2016) is designed to address this knowledge gap. The eastern half of the Schreiber-Hemlo greenstone belt (i.e. east of the Coldwell complex) is much better understood (Muir 2000) and consists of three main units. The lowermost unit is dominated by tholeiitic metavolcanic rocks that are intercalated with lesser amounts of mafic to ultramafic intrusions and flows, and is older than 2697 Ma. This is overlain by calc-alkalic metavolcanic rocks (flows and pyroclastic rocks) and related intrusions that are between 2695 and 2688 Ma. This metavolcanic rock sequence is intercalated with metasedimentary rocks and is eventually superseded by an overlying metasedimentary rock dominated sequence (Muir et al. 1999). The greenstone belt is bounded to the north and south by earlier (ca. 2720 Ma), gneissic granitoid rocks of the Black-Pic and Pukaskwa batholiths (see Figure 2), and has been intruded internally and along its margins by younger granitoid plutons ranging in age from 2697 to 2677 Ma. These include the ca. 2697 Ma Dotted Lake pluton, the ca. 2690 to 2684 Ma Cedar Lake pluton, Cedar Lake stock and Heron Bay pluton and the ca. 2679 to 2677 Ma Gowan Lake, Musher Lake and Bremner plutons (Beakhouse 2001). 45 �Figure 2. Generalized geology of the eastern half of the Schreiber-Hemlo greenstone belt (from Muir and Smyk 2006). The eastern Schreiber-Hemlo greenstone belt has the morphology of an open synclinorium with complex internal structural patterns. It is interpreted to have undergone 2 main stages of structural deformation (Muir et al. 1999; Jackson 1998). Each deformation stage occurred over an extended period of time and may have consisted of multiple events (i.e., as documented by Muir (1997, 2003) in the immediate vicinity of the Hemlo gold deposit). The first phase of deformation (D1R) affected rocks younger than 2693 Ma and resulted in the development of penetrative foliation that is generally subparallel to stratigraphy and intrusion boundaries. The second regional deformation event (D2R) affected rocks as young as 2675 Ma and is considered to have been responsible for the development of map-scale folds with upright axial planar fabrics (Jackson 1998). The eastern Schreiber-Hemlo greenstone belt is dominated by westward-plunging structural elements that suggest that the portion of the belt adjacent to the Coldwell complex exposes a shallower structural level of the belt than the eastern end (Muir et al. 1999; Jackson 1998). This 46 �interpretation is consistent with the general eastward increase in metamorphic grade from greenschist facies in the Heron Bay area, to amphibolite facies in the Hemlo area (Thompson 2006). Numerous high strain zones have been documented in the vicinity of the Hemlo gold deposit, with the most prominent of these being the Hemlo fault zone (Muir 1997). This high-strain zone may represent a portion of a regional-scale boundary fault that extends along the entire length of the greenstone belt between Heron Bay and Hemlo (Williams et al. 1991; Muir et al. 1999). GEOLOGY OF THE HEMLO GOLD DEPOSIT The Hemlo gold deposit is located 35 km east of Marathon in the south-central portion of the eastern Schreiber-Hemlo greenstone belt (see Figure 2). The following overview of the deposit geology is excerpted from Muir et al. (1995). The Hemlo deposit is situated within supracrustal rocks in a southern bifurcated segment of the eastern part of the Schreiber-Hemlo greenstone belt. Page (1947a, 1947b; 1948; 1949) was the first geologist who recognized the Hemlo Fault as an important structural feature and identified the close co-planar relationship the fault had with the Lake Superior Shear Zone. He also recognized that all the gold discoveries at that time were hosted by the Lake Superior Shear Zone, and stated that the zone had been traced on surface for over 12 kilometres. Page also noted that the gold mineralization was associated with felsic porphyritic bodies and that the emplacement of these porphyries was related to a major structure which he termed the "Heron Bay-Hemlo Break". The Hemlo deposit largely lies at or near the contact between felsic to intermediate quartz-feldspar-phyric rocks (pyroclastic and subvolcanic(?) varieties) and metasedimentary rocks. Here, the rocks generally strike at 290o to 295o and dip between 60o and 70o to the northeast. The Hemlo deposit is presently interpreted by the authors as being hosted within 290ostriking, highly strained, transposed, and juxtaposed, lithotectonic supracrustal segments, which lie in a generally east-striking greenstone belt. The deposit cannot be demonstrated to be stratiform or stratabound. Sporadically distributed, anomalous gold mineralization has been noted, about several kilometres southeast and east-southeast of the Hemlo deposit on the Lac Minerals Limited, White River property, as being spatially associated with sericitic and pyritic rocks within what is interpreted as a brittle-ductile shear zone (Pan and Fleet 1988, 1989, 1990; Pan 1990). Underground mapping and drilling have demonstrated the existence of parallel mineralized zones within both the metavolcanic and metasedimentary rocks, as well as mineralized zones which transect the metavolcanic-metasedimentary contact. The Hemlo deposit orebodies, collectively, extend for a strike length of about 3.7 km, a depth of 1.35 47 �km, and an approximate down-plunge distance of 2.5 km. The main mineralized zone extends for a strike length of about 2.9 km, and a down-dip distance of 2.5 km (Harris 1989). The thickness of the main mineralized zone ranges from about 2 m in the David Bell Mine (Burk et al. 1986) to 50 m in the Williams Mine (Walford et al. 1986). Several types of ore are delineated in each of the 3 mines, based largely on the predominant mineral(s) and/or textures present. Commonly, because of extensive metasomatism and deformation, the mineralized zones comprise rocks of equivocal protolith(s). Alteration, collectively, is in the form of widely various degrees of microclinization, sericitization, biotitization, silicification, carbonatization, albitization, pyritization, and tourmalinization. Significant amounts of barite of equivocal origin are locally present. Bright green vanadian muscovite (Harris 1989) is commonly present in the altered rocks, as is molybdenite. At least 2 ages of quartz veins can be found within the ore zones: some veins display considerable folding, attenuation, boudinage, and dismemberment, whereas others display minimal deformation. In some cases, outside the ore zone, there are numerous quartz veins which tend to display a lower degree of deformation. Collectively, the ores are enriched in Au, Mo, Sb, Hg, As, Tl, V, and Ba. Gold is commonly disseminated along with molybdenite. Native gold grains are mercury rich and occur along quartz-feldspar and pyrite grain boundaries and fractures, as well as inclusions in, or rimmed with, several varieties of sulphide minerals including, rarely, pyrite and molybdenite (Harris 1989). Visible gold is not common overall, but does occur within quartz veins in feldspathized, molybdenite-bearing rocks, along molybdenite-green-mica-bearing fractures, in stibnite- and cinnabar-bearing quartz pods, and rarely in fractures in some of the plagioclase-porphyritic dikes. Molybdenite is the second most abundant sulphide, after pyrite, and occurs as fine- to very fine-grained, foliation-parallel blades, euhedral crystals, and platy masses mostly in association with silicate minerals, chiefly feldspar and quartz (Harris 1989). Previous field guides for the Hemlo area have largely focussed on the Hemlo “Main Zone,” which continues to contribute to the production of the Williams Mine, and is known as the “B Zone” in current mine terminology. However, much of the current mine production is obtained from a distinct ore body known as the “C Zone.” The following description of the C Zone is excerpted from Langlais and Barber (2015). The C Zone represents multiple sub-parallel lenses of irregular, generally narrow, gold mineralization. C Zone ore is stratigraphically different from the main zone and occurs in two broad geological domains, the porphyritic felsic metavolcanics and the intermediate to felsic volcaniclastic sediment unit. The open pit is located within the C Zone. 48 �The general stratigraphy from south to north is Lower Metasedimentary rocks, Porphyritic Felsic Metavolcanics (Moose Lake Porphyry), Quartz Eye Muscovite Schist, Intermediate to Felsic Volcaniclastic Sediments (fragmental unit) and the Upper Metasedimentary rock sequence. Lower and upper denote the relative structural positions of the metasedimentary rock units as the younging directions are unclear. All of the major rock units are highly deformed with multiple events of deformation. Structural geology is complex. Rocks in the deposit area exhibit high strain. At the deposit scale, rocks in the area are tightly isoclinally folded. Most of the ore bodies occur on one or more limbs of these folds. Local drag folding can be seen in the ore. Occasional transverse faults offset ore and wall rock units up to a few meters, and there is some shearing along major contacts. Regional metamorphism is up to amphibolite grade. The deposit has also been cut by a number of north-south trending diabase and lamprophyre dikes which post-date mineralization. Although much research has been carried out in the Hemlo area since gold production commenced in 1985, it is interesting to note that considerable debate continues around the application of a genetic model to the Hemlo gold deposit (especially for the Main Zone). Epithermal, intrusion-related (i.e., porphyry) and shear zone-hosted mesothermal models have all been suggested to explain the origins of this enigmatic deposit, with none having achieved widespread acceptance Muir (2002). Irrespective of the model used to explain its origins, it is clear that the Hemlo deposit’s current morphology is largely related to deformation associated with the Lake Superior shear zone. Hemlo Gold deposit Area Exploration and Development History The chronology of events leading up to the discovery and development of the Hemlo mines is condensed from Muir et al. (1995). The subsequent development summary is largely based on Langlais and Barber (2015), with additional information obtained from the Thunder Bay South District Resident Geologist’s files. 1869: Gold was discovered by Moses Pee-Kong-Gay near the present town of Heron Bay. 1920's: J. LeCours sank test pits on a mineralized shear zone near Hemlo station, 6 km southwest of the Hemlo deposit. Assays of up to 4.16 ounces gold per ton were reported. At about the same time, a group of claims was staked on a small quartz vein that returned low gold assay values just to the north of railway mile post 38 (measured from White River). 1930-31: J.E. Thomson mapped the area for the Ontario Department of Mines. 1937: Bowhill Mines shipped a 500 lb. (227 kg) test sample from the Heron Bay area that returned 0.30 ounce gold per ton and 1.53 ounces silver per ton. 49 �1944-46: Prospector Peter Moses discovered a siliceous, mineralized shear zone approximately one-half mile (0.8 km) north of mile post 37 on the railway with samples returning assays up to 0.415 ounce gold per ton. Harry Ollmann and Dr. Jack K. Williams staked 11 claims in the discovery area (Ollmann-Williams property). Gold values were encountered in a large shear zone and stripping, trenching, and diamond drilling were completed. The claims were subsequently patented in 1947. After Ollmann died in December 1947, the claims were placed in Williams’ name in trust by mutual consent. 1946-50: Adjoining claims were staked by consulting geologist Trevor Page. These claims, together with others staked by associates including Moses Fisher (J.E. Thomson's guide), subsequently became part of the Lake Superior Mining Corporation Limited property. Samples collected by Page assayed up to 0.13 ounce gold per ton. Lake Superior Mining Corporation carried out mapping, trenching, chip and channel sampling, and X-Ray diamond drilling on the Lake Superior shear zone. By 1950, a mineralized zone containing 31 543 tons to a depth of 300 feet (91 m) with a calculated width of 8.8 feet (2.7 m) and a cut grade of 0.22 ounce gold per ton was outlined. 1951: The Lake Superior property was optioned by Teck-Hughes Gold Mining Limited. 6 diamond drill holes totalling 2733 feet (833 m) were completed to add to the over 6000 feet of drilling completed to that time by Lake Superior Mining Corporation Limited. The size of zone No. 1 (formerly the 'A' zone) was increased to 76 653 tons at a grade of 0.27 ounce gold per ton. 1957: Teck Exploration Company Ltd. drilled 7 “packsack” drill holes totalling 289 feet (88 m). 1957: Bartley and Page wrote a geological report on the Hemlo area for the Canadian Pacific Railway. 1958: Cusco Mines Ltd. optioned the Lake Superior claims and carried out diamond drilling to test the main mineralized zone. 1960s: The Lake Superior property was staked intermittently by prospectors during the 1960's. 1973: The Lake Superior property was staked by J.E. Halonen for Ardel Explorations Ltd. Three holes totalling 789.9 feet (241 m) were drilled and the deposit tonnage was increased to 150 000 tons grading 0.21 ounce gold per ton above 60 feet (18 m) depth. 1976-77: Claims were staked by R.G. Newman to the west of the Williams property. The claims were investigated by Copper Lake Explorations Ltd. (soil and bedrock geochemical sampling). 50 �1977-78: T.L. Muir of the Ontario Geological Survey mapped the Heron Bay and Hemlo areas, in 1977 and 1978, respectively (Muir 1982a, 1982b). Muir reported an occurrence (0.32 oz/ton Au), presently referred to as the “Highway Zone,” in altered felsic metavolcanic rocks several kilometres west of the main Hemlo deposit. This occurrence is probably in the vicinity of the mile post 38 discovery of the 1920's. Claim stakers and explorationists would later base much of their land acquisition during the staking rush on Muir's maps. 1979: Prospectors Donald McKinnon and John Larche staked the claims surrounding the 11 patented Williams claims. 1980: Corona Resources Ltd. (later International Corona Resources Limited) optioned the McKinnon and Larche claims and completed preliminary linecutting and geophysical surveys. 1981: Corona began a $600 000 diamond drilling program in January. In March, R. Hughes and F. Lang optioned 156 claims which lie to the east and west of the Williams and Corona properties. These claims were put into the holding of their companies, Golden Sceptre Resources Ltd. and Goliath Gold Mines Ltd., who subsequently relinquished controlling interest to Noranda Exploration Company Ltd. In May, representatives of LAC Minerals Ltd. visited Corona's drill site and exchanged information pursuant to a possible joint-venture agreement. While negotiations with Williams' widow in Maryland for the Williams property were ongoing, diamond drilling was stepped back to the east of the outlined deposit. Drill hole 76 intersected a 10.5-foot (3.2 m) section grading 0.209 ounce gold per ton at a depth of 336.5 feet (102.5 m). This new, separate zone was the main Hemlo orebody. By August, 120 drill holes totalling 43 000 feet (13 106 m) had delineated 750 000 tons of rock grading 0.10 ounce gold per ton in the 'West' zone and had begun to indicate the much larger reserves of the 'East' or main zone. The Hemlo gold rush, ultimately involving 180 companies, ensued. Both Corona and LAC had been actively negotiating for the Williams property. In July, Mrs. Williams accepted LAC’s offer. Corona, citing a breach of a fiduciary agreement, sued LAC for ownership of the Williams claims. Teck Corporation subsequently entered into a joint venture agreement with Corona in December to develop what would become the David Bell Mine. 1982: LAC announced the discovery of the deposit on their property, which would become known as the Page-Williams Mine. Drilling by Goliath Gold Mines intersected the northward-dipping extension of the ore zone. The Goliath part of the deposit became, after a joint venture with Noranda Exploration Company Limited, the Golden Giant Mine. 51 �1985: The David Bell, Golden Giant and Page-Williams mines commenced gold production. 1986-87: The Supreme Court of Ontario awarded the Page-Williams Mine to International Corona Resources Ltd. LAC appealed the decision to the Ontario Court of Appeal but continued to operate the mine under conditions imposed by the court. The Ontario Court of Appeal upheld the earlier decision in October, 1987. The Supreme Court of Canada later granted LAC the right to appeal the provincial court ruling. 1987: Golden Sceptre Resources Ltd., Goliath Gold Mines Ltd., and Noranda Minerals Inc. amalgamated their holdings and formed Hemlo Gold Mines Inc. 1989: The Supreme Court of Canada awarded the Page-Williams Mine, Canada's largest gold producer, to Corona who subsequently re-named it the Williams Mine (the mine operates under the name Williams Operating Corporation). 1991: Homestake Mining Corporation purchased the assets of International Corona Resources, including their interest in the Williams and David Bell Mines. 1992: Noranda Minerals Inc. transferred ownership of the Golden Giant Mine to Hemlo Gold. 1996: Battle Mountain Canada Ltd. acquired the Golden Giant Mine. 1998-99: Williams Operating Corporation acquired the surface and mineral rights to the Sceptre claims from Battle Mountain Canada to the 9450 elevation of the Williams Mine grid in 1998. In 1999, Williams also acquired the surface and mining rights on the Horizon claims from Battle Mountain Canada to the 10150 elevation of the Williams Mine grid. These acquisitions would permit pit expansion to the west, and allow evaluation of underground mining of the down dip extension of the C-Zone pit. 1999: Homestake’s interest in the Williams and David Bell Mines was purchased by Barrick Gold Inc. in 1999. Milling operations ceased at the David Bell Mine when ore processing was transferred to the Williams mill. 2001: Ownership of Golden Giant changed to Newmont Canada Ltd. 2002: Williams acquired the surface and mineral rights from surface to the 10150 level on lease 273 and the remainder of lease 274 from Newmont Canada Ltd., providing an area for barren waste stockpiles from the expanded pit. 2006: Williams acquired the surface and mineral rights on lease 106623 from Newmont Canada Ltd. This acquisition allowed Williams to mine C Zone mineralization above the 9450 level as well as the down dip extension of the C Zone mineralization on the Interlake property. 52 �2006: The Golden Giant Mine became the first Hemlo mining operation to close. The mine produced a total of 6,780,373 ounces of gold. 2008: Newmont and Williams entered into an agreement to allow Williams to extend its underground mining operations on the Williams property through a 60 m restricted area (Boundary Pillar). 2009: Barrick Gold acquires Teck’s interests in the Williams and David Bell mines, giving Barrick sole ownership. 2010: Barrick acquires claims from Newmont that include the Golden Giant Mine workings. 2014: David Bell Mine closed. 2015: Barrick acquired additional claims from Newmont that are located immediately to the north and west of the Williams Mine. These acquisitions have opened up new opportunities for ore body expansion and exploration. Barrick’s land holdings as of August 2016 are shown on Figure 3. 2017: The Williams Mine is currently owned by Barrick Gold Corporation and continues to produce from open pit and underground operations. 2016 production was 235 000 ounces Au from 3 408 000 tonnes milled (Barrick Gold Corporation, Q4 and Year-End Mine Statistics, February 15, 2017). 53 �Figure 3. Barrick Gold Corporation’s Hemlo Mine area land holdings (from Williams Mine Closure Plan Amendment, August 2016). Hemlo Gold Deposit Production, Reserves and Resources Cumulative gold production from the Hemlo gold camp to the end of 2016 has been 21 667 271 ounces Au (worth $26 billion in $US at today’s gold price of $1200/oz) from 108 983 846 tonnes of milled ore. The following graph (Figure 4) illustrates annual production from the three mines between 1985 and 2016. Production peaked at approximately 1.35 Moz in 1990. Proven and Probable Reserve figures for the Williams Mine as of December 31, 2016, totalled 25 782 000 tonnes at a grade of 1.92 g/t Au for a total of 1 588 000 ounces Au. Measured and Indicated Resources currently stand at 58 897 000 tonnes at a grade of 0.908 g/ton Au for a total of 1 720 000 ounces Au (Barrick Gold Corporation, 2016 Mineral Reserves and Mineral Resources, February 15, 2017). 54 �Figure 4. Graph illustrating annual gold production from the Hemlo camp to the end of 2016. Note that between 2009 and 2014 only the combined production statistics for the Williams and David Bell mines were published. These combined statistics were assigned to the Williams mine for the purposes of this graphic. Figure 5. Long section view of the Hemlo deposit illustrating the location of the orebody (mined-out and reserves), mineral resources and areas of exploration potential along strike and down-plunge toward the west (from Barrick Gold Corporation, investor day presentation, February 22, 2016). 55 �Field Trip Stops Most of the field trip stop descriptions contained in this guide are mildly edited excerpts from an unpublished Ontario Geological Survey field trip guide that was prepared by Muir and Smyk (2006). Many of the coloured maps (unless otherwise noted) are also excerpted from that field guide. Field trip locations for the Heron Bay area are shown on Figure 6, while Hemlo area stops are shown on Figure 9. Figure 6. Geological map illustrating field trip stop locations (white triangles) in the Heron Bay area (geology from Ontario Geological Survey 2011) 56 �Stop H1: Pillowed and massive mafic metavolcanic rocks UTM Zone 16, 552024E, 5393482N From Marathon, travel 6.8 km southeast on Highway 17 from the corner of Peninsula Road to the intersection with Highway 627. Then travel south for 1.7 km along highway 627 to an outcrop exposure on the west side of the highway, at a power line crossing. Figure 7. Pillowed mafic metavolcanic rocks at stop H1. At this stop are several small outcrops that display sections of a steeply dipping, thick mafic, tholeiitic flow. The more northerly ones consist of "massive," medium- to fine-grained basalt. One outcrop displays what appears to be a dike with a highly irregular orientation. Although large amphibole porphyoblasts give the impression that the rock is somewhat gabbroic, grain size decreases upwards through the flow to its aphanitic top, where small pillows and autoclastic breccias are developed. Although the pillows are somewhat flattened (see Figure 7), they have well-preserved selvages that indicate that the top direction is toward the south. The most southerly outcrop exposes a section that displays possible flow banding and what appears to be a flow top breccia or pillow breccia. Minor quartz + carbonate veins are present, particularly in the main outcrop. 57 �Stop H2: Felsic pyroclastic rocks and “Heronite” dike Continue 5.6 km south along Highway 627 (and through the community of Heron Bay) to an outcrop located on the west side of the highway. This stop is located approximately 300 m past the intersection with a dirt road that heads east from the highway. UTM Zone 16, 553640E, 5388442N Figure 8. Analcite tinguaite (heronite) dike cross-cutting felsic pyroclastic rocks at stop H2. This stop provides an opportunity to view some of the felsic to intermediate pyroclastic rocks that occur in the immediate vicinity of Heron Bay. The metavolcanic rocks in this outcrop exposure have been cross-cut by an approximately 1 m wide Mesoproterozoic alkalic mafic dike that is likely to be related to the alkalic rocks of the Coldwell complex (see Figure 8). The Heron Bay area pyroclastic rocks are quartz-plagioclase-phyric, and include pyroclastic breccia, tuff-breccia, lapilli-tuff, tuff and crystal tuff (Muir 1982). These rocks are mostly calcalkalic dacite (intermediate), with some rhyolite breccias occurring near the lakeshore of Heron Bay). Overall, the fragments are subrounded to subangular, heterolithic in texture and composition, commonly more felsic than the matrix, and quartz-feldspar phyric. Some of the 58 �fragments are more mafic than the matrix and include intermediate, feldspar ± quartz-phyric rocks, and mafic, aphyric rocks. Bluish quartz phenocrysts are locally common. The matrix consists of feldspar, quartz, sericite, ± chlorite. These rocks were dated at 2695 ± 2 Ma by Corfu and Muir (1989). The alkalic mafic dike exposed in this outcrop has been classified as analcite tinguaite (heronite). A number of dikes with this composition occur in the Heron Bay area and were first described by Coleman (1899; 1900). Interesting features include the presence of ocelli within the dike and fluorite mineralization in the metavolcanic rocks near the dike contact. Stop H3: Deformed and altered felsic rocks along the shoreline of Heron Bay Turn around and travel north on Highway 627 for 1.3 km back to the community of Heron Bay and turn west onto a gravel road just before the railway crossing. Then travel 1.7 km to a boat launching site on the shore of Heron Bay, Lake Superior. UTM Zone 16, 551238E, 5388997N The felsic metavolcanic rocks exposed along the shoreline at this stop provide an example of some of the deformation and alteration effects that are associated with the Heron Bay deformation zone. The following description of the Heron Bay deformation zone is excerpted from MacTavish and Osmani (1996). The 'Heron Bay Deformation Zone' (HDZ) is a strong, locally intense, northeasterly trending, roughly 900 m wide zone, consisting of numerous, discrete, anastomosing shears, faults, and lineaments. Shearing has deformed the host rocks into a highly variable assemblage of quartz-sericite-carbonate±chlorite schist. Hematization, silicification and iron carbonatization are locally common. Intermediate to felsic hypabyssal intrusive rocks (eg. porphyries) are often emplaced along the margins of the zone in a region of more brittle-ductile deformation. These dykes/sills are shallow dipping, and with the exception of the margins of a few bodies, are undeformed. This suggests that they were emplaced during the later stages of the regional deformation events. A number of gold occurrences are found in association with the Heron Bay deformation zone. (e.g. Heron Bay Mine (Peekongay) and Bowhill Mines occurrences; Patterson 1984) One of those occurrences, known as the Screamer Zone is located approximately 200 m north-northwest of this stop. Gold mineralization at the Screamer Zone occurs in two variably brecciated quartzcarbonate veins. These veins occur within sheared and iron-carbonatized mafic metavolcanic host rocks at the contact with a feldspar porphyry dike. The dike itself intruded along the contact between mafic and felsic to intermediate metavolcanic rocks (MacTavish and Osmani 1996). 59 �60 Figure 9. Geological map of the Hemlo gold deposit area showing field trip stop locations (white triangles). Stops H4 and H15 are approximately 2.25 km east and west of the map limits respectively. Geology from Muir (2002). �Stop H4: Cedar Lake pluton Return to Highway 17 and then proceed east. After travelling for approximately 33 km along Highway 17, and just after passing the Williams Mine, you will see the intersection with Highway 614. Continue eastward along Highway 17 past this intersection for another 1.4 km to a location with road cuts on both sides of the highway. UTM Zone 16, 585227E, 5395748N The Cedar Lake pluton consists of massive to very weakly foliated medium-grained, microclinemegacrystic, hornblende-biotite granodiorite that contains mafic/ultramafic clots or xenoliths and diorite to monzodiorite inclusions and dikes (varies with location in pluton). The diorite and monzodiorite inclusions and dikes display a variety of relationships with the granodiorite including back-veining by the granodiorite. These relationships suggest a co-magmatic relationship (Beakhouse, 2001). The mafic-ultramafic and dioritic inclusions are commonly preferentially oriented, generally parallel or subparallel to the contact of the pluton (in a broad sense) and to the foliation in the granodiorite where it is discernible. The foliation locally deflects around the inclusions. The foliation may be related to original flow foliation; synchronous D2 deformation (i.e., syntectonic pluton); and/or a later D3 deformation. The pluton here has been intruded by a few intermediate-composition Archean dikes and a Proterozoic biotite lamprophyre dike. The Archean dikes are locally sheared with an apparent dextral sense. The granodiorite at Stop H4 was initially dated at 2688 +3 Ma (Corfu and Muir 1989). A more recent sample provided a somewhat younger age of 2680 ±1 Ma (Beakhouse and Davis, 2005). Stop H5: Cedar Lake Pluton western contact with metasedimentary rocks Turn around vehicle (there is a gravel road on the south side of the highway that can be used as a turn-around site just east of stop H4) and travel west along Highway 17 for 2.6 to the Yellow Brick Road sign. UTM Zone 16, 582891E, 5394626N At this stop, the Cedar Lake pluton consists of a relatively magnetite-rich, fine- to mediumgrained, biotite-hornblende granodiorite, which is massive except for a very weak foliation within several metres of the contact. A plagioclase-phyric, hornblende-biotite sheet exists to the southwest (Figure 10). The granodiorite at Stop H5 is intruded by many aplite and pegmatite dikes, some of which are composite: generally, pegmatite tends to crosscut aplite. The dikes typically terminate against the country rock schist, although some display ductile deformation within the contact granodiorite. The strike of the dikes within the granodiorite tends to “rotate” clockwise toward the contact, consistent with a dextral component of displacement at the contact. To the north-northwest of here, dextral shear along the contact is related to D2, even though D2 is an overall sinistral event. 61 �The country rocks within about 9 m of the contact consist of D3-crenulated mafic schist containing several locally dismembered, aplitic and granodioritic dikelets and stringers, which display refolded folds. This refolded nature of country rock layering and/or dikes in the immediate contact aureole is common with at least the Cedar Lake pluton and the Pukaskwa Batholith. For approximately the next 150 m westward, the country rocks consist mostly of various sets of metawacke and metasiltstone “packages”, with locally isoclinally folded layering. There are numerous mafic to felsic dikes and several swarms of dikes, generally granitic. Boudinage in dikes is locally present. The granitoid dikes consist mostly of weakly foliated granodiorite, which is similar in grain size and composition to the marginal phase of the Cedar Lake Pluton, as well as some plagioclase-porphyritic dikes. Within this sequence of turbiditic sedimentary rocks is a thick, composite sheet (Figure 10) of foliated, medium-grained, plagioclase-porphyritic, hornblende-biotite granodiorite (2687 ± 3 Ma; Corfu and Muir 1989), and foliated, finer-grained, biotite-hornblende granodiorite, both of which were intruded by aplitic dikes and subsequently by quartz veins. At the structurally lower (western) contact of this sheet, apophyses of the porphyritic granodiorite are folded about a less steeply dipping, S2-like foliation, with attendant attenuation and boudinage which has taken place in 2 dimensions. The folds have been interpreted by others as F2 folds (i.e., dikes predated F2), but it is more likely that they represent a local contact strain phenomenon related to a progressive and complex D2 event. Figure 10. Sketch map of outcrops at the Cedar Lake pluton western contact (from Muir et al. 1995). Note that the number labels shown on this figure denote the 1995 field trip stop numbers. 62 �Stop H6: Deformed metaconglomerate Travel 1.5 km to an outcrop located on the north side of the highway approximately 150 m past the access road to the former David Bell Mine site. UTM Zone 16, 581605E, 5393852N The main outcrop in this set of 3 small outcrops consists mostly of 2 metaconglomerate “layers” (one with predominantly cobbles-boulders; the other with predominantly pebbles), separated by a medium- to coarse-grained wacke locally with entrained pebble- to cobble-sized clasts. This represents a weak, remnant bedding (S0/S1). Flattened clasts tend to be aligned parallel to S2, and oriented lengthwise slightly clockwise with respect to the crude layering, consistent with this unit being on the northeast limb of the northwest-closing Williams fold (see Figure 9). This fold is part of the large-scale S-shaped Williams-Teck fold pair, formed during the D2 sinistral shear event and best delineated by the Moose Lake volcanic complex (yellow unit; Figure 9). Note that some of the clasts have been deformed by F3 folds (see Figure 11). Figure 11. Deformed conglomerate (F3 fold) at Stop H6. 63 �Stop H7: Cedar Creek fault zone area Travel 300 m west along Highway 17 to a series of outcrops on the north side of the highway. UTM Zone 16, 581337E, 5393743N 7A 7C 7B Figure 12. Sketch map of outcrops at stop H7 (from Muir et al. 1995). Outcrop H7A: Folded, sheared, porphyroblastic metasedimentary units Compositional layering in this outcrop is generally well defined and likely reflects modified original bedding. Folded layering defines F2 folds and retrograded porphyroblasts (cordierite?) are aligned parallel to S2. Other porphyroblast species found include garnet, and anthophyllite / cummingtonite. Evidence of superposed D3 dextral shear consists of: spaced shear bands and microfabrics with the apparent appropriate geometry and sense of fabric deflection. Some small Z-shaped folds are ambiguous F2 or F3 folds. Note that the orientation of the axial plane of the main F2 fold here is about 270o, whereas the overall orientation of the Williams fold axial plane is about 290o, possibly consistent with some back-rotation during D3. Some of the amphibolerich (“calc-silicate”) layers reveal evidence of alteration, commonly on both sides of the layers, and this evidence increases toward the Barren Sulphide zone (Outcrop 7B). This suggests that all of the rocks may have been altered. Toward this zone, there is a general change to more thinly laminated pelitic(?) sedimentary rocks, which also show more evidence of alteration (pyrite, muscovite). An increase in the degree of strain toward the zone is evident from the presence of tighter F2 folds and boudinage in calc-silicate layers and a mafic dike (Figure 13). Here the tight F2 folds display S-shaped asymmetry. 64 �Figure 13. Geological map of outcrop area H7A (after Muir 1990). Figure 14. F2 fold closure at Outcrop H7A. 65 �Outcrop H7B: Barren Sulphide Zone (Cedar Creek fault zone) This zone (at least 15 m thick) was also known in the earlier days as the Sucker zone. This is because it had all of the appearances of being a good candidate for a mineralized “horizon,” but in fact is essentially barren. The significant degree of oxidation precludes many unambiguous observations, but it appears to comprise sheared, pyritiferous, quartz-feldspar-sericite schist with small amounts of green mica and irregularly distributed, trace amounts of gold. The rock is possibly derived from the hanging wall pelitic/wacke metasediments (Outcrop H7A). Shearing is quite likely due to D3 based on fabric deflection, geometry of lozenges and the presence of inferred shear bands. Earlier involvement of D2 is possible. The Barren Sulphide zone is inferred to coincide with the Cedar Creek fault zone, one of several high-strain zones along this stretch of highway. Outcrop H7C: Reworked felsic volcaniclastic rocks These rocks consist of somewhat rusty weathering, layered, feldspathic, quartz-crystal-bearing rocks, interpreted to be derived from reworked felsic volcanic detritus of the Moose Lake volcanic complex (MLVC; Figure 9) which, in part, is spatially associated with the Hemlo gold deposit. The rocks do not appear to be the protolith to most if any of the muscovitic schists in the Barren Sulphide zone. Minor disseminated pyrite and rare green (vanadian?) muscovite are locally present in the structurally upper parts of this unit, indicating some alteration has taken place. Stop H8: Bedded felsic fragmental rocks Cross to the south side of the highway (while carefully watching for traffic) and walk a short distance to a low outcrop that is adjacent to the highway. UTM Zone 16, 581210E, 5393696N This is an outcrop of layered, quartz-plagioclase-phyric, heterolithic, felsic fragmental rocks that display a variety of clast sizes. The clasts are flattened parallel to S2 and are oriented counterclockwise with respect to layering (consistent with being on the central limb of the WilliamsTeck fold pair). Stop H9: Felsic fragmental and pelitic sedimentary rocks Carefully re-cross the highway and walk 100 m west to an outcrop on the north side of the highway. UTM Zone 16, 581099E, 5393719N The structurally lower part of Stop H9 consists of an enigmatic unit inferred to be altered, felsic fragmental rocks of volcanic origin. Alteration here is uncharacteristic, showing parts that are greenish, charcoal grey, and pink. Lenses are locally discernible, in part defined by the 66 �distribution of the coloured minerals. Staining for K-feldspar shows microcline tends to be locally distributed around some of the mafic lenses and along some of the dominant cleavage. Note the high degree of strain in this outcrop (likely another, currently unnamed, high-strain zone). The structurally upper part of this outcrop, which displays a sharp, locally slightly discordant contact with the felsic rocks, consists of hard, thinly laminated pelitic rocks that contain abundant staurolite, garnet and sillimanite (fibrolite). A very tight east-closing F2 fold in the layering is evident, similar in style to that in the pelitic rocks of Outcrop H7A. A possible westclosing fold (south part of unit) is enigmatic but, if present, would make this a Z-shaped fold pair. Some staurolite crystals have overgrown and incorporated the laminations. Clockwise rotation of many staurolite crystals, on both limbs of the F2 fold, as well as the orientation of fibrolite, locally about staurolite and counterclockwise to laminations, are inferred as D3-related features. Reddish alteration along undeformed fractures may be related to common alteration (hematitization and/or epidotization) found near diabase dikes, one of which lies at the west end of the outcrop. Stop H10: Main Mineralized Zone Travel 450 m west along Highway 17 to a low outcrop in the ditch on the north side of the highway. UTM Zone 16, 580652E, 5393727N This outcrop exposes the gold-mineralized rocks of what had been previously termed the West or Lake Superior zone. Although not directly connected with the main Hemlo orebody (aka “B” zone) it represents both its on-strike and up-dip projection that was the focus of the vast majority of exploration efforts prior to the discovery of the main orebody in May, 1981. Figure 15 attempts to indicate various features that were visible several years ago. Close examination near the westernmost channel sample (outcrop “B”) reveals highly strained pyritiferous fragmental rocks with, among other things, sparse green (vanadian) muscovite (gMs) lenses and a general grey to bluish grey colour in parts of the matrix due to finely disseminated molybdenite, which is generally a “pathfinder” to gold in this deposit. Grab and channel sampling has returned up to 6 g/t Au. The outcrop contains appreciable barite crystals (Davis and Lin 2003). The high degree of strain in this fragmental rock reflects that these rocks lie within the Lake Superior Shear zone, one of several high-strain zones in the Hemlo deposit area. This high-strain zone is up to 50 m thick. Complex fabric relationships with a possible chronologic interpretation are indicated in the inset at the upper part of Figure 15 pertaining to outcrop “B”. F3 folds (“flanking structures”) can be seen in the southeast part of outcrop “B”. 67 �About 50 m west along the highway ditch is outcrop “A”, which displays the more “massive” quartz-plagioclase porphyry (QPP) that structurally underlies the Main Mineralized zone. Also evident is an example of one of many (in the area) swarms of plagioclase-phyric dikes. Note that the dikes are deformed: extension followed by shortening, reflecting boudinage related to D2 predated shortening related to D3. Also note that in these outcrops and all other ones already seen and to be seen later today, that quartz veins are generally few and small; there typically being as many, if not more, small pods or lenses of quartz as there are veins. Full interpretation of kinetics is commonly enigmatic. This attests to the complex history of strain. Figure 15. Main Mineralized Zone (also known as West Zone) geology (after Muir and Smyk 2006). 68 �Stop H11: Inter-ore zones metasedimentary rocks. Travel 180 m west along Highway 17 to a pair of outcrops on the north side of the highway that are bisected by a road that formerly accessed the Williams A Zone open pit. UTM Zone 16, 580471E, 5393729N The outcrop to the east of the A Zone pit road (see Figure 16) displays relatively feldspathic, turbiditic sedimentary layers, with well-preserved grain-size gradation indicating overturned, south-southwest facing units. The well-preserved nature of this outcrop, contrasts with the adjacent rocks (hanging wall and footwall) and suggests that it may represent a low-strain lithon. To the west of the A Zone road, the sediments are darker grey and locally contain magnetitebearing layers and discrete folds. At the western end of the outcrop, the layering is interpreted as transposed. The increase in degree of strain over an inferred 5 to 10 m across strike between outcrops is considerable and is inferred to reflect, in part, the Moose Lake fault zone. At least 2 types of feldspar porphyry dikes are exposed in this outcrop. Out of interest, the structurally lower one, locally called '‘popcorn” porphyry because of its coarser plagioclase phenocrysts, was used in the early stages of exploration as a cutoff marker unit for drilling collared to the north. Note that, ironically, the Lower Mineralized zone occurs structurally below this dike, and is exposed about 110 m further west along the highway. Figure 16. Sketch map of outcrops at Stop H11 (from Muir et al. 1995). The number labels shown on this figure denote the 1995 field trip stop numbers. The A Zone pit can be seen through the fence located just to the north of the outcrops at this stop. Figure 17 illustrates the pre-mining geology of the surface expression of the A Zone. The ore zone occurs at the contact between metasedimentary and fragmental rocks. 69 �Figure 17. Pre-mining geological map of the A Zone. The approximate outline of the ore zone is shown in red (from Muir 2002; inset showing ore outline derived from Walford et al. 1986). Stop H12: Up-dip projection of the Lower Mineralized Zone. Travel 110 m west to another outcrop on the north side of the highway. UTM Zone 16, 580311E, 5393727N This outcrop is the up-dip projected zone of alteration associated with the actual Lower Mineralized Zone (which occurs at about 900 m depth here). However, the rocks at surface are altered, and locally contain low gold values, minor amounts of green (vanadian?) muscovite, pyrite, medium to coarse-grained tourmaline, muscovite, and rarely microcline porphyroblasts. Barite veinlets and layer-parallel seams have been reported (Schnieders, pers. com. 1994). Magnetite-bearing wacke, which is at the north part of the outcrop (see Figure 18), becomes muscovitic and possibly silicified adjacent to the quartz-plagioclase porphyry (QPP of the Moose Lake Volcanic Complex). The contact between the altered sedimentary rocks and the QPP is locally at an uncharacteristically high angle to the layering in part of the outcrop (folded? crosscutting?). Granodioritic, feldspar porphyritic, and intermediate to mafic dikes occur here. 70 �Figure 18. Sketch map of the up-dip projection of the Lower Mineralized Zone (from Muir et al. 1995). The number label shown on this figure denotes the 1995 field trip stop number. Stop H13: Hemlo Fault zone. Travel 1.5 km west along highway 17 (past the Williams Mine entrance road and tailings line overpass) to a steep road cut on the north side of the highway. UTM Zone 16, 578843E, 5393791N This exposure shows part of the fault zone (which is probably up to tens of metres thick), and features the structurally lower amphibolite/gneissic amphibolite (inferred to be derived from pillowed and massive mafic volcanic units, based on outcrops to the west) and the structurally overlying feldspathic sedimentary rocks. Mafic-ultramafic dikes occur largely as chlorite ± actinolite ± talc phyllonites, mostly within the amphibolitic rocks, and provide much of the evidence for relatively late D3 dextral shear, as can be seen by fabric relationships. Also occurring here are a few intermediate dikes (presently biotite schists) and a plagioclase-phyric dike swarm. Very-coarse-grained tourmaline crystals and crystal “clots” are associated with a feldspar ± quartz dike or vein. The outcrop, featured in Figure 19, was modified in 1989 (postmapping by Smart 1988) by roadway blasting. 71 �Figure 19. Geology of the Hemlo fault zone at Stop H13 (modified from Smart 1988). Stop H14: Highway Zone Travel 1.3 km west along Highway 17 to a large outcrop area on the north side of the highway. UTM Zone 16, 577540E, 5393717N This outcrop (see geological map, Figure 20) demonstrates the heterogeneous strain that occurs even away from the Hemlo gold deposit. It presumably represents the structurally upper part of the tens-of-metres thick Hemlo fault zone. The north part of the outcrop displays disrupted layering in feldspathic siltstone and arenite with S-shaped folds. As one approaches the highway (and hence the Hemlo fault zone), the units change to: 1) quartz-plagioclase-phyric, heterolithic and monolithic, felsic fragmental rocks, displaying contacts tightly interfolded(?) with wacke The felsic fragmental rocks are pyritiferous, erratically auriferous and locally contain green (vanadian?) muscovite; and then 2) wacke-siltstone which displays well-developed laminations that are thinly spaced and inferred to be derived, in part, from a combination of primary layering and compositional differentiation along the main cleavage. Very tight folds (south of the diabase dike) with S-shaped asymmetry are evident. One interpretation is that there is a composite S1/S2 fabric that is locally preserved and oriented 72 �slightly clockwise to the dominant Sm (mylonitic) fabric. The S3 flattening fabric (micaceous) can be locally seen within the Sm cleavage, counter-clockwise to it. The contact with the amphibolitic and mafic-ultramafic rocks seen in Stop H13 presumably lies to the south of the highway. Figure 20. Geological map of the Highway Zone at Stop H14 (modified from Muir et al. 1995). 73 �Stop H15: Homestake F3 fold. Travel 3 km west along Highway 17 to a high road cut on the south side of the highway. UTM Zone 16, 574529E, 5393489N This outcrop (see geological map, Figure 21) is possibly north of the Hemlo fault zone, but ambiguity abounds. Several features in this otherwise over-lichened outcrop are of note: 1) Rare, calc-alkalic or “alkalic” pillows with feldspar phenocrysts and hornblende crystals, locally with amygdaloidal/vesicular textures; 2) Abrupt strain partitioning from slightly strained pillows (near the highway) to moderately strained pillows (D2) over a distance of less than 1 m southward; 3) A related volcanic breccia or volcaniclastic unit (fragments with similar composition and textures); 4) Adjacent feldspathic arenite and/or altered wacke; 5) A short, thin gossan, which returned 103 ppm Au and 100 ppm Mo (Resident Geologist’s files, Thunder Bay South District) – the highest molybdenite results outside of the Hemlo gold deposit proper and similar to some feldspathized tholeiitic pillows “along strike” to the west of the deposit; 6) Varieties of variably deformed metawacke and metasiltstone, locally with garnet ± magnetite ± staurolite porphyroblasts; 7) Examples of S2 and F2 structural elements overprinted by S3 and F3 structural elements (e.g., folded S2); 8) Numerous variably deformed (folded, transposed) dikes such as mafic schists; mafic biotite lamprophyre schists ± mafic-ultramafic xenoliths; a mafic net-veined or backveined dike (mixture of previous 2 types?); and 2 types of intermediate dikes; 9) The xenolith-rich parts of the lamprophyre dikes are very similar to some of the diamondiferous intrusions found in the Wawa greenstone belt. 74 �Figure 21. Detailed geology of the Homestake F3 fold exposure (modified from Muir et al. 1995). 75 �References Barrick-Hemlo Williams Operating Corporation and Amec Foster Wheeler Environment and Infrastructure 2016. Williams Mine closure plan amendment; Ministry of Northern Development and Mines, Thunder Bay Mineral Development and Lands Branch Office, Mine Closure Plan Files, 207 p. Beakhouse, G.P. 2001. Nature, timing and significance of intermediate to felsic intrusive rocks associated with the Hemlo greenstone belt and implications for the regional geological setting of the Hemlo gold deposit; Ontario Geological Survey, Open File Report 6020, 248p. Beakhouse, G.P. and Davis, D.W. 2005. Evolution and tectonic significance of intermediate to felsic plutonism associated with the Hemlo greenstone belt, Superior Province, Canada; Precambrian Research, v.137, p.61-92. Burk, R., Hodgson, C.J., and Quartermain, R.A. 1986. The geological setting of the Teck-Corona Au-MoBa deposit, Hemlo, Ontario, Canada; in Proceedings of gold '86, an international symposium on the geology of gold, Toronto, Ontario, 1986, p.311-326. Coleman, A.P. 1899. Dyke rocks near Heron Bay; Ontario Bureau of Mines, Vol.8, Pt.2, p. 172-174. Coleman, A.P. 1900. Heronite or analcite tinguaite; Ontario Bureau of Mines, Vol.9, p. 186-191. Corfu, F. and Muir, T.L. 1989. The Hemlo–Heron Bay greenstone belt and Hemlo Au-Mo deposit, Superior Province, Canada, 1. Sequence of igneous activity determined by zircon U-Pb geochronology. Chemical Geology (Isotope Geoscience Section), 79: 183-200. Davis, D.W. 1998. U-Pb zircon and titanite geochronology; Part 3, in Geological setting of the Hemlo gold deposit; an Interim Progress Report, Jackson, S.L., Beakhouse, G.P. and Davis, D.W. (eds), Ontario Geological Survey, Open File Report 5977, p.1-10. Davis, D.W. and Lin, S. 2003. Unraveling the geologic history of the Hemlo Archean gold deposit, Superior Province, Canada: a U–Pb geochronological study; Economic Geology, v.98, p.51-67. Harris, D.C. 1989. The mineralogy and geochemistry of the Hemlo gold deposit, Ontario; Geological Survey of Canada, Economic Geology Report 38, 88p. Jackson, S.L. 1998. Stratigraphy, structure and metamorphism; Part 1, in Geological setting of the Hemlo gold deposit; an Interim Progress Report, Jackson, S.L., Beakhouse, G.P. and Davis, D.W. (eds), Ontario Geological Survey, Open File Report 5977, p.1-71. Langlais, A. and Barber, R. 2015. Barrick Gold Inc. work assessment report, Bomby Township, Thunder Bay Mining District; Thunder Bay South Resident Geologist District, Assessment Files, AFRO number 2.56670, 75p. MacTavish, A. and Osmani, I. 1996. Geological report for the Toothpick West and East properties, Heron Bay project, Pic Township, northern Ontario; Thunder Bay South Resident Geologist District, Assessment Files, AFRO number 2.16854, 22p. 76 �Magnus, S.J. and Arnold, K.A. 2016. Geology and mineral potential of the western Schreiber–Hemlo greenstone belt; in Summary of Field Work and Other Activities, 2016, Ontario Geological Survey, Open File Report 6323, p.11-1 to 11-17. Magnus, S.J. and Walker, J. 2015. Geology and mineral potential of Walsh, Tuuri and Syine Townships, Schreiber-Hemlo greenstone belt; in Summary of Field Work and Other Activities 2015, Ontario Geological Survey, Open File Report 6313, p.14-1 to 14-12. Muir, T.L. 1997. Precambrian geology, Hemlo gold deposit area; Ontario Geological Survey, Report 289, 219p. Muir, T.L. 2000. Geological compilation of the eastern half of the Schreiber–Hemlo greenstone belt; Ontario Geological Survey, Map 2614, scale 1:50 000. Muir, T.L. 2002. The Hemlo gold deposit, Ontario, Canada: principal deposit characteristics and constraints on mineralization; Ore Geology Reviews, v.21, p.1-66. 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 Sciences, v.40, p.395-430. Muir, T.L., Jackson, S.L. and Beakhouse, G.P. 1999. The regional framework of the Hemlo Gold Deposit; in Summary of Field Work and Other Activities 1999, Ontario Geological Survey, Open File Report 6000, p.15-1 to 15-6. Muir, T.L., Schnieders, B.R. and Smyk, M.C. 1995. Geology and gold deposits of the Hemlo area revised edition; Institute on Lake Superior Geology, 41st Annual Meeting, Marathon, ON, 1995, v.41, part 2d, 120p. Muir, T.L. and Smyk, M.C. 2006. Hemlo greenstone belt: Hemlo gold deposit area and Heron Bay area; unpublished OGS Precambrian Geoscience Section field trip guide, May 2-3, 2006, 28p. Ontario Geological Survey 2011. 1:250 000 scale bedrock geology of Ontario; Ontario Geological Survey, Miscellaneous Release—Data 126–Revision 1. Page, T.W. 1947a. A report on the Ollmann-Williams group of claims, Hemlo, Ontario; assessment files, Resident Geologist's office, Schreiber--Hemlo District, Thunder Bay, 8p. Page, T.W. 1947b. Report on the property of the Lake Superior Mining Corporation Ltd.; assessment files, Resident Geologist's office, Schreiber--Hemlo District, Thunder Bay, 1p. Page, T.W. 1948. A report on the property of Lake Superior Mining Corporation Ltd., Hemlo Area, Ontario; assessment files, Resident Geologist's office, Schreiber--Hemlo District, Thunder Bay, 23p. Page, T.W. 1949. Report on the properties of Lake Superior Mining Corporation Limited; assessment files, Resident Geologist's office, Schreiber--Hemlo District, Thunder Bay, 4p. Pan, Y. 1990. Metamorphic petrology and gold mineralization of the White River gold prospect, Hemlo area, Ontario; unpublished PhD thesis, University of Western Ontario, London, Ontario, 256p. 77 �Pan, Y. and Fleet, M.E. 1988. Metamorphic petrology of the White River gold prospect, Hemlo area, Ontario; in Geoscience Research Grant Program, Summary of Research 1987-1988, Ontario Geological Survey, Miscellaneous Paper, p.164-176. Pan, Y. and Fleet, M.E. 1989. Metamorphic petrology and gold mineralization of the White River gold prospect, Hemlo area; in Geoscience Research Grant Program, Summary of Research 1988-1989, Ontario Geological Survey, Miscellaneous Paper 143, p.42-52. Pan, Y. and Fleet, M.E. 1990. Metamorphic petrology and gold mineralization of the White River gold prospect, Hemlo area; in Geoscience Research Grant Program, Summary of Research 1989-1990, Ontario Geological Survey, Miscellaneous Paper 150, p.13-26. Patterson, G.C. 1984: Field Trip Guidebook to the Hemlo Area; Ontario Geological Survey, Miscellaneous Paper 118, 33p. Smart, P. 1988. A lithological study along the Hemlo fault; unpublished Hon. BSc thesis, Queen's University, Kingston, Ontario, 43p. 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, 80p. Walford, P., Stephens, J., Skrecky, G., and Barnett, R. 1986. The geology of the 'A' Zone, Page-Williams Mine, Hemlo, Ontario, Canada; in proceedings of Gold '86, an international symposium on the geology of gold, Toronto, Ontario, 1986, p.362-378. 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. 78 �Field Trip 2 Unusual Archean Diamond-bearing rocks of the Wawa Area by Ann C. Wilson Ministry of Northern Development and Mines, Resident Geologist’s Program, Ontario Geological Survey, Timmins, Ontario On the cover (clockwise from top): Giant lower crustal to upper mantle xenoliths, Enigma Property, Menzies Township, Oasis Diamond Corporation Inc.; Diamonds from the Festival Property (photo courtesy of Pele Mountain Resources Inc.); Sandor Diamond Occurrence, Highway 17, Spider Resources Inc. & KWG Resources Inc.; Heterolithic diamond-bearing breccia, Engagement Zone, Northern Sierra Minerals Corporation 79 �Unusual Diamond-bearing rocks of the Wawa Area Introduction In 1991, local prospector C. “Mickey” Clement recovered at least three alluvial diamonds from the Michipicoten River. Two of the stones were sent to the Department of Mineralogy, Royal Ontario Museum, where they were identified as industrial-grade diamonds with weights of 1.05 and 1.13 carats. Both stones were described as frosted and graphite-inclusion riddled. In 1995, prospector Sandor Surmacz and geologist Marcelle Hauseux recovered diamonds from a bedrock occurrence on the Trans Canada Highway, approximately 20 km north of the town of Wawa. An 18.1-kg bulk sample of a xenolith-rich lamprophyre yielded 1 macrodiamond and 5 microdiamonds. All but one was gem quality. Since then, more than 50 occurrences of diamondiferous bedrock have been reported in an area of approximately 30 km2 in size, centred approximately 20 km north of the town of Wawa. The occurrences are hosted within a sequence of unusual, Archean-aged, heterolithic breccias. Historically, this sequence of rocks has received little exploration interest and was considered to have little economic significance. This field trip will focus on exposures of a series of foliated lamprophyre dikes and associated heterolithic breccias outcropping in Lalibert, Leclaire, Menzies and Musquash townships. This field trip guide represents a summary of information available at the time of writing and should not be considered the final analysis of these rock types. Much more research is required on these rocks. Active exploration and research is still underway on the properties included in this field guide. Given the limitations of time, the field trip will visit only some of the more accessible properties. Bear in mind that when visiting active exploration or mine properties, permission must be granted by the property owner. Current ownership information can be obtained from the Resident Geologist’s Office in Timmins, or the District Geologist’s Office in Sault Ste Marie, Ontario. Geological Overview of the Michipicoten Greenstone Belt The Wawa region lies within the Wawa subprovince of the Canadian Shield. The Michipicoten greenstone belt extends inland for approximately 150 km from the Lake Superior shore and has an average width of 38 km. The greenstone belt consists of supracrustal rocks of Archean age. Younger Archean granitic rocks surround the greenstone belt. Figure1 shows a generalized geological map of the Michipicoten greenstone belt. The oldest volcanic cycle is approximately 2900 Ma and is of limited distribution. This cycle is best developed in Esquega Township along the southern flank of the supracrustal assemblage. Portions of this metavolcanic cycle extend into eastern McMurray and western Lastheels townships. The base of this volcanic cycle consists of massive to pillowed komatiitic flows intruded by mafic sills. The ultramafic rocks are overlain by intermediate to felsic tuffs capped by thinly bedded chert-magnetite-sulphide iron formation (Judith-Kathleen Iron Formation). Intermediate to felsic metavolcanic tuffs below the Judith iron formation have been dated at 2889 ± 9 Ma (Turek et al. 1992). Overlying the 2900 Ma cycle is a 2750 Ma volcanic cycle. This volcanic cycle is predominately composed of intermediate to felsic tuffs, breccias and flows. Porphyritic and spherulitic flows are not common and most of the intermediate to felsic metavolcanic rock is fragmental. The base of this cycle consists of high magnesium and high iron tholeiitic massive and pillowed flows. It lies conformably atop the JudithKathleen iron formation at the east end of Wawa Lake, but the basal unit is poorly exposed elsewhere. Overlying the mafic metavolcanic rocks is a sequence of heterolithic, intermediate to mafic breccia that has been traced for a distance of over 13 kilometers. This unit is in turn, overlain by a thick section of intermediate to felsic tuffs, breccias and massive flows that reaches a maximum thickness of approximately 2000 m below the Helen Iron Formation. The upper part of the intermediate to felsic metavolcanic rocks has been dated at 2749 ± 2 Ma (Turek et al. 1992). The Michipicoten (Helen) iron formation caps this 80 �81 Figure 1. Generalized geological map of the Michipicoten greenstone belt showing some of the diamond occurrences (modified after Stott et al. 2002). �this volcanic cycle. It commonly exceeds 100 m in thickness and, in the vicinity of the past-producing Helen Mine; it has been tectonically thickened to over 300 m. This iron formation was the source of virtually all commercial iron ore production in the Wawa area from 1898 to 1998. The youngest metavolcanic rocks in the area are those of the 2700 Ma volcanic cycle. These rocks underlie approximately fifty-percent of the Michipicoten greenstone belt and are found in the central and northern parts of the belt. The basal unit is composed of massive and pillowed mafic amygdaloidal flows that attain a maximum thickness of 5.5 km in the Goudreau area. This unit is overlain by intermediate to felsic metavolcanic rocks, or their stratigraphic equivalent, the Doré metasedimentary rocks. These metavolcanic rocks are typically composed of tuffs and monolithic and heterolithic breccias. Quartz + feldspar crystal tuff and an intermediate tuff from this volcanic cycle returned a U-Pb zircon age of 2701 ± 8 Ma (Turek et al. 1992) and 2701.4 ± 2.1 Ma (Ayer et al. 2003). The intermediate to felsic tuffs interdigitate with clastic metasedimentary sequences that include cross-bedded sandstone and a tonalite cobble conglomerate (Doré conglomerate). Corfu and Sage (1987, 1992) reported an age of 2698 ± 2 Ma for a tonalite clast in the Doré conglomerate and maximum ages of 2680 ± 3 and 2682 ± 3 Ma for sedimentary sequences in northern and central parts of the Michipicoten greenstone belt. Geochronological and structural evidence indicates that sedimentation continued after cycle 3 volcanism and predated a major folding and faulting event. Arias (1996) noted that the rocks comprising cycle 3 in the central part of the Michipicoten greenstone belt are upside down and represent the overturned limb of a belt-scale recumbent nappe fold. This inverted limb has been refolded and imbricated by subsequent southverging thrust faults, which caused local repetition of the stratigraphic sequence (Wilson 2004). Felsic plutonism occurred synchronous with all of the major volcanic cycles and continued after volcanism ceased at Wawa. Plutonic rocks associated with cycle 1 volcanism include the Murray-Algoma porphyry (2881± 6 Ma) and the Regnery biotite granite of the Hawk Lake granitic complex (2888 ± 2 Ma). Both intrusions are situated in Esquega Township. The Jubilee granitic stock, located in McMurray Township, was dated at 2745 ± 3 Ma and is coeval with cycle 2 volcanism. Plutons associated with cycle 3 volcanism range in composition from tonalite through granodiorite and granite and have ages ranging from 2698 to 2693 Ma. These plutons are located south and west of the Michipicoten greenstone belt (Stone and Semenyna 2004). The Kapuskasing Structural Zone extends east from the shore of Lake Superior, northeast through Kapuskasing and into the Hudson Bay Lowland. Local features interpreted to be associated with it include northeast-striking Proterozoic lamprophyre dikes (Sage 1994; Morris 1999). Lamprophyre dikes of middle Proterozoic age are common in the region south of the Wawa-HawkManitowik Lake Fault and rare to non-existent north of the fault. They are carbonate, biotite, and sometimes olivine-rich and usually less than 1.0 m in width. The dikes generally strike northeast. Dikes exposed in McMurray Township commonly have blue to blue-green sodic amphibole developed in their wall rocks. This mineral has been interpreted to be a product of fenitization. These dikes are likely spatially and temporally related to the emplacement of the Keewenawan-age Firesand River Carbonatite (Sage 1994). North of the Wawa-Hawk-Manitowik Lake Fault, in the area extending west from the former Magpie Mine (Leclaire Township), to the east side of the Dickenson Lake Stock (Lalibert Township), there are a series of what appear to be narrow, biotite-amphibole-rich dikes that have been interpreted as Archean lamprophyres. These dikes commonly have large, rounded inclusions (lower crust to upper mantle-derived xenoliths) up to 3.0 m in size, the centres of which are often completely altered to radiating clusters of actinolite crystals. Titanite from the matrix of one of these dikes returned an age of 2703 ± 42 Ma (Sage 2000). The date is interpreted to be a minimum age of intrusion. Subsequent dating of a zircon from a gneissic xenolith from the same dike returned an age of 2684.9 ± 1.4 Ma (Ketchum, Kamo and Davis 2003). 82 �Several narrow (<10 m) kimberlite dikes also intrude the Michipicoten greenstone belt. These dikes are largely restricted to the eastern part of the greenstone belt and are spatially related to the northeast-trending Kapuskasing Structural Zone. Several of these dikes have been dated. Intrusions K115 and K121, located in Isaac Township, returned an average 87Rb/86Sr age of 1097 ± 7 Ma (Kaminsky et al. 2002). A kimberlite dike intersected in Pele Mountain Resources Inc.’s drill hole 97-34, drilled in Riggs Township, returned a 207 Pb/206Pb age of 1197 ± 24 Ma (R. P. Sage, Ontario Geological Survey, unpublished, 2000; Wilson 2004). Quaternary Geology All Quaternary deposits within the Wawa area were deposited during the Late Wisconsin by the Labrador sector of the Laurentide Ice Sheet. Peat, recovered from a bog located within the surface of a terrace associated with the highest glacial lake in the Lake Superior basin, was radiocarbon dated at 9759 ± 170 BP. A caribou antler, recovered from near the surface of the delta of glacial lake Minong III, yielded a radiocarbon date of 8820 ± 80 BP (Morris 2001). Bedrock striae indicate that there were two prominent ice flow directions. The oldest and most pervasive ice flow was south to southwest (159° - 240°). A later, weaker ice flow was to the southwest and west (220° - 290°). The younger set of striae was formed during the latter stages of glaciation as the ice sheet began to thin and bedrock topography began to influence the direction of ice flow. Much of the overburden consists of a thin (< 1 m), discontinuous till veneer draped over the bedrock. Most of the area's thicker surficial deposits are located within bedrock controlled valleys where glaciolacustrine waters from the Lake Superior basin covered the area. In several of these valleys, the glaciofluvial material reaches thicknesses up to 32 m (Morris 2001). Structural Geology of the Diamond-bearing Rocks The greatest concentration of diamond-bearing rocks in the Michipicoten greenstone belt is constrained to a roughly 30 km2 block of land coinciding with a D1 recumbent nappe identified by Arias (1996). The nappe lies immediately north and adjacent to the Kapuskasing Structural Zone and has affected the two youngest rock sequences in the greenstone belt. In cross section, a generalized structural section across the greenstone belt would be upside down and represents the overturned limb of a belt-scale recumbent fold. This inverted limb of the nappe fold has been refolded and imbricated by subsequent south-verging thrust faults. The result is that the geometry of the rocks hosting the diamondiferous bedrock is that of an inverted anticline (Arias and Helmsteadt 1990). The effect is to create a tectonic repetition of the diamond-bearing sequence of rocks. In simplified terms, the rocks hosting the diamonds in the Wawa area can be considered as a single, overturned fold limb that has been faulted along a series of thrust zones. As a result, the diamond-bearing breccias, the lamprophyres associated with them and the surface onto which these metavolcanic rocks were deposited has been repeated at least four times (Walker 2002). The western limit of the nappe is likely the Dickenson Lake fault that passes along the west side of the Dickenson Lake stock. Extrusive lithologies similar to those hosting diamonds were recognized to the east of the stock by Sage (1993) during reconnaissance mapping. To date, no diamondiferous occurrences have been discovered to the west of the Dickenson lake fault. The eastern limit of the nappe is the Marsden Lake fault. Prior to 2002, rocks favourable to hosting diamonds had not been observed east of this structure (Wilson 2004). However, in 2002, Oasis Diamond Exploration Inc. made a discovery of diamonds on the east shore of the Magpie River and in 2004, diamondiferous occurrences of bedrock were found on the west shore of the Magpie River in Chabanel Township. These discoveries suggest that the potential for these host rocks extends farther eastward than previously believed. Figure 2 shows a graphic representation of the nappe structure. 83 �Figure 2. Composite structural section through the central part of the Michipicoten greenstone belt. Section X-Y from south-central Corbiere Tp. (Josephine Iron Range) to Andre Lake central Corbiere Tp. Section Y-Z is a schematic section from west central Lalibert Tp. The sketch in the lower left explains the present configuration of the belt as a regional nappe fold (F1) refolded about F2. Imbricate thrusts are considered related to Fs (Arias 1996). Description of the Diamond-bearing Rocks The diamond-bearing breccia and associated lamprophyre is broadly distributed throughout Lalibert, Leclaire, Menzies and Musquash townships. On-going mapping by Pele Mountain Resources Inc. on the Festival property and by Nathalie Lefebvre on the GQ Property has helped to refine the classification of these rocks. Systematic exploration and sampling suggest that the individual diamond occurrences are part of a much larger suite of rocks and that diamonds occur primarily within discrete layers at the base of diamond-bearing zones. North-northwest-trending diamond-bearing zones of breccia and lamprophyre are up to 1500 m in length and up to 800 m in width (Pele Mountain Resources Inc., press release, January 18, 2005). The breccia forms thick units (maximum true thickness is approximately 110 m) dipping to the northeast 30°. The lateral extent and thickness of the breccia unit is not well constrained, owing to the large-scale regional folding and thrusting (Lefebvre 2004). Figure 3 provides a detailed map of the southwest corner of the Festival Property showing a recent interpretation of these diamond-bearing zones. The diamond-bearing rocks can be visually subdivided into two classes, lamprophyre (dikes and bodies of indeterminate morphology) and heterolithic or polymict breccias. It is often difficult to differentiate between the two classes since the lamprophyre dikes frequently contain an assortment of inclusions that 84 �85 Figure 3. Detailed geology of the southwestern corner of the Festival Property (modified from Pele Mountain Resources, Inc. press release, January 18, 2005). The units shown in green consist of mafic to intermediate massive and pillowed flows, breccias and tuffs (Vaillancourt 2005b, Sage et al. 1982) �give them the appearance of breccia. The lamprophyre dikes cut the breccia units. Both lithologies have been metamorphosed to upper greenschist facies. The breccia primarily consists of angular, pebble-sized, lithic fragments, mainly of volcanic composition, contained within a green to grey fine-grained matrix. The matrix grain size ranges from < 2 mm to 1 mm. At least eleven distinctive types of lithic fragments have been observed in the breccia and the fragments are irregularly distributed throughout the breccia. The clast population is primarily derived from rocks with which the breccia is intercalated. Most typically these clasts are mafic and felsic metavolcanic rocks and intermediate to mafic intrusive rocks. Other clast types include fragments of clast-supported breccia within matrix supported breccia, fragments of earlier matrix-supported breccia with fewer than 5% fragments and coated lithic fragments (Lefebvre 2004). The breccia is characteristically massive, unstratified and poorly sorted with clast size ranging from sand to boulders up to 9 m. Primary sedimentary structures such as bedding and crude grading are rare (Lefebvre 2004). Petrographic work by Lefebvre (2004) on the breccias identified a typical fragmental texture within a mineralogically variable matrix. Typically the groundmass is dominated by actinolite, but chlorite and biotite dominated groundmass are locally predominant. Juvenile magmatic material also was observed as discrete fragments and rims on other clasts in the breccia. Its petrography is distinct from the breccia groundmass. The juvenile magmatic material contains more abundant actinolite grains; fewer epidote and fine-grained plagioclase grains and more fine-grained oligoclase and muscovite; and more oscillatoryzoned hornblende. Most exploration efforts over the past few years have concentrated on the breccias because they are considered to have the best potential for hosting commercial diamond deposits. Over the course of the last few years, explorationists have subdivided the diamond-bearing breccias into three separate facies. These facies are volcanic (pyroclastic), subvolcanic/intrusive breccia and hypabyssal facies. A variable, but distinctive proportion and composition of fragments and/or xenoliths characterizes each facies (Wilson 2004). The volcanic facies contains breccia, lapilli- and ash sized fragments and consists of medium to thickly bedded pyroclastic air-fall deposits. They are characterized by angular to sub-angular Archean supracrustal fragments, some hypabyssal fragments and rare lower crustal to upper mantle xenoliths. The subvolcanic/intrusive breccia facies are the most variable in texture and appearance. The rock is characterized by observed intrusive relationships, a high proportion of fragments and a close proximity to the volcanic facies. The fragment characterization is variable. The facies can contain all or some of the following fragment types: supracrustal fragments, crustal fragments and lower crustal to upper mantle xenoliths. This facies was once included with the lamprophyres but is now interpreted by various exploration companies as a debris flow. The hypabyssal facies hosts variable proportions (<25%), of sub-rounded to rounded mantle xenoliths, as well as minor proportions of gneiss and/or trondhjemite fragments. This facies also was originally classified as lamprophyre but is now considered to be part of the debris flows (Lefebvre et al. 2003). The brecciated unit(s) could also be a single or multiple diatreme(s) localized within Lalibert, Leclaire, Menzies and Musquash townships. A tectonic repetition of the diamond-bearing diatreme(s) has been achieved through the regional deformation events described previously. This regional deformation may help to explain the layered appearance observed at some of the diamond occurrences (J. Ayer, Ontario Geological Survey, personal communication 2006). Lamprophyre occurs as narrow dike-like intrusions or in bodies of indeterminate morphology cross-cutting or intercalated with local country rocks. Dikes range in width from 50 cm to 2 m and may display 2-3 cm offshoots in some locations. Contacts with the host rocks vary from sharp and straight to highly irregular. No variation in grain size within the dikes has been observed and the no variation in colour or mineralogy 86 �has been observed within country rock adjacent to the dikes. The lamprophyre is fine-grained, grey in colour and contains approximately 5-10% subrounded to subangular fragments. The fragment population is dominated by actinolite-rich monomineralic rocks, or by biotite-rich greenstone and hornblende-rich ultramafic rocks (Lefebvre 2004). Lamprophyre is differentiated from the breccia by: a lower clast content; a predominance of highly altered, coarse-grained actinolite fragments; scarcity of wall rock fragments; the rounded shape of the xenoliths; and the presence of a weaker fabric (Lefebvre 2004). The lamprophyre dikes post-date all other lithologies. The lamprophyre with indeterminate morphology predates most of the host lithologies since fragments of each are found within these lamprophyres. However, in some locations, lamprophyre fragments have been observed within intermediate and mafic intrusive rocks near the contact with lamprophyre (Lefebvre 2004). Fragments within lamprophyre dikes commonly have a biotite-rich rim enclosing the xenoliths and the fragments found in the lamprophyres of indeterminate morphology do not. The lamprophyres of indeterminate morphology also show positive relief of less severely weathered xenoliths more so than the lamprophyre dikes. Lastly, the lamprophyre dikes have a less variable fragment lithology (Lefebvre 2004). The lamprophyre is petrographically distinct from the breccia. It contains a lower abundance of clasts and fewer clast types. Unlike the breccia, the lamprophyre contains no juvenile magmatic material and oscillatory-zoned hornblende grains are rare. Detailed petrographic descriptions of the lamprophyre can be found in Lefebvre (2004). Relationship between diamond content and lithology Microdiamonds have been recovered from a wide array of breccias and lamprophyres in the Wawa area. Between 2003 and 2004, the Ontario Geological Survey (OGS) investigated a number of diamond occurrences during a mapping program conducted in Menzies and Musquash townships. Closer look at locations where bulk samples had been collected suggested that the bulk samples probably included more than one rock type. Diamonds were recovered from each of these bulk sample sites, but it would be difficult to establish from which of the rock types the diamonds were recovered. The OGS undertook a limited sampling program to further investigate the diamond content of specific lithologies (Vaillancourt et al. 2005a). The OGS collected three small samples (95.21 kg total weight) from three different lithological units at the Cristal and Genesis diamond occurrences. These occurrences are on the Festival Diamond Property owned by Pele Mountain Resources Inc. Results from this limited sampling program reinforce the observation that microdiamonds are not restricted to a single unit. Microdiamonds were recovered from heterolithic breccia, both with and without ultramafic magma pockets, and from a fragment-free ultramafic dike. There is the possibility, however, that the diamonds recovered from the ultramafic dike are xenocrysts derived from diamond-bearing host breccia (Vaillancourt et al. 2005a). The OGS concluded that the results from only three samples are not sufficient to draw irrefutable conclusions regarding the location of the microdiamonds. Collection and analysis of more, well constrained samples is necessary to further refine the diamond potential of specific host rocks. In 2005, Spider Resources Inc. and KWG Resources Inc. conducted a similar bedrock sampling program to investigate the diamond-bearing potential of specific lithologies. Separate representative samples (approximate weight 16 kg each) of the matrix, xenoliths and run of mill (ROM) portions of the bedrock were collected from the Wawa Diamond Project and were sent for caustic dissolution. The results are as follows: matrix sample returned 67 diamonds (0.008 total ct), xenolith sample returned 244 diamonds (0.051 total ct) and the ROM sample returned 86 diamonds (0.006 total ct) (Spider Resources Inc., press release, February 20, 2006). 87 �Geochronology In conjunction with recent geological mapping in the area, the Ontario Geological Survey has been conducting geochronological work to help understand the nature and timing of the diamondiferous units and their host rocks within the Michipicoten greenstone belt. A felsic volcanic horizon hosting diamondiferous units returned a 207Pb/206Pb age of 2701.4 ± 2.1 Ma. Maximum 207Pb/206Pb ages of 2685.1 ± 1.0 Ma and 2684.9 ± 1.4 Ma have been returned for diamondiferous lamprophyre dikes cutting the Catfish assemblage (2.7 Ga) intermediate to felsic metavolcanic rocks in Lalibert and Menzies townships (Ayer et al. 2003). A second sample of felsic lapilli tuff, part of the Catfish assemblage, adjacent to the Moet Occurrence contains zircons that returned a 207Pb/206Pb age of 2698.7 ± 1.1 Ma (Vaillancourt et al. 2004). A sample was collected from the diamondiferous breccia at the Moet Occurrence in order to determine the age of brecciation. Five zircons were analyzed. The three oldest ages are 2687 ± 2 Ma, 2683 ± 2 Ma and 2681 ± 2 Ma. The two youngest zircons cave results that precisely overlapped one another at 2679.2 ± 2.1 Ma (Vaillancourt et al. 2004). Since this is the youngest zircon age obtained from the breccia, it represents either the time of cystallization or emplacement of the body if the zircons are magmatic, or a maximum time of emplacement if the zircons are xenocrystic (Vaillancourt et al. 2005a). If the zircons are xenocrystic, this age must still be close to the time of breccia emplacement since a lamprophyre dike from the GQ property returned a date of 2673 ± 8 Ma from titanite (R. P. Sage, Ontario Geological Survey, unpublished 2000). The data indicate that the felsic metavolcanic rocks hosting the diamondiferous breccias are part of the Catfish assemblage. The maximum age for the diamondiferous breccias and the associated dikes is less than 2680 Ma. These absolute age constraints indicate that the breccias are not volcaniclastic units belonging to the Catfish assemblage (Vaillancourt et al. 2005a). Zircons from a sample of felsic lapilli tuff from northwestern Menzies township returned an age of 2736.0 ± 0.8 Ma which is taken to represent the age of eruption and crystallization of the tuff. This age clearly indicates that the volcanic package underlying the iron formation in the western part of Menzies Township is part of the Wawa assemblage (2.75 Ga) and brackets the uppermost part of the assemblage at 2736 Ma (Vaillancourt et al. 2005a). Geochemistry of the Diamond-bearing Rocks Both Williams (2002) and Lefebvre (2004) conclude that the whole rock major element geochemistry is consistent with a calc-alkaline classification for both the lamprophyres and the associated breccia. Both authors also noted that the compositions of chromite in the Wawa metavolcanic rocks are in the range typical for lamprophyres and dissimilar to those in kimberlites and lamproites. Whole rock geochemistry for the diamond-bearing rocks is tabulated in Sage (2000), Williams (2002), Lefebvre (2004), Stone and Semenyna (2004) and Vaillancourt et al. (2005c). Sage and Williams’ work is specific to the diamond-bearing and non diamond-bearing lamprophyres. Work by the other authors relates to both the breccia and the lamprophyres. Whole rock geochemistry for kimberlites of the Wawa area can be found in Kaminsky et al. (2002). Diamond characteristics Lefebvre (2004) undertook a study examining a parcel of 80 macrodiamonds recovered from the volcaniclastic breccia on the GQ Property. Results from this work are summarized below and in De Stefano et al. (2006). Additional work on the morphology of the Wawa diamonds can be found in Stone and Semenyna (2004). Stachel et al. (2004) summarize results of analysis conducted on diamonds from the Genesis and Cristal diamond occurrences presently held by Pele Mountain Resources Inc. 88 �Shape: The diamonds display a highly variable primary growth form. Most of the diamonds are either octahedral aggregates (44% of the population) or single octahedral crystals (26%). Single cubic and cubicoctahedral crystals and their aggregates, as well as macles form the remainder of the population. Fortyeight percent of the diamond population is single crystals and only 28 diamonds could be evaluated for crystal regularity. The majority of the diamonds also display some degree of distortion. Colour and transparency: The diamonds included in this study are classified into colourless, brown, grey, black, yellow and white. No pink, green, violet or blue diamonds were observed. The colour distribution within the population is: colourless (48%), heterogeneous (24%), yellow (11%), black (3%), brown (10%) and grey (3%). The heterogeneity in colour is observed only in aggregates. The diamond population consists of 48% transparent crystals, 25% translucent crystals, 14% opaque crystals and 14% is a combination of opaque and translucent crystals. Transparent crystals are typically colourless and also comprise a few yellow octahedral single crystal and coarse aggregates as well as macles. Translucent crystals comprise all possible primary crystal forms and colours. Opaque crystals are mostly fine-grained aggregates which have black body colouring. Resorption: Generally speaking, the diamonds have experienced low degrees of resorption. Only 21% of the diamond population displays extensive resorption. Some crystals (14%) exhibit non-uniform resorption where one part of the crystal is more strongly resorbed than another. Inclusions: Mineral inclusions were identified in 58% of the diamonds. Both primary and secondary inclusions were observed. The mineralogy of the recovered primary inclusions is listed in descending order of abundance: olivine (Fo92 and Fo89), clinopyroxene (omphacite), plagioclase (albite and An-rich), orthopyroxene (En93) and Fe-Ni sulphide (pentlandite). Cathodoluminescence: The relative abundances of cathodoluminscence (CL) colours for the Wawa diamonds are: orange-red (46%), yellow (28%), orange-green (10%), green (6%), and other non-uniform colours (10%). None of the 69 diamond examined displayed the more common blue CL. Impurities: Fournier Transform Infrared (FTR) spectrometry was used to investigate the nitrogen and aggregation states for 41 diamonds. The majority of the diamonds have low nitrogen contents, < 300 ppm. The diamonds show two modes of nitrogen aggregation suggesting mantle storage at 1100 - 1170° C. Diamonds from the Genesis occurrence are almost exclusively cubes including some fragmented, twinned and moderately resorbed cubes. Most of the crystals contain clouds. Fully transparent stones are dominantly brown although colourless stones also are common; one diamond was yellow in colour. Nitrogen concentrations range from below detection (<10 ppm) to 600 atomic ppm. Nitrogen aggregation is very low (Stachel et al. 2004). Diamonds from the Cristal occurrence range from un-resorbed octahedra to highly resorbed dodecahedra. Octahedral and weakly resorbed octahedral stones dominate the population. About 25% of the population are irregular crystals, macles are common (15%) and about 5% of the diamonds show cubo-octahedral growth. The stones fall into two dominant colour classifications, colourless and a range of brown colouration. Nitrogen contents range from <10 – 560 ppm, but with only one exception nitrogen is ≤170 ppm. Nitrogen aggregation varies between 0 and 97% B-centre. Olivine is the most common mineral inclusion, followed by pyrope garnet and Mg-chromite (Stachel et al. 2004). Origin of the Diamond Deposits Based on published data on the diamond-bearing rocks at Wawa and Cobalt, Wyman et al. (in press) suggest that the tectonic setting of the deposits and nature of the host rocks indicate that the diamonds may be derived from the asthenospheric wedge and subducted slab at shallow depths (100 – 160 km) rather than the deep keels of Archean cratons associated with traditional diamond deposit types. Models of lowtemperature Phanerozoic diamond formation in active subduction zones, or rapid uplift and emplacement of peridotite massif occurrences, can be adapted to the Archean deposits. The stability field of diamonds in 89 �most Phanerozoic scenarios may be too deep to be accessed by the lamprophyric magmas. Shallow subduction, as proposed for these occurrences of adakitic-type rocks in the Wawa subprovince, could generate two different diamond stability windows at sufficiently shallow depths to account for their presence in lamprophyric magmas. Wyman et al. (in press) states that any tectonic model for these Archean diamond occurrences must address several requirements. These requirements include 1. a deep source for oxidized metasomatic fluids that is activated prior to lamprophyre emplacement 2. a mechanism to isolate this isotopically aged and depleted source for tens or hundreds of millions of years until it is heated in the mantle during orogeny 3. a hybridized mantle source for primitive, hydrous, shoshonitic lamprophyres 4. sustained cold finer effect in the mantle to establish a shallow-mantle diamond stability window Two theories of diamond origin are postulated by De Stefano et al. (2006). Both a cratonic and orogenic model of diamond formation are discussed in an effort to rationalize the observed diamond characteristics. The authors conclude that neither model fully explains all of the observed characteristics. 90 �Field Trip Stops Field Trip Road Log Stop 1 2 3 4 5 6 7 8 Locality Intersection of Hwy 101 and Hwy 17 Take Highway 17 north km 0 Catfish Road forestry road – turn east 19.2 GQ Diamond Discovery Northern Sierra Minerals Corporation Area B Northern Sierra Minerals Corporation Engagement Zone 23.3 23.9 29.1 Return to Highway 17 reset odometer Drive north to access road – turn east Park Walk eastward along trail Moet Occurrence, Festival Property 0 6.2 6.6 7.6 7.6 Return to Highway 17 reset odometer Drive north, park on shoulder of highway Sandor Diamond Occurrence Continue north on Highway 17 to intersection of Highway 519, turn right 0 4.3 4.3 12.6 Safely turn in parking area and return to Intersection of Hwy 17 and 519 reset odometer Drive south on Highway 17, park on shoulder of road Dubreuilville Dike Continue south on Highway 17 Turn left into access road to gravel pit, park Walk south approximately 150 m Monchiquite Dike Drive north on Hwy 17 to Wawa Wawa Motor Hotel 0 3.1 3.1 44.9 45 53.2 STOP 1 - GQ Diamond Discovery Site Northern Sierra Minerals Corporation Area A UTM co-ordinates – 0665570E 5333291N NAD83 Several outcrops of diamondiferous breccia outcrop on the west side of a forestry road. This exposure is an example of the hypabyssal facies of the three identified diamond-bearing units. The rock cut displays the apparently conformable nature of these “lamprophyre” dikes. The most notable features of these outcrops are the actinolite-rich nature of the matrix and the presence of biotite-rich reaction rims around the xenoliths. It is frequently difficult to distinguish these dikes from the mafic to intermediate agglomeratic and tuffaceous host rocks. In the vicinity of the discovery area, located at the south end of Area A, the diamondiferous breccias are arrayed linearly along the logging road where the topography indicates a 5 – 10 m thick, northwest-trending dike (Cavey 2002). A compilation of the geology of the GQ Property is shown in figure 4. Figure 5 provides a compilation of the diamond occurrences on the GQ Property. 91 �Figure 4. Geological compilation map of the GQ Property, Musquash Township, Northern Sierra Minerals Corporation (Cavey 2004). 92 �Figure 5. Occurrences of diamondiferous bedrock on the GQ Property, Musquash Township, Northern Sierra Minerals Corporation (Cavey 2004). 93 �A relatively precise 207Pb/206Pb age of 2674 ± 8 Ma was returned from a sample collected in 2000 by R. P. Sage (R. P. Sage, Ontario Geological Survey, unpublished 2000). The sample analysed was titanite. The titanite grains are considered to be a primary mineral and not xenocrystic. In thin section, the rock is characterized by a green, medium-grained, granoblastic to decussate groundmass of actinolite, biotite, chlorite, plagioclase and accessory minerals. Amphibole or biotite macrocrysts up to 1 mm are common (Stone and Semenyna 2004). Local prospectors T. Nicholson, J. Robert and M. Tremblay made the discovery in the fall of 1999. The first two bedrock samples (63.4 kg and 70.5 kg) collected were processed by Kennecott Canada Exploration Inc. in their Thunder Bay laboratory. According to Kennecott’s report, the 63.4 kg sample yielded 45 diamonds, of which 10 were macro diamonds and 35 were microdiamonds. One of the macro diamonds measured 1.01 mm in one dimension. The 70.5 kg sample yielded 9 microdiamonds. All stones were white in colour and transparent in clarity. Duplicate samples were collected by Band-Ore Resources Ltd. in early 2000 and were processed at SGS Lakefield Research Limited. A 54.6 kg sample yielded 98 microdiamonds. A confirmation sample from the same area yielded 98 microdiamonds from a 54.6 kg sample. In 2000, Band-Ore Resources drilled 3 short holes (75 m total) at the discovery site. Table 1 details the diamond recovery results from the drill program. Only partial intervals from drill hole DDH GQ-00-3 were submitted for microdiamond recovery since portions of the core were used for thin sections, microprobe analysis and display purposes. In total 5 diamonds were recovered from DDH GQ-00-3, including one champagne coloured macro diamond and one white microdiamond from a sample weighing 7.5 kg. Drill sections for DD GQ-00-01 through 03 are shown on figure 6. Table 1. Diamond recovery results from 2000 Band-Ore Resources Ltd. diamond-drilling program Drill Hole DDH GQ-00-1 DDH GQ-00-2 Sample No. Sample Size (kg) No. Macro Diamonds No. Micro Diamonds Sample 1A 63.35 1 Sample 1B 30.17 10 Sample 1C 28.51 6 Sample 2A 37.52 1 30 Sample 2B 30.97 434 Sample 2C 28.51 30 results compiled from Band-Ore Resources Ltd. press releases 2000 To date, the discovery site has yielded 746 diamonds, including 15 macro diamonds, from sample material weighing 785 kg. The largest diamond recovered exceeds 1.0 mm in size and the majority of the stones are gem quality, white, clear and transparent. STOP 2 - Northern Sierra Minerals Corporation Area B UTM co-ordinates – 0665425E 5334748N NAD83 The exposure on the east side of the forestry road provides an excellent exposure of the subvolcanic/intrusive breccia facies. Subrounded to rounded xenoliths dominate the vertical exposure (Figure 7). Field relationships between the intrusive breccia and other heterolithic breccias can be observed in several outcrops along the road. Texturally, the subvolcanic/intrusive facies may resemble both the hypabyssal facies and the intrusive heterolithic breccias. The facies consists of mica and amphibole phenocrysts (<2mm) in a groundmass of mica, actinolitic amphibole and lesser albite, carbonate, sphene and oxides. Alteration includes variably chloritized mica while the other phenocrysts have been extensively altered to varying proportions of mica, albite and actinolitic amphibole. 94 �Figure 6. Drill hole sections GQ-00-01, GQ-00-02 and GQ-00-03, GQ Property, Musquash Township, Northern Sierra Minerals Corporation (Cavey 2002). 95 �Figure 6 cont’d. Drill hole sections GQ-00-01, GQ-00-02 and GQ-00-03, GQ Property, Musquash Township, Northern Sierra Minerals Corporation (Cavey 2002). 96 �Figure 7. Stop 2 - Northern Sierra Minerals Corporation Area B – rounded lower crustal to upper mantle xenoliths Stone and Semenyna (2004) completed a petrographic examination of one of the ultramafic xenoliths from this site. The sample was dominated by a coarse, radiating to decussate, clear to pale green amphibole of tremolitic to magnesium-rich actinolite composition. Carbonate occurs locally and biotite is concentrated at the rims of the xenolith. Band-Ore Resources Ltd. discovered area B in 2000. Thirty-three (33) reconnaissance samples were collected from this area and a total of 273 diamonds was recovered from 352 kg of material (Cavey 2002). One 24 kg sample returned 126 microdiamonds (1.37 mg total weight). No macro diamonds were recovered from Area B. Band-Ore Resources Ltd. completed only a reconnaissance sampling program, minor stripping and trenching in this area. The Barnett Zone lies approximately 1.6 km to the northwest. It was discovered in the fall of 2001 by Kennecott Canada Exploration Inc. who completed a limited program of mechanical stripping, channel sampling and washing of outcrops over the area. A total of 27 outcrop channel samples (270 kg) were collected. The channel samples returned 330 microdiamonds and 3 macro diamonds. A single 24 kg sample of heterolithic breccia from the Barnett Zone returned 3 macrodiamonds and 123 microdiamonds. A total of 273 diamonds (261 microdiamonds and 12 macrodiamonds) were recovered from 34 samples (352 kg) collected between September 2000 and July 2001 by Kennecott Canada Exploration Inc. in Area B (including the Barnett Zone). 97 �STOP 3 - Northern Sierra Minerals Corporation Engagement Zone Bulk Sample Site UTM co-ordinates – 0667760E 5336073N NAD83 The main outcrop exposure consists of medium to thickly bedded pyroclastic air-fall deposits and debris flows described as heterolithic tuff-breccias that grade upwards to lapilli-tuff and tuff. The groundmass of the extrusive phase is fine-grained with variable proportions of relatively small (<2mm) altered phenocrysts, including chloritized mica. The groundmass consists primarily of actinolitic amphibole with rare to minor mica or granular albite. The pyroclastic rocks are mineralogically and compositionally similar to intrusive varieties, but have a significantly higher proportion of mica phenocrysts. Both diamond-bearing intrusive and extrusive rocks host significant proportions of fragments derived from the local country rock. Rare to minor, deep crustal and upper mantle xenoliths, such as banded gneiss and extensively altered talcose ultramafic xenoliths are present. Fresh mantle rocks, such as lhertzolite, harzburgite and eclogite have not been identified. The matrix material is typically fine-grained, green, weakly foliated actinolite schist. Albite is present, although it is less abundant (<10%) than in the lamprophyre dikes and implies a more ultramafic composition for the breccia matrix than for the lamprophyre dikes. Titanite is fairly abundant and calcite, epidote, apatite and sulphide minerals occur locally. Macrocrysts of actinolite are commonly observed. Rare macrocrysts of amphibole also are observed and are frequently altered to actinolite. The actinolite macocrysts are probably metamorphic in origin, whereas the amphibole macrocrysts may represent accidental or cognate crystals (Stone and Semenyna 2004). The Engagement Zone has a minimum strike length of 335 m and a horizontal width in excess of 75m. The zone strikes northwesterly and has a shallow dip to the northeast. This zone may represent the southeast extension of Pele Mountain Resources Inc.’s Cristal diamond occurrence located approximately 2 km to the northwest. In 2003, a 0.72 carat macrodiamond was recovered from a bulk sample collected from the Cristal. The geology of the Engagement Zone and sample locations are shown in Figure 8. A simplified cross section of the Engagement Zone is shown in Figure 9. Band-Ore Resources Ltd. discovered this zone in January 2001. A 16 kg sample from a single angular boulder of diatreme breccia returned 128 microdiamonds. Four subsequent samples (96 kg total weight) returned 5045 microdiamonds and 65 macrodiamonds. In 2001 a mini-bulk sample weighing 12.5 tonnes was collected under the supervision of Kennecott Canada Exploration Inc. and shipped to the Saskatchewan Research Council. The largest diamond recovered from this sample was a 0.254 carat, broken, white octahedral stone. Two additional bulk samples were collected in 2003. A 22 tonne sample tested an area where 6 channel samples (63 kg total) recovered 1752 stones. A 20 tonne sample tested an area where 5 channel samples weighing 41.8 kg returned 552 stones. The results from these bulk samples are shown in Table 2. Table 2. Diamond results from Engagement Zone bulk samples (2001) Occurrence Sample Weight Sieve +1mm Sieve +2mm Sieve +3mm Sieve +5mm Sieve +6mm Engagement Zone East 22.1 tonnes 1 4 4 Engagement Zone West 20.4 tonnes 2 3 3 2 1 Total Diamonds Total Carat Weight 12 0.375 8 0.155 results compiled from Band-Ore Resources Ltd. press releases 2004 To date, exploration on the Engagement Zone has included an orientation geochemical survey, geological mapping, channel sampling, trenching, bulk sampling and a 9-hole (1775 m) diamond drilling program designed to test the strike continuation of the zone. The drilling program demonstrated that thick diamondiferous breccia deposits can feather out and thin to a few centimeters thickness (Cavey 2003). 98 �Figure 8. Stop 5 – Geology and sample locations at the Engagement Zone, GQ Property, Northern Sierra Minerals Corporation (Cavey 2002) 99 �Figure 9. Simplified cross section through the Engagement Zone looking northwest (Cavey 2004). STOP 4: - Moet Occurrence, Festival Property Pele Mountain Resources Inc. UTM co-ordinates – 0662709E 5338009N NAD83 The Moet occurrence is a large stripped outcrop that extends in a north-south orientation across the forestry road. The outcrop displays all three facies of diamond-bearing bedrock exposed over an area 500 m by 300 m. It is hosted within fine- to medium-grained mafic metavolcanic rocks with closely associated intermediate to felsic metavolcanic rocks and metasediments. The volcanic facies was found concentrated within a series of outcrop exposures along the west side of a 5-8 m north-trending ridge and the subvolcanic/intrusive breccias and hypabyssal rocks are present in several outcrops east of the volcanic facies. The subvolcanic/intrusive breccias are hosted in the metavolcanics and the volcanic facies appears to overlie these rocks, and are in turn overlain by intermediate to felsic metavolcanic rocks. The fragments 100 �within the volcanic facies consist of country rock fragments with lesser crustal fragments and lower crust to upper mantle xenoliths. The hypabssyal facies displays primarily mantle xenoliths (Walker 2003). A detailed geological map of the Moet locality is shown in Figure 10. Figure 10. Detailed geological map of the Moet Occurrence, Festival Property, Pele Mountain Resources Inc. (Kjarsgaard et al. 2003). The access trail to the outcrop passes through a sequence of intermediate to felsic tuffs and tuffaceous breccias. The composition of the fragments and that of the matrix are highly variable from felsic to intermediate and a combination of felsic matrix with intermediate fragments and vice versa is not uncommon. At a distance of approximately 1km, the trail passes through a sequence of mafic to intermediate tuffs and lapilli tuffs (Vaillancourt et al. 2005b). An age date of 2698 ±1 Ma was returned from sample of the felsic metavolcanic rocks at the occurrence. A sample of the breccia returned an age date of 2680 Ma (J. Ayer, Ontario Geological Survey, personal communication, 2004). Discovered by Pele Mountain Resources Inc. in 2001, the occurrence initially gained interest because the breccia has a size distribution of diamonds that includes coarser sized diamonds from relatively small samples. For example, an 8 kg sample collected in 2001 recovered a total of 9 diamonds, 4 of which were in the +600 mesh fraction. Diamonds are consistently recovered from both the volcanic and subvolcanic facies at this showing. A summary of the results of the 2001 and 2002 sampling of the occurrence by Pele Mountain Resources Inc. is found in Table 3. 101 �Table 3. Summary of the diamond results from the 2001-02 sampling of the Moet Occurrence Facies Weight (kg) <425 mesh >425 mesh volcanic volcanic hypabyssal hypabyssal volcanic volcanic hypabyssal subvolcanic breccia 117 16 24 8.6 9.3 14.1 9.3 32 48 2 0 0 7 12 0 30 0 1 0 0 0 5 0 1 total diamonds 48 3 0 0 7 17 0 31 results compiled from Pele Mountain Resources Inc. press releases 2002 Further exploration was conducted at the occurrence in 2003 when DeBeers Canada Exploration Inc. completed a detailed airborne geophysical survey, stripping, mapping and sampling program. A 47.8tonne bulk sample from the site returned 5 diamonds with a total carat weight of 0.13. All diamonds were recovered from the +1 to +3 sieve class screens (Pele Mountain Resources Inc., press release, March 17, 2004). STOP 5: - Sandor Diamond Occurrence Spider Resources Inc. & KWG Resources Inc. UTM co-ordinates – 0659805E 5342191N NAD83 The Sandor occurrence is the first confirmed occurrence of diamonds in bedrock in the Wawa area. The occurrence is located in a 4 m high road cut on the east side of the Trans Canada Highway (Highway 17). The dike is approximately 5 m wide, steeply dipping and strikes roughly parallel to the regional schistosity at 120°. The dark, greenish-grey rock weathers olive grey, is highly fractured, moderately carbonatized and is non-magnetic. It is composed of up to 40% actinolite replaced mantle xenoliths and supracrustal xenoliths. Towards the margins of the dike xenoliths are less common and the rock grades into an adjacent micaceous dike. Only remnants of the dike remain in situ. The dike is hosted by gabbros and intermediate to felsic crystal tuffs. A (308.6 kg) sample of the dike, collected by Spider Resources Inc. in 1997, returned a total of 97 diamonds comprising 1 commercial stone, 13 macrodiamonds and 83 microdiamonds. A short walk into the forest from the top of the outcrop leads to a larger stripped area where field relationships between the host gabbro and the dike can be observed. A second, xenolith-bearing dike (occurrence LAL-3) is located at the north end of the outcrop. This dike is 2 m wide and closely resembles the Sandor occurrence. A 34.6 kg sample of this dike contained 1 microdiamond. A detailed geological map of the Sandor occurrence is shown in Figure 11. A compilation map of the geology of the Spider Resources Inc. and KWG Resources Inc. property is shown in Figure 12. Using normative mineralogy, Sage (2000) concluded that this dike should be classified as a spessartite. A spessartite is defined as a lamprophyre composed of phenocrysts of green hornblende or clinopyroxene in a groundmass of sodic plagioclase with accessory olivine, biotite, apatite and opaque oxides. Titanite and rutile from the matrix of the Sandor dike returned an age of 2703 ± 42 Ma (Sage 2000). The date is interpreted to be a minimum age of intrusion. Subsequent dating of a zircon from a gneissic xenolith from the Sandor dike returned a 207Pb/206Pb age of 2684.9 ± 1.4 Ma (Ketchum, Kamo and Davis 2003). Spider Resources Inc. and joint venture partner KWG Resources Inc. have taken 3 mini-bulk samples along the 1 km strike extent of the dike since the fall of 2001. The three bulk samples had a combined weight of 7.61 tonnes and returned 11 commercial stones and 9 macro diamonds. These samples were tested only for macro diamond and commercial diamond content (Spider Resources Inc., press releases, February and March 2002). 102 �Figure 11. Detailed geology of the Sandor Occurrence, Spider Resources Inc. & KWG Resources Inc. (Kjarsgaard et al. 2003). Figure 12. Generalized geology of Wawa Project, Spider Resources Inc. and KWG Resources Inc. (Spider Resources Inc. and KWG Resources Inc., CD-ROM presentation, update February 24, 2004) 103 �STOP 6: - Dubreuilville Dike - Xenolith-rich lamprophyre Spider Resources Inc. & KWG Resources Inc. UTM co-ordinates – 0657251E 5347023N NAD83 The dike outcrops in a 1.5 m high exposure on the west side of the Trans-Canada Highway (Highway 17). In the area between the former Magpie Iron Mine in Leclaire Township to approximately the eastern contact of the Dickenson Lake Stock, there are a number of exposures of unusual dike rocks. These rocks are characterized by prominent round, to elliptical inclusions of actinolite or actinolite plus talc. Xenoliths are altered to fine-grained actinolite with or without talc, and some display zoning from talc core to an actinolite rim. The actinolite inclusions may consist of prismatic green crystals as large as 8 cm in length, which may be randomly oriented or radiating inward towards the core. The inclusions containing talc consist of a talc core with the prismatic to acicular actinolite projecting radially inward towards the core. The xenoliths are believed to represent at least two original mafic compositions which are likely to be originally of lower crust origin or deeper. A weakly developed regional schistosity crosses the dike implying an Archean age (Sage 1993, 2000). An example of one of the lower crustal to upper mantle xenoliths is shown in Figure 13. Figure 13. Lower crustal to upper mantle xenolith, Dubreuilville Dike Stop 6, Highway 17 North. This dike may be the one described by Higgins (1986). He reported that the dike consists of 60% euhedral amphibole, 20% biotite replacing amphibole and 15%plagioclase. Minor sphene and opaque minerals are present and chromite is reported from the core of the talc-bearing clasts. The bulk composition of the nodules is reportedly pyroxenite, but Higgins did not indicate whether their source was mantle or crust. The rounded outline of the clasts may reflect magmatic erosion during transport and the present mineralogy is the product of regional metamorphism (Higgins 1986). 104 �STOP 7: - Monchiquite Dike Michipicoten Post Provincial Park UTM co-ordinates – 0663600E 5309223N NAD83 On the west side of Trans Canada Highway (Highway 17) two, 1-metre wide and one, 3-metre wide monchiquite dikes outcrop at the base of a 3 m high outcrop. The dikes are parallel, strike 060° and crosscut a portion of the Mission Stock. A 4 m wide, east-trending microsyenite dike also can be observed cutting the granodiorite and the monchiquite at this location. The north-northwest-trending Trembley Fault cuts across the north end of the outcrop (Massey 1985). These lamprophyres are most commonly seen in road cuts along the highway and along the shore of Lake Superior. Typically, they are black when fresh and weather to an orange-brown colour. The dikes are usually narrow and are typically less than 1 m wide. Biotite-phyric and olivine-phyric varieties are most common, but pyroxene and feldspar phenocrysts also are observed. Often the dikes show evidence of multiple and composite intrusion, sometimes with tin screens of country rock trapped within the dikes. Lamprophyre dikes south of the Michipicoten River typically have a northeast trend (Massey 1985). Lamprophyre dikes are commonly found cross cutting all lithologies south of the Wawa – Hawk – Manitowik Lakes Fault. Because of their ease of weathering they are infrequently seen in outcrop. However, the dikes are frequently found in underground mine workings. These dikes have a number of macrocrystic resemblances to the dikes seen here. As early as 1927, geologists working in the area had noted a similarity between these lamprophyre dikes and kimberlite (Gledhill 1927). Recent petrographic and mineralogical studies on several dikes in both McMurray and Lendrum townships have suggested that some of the geochemistry falls within the classification of type II kimberlites (orangeites) (Barnett 2001). To date, no diamonds have been recovered from these dikes. These lamprophyre dikes are probably Proterozoic in age and are interpreted to represent Proterozoic alkalic magma emplacement into structures related to the Kapuskasing Structural Zone, perhaps consanguineous with the nearby Firesand Creek carbonatite complex. Rocks from the Firesand Creek carbonatite complex have a U-Pb date of 1078 ± 2.4 Ma (Sage 2000). STOP 8: - Diamondiferous Conglomerates north of Wawa This unit, referred to as the Leadbetter Conglomerate, is the main diamond-bearing unit on the property; it is comprised of clast supported plymictic conglomerate that is poorly sorted. Predominantly massive, the unit is rarely stratified and when it is, the bedding varies in thickness from 10-30 centimetres to over 1 meter with no obvious graded bedding. The sdeiments range in clast size from mud-sized particles to bould-sized clasts which are sub-rounded to rounded, flattened and highly stretched. The matrix is very fine-grained and chlorite rich with a minor amount of quartzofeldspathic material. The clasts predominantly consists of volcanics, with a minor amount of non-volcanics (siltstone and chert). The conglomerate appears to pinch out to the northwest in the project area, but thickens dramatically in the central andeaster parts of the project area where surface exposures give an indicated thickness of up to 300 m. Deformation of this unit has resulted in stretched or elongated clasts. The degree of stretching is variable suggesting that there are local zones of intense deformation. The diamondiferous conglomerate is interpreted to represent valley fill deposits in the proximal reaches of an alluvial fan comples (Wendland, 2009) with the matrix supported cobble and boulder conglomerates forming a debris flow sequence. An ultramafic component in the source for the matrix component of the of the conglomerate is indicated by immobile element geochemistry. The fact that a distinct suite of diamond indicator minerals consisting of chromite, olivine, and rare ilmenite, pyrope garnet and chrome diaopside, have been recovered from the conglomerate makes it unique incomparison to the sparsely diamondiferous Wawa volcaniclastics located to the north which contain a paucity of conventional indicator minerals (Verley 2009). 105 �STOP 9: - Contemplation of the rocks on the fireplace at the Wawa Motor Hotel Acknowledgments The author would like to thank Wayne O’Connor, Northern Sierra Minerals Corporation, Al Shefsky, President, Pele Mountain Resources Inc. and Neil Novak, Vice-President of Exploration, Spider Resources Inc. for permission to access properties described in the field guide. Editorial comments by R. P. Sage also were appreciated during the preparation of the field guide. My thanks also go to Glenn Seim for his technical assistance in the preparation of the guidebook. 106 �Bibliography Arias, Z. G. 1996. Structural evolution of the central Michipicoten greenstone belt, Superior Province, Wawa, Ontario, Canada; unpublished MSc thesis, Queen’s University, Kingston, Ontario, 140p. Arias, Z. G. and Helmstaedt, H. 1990. Structural evolution of the Michipicoten (Wawa) greenstone belt, Superior Province: Evidence for an Archean fold and thrust belt; in Geoscience Research Grant Program, Summary of Research 1989-1990, Ontario Geological Survey, Miscellaneous Paper 150, p. 107-114. Ayer, J. A., Conceição, R. V., Ketchum, J. W. F., Sage, R. P., Semenyna, L. and Wyman, D. A. 2003. The timing and petrogenesis of diamondiferous lamprophyres in the Michipicoten and Abitibi greenstone belts; in Summary of Field Work and Other Activities 2003, Ontario Geological Survey, Open File Report 6120, p. 10-1 to 10-9. Barnett, R. L. 2001. Fletch Kimberlite correspondence, Report of Work, Matchinameigus and Fletch Properties, Dolson & Echum Townships, Sault Ste Marie Mining Division, Northern Ontario; Timmins Resident Geologist’s Office, Dolson Township, assessment file WP–Dolson–4, unpaginated. Cavey, G. 2002. Geological report on the Wawa properties for Band-Ore Resources Ltd. Sault Ste Marie Mining Division, Ontario; unpublished NI 43-101 report, 50p. Cavey, G. 2003. Updated geological report on the Wawa property for Band-Ore Resources Ltd. Sault Ste Marie Mining Division, Ontario; unpublished NI 43-101 report, 35p. Cavey, G. 2004. Summary geological report on the GQ Property for Band-Ore Resources Ltd. Sault Ste Marie Mining Division, Ontario; unpublished NI 43-101 report, 39p. Corfu, F. and Sage R. 1987. A precise U-Pb zircon age for a trondhjemite clast in Doré conglomerate, Wawa, Ontario; in Proceedings and Abstracts, Institute on Lake Superior Geology Annual Meeting, v. 33, p. 18. Corfu, F. and Sage, R. 1992. U-Pb age constraints for deposition of clastic metasedimentary rocks and late-tectonic plutonism, Michipicoten belt, Superior Province; Canadian Journal of Earth Sciences, v. 29, p. 1640-1651. De Stefano, A., Lefebvre, N. and Kopylova, M. 2006. Enigmatic diamonds in Archean calc-alkaline lamprophyres of Wawa, southern Ontario, Canada; Contributions to Mineralogy and Petrology, on-line edition, January 5, 2006. Gledhill, T. L. 1927. Michipicoten Gold Area, District of Algoma; Ontario Department of Mines, Annual Report, 1927, v. 36, pt. 2, p. 1-49. Higgins, M. D. 1986. Nodule-bearing spessartite (lamprophyre) dike near Wawa, Northern Ontario; in Program with Abstracts GACMAC-GCU-AGC-AMC-UCG, vol. 11, p. 81. Kaminsky, F. V., Sablukov, S. M., Sablukova, L. I, Shchukin, V. S. and Canil, D. 2002. Kimberlites from the Wawa area; Canadian Journal of Earth Sciences, v. 39, p. 1819-1838. Ketchum, J., Kamo, S. and Davis, D. 2003. U-Pb ages from the Superior and Grenville Provinces of Ontario; unpublished report of the Jack Satterly Geochronology Laboratory, Toronto. Kjarsgaard, B. A., McClenaghan, M. B., Boucher, D. R. and Kivi, K. 2003. Kimberlites and ultrabasic rocks of the Wawa, Chapleau, Kirkland Lake and Lake Timiskaming Areas; in VIIIth International Kimberlite Conference, Northern Ontario Field Trip Guidebook, B. A. Kjarsgaard (ed) p. 1-37. Lefebvre, N. S. 2004. Petrology, volcanology, and diamonds of Archean calc-alkaline lamprophyres, Wawa, Ontario, Canada; unpublished MSc thesis, The University of British Columbia, Vancouver, British Columbia, 265 p. Lefebvre, N., Kopylova, M., Kivi, K. and Barnett, R. 2003. Diamondiferous volcaniclastic debris flows of Wawa, Ontario Canada; long abstract prepared for VIIIth International Kimberlite Conference, Victoria, British Columbia, Canada. Massey, N. W. D.1985. Geology of the Mishewawa Lake Area, District of Algoma; Ontario Geological Survey, Open File Report 5532, 167p. Morris, T. F. 1999. Overburden as a media for kimberlite, base metal and gold exploration, Wawa region, northeastern Ontario; Geological Association of Canada – Mineralogical Association of Canada, Joint Annual Meeting, GAC-MAC Sudbury 1999, Field Trip B6 guidebook, 67p. Morris, T. F. 2001. Quaternary geology of the Wawa Area, Northeastern Ontario; Ontario Geological Survey, Open File Report 6055, 67p. 107 �Sage, R. P. 1993. Geology of Killins, Knicely and Lalibert townships, District of Algoma; Ontario Geological Survey, Open File Report 5589, 141p. Sage, R. P. 1994. Geology of the Michipicoten greenstone belt; Ontario Geological Survey, Open File Report 5888, 592p. Sage, R. P. 2000. The “Sandor” diamond occurrence, Michipicoten greenstone belt, Wawa, Ontario: A preliminary study; Ontario Geological Survey, Open File Report 6016, 49p. Sage, R. P., England, D., Calvert, T., Oudkerk, G., Worona, R. and Kusciusko, R. 1982. Precambrian geology of Musquash Township, Algoma District; Ontario Geological Survey, Map P.2556, scale 1:15 840. Stachel, T., Blackburn, L., Kurszlaukis, S., Barton, E. and Walker, E. C. 2004. Diamonds from the Cristal and Genesis volcanics, Wawa area, Ontario; in Abstracts of Talks & Posters, 32nd Annual Yellowknife Geoscience Forum, 16-18 November 2004, p. 74-75. Stone, D. and Semenyna, L. 2004. Petrography, chemistry and diamond characteristics of heterolithic breccia and lamprophyre dikes at Wawa, Ontario; Ontario Geological Survey, Open File Report 6134, 39p. Stott, G. M., Ayer, J. A., Wilson A. C. and Grabowski, G. P. B. 2002. Are Neoarchean diamond-bearing breccias in the Wawa area related to late-orogenic alkalic and “sanukitoid” intrusions?; in Summary of Field Work and Other Activities 2002, Ontario Geological Survey, Open File Report 6100, p. 9-1 to 9-10. Turek, A., Sage, R. P. and Van Schmus, W. R. 1992. Advances in the U-Pb ziron geochronology of the Michipicoten greenstone belt near Wawa, Ontario; Canadian Journal of Earth Sciences, v. 27, p. 649-656. Vaillancourt, C., Ayer, J. A., Zubowski, S. M. and Kamo, S. L. 2004. Synthesis and timing of Archean geology and diamond-bearing rocks in the Michipicoten Greenstone Belt: Menzies and Musquash townships; in Summary of Field Work and Other Activities 2004, Ontario Geological Survey, Open File Report 6145, p. 6-1 to 6-9. Vaillancourt, C., Ayer, J. A. and Hamilton, M. A. 2005a. Synthesis of Archean geology and diamond-bearing rocks in the Michipicoten greenstone belt: Results from microdiamond extraction and geochronological analyses; in Summary of Field Work and Other Activities 2005, Ontario Geological Survey, Open File Report 6172, p. 8-1 to 8-11. Vaillancourt, C., Dessureau, G. R. and Zubowski, S. M. 2005b. Precambrian geology of Menzies Township; Ontario Geological Survey, Preliminary Map P.3366, scale 1:20 000 Vaillancourt, C., Zubowski, S. M. and Dessureau, G. R. 2005c. Lithogeochemical data and field photographs for the Wawa area: Menzies and Musquash Townships; Ontario Geological Survey, Miscellaneous Release – Data 151. Verley, C. G.. 2009. 2009 Update of Activities on the Leadbetter Diamond Project; National Instrument-43-101, for Dianor Resources Inc. Ontario, 78p. Walker, E. C. 2002. Diamond deposits of the Festival Property, Wawa, Ontario; Report prepared for Pele Mountain Resources Inc. under National Instrument 43-101, 41p. Walker, E. C. 2003. Diamond deposits of the Festival Property, Wawa, Ontario; Report prepared for Pele Mountain Resources Inc. under National Instrument 43-101, 39p. Wendland, C. 2010. Diamondiferous Mass-Flow and Traction Currenct Deposits in a Neoarchean Fan Delta, Wawa Area, Superior Province unpublished MSc thesis, Lakehead University, 102 p. Williams, F. 2002. Diamonds in late Archean calc-alkaline lamprophyres Ontario, Canada: Origins and implications; unpublished BSc thesis, University of Sydney, Sydney, Australia, 82 p. Wilson, A. C. 2004. Diamond exploration targets, Michipicoten greenstone belt; Canadian Institute of Mining Bulletin, v. 97, no. 1077, p. 41-46. Wyman, D. A., Ayer, J. A., Conceição, R. V., and Sage, R. P. in press. Mantle processes in an Archean orogen: evidence from 2.67 Ga diamond-bearing lamprophyres and xenoliths, Lithos. 108 �Field Trip 3 Geology of the Wawa gold project Jean-Francios Montreuil, Quentin Yarie, and Conrad Dix Red Pine Exploration Ltd, Toronto, Ontario 109 �1.0 Introduction The fieldtrip on the Wawa Gold project combines an overview of the exploration and mining history in the Wawa Gold Camp and of the geology of the various mineralized structures of the property. The field trip is centered on the historic Surluga Road, along which most of the historic gold mines of the Wawa Gold Camp were exploited. The Wawa Gold Project is located 2 km east of the town of Wawa in Ontario and has a long exploration history that begun in the 1860s and has been discontinuously explored and developed since. This long period of activity resulted in the exploitation of 9 gold mines with preserved records of production (Fig. 1-1 and Table 1-1; Rupert, 1997); as well as many other small-scale mining operations without production records and the digging of numerous pits and the sinking of many shafts (Abandoned Mines Information System Database, 2017). The main gold concentration defined so far on the Wawa Gold Project is the Surluga Deposit a NI 43-101 compliant inferred resource of 1,088,000 ounces @ 1.71 g/t gold (0.5 g/t gold cut-off; Ronacher et al., 2015; Table 1-2). The inferred Surluga Deposit resource is entirely hosted within a brittle-ductile shear zone named the Jubilee Shear Zone. Recently discovered gold zones not identified in the immediate vicinity of the inferred resource are not incorporated in the current resource model. Table 1-1 Historic gold mine and gold production that were active on the Wawa Gold Project Mine Tonnes Milled Gold Grade (g/t) Gold Recovered (Ounces) Mariposa Grace+Darwin Parkhill Van Sickle Cooper Jubilee Minto Surluga 8 41,302 114,096 8,372 4,435 107,930 57,335 86,082 72.99 13.27 14.81 6.34 11.42 4.29 12.56 3.12 19 17634 54298 1710 1627 Totals 419,560 9.04 120,093 Production period(s) 1902-1903 1900-1903, 1907-1910, 1934-1937 1929-1938 1933-1936 1897-1900, 1926-1939 36178 8626 1964-1969 Table 1-2 Mineral resource statement* of the Surluga Deposit effective May 26th, 2015 estimated by SRK consulting (Canada) Inc. Resource Category Cut-off Gold (g/t) Quantity (‘000 t) Grade Gold (g/t) Contained Metal Gold (‘000 oz) Inferred** Pit-Constrained 0.4 10,239 2.05 676 Outside Pit-constrained 0.4 8,630 1.07 298 Underground 2.5 955 3.73 114 Total 0.5 19,824 1.71 1,088 * Mineral resources are not mineral reserves and have not demonstrated economic viability. All figures are rounded to reflect the relative accuracy of the estimate. Composites have been capped where appropriate. ** Open pit mineral resources are reported at a cut-off grade of 0.40 g/t gold in relation with a conceptual pit shell constructed by SRK. Underground mineral resources include classified modelled blocks below the conceptual pit shell and above a cut-off grade of 2.50 g/t gold. Cut-off grades are based on a gold price of US$1,250 per ounce, a gold recovery of 95 percent and a $US:$CAD exchange rate of 0.95. 110 �Figure 1-1 - Map showing the main historic mines as well as some of the main shafts and pits of the Wawa Gold Project 2.0 Overview of the property history 2.1 Discovery period - 1897 to 1910 The Wawa area has been explored for gold since the 1860s and gold was first discovered by William Teddy's wife in 1897 at Mackay Point along the south shore of Wawa Lake (Rupert, 1997; Frey, 1987). A staking rush followed the discovery and benefited from the change in claim staking adopted by the Ontario Government to encourage staking in 1895 (MacMillan and Rupert, 1990). This early rush period resulted in multiple discoveries. Attempts to produce gold from bedrock started in 1897 with the sinking of many shafts and the digging of many test pits throughout the property. Between 1897 and 1898, gold was identified on the Jubilee Shear Zone and a 103-foot shaft was sunk by the Great Northern Mining Company Ltd. in the sericite schists 111 �characteristic of the structure west of Jubilee Lake (Sage, 1994). Gold values encountered in that shaft were described as very low and the shaft was abandoned. In 1897, the Minto Mine was discovered by S. Berailldt who sold it to D. Tisdale. A 130-foot inclined shaft was sunk and the Minto mine operated until 1900. The Hornblende Shear Zone, located 300 metres west of the Jubilee Shear Zone, was discovered in 1899 by Mr. Peter Nissen. Two inclined shafts of 22 and 32 feet were sunk in the structure and in 1900, a test mill was constructed near Hornblende Lake by the Hornblende Mining Company. In 1902 and 1903, the Mariposa Gold Company sunk the 208-foot Mariposa shaft inclined at 80°NE in the footwall of the Mariposa Vein with two drifted levels at 100 and 200 feet (Sage, 1994). Gold production in the discovery period started on a larger scale in 1900 following the discovery of the Grace vein. The Algoma Commercial Company sunk a 304-foot shaft and extracted 6,097 tons of ore between 1900 and 1903 after which, commercial gold production ceased (Sage, 1994). The Lepage Gold Mining Company resumed production in the Grace Mine between 1907 and 1910 and produced 4,260 tons of ore (Fig. 2-1). Figure 2-1 - Grace Mine in 1908 Source: http://www.michiwawa.ca/albums/wawa/goldfields/goldfield_one.htm 2.2 Peak of mining activity - 1925 - 1938 During the period between 1910 and 1925, the Wawa Gold Project went into an exploration and production hiatus, with many land transactions between different parties and brief periods of field activity (Sage, 1994). The period extending from the mid-late 1920s to the late 1930s then saw the peak of mining activity on the property, with several mines in operation. By the late 1930s, 15 mines produced gold in the Wawa area (Frey, 1987). On the Wawa Gold Project, production records exist for eight of the mines that produced during that period (Cooper, Minto, Jubilee, Parkhill, Grace+Darwin, Mariposa and Van Sickle; Figure 1-1; Tables 1-1). The Cora vein, located in the Jubilee Shear Zone, was also briefly mined in the 112 �Cora shaft in 1927. The other larger mine from that period that produced gold from the Jubilee Shear Zone was the Jubilee Mine that produced 107,930 tonnes @ 4.29 g/t gold. The largest producer of that period was the Parkhill Mine that produced 54,298 ounces of gold from 114,096 tonnes @ 14.81 g/t gold between 1929 and 1938 (Table 1-1; Figure 2-2). Figure 2-2 - Mining town of Parkhill inhabited during the operation of the Parkhill Mine Source: http://www.michiwawa.ca/albums/wawa/goldfields/goldfield_one.htm 2.3 Surluga Mine discovery and first mining operation - 1960 to 1980 Exploration and development activity substantially slowed down in the 1940s and the 1950s, but resumed in 1960 when Tom Surluga and W.D. Sutherland drilled 25 holes between 1960 and 1961 that resulted in the discovery of the Surluga Deposit (Table 5-7; Sage, 1994). In 1962, Sutherland and company formed Surluga Gold Mines Limited to continue the development of the Surluga Deposit (Table 5-7). Between 1964 and 1968, Surluga Gold Mines Limited sank a 950-foot shaft with 7 levels spaced by 150 feet that formed the Surluga mine. In 1967, Surluga Gold Mines Limited constructed a 750 ton per day mill on the property and started an extensive underground drilling program (Amalgamated reports, 41N15NE0036). The mill was in operation in 1968 and 1969 but, inadequate definition of reserves in the mine before its construction precluded its optimal operation. The mill was shut down in 1969 and underground development continued between 1969 and 1971 with the objective of defining enough reserves to operate the mill (Amalgamated reports, 41N15NE0036). In 1973, under strong debt pressure, the Surluga Gold Mine company was re-organized as Pursides Gold Mines that conducted an extensive exploration program between 1973 and 1975 (Amalgamated reports, 41N15NE0036). Pursides Gold Mines ultimately went bankrupt and was re-organized as Citadel Gold Mines Inc. ("Citadel") in 1980. 2.5 Second development of the Surluga Mine by Citadel Gold Mines - 1986 to 1991 In 1986, before additional reserves were defined and the exploration model updated, the Surluga mine was dewatered, the Surluga mine shaft refurbished, the mill reconstructed, and a 3 year program of surface and underground drilling and geological mapping started on the Surluga Deposit (Rupert, 1997). One exploration success is the discovery in 1989 of the Old Tom zone in the southernmost part of the 113 �Surluga Deposit. During that period, Citadel also undertook an extensive exploration program of its property to find additional gold to feed the newly refurbished mill (Rupert, 1989a, b; Rupert and Leroy, 1989). This included diamond drilling of Root and Cooper-Ganley vein systems, stripping, trenching, channel sampling and geological mapping, as well as many airborne and ground geophysical surveys. In 1987, Citadel purchased from Duraine, the Parkhill and Grace-Darwin Mine properties (Rupert, 1997). Development and exploration in the Surluga Deposit however stopped again in 1989 because of the mill inefficiency, lack of defined reserves to feed the mill, the un-optimized design of the mine, including the difficulties of mechanizing production and problems with dilution control because of the cryptic boundaries of the high-grade zones (E. Hoffman, pers. comm.). Figure 2-3 Surluga mine in 2002 before site reclamation in the closure plan Source: http://www.michiwawa.ca/albums/wawa/goldfields/goldfield_one.htm 2.6 Change of the exploration model for the Surluga Deposit - 1990 to 1996 In 1996, while reviewing the available data on the Surluga Deposit, Bowdidge (1996) postulated that the Jubilee Shear Zone is a large-scale structure up to 150 feet thick and contains widespread low grade mineralization; and evaluated that a substantial resource of low-grade mineralization may exists in structure. This evaluation changed the perception of the property and forms the basis of the exploration model now used by Red Pine Exploration. 114 �3.0 Geology of the Wawa Gold Project 3.1 Regional Geology The Wawa Gold Project is in the southern part of the Michipicoten greenstone belt, one of two greenstone belts that form the Wawa Subprovince of the Superior Province (Fig. 2-1). The Michipicoten greenstone belt was formed by three cycles of mafic to felsic volcanism associated with concomitant subvolcanic intrusions (Sage, 1994). Zircon U-Pb ages date volcanic Cycle 1 to 2.9 Ga, volcanic Cycle 2 to 2.75 Ga, and volcanic Cycle 3 to 2.7 Ga (Turek et al., 1992; Sage, 1994). In the southern part of the Michipicoten greenstone belt, two large intrusive complexes were emplaced during cycles 1 (Hawk Lake Granitic Complex) and 2 (Jubilee Stock). These syn-volcanic intrusions have been interpreted to delineate the centres of calderas and to be the intrusive equivalent of their host felsic to intermediate volcanic rocks (Sage, 1994). Like other greenstone belts within the Superior Province, the Michipicoten greenstone belt contains basaltic to komatiitic volcanic rocks. Throughout most of the Michipicoten Greenstone Belt, the hiatus between volcanic Cycles 2 and 3 was marked by the deposition of large banded iron formations. In the Wawa area, from their discovery in 1898 to the closure of the last iron mine of the region in 1998, these iron formations were explored and mined over a 100 year period and were at one point the largest source of iron in Canada. All the rocks of the Michipicoten greenstone belt are metamorphosed at greenschist facies and its volcano-plutonic sequences have been repeatedly deformed and folded. Post-Archean magmatism includes diabase dikes and the emplacement of the Firesand River Carbonatite intruded along the Wawa-Hawk Lake-Manitowik Lake Fault System. The Wawa Gold Project is located within the southern part of the Michipicoten greenstone belt (Sherman, 2005). A prominent structure in the southern Michipicoten greenstone belt is the Wawa-Hawk Lake-Manitowik Lake Fault System, which defines the boundary between a lamprophyre-rich domain to the south and lamprophyre-free domain to the north (Figure 2-1). The emplacement of the Firesand River Carbonatite along the Wawa-Hawk Lake-Manitowik Lake Fault System suggests that the fault is deep-seated, whereas the location of the Jubilee Stock and Hawk Granite Complex along the fault indicate that it may follow an older structure active during the formation of the greenstone belt. 115 �Figure 3-1 - Regional Geology of the Michipicoten Greenstone Belt and location of the Wawa Gold Project within the belt Geology from Ontario Geological Survey (2011) open file MRD 126; Showing and deposit locations from Ontario Geological Survey (2014) Mineral Deposit Inventory 3.2 Property Geology The Wawa Gold Project occurs in a mafic to felsic volcano-plutonic sequence formed during the second cycle of volcanism in the Michipicoten Greenstone Belt (Sage, 1994). 3.2.1 Jubilee Stock The main intrusive complex of the property is named the Jubilee Stock that represents a key geological unit of the property for gold mineralization as all the largest historic mines (Darwin-Grace, Jubilee, Minto, Surluga, Parkhill) and most of the known gold showings are located within or at the margins of the Jubilee Stock (Fig. 3-2). The compositional and geometrical complexity of the Jubilee Stock, comprising many contacts zones between rocks of different rheology, are interpreted to be important controls on the geometry and distribution of the gold zones. 116 �Figure 3-2 - Geology map of the Wawa Gold Property from Ronacher et al. (2016) 117 �The Jubilee Stock is a composite calc-alkaline to transitional intrusive complex formed by many individual intrusions compositionally ranging from gabbroic-dioritic to granitic (Figs. 3-3a, b and 4a, b). Based on U-Pb ages on zircons (Sullivan et al., 1985), the coarser-grained intrusions of the Jubilee Stock was emplaced around 2,745 ± 3 Ma, whereas one porphyritic intrusions located at the margins of the stock was dated at 2,742 ± 6 Ma (Turek et al., 1992). Within error, these ages are contemporaneous to those of its host volcanic sequence dated at 2,746 ± 11Ma and 2,744 ± 10 Ma (Turek et al., 1992). The main intrusive facies of the Jubilee Stock include: medium- to coarse-grained "diorite" (key unit historically used to define the Jubilee Stock), porphyritic intrusions with variable phenocryst assemblages (biotite, feldspar-biotite, feldspar, quartz-feldspar) generally emplaced at the margins of the intrusive complex, fine- to medium-grained mafic intrusions (gabbroic), zones of magma mixing and mingling where mafic to intermediate-felsic magmas intrude each other; and broad zones of intrusive breccia developed at the contact between the intrusive complex and its host volcanic rocks, but also along the contact zones between individual intrusions of the Jubilee Stock. Mapping by Sage et al. (1982) shows that the core of the Jubilee Stock is curved-shaped into a sigmoid form, its long axis oriented at 20°, and has an approximate 6 x 1.3 kilometres surface expression. The actual extent of the Jubilee Stock remains however ill-defined; as the porphyritic intrusions typically forming the margins of the intrusive complex are texturally similar to some of the crystal-rich volcanic rocks of the volcanic sequence and their classification as intrusive or volcanic varied according to the generations of workers on the property. Recent re-mapping by Red Pine indicates that the surface extent of the Jubilee Stock is larger than the core zone currently illustrated on the geology maps of the property. Medium-grained dioritic to granitic intrusions (Figures 3-5 and 3-6) Medium-grained to coarse-grained dioritic to granitic intrusions form the diagnostic core zone of the Jubilee Stock. Historically to simplify the nomenclature of those units, all the operators of the property described the medium- to coarse-grained intrusions of the Jubilee Stock using diorite as a generic term. However, chemically based on the Pearce (1996) diagram, the coarser grained intrusions of the Jubilee Stock vary compositionally between diorite and granite/granodiorite (Fig. 3-3a). This is supported by the petrographic work from Sage (1994) that indicates some of the intrusions have modal 10-30% quartz, 4055% plagioclase and 10-20% biotite, which indicates granitic composition. Porphyritic intrusions (Figure 3-7) Many porphyritic intrusions surround the core of Jubilee Stock and were hypothesized by Sage (1994) to occupy the ring fracture of a large caldera centered on the Jubilee Stock. In the contact zones between different intrusions, the porphyritic intrusions are often intermixed together and with intrusions of the medium- to coarse-grained diorite. The main primary phenocrysts assemblages observed in the porphyritic units are: feldspar, biotite-feldspar, quartz-feldspar and quartz. A compositional continuum and visual gradation between the medium- to coarse-grained diorite and intrusions of the feldspar-phyric, biotite-feldspar-phyric and biotite-phyric units are commonly observed, indicating the likely coeval emplacement of those units. Because of the variability in the mapping and logging of the porphyritic units, the porphyritic units of the Jubilee Stock remain undivided. The porphyritic intrusions are compositionally similar to the Jubilee Stock (Fig. 3-4). 118 �Figure 3-3 Geochemical classification diagram of the intrusions typical of the core of the Jubilee Stock. A) Zr/Ti versus Nb/Y discrimination diagram from Pearce (1996); B) Th/Yb discrimination diagram for discrimination of magmatic affinities from Ross and Bédard (2009) 119 �Figure 3-4 Geochemical classification diagram of the porphyritic intrusions observed in the Jubilee Stock. A) Zr/Ti versus Nb/Y discrimination diagram from Pearce (1996); B) Th/Yb discrimination diagram for discrimination of magmatic affinities from Ross and Bédard (2009) 120 �Figure 3-5 Medium- to coarse-grained facies of the Jubilee Stock "diorite" near the contact with the volcanic units containing enclaves of volcanic rocks Figure 3-6 - Typical Jubilee Stock "diorite" in forming the core of the intrusive complex 121 �Figure 3-7 Feldspar-quartz porphyritic intrusion exposed near the Surluga Deposit Silicified/Albitized unit (Figure 3-8) This unit corresponds to rock altered by a strong to intense sodic-silica alteration and encompasses altered diorite, volcanic units and porphyritic intrusions. This unit prevails in certain zones of the Wawa Gold Corridor and may correspond to the hornfelsed units described by Sage (1994) as occurring along some of the contacts between the Jubilee Stock and the volcanic rocks. In zones of intense alteration, the primary textures of the host rocks are generally destroyed, the unit becomes quite homogeneous and protolith identification is difficult. In the transitional zones, strong alteration fronts are seen to replace the host units. The predominant precursor unit is most likely fine-grained volcanic units intruded by the Jubilee stock in which albitization was preferentially partitioned. 122 �Figure 3-8 Albitized unit formed near the contacts between the Jubilee Stock and the volcanic units Intrusive breccias (Figures 3-9 and 3-10) Many of the contact zones between the intrusions of the Jubilee Stock, but also between different intrusive facies of the stock, are characterized by the formation of intrusive breccia zones. The breccia cement is typically composed of the coarser-grained facies granitic intrusions, whereas the fragments, predominantly of volcanic origin, are fine- to very fine-grained, vary considerably in size, ranging from a few millimetres to tens of metres, and some are partially assimilated by the dioritic magma. A report by Sage (1994) and noted by Red Pine geologists, this is making the mapping of this unit, especially in drill cores, particularly challenging. Figure 3-9 Surface exposure of the intrusive breccia formed at the contact between the Jubilee Stock medium- to coarse-grained diorite and the volcanic units 123 �Figure 3-10 Intrusive breccia texture in drill hole and melanocratic feldspar-phyric unit in the contact zone between the Jubilee Stock coarse-grained diorite and the volcanic units 3.2.2 Gabbroic rocks Tholeiitic to calc-alkaline mafic-intermediate to mafic intrusions were documented by Red Pine in the Jubilee Stock (Fig. 3-11). Visually, the mafic intrusions were described and discriminated based on the grain size of the core of the intrusion and the absence or presence of a porphyritic texture with feldspars being the main phenocrysts. The mafic intrusions observed in the Jubilee Stock include: coarse-grained gabbros pertaining to the tholeiitic suite (Fig. 3-12), fine-grained gabbros pertaining to the tholeiitic suite (Figure 3-13), and feldspar-phyric very fine-grained gabbro to gabbro-diorite generally pertaining to the calc-alkaline suite (Figure 3-14). Both the tholeiitic and the calc-alkaline mafic intrusions are deformed in the gold-bearing structures of the Wawa Gold Corridor. Walker (2011) also recognized, based on the observation of magma mixing textures between felsic and a mafic magmas in the Jubilee Stock, that some of the mafic intrusions are comagmatic with the stock. This observation is supported by the occurrence of calc-alkaline mafic intrusions in the Jubilee Stock. The intersection between some intrusions of the tholeiitic suite and the gold-bearing structure was also observed to form zones of preferential gold enrichments. In some of the mafic intrusions of the calc-alkaline suite in the Surluga Deposit, Ni-Cu mineralization can occur as disseminated cluster of pyrrhotite-chalcopyrite, the pyrrhotite likely intermingled with pentlandite (Fig. 3-14). The largest mafic intrusion on the property is centered on Reed Lake and forms the Reed Lake maficultramafic complex which is composed of diorite, quartz-gabbro, leuco- to mela-gabbro and pyroxenite (Fig. 6-2). Sage (1994) inferred that the combined trends of the long axis of 315° for the Reed Lake and 20° for the Jubilee Stock may suggest there were emplaced in a conjugate fracture system and that they are possibly petrogenetically related. 124 �Fig. 3-11 Geochemical classification diagram of the mafic and mafic/intermediate intrusions observed in the Jubilee Stock. A) Zr/Ti versus Nb/Y discrimination diagram from Pearce (1996); B) Th/Yb discrimination diagram for discrimination of magmatic affinities from Ross and Bédard (2009) 125 �Figure 3-12 Coarse-grained tholeiitic gabbroic intrusion in the Jubilee Stock Figure 3-13 Fine-grained tholeiitic gabbro dyke in the Jubilee Stock 3.2.3 Volcanic units For most of the Wawa Gold Project where large surface mapping and drilling programs have been conducted, the descriptions of the volcanic units are constantly evolving depending of the geologist, exploration model, and time period. In many cases the sub-volcanic porphyritic intrusions, part of the Jubilee Stock, and the volcanic units, are confused and their classification inter-changed. To add to the difficulties of recognizing the volcanic units, in historic logs, in adjacent drill holes, some of the goldbearing structures are variably described as sedimentary rocks, fragmental volcanic units, volcanic flows and porphyritic intrusions. 126 �3.2.4 Lamprophyre dikes Lamprophyre dikes are pervasive throughout the Wawa Gold Project and at least two generations of lamprophyre exists. One generation is late-stage and cut all the gold mineralized zones of the property. Dikes of that generation are black, porphyritic, medium-grained and strongly magnetic with a blue amphibole alteration halo. A possible set of lamprophyre is likely older. Dikes of that set are generally smaller, their primary mineralogy is partially to completely replaced, which gives them a dark- to palegreenish color. One dike of this set is also possibly gold mineralized, indicating that some of the lamprophyre dykes could have been emplaced prior to the formation of the gold system. A few carbonatite dikes, likely related to the Firesand Carbonatite located a few hundred metres east of the northeastern corner of the property, were also observed in drill holes in the Surluga Deposit. 3.3 Late brittle faulting The main brittle fault of the Wawa Gold Project is the NW-oriented and subvertical Parkhill Fault. Following Sage (1994), the Parkhill Fault is the southeastern extension of the northwest-striking Black Trout Lake Fault and is seen to truncate the Wawa Gold Corridor, which likely resurfaces to the south in the Darwin Shear Zone. The age of the Parkhill Fault remains uncertain and its intrusions by gabbroic rocks interpreted to be Archean indicate that it is possibly a long lived-structure in the area, even possibly formed during the evolution of the gold system. The interpreted late movement along the Parkhill Fault is left-lateral with an unknown vertical component. 4.0 Gold zones of the Wawa Gold Project 4.1 Attributes of the gold zones of the Wawa Gold Project Gold mineralization is obvious throughout the Wawa Gold Project and is spatially related to the numerous shear zones, fractures and replacement zones of variable strike and dip. The Surluga Deposit forms the largest gold concentration currently defined on the Wawa Gold Project. The deposit is entirely hosted in the Jubilee Shear Zone that crosscut the various intrusive rocks of the Jubilee Stock. The Jubilee Shear Zone is a brittle-ductile structure consisting of a number of parallel, ~300–900 m long en-echelon segments that strikes northeast (018–034°) and dips (25–55°) to the southeast (Figs. 4 to 6). Its width ranges from 9 m to 75 m and extends from Wawa Lake to the northwest-trending Parkhill Fault (3.2 km). Helmstaedt (1988) interpreted that the gold-mineralized quartz veins predate some of the brittle-ductile shearing in the Jubilee Shear Zone and that the geometry of the high-grade zone of the deposit is controlled by a strong stretching lineation in the shear zone. Syn-deformation folding in the structure may also increase the thickness of the mineralized zone as parasitic folds, generally present in the thickest mineralized zones of the structure (Figure 6-19). Red Pine Exploration demonstrated that the Jubilee Shear Zone is part of a larger deformation corridor that includes many other gold-bearing structures and zones of gold mineralization. This larger deformation corridor, in which tectonic foliations oriented NNE to NE (20°–45°) and dipping between 30° and 80° and stretching lineation trending 160°-190° and plunging 20°-35° are systematically observed (Fig. 7), was named the Wawa Gold Corridor. In addition to the Jubilee Shear Zone, the main goldbearing structures of the Wawa Gold Corridor includes: the Darwin Shear Zone (interpreted southern extension Jubilee Shear Zone south of Parkhill Fault), the Grace Deformation Zone, the Hornblende shear zones (Hornblende Upper and Lower), the Parkhill Mine Shear Zone, the Minto shear zones (Minto A, B 127 �and C), the Surluga Road Shear Zone, and the William gold zones. From north to south, the Wawa Gold Corridor is continuously defined between Wawa Lake and the Darwin-Grace Mine. Laterally, the Wawa Gold Corridor is defined between the Hornblende Shear Zone and the Minto B Shear Zone north of the Parkhill Fault, and between the Darwin Shear Zone and the Darwin-Grace Mine, south of the Parkhill Fault (Figs. 1, 3, 8-10). Throughout the Wawa Gold Corridor, gold mineralization predominantly occurred in brittle-ductile shear zones, but also occurs in micro-breccias formed into albitized zones of the Jubilee Stock intrusions, and into network of quartz tension veins. Although the main orientation of the tectonic fabrics defining the Wawa Gold Corridor and some of its main gold zones is NNE (Hornblende, Jubilee, Minto B, Darwin), additional orientations exists for the corridor gold-bearing shear zones. This includes ENE-oriented shear zones (Minto C, Parkhill Mine Shear Zone, Mickelson Shear Zone, Root Vein structure) and WNW-oriented shear zones (Grace Deformation Zone, Minto A shear Zone). Domains of L-tectonite controlled by the stretching lineation trending 160°190° and plunging 20°-35° without strong planar fabric development are also relatively common. Figure 4 - Cora Shaft into the Jubilee Shear Zone 128 �Figure 5 - Brittle-ductile shearing and tight parasitic folding of auriferous quartz veins in the Jubilee Shear Zone Figure 6 - Zone of brittle deformation in the Jubilee Shear Zone 129 �Figure 7 - Characteristic stretching lineation of the Wawa Gold Corridor preferentially partitioned in a mafic dike (William Gold Zone) Figure 8 - Brittle-ductile deformation in the Minto B Shear Zone 130 �Figure 9 - Intersection of the Minto A Shear Zone, related to the Minto and Parkhill mines, and the Jubilee Shear Zone 131 �Figure 10 - Gold mineralization in the Grace Deformation Zone related to the historic Darwin-Grace Mine 132 �Figure 6-19 - Folded high-grade quartz shear vein in the Mickelson Shear Zone Channel sample contains 69.5 g/t gold over 0.7m In most of the gold zones on the property, key visual indicators of gold mineralization include the presence of sulfides, potassic alteration (sericitic or biotitic) and development of quartz veins with accessory sulfides (Fig. 11). The main style of gold mineralization in the Wawa Gold Corridor, in both the shear zones and the replacement zones, principally occurs as free gold associated with pyrite, accessory to minor pyrrhotite and minor to absent chalcopyrite deposited with silicification and sericitic alteration, and with or without quartz veining. Moderate to strong tourmalinization also occurred in some of those gold zones. The modal content of sulfides is generally around 0.2-3%, and locally up to 5-10% in the higher grade zones, which are almost systematically associated with quartz veins. Another style of gold mineralization is characterized by free gold associated with abundant arsenopyrite with accessory to minor pyrite and/or pyrrhotite deposited with chlorite+biotite alteration, weak to moderate-strong sericite alteration and with or without quartz veining. Arsenopyrite in those gold zones can occur in modal content exceeding 3% (Grace Deformation Zone, Minto C shear zone). In the Jubilee Shear, Hornblende and Minto A+B shear zones, arsenopyrite-bearing or pyrite-bearing gold zones occur in close spatial association. Many zones with a transitional metal signature between the As-rich and other metal assemblages also occur in those structures (Fig. 12). In the transitional zones, pyrite is the main sulfide, but these zones exhibit a variable arsenic content between 100-1000 ppm. In the Grace and Minto C shear zones, arsenopyrite-rich gold mineralization predominates and is associated with abundant quartz veining in the higher grade zones of the structure (Fig. 10). In the Minto C shear zone, quartz veining is also relatively minor to absent; and does not seem to be the main control on gold concentration as highgrade gold in the structure is not spatially associated with quartz veining like in the Grace Deformation Zone (Fig. 13). 133 �Figure 11 - Grey quartz vein with pyrite typical of the higher-grade zones of the pyritic gold zones of the Surluga Deposit Drilling intersection contains 18.62 g/t gold over 0.48 m A rarely observed sulfide paragenesis formed of pyrite with accessory sphalerite and galena deposited in quartz veins also occur in the Wawa Gold Corridor, and was so far only observed in the Surluga Deposit. However chemically Pb and Zn enrichments are characteristic of many of the property gold zones (Fig. 12). The main network of gold-bearing tension veins recognized by Red Pine in the Wawa Gold Corridor is preferentially formed in the medium- to coarse-grained diorite and is not associated with obvious shearing (Fig. 13). Individual tension veins of the network are moderately to steeply dipping (50-80°) to the WNW to NNE and typically contain accessory tourmaline, accessory pyrite and pyrrhotite, minor chalcopyrite and locally visible gold. The variable development of tectonic fabrics in the tension veins of the property indicate a pre- to syn-tectonic formation of the gold mineralized tension veins. 134 �Figure 12 - PC1 vs PC2 biplot of log-centered transformed concentration of metals to illustrate the diversified metal assemblages observed in the gold zones of the Wawa Gold Corridor Analyses from 326 drill core samples with Au > 1 g/t; because of the large number of analyses below detection, Ag, Se and Te were not used for the PCA. Figure 12 - High-grade core of the Minto C Shear Zone formed after a strongly sericitized and silicified quartz-feldspar porphyry Grab samples from this arsenopyrite-rich shear zone without ubiquitous quartz veins contains 17 g/t and 5.51 g/t gold 135 �Figure 13 - Quartz tension vein with visible gold in the footwall of the Surluga Deposit Drilling intersection contains 53.2 g/t gold over 1 m The William gold zones form a style of gold mineralization that contrasts with the shear-hosted gold zones of the Wawa Gold Corridor and were first identified during the fall 2015 drill program. In the William gold zones, tectonic fabrics (planar and linear) are absent to locally strong, hydrothermal alteration spatially related to gold deposition is weak to strong and mineralization is associated with pervasive micro-brecciation of the albitized host rock (Fig. 13). The main geometrical controls hypothesized for the William gold zone mineralized zone are: weak ENE-striking and shallowly dipping (20°-35°) shearing and the stretching lineation of the Wawa Gold Corridor that shallowly plunges to the ESE. An elevated arsenic background is diagnostic of these zones and likely relates to the finely disseminated pyrite interpreted to be related to gold mineralization. Because of the weak visual indicators associated with William-like mineralized zones, they were generally missed by all previous operators of the property. Figure 13 - William-like mineralization in the Jubilee Shear Zone hangingwall Drilling intersection contains 1.86 g/t gold over 1.15 metre Three main controls on the spatial distribution and geometry of the gold zones were defined in the Wawa Gold Corridor and include: • • • rheological contrasts between the various intrusive phases of the Jubilee Stock to partition and focus deformation and fluid circulation (Fig. 14); chemical contrast between units of the Jubilee Stock and more specifically the contact zones between gabbroic intrusions and the other intrusion types of the Jubilee Stock to partition and focus deformation and fluid circulation, and also to act as a chemical trap for gold; and the stretching lineation characteristic of the shear zones of the property. 136 �Figure 14 - Strain partitioning in the Minto B shear zone where shearing is preferentially partitioned in the mafic rocks whereas the dioritic domain remains weakly deformed 4.2 Hydrothermal alteration associated with gold mineralization Many mineralogical changes are associated with the formation of the gold-bearing shear zones. In mafic rocks, progressive chloritization and carbonatation (calcite and ankerite) with subsidiary tourmalinization and mariposite formation indicate increasing proximity to the gold-bearing shear zones is (Fig. 2). In the intermediate to felsic precursors, the mineralogical changes are progressive sericitization, silicification and carbonatation. In intermediate to mafic rocks, magnetite is also deposited in the shoulders of the goldbearing structures and can form haloes extending 25-30 metres away from the gold-bearing structure. In the Wawa Gold Corridor the hangingwall of the Jubilee Shear Zone the biotite alteration forms a distal halo extending tens of metres away from the structure that gets progressively chloritized with increasing proximity to the shear zone. Barren sericitization in sheared intermediate to felsic precursor rocks and barren chloritization and carbonatation in sheared gabbroic rocks also occurs in the gold-bearing structures of the property. A pervasive hydrothermal imprint related to the emplacement of swarms of lamprophyre dykes is present throughout the property. Mineralogically it leads to the formation of riebeckite in the contact zones of the lamprophyre dykes as well as networks of iron carbonate (pos. siderite) veins with K-feldspar selvages. 137 �5. Field Trip Stops Figure 5-1 Stops of the fieldtrip on the Wawa Gold Project Stop 1: Cora Shaft and Jubilee Shear Zone Coordinates: UTM 16 NAD83 667930mE, 5316243mN This stop provides an overview of the early mining history in the Jubilee Shear Zone, current host of the inferred resource estimated for the Wawa Gold Project, and what is now the Surluga Deposit. This stop is located just south of the small peninsula in Jubilee Lake where the historic Jubilee Mine was operated 138 �between 1926 and 1938 (Fig. 5-1), and where the Cora Shaft was sunk on a quartz vein in the Jubilee Shear Zone in 1927. The Cora Shaft outcrop provides a good overview of the structural complexity of the Jubilee Shear Zone, of the geometry of some of the parasitic folds in the shear zone, and on the higher grade quartz veins formed in the shear zone. Individual grab samples from the Cora Shaft quartz vein contain up to 50.8 g/t gold. Figure 5-1 Jubilee Mine during operation in the 1930s Source: http://www.michiwawa.ca/albums/wawa/goldfields/goldfield_one.htm 139 �Figure 4 Exposure of the Jubilee Shear Zone at the Cora Shaft Stop 2: Jubilee Shear Zone Southern Extension Coordinates: UTM 16 NAD83 667723 mE, 5315771 mN This stop illustrates the control of the zones rheological contrasts between different intrusions of the Jubilee Stock has on strain partitioning, fluid circulation and gold concentration in the Jubilee Shear Zone. The Jubilee Shear Zone on the outcrop traverses a contact zone between a mafic dyke and the Jubilee Stock "diorite". Brittle-ductile deformation is strongly partitioned into the mafic rocks in the shear zone, whereas quartz veining and gold concentration preferentially occur along the contact zones between the deformed mafic rocks and the "diorite". Channel sample from the contact zone contains 6.74 g/t gold over 1 metre. 140 �Figure 5 Exposure of the southern extension of the Jubilee Shear Zone Stops 3 to 7 - Wawa Gold Corridor transect This series of stops illustrates some of the main gold zones and gold mineralization types of the Wawa Gold Corridor in the northernmost extension of the Surluga Deposit. The transects begins in the Jubilee Shear Zone and also covers some the various intrusive facies of the Wawa Gold Corridor. Stop 3: Jubilee Shear Zone Northern Extension Coordinates: UTM 16 NAD83 668273 mE, 5317339 mN This stop provides an overview of the Jubilee Shear Zone in the northernmost extension of the Surluga Deposit. The trench also exposes the brittle to brittle-ductile deformation regime that were active in the Jubilee Shear Zone. Stop 4: Tholeiitic Gabbro and Jubilee Stock "diorite" Coordinates: UTM 16 NAD83 668231 mE, 5317362 mN This stop shows at surface, one of the tholeiitic gabbro dyke on the property. The intersection between that gabbro dyke and the Jubilee Shear Zone spatially correspond to one of the main high-grade zones defined in the Surluga Deposit. Gabbro dykes of the tholeiitic series are considered as having a critical role on the spatial distribution of the higher-grade zones in the gold-bearing structures of the property. 141 �Figure 6 Contact between the tholeiitic gabbro and an intermediate intrusion of the Jubilee Stock Stop 5: Surluga Road Shear Zone Coordinates: UTM 16 NAD83 668130 mE, 5317400 mN This stop shows one of the shear zone, parallel to the Jubilee Shear Zone, discovered by Red Pine in the Wawa Gold Corridor. Similar to the other structures on the property, the intersection between the structure and mafic/diorite contacts forms a preferential zone for gold enrichment and quartz veining. The shear is adjacent to the Surluga Road, but was not tested for gold before the exploration program of Red Pine during the summer of 2016. Channel sampling on the exposed outcrop indicate that the shear zone contains 1.59 g/t gold over 7 metres. 142 �Figure 7 Exposure of the Surluga Road Shear Zone next to the Surluga Road Stop 6: Hornblende Shear Zone Coordinates: UTM 16 NAD83 668054 mE, 5317474 mN The Hornblende Shear Zone is one of the large gold-bearing structures in the footwall of the Surluga Deposit and two contrasting styles of mineralization are visible on the exposure. The northern part of the exposure is characterized by a high-strain domain with low quartz vein density and pervasive dissemination of pyrite. The southern domain is characterized by weak strain intensity, more abundant quartz veining. Drilling in the northern domain hole HS-15-28 contains 1.25 g/t gold over 15.35 metres, whereas hole HS-15-27, drilled in the southern domain, contains 3.11 g/t gold over 8.1 metres. 143 �Figure 8 Hornblende Shear Zone exposure Stop 7: William Gold Zone Coordinates: UTM 16 NAD83 668093 mE, 5317233 mN This stop illustrates the cryptic style of gold mineralization associated with the William gold zones. The controls on the spatial distribution of the gold on the outcrop, and within the William gold zones in general, remain to be clearly established. Tectonic fabrics of the Wawa Gold Corridor are moderately to weakly developed on the outcrop and well-evident in the mafic dykes in which a strong stretching lineation is developed, but the relation between the tectonic fabric and gold distribution could not be unequivocally established. Stop 8: Root Vein Coordinates: UTM 16 NAD83 668775 mE, 5318459 mN This stop illustrates a gold-bearing quartz vein formed in a ENE-oriented shear zone. The overall style of gold mineralization characteristic of the Root Vein is similar to the structures mined in the Parkhill and Van Sickle mines. The outcrop also exposes at surface an example of a lamprophyre dyke that is part of the lamprophyre dyke swarm of the Wawa Gold Property; and provides good exposures of the porphyritic and coarser-grained intrusive facies of the Jubilee Stock, and of the tension veins formed in the Wawa Gold Corridor. 144 �Figure 9 Core of the Root vein Length of channel sample is 1.5 metres Stop 9 Darwin-Grace Mine site and exposure of Grace Deformation Zone UTM Coordinates: UTM 16 NAD83 668077 mE, 5313382 mN This stop illustrates the site of the historic Darwin-Grace mine and an exposure of the Grace Deformation Zone. The exposure of the Grace Deformation Zone illustrates the strong brittle-ductile deformation and arsenopyrite flooding with quartz veining characteristic of that structure. Grab samples collected from the Grace Deformation on the outcrop contain between 1.27 g/t and 18.4 g/t gold. Stop 10 Gold zones of the Wawa Gold Corridor (Red Pine Exploration core shack) This stop, at the Red Pine core shack, will present a suite of drill core intersections to illustrate the main gold zones discovered so far in the Wawa Gold Corridor. This will include drill intersections from: the Jubilee Shear Zone, the Hornblende Shear Zone, the William gold zones, the Minto shear zones and the Grace Deformation Zone, and some of the gold-bearing tension veins. 6. References Amalgamated Reports, Ontario: Ontario Ministry of Northern Development and Mines Assessment Report No. 41N15NE00036, 103 p. Bowdidge, C., 1996, Mineralization in the Jubilee Shear Zone - Re-appraisal as a large-tonnage, lowgrade bulk-mineable underground resource. Goldbrook Exploration Inc., 15 p. 145 �Frey, E.D., 1987, Geology of Wawa area gold mineralization: Institute of Lake Superior Geology Field Trip Guidebook, v. 33, Part 2, 31 p. Helmstaedt, H., 1988, Structural observations in the Surluga and Jubilee mines, Citadel Gold Mines Inc., Wawa, Ontario: Report for Citadel Gold Mines Inc., 29 p. MacMillan, D. and Rupert, R.J., 1990, Exploration Report -- Geological Mapping in the Vicinity of the Grace- Darwin, Parkhill and Minto Mines: Report for Citadel Gold Mines Inc., 61 p. Ontario Geological Survey 2011. 1:250 000 scale bedrock geology of Ontario; Ontario Geological Survey, Miscellaneous Release–Data 126 - Revision 1. Ontario Geological Survey 2014. Mineral Deposit Inventory—2014; Ontario Geological Survey. Pearce, J.A., 1996, A user guide to basalt discrimination diagrams, in Wyman, D.A., ed., Trace element geochemistry of volcanic rocks: Application for massive sulfide exploration: Geological Association of Canada, Short Course Note 12, p. 79–113. Ross, P.-S. and Bédard, J.H., 2009, Magmatic affinity of modern and ancient subalkaline volcanic rocks determined from trace element discriminant diagrams. Canadian Journal of Earth Sciences, v. 46, p. 823−839. Rupert, R.J., 1989a, Citadel Gold Mines Inc. Report on Magnetometer Survey Block B West of Firesand River, McMurray Township, Ontario: Ontario Ministry of Northern Development and Mines Assessment Report No. 41N15NE0023, 20 p. Rupert, R.J., 1989b, Citadel Gold Mines Inc. Report on Magnetometer Survey South Part of Block A at Deep Lake: Ontario Ministry of Northern Development and Mines Assessment Report No. 41N15NE0021, 14 p. Rupert, R.J., 1997, Exploration report on the Wawa area properties of Citadel Gold Mines Inc., Report for Citadel Gold Mines Inc., 51. Rupert, R.J. and Leroy, A., 1989, Citadel Gold Mines Inc., Technical Reports OMEP Project No. OM887- C-254: Ontario Ministry of Northern Development and Mines Assessment Report No. 42C02SE0220, 465 p. Sage, R.P., 1994, Geology of the Michipicoten greenstone belt: Ontario Geological Survey Open File Report 5888, 592 p. Sage, R.P., Sawitsky, E., Turner, J., Leeselleur, P., and Sagle, E., 1982, Precambrian Geology of McMurray Township, Wawa Area, Algoma District. Ontario Geological Survey, Preliminary Map Scale 1:15 840, P. 2441. Sherman, B., 2005, Illustrated Information to Accompany an Independent Assessment of the Mineral Exploration Potential of the Surluga Property of Citadel Gold Mines Inc., at Wawa, Ontario: Report for Citadel Gold Mines Inc., 48 p. 146 �Sullivan, R. W., Sage, R. P., and Card, K. D. 1985. U-Pb zircon age of the Jubilee stock in the Michipicoten greenstone belt near Wawa, Ontario. In Current research, part B. Geological Survey of Canada, Paper 85-1B, pp. 361-365. The Wawa History Album, 2009, http://www.michiwawa.ca/albums/wawa/goldfields/goldfield_one.htm, website consulted March 27-28th, 2017 Turek, A., Sage, R.P., and Van Schmus, W.R., 1992, Advances in the U-Pb zircon geochronology of the Michipicoten greenstone belt, Superior Province, Ontario. Canadian Journal of Earth Sciences, v. 29, p. 1154-1165. Walker, E.C., 2011, 2011 Prospecting, Geology, and Sampling Program: Re-Appraisal of an Old Gold Camp. Ministry of Northern Development and Mines, Assessment Report N. 20010231, 65 p. 147 �Field Trip 4 Geology of the Island Gold Mine Doug MacMillan, Simon Comtois-Urbain, Harold Tracanelli Richmont Mines Inc.- Island Gold Exploration Department Dubreuilville, Ontario 148 �Introduction This field trip will provide participants with an overview of the local property geology of the Island Gold Mine which is situated 83 kilometers north-west of Wawa. The trip will start at the Island Gold Mine coreshack where core will be displayed from ore intercepts within the mine horizon across a 2 kilometer wide strike length and then the visitation of outcrop exposures along a 4 kilometer segment of the immediate mine stratigraphy. Elements of the Goudreau Lake Deformation Zone brittle-ductile deformation which include shearing, folding and veining will be seen through many of the exposures in the area including the gabbro sill hosting the Kremzar Deposit, highly Z folded Michipicoten Iron formation at the Morisson No. 1, the exposed south east extension of the Island Gold mine structure and within the Webb Lake Sill and Stock which hosts the Magino Mine and Argonaut Gold open pit resource. The Goudreau-Lochalsh area is a historic gold camp with the first gold discovery in 1918 by D. J. McCarthy and Mr. W. J. Webb within the Webb Lake Stock and later in 1926 by Patrice Kremzar on what is now the Richmont Island Gold Mine Property. In the 1930’s the Cline Mine was in production 8 kilometers east of the Island Gold mine and eventually produced 63,000 ounces of gold. Regional Geology The Island Gold deposit located in the Wawa Sub-Province of the Superior Province of the Canadian Shield. The Wawa Subprovince is composed of numerous east-west trending sublinear supracrustal belts of greenstone enclosed by larger regions of granitoid rocks. Greenstone belts are dominated by metavolcanic and metasedimentary rocks ranging in age from 2.9 to 2.7 Ga (Percival, 2012). The Island Gold Deposit occurs on the north central flank of the Michipicoten greenstone belt which has produced over 3 million ounces of gold since the first `gold rush’ in Wawa in the late 1890’s. The greenstone belt has dimensions of 145 km by 45 km in dimensions and lies 150 km south east of the prolific Hemlo Mining Camp. It has been subdivided into three cycles of bimodal volcanism aged 2.9, 2.75 and 2.7 Ga respectively, referred to as the Hawk, Wawa and Catfish assemblages (Turek, 1994, Sage 1994). Volcanism in the lower and upper cycles are interpreted by Sage (1994) to have been Hawaiian and Plinian types respectively. Each of the three cycles is capped by pyrite bearing iron formation with an unconformity separating the volcanic rocks from the overlying iron formation in cycles 2 and 3. The Dore metasediments, composed of wackes and conglomerates, unconformably overlie the 2.7 Ga cycle and are the youngest rocks in the belt. The metamorphic grade varies from upper greenschist to lower amphibolite at the margins of the belt in proximity to the external granitiod rocks. The belt is interpreted to consist originally of a monoclonal sequence that was subsequently underwent complex thrusting, folding and shearing (Sage 1994). Arias and Helmstaedt (1990) interpret the present configuration of the belt as a regional nappe fold refolded about an F2 axis and then imbricately thrusted into numerous slices. 149 �Figure 1: Michipicoten Greenstone Belt (MGB) Figure 2: MGB Geology and Structure 150 �The main structural control on most Au occurrences in the Goudreau-Lochalsh area in which the Island Gold deposit is situated is the Goudreau Lake Deformation Zone (GLDZ). This comprises a 30 kilometer long by 4.5 kilometer wide deformational structure described by Heather and Arias (1992) as a dextral oblique slip structure occurring near the interface between the Wawa and Catfish volcanic cycles. The deformation zone has a gentle sigmoidal shape and is not a focused single structure but a composite of smaller scale structures with development of deformation being more controlled by hosting geological units such as formational contacts where rock competency contrasts exist and less by external regional stress. (Dube B., and Gosselin, P., 2007). The Cradle Lake Deformation is situated south of the GLDZ on the south arm of the Goudreau Lake Anticline near the interface of cycle 2 and 3 volcanic assemblages. The deposits of the Goudreau Lochalsh camp have a strong spatial correlation with a variety of felsic intrusive rocks including trondhjemite stocks and sills, felsic porphyries and dyke complexes which is common to many Superior Province gold deposits (Hodgson and MacGeehan, 1982). Exceptions occur including the Kremzar Deposit (47,000 oz. 1988-90) which is hosted by a regional gabbroic sill. The Island Gold deposit is situated within the cycle 2 volcanic assemblage however there is close spatial association with both mafic, felsic dykes and massive felsic volcanic rocks within the mineralized gold bearing corridor. Competency contrasts and rheologic variations between the more and less competent units provide planes of weakness along which shearing can develop, structural traps can open and gold bearing veins can subsequently be formed. The iron rich chemistry of proximal mafic dykes are also considered to assist in gold formation by providing a chemical trap in the Island Gold system. Other factors include the recognition of an antiform on the north limb of the Goudreau Lake Anticline in which the Island Gold mine sequence appears to be situated in adding to structural complexity of parasitic folding and flexural slippage and associated shearing and thrusting along fold limbs. Figure 3: Goudreau Lake Deformation Zone (GLDZ) 151 �Figure 4: Island Gold Stop Locations STOP 1: The Island Gold Mine Coreshack Location: UTM NAD83 16U 690538mE, 5353660mN Description: Since the beginning of production in 2007 at the Island Gold Mine Richmont Mines has produced 2.1 MT of ore at 6.46 g/t totaling 433,000 ounces of gold The Island Gold Mine is an Archean Lode gold deposit and consists of seven main narrow sub-parallel east-northeast striking, steeply south dipping, stacked quartz vein systems carrying gold mineralization within envelopes of intense sericitization, silicification and carbonatization with 2% to 5% pyrite which can be up to 8 meters wide. (Adam et al., 2015). The mineralized zones are sub parallel and can anastomize thereby merging or splitting along strike. Main veins styles include laminated veins with a defined ribbon appearance (Fig.7), sheeted veinlet zones composed of millimeter to centimeter scale quartz carbonate stringers, massive smokey greyish veins (Fig.5) and several types of milky white extensional veins which can appear a blow outs, gashes or horizontal ladder veins. Quartz veins at depth can be over four meters wide (Fig.7) The mineralized corridor increases from 50 meters wide above the 400L to over 100 meters at depth. Gold mineralization discovered at depth now constitute the principal source of ore and represent the down dip continuation of the mineralized zones which were mined before 2016. As of December 31, 2016, total Proven and Probable Reserves at the Island Gold Mine were 2.5 MT at a grade of 9.17 g/T for 752,200 gold ounces. The estimated Measured and Indicated Resources totaled 0.47 MT at a grade of 5.94 g/T for 91,450 ounces. Estimated Inferred Resources totaled 3.0 MT at a grade of 10.18 g/T for 995,700 ounces. 152 �Diamond drill core will be displayed from intercepts across approximately 2 kilometers of the mine horizon which will include intercepts of mineralization, quartz veining and alteration together with accompanying longsection and cross sections for location and geology. Core representatives of footwall and hangingwall rock will be presented as well as representative rocks types within the sequence. Figure 5: C zone quartz vein Figure 6: GD-14-01C, 19.9 g/t over 3.9 meters true width, -1250 meters 153 �Figure 7: Isometric Diagram Island Gold Mine Looking North East Figure 8: 800 Level quartz veins on 4 meter wide face (42.25g/t – 325.49 g/t uncut) 154 �STOP 2: Heritage Outcrop Location: NAD83 16U 690478mE, 5353601mN Description: The Heritage outcrop is situated in the younger northerly Catfish Volcanic Assemblage (2.9 Ga) which is primarily mafic in character facing north, dipping steeply north and striking 060°-070°. This outcrop is the discovery outcrop of the Kremzar Mine which was in production by Canamax between 1988-1990 and produced a total of 47,000 ounces at a grade of ?. The Kremzar deposit is hosted by a regional gabbroic sill which extends across the property to the Maskinonge Lake fault to the east. Gold was first discovered on the property in 1926 by Patrice Kremzar in a piece of float on the old Pic road. The property contains over 17 gold showings of varying economic potential. The Kremzar orebody occurs within sheared and altered gabbroic intrusive rocks. Kwok (1986) describes the alteration as an outer chlorite-biotite-carbonate zone and an inner zone dominated by sericitebiotite-potassium feldspar-Fe-carbonate. The mineralized sheared areas of rock weather a rusty brown. The main Kremzar mineralization is comprised of two sheared hosted quartz vein systems. The main mineralized zone called the R zone is situated at the west of the exposure and occurs within a 120-140° striking shear zone (dextral movement), dips steeply southwest and plunges 70° north east. A secondary B zone is situated to the north west of the exposure. Outside of the main mineralized shear zone containing the R and B zones the gabbroic rocks have undergone a complex system of brittle fracturing with fracture controlled Fe carbonate alteration and quartz filled fractures dominate the veining with the primary orientation at 30° (sinistral movement) and a secondary orientation of 120-140°, both orientations may contain anomalous gold values. Sharp boundaries exist between the altered sheared rock corridor and the surrounding unaltered gabbro which bound them. Figure 9: Heritage Outcrop R zone 155 �Figure 10: Heritage Outcrop Map A – intensely sheared and altered gabbro B – Undeformed gabbro C – clot size coarse grained amphibole and plagioclase crystals D – Diabase 160° azimuth E- Outer alteration zone, cl-bt-cb F – Inner alteration zone se-bt-Kspar-fe cb G – sharp boundary between alteration and relatively unaltered rocks H – 120°-140° striking shear zone I – R zone vein J – R zone K – B zone L – R and B zone appear to converge M – brittle fracturing N – myriad of narrow quartz filled fracture fillings O – strong fe carbonate P – brittle fractures strongly disrupted in ductile shear 156 �STOP 3: Morrison No.1 Iron Formation Location: NAD83 16U UTM 690775mE, 5352598mN Description: This stop is part of the Goudreau Iron Range, a Michipicoten type iron formation, which occurs at the interface between Cycle 2 (Wawa Assemblage, 2.75 Ga) and Cycle 3 (Catfish Assemblage, 2.9 Ga) volcanics. The iron formation is in sharp conformable contact with the southerly crystal tuffs of the Wawa Volcanic assemblage which it over lies. The Goudreau iron formation is a thickly bedded exhalative chemical sediment which can contain a minimum of five facies (Sage 1994) including a lower siderite or calcite unit, pyrite unit, chert-magnetite unit, chert-wacke unit and an uppermost argillitepyrite-graphite. At the eastern outcrop of the Morrison No.1 a stratigraphic sequence of siderite bearing ironstones (10m) overlain by massive pyrite (3m) in turn overlain by chert-pyrite-magnetite beds (20m) at the top (north) can been seen. Oxygen isotope data indicates iron formation deposition occurred under 200° C (LeSeeleur, 1980) suggesting a distal low temperature regime of formation and that depositing fluids were too low to transport base metals (Edmond et al, 1974). Randomly oriented chloritoid porphyroblasts within the iron formation are prominent and range up to 1 cm in diameter occurring pyroclastic fragments as well as the footwall felsic volcanic rocks to the south. Chlortiod alteration is a common alteration mineral in the Wawa iron ranges. Chloritoid bearing pyroclastic fragments within the overlying the Morrison iron formation are present and common in outcrop. Lockwood (1983,1986)) determined chloritoid-bearing rocks where a stratiform alteration pattern resulted from unfocused fluid discharge and overall low water/rock ratios under low pressure in subaqueous to subaerile conditions. Sage suggests that regionally the great strike extent of chloritoid bearing units indicates a hydrothermal system controlled by rock permeability along a broad aquifer. Shears zones are present throughout the outcrop as narrow discrete zones displaying dextral movement. S and Z symmetry folds are also present in the Morrison No.1 exposure. Z folds may be interpreted as dextral shear folds and the S folds possibly suggesting parasitic folding related to a possible antiformal structure in the area. Minor folds in the iron formation are present and plunge shallowly to the west. Other exposure features of note include boudinaged mafic dykes which `float’ within less competent siderite ironstones but which appear to be one original mafic dyke unit which has been tightly Z folded. The Morrison No.1 has as well been highly Z folded in the immediate area as depicted by geophysical interpretation. Regionally along strike however the iron formation can be highly transposed or distended. The gold mineralization is fracture controlled and quartz filled fractures can be seen developing at the west end of the out crop area. A historic non-compliant 43-101 resource for the Morrison #1 is about 540,000 tons @ .05 g/t gold (T. Foster, 1982). Figure 11: Chloritoid in Mafic Dykes Figure 12: Sulphide Facies Iron Formation 157 �Figure 13: Morrison No.1 West Zone A – footwall volcanic rocks B – abundant chloritoid C – boudinaged mafic dyke in siderite ironstone D – py bearing ironstone + pervasive Fe-carbonate E – strongly siderized tuffaceous rocks F – discrete shears and dextral displacement G – Z symmetry folds H – S symmetry folds I – crenulation cleavage suggesting oblique north side up dextral displacement J – siderite overlain by pyrite overlain by chert-pyrite-magnetite at the top K – narrow quartz veinlets crosscut iron formation 158 �Stop 4: South East Mine Structure Stripped Area Location: NAD83 16U 692318mE, 5352439mN Description: This stripped area is situated at the south eastern end of Goudreau Lake. It represents the only exposed segment of the Island Gold mine structure to date as the producing mine is presently situated beneath Goudreau Lake. The stripped exposure consists of a sequence of fragmental volcanic rock units succeeded to the south by a highly sheared, friable zone of sericitized felsic rocks in turn succeeded by massive felsic volcanic rock units. Local mafic dykes can occur throughout the sequence. Quartz veining is present locally and can be up to a meters in width. The shear has been correlated with the eastern strike extension of the Island Gold mine structure. At this location the structure is dipping steeply north and is striking approximately 70°. Felsic volcanic rocks in the southern part of the exposure area are a generally massive, fine grained, white weathered type and can contain clear colorless rounded quartz eyes up to 5%. The felsic volcanic rocks within the mine sequence to the west have a rock chemistry that straddles the border of the Rhyodacite-Dacite and Andesite field in the Winchester and Floyd (1977) diagram (T.Ciufo, 2017). The felsic rocks place in the dacite field of a TAS diagram (T.Ciufo, 2017). The fragmental volcanic rock unit in the north part of the stripped exposure is generally oligomictic, matrix supported and not well sorted. Variable amounts of whitish lensoid shaped lapilli fragments range in size from 4mm to several centimeters with compositions from felsic to intermediate with rare mafic clasts on occasion. Locally larger subangular to angular fragments up to 20 cm are present. The chloritoid alteration can commonly be seen within the fragments and matrix. Chloritoid bearing fragments can locally been seen suspended in chloritic units of either sediment or dyke rocks. Rock competency contrast is suggested to have created planes of weakness and therefore promoted shear propagation along these boundaries as one part of the mechanism in the gold forming process at Island Gold. In the exposure this seems to be demonstrated as a highly sheared felsic rock zone occurs between a southerly massive felsic volcanic unit to the south and a `less competent’ volcanic breccia unit to the north. Figure 14: Island Gold Mine Structure SE of Goudreau Lake 159 �Stop 5: Webb Lake Sill (optional) Location: NAD83 16U 689841mE, 5351605mN Description: The Webb Lake Stock is a trondhjemite intrusion situated 250 meters north of the Island Gold Mine structure. The stock has been mapped by Sage (1994) as being two rock masses composed of a southerly and northerly lobe which are divided by an intervening unit of a fine grained mafic intrusive or possibly metavolcanic. The main body of the stock extends for a strike length of over 2 km but continues into the Richmont Property to the east with a more sill like geometry traceable for 5 kilometers. The Webb Lake Stock dips 60-70° north and strikes approximately 70°. The intrusive is generally fine grained, hypidiomorphic granular with 65% plagioclase and 35% quartz with minor carbonate, chlorite and tourmaline. Sericite is common and potassium feldspar is rare. The outcrop exposure exhibits Z folds which display shallow 240° oriented fold hinges and north dipping axial planes. This structure is interpreted (K. Jellicoe, 2017) to have formed during D2 deformation during a NW-SE south-up dextral transpression. A late extensional tourmaline vein is present on the outcrop which cross cuts a previously developed D1 foliation as floods the planes of foliation. Figure 15: Z fold on south flank of Webb Lake Sill – Vent Raise Outcrop 160 �STOP 6: Webb Lake Trondhjemite Stock (Optional) Location: NAD83 U16 689416mE, 5351240mN Description: This stop is on the Webb Lake stock and hosting the gold mineralization at the Magino deposit. Gold was first found by Mr. D. J. McCarthy and Mr. W. J. Webb in 1917. Inferred reserves are estimated at 105.4Mt at 0.89g/t Au. The long axis of the north dipping stock is parallel to the regional supracrustal stratigraphy and has a strike length of approximately 2 kilometers and widths up to 300 m. The stock was emplaced in mafic volcanic rocks from the lower part of the Catfish assemblage (Cycle 3). It is called a granodiorite by Argonaut (43-101, 2016) or a trondjhemite by Sage (1994). The gold is hosted in numerous quartz veins and in quartz-flooded bands, parallel to the Goudreau Lake Deformation Zone (Argonaut 43-101, 2016). Silicification, sericitisation and an increase in pyrite content are typical around the mineralized veins and quartz flooded bands. This outcrop exhibits a mostly massive intrusive with numerous thin more ductile bands, and a few thin mineralized quartz veins that can be followed on a few meters. Figure 16: Webb Lake Stock Stop Locations 161 �Figure 17: Webb Lake Trondhjemite, small scale auriferous veins Figure 18: Webb Lake Trondhjemite C/S fabric Stop 7: Iron Formation (Optional) Location: NAD83 U16 688700mE, 5350900mN Description: Upper level of the Michipicoten Iron Formation containing banded layers of chert, magnetite, pyrite with intercalated argillaceous cycle three mafic volcanic rocks. The Webb Lake stock is located less than 30 meters north. 162 �References Adam, D., Bastien, J., Belisle, M., and Poirer, S. 2015 Technical Report and Preliminary Economic Assessment for the Island Gold Lower Zones (according to National Instrument 43-101 and Form 43101F1). Richmont Mines Incorporated, Rouyn-Noranda. Arias, Z., and Helmstaedt, H., 1990. Structural evolution of the Michipicoten (Wawa) greenstone belt, Superior Province, in Geoscience Research Grant Program, summary of research 1989-1990, Edited by V.G.Milne, Ontario Geological Survey, Miscellaneous Paper 150, p. 107-114. Dube, B., and Gosselin, P., 2007. Greenstone-Hosted quartz veins deposits, in Goodfellow, W.D., ed., Mineral Deposits of Canada: A synthesis of Major Deposit Types, District Metallogency, the Evolution of Geological Provinces, and Exploration Methods: Gelogical Association of Canada, Mineral Deposits Division, Special Publication No.5, p.49-73. Heather, K.B., and Arias, Z. 1992. Geological and Structural Setting of Gold Mineralization in the Goudreau-Lochalsh Area, District of Algoma. In Summary of Field Work and Other Activities 1987, by the Ontario Geological Survey. Editied by R.B. Barlow, M.E. Cherry, A.C. Colvine, B.O. Dressler and O.L. White. Ontario Geological Survey, Miscellaneous Paper 137, pp.155-162. Hodgson, C.J. and MacGeehan, P.J., 1982, A review of the geological characterisitics of ‘gold only’ deposits in the Superior Province of the Canadian Shield, in Hodder, R.W. and Petruk, W., eds., Geology of the Canadian Gold Deposits: Canadian Mining and Metallurgy, Special Volume 24, p.211-228. Kwok, Kai-Ming, 1986. Gold Mineralization in Relation to Potassic Alteration, Kremzar property, Finan Township, District of Algoma, Ontario, Canada, unpublished BSc thesis, Laurentian University, Sudbury Ontario, 107p. Lockwood, M.B., 1986. The Petrogenetic and Economic Significance of Chloritoid in the Wawa Greenstone Belt; unpublished Master of Science Thesis, Carelton University, Ottawa, 220p. Percival, J.A., Skulski, T., Sanborn-Barrie, M., Stott, G.M., Leclair, A.D., Corkery, M.T., and Boily, M. 2012. Geology and tectonic evolution of the Superior Province, Canada. Chapter 6 In Tectonic Styles in Canada: The LITHOPROBE Perspective. Editied by J.A. Percival, F.A. Cook and R.M. Clowes. Geological Survey of Canada, Special Paper 49, pp. 321-378. Sage, R.P., 1994. Geology of the Michipicoten Greenstone Belt. Ministry of Northern Development and Mines, Ontario Geological Survey, Open File Report 5888. Turek, A., Smith. P.E., and Van Schmus, W.R. 1984. U-Pb zircon ages and the evolution of the Michipicoten plutonic-volcanic terrane of the Superior Province, Ontario. Canadian Journal of Earth Sciences, 29: 1154-1165. 163 �FIELD TRIP 5 Geology of the Renabie Mine area, Michipicoten greenstone belt Lise Robichaud (Ontario Geological Survey) Jordan McDivitt (Laurentian University) Introduction This field trip focuses on the northeast portion of the Michipicoten greenstone belt and includes the past-producing Renabie Mine, situated within the Missanabie–Renabie gold district (Figure 1). The bedrock geology of the field stops is based on recent mapping of the Renabie Mine area completed by the Ontario Geological Survey (Robichaud, McDivitt and Trevisan 2017; Robichaud et al. 2016; Robichaud et al. 2015; Robichaud, McDivitt and Trevisan 2015; Robichaud and McDivitt 2014). Additionally, detailed outcrop maps for Stops 3, 4, 5, and 15 are modified from a recently completed MSc thesis: Gold mineralization in the Missanabie–Renabie district of the Wawa Subprovince (Missanabie, Ontario, Canada) (McDivitt 2016a). An accompanying Miscellaneous Release of Data (McDivitt 2016b) also provides additional information. Readers should consult publications referenced in this guidebook for additional information and references. The order and number of stops that will be visited during this field trip will be determined by access, time, and weather concerns. Regional Geology The Michipicoten greenstone belt consists of successions of Archean metavolcanic and metasedimentary rocks intruded by Archean granitic rocks (Turek, Smith and Van Schmus 1982) and younger Proterozoic mafic dikes. The supracrustal rocks of the belt have been previously subdivided into 3 distinct volcanic cycles: 2900 Ma, 2750 Ma and 2700 Ma (Sage and Heather 1991; Heather and Arias 1992; Turek, Smith and Van Schmus 1982, 1984; Turek, Van Schmus and Sage 1988). New geochronological data from recent mapping suggest that there is a previously undocumented volcanic cycle ranging in age from 2731 to 2723 Ma (Kamo 2014, 2015, 2016), which indicates that the volcanic history is more continuous than was previously believed. The Michipicoten greenstone belt has been previously interpreted as a continuation of the Abitibi greenstone belt west of the Kapuskasing Structural Zone (Ayer et al. 2010). In comparison, the Abitibi greenstone belt consists of stratigraphically continuous sequences of Archean metavolcanic rocks and metasedimentary rocks ranging from earlier than 2750 Ma to 2695 Ma followed, in Ontario, by 2 sedimentary basins: Porcupine-type at 2690 to 2682 Ma and Timiskaming-type at 2676 to 2670 Ma (Ayer et al. 2002; Ayer et al. 1999a; Ayer et al. 1999b; Ayer, Ketchum and Trowell 2002; Ayer et al. 2005). The second (2750 Ma) and third (2700 Ma) volcanic cycles of the Michipicoten greenstone belt, as well as the 2731 to 2723 Ma volcanism, may represent time-equivalent volcanism similar to the Abitibi greenstone belt; however, no volcanism of 2900 Ma (equivalent to the first cycle) has been described in the Abitibi greenstone belt. Geological Setting of the Renabie Mine Area This field trip targets the northeast portion of the Michipicoten greenstone belt including the pastproducing Renabie Mine. This area is composed of metamorphosed mafic to felsic volcanic and volcaniclastic rocks, interspersed with metasedimentary rocks. Large-bodied, felsic to intermediate granitoids intrude the Archean supracrustal rocks. These include the Missinaibi Lake batholith, which 164 �Figure 1. Simplified geological map of the northeast part of the Michipicoten greenstone belt based on recent mapping by the Ontario Geological Survey (Robichaud, McDivitt and Trevisan 2017; Robichaud et al. 2016; Robichaud et al. 2015). Rennie, Leeson, Stover and Brackin townships are included in the map area. Younging direction symbol indicates stratigraphic facing. Field trip stop locations are indicated on the map. The Renabie road and the Renabie Mine are also denoted for reference purposes. All UTM co-ordinates are provided in Zone 17 using NAD83. 165 �dominates the eastern portion of the area, the Wabatongushi Lake granitoid complex, which occupies the northern portion of Rennie Township, and the Ash Lake pluton, which occupies the southwest portion of Stover Township. Archean felsic stocks, sills, and dikes, as well as Proterozoic gabbroic dikes, are also intrusive to the Archean supracrustal rocks (see Figure 1). The only past-producing gold zones in the Missanabie–Renabie gold district are located in the Renabie Mine area, within tonalite of the Missinaibi Lake batholith near its contact with metavolcanic rocks of the Michipicoten greenstone belt (see Figure 1). The gold zones were mined at the Renabie underground mine and at the C-zone, Nudulama, and Braminco open pits. Mining occurred over a 50 year period, from 1941–1991, and produced roughly 1.1 million ounces of gold at a grade of approximately 6.6 g/t (Turek et al. 1996; Callan and Spooner 1998). The mineralized zones from the Renabie Mine to the Nudulama East outcrop define an east-southeast trend, referred to as the “Renabie trend”, which strikes close to perpendicular to the regional foliation. Assay-defined ore shoots along the Renabie trend have eastern to east-southeastern trends and a moderate westerly plunge (McDivitt 2016a). The Braminco zone, which is located 1.1 km south of the Renabie trend, differs in orientation: the zone is parallel to the regional foliation, which strikes north-northwest and dips steeply to the west. Ore shoots within the Braminco zone plunge moderately to the west, similarly to those along the Renabie trend (Callan and Spooner 1998). Metavolcanic Rocks Mafic metavolcanic rocks are the dominant lithology in the central portion of the area and are interlayered with felsic to intermediate metavolcanic rocks and metasedimentary rocks (see Figure 1). Massive flows are predominant, but pillowed flows are observed locally. The pillows are well to poorly developed and are up to 1 m in diameter. The mafic metavolcanic flows are dark grey to black on fresh surfaces and are typically fine-grained and well foliated. The mafic metavolcanic rocks are typically characterized by greenschist-facies metamorphic mineralogy, although, near the margin of the felsic to intermediate intrusive rocks, the metamorphic mineralogy is representative of amphibolite facies. Where the Archean supracrustal rocks are wedged between the Missinaibi Lake batholith and the Wabatongushi Lake granitoid complex, the rocks are locally migmatized. Felsic to intermediate metavolcanic rocks occur largely in the southern half of Rennie Township and extend south into Stover and Brackin townships (see Figure 1). They are generally fine- to medium-grained volcaniclastic rocks dominated by tuffs and crystal tuffs. Tuff-breccias occur locally in the northern part of Stover Township. The fresh surface of all these felsic to intermediate rocks is light grey and weathers to a lighter grey to beige colour. Bedding is rarely observed and ranges in thickness from finely laminated (a few millimetres) to thickly bedded (up to 1 m). Turek et al. (1996) reported a U/Pb age of 2740±8 Ma for felsic metavolcanic rocks in the southeastern corner of Rennie Township; a U/Pb age of 2730.9±1.2 Ma for the same rocks was reported by Kamo (2016) and Robichaud, McDivitt and Trevisan (2017). A U/Pb age of 2728.6±1.1 Ma was reported for the felsic metavolcanic rocks in Brackin Township (Kamo 2015; Robichaud et al. 2015). A younger U/Pb age of 2704.6±2.1 Ma for the more northern package of felsic metavolcanic rocks, in the north-central portion of Rennie Township was reported by Kamo (2016) and Robichaud, McDivitt and Trevisan (2017). Metasedimentary Rocks Clastic metasedimentary rocks are generally restricted to the southwestern part of Rennie Township, extending to the central part of Stover Township although minor occurrences can also be observed in central Rennie Township. These rocks consist of buff grey, quartz-rich, thinly bedded siltstone, sandstone, and conglomerate. Siltstone is the dominant sedimentary unit, but sporadic beds of sandstone and conglomerate also occur. The conglomerate is matrix-supported, but contains large cobbles of predominantly tonalitic composition with a few mudstone and/or siltstone cobbles. Bedding thickness 166 �varies between sediment types: the siltstone tends to be thinly to thickly laminated, the sandstone beds range from a few decimetres to a metre in thickness, and the conglomerate beds are typically several metres thick. Facing reversals are implied by cross-bedding and graded bedding observed locally in some of the sedimentary rocks, indicating that the area is folded. A U/Pb age of 2695±3 Ma for conglomerates in the northwestern corner of Stover Township was reported by Davis (2016). Iron formation is defined primarily by geophysical evidence (Ontario Geological Survey 1999, 2002a, 2002b, 2003, 2011). Iron formation extends predominantly from the southwestern part of Rennie Township to the central part of Stover Township. Minor iron formation also occurs in east-central Rennie Township and in the southern portion of Brackin Township. The iron formation typically consists of magnetite-rich layers interlayered with siltstone or sandstone, forming beds that range in thickness from thin laminations to thin beds (≤10 cm). Disseminated sulphides were also noted within some of the clastic layers. Archean Intrusions The most extensive Archean intrusion in the map area is the Missinaibi Lake batholith (see Figure 1). It occupies most of the eastern half of the map area and is primarily composed of medium-grained tonalite to granodiorite with minor occurrences of granite. The main mafic mineral is typically biotite, but the presence of hornblende is noted in varying amounts locally. The batholith is generally moderately to well foliated, although some areas are characterized by weak fabric development. Kamo (2015) reports a U/Pb age of 2720.8±1.4 Ma for a tonalitic phase of the Missinaibi Lake batholith in Brackin Township. Turek et al. (1996) reported a U/Pb age of 2741±21 Ma for a tonalitic phase of the Missinaibi Lake batholith in Leeson Township. Younger phases of the Missinaibi Lake batholith occur as plutons, dikes, and stock-like features. The largest occurrence of these younger phases is at the boundary between Leeson and Brackin townships near Crooked Lake (see Figure 1). Younger phases of tonalitic composition tend to be finer grained than the main phase of the batholith and, in some instances, display the regional foliation, although other examples are noted to be massive. A sample of one of these younger tonalitic phases returned an age of 2688±2 Ma (Kamo 2015; Robichaud et al. 2015). The northern portion of Rennie Township is occupied by the felsic intrusive rocks of the Wabatongushi Lake granitoid complex (see Figure 1). These rocks vary in composition from tonalite to granodiorite and typically contain less biotite than rocks from the Missinaibi Lake batholith. There is a 500 m thick boundary between the granitoid complex and the mafic metavolcanic rocks to the south. This boundary is characterized by lit-par-lit injection of the mafic metavolcanic by felsic intrusive rocks. A U/Pb age of 2700±1 Ma for the Wabatongushi Lake granitoid complex was reported by Kamo (2016) and Robichaud, McDivitt and Trevisan (2017). Turek et al. (1996) reported a U/Pb age of 2688 Ma ±14 Ma for the same intrusion. The Ash Lake pluton (see Figure 1) occupies the southwestern portion of Stover Township and is composed primarily of medium-grained tonalite to granodiorite with minor occurrences of granite. The main mafic minerals are typically biotite ± hornblende. The rocks are generally massive to moderately foliated, although the margins of the pluton are characterized by stronger foliation development. Turek et al. (1996) reported a U/Pb age of 2679±5 Ma for the Ash Lake pluton in West Township, situated to the west of Stover Township. Frarey and Krogh (1986) reported a U/Pb age of 2684.5±2.7 Ma for the Ash Lake pluton in Copenace Township, located southwest of Stover Township. The Rennie Lake stock, which occurs in the southwest portion of Rennie Township (see Figure 1), is composed of massive granitic to granodioritic rocks. The main mafic mineral is amphibole with minor biotite. The rocks are light pink on fresh surfaces and weather to creamy pink. A U/Pb age of 2678±4 Ma for the Rennie Lake stock was reported by Davis (2016) and Robichaud, McDivitt and Trevisan (2017). Turek et al. (1996) reported a U/Pb age of ca. 2668 Ma (from a single zircon) for the Rennie Lake stock. 167 �The Ruby Lake stock is interpreted to be temporally related to the Rennie Lake stock. The Ruby Lake stock, which occurs in the southeast portion of Stover Township (see Figure 1), is composed of massive, nonfoliated granitic to granodioritic rocks. The core of the Ruby Lake stock commonly displays potassium feldspar megacrysts, but the texture has also been noted near the intrusion’s contacts. The main mafic mineral is amphibole with minor biotite. The stock is light pink on fresh surfaces and weathers from creamy to dark pink. Turek et al. (1996) reported a U/Pb age of 2661±11 Ma for the Ruby Lake stock. Several smaller mafic and ultramafic intrusions occur in the supracrustal rocks in the area (see Figure 1). The mafic rocks are typically gabbroic in composition and are medium- to coarse-grained, foliated, and typically amphibolitized. The ultramafic rocks differ in composition (from peridotite to pyroxenite and hornblendite) and are variably serpentinized. They are often massive, fine- to coarse-grained, and sometimes display large euhedral crystals of pyroxene or hornblende. Proterozoic Intrusions Proterozoic mafic dikes are late features in the area. Most dikes trend north-northwest; however, a few trend northeast. The Proterozoic dikes generally have sharp, linear aeromagnetic signatures and the dikes shown in Figure 1 are delineated mainly by using airborne magnetic data (Ontario Geological Survey 1999, 2002a, 2002b, 2003, 2011). They are predominantly fine- to medium-grained gabbros, which commonly display diabase texture, and occasionally contain plagioclase phenocrysts or glomerocrysts. The dikes sometimes contain trace amounts of pyrite. Based on their orientation, the north-northwest-trending dikes are interpreted to be part of the Matachewan dike swarm (2454 Ma: Osmani 1991; 2473 Ma: Heaman 1997) and the northeast-trending dikes have been interpreted to be part of the Biscotasing dike swarm (2150 Ma: Osmani 1991). Structural Geology Primary layering (S0) is observed in the form of sedimentary bedding and pillowed flows. Asymmetrical layering, such as graded beds and cross-beds, are sometimes observed within the area. In the eastern portion of the map area, along the boundary with the Missinaibi Lake batholith, facing is to the west. In the northern mafic metavolcanic rocks, just south of the Wabatongushi Lake granitoid complex, facing is to the southwest. The clastic metasedimentary rocks in the southwest corner of Rennie Township face northeastward. The earliest foliation (S1) is only locally defined and is consistently overprinted by the dominant regional foliation (S2). Where it is observable, the S1 foliation is axial planar to F1 folds, which are defined by mineralized laminated quartz veins. Both the S1 foliation and F1 folds are varied in orientation resulting from effects of later overprinting deformation. The S1 foliation and F1 folds are overprinted by tight, generally upright, F2 folds. The regional S2 foliation is axial planar to the F2 folds and is northwest to north-northwest striking with a steep westerly dip in the Renabie Mine area. At the Pileggi No. 1 outcrop, in the northwest corner of Stover Township, the S2 foliation is subvertical and strikes west-northwest (Figure 1). The S2 foliation is well-developed in the Missinaibi Lake batholith, where it is best identified by differentiated domains of biotite, and in supracrustal rocks of the Michipicoten greenstone belt, where it parallels primary layering (S0). The S2 foliation also parallels the intrusive contacts of the supracrustal rocks with the Missinaibi Lake batholith and Ash Lake pluton. A regional L2 mineral stretching lineation is developed in association with the S2 foliation. In the Renabie Mine area the L2 lineation parallels the orientation of ore shoots and has a moderate to steep westerly plunge. 168 �Late, east- to east-southeast-striking, steeply south-dipping shear zones overprint the S2 foliation along the Renabie trend. These shear zones are host to the gold-bearing laminated quartz veins that were the main focus of gold production in the Renabie Mine area. The shear zones formed during a D3 deformation event and are characterized by a strong internal S3 foliation developed within the quartz-sericite-pyrite alteration halos that surround the laminated veins. Shear sense indicators record sinistral-reverse, oblique slip movement along the shear zones during the D3 event. The mineralized D3 shear zones are exposed along the Renabie trend from west to east at the C-Zone, Nudulama, and Nudulama East outcrops (McDivitt 2016a) (see Figure 1). Within the shear zones along the Renabie trend, S3 defines east-northeast-trending, steeply plunging Zshaped drag folds. These folds record a late dextral transcurrent reactivation of the shear zones during a D4 deformation event. Similar Z-shaped F4 folds overprint the S2 foliation at the Pileggi No. 1 outcrop (see Figure 1), suggesting that a late dextral reactivation of mineralized zones with an easterly trend is a recurrent feature in the district (McDivitt 2016a). The study area was also affected by late brittle faulting (see Figure 1). The presence of these late faults is primarily supported by offsets revealed through mapping, topography, and by geophysical interpretation. There are several major faults interpreted in the area, some of which show displacements of up to 1 km. A major sinistral fault trends north-northwest from the southeast corner of Stover Township to the southcentral edge of Rennie Township, transecting the Ruby Lake stock and showing a displacement of approximately 1 km. Another major fault runs underneath Crooked Lake and shows dextral displacement of just under 1 km; however, it does not transect the Ruby Lake stock. There are several faults extending southwestward from the northwest corner of Leeson Township towards the Rennie Lake stock. Some show dextral displacement of 1 km, whereas others do not display obvious offsets. 169 �Field Trip Stops Notes: 1) Some of these outcrops are adjacent to open pits; please refrain from crossing fenced areas to access outcrop. 2) This group is unlikely to visit all of the stops described below during this trip. A number of stop descriptions are included in this guide to provide context and for future visits to the region. Permission must be granted from mining companies for future access. 3) The order and number of stops that will be visited during this field trip will be determined by access, time, and weather concerns. These factors may preclude visiting some stops, and alternative locations may be substituted. 4) All locations are given in NAD 83, Zone 17N UTM coordinates. Stop 1: Braminco open pit Location: UTM Zone 17, 288468E 5360854N The Braminco open pit (see Figure 1) is hosted in the Missinaibi Lake batholith near the contact with the mafic metavolcanic rocks of the Michipicoten greenstone belt. It consists of a shear zone that is 10-15 m wide, striking 135-145° and dipping moderately to the southwest (Sage and Heather 1991). Mineralization at Braminco is similar in character to mineralization along the Renabie trend and consists of shear zone-hosted laminated quartz veins associated with quartz-sericite-pyrite alteration zones and late, hematite-bearing alteration assemblages. However, the Braminco shear zone differs in orientation from the Renabie trend shear zones, which are more easterly in trend. The Braminco shear zone is interpreted to have formed as a sinistral D2 shear zone that underwent a dextral reactivation during D3 (McDivitt 2016a). It was mined primarily for its silica, but gold was also recovered (Wilson, A., OGS, personal communication, 2012). Stop 2: Baltic D outcrop Location: UTM Zone 17, 289990E 5360020N The Baltic D outcrop (see Figure 1) contains gold-bearing laminated quartz veins (Photo 1A) surrounded by sericitized, sulphide-bearing wallrock (Photo 1B) that formed after biotite tonalite of the Missinaibi Lake batholith. The laminated quartz veins are shallow in their dip and define isoclinal, near-recumbent F1 folds. A continuous S1 cleavage is axial planar to the folds and is well-developed within the sericitized wallrock. The F1-folded veins and S1 cleavage are refolded by upright, open, northwest-striking F2 folds (McDivitt 2016a). The regional S2 foliation is axial planar to the F2 folds. The S2 foliation strikes northwest and dips steeply west; it is well-developed in the biotite tonalite surrounding the sericitized wallrock marginal to the veins. The regional L2 stretching lineation occurs in association with S2 foliation and the F2 folds. The lineation plunges moderately northwest, subparallel to the hinges of the F2 folds (McDivitt 2016a). 170 �A B Photo 1. A) Overview photograph of the Baltic D outcrop displaying shallowly-dipping laminated quartz veins. Compass for scale is 6.9 cm wide, with sighting arm pointing north. B) Laminated quartz vein (LQV) adjacent to quartz-sericite-pyrite (QSP) schist, within which S1 is developed. Pencil for scale is 0.8 cm wide and points north. Stop 3: Nudulama East outcrop Location: UTM Zone 17, 289028E 5361881N The Nudulama East outcrop (Figure 2) is the easternmost outcrop defining the Renabie trend (see Figure 1). The outcrop is situated within the Missinaibi Lake batholith, approximately 2 km to the east of the contact with the Michipicoten greenstone belt. Mineralization is characterized by laminated quartz veins (Photo 2A) hosted within steeply dipping, east-southeast-trending D3 shear zones. The shear zones are defined by strongly foliated (S3) quartz-sericite-pyrite schist. Both the laminated veins and schist are mineralized. The veins typically contain 2-5% pyrite with minor chalcopyrite, molybdenite, galena, sphalerite, and rare visible gold. The surrounding schist contains similar amounts of pyrite, but other sulphide minerals are rare to absent. In addition to the laminated veins, late quartz ± chlorite ± epidote ± potassium-feldspar veins associated with hematite-bearing alteration assemblages occur at Nudulama East (Photo 2B, C). Where these veins are abundant, biotite within the host tonalite is pervasively chloritized, and the rock displays a distinct reddish hue because of the presence of hematite. A large quartz-chlorite breccia body is associated with the hematite-bearing alteration zone and cuts across the southern mineralized shear zone (Photo 2D, E). The breccia contains clasts of laminated quartz veins and sheared, hematite-altered tonalite enclosed within a matrix of vuggy quartz locally infilled by chlorite (Photo 2F) (McDivitt 2016a). At Nudulama East, the S2 foliation is oriented roughly perpendicular to the trend of the mineralized D3 shear zones. The S2 foliation and F2 folds defined by laminated quartz veins and felsic dikes are overprinted by F3 folds and transposed into parallelism with the D3 shear zones. The F3 folds have an axial planar S3 foliation, which becomes the main foliation within the shear zones. Mutual overprinting relationships observed between hematite-associated veins and late D3 structures suggest that the hematiteassociated veins are syn-D3 in timing (McDivitt 2016a). Neither the hematite-associated veins nor the quartz-chlorite breccia contain significant gold mineralization. 171 �Figure 2. Detailed geological map of Nudulama East, modified from McDivitt (2016). Lower-hemisphere, equalarea stereographic plots showing the L2 stretching lineation and poles to the S2 and S3 foliations. n = number of measurements. 172 �A B C D E F Photo 2. Field photographs of the Nudulama East outcrop. A) Laminated quartz vein. Pencil for scale is 0.8 cm wide and points north. B) L2-lineated, hematite-bearing altered tonalite. Pencil for scale is 0.8 cm wide and points north. C) Planar hematite-association alteration vein containing quartz-epidote infill mineralogy. Pencil for scale is 0.8 cm wide and points north. D) Hydrothermal breccia with fragments of foliated, hematite-altered tonalite surrounded by a quartz-chlorite matrix. E) A well-defined contact between a laminated quartz vein (LQV) vein within a quartz-sericite-pyrite shear zone, and a quartz-breccia vein containing clasts of hematite-bearing altered tonalite. Compass for scale is 7.1 cm wide, with sighting arm pointing north. F) Irregular-shaped hematiteassociated alteration veins with quartz-chlorite infill mineralogy. Well-developed hematite-bearing alteration zones surrounds the veins. Pencil for scale is 0.8 cm wide and points north. 173 �Stop 4: Nudulama – surrounding outcrop Location: UTM Zone 17, 288623E 5361923N The Nudulama outcrop (Figure 3) is located approximately 400 m west of the Nudulama East outcrop along the Renabie trend (see Figure 1). The outcrop is situated within the Missinaibi Lake batholith, approximately 1.5 km to the east of the contact with the Michipicoten greenstone belt. Mineralization at Nudulama is characterized by laminated quartz veins within an approximately 10 m wide D3 shear zone that consist of strongly foliated quartz-sericite-pyrite schist (Photo 3A, B) (McDivitt 2016a). The laminated veins comprise very fine-grained saccharoidal quartz layers separated by thin sericitic laminations. The veins typically contain up to 5% pyrite with minor chalcopyrite, molybdenite, galena, sphalerite, and rare visible gold; the schist contains similar quantities of pyrite, but a relative absence of other sulphide minerals is noted. Both the laminated veins and schist consistently return significant gold values (i.e., >1 ppm), with higher grade samples containing in excess of 10 ppm gold. Late quartz ± chlorite ± epidote ± potassium-feldspar veins associated with hematite-bearing alteration assemblages occur at Nudulama, but they are less abundant than at Nudulama East. Steeply dipping, Z-shaped drag folds that trend east-northeast are defined by the S3 foliation within the mineralized shear zone. These folds formed during a late dextral transcurrent reactivation of the shear zone during a D4 deformation event (McDivitt 2016a). Figure 3. Detailed geological map of Nudulama, modified from McDivitt (2016). Lower-hemisphere, equal-area stereographic plots of the L2 stretching lineation and poles to S2 and S3 foliations. n = number of measurements. 174 �A B Photo 3. Field photographs of the Nudulama outcrop. A) Mineralized zone shows a laminated quartz vein surrounded by quartz-sericite-pyrite schist. Compass for scale is 6.9 cm wide, with sighting arm pointing north. B) East-facing, vertical wall at the Nudulama open pit (~10 m wide), displaying the same mineralized zone as in A. Photo taken looking north. Stop 5: C-Zone outcrop Location: UTM Zone 17, 287890E 5362050N The C-Zone outcrop (Figure 4) is located approximately 750 m west of the Nudulama outcrop along the Renabie trend (see Figure 1). The outcrop occurs within the Missinaibi Lake batholith, approximately 750 m to the east of the contact with the Michipicoten greenstone belt. Mineralization at the C-Zone outcrop is similar in character to mineralization at Nudulama and Nudulama East outcrops: it consists of laminated quartz veins (Photo 4A, B) hosted within an east-trending, steeply dipping D3 shear zone of variable width (5-10 m). The shear zone consists of strongly foliated (S3) quartz-sericite-pyrite schist. Both the laminated veins and the schist are mineralized. The schist contains 2-5% pyrite; the veins contain similar amounts of pyrite but can also have small amounts of chalcopyrite, molybdenite, galena, sphalerite, and rare visible gold (McDivitt 2016a). A Photo 4. Field photographs of the C-Zone outcrop. A) The main laminated quartz vein. The hammer head points to the east (the hammer is 70 cm long). B) A close up photograph of a laminated quartz vein. Compass for scale is 6.9 cm wide, with sighting arm pointing north. B 175 �Figure 4. Detailed geological map of the C-Zone, modified from McDivitt (2016). Lower-hemisphere, equal-area stereographic plots of the L2 stretching lineation and poles to S2 and S3 foliations. n = number of measurements. Stop 6: No. 2 Shaft and contact outcrop Location: UTM Zone 17, 287246E 5361990N The site of the former Renabie Mine is the westernmost outcrop defining the Renabie trend. At the now defunct site of the No. 2 shaft (see Figure 1), the Shaft Fault offsets the contact between the Archean supracrustal rocks of the Michipicoten greensone belt and the Missinaibi Lake batholith. The fault is not observed at surface and was primarily defined from underground workings. This fault, and others parallel to it, are defined as late, north-trending brittle faults that cut across the Renabie trend. These faults generally appear to have a sinistral offset. The Shaft Fault has been interpreted as an oblique-slip fault having a sinistral strike-slip component as well as a west-side-up (reverse), dip-slip component (Callan and Spooner 1998). On the hill to the south of the No. 2 Shaft, the contact between the mafic metavolcanic rocks of the Michipicoten greenstone belt and tonalite of the Missinaibi Lake batholith can be observed. The contact trends roughly 140°, dips 70° to the southwest and parallels the regional S2 foliation. 176 �Stop 7: Ultramafic metavolcanic rocks Location: UTM Zone 17, 284487E 5366237N Ultramafic metavolcanic rocks (Photo 5) (see Figure 1) are seldom observed in the area. This outcrop displays talc and chlorite alteration and also contains trace amounts of pyrite. The foliation parallels the contact of the supracrustal rocks with the Wabatongushi Lake granitoid complex, which is situated roughly 1.4 km north of the outcrop. Photo 5. Highly altered ultramafic metavolcanic rocks. Stop 8: VMS-style alteration Location: UTM Zone 17, 285343E 5364967N This is a steep outcrop located on the west side of the road (see Figure 1). The main lithology consists of felsic tuffs (Photo 6A) with areas of crystal tuff. At the top of the exposure a thick (~10-20 cm) band of coarse-grained amphibole and garnet can be observed (Photo 6B). Disseminated pyrite can also be found within the alteration area. B A Photo 6. A) Felsic metavolcanic rocks. B) Amphibole and garnet alteration with disseminated pyrite. 177 �Although geochemical analysis did not return particularly good assay values for this outcrop, it does bear some similarities with the Conboy Lake occurrence (MDI42B05NW00021; Ontario Geological Survey 2017). The Conboy Lake occurrence (282745E 5366813N) is located in north-central Rennie Township, approximately 3 km to the northwest of this location. It is an historic zinc, silver and copper occurrence with minor gold mineralization, and is classified in the OGS Mineral Deposit Inventory (MDI) as a developed prospect with reserves. It has a long history of sporadic mineral exploration between 1939 and 2010, as summarized in Ontario Geological Survey (2017). The Conboy Lake occurrence is found within younger felsic metavolcanic rocks (2704.6±2.1 Ma; Kamo 2016; Robichaud, McDivitt and Trevisan 2017) that display pervasive sericite alteration. The zinc mineralization occurs as thick layers of massive sphalerite with disseminated pyrite and chalcopyrite (Robichaud, McDivitt and Trevisan 2015; Riley 1971, p.45-49; Ontario Geological Survey 2017). Stop 9: Felsic crystal tuff Location: UTM Zone 17, 285440E 5362893N The felsic crystal tuffs (see Figure 1) at this outcrop are typical of the area. They are generally fine- to medium-grained with coarser crystals of either quartz or plagioclase. The fresh surface is light grey and weathers to a lighter grey to beige colour. Bedding is rarely observed in the felsic metavolcanic rocks and this outcrop is no different. This outcrop was sampled for geochronology in 2015, with a U/Pb age of 2730.9±1.2 Ma reported by Kamo (2016). Stop 10: Felsic tuff-breccia Location: UTM Zone 17, 283838E 5361108N Felsic tuff-breccias (Photo 7) (see Figure 1) are seldom observed in the area and are very similar to the conglomerates, but differ in their monolithic clast composition. The clasts are composed of felsic metavolcanic material and are angular. In some areas, the clast edges seem to be broken in situ, making it a mosaic breccia. Photo 7. Felsic tuff breccia. Compass for scale is 7.1 cm wide, with sighting arm pointing north. 178 �Stop 11: Pillowed mafic flows Location: UTM Zone 17, 279434E 5361230N This outcrop (see Figure 1) is located approximately 50 m to the north of the Renabie road; it may need to be cleared of the brush to see the features. The outcrop consists of mafic lava flows. Pillows can be observed, ranging in size from 30 to 60 cm. Stop 12: Rennie Lake turnoff conglomerate Location: UTM Zone 17, 279434E 5361230N This polymictic conglomerate is situated on the Renabie road (Photo 8). The conglomerate is matrix supported, but contains large cobbles of predominantly tonalitic porphyry with a few mudstone and siltstone cobbles. The conglomerate is highly foliated and the clasts show an elongation fabric that parallels bedding. Cross-bedding is observed in the more sandy layers and indicates younging to the northeast. A U/Pb age of 2695±3 Ma for the sandy portion of this outcrop was reported by Davis (2016). Photo 8. Highly foliated conglomerate. Compass for scale is 7.1 cm wide, with sighting arm pointing north. Stop 13.1: Baltimore Lake metasedimentary rocks and diabase dike Location: UTM Zone 17, 279076E 5361116N The Baltimore Lake metasedimentary rocks consist of buff-grey, quartz-rich, thinly bedded siltstone intercalated with lesser sandier layers. Small staurolite grains can be observed in some areas. Graded bedding (Photo 9A) can be observed on the outcrop and indicates facing is to the northeast. The diabase dike is located at the southern end of the outcrop, near the water. The dike is interpreted to be of the Matachewan swarm because of its north-trending orientation. The contact between the metasedimentary rocks and the dike can be observed near the water’s edge. The dike contains numerous plagioclase phenocrysts, and in one area, several large glomerocrysts (Photo 9B). On the southern shore of Baltimore Lake, the metasedimentary rocks are more deformed. They are folded, making primary features difficult to discern (Photo 9C, D). The metasedimentary rocks are predominantly sandstones that are finely interbedded with silty layers, but some of the sandstone beds are up to 1 m in thickness. Conglomerates can also be observed on the southern shore of the lake. 179 �A B C D Photo 9. Field photographs of the Baltimore Lake area. A) Graded bedding indicates facing is to the top of the photo. Top of photo is to the northeast. B) Matachewan dike with plagioclase glomerocrysts. C) Interbedded sandy and silty layers showing parasitic axial planar folding. D) Large synclinal fold. Compass for scale is 7.1 cm wide, with sighting arm pointing north. Stop 13.2: Baltimore Lake metasedimentary rocks Location: UTM Zone 17, 278836E 5361004N Approximately 250 m west of the last outcrop sits another large exposure of the same metasedimentary rocks. A very well-polished surface occurs in the middle of the Renabie road and offers the best exposure; however, there are many areas of interest across the outcrop. The metasedimentary rocks consist of buffgrey, quartz-rich, thinly bedded siltstone and sandstone. Siltstone is the dominant sedimentary unit, but sporadic beds of sandstone also occur. The siltstone tends to be thinly to thickly laminated, and the sandstone ranges from a few decimetres to 1 m in thickness. Stop 14.1: Iron formation Location: UTM Zone 17, 277276E 5361723N From the Renabie road, go north past the quarry for approximately 1.3 km until a small east-trending trail is reached. Follow this trail for 1 km on foot (or by ATV); the outcrop will be on the south side of the trail. 180 �The iron formation consists of magnetite-rich layers interbedded with sandstone (Photo 10A, B). Folds can be observed within the banded iron formation, and are especially visible in the central cavity where the third dimension can be observed. Disseminated pyrite and chalcopyrite are also noted. B A Photo 10. A) Banded iron formation displaying magnetite-rich layers interbedded with sandstone. B) Folded banded iron formation. Stop 14.2: Iron formation Location: UTM Zone 17, 276889E 5359035N This iron formation is more readily accessible than that at the previous stop, but is not as well-exposed. This outcrop is located approximately 100 m down the road leading to the Pileggi No. 1 outcrop (Figure 1). There is also another small exposure 200 m further down the road. The outcrop is dominated by the presence of gabbro; the iron formation occupies a small portion (< 1 m2) of this outcrop. The iron formation consists of magnetite-rich layers interbedded with sandstone. Stop 15: Pileggi No. 1 outcrop Location: UTM Zone 17, 277677E 5358487N The Pileggi No. 1 outcrop (Figure 5) occurs approximately 120 m north of the main road (Figure 1). There is a small access trail that can be used to walk up to the outcrop. The outcrop consists of 2 separate exposures (A and B) that connect over a small ridge (see Figure 5 inset). The outcrop consists of metavolcanic rocks of the Michipicoten greenstone belt that have been intruded by feldspar-phyric and amphibole-phyric dikes. Matachewan dikes intrude the metavolcanic rocks. Gold mineralization occurs within deformed laminated quartz veins that returned values up to 22.8 ppm. The laminated quartz veins are overprinted by isoclinal F1 folds associated with an S1 axial planar cleavage. The F1 folds and S1 cleavage formed during an early deformation event, which predated the development of the regional S2 foliation (Photo 11A). The S2 foliation (Photo 11B), which strikes 181 �182 Figure 5. Detailed geological maps of Pileggi No. 1 north (A) and south (B) exposures, modified from McDivitt (2016). Inset map shows the relative location of the two exposures. �approximately 100° and dips steeply, is axial planar to an upright, shallowly plunging F2 fold that occupies roughly half the width of the outcrop. The F2 fold overprints the F1 folded laminated veins and the S1 cleavage. Parasitic folds in the hinge and limbs of the larger F2 fold vary in plunge from subhorizontal to moderately plunging. The S2 foliation is manifested as a disjunctive or crenulation cleavage in the hinges of F2 folds, and as a slaty transposition cleavage along the limbs of the folds. Zshaped drag folds and northwest-trending, subvertical quartz tension gashes overprint the S2 foliation and the laminated veins (Photo 11C). The Z-shaped drag folds trend east-northeast and are steeply plunging. The quartz tension gashes are either straight or Z-shaped sigmoidal. The drag folds and the quartz tensions gashes are attributed to a late dextral reactivation of the mineralized zone during the D4 deformation event (McDivitt 2016a). The variable shape of the tension gashes indicates that they were emplaced throughout the D4 event, with the Z-shaped tension gashes being older than the straight ones. Unlike the laminated veins, the late tension gashes are not gold bearing. B A C Photo 11. Field photographs of the Pileggi No. 1 outcrop. A) Laminated quartz vein overprinted by isoclinal F1 fold and tight F2 folds. The S1 and S2 cleavages are axial planar to F1 and F2 folds, respectively. Facing is to the west. B) F2 folds defined by laminated quartz veins. Note the opposing plunge directions of some of the F2 fold hinges. Pencil for scale is 0.8 cm wide and points north. C) A north-west trending quartz-tension gash. Marginal to the vein, S2 defines a Z-fold, and the vein itself displays Z-asymmetry; both features are supportive of vein emplacement during dextral shearing. Pencil for scale is 0.8 cm wide and points north. Acknowledgements Particular thanks are extended to Ann Wilson and Joseph Walker, for agreeing to lead the field trip in the absence of the authors. The authors appreciates all the support from the staff of the Sault Ste. Marie and Timmins Resident Geologist offices, in particular Anthony Pace and Ann Wilson. Patrick Gervais is thanked for editing figures for this guidebook. Sonia Préfontaine and Marg Rutka are thanked for reviewing the text. 183 �REFERENCES Ayer, J., Amelin, Y., Corfu, F., Kamo, S., Ketchum, J., Kwok, K. and Trowell, N. 2002. Evolution of the southern Abitibi greenstone belt based on U-Pb geochronology: Autochthonous volcanic construction followed by plutonism, regional deformation and sedimentation; Precambrian Research, v.115, p.63-95. Ayer, J.A., Goutier, J., Thurston, P.C., Dubé, B. and Kamber, B.S. 2010. Tectonic and metallogenic evolution of the Abitibi and Wawa subprovinces; in Summary of Field Work and Other Activities 2010, Ontario Geological Survey, Open File Report 6260, p.3-1 to 3-6. Ayer, J.A., Ketchum, J.W.F. and Trowell, N.F. 2002. New geochronological and neodymium isotopic results from the Abitibi greenstone belt, with emphasis on the timing and the tectonic implications of Neoarchean sedimentation and volcanism; in Summary of Field Work and Other Activities 2002, Ontario Geological Survey, Open File Report 6100, p.5-1 to 5-16. Ayer, J.A., Thurston, P.C., Bateman, R., Dubé, B., Gibson, H.L., Hamilton, M.A., Hathway, B., Hocker, S.M., Houlé, M.G., Hudak, G.J., Ispolatov, V., Lafrance, B., Lesher, C.M., MacDonald, P.J., Péloquin, A.S., Piercey, S.J., Reed, L.E. and Thompson, P.H. 2005. Overview of results from the Greenstone Architecture Project: Discover Abitibi Initiative; Ontario Geological Survey, Open File Report 6154, 125p. Ayer, J.A., Trowell, N.F., Amelin, Y. and Corfu, F. 1999a. Geological compilation of the Abitibi greenstone belt in Ontario: Toward a revised stratigraphy based on compilation and new geochronology results; in Summary of Field Work and Other Activities 1998, Ontario Geological Survey, Miscellaneous Paper 169, p.14-24. Ayer, J.A., Trowell, N.F., Madon, Z., Kamo, S., Kwok, Y.Y. and Amelin, Y. 1999b. Compilation of the Abitibi greenstone belt in the Timmins–Kirkland Lake area: Revisions to stratigraphy and new geochronological results; in Summary of Field Work and Other Activities 1999, Ontario Geological Survey, Open File Report 6000, p.4-1 to 4-14. Callan, N.J. and Spooner, E.T.C. 1998. Repetitive hydraulic fracturing and shear zone inflation in an Archean granitoidhosted, ribbon banded, Au-quartz vein system, Renabie area, Ontario, Canada; Ore Geology Reviews, v.12, p.237-266. Davis, D.W. 2016. Geochronology of rocks from northwest Ontario 2015-16. Part 2: LA-ICP-MS Geochronology, internal report for the Ontario Geological Survey; Jack Satterly Geochronology Laboratory, University of Toronto, Toronto, Ontario, 100p. Frarey, M.J. and Krogh, T.E. 1986. U-Pb zircon ages of late internal plutons of the Abitibi and eastern Wawa subprovinces, Ontario and Quebec; in Current Research, Part A, Geological Survey of Canada, Paper 86-1A, p.43-48. Heaman, L.M. 1997. Global mafic magmatism at 2.45 Ga: Remnants of an ancient large igneous province?; Geology, v.25, p.299-302. Heather, K.B. and Arias, Z. 1992. Geological and structural setting of gold mineralization in the Goudreau–Lochalsh area, Wawa gold camp; Ontario Geological Survey, Open File Report 5832, 159p. Kamo, S.L. 2014. Report on U-Pb CA-ID-TIMS geochronology on volcanic and plutonic rocks, Superior and Grenville provinces, Ontario; internal report for the Ontario Geological Survey, Jack Satterly Geochronology Laboratory, University of Toronto, Toronto, Ontario, 27p. ——— 2015. Report on U-Pb ID-TIMS geochronology on volcanic and plutonic rocks, Superior and Grenville provinces, Ontario; internal report for the Ontario Geological Survey, Jack Satterly Geochronology Laboratory, University of Toronto, Toronto, Ontario, 48p. ——— 2016. Part A: Report on U-Pb ID-TIMS geochronology for the Ontario Geological Survey: Bedrock mapping projects, Ontario, internal report for the Ontario Geological Survey; Jack Satterly Geochronology Laboratory, University of Toronto, Toronto, Ontario, 48p. 184 �McDivitt, J.A. 2016a. Gold mineralization in the Missanabie–Renabie district of the Wawa Subprovince (Missanabie, Ontario, Canada); unpublished MSc thesis, Laurentian University, Sudbury, Ontario, 175p. ——— 2016b. Gold mineralization in the Missanabie–Renabie district of the Wawa Subprovince: Geochemical data and photographs; Ontario Geological Survey, Miscellaneous Release—Data 339. Ontario Geological Survey 1999. Single master gravity and aeromagnetic data for Ontario, Geosoft® format; Ontario Geological Survey, Geophysical Data Set 1036. ——— 2002a. Ontario airborne geophysical surveys, magnetic data, grid data, Geosoft® format, Kapuskasing– Chapleau area; Ontario Geological Survey, Geophysical Data Set 1040b—Revised. ——— 2002b. Ontario airborne geophysical surveys, magnetic data, profile data, Geosoft® format, Kapuskasing– Chapleau area; Ontario Geological Survey, Geophysical Data Set 1040d—Revised. ——— 2003. Ontario airborne geophysical surveys, magnetic and electromagnetic data, grid and profile data, Geosoft® format, Wawa area; Ontario Geological Survey, Geophysical Data Set 1009b. ——— 2011. Ontario airborne geophysical surveys, magnetic and electromagnetic data, grid and profile data (ASCII and Geosoft® formats) and vector data, Magpie River–Missinaibi Lake area—Purchased data; Ontario Geological Survey, Geophysical Data Set 1237. ——— 2017. Mineral Deposit Inventory; Ontario Geological Survey, Mineral Deposit Inventory (March 2017 update), online database. Osmani, I.A. 1991. Proterozoic mafic dike swarms in the Superior Province of Ontario; Chapter 17 in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 1, p.661-681. Riley, R.A. 1971. Glasgow, Meath, and Rennie townships, Algoma and Sudbury districts, Ontario; Ontario Geological Survey, Map 2210, scale 1:31 680. Robichaud, L. and McDivitt, J. 2014. Geology and mineral potential of Brackin Township, Michipicoten greenstone belt; in Summary of Field Work and Other Activities 2014, Ontario Geological Survey, Open File Report 6300, p.51 to 5-11. Robichaud, L., McDivitt, J.A., Krystopowicz, N.J. and Szumylo, N. 2015. Precambrian geology of Brackin Township, Michipicoten greenstone belt; Ontario Geological Survey, Preliminary Map P.3797, scale 1:20 000. Robichaud, L., McDivitt, J.A. and Trevisan, B.E. 2015. Geology and mineral potential of Rennie and Leeson townships, Michipicoten greenstone belt; in Summary of Field Work and Other Activities 2015, Ontario Geological Survey, Open File Report 6313, p.5-1 to 5-11. ——— 2017. Precambrian geology of Rennie and Leeson townships, Michipicoten greenstone belt; Ontario Geological Survey, Preliminary Map P.3803, scale 1:20 000. Robichaud, L., Walker, J., West, S.M. and Nywening, A. 2016. Geology and mineral potential of Stover Township, Michipicoten greenstone belt; in Summary of Field Work and Other Activities, 2016, Ontario Geological Survey, Open File Report 6323, p.5-1 to 5-10. Sage, R.P. and Heather, K.B. 1991. The structure, stratigraphy and mineral deposits of the Wawa area; Geological Association of Canada–Mineralogical Association of Canada–Society of Economic Geologists, Joint Annual Meeting, Toronto 1991, Field Trip A6, 118p. Turek, A., Heather, K.B., Sage, R.P. and Van Schmus, W.R. 1996. U–Pb zircon ages for the Missanabie–Renabie area and their relation to the rest of the Michipicoten greenstone belt, Superior Province, Ontario, Canada; Precambrian Research, v.76, p.191-211. 185 �Turek, A., Smith, P.E. and Van Schmus, W.R. 1982. Rb–Sr and U–Pb ages of volcanism and granite emplacement in the Michipicoten belt, Wawa, Ontario; Canadian Journal of Earth Sciences, v.19, p.1608-1626. ——— 1984. U/Pb zircon ages and the evolution of the Michipicoten plutonic–volcanic terrane of the Superior Province, Ontario; Canadian Journal of Earth Sciences, v.21, p.457-464. Turek, A., Van Schmus, W.R. and Sage, R.P. 1988. Extended volcanism in the Michipicoten greenstone belt, Wawa, Ontario; abstract in Geological Association of Canada–Mineralogical Association of Canada, Joint Annual Meeting, St. John’s 1988, Program with Abstracts, v.13, p.A127. 186 �Field Trip 6 Kapuskasing Structural Zone and the Borden Lake Gold Deposit Pierre Bousquet M.Sc., P.Geo. Resident Geologist Program, Ontario Geological Survey, Timmins, Ontario and Jason Rickard M.Sc., P.Geo Senior Geologist for Goldcorp Inc.; Timmins, Ontario Introduction The Kapuskasing Structural Zone is a window into the continental crust, which was investigated thoroughly during the past 30 years. Due to its nature, it was suspected to be barren of any mineralization typical of greenstone belts. Recent findings raised more questions about it, and gave geoscience a black eye, especially after the discovery of the Borden Lake gold deposit. This field trip will be an overview of the Kapuskasing Structural Zone (KSZ). The KSZ was the subject of an ILSG field trip back in 1987 (Percival 1987). This field trip will take place within reasonable distance of Highway 101. As a precautionary note, be aware of the traffic when exiting the vehicles and while crossing or walking along the highway or any roads. Field trip participants should also use caution on outcrops and ditches since they might prove slippery and steep. Exploration and mining history Early explorers were mostly looking at the greenstone belts on either side of the Kapuskasing Structural Zone. However, within the zone, exploration was mostly aimed at the carbonatite intrusions. In the late 1950’s, the Nemegosenda Carbonatite Complex was explored by the Dominion Gulf Company. The company performed an aeromagnetic survey which showed a circular anomaly on the east side of Nemegosenda Lake. This anomaly was, in fact, the Nemegosenda Alkaline Complex. Through clever use of a ground magnetic survey in an area where outcrops were scarce, the Dominion Gulf Company singled out highs which required follow-up drilling. From 1954 to 1959, the company drilled over a hundred holes and even opened an adit of 190 metres into the main mineralized zone (D-Zone) which contained niobium (Archibald, 2008). The company also completed diamond drilling in the South-East Area, located 2 km southeast from the adit, which shows endowment in rare earth elements. The property was taken over by Musto Explorations Ltd. in 1987, who pursued the exploration on the South-East Area by retrieving core drilled in the fifties, split and re-assayed the core. The results were similar to those obtained in the fifties. In the early nineties, Placer Explorations did a re-plotting of the D zone and South East Area. These plots were used to calculate a block model of the D zone. In 2008, Sarissa Resources Inc. acquired the Nemegosenda project, and re-evaluated the historic data. Sarissa Resources followed up with a series of diamond drill holes to confirm its location in the D zone. In the fall of 2016, the project was sold to Indo Global Exchange(s) Pte, Ltd. 187 �The Lackner Lake carbonatite alkalic complex is another area which has undergone exploration in the past sixty years. Two patented claims, in McNaught Township, uncovered magnetite mineralization discovered over a century ago. In 1949, prospectors discovered strong radioactivity on these claims. Nemegos Uranium Corporation was formed to explore these and nearby claims. The exploration concentrated on developing apatite, magnetite, and uranium ore deposits. In 1951, an aeromagnetic survey was flown by Dominion Gulf Company, who followed up with staking anomalies located on the east flank of the structure. At the same time, niobium was discovered in samples assayed by Nemegos Uranium Corporation, but the results were not followed up. In 1954, Multi-Minerals Limited demonstrated the extensive niobium-bearing mineralization on the claims formerly owned by Nemegos Uranium Corporation. The company trenched and drilled in the southwest corner of the complex, which uncovered three zones of niobium mineralization. In 1970, Multi-Minerals Limited optioned one zone to Fetio Industrial Developments Limited who proceeded with an evaluation of possible production of titanium-iron and phosphate concentrates. The company shipped 1500 tons of concentrate to the United State for metallurgical testing. The property was then optioned to Mertec Resources Development in 1974, which was terminated in 1975. During 1978 to 1979, Multi-Minerals Limited drilled 6 holes totalling 2176 feet to test uranium potential of various mineralized zones outlined by previous work. 6378366 Canada Inc. and 6070205 Canada Inc. went back to compile data on the Lackner Lake carbonatite in 2008. A series of exploration projects were undertaken, including prospecting, sampling, ground radiometric survey and assaying. It was followed by an aerial magnetic and radiometric survey by Rare Earth Metals Inc. in 2009. The most recent work was completed in 2014 by Gold Crossing Limited and International Explorers & Prospectors Inc. who completed radiometric surveys, geological mapping, sampling, and assaying of rock and past cores. The Shawmere Anorthosite complex was explored back in the 1990’s. Purechem Ltd. did geological mapping, geochemistry, petrography and bulk sampling from 1993 to 1999 for aluminium silica. The property was then purchased by Southern Africa Corporation in 2001, which bulk sampled and performed more tests. In 2002-2007, Avalon Ventures Ltd. acquired the property and completed bulk sampling and sent more than 1000 tons of material for furnace trials and processing of anorthosite. The company, now under the name of Avalon Advanced Metals Inc., still holds the property. The Borden Lake gold deposit was initially explored in the early 1990’s by M. Tremblay. The prospector, with the help of Jacques Robert, completed a series of exploration projects including prospecting, sampling, electromagnetic survey and stripping in an area which featured Timiskaming age conglomerates. Getting positive results, the claims were acquired by Probe Mines Limited in 2010. From 2010 to 2013, Probe Mines Limited drilled over 250 holes on the property. Probe Mines Limited released an indicated resource estimate showing 9,262,000 tonnes at a grade of 5.39 g/t gold, and inferred resources estimate of 3,034,000 tons at a grade of 4.37 g/t gold for an underground operation (cut-off of 2.5 g/t gold), and an indicated 70,301,000 tons at a grade of 1.03 g/t gold and an inferred 247,000 tonnes at a grade of 0.8 g/t gold for an open pit operation (cut-off of 0.5 g/t gold; Dzick 2014). Goldcorp Inc. acquired the property by absorbing Probe Mines Limited in the summer of 2013. Goldcorp Inc. is currently drilling and planning for the possible opening of a mine in the near future. The latest resource estimate show 3.02 Mt at a grade of 5.77 g/t gold of indicated resources, 2.03 Mt at a grade of 5.49 g/t gold of inferred resources (Goldcorp Inc. 2016). 188 �Geologic Setting The Kapuskasing Structural Zone (KSZ) is located in the Superior Province, cutting it in two. It trends in a NNE direction, starting south of James Bay and dying off gradually south of Chapleau. While the rocks on each side are of greenschist to amphibolite facies, mostly composed of volcano-sedimentary belts with intrusives, the rocks of the KSZ are striking in contrast since they are of amphibolite to granulite facies. The KSZ is also characterized by a strong positive gravity and aeromagnetic anomalies that are trending NNE. The west contact of the KSZ is transitional while the east contact is outlined by the Ivanhoe Lake cataclastic zone. The structures within the zone are at an almost right angle with those from the surrounding belts, as pictured in aeromagnetic surveys (Figure 1). The rock compositions within the structure consist of alternating bands of mafic gneiss, paragneiss, tonalitic gneiss and dioritic rocks which are elongated to the northeast (Percival 1981). Two discrete bodies of anorthosite form the Shawmere anorthositic complex (Thurston, Siragusa and Sage, 1977). All the rocks in the KSZ were deformed and metamorphosed to high-grade, and the contact relations among units are still blurry in understanding (Percival 1981). Paragneiss is characterized by garnet, biotite, plagioclase and quartz. It is thought to be of sedimentary origin (Percival 1981). These paragneissic rocks contain up to 20% concordant tonalitic layers, and locally contain hypersthene. Paragneisses were observed to be richer in biotite southeast of the Shenango complex. A small unit of paragneiss west of Tom Smith Lake contains up to 10% muscovite as well as carbonate layers, while a small unit southeast of Carty Lake contains up to 5% graphite (Percival 1981). South of Nemegosenda Lake, along Highway 101, layers of varying composition on the 10-50 cm scale in fine-grained, biotite-poor, biotite-plagioclase-quartz gneiss hints that these rocks have an arkose protolith (Percival 1981). A recurrent component of many paragneiss outcrops are enclaves of mafic gneiss. These enclaves are comparable in composition and texture to the bigger mafic gneiss belts. The variation in composition of the paragneiss suggests an original sedimentary facies change (Percival 1981). Mafic gneiss is distinguished by garnet-clinopyroxene-hornblende-plagioclase-quartz mineral assemblages and generally incorporates concordant tonalite layers (Percival 1981). Centimeter thick layers are made by different proportions of minerals. Discrete 2-4 km wide mafic gneiss belts are found in the Borden-Hellyer Lakes area, in a body adjacent south of the main anorthosite body. Dioritic rocks occur dominantly within paragneiss belts. These homogeneous meta-igneous bodies consist of medium-to-coarse-grained hornblende-biotite-plagioclase rocks with up to 10% quartz as well as some clinopyroxene, orthopyroxene and rare garnet (Percival 1981). The texture of the dioritic rocks varies from foliated to flaser or augen, to gneissic. The largest body occurs in the Chapleau-Nemegosenda Rivers area, which also shows gabbro, hornblendite and pyroxenite layers from 10 cm to 2 m in width. Veinlets of quartz-monzonite constitute 15% of outcrop in the belt northeast of the Lackner complex, which possesses clinopyroxene-bearing leucosome (Percival 1981). 189 �Figure 1: Aeromagnetic map of the Kapuskasing Structural Zone (modified from Gupta 1991). 190 �Tonalitic rocks outcrop as discrete belts of gneissic and xenolithic bodies south of the Shawmere anorthosite complex (Percival 1981). Southeast and south of Carty Lake, there is a body of coarse garnethornblende-biotite-plagioclase-quartz gneissic tonalite hosting mafic and ultramafic granulite enclaves. Inclusions ranging from fist-sized to 1 m, constitute 15% of the rock and are of layered mafic gneiss, amphibolite, and garnet-orthopyroxene-hornblende-biotite rock. Schlieren and units of paragneiss up to 1 km wide are common. The proportion of garnet diminishes and the composition becomes granodioritic to the southwest along the belt. Enclaves encountered are mostly amphibolites with rare bright green cummingtonite-hornblende-biotite rocks (Percival 1981). Three eastward-thinning belts, between Nemegosenda and Lackner Lakes, are comprised of medium-to-coarse grained, thinly-banded gneissic tonalite (Figure 2) containing hornblende, biotite, plagioclase and quartz in variable proportions (Percival 1981). Figure 2: Thinly-banded gneissic tonalite The Shawmere anorthosite complex consists of a large, oval mass of anorthosite to anorthositic gabbro lying adjacent to and ESE of the KSZ underlying an area of 800 km2, and a satellite Y-shape mass underlying an area of 90 km2 (Thurston, Siragusa and Sage, 1977). A strongly foliated quartz diorite and monzonite underlying an area of 260 km2 sits in between the two masses of anorthosite. The complex has an overall dimension of 84 km by 24 km, its main axis pointing in an ENE direction. The complex is divided into zones of contrasting structural and compositional characteristics: a megacrystic gabbroic anorthosite zone, an anorthosite zone, a banded zone and a border zone (Riccio 1981a , Riccio 1981b). The central portion of the complex is occupied by the megacrystic gabbroic anorthosite zone, which extends to the northeastern margin of the intrusion (Riccio 1981a, Riccio 1981b). It is dominated by 191 �gabbroic anorthosite with subordinate anorthosite, anorthositic gabbro and gabbro, and minor melanogabbro (Riccio 1981a, Riccio 1981b). Rocks within this zone have large (1 to 50 cm) phenocrysts (Percival 1981). Grains of pyroxene with amphibole or garnet coronas are common within this zone (Percival 1981). Olivine appears rarely, at the center of orthopyroxene grains (Percival 1981). The northwestern part of the intrusion shows the anorthosite zone, which consists of granular aggregates of plagioclase, with minor amphibole and rare garnet (Riccio 1981a, Riccio 1981b). The southeastern margin of the complex is where the banded zone appears. It is characterised by bands of 1-30 cm thick of pure anorthosite, clinopyroxene-hornblende-plagioclase gabbro, sphene-garnet-amphibole-plagioclasequartz rock, garnet-rich garnet-hornblende-quartz rock and minor ultramafic rocks (Riccio 1981 a, Riccio 1981b). The 50-100 m thick border zone separates anorthositic rocks from country rocks in the CartyHarold Lake area and along the northwestern margin of the complex (Riccio 1981a, Riccio 1981b). This zone consists of foliated to gneissic garnetiferous amphibolite with 5-20% concordant to discordant tonalite bands (Percival 1981). Mafic dikes have been observed in the KSZ, in two different swarms (Percival 1981). The Kapuskasing swarm is characterized by ENE-striking, SE-dipping dikes of 1-10 m wide, consisting of sparsely plagioclase-porphyritic, medium-to-fine-grained, ophitic, green-grey gabbro (Percival 1981). The second swarm is of NE-trending dikes with pitted weathering rusty surface which may be part of the Abitibi swarm (Percival 1981). In the Kapuskasing Structural Zone, four alkalic rock-carbonatite complexes are identified. Three are located within 20 km of Borden Lake: Borden, Nemegosenda and Lackner Alkalic complexes. The fourth complex is the Shenango (Figure 2). All four are easily noticeable on aeromagnetic surveys, appearing like round highs on the maps. The Shenango Alkalic complex contains different rock suites than the Borden, Nemegosenda and Lackner complexes. The Shenango contains diorites, monzodiorites, quartz monzonites and granites (Sage 1987c), which differs from the other carbonatites found in the other intrusives of similar age; it is a silica-saturated series of rocks (Sage 1987c). Figure 3: Alkalic complexes of the Kapuskasing Structural Zone 192 �The Nemegosenda Lake Alkalic Rock Complex is made of fine-to-medium-grained syenitic to malignitic rocks which are limited along the southern border of the complex by an arcuate mass of coarse-grained nepheline syenite (Sage 1987b). The complex is contained within a fenitized envelope produced by metasomatism by alkali-iron-rich aqueous fluids from the crystallizing magma (Parsons 1961). Ijolites occur in the northwest corner of the complex, and a mass of gabbro rests along the northwest margin and an isolated band along the east flank. Parsons (1961) noted fresh unmetamorphosed nature of the gabbro units but classified them as country rocks. Sage (1987b) believed that they are an early phase of the alkalic magmatism, perhaps analogous to the gabbroic margins observed at the Port Coldwell and Killala Lake complexes located north of the northeast corner of Lake Superior. The Borden Township Alkalic Rock Complex intruded Archean biotite-quartz-plagioclase gneisses which show fenitization in proximity to the carbonatite intrusion (Sage 1987a). According to Sage (1987a), sovite, silicocarbonatite and fenitized wall rock alternate throughout the length of the examined drill core. It suggests that the complex likely represents a succession of sovite and silicocarbonatite cone sheets emplaced into brecciated biotite-quartz-plagioclase gneisses (Sage 1987a). The Lackner Lake Alkalic Rock Complex consists of core and peripheral nepheline syenites, separated by a medial, arcuate, partial ring of alkali mafic rocks like ijolite and malignite (Sage 1988). The leucocratic coarse to very coarse-grained, nepheline syenites of the core and periphery cannot be differentiated by mineralogy, textural or crosscutting field relations (Sage 1988). In proximity to the arcuate band of mafic rocks, the syenites are strongly to weakly trachytoid texture, are locally finer-grained, and contain abundant subangular to subrounded mafic xenoliths. The xenoliths, commonly biotite-rich, are derived from the more mafic phases of the complex which are cut by the nepheline syenites (Sage 1988). Dikes of magnetite-apatite cut both the mafic and syenitic phases of the complex, which is also cut by lamprophyre dikes (Parsons 1961). Cataclastic rocks are present in the transition between the Western Abitibi subprovince and the Kapuskasing Structural Zone, especially in the southern Ivanhoe Lake-Ivanhoe River area. The area is characterized by mylonite and cataclasite (Percival 1981). Pseudotachylite and ultra-cataclasite veinlets are developed within the high-grade rocks of the KSZ, including anorthosite, northwest of Ivanhoe Lake (Riccio 1981a, b). The Ivanhoe Lake cataclastic zone, the zone affected by cataclasis, is 1-2 km in width and may have more than one discrete fault zone (Percival 1981). Structure The rocks of the Kapuskasing Structural Zone are characterized by gneissosity, defined by small to major differences in mineralogical composition of individual bands ranging from 1 to 90 cm in thickness (Thurston et al. 1977). The orientation of lithological contacts and gneissosity make up the prominent east-northeast structural grain of the Kapuskasing structural zone. Gneissosity in all rock types is folded or warped about gently-plunging (0-25°) northeast-trending axes (Percival 1987). The folds vary from northwest-facing monoclinal flexures to isoclinal with consistent "Z" sense asymmetry when viewed toward the east. Axial surfaces are rarely accompanied by a foliation defined by flattened quartz grains. The trend of fold axes and lineations is northeast-southwest throughout this part of the Kapuskasing zone, but plunge direction varies on a regional scale from dominantly southeasterly in the south to northeasterly in the north. Between these areas, lineations are within 10° of horizontal and abrupt changes in plunge 193 �direction occur on the 100 m scale. Both regional and local plunge reversals can be related to a gently southeast-plunging warp axis (Percival 1987). Plausible Explanations To this day, the Kapuskasing Structural Zone has seen various explanations for its presence. The origin of the structure has been interpreted as “thinning of the granitic upper crustal layer” (Garland 1950), Mid- to Late Proterozoic rifting (MacLaren et al. 1968; Bennett et al. 1967; Burke and Dewey 1973; Thurston et al. 1977) a suture between the western and eastern Superior Province (Wilson 1968), dextral transcurrent faulting (Goodings and Brookfield 1992), and an east-verging thrust exposing an oblique crustal cross section (Percival and Card 1983; Percival 1986). Percival and West (1994) criticize these models since no simple model is capable of answering all the present features. The suture model does not account for the correlativity in age and geology between the Abitibi and Wawa subprovinces. A horst model describes the geometry of the blocks within the KSZ, but the third dimension shows a better consistency with a compressional origin than extension. Also, the KSZ existed before the emplacement of the carbonatites and alkali rock complexes around 1100 Ma. The dextral transcurrent model is based on northeasterly aeromagnetic and map trends in the KSZ. Other explanations for these trends are possible, and the scale of any transcurrent displacement seemed to be minor (< 20km). The east-verging Early Proterozoic thrust fault seems to explain many features of the KSZ. However, the Matachewan dike swarm displacement noted along the southern end of the KSZ does not seem possible given its semicontinuous nature, which shows 55-70 km west-over-east apparent movement (Percival and West 1994). Percival and West (1994) use a model which follows this explanation: “Supracrustal rocks of the Kapuskasing Zone (2750-2700 Ma) were buried by younger supracrustal rocks, tectonic shortening, and intrusion of mid-crustal tonalities (2700-2660 Ma). Metamorphism began during this period and continued in response to magmatic heat and crustal collapse (2660-2625 Ma). The Borden Lake conglomerate was deposited in a pull-apart basin and transported into the hot deep crust as a sliver along a downward vector on a dominantly transcurrent fault. Kapuskasing rocks remained at depth, where they cooled slowly and underwent intermittent minor deformation (2625-2585 Ma). The Superior Province was eroded by 10 km on average, elevating Kapuskasing levels from approximately 30 to 20 km. Incipient breakup of the Superior craton (2500-2450 Ma) was recorded as new mineral growth at deep structural levels and by Matachewan dyke injection. A few additional kilometres of erosion preceded intrusion of Kapuskasing dykes (2040 Ma) into the intact crustal section. At approximately 1.9 Ga, when Kapuskasing levels had cooled to <300oC, stresses caused by plate collisions at the Superior margin were transmitted into the interior in the form of dextral transpression along northeast-trending and northwest-over-southeast thrusting, elevating Kapuskasing-level rocks […]. Formation of a crustal root accommodated shortening in the lower crust […]. Northwest-dipping normal faults and a conjugate set of strike-slip faults broke the Kapuskasing structure into separate blocks with variable geometry. During the relaxation phase, isostatic rebound reduced topography on the root and probably produced a few more kilometres of uplift along steep structures not coincident with the Ivanhoe Lake Fault.” The recent discovery of the Borden Lake deposit shows that perhaps the story is wrong; maybe we are looking at mineralization put into place prior to the uplift. 194 �Figure 4: Field trip stops Stop 1: Shawmere Anorthosite Zone: 17, Easting: 366922 Northing: 5332558 Access: Take the Warren-Carty main haul road. Travel down the road for about 10 km. The site is accessible from the road through a wet overgrown section which leads to the bulk sample pit created by Avalon Ventures Ltd. back in 2007. The rocks in the bulk sample pit (Figure 5) are a very coarse-grained anorthosite with 1-4 mm diameter grains, with sporadically some 3 cm diameter porphyroblasts although most are in the 1 cm diameter range. The porphyroblasts are xenomorphic, poikiloblastic and disseminated throughout the rocks. The grains in the matrix are equant and may be polygonized. 195 �Figure 3: Shawmere Anorthosite pit Stop 2: Sandy Outcrop Zone: 17, Easting: 360841 Northing: 5318167 The outcrop shows granitic gneiss, with melanosome and leucosome in thin beds of 1 to 3 cm in thickness. The melanosome consists of amphiboles, biotite and calcic feldspars, while the leucosome consists of quartz, potassic and sodic feldspars. The folded and wavy beds underline migmatization of the rock. A diabase dike crosses the outcrop with seeming difficulty. Stop 3: Borden Turnaround Outcrop Zone: 17, Easting: 340126 Northing: 5309885 The outcrop (Figure 6) shows a seemingly partially undifferentiated granitic migmatite. Dark paleosomes are seen on the outcrop, composed of biotite and amphiboles (restites). Garnets are rather invisible, some trace sulphides are present. Behind the outcrop, on boulders broken off during the road construction, it is possible to see an “augen” (Figure 7) made out of a piece of possibly chert, which may suggest that the protolith may be a metasediment. 196 �Figure 4: Turnaround Outcrop Figure 5: "Augen" made of possibly chert, Turnaround Outcrop. Notice the zoning in the eye, which is perpendicular to the gneissossity of the rock. 197 �Stop 4: Borden Lake Conglomerate Zone: 17, Easting: 330441 Northing: 5305051 The Borden Lake Conglomerate is a deformed metaconglomerate (Figure 8) which was dated at 2664 ± 12 Ma (Percival et al. 1981), using tonalitic cobbles. The age date of the zircons is of a later deformationmetamorphic event. Some texts refer to it as a “pebble” conglomerate, but most clasts in the outcrop are more in the “cobble” range of sizes. The rock is vastly clast supported with a 10-15% matrix of quartzbiotite-hornblende-garnet. The cobbles are made of felsic metavolcanics, metasediments, granodiorite, tonalite, plagioclase-porphyritic meta-andesite and amphibolite, with some hornblendite and quartz veins. The foliation is gently north dipping. They are elongated, forming rods lineated at 075oN, and a plunge of 026o (Figures 8 and 9). The metaconglomerate is accompanied spatially by an amphibolite and paragneiss to the south of the highway. Somehow, the amphibolite is the host of the gold mineralization of the Borden Lake deposit. To the south of the outcrop, towards the highway, lies a felsic porphyry unit (Figure 10). The unit is grey in appearance with white “eyes” of quartz porphyroblasts. The porphyroblasts are up to 1 cm in diameter. Reddish garnets can be seen on the outcrop. Figure 6: Borden Lake conglomerate, with elongated cobbles 198 �Figure 7: Borden Lake conglomerate, down plunge view Figure 8: Quartz porphyry, with quartz eyes up to 1 cm in diameter. 199 �Stop 5: Chapleau Truck Stop Outcrop Zone: 17, Easting: 321942 Northing: 5294079 The roadcut (Figure 11) shows sub-vertical orientation of mafic xenoliths in a medium grained tonalite, showing a “salt & pepper” texture. A pegmatitic aplite vein (~10 cm in thickness) crosscuts the outcrop. The abundance of mafic xenoliths seems to increase towards the south of the outcrop along the highway. Figure 9: Truckstop outcrop with subvertical xenoliths and cut by a dark pink aplite vein Stop 6: Borden Lake Deposit The Borden Lake gold deposit lies at the intersection of the Wawa sub-province, the Kapuskasing Structural Zone (KSZ) and the Abitibi sub-province, specifically within the southernmost limits of the KSZ (Dzick 2014). It is located on a peninsula of Borden Lake about 20km from the town of Chapleau. The deposit dips at 40-45° to the north-northeast and strikes to the west-northwest. This trend is bound to the north by an extensive package of metaconglomerates and intersected by a significant east-west striking fault. To the south it is bound by a similar package of metaconglomerates, which then grades into various garnet-boitite gneisses, amphibolites and granulite-grade gneisses. The gold mineralization within the deposit is typically characterized by a higher-grade core surrounded by a lower-grade envelope, within a package of volcano-metasedimentary rocks of variable composition. The main sulphides are pyrite and pyrrhotite, with the latter typically dominating within the gold zone. The west-northwest portion of the deposit is generally lower grade with some higher grade pods. The east-southeast portion of the deposit hosts the High-Grade Zone (Dzick, 2014). 200 �Figure 10 201 �An interpreted geological drill section (Figure 12) intercepts the High-Grade Zone (HGZ) at the south eastern portion of the deposit. The general hanging wall sequence of rocks is an intermixed package of felsic intrusive (quartz-metadiorite to diorite) sills/dikes with felsic metavolcanic-metasedimentary gneisses. This upper sequence of felsic rocks is cross-cut or intermixed with a coarse grained porphyroblastic metagabbro (locally termed amphibole felsic gneiss) and amphibolite units. Sulphide mineralization in the hanging wall is dominantly pyrite in amounts ranging from trace to 1%. The appearance of pyrite and pyrrhotite mineralized amphibolites marks the transition from the hanging wall to the beginning of the gold-bearing zone and the start of the low-grade gold envelope. The low-grade envelope (Figure 13) is typically characterized by intermixed felsic gneisses and amphibolites with an overall increase in sulphide mineralization to 1-2% pyrite/pyrrhotite. The increase in sulphide content generally coincides with a higher degree of strain and development of a foliation that becomes better defined and more intense down sequence. The transition from the low-grade envelope to the beginning of the high-grade core is marked by the presence of a unit that is locally termed the granitic gneiss. The highest grade portions of the gold zone correlate with deformed quartz veins and quartz pegmatites hosted within small sections of garnet-biotite gneiss and amphibolite. All main sequence units within the core can be silicified to varying degrees, with the units proximal to the quartz veins and pegmatites generally undergoing the strongest intensity of alteration. Visible gold is most often associated with quartz veining and areas of intense silicification. Figure 11 202 �The Borden Lake deposit is located on a peninsula on Borden Lake. A volcano-sedimentary package of various composition hosts the gold mineralization, which occurs as a wide zone of disseminated and fracture-controlled sulphides. Pyrite and pyrrhotite are the main sulphides present, with pyrite being the most common. Gold occurs in a high-grade core surrounded by a low to moderate grade envelope, accompanied by minor silver. The grade of the core seems to be improving towards the southeast where it becomes the high-grade zone (HGZ) with average grades above 2.5 g/t gold (Dzick, 2014). The northwest section of the deposit is characterized by sporadic silicification without lithological control or quartz veining. The southeast section, on the other hand, has a well-developed hydrothermal system with quartz flooding and potassic alteration which defines the HGZ. Various host rocks contain the mineralization, usually dominated by metasedimentary horizons and subordinate intrusives of felsic to intermediate composition, who display feldspathic, chloritic and biotitic alterations. Outcrops in the northwest rarely show visible gold, which is the opposite of the HGZ in the southeast, especially in the quartz-rich core. That core is present in the low-grade zone, sometimes attaining very high grades like the HGZ (Dzick, 2014). The deposit showed continuity and has been consistently intersected along strike, reaching a length of 3.7 km (Dzick, 2014). The deposit remained open both to the northwest and southeast directions. The dip and plunge of the deposit are to the northeast and shallow southeast respectively. Gold mineralization seemed to be controlled by a ductile shear zone, which is more apparent in the HGZ. The mineralized zone has been confirmed to a vertical depth of approximately 650 m, and is up to 120 m wide (Dzick, 2014). References Archibald, J.C. 2009. Technical Report on the Nemegosenda Property for Sarissa Resources Inc; Biliken Management Services Inc. Filed on SEDAR.com Bennett, G., Brown, D.D., George, P.T., and Leahy, E.J. 1967. Operation Kapuskasing. Ontario Department of Mines, Miscellaneous Paper 10, 98p. Burke, K., and Dewey, J.F. 1973. Plume-Generated Triple Junctions: Key Indicators in Applying Plate Tectonics to Old Rocks. Journal of Geology, Vol. 81, p. 406-433. Dzick, W. 2014. Probe Mines Limited: Mineral Resources Estimate Update, Borden Lake Project. NI 43101 technical report, June 10, 2014. 179 p. Garland, G.D. 1950. Interpretation of Gravimetric and Magnetic Anomalies on Traverses in the Canadian Shield of Northern Ontario. Publications of the Dominion Observatory (Ottawa), Vol. 16, part 1. Goldcorp 2016. Reserves and Resources estimate table. http://s1.q4cdn.com/038672619/files/doc_downloads/2016/oct/Reserves-and-Resources-TableWebsite_FINAL.pdf Goodings, C.R. and Brookfield, M.E. 1992. Proterozoic Transcurrent Movements along the Kapuskasing Lineament (Superior Province, Canada) and their Relationship to Surrounding Structures. Earth-Science Reviews, Vol. 32, p147-185. 203 �Gupta, V.K. 1991. Shaded image of total magnetic field of Ontario, east-central sheet; Ontario Geological Survey, Map 2586, scale 1:1 000 000. Parsons, G.E. 1961. Niobium-Bearing Complexes East of Lake Superior; Ontario Geological Survey, Geological Report 3, p.33-50, Map 2007, Scale 1 inch to ¼ mile. Percival, J.A. 1981. Geological evolution of part of the central Superior Province based on relationships among the Abitibi and Wawa subprovinces and the Kapuskasing structural zone (Ph.D. Thesis). Queen’s University, Kingston 300 p. Percival, J.A. 1987. The Kapuskasing Uplift: Archean Greenstones and Granulites. Institute on Lake Superior Geology Thirty-Third Annual Meeting; Wawa, Ontario. Vol. 33, Part 5, 54p. Percival, J.A., and Card, K.D. 1983. Archean Crust as Revealed in the Kapuskasing Uplift, Superior Province, Canada. Geology, Vol. 11, p. 323-326. Percival, J.A. and West, G.F. 1994. The Kapuskasing Uplift: A Geological and Geophysical Synthesis. Canadian Journal of Earth Sciences, vol. 31, p1256-1286. Riccio, L. 1981a. Precambrian Geology of the Shawmere Anorthositic Complex (North), District of Sudbury; Ontario Geological Survey Preliminary Map P. 2383, Geological Series, Scale 1:15 840. Geology 1979. Riccio, L. 1981b. Precambrian Geology of the Shawmere Anorthositic Complex (South), District of Sudbury; Ontario Geological Survey Preliminary Map P. 2384, Geological Series, Scale 1:15 840. Geology 1979. Sage, R.P. 1988. Geology of Carbonatite – Alkalic Rock Complexes in Ontario: Lackner Lake Alkalic Rock Complex, District of Sudbury; Ontario Geological Survey, Study 32, 141p. Sage, R.P. 1987a. Geology of Carbonatite – Alkalic Rock Complexes in Ontario: Borden Township Carbonatite Complex, District of Sudbury; Ontario Geological Survey, Study 33, 62p. Sage, R.P. 1987b. Geology of Carbonatite – Alkalic Rock Complexes in Ontario: Nemegosenda Lake Alkalic Rock Complex, District of Sudbury; Ontario Geological Survey, Study 34, 132p. Sage, R.P. 1987c. Geology of Carbonatite – Alkalic Rock Complexes in Ontario: Shenango Township Alkalic Rock Complex, Districts of Sudbury and Algoma; Ontario Geological Survey, Study 35, 119p. Thurston, P.C., Siragusa, G.M., and Sage, R.P. 1977. Geology of the Chapleau Area, Districts of Algoma, Sudbury and Cochrane; Ontario Geological Survey, GR157, 293p. Wilson, J.T. 1968. Comparison of the Hudson Bay Arc with some Other Features. In Science, History and Hudson Bay. Edited by C.S. Beals and D.A. Shenstone. Department of Energy, Mines and Resources, Ottawa, p1015-1033. 204 � 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, 2017 Description An account of the resource Institute on Lake Superior Geology. Wawa, Ontario. May 8-12, 2017. 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 2017 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/e5e2a3aeea1565339a75ff20b1c88207.pdf 2f72c773c08031a7b9bd02c23a9e41a6 PDF Text Text Institute on Lake Superior Geology 64th ANNUAL MEETING May 15-18, 2018 Iron Mountain, Michigan Hosted by: LAUREL G. WOODRUFF, WILLIAM F. CANNON AND ESTHER K. STEWART CO-CHAIRS U.S. GEOLOGICAL SURVEY WISCONSIN GEOLOGICAL & NATURAL HISTORY SURVEY Proceedings Volume 64 Part 1 – Program and Abstracts Edited by Esther K. Stewart �64th INSTITUTE ON LAKE SUPERIOR GEOLOGY VOLUME 64 CONSISTS OF: PART 1: PROGRAM AND ABSTRACTS PART 2: FIELD TRIP GUIDEBOOK TRIP 1: ARCHEAN AND PALEOPROTEROZOIC GEOLOGY OF THE FELCH DISTRICT, CENTRAL DICKINSON COUNTY, MICHIGAN TRIP 2: GEOLOGY OF THE HEMLOCK FORMATION TRIP 3: GEOLOGY AND IRON ORES OF THE MENOMINEE IRON RANGE, DICKINSON COUNTY, MICHIGAN TRIP 4: GRANITOID ROCKS OF THE PEMBINE-WAUSAU TERRANE IN NORTHEASTERN WISCONSIN Reference to material in Part 1 should follow the example below: Authors, 2018, abstract title, 64th Institute on Lake Superior Geology Proceedings, v. 64, Part 1, Program and Abstracts, p. xx. Proceedings Volume 64, Part 1: Program and Abstracts, and Part 2: Field Trip Guidebook are published by the 64th Institute on Lake Superior Geology and distributed by the Institute Secretary: Peter Hollings Department of Geology Lakehead University Thunder Bay, ON P7B 5E1 CANADA peter.hollings@lakeheadu.ca Some figures in this volume were submitted by authors in color, but are printed grayscale to conserve printing costs. Full color imagery will appear in the digital version of the volume when it is available on-line at: http://www.lakesuperiorgeology.org ISSN 1042-99 i �Part 1: Program and Abstracts Table of Contents Institutes on Lake Superior Geology, 1955-2018 iii v Sam Goldich and the Goldich Medal Goldich Medal Guidelines vii Goldich Medalists and Goldich Medal Committee ix Citation for Goldich Medal Award to Val Chandler x Honoring the Pioneers of Lake Superior Geology xii Memoriam to William D. Addison xiii Eisenbrey Student Travel Awards xiv Joe Mancuso Student Research Awards xv Doug Duskin Student Paper Awards and Award Committee xvi Board of Directors, Local Committee, and Session Chairs xix Field Trip Leaders xx Corporate and Individual Sponsors of Student Travel and Registration xxi Report of the Chair of the 633rd Annual Meeting xxii Technical Program xxvi Poster Presentations xxxiii Abstracts xxxvi ii �Institutes on Lake Superior Geology, 1955-2018 95 o o 85 o Wabigoon subprovince90 o 80 48 o Wawa-Abitibi subprovince 48o Wawa-Abitibi subprovince o 45 45o Minnesota River Valley subprovince MEETING LOCATIONS Phanerozoic Mesoproterozoic Map by Mark Jirsa 95o Paleoproterozoic o 90 85o Archean Superior Province # 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 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 �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 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 2018 GOLDICH MEDAL RECIPIENT Val Chandler Goldich Medal Committee Serving through the meeting year shown in parentheses. Shannon Zurevinski (2015-2018) Lakehead University Klaus Schultz (2016-2019) U. S. Geological Survey Dan England (2017-2020) Eveleth Fee Office ix �Citation for the Goldich Medal Recipient to Val W. Chandler Val W. Chandler, geophysicist extraordinaire, has been my friend and professional colleague for almost forty years. We have worked together on projects too numerous to mention, beginning in 1979 only a few weeks after Val escaped to the blissful cool of Minnesota after a brief stint at Amoco, Inc. in the heat and humidity of Houston, Texas. His personal accomplishments and contributions to diverse joint projects at the Minnesota Geological Survey, and beyond, are remarkable in their scientific breadth. It is a privilege for me to be Val’s citationist for the 2018 Goldich Medal. Val was born and raised in Indianapolis, Indiana. After graduating from high school in 1967, having excelled academically and also athletically in track and field, he entered Indiana University. There he majored in geology and continued his athletic career as a “weight-man” on the IU varsity track team. In 1970 he won the Big Ten Conference championship in discus and placed second in shot-put. Fortunately for us, he declined the overtures of professional football scouts attracted by his impressive size and strength and instead decided to pursue a graduate education in the earth sciences at Indiana, where he obtained an M.S. in geophysics, and at Purdue, where he acquired a Ph.D. in geophysics in 1978 under the tutelage of Prof. Bill Hinze. His graduate work in both universities involved extensive practical applications of magnetic and gravity methods. Val’s professional contributions to understanding the geological framework of Minnesota and the greater Lake Superior region can be subdivided into three main parts. His first challenge was to plan and then supervise the production of a state-wide, high-definition aeromagnetic map of Minnesota. That project, spread over roughly 12 years, involved negotiating contracts with private-sector geophysical mapping firms, performing quality-control tests of the data, writing progress reports to university and governmental administrators, and securing funding for successive segments of the project from the Minnesota legislature. More or less coincident with all of this, Val oversaw a parallel effort to complete a high-quality gravity survey of the state that involved faculty and students from the University of Minnesota and Northern Illinois University and included important contributions of data from private-sector sources. The net result of these efforts was a set of digital potential-field geophysical maps of the state that were widely acknowledged to be among the very best in North America. After the geophysical mapping of Minnesota was essentially finished, about 1992, Val devoted more and more of his time to geological interpretation of the geophysical data. In this work he collaborated in various ways with geologists in the MGS, such as myself, Mark Jirsa, Jim Miller, and Terry Boerboom, and with many geologists in adjacent states and provinces. Furthermore, he contributed to important national and international geophysical projects such as the development of the gravity anomaly map of North America (1988) and the magnetic anomaly map of North America (2002). All along, Val was assiduous in applying the latest technological advancements to the presentation and interpretation of geophysical data. Among the techniques he perfected is the so-called “SMOG” presentation in which gravity and magnetic anomalies are combined. The SMOG acronym means Superimposed Magnetics On Gravity. A SMOG map shows the first vertical derivative of the magnetic signature (typically in grayscale) draped over the second vertical derivative of the gravity signature (typically shown in bright colors). The value of modern computing power in producing x �these maps and other analytical tools cannot be overemphasized, and Val’s efforts in developing and improving computational applications, such as SMOG maps and various digital modeling methods, have proven to be powerful aids to the geologic mapping of Precambrian terranes beneath glacial cover in Minnesota and the rest of the Lake Superior region. As we all know, the Precambrian rocks of the Lake Superior region host a wealth of metallic mineral resources, and the potential for discovering and developing future economically viable Precambrian mineral deposits in covered areas has long been an attractive possibility to exploration companies and politicians. Indeed, that possibility was emphatically presented to Minnesota policy makers in the late 1970s, during a deep recession in the Minnesota iron-mining industry. It gave rise to a push for “minerals diversification” and created a political environment in which the importance of geophysics to the diversification effort could be successfully argued. That set of conditions brought Val to us, and his presence has produced dividends. Today we can make much better geologic maps of Precambrian terranes than we could before digital aeromagnetic and gravity maps became a reality, and consequently can make more credible assessments of mineral potential. In recent years, however, the sense of urgency expressed in the public and political sectors has changed. Clean water, especially clean groundwater, has supplanted metals as the political “ore of choice”. This is reality. Val, in the third chapter of his career, has pivoted from the geophysical interpretation of Precambrian rocks to the pursuit of techniques that aid three-dimensional hydrogeologic mapping of Quaternary glacial deposits. He has applied passive seismic methods to the estimation of sediment thickness above sub-Quaternary bedrock, an important parameter in aquifer delineation and groundwater management. He continues to perfect passive seismic techniques and works in close cooperation with soft-rock stratigraphers and hydrogeologists at MGS and affiliated state agencies. Last but not least, Val is a teacher. He is an adjunct professor of geophysics in the school of earth sciences at the University of Minnesota. He has taught various undergraduate-level geophysics courses over the years and advised or co-advised several graduate students pursuing M.S. or Ph.D. degrees. He has long been an advocate for advancing the understanding and sensible application of science in the public sphere. Val continues to be fascinated by the geology and geological resources of the Lake Superior region, both solid and liquid. His enthusiasm and his professional contributions to our collective understanding of this area unquestionably qualify him to join the ranks of Goldich Medal recipients. It is my distinct pleasure, therefore, to present Val W. Chandler to the Institute as its 2018 recipient of the Samuel S. Goldich Medal for “Outstanding contributions to the geology of the Lake Superior region”. Submitted by David Southwick Director Emeritus Minnesota Geological Survey xi �Honoring the Pioneers of Lake Superior Geology (Adopted by the Board of Directors, 2016) Preamble At the suggestion of Gene LaBerge, the 2016 executive board agreed to implement a program to recognize historic pioneers in the understanding of geology in the Lake Superior region. Beginning with the 2017 annual meeting, nominations will be accepted from the membership for geologists whose work was conducted primarily before inception of the institute in 1955. Biographical sketches of those pioneers will be presented at future annual meetings so that all might appreciate the value of their contributions. Selection of nominees will be decided in part by the organizing committee of each year's annual meeting, in consultation with the Board, to ensure equitable geographic representation in the selection process. Award Guidelines 1) Nominations from the membership will be submitted via the Institute web site and forwarded to the Chair of the next Annual Meeting. The nominations will be no more than half a page in length and will summarise the contribution of the nominee. 2) The Organising Committee will select one or two individuals to be highlighted at the next Annual meeting and submit those names to the Board for approval. 3) The nominator will be requested to prepare a brief presentation to be given during the next annual meeting with a summary to be included in the Proceedings volume. 4) Unsuccessful nominations will be kept by the Secretary for two years and forwarded to the next meeting Chair; these nominations may be resubmitted at a later date. The Board will review this award every five years. Pioneers of Lake Superior Geology 2017 Douglass Houghton (1809-1845) 2018 not presented xii �In Memoriam William D. Addison October 25, 2017- a day of great loss for The Institute on Lake Superior Geology, its members, and countless others whose professional and personal lives were deeply influenced by Bill Addison. Bill’s many and varied accomplishments are impossible to fully enumerate in a short remembrance. Bill, born in Toronto, lived much of his younger years in Thunder Bay before earning his B.Sc. in Forestry and M.Sc. in Fisheries Biology from the University of Toronto, where he met Wendy Livingston, who would become his wife. Bill and Wendy settled in Thunder Bay, where Bill worked as a fisheries biologist before joining Wendy in a teaching career at Westgate High School, where he taught Biology, Chemistry, and Geology for nearly 30 years. The Institute, and the broader geological community, know Bill for the discovery, made with long-time colleague and co-investigator Greg Brumpton, of the layer of meteor impact debris that was spread across the Lake Superior region because of the great Sudbury impact. Bill first presented the documentation of the impact layer near Thunder Bay in 2005, at the 51st Annual Meeting of the Institute. Papers in the journal “Geology” and a Geological Society of America Special Paper soon followed and led others to discover the debris layer at many other localities around Lake Superior. Bill and Greg received world-wide recognition for their discovery, which spurred a flurry of research by an international group of Earth scientists that continues today. Bill’s search for the Sudbury layer was, remarkably, only one of his many interests in the natural world, although one that he and Greg pursued with great patience and diligence for more than a decade before their final success. Bill is one of a select few to receive both the Goldich Medal and Homer Award from the Institute-- the Goldich shared with Greg Brumpton, recognizing their work on the Sudbury impact-- the Homer entirely a recognition of Bill’s own (mis)deeds. Future generations, to their loss, will know Bill as a name and author of groundbreaking geologic research. But the man-- larger-than-life, congenial, gregarious, and generous, that so many of us had the pleasure of knowing, if for only too short a time, should be remembered and celebrated as well. You could not know Bill for long without feeling that you had made a great new friend—and you would be right. He had seemingly unlimited space in his life and heart for friendship and kindness. He loved sharing his many unique experiences through his raconteurial skills, and had a seemingly limitless trove of fascinating tales of his adventures. Bill was a true lover of nature and supporter of its preservation. He and Wendy traveled the back roads and trails of the world celebrating both its natural beauty and cultural history. A fortunate group of friends received his “Epistles” from the road, an authoritative diary of daily discoveries, beautifully illustrated by his exceptional photography. A life well-lived to the fullest, two loving, accomplished daughters, Michelle and Kirsten who blessed him with four grandchildren, lasting scientific contributions, and a host of friends and colleagues who were fortunate to have known him--this is the legacy of William D. Addison xiii �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. xiv �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 four students to be granted research funding of $500.00 each from the Joe Mancuso Student Research Fund. The awardees were: Chanelle Boucher Lakehead University, MsC, Dept. Geology, cbouche2@lakeheadu.ca TOPIC: Komatiitic units within the Lake of the Woods Greenstone Belt Dustin Andrew Liikane University of Toronto, PhD, Dept. Earth Sciences, dustin.liikane@mail.utoronto.ca TOPIC: Controls on the timing and localization of mineralized intrusions within the Midcontinent Rift Jacqueline L. Drazan University of Minnesota-Duluth, MsC, Dept. Earth and Environmental Sciences, draza004@d.umn.edu TOPIC: Can silicon isotopes of quartz be used to determine chert petrogenesis in VMS-hosting systems in the ~2.7 Ga Abitibi Greenstone Belt, Canada? Margaret Upton University of Minnesota-Duluth, MsC, Dept. Earth and Environmental Sciences, upton040@d.umn.edu TOPIC: Alteration mineral zonation and geochemical characteristics of the Back Forty Deposit, MI—A replacement-style zinc- and gold-rich volcanogenic massive sulfide deposit xv �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. 2018 Student Paper Awards Committee Latisha Brengman – University of Minnesota-Duluth Robert Cundari – Ontario Geological Survey Esther Stewart – Wisconsin Geological & Natural History Survey xvi �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 Rob Cundari (2015-2018) – Ontario Geological Survey Pete Hollings - Secretary (2017-2020) – Lakehead University Mark Jirsa – Treasurer (2015-2020) – Minnesota Geological Survey Local Committee Tom Mroz- BSGE, MSPG, CPG Session Chairs Marcia Bjørnerud - Lawrence University Ben Drenth - U.S. Geological Survey John Esch - Michigan Department of Environmental Quality Daniel Holm - Kent State University Suzanne Nicholson - U.S. Geological Survey Dean Peterson - Natural Resources Research Institute Amy Radakovich - Minnesota Geological Survey Shannon Zurevinski - Lakehead University xix �Field Trip Leaders Field trips have been the mainstay of the ILSG since its inception 64 years ago. We want to give a special thanks to the field trip leaders who volunteered their time and talent in carrying that tradition forward. 1) Archean and Paleoproterozoic Geology of the Felch District, Central Dickinson County, Michigan Bill Cannon, Klaus Schulz, Robert Ayuso – U.S. Geological Survey Tom Mroz – BSGE, MSPG, CPG 2) Geology of the Hemlock Formation Tom Waggoner – Consulting Geologist 3) Geology and Iron Ores of the Menominee Iron Range, Dickinson County, Michigan Tom Mroz – BSGE, MSPG, CPG Bill Cannon – U.S. Geological Survey 4) Granitoid rocks of the Pembine-Wausau Terrane in northeastern Wisconsin Klaus Schulz – U.S. Geological Survey Marcia Bjørnerud – Lawrence University xx �Sponsors The following organizations and individuals made general contributions to the 64th Annual Meeting. We thank them for their commitment to the Institute on Lake Superior Geology. All of the funds contributed this year go toward supporting student travel and registration. INDIVIDUAL CONTRIBUTORS TO STUDENT TRAVEL SCHOLARSHIP MARY KAY ARTHUR STEVEN BAUMANN L. GORDON MEDARIS, JR. With an especially generous donation provided by RON SEAVOY xxi �REPORT OF THE CHAIRS OF THE 63rd ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY WAWA, ONTARIO The 63rd Institute on Lake Superior Geology (ILSG) was held in Wawa, Ontario, Canada on May 8-12, 2017 with headquarters at the Michipicoten Memorial Community Centre. This was only the second time in the 63 year history of ILSG that the annual meeting has been held in Wawa, the first time in 1987. The meeting was held completely during regular work days (M-F) with technical sessions on Wednesday and Thursday breaking with past tradition of technical sessions being on Thursday and Friday with post-meeting field trips on Saturday. The meeting was cochaired by Anthony Pace (Ontario Geological Survey, Ministry of Northern Development and Mines, Sault Ste Marie, Ontario), Ann Wilson (Ontario Geological Survey, Ministry of Northern Development and Mines, Timmins, Ontario) and Ted Bornhorst (A. E. Seaman Mineral Museum, Michigan Technological University, Houghton, Michigan). Margaret Hanson (A. E. Seaman Mineral Museum, Michigan Technological University, Houghton, Michigan) was registrar for the meeting and co-editor and co-compiler of the two parts of the Proceedings Volume. The meeting was attended by a total of 135 registrants, which exceeded our expected 100 registrants. This included 37 students which is near 30 %, similar to but slightly less than student participation in recent past meetings. The technical sessions and field trips are equally important components of the annual ILSG meeting. The two days of technical sessions included a total of 28 oral presentations (15 by students) and 22 poster presentations (11 by students). The oral presentations included a wide variety of geologic topics from across the Lake Superior region. They were organized to provide a mix of professional and student presentations rather than by themes. There were no student oral presentations scheduled on the afternoon of the second day to facilitate the decision making for the Doug Duskin Student Paper Awards. A separate block of time was set aside during the technical sessions for poster presentations rather than the past practice of poster sessions being held during the social and coffee breaks. The best student oral presentation was by Ross Salerno (University of Minnesota, Duluth) who presented on the Vermilion Granitic Complex of northern Minnesota; the best student poster presentation was by Morgan Sanger (University of Wisconsin, Madison) who presented on seismic interpretation of the Midcontinent rift. We are especially grateful to the members of the Student Paper Awards Committee who must attend each and every talk and truly listen to them! Each year overall presentations by students are improving and this makes the task of identifying the best among them more and more difficult. We thank Mark Puumala (Ontario Geological Survey), Amy Radakovich (Minnesota Geological Survey), and Laurel Woodruff (U. S. Geological Survey) for being willing to judge the student papers. Field trips are an essential and important part of the ILSG annual meeting. All of the field trips were filled to capacity with 85 participating in at least one field trip. There were six field trips for the Wawa meeting, three pre-meeting and three post-meeting. The pre-meeting field trip #1 was a two day trip on May 8 and 9, 2017 that was based in Marathon, Ontario, about 190 km driving distance northwest of Wawa: Archean and Proterozoic geology of the Marathon-Hemlo area led by Allan MacTavish (Panoramic PGMs Canada Ltd), Mark Puumala, Mark Symk, and Tom Muir (Ontario Geological Survery), David Good (University of Western Ontario), and John McBride (Stillwater Canada Inc.). The other two pre-meeting field trips, #2 and #3, were one day xxii �on May 9, 2017 based out of Wawa: More unusual diamond-bearing rocks of the Wawa area led by Ann C. Wilson (Ontario Geological Survey) and Geology of the Wawa gold project led by Jean-Francois Montrueuil, Quentin Yarie, and Conrad Dix (Red Pine Exploration Ltd.). Two of the three post-meeting field trips (#4 and #5) were one day on May 12, 2017 based out of Wawa: Geology of the Island Gold Mine led by Doug MacMillan, S. Comtois-Urban, and Harold Tracanelli (Richmont Gold Mines Ltd.) and Geology of the Renabie area led by Lise Robichaud (Ontario Geological Survey) and Jordan McDivitt (Laurentian University). The other postmeeting field trip (#6) was for one day on May 12, 2017 but relocated late afternoon for overnight in Chapleau, Ontario: Kapuskasing structural zone and Borden Lake Gold deposit led by Pierre Bousquet (Ontario Geological Survey) and Jason Rickard (Goldcorp Inc.). The annual ILSG banquet was held at the Michipicoten Memorial Community Centre on Wednesday evening, May 10 and was attended by 81 individuals. The attendees were treated to a home cooked banquet meal followed by awarding of the the 2017 Goldich Medal to Philip Fralick of Lakehead University. Mark Smyk (Ontario Geological Survey) presented a summary of Phil's contributions to the understanding of Lake Superior geology to banquet attendees prior to awarding him the Goldich Medal. Phil has made significant contributions to the ILSG since 1985; he co-chaired the annual meeting in 2000 and has contributed to more the 75 ILSG abstracts and field trip guidebooks. The banquet presentation was by Johanna Rowe (historian and author from Wawa) who enlightened us on people involved in the long mining history of the Michipicoten area. Several in the local community came to the talk including Mickey Clement who was the first person to bring a sample of diamonds to the Wawa field office; the attendees gave him a round of applause. At the 2016 Board of Directors meeting in Duluth the board adopted a new award, Pioneer of Lake Superior Geology, at the suggestion of Gene LaBerge, 1995 Goldich Medalist and Chair of the 1984 ILSG. The co-chairs selected Douglass Houghton (1809-1845) as the first ILSG "Pioneer of Lake Superior Geology." As the first formal presentation, Ted Bornhorst introduced the new award program and encouraged nominations to be sent to the 2017 co-chairs and followed his introduction of the award by a biographical sketch of honoree Douglass Houghton focused on the attributes that led to his success at such a young age. Pioneers of Lake Superior Geology have contributed to the understanding of geology in the Lake Superior region primarily before the inception of the ILSG in 1955. The Eisenbrey Student Travel Awards are supported by the Institute on Lake Superior Geology and by generous donations by corporations, societies, and individuals. A total of $2,500 US was awarded to students with varying amounts based on distance from Wawa, being senior author on a presentation, traveling with another senior author student presenter, and/or traveling with another student. Only students who applied by the deadline were given an award. This year we thank Argonaut Gold, Geological Society of Minnesota, Mary Kay Arthur, Gordon Medaris, and Ron Seavoy for providing funds, in addition to ILSG, to help us support student participation in the annual meeting of ILSG. The following students were provided financial assistance to attend the meeting in Wawa: Stephen Hanson, Ann Hunt, Ross Salerno, Margaret Upton (University of Minnesota, Duluth), Munira Afroz, Kira Arnold, Brittany Ramsay (Lakehead University), Morgan Sanger, Luke Schranz (University of Wisconsin, Madison), Juliana Olsen-Valdez, and David Wilkes (Lawrence University). xxiii �The Institute’s Board of Directors met on Thursday May 5th to discuss the business of the Institute. The meeting was attended by meeting co-chairs Anthony Pace, Ann Wilson, Theodore Bornhorst, Rob Cundari (2018), Jim Miller (2017), Treasurer Mark Jirsa, Secretary Peter Hollings and guests Esther Stewart, Laurel Woodruff, and Bill Cannon. Secretary Hollings took the minutes of the Board meeting that are as follows: 1. Accepted report of the Chairs for the 62nd ILSG, Duluth, Minnesota; as printed in the Proceeding Volume (Miller), and minutes of last Board meeting, May 5, 2016 (Hollings) 2. Received, discussed, and accepted 2015-2016 ILSG Financial Summary (Jirsa). 3. Received, discussed, and accepted 2015-2016 report of the Secretary (Hollings). 4. Approved Anthony Pace as on-going ILSG Board member. 5. Discussed and approved renewal of Mark Jirsa as Institute Treasurer (end of term 2020). This was later approved by a vote of the membership. 6. Approved Iron Mountain as the site for the 64th annual ILSG meeting. The meeting will be hosted by Esther Stewart, Laurel Woodruff and Bill Cannon. 7. There was discussion as to future meeting locations in Wisconsin with suggested possibilities of Wisconsin Dells and Terrace Bay. Discussion of other locations included mention of Sudbury. 8. Discussed and approved replacing Helene Lukey as the “member from industry” on Goldich Committee (end of term 2017) with Dan England 9. Discussed the possibility of co-hosting ILSG 2020 with the NCGSA meeting – item was tabled pending further discussion with NCGSA organisers. 10. It was agreed that Amy Radakovich (MGS) would be the second signatory on the ILSG accounts. 11. It was agreed that the local chairs would have the final decision as to whether or not to allow silent auctions in support of the ILSG or affiliated student groups. 12. The topic of the A. E. Seaman Mineral Museum serving as registrar (by electronic and check payment) instead of the meeting Chairs running registration through an outside paid service such as Eventbrite was introduced for discussion by Ted Bornhorst. It was agreed that Ted would provide an estimate of the costs involved. 13. It was agreed that the hosting of the ILSG volumes would be relocated from the PRC server to Hollings’ account on Lakehead University servers. Hollings to complete the move ASAP. The dedication and perseverance of the local businesses played an important role in the success of the 2017 ILSG meeting. The co-chairs thanked the community of Wawa through a letter to the Mayor of Wawa, Ron Rody: The staff of the Wawa Economic Development Corporation for providing us with a list of motel accommodations, community contacts and providing all participants with a bag Wawa souvenirs. The staff of the Michipicoten Community Centre, who provided us with the venue to host this event and the staff that worked the bar during our evening social and banquet. We express our sincere gratitude towards Judy Moore and her staff, who catered the event. Many who attended complimented on the food and service she provided. A job well done! The staff of the local Subway shop, who provided the lunches for 5 of the 6 geological field trips during the week. Larry Lacroix of Lloyd’s of Wawa who provided the school bus transportation that was needed for the geological field trips throughout the Wawa and Chapleau areas. Matt Larrett from Michipicoten High School, who provided us with the speaker system for the two days of technical sessions. We thanked the local motels and lastly, Johanna Rowe, who was our guest speaker at the banquet. xxiv �We the 2017 co-chairs would like to again thank all those who continue to make ILSG one of the best regional geoscience meetings in North America: participants, presenters, field trip leaders, session chairs, best student paper committee members, Goldich committee members, ILSG Board members and the incoming 2018 chairs. We appreciated all of support and positive comments about the Wawa meeting and look forward to seeing many of you at the 2018 ILSG in Iron Mountain. Respectfully submitted, Theodore J. Bornhorst, Anthony Pace, and Ann C. Wilson Co-chairs, 63rd Institute on Lake Superior Geology xxv �TECHNICAL PROGRAM TUESDAY MAY 15, 2018 Field trips 1 and 2 begin and end at the Pine Mountain Lodge, Iron Mountain, Michigan 8:00 am - 5:00 pm PRE-MEETING FIELD TRIPS 1) Archean and Paleoproterozoic Geology of the Felch District, Central Dickinson County, Michigan Bill Cannon, Klaus Schulz, Robert Ayuso - U.S. Geological Survey Tom Mroz – BSGE, MSPG, CPG 2) Geology of the Hemlock Formation Tom Waggoner – Consulting Geologist 4:00 pm - 10:00 pm Registration (Pine Mountain Lodge) 7:00 pm - 10:00 pm Welcoming Reception (Pine Mountain Lodge) Poster Session (Pine Mountain Lodge) WEDNESDAY MAY 16, 2018 7:30 am – 11:30 am Registration (Pine Mountain Lodge) 8:00 OPENING REMARKS (Pine Mountain Lodge) Laurel Woodruff, Bill Cannon, Esther K. Stewart, Co-Chairs, 2018 ILSG TECHNICAL SESSION I Session Chairs: Shannon Zurevinski – Lakehead University Ben Drenth – U.S. Geological Survey 8:10 Christian Schardt and Mady David High-technology metal behavior in ore-forming environments and its implication for the Vermilion District, northern Minnesota 8:30 Andrea Reed Pilot study results for potential lithium mineralization on state-managed mineral rights in Minnesota xxvi �8:50 *Matthew W. Matko and Christian Schardt Microanalysis of rock and mineral textures and its relationship to mineralization and ore comminution 9:10 Jeffrey L. Mauk Geochemical signatures of hydrothermal alteration in clastic sedimentary rocks: theory, recognition, and application 9:30 COFFEE BREAK 9:50 Robert Cundari, Mark Smyk, Dorothy Campbell, and Mark Puumala Possible emplacement controls on diamond-bearing rocks north of Lake Superior 10:10 *Joseph Rasmussen, Esther Kingsbury Stewart, John Skalbeck, and Madeline Gotkowitz Modeling the Precambrian topography of Columbia County, Wisconsin using twodimensional models of gravity and aeromagnetic data and well construction reports 10:30 David Southwick, Val Chandler, and Mark Jirsa Geophysical, structural, and tectonic interpretation of the Yellow Medicine and Appleton shear zones, SW Minnesota and SE South Dakota: A work in progress 10:50 Val Chandler, David Southwick, and Mark Jirsa Recent gravity and magnetic investigations of the Minnesota River Valley Subprovince: New insights into ancient problems 11:10 Benjamin J. Drenth, Laurel G. Woodruff, Klaus J. Schulz, William F. Cannon, and Robert A. Ayuso On the source(s) of the Felch-Arnold gravity anomaly, Upper Peninsula, Michigan 11:30 End of Technical Session I 11:30 LUNCH BREAK ILSG BOARD OF DIRECTORS MEETING TECHNICAL SESSION II Session Chairs: Dean Peterson – Natural Resources Research Institute Suzanne Nicholson – U.S. Geological Survey 1:00 *Kira Arnold, Pete Hollings, Seamus Magnus, Shannon Zurevinski, and Robert Creaser Geology and geochemistry of the Terrace Bay Batholith, N. Ontario xxvii �1:20 *Simon Dolega and Philip Fralick Geochemistry of shallow and deep water Archean meta-iron formations and their post depositional alteration in Western Superior Province, Canada 1:40 *Victoria Stinson and Y. Pan Neoarchean to Paleoproterozoic reconstructions using metamorphic core complexes as evidence of continental transform plate motion and their implications in Archean tectonics 2:00 Wouter Bleeker Archean BIF clasts vs. Paleoproterozoic jasper clasts? The proof is in the pudding (stone) 2:20 COFFEE BREAK 2:40 Paul Eger, Courtnay Bot, Dave Meineke, and Dave Adams What to do after the bull has left the china shop- Picking up the community relation pieces 3:00 *Vittoria Smith and Shannon Zurevinski Petrology and 11B composition of tourmaline within the 2685 Ma Ghost Lake Batholith and Mavis Lake Pegmatites 3:20 Klaus J. Schulz, William F. Cannon, and Laurel G. Woodruff Geochemistry of mafic rocks in Dickinson County, Michigan: Evidence for ~2.1 Ga Rifting 3:40 Thomas W. Buchholz, Alexander U. Falster, and Wm. B. Simmons Possible alumotantite from the Nine Mile pluton, Wausau Complex, Marathon County, WI. 4:00 POSTER VIEWING- AUTHORS WILL BE PRESENT AT THEIR POSTERS 5:00 END OF TECHNICAL SESSION II 6:00 RECEPTION AND CASH BAR (Pine Mountain Lodge) 7:00 ANNUAL BANQUET (Pine Mountain Lodge) • Announcement of 65th Annual Meeting Location • • 2018 Goldich Award Presentation to Val Chandler Banquet Presentation - Nancy Langston (Michigan Technological University) Presentation title: Sustaining Lake Superior xxviii �THURSDAY MAY 17, 2018 8:00 OPENING REMARKS, UPDATES (Pine Mountain Lodge) Laurel Woodruff, Bill Cannon, Esther K. Stewart, Co-Chairs, 2018 ILSG TECHNICAL SESSION III Session Chairs: Amy Radakovich – Minnesota Geological Survey Daniel Holm – Kent State University 8:10 Daniel Holm, Terrence J. Boerboom, and Scott Scheiner Reinterpretation of the ages of deposition and folding of Animikie Basin metasedimentary units in east-central Minnesota 8:30 Joshua J. Schwartz, Esther Kingsbury Stewart, and L. Gordon Medaris Jr.+ Detrital zircons in the Waterloo Quartzite, Wisconsin: Implications for the ages of deposition and folding of supermature quartzites in the Southern Lake Superior Region 8:50 Brad Gottschalk, Caroline Rose, and M. Carol Mccartney Geologic history meets the web – online data of the Lake Superior Division of USGS 9:10 William J. Hinze Mapping the Midcontinent Rift System 9:30 COFFEE BREAK 9:50 Jennifer Smith, Wouter Bleeker, Dean Rossell, and Justin Laberge Compositional and geochemical characteristics of the Crystal Lake intrusion, Ontario 10:10 Sean O’Brien, Pete Hollings+, and Jim Miller Geology of the Crystal Lake Gabbro and the Mount Mollie Dyke, Midcontinent Rift, Northwest Ontario 10:30 *Dustin A. Liikane, Wouter Bleeker, Mike Hamilton, Sandra Kamo, Jennifer Smith, Peter Hollings, Robert Cundari, and Michael Easton Controls on the localization and timing of mineralized intrusions within the ca. 1.1 Ga Midcontinent Rift system 10:50 David Good Petrogenesis of mafic magmatism in the Coldwell Complex Part 1. Geochemical model to explain origin of metabasalt by partial melting in the SCLM xxix �11:10 Evgeniy Kulakov, Theodore J. Bornhorst+, Chad Deering, and James B. Moore The youngest magmatic activity of the Midcontinent Rift at Bear Lake, Keweenaw Peninsula, Michigan 11:30 End of Technical Session III 11:30 LUNCH BREAK TECHNICAL SESSION IV Session Chairs: Marcia Bjørnerud – Lawrence University John Esch – Michigan Department of Environmental Quality 1:00 Kelli McCormick, Kevin Chamberlain, and Colin Paterson An 1149 Ma U-Pb baddeleyite crystallization age and geochemistry of gabbroic intrusions at the southwestern margin of the Superior Craton, southeastern South Dakota 1:20 Jim DeGraff and B.T. Carter Thrust Kinematics of the Keweenaw Fault North of Portage Lake, Michigan 1:40 John A. Yellich Michigan Geological Survey six years after assignment to Western Michigan University, where are we today? 2:00 John M. Esch LiDAR Revolutionizing Geological Mapping 2:20 COFFEE BREAK 2:40 Dean M. Peterson Assembling Minnesota: Integration of 140 years of government, academic, and industry geologic studies into a seamless statewide GIS database 3:00 Mark A. Jirsa and others On-going geologic mapping in Minnesota’s Arrowhead Region by the Minnesota Geological Survey 3:20 John M. Esch, Alan Kehew, Sebastian Huot, and John Yellich Surficial geology of the Iron Mountain 7.5 Minute Quadrangle, Dickinson County, Michigan, Florence & Marinette Counties, Wisconsin xxx �3:40 Phil Larson, George Hudak, Al Mactavish, Peter Hinz, Amy Radakovich, Juk Bhattacharyya, Paula Engelhardt, Steve Engelhardt, Brigitte Gelnias, David Good, Emily Gorner, Sheree Hinz, Peter Jongewaard, Deb Kroch, Matt Svensson, and Andrew Tims Land of fire and ice: Summary of the 2017 ILSG field trip to Iceland 4:20 BEST STUDENT PAPER AWARDS STUDENT TRAVEL AWARDS 4:40 END OF TECHNICAL SESSIONS * 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. xxxi �FRIDAY MAY 18, 2018 8:00am – 5:00pm POST-MEETING FIELD TRIPS Field trips 2 and 3 begin and end at the Pine Mountain Lodge, Iron Mountain, Michigan 3) Geology and Iron Ores of the Menominee Iron Range, Dickinson County, Michigan Tom Mroz – BSGE, MSPG, CPG Bill Cannon – U.S. Geological Survey 4) Granitoid rocks of the Pembine-Wausau Terrane in northeastern Wisconsin Klaus Schulz – U.S. Geological Survey Marcia Bjørnerud – Lawrence University xxxii �POSTER PRESENTATIONS ANDERSON, Eric, SCHULZ, Klaus, DRENTH, Benjamin, CANNON, William, and QUIGLEY, Thomas New gravity and high-resolution aeromagnetic data provide insights into Precambrian geology in the eastern Pembine-Wausau terrane *ASHAUER, Zachary, CURRIER, Ryan, NORFLEET, Mark Textural analyses of rapakivi mantles: Evidence for semi-selective replacement in Proterozoic rapakivi granites AYUSO, R.A., SCHULZ, K.J., CANNON, W.F., WOODRUFF, L.G., VAZQUEZ, J.A., FOLEY, N.K., and JACKSON, J. New U-Pb zircon ages for rocks from the Granite-Gneiss Terrane in Northern Michigan: Evidence for events at ~3750, 2750, and 1850 Ma *BLOTZ, Kaelyn E., LODGE, Robert W.D. Ore petrography of the Flambeau volcanogenic massive sulfide deposit, northwestern Wisconsin: Implications for hydrothermal fluid composition BOERBOOM, Terrence J. Fault-controlled dike emplacement in the Grand Marais, Minnesota area *DRAZAN, Jacqueline, BRENGMAN, Latisha, FEDO, Christopher Preliminary petrographic and geochemical investigation of silicified volcanic rocks and silica-rich exhalative rocks from the ~2.7 Ga Abitibi Greenstone Belt, Canada EASTON, Robert M. GEON 12 to 11 history of the Lake Superior Region and speculation about the relationships between the Midcontinent Rift and the Grenville Orogen ESCH, John M, KEHEW, Alan, HUOT, Sebastien, YELLICH, John Surficial geology of the Iron Mountain 7.5 Minute Quadrangle, Dickinson County, Michigan, Florence & Marinette Counties, Wisconsin *FITZPATRICK, William, HOOPER, Robert, and LODGE, Robert, Gélinas, Brigitte Mineral chemistries of the Tower Mountain Intrusive Complex Au-deposit, Ontario GRAUCH, V.J.S., BEDROSIAN, Paul A., STEWART, Esther Kingsbury, and HELLER, Samuel Inferences on the subsurface distribution of Oronto and Bayfield Groups north and west of the Douglas Fault, Northwestern Wisconsin xxxiii �GREEN, Carlin J., SEAL, Robert, R., II, CANNON, William F., PIATAK, Nadine, and MCALEER, Ryan J. Origin, distribution, morphology, and chemistry of amphiboles in the Ironwood IronFormation, Gogebic Iron Range, Wisconsin, U.S.A. *HAFFTEN, Doug and RADWANY, Molly Geothermobarometry of a Precambrian amphibolite from Cornell WI *HANNACK, Gina, and RADWANY, Molly Hornblende-plagioclase thermometry of the Eau Claire River Complex, western Wisconsin *HONE, Samuel V. and ZIEG, Michael J. Olivine crystal size distribution in the Black Sturgeon Sill, Nipigon, Ontario *JACOBSON, Regan E., LODGE, Robert W.D Reconstructing Paleoproterozoic volcanism in northwestern Wisconsin: Geochemistry of the Flambeau Cu-Zn-Au Mine JIRSA, Mark A., STARNS, Edward C., and SCHMITZ, Mark D. Geology and geochronology of the 2006 Cavity Lake forest fire area, Boundary Waters Canoe Area Wilderness, NE Minnesota KINGSBURY STEWART, Esther, STEWART, Eric D., and ROUSHAR, Kathy New bedrock geologic mapping of Dodge County, Wisconsin provides evidence for Paleozoic reactivation of Precambrian structures *LIIKANE, Dustin A., BLEEKER, Wouter, HAMILTON, Mike, KAMO, Sandra, SMITH, Jennifer, HOLLINGS, Peter, CUNDARI, Robert, and EASTON, Michael Controls on the localization and timing of mineralized intrusions within the ca. 1.1 Ga Midcontinent Rift system MATTOX, Stephen, BOLHUIS, Chris, and SOBOLAK, Christina Using credit-by-exam to connect advanced high school geology courses to university geology departments: Current status of a state-wide program in Michigan *OLSEN-VALDEZ, Juliana and BJØRNERUD, Marcia The Brussels Hill Structure, Door County, Wisconsin: Impact crater, diatreme or other? *OLSON, Maile J., LODGE, Robert W. D. Komatiite-hosted nickel-copper mineralization potential in the eastern Shebandowan Greenstone Belt, Ontario, Canada * ROSE, Katharine; ESSIG, Espree, and THAKURTA, Joyashish Variation trends in sulfur isotope ratios at the Eagle and East Eagle intrusions and the surrounding country and basement rocks of the Baraga Basin, Upper Peninsula, Michigan xxxiv �*RUPP, Kevin, THAKURTA, Joyashish, and MAHIN, Robert Preliminary investigation of the East Eagle Intrusion Gabbro in Marquette County, Michigan. * TYRRELL, C.W., HUBBELL, G.E., and DEGRAFF, J.M. Keweenaw Fault geometry and kinematics along Bête Grise Bay, Michigan *UPTON, Margaret, SCHARDT, Christian, HUDAK, George, QUIGLEY, Eric Alteration mineral zonation and geochemical characteristics of the Back Forty Deposit, MI; a replacement-style zinc- and gold-rich volcanogenic massive sulfide deposit * VALL, Kathryn G., STEINMAN, Byron A., POMPEANI, David P., SCHREINER, Kathryn M., DEPASQUAL, Seth Reconstruction of paleoenvironmental conditions and temporal patterns of ancient mining on Isle Royale using biogeochemical analyses of lake sediment YELLICH, John A. Michigan Geological Survey Six years after assignment to Western Michigan University, Where are we today? ZIEG, Michael J. and HONE, Samuel V. The Origin of Layering in the Olivine Zone, 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. xxxv �ABSTRACTS xxxvi �New gravity and high-resolution aeromagnetic data provide insights into Precambrian geology in the eastern Pembine-Wausau terrane ANDERSON, Eric1, SCHULZ, Klaus2, DRENTH, Benjamin1, CANNON, William2, and QUIGLEY, Thomas3 1 US Geological Survey, MS 964, PO Box 25046, Denver, CO 80225 USA 2 US Geological Survey, 12201 Sunrise Valley Dr., MS 954, Reston, VA 20192 USA 3 Great Lakes Exploration Inc., Menominee, MI 49858 USA The Pembine-Wausau terrane represents a major Paleoproterozoic belt of metavolcanic and intrusive rocks that formed in an island-arc setting at the southern limit of the Archean Superior Craton (Schulz and Cannon, 2007). The island-arc complex was accreted to the continental margin along the Niagara fault zone during the Penokean orogeny and subsequently intruded by syn- to post-tectonic granitoids. The terrane is known to host a number of significant volcanogenic massive sulfide deposits including the Back Forty deposit. Limited outcrop makes bedrock mapping difficult. In 2016, the USGS contracted a high-resolution aeromagnetic survey over parts of the Pembine-Wausau terrane. The data were collected along north-south flight lines spaced 150 m at a nominal height of 80 m. These data, along with existing and in-fill gravity stations and physical property measurements, are helping improve Precambrian bedrock maps (Figure 1; Sims, 1990; Sims and Schulz, 1993). The complete (terrain-corrected) Bouguer gravity anomaly map shows strong gradients, indicating significant lateral variations in bedrock density. A west-northwest trending linear high with ~5 mGal amplitude occurs along splays of the Niagara fault and correlates with mapped mafic-ultramafic rocks having measured density of ~2.90 g/cm3. South of the Niagara fault zone, a strong gravity gradient trends east-west along the southern mapped extent of the McAllister Formation that consists of basaltic and andesitic rocks with a density of ~2.91 g/cm3. South of the gradient is a linear low that expands to the south and east beneath Phanerozoic cover. A broad high with amplitude of ~15 mGal occurs over the southern extent of the Athelstane and Amberg granites. However, the anomaly differs from geologic map patterns and aeromagnetic magnetic anomalies and so the source is not well understood. A standard reduction-to-pole (RTP) transformation was applied to the aeromagnetic data to better align anomalies with causative sources. The RTP map shows broad positive anomalies with amplitude around 2000 nT north of the Niagara fault. Between fault splays is a series of west-northwest trending linear highs with amplitude around 1000 nT. The linear features are 1.5 to 3 km-long and 500 m-wide; some of these correlate with mapped mafic-ultramafic rocks. The Pembine ophiolite rocks are well imaged by ~2000 nT anomaly high; several additional high amplitude anomalies occur within the Quinnesec Formation. A west-northwest trending aeromagnetic gradient is observed at the southern extent of the McAllister Formation that broadly parallels the strong gravity gradient. To the south are linear north-south to northnortheast trending magnetic highs that extend for more than 10 km. These linear features have 1 �amplitudes between 5 and 60 nT. Near Amberg, they are associated with diabase dikes (magnetic susceptibilities of about 15 x 10-3SI) that cut the late tectonic Athelstane Quartz Monzonite. At the southern end of the mapped Athelstane and Amberg granites is an oval magnetic high that trends northeastward contradictory to geologic map patterns. The amplitude (~325 nT) is significantly less than the anomalies observed over the mafic-ultramafic rocks to the north, suggesting a different source rock composition. The tilt and first vertical derivative maps were derived from the RTP data to accentuate near surface and subtle magnetic features. Both maps show linear trends that change orientations proximal to gravity gradients. The prominent north-south to northeast trending magnetic lineaments do not appear to extend much beyond the gravity gradient into the McAllister Formation. In addition, these lineaments appear to have several subparallel northeast trending discontinuities. Magnetic lineaments in the Pemene Formation parallel mapped trends in the volcanic rocks. Near the Niagara fault the extent of the west-northwest trending magnetic lineaments is much better resolved in the derivative maps than in the RTP maps. Figure 1: Geologic map (A) and reduced-to-pole (RTP) anomaly map (B) of the study area. References Schulz, K.J. and Cannon, W.F., 2007. The Penokean orogeny in the Lake Superior region, Precambrian Research, 157: 4-25. Sims, P.K., 1990. Geologic map of Precambrian rocks of Iron Mountain and Escanaba 1° x 2° quadrangles, northeastern Wisconsin and northwestern Michigan, U.S. Geological Survey Miscellaneous Investigations Series, Map I–2056, scale 1:250,000. Sims, P.K., and Schulz, K.J., 1993. Geologic map of Precambrian rocks of parts of Iron Mountain and Escanaba 30’ x 60’ quadrangles, northeastern Wisconsin and adjacent Michigan, U.S. Geological Survey Miscellaneous Investigations Series, Map I–2356, scale 1:100,000. 2 �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 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. 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). This study describes and classifies the Terrace Bay batholith in order to investigate its petrogenesis and related gold and base metal mineralization. The core of the Terrace Bay Batholith is a massive, homogeneous equigranular and locally quartz porphyritic granodiorite. The granodiorite typically consists of medium- to coarsegrained quartz and feldspar phenocrysts with a groundmass of fine-grained feldspars, quartz, amphibole, biotite and disseminated magnetite and sulphide minerals. Multiple outcrops in the center of the batholith host very coarse-grained phenocrysts of feldspar, ranging in size from 1 to 3 cm. An outcrop of diorite was found in the center of the pluton, composed of medium-grained amphibole and plagioclase, with very few quartz crystals. Some areas of the diorite outcrop are monzodioritic, with over 5% potassium feldspar. Thick overburden which covers the contacts with the granodiorite making their relationship uncertain, but the diorite likely represents either a more mafic phase of the granitic magma or possibly an autolith. Geochemically the granodiorite is classified as I-type granite. It has a calc-alkaline signature, characteristic of rocks formed above a subduction zone. On a primitive mantlenormalized trace element profile the samples are LREE enriched with unfractionated HREE and prominent negative Nb-Ti anomalies. The diorite shows a similar trend to the granodiorite suggesting that it was formed from a similar if not the same source. Two distinct alteration styles have been observed in the pluton; a common pervasive potassium-hematite alteration and a less common chlorite and epidote alteration. The chlorite– epidote variety is an intense alteration but is restricted to veins and dykes. The potassiumhematite alteration has been observed across the batholith. In most cases, the groundmass is obliterated and composed of fine- to very fine-grained hematite and potassium feldspars. Phenocrysts of quartz are typically unaltered but relict feldspars have been sericite altered. The chlorite-epidote alteration is generally composed of fine-grained chlorite and epidote in the groundmass with quartz phenocrysts and relict sericite-altered feldspars. The pluton is crosscut by quartz carbonate veins, which locally contains black tourmaline along the vein contacts. Mineralization in the quartz veins includes pyrite, chalcopyrite, galena, molybdenite and arsenopyrite. In contrast, the pluton typically hosts only pyrite and molybdenite. Generally the molybdenite present in the granite is disseminated, but has been 3 �found to occur in coarse pods up to 3 cm wide. Occurrences of molybdenum mineralization are spatially correlative with the gold mineralized occurrences, which are most commonly located in quartz-veined and altered zones near the contacts of the pluton. A molybdenite sample yielded a mineralization age of 2671 +/- 12 Ma. 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. and Magnus, S.J. 2017. Geology and mineral potential of the Terrace Bay pluton, western Schreiber–Hemlo greenstone belt; in Summary of Field Work and Other Activities, 2017, Ontario Geological Survey, Open File Report 6333, p.12-1 to 12 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. 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. 485-539. 4 �Textural Analyses of Rapakivi Mantles: Evidence for Semi-Selective Replacement in Proterozoic Rapakivi Granites ASHAUER, Zachary1, CURRIER, Ryan1, NORFLEET, Mark1 1 Natural and Applied Sciences, University of Wisconsin Green Bay, 2420 Nicolet Dr. Green Bay, Wisconsin 54311 Rapakivi granite complexes are commonly associated with caldera forming eruptions (Karell et al., 2014). Characteristic of these granites is the rapakivi texture, the mantling of plagioclase on K-feldspar meagcrysts. First described in scientific literature by Sederholm in 1891, consensus on a model for rapakivi texture formation remains elusive. Textural analyses that help constrain the mechanism of formation are presented here. The textural analysis consists of a systematic survey of mantle thicknesses in relation to the mantled feldspar radius, and was conducted on two classical rapakivi systems: the Wolf River Batholith (1.48-1.46 Ga; Dewane and Van Schmus, 2007) Wisconsin and the Wiborg Batholith (1.65-1.62 Ga; Rämö, 1991) southeastern Finland. Mantle analysis samples were obtained from dimension stone slabs of the Waupaca Wiborgite in the Wisconsin Capitol Rotunda and of Ylämaa Wiborgite (Baltic Brown) slabs from five Green Bay area businesses. The Wolf River and Wiborg batholiths are A-type intrusive bodies underlying areas >9,000 km2, containing wiborgite variety of rapakivi granite, Waupaca Wiborgite and Ylämaa Wiborgite, respectively. Wiborgite is a porphyritic granite containing ovoidal megacrysts of Kfeldspar ranging upwards of 6 cm in diameter, with over 50% of megacrysts mantled by 1-4 mm of plagioclase. The Waupaca Wiborgite contains a greater population of euhedral K-feldspars than the Ylämaa Wiborgite where nearly all crystals are ovoidal in shape. Mantled feldspar cores and mantles were outlined by hand from high-resolution pictures of slabs. Mantle and core area dimensions were calculated using image analysis software Image J (Schneider et al., 2012) and converted to respective radii for comparison. Results illustrate a trend of thickest mantles developing on the middle size class of crystals, which is consistent across all samples and the two separate systems (Figure 1). Data density plots stretch out along the x-axis; implying larger radius crystals generally have smaller thickness mantles. To properly interpret results, a model was produced replicating variable mantle and crystal radii size scenarios observed from a 2D slice of mantled spheres. The model evaluates three scenarios of mantle thickness in relation to increasing mantled feldspar radius: (1) mantle thickness is variable with crystal radius, (2) mantle thickness is a consistent proportion of crystal radius, and (3) mantle thickness decreases with increasing crystal radius. Scenarios 1 and 2 overestimate mantle thicknesses and display data distributions inconsistent with mantle analysis results (Figure 2). Scenario 3 closely resembles mantle analysis results showing thickest mantles occur on the middle size class of crystals and concentrate closest to the x-axis. Mantle analysis coupled with theoretical modeling suggests mantle thickness has dependence on mantled feldspar size. This is interpreted as forming within a contact melt zone, driven by underplating of hot magma, which resulted in vigorous stirring once a tipping point is reached through buoyant instability. This model thus suggests dissolution-controlled replacement mantle growth in an up-temperature regime, consistent with caldera volcanism. 5 �Figure 1. Mantle thickness (radius of mantled feldspar – radius of core feldspar) plotted against radius of mantled feldspar. A-E independent slabs of Ylämaa Wiborgite, F compiled slabs of Waupaca Wiborgite. Notice middle size class of crystals generally have thickest mantles. Figure 2. Mantle thickness (radius of mantled feldspar – radius of core feldspar) plotted against radius of mantled feldspar. (A) Random thickness mantle scenario, (B) consistent proportion of mantle thickness to mantled feldspar size, and (C) mantle thickness decreases with increasing mantled feldspar size. Notice y-axis scale is double that of mantle analysis results. References: Dewane, T.J., Van Schmus, W.R., 2007. U-Pb geochronology of the Wolf River batholith, north-central Wisconsin: Evidence for successive magmatism between 1484 Ma and 1468 Ma. Precambrian Research 157, 215-234. Karell, F., Ehlers, C., Airo, M., 2014. Emplacement and magmatic fabrics of rapakivi granite intrusions within Wiborg and Aland rapakivi granite batholiths in Finland. Tectonophysics 614, 31-43. Rämö, O.T., 1991. Petrogenesis of the Proterozioic rapakivi granites and related basic rocks of southeastern Fennoscandia: Nd and Pb isotopic and general geochemical constraints. Geological Survey of Finland, Bulletin 355, 161p. Schneider, C.A., Rasband, W.S., Eliceiri, K.W., 2012. NIH Image to ImageJ: 25 years of image analysis. Nature methods 9 (7): 671-675. Sederholm, J.J., 1891. Ueber die finnländischen Rapakiwigesteine. Tschermak's Miner. Petrograh. Mitth. 12, 1-31. 6 �New U-Pb Zircon Ages for Rocks from the Granite-Gneiss Terrane in Northern Michigan: Evidence for Events at ~3750, 2750, and 1850 Ma AYUSO, R.A.1, SCHULZ, K.J.1, CANNON, W.F.1, WOODRUFF, L.G.2, VAZQUEZ, J.A.3, FOLEY, N.K. 1, and JACKSON, J. 1 1 U.S. Geological Survey, Reston, VA 20192, 2 U.S. Geological Survey, Mounds View, MN 55112, 3 U.S. Geological Survey, Menlo Park, CA 94025 Early Archean rocks are part of the granite-gneiss terrane along the southern margin of the Superior craton [1]. Recently, we reported preliminary zircon age data from the Carney Lake gneiss in the granitegneiss terrane of northern Michigan that indicated an Eoarchean component ca. 3750 Ma [2]. Here we report additional sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon ages for the Carney Lake gneiss that further document the Eoarchean component. We also report new U-Pb zircon ages for the Hardwood gneiss complex, the Peavy Pond complex, and a porphyritic red granite from northern Michigan. Two samples were collected from the Carney Lake gneiss. Zircons were obtained from a granitic K-feldspar-bearing gneiss that is locally pegmatitic; the zircons range from anhedral to subhedral, contain complex irregular growth zoning, and display multiple growth rims. Zircons also were obtained from a banded and folded gray to red granitic gneiss; the zircons are slightly rounded to subhedral. One sample each was collected from the Hardwood gneiss complex, Peavy Pond complex, and porphyritic red granite. The Hardwood gneiss sample is a fine-grained, layered garnet-pyroxene-quartz-magnetite gneiss (granulite grade) that contains brown and mostly anhedral zircons. The Peavy Pond sample is a mediumgrained granite with honey-colored euhedral to subhedral zircons characterized by doubly terminated prisms. The porphyritic red granite is foliated, contains K-feldspar augen, and has clear to pale brown subhedral zircons that commonly display igneous oscillatory bands. SHRIMP U-Pb data were obtained on handpicked zircons. All peaks, including U, Th, Pb, REE, Hf, Ti, and Y, were measured sequentially. Raw data were reduced using the Squid 2 and Isoplot programs [3, 4]. Figure 1: A. Concordia diagram for 129 spot analyses from zircons in the Carney Lake gneiss. B. Concordia diagram for 56 spot analyses from zircons in the Hardwood gneiss. On a Concordia diagram, U-Pb data for Carney Lake show clusters with points ranging from concordant to discordant (Fig. 1A). The predominant data cluster of nearly concordant points has an intercept ca. 2750 Ma; a smaller concentration of nearly concordant analyses occurs at ca. 3750 Ma. One possible data 7 �alignment spans from an upper intercept age of ca. 3750 Ma to the lower intercept age of ca. 2750 Ma; a second possible alignment spans a range from 2750 Ma toward an imprecisely defined intercept around 1000 Ma (Fig. 1A). The ca. 3750 Ma age on zircon cores from the Carney Lake gneiss is evidence of an Eoarchean component in the granite-gneiss terrane (Fig. 1A). The gneiss was affected by igneous and thermal events at ca. 2750 (and younger), which resulted in new zircon crystallization, recrystallization, and formation of overgrowths. U-Pb zircon dates for the Hardwood gneiss yielded evidence of a Neoarchean component (concordant spot analyses) ca. 2750-2500 Ma as well as younger dates ca. 1900 Ma (Fig. 1B). U-Pb data for the Peavy Pond complex range from concordant to discordant and plot along a trend intercepting Concordia at ca. 1850 Ma (a small data cluster plots at ca. 2600 Ma) (Fig. 2A). The majority of spots for the red granite is concordant or plots adjacent to Concordia at ca. 2099 Ma (age of crystallization) (Fig. 2B). Figure 2: A. Concordia diagram for 32 spot analyses of zircons from the Peavy Pond complex. B. Concordia diagram for 18 spot analyses of zircons from the porphyritic Red Granite. The zircons show typical REE chondrite-normalized patterns (LREE-depleted, HREE-enriched), negative Eu anomalies, and positive Ce anomalies. The Carney Lake gneiss zircons have the most diverse REE patterns and widely variable Eu and Ce anomalies. The Hardwood gneiss also has diverse REE patterns. Trace element ratio plots (e.g., U/Yb vs. Hf) [5] suggest a continental magmatic origin for zircons from the Carney Lake gneiss, Hardwood gneiss, and Peavy Pond complex. A continental arc (or enriched mantle?) is implicated for zircons from the red granite. The ca. 3750 Ma age of the Carney Lake gneiss documents the presence of an Eoarchean component in northern Michigan. The ca. 2750 Ma of the Hardwood gneiss indicates the contribution of a Neoarchean component in the region. Igneous intrusive events occurred ca. 2750, 2099, and 1850 Ma. There is no evidence for an older Archean component in the porphyritic red granite. References [1] Peterman, Z.E., Zartman, R.E., and Sims, P.K., 1980: Geol. Soc. America Sp. Paper 182, p. 125–134. [2] Ayuso, R.A., Schulz, K.J., Cannon, W.F., Woodruff, L.G., Vazquez, J.A., and Jackson, J., 2017, Institute on Lake Superior Geology, Proceedings 63rd Annual Meeting, part 1, p. 9-10. [3] Ludwig, K.R., 2009: SQUID 2, Berkeley Geochronology Center Special Publication no. 5, 110 p. [4] Ludwig, K.R., 2012: Isoplot 3.75, Berkeley Geochronology Center Special Publication no. 5, 75 p. [5] Grimes, C.B., Wooden, J.W., Cheadle, M.J., and John, B.E., 2015, Contrib. Min. Pet., 170: 46. 8 �Archean BIF clasts vs. Paleoproterozoic jasper clasts? The proof is in the pudding (stone) BLEEKER, Wouter Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario, K1A 0E8 Email: wouter.bleeker@canada.ca The ca. 2.50-2.25 Ga Huronian Supergroup (Bennett et al., 1991; Young et al., 2001), a largely intra-cratonic rift and regional cover sequence overlying the southern Superior craton (Bleeker and Ernst, 2006), contains one of the more important records of Paleoproterozoic Earth evolution. Among other things, it contains a superb volcanic rift sequence including subaerial and subaqueous basalt flows, and the Copper Cliff Rhyolite and its subvolcanic pluton, the Creighton Granite, now precisely dated at 2459±7 Ma (Bleeker et al., 2015). This lower rift sequence is terminated by conglomerate/sandstone and greywacke turbidites of the Matinenda and McKim formations, the former hosting important detrital pyrite and uraninite concentrations that have been mined for uranium in the Elliott Lake area (Roscoe, 1969). Overlying this lower rift sequence are several cycles of conglomerate, sandstone and silt- to mudstone, and back to sandstones, three of which start out with glacial diamictites. Minor erosional disconformities or unconformities are present at the base of these cycles. Diamictites of the second cycle are overlain by carbonates of the Espanola Formation, which may represent cap carbonates and, thus, may constitute a globally important marker horizon. The third and aerially most extensive of these cycles starts with the Gowganda Formation, which consists of glacial diamictites at its base and hosts the first red-bed sandstones of the Huronian toward its top. The Gowganda Formation is overlain by white to red sandstones and conglomerates of the Lorrain Formation, which hosts a conspicuous member of red jasper clast conglomerate, locally known as “puddingstone” (Fig. 1). The red jasper clasts, typically 0.5-5.0 cm in size, in a white to off-white quartzdominated matrix, make for an attractive rock type that Figure 1: Top, red jasper clast in is sought after as a decorative stone. For the last century, these jasper clasts have “puddingstone”, very fine-grained and with delicate lamination and textures, and no been interpreted as being derived from Archean metamorphic minerals (no magnetite). Bottom, basement to the north, particularly the ca. 2720-2725 typical recrystallized Abitibi BIF, at about the Ma, black to sometimes red, banded iron formations same scale, with recrystallized texture, a fabric, and metamorphic magnetite. (BIFs) of the Abitibi greenstone belt. Good exposures 9 �of such BIFs occur in the Timmins area, around Temagami, and in Wawa. They have been mined for iron ore at a number of localities including Temagami (Sherman Mine), south of Kirkland Lake (Adams Mine), and Wawa (Helen Mine). All of these BIFs have seen pervasive deformation and low grade metamorphism, and contain abundant magnetite. Several observations lead me to question this interpretation: the jasper clasts of the Huronian puddingstone are often angular, i.e. more or less proximal; in typical puddingstone they suddenly become a dominant clast type, again suggesting a proximal source; there are few if any real BIF clasts; the jasper clasts are extremely fine-grained and delicately textured (Fig. 1) and do not contain magnetite, unlike BIF samples from the Abitibi which are noticeably more recrystallized (an order of magnitude coarser in grain size) and invariably contain metamorphic magnetite (Fig. 1). I conclude that the conspicuous jasper clasts of Lorrain puddingstone are not of Archean derivation, but rather represent penecontemporaneous reworking of otherwise poorly preserved Huronian jasper deposits, possibly associated with a minor volcanic or hydrothermal centre that has not yet been identified. Given that the occurrence of puddingstone is strongly concentrated in, if not unique to, the area around Bruce Mines, the source jasper beds were likely local deposits restricted to that part of the Huronian basin, possibly the fine-grained siliceous siltstone and associated layers that have been referred to in some of the early papers on the Huronian as “Bruce Mines Jasper” (Collins, 1925). The jasper clasts constitute a range from dark red to pure white chert, all of which show delicate textures and layering. Among the white chert-like clasts some resemble unrecrystallized agate, also suggesting deep weathering and reworking of Paleoproterozoic volcanic units containing agate nodules. Rare accompanying clasts of quartz porphyry may allow dating of this part of the Huronian succession. If indeed Lorrain-age jasper, these clasts could host important information about the ambient environment at ca. 2.35 Ga. References Bennett, G., Dressler, B.O., Robertson, J.A., 1991. The Huronian Supergroup and associated intrusive rocks. In: Geology of Ontario, Part 1, P.C. Thurston, H.R. Williams, R.H. Sutcliffe and G.M. Stott (eds.), Ontario Geological Survey, p. 549–591. Bleeker, W., and Ernst, R.E., 2006. Short-lived mantle generated magmatic events and their dyke swarms: The key unlocking Earth's palaeogeographic record back to 2.6 Ga. In: Dyke Swarms—Time Markers of Crustal Evolution, E. Hanski, S. Mertanen, T. Rämö, and J. Vuollo (eds), A.A. Balkema, Rotterdam, The Netherlands, p. 3-26. Bleeker, W., Kamo, S.L., Ames, D.E., and Davis, D., 2015. New field observations and U-Pb ages in the Sudbury area: toward a detailed cross-section through the deformed Sudbury Structure. In: Geological Survey of Canada, Open File 7856, p. 151–166. Collins, W.H., 1925. North shore of Lake Huron. Geological Survey of Canada, Memoir 153, 160 p. Roscoe, S.M., 1969. Huronian rocks and uraniferous conglomerates in the Canadian Shield. Geological Survey of Canada, Paper 68-40, 205 p. Young, G.M., Long, D.G., Fedo, C.M., and Nesbitt, H.W., 2001. Paleoproterozoic Huronian basin: product of a Wilson cycle punctuated by glaciations and a meteorite impact. Sedimentary Geology, vol. 141, p. 233-254. 10 �Ore Petrography of the Flambeau volcanogenic massive sulfide deposit, northwestern Wisconsin: Implications for hydrothermal fluid composition BLOTZ, Kaelyn E., LODGE, Robert W.D. Department of Geology, University of Wisconsin-Eau Claire, Eau Claire, WI 54701 The Paleoproterozoic Flambeau Cu-Zn-Au volcanogenic massive sulfide (VMS) deposit is located near the town of Ladysmith, Wisconsin within the Pembine-Wausau Terrane of the Penokean Orogeny (Schulz and Cannon, 2007) and is one of at least 13 other VMS deposits that have been identified in the state (DeMatties, 1994). The Flambeau is the only deposit to have been mined in Wisconsin after it was discovered by Kennecott Minerals Company in 1968 and was exploited due to the unusual grade of the orebody in the supergene enriched cap (May and Dinkowitz 1996). Mining began in 1993 and lasted until 1997 when extraction of the supergene enriched cap was completed. The high-grade copper ore body produced nearly 1.8 million tons of ore with an average of 10% copper and 0.18 ounces of gold per ton before the open pit was completely refilled and the site was reclaimed (Jones and Jones, 1999). The hypogene geology of the Flambeau deposit is characterized by massive to semi-massive Cu-Zn-Pb sulfides hosted in altered intermediate-felsic rocks that were metamorphosed into chlorite-andalusite-biotite schists. Before metamorphism occurred, these rocks were formed in a submarine hydrothermal system and their compositions can provide insight into the mechanisms of gold enrichment at the Flambeau mine. This study focuses on the identification of trace minerals and mineralogical variations within the ore zone at the Flambeau deposit. Samples were collected from drill core stored at the Wisconsin Geologic and Natural History Survey core repository and were then processed into polished thin sections. There are two main types of primary ore: a massive pyrite-chalcopyrite dominated assemblage and a weakly banded sphalerite-pyrite-galena dominated assemblage (May and Dinkowitz, 1996). Using scanning electron microscopy-energy dispersive spectroscopy, trace ore minerals identified in the ore zone include tellurides (hessite, altaite, tsumoite, bismuth), electrum, arsenopyrite, acanthite, bismuthinite, cassiterite, monazite, and an unnamed tungsten mineral. The presence of these minerals is important in determining the physical and chemical characteristics of the hydrothermal fluids since these trace minerals form under specific hydrothermal conditions. The relative abundance of the trace minerals, coupled with the anomalous Cu-enrichment in the Flambeau felsic-intermediate dominated strata, may indicate that this is not a traditional VMS deposit. Preliminary data suggests that there may have been magmatic fluids present in the seawater-dominated hydrothermal system. This interpretation is supported by geochemical characteristics of the alteration assemblages (Blotz et al. 2018). Mass balance calculations suggest a sericite-silica dominated assemblage consistent with argillic alteration. Based on these observations, the Flambeau deposit is possibly an example of a hybrid VMS-epithermal system. 11 �Figure 1: A) Silver telluride throughout pyrite grains and grain boundaries. B) Bismuth telluride within pyrite grain. C) Gold electrum within chalcopyrite. D) Acanthite within sphalerite. Blotz, K.E., Fredrickson, E.T., Lodge, R.W.D., 2018, Characteristics of ore and alteration mineral assemblages at the Flambeau volcanogenic massive sulfide deposit, northwestern Wisconsin. Geological Society of America-North Central Annual Meeting. DeMatties, T.A., 1994, Early Proterozoic Volcanogenic Massive Sulfide Deposits in Wisconsin: An Overview: Economic Geology, v. 89, p. 1122-1151. Jones, C.L., and Jones, J.K., 1999, The Flambeau Mine, Ladysmith, Wisconsin: The Mineralogical Record, v. 30, p. 107-131. May, E.R., and Dinkowitz, S.R., 1996, An Overview of the Flambeau Supergene Enriched Massive Sulfide Deposit: Geology and Mineralogy, Rusk County, Wisconsin, in LaBerge, G.L., ed., Volcanogenic Massive Sulfide Deposit of Northern Wisconsin: A Commemorative Volume: Institute on Lake Superior Geology Proceedings, v. 2, part 2, p. 67-93. Schulz, K.J., and Cannon, W.F., 2007, The Penokean orogeny in the Lake Superior region: Precambrian Research, v. 157, p. 4-25. 12 �Fault-controlled dike emplacement in the Grand Marais, Minnesota area BOERBOOM, Terrence J., Minnesota Geological Survey Over the past few years bedrock field mapping projects have been undertaken in the area around Grand Marais, northeastern Minnesota; these were funded in part by the USGS Statemap mapping program. The most recent round of field work, completed in 2017, was in the Mark Lake 7.5’quadrangle (Fig.1), near the southern margin of the ca. 1,099 Ma Eagle Mountain granophyre (EMG). This work has delineated a network of diabase dikes and sills, some of which were apparently intruded along faults, as evidenced by disruption of a distinctive quartz- and feldspar-phyric rhyolite (QFPR) unit, which in one block has been folded into a southwest-plunging syncline (Figs. 1 and 2). The correlation of the QFPR between the blocks is strengthened by an underlying thin unit of a distinct sparsely but coarsely porphyritic andesite flow along most of its length (Fig. 2). The extent of the QFPR is much greater than previously recognized; it may correlate with the Devil’s Kettle Rhyolite to the east, but correlation of this as well as other interlayered mafic to intermediate volcanic rocks is difficult owing to gaps in outcrop, structural complications, and intervening intrusions. The mafic volcanic rocks within several km of the EMG are typically quite metamorphosed/altered, particularly the more primitive ophitic basalt varieties, and the amygdules contain epidote, local fine fibrous amphiboles, and rarely garnet along with the usual mixtures of chlorite, calcite, and quartz. The diabase dike/sill complex is continuous with the Lake Clara diabase, as previously mapped by the author to varying degrees to the west and southwest of the Mark Lake quadrangle. The Lake Clara complex generally forms an arc from northeast to east-west that mimics the synformal shape of the Sawbill Lake intrusion (Brooker and Miller, 2012), which is part of the Brule-Hovland complex (Fig. 1), and also the general curvature of the volcanic pile. However, several northwest-trending diabase offshoots imply that emplacement was locally controlled by preexisting faults, as the QFPR is clearly offset across these northwest dikes. The margins of the dikes, particularly those of northwest orientation, are flanked by thin zones of intermediate intrusive rocks that commonly show quench textures, and the central part of one of the thickest northwest dikes contains a zoned pod that ranges from ferromonzodiorite at the edge to granophyre in the center (Fig. 2). All of the intermediate to felsic phases associated with the diabase, including the granophyre pod, contain small glassy ‘quartz eyes’ interpreted to be xenocrystic grains derived from melted QFPR implying that melting of rhyolite may have also taken place at some depth. Another small, northwest-trending hybrid dike (NE corner of Figure 2) consists of red fine-grained felsite that contains comagmatic cm-to m-sized, scallop-edg0ed intermediate to mostly mafic enclaves, in nearly equal proportions of felsic to mafic material. The red felsite matrix contains abundant quartz and feldspar phenocrysts, and is essentially identical to the nearby QFPR. This hybrid dike is adjacent to another ‘felsite’ dike that is enclave free, and has only small feldspar phenocrysts. Both are oriented to the northwest, nearly perpendicular to the strike of the hosting volcanic rocks, and are believed to be related to the main Lake Clara diabase dike set (Figure 2). These are outside of the main diabase swarm, but are consistent with melting of the rhyolite at depth and commingling with mafic magma prior to upward movement along fault zones, along smaller incipient faults outside of the main swarm. REFERENCE: Brooker, B.P, and Miller, J.D.,Jr, 2012, Bedrock geologic map of the Sawbill Lake intrusion, Cook County, Minnesota, University of Minnesota Duluth Precambrian Research Center; scale 1:24,000. 13 �Figure 1. Regional geologic context of the Lake Clara diabase complex. Unlabeled areas – Keweenawan volcanic rocks, undivided. Outline of Mark Lake quadrangle shown; geology of the south half of this map from varied 1:24,000 scale maps by Boerboom and others; north half from MGS map S-21. Figure 2. Northern third of the Mark Lake quadrangle showing offset of QFPR across NW trending diabase dike offshoots, marginal intermediate hybrid rocks, and central felsic granophyre pod. 14 �Possible Alumotantite from the Nine Mile pluton, Wausau Complex, Marathon County, WI. BUCHHOLZ, Thomas W.1, FALSTER, Alexander U. 2, and SIMMONS2, Wm. B. 1 1140 12th Street North, Wisconsin Rapids, Wisconsin 54494; 2Maine Mineral and Gem Museum, PO Box 500, 99 Main Street, Bethel, Maine 04217. The Nine Mile granite and quartz monzonite pluton is the youngest (≈1505 Ma, Dewane & Van Schmus, 2007) and most silicic of the four intrusions comprising the Wausau Syenite Complex. The Red Rock Granite northeast gravel pit is located in the south-western portion of the Nine Mile Pluton. Pegmatites and aplites are uncommon in this portion of the pluton, but in 2015 a small aplitepegmatite was exposed in the western working face of the northern portion of the pit, and samples were recovered from the talus below the exposure. The arch-shaped dike was approximately 20 cm thick, with a thin pegmatitic zone measuring approximately 5-6 cm thick near a margin of the dike. The center of the pegmatite in several samples had a thin (<0.5 cm) discontinuous band of fine-grained albite. Occurring adjacent to and within the albite band were small zircons, small crystals of columbite-group minerals, and very small (300-400µm) brownish grains of an unusual non-fluorescent niobium-bearing alumotantalate mineral. Associated minor minerals include: almandine-spessartine, columbite-(Fe), tapiolite-(Fe), zircon, hafnian zircon, zoned microlite-pyrochlore and U-rich pyrochlore, betafite, xenotime-(Y), ilmenite, monazite(Ce), and thorite. Chemical analysis of this alumotantalate yields a formula of (Al 0.986 Fe2+ 0.021 Mn2+ 0.001 ) Σ1.008 (Ta 0.803 Nb 0.147 Ti 0.062 ) Σ1.012 O 4.000 which is essentially identical to alumotantite (AlTaO 4 ). The stoichiometry of simpsonite, Al 4 Ta 3 O 18 (OH), effectively rules this species out, and there are no other known alumotantalates. X-ray diffractometry is needed to further confirm the presence of alumotantite, but paucity of material precludes this. Pegmatites of the Nine Mile Pluton are anorogenic in origin; and typically such pegmatites lack Tadominant phases. However, as we have previously reported, Nine Mile pluton pegmatites often contain late-stage Ta-enrichment, resulting in the formation of various Ta-dominant phases, including tantalite-(Mn), tapiolite-(Fe), and microlite. The occurrence of ‘alumotantite’ is noteworthy considering the overall metaluminous nature of the NYF pluton. It seems likely this occurrence resulted from a process of very late-stage fractionation similar to the processes that produced the high-Ta species in other pegmatites in the pluton. The lack of dark mica (annite or siderophyllite) in these dike samples suggests availability of Fe was probably somewhat limited, and the small amount of Fe available for interaction with late-stage fluids was likely consumed in the formation of columbite-(Fe), tapiolite-(Fe), almandine-spessartine and ilmenite, while crystallization of almandine-spessartine suggests development of a peraluminous environment. Pyrochlore, microlite, monazite and albite crystallization probably also reduced concentrations of 15 �Ca, Na, U and other elements. Lacking other cations, remaining Al combined with residual Ta and Nb to crystallize small amounts of probable alumotantite. Another possibility for increased Al availability may be a greisenization trend such as has been observed in one location in the pluton where abundant topaz was found. In an attempt to recover additional material, the site was revisited in June 2017. Little dike material was accessible in the pit wall due to slumping, but aplite-pegmatite samples were recovered from the adjacent floor of the pit. Due to the virtual absence of aplites and pegmatites in this area of the pluton, it is very probable that the samples originated from the same dike as those hosting the probable alumotantite. None of the samples exhibited the thin aplite band noted previously. However, examination of heavy mineral separates from the samples revealed a somewhat similar mineral assemblage partially reflecting the above association. The 2017 samples contained small crystals of dark mica (annite or siderophyllite, unlike the 2015 material with the thin albite band), along with Nb-bearing ilmenite, a Nb-bearing TiO 2 phase, pyrochlore, betafite, monazite, Hf-enriched zircon and columbite-(Fe). Notable is the lack of microlite, tapiolite-(Fe), almandine-spessartine, and probable alumotantite, suggesting the extreme fractionation that produced the unusual phases recovered in 2015 was restricted to a small portion of the dike. REFERENCE: Dewane, T. J., Van Schmus, W. R. (2007): U-Pb geochronology of the Wolf River batholith, north-central Wisconsin: Evidence for successive magmatism between 1484 Ma and 1468 Ma. Precambrian Research, V. 157, pp. 215-234. 16 �Recent gravity and magnetic investigations of the Minnesota River Valley Subprovince: New insights into ancient problems CHANDLER, V.W., SOUTHWICK, D. L., and JIRSA, M. A. Minnesota Geological Survey, University of Minnesota, 2609 Territorial Rd, St. Paul, MN 55114 U.S.A. Geophysical studies have been a long-standing companion to geologic investigations of the gneissic Minnesota River Valley (MRV) subprovince, due in part to limited outcrops and drill cores. Over the last decade various geophysical investigations conducted as part of state and academic programs have provided new insights into the crustal structure and evolution of this ancient and somewhat enigmatic component of the Archean Superior Province. Gravity and magnetic methods have continued to dominate geophysical studies of the MRV subprovince. Recent compilations of regional-scale gravity and magnetic grids and maps have assisted in extending MRV geology into eastern South Dakota (McCormick 2010 a, b; Southwick and others, 2018). At higher resolution, derivative-enhanced grids of gravity and magnetic data have been used extensively for bedrock mapping, which is being conducted in support of the County Geologic Atlas (CGA) Program of the Minnesota Geological Survey. These studies have added considerable detail regarding the internal geology of the blocks comprising the MRV subprovince — which from north to south are the Benson, Montevideo, Morton, and Jeffers. The enhanced gravity and magnetic data have been especially useful in 1:100,000-scale mapping of compositional variations and fold patterns within the gneissic blocks. The enhanced gravity and magnetic grids have also been very helpful in detailed mapping of the structural discontinuities that bound the blocks, including the Great Lake Tectonic Zone, the Appleton Shear zone, the Yellow Medicine shear zone, the Brown County lineament, and the Spirit Lake tectonic zone (SLTZ). Model studies of gravity and magnetic data along selected profiles have been useful in compiling geologic cross-sections for CGA mapping. Similar to earlier modeling at lower resolution, the newer models indicate that the three northernmost structural discontinuities of the MRV subprovince can be suitably approximated by slab-like sources that dip moderately to steeply northwards. Modeling of the interior of the MRV blocks is considerably more challenging; complex fold patterns and strong anomaly interference make interpretation difficult, especially with regard to determining the subsurface geometry of individual anomaly sources. In addition, outcrop evidence in the Minnesota River Valley indicates shallow structural dips of lithologic units locally that may obfuscate geophysical modeling. Nonetheless, model studies in these areas can still be useful for estimating the general range and spatial distribution of density and magnetization values for upper crustal rocks, resulting in improved lithologic identification and mapping. Gravity and magnetic modeling reveals significant differences between the SLTZ and the other structural discontinuities of the MRV subprovince. Firstly, the SLTZ, which forms the southern terminus of the MRV subprovince, is interpreted to dip southwards not northwards. Secondly, using values that are consistent with existing rock property data, most anomaly 17 �signatures of the MRV subprovince can be accommodated by sources within the shallow crust (<10 km. depth), but a prominent magnetic minimum that extends along SLTZ may involve much of the crustal section. Physical property data are not available for lower crustal rocks in Minnesota, but studies elsewhere of long-wavelength magnetic anomalies, crustal xenoliths, and crustal thicknesses indicate that the lower crust of cratonic areas typically is strongly magnetic, most likely reflecting enrichment of magnetite in granulite facies rocks (Langel and Hinze, 1998). Assuming reasonable levels of magnetization for the lower MRV crust (~3.5 SI), much of crustal section to the southeast of the SLTZ is interpreted to be non-magnetic. This apparent loss of crustal magnetization might reflect the deep emplacement of non-magnetic rocks, such as Paleoproterozoic metasedimentary rocks along the tectonic zone, or destruction of magnetic oxides via fluids moving along and above the tectonic zone. Evidence for the former possibility is available from recent magnetotelluric studies, where a conductive zone has been imaged along the southeastern edge of the SLTZ at mid- to deep- crustal levels (Bedrosian, 2016; Yang and others, 2015). Bedrosian suggested that the conductive zone might reflect Paleoproterozoic metasediments, which are known to be associated with prominent conductivity anomalies further north, where these rocks lie at or near the surface. Given the success so far for gravity and magnetic studies of the MRV subprovince, it seems likely that these data will continue to be useful for geologic studies for many years to come. REFERENCES Bedrosian, P. A., 2016, Making it and breaking it in the Midwest: Continental assembly and rifting from modeling of EarthScope magnetotelluric data, Precambrian Research, v. 278, p. 337-361. Langel, R. A., and Hinze, W. J., 1998, The magnetic field of the earth’s Lithosphere, Cambridge University Press, p. 263-268. McCormick, K.A., 2010a, Precambrian basement terrane of South Dakota: South Dakota Geological Survey Program Bulletin 41, 37p. McCormick, K.A., 2010b, Plate 1: Terrane map of the Precambrian basement of South Dakota: South Dakota Geological Survey Program Bulletin 41, External pdf file, compilation scale 1:1,000,000. Southwick, D. L., Chandler, V. W., and Jirsa, M. A., 2018, Geophysical, structural, and tectonic interpretation of the Yellow Medicine and Appleton shear zones, SW Minnesota and SE South Dakota: A work in progress, Institute on Lake Superior Geology 64th Annual Meeting, Part 1, Program and Abstracts, this volume. Yang, B., Egbert, G. D., Kelbert, A., and Naser, M.M., 2015, Three-dimensional electrical resistivity of the north-central USA from EarthScope long period magnetotelluric data, Earth and Planetary Science Letters, v. 422, p. 87-93. 18 �Possible Emplacement Controls on Diamond-Bearing Rocks North of Lake Superior CUNDARI, Robert1, SMYK, Mark1, CAMPBELL, Dorothy1 and PUUMALA, Mark1 1 Resident Geologist Program, Ontario Geological Survey, Ministry of Northern Development and Mines, 435 James St. S., Suite B002, Thunder Bay, ON, P7E 6S7 Canada The recent discoveries of a number of diamond-bearing, ultramafic rocks, including kimberlite, in Archean country rocks of the Superior Province, north of Lake Superior, has provided insights into lithotectonic controls on their emplacement and suggest potential for further discoveries. The Permian to Triassic Pagwachuan kimberlites, 100 km north of Marathon, were discovered by De Beers Canada Inc. in 2015-16 within Neoarchean metasedimentary rocks of the Quetico Subprovince. Five separate kimberlites range in size from 0.5 to 2.5 ha and are reportedly multi-phase, complex pipes (Delgaty et al. 2017). The Paleoproterozoic Rabbit Foot kimberlite, 50 km east-southeast of Marathon, has been explored in recent years by Rio Tinto Canada Diamonds Exploration Inc. It intrudes Neoarchean granitoids of the Pukaskwa batholithic complex and deformed metasedimentary rocks that are probably related to rocks of the Schreiber-Hemlo greenstone belt (Brett and Russell 2016). A number of diamondiferous ultramafic rocks of unknown age have been discovered within the TransSuperior Tectonic Zone (TSTZ) west of Marathon. In 2007, the Madonna alnöite ultramafic lamprophyre dyke was discovered 35 km northwest of Marathon by Rudy Wahl, who also discovered the nearby Prairie Lake paralamproite. Although hosted by Neoarchean rocks, these ultramafic intrusive rocks are spatially associated with Midcontinent Rift (MCR)-related and TSTZ-hosted intrusions, such as the Coldwell and Killala Lake alkalic complexes, the Prairie Lake carbonatite. The Chipman Lake fenites and carbonatites also occur within the northern extension of the TSTZ (Sage 1991), north of the Pagwachuan kimberlites. The Ripple Lake diatreme and associated lamprophyre dykes occur immediately west of the Coldwell complex and are likely associated with the TSTZ. They have also been the focus of diamond exploration. The TSTZ is a north-northeast-trending fault zone that extends for at least 600 km and is locally referred to as the Thiel fault (Sage 1991). The major MCR-related alkalic intrusions emplaced along the TSTZ cluster around 1.0 Ga, although Sage (1983) identified lamprophyre dikes on the Slate Islands emplaced at approximately 300 Ma. The Gravel River fault, traced for over 200 km, is a northeast- to eastnortheast-striking regional fault system that displays an oblique sinistral sense of transcurrent motion (Williams 1989). Several structures related to the TSTZ, the Gravel River fault and a number of northwest-trending faults intersect in the vicinity of the Pagwachuan kimberlite pipes. The discovery of five new kimberlite pipes in the Pagwachuan Lake area highlights the potential for further discovery in the region. In the Geraldton area, a number of discrete magnetic anomalies resemble anomalies related to known kimberlite pipes. The confluence of major, intersecting structures (e.g. TSTZ and Gravel River fault) are proven to be effective pathways for deep-seated magmas, tapping melts well within the diamond stability field. These fault systems are shown to have been activated for extended periods of time (i.e. MCR-related alkalic intrusive rocks ca. 1.1 Ga and the Pagwachuan kimberlite swarm ca. 220 to 252.9 Ma). The occurrence of the Paleoproterozoic Rabbit Foot kimberlite (ca. 1945 Ma; Brett and Russell 2016) suggests that large, crustal-scale faulting and magmatism is long-lived in this part of 19 �the Superior Province, although obvious lithotectonic controls are not as yet identified. Recent discoveries of diamondiferous rocks north of Lake Superior demonstrate its potential and suggest that further discoveries will be made. Figure 1. Geological map showing the location of the Pagwachuan kimberlite cluster and other kimberlitic and ultramafic rocks mentioned in the abstract. Approximate traces of the Gravel River fault after Williams (1989). The abbreviation “TSTZ” indicates the approximate location of the Trans-Superior Tectonic Zone. All UTM co-ordinates provided in NAD83, Zone 16. Bedrock geology from Ontario Geological Survey (2011). References Brett, C.R. and Russell, S. 2016. Indicator mineral and soil geochemical sampling of quaternary cover and microdiamond, indicator mineral, and geochronology of ultramafic intrusive rocks, Oskabukuta property, Ontario, Thunder Bay Mining District; Thunder Bay South District, Assessment Files, AFRO report number 2.56539, 104p. Delgaty, J., Fulop, A., Seller, M., Hartley, M., Zayonce, L., Januszczak, N. and Kurszlaukis, S. 2017. Ontario’s newest kimberlite cluster – the Pagwachuan cluster; poster abstract in 11th International Kimberlite Conference, Gaborone, Botswana, September 18–22, 2017, Extended Abstract No.11IKC-4517, 4p. Ontario Geological Survey 2011. 1:250 000 scale bedrock of Ontario; Ontario Geological Survey, Miscellaneous Release—Data 126–Revision 1. Sage, R.P. 1983. Geology of the Slate Islands; Ontario Geological Survey, Open File Report 5435, 333p. ——— 1991. Alkalic rock, carbonatite and kimberlite complexes of Ontario, Superior Province; Chapter 18 in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 1, p.683709. Williams, H.R. 1989. Geological studies in the Wabigoon, Quetico and Abitibi–Wawa subprovinces, Superior Province of Ontario, with emphasis on the structural development of the Beardmore–Geraldton belt; Ontario Geological Survey, Open File Report 5724, 189p. 20 �Thrust Kinematics of the Keweenaw Fault North of Portage Lake, Michigan DeGraff, J.M.1 and Carter, B.T. 2 1 Michigan Technological University, Houghton, MI 49931 2 Consultant, Houston, TX 77027 The Keweenaw Fault (KF) is the most significant fault of the Midcontinent Rift System (MRS) based on its length of 350 km (1), postulated net slip of 9 km (2), and thrusting of copper-bearing Portage Lake Volcanics (PLV, 1.1 Ga) over younger Jacobsville Sandstone (JS) (Fig. 1). This large fault and others figure prominently in ideas about MRS development (3-4) and copper deposits mined until recently along the Keweenaw Peninsula (5). Ideas about the KF (6-7, USGS 1950s maps) largely date to before modern concepts about thrust faults and before advances in cross-section modeling. As a result, many aspects of the fault’s geometry, kinematics, and timing remain unclear or are simply outdated. The prevailing view (3-5) is that the KF began as a steep, rift-bounding, normal fault during crustal extension and later was inverted during compression. This scenario would produce a fault dipping > 45° NW, however published maps and cross-sections show the KF dipping ≤ 45⁰ NW for much of its length and often < 30⁰ (6, USGS 1950s maps). PLV layers locally exhibit slip along their boundaries (6, 8) and near the fault they generally parallel its surface. These observations suggest a thrust fault system detached along PLV layers (Fig. 1b), which is inconsistent with direct inheritance from a rift-bounding normal fault. North of Portage Lake (Fig. 2) good fault exposures occur along crosscutting valleys (5-7), and mining drill holes and workings provide good local control on PLV stratigraphy and structure. One transect northeast of Houghton (Fig. 2, Loc. 1) crosses the Keweenaw and Hancock faults, which bound an anomalous area of gently dipping to horizontal PLV layers (2). USGS geologists interpreted the KF as dipping 22⁰ NW at the surface and possibly connecting to a steeper Hancock Fault. Our data compilation and kinematic modeling show that this geometry can be replicated by thrust motion of a detached master fault, the Hancock Fault being an imbricate thrust with increased dip in its hanging wall. A second transect west of Lake Gratiot (Fig. 2, Loc. 2) crosses the KF and another one to the northwest, possibly analogous to the Hancock Fault but less well defined. USGS geologists described horizontal to shallow dipping PLV layers between these faults based on surface and drill hole data (2). Furthermore, USGS maps show that the KF trace has a prominent reentrant of JS into the area of overthrust PLV layers, which implies a nearly horizontal fault surface based on our 3-point calculations. Forward kinematic modeling of these relationships suggests that the KF propagated upward from a deep detachment and reached a shallow detachment near the top of the JS. The northwest fault may represent an out-of-sequence cutoff of the leading edge of the thrust sheet near the top of a major ramp. We suggest that the KF began as a thrust fault during a post-rift compressional event, its initiation point possibly controlled by deeper, precursor, normal faults. This ongoing research raises many questions answerable with further work. Objectives are to determine layer and fault geometry at the onset of faulting, to infer deformation history of layers displaced by fault motion, and to define subsurface relationships between the Keweenaw and nearby faults. The ultimate goal is to define tectonic conditions leading to origin and evolution of the KF and other major faults in the region. 21 �Figure 1: (a) Major rock units and faults in the Lake Superior area; KF = Keweenaw Fault, DF = Douglas Fault, IRF = Isle Royale 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 focus area of Figure 2. (b) Cross-section along A-A’ in map showing PLV (red-orange) offset about 9 km by the Keweenaw Fault, and JS (tan) locally deformed in the footwall (2). Figure 2: Focus area north of Portage Lake (adapted from 2). Major faults shown as dark red traces. 1) Dover Creek transect with smaller Hancock Fault northwest of the Keweenaw Fault. 2) Bruneau Creek transect with unnamed fault northwest of the Keweenaw Fault. References 3. 4. 5. 6. 7. 8. 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. 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. 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. Stein, C.A., Kley, J., Stein, S., Hindle, D., and Keller, G. R., 2015, North America’s Midcontinent Rift: When rift met LIP: Geosphere, v. 11, no. 5, p. 1607-1616. Bornhorst, T.J. and Barron, R.J., 2011, Copper deposits of the Western Upper Peninsula of Michigan: Geological Society of America, Field Guide 24, p. 83-99. Butler, B.S. and Burbank, W.S., 1929, The Copper Deposits of Michigan: USGS Prof. Paper 144, 238 p. Irving, E.D. and Chamberlin, T.C., 1885, Observations on the Junction between the Eastern Sandstone and the Keweenaw Series on Keweenaw Point, Lake Superior: Bull. U.S. Geol. Survey No. 23, U.S. Government Printing Office, Washington, D.C., 58 p. 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. 22 �Geochemistry of Shallow and Deep Water Archean Meta-Iron Formations and their Post Depositional Alteration in Western Superior Province, Canada DOLEGA, Simon1 and FRALICK, Philip1 1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario P7B 5E1 Canada (sdolega@lakeheadu.ca) One purpose of studying banded meta-iron formations is to determine the chemical composition of seawater in the Archean ocean and the oxygen content of the Archean oceanicatmospheric system. Geologists use the geochemistry of meta-iron formations to make interpretations on the chemical conditions in the Archean. However, most scientists neglect the possibility of post-depositional alteration affecting the element geochemistry preserved in the meta-iron formations. This thesis explores the role of post-depositional mechanisms on the geochemistry of four banded meta-iron formations. The four different locations hosting Archean meta-iron formations chosen for this study include: meta-iron formations from the Beardmore/Geraldton greenstone belt of the Eastern Wabigoon Domain, Lake St. Joseph greenstone belt of the Uchi Domain, North Caribou greenstone belt of the North Caribou Terrane and Shebandowan greenstone belt of the Wawa Subprovince. The meta-iron formations from the Beardmore/Geraldton and Lake St. Joseph greenstone belts are interpreted to be deposited in a shallow water setting, while meta-iron formations from the North Caribou and Shebandowan greenstone belts are interpreted to be deposited in deeper water environments. This thesis also investigated ocean stratification by comparing the geochemistry of shallow and deep meta-iron formations. The main source iron and silica to the oceans was hydrothermal venting fluids. Iron and silica precipitated out of seawater as iron oxyhydroxides and amorphous silica, which deposited through cyclical processes. Elements dissolved in the Archean ocean were adsorbed onto iron oxyhydroxides and silica during deposition. Crystallization of quartz, magnetite and hematite occurred during diagenesis and magnetite continued to grow during progressive metamorphism. The lack of cerium anomalies, significant Y/Ho ratio values greater than average shales and the non-significant amount of authigenic chromium preserved in the meta-iron formations suggests that the oceans were anoxic. Therefore, in the Archean there was no significant oxygen stratification between the shallow and deeper water environments. Significantly most of the elements were derived from multiple sources, including the siliciclastic phase, seawater or hydrothermal venting fluids, at various proportions. Al 2 O 3 , TiO 2 , Th, V, Nb, U, REEs and Y were determined to be immobile during post-depositional alteration. The rest of the elements may have been isochemical during post-depositional alteration or may have been mobilized during post depositional alteration. 23 �Mobility during diagenesis is clearly exhibited by sodium and potassium in the meta-iron formation samples from the Beardmore/Geraldton, Lake St. Joseph and North Caribou greenstone belts. Sodium was relatively immobile, and potassium was mobilized in the magnetite- and magnetite/grunerite-dominated meta-iron formations during diagenesis. Potassium was relatively immobile, and sodium was mobilized in the hematite-, jasper- and chert-dominated meta-iron formations during diagenesis. If most of the elements remained relatively immobile during post-depositional alteration, then the ocean compositions in the Archean were heterogeneous. Shallow waters were more enriched in K 2 O, Rb and LREEs, while the deeper waters were more enriched in Cs, Na 2 O, CaO, MnO and HREEs. However, if the assumption that these elements were immobile is false, then the meta-iron formation does not preserve the ocean chemistry of the ancient ocean. 24 �Preliminary petrographic and geochemical investigation of silicified volcanic rocks and silica-rich exhalative rocks from the ~2.7 Ga Abitibi Greenstone Belt, Canada DRAZAN, Jacqueline1, BRENGMAN, Latisha1, FEDO, Christopher2 1 Department of Earth and Environmental Sciences, University of Minnesota-Duluth, 1114 Kirby Dr., Duluth, MN, 55812; 2Department of Earth and Planetary Sciences, University of Tennessee, 1621 Cumberland Avenue, 602 Strong Hall, Knoxville, TN 37996 USA. The ~2.7 Ga Abitibi greenstone belt (AGB) is a well-preserved volcanic arc terrane dominated by mafic and felsic volcanic lithologies interstratified with siliceous chemical sedimentary rocks (namely chert and iron formations) and less prevalent clastic rocks (Mueller et al., 2009; Thurston et al., 2008; Gibson et al., 1983). Regionally, the terrane is characterized by sub-greenschist facies metamorphism and locally high SiO2 concentrations due to silicification (post-depositional addition of silica phases; Brengman and Fedo, 2018; Gibson et al., 1983). Typically, silica-rich fluids permeate porous ash, tuffaceous material, or individual volcanic flow units during hydrothermalism to produce silicified rocks of varying composition and SiO2 content. Under conditions of minor replacement, primary textures preserve (ie- phenocrysts, glass shards, amygdules, pumice fragments, volcaniclastic features), allowing field identification of silicified rocks. However, in volcanogenic massive sulfide producing systems, hydrothermal replacement leads to mineralogical and geochemical changes of protolith volcanic rocks, often obscuring primary volcanic textures in the process. This leaves behind a rock with excess SiO2 lacking features indicative of the rocks’ initial genesis. Within these geologic settings, silicified rocks can be difficult to distinguish from other siliceous chemical sedimentary rocks, which precipitate directly from seawater and/or mixed hydrothermal fluids forming discrete units (e.g. Brengman and Fedo, 2018; Thurston et al., 2008). Results from preliminary studies show that the silicon isotope composition of quartz differs between the two siliceous rocks (Brengman et al., 2016). Here we present initial results from a preliminary geochemical investigation of wellpreserved silicified volcanic rocks and an associated exhalite from the Amulet Rhyolite locality near Rouyn-Noranda, QC. We aim to provide a geochemical framework for interpreting preliminary silicon isotope isotopic data of the same rocks used as a tool to differentiate siliceous volcanic rocks from siliceous chemical sedimentary rocks. Composition and texture of the Amulet Rhyolite samples were determined using scanning electron microscopy, transmitted and reflected light microscopy, inductively coupled plasma optical emission spectrometry, and inductively coupled plasma mass spectrometry. The primary volcanic mineral assemblage consists of feldspar, chlorite, and amphiboles, with prevalent zones of epidote and quartz alteration. Based on geochemistry (SiO2 = 55.08–73.1 wt.%; Al2O3 = 12.11–15.36 wt.%; CaO = 0.79–7.67 wt.%; Na2O = 0.18–5.11 wt.%; K2O = 0.2– 5.36 wt.%; Fe2O3(t) = 4.13–18.53 wt.%; MgO = 0.8–5.47 wt.%), mineralogy (amphibole and feldspar micro-phenocrysts, glassy groundmass replaced by chlorite), and texture (quartz-filled amygdules, aphanitic matrix), samples classify as basalts and andesites, with an overabundance of quartz (Figure 1a, b). Mineralogically, samples show alteration features similar to other localities within the AGB: abundant mega- and micro-quartz alteration and patchy epidote alteration with minor dispersed carbonates (Figure 1a,b; Brengman and Fedo, 2018). Overlying one of the amygdaloidal pillowed basalt units is the marker “A” exhalite unit, thought to represent exhalative precipitation (Gibson et al., 1983; Figure 1c,d). The exhalite unit is 25 �characterized by fine banding (Figure 1d) and is principally composed of microcrystalline quartz with minor aluminous mineral phases (Figure 1d). Geochemically this unit is distinct from local volcanic rocks, with higher SiO2 (78.03 wt.%) and K2O content (5.36 wt.%), lower Al2O3 (10.64 wt.%), CaO (0.37 wt.%), and Na2O content (2.33 wt.%) content, and significantly lower Fe2O3(t) and MgO content (1.69 and 0.04 wt.% respectively). Due to the level of preservation, the exhalite unit and underlying silicified volcanic rocks can be differentiated based on petrography and geochemistry making them a good test locality for studying the silicon isotope variability between the two siliceous rock types. These initial geochemical results provide the framework for future silicon isotope analyses on the same sample suite. Figure 1. Representative samples photographs (field and photomicrographs) from samples of the Amulet Rhyolite. (a) Cross-polarized light photomicrograph of amygdaloidal basalt (quartz-filled with chalcopyrite centers). (b) Cross-polarized light photomicrograph of quartz altered andesitic volcanic rock with megaquartz alteration patch. (c) Field photograph of exhalite contact with underlying pillowed basalt unit. (d) Cross-polarized light image of finely banded exhalite unit. REFERENCES Brengman, L.A. Fedo, C.M., Whitehouse, M.J., 2016. Micro-scale silicon isotope heterogeneity observed in >3.7 Ga Isua Greenstone Belt, SW Greenland. Terra Nova: 28, p. 70-75. Brengman, L.A., Fedo, CM., 2018. Development of a mixed seawater-hydrothermal fluid geochemical signature during alteration of volcanic rocks in the Archean (~2.7 Ga) Abitibi Greenstone Belt, Canada. Geochimica et Cosmochimica Acta: 227, p. 227-245. Gibson, H.L., Watkinson, D.H., Comba, C.D.A, 1983. Silicification: Hydrothermal Alteration in an Archean Geothermal System within the Amulet Rhyolite Formation, Noranda, Quebec. Economic Geology: 78, p. 954971. Mueller, W.U., Stix, J., Corcoran, P. L., Daigneault, R., 2009. Subaqueous calderas in the Archean Abitibi greenstone belt: An overview and new ideas. Ore Geology Reviews: 35, p. 4-46. Thurston, P.C., Ayer, J.A., Goutier, J., Hamilton, M.A., 2008. Depositional Gaps in Abitibi Greenstone Belt Stratigraphy: A Key to Exploration for Syngenetic Mineralization. Economic Geology: 103, p. 1097-1134. 26 �On the source(s) of the Felch-Arnold gravity anomaly, Upper Peninsula, Michigan DRENTH, Benjamin J.1, WOODRUFF, Laurel G.2, SCHULZ, Klaus J.3, CANNON, William F.3, and AYUSO, Robert A.3 1 U.S. Geological Survey, PO Box 25046, MS 964, Denver, CO, 80225 2 U.S. Geological Survey, 2280 Woodale Drive, St. Paul, MN, 55112 3 U.S. Geological Survey, 12201 Sunrise Valley Dr., MS 954, Reston, VA, 20192 Away from the Midcontinent Rift, gravity highs in the Upper Peninsula of Michigan are normally attributed to rocks of the Paleoproterozoic Marquette Range Supergroup. In particular, dense iron formations and mafic volcanic rocks of the Menominee Group produce gravity highs where they reach significant thicknesses, and are juxtaposed against lower-density Archean rocks and sedimentary rocks of the Marquette Range Supergroup (e.g., Klasner at al., 1985). A 13 mGal, E-W trending gravity high lies over Archean and Paleoproterozoic rocks in the eastern part of the Felch trough area near the town of Felch, and extends ~25 km eastward over Paleozoic sedimentary rocks to the vicinity of the town of Arnold (Fig. 1). Bacon (1956) suggested the source is dense units of the Paleoproterozoic Marquette Range Supergroup. However, subsequent mapping (James et al., 1961) in the Felch trough area showed that Archean, not Paleoproterozoic, rocks dominate the area of Precambrian exposures that coincide spatially with the gravity high (Fig. 1). New ground gravity data and density measurements, as well as inspection of spatial relations between gravity anomalies and geologic mapping, show that the most likely candidates for the source of the Felch-Arnold gravity anomaly are the Six-Mile Lake Amphibolite and the Hardwood Gneiss (both long assumed to be Archean). Archean granites and gneisses form most of the surrounding rocks and have mean density of 2700 kg/m3. The Six-Mile Lake Amphibolite (mean density 3020 kg/m3) and the Hardwood Gneiss (mean density 2880 kg/m3) present the best spatial correspondence with the anomaly, and each could have a plausibly large subsurface volume to account for the eastward extension of the anomaly over Paleozoic sedimentary rocks. Other units in the area lack either the density, volume, or spatial distribution required to be candidates for the source. Geochemical similarities between the Hardwood Gneiss and Six Mile Lake Amphibolite suggest that those two units may be related (Schulz et al., this volume). Further, a new radiometric date on the Hardwood Gneiss of ~2.75 Ga (Ayuso et al., this volume) confirms the long-assumed Archean age for that unit. References Ayuso, R.A., Schulz, K.J., Cannon, W.F., Woodruff, L.G., Vazquez, J.A., Foley, N.K., and Jackson, J., this volume, 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, Part 1: Program and Abstracts, v. 64. Bacon, L.O., 1956, Relationship of gravity to geological structure in Michigan's Upper Peninsula, in Snelgrove, A.K., ed., Geological Exploration: Institute on Lake Superior Geology 2nd Annual Meeting, Houghton, Michigan, p. 54-58. 27 �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. 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., this meeting, Archean and Paleoproterozoic geology of the Felch District, central Dickinson County, Michigan: Institute on Lake Superior Geology 64th Annual Meeting Field Guide. 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. Klasner, J.S., King, E.R., and Jones, W.J., 1985, Geologic interpretation of gravity and magnetic data for northern Michigan and Wisconsin, in Hinze, W.J., ed., The Utility of Regional Gravity and Magnetic Anomaly Maps, Society of Exploration Geophysicists, p. 267-286. Schulz, K.J., Cannon, W.F., and Woodruff, L.G., this volume, Geochemistry of mafic rocks in Dickinson County, Michigan: Michigan: Institute on Lake Superior Geology, Part 1: Program and Abstracts, v. 64. Figure 1: Left: Bedrock geology, after James et al. (1961), Bayley et al. (1966), Cannon and Ottke (1999), Ayuso et al. (this volume), and Cannon et al. (this meeting). Right: Complete Bouguer gravity anomalies. Inset (lower right) shows location of study area. 28 �GEON 12 to 11 history of the Lake Superior Region and speculation about the relationships between the Midcontinent Rift and the Grenville Orogen EASTON, Robert Michael1 1 Earth Resources and Geoscience Mapping Section, Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, Ontario P3E 6B5 mike.easton@ontario.ca This presentation was inspired by some of the questions posed in the recent Geological Association of Canada Howard Street Robinson Lecture Tour presentation by Dr. P. Hollings on the “Metallogeny and Magmatism of the 1.1 Ga Midcontinent Rift”. It will focus on 3 specific topics related to the evolution of the Midcontinent Rift and whether or not the rift is the result of a typical mantle plume event. These are: 1) regional Geon 12 events that may have had an effect on localizing the rift, 2) the apparent time gap between major tectonic events in the Grenville Orogen in central North America during Geon 11 and the onset of magmatic activity in the Midcontinent rift, and 3) an alternative tectonic setting for generating a Large Igneous Province (LIP) without an active “hotspot” or mantle plume. Regional Geon 12 Events There were 2 attempted rifting events in North America that occurred to the northwest and the east of the Midcontinent Rift during Geon 12. Both produced large, radiating dike swarms, and near their centres, a variety of layered mafic intrusions. These are the circa 1267 Ma Mackenzie dike swarm (LeCheminant and Heaman 1989) and the circa 1238 Ma Sudbury dike swarm (Krogh et al. 1987). The centre of the latter swarm is now buried beneath the Grenville Orogen in western Quebec, approximately 1,800km east of the centre of Lake Superior. The Midcontinent Rift developed in the area between these 2 earlier rifting attempts. Is its location between these 2 earlier “plumes” significant? The Grenville Orogen (Ontario region) during Geon 11 The Grenville Orogen in Ontario is divided into 3 major segments (Carr et al. 2000): 1) the Laurentian margin (which consists of para-autochthonous and para-allochthonous rocks that developed on the margin of Laurentia during the Archean to Mesoproterozoic), 2) the Composite Arc Belt (CAB), which consists of a variety of arc fragments that developed somewhere outboard of North America between 1300 to 1220 Ma and which were stitched together between 1220 and 1190 Ma), and 3) the Frontenac-Adirondack belt (FAB), a continental platform to continental arc that was the site of voluminous anorthosite-mangeritecharnockite-granite (AMCG) magmatism between 1190 and 1140 Ma, and which was stitched to the southern margin of the Composite Arc Belt at circa 1160 Ma (Carr et al. 2000). It is unclear if the Laurentian Margin and the co-joined CAB and FAB were proximal to one another prior to circa 1080 Ma (see discussion in Carr et al. 2000). It is possible also that they were distal to Laurentia at the time the Midcontinent Rift was active. The Ottawan (or “Grenville”) orogeny is the onset of Grenville-wide metamorphism and deformation across all 3 segments of the Grenville in Ontario and is the result of Himalayan-style continent-continent collisional event. Grenville-wide metamorphism and deformation occurs across all 3 segments of the Grenville in Ontario during the Ottawan orogeny. In Ontario, there are remarkably few U-Pb ages (Ontario Geochronology Inventory 2018), and certainly no major magmatic, metamorphic or pervasive deformational events, between the end of AMCG magmatism at circa 1140 Ma and the start of the Ottawan orogeny, a period of approximately 55 million years (1140-1085 Ma). This Ontario Grenville time gap almost exactly coincides with the magmatic time span encompassed by the Midcontinent Rift (Abitibi and other dikes at circa 1140 Ma to late felsic volcanism at circa 1085 Ma on Michipicoten Island). Interestingly, this time gap does not exist in the Grenville in West Texas, where the period 11301110 Ma was characterized by high-grade metamorphism, deformation, and thrusting (Moser et al. 2008), however this was occurring approximately 2,000 km south-southeast of the Midcontinent Rift. 29 �Possible Tectonic Setting An important aspect of the geology of the Lake Superior region is that from circa 1900 to 1350 Ma the southern margin of Laurentia was the loci for repeated arc development, similar to the west coast of North America during the Mesozoic to Cenozoic. The culmination of this arc activity was the formation of a large Andean-style arc, represented by the magmatic rocks of the Eastern Granite-Rhyolite Province and its equivalents (circa 1480-1350 Ma) in the Laurentian Margin of the Grenville Orogen in Ontario. This almost 500 million year period of subduction beneath Laurentia would have greatly modified the lithosphere beneath Laurentia, and left remnants of partly subducted slabs in the mantle beneath the margin of Laurentia. Consequently, the recent history of western North America may provide some insights into how the Midcontinent Rift have formed. Fouch (2012) and Zhou et al. (2018) provide an alternative to the “hotspot” model for the development of the Columbia River basalts, the Snake River Plain, and Yellowstone; one that does not require a “hotspot” or classic mantle plume. Their model involves asthenosphere upwelling following slab-breakoff after subduction of the Farallon and Juan du Fuca plates beneath western North America. There are some similarities between this scenario and the development of the Midcontinent Rift. First, in both areas, there was a long period of arc magmatism prior to the onset of LIP magmatism. Second, magmatism in both areas occurred over a long time interval (more than 30 million years), was geographically widespread, and toward the waning stages became localized with a greater abundance of felsic magmas. Third, the model does not result in radiating dike swarms characteristic of many mantle plumes. Finally, in the case of Yellowstone, this upwelling may eventually lead to a pseudo-plume — something which would explain the latter stages of the Midcontinent Rift and the plume-like geochemistry of the magmas. What is not known is if a transform fault setting is needed after the end of subduction for the down-going slab to break-off and result in asthenosphere upwelling. This was the case for western North America, but presently cannot be confirmed to have occurred in the Ontario Grenville. Nonetheless, a transform would be one means whereby events in the Grenville realm could be isolated from events in Laurentia for a protracted period of time. Even if the tectonic setting envisioned by Fouch (2012) and Zhou et al. (2018) is a viable model for explaining the development of the Midcontinent Rift, there is a key difference. Unlike western North America, in the Mesoproterozoic a continent-continent collision occurred not long after the LIP event was initiated, effectively shutting the system down, likely prematurely. However, this extensive pre-heating of the lower crust in the Lake Superior region may have well been responsible for the ductile and longlived metamorphism present in the Ontario Grenville, as suggested by Carr et al. (2000). References Carr, S.D., Easton, R.M., Jamieson, R.A., and Culshaw, N.G. 2000. Geologic transect across the Grenville Orogen of Ontario and New York; Canadian Journal of Earth Sciences, v.37, p.193-216. Fouch, M.J. 2012. The Yellowstone Hotspot: Plume or Not? Geology, v.40, p.479-480. Krogh, T.E., Corfu, F., Davis, D.W., Dunning, G.R., Heaman, L.M., Kamo, S.L., Machado, N., Greenough, J.D. and Nakamura, E. 1987. Precise U-Pb isotope ages of diabase dikes and mafic to ultramafic rocks using trace amounts of baddeleyite and zircon; in Mafic dike swarms, Geological Association of Canada, Special Paper 34, p.147-152 LeCheminant, A.N. and Heaman, L.M. 1989. Mackenzie igneous events, Canada: Middle Proterozoic hotspot magmatism associated with ocean opening; Earth and Planetary Science Letters, v.96, p.38-48. Moser, S., Helper, M. and Levine, J. 2008. The Texas Grenville Orogen, Llano Uplift, Texas; Fieldtrip guide to the Precambrian Geology of the Llano Uplift, central Texas, Geological Society of America, Annual Meeting 2008, Houston, Texas, 54p. Zhou, Q., Liu, L. and Hu, J. 2018. Western US volcanism due to intruding oceanic mantle driven by ancient Farallon slabs; Nature Geoscience, v.11, p.70-76. 30 �What to do after the bull has left the china shop- Picking up the community relation pieces EGER, Paul1, BOT, Courtnay1, MEINEKE, Dave1 and ADAMS, Dave2 1 Global Minerals Engineering LaPointe Iron Company 2 Today successful mining projects must meet the “triple bottom line”; economic, environmental and social. In 2010 Gogebic Taconite (GTac) acquired a lease option to develop an open pit taconite mine in northwestern Wisconsin. Although the company had mining experience, it was limited to coal near existing mines with minimal opposition. GTac’s proposed development was viewed as either an economic boon or an environmental disaster. GTac’s decision to focus on lobbying at the state level mobilized opposition from the local, environmental and tribal communities. After spending millions of dollars in additional resource evaluation and environmental studies, GTac decided to abandon the project in 2015. When the option to lease was terminated in 2015, LaPointe decided it was important to rebuild local partnerships and begin to develop sound data so that future decisions could be based on science and not fear. These efforts include support for scientific studies, baseline regional water quality monitoring and periodic meetings with community leaders. 31 �Surficial Geology of the Iron Mountain 7.5 Minute Quadrangle, Dickinson County, Michigan, Florence & Marinette Counties, Wisconsin ESCH, John M1, KEHEW, Alan2, HUOT, Sebastien3, YELLICH, John4 1 Michigan Dept. of Environmental Quality, Office of Oil, Gas, and Minerals, P.O. 30256, Lansing, MI 48909, 2 Department of Geological and Environmental Sciences, Western Michigan University, Kalamazoo, MI 49008, 3 Illinois State Geological Survey, Prairie Research Institute, University of Illinois at Urbana-Champaign, Champaign, IL 61820, 4 Michigan Geological Survey, Western Michigan University, Kalamazoo, MI 49008 The Iron Mountain 7.5 minute quadrangle lies within complex glacial deposits of the Green Bay Lobe of the Laurentide Ice Sheet. In 2017 the Michigan Geological Survey mapped the quad as part of a USGS STATEMAP project. Surficial mapping was greatly aided by the availability of LiDAR elevation data. This mapping has provided new, detailed information on the surficial landforms and deposits as well as relationships between the glacial deposits and the underlying bedrock. The complexity of the glacial deposits is in part due to high relief on the bedrock surface and complexity of underlying bedrock formations and structure. Three icemargins were mapped across the quad. A deep bedrock trough mapped as part of an earlier environmental investigation was further defined as well as the bedrock topography and drift thickness mapped across the quad. In addition, the mapping identified ice-walled lake plains, eskers, drumlins and terraces that were not previously mapped. A new OSL age date was obtained for an outwash deposit. The sediments include diamicton (till), sand and gravel, boulders and interbedded silt and clay. The glacial deposits are late Wisconsinan (about 14,500 cal yr BP to 12,500 cal yr BP) in age. Within the Iron Mountain Quad, the west-northwesterly ice flow direction is reflected in the numerous drumlins on the uplands and streamlined bedrock hills. Rock drumlins occur within the quad and others are exposed in the bottom of some sand and gravel pits. The elevation across the map ranges from 919 feet above mean sea level (AMSL) along the Menominee River to 1572 feet AMSL at the top of Millie Hill. Three distinct bedrock-controlled uplands occur north of the Menominee River: Pine Mountain, Millie Hill and Trader Hill. These uplands are mostly cored by diamicton and strewn with boulders. These boulders extend to depth into the subsurface based on the local water wells logs and the three borings drilled for this project. The diamicton is mostly reddish brown to brown. Although the larger foundation for these uplands is bedrock controlled, drift up to 140 feet thick occurs in places. Three ice margins are interpreted within the map. From west to east they are the Winegar-Sagola-Early Athelstane Moraine (about 14,500 cal yr BP), the Middle Athelstane ice margin and the Marenisco-Late Athelstane ice margin (about 13,000 cal yr BP). Under the City of Kingsford lies an extensive pitted outwash plain with elevations ranging from 1140 to 1120 feet AMSL formed west and south of the Middle Athelstane ice margin. An OSL age 12,600 ± 1,000 cal yr was obtained from a sample taken from the edge of this outwash plain. This outwash plain overlies a lacustrine sequence of mostly silts, clays, sands 32 �and some gravels that is as much as 300 feet thick. This lacustrine sequence overlies a deep bedrock trough under the City of Kingsford. A later, short term fluvial event likely occurred over this outwash surface, as evidenced by the sharp, steep, wave cut or fluvially cut scarp at 1140 feet AMSL along the northern side of this surface. As the Green Bay Lobe ice retreated to the east, there must have been successive lowering of the outwash outlets to the south and southeast because of the step-like lowering of the outwash terraces to east along the Menominee River. The lowest of these terraces to the east is 180 feet lower than the large outwash plain to the west. A passive seismic instrument using the Horizontal-Vertical Spectral Ratio (HVSR) method was used to gather additional bedrock control for data on bedrock topography and drift thickness. This technique uses the horizontal-to-vertical spectral ratio method to record ambient seismic noise with 3-component geophones. HVSR calibration readings were gathered at 15 wells and borings of known bedrock depth. These data were used to develop a local HVSR bedrock depth calibration curve. Exploration readings were taken at 44 locations within the map. Very good bedrock depth estimates were made in the outwash areas of the map. In the upland morainal area, however, the method yielded depth estimates that were much too shallow relative to the local bedrock elevation of the area. This disconnect is likely due to buried, overconsolidated dense glacial till which was encountered at depth in the three borings drilled for this project. The HVSR bedrock depth estimates at these three borings match well with the depths to the top of the dense till. A significant gamma-ray log kick was also seen in the borings at or near the top of this dense till. Although the glacial deposits in the Iron Mountain Quadrangle average 40 feet thick, numerous bedrock outcrops exist. The drift is maximally 363 feet over the deep bedrock trough in Kingsford. In many places, the land surface topography is controlled not by the glacial deposits, but by the underlying bedrock and bedrock structure. One important exception is a pronounced buried deep bedrock trough that underlies the large pitted outwash plain in Kingsford. Another buried bedrock trough underlies the lowland along the Menominee River in the southeastern part of the map. A poorly defined bedrock low connects the two troughs north of the Menominee River. There is high relief on the bedrock surface ranging from 730 feet to 1530 feet AMSL across the map. Bedrock outcrops and mounds appear throughout the area, even where nearby borings show over 100 feet to bedrock. The bedrock geology exposed at the surface and underlying the glacial deposits in the Iron Mountain Quadrangle is very complex and has had a significant and controlling effect on the overlying glacial deposits. Underlying the quad are Precambrian complexly faulted and folded Precambrian metasedimentary rocks and metavolcanics and granitic intrusions as well as much later Cambrian Sandstones. References Esch, J.M., and Kehew, A.E., 2017, Surficial Geology of the Iron Mountain 7.5 Minute Quadrangle, Dickinson County, Michigan, Florence & Marinette Counties, Wisconsin, Michigan Geological Survey, Surficial Geologic Map Series SGM-17-04, scale 1:24000. 33 �LiDAR Revolutionizing Geological Mapping ESCH, John M Michigan Department of Environmental Quality, Oil, Gas & Minerals Division, Constitution Hall 2nd Floor South, 525 West Allegan Street, Lansing, Michigan, 48933 LiDAR (Light Detection And Ranging) has fundamentally changed how we view and interpret the landscape and has revolutionized geological mapping. Often subtle features can be seen in the LiDAR topography data that are not visible on aerial photography, topographic maps, or digital elevation models (DEMs). LiDAR is an optical remote sensing technology that emits intense, focused beams of Light at the ground and measures the time it takes for the reflections to be detected by a sensor. This results in a densely spaced (QL2): 2 points/meter network of highly accurate georeferenced elevation points called a point cloud. These elevation points are classified as to what the LiDAR pulse was reflected off (ground, vegetation, water, buildings or other objects). The ground elevation points are used to produce highly accurate Digital Elevation Models that can be used to generate three-dimensional representations of the Earth’s surface and its features. Elevation accuracies are on the order of 10 centimeters. Airborne LiDAR is the most common, but there is also terrestrial LiDAR and bathymetric LiDAR. Terrestrial LiDAR can be used for mapping high cliff and quarry faces to create a virtual outcrop. The most useful airborne LiDAR product is the bare earth digital elevation model (DEM). Other common deliverable LiDAR products are a classified point clouds and intensity Images. A LiDAR attribute that may be of value for geologists is the intensity of the returned pulse, which is the strength of the return or how strongly the laser pulse was reflected back to the sensor. This is usually presented a greyscale .tif image and may be useful for mapping soft ground (wetlands) vs hard ground (potentially bedrock outcrops). Common LiDAR derivative products include DEM hillshade, digital surface models, shaded relief, contours and automated building extraction. The higher resolution topography advantage of 0.6 meter LiDAR DEMs over existing 30 and 10 meter DEMs is obvious. This very dense data coverage allows for seeing subtle geologic features, and cultural features like curbs, plow furrows, and two-tracks. It can also can be used to see what is under tree canopy. Bedrock outcrops often appear distinct from the surrounding topography in the LiDAR data. This is valuable for mapping in remote areas with little known exposure. Hydrologic features like streams, valleys and subtle erosional features are more accurately and easily seen using LiDAR. Many more karst feature like sinkholes, disappearing streams, and solution enhanced joint areas have been identified using LiDAR. Subtle glacial features like ice-walled lake plains are almost never seen on aerial photos or topographic maps 34 �(except for large ones) and were rarely identified in Michigan prior to LiDAR. With LiDAR they are relatively easy to see. Other subtle glacial features seen using LiDAR, but sometimes too small to be seen on topographic maps include small eskers, drumlins, flutes, subtle terraces, fans, deltas, small sand dunes, paleo-shorelines, ice margins, bars, pendants, and erosional scarps. These previously undetected landforms may fundamentally change how one interprets the area geology. Pits and other excavations as well as scarps are easily seen using LiDAR and are helpful for places to investigate. LiDAR allows geologists to be more efficient in the field by allowing them to see the landscape how it really is before going out in the field. It also helps in fine tuning and focusing field work in specific area, features and landforms. It also allows them to map subtle features that may not be accessible due to land ownership permissions. This presentation will show the widely varying applications of LiDAR for geological mapping across Michigan. A B Figure 1 (A) Northwest Albion, Michigan, 7.5 Minute Topographic Map, USGS 1980. 10 Foot Contour Interval. (B) Northwest Albion Area, Calhoun County LIDAR Shaded Relief Map, clearly indicating esker trending SW-NE across map. From Carswell (2014) and Esch (2013). References Carswell, W.J., Jr., 2014, The 3D Elevation Program—Summary for Michigan (ver. 1.2, June 29, 2015): U.S. Geological Survey Fact Sheet 2014–3107, 2 p., Esch, J. M., 2013, Surficial geology of the Northwest Albion 7.5 minute quadrangle, Calhoun County Michigan, Michigan Geological Survey, Surficial Geologic Map Series SGM-1302, scale 1:24,000. 35 �Mineral Chemistries of the Tower Mountain Intrusive Complex Au-Deposit, Ontario FITZPATRICK, William1, HOOPER, Robert1, and LODGE, Robert1, Gélinas, Brigitte2 1 Dept. of Geology, University of Wisconsin Eau Claire, 105 Garfield Av., Eau Claire, WI 54701 Dept. of Geology, Lakehead University, 955 Oliver Rd, Thunder Bay, ON, P7B5E1 2 Hydrothermal fluid systems driven by magmatic activity are some of the most important ore forming processes for many metallic minerals. This project involves characterization of hydrothermal alteration associated with the Archean Tower Mountain Intrusive Complex and its related Au deposit within the Shebandowan Greenstone Belt. The Tower Mountain Intrusive complex (TMIC) consists of a ~1.5km2 monzonite stock cored by a later diorite intrusion. Cross cutting both units are small dikes of monzonite porphyry and meter-scale areas of hydrothermal brecciation. The TMIC intrudes Timiskaming-type assemblages of calc-alkaline volcanic rocks and conglomeritic alluvial-fluvial sedimentary. The main monzonite and syenite stocks have been U-Pb dated to 2690 ± 3 Ma while calc-alkaline volcanics elsewhere in the Shebandowan greenstone belt similar to TMIC host rocks were dated to 2690 Ma (Corfu and Scott, 1998). The contemporary U-Pb dates as well as the lack of a surrounding contact metamorphic zone implies that the Tower Mountain Intrusive complex was syn-volcanic and emplaced at relatively shallow depth (Carter, 1990). The hydrothermal alteration at TMIC requires at least a two stage process. The first alteration results in initial oxidation followed by a mineralizing event with lower fO2. Mineral chemistry data has been collected from numerous primary and alteration minerals using a SEM/EDS detector. The rocks have been heavily altered and few primary minerals remain except for scattered patches of Mg rich-hornblende, Mg-rich augites, magnetite-ilmenite intergrowths and minor monazite, zircon. All primary feldspars have been altered to end-member alkali feldspars (albite and k-spar). The first hydrothermal alteration event resulted in oxidation of the magmatic Fe-Ti oxides with production of hematite and secondary rutile. Other minerals associated with this alteration event include: titanite, epidote, fluoro-apatite (e.g. Ca 4.5 Mn,Fe .5 (PO 4 ) 3 F), and Mg-rich phengite, Mgchlorite which petrographically are seen as sericitization of magmatic alkali feldspars and mafic minerals. Some of the epidotes from this alteration episode have LREE enriched epidote rims and some of the phengites have fluorine in the hydroxyl-site (K 1.7 Na .3 )(Al 3 Mg .8 Fe .2 )(AlSi7 )(OH 3.9 ,F .1 ). This first alteration episode resulted in pervasive pink hematite staining and green (epidote) alteration seen in outcrop (Gélinas et al., 2016). Also related to this alteration episode are scattered, small (~3µm2) barite and celestine grains. The rocks were subsequently altered by a sulfidizing fluid which results in hematite replacement by pyrite, chalcopyrite, and pyrrhotite. Sulfidizing fluid alteration also results in observed Fe-rich chlorite, ferroan-dolomite and the gold mineralization. This model is consistent with Au being transported as an Au-bisulfide complex and precipitated along with the sulfide minerals. Relating the fluid history described above to lithologies seen in the field, the first oxidizing phase of alteration is likely to have initiated with intrusion of the monzonite stocks. Fluids related to alkaline magmatism have long been known to have an association with oxidizing, hematitic alteration (e.g. Greenberg, 1986) and also carbonatizing, halide and LREE-rich metasomatism (e.g Wooley, 2003). Evidence observed in thin section indicates that introduction of reducing, sulfidizing fluids and Au-mineralization is related to later intrusion of the monzonite porphyry 36 �dikes. No magnetite is observed in the monzonite porphyry whereas all other samples contained magnetite in various states of decay along with pyrite. This indicates that sulfidizing fluids were more active in the monzonite porphyry, possibly because of closer spatial association to the intruding magmas. ru A B ru+ttn Ksp Ksp mt ap ttn Mg-chl ab ab hb Mg, Fe chl C D py phen mt ab ttn ab cpy py po ab Ksp Fe-chl Figure 1: A: Magmatic hornblende and magnetite in matrix of Mg-chlorite, albite and k-spar. From monzonite. B: Rutile, titanite clusters with scattered apatite in mg-chlorite, phengite, albite, kspar matrix. From monzonite. C: Highly corroded magnetite separated by Fe-chlorite from pyrite in equilibrium. From hydrothermal breccia. D: Zoned pyrite grain containing rim of pyrrhotite and inclusions of chalcopyrite. From monzonite porphyry. ru: rutile, mt: magnetite, Fe/Mg chl: Iron or Magnesium rich chlorite, py: pyrite, hb: hornblende, Ksp: k-spar, ttn: titanite, cpy: chalcopyrite, po: pyrrhotite Carter, M.W., 1990, Geology of Forbes and Conmee townships, Ontario Geological Survey, Open File Report 5726. Gélinas, B.R., Lodge, R.W.D., Gibson, H.L., 2016, Characterization of the Mineralization and Alteration at Tower Mountain, Conmee Township, Shebandowan Greenstone Belt, Ontario, Ontario Geological Survey, Miscellaneous Release - Data 330. Greenburg, J., 1986, Magmatism and the Baraboo Interval: Breccia, Metasomatism and Intrusion: Geoscience Wisconsin, v. 10, p. 96-112. Woolley, A., 2003, Igneous Silicate Rocks Associtated with Carbonatites: Their Diversity, Relative Abundances and Implications for Carbonatite Genesis: Periodico Mineralogia, v. 72, p. 9-17. 37 �Petrogenesis of mafic magmatism in the Coldwell Complex Part 1. Geochemical model to explain origin of metabasalt by partial melting in the SCLM GOOD, Dave Department of Earth Sciences, Western University, London, ON N6A 5B7 Canada Data for basalt and intrusive mafic rocks from the early stage of evolution of the Midcontinent Rift show well-defined trends for trace element abundances that are consistent with variable degrees of partial melting in a mantle plume source and subsequent fractional crystallization. For example, in a plot of La vs. Zr, early MCR basalt data plot in a field that spans compositions from E-MORB to OIB. The La/Zr values increase from about 0.07 to 0.18 with increasing La, as expected given the relative incompatibility of these elements. Deviations from the E-MORB to OIB-like compositions are explained by interaction of the magma with either SCLM or continental crust during ascent, or by varying depths of partial melting. A comparison of the early stage MCR rocks to the Coldwell mafic rocks shows significant differences between the two groups. First, the ranges of trace element concentrations for Coldwell rocks is one to two orders of magnitude greater than for the entire suite of MCR rocks. Second, Coldwell rocks show significant LREE enrichment relative to HFSE (example, very high La/Zr). Remarkably, Th/Nb for the Coldwell rocks are near mantle values (0.12) signifying enriched LREE is not a result of crustal contamination. It is difficult to explain these differences between Coldwell and MCR rocks by processes such as partial melting or crystal fractionation and some other explanation is required for the decoupling of these highly incompatible trace elements. Trace element abundances for Coldwell metabasalt units 1 to 3b are some of the lowest values in the MCR. Spider diagram patterns for the metabasalt and gabbro units are characterized by strong negative Nb and Zr anomalies that resemble patterns observed for mantle xenoliths from the SCLM at numerous locations, including Bir Ali, Yemen. 38 �A geochemical model that supports generation of Coldwell metabasalt by partial melting of a hypothetical SCLM source was proposed by Good and Lightfoot. The source composition is based on mantle xenolith data from Bir Ali, Yemen and was synthesized by mixing lherzolite with a very small fraction (1-2%) of secondary clinopyroxene or amphibole. This model can explain many features of the Coldwell metabasalt units, however, at this stage, trace element abundances are too low to predict the nature of metasomatism (carbonatite or alkaline) in the source. References Cundari, Robert, 2012. MSc thesis, Lakehead University, 142 pages Davis, Sarah, 2016. BSc thesis, Lakehead University, 63pages Good D.J. and Lightfoot P.C., Submitted to CJES, Feb 2018 Good, D.J., Cabri, L.J. and Ames, D.E., 2017, Ore Geology Reviews, v. 90, p.748-771 Good, D.J., Epstein, R., McLean, K., Linnen, R.L. & Samson, I.M., 2015, Econ Geol v.110, p.983-1008 Keays, R.R. and Lightfoot P.C., 2015, Econ Geol v. 110, p. 1235-1267. Sgualdo, P., Aviado,K., Beccaluva, L., Bianchini, G., Blichert-Toft , J., Bryce, J.G., Graham, D.W., Natali, C. and Siena, F. 2015. Tectonophysics, v. 650, p. 3-17. Sage, R.P. 1994, OGS, Open File report 5888, 592p. Sun, S.S., and McDonough, W.F. 1989. Geological Society, London, Special Publications v. 42, p. 313-345. 39 �Geologic history meets the web – online data of the Lake Superior Division of USGS GOTTSCHALK, Brad, ROSE, Caroline, and MCCARTNEY, M. Carol Wisconsin Geological and Natural History Survey, University of Wisconsin – Extension, caroline.rose@wgnhs.uwex.edu Working out of their headquarters in Madison, Wisconsin from 1882 to 1922, USGS Lake Superior Division geologists laid the groundwork for all subsequent investigations of this region’s Precambrian rocks. Nine monographs, four bulletins, and a professional paper describe the findings of those early geologists. The physical samples and paper records they collected and used to produce those publications comprise the Lake Superior Legacy Collection held by the Wisconsin Geological and Natural History Survey (WGNHS, the Survey). The Survey began digitizing the Lake Superior collection in 2011, when the UW Digital Collection scanned field notes written by Charles Van Hise. The collection contains: more than 30,000 hand samples; over 13,000 thin sections (photographed in plane- and cross-polarized light); 467 field notebooks (321 of which have been scanned); 67 maps; and, 35 catalogs of specimens, lithological descriptions, and more. The web application links all of these components together, presents this image-rich dataset in a visual way, and also provides some historical context. http://data.wgnhs.uwex.edu/lake-superior-legacy/index.html Figure 1: Through the interactive map, samples can be found by location, rock type, sample notes, sample number, field notebook number, and other attributes. 40 �In the Lake Superior Legacy Collection application, researchers and historians can: search for hand sample locations on an interactive map; browse a gallery of thin sections and a list of field notebooks; view and zoom into high-resolution thin section photographs; examine hand-drawn topographic and geologic maps; and review the hand-written field notebooks and lithological descriptions of the pioneering geologists who collected these physical samples. More importantly, they can relate a hand sample to its thin sections and to its location. They will also find the notebook and page number where any specific sample is described – with a link to the online scanned version of those notes. Figure 2: Browse thin section photographs in the gallery; use the viewer to zoom and to fade between plane- and cross-polarized light; follow the link to view details for the related hand section. This long-term data preservation project, completed with the help of the USGS National Geological and Geophysical Data Preservation Program, is allowing today’s geologists to gain access to the work of the early giants, Roland Irving and Charles Van Hise. Additionally, we have been able to present this collection of beautiful thin sections and hand-written notes in a visually appealing application that shares the beauty and history of Lake Superior geology online. 41 �Inferences on the Subsurface Distribution of Oronto and Bayfield Groups North and West of the Douglas Fault, Northwestern Wisconsin GRAUCH, V.J.S.1, BEDROSIAN, Paul A.1, STEWART, Esther Kingsbury3, and HELLER, Samuel2, 1 U.S. Geological Survey, MS 964, Federal Center, Denver, CO, 80225 U.S. Geological Survey, MS 939, Federal Center, Denver, CO, 80225 3 Wisconsin Geological and Natural History Survey, 3817 Mineral Point Rd., Madison, WI 53705 2 The period of extensive sedimentation that largely post-dated magmatic activity related to the 1.1 Ga Midcontinent rift is recorded by the Oronto and overlying Bayfield Groups in northwestern Wisconsin and correlative units in neighboring Minnesota (Fig. 1). The reversesense Douglas fault juxtaposes the two Groups: Oronto Group rocks form a syncline within the St. Croix Horst to the south and east, and the gently inclined Bayfield Group sandstones are to the north and west (Fig. 1). Previous subsurface interpretations north and west of the Douglas Fault have come from geophysical studies (e.g., Ocala and Meyer, 1973; Allen et al., 1997), which relied on recognizing characteristic densities and seismic velocities derived from modeling or sample measurements (Halls, 1969) to identify formation divisions within the Groups. Formations within the Bayfield and Oronto Groups have characteristic velocities and densities that both increase with older age. We have reassessed the previous geophysical work in northwestern Wisconsin using recently obtained digital access to proprietary seismic-reflection data, preliminary results from new airborne electromagnetic (AEM) survey lines, and a reexamination of characteristic seismic velocities correlated to geologic units from legacy refraction data, borehole logs and drill core in the Ashland Syncline area (Fig. 1). Our findings are summarized as follows (refer to Fig. 1). Data processing tests of LS-10 seismic line (westernmost Lake Superior) to find the velocity function that images the sharpest reflections suggest that the top ~2 km is composed of low-velocity (3.2 to 3.7 km/sec) rocks overlying igneous basement. This finding is in general agreement with previous refraction studies (e.g., Ocala and Meyer, 1973), who attribute the low velocities to the Bayfield Group (typical range 2.74-3.5 km/sec from Mooney et al., 1970). However, our geologic correlations from the well data indicate that the upper part of the Freda Sandstone (upper Oronto Group) has velocities from 3.2 to 3.9 km/sec, which overlap with the range expected for Bayfield Group. The low velocities here compared to Freda Sandstone elsewhere are likely caused by the greater volume of siltstones and shales (Halls, 1969). Comparison of lithology and unit picks in drill core to resistivity sections from the AEM lines show consistent correlation of low resistivities (10-50 ohm-m) with siltstones and shales of the upper Freda Sandstone and of moderate to high resistivities (>200 ohm-m) with sandstones of the Bayfield Group. Using these results, we interpret the AEM resistivity sections to show a lakeward thinning wedge of Bayfield sandstones that appear to terminate near the northern shore of the Bayfield Peninsula. The wedge overlies low resistivities typical of the shales and siltstones of the Freda Sandstone. The sequence of velocities found from a refraction site near onshore seismic-reflection line SEI-1 has a velocity-depth pattern generally compatible with overall variations in the velocities observed for Oronto Group units within the Terra-Patrick well (Dickas and Mudrey, 1999). We thus can roughly correlate the refraction velocity profile to reflection packages in 42 �SEI-1 and extrapolate the interpretation to LS-10. Combined with low-resistivities observed below 500 m depth where an AEM line crosses SEI-1, we infer that about 500 m of Bayfield Group overlies about 3.5 km of Oronto Group along SEI-1. Thus, the ~2-km thick, low-velocity, low-resistivity sedimentary section under LS-10 indicates that lower Oronto (Copper Harbor and probably Nonesuch) units are missing in the westernmost part of Lake Superior and Freda strata directly overlie igneous basement. This conclusion is supported by Allen et al. (1997), who used a different line of reasoning from seismic reflection line LS-8. The results imply that lower Oronto strata were either not deposited or were eroded from the area under the westernmost lake up until upper Freda time, whereas the areas to the southeast were accumulating sediments throughout the entirety of Oronto and Bayfield times. References 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: Geological Society of America Special Paper 312, p. 47-72. Dickas, A.B., and Mudrey, M.G., Jr., 1999, Terra-Patrick #7-22 deep hydrocabron test, Bayfield County, Wisconsin: Wisconsin Geological and Natural History Survey Miscellaneous Paper 97-1, 117 pp. Halls, H.C., 1969, Compressional wave velocities of Keweenawan rock specimens from the Lake Superior region: Canadian Journal of Earth Sciences, v. 6, p. 555-568. McGinnis, L.D., and Mudrey, M.G., Jr., 2003, Seismic reflection profiling and tectonic evolution of the Midcontinent rift in Lake Superior: Wisconsin Geological and Natural History Survey MP 91-2. Mooney, H.M., Farnham, P.R., Johnson, S.H., Volz, G., and Craddock, C., 1970, Seismic studies over the Midcontientn gravity high in Minnesota and northwestern Wisconsin: Minnesota Geological Survey Report of Investigations 11, 191 pp. Ocala, L.C., and Meyer, R.P., 1973, Central North American Rift System, 1. Structure of the axial zone from seismic and gravimetric data: Journal of Geophysical Research, V. 78, no. 23, p. 5173-5194. Stewart, E.K., and Mauk, J.L., 2017, Sedimentology, sequence-stratigraphy, and geochemical variations in the Mesoproterozoic Nonesuch Formation, northern Wisconsin, USA: Precambrian Research, v. 294, p. 111-132. Figure 1: Regional geology and index map locating geophysical and drillhole information. Modified from Stewart and Mauk (2017). USGS – U.S. Geological Survey. WGNHS – Wisconsin Geological and Natural History Survey 43 �Origin, distribution, morphology, and chemistry of amphiboles in the Ironwood IronFormation, Gogebic Iron Range, Wisconsin, U.S.A. GREEN, Carlin J., SEAL, Robert, R., II, CANNON, William F., PIATAK, Nadine, and MCALEER, Ryan J. U.S. Geological Survey, MS 954, Reston, VA 20192 The Ironwood Iron-Formation, located in the Gogebic Iron Range in Wisconsin, is one of the largest undeveloped taconite resources in the United States. Interest in the development of this resource is complicated by potential environmental and health effects related to the presence of amphibole minerals in the Ironwood Iron-Formation, a consequence of Mesoproterozoic contact metamorphism. The purpose of this study is to provide mineralogical information about these amphiboles to aid regulatory, medical, and mining entities in their evaluation of this potential resource. Optical microscopy, X-ray diffraction, scanning electron microscopy, and electron microprobe analysis techniques were utilized to study the origin, distribution, morphology, and chemistry of amphiboles in the Ironwood IronFormation. The development of amphiboles from Fe-carbonates and Fe-phyllosilicates at temperatures of approximately 300 -340º C has long been recognized as a result of regionally extensive contact metamorphism of the Ironwood Iron-Formation by the Mellen Intrusive Complex, however amphiboles related to the emplacement of diabase or gabbro dikes and sills in low-grade iron-formation were also recognized in this study area. Amphiboles in the Ironwood Iron-Formation most commonly developed in massive and prismatic habits, and locally assumed a fibrous habit. Fibrous amphiboles were locally recognized in the two potential ore zones of the Ironwood Iron-Formation, but were not observed in the portion considered to be waste rock. Massive and prismatic amphiboles show a wide range of Mg# values (0.06 to 0.87), whereas Mg# values of fibrous amphiboles are restricted from 0.14 to 0.35. Factors that influenced the compositional variability of amphiboles in the Ironwood Iron-Formation may have included temperature of formation, the presence of coexisting minerals, morphology, bulk chemistry of the iron-formation, and variations in prograde and retrograde metamorphism. 44 �Geothermobarometry of a Precambrian amphibolite from Cornell WI HAFFTEN, Doug and RADWANY, Molly Department of Plant and Earth Sciences, University of Wisconsin – River Falls, 410 S 3rd Street, River Falls, WI Garnet amphibolite at Cornell, Wisconsin is part of the Chippewa amphibolite complex, a group of amphibolite-facies rocks with diverse protoliths, outcropping in the valley of the Chippewa River and its tributaries in western Wisconsin (Laberge and Myers, 1984). Metamorphism of the region occurred in Precambrian time, due to the Penokean orogeny (Schulz and Cannon, 2007). The Cornell amphibolite contains an ideal mineral assemblage for estimating both temperature and pressure of metamorphism, making it useful for understanding the effect of the Penokean Orogeny on the Marshfield Terrane. We obtained whole-rock geochemical data for the sample using a Bruker S8 Tiger X-ray fluorescence spectrometer at University of Wisconsin – Eau Claire. Ratios of immobile trace elements (Zr/Ti=0.02 and Nb/Y=0.14) indicate a mafic-intermediate protolith for the amphibolite. Petrographic analysis of the Cornell amphibolite reveals an assemblage of 20% hornblende, 45% plagioclase, 20% quartz, and 15% garnet with accessory apatite and zircon. We used a JEOL 8900 electron microprobe at the University of Minnesota to obtain mineral compositions. Which revealed ferro-tschermakitic hornblende, almandine-rich garnet and plagioclase An 40-65 . To determine the temperature of metamorphism, we used the Holland and Blundy (1994) hornblende-plagioclase thermometer. The results of this method suggest temperatures between 606-646°C. To determine the pressure of metamorphism, we applied the Kohn and Spear (1990) garnet-hornblende-plagioclase-quartz geobarometer. The range of pressures determined using this method is 5.74-6.64 Kbar. These results reveal more specific information about the Chippewa amphibolite complex and the dynamic Precambrian past of Wisconsin. REFERENCES Holland, T and Blundy, J, 1994, Non-ideal interactions in calcic amphiboles and their bearing on amphibole-plagioclase thermometry, Contributions to Mineralogy and Petrology, Vol. 116, p. 433447 Kohn, M.J. and Spear, F.S., 1990, Two new geobarometers for garnet amphibolites, with applications to southeastern Vermont, American Mineralogist, Vol. 75, p. 89-96 Laberge, G.L. and Myers, P.E., 1984, Two early Proterozoic successions in central Wisconsin and their tectonic significance, Geological Society of America Bulletin, Vol. 95, p. 246 Schulz, K.J. and Cannon, W.F., 2007, The Penokean orogeny in teh Lake Superior region, Precambrian Research, Vol. 157, p. 4-25 45 �Figure 1: Backscatter electron image showing the main four phases in the Cornell amphibolite. Mineral compositions were used to determine pressure and temperature of metamorphism. Grt: garnet, Pl: plagioclase, Ts: tschermakite, Qz: quartz. 46 �Hornblende-Plagioclase thermometry of the Eau Claire River Complex, western Wisconsin HANNACK, Gina, and RADWANY, Molly Department of Plant and Earth Sciences, University of Wisconsin River Falls, 410 S. 3rd Street River Falls, WI The Eau Claire River Complex is a metamorphosed and deformed layered mafic intrusion that outcrops in Big Falls County Park near Eau Claire, Wisconsin. The unit is part of the larger Chippewa amphibolite complex within the Precambrian Marshfield Terrane (Cummings, 1984). The unit is characterized by a compositional layering that alternates between mafic and feldspathic compositions. We distinguish two types of rock - mafic amphibolites, containing ~80% hornblende, and feldspar-rich amphibolites, containing ~15% hornblende. The vertical, north-south striking foliation is defined by compositional layers, likely inherited from primary, igneous layering. Lineations, defined by hornblende, are at a high angle to compositional layering (approximately east to west) and represent crystallization of hornblende during metamorphism of the complex. Cummings (1984) established field evidence of a granulite facies metamorphic event represented by garnet porphyroblasts. Overprinting occurred during a second event at amphibolite facies and resulted in pseudomorphism of garnet by hornblende. The second event was pervasive and accompanied by dynamic recrystallization of plagioclase. Finally, there was a third, greenschist facies metamorphic event, resulting in crystallization of epidote, zoisite, biotite, and chlorite. Mafic amphibolites consist of approximately 80% hornblende, 15% plagioclase and 5% accessory minerals ilmenite and apatite. The feldspar-rich amphibolites are comprised of 80 to 85% plagioclase, 10 to 15% hornblende and 5% ilmenite and apatite. Greenschist facies retrograde assemblage consists of minor epidote, zoisite, chlorite, and biotite. Pyrite also occurs in several samples, especially in association with epidote veins. We determined mineral compositions for one mafic amphibolite using a JEOL 8900 electron microprobe at University of Minnesota. Plagioclase compositions range from An 42 to An 85 . The amphibole present is Figure 1: Microprobe image containing hornblendemagnesio-hornblende with Mg/(Mg+Fe) plagioclase thermometer pairs between 0.55 to 0.59. Microprobe data 47 �was utilized in the application of the Holland & Blundy (1994) edenite-richterite thermometer (Figure 1). We chose this thermometer based on the minor amount of quartz found in petrographic analysis. Results of this thermometer show that the Eau Claire Complex underwent upper amphibolite facies metamorphism at temperatures ranging from 719 °C to 769 °C (assuming P of 10 kb; Figure 2). Our results provide insight into the mineralogic and structural effects of the Penokean Orogeny on the crystalline rocks of the Marshfield terrane and a quantitative estimate of thermal conditions during this collisional event. Figure 2: Temperatures of metamorphism of the Eau Claire River Complex, as determined by application of the Holland and Blundy (1994) edenite-richterite thermometer. References Cummings, ML, 1984, The Eau Claire River Complex: a metamorphosed Precambrian mafic intrusion in western Wisconsin, Geological Society of America Bulletin, Vol. 95, p. 75-86 Holland, T and Blundy, J, 1994, Non-ideal interactions in calcic amphiboles and their bearing on amphibole-plagioclase thermometry, Contributions to Mineralogy and Petrology, Vol. 116, p. 433-447 48 �Mapping the Midcontinent Rift System Hinze, William J. Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN 47906 Mapping the ca. 1100 Ma old Midcontinent Rift System (MRS) which is arguably the most significant non-orogenic structure of the North American midcontinent has been an important, but challenging objective for the past half century because it is hidden for most of its ~2500 km length by relatively flat-lying Phanerozoic sedimentary rocks. Even where it is crops out in the Lake Superior region, it is largely covered by sedimentary rocks of late-stage Keweenawan rift basins and Pleistocene glaciation sediments. Mapping of the buried rift system is based on interpretation of gravity and magnetic anomaly data that are not definitive, poorly distributed deep seismic profiling, and limited basement drill holes. As a result, uncertainty exists in the geographic location, extent, and configuration of the buried MRS. Additional ambiguity is caused by confusion in defining the required characteristics of continental rifts. A review of the available data on the MRS and Mesoproterozoic rocks of the North American midcontinent provides insight into the likely geographic location, configuration, and extent of the rift system and identifies portions of the rift’s configuration that are most problematic. Generally, the MRS is shown extending in a southerly-open arc along several rift units from the western end of Lake Superior into Kansas and a shorter, eastern, branch of less intense rifting that continues south from the eastern end of the Lake into southeastern Michigan. Studies of basement rocks from Kansas southwestward indicate that magmatic activity similar in age to the MRS occurred in this region as it did broadly over the present-day midcontinent. However, no rift basins have been found or are indicated by geophysical and deep drilling data to the south of Kansas or elsewhere where magmatic activity occurred outside of the trend of the MRS. Perhaps this magmatic activity is evidence of incipient rifts, that is regions where intrusions have accompanied lithospheric extension which failed to reach an intensity where surface faulting and rift basins developed. The termination of the other branch of the rift in Michigan is complicated by its intersection with geologic structures resulting from the Grenville orogeny whose latest activity is slightly younger than the MRS. Gravity and magnetic anomalies suggest that the MRS terminates at the Grenville Front in southeastern Michigan, but originally it may have extended to approximately the border with Ontario. Late stage Grenvillian overthrusts may overlie the extreme terminus of the MRS in Michigan. North/south striking gravity positive anomalies extending through Ohio along the eastern margin of the Grenville Front perhaps as far as Alabama have fostered the hypothesis that the rift system extends south from southeastern Michigan. The positive gravity anomalies have been purported to be the locus of rift basins of volcanic rocks. However, because rift basins do not occur at the basement surface coincident with the gravity highs, they have been interpreted as relic rifts that exist beneath Grenvillian overthrusts that post-date the MRS. Unfortunately, seismic reflection profiling does not confirm the presence of these rifts beneath the overthrusts observed 49 �in the seismic reflection data. Alternatively, integrated interpretation of the Grenville Front Tectonic Zone gravity and magnetic anomalies, seismic reflection profiling, and basement drillhole samples indicate that the Grenville Front positive gravity anomalies are caused by upthrusted high-grade metamorphic rocks from upper and mid-level crustal rocks of the Grenville orogen. According to the latter interpretation the lithic-arenites, the Middle Run formation, that occur unconformably below the Paleozoic sedimentary formations west of the Grenville Front in Ohio and adjacent states were deposited in a foreland basin primarily from erosion of the Grenville highlands to the east rather than in a marginal late-stage rift basin adjacent to a rift trough. This conclusion is supported by reported dating of detrital zircons from the Middle Run formation. Especially problematic in mapping the MRS is the location of the eastern margin of the terminus of the rift in the Northern Peninsula of Michigan and the connection of the Lake Superior Rift with the Cross-Michigan Rift segment to the south. A “third branch” of the MRS remains elusive, but the most likely candidate is the Nipigon Embayment that did not develop into a rift basin such as found beneath Lake Superior and the eastern and western branches of the MRS. 50 �Reinterpretation of the ages of deposition and folding of Animikie Basin metasedimentary units in east-central Minnesota HOLM, Daniel1, BOERBOOM, Terrence J.2, and SCHEINER, Scott1 1 Dept of Geology, Kent State University, Kent OH 44224 dholm@kent.edu 2 Minnesota Geological Survey, boerbo001@umn.edu Animikie Basin (AB) sedimentary rocks in Minnesota have historically been interpreted solely as Penokean (1875-1835 Ma) foredeep deposits. Yet detrital zircons as young as ca. 1770 Ma (Heaman and Easton, 2005) in the northern reaches of the AB (Rove Formation in Ontario) indicate that deposition in the upper part of the basin may have occurred during Yavapai orogenesis (1800-1700 Ma). Much of the AB sequence is only very weakly metamorphosed and mildly deformed. However, along its southern margin in east-central Minnesota, deformed AB sedimentary rocks show an increase in metamorphism and strain southward toward a mid-crustal plutonic-gneiss dome terrane largely exhumed during Yavapai orogenesis. Holst (1984) recognized two distinct structural zones; a northern once-deformed region characterized by upright folds and a single well-developed cleavage, and a southern twice deformed terrane characterized by refolded recumbent fold nappes and two cleavages. Holst interpreted the map trace separating these two deformation zones to be a Penokean thrust fault (Fig. 1A) and assumed all of the sedimentation and deformation to be Penokean. Given the wealth of geologic and geochronologic data which now document Yavapai-age magmatism, metamorphism, sedimentation and deformation overprinting the Penokean orogeny, we reinterpret Holst’s Line as a possible Yavapai age angular unconformity that separates the once/twice-deformed units (Fig. 1B), implying that only the southern terrane sediments and the early recumbent nappes are Penokean. In this model, rapid exhumation of the entire Penokean/Yavapai internal zone resulted in rapid erosion rates and renewed Yavapai orogenic sedimentation into the Animikie Basin followed by folding of both sedimentary sequences. Bedrock mapping, geophysical data, and geochemical/isotopic analyses of the metasedimentary rocks along the southern margin of the AB in Carlton County Minnesota, briefly described below, are at least consistent with this new hypothesis. A B Fig. 1. Schematic synopsis of tectono-sedimentary interpretations along the southern margin of the Animikie Basin, east-central Minnesota. A: Late Penokean thrust fault cuts Penokean Animikie foredeep deposits (after Holst, 1984). B: Yavapai age unconformity separates Penokean (south) from Yavapai orogenic deposits (north). Remapping and relogging of cores and cuttings (Boerboom, 2009) and aeromagnetic data reveals lithologic differences south and north of Holst’s Line. The southern units are characterized by a moderate gravity high and a belt of discontinuous aeromagnetic anomalies interpreted as ‘sulfidic horizons’ with large cubic pyrite porphyroblasts. The sulfidic horizons may be similar to those at the base of the Baraga basin in Michigan (the Bijiki Iron Formation) and possibly to portions of the iron rich layers near in the Cuyuna South Range. The sulfidic horizons are absent north of Holst’s Line. 51 �Geochemical analyses of samples collected across the contact reveal a concentrated grouping of trace element data from the southern samples and a larger spread from the northern samples, suggesting a more variable source for the northern sedimentary sequence. More importantly, Nd isotopic data across the contact reveal a more juvenile source for rocks south of (i.e., below) the contact (ƐNd (T)1.77Ga = 9.2 and 1.2) and an older more enriched Archean source directly north of (i.e., above) the contact (ƐNd (T)1.85Ga = -0.4 and -3.9). We interpret the southerly sequence to be Penokean and derived from the newly accreted juvenile arc terrain. However we propose that the northerly sequence is Yavapai in age and derived from an older more enriched Archean or mixed source such as is presently exposed in the plutonic-gneiss dome corridor. We propose a simplified model for tectono-sedimentary formation of the AB (Fig. 2). At the base of the basin the Penokean unconformity is shown as a nonconformity above Archean basement. To the south, the basal Penokean unconformity becomes an angular unconformity separating pre-Penokean sedimentary and volcanic rocks to the south from Penokean foredeep rocks to the north. Only the southern portion of the AB closest to the orogenic zone experienced Penokean deformation. During the Yavapai orogeny, the southern margin of the basin experienced uplift and erosion, followed by Yavapai orogenic deposition resulting in the formation of a disconformity in the north and a Yavapai angular unconformity in the south. Rapid Yavapai age exhumation of the plutonic-gneiss dome terrane led to copious amounts of sediment being shed into the basin. Near the end of the Yavapai orogeny, deformation resulted in folding of the Yavapai foredeep deposits and refolding of Penokean and pre-Penokean rocks in the south. Deformation waned to the north away from the orogenic zone. If correct, our reinterpretation has important ramifications for interpreting the inventory of structures in the upper Great Lakes region. For instance, the sedimentary rocks and the late open upright second-generation folds exposed at Thomson Dam, may both be manifestations of Yavapai orogenesis and not classic features of Penokean orogenesis. 1770 Ma zircons -εNd +εNd Fig. 2. Simplified schematic synopsis of tectono-sedimentary formation of the Animikie Basin in east-central Minnesota. Boerboom, T.J., 2009, Plate 2 – Bedrock Geology, pl. 2 of Setterholm, D.R., Project manager, Geologic Atlas of Carlton County, Minnesota, Minnesota Geological Survey County Geologic Atlas Series C-19, pt. A, 7 pls, scale 1:100,000. Heaman, L.M., and Easton, R.M., 2005, Proterozoic history of the Lake Nipigon area, Ontario, constraints from UPb zircon and baddeleyite dating (Abs.): Institute on Lake Superior Geology 51, Part 1 – Program and Abstracts, p. 24-25. Holst, T.B., 1984, Evidence for nappe development during the Early Proterozoic Penokean Orogeny, Minnesota: Geology, v. 12, p. 135-138. 52 �Olivine Crystal Size Distribution in the Black Sturgeon Sill, Nipigon, Ontario HONE, Samuel V. and ZIEG, Michael J. Department of Geography, Geology, and the Environment, Slippery Rock University, Slippery Rock, PA 16057 Recent years have seen widespread acceptance of the idea that igneous intrusions are often emplaced in multiple phases or pulses (Miller et al., 2011). To test whether textural data could be used to distinguish between these pulses, we examined olivine from the Black Sturgeon sill, a 256 m thick diabase intrusion located southwest of Lake Nipigon in Ontario. We collected samples from a continuous drill core through the sill and subdivided it into several zones using modal mineralogy and textural data. The most primitive and olivine-rich zone is from 120-200 m above base. In this study, we investigated the use of crystal size distributions (CSDs) to discriminate between distinct populations of olivine in this olivine-rich zone. CSDs are a well-established method of quantifying the textures of igneous rocks (Zieg and Marsh, 2002). We performed this analysis by collecting images from 42 samples throughout the olivine zone and stitching them together into large mosaics. We manually traced at least 200 olivine crystals from each sample, then entered the list of crystal sizes into the software package CSDCorrections 1.5 (Higgins, 2000) to calculate the CSDs. The slope and intercept of best-fit lines to the CSDs are used to quantify the texture (Zieg and Marsh, 2002). Two typical CSDs with best-fit lines are shown in Figure 1. Cluster analysis of the slope-intercept data reveals four well-defined textural groups, which could correspond to distinct populations of olivine (Fig. 2). These groups are typically found in separate parts of the olivine zone (Fig. 3), with sharp boundaries between the different groups. As an example, the textures of samples 264 and 265.5 are clearly qualitatively and quantitatively distinct (Fig. 1), even though they are found within 1.5 m of each other. Using the slope and intercept of CSDs, along with cluster analysis, we can identify separate populations of olivine in the olivine zone of the Black Sturgeon sill. While we have not found any consistent variation within groups, the breaks between the populations are sharp and coincide roughly with compositional changes (Zieg and Hone, this issue). We interpret these breaks as evidence of the episodic emplacement of the Black Sturgeon sill. CSDs have proved an effective complement to other methods in identifying discrete magma pulses in this sill. The episodic nature of magma emplacement in the Black Sturgeon sill also raises the possibility that other layered mafic intrusions formed by a similar process. References Higgins, MD, 2000. Measurement of crystal size distributions. American Mineralogist, 85: 1105-1116. Miller, CF, Furbish, DJ, Walker, BA, Claiborne, LL, Koteas, GC, Bleick, HA, and Miller, JS, 2011. Growth of plutons by incremental emplacement of sheets in crystal-rich host: Evidence from Miocene intrusions of the Colorado River region, Nevada, USA. Tectonophysics, 500: 65-77. Zieg, MJ, and Marsh, BD, 2002. Crystal size distribution and scaling laws in the quantification of igneous textures. Journal of Petrology, 43, 1: 85-101. 53 �1 mm 1 mm Figure 1. Representative textures. a) Photomicrograph of sample 264(5% olivine). b) CSDs of samples 264 and 265.5. c) Photomicrograph of sample 265.5 (26.5% olivine). The finer-grained texture (c) has a higher intercept and steeper slope. Figure 2. Identification of four textural groups based on the CSD slope and intercept. Figure 3. CSD slope (a) and intercept (b) profiles through the olivine zone. 54 �Reconstructing Paleoproterozoic volcanism in northwestern Wisconsin: Geochemistry of the Flambeau Cu-Zn-Au Mine JACOBSON, Regan E., LODGE, Robert W.D. Department of Geology, University of Wisconsin – Eau Claire: Eau Claire, WI 54702-4004 The Flambeau mine is located 1 mile southwest of the town of Ladysmith, located in Rusk County, WI. The mine is a part of the Wisconsin Magmatic Terrane and is a group of volcanic and plutonic rocks that formed during volcanism associated with the accretion of the Pembine-Wausau terrane onto the southern end of the Superior craton during the Paleoproterozoic Penokean orogeny (May and Dinkowitz, 1996). The Flambeau is one of a number of volcanogenic massive sulfide (VMS) deposits, but it is the only one that has been mined. The mined portion of Flambeau deposit is part of an enriched zone where relatively little is known about the original hypogene geology. The ore-hosting rocks consists of metamorphosed variably-altered volcanic rocks and cherty iron-formations that are now sericite to quartz-sericite schists, and biotite-andalusite-chlorite schists (DeMatties, 1994). The Flambeau was mined from 1993-1997 and produced 181,000 tons of copper, 334,000 ounces of gold, and 3.3 million ounces of silver contributing over a billion dollars in state revenue. Mining of the enriched orebody was completed in 1997 (Jones and Jones, 1999). Since then, the site has been reclaimed and revegetated. Therefore, the only remaining rock for the Flambeau site is preserved in drill core stored at the Wisconsin Geological & Natural History Survey core repository. VMS ore deposits are formed in submarine environments where high temperature hydrothermal fluids react with cold sea water to cause the precipitation of sulfide minerals. Usually, the characteristics of the volcanic system influence the composition of ore and alteration mineral assemblages. However, ongoing studies of Flambeau ores and hydrothermal alteration do not align with previous interpretations for the Flambeau volcanic system (Blotz et al. 2018). Historic research focuses largely on the mined portions of the deposit as the remainder of the strata is covered by thick Quaternary glacial deposits. This study utilizes major and trace element geochemistry of least-altered host rocks to assess the magmatic and tectonic affinity of the rocks hosting the Flambeau deposit. This study is the first trace rare earth data set for the Flambeau deposit. Assessing the magmatic affinity of these rocks will provide insight to the petrogenesis of arc magmatism/collision and the magmatic/tectonic controls on VMS mineralization during ocean-continent collision. Previous stratigraphic interpretations of the hangingwalll rocks indicate three units consisting of quartz-augen schist, metadacite, and chlorite schist (May and Dinkowitz, 1996). Geochemical data produced in this study indicates that the Flambeau deposit is primarily hosted by intermediate volcanics locally interleaved with quartz-phyric rhyolitic rocks (Fig. 1). Preliminary data reveal a bimodal distribution and all show arc-like characteristics on primitive-mantle normalized plots with light REE enrichment and negative Nb and Ti anomalies. Felsic rocks show FI to FII type (Lesher et al. 1986) trace element characteristics inicative of formation at moderate crystal depths. Ongoing trace element analysis will more clearly document the magmatic affinity of this volcanic suite. 55 �Figure 1 This shows Zr/Ti and Nb/Y ratios of units 3a, 5, and 2a. These ratios are representative of the host rocks in the Flambeau VMS deposit. Plot from Pearce (1996). References Dematties, T.A., 1994, Early Proterozoic Volcanogenic Massive Sulfide Deposits in Wisconsin; an overview: Economic Geology, v. 89, p. 1122–1151, doi: 10.2113/gsecongeo.89.5.1122. Jones, E.L., and Jones, J.K., 1999, The Flambeau Mine, Ladysmith, Wisconsin: The Mineralogical Record, v. 30, p. 107-131 May, E.R., and Dinkowitz, S.R., 1996, An Overview of the Flmabeau Supergene Enriched Massive Sulfide Deposit: Geology and Mineralogy, Rusk County, Wisconsin, in LaBerge, G.L., ed., Volcanogenic Massive Sulfide Deposit of Northern Wisconsin: A Commemorative Volume: Institute on Lake Superior Geology Proceedings, v. 2, part 2, p. 67-96 Lesher, C.M., Goodwin, A.M., Campbell, I.H., Gorton, M.P., 1986. Trace-element geochemistry of ore-associated and barren, felsic metavolcanic rocks in the Superior Province, Canada. Canadian Journal of Earth Sciences 23, 222-237. Blotz, KE, Fredrickson, ET, Lodge, RWD. (2018) Characteristics of ore and alteration mineral assemblages at the Flambeau volcanogenic massive sulfide deposit, northwestern Wisconsin. Geological Society of America, Abstracts with Programs. Pearce, J.A., 1996. A user's guide to basalt discrimination diagrams. In: Wyman, D. A. (Eds.), Trace element geochemistry of volcanic rocks; applications for massive sulphide exploration. Geological Association of Canada, short Course Notes, p. 79-113. 56 �On-going Geologic Mapping in Minnesota’s Arrowhead Region by the Minnesota Geological Survey JIRSA,* Mark A., Minnesota Geological Survey (MGS) (jirsa001@umn.edu) *The large number of collaborators engaged in various components of this endeavor precludes complete acknowledgement here. Consult MGS Open-File Report OFR2016-04 for authorship details. This presentation describes geologic mapping by the MGS in northeastern Minnesota, with an emphasis on the bedrock geology. We are in year 3 of a 6-year process to create County Geologic Atlases for St. Louis and Lake Counties. The “Arrowhead Project” area includes parts of the Boundary Waters Canoe Area Wilderness, Voyageurs National Park, Superior National Forest, and several State forests. It also encloses the easternmost part of the Mesabi Iron Range, the “Cu-Ni District,” and the Duluth metropolitan area (the 4th largest in the state). County atlases contain maps and other imagery depicting bedrock geology, geophysical data, bedrock geochronology, bedrock topography, depth to bedrock, surficial sediments, and subsurface sediment layers; together with the extensive digital data sets used to construct them. The atlases are designed to provide regional 4-D geologic framework in digital and print formats to support ongoing and future studies related to land, water, and mineral resources. Figure 1. Generalized map of northeastern Minnesota showing the location and geologic setting of county-scale mapping (modified from MGS State Map Series S-21). Because these are two of the largest counties in the state, we’ve divided them for mapping purposes into subareas referred to here as the Central, Southern, and Northern Arrowhead. Work in each subarea involves 1 or more seasons of field mapping by 5 geologists, rotary-sonic drilling, trenching, and acquisition of drill hole, petrographic, geochronologic, and geophysical data. Work in the Central Arrowhead subarea is complete (e.g., Jirsa and others, 2017), and components of the other subareas are in various stages of completion. Preliminary products for all subareas are published as they become available in MGS Open-File Report OFR2016-04. This open-file report will remain the primary repository for on-going mapping. Once preliminary work in all subareas is published, the data will be recombined into county geologic atlases. Creation of these products is a team effort, involving 14 staff members from MGS, and 57 �several from partner agencies. Staff of the Minnesota Department of Natural Resources will use these maps and data sets to conduct regional groundwater studies, including assessments of flow systems, aquifers, groundwater chemistry, and an assessment of sensitivity to pollution, to produce Part B of the County Geologic Atlases. Support is provided by the U.S. Geological Survey National Cooperative Geologic Mapping Program, the Environment and Natural Resources Trust Fund (as recommended by Legislative-Citizens Commission on Minnesota Resources—LCCMR), and the Boards of Commissioners of St. Louis and Lake Counties. The region’s bedrock includes portions of three subprovinces of the Archean Superior Province, Paleoproterozoic strata including the Biwabik Iron Formation, and Mesoproterozoic volcanic and intrusive rocks of the North Shore Volcanic Group and Duluth Complex (Fig. 1). The latter hosts polymetallic mineral deposits under consideration for new mining. The bedrock in much of the region has been mapped to varied levels of detail in the past, driven in part by minerals exploration. Despite this, our current effort identified many areas that escaped previous mapping or were mapped in minimal detail, and it attempts to fill those voids and integrate the disparate sources of information. Mapping is conducted primarily at 1:24,000-scale, with printable products that are generalized from the companion digital data sets to scales of 1:100,000 to 1:200,000. One of the more geologically interesting aspects of recent work is the recognition that chemical weathering of bedrock prior to glaciation played a fundamental role in shaping the region’s topography, bathymetry, hydrogeology, and ecology. In this region where bedrock is at and close to the land surface, recently acquired empirical evidence indicates that differential erosion of saprolitic bedrock reflects both the compositional and structural attributes of the rock. Essentially, the bedrock surface in much of the area reflects the somewhat transitional boundary between fresh and weathered rock. In areas where outcrop, drill hole data, and access are limited, lidar imagery can be employed to infer both compositional and structural trends in bedrock. In addition, the presence of varied thicknesses and compositions of saprolite likely contributes to hydrogeologic characteristics, though further study is needed. The most recent bedrock mapping (Northern Arrowhead subarea) includes a component of geochronologic analyses. Although the results are not yet published, all the samples submitted are inferred to be Neoarchean—an era of rocks in Minnesota historically lacking extensive highresolution geochronologic data. Three main temporal objectives are attempted with these samples: 1) establish ages of successor basin deposits in the Wawa subprovince of the Superior Province; 2) establish ages of intrusions emplaced into several geologic settings; and 3) establish ages of major neosomatic components of migmatitic rocks that comprise the Quetico subprovince. If successful, these data will refine the temporal framework for deposition, magmatism, deformation, and metamorphism that will contribute to understanding the region’s tectonic evolution. As with all products derived from the Arrowhead project, the geochronologic results will be published in the open-file report mentioned above. REFERENCE Jirsa, Mark A., Boerboom, Terrence J., Radakovich, Amy L., Chandler, Val W., Peterson, Dean M., Schmitz, Mark D., Dengler, Elizabeth L., Wagner, Kaleb G., Lively, Richard S., and Setterholm, Dale R., 2017, Geologic mapping in the Central Arrowhead Area, northeastern Minnesota: 63rd Institute on Lake Superior Geology Proceedings, v. 63, Part 1, Program and Abstracts, p. 46-47. 58 �Geology and geochronology of the 2006 Cavity Lake forest fire area, Boundary Waters Canoe Area Wilderness, NE Minnesota JIRSA1, Mark A., STARNS2, Edward C., and SCHMITZ3, Mark D. 1 Minnesota Geological Survey, 2609 W. Territorial Road, St. Paul, MN 55114-1009 ConocoPhillips Alaska, Inc. 700 G St., Anchorage, AK 99501 3 Department of Geosciences, Boise State University, 1910 University Drive, Boise, ID 83725-1535 2 The bedrock geology in this part of the Boundary Waters Canoe Area Wilderness (BWCAW) is incredibly diverse and remarkably well exposed. Parts of the area were mapped to varied levels of detail in the 1930’s, and more recently in the 1970’s and 1980’s, with efforts focused primarily along waterways. A devastating wind storm in 1999 flattened trees in much of the region, and a delayed result in 2006 was the Cavity Lake forest fire. The fire exposed bedrock and allowed comparatively unencumbered access to interior parts of the map area, creating a unique and time-sensitive opportunity for mapping. Field work and compilation of prior mapping was conducted in 2007-2008 with funding from the USGS National Cooperative Geologic Mapping Program, and a preliminary map was produced (Jirsa and Starns, 2008). That map has been revised to incorporate subsequent geochronologic analyses and field work by the lead author and students of University of Minnesota, Duluth, Precambrian Research Center field camps (2007-2015). The map at scale 1:24,000, and companion data are published as Minnesota Geological Survey Miscellaneous Map M-193 (Jirsa and others, 2017). One previously unpublished geochronologic date by coauthor Schmitz is presented on the map and discussed here. Figure 1. Generalized bedrock geology of northeastern Minnesota showing the location of Cavity Lake map area (bold black outline). Inset map shows location of Neoarchean rocks within the Wawa subprovince of the Superior Province. M-193 portrays bedrock that represents crustal evolution spanning the Neoarchean to the Mesoproterozoic (Fig. 1), with an emphasis on structural and stratigraphic relationships in the Neoarchean portion. Neoarchean greenstone-granite terrane of the Wawa subprovince of Superior Province is represented by a succession of mostly mafic to ultramafic metavolcanic and hypabyssal intrusive rocks (ca 2700 Ma); unconformably overlain by hornblende-phyric, calc-alkalic volcanic and volcaniclastic rocks (ca 2690 Ma; newly published age), and intruded by the Saganaga Tonalite (also ca 2690 Ma). This succession was uplifted, chemically weathered (Driese and others, 2011), and subaerially 59 �eroded to provide detritus to one or more successor basins. Based on correlation with Neoarchean terrane along strike in Canada (Corfu and Stott, 1998), the latter sequence of clastic strata is thought to have been deposited at about 2684-2682 Ma—after emplacement of the Saganaga Tonalite (2690 Ma), and before the primary regional deformation and metamorphic event at about 2680 Ma (Boerboom and Zartman, 1993). All of these rocks were cut by mafic dikes inferred from field relationships to be both Paleoproterozoic and Mesoproterozoic. The Neoarchean rocks and some dikes are unconformably overlain by Paleoproterozoic metasedimentary strata of the Animikie Group (ca 1880-1830 Ma), which includes the Gunflint Iron Formation. The uppermost layers of iron-formation are intensely deformed and overlain east of this map area by thin lenses of ejecta from a meteorite impact that occurred near Sudbury, Ontario, at ca 1850 Ma (Jirsa and others, 2011). Mesoproterozoic rifting is manifest in hypabyssal dikes and sills known collectively as the Logan intrusions (ca 1115 Ma), and several intrusive phases of the Duluth Complex (ca 1100 Ma) emplaced into both Neoarchean and Proterozoic rocks. One of the most notable geologic features in this region is the local preservation of 4 major unconformities; two within Neoarchean rocks, and two in and at the base of Paleoproterozoic strata. The Neoarchean rocks in the central Boundary Waters Canoe Area Wilderness are inferred here to comprise a Timiskaming-type extensional basin and its apparent wall- and floor-rocks. The geologic units in the region were parceled by Gruner (1941) into eight structural segments separated by anastomosing shear and fault zones, and this map exposes parts of the eastern four of those segments. Although rock types are comparatively pristine within each segment, correlation of units from one fault-bounded block to another is challenging. Each block was uplifted, down-dropped and tilted differently, which results in different stratigraphic levels of exposure and repetition of strata locally. This map attempts to “unstrain” the rocks within each segment to reveal stratigraphic variations that may reflect fluctuations in basin geometry and progressive erosional dissection of basin wall rocks. This contributes to a regional tectonic model that involves basin development during late stages of terrane accretion. REFERENCES Boerboom, T.J., and Zartman, R.E., 1993, Geology, geochemistry, and geochronology of the central Giants Range batholith, northeastern Minnesota: Canadian Journal of Earth Sciences, v. 30, p. 2510-2522 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. Driese, S.G., Jirsa, M.A., Ren, M., Sheldon, N.D., Brantley, S.L., Parker, D., and Schmitz, M., 2011, Neoarchean paleoweathering of tonalite and metabasalt: Implications for reconstructions of 2.69 Ga early terrestrial ecosystems and paleoatmospheric chemistry: Precambrian Research, v.189, p. 1-17. Gruner, J.W., 1941, Structural geology of the Knife Lake area of northeastern Minnesota: Geological Society of America Bulletin, v.52, p.1577-1642. Jirsa, M.A., Fralick, P.W., Weiblen, P.W., and Anderson, J.L.B., 2011, Sudbury impact layer in the western Lake Superior region, in Miller, J.D., Hudak, G.J., Wittkop, C., and McLaughlin, P.I., eds., Archean to Anthropocene: Field Guides to the Geology of the Mid-Continent of North America: Geological Society of America Field Guide 24, p. 147–169. Jirsa, M.A., and Starns, E.C., 2008, Preliminary bedrock geologic map of the 2006 Cavity Lake fire area, parts of Ester Lake, Gillis Lake, Munker Island, and Ogishkemuncie Lake 7.5-minute quadrangles, northeastern Minnesota: Minnesota Geological Survey Open-File Report OF08-05, scale 1:24,000. Jirsa, M.A., Starns, E.C., and Schmitz, M.D., 2017, Bedrock geology of the 2006 Cavity Lake fire area, Boundary Waters Canoe Area Wilderness, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map M-193, scale 1:24,000. 60 �The youngest magmatic activity of the Midcontinent Rift at Bear Lake, Keweenaw Peninsula, Michigan KULAKOV, Evgeniy1, BORNHORST, Theodore J.2, DEERING, Chad3, and MOORE, James B.4 1 Centre for Earth Evolution and Dynamics, University of Oslo, Oslo, Norway A. E. Seaman Mineral Museum, Michigan Tech, Houghton, MI 49931 3 Department of Geological and Mining Engineering and Sciences, Michigan Tech, Houghton, MI 49931 4 Moore Rock Farm, Keweenaw Peninsula, MI 2 The Bear Lake igneous body is located near Michigan's McLain State Park in the southwest side of the Keweenaw Peninsula (sections 24 and 25, T56N, R34W) and represents the youngest known magmatic activity of the Midcontinent Rift. Bear Lake was mapped as an intrusive rhyolite (Cornwall and Wright, 1956) cross-cutting the Freda Sandstone of the Oronto Group. It is a dark gray to red colored fine-grained igneous rock with micro-phenocrysts of K-feldspar, biotite, hornblende, quartz, apatite, and iron oxides and is flow-banded in many outcrops. Bear Lake is, however, not a rhyolite, but instead intermediate in composition with an average (N=6) of 59.1 wt. % SiO 2 , 1.2 wt. % Na 2 O and 6.6 wt. % K 2 O. This composition falls in the trachyandesite field of LeMaitre for fresh rocks (2002 Cambridge University volcanic rock classification) and would be further subdivided as a latite. While altered, the alkaline tendency of the rock is confirmed by the high concentration of the immobile elements with 1273 ppm Zr, 270 ppm La, and 496 ppm Ce. The grain size and flow banding texture is consistent with the igneous body having formed either as a shallow intrusive or an extrusive flow. The interpretation that this is an extrusive deposit is supported by the results of drilling within the body by Johnson et al. (1980). They described glacial overburden underlain by 9 m of highly altered fragmental rocks in turn underlain by 8 m of siltstone and coarse-grained arkose over the igneous rock body. The contact between the igneous body and beds of the Freda Sandstone is not exposed. Those beds nearest to the contact are altered with visible calcite. The igneous body is variably altered and contains veinlets of calcite, quartz, and heulandite, which are more prominent near an exploration shaft in the west central side of the body dug about 1917. Strong conductors and anomalous copper content of about 190 ppm (Snider and Parker, 1979) led to geophysical surveys and drilling (Johnson et al.,1980). Minor amounts of native copper were reported in the drill core (Johnson et al., 1980). Importantly, establishing a geologically meaningful absolute age for the Bear Lake latite would constrain the rate of rift-centric sedimentation and the true end of known rift magmatism. Early attempts at establishing an age were discussed by Morey and Van Schmus (1988) who reported a Rb-Sr age of 1062+/-34 Ma compared to 1007+/-25 Ma by Chaudhuri (1975), but concluded the Rb-Sr isotopic system did not represent the emplacement age. Subsequent dating has shown that the 1060 Ma age is comparable to the 1060 to 1050 Ma age of widespread hydrothermal alteration associated with the native copper deposit (Bornhorst et al., 1988). In 1984, Bornhorst and a student, D. Wall, completed field work along Seven Mile Creek which bisects the body near the contact with the dual objective of developing a better understanding of the age relationship with the Freda Sandstone, intrusive versus extrusive origin and selecting a 61 �sample for zircon U-Pb dating. A sample was submitted to the Royal Ontario Museum for zircon separation and U-Pb dating. However, petrographic observations of the zircons revealed that they were likely xenocrysts and, therefore, no follow up analytical work was completed. Recently, the University of Oslo was able to extract three small zircons that yielded a statistically consistent concordia age of 1091.4 +/-1.7 Ma by ID-TIMS. The Bear Lake body intersects the Freda Sandstone and is about 1,500 m above the base of the Nonesuch Shale (Fig. 1) dated at 1081 +/- 9 Ma (Pb-Pb isochron; Ohr, 1993). Stratigraphically about 1,000 m under the Nonesuch is the Lake Shore Traps dated at 1087.2+/-1.6 Ma (U-Pb age on zircons; Davis and Paces, 1990). Fairchild et al. (2017) reported a 1.6 Ma younger age for the Lake Shore Traps and also reported an age of about 1084 Ma for rocks of comparable stratigraphic position from Michipicoten Island. Thus, the 1091 Ma age on the Bear Lake igneous body is not consistent with other published radiometric ages and, therefore, is not geologically meaningful; i.e. the zircon grains are xenocrysts as suspected decades prior. Figure 1: Stratigraphic column. We have not yet exhausted our efforts to obtain a geologically meaningful radiometric age for Bear Lake and continue to search for primary igneous zircons. References Cited Bornhorst, T.J., Grant, N.K., Obradovich, J.D., and Huber, N.K., 1988, Age of native copper mineralization, Keweenaw Peninsula, Michigan: Economic Geology, v. 83, p. 619-625. Chaudhuri, S., 1975, Geochronology of upper and middle Keweenawan rocks of Michigan: of the 21st Annual Institute on Lake Superior Geology, Proceedings, v. 1, p. 32. Cornwall, H. R., and Wright, J. C., 1956, Geologic map of the Hancock quadrangle, Michigan: U. S. Geological Survey Mineral Investigations Field Studies Map MF 46. 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. 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, no. 1., p. 117-133. Johnson, A., Parker, B., Snider, D., Van Alstine, J., 1980, Petrology of the Bear Lake intrusive, Keweenaw Peninsula, Michigan: 26th Institute on Lake Superior Geology Proceedings, v.1, p. 67. Morey, G. B., and Van Schmus, W. R., Correlation of Precambrian rocks of the Lake Superior region, United States: U. S. Geological Survey Professional Paper 1241-F, 32p. Ohr, M., 1993, Geochronology of diagenesis and low-grade metamorphism in pelites: Ph.D. dissertation, The University of Michigan, Ann Arbor, MI.,161 p. Snider, D. W., and Parker,, B. K., 1979, Geochemical and geophysical anomalies associated with the Bear Lake intrusive, sections 24 and 25, T56N, R34W, Houghton County, Michigan: 25th Institute on Lake Superior Geology Proceedings, v. 1, p. 38. 62 �Land of Fire and Ice: Summary of the 2017 ILSG Field Trip to Iceland LARSON1, Phil; HUDAK2, George; MACTAVISH3, Al; HINZ4, Peter; RADAKOVICH5, Amy; BHATTACHARYYA6, Juk; ENGELHARDT7, Paula; ENGELHARDT8, Steve; GELNIAS9, Brigitte; GOOD10, David; GORNER9, Emily; HINZ4, Sheree; JONGEWAARD11, Peter; KROCH, Deb; SVENSSON10, Matt; TIMS12, Andrew 1 Vesterheim Geoscience, PLC, Duluth, MN Natural Resources Research Institute, University of Minnesota - Duluth, MN 3 Panoramic PGMs (Canada) Limited, Thunder Bay, ON 4 Ontario Ministry of Northern Development and Mines, Thunder Bay, ON 5 Minnesota Geological Survey (MGS), University of Minnesota-Twin Cities 6 Department of Geography, Geology and Environmental Science, University of Wisconsin – Whitewater 7 HydroGeo Solutions LLC, Green Bay, WI 8 Green Bay, WI, independent photographer 9 Department of Geology, Lakehead University, Thunder Bay, ON 10 Department of Earth Sciences, Western University, London, ON N6A 5B7 Canada 11 Cliffs Natural Resources - Retired 12 Northern Mineral Exploration Services, Thunder Bay, ON 2 ABSTRACT The Institute on Lake Superior Geology recently conducted a field trip to Iceland between July 26 and August 5, 2017. The 11-day trip was led by Phil Larson, George Hudak, Al MacTavish, and Peter Hinz, and included 16 participants, including 10 professional geologists and 4 graduate students. The trip traversed roughly the south one-half of the island (Fig 1). Stops covered a wide variety of topics – from volcanology, igneous petrology, and magmatic-hydrothermal ore deposits to the glacial geology and geomorphology of Iceland. Iceland is the subaerial expression of the crust that forms the floor of the Atlantic Ocean. Though all of its rocks are younger than about 25 Ma, the geology of Iceland bears many similarities to that of the 1.1 Ga Midcontinent Rift in North America. The Mid-Atlantic Ridge steps ~100 km east across the island and allowed us to see a modern-day expression of rift-related volcanic and hypabyssal intrusive mafic (to rarely felsic) rocks, which continue to form today. The onset of the most recent Ice Age in the Pleistocene created spectacular ice sheets, valley glaciers, and glacial sediment deposits, as well as the unique landforms reflecting the interaction of volcanism and glacial ice, all of which we observed on the trip (Thordarson & Hoskuldsson, 2014). 63 �Figure 1. Shaded relief map (Google Images, 2017) of Iceland, showing glaciers in white. Colored lines show the 11-day trip route, starting and ending in Reykjavik. This presentation summarizes highlights from our trip and draws comparisons of Iceland’s geology to the Midcontinent Rift. Featured highlights of the bedrock geology will include both subaerial and subglacial volcanic deposits such as mafic tuffs, pillowed basalts, cinder cones, columnar basalts, peperites, pillowed dikes, pumice and ash deposits, moberg, tuyas, and spectacular aa and pahoehoe fields. We will discuss the surface expression of the Mid Atlantic Ridge at the Krafla Lava Fields, the Snaefellsnes Peninsula Volcanic Zone, and the historic Láki Flow. The presentation will also showcase vast outwash plains created by glacial jokulhaups, moraine deposits, proglacial lakes, and crevasse fields. REFERENCES Thordarson, T., & Hoskuldsson, A. (2014). Iceland Second Edition (2nd ed., Classic Geology in Europe 3). Edinburgh: Dunedin Academic. 64 �Controls on the localization and timing of mineralized intrusions within the ca. 1.1 Ga Midcontinent Rift system LIIKANE, Dustin A.1, BLEEKER, Wouter2, HAMILTON, Mike3, KAMO, Sandra3, SMITH, Jennifer2, HOLLINGS, Peter4, CUNDARI, Robert5, and EASTON, Michael6 1 Department of Earth Sciences, University of Toronto, Toronto, Ontario; dustin.liikane@mail.utoronto.ca Geological Survey of Canada, Ottawa, Ontario 3 Jack Satterly Geochronology Laboratory, Dept. of Earth Sciences, University of Toronto, 22 Russell St., Toronto, Ontario, Canada, M5S 3B1 4 Department of Geology, Lakehead University, 955 Oliver Rd, Thunder Bay, Ontario, Canada, P7B 5E1 5 Ontario Geological Survey, 435 James Street South, Thunder Bay, Ontario, P7E 6S7 6 Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, Ontario, Canada, P3E 6B5 2 The 1.1 Ga Mid-Continent Rift (MCR) represents one of the largest and best-preserved intra-continental rift systems of Precambrian age (Davis and Green, 1997; Miller and Nicholson, 2013; Swanson-Hysell et al., 2014; Bleeker et al., 2018). It is host to the Duluth Complex (second largest layered intrusion in the world), which contains significant low-grade mineralization of Ni-Cu-Co and platinum group elements (PGEs), and possibly reef-style PGE mineralization. Higher-grade Ni-Cu mineralization has also been identified within the rift, localized in smaller, conduit-type intrusions (e.g., Eagle deposit). Numerous mineralized intrusions are associated with the MCR on both sides of the border (e.g., Coldwell Complex near Marathon, Ontario; Tamarack near Duluth, Minnesota; Eagle near Marquette, Michigan), with many being actively explored by a number of companies. Many intrusions related to the MCR have been dated by U-Pb methods (Figure 1); however, many of the ages were obtained prior to the introduction of routine chemical abrasion techniques on single zircon grains. This improvement often leads to better precision and accuracy, allowing for sub-million-year age resolution. With this technique, we can better constrain (for the first time, in some cases) the age of emplacement of several MCR-related intrusions. This will allow us to understand the dynamics of the MCR’s plumbing system, and how it evolved over time. At the deposit scale, the high-precision ages may allow us to recognize whether these intrusions are long-lived conduits or intrusions emplaced in one single magmatic pulse. Furthermore, high-precision ages, along with lithogeochemistry, will allow us to link these individual intrusions to distinct stages of the flood basalt sequence. It may also reveal temporal constraints on the formation of mineralized intrusions within the MCR. 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: in Targeted Geoscience Initiative: 2017 report of activities, v. 2, (ed.) N. Rogers; Geological Survey of Canada, Open File 8373, p. 15–27. https://doi.org/10.4095/306594. 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(4), p. 476–488. doi:10.1139/ e17-039. 65 �Miller, J., and 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 copper-nickel-platinum group element deposits of the Lake Superior Region: Edited by Miller, J. Precambrian Research Center Guidebook, v. 13-01, p. 1–49. Swanson-Hysell, N.L., Burgess, S.D., Maloof, A.C., Bowring, S.A., 2014, Magmatic activity and plate motion during the latent stage of Midcontinent Rift development: Geology, v. 42, p. 475-478. Figure 1: Summary map of the Midcontinent Rift (adapted from Miller and Nicholson, 2013, and references therein), highlighting all of the rift-related intrusions. Undated or poorly dated intrusions, and/or ages that are problematic, are identified by stars with yellow outline. High-precision U-Pb ages on volcanic rocks are shown for reference. Summary of lithostratigraphic columns from across the Midcontinent Rift are integrated into this map nearest to their approximate geographic locations (adapted from Swanson-Hysell et al., 2014, and references therein). For complete references to all the ages, see Bleeker et al. (2018). 66 �Microanalysis of rock and mineral textures and its relationship to mineralization and ore comminution MATKO, Matthew W. , and SCHARDT, Christian Department of Earth and Environmental Sciences, University of Minnesota Duluth, 229 Heller Hall, 1114 Kirby Drive, Duluth, MN 55812 Research on ore bodies has typically focused on macro-scale processes, such as fluid migration, chemical and metal transfer, as well as physical and chemical changes (Cathles, 1981; Zientek, 2012). The results of this research are then typically been applied to large-scale features based on field observations, laboratory experiments, and theoretical assumptions to create models for ore deposits. However, it is not well understood how micro-scale properties such as: mineral grain size, grain shape, grain orientation, or fracture characteristics may influence various ore deposit formation. Mineralization styles such as: disseminated, next-texture, porphyry, vein-style may be partially controlled, if not dictated, by these features. It is therefore important to gain a better understanding of the role of these features in larger-scale processes. To obtain small-scale rock property information, multiple analytical methods (x-ray computed tomography - XRCT, Electric Pulse Disaggregation - EPD, Mineral Liberation Analysis - MLA) were used to examine selected ore samples (porphyry, Mississippi Valley type, volcanic massive sulfide, liquid magmatic sulfides). Sample cores were scanned using XRCT and spatial reconstructions were produced using 3D image processing software. This technique yields in-situ information such as grain size parameters, porosity distribution, or spatial orientation of mineral aggregates. Hand samples were disaggregated into individual mineral grains using EPD by sending repeated electric pulses through the material, causing mineral separation preferentially along mineral grain boundaries. This technology allows material separation while preserving mineral grain morphology (Cabri et al., 2008) and may provide a true alternative to traditional methods of ore comminution. The resulting aggregate material was analyzed with scanning electron microscopy using MLA software. This software yields mineral liberation data from the EPD technology, in addition to grain shape, size, mineral associations, and mineral abundances. Results show that XRCT data can be utilized to locate small-scale melt migration pathways such as micro-fractures (fig. 1a) in addition to mineral grain morphology and size distribution. Identification of these micro-scale features has the potential to greatly assist in our understanding of how ore textures and mineral deposits form. In addition to visual analysis of insitu material, data can be exported to construct vector graphs, which enable the visualization of ore grain orientations to determine existing ore grains orientation patterns within the rock (fig. 1b). Results also indicate very good mineral separation of silicates from sulfides using EPD compared to traditional mechanical processing, especially at < 250-150 µm. Separation efficiency was confirmed by running MLA on the samples, where it was determined that the average liberation yield for ores present ranged from 70 % to 80 % (fig. 1c). There does appear to be some dependence on ore grain size and deposit type that can affect these values. EPD technology offers unique opportunities for ore processing such as the pre-weakening of material before being subjected to more traditional methods of crushing, or an initial ore-silicate separation phase to remove the bulk of the gangue material. 67 �Figure 1 3D reconstruction of Cu-sulfide ore from a porphyry deposit that highlights a planar feature created by mineralization within a micro-fracture (a). Data for particles within the planar feature graphed to display long axis grain orientation (b). A composite image showing a representative sample of sulfide mineralization from disaggregated Eagle Mine material using MLA software. It highlights the high degree of Cu-sulfide liberation from silicates while also showing minor Cu-sulfides association with Ti-oxides (c). References Cabri, Louis J., Rudashevsky, N. S., Rudashevsky, V. N., & Oberthür, T. (2008). Electric-pulse disaggregation (Epd), hydroseparation (Hs) and their use in combination for mineral processing and advanced characterization of ores. Canadian Mineral Processors, 40th Annual Meeting, Ottawa, Proceedings. v. 211, p. 211-235. Cathles, L.M. (1981) Fluid flow and ore genesis of hydrothermal ore deposits: Economic Geology 75th Anniversary Volume, p. 424 – 457 Zientek, M.L. (2012) Magmatic Ore Deposits in Layered Intrusions - Descriptive Model for Reef-Type PGE and Contact-Type Cu-Ni-PGE Deposits: U.S. Geological Survey Open File Report 2012-1010, p. 48 68 �Using Credit-by-Exam to Connect Advanced High School Geology Courses to University Geology Departments: Current Status of a State-wide Program in Michigan MATTOX, Stephen1, BOLHUIS, Chris 2, and SOBOLAK, Christina 3 1 Department of Geology, Grand Valley State University, Allendale, MI, 2Hudsonville High School, Hudsonville, MI; 3St. Fabian Middle School, Farmington Hills, MI To address the national shortage of geologists and to diversify the geoscience workforce we are constructing a seamless path from rigorous high school geology classes taught by well-trained teachers to geoscience departments at state universities and colleges. This program is modeled on an existing high school – university collaboration that has added a significant number of students to the career pipeline. Although geologists have vigorously lobbied for an AP course and exam in geology, the College Board has resisted because they perceive that demand will be low. The lack of an AP course has made it difficult for those high schools nationwide that offer advanced (i.e., college-level content) geology courses to obtain appropriate recognition for their students’ accomplishments. Mattox designed a test, with input from my GVSU peers (and reviewed by faculty at eight other university geology departments), that includes 70 multiple choice, 10 essay questions, a map test with skills and landforms, and a rock and mineral exam (essentially what we use in our introductory physical geology course). We give the exam over 5-6 hours on two different days. The support of two NSF grants allowed us to build the network of universities and then extend the size of the program. NSF supported ended in 2016. Universities awarding credit: Central Michigan, Eastern Michigan, Grand Valley, Lake Superior State, Michigan Tech, Northern Michigan, University of Michigan-Dearborn, Wayne State University, Western Michigan, and Hope College. Montana State has awarded credit to two students. Mattox has started new discussions with Ferris State University and nearby 2YCs: Muskegon Community College and Grand Rapids Community College. Additional colleges and universities are invited to join each year. Participating colleges sign a MOU to award credit. Since 2001, 1334 students have taken a rigorous high school geology/Earth science course. Of these 777 students have passed, about 58%. With NSF support the program has grown and now about 250 students are tested each year at 9 or 10 high schools. Commonly, 20 students request credit and 5-7 start university as declared geology or Earth science majors, usually at CMU, GVSU, MTU, NMU, or WMU. Insights from this project include: • • • Administrators were surprisingly receptive to and supportive of a rigorous high school geology credit by exam course. Ideally teachers with a B.S. in Geology and additional course work or M.S. teach the course. However, motivated science teachers without an Earth Science B.S. can be successful. The high school course can be a single semester, two trimesters, or a full year. 69 �• • • • Pass rates tend to improve over 3-4 years and then stabilize. Some schools never had a student pass. About two new high schools join the program each year. Currently, high school teachers are working to align the Next Generation Science Standards to the content/skills of the college physical geology class. We have demonstrated the feasibility of establishing a statewide network of universities to award college credit for passing a credit by exam during a high school geology course. Growth of credit-by-exam in Michigan over the last six years. High schools in the program: HUD (Hudsonville); GH (Grand Haven); GPS (Grosse Point South); OK (Okemos); BR (Black River); DA (Dream Academy); DIT (Detroit Institute of Technology); HF (Henry Ford Academy); MA (Multicultural Academy); PHS (Pioneer); HHS (Huron); SHS (Sturgis); RHS (Roscommon HS); WOHS (West Ottawa HS); FHC (Forest Hills Central); and KH (Kenowa Hills). This material is based upon work supported by the National Science Foundation under OEDG Grant No. NSF 08-605 1006652. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. 70 �Geochemical signatures of hydrothermal alteration in clastic sedimentary rocks: theory, recognition, and application MAUK, Jeffrey L. 1 USGS, MS-973 Denver Federal Center, P O Box 25046, Denver, CO 80225-0046, USA Much of the terminology to describe hydrothermal alteration came from classic studies of porphyry copper deposits, which popularized terminology such as potassic, argillic, advanced argillic, phyllic, and propyllitic (e.g., Lowell and Guilbert, 1970; Sillitoe, 2010, and references therein). This terminology was developed for plutonic and volcanic igneous rocks that, when fresh, contain unaltered feldspar and mafic minerals. The main driver of hydrothermal alteration, hot water, promotes reactions that convert feldspar and mafic minerals to phyllosilicate minerals. In contrast, clastic sedimentary rocks form from weathered material, and that weathering results in degradation or destruction of feldspar and mafic minerals to form clay minerals such as smectite. During diagenesis, these clay minerals transform to higher rank clay minerals, such as interstratified illite-smectite and illite. Diagenetic reactions can ultimately lead to formation of micas such as muscovite. Therefore, normal weathering and diagenetic reactions destroy many igneous minerals, and form minerals that are similar to those that occur in hydrothermally altered igneous rocks. This can make it difficult to recognize hydrothermal alteration in sedimentary rocks. This abstract describes some key styles of hydrothermal alteration, and evaluates geochemical methods that can help to identify these types of alteration in clastic sedimentary rocks. Chemical sedimentary rocks, such as limestone and dolomite, are not considered. In igneous rocks, propylitic alteration commonly covers extensive areas; it is characterized by chlorite, epidote, and calcite, with minor pyrite. Chlorite, calcite, and pyrite are common diagenetic minerals, so propylitic alteration is an excellent example of a style of alteration that is readily identifiable in igneous rocks, but difficult to impossible to recognize in sedimentary rocks. Furthermore, geochemical studies of altered igneous rocks show that major elements are relatively immobile during propylitic alteration, and the main components that are gained are sulfur and carbonate. Again, because carbonate minerals and pyrite are common in sedimentary rocks, geochemical analyses are unlikely to provide diagnostic evidence of propylitic alteration. In contrast, alkali metasomatism, which includes K and Na alteration, can produce diagnostic minerals and distinct whole rock geochemical compositions. These reactions can produce Kfeldspar albite, illite, or Na-mica, or a combination of these minerals. Mass changes associated with K and Na metasomatism can be evaluated graphically using plots of molar (2Ca + Na + K)/Al versus molar K/Al to evaluate K-metasomatism, and molar (2Ca + Na + K)/Al versus molar Na/Al to evaluate Na-metasomatism. These plots allow identification of important hydrothermal minerals, and reflect alteration processes by showing trends from unaltered toward altered rocks. Sodium metasomatism is a hallmark of many sediment-hosted Cu deposits, but albite is also a common diagenetic mineral, so geochemical testing must include a sufficiently large suite of rocks to test how common and widespread albite is on a regional basis, and whether Na metasomatism stems from diagenesis or mineralization, or both. Phyllic alteration is characterized by quartz, sericite, and pyrite. Where intensely and pervasively developed, this alteration produces white rocks that are rich in pyrite; the pyrite may form up to 10% of the volume of the rock. Phyllic alteration is accompanied by leaching of Mg, Na, and Ca, and enrichment of K and S, so it is readily characterized by major element and S data. 71 �Argillic alteration can be texturally destructive, and produces clay minerals such as montmorillonite, illite, cholorite, and kaolinite. Advanced argillic alteration is very texturally destructive; it results from more aggressive acid leaching of rock that produces quartz or vuggy silica, plus alunite and kaolinite. In both argillic and advanced argillic alteration, Mg, Fe, Ca, Na, and K are leached from the rock. Silica may appear to be enriched, but that is due to loss of other elements rather than actual addition of SiO 2 . Silicification is the addition of silica to the rock, typically as quartz in veins, or in pore-filling cement, or both. In some cases, quartz replaces precursor minerals in the rock. All styles of silicification may be well-developed in host rocks around hydrothermal veins. Recognition and quantification of veins is relatively easy, but identification of silicification by pore filling or mineral replacement is more difficult. This is best exemplified in sandstones and quartzites, where beds that are naturally coarser-grained and more well-sorted would be harder and may have a more vitreous luster, leading them to be classified as silicified. Alas, geochemistry offers little assistance here, because the natural variability of clastic sedimentary rocks ensures a wide range of SiO 2 concentrations, and it is exceptionally difficult to document Si gain except under extreme conditions. Sulfidation is the addition of sulfide minerals—typically pyrite—to rock. This is common around many hydrothermal veins, and can be intense and pervasive around sediment-hosted massive sulfide deposits. Sulfur addition is readily characterized by whole rock geochemistry, provided that, as noted above, the addition of sulfur is sufficient to exceed background concentrations from diagenetic pyrite. Bleaching is used where rock is a lighter color than normal, and has two main styles: (1) a general lightening in color, such as dark green to light green, and (2) changing of a red rock to a white rock. The former is common around some hydrothermal veins. The latter occurs in some sediment-hosted copper deposits, and is spectacularly displayed in redbeds in the four corners region of the U.S., where the bleached zones reflect pathways of oil and gas migration. Bleaching is a nearly isochemical process, although in some cases (2) results in Fe loss. Carbonate alteration is common and widespread around many hydrothermal veins, and around many stratiform and stratabound orebodies. Some deposits show pronounced zonation of carbonate minerals, from Fe-rich near deposits, to Ca-rich in distal areas. The carbonate alteration can occur as veins and veinlets, particularly in the inner alteration zones, but disseminated carbonate is more common. Carbonate alteration can be quantified by geochemical analyses of carbonate C, provided that the addition of carbonate is sufficient to exceed background concentrations from diagenetic carbonate. In summary, whole rock geochemistry can provide a means to evaluate and quantify many, but not all types of hydrothermal alteration. Major element analyses, plus total S, carbonate C, and organic C, are the most important for geochemical evaluation of clastic sedimentary rocks. The most significant sediment-hosted deposits in the Midcontinent Rift are sediment-hosted Cu deposits, and for those, alkali metasomatism is the most promising alteration indicator. References Lowell, J. D., and Guilbert, J. M., 1970, Lateral and vertical alteration-mineralization zoning in porphyry ore deposits: Economic Geology, v. 65, p. 373-408. Sillitoe, R. H., 2010, Porphyry copper systems: Economic Geology, v. 105, p. 3-41. 72 �An 1149 Ma U-Pb baddeleyite crystallization age and geochemistry of gabbroic intrusions at the southwestern margin of the Superior Craton, southeastern South Dakota McCORMICK, Kelli1, CHAMBERLAIN, Kevin2, and PATERSON, Colin3 1 Department of Mining Engineering and Management, South Dakota School of Mines and Technology, Rapid City South Dakota 57701; 2Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82071, Faculty of Geology and Geography, Tomsk State University, Tomsk 634050 Russia; 3Department of Geology and Geological Engineering, South Dakota School of Mines and Technology, Rapid City South Dakota 57701 Gabbroic intrusions have been intersected in drill holes in southeastern South Dakota along the southwestern margin of the Superior Craton (Fig. 1). In order to better constrain the ages of the basement terranes in this region, several samples from one set of intrusions, the Corson diabase, were analyzed for the presence of datable minerals. For this study, samples of one Corson intrusion analyzed by Meyers (2013) was sent for mineral separation. Approximately 40 baddeleyite grains were separated by U. Söderlund (Lund University). Dating of the baddeleyite was by U-Pb isotope dilution thermal ionization mass spectrometry at the University of Wyoming. A U-Pb baddeleyite crystallization age of 1149.4 + 7.3 Ma from this Corson diabase sample (McCormick et al., 2017) is interpreted to represent an early stage of the Midcontinent Rift (MCR). McCormick et al. (2017) suggest that Corson diabase intrusions represent a failed rift arm (Fig. 1). The inferred NE trend of the Corson diabase, considered together with the trends of other possible MCR-related intrusions, are also consistent with the model of a mantle plume origin for the MCR (Hutchinson et al., 1990). Sampling of another Corson diabase core for U-Pb mineral dating is in progress. Several samples from two cores (03-W-01 and 03-W-02) intersecting a large gabbroic intrusion(s) near the town of Wakonda (Fig. 1) were analyzed in this study for the presence of datable minerals. A sample from 03-W-01 sent for mineral separation yielded approximately 50 baddeleyite grains, some more than 100 µm. Dating of these minerals is in progress. In conjunction with the dating, samples were taken from the Wakonda cores for geochemical analysis and compared with existing geochemistry of Corson diabase. The Wakonda and Corson samples are tholeiitic and generally olivine normative. A plot of TiO 2 vs Mg# (Fig. 2) shows the Wakonda gabbros to be somewhat geochemically distinct from the Corson diabase, but similar to MCR intrusions around the Thunder Bay region. REFERENCES: Cundari, R.M., Carl, C.F.J., Hollings, P. and Smyk, M.C., 2013, New and compiled whole-rock geochemical and isotope data of Midcontinent Rift-related rocks, Thunder Bay Area. Ontario Geological Survey Miscellaneous Release—Data 308. McCormick, K. A., Chamberlain, K. R, and Paterson, C. J., 2017, U–Pb baddeleyite crystallization age for a Corson diabase intrusion: possible Midcontinent Rift magmatism in eastern South Dakota. Can. J. Earth Sci., Published at www.nrcresearchpress.com/cjes on 10 October 2017, 7 p. Myers, J., 2013. A petrographic analysis of mafic intrusions of an unknown age from Southeastern South Dakota. Unpublished senior thesis, Department of Geology and Geological Engineering, South Dakota School of Mines and Technology: 17 p. Hutchinson, D.R., White, R.S., and Cannon, W.F., and Schulz, K.J., 1990. Keweenaw hot spot: geophysical evidence for a 1.1 Ga mantle plume beneath the Midcontinent rift system. J. of Geophys. Res., 95, p. 10,869-10,884. 73 �Figure 1: Map encompassing the intrusions discussed in this study. Diamonds are Corson diabase, triangles are other gabbros. MCR = Midcontinent Rift; SBZ = Superior boundary zone; SLTZ = Spirit Lake tectonic zone; SQ = Sioux Quartzite (subsurface extent); SRA = Superior rift arm (proposed). Becker embayment after the NICE working group. Figure 2: TiO 2 vs Mg# plot of southeastern South Dakota gabbroic intrusions and Thunder Bay area MCR intrusions from Cundari et al., 2013. Filled diamonds are Corson diabase, filled squares are Wakonda gabbro core 03-W-01, and large filled circles are Wakonda gabbro core 03-W-02. 74 �Geology of the Crystal Lake Gabbro and the Mount Mollie Dyke, Midcontinent Rift, Northwest Ontario O’BRIEN, Sean1, HOLLINGS, Pete1, and MILLER, Jim2 Department of Geology, Lakehead University, 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada; 2Department of Earth and Environmental Sciences, University of Minnesota - Duluth, 1114 Kirby Drive, 223 Heller Hall, Duluth, MN 55812. 1 The Crystal Lake Gabbro (CLG) is a Y-shaped, up to 750 m wide, layered mafic intrusion with a 5 km long northern limb and a 2.75 km long southern limb, with localized Cu-Ni mineralization. The Mount Mollie Dyke (MMD) is an arcuate, 60 to 350 m wide, macrodyke that lies on trend east of the CLG and extends for 35 km toward Lake Superior. Both intrusions are part of the 1.1 Ga Midcontinent Rift (MCR) and were emplaced into the Paleoproterozoic Rove Formation of the Logan Basin, approximately 50 km south of Thunder Bay. Current U-Pb age determinations imply a ~10 m.y. age difference with CLG being formed at 1099.6 ± 1.2 Ma and the MMD being formed at ~1109.3 ± 6.3 Ma (Heaman et al., 2007; Hollings et al., 2010). However, this age difference is at odds with both intrusions being normally polarized (an attribute of MCR rocks younger than 1102 Ma; Davis and Green, 1997) and their being on trend with each other. The CLG profiled in a drill core from its southern limb can be broadly divided into Upper, Main, and Lower Zones with further subdivisions of the Main and Lower Zones based largely on geochemistry. The Lower Zone occurs between two xenoliths of an early MCR (~1115 Ma) plagioclase porphyritic Logan Sill diabase. The Lower Zone consists of subophitic to ophitic troctolite, augite troctolite, and olivine gabbro and can be subdivided into an upper and basal marginal subzone as well as an interior subzone. Both marginal subzones host disseminated sulphides. An overall bottom-up-directed fractional crystallization of the Lower Zone is suggested by the progressive decrease in Fo content of olivine, Mg# of clinopyroxene, and whole-rock MgO upsection. Above the upper Logan Sill xenolith, the Main Zone similarly consists of subophitic to ophitic troctolite, augite troctolite, olivine gabbro, and gabbro. Petrography, lithogeochemistry, and mineral composition was used to subdivide the Main Zone into five subzones: a basal marginal subzone, upper margin subzone, and three interior cycles that display cryptic variations indicative of fractional crystallization and magma recharge events. Like the margins of the Lower Zone, the Upper Zone as well and the basal marginal subzone of the Main Zone contain disseminated sulphides and are characterized by relatively high Fo content olivine and low incompatible trace element concentrations. These mineralized zones are interpreted to have crystallized from the same initial pulse of magma into the CLG, which was sulphur-saturated. Cyclical cryptic variations in the internal subzone of the Main Zone are interpreted to indicate upward directed fractional crystallization, interrupted by emplacement of additional magma pulses into the core of the intrusion. All rocks of the Main Zone are olivine and plagioclase orthocumulates indicating that fractional crystallization was not particularly efficient. Throughout the evolution of the CLG, the differentiation of the magma was limited as it did not result in clinopyroxene and Fe-Ti oxide becoming cumulus phases. This was likely due to magmatic recharge and inefficient fractional crystallization. 75 �Texturally and geochemically, the MMD can be broadly divided into an Upper and Main Zones, with a subdivision of the Main Zone into an upper and lower sequence and a pegmatitic segregation subzone. The Upper Zone consists of ferrodiorite and likely represents the end product of extensive fractionation. The Main Zone is characterized by troctolite, augite troctolite, olivine gabbro, and gabbro with MgO, CaO, Al 2 O 3 , and Ni concentrations decreasing upwards and SiO 2 , TiO 2 , K 2 O, Na 2 O, P 2 O 5 , and incompatible trace element concentrations increasing, consistent with bottom-up fractional crystallization. Strong differentiation of the MMD magma is indicated by the habit change of clinopyroxene from ophitic (intercumulus) to granular (cumulus), which is the basis for the subdivision of the lower and upper sequences. The lower sequence of the Main Zone also hosts a 24 m thick interval containing 1 to 2 m wide gabbroic pegmatite layers. These pegmatites are interpreted to be the result of localized enrichment of magmatic volatiles. The presence of an evolved core in the MMD surface expression, coupled with the mineral composition of olivine, plagioclase, and clinopyroxene, remaining at relatively constant Fo, An, and Mg# values, respectively, below the pegmatitic layers suggests that there was some degree of lateral crystal fractionation as well as bottom up fractionation. The well-defined fractionation sequence as well as an absence of abrupt geochemical changes suggests that the MMD fractionally crystallized from a single pulse. Liberation of external sulphur from the surrounding Rove Formation, is suggested by the greater than mantle S/Se values as well as δ34S values between +4.0 and +21.0‰ of the sulphides within the CLG. The addition of external sulphur evidently resulted in sulphur saturation during initial emplacement of the CLG magmas. Primitive mantle normalized multi-element diagrams and trace element ratios provide supporting evidence for a localized shallow level of crustal contamination, as well as a deeper more widespread contamination component of both the CLG and MMD magmas. The estimated parental magma compositions and average primitive mantle normalized trace element concentrations of the CLG and MMD suggest that they shared similar, if not the same, magma source. The CLG parental magma was slightly more evolved than the MMD suggesting that the magmas were sourced from a fractionating staging chamber. The estimated parental magma compositions of the CLG and MMD closely resemble those of the Layered Series intrusions of the Duluth Complex, supporting previous speculation that the CLG may be a satellite intrusion of the Duluth Complex. Despite current geochronology data to the contrary, the results of this study strongly suggest that the CLG and the MMD are petrogenetically linked, if not parts of the same intrusive system. REFERENCES 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 34, 476-488. Heaman, L., Easton, R., Hart, T., Hollings, P., MacDonald, C. and Smyk, M. (2007) Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon region, Ontario. Canadian Journal of Earth Sciences 44, 1055-1086. Hollings, P., Smyk, M., Heaman, L.M. and Halls, H. (2010) The geochemistry, geochronology and paleomagnetism of dikes and sills associated with the Mesoproterozoic Midcontinent Rift near Thunder Bay, Ontario, Canada. Precambrian Research 183, 553-571. 76 �The Brussels Hill Structure, Door County, Wisconsin: Impact crater, diatreme or other? OLSEN-VALDEZ, Juliana and BJØRNERUD, Marcia Geology Department, Lawrence University 711 E Boldt Way, Appleton, WI 54911 USA Brussels Hill (44.759°N, 87.593°W) is a localized area of intensely fractured and faulted bedrock in a region of otherwise undeformed lower Silurian dolostone. It was first identified as a geologic anomaly by Kluessendorf (2011), who suggested that the site was an impact crater. More recently, Lawrence University students have carried out geological and geophysical surveys of Brussels Hill (e.g., Zawacki & Bjørnerud, 2014), and in 2017 we obtained a ca.100 m core from the center of the structure. While some characteristics of the site are consistent with an eroded impact crater, others are at odds with this hypothesis. The area of disturbed rock coincides with a distinctive, nearly circular, flat-topped topographic high, ca. 2 km in diameter, which stands 40 m above the surrounding landscape and is ringed by rugged, tree-covered slopes. Around the edges of the hill, most prominently on the north side of the structure, the Silurian bedrock dips gently (15-20°) inward. Glacial till lies above the dolostone at the top of the hill. A quarry near the central part of the disturbed area provides excellent three-dimensional exposures of the most intensely deformed bedrock. In the quarry, bedding orientations vary dramatically over distances of meters. Coherent structures are difficult to discern, and the rocks are fragmented at every scale. In places, primary layering can be traced for tens of meters, while elsewhere the rocks are pervasively brecciated to centimeter- or smaller-sized clasts. Some breccias are monomict but most are polymict, containing dolostone and chert clasts with a variety textures and hues. Both types of breccia commonly contain subspherical vugs 1-5 mm in diameter. The breccias lack any sort of internal stratigraphy and are thus unlikely to represent fall-back ejecta, as first suggested by Kluessendorf (2011). No shatter cones have been observed, even though the finely crystalline host dolostone would be favorable for their formation. Silurian dolostone is the only bedrock normally exposed in this area, but meter-scale lensoid and tabular bodies of fine-grained glauconite-bearing sandstone occur at Brussels Hill. These sandstones have a carbonate matrix with spherical, sub-mm-scale voids. Many also have fine laminae that parallel the edges of the bodies. Most of the grains in the sandstones are highly rounded, but about 15% have unusual shapes: shard-like, crescentic, and irregularly concavo-convex. No shock lamellae have been observed in the quartz grains. The mature, rounded grains and presence of glauconite suggest that these sands were derived from Cambrian or possibly Ordovician strata that normally lie 350 to 400 m in the subsurface in Door County, and their presence rules out a karstic collapse origin for the disturbance. We conducted a gravity survey of Brussels Hill using a LaCoste-Romberg gravimeter and Trimble differential GPS system (Edwards, 2016). N-S and E-W transects with readings every 200 m were made across the hill, extending on the north and west into areas where bedrock is undisturbed. Using the GPS ‘base and rover’ system, station locations were sited to 1 cm precision. After free-air, terrain, and tidal corrections of the time-stamped and geolocated data, the resulting Bouguer anomaly map revealed a small but significant positive gravity anomaly of 0.85 mGal near the center of the hilltop. Because the brecciated rocks exposed at the surface have a lower density (ca. 2.5 g/cm3) than the surrounding pristine dolostone (2.8 g/cm3), the observation of a positive gravity indicates that there must be relatively dense rocks in the subsurface below the center of the Brussels Hill structure. 77 �We have recently logged a 103-m drill core from the quarry, obtained in cooperation with the Wisconsin Geological and Natural History Survey. The rocks in the core are brecciated to varying degrees to a depth of about 70 m, and the core intercepted the Upper Ordovician Maquoketa Shale at about 66 m. The brecciated zones are similar to those exposed at the surface, with vug size broadly correlated with the size of clasts. Thin sections of apparently intrusive veins of brecciated material show crude size sorting. Chert clasts in the core have a wide range of colors – not only white and grey, common in the Silurian units -- but also beige, green, dark red and brown, perhaps from Ordovician strata. Some brown cherts are shattered dilatantly in a manner that suggests explosive decompression. Collectively, these observations point to the involvement of a gas phase that forcefully propelled broken rock upward from significant depth and into a complex network of fractures. The biggest surprise from the drilling was that the lowest rocks in the core – about 30 m of the Maquoketa Shale -- are almost undeformed. This was unexpected, given that material from underlying Cambrian/Ordovician units is intermingled with the Silurian dolostones above; the exotic sandstones and cherts must have passed through the Maquoketa level en route to the surface. Our provisional interpretation is that the Maquoketa Shale, which acts as a regional aquitard in the modern groundwater system, behaved in a similar way in the face of the gas pressures during the explosive event at Brussels Hill. In an impact scenario, carbon dioxide released by shock-related devolatilization of carbonate rocks in the deep subsurface may have been trapped by the low-permeability Maquoketa Shale, which then failed locally, providing isolated conduits for deep-seated rocks to be brought to the surface by over-pressured gases. The drilling site was apparently not one of those spots. However, recent experiments on carbonates in shock metamorphism (Bell, 2016) show that the pressures required for devolatilization of calcite and dolomite exceed 20 GPa – higher than the transient pressures needed to form shatter cones and planar deformation features in quartz (ca. 8 and 12 GPa, respectively), which are absent at Brussels Hill. Other inconsistencies with the impact hypothesis are 1) the inward dips of the beds around the disturbed zone and 2) the height of the hill, which at 40 m is about 10 times higher than the expected central uplift for a crater of 2-3 km diameter (Cintala & Grieve, 1992). We therefore speculate that the disturbance was caused by an overpressured gas phase that came from below, perhaps from a kimberlite or similar intrusion. In this case, the gases that formed the vuggy rocks and carried rocks up from lower stratigraphic levels may have left a distinctive geochemical signature. Preliminary XRF and cathodoluminescence analyses do suggest that some of the carbonate material in the vuggiest breccias and intrusive sandstone bodies is chemically distinct, with elevated Sr values and blebby occurrences of calcite (in rocks that are otherwise entirely dolomitic). The Brussels Hill structure lies about 160 km south of the Jurassic Lake Ellen kimberlite in Iron County, MI (Cannon & Mudrey, 1981; Zartman et al., 2013). References cited Bell, M., 2016. Meteoritics and Planetary Science 51, 619-46. Cannon, W.F. & Mudrey, M., 1981. USGS Circular 842. Cintala, M. & Grieve. R., 1992. Geol. Soc. Am. Special Paper 293, 51-60. Edwards, K., 2016. Lawrence University Senior Thesis, unpub. Kluessendorf, J., 2011. Geol. Soc. Am. Abstr. 43.1, 117. Zartman et al., 2013. Journal of Petrology, 54, 575-608. Zawacki, E. & Bjørnerud, M., 2014. Geol. Soc. Am. Abstr. 46.6, 707. 78 �Komatiite-hosted nickel-copper mineralization potential in the eastern Shebandowan Greenstone Belt, Ontario, Canada OLSON, Maile J1., LODGE, Robert W. D1., and HINZ, Sheree2 1 Department of Geology, University of Wisconsin-Eau Claire, Eau Claire, WI 54702-4004 2 Ontario Geological Survey, Thunder Bay, ON, Canada, P7E 6S8 Komatiite-hosting strata in Archean greenstone belts are important exploration targets because of the potential for hosting nickel-copper (Ni-Cu) magmatic sulfides (e.g. Houlé 2011). The 2.72 Ga Shebandowan greenstone belt, which is part of the Wawa-Abitibi terrane (Stott et al. 2010), has known to host such deposits in the western part of the belt at the Shebandowan Nickel Mine (Morton 1982). Despite abundant komatiite deposits in the Eastern part of the belt, no other magmatic sulfide deposits have been discovered in this region to date. Our research continues to refine geochemical and textural data that provides evidence of assimilation and supports interactions between komatiites and silica- and sulfur-rich sedimentary rocks. This project explores the potential for Ni-Cu deposits in this region. Furthermore, petrographic and geochemical study of these rocks can improve our limited understanding of Archean tectonic processes. Komatiites were formed during the Archean when young Earth had enough heat to produce large volumes of mantle-derived magmas. Because these komatiitic melts have such an extremely high temperature, they thermally mechanically erode the base of the flow deposit, carving out a channel for itself, giving the melt the ability to assimilate the host rock. When ultramafic magmas assimilate sulfur-bearing crustal sedimentary rocks, they can form Ni-CuPGE deposits. Detailed field mapping of bedrock exposures was completed in the Bateman property exploration trenches, dug in 2008 by Linear Metals Corporation and expands on previous research by Hinz and Hollings (2015). These exploration trenches improved and expanded the available outcrop surface, giving the opportunity to observe the stratigraphy of komatiite flows and how they interact with the surrounding strata. Original textures have largely been preserved in this area due to minimal deformation and metamorphism so textural evidence can be used as a good indicator of komatiite-sediment interaction. Preliminary results from textural, petrographic, and geochemical analyses provide indication of komatiite-sediment intermingling and the presence of Ni-Cu-PGE sulfides. Fig. 1.A-B shows komatiite-sediment contact textures in outcrop with the lighter colored chert being brecciated in contact with the ultramafic flow. The bedding in the chert are being truncated and the edges of the breccia fragments are rounded and potentially thermally eroded (Fig. 1.A). Transmitted-light petrography, and whole rock geochemical analyses were used on the collected samples. Fig. 1.B shows the chert and komatiite have a very chaotic contact zone with arms and irregular blobs of each rock type. In some areas, the contact is diffuse. Other textures include variolitic glassy margins, rounded sedimentary inclusions, and disruption of chert laminations, suggesting the two rock types had interactions prior to lithification. Fig. 1.C-D are petrographic photos of thin sections from the samples and have jigsaw brecciation as well as rounded edges of brecciated sediment clasts from komatiite assimilation. Fig. 1.D also has a diffuse contact that distinctly presents evidence for komatiite-sediment mingling and potential partial melting of the sedimentary rock. The irregularity of the contact and brecciation expresses that the fracture mechanism is not tectonic but is due to hot komatiites shattering colder 79 �sediments. Geochemical diagrams show the compositional array of komatiites deflects towards the calc-alkalic part of the diagram which is unusual for these magmatic suites and is likely caused by contamination. Melt modeling diagrams showing komatiite-sediment interactions with potential melt compositions display geochemical ranges that cannot be explained by fractional crystallization but instead seems to have a mixing pattern with the sedimentary rock. Figure 1: A-B) Outcrop photographs illustrating the various contact relationships between light colored metasedimentary and dark colored mafic to ultramafic units exposed in the Bateman Property trenches. C-D) Petrographic photographs illustrating the same contact relationships. References Stott, G.M., Corkery, M.T., Percival, J.A., Simard, M. and Goutier, J. 2010. A revised terrane subdivision of the Superior Province; in Summary of Field Work and Other Activities 2010, Ontario Geological Survey, Open File Report 6260, p.20-1 to 20-10. Hinz, S. and Hollings, P. 2015. Preliminary description of the ultramafic metavolcanic rocks in the eastern part of the Shebandowan greenstone belt, northwestern Ontario; in Summary of Field Work and Other Activities 2015, Open File Report 6313, p. 16-1 to 16-7. Houlé, M.G., Lesher, C.M., 2011. Komatiite-associated Ni-Cu-(PGE) mineralization in the Abitibi Greenstone Belt, Ontario. Reviews in Economic Geology 17, 89-121 Morton, P., 1982. Archean volcanic stratigraphy, and petrology and chemistry of mafic and ultramafic rocks, chromite, and the Shebandowan Ni-Cu Mine, Shebandowan, northwestern Ontario. Carleton University, p. 346. 80 �Assembling Minnesota: Integration of 140 Years of Government, Academic, and Industry Geologic Studies into a Seamless Statewide GIS Database PETERSON, DEAN M.1 1 Natural Resources Research Institute, University of Minnesota Duluth, 5013 Miller Trunk Highway, Duluth, Minnesota 55811-1442. dmpeters@d.umn.edu Over the last 140 years, the search for ores and the mining of mineral deposits has played a huge role in revealing the geology of Minnesota. Dozens of companies utilized classic exploration techniques (geological mapping, geophysical surveying, geochemical studies, test pitting, drilling, and shaft sinking) to target, develop, and mine ore deposits. The early successes (1880s) of these endeavors drove home to forward thinking individuals in government and at the University of Minnesota the need to understand and characterize the geology of the state in a broad context. These developments included regulations to manage lands and archive company data, long-lived programs in mineral processing and metallurgy research, and dedicated programs to map the bedrock geology of the state (Figure 1). All told, a vast amount of information exists on the geology of Minnesota in archives of state agencies, at colleges and universities, and within the United States (USGS) and Minnesota geological surveys (MGS). However, much of this information is currently still archived in file cabinets in analog form (paper maps, documents, folios), though vast amounts of these data have been scanned and are accessible online. Although great strides have been made to integrate these historic datasets into ongoing digital geologic products, major gaps exist and the standardization of how to capture and digitally archive the geologic facts these data hold is by no means complete. To encourage the development of, and risk assessment tools for, an environmentally sound mining industry, government agencies need to put forth both attractive and competitive policies as well as robust geological information. This is particularly true for the mineral exploration component of the mining industry, for without exploration activities the eventual development and extraction of minerals will not take place. Therefore, the University of Minnesota’s Natural Resources Research Institute (NRRI) has begun a copyrighted internal initiative to create a seamless digital GIS compilation (Table 1) that preserves, integrates, and interprets all of the known and trusted bedrock geological data for the entire state of Minnesota, i.e., Assembling Minnesota. The completion of such a compilation in a format prepared for integrated modeling via spatial analysis is a formidable task, and in the end can take many years to complete. This geological compilation is designed to preserve the observed facts generated by geologists/geophysicists/ geochemists in the field over the last 140 years in a way that future geologists may use to make new geological interpretations long into the future. Figure 1. Timeline of outcrop mapping and mineral exploration/development in Minnesota. 81 �Table 1. Listing of the current datasets in the Natural Resources Research Institute’s, Assembling Minnesota geological GIS compilation, © 2018 Regents of the University of Minnesota. All rights reserved. 82 �Modeling the Precambrian topography of Columbia County, Wisconsin using twodimensional models of Gravity and Aeromagnetic data and Well Construction Reports RASMUSSEN, Joseph1, KINGSBURY STEWART, Esther2, SKALBECK, John1, and GOTKOWITZ, Madeline2 1 University of Wisconsin-Parkside, 900 Wood Road, Kenosha, WI 53141 Wisconsin Geologic and Natural History Survey, 3817 Mineral Point Road, Madison, WI 53705 2 The Cambrian-Ordovician aquifer is the primary source of groundwater for high-capacity wells across much of Wisconsin. This prominent groundwater system is over 2000 feet thick in some areas and is impacted by the underlying crystalline Precambrian basement (Leaf et al., 2014), which includes many irregularities, the most prominent of which is the Baraboo Syncline of Columbia and Sauk Counties. This project is a continuation of previous work done in Dodge and Fond du Lac Counties by the University of Wisconsin-Parkside and the Wisconsin Geological & Natural History Survey (MacAlister et al., 2016). The goal of this project is to produce an updated Precambrian topographic map of Southern Wisconsin by using gravity and aeromagnetic data to interpret Precambrian topography away from outcrop and boreholes. This will improve definition of the lower extent of the aquifer, aiding water supply management efforts. Modeling of gravity and aeromagnetic data from the United States Geological Survey (Snyder and Daniels, 2002) was conducted using GM-SYS 3D modeling software in Geosoft Oasis Montaj. Grids of subsurface layers were created from the data and constrained by well and drilling records as well as outcrop maps that were digitized using ArcMAP (Dalziel and Dott, 1970). The Precambrian basement underlying Columbia County is comprised of ca 1.75 Ga granites and rhyolites that are non-conformably overlain by <1.71 Ga quartzite, slate, and ironformation of the Baraboo interval (Medaris et al., 2003). The Baraboo interval metasedimentary rocks and underlying granite and rhyolite was subsequently folded and faulted (e.g. Dalziel and Dott, 1970). The folded layer of iron-formation provides a telltale signatures that aids construction of geophysical models because it has an average magnetic susceptibility of 53000 µcgs, compared to the average susceptibility of the rest of the bedrock of around 1500 µcgs. We use geologic mapping and cross-sections, drill core, magnetic susceptibility and density measurements, petrography and geochemistry, and well construction reports to refine physical modeling constraints. Preliminary results indicate (1) regional Precambrian geology may be interpreted from geophysical data and (2) Precambrian topography is controlled by Precambrian geology and is therefore somewhat predictable. 83 �Figure 1 – Gravity (left) and Aeromagnetic (right) anomaly maps of Columbia County showing the location of 2D models. Dalziel, I. W. D. and Dott, R. H.,1970. Geology of the Baraboo District, Wisconsin: A description and field guide incorporating structural analysis of the Precambrian rocks and sedimentological studies of the Paleozoic strata. Wisconsin Geological and Natural History Survey Information Circular 14. Leaf, A. T., Gotkowitz, M. B., and Dunning, C. P., 2014. A groundwater flow model for Columbia County, Wisconsin. Wisconsin Section of American Water Resources Association Program and Abstracts, Wisconsin Dells, p. 69. Macalister, E. A., Skalbeck, J. D. and Stewart, E. K., 2016. Estimating the subsurface basement topography of Dodge County, Wisconsin using three dimensional modeling of gravity and aeromagnetic data. AGU abstract 184894. Medaris, L.G., Singer, B.S., Dott, R.H., Naymark, A., Johnson, C.M., Schott, R.C., 2003. Late Paleoproterozoic climate, tectonics, and metamorphism in the southern Lake Superior region and Proto-North America: Evidence from Baraboo interval quartzites. Journal of Geology 111, 243257. Snyder, S. L. and Daniels, D. L., 2002. Wisconsin Aeromagnetic and Gravity Maps and Data: A website for distribution of data. USGS Open File Report 02-493. 84 �Pilot study results for potential lithium mineralization on State-managed mineral rights in Minnesota REED, Andrea Minnesota Department of Natural Resources, 1525 3rd Avenue East, Hibbing, MN 55746 The Minnesota Department of Natural Resources (DNR) manages mineral rights on approximately 12 million acres of land. The royalties and rentals generated from these lands help fund Minnesota’s School and University Trusts, the state General Fund, and local governments. To improve the earnings for these entities, the DNR maintains and collects mineral exploration data. The DNR also seeks opportunities to diversify Minnesota’s nonferrous mineral portfolio. With lithium recognized as an element critical for clean energy development (U.S. Department of Energy 2010) and an increasing demand for it, the DNR decided to conduct a pilot study on the potential for lithium occurrences in Minnesota. Of the different lithium deposit types, granitic pegmatites seem to show the most promise of hosting lithium occurrences in Minnesota. Igneous and metamorphic rocks cover a significant portion of the state and are the host rocks for pegmatites (London 2008). Pegmatites host known lithium occurrences in the Quetico Subprovince (the Georgia Lake pegmatites) and Wabigoon Subprovince (Mavis Lake pegmatite group) in Ontario (Selway et al. 2005). A minor amount of lithium is known to occur in the abandoned Rader Mine near Lake of the Woods, on the Minnesota side of the Wabigoon Subprovince (Zamzow & Morey 1991). The pilot study site is located in the Quetico Subprovince on Public School Trust Land northeast of Orr, MN. Little was initially known about the site other than it contained a large pegmatitic granite outcrop. Bulk samples (roughly 4 kg each) of rock were taken from a single pegmatite dike to determine the type of granite and identify the presence of lithium and other trace elements. Small chip samples of feldspar were taken from multiple places along the length of multiple dikes to assess overall fractionation trends. Similar methods are generally accepted for rare element-bearing granitic pegmatite exploration (e.g., Černý 1991, Selway et al. 2005). In addition, nearby glacial till was sampled to see if Laser-Induced Breakdown Spectroscopy (LIBS) could be used to link sand fraction sediments to the pegmatite outcrop. The results for the pegmatite sampling are presented here. Three pale pink monzogranite-pegmatite dikes were identified in the course of fieldwork. The dikes range from 1 to 15 meters in width in outcrop, strike slightly north of east, and have variable apparent dips to the south. Textures range from aplitic to pegmatitic (up to 10 ⨯ 20 cm crystals), with the most common grain size being medium- to coarse-grained granite. Mineralogy is principally composed of feldspars and quartz with trace amounts of magnetite, biotite, and apatite (in order of decreasing abundance). Using the methods and descriptions of Frost et al. (2001), Whalen et al. (1987), London (2008), Černý (1991), whole rock analysis of the bulk samples indicate a mixed A- and I-type signature (tending more towards A-type) and a weakly peralkaline to metaluminous nature, suggesting the sampled dike should be categorized as an NYF granite. Trace element analysis revealed low lithium and REE content, slightly increasing fractionation to the west in the Rb/Sr, Rb/Ba, and La/Yb ratios, confirmation of the non-peraluminous nature of the dike in the Zr/Hf ratio, and confirmation of the A-type signature in the behavior of the REE pattern. Electron microprobe analysis of k-feldspar in collected perthite chip samples reveals that the ratios of Rb/Ba, Rb/Sr, and K/Rb, as well as the distribution of P, (London 2008, London & 85 �Černý 1990) show a strong increasing fractionation trend of this dike set to the northwest. It also shows a slight increasing fractionation trend to the west, confirming the trend pattern seen in the bulk samples. In general, the fractionation trend of these granitic dikes is oriented approximately perpendicular to their strike. Overall, the results suggest that lithium is unlikely to be a significant component in any rocks related to this specific granitic system, even if the identified fractionation trend were to be followed beyond the bounds of the pilot site. References Černý, P. (1991). Rare-element granitic pegmatites. Part II: regional to global environments and petrogenesis. Geoscience Canada, 18(2), pp. 68-81. Frost, B., Barnes, C., Collins, W., Arculus, R., Ellis, D., and Frost, C. (2001). A geochemical classification for granitic rocks. Journal of Petrology, 42(11), pp. 2033-2048. London, D. (2008). Pegmatites. Special publication 10 of The Canadian Mineralogist. Québec, QC: Mineralogical Association of Canada. 347 p. London, D. and Černý, P. (1990). Phosphorus in alkali feldspars of rare-element granitic pegmatites. The Canadian Mineralogist, 78, pp. 771-786. Selway, J., Breaks, F., and Tindle, G. (2005). A review of rare-element (Li-Cs-Ta) pegmatite exploration techniques for the Superior Province, Canada, and large worldwide tantalum deposits. Exploration and Mining Geology, 14(1-4), pp. 1-30 U.S. Department of Energy (2010). 2010 Critical Materials Strategy Summary. U.S. Department of Energy. 4 p. Retrieved from: https://energy.gov/sites/prod/files/edg/news/documents/Critical_Materials_Summary.pdf Whalen, J., Currie, K., and Chappell, B. (1987). A-type granites: geochemical characteristics, discrimination and petrogenesis. Contributions to Mineralogy and Petrology, 95 pp. 407419. Zamzow, C., and Morey, G. (1991). M-074 Reconnaissance geologic map of the Northwest Angle, Lake of the Woods County, Minnesota. Minnesota Geological Survey. Retrieved from the University of Minnesota Digital Conservancy, https://conservancy.umn.edu/handle/11299/60043 86 �Variation trends in sulfur isotope ratios at the Eagle and East Eagle intrusions and the surrounding country and basement rocks of the Baraga Basin, Upper Peninsula, Michigan ROSE, Katharine1; ESSIG, Espree2 and THAKURTA, Joyashish1 1 Department of Geological and Environmental Sciences, Western Michigan University, 1903 W. Michigan Ave. Kalamazoo, MI 49008 2 Eagle Mine, Lundin Mining Corporation, 4547 County Road, Champion, MI 49814 The Eagle Ni-Cu sulfide deposit in Marquette County, Michigan is a magmatic sulfide deposit composed of massive, semi-massive and disseminated sulfide minerals hosted in conduit-shaped peridotitic intrusive rocks in the Baraga Basin (Ding et al., 2010). The intrusion has been dated at 1.1 Ga and has been interpreted to be a part of the magmatism associated with the Mesoproterozoic Midcontinent Rift event. Although the ore-grade Ni-Cu sulfide mineralization is located in the sulfide rich part of the intrusive bodies (Figure 1 and 2), relatively small amounts of sulfide minerals are dispersed throughout the intrusion and in the immediate country rocks of the metamorphosed Paleoproterozoic Michigamme Formation and further in the Archean granite-gneiss which forms the basement rock of the Baraga Basin area of UP Michigan (Ding et al., 2012; Hinks, 2016). δ34S ‰ (V-CDT) values have been determined from sulfide minerals of the Eagle and East Eagle intrusions. The distribution trends in the isotope ratio has been studied with respect to spatial directions as well as rock compositions. Peridotitic Rocks Semimassive Sulfides Michigamme Formation Peridotitic Rocks Michigamme Formation Semi-massive and Massive Sulfides Gabbro Massive Sulfides Archean Basement Granite-Gneiss Figure 1: 3-D model of main Eagle intrusion, drill holes EA0300 and EA03301, looking to the east. (Source: Eagle Mine) Figure 2: 3-D model of Eagle East intrusion, looking to the north, with drill hole 17EA364 and 17EA364A. (Source: Eagle Mine) 87 �Based on previous (Hinks, 2016) and present studies, δ34S values of pyrrhotite, chalcopyrite and pentlandite in the massive, semi-massive and disseminated sulfides vary within a range of 0‰ to 5‰. Disseminated pyrite and pyrrhotite in Michigamme Formation slates display δ34S values from 6‰ to 32‰. Disseminated pyrite grains in the Archean basement rocks also display a wide range of δ34S values from -11‰ to 7‰. The distribution of δ34S data in the country and basement rocks, when observed from a directional standpoint along depths of drill-cores show a variation from 10‰ to 2‰ towards the intrusion from the surrounding rocks. However, variations in δ34S values are more distinct from one rock type to another. New data from this study (Table 1) show that within the spatial domain of the sulfide deposit the δ34S changes are more uniform and within a narrower range between 2‰ and 3‰ for the Eagle intrusion. Drill Hole Sample ID δ34S‰ 17EA364 KGR-03-a 32.6 17EA360D KGR-18-a 2.6 17EA360D KGR-18-b 2.5 17EA360D KGR-26-a 6.1 EAUG 0300 KGR-37-a 2.6 EAUG 0300 KGR-37-b 2.5 EAUG 0300 KGR-38-a 2.9 EAUG 0300 KGR-38-b 2.8 EAUG 0300 KGR-39-a 3.1 EAUG 0300 KGR-39-b 3.0 EAUG 0300 KGR-40-a 3.0 EAUG 0300 KGR-41-a 2.9 MF-Michigamme Formation SMSU-Semi-Massive Sulfide Unit BSMT-Archean Basement Granite Unit MF Gabbro Gabbro BSMT SMSU SMSU SMSU SMSU SMSU SMSU SMSU SMSU Table 1: Sulfur isotope ratios determined from sulfide minerals in the Eagle intrusion, the surrounding country, and basement rocks of the Baraga Basin. REFERENCES Ding, X., C. Li, E. M. Ripley, D. Rossell, and S. Kamo, 2010, The Eagle and East Eagle sulfide orebearing mafic ultramafic intrusions in the Midcontinent Rift System, upper Michigan: Geochronology and petrologic evolution, Geochem. Geophys. Geosyst., 11, Q03003, doi:10.1029/2009GC002546. Ding, X., E.M. Ripley, S.B. Shirey, C. Li (2012), Os, Nd, O and S isotope constraints on country rock contamination in the conduit-related Eagle Cu-Ni-(PGE) deposit, Midcontinent Rift System, Upper Michigan: Geochim. Cosmochim. Acta, 89, pp. 10-30. Hinks, B., 2016, Geochemical and petrological studies on the origin of nickel-copper sulfide mineralization at the Eagle intrusion in Marquette County, Michigan, MS Thesis, Western Michigan University 88 �Preliminary Investigation of the East Eagle Intrusion Gabbro in Marquette County, Michigan RUPP, Kevin1, THAKURTA, Joyashish1, and MAHIN, Robert2 1 Department of Geosciences, Western Michigan University, 1903 W. Michigan Ave. Kalamazoo, MI 49008 2 Eagle Mine, Lundin Mining Corporation, 4547 County Road, Champion, MI 49814 The Eagle deposit is a high-grade, mafic to ultramafic Ni-Cu-bearing sulfide deposit located in Michigan’s Upper Peninsula in Marquette County. The Eagle and East Eagle intrusions are associated with the ~1.1 Ga Midcontinent Rift System and are also associated with the east-west trending Marquette-Baraga dike swarm. Current proven and probable reserves for Eagle are 4.8 million tonnes with an average grade of 2.8% Ni, 2.4% Cu, 0.1% Co, 0.3 gpt Au, 3.4 gpt Ag, 0.7 gpt Pt, and 0.5 gpt Pd (Clow et al., 2017). Recent drilling programs have intersected a vertical gabbroic rock unit in contact with the high-grade mineralization zone of the East Eagle conduit (figure 1). Initial geochemical analysis indicate that the gabbro is depleted in Cu and PGE and becomes more enriched in MgO and FeO with depth. Intrusions depleted in metals often overlie massive sulfide deposits due to the preferential accumulation of metals within sulfide minerals. This is significant in that the gabbroic unit could indicate another massive sulfide deposit at the base of the intrusion. This study attempts to determine the relationship between the gabbroic unit and the known Eagle intrusions based on petrological and geochemical data. Primary objectives of this project are: (1) age determinations using the U-Pb zircon/baddeleyite method, (2) comparison of the gabbroic samples with Eagle and East Eagle based on petrography, whole and trace element geochemistry, and mineral compositions, and (3) comparison of sulfur isotope values with the known values for the Eagle and East Eagle intrusions. The Eagle intrusions were radiometrically age dated and determined to be 1107.3 ± 3.7 Ma (Ding et al, 2010). If these ages are similar, the prospect for sulfide mineralization in a lower staging chamber will be heightened. Previous geochemical studies on the Eagle intrusion show FeO/MgO ratios and the Al 2 O 3 contents of parental magmas to be within the range of picritic basalts erupted during early-stages of the Midcontinent Rift. Whole and trace element geochemical analysis, along with microprobe analysis, will aid in determining the genetic relationship between Eagle and the gabbroic unit. Textural and isotopic characteristics of disseminated sulfides hosted within the gabbro will also be analyzed using reflected light microscopy and a Delta V Mass Spectrometer. Preliminary samples show high degrees of sericitic, propylitic, and carbonate alterations which decrease with depth away from the East Eagle intrusion. Pervasive alteration in many of the samples makes distinguishing individual mineral phases and textures difficult, but primary relict textures (mainly olivine) are seen throughout the samples. Most samples resemble a medium to fine-grained, olivine magnetite gabbro. Plagioclase (50-60%) occurs as subprismatic to lath-like grains that are moderately to strongly altered (up to 60% alteration minerals) to sericite. Olivine (2-8%) are distinguish by subprismatic to subhedral relict grains that altered (90100% alteration minerals) to serpentine and iron-rich oxides. Subprismatic pyroxenes (10-20%) show varying degrees of chlorite alteration to chlorite. Disseminated sulfide mineralization is observed with major sulfide minerals consisting of pyrite, pyrrhotite, chalcopyrite, and pentlandite. Electron microprobe analysis is needed to determine the specific mineral compositions. 89 �Elev (z) -600 Massive sulfides Peridotite gabbro -800 Archean basement -10000 Figure 1: A North-facing 3D-model of the gabbroic unit adjacent to the massive sulfide deposit of East Eagle. Drill core 17EA360 is shown intersecting the gabbroic unit and continuing down through the Archean Basement (image courtesy of Lundin Mining Corporation. Special thanks to Espree Essig and the exploration team) REFERENCES Clow, G. G., Lecuyer, N. L., Rennie, D. W., Scholey, B. J. Y. (2017) NI 43-101 Technical Report on the Eagle Mine, Michigan, USA. Report for Lundin Mining Corporation, dated April 26, 2017, pp. 1-306. Ding, X., C. Li, E. M. Ripley, D. Rossell, and S. Kamo (2010), The Eagle and East Eagle sulfide orebearing maficultramafic intrusions in the Midcontinent Rift System, upper Michigan: Geochronology and petrologic evolution, Geochem. Geophys. Geosyst., 11, Q03003, doi:10.1029/2009GC002546. 90 �High-technology metal behavior in ore-forming environments and its implication for the Vermilion District, northern Minnesota. SCHARDT, Christian and DAVID, Mady Department of Earth and Environmental Sciences, University of Minnesota-Duluth, 1114 Kirby Dr. Duluth, MN 55812 High-technology metals (HTM), such as In, Ge, Ga, and Tl, are increasingly important for essential industrial applications as well as renewable energy technology. They typically occur in very low concentrations (~ 1 ppm; Terashima 2001) and may reach concentrations of up to 0.15 % in some deposits (e.g., Li et al., 2015). As they do not form ore minerals, they substitute for other metals (Cu, Zn, Sn) in ore mineral such as sphalerite, chalcopyrite, and stannite (Johan, 1988, Pavlova et al, 2015). As a consequence, these metals are sourced as byproducts from other ore deposits (Ishihara and Endo, 2007; Pavlova et al. 2015). While the formation of these ore deposits is relatively well understood, it remains unclear why these metals are restricted to certain ore deposits. This is due to our poor understanding of their general thermodynamic behavior, sourcing, transport, and enrichment mechanisms in selective ore deposition environments. In fact, there is a surprising lack of data regarding the concentration of these metals in various geological settings and their sourcing in ore-forming systems. To better understand the behavior of these metals and gain insight into their enrichment, crustal abundances and concentrations in other sources (seawater, rivers, hydrothermal fluids) were collected along with available data from deposits with confirmed HTM enrichment (volcanogenic massive sulfides, Mississippi Valley Type, Sedimentary-Exhalative, tin granites). In addition, existing geochemical data from the Vermilion district (Peterson, 2001), assumed to host potential massive sulfide mineralization, have been supplemented by available till analysis provided by Larson (2018), and new analysis of various drill holes located within in the Vermilion district. In, Ge, Ga, and Tl show lowest average values in seawater and river waters (< 1 ppb to 0.5 ppm), well below average crustal abundances (up to 15 ppm; see figure 1.). Hydrothermal fluids show concentrations between 0.1 and 10 ppm, close to average crustal averages. HTM values for continental crust (metamorphic, sedimentary, magmatic) show In having the lowest average values (0.1 - 0.5 ppm), increasing to 0.7 ppm for Tl, followed by Ge (1.4 ppm) and Ga (18 ppm; see figure 1). Limited data for the Vermilion district and the Duluth Complex are comparable to trends of other magmatic and volcanic averages, respectively. Results suggest that these HTM are not typically enriched in surface waters (< 1 ppm). Data for hydrothermal fluids show Tl enrichment (up to 20-fold) while In shows average concentrations. Ge and Ga, however, are significantly depleted (Ge: 5-fold; Ga: 2-fold) compared to average crustal abundances. This poses the question where and how HTM get enriched to values recorded in some sulfide deposits (≥ 1000 ppm) and why this process seems to be restricted to certain geological environments. To study this issue, geochemical data from HTM-bearing ore deposits have been analyzed to determine if differences in the substitution behavior of HMT exist between different ore deposit types. Initial results indicate differences in the substitution behavior of HTM in host rocks (inset figure 1) as well as ore minerals (not shown), which may point to a) variable HTM sourcing, b) mineralization conditions, and/or c) different hydrothermal fluid chemistries as a function of formation environment. Further analysis is underway to determine if this also applies to the Vermilion district and its potential to host significant HTM concentrations. 91 �Figure 1 Minimum, average, and maximum concentrations of In, Tl, Ge, and Ga in crustal rocks and fluids. The Vermilion district and the Duluth Complex show patterns similar to other volcanic and magmatic rock data (not shown). Inset: Cu-Ga-Zn ternary plot for whole-rock data from major HTM-bearing deposits (VMS - volcanogenic massive sulfides). Differences exist between mafic/felsic volcanic, plutonic (tin deposits), and sedimentary settings (siliciclastic). Similar trends are also observed for other element combinations, including non HTM elements. References Ishihara, S., and Endo, Y. (2007) Indium and other trace elements in volcanogenic massive sulfide ores from the Kuroko, Besshi and other types in Japan. Bulletin of the Geological Survey of Japan, v.58, p. 7 - 22 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 Larson, P., 2018, personal. communication Li, Y., Tao, Y., Feilin, Z., Mingyang, L., Feg, X., and Xianze, D., 2015, Distribution and existing state of indium in the Gejiu Tin polymetallic deposit, Yunnan Province, SW China. Chinese Journal of Geochemistry, v. 34, p. 469 - 483 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 Peterson, D.M., 2001, Development of Archean Lode-Gold and Massive Sulfide Deposit Exploration Models using Geographic Information System Applications: Targeting Mineral Exploration in Northeastern Minnesota from Analysis of Analog Canadian Mining Camps; University of Minnesota Ph.D. thesis, 503 pages, 12 plates, 1 CD-ROM. 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 92 �Geochemistry of mafic rocks in Dickinson County, Michigan: Evidence for ~2.1 Ga Rifting SCHULZ, K.J.1, CANNON, W.F.1, and WOODRUFF, L.G.2, 1 U.S. Geological Survey, Reston, VA 20192, 2 U.S. Geological Survey, Mounds View, MN 55112 Mafic rocks of purported Archean and Paleoproterozoic age are a significant and widespread component of the bedrock geology in Dickinson County, Michigan (James et al., 1961). For this study we have sampled mafic rocks that occur in the Carney Lake Gneiss and other Archean gneisses of the county as well as mafic rocks in the Dickinson Group and the Hardwood Gneiss. Field relations of the mafic rocks in the Archean gneisses are often ambiguous; some are clearly dikes but whether they are Archean or Paleoproterozoic in age is often uncertain. Mafic rocks in the Carney Lake Gneiss and other Archean gneisses in the region range from highly deformed amphibolite inclusions in granitic gneiss to less deformed “salt and pepper” amphibolites to metadiabase dikes with no penetrative fabric and lower metamorphic grade. We have analyzed ten samples of the “salt and pepper” amphibolites and found two basaltic compositional types. Group 1 samples, two of which are from identified dikes, have relatively low MgO (~4 to 6 wt. %), moderately fractionated incompatible trace element patterns, and negative Nb-Ta anomalies on a primitive mantle normalized (PMn) trace element plot (Figure 1A). In contrast, Group 2 samples, for which field relations are ambiguous, have higher MgO (~6 to 10 wt. %), lower trace element contents than the first group, and distinctive flat PMn trace element patterns (Figure 1A). The Dickinson Group is composed of the basal East Branch Arkose overlain by the Solberg Schist and Six Mile Lake Amphibolite (James et al., 1961). Two samples of amphibolite collected along strike in the East Branch Arkose have very similar tholeiitic basalt compositions characterized by moderately enriched light REE and no Nb-Ta anomalies on a PMn trace element plot (Figure 1B). A sample of a metadiabase dike cutting Archean granitic gneiss north of the East Branch Arkose sample location and an amphibolite from a road cut to the south near Felch are similar in composition except for a positive Th anomaly when normalized to primitive mantle, which is likely the result of crustal contamination. Samples of mafic rocks from the Solberg Schist range from basalt to andesite (~45 to 56 wt. % SiO2; ~4 to 12 wt. % MgO), are more enriched in light REE than the amphibolite in the East Branch Arkose, and have negative Nb-Ta anomalies on a PMn trace element plot (Figure 1B). Samples of the Six Mile Lake Amphibolite, in contrast to the amphibolites in the East Branch Arkose and Solberg Schist, have much lower trace element contents and flat PMn trace element patterns much like the Group 2 amphibolites sampled in the Carney Lake Gneiss (Figure 1B). In addition, a large metagabbro body and a dike sampled in the Solberg Schist are similar in composition to the Six Mile Lake Amphibolite. This supports the interpretation that the Six Mile Lake Amphibolite is the upper, youngest part of the Dickinson Group (James et al., 1961). Samples of mafic gneiss in the Hardwood Gneiss complex are generally similar in composition to the Six Mile Lake Amphibolite with similar low trace element contents and 93 �relatively flat PMn trace element patterns (Figure 1C). One mafic gneiss sample is enriched in light REE and has a large negative Nb-Ta anomaly that is likely the result of contamination by felsic crustal rocks. Three Paleoproterozoic dike swarms, the Marathon, Kapuskasing, and Fort Frances, which outcrop around the northern margin of Lake Superior and range in age from 2126 to 2067 Ma, are attributed to a long-lived mantle plume event that accompanied rifting along the southern margin of the Superior craton (Halls et al., 2008). Like the mafic rocks in the Dickinson Group, the older dikes (Marathon and Kapuskasing) show enriched and fractionated incompatible trace element patterns while the youngest (Fort Frances) are relatively depleted and have flat trace element patterns. The overlap in composition of the mafic rocks sampled in Dickinson County with the Paleoproterozoic dikes on the north side of Lake Superior suggests the Dickinson County mafic rocks also may be related to the final rifting of the Superior and Wyoming cratons. This is supported by the presence of 2.1 Ga detrital zircons in the East Branch Arkose (Craddock et al., 2013). Figure 1. Primitive mantle normalized trace element patterns for mafic rocks from Dickinson County. References Craddock, J.P., Rainbird, R.H., Davis, W.J., Davidson, Cam, Vervoort, J.D., Konstantinou, Alexandros, Boerboom, Terry, Vorhies, Sarah, Kerber, Laura, and Lundquist, Becky, 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. 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 margin of the North American Superior Province: Precambrian Research, v. 162, p. 327–353. 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. 94 �Detrital Zircons in the Waterloo Quartzite, Wisconsin: Implications for the Ages of Deposition and Folding of Supermature Quartzites in the Southern Lake Superior Region SCHWARTZ, Joshua J.1, STEWART, Esther K.2, and MEDARIS, L. Gordon Jr.3 1 Geological Science, California State University, Northridge, California 91330 Wisconsin Geological and Natural History Survey, Madison, Wisconsin 53705 3 Department of Geoscience, University of Wisconsin–Madison, Madison, Wisconsin 53706 2 Proterozoic supermature quartzites of the Baraboo Interval are a prominent and significant Precambrian feature of the southern Lake Superior region, covering an area of ~175,000 km2. The Waterloo Quartzite in SE Wisconsin has long been correlated with other quartzites of the Baraboo Interval, based on similarities in sedimentary characteristics, geological setting, and chemical composition. Metapelites in the Baraboo and Waterloo sequences are among the most chemically mature sedimentary rocks in the geological record, having Chemical Indices of Alteration of 99.6 and 97.6, respectively. Although similar to the Baraboo Quartzite in many respects, the Waterloo Quartzite differs in having experienced pervasive K–metasomatism (Table 1). In addition, axial-planar cleavage is more strongly developed in quartzite at Waterloo compared to Baraboo, and Waterloo exhibits a higher grade of metamorphism, with Waterloo metapelite containing andalusite (amphibolite facies) and Baraboo metapelite containing pyrophyllite (greenschist facies). Seven samples of Waterloo quartzite and pebbly quartzite in Dodge county were collected for detrital zircon analysis to evaluate sources of sediment and to determine possible relationships to other supermature quartzites in the region. Samples include five quarried blocks in the Michels Materials Waterloo Quarry and individual samples from outcrops at Hubbleton and Mud Lake. Bedding orientations overlain on an aeromagnetic anomaly map of the area suggest that quartzite at the Michels quarry may be in a lower stratigraphic position than those at the Hubbleton and Mud Lake localities. A relative probability plot for detrital zircons (filtered for dates <10% discordant) in the Waterloo Quartzite at the Michels quarry displays a strong geon 16 (Mazatzal) and geon 17 (Yavapai) signal, and diminished geon 18 (Penokean) and geon 25–27 (Algoman) signals (Fig. 1A). Maximum Ages of Deposition calculated from the youngest statistically homogenous population (MSWD ≤ 1.0) are 1759±13 (n=21), 1694±11 (n=36), 1671±14 (n=21), 1669±14 (n=21), and 1643±11 Ma (n=42). In contrast, Waterloo quartzites at Hubbleton and Mud Lake are characterized by an absence of geon 16 zircons, a pronounced Penokean population, and a subdued, but distinct Algoman population (Fig. 1A). Quartzites in the Baraboo Range, Sauk County, consist of a stratigraphically lower fluvial facies and an upper shoreface marine facies, whose detrital zircon populations differ from each other (Fig. 1B). Fluvial quartzites display a predominant geon 17–18 signal, and shoreface marine quartzites contain a pronounced geon 18–19 population and a distinct geon 25-27 population. 95 �The Algoman, Penokean, and post– Penokean (Yavapai) populations of detrital zircons in the Baraboo quartzites are consistent with derivation from the proximal post– Penokean Montello Batholith and more distal, northerly Penokean and Archean basement; postPenokean zircons are more abundant in the stratigraphically lower fluvial facies than in the higher shoreface marine facies, which was deposited after burial of the 1750 Ma Montello Batholith. Detrital zircons in Waterloo quartzites at Hubbleton and Mud Lake were also derived from the proximal Montello Batholith and more distal, northerly Penokean and Algoman terranes. In contrast, Waterloo Quartzite at the Michels quarry is characterized by a relative abundance of geon 16 zircons and a pronounced population peak at 1700 Ma. Clearly, the provenance of this quartzite was different than that of the other analyzed quartzites. Geon 16 juvenile crust is absent in the southern Lake Superior region, but occurs in the subsurface south of Wisconsin as part of the transcontinental 1.68–1.60 Mazatzal belt (Whitmeyer and Karlstrom, 2007). Thus geon 17 and geon 16 zircons in quartzite at the Michels quarry were likely derived by northerly transport from the proximal Yavapai terrane and more distal Mazatzal terrane to the south. The shift in detrital zircon age populations between the Michels quarry and Hubbleton and Mud Lake outcrops reflects a change in transport direction from north to south. Deposition of quartzite at the Michels quarry is bracketed between ca. 1643 Ma, its youngest maximum age of deposition, and 1452 Ma, the 40Ar/39Ar cooling age of muscovite in folded metapelite (Medaris et al., 2003). Because the Michels quarry lies stratigraphically below the Hubbleton and Mud Lake outcrops, the depositional age for quartzites at these localities must also be no older than ca. 1640 Ma. The extreme chemical maturity of Waterloo metapelite requires derivation from a region of subdued relief that experienced intense chemical weathering. Such chemical maturity, combined with geon 16 Maximum Ages of Deposition for quartzite at the Michels quarry, is consistent with deposition of the Waterloo Quartzite after the 1630 Ma Mazatzal Orogeny, with depositional space for the thick (~1000 m) quartzite sequence being provided by post-Mazatzal rifting. If this scenario is correct, it requires that deformation and folding of the Waterloo Quartzite occurred during the geon 14 Wolf River tectonomagmatic event. References Medaris et al., 2003, Journal of Geology, v. 111, p. 243–277. Van Wyck and Norman, 2004, Journal of Geology, v. 112, 305-315. Whitmeyer and Karlstrom, 2007, Geosphere, v. 3, 220-259. 96 �Compositional and geochemical characteristics of the Crystal Lake intrusion, Ontario SMITH, Jennifer1, BLEEKER, Wouter1, ROSSELL, Dean2 and LABERGE, Justin2 1 2 Geological Survey of Canada, 601 Booth Street, Ottawa, Canada; email:jennifer.smith6@canada.ca Rio Tinto Exploration Canada Inc. 1300 Walsh Street, Thunder Bay, Canada The 1.1 Ga failed rift system hosts a range of mafic-ultramafic, carbonatitic and alkaline intrusions (Bleeker et al., 2018), many of which are actively being explored for a range of commodities (e.g., Ni, Cu, PGE, Co, Cr, V, Nb). The discovery of the high grade, massive sulphide, Ni-Cu Eagle deposit in 2002, has resulted in a surge of exploration activity and interest in the Ni-Cu-PGE potential of the MCR. Early rift (1117 to 1106 Ma) conduit-type, ultramafic intrusions (e.g., Tamarack, Eagle), remain the most attractive but challenging exploration targets (Heaman et al., 2007). The 1099±1 Ma Duluth Complex and similar large, sheet-like intrusions (e.g., Sonju Lake, Mellen Complex, Echo Lake, Crystal Lake, Coldwell Complex) still remain prospective, although typically contain lower metal tenors (Ripley, 2014). The 1099.1±1.2 Ma (Heaman et al., 2007) Crystal Lake layered intrusion, located 47 km southwest of Thunder Bay, contains low-grade Ni-Cu-PGE sulphide mineralisation and uneconomical chromite occurrences (Geul, 1970; Smith & Sutcliffe, 1987). Although mineralisation was first discovered in the 1950s and has been extensively explored since, the intrusion remains a prospective exploration target with Rio Tinto undertaking more recent drill programs in 2014-15 (Figure 1). This intrusion outcrops as a prominent Y-shaped body, intruding S-bearing shales, argillites and greywackes of the Paleoproterozoic Rove Formation. Geochemically, the Crystal Lake Figure 1. Crystal Lake intrusion and location of Rio Tinto’s 2014intrusion can be distinguished from the more 2015 boreholes. Adapted from Geul 1970. primitive conduit-type bodies by: olivine composition (Fo 51-79 ), low Ni/Cu and Pt/Pd ratios (<1), higher REE abundances, LREE enrichment and minimal fractionation of HREEs (Gd/Yb <2; Thomas, 2015). Previous work divided the intrusion into four discrete zones (Smith & Sutcliffe, 1987). The Basal Zone contains an aphanitic chill zone, with inclusions of S-bearing Rove sedimentary rocks. The overlying Lower Zone is characterised by medium to pegmatitic, vari-textured gabbro with irregular, coarse segregations of Cr-bearing leucogabbro and anorthosite. The Middle Zone marks the beginning of phase layering and comprises four magmatic cycles. Each cycle corresponds to an influx of magma (Cogulu, 1993a) and consists of a basal Cr-spinel bearing troctolite/olivine gabbro and an upper anorthositic gabbro. The Cr-spinel occurs in discrete layers and is recognised within orthocumulate and adcumulate rocks where it constitutes 8 to 36 modal%, respectively. Compositional differences in Cr-spinel occurrences have been attributed to the effects of in-situ re-equilibration (Cogulu, 1993a). The Upper Zone is marked by the disappearance of Cr-spinel and anorthositic layers. This unit consists of coarse-grained olivine gabbro and medium-grained troctolite. Low-grade, Ni-Cu-PGE sulphide mineralisation is developed throughout the Lower and Middle Zones, with the Upper Zone barren of sulphides. The Lower Zone is characterised by disseminated to massive sulphides which are mainly concentrated towards the basal contact and within late pegmatitic zones. The association of sulphides with pegmatitic 97 �phases is not unique to the Crystal Lake intrusion, also being recognised in the Coldwell Complex and other world-class deposits (e.g., Merensky Reef). Cogulu (1993b) noted that pyrrhotite dominates the basal assemblages. Middle Zone sulphides are disseminated and closely associated with volatiles. Here, assemblages are Cu-rich with lesser proportions of pentlandite and pyrrhotite (Cogulu, 1993b). From Rio Tinto’s 2014-15 dill holes the following observations are made. The Lower and Middle Zones are characterised by low Pt/Pd (<0.3), Ni/Cu (<1) and mantle-like Cu/Pd values (103-104). Whilst the Ni/Cu increases into the Middle Zone, the Cu/Pd ratio decreases along with incompatible element concentrations. The Upper Zone is more homogeneous with higher Pt/Pd (0.5-1), Ni/Cu (often >1), and higher than mantle Cu/Pd ratios (>104). Ni/Cu decreases through the Upper Zone whilst Pt/Pd, Cu/Pd, and incompatible elements increase. The implications and cause of these geochemical trends has yet to be fully constrained. The addition of crustal S is considered critical in the genesis of many of the MCR Ni-Cu sulphide deposits (Ripley, 2014). Preliminary δ34S data indicate a strong crustal component throughout the Crystal Lake intrusion with δ34S ranging from 1.4-16.5‰ (Thomas, 2015). The majority of data resides outside the mantle range of 0±2‰. Thomas (2015) argues that the intrusion was emplaced as a series of S-saturated magma pulses, with δ34S variability attributed to contamination by different S-bearing horizons. S/Se ratios however, are more consistent with in-situ contamination with a footwall influence evident in the Lower Zone. The Middle and Upper Zones exhibit lower than mantle S/Se ratios, showing no evidence of a crustal control, which various processes may have masked (e.g., S-loss, upgrading, increased R-factor). To date, no proposed model accounts for all of these features. The Crystal Lake intrusion remains an interesting deposit. Whilst the mineralisation shows many parallels to those observed at the Duluth and Coldwell Complexes, various questions remain regarding the source characteristics and range of magmatic processes involved in their development. References Bleeker, W., Liikane, D.A., Smith, J., et al. 2018. Activity NC-1.3: Controls on the localisation and timing of mineralised intrusions in intra-continental rift systems, with a specific focus on the ca. 1.1 Ga Midcontinent Rift (MCR) system. Geological Survey of Canada, Open File 8373, 15-27. Cogulu, E.H., 1993a. Factors controlling postcumulus compositional changes of chrome spinels in the Crystal Lake intrusion, Thunder Bay, Ontario. Geological Survey of Canada, Open File 2748. Cogulu, E.H., 1993b. Mineralogy and chemical variations of sulphides from the Crystal Lake Intrusion, Thunder Bay, Ontario. Geological Survey of Canada, Open File 2749. Geul, J.C., 1970. Geology of Devon and Pardee Townships and the Stuart Location. Ontario Department of Mines, Geological Report 87. Heaman, L.M., Easton, R.M., et al., 2007. Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon region, Ontario. Canadian Journal of Earth Sciences, 44(8), 1055-1086. Ripley, E.M., (2014). Ni-Cu-PGE mineralisation in the Partridge River, South Kawishiwi, and Eagle intrusions: A review of contrasting styles of sulphide-rich occurrences in the Midcontinent rift system. Economic Geology, 109(2), 309-324. Smith, A.R., & Sutcliffe, R.H., 1987. Keweenawan intrusive rocks of the Thunder Bay area. Ontario Geological Survey Miscellaneous paper 137. Thériault, R.D., Barnes, S-J., & Severson, M.J., 1997. The influence of country rock assimilation and silicate to sulphide ratios on the genesis of the Dunka Road Cu-Ni-PGE deposit, Duluth Complex. Canadian Journal of Earth Science, 34, 375-389. Thomas, B., 2015. Geochemistry, sulphur isotopes and petrography of the Cu-Ni-PGE mineralised Crystal Lake Intrusion, Thunder Bay, Ontario. M.Sc. Thesis. 98 �Petrology and 11B Composition of Tourmaline within the 2685 Ma Ghost Lake Batholith and Mavis Lake Pegmatites SMITH, Vittoria and ZUREVINSKI, Shannon Department of Geology, Lakehead University, 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada The Ghost Lake Batholith (GLB) and derived Mavis Lake Pegmatite group are an example of a granite-pegmatite system in which both the parent granite and least- to- most evolved pegmatites are visible and accessible. The GLB has been divided into eight internal units based on mineralogy and texture, while the Mavis Lake Pegmatite group is divided into three broad zones based on the mineralogy of the pegmatite bodies (Breaks and Moore, 1992). Samples collected from the biotite granite phase (GLB-3) of the Ghost Lake Batholith have the mineral assemblage typical of an S-type peraluminous granite. The granite mineralogy is made up of mostly quartz, albite, potassium feldspar and biotite with accessory muscovite, garnet, zircon, and blue apatite. Pegmatitic segregations within the parent granite consist of potassium feldspar, quartz, and biotite, and accessory garnet. Apatite within this unit is typically associated with or included directly within the biotite and grains analyzed via SEM have been found to be LREE-enriched. Pegmatite bodies within the beryl-columbite zone of the Mavis Lake Pegmatite group are hosted within mafic metavolcanic rocks and show considerable variation in mineralogy and texture. Pegmatites range from potassic to albitic, and garnet is occurring as an accessory phase. Tourmaline is common in various pegmatitic units within the beryl-columbite and spodumene-beryl-tantalite zone. Due to its commonality within the Mavis Lake group, tourmaline has been sampled and studied using major and trace element techniques to assess its usefulness as an indicator of fractionation between and within individual pegmatite bodies. Tourmaline core compositions within the pegmatite throughout the Mavis Lake Group range between schorlitic in composition to dravitic (Fig.1). In the case of the Taylor emerald occurrence, the tourmaline species within the pegmatite range from dravitic in the border zone, to schorlitic within the pegmatite body, reflecting a decrease in Fe and an increase in Mg from rim to core. Tourmaline zoning profiles from tourmaline within the Taylor emerald occurrence show significant substitution between Fe and Mg in the Y- site and an inverse relationship between increasing Na contents and decreasing vacancies within the crystals X-site (Fig.2). Similarly, tourmaline from a nearby potassic pegmatite show similar progression in decreasing Na from core to rim with an increasing amount of vacancies. In contrast, the Fe contents in the Y-site steadily increase from core to rim, and Mg shows a closely inverse reaction to the Al contents, suggesting a proton-loss substitution. Boron isotope data collected in situ via SIMS report δ11B values from pegmatites within the contact beryl zone between 8.1‰ to 13.9‰. Variable mineralogy and major- and trace-element mineral chemistry within the Beryl-columbite zone suggest that the degree of host-rock interaction highly influences the tourmaline chemistry. This supports previous work by Breaks and Moore (1992), who had previously suggested that the high Mg contents within the Taylor pegmatites may be related to metasomatic transfer with the metavolcanic host units. While tourmaline is widely considered a petrogenetic indicator for the degree of fractionation within pegmatitic systems, this concept does not seem to apply to a system like Mavis Lake where tourmaline is restricted to the border zones of the pegmatites and is highly influenced by host-rock interaction. 99 �Figure1. Tourmaline speciation diagram for tourmaline within the Mavis Lake Group following the classification scheme of Henry et al., 2005. Fig. 2: Tourmaline zoning profiles for the X and Y sites within a euhedral crystal of tourmaline in potassic pegmatite. REFERENCES: BREAKS, F.W. AND MOORE, J.M., 1992. "The Ghost Lake batholith, Superior Province of northwestern Ontario: a fertile, S-type, peraluminous granite-rare-element pegmatite system." Canadian Mineralogist v. 30, p.835-835. 100 �Geophysical, structural, and tectonic interpretation of the Yellow Medicine and Appleton shear zones, SW Minnesota and SE South Dakota: A work in progress SOUTHWICK, David, CHANDLER, Val, and JIRSA, Mark Minnesota Geological Survey, University of Minnesota, 2609 Territorial Rd, St. Paul, MN 55114 U.S.A. The Yellow Medicine and Appleton shear zones (YMSZ and ASZ) are prominent geophysical features of the Minnesota River Valley (MRV) subprovince of the Superior craton. Maps of the first vertical derivative of the magnetic anomaly and the second vertical derivative of the gravity anomaly show that the two zones converge into a single strand in east-central South Dakota, and that the combined fault strand continues west-southwest as least as far as the east margin of the Paleoproterozoic Trans-Hudson orogen. Faulting in the YMSZ and ASZ is thought to have begun in the Sacred Heart accretionary event (ca. 2600 Ma) in which the MRV subprovince was amalgamated to the south margin of the Superior craton. Fault motion may have peaked during Yavapai tectonism, between ca. 1785 and 1775 Ma, in concert with a major episode of granitic magmatism and orogenic uplift. The Minnesota segment of the YMSZ consists of an axial zone where there are multiple anastomosing sub-zones of concentrated fault damage and km-wide flanking zones of dispersed fault damage. The axial zone is well defined geophysically; the zones of dispersed fault damage are not. Drill cores reveal that the axial zone contains heterolithic crush breccia, fine crush breccia, crush microbreccia, protocataclasite, cataclasite, and protomylonite that were derived from identifiable quartzofeldspathic orthogneiss, foliated garnet-quartz-hornblende paragneiss, amphibolite, and plagiogranite. Graphite-rich fault rocks encountered in boreholes toward the east end of the YMSZ, near the west-northwest- verging tectonic front of the Penokean orogen, may be tectonically dismembered slices of Penokean metasedimentary rocks caught up in Yavapai faulting. Pseudotachylyte is relatively abundant in the axial zone of the YMSZ and in the narrow faults in the flanking zones of dispersed fault damage (Craddock and Magloughlin, 2005). Diabase dikes of the Kenora-Kabetogama/Fort Frances swarm (ca. 2070 Ma) are offset by and/or terminated against the YMSZ and the ASZ, whereas hornblende andesite and ferrodiorite dikes that cut the 1792 Ma and younger intrusions of the composite East-Central Minnesota Batholith (ECMB) transect the YMSZ and ASZ without deviation. A U-Pb zircon age of ca. 1780 Ma inferred for one of the hornblende andesite dikes (Schmitz et al., 2018, in prep.) limits the timeframe of geophysically discernable fault motion to the period between 2070 Ma (pre-Penokean) and 1780 Ma (mid-Yavapai). Geophysical patterns suggest that a considerable component of fault displacement on the YMSZ system was left-lateral strike slip that on a regional scale shifted the Morton block, south of the YMSZ, eastward relative to the Montevideo block on the north. We speculate that this displaced a NNW-trending piece of the southern Trans-Hudson orogen eastward from central South Dakota into far WSW Minnesota, where NNW geophysical trends are evident and as yet unexplained. This regional interpretation is based on potential-field images that were upward-continued to five km in order to even out resolution differences among the various data sets that were compiled in the source magnetic and gravity maps of North America (North American Magnetic Anomaly Group (NAMAG), 2002; Committee for the Gravity Anomaly Map of North America, 1988). Our interpretation in eastern South Dakota is submitted as an alternative to an earlier interpretation presented by McCormick (2010a, b) that was based on the original unleveled magnetic and gravity maps. 101 �Vertical displacement on the YMSZ, up on the north, is indirectly inferred from Yavapai K-Ar ages from rocks in the Montevideo block and the absence of K-Ar ages younger than late Neoarchean in rocks in the Morton belt (Goldich et al., 1961). These observations suggest that K-Ar systematics were reset in Montevideo rocks that were hotter and deeper in the crust prior to Yavapai convergence and associated uplift above the north-dipping YMSZ, whereas the Morton rocks remained higher in the crust and relatively cool. The possibility of mafic underplating having had a role in Montevideo reheating and uplift in mid-geon 17 is suggested by the observed higher-gravity signature of the Montevideo block, particularly toward its east end. REFERENCES Committee for the Gravity Anomaly Map of North America, 1988, Gravity anomaly map of North America: Geological Society of America, 5 sheets, scale 1:5,000,000. Craddock, J.P., and Magloughlin, J.F., 2005, Calcite strains, kinematic indicators, and magnetic flow fabric of a Proterozoic pseudotachylyte swarm, Minnesota River valley, USA: Tectonophysics, v. 402, p. 153-168. Goldich, S.S., Nier, A.O., Baadsgaard, H., Hoffman, J.H., and Krueger, H.W., 1961, The Precambrian geology and geochronology of Minnesota: Minnesota Geological Survey, Minneapolis, Minnesota, Bulletin 41, 193 p. McCormick, K.A., 2010a, Precambrian basement terrane of South Dakota: South Dakota Geological Survey Program Bulletin 41, 37p. McCormick, K.A., 2010b, Plate 1: Terrane map of the Precambrian basement of South Dakota: South Dakota Geological Survey Program Bulletin 41, External pdf file, compilation scale 1:1,000,000. North American Magnetic Anomaly Group (NAMAG), 2002, Magnetic anomaly map of North America: U. S. Geological Survey Open File Report OFR 02-414 (On line only) (http://pubs.usgs.gov. /of/2002/of02-414/) Schmitz, M.D., Southwick, D.L., Bickford, M.E., Mueller, P.A., and Samson, S.D., 2018, in prep., Neoarchean and Paleoproterozoic events in the Minnesota River Valley subprovince, with implications for southern Superior craton evolution and correlation: Submitted to Precambrian Research March 2018. 102 �New bedrock geologic mapping of Dodge County, Wisconsin provides evidence for Paleozoic reactivation of Precambrian structures KINGSBURY STEWART, Esther, STEWART, Eric D., and ROUSHAR, Kathy Wisconsin Geologic and Natural History Survey, 3817 Mineral Point Road, Madison, WI 53705 We present a preliminary 1:100,000-scale bedrock geologic map of Dodge County, Wisconsin (fig. 1). Bedrock is mostly buried beneath 6 to 18 meters (20 to 60 feet) of glacial deposits that locally exceed 60 meters (200 feet) within bedrock channels in the eastern part of the county. Due to the significant glacial deposits, mapping is based on integration of geophysical data (gravity and aeromagnetic anomaly data, passive seismic readings), subsurface data (drill core, downhole geophysical logs, geologic logs based on cuttings from municipal wells, and well construction reports of private water wells), and observations collected at sparse outcrops and quarries. To produce the map, we interpolated a bedrock elevation surface from the top of bedrock contact recorded in 3,831 geolocated wells. Depth-structure maps of the base of Paleozoic map units and the top of the Precambrian surface were gridded from map unit contacts picked in 882 wells. The intersection of the depth-structure maps and the bedrock elevation surface defines map unit contacts. The bedrock geology is comprised of a Precambrian bedrock surface characterized by regional-scale folding and topographic relief overlain by upper Cambrian siliciclastics and Ordovician through Silurian dolostone and siliciclastics. The Paleozoic section thickens from west to east towards the Michigan Basin such that western Dodge County is underlain by the Cambrian through Middle Ordovician sandstone and dolostone while eastern Dodge County is underlain by Silurian dolostone of the Niagara Escarpment. Results from the first three years of this four-year effort clarify the stratigraphy and structure of the Precambrian units as well as the influence of Precambrian structure on deposition of the overlying Paleozoic sediments. The Precambrian rocks include folded metasediments of the Baraboo interval (<1.7 Ga) that were intruded by ca. 1.4 Ga granite (Medaris et al., 2011). A bedrock core drilled as part of the mapping effort encountered a likely altered banded ironformation that is known to be present ~40 miles (64km) to the northwest within the Baraboo interval stratigraphy. We tie this core to a characteristic, curvilinear aeromagnetic anomaly and extrapolate to calibrate the regional aeromagnetic data in Dodge County and thus map the distribution of Precambrian units. Map patterns of the Precambrian surface demonstrate that the Baraboo-interval metasediments were folded into east-northeast-trending, doubly-plunging anticlines and synclines with ~30km (18.6 mile) wavelength. Map patterns further demonstrate that the Waterloo quartzite, which outcrops in a broad syncline in southwestern Dodge County, is distinct from, and likely stratigraphically above, the Baraboo quartzite. Precambrian topography was mostly infilled by Cambrian sandstone such that the thickness of the Cambrian Elk Mound Group sandstone can vary by >82 meters (270 feet) over several miles while the thickness of the overlying Cambrian Tunnel City and Trempealeau Groups are relatively consistent. The Paleozoic units were then folded into broad, east-west trending, gentle anticlines and synclines with lengths of 13.6 km (8.5 miles) to 40 km (25 miles), widths of about 8 to 10.5 km (5 to 6.5 miles), and amplitudes of 20 to 100 meters (65 to 328 feet). Data from well cuttings and drill core suggest faulting locally uplifted the Precambrian basement through early Ordovician Prairie du Chien Group. The overlying Middle Ordovician Ancell Group unconformably overlies the Prairie du Chien Group. The overlying Sinnipee Group is gently folded with no clear evidence for fault offset. Sulfide mineralization is present throughout the Paleozoic section in Dodge 103 �County and is preferentially located along faults near fold axes (Brown and Maas, 1992, this study). Fold geometry and preferential sulfide mineralization along fold limbs observed in Dodge County is similar to fold geometry and mineralization reported by Heyl et al. (1959) for the Upper Mississippi Valley Lead-Zinc District, suggesting similar controls on deformation and mineralization for southwestern and southeastern Wisconsin. Figure 1. Generalized 1:1,000,000-scale bedrock geologic map of Dodge County showing data sources for 1:100,000-scale mapping. Inset map locates Dodge County (blue) in Wisconsin. Modified from Mudrey et al. (1982). References Brown, B. A. and R.S. Maas. 1992. A reconnaissance survey of wells in eastern Wisconsin for indications of Mississippi Valley Type Mineralization: Wisconsin Geological and Natural History Survey Open File Report 92-3, 31p. Heyl A. Jr., A.R. Agnew, E.J. Lyons, and C.H. Behre Jr. 1959. The geology of the Upper Mississippi Valley ZincLead District: US Geological Survey Professional Paper 309, 310p. Medaris, L.G., Jr., R.H. Dott, Jr., J.P. Craddock, and S. Marshak. 2011. The Baraboo District- A North American classic in Miller, J.D., Hudak, G.J., Wittkop, C., and McLaughlin, P.I., eds., Archean to Anthropocene: Field Guides to the Geology of the Mid-Continent of North America: Geological Society of America Field Guide 24, p. 63-82. Mudrey, M.G., Jr., Brown, B.A., and Greenberg, J.K. 1982. Bedrock geologic map of Wisconsin: Wisconsin Geological and Natural History Survey State Map 18, scale: 1:1,000,000. 104 �Neoarchean to Paleoproterozoic reconstructions using metamorphic core complexes as evidence of continental transform plate motion and their implications in Archean tectonics STINSON, V.R. and PAN, Y. Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, SK S7N 5E2 Canada Paleotectonic reconstructions use geological, geophysical, and paleontological information to piece together cratons, continents, and supercontinents. Metamorphic core complexes commonly have transform, transpression, and transtension components which are underrepresented in paleotectonic reconstructions and may also be used to provide evidence of subduction, collision, and exhumation. Due to the nature of the medium to high-grade metamorphic and felsic plutonic igneous lithologies in the footwall they are typically well-preserved and yield robust minerals used in geochronology. In this study we have combined literature review and field mapping, geochronology, and petrology to investigate the potential for reconstructing Archean cratons in transpressive to trantensional tectonic settings in the Neoarchean. This study recommends the use of metamorphic core complexes as evidence for transpressionaal to transtensional plate motions in paleotectonic plate reconstructions including reconstructions for the Proterozoic and Archean eons as data is sparse or poorly preserved. Further multi-disciplinary tectonic studies are necessary to broaden our understanding of Archean tectonics and by using metamorphic core complexes as analogues for transform plate boundaries we may greatly enhance paleotectonic reconstructions. The paleo-northeast-directed oblique collision between the Minnesota River Valley terrane with the southern Superior craton in the Neoarchean created predominantly dextral transpression in the Minnesota River Valley terrane and regional sinistral and dextral transpression to local sinistral transtension throughout the southern Superior craton. The rigid, Paleoarchean to Neoarchean Minnesota River Valley terrane collided into the recently formed, and rheologically weaker, southern Superior craton forming sinistral-oblique regional structures in the western Superior craton to the formation of metamorphic core complexes the eastern Superior craton suggesting transtension increased towards the east. This tectonic evolution from the formation of 2.700 Ga and 2.67 Ga MORB and arc to subduction to collision at 2.65-2.63 Ga to exhumation and lateral escape at 2.63 to 2.60 Ga is present in numerous Archean cratons worldwide with indistinguishable lithologies, structures, igneous and metamorphic petrology, geochemistry, geochronology, and mineral deposits. Evidence of this collision to transtensional tectonics and strike-slip deformation in the eastern Superior craton is preserved in the Archean cratons worldwide. Evidence of collision is preserved in the Paleoarchean to Neoarchean Minnesota River Valley terrane, Wyoming, Gawler, and West Antarctica and transpression to sinistral transtension due to lateral escape is preserved in Neoarchean Baltica (Central Kola Belt), Zimbabwe and Kaapvaal (Limpopo Belt), Yilgarn (Tropicana Gneiss), Eastern Antarctica, North China and Dharwar cratons. The development of craton, continental, or supercontinent breakup may have been triggered due the subduction of a transform plate boundary in the Wawa and Abitibi subprovinces, the size or rheology contrast of the colliding plates, the angle of the collision, and the formation of the metamorphic core complexes, lack of decoupling between the mantle and lithosphere boundary triggering mafic dyke swarms, plutonism, and (super) continental breakup. 105 �Keweenaw Fault Geometry and Kinematics along Bête Grise Bay, Michigan Tyrrell, C.W.1 Hubbell, G.E. 1, and DeGraff, J.M. 1 1 Michigan Technological University, Houghton, MI 49931 The Keweenaw Fault (KF) extends 350 km along the southern margin of the Midcontinent Rift System (MRS) from northwestern Wisconsin to near the tip of the Keweenaw Peninsula in Michigan (1). Reverse movement on the fault has thrust and tilted Portage Lake Volcanics (PLV, 1.1 Ga) over younger Jacobsville Sandstone (JS) (Fig. 1). The northeast portion of the fault near Keweenaw Point has been a matter of some interest since the USGS mapping campaign of the 1950s. Based on geophysical evidence, some have proposed that the fault continues offshore along an arc curving to the right by 90° to a southeasterly direction (1, 3-4). The farthest northeast location where the Keweenaw Fault can be directly examined is along the south side of the Keweenaw Peninsula from Bête Grise Bay eastward (Figs. 1, 2A). Here USGS maps from the 1950s show five shoreline areas where PLV and JS strata are juxtaposed (56). Our detailed mapping under the USGS EdMap program reveals that the previously mapped fault trace, based in part on aeromagnetic data and showing all PLV-JS contacts as faulted, oversimplifies the geologic relationships in this area. The anomalously sinuous, single fault trace mapped in the 1950s consists of at least five fault segments, generally striking ESE and forming a left-stepping pattern along the shoreline (Fig. 2B). At least three PLV-JS contacts previously mapped as faulted instead exhibit an unconformity between basal JS strata and older PLV lava flows. At one location, slightly deformed JS strata unconformably overlie fault breccia and gouge cutting PLV strata, indicating that one period of major slip on this KF segment occurred before local JS deposition. At other locations, JS strata are clearly cut and deformed along faulted contacts with PLV lavas, providing evidence for a second period of slip on the KF system after some or all JS deposition. Along shore near the Bare Hill rhyolite, PLV strata dip moderately to steeply SE to SSE for at least 3 km, a reversal of normal northerly dip that suggests an anticline developed north of this KF segment. Faulted PLV-JS contacts in the area generally dip > 80° N but locally dip steeply south. Geologic relationships across one fault segment suggest a significant component of dextral strike slip, and secondary faults have surface markings that indicate a mix of strike-slip and dip-slip motion. Ongoing work to quantify these relationships is designed to determine the degree of strikeslip to dip-slip partitioning along this portion of the KF system. The regional trace of the KF changes direction by over 70⁰ from NNE near Houghton to ESE at Bête Grise Bay, which mimics the change in strike of PLV layers over the same distance (Fig. 1). Paleomagnetic work (7-8) suggests that this direction change is a primary geometric attribute of the fault and not a result of bending around a vertical axis. Large crustal-scale faults often curve and split into segments near their terminations (9-10). Our mapping results thus imply that the KF system may terminate near the end of the peninsula in a series of fault splays, possibly transferring slip to other faults farther east. We hypothesize that slip on the KF changes from dominantly reverse dip-slip movement along its NE-trending portion near Houghton to dominantly dextral strike-slip near the tip of the Keweenaw Peninsula, and that slip magnitude decreases over this same distance. Acknowledgements: We appreciate the USGS funding this work and the timely field visit and comments last year by Bill Cannon, Klaus Schulz, and Laurel Woodruff, which does not imply their agreement or disagreement with these results. 106 �Figure 1: Keweenaw Peninsula where Portage Lake Volcanics are thrust over Jacobsville Sandstone. Black rectangle along the Keweenaw Fault near the tip of the peninsula marks focus area of Figure 2. (adapted from 2). Figure 2: Focus area along the Keweenaw Fault from Bête Grise Bay eastward (adapted from 5-6). Major faults shown as dark red traces. A) USGS maps from 1950s. B) Status of current fault mapping overlaid on prior maps. 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. 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. 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. 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. 5. 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. 6. 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. 7. Hnat, J.S., van der Pluijm, B.A., and van der Voo, R., 2006, Primary curvature in the Mid-Continent Rift: Paleomagnetism of the Portage Lake Volcanics (northern Michigan, USA): Tectonophysics, v. 425, p. 71–80. 8. Kulakov, E.V., Smirnov, A.V., and Diehl, J.F., 2013, Paleomagnetism of 1.09 Ga Lake Shore Traps (Keweenaw Peninsula, Michigan): new results and implications: Can. J. Earth Sci., v. 50, no. 11, p. 1085-1096. 9. Boyer, S.E. and Elliott, D., 1982, Thrust systems: AAPG Bulletin, v. 66, p. 1196-1230. 10. Brozovic, N. and Burbank, D.W., 1999, Dynamic fluvial systems and gravel progradation in the Himalayan foreland: Geological Society of America Bulletin, v. 112, no. 3, p. 394-412. 107 �Alteration Mineral Zonation and Geochemical Characteristics of the Back Forty Deposit, MI; a Replacement-style Zinc- and Gold-rich Volcanogenic Massive Sulfide Deposit UPTON, Margaret1, SCHARDT, Christian1, HUDAK, George2, QUIGLEY, Eric3 1 Department of Earth and Environmental Sciences, University of Minnesota - Duluth, 1114 Kirby Drive, 223 Heller Hall, Duluth, MN 55812 2 Natural Resources Research Institute, University of Minnesota - Duluth, 5013 Miller Trunk Hwy, Duluth, MN 55811 3 Project Geologist, Aquila Resources, 414 10th Avenue, Menominee, MI 49858 The Aquila Resources Back Forty zinc- and gold-rich polymetallic volcanogenic massive sulfide (VMS) deposit is located adjacent to the Menominee River near Stephenson in the Upper Peninsula of Michigan. VMS deposits are created in submarine environments when heated seawater circulates through oceanic crust and precipitates base and precious metals at or near the seafloor due to both cooling and neutralization of the ore fluid. In the process, host rock mineralogy and geochemistry are modified by both downwelling and upwelling hydrothermal fluids, which produces distinct alteration mineral assemblages and metasomatic changes within the host rock (Shanks and Thurston, 2012). Alteration mineral assemblages and their spatial distribution can be used to unravel the geochemical evolution of the system, and help locate mineralization. The relationship between host rock and alteration mineralogy is not well understood or documented at the Back Forty Deposit but essential for understanding its genesis. The main objectives of this work are to identify physical, mineralogical, and geochemical characteristics of hydrothermal alteration mineral assemblages associated with the Back Forty Deposit. Alteration mineral and geochemical characterization include drill core logging, lithogeochemical, petrographic, and SEM analysis to better understand detailed mineral assemblages and mineral species’ chemical attributes. Core from nine drill holes (~ 2,950 meters), were logged to identify alteration mineral assemblages, intensity, and their textural characteristics. The deposit, hosted in felsic pyroclastic rocks, shows mostly sericite alteration, which was used to establish an alteration intensity scale of 1-4 (1: weak, 4: intense). Major alteration mineral assemblages observed were sericite ± silica ± chlorite. Sericite alteration is pervasive throughout the deposit (2-3) with silica alteration intensity ranging from 1-2 and a few areas of silica flooding (3-4). Weak chlorite alteration occurred throughout the deposit within the host rhyolite crystal tuff units as spotty chlorite associated with sulfide mineralization (1-2). Whole rock and trace element lithogeochemistry will be evaluated using the alteration box plot (Large et al., 2001) and mass balance analysis, such as the ISOCON method (Grant, 1986; see fig. 1). Results will be essential to assess quantitative chemical changes associated with alteration mineral assemblages and their spatial distribution to identify likely hydrothermal fluid flow pathways and mineralization vectors within the deposit. Using petrographic analysis as well as structural and lithological data, cross sections identifying alteration mineral zonation and its relative extent will be created to determine the relationship between massive sulfide mineralization and alteration mineral assemblage presence and intensity. From these results, it 108 �may be possible to use alteration mineralogy and geochemistry to determine the direction of hydrothermal fluid flow associated with mineral deposition and aid in future exploration efforts to locate additional mineralization on the Back Forty Deposit property, as most VMS deposits occur in clusters (Galley et al., 2007). Concentration of Intense Sericite Altered Sample, 108410 50 Hg Ta 40 Sn Yb V Hf K2O TiO2 SiO2 W 30 Zr Al2O3 Ga P2O5 Nb Ni Mo Ba 20 Y As MgO Pb S Ag Co 10 Fe2O3(T) 0 Na2O Cr 0 10 Sr Au MnO Cu 20 30 CaO 40 CO2 Zn 50 Concentration of Least Altered Sample, LK-348 Figure 1. ISOCON plot of selected elements used to compare elemental gains and losses between least and most altered samples. Isocon line of best fit is defined by immobile elements (Hf, Nb, Ta, Zr). Components above the line are enriched; below are depleted (modified from Ross, 2011). References Aquila Resources, 2017. Back Forty: Zinc- and Gold-rich Deposit. http://www.aquilaresources.com/projects/backforty-project/#! (accessed March 2018). Galley, A., Hannington, M., Jonasson, I., 2007. Volcanogenic Massive Sulphide Deposits. Geological Survey of Canada, Special Publication 5, p. 141-161. Grant, J. A., 1986. The Isocon Diagram: A Simple Solution to Gresens' Equation for Metasomatic Alteration. Economic Geology, v. 81, p. 1976-1982. Large, R. R., Gemmell, B.J., Paulick, H., 2001. The Alteration Box Plot: A Simple Approach to Understanding the Relationship between Alteration Mineralogy and Lithogeochemistry Associated with Volcanic-Hosted Massive Sulfide Deposits. Economic Geology, v. 96, p. 957-971. Shanks, W.C.P., Thurston, R., 2012. Volcanogenic Massive Sulfide Occurrence Models. USGS Scientific Investigations Report 2010–5070–C, 363 p. Ross, C., Hudak, G., Morton, R., Quigley, T., and Mahin, B., 2011, Preliminary stratigraphy and physical volcanology associated with the Paleoproterozoic Back Forty VMS deposit, Menominee County, Michigan [abstract/poster]: Institute on Lake Superior Geology, v. 57, Part 1, p. 70-71. 109 �Reconstruction of paleoenvironmental conditions and temporal patterns of ancient mining on Isle Royale using biogeochemical analyses of lake sediment VALL, Kathryn G.1, STEINMAN, Byron A.1, POMPEANI, David P.2, SCHREINER, Kathryn M.3, DEPASQUAL, Seth4 1 Earth and Environmental Sciences, Large Lakes Observatory, University of Minnesota Duluth Department of Geography, Kansas State University, Manhattan, KS 66506 3 Chemistry, Large Lakes Observatory, University of Minnesota Duluth 1049 University Dr, Duluth MN 55805 4 Cultural Resources, Isle Royale National Park, 800 E Lakeshore Dr, Houghton MI 49931 2 Isle Royale and the Keweenaw Peninsula of Michigan are home to some of the oldest examples of native North American metalworking and land use. The overarching objective of this research is to produce a reconstruction of the timing, spatial patterns, and environmental impacts of mining activities on Isle Royale through sedimentological and biogeochemical analysis of lacustrine sediments. We also seek to produce a parallel record of paleoenvironmental conditions in order to assess the potential impacts of environmental change on ancient mining cultures. In 2016, we collected a 7.5 m long sediment core sequence from Lily Lake on Isle Royale, MI. Lily Lake lies approximately 100 m above the current water level of Lake Superior, and formed approximately ~11,000 years before present following the retreat of the Laurentide ice sheet. Lily Lake has been exposed to very little human land use change relative to other lakes on Isle Royale (e.g. there are no ancient mine pits in the immediate catchment), and thus is well suited for reconstructing past environmental changes. We analyzed weakly sorbed metal concentrations using ICP-MS to test hypotheses on the timing and transport mechanisms of potential metal pollution derived from ancient mining activities. In addition, we conducted EAIRMS analysis (including carbon/nitrogen ratios, and the isotopic composition of organic C and N) on bulk organic sediment to provide a record of natural paleoenvironmental changes. Preliminary results from the metals analysis provide evidence of Middle Archaic mining activity that is temporally consistent with radiocarbon dated artifacts and similar evidence from other lakes located adjacent ancient mine pits on Isle Royale and the Keweenaw Peninsula of Michigan. Additional work is required to assess the relative influence of natural versus anthropogenic processes that may have influenced metal concentrations in Lily Lake sediment and to determine a transport mechanism for the putative mining related pollution. This study will provide a record of spatial/temporal patterns of mining activity and paleoenvironmental change in the Great lakes region that will aid in our understanding of large scale continental climate patterns, environmental responses, and the potential influence of climate/environmental variability on ancient land use and mining practices. 110 �Michigan Geological Survey Six years after assignment to Western Michigan University, Where are we today? John A. Yellich, CPG, Director, Michigan Geological Survey The Michigan Geological Survey functionality was reduced in 1978 to conducting minimal research, scientific publications and data management. For the next 30 years, Michigan went through multiple oil and gas booms and busts, Superfund authorization, Leaking Underground Storage Tanks, Brownfields, some mining development, yet no funding for a functioning geological survey. Where could you go to get up to date geologic research or information? What was and is still being used are special publications from the USGS, associations or academia and a 1982 Surficial Geological mapped based on 1915 field mapping, 1955 updates and a color change with some soils in 1982, this is 1915 surface geology only. The Michigan Geological Survey (MGS) was assigned in 2011, by legislation from the DEQ - Office of Oil Gas and Minerals to the Geological and Environmental Sciences (GES) Department at Western Michigan University (WMU), with no funding. MGS has functioned at the GES Department with WMU funding for two years, grants and a Special Appropriations (SA) from the Michigan Legislature in 2016, and has strived to establish a scientific value of a functioning geological survey by presenting programs and projects associated with the current day natural resources needs of Michigan. MGS surveyed the stakeholders and has been assessing some of the components of the noted societal needs, an integral segment of any survey today. A functioning geological survey is not the same as it was 25 or more years ago. Consequently, the users of geologic based data are not just the geoscientist, but regulators, county planners and development organizations, engineers, environmental scientists, extractive and land development industries, citizen scientists, anyone that has “boots” that touch the ground. MGS has completed a significant portion of the demonstration process and presented results to all the State of Michigan functional departments and has received letters of recommendation to support the continued geological research and mapping efforts conducted to date. MGS was also recognized and a resolution submitted to the Governor by the twelve sovereign Michigan tribes as needing a funded functioning geological survey to map and assess the water resources of Michigan. Michigan and many other states have a new contaminant, PFAS, and MGS has presented a geological approach to assess the aerial magnitude of this impact, geology. All these products and projects are scientific societal needs, however, at this time, the Legislature and Governor’s office has not seen fit to have an annual funding mechanism for the Michigan Geological Survey. The Natural Resources of Michigan are and have been an economic foundation and provided societal benefits to Michigan since 1840’s, over a 175 years. The identification and protection of these resources needs sufficient geologic information to assess, protect and manage all the components associated with any natural resources. 111 �Since 2011, the MGS has published 14 quadrangles (4-UP; 10-LP) with four in process within those 6 years, having one full time staff, some faculty and two contractors. There are critical need areas of Michigan that need to be mapped, but it takes a commitment by the State and requests by society for funding. MGS has provided geologic guidance on water and chloride issues in Ottawa County and MGS projects and research have strongly supported geological science in all aspects of identifying, managing and accessing the water resources of Michigan. MGS was instrumental in support of Statewide airborne LiDAR and encouraged Michigan to develop a program to contact the users and identify the benefits. This airborne effort was then done at a reduced cost and with nearly half of the State flown we now have LiDAR that will provide greater benefits to scientists and the public. MGS initiated and completed research utilizing standard geophysical methods and is utilizing new methods and remote sensing to support the 3D mapping projects and derivative geological products. Tromino Passive Seismic, NASA Gravity Recovery and Climate Experiment (GRACE) and Interferometry to assess, bedrock depth and topography, water storage and surface movements, respectively. For example, a City of Portage bedrock valley mapping for water resources, GRACE projections of increased water storage in Cass, St. Joseph, Kent and Ottawa counties has presented scientific research projects that support Michigan natural resources and yet no full time funding. These successful scientific demonstrations have also supported students in MS theses and PhD dissertations. MGS has strongly supported regionally and in Washington, DC the USGS geologic, geophysical and FEDMAP programs for airborne and ground surveys to assess the buried geology, near surface geology, water resources and shoreline stability issues of the Great Lakes areas, to name a few. The geologic community has a voice that has not been loud enough to be heard in Lansing and also, Washington, DC. Geoscientists must tell everyone that to understand our world today and tomorrow, we need geology. For example here we are in Michigan six years later, not knowing if we have sufficient water resources for some areas, questioning scientific data with “Wikipedia” type information, needing an updated geologic map of the UP bedrock and glacial systems and you as geoscientists not loudly proclaiming that validated geology needs to be done in priority areas of Michigan. You are the experts, what should the Geological Survey be doing? 112 �The Origin of Layering in the Olivine Zone, Black Sturgeon Sill, Nipigon, Ontario ZIEG, Michael J. and HONE, Samuel V. Department of Geography, Geology, and the Environment, Slippery Rock University, Slippery Rock, PA 16057 Layering in mafic intrusions is one of the most interesting, but also most controversial, aspects of igneous petrology. In this study, we explore the role of phenocrysts (entrained crystal cargo) versus in-situ fractionation in controlling layer development. Samples were taken from a continuous drill core through the Black Sturgeon sill (BSS), a 250 m mafic intrusion with a welldeveloped olivine zone from 120-200 m above its base. We analyzed bulk-rock geochemistry, modal mineralogy, and textures at 0.5 m intervals through this range, then used the resulting data set to investigate the petrogenetic processes responsible for the geochemical and petrographic variations in this part of the sill. Principal component analysis was used to characterize and summarize variations in the standardized and log-normalized major and minor oxide abundances. The first three principal components are the most significant, accounting for over 90% of the system variance. Based on these components, the rocks in the olivine zone fall into four distinctive compositional groups (Fig. 1). We interpreted the petrogenetic significance of the principal components by comparing them to trace elements, modes, norms, textures, and fabrics. In this initial comparison, we identified strong correlations between: the first principal component (PC1) and Ni-Sr (Fig. 2a); the second principal component (PC2) and incompatible element abundances, particularly Cu (Fig. 2b); the third principal component (PC3) and Sc (Fig. 2c). Thus, we conclude that PC1 is controlled by the ratio of olivine to plagioclase, PC2 is controlled by the abundance of a fractionated interstitial liquid component, and PC3 is controlled by augite abundance. Three fundamental observations are critical for understanding the significance of our results. (1) The olivine zone consists of four segments (a-d) defined by variations in the first principal component (Fig. 3a), which reflect the relative importance of olivine and plagioclase. (2) The olivine zone has a single coherent Z-shaped profile for PC2, controlled by smooth variations in incompatible element abundances (Fig. 3b). (3) PC2 and PC3 are positively correlated in Segments a, c, and d; they are negatively correlated in Segment b (Fig. 3c). Each of the four segments represents a distinct batch of magma, with its own characteristic phenocryst assemblage. The average ratio of olivine to plagioclase generally increased upwards, suggesting either an upward increase in source primitivity or “subcretion” of increasingly evolved magma pulses. All segments intruded rapidly compared to solidification time; after emplacement was complete, crystal-mush compaction drove evolved interstitial liquids from Segment b up into Segment d. The relationships between PC2 and PC3 suggest that augite crystallized after compaction-driven redistribution of evolved liquids in Segments a, c, and d. In Segment b, however, it was part of the crystal cargo, and Sc was not depleted (as Cu was) by the expulsion of interstitial liquids. In conclusion, the emplacement of multiple pulses of magma, each entraining a unique crystal cargo, controlled the basic layering structure and the major-oxide variability of the BSS olivine zone. Compaction-driven redistribution of interstitial liquids significantly modified trace element abundances, producing cryptic compositional layering incongruent with the modal assemblages. Although our results only address the formation of layering in this specific intrusion, the procedures we have developed can be applied to any system. 113 �Figure 1. Compositional groups. Four compositional groups can be distinguished. Figure 2. Interpretation of PCs. (a) PC1 reflects the olivine:plagioclase ratio. (b) PC2 reflects incompatible abundance. (c) PC3 reflects augite abundance. Figure 3. Stratigraphic profiles. (a) The olivine zone can be divided into four distinct segments. (b) Incompatible element abundances suggest compaction-driven redistribution of interstitial liquids. (c) Augite is correlated with incompatibles in Segments a, c, and d, but not in Segment b. This suggests that augite was a phenocryst phase in Segment b, but not in Segments a, c, or d. 114 � https://digitalcollections.lakeheadu.ca/files/original/b037a57a11eb6107fb3048857e8d3eee.pdf 399d169e2e63eb43833bac42fcefd842 PDF Text Text �Institute on Lake Superior Geology 64th ANNUAL MEETING May 15-18, 2018 Iron Mountain, Michigan SPONSORED BY: U.S. GEOLOGICAL SURVEY AND WISCONSIN GEOLOGICAL AND NATURAL HISTORY SURVEY Meeting Co-Chairs Laurel Woodruff, William Cannon, and Esther Stewart Proceedings Volume 64 Part 2: Field Trip Guidebooks Compiled by William F. Cannon Cover Photos: 1. Sandstone of Munising Formation (Upper Cambrian) lying on the Vulcan Iroin-formation at Groveland Iron Mine. Seen on trip 1. Photo by William Cannon. 2. Pillowed basalt of the Hemlock Formation at Way Dam, Michigan. Seen on Trip 2. Photo by Thomas Waggoner., 3. Dave’s Falls on the Pike River near Amberg, Wisconsin. Bedrock is the Athelstane Quartz Monzonite cut by diabase dikes. Seen on trip 4. Photo by William Cannon. 4. Quinnesec Iron Mine on the Menominee Iron Range, Michigan. Seen on Trip3. Photo by William Cannon. �64TH INSTITUTE ON LAKE SUPERIOR GEOLOGY VOLUME 64 CONSISTS OF: PART 1: PROGRAM AND ABSTRACTS PART 2: FIELD TRIP GUIDEBOOK TRIP 1: ARCHEAN AND PALEOPROTEROZOIC GEOLOGY OF THE FELCH DISTRICT, CENTRAL DICKINSON COUNTY, MICHIGAN TRIP 2: GEOLOGY OF THE HEMLOCK FORMATION TRIP 3: GEOLOGY AND IRON ORES OF THE MENOMINEE IRON RANGE, DICKINSON COUNTY, MICHIGAN TRIP 4: GRANITOID ROCKS OF THE PEMBINE-WAUSUA TERRANE IN NORTHEASTERN WISCONSIN Reference to material in Part 1 should follow the example below: Authors, 2018, abstract title, 64th Institute on Lake Superior Geology Proceedings, v. 64, Part 1, Field Trip Guidebook, p. xx. Proceedings Volume 64, Part 1: Program and Abstracts, and Part 2: Field Trip Guidebook are published by the 64th Institute on Lake Superior Geology and distributed by the Institute Secretary: Peter Hollings Department of Geology Lakehead University Thunder Bay, ON P7B 5E1 CANADA peter.hollings@lakeheadu.ca Some figures in this volume were submitted by authors in color, but are printed grayscale to conserve printing costs. Full color imagery will appear in the digital version of the volume when it is available on-line at: http://www.lakesuperiorgeology.org ISSN 1042-9964 ��Part 2: Field Trip Guidebooks Table of Contents Trip 1: Archean and Paleoproterozoic Geology of the Felch District, Central Dickinson County, Michigan 1 Trip 2: Geology of the Hemlock Formation 39 Trip 3: Geology and Iron Ores of the Menominee Iron Range, Dickinson County, Michigan 69 Trip 4: Granitoid Rocks of the Pembine-Wausua Terrane in Northeastern Wisconsin 107 �FIELD TRIP 1 Tuesday May 15, 2018 ARCHEAN AND PALEOPROTEROZOIC GEOLOGY OF THE FELCH DISTRICT, CENTRAL DICKINSON COUNTY, MICHIGAN William F. Cannon, Klaus J. Schulz, Robert A. Ayuso, U.S. Geological Survey Thomas H. Mroz, BSGE, MSPG, CPG INTRODUCTION This trip examines the stratigraphy, structure, and economic geology of Precambrian rocks near the town of Felch in central Dickinson County of northern Michigan. The location of the area is shown on Figure 1 relative to other well-documented structures and iron ranges of the region. Precambrian rocks range in age from Archean to Paleoproterozoic, and outliers of Cambrian sandstones are also widespread. Relationships along the basal Cambrian unconformity are included in the trip. Much of the interest in the area, both geologically and economically, has been focused on the Felch trough (Figure 2) where Paleoproterozoic rocks of the Chocolay and Menominee Groups form a complex syncline that is infolded and infaulted with Archean gneisses. Minor amounts of iron ore were produced in the early history of the area and a major concentrating-grade mine, the Groveland, was active in the 1960’s and 1970’s. The geology of the area was mapped and described in detail by the U.S. Geological Survey in 1950’s and published in 1961 (James et al., 1961). The information in that report is the basis for much of the descriptive material in this guide. A few more recent studies have added significant additional data and geochronological constraints that clarify some of the relationships between various rock units and the events that formed them. An underlying theme of the trip is that a substantial amount of additional research is warranted to fully understand this complex region and that relatively abundant outcrops, along with newly acquired high-resolution geophysical data, make this a very attractive region for a wide variety of research. Stratigraphy. The long-accepted stratigraphic sequence of Archean and Paleoproterozoic rocks that was defined by James et al. (1961) requires some modification based on more recent radiometric age determinations. Some of these are discussed in more detail below in individual stop descriptions. A tentative correlation chart in Figure 3 reflects these suggested changes, but additional work seems warranted to clarify some aspects before proposing formal changes to stratigraphic nomenclature and correlations. 1 �Figure 1. Generalized geologic map of part of the Upper Peninsula of Michigan showing the location of central Dickinson County in relation to major structures and iron ranges of the region. Modified slightly from James et al. (1961, plate 1). The oldest rocks of the region are complex gneisses of the Carney Lake Gneiss and other probably correlative units in structurally separated fault blocks. They have long been considered Archean. Recent geochronology of samples collected south of the fieldtrip area using the USGS/Stanford Sensitive High Resolution Ion Microprobe (SHRIMP) produced U-Pb data on zircons that confirm an Archean age (Ayuso et al., 2017; 2018). Two samples were collected for radiometric dating from the southern half of the Carney Lake complex: 1) sample 1 is from a granitic K-feldspar-bearing gneiss that is locally pegmatitic; 2) sample 2 is from a banded and folded gray to red granitic gneiss. Abundant zircons (70-200) were obtained from sample 1 that range from anhedral to subhedral, contain complex igneous and irregular growth zoning, and multiple growth rims; these zircons have irregular to pyramidal overgrowths. The zircons from sample 2 range from slightly rounded to subhedral and are otherwise mostly similar to zircons from sample 1. A total of 129 cores and rims were analyzed. Individual zircons have older ages near their cores (mostly discordant) and younger ages near their rims (Figure 4A). 2 �3 �Figure 3. Proposed correlation chart for the area of Field trip 1. Modifications from previous correlations are based on recent radiometric determinations that provide direct ages for some units and place constraints on the ages of others. On a concordia diagram, U-Pb data plot as clusters of data points ranging from concordant to discordant and suggest several chords and intercepts that are common to both samples from the Carney Lake Gneiss (Figure 4B). That study identified cores of individual zircons as old as 3.8 Ga. The most common age for individual zircons and for rims on older grains is about 2.75 Ga and records a younger major event in the late Archean. James et al. (1961) defined the Dickinson Group, consisting of the East Branch Arkose, Solberg Schist, and Six Mile Lake Amphibolite, as Archean based on field relationships between the Six Mile Lake Amphibolite, the upper formation of the group, and presumed Archean gneisses to the south. However, U-Pb dating of detrital zircons in the East Branch Arkose has shown that it must be 2.1 Ga or younger (Craddock et al., 2013). We suggest that the Six Mile Lake Amphibolite is also Paleoproterozoic because of chemical similarities between it and other Paleoproterozoic mafic rocks. The Archean age proposed by James et al. (1961) was based on field relationships 4 �between amphibolites and Archean granites and gneisses that require further evaluation. The Solberg Schist is intruded by a gabbro that has the distinctive composition of the Six Mile Lake Amphibolite (see Stop 6 for discussion). James et al. (1961) described the contact between the East Branch Arkose and Solberg Schist as transitional. Further study of the Dickinson Group is clearly required to resolve the age relationships of its three formations. Figure 4. A- BSE (back scatter electron) image of a zircon from the Carney Lake Gneiss showing ages of four analyzed spots. B- Concordia diagram for 129 spot analyses from zircons in the Carney Lake Gneiss. A distinctive granite in the area referred to informally as the “porphyritic red granite” by James et al. (1961), has now been dated using SHRIMP U-Pb on clear and pale brown zircons at 2.1 Ga (Ayuso et al., 2018; and discussion of Stop 11). The porphyritic red granite provides a local source for the relatively common detrital zircons of that age found in the East Branch Arkose of the Dickinson Group (Craddock et al., 2013). These zircons also suggest an erosional interval after 2.1 Ga to unroof the granite prior to deposition of the Dickinson Group. The Dickinson Group is overlain by the Hemlock Volcanics northwest of the field trip area. Those volcanics are dated at 1.875 Ga (Schneider et al., 2002) and establish a minimum age for the Dickinson Group. The Chocolay Group, consisting of the glaciogenic Fern Creek Formation, Sturgeon Quartzite, and Randville Dolomite, is well constrained to have been deposited between 2.3 Ga, the age of detrital zircons within it, and 2.1-2.2 Ga, the age of xenotime cement in the sediments (Vallini et al., 2006). The Menominee Group, including the classic stratigraphy of the Vulcan Iron-formation and underlying Felch Formation, is nowhere in contact with the Dickinson Group although the two are not far separated in the Felch area (see Figure 2). They may be largely time equivalent in spite of rather pronounced lithologic differences. The Michigamme Formation lies at the top of the stratigraphic succession across the region and has been widely interpreted to have an unconformable contact with the underlying units. The Michigamme and underlying Hemlock Volcanics are intruded by the Peavy Pond Complex to the 5 �northwest of the field trip area. A recent SHRIMP U-Pb date on honey-colored euhedral to subhedral zircons for the Peavy Pond Complex yielded an age of 1.85 Ga (Figure 5) (Ayuso et al., this volume). The results place a minimum age on at least the lower part of the Michigamme Formation. This presents something of an enigma in that elsewhere in the Upper Peninsula the Sudbury impact layer, which was deposited at 1.85 Ga, lies at or near the base of the Michigamme Formation (Cannon et al., 2010). It seems, therefore, that rocks assigned to the Michigamme Formation in the Felch area could be substantially older than the Michigamme Formation in much of the larger region of the Upper Peninsula. Figure 5. A- BSE (back scatter electron) image of a zircon from the Peavy Pond Complex showing the SHRIMP U-Pb age of an analyzed spot. Bright spots are residual gold coating from SHRIMP analyses. B- Concordia diagram for 32 spot analyses of zircons from the Peavy Pond Complex. Tectonics. Rocks in the Felch region record multiple orogenic events from 3.8 Ga to younger than 1.83 Ga. Archean rocks are very complexly deformed and metamorphosed by at least two Archean orogenies, one at 3.8 Ga and a later 2.75 Ga event. They also record one or more deformational and metamorphic events in the Paleoproterozoic. Although the work of James et al. (1961) mapped and described much of the Archean of the area, detailed structural studies have not been done, and much remains to be learned about the sequence of rock-forming and deformational events recorded in these complex gneisses. More study has been devoted to the tectonics of Paleoproterozoic rocks and at least an outline of their tectonic history is known, although much additional detail seems likely to be decipherable with futther examination. Only two published structural studies based on substantial new field data have been published since 1961. Ueng and Larue (1988) defined four tectonic terranes and six phases of deformation within Paleoproterozoic strata. Klasner and Sims (1993) proposed a somewhat different sequence and suggested that the set of faults that dominate the map pattern near Felch were backthrusts formed in later phases of the Penokean orogeny at about 1.85 Ga. Both of those studies were conducted prior to significant geochronologic constraints determined over the past two decades. They ascribed all deformational events to the Penokean orogeny, 6 �which by that definition was a prolonged and complex event including both lateral and horizontal shortening phases. More recently, several geochronologic studies show that the tectonic sequence is much more complex and that the region bears the imprint of two other orogenies after the Penokean. A sequence of Archean-cored gneiss domes surrounded by Paleoproterozoic sedimentary and volcanic rocks across central Minnesota, northern Wisconsin, and northern Michigan, once thought to be late Penokean structures, was documented to have formed in mid geon 17 (Schneider et al., 2004). This gneiss dome corridor was interpreted to have been related to subduction during the Yavapai orogeny at about 1.75 Ga. Archean masses near Felch are part of the gneiss dome corridor and, although now dismembered by later faulting, record that Yavapai basement doming event rather than Penokean deformation. A distinctive aspect of the Felch region is a set of easterly and ENE-trending faults that cut all other structures (see Figure 2). They result in a sequence of fault slices that dominate the map pattern of the region. A comparable array of such faults is not known elsewhere in the region. This faulted domain is bounded on the north by the Bush Lake fault, across which there is a sharp discordance in structural trends, nearly 90° in places. The age of the faulting can be constrained by the fact that it offsets the 1.85 Ga Peavy Pond Complex to the west of the field trip area. It also has been shown to offset metamorphic isograds (Attoh and Klasner, 1989) of the Peavy node where peak metamorphism has been dated at 1.83 Ga (Schneider et al., 2004; Holm et al., 2007). Attoh and Klasner (1989) also documented a change in metamorphic pressures across the Bush Lake fault with peak pressures of 4.8 kbar to the south and 3.3 kbar to the north. This suggests that fault motion was south side up with vertical displacement on the order of five kilometers. This set of faults also partly dismembers the gneiss dome structures related to the mid-geon 17 Yavapai orogeny and suggests that they are post-Yavapai structures. An imprint of the geon 16 Mazatzal orogeny in this region was documented by Romano et al. (2000), who showed a heating event of 300-350oC across the Felch region in late geon 16. Whether this heating was accompanied by deformation has not been determined in the Felch region, but to the south, in Wisconsin, strong deformation of Baraboo interval quartzites shows conclusively that strong Mazatzal-aged deformation was widespread. It seems reasonable, with our present level of understanding of the tectonics of the Felch region, that the easterly-trending set of faults is the northernmost manifestation of Mazatzal deformation. Metamorphism. In a classic work on regional metamorphism, James (1955) identified three metamorphic nodes within Paleoproterozoic rocks in the Upper Peninsula of Michigan. One of those, the Peavy node, encompasses the area of this field trip. Most stops lie within the staurolite zone and the effects of recrystallization are obvious in outcrop. Since James’ work, radiometric dating and more modern metamorphic petrology studies have further defined the character of the metamorphism of the region. The age of peak metamorphism of the Felch node has been determined as circa 1.83 Ga from dates derived from metamorphic monazite (Schneider et al., 2004; Holm et al., 2007) measured in Archean gneiss near Foster City. Peak temperature and pressure of metamorphism were about 600-650oC and 5 kbars (Attoh and Klasner, 1988). A younger amphibolite facies metamorphism was documented at 1.78-1.74 Ga related to Yavapai accretion and gneiss doming (Holm et al., 2007). Much older metamorphism, probably culminating at about 2.75 Ga, is evident within Archean rocks, many of which are gneisses and migmatites. The Hardwood Gneiss, seen at Stop 7, 7 �records granulite facies conditions with estimated pressures of 8.2-11.6 kb and temperatures of ~770°C for an initial event, and conditions of 6.0-10.1 kb and temperatures of 610-740oC for a second event inferred to be Paleoproterozoic but “pre-Penokean” (Peterson and Geiger (1990). Economic geology. Iron, mostly related to the Vulcan Iron-formation, has been the principal commodity of interest in the Felch region. Early exploration and attempts to develop mines are summarized by James et al. (1961). The first indication of ore in the Felch trough was a description of a ridge of high grade iron ore in eastern Iron County by Jacob Houghton and reported to William Burt, a government land surveyor, in Marquette County in 1846 and was recorded in Senate Documents for the 31st Congress (Jackson, 1849). At what later became the Groveland mine a veneer of oxidized Vulcan Iron-formation was found on the south side of the ridge that drew the attention of early developers. But once operations started stripping a pit, the veneer was found to be only several feet thick and did not extend to depth as originally thought. The first development was an economic disaster as monies had been invested in town sites and the extension of a railroad to the Escanaba iron furnaces. Being not far from more prospective areas such as the Menominee Range to the south, the Felch area was heavily prospected and numerous attempts at mining occurred between 1880 and 1913. Only four mines of any significance were developed in the Felch area and produced a total of about 625 million tons of ore, mostly of low grade. These direct-shipping ores were soft masses of hematite and goethite that were found directly beneath the unconformity between the Vulcan Iron-formation and overlying Cambrian sandstones. They are widely accepted to be paleosupergene deposits formed by late Precambrian and/or early Cambrian weathering. The Felch region differs from much more productive nearby regions, such as the Menominee Range and Iron River-Crystal Falls district, in being substantially metamorphosed. Rocks, including the Vulcan Iron-formation, are coarse-grained as a result of metamorphic recrystallization and thus are less susceptible to the paleo-weathering than comparable iron-formations in other nearby districts where large paleosupergene ore bodies were formed. With the advent of large-scale concentrating and pelletizing technology in the 1950’s, portions of the Vulcan Iron-formation became targets for concentrating-grade ore production. The Groveland area proved to have sufficient tonnage of iron-formation accessible by open-pit mining to allow development of the Groveland mine, a significant iron-producer in the 1960’s and 1970’s. The potential for mineral production in this area has dropped in recent decades as exploration projects, such as for uranium and diamonds, have failed to find deposits of current economic viability. There are active gravel pit operations, reprocessing of Groveland Mine waste rock dumps for crushed stone, and small quarries in the Randville Dolomite for decorative stone. This limited activity is the current extent of mineral resource exploitation in central Dickinson County 8 �FIELD TRIP STOPS Stop 1. Groveland mine. (45.988°N, 87.981°W) The geology within the long-abandoned open pit of the Groveland mine is not accessible for field trips because of flooding, slumping of pitwalls, and safety concerns of current owners. The mine property is fenced and is not accessible to the public. We have been given access to the property to examine material on large waste-rock piles that show good examples of the various lithologies of the Vulcan Ironformation, and provide views into parts of the flooded pit. Figure 9 shows the present surficial character of the mine area and the location of the waste piles available for observation and sampling. Figure 6. The Groveland Mine has a long history that began in the late 1880’s and continued into the 1980’s. This view, probably from the 1970’s, is looking south with the plant in the upper central part of the air photo. Source M. A. Hanna Company. Archives of the Michigan Department of Environmental Quality. History. Mining of iron ore at Groveland began as an underground operation in 1891 on outcrop of the Vulcan Iron-formation identified in 1846 by assistants to William Burt, the government land surveyor. The operation was abandoned after only a few years of operation due to the lack 9 �of direct shipping ore. The mine was reopened in 1901 and mined for four years by Corrigan, McKinney & Company. In 1907 the mine was again started up by the Groveland Mining Company. and had production through a 294 foot deep, three compartment shaft with levels at 70, 140,` and 210 feet. Iron content remained a problem and production ceased in 1913. It was reported that the last shipment had unacceptable iron content and was dumped into Lake Erie. Several companies gained ownership of the properties and in 1926 test pits and trenches were completed by an independent developer, Mr. R. M. Adams. In 1948 the properties were consolidated through leases by M. A. Hanna Co. and in 1951 the Grovelend became their first taconite project. A pilot project ran for six months to develop grinding and concentration processes of the jasper ores, but it took seven years to develop a viable process to treat the complex ore, which is unique because of its mineralogy and metamorphism. The ore is very coarse grained and was defined by the operators as three types; magnetite, magnetite silicate, and specular hematite (Figure 7). In 1957 construction of the concentrating and pelletizing plant began and in 1959 the plant became operational with an output of 700,000 tons of concentrate annually. The mine became the second concentrating grade (taconite) operation in Michigan, following closely the opening of the Humbolt mine on the Marquette range. In 1963 a traveling grate pellet plant was completed with a capacity of 1.25 million tons per year. The $35 million expansion also included a concentrator upgrade and production was increased to 1.6 million tons annually. In 1968 a fourth line was added and resulted in an annual capacity of 2 million tons. 1977 was a record year with the output of iron concentrate reaching 2.1 million tons. Overall, the expenditures totaled $70 million, and employed 530 resulting in an investment of $132,000 per employee. Annual payroll was $12.5 million and taxes provided $1.332 million of revenue to the state and local governments. Operating services and supplies were $31.5 million for the local economy. In 1980 the mine closed after producing about 36 million tons of pelletized iron concentrate. Portions of the fresh water ponds have been developed into recreational fishing areas for the public. Geology. The most complete geologic description of the Groveland deposit is by Cumberlidge and Stone (1964), two geologists with M.A. Hanna Mining Company, and was based on extended observations as the present pit was developed. They showed that the ore body formed the keel of a complex doubly plunging syncline that was overturned to the south. The thickest extent of the Vulcan Iron-formation was along the south limb as shown in the plan map and cross section, (Figure 8). The north overturned limb was capped by Cambrian sandstone prior to stripping, and initial underground development was probably in the iron-formation subcrop below the sandstone contact. The Vulcan Iron-formation is divided into three informal members at the Groveland mine (Figure 8): 1) the lower Vulcan consisting mostly of hematitic jasper, 2) the middle Vulcan is dominantly even-bedded magnetite-silicate iron-formation with lesser hematitic jasper, and 3) the upper Vulcan Iron-formation is uniformly bedded magnetite-silicate iron-formation. The mine is located in the staurolite zone of regional metamorphism, and iron silicates in the Vulcan Iron-formation are commonly coarse-grained reflecting that intense recrystallization. The most common silicate minerals identified by Cumberlidge and Stone (1964) are Ca-Mg hornblende, tremolite, and actinolite. Pyroxene, biotite, and cummingtonite are less abundant and garnet is rare. Some representative photographs of the geology of the mine area is shown in Figure 7. 10 �Figure 7. Photographs from the Groveland mine. A- Unconformity between flat-lying sandstone of the Munising Formation and weathered Vulcan Iron-formation on the north wall of the Groveland pit. B- Folded jasper–specularite iron-formation, C-Lenticular beds of oolitic jasper with specularite interbeds, D-Silicate-magnetite iron-formation with large sheaves of amphibole. . 11 �Figure 8. Geologic map and cross section (with magnetic and gravity profiles) of the Groveland mine area. The Groveland pit was developed in the thickest part of the Vulcan Iron-formation in the eastern part of the map. Source: M.A. Hanna Mining Company from archives of the Michigan Department of Environmental Quality. Stratigraphic section summarizes descriptions of the Vulcan Iron-formation and related units as reported in Cumberlidge and Stone (1964) provided by Thomas Waggoner (personal communication). Unit names are informal mine terminology. . 12 �Figure 9. False color LiDAR image of the area of the Groveland mine showing the location of Stop 1 and outcrops as mapped by James et al. (1961) prior to mine development. 13 �Figure 10. Location of Stops 2 and 3 and the location of holes drilled for uranium exploration near Stop 2. Image from Google Earth. Stop 2. Archean gneiss at Gene’s Pond. (46.058o N, 87.855 o W) At the boat launch site at the west end of Dixon Road are several outcrops of Archean granitic gneiss and mafic dikes that cut the gneiss. The granitic rocks are mostly plagioclase porphyritic rocks with a moderately to well developed, nearly vertical shear foliation (Figure 11A). The mafic dikes cut the foliation. They appear largely massive and undeformed in outcrop but are thoroughly amphibolitized and the amphiboles show a weak alignment in thin section (Figure 11B). Based on landforms, we interpret that there are outliers of Munising Formation (Upper Cambrian sandstone) both east and west of this locality, and further interpret that the present land surface here is very nearly the exhumed unconformity at the base of the Cambrian. The unusual reddish hue of much of the granite may be a reflection of weathering or alteration along the unconformity (see Figure 11A and 12). 14 �Figure 11. A- Moderately sheared Archean granite. B- Massive mafic dike with blocky fracture. Figure 12. Photomicrographs of sheared granitic rock at Stop 2. Rock is mostly plagioclase with moderately developed cataclastic textures. Nearly all plagioclase grains are stained with submicroscopic hematite (?). A- Plane polarized light, B- Crossed Nichols. A rather extensive exploration effort for unconformity type uranium deposits was undertaken here in the early 1980’s by Minatome Corporation (Hunter, 1986; Lehman, 1987). This included twenty shallow drill holes shown on Figure 10, one of which was only a few tens of meters south of these outcrops. Uranium, occurring as pitchblende, was found as open-space fillings along with calcite, hematite, and minor chlorite. The drilling defined an E-W brittle fault that dips about 60o north and is subparallel to an older mylonitic fault. The mineralized assemblage heals breccias that are most common in the hanging wall of the brittle fault. The drill holes were located to test the depth extent of surface radioactive occurrences but found that no mineralization extended more than 85 m below the surface and that the surface extent of mineralization of individual occurrences was about of equal vertical dimensions. U-Pb dating of the pitchblende (Lehman, 1987) yielded a range of results, all of which were of Paleozoic or younger age. A likely conclusion is that the mineralization formed in Paleozoic or younger times just below the unconformable contact of Cambrian sandstone and Archean gneiss. This exploration and its results have never been described in detail, but drill logs and core for most of the holes are available for study at the Michigan Geological Sample Repository in Harvey, Michigan. 15 �Stop 3. Randville Dolomite at Gene’s Pond. (46.072 o N, 87.866 o W.) A small lakeside outcrop just south of the boat launch at Gene’s Pond public access site displays many of the typical features of the Randville Dolomite in the northern part of the Felch area. In much of the Felch area strong metamorphism has converted the Randville Dolomite into coarsely crystalline white to gray marble. Much of the primary structure is obliterated. However, Stop 3 lies north of the highest grade metamorphic zones and metamorphic recrystallization is only minor with abundant primary features preserved. Here, the Randville underlies a large area between the Bush Lake fault on the north and the Norway Lake fault on the south. It has been described in some detail by Clark (1961, Chapter C of James et al., 1961). Unfortunately, the more illustrative Randville outcrops described in detail by Clark are not easily accessible for field trips. The small outcrop at Stop 3 is shallowly dipping and well bedded dolomite with undulose, generally upward-domed, bedding that is likely stromatolitc structures (Figure 13) Figure 13. Randville Dolomite at Stop 3 showing undulose bedding, probably reflecting weakly developed stromatolitic mounds. The general description of the Randville provided by Clark (1961, p. 107-109) is “The Randville dolomite is apparently divisible into three members: (1) an upper member and (2) a lower member of dolomite with minor interbedded slate, separated by (3) a slate member with minor interbedded dolomite…. The total thickness is more than 800 feet in the Norway Lake area. Neither the top nor the bottom of the formation is exposed, and the character of the rocks that immediately underlie and overlie the Randville dolomite is not known. …… Most of the dolomite 16 �is massive to thin bedded, and stromatolites (algal structures) and intraformational conglomerate are common. The dolomite is light gray to red on fresh surfaces and weathers white to light brown. It has a fine sugary texture. Grains of quartz sand, most of which show undulatory extinction, are abundant in some beds and in some places comprise more than 50 percent of the rock. No oolites were found. The stromatolites, in sections normal to bedding planes, are concentrically banded structures with domal or columnar form, and in sections parallel to bedding planes they are concentrically banded elliptical forms. Most are 1 to 3 inches in diameter and 2 to 6 inches high. The banding of the stromatolites is convex upward, providing a reliable criterion for tops of beds. Where the structures are partly replaced by chert the forms are accentuated on weathered surfaces. Intraformational conglomerates or breccias are present ….. Most of the pebbles are dolomite, but a few pebbles of dolomitic slate occur. The pebbles are 1 to 4 inches in diameter and are well rounded. No strong dimensional orientation is evident. The matrix is dolomite with intermixed coarse quartz sand.” The slates, as described by Clark (1961) are dark gray to gray-green and are composed largely of sericite with some quartz, chlorite, and microcline. Graded beds are common. Stops 4, 5, and 6. The Dickinson Group The three formations originally defined as the Dickinson Group (James et al., 1961), 1- the East Branch Arkose (Stop 4), 2- the Solberg Schist (Stop 5), and 3- the Six Mile Lake Amphibolite (Stop 6) will be examined from north to south, the originally interpreted stratigraphic order. Stratigraphy and age- The Dickinson Group was originally defined by James et al. (1961) to include three formations, from presumed oldest to youngest, the East Branch Arkose, the Solberg Schist, and the Six Mile Lake Amphibolite, which they interpreted to be in conformable contact with each other. The East Branch Arkose is a sequence of arkosic conglomerate and sandstone with interbedded mafic volcanic rocks. The Solberg Schist consists of finer clastic and volcanic rocks with at least one interbedded banded iron-formation, the Skunk Creek Member. The Six Mile Lake Amphibolite was interpreted to be highly metamorphosed mafic volcanic rocks with abundant granitic intrusions. James et al. (1961) provided detailed geologic maps and descriptions of each unit. This discussion is based largely on their work. Each unit has a maximum thickness of 2,000 to 4,000 feet, and James et al. (1961) state that a total thickness of 10,000 to 12,000 feet for the group is indicated. This original work was done before radiometric ages were available so relative ages were based on field relationships. The supposed lower formation of the group, the East Branch Arkose, seems clearly to lie unconformably on Archean gneisses to the north as indicated by numerous gneissic pebbles in conglomerates of the East Branch Arkose. To the south, James et al. (1961) believed that the Six Mile Lake Amphibolite was intruded and altered by a large batholith of Archean age, so ascribed a late Archean age to the group. More recently, the acquisition of radiometric data has modified the permissible age range for the Dickinson Group. All units of the group were recrystallized during regional metamorphism related to the Peavy metamorphic node (James, 1955). That metamorphism has been dated at 17 �approximately 1.83 Ga, the age of metamorphic growth of monazite (Holm et al., 2007), thus providing a minimum age for the group. Detrital zircons reportedly from the East Branch Arkose show a spectrum of ages (Craddock, et al, 2013) that includes a strong, well defined, peak at approximately 2.1 Ma which would establish a maximum age for the East Branch. Unfortunately the coordinates given by Craddock et al. (2013) for the sample correspond to a roadcut of Solberg Schist, not East Branch Arkose, putting some uncertainty on interpretation. But if the Solberg is conformable with the East Branch, as interpreted by James et al. (1961) the results still provide a constraint for the group as a whole even if the sample is from the Solberg. Thus, the available radiometric age constraints place the East Branch Arkose and possibly Solberg Schist sedimentation and volcanism within the range of 2.1 to 1.83 Ga, similar to other Paleoproterozoic strata of the region. We suggest a reinterpretation of the Dickinson Group in which it is entirely Paleoproterozoic. The age of the Six Mile Lake Amphibolite is problematic in this interpretation in that James et al. (1961) described a southward transition of amphibolites into gneisses that are clearly of Archean age. Whether these amphibolites are a part of the Six Mile Lake or some older sequence is not clear at present and requires further evaluation. Lithology- The following lithologic descriptions are summarized entirely from the detailed descriptions provided in James et al. (1961). East Branch Arkose: As described by James et al., (1961. p. 13-14), “The formation consists of thick-bedded arkose with many beds of coarse conglomerate, interbedded with metamorphosed tuffs and basic volcanic flows. The conglomerates, though not the dominant rock type, are the most striking feature of the formation. The beds typically are 10 to 30 feet in thickness. The pebbles in the conglomerate have been drawn out into lenses that, on a horizontal surface across the nearly vertical beds, have a length-to-width ratio of about 3:1. In most parts of the area this shearing is parallel to bedding, that is, eastward but in a few places it is at an angle. Linear structure is not pronounced; most of the flattened pebbles have a length in vertical section about equal to that in horizontal section. In a few places a nearly vertical linear structure marked by grooving of the pebbles is evident. Vitreous quartzite is the dominant rock type among the pebbles, with granite gneiss, slate or schist, and quartz being of lesser abundance. Some of the quarzite pebbles show well-defined bedding. The arkose is pink to gray, massive, and abundantly cross-bedded. In many outcrops and in hand specimens it closely resembles a granite gneiss, but the well-defined crossbedding is complete proof of its sedimentary origin. In the more southerly outcrops of the East Branch arkose, dark-gray fine-grained tuffs are interbedded with the arkose; the rock consists principally of quartz and untwinned feldspar, with scattered grains of epidote, biotite, and carbonate. Rounded grains of opalescent quartz are present in some layers. Metabasalt flows are not uncommon. The rock is black and hornblende-rich. In the outcrops in the NW sec. 17, T. 42 N., R. 28 W., the originally scoriaceous top of one of these flows can be seen. Some of the metamorphosed flows are moderately magnetic and give rise to the aeromagnetic anomalies shown on the general map of the district.” Solberg Schist- James and et al. (1961, p. 17-18) describe the Solberg Schist as follows. “The Solberg schist lies immediately south of the East Branch arkose. A considerable amount of interbedding of arkose and schist is evident in the outcrops, so that location of the contact between the units is somewhat arbitrary. . . . The more northerly exposures of the unit consist 18 �chiefly of dark fine-grained hornblende and biotite schists. Locally, muscovite is an abundant constituent. Some outcrops show a banding, essentially parallel to the foliation, which may represent original layering. In one place, near the south edge of sec. 13, T. 42 N., R. 29 W., the schist is interbedded with coarse clastic material similar to rock of the East Branch arkose. The more southerly exposures of the Solberg schist consist of quartz-mica schist, parts of which might be better termed micaceous quartzite. This rock is exposed in the north part of sec. 24, T. 42 N., R. 29 W., and secs. 21 and 2, T. 42 N., R. 28 W. In general, the rock is massive, gray, and well banded. The banding, which consists of alternation of quartzitic and micaceous layers, almost certainly represents original bedding. Linear structure is strongly developed in all the schists, especially in the hornblendic varieties. It is marked by orientation of hornblende needles and by biotite. The lineation is in the plane of foliation and in general plunges eastward at a low angle.” The Skunk Creek member of the Solberg schist is a bed of iron-formation. This bed gives rise to a very strong magnetic anomaly, by means of which the iron-formation can be traced in a belt across most of the map area. . . .The Skunk Creek member has been penetrated by drill holes in several places. It is chiefly from this drill core that information concerning the lithologic character has been obtained, although none of the holes has cut the entire unit. The distinctive part of the formation is a thin-banded rock consisting of alternating layers of granular quartz (probably originally chert), magnetite, and various mixtures of hornblende, biotite, grunerite, garnet, and epidote. This material grades into biotite-hornblende schist, containing magnetiterich layers, by interbedding at both the upper and lower contacts. The iron-formation is cut by many thin dikes of coarse pegmatite; garnet, and tourmaline are commonly developed near the contacts. The thickness of the Skunk Creek member is somewhat uncertain because of the small amount of data available, but it is about 100 feet.” Six Mile Lake Amphibolite- James et al. (1961, p. 18-19) described the Six Mile Lake Amphibolite as follows. “In outcrops the amphibolite is a dark almost black massive fine- to medium-grained rock in which hornblende is the major constituent. In thin section feldspar (andesine or oligoclase) is abundant; in hand specimens it is less noticeable but gives a faint salt-and-pepper appearance to the rock, especially on surfaces broken across the foliation. Compositional layering is evident in some places, but in general the rock is homogeneous. The more southerly outcrops approach banded gneiss in character. Foliation parallel to the compositional layering is generally present, but may be subordinate to a strong linear structure that characterizes the rock. The lineation, which is marked by orientation of hornblende needles, plunges eastward at a low angle. In almost every outcrop the amphibolite is cut by dikes, pods, or irregular bodies of younger pegmatite.” Structure- The Dickinson Group appears to form a thick south-facing monocline that dips vertically to steeply southward. Virtually all the numerous stratigraphic top determinations from cross bedding in the East Branch are south facing and show no indication of fold repetition (James et al., 1961). Such determinations are lacking in the Solberg Schist, but based on the presumed conformable relationship to the East Branch, at least the lower portions of the Solberg seem likely to be south-facing. A series of magnetic anomalies, most notably that produced the Skunk Creek Member, can be followed for many kilometers as markedly straight traces, 19 �indicating that large scale folds within the Solberg are lacking on the scale of the James et al. study and that it, like the East Branch, is entirely south-facing. On a smaller scale, a strong schistosity is developed in both units and outcrop-scale folds with shallow plunges are fairly common. Whether these features reflect a much larger structure, only partly preserved, or are the largest-scale structures that formed as the units were rotated to their near-vertical orientation is a matter for speculation with our present understanding of the region. Correlation and tectonic setting of deposition- The radiometrically indicated age range of the Dickinson Group suggests that it is a possible time-correlative of the Menominee and/or Baraga Groups exposed nearby and across the southern Lake Superior region. Volcanic rocks interlayered with iron-formations of the Menominee Group were formed at 1.875 Ga (Schneider et al., 2002) and a diabase sill intruded into iron-formation in the Marquette iron range has been dated at about 1.890 Ga (Peitrzak-Renaud and Davis, 2014). The ejecta layer from the Sudbury impact at 1.850 Ga lies near the upper contact of the Menominee Group and the base of the overlying Baraga Group (Cannon et al., 2010). Post tectonic granite plutons that intruded the Baraga Group at 1.833 Ga (Schneider et al., 2001) provide a minimum age. It is possible, therefore, that the Dickinson Group was also deposited during or slightly before the MenomineeBaraga sequence, but its lithology and apparent tectonic setting are unmatched by any other sequence in the region. The combined stratigraphic thickness of 2-3 kilometers of predominantly clastic sediments with interlayers of mafic volcanic rocks argues for deposition in a rapidly subsiding basin in which a generally fining-upward sediment sequence was deposited. The East Branch Arkose, based on lithology and bedding features, seems clearly to be fluvial. Yet, the occurrence of banded cherty iron-formation in the Solberg Schist argues that the basin evolved into marine conditions by the later parts of deposition of that formation. It seems reasonable that the Dickinson Group was deposited in an extensional rift basin in the back-arc basin phase of the Penokean orogeny at about 1875 Ma as defined by Schulz and Cannon (2007). Stop 4. East Branch Arkose. (46.044o N, 87.840o W) Good exposures of the East Branch Arkose can be seen just west of Spring Hill Road. Please respect private property immediately south of the area examined by this trip. The area was mapped in detail (James et al., 1961) and a portion of the map is reproduced in Figure 14. All lithologies are exposed here including coarse, cross-bedded arkose (Figure 15A), quartzite pebble conglomerate (Figure 15b, c, d) and massive basalt. They can be seen in sequence along a short south-to-north transect. All units have nearvertical dips and face south. Slightly to the west, a set of diabase dikes cuts the units at a low angle. Of interest in the conglomerates is the great preponderance of pebbles of white to pink quartzite, a small percentage of which have well preserved bedding. The only known source for such clasts in the region is the Sturgeon Quartzite, which is the basal unit of the Paleoproterozoic sequence in most of the area. Pebbles of granite are common but make up only a small percentage of the pebble-sized clasts. They were used by James et al. (1961) as further evidence of an Archean source, but considering the new age for the porphyritic red granite of 2.1 Ga, the clasts could be from that unit rather than the Archean. Quartzite pebbles typically have flattened shapes that are more likely to be original shapes rather than caused by deformational flattening. A suggestion of imbricate structures can be seen locally. The abundant sand-sized grains of K- 20 �feldspar in the arkosic beds further indicate a granitic (Archean or porphyritic red granite?) source for much of the formation. Figure 14. Map of the outcrop area of East Branch Arkose from James et al., (1961). Darker shades are areas with abundant outcrops; lighter shades are covered. The approximate transect for the field trip is shown near the east edge of the area and is about 150 meters west of Spring Hill Road. 21 �Figure 15. Photographs of the East Branch Arkose. A- Cross-bedded coarse-grained arkose. BConglomerate consisting mostly of quartzite pebbles. Imbricate pebbles indicate current flow from right to left. C- Conglomerate with clast of quartzite with relict bedding. D- Close-up of conglomerate containing both quartzite and granite pebbles. Two samples of the basalt (amphibolite) collected along strike have very similar tholeiitic basalt compositions (~7% MgO, ~50% SiO2) with intermediate TiO2 (~1.4-1.9%) and FeOtotal (~12.5%). The trace elements are characterized by moderately enriched light rare earth elements (REE) and no negative Nb-Ta anomaly when normalized to primitive mantle (Figure 16). A sample of one of the dikes cutting the Archean granitic gneiss at Stop 2 has a trace element content identical to the basalt in the East Branch Arkose except for an enrichment in Th (Figure 16B); it also has higher SiO2 (~54%) than the basalt and may have been contaminated by the granitic gneiss. An amphibolite sampled in a road cut on the west side of Felch also has a composition very similar to the basalt in the arkose (Figure 16). The East Branch mafic rocks are very similar in composition to basalts found in continental rifts. 22 �Figure 16. Chondrite normalized REE plot (A) and primitive mantle normalized trace element plot (B) of basalt in the East Branch Arkose, a metadiabase dike in Archean granitic gneiss at Stop 2, and an amphibolite (sill?) from a road cut on the west side of Felch. Stop 5. Solberg Schist. (46.04oN, 87.82oW) This roadside outcrop on west side of County Road 581 is representative of much of the Solberg Schist. The rock is interlayered amphibolite and biotite-garnet schist with a prominent near-vertical foliation. The unusual ribbed appearance of the outcrop surface may reflect subtle bedding although lithologic changes across the ribs are not obvious (Figure 17B). If they are bedding, the outcrop displays a westward-plunging antiform. Radiometric data on detrital zircons are reported by Craddock et al. (2013) from a location whose coordinates correspond to this outcrop. The spectrum of ages ranges from about 3.8 Ga to about 2.1 Ga. The significant peak of ages at 2.1 Ga places a maximum age on the unit. Unfortunately the sample was described as East Branch Arkose by Craddock et al. (2013), so there is some uncertainty as to where the dated sample was collected and what it represents. This outcrop, and many others, show intense, small-scale deformation structures and commonly nearvertical foliation and bedding. Although these suggest that the unit is complexly deformed and may include significant repetition of stratigraphy, the disposition of the Skunk Creek Member is enigmatic in that it has a nearly straight outcrop trace for more than 20 kilometers (see Figure 2) and shows no indication of fold repetition. 23 �Figure 17. Outcrop photographs of the Solberg Schist. A-Well-bedded schist with interlayers of micaceous schist and more quartzose layers. Thin stringers of granite in upper half. BShallowly-dipping beds (?) cut by vertical foliation. Compositionally, the Solberg mafic schist ranges from basalt (~6-12% MgO, ~45-50% SiO2) to andesite (~4% MgO, ~56% SiO2) with relatively high TiO2 (~1.7-2.4%) and FeOtotal (~13-16%). Samples have steep light REE-enriched chondrite normalized patterns and negative Nb-Ta anomalies when normalized to primitive mantle (Figure 18). They are compositionally distinct from the amphibolites in the East Branch Arkose and the Six Mile Lake Amphibolites. Figure 18. Chondrite normalized REE plot (A) and primitive mantle normalized trace element plot (B) for samples of the Solberg Schist. Field for basalt of the East Branch Arkose and related amphibolites (shown in Figure 16) shown for comparison. 24 �Stop 6. Six Mile Lake Amphibolite. (46.02oN, 87.846oW) A small exposure on the west side of Wickman’s Marsh Road is typical of the Six Mile Lake Amphibolite. Here it is a rather uniform schistose amphibolite with small deformed granitic stringers. It is cut by two undeformed pegmatite dikes (Figure 19) The Six Mile Lake Amphibolite was originally described and named by James et al. (1961), as summarized above, who placed it as the uppermost formation in the Dickinson Group and ascribed an Archean age. As also discussed above, that is now in question because of 2.1 Ga detrital zircons in the lower part of the group. At least a portion of the rocks included in the Six Mile Lake by James et al. (1961) are intruded by granitic rocks of Archean age and appear to grade southward into banded gneiss the makes up much of the Archean in that area. Whether these latter amphibolites are truly a part of the Six Mile Lake or are an older amphibolite unit that lies adjacent to the Six Mile Lake is a subject for further evaluation. Compositionally the Six Mile Lake Amphibolite is a tholeiitic basalt characterized by low TiO2 (<1.5 wt. %) and trace element content. Unlike most of the amphibolites in the region, which are characterized by enriched light REE and negative Nb and Ta anomalies when normalized to primitive mantle, the Six Mile Lake Amphibolite has a flat chondrite normalized REE pattern and no Nb and Ta anomalies (Figure 20). It should be noted that the large metagabbro body in the Solberg Schist has a similar composition to that of the Six Mile Lake Amphibolite (Figure 20). Amphibolites from the Carney Lake Gneiss complex and the Hardwood mafic gneiss also have compositions similar to the Six Mile Lake Amphibolite (Figure 20). Figure 19. Six Mile Lake Amphibolite. A.- Hornblende schist with granitic stringers. B.- Schist cut by pegmatite dikes. 25 �Figure 20. Chondrite normalized REE plot (A) and primitive mantle normalized trace element plot (B) for samples of Six Mile Lake Amphibolite and related rocks. Fields for some other amphibolites from the region shown for comparison Stop 7A. Mafic granulite of the Hardwood Gneiss. (45.969oN, 87.711oW) Roadcuts on both the north and south sides of Highway M-69 are good examples of typical granulite of the Hardwood Gneiss. The rocks consist of various assemblages of hornblende, pyroxene, plagioclase, microcline, and garnet. They are very strongly foliated (Figure 21A) and, in places, show prominent compositional layering (Figure 21B). The Hardwood Gneiss was recognized and named by James et al. (1961) as an unusual unit of very highly metamorphosed rocks that are exposed over an area of only about 5 square kilometers at the eastern edge of their study area (see Figure 2). To the south, the Hardwood is in fault contact with the Paleoproterozoic Michigamme Formation. To the west and north it is in contact with Archean granite and granitic gneisses, but the nature of the contact is not known. The area of exposure is bounded on the east by Cambrian and younger strata. It is quite possible that the Hardwood underlies a substantially larger area beneath that cover. The general structure of the Hardwood in the area of exposure is the keel of a gently east-plunging synform defined by the gneissic foliation, so it is likely that the gneisses continue eastward in the pre-Cambrian basement. James et al. (1961, p.22-23) described the Hardwood as follows “In general, the gneiss is strongly layered, with individual layers ranging from a fraction of an inch to a few feet in thickness. The dominant rock type is a dark-medium-grained gneiss composed of hornblende, plagioclase, and pyroxene. Interlayered with this rock are beds of dark gneiss but of different grain size, beds of dark vitreous-lustered rock with alternating light and dark laminae, garnetquartz-mica schist, and light-colored rock that resembles quartzite. Some of the layers are rich in magnetite. …. The gneiss appears to have been originally a volcanic sequence, at least in part tuffaceous, with some inter- bedded sedimentary rocks, intruded by gabbro sills. The rocks have been dynamothermally metamorphosed under conditions that resulted in the alteration of most of the original pyroxene in the igneous facies to hornblende and garnet, and the development of mica, hornblende, and plagioclase in rocks that appear to have originally been acidic volcanics. The Hardwood gneiss, as seen in the exposures, is folded along axes that plunge eastward at low angles, and the general structure appears to be an eastward plunging syncline.” 26 �Figure 21. Hardwood Gneiss at stop 7A. A- Typical mafic granulite with strong, somewhat anastomosing, foliation. B- Straight-banded granulite gneiss. Compositional layering is expressed as variations in plagioclase:mafic mineral ratio. C and D are photomicrographs of two contrasting textures. C- Granoblastic textured interlayered quartz-microcline-plagioclase rock (center) and hornblende with minor (ortho?)pyroxene. D- Foliated rock with pyroxene (larger light grains) and small garnets (dark layer near top) in quartz-sericite matrix. Additional lithologies that have been included in the Hardwood gneiss are metasediments (Stop 7B), including quartzite and pelitic schist. The schists include garnet-biotite-sillimanite-kyanite mineral assemblages. Chemical analyses of mafic rocks in the Hardwood Gneiss are generally similar to those of the Six Mile Lake Amphibolite with similar low TiO2 and trace element content (Figure 22). One mafic gneiss sample has enriched light REE and a large negative Nb-Ta anomaly is likely the result of contamination by felsic crustal rocks. 27 �Figure 22. Chondrite normalized REE plot (A) and primitive mantle normalized trace element plot (B) for mafic gneiss samples from the Hardwood Gneiss. Field for Six Mile Lake Amphibolite shown for comparison. Metamorphism of the Hardwood Gneiss was studied by Peterson and Geiger (1990) who determined conditions of its metamorphism based on mineral assemblages. They defined two distinct episodes of metamorphism. Geothermobarometry indicates conditions of 8.2-11.6 kbar and ~770°C for the earliest event, and conditions of 6.0-10.1 kbar and 610-740oC for the second. They proposed that the older event was Archean and contemporaneous with a high-grade metamorphic event recorded in the Minnesota River Valley. They interpreted the younger event as probably Paleoproterozoic and pre-Penokean, with metamorphic conditions more intense than those generally ascribed to the Penokean orogeny in Michigan. Although they recognized both events in the typical layered gneisses, only the second event is recorded by the metasedimentary units. These extremely high metamorphic pressures are unique in the southern Lake Superior region and are comparable to or slightly higher than those of the Kapuskasing structure in Ontario. For comparison, the metamorphic pressures interpreted for the Peavy node, the nearest Paleoproterozoic metamorphic node, are less than 5 kbars (Attoh and Klasner, 1989). The metamorphic conditions recorded by the Hardwood Gneiss are equivalent to temperatures and pressures of the lowermost crust. The Hardwood Gneiss has been widely accepted to be of Archean age and to be part of the gneiss complex that forms the basement for the Paleoproterozoic sedimentary and volcanic sequence, although no radiometric ages have been previously determined. Our new SHRIMP UPb zircon data reveal a group of concordant to nearly concordant zircon spot analyses at ca. 2750-2500 Ma (Ayuso et al., 2018) documenting a Neoarchean component (Figure 23). A second group of spot analyses (Figure 23) document a younger period of zircon growth ca. 1900 to 2200. A thermal event of this age has not been recognized previously in the region. Metamorphism of the Peavy node, which encompasses the area of the Hardwood Gneiss, has been dated at ca. 1830 Ma at outcrops within a few kilometers west of the Hardwood (Schneider et al., 2004; Holm et al., 2007) using Pb-Pb ages of monazite. Furthermore, metamorphic pressures of the Peavy node are estimated at ca. 5 kb (Attoh and Klasner, 1988) in contrast to the much higher pressures determined for the Hardwood (Peterson and Geiger (1990). An additional significant feature of the Hardwood Gneiss is that we have detected no Eoarchean components, unlike the nearby Carney Lake Gneiss, which has yielded numerous Eoarchean spot analyses. Thus, the Hardwood Gneiss is presently enigmatic in terms of its parentage, metamorphic history, and kinematics of emplacement relative to surrounding rock units. 28 �Figure 23. A- BSE (back-scatter electron) image of two anhedral zircons from the Hardwood Gneiss showing SHRIMP U-Pb analyzed dates for two spots. B- Concordia diagram for 56 spot analyses for the Hardwood Gneiss. Stop 7B. Pelitic schist of the Hardwood Gneiss. (45.967oN, 87.729oW) A low roadcut on the west side of Swan Peterson Road, just north of its intersection with M-69 is garnet- biotitesillimantie-kyanite schist. These are among the structurally (stratigrahically?) lowest parts of the Hardwood Gneiss. Garnet porphyroblasts are common (Figure 24A). A strongly developed foliation dips about 30 degrees east. White elongate masses (Figure 24B) consist of quartz and both kyanite and sillimanite. They are elongated within the foliation but have slightly flattened shapes in cross section (Figure 24B). They define a prominent lineation that plunges about 35° east. Figure 24. A- Photomicrograph of biotite schist with porphyroblasts of eudedral to subhedral garnet. B- Hand sample of schist. Upper surface is the schistose parting and shows elongate somewhat contorted white masses of quartz and aluminosilicates, both sillimanite (fibrolite) and kyanite. Cut front face shows that these are somewhat flattened, rod-like masses that define a prominent lineation along fold axes. 29 �Stop 8. Cambrian/Paleoproterozoic unconformity. (45.997oN, 87.841oW) Roadcuts on the north side of Highway M-69 west of the village of Felch expose the unconformity between the Munising Formation (Cambrian) and Paleoproterozoic strata of the Vulcan Iron-formation and Randville Dolomite. Along most of the roadcut nearly flat-lying sandstone of the Munising Formation is at the base of the exposures (Figure 25A), but the higher hills to the north are underlain by Paleoproterozoic rocks indicating that substantial topography existed along the unconformity. Near the west end of the outcrop (Figure 25B) the unconformity is a steeply eastward dipping contact across which the flat-lying Munising abuts against a topographic knob composed of clasts of Vulcan Iron-formation, some of which have indications of secondary iron enrichment (Figure 25C). Munising sand fills the voids in the rubble. The material appears to have been a loosely packed pile of iron-formation rubble at the time of Cambrian transgression. Voids were filled with sand that infiltrated the pile. The angular shape of clasts indicates that material was not transported any substantial distance. The material may have been talus accumulated along the base of a south-facing slope. Farther east along the roadcut the lower part of the exposures are again angular rubble of both Vulcan Iron-formation and Randville Dolomite with voids filled with Munising sand. Exposures higher on the hill just to the north are entirely the Paleoproterozoic units indicating that the rubble is a thin carapace of material lying against a steep south-facing slope that existed during Cambrian transgression. In some places the rubble has intervals of well-bedded sandstone within it (Figure 25D); an indication that the rubble was being delivered to the site during sandstone deposition. These relationships suggest that the rubble of Paleoproterozic rocks was a talus deposit along the base of a marine shoreline cliff during deposition of the Munising Formation. The relationships seen here between Precambrian rocks and the Cambrian sandstone emphasize that considerable topography existed on the surface over which the Cambrian seas transgressed across the area. Similar relationships are seen on Field Trip 3 along the Menominee Range. The relationships also suggest that outliers of Cambrian sandstone may be much more widespread under low areas than has been shown on previous maps, and that outcrops of Precambrian rocks on the present land surface were likely only at shallow depth beneath the unconformity before being exposed by younger erosion. Much of the topography presently seen over Precambrian areas is likely to be largely relict topography of the Cambrian landscape. 30 �Figure 25. A- Flat-lying red sandstone of the Munising Formation (foreground) passing laterally into rubble of Vulcan Iron-formation at far left. View looking northwest. B- Steeply dipping unconformity shown by dashed line between Munising Formation (right) and iron-formation rubble (left). C- Clast of specularite-jasper iron-formation within rubble surrounded by Munising sand. D- Rubble of iron-formation and dolomite with thin, nearly horizontal interbeds of Munising Formation. Stop 9. Randville Dolomite and post-Cambrian breccia. Along highway M-69, near the intersection with County Road 11, a roadcut on the north side of the highway exposes coarsely recrystallized carbonate and quartz. We interpret this to be the Randville Dolomite which, in previous mapping (James et al., 1961), was traced to within about 500 meters of this newer roadcut. An unusual feature of this exposure is a zone of breccia about 10 meters wide composed of fragments of both the Randville and of the Munising Formation. The Munising Formation occurs as coherent clasts of red sandstone (Figure 26), not unconsolidated sediments as at Stop 8, indicating that the breccia formed sometime after deposition and lithification of the Munising Formation. The Randville Dolomite occurs as angular clasts as large as about a meter diameter. The cause of this late brecciation is not clear. One possibility is that it is a karst collapse breccia formed by solution of the Randville. The Munising fragments must have been transported downward at least a few meters assuming that the Cambrian unconformity was originally slightly above the present land surface. Another possibility is that the breccia is related 31 �to kimberlite intrusion, although we have found no indication of igneous rocks in the breccia matrix. Post-Ordovician kimberlites are known at numerous localities in the region and most have clasts of Ordovician carbonates that have fallen downward in the pipes. (See discussion in Field Trip 2 of this volume). Other areas of disturbed Cambrian sediments have been described in the area (James et al., 1961). The Munising has been observed with dips as high as 60o locally. Most of these disturbed areas are along Precambrian faults and were interpreted to be caused by Paleozoic or later reactivation of those faults. The faults were inferred and, in one instance documented, to have normal offset. As mapped by James et al. (1961), a fault does pass about 100 meters south of this exposure so there is some possibility that the brecciation seen here is simply related to reactivation of that fault. . Figure 26. Sandstone of the Munising Formation with steep eastward dip in carbonate breccia. Stop 10. Banded gray gneiss. (46.004oN, 87.900oW) The term “banded gray gneiss” was applied informally to a belt of Archean rocks that occur mostly immediately north of the Felch trough. Roadcuts on both sides of Highway M-69 display representative lithologies of this unit. (Note that the present position of M-69 is substantially different from that shown in James et al, (1961) because of relocation related to the Groveland mine development.) This exposure is migmatitic gneiss composed mostly of mafic material with minor granitic stringers (Figure 27A). Most layering is nearly vertical and numerous tight isoclinal folds are present (Figure 27B). An unusual feature is a large inclusion of coarse-grained plagioclasehornblende gneiss shown on the south cut. The inclusion has a strong foliation that lies at a distinctly lower angle than that of the surrounding gneiss. 32 �Figure 27. A- Typical amphibolitic gneiss with numerous granitic stringers. Many stringers are undeformed and are apparently post- or late-tectonic injections or segregations. B- Tightly folded migmatitic gneiss. The general description of the banded gray gneiss in this vicinity, given by James et al. (1961, p. 121-122) is as follows: “Most of the gneiss is light gray, or alternating light and dark gray, and is thinly layered. The rock typically contains thin rather discontinuous biotitic and hornblendic layers, not more than a millimeter thick, alternating with quartz-feldspar layers several millimeters thick. Both on fresh breaks and on weathered surfaces the gneiss is somewhat mottled as a result of segregation of feldspar into patches a few millimeters across and 5 or 6 mm long. Near some dikes of granite pegmatite, the gneiss contains layers a few millimeters wide composed of pinkish feldspar, some of which cut across the gneissic layering. In some places, particularly near the southern margin of the gray gneiss belt, the gneiss contains lenses and pods of amphibolite as much as 100 feet thick and a quarter of a mile or more long. The light-colored layers of the gneiss are composed chiefly of feldspar, quartz, and biotite. The texture is exceedingly irregular and the abundance of the various constituents varies widely from place to place. The larger grains quartz, plagioclase, and microcline vary in size from about 0.5 mm to 1.5 mm. The microcline, which usually occurs as smaller crystals than the other constituents, is unaltered. The plagioclase in two specimens is oligoclase, whereas in a third it is probably albite. Other minerals, present in small quantities, are muscovite, chlorite, epidote, zircon, leucoxene, apatite, and iron oxides. The amphibolite that makes up the dark layers and pods in the banded gray gneiss is virtually identical to the Six-Mile Lake amphibolite previously 33 �described. The rock is medium grained, with strong preferred orientation of the minerals. Hornblende, plagioclase, and quartz are the chief constituents.” An analysis of a sample of the gray gneiss from this road cut indicates it is basaltic with ~50% SiO2, ~5% MgO, and relatively high TiO2 (~1.9%) and FeOtotal (~14.5%). The trace elements are characterized by enriched light REE and negative Nb-Ta anomaly when normalized to primitive mantle (Figure 28). The mafic gray gneiss composition overlaps with that of the Solberg Schist (Figure 28) although the Solberg is likely of Paleoproterozoic age. Figure 28. Chondrite normalized REE plot (A) and primitive mantle normalized trace element plot (B) for mafic gray gneiss. Field for Solberg Schist is shown for comparison. Stop 11. Porphyritic red granite. (46.009oN, 88.061oW) Roadcuts on both the east and west sides of Highway M-95 are examples of typical porphyritic red granite, which was recognized as a map unit by James et al. (1961). The porphyritic red granite is a ferroan potassium-rich granite with A-type within-plate chemical characteristics. As mapped, it occurs in two elliptical bodies as shown on Figures 2 and 29. However, outcrops in the area are very sparse and the margins of the granite are poorly constrained. Likewise, its relationship to surrounding units is not clear. The surrounding area was designated “Dickinson group undivided” based largely on the westward extension of magnetic anomalies, mostly in the Solberg Schist, from areas of better exposure to the east. Because the magnetic anomaly produced by the Skunk Creek Member, a medial bed in the Solberg Schist, passes to the south, it is likely that the porphyritic red granite is surrounded by the lower parts of the Dickinson Group. Whether the granite bodies are intrusive into the Dickinson Group or are domes of pre-Dickinson basement is not clear from available exposures. The foliation is approximately parallel to the inferred margins of the granite bodies and dips steeply outward from their centers indicating that they are domal structures. As described by James et al. (1961), “The rock is generally homogeneous and coarse grained. Inclusions are rare, but dark schlieren and compositional layering locally are present. . . . Lenticular to tabular pink feldspars about half an inch long are abundant and impart a porphyritic appearance to the rock. Some of the feldspars are euhedral, but most are in fact augen, and on horizontal surfaces a foliation produced by oriented feldspars is faint to distinct. In vertical sections the structure is easily seen. In most exposures, steeply plunging lineation, 34 �marked by orientation of both microcline augen and biotite, is well developed, and in places it is the dominant structure. The feldspars form about two-thirds of the rock; quartz and biotite make up the remaining onethird. Quartz itself makes up 10 to 20 percent of the rock. The cores of many of the microcline phenocrysts are white or colorless and are mantled with feldspar that is salmon pink or red. In thin section the microcline crystals have indefinite borders against a finer grained aggregate of microcline, oligoclase that is reddish and kaolinized, and quartz. The relationships suggest granulation of the borders of original tabular microclines followed by recrystallization of the granulated material.” The preferred interpretation of James et al. (1961) was that the granite is pre-Dickinson group and therefore likely to be of Archean age, based on their assignment of an Archean age for the Dickinson Group. A new radiometric age (SHRIMP U-Pb data for zircon) determined for the granite (Ayuso et al., 2018) is ca. 2.099 Ga. (Figure 30). This well constrained age is rather surprising in that no other granites of comparable age are known in the region. Granites of this age may provide a local source for the rather abundant 2.1 Ga detrital zircons reported from the nearby Dickinson Group (Craddock et al., 2013). The date further strongly suggests that the porphyritic red granite is the basement on which the Dickinson Group was deposited and was uplifted in domal structures after Dickinson deposition. Figure 29. A portion of Plate 2 west from James et al. (1961) showing two bodies of porphyritic red granite occurring as cores of domes surrounded by undivided strata of the Dickinson Group. Red dots are aeromagnetic anomalies and red lines are inferred connections of magnetic beds between measurement points, solid where probable, dashed where uncertain. 35 �Figure 30. A- BSE (back scatter electron) image of zircon grain from the porphyritic red granite showing age of spot analyzed by SHRIMP. B- Concordia diagram for 18 spot analyses of zircons from the porphyritic red granite. References Attoh, K. and Klasner, J.S., 1989, Tectonic implications of metamorphism and gravity field in the Penokean orogeny of northern Michigan, Tectonics, v. 8, p. 911-933 Ayuso, R.A., Schulz, K.J., Cannon, W.F., Woodruff, L.G., Vasquez, J.A., and Jackson, J., 2017, Evidence for the presence of Eoarchean crust in northern Michigan, Institute on Lake Superior Geology, Proceedings of 63rd annual meeting, part 1: Program and abstracts, p. 910. 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. 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, Geological Society of American Bulletin, v. 122, p. 50-75. Clark, L.D., 1961, Chapter C, Precambrian geology of the Norway Lake area: in James, H.L., Clark, L.D., Lamey, C.A., and Pettijohn, F.J., Geology of central Dickinson County, Michigan, U.S. Geological Survey Professional Paper 310, p. 97-113. 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. 36 �Cumberlidge, J.T., and Stone, J.G., 1964, The Vulcan Iron-formation at the Groveland Mine, Michigan, Economic Geology, v. 59, p. 1090-1106. 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. 106126. Hunter, J., 1986, Uranium mineralization at the Felch prospect, Upper Peninsula, Michigan, United States of America (summary): Uranium deposits in magmatic and metamorphic rocks, Proceedings of a technical committee meeting, Salamanca, p.213-215. Jackson, C. T., 1849, Report on the geological and mineralogical survey of the mineral lands of the United States in the State of Michigan, U.S. 31st Cong., 1st sess., S. Doc. 1, p. 371-935. James, H.L., 1955, Zones of regional metamorphism in the Precambrian of northern Michigan, Geological Society of America Bulletin, v. 66, p. 1455–1488. 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. Klasner, J.S., and Sims, P.K., 1993, Thick-skinned, south-verging backthrusting in the Felch and Calumet troughs area of the Penokean orogeny, northern Michigan, U.S. Geological Survey Professional Paper 1904-L, 28 p. Lehman, G.A., 1987, U-Pb dating of pitchblende from Dickinson County, upper Michigan, suggests reactivation of Precambrian structures during formation of the Michigan basin, Proceedings of the Institute on Lake Superior Geology, part 1, Proceedings and Abstracts, v. 33, p. 37-38 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. Peterson, J.W., and Geiger, C.A., 1990, The Harwood gneiss: evidence for high P-T Archean metamorphism in the Southern Province of the Lake Superior region, Journal of Geology, v. 98, p. 273-281. Pietrzak-Renaud, N, and Davis D., 2014, U-Pb geochronology of baddeleyite from the Belleview metadiabase: age and geotectonic implications for the Negaunee Iron-Formation, Michigan: Precambrian Research, v. 250, p. 1-5. Romano, D., Holm, D., and Foland, K., 2000, Determining the extent and nature of Mazatzalrelated overprinting of the Penokean orogenic belt in the southern Lake Superior region, north-central USA, Precambrian Research, v. 104, p. 25–46, doi: 10.1016/S03019268(00)00085-1. 37 �Schneider, D., Bickford, M., Cannon, W., Schulz, K., and Hamilton, M., 2002, Age of volcanic rocks and syndepositional iron-formations, Marquette Range Supergroup: implications for the tectonic setting of Paleoproterozoic iron-formations of the Lake Superior region, Canadian Journal of Earth Sciences, v. 39, p. 999–1012, doi: 10.1139/E02-016. Schneider, D.A., Holm, D.K., O’Boyle, C., Hamilton, M., and, Jercinovic, M., 2004, Paleoproterozoic development of a gneiss dome corridor in the southern Lake Superior region, USA, in Whitney, D.L., Teyssier, C., and Siddoway, C.S., eds., Gneiss domes in orogeny, Geological Society of America Special Paper 380, p. 339–357. Schulz, K.J., and Cannon W.F., 2007, The Penokean orogeny in the Lake Superior region, Precambrian Research, v. 157, p. 4-25. Ueng, W.C., and Larue, D.K., 1988, The early Proterozoic structural and tectonic history of the south central Lake Superior region, Tectonics, v. 7, p. 369-388. Vallini, D.A., Cannon, W.F., and Schulz, K.J., 2006, Age constraints for Paleoproterozoic glaciation in the Lake Superior Region: detrital zircon and hydrothermal xenotime ages for the Chocolay Group, Marquette Range Supergroup, Canadian Journal of Earth Sciences, v. 43, p. 571-591. 38 �FIELD TRIP 2 Tuesday May 15, 2018 GEOLOGY OF THE HEMLOCK FORMATION Tomas Waggoner, Consulting Geolo Email: thomaswaggoner@hotmail.comgist INTRODUCTION The one day field trip will make eleven stops to examine the major rock types that make up or impact the 1,874 Ma Paleoproterozoic Hemlock Formation (Figure 1). The ~30,000 foot thickness of the primarily tholeiitic basalt is particularly rich in iron oxides and could easily have provided both the iron and silica incorporated in major portions of the Lake Superior type iron formations. The first stop will examine the Lake Ellen Kimberlite which is the only easily accessible kimberlite in the Upper Peninsula Kimberlite District and it also intrudes the Hemlock Formation. The differentiated West Kiernan sill will be visited including the ultramafic lower unit and the base metal rich differentiate near the base of the gabbro unit. The upper transition will be examined where plagioclase levels approach 80% and contains significant titaniferous magnetite, stilpnomelane and apatite. Other stops will illustrate rhyolite, volcanic conglomerates, amygdaloidal and pillowed basalts. Also, one of the stops will examine the Mansfield iron member which is one of several areal restricted iron formations present in the Hemlock. 39 �General Geology The ~2.7 Ma Margeson Creek Gneiss in the center of the Amasa Uplift is overlain by the Randville Dolomite of the Chocolay Group and is equivalent to the Kona Dolomite on the Marquette Range and the Badriver Dolomite on the Gogebic Range. Dating by Vallini and others (2006) indicates the age of the Sturgeon quartzite under the Randville dolomite in the Iron Mountain area is 2.32.2 Ga. The Randville dolomite directly underlies the Hemlock Formation and exhibits several lithologies. The most pervasive lithologic type is a sandy dolomite while a coarse orange arkose, not present in the Randville equivalents elsewhere, is common at the southern portion of the Amasa Uplift. The age of the Randville is at least 300 million years older than the 1874 Ma Hemlock Formation (Schneider and others, 2002) . The basal conglomerate and quartzite units present in the Chocolay Group of the Marquette Range and Menominee Range are absent around the Amasa Uplift. At the south end of the Amasa Uplift the Randville is approximately 1800 feet thick and consists of sandy dolomite, feldspathic quartzite, arkose and sericite argillite. Some authors have postulated that the Randville has pinched out on the north end (Cannon and others, 1976; Foose, 1981). However, subsequent drilling in section 28, T. 46 N., R. 33 W indicates the dolomite is present on the north end, but that same drilling was not extensive enough to identify lithologic types or thickness. Overburden depths on the north end of the Uplift can vary from 50 feet to over 560 feet with no reported outcrops. During the early Penokian orogeny the Pembine-Wausau terrane (Figure 2) was already accreted to the Superior Provence by 1875 Ma (Figure 2). Back-arc extensional basins south of the Niagara fault contains numerous volcanogenic massive sulfide deposits (VMS). Rifting on the continental margin produced a basin(s) into which banded iron formations formed coeval with volcanism. The age of banded iron formation deposits and the intra-arc rift massive sulfide deposits in the 40 �Pembine-Wausau terrane are both ~1874 Ma. One of the largest volcanic centers is the Hemlock Formation, located in Iron County, Michigan. The volcanic pile achieved a thickness in excess of 30,000 feet west of the Amasa Uplift and thins away in all directions (Figure 3). It is estimated the footprint exceeds 2,800 square miles. Geophysics A total field positive magnetic anomaly around the west side of the Amasa Uplift is caused by increased magnetite content in the upper 6000 feet of the Hemlock basalts (Figure 3). The Amasa Iron-formation is essentially non-magnetic as primary iron minerals have all been oxidized to hematite and in some cases enriched to iron ore that was mined early in the last century. Major conductivity zones are associated with graphitic slates internal to the volcanics and the overlying graphitic Michigamme slates. The presence of disseminated sulfides in the lower portion of the normal gabbro of the West Kiernan sill produce minor EM anomalies. Figure 3. Increase in iron content in basalt near the top of the Hemlock Formation 41 �Figure 4. Plan geology map showing the sub crop of the Hemlock Formation (Xm2) and the later thrust faulting. Field trip area is defined by the red box. (After USGS Map I-2356) Generally the exposed Archean portion of the Amasa Uplift exhibits low gravity readings. Most of the Hemlock sub crop area exhibits a muted gravity contrast. Positive gravity values are associated with the Riverton Iron-formation in the Iron River-Crystal Falls allochthon and an area northwest of the Amasa Uplift. A Bouguer ground gravity map with limited field stations of the Amasa Uplift area show an increase in the gravity field toward areas of thicker basalts of the Hemlock Formation, especially where the increased magnetite content increased the specific gravity of the rock. After deposition of the Michigamme Formation the continued northern push by the Wisconsin Magmatic terrane produced a number of east-west thrust fault panels replicating the stratigraphy in each panel (Figure 4). Hemlock Formation The Hemlock Formation was named (Clements, 1899) for volcanic rocks found near the Hemlock River west of the Amasa Uplift. The Hemlock belong to the Menominee Group of the Marquette Range Supergroup. The volcanics and sediments were deposited subaqueously and are believed to be terrigenous sedimentary sourced (Johnson, 1975; Dann, 1978; Ueng, 1987; and Beck, 1991). Beck (1991) described the volcanics as continental flood basalts. Principle lithologies listed in order of decreasing abundance are: basalt, vocaniclastics (referred to as agglomerates, hyaloclastites and breccias), rhyolite, graphitic slates and small iron formation members (one, the Bird, is an iron oxide chert and the other, the Mansfield, is a carbonate chert). 42 �The base of the Hemlock Formation rests unconformably on the Randville Dolomite except in the vicinity of Michigamme Mountain where the base of the Hemlock rests on a small deposit of unique quartzite composed of angular quartz and chert clasts along with minor massive chert. Portions of the quartzite have been replaced by magnetite and specularite while some of the chert contains secondary specularite that replaced the chert yielding a crude banding appearance. The Hemlock tholeiitic basalt magma (Figure 5) by its reduced nature, concentrated iron in the residual magma, principally as magnetite. Replacement iron oxides are found in the small clastic unit located on the southeast portion of the Amasa Uplift at the base of the Hemlock Formation. Iron oxide concentrations, principally magnetite, are found sporadically in the basaltic portion of the Hemlock and in the upper ~6000 feet of the basalts. Following active volcanism banded iron formations were formed (Amasa [west side of Uplift] and Fence River [east side of the Uplift]). There are a number of intrusive sills found within the Hemlock Formation. One, the West Kiernan sill, is approximately thirteen miles long and one mile thick as measured at outcrop. It is differentiated into four distinct units plus a chilled diabase contact zone. Relatively thin, but internally layered sills like the West Kiernan sill, consisting of basal peridotite overlain by pyroxenite, gabbro, and granophyre, are unusual in that injection of relatively crystal-free magma does not appear to form well layered, well-differentiated intrusions (March, 2006). For example, the large Sudbury impact melt sheet (3 km x 200 km; volume of ~30,000 km3) shows no sign of layering and very little sign of differentiation while the more than 300 m thick Palisades sill consists only of an olivine-enriched lower layer (~3 m thick), diabase, ferrodiabase, and granophyre (Walker, 1969). However, similarly layered and differentiated sills like the West Kiernan sill have been described from other Precambrian terranes including the Archean Vermilion district in Minnesota (Schulz, 1982), the Abitibi greenstone belt in Ontario (MacRae, 1969), the Eastern Goldfields region in Western Australia (Williams and Halberg, 1973) and the 43 �Barberton Mountain Land in South Africa (Anhaeusser, 1985). For the Archean examples, the layered sills appear to reflect crystallization from an iron-rich picrite magma (Schulz, 1982). Whether the West Kiernan sill is also related to a high-iron picrite magma is unclear. Until the late1980’s the volcanic extrusives (Badwater Greenstone) around the Iron River-Crystal Falls District were considered to be a distinct and separate mafic extrusive unit from the Hemlock. Several papers suggested that Badwater Greenstone was actually the same unit as the Hemlock based on similarity of rock types, textures and geochemistry (Dann, 1978, 1979). In the mid-1980’s several USGS personnel were looking south across the Paint River and noticed a pink colored rock beneath the greenstone and identified it as the Saunders Formation (Randville equivalent). This suggested that the entire Iron River-Crystal Falls district could be an allocthon (detachment fault) that had been forced northward up and over the underlying younger Michigamme Formation. In addition it was recognized that the Paint River Group was not a younger sequence than the Baraga Group but rather a fault repetition with the slates over the Riverton Iron Formation equivalent to the Michigamme slates (Figure 6). Regional metamorphism produced during the Penokean orogeny in the area of the field trip falls either in the chlorite or biotite isograd (Weir, 1986). Hemlock Formation as a Flux Source for portions of Iron Formations in the Lake Superior District Intermittingly, during and at cessation of active volcanism the magma source continued to yield significant quantities of iron, silica, phosphorus and carbon dioxide forming a large volume super plume (Figure 7) containing both instantly crystalized iron oxides and amorphous silica along with significant soluble Fe++ that spread in the ocean over thousands of square kilometers. Both 44 �magnetite and specularite were produced at the vent site(s) under equilibrium conditions that allowed instant crystallization, much like the sulfide particulate matter associated with black smoker vents. Very fine specular hematite is found at the core of much of the magnetite in banded iron formations worldwide (Han, 1966, 1978, 1988). The iron oxides (i.e. microplaty-specular hematite and magnetite) iron carbonate and amorphous silica settled to the sea floor (possibly on seasonal changes) forming alternating bands of chert and iron minerals with thickness based on flux available from the source, current directions and saturation conditions at the deposition site. Near shore banded iron formations exhibit granular textures, cross bedding and occasional reef development of stromatolites while deeper deposits are lithic without oolitic or granular layers or significant traces of biogenic activity. Ancient VMS (e.g. New Brunswick #3 and 6; Austin Brook; Manitowadge, Ontario; Lokken, Norway; Bending Lake, Ontario) and SED-EX (e.g. Little Commonwealth and Dunkel exploration, Wisconsin) deposits, with associated banded iron formations, have modern day analogues (without banded iron formations) at spreading plate centers where iron is precipitated as an oxyhydroxide ostensibly converted to hematite and magnetite at a later period. Oxyhydroxides do not convert to specularite or magnetite under normal natural conditions. Current sea floor discharge systems yield small sea floor deposits from small plumes under predominantly oxidizing conditions. Large systems capable of producing sufficient silica and iron for the creation of banded iron formations would require a supersized plume extending over large areas. Also, these large systems would create a significant reduced environment where early formed iron compound particles can scavenge additional iron. Sea floor ventings studied since discovery in the 1970’s are useful in observing the mechanisms in play in the formation of iron oxides and silica near the vent sites. Lake Nyos in Cameroon (Ozawa, 2016) illustrates the formation of crystalline siderite from a vent source high in Fe+2 and CO2 under a slightly acid environment. This would suggest that a plume environment at the bottom of the sea can quickly 45 �precipitate iron carbonate along with amorphous silica which can separate into chert and iron rich layers (Krapez, 2006). Conditions impacting the type and nature of the banded iron deposits produced include:  Plume buoyancy.  Hydrothermal water temperature, pH and Eh.  Amount of iron and silica flux including CH4, CO2, H2S and fluorine.  Discharge conditions including sea water temperature, salinity and pressure (depth).  Discharge and plume environment, oxidation and reduction conditions, plume size/thickness.  Size of the igneous flux source  Definable presence of iron and silica available to produce enough flux to form iron formations. FIELD TRIP STOPS Ten of the eleven stops are shown in Figure 8 below. Figure 8. Hemlock field trip stops 1-7 and 9-11 Kiernan and Lake Mary Quadrangle 46 �Stop 1. Lake Ellen Kimberlite UTM: 46o 10.478 N 88o 10.587 W, SWSW Sec. 27, 44-31 Gair (1956) in USGS Bulletin 1044, plate 2 noted a small magnetic anomaly in the SWSW of Section 27, T. 44N, R. 31 W. and made an observation that the magnetic high was caused by magnetite in the glacial till. In 1971 William Spence and Klaus Schulz discovered the Lake Ellen kimberlite (also referred to as Site 10) under thin soil cover while conducting base metal exploration for industry. Cannon (1981) wrote a report on the occurrence. Crystal Exploration acquired control of the property and conducted geophysics (Figure 9), drilling, trenching, bench testing and analytical work. This activity spurred other companies to conduct air and ground magnetics, soil and stream surveys, land leasing and follow-up drilling. Shapes of kimberlites are usually round or elliptical and can cover an area from a few acres and up to 200+ acres. The Lake Ellen kimberlite is approximately 600 feet in diameter (Figure 9). Most of the kimberlites in the district have a similar physical appearance (Figure 10). Depth of burial and degree of weathering can make geophysical prospecting difficult. The Lake Ellen pipe has a variable magnetic signature. Unweathered kimberlites contain minor magnetite which can be oxidized to hematite near surface and lose their magnetic properties. Where weathering produced clays or where epiclastic units are present the electromagnetic techniques can be an effective location tool. Inhomogeneity can mask certain parts of a single pipe. In addition to geophysical techniques, soil or stream sampling can be an effective method of discovery because the Lake Ellen is overlain by a thin soil cover. An excavation pit used for both bulk sampling and woods road construction constitutes the focus of the first stop. 47 �Figure 10. Kimberlite V-28-c-1 in Sec. 2, T. 38 N., R. 27 W. showing lapilli, xenocrysts and xenoliths of Ordovician limestone. The Lake Ellen kimberlite intruded the steeply dipping Hemlock rhyolite and Fence River Ironformation (Figure 11). Dating of inclusions found within diamonds mined in Australia, South Africa and Botswana (Kirkley, 1992) found the age of diamond formation was generally older than the kimberlites or lamproites themselves. It can be postulated that any diamonds present in the Upper Peninsula kimberlite district are also older. 48 �The known kimberlite district extends from the Michigamme Reservoir in the northwest to Powers in the southeast, a distance of 62 miles. The oval outline of the kimberlite district has a width of approximately 19 miles (Figure 12). Figure 12. Plan map of the Michigan Upper Peninsula Kimberlite District. A subset of purple chromium rich garnet xenocrysts can be used to project the potential of any kimberlite to contain diamonds. Orange, red-orange and light orange garnets are from eclogite xenoliths while purple, red and pink garnets fall within peridotite affinity. The Lake Ellen kimberlite has very few indicator garnets, 3 of a population of 178 (McGee, 1988) or 1.7% indicating a lower probability of containing diamonds. A 180 short ton bulk sample produced four diamonds which in quality and number make the Lake Ellen kimberlite uneconomic. None of the discovered kimberlites in the field have economic concentrations of diamonds. Xenoliths and xenocryst compositions indicate the diatreme originated in the upper mantle. Based on data obtained from garnets the calculated equilibrium temperatures range from 950o-1100o C. The kimberlite originated from approximately 140-160 km below the surface (Griffin, 2004). Age dating of Site 73 Kimberlite emplacement yielded a zircon age of 155 Ma while a K-Ar determination on phlogopite yielded a 190 Ma age indicating a Jurassic Period (138-205 Ma) emplacement. Granulite whole rock data suggest two age groups with affinities to tholeiitic basalts. Trace element analyses suggest one group is of Archean derivation while a second group is aligned with the Keweenawan extrusive rocks of the Mid-Continent Rift (Zartman and others, 2012). 49 �Stop 2a. Basal Hemlock Formation mineralized quartzite (Figure 13) UTM: 46o 9.545N 88o 11.087W, NENE Sec. 4, 43-31 Stop 2 will concentrate on Michigamme Mountain, a local topographic high (Figure 13). The principle rock at this location is a unique mineralized quartzite of limited areal extent. Gair (1956) named the quartzite Goodrich with the younger Hemlock overlying the quartzite. We now know that the age of the Hemlock (1.84 Ga) matches the age of the major iron formations in the Lake Superior region. The quartzite is composed of sub-angular quartz and granular fragments of chert. The quartzite overlies a thin massive chert. A later hydrothermal influx of specularite (Figure 14) and magnetite (Figure 15) associated with potassic and silica enrichment replacing some of the quartz and chert in the quartzite. Subsequent surface supergene oxidation converted some of the magnetite to martite. Soluble iron values are generally below 20% but can exceed 50%. At this location we are standing on an east-west structural anticline that may possibly be a chert dome. Stop 2b. Specularite chert from test pits. UTM: 46o 9.583N 88o 11.115W, SESE Sec. 33, 44-31 In the saddle between the two mineralized quartzite topo highs is a massive chert with secondary specularite (Fig. 13). You will note quite a bit of cross cutting specular hematite suggesting the iron oxide was emplaced in the chert at a later time. From Stops 2a & b you can see the highest elevation on Michigamme Mountain 80 yards to the southwest. The topographic high outcrop is quartzite with only minor magnetite and the occasional quartz vein containing micaceous selvages along the contacts 50 �Figure 14. Specularite chert from saddle on Michigamme Mountain Figure 15. Martite after magnetite with quartz/ adularia vein from Michigamme Mountain. Stop 2c. Hydrothermally enhanced rhyolite UTM: 46o 9.581N 88o 11.116W, SWSE Sec. 33, 44-31 While ascending Michigamme Mountain note the occasional black rock both to the north and underfoot. This is the same magnetite, now martite, quartzite to be seen at Stop 2a. The quartzite is overlain by a rhyolite flow representing the basal Hemlock Formation present along the eastern outcrop area of the Amasa Uplift. The rhyolite has quartz eyes and is typically pink on outcrop and orange on fresh surface (Figure 16 and 17). The orange color is primarily due to the addition of potassium feldspar. Chemistry of most of the rhyolite in the Hemlock shows the difference in K-spar (Table 1). Slight additional silica is provided by numerous random quartz veins that can be found in both the rhyolite (Figure 17) and underlying quartzite. Some of the veins in the quartzite contain selvages of micaceous hematite. It is suspected the potassium and silica were introduced at the time both magnetite and specularite replaced portions of the chert and quartzite at the base of the Hemlock. Further support of this timing is noted by the absence of alteration of overlying tuff and agglomerate at this stop. Figure 16. Rhyolite outcrop showing quartz veins Figure 17. Enhanced alkali/silica rhyolite porphyry 51 �Oxide SiO2 Al2O3 Fe2O3 FeO MgO Ca0 Na2O K2O TiO2 P2O5 Alt rhyolite*1 73.5 12.2 2.3 .22 .31 .21 .23 9.85 .99 .08 99.9 Unaltered rhy.*2 72.7 10.1 1.3 3.1 l.8 1.4 1.0 3.9 .47 .08 95.9 Table 1. Comparison of altered and unaltered rhyolite within the Hemlock Formation. *1 USGS Bull. 1044 p. 54 *2 USGS Bull. 1226 p. 21 Potassium values are twice those of other Hemlock rhyolite flows (see Table 1). Gair and Wier (1956, p. 53) “The acid volcanic rocks in the western part of the quadrangle (Kiernan-Sec. 36, T. 44 N., R. 32 W; Sec. 5, T. 43 N., R. 31 W) is much fresher and poorer in feldspar than the rock in the vicinity of Michigamme Mountain”. Table 1 shows the chemical difference between fresh and altered rhyolite. Chemistry of most of the Hemlock rhyolites matches that of the granophyre of the West Kiernan Sill. Ueng (1988) suggested the rhyolite formed in the magma chamber by crystal fractionation similar to the West Kiernan Sill differentiation that produced the granophyre. It can be speculated that the iron oxide addition to the underlying quartzite and chert occurred after extrusion of the rhyolite and prior to the next basaltic eruption. The overlying mafic tuff and agglomerate do not show any alteration beyond the generation of chlorite common to most of the Hemlock Formation indicating the oxide event occurred before emplacement of the basaltic rocks. On our trip returning to the vehicle, we will examine test pit material that represents fine specularite replacing the host quartzite. Stop 2d. Agglomerate (optional) UTM: 46o 9.692N 88o 10.985W, SESE Sec. 33, 44-31 A significant portion of the Hemlock is composed of fragmental tholeiitic basalt variously referred to as agglomerate, breccia or conglomerates (Figures 18-19) depending on visual degree of sorting or angularity. This stop has been highly weathered and is covered by moss and lichens. A more photogenic opportunity will be afforded at Stop 8. Figure 18. Fine grained volcanic conglomerate. Figure 19. Coarse grained agglomerate (hyaloclastites) breccia. 52 �Stop 3. East Kiernan Sill (optional) UTM: 46o 8.828N 88o 12.568W, SWSE Sec. 5, 43-31 The East Kiernan sill is a much smaller intrusion than the Western Kiernan sill. It is primarily an undifferentiated gabbro containing 50-80% prismatic hornblende after augite, albite, titaniferous magnetite, and apatite with less than 5% quartz. The plagioclase has been saussuritized and some hornblende converted to chlorite while the oxides have been converted to titanite and rutile. A number of gabbro samples of both Kiernan sills have been analyzed for PGEs (Bornhorst, 1990). None of the samples show values above background. Stop 4a. Mansfield Mine Location Monument UTM: 46o 6.835 N 88o 13.076 W, NWNW Sec. 20, 43-31 In 1889 the Mansfield Mining Co. took a lease on the Mansfield natural iron ore deposit from J.M. Longyear. Bessemer iron ore (<.045% P) was mined from six levels down to 435 feet below surface. Most of the workings were under the Michigamme River. Figure 20. Sign and plaque commemorating the Mansfield Mine Site disaster of 1893 On the evening of Sept. 27, 1893 a lower level pillar gave way causing the rock above to cave to surface flooding the mine workings with water from the Michigamme River. Twenty seven miners lost their lives in an instant. In 1897 the DeSoto Iron Co. of Springfield, IL bought the mine, redirected the river channel and reopened the mine. The Oliver Mining Co. (now USX) operated the mine from 1911 until the exhaustion of iron ore in 1913 at which time the workings had reached the 17th level about 1480 feet below the collar. A total of 1,462,504 long tons (LT) were mined between 1890 and 1913. In 1983 the mine site was designated the Mansfield Mine Location Historic District. A plaque with the names of the 27 men who died in the disaster marks the site (Figure 20). A few restored buildings mark the site of the mining community. 53 �Stop 4b. Mansfield Iron bearing slate member and agglomerate/volcaniclastics. UTM: 46o 6.945’ N 88o 13.185’W, NWNW Sec. 20, 43-31 The vertical #2 shaft went through fine volcanic conglomerate of the Hemlock and can be found on the mine dump. The slate portion is also represented on the dump (Figure 21) along with the iron formation. The Mansfield iron member has a limited areal extent of about 4 miles on strike. It was originally a siderite chert with soluble irons ranging from 10-47.3% that averaged ~25%. Near the contact with the intrusive West Kiernan Sill the Mansfield iron member was metamorphosed to a magnetite stilpnomelane chert. At the Mansfield Mine location the original siderite chert was far enough from the sill contact to avoid metamorphic alteration, but it did experience supergene oxidation and enrichment (Figure 22) with ore grades averaging about 52.8% natural iron with less than .045% P making it Bessemer type ore. Figure 21. Graphitic slate with pyrite cubes Figure 22. Direct shipping hematite ore with gypsum(white arrow). Stop 5. Steeply dipping Hemlock pillowed and massive basalt-Hemlock Falls Dam Site. UTM: 46o 7.840 N 88o 13.504 W, SESE Sec. 7, 43-31 The bulk of the Hemlock Formation is composed of massive, amygdaloidal and pillowed basalts and volcaniclastic rocks of the same composition. On the western edge of the Hemlock Falls Dam abutment is a large outcrop of pillow basalt dipping steeply to the west. Directly overlying the pillowed basalt is massive basalt as seen on the left side of Figure 23. 54 �Figure 23. Steeply dipping tholeiitic pillowed basalts at the west abutment of the Hemlock Falls Dam site. Stop 6. Amygdaloidal basalt of the Hemlock Formation UTM: 46o 6.461 N 88o13.931 W, NWNW Sec. 20, 43-31 On the north side of the County Road is a low outcrop of broken amygdaloidal basalt porphyry (Figure 24) with abundant gas vesicles filled with secondary albite quartz and carbonate. Some parts of the basalt contain a few per cent sulfides, mostly pyrite. Amygdaloidal basalts are not as abundant as massive basalts or agglomerates. 55 �Figure 24. Polished surface of amygdaloidal basalt at Stop 6 DIFFERENTIATED WEST KIERNAN SILL The West Kiernan Sill is approximately 13 miles long and averages 1.1 miles in thickness. It is designated a sill in that most contacts are concordant with the Hemlock units. Regional metamorphism for the entire sill is in the greenschist metamorphic zone. The sill can be divided into four distinct rock units: a basal serpentinized peridotite (Stop 11), a thick normal gabbro, a transition gabbro and, occasionally, an upper granophyre (Figure 25). A chill zone in the contact area is usually a diabase. The basal peridotite is composed of serpentine with aggregates of tremolite, talc and magnetite with minor amounts of carbonate and chlorite. Overlying the serpentine is a 900-1200 m thick normal gabbro. An increase in size and amount of plagioclase is noted from the base to the top of the unit (Figure 25). Alteration stilpnomelane is noted in the upper portion of the unit and becomes common in the overlying transition gabbro. The transition (Stop 7b) zone contains spotty concentrations of titaniferous magnetite that show up as magnetic bullseyes on airborne magnetic survey maps. The granophyre looks like a medium grained granite with minor or missing mafic minerals. Minerals present include albite, oligoclase, microcline and orthoclase with occasional needle of magnetite. Calcite is also pervasive. Geochemical analyses of the sill units are given in Table 2 along with analyses of the Hemlock basalt. 56 �1 2 3 Transition Oxide Peridotite*1 Gabbro*1 Gabbro*1 SiO2 39.01 44.94 57.2 Al2O3 6.56 19.70 10.8 Fe2O3 11.49 1.72 8.8 FeO 5.30 7.40 7.5 MgO 23.84 8.91 2.7 CaO 3.57 9.22 4.8 Na2O -1.94 2.8 K2O .02 .36 1.1 TiO2 2.04 .73 2.9 P2O5 .19 .09 .76 MnO .11 .13 .26 CO2 .08 .45 .37 H2O 7.10 4.56 3.90 Total 99.97 100.26 98.90 4 5 High Iron*2 34.1 3.4 13.1 22.3 8.5 7.6 .13 .10 5.42 .38 .38 3.0 98.40 Granophyre 69.12 12.83 .78 5.85 1.38 .70 1.72 3.44 .73 .11 .05 .70 2.25 99.75 *1 Bayley, 1959, Amer. Jour. of Sci. v. 257, p. 428 (col. 1-3) *2 Wier, 1967, USGS Bull. 1226, p. 35 (NW ¼ Sec. 12, 43-32) *3 Foose, 1981, N=7 *4 Foose, 1981, N=10 agglomerate/volcaniclastics Table 2. West Kiernan Sill-Whole rock analyses 57 7 Hemlock Basalt*3 48.2 14.6 6.7 7.8 5.9 5.8 3.03 .92 2.21 .30 .17 .71 2.64 98.98 8 Hemlock Agglomerate*4 47.6 14.4 5.4 7.6 6.6 7.8 3.29 .70 2.15 .19 .17 .70 2.57 99.17 �Based on the similarities in geochemistry between the West Kiernan and the Hemlock extrusives (Ueng, 1987) concluded both major igneous rocks were co-magmatic in origin. He further stated crystal fractionation in the magma chamber was responsible for the occasional rhyolite flows. Xenoliths of Hemlock are present in both the East and West sills (Bayley, 1959; Wier, 1967). Stops 7a-b-c will allow us to examine the transition gabbro (7c) containing abundant magnetite and the overlying granophyre (7a). Whole rock assays for these outcrops are shown on Table 3. Note the major increase in silica, sodium and potassium coupled with a significant reduction in iron oxides and titanium in the granophyre. The two rock types represent a significant chemical change during late stage differentiation. OXIDE A*1 B*2 SiO2 34.1 51.0 Al2O3 3.4 16.3 Fe2O3 13.1 1.4 FeO 22.3 9.2 MgO 8.5 8.5 CaO 7.6 10.1 Na2O .13 1.83 K2O .10 .68 H2O 3.0 Ti02 5.42 .85 P2O5 .38 .05 CO2 <.05 MnO .38 .14 S .38 *1 test pit with magnetite in transition gabbro. USGS Bulletin 1226, p. 35 *2 Fox M.S. thesis sample A5P outcrop near railroad cut. Table 3. Whole rock analyses upper Kiernan Sill transition gabbro near granophyre contact (Stop 7). Stop 7a. West Kiernan Sill granophyre UTM: 46o 8.545 N 88o 15.742 W, NWNW Sec. 12, 43-32 The outcrop is typical of the granophyre phase of the West Kiernan Sill. ferromagnesium mineralization is present and it does resemble a granite. Very little Stop 7b. Coarse grained transition gabbro UTM: same as 7a. This location is very coarse grained and contains around 80% plagioclase, typical of the upper part of the transition gabbro. The apatite is of the fluorine variety. Stop 7c. Transition gabbro with magnetite UTM: 46o 8.508 N 88o 15.717 W, NWNW Sec.12, 43-32 These test pits are near the top of the transition gabbro where abundant titaniferous magnetite is common. Additional minerals include stilpnomelane/bannisterite, ilmenite, biotite, augite, 58 �feldspar, chlorite and ferrohornblende (Figure 26-27). Ueng (1988) speculated that magnetite, ilmenite and apatite crystalized during fractionation in the slowly cooled sill. Figure 26. Skeletal titaniferous magnetite in transition gabbro. Figure 27. Gabbro porphyry with high titaniferous magnetite content. Stop 8. Top of Hemlock with increased magnetite UTM: 46o 14.923’ N 88o 26.206’ W, Sec. 4, 44-33 Figure 28. Hemlock agglomerate with iron oxide values above 20% mostly as magnetite. Stop 8 represents the upper 6000 feet of the Hemlock. Fine grained pillow and fragmental tholeiitic basalts (Figure 28) predominate and contain up to 25% FeO with most of the iron over 12% FeO as magnetite. 59 �Stop 9. West Kiernan Sill-copper/nickel gabbro UTM: 46o 4.923 N 88o 12.644 W, SENW Sec. 32, 43-31 The majority of the sill consists of a normal gabbro. Plagioclase and augite were the major minerals in the rock. The augite has been altered to hornblende. At this stop the gabbro outcrop contains disseminated pyrrhotite, chalcopyrite and pentlandite (Figure 29). The outcrop exhibits a typical northern climate gossan with shallow oxidation representing low grade disseminated sulfides. A grab sample from this outcrop assayed 0.35% copper/nickel. Figure 29. Polished slab showing Chalcopyrite-pentlandite-pyrrhotite gabbro from the West Kiernan sill. Stop 10. Magnetite bombs in the Hemlock Formation UTM: 46o 5.191’ N 88o 12.015’ W In 2005 Prime Meridian Resources drilled a coincident TEM and magnetic anomaly target in the Hemlock Formation east of the Michigamme River and just north of highway M-69. The magnetic anomaly is due to concentrations of magnetite and the conductor anomaly is due to increased amounts of pyrite and chalcopyrite in some parts of the magnetite rich zone. The hole identified as KS-102-2 penetrated alternating chert, tuffs and amygdaloidal basalts bottoming in magnetite rich basalts resembling volcanic ejecta or bombs (Figure 30). 60 �Figure 31. Photo of the outcrop of magnetite bombs with the up direction to the top. Field trip participants will examine diamond drill core representing the siliceous zone and the magnetite rich zone at the bottom of the hole. The best outcrop example is approximately 1 mile round trip from this location, however, the outcrop is now encrusted in lichen obscuring the salient features seen in the Figure 31. We will examine core from DDH KS-102-2 which exhibits excellent features of the magnetite bombs (Figure 32). 61 �Figure 32. Diamond drill core showing magnetite bombs with albite and calcite filling amygdules. Figure 33. Polished bomb showing amygdules and groundmass magnetite and titanite. Individual fragments range in size from 5-7 inches long and 1-2 inches thick and contain variable magnetite concentrations in the groundmass ranging from 15-70% (Figure 33). The groundmass consists of albite, titanite and magnetite. The amygdules have been filled with albite, epidote, calcite and ilmenite. Both the magnetite and titanite are absent from the vesicles supporting the concept that the magnetite was primary and was not involved with later hydrothermal overprint. The size of the magnetite is bimodal with two distinct morphologies present. The first are the small euhedral individual crystals of magnetite while the second type can best be described as aggregates of anhedral magnetite. The distribution of the magnetite in the groundmass is not uniform. In some areas the amount of magnetite approaches 70%. It is suggested the crystallization had occurred by the time it was expelled from the vent. The magnetite was observed to increase in size and amount near the edges of vesicles and between large vesicles. The fine interstitial material between the bombs is devoid of magnetite and titanite indicating a different source for the fine grained interstitial material. It has been suggested that what appears to be bombs are in reality very small flattened pillows and not bombs at all. This interpretation is a possibility, however, the lack of magnetite in the fine grained material between fragments would be difficult to reconcile pillow creation. One more significant fact is the magnetite is rather pure and does not contain any titanium suggesting that element separation occurred in the magma chamber. Ueng (1988) also described rhyolite bombs in the central portion of the Hemlock Formation. Stop 11. Serpentinized peridotite at the base of the West Kiernan sill UTM: 46o 4.894 N 88o 11.832 W, SWNW Sec. 33, 43-31 Stop 11 represents the basal peridotite of the West Kiernan Sill that has undergone extensive serpentinization. The roadside outcrop on the north side of M-69 exhibits fuzzy outlines of altered phenocrysts of pyroxene. Ueng (1988) did not observe any relic olivine. The process of serpentinization creates abundant secondary magnetite as evidenced by the strong magnetic attraction exhibited by the outcrop. Additional alteration minerals include tremolite, talc, 62 �carbonate, chlorite and some actinolite. Chemistry of the ultramafic rocks are shown in Table 2 where the MgO values are about 23.8%. Acknowledgements I would like to thank Klaus Schulz and Bill Cannon for their constructive conversations and comments on the guide content. Appreciation is expressed to two other reviewers for their helpful review. I would also like to thank Stacy Saari for work on several illustrations. Appreciation is extended to the multiple private land owners for allowing access to several of the field trip stops. During the poster session a display of North American kimberlite samples from the Doug Duskin collection will be on display for comparison with the Lake Ellen kimberlite visited on this field trip. The bibliography at the end of the field trip guide contains many more references than cited in the text to aid anyone who wishes to delve further into the subject in greater detail. Bibliography-Hemlock Formation Aldrich, L.T., Davis, G.L., and James, H.L., 1965, Ages of minerals from metamorphic and igneous rocks near Iron Mountain, Michigan: Journal of Petrology, v. 6, p. 445-472. Anhaeusser, C.R., 1985, Archean layered ultramafic complexes in the Barberton Mountain Land, South Africa in Ayers, L.D., Thurston, P.C., Card, K.D., and Weber, W. eds., Evolution of Archean supracrustal sequences: Geological Association of Canada Special Paper 28, p. 281-301. Banks, P.O., and Van Schmus, W.R., 1971, Chronology of Precambrian rocks of Iron and Dickinson Counties, Michigan: Institute on Lake Superior Geology abs, v. 17, p. 9-10. Bartlett, W.A., et al, 1976, Distribution of sulfur in the West Kiernan Sill, Iron County, Michigan: Bowling Green State University MS thesis, 87p. Bartlett, W.A., Lougheed, M.S., Mancuso, J.J., and Walter, L.J., 1976, Distribution of sulfur in the West Kiernan Sill, Iron County, Michigan: Institute on Lake Superior Geology abs, v. 22, p. 6. Bartlett, W.A., 1976, Distribution of sulfur in the West Kiernan Sill, Iron County, Michigan: Bowling Green State University M.S. thesis, 87 p. Barovich, K.M., Patchett, P.J., and Peterman, Z.E., 1987, Origin of the 1.9-1.7 Ga Penokean continental crust of the Lake Superior region abs: Eos, v. 68, p. 1547. Bayley, R.W., 1959, A metamorphosed differentiated sill in Northern Michigan: American Journal of Science, v. 257, p. 408-430. Bayley, R.W., 1959, Geology of the Lake Mary Quadrangle Iron County, Michigan: U.S Geological Survey Bulletin 1077, 112 p. 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 573, 96 p. Baxter, D.A., and Bornhorst, T.J., 1988, Multiple discrete mafic intrusion of Archean to Keweenawan age, western Upper Peninsula, Michigan: Institute on Lake Superior Geology abs, v.34, p. 6-8. Beck, J.W., 1984, Nd and Sm isotopic studies of the Quinnesec and Hemlock Formations in northeastern Wisconsin and adjacent Michigan: Lake Superior Geology abs, v. 30, p. 1-2. Beck, J.W., 1988, Implications for early Proterozoic tectonics and the origin of continental flood basalts based on combined trace element and neodymium/strontium isotopic studies of mafic igneous rocks of the Penokean Lake Superior belt, Minnesota, Wisconsin and Michigan: University of Minnesota Ph.D. thesis, 262 p. Beck, J.W., and Murthy V.R.., 1991, Evidence for continental crystal assimilation in the Hemlock Formation flood basalts of the early Proterozoic Penokean orogeny, Lake Superior region: U.S. 63 �Geological Survey Bulletin 1904 I, p. 101-125. Bornhorst, T.J., and Baxter, D.A., 1990, Reconnaissance evaluation of platinum group elements in selected Precambrian rocks of the western Upper Peninsula, Michigan: Michigan Department of Natural Resources Geological Survey Division: Geology Report 90-2, 39 p. Cambray, F.W., 1978, Plate tectonics as a model for the environment of deposition and deformation of the early Proterozoic (Precambrian X) of Northern Michigan: Geological Society of America abs, p. 7 Cannon, W.F., 1973, The Penokean orogeny in northern Michigan in Huronian stratigraphy and sedimentation: Geological Society of America Special Paper 12, p. 251-271. Cannon, W.F., and Klasner, J.S., 1976, Geologic map and geophysical interpretation of the Witch Lake Quadrangle Marquette, Iron and Baraga Counties, Michigan: U.S. Geological Survey Map I-987, scale 1:62,500. Cannon, W.F., 1983, Mineral resource assessment of the Iron River 1o x 2o Quadrangle, Michigan and Wisconsin: U.S. Geological Survey Circular 887, 21 p. Cannon, W.F., 1985, Mineral-resources map of the Iron River 1o x 2o Quadrangle, Michigan and Wisconsin: U.S. Geological Survey Map I-1360-A, scale 1:250,000. Cannon, W.F., 1986, Bedrock geologic map of the Iron River 1o x 2o Quadrangle, Michigan and Wisconsin: U.S. Geological Survey Map I-1360-B, scale 1:250,000. Cannon, W.F., 1986, Structural and tectonic map of the Iron River 1o x 2o Quadrangle, Michigan and Wisconsin: U.S. Geological Survey Map I-1360-D, scale 1:250,000. Chartier, R., 1985, The texture and mineralogy of the Lake Ellen kimberlite, Crystal Falls, Michigan USA: Institute on Lake Superior Geology abs, v. 31, p. 10. Clements J.M., and Smyth, H.L., 1899, The Crystal Falls Iron-Bearing District of Michigan: U.S. Geological Survey Monograph 36, 512 p. Cogan, M.J., 1993, Primary and secondary Bouguer gravity trend analysis and structural implications for early Proterozoic Kiernan sills, Iron County, Michigan: Geological Society of America, v. 25, p. 13. Cudzilo, T.F., 1978, Geochemistry of early Proterozoic igneous rocks, northeastern Wisconsin and upper Michigan: University of Kansas Ph..D dissertation, 202 p. Cummings, M.L., 1978, Metamorphism and mineralization of the Quinnesec Formation, northeastern Wisconsin: University of Wisconsin Ph.D. thesis, 190 p. Dann, J.C., 1978, Major-element variation within the Emperor Igneous Complex and the Hemlock and Badwater volcanic Formations: Michigan Technological University M.S. thesis, 159 p. Dann, J.C., 1978, Major-element variation within the Emperor Igneous Complex and the Hemlock and Badwater volcanic Formations: Institute on Lake Superior Geology abs, p. 15. DeMatties, T.A., Rowell, W.F., Munroe, J.F., 2007, An evaluation of the Prime Meridian midcontinent nickel-copper exploration program: Technical Report: Prime Meridian Resources, 538 p. Dutton, C.E., and Linebaugh, R.E., 1967, Map showing Precambrian geology of the Menominee IronBearing District and vicinity Michigan and Wisconsin: U.S. Geological Survey Map I-466, scale 1:125,000. Dutton, C.E., 1971, Geology of the Florence area, Wisconsin and Michigan: U.S. Geological Survey Professional Paper 633, 54 p. Foose, M.P., 1981, Geology of the Ned Lake Quadrangle, Iron and Baraga Counties, Michigan: U.S. Geological Survey Map I-1284, scale 1:62,500. Fox, T.P., 1983, Geochemistry of the Hemlock metabasalt and Kiernan sills, Iron County, Michigan: Michigan State University M.S. thesis, 73 p. Gair, J.E., and Wier, K.L., 1956, Geology of the Kiernan Quadrangle Iron County, Michigan: U.S. Geological Survey Bulletin 1044, 88 p. Graff, C.W., 1982, Iron-enriched basaltic fragmental rocks erupted in a shallow subaqueous environment, the Hemlock Formation, Amasa Quadrangle, Michigan: Institute on Lake Superior abs, v. 28, p. 11. 64 �Greenberg, J.K., and Brown, B.A., 1983, Lower Proterozoic volcanic rocks and their setting in the southern Lake Superior district, in Medaris. L.G., Jr., ed., Early Proterozoic geology of the Lake Superior region: Geological Society of America Memoir 160, p. 67-84. Han, T.M., 1966, Textural relations of hematite and magnetite in some Precambrian metamorphosed oxide iron-formations: Economic Geology, v. 61, p. 1306-1310. Han, T.M., 1978, Microstructures of magnetite as guides to its origin in some Precambrian iron formations: Fortschr. Mineral, v. 56, p. 105-142. Han, T.M., 1988, Origin of magnetite in Precambrian iron-formations of low metamorphic grade in Proceeding of the Seventh Quadrennial IAGOD Symposium: E. Schweizerbart’sche Verlagsbuchhandlung, D-7000 Stuttgart 1: p. 641-656. Heran, W.D., and Smith B.D. 1980, Description and preliminary map of airborne electromagnetic survey of parts of Iron, Baraga, and Dickinson Counties Michigan. U.S. Geological Survey Open File Report 80-297, 8 p. Hoffman, J.D., 1984, Nickel distribution in B-horizon soils, Iron River 1o x 2o Quadrangle, Michigan and Wisconsin: U.S. Geological Survey Map I-1360-K, scale 1:250,000. Hoffman, P.F., 1987, Early Proterozoic foredeeps, foredeep magmatism, and superior type iron formations of the Canadian shield, in Kroner, A., ed., Proterozoic lithospheric evolution: American Geophysical Union Geodynamics Series, v. 17, p. 85-98 Hotz, P.E., 1953, Petrology of granophyre in diabase near Dillsburg, Pennsylvania: Geological Society of America Bulletin, v. 64, p. 675-704. James, H.L., 1955, Zones of regional metamorphism in the Precambrian of northern Michigan, Geological Society of America Bulletin, v. 66, p. 1455-1457. James, H.L., Dutton, C.E., Pettijohn, F.J., and Wier, K.L., 1968, Geology and ore deposits of the Iron River-Crystal Falls District, Iron County, Michigan: U.S. Geological Survey Professional Paper 570, 134 p. James, H.L., 1958, Stratigraphy of pre-Keweenawan rocks in parts of northern Michigan: U.S. Geological Survey Professional Paper 314-C, 44 p. Johnson, D.J., 1975, Petrology of a portion of the Hemlock Formation, Iron County, Michigan: Michigan Technological University MS thesis, 51 p. Johnson, D.J., 1975, Petrology and tectonic setting of the Hemlock Formation, Iron County, Michigan: Institute on Lake Superior Geology abs, v. 21, p. 3 King, E.R., 1987, Aeromagnetic map of the Iron River 1o x 2o Quadrangle, Michigan and Wisconsin: U.S. Geological Survey Map I-1360-F, scale 1: 250,000. Kirkley, M.B., Gurney, J.J., and Levinson, A.A., 1992, Age, origin and emplacement of diamonds: a review of scientific advances in the last decade: CIM Bulletin, v. 84, p. 48-57. Klasner, J.S., Ojakangas, R.W., Schulz K.J., and Laberge, G.L., 1988, Evidence for development of an early Proterozoic overthrust-nappe system in the Penokean orogeny of Northern Michigan: Institute on Lake Superior Geology abs, v. 34, 56-57. Klasner, J.W., and Jones, W.J., 1989, Bouguer gravity anomaly map and geologic interpretation of the Iron River 1o x 2o Quadrangle, Michigan and Wisconsin: U.S. Geological Survey Map I-1360-E, scale 1:250,000. Krapez, B., Barley, M.E., and Pickard, A.L., 2003, Hydrothermal and resedimented origins of the precursor sediments to banded iron formation: sedimentological evidence from the early Paleoproterozoic Brockman Supersequence of Western Australia: Sedimentology, v. 50 p. 979-1011. Kruger, C.L., 1967, Aeromagnetic map of the Crystal Falls Quadrangle and part of the Florence Quadrangle, Iron County, Michigan: U.S. Geological Survey Map GQ-607, scale 1:62,500. Kruger, C.L., 1967, Aeromagnetic map of the Perch Lake Quadrangle, Houghton, Baraga and Iron Counties, Michigan: U.S. Geological Survey Map GP-600, scale 1:62,500. Kruger, C.L., 1967, Aeromagnetic map of the Ned Lake Quadrangle and part of the Witch Lake Quadrangle, Iron, Baraga and Marquette Counties, Michigan: U.S. Geological Survey 65 �Map GP-609, scale 1:62,500. Larue, D.K., and Sloss, L.L., 1980, Early Proterozoic sedimentary basins of the Lake Superior region: Geological Society of America Bulletin, pt. II, v. 91, p. 1836-1874. Larue, D.K., 1983, Early Proterozoic tectonics of the Lake Superior region-tectonostratigraphic terranes near the purported collision zone in Medaris, L.G., Jr., Early Proterozoic geology of the Lake Superior region: Geological Society of America Memoir 160, p. 33-47. Larue, D.K., and Ueng, W.L., 1985, Florence-Niagara terrane-an early Proterozoic accretionary complex, Lake Superior region: Geological Society of America Bulletin, v. 96, p. 1179-1187. MacRae, N.D., 1969, Ultramafic intrusions of the Abitibi area, Ontario: Canadian Journal of Earth Sciences, v. 6, p. 281-303. March, B.D., 2006, Dynamics of magmatic systems: Elements, v. 2, p. 287-292. Ozawa, A., Ueda, A., “Fantong, W.Y., Anazawa, K, Yoshida, Y, Kusakabe, M., Ohba, T., Tanyileke, G., and Hell, J.V., 2016, Rate of siderite precipitation in Lake Nyos, Cameroon, geochemistry and geophysics of active volcanic lakes: Ohba, Capaccioni and Caudron, eds., Geolgical Society, London, Special Publications 437. Paddock, D.R., 1982, A Gravity investigation of eastern Iron County, Michigan: Michigan State University M.S. thesis, 110 p. Peterson, W.L., 1985, Surficial geologic map of the Iron River 1o x 2o Quadrangle, Michigan and Wisconsin: U.S. Geological Survey Map I-1360-C, scale 1:250,000. Rehfuss, I.L., 1912, The Bird mine section: University of Wisconsin B.A. thesis, 49 p. Ruotsala, A.P., 1974, Composition and tectonic setting of middle Precambrian lavas, Crystal Falls area, Michigan: Institute on Lake Superior Geology abs, v. 20, p. 28. Schneider, D.A., Bickford, M.E., Cannon, W.F., Schulz, K.J. and Hamilton, M.A., 2002, Age of volcanic rocks and syndepositional iron formations, Marquette Supergroup: implications for the tectonic setting of Paleoproterozoic iron formations of the Lake Superior region: Canadian Journal of Earth Science, v. 39, p. 999-1012. Schulz, K.J., 1982, Magnesian basalts from the Archean terrains of Minnesota, in Arndt, N.T., and Nisbet, E.G., eds, Komatiites: London, George Allen and Unwin, p. 171-186. Schulz, K.J., and Cannon, W.F., 2007, The Penokean orogeny in the Lake Superior region: Precambrian Research, v. 157, p. 4-25. Schulz, K.J., and Cannon, W.F., 2008, Synchronous deposition of Paleoproterozoic superior-type banded iron formations and volcanogenic massive sulfides in the Lake Superior region: implications for the tectonic evolution of the Penokean orogeny: Geological Society of America, abs. w/programs, v. 40, p. 387. Schulz, K.J., 1984, Early Proterozoic Penokean rocks of the Lake Superior region: geochemistry and tectonic implications: Institute on Lake Superior Geology abs, v. 30, p. 65-66. Sims, P.K., 1992, Geologic map of Precambrian rocks, southern Lake Superior region, Wisconsin and northern Michigan: U.S. Geological Survey Map I-2185, scale 1:500,000. Stahl, S.D., et al, 1993, Primary and secondary Bouguer gravity trend analysis of the Kiernan sills area, Iron County, Michigan: implications for early Penokean tectonics: The Geology of Michigan and its Geological Resources Symposium III, MI DNR. Ueng, W.C., Larue, D.K., and Sedlock, R.L., 1984, The early Proterozoic tectonic history of the southcentral Lake Superior region: Institute on Lake Superior Geology field trip guide, v. 30, p. 1-22. Ueng, W.C., Larue, D.K., and Sedlock, R.L., 1988, Geochemistry and petrogenesis of the early Proterozoic Hemlock volcanic rocks and the Kiernan sills, southern Lake Superior region: Canadian Journal of Earth Science, v. 25, p. 528-546. Vallini, D.A., Cannon W, F, and Schulz, K, J., 2006, Age constraints for Paleoproterozoic glaciation in the Lake Superior region: detrital zircon and hydrothermal xenotime ages for the Chocolay Group, Marquette Range Supergroup: Canadian Journal of Earth Science, v. 43, p. 571-591. Waggoner, T. D., Duskin, D., Karakus, M., and Gartner, J., 2011, Pyroclastic magnetite bombs in 66 �the Hemlock Formation, Iron County, Michigan: Institute on Lake Superior abs, v. 57, p 93-94. Walker, F., 1969, The Palisades Sill, New Jersey-a reinvestigation: Geological Society of America Special Paper 111, 178 p. Wier, K.L., 1967, Geology of the Kelso Junction Quadrangle Iron County, Michigan: U.S. Geological Survey Bulletin 1226, 47 p. Wier, K., 1986, Metamorphic map of the Iron River 1o x 2o Quadrangle, Michigan and Wisconsin: U.S. Geological Survey Map I-1360-G, scale 1:250,000. Williams, D.A.C. and Halberg, J.A., 1973, Archean layered intrusions of the eastern Goldfields region, Western Australia: Contribution to Mineralogy and Petrology, v. 38, p. 45-70 Zeitz, I., and Kirby, J.R., 1971, Aeromagnetic map of the western part of the northern peninsula, Michigan, and part of northern Wisconsin: U.S. Geological Survey Map GP-750, scale 1:250,000. __________, 1978, Aerial gamma ray and magnetic survey peninsula portion, Hancock Quadrangle Wisconsin, Minnesota, Michigan, Iron River Quadrangle, Michigan Wisconsin and Marquette Quadrangle: Michigan, v. 1 Final DOE Report. __________, 1980, Airborne electromagnetic map and magnetic survey of parts of the upper Peninsula Of Michigan and Northern Wisconsin: U.S. Geological Survey Open File Report 81-577, Scale 1:24,000. Bibliography-Lake Ellen Kimberlite. Cannon, W.F., and Mudrey, M.G., 1981, The potential for diamond-bearing kimberlite in northern Michigan and Wisconsin: USGS Circular 842, 15 p. Carlson, S.M., and Floodstrand, W., 1994, Michigan kimberlites and diamond exploration techniques: Institute on Lake Superior Geology field trip guide, Part 4, p. 1-15. Chartier, T., 1985, The texture and mineralogy of the Lake Ellen kimberlite: Institute on Lake Superior Geology abs, v, 31, p. 10-11. Clements, B., 2017, The Canadian diamond business: 25 years and going strong: SEG Newsletter, p. 1 and 12-17. Griffin, W.L., O’Reilly, S.Y., Doyle, B,J., Pearson, N.J., Coopersmith, H., Kivi, K., Malkovets, V. and Pokhilenko, N., 2004, Lithosphere Mapping beneath the North American Plate: Lithos, v. 77, p. 873-922. Hausel, W.D., 1998, Diamonds and mantle source rocks in the Wyoming craton with a discussion of other U.S. occurrences: Wyoming State Geological Survey Report of Investigations 53, 93 p. Head, J.W., and Wilson, L., 2002, Diatremes and kimberlites I: definition, geological characteristics and association: Micro Symposium 36 (MSO32), 2 p. Jarvis, W., and Kalliokoski, J, 1988, Michigan kimberlite province: Institute on Lake Superior Geology abs, v. 34, p. 46-48. Jarvis, W., 1993, Michigan kimberlites: an update: abs, 61swt PDAC, Paper M-10. Kirkley, M.B., Gurney, J.J., 1992, Age, origin and emplacement of diamonds: a review of scientific advances in the last decade: CIM Journal, v. 84, p. 48-57. Kjarsgaard, B.S., 2007, Kimberlite pipe models: significance for exploration, ore deposits and exploration technology in Proceeding of Exploration 07: Fifth Decennial International Conference on Mineral Exploration, Milkereit, B., ed.: Paper 46 p. 667-677. McGee, E.S., and Hearn, B.C., 1983, Lake Ellen kimberlite, USA: U.S. Geological Survey Open File Report 83-156, 34 p. McGee, E.S., and Hearn, B. C., 1984, The Lake Ellen kimberlite, Michigan, U.S.A.: in Kimberlites. I: kimberlites and related rocks, Korprobst, J., ed., p. 143-154. 67 �McGee, E.S., 1987, Garnet xenocryst analysis potential for diamonds in Williams kimberlite, north central Montana and the Lake Ellen kimberlite, northern Michigan: U.S. Geological Survey Open File Report 87-418, 15p. McGee, E.S., 1988, Potential for diamonds in kimberlites from Michigan and Montana as indicate by garnet xenocryst composition: Economic Geology, v. 83, p. 428-432. Paces, J.B., and Taylor, L.A., 1990, Petrography, mineral chemistry and geothermobarometry of mafic granulite and eclogite nodules from upper Michigan kimberlites: Institute on Lake Superior Geology abs, v 36, p. 82-84. Quigley, P.O., 2007, Michigan kimberlites revisited: new mineral, chemical and petrographic analysis: Institute on Lake Superior Geology abs, v. 53, p. 63. Scully, K.R., Canil, D., and Schulze, D.J., 2004, The lithospheric mantle of the Archean Superior Provence as imaged by garnet xenocryst geochemistry: Chemical Geology, v. 207, p. 189-221. Skillings, D., 1995, Crystal Exploration Inc. continuing to evaluate diamond potential of Michigan’s upper peninsula and in Wisconsin: Skillings Mining Review, v. 84, p. 5-7. Zartman, R.E., Kempton, P.D., Paces, J.B., Downes, H., Williams, I.S., Dobosi, G., and Futa, K., 2013, Lower-Crustal xenoliths from Jurassic kimberlite diatremes, upper Michigan (USA): evidence for Proterozoic orogenesis and plume magmatism in the lower crust of the southern Superior Province: Journal of Petrology, v. 54, p. 575-608. ________, 1992, Petrography of the Crotch Lake kimberlite, Michigan: Exmin document on file at the Michigan DEQ Core Library. ________, 1993, Ashton Mining of Canada Inc. Annual Report, 21 p. 68 �FIELD TRIP 3 Friday, May 18, 2018 GEOLOGY AND IRON ORES OF THE MENOMINEE IRON RANGE, DICKINSON COUNTY, MICHIGAN Thomas H. Mroz, BSGE, MSPG, CPG William F. Cannon, Klaus J. Schulz, Robert A. Ayuso, U.S. Geological Survey INTRODUCTION The Menominee Iron Range was visited on a previous ILSG field trip in 2003. This trip differs substantially from that previous trip and visits mostly new localities with only three exceptions. The following introductory material is reproduced in large part from the 2003 Guidebook (LaBerge et al., 2003). The trip visits most of the stratigraphic units of the area including the Carney Lake Gneiss, the Archean basement on which Paleoproterozoic sedimentary rocks were deposited. The emphasis of this trip is the Paleoproterozoic section and most stratigraphic units will be seen. Several stops are focused on the Vulcan Iron-formation, the principal iron-bearing unit of the Range. Both the unaltered formation and the secondary ores that formed within it will be seen. The Menominee Iron Range, one of the principal iron producing districts of the Lake Superior region, produced about 260 million tons of high grade iron ore between 1873 and 1946, but has been inactive since. The ores were secondary concentrations of iron oxides and hydroxides within the Paleoproterozoic Vulcan Iron-formation. The ores are generally believed to be paleosupergene concentrations that formed on the Cambrian surface and were covered by late Cambrian sandstone of the Munising Formation. The range also lies very near the Niagara fault zone, the paleosuture between the Superior craton and the accreted Pembine-Wausau arc terrane of northern Wisconsin, and bears the imprint of the strong deformation produced during the accretion. Stratigraphy. The presently accepted stratigraphic terminology for the Menominee Iron Range was developed by Bayley et al. (1966) and modified only slightly since. The stratigraphic relationships are shown in Figure 2, which is modified from Bayley et al. (1966) to reflect changes in terminology and radiometric age determination since that publication. Precambrian rocks range in age from Archean to Mesoproterozoic. They are capped by Late Cambrian sandstones, which occur as numerous outliers and underlie most of the higher ridges along the Range 69 �Figure 1. Generalized geologic map of the Menominee Iron Range showing the location of the field trip stops. . Figure 2. Sequence of formations in the Menominee Iron Range. Modified from Bayley et al. (1966, table 6). 70 �The following summary draws heavily on previous descriptions by Bayley et al. (1966), who provided the most recent comprehensive study of the Range, and LaBerge et al. (2003), who prepared the 2001 ILSG fieldtrip guidebook for the Range. The Precambrian stratigraphic units can be divided into four principal sequences ranging from Archean to Mesoproterozoic. Radiometric age determinations conducted since the previous ILSG guidebook provide new clarity on absolute ages of these sequences. Archean- Archean rocks form the basement on which the Paleoproterozoic rocks of the Menominee Iron Range were deposited. They underlie a large area north of the Range and are known as the Carney Lake Gneiss. The Carney Lake is a complex, multilithic assemblage of mostly granitic and lesser mafic rocks, which are described more fully in the description of Stop 1. The most recent rock-forming period of the complex was at about 2.75 Ga, but recent age determinations (Ayuso et al., 2017; this volume) have identified cores of zircon grains as old as about 3.8 Ga indicating a very extended history within these rocks. Paleoproterozoic- The Paleoproterozoic rocks of the Menominee Iron Range are entirely sedimentary and together make up the Marquette Range Supergroup. Three individual groups are present; from oldest to youngest they are the Chocolay, Menominee, and Baraga Groups. Each group is separated by unconformities and the Supergroup, as well, lies unconformably on the Archean Carney Lake Gneiss. Chocolay Group- The Fern Creek Formation, Sturgeon Quartzite, and Randville Dolomite make up the Chocolay Group. The basal formation, the Fern Creek Formation, is a glaciogenic unit that is preserved only sporadically in the area. Stop 2 is one of the best localities to study it. The stop has been well described by R.J Ojakangas (LaBerge et al., 2003). The medial unit of the Chocolay Group, the Sturgeon Quartzite (see Stop 2 description), forms a continuous blanket of orthoquartzite throughout the area and lies directly on the Carney Lake Gneiss in areas devoid of the Fern Creek. A sericitic unit at the top of the Fern Creek (see Stop 2 description) may be a paleosol indicating a period of weathering between deposition of the Fern Creek and the Sturgeon. The Randville Dolomite, the uppermost unit of the Chocolay Group, is an extensively exposed shallow-water carbonate unit containing numerous stromatolite horizons and other indications of shallow or intertidal deposition. Radiometric ages of detrital zircons and authigenic xenotime have bracketed the depositional age of the Chocolay Group between 2.2 and 2.3 Ga and support its correlation with lithologically similar units in the Huronian Supergroup of Ontario (Vallini et al., 2006). Menominee Group- The Menominee Group is composed of the basal Felch Formation and overlying Vulcan Iron-formation, the major iron-bearing member of the Menominee Iron Range. The group lies unconformably on the Chocolay Group, although it is structurally concordant with the older rocks in most places. Bayley et al. (1966) describe several localities where there is an angular discordance between the Felch and Vulcan; where there is a basal conglomerate in the Felch Formation it is composed in large part of clasts of the Randville. The absolute age of the Menominee Group in this area has not been determined but regional relationships provide some constraints. To the northwest, the Hemlock Volcanics, part of the Menominee Group and interlayered with iron-formation probably approximately coeval with the Vulcan, have been dated at 1.87 Ga (Schneider et al., 2002). That date indicates that more than 300 million years likely separate the end of Chocolay Group deposition and the deposition of the Menominee Group. 71 �Felch Formation- The Felch Formation is a sericitic slate and quartzite unit that overlies the Randville Dolomite. It consists of thin-bedded sericitic slate and phyllite and intercalated thinbedded quartzite, with the quartzite layers being more prevalent near the top of the formation (Bayley et al., 1966). It is about 100 feet thick on the south range but is as much as 500 feet thick on the north range. Bayley et al. (1966) considered the Felch Formation to be correlative with the Ajibik Quartzite and Siamo Slate of the Marquette district and the Palms Formation of the Gogebic district. The Felch Formation is conformable and gradational with the overlying Vulcan Iron-formation. Vulcan Iron-formation- The Vulcan Iron-formation is the major iron-bearing unit of the Menominee district. It is well known from numerous mines and drill holes, but generally is not well exposed in natural outcrops. The iron-formation is divided into four units, two composed mainly of granular iron-formation and two composed of slate and slaty iron-formation. In succeeding order the units are the Traders Iron-bearing Member, the Brier Slate, the Curry Ironbearing Member, and the Loretto Slate. They are described in detail by Bayley et al. (1966). The Traders and Curry Members contain layers of granular jasper alternating with layers of magnetite and hematite. The Brier and Loretto Members are mainly laminated siliceous iron-rich slate, which locally contains laminae of detrital quartz, feldspar, micas, zircon, and tourmaline. According to Dutton (1958), the iron-formation is about 1,000 feet thick, of which about 730 feet is ferruginous slate (Brier Slate - 330 feet, Loretto Slate - 400 feet) and 270 feet is granular ironformation (Traders - 100 feet, Curry - 170 feet). The stratigraphy within the Vulcan is seen well at Stops 3, 7, and 8. A detailed stratigraphic section (from oldest to youngest) from the Curry Mine located between the towns of Vulcan and Norway shows that above the Randville Dolomite there is a 20 foot conglomerate consisting of novaculite (dense, fine-grained siliceous rock resembling chert) boulders and smaller angular fragments cemented by ferruginous silica. A similar rock is seen at Stop 8. A fault breccia occurs in two zones bordering a dolomitic slate, over a 25-foot interval. Above the fault breccia is 26-foot interval of vitreous quartzite and then 69 feet of Felch Formation that is divided into several horizons including quartz slate, blocky green slate, massive brown chert, blocky brown talcose slate, shaly brown talcose slate, and topped by the “Trader’s quartzite” (informal terminology). The Vulcan Iron-formation lies on top of the quartzite and is 111 feet thick with several horizons noted; ferruginous slaty iron-formation, wavy-bedded red cherty iron-formation, massive wavy iron-formation with brownish red chert lenses, even-bedded iron-formation with brown chert beds, and the uppermost unit is a massive brown granular chert horizon. The Brier slate is in fault contact with the Traders Iron-bearing Member. It is grey to brown (oxidized) laminated slate, 100 feet thick that is also in fault contact with the Curry Iron-bearing Member. The Curry is 158 feet thick with a thin basal slaty phase and thick even-bedded, dark reddish purple granular chert with specular hematite laminae and in cross fractures. The Loretto Slate is the next formation horizon at about 45 feet thick and bounded by sheared contacts. It is a dark brown, thinly laminated, blocky ferruginous slate. The “Hanbury slate” (Michigamme Formation) overlies the Loretto and in the 5th evel of the Curry mine consists 405 feet of ferruginous mottled red and white slate, then a greenish grey thinly laminated slate with a high chlorite content, and finally a soft pyritic black carbonaceous slate with graphite on shear planes. This stratigraphic section from the Curry Mine is the most complete sequence known for the Range and was developed by Penn Iron Mining Company geologists. 72 �Baraga Group- The Baraga Group consists of a single unit, the Michigamme Formation. The belts underlain by the Michigamme Formation are very poorly exposed, which accounts, at least in part, for the lack of detailed mapping of what may well be otherwise discernible map units. According to Bayley et al. (1966), the Michigamme Formation consists chiefly of slate, especially quartzose, micaceous, and graphitic varieties, subgraywacke, quartzite, conglomerate, dolomite, dolomitic quartzite, and some iron-formation. More recent exploration drilling also has identified units of mafic volcanic rocks. An unconformity between the Michigamme and underlying Vulcan Iron-formation is indicated by the presence of basal a conglomerate, reported from a few localities, that contains clasts of iron-formation and other Menominee and Chocolay Group lithologies, and by regional truncation of pre-Baraga Group units beneath the basal Michigamme units. Although the Michigamme Formation lies on the Vulcan Iron-formation along both the north and south ranges, the Vulcan is largely absent to the north. The stratigraphic section bounding the Archean Carney Lake Gneiss consists of only the Chocolay and Baraga Groups, with the Menominee Group absent except for the extreme eastern end of the area. These relationships suggest that a topographic high existed to the north of the Menominee Range during or shortly after the time of Menominee Group deposition. Mesoproterozoic- The only rocks of Mesoproterozoic age are thin dikes of unmetamorphosed and undeformed diabase of probable Keweenawan age. They are known mostly where they cut the Carney Lake Gneiss. Typical dikes are only a meter or two wide and commonly have chilled margins against the rocks into which they are intruded. Structure. The Menominee iron district (Figure 3) is a south-facing homocline of Paleoproterozoic strata in which stratigraphic repetitions are created by two major faults and by folding internal to fault slices (Bayley et al., 1966). The faults cut the folds longitudinally, approximately along the fold axes, repeating the Paleoproterozoic sequence three times. The structural elements of the Range are shown in Figure 3, reproduced from Bayley et al. (1966, figure 22). The 3-D geometry is shown in Figure 4, reproduced from Bayley et al. (1966, figure 23). On the north, the Carney Lake Gneiss forms the core of a broad anticlinal structure (Figure 3). The Paleoproterozoic strata lie unconformably on the gneiss and dip steeply to the south or are overturned (as at Stop 2) and dip steeply north and face south. Interestingly, on this northernmost sequence of strata the Menominee Group, including the Vulcan Iron-formation, is absent and the Michigamme Formation lies directly on the upper unit of the Chocolay Group. This suggests that there was uplift in the area of the Carney Lake Gneiss concurrent with or shortly after deposition of the Menominee Group, creating a topographic high. In the south, the Paleoproterozoic strata are repeated twice by major faults to form the two ranges of the district. These faults were named the North Range fault and South Range fault by Bayley et al. (1966). The faults have steep dips at the present level of exposure and consistently show southside-up displacement. More recent interpretations (e.g., Sims and Schulz, 1993) consider them to have been thrust faults, which were steepened by continued shortening of the thrust panels. The rocks in the hanging wall (south side) of these faults have no indications of Archean basement rocks, in contrast to the area immediately to the north where the Carney Lake Gneiss is an integral part of the structure. The north range and south range panels may be allochthons detached from basement and thrust northward over the more autochthonous sequence of the northern part of the district. The Menominee Range is bounded on the south by the Niagara fault, along which it is in contact with volcanic rocks of the Wisconsin magmatic terranes. 73 �Figure 3. Structural elements of the Menominee Iron Range from Bayley et al. (1966, figure 22). The “south fault” is now referred to as the Niagara fault and is recognized as the suture between continental margin assemblages to the north and the accreted Wisconsin Magmatic Terranes to the south. 74 �Figure 4. Block diagram showing distribution of stratigraphic units of the Menominee Iron Range, from Bayley et al. (1966, figure 23). Iron deposits Iron ore was discovered in the Menominee district in 1848 by two explorers, J.W. Foster and S.W. Hill, according to Winchell (1895). However, iron mining did not begin until 1870, when N.P. Saxton started digging pits and trenches on the site of the Breene Mine, with the first ore being shipped in 1873 (Bayley et al., 1966). All the major mines had been opened by 1878. Production continued until 1946, with a total production from the district of approximately 85,000,000 tons (Bayley et al., 1966). Seven mines produced nearly 77,000,000 tons of ore, with a majority of the production from the district coming from three mines, the Chapin (27,500,000 tons), the Penn (21,700,000 tons) and the Aragon (11,200,000 tons) (Dutton, 1958). Production from the Chapin Mine ended in 1934 with a major collapse of the workings. The subsidence from this collapse formed the lake on the north side of Iron Mountain. A causeway across the lake now carries the traffic on Highways US 2 and US 141. Ore from the district was hauled by rail to Escanaba, Michigan; from there it was carried by boat to steel mills on the lower Great Lakes. The majority of the ore shipped from the district was high-grade (>50 % Fe) natural iron ore. Some ‘siliceous hematite’ ore (40-50% Fe) was produced from the Millie Pit and the Traders Pit. The Traders ore was used at one time for an experimental project at the Ardis Furnace where an attempt was made to high grade the ore utilizing high temperature roasting. The ruins of the Ardis Furnace are now on the National Register of Historic Places and can be visited near downtown Ironwood. The last of the mines in the area was the Groveland Mine in the Felch trough that operated until the early 1980’s using beneficiation methods to process both hematite and magnetite ore with complex iron silicates to produce a concentrate. 75 �Although the iron-formation in the Menominee district was studied as a possible source of beneficiating ore ("taconite ore"), no commercial operation has been undertaken. In the early 1950’s the Oliver Mining Company completed extensive diamond drilling and evaluation of underground ore reserves in the Vulcan – Norway portion of the Range and had designed open pit operations to extract ore from the Curry and Traders Iron-bearing Members. The area included the Curry, Brier Hill, and Aragon shafts which were the deepest mines on the Range at over 2000 feet. FIELD TRIP STOPS Stop 1: Carney Lake Gneiss (45.873°N, 87.86°W) The Carney Lake Gneiss occurs north of the Menominee Iron Range and forms the Archean basement on which the Paleoproterozoic strata of the Range were deposited. The Carney Lake Gneiss was defined and described by Bayley et al. (1966) who mapped the unit in some detail and published maps and lithologic descriptions of it, but did not attempt to decipher its obviously highly complex internal history. The general descriptions of the Carney Lake below are extracted from that publication. According to Bayley et al. (1966, p. 20-29) “Granitic gneiss constitutes about 85 percent of the Carney Lake Gneiss; of the remainder, about 5 percent is granodiorite and syenite dikes, and about 10 percent is inclusions of older rock. The gneiss is not uniform in composition or appearance, but varies from a gray plagioclase-biotite gneiss to red microcline-biotite gneiss. For the purpose of discussion these types will be designated gray gneiss, composite gneiss, and red gneiss, respectively.” “The gray gneiss probably constitutes about 25 percent of the complex collectively called the Carney Lake Gneiss. It is most abundant in the northern half of the complex where it contains many amphibolite inclusions. In thin sections the gray gneiss shows abundant plagioclase, quartz, and biotite. The foliation is well shown by aligned biotite, and also by the plagioclase and quartz, which are arranged in subparallel elongated grains and in lenticles. Cataclastic structures are common.” “The composite gneiss constitutes at least 70 percent of the complex. It is present almost everywhere, but is more abundant in the southern half of the area, where it contains minor patches of red gneiss and many inclusions of biotite schist. … The grain size of the composite gneiss ranges from medium to coarse. The gneiss is streaky and consists of red and gray elements, the red parts composed of pink microcline and quartz, the gray parts chiefly of plagioclase and biotite, which are the same minerals that constitute the gray gneiss. At some places the red part forms patches and streaks within the gray, at others the gray is enveloped by the red, and at still others the two elements form alternating layers. The red part commonly occurs as veins or layers of coarse pegmatite which cut across the foliation or bifurcate and join other layers. Here and there veins and layers of the red material swell and form large pods of pegmatite which fade transitionally into the gray rock. Pegmatite pods may also pinch and swell along the strike of the foliation of the gneiss. Locally they stop abruptly against the gray rock, only to appear again further along the strike. … The red gneiss is medium grained and weakly 76 �foliated. Fresh specimens are pink or red, whereas weathered specimens are brownish pink. Like the composite gneiss, the red gneiss consists of two components of different age, an older part composed of extensively altered plagioclase, and a younger part that consists of quartz, microcline, muscovite, and minor amounts of albite, but the red gneiss generally contains more quartz and microcline and less biotite and plagioclase than the other gneisses.” “Granodiorite occurs as rare dikes that are most abundant in the southern half of the complex and are more likely to be closely associated with the composite gneiss than the other types. The granodiorite is massive, equigranular, pink, medium to fine grained, and brownish-pink weathering.” “Inclusions in the Carney Lake Gneiss constitute less than 10 percent of the complex, but bear importantly on the character and origin of the gneiss. They consist of amphibolite, biotite schist, and metasedimentary rocks. The inclusions are clearly older than the gneiss and may represent ( 1) engulfed parts of the pre-gneiss Dickinson Group which occurs to the north of the complex consists, in part, of a metamorphosed series of basic tuffs, graywacke-type deposits, and basaltic flows (James et al., 1961), or ( 2) engulfed parts of the Quinnesec Formation which occurs to the south of the complex and consists of metavolcanic rocks, greenstone, amphibolite, and schist.” (Note: Radiometric ages determined since the Bayley et al. (1966) report indicate that both the Dickinson Group and Quinnesec Formation are younger than the Carney Lake Gneiss.) “The field relations show that (1) the gray gneiss grades into composite gneiss, into inclusions of amphibolite, and, more rarely, into inclusions of biotite schist, (2) this gneiss exhibits sharp contacts against the inclusions and appears as dikes in them, (3) the composite gneiss grades into red gneiss and into biotite schist, and (4) both the composite gneiss and the red gneiss contain inclusions of biotite schist and occur as dikes and stringers in some of the inclusions. Further, the gneisses are cut by red granodiorite dikes; one of these dikes, in turn, is cut by a late middle Precambrian metadiabase dike, and another metadiabase dike contains an inclusion of granodiorite. The relations of the syenite, which is known only in the southeast corner of the complex, and the grandiorite dikes are not clear, but the lack of foliation of the granodiorite dikes and the slight foliation of the syenite may indicate that the syenite was emplaced before the granodiorite dikes.” The descriptions in Bayley et al. (1966) and the relatively cursory examination that we have so far conducted in the Carney Lake Gneiss make it obvious that these rocks contain a very extended and complex history. In particular, our recent documentation of zircon grains with cores as old as 3.8 Ga (Ayuso et al, 2017; this volume) indicates that these rocks are undoubtedly part of the Gneiss Terrane defined by Sims et al. (1980) and contain vestiges of Eoarchean crust. Geochronology of samples of the Carney Lake Gneiss done using the USGS/Stanford Sensitive High-Resolution Ion Microprobe (SHRIMP) produced U-Pb data on zircons that confirms an Archean age (Ayuso et al., 2017; this volume). Two samples were collected for radiometric dating from the southern half of the complex: 1) sample 1 is from a granitic K-feldspar-bearing gneiss that is locally pegmatitic; 2) sample 2 is from a banded and folded gray to red granitic gneiss. Abundant zircons (70-200) were obtained from sample 1 that range from anhedral to subhedral, contain complex igneous and irregular growth zoning, and multiple growth rims; 77 �these zircons have irregular to pyramidal overgrowths. The zircons from sample 2 range from slightly rounded to subhedral and are otherwise mostly similar to zircons from sample 1. One hundred and twenty nine analyses of cores and rims were obtained. Individual zircons have older ages near their cores (mostly discordant) and younger ages near their rims. On a concordia diagram (Figure 5), U-Pb data plot as clusters of data points ranging from concordant to discordant and suggest several chords and intercepts that are common to both samples from the Carney Lake Gneiss (Figure 5). That study identified cores of individual zircons as old as 3.8 Ga. The most common age for individual zircons and for rims on older grains is about 2.75 Ga and records a younger major event in the late Archean (Figure 6). Figure 5. A-BSE (back scatter electron) image of a zircon from the Carney Lake Gneiss showing ages of four analyzed spots. B- Concordia diagram for 129 spot analyses from zircons in the Carney Lake Gneiss. 78 �Figure 6. Histogram showing the distribution of individual SHRIMP spot analyses of zircons from the Carney Lake Gneiss. Shaded areas are number of analyses. Solid line is relative probability. The gross structure within the Carney Lake Gneiss as mapped by Bayley et al. (1966) is a dome elongated in an east-west direction as defined by foliation and compositional layering of the gneisses. However, the age of the doming event is uncertain. Basal Paleoproterozic strata surrounding the dome are generally steeply dipping, in part overturned, and mostly concordant with the contact with the Carney Lake, indicating that much of the doming post-dates deposition of those strata that are as young as about 1850 Ma. Thus, much of the internal structure of the Carney Lake likely has a strong Paleoproterozic imprint superimposed on a complex set of Archean structures. A series of outcrops along a powerline east of Norway Truck Road provides a good example of various lithologies and structural complexity of the Carney Lake Gneiss. Figure 7 shows locations for outcrops and locations of photos in Figure 8. In general, the gneisses are more amphibolitic to the west and become progressively more granitic to the east, although a great deal of finer-scale complexity also is seen here. A few thin (1-3 m) mafic dikes that are younger than the complex gneissic structure can also be seen. 79 �Figure 7. Map showing location of outcrops of Carney Lake Gneiss along a powerline and location of photographs shown in Figure 8. 80 �Figure 8. Photographs of Carney Lake Gneiss along powerline. Locations shown on Figure 7. A-Amphibolitic gneiss cut by weakly deformed pegmatites. B-Contorted granitic gneiss with amphibolite inclusions. C-Amphibolitic gneiss with granitic stringers. D-Granitic gneiss with 81 �moderately dipping foliation. E-Foliated amphibolitic gneiss cut by undeformed pegmatite. FBanded gneiss with intrafolial isoclinal folds. Stop 2: Sturgeon River locality. Archean basement, Fern Creek Formation, and Sturgeon Quartzite. (45.784°N, 87.789°W) (This description is modified slightly from a previous field trip guide (LaBerge et al., 2003) that was written by Richard Ojakangas. This locality also has been modified in recent years by removal of a previous hydroelectric dam and draining of the impoundment. As a result, some additional outcrops have been created, especially of the Carney Lake Gneiss, but these are not the focus at this stop. The following description refers to the dam for locational purposes and is still useful in that vestiges of the dam can still be seen.) The Fern Creek Formation and Sturgeon Quartzite are the lower two formations of the Chocolay Group. The group is discontinuous and has been recognized in parts of the Marquette Iron Range and Gogebic Iron Range as well as here in the Menominee Range. The age of the group is bracketed between about 2.2 and 2.3 Ga based on ages of detrital zircon grains and hydrothermal xenotime (Vallini et al., 2006). The group appears to be equivalent to lithologically similar formations in the Huronian Supergroup in Ontario. Although the Sturgeon Quartzite is essentially continuous along the Menominee Range, the Fern Creek is preserved only locally, one of the best exposures being at this stop. Sericitic sediments near the top of the Fern Creek have been interpreted to be reworked paleosols formed prior to deposition of the Sturgeon Quartzite as discussed below (originally in Ojakangas’ stop description). If true, there is a disconformity between the Fern Creek and Sturgeon, which may account for the very limited preservation of the Fern Creek. Here the Sturgeon River has cut a deep gorge through the Sturgeon Quartzite; the formation was named for this locality. This small area has been well studied, especially because of the presence of the Archean-Paleoproterozoic contact at the dam. The area has been described by Credner (1869), Brooks (1873), Rominger (1881), Irving (1890), Bayley (1904), Lamey (1937), Pettijohn (1943), and Trow (1948). Substop 1. Walk past the gate to the end of the road at the powerhouse and dam. We will traverse back up the road to the vehicles, thus observing the rock units in stratigraphic sequence. The dam was constructed on Sturgeon River Falls, which was held up by a thick mafic dike that can be seen in the woods off the east end of the dam. The unconformity between the Archean Carney Lake Gneiss and the Paleoproterozoic Fern Creek Formation can be seen in a small ground-level exposure adjacent to the dam (Figure 9). The lowest bed in the Fern Creek is a diamictite at this spot, whereas a short distance to the west on the river bottom by the power station, the lowest unit is arkosic sandstone with rare oversized stones. 82 �Figure 9. Unconformity at Sturgeon Dam. Hammer head rests on Archean Carney Lake Gneiss and hammer handle is on basal diamictite of the Fern Creek Formation. Nearby in the river bottom, the basal unit of the Fern Creek is arkosic sandstone with rare dropstones, illustrated in Figure 11. Figure 10. Stratigraphic column at the Sturgeon River locality. SQ at the top of the column designates Sturgeon Quartzite. 83 �Figure 11. Granitic dropstone in lowest sandstone of the stratigraphic column. Note that the stone has pierced and bowed down the underlying strata. Figure 10 is a measured column of the Fern Creek Formation. The lower 25 m are well exposed when there is no water in the channel. Note that this portion of the formation consists of five beds of diamictite (matrix-supported conglomerate) as thick as 2.5 m, three arkosic sandstone beds as thick as 2.6 m with rare oversized stones, stacked arkosic sandstone beds with minor intercalated siltstone and argillite laminae, an argillite bed 4.5 cm thick, and a 15 cm conglomerate. Interestingly, the well-exposed section seen in the river bottom is not found on the west bank of the river; only 1½ m of conglomeratic rock is present there. Apparently, the more complete section is preserved in a topographic low on the Archean surface. However, faulting may be a factor as well, for weathered pyrite is present along a fault between the Archean basement and the Fern Creek west of the powerhouse. The middle 25 m of the Fern Creek Formation is relatively poorly exposed; Figure 10 shows this portion consisting of conglomerate, graywacke sandstone with oversized stones, and arkosic sandstone with oversized stones. Interpretation: This is a glaciogenic sequence. The diamictites may be thin tills deposited beneath glacial ice, but more likely are debris flow deposits as suggested by one diamictite bed that grades upward into sandstone. Some of the conglomeratic beds are difficult to clearly classify as 84 �either matrix-supported or clast-supported. One 20 cm bed at the 15 m level in the section is graded from medium sand to clay, suggestive of a turbidity current mechanism. Several of the oversized stones in the sandstone and greywacke beds show either a bowing down of the underlying laminae or an actual penetration, indicating that the stones were dropped into the basin from above and are indeed dropstones. Other lonestones may be dropstones, too, but clear evidence is lacking. The likely mechanism for deposition of dropstones is release from melting icebergs or from a floating glacier. Substop 2: The 25 m section between 50 and 75 m on Figure 10 is intermittently exposed on the west bank of the river, but this area is usually inaccessible because of high water. It includes beds of sericitic quartzite interbedded with sericite schist. The sericitic nature of this interval is illustrated by a small road-level outcrop between the road and the river just north of the quartzite ridge. This is a sericitic quartz pebble conglomerate with sericite clay chips, some reddish rather than yellow-green in color. Interpretation: This sericitic portion of the column is interpreted as a reworked paleosol that formed on the Fern Creek Formation during a warm climatic period that followed glaciation. Trow (1948) first suggested that this might be a paleosol. Substop 3: Sturgeon Quartzite ridge. Note that the bedding is slightly overturned towards the south, and that cross-bedding indicates that stratigraphic tops are to the south. Cross-bedding is of both trough and planar types. According to Trow (1948), the general cross-bedding indicates a paleocurrent trend from the northwest toward the southeast. Since the original field trip guidebook was prepared (LaBerge et al., 2003), geochronological studies (Vallini et al., 2006) have constrained the age of the Sturgeon Quartzite, and by inference of the underlying Fern Creek Formation. Most detrital zircons have ages between 2.5 and 2.7 Ga, but there is also a well-defined cluster of ages at about 2.3 Ga, thus providing a maximum age of deposition. Xenotime overgrowths on zircon grains are as young as 2.1 Ga and define a minimum age. These ages are consistent with age ranges determined for equivalent units in the Marquette and Gogebic Iron Ranges. Interpretation: Abundant asymmetrical ripple marks have low ripple indices (wave length/ripple height) indicative of deposition by water rather than by wind. The beds are generally of even thickness, indicative of a shallow marine rather than a fluvial environment of deposition. Stop 3. Underground tour of the Iron Mountain Iron Mine. (45.782°N, 87.864°W) The Iron Mountain Iron Mine has been in operation as a tourist locality for 60 years and thousands of people have enjoyed this historic site (Figure 12). The #2 adit is one of three exploration tunnels driven perpendicular to the strike of the Vulcan Iron-formation in search of high-grade (>50% Fe) ore in the early 1870’s along the south side of Brier Hill (Figure 13). The #1 adit was driven north to intercept at depth high grade ore that cropped out at the surface to the west of #2 adit. The first iron ore production came from this operation which was a combination of open pit and underground mining. The exploration then turned east along strike to explore the Traders Ironbearing Member of the Vulcan Iron-formation, but lost the member due to longitudinal faulting. The #2 adit was completed and crossed several faults and duplication of beds to find the Traders Iron-bearing Member 1,000 feet into the hillside. (Figures 14, 15). Further exploration sited the #3 adit a half a mile to the east and intercepted the complete section of “Hanbury Slate” 85 �(Michigamme Formation of present usage), Curry Iron-bearing Member, Brier Slate Member, and Traders Iron-bearing Member. The #3 East Vulcan Shaft was located based on finding high grade ore at this locality. Figure 12. The gateway to paradise. Portal to thousand-foot-long adit of the Iron Mountain Iron Mine. The tour will allow for the observation of a complete stratigraphic section from the “Hanbury Slate” (Michigamme Formation) at the entrance of the adit, through the Loretto Slate, Curry Iron-bearing Member into the Brier Slate Member (duplicated by folding and faulting), then through the Traders Iron-bearing Member and terminating at the “Footwall Slate”, which here is a breccia. These explorations were all drilled with hand-held drill bits and sledge hammers. Black powder was used for blasting and all material was hand loaded into tram cars and pulled with mules. The tunnel next was turned west to follow the contact of the Traders Iron-bearing Member and the footwall, but only goes about 70 feet before it intercepted another fault. The iron formation is brecciated and it is believed that the tunnel is near the subcrop of the formation with the overlying Cambrian sandstone because much of it is filled with friable sand through this section. The tunnel meanders a little and gains some elevation while intercepting brecciated and shallow dipping broken banded iron-formation as it comes to an intersection where tunnels go off in three directions (Figure 14). 86 �The tunnels to the north and west explore a wedge of Traders Iron-bearing Member and outlined a block showing low angle dips to the south. At this point a small ore body was found and extracted leaving a stope with broken slabs of rock. Along the south edge is a decline filled with spring water that wraps around a pillar. The stope connects to a shaft about 30 feet deep that was used to determine if more ore occurred at depth. The west tunnel explores the Traders Ironbearing Member about 500 feet along the East Vulcan longitudinal fault and terminates at the zone of caving to the west. The south tunnel crosses the strike of the Traders Iron-bearing Member and Brier Slate Member again and intercepts another fault where high grade ore is located. This passage was developed from the Old Central Shaft which was sited near the #1 adit. The extraction of ore at this location using sublevel caving methods left this large void (stope) as the ore was removed from below and the waste rock allowed to settle and partially prop up the workings. Several items should be noted: In the large stope, the unconformity between the iron formation, which strikes N 75° W and dips 70° SW, and the horizontal sandstone is striking. The ground water rain can be heard when it is quiet. The mined bottom of this ore block is about 600 feet to the southeast where it is cut by the large longitudinal fault and offset to the south. At that point in the operations, a ‘New’ Central shaft (Figure 18) was completed and used to extract ore to the 1200 foot level and develop ore to 1,600 feet toward East Vulcan #4 shaft. That was the extent of mining at the end of WWII when it became too costly to mine these orebodies without significant upgrades to equipment. Mines to the west: the West Vulcan, Curry, Brier Hill, and Aragon, were connected by tunnels to this property and went to depths of 2,400 feet. Figure 13. Plat of the Central Vulcan are in 1938. Geological interpretation and mine sites including subsidence area. 87 �Figure 14. Geological interpretation of the Iron Mountain Iron Mine adit area at the ‘Tunnel Level”, which correlates to the 1st Level of the old Central Vulcan Mine. Based on a blueprint from the Penn Iron Company, circa 1900 (see Figure 15). Walking tour, shown in heavy black line begins at the portal. Figure 15. Blueprint of Penn Mining Company adit #2. The initial straight section of the adit trending N 20o E, is 1,000 feet long for a scale reference. Note the offset of the Traders IronBearing Member on this image and Figure 14 and the location of the ore relative to fault geometry. When compared to other mines in the Range, this structure displays east dipping fault control where all mines to the west have west-dipping ore bodies. 88 �Figure 16. Traders Iron-bearing Member, shattered martite-jasper displaying 30o dip to the south into the wall. A carbide light for scale. Figure 17. Quartz druze on Jasper Iron Formation from the Central Vulcan Mine. Specimen was collected from a retaining wall on the north side of the west parking lot at the shaft. The wall contained several specimens of trace minerals that cement brecciated martite ore including calcite and pyrite. Only the quartz specimens survived the weathering since closure in 1946. The minerals occurred in “water courses”, in the ore bodies, usually in vertical channels as described by local miners. 89 �Figure 18. The New Central Shaft, Vulcan, Michigan. View looking west. Old Central shaft, circa 1877, was due north 1,500 feet’ (500m) and the underground workings connected at the 6th and 9th levels between the shafts. Stop 4. Randville Dolomite. (45.806°N, 87.951°W) The Randville Dolomite, the youngest formation of the Chocolay Group, is exposed extensively in the region. It is well described by Bayley et al. (1966, p. 35) from which the following description is excerpted. “Massive clastic dolomite makes up a large part of the Randville Dolomite and is closely associated with thick- and thin-bedded sandy dolomite, dolomitic and quartzose slate and phyllite, and pebbly dolomite conglomerate. Thick beds of nearly pure crystalline dolomite are present in some areas and probably make up an important part of the formation. A most distinctive rock type in the formation shows algal structures (stromatolites). These are domical, 1-3 inches high, 3-12 inches in diameter, and composed of nested laminae of pure dolomite. The algal structures occur nearly every place in the district where the dolomite is exposed. They form reefs as much as 50 feet thick and of great but undetermined linear extent. They are also present in the Randville Dolomite of central Dickinson County (James et al., 1961) and in the Kona Dolomite of the Marquette district. As pointed out by James, stromatolite structures are also reported in nearly all dolomite of late Precambrian age-in the western United 90 �States and Canada, Australia, South Africa, and Fennoscandia-and most geologists now accept the view that they represent fossil algal colonies. In the mapped area the algal dolomite is usually associated with thin-bedded sandy and conglomeratic dolomite of shallow-water deposition. This general association may be best observed in the outcrop area southeast of Lake Antoine, where algal dolomite, ripple-marked sandy dolomite, and thin dolomite beds showing mud cracks occur together.” At Stop 4, an active gravel/stone operation, recent activities have exposed conglomeratic dolomite. The rock is a poorly sorted, thick-bedded, intraformational conglomerate composed almost entirely of clasts of dolomite as much as about 10 cm diameter. Clasts visible in hand specimen range down to sand-sized grains. All clasts are composed of very fine-grained gray to pinkish dolomite. The matrix is somewhat darker, coarser-grained dolomite. The rock appears to be an intraformational conglomerate and we have seen no exotic clasts that would indicate clastic input for a distant source. We also have not seen any clearly biogenic features here. The total thickness of this conglomeratic unit was not exposed in September, 2017, but it appears to be at least 10 meters thick. Figure 19 illustrates the typical lithology. Figure 19. Conglomerate composed entirely of clasts of Randville Dolomite. Stop 5. Michigamme Slate. (45.777° N, 87.889° W) Brickyard Road, Norway Michigan. Exposures are on private property to which we have been granted access for this field trip. The bedrock ridge south of Hanbury Lake in sections 15 and 16, T. 39 N., R 29 W. contains the most extensive exposures of the Michigamme Slate on the Menominee Range. These rocks have been informally referred to as the Hanbury Slate in some early reports, but were renamed the Michigamme Slate by Bayley et al. (1966). 91 �Figure 20. Geologic map of the Michigamme Slate in the Hanbury Lake area. The figure shows parts of plates 2 and 3 of USGS Professional Paper 513 (Bayley et al., 1966). The area is entirely underlain by Michigamme Slate except for a few bodies of intrusive metagabbro (pCmg). Areas of outcrop are shown by darker shade. Various lithologies of the Michigamme are indicated as sl-slate, qtz-quartzite, dolo-dolomite, gw-graywacke. Stop 5 is near the west end of the ridge where lithologies change from highly sheared greywacke slate to dolomitic slate intruded by metagabbro. Folds indicate a duplication of strata that impacts true thickness estimates of the Michigamme Slate in the southern portion of the Range. Complex folding of the sediments is evident in outcrop along with a significant change in lithology which includes greywacke, quartzite, carbonate, and pyritic carbonaceous shale. Folds have vertical to steep southward-dipping axial planes as indicated by the prominent foliation, and plunge from 30-40° to the east. In USGS Monograph XLVI (Bayley, 1904), a detailed description of the area describes both large (meters) to small (centimeters) scale folding that exhibits strike and dips normal to the regional strike of the range (N 75° W). On the west and north ends the area the slate is cut by metadiabase dikes. These dikes are likely the same age as those identified in the mine workings at the Penn Mines Central Shaft, and at the Cyclops and Norway mines open pits on Norway Hill. Descriptions and discussion of the area from USGS Professional Paper 513 (Bayley et al., 1966, p. 60) follows: Dolomitic rock.- Dolomitic quartzite, dolomitic shale, and dolomite occur chiefly in the broad belt of outcrops south of Hanbury Lake. Dolomitic quartzite occurs south of Hanbury Lake only, where it is associated with dolomitic slate and dolomite and with intrusive metagabbro. The quartzite beds appear to be confined to the eastern three-fourths of the group of outcrops south of the lake, probably because the quartzite beds lens out to the west or are doubled back in a fold. Numerous minor folds in the slate show small areas where the beds strike north across the overall northwest foliation, and folding is thus indicated as the more likely cause of the limited distribution of the quartzite. 92 �The dolomitic quartzite is dark grey or, if encrusted with limonite, brown. The beds are 1-10 feet thick and commonly show quartz-filled cross-fractures that do not enter the adjacent slate beds. A distinctive characteristic of the quartzite is the presence of chips of black slate as much as 6 inches long in most beds. The rock is made up of about equal parts of well-rounded and wellsorted quartz grains and dolomite, and trace amounts of carbonaceous dust. The quartz grains all show undulatory extinction when viewed under the microscope, a feature probably inherited from the source rocks inasmuch as the quartzite does not appear to be deformed internally. The only exposed dolomite in the formation is confined to a belt of outcrops trending northwest from south of Hanbury Lake. The northwestern most rocks on the belt are dolomitic slates which outcrop in secs. 4 and 5, T. 39 N., R. 30 W. South of Hanbury Lake the dolomite is light colored, banded, and somewhat slaty: it occupies the north part of the group of outcrops. The best exposed rock is at the lakeshore. The beds are folded, and in the northern-most outcrop the general strike is nearly normal to the trend of the outcrop belt; The dips are low to the southeast, because these beds are on the crest or in thee trough of a minor fold. The lesser plications and folds on the beds plunge at low angles, less than 30 degrees SE. West of Hanbury Lake, in the south parts of secs. 7 and 8, T. 39 N., R. 29 W., are outcrops of siliceous and dolomitic grey slate and rather thick-bedded siliceous gray dolomite. A rind of limonite that coats the exposed surfaces of the dolomite indicates that the carbonate is probably ferruginous, an observation previously made by W. S. Bayley, who reported the chemical analysis shown in Table 29. On the assumption that all the iron , magnesia, and lime form carbonates, W. S. Bayley gives the composition of the carbonate as about 9 percent FeCO3, 41 percent MgCO3, and 50 percent CaCO3. Stop 6. Niagara fault splay at Piers Gorge. (45.759°N, 87.942°W) (text reproduced from 2003 ILSG guidebook. Laberge et al., 2003) Rocks exposed along the Menominee River at Piers Gorge are almost certainly a branch of the Niagara fault zone and represent one of the few exposures of the fault zone. This location is about one kilometer south of the mapped trace of the Niagara fault. The hill lying north of the gorge, but still south of the mapped fault, is underlain by metagabbro that is much less deformed than the rocks in the gorge. These relationships indicate that strain along the fault zone was distributed very heterogeneously and concentrated in discrete zones of very high strain surrounding islands of weakly deformed rocks. The rocks in the gorge are highly foliated and lineated quartz-sericite schists and chloritic schists, probably developed from felsic and mafic volcanic rocks. Felsic and mafic volcanic rocks with only weak foliation, along with mafic sills with little internal deformation, are exposed on both sides of this strongly foliated zone. Metagraywacke of the Marquette Range Supergroup is exposed in Norway, about 2 miles north of this locality, and volcanic and plutonic rocks of the Wisconsin magmatic terranes are exposed along the Menominee River in this area. The foliation here strikes N 80-85° W and dips 80-85° N. and has a stretch lineation that plunges 60-65°, N 85° W. As the recognized boundary between the dominantly sedimentary rocks of the Marquette Range Supergroup to the north and the Wisconsin magmatic terranes to the south, the Niagara fault zone is commonly referred to as a suture. However, it lacks some features 93 �(such as a mélange) that are typical of suture zones. Geophysical evidence (Attoh and Klasner, 1989; and LaBerge and Klasner, 2001) suggests that thinned continental crust of the Superior craton has been overridden by the Wisconsin magmatic terranes, and extends in the subsurface for 10-50 miles south of the Niagara fault zone. If this is the case, the Niagara fault zone may be the frontal thrust on which oceanic rocks of the Wisconsin magmatic terranes overrode the continent margin assemblage of the Marquette Range Supergroup. Continued compression of the suture zone resulted in the steepening of the thrust surfaces into their present, nearly vertical orientation. Figure 21. Schist in Niagara fault zone at Piers Gorge. A- Highly foliated schist along north bank of the Menominee River. B- Nearly vertical foliation with modern slump toward river producing a spurious shallower foliation. Both photos from 2003 ILSG field trip. Stop 7. Quinnesec Mine (45.810°N, 87.991°W) The abandoned workings of the Quinnesec mine (known locally as the Devil’s Icebox) are mainly in the Traders Iron-bearing Member of the Vulcan Iron-formation. The property is fenced and accessible by arrangement with the property owner. The mine lies on the overturned north limb of a second-order syncline (Figure 22). The Precambrian strata at the mine dip about 60° north, but face southward, inasmuch as the Brier Slate Member of the Vulcan is along the south side of the excavated approach to the mine, and the Felch Formation is along the north wall of the workings. Cross-sections through the Vivian Mine immediately to the west of Stop 7 show the geometry of the westward plunging folds and the overturned structure we see at the mine exposure. The ore is specular hematite and jasper and the brecciated subcrop was considered an ore where it has been reworked in a shoreline environment during the Cambrian Period. 94 �The mine workings provide an exceptional view of the unconformity and basal Cambrian sandstone overlying the mine workings along the north side of the hill (Figure 24 A, B). The basal portion of the sandstone contains numerous angular slabs of oxidized iron-formation, iron ore, and slate in a sandy matrix (Figure 24 C). Clearly, this area was a small island as the Cambrian sea advanced over the area. Cross sections (such as Figure 23) also show the steep local relief that existed on the Precambrian erosional surface. The complex folding and duplication of beds made for a more resistant area of iron-formation that likely led to the development of a topographic high. The clasts of iron ore in the basal conglomerate also indicate that the ore here was formed before the Cambrian sea covered the area. Figure 22. Geologic map of the Quinnesec Mine and vicinity showing that the workings were developed in the overturned northern limb of a small syncline. From LaBerge et al. (2003, based on mapping by Bayley et al. (1966)). 95 �Figure 23. A portion of plate XXX from U.S. Geological Survey Monograph XLVI (Bayley, 1904). 96 �Figure 24. Photographs of the Quinnesec Mine workings. A- View looking west into the mine workings. The Traders Iron-bearing Member of the Vulcan Iron-formation dips steeply north and faces south. The unconformity with the overlying Munising Sandstone (Late Cambrian) is well exposed and shows steep topography that existed on the Precambrian units during Cambrian marine transgression. Photo by Thomas Waggoner. B- Close-up view of the unconformity. C- Basal breccia of the Munising Sandstone, probably talus deposited at the base of the paleoescarpment formed by the Vulcan. Large clasts are entirely iron-formation, many of which show secondary iron enrichment. Lighter matrix is quartzose sand. 97 �Stop 8. Keel Ridge area (45.810°N, 88.028°W) The area of the previous Keel Ridge mine is now operated for crushed stone as well as being excavated for future business development. The Keel Ridge mine was one of the earliest mines opened on the Menominee Range in 1880, but only produced until 1899 with a total production of 93,101 tons. The mine was located just to the northwest of the large stripped area we will examine at this locality. The stratigraphic section exposed is from the Randville Dolomite on the north to the Michigamme Slate (“Hanbury Slate”) on the south and includes an excellent cross section of various members of the Vulcan Iron-formation. Although the area is easily accessible from U.S. 2 and the various units of the Vulcan Iron-formation can be observed and sampled, it is private property and should not be entered without permission of the owner. Figure 25. The Keel Ridge mine area. Geologic units are as indicated in USGS Professional Paper 513, Plate 1 (Bayley et al., 1966). Excavation reveals the northwest-striking formations that include the upper part of the Felch Formation (“Traders Slate”), Traders Iron-bearing Member, Brier Slate, and Michigamme Slate (“Hanbury Slate”). The formations face to the south and dip to the north at 80o- 90o. 98 �Figure 26. View looking east on east side of exposure showing the upper part of the Felch Formation (“Traders Slate”) and conformable contact with the Traders Iron-bearing Member of the Vulcan Iron-formation. Dips are vertical to overturned toward the south. The northernmost exposures are of the Randville Dolomite, which here consists of a breccia of angular siliceous material (chert?) in a siliceous carbonate matrix. This may be a residual accumulation of chert nodules and beds formed by solution of the carbonates of the Randville on the long-lived erosion surface, now expressed as the unconformity between the Randville and the Felch Formation. The next outcrop to the south is the Felch Formation (“Traders Slate”) which is a rare exposure of this unit in the entire range. Bayley (1904) referred to the distinctive sericitic slate or quartzite found at the top of the Felch Formation as either the “Traders Quartzite” or the “Traders Slate” depending on the predominant lithology. The formation is described in USGS Professional Paper 513 (Bayley et al., 1966, p. 38) as: “Lithology of the Felch Formation is remarkably uniform throughout the length of the south iron range, but variable along the north range. On the south range the formation is about 100 feet thick and consists of thin-bedded sericite slate and phyllite, and intercalated thin-bedded quartzite. The quartzite layers appear to be prevalent in the upper part of the formation, and a thin (4 in. to 3 ft.) key bed of dark ferruginous quartzite, the so-called “Traders quartzite,” is commonly present near the top of the formation. The fine-grained clastic rocks which make up the major part of the formation on the south iron range include slate, phyllite, siltstone, and schist. All these rocks show minor differences imposed during deposition and modifications imposed by later deformation and low-rank metamorphism, but they bear a close outward resemblance to one another and show a common mineralogy. 99 �They are predominantly thin-bedded rocks, the layers commonly less than 1 cm thick, and most show bedding-plane fissility. Cleavage surfaces of phyllite are lustrous and spangled with tiny plates of white mica. On fresh surfaces the rocks are gray to greenish gray, but where weathered they may be pale green, red, or light buff, or almost white and mottled with red; color banding is not conspicuous. The chief mineral components of all fine-grained types are pale green sericite and quartz; minor components are feldspar, chlorite, biotite, hematite, and magnetite. Most of the rock layers are composed of about equal parts of the two chief components, but layers composed predominately of one or the other are common. Medium to coarse well-rounded grains of quartz and potassic feldspar, commonly visible to the unaided eye, are scattered throughout many specimens of the slaty rock and form wafer-thin discontinuous quartzite stringers between the slaty layers. These latter characteristics of the slate are useful but not infallible criteria for identifying Felch strata in the field. The prevailing texture of the rocks is micro-schistose. In some specimens the quartz grains as well as the sericitic groundmass are elongated in the plane of schistosity, which at most places parallels the bedding.” Several tens of feet are exposed at this location including the sharp conformable contact with the overlying Traders Iron-bearing Member. The two members of the Vulcan Iron-formation that were most significant economically are the Traders and Curry Iron-bearing Members. They were described in USGS Professional Paper 513 (Bayley et al., 1966, p. 43) as follows: “The rocks of the Traders and Curry members are iron formation, which has been defined by James (1954, p. 239) as “a chemical sediment, typically in bedded or laminated, containing 15% or more of a layer of sedimentary origin, commonly but not necessarily containing layers of chert.” The iron-formation of the Vulcan is thin bedded in commonly laminated, but it does not display uniformity in the thickness of the beds. Individual beds generally range from 1 mm to 30 cm in thickness. As a rule, beds of granular jasper alternate with beds composed chiefly of oxides of iron, principally hematite Fe2O3 (69.94% iron), and a lesser amount of magnetite Fe3O4 (72.4% iron). Almost all of the iron-rich layers contain a small amount of crystalline quartz, and at some places dolomitic carbonate and chlorite as well. The iron-formation usually is dark. Viewed from a distance it commonly appears dark gray or reddish-brown, but at close range it appears as a medley of deep red or maroon, metallic gray, and black. If much oxidized, hues of orange and red are dominant. Most jasper beds are maroon (liver colored) or red. They are generally thicker than adjacent iron rich beds and most are uniformly straight bedded, but irregular beds and lenticular beds are common. The jasper beds are composed chiefly of red jasper granules, specular hematite, magnetite, and metachert (a fine-grained mosaic of crystalline quartz). The granular character of most jasper beds can be seen by the unaided eye, but a wetted surface and a hand lens are helpful. In their primary state the jasper granules are a mixture of amorphous silica and red iron oxide. In their primary state the jasper granules are a mixture of amorphous silica and red iron oxide (fig. 13). In their characteristic crystallized state, the iron oxide is specular hematite, magnetite, or both, and the silica is crystalline quartz (fig 14). Most jasper beds contain, in addition to jasper granules, ooliths which are made up of concentric layers of red amorphous hematite and silica 100 �about a nucleus of quartz or jasper. Re-crystallized ooliths form the same products as the granules. The granules, in shape, size, and appearance, resemble the greenalite granules, that are so characteristic of the Biwabic Iron-Formation of the Mesabi Range. They may represent the analog of the greenalite granules, formed under oxidizing conditions.” Figure 27. Figure 13 from Bayley et al., 1966, p. 44. “Photomicrograph showing jasper granules in a metachert matrix. The very dense granules are bright-red noncrystalline jasper. The gray (salt-and-pepper) granules show an early stage of crystallization-segregation of iron oxide as hematite or magnetite from silica. Ordinary light.” 101 �Figure 28. Figure 14 from Bayley et al., 1966, p. 45. “Photomicrograph showing several types of crystallization of japer granules. Quartz, white; specular hematite and magnetite, gray or black. Note the wide variation in the iron to silica ratios from one granule to another. The black spicules are specular hematite, and the square to rectangular sections are magnetite. Most granules pictured contain both iron minerals. The mottled granule (top center) represents an early stage of crystallization and segregation. The shapes of the iron oxide segregations suggest incipient specular hematite. Plane-polarized light.” The Brier Slate is in fault contact with the Traders Iron-bearing Member and is oxidized at this location. The Curry Iron-bearing Member is missing either because of faulting or nondeposition. This interpretation is based on the formations exposed in the underground workings of the Keel Ridge mine. The Michigamme Slate (“Hanbury Slate”) is in fault contact with the Brier Slate and is best exposed on the west side of the excavation. The outcrop shows significant shearing and oxidation of the slate with probable duplication of section to the south Presented next are detailed descriptions of the Traders Iron-bearing Member, the Curry Ironbearing Member, and the Brier Slate by Oliver Mining Company geologists from crosscuts in the mines east of this locality. Comments in their archived records state that the direct shipping ores (+50% Fe) from the various mines could be identified by their physical and mineralogical characteristics. 102 �Traders Iron-bearing Member. A detailed description of the Traders Iron-bearing Member from the Penn Mining Co. to the east shows a measured section of 112’, less than the average 132’ measured in sections in the south range by the USGS. An abrupt conformable contact with the underlying “Traders Quartzite” occurs in the mine. Basal 3 feet: Ferruginous slate or slaty iron-formation consisting of a micaceous, medium-grained, even-bedded unit, with a high silica content slate. Iron content, 2632%. Earthy hematite occurs with specular hematite scattered throughout. Next 12 feet: Wavy-bedded jasper iron-information with fine-grained red (liver colored) chert bands. Beds consist of thin even slaty specular hematite laminae (not readily cleavable), and long narrow (1/4” – 4”), lenses of fine grained to saccharoidal red (liver colored) chert. Grades gradually to a granular chert phase (next layer), by change of color and finer grain size. Iron content 33-40%. Next 18 feet: Massive wavy bedded iron formation with reddish brown granular chert lenses. A fairly massively, bedded jasper. Thin laminae of slaty specular hematite (not readily cleavable) occur with heavy lenses of reddish-brown saccharoidal to granular chert. Top grades into overlying even-bedded iron-formation by decrease in the amount of granular chert. Bottom grades into red chert phase by a gradual change of color and becomes finer grained while the specular hematite background remains the same. Iron content is 34-40%. Next 64 feet: Even-bedded iron-formation with dark brown, fine-grained chert. (and occasional granular chert lenses). Even-bedded cherty iron-formation composed of thin (1/64” to 1/4”) laminae of slaty specularite with narrow (1/4”) lenticular bands of darkbrown, fine-grained chert. Near the base occur heavier bands of the dark brown, finegrained chert up to 2 inches in thickness which disappear toward the top and bedding there becomes uniformly thin and quite even. One section shows one of these heavy finegrained chert bands. Another section shows a typical thin, even-bedding of the higher phase. Occasional heavy lenses of dark, reddish-brown, granular chert occur frequently near the base where this unit grades into the granular chert below but they become rarer in the upper portion. Iron content is 34-43%. Next 15 feet: Massive dark granular chert. Massive, irregularly bedded, lean, granular chert, dark-brownish-purple in carbide light. Contains very little slaty ferrugenous matter and the chert is all granular with many red jasper granules. Specularite occurs in thin irregular veinlets through the body of the chert. Iron content 25-33%. Curry Iron-bearing Member. A measured section from this locality is 158 feet in fault contact with Brier Slate. Basal 14 feet: Slaty basal phase: Even-bedded blocky, dark-brown, siliceous, slaty ferruginous rock, containing very little free chert. Laminations are 1/8 inch- to 1/2 inchthick and consist of slaty brown, hematite, becoming bluer with increase of specularite toward the top. What free chert exists is purplish granular Curry-type. This horizon seems favored for ore concentration. Iron content is 32-33%. Next 144 feet: Cherty phase: It is a heavy bedded, straight-bedded blocky specular cherty iron-formation with groups of thin, even, rich specular laminae alternating with 103 �irregular lenses (1/4 inch to 4 inches thick) of dark reddish-purple granular chert, shot through with the regular veinlets and mottles of specularite. The chert is invariably granular and dark reddish purple in color. Iron content is 32-43%. Brier Slate. The Brier Slate separates the Traders and Curry Iron-bearing Members in this mine cross-section. The slate is 104 feet thick and displays contacts that are faulted with both ironformations. The Brier Slate is a soft, fine to medium grained, thinly laminated, blocky, ferruginous slate. The color is very dark with a white streak where it is unoxidized, but it is generally oxidized to a chocolate brown color with a dull red streak. Bedding laminae are thin but prominent, especially near the bottom, and grain size varies by laminae, with the coarser grains in the heavier laminae. A coarse phase occurs near the middle. Concentration may produce higher iron analysis locally, but never an ore body. Iron content averages 23%, varying between 15 to 30%. References Attoh, K., and Klasner, J.S., 1989, Tectonic implications of metamorphism and gravity field in the Penokean orogen of northern Michigan, Tectonics, v. 8, p. 911-933. Ayuso, R.A., Schulz, K.J., Cannon, W.F., Woodruff, L.G., Vasquez, J.A., and Jackson, J., 2017, Evidence for the presence of Eoarchean crust in northern Michigan, Institute on Lake Superior Geology, Proceedings of 63rd annual meeting, Part 1: Program and abstracts, p. 910. 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-gness terrane in northern Michigan, Institute on Lake Superior Geology, Proceedings of 64th annual meeting, Part 1: Program and abstracts. Bayley, W.S., 1904, The Menominee iron-bearing district of Michigan, U.S. Geological Survey Monograph XLVI, 513 p. 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. Brooks, T.B., 1873, Iron-bearing Rocks (Economic), Michigan Geological Survey, v. 1, Pt. 1, Chapters 1-4, 319 p. Credner, H., 1869, Die vorsilurischen Gebilde der "obern Halbinsel von Michigan" in Nord Amerika, Deutsche Geologische Gesellschaft, v. XXI, p. 51 6-554. Dutton, C.E., 1958, Precambrian geology of parts of Dickinson and Iron Counties, Michigan, Field Guide for Michigan Basin Society, 44 p. Irving, R.D., 1890, The greenstone schist area of the Menominee and Marquette regions of Michigan, explanation and historical notes, U.S. Geological Survey Bulletin 62, 241 p. 104 �LaBerge, G.L., and Klasner, J.S., 2001, Geology and tectonic significance of Early Proterozoic rocks in the Monico area, northern Wisconsin, U.S. Geological Survey Miscellaneous Investigations Series Map 1-2739, scale 1:24,000. LaBerge, G.L., Cannon, W.F., Schulz, K.J., Klasner, J.S., and Ojakangas, R.W., 2003, Paleoproterozoic stratigraphy and tectonics along the Niagara suture zone, Michigan and Wisconsin, 49th Annual Meeting of the Institute on Lake Superior Geology, Part 2 Field Trip Guidebook, 107 p. Lamey, C.A., 1937, Republic Granite or basement complex, Journal of Geology, v. 46, p. 48751. Pettijohn, F.J., 1943, Basal Huronian conglomerates of Menominee and Calumet districts, Michigan, Journal of Geology, v. 51, p. 387-397. Rominger, C., 1881, Menominee Iron Region: Michigan Geological Survey, v. IV, p. 190-192. Schneider, D.A., Bickford, M.E., Cannon, W.F., Schulz, K.J., and Hamilton, M.A., 2002, Age of volcanic rocks and syndepositional iron formations, Marquette Range Supergroup: implications for tectonic setting of Paleoproterozoic iron formations of the Lake Superior region, Canadian Journal of Earth Sciences, v. 39, p. 999-1012. Sims. P.K., Card, K.D., Morey, G.B., and Peterman, Z.E., 1980, The Great Lakes tectonic zone-a major crustal structure in central North America, Geological Society of America Bulletin, v. 91, p. 690-698. Sims, P.K., and Schulz, K.J., 1993, Geologic map of Precambrian rocks in parts of Iron Mountain and Escanaba 30' X 60' quadrangles, northeastern Wisconsin and adjacent Michigan, U.S. Geological Survey Miscellaneous Investigations Series Map 1-2356, scale 1:100,000. Trow, J.W., 1948, The Sturgeon Quartzite of the Menominee district, Michigan, Ph.D. thesis, Chicago, Illinois, University of Chicago, 60 p. Vallini, D.A., Cannon, W.F., and Schulz, K.J., 2006, Age constraints for Paleoproterozoic glaciation in the Lake Superior Region: detrital zircon and hydrothermal xenotime ages for the Chocolay Group, Marquette Range Supergroup, Canadian Journal of Earth Sciences, v. 43, p 571-591. Winchell, H.V., 1895, Historical sketch of the discovery of mineral deposits in the Lake Superior region, Geological and Natural History Survey of Minnesota, 23rd Ann. Report, p. 116-155. 105 �FIELD TRIP 4 Friday May 18, 2018 GRANITOID ROCKS OF THE PEMBINE-WAUSAU TERRANE IN NORTHEASTERN WISCONSIN Klaus J. Schulz, U.S. Geological Survey With a contribution from Marcia Bjornerud, Lawrence University INTRODUCTION This trip examines granitoid rocks of the Pembine-Wausau terrane that are exposed in northeastern Wisconsin. The Pembine-Wausau terrane is one of the Paleoproterozoic magmatic arcs that comprise the internal domain of the Penokean orogen (Figure 1; Schulz and Cannon, 2007). The terrane consists of mafic to felsic volcanic rocks ranging from tholeiitic to calcalkaline in composition, subordinate sedimentary rocks, and granitoid intrusive rocks of largely calc-alkaline affinity. It was accreted to the southern margin of the Archean Superior craton beginning about 1,875 Ma along a paleosuture now marked by a major ductile deformation zone, the Niagara fault zone (Sims et al., 1985). The rocks in northeastern Wisconsin have been key to understanding the stratigraphic and tectonic evolution of the Pembine-Wausau terrane and the nature of the Penokean orogeny. Rocks in the area are fairly well exposed, especially compared to other areas in northern Wisconsin. Granitoid rocks constitute nearly half of the outcropping rocks of the area and are mainly granodiorite and tonalite, but include gabbro, diorite, and granite (Sims and Schulz, 1993). An older suite, ranging in age from about 1,890–1,870 Ma, is dominantly calcic to calcalkaline and appears to be cogenetic with the volcanic arc magmatism, while younger, 1,860– 1,840 Ma, calc-alkaline to alkaline plutons are broadly contemporaneous with collision of the Pembine-Wausau terrane with the Superior craton margin (Sims et al., 1992). Younger posttectonic intrusions, emplaced at about 1,835 and 1,760 Ma, consist of alkali-feldspar granite suites (Sims et al., 1993). In the area of the field trip (Figure 2) most of the exposed granitoid rocks are part of the Dunbar dome, an irregular, asymmetrical structure ~470 km2 in area, composed of gneiss, migmatite, amphibolite, and foliated to unfoliated granitoid rocks mantled by steeply dipping sedimentary and volcanic rocks of Paleoproterozoic age (Sims et al., 1992). The Dunbar dome is one of several roughly correlative domes in northern Wisconsin (Morey et al., 1982) which have less well-exposed gneiss and granitoid rocks with comparable isotopic ages (Sims and Peterman, 1980; Sims et al., 1989) and chemical compositions (Sims et al., 1993). In the Dunbar dome, compositionally varied gneisses, assigned by Cain (1964) to the Dunbar Gneiss, are intruded by five major plutons named the Marinette Quartz Diorite, Newingham 106 �Tonalite, Hoskin Lake Granite, Spikehorn Creek Granite, and Bush Lake Granite (Sims et al., 1992). This field trip will examine exposures of each of the major units of the Dunbar dome except the Bush Lake Granite. Detailed descriptions of these major units including their petrography, structure, geochemistry, age, and petrogenesis are given in Sims et al., 1984; 1985; and 1992 (the 1984 reference is available for free download from the ILSG website http://www.lakesuperiorgeology.org/; the 1992 reference is available for free download from the USGS website https://www.usgs.gov/products/publications/official-usgs-publications). In addition to the granitoid units of the Dunbar dome, the field trip will examine exposures of the Twelve Foot Falls Quartz Diorite, a subvolcanic intrusion that is comagmatic with calc-alkaline volcanic rocks in the Quinnesec Formation (Sims et al., 1992) as well as the post-tectonic (~1,835 Ma) Athelstane Quartz Monzonite and Yavapai-age (~1,750 Ma) Amberg Granite which occur south of the Dunbar dome (Sims, 1990; Sims et al., 1993). Sims et al. (1992) concluded that the granitoid rocks within and outside the Dunbar dome were derived from different sources based on their contrasting chemistry, and presumably were developed in different tectonic environments, and were subsequently superposed tectonically. They attributed the Newingham Tonalite and Twelve Foot Falls Quartz Diorite to the subduction processes that formed the volcanic arc represented by the Quinnesec volcanic rocks. In contrast, the intrusions within the dome, which range from syn- to post-tectonic, were attributed to melting of continental lithosphere during collision of the arc with the continental margin of the Superior craton. However, these conclusions need to be reevaluated in light of acquired Nd isotope data (Van Wyck and Johnson, 1997; Schulz and Ayuso, 1998) and new understanding of the processes that produce granitoid rocks in orogenic belts (Hildebrand and Whalen, 2017). Although the granitoid rocks of the Dunbar dome range in composition from calcic tonalite to calc-alkaline granodiorite to alkali-calcic quartz diorite, they have a surprisingly small range of enriched ɛNd values centered on 0 and depleted mantle model ages of ~2.0 to 2.2 Ga. The Dunbar Gneiss has the most negative ɛNd(1,860) values of -2.1 to -3.4 and the Hoskin Lake Granite the most positive ɛNd(1,835) value of +1.71; the Marinette Quartz Diorite and Newingham Tonalite have similar ɛNd(1,860) values near 0 (+0.12 and +0.39, respectively). Only the Twelve Foot Falls Quartz Diorite, which is a subvolcanic intrusion, has a strongly positive ɛNd(1,900) value of +4.54 indicating derivation from a long-term light rare earth element (REE) depleted source. The narrow range of enriched ɛNd values for the Dunbar dome granitoids is unlikely to be the result of crustal contamination as it would be highly fortuitous for granitoids of such varying chemistry to all have similar degrees of crustal contamination. Instead, the narrow range in ɛNd values is more likely a characteristic of the source from which the granitoids were derived. In addition, the data indicate that the Newingham Tonalite is likely not a syn-volcanic intrusion, as suggested by Sims et al. (1992), but rather is a syn-collisional intrusion and part of the Dunbar dome suite. A characteristic feature of the Dunbar dome granitoids is that they are relatively enriched in Ba, K, Nb, Rb, Sr, Ta, and Th, and have steep, light REE-enriched patterns (Sims et al., 1992). Recently, Hildebrand and Whalen (2017) examined the geochemistry of a number of major 107 �Cordilleran-type batholiths including the Sierra Nevada, Peninsular Range, Idaho-Montana, and Cascades-Coast Plutonic Complex of North America among others. They noted a clear compositional distinction between plutons generated as syn-volcanic intrusions during subduction and those emplaced as syn- to post-tectonic intrusions during slab failure (breakoff). In particular, they showed that magmas generated during slab failure have relatively high Nb/Y, Sr/Y, and Sm/Yb ratios (Figure 3) as well as evolved radiogenic isotopes. These characteristics are shown by intrusive rocks ranging from gabbro to granite and calcic to alkaline in composition (Hildebrand and Whalen, 2017). They concluded that the distinctive whole-rock geochemistry, as well as radiogenic and stable isotope compositions, of slab failure magmas involve only minor amounts of crustal contamination and are derived mainly from plagioclaseabsent melting of garnet-bearing rocks in the mantle (for example, garnet pyroxenite, eclogite, and/or subcrustal lithosphere). As seen in Figure 3, the Dunbar dome granitoids, including the Newingham Tonalite, plot in the fields defined by Hildebrand and Whalen (2017) for slab failure magmas. Figure 1. Generalized geologic map of the Penokean orogen in the Lake Superior region showing approximate location of Figure 2 (from Schulz and Cannon, 2007). 108 �Figure 2. Geologic map for a portion of northeast Wisconsin showing the locations of the field trip stops (from Sims and Schulz, 1993). 109 �Figure 3. Plots of Nb vs Y (A), La/Sm vs Sm/Yb (B), Nb/Y vs Sr/Y (C), and La/Yb vs Sr/Y (D) for samples from the Dunbar dome and Twelve Foot Falls Quartz Diorite. Fields after Hildebrand and Whalen (2017). FIELD TRIP STOPS Stop 1. Hoskin Lake Granite (Outcrop on north side of County road N; 45.764° N., 88.071° W.; Note that the outcrop is on private property and permission from the owner is required) The Hoskin Lake Granite is an arcuate, convex-northward body of granite on the north margin of the Dunbar dome characterized by (1) pink to gray, medium- to coarse-grained inequigranular granite with large, oriented, tabular potassium feldspar crystals (Figure 4), (2) abundant inclusions of mafic-intermediate volcanic rocks of the Quinnesec Formation, and (3) late, euhedral crystals of potassium feldspar that lie athwart to an older foliation and, at least locally, transect centimeter-thin quartz veins. As noted by Cain (1964), the southern margin of the granite appears to be gradational into rocks assigned to the Marinette Quartz Diorite and evidence for K-metasomatism along the border of the two units is compelling. To the east, the Hoskin Lake Granite appears to grade into the post-tectonic Spikehorn Creek Granite. Although different in appearance, the two granites have similar compositions (Sims et al., 1992). Excellent descriptions of the Hoskin Lake Granite are given in Bayley et al. (1966) and Sims et al. (1992). An Nd isotope analysis of one sample of the Hoskin Lake Granite gave a ɛNd (1,835) = +1.71 110 �with a depleted mantle model age of 1.99 Ga (Schulz and Ayuso, 1998), suggesting derivation from a slightly light REE depleted source. Figure 4. Hoskin Lake Granite. Stop 2. Marinette Quartz Diorite (Railroad cut on County road O; 45.747° N., 88.033° W.) Note: Access to this railroad grade is strictly prohibited without prior approval of the owner. The railroad cut shows metamorphosed Marinette Quartz Diorite cut by dikes of Hoskin Lake Granite and leucogranite. The Marinette Quartz Diorite is a large, layered sill-like intrusive body that was emplaced in the contact zone between the Dunbar Gneiss and the Quinnesec Formation and the intrusive Newingham Tonalite. It is dominantly composed of quartz diorite and diorite with moderately high biotite (~10 to 20%), hornblende (trace to 30%), and sphene (trace to 6%) contents. In the north-central part of the Dunbar dome, the rocks contain variable amounts of potassium feldspar and are interpreted as hybrid rocks reflecting post-crystallization Kmetasomatism (Sims et al., 1992). In the eastern part of the body, which is relatively unmetamorphosed, the Marinette Quartz Diorite is a dark gray to black, medium-grained, hypidiomorphic granular rock with well- to ill-defined layering and generally lacks a penetrative foliation; rare clinopyroxene occurs as relicts in the cores of some hornblende crystals. South of Dunbar, the Marinette Quartz Diorite is medium to dark gray, mesocratic, layered quartz diorite and diorite cut by abundant granite pegmatite and aplite dikes (Figure 5). Based on the presence of mineralogical layering and geochemistry, the Marinette Quartz Diorite is interpreted as dominantly cumulate rocks derived from an alkaline mafic to intermediate magma with within plate–syn-collisional compositional characteristics (Sims et al., 1992). Uranium-lead zircon dating of the Marinette Quartz Diorite gives an age of 1,862±15 Ma, which overlaps the age of the Dunbar Gneiss that it intrudes (Sims et al., 1992). Neodymium isotope analyses of the Marinette Quartz Diorite show ɛNd(1,860) = +0.7 to +0.12 with a depleted mantle model age of ~2.1 Ga (Barovich et al., 1989; Schulz and Ayuso, 1998), suggesting derivation from an enriched source. 111 �Figure 5. Marinette Quartz Diorite south of Dunbar cut by granite pegmatite dikes. Stop 3. Marinette Quartz Diorite (Brown Spur road going east from County road O; 45.713° N., 88.011° W.) Although there appears to be no actual outcrop of Marinette Quartz Diorite at this stop, there are numerous angular boulders of black, medium- to coarse-grained diorite to quartz diorite typical of the eastern, less metamorphosed part of the intrusion. Stop 4. Newingham Tonalite (At the intersection of Highway 8 and 1 Mile Road, north side of road; 45.628° N., 88.096° W.) The Newingham Tonalite forms a large body, ~75 km2 in area, that intrudes the volcanic rocks of the Quinnesec Fomation along the southeast margin of the Dunbar dome (Figure 2). The contact zone is at least 100 m wide, and consists of interlayered tonalite bodies and generally angular amphibolite (metabasalt) inclusions. Near the contact with the Marinette Quartz Diorite, the Newingham Tonalite has been largely converted to granodiorite or granite by post-crystallization addition of potassium feldspar forming a hybrid, megacrystic facies of the Newingham Tonalite (Sims et al., 1992). The Newingham Tonalite is mostly a uniform light gray, medium-grained, slightly porphyritic rock (Figure 6) that generally has a good secondary foliation except in the eastern portion north of Pembine. It is locally cut by dikes of slightly porphyritic tonalite and, occasionally, granite pegmatite. The Newingham Tonalite has the compositional characteristics of high Al2O3-type tonalite-trondhjemite suites including high Al2O3 (>15 wt.%) and Sr (>600 112 �ppm), low K and Rb, and steep REE patterns depleted in heavy REE (La/Yb = 60-90) (Sims et al., 1992). Uranium-lead zircon dating of the Newingham Tonalite gives an imprecise age of 1,861±40 Ma (Sims et al., 1992). Neodymium isotope analysis of one sample of the Newingham Tonalite gave a ɛNd(1,860 Ma) = +0.39 with a depleted mantle model age of 2.09 Ga (Schulz and Ayuso, 1998), suggesting derivation from an enriched source or contamination by older crustal rocks. Figure 6. Newingham Tonalite. Stop 5. Dunbar Gneiss (Intersection of Highway 8 with County road U, west side of road; 45.655° N., 88.199° W.) Exposed here is a low outcrop of mainly megacrystic granite gneiss that contains rafts of amphibolite and is intruded by granite pegmatite. The gneiss has a pervasive steeply dipping N. 50° W. foliation. The Dunbar Gneiss generally consists predominantly of gray biotite gneiss, which is layered at scales ranging from a few centimeters to several meters reflecting differences in the amount and kind of major minerals as well as differences in grain size (Figure 7). Granite pegmatite and aplite intrude the Dunbar Gneiss particularly in the western part of the dome and can compose more than 50 percent of the outcrop. As described by Sims et al. (1992, p. 7)….”The biotite gneisses are mylonitic rocks. They have a dominant xenomorphic granular (granoblastic) texture and a penetrative foliation expressed by oriented biotite and, less commonly, by elongate and flattened aggregates of quartz and plagioclase that are generally subparallel to compositional layering….The biotite gneisses have a moderate range in composition from layer to layer; their average and modal composition is granodiorite. Plagioclase (calcic oligoclase-andesine) and quartz are the principal minerals, potassium feldspar varies from 0 to about 30 percent, and biotite generally composes from 10 to 20 percent 113 �of the rocks. Myrmekite and myrmekitic plagioclase are abundant….Sphene (titanite) is the principal accessary mineral and comprises as much as 2 percent of the rock.” The Dunbar Gneiss is calc-alkaline in composition with intermediate SiO2 contents (~62 to 72 wt.%), moderately high Al2O3 (~14 to 17.5 wt.%), and high K2O (2.4 to 5.1 wt.%). It also is enriched in Ba, Nb, Rb, light REE, Ta, and Th, and has steep REE patterns with depleted heavy REE (Sims et al., 1992). A sample of Dunbar Gneiss just north of this outcrop gave a U-Pb zircon concordia upper intercept age of 1,862±5 Ma (Sims et al., 1992). Neodymium isotope analyses by Barovich et al. (1989), Schulz and Ayuso (1998), and Van Wyck and Johnson (1997) gave very similar results with ɛNd(1,860) = -2.1 to -3.4 and depleted mantle model ages of 2.20 to 2.41 Ga. The isotope data suggest the protolith of the Dunbar gneiss was derived from an enriched source. Figure 7. Dunbar Gneiss and granite pegmatite. Stop 6. Dunbar Gneiss (Spur Lake Road going west from County road U; 45.684° N., 88.232° W.) The exposures on the east side of the road consist of compositionally layered biotite gneiss and lesser amphibolite intruded by megacrystic biotite gneiss, granite pegmatite, and aplite. All rocks are deformed and have a vertical N. 50-55° W. foliation. Stop 7. Twelve Foot Falls Quartz Diorite and mylonite, ultramylonite, and pseudotachylyte along the Twelve Foot Falls shear zone The Twelve Foot Falls Quartz Diorite is an elongate, east-west trending body that intrudes and locally contains inclusions of metavolcanic rocks of the Quinnesec Formation south of the Dunbar dome (Figure 2). The quartz diorite is generally massive in the eastern part, but becomes foliated towards the west. At the type locality at Twelve Foot Falls on the north branch of the Pike River, the quartz diorite has been intensely sheared by the Twelve Foot Falls shear zone and is mainly a mylonitic gneiss. As described by Sims et al. (1992, p. 43)….”Outside the shear zone, the quartz diorite is medium to coarse grained and is characterized by subhedral plagioclase (sodic andesine) crystals as much as 1 cm long, smaller subhedral hornblende 114 �crystals, in part pseudomorphic after pyroxene, and anhedral crystals of blue quartz as much as 1 cm in diameter. Microcline locally occurs as a late interstitial mineral. The primary texture is hypidiomorphic granular, but finer grained secondary textures are superposed on it at many places. Characteristically, the rock is considerably retrograded: plagioclase is partly to largely altered to epidote and albite, and hornblende is partly altered to biotite, epidote, and chlorite. Other alteration minerals are sphene, opaque oxides, and calcite.” The quartz diorite has a calc-alkaline andesite (SiO2 = 57 wt.%) composition with low TiO2 (0.44 wt.%) and high field strength element contents (Sims et al., 1992). It is similar in composition to andesite volcanic rocks in the Quinnesec Formation and is interpreted to be a subvolcanic intrusion. A sample dated by Schulz and Schneider (2005) gave a U-Pb zircon concordia upper intercept age of 1,889±6 Ma age. This age shows that the volcanic rocks of the Quinnesec Formation are significantly older than the intrusive rocks of the Dunbar dome and places a minimum age on the Pembine ophiolite present within the Quinnesec Formation (Schulz and Schneider, 2005). An Nd isotope analysis of the quartz diorite gave a ɛNd(1,900) = +4.54 with a depleted mantle model age of 1.87 Ga (Schulz and Ayuso, 1998) indicating a depleted source and no crustal contamination. The strongly positive ɛNd value for the quartz diorite is similar to that determined for the mafic volcanic rocks of the Quinnesec Formation (Beck and Murthy, 1991). (Material below and Stops 7a and 7b contributed by Marcia Bjornerud, Lawrence University) The Twelve Foot Falls shear zone (Sims, 1990) can be traced for at least 20 km along strike in northwest Marinette County, Wisconsin, from Twelve Foot Falls County Park on the north branch of the Pike River to just south of Kidd Lake. The timing of displacement on the shear zone is only broadly constrained; the shear zone transects the Twelve Foot Falls Quartz Diorite (1,889±6 Ma) as well as the metavolcanic Quinnesec Formation, and it lies immediately south of the 1,862±5 Ma Dunbar Gneiss (Sims et al., 1992). The vertical to steeply northeast-dipping foliation and mylonite bands in the Twelve Foot Falls Quartz Diorite are broadly parallel to the foliation in the southern part of the Dunbar dome, but the Twelve Foot Falls shear zone does not cut through the dome itself. Sims (1990) suggested that the northern part of the Amberg Granite (U-Pb zircon age 1,752±8 Ma) also is transected by the shear zone; if so, the zone would have developed during or after the Yavapai orogenic cycle. The sense of slip also is poorly constrained; a weak down-dip lineation points to dip-slip motion, but it is not clear whether the slip sense was reverse – which would suggest activity synchronous and sympathetic with convergence on the Niagara Fault – or normal, which would indicate slip related to late-orogenic relaxation. Stop 7a: Twelve Foot Falls County Park (45.579° N., 88.137° W.) Note that Marinette County parks require an entrance fee; use self-service registration box in main parking area. The main falls are visible from the parking area across a small pool in the Pike River. Follow the narrow foot path north of the picnic area to reach the outcrop adjacent to the falls, where the Twelve Foot Falls Quartz Diorite is well-exposed. The rock there is strongly foliated and locally mylonitized, and both the foliation and mylonitic fabric are defined by bands of quartz and feldspar alternating with aligned hornblende crystals (partly regressed to chlorite), indicating that 115 �the overall schistosity and localized zones of high strain formed at peak metamorphic (amphibolite facies) conditions. Depending on water levels, there is another area of exposed rock about 220 m downstream from Twelve Foot Falls, just above Eight Foot Falls. There, dark, branching discordant veins 0.3-0.5 cm wide and 10-15 cm long cut across the foliation in the host rock (Figure 8). In thin section, the veins are found to contain a mesh of fine retrograded hornblende (?) crystals with high aspect ratio, arranged with no preferred orientation in a non-crystalline matrix that is dark in plane polarized light. These macro- and micro-scale characteristics suggest that the veins represent devitrified pseudotachylyte injection veins – frictional melt generated on a fault plane during seismic slip and injected as ‘hydro’-fractures into the surrounding rock (Nadziejka and Bjørnerud, 2014; Larson and Bjørnerud, 2017). Significantly, the pseudotachylyte material can be seen in both outcrop and thin section to have been cut by, and in places incorporated into, the mylonitic bands. This indicates that brittle seismic failure occurred at least once while the rocks were still at depths and temperatures where crystal plastic deformation was predominant. We have also found small amounts of pseudotachylyte at Eighteen Foot Falls, about 1 km upstream from Twelve Foot Falls, and at Dave’s Falls near Amberg. Thin sections of specimens from Eight Foot Falls also show mutually cross-cutting relationships between plastically deformed quartz veins and pseudotachylyte (Figure 9). This indicates that brittle tensile fracture and fluid flow occurred in alternation with seismic failure and ductile deformation. Some of the quartz veins contain significant amounts of pyrite. In addition, the foliation is in places transected by discontinuous cm-long veins in which hornblende and quartz occur as fibrous crystals perpendicular to the walls. The crystals have growth bands and fluid inclusion planes oriented parallel to the vein walls. These features suggest that the veins formed by the ‘crack-seal’ mechanism, in which cyclic fluid pressure variations cause hydrofracturing and incremental mineral growth. Crack-seal veins are most commonly found in the shallow upper crust; the fact that hornblende is one of the vein-filling minerals indicates that in this case, the process occurred at greater depth and higher temperatures. In combination, these observations provide an exceptional glimpse into the complex interplay of deformation mechanisms and fluid flow in the middle crust during an orogenic event. Large earthquake ruptures apparently penetrated downward into rocks that were otherwise at temperatures high enough for full crystal plasticity. Such mutually cross-cutting relationships between mylonites and pseudotachylytes have been reported from only a small number of sites around the world (Sibson and Toy, 2006). Strain incompatibilities related to these ruptures may have caused dilatancy and large fluid pressure gradients that led to the formation of quartz-pyrite and quartz-hornblende veins. 116 �Figure 8. Outcrop photos of pseudotachylyte and mylonite in the Twelve Foot Falls Quartz Diorite at Eight Foot Falls. Brunton compass and pencil indicate scale. White arrows show places where pseudotachylyte has been offset along the mylonitic foliation. Although the apparent offset is left lateral, lack of three-dimensional exposure makes true slip vector difficult to determine. 117 �Figure 9. Photomicrograph (cross-polarized light) of finely recrystallized quartz diorite host rock cut by a quartz vein that was in turn intruded by pseudotachylyte (altered entirely to clinochlore). Scalloped edge of quartz grain in middle of image is consistent with melting. Vein quartz shows undulatory extinction, indicating that plastic deformation followed or alternated with brittle fracture. Stop 7b: Powerline exposure of Twelve Foot Falls Shear Zone (From the intersection of Twelve Foot Falls Road and Forest Road 513, drive 1.3 km (0.8 miles) west on 513 to an open area where a major powerline crosses the road; 45.584° N., 88.156° W.) This site provides a glimpse of the strain heterogeneity typical of the Twelve Foot Falls shear zone. The textural character of the Twelve Foot Falls Quartz Diorite ranges from igneous to ultramylonitic, in some cases over distances of centimeters. The foliation and mylonitic bands dip about 80° NE. In places, a weak down-dip mineral lineation is discernible. In thin section, feldspar porphyroclasts show extremely long tails – suggesting very high shear strains – but no consistent asymmetry that would allow the sense of shear to be determined unambiguously. Stop 8. Athelstane Quartz Monzonite and Amberg Granite (U.S. Highway 141 and Black Sam Road just north of Amberg; 45.517° N., 87.996° W.) This pavement outcrop consists of Athelstane Quartz Monzonite cut by dikes of Amberg Granite (Figure 10; Medaris et al., 1973). The Athelstane Quartz Monzonite intrudes felsic volcanic rocks (Beecher Formation) north of this stop and extends for considerable distance both to the west and south (Sims, 1990). It is a pink medium- to coarse-grained granite to granodiorite with 118 �allotriomorphic granular texture and 5 to 10 percent biotite and(or) hornblende (Sims et.al., 1993). Typically, the quartz monzonite has a clotty appearance due to the interstitial nature of the mafic minerals (Figure 10). Small metavolcanic inclusions are present locally. The Athelstane Quartz Monzonite is mildly peraluminous and has high SiO2 (68–77 wt.%), intermediate Al2O3 (12–15 wt.%), K2O greater than Na2O, and enrichment in iron (FeOt/(FeOt + MgO) = ~0.9) (Sims et al., 1993). Samples plot in within-plate and syn-collisional fields on trace element tectonic discriminant diagrams (Sims et al., 1993). A U-Pb zircon age of 1,835±15 My was determined on a sample from a large quarry south of this stop (Sims, 1990). This age overlaps with that determined for the Spikehorn Creek Granite on the northeast side of the Dunbar dome (Sims et al., 1992). Barovich et al. (1989) determined a ɛNd(1,835) = +1.1 with a depleted mantle model age of 2.07 Ga on a sample of the quartz monzonite. The positive ɛNd value is similar to that determined for the Hoskin Lake Granite in the Dunbar dome (see Stop 1) and suggests derivation from a similarly light REE depleted source. The Amberg Granite, seen here as dikes cutting the Athelstane Quartz Monzonite, is gray, fineto medium-grained, with a hypidiomorphic granular texture and has biotite as the major ferromagnesian phase. It also occurs in at least three intrusive bodies within the Athelstane Quartz Monzonite (Sims, 1990). It has a U-Pb zircon age of 1,752±8 Ma (Sims, 1990); it is one of a number of small plutons of this age found across northern Wisconsin (Sims et.al., 1993). These ~1,750 Ma plutons are coeval with the anorogenic granite-rhyolite terrane in south-central Wisconsin (Anderson et al., 1980; Smith, 1983). A sample of the Amberg Granite has an ɛNd(1,750 Ma) = -0.91 and a depleted mantle model age of 2.17 Ga (Schulz and Ayuso, 1998), suggesting derivation from an enriched source. Figure 10. Athelstane Quartz Monzonite cut by dikes of Amberg Granite. 119 �Stop 9. Athelstane Quartz Monzonite and mafic dikes (Dave’s Falls County Park just south of Amberg; 45.497° N., 87.989° W.) Excellent exposures of the Athelstane Quartz Monzonite occur on both sides of the Pike River. The Athelstane is cut by mafic dikes (~3 to 20 m wide) that strike about north-south. The dikes, at least three of which are exposed in the park, weather recessively relative to the Athelstane (Figure 11). The dikes have an andesitic composition (SiO2 ~53 to 55 wt.%) with low MgO (~2.5 to 3.5 wt.%) and TiO2 (~1.8 wt.%) contents, enriched light REE chondrite normalized patterns, and negative Nb-Ta and Ti anomalies on a primitive mantle normalized trace element plot (Figure 12). Dikes with similar composition have been observed cutting outcrops of the Twelve Foot Falls Quartz Diorite and in drill core cutting the Back Forty massive sulfide deposit in Michigan (Schulz, unpublished data). The dikes are post-1,835 Ma and pre-Keweenawan in age, but their actual age is not known. Figure 11. Mafic dike (in valley, looking north) cutting the Athelstane Quartz Monzonite. 120 �Figure 12. Chondrite normalized rare earth element plot (A) and primitive mantle normalized trace element plot (B) for andesite dikes from northeastern Wisconsin (Schulz, unpublished data). Stop 10. Spikehorn Creek Granite (East side of U.S. Highway 8 at intersection with Morgan Park Road; 45.703° N., 87.981° W.) The road cut shows the Spikehorn Creek Granite with metabasalt inclusions (Figure 13). As described in Sims et al. (1992, p. 27-28)…”The Spikehorn Creek Granite is a gray to pinkishgray, fine- to medium-grained rock containing scattered anhedral potassium feldspar grains as much as 2 cm in diameter. It is generally massive, but locally (especially near the margins of the body), it bears a mylonitic foliation expressed mainly by recrystallized quartz leaves and oriented biotite….The granite has sharp intrusive contacts against the Quinnesec volcanic rocks and the Marinette Quartz Diorite, and small ramifying dikes intrude these rocks for distances as much as 400 m from the contact….The Spikehorn Creek Granite (of the Niagara lobe) and the Hoskin Lake Granite are compositionally similar except that the Hoskin Lake has slightly higher K2O content.” 121 �Figure 13. Spikehorn Creek Granite with angular metabasalt inclusions. References Anderson, J.L., Cullers, R.L., and Van Schmus, W.R., 1980, Anorogenic metaluminous and peraluminous granite plutonism in the Mid-Proterozoic of Wisconsin, USA, Contributions to Mineralogy and Petrology, v. 74, p. 311–328. Barovich, K.M., Patchett, J.R., Peterman, Z.E., and Sims, P.K., 1989, Origin of 1.9 - 1.7 Ga Penokean continental crust of the Lake Superior region, Geological Society of America Bulletin, v. 101, p. 333–338. Bayley, R.W., Dutten, 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. Beck, W., and Murthy, V. R., 1991, Evidence for continental crustal assimilation in the Hemlock Formation flood basalts of the Early Proterozoic Penokean Orogen, Lake Superior region, U.S. Geological Survey Bulletin 1904-I, 28 pp. Cain, J.A., 1964, Precambrian geology of the Pembine area, northeastern Wisconsin Papers of Michigan Academy of Science, Art, and Letters, v. 49, p. 81–103. Hildebrand, R.S., and Whalen, J.B., 2017, The tectonic setting and origin of Cretaceous batholiths within the North American Cordillera–the case for slab failure magmatism and its significance for crustal growth, Geological Society of America Special Paper 523, 113 p. 122 �Larson, M., and Bjørnerud, M., 2017, Seismic slip, mylonitization and fluid flow along the Penokean Twelve Foot Falls shear zone, Marinette Country, northeastern Wisconsin, Institute on Lake Superior Geology, Part 1: Program and Abstracts, v. 63, p. 56–57. Medaris, L.G., Van Schmus, W.R., Lahr, M.M., Myles, J.R., and Anderson, J.L., 1973, Guidebook to the Precambrian geology of northeastern and northcentral Wisconsin, 19th Institute on Lake Superior Geology, Wisconsin Geological and Natural History Survey, 86 p. Morey, G.B., Sims, P.K., Cannon, W.F., Mudrey, M.G., Jr., and Southwick, D.L., 1982, Geolgical map of the Lake Superior region, Minnesota, Wisconsin, and northern Michigan,Minnesota Geological Survey State Map Series S-13, scale 1:1,000,000. Nadziejka, B., and Bjørnerud, M., 2014, Petrographic characterization of the Penokean Twelve Foot Falls shear zone, Marinette County, WI, Evidence for coeval ductile and seismic behavior: Institute on Lake Superior Geology, Part 1: Program and Abstracts, v. 60, p. 91– 92. Schulz, K.J., and Ayuso, R.A., 1998, Crustal recycling in the evolution of the Penokean orogen: Isotopic evidence for Archean contributions to crustal growth in the Pembine-Wausau terrane, northern Wisconsin, Institute on Lake Superior Geology, Part 1: Program and Abstracts, v. 44, p. 113–114. Schulz, K.J., and Schneider, D.A., 2005, Age constraints on the Paleoproterozoic Pembine ophiolite-island arc complex and implications for the evolution of the Penokean orogeny, Geological Society of America Abstracts and Program, v. 37, no. 5, p. 4. Schulz, K.J., and Cannon, W.F., 2007, The Penokean orogeny in the Lake Superior region, Precambrian Research, v. 157, p. 4–25. Sibson, R.H., and Toy, V.G., 2006, The habitat of fault‐generated pseudotachylyte: Presence vs. absence of friction‐melt, in Abercrombie, Rachel, McGarr, Art, Di Torro, Giulio, and Kanamori, Hiroo, eds., Earthquakes: Radiated Energy and the Physics of Faulting: American Geophysical Union Monograph 170, p. 153–166. Sims, P.K., 1990, Geologic map of Precambrian rocks of the Iron Mountain and Escanaba 1° x 2° quadrangles, northeastern Wisconsin and northwestern Michigan, U.S. Geological Survey Miscellaneous Investigations Series, Map I–2056, scale 1:250,000. Sims, P.K., and Peterman, Z.E., 1980, Geology and Rb-Sr age of lower Proterozoic granitic rocks, northern Wisconsin, in Morey, G.B., and Hansen, G.N., eds., Selected studies of Archean gneisses and lower Proterozoic rocks, Southern Canadian Shield: Geological Society of America Special Paper 182, p. 139–146. Sims, P.K., and Schulz, K.J., 1993, Geologic map of Precambrian rocks of parts of Iron Mountain and Escanaba 30’ x 60’ quadrangles, northeastern Wisconsin and adjacent Michigan, U.S. Geological Survey Miscellaneous Investigations Series, Map I–2356, scale 1:100,000. Sims, P.K., Schulz, K.J., and Peterman, Z.E., 1984, Guide to the geology of the Early Proterozoic rocks in northeastern Wisconsin, Institute on Lake Superior Geology, Field Trip 1, 93 p. Sims, P.K., Peterman, Z.E., and Schulz, K.J., 1985, The Dunbar Gneiss-granitoid domeimplications for Early Proterozoic tectonic evolution of northern Wisconsin, Geological Society of America Bulletin, v. 96, p. 1101–1112. 123 �Sims, P.K., Van Schmus, W.R., Schulz, K.J., and Peterman, Z.E., 1989, Tectono-stratigraphic evolution of the Early Proterozoic Wisconsin magmatic terranes of the Penokean Orogen, Canadian Journal of Earth Sciences, v. 26, p. 2145–2158. Sims, P.K., Schulz, K.J., and Peterman, Z.E., 1992, Geology and geochemistry of Early Proterozoic rocks in the Dunbar area, northeastern Wisconsin,: U.S. Geological Survey Professional Paper 1517, 65 p. Sims, P.K., Schulz, K.J., DeWitt, E., and Brasaemle, B., 1993, Petrography and geochemistry of Early Proterozoic granitoid rocks in Wisconsin magmatic terranes of the Penokean orogen, northern Wisconsin—a reconnaissance study, U.S. Geological Survey Bulletin 1904-J, 31 p. Smith, E.I., 1983, Geochemistry and evolution of the early Proterozoic, post-Penokean rhyolites, granites, and related rocks of south-central Wisconsin, U.S.A., in Medaris, L.G., Jr., ed., Early Proterozoic geology of the Great Lakes region: Geological Society of America Memoir 160, p. 113–128. Van Wyck, Nicholas, and Johnson, C.M., 1997, Common lead, Sm-Nd, and U-Pb constraints on petrogenesis, crustal architecture, and tectonic setting of the Penokean orogeny (Paleoproterozoic) in Wisconsin, Geological Society of America Bulletin, v. 109, p. 799– 808. 124 � 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, 2018 Description An account of the resource Institute on Lake Superior Geology. Iron Mountain, Michigan. May 15-18, 2018. 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 2018 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English