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Geology and Genesis of the Duluth Complex and Its Ore Deposits

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Geology and Genesis of the Duluth Complex and Its Ore Deposits JIM MILLER PRECAMBRIAN RESEARCH CENTER DEPT. OF GEOLOGICAL SCIENCES UNIVERSITY OF MINNESOTA DULUTH Minnesota Ground Water Association Spring Meeting May 6, 2010 Talk Outline • History of Geologic Mapping • Evolution of the Midcontinent Rift •Geology & Genesis of the Duluth Complex •Cu-Ni-PGE Mineralization Stillwater Complex, Montana Skaergaard, East Greenland Sudbury Complex, Second Ontario DuluthDuluth Largest Complex Great Complex Dyke, Gabbro Zimbabwe Complex in Bushveld Complex, the World South Africa Geology of Minnesota circa 1872 Minnesota Geological Survey 1872 - 1900 N.H. Winchell The Grout and F.F. Grout Schwartz Era 1911-1958 G.M. Schwartz Saganaga Lake, Aug. 1922 “Chicken-track” Reconnaissance Mapping Sonju Lake area geology 1939 Grout, 1918 Geology of Northeast Minnesota circa 1932 The Quadrangle Mapping Era 1961 - 1982 P.K. Sims, MGS Director 1961-1973 Matt Walton, MGS Director, 1973-1987 J.C. Green – North Shore Volcanics P.W. Weiblen - Bald Eagle Int, Long Is. Lk. W.C. Phinney - NW Duluth Complex D.M. Davidson Jr. - E Duluth Complex B. Bonnichsen - S Duluth Complex, Babbitt Ph.D. Theses R.B. Taylor – Duluth area H.D. Hathan – Gunflint Trail area E.A. Mathez – Logan Sills M.G. Mudrey – Pigeon Point R.W. Cooper – Babbitt/Harris Lake area M.P. Foose – Harris Lake area MGS Misc. Map M-1 M.S. Theses 1964, R.B. Taylor N.W. Jones – Hovland Db, Logan Sills J.A. Kilburg – Ely’s Peak J.R. Burnell – Brule Lake R.J. Stevenson – Sonju Lake Intrusion N.M. Pope – Silver Cliff/Lafayette Bluff John Green Bill Phinney Paul Weiblen Gooseberry Falls, 1996 Gabbro Lake 15’ Quadrangle MGS Miscellaneous Map M-2 Green, Phinney, & Weiblen, 1966 massive sulfide, basal DC Phinney Nathan/Mathez Bonnichsen Davidson Two Harbors Sheet J.C. Green 1982 Aeromagnetic Survey of NE Minnesota 1979-1982 First Vertical Derivative of Aeromagnetic Data Shallow Drilling Program 1993-1995 Well Exposed Poor Exposure No Exposure Compilation of the Geologic Picture 2001 MGS Miscellaneous Map M-119, 2001 MGS Report of Investigations 58 2002 Geologic Mapping since 2001 The need for an update _______________ THE MIDCONTINENT RIFT _______________ An attempt at continental rifting 1.1 billion years ago A. Rift Magmatism Basalt The Evolution of Gabbro Flows the Midcontinent Crust Rift Mantle 1,109-1,086 Ma Mantle Plume B. Sediment Infilling Sandstone 1,090-900 Ma C. Compression 1,000-900 Ma GLIMPCE Seismic Line A from Trehu et al., 1994 Bimodal Magmatism of the Midcontinent Rift From Miller & Vervoort (1996) Litho/Chronostratigraphy of the North Shore Volcanic Group From Davis and Green (1997) THE DULUTH COMPLEX Felsic Series 1109-1106 Ma (Vervoort et al., 2007) Poplar Lake Intrusion Crocodile Lake Intrusion Early Gabbro Series 1107 Ma Paces & Miller, 1993 Davis and Green, 1997 Anorthositic Series troctolitic anorthosite (TA) poikilitic olivine anorthosite (poA) 1099 Ma (Paces & Miller, 1993) olivine leucogabbro (oLG) mgTA gabbroic anorthosite (GA) cgTA troctolite 1099 Ma (Paces & Miller, 1993) Layered Series CUMULUS STRATIGRAPHY IN LS INTRUSIONS Pl+Cpx+Ox+Ol+Ap+Gp layered troctolite OLI – Osier Lake Intrusion Pl+Cpx+Ox+/-Ol DLS – Layered Series at Duluth BEI – Bald Eagle Intrusion WLI – Wilder Lake Intrusion GLI – Greenwood Lake Intrusion BLI – Boulder Lake Intrusion Pl+Ol WMI – Western Margin Intrusion SKI – South Kawishiwi Intrusion TI – Tuscarora Intrusion L1 – Lake One Intrusion Ol PRI – Partridge River Intrusion X-bedded olivine gabbro Beaver Bay Complex 1096 Ma (Paces & Miller, 1993) Topography and Geology in the Tettegouche State Park Area Anorthosite Inclusions in the Beaver River Diabase Origin of Anorthositic Rocks of the Duluth and Beaver Bay Complexes Contact Zone Cu-Ni-PGE Sulfide Deposits of the Duluth Complex Average Grades Cu – 0.6% Ni - 0.2% Co – 0.005% Pd+Pt+Au – 500-1000 ppb Creation of the Cu-Ni-PGE Sulfide Deposits of the Duluth Complex Cu S Ni S Co Pd + Pt Au Dominant Mineralogy of Magmatic Cu-Ni Sulfide Deposits Pyrrhotite – FeS1-x Chalcopyrite - CuFeS2 Pentlandite - (Fe,Ni)9S8 Bornite - Cu5FeS4 Talnakhite – Cu9(Fe,Ni)8S16 Chalcocite - Cu2S Cobaltite - CoAsS Mineralization Model for the Basal Intrusions of the Duluth Complex Styles of Mineralization after Peterson (2001) • Open (early) – vertically extensive (> 450 meters) – low - mod Cu-Ni and very low Au+PGE – Cu-Ni grades typically increase towards the basal contact • Confined (late) – vertically restricted (< 150 meters) – mod - high Cu-Ni and mod to high Au+PGE – Cu-Ni grades typically are the highest near the top of the mineralized zone Spruce Road, Open Style Maturi Deposit, Confined Style Current Exploration Activity Polymet –Northmet Teck American –Mesaba Franconia – Birch Lake, Maturi, Spruce Rd Duluth Metals – Nokomis, Dunka Pit Encampment– South Filson Cr. A World Class Ore Deposit That Someday Will Be Mined (Eckstrand and Hulbert, 2007).
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