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Geochemical Investigation of the Origin of the Back Forty

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Geochemical Investigation of the Origin of the Back Forty GEOCHEMICAL INVESTIGATION OF THE ORIGIN OF THE BACK FORTY VOLCANOGENIC MASSIVE SULFIDE DEPOSIT IN MENOMINEE COUNTY, MICHIGAN Anthony Boxleiter1, Joyashish Thakurta1, and Thomas O. Quigley2 1. Department of Geosciences, Western Michigan University, Kalamazoo, MI 49008, [email protected]; 2. Aquila Resources Inc., Menominee, MI 49858 Introduction Volcanogenic Massive Sulfide (VMS) deposits constitute one of the world’s greatest sources of copper, zinc, lead, silver, gold, and a wide range of by-products including tin, cadmium, antimony, and bismuth (Kearney, 2003). The metals we mine on earth from ore-bearing deposits, such as VMS deposits, are vital to everyday life from the vehicles we drive to the homes we build. The first step involved in unlocking the value of metal on earth begins with geologic exploration to locate these deposits within the earth. This begins with field work conducted by geologists Michigan involving mapping, rock sampling and analysis. VMS deposits form in oceans, where convective circulation of seawater is driven through the seafloor by local or regional magmatic heat sources. This super-heated ocean water leaches components out of the footwall volcanic rocks, carrying metals and sulfur. When these fluids interact with cold, ambient seawater, decreasing temperature and increasing pH causes precipitation of base and precious metals as sulfide minerals at venting zones of within the seafloor. Deposits are classified based on host rock compositions: mafic, bimodal-mafic, mafic-siliciclastic, bimodal- felsic, and bimodal-siliciclastic (Barrie and Hannington, 1999). It is important to classify VMS deposits based on these types, allowing for comparison between similar deposits based on tectonic settings, geographic occurrence, and metal content. Wisconsin Fig. 1: Locations of VMS deposits along the E-W trend of Back Forty 3-D model including two planes containing the Penokean Volcanic Belt in northern Wisconsin. The drill holes LK-150, 166, 171, 451, and 127 sampled in Back Forty is the easternmost deposit of this trend and is this research (Aquila Resources, Inc.) the only VMS deposit found in the Michigan Upper Peninsula (Thakurta and Quigley, 2013) Fig. 3: Massive (red) and stringer (yellow) sulfide textures in The Back Forty core samples from the main zone and occurrence within the host rocks (right) The Back Forty VMS deposit is bimodal-felsic or Kuroko-style VMS deposit. This type is defined by having >50% felsic volcanic rocks, and <15% siliciclastic rocks in the host rock succession (Barrie, 2007). The Back Forty VMS deposit is located in Menominee County, Michigan. Several 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 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 Thin-Section Micrographs in Reflected Light characteristics, two of which are found to host sulfide mineralization (Quigely et al., 2008). Above Left: massive sulfide texture, (a) abundant partially reabsorbed pyrite (cream) in (b) sphalerite (medium grey), Crandon (Zn-Cu) Flambeau (Cu) Bend (Cu) Lynne (Zn-Pb-Cu) Back Forty (Zn-Cu) Objectives (c) euhedral arsenopyrite (white/light grey) top middle. Scale 20 bar: 1 mm. Above Right: massive sulfide texture, (a) abundant semi- 15 • Geochemically classify host rocks based Tivey, 1998 course pyrite (cream), (b) sphalerite (medium grey), (c) on minor, major, and trace element chalcopyrite (yellow). Scale bar: 1 mm 10 geochemistry (Fig.2) • Establish a relationship between sulfide ore mineral occurrence and the textural Frequency 5 characteristics within the host rock (Fig. 3) 0 • Establish the occurrence and textural characteristics of sulfide minerals within -5 -4 -3 -2 -1 0 1 2 3 4 5 the host rock to provide a relative timeline δ34S (‰VCDT) (Woodruff, USGS; Lynne, Flambeau, and Bend values) Sulfide (Myers, 1983; Crandon values) of volcanism and mineralization/hydrothermal infiltration (from Schulz, 2007) events. (Aquila Resources) • Acquire sulfur isotope values from sulfide minerals in each of the five major zones of the Back Forty deposit containing massive Back Forty Generalized sulfide mineralization, and use these Stratigraphic Column values to model the source of the sulfur. • Integrate all the available data to generate a model for formation of the Back Forty VMS deposit (Fig.4) Fig. 2: Host rocks of the Back Forty have been identified as primarily rhyolitic to rhyodacitic in composition based on SiO2 (from Schulz, vs Zr/TiO2 ratios (Thakurta and Quigley, 2013) 2007) References Sulfur Isotope Analysis 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. Barrie, C. T., and Hannington, M. D., 1999, Volcanic-associated Massive Sulfide Deposits: Processes and Examples in Modern and Ancient Settings: Introduction: In Reviews in Economic Geology Volume 8: Volcanic-Associated Massive Sulfide Deposits: Processes and Examples in Modern and Ancient Settings, Barrie, C. T., and Hannington, M. D., editors, p. 1-11. Barrie, C.T., 2007, Petrography and mineral chemistry of the Back 40 VMS deposit, Menominee County, Michigan: Initial observations, An interim report for Aquila Resources Sulfur isotope values characterize the distribution of sulfide minerals in the Back Forty deposit and model the origin of 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 34 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 sources. Magmatic or mantle derived sulfur has δ S values of 0 ± 2‰-VCDT (Ripley and Li, 2003). Biologically derived 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 34 Hannington, M., John W. Jamieson, Boswell A. Wing, and James Farquhar, 2006. EVALUATING ISOTOPIC EQUILIBRIUM AMONG SULFIDE MINERAL PAIRS IN ARCHEAN ORE DEPOSITS: CASE STUDY FROM THE sulfur would produce depleted or negative δ S values, resulting locally from microbial reduction of seawater sulfate (Taylor KIDD CREEK VMS DEPOSIT, ONTARIO, CANADA. Society of Economic Geologists, Inc. Economic Geology, v. 101. pp. 1055-1061. 34 Huston, David L., Pehrsson, S., Eglington, Bruce M., and Zaw, K. 2010. The Geology of Volcanic-Hosted Massive Sulfide Deposits: Variations through Geologic time with Tectonic Setting: Economic Geology v. 105, pp. 571- et al., 2010). Sulfur derived from heavy contemporaneous seawater sulfate would produce enriched or positive δ S values 591 Keaney, M.K., 2003. Volcanic-Associated Massive Sulphide Deposits for Billiken Management Services. Toronto, Ontario. June 12. 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. (Strauss and Schieber, 1990). Sulfur isotope values obtained from 25 sulfide ore minerals from the five major zones of 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, 34 v. 23, p. 222-237. sulfide mineralization within the Back Forty indicate a primarily mantle derived source of sulfur. δ S values range from Myers, Lesley Louise, 1983. Geochemistry of the Crandon Massive Sulfide Deposit, Wisconsin: Sulfur Isotope and Fluid Inclusion Data. M.S. Thesis – UWc-Madison. Sulfide 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 Ripley, Edward M. and Chusi Li, 2003. Sulfur Isotope Exchange and Metal Enrichment in the Formation of Magmatic Cu-Ni-(PGE) Deposits. Economic Geology Vol. 98, 2003, pp. 365-641 -2.53 to 3.92‰-VCDT with an average value of 0.94‰ and a standard deviation of ± 1.42‰. In Archean to Paleoproterozoic 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 poster abstract deposits, such as deposits formed during the Penokean Orogeny, the bulk of the sulfur was derived by leaching rocks (i.e. Schulz, K.J. and Cannon, W.F., 2007. The Penokean Orogeny in the Lake Superior region, Precambrian Research, 157, 4-25 Schulz, K.J., 2007. Sulfide Deposits
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