Characteristics of ore and trace mineral assemblages at the Flambeau volcanogenic massive sulfide deposit, northwestern Wisconsin

Kaelyn E. Blotz and Dr. Robert W.D. Lodge Center of Ore Research and Exploration Department of Geology, University of Wisconsin - Eau Claire

Introduction Flambeau Ore Mineralogy Figure 1: The Acanthite - Ag S Acanthite - Ag S Flambeau Cu-Zn-Au 2 2 Mine A B The Paleoproterozoic Flambeau Cu-Zn-Au volcanogenic massive Ac Py Sp sulfide (VMS) deposit Ac Py Figure 5: Sphalerite-Rich Ore is located within the Scanning electron microscopy was utilized Pembine-Wausau in order to characterize trace mineral Terrane of the Sp assemblages within the Flambeau deposit. Penokean Orogeny Using energy dispersive spectroscopy, many near the town of trace minerals were identified that are Ladysmith, Wisconsin commonly associated with the hybrid (Schulz and Cannon, epithermal-VMS model. 2007). Of the many Sphalerite-rich ore showed trace minerals VMS deposits in the C D primarily composed of silver. Silver telluride Penokean Orogen, Sp (hessite) and acanthite (images A & B) were the Flambeau is the Py common as well as stibnite (image C). only one to have been Arsenopyrite was also identified (image D). mined. Mining began in 1992 and lasted until 1997 when Sb Silver minerals were typically associated with extraction of the supergene enriched cap was completed. The sphalerite and pyrite. Most trace minerals high-grade copper ore body produced nearly 1.8 million tons typically occurred within fractures or grain of ore before the open pit was completely refilled and the site Ar boundaries in sphalerite and less commonly was reclaimed. pyrite. Sp Sp Figure 2: Drill Mineral Abbreviations for Image 5: Acanthite (Ac), core samples Arsenopyrite (Ar), Pyrite (Py), Sphalerite (Sp), Stibnite (Sb) showing the two main types of Stibnite - Sb2S3 Arsenopyrite - FeAsS primary ore Hessite - Ag Te Bismuthinite - Bi S Since the site 2 2 3 has been A B completely reclaimed, there Py is no bedrock Py exposed in the Figure 6: Chalcopyrite-Rich Ore immediate area C halcopyrite-rich ore, the dominant ore and the only type, showed a greater abundance of remaining Hs Bi tellurides compared to the sphalerite-rich ore. material from the Silver (image A) and bismuth tellurides were deposit is stored Cp common along pyrite grain boundaries and as at a core micron-scale inclusions with the occasional repository operated by the Wisconsin Geologic and Natural lead telluride. Inclusions of bismuthinite were History Survey. This study focuses on the trace mineral identified most commonly within pyrite (image abundances and mineralogical variations of the ore and alteration C D Cp B). Electrum (images C & D) was found only zones at the Flambeau deposit. In total, there have been 52 thin Py within chalcopyrite-rich samples and was sections analyzed from the deposit. typically found within chalcopyrite. Py Other trace minerals identified include acanthite, bismuthinite, cassiterite, monazite, and an unnamed tungsten mineral. The VMS Models El Cp Figure 3: Standard VMS Model majority of these trace minerals are The Standard VMS model El commonly associated within hybrid consists of lenses containing epithermal-VMS deposits. polymetallic massive sulfides Mineral Abbreviations for figure 6: Bismuthinite (Bi), that form near the seafloor in Chalcopyrite (Cp), Electrum (El), Hessite (Hs), Pyrite (Py), submarine volcanic environments in extensional Electrum - Au, Ag Electrum - Au, Ag tectonic settings. The deposits Galley et al., 2007 form from metal-enriched fluids Conclusion References associated with seafloor hydrothermal convection. All VMS deposits contain variable amounts of Dubé, B., et al., 2007, Gold-rich volcanogenic massive sulphide deposits, The characteristics of the hydrothermal fluids are typically trace minerals, however the characteristics and Geological Survey of Canada. weakly acidic in reducing environments with temperatures abundances of the minerals can be important in Franklin, J.M., et al., 2005, Volcanogenic Massive Sulfide Deposits, ranging from ~140°C to >400°C. This results in the formation of Society of Economic Geologists, Inc.,100th Anniversary Volume, p. determining the original hydrothermal environment in 523-560. alteration zones with varying compositions and sizes. The which the deposit formed. The relative abundance Galley, A., et al., 2007, Volcanogenic massive sulphide deposits in alteration zones are characterized by epidotization, silicification, and characteristics of the trace minerals, along with Goodfeelow W.D., ed., Mineral Deposits of Canada, Special Publication 5, p. 141-161. or Mg-Fe metasomatism. the Cu-enrichment in the Flambeau bimodal felsic May, E.R. and Dinkowitz, S.R., 1996, An overview of the Flambeau lithostratigraphy (Franklin et al, 2005), may indicate supergene enriched massive sulfide deposit: geology and mineralogy, Figure 4: Gold-Rich VMS Model that this is not a traditional VMS deposit. One Rusk County, Wisconsin, in LaBerge, G.L., ed., Volcanogenic Some gold-rich VMS deposits Massive Sulfide Deposit of Northern Wisconsin: A Commemorative mechanism for Au and/or Ag-enriched VMS is if the Volume: Institute on Lake Superior Geology Proceedings, v. 2, part 2, are interpreted to be a hydrothermal fluid has a magmatic component. Franklin et al., 2005 p. 67-93. combination of both standard Based on the alteration assemblages observed in Schulz, K.J. and Cannon, W.F., 2007, The Penokean orogeny in the Lake Figure 10: Solubility Curves for Cu, Zn, Superior region: Precambrian Research, v. 157, p. 4-25. VMS systems and epithermal the Flambeau, the deposit formed in an environment and Au deposits. They share a similar with lower pH and higher temperatures than are This phase diagram shows the solubility volcanic stratigraphy and typical of normal seawater-dominated hydrothermal Dubé et al., 2007 of Cu, Zn, and Au under typical sea- tectonic setting to standard VMS systems. The alteration data, coupled with the floor VMS systems. Numbers on curves Acknowledgements compositions and mineral-associations of trace deposits but their alteration and mineralization style differs. represent ppm for base metals and ppb The authors would like to thank the financial support from Most Au-VMS deposits are characterized by metamorphosed minerals and unusual Cu-enrichement suggests that for Au. Au is shown as two sets of ORSP through the Bluegold Commitment Fund. SEM-EDS advanced argillic and massive silicic alteration, which indicates there may have been magmatic fluids present in the curves to represent the differences analyses were completed at the Material Science Center at seawater-dominated hydrothermal system, likely UWEC. Logistical support and financial from the WGNHS was an oxidized low-pH hydrothermal fluid that differs significantly between chloride complexed gold and making the Flambeau deposit a hybrid VMS- provided during sample collection from the core repository in from the mainly reduced, near neutral to weakly acidic fluids bisulfide-complexed gold (Franklin et epithermal system. Mount Horeb, WI. typical of most standard VMS deposits (Dubé et al., 2007). al., 2005).