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Hydrothermal Footprint of Carlin-type Gold Deposits at the District Scale: Jerritt Canyon Mining District, Elko County, Nevada
AuthorPatterson, Lucia M.
AdvisorMuntean, John L.
Geological Sciences and Engineering
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Carlin-type gold deposits in Nevada are among the leading gold producing deposits in the United States and constitute 8% of world production, making the United States one of the leading gold producing countries in the world. The Jerritt Canyon district, located in northeastern Nevada, hosts several Carlin-type deposits and has produced over 7 million ounces of gold.Many studies have been completed documenting the hydrothermal footprint of Carlin-type deposits at the deposit scale. However, maps depicting the hydrothermal footprint of a Carlin-type deposit at the district scale are lacking. The extent of the hydrothermal footprint of Carlin-type deposits is unclear. Maps depicting hydrothermal products such as alteration, veining, geochemistry, and isotopic data are regularly used in epithermal and porphyry deposits as they commonly form distinct patterns around ore. Maps depicting the hydrothermal footprint of Carlin-type deposits could be used in a similar fashion for exploration and could improve the existing model for these deposit types.In 2000, Anglo Gold collected a data set consisting of multielement analyses from 6,416 drill holes, from one stratigraphic interval at the bottom 5 feet of the Roberts Mountains Formation, directly above the Saval Discontinuity in the Jerritt Canyon district. The lithology along the base of the Roberts Mountains Formation is predominantly calcareous siltstones; thus, the data set eliminates the effect that variable lithology may have on a hydrothermal footprint. Mapping and core-logging completed during this study are consistent with the interpretation that the Saval discontinuity is a subaerial paleokarst that has been locally structurally activated. In addition, Yukon-Nevada Gold supplied their data set which consisted of previously logged geology corresponding to the Anglo Gold data set. A sub-set of holes were re-logged to document and obtain data on the Saval discontinuity, and to generate maps depicting patterns of alteration, veining, mineralization, carbonate isotopes, and clay distribution with respect to known mineralization. The geochemical data set was analyzed visually and statistically to document associations between gold and other elements.Analysis of the geochemical data set has defined two groups of elements. The first group, which consists of gold, arsenic, mercury, thallium, and tellurium, is the elemental suite most closely associated with the Carlin-type gold mineralization. The Carlin-type elements are spatially and statistically associated with each other and are elevated within and outboard of the vertical projection of known gold deposits. The second group is composed of elements common in black shales and consists of two subgroups. The first subgroup consists of uranium, phosphorous, lanthanum, and gallium; and the second group is comprised of zinc, uranium, molybdenum, vanadium, nickel, copper, cadmium, and silver. The black shale elements are statistically and spatially associated with each other and do not exhibit any relationship with the Carlin-type elements.Re-logged geology, previously logged geology, and XRD analysis all indicate that silicified and decarbonatized rock, along with quartz veining are localized in areas within or near the vertical projection of the gold deposits. δ18O values from rock carbonate are depleted within and around the vertical projections of the gold deposits, whereas δ13C values show no pattern with respect to mineralization. Ferroan dolomite and ankerite identified by carbonate staining and XRD analysis exhibit the most continuous halo to mineralized areas.The key findings from this study were identification of major centers of fracture controlled fluid upwelling responsible for Carlin-type mineralization, a pre-ore black shale elemental signature, and a suite of vectoring tools which could aid in the exploration of Carlin-type deposits. The major center of upwelling fluids appears to be under the Steer/Saval/SSX area as evidenced by pervasive elevated values of the Carlin-type elements and depletions in δ18O values. The pervasive, spotty or irregular distribution of the Carlin-type elements and, especially, logged alteration features, in and around the deposits at Jerritt Canyon, suggests complex fracture control of the hydrothermal fluids. The group one black shale elemental signature (P, U, La, Ga) reflects formation of apatite by upwelling oceanic waters during a transgressive cycle. However, the group two black shale signature most likely originated from the migration of basinal brines and petroleum under conditions similar to MVT deposits. Previous petrographic work has documented an intimate association between potential iron bearing dolomite and MVT mineralization indicating that the basinal brine responsible for MVT mineralization would have also been capable of producing ferroan dolomite, which would have been a critical rock preparation event for later Carlin-type mineralization. The most important exploration tools to come out of this study are the identification of halos of ferroan dolomite, depleted δ18O values in rock carbonate, and continuous, elevated, arsenic, mercury, thallium, and to a lesser extent, tellurium concentrations around the vertical projection of the gold deposits and the zone of gold values greater than 300 ppb along the Saval.