Igneous Geochemistry and Geochronology of the Spruce Mountain District, Elko County, Nevada
AdvisorThompson, Tommy B
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Spruce Mountain is a polymetallic mining district located at the southern end of the Pequop mountains in Elko County, Nevada. Historic production of lead, zinc, silver, copper, and gold came from replacement mantos, veins, and skarns. These deposits are spatially associated with porphyry intrusions located throughout the district. Exploration efforts have since revealed the presence of Carlin-type gold and porphyry molybdenum mineralization.Geologic mapping, petrography, geochemical analysis, and radiometric dating were used to describe each of the igneous intrusions within the district. Subduction-related magmatism occurred in three pulses during the late-Jurassic and mid-Eocene. Jurassic magmatism is represented by two intrusions of medium-grained rhyolite and dacite porphyry emplaced at 155 and 156 Ma, respectively. The more voluminous Eocene magmatism comprises five units of dikes and stocks of porphyritic rhyolite, granite, and aplite. Eocene intrusions were emplaced at 37.5 and 39.1 Ma. Fine to medium-grained andesite, trachyandesite, and lamprophyre dikes are also present although their absolute and relative ages remain unknown.Magmatism within the district is representative of northern Nevada in terms of age, formation, and composition. Jurassic and Eocene intrusions were emplaced during the most active periods of Jurassic and Eocene magmatism in northern Nevada. Both events were subduction-related and produced high K calc-alkaline melts which may have differentiated in one or more large magma chambers underlying the district. Rhyolite and dacite represent 70% of all Jurassic intrusions in northern Nevada while rhyolite represents 60% of the Eocene intrusions in northern Nevada.Intrusions within the district closely resemble those associated with Carlin-type gold deposits along the Carlin trend and the porphyry molybdenum deposit at Mt. Hope. Similar age, genesis, composition, and alteration suggest the large scale processes responsible for magmatism were related in each of the three locations. Because of these similarities, the Spruce Mountain district is prospective ground for economic deposits of both styles of mineralization.