Buffalo Canyon: An Oligocene Greisen-Like Intrusion-Related Gold Deposit in the Union Mining District, Nye County, Nevada
AdvisorMuntean, John L
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Reduced intrusion-related gold (RIRG) deposits have been documented around the globe, and have been best studied in Alaska and Yukon, but few examples have been documented in Nevada. One occurrence in Nevada is the Jurassic Bald Mountain deposit in eastern Nevada; however, for most Nevada occurrences there has been insufficient work that establishes a genetic connection to intrusions or determines their age. Buffalo Canyon is a gold prospect located near the historic Berlin mining area in northern Nye County, Nevada, where gold, silver, copper, lead and zinc were mined in the past from mesothermal quartz veins. Buffalo Canyon contains the more recently discovered Everson prospect, a poorly defined gold deposit, which has characteristics of RIRG deposits. The district includes a series of intrusions emplaced into Triassic metasedimentary and metavolcanic rocks. These previously undated intrusions vary in composition from diorite to granodiorite and granite. U-Pb dating of zircons by LA-ICPMS has revealed distinct intrusive events in the Jurassic, Cretaceous, and Oligocene. Metaluminous, biotite-pyroxene diorite stocks to porphyritic granodiorite dikes have been dated at 162.03 + 0.91 Ma and 158.72 + 0.94 Ma, respectively. Peraluminous, ilmenite-bearing leucogranite plugs and dikes have been dated at 81.96 + 0.43 Ma and 82.9 + 0.30 Ma, respectively. A variety of Oligocene dikes and small plugs that include quartz-monzodiorite, quartz monzonite, dacite and granite are metaluminous to peraluminous and locally ilmenite-bearing. Four dates of Oligocene intrusions fall within error of each other. They yielded dates of 25.13 + 0.29, 24.9 + 0.30, 24.8 + 0.40 Ma and 24.6 + 0.30 Ma. Country rocks at Buffalo Canyon, particularly those near Jurassic intrusions, are metamorphosed to biotite hornfels. Biotite hornfels is overprinted by sodic-calcic alteration that is likely related to Jurassic intrusive activity, and consists primarily of actinolite + albite in veins and disseminated patchworks. Mesothermal quartz veins have a Au-Ag-(Sb-Pb-As-Cu) geochemical signature, are composed predominantly of moderately deformed, recrystallized quartz and contain liquid CO2-bearing fluid inclusions. Mesothermal quartz veins are hosted within Jurassic intrusions and are post-Luning-Fencemaker thrust (mid-Jurassic) deformation. Small volumes of Oligocene intrusive rocks are variably altered to muscovite-tourmaline or propylitic assemblages. Base-metal sulfide mineralization occurs as veins and replacements, both of which are associated with tourmaline as a dominant gangue mineral. Base-metal sulfide mineralization has a Ag-Pb-Zn-Sb-As-(Au-Bi-Cu-Mo) signature. Free gold within low sulfide, 1-10cm thick, sheeted, crack-seal quartz veins with locally developed sericite-tourmaline envelopes define the bulk of the Everson gold mineralization. Other gangue minerals in these veins include muscovite, tourmaline and fluorite. The veins have a Au-Te-(Mo-Bi-Sn) signature and high Au:Ag ratios (>1:1). The quartz in these gold-bearing veins contain common hypersaline brine and coexisting vapor-rich fluid inclusions, indicative of trapping within the two-phase brine + vapor field. Combined with estimated thicknesses of older and coeval Oligocene volcanic rocks, the fluid inclusions suggest the deposit formed at <~3 km paleodepth. The Everson-related quartz veins also cross-cut mapped Jurassic intrusive rocks, sodic-calcic veins, base-metal sulfide mineralization, and Oligocene granitic intrusions. Muscovite and tourmaline + fluorite associated with alteration and mineralization suggest a greisen-like style of alteration and mineralization. The preponderance of evidence suggests that most gold mineralization at Buffalo Canyon is Oligocene in age, related to a felsic intrusion that only crops outs locally, but is likely extensive in the subsurface. Buffalo Canyon bears strong similarities to typical RIRG systems, and there are likely additional unknown examples within Nevada. A significant outcome of this study is that Buffalo Canyon formed in the Oligocene, related to volcanism during slab rollback that led to the ignimbrite flare-up in the central Nevada. Though most RIRG systems in Nevada could indeed be Mesozoic, the research on Buffalo Canyon opens the possibility that similar deposits may also be related to much younger intrusions that may be largely covered by coeval Tertiary volcanic rocks.