GEOLOGIC SETTING, GEOCHEMISTRY, AND GEOCHRONOLOGY OF EPITHERMAL GOLD-SILVER DEPOSITS IN THE SEVEN TROUGHS DISTRICT, NORTHWEST NEVADA

The Seven Troughs district, near Lovelock, NV, contains small, high-grade low-sulfidation epithermal Au–Ag vein deposits temporally and spatially related to a bimodal suite of middle Miocene volcanic rocks. The district produced about 158,468 oz Au and 995,876 oz Ag from 152,339 tons of ore in the early 20th century. Ore consisted of quartz-adularia veins containing coarse electrum, naumanite, and minor base metal sulfide minerals. Volcanic host rocks form a rhyolite flow-dome complex, intercalated with volcaniclastic sedimentary rocks and basalt to andesite lava flows and dikes, that overlies and intrudes older tuffs and Mesozoic basement. Despite an overall bimodal composition (49–61 and 71–77 wt. % SiO₂), the volcanic rocks have characteristics of both calc-alkaline and tholeiitic suites. Notably, mafic rocks are compositionally similar to tholeiitic basalts related to the northern Nevada rift, whereas many rhyolites have more calc-alkaline compositions and contain biotite and/or hornblende phenocrysts. The volcanic rocks were emplaced into a narrow (~2.5 km wide) NNE–trending graben formed during syn-magmatic extensional faulting. Veins are controlled primarily by N– to NNE–striking normal faults, many of which also are intruded by basalt dikes. Biotite and sanidine ⁴⁰Ar/³⁹Ar dates (FCT=28.02 Ma) on pre- and post-mineral rhyolites range from 14.41±0.09 to 13.78±0.06 Ma (5 samples), a basalt dike is 14.1±0.2 Ma, and adularia from the Mazuma vein is 13.82±0.02 Ma. The coeval volcanic rocks and low-sulfidation deposits at Seven Troughs are distinctly younger than most similar epithermal deposits in the northern Great Basin (NGB) (mostly 16.5 to 15 Ma; e.g., Midas, Sleeper, Buckskin-National, Hog Ranch, Ivanhoe). In addition, Seven Troughs lies along one of the magnetic anomalies suggested to have formed by rifting and mafic magmatism related to the Yellowstone hot spot at about 16.6 Ma (Glen and Ponce, 2002, Geology); our dates suggest that faulting and magmatism at Seven Troughs is about 2 m.y. younger than the age inferred for this feature. Compositional and age differences between Seven Troughs and other middle Miocene rhyolite-hosted epithermal deposits suggest that the Seven Trough deposits may be more closely related to Western Cascades calc-alkaline magma than other rhyolite-hosted deposits in NGB.