MULTIPLE EPISODES OF MAGMATISM, QUARTZ-ALUNITE ALTERATION, AND ADULARIA-SERICITE PRECIOUS METAL MINERALIZATION, WESTERN VIRGINIA RANGE, NEVADA

The western Virginia Range, Nevada, site of the world-class Comstock lode, had a protracted mid-Miocene history of magmatic and hydrothermal events that spanned ~6 Ma. New mapping and 40Ar/39Ar geochronology near Virginia City have refined the stratigraphy and ages of eruptive rocks, adularia-sericite precious-metal veins, and quartz-alunite alteration cells into sequential as well as concurrent events. The earliest magmatic episode (lower Alta Fm., 18.20±0.16 to 17.58±0.11 Ma; n=5) produced pl-hbl-px-phyric andesite lavas and did not cause any major hydrothermal event. The second episode (upper Alta, 16.06±0.13 to 15.13±0.10 Ma; n=6) also produced pl-hbl-px andesites and culminated with intrusion of the slightly more silicic, biotite-bearing Davidson granodiorite (15.22±0.06 Ma). Several quartz-alunite hydrothermal alteration cells near Virginia City (15.4±0.4 to 15.06±0.07 Ma, alunite; n=4), comprising cubic kilometers of rock altered by disproportionation of SO2, were contemporaneous with upper Alta-Davidson magmatism. Stable isotope data indicate that alunite of these cells is magmatic-hydrothermal. A third igneous episode produced both lavas and hypabyssal intrusions (Kate Peak; 14.79±0.10 to 14.30±0.10 Ma; n=9) and marked a transition to more silicic magmatism (pl-hbl-bi±qtz-phyric andesite-dacite). Still younger magmatic episodes are indicated by one date on an intrusion (12.83±0.09 Ma), which is petrographically similar to Kate Peak rocks, and by published ages to the northeast. A quartz-alunite hydrothermal alteration cell north of Virginia City is 13.0±0.6 Ma (alunite).

The NNE-striking Comstock lode-Silver City precious-metal veins, which resulted from circulation of reduced hydrothermal fluids, formed at 14 Ma (14.08±0.06 to 13.97±0.04 Ma; adularia, n=4). The Occidental-Flowery veins, which parallel the Comstock lode 3-5 km to the east, are distinctly younger (13.45±0.04 to 13.26±0.04 Ma, adularia; n=5), verifying earlier K-Ar and thermometric analyses that documented separate hydrothermal systems. We have not yet found any igneous activity that was contemporaneous with either precious-metal system.