Epithermal Deposits: Geological characteristics and Genetic Processes for Exploration & Discovery, Comstock District

The Comstock district, in the southern Virginia Range of western Nevada, was the first major silver-gold discovery in the United States. In the district, Early Mesozoic meta-igneous and metasedimentary rocks intruded by Late Mesozoic granitic plutons are unconformably overlain by Oligocene to Early Miocene silicic ash-flow tuffs and Early to Middle Miocene andesites with accompanying intrusions of andesite-diorite stocks. High sulfidation alteration accompanied the latter stages of this magmatism. After a brief hiatus, andesite dikes were intruded concurrently with east-side-down normal faulting and renewed andesitic volcanism. Intermediate sulfidation precious metal mineralization of the Comstock and Silver City Lodes appears to be related to the latter stages of this magmatic activity. The intermediate sulfidation Occidental Lode and high sulfidation alteration to the north of the district formed somewhat later with no known associated magmatism. Pliocene to Recent reactivation of the major structures has down-dropped and preserved from erosion much of the high level alteration while exposing near the surface much of the ore mineralization.
High sulfidation alteration includes quartz-alunite, quartz-pyrophyllite-diaspore, quartz-illite, and quartz-sericite assemblages in several centers of alteration. Quartz-adularia stockwork veining, characteristic of the ore zone, passes upward and downward into calcite veining. Pyrite-rich anhydrite-quartz-illite alteration lies above some of the veins, but is unclear as whether it is high or low sulfidation related. Epidote-bearing propylitic alteration forms belts along the intermediate sulfidation veins. Anomalous enrichment of Ag, Au, As, and TI characterize oxidized lode mineralization, whereas the high-level hanging wall acidic alteration is characterized by enrichment of TI, Te, As, B, F, Hg, Mo, Pb, and Sb. A district wide depletion of Zn, Co, Ni, and Mn in surface exposures may result from either hypogene or supergene leaching.