Abstract

Two types of epithermal precious and base metal deposits hosted primarily by Tertiary volcanic rocks have been distinguished through a detailed examination of the characteristics of 16 well-documented epithermal districts. These characteristics include the ore, gangue, and alteration mineral assemblages, the spatial and temporal distributions of these assemblages, host-rock composition, age relations between ore deposition and emplacement of the host rock, size of the district, temperatures of mineral deposition, chemical composition of the fluids, paleodepth estimates, origin(s) of the fluids, and regional geologic setting. The relative value of each of these features for distinguishing epithermal precious and base metal deposits is estimated. The vein and alteration mineral assemblages are the most important factors, and age relations between ore deposition and emplacement of host rock also appear to be significant. The two major types are designated the acid-sulfate type (Goldfield, Nevada) and the adularia-sericite type (Creede, Colorado; Round Mountain, Nevada). Both types have gold-rich and silver-rich subsets. Base metal contents in the acid-sulfate-type deposit are relatively high and relatively copper rich; there is a wide range in the base metal contents in the adularia-sericite-type deposit, although most are relatively copper poor. Adularia-sericite-type deposits are considerably more abundant than acid-sulfate-type deposits.The two types of deposits appear to form under similar pressure-temperature conditions but in different geological and geochemical environments in ancient geothermal systems. The acid-sulfate-type deposit forms in the root zones of volcanic domes from acid waters that contain residual magmatic volatiles. The adularia-sericite-type deposit forms in a geothermal system where surficial waters mix with deeper, heated saline waters in a lateral flow regime, high above and probably offset from a heat source at depth; neutral to weakly acidic, alkali chloride waters are dominant.

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