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Abstract

Research in the past two decades on epithermal gold and silver deposits has led to the recognition of three principal varieties—volcanic-rock-related adularia-sericite(illite) type, alunite-kaolinite ± pyrophyllite type, and sedimentary-rock-hosted Carlin-type deposits. A review of the salient characteristics of each variety and the state of our understanding about the genesis is best accomplished through well-studied examples. Creede, Colorado, Bodie, California (adularia-sericite(illite) types), and Summitville, Colorado (alunite-kaolinite ± pyrophyllite type), illustrate the tectonic setting, alteration and mineralization, and environment of ore deposition of volcanic-rock-related systems. Carlin, Getchell, and Jerritt Canyon, Nevada, serve as good examples of the same characteristics in sedimentary-rock-hosted Carlin-type systems.

There are many similarities among the different varieties of epithermal deposits including the predominance of a meteoric water component throughout the ore-forming stages in the volcanic-rock-related systems and in the intermediate to later stages of the Carlin-type systems; the predominance of large-scale, near-neutral pH alteration assemblages in the deeper parts of the hydrothermal systems; and the time-space relations which indicate heat energy input from intrusive rocks at differing levels in both the volcanic-rock and Carlin-type systems.

The above factors allow a common genetic framework to be established for epithermal deposits. The general model involves interaction of deeply penetrating ground waters and magmatic vapor derived from contemporaneous intrusions. In the large-scale systems responsible for Carlin-type deposits, connate brines also play an important role. The transport capability of deep fluids in epithermal hydrothermal systems may be shown to be dependent largely on the H2S content of the deep fluid, and in turn, through a series of fluid-mineral equilibria to depend strongly on temperature and inversely on CO2 content. Both factors relate to the source strength of magmatic or other evolved fluid at depth in the convection systems. This same common geochemical and hydrodynamic framework provides a basis for the discussion of depositional regimes and the real distinctions between the types of epithermal deposits observed in the field.

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