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Abstract

An epithermal ore deposit is defined as a relatively near-surface deposit formed in a hydrothermal system under low to moderate pressure and a temperature range below about 300°C (Barrett, 1985). This concise definition is a restatement of Lindgren's characteristics of hydrothermal systems of “epithermal” character. A modification of Lindgren's characteristics is tabulated in Table 9.1. These characteristics are both physical and chemical, and we will, in this and the following paper (Berger and Silberman, 1985, this volume), attempt to relate them.

Epithermal lode deposits in the Circum-Pacific region produce approximately 30 million grams of gold annually (Giles and Nelson, 1982) and a larger, but indeterminate, amount of silver. Many epithermal deposits are closely associated with convergent plate boundaries related to present and relatively recent regimes of plate tectonic interaction (Giles and Nelson, 1982; Sawkins, 1984). These mobile regions of the earth's crust are characterized by recent volcanism, high heat flow and tectonic activity, and by the presence of active and recently active geothermal fields, some of which have deposited precious metals and associated metals (Table 9.1) in similar concentrations (but not volumes) to those found in the epithermal ore deposits (Weissberg et al., 1979; Henley, 1985, this volume).

The understanding of processes that occur during the formation of epithermal ore deposits has been advanced in the recent past by the suggestion that these ore deposits are essentially fossil geothermal systems (e.g., White, 1955, 1981 ; White, 1974; Wetlaufer et al., 1979; Henley and Ellis, 1983; Henley, 1985, this volume).

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