Relationship of Trace-Element Patterns to Alteration and Morphology in Epithermal Precious-Metal Deposits
Miles L. Silberman, Byron R. Berger, 1985. "Relationship of Trace-Element Patterns to Alteration and Morphology in Epithermal Precious-Metal Deposits", Geology and Geochemistry of Epithermal Systems, B. R. Berger, P. M. Bethke
Download citation file:
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).
Figures & Tables
Geology and Geochemistry of Epithermal Systems
In the context of exploration for epithermal deposits, why study geothermal systems at all? After all, not one exploited system to date has been shown by drilling to harbor any economically significant metal resource--but then until recently not one had been drilled for other than geothermal energy exploration.* The latter involves drilling to depths of 500-3000 meters in search of high temperatures and zones of high permeability which may sustain fluid flow to production wells for steam separation and electricity generation. In many cases such exploration wells have discovered disseminated base-metal sulfides with some silver and argillic-propylitic alteration equivalent to that commonly associated with ore-bearing epithermal systems (Browne, 1978; Henley and Ellis, 1983; Hayba et al., 1985, this volume). In general, however, geothermal drilling ignores the upper few hundred meters of the active systems and drill sites are situated well away from natural features such as hot springs or geysers, the very features whose characteristics (silica sinter, hydrothermal breccias) are recognizable in a number of epithermal precious-metal deposits (see, for example, White, 1955; Henley and Ellis, 1983; White, 1981; Berger and Eimon, 1983; Hedenquist and Henley, 1985; and earlier workers such as Lindgren, 1933). Knowledge of the upper few hundred meters of active geothermal systems is scant and largely based on interpretation of hot-spring chemistry. Tantalizingly, in a number of hot springs, transitory red-orange precipitates occur which are found to be ore grade in gold and silver and which carry a suite of elements (As, Sb, Hg, Tl) now recognized as characteristic of epithermal gold deposits (Weissberg, 1969).