Relationships of Trace-Element Patterns to Geology in Hot-Spring-Type Precious-Metal Deposits
Published:January 01, 1985
Byron R. Berger, Miles L. Silberman, 1985. "Relationships of Trace-Element Patterns to Geology in Hot-Spring-Type Precious-Metal Deposits", Geology and Geochemistry of Epithermal Systems, B. R. Berger, P. M. Bethke
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Those epithermal precious-metal deposits where ore was precipitated within 100-300 m of the earth's surface such that the direct interaction of hydrothermal fluids with the surface is a major cause of ore-mineral precipitation in the upper part of the system make up the subclass known as hot-spring-type deposits (Berger and Eimon, 1983; Berger, 1985). The deposits were emplaced as small veins, stockworks, and explosive breccias in association with non-marine volcanism, generally calc-alkaline in composition. Henley (1985b, this volume) and Hayba et al. (1985, this volume) prefer to not separate hot-spring deposits as a separate class or subtype of epithermal...
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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).