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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, 1985a; 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)Now recogpized as characteristic of epithermal gold deposits (Weissberg, 1969).

The term “hydrothermal” encom passes al l types of hot-water phenomena in the earth’s crust although most commonly the term is used in reference to those associated with impressive geyser activity, aesthetically attractive hot pools, etc. These features are most common in volcanic areas such as Yellowstone National Park, U.S.A., Iceland, or in the Taupo Volcanic Zone of New Zealand, but other terranes also host hydrothermal activity even though subsurface temperatures may be relatively low and surface features less impressive. Warm springs in the Rocky Mountains, the European or New Zealand Alps, or in the sedimentary massifs of central Europe are examples, and it is clearly important for mineral exploration to discriminate these types of systems from those in more favorable geological environments.

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