Fluid-Inclusion Systematics in Epithermal Systems
Fluid-inclusion analyses have provided some of the most useful information for determining the physical and chemical environments of mineral formation. The purpose of this chapter is to describe those fluid-inclusion characteristics which serve to distinguish relatively near-surface, epithermal formation conditions from deeper and, potentially, higher temperature formation conditions, and to discuss several techniques and problems which are specific to fluid inclusions trapped in the epithermal environment. A detailed summary and critique of fluid-inclusion literature related to epithermal systems has not been attempted. For this information the reader is referred to the recent compilations of Buchanan (1981), Heald-Wetlaufer et al. (1983), Roedder (1984), and Hedenquist and Henley (1985). Moreover, we have not attempted to relate any particular fluid-inclusion characteristic to a specific type or stage of mineralization, because an adequate data base to do so does not presently exist.
This presentation is limited to two subjects--the petrography and petrology of fluid inclusions from the epithermal environment--and is intended to provide the explorationist with a basic understanding of the criteria for recognizing and interpreting inclusions trapped in this environment. Two important topics will be discussed in detail: (1) the identification and interpretation of fluid inclusions trapped from boiling fluids, and (2) the identification of gases (mainly CO2) in fluid inclusions and the effect of volatiles on calculated pressures and depths of trapping. We will not, however, discuss the important chemical consequences of boiling and dissolved volatiles, as these subjects are covered in detail in other chapters in this volume (see Henley)
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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).