Light Stable-Isotope Systematics in the Epithermal Environment
Cyrus W. Field, Richard H. Fifarek, 1985. "Light Stable-Isotope Systematics in the Epithermal Environment", Geology and Geochemistry of Epithermal Systems, B. R. Berger, P. M. Bethke
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Stable-isotope geochemistry has made important contributions to the widely acknowledged “renaissance” in the earth sciences for more than three decades. This status may be ascribed both to theoretical and practical considerations. First, the isotopic species of an element may be fractionated (partitioned unequally) between two or more coexisting phases because of mass-dependent differences in their chemical and physical behaviors, and the amount of such fractionation normally varies inversely with temperature and independently of pressure. Accordingly, the isotopic abundances of an element may serve to define the mechanisms of formation, thermal environment, and provenance of rocks, minerals, and fluids. Second, the analytical procedures now available render most geologic materials well suited for routine and rapid isotopic measurements. Some important milestones of the 1930's and 1940's leading to our present understanding include the discovery of deuterium and formulation of the theoretical basis for stable-isotope fractionation by Harold C. Urey and colleagues at the University of Chicago and the development of improved mass spectrometers by Alfred O. Nier at the University of Minnesota. The subsequent construction of laboratory facilities elsewhere was commonly directed by graduates and associates of these pioneers and their respective institutions.
As of today, the literature relevant to stableisotope geochemistry is voluminous and far beyond the scope of this topical overview. Most investigations, apart from those concerned with theory or laboratory experimentation, have been focused on one or more of the following objectives: (1) the conditions and mechanisms of rock or mineral formation; (2) the sources of magma, sediment
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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).