Boiling, Cooling, and Oxidation in Epithermal Systems: A Numerical Modeling Approach
Mark H. Reed, Nicolas F. Spycher, 1985. "Boiling, Cooling, and Oxidation in Epithermal Systems: A Numerical Modeling Approach", Geology and Geochemistry of Epithermal Systems, B. R. Berger, P. M. Bethke
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Some active geothermal systems are currently depositing gold, silver, and base metals, and most “epithermal” ore deposits formed in once-active geothermal systems (e.g., White, 1981; Henley, 1985, this volume). Boiling of hot (l00°-300°C) ground water in such systems is a process of fundamental significance because it fixes temperature gradients (e.g., White et al., 1971; Muffler et al., 1971; Henley and Ellis, 1983) and causes precipitation of sulfide, carbonate, and silicate minerals (e.g., Buchanan, 1981; Berger and Eimon, 1983). The gas phase, including H2O, CO2, and H2S, when condensed and oxidized near the surface, produces acid waters that generate argillic alteration of rocks and which may trigger deposition of precious metals. The geologic and hydrologic framework of a boiling geothermal system is depicted in Figure 11.1, based in part on White et al. (1971), Henley and Ellis (1983), Berger and Eimon (1983), and Steven and Eaton (1975). Figure 11.2 corresponds to Figure 11.1, showing in flow-diagram form the chemical components and processes in the hydrothermal system. These include boiling (A, Figs. 11.1 and 11.2), condensation of the boiled gas in rock (B), oxidation of the gas by the atmosphere (C), condensation followed by oxidation of the gas in cool, fractured ground (D), mixing of acid ground waters with the boiled liquid (E), and mixing of cold ground water with the boiled liquid (F). All of these processes shape the chemistry of geothermal systems and several of them are responsible for ore formation in epithermal systems. We present here some results
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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).