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Abstract

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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