Abstract

The andesitic volcanoes Kirishima, Kyushu, and Esan, Hokkaido, discharge 94 to 225 °C HCl-bearing vapors from summit fumaroles. At Kirishima a geothermal system exists on the flanks of the large volcanic massif (1500 m relief and 12 km radius). In contrast, Esan is a small dome (600 m relief and 1 km radius) with ephemeral hot springs. The chemical and isotopic compositions of the fumarole gases and condensates, and of waters from hot springs, indicate that Esan discharges are dominated by magmatic water and gases, whereas those at Kirishima are mainly meteoric. The Kirishima geothermal system contains acid fluids that are neutralized by interaction with the host rock and dilution by meteoric ground water; the acidity is probably of magmatic origin. A large ground-water carapace at Kirishima condenses a majority of magmatic volatiles and metals before they can discharge to the surface, in contrast to Esan, where the volatiles (including metals) degas to the atmosphere. This suggests that a meteoric system may be necessary to condense metals in this high-level volcanic environment to provide a situation conducive to hydrothermal mineralization at epithermal depths.

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