Abstract

The Satsuma Iwojima volcano, southwest Japan, degasses 5 x 106 t/yr H2O, 9 x 10 4 t/yr S, and 6 x 104 t/yr Cl from high-temperature (≤880 °C) fumaroles atop a now-altered rhyolite dome that erupted 1200 yr ago. Acidic hot springs (pH 1.1-1.8) discharge ∼20 x 106 t/yr H2O to the sea; this water is composed of ∼1 part magmatic vapor absorbed by 6 parts meteoric ground water. The Cl and SO4 in solution originate from the vapor, whereas cation components are derived largely by dissolution of the rhyolite. The flux of Pb, Zn, Cu, and Mo in the vapor and acidic springs ranges from 0.1 to 10 t/yr each, whereas the Au flux is 10-5 and 10-3 t/yr, respectively. The low concentrations of NaCl and metals in the vapor are due to the condensation of a hypersaline liquid from the vapor during ascent and depressurization, meaning that the atmospheric-pressure vapor does not reflect the composition of the fluid exsolving from the magma. Neither this low-pressure vapor nor the acidic waters can account for high-sulfidation Cu-Au ore deposits deduced to have formed in this environment; such mineralization requires the subsequent ascent of a metal-rich fluid.

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