Zoning structure of acidic hydrothermal alteration products and temporal and spatial variation of hydrothermal fluids

Abstract

Alteration of the pyroxene-amphibole andesite of Garan-dake volcano, Oita, occurs by the acidic hydrothermal fluid to form cristobalite. Hand specimens with unaltered/weakly altered core and cristobalite crust show various sequences of intermediately altered layers including alunite, pyrite, kaolinite, goethite and hematite. These alteration products suggest that the hydrothermal fluids change their geochemical conditions temporally and spatially. Especially, sulfur and/or iron-bearing mineral species are influenced by the fluctuations of the redox conditions in the hydrothermal fluids.