Glassy two-pyroxene andesite, which distributed in the vicinity of Bonten-yama, was subjected to intense alteration, presumably originated from solfataric activities.
The altered rocks consist mainly of silicified and pyrophyllitized parts, which occur in irregular shapes, bounded with each other.
The silicified part is composed dominantly of microcrystalline quartz grains, though it looks like porous breccia. The pyrophyllitized part, which assumes white clay appearance, are mostly composed of pyrophyllite, diaspore and quartz, associated with small amounts of rutile, alunite and iron minerals (hematite and goethite).
Porphyritic texture of original andesite are not preserved in either of the altered part.
Seeing from the chemical compositions of the original. and altered rocks, it seems that the type of alteration in Bonten-yama is characterized by acid leaching process at high temperature. From the geological features, it is likely that the deposits were formed near the surface during the alteration.
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