Hypogene Acid Alteration at Unzen Jigoku Steaming Ground, Unzen Volcano, Kyushu, Japan

Abstract

Advanced argililic alteration zones (Alinite zone, and Kaolin zone) are dominant in and around the Unzen Jigoku due to the present acid-sulfate steam-heated waters. However, silicified rock mainly composed of quartz is distributed at the Old-Hachiman area in the Jigoku. Fluid inclusions from quartz veins in the rock suggest that the rock was formed in two phase conditions under 170 to 220°C at least a depth of about 100m. Moreover, dickite occurs from a kaolin zone surrounding the silicified rock at the southwest of the rock. Some of sulfur isotope data of alunite minerals occurred near the silicified rock indicate a hypogene origin, although many of alunite minerals were formed under steam-heated water. Such alteration features conclude that the silicified rock at the Old-Hachiman area was formed under a volcanic acid fluid environment about 200°C, which was eroded out at least 100m and located at the present surface.