Clay Minerals Distribution in the Ibusuki Area, Kagoshima Prefecture

Many hydrothermal alteration zones exist in the Ibusuki area, Kagoshima Prefecture. These alteration zones were formed within Ata caldera at the southern end of Kyushu. The original rocks of these altered ones are the in-filling volcanic rocks of the caldera. The altered zones were formed after the eruption of Ata pyroclastic flow which occurred 43, 910 years ago. Volcanic rocks erupted after the Ikeda pyroclastic flow deposits were scarcely altered. Therefore, these altered zones were probably formed after the eruption of the Ata pyroclastic flow and before the Ikeda pyroclastic flow. Clay minerals in some alteration zones were likewise studied. Altered minerals such as cristobalite, tridymite, alunite, kaolinite, 10 Å-halloysite, boehmite, heulandite and some iron minerals were observed in this area. Some of these zones were divided into three zones characterized by the presence of altered minerals. These are cristobalite zone, kaolinite zone and smectite zone from the inner silicified part to the outer part.