The distribution of hydrothermal alteration zone and acidic alteration by hot spring water in Unzen Jigoku, Nagasaki Prefecture, Japan

Abstract

Unzen Jigoku steaming field is a geothermal manifestation area at the southwest of Unzen volcano on the Shimabara Peninsula. We investigated the hydrothermal alteration for revealing the history of Unzen Jigoku alteration zone by geothermal activity, using XRD analysis and FE-SEM-EDS analysis of alteration minerals. In addition, we analyzed chemical composition of hot spring water. The Unzen Jigoku steaming field has two different types of alteration zones: silicified zone (containing quartz or cristobalite), and acidic alteration zone (containing alunite and kaolin minerals). Silicified alteration zone is subdivided into quartz-dominant zone and cristobalite-dominant zone. The quartz-dominant zone is considered as formed at the underground situations with 170-200 °C conditions based on the occurrence of quartz and anatase. The acidic alteration zone overlaps a part of the silicified alteration zone. In addition, the distribution of acidic alteration zone corresponds to the area of active fumaroles. Chemical composition of the hot spring waters is characterized by low pH (= 1.96-6.19), SO₄-rich (>187 ppm), and Cl-poor (<7 ppm), which is typically recognized in steam-heated water in a geothermal manifestation. Saturation indices for acidic alteration minerals (alunite and kaolin minerals) were calculated from the obtained chemical composition of hot spring water. The indices showed oversaturation of acidic alteration minerals at the condition of surface of the steaming field. Our analytical results suggest that the acidic alteration is associated with the present hot spring activity. On the other hands, the silicic alteration would be attributed to occur at higher temperature condition, probably during this region had been beneath the surface of the ground.