Untitled research work

Hydrothermal rock alteration in the eastern Hachimantai geothermal area including Toshichi, Matsukawa and Kakkonda, Iwate Prefecture can be divided into two types; alkaline and acidic alteration types. The alkaline alteration type characterized by the presence of chlorite associated with sericite and mixed-layer mineral of sericite and montmorillonite regionally occurs along the valley of the Kakkonda River including the Kakkonda geothermal field (hot water field). The distribution area of sericitic minerals within the area of this type probably shows the upflow area of hydrothermal solutions. The acidic alteration type characterized by minerals such as pyrophyllite, alunite and kaolinite with sudoite, tosudite, dickite, sericite, zunyite and diaspore are developed in the Matsukawa geothermal field (vapor-dominated field) which is located only 7km northeast of the Kakkonda geothermal field. The hydrothermal systems in the Kakkonda and Matsukawa geothermal fields appear to be derived from the same deep-seated hot water beneath this area. The Toshichi geothermal field which is covered by kaolinite and alunite zones is estimated to be of a hot water system because alkaline alteration zone is developed beneath this field.