Hydrothermal rock alteration of the Tamagawa Welded Tuff (Part 2)

The hydrothermal rock alteration of the Tamagawa Welded Tuff (1.0 to 2.0 Ma in age) which unconformably covers the Neogene Tertiary sediments is studied to assess the geothermal resources in the Hachimantai geothermal field, northern Japan. The rock alteration is classified into two types; alkaline and acidic alterations. Chlorite zone formed by alkaline alteration is regionally distributed extending NW to SE direction in the central part of the studied areas. Acidic alteration zones characterized by the presence of kaolin, alunite and pyrophyllite, however, are locally distributed around present thermal manifestations such as Toshichi and Takinoue spas. The distribution of chlorite zone shows the regional activities of alkaline hydrothermal solutions in the Tamagawa Welded Tuff. The remarkable distribution of sericitic minerals within the ehlorite zone at the upper stream area of Kakkondagawa river indicates that the area may have been the center of the alkaline hydrothermal activities related to the volcanism which formed the Tamagawa Welded Tuff.