Hydrothermal Rock Alteration of Matsukawa Geothermal Area, Iwate Prefecture

Abstract

The three-dimensional structure of a hydrothermally altered mass was revealed by deep drillings for the exploitation of the geothermal resources in Matsukawa. The mass comprises five zones of different mineral. assemblages, i. e. weakly altered zone, montmorillonite zone, kaolin zone, alunite zone and pyrophyllite zone, respectively. The three zones, montmorillonite, kaolin and alunite zones, are zonallyarranged. The followings are discussed:
1. The altered mass as a whole is rather vertically elongated reaching to 720m in depth (from the restored surface at the time of main humarolic activity). The shallower part above the 340m level shows a asymmetrical zoning up to the surface. In the deeper part is recognized a symmetrical zoning from conduit to periphery.
2. The amount of discharge of fluid was probaly larger in the deeper part than in the shallower part. The quantities of migrated elements as a whole were generally larger in the shallower part than in the deeper part, although the tendency of migration is similar.