Abstract
Hydrothermal alteration mineral assemblages were determined by X-ray diffraction pattern of samples from two deep boreholes, which were drilled at Nebukawa and Miyashita, southeast flank of the Hakone volcano. Both boreholes reached the basement (Hayakawa tuff breccias and Yugashima formation) thorough Hakone volcanic rocks. Characteristic mineral assemblages are smectite, swelling chlorite, chlorite, chlorite + epidote + wairakite in descending order in the Nebukawa borehole. In the Miyashita borehole, they are smectite, chlorite, and chlorite + epidote, in descending order. Basically, these changes in mineral assemblages are results of temperature gradient in neutral to basic hydrothermal activity. The difference in two boreholes probably comes from the difference in chemistry of hydrothermal fluid, in particular, CO2 concentration.
