Mutsu-Hiuchidake Volcano has an erosion caldera which shows a horseshoe-like geomorphological feature toward east. Tertiary strata as a basement are distributed in the caldera. Tertialy strata and a part of pyroclastic deposits of the Mutsu-Hiuchidake Volcano have altered strongly to moderately by hydrothermal activities related to the volcanism. This study is to clarify a progress history of the alteration by using a geological mapping, thermoluminescence (TL) dating, x-ray diffraction analysis, an infrared reflection absorption analysis and a fluid inclusion study.
      Highly altered zone is recognized in the area of midstream to upstream along the Ohakagawa and the Koakagawa within erosion caldera. The argillic alteration zone surround a silicification zone in the highly altered area shows a circular distribution. The strongly altered areas are along NNW-SSE to NNE-SSW fractures. The alteration areas were divided into the smectite, kaolinite, alunite and pyrophyllite zones.
      The kaolinite and alunite zones give the TL ages of quartz 67 ± 13 ka (KG-5), 88 ± 18 ka (OG-4) and 91 ± 23 ka (OG-1). The smectite zone within the argillic alteration zone of outside of collapse caldera, yield the ages 752 ± 215 ka (SO-2) and 615 ± 197 ka (KG-1). These TL ages suggest the hydrothermal activity end at 70 to 90 ka.
      The existence of pyrophyllite suggests that hydrothermal temperatures were 200 to 250 °C in these area. This is supported from the homogenization temperatures of fluid inclusions in calcite, 242 °C in average.