Stratigraphy and hydrothermal alteration based on well data from the Oku-aizu geothermal system, Japan. Bull

摘要

The stratigraphy and hydrothermal alteration around the Okuaizu geothermal system were studied in core and cuttings from geothermal wells drilled by NEDO and Okuaizu Geothermal Co., Ltd.The basement is thought to be composed of a pre-Tertiary granodiorite and/or sedimentary rocks. Early to middle Miocene formations (Oh-hizawa, Takizawagawa, Ogino and Urushikubo Formation) consist mainly of rhyolitic to dacitic lavas, pyroclastic rocks intercalated with clastic rocks, and minor basaltic rocks, which are considered to unconformably overlie the basement. The late Miocene caldera-related formation (Iriyamazawa Formation) consisting of dacitic ash flow tuffs, debris avalanche deposits and lake sediments, unconformably overlies the early to middle Miocene formations. Pleistocene lacustrine sediments and rhyolitic lava domes (Sunagohara Formation), formed in a caldera environment, unconformably cover the Miocene formations.Five alteration zones can be distinguished using clay minerals assemblages : chlorite + sericite zone (PR), chlorite/smectite or sericite/smectite mixed clay zone (ML), smectite + /-zeolite zone (SZ), kaolinite + /-sericite zone (K) and sericite zone (S).Based on the stratigraphy, present temperature and the spatial distribution of each alteration zone, three stages of hydrothermal alteration can be recognized in the Okuaizu geothermal district:1) middle Miocene submarine hydrothermal alteration, 2) late Miocene caldera related hydrothermal alteration, and 3) hydrothermal alteration associated with the present geothermal system. 1) is characterized by a widespread PR zone in the early Miocene formations. 2) is limited to the Iriyamazawa Formation deposited in a 7 Ma caldera. The PR and ML zones are seen in the lower part, and the SZ zone in the upper part of the caldera, separated spatially from similar alteration zones in the early Miocene formations. 3) has a spatially zoned distribution of the alteration zones depending on the present temperture in addition to some other alteration zones. A zonal distribution of the SZ zone in the upper low-temperature portion, the PR zone in the lower high-temperature portion and the ML zone in the intermediate portion is observed. In addition, the sporadic K zone, which was probably formed by acidic fluids generated by the interaction of oxidized underground water with H_2S and CO_2 gases released from the geothermal fluid during boiling, and the S zone around a high-temperature reservoir and adjacent to the K zone are observed. Anhydrite is found in the high-temperture portion (>100 to 150 deg C) and various carbonate minerals are also common within the system.