Alteration mineralogy and whole rock geochemistry of the Miocene Takizawagawa Formation distributed around the Okuaizu geothermal system were studied using bulk XRD techniques, bulk chemical analysis using XRF, NAA and ICP, and thin sections observation. This study aims to reveal the hydrothermal alteration zoning and to estimate gains and losses of m司orand minor elements in bedrocks around the reservoir depth of the Okuaizu geothermal system during the modern geothermal activity. The Takizawagawa Formation, which is composed of Miocene rhyolitic to dacitic lavas and pyroclastic sediments, generally has undergone regional diagenetic alteration and overprinting local hydrothermal alteration both formed in Tertiary. In the Okuaizu geothermal system, the modern hydrothermal alteration overprints the Tertiary diagenetic and hydrothermal alteration. Regional diagenetic and overprinting local hydrothermal alteration is mostly characterized by illite and chlorite (illite-chlorite (IC) zone). In the reservoir depth of the modern geothermal system, following four alteration zones are recognized based on the distribution of clays, carbonates and sulfates. 1)Illite-chlorite+ (IC+) zone 2)Illite (I) zone 3)Kaolinite (K) zone 4)Mixed-layered clay mineral (ML) zone--- ---Judging from the result obtained in this study on hydrothermal alteration, combined with the published geological, hydrological and geochemical data, the following genetical model is proposed for each alteration zone in the modern geothermal system. 1)Illite-chlorite+ (IC+) zone is formed during large-scale infiltration of the deeper high temperature geothermal fluid with higer C02 content. 2)Illite (I) zone is formed by local infiltration of relatively shallower geothermal fl凶dwith boiling. 3)Kaolinite (K) zone is formed by relatively low pH shallower underground water genereted by C02 injection, which is brought by boiling of the deeper geothermal reservoir. 4)Mixed-layered clay mineral (ML) zone is formed as a peripheral phase of the Kaolinite zone.

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The Mt. Wasso Moonstone Rhyolitic Welded Tuff (hereafter referred to as Mt. Wasso Rhyolite) is assigned to the lowermost part of the Miocene Hokuriku Group in the eastern part of southwest Japan. In an effort to clarify the rotational movements of southwest Japan associated with opening of the Japan Sea back-arc basin, we conducted paleomagnetic measurements and fission-track (FT) dating of the Mt. Wasso Rhyolite. Stable site-mean paleomagnetic directions and zircon FT ages were determined for five and three sites, respectively. The FT ages range between 20 and 22 Ma, and accord with a previous age determined by Rb-Sr whole rock isochron method. The numerical ages around 20 Ma correspond to turning of a hydrothermal alteration event, which may have caused remagnetization of the Mt. Wasso Rhyolite. Tilt-corrected paleomagnetic directions are clustered with a large easterly deflection (ca. 50° ) , indicating a clockwise rotation of the study area since 20 Ma. Compared with paleomagnetic directions of the Iozen Formation (15～16 Ma) in adjoining hills, the present result implies that the study area did not significantly rotate during the early Miocene.

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The Tanmai pyrophyllite deposit, Quang Ninh Province, was studied geologically and mineralogically. The deposit consisting of five pyrophyllite bodies, is classified into five alteration zones: kaolin, pyrophyllite, siliceous, diaspore and alunite. All of them are characterized by low iron content. The original rocks are volcanics of rhyodacitic composition such as tuff, volcanic breccia, lava and sheeted intrusive rocks. Siliceous zone originated from felsic intrusive rocks and massive lava. The sequence of the alteration is estimated as follows: kaolin zone formed earlier under lower temperature, and pyrophyllite-and siliceous-zones followed at higher temperature. The contrast in the permeability of the original volcanics resulted in the difference of mineral composition; a pyrophyllite zone from high permeability and a siliceous zone for low permeability units. The diaspore zone has formed as fissure-flling veins selectively in the siliceous zone. This sequence of the formation probably progresses with temperature. Vein-filling diaspore + pyrophyllite in the siliceous zone has precipitated in a hydrothermal solution under decreasing P H₂O caused by vertical fissure systems. The brittle nature of the siliceous zone has developed a vein-filling diaspore. The pyrophyllite zone being the majority of the Tanmai deposit has formed at a temperature of 260-290°C, estimated by the experimental data. Finally, alunite precipitated in the declining stage of hydrothermal activity.

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