Sendachiyama clay deposits which consist of kaolinite, halloysite, sericite and associated quartz, occur in hydrothermally argillized zone of andesite and green tuff of Miocene age. Since quartz veining seems to be genetically related with the clay minerals, filling temperatures, salinities and chemical compositions of fluid inclusions in quartz were studied by means of the heating-and cooling-stage and IMA.
Filling temperatures range from 200°C to 300°C and salinities are less than 6 wt.% NaCl equivalent con-centration. Ratios of K/Na of solutions in inclusions are in order of from 0.1 to 1.0. Average value of filling temperatures of the samples from kaolinite-rich zone is 261°C and the one from halloysite-rich zone is 236°C. Judging from the fact that the thermal variation with depth is not recognizable, ascending thermal waters were not diluted with meteoric water of low temperatures as far as the present level of the deposit is concerned. The data obtained suggest not only the conditions of the formation of this deposit, but also the conditions of wall-rock alteration of kaolinite-halloysite-sericite-quartz combination.

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The Mitsuishi area, Okayama Prefecture, is noted for the most active Roseki production in Japan. The area mainly produces pyrophyllite and sericite clays and rocks, and partly pyrophyllite-kaolinite clay with diaspore.
The area consists mainly of the upper Paleozoic sedimentary rocks and the late Cretaceous rhyolitic rocks. The latter can be divided into two formations; the Lower and the Upper. The Lower Formation is hydrothermally altered extensively and cut by numerous faults of post-alteration. The Formation is originally composed of acidic welded tuff and a few intercalations of bedded tuff-tuffaceous shale alternation. The Upper Formation consists of rhyolitic and dacitic welded tuffs accompanied by tuffaceous shale beds. Except for a small part, the Upper Formation is not subjected to hydrothermal alteration. In addition, a thin conglomerate bed consisting of breccias of variously altered rocks was found near the basal part of the Upper Formation. These facts evidently show that the Roseki deposits were formed after the deposition of the Lower Formation. In view of the almost concordant relation between both Formations, hydrothermal alteration must have taken place almost near the surface.
The altered rocks in the Lower Formation can be classified into the following altered zones; silicified zone, quartz-sericite rock zone, pyrophyllite or sericite clay zone, weakly argillized zone and weakly altered zone in the order from the center to the margin. Such a zonal arrangement of altered rocks suggests that the country rocks, most of which are acid pumiceous welded-tuffs, were altered by strongly acid hydrothermal solution. It is reasonably considered that initial gas and solution containg H₂S and others were oxidized near the surface and formed hydrothermal sulfuric acid solution.

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Berthierite, a rather uncommon mineral, was found in geodes of quartz-manganocalcite vein at the 450mL, W42 point of Toyoha Mine, Hokkaido. This vein crosses the Tajima vein with almost the same trend as that of the Soya vein, representative of so called NW-SE system.
Berthierite and co-existing minerals were investigated by means of ore microscope, transmission microscope with heating stage, X-ray diffractometer and EPMA, and three stages of mineralization were discriminated as follows:
1: Pyrrhotite stage (pyrrhotite-quartz)
2: Pyrite stage (pyrite-arsenopyrite-quartz-calcite)
3: Berthierite stage (berthierite-arsenopyrite-native arsenic-Pb, Sb sulfosalt-quartz-calcite) Filling temperatures of fluid inclusions in the quartz of the first stage were within the range from 175°C to 210°C. On the basis of these data and the thermochemical data given by BARTON (1969, 1971), it is suggested that the formation temperature of the vein was about 200°C at the first stage and decreased to about 150°C at the last stage, and that the sulfur fugacity in the vein was approximately 10 ^-17 atm during the mineralization.

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The Neogene strata of the Green Tuff region have a thickness of several thousand meters, of which pyroclastic sediments constitute the dominant part. Authigenic mineral zones in the Neogene strata are arranged in descending order: fresh glass zone, clinoptilolite-mordenite zone, analcime zone, and albite zone.
Coals and finely dispersed coaly particles in the sediments were used for measuring the vitrinite reflectivity. The reflectivity measurements at the boundary between fresh glass and clinoptilolite-mordenite zones are about 0.3%. Present data are insufficient to determine vitrinite reflectivity at the top of analcime and albite zones. Past temperature estimated from the vitrinite reflectivity measurements in each zone is within the limits of the temperature stability of the diagnostic minerals in each zone. In general, the vitrinite reflectivity in the Miocene sediments of the Green Tuff region is higher than that of coals in the Paleogene coalfields which are of commercial importance in Japan. The higher reflectivity of the Green Tuff region is due to the volcanic activities in the Neogene time.

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