1979 Volume 29 Issue 154 Pages 83-95
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.
