GeoRepHiroshimaUniv_12_1.pdf 25.5 MB
The Pyrohyllite Deposits in the Chugoku Province, West Japan
In the world Japan and U.S.A. are the most productive countries of pyrophyllite. By far the greater part of the pyrophyllite production in Japan is produced in Chugoku Province, West Japan. Numerous deposits of pyrophyllite are found in the province, among which Mitsuishi district in Okayama prefecture and Shokozan district in Hiroshima prefecture are excellent. The deposits are all conceived in a volcanic complex. The complex consists of andesite, dacite, rhyolite, and their tuffs and occasionally intercalates thin layers of shale and sandstone, among which the tuffaceous rocks exclusively predominate.
The deposits are lenticular or layer shaped and almost concordantly contained in the definite horizons of the rhyolitic tuffs.
A roughly symmetrical zonal arrangement of minerals are found in the deposits. The main aluminous minerals in the deposits are pyrophyllite, diaspore, corundum, andalusite, dickite (ka-olinite), boehmite, and sericite. Quartz, pyrite, hematite, and chlorite are almost always present. Except pyrite and hematite associated ore minerals are very scanty, and are stibnite and cinnabar.
The deposits are formed in the near surface environment, replacing rhyolitic tuffaceous rocks, by the post volcanic hydrothermal or exhalative action related to the volcanic activities of the volcanic complex. Some high temperature aluminous minerals in the deposits may be produced in high temperature exhalative conditions.
The geological age of the mineralization is considered almost contemporaneous to that of the volcanic complex. According to many Japanese geologists, the age of the volcanic complex is considered to be the Cretaceous.
The Upper Cretaceous ～ Lower Tertiary intrusive granitic rocks associating numerous heavy ore mineral deposits is broadly distributed in the province. Although some lower grade of thermal metamorphism are sporadically observed in the volcanic complex, not only the accurate thermal metamorphic effects by these plutonics are not ascertained in the pyrophyllite deposits, but also any relation between the mineralizations of the heavy ore minerals and the pyrophyllite deposits is not ascertained.