Abstract
Numerous pyrophyllite (“Roseki”) deposits occur in Cretaceous volcanic rocks distributed in Chugoku province, Southwest Japan. They are developed mainly in three areas; Abu-Susa (Yamaguchi Prefecture), Shokozan (Hiroshima Prefecture) and Mitsuishi areas (Okayama prefecture). K-Ar ages were determined for illite-bearing ores collected from “Roseki” deposits (Tsuchihashi, Yagi and Shinagawa mines) distributed in the Mitsuishi district. In particularly, specimens of various modes of occurrence (silicified, spotted, illiterich, vein-form, vein and pyrophyllite bearing) were mainly collected from Tsuchihashi deposit. The Tsuchihashi, Yagi and Shinagawa deposits are dated to be 73-80Ma, 76.9-78.7Ma and 77.6Ma, respectively.
The younger specimens are richer in K-ion than them of older. The latest stage of the alteration were confirmed in clay veins. Hydrothermal activity of the Mitsuishi district continued for almost lOMa in the Late Cretaceous period.
