1988 Volume 38 Issue 210 Pages 357-366
K-Ar ages were determined for sericite-bearing ores collected from pyrophyllite ("Roseki") deposits distributed in the Chugoku district; Yanoshokozan, Sankin, Kumano and Hohro (Hiroshima Prefecture), Ohguni (Shimane Prefecture) and Naburi (Yamaguchi Prefecture). The results show that the formation ages of these ore deposits are ranging from 62 to 87 Ma. The Hohro and Naburi deposits are dated to be 84.0 Ma and 87.1 Ma, respectively, which correspond both to the ages of the host rock volcanism (Takada rhyolitic rocks). However, the Ohguni, Yanoshokozan, Sankin and Kumano deposits are dated to be 61.4 Ma, 66.6-66.8 Ma, 72.7 and 77.5 Ma, respectively. It is noted that the formation ages of these ore deposits are younger than those of the host Takada and Kisa volcanics. Moreover, the determined ages are continuously distributed in the range between Late Cretaceous to Early Palaeogene periods. Thus, the inferred intimate geological association of the "Roseki" deposit with the host volcanics established in the previous literatures should be re-examined.
