A guide to exploration for the Roseki deposits

Abstract

Since 1960, the pyrophyllite-kaolinite (Roseki) deposits of the Chugoku region, the Inner Zone of Southwest Japan, have been investigated by Dr. Kinosaki, former professor of Hiroshima University, and his co-workers. Many fundamental data on the geology, hydrothermal alteration, and mineralogy for the development and utilization of the Roseki deposits and their products have been accumulated. That is, almost all of the Roseki deposits were formed in roughly concordant with the bedding planes of the tuff intercalated with lucustrine sediments and acidic volcanics during the Upper Cretaceous Period. The extensive reactions between host rocks and sulfuric acid solutions gave rise to a zonal arrangement of the hydrothermal minerals. The main constituent minerals, excluding pyrophyllite and kaolinite, are different in each Roseki district, such as those of Mitsuishi, Shokozan, Abu, and so on. The variations of the alteration zonings and main mineral assemblages may reflect the difference in the host rocks and the physico-chemical conditions of formation of the Roseki deposits. Such fundamental data as 1) stratigraphic control, 2) vertical zoning and 3) effect of host rock composition, were successfully applied to the exploration in the past years. A few case studies are described in this paper.