Kaolinization related to the Cretaceous acid volcanism at the Fukuyama mine, Hyogo prefecture

Abstract

Hyogo prefecture is noted for occurrence of several kaolinic roseki deposits related to the Cretaceous acid volcanics, which are widely distributed through Chugoku province from Hyogo to Yamaguchi prefectures.
The Fukuyama mine, one of them, produces purely kaolinic roseki and quartz-kaolin rocks (siliceous roseki). The former is used for high-grade refractories, and the latter for insecticide carriers. The mode of occurrence of the kaolinic roseki deposit and the genetic relation to the acid volcanisms were investigated.
The area is underlain by rhyolite flow, rhyolitic tuff breccia, dacite intrusive, debris deposits and rhyolitic welded tuff in the ascending order. The roseki deposit mainly occurs in the dacite intrusive and the adjcent rhyolitic tuff breccia, which is possibly explosion breccia. The underlying rhyolite is only weakly altered, and the uppermost welded tuff is almost fresh. The debris deposit consists mostly of variously altered and poorly sorted dacite breccias. It overlies the altered dacite and tuff breccia discordantly.
The purely kaolinic roseki occurs as veins, less than 5 meters in width, in the dacite intrusive, and is accompanied by thin diaspore veinlets at the central part. The roseki is composed mostly of dickite and wellordered kaolinite and accompanied by pyrite dissemination in places. At the outer-side of the roseki veins, the siliceous roseki occurs extensively. It consists mainly of variable ratios of quartz and kaolin minerals occasionally accompanied by mica clay minerals and interstratified mica and expandable minerals. All of the altered rocks are intruded by a felsic dike, which is also more or less altered.
The process of the kaolinization probably has a close relation to a series of local acid-volcanisms as stated below:
(1) Extrusion of rhyolite lava flow.
(2) Deposition of explosive rhyolitic tuff breccia.
(3) Intrusion of dacitic magma near the surface.
(4) Acid hydrothermal alteration mainly in the dacite intrusive and the adjacent tuff breccia, and continuous rising of the dacite intrusive up to the surface to form possibly an altered dacite dome.
(5) Break down of the altered dome to cause the debris deposits, and finally the deposition of the welded tuff, by which the roseke deposits were covered to be preserved.