Geology and vein system

A detailed general geology of the Hishikari district has been done by many investigators (Abe et al., 1986 MMAJ and SMM, 1987 NEDO, 1991 Izawa et al., 1993). The general geology of this district is briefly described below. 

The district is composed of sedimentary rocks of the pre-Paleogene Shimanto Supergroup (dominantly shale and sandstone) and Quaternary andesitic and dacitic volcanic rocks. The Shimanto Supergroup is comprised of shale, sandstone and their alternations. Although no fossil data are available, the age of sedimentation is thought to be middle to upper Cretaceous age from its lithology (Izawa et al., 1990). The Shimanto 

Supergroup rocks in the Hishikari district suffered hydrothermal alteration. Chlorite, quartz and sericite occur abundantly near the veins. The other constituents are pyrite, albite, calcite and organic matter. 

Quaternary volcanic rocks unconformably overlie the Shimanto Supergroup. Quaternary volcanic rocks consist of Hishikari Lower Andesites (0.98-1.62 Ma), Hishikari Middle Andesites (0.78-0.79 Ma), Kurozonsan Dacites (0.95-1.56 Ma), Shishimano Dacites (0.66-1.6 Ma), and Hannyaji Welded Tuff (0.56-0.731 Ma) (Izawa et ah, 1990, 1993). 

The Hishikari Upper Andesites overlying the Hishikari Lower Andesites and the Shishimano Dacites consist of hypersthene-augite andesite lava flows and their pyroclastic rocks. 

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