Pustaka Ilmiah Universitas Padjadjaran

Abstrak

Distribution and Characteristics of Epithermal Gold-Silver Mineralization of the Southwestern and Northeastern Hokkaido, Japan

Hokkaido is the second largest and northernmost island among the four main islands in Japan. The Neogene Tertiary is the important period in regard to the metallogeny of southwestern and northeastern Hokkaido. The gold-silver deposits in this district were one of the valuable mining districts in Japan. Hokkaido is divided into three geologic units from a geotectonic view point, namely west, central and east Hokkaido which are bounded by the Sapporo — Tomakomai lowland belt and the eastern margin of Tokoro — Toyokoro tectonic belt (Minato et al., 1965). The West Hokkaido Metalogenic Province (WHMP) epithermal vein-type Au-Ag is related to terrestrial volcanism of the northeast Japan inner arc, while hydrothermal mineralization of the Kitami region at northeastern Hokkaido is related to Middle to Late Miocene back-arc volcanism of the Kuril arc. Most of the ores from the deposits around southwestern Hokkaido is massive and occurs in veins, bedded deposits and steep irregular replacement bodies near feeder fissures that strike around N50-70°E, and cut calc-silicate rock with light-colored and medium-grained texture. Neogene Tertiary and that of Late Neogene Tertiary to Quaternary mineralization periods are overlapping together in the green tuff regions. The northeastern Hokkaido veins-type mineralization strikes mainly in the E-W or NE-SW direction. Watanabe (1986) suggested that the region had experienced an E-W trending compressive stress. The basement rocks of Cretaceous age in this area are the Hidaka group comprising chiefly sandstone and shale, intercalated with conglomerate, acidic tuff, chert and limestone. The veins and rare massive silicified body of gold-silver ore deposits in this area are associated with clay at the upper part of the tuffaceous Konomai group.