Study of hydrothermal mineralization in Otaru district, Hokkaido, with special reference to acid sulfate alteration.

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The Sea of Japan side of the northern part of Japanese Islands forms a geologic province called the Green Tuff region where occur many ore mineralizations including gold-silverveins, kuroko deposits, and base metal veins. Otaru is located at the northern end of the Green Tuff region. Akaiwa district was main target of this project and Asari district, 13 km southeast of Akaiwa was also studied.Mt. Shimo-akaiwa(279m above the sea) consists of intensely silicified volcanic rock (andesite?) containing alunite sporadically. At sea-shore, a silicified rock is surrounded by zoned alteration, argillic(mainly dickite) zone, illite(sericite)-smectite zone, chlorite zone, and propylite. The silicified rock contains diaspore crystals up to 2mm in size in the pore. In the argillic zone contains pyrophyllite and/or zunyite. This distribution of minerals seems to be resulted from oxidation of sulfur-containing acid hydrothermal solution."Breccia pipes" extend vertically. Inverse grading of silicified clast in a pipe suggests that it was formed by vigorous movements of ascending fluid. "Layered parts" are composed of fine-grained powders of quartz and alunite with normal size grading and seem to be formed in small hot lakes.Geothermometry using homogenization temperature of fluid inclusion, sulfur isotope fractionation between sulfide and sulfate minerals, and mineral stability gave 200-250ﾟC.Two K-Ar ages, 9.7*0.5 and 10.0*0.5 Ma have been obtaind from Akaiwa, and two, 6.1*0.6 and 4.8*0.4 from Asari, all for alunite. Considering younger ages reported from Toyoha, south of Asari, the hydrothermal activity migrated southwards since the Miocene age.