HYDROTHERMAL ALTERATION ASSOCIATED WITH THE PLIOCENE MUINE VOLCANO AND BASE-METAL MINERALIZATION AT THE TOYOHA DEPOSIT, NORTHEAST JAPAN

Muine volcano, a Pliocene calc-alkaline andesite volcano in northeast Japan, is characterized by a composite volcanic body associated with phreatic craters and landslide and debris flow deposits. Examination on the hydrothermal alteration at the surface near the volcano identified three different alteration zones; 1) Muine zone near the center of the volcano, characterized mainly by sporadic occurrence of low-temperature advanced argillic assemblage fringed with argillic alteration, 2) Toyoha argillic and propylitic zone, encompassing ore veins of the Pb-Zn-Ag Toyoha deposit on the north of the Muine zone, and 3) Yunosawa argillic and advanced argillic zone, corresponding to the present geothermal area east of the Toyoha zone. Sulfur isotopic analysis indicates that the d34S values of alunite in the Muine and Yunosawa zones are variable from -4.1 to +31.5 per mil. The Muine zone includes relatively low (9 to17 per mil) and high (23 to 31 per mil) value alunites, which are interpreted to have formed in magmatic-steam and magmatic-hydrothermal environments, respectively. The -4.1 per mil alunite in the Muine zone, and -2.8 per mil alunite in the Yunosawa zone are interpreted as forming in a supergene and steam-heated environment, respectively. Published d34S values of sulfide minerals in the Toyoha zone are constant from 5 to 9 per mil, with an average of 7 per mil, suggesting that reduced aqueous sulfur species were dominant over oxidized sulfur species in the ore fluids. The different hydrothermal mineral assemblages and environments of the Muine, Toyoha and Yunosawa zones are interpreted to have formed by immiscible vapor and brine exsolved from shallow-level (2-3 km deep) magmatic intrusions beneath the Muine volcano. The SO2-dominant vapor ascended to cause the advanced argillic and argillic alteration with phreatic eruptions in the Muine zone, which promoted the volcano collapse. The metal-rich brine flowed laterally to the north to the Toyoha zone, where the brine ascended through fractures and mixed with meteoric water, resulting in sulfide deposition with propylitic and argillic alteration. Fluid boiling in the Toyoha zone released H2S-enriched vapor, which was oxidized in the steam-heated environment in the Yunosawa zone as well as a part of the Toyoha zone.