Obira deposit

Obira deposit is located at south Kyushu. It occurs in Shimanto Group rocks composed of shale, and limestone, associated with middle Miocene granitic intrusive rocks. Polyascendant zoning and mineralization are observed in the Obira deposit. From early- to late-stage mineralizations are  

Occurrence of pyrrhotite, arsenopyrite and high iron content of sphalerite indicate low fs,2 and /02 conditions. Decrepitation temperatures of fluid inclusions in quartz from Sn-Cu-As stage are 403-322°C (Enjoji and Takenouchi, 1976). 

Distinct features of the above-mentioned polymetallic deposits are summarized as 

Temperature and sulfur fugacity estimated from iron and zinc partitioning between coexisting stannite and sphalerite and coexisting stannoidite and sphalerite 

Stannite is the most common tin sulfide mineral in the ore deposits associated with tin mineralization. This mineral sometimes contains appreciable amounts of zinc, together with iron. Several workers have suggested that the zinc and iron contents of stannite are related to temperature. With respect to the study of the phase relationships in the pseudobinary stannite-kesterite system. Springer (1972) proposed zincic stannite as a possible geothermometer mainly based on the chemical compositions of the two exsolved phases (stannite and kesterite). Nekrasov et al. (1979) and Nakamura and Shima (1982) experimentally determined the temperature dependency of iron and zinc partitioning between stannite and sphalerite. 

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It is generally accepted that Kuroko deposits formed under the submarine environment, while polymetallic vein-type deposits in central and Northwest Japan (Ashio, Tsugu, Kishu, Obira, etc.) under the subaerial environment. 

In contrast, in Southwest Japan, polymetallic veins (so-called xenothermal-type deposits in the sense of Buddington (1935) or subvolcanie hydrothermal type in the sense of Cissartz (1928, 1965) and Schneiderhohn (1941, 1955) occur. Examples of these deposits are Ashio, Tsugu, Kishu and Obira. All these vein-type deposits have formed at middle Miocene age in western part of Tanakura Tectonic Line under subaerial environment. In these deposits, many base-metal elements (Sn, W, Cu, Pb, Zn) and small amounts of Au and Ag are concentrated. These deposits are associated with felsic volcanic and plutonic rocks along the Median Tectonic Line (MTL) or south of MTL. 

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