Obayashi mine in Ohnan-cho, Shimane Prefecture, was operated as silver mine since 1601, and closed in 1966. In this study, we investigated source minerals of silver in Obayashi mine. Obayashi mine is a vein-type deposit occurring in rhyolite-dacite lavas, pyroclastic rocks, tuffaceous sandstone and shale of Cretaceous in age. Major ore minerals are pyrite and arsenopyrite, and the associated minerals are sphalerite, galena, chalcopyrite, pyrrhotite and marcasite. Euhedral to subhedral pyrite and euhedral arsenopyrite contain droplet-like galena, chalcopyrite and pyrrhotite. Crystallization sequence of euhedral pyrite (pyrite I) -> galena+chalcopyrite -> pyrite (pyrite II) -> arsenopyrite+galena is also observed. Some sphalerite grains have chalcopyrite disease. Very minute silver minerals (less than 1 μm), such as native silver, argentite, electrum, unidentified Ag2SbS2 and Cu-Ag-Sb-S phases, occur in pyrite and sphalerite grains.
