The Kamioka mine is a representative lead-zinc skarn deposit in Japan which was formed mainly by the replacement of limestone, a member of the Hida metamorphic rocks. The mine is composed of three main deposits, Mozumi, Maruyama and Tochibora from the north to south.
The Sakonishi district is located at 2km south-east of the Mozumi deposit. The Kita 20 you fault which is cosidered as one major conduit of ore fluids responsible for the formation of the Mozumi deposit intersects the district. Although little ore exposure has been detected at the surface, a promising zinc deposit was discovered at vertical depth of 250m below the surface. The downhole length of the ore is 44.4m with an average grade of 13.4% Zn, 0.02% Pb and 7g/t Ag. The deposit develops with NE-SW strikes and steep dips.
The ores in the Sakonishi district are classified on the basis of the mode of occurrence into three categories, namely "disseminated type", "Mokuji type" and "Network (small vein) type". The hopeful ore is mainly composed of the disseminated type ore with sphalerite and minor amounts of galena, chalcopyrite and pyrite. Epidote, chlorite and hematite are as gangue minerals distributed in the Inishi rocks. The mineralization is considered to have been closely related to the activity of the Kita 20 you fault, the Atostugawa 1 you fault and porphyrite dyke.
The homogenization temperature of fluid inclusions in the sphalerite of the disseminated type ore is low ranging from 90°C to 180°C. The sphalerite varies in colour dependent on the iron content and shows "chalcopyrite disease". The limestone core samples which were taken near the orebody display more depletion of oxygen isotope ratios than the unaltered limestones. This indicates that the hydrothermal fluid responsible for the mineralization was of meteoric water origin similarly to other deposits in the Kamioka mine.
It is assumed from these observations that the mineralization in the Sakonishi district took place at two stages, (1) the formation of the Mokuji ore from the pyrometasomatic fluid along the Kita 20 you fault, and (2) that of the disseminated and network ore under hydrothermal conditions. Further exploration will provide additional informations on the rigorous model responsible for the high-grade zinc mineralization.