The Ôgane mine, situated to the east of the Suttsu Bay, is one of several mines located in the mineralized district of Cenozoic era in southwestern Hokkaido. In this area, green tuff and tuff breccia are intercalated in the middle part of a Miocene igneous series consisting of quartz porphyry, hornbende andesite, propylite and two-pyroxene andesite. Ore mineralization took place in the igneous rocks and tuff breccia along an anticlinal fold-axis.
The ore deposits of the Ôgane mine are epithermal gold-silver veins. On the basis of minerals forming them the veins are classified as follows: quartz-rhodochrosite veins, quartz-sulphide veins, telescoped chalcopyrite-sphalerite-galena veins and barite-quartz veins. These veins are in masses of propylite, two-pyroxene andesite and green tuff breccia, and are located in the inner part of a dome, structure on the anticlinal fold-axis.
The Nishitani-vein group, which is one of four groups in the Ôgane mine, consists of three parallel veins deposited in propylite. The propylite surrounding the Nishitani vein group has a zonal structure that includes a chlorite-carbonate zone, an albite-sericite zone and a sericite (?)-quartz zone. This structure probably was formed during the late stages of propylitization.
The some width of wall rock was, in general, altered strongly: the altered rocks are subdivided into the following three rock facies: a carbonate-chlorite rock, a quartz-sericite rock and a sericite(?)-quartz rock. These altered facies must have been important channelways for the solution, The ore minerals of the veins are pyrite, chalcopyrite, sphalerite, galena, tetrahedrite, native gold, electrum and silver minerals; gangue minerals are quartz, adularia, rhodochrosite and barite,
It is inferred that ore mineralization had three phases as the veins can be divided into a sulphide zone, a rhodochrosite zone and a barite zone. The sulphide zone includes great numbers of gold particles; the rhodochrosite zone generally has lower gold values. The particles of native gold are present as round grain sand are closely associated with chalcopyrite and sphalerite.
After the ore mineralization the Nishitani-vein group was disrupted by strike and oblique faults. The faulting which took place was closely related to location and control of the ore deposition.