Ore deposits of the Koryu mine, which is located near the Shikotsu Lake, Hokkaido, are gold and silver veins of epithermal type. They develop in black hard mudstone of the Miocene age and are composed of mainly quartz associated with some amounts of johannsenite, adularia, chlorite, vermiculite, manganoan calcite, pyrite, chalcopyrite, sphalerite, galena, hematite, electrum and silver minerals such as aguilarite, pearceite, polybasite, pyrargyrite, proustite, miargyrite, unknown Ag-Sb-As sulfosalt mineral and silver bearing tetrahedrite etc. The ore veins show crustified banding structure which consists of bands of quartz, johannsenite, adularia, chlorite, vermiculite, silver minerals and sulfide minerals. Many druses and vugs are found in the central portion of the quartz veins. Many kinds of small crystals of adularia, manganoan calcite, aguilarite, pearceite, pyrargyrite, miargyrite, unknown Ag-Sb-As sulfosalt mineral and tetrahedrite etc. besides quartz occur in the druse. Electrum is found in intimate association with chalcopyrite, aguilarite, pearceite, sphalerite, galena and pyrite, and its composition is from 38.5 to 70.0 at. % Ag. While that of native silver is 99.9 at. % Ag. The composition of aguilarite has a range from 14 to 41 mole % Ag2Se. According to Petruk et al. (1974), aguilarite having such compositions as above is not homogeneous and associates with acanthite. However, aguilarite as above from this mine is monophase with monoclinic cell. Pearceite and polybasite series minerals appear as rim of aguilarite or in association with electrum, chalcopyrite and sphalerite. Compositions of pearceite series are from 0 to 100 at. % in Sb/(As+Sb) ratio showing completely continuous solid solution of the series. Also, those of pyrargyrite and proustite series minerals have a wide range from 0 to 98 at. % in Sb/(As+Sb) ratio. It indicates that pyrargyrite-proustite solid solution is stable at low temperatures, although Toulmin (1963) suggested that the solid solution becomes immiscible at temperatures below 300°C. Tetrahedrite assembled with chalcopyrite and pyrargyrite in the druse contains silver from 24.1 to 25.3wt%. The homogenization temperatures and salinities of fluid inclusions in quartz are from 240° to 270°C and from 0.0 to 1.4wt% NaCl equivalent, respectively. Temperatures and sulfur fugacities of gold-silver mineralization in the Koryu mine are estimated as 175° to 240°C, and -15.5 to -ll.5 in log fs2 (atm), respectively, from the data of compositions of electrum with aguilarite and FeS contents of sphalerite with pyrite.