It was theoretically derived that the iron content of sphalerite, silver content of electrum and selenium contents of argentite and galena, which all coexist with pyrite, are related to temperature, and selenium and sulfur fugacities. Based on this relation and analytical data on these coexisting minerals, selenium and sulfur fugacities and formation temperature for the selenium-rich gold-silver vein-type deposits were deduced. The temperature estimated from this assemblage (ca. 150–300°C) is in agreement with the homogenization temperature for fluid inclusions in the Kushikino and Takatama deposits.