The Kosaka mine is a productive Cu-Pb-Zn-Au-Ag mine in Japan and is located at the eastern margin of the Hokuroku mining district in northeast Honshu Island. The ore deposits of the mine are divided into three main groups from north to south, namely, the Motoyama, Uchinotai, and Uwamuki ore deposits. Each ore deposit has both massive stratiform and underlying network or disseminated, siliceous orebodies. The geologic features of each deposit are described.The iron content of sphalerite coexisting with pyrite from the Uchinotai-nishi and Uwamuki No. 4 ore deposits never exceeds 5 mole percent FeS. The iron content of sphalerite from the latter and two other Kuroko deposits decreases gradually upward from stockwork to stratiform orebodies. This vertical zoning may be a common feature of the Kuroko deposits, although those from the Uchinotai-nishi ore deposit show a minimum iron content at the boundary between the black and yellow ore zones.Mixing of sea water with the ascending hydrothermal solution causes an increase in oxygen fugacity and a simultaneous temperature decrease in the ore-forming environment. They are the main causes of ore deposition. Deposition of barite is likely to be the result of an increase in activity of metastable sulfate ions which are added from sea water to the reducing ore-forming system.A magmatic origin of the ore-forming fluid is also proposed on the basis of fluid inclusion data and geologic evidence.