Polymetallic mineralization at the Nakakoshi copper deposits, central Hokkaido, Japan

Polymetallic mineralization at the Nakakoshi deposits, Kamikawa town, centr al Hokkaido, occur as fracture-filling veins in Cretaceous slate of the Hid aka Supergroup. Ten veins have been recognized in NE-SW and E-W directions. Sericite in altered slate which is the host of the deposits, was dated at 31.1 Ma, Oligocene in age. No.9 Vein consists of massive chalcopyrite ore with various kinds of minera ls such as pyrite, pyrrhotite, arsenopyrite, sphalerite, tetrahedrite, Ag-m inerals and Cu-Zn-Fe-In-Sn-S minerals, quartz and sericite. Chalcopyrite an d pyrite contain sphalerite star and sphalerite with chalcopyrite emulsions . Maximum indium contents of sphalerite and the Cu-Zn-Fe-In-Sn-S minerals a re 1.8 and 16.3 wt%, respectively. The sulfur isotopic ratios, delta(34)S o f ore minerals, range from -12.9 to -9.6 parts per thousand. Formation temp eratures of the sulfide minerals are estimated as 300 similar to 500 degree s C, based on the paragenesis and chemical compositions of the minerals. Bulk In contents (0.02%), and mineral assemblages of the Nakakoshi deposits are similar to those of the reduced type veins of the Toyoha deposits. How ever there is distinct difference between the two ore deposits in the delta (34)S ratios of the ore minerals; the averaged values are -10.9 parts per t housand at the Nakakoshi and 5.2 parts per thousand at the Toyoha. It is su ggested that the Nakakoshi deposits were formed by ore fluids liberated fro m ilmenite-series granitoids under higher temperature than the Toyoha veins , which were formed by fluids liberated from a magnetite-series magma.