Abstract
Massive sulfide ores from the Shimokawa mine have been studied to approach the ore genesis. A number of polished sections covering almost all part of the Shimokawa orebodies were observed microscopically and the vicissitude of the ore mineral assemblages in the orebodies has been revealed. To consequent, two phases of mineralization related to the formation of the Shimokawa ores have been distinguished, i.e. the early phase mineralization is characterized by precursory cubic or colloform pyrite (py(I)), indicating that it was created in an oceanic ridge as an extrusive magmatic activity, whereas the subsequent one is marked by the presence of chalcopyrite-pyrrhotite assemblage associated with sphalerite, worm-like pyrite (py(II)), cubanite, mackinawite and cobaltpentlandite as accessory minerals. Pyrrhotite has been divided into three types; a) hexagonal, b) monoclinic and c) hexagonal-monoclinic mixture. Hexagonal pyrrhotite tends to occur in lower level and monoclinic one in upper level of the orebodies, while hexagonal-monoclinic mixture occpcupies the intermediate part.
Chemical analyses of some ore minerals have disclosed that Ni in pyrite and pyrrhotite, and Ni, Co in chalcopyrite and cubanite are almost nil, mackinawite shows a notable substitution between Co and Cu, Co/Ni of monoclinic pyrrhotite is extremely high, Co/Ni of cobaltpentlandite is higher in the ores at upper level.
Geochemical profile of the Shimokawa orebodies led us to a conclusion that there is a geochemical zonation suggesting the ore-forming conditions of later phase mineralization varies methodically from southern bottom toward northern top of the orebodies.
