Kuroko deposits: their geology, geochemistry and origin

Abstract

Synopsis The Kuroko deposits of Japan are predominantly massive, stratiform sulphide ores with associated underlying stockwork ores of Miocene age in the Green Tuff volcanic sequences. They formed during a very limited period around 13 m.y. ago in association with the late-stage felsic rocks of the mafic to felsic volcanic cycle that began in the late Oligocene or early Miocene. They occur in narrow zones parallel to island arcs. The Benioff zone is at about 150-km depth below the Kuroko zones, which were areas of high heat flow during the Miocene. The major constituents of the deposits are pyrite, chalcopyrite, sphalerite, galena, barite and quartz. They are zoned vertically. Gypsum and/or anhydrite commonly occur as separate bodies beside the sulphide-barite bodies. It is thought that the ores were precipitated on the Miocene sea-floor from saline solutions in a weakly acidic, reducing environment at about 200–250°C as a result of falling temperatures. The zoning of minerals may have resulted from changes in density of the ore solution-sea water mixtures as precipitation proceeded. Studies of light stable isotopes suggest that the mineralizing solutions were predominantly coeval sea water. Lead isotope data, together with base-metal contents of the host rocks, however, indicate that leaching by circulating sea water cannot have been the dominant mechanism. So, magmatic water probably played an important role in the mineralization. The following model is therefore proposed. About 20–25 m.y. ago, as the Pacific plate was being subducted, a large amount of basic to intermediate magma was generated and differentiated in the lower crustal region. Magmatic fluids produced at the last stage of differentiation, about 13 m.y. ago, rose to the sea-floor and, on mixing with sea water, produced the Kuroko deposits. Subsidence of the sedimentary basins before the felsic volcanism could have been the result of volume decrease during solidification of underlying magma.