Why No Porphyry Copper Deposits in Japan and South Korea?

This analysis concludes that there is little realistic likelihood of discovering large, high-grade porphyry Cu deposits in Japan and South Korea because of an unusual combination of geological factors. First, weak inter-plate coupling, trench retreat, and extension - a tectonic regime unfavorable for porphyry Cu formation - appears to have characterized the convergent margin during much of Cretaceous through Neogene time. Second, Cretaceous through Quaternary magmatism involved widespread ash-flow caldera development, a volcanic style that suppresses porphyry Cu formation because the all-important Cu-bearing magmatic fluids - if present - would be explosively discharged during silicic ignimbrite eruption and lost to the atmosphere. Third, due to the trench retreat, crustal profiles beneath Japan are dominated by accretionary complexes containing chemically reduced lithologies, which decrease the redox state of subduction-related magmas during their ascent. The most reduced intrusions belong to the ilmenite-series, which may lock up chalcophile elements at depth and produce magmatic fluids that are incapable of transporting the Cu required for significant porphyry deposit formation. Fourth, many of the caldera-related magmatic rocks, including the principal intrusions, are not only derived by partial melting of reduced crust but are commonly also highly fractionated; hence, any associated mineralization tends to be Mo +/- W-rich rather than Cu dominated. Fifth, intrusive rocks with high Sr/Y, typical of hydrous magmas responsible for porphyry Cu belts worldwide, appear to be present only locally. Sixth, two important magmatic and metallogenic tracts in Japan, the Kuroko-bearing Green Tuff belt and Kitami low-sulfidation epithermal Au region, are characterized by bimodal magmatism, which worldwide is compositionally unsuited to porphyry Cu generation. Seventh, the Pliocene-Quaternary andesitic-dacitic volcanoes that could be the surface expressions of porphyry Cu systems are mostly too shallowly eroded to reveal underlying deposits should they exist. Consequently, few magmatic suites in Japan and South Korea provide propitious environments for porphyry Cu exploration, with the last hope believed to lie at depth below late Miocene-Pliocene advanced argillic lithocaps, several of which contain relatively minor high-sulfidation epithermal Au mineralization. However, even there, the reduced crust may have militated against effective Cu transport into the porphyry environment. The magmatic and metallogenic factors that downgrade the porphyry Cu potential of Japan and South Korea are believed to be equally applicable to other subduction-related magmatic arc segments worldwide.