Porphyry Copper Potential in Japan Based on Magmatic Oxidation State

Abstract

AbstractThe formation of large porphyry Cu deposits requires shallow intrusion of felsic stocks that are injected into the focused sites of the mineralization. The parental magmas for these igneous rocks are oxidized, with a fO2 above FMQ + 2 (2 logarithmic units above fO2 of fayalite‐magnetite‐quartz buffer), which enables the magmas to transport large amounts of S and metals. The Japanese islands contain continental and island‐arc magmatic belts of various ages, from Mesozoic to recent, that were formed in response to westward subduction of oceanic plates. Mesozoic–Paleocene magmatism formed arcs on the eastern margin of Eurasia before the opening of the Sea of Japan. The back‐arc rifting that formed the Sea of Japan separated these igneous rocks from Eurasia, with their relics exposed as volcano–plutonic belts in Japan. Among these, the San‐in belt is the only representative of an upper crustal section that includes magnetite‐bearing granitic rocks. The opening of the Sea of Japan was accompanied by the asthenospheric mantle upwelling and eruption of oceanic magma. This was followed by extensive Miocene submarine volcanic activity that formed the Green Tuff belt. The current configuration of the Japanese islands was established in Pliocene. Young arcs, such as the Kuril, Izu‐Bonin, and Ryukyu arcs, are largely composed of mafic magmas, with accompanying asthenospheric upwelling in back‐arc extension. As the asthenospheric mantle and associated magmas have a reduced fO2, below FMQ (fayalite‐magnetite‐quartz buffer), continual upwelling of asthenospheric mantle creates limited areas of potential porphyry Cu mineralization in Japanese islands. Possible prospective areas include young stratovolcanoes in Honshu and Kyushu islands. Fertile rocks that may be associated with porphyry Cu mineralization can be identified by evaluating their magmatic oxidation conditions through mineralogical and mineral chemistry studies.