AbstractThe role of potassic alteration and associated quartz veins for porphyry Cu mineralization in two representative porphyry systems (El Salvador, Chile, and Grasberg, Indonesia) was examined to evaluate the potential of porphyry Cu mineralization in Japan. The reactions between the aqueous magmatic–hydrothermal fluids and host rocks during potassic alteration at El Salvador and Grasberg includes (i) reduction of SO42− in the aqueous fluids due to oxidation of ferrous iron in mafic minerals and (ii) disproportionation of SO2 in the fluids by elevated concentrations of Ca2+ leached by the fluids from igneous plagioclase and hornblende. These reactions resulted in an increase of reduced S species (H2S) in the fluids and promoted Cu sulfide precipitation during the potassic or subsequent chlorite–sericite alteration in the temperature range of 600–400°C.Although numerous magnetite‐series, calc‐alkaline intrusive stocks of dioritic or granodioritic composition are exposed in the Sanin belt of the Cretaceous–Paleogene Southwestern Japan arc and late Miocene Northeast Japan arc, potassic alteration represented by abundant hydrothermal biotite, magnetite, and K‐feldspar is uncommon in these rocks. However, the intrusive rocks may have undergone molybdenite mineralization in association with phyllic alteration, represented by quartz, sericite and pyrite, or chalcopyrite in chlorite–sericite alteration in the subepithermal and epithermal environments at <400°C. The absence of porphyry Cu deposits in the Japanese islands in petrochemically and compositionally favorable intrusive rocks is ascribed to the lack of early quartz veins and poor development of potassic alteration that prevented Cu sulfide precipitation at high temperatures. The lack of early quartz veins and scarcity of potassic‐altered intrusive rocks in Japan is explained by the noncompressional stress field throughout the Japanese islands, which was incapable of maintaining lithostatic pressures during fluid exsolution from the intrusive magmas.