LAUSR

Abstract Oxygen fugacity ( f O 2 ) is a key factor that controls the formation of porphyry Cu deposits. Porphyry Cu deposits are typically oxidized, but when and how porphyry magmas gain their high oxygen fugacity signatures, and how oxygen fugacity controls porphyry mineralization, remains obscure. To trace the origin of the high oxygen fugacity in porphyry Cu deposits, we determined trace element compositions and U-Pb ages of magmatic and inherited zircon from Dexing porphyry Cu deposit, calculated Ce 4+ /Ce 3+ of zircons and estimated the oxygen fugacity of their parental magmas. The Ce 4+ /Ce 3+ ratios of Middle Jurassic (∼170 Ma) magmatic zircons are high (550 on average), whereas the Ce 4+ /Ce 3+ ratios of inherited zircons (200–880 Ma) are much lower (263 on average). The relationship suggests that the Dexing porphyry magma was highly oxidized when the Jurassic magmatic zircons crystallized (estimated f O 2 : ΔFMQ + 0.7 (±1.3) – ΔFMQ + 1.9 (±1.3)), which cannot be attributed to partial melting of the Neoproterozoic arc crust (estimated f O 2 : ΔFMQ − 2.4 (±1.1) – ΔFMQ + 0.7 (±1.2)) as proposed previously. Instead, the high f O 2 (ΔFMQ + 1.5) of these magmas is a primary feature of the latest magmatism. Sulfate rather than sulfide is the dominant species at f O 2  > ΔFMQ + 1.5. In general, the behavior of Cu and other chalcophile elements is controlled by sulfide, while the sulfur speciation is controlled by oxygen fugacity. Previous modeling results show that partial melting of mantle peridotite under high oxygen fugacity (even at >ΔFMQ + 1.5) cannot form Cu-rich magmas, which plausibly explains the lack of porphyry Cu deposits in normal arc rocks. This is because mantle peridotite has low Cu and S contents. Our modeling shows that partial melting of subducted oceanic crust, under oxygen fugacities higher than ΔFMQ + 1.5, is favorable for producing primary magmas with Cu contents sufficiently high for porphyry mineralization, which plausibly explains the close relationship between porphyry Cu deposits and oxidized magmas with adakitic affinities.