Oxygen Fugacity and Porphyry Mineralization: A Zircon Perspective of Dexing Porphyry Cu Deposit, China

Oxygen fugacity (fO2) 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 Ce4+/Ce3+ of zircons and estimated the oxygen fugacity of their parental magmas. The Ce4+/Ce3+ ratios of Middle Jurassic (∼170Ma) magmatic zircons are high (550 on average), whereas the Ce4+/Ce3+ ratios of inherited zircons (200–880Ma) are much lower (263 on average). The relationship suggests that the Dexing porphyry magma was highly oxidized when the Jurassic magmatic zircons crystallized (estimated fO2: ΔFMQ+0.7 (±1.3) – ΔFMQ+1.9 (±1.3)), which cannot be attributed to partial melting of the Neoproterozoic arc crust (estimated fO2: ΔFMQ−2.4 (±1.1) – ΔFMQ+0.7 (±1.2)) as proposed previously. Instead, the high fO2 (ΔFMQ+1.5) of these magmas is a primary feature of the latest magmatism. Sulfate rather than sulfide is the dominant species at fO2>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.