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Oxygen fugacity (fO2) is a critical factor that controls the formation mechanisms of the porphyry Cu deposits. However, the differentiation of the fO2 in post-collisional Au-mineralized and Cu-mineralized porphyry intrusions remains unclear. The post-collisional Jinshajiang–Ailaoshan metallogenic belt hosts several Eocene Cu–Au–Mo porphyry systems including the Yulong Cu, Machangqing Cu, Chang'anchong Cu, and Beiya Au deposits and contains several barren porphyry intrusions, such as the Jianchuan, Putaishan, and Bailiancun granite porphyries. This study compares the zircon chemistry, magmatic fO2, and petrogenesis among the Cu-mineralized, Au-mineralized, and ore-barren porphyries. Our results show that the fO2 of the Cu- and Au-porphyry systems (average ΔFMQ = + 1.59 ± 0.77) is higher than that of the ore-barren (average ΔFMQ = − 0.43 ± 0.91) granite porphyry. Notably, compared to the Au-mineralized porphyries (average ΔFMQ = + 0.99 ± 0.70), the Cu-mineralized porphyries crystallized from more oxidized magma (average ΔFMQ = + 1.89 ± 0.60). All the Cu-mineralized, Au-mineralized, and ore-barren porphyry intrusions originated from partial melting of a thickened lower crust with minor input from the enriched lithospheric mantle-derived melts. The lithosphere of the Au-mineralized and ore-barren porphyry intrusions was influenced mainly by the slab-derived fluids, whereas the lithosphere of the Cu-mineralized porphyry intrusions was metasomatized by slab-derived melts. Therefore, we propose that the parental magmas for the Cu–mineralized porphyry possibly inherited the high oxidation state of the slab-derived melts and then contributed to the high fO2 of the Cu-mineralized granite porphyries. Collectively, the magmatic processes at deep levels played an essential role in determining the fO2 of the granite porphyry, thus controlling the mineralization diversity. [ABSTRACT FROM AUTHOR]

The Beiya giant porphyry Au deposit is the largest Cenozoic Au deposit in the Jinshajiang Cu–Mo–Au metallogenic belt. Many granite porphyries of ~36 Ma were exposed in the Beiya deposit, including the Wandongshan (WDS) and Hongnitang monzogranite porphyries (HNT–MPs) with economically important Au metal resources, the uneconomic WDS biotite MP, and Putaishan granite porphyry (GP), and subeconomic MPs at Nandaping (NDP) and Matouwan (MTW) prospects. To reveal the fundamental factors that lead to the different fertility of these granitic porphyries, this study uses the biotite and zircon geochemical compositions to evaluate their petrogenetic and magmatic conditions (e.g., F content, pressure, fO2). The results show that biotites from economic WDS and HNT–MPs exhibit moderate F contents (0.85%–2.16%), low n(FeT)/n(FeT + Mg) ratios (average of 0.36), and shallow emplacement depths of 0.19–1.76 km. In contrast, the uneconomic granite porphyries are characterized by variable F contents (0.17%–2.75%), high n(FeT)/n(FeT + Mg) ratios (average of 0.43), and deep emplacement depths of 2.76 to 5.02 km. Moreover, the economic WDS and HNT–MPs display higher average zircon fO2 (ΔFMQ [fayalite–magnetite–quartz] of +0.88) than uneconomic granite porphyries (ΔFMQ of −0.19), further confirming a higher fO2 of the economic granitic porphyries. These features indicate that magmatic fO2 is a critical factor controlling Cu–Au mineralization at the Beiya Au deposit. In comparison with the economic WDS and HNT–MPs, the subeconomic MPs in the MTW and NDP prospects exhibit higher average biotite n(FeT)/n(FeT + Mg) ratios of 0.47 and 0.45, and lower average zircon ΔFMQ of −0.05 and −0.23, suggesting lower magmatic fO2 and thus have relatively weak potentials to form Cu–Au mineralization. Notably, the economic WDS and HNT–MPs have higher biotite XMg ratios than those of the subeconomic and uneconomic granitic porphyries, indicating that the economic MPs were formed by the partial melting of mixed lower crustal materials with more mantle‐derived materials. Therefore, the addition of mantle‐derived materials played an important role in determining the fO2 of the granitic porphyry, thus primarily controlling its fertility of Cu–Au mineralization. [ABSTRACT FROM AUTHOR]