Redox states and genesis of Cu- and Au-mineralized granite porphyries in the Jinshajiang Cu–Au metallogenic belt, SW China: studies on the zircon chemistry

Oxygen fugacity ( f O 2 ) is a critical factor that controls the formation mechanisms of the porphyry Cu deposits. However, the differentiation of the f O 2 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 f O 2 , and petrogenesis among the Cu-mineralized, Au-mineralized, and ore-barren porphyries. Our results show that the f O 2 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 f O 2 of the Cu-mineralized granite porphyries. Collectively, the magmatic processes at deep levels played an essential role in determining the f O 2 of the granite porphyry, thus controlling the mineralization diversity.