Evolution of oxygen fugacity and copper in the Mesozoic Shanyang porphyry groups, South Qinling

Porphyries, closely associated with the copper ore deposits in orogenic belts, are usually considered to have high oxygen fugacity and display high Sr/Y ratios. However, it is still ambiguous whether the high oxygen fugacity and the enrichment of copper are inherited from magma source or obtained by magmatic processes, and the linkage between the high Sr/Y magmas and the formation of porphyry Cu deposits remains unclear. To address these issues, an integrated study of zircon geochronology and oxygen fugacity, petrography, geochemical and Sr-Nd-Hf isotopic compositions for was carried our for the Shanyang porphyry groups from the South Qinling Orogen, Central China. The crystallization ages for the Shanyang porphyries range from ca. 152 to 140 Ma. Our results suggest that the Shanyang porphyry groups are high Ba-Sr granitoids, rather than adakitic rocks, and there is no inevitable connection between high Sr/Y magma and the formation of PCDs. Their parental magmas were derived from partial melting of enriched heterogeneous lithospheric mantle that had been metasomatized by fluid or melt released from the previous subducting slab. Through magma differentiation, the Shanyang porphyry magmas changed from the oxidation state (ΔFMQ +1 to +2) to the reduced state (ΔFMQ +1 to -0.5). The redox condition of magma may be very different from its source and can be shifted remarkably during magmatic evolution that caused by fractional crystallization of garnet in the deep crust and magma degassing in the shallow upper crust. Remelting of the early formed sulfides and gas-brine reactions could enrich copper in the exsolved volatile fluid. Furthermore, the periodic and long-lived magmatic-hydrothermal systems in the shallower magma reservoirs play a critical role in the formation of porphyry Cu deposits.