Abstract: The Wenquan porphyry Mo deposit, containing 0.12 Mt Mo at average Mo grade of 0.048%, is closely associated with the concentrically zoned multiphase Wenquan batholith in the West Qinling Orogen. The Wenquan batholith consists of five units: biotite granite(Ⅰ), biotite monzogranite porphyry(Ⅱ), porphyritic monzogranite(Ⅲ), monzogranite porphyry(Ⅳ) and porphyritic syenogranite(Ⅴ). The disseminated and veinlets Mo mineralization is genetically associated with the biotite monzogranite porphyry and porphyritic monzogranite phases and hosted mafic enclaves. We calculated the oxygen fugacity (f(O2)) of the multiphase Wenquan batholith by using a relation for cerium redox equilibria in silicate melts and an internally-consistent model for zircon-melt partitioning of Ce. The ore-bearing porphyries had average f(O2) of ΔFMQ=1.35 and ΔFMQ=1.38 for units Ⅱ and Ⅲ, respectively, while the barren porphyries had lower f(O2) values of ΔFMQ=-0.61 and ΔFMQ=-0.73 for unit Ⅰ and Ⅳ, respectively. We therefore proposed that oxidized magmas can be associated with enriched mantle. Molybdenum can be extracted from high oxygen fugacity melt in the Paleozoic Mo-rich sediments in the Qinling Orogen. The increasing of oxygen fugacity is furthermore beneficial to concentrating Mo in the residual melt, favorable for Mo mineralization.

Key words: oxygen fugacity, Wenquan Mo deposit, porphyry ore deposit, West Qinling Orogen