High fO(2) conditions characterize the majority of global porphyry copper deposits and contain highly oxidized minerals, such as magnetite and anhydrite. In contrast, the No. 1 porphyry Cu-Au deposit in the Xiongcun district (Tibet, China) has abundant pyrrhotite, reduced fluids (CH4 >> CO2), and a relative lack of highly oxidized minerals, which are indicative of low fO(2) conditions. Scanning electron microscopy, fluid inclusion, C-H-O-He Ar isotopes, and whole-rock organic carbon contents and isotope analysis were used to constrain the evolution of ore-forming fluid, the origin of CH4 and metal deposition mechanisms for the No. 1 deposit. The He Ar isotopic compositions (He-3/He-4 = 0.11-0.96 Ra, Ar-40/Ar-36 = 418.7-2920.2) suggest that the ore forming fluids predominantly derived from crust source with minor mantle input. The H-O isotopic analysis results (delta O-18(H2O) = 1.8 to + 5.2 parts per thousand, delta D = -106 to -89.9 parts per thousand) indicate that the ore-forming fluids were derived from a magmatic source that mixed with some meteoric waters. The element compositions of zircons and fluid/ melt inclusions from the mineralized Middle Jurassic quartz diorite porphyry reveal that the primary magma was characterized by high log fO(2) (> NNO) conditions. The quartz diorite porphyry intruded into the carbon-bearing wall rocks produced abundant CH4 by thermal decomposition of organic matter (delta C-13(CH4) = -26.3 to -28.5 parts per thousand), which changed the redox state of the porphyry copper system from oxidized to reduced condition. Ore elements were deposited via fluid boiling as a consequence of the rapid decrease in temperature and pressure.