High-oxidation-governed fluid evolution in the Naruo porphyry Cu-Au deposit, Tibet, China

The Naruo porphyry Cu-Au deposit is the third discovered large deposit in the Duolong metallogenic district. 'Previous research has mainly focused on the geochemistry of the ore-bearing granodiorite porphyry; however, the metallogenesis is also poorly understood. Based on mapping using outcrops and geological drill cores, the alteration zones were outlined and the sequence of veins was also identified. Furthermore, fluid inclusion microthermometry and Raman composition analyses were conducted. A boiling fluid system is indicated by the coexisting aqueous, brine and vapor inclusions in all vein types. Daughter-metal minerals bearing brine inclusions are the major fluid inclusion types in all veins, suggesting that the Naruo porphyry Cu-Au deposit was formed by high-salinity fluids; thus, the measured data of the brine inclusions are mainly used for the interpretation of fluid evolution. In the early quartz-magnetite veins, brine fluid inclusions exhibit high homogenization temperatures (greater than 500 degrees C) and diverse salinities (30.1-61.5 wt% NaCl equiv), while in the later magnetite-absent, chalcopyrite-bearing quartz-sulfide veins, the homogenization temperatures of aqueous and brine inclusions are below 440 degrees C, suggesting that the magnetite mostly precipitated at temperatures higher than 440 degrees C. In the even later quartz-pyrite +/- chalcopyrite veins, the homogenization temperatures of the brine inclusions are mainly below 300 degrees C, implying that the temperature range from 440 degrees C to 300 degrees C is the main copper mineralization interval. The precipitation temperature of chalcopyrite fits the fact that the copper ore-body is present within the potassic and early phyllic alteration zones. Hematite and magnetite are common within high-salinity inclusions in all vein types, indicating the high oxidation property of the ore-forming fluid. High oxidation facilitates the scavenging of sulfur and chalcophile metals into magmatic-hydrothermal systems. Moreover, hematite or magnetite deposition can facilitate the reduction of sulfate to sulfide and decrease pH values, ultimately affecting sulfide precipitation and hydrothermal alteration. The constant high oxidation is crucial for the metallogenesis of the Naruo porphyry Cu-Au deposit.