Alteration and mineralization of the giant Pulang porphyry Cu-Au deposit, southwest China: Evidence from biotite mineralogy

The Pulang porphyry Cu-Au deposit is the largest Indosinian porphyry Cu deposit in the Geza porphyry belt. Magmatic and hydrothermal biotite crystals are widely distributed across ore bodies in the central part of the deposit. Determining the compositions of these biotite crystals can help us to understand the ore-forming fluids and processes. Here we present chemical analyses of biotite from samples collected from the ZK0406 borehole, which traverses the center of the ore bodies and the various alteration zones. We identified three types of biotite (types I, II, and III). Type I biotite is primary magmatic biotite and is surrounded by limited chalcopyrite. Type II biotite is alterated biotite surrounded by chalcopyrite and pyrite. The edges and cleavage planes of type II biotite are commonly replaced by chlorite. Type III biotite is hydrothermal biotite distributed in porphyries along fissures and is associated with large amounts of chalcopyrite and pyrite. All three biotite types are Mg-biotite. Geothermometer and oxybarometer results using the compositions of the type I biotite crystals suggest that the magmas in the Pulang deposit had temperatures of 720-766 degrees C and oxygen fugacities of FMQ+2.3 to FMQ+3.2. The melt/fluid fugacities estimated using the compositions of the three biotite types suggest that the fluids present during the Potassic alteration stage may have had the high transport capacity for Cu and Au in the Pulang deposit. As a result of alteration, Al and Fe were enriched in the type I biotite, whereas Si, Ti, Mg, and Na were leached, then foremed type II biotite. Type III biotite crystals have high Cu and K contents and formed during Potassic alteration. Combining these data with field observations suggests that the Cu enrichment was closely related to the type II and III biotite, which formed during the potassic-silicate alteration stage; i.e., the major ore-forming stage of the Pulang deposit.