Platinum-group elements, zircon Hf-O isotopes, and mineralogical constraints on magmatic evolution of the Pulang porphyry Cu-Au system, SW China

Abstract

A new model of porphyry Cu system, which involves pre-enrichment of Cu through sulfide accumulation near the mantle-crust boundary, has been recently proposed to account for variations of Cu endowment in different magmatic arcs. To test this model and explore the possible controlling factors of porphyry Cu metallogeny, we studied the mineralogy and platinum-group elements (PGE) geochemistry of the giant Pulang porphyry Cu-Au system in the Sanjiang Region, SW China. The Late Triassic Pulang intrusive complex comprises five phase of porphyrtic stocks and dikes (ca. 217–212 Ma), which intruded a slightly older (ca. 230–218 Ma) volcanic-sedimentary sequence of the Tumugou Formation. Both the intrusive complex and the related volcanic wall rocks have similar PGE concentrations, Pd/Pt ratios and primitive mantle-normalized PGE patterns, indicating that no discernible sulfide cumulates were lost or gained during magmatic differentiation. The variation in PGE concentrations between different types of rocks at Pulang is probably ascribed to magma mixing between mafic and felsic end-members.Two types of amphibole, i.e., high-Al (Al2O3 = 9.4–11.8 wt%) and low-Al (5.9–7.3 wt%) types, have been identified in the Pulang complex. It is estimated that the magma equilibrated with the high-Al amphibole may have contained up to 6.2 wt% H2O, and was formed under 334–538 MPa (ca. 11–18 km deep eqv.) with oxygen fugacity (fO2) between NNO + 0.2 and NNO + 1.5. In contrast, the magma that equilibrated with the low-Al amphibole was likely formed at a much shallower depth (89–202 MPa, ca. 3–7 km eqv.), and was more oxidized (NNO + 1.0–NNO + 1.7) with a low H2O content (4.2–4.9 wt%). High fO2 would increase sulfur solubility in the magma, thus prohibiting early removal of metals via sulfide precipitation. In addition, the higher H2O content may have enhanced volatile exsolution from the evolving magmas at Pulang. We suggest that oxygen fugacity and H2O content of the magma are two critical factors that controlled the Pulang porphyry Cu-Au metallogeny.