Platinum-group element geochemistry of the Hongge Fe–V–Ti deposit in the Pan-Xi area, southwestern China

Abstract

Mafic and ultramafic intrusions in the Pan-Xi area along the western margin of the Yangtze block, southwestern China, are spatially associated with the Late Permian Emeishan flood basalts of the Emeishan Large Igneous Province. The Hongge layered intrusion is one of the plutonic bodies of this province and hosts a giant Fe–V–Ti deposit. This intrusion has three zones: a lower, olivine clinopyroxenite zone, a middle, clinopyroxenite zone, and an upper, gabbro zone. Each of these zones consists of one or two compositional cycles, which have distinct Mg# values, TiO2 concentrations and total REE contents. The middle clinopyroxenite zone and upper gabbro zone contain thick (14–84 m) magnetite-mineralized layers. Samples from the lower olivine clinopyroxenite zone and middle clinopyroxenite zone of the Hongge intrusion are enriched in platinum and palladium relative to iridium and ruthenium. Rocks of the lower olivine clinopyroxenite zone have Pd/Ir ratios (1.8–22.3) lower than those of the middle clinopyroxenite zone (6.2 to 83 in its lower part and 3.6 to 49 in its upper part). The Pd/Ir ratios increase progressively upwards in each cyclic unit. Chromite is the major phase controlling concentrations of iridium and ruthenium. The Cu/Pd ratios at the bottom of each cyclic unit are close to that of the mantle and increase upwards. This variation suggests that each cyclic unit represents a new batch of magma, and sulfide mineral segregation removes PGE (platinum-group elements), Ni and Cu. Each cyclic unit in the Hongge layered intrusion might have resulted from crystal fractionation and mixing between a primary and evolved magma. The high Cu/Pd, Ti/Pd, Ni/Pd and Cu/Ir ratios of the intrusion suggest that the sulfide mineral segregation may have played an important role in PGE differentiation. If this interpretation is correct, then there is a potential to find economic PGE-rich horizon in the Hongge-type intrusions in the region.