Chalcophile element geochemistry and petrogenesis of high-Ti and low-Ti magmas in the Permian Emeishan large igneous province, SW China

Abstract

Sulfide-poor mafic layered intrusions, sills/dykes and lava flows in the Funing region, SW China, are part of the ~260 Ma Emeishan large igneous province. They belong to either a high-Ti group (TiO2 = 1.6-4.4 wt%) with elevated Ti/Y ratios (351-1,018), or a low-Ti group (TiO2 < 1.2 wt%) with low Ti/Y ratios (133-223). This study investigates the role of fractionation of olivine, chromite and sulfide on the distributions of chalcophile elements, Ni, Cu and PGE, of the high-Ti and low-Ti group rocks at Funing. The high-Ti group rocks contain 1.6-5.3 ppb Pt + Pd, 0.06-0.43 ppb Ir and 0.01-0.13 ppb Ru, and show relative constant (Cu/Pd)PM ratios (4.0-9.7) and a negative correlation between Ni/Pd and Cu/Ir ratios. Fractionated IPGE/PPGE patterns and very negative Ru anomalies of the high-Ti group rocks, together with low Fo values (59-62 mol%) of olivine, indicate that the high-Ti magmas may have experienced fractionation of olivine and chromite under S-undersaturated condition. Based on the PGE concentrations, the low-Ti group rocks can be further divided into two subgroups; a high-PGE low-Ti subgroup and a low-PGE low-Ti subgroup. The high-PGE low-Ti group rocks are rich in MgO (10-20 wt%), but Fo values of olivine from the rocks are low (74-76 mol%). The rocks contain highly variable PGE (Pt + Pd = 1.7-88 ppb, Ir = 0.05-1.3 ppb), Ni (179 -1,380 ppm) and Cu (59-568 ppm). They have Cu/Zr ratios >1, low (Y/Pd)PM ratios (0.2-7.1) and nearly constant (Cu/Pd)PM ratios (1.5-3.8). The even and parallel chalcophile element patterns of the high-PGE low-Ti subgroup rocks are likely a result of olivine-dominated fractionation under S-undersaturated condition. The low-PGE low-Ti group rocks have low MgO (4.5-8.9 wt%) and very poor PGE (Pt + Pd 0.5-1.6 ppb, Ir 0.004-0.02 ppb) with low Cu/Zr ratios (0.1-0.5), high (Y/Pd)PM (26-70) and variable (Cu/Pd)PM ratios (2.8-14). The trough-like chalcophile element patterns of the low-PGE low-Ti subgroup rocks indicate that the magmas were sulfide saturation and sulfide melts were extracted from the magmas. The extracted sulfide melts might be potential Ni-Cu sulfide ores at depth in the Funing region. © 2010 Springer-Verlag.