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

摘要

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 (TiO_2 = 1.6–4.4 wt%) with elevated Ti/Y ratios (351–1,018), or a low-Ti group (TiO_2＜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.