Platinum-group element (PGE) geochemistry of the Emeishan basalts in the Pan-Xi area, SW China

Abstract

The Longzhoushan and Ertan basalts in the Pan-Xi area were generated by relatively low degrees of partial melting (< 7%), during which Ir, Ru, and Rh are compatible while Pt and Pd are incompatible. Most of these samples have high PGE concentrations and much higher Pd/Ir and Cu/Pd ratios than those of primitive mantle. A small amount of sulfides (∼0.007%) remained in the mantle source during partial melting, whereas part of the magmas reached sulfide saturation and segregated PGE-rich sulfides (∼0.020%–0.033%) during ascent period. Osmium, Ir, and Pt could form platinum-group metallic alloys hosted in chromites or sulfides during fractional crystallization process, leading to the decoupling of Pt and Pd contents and considerable variations of Ir contents. Sulfides segregated from the basaltic magmas may provide important material sources for the formation of the Cu-Ni-PGE deposits (mineralization) in the mafic-ultramafic intrusions near the magma conduits.