Discovery of High-Os Picrites in the Large Emeishan Igneous Province and Its Geological Significance

We report concentrations of platinum group elements (PGE: Os, Ir, Ru, Rh, Pt and Pd) for picrite-basalt lavas of the large Emeishan igneous province. The absolute concentrations of Os in most of the Emeishan picrites are much higher than in komatiites, and in estimated primitive mantle. In addition, they have super-chondritic Os/Ir ratios (2.84～3.88). The unusually high Os/Ir ratios suggest that the picritic magmas may have been contaminated by black shales. Modeling shows that a 7% partial melt of primitive mantle containing 0.01% of sulfides can account for the observed Os, Ir and Ru concentrations in the most primitive picrite if the melt is contaminated by 10% of black shale en route to the surface. The similarities in Os concentrations and other geochemical characteristics between the Emeishan province and Siberian Traps imply that they might be derived from a super-plume, which originated from mantle-core boundary. The large Emeishan igneous province has most of the features required to form Noril'sk-style Ni-Cu-PGE sulfide deposits; therefore, its potential for large magmatic sulfide deposits must be considered.