Origin of extremely PGE-rich mafic magma system: An example from the Jinbaoshan ultramafic sill, Emeishan large igneous province, SW China

Abstract

The ~ 260 Ma Jinbaoshan sill is part of the Permian Emeishan large igneous province in SW China and is mainly composed of wehrlite and hosts a PGE deposit containing ~ 45 tonnes of Pt and Pd with ore grades ranging from 1 to 5 ppm Pt + Pd. The sill is composed of several PGE-rich horizons at different stratigraphic heights. A ~ 5 m-thick PGE-rich horizon at the base of the sill, which is underlain by a chromite-rich, PGE-poor layer, accounts for 44% of the total ore reserve. The Jinbaoshan sill is rich in PGEs with Ir ranging from 1.7 to 138 ppb, Ru from 0.9 to 39 ppb, Rh from 3.1 to 133 ppb, and a total Pt and Pd from 76 to 5371 ppb. All the rocks from the sill display primitive mantle-normalized chacophile element patterns enriched in PGEs relative to Ni and Cu. The low Cu/Pd ratios (20 to 1938) of the rocks relative to the primitive mantle (Cu/Pd = 7000) imply that there was no sulfide fractionation of the magmas. The PGE enrichment of the sill indicates that it formed from extremely PGE-rich magmas. Modeling results show that such PGE-rich magmas likely resulted from a multistage-dissolution upgrading process in an open magma conduit system. Large amounts of chromite crystallization triggered sulfide saturation when PGE-rich magmas emplaced into the upper magma chamber of the system. Settling and sorting of chromite and most sulfide liquids along with olivine resulted in the formation of the largest PGE-rich horizon at the base of the sill. The rest minor sulfide liquid accumulated to form PGE-rich horizons in the middle part of the sill aided by turbulence and convection within the magma chamber.