Platinum-group element geochemistry of ultramafic rocks in Maowu,Dabie Mountains

Abstract：The contents of platinum group elements (PGE; Ir, Ru, Rh, Pt, Pd) in orthopyroxenite and garnet websterite in Maowu have been determined using the nickel sulfide fire assay, combined with ICP-MS. Analysis indicates that： the PGE abundances are not related to rock types, and the primitive mantle-normalized PGE patterns have negative slopes, showing the fractionation of Pd and Ir. The formation of these PGE characters was constrained by the presence of PGE in rocks. Analysis of the PGE-carrier phases shows that PPGE are concentrated in Cu-rich sulfides, while IPGE occur as quasi-residual, insoluble monosulfide solid solution. Crustal contamination also contributed to the fractionation of platinum-group elements. In addition, the presence of fluids during rock formation resulted in selective mobilization of Pt and Pd. Therefore the combined action of monosulfide solid solutions and fluids produced the characteristics of PGE distribution in quasi-residual pyrolite of the Maowu ultramafic rocks.