Differentiation of platinum-group elements (PGE) and of gold during partial melting of peridotites in the lherzolitic massifs of the Betico-Rifean range (Ronda and Beni Bousera)

Abstract

The Ir, Ru, Rh, Pt, Pd and Au contents of 28 peridotites of the Ronda and Beni Bousera lherzolitic massifs were determined using ICP-MS after hydrolytic extraction. Mantle-normalized NiPGECu distribution in these peridotites varies from almost flat patterns to patterns depleted in Pt, Pd and Cu. This distribution is principally controlled by partial melting which governs the differential behaviour of the PPGE (Pt, Pd) and Cu with respect to the IPGE (Ir, Ru) and Ni. The PPGE and the Cu are extracted and concentrated in silicate melt as a dissolved sulphide complex. On the other hand, the IPGE are not mobilized and are retained in the residues, probably in intergranular refractory phases. Rh behaves like a chalcophile element in the Beni Bousera peridotites (as do Pt and Pd), unlike the Ronda peridotites. The oxygen fugacity ( f O 2 ) conditions (determined by using a solid electrolyte cell) and the low Rh content in the spinel prevent any suggestion of incorporation of this element into the spinel as assumed by the thermochemical data. Unlike the Beni Bousera peridotites, the non-chalcophile behaviour of Rh in the Ronda peridotites tends to imply lower sulphur fugacity ( f( S 2 ). The absence of a ‘pinel effect’ on the incorporation of Ir, Ru and Rh is probably related to the petrogenetic history of this mineral. It could represent the heritage of pre-existing phases which have no influence on the partitioning of these elements within the mantle.