Chalcophile-siderophile element systematics and regional-scale magmatic percolation in the Ronda peridotite massif (Spain)

Research areas:
Upper mantle, Peridotites, Platinum-group elements, Base metal sulfides, Melt percolation
Eighteen samples from the south-western part of the Ronda lherzolitic massif have been analysed with bulk-rock and in-situ techniques to unravel the fate of Cu, platinum-group element and chalcogens (S, Se) in a middle Proterozoic subcontinental lithospheric mantle segment variably overprinted by kilometre-scale porous flow percolation of asthenosphere-derived silicate melts during the Alpine orogeny. Chalcogen elements (S, Se Cu) and PGE systematics fit well the published data base for orogenic mantle peridotites as a whole. Positive correlations between S, Se, Cu and fertility result from progressive removal of sulfides from the mantle at increasing degrees of melting. A few harzburgites preserved the S-, Se- and Pd-depleted (Pd/Ir < 1)-chondrite normalized PGE patterns of residues after partial melting. The Oligocene thermo-mechanical erosion event triggered secondary partial melting of the sulfide phase liberating a Cu-Ni-rich sulfide melt now identified through Cu-Ni-rich sulfide inclusions (pentlandite + chalcopyrite ± bornite) and a wide range of intergranular Cu-rich sulfides and alloys. Residual monosulfide solid solution (coexisting with refractory platinum group minerals (laurite-erlichmanite) in spinel tectonites) survived throughout all of the tectonometamorphic domains, thus preserving relative and absolute abundances of compatible platinum group elements (Os, Ir, Ru, Rh). Local scale redistribution of the Cu-Ni-rich sulfide melt accounts for the enrichment/depletion trends of Te, Pd, Pt, Au, Ag, Cu identified in granular peridotites. On cooling, the sulfide melt produced pentlandite by reacting with monosulfide solid solution at T < 870 °C while transfering incompatible elements that generated micrometric Pt-Cu-Te-As-bearing platinum-group mineral inclusions (e.g. malanite CuPt2IrS4, moncheite PtTe2, Pt arsenide, secondary Pt–Cu alloys), in addition to superchondritic Pd/Ir and nearly chondritic Se/Te identified in mantle sulfides of metasomatic origin. By contrast, micrometer-sized Au particles identified in Au-enriched spinel harzburgites are clearly primary precipitates from volatile-rich small melt fractions.