New data on base metal mineralogy and platinum-group minerals in the Ojen chromitites (Serrania de Ronda, Betic Cordillera, southern Spain)

Three small Cr-rich chromitites occurrences (62 < Cr# < 84) from the Ojen lherzolitic massif (south Spain) were studied for base metal mineralogy, bulk rock PGE (platinum group elements) abundances and platinum-group minerals (PGM). A great variety of base metal sulfides (BMS), Ni arsenides, Ni sulfarsenides and Ni-sulfobismuthides are reported. Bulk-rock PGE contents span more than two orders of magnitude. Two occurrences display the typical PGE pattern of ophiolitic chromitites (high contents of Os, Ir, Ru and Rh and depletion in Pt and Pd; SigmaIPGE = 114.1-628.3 ppb; IPGE = Os, Ir, Ru). Another chromitite, the most chromiferous, is very poor in IPGE and Rh, while displaying a positively-slopping Primitive Mantle-normalized pattern from Pt to Pd and Au. PGM inclusions are abundant in the PGE-rich chromitites, consisting in laurite-erlichmanite, Ir-Pt-Rh sulfarsenides, Ru-Os sulfarsenides, Pt arsenides (sperrylite), Pd antimonide (stibiopalladinite) and Pt-Fe and Os-Pt alloys. The igneous crystallization sequence starts with laurite (containing appreciable amount of sulfarsenide in solid solution), erlichmanite, Ru-Os sulfarsenides and Ir-Rh sulfarsenides, followed by lr-Pt-Rh sulfarsenides, Pt-sulfarsenide and the other Pt-minerals (sperrylite, Pt3Fe, Pt-Os alloys). The high As activity during the crystallization of chromite prevented the formation of Os-Ir-Ru alloys. Stibiopalladinite is a late magmatic phase. Pd and Au in the PGE-poor chromitite are accounted for the high modal abundance of intergranular Ni arsenides. PGM-BMS textures indicate late-segregation for the BMS, probably from late-magmatic fluids. The abundance of As minerals and the presence of Bi- and Sb-bearing. phases in the Ojen chromitites confirm that volatile-rich, small-melt fractions have migrated through the Ojen lherzolitic massif just before its tectonic emplacement into the continental crust.