Platiniferous chromitite and the tectonic setting of ultramafic rocks in Cabo Ortegal, NW Spain

High-Cr number (Cr#) chromitites with exceptional concentrations of platinum (up to 7000 ppb) and palladium (up to 4400 ppb) occur in sulphide-poor dunites exposed along the western side of the ultramafic Herbeira Massif at Cabo Ortegal, NW Spain. The Pt-Pd enriched chromitites are restricted to the exposed top of the c. 800 m thick Herbeira Layered Complex, which comprises two dunite-dominated units (lower and upper dunites) separated by a garnet-bearing pyroxenite-dominated unit. The Herbeira Layered Complex is interpreted as lower crustal and is underlain by high-Al spinel harzburgite interpreted as mantle tectonite. Dunite lenses within this underlying tectonized harzburgite also contain high-Cr# chromitites, but these are Pr-Pd depleted (Pt and Pd have been strongly fractionated into the overlying Herbeira Layered Complex). Highest Pt and Pd concentrations are restricted to the top of the upper dunite, where a negative Ru anomaly also becomes apparent. The Cabo Ortegal ultramafics form part of a high grade terrane with HP granulites, garnet pyroxenites and eclogites, and are similar to PGE-rich chromite concentrations in are root complexes in Pakistan (Jijal) and Alaska (Talkeetna and Tonsina). The Cabo Ortegal chromitites are interpreted as having crystallized from a sulphide-poor, mantle-derived suprasubduction melt deep within an are massif subsequently thrust over the Gondwanan continent during Variscan collision.