Abstract

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 Pt–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 arc 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 arc massif subsequently thrust over the Gondwanan continent during Variscan collision.

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