Caracterización geoquímica y distribución de la mineralización de Cr y EGP en los macizos ultramáficos de Cabo Ortegal (NO España)

Abstract

Chromitites with up to 13,375 ppb of total PGE occur in layered dunites in the ultramafic massifs of Cabo Ortegal (NW Spain). These ultramafics comprise mantle harzburgites (with dunite pods) overlain by a (crustal) layered complex formed by a lower dunite, a pyroxenite, and an upper dunite. Spinel-rich areas and chromitite in dunite pods are present in the basal harzburgites. Chromitite layers occur only rarely in the pyroxenite, more commonly in the lower dunite (increasing towards the top), and most commonly in the upper dunite. The thickest chromitite layers (50 cm thick) occur at the top of the upper dunite where the highest concentrations of PGE are also reached. The repetition of dunite above and below the pyroxenite in the layered intrusion, with both dunite units showing upward increasing abundances of chromite, Pt and Pd, suggests magma replenishment during a multiple intrusion history. The presence of AI-rich spinels in both harzburgites and pyroxenites, together with other suggestions of high pressure conditions such as high values of Fe and Cr in chromites, an abundance of garnet in some pyroxenite layers, and association with high pressure granulites and eclogites, indicate a mantle-crustal interface in an arc environment.