Mineralogía y comportamiento geoquímico del oro en los macizos lherzoliticos Betico-Rifeños (España y Marruecos)

Abstract

The Lherzolite massifs from the Serranía de Ronda and Beni Bousera are pieces of the upper mantle emplaced in the continental crust. Before their crustal emplacement, the Lherzolite suffered several partial melting events, giving rise to depleted peridotites (harzburgites and dunites) as well as to pyroxenites and magmatic Cr, Cr-Ni and (Fe-Ni-Cu) sulfide ores. The average Au content are strongly concentrated in the mineralizations, especially in the Cr-Ni ores, in contrast with the usual behaviour of the noble metals in magmatic ores related to mafic-ultramafic rocks. Au, Pt and Pd are concentrated in the Ni arsenides where most of the former elements are hidden in solid solution in the niccolite. The alteration of niccolite produces the exsolution of native Au and alloys of Au-Cu and Au-Cu-Ag. In most (Fe-Ni-Cu) sulfide ores, the Au content is very low. However, the bodies with the highest Cu/Ni ratio and with hydrothermally altered wall rocks are enriched in Au. The geochemical behaviour of noble metals suggests that the parental partial melts of the «ore magma» were generated from a depleted mantle source and originally, they were S- and As-undersaturated and enriched in noble metals. During ore formation, As played an important role as collector of Au (as well as of Pt and Pd); therefore, the earlier As saturation of the liquids and the subsequent crystallization of the Ni arsenides produced Au-, Pt- and Pd-depleted (Fe-Ni-Cu) sulfide ores.