Condicionantes metalogenéticos para Ni-Cu em sulfetos do corpo metabásico do Córrego do Oliveira, Região Serrana do RJ

Abstract

The Corrego do Oliveira body, located between Duas Barras and Sumidouro (RJ), is a mafic-ultramafic sill intruded in the Rio Negro Complex orthogneisses near to the contact with quartzites and calc-silicate rocks of the Andrelandia Metasedimentary Sequence. The presence of disseminated sulfides and the local geological setting suggests the possibility of a metallogenetic system characterized by Ni-Cu-(PGE) sulfides. Geological mapping together with geochemical, petrographic and mineral chemistry studies have been undertaken in order to characterize the mafic-ultramafic body and its mineralization. According to the structural style, the body is composed of rocks of two facies: the Oriented Facies and the Isotropic Facies. The latter preserves part of the igneous record partially superimposed by metamorphic paragenesis and textures. The negatives anomalies of Nb and Ti and the positive anomaly of Pb, together with the rare earth elements patterns indicate a genetic correlation with the Rio Negro Complex rocks. The high Mg content (11.3 15.4 %) and the Mg/Fe ratios in orthopyroxene (60 75) indicate moderately primitive compositions with relatively high degree of melting in the source, which is related to the high availability of Ni and other chalcophile metals. Petrographic studies together with mineral chemistry analysis confirmed the presence of chalcopyrite and pentlandite as zones and exsolutions flames within the pyrrhotite. The enriched δ34S isotopic composition (+4.50 / +6.21) and the anomalous high contents of SiO2 (50.8 52.3 %) suggest assimilation of crustal rocks adding silica and/or sulfur to the system, which may have conditioned sulfur saturation and, consequently, the formation of the sulfide liquid. The presence of sulfides as inclusions in mafic minerals, the existence of exsolved phases of Ni and Cu and the Cu/Zr ratio of the associated barren-sulfide rocks suggest that the segregation of the sulfide liquid occurred before, or simultaneously with, the crystallization of the silicates. The evidences confirm the existence of Ni- and Cu-rich sulfides setting the metallogenetic system and suggesting a potential for this type of deposit. No evidence of the presence of PGE were found. Further studies regarding the crystallization age and the distribution of the mafic and ultramafic zones in the Corrego do Oliveira body are in progress for a better understanding of the metallogenetic system and for the determination of possible accumulation zones of sulfides. The possibility of this kind of prospects in similar geological contexts is an interesting opportunity for the mineral research and the economy of the Rio de Janeiro State, as much as for the national economy.