Coupled substitutions in PGE-enriched cobaltite: new evidence from the Rio Jacare layered complex, Bahia state, Brazil

摘要

Microcrystals of platinum- group element ( PGE)- bearing cobaltite occur in the Gulcari A deposit of vanadiferous titanomagnetite in the lower zone of the Rio Jacare mafic- ultramafic layered intrusion, Brazil. Aggregates of cobaltite and sperrylite are cluster- like and developed generally along the boundary of Fe- Ti oxide grains with deuteric silicates. Our observations of cryptic zoning, compositional variability and interelement correlations are based on 37 analytical points ( wavelength- dispersion spectrometry mode) of cobaltite, and indicate that Ir and Rh behave uniformly with Ni and antipathetically with Co which, in turn, correlates directly with S content. Iridium, Rh and Ni apparently substitute for Co in the As- enriched grain core, and the substitution mechanism invokes solid solution with a cattierite- type molecule: ( Ni + Ir + Rh) + ( AsS) = Co + ( S-2). The PGE- bearing cobaltite probably crystallized as a primary phase at 500 to 300 degrees C, from microvolumes of a late fluid phase. The observed enrichment in S and decrease in the As: S ratio at the cobaltite grain margins is a reflection of the increase in sulfur fugacity ( fS2) with decrease in temperature of crystallization.