PLATINUM-GROUP MINERAL ASSEMBLAGES AND CHROMITE COMPOSITION IN THE ALTERED AND DEFORMED BACURI COMPLEX, AMAPA, NORTHEASTERN BRAZIL

Abstract

In the Bacuri complex, Amapa, Brazil, there is good evidence for Pd mobility both during serpentinization and lateritization. In non-lateritized, serpentinized samples, Pd bismuthide occurs with chalcopyrite in chlorite-filled veins that cross-cut serpentine and chromite probably only a few tens of micrometers from the source of the platinum-group elements (PGE). Low Pt/Pd values of less than one are characteristic of magmatic values, and sporadic higher values of up to 4 indicate partial removal of Pd. The highest Pt/Pd values (up to 26) occur in laterites, where Pt remains but Pd has been extensively removed. The PGE are concentrated in non-lateritized rock-types including chromitite, serpentinite containing disseminated chromite, and sulfide-bearing serpentinite. The highest whole-rock PGE concentrations are 166 ppb Pt and 609 ppb Pd. Osmium, Ir and Ru concentrations in the chromitite can be attributed to the presence of laurite and irarsite, whereas sulfide-bearing serpentinite contains sperrylite. Pd–Bi tellurides, including sobolevskite and michenerite, are the most common platinum-group minerals. They commonly occur with pentlandite and (less common) pyrrhotite, typically partially altered to millerite, magnetite, pyrite, nickeloan pyrrhotite, maucherite and gersdorffite, all of which form less than 1% of these rocks. Chromitite in folded layers 3–4 m thick has Cr/(Cr + Al) values of 69.0–84.1, Mg/(Mg + Fe2+) values of 27.6–51.4, and TiO2 values of up to 1.69%, compositions most typical of chromite from a stratiform complex of continental origin.