Platinum-group mineral assemblages and chromite composition in the alteredand deformed Bacuri complex, Amapa, northeastern Brazil

In the Bacuri complex, Amapa, Brazil, there is good evidence for Pd mobilit y both during serpentinization and lateritization. In non-lateritized, serp entinized samples, Pd bismuthide occurs with chalcopyrite in chlorite-fille d veins that cross-cut serpentine and chromite probably only a few tens of micrometers from the sourer of the platinum-group elements (PGE). Low Pt/Pd values of less than one are characteristic of magmatic values, and sporadi c higher values of up to 4 indicate partial removal of Pd. The highest Pt/P d values (up to 26) occur in laterites, where Pt remains but Pd has been ex tensively removed. The PGE are concentrated in non-lateritized rock-types i ncluding chromitite, serpentinite containing disseminated chromite, and sul fide-bearing serpentinite. The highest whole-rock PGE concentrations are 16 6 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 sobo levskite and michenerite, are the most common platinum-group minerals. They commonly occur with pentlandite and (less common) pyrrhotite, typically pa rtially altered to millerite, magnetite, pyrite, nickeloan pyrrhotite, mauc herite and gersdorffite, all of which form less than 1% of these rocks. Chr omitite 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%, composi tions most typical of chromite from a stratiform complex of continental ori gin.