Platinum-group minerals in the Limoeiro Ni-Cu-(PGE) sulfide deposit, Brazil: the effect of magmatic and upper amphibolite to granulite metamorphic processes on PGM formation

The Limoeiro Ni-Cu-(platinum-group elements (PGE)) deposit is a recent discovery associated with an igneous tubular conduit system in northeastern Brazil. Representative ores from the deposit have been used for platinum-group minerals (PGM) identification and for PGE in base metal sulfides (BMS) quantification. Ninety-eighty percent of the PGM in the massive sulfide ores is homogeneous Pt-Ni-Bi-bearing merenskyite (PdTe2) enclosed primarily by pyrrhotite, suggesting that it is formed by exsolution from monosulfide solid solution (MSS). Merenskyite gradually but systematically becomes poorer in Pt and Ni with increasing fractionation, which is interpreted to reflect a transition to a more evolved sulfide liquid that segregated in the eastern parts of the intrusion. In massive sulfide ores, merenskyite forms unusually large (up to 5000 mu m(2)) euhedral grains, commonly in contact with spherical silicate inclusions. BMS hosts 12-16 % of the Pd, with the remainder hosted by PGM, which is interpreted to indicate that merenskyite recrystallized from a PGE-bearing bismuthotelluride metamorphic melt formed during high-grade metamorphism. Sperrylite (PtAs2) is the second most abundant PGM (18 % of PGM in disseminated ore) and in contrast to merenskyite occurs mainly as very small (median of 25 mu m(2)) inclusions in high-temperature silicates and oxides, interpreted to have crystallized at high temperatures directly from sulfide blebs that formed and were transported within the Limoeiro magma conduit.