PLATINUM-GROUP ELEMENTS IN THE MERENSKY REEF .1. PGE IN SOLID-SOLUTION IN BASE-METAL SULFIDES AND THE DOWN-TEMPERATUUE EQUILIBRATION HISTORY OF MERENSKY ORES

The platinum-group elements (PGE) in base metal sulfides (BMS) of the Merensky reef are mostly close to the detection limit of the proton mi croprobe. The only phase that accommodates appreciable PGE is pentland ite. Total average PGE plus Au grades of the sulfide fraction of the M erensky reef are about 500 ppm, We estimate the modal proportions of t he major BMS to be around 53 percent pyrrhotite, 25 percent pentlandit e, and 22 percent chalcopyrite (ignoring minor phases). Using this est imate, we calculate by how much the sulfides are oversaturated with re spect to individual PGE. With respect to Pt, the sulfides are many tim es oversaturated, i.e., nearly all Pt occurs as discrete PGE phases. W ith regard to Pd the sulfides are oversaturated by about a factor of t wo. The Ru and Rh levels are at and below saturation levels. Available experiments suggest that the entire PGE content of the sulfide fracti on can easily be accommodated in solid solution in BMS at temperatures as low as 500 degrees C. The fact that the BMS are oversaturated with most PGE thus indicates that the sulfides have continued to exsolve P GE below that temperature. Calculated sulfur fugacities indicate that f(s2), is controlled by silica activity, as expected in high-temperatu re ores, suggesting that metal/sulfur ratios of the ore may not have c hanged much since complete solidification of the intercumulus silicate melt of the Merensky reef. All sulfides investigated have cooled belo w the maximum temperature of pentlandite-pyrite coexistence, which exp eriments place at 250 +/- 30 degrees C. Final closure temperatures of the sulfide-PGE mineral assemblages, approximated by extrapolating the pentlandite-pyrrhotite solvus beyond its experimentally determined ra nge, are possibly as low as 80 to 90 degrees C.