Magmatic platinum-group mineral nanoparticles and partitioning in natural magmatic sulfide liquids

Recent experimental studies have shown that PGE form nanoparticles (crystals and melt droplets) in synthetic sulfide liquids at magmatic temperatures. Because PGE can exsolve from sulfide phases during cooling, the occurrence of platinum-group mineral (PGM) nanoparticles (NPs) within base metal sulfides in natural sulfide ores does not constitute empirical proof of their existence at magmatic temperatures. This investigation provides empirical evidence that PGE form PGM nanoparticles in natural magmatic sulfide liquids. Cobaltite-gersdorffite solid solution (CGSS) from the Yangliuping Ni-Cu-PGE deposit in SW China crystallized from very PGE-rich immiscible arsenide melt droplets. The PGE contents measured in the CGSS are significantly higher than the values estimated from equilibrium partitioning between sulfide and arsenide melts. We locate PGE-sulfarsenide (irarsite-hollingworthite-platarsite solid solution, (Ir-Rh-Pt)AsS; IRPAS) nanoparticles (NPs) embedded in CGSS. Transmission electron microscope (TEM) observations reveal that the IRPAS NPs exhibit chemical zoning and display distinct crystallographic orientations relative to the host CGSS. Structural and compositional features of the IRPAS NPs suggest that they nucleated in the sulfide melt during the magmatic stage before the arsenide melt segregated from the sulfide liquids. The physical incorporation of the IRPAS NPs into the arsenide melt accounts for the exceptionally elevated PGE levels in CGSS. Magmatic PGE-NPs play an important role in controlling PGE partitioning in sulfide liquids and during the generation of PGE-rich ores.