Implications of nano- and micrometer-size platinum-group element minerals in base metal sulfides of the Yangliuping Ni-Cu-PGE sulfide deposit, SW China

The concentrations of platinum-group elements (PGE) and semimetal elements (As, Sb, Se, Te and Bi) in the base metal sulfides from the Yangliuping deposit were determined using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS). Mass balance calculation reveals that the base metal sulfides only contain < 40 wt% Os, < 15 wt% Ir, < 25 wt% Pd and negligible Pt, Rh and As, as solid solution. Such PGE percentages in the base-metal sulfides as solid solution are evidently lower than in the sulfides from the Jinchuan, Norilsk and Sudbury Ni-Cu-(PGE) sulfide deposits. Euhedral shape and similar chemical composition of the nanometer-size PGE-arsenides and sulfarsenides in the base-metal sulfides suggest that they crystallized from the sulfide melt before the crystallization of monosulfide solid solution (MSS) and intermediate solid solution (ISS). It is proposed that the nanometer-size PGE-arsenides and sulfarsenides formed from PGE-As molecular or polymolecular clusters in the sulfide liquid at high temperature. The PGE-As molecular or polymolecular clusters prevent As and PGE from partitioning into the base metal sulfide lattice and tend to form discrete nanometer-size PGE-arsenides and sulfarsenides. Thus, semimetal elements, particularly As, play an important role on behaviour of PGE during solidification of magmatic sulfide liquids.