Behavior of palladium during fractionation of sulfide liquid: New constraints from the Kalatongke Cu-Ni sulfide-bearing intrusion in the central Asian orogenic belt, NW China

Abstract

The distribution of platinum-group elements (PGE; Os, Ir, Ru, Rh, Pt, and Pd) in base metal sulfides (BMS) of magmatic Ni-Cu-(PGE) sulfide deposits has been extensively studied. However, the origin of Pd in pentlandite and the role of Te, As, Bi, Sb, and Se (TABS) in collecting PGE during fractionation of sulfide liquid have not yet been well constrained. Representative massive sulfide ores of the Kalatongke Cu-Ni sulfide-bearing mafic intrusion in the central Asian orogenic belt (CAOB) were selected to identify platinum-group minerals (PGM) using a scanning electron microscope (SEM) and to investigate the distribution of PGE and TABS in the BMS using in situ laser-ablation inductively coupled-plasma mass spectrometry (LA-ICP-MS). The results show that concentrations of Os, Ir, Ru, Rh, and Pt in the BMS are all below or close to the detection limits and only pentlandite contains 0.1 to 3.6 ppm Pd. Three generations of pentlandite are recognized in the massive sulfide ores; i.e., coarse-grained, granular pentlandite that crystallized earlier from monosulfide solid solution (MSS) prior to crystallization of pyrrhotite, loop-textured pentlandite around pyrrhotite that crystallized after pyrrhotite, and pentlandite that crystallized from evolved Fe-rich MSS and is associated with fine-grained myrmekitic magnetite. Granular pentlandite contains higher Pd (1.4 to 3.6 ppm) than that of loop-textured pentlandite (0.8 to 1.7 ppm Pd) and pentlandite associated with myrmekitic magnetite (0.1 to 0.5 ppm Pd). Given that Pd is incompatible to MSS during fractionation of sulfide liquid and is preferentially concentrated to residual sulfide liquid, we propose that the distribution of Pd in pentlandite may be attributed to rapid diffusion of Pd from the residual sulfide liquid into MSS/pentlandite on cooling and that Pd concentrations in the three generations were likely controlled by their crystallization sequence from MSS. In addition, LA-ICP-MS elemental mapping for BMS shows that TABS are not preferentially concentrated in chalcopyrite and euhedral Ni-merenskyite [(Pd,Pt)(Te,Bi)2] grains are mainly enclosed within pyrrhotite (n = 187) rather than chalcopyrite (n = 19). The random distribution of Ni-merenskyite in the BMS is supportive of an idea that Pt and a portion of Pd were likely stabilized by the presence of Te and Bi ligands in sulfide liquid and Te-Bi-Pd-Pt associations were then randomly distributed in MSS and intermediate solid solution (ISS) during fractionation of sulfide liquid. This study shows that the formation of TABS-bearing PGM in the massive sulfide ores of the Kalatongke intrusion was not controlled by fractionation of sulfide liquid, providing a new case that the behavior of PGE is mainly controlled by TABS rather than their partitioning coefficients in sulfide liquid.