Occurrence and magmatic origin of platinum-group minerals in the Hongge layered intrusion, SW China

Platinum-group elements (PGEs) are typically hosted in the base metal sulfides (BMS) and platinum-group minerals (PGMs) of magmatic Ni-Cu-(PGE) sulfide deposits. The PGM may crystallize directly from sulfide melt, via exsolution from the BMS on sub-solidus cooling or recrystallization from them during metamorphism and hydrothermal overprints. Nevertheless, the distribution and origin of the major species of PGMs in S-poor mafic-ultramafic rocks, e.g., many layered intrusions hosting Fe-Ti-V-Cr oxide deposits, remain unclear. The origin of PGMs in these rocks can be determined by their paragenesis and crystallographic orientations. Here, we systematically investigate the major species of PGMs in the Hongge layered intrusion (SW China), and examine the distribution of PGMs and the orientation relationships between PGMs and host minerals using the FIBHRTEM techniques. A total of 52 grains of PGMs are identified from the olivine clinopyroxenite of the lower zone and Fe-Ti oxide-bearing olivine clinopyoxenite in the middle zone of the intrusion. They mainly include Ptarsenide and Pd(Bi,Te,Sb) compounds. Pt-arsenide mainly occurs as discrete euhedral sperrylite (PtAs2). The TEM analyses reveal that sperrylite and adjacent pyrrhotite have no specific crystallographic orientation relationships and form incoherent grain boundaries at their contact, indicating that the euhedral sperrylite likely crystallized directly from silicate melt and attached to the BMS and other coexisting minerals, rather than exsolved from precursor PGE-bearing BMS. The Pd(Bi,Te,Sb) compounds present as anhedral grains of froodite (PdBi2), sobolevskite [Pd(Bi,Te)], merenskyite (PdTe2), and stibiopalladinite (Pd5Sb2). These grains are either enclosed within, attached to, or interstitial to the BMS, Fe-Ti oxides and silicates, suggesting that they were unlikely exsolved solely from PGE-bearing BMS. The HRTEM images reveal that the anhedral Pd(Bi,Te,Sb) compounds can be monocrystals or polycrystal. Monocrystal Pd(Bi,Te,Sb) compounds were not subjected to postcrystallization modification, and polycrystal Pd(Bi,Te,Sb) compounds are composed of numerous randomly distributed crystals. Given the anhedral grain shape, it is likely that the Pd(Bi,Te,Sb) compounds can crystallize to form different types of Pd-bearing PGMs from early precipitated Pd-bearing Bi(Te,Sb) melts. This study shows that the presence of Te, As, Bi, Sb and Sn (TABS) can play a crucial role in scavenging PGEs from S-undersaturated mafic magmas. Being long neglected, discrete PGMs, rather than PGE-bearing BMS, are likely the major endowment of PGEs in the Hongge layered intrusion.