Thermodynamic conditions control the valences state of semimetals thus affecting the behavior of PGE in magmatic sulfide liquids

Abstract

In magmatic Nickel-Copper-Platinum-group elements (Ni-Cu-PGE) sulfide deposits, PGE occur as solid solution in base metal sulfides (BMS) or occur as platinum-group minerals (PGM). In different deposits, such as the world-class deposits at Jinchuan and Noril’sk district which formed from sulfide liquids with different oxygen fugacity (fO2), the proportion of PGE forming PGMs and entering BMS as solid solution are variable and the PGMs are different in mineral species and morphology. The early-cumulated massive ores from the two deposits were analyzed using Electron Probe Microanalysis (EPMA), Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS), Scanning Electron Microscope (SEM) and Focused Iron Beam and Transmission Electron Microscope (FIB-TEM). Combined with whole-rock PGE and semimetals data, this contribution discusses the effect of thermodynamic conditions (mostly fO2) and semimetals on the distribution of PGE and the mineral species and morphology of PGM in magmatic sulfide deposits. The Noril’sk massive ores commonly contain 5–12 vol% of magmatic magnetite whereas magnetite is absent in the Jinchuan massive ores, which suggests that the sulfide liquid at Noril’sk had a higher oxygen fugacity (fO2) compared to the sulfide liquid at Jinchuan. In the Jinchuan massive ores, only ∼ 39 % Ir and 37 % Rh are presented in solid solution within BMS. Meanwhile, many (Ir-Rh-Pt)AsS crystals with grain sizes of 200 nm to 2.5 μm were recognized enclosed in BMS grains and cobaltite-gersdorffite solid solution (CGSS) or rarely located at BMS grain boundaries. Microstructures of the (Ir-Rh-Pt)AsS minerals suggests that they probably crystallized from the sulfide liquid together with MSS. In contrast, more than 63 % Ir and 89 % Rh are presented in solid solution within BMS and no (Ir-Rh-Pt)AsS crystal was found in the Noril’sk massive ores. On the other hand, arsenides, stannide, antimonides, tellurides and bismuthides of Pd and/or Pt (Pt-Pd-semimetal minerals) were found in the Jinchuan and Noril’sk massive ores. This study suggests that thermodynamic conditions (especially fO2) control the valence states of semimetals (especially As) affecting the behavior of PGE in sulfide liquids. A relatively low fO2 of sulfide liquids leads to a relatively high Asn-/Asn+ ratio, which is atomical ratio of the anion to cation of As (n represents valence number), and results in relative more Ir-AsS, Rh-AsS and Pt-As pre-nucleation clusters. These pre-nucleation clusters were enriched in or near the migrating MSS grain boundaries and finally reaching oversaturation and nucleation of (Ir-Rh-Pt)AsS crystals occurred. In contrast, a relatively high oxygen fugacity (fO2) of sulfide liquids results in a low Asn-/Asn+ ratio and Ir-AsS, Rh-AsS and Pt-As pre-nucleation clusters forming a negligible amount of (Ir-Rh-Pt)AsS minerals and facilitating Ir and Rh to enter MSS crystal lattice. As for the deposits formed from sulfide liquids with a relatively low fO2, such as the deposits at Jinchuan, Yangliuping and Sudbury, the fractional crystallization of MSS together with (Ir-Rh-Pt)AsS minerals resulted in PGE differentiation.