Platinum-group minerals and Fe–Ni minerals in the Sartohay podiform chromitite (west Junggar, China): Implications for T–pH–fO2–fS2 conditions during hydrothermal alteration

Abstract

Complex assemblages consisting of native iron + pentlandite, pentlandite + heazlewoodite + magnetite, heazlewoodite + magnetite + platinum-group minerals (PGM), and millerite ± heazlewoodite are associated with three types of altered chromites (partly altered chromite, double-rimmed chromite, and porous chromite) discovered in the Sartohay podiform chromitite hosted by the Sartohay ophiolitic mélange in west Junggar of China. We propose a two-stage genetic model to explain the chromite alteration processes based on detailed petrographic observations and thermodynamic calculations. Reactions between primary chromite and olivine formed porous Fe2+-rich chromite and chlorite patches under low-temperature, reducing conditions. Afterwards, oxidizing Fe3+- and CO2-rich fluids derived from host serpentinite were incorporated into the Fe2+-rich chromite, shifting its composition to heterogeneous Fe3+-rich chromite (ferrian chromite). Thermodynamic calculations suggest that the co-evolution of fO2 and fS2 played a crucial role in controlling the formation and/or transformation of the Fe–Ni minerals coevally with the sulfides, antimonides, and alloys of platinum-group elements (PGE). Pd and Pt precipitated as sulfides, while Ir precipitated as an alloy when log fS2 ranged from –16.0 to –14.2 within the heazlewoodite stability field. Compared with the host serpentinite, the fO2 in the Sb-bearing fluid equilibrated with chromitite was low enough to decrease the Pd solubility, leading to co-precipitation of Sb and Pd.