Ore Genesis of the Baimashigou Porphyry Cu (Mo–Au) Deposit: Constraints from Sulfide Trace Element and S Isotope Analyses

The Baimashigou porphyry Cu (Mo–Au) deposit is located in the eastern segment of the Central Asian Orogenic Belt. In situ elemental and S isotopic analyses of sulfides indicate that distinct Au mineralization occurred concurrently with Cu and Mo mineralization in the deposit. Au occurs mainly as discrete particles hosted in carrollite, or as inclusions within both carrollite and pyrite, particularly in Py3-2 of stage 3. Geochemical data from magnetite and chalcopyrite record a gradual temperature decline during ore formation, with fluids sourced mainly from magmatic–hydrothermal sources. Sulfide δ34S values range from 0.10 to 1.62‰, consistent with a magmatic S origin. The ore-forming process is divided into four stages. Stage 1 veins comprise mainly K-feldspar and Magnetite. Sulfide precipitation, especially chalcopyrite and pyrite, becomes evident in Stage 2. Predominant precipitation of chalcopyrite and molybdenite occurred during Stage 3, whereas Stage 4 is dominated by calcite and fluorite. During Stages 2–3, Co preferentially partitioned into carrollite rather than pyrite. Based on S isotope thermometry, the equilibrium temperatures of Stage 3 are calculated to be 329–359°C, corresponding to mineralization pressures of 118–170 bar after Stage 3 correction. The persistent increase in δ34S values during mineralization suggests that hydrogen sulfide loss likely played a key role in driving sulfide precipitation.