Coupled phase separation and fluid cooling trigger gold precipitation in the Xiakanmucang reduced intrusion-related gold deposit, China

Abstract The mechanism of gold precipitation in reduced intrusion-related gold deposits (RIRGDs) remains contentious, with ongoing debate over whether this process is predominantly controlled by fluid immiscibility (phase separation) or by the temperature-driven destabilization of gold complexes. This study presents a comprehensive investigation into the gold precipitation mechanisms in RIRGDs, encompassing petrography, geochronology, trace element geochemistry, and fluid inclusion microthermometric study of the Xiakanmucang deposit, China. Two stages of mineralization have been identified in the deposit: the tourmaline-quartz-arsenopyrite-pyrite-chalcopyrite-tetrahedrite-pyrrhotite-monazite assemblage (stage 1), and the quartz-calcite-pyrite-arsenopyrite-stibnite-sphalerite-galena-chalcopyrite association (stage 2). The mineralization of the Xiakanmucang deposit occurred at 239.3 ± 2.3 Ma as determined from stage 1 hydrothermal monazite U-Pb geochronology, consistent with the zircon U-Pb age of the reduced ilmenite-series granite host, which formed at 239.5 ± 0.5 Ma. The Au content of pyrite associated with stage 1 (Py1) varies from below detection limit (BDL) to 0.29 ppm with a median of 0.03 ppm, and the Au content of stage 1 arsenopyrite (Apy1) ranges from BDL to 1.84 ppm with a median of 0.04 ppm. Py1 shows a narrow range of δ34S values from −6.8‰ to −4.7‰ with a median of −5.4‰. The halite-rich H2O-NaCl ± CH4 inclusions (~48.26 wt% NaCl equivalent [equiv.]) in stage 1 were trapped together with low-salinity (~7.86 wt% NaCl equiv.), vapor-rich inclusions at temperatures of ~403 °C and at a depth of ~2.5 km under lithostatic conditions, indicative of phase separation. Stage 2 pyrite (Py2) contains relatively high Au content, with values ranging from 0.01 ppm to 15.99 ppm and a median of 1.30 ppm, and stage 2 arsenopyrite (Apy2) Au content varies from 0.24 ppm to 164.12 ppm, with a median of 11.51 ppm. Py2 exhibits a narrow range of δ34S values, from −6.6‰ to −3.2‰, with a median of −5.7‰. Fluid inclusion petrography and microthermometry results reveal that stage 2 is dominated by two-phase aqueous inclusions, exhibiting relatively lower homogenization temperatures (~292 °C; 307 °C pressure corrected) and lower salinities (~8.68 wt% NaCl equiv.) compared to stage 1. Phase separation in stage 1 promoted low-grade gold precipitation accompanied by extensive sulfide formation. Fluid cooling in stage 2, driven by adiabatic process and continuous volatile loss, promoted high-grade gold precipitation. This study demonstrates the respective roles of phase separation and fluid cooling in controlling low-grade and high-grade gold precipitation during the ore-forming processes of RIRGDs.