Fluid evolution and genesis of the Zhuanghuhe Au–Sb mineralization in the Duobaoshan orefield, Northeast China: Evidence from fluid inclusions and H–O–C isotopes

The newly discovered Zhuanghuhe Au–Sb occurrence is located 20 km north of the famous Duobaoshan Cu–Mo deposit, it is the first gold–antimony deposit discovered in the Duobaoshan orefield. The hydrothermal quartz vein-type orebodies occur in the Middle Ordovician Duobaoshan Formation and are controlled by NNW and ENE compressional faults. Four mineralization stages are identified: stage I quartz + pyrite + arsenopyrite, stage II quartz + polymetallic sulfides, stage III quartz + stibnite, and stage IV quartz + calcite. Fluid inclusions (FIs) in quartz and calcite veins include six types: types I (two-phase aqueous), IIa (aqueous–carbonic; the volume of the carbon phase <50 %), IIb (aqueous–carbonic; the volume of the carbon phase >50 %, with more CO₂ than CH₄), IIc (aqueous–carbonic; the volume of the carbon phase >50 %, with more CH₄ than CO₂), IIIa (carbonic, with more CO₂ than CH₄) and IIIb (carbonic, with more CH₄ than CO₂). FIs of stage I are types I, IIa, IIb and IIIa, with homogenization temperatures of 279–311 °C and salinities of 4.87–11.84 wt%, indicating a medium-temperature low-salinity immiscible NaCl–H₂O–CO₂ fluid system. Stage II contains all FI types with final homogenization temperatures of 233–288 °C and salinities of 4.94–8.67 wt%, indicating a medium- to low-temperature low-salinity NaCl–H₂O–CO₂–CH₄ system. FIs of stage III contain types I and IIa with homogenization temperatures of 193–240 °C and salinities of 3.85–6.63 wt%, belong to a low-temperature low-salinity NaCl–H₂O–CO₂ ± CH₄ system. FIs of stage IV contain only type I with homogenization temperatures of 158–212 °C and salinities of 2.56–4.01 wt%, indicating a NaCl–H₂O system. The H–O–C isotope data show that the NaCl–H₂O–CO₂ fluids of stage I (δD = −98 to −105.7 ‰, δO_H2O = 5.7 to 6.9 ‰, δ¹³C_CO2 = −11.9 to −11.1 ‰) were derived from a magmatic system, the fluids of stage II (δD = −85.7 ‰, δO_H2O = 2.8 ‰, δ¹³C_CO2 = −14.2 ‰, δ¹³C_CH4 = −32.5 to −31.9 ‰) were characterized by the addition of CH_4, which might have come from the reaction between the original ore-forming fluids and carbonaceous slate of the Duobaoshan Formation. The fluids in stage III (δD = −90.2 to −89.6 ‰, δO_H2O = −9.1 to −6.1 ‰) and stage IV (δD = −110.6 to −96.3 ‰, δO_H2O = −13.0 to −12.6 ‰) were characterized by mixing of meteoric water. According to the geology, fluid inclusion and H-O-C stable isotope evidences, the initial magma-derived NaCl-H₂O-CO₂ fluids were immiscible in stage I and then the process of mixing with CH₄-bearing fluids in stage II, resulting in the gold precipitation; the precipitation of antimony was mainly related to the temperature drop and the mixing of meteoric water in stage III.