Nature, source, and evolution of the ore-forming fluids in the Dunbasitao gold deposit, East Junggar, China: Constraints from geology, fluid inclusions, and C-H-O isotopes

The origin of CO₂-dominated fluids in orogenic gold deposits is usually mired in controversy. The Dunbasitao gold deposit is the largest among several gold deposits along the Armantai suture zone, East Junggar, China. Orebodies are hosted in volcano-sedimentary rocks of the Lower Carboniferous Jiangbasitao Formation and porphyritic quartz diorite and are controlled by shear zones. Based on the cross-cutting relationship of veins, the hydrothermal process can be divided into the early, middle, and late stages and the gold only occurred in the middle stage. Seven types of fluid inclusions (FIs) were identified in the Dunbasitao deposit: Type-1 FIs (L_H2O + L_CO2 or L_H2O + L_CO2 + V_CO2) that homogenized to H₂O phase or show critical homogenization; Type-2 FIs (L_H2O + L_CO2 or L_H2O + L_CO2 + V_CO2) that homogenized to CO₂ phase; CO₂-dominated Type-3 FIs (L_CO2 or L_CO2 + V_CO2); Two-phase aqueous Type-4 FIs (L_H2O + V_H2O) that homogenized to liquid phase; Type-5 FIs are composed of three phases (L_H2O + V_H2O + Solid), which homogenized to liquid phase; Type-6 and Type-7 FIs refers to FIs composed of a single liquid or vapor H₂O phase, respectively. Microthermometry and laser Raman spectroscopy (LRS) analyses show that the initial ore-forming fluids in the early stage were moderate-T_h (326–342 °C), low salinity (2.4 to 4.7 wt% NaCl equiv), and homogenous H₂O−CO₂−NaCl liquid fluids, containing 13–33 mol% CO₂ and a small amount of CH₄, H₂S, and N₂. The calculated δ¹⁸O_H2O (V-SMOW) values are 7.5–8.5 ‰, δ¹⁸D_H2O (V-SMOW) values range from −100 to −96 ‰, and δ¹³C_fluid (PDB) range from −3.8 to −2.5 ‰. In the middle stage, two typical immiscible fluid inclusion assemblages (FIA.18 and FIA. 27) composed of both Type-1 and Type-2 FIs were measured. The salinity of FIA.18 is 3.3 wt% NaCl equiv. and T_h is 320 °C. For FIA.27, the salinity is 2.2 wt% NaCl equiv. and T_h is 298 °C. The salinities of Type-4 FIAs vary from 5.9 to 14.4 wt% NaCl equiv., significantly higher than Type-1 and Type-2 FIs (1.7 to 3.5 wt% NaCl equiv). The salinities of three isolated Type-5 FIs (halite-bearing) range from 30.3 to 32.9 wt% NaCl equiv. and T_h vary from 257 to 277 °C. These facts indicate extreme unmixing between carbonic and aqueous fluids in the middle stage, which led to gold precipitation. The Type-3 FIs represent the salt-free, H₂O-lost, CO₂ end-member composition, whilst the Type-5 FIs and high-salinity Type-4 FIs represent the highly saline aqueous end-member composition. Late-stage fluids are initially liquid, very low-salinity (1.4 to 2.6 wt% NaCl equiv), and low-T_h (176–233 °C) aqueous fluids. Two immiscible carbonic FIAs from the middle stage yield a trapping pressure (P_t) of 86.9–133.9 MPa. Considering the rapid pressure fluctuations during fluid immiscibility, using hydrostatic or lithostatic pressure alone cannot get the real mineralization depth, Therefore, we use the lithostatic pressure for the P_t maximum and hydrostatic pressure for the P_t minimum and calculated a mineralization depth of 5.3–8.9 km, which doesn't represent the change but uncertainty for depth. Combining the characteristics of ore-forming fluids with the hydrothermal alteration and mineral compositions of ores, the Dunbasitao gold deposit is determined as a mesozonal orogenic gold deposit and there is considered to be prospecting potential in the deep.