Genesis of the Yu'erya gold deposit, East Hebei, China: Insights from geology and fluid inclusions

The Yu'erya gold deposit is a large-scale gold deposit in eastern Hebei. However, the ore genesis is controversial because the gold orebody is closely related to biotite granite in time and space. The hydrothermal processes of the Yu'erya gold deposit can be divided into early quartz (Q1)-pyrite stage, middle quartz (Q2)-sulphide stage and late quartz (Q3)-calcite stage. The Q1 is subdivided into Q1-1 and Q1-2 generations by SEM-CL images, and Q2 is subdivided into Q2-1 and Q2-2 generations. Three types of fluid inclusions (FIs), including aqueous-carbonic type (C-type), aqueous-rich type (W-type) and vapour-rich type (V-type), were observed in the Yu'erya gold deposit. The Q1-1 is dominated by C-type inclusions, and the other quartz is dominated by W-type inclusions. Laser Raman analysis shows that the initial ore-forming fluids mainly belong to the CO₂-H₂O-NaCl system and gradually transferred to the H₂O-NaCl system. The C-type inclusions of Q1-1 show the salinity of 3.1–10.1 (av. 6.9 ± 1.7) wt.% NaCl eqv., and the total homogenization temperature (T_h) of 305–391°C. The homogenization temperature decreases gradually from the early to the late stage, with insignificant change in salinity, indicating that fluid cooling is the main precipitation mechanism. Trapping pressures of C-type FIs are estimated at 206–284 MPa and 203–211 MPa for the early and middle stages, corresponding to the mineralization depths of 7.2–9.9 km and 7.1–7.4 km, respectively. The FIs in the early stage indicate that the initial ore-forming fluids are characterized by low salinity and CO₂ richness, which are different from those of the intrusion-related gold deposits, but consistent with the orogenic gold deposits.