Origin and evolution of the ore-forming fluids of the Bajiazi gold deposit in the Jiapigou gold ore belt, NE China: Constraints from fluid inclusions, H–O–S isotopes, and pyrite trace element concentrations

•The ore-forming fluids formed in a CO2–H2O–NaCl ± CH4 ± N2 hydrothermal system.•Fluid immiscibility mechanism is the key factor for the precipitation of Au and other sulfides.•H-O-S isotope signatures indicate that the initial ore-forming fluids and materials were derived from magmatic sources.•The high Co/Ni values of pyrite are consistent with a magmatic–hydrothermal origin.•The Bajiazi gold deposit is a magmatic–hydrothermal ore deposit.