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

Abstract

The Bajiazi gold deposit, located within the southeastern Jiapigou gold ore belt (JGB) along the northeastern corner of the North China Craton (NCC), contains > 6 tons of gold at an average grade of 24 g/t. More than 20 total auriferous quartz veins are hosted in the Neoarchean basement gneiss and amphibolite and are spatially and temporally associated with Mesozoic intermediate-acid dikes. The distribution of the gold orebodies is mainly controlled by NNE-trending brittle–ductile shear zones and faults. Three hydrothermal stages were identified, namely, quartz–pyrite (stage I), quartz–gold–polymetallic sulfide (stage II), and quartz–carbonate (stage III). Three types of fluid inclusions (FIs) were identified: H2O–NaCl (W-type), CO2–H2O (C-type), and pure CO2 (PC-type) FIs. From stage I to stage III, the FI suite changes from W-type and C-type in stage I to all three types in stage II and to only W-type in stage III. The FIs in the quartz of stages I to III mainly homogenized at temperatures (Th) of 283–395 °C, 183–287 °C and 116–186 °C, respectively, with salinities of 3.87 to 13.01 wt% NaCl equiv, 4.65 to 18.55 wt% NaCl equiv, and 3.83 to 9.68 wt% NaCl equiv, respectively. The ore-forming fluids of the Bajiazi gold deposit evolved from a H2O–NaCl–CO2 system in stages I and II to a H2O–NaCl system in stage III. Fluid immiscibility is an important mechanism leading to rapid precipitation of sulfides and gold. H–O–S isotope signatures indicate that the initial ore-forming fluids and materials were originally derived from magmatic sources. Electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) data on pyrite grains show that abundant gold mineralization occurred in the middle hydrothermal stage. The high Co/Ni ratios of pyrite are consistent with a magmatic–hydrothermal origin. The results, combined with previous regional studies, indicate that the Bajiazi gold deposit is a mesothermal magmatic–hydrothermal lode gold deposit.