Genesis of the Wulong gold deposit, northeastern North China Craton: Constraints from fluid inclusions, H-O-S-Pb isotopes, and pyrite trace element concentrations

Abstract

The Wulong vein-type gold deposit, located along the northeastern margin of the North China Craton (NCC), contains > 80 tonnes of gold at an average grade of 5.35 g/t. A total of 380 auriferous quartz veins are hosted in Late Jurassic gneissic two-mica granite and Early Cretaceous granodiorite. A paragenetic sequence, representing three hydrothermal stages, has been identified: (1) quartz ± pyrite; (2) quartz-polymetallic sulfide; and (3) quartz-calcite ± pyrite. Petrography, fluid inclusion microthermometry, and laser Raman spectroscopy revealed three types of fluid inclusions: CO2 ± CH4 (type I), H2O-CO2 ± CH4 (type II), and H2O-NaCl (type III). Fluid inclusion data indicate that during mineralization, fluid temperatures evolved from 283 to 395 °C to 219–328 °C to 144–255 °C, and salinities varied from 0.70 to 8.95 wt% to 0.18–7.86 wt% to 0.18–4.96 wt% NaCl equivalent. Quartz yielded δ18OH2O-SMOW and δDSMOW values of 0.9–7.3‰ and −65‰ to −48‰, respectively, indicating that the ore-forming fluids were derived from magmatic water and mixed with meteoric water during the later stages of mineralization. Sulfides have δ34SCDT sulfur isotopic compositions ranging from 0.9‰ to 3.9‰, suggesting that the S was derived from a magmatic source. Pb isotopic compositions of sulfides (206Pb/204Pb = 17.488–17.718, 207Pb/204Pb = 15.565–15.631, and 208Pb/204Pb = 38.190–38.766) indicate that the Pb was derived primarily from the upper crust, with minor input from the mantle. In situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) trace element analyses of three generations of pyrite (Py1, Py2, and Py3) reveal that gold concentrations in Py2 are higher than those in Py1 and Py3, which indicates that gold mineralization occurred primarily during the middle stage. High Co/Ni ratios of pyrite are consistent with a hydrothermal origin. We suggest that the Wulong gold deposit formed during lithospheric thinning associated with destruction of the NCC, this process was triggered by subduction of the Paleo-Pacific plate during the Early Cretaceous.