Genesis of the Shirengou gold deposit, northern North China Craton, based on zircon U-Pb, fluid inclusion, sulfide compositional and S isotope data

The newly discovered vein-type Shirengou gold deposit on the northern edge of the North China Craton contains at least 8.69 tons of Au at an average grade of 9.80 g/t. Six auriferous quartz veins have been identified within Upper Jurassic rhyolitic breccia-bearing crystal-lithic tuff, and they follow NW-trending secondary faults. Gold mineralization was closely related to silicification, with pyrite as the predominant ore mineral. Four mineralization stages can be discerned, involving Stage I milky quartz, Stage II quartz-pyrite, Stage III gray quartzpolymetallic sulfide, and Stage IV quartz-calcite +/- pyrite. Zircon U-Pb dating of a pre-ore porphyryritic rhyolite dike sets an upper age limit on the gold mineralization of -131.1 Ma. Two-phase and single-phase aqueous inclusions were observed in quartz related to all four mineralization stages. From Stage I to Stage IV, the fluid inclusion homogenization temperatures decrease from respectively 257-340 degrees C to 230-277 degrees C, 201-250 degrees C and 161-200 degrees C, with corresponding salinities of 3.39-9.21, 2.24-8.00, 1.22-7.59, and 0.18-3.55 wt % NaCl equivalent, respectively. Thus, the ore-forming fluid can be described as moderate-high temperature, moderate-low salinity H2O-NaCl fluids. The delta 34S values of sulfides vary from 0.0 to 5.3 parts per thousand, indicating that the S was derived primarily from magmatic source(s), with some contribution from the metamorphic rocks of the surrounding Jianping Group. In-situ LA-ICP-MS trace element analysis of sulfides, together with our new age data, suggest that the ore-forming fluid was initially of Early Cretaceous magmatic-hydrothermal origin, and subsequently modified by interaction with the wall rocks of the Jianping Group, extracting the ore-forming metals from these rocks into the fluids.