Isotope and fluid inclusion geochemistry and genesis of the Qiangma gold deposit, Xiaoqinling gold field, Qinling Orogen, China

Abstract

The Qiangma gold deposit is hosted in the >1.9Ga Taihua Supergroup metamorphic rocks in the Xiaoqinling terrane, Qinling Orogen, on the southern margin of the North China Craton. The mineralization can be divided as follows: quartz-pyrite veins early, quartz-polymetallic sulfide veinlets middle, and carbonate-quartz veinlets late stages, with gold being mainly introduced in the middle stage. Three types of fluid inclusions were identified based on petrography and laser Raman spectroscopy, i.e., pure carbonic, carbonic-aqueous (CO2–H2O) and aqueous inclusions.The early-stage quartz contains pure carbonic and CO2–H2O inclusions with salinities up to 12.7wt.% NaCl equiv., bulk densities of 0.67 to 0.86g/cm3, and homogenization temperatures of 280−365°C. The early-stage is related to H2O–CO2±N2±CH4 fluids with isotopic signatures consistent with a metamorphic origin (δ18Owater=3.1 to 5.2‰, δD=−37 to −73‰). The middle-stage quartz contains all three types of fluid inclusions, of which the CO2–H2O and aqueous inclusions yield homogenization temperatures of 249−346°C and 230−345°C, respectively. The CO2–H2O inclusions have salinities up to 10.9wt.% NaCl equiv. and bulk densities of 0.70 to 0.98g/cm3, with vapor bubbles composed of CO2 and N2. The isotopic ratios (δ18Owater=2.2 to 3.6‰, δD=−47 to −79‰) suggest that the middle-stage fluids were mixed by metamorphic and meteoric fluids. In the late-stage quartz only the aqueous inclusions are observed, which have low salinities (0.9−9.9wt.% NaCl equiv.) and low homogenization temperatures (145−223°C). The isotopic composition (δ18Owater=−1.9 to 0.5‰, δD=−55 to −66‰) indicates the late-stage fluids were mainly meteoric water.Trapping pressures estimated from CO2–H2O inclusions are 100−285 MPa for the middle stage, suggesting that gold mineralization mainly occurred at depths of 10km. Fluid boiling and mixing caused rapid precipitation of sulfides and native Au. Through boiling and inflow of meteoric water, the ore-forming fluid system evolved from CO2-rich to CO2-poor in composition, and from metamorphic to meteoric, as indicated by decreasing δ18Owater values from early to late. The carbon, sulfur and lead isotope compositions suggest the hostrocks within the Taihua Supergroup to be a significant source of ore metals. Integrating the data obtained from the studies including regional geology, ore geology, and fluid inclusion and C–H–O–S–Pb isotope geochemistry, we conclude that the Qiangma gold deposit was an orogenic-type system formed in the tectonic transition from compression to extension during the Jurassic−Early Cretaceous continental collision between the North China and Yangtze cratons.