Geology, geochemistry and ore genesis of the Wenyu gold deposit, Xiaoqinling gold field, Qinling Orogen, southern margin of North China Craton

摘要

The Wenyu giant gold deposit is hosted in the Precambrian Taihua Supergroup metamorphic rocks within the Xiaoqinling terrane (Qinling Orogen), on the southern margin of the North China Craton. The mineralization can be divided into three stages: quartz-pyrite veins early, quartz-sulfide veins middle (main), and carbonate-quartz veinlets late, with gold being mainly introduced in main stage. Quartz formed in two earlier stages contains three compositional types of fluid inclusions, i.e. pure CO_2, CO_2-H_2O and NaCl-H_2O, but the late-stage minerals only contain the NaCl-H20 inclusions. The inclusions in quartz formed in the early, main and late stages yield total homogenization temperatures of 262-417℃, 236-407℃ and 114-239℃, respectively, with salinities no higher than 13 wt.% NaCl equiv. Trapping pressures estimated from CO_2-H_2O inclusions are 139-399 MPa and 111-316 MPa in the early and main stages, corresponding to mineralization depths of 14 km and 11 km, respectively. 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 CO_2-rich to CO_2-poor in composition, and from metamorphic to meteoric, as indicated by decreasing δ~(18)O_(water) 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, fluid inclusion and C-H-O-S-Pb isotope geochemistry, we conclude that the Wenyu 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.