Geology, geochemistry and genesis of the Huachanggou gold deposit, western Qinling Orogen, central China

摘要

The large Huachanggou gold deposit, on the southern margin of the Mian-Lue Suture in western Qinling Orogen (central China), is hosted in the Devonian Sanhekou Group composed of spilite, limestone and phyllite. The mineralization is considered to have occurred in three stages:(1) Early quartz-pyrite veins, (2) Middle quartz-sulphides veins, and (3) Late carbonate-quartz veinlets, with gold being introduced mainly in the second stage. Quartz formed in the two earlier stages contains four types of fluid inclusions, namely (1) Pure CO_2, (2) CO_2-H_2O, (3) Solid-bearing and (4) NaCl-H_2O, whereas the late-stage minerals contain only the NaCl-H_2O inclusions. The inclusions in quartz formed in the early-, middle- and late stages have yielded total homogenization temperatures of 272-385 °C, 232-395 °C and 118-228 °C, respectively, with salinities no higher than 12 wt.% NaCl equiv. For the middle-stage, trapping pressures estimated from the CO_2-H_2O inclusions for the altered spilite-type (123-326 MPa) and quartz vein-type (132-342 MPa) orebodies correspond to mineralization depths of 12 km and 13 km, respectively. We suggest that fluid boiling and mixing may have caused rapid precipitation of metallic sulphides and native gold. Through boiling and influx of meteoric water, the ore-forming fluid system has evolved from CO_2-rich to CO_2-poor and from metamorphic to meteoric, as indicated by the decreasing δ~(18)O_(water) values from the early- to late stages. Integrating the evidence obtained from regional geology, ore deposit geology, fluid inclusion and C-H-O isotope geochemistry, we conclude that the Huachanggou gold deposit is best ascribed to be an orogenic-type system formed in the tectonic setting of the north-vergent oceanic plate subduction along the Mian-Lue Suture during the Late Triassic.