Discovery and significance of native gold in the Zhaishang gold deposit, southern Gansu, China

Located in the western part of the Min-Li metallogenetic belt, western Qinling Mountains, the Zhaishang gold deposit is a recently discovered large Carlin-type disseminated gold deposit. Examination of ores by optical microscope, electron probe, X-ray surface scanning, chemical analyses from the deposit, has revealed some native gold minerals. Under the reflecting microscope, the native gold minerals have a variety of forms such as oval, flake, triangular, rod-like, filamentous, irregular, which range in size from 5 to 150 μm. Electron microprobe analysis of seven points on native gold gave the results as follows: Au 88.23% to 92.73% (91.29 on average )，Ag 7.41% to 9.08% (8.08 on average), which range in fineness from 905 to 926. The carriers are arsenian pyrite, galena, coloradoite and ankerite. Gold is embodied as intergranular, fissure-filling, and interlocking form. Dissolution of ferroan limestone in the host rocks and sulfidation of the dissolved iron by H2S introduced by ore fluids were the most important depositional mechanism for visible gold grains in the Zhaishang gold deposit.