Characteristics of mineral association and mechanism of Au precipitation in the Zhaishang gold deposit, southern Gansu

Located in the western part of the Minxian County-Lixian County metallogenic belt, West Qinling Mountains, the Zhaishang gold deposit is a recently discovered large-sized Carlin-type disseminated gold deposit. Examination of ores by optical microscope, electron microprobe and X-ray surface scanning has revealed that the ore deposit is quite complex in mineralogy. Up to now more than 40 minerals have been identified. These minerals include not only sulfides and sulfosalt of Cu, Pb, Zn, As, Mo, Ni and Sb-bearing minerals, a variety of oxides, sulfate, carbonate, and tungstate minerals but also visible gold grains, tellurides, native metals, and polymetallic intermetallic compounds. Among the most common minerals there are native gold, pyrite, chalcopyrite, tetrahedrite, galena, sphalerite, stibnite, quartz, calcite, ankerite, siderite, and barite. The identification of some rare minerals in the Carlin-type gold deposits such as chalcostibnite, bournonite, zinkenite, molybdenite, coloradoite, melonite, native metals, Cu-Zn-Ni-Sn-Fe intermetallic compounds and scheelite is of great significance. The diversity of mineral species in the ores and the occurrence of visible native gold minerals constitute outstanding features of the gold deposit. Dissolution of ferruginous limestone in the host rocks and sulfidation of the dissolved iron by H2S introduced by ore fluids were not only the most important depositional mechanism for visible gold grains but also the advantageous conditions for gold enrichment in the Zhaishang gold deposit.