OCCURRENCE AND DISTRIBUTION OF GOLD IN THE QIULING CARLIN-TYPE GOLD DEPOSIT, WESTERN QINLING OROGEN, CHINA

The Qiuling gold deposit is located in the western Qinling Orogen and has mineralization characteristics typical of Carlin-type gold deposits in Nevada. The deposit is hosted in the Upper Devonian Nanyangshan Formation and Lower Carboniferous Yuanjiagou Formation, both consisting of various amounts of siltstones, argillaceous siltstones, silty shales, and argillaceous limestones. Gold mineralization is strictly structurally controlled, and ore bodies are mostly hosted in the northeast- and northwest-striking faults.

The main metallic minerals are arsenopyrite, arsenian pyrite, and minor stibnite, intergrown with alteration minerals dominated by quartz, calcite, sericite, and ankerite. There are three types of pyrite: framboid pyrite, fine-grained pyrite (< 100 μm), and coarse-grained pyrite. The fine-grained pyrite is commonly composed of an arsenic-poor core and arsenian overgrowths. Laser ablation ICP-MS analyses revealed that the arsenian rims of the fine-grained pyrite contain 17 to 83 ppm Au, whereas both the arsenic-poor cores and the coarse-grained pyrite commonly have low Au contents (<12 ppm). Gold concentrations up to 118 to 800 ppm were sporadically identified in the framboid pyrite, in the cores of fine-grained pyrite, and in the coarse-grained pyrite grains as well. Arsenopyrite contains 160 to 1799 ppm Au. Electron microprobe, laser ablation ICP-MS analyses, and scanning electron microscopy results indicate that gold occurs predominantly as solid solution in arsenopyrite and arsenian pyrite, but nanoparticles of native gold are also identified. Visible gold grains (0.3-6 μm) are locally present within sericite, quartz, and calcite that may have formed by leaching and recrystallization of solid solution of gold once locked in the arsenian pyrite and arsenopyrite.

The results presented here indicate that the Qiuling gold deposit was a result of three stages of gold mineralization: (1) pre-enrichment of gold in diagenetic pyrite in the host rocks, (2) sulfidation of the host rocks by acidic, auriferous, and sulfur- and arsenic-rich fluids, and (3) hydrothermal overprints of the early precipitated arsenian pyrite and arsenopyrite that caused deposition of the visible gold grains. Stage 2 was the most economically important in the formation of the Qiuling gold deposit.