MA Ke-zhong, CHEN Mao-hong*

(Institute of Mineral Resources, Chinese Academy of Geological Science，Beijing 100037, China)

Abstract: The Baiyong and Baipeng gold deposits are two small Carlin type gold deposits in the Northwest Guangxi. Compared with Mingshan and Jinya gold deposits in the same area, they are characterized by the gold-bearing minerals of coarse-grained pyrites and arsenopyrites with their sizes of pyrite and arsenopyrite grains up to 10－15 mm and about 15 mm, respectively. Coarse arsenopyrite is often disseminated or intersected with pyrite. Electron microprobe analytical results show that pyrites of the Baiyong gold deposit are characterized by high As contents (0.63%－2.35%), low contents of S and Fe, and negative correlation between As and S, similar to those of the common Carlin type gold deposits. However, the arsenic-bearing pyrite has homogeneous structure with no obvious element zoning, indicating that the coarse-grained pyrite in the Baiyong gold deposit was formed in a relatively stable environment of hydrothermal fluid. The arsenic-bearing pyrite in the Baipeng gold deposit has zoning structure with low arsenic content in the core and high arsenic content (2.309%) in the 100－200 μm wide rim, which is different from those (narrow and dense zones of about 5 μm in width) of the pyrite in the Mingshan gold deposit. It indicates that the pyrite of the Baipeng gold deposit may be formed in an environment between the formation environment of the Baiyong gold deposit and that of the Mingshan gold deposit, suggesting that the wide high-arsenic zone of pyrite in the Baipeng deposit could be formed due to the relatively long activity cycle and sufficient supply of the hydrothermal fluid. The results of EPMA analysis show that Au in the Baiyong and Baipeng gold deposits occurred in arsenic-bearing pyrites and arsenopyrites in the form of solid solution (Au+), and the Au in ore-forming fluid was unsaturated. Because the growth of arsenic-bearing pyrite is under relatively stable environment in sufficient fluids, there are fewer vacancies and lattice defects on the surface of pyrite crystals without enough room to be occupied by gold. In addition, a large amount of iron is provided from the Fe-dolomite in the strata for the formation of pyrite. This further resulted in the low possibility of surficial vacancies and lattice defects of pyrite crystals during their formation process, and ultimately led the very low gold content in pyrite. At the same time, As contents of arsenian pyrites in the Baiyong and Baipeng gold deposits are generally lower than those of arsenian pyrites in typical Carlin type gold deposits, which leads to the lack of "catalyst" for Au entering into pyrite, resulting in the low gold grade of ores in these two gold deposits. This suggests that coarse grained pyrite and arsenopyrite are not very favorable for the formation of Carlin type gold deposits.

Keywords: Baiyong gold deposit; Baipeng gold deposit; occurrence state of gold; mineralogy

ACTA MINERALOGICA SINICA Vol. 40, No.5, 2020, page 601-614