Characteristics of mineralization and gold occurrence in Shaxi porphyry copper-gold deposit, central Anhui, China

Abstract

Through field geological surveys, mineralogical observation by microscopy, compositional analyses on minerals using EPMA, geochemical analyses on Cu-Au-bearing intrusive rocks, fluid inclusion and isotope studies, and element distributions detected by PIXE system, the characteristics of mineralization and gold occurrence in the Shaxi porphyry copper-gold deposit (Central Anhui Province, East China) have been fully investigated. The results of the study are summarized as the follows: (1) Altogether 5 types of copper (gold) ore can be detected, namely chalcopyrite ore, Cu-bearing pyrite ore, magnetite-chalcopyrite ore, pyrite-bornite-chalcopyrite ore and chalcopyrite-molybdenite ore; (2) The assemblage of ores mainly consists of chalcopyrite, pyrite, psedomorph hematite, bornite, magnetite, chalcocite, covellite and arsenopyrite; the non-metal minerals are quartz, K-feldspar, calcite, gypsum, anhydrite; (3) Liquid-vapour conditions have been documented and show that fluid inclusions frequently occur in quartz veins accompanied by pyritization-chalcopyritization-gold mineralization, mostly consisting of a coexisting liquid-gas phase and some are only with pure liquid phase. The ore-forming temperature is between 230-350°C; (4) Sulfur isotope studies show that the δ 3 4 S values are between 0.28-0.80 per mills for most of sulfides, δ 3 4 S values of chalcopyrite are somewhat homogenous those of pyrites; (5) The gold occurred in micro-inclusion sizes (<1 μm), heterogeneously distributed in chalcopyrite and pyrite in the ore deposit. The Au mineralization is positively correlated with As, S, Cu, Fe, Zn and Ti in chalcopyrite and pyrite. (6) The major controlling deposition of Au in the region is probably the oxidation potential of the red sedimentary rocks of shale and fine grain sandstone may be crucial as a geochemical barrier to the reducing fluid. Some fine-grained crystals of pyrite and pyrrhotite formed under conditions of very rapid crystallization from the hydrothermal solutions, in which the content of H 2 S decrease rapidly accompanied by destabilization of Au disulfides complexes. This study has provided the first evidences for the theoretical study of gold mineralization as well as its possible utilization during the mining process of the porphyry copper-gold deposit.