Metallogenic mechanism of the Wangu gold deposit, Jiangnan orogenic Belt: Constraints from the textures and geochemical compositions of gold-bearing pyrite

The Wangu gold deposit, with dominant mineralized slate, sulfide- quartz veins and breccia-styles mineralization, is one of the most representative gold deposits in the Jiangnan Orogen, southern China. The gold mineralization in the deposit can be divided into five stages, the pre-gold barren quartz Stage I and barren quartz- sericite- scheelite Stage II, syn-gold smoke gray quartz- sericite- arsenopyrite (Apy-1)- pyrite (Py-1)- gold Stage III, smoky gray quartz- sericite- pyrite (Py-2)- arsenopyrite (Apy-2)- gold- polymetallic sulfide Stage IV, and post-gold quartz- calcite Stage V. Pyrite can be divided into two generations, Py-1 (Stage III) and Py-2 (Stage IV). According to the mineralization styles, pyrite in the deposit can be divided into pyrite in mineralized slate (Py-a), pyrite in sulfide- quartz veins (Py-b) and pyrite in breccia-style ores (Py-c).According to the microscopic and backscattering microstructure observation of the two generations pyrite, Py-1 shows uniform dark color, porous microstructure, with a small amount of polymetallic sulfide in pores. Py-2 shows uniform bright color, smooth crystal surface, and a few Py-2 wraps Py-1 showing a zoned texture. Based on Laser Ablation Inductively Coupled Plasma Mass Spectrometer (LA-ICP-MS) trace element spot analysis, mapping and Laser Ablation Multicollector Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICP-MS) sulfur isotope analysis, Py-1 is rich in Co, Ni, Pb and Zn, poor in As and Au, and has high sulfur isotope composition (avg: −8.43 ‰). Py-2 is rich in As and Au, but low in Co, Ni, Pb and Zn, and relatively low in sulfur isotope composition (avg: −9.96 ‰), indicating that the ore-forming physico-chemical condition changed from reducing to oxidizing from Stage III to Stage IV. Furthermore, pyrite formed in different mineralization styles show distinct textural and geochemical characters. Py-a is mostly euhedral to subhedral and disseminated, while Py-b is generally subhedral particle forming aggregate. Py-c is mostly anhedral fine grainy. Elements As, Au, Co, Ni and Pb of Py-a are higher than that of Py-b and Py-c, while Ag and Zn are lower. Meanwhile, the δ34S values of Py-a (−11.51 ∼ 4.98 ‰) is higher than Py-c (−18.26 ∼ -5.87 ‰) and Py-b (−29.57 ∼ -7.69 ‰), which reflects a more open and oxidizing condition of sulfide- quartz veins system. Trace element spot analysis and mapping of pyrite show that gold mainly exists in pyrite in the form of solid solution, accompanied by a small amount of natural gold and nano-sized gold particles.The coupled relationship between Au and As in pyrite, and the significant solid solution of gold in pyrite, indicates the destabilization of gold-sulfur complexes in the fluid and the which formed are of gold into the pyrite lattice. This demonstrates that sulfidation is the dominant precipitation mechanism of invisible gold in the deposit. Sulfide- quartz veins undergo repeated “crack-seal” processes during the cyclic fluctuation of fluid pressure, which have formed “laminated” texture, and pyrite with high Ag content and oscillatory zoning. Abrupt drop in fluid pressure induced the escape of a large amount of gas, and fluid phase separation, which further resulted in the precipitation of native gold.