Process and Mechanism of Gold Mineralization at the Zhengchong Gold Deposit, Jiangnan Orogenic Belt: Evidence from the Arsenopyrite and Chlorite Mineral Thermometers

The Zhengchong gold deposit, with a proven gold reserve of 19 t, is located in the central part of Jiangnan Orogenic Belt (JOB), South China. The orebodies are dominated by NNE- and NW- trending auriferous pyrite-arsenopyrite-quartz veins and disseminated pyrite-arsenopyrite-sericite-quartz alteration zone, structurally hosted in the Neoproterozoic epimetamorphic terranes. Three stages of hydrothermal alteration and mineralization have been defined at the Zhengchong deposit: (i) Quartz⁻auriferous arsenopyrite and pyrite; (ii) Quartz⁻polymetallic sulfides⁻native gold⁻minor chlorite; (iii) Barren quartz⁻calcite vein. Both invisible and native gold occurred at the deposit. Disseminated arsenopyrite and pyrite with invisible gold in them formed at an early stage in the alteration zones have generally undergone syn-mineralization plastic-brittle deformation. This resulted in the generation of hydrothermal quartz, chlorite and sulfides in pressure shadows around the arsenopyrite and the formation of fractures of the arsenopyrite. Meanwhile, the infiltration of the ore-forming fluid carrying Sb, Cu, Zn, As and Au resulted in the precipitation of polymetallic sulfides and free gold. The X-ray elements mapping of arsenopyrite and spot composition analysis of arsenopyrite and chlorite were carried out to constrain the ore-forming physicochemical conditions. The results show that the early arsenopyrite and invisible gold formed at 322⁻397 °C with lgf(S2) ranging from −10.5 to −6.7. The crack-seal structure of the ores indicates cyclic pressure fluctuations controlled by fault-valve behavior. The dramatic drop of pressure resulted in the phase separation of ore-forming fluids. During the phase separation, the escape of H2S gas caused the decomposition of the gold-hydrosulfide complex, which further resulted in the deposition of the native gold. With the weakening of the gold mineralization, the chlorite formed at 258⁻274 °C with lgf(O2) of −50.9 to −40.1, as constrained by the results from mineral thermometer