Ore-forming fluid evolution and gold precipitation mechanism at Huangjindong gold deposit, southern China: Insights from fluid inclusions and trace elements in quartz

Abstract

The representative Huangjindong gold deposit with >80 t gold is controlled by the NNE-NE-trending Changsha-Pingjiang Fault, in the central Jiangnan Orogen, south China. Petrography, microthermometry and Laser Raman studies of fluid inclusions in quartz from the deposit show that (1) two types of fluid inclusions are present (Type 1 H₂O-CO₂ inclusions, Type 2 aqueous inclusions) in all three stages. The homogenization temperature decreased through time with 330 ∼ 192 °C, 253 ∼ 180 °C and 232 ∼ 161 °C for the three stages, respectively; (2) The initial ore-forming fluids belong to CO₂-NaCl-H₂O system, with H₂O-CO₂-CH₄-bearing volatile gas, medium to high temperature (192 ∼ 330 °C), low-salinity (2.2 ∼ 8.9 wt% NaCl eqv.) and low density (0.745 ∼ 0.988 g/cm³). In-situ LA-ICP-MS trace element analysis of quartz of different stages show that the syn-gold auriferous quartz has higher trace element contents and mineral inclusions compared to the pre- and post-gold barren quartz. The initial deeply-sourced fluids migrated to the EW-NWW-trending third-order faults related to the EW-NWW-trending anticlines and synclines, forming large-scale milky barren quartz veins during the pre-gold stage at ≥330 ∼ 192 °C. During the mineralization, the characteristics of disseminated, quartz-sulfide veins and brecciate ores, phase separation characteristics showed by fluid inclusions, and the high content of trace elements indicate that fluid immiscibility and fluid-wallrock interaction triggered the instability of ${Au\left(HS\right)}_2^{-}$ and precipitation of Au, forming gray auriferous quartz-polysulfides veins that overprinted the early barren quartz veins at ≥253 ∼ 180 °C. Minor white quartz-carbonate veins formed under relatively lower temperature of ≥232 ∼ 161 °C and pressure marking the end of the ore-forming hydrothermal event.