Ore-forming mechanism and physicochemical evolution of Gutaishan Au deposit, South China: Perspective from quartz geochemistry and fluid inclusions

Abstract

The Gutaishan is an important gold deposit of the Jiangnan Orogen in South China. The deposit is characterized by multiphase alteration/mineralization, including barren quartz (pre-ore Stage I) quartz + dolomite + pyrite + arsenopyrite (Stage II), quartz + dolomite + native gold + pyrite + arsenopyrite + stibnite (Stage III), and quartz + dolomite ± pyrite (Stage IV).Cathodoluminescence (CL) observations show that each of Stage II and III quartz comprises two sub-stages (Stage II: QII1 and QII2; Stage III: QIII1 and QIII2), and Stage IV quartz (QIV) contains no sub-stages. QIII1 has distinct core-rim texture, with the CL intensity decreasing from core to rim. QII1, QII2, QIII2 and QIV are texturally more homogeneous. Variations of aluminium concentration correlate with the CL intensity and reveal the determinated of pH in fluids during Stage II and III, with marked decline to Stage IV. Fluid inclusions (FIs) trapped in quartz are liquid-rich and CO2-rich, suggesting a CO2-H2O-NaCl fluid system. Fluid inclusion microthermometry data suggest that the ore-forming fluids were under conditions of medium-low salinity (Stage II: 1.02–11.46 wt% NaCl equiv.; Stage III: 0.83–9.84 wt% NaCl equiv.; Stage IV: 4.39–8.10 wt% NaCl equiv.) and temperature (Stage II: 190–285 °C; Stage II: 170–270 °C; Stage II: and 163–240 °C), consistent with the low Ti content (<10 ppm) in the quartz. Fluid boiling, coupled with decrease of temperature, may have controlled gold precipitation at Gutaishan, based on the coexistence of liquid-rich and CO2-rich FIs with similar homogenization temperatures and large salinity variations. Integrating regional tectonic evolution, previous H–O–S–Pb data, and close temporal and spatial relationship with the Baimashan granite rocks, we propose that the Gutaishan gold mineralization may have been genetically linked to the Mesozoic magmatism related to the Baimashan granite rocks formed in an extensional setting during subduction of the Paleo-Pacific plate.