Trimodal distribution of CO2-bearing fluid inclusions in the gold — silver-bearing quartz veins of the Phuoc Thanh area, Central Vietnam: Its implication to the Au−Ag precipitation

Abstract

CO2-rich fluid inclusions from quartz veins provide information about pressure-temperature conditions, as well as effervescence, in the gold-silver-bearing quartz veins in the Phuoc Thanh area, Central Vietnam. Homogenization temperatures of fluid inclusion are bimodal, and support textural evidence that mineralization occurred in two stages. 90% of the fluid inclusions in the quartz are CO2-bearing. The trimodal distribution of CO2 mole fraction of the inclusions are common in the area, which indicates the CO2 unmixing, resulting the separation of the high (type IIb) and low (type IIa1) XCO2 fluids from the parental fluid (type IIa2) with intermediate XCO2. The precipitation of associated sulfide and Au−Ag minerals is related to CO2 effervescence with a concomitant decrease in sulfur activity. The temperature of mineralization is estimated as the range from 300 to 380°C in stage I and from 230 to 300°C in stage II. Pressure of ore formation ranged from 350 to 500 bars during both stages.