Fluid immiscibility and gold deposition in the Xincheng deposit, Jiaodong Peninsula, China: A fluid inclusion study

The Xincheng gold deposit, located in west Jiaodong Peninsula in southeast North China Craton, is a representative mesothermal lode deposit hosted in Late Mesozoic granitoids in Jiaodong. Gold mineralization occurs as disseminated- and stockwork-style ores within the hydrothermal breccias and cataclastic zones controlled by the Jiaojia fault, whereas echelon tensile auriferous veins hosted in the NE- and NNE-trending subsidiary faults cutting the granitoids occur subordinately. According to crosscutting relationships and mineral paragenesis, four paragenetic stages were identified, which are pyrite–quartz–sericite (stage 1), quartz–pyrite (stage 2), quartz–polysulfide (stage 3) and quartz–carbonate (stage 4). Gold was deposited during the quartz–pyrite and quartz–polysulfide stages. On the basis of microthermometry and Raman spectroscopy on fluid inclusions contained within the quartz veins from stages 2 and 3, three types of fluid inclusions were recognized: (1) type 1 H2O–CO2 inclusions that show high temperatures (ca. 260 °C), low salinities (2.4–8.9 wt.% equiv. NaCl) and variable XCO2 (0.03 to 0.20), (2) type 2 aqueous inclusions with medium temperatures (ca. 220 °C) and low to moderate salinities (3.1–13.3 wt.% equiv. NaCl); (3) type 3 pure CO2 inclusions with a carbonic phase density of 0.712 ± 0.03 g/cm3. Types 1 and 2 inclusions appear in the same growth phase of the quartz grains from the breccias and tensile auriferous veins. These coexisting inclusions are likely formed by fluid immiscibility due to unmixing from a single homogeneous H2O–CO2 parent fluid at trapping P–T conditions of 221 to 304 °C (average 261 ± 19 °C) and 780 to 2080 bar. The fluid immiscibility is interpreted to be initiated by fluid pressure decrease at ca. 300 °C. The ore-fluid P–T–X conditions of the Xincheng gold deposit are the same as those for mesothermal deposits. Gold was most probably transported as a Au(HS)2− complex at Xincheng. Fluid immiscibility over the temperature interval of 221–304 °C resulted in significant H2S loss from the hydrothermal solution, thereby reducing Au(HS)2− solubility with concomitant deposition of gold. The mineralizing process of the granitoid-hosted Xincheng lode-gold deposit is likely related to the fluid immiscibility.