Ore-forming fluids associated with granite-hosted gold mineralization at the Sanshandao deposit, Jiaodong gold province, China

The Sanshandao gold deposit, with total resources of more than 60 t of gold, is located in the Jiaodong gold province, the most important gold province of China. The deposit is a typical highly fractured and altered, disseminated gold system, with high-grade, quartz-sulphide vein veinlet stockworks that cut Mesozoic granodiorite. There are four stages of veins that developed in the following sequence: (1) quartz-K-feldspar-sericite; (2) quartz-pyrite = arsenopyrite; (3) quartz-base metal sulfide; and (4) quartz-carbonate. Fluid inclusions in quartz and calcite in vein/veinlet stockworks contain C-O-H fluids of three main types. The first type consists of dilute CO₂ H₂O fluids coeval with the early vein stage. Molar volumes of these CO₂-H₂O fluid inclusions, ranging from 50-60 cm³/mol, yield estimated minimum trapping pressures of ≥13 kbar. Homogenization temperatures, obtained mainly from CO₂ H₂O inclusions with lower CO₂ concentration, range from 267-375 °C. The second inclusion type, with a CO₂-H₂O ± CH₄ composition, was trapped during the main mineralizing stages. These fluids may reflect the CO₂-H₂O fluids that were modified by fluid/rock reactions with altered wallrocks. Isochores for CO₂-H₂O ± CH₄ inclusions, with homogenization temperatures ranging from 204 325 °C and molar volumes from 55 to 70 cm³/mol, provide an estimated minimum trapping pressure of 1.2 kbar. The third inclusion type, aqueous inclusions, trapped in cross-cutting microfractures in quartz and randomly in calcite, are post-mineralization, and have homogenization temperatures between 143 228 °C and salinities from 0.71-7.86 wt% NaCl equiv. Stable isotope data show that the metamorphic fluid contribution is minimal and that ore fluids are of magmatic origin, most likely sourced from 120-126 Ma mafic to intermediate dikes. This is consistent with the carbonic nature of the fluid, and the crosscutting nature of those deposits relative to the host Mesozoic granitoid.