Geochemistry of the ore-forming fluids in gold deposits from the Taihang Mountains, northern China

Gold deposits in the Taihang Mountains, northern China, mainly consist of quartz sulfide veins in granitoid plutons. This paper describes the geological setting of the gold deposits, and presents the results of microthermometric, Fourier transform infrared spectra, and stable isotope analyses of ore-forming fluids for the purpose of examining, the characteristics of these fluids. The ore-forming fluid was of high temperature (up to 380 degreesC) and high salinity (33-41 wt% NaCl equiv.), represented by type I inclusions (with daughter minerals). This fluid evolved to low salinity at low temperatures recorded in type II (liquid-rich) and III inclusions (vapor-rich). Primary type II inclusions coexist with type III inclusions in quartz. Type III inclusions have almost the same homogenization temperatures as type II inclusions. This probably reflects boiling. The secondary fluid inclusions homogenized at lower temperatures, and have lower salinities than primary inclusions. Based on microthermometric data. we propose that the high-temperature fluid that separated from residual magma corresponded to the ore-forming fluid represented by type I inclusions. This fluid mixed with meteoric water in the upper part of the granitic pluton and was diluted. The diluted fluid boiled, probably due to abrupt pressure decrease. and formed liquid-rich type II inclusions and vapor-rich type III inclusions. The deposition of sulfide minerals and gold probably occurred during boiling.