Abstract: The Jiapigou gold ore concentration area, located in the collage section between the northeastern margin of the North China craton and the Xing’an-Mongolian orogenic belt, is one of the most important gold-producing areas in China. However, the origin of the ore-forming fluids and genetic types of the gold deposits in the Jiapigou gold ore concentration area  are still widely disputed. Moreover, there is still a chronic lack of systematic research on evolution of the ore-forming fluids and ore-forming mechanism. To address the problems above, this paper selects the most representative Sandaocha gold deposit in the Jiapigou gold ore concentration area  as an example to carry out a detailed analysis of fluid inclusions, laser Raman spectroscopy and H-O isotopes. Four stages of mineralization have been identified in the Sandaocha gold  deposit: (Ⅰ) milky quartz, (Ⅱ) quartz-pyrite, (Ⅲ) quartz-polymetallic sulfide, and (Ⅳ) quartz-carbonate. The stages Ⅱ and Ⅲ represent the main gold mineralization. Three types of primary fluid inclusions (FIs) are identified in quartz formed at different stages: NaCl-H2O-CO2 (C-type), NaCl-H2O (W-type) and pure CO2 (PC-type). The early-stage quartz contains C- and W-type FIs, which have homogenization temperatures of 283-411 ℃ and salinities of 4.26%-17.48% NaCl equiv. The main-stage quartz contains all three types of FIs, with homogenization temperatures of 210-288 ℃ and salinities of 2.07%-15.76% NaCl equiv. The late-stage quartz contains only W-type FIs with homogenization temperatures of 131-210 ℃ and salinities of 2.57%-14.04% NaCl equiv. The H-O isotope results indicate that the initial ore-forming fluid is the mixture of magmatic water and a small amount of meteoric water, and the meteoric water was continuously added to the ore-forming fluids during mineralization. The ore-forming fluid system evolved from a moderate-temperature and moderate- to low-salinity NaCl-H2O-CO2 system in the early stage to a NaCl-H2O system with a low temperature and salinity in the late stage. Water-rock interactions generally occurred between ore-forming fluids and metamorphic wall-rocks during fluids migration. The sudden decompression and the addition of meteoric water led to fluid immiscibility and the exsolution of CO2, H2S, and other volatiles. This process destroyed the stability of Au-S complexes and facilitated the precipitation of gold and other ore-forming elements. Comprehensive studies of geology, fluid inclusions and H-O isotopes confirm that the Sandaocha gold deposit is a mesothermal hydrothermal vein gold deposit.

Key words: fluid inclusion, H-O isotope, ore-forming fluid, Sandaocha gold deposit, Jiapigou gold ore concentration area