Origin and evolution of the ore-forming fluids of the Erdaogou and Xiaobeigou gold deposits, Jiapigou gold province, NE China

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The Jiapigou gold province, located in the eastern part of the northern margin of the North China Craton, includes the Erdaogou and Xiaobeigou gold deposits, which are hosted in amphibolites, amphibolitic gneisses, and tonalite-trondhjemite-granodiorite gneisses of the Jiapigou Group. The gold deposits occur as gold-bearing quartz veins and disseminated- and veinlet-type mineralization. Orebodies are strictly controlled by NW–SE -trending secondary faults. Four stages of mineralization have been identified in the Erdaogou gold deposit: (1) milky quartz, (2) pyrite-quartz, (3) native gold-quartz-pyrite, and (4) quartz-carbonate. In the Xiaobeigou gold deposit, five stages of mineralization have been identified: (1) milky quartz, (2) pyrite-sericite-quartz, (3) pyrite-quartz, (4) native gold-quartz-pyrite, and (5) quartz-carbonate. Three fluid inclusion populations have been identified in the deposits: CO2-H2O (C-type), aqueous (W-type), and pure CO2 (PC-type). C-type, W-type, and PC-type fluid inclusions are observed in the Erdaogou gold deposit, whereas C-type and W-type fluid inclusions are dominant in the Xiaobeigou gold deposit. Microthermometric data indicate that the fluid inclusions of the deposits homogenize at 128–370°C (0.18–15.35wt.% NaCl equivalent). Pressures estimated from fluid inclusion data cluster into two populations at 140–165MPa and 60–80MPa, indicating that mineralization occurred at a depth of 5.1–8.0km. The O-isotope ratios (δ18OH2O) of ore-forming fluids in the Xiaobeigou gold deposit range from −1.84‰ to −1.14‰, with δD of −102‰, whereas late-stage fluids have δ18OH2O of −9.40‰ to −9.20‰, and δD of −78‰. Integrated C-H-O-S-Pb-isotope data suggest that the initial ore-forming fluids of the deposits consisted of magmatic water produced by the partial melting of lower crustal material containing a mantle component. Water-rock interaction occurred in the early mineralization stage. Fluid immiscibility resulted in rapid gold deposition. Regional and deposit geology, fluid inclusions, and stable isotopes indicate that the Erdaogou and Xiaobeigou gold deposits are mesothermal, related to subduction of the Paleo-Pacific plate. The Jiapigou gold province has significant potential for deep-seated mineralization, and should be the focus of future exploration.