Genesis of the Xiaobeigou gold deposit in the Jiapigou gold district, Northeast China: Constraints from fluid inclusions, quartz H-O isotopes, and pyrite in situ sulfur isotope and trace element compositions

The Xiaobeigou gold deposit (>20 t @ 17.36 g/t) is located in the Jiapigou gold district (JGD), Northeast China, representing a large-sized quartz vein-type gold deposit hosted by Neoarchean basement gneiss and is related to Mesozoic intrusions in space and time. Four hydrothermal stages were identified from early to late, namely, (1) barren quartz stage, (2) quartz-pyrite stage, (3) gold-quartz-polymetallic sulfide stage, and (4) quartz-carbonate stage. Three main types of fluid inclusions (FIs) have been identified: water-rich (H2O-NaCl; W-type), mixed aqueous-carbonic (CO2-H2O-NaCl; C-type) and pure carbonic (CO2; PC-type) FIs. The early-stage quartz contains W- and C-type FIs that yield moderate homogenization temperatures (270-379 degrees C) and moderate-low salinities (4.82-17.87 wt% NaCl equivalent). The main-stage quartz grains contain all three types of FIs that yield medium-low temperature (179-263 degrees C) and moderate-low salinity (2.82-9.86 wt% NaCl equivalent). Only Wtype FIs are recognized in late-stage quartz grains, which have low temperatures (133-183.C) and low salinities (1.91-7.73 wt% NaCl equivalent). The initial ore-forming fluid of the Xiaobeigou gold deposit was a moderate homogenization temperature and moderate-low salinity CO2-H2O-NaCl +/- CH4 +/- N-2 hydrothermal fluid system. Fluid immiscibility is the main mechanism leading to the precipitation of gold and other metal sulfides. The H-O isotopic values (delta(OH2O)-O-18 = 6.87 to 5.90%; dD = 102 to 78%) and composition of the quartz suggest that the initial ore-forming fluids consisted mainly of magmatic water mixed with meteoric water during the late stage of the mineralization process. The S isotope data (delta S-34 = 3.2 to 4.4%, avg. 3.6%) indicate that the ore-forming materials are magmatic sources. EPMA and LA-ICP-MS data show that a large amount of gold mineralization mainly occurs in the main ore-forming stage. The high Co/Ni ratio indicates that the pyrite may be a magmatichydrothermal source. Taking into account all available data and regional geological history, we suggest that a magmatic-hydrothermal source for the Xiaobeigou gold deposit in the JGD and the ore formation is related to the subduction of the Paleo-Pacific Plate beneath Eurasia.