Research On Ore-forming Fluids Of Gold Deposits In Jiapigou-Haigou Gold Belt, Jilin Province And Deep-seated Metallogenic Assessment

The Jiapigou-Haigou gold belt is located in the eastern part of the northern margin of the North China Craton. The tectonic history of the region is dominated by the evolution of the Paleao-Asian Ocean and several micro-continental blocks collided together making up the eastern part of Central Asia Orogenic Belt. It has undergone tectonism related to the Paleao-Pacific tectonic domain since the Mesozoic. As such, the region records numerous tectono-magmatic events and contains a significant number of hydrothermal Au mineralization, which has been concerned by many domestic and international geologists for a long time. Erdaogou, Xiaobeigou and Haigou gold deposits are the most representative ones here. The Erdaogou and Xiaobeigou gold deposits are located in the northwestern part of the gold belt, which are the most important part of the Jiapigou gold Au ore-concentrated area, as well as the Haigou gold deposit is in the southeast. The three gold deposits occurring as gold-bearing quartz vein and/or disseminated- and veinlet-style mineralization, alternately, are strictly controlled by secondary faults. The Erdaogou and Xiaobeigou gold deposits are hosted in the Jiapigou Group composed of amphibolite, amphibolitic gneiss, and granitic gneiss, and the Haigou gold deposit is mostly hosted in the monzonite and monzogranite, partially in Seluohe Group. Four stages of mineralization in the Erdaogou gold deposit have been identified: milky quartz, pyrite-quartz, native gold-quartz-pyrite, and quartz-carbonate; in the Xiaobeigou gold deposit, five detailed stages of mineralization have been divided: milky quartz, pyrite-sericite-quartz, pyrite-quartz, native gold-quartz-pyrite, and quartz-carbonate; in the Haigou gold deposit, there are four detailed stages of mineralization: milky quartz, pyrite-quartz, native gold-quartz-pyrite, and quartz-carbonate.Three main types of fluid inclusion are observed within the Erdaogou gold deposit: CO2-H2O(C-type), aqueous(W-type), and pure CO2(PC-type). Microthermometric data indicate that the FIs associated with metallogenesis homogenize at 128–370°C(cluster in a range of 190–270°C in the main stage), yielding a salinity of 0.18–15.35 wt.% NaCl eqv., and a density of 0.76–0.89 g/cm3. The estimated pressures are clustered into two groups of 140–165 MPa and 60–80 MPa, yielding a metallogenic depth of 5.1–8.0 km. The ore-forming fluids belong to medium- to low-temperature, medium- to low-salinity, low density and CO2-inrich CO2-H2O-NaCl system. In the Xiaobeigou gold deposit, C-type and W-type FIs are dominated and few PC-type FIs can be observed. Microthermometric data indicate that the FIs associated with metallogenesis homogenize at 144–348°C(cluster in a range of 190–270°C in the main stage), yielding a salinity of 1.02–11.03 wt.% NaCl eqv., and a density of 0.76–0.96 g/cm3. The estimated pressures are in a range of 60–175 MPa, which gradually reduce from early stage to late, yielding a metallogenic depth of 5.1–8.0 km. The ore-forming fluids belong to medium- to low-temperature, medium- to low-salinity, low density and CO2-inrich CO2-H2O-NaCl system. In the Haigou gold deposit, three main types of fluid inclusion are observed. Microthermometric data indicate that the FIs associated with metallogenesis homogenize at 145–410°C(cluster in a range of 190–270°C in the main stage), yielding a salinity of 1.23–11.93 wt.% NaCl eqv., and a density of 0.49–0.93 g/cm3. The estimated pressures are in a range of 90–275 MPa, which gradually reduce from early stage to late, yielding a metallogenic depth of 5.1–9.8 km. The ore-forming fluids belong to medium- to low-temperature, medium- to low-salinity, low density and CO2-inrich CO2-H2O-NaCl system. The results of the analysis of fluid inclusion gaseous phases by laser Raman spectroscopy within the Erdaogou, Xiaobeigou, and Haigou gold deposits indicate that the main volatiles within fluid inclusions trapped during the mineralization are CO2 and H2 O, commonly containing variable traces of N2, CH4, CO, and H2, reflecting a reducing condition.The Xiaobeigou gold deposit hydrogen and oxygen isotope data in this study show that the ?18OH2O values vary between –1.84 ‰ and –1.14 ‰, with the ?D value of –102 ‰ in the main stage, and the ?18OH2O values vary from –9.40 ‰ to –9.20 ‰, with the ?D value of –78 ‰ in the late stage. The hydrogen and oxygen isotope data show that the ?18OH2O values vary between 6.7 ‰ and 8.4 ‰, with the ?D value varying between –80 ‰ to –64 ‰ in the early stage. Integrated C-H-O-S-Pb isotope analysis indicates that the initial ore-forming fluids of the three gold deposits are mainly magmatic water in original derived from magmas produced by partial melting of the lower crust with mantle input. The intensive water-rock interaction is widespread in the early stage. Fluid immiscibility and mixing are the two most important factors resulting in rapid gold and other sulphides deposition. The ore-forming fluids subsequently evolved into meteoric waters in the late stage.The geochronology researches in the Haigou gold deposit show that it formed during the Middle Jurassic, as evidenced by fluid inclusion 40Ar/39 Ar dating(172–170 Ma), hydrothermal sericite 40Ar/39 Ar dating(161.9±1.3 Ma), and diorite porphyrite zircon U-Pb dating(131–122 Ma). The Huangnihe monzonitic granite U-Pb age is 167±3 Ma, as well as the Dapuchaihe quartz diorite U-Pb age is 164.9±0.8 Ma. Combined with the geochronology data within the Jiapigou-Haigou gold belt, gold metallogenic event in the Jiapigou area occurred during ~200 Ma and the Haigou gold deposit was formed in the Early Jurassic(161 Ma), which coincided with those of the emplacement of the intrusives.Integrating the evidences obtained from regional geology, ore geology, fluid inclusion, geochronology, and stable isotope geochemistry, it is concluded that the Erdaogou, Xiaobeigou and Haigou gold deposits are best ascribed to be mesothermal deposits formed in a paleao-Pacific plate subduction-related environment(intracontinental block collision-related environment). During the late Indosinian, it was a post-collision tectonic setting in the study area. In the Yanshanian epoch, this area is dominated by the subduction of the palaeo-Pacific plate beneath the Eurasian continent leading causing the intracontinental blocks collision. All the three gold deposits formed during the transition from compressive to extensional tectonics. In addition, all the three gold deposits have wide stable mineralization settings. With a comparison of the typical mesothermal gold deposit, this area has significant potential for deep-seated mineralization, an area that should be the focus of future exploration.