Meijuan YAO¹, Junfeng SHEN¹ & Huapeng NIU^2
1State Key Laboratory of geological processes and mineral resources, China University of Geosciences, Beijing 100083, China. yaomeijuan1016@126.com
²China University of Petroleum, Beijing 102249, China

FLUID EVOLUTION AND ORE GENESIS OF THE QIANHE GOLD DEPOSITE, WESTERN HENAN PROVINCE, CENTRAL CHINA

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The Qianhe structure-controlled orogenic-mesothermal gold deposit is located in the Late Archean to Early Proterozoic metamorphosed volcanic and sedimentary rocks in the south margin of North China craton (NCC). Three stages of hydrothermal ore-forming processes are recognized, Early (E), Middle (M) and Late (L), characterized by coarse pyrite-quartz veins, fine pyrite-polymetallic sulfide veins and quartz-carbonate veins, respectively. Gold typically occurs as fracture-fillings associated with chalcopyrite and galena in M-stage. Fluid inclusions (FI) were examined in quartz from E-, M- and L-stage samples. FI petrography and microthermometric results suggest that three types of fluid inclusions are present at the deposit. Tm_CO2 and Th_CO2 in M-stage quartz range from -59.7 to -57.2°C and from 15.4 to 22.1°C, which are lower than those in E-stage quartz of ranging from -57.1 to -56.6°C and from 16.3 to 25.7°C. CO₂-rich inclusions have similar Tm_CO2 and Th_CO2 to those for CO₂-H₂O inclusions. CO₂-H₂O inclusions show salinities of 5.51 to 10.04 wt.% NaCl equivalent and Th of 242 to 336°C in E-stage quartz, and show salinities of 3.52 to 8.66 wt.% NaCl equivalent and Th of 213 to 240°C in M-stage quartz. Aqueous inclusions have a decreasing trend of salinities and Th from E-stage through M-stage to L-stage. Homogenization pressures of FI are estimated ranging from 367 to 872×10⁵ Pa. The M-stage fluid has the lowest contents of ions (e.g., SO₄^2-, Cl^-, K⁺) and (K+Na)/(Mg+Ca) but highest CO₂/H₂O ratios. The change in ore-forming fluids from K₂SO₄ type to NaCl type indicates the superposition of two hydrothermal mineralizing events. Together with the alkaline and reducing conditions, as indicated by decreased pH and increased Eh values, is most conducive to the deposition of polymetallic sulfides and native Au and electrum. Unlike in E-stage, FI in M- and L-stage quartz shows evidence of fluid immiscibility. The fact that most of gold is associated with M-stage quartz and sulfide but not with that of E-stage suggests that gold deposition occurs at the M-stage of fluid immiscibility. The sudden phase separation led to the deposition of large amounts of gold at the Qianhe deposit.