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Abstract
The authors discuss the ore-forming temperatures, orebody size and mineralization stages of the Qianhe gold deposit by using the thermoelectricity and decrepitation temperatures of pyrite from the deposit and evaluate the ore prospects at depths in the ore district by using mineralogical mapping. The study results show that the ore-forming temperatures of the deposit range from 222.3 to 360.1°C, indicating that the deposit is a mesothermal one. The thermoelectric properties of pyrite in all the levels are of the N type, with the total content reaching 81.4%. Spatially, the degree of erosion of the orebody shows the rhythmic feature, i.e. being from high to low and again to high, which indicates that the gold orebody is persistent form the 200 to 520 m levels and has a tendency to continue downward. There are mainly three types of decrepitation peak for pyrite, which correspond to the early, middle and late mineralization stages respectively.
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. CO 2-rich inclusions have similar Tm CO2 and Th CO2 to those for CO 2-H 2O inclusions. CO 2-H 2O 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 872×10 5 Pa. The M-stage fluid has the lowest contents of ions (e.g., SO 42-, Cl -, K +) and (K+Na)/(Mg+Ca) but highest CO 2/H 2O ratios. The change in ore-forming fluids from K 2SO 4 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.
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