Geochronological framework and Pb, Sr isotope geochemistry of the Qingchengzi pb-Zn-Ag-Au orefield, Northeastern China

摘要

The Qingchengzi orefield in northeastern China, is a concentration of several Pb-Zn, Ag, and Au ore deposits. A combination of geochronological and Pb, Sr isotopic investigations was conducted. Zircon SHRIMP U-Pb ages of 225.3 +- 1.8 Ma and 184.5 +- 1.6 Ma were obtained for the Xinling and Yaojiagou granites, respectively. By step-dissolution Rb-Sr dating, ages of 221 +- 12 Ma and 138.7 +- 4.1 Ma were obtained for the sphalerite of the Zhenzigou Zn-Pb deposit and pyrargyrite of the Ag ore in the Gaojiabaozi Ag deposit, respectively. Pb isotopic ratios of the Ag ore at Gaojiabaozi (~(206)Pb/~(204)Pb = 18.38 to 18.53) are higher than those of the Pb-Zn ores (~(206)Pb/~(204)Pb = 17.66 to 17.96; Chen et al. [Chen, J.F., Yu, G., Xue, C.J., Qian, H., He, J.F., Xing, Z., Zhang, X., 2005. Pb isotope geochemistry of lead, zinc, gold and silver deposit clustered region, Liaodong rift zone, northeastern China. Science in China Series D 48, 467-476.]). Triassic granites show low Pb isotopic ratios (~(206)Pb/~(204)Pb = 17.12 to 17.41, ~(207)Pb/~(204)Pb = 15.47 to 15.54, ~(208)Pb/~(204)Pb = 37.51 to 37.89) and metamorphic rocks of the Liaohe Group have high ratios (~(206)Pb/~(204)Pb = 18.20 to 24.28 and 18.32 to 20.06, ~(207)Pb/~(204)Pb = 15.69 to 16.44 and 15.66 to 15.98,~(208)Pb/~(204)Pb = 37.29 to 38.61 and 38.69 to 40.00 for the marble of the Dashiqiao Formation and schist of the Gaixian Formation, respectively). Magmatic activities at Qingchengzi and in adjacent regions took place in three stages, and each contained several magmatic pulses: ca. 220 to 225 Ma and 211 to 216 Ma in the Triassic; 179 to 185 Ma, 163 to 168 Ma, 155 Ma and 149 Ma in the Jurassic, as well as ca. 140 to 130 Ma in the Early Cretaceous. The Triassic magmatism was part of the Triassic magmatic belt along the northern margin of the North China Craton produced in a post-collisional extensional setting, and granites in it formed by crustal melting induced by mantle magma. The Jurassic and Early Cretaceous magmatism was related to the lithospheric delamination in eastern China. The Triassic is the most important metallogenic stage at Qingchengzi. The Pb-Zn deposits, the Pb-Zn-Ag ore at Gaojiabaozi, and the gold deposits were all formed in this stage. They are temporally and spatially associated with the Triassic magmatic activity. Mineralization is very weak in the Jurassic. Ag ore at Gaojiabaozi was formed in the Early Cretaceous, which is suggested by the young Rb-Sr isochron age, field relations, and significantly different Pb isotopic ratios between the Pb-Zn-Ag and Ag ores. Pb isotopic compositions of the Pb-Zn ores suggest binary mixing for the source of the deposits. The magmatic end-member is the Triassic granites and the other metamorphic rocks of the Liaohe Group. Slightly different proportions of the two end-members, or an involvement of materials from hidden Cretaceous granites with slightly different Pb isotopic ratios, is postulated to interpret the difference of Pb isotopic compositions between the Pb-Zn-(Ag) and Ag ores. Sr isotopic ratios support this conclusion. At the western part of the Qingchengzi orefield, hydrothermal fluid driven by the heat provided by the now exposed Triassic granites deposited ore-forming materials in the low and middle horizons of the marbles of the Dashiqiao Formation near the intrusions to form mesothermal Zn-Pb deposits. In the eastern part, hydrothermal fluids associated with deep, hidden Triassic intrusions moved upward along a regional fault over a long distance and then deposited the ore-forming materials to form epithermal Au and Pb-Zn-Ag ores. Young magmatic activities are all represented by dykes across the entire orefield, suggesting that the corresponding main intrusion bodies are situated in the deep part of the crust. Among these, only intrusions with age of ca. 140 Ma might have released sufficient amounts of fluid to be responsible for the formation of the Ag ore at Gaojiabaozi.