Rb-Sr isochron age and isotopic compositions of H, O, S and Pb of the Gaozhuang gold deposit, Xixia County, Henan Province: Identification of intraplate orogenic metallogenesis in northern Qinling Mountain

The Gaozhuang gold deposit is one of the important Au deposits in Au polymetallic ore concentration area of northern Qinling Mountain, Henan Province. The Rb-Sr isochron age and H, O, S and Pb isotopes of ores from the Gaozhuang gold deposit were analyzed so as to constrain Au mineralization formation age and discuss the source of ore metals and the controlling factor of deep ore fluids discharge as well as the Au mineralization potential. The ⁸⁷Rb/⁸⁶Sr and ⁸⁷Sr/⁸⁶Sr ratios of pyrite and quartz from 7 ore samples are in the range of 0.0709-5.963 and 0.711139-0.721982, respectively. The initial ⁸⁷Sr/⁸⁶Sr ratios are in the range of 0.711025±0.000052 and the Rb-Sr isochron age is (129.8±1.4)Ma (MSWD=1.3), almost consistent with the age of the Yanzhen granitic intrusive and the Gaozhuang granitic aplite. It is suggested that they were nearly formed at the same time of -130Ma in early Cretaceous. The δ¹⁸O_V-SMOW and δD_V-SMOW values of 3 quartz samples are 11.6‰-12.1‰ and -60.9‰—-42.5‰, respectively. Their δ¹⁸O_H2O values range from 2.16‰ to 2.66‰, suggesting that ore-forming fluids of the Gaozhuang gold deposit were a mixed source. The δ³⁴S_V-CTD values of 6 pyrite samples are from -2.8‰ to -1.6‰ with a tower shape in the δ³⁴S histogram, which indicates a mantle source. The ratios of ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb of 7 samples are 17.549-18.278, 15.461-15.619 and 37.593-38.428, respectively. Their μ, Δα, Δβ and Δγ are 9.30-9.51, 59.52-75.10, 11.44-19.80 and 30.38-37.99. In ²⁰⁶Pb/²⁰⁴Pb-²⁰⁷Pb/²⁰⁴Pb and ²⁰⁶Pb/²⁰⁴Pb-²⁰⁸Pb/²⁰⁴Pb diagrams, sample points of the Gaozhuang gold deposit are close to the orogenic line. They fall into the crust-mantle subduction zone mixed lead source and the orogenic belt lead source in Δβ-Δγ of Pb isotopic diagram. It is suggested that metallogenic materials of the Gaozhuang gold deposit were derived from the lower crust and the upper mantle, with the addition of the upper crust materials in the ore-forming process. The formation of the Gaozhuang gold deposit was closely related to Yanshanian intraplate (intracontinental) orogenesis of northern Qinling Mountain. The Gaozhuang gold deposit, the Yanzhen granitic intrusive and the Gaozhuang granitic aplite are all products of regional lithosphere delamination. Deep fluids were rapidly discharged, and diverse sources of metallogenic materials at different depths were connected in the process of regional lithosphere delamination. The Gaozhuang gold deposit was formed when the mixed fluids stopped moving in shallow crust. It is suggested that the deep part beneath the Gaozhuang gold deposit and the Yanzhen area possessed endogenetic metallogenic potential with rapid discharging of deep fluids controlled by regional lithosphere delamination.