Zircon U−Pb age and geochemical characteristics of biotite monzonitic granite and mineralization background in Xiaodachuan Pb−Zn−Cu deposit of Inner Mongolia, the southern Great Khingan Range

Objective The newly discovered Xiaodachuan Pb−Zn−Cu deposit in the southern section of the Greater Khingan Range occurs in the biotite monzonitic granite body and is spatially affected by the late Yanshanian intrusive rocks. The age of diagenesis and mineralization, petrogenesis and metallogenic geological background are still unclear. The study can provide a new basis for the metallogenic regularity of lead and zinc in the area.

Methods Samples of biotite monzogranite from the Xiaodachuan Pb−Zn−Cu deposit in the southern section of the Greater Khingan Range were collected for petrographic study, zircon U−Pb dating, whole rock geochemical analysis, and Sr−Nd isotopic investigations.

Results Research shows that the zircon U−Pb ages of the biotite monzonitic granite are 135.9±0.8 Ma and 134.9±0.8 Ma, indicating that the Pb−Zn mineralization occurred in the Early Cretaceous. The rock geochemical characteristics show that the biotite monzonitic granite is rich in SiO₂(71.29%~72.92%), K₂O＋Na₂O(7.17%~7.89%), Al₂O₃(13.35%~14.48%), poor in MgO(0.61%~0.64%) and CaO(1.24%~1.73%), enriched in Nb, Ta, Zr, Hf and Th elements, depleted in Ba, K, Sr, P and Ti elements, and the rare earth distribution pattern is “seagull” type. The δEu value is 0.35~0.47, and the negative Eu anomaly is obvious. It belongs to high−potassium calc−alkaline and peraluminous A₂−type granite. The Sr−Nd isotope characteristics show that the (⁸⁷Sr/⁸⁶Sr)_i value is 0.70545~0.70548, the ε_Nd(t) value is −1.7 ~ −0.3, and the two−stage model age of Nd isotope is 1071~957 Ma.

Conclusions Combined with the chronological and geochemical characteristics, it is considered that the diagenetic material is a mixture of new crust and mantle−derived material and is contaminated by the upper crust. The Xiaodachuan biotite monzonitic granite was formed in an extensional environment after the closure of the Mongolia−Okhotsk Ocean, which also led to the occurrence of diagenesis and mineralization in the region.