Genesis of the early mesozoic granitoids at the Hardat Tolgoi Ag-Pb-Zn deposit in East Ujimqin Banner, Inner Mongolia, NE China: Insights from whole-rock geochemistry, zircon U-Pb-Hf isotopes, and Pb-Si systematics

Abstract

The Hardat Tolgoi Ag-Pb-Zn deposit is located in the Erenhot-East Ujimqin polymetallic metallogenic belt, on the western part of the Great Xing’an Range, Inner Mongolia, northeast China. The Ag-Pb-Zn veins are mainly hosted by the argillaceous siltstone and feldspar quartz sandstone of Aobaotinghundi Formation, which show close genetic relationship with the granitoids at Hardat Tolgoi (i.e., biotite granite, syenogranite, and monzogranite). New LA-ICP-MS zircon U-Pb dating yielded the emplacement ages of 196.1 ± 5.8 Ma for biotite granite, 192.3 ± 6.0 Ma for syenogranite, and 194.8 ± 1.7 Ma for monzogranite, respectively, indicating that they were formed in the Early Jurassic. Geochemically, these granitoids are characterized by high SiO2, Na2O + K2O, and low MgO and TiO2 concentrations, belonging to high-K calc-alkaline series. They exhibit relatively flat REE patterns, with pronounced negative Eu anomalies (EuN/EuN* = 0.004–0.015), enriched in LREEs and LILEs (e.g., Rb, U, and Pb), and depleted in Ba, Sr, Nb, P, and Ti. Zircon in situ Hf isotopic analyses yield positive εHf(t) values ranging from + 5.7 to + 11.8, corresponding relatively young Hf isotopic crustal model ages of 452 to 843 Ma. The granitoid samples have the 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of 18.950 to 19.444, 15. 573 to 15.615, and 38.487 to 38.789, corresponding to initial Pb ratios of 18.483 to 18.960, 15.552 to 15.662, and 37.905 to 38.193, respectively, and yield the δ30Si values between − 0.4 and − 0.1‰, similar to the quartz separates (−0.7 to − 0.1‰). The available geochemical and isotopic data imply that the source magma at Hardat Tolgoi was most likely derived from partial melting of juvenile lower crust. Combined our whole-rock geochemical and Hf − Pb − Si isotopic data with regional tectono-magmatic evolution, it is proposed that the Early Jurassic magmatism at Hardat Tolgoi was probably attributed to the southeastward subduction of the Mongol − Okhotsk oceanic plate beneath the Erguna massif, and their generation was favorable to the Hardat Tolgoi Ag-Pb-Zn ore formation.