⁴⁰Ar-³⁹Ar Dating of Muscovite from the Zhaojinggou Nb-Ta Polymetallic Deposit in Wuchuan County of Inner Mongolia and Its Geological Implications

The Zhaojinggou Nb-Ta polymetallic deposit in Inner Mongolia is a newly discovered large rare metal ore deposit. The reserve of Nb-Ta oxides is more than 8200 tons. Previous studyfocus on geological features of this deposit, but they the precise mineralization age remains unknown. In this paper, muscovite in amazonite granite pegmatite was used to constrain the timing of mineralization using ⁴⁰Ar-³⁹Ar isotope method. Results show that Nb-Ta mineralization is mainly hosted in alkali-feldspar granite, alkali-feldspar aplite, greisen, and amazonite granite pegmatite. According to the geologicalrelationship between Nb-Ta minerals and muscovite, we can infer that Nb-Ta minerals and muscovite are formed at the same time. Thus, the muscovite plateau ages of 123.57±0.66 Ma and 124.0±2.0 Ma can represent the mineralization age. Results show that the Zhaojinggou Nb-Ta polymetallic deposit was formed at late Yanshanian (or at least Early Cretaceous), the ⁴⁰Ar-³⁹Ar age have bigger difference with the zircon age, which is of great importance for ore genesis. Combining with regional geology, the author thinks that an essential rare-metal mineralization happened in the middle part of inner Mongolian axis on the north margin of north china platform during the late Yanshan movement.