Genesis and geochronology of the Bayanteeg Li-mineralized pegmatite in the Idermeg terrane, central Mongolia

Abstract

We present the first petrographic, lithogeochemical, and geochronological study of the Bayanteeg LCT pegmatite located in Idermeg terrane, central Mongolia, and interpret the findings within the geodynamic setting. The pegmatite extends over 140 m with a width of 1.3 m and unknown depth within Neoproterozoic gneiss. The pegmatite contains plagioclase, quartz, and lepidolite with minor K-feldspar, spodumene, muscovite, and topaz, and accessory amounts of cassiterite, amblygonite, columbite-tantalite, monazite, zircon, apatite, and fluorite. Locally, minor secondary quartz and lepidolite occur interstitially between plagioclase and quartz and along the edges of primary lepidolite, respectively, implying late-stage hydrothermal influence. Lithogeochemical data show that the pegmatite contains 0.3–1.12 wt% Li, 256–1285 ppm Cs, and 59–522 ppm Ta. Monazite U-Th-Pb geochronology yielded an age of 144.9 ± 2.8 Ma while cassiterite yielded a U-Pb age of 134.8 ± 1.4 Ma. Lepidolite yielded ⁴⁰Ar/³⁹Ar plateau age of 131.25 ± 0.3 Ma. These age results fall during the geodynamic evolution of an intracontinental extension accompanied by the exhumation of metamorphic core complexes and extensive magmatism in the eastern Central Asian Orogenic Belt. These events occurred due to a combination of gravitational collapse resulting from lithospheric delamination and asthenospheric upwelling. The geodynamic setting during the pegmatite emplacement implies abnormally hot conditions, ruling out the possibility of anatectic origin. The pegmatite dike with elevated concentrations of Be, Ga, Rb, Nb, Sn, Cs, Ta, and Tl supports a granitic origin with a hidden parental granite at depth. The fact that the Idermeg terrane contains several LCT pegmatites implies an important exploration target for Li exploration.