GENESIS AND GEOCHRONOLOGY OF THE BAYANTEEG LI-MINERALIZED PEGMATITE IN THE IDERMEG TERRANE, CENTRAL MONGOLIA

Lithium, which is classified as a ‘critical mineral’ by the United States and other countries, is an important component for renewable energy technologies. Some of the main sources of Li are Li-, Cs-, and Ta-rich pegmatites (i.e., LCT pegmatites). Here we present the results of an ongoing study of the Bayanteeg Li pegmatite that is located in the Idermeg terrane, Central Mongolia. The study aims (1) to determine the timing of the pegmatite formation, and (2) to provide insights into the source of the LCT pegmatite and the formation mechanism, as well as the tectonic setting that facilitated the LCT pegmatite formation. The pegmatite body is hosted within the Neoproterozoic Oortsog Formation. The pegmatite body extends over 140 m with a width of up to 1.3 m. Chemical analysis show that the pegmatite contains 1.48-2.15% LiO₂, 0.1% BeO, 200 ppm Ta₂O₃ and 100 ppm Nb₂O₅. Pegmatite samples from Bayanteeg consist of plagioclase, quartz, lepidolite, K-feldspar, and muscovite with accessory amounts of cassiterite, topaz, beryl, columbite-tantalite, monazite, zircon, apatite, and fluorite. Secondary albite and quartz are locally observed interstitially between primary plagioclase and quartz while secondary lepidolite occurs locally along the edges of primary lepidolite indicating late-stage hydrothermal alteration. Six monazite grains with homogeneous texture without any patterns were dated using field emission (FE) EMPA to determine the pegmatite formation age. The weighted mean age of total 41 measurements is defined at 144.0±3.3 Ma with 95% confidence level and MSWD=0.65. Additionally, ⁴⁰Ar/³⁹Ar plateau age for lepidolite yielded 131.25±0.28 Ma with MSWD = 1.4 as a mean value of two different measurements with each plateau age comprising of 100% ³⁹ArK. These ages indicate that the Bayanteeg deposit formed during an episode of crustal thinning that occurred between 145 Ma and 135 Ma that follow the crustal thickening that occurred from 165 Ma to 145 Ma in the Idermeg terrane and eastern Mongolia. Because of the timing of the pegmatite emplacement, we argue that the Bayanteeg LCT pegmatite was sourced from Idermeg terrane basement rocks upon decompression melting during the post-collisional extension.