Tourmaline Mineralogy, ⁴⁰Ar-³⁹Ar Dating and Implications for the 509 Daobanxi Lithium Deposit in Xinjiang Province

The Dahongliutan ore district in West Kunlun, one of the most important super-large pegmatite-type lithium mineralization zones in China, has attracted significant attention for the mechanisms of rare metal enrichment. Previous studies have conducted detailed research on the Li-rich spodumene-bearing pegmatites in the area. However, the genesis of the widely exposed, Li-poor tourmaline-bearing pegmatites and their relationship with lithium-rich pegmatites remains debated. This study focuses on the petrography and tourmaline mineralogy of the Li-poor pegmatites in the super-large 509 Daobanxi lithium deposit and successfully obtains a new tourmaline ⁴⁰Ar-³⁹Ar isotope age. The results show that: ① the Li-poor garnet-tourmaline-bearing pegmatite, mainly composed of plagioclase, quartz, alkali feldspar, muscovite, tourmaline, and minor garnet; ② the composition of the tourmaline is relatively homogeneous and characterized by high FeO^T (11.19%–13.24%), low CaO (0.06%–0.29%), MgO (0.02%–0.10%), and Na₂O (0.69%–1.12%) contents, belonging to the schorl subgroup. Comparable to the tourmaline characteristics of two-mica granite and spodumene pegmatite in the area, the studied tourmaline is of typical magmatic-hydrothermal tourmaline. The tourmaline composition is mainly controlled by the (^X□, Al) (Na, R²⁺)₋₁ substitution; ③ the ⁴⁰Ar-³⁹Ar plateau age of the tourmaline is (229.8±0.4) Ma, indicating that the Li-poor pegmatites, biotite granites, and Li-rich pegmatites are products of single magmatic-hydrothermal activity. Based on the characteristics of the pegmatite occurrence, it is proposed that the Li-poor pegmatites represent an early-stage melt poor in rare metals, while the Li-rich ones are more evolved magmatic-hydrothermal products enriched in rare metals. The crystallization of Li-poor minerals (garnet, tourmaline, feldspar, etc.) in the pegmatites near the intrusion facilitated further enrichment of rare metals in the residual melt/fluid, leading to the distribution of pegmatites with Li-poor zones near the intrusion and Li-rich zones further away.