Unprecedented tantalum enrichment of zircon through stepwise crystallization in Li-Pegmatite, Uljin, South Korea

This study investigates the unprecedented enrichment of tantalum (Ta) and the stepwise crystallization of zircon in the Li-pegmatite from the Boam mine area, Uljin, South Korea. In the studied pegmatite, three types of zircon—high-uranium (U) zircon, high-tantalum (Ta) zircon, and high-hafnium (Hf) zircon were identified in correspondence with the melt evolution stage. Remarkably, the Ta-rich zircon exhibited an exceptionally high Ta concentration of up to 6.31 wt%, the highest ever reported. The findings suggest that the Ta-rich zircon is crystallized under the influence of an extremely fractionated, Li-, F-enriched and P-poor melt. Considering that the studied pegmatite originates from a Ta-depleted melt, this phenomenon indicates that Ta enrichment in zircon depends more on the melt composition rather than the Ta content in the melt. Furthermore, since the aqueous fluid expelled from the melt facilitates the recrystallization of structurally unstable zircon, fluid influence should be minimized for the preservation of Ta-rich zircon. These findings offer critical insights for behavior of tantalum and zirconium during formation of rare-metal pegmatites.