Mineralogical and geochemical signatures of zircons recording magmatic to hydrothermal processes in the Li-pegmatite system at the Boam deposit, South Korea

In this study, we conduct a micro-analysis of zircons from the Li-bearing Boam pegmatite deposit in Uljin, South Korea, with the aim of understanding the specific evolution process of the pegmatitic melt. All of the zircons observed in the pegmatite have identical textural features (T1, T2, and T3) and distributions of major/minor elements. The primary domain T1 is identified by a relatively bright ring or band within oscillatory and sector zoning in backscattered electron and cathodoluminescence images. The concentrations of UO₂ and HfO₂ reach 0.81 wt% and 12.88 wt%, respectively. These results indicate that the zircons are derived from highly evolved melts of magmatic origin. However, the chondrite-normalized rare earth element (REE) patterns categorize these zircons into type-A and type-B groups. This distinction can be explained by a model in which each type of zircon grows from separate melts. In type-A zircons, T1 has positive Ce anomalies and a steeply increasing REE pattern from light REEs (LREEs) to heavy REEs (HREEs), suggesting crystallization from a silicic melt with a high silicate content and a relatively low H₂O content. On the other hand, type-B zircons have a negative Ce anomaly at T1, with a flat REE pattern due to slight LREE enrichment, indicating crystallization from an aqueous melt with a low silicate content and a high H₂O content. T2 is recognized as a relatively pure zircon composition with a secondary texture that recrystallized after T1 crystallization. The embayment/inward-penetrating textures and mineral inclusions related to the mineral assemblage in the pegmatite means T2 has the signature of hydrothermal alteration via F-rich fluid exsolved from these melts. This domain formed via a coupled dissolution–reprecipitation process that selectively recrystallized the primary domains with a very high U and Th content. This domain exhibits a higher LREE content compared to T1 in both types of zircon, suggesting that the hydrothermal fluid is enriched in the LREEs due to their incompatible characteristics and the selective retention of REEs due to their similar ionic radii. T3 displays a mottled/porous texture and the enrichment of non-formula elements such as Ca, Fe, and Al, indicating that this domain has undergone a typical diffusion–reaction process due to metamictization. Considering the time required for the accumulation of radiation damage, this domain is not believed to have formed during the pegmatite crystallization stage but rather to have formed via the infiltration of fluids of external origin after geological-scale time. Overall, the crystallization and subsequent alteration of these zircons serve as an indicator of the overall evolutionary history of the pegmatite. Our findings specifically suggest that the zircons in the Boam deposit have been crystallized in a melt–melt immiscibility process involving a silicic melt and an aqueous melt and subsequently recrystallized by a hydrothermal fluid exsolved from the immiscible melts.