Fingerprinting hydrothermal alteration associated with pegmatite-type Li-(Be) mineralization using titanite

• Titanites from the Bayankala Group exhibit hydrothermal origin characteristics. • The titanite U-Pb ages are consistent with the regional Li-mineralization epoch. • Titanite chemistry records significant enrichment of ore-forming elements (REE, Nb-Ta-Sn, and Li) in the hydrothermal fluids. The role of hydrothermal fluids in the formation of lithium-cesium-tantalum (LCT) pegmatites, particularly their contribution to mineralization, remains a topic of debate. To address this, we conducted an integrated in situ geochemical and U-Pb geochronological study of hydrothermal titanite from the Dahongliutan district, NW China. Hydrothermal titanite, accompanied by chlorite, epidote, and calcite, occur in the host quartz-schist adjacent to both barren and Li-mineralized pegmatites. These titanites exhibit textures (homogeneous or patchy zoning) and geochemical signatures (elevated F and Al, LREE depletion, low Th/U ratios) characteristic of a hydrothermal origin. Crucially, titanite U–Pb ages of 214.1 ± 5.8 Ma (from the host of a Li-mineralized pegmatite) and 202.8 ± 6.8 Ma (from the host of a barren pegmatite) overlap with respective mineralization (cassiterite: ∼211 Ma) and magmatic (zircon: ∼201 Ma) ages within analytical error. This provides direct evidence that hydrothermal alteration was synchronous with pegmatite emplacement, not a later, unrelated event. Compared to titanite associated with barren pegmatites, those related to Li-mineralized dikes are systematically enriched in Li, F, Nb, Ta, and Sn. This compositional fingerprint demonstrates that the hydrothermal fluids were intrinsically metal-fertile and directly sourced from the crystallizing, rare-metal-rich melts. Our results unequivocally highlight the essential contribution of magmatic-hydrothermal fluids to metal concentration in LCT-type pegmatite systems and establish hydrothermal titanite as a robust pathfinder mineral for rare-metal exploration.