Mineralogical tracers for magmatic and hydrothermal controls on Nb-Ta-Li mineralization: a case study from the Hailuoling granite system, Southern China

Abstract

The relative roles of magmatic versus hydrothermal processes in Nb-Ta-Li mineralization within highly evolved granites remain debated. The Hailuoling pluton comprises a magmatic-hydrothermal sequence from monzogranite (139.1 ± 1.2 Ma) through albite granite (137.0 ± 1.3 Ma) and pegmatite to greisen (137.0 ± 1.6 Ma), as dated by U-Pb geochronology on cassiterite/columbite. During the magmatic stage, columbite-group minerals evolve compositionally from columbite-(Fe) through columbite-(Mn) to tantalite-(Mn) and microlite, while siderophyllite exhibits progressively increasing Li, F contents and Ta/(Nb + Ta) ratios. Geochemical and mineralogical evidence suggests that the Nb-Ta mineralization [(Nb + Ta) = 200–300 ppm] in the albite granite mostly results from fractional crystallization of the monzogranite. The magmatic-hydrothermal transition stage features tantalite-(Fe) overgrowths on primary columbite-(Mn) and tantalite-(Mn), suggesting a metasomatic Nb-Ta mineralization episode triggered by hydrosaline melt separation from the fractionated albite granite/pegmatite system. The composition and mineral relics in greisen indicate fluid-rock interaction between the precursor monzogranite and acidic, Li-F-Fe-rich fluids exsolved from the evolving granitic melt. This process drives greisenization associated with elevated Li mineralization (Li ≈ 1500 pm) and minor Nb-Ta enrichment, which is marked by hydrothermal precipitation of Li-rich siderophyllite, W-bearing cassiterite, wolframite, and columbite-group minerals with decreased Ta/(Nb + Ta) and Mn/(Fe + Mn) ratios. This study concludes that fractional crystallization dominates the economic Nb-Ta mineralization and the initial Li enrichment in the Hailuoling deposit, while hydrosaline melt segregation and subsequent fluid exsolution contribute to subordinate metasomatic Nb-Ta enrichment and major Li mineralization.