Controls on Nb–Ta Mineralization in a Highly Fractionated Albite Granite (Tailaihua, Great Xing’an Range, Northeast China): Magmatic Fractionation and Early Onset of Fluid Exsolution

The Tailaihua Nb–Ta–Be deposit in the southern Great Xing’an Range, Northeast China, is hosted in a highly fractionated peraluminous albite granite. New monazite U–Pb dating yields an emplacement age of ca. 147 Ma, and positive eNd(t) values with young two-stage Nd model ages indicate derivation by partial melting of Phanerozoic juvenile crust. Whole-rock geochemistry reveals extreme enrichment in SiO₂ and Rb, strong depletion in Sr, Ba, and Eu, and pronounced rare-earth element tetrad effects, together with widespread replacement textures in muscovite and columbite-group minerals (CGM), documenting intense fractional crystallization and melt–fluid interaction during magmatic–hydrothermal evolution. Niobium–tantalum mineralization is dominated by magmatic and magmatic–hydrothermal transition processes. Early CGM cores formed via fractional crystallization of evolved granitic melt, whereas Ta-rich rims record overgrowth and modification by interacting residual melt and exsolved fluids during the magmatic–hydrothermal transition. Tungsten mineralization occurs in three successive generations: W-rich fluid replacement of early CGM forms wolframoixiolite and Nb–Ta-rich wolframite (Wlf-1); intensive water–rock interaction precipitates greisen-hosted wolframite (Wlf-2); and decreasing P–T in fractures generates quartz–wolframite veins (Wlf-3) during the post-magmatic hydrothermal stage. Identical core–rim CGM textures in both granite and greisen demonstrate that large-scale fluid exsolution initiated early during granitic melt evolution, overlapping temporally with fractional crystallization and melt–fluid interaction, rather than occurring as a discrete late-stage event. The timing of fluid exsolution from melt represents a key control on the Nb–Ta mineralizing potential of exsolved fluids. This study provides a general model for Nb–Ta mineralization applicable to W–Be-mineralized albite granite systems.