F-dominant versus B-dominant granite–greisen systems of Degana–Balda, NW India: controls on endogreisen development and wolframite deposition

In peraluminous, rare-metal granites associated with tungsten (W) mineralization, greisen alteration is common. However, the economic locus of wolframite [(Fe,Mn)WO₄] varies between endogreisen within the granite and quartz-dominated vein systems hosted by granite and/or country rocks. This study evaluates how feldspar availability, halogen-controlled melt–fluid evolution, and Fe–W mass transfer govern the degree of endogreisen development and wolframite localization. We compare two Neoproterozoic granite-associated ore systems from the W (±Sn) province of NW India: Degana and Balda. At Degana, the abundance of magmatic topaz indicates an F-rich late-magmatic evolution and high effective F activity in the melt–fluid system. We interpret Na–F (±Na–Al–F) complexing to have reduced early albite saturation, favouring a sodic, albite-rich residual granite. Metasomatic textures record pervasive potassic overprinting along feldspar-rich reaction fronts, which pre-conditioned the granite for subsequent low-pH greisen alteration. Feldspar-destructive reaction fronts then produced pervasive quartz–muscovite endogreisen with secondary topaz and fluorite, within which wolframite precipitated. Isocon-based whole-rock mass-balance constraints indicate net Fe addition and strong alkali loss (ΔCi/Ci⁰ ≈ +0.82 for Fe; ≈ –1.32 for Na+K), together with enrichment of W and granitophile elements (Li, Sn, Rb). These gains and losses are consistent with their transport by granite-derived magmatic–hydrothermal H₂O–CO₂ brines of moderate–high salinity (~12–22 wt% NaCl equiv.).

Balda represents a contrasting end-member in both melt evolution and hydrothermal halogen budget. Magmatic topaz is scarce, implying lower effective F activity during late differentiation than at Degana; early feldspar stability and fractionation yielded a comparatively feldspar-poor granite. Reduced feldspar buffering capacity, together with limited F-assisted feldspar hydrolysis, restricted both potassic and subsequent greisen overprinting, producing discontinuous, weakly developed tourmaline-rich endogreisen that lacks hydrothermal topaz and fluorite. Despite this, Balda endogreisen records pronounced Fe addition and rare-metal enrichment relative to unaltered granite, with Fe and associated metals (W–Sn–Li) hosted in Fe-rich micas and tourmaline. Wolframite is not observed in either endogreisen within the granite or exogreisen developed in metapelitic country rocks; instead, it is confined to metapelite-hosted quartz–tourmaline veins, where decompression-driven immiscibility of H₂O–CO₂ fluids and wall-rock buffering likely increased pH and promoted wolframite saturation. Together, Degana and Balda demonstrate that Fe and rare-metal enrichment in endogreisen is common but not sufficient for wolframite precipitation. Economic endogreisen-hosted wolframite mineralization requires concurrence of F-assisted feldspar destruction with adequate feldspar buffering capacity—conditions better expressed in the F-dominant Degana system than in the tourmaline-rich (B-dominant) Balda system.