Cassiterite as a pathfinder to Li mineralization in pegmatites: insights from the Murong deposit, NW China

The Murong deposit, the largest single-vein Li pegmatite in Asia, hosts a > 100 m thick principal ore vein, yet its emplacement age and the mechanisms of post-intrusion Li enrichment remain unresolved. Integrating columbite-group mineral (CGM) and cassiterite U–Pb geochronology, cassiterite geochemistry, and high-resolution cathodoluminescence (CL) imaging of cassiterite and spodumene, this study reconstructs the evolution of the zoned Murong pegmatite. The internal zonation is described here as an outer zone, comprising a centimetre-scale fine-grained border subzone and a thicker wall subzone, followed inward by an intermediate zone and a Li-rich core zone. Cassiterite is divided into five operational textural–compositional populations rather than five automatically discrete genetic generations: dark homogeneous Cst1 and dark oscillatory-zoned Cst2 in the outer zone, gray homogeneous Cst3 in the intermediate zone, gray oscillatory-zoned Cst4 in the core and intermediate–core transition, and late bright Cst5 domains. Hyperspectral CL imaging further distinguishes early magmatic–hydrothermal transitional spodumene crystals from later hydrothermal spodumene–quartz veinlets, recorded most clearly by Cst4 and Cst5, respectively. Cassiterite U–Pb ages from all zones consistently indicate ca. 193 Ma, constraining pegmatite emplacement and coeval Li mineralization. In contrast, CGM U–Pb ages are systematically older (∼202–204 Ma), a discrepancy attributed to matrix effects during microbeam analysis, supported by differences in Ta/(Nb + Ta) between the analyzed CGM and reference materials. From the outer to core zones, cassiterite becomes depleted in Nb, Ta, Fe, Zr, Hf, Ti, and Y, but Ta/(Nb + Ta) and Zr/Hf do not follow the monotonic trends expected for closed-system fractional crystallization. This non-monotonic behavior indicates that accessory-mineral crystallization, phase-specific partitioning, and melt–fluid immiscibility overprinted the simple fractional-crystallization signal. Together with reported melt inclusions indicating melt-fluid immiscibility, the high Li content of Cst4 identifies the magmatic–hydrothermal transition as the principal stage of extensive spodumene mineralization. These findings show that cassiterite CL textures and geochemistry can trace the chemical evolution and metallogenic history of spodumene pegmatites.