Cassiterite features and lifespan of Limu granite Ta-Nb-Sn-W ore-forming system in Guangxi, South China

Abstract

Rare metal deposits are clearly related to highly evolved granites. The Limu LiF granite complex hosts early quartz vein-hosted WSn ore and late stage disseminated Ta-Nb-Sn ± W ore. To elucidate unique characteristics of cassiterite from Ta-Nb-Sn-W ore-forming system and the link between magmatic evolution and NbTa mineralization, we analyzed textures and trace element compositions of cassiterites from both ore types, and conducted LA-ICP-MS UPb dating on cassiterites and columbite group minerals (CGMs). Both vein and disseminated cassiterites exhibit low Fe and high (Nb + Ta) contents with molar (Nb + Ta)/Fe ratios (>7), significantly exceeding those of typical Sn-granite cassiterites (<2). Late disseminated cassiterite displays high Nb and Ta oscillatory zoning eroded by low NbTa domains, and intergrows with CGMs indicating a dissolution-reprecipitation process via hydrosilicate liquid during crystallization. Contrastingly, low and constant Zr/Hf (~4) suggest Zr and Hf remain stable in the hydrosilicate liquid. We proposed that high molar (Nb + Ta)/Fe ratios and eroded structures in cassiterites may indicate the potential NbTa ore. Early quartz-vein cassiterites yielded ages of 215.5 ± 3.5 Ma, while late disseminated ores produced ages of 214.3 ± 2.4 Ma (cassiterite) and 212.7 ± 1.1 Ma (CGMs). Combined with previous zircon ages from early-stage granite (230–227 Ma), the Limu magmatic-hydrothermal system spans 15–16 Myrs. This long-lived magmatic-hydrothermal system depleted in Fe and enriched in (Nb + Ta), driven by continuous mantle-derived heat, likely underwent early fluid exsolution, triggering quartz vein WSn ores, followed by further melt evolution forming Nb-Ta-rich hydrosilicate liquid, which we consider a prerequisite for NbTa enrichment and evolved to generate the late disseminated Ta-Nb-Sn ± W ore.