Topaz–albite granites and rare-metal mineralization in the Limu District, Guangxi Province, southeast China

The topaz–albite granites of the Limu district are ultra-acidic, peraluminous, Li–F–Na-rich and Sn–Ta–Nb-mineralized. A distinct vertical zonation is developed in the granite stocks. There is an upward, systematic transition from leucocratic microcline-albite granite, through albite-microcline granite, topaz–albite granite, pegmatite stockscheider and layered pegmatite–aplite dikes, to K-feldspar–quartz veins and lepidolite–fluorite stringers in the country rocks. Snow-ball textures, homogeneous distribution of rock-forming and accessory minerals, disseminated mineralization, and melt inclusions in quartz, topaz, and albite are typical features indicative of their crystallization from the late stage Li–F–Na-rich and Sn–Ta–Nb-bearing residual granitic melts at a higher intrusion level. A comparison with rare-metal-bearing pegmatite, ongonite, topaz rhyolite and obsidian glass from other regions shows the worldwide existence of these specialized residual melts. Their emplacement and crystallization in a variety of geological environments result in the formation of a series of chemically similar rocks with different petrographic textures and mineral associations. The topaz–albite granites and associated mineralization in the Limu district provide a good example of highly evolved magmatic fractionation in the F-rich granite system and fluid/melt partitioning behavior of rare-metal elements during magmatic-hydrothermal evolution.