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, peralumino us, 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 grani te, topaz-albite granite, pegmatite stockscheider and layered pegmatite-apl ite dikes, to K-feldspar-quartz veins and lepidolite-fluorite stringers in the country rocks. Snow-ball textures, homogeneous distribution of rock-for ming and accessory minerals, disseminated mineralization, and melt inclusio ns in quartz, topaz, and albite are typical features indicative of their cr ystallization from the late stage Li-F-Na-rich and Sn-Ta-Nb-bearing residua l granitic melts at a higher intrusion level. A comparison with rare-metal- bearing pegmatite, ongonite, topaz rhyolite and obsidian glass from other r egions shows the worldwide existence of these specialized residual melts. T heir emplacement and crystallization in a variety of geological environment s result in the formation of a series of chemically similar rocks with diff erent petrographic textures and mineral associations. The topaz-albite gran ites and associated mineralization in the Limu district provide a good exam ple of highly evolved magmatic fractionation in the F-rich granite system a nd fluid/melt partitioning behavior of rare-metal elements during magmatic- hydrothermal evolution.