Lithium isotope fractionation during magmatic differentiation and hydrothermal processes in rare-metal granites

Lithium isotope composition is potentially an effective geochemical tracer for hydrothermal processes and magmatic differentiation associated with rare-metal granitic rocks. The Yashan and Xihuashan plutons in South China are extraordinarily Li-F rich rare-metal granites that contain niobium-tantalum and tungsten deposits, respectively. As a moderately incompatible trace element in a felsic melt system, Li notably increases from protolithionite granite and Li-mica granite (88.7-175 mu g/g) to topaz-lepidolite granite (7430-8080 mu g/g) in the Yanshan pluton. Despite a large variation in Li concentrations, the delta Li-7 values of the Yashan pluton vary within in a narrow range from -1.5 parts per thousand to 1.5 parts per thousand. In contrast, the delta Li-7 values of the Xihuashan pluton notably increase from biotite granite and two-mica granite (-0.2 parts per thousand to +0.7 parts per thousand) to muscovite granite (+1.9 parts per thousand to +4.4 parts per thousand) with much less variation in the Li concentrations (37.8-209 mu g/g), which is best explained by the high diffusion rate of Li-6 relative to Li-7 during disequilibrium fluid-rock interaction. The Xihuashan greisen has negative delta Li-7 values (from -2.7 parts per thousand to -2.1 parts per thousand), which are attributed to extensive fluid-rock interaction in an open system. Lithium isotope fractionations are consistent with a diversity of mineralization in the rare-metal granitic rocks. Tungsten mineralization is likely associated with an open hydrothermal process. Fluid-rock interaction has a much stronger effect on Li isotope fractionation than does magmatic differentiation in a highly evolved magmatic system. Ta-Nb mineralization is related to the magmatic differentiation in a closed magmatic-hydrothermal system. The exsolution of a supercritical fluid during magmatic differentiation and fluid-rock interaction in a closed magmatic-hydrothermal system is insufficient for producing notable Li isotope fractionation. (C) 2018 Elsevier Ltd. All rights reserved.