Barium isotope evidence for a magmatic fluid-dominated petrogenesis of LCT-type pegmatites

Abstract

Understanding the petrogenesis of granitic pegmatites associated with Li mineralisation is fundamental in constraining rare metal enrichment mechanisms. However, there is still significant controversy surrounding the central issue of the nature of pegmatite-forming liquids. Here, we report Ba isotope data for a 3000 m borehole in the Jiajika pegmatite field that hosts the largest hard rock-type Li deposit in Asia. The pegmatites exhibit markedly lower δ^138/134Ba than the wall rocks and the average upper continental crust, which cannot be attributed to low degree anatexis of metasedimentary or crystal fractionation of highly evolved granitic magmas. Instead, modelling suggests that their low δ^138/134Ba most likely results from substantial involvement (10–40 %) of isotopically light hydrothermal fluids exsolved from the underlying magmatic reservoirs. In the context of transcrustal magmatic system, these magmatic fluid components may be a key medium in extracting, concentrating, and transporting Li to provide a material source for mineralisation in granitic pegmatites.