Geochronology, petrology, and lithium isotope geochemistry of the Bailongshan granite-pegmatite system, northern Tibet: Implications for the ore-forming potential of pegmatites

Abstract

Identification of possible factors that affect Li-mineralizing potential is important to the understanding of the genesis of Li pegmatites. This study provides a comprehensive characterization of the recently discovered Bailongshan Li pegmatite district based on geochronological, petrological, mineralogical, thermal modeling, and Li isotopic data. Columbite-(Fe) U–Pb dating of two pegmatite dikes indicates they were emplaced at 212.3 ± 0.9 and 213.9 ± 0.7 Ma. The spodumene-absent pegmatite dikes yield substantially higher δ7Li values (+2.0‰ to +4.9‰) than spodumene-bearing dikes (−1.9‰ to +0.8‰). Modeling of Li isotopic fractionation during fractional crystallization, fluid exsolution, and diffusion after pegmatite emplacement indicates that the lighter Li isotopic compositions of spodumene-bearing pegmatites are attributable to fluid exsolution or diffusion-driven fractionation of short duration, whereas the heavier isotopic compositions of spodumene-absent pegmatites can be attributed to longer-duration, diffusion-driven fractionation. Together with the results of thermal modeling, we suggest that the cooling rate of pegmatite dikes was an important factor controlling the Li isotopic compositions of pegmatites and slower cooling caused heavier Li isotopic compositions. It may have important implications for the ore-forming potential of Li in pegmatites that high cooling rate is beneficial for forming Li pegmatites.