Extreme enrichment of lithium in granitic pegmatite by liquid immiscibility

Abstract

Single-stage partial melting modeling of meta-sediments or extreme fractional crystallization is inadequate for producing melts with the required Li content (>5000 ppm) for spodumene saturation. Therefore, multi-stage partial melting or fractional crystallization processes have been simulated to achieve the necessary Li content. However, these modeling approaches have overlooked the phenomenon of liquid immiscibility, which has been widely observed in the formation of spodumene-rich granitic pegmatites through studies of melt/fluid inclusions. In this study, we investigate the contribution of liquid immiscibility to the enrichment of lithium in granitic pegmatites by conducting elemental micro-analysis and melt inclusion Raman mapping on multi-stage apatite (Ap) samples from the Dangba large spodumene pegmatite deposit (NW China). The apatite samples are categorized into magmatic Ap1, transitional Ap2, hydrothermal Ap3a, and Ap3b based on the presence of exclusive melt, coexisting melt/fluid, liquid-rich, and vapor-rich fluid inclusions, respectively. The observed increase in Li content from Ap1 (median 43.4 ppm) to Ap2 (median 92.1 ppm) can be attributed to melt-melt immiscibility and subsequent fluid exsolution, as evidenced by the occurrence of coexisting water-rich/-poor melt and fluid inclusions. Furthermore, the significant surge in Li content from Ap2 to Ap3a (median 364 ppm) may result from fluid-fluid immiscibility, supported by the presence of coexisting liquid- and vapor-rich fluid inclusions, complementary negative (Ap3a) and positive (Ap3b) Eu anomalies in the chondrite-normalized rare earth element (REE) patterns, and the lowest Li contents found in Ap3b (median 12.3 ppm) among all apatite types. Considering the partition coefficients of Ap-melt (DLiAp/melt < 0.05) and Ap-fluid (DLiAp/fluid << 0.05), both the melts and fluids associated with Ap2 and Ap3a contain sufficient Li content for spodumene saturation. In conclusion, our findings highlight the significant role of liquid immiscibility in the extreme enrichment of lithium during the formation of granitic pegmatites.