Lithium supernormal enrichment via supercritical fluids: Fluid inclusion evidence from the Kaluan spodumene pegmatites in Altai, northwestern China

Unlike the zoned granitic pegmatites developed in the Altai orogenic belt (central Asia), the spodumene pegmatites identified in this study are characterized by unzoned, homogeneous fabrics and whole-dike lithium (Li) mineralization. These features indicate that the pegmatites did not undergo continuous fractional crystallization and that spodumene was saturated in pegmatite-forming melts and/or fluids after emplacement. However, the mechanisms responsible for Li supernormal enrichment and whole-dike Li mineralization in such spodumene pegmatite remain unknown. In this study, we selected two whole-dike Li-mineralized spodumene pegmatite (abbreviated as WDLP) dikes, including the Kaluan No. 806 (KLA806) and Kukalagai No. 650 (KKLG650) pegmatites, and one highly differentiated and zoned pegmatite, specifically the Koktokay No. 3 (KKTK3) pegmatite, from the Chinese Altai for a systematic comparative analysis. In situ chemical composition analyses of fluid inclusions hosted in quartz and beryl reveal that the pseudosecondary fluid inclusion assemblages (FIAs) in the KLA806 and KKLG650 pegmatites have anomalously high Li concentrations (4000−14,000 ppm) and low K/Rb ratios (<20), which are distinctly different from those of the KKTK3 pegmatite (3000−6000 ppm Li, K/Rb ratios of 30−190). Pressure-temperature isochores calculated for primary and pseudosecondary quartz-hosted FIAs yield emplacement pressures of 3.1 ± 0.4 kbar for the KLA806 pegmatite and 2.9 ± 0.4 kbar for the KKLG650 pegmatite, corresponding to emplacement depths of 11.5 ± 1.5 km and 10.8 ± 1.5 km, respectively. These values are significantly shallower than those of the KKTK3 pegmatite (3.8 ± 0.6 kbar, 14.1 ± 2.0 km). Petrographic observations show that spodumene in the KKTK3 pegmatite is extensively metasomatized by late-stage exsolved fluids with the formation of quartz, muscovite, secondary spodumene, and other Li-bearing minerals. In contrast, the KLA806 and KKLG650 pegmatites display neither metasomatism nor recrystallization of spodumene. These results suggest that spodumene in the WDLP crystallized immediately under Li-(over)saturated conditions in H2O-rich fluids after emplacement. We therefore propose that Li was supernormally enriched in supercritical fluids due to the differentiation process in a deep magma chamber, which gave rise to the formation of the WDLP through upward migration into the shallow crust. Given the widespread occurrence of the WDLP in China (e.g., in the Altai, Songpan−Ganzi−West Kunlun, and Himalaya orogens) and their significant Li mineralization, we conclude that the WDLPs represent optimal exploration targets for future Li prospecting.