Research progress of lithium and lithium isotopes in coal-bearing strata

Significance Lithium (Li), as a strategic metal in emerging industries, has become a significant geochemical tracer due to its pronounced isotopic fractionation effect. In recent years, lithium deposits in coal-bearing strata have emerged as a focus area in the exploration of strategic metal mineral resources. Investigating the composition and variation of lithium isotopes in coal-bearing strata helps to elucidate the sources, migration, and enrichment processes of lithium, providing a theoretical foundation for the exploration and development of lithium deposits in coal-bearing strata. Advances This study summarizes the research progress of lithium and its isotopes in coal-bearing strata from three aspects: basic characteristics, isotope composition and fractionation mechanisms, analytical methods for lithium content determination, extraction and separation techniques, and lithium isotope analysis. The findings indicate that lithium is widely distributed in the mantle and crust, exhibiting strong fluid activity. The two natural stable isotopes (⁶Li and ⁷Li) display significant fractionation effects due to differences in diffusion rates and relative mass, making them key geochemical tracer tools. Lithium deposits in coal-bearing strata are mainly found in the Carboniferous-Permian coal-bearing strata of North China and the Late Permian coal-bearing strata of South China. Lithium is primarily hosted in secondary clay minerals, and its enrichment is influenced by various factors such as sedimentary diagenesis, microbial activity, tectonic movements, magmatic hydrothermal activity, and groundwater migration. Lithium isotope fractionation in coal-bearing strata is mainly influenced by factors such as temperature, weathering, metamorphism, and the formation of secondary clay minerals. The methods for determining lithium content in coal-bearing strata samples have become relatively mature, and high-precision lithium isotope testing technologies have made the widespread application of lithium isotopes possible. MC-ICP-MS has been preliminarily applied to the study of lithium isotope fractionation mechanisms in coal-bearing strata, but in situ micro-area lithium isotope testing technology is still in the exploratory stage. Due to the complex components and structures of lithium-bearing carriers in coal-bearing strata, there is an urgent need to develop in-situ analytical standard samples for lithium isotopes in coal-bearing strata and to establish standardized analytical protocols. The focus of the separation and extraction technology for lithium resources in coal-bearing strata is improving leaching efficiency and the purification and recovery of lithium from the leachate. Prospects Current research on the fractionation mechanisms of lithium isotopes in coal-bearing strata remains insufficient, with several limitations, including a superficial understanding of the fractionation mechanisms, the quality discrimination effect in testing methods, and the lack of in situ analysis standard samples and simulation experiment validation. Future research trends for lithium and its isotopes in coal-bearing strata include the study of the dynamic processes and enrichment mechanisms of lithium migration in coal-bearing strata, the development of high-precision lithium isotope testing and analysis technologies, the investigation of the coupling mechanism between lithium isotope fractionation and sedimentary thermal evolution processes, and the separation, extraction and recovery of lithium resources in coal-bearing strata.