Mineralogical characteristics, formation processes, and prospecting potential of Early–Middle Triassic lithium-rich claystone in the Sichuan Basin, South China

Abstract

Early to middle Triassic sedimentary units within the Sichuan Basin of southwest China contain extensive occurrences of lithium (Li)-rich clay layers (the so-called “Green bean rock”, or GBR) that have significant economic potential as a result of their substantial scale. However, a comprehensive understanding of the genesis and processes involved in the formation of these clay layers remains elusive. This study presents the results of a systematic investigation of three typical Li-rich clay profiles within the basin, using X-ray diffraction (XRD) and scanning electron microscopy (SEM) for mineralogical characterization and analysis. The presence of micron-scale primary high-temperature quartz and iron microspherules suggests that these Li-rich clay deposits have a volcanic origin. XRD analysis indicates that bulk rock samples of GBR are predominantly composed of quartz, clay minerals, and either calcite or gypsum, with the clay mineral assemblage dominated by illite and/or illite/smectite (I/S). Specifically, the GBR of the northeastern Sichuan Basin has a higher level of illitization compared to its southern counterpart, representing differences in sedimentary facies that are also found in other Li environments such as the Li deposits of Nevada. The northeastern GBR contains gypsum and clay minerals, whereas the southern counterpart consists mainly of carbonate lithologies with minor amounts of clay. This indicates that the sedimentological controls on the formation of clay mineral assemblages also influence the formation of clay-type Li deposits and Li enrichment processes. The composition of the clay components is indicative of probable derivation from smectite during diagenesis after originally being generated by the submarine modifications of volcanic materials. The later formation of I/S formation was also most likely the result of the illitization of volcanogenic smectite. The widespread presence of clay-type Li deposits indicates extensive eruptions of volcanic ash during Early to Middle Triassic volcanic events. The Li trapped within the illite and I/S is thought to have been sourced from volcanogenic materials via transportation in a composite fluid comprising meteoric, porewater, and hydrothermal fluids. These economically significant Li-rich clay minerals, found within a marine basin, represent a promising target for lithium exploration as a result of the extractable metal content. This research provides new insights into the processes involved in the formation mechanisms of clay-type lithium deposits and discusses the prospecting potential of volcanic clay-type Li deposits in the Sichuan Basin, contributing to the critical assessment of the Li resources needed for the energy transition and sustainable energy initiatives.