Paleoenvironmental Controls and Economic Potential of Li-REY Enrichment in the Upper Carboniferous Coal-Bearing “Si–Al–Fe” Strata, Northeastern Qinshui Basin

Critical metals in coal-bearing strata have recently emerged as a frontier hotspot in both coal geology and ore deposit research. In the Upper Carboniferous coal-bearing “Si–Al–Fe” strata (Benxi Formation) of the North China Craton (NCC), several critical metals, including Li, Ga, Sc, V, and rare earth elements and Y (REY or REE + Y), have been discovered, with notable mineralization anomalies observed across northern, central, and southern Shanxi Province. However, despite the widespread occurrence of outcrops of the “Si–Al–Fe” strata in the northeastern Qinshui Basin of eastern Shanxi, there has been no prior report on the critical metal content in this region. Traditionally, the “Si–Al–Fe” strata have been regarded as a primary source of clastic material for the surrounding coal seams of the Carboniferous–Permian Taiyuan and Shanxi Formations, which are known to display critical metal anomalies (e.g., Li and Ga). Given these observations, it is hypothesized that the “Si–Al–Fe” strata in the northeastern Qinshui Basin may also contain critical metal mineralization. To evaluate this hypothesis, new outcrop samples from the “Si–Al–Fe” strata of the Benxi Formation in the Yangquan area of the northeastern Qinshui Basin were collected. Detailed studies on critical metal enrichment were assessed using petrographic observations, mineralogy (XRD, X-ray diffractometer), and geochemistry (XRF, X-ray fluorescence spectrometer, and ICP-MS, inductively coupled plasma mass spectrometer). The results indicate that the siliceous, ferruginous, and aluminous rocks within the study strata exhibit varying degrees of critical metal mineralization, mainly consisting of Li and REY, with minor associated Nb, Zr, and Ga. The Al2O3/TiO2, Nb/Y vs. Zr/TiO2, and Nb/Yb vs. Al2O3/TiO2 diagrams suggest that these critical metal-enriched layers likely have a mixed origin, comprising both intermediate–felsic magmatic rocks and metamorphic rocks derived from the NCC, as well as alkaline volcaniclastics associated with the Tarim Large Igneous Province (TLIP). Furthermore, combined geochemical parameters, such as the CIA (chemical index of alteration), Sr/Cu vs. Ga/Rb, Th/U, and Ni/Co vs. V/(V + Ni), indicate that the “Si–Al–Fe” strata in the northeastern Qinshui Basin were deposited under warm-to-hot, humid climate conditions, likely in suboxic-to-anoxic environments. Additionally, an economic evaluation suggests that the “Si–Al–Fe” strata in the northeastern Qinshui Basin hold considerable potential as a resource for the industrial extraction of Li, REY, Nb, Zr, and Ga.