Geochemistry and Lithium Enrichment Modes of the No. 8 Coal, Late Carboniferous, Eastern Ordos Basin

Recent studies have highlighted the presence of lithium enrichment in coals within the Ordos Basin, which has garnered significant attention due to the potential economic value. However, most research has concentrated primarily on the coal seams of the Taiyuan and Shanxi formations, with limited reports on the Benxi Formation. To address this gap, our study focused on elucidating the geochemical characteristics and origins of the lithium enrichment in the No. 8 coal of the Benxi Formation through the use of optical microscopy and inductively coupled plasma mass spectrometry (ICP-MS). The results showed that the No. 8 coal was a bituminous coal, characterized by medium ash yield, low volatile matter, high total sulfur content and medium vitrinite proportion. The No. 8 coal was enriched in Li (average 91.7 ppm, CC = 6.55) and Zr (average 191 ppm, CC = 5.30), the unusual enrichment of Li being primarily located in the middle of the coal. The minerals in the coal were predominantly clay minerals, along with minor amounts of pyrite, quartz and calcite. The occurrence mode of Li in the No. 8 coal was associated with aluminosilicate minerals, presumably kaolinite. Based on geochemical characteristics, the sedimentary source of the No. 8 coal was intermediate-felsic volcanic rock from the Yinshan oldland. The enrichment of Li can be attributed to the supply of terrestrial debris from the Yinshan oldland and the depositional environment. Our analysis identifies three distinct stages of lithium enrichment, emphasizing the critical role played by the terrestrial debris, as well as the acidic to partially reducing conditions, in facilitating this process. In conclusion, our study sheds light on the mechanisms underlying lithium enrichment in the No. 8 coal of the Benxi Formation, highlighting the significance of geological factors in shaping the distribution and concentration of critical metals in coal.