Carbonaceous Shale Deposits as Potential Unconventional Sources for Rare Earth Elements at the Witbank Coalfield, Permian Vryheid Formation, South Africa

Carbonaceous shale has garnered significant interest as a viable alternative source of rare earth elements (REEs) besides conventional REE-bearing ores. This study characterized rare earth element + Yttrium+ Scandium (REYs) enrichment in the 11 core samples of carbonaceous shale (7) and coal (4) collected from Arnot Mine. Major elements of the studied carbonaceous shale (CS) and coal showed high amounts of SiO2, Al2O3, and Fe2O3, indicating a high content of aluminosilicate and iron-rich minerals. The plots Na2O + K2O against SiO2 suggested alkali granite, granite, and granodiorite provenance sources for the studied shale and coal. The samples showed enrichment in low and heavy rare elements crystallized from a low potassium tholeiitic and medium calc-alkaline magma based on the plots of LaN/YbN and K2O vs. SiO2. The mineralogical and maceral analysis revealed the dominant presence of kaolinite (15%–45%), and it was suggested as the cation exchange site resulting from the isomorphous substitution of Al3+ for Si4+. Additionally, siderite was suggested as one of the REY hosts due to the Fe3+ site forming a complex with the REE3+ ions. Furthermore, the samples were classified as lignite to sub-bituminous coal category with dominant minerals including kaolinite, quartz, and siderite. The outlook coefficient (Coutl) of REY in CS revealed a promising area for economically viable, having two enrichment types, including low (La, Ce, Pr, Nd, and Sm) and heavy (Ho, Er, Tm, Yb, and Lu). The EuN/EuN* and CeN/CeN* ratio for the current studied samples exhibited a weak negative to no anomaly, and most of the studied samples were characterized by distinctive positive Gd anomalies derived from sediment source regions weathered from alkali granite, granite, and granodiorite provenance formed from a low potassium tholeiitic and medium calc-alkaline magma.