Abstract
Coal is a potential alternative source for rare earths, including Yttrium and Scandium (REY+Sc), which are used in modern technologies. Research on the concentration and distribution of REY+Sc has been conducted in the Karoo Basin, and results confirming the presence of the elements were obtained. However, further research is required to determine whether similar findings occur in another part of the coal basin, and to further interrogate the host minerals and REY+Sc occurrences. To this end, a single borehole was collected from the Delmas portion of the Witbank Coalfield, Manungu Colliery.
This study investigates the concentration and origin of REY+Sc within the coal horizons and associated clastic sediments hosted in the Vryheid Formation, Witbank Coalfield. A total of 28 samples, representing all lithological units identified through detailed core logging, were selected for analysis. Petrographic examination and proximate analysis were undertaken to characterise the samples, while inductively coupled plasma mass spectrometry (ICP-MS) was employed to quantify REY+Sc concentrations. X-ray diffraction (XRD) and X-ray fluorescence (XRF) were used to refine the mineralogical and geochemical framework and scanning electron microscopy-energy dispersive x-ray spectrometry (SEM-EDS) provided insights into the modes of occurrence of the constituent minerals.
The stratigraphy includes carbonaceous shales, sandstones, and 3 coal seams (the No. 2, 3 and 4 seams). The organic matter in seams 3 and 4 displays a noticeably brighter maceral composition compared to that in Seam 2. Vitrinite reflectance values from Seam 2 increase upward toward the seam roof, suggesting progressive thermal alteration. The highest vitrinite reflectance values, ranging between 1.2 and 1.5 %RoVmr, were recorded in Seams 3 and 4 as well as in samples 1 and 2. These elevated reflectance values suggest localized thermal influence, possibly resulting from the presence of igneous intrusions near the base and top of the stratigraphic sequence.
The total REY+Sc concentrations ranged from 40 to 840 ppm, with 23 samples showing values higher than the average values reported for the Upper Continental Crust (UCC =179.4 ppm), Chinese (137.9 ppm), USA (62 ppm), and World coals (72.4 ppm). The results show higher REY+Sc concentrations in the clastic lithologies (150-
iv
840 ppm) compared to the coal seams. The highest REY+Sc concentrations for the coal samples are from the No. 2 Seam (35 - 280 ppm). The studied Vryheid Formation samples are rich in light-REY+Sc, followed by medium-REY+Sc, with heavy-REY+Sc being the least abundant, following typical global patterns. When normalised to the UCC, the samples show a high enrichment for Ce, Nd, Sm, Gd, Dy, Er, and Y, alongside a weakly negative Ce anomaly, as well as a positive Eu anomaly.
The XRD results revealed a high concentration of kaolinite, quartz, carbonates (calcite and dolomite) and pyrite, the latter being least abundant. Other minerals included anatase, chlorite, diopside, magnetite, microcline, muscovite, siderite, and smectite, with higher concentration observed towards the top of the stratigraphy. In addition to these minerals, SEM-EDS revealed the presence of allanite, REY+Sc rich- silicates, zircons, and zirconolites.
Pearson’s correlation coefficients indicate strong positive relationships among all REY+Sc groups with kaolinite, and Al₂O₃, TiO₂, and ZrO. The MREEs show strong correlations with anatase, while the HREEs display significant associations with calcite, siderite, diopside, and epidote, as well as with CaO, MgO, MnO, and ZrO₂. The results further indicated both direct and indirect occurrences of REY+Sc, with direct occurrences representing REY+Sc hosted within discrete mineral phases such as zircons and titanites, and indirect occurrences involving REY+Sc adsorbed onto or substituted within clay minerals and organic matter.
Two principal modes of REY+Sc enrichment were identified based on UCC normalised REY+Sc ratios, mineralogical observations, and geochemical correlations: (1) terrigenous enrichment, supported by L-type LaN/LuN ratios (>1), associations with kaolinite and Al₂O₃, and the presence of fine-grained REY+Sc minerals in samples 1–4; and (2) hydrothermal enrichment, indicated by H-type LaN/LuN ratios (<1) in samples 5–28, elevated thermal alteration features, and strong correlations between HREEs and hydrothermal alteration minerals such as siderite, diopside, and epidote.