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Petrological and mineralogical features are key for inferring palaeoenvironmental and sedimentary conditions of coal-bearing sediments. Following the logging of a borehole core, this study reports on the organic petrology and mineralogy of the A and B Seams, Ermelo Coalfield, Main Karoo Basin (MKB), South Africa. Seventeen coal samples collected from the core, differentiated based on changes in lithotypes, were assessed using detailed petrography (macerals, microlithotypes, and mean random vitrinite reflectance), quality analyses (proximate, elemental, and gross calorific value), and mineralogy in order to deduce depositional environments for the Ermelo coals. The lithologies present in the core were typical of the MKB, and the coals were inertinite- and mineral matter-rich, apart from samples occurring at the base of the seams. The latter samples also had a higher proportion of structured inertinite macerals (fusinite and semifusinite) and lower ash yields. The rest of the samples had higher inertodetrinite contents, occurring in association with detrital quartz and kaolinite, resulting in higher ash yields. The formation of each coal seam thus commenced with the prevailing conditions conducive to the preservation of structured macerals and concluded with the accumulation of predominantly inertodetrinite. Select samples from the B Seam also had durite, an association of inertodetrinite and liptinite (including alginite). The co-occurrence of inertodetrinite and the detrital minerals alongside alginite suggests two depositional episodes. The first episode relates to the formation of primary inertinite macerals through charring, followed by fragmentation and redeposition as inertodetrinite. This latter process likely also reflects seasonal flooding, resulting in fluctuating water levels within the palaeomire. The second episode was quieter, occurring in deeper waters, and conducive for the growth of algae. An allochthonous origin is thus proposed for inertodetrinite and the associated detrital minerals, whereas alginite deposition was autochthonous. The cyclical upward-fining and upward-coarsening facies of the MKB were observed in the core, indicative of fluvial and deltaic palaeoenvironments, respectively. The fluvial facies were generally associated with the occurrence of structured vitrinite and inertinite macerals, suggesting in-situ formation, and mostly occur at the base of the coal seams. In contrast, the inertodetrinite-rich samples appear to be related to the overlying deltaic deposits. • An Ermelo core intersecting the A and B Seams is described. • Inertinite is common in the coals, though vitrinite is higher at the base of the seams. • In some B Seam samples, alginite occurs with inertodetrinite reflecting autochthonous and allochthonous deposition. • Structured macerals are associated with fluvial facies and inertodetrinite with deltaic facies. [ABSTRACT FROM AUTHOR]

The response of inertinite-rich Witbank coals (South Africa) to rapid heating was evaluated using petrography, selected Nuclear Magnetic Resonance (NMR) and X-Ray Diffraction (XRD) structural parameters. A total of thirteen (13) coal samples were collected from the No. 2 Seam; a set of six (6) samples from the western and the eastern sides of a ~ 0.4 m thick dolerite dyke emplaced into the coal seam at 0.25 m intervals. An unaltered coal (Un-C; the 13th sample), collected away from the dyke but within the same seam at the mine, was included for comparison. All 13 coal samples were dominated by inertinite. Semifusinite was most abundant in the samples from the western side of the dyke, whereas the proportion of inertodetrinite was highest in samples from the eastern side. Due to very low vitrinite contents, mean random total reflectance (%RoTmr) was used to compare the maturity of the inertinite-rich coals. A decrease in volatile matter was correlated with an increase in mean random total reflectance (%RoTmr; measured on semifusinite, inertodetrinite, and vitrinite), the latter used as a potential maturity indicator for the thermally altered inertinite-rich coals. Mean random total reflectance rose from a background value of 1.52%RoTmr (for Un-C) to 2.83%RoTmr for the coal sample at the western coal-dyke contact (0.0 m). The corresponding reflectance values for the thermally altered coals from the eastern side were less consistent, decreasing closest to the dyke (1.93%RoTmr for the sample at 0.0 m); this trend is also reflected in the NMR and XRD parameters. Eight (8) samples were selected for the NMR analysis, representative of the maceral composition and maturities of the coals investigated. For Un-C, the fraction of aromatic carbons (fa) was 0.88 (average number of carbons per aromatic cluster (C) = 17), and the interplanar spacing (d002) was 3.48 Å (with a crystallite height of 22.09 Å). For the thermally altered coals from the western side, fa decreased from 0.96 at the coal-dyke contact (C = 26 for sample at 0.0 m) to 0.92 for the sample at 1.25 m from the dyke (C = 20). For samples from the eastern side, fa increased from 0.91 (C = 19 for the sample at 0.25 m) to 0.93 (C = 23 for sample at 0.5 m), before decreasing to 0.89 further from the dyke (C = 18 for sample at 1.25 m). In general, the d002 values for the thermally altered coal samples were larger than for graphite. Similarly, only marginal increases in crystallite heights were observed. Although the dyke induced coal maturation and the resultant increase in fa , the overall lack of graphitic order in the thermally altered coals was ascribed to the limited thermal energy provided by the relatively thin dyke; increased heat input would be required to effect significant structural changes in the inertinite-rich coals. Differences between the two sets of thermally altered coal samples were attributed to the emplacement dynamics of the igneous intrusion, which require further investigation. [ABSTRACT FROM AUTHOR]

The increased global demand for rare earth elements (REEs) from unconventional sources has led to increased interest and research in coal deposits. South Africa has extensive coal resources, but understanding of the concentration, mode of occurrence, and distribution of REEs in the coals and associated sediments is limited. This study aims to assess selected coal samples obtained from an exploration project underway in the Waterberg Coalfield (South Africa) to determine the concentration and mode of occurrence of REEs, and to gain insight into the behaviour of REEs during density fractionation. Density fractionation of coal is considered a pre-concentration method prior to REE extraction. Samples were selected from zones 8, 7, 4, and 3, spanning the Grootegeluk and Vryheid Formations of the Waterberg Coalfield. Density fractionation was conducted at relative densities (RD) of 1.3 to 1.8, providing float (F) and sink (S) products. Five parent and thirty density-fractionated coal samples were characterized using proximate analysis, petrography, X-ray diffraction (XRD), and the concentration of REEs determined using Inductively Coupled Plasma- Mass Spectrometry (ICP-MS) (after microwave digestion). The samples are classified as medium-rank C bituminous coals with variable maceral composition. The dominant minerals are kaolinite and quartz, occurring as both syngenetic and epigenetic phases. The total REY (REEs + yttrium) concentrations range from 45.1 to 389.2 ppm, higher than for world coals (except sample 8H F1.30, REY = 45.1 ppm), and generally higher than the average for Chinese coals. Float products obtained at 1.40 to 1.80 RD show greater variation of light REY (LREY) compared to medium REY (MREY) and heavy REY (HREY), whereas float fractions at 1.30 and sink fractions at 1.80 RD show no distinct enrichment patterns. Although the results suggest a relationship between the concentration of LREY with float fractions obtained between 1.40 and 1.80 RD, pre-concentration using density fractionation generally shows limited benefits for these coals. Ashing of the coals may be more effective in concentrating the REEs and further investigations are therefore required. • The REY concentrations range from 45.1 to 389.2 ppm, comparable to or higher than that of world coal average values. • Density fractionated 1.4–1.8 float products show a greater variation of light REY (LREY) compared to medium REY (MREY) and heavy REY (HREY). • Pre-concentration by density fractionation may not be the best REY concentration process for these Waterberg coal samples. [ABSTRACT FROM AUTHOR]