Geology, mineralogy and the occurrence of rare earth elements in the coal deposit at the Uitkomst Colliery (Utrecht Coalfield), Kwazulu-Natal, South Africa

Abstract

The Utrecht Coalfield is one of the coalfields of the Main Karoo Basin, which is currently underexplored and there is no information regarding the REEs concentration. Before extracting the REEs, it is significant to examine the distribution and concentration of REEs within the host rock, in this case, coal. The purpose of the study is to explore the mineralogical and geochemical characteristics and occurrence of REEs in the coal deposits at the Utrecht Coalfield (Uitkomst Colliery). Seventeen samples from three different seams (Gus, Alfred, and Fritz) were analysed using petrography, ultimate analysis, proximate analysis, and X-ray diffraction (XRD) analysis. The REEs concentration in the samples was determined by inductively coupled plasma (ICP-MS) techniques, whereas scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) as a direct method and Pearson’s correlation as an indirect method was used to determine the mode of occurrence of the REEs in the samples from the Utrecht Coalfield. The Vryheid Formation in the Utrecht Coalfield consists of sandstones, siltstones, shale, and mudstone, with minor coal seams. The average thickness of the Eland, Fritz, Alfred, and Gus seams is 0.30 m, 0.21 m, 1.21 m, 1.12 m, and 1.26 m, respectively. The samples were dominantly inertinite rich with lesser liptinite and vitrinite macerals and were categorised as medium-rank- C. The Gus, Alfred and Fritz seams had an average ash content of 14.9%, 29.9% and 18.4%, respectively. Few of the samples were low sulphur coals whereas others were medium sulphur coals. The dominant minerals detected by the XRD are quartz and kaolinite with lesser pyrite. The sum concentration of REEs ranged from 23.15 to 173.50 ppm, with an average of 101.43 ppm. The average REEs concentration for the Gus Seam was 79.47 ppm, 120.79 ppm for the Alfred seam, and 137.63 ppm for the Fritz seam. The samples had REEs concentrations higher than that of Worlds and USA Coals except for samples CMB-L1, CB2-L1, CB1-L2, CB2-L2, and CB2-S1. Furthermore, samples CB1-L1, CMB-D, JCBH08B, and JCBH20 had REEs concentrations higher than that of Chinese Coals. The Pearsons’ correlation indicated a mixed inorganic-organic affinity of REEs as shown by the positive correlation. The SEM-EDS analysis in the samples revealed that the REEs have inorganic and organic affinities. The coal-forming environment was weakly reducing to oxidizing, favouring REEs enrichment. The provenance is predominantly a felsic source region, likely a mixture of Archean and post-Archean rocks, including sedimentary sources with granitic, tholeiitic, and alkalic basalt. The Utrecht Coalfield is not promising for REEs extraction.