Geological and geochemical characterisation of the occurrence Of rare earth elements in coal and carbonaceous shale at the Vele Colliery (tuli coalfield) in Limpopo Province, South Africa

Abstract

The Tuli Coalfield (Vele Colliery) is considered to be the smallest coalfield in Limpopo Province that is explored for the purpose of coking coal. As a result, there is still yet to be a study that investigates the occurrence of REEs in coal and carbonaceous shale in the Tuli Coalfield. Therefore, this study aims to investigate the geology, mineralogy, and occurrence of rare earth elements (REEs) in carbonaceous shale and coal of the Tuli Coalfield. To understand the geology and characteristics of coal and carbonaceous shale core logging, petrography, proximate analysis and ultimate analysis were used. The x-ray diffraction (XRD) analysis, scanning electron microscopy with energy dispersive x-ray (SEM+EDX) analysis and inductively coupled plasma-mass spectrometry (ICP-MS) were also utilized for mineralogy of coal and carbonaceous shale and concentration of REEs. The stratigraphy of the Vele Colliery is made up of three seams, namely the top seam, middle seam and bottom seam from the Madzaringwe Formation. The identified seams have coal-bearing strata of interlayered black carbonaceous shales, greyish mudstone, and sandstone. Major intrusions are present at the top seam and middle seam. Vitrinite macerals dominate the organic composition of the coals, ranging from 59.5-96.9 mmf.vol.%. The common sub-maceral of vitrinite, collotelinite, has a total count that ranges from 52.4 to 74.9 mmf.vol.%. The dominance of collotelinite suggests that the coals are mostly derived from parenchymatous and woody tissues of roots, stems, barks and leaves. Shales vary in organic composition, with over half of the samples dominated by inertinite maceral that ranges from 48.7 to 80.4 mmf.vol.%. Fusinite, an abundant sub-maceral of inertinite found in the shales, is probably the result of fire or degrading processes. The moisture content, volatile content and fixed carbon of the coals, respectively range between 0.8-1.9%, 6.5-31.1% and 25.7-51.0%. Furthermore, ash content of the coals varies from 21.2 to 51.4%. Contrary to coal, the shales exhibit high ash content between 67.7 and 86.2% and lower fixed carbon ranging from 2.4-15.7%. The moisture and volatile matter of carbonaceous shale values range between 0.5-1.2% and 4-29.7%, respectively. The coals have low to intermediate values of total sulphur (S), hydrogen (H), nitrogen (N) and oxygen (O). Carbon (C) value is high in the coals, with higher values observed in the altered coals. The proportion of S, C, H, and N is low while oxygen content is high in carbonaceous shales. The vitrinite reflectance results show that the fresh (non-intruded) coals are medium rank C bituminous (Rovmr: 0.61-0.92). The intruded coals are medium-rank B bituminous coals (Rovmr: 1.12-1.3%) and high-rank C anthracite (Rovmr: 2.80%). The higher vitrinite reflectance of intruded coals is possibly due to temperature, time and distance impact of the intrusions. The mineralogy of the fresh coals is predominated by quartz and kaolinite while the altered coals show high content of dolomite and calcite. On the other hand, carbonaceous shales compositions show kaolinite, quartz and plagioclase abundance. Monazite and xenotime occur as REE-bearing phosphate minerals of the coals. In addition, monazite is a host of light rare earth elements (LREEs) of cerium (Ce), lanthanum (La), and neodymium (Nd), while xenotime bears yttrium (Y) and medium rare earth elements (MREEs). The total average concentrations of REEs in the studied coals and carbonaceous shales is 102,19 mg/kg and 168,65 mg/kg, respectively. The samples studied exhibit LREEs enrichment as opposed to heavy rare earth elements (HREEs). Furthermore, europium (Eu) and cerium (Ce) have shown weakly negative anomalies that suggest REEs are derived from felsic rocks. The sediment-parent rocks are mainly sedimentary rocks, and these sediment-parent rocks have granite and gneissic provenance characteristics. The source rocks for coal are likely to be post-Archaean rocks, favourably from Lebombo Granite. Then again, carbonaceous shales have sources from the Archaean age which are possibly from Palmietfontein and Mount Dowe. The percentage of critical elements (REYdef.rel) and outlook coefficient between (Coutl) classify coal and carbonaceous shale in an economically promising region as ore deposits of REEs. Moreover, the REEs from this region are higher than some international deposits. The study suggests that coal and carbonaceous shales may be potential alternatives of REEs.