Organic petrographic and mineralogical composition of the No. 6 coal seam of the Soutpansberg Coalfield, South Africa: Insights into paleovegetation and depositional environment

This study investigates the paleodepositional conditions of the No.6 Seam of the Madzaringwe Formation in Makhado and Voorburg south area of the Soutpansberg Coalfield(Limpopo Province,South Africa)utilizing organic petrography and inorganic geochemical proxies.The coals are predominantly high-volatile bituminous B-A rank with high ash yields(avg.36.1 wt%),characterized by high-vitrinite(~41.5 vol%),moderate-to-high inertinite(9.8 vol%–33.7 vol%)and low liptinite(~2.3 vol%).The distribution of inertinite varies among different coal horizons(from bottom-lower to middle-upper),suggesting differential oxidation conditions and/or paleofire occurrence.Vitrinite-to-inertinite(V/I)ratio,tissue preservation–gelification index(TPI–GI),and groundwater–vegetation index(GWI–VI)plots,indicate that the peat-forming forest-swamp vegetation accumulated under mesotrophic-to-rheotrophic hydrological conditions.The presence of structured macerals(i.e.,telinite,collotelinite,fusinite,and semifusinite)suggests well-preserved plant tissues,whereas framboidal pyrite and sulphur content(0.24 wt%–2.16 wt%)point to brackish-water influence at the peat stage.The coals contain quartz,kaolinite,siderite,muscovite,dolomite,calcite,and pyrite minerals,most of which were likely sourced from felsic igneous rocks.The Al/(Al+Fe+Mn)and(Fe+Mn)/Ti ratios for the studied samples range between 0.24–0.97 and 0.57–70.10,respectively.The ratios,Al–Fe–Mn plot,and presence of massive botryoidal-type pyrite imply some influence of meteoric waters or fluids from hydrothermal activity post-deposition.Moreover,the chemical index of alteration(CIA:98.25–99.67),chemical index of weathering(CIW:92.04–97.66),and A–CN–K ternary diagram suggest inorganic matter suffered strong chemical weathering,indicating warm paleoclimatic conditions during the coal formation.

Petrographic composition of coal within the Benue Trough,Nigeria and a consideration of the paleodepositional setting

The petrographic composition of Cretaceous-age coals hosted in the Benue Trough,Nigeria is presented and discussed in terms of the paleodepositional settings that infuenced the coal-bearing formations.The Benue Trough is a failed arm of the triple junction of an inland sedimentary basin that extends in a NE-SW direction from the Gulf of Guinea in the south,to the Chad Basin in the north.A total of twenty-nine(29)coal samples were obtained from nineteen coal localities in the Upper(UBT),Middle(MBT),and Lower Benue Trough(LBT).The high average volatile matter yield,low average ash yield,high calorifc value(24.82 MJ/kg,on average),and low sulphur values indicate good quality coal deposits.The organic matter is dominated by vitrinite,reported at an average of 59.3%by volume(mineral-matter free).Variation was noted in the inertinite content across three sub-regions.Liptinite macerals were not commonly observed in the studied samples and were absent in the MBT samples.Coal facies studies decipher the paleoenvironmental conditions under which the vegetation accumulated.Indices commonly used are the gelifcation index(GI),tissue preservation index(TPI),ground water index(GWI and variations),vegetation index(VI),and wood index(WI).Comparing the array of coal facies models applied,the MBT samples difer from the UBT and LBT samples,concurring with the coal quality data.The UBT and LBT coals formed in an upper deltaic to drier piedmont plane depositional environment,while the MBT coal formed in a lower deltaic marsh to wet forest swamp depositional environment.All samples indicate an ombrotrophic paleomire.In view of the modifed equations and the plots used,interpreting depositional environments from just a single model is not reliable.