Geochemical and Mineralogical Secrets of Cretaceous Coals in the Upper Benue Trough, Nigeria

Abstract

The accelerating global demand for strategic minerals and rare earth elements (REEs), 
compounded by geopolitical supply constraints and market volatility, has intensified 
exploration of unconventional resource streams, including coal and coal combustion by
products. Beyond their traditional role as energy resources, certain coal deposits have 
emerged as potential repositories of critical elements of economic and technological 
importance. This study presents a comprehensive mineralogical and geochemical 
characterization of Cretaceous coal deposits from the Upper Benue Trough (UBT), 
Nigeria, thereby contributing essential baseline data to the limited body of literature on 
strategic element enrichment in West African coals. Seven representative coal samples 
collected from four distinct localities were analyzed using thermogravimetric analysis 
(TGA), bomb calorimetry, X-ray diffraction (XRD), X-ray fluorescence (XRF), and 
inductively coupled plasma–mass spectrometry (ICP-MS). Proximate analysis yielded mean 
moisture content of 6.8 wt.%, volatile matter of 27.7 wt.%, fixed carbon of 48.4 wt.%, 
and ash yield of 17.6 wt.%, indicating moderate-rank coal characteristics. The average 
calorific value of 24.11 MJ/kg, coupled with low sulfur content (0.85%, dry ash-free 
basis), suggests favorable combustion performance with comparatively reduced 
environmental risk. Ultimate analysis revealed average elemental concentrations of 
carbon (74.30 wt.%), hydrogen (5.88 wt.%), nitrogen (1.72 wt.%), and oxygen (16.46 wt.%), 
consistent with sub-bituminous to high-volatile bituminous classification. Mineralogical 
assemblages are dominated by kaolinite and quartz, with subordinate occurrences of 
gismondine, cadmium cyanide, and zinckenite. Geochemical data demonstrate enrichment 
in Ba, Mn, Cr, Cu, Sr, Zn, Zr, and light REEs including La, Ce, Nd, Sm, and Eu. The combined 
mineralogical and trace element signatures indicate that UBT coals represent a viable 
secondary source of critical minerals. Strategic recovery from coal ash residues could 
therefore provide an economically feasible and environmentally adaptive complement to 
conventional hard-rock REE exploitation.