Distribution, mode of occurrence, and significance of rare-earth elements in coal from Samaleswari open cast coal blocks, Odisha, India with their provenance and paleodepositional environment

Abstract

This study aims to thoroughly examine the distribution, concentration, and occurrence of a broad selection of REEs and major elements in coal samples from Samaleswari coal block, IB valley, Odisha, India using advanced geochemical tools. A total of 85 coal samples from four boreholes were characterized using proximate analysis and calorific value measurements. The corresponding ash samples were examined using X-ray Diffraction (XRD), X-ray Fluorescence (XRF), Electron Probe Microanalyzer (EPMA), and Inductively Coupled Plasma Mass Spectrometry (ICP-MS).The coal ash samples were delineated to be rich in SiO2 (42-65%) along with a lower proportion of Al2O3 (24-41%), Fe2O3 (5-6%), and TiO2 (1-2%). The average concentration of REEs in the four boreholes was found to be 510 ppm, with the distribution pattern being Ce > La > Nd > Y > Pr > Sc > Sm > Gd > Dy > Er > Yb > Eu > Ho > Tb > Tm > Lu. The concentration of critical REEs varied in the range of 100-300 ppm, and outlook coefficient (C-outl) ranged from 0.7 to 1.0, demonstrating our coal seams' suitability as a promising source of REEs. A higher concentration of REEs in BH1 was correlated with a relatively higher proportion of apatite. The increment in REE concentration with depth was speculated to be due to their close association with hematite, especially in BH1 and BH2. A strong positive correlation of all the REEs with clay minerals and specifically TiO2 reinstates their abundance in fine-grained clastic sedimentary rocks which are not significantly affected by weathering. Point analysis using EPMA and Scanning Electron Microscope-Energy Dispersive Analysis X-ray (SEM-EDAX) confirmed the co-existence of La, Ce, Pr, and Nd in the Fe-containing aluminosilicate matrix. In addition, the major oxide ratio and their plot entail terrestrial depositional environment in the basin during coal formation and deposited in semi-arid climatic conditions.