Geochemistry of Rare Earth Elements in Lower Gondwana Coals of the Talchir Coal Basin, India

Abstract

This study investigated the concentration, distribution, and modes of occurrences of rare earth elements in coal from the Talchir coal basin, Odisha State. This basin comprises two coal bearing Formations i.e. Barakar and Karharbari. Samples, collected from one borehole, were analyzed by X-ray diffraction, X-ray fluorescence, electron microprobe and inductively coupled plasma mass spectrometry (ICP-MS). The coals are medium to high volatile, high ash and low sulphur. The Rare Earth Elements and Yttrium (REY) of the 34 coal samples varies from 29.6 ppm to 179.4 ppm with an average value of 91.0 ppm. The average value of total REY of the Barakar Formation (53.6 ppm) is considerably lower than the average value of Karharbari Formation (127.4 ppm). The average of the ratio of light to heavy rare earth elements (LREE/HREE) is also much higher in the Karharbari Formation (8.1) than in the Barakar Formation (4.4). Coals from both Formations show negative europium anomalies while only coals from the Karharbari Formation show a positive cerium anomaly. These differences indicate differences in depositional conditions between the coals of the two Formations within the coal basin. Through EPMA analysis, REY-bearing phosphates are found to be present in this coal in clay minerals. Monazites are found in Mg aluminosilicate matrix as dispersed minerals which may be detrital and monazite-bearing chlorite indicates that the monazites are formed by hydrothermal alteration during coalification. Ag, Ba and Zr are found as replacements or substitution in the REY phosphates.