Elemental geochemistry and mineralogy of coals and associated coal mine overburden from Makum coalfield (Northeast India)

Abstract

The insufficient research on the nature and mode of occurrence of minerals and other inorganic elements of the Northeast Indian coal deposits has resulted in lack of proper utilization, assessment and mitigation of environmental degradation. The mineralogical and geochemical aspects of these coals were studied through the techniques such as optical microscopy, low temperature quantitative X-ray diffraction (LTA-XRD), scanning electron microscopy energy dispersive spectroscopy (SEM–EDS), inductively coupled mass spectroscopy (ICP-MS) and chemical analysis techniques. The leaching behavior of the elements including REEs in presence of water was studied. The high organic sulfur (>75 % of total sulfur) and low ash yields were observed in these coals, varying with seam depth. The dominance of vitrinite (94.1–94.3 %) with an R max 0.72–0.78 % in these coals along with their dominance within the lean organic fraction in the coal mine overburden (OB) samples was reported. The major oxides present in these coals and their associated overburden samples were studied through XRF analysis. Various mineral phases were identified in both coal and OB samples such as dawsonite, pyrite, diaspore, marcasite, gypsum, thenardite berzeliite (garnet), etc. The SEM–EDS analysis indicated the presence of siderite, pyrite, and kaolinite, quartz, illite, etc., in the coal, OB and leached samples. The distribution of concentrations of the 48 trace and rare earth elements (REEs) varying with seam depth were normalized by upper continental crust (UCC) and found to be lower in comparison to global, Chinese, and USA coals. The statistical analysis on the trace and rare earth elements in the coals was performed to know their mutual correlations.