Trace element geochemistry and stable isotopic (δ13C and δ15N) records of the Paleocene coals, Salt Range, Punjab, Pakistan

Abstract

The Paleocene coals of the Salt Range in the Punjab Province of Pakistan have great economic potential; however, their trace element and stable isotopic characteristics have not been studied in detail except for a few sporadic samples. In this study, a total of 59 coal samples of which 14 are obtained from open cast mines have been investigated for elemental composition and δ13C-δ15N isotopic signatures. Average contents of trace elements such as Co, Cr, Cu, Pb, Sr, Th, U, V, and Zn are 7.4, 41.7, 11.2, 12.5, 90.2, 4.0, 1.9, 128, and 31.1 mg/kg, respectively. These values, when compared with the World Coal Clarke values, were relatively higher in low-rank coals in comparison with Clarke values for brown coals. Likewise, As (20.4 mg/kg), Co (6.6 mg/kg), Cr (22.4 mg/kg), Cu (13.3 mg/kg), Pb (19.2 mg/kg), Sr (154.7 mg/kg), Th (2.5 mg/kg), V (47.8 mg/kg), and Zn (75.1 mg/kg) were significantly higher in the sub-bituminous to bituminous coals of the Salt Range. Mineralogical analysis, based on X-ray diffraction and energy dispersive X-ray spectroscopy, revealed that the studied samples contain illite, kaolinite calcite, gypsum, pyrite, and quartz. Elemental affinity with organic and inorganic phases of coals calculated by an indirect statistical approach indicated a positive association of ash content with Ag, Al, Co, Cr, Cs, Cu, Mn, P, Rb, Pb, Th, U, and V, suggesting the presence of inorganic components in studied coals. However, As, Fe, Sr, and Zn exhibit negative correlations that imply their association with the organic fraction. The δ13C and δ15N isotopic range and average values for 12 coal samples were − 24.94‰ to − 25.86‰ (−25.41‰) and − 2.77‰ to 3.22‰ (0.96‰), respectively, reflecting 3C type modern terrestrial vegetation were common in the palaeomires of studied coal seams. In addition, the trivial variations of 0.92‰ and 0.45‰ among 13C and 15N values can be attributed to water level fluctuations and plant assemblies.