Effects of contact metamorphism on δ¹³C, δ¹⁵N, and XPS nitrogen functional forms of inertinite-rich Witbank coals (South Africa): Indications for possible alteration by hydrothermal fluids

In this study, δ¹³C, δ¹⁵N, and nitrogen functional forms were used to evaluate the effects of contact metamorphism on inertinite-rich Main Karoo Basin coals (South Africa). Eighteen samples were collected at 0.25 m intervals from the western and eastern sides of a ∼ 2.6 m thick dolerite dyke emplaced into the No. 2 Seam; an unaltered coal from the same seam was included as a control (Witbank Coalfield). Inertinite (semifusinite and inertodetrinite) dominated the organic composition of the samples. δ¹³C and δ¹⁵N for the unaltered coal were − 21.1‰ VPDB and 1.5‰ Air, respectively. For the intruded coals from the western side, δ¹³C ranged between −22.4 and −21.4‰ VPDB, and δ¹⁵N between 1.2 and 2.6‰ Air. The δ¹³C for samples from the eastern side ranged between −21.8 and −20.7‰ VPDB, and between 1.8 and 3.0‰ Air for δ¹⁵N. Differences in δ¹⁵N between the unaltered and the intruded coals were generally more significant than in δ¹³C, especially for samples from the eastern side. The positive shifts in δ¹⁵N may reflect reactions between organic nitrogen in coal and mobile nitrogen transported by hydrothermal fluids. The absence of a relationship between δ¹⁵N and distance from the dyke, may reflect secondary, localized contact metamorphism due to hydrothermal fluid migration within the intruded seam. The circulation of a fluid transporting ¹⁵N-rich moieties, apparently more active on the eastern side, was supported by the higher ash yields, δ¹⁵N, and N_total/C_organic ratios. The correlation between N_total/C_organic and ash yields suggests deposition of NH₄⁺ − bearing inorganic components preferentially into the eastern side. Similar to the unaltered coal (N-5 = 47.24 at.%), N-pyrrolic was the dominant nitrogen functional form of the intruded coals, reflecting the developmental pathways for the inertinite macerals as opposed to the contact metamorphism. The low proportion of graphitic (quaternary) nitrogen, irrespective of sample distance from the dyke, suggests metamorphic temperatures lower than 900 °C.