Using δ15N and δ13C and nitrogen functionalities to support a fire origin for certain inertinite macerals in a No. 4 Seam Upper Witbank coal, South Africa

•Inertinite- and vitrinite-rich sub-samples were prepared using density separation.•The coal samples were compared using δ15N and δ13C and nitrogen functionalities.•Coal formation pathways were inferred for the dominant macerals of each sample.•Low 14N and high N-quaternary for the vitrinite-rich imply bacterial degradation.•Higher 14N and N-pyrrolic values for the inertinite-rich sample suggest charring.