Metamorphism of mineral matter in coal from the Bukit Asam deposit, south Sumatra, Indonesia

The coal of the Miocene Bukit Asam deposit in south Sumatra is mostly sub-bituminous in rank, consistent with regional trends due to burial processes. However, effects associated with Plio–Pleistocene igneous intrusions have produced coal with vitrinite reflectance up to at least 4.17% (anthracite) in different parts of the deposit. The un-metamorphosed to slightly metamorphosed coals, with Rv_max values of 0.45–0.65%, contain a mineral assemblage made up almost entirely of well-ordered kaolinite and quartz. The more strongly heat-affected coals, with Rv_max values of more than 1.0%, are dominated by irregularly and regularly interstratified illite/smectite, poorly crystallized kaolinite and paragonite (Na mica), with chlorite in some of the anthracite materials. Kaolinite is abundant in the partings of the lower-rank coals, but is absent from the partings in the higher-rank areas, even at similar horizons in the same coal seam. Regularly interstratified illite/smectite, which is totally absent from the partings in the lower-rank coals, dominates the mineralogy in the partings associated with the higher-rank coal beds. A number of reactions involving the alteration of silicate minerals appear to have occurred in both the coal and the associated non-coal lithologies during the thermal metamorphism generated by the intrusions. The most prominent involve the disappearance of kaolinite, the appearance of irregularly interstratified illite/smectite, and the formation of regular I/S, paragonite and chlorite. Although regular I/S is identified in all of the non-coal partings associated with the higher-rank coals, illite/smectite with an ordered structure is only recognised in the coal samples collected from near the bases of the seams. The I/S in the coal samples adjacent to the floor of the highest rank seam also appears to have a greater proportion of illitic components. The availability of sodium and other non-mineral inorganic elements in the original coal to interact with the kaolinite, under different thermal and geochemical conditions, appears to be the significant factor in the formation of these new minerals, and distinguishes the mineralogical changes at Bukit Asam from those developed more generally with rank increases due to burial, and from the effects of intrusions into coals that were already at higher rank levels.