Petrological, mineralogical, and geochemical compositions of the Eocene coals from Senakin Peninsula, South Kalimantan, Indonesia

Samples from the Senakin coal bed (Eocene age, Pasir Sub-basin) were analyzed to study the concentration of critical elements and investigate genetic types and enrichment processes. Of particular interest was to determine if an overlying mafic volcanic unit dated to the same age as the overlying Eocene sediments and tonstein layers withing the coal, affected the geochemical composition of the coal bed. Five partings were identified as being mafic-alkali tonsteins. The lower and upper tonsteins are interpreted to originate both from volcanic ash-fall material intermingled with water-borne detrital sediments. The middle tonstein is thought to have originated primarily as an ash-fall. A consistent feature of all partings, however, is that they are characterized by a graupen texture with additional vermicular and vitroclastic textures. The coal samples from the Senakin coal bed are dominated by vitrinite (64 vol%) with subordinate proportions of liptinite (29 vol%), and inertinite (7 vol%). Optical and SEM-EDX measurements of the coal samples showed that the inorganic material is dominantly pyrite, marcasite, quartz, kaolinite, illite, apatite, calcite, Ti oxides, Ti-V-bearing minerals, and a Mo-bearing mineral. Some coal samples from the Senakin coal bed have total rare earth element and yttrium (REY) concentrations up to 283.53 ppm. Furthermore, coals are enriched in REY and some trace elements (e.g., Zr (Hf), Nb (Ta), Ga, Sr, and V). It is hypothesized that the critical enrichments of the Senakin coal bed may be controlled primarily by three factors: (1) mafic-intermediate volcaniclastic rocks whose provenance are possibly from the Pitap and Haruyan Formations, (2) marine water infiltration from overlying sediments into recently buried peat, and/or (3) the occurrence mafic-alkali tonstein layers.