Petrography and inorganic geochemistry of the Rosebud coal seam at the Absaloka, Big Sky and Rosebud mines, Powder River Basin, Montana

Quantitative maceral and microlithotype analyses, huminite reflectance measurements and inorganic constituent analyses of the samples of the Rosebud subbituminous coal seam from the Absaloka, Big Sky and Rosebud Mines were used in the interpretation of the depositional environment of the seam and for prediction of ash formation during combustion. The mean huminite reflectance varies from 0.31% at the Rosebud Mine to 0.34% at the Absaloka Mine and 0.39% at the Big Sky Mine, indicating that the coal rank is transitional from lignite to subbituminous C. Despite the variation in huminite reflectance, samples from the three mines are similar in mean content of huminite (76 to 81 vol%), liptinite (4 to 6 vol%) and minerals (3 to 4 vol%), but somewhat variable in inertinite (10 to 16 vol%). A direct correlation of mean huminite reflectance with mean huminite abundance and an inverse correlation with mean inertinite abundance suggests a possible correlation . between coalification and floral precursors.Within-seam variations in petrographic composition and huminite reflectance are high. Lithologic layering with similar sequence and thickness in the Absaloka and Big Sky profiles is delineated by petrographic variation, suggesting similar geological histories for the two sites. A depositional environment transitional between reed marsh and forest swamp in a fluvial-deltaic system, is suggested for the Rosebud seam based on interpretation of microlithotype and maceral compositions.Clay minerals, quartz and calcite are the most abundant mineral phases in the Rosebud seam. Except for the top 0.3 m, the seam at the Big Sky Mine has mean pyrite content as low as 0.4 wt%. A phosphate mineral, crandallite(?), occurs as an abundant authigenic mineral in some layers. Crandallite and associated kaolinite, quartz, zircon, rutile and apatite, may have originated from volcanic ash.Maceral-associated inorganic elements have significant abundance in ulminite, fusinite and detrogelinite. The most abundant element is Ca, followed by Al, S, Mg, Si, Cl and Na. These elements in macerals are interpreted either as ions associated with carboxylate groups or as coordination complexes in the maceral molecular structure and are a major contributor to low-temperature fouling deposits in utility boilers firing the Rosebud coal.