Geochemical evaluation of calcareous coal horizons, Gebel Crystal, Western Desert, Egypt: Paleoenvironmental implications

Abstract

Crystal Mountain (also called Gebel Crystal) is a famous karst feature in the Western Desert of Egypt. It is a huge, unroofed cave composed of chalky limestone that constitutes the host rock of the Upper Cretaceous Khoman Formation. The cave filling consists of alternating irregular strata composed of calcareous coal horizons with complex sinkholes, detrital material, calcite crystals, and red sandstones. These coal horizons have attracted the attention of many scientists to explain the depositional environment, coal types, and ash minerals. However, there are no recent studies about the detailed paleoenvironmental conditions of these coal horizons. Accordingly, different calcareous coal horizons from the Gebel Crystal were examined to determine the paleoenvironment of the deposits. Coal samples were subjected to mineralogy and organic geochemistry to determine the nature, origin and density of the vegetation cover paleoenvironment. In addition, analyses of major and trace elements of the studied coal samples were performed to determine the different paleoenvironmental conditions of the deposits, such as paleoclimate, paleosalinity, organic matter origin, and paleo-redox conditions. The results reveal that the calcareous coal is a very low-grade coal (average ash content 40.96 wt%) with a low-rank lignite type (soft brown coal), which belongs to the ortho-lignite and/or lignite A and B facies. Type III kerogen was observed to be the main precursor of the humic peat, lignite, and sub-bituminous coal, and organic matter maturation suggests immature diagenesis stage. The immature diagenesis stage is consistent with the concept that excludes a hydrothermal solution for the formation of calcite in the coal samples. The dominance of inertinite (49.2%) and liptinite (32.4%) over huminite (18.3%) as macerals indicates that the desiccation phase took place in the mire prior to final preservation. Paleoenvironmental indices such as tissue preservation index (TPI), gelification index (GI), vegetation index (VI), ground water index (GWI) and facies diagrams suggest the dominance of marsh vegetation in the limno-telmatic, rheotrophic mire system. Piedmont alluvial plain and back-barrier with freshwater supply are inferred as the depositional sites for the studied calcareous coal formation. The mineralogical composition of the coal horizons consists of carbonate (calcite and manganocalcite), quartz, and evaporite (halite and anhydrite) minerals. The results of the paleoenvironmental indicators (e.g., Sr/Ba, Rb/K₂O, Sr/Cu, V/(V + Ni), V/Cr, Ni/Co, V/Ni, Co/Ni, sulfur, and C-value) reveal that freshwater conditions were dominant during the deposition of the coal horizons. The prevailing paleoclimate was an arid climate, which was responsible for the formation of the evaporite minerals. The organic matter originated from terrestrial sources of plant material, and the coal horizons were deposited under suboxic redox conditions.