Mineralogy and geochemistry of the Middle Jurassic coal in the southern Ordos Basin, China: Responses of element geochemical behavior to climatic changes

The Jurassic was characterized by a fluctuating climate. The coal-bearing strata deposited in North China during this period are possibly one of the useful recorders for palaeoenvironment and palaeoclimate. This study investigates the sources of inorganic components in the Middle Jurassic coal from the southern Ordos Basin and discusses the geochemical characteristics of these coals in response to changes in the Middle Jurassic climate. Specifically, the petrological, mineralogical, and elemental geochemical characteristics of the No. 4 coal from the Tingnan mine were examined. These results reveal that the No. 4 coal is characterized by low-ash, medium‑sulfur, and high-inertinite contents. Compared with the values of common Chinese coals and world hard coals, respectively, the contents of most major element oxides and trace elements are depleted. The discrimination diagrams (including Al2O3/TiO2, Al2O3/TiO2 vs. Zr/TiO2, and Al2O3/TiO2 vs. Nb/Yb) indicate that inorganic detrital materials in the coals mainly come from intermediate-felsic rocks in the Qinling Orogenic Belt. Authigenic kaolinite may have formed in acidic conditions. The M-type enrichment and positive Gd anomalies indicate acidic water circulation within the basin. The modes of occurrence of some minerals, such as calcite, dolomite, and pyrite, suggest the injection of diverse solutions during the epigenetic stage. Geochemical indicators, including Ga/Rb, Sr/Ba, Th/U, and δ13C, indicate deteriorating environmental conditions and a gradual transition towards aridity. The high-frequency fluctuation of δ13C values in the lower part of the No. 4 coal indicates an unstable climate during the early stage of peat deposition. The δ13C values in the coals experience a significant positive excursion, indicating increasing arid conditions. Notably, the petrological and geochemical characteristics of the No. 4 coal from the southern and northern margins of the Ordos Basin provide information on changeable and oscillating climates (arid and humid) during the Middle Jurassic. •The Qinling Orogenic Belt is the main erosion source area for the Tingnan coal.•The Tingnan coal underwent the injections of multiple solutions.•Increased δ13C and changed sensitive elemental ratios indicate a more arid climate.•Arid climate may contribute to subsequent global activities such as marine anoxia and greenhouse effects.