Variations in stress-sensitive minerals and elements in the tectonic-deformation Early to Middle Permian coals from the Zhuxianzhuang mine, Anhui Province

The variations in stress-sensitive minerals and elements in tectonically deformed coals (TDCs) were revealed in this paper. Results show that the clay minerals, carbonates and sulfides in the Middle to Lower Permian TDCs of the no. 8 and no. 10 coal seams of the Zhuxianzhuang mine are dominated by kaolinite, iron dolomite and calcite, and pyrite, respectively. The contents of Th are significantly higher than U, indicating that the sedimentary paleo-environment was almost independent of seawater and was characterized by hypoxia during peat accumulation periods. The light rare earth elements (LREEs) are enriched, yielding (La/Lu)N values of 1.00–12.30 and slightly positive Ce anomalies in the no. 8 (0.95–1.44, δCe/δEu = 1.30–2.62) and no. 10 coal seams (1.02–1.31, δCe/δEu = 1.66–2.17). The cluster results show that stress-sensitive elements in TDCs can be classified as aggregation-type (Si, Al, Ti, K, Fe, Sc, Li, Rb, Nb, Ag, Cd, Ta, Tl, Bi, Th, U, W), dissipation-type (N, H, Cl, Mo, Ni, Y, Sr), and mutant-type (O, Na, Ca, Be, Mn, Co, Ga, Ge, Pb, Bi, As). The enrichment effects of ductile deformations for major elements are more significant than those of brittle deformations. The combinations of (Nb, U), (Th, Rb), and (Li, Sc, Ag, Ta) are good brittle-, ductile-, and brittle-ductile enrichment-type elements, respectively, which can be used as good indicators of the coal's tectonic deformation. Sr records higher loss rates in brittle TDCs than in wrinkle coals. The Mo content gradually decreases with increasing tectonic deformation intensity, which is related to the dynamic metamorphism caused by tectonic deformation. Nucleophilic clay mineral elements (Be, Ga, Pb, Ge) in the no. 8 and no. 10 coal seams record similar trends and are highest in scaly coals, indicating that strong shearing effects can promote the enrichment of these elements. The contents of LREE and heavy rare earth elements (HREEs) both increase with the increasing tectonic deformation intensity. For various TDCs, LREE record higher degrees of fractionation than the HREE.