Geochemistry and mineralogy of Permo-Carboniferous coals from the Fugu mining district, Shaanxi Province, China: Implications for the evolution of the northeastern Ordos Block

Abstract

The northeastern Ordos Block, a pivotal transition zone between the stable North China Craton and the reactivated Yinshan Orogenic Belt, preserves critical records of intra-cratonic responses to the closure of the Paleo-Asian Ocean. However, the Permo-Carboniferous tectonic evolution of the northeastern Ordos Block is debated due to the conflicting sediment source interpretations. This study reports the systematic geochemical and mineral characteristics of three coal seams (Nos. 9, 4, and 2 upward) of the Permo-Carboniferous period collected from the Fugu mining district, northeastern Ordos Block, China, and focuses on the evolution therein. The studied coal samples exhibited slight enrichments in Th, Zr, Hf, Ta, Li, and Ga compared with the word hard coals. The Al₂O₃/TiO₂ ratio, Zr/Sc and Th/Sc plots, REY patterns, and Sr-Nd isotope analyses suggested that the elevated trace elements in three coals originated from the Benxi Formation bauxite in the northeastern Ordos Block. In the No. 9 coal (lower section of the Taiyuan Formation), the alteration in the provenance supply, as traced by variations of the high field strength elements (HFSEs), Al₂O₃ contents, Al₂O₃/TiO₂ ratios, and presence of Al-hydroxides, indicated that the northeastern Ordos Block underwent a rapid and brief uplift during the early depositional period of the No. 9 coal, followed by a sharp subsidence, and finally a gradual uplift throughout the middle to late depositional periods. The medium–high sulfur content, high Sr/Ba ratio, and negative correlation between Sr/Ba and ash yields in the No. 9 coal reflected a mixed intrusion of freshwater and seawater to the Ordos Block during this period. In the No. 4 coal (lower section of the Shanxi Formation), relatively low contents of the HFSEs and Al₂O₃ suggested the minimal terrigenous input from the Benxi Formation bauxite, indicating inconspicuous uplift occurring in the northeastern Ordos Block during the initial stage of peat accumulation. Subsequently, a gradual increase in these elements, accompanied by occurrence of minerals like Al-hydroxides, rutile, and zircon, indicated an enhanced supply from Benxi Formation bauxite, suggesting a gradual uplift. After peat accumulation, a decrease in these elements implied a diminishing influence of Benxi Formation bauxite and subsequent subsidence of the northeastern Ordos Block. In the No. 2 coal (upper section of the Shanxi Formation), variations in the supply of elements (the HFSEs and Al₂O₃) and anatase from the Benxi Formation bauxite indicated vigorous tectonic activity in the northeastern Ordos Block, with a short-lived subsidence during the initial stage, then a slow uplift, and finally sharp uplift and subsidence during the middle to late stages.