Enrichment mechanisms of rare earth elements in Early Permian coal from the Kaiping coalfield and indicative significance for coal-forming environments

The Early Permian witnessed significant shifts in both the paleogeographic location and paleoclimatic conditions of the North China Craton, leading to the formation of extensive coal deposits. This study investigates the No. 7 and No. 8 coal seams of the Lower Permian Damiaozhuang Formation in the Kaiping Coalfield, located on the northeastern margin of the North China Craton. Through integrated analyses using Gas Chromatography-Mass Spectrometry, X-ray Diffraction, Inductively Coupled Plasma Mass Spectrometry, and Electron Probe Microanalysis, this study elucidates the enrichment mechanisms of rare earth elements (REEs) and reconstructs the paleoenvironmental conditions of coal formation. The results indicate that the No. 7 and No. 8 coal seams exhibit significant concentration in REEs, with average concentrations of 319.38 ppm and 690.27 ppm, respectively. The ratios of (La/Lu) N , (La/Sm) N , and (Gd/Lu) N indicate a light RRE (LREE)-enriched pattern. Monazite, identified as discrete grains disseminated within clay minerals, serves as the host mineral for REEs in coal deposits. The enrichment of REEs in coal is attributed to the combined influence of synsedimentary processes, REE-bearing minerals, organic matter, and clay minerals. Geochemical proxies, including Sr/Ba, Cu/Zn, Ni/Co, and δEu values, together with the ternary diagram of fluorene compounds and cross-plots of alkyl dibenzothiophenes/alkyl dibenzofurans versus Pr/Ph ratios, indicate an oxidizing freshwater depositional environment. Furthermore, Sr/Cu ratios exceeding 5 suggest a warm to hot paleoclimate during the coal formation. The bimodal distribution pattern observed in the mass chromatograms of n -alkanes, combined with parameters such as Pr/C 17 , Ph/C 18 , Carbon Preference Index, and Odd-to-Even Predominance, indicates that the organic matter was derived predominantly from higher plants, with a minor contribution from aquatic organisms. The sedimentary provenance is primarily identified as felsic rocks. This study elucidates the enrichment mechanisms of REEs in coal and reconstructs the associated coal-forming environment, providing a theoretical basis for the exploration and identification of REE-enriched coals.