In deep underground environments, temperature is one of the key factors affecting the geochemistry behaviors of rare earth elements (REE) in organic-rich fluid. However, the influence of temperature on the interaction between humic acids (HA) and REE is not well known. In the present study, the influence of temperature on the HA–REE-binding behavior was evaluated based on heating experiments of REE-doped HA solution. Lignite-extracted HA and REE-binding experiments were conducted over a temperature range of 20 to 200 °C to quantify HA–REE complexation and the influence of temperature on HA binding sites. Results showed that increasing temperature and decreasing [REE]/[HA] ratio cause an increase of K_d value (the partition coefficient of REE between HA and aqueous solution). During heating $K_{\mathrm{d}}^{\mathrm{REE}}$ $K_{\mathrm{d}}^{\mathrm{REE}}$ patterns gradually change from middle REE-enriched-type (M-type) at 20 °C to light and middle REE-enriched-type (L-M-type) at 50 and 100 °C, and to light REE-enriched-type (L-type) at 150 °C and 200 °C. The increase of REE bonded with HA and modifications of $K_{\mathrm{d}}^{\mathrm{REE}}$ patterns during the thermal treatment may be attributed to the increase of REE-binding sites, especially carboxylic sites, as a consequent of HA decomposition. This study provides a glimpse into the HA–REE-binding behaviors in the deep underground environment, which may shed light on the geochemical characteristics of REE in some organic-bearing rocks, and their changes during the coalification process.

The Qinshui Basin is located in the southeast of Shanxi Province, China. It is one of the most abundant coal resources from Permo-Carboniferous North China. It is rich in coal and coalbed methane resources. However, the accumulation of coalbed methane is complex and the enrichment law has not been fully understood because of the high heterogeneity of coal reservoirs in the Qinshui Basin. The examination of dissimilarities between tectonically deformed coals (TDCs) and primary coals at multiple scales holds paramount importance in advancing our understanding of the occurrence and flow patterns of coalbed methane, and in providing guidance for exploration efforts. In the present study, the samples from the Jincheng Mine, Qinshui Basin, were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), CO₂ gas adsorption and 3D X-ray micro-computed tomography. The results showed that the dominant minerals in coal were illite, kaolinite, and calcite, with minor amounts of quartz and ankerite. In comparison to primary coal, tectonism could increase the microfractures density of type A (the fracture of width ≥ 5 μm and length > 10 mm) in TDCs. In CO₂ gas adsorption in mylonite coal, it was observed that the volume of micropores (<2 nm) was significantly reduced leading to a decrease in gas adsorption capacity. The result of Micro-CT scanning revealed that the minerals occurred as veins in primary coal, but as irregular aggregates in TDCs. Moreover, tectonism had a staged impact on fracture structure, which was initially closed in cataclastic coal and then formed into granulated coal during the tectonic evolution. The effects of tectonism on coal structure had an impact on the connectivity of micropores at the micrometer scale by the destruction of the pore throat structure, increasing the heterogeneity of the reservoir. These findings help to better understand the changes in TDC structure at different scales for developing effective strategies for coalbed methane exploration and production.

Coal-bearing strata in the Ningdong Coalfield (western Ordos Basin) contain elevated concentrations of critical metals; however, the processes governing their enrichment remain incompletely understood. This study integrates zircon U–Pb geochronology, LA-ICP-MS elemental mapping, sequential chemical extraction, and mineralogical and bulk geochemical analyses to constrain the provenance, host phases, and enrichment mechanisms of Li, REY, and Ga in Late Carboniferous–Early Permian coals from the Ningdong Coalfield. The No. 5 and No. 9 coals are medium-ash bituminous coals significantly enriched in Li and rare earth elements and yttrium (REY) relative to world coal averages. Zircon U–Pb ages reveal distinct provenance for the two coal seams: the No. 5 coal records a dominant, proximal volcanic source linked to the Central Asian Orogenic Belt (CAOB) at ca. 298 Ma, whereas the No. 9 coal reflects mixed contributions from both the CAOB and the Alxa Block. Tonsteins and dispersed pyroclastic materials, interpreted as altered alkaline to peralkaline volcanic ash, constitute the principal source of critical elements. Titanium-bearing vermicular kaolinite formed through in situ alteration of volcanic ash exhibits the highest Li concentrations among the host phases, whereas REY are mainly concentrated in authigenic goyazite within phosphorus-rich horizons. Coal benches adjacent to tonsteins and partings display higher concentrations of Li, REY, and Ga than the associated non-coal lithologies, indicating leaching and redistribution of these elements by acidic diagenetic fluids. The weighted mean Li2O concentrations in coal ash reach 861 μg/g and 871 μg/g for the No. 5 and No. 9 coals, respectively, exceeding the proposed recovery threshold (800 μg/g); rare earth oxides (REO) contents locally attain ~5200 μg/g, substantially surpassing the cut-off grade (1000 μg/g); and Ga in the No. 9S coal ash averages 157 μg/g, exceeding the economic cut-off (100 μg/g). These results demonstrate that coupled alkaline volcanic ash input and acidic leaching exert the primary control on critical-element enrichment in the Ningdong Coalfield, suggesting that critical-element mineralization is regionally pervasive rather than locally confined.