## 3 Samples and Analytical Methods  
Twelve samples were collected following the American Society for Testing and Materials Standard ASTM D4596-22 (2022) [31] via slot sampling at the Gaoping Coal Mine (Figure 1), including one roof, one floor, one parting (15R, 15F, and 15P), and nine coal samples (C1-C9). After sample collection, the samples were immediately placed into sealing bags to prevent contamination and oxidation. According to the Chinese National Standard GB/T 16773-2008 [32], polished coal sections can be prepared to observe macroals under a microscope and determine the vitrinite reflectance. Powdered samples with sizes less than 74 \(\upmu\)m, were prepared for coal quality analysis, geochemistry analysis, and X-ray diffraction (XRD) analysis. Vitrinite reflectance was determined according to the American Society for Testing and Materials Standard ASTM D2798-21 (2021) [33] using a Leica DM4P microscope.  
Figure 1: (**A**) Map of coal-bearing regions in China (modified from reference [30]); (**B**) Regional geographical map of research region; (**C**) Stratigraphic column section of the Gaoping coal-bearing strata.  
Low-temperature oxygenated plasma ashing of the coal powder samples was performed using K1050X plasma. The low-temperature ash and non-coal mineral components were analyzed by XRD (Bruker D8 Discovery). The test 20 angle was 5\({}^{\circ}\)-70\({}^{\circ}\) with a step of 0.02\({}^{\circ}\) and a count time of 0.75 s/step. These tests and experiments were conducted at the Shanxi Key Laboratory of Fine Exploration of Coal-based Critical Mineral Resources. In quantitative analysis, the major diffraction peaks (\(d\)-values and intensities) were compared with PDF standard cards to determine the major mineral compositions. The calculations were based on the formula [34; 35; 36]:  
\[W_{A}=(I_{A}/K_{A})/(I_{A}/K_{A}+I_{B}/K_{B}+I_{C}/K_{C}+\ldots\ldots)\times 1 00\%\]  
where \(W_{A}\), \(I_{A}\), and \(K_{A}\) are the weight percentage, the intensity of the most intense peak, and the reference intensity ratio (\(K\)) of the mineral phase to be determined, respectively. Symbols such as \(A\), \(B\), and \(C\) represent different minerals.  
The major element test was conducted according to the Chinese National Standard GB/T 14506.28-2010 [37], and each powdered sample was weighed to 1 g and prepared using the melting method, with anhydrous lithium tetraborate and lithium chloride as the mixing solvents, ammonium nitrate as the oxidant, and a small amount of lithium bromide as the flux. The mass ratio of sample to flux was 1:8, and the sample was melted at 1200 \({}^{\circ}\)C for 10-15 min to prepare a glass sheet. The glass sheet was evaluated using an X-ray fluorescence spectrometer (Axios-mAX). The trace element test was conducted according to the Chinese National Standard GB/T 14506.30-2010 [38], and each powder sample was dissolved in a mixture of hydrofluoric acid (HF) and nitric acid (HNO\({}_{3}\)) (approximately 0.05 g) and then diluted with HNO\({}_{3}\) in a sealed tube up to 50 mL. The trace elements in solutions were analyzed by inductively coupled plasma mass spectrometry (NexION300D plasma mass spectrometer). Major and trace element tests were completed at the Analytical Laboratory of Beijing Research Institute of Uranium Geology.  
Scanning electron microscopy (SEM) images were obtained using a JEOL JSM-5610 LV scanning electron microscope coupled with an energy-dispersive X-ray spectrometer (EDS) at the Chinese Academy of Geological Sciences. The operating conditions included an accelerating voltage of 20 kV and a spot size of 38 \(\upmu\)m.  
Infrared spectroscopy (IR) was performed using a Nicolet 6700 FTIR spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). The dried potassium bromide and powder samples were mixed at a ratio of 200:1, thoroughly ground, and compacted into tablets under a pressure of \(\sim\)30 MPa for 1 min. Spectral scanning resolution was set at 4 cm\({}^{-1}\), spanning the wave number range of 4000-400 cm\({}^{-1}\).