Modes of Occurrence of Gallium in Al-Rich Coal: A Case Study of Permian Coal in Jungar Coalfield

High Resolution Image Download MS PowerPoint Slide Al-rich coal deposits serve as a strategic alternative source for Gallium (Ga), a critical metal essential for the semiconductor industry. While boehmite is a well-known host for Ga in Al-rich coals, the modes of occurrence of Ga in boehmite-free, Al-rich coals remain poorly understood. This study investigates the mineralogy and in situ geochemistry of the Permian No. 4 coal seam from the Huangyuchuan Mine, Jungar Coalfield (Ordos Basin, China), employing a combination of optical microscopy, Scanning electron microscopy (SEM)-EDS, LA-ICP-MS, and TOF-SIMS. Bulk geochemical analysis reveals that the studied coal samples have average Al 2 O 3 and Ga concentrations of 10.43 wt % and 15.96 ppm, respectively. Within this sample set, a strong positive correlation between Al 2 O 3 and Ga (Pearson coefficient r = 0.89) is observed for the majority of samples, excluding the anomalous sample 4–12. This specific sample (sample 4–12) exhibits a significantly lower Al 2 O 3 content (3.66 wt %) but a high Ga concentration (19.7 ppm), deviating from the established inorganic-hosted trend and serving as a critical specimen to investigate the organic affinity of Ga. Five kaolinite morphotypes were identified: cell-filling, epigenetic layered, epigenetic clay-like, clastic, and vermicular. However, contrary to the bulk geochemical correlation, in situ mapping via TOF-SIMS and LA-ICP-MS reveals that Ga is decoupled from authigenic aluminosilicates at the microscale. Instead, Ga exhibits a strong affinity for the organic matrix (specifically collotelinite) rather than secondary kaolinite phases. Geochemical proxies (e.g., Al 2 O 3 /TiO 2, Sr/Ba, V/Ni) indicate the coal formed in an anoxic, freshwater peat-forming environment. We propose that Ga was initially liberated from the weathering of terrigenous feldspathic detritus and subsequently adsorbed by organic matter during peat accumulation, which limited its remobilization into later-stage authigenic minerals. This study highlights that while inorganic minerals govern Ga distribution broadly, organic associations act as a crucial complementary mechanism for Ga enrichment, particularly in the low-ash intervals of Al-rich coal systems lacking boehmite.