Enrichment of Al-Li-Ga-Nb(Ta)-Zr(Hf) in the Permo-Carboniferous coal-bearing sequences of the Jungar Coalfield, northern China

Coal-hosted critical metal deposits have emerged as a viable alternative to conventional critical metal resources and have garnered significant attention in recent years. Notable enrichments of critical metal assemblages, including aluminum, gallium, lithium, and rare earth elements and yttrium (REY) have been identified in several Permo-Carboniferous coal measures within the Jungar Coalfield and across other coalfields in the Ordos Basin, northern China. Using a number of analytical techniques such as X-ray diffraction analysis (XRD), scanning electron microscopy (SEM-EDX), the TIMA-X integrated mineral analyzer, X-ray fluorescence spectroscopy (XRF), inductively coupled plasma-mass spectrometry (ICP-MS), and LA-ICP-MS, this study systematically investigates the mineralogical and geochemical characteristics of three economically significant coal seams (Nos. 3, 6 upper, and 6 coal) in the Laosangou exploration area of the northern Jungar Coalfield. The study focuses on the modes of occurrence, enrichment mechanisms, and economic potential of critical metal assemblages within the coal deposits.

Compared to the average for world hard coals, Al-Ga-Li-Nb(Ta)-Zr(Hf) assemblages are generally enriched in the No. 3 coal of the Shanxi Formation and Nos. 6 upper and 6 coals of the Taiyuan Formation, and exhibit much higher values in the lower portion of the No. 6 coal adjacent to or within the tonsteins. Aluminum, Ga, and Li in the studied coals are primarily hosted in kaolinite and/or boehmite, while Nb, Ta, Zr, and Hf primarily occur in zircon and/or anatase embedded within kaolinite. The critical metals in the studied coals all demonstrate promising economic potential and could potentially be extracted from the coal-combustion fly ashes.

The enrichment of the Al-Li-Ga-Nb(Ta)-Zr(Hf) assemblages in these coals is attributed to contributions from both terrestrial sediment sources and pyroclastic materials, as well as hydrothermal fluid injections. The sediment provenance of the studied coals includes inputs mainly from the Yinshan Oldland, northern Trans North China Orogens (TNCO), and bauxite-rich weathering crusts of the Benxi Formation, which served as initial sources of Al, Li, Ga, and other critical metals. Additionally, the intermediate-felsic alkaline volcanic ash enriched in Li, Ga, Nb, Ta, Zr, and Hf along with the injection of acid groundwater or low-temperature hydrothermal fluids further facilitated the enrichment of these critical metals.