Sedimentary environment controls on critical metals enrichment in late Permian coal measures in western Guizhou, China

Abstract

Critical metals are, as the name suggests, critical to modern society and understanding how and where they form is essential for targeted exploration for new resources. In the late Permian, the input of alkaline volcanic ash from the Emeishan Large Igneous Province (ELIP) led to the formation of multiple Nb–Zr–REY (rare earth elements and Y)–Ga-enriched coal seams (called metalliferous coals) within the coal measures (middle-lower Longtan Formation) in western Guizhou, which exhibiting exceptionally high natural gamma-ray log (GR) positive anomalies. Ore beds are characterized by GR values > 2.0 pA/kg in rock layers and > 1.6 pA/kg in adjacent coal seams. The sedimentary environment of deposition controls both the formation of ore beds and the enrichment of critical metals within them. Ore beds are distributed across four facies: floodplain paleosols, freshwater peat mires, marine-influenced peat mires, and lagoons, with four corresponding lithological associations: tonstein–coal, paleosol–coal, coal, and carbonaceous mudstone. The highest concentrations of critical metals are found in tonstein–coals that formed in marine–influenced peat mire environments. Leaching by acid rain and meteoric water, adsorption effects of peat and clay minerals, the presence of acidic and reducing environments, seawater incursion, and the effects of plant growth all play a role in enhancing critical metals enrichment. The coal ash from metalliferous coals contains average concentrations of Nb, Zr, REY, and Ga of 813.80 μg/g, 5,178.58 μg/g, 1,255.21 μg/g, and 117.01 μg/g, respectively. The number of minable seams is one or two, with an average thickness of 1.98 m. The critical metals reserves in minable metalliferous coal ash are estimated at 7.81 × 10⁵ t (Nb), 4.97 × 10⁶ t (Zr), 1.21 × 10⁶ t (REY), and 1.12 × 10⁵ t (Ga). However, the coal ash is enriched not only with critical metals but also with elements harmful to health such as Pb, Cr, Th, and U, along with abundant nano–scale quartz particles, posing substantial threats to human health. Our findings represent a valuable contribution to exploration for critical metals in sedimentary successions and furthermore demonstrate potential for efficient and clean utilization of coal ash as a metal resource in an increasingly circular economy.