The influence of terrestrial plants on the enrichment of critical metals in coal

The terrestrialization of plants during the Paleozoic enabled the earliest peat-forming wetlands initiating coal accumulation in the Middle to Late Devonian. Beyond their role in organic carbon burial, terrestrial plants created the geochemical conditions for peat accumulation and coalification, thereby allowing the incorporation of metals within coal. Metals in coal have multiple sources and modes of occurrence and mineralisation can occur at various stages. Later-stage processes in coal formation have received considerable attention whereas the role of precursor plant material in early-stage metal accumulation remains underestimated. This study synthesizes current knowledge of the early stage processes for metal enrichment in coal and compares the roles of plant biomass, organic matter maturation and hydrothermal processes on the redistribution of metals in coal. Although critical metals (e.g. Ge, U, rare earth elements) are the focus, other metals are also discussed, as they broadly exhibit similar behaviour during peat accumulation, diagenesis, coalification and post-diagenetic stages. Terrestrial plants are the primary precursors of coal and establish the initial metal distribution, with metal uptake by plants (bioassimilation) and by humic acids in peat that interact with associated sediments and volcanic ashfalls. In both peat and low-rank coals humic substances provide potential metal-binding sites primarily through chelation. Organic association is especially significant in low-rank coals where most metals are bound to organic matter. The stability of these associations depends on the chemical environment and binding energies of specific functional groups, as demonstrated by density functional theory (DFT) calculations. Several world-class Ge deposits occur in low-rank coals, with Ge and associated metal(loid)s (e.g. W, U, As) being mainly bound organically in chelated form. These metals are progressively released from organic matter as coal rank increases, due to the loss of oxygen-bearing functional groups, and their concentrations in high-rank coals are generally negligible. Instead, these metals reform as submicron to nano-scale mineral phases, that are commonly reported as organically bound metals in high-volatile bituminous coals, for example by selective leaching studies. In some coals, post-depositional hydrothermal fluids migrating via permeable conduits such as coal cleats, fractures and porous inertinite layers are the principal control on metal enrichment, exceeding the influence of original peat composition. Nevertheless, metal bioassimilation and complexing by humic substances in peats represent two pathways of metal enrichment in coal, alongside other geological processes.