241 / 2025-04-15 18:42:05

Terrestrial Plant Control on the Enrichment of Critical Metals in Coal-Bearing Strata

coal,Terrestrial Plant,critical metals,organic matter,Hyperaccumulation,Post-Depositional Processes

Theme 3: Earth-Life Co-evolution Through Time > Session 3c: Life-Mineral Interactions during the Plant Terrestrialization

Although the metal content in most coals is relatively low, under certain conditions, some critical metals can be enriched in coal, reaching or even exceeding the grades of traditional critical metal ore deposits. Typical examples include gallium, germanium, rare earth elements, lithium and uranium. The inorganic matter in coal is characterized by multiple sources, and accordingly, previous studies have attributed the enrichment of critical metals to factors including the terrestrial material in the erosional source areas, contemporary volcanic ash, leaching by groundwater and other solutions, and post-diagenetic hydrothermal processes. However, the contribution of coal-forming plants, which are the primary precursors of coal, to the inorganic geochemistry of coal is still not well understood. Moreover, there is little discussion on the coupling effects of multiple factors and the organic matter formed by coal-forming plants.

Hyperaccumulation by certain plant species, determined by the maximum levels of an element a plant can safely take up from the soil without causing harm to its tissues, is not uncommon. The presence of hyperaccumulator plants among modern species suggests that this mechanism likely existed among peat-forming plants in the geological past. In some coal seams, especially those where coal occurs near the bottom or within claystone partings, there is a relative enrichment in heavy rare earth elements. This may reflect a biological pattern of rare earth element uptake and transfer from roots to stems and eventually to leaves in coal-forming plants. Conversely, in coal seams formed in raised peat bogs-where rainwater is the primary nutrient source-rare earth elements tend to be depleted, indicating that as peat accumulates, plant roots may lose contact with inorganic substrates, thus limiting the uptake and enrichment of these elements.

Hyperaccumulation is only one of several mechanisms contributing to the organic association of inorganic elements in coal. Following plant death and burial, the coalification process initiates a long transformation of plant tissues. During the peat and low-rank coal stages, plant-derived organic matter retains abundant functional groups capable of binding critical metals that may originate from multiple processes. Recent research highlights the role of hydroxyl and carboxyl groups in the fixation and mineralization of trace elements, such as Ge, W, As, Be, and U, within the organic matter of peat and low-rank coals. However, as coal matures to higher ranks, such as bituminous coal and anthracite, the progressive loss of oxygen-containing groups leads to the expulsion of metals from the organic matrix as the binding sites disappear.

The mobilization of metals during coal's thermal maturation represents a key process influencing the redistribution and potential secondary enrichment of critical elements. In parallel, the physical structure of coal-including cell cavities, cleats, fractures, and pore systems-plays a crucial role in metal retention and migration pathways. Post-maturation processes can further alter the original metal distribution, complicating interpretations of enrichment patterns. Compounding these challenges are the inherent difficulties in tracing the composition of precursor plant material and distinguishing plant-derived signatures from those overprinted by diagenetic and post-diagenetic transformations. Together, these factors underscore the complexity of understanding critical metal enrichment in coal and highlight the need for integrated studies that account for both organic and inorganic processes across the entire coal-forming continuum.