Lichen as a Potential Exploration Tool for Rare Earth Element–enriched Carbonatites

The increased demand for rare earth elements (REE) associated with the global transition to low-carbon energy systems requires the development of new exploration tools. Lichens, symbiotic associations of fungi and algae or cyanobacteria, represent promising biological indicators in mineral exploration owing to their visibility, substrate specificity, and environmental resilience. Through detailed field mapping and preliminary geochemical investigations in the Bayan Obo deposit, we identified that the lichen Xanthoria elegans selectively grows on REE-enriched carbonatites. This lichen contains exceptionally high REE concentrations (ΣREE = 3,500–17,000 ppm), which correlate closely with the REE contents of its substrates. It also exhibits Sr and Pb isotope compositions indistinguishable from those of its host carbonatites. The strong spatial and chemical affinity of X. elegans for carbonatites, combined with its scarcity on other lithologies, supports it as a reliable and effective field indicator in this region and its potential in analogous environments elsewhere. When integrated with remote-sensing techniques, this new biological proxy could offer a rapid, low-cost, and environmentally friendly means to detect REE-bearing carbonatites, offering a valuable complement to traditional exploration strategies.