An in situ, micro-scale investigation of inorganically and organically driven rare-earth remobilisation during weathering

Abstract

At present, a significant portion of rare-earth elements (REEs) are sourced from weathering profiles. The mineralogy of the protolith plays an important role in controlling the fate of REEs during weathering, as accessory minerals contain the bulk the REE budget in most rocks, and different minerals vary in their susceptibilities to weathering processes. REE supergene deposits (‘adsorption clay deposits’) are associated with deep weathering in tropical environments, which often precludes characterisation of the incipient steps in REE liberation from their host minerals in the protolith. Here we have targeted a weathered REE-enriched lithology from a sub-arid environment undergoing relatively rapid uplift, namely the Yerila Gneiss from the Northern Flinders Ranges, Australia, where regolith was shallow or absent and parent rock material had yet to completely break down. Results from X-ray fluorescence mapping, scanning electron microscopy (SEM), SEM-focussed ion beam milling (FIB-SEM), inductively-coupled plasma mass spectrometry (ICP-MS) and laser ablation ICP-MS highlight the migration pathways of REEs and associated U and Th from allanite-(Ce) grains that are the main REE host within Yerila Gneiss material. Migration of light REEs and Th away from the allanite-(Ce) grains via radial cracks resulting from allanite-(Ce) metamictisation was interpreted to result from weathering, as Ce is partially present in its tetravalent oxidation state and Th mobility is most easily explained by the involvement of organic ligands. FIB-SEM provides further evidence for the importance of biogenic processes in REE+U/Th mobility and fractionation in uranothorite-associated spheroidal structures associated with the weathering of allanite-(Ce). Organic carbon was also found in association with a xenotime-(Y) grain; in this case, REE liberation is most likely a by-product of biogenic phosphate utilisation. These results highlight that local controls (at mineral interfaces) mediated by biota and/or biogenic organic matter can control the initiation of REE (+Th,U) mobilisation during weathering.

Insights into the Selective Transformation of Ceria Sulfation and Iron/Manganese Mineralization for Enhancing the Selective Recovery of Rare Earth Elements

“…Scanning electron microscopy–energy-dispersive spectrometry (SEM-EDS) analysis of the tailing (Figure S3) also indicated the coexistence of Ce and Fe at various spots. Several studies have confirmed the high elemental affinity among Ce, Fe, and Mn in geochemical processes, such as the formation of regolith-hosted REE deposits, REE-rich deep-sea sediments, and hydrogenetic ferromanganese crusts . Considering the weight percentage and REE concentration of the fractions, the particles in the size range of +0.08–0.15 mm with the approximate concentration of 982.3 mg kg –1 were selected for further recovery experiments, and the details of individual REE concentrations and main chemical composition are listed in Table S1.…”

High-Grade REE accumulation in regolith: Insights from supergene alteration of an apatite-rich vein at the Kapunda Cu mine, South Australia

“…Factors that control how the REEs are mobilized, transported or deposited in geological fluids include pH, temperature, ligand activities (i.e., of chloride, sulfate, fluoride, carbonate, phosphate and organic species), the availability of oxidising or reducing agents, crystallographic controls and adsorption onto clays or suspended particulate matter (Aubert et al 2001;Borst et al 2020;Li et al 2022a, b;Liu et al 2018;Liu et al 2017;Louvel et al 2022;Migdisov et al 2016;Nelson et al 2003). Many of these factors are challenging to constrain in regolith due to the poor preservation of fluid inclusions, small mineral grain-sizes and dynamic (bio)geochemical processes (Kalintsev et al 2021). Nevertheless, certain minerals and textures can provide insight into the conditions of weathering.…”

“…Particularly, there is an increasing focus on the processes that govern REE behaviour in supergene systems (Andersen et al 2017;Borst et al 2020;Cook et al 2023;Hutchinson et al 2022;Li et al 2022a, b;Wu et al 2023). This is due to the complexities introduced by chemical weathering, overprinting, primary mineral destruction and multiple REE fractionation processes that result from combinations of aqueous complexation with organic and inorganic ligands, pH-Eh variation, biota, climate, authigenic mineral precipitation, clay mineral adsorption, and groundwater flow (Andersen et al 2017;Borst et al 2020;Chapela-Lara et al 2018;Cook et al 2023;Kalintsev et al 2021;Ondrejka et al 2018;Ram et al 2019;Wu et al 2023;Zhukova et al 2021). Supergene processes also enriched some of the world's most important REE deposits, such as Mt.…”

The photogeochemical cycle of Mn oxides on the Earth's surface

“…Interestingly, on Earth's surface, rare earth elements (REEs) are slightly enriched in mineral coatings compared with the underlying bedrock (Thiagarajan and Lee, 2004;Goldsmith et al, 2014;Xu et al, 2019). Particularly, the enrichment degree of cerium (Ce) in mineral coatings can reach 12 times, and Ce is concentrated in Mn-rich regions (Kalintsev et al, 2021). The occurrence of Ce is either in the form of interlayer cations in birnessite, or as Ce(OH) x nanoclusters adsorbed on corners of birnessite layers.…”