Tobias Bamforth 1,2, Caroline Tiddy 1, Ignacio Gonzalez-Alvarez 2,3, Eric Whittaker 4, Leon Faulkner 5
1Future Industries Institute (FII), University of South Australia (UniSA), Adelaide, Australia; 2CSIRO, Mineral Resources Discovery Programme, Kensington, Perth, Australia; 3CET, University of Western Australia, Crawley, Perth, Australia; 4Terramin Australia Ltd., Adelaide, Australia; 5Environmental Copper Recovery Ltd., Adelaide, Australia
The negatively-charged surfaces of clay minerals demonstrate an excellent capacity for the adsorption of rare-earth elements (REE’s) in regolith. Often, this is observed during the formation of secondary phases such as kaolinite and halloysite (Borst et al. 2020. Nat. Comm.), though alternate clay minerals display a similar capacity for REE sorption across low-temperature environments. For instance, ferrihydrite (5Fe2O3 · 9H2O) is an amorphous clay mineral that is widely acknowledged as an excellent REE sorbent in hydrospheric systems. Thus far, evidence for its potential in association with high-grade REE accumulation has been limited; however, new data from the Kapunda Cu mine, South Australia, suggests that even in the presence of co-existing kaolinite, ferrihydrite may act as a principal sorbent during lanthanide fixation.
Petrographic analysis of weathered vein samples from the Kapunda mine, which exhibit total rare-earth oxide (TREO) concentrations of 17.1 wt%, demonstrate that interstitial aggregates of REE phases monazite and rhabdophane are spatially associated with ferrihydrite particulates. In comparison, a negative correlation is observed between REE minerals and crystalline phases of goethite, hematite and kaolinite. Ferrihydrite formed during the acidic dissolution of primary pyritic minerals, which is evidenced by cubic void spaces throughout the sample. In addition, geochemical evidence from Ce/U valency modelling suggests that weathering fluids were highly acidic (pH 1 – 3) and moderately oxidising (Eh 0.7 – 0.9) in nature. REE mobilisation was likely facilitated by the formation of sulphate complexes, which is supported by geochemical whole-rock profiles that exhibit low chondrite-normalised La/Yb ratios (Migdisov et al. 2016. Chem. Geol.). Chondrite-normalised plots also exhibit Pr > Nd > Ce > La enrichment, which compliment previous conclusions on the ability of ferrihydrite to selectively adsorb the valuable REE’s Pr and Nd (Bau. 1999. Geochim. Cosmochim. Acta.). Phosphate ions were sourced from primary sedimentary apatite, which is evidenced by the pseudomorphic replacement of hexagonal crystals by secondary REE-baring phases. This advocates for the role of ferrihydrite as a catalyst following the co-adsorption of REE’s and PO42- (Arai and Sparks. 2001. J. Colloid Interface Sci.), and explains the observed spatial boundaries of lanthanide accumulation, as apatite is not expressed in notable quantities throughout the bulk of the Cu deposit.
Results demonstrate the potential of ferrihydrite as a principal sorbent for economic REE mineralisation, when considering: (1) its relative pervasiveness across global regolith profiles; (2) its crystal structure, surface area and adsorption capacity and; (3) its potential ability to selectively adsorb the valuable light rare-earth elements Nd and Pr. Additionally, this work emphasises the importance of apatite as a phosphate source in REE-mineralising systems, and advocates for the increased consideration of ferrihydrite as a potential catalyst for REE accumulation as the demand for these critical metals escalates under the development of renewable technologies.
Tobias studied BSc Geology at the University of Southampton, before completing an MSc in Global Management of Natural Resources at University College London. During this time, he completed a mineral exploration studentship at UniSA and CSIRO, and is now studying a PhD in Geochemistry at Murdoch University, Perth.