The fundamentals of rare earth element ion adsorption clay deposits: A mineral systems approach for exploration

The exponential growth of demand for ‘green-technologies’ requires significantly increased production of critical elements, including rare earth elements (REE). Some of the most significant (and largest) REE deposits are associated with carbonatites. However, carbonatites are predominantly light-(L)REE-enriched, which has implications for meeting global heavy-(H)REE demand. As a result, REE ion adsorption clay deposits (IACD), which are examples of intense weathering, have sparked international interest as a HREE source (~80 % of global HREE are sourced from IACD). Therefore, this study presents a comprehensive review of REE IACD to understand their constraints, global distribution, and main features while applying a mineral systems approach.

The REE source for IACD, although typically granitic, is more diverse than traditionally thought, with the weathering of local igneous, metamorphic, and sedimentary rocks and external fluids (e.g., hydrothermal fluids and basinal brines) and lithologies (e.g., transport of weathering constituents rather than an in-situ source) supplying the REE required for IACD formation. Following the weathering of REE-rich source material, REE are liberated and mobilised in the weathering profile through pH-dependent complexation with ligands (e.g., F⁻, CO₃²⁻, SO₄²⁻, PO₄³⁻) or as hydrated REE species. The nature of the source (e.g., relative LREE- or HREE-enrichment) and fluids within the weathering profile (e.g., pH and ligand concentrations) control REE fractionation and relative LREE and HREE enrichment of a IACD. Once mobilised, REE are adsorbed out of solution and enriched onto clay minerals (e.g., kaolinite and halloysite), a process strongly controlled by pH and the physicochemical characteristics of the clays present, with REE adsorption most favourable under circumneutral conditions. To preserve REE enrichment (and IACD formation) through clay adsorption, a low erosional setting is required. Climates with excessive rainfall (e.g., tropical humid climates) may be problematic for REE IACD preservation through geological time, where excessive rainfall results in clay dissolution and saprolite collapse. The conceptual model provided in this study develops a framework that will be built upon in the coming years as our knowledge of these deposit types and global exploration continues.