The genesis of regolith-hosted rare earth element and scandium deposits: Current understanding and outlook to future prospecting

Abstract

Rare earth elements (REE) and scandium (Sc) have become strategical and critical resources to nations worldwide. Currently, regolith-hosted REE deposits in South China and the neighbouring regions provide more than 90% of the global heavy REE (HREE) production. Recent discoveries highlight the potential of exploitable Sc resources from regolith-hosted deposits. Hence, knowledge about the nature and ore-forming processes of these REE and Sc deposits is important to facilitate identification of favourable geological and geomorphological environments and thus to guide exploration of these deposits. Previous studies have confirmed that felsic and mafic-ultramafic rocks are favourable parent rocks for the formation of REE and Sc deposits, respectively. Apparently, these parent rocks are already enriched in REE and Sc, both geochemically and mineralogically, during their petrogenesis. Favorable protolith for the formation of regolith-hosted REE deposits includes granites, syenites, and volcanic rocks with most of the REE hosted in weathering susceptible minerals, for example, synchysite, gadolinite, and hingganite for the HREEs, and allanite, titanite, apatite for the LREEs. Clinopyroxene-rich mafic-ultramafic rocks are ideal protolith for the formation of regolith-hosted Sc deposits. During weathering, REE are liberated during decomposition of the parental minerals and mainly sorbed on clay minerals, notably kaolinite and halloysite. A critical transformation from poorly crystallized, nano-sized halloysite and kaolinite to much more crystalline and larger vermicular kaolinite is observed with progressive weathering. Adsorption capacity of the clay minerals significantly decreases in this transformation, thereby, REE adsorption becomes unfavorable and desorption dominates in the shallow soils, whereas efficient adsorption on the clay minerals could take place in deep regolith. Weathering behavior of Sc is different from the REE. A case study from Australia reveals that Sc is initially sorbed on smectite after decomposition of parental Sc-rich clinopyroxene. Subsequently, Sc is sorbed on neoformed Fe oxyhydroxides, thereby efficiently retain in the shallow Fe-rich duricrust, when smectite is dissolved in more advanced weathering. Mobilization, re-distribution, and accumulation of REE and Sc to form deposits are closely related to the groundwater flow and adsorption-desorption processes of these elements on the supergene minerals. Transport of REE would be dominated as free cations in shallow, acidic soils but the importance of REE-carbonate complexes dramatically increase with depth, facilitating the mobilization of HREE. Moreover, external elemental input from local groundwater would result in precipitation of supergene REE minerals, for example chernovite-(Y) at the Zudong deposit in South China. Supergene processes are key in controlling the formation and preservation of ore bodies. Alternating episodes of intense and weak erosion are favourable to form giant regolith-hosted deposits, whereas an equilibrium state between weathering and erosion also plays a role in the ore formation. Intense erosion would accelerate chemical weathering and liberation of REE and Sc from the protolith for ore formation, whereas intervals of minimal erosion are also required for preservation of thick ore bodies. Pre-concentration of REE and Sc in parent rocks, and favourable supergene environments for the ore formation and preservation remain for further studies. Key scientific questions awaited to be solved include the petrogenesis of geochemically and mineralogically favorable protolith as ground preparation for the supergene ore formation and the concentration mechanism of REE and Sc in this process. Moreover, it is crucial to decipher the occurrence of REE and Sc in different clay minerals and Fe oxyhydroxides in these deposits, and the associated sorption-desorption mechanism. Application of visible light-infrared spectroscopy to the exploration of regolith-hosted REE and Sc deposits deserves particular attention in the future endeavour.