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 sorptiondesorption mechanism. Application of visible light-infrared spectroscopy to the exploration of regolith-hosted REE and Sc deposits deserves particular attention in the future endeavour.

稀土和钪是世界各国竞相争夺的战略矿产资源. 世界上90%以上的重稀土来自中国南方及邻区的风化壳型稀土矿床, 而全球各地也有越来越多风化壳型钪矿床被发现, 提供了潜在可用的钪资源. 因此, 风化壳型矿床的特点和成矿过程亟需更多的研究以助于圈定有利的地质地貌环境, 开展稀土和钪的勘查找矿工作. 前人研究表明, 风化壳型稀土和钪矿床分别主要源于长英质岩浆岩和基性-超基性岩的风化, 矿体成分对母岩有很强的继承性, 暗示成矿母岩在成岩过程中已经发生稀土和钪的预富集作用. 风化过程中, 稀土和钪分别主要吸附在黏土矿物和铁氢氧化物中. 这些元素的活化-迁移与地下水的活动和它们在次生矿物中的吸附和解吸密切相关, 并最终导致这些元素富集成矿. 表生地质过程对风化壳型矿床的成矿和保存有重要的制约, 强弱侵蚀间歇交替对厚大矿体的发展有利, 而风化作用与剥蚀作用之间存在的某种平衡亦可能对这类矿床的形成有重要影响. 未来仍需更多研究关注成矿母岩中元素的预富集机制、矿体形成及保存有利的表生条件、元素赋存状态等问题. 此外, 可见-近红外光谱技术在找矿上的应用也是未来有关研究的一个重要方向.