Regolith-hosted rare earth element deposits

Common landscape in South China: low-lying areas with densely vegetated low hills[10]

The morphology of the deposits in South China (southern Jiangxi, southwestern Fujian, northern Guangdong and northwestern Guangxi in particular) is determined by both regional and local factors.[11]

Regionally, the deposits are generally found in areas with low topography, low hills lower than 500 m in elevation.[10] Moreover, as located in subtropical area, South China has a warm and humid climate. Therefore, the deposits are often densely-vegetated.[1]

Locally, the deposits tend to form along ridges, rather than valleys. Thicker weathering crust, together with its associated orebody is found along ridges while its thickness decreases valley-wards or down slope.[15]

The whole weathering crust can range from 30 to 60m in thickness,[16][14] depending on its local structural, geomorphological, and hydrogeological conditions. In general, the deposit can be divided into 4 layers with accordance to its weathering intensity. Taking a 25-m-thick weathering crust as an example, its weathering profile is as illustrated as below:

Weathering Profile of Regolith-hosted REE Deposits. (A) Humic layer. (B) Completely weathered layer. (C) Strongly weathered layer. (D) Weathering front. (E) Unweathered rock.[1]

REE enrichment generally occurs as a 5-to-10-meter-thick zone between the completely weathered layer and strongly weathered layer[12] and it is targeted for commercial mining. Compared to other REE deposits, regolith-hosted rare earth element deposits are substantially low-grade[3] (containing 0.05-0.3 wt.% extractable REEs[13]). Nevertheless, due to its easy extraction method, low processing costs and large abundance, the orebodies are economic to be extracted.[18]

In terms of tectonic settings, no obvious trend in tectonic setting that favours the formation of regolith-hosted rare earth element deposits is observed.[1] Geologists had long believed that among different magma sources, magmas originated formed anorogenic (non-mountain building) and anhydrous settings (e.g. divergent plate boundaries), which consequently form A-type granites, are REEs-enriched.[21] This is because lower degree of partial melting in this tectonic setting favours the enrichment of the REEs, which are incompatible and tend to melt preferentially. However, from field observations, A-type granite is not outstandingly REEs-enriched (in Total REE %). Instead, it is similar to I-type granites (sourced from magma of partially-melted igneous rocks) and S-type granites (sourced from magma of partially-melted sedimentary rocks), which are originated from orogenic (mountain-building) settings (e.g. convergent plate boundaries).[1]

In terms of geological times, these REEs-enriched granites which formed evenly over a wide geological time period (i.e. from Ordovician to Cretaceous),[22] showing that these deposits are not formed in special environments related to any major geological events.[1]

In general, parental rocks of regolith-hosted rare earth element deposits are felsic igneous rocks (e.g. granite, rhyolite, rhyolitic tuff etc.), which are associated with granitic magmatism and volcanism in subduction system.[23] During magma crystallisation, LREE and HREE are primarily enriched in granitoids through two separate mechanisms.[19]

• 

  Evolutionary diagram showing the first stage of LREE-enrichment process by magma differentiation.[1]
  LREE enrichment: LREE-enriched granitoids are formed by magma differentiation, which progressively fractionates magma composition into chemically-distinctive layers during its cooling process.[1] As REEs are incompatible elements (less preferred to incorporate into the structure of solidifying crystals), they remain as melt in magma chamber until the last stage of cooling. Therefore, the last and the uppermost fraction of granitoid is highly REEs-enriched.[1]
• 

  Evolutionary diagram showing the first stage of HREE-enrichment process by auto-metasomatism.[1]
  HREE enrichment: HREE-enriched granitoids are formed by auto-metasomatism. It is a process of chemical alterations of recently crystallised felsic magma by the left-over hydrothermal fluid (e.g. water, CO₂ etc.) at the later stage of magma crystallisation.[19] During the chemical alteration, through various chemical reactions with hydrothermal fluids, HREEs are then introduced into secondary minerals along veinlets.[19]

Secondary process (i.e. weathering) is essential in further enrichment of HREE-/LREE-rich granitoid. It turns the granitoid to an economically-extractable orebody. Therefore, warm and humid climate, together with slightly acidic soil in subtropical zones favour the formation of regolith-hosted rare earth element deposits. A combination of intense chemical, physical and microbiological weathering[24] allows the removal of REEs in upper, more acidic completely weathered layer, downward migration through rainwater and eventually, deposition (and concentration) at lower, less acidic moderately weathered layer.[16][25][26][27] (REE forms a more stable complex in soil with higher pH).[20]

In addition, intense weathering in subtropical areas (i.e. South China) continuously removes significant volume of overlying materials from the in-situ weathering system, which is a process called denudation.[1] In response to the mass removal, exhumation (an isostatic-uplifting process which deep-seated rock is brought to the land surface) occurs and hence, replenishes materials for on-going denudation.[16][28] Thus, the dynamic equilibrium system between denudation and exhumation further facilitates the development of thicker weathering profile as well as the accumulation of REEs.[1]

Abundant rainwater causes rapid infiltration and leaching of REEs to layer B and C. (Figure t₁) Moreover, secondary processes (i.e. weathering and erosion) remove upper layers of materials.(Figure t₂) In response to the ongoing removal of over-laying landmass, isostatic-uplifting process (exhumation) occurs continuously.[16][28] (Figure t₃) Hence, the dynamic equilibrium system between denudation and exhumation further facilitates the accumulation of REEs.[1] (Figure t₄)

Exchangeable phase (i.e. Clay-REE) accounts for 60-90% of the total REE content in the deposits. In this phase, REEs occur as mobile cations (i.e. REE³⁺), hydrated cations (i.e. [REE(H₂O)_n]³⁺or a part of positively charged complexes,[30] which are adsorbed (physically adhered by weak electrostatic attraction) at sites of permanent negative charge on clay minerals[29] (e.g. kaolinite, halloysite, illite etc.) Thus, REEs can be recovered and extracted easily by ion-exchange leaching with dilute electrolyte.[31]

Mineral phases (i.e. REE-hosting minerals) account for 10-30% of the REE content in the deposits.[30] REEs are incorporated in accessory minerals[3] (i.e. Bastnäsite (REE)(CO₃₎F, Monazite (REE)PO₄ and Xenotime (Y,REE)PO₄) as a part of crystal lattice.[32] As REEs are held by chemical bonds, alkaline bake or acid leach is required to decompose and extract REEs.[30]

In the early 1970s, batch leaching using sodium chloride solution (NaCl) was carried out in the extraction of REEs. Firstly, REE-ores were extracted and sieved by open-pit mining. Then, they are leached in barrels with ~1M NaCl solution and precipitated with oxalic acid (C₂H₂O₄).[25]

However, mining scale was highly limited by batch leaching (or bath leaching in late 1970s, using concrete pools instead of barrel) while high concentration of lixiviant could only produce low yield product with poor product quality (<70% of REE in concentration).[25] These drawbacks surpassed the originals benefits of this kind of deposits (i.e. short processing time and extremely low costs).[25]

In 1980s, batch and heap leach using ~0.3M ammonium sulphate solution ((NH₄)₂SO₄) was developed. REEs-bearing soil was mined from orebodies and piled up on a flat leak-proof layer with a collecting dish at the bottom. (NH₄)₂SO₄ solution was then injected on top of the soil and allowed for leaching. After 100 to 320 hours, REE extraction (with purity up to 90%)[25] was collected for final processing.[35]

Due to stronger desorption capability of NH₄⁺ compared with Na⁺,[13] the technology had an improved final product quality and a reduction in lixiviant consumption. Hence, it had been used as a primary model of REE leaching process in the following 30 years.[25]

Lixiviant (Leaching solution) is injected through plastics pipes, which are inserted into a leach hole.

In the last three decades, intense use of batch and heap leaching has posed a devastating and irreversible effect on the environment as well as the ecosystem in South China. Unregulated disposal of waste has also brought health problems to the residents near the mines.[35] Thus, a compulsory in-situ leaching technology was implemented in 2011 to minimise aforementioned adverse effects.[36]

In-situ leaching technology requires comprehensive geological survey of local hydrogeological structure, rock joints pattern and ore characteristics in order to design a catchment area for the leaching process. Then, vertical leaching holes (0.8 m in diameter and 2 to 3 m apart)[13] are drilled to reach the top of the REE-enriched layer (B) (1.5 to 3 m in depth) to allow injection of pressurised lixiviant (i.e. ~0.3M (NH₄)₂SO₄). Finally, the REEs-loaded leaching solution is collected by recovery ponds at the bottom of orebody for final processing.[13]

Recently, researchers have been developing various techniques to increase the yield of leaching REE.[37] Bioleaching, a technique where REEs are solubilised by microbial activities or by-products of microbial metabolism, is actively studied as a greener alternative to the current method, which has been serious pollution to the environment.[38] In terms of extraction effectiveness, some studies have reported that the recovery of REE by bioleaching could vary from less than 1% to nearly 90%.[38] Thus, further understanding of the bioleaching mechanism is required before it is commercially practised.[37]