A comparison of global rare earth element (REE) resources and their mineralogy with REE prospects in Sri Lanka

Highlights

• Prospect new REE resources is essential considering their status as strategic elements.

• Sri Lanka has a favorable geological setting implying the presence of REEs.

• The Pulmoddai mineral sand deposit is the most potential RE resource in Sri Lanka.

• Carbonatites in Eppawala, alluvial deposits and pegmatites are the other RE prospects.

Abstract

Rare earth elements (REEs), a group of 17 elements comprises 15 lanthanides, scandium and yttrium, are largely attracting the world’s attention due to their importance in a wide variety of advanced technological applications. Global REEs production is mainly sourced from resources, such as carbonatites, alkaline igneous rocks, placers, laterites, and ion-adsorption clays. Recently, REE demand has been escalating, especially due to the REE applications in renewable energy and defense sectors, expecting a worldwide shortage of REE supply in the future. Therefore, REEs have been widely accepted as strategic elements in the world, which compels to prospect for new and alternative REE resources. In this context, Sri Lanka has a favorable geological setting which implies the presence of REE mineralization. Previous geochemical studies in Sri Lanka have reported significant concentrations of REEs in different geological formations and mineral resources. Accordingly, Pulmoddai and other beach placer deposits, Eppawala carbonatite, alluvial placer deposits, and pegmatites have been identified as potential REE resources in Sri Lanka. Monazite, apatite, allanite, and zircon are the primary rare earth (RE) minerals found in the preceding resources. The Pulmoddai mineral sand deposit is considered as the most potential REE resource in the island, which is enriched in monazite containing more than 61% of light rare earth elements (LREEs). Similarly, Eppawala carbonatite contains high concentrations of LREEs. However, despite their significant REE enrichments, to date, no attempt has been made to recover these REE prospects, which essentially conceals their potential of catering for both local and global REE supply chains.

Introduction

In 1787, a unique black mineral was found in a small quarry in Sweden containing a series of new kinds of elements, lately named as rare earth elements (REEs). It was the world’s first ever discovery of REEs and cerium (Ce) was the first rare earth element (REE) isolated in 1803 (Pecharsky and Gschneidner Jr, 1999). Since then, more than 250 rare earth (RE) minerals have been identified in different geological formations and mineral resources, worldwide. REEs are a set of seventeen chemical elements comprising 15 lanthanides, yttrium (Y) and scandium (Sc) (Balaram, 2019). REEs are typically divided into two sub-groups, namely light rare earth elements (LREEs) from lanthanum (La) to samarium (Sm) and heavy rare earth elements (HREEs) from europium (Eu) to lutetium (Lu) plus Y. However, Sc does not belong to any of these categories, as it does not occur in the same deposits as the other REEs (Van Gosen et al., 2014).

REEs are not as rare as their name suggests. They are lithophile elements (elements enriched in the earth’s crust), which have a total crustal abundance of 220 ppm (Table 1), even higher than that of carbon (200 ppm). Moreover, some REEs are more abundant than most of the frequently used industrial metals, such as copper (Cu), cobalt (Co), lead (Pb), and tin (Sn) (Gupta and Krishnamurthy, 2005). According to Table 1, La, Ce, Y, and neodymium (Nd) are the most abundant REEs in the earth’s crust, whereas promethium (Pr) is the rarest of all REEs. Nevertheless, the commercial availability and their metallurgical usage do not depend on its crustal abundance. They depend on factors, such as (1) the degree of natural enrichment of a metal into the ore deposit (2) the relative feasibility of separating the ore from the host rock, and (3) the feasibility of extracting the metal from the ore. Since REEs do not satisfy the aforementioned requirements, they are termed as “rare” (Hampel and Kolodney, 1961).

REEs exhibit unique chemical, physical, magnetic, and luminescent properties, due to their different atomic structures and states (Zepf, 2013, Dushyantha et al., 2020). These properties promote a multitude of technological advantages, such as low energy consumption, high efficiency, miniaturization, speed, durability, and thermal stability. Therefore, REEs are commonly used in modern high technological applications, such as rechargeable batteries, autocatalytic converters, super magnets, LED lighting, fluorescent materials, and solar panels (Balaram, 2019). This immense consumption of REEs in the advancement of modern high-tech and green technologies has resulted in a rapid growth in the REE demand over the last two decades (Mancheri et al., 2019). Moreover, it has been predicted that the demand for REEs could increase by 7–8% annually (Kingsnorth, 2016). Since global production is presented in terms of rare earth oxides (REOs), global production estimation of REOs has increased approximately from 100 to 200 thousand metric tonnes over the period of 2010–2018, and it is forecasted to increase over 250 thousand tonnes by the year 2025 (Fig. 1).

Based on the estimations in 2019, China holds over 79% of the world’s mined production and 37% of the world’s REE reserves (Terry, 2019). Such a high share of the supply can be risky, since it creates a monopoly in REE industry. China, for example, restricted REE exports to Japan over a territorial rivalry, in 2010 and it triggered an increase of REE prices and panic buying. However, lately in 2010, China removed the supply restriction imposed on Japan and lowered their REE export quotas. Due to China’s monopoly and the escalating demand of REEs, it has been forecasted that a significant supply risk for REEs may arise in the near future. Therefore, REEs have been considered as the critical elements, especially HREEs, such as dysprosium (Dy), terbium (Tb), and Y (Balaram, 2019).

Secondary REE resources, such as e-waste, phosphogypsum, and mine tailings have become promising sources; however, their contribution to cater the ever-increasing REE demand is not sufficient. Therefore, natural REE resources are continued as the major source of REEs (Binnemans et al., 2013). REEs in natural RE resources do not exist as individual metals due to their high reactivity, and thus, they are found in various minerals. Among all the RE minerals that are identified to date, only a few minerals, such as bastnaesite, monazite, and xenotime (Table 2) are commercially used to produce REEs (Dostal, 2017).

Currently, bastnaesite is considered as the primary mineral of the world’s REE production since it is abundantly found in the world’s largest RE mines: Bayan Obo in China and the Mountain Pass deposit in the USA. Bastnaesite contains approximately 75% of REOs and considered as the primary source of LREEs. Bastnaesite mainly occurs in vein deposits, contact metamorphic zones, and pegmatites, whereas carbonate – silicate rocks with alkaline intrusive, quartz veins, fluorite-bearing veins, and breccia fillings in Permian sandstone are also found to be the host rocks of this mineral (Gupta and Krishnamurthy, 2005, Jordens et al., 2013, Voncken, 2016). Monazite is widely distributed in the world, occurring in different geological environments and mineral deposits. It contains approximately 70% of REOs comprising 10–40% La₂O₃, 4–12% ThO₂, 20–30% Ce₂O_3, and high amounts of Nd, Pr, and Sm (Thompson et al., 2012). Although monazite is considered as a major source of LREEs, their extraction has become a difficult process due to the high enrichment of radioactive elements, such as thorium (Th) and uranium (U) (Voncken, 2016, Xaba et al., 2018). Monazite mainly occurs in beach placer deposits, usually in high concentrations, while it is also found as an accessory mineral in granite, gneiss, and other igneous and metamorphic rocks (Wall, 2014). According to the estimations in 2005, about 10.21 million tonnes of terrestrial monazite placer deposits were found in the world, and those are distributed among China, USA, and India in percentages of 36%, 13%, and 3%, respectively (Pandey, 2011). In addition, other key RE minerals are listed Table 2.

In recent years, a necessity of exploration for new and alternative sources of REEs was emerged due to the supply risk of REEs and consequently, many new deposits have now been discovered (e.g. Bokan-Dotson and Bear Lodge in the USA, Hoidas Lake in Canada, Norra Karr in Sweden) (McLemore, 2015, Balaram, 2019). In this context, Sri Lanka has a geological setting that is favourable for REE resources. Previous geochemical studies in Sri Lanka disclose clear evidence of significant concentrations of REEs in different geological environments and resources. Therefore, this paper provides a critical overview on potential REE resources of Sri Lanka from a geological point of view. Their mode of occurrence is discussed in detail, including comparisons to the known types of REE resources in the world. This comparison will be useful to understand the significance of Sri Lankan REE resources and develop future exploration studies to appraise the economic viability.