Understanding rare earth elements in heavy mineral sand systems

Abstract

Heavy mineral sands (HMS) represent an alternative major resource of Rare Earth Elements (REE) deposits with over 90 % of these formed within the past 66 Myr. These deposits exhibit a diverse range of mineral compositions (i.e. ilmenite, rutile, zircon, monazite, xenotime, and garnet) and can be formed from a wide range of geological and surficial processes. However, few studies examined the in-depth geological characteristics and exploration strategies to specifically target HMS mineralisation, and particularly none have specifically focussed on the REE component of these deposits. Here we show a total of 1173 REE-HMS deposits that exhibit diverse heavy mineral compositions. The largest REE-HMS deposits show an average heavy mineral grade of 7 % (SD = 12) and an average resource size of 1337 Mt. (SD = 1932). REE-HMS deposits commonly concentrate along Quaternary coastlines that cluster within a 100 km radius and predominate in tropical humid areas within 25 degrees north and south of the Equator. Yet, 40 % of large REE-HMS deposits formed during the Neogene, Paleogene, and Cretaceous situated inland from modern coastlines. Another feature of REE-HMS clusters shows that coastal areas that are protected from major global oceanic currents show a higher prevalence than regions that remain unprotected. REE-HMS deposits originate from rocks found in high-grade metamorphic belts, orogenic belts, and cratons, transported by wind and well-developed river systems, particularly in tropical and humid climates. This study highlights the importance of geological, climatic, and surface process factors in shaping the distribution of REE-HMS deposits. Furthermore, we also consider socioeconomic factors, such as land accessibility, politics, exploration maturity, and market demand playing crucial roles. A mineral exploration approach is proposed that emphasises the use of specific datasets and exploration methodologies tailored to the footprints of REE-HMS deposits. Overall, this study's findings contribute to a better understanding of REE-HMS deposits, providing insights for future exploration and resource management.