Rare Earth Elements (REEs) are critical for the development of future technology and the maintenance of our modern infrastructure. The REE market is currently a hot topic as the world depends predominantly on China (~70%) for the global supply of REEs (Liu et al., 2023). However, new policies recently introduced by the USA, most notable the REEShore and the Inflation Reduction Acts aim to diversify the production of REEs away from China. In this article we will discuss what REEs are and why are they important. Then we will explore the current metallogenic model for two of the most significant REE deposit types in China, the carbonate-related Bayan Obo REE-Nb-Fe deposit and the regolith-hosted Zudong HREE deposit.
What are Rare Earth Elements and how are they used?
REEs comprise the 15 elements of the lanthanide series: lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and lutetium (Lu) (Liu et al., 2023). Yttrium (Y) and scandium (Sc) are often grouped alongside the lanthanides and referred to as REEs. The REEs are often divided into the two informal sub-divisions termed light rare earth elements (LREEs), which comprises the elements La to Gd, and heavy rare earth elements (HREEs), which comprises the elements from Tb to Lu & Y (Liu et al., 2023). In this brief introduction we will explore a single common use of the REEs. If the reader is interested further in the use of REEs in our modern society, they are directed to the following webpage (https://pubs.usgs.gov/fs/2014/3078/pdf/fs2014-3078.pdf). A currently significant use of REEs is in the creation of REE-alloys (Nd, Sm, Gd, and Pr) to form permanent magnets for electric motors (Duchna and Cieślik, 2022). As REE magnets comprise a magnetic field that can range from 2-7 times stronger than a normal iron magnet (Duchna and Cieślik, 2022), their implementation can produce more efficient electric motors. These electric motors are used in a wide variety of applications including wind turbines, electric cars, hydro-electric dams as well as E-bikes and scooters. In fact, a common 1 MW wind turbine contains 216 kg of Nd and 17 Kg of Dy (Verma et al., 2022).
Rare Earth Element deposits of China
There is a range of different REE deposit types in China, including carbonatite-related, clay-hosted, placer, coal-hosted, marine sedimentary and sedimentary metamorphic types (Yin and Song, 2022). However, the two economically significant deposit types discussed in this article include: (1) Carbonatite-related, which is discussed with an emphasis on the Bay Obo carbonatite-related REE-Nb-Fedeposit and: (2) Regolith-hosted type, which is discussed with an emphasis on the Zudong HREE deposit.
Bayan Obo Carbonatite Related REE-Nb-Fe Deposit
The distribution of carbonatite related REE deposits in China is restricted to orogenic belts along the margins of cratons or within continental margin rifts (Yang and Woolley, 2006). The Bayan Obo REE-Nb-Fe deposit is located within the Inner Mongolia Province and is situated within the Bayan Obo continental margin rift, in the northern region of the North China Craton (Smith et al., 2015; Yang et al., 2017). The Bayan Obo deposit is the largest known REE deposit in the world, and the second largest Nd deposit in the world (Smith et al., 2015). This deposit single handedly accounts for 92% of Chinas LREE resources and 66% of Chinas Nb resources (Fan et al., 2016). The Bayan Obo deposit is dominated by the REEs Ce (~50%), La (~27%), Nd (~15%) and Pr (~5%) (Fan et al., 2016).
Within the vicinity of the deposit, there are nearly 100 mapped predominantly NE to NW-trending carbonatite dykes that range from 0.5 – 2 m in width and 10 – 200 m length (Fan et al., 2014). The dykes are composed of calcite and dolomite with minor apatite, monazite, barite, bastnaesite and magnetite (Yang et al., 2003). Geochronology performed on the carbonatite dykes indicate a crystallisation age of ca. 1420 Ma (Fan et al., 2014).
The Bayan Obo deposit is divided into three distinct orebodies, the West, Main and East. The west orebody is mainly hosted within dolomite (Fan et al., 2016). The origin of the host dolomite is still debated and is interpreted to be either a sedimentary dolostone (Lai et al., 2012) or a large carbonatite intrusion (Le Bas et al., 1997). The main and east orebodies are hosted between the boundary of the dolomite and a K-rich slate unit (H9), which belongs to the Bay Obo Group (Fan et al., 2016; Yang et al., 2017). The mineralogy of the deposit is highly complex and hosts >170 minerals with 20 distinct REE-bearing minerals and 20 Nd-bearing minerals (Zhang and Tao, 1986). There are three types of ore types present in the deposit and are separated based on their mineralogy and their macroscopic texture (Fan et al., 2016; Smith et al., 2015). The first ore type consists of disseminated monazite with lesser dolomite, ankerite and magnetite, which occupy small-scale fractures and occur as an interstitial component throughout the protolith (Smith et al., 2015; Yang et al., 2017). The second ore type consists of banded Fe and REE mineralisation (Smith et al., 2015; Yang et al., 2017). Banded Fe & REE mineralisation is composed of magnetite, which has partially been altered to hematite, and monazite, which has been overprinted by bastnäsite, of which is overprinted by subsequent Ca–REE fluorocarbonates (Chao et al., 1992; Fan et al., 2016; Yang et al., 2017). The last ore type is present as cross-cutting veins that are predominantly concentrated within the shallow regions of the Main and East deposits, and are dominated by aegirine with lesser fluorite, huanghoite, albite, calcite, biotite, and pyrite (Chao et al., 1992; Fan et al., 2016).
There has been a wide variety of geochronological techniques applied to the Bayan Obo deposit, which produce ages ranging from ca. 1300–400 Ma. Initial REE mineralisation is interpreted to have been deposited at ca. 1400–1300 Ma by a magmatic-hydrothermal fluid, coeval with the formation of nearby carbonatite magmatism (Yang et al., 2011a). On a larger scale, the carbonatites, and therefore the Bayan Obo REE deposit are associated with rifting along the Bayan Obo rift. This event resulted in a low degree of partial melting of the mantle lithosphere at ca. 1400–1200 Ma, correlating with the final breakup of the Columbia Supercontinent (Yang et al., 2011b). The ore deposit was subsequently deformed and metamorphosed from 430-400 Ma, which resulted in the distinct banded characteristic of the banded ore type, and the remobilisation of pre-existing REE mineralisation to form Ca–REE fluorocarbonates and the cross-cutting REE-bearing veins (Smith et al., 2015).
The Zudong Regolith Hosted HREE Deposit
Regolith hosted REE deposits dominate the HREE production in China (Yin and Song, 2022). Regolith hosted deposits can be categorised into residual and ion-adsorption type deposits (Yin and Song, 2022). Residual deposits are composed of primary and/or secondary REE-bearing minerals (such as monazite, xenotime & bastnäsite), while ion-adsorption deposits consist of REEs that are adsorped onto weathered clay material. For regolith hosted REE deposits, the abundance of HREEs/LREEs is primarily the function of the parent material (Li et al., 2017).
The Zudong HREE deposit has been mined for over fifty years (Li et al., 2019). The parent material for this deposit is interpreted to be the 168.2 ± 1.2 Ma Zudong granite (Zhao et al., 2022). The Zudong granite is positioned along a NNE-SSW trending fault and is exposed over an area of 32.5 km2(Dianhao et al., 1989; Zhao et al., 2022). The Zudong granite consists of two main rock types, a biotite K-feldspar granite and a muscovite K-Na-feldspar granite (Dianhao et al., 1989). Interestingly, both granites have similar concentrations of ~260 ppm REEs (Dianhao et al., 1989). The muscovite K-Na-feldspar granite is interpreted to be the product of the late stage autometasomatism (which means it was recrystallised by its last water rich liquid fraction) of the biotite-K-feldspar granite, resulting in a relative enrichment of HREEs (Dianhao et al., 1989). The mineralogy of the muscovite granite consists of quartz, K-feldspar, muscovite, and albite with trace abundances of fluorite, synchysite-(Y), zircon, monazite, gadolinite, xenotime, aeschynite-(Y), pyrochlore, columbite, parasite, yttrialite, hingganite-(Y), and fergusonite (Dianhao et al., 1989; Li et al., 2017).
Weathering and remobilisation of the REEs from the muscovite granite occurred via a slightly acidic to near-neutral fluid (pH values of 5.4 to 6) (Li et al., 2019). The REEs that form the clay-hosted Zudong deposit are predominantly sourced from the breakdown of the REE-bearing minerals synchysite-(Y), gadolinite-(Y), hingganite-(Y), and yttrialite-(Y) (Li et al., 2019, 2017). Weathering of REE minerals in the B horizon is accompanied by the alteration of Na-K feldspar minerals into a variety of clay minerals, most importantly halloysite (Li et al., 2019). Unlike kaolinite, which has a relatively flat crystallography, halloysite has a tube-like crystallography, and therefore a significantly higher specific surface area. This characteristic allows a high abundance of REEs to be adsorped onto the halloysite. With continued weathering the in the upper portions of the B profile, halloysite is readily converted into kaolinite (Li et al., 2019). As kaolinite has a lower specific surface area and a lower cation exchange ability, the conversion process releases much of the adsorped REEs, which are released into the regolith and often mobilised back into the deeper portions of the soil profile where they and re-adsorbed by halloysite or precipitate as chernovite-(Y) (Li et al., 2019).
Primary protolith REE minerals including, xenotime-(Y), zircon and the aeschynite/euxenite-groups are largely preserved in the lower B to upper C profile, resulting in a relatively HREE-rich residue in the regolith (Li et al., 2019). Combined, the processes outlined above have resulted in the formation of the Zudong HREE deposit which is composed of ~65% adsorped REEs, ~15-20% REEs in the form of chernovite-(Y), and ~15-20% in the form of primary protolith REE minerals (Li et al., 2019).
In conclusion, China has is the largest global producer of Rare Earth Elements (REEs), which are essential for various technological advancements and the maintenance of our current infrastructure. The country's dominance in REE production can be attributed to the exploitation of various deposit types, with carbonatite-related REE and regolith-hosted HREE deposits being the most economically significant. In this blog we have briefly summarised the metallogenic models for each deposit focussing on the Bayan Obo REE-Nb-Fe deposit and the regolith-hosted Zudong HREE deposit.
References
Balaram, V., 2019. Rare earth elements: A review of applications, occurrence, exploration, analysis, recycling, and environmental impact. Geoscience Frontiers 10, 1285–1303.
Chao, E.C.T., Back, J.M., Minkin, J.A., Yinchen, R., 1992. Host-rock controlled epigenetic, hydrothermal metasomatic origin of the Bayan Obo REEFe-Nb ore deposit, Inner Mongolia, PRC. Applied Geochemistry 7, 443–458.
Dianhao, H., Chengyu, W., Jiuzhu, H., 1989. REE geochemistry and mineralization characteristics of the Zudong and Guanxi granites, Jiangxi Province. Acta Geologica Sinica‐English Edition 2, 139–157.
Duchna, M., Cieślik, I., 2022. Rare Earth Elements in New Advanced Engineering Applications.
Fan, H.-R., Hu, F.-F., Yang, K.-F., Pirajno, F., Liu, X., Wang, K.-Y., 2014. Integrated U–Pb and Sm–Nd geochronology for a REE-rich carbonatite dyke at the giant Bayan Obo REE deposit, Northern China. Ore Geol Rev 63, 510–519.
Fan, H.-R., Yang, K.-F., Hu, F.-F., Liu, S., Wang, K.-Y., 2016. The giant Bayan Obo REE-Nb-Fe deposit, China: controversy and ore genesis. Geoscience Frontiers 7, 335–344.
Lai, X., Yang, X., Liu, Y., Yan, Z., 2016. Genesis of the Bayan Obo Fe–REE–Nb deposit: evidences from Pb–Pb age and microanalysis of the H8 formation in Inner Mongolia, North China Craton. J Asian Earth Sci 120, 87–99.
Lai, X., Yang, X., Sun, W., 2012. Geochemical constraints on genesis of dolomite marble in the Bayan Obo REE–Nb–Fe deposit, Inner Mongolia: implications for REE mineralization. J Asian Earth Sci 57, 90–102.
Le Bas, M.J., Spiro, B., Xueming, Y., 1997. Oxygen, carbon and strontium isotope study of the carbonatitic dolomite host of the Bayan Obo Fe-Nb-REE deposit, Inner Mongolia, N China. Mineral Mag 61, 531–541.
Li, M.Y.H., Zhou, M.-F., Williams-Jones, A.E., 2019. The genesis of regolith-hosted heavy rare earth element deposits: Insights from the world-class Zudong deposit in Jiangxi Province, South China. Economic Geology 114, 541–568.
Li, Y.H.M., Zhao, W.W., Zhou, M.-F., 2017. Nature of parent rocks, mineralization styles and ore genesis of regolith-hosted REE deposits in South China: An integrated genetic model. J Asian Earth Sci 148, 65–95.
Liu, S.-L., Fan, H.-R., Liu, X., Meng, J., Butcher, A.R., Yann, L., Yang, K.-F., Li, X.-C., 2023. Global rare earth elements projects: New developments and supply chains. Ore Geol Rev 105428.
Smith, M.P., Campbell, L.S., Kynicky, J., 2015. A review of the genesis of the world class Bayan Obo Fe–REE–Nb deposits, Inner Mongolia, China: Multistage processes and outstanding questions. Ore Geol Rev 64, 459–476.
Verma, S., Paul, A.R., Haque, N., 2022. Assessment of Materials and Rare Earth Metals Demand for Sustainable Wind Energy Growth in India. Minerals 12, 647.
Yang, K.-F., Fan, H.-R., Santosh, M., Hu, F.-F., Wang, K.-Y., 2011a. Mesoproterozoic carbonatitic magmatism in the Bayan Obo deposit, Inner Mongolia, North China: Constraints for the mechanism of super accumulation of rare earth elements. Ore Geol Rev 40, 122–131.
Yang, K.-F., Fan, H.-R., Santosh, M., Hu, F.-F., Wang, K.-Y., 2011b. Mesoproterozoic mafic and carbonatitic dykes from the northern margin of the North China Craton: implications for the final breakup of Columbia supercontinent. Tectonophysics 498, 1–10.
Yang, X., Lai, X., Pirajno, F., Liu, Y., Mingxing, L., Sun, W., 2017. Genesis of the Bayan Obo Fe-REE-Nb formation in Inner Mongolia, north China craton: a perspective review. Precambrian Res 288, 39–71.
Yang, X.-M., Yang, X.-Y., Zheng, Y.-F., Le Bas, M.J., 2003. A rare earth element-rich carbonatite dyke at Bayan Obo, Inner Mongolia, North China. Mineral Petrol 78.
Yang, Z., Woolley, A., 2006. Carbonatites in China: A review. J Asian Earth Sci 27, 559–575.
Yin, J.N., Song, X., 2022. A review of major rare earth element and yttrium deposits in China. Australian Journal of Earth Sciences 69, 1–25.
Zhang, P., Tao, K., 1986. Mineralogy of Bayan Obo.
Zhao, X., Li, N.-B., Huizenga, J.M., Zhang, Q.-B., Yang, Y.-Y., Yan, S., Yang, W., Niu, H.-C., 2022. Granitic magma evolution to magmatic-hydrothermal processes vital to the generation of HREEs ion-adsorption deposits: Constraints from zircon texture, U-Pb geochronology, and geochemistry. Ore Geol Rev 146, 104931.