Verplanck reported in 2017 that groundwater plays an important role in the formation of many rare earth element (REE) deposits. Groundwater and associated hydrothermal fluids can transport these elements and with changes in fluid composition or properties, the REEs can be removed from solution by adsorption or precipitation of REE-rich minerals. Currently, REEs are primarily produced from two types of ore deposits, ion adsorption clay deposits (located in southern China) and carbonatites.
Ion adsorption clay deposits in southern China form by lateritic weathering of underlying granites. Groundwater is responsible for the mobilization, transport, and deposition of the REEs in forming these ore deposits. Carbonatites are a relatively rare type of igneous rock that contain the highest concentrations of REEs of any igneous rocks. Ore-grade REE enrichment in carbonatites can result from crystallization of REE mineral phases directly from the magma or by hydrothermal processes. One carbonatite-related REE ore deposit, the Mount Weld deposit (Australia), was formed by lateritic weathering of the underlying carbonatite.
Rare earth elements (REEs) are not as rare in nature as their name implies, but economic deposits are not common and few deposits have been large REE producers. In the past 20 years, demand for REEs has increased dramatically because of their applications including chemical catalysts, metal alloys, glass polishing, and permanent magnets. Because of their unique properties, REEs are essential for use in high tech devices including smart phones, digital cameras, computer hard disks, fluorescent and light-emitting-diode (LED) lights, flat screen televisions, computer monitors, and electronic displays.
The REE group is composed of 15 elements that range in atomic number from 57 (lanthanum) to 71 (lutetium), and yttrium (atomic number 39) is commonly regarded as a REE because of its chemical and physical similarities and affinities. Scandium (atomic number 21) is chemically similar to, and thus sometimes included with, the REEs, but it does not occur in economic concentrations in the same geological settings as the other REEs and will not be discussed further. Rare earth element deposits are generally enriched in either light REEs (LREEs; La-Gd) or heavy REEs (HREEs; Tb-Lu).
The present global production of REEs comes from only a few sources. Two ore-deposit types account for the vast majority of world’s REE production. A few carbonatite-related deposits provide the world’s LREEs, and numerous small ion adsorption clay deposits in southern China provide the world’s production of HREEs. Carbonatites are a relatively rare type of igneous rock composed of greater than 50 vol.% primary carbonate minerals, dominantly calcite and/or dolomite, and contain the highest concentrations of REEs of any igneous rocks.
Chondrite-normalized REE patterns for carbonatites display extreme LREE enrichment, no Eu anomaly and a steep pattern with a negative slope. Ore-grade enrichment of REEs in carbonatites can occur by primary magmatic crystallization of REE-rich mineral phases or by late-stage fluid mobilization and consequential REE-phase crystallization. In one deposit currently being mined, Mount Weld, the REE ore-grade enrichment is due to lateritic weathering of an underlying carbonatite. The second deposit type, ion adsorption clay deposits in southern China, form by tropical weathering of granites where the REEs are released to solution from minerals within the granite and a significant fraction are adsorbed on to clays in the weathered zone. Chondrite-normalized REE patterns of these deposits vary due to a number of factors that will be discussed in the following sections. Compared to other REE deposits, these deposits are relatively low grade but are economic to mine because they can be enriched in the more valuable HREEs and are relatively inexpensive to mine.
Groundwater can play a significant role in ore-grade enrichment of REEs in both carbonatite and ion adsorption clay deposits. This paper will review the fluid-driven processes involved in REE deposit genesis. Similar to many metals, the behavior of REEs in aqueous solutions can be described as a competition between the formation of stable aqueous complexes and the tendency of REEs to partition to solid phases either by adsorption or (co)precipitation. Solution chemistry plays an important role with the dominant inorganic ligands being carbonate, phosphate, sulfate, hydroxide, chloride, and fluoride. Other factors that may affect partitioning between solid and aqueous phases include: pH, temperature, redox conditions, and the composition of solid phases.
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