Linnen, and three co-authors, contributed a chapter (i.e., 13.21) in 2014 to the Treatise on Geochemistry [(2nd Edition), Volume 13, 2014, pp. 543-568] and a paper that rare-element mineral deposits can contain economic concentrations of lithophile elements. There is no strict definition on what elements constitute these deposits. Some publications include alkaline and alkaline earth elements such as Li, Rb, Cs, and Be, and the metals Sc, Sn, and W as rare elements, but this chapter is restricted to Y, the rare-earth elements (REE, La to Lu), Zr, Hf, Nb, and Ta. The rare elements are not particularly rare, but one feature that they share is that they can be difficult to separate (i.e., separate individual REE, Hf from Zr and Ta from Nb).
The estimated abundances of Zr, Hf, Nb, and Ta in the upper continental crust are 193, 5.3, 12, and 0.9 ppm, respectively, which is slightly higher than in the bulk continental crust, 132, 3.7, 8, and 0.7 ppm, respectively (See Chapter 4.1). These concentrations are much higher than those estimated for the primitive mantle, 10.8 ppm Zr, 0.300 ppm Hf, 0.588 ppm Nb, and 0.040 ppm Ta (see Chapter 3.1). For comparison, the concentration of Cu in the upper continental crust and in primitive mantle is 28 and 20 ppm, respectively (See Chapter 3.1). The distribution of REE is similar.
In the upper continental crust, the concentrations of Y and two of the light REE (LREE), La and Ce, are 21, 31, and 63 ppm, respectively, whereas their concentrations in the bulk continental crust are 19, 20, and 43 ppm, respectively, and in the primitive mantle are 4.37, 0.686, and 1.786 ppm, respectively (See Chapter 3.1). The abundance of REE decreases with increasing atomic number (following the saw-toothed Oddo–Harkins rule, see (more)) and the heavy REE (HREE), for example Yb and Lu, have concentrations of 1.96 and 0.31 ppm, respectively, in the upper continental crust, 1.9 and 0.3 ppm, respectively, in the bulk crust, and 0.462 and 0.071 ppm, respectively, in primitive mantle (See Chapter 3.1).
Typical ore grades for these elements range from several hundred parts per million in the case of Ta to a few weight percent in the case of Zr, Nb, and REE (commonly reported as total rare-earth oxide, TREO). Thus, the enrichment factors from primitive mantle to ore deposit range from ∼1000 for Zr to 50,000 for Nb. All of the rare elements considered here share several characteristics. In igneous environments, they are generally incompatible (partition to the melt over minerals) and are typically concentrated in accessory phases. Consequently, these elements are enriched in melts that result either from very low degrees of partial melting or from extreme fractionation. This includes carbonatites, peralkaline granites and silica-undersaturated rocks, and peraluminous granites and pegmatites. The above behavior also explains why these elements are enriched in the crust. The abundance of the REE in primitive mantle, bulk continental crust, and upper continental crust are typically normalized to the CI chondrite. Primitive mantle shows a flat profile, with values of approximately two. The strong incompatible behavior of the LREE (La to Eu) compared to HREE (Gd to Lu) is clearly visible for the continental crust, as is the enrichment of Zr–Hf and Nb–Ta.
As a group, the rare elements are relatively insoluble in most aqueous fluids and are commonly used as immobile elements in calculations designed to estimate mass changes of elements in hydrothermally altered rocks. However, there is also abundant evidence that the rare elements are mobile in fluids with specific ‘hard’ ligands and one of the challenges in understanding rare-element deposits is being able to identify magmatic and metasomatic processes and evaluate their relative importance as ore-forming processes.
Rare elements include rare-earth elements (REE), Nb, Ta, Zr, and Hf. As a group, these elements are concentrated in the crust and are incompatible (remain in the melt rather than substitute into silicate minerals). Carbonatites host the most important deposits of REE and Nb, although peralkaline granites and silica under saturated rocks also host deposits of these elements. By contrast, Ta is concentrated in peraluminous granites and pegmatites. The former types of deposits are characterized by rift and anorogenic tectonic settings, whereas the latter, most typically, are in collisional settings.
Rare elements are concentrated by magmatic processes; in silicate melts one of the most important controls on rare element mineral solubility is the (Na + K)/Al ratio of the melt. Their solubilities are also temperature dependent and both these parameters may control magmatic ore deposition. Fluid–melt partition coefficients are typically low, but nonetheless the grades in many rare-element deposits are highest where hydrothermal alteration is strongly developed. Rare elements are hard acids in the Pearson sense and they prefer to bond electrostatically to form aqueous complexes with hard bases such as F− and OH−, as well as SO42 −, CO32 −, and PO43 −. Fluid mixing may be the principal control of mineralization in hydrothermal systems.
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