Chakhmouradian and Zaitsev reported in 2012 that out of some 50 advanced rare earth exploration projects currently active outside China, 20 target carbonate igneous rocks (carbonatites) or their weathering products, whereas at least 13 other projects are focused on various alkaline silicate rocks. These statistics reflect the importance of igneous deposits in the economics of rare earth elements (REEs). Indeed, between the 1960s and 1995, these deposits accounted for over 50% of the global REE production. In the early 1990s, 30–35% of the total production of ca 50,000 tonnes of total rare earth oxides (REOs) came from the carbonatite-hosted Mountain Pass deposit in California, 5–10% from Chinese carbonatites, and 10–15% from loparite mines in nepheline syenites at Lovozero, Russia.
The authors discussed the most significant types of igneous REE deposits and provides an overview of their sources, their evolution, and their geological, geochemical, and mineralogical characteristics. Rare-earth deposits in igneous rocks can be grouped into five distinct categories differing in the provenance and evolution of the magma and in the rock types hosting mineralization:
1) carbonatites,
2) peralkaline silica-undersaturated rocks,
3) peralkaline granites and pegmatites,
4) pegmatites associated with sub- to metaluminous granites, and
5) Fe oxide–phosphate deposits.
The authors emphasized the first three categories because they are far more important from a practical standpoint. To complement the information provided elsewhere in this issue by Kynicky et al. (2012), Mariano and Mariano (2012), and Williams-Jones et al. (2012), The authors focused on localities outside China and North America, mostly on Russian occurrences on the Kola Peninsula, and eastern Siberia, including discussion of Tomtan and Chuktukan, but also mentioned:
- Nambia at Lofdah
- Norway at Fen
- Afghanistan at Khanneshin
- Australia at Weld Mountain
- India at Amba Dongar
- Brazil at Barrado Hapirapva
- Malawi at Tundulu & Kanganhunde
- Tasmania at Wigu Hill
The authors concluded that the current (2012) understanding of REE deposits is too fragmentary and too “crustal” (i.e. lacking in depth) for the development of an integrated metallogenic model in any foreseeable future. Geologists are just beginning to appreciate the scale and significance of the mantle processes responsible for rare-element transport and enrichment in parts of the lower continental crust and subcontinental lithosphere.
Another highly speculative area is the behavior of rare earths in those natural systems whose complexity has thus far eluded all experimental and thermodynamic attempts
at modeling their evolutionary pathways. For example, geologists and geochemists know very little (as of 2012) about the relative stability of different REE–Nb–Ta oxides in silicate magmas and REE partitioning between carbonate minerals and melts.
Equally obscure (and underappreciated) is the fate of Y and lanthanides in syn-emplacement contact metasomatism, post-magmatic deformation, and metamorphism. Needless to say, work in any of these or other areas will offer great intellectual rewards to researchers willing to embrace the challenge.
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