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Van Tuyl Lecture: A. E. Williams-Jones, McGill University, Montreal, Quebec

January 30 @ 4:00 pm - 5:00 pm

A. E. Williams-Jones, McGill University, Montreal, Quebec

Berthoud 241, 4-5PM

The Rare Earth Elements: A Tale of Plumes, Magmas and Aqueous Fluids

Abstract: Because of their unusual magnetic, photoelectric and electrical properties, the rare earth elements (REE) are of strategic importance to the advanced technologies of developed countries, particularly their green technologies. Currently China produces over 80% of the REE and is reserving a sizable part of this production for its own consumption. There is thus a pressing need for new REE resources in other parts of the World.    The REE comprise the lanthanides plus Y and Sc and are subdivided into light (LREE; La – Gd) and heavy (HREE; Tb – Lu + Y + Sc) subclasses. Among these metals, Nd, Pr, Eu, Dy, Tb and Y are deemed critical rare earths because of their great importance to clean energy and high supply risk

Economic or potentially economic REE deposits are hosted by or genetically associated with alkaline igneous rocks and carbonatites derived from a carbonated, fluorine- and REE-enriched mantle; the LREE favour carbonatites and the HREE alkaline silicate rocks. With the exception of Sc, the REE are incompatible elements (large charge to radius ratio) and are thus concentrated by small degrees of melting of the mantle and very high degrees of fractional crystallisation. They reach economic concentrations in the final magma residues, where they crystallise as REE silicate, carbonate or phosphate minerals, many with complex stoichiometry. In contrast to the other REE, Sc has a small ionic radius, intermediate to those of Mg and Fe, and concentrates in ferromagnesium minerals, particularly clinopyroxenes. Recently, it has been recognised that small volumes of fluoride-rich silicate magmas may separate from the silicate magma and concentrate the REE by a factor of up to two orders of magnitude.

Hydrothermal mobilisation may be essential to the economic viability of many REE deposits. Indeed, the World’s largest deposit, Bayan Obo, China, is dominantly hydrothermal. The REE in this deposit are concentrated as monazite-(Ce) and bastnäsite-(Ce), except for Sc, which is concentrated in aegirine, a clinopyroxene; 90% of the World’s Sc supply is from Bayan Obo. Chloride complexes are the main species responsible for REE transport, although in some environments, sulphate complexes may also be important. Concentration of the REE to exploitable levels occurs when acidic REE-Cl-bearing fluids interact with rocks having high pH-buffering capacity or mix with cooler, more alkaline fluids. Fractionation and concentration of the HREE, however, may be favoured by interaction with rocks of low pH buffering capacity.

Approximately 90% of the World’s supply of HREE comes from ion adsorption REE deposits, in which the REE (except Sc) released by the weathering of A-type (alkaline) granites are adsorbed by kaolinite group minerals in a lateritic regolith. The REE are transported as carbonate or bicarbonate complexes, adsorb to halloysite (a metastable kaolinite group mineral) in the upper part of the profile, are desorbed during its transformation to kaolinite, transported deeper in the profile and adsorb again (on kaolinite) in response to increasing pH in the lower B and upper C soil horizons, where they achieve economic concentrations. A similar process is envisaged for Sc, which reaches economic concentrations in laterites developed over the clinopyroxenites of Alaskan-type (alkaline) ultramafic complexes.

This presentation reviews the history of the discovery of the REE, their properties and their uses. It then proceeds to an examination of the processes that lead to their eventual concentration to exploitable levels in the Earth’s crust. Using knowledge gained from studies of several world-class deposits, the conclusion is drawn that economic concentrations of the REE are developed through a complex interplay of igneous, hydrothermal and in some cases chemical weathering processes in parts of the World where appropriate tectonic settings, emplacement conditions and occasionally even climatic conditions fortuitously align.


January 30
4:00 pm - 5:00 pm
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