From REE-Enrichment to REE Deposits – An Overview


From REE-Enrichment to REE Deposits – An Overview

Möckel, R.; Kempe, U.; Gutzmer, J.

Despite their name, rare earth elements (REE, La–Lu) are not rare. As a matter of fact, they are three orders of magnitude more abundant in the Earth’s Crust than for example gold (e.g. Wedepohl, 1995). Ore-forming processes produce local enrichments in which the concentration of gold may exceed its average crustal abundance by a factor of 1000 – or more. Rare earth elements, in contrast, are widely and evenly disseminated in the Earth’s Crust – they are rarely enriched to form economic concentrations that may then be exploited as ore deposits.
Despite being rather scarce, REE enrichments may form by a number of geological processes. Consequently, a number of classifications exist in literature for REE deposits. Broadly, REE deposits of magmatic affiliation can be distinguished from REE deposits of hydrothermal and sedimentary origin. The latter include marine (heavy mineral) placer deposits, residual ion adsorption clays (IACs) as well as highly REE-enriched lateritic caprocks developed at the expense of REE-enriched protoliths. Magmatic deposits include, most importantly, REE enrichment in igneous rocks such as alkaline to peralkaline and carbonatite rocks, as well as rare metal enriched pegmatites. The least common type of REE deposit is of hydrothermal origin, including monazite and/or xenotime-rich vein and breccia-type mineralization. Combinations of above-mentioned ore-forming processes are known or under discussion (e.g. Drew et al. 1990, Dostal et al. 2014, Kempe et al. 2015), resulting in complex mineralisation styles, complex mineral assemblages and multi-stage paragenetic sequences. It is therefore not surprising that the assignment to one or another type is ambiguous in some cases.
Despite obvious genetic complexity and a multitude of ore-forming environments, more than 90% of the world’s recent REE production stems from (virtually) monomineralic ores, including the Bayan Obo deposit (China), the Mountain Pass deposit (USA) and the Mt. Weld deposit (Australia). The former two are pristine magmatic carbonatites, whereas the latter is a lateritic deposit formed by enrichment of a carbonatite intrusion (Long et al. 2010). Bastnaesite, a REE-fluoride-carbonate, is the only important ore mineral at Mountain Pass, monazite, a REE phosphate clearly predominates at Mt. Weld, whereas both of these common REE-minerals are widespread at Bayan Obo.
REE mineral-bearing placer deposits are well known in the marine environment. They always contain monazite as the quantitatively most important REE-mineral, with xenotime, a Y-HREE phosphate closely related to monazite, a distant second. Such placer deposits are, however, currently only exploited for REE in India.
Needless to say, all of the deposit types described above are highly enriched only in light REE (LREE), whereas the highly coveted heavy REE (HRREE) are present only in very minor concentrations. HREE are mostly extracted from ion adsorption clay deposits in southern China. These deposits are the product of intensive chemical weathering of only mildly REE-enriched granites in a warm and humid climate. REE are remobilized during weathering and adsorbed to the surface of clay minerals. There are thus no clear REE minerals present – and exploitation is by in-situ leaching rather than conventional mining (Orris & Grauch, 2002).
Because demand in particular for HREE for high tech applications is rapidly increasing, other REE occurrences with more exotic and more complex mineralogies have been have recently come into focus. This includes alkaline igneous complexes, e.g., in Russia (Lovozero), Canada (Strange Lake, Thor Lake), Sweden (Norra Kärr) and several localities in Greenland. Some of these deposits are hydrothermally altered, leading to considerable increase of REE contents and other valuable elements. Such processes of hydrothermal alteration do not only result in an increase of REE concentration, also a complex assemblage of REE minerals (e.g. Kempe et al. 2015).

References:

Dostal, J., Kontak, D.J. and Karl, S.M. (2014): The early jurassic Bokan Mountain peralkaline granitic complex (southeastern Alaska): Geochemistry, petrogenesis and rare-metal mineralization. Lithos 202–203:395–412
Drew, L.J., Qingrun, M. and Weijun, S. (1990): The Bayan Obo iron-rare-earth-niobium deposits, Inner Mongolia, China. Lithos 26:43–65
Kempe, U., Möckel, R., Grauner, T. Kynicky, J. and Dombon, E. (2015): The genesis of Zr-Nb-REE mineralisation at Khalzan Buregte (Western Mongolia) reconsidered. Ore Geology Reviews 64: 602–625
Long, K.R., Van Gosen, B.S., Foley, N.K. and Cordier, D. (2010): The principal rare earth element deposits of the United States – A summary of domestic deposits and a global perspective. USGS Scientific Investigations Report 2010-5220
Orris, G.J. and Grauch, R.I. (2002): Rare earth element mines, deposits, and occurences. USGS Open-File Report 02-189
Wedepohl, K.H. (1995): The composition of the continental crust, Geochimica et Cosmochimica Acta, 59(7):1217–1232

Keywords: rare earth elements

  • Invited lecture (Conferences)
    AREE 2015 - Analysis of Rare Earth Elements Methods and Applications International Colloquium and Exhibition, 05.-06.10.2015, Kleve, Deutschland

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