Sunday, November 15, 2015

Geological Setting of Rare Earth Elements in Europe

Despite their name, the rare earth elements (REE) are not all particularly rare in the earth's crust; the name reflects the difficulty of separating them into the native metals, and the fact that some members of the group are indeed rare (Chakhmouradian and Wall, 2012). They occur in small amounts in all parts of the Earth's crust in a wide range of tectonic settings, and are widely used for petrogenetic studies. The total concentration of REE in average bulk continental crust is c.125 ppm; (Rudnick and Gao, 2004). Development of a potentially economic rare earth element resource requires that they are concentrated signi fi cantly above these background levels, typically to percentage concentrations of total REE. Enrichment of the REE may occur through primary processes such as magmatic processes and hydrothermal fluid mobilisation and precipitation, or through secondary processes that move REE minerals from where they originally formed, such as sedimentary concentration and weathering. Natural rare earth element deposits and occurrences may thus be divided into primary (high-temperature) and secondary (low-temperature) deposit types.

The most important primary deposits with high grade and tonnage are typically associated with alkaline–peralkaline igneous rocks and carbonatites formed in extensional intracontinental rifts (Chakhmouradian and Zaitsev, 2012; Wall, 2014). Primary REE concentrations can also be formed in a range of other geological settings, often associated with granites and pegmatites or with hydrothermal systems, and more rarely in metamorphic or diagenetic settings. Erosion or weathering of any of these primary enrichment types may produce secondary deposits such as placers and ion adsorption deposits (Wall, 2014). In a global context, the bulk of the world's REE are currently derived from carbonatites, notably Bayan Obo in China; these deposits are typically high-grade, but LREE-dominated (Chakhmouradian and Wall, 2012; Wall, 2014). REE deposits associated with alkaline igneous rocks are typically lower grade but with larger tonnage and a higher content of the most critical HREE (Wall, 2014).

Significant localities or groups of localities described
Tabel 1. Significant localities or groups of localities described in this paper, classified as resource (those with a formal REE resource estimate compliant with the JORC or NI-43–101 reporting codes); deposit (those for which an economic resource is likely to be present and may be identified by future exploration); occurrence (those in which the REE are enriched but which are unlikely to be economic); and by-product (those in which the REE could be economic as a by-product of another commodity).

Formation of REE deposits in alkaline to peralkaline igneous rocks and carbonatites is typically due to magmatic and/or hydrothermal processes (Wall and Mariano, 1996; Kogarko et al., 2002; Salvi and Williams-Jones, 2006; Schilling et al., 2011; Sheard et al., 2012; McCreath et al., 2012). Alkaline silicate and carbonatite magmatism are associated with small degrees of partial melting of enriched mantle, potentially derived either from metasomatised lithospheric mantle or from mantle plumes (Fig. 1), or from interaction between the two (Downes et al., 2005; Wilson and Downes, 2006; Ernst and Bell, 2010).

The main environments of alkaline igneous rocks and carbonatites
Fig 1.Schematic diagram to illustrate the main environments of formation of alkaline igneous rocks and carbonatites, major hosts of many REE deposits.

Further evolution of these small-degree partial melts in a near closed system is typically needed to produce highly evolved igneous rocks enriched in REE minerals. Notably, many important REE deposits are associated with extremely peralkaline igneous rocks containing complex Na–K–Ca–(Fe, Zr, Ti) silicates such as eudialyte group minerals and aenigmatite that are commonly also enriched in the REE; such rocks are termed ‘agpaitic’ (Sorensen, 1997; Marks et al., 2011). In contrast, in most other felsic igneous rocks the REE are hosted in accessory minerals such as zircon, allanite, apatite, and monazite, and these rocks are termed ‘miaskitic’. Key REE minerals within carbonatites include bastnäsite, parisite, synchysite, monazite, pyrochlore and many others ( Wall and Mariano, 1996 ). Major REE-bearing minerals found in European deposits are listed in Table 2.

Main REE-bearing minerals and mineral groups
Table 2. Table of all the main REE-bearing minerals and mineral groups found in the European deposits and occurrences described here. Note that many of these groups contain a range of individual mineral species (Wall, 2014), for example the monazite group includes monazite-(La), monazite-(Ce), monazite-(Nd), and monazite-(Sm). For simplicity, these variations are encompassed as REE (LREE/HREE) in the formulae given here. Mineral groups are marked with an asterisk.

The main REE metallogenetic provinces in Europe (Fig. 2) are those areas where extensional tectonics and introduction of enriched mantle melts have produced alkaline silicate and carbonatite magmatism. Major REE deposits are currently known where the plutonic complexes at depth in continental rift zones have been exposed by erosion ( Goodenough et al., 2014 ). The most notable of these are the Mesoproterozoic Gardar Province of southwest Greenland ( Upton et al., 2003 ), and the Protogine Zone, a major, multiply reactivated, in part extensional structure in southern Sweden (Åberg, 1988; Andréasson and Rodhe, 1990). Both of these zones currently host advanced REE exploration projects. Several intracontinental rift-related provinces of Palaeozoic age occur in Europe, including the Devonian Kola Alkaline Province, which extends from Russia into Finland, and the Permo-Carboniferous Oslo Rift in Norway. The Kola Alkaline Province contains some large peralkaline igneous complexes ( Downes et al., 2005 ) that represent major Russian REE resources, but these lie outside the geographical scope of this paper.

Map of Europe of the key REE metallogenetic belts
Fig 2. Overview map of Europe showing the approximate extent of the key REE metallogenetic belts described in this paper. Notable REE deposits and occurrences that do not fall within a distinct belt and are not shown on other maps are indicated by symbols.

Some episodes of European rifting have progressed to continental break-up and development of a new ocean, notably the formation of the Iapetus Ocean during the Neoproterozoic (Svenningsen, 2001), and the opening of the North Atlantic from the Jurassic into the Cenozoic (Saunders et al., 2013). Such rift phases are typically associated with large volumes of magmatism, but central complexes with alkaline compositions are rare, although isolated carbonatite bodies and dyke swarms are known.

Localised rifting and alkaline magmatism have developed periodi cally across much of central and southern Europe from the Triassic into the Cenozoic, both to the north of the Alpine collision zone and around the margins of the Mediterranean (Wilson and Downes, 2006). In these areas, alkaline volcanic rocks are typically exposed at the surface; the central complexes that might contain signi fi cant primary REE resources are still likely to be hundreds of metres to kilometres below the surface. In general, the major known potential for REE resources around the Mediterranean is dominated by secondary deposits such as bauxites.

Potential REE deposits can also be associated with magmatic and hy- drothermal activity in other tectonic settings away from intracontinental rift zones. The most notable of these occur in the Palaeoproterozoic Bergslagen province in Sweden, including the Bastnäs deposits where the LREEs were fi rst discovered. These deposits are considered to have formed through reaction of carbonates with fl uids derived from subduction-related magmas (Holtstam et al., 2014; Jonsson et al., 2014). There are a number of other areas in Europe where alkaline magmatism has developed towards the end of an orogenic cycle, such as in the Caledonides, and these areas may also contain localised REE enrichments (Walters et al., 2013).

No significant secondary REE deposits with high tonnage and high grade are currently known in Europe, but at a number of localities erosion and weathering processes have formed low-grade REE concen- trations that have economic potential because of their relative ease of processing. These include heavy mineral placers, particularly along the Mediterranean coastlines, and bauxites in many parts of southern Europe. In China, and other parts of the world that have experienced tropical weathering, the REE are known to be enriched in weathered ion adsorption clay deposits (Kynicky et al., 2012). However, studies of weathered granitic rocks in Europe have shown no evidence of REE upgrading during the weathering process (Hohn et al., 2014).

Written by: K.M. Goodenough, 2015 (in journal ore geology reviews : Europe's rare earth element resource potential). Editor : Flyshgeost.

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