Enrichment of Rare Earth Elements during magmatic and post-magmatic processes: a case study from the Loch Loyal Syenite Complex, northern Scotland

Abstract

Concern about security of supply of critical elements used in new technologies, such as the Rare Earth Elements (REE), means that it is increasingly important to understand the processes by which they are enriched in crustal settings. High REE contents are found in syenitedominated alkaline complexes intruded along the Moine Thrust Zone, a major collisional zone in north-west Scotland. The most northerly of these is the Loch Loyal Syenite Complex, which comprises three separate intrusions. One of these, the Cnoc nan Cuilean intrusion, contains two mappable zones: a Mixed Syenite Zone in which mafic melasyenite is mixed and mingled with leucosyenite and a Massive Leucosyenite Zone. Within the Mixed Syenite Zone, hydrothermal activity is evident in the form of narrow altered veins dominated by biotite and magnetite; these are poorly exposed and their lateral extent is uncertain. The REE mineral allanite is relatively abundant in the melasyenite and is extremely enriched in the biotite-magnetite veins, which have up to 2 % total rare earth oxides in bulk rock analyses. An overall model for development of this intrusion can be divided into three episodes: (1) generation of a Light Rare Earth Element (LREE)-enriched parental magma due to enrichment of the mantle source by subduction of pelagic carbonates; (2) early crystallisation of allanite in melasyenite, due to the saturation of the magma in the LREE; and (3) hydrothermal alteration, in three different episodes identified by petrography and mineral chemistry, generating the intense enrichment of REE in the biotite-magnetite veins. Dating of allanite and titanite in the biotite-magnetite veins gives ages of c. 426 Ma, overlapping with previously published crystallisation ages for zircon in the syenite.

The evolution of REE mineralisation within the Ditrău Alkaline Complex, Romania: interplay of magmatic and hydrothermal processes

“…In the early crystallising magmas of the Ditrău Complex, the REE behaved compatibly and were incorporated into cumulus minerals such as titanite and apatite. This indicates early REE saturation in the magma, with the REE removed from the melt before the more evolved lithologies crystallise; similar early crystallisation of REE-bearing minerals is also observed in the Loch Loyal Syenite Complex in the UK (Walters et al, 2013).…”

The Early Jurassic Bokan Mountain peralkaline granitic complex (southeastern Alaska): Geochemistry, petrogenesis and rare-metal mineralization

“…These rocks frequently display a distinct enrichment in rare earth elements (REE), high field strength elements (HFSE) and radioactive elements (uranium and thorium) which, in some cases, are of economic significance (e.g., Chakhmouradian and Zaitsev, 2012;Richardson and Birkett, 1996;Salvi and Williams-Jones, 2004). Controversy remains, however, regarding the origin of these silica-oversaturated alkaline magmas, as well as whether the contained mineralization (HFSE, REE, Th and U) is due to extensive fractional crystallization or is associated with the occurrence of late-stage magmatic-hydrothermal processes, which are common in these systems (e.g., Marks et al, 2003;Salvi and Williams-Jones, 2004;Walters et al, 2013).…”

Evidence for dissolution-reprecipitation of apatite and preferential LREE mobility in carbonatite-derived late-stage hydrothermal processes

“…Dead-time was typically around 25%. Quantitative analyses were calibrated using a combination of pure element and mineral standards using the method detailed in Walters et al (2013). Duplicate analyses of an internal apatite standard using the CI EPMA and, separately, the FEI EDS indicate a relative error of ±1% for these analyses, respectively.…”