Rare earth mineralisation in the Cnoc nan Cuilean intrusion of the Loch Loyal Syenite Complex, northern Scotland

A.S. Walters1*, H.S.R. Hughes2, K.M. Goodenough3. A.G. Gunn1 & A. Lacinska1 *abig@bgs.ac.uk1British Geological Survey, Keyworth 2School of Earth & Ocean Sciences, Cardiff University 3British Geological Survey, Edinburgh

3. REE and trace element geochemistry

Melasyenites are enriched in total rare earth oxide (TREO) relative to the leucosyenites with average values of 3800 ppm and 1400 ppm respectively. The highest contents, up to 20 000 ppm TREO (2%), are found in narrow biotite-magnetite-rich veins identified in a single stream section near the eastern margin of the intrusion. All lithologies are light rare earth element (LREE) dominated (Figure 2) with high concentrations of Ba and Sr and low levels of Nb and Ta (Figure 3).

Figure 2: TREO distribution in Cnoc nan Cuilean lithologies

6. ConclusionsThe Cnoc nan Cuilean intrusion of the Loch Loyal Complex is enriched in REE compared to similar intrusions in Scotland with values of up to 2% TREO.All lithologies are LREE dominated and the main REE-bearing phase is allanite.A preliminary genetic model involves crystallisation of a REE-enriched melasyenite magma followed by later remobilisation and upgrading of REE within biotite-magnetite veins. This model may be a basis for continuing exploration in Scotland or, by furthering our understanding of how REE are enriched, can be applied to similar settings elsewhere.

References[1] Hughes H.S.R. (2011) The geology and rare earth element metallogenesis of the Cnoc nan Cuilean intrusion, Loch Loyal Complex, NW Scotland. Unpublished MSc thesis, Camborne School of Mines, University of Exeter.

Mineral abbreviationsAp: Apatite; Aln: Allanite; Aln alt: Allanite alteration products Cpx: Clinopyroxene; Kfs: K-feldspar; Mag: Magnetite; Mg v: Microgranite vein; REE-Sr C: REE-Sr carbonate; Ttn: Titanite; Thr: Thorite

Aln 1

1. Crystallisation of a REE-enriched melasyenite magma: formation of early apatite, clinopyroxene and titanite.

2. Crystallisation of all magmatic phases: allanite forms as rims round apatite and interstitial to earlier minerals.

4. Late magmatic veining: microgranite veining cross-cuts and offsets earlier allanite veinlets. This magmatism also introduces new fluids.

5. Biotite-magnetite veins: recrystallisation of primary magmatic minerals. Fluids remobilise and redistribute REE and trace elements from the syenites becoming concentrated in biotite-magnetite veins.

6. Alteration and fracturing: allanite altered to a REE-silicate phase and fracturing. Fractures are infilled by REE-Sr-bearing carbonate veinlets.

5. Genetic model

3. Cooling of the solid system: syenite fractures due to contraction and circulation of fluids. Fractures are infilled with allanite.

Cpx

Ap

Ttn

Aln 1

Aln 2

Mg v

Ap

Aln 3

CpxMag

Aln alt

REE-Sr C

4. REE-bearing minerals

Allanite is the dominant REE-bearing mineral, and is present in all major magmatic lithologies and the biotite-magnetite veins. Three generations have been identified: 1. Late-magmatic allanite rimming apatite (Figure 4a)2. Allanite micro-veinlets cross-cutting the syenite (Figure 4b)3. Allanite in biotite-magnetite veins (Figure 4c)TREO concentrations of the different allanite generations are similar, averaging 22%. The vein-hosted allanite has been altered, locally leading to a net enrichment of up to 50% TREO (Figure 4d).

Figure 4a: Late-magmatic allanite (BSEM)

Figure 4b: Allanite micro-veinlets (BSEM)

Figure 4c: Vein-hosted allanite (BSEM)

Figure 4d: Altered allanite (BSEM)

Ap

Aln rim Cpx

Cpx

Kfs

Aln veinlet

Cpx

Thr

Aln

Ap

Aln

Aln alt

Cpx Cpx

Ap

2. The Loch Loyal Complex

The Loch Loyal Syenite Complex comprises three intrusions: Ben Loyal, Beinn Stumanadh and Cnoc nan Cuilean (Figure 1). The Cnoc nan Cuilean intrusion, which covers an area of about 3 km2, can be subdivided into two zones: a Mixed Syenite Zone (MSZ) and a later Massive Leucosyenite Zone (MLZ). Evidence from field mapping and 3D-modelling suggests that melasyenites were passively emplaced to form a lopolith concordant with the Moine and Lewisian country rocks. A later episode of leucosyenitic magmatism caused mixing and mingling with the melasyenite forming the MSZ. Continued intrusion of leucosyenite melts then formed the MLZ [1].

Figure 1: Geological map of the Loch Loyal Complex

Ben Loyal

Cnoc nan Cuilean

Beinn Stumanadh

Loch Loyal Fault

Figure 3: Spider diagram for Cnoc nan Cuilean lithologies

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Ba Rb Th K Nb Ta La Ce Sr Nd P Sm Zr Hf Ti Tb Y Tm Yb

Rock

/cho

ndrit

e

LeucosyeniteMelasyeniteBiotite-magnetite veins

1. Introduction

Global concerns about security of rare earth element (REE) supply have generated interest in identifying new deposits and understanding the processes by which they form. BGS studies on potential indigenous resources focus on the Caledonian alkaline intrusive complexes of north-west Scotland. The highest REE values have been found in the Cnoc nan Cuilean intrusion of the Loch Loyal Complex.

Study area