Although
there has been no systematic evaluation of REE resources in the
British Isles, REE enrichment is known to occur in a range of
geological settings. The British Isles have a long and complex
geological history, and it is likely that further examples of REE
mineralization remain to be discovered.
Geology of the British Isles
The
geology of the British Isles varies from Archaean rocks of the North
Atlantic craton in the far northwest of Scotland, through
Proterozoic and Palaeozoic sequences to Mesozoic sedimentary rocks in
the southeast of England. The British Isles have experienced a
number of orogenic events, most notably the Caledonian Orogeny in the
north and the Variscan orogeny in the south. Alkaline magmatism
associated with post-collisional and extensional settings is
widespread, particularly in Scotland, Northern Ireland, and southwest
England, and hosts the most significant known examples of REE
mineralization. Some REE mineral occurrences are also found in areas
of hydrothermal vein mineralization better known for lead and zinc
exploitation. Secondary REE mineralization formed by diagenetic
processes have been recognized in Palaeozoic sedimentary rocks in
Wales and southern England.
REE in alkaline igneous rocks and carbonatites
The
most significant enrichments of the REE known in the British Isles
are in north west Scotland, in Silurian alkaline igneous intrusions
that were emplaced at the end of the Caledonian Orogeny. In the
Loch
Loyal syenite complex,
veins of hydrothermally altered mafic syenite that are rich in
allanite and apatite contain up to 2 wt per cent total rare earth
oxides (Walters et al., 2013). During the Palaeogene, rift-related
igneous complexes associated with the opening of the North Atlantic
developed in western Scotland and Northern Ireland. Occurrences of
REE-bearing minerals such as fergusonite and gadolinite are found in
several of these complexes, most notably in the Northern
Arran granite in
Scotland and in the Mourne
Mountains of
Northern Ireland, where REE minerals are locally concentrated in
alluvial sediments derived from the granites. Carbonatites are rare
in the British Isles, with the most important being the Permian
Beara-Allahies
carbonatite in
western Ireland (Brady and Moore, 2012). This has previously been
mined for copper, and has minor enrichment in REE.
REE in hydrothermal systems
REE
minerals have been identified within lead-zinc-fluorite veins in
the North
Pennine orefield of
northern England, where they are associated with bismuth-bearing
quartz veins. The mineral assemblage includes the fluorcarbonates
synchysite and bastnäsite, as well as monazite and xenotime.
REE in nodular monazites
Monazite
nodules are found in Lower Palaeozoic sedimentary rocks of central
Wales and also in the Variscan sedimentary succession of southwest
England. These nodules were formed through diagenetic processes
during sediment compaction (Milodowski and Zalaciewicz, 1991) and
have been further concentrated as placers in modern alluvial
sediments. They show zonation of the light REE and heavy REE and are
unusually high in europium but low in thorium. However, further work
would be needed to assess their economic importance.
Key references
Brady,
A E, and Moore, K R. 2012. A mantle-derived dolomite
silicocarbonatite from the southwest coast of Ireland. Mineralogical
Magazine 76(2),
357–376.
Hyslop,
E K, Gillanders, R J, Hill, P G, and Fakes, R D. 1999. Rare-earth
bearing minerals fergusonite and gadolinite from the Arran granite.
Scottish
Journal of Geology,
vol. 35, p65–69.
Milodowski,
A E, and Zalasiewicz, J A. 1991 Redistribution of rare earth
elements during diagenesis of turbidite/hemipelagite mudrock
sequences of Llandovery age from central Wales. Geological
Society Special Publication 57,
101–12.4
Walters,
A S, and Lusty, P. 2011. Rare Earth Elements. British
Geological Survey commodity profile.
Walters,
A S, Goodenough, K M, Hughes, H S R, Roberts, N M W, Gunn, A G,
Rushton, J, and Lacinska, A. 2013. Enrichment of Rare Earth Elements
during magmatic and post-magmatic processes: a case study from the
Loch Loyal Syenite Complex, northern Scotland. Contributions
to Mineralogy and Petrology 166,
1177–1202.
