Relatively
few REE occurrences are known in France, although the presence of
alkaline magmatism in the Massif Central, the Pyrenées and Corsica
raises the possibility that further occurrences remain to be
discovered. Presently, the only relatively well documented REE
deposits are monazite nodules of detrital origin in shales from
Central Brittany.
Placers of Brittany
The
most significant REE enrichments are located in low grade
metamorphosed black shales of Dinantian
(Châteauneuf-du-Faou,
Corlay)
or Ordovician age (le
Grand-Fougeray, Châteaubriand),
and may also be found in Neoproterozoic basement adjacent to the
Ordovician series (Craon).
These
shales commonly contain 50 to 200 g/t of monazite nodules.
However, from an economic point of view, only placers where these
nodules have been concentrated are likely to be of interest.
Le Grand-Fougeray Placer
Le
Grand-Fougeray monazite placer, derived from shales of the Ordovician
Traveusot Formation, is the largest and the best documented of the
placers in Brittany. Estimated tonnages in the alluvium of the Aron and
Gras rivers are modest (1900 t of monazite in clayey gravels with a
grade of 2 to 2.6 kg of monazite per ton, and a cut-off at 1kg/t).
However, this monazite is exceptionally rich in europium (0.2 to 1
percent Eu2O3); it also contains neodymium (13 to 14 percent Nd2O3)
and is poor in thorium (<1 percent ThO2).
Exploitation
tests were carried out at a site in the Aron river in 1967–1968,
and 78 tons of monazite were extracted and marketed. New analyses
have indicated interesting grades in dysprosium (0.25 percent Dy)
and terbium (0.1 percent Tb).
Other
occurrences
Similar
layers of shales with monazite nodules and associated placers, but of
lesser interest, are also known in the Mayenne region in the southern
Massif Central, in the Ardennes and in the French Pyrenées (Arize
massif) (Lacomme et al., 1993). Interesting sedimentary occurrences
include Le Vigan, in the Cévennes, where Cambrian dolomites contain
thick but discontinuous phosphate-enriched layers with 0.1–0.22 per
cent uranium, 0.1–0.18 percent thorium and 0.17–0.25 percent
REE. In the Montagne Noire, approximately 30 cm thick layers of
limestone and black shale are intercalated within Cambrian
fine-grained detrital marine sediments, and contain clasts and
phosphate-enriched nodules with 5–20 parts per million (ppm)
thorium, 10–40 ppm uranium and 0.03–0.21 percent REE (Laval et
al., 1990).
The
majority of other known REE occurrences in France are of
mineralogical interest rather than having likely resource potential
(Massif Central, Armorican Massif, Estérel, Pyrenées). The Trimouns
deposit at Luzenac in the central Pyrenées is a world-class talc
deposit, where bastnäsite and several rare earth minerals have been
described. Disseminated occurrences of rare earth minerals are also
known in various intrusive complexes (syenites in the bay of Morlaix
in Brittany, alkaline granite of the Evisa region in Corsica) and
volcanic formations (e.g., Cantal).
Key references
Donnot,
M, Guigues, J, Lulzac, Y, Magnien, A, Parfenoff, A, and Picot,
P.1973.Un nouveau type de gisement d'europium : la monazite grise á
europium en nodules dans les schistes paléozoïques de
Bretagne.Mineralium
Deposita (in
French), 8, 7–18.
Guigues,
J, and Devismes, P.1969.La prospection minière á la batée dans le
Massif Armoricain : méthodes, résultats, atlas
minéralogique.Mémoire
du BRGM No.
71 (in French) 167 p.
Lacomme,
A, Hottin, A-M, and Laval, M.1993. La monazite grise du massif de
l'Arize (Pyrénées françaises) Report
BRGMR
37041 GEO-SGN 93 (in French), 48 p.
Laval,
M, Prian, J-P, Hottin, A-M, and Orgeval, J-J. 1990. Les terres rares
dans les formations cambriennes des Cévennes et de la Montagne Noire
(Massif central français). Report
BRGM R31672
DEX-DAM-90 (in French), 40 p.
