|
La Crouzille District - Margnac, Fanay, Le Fraisse |
|
|
France |
| Main commodities:
U
|
|
 |
|
|
 |
Super Porphyry Cu and Au
|
IOCG Deposits - 70 papers
|
All papers now Open Access.
Available as Full Text for direct download or on request. |
|
|
The La Crouzille uraniferous district and the Margnac, Fanay and Le Fraisse mines constitute the main intragranitic uranium ores in France. They are located approximately 20 km north of Limoges in Haute-Vienne, northern France.
The uranium deposits of the La Crouzille district are located within the Saint-Sylvestre two mica granite forms the central part of a larger Saint-Sylvestre granitic massif within the north-west part of the Massif Central. The granitic massif is surrounded by metamorphic suites composed of mica-schists, plagioclase gneisses and quartz-feldspar gneisses and leptynites that wre deposited at around 600 Ma. Granitic rocks were emplaced in two periods, at 515 to 470 Ma and at 450 to 430 Ma. Both the surrounding intruded sequences and the granites were subjected to peak metamorphism with local anatexis and melting between 380 and 360 Ma which produced the Saint-Sylvestre granitic massif from ananatectic magma.
The Saint-Sylvestre granitic massif is composed of the Brame biotite-sillimanite granite to the west and the Saint-Sylvestre two mica granite to the east. The two have only minor chemical differences, although they are mineralogically and texturally different.
The Brame granite is foliated and the orthoclase-sillimanite-biotite assemblage implies catazonal crystallisation. In contrast the Saint-Sylvestre two mica granite is not foliated and has a mesozonal biotite-muscovite association. These granites have been interpreted to have been two separate phases, although Leroy (1978) cites evidence that they represent two facies of the same granite, with the muscovite of the Saint-Sylvestre two mica granite being secondary. The lower facies which contains quartz-biotite-sillimanite-orthoclase-oligoclase was foliated during the P2 phase of folding. The upper facies, which contains quartz-biotite-muscovite-orthoclase-albite is interpreted to have formed from late magmatic and deuteric alteration of the lower facies. Muscovite and albite increase upwards. It is further interpreted that the magma was formed by anatexis during the peak metamorphism at 360 Ma and recrystallised within the enclosing metamorphics during the P2 folding from 360 to 350 Ma. At 336 Ma a fine two-mica andalusite granite intruded the Brame granite followed by late magmatic and then deuteric alteration (producing muscovite and albite) in the upper part of the massif.
Two fault systems cut the Saint-Sylvestre granite, namely: i). The earliest east-west to WNW and north-south to NNE fault sets which control phyllic alteration of the two-mica granite and the emplacement of 285 ±10 Ma lamprophyre and microgranite dykes into the two mica granite. At the intersections of these fault sets, the alteration forms a pipe like structure. ii). A subsequent faulting system which cuts and displaces the previous structures and dykes, which trends NW controls the uranium veining.
Two styles of mineralisation are distinguished in the district:
i). Vein-type mineralisation - generally occurring as brecciated veins with pitchblende, pyrite and microcrystalline hematitic quartz cemented with muscovite argyllic minerals, black oxides and rare fluorite. At Le Fraisse, veins dip at around 70° are 100 to 200 m in strike length, up to 300 m in vertical extent and from a few cm to 20 m in thickness, but average around 3 m in width (Hall, 1987).
ii). Disseminated mineralisation, accounting for 40% of the ore at Margnac and 30% at Fanay. The mineralisation occurs in mica alteration pipes (known as episyenites) and veins, and in brecciated granite and feldspar episyenites. "Episyenite' pipes may be several to several tens of metres in diameter. Pitchblende is the primary uranium mineral, accompanied by pyrite, coating rock-forming minerals and in fractures/cleavage. Fine crystalline quartz, associated with hematite and marcasite and rare prismatic coffinite is also added to the rock mass. Late fluorite, barite and calcite are also observed in the zones of disseminated mineralisation.
Where veins are in contact with the lamprophyre dykes within the mine the lamprophyres are altered and grades are enhanced.
The deposits of the La Crouzille district were reported by Leroy (1978) as having contained 22 950 tonnes of U, or 27 000 tonnes of U3O8.
The most recent source geological information used to prepare this decription was dated: 1978.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
|
|
|
|
Ballouard, C., Poujol, M., Mercadier, J., Deloule, E., Boulvais, P., Baele, J. M., Cuney, M. and Cathelineau, M., 2018 - Uranium metallogenesis of the peraluminous leucogranite from the Pontivy-Rostrenen magmatic complex (French Armorican Variscan belt): the result of long-term oxidized hydrothermal alteration during strike-slip deformation: in Mineralium Deposita v.53, pp. 601-628.
|
Leroy J, 1978 - The Margnac and Fanay uranium depoits of the La Crouzille District (western Massif Central, France): geologic and fluid inclusion studies: in Econ. Geol. v73 pp 1611-1634
|
|
Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
|
Top | Search Again | PGC Home | Terms & Conditions
|
|
