French Massif Central Uranium - St. Sylvestre, Millevaches, Western Marche, Bernardan


Main commodities: U
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The north-western part of the French Massif Central represents the largest uranium district in Europe, having produced more than 30 000 tonnes of uranium since 1947.   Three peraluminous granite complexes are of economic significvance in this region, namely the St. Sylvestre, Millevaches and the Western Marche complexes.

The Bernardan deposit is within the Hercynian Western Marche Complex, hosted by two mica leucogranite.   The granites of the host complex intrude Palaeozoic biotite migmatites and gneisses.   The Western Marche Complex is one of a suite of synkinematic peraluminous intrusions emplaced along the structural extension of the South Armorican Shear Zone.   It comprises 1). a late Devonian (365 ±10 Ma) biotite rich granite to the west, and 2). the two mica leucogranite with various facies which hosts the mineralisation and is equated with the aluminopotassic French Variscan granites of 340 to 300 Ma age.   The St Sylvestre Granite, which also belongs to this group, is dated at 325 to 315 Ma.   The two mica granites are regarded as being "fertile" as they average 12.5 ppm U, most of which is present as leachable uraninite.   Late deuteric alteration affected the mineralised granites to produce pervasive sericitisation of plagioclase, pervasive chloritisation of biotite and the development of perthite.

The mineralised area is influenced by two main faults, one of which, a major 120° trending regional feature 2 km north of the Bernardan deposit, separates the granite complex from the Palaeozoic sequence.

The Bernardan deposit occurs as disseminated mineralisation within a vuggy granite termed an "episyenite".   Some 10 bodies have been identified, all structurally controlled within irregular episyenite pipes, lenses or vertically elongated pods.   The episyenites are caused by the leaching of quartz from the primary granite, but with preservation of the original texture.   The resultant rock has a significant 20 to 30% porosity and permeability.   At Bernardan the episyenites are within a schistose granite and are controlled by a dense network of superimposed fractures on all scales.   The uranium ore occurs as either autunite-gummite (oxidised) or pitchbelnde-coffinite (reduced) adsorbed onto clay minerals.

Production from Bernardan to 1997 was approximately 4300 tonnes of U from ore with an average grade of 0.53% U.   The deposit was initially mined by open cut to a depth of 120 m, before going underground, where by 1997 it was being worked at a depth of 350 m.

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 1997.    
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.

  References & Additional Information
   Selected References:
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.
Patrier P, Beaufort D, Bril H, Bonhomme M, Fouillac A M, Aumaitre R  1997 - Alteration-mineralization at the Bernardan U deposit (western Marche, France): the contribution of alteration petrology and crystal chemistry of secondary phases to a new genetic model: in    Econ. Geol.   v92 pp 448-467

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 takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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