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Coutras |
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France |
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U
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
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The Coutras tabular, sandstone-type uranium deposit is located north of the Aquitaine Basin, approximately 50 km north-east of Bordeaux in western France.
Mineralisation is hosted by unconsolidated Middle Eocene arenaceous sediments of variable colour deposited in a continental environment. The Tertiary continental sequence that hosts the mineralisation was deposited over a sequence of Cretaceous limestones and followed a period of severe weathering during the Paleocene. During the Tertiary, Hercynian rocks of the Massif Central, with their sedimentary and lateritic cover were uplifted and eroded. These were re-deposited as sediments that host the ore at Coutras, localised within a funnel basin, bounded by two anticlines, where fluvial drainages from the north and east converged. A subsequent marine transgression from the west was associated with the deposition of the immediate host sequence. The uranium mineralisation parallels the neritic zone and is located at the transition facies between marine and fluviatile sedimentation. Four facies are mapped within the ore bearing formation, namely i). lagoonal, ii). delta plain, iii). fluvial and iv). lacustrine to pro-delta. The uranium mineralisation is principally concentrated within the delta plain facies where organic matter is abundant, although local uranium anomalies are also known form the lagoonal facies.
The deltaic facies comprise a truncated, upward thinning sequence with eroded contacts, with dominant unconsolidated, moderately to poorly sorted, medium- to coarse -grained sand and clay layers, characterised by numerous clay galls, wood fragments and chevron crossbeds. The detrital components of the host sediments mainly include mono- and poly-crystalline quartz, feldspar and granitic and metamorphic rock fragments derived from the French Massif Central to the east, set within a phyllosilicate matrix. Pore constitute around 45% of the sandstones. The sands are sub-arkosic to sub-lithic arenites. K feldspars and plagioclase are stained by iron oxides and are partly altered to clay
The ore lenses occur at the surface in the north, and dip to the south to a depth of 100 m in the south. They are distributed over a NNW elongated interval some 20 km long and 1 to 3 km in width, and occur as tabular black lenses. Macroscopically the ore is composed of dark sands and clays that contain abundant particles of organic matter, coalified and silicified wood fragments up to 0.5 m in diameter. The uranium ore forms in dark tabular lenses with a thickness ranging from a few decimetres to a few metres in thickness and with lateral dimensions of from a few metres to a few tens of metres. The bodies occur at a range of levels within the host sequence and are usually concentrated above thick and continuous clayey layers and crosscut bedding. The uranium is disseminated in the clay matrix, concentrated preferentially on carbonaceous matter which occurs as fragments is as fine as µm scale. Pyrite crystals occur near and in organic fragments as euhedral crystals and framboids, generally less than 20 µm across. Larger coalified wood fragments contain high uranium grades (up to 20% U).
There is a good correlation between uranium, sulphur and total carbon, with early diagenetic accumulation of uranium in a permeable, organic rich and slightly acid environment. Uranium is mainly fixed in organics as organo-uranyl compounds. Uranium minerals detected include coffinite, pitchblende and secondary hexavalent U minerals.
The deposit contained 20 000 tonnes of U metal at a grade of 0.1% U.
For more detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 1988.
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.
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Meunier J D, Trouiller A, Brulhet J, Pagel M 1989 - Uranium and organic matter in a paleodeltaic environment: The Coutras deposit (Gironde, France): in Econ. Geol. v84 pp 1541-1556
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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.
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