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Franceville, Okla, Mounana, Boyindzi, Okelobondo, Bangombe, Mikouloungou, Kiene

Gabon

Main commodities: U
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Sediment hosted uranium deposits are found within the Franceville Basin of Gabon, Africa, with major deposits on the north-western edge of the basin at Mounana ( 9.6 Mt @ 4 kg/t U3O8; 38 400 tonnes of U3O8), Okla (18 000 tonnes U3O8) and a series of other similar deposits including Boyindzi (3100 tonnes U3O8), Okelobondo (7400 tonnes U3O8).   Less important deposits are found in the central part of the basin at Bangombe (3000 tonnes U3O8), Mikouloungou (4700 tonnes U3O8) and Kiene.

The intracratonic Franceville Basin contains a 1000 to 4000 m thick Palaeoproterozoic sequence bounded to the NE by the 'North Gabon Craton' and to the SW by the associated Chaillu Block.   The overall depository is subdivided into a number of sub-basins by basement ridges.   The succession within the basin comprises, from the base,

Mabinga Sandstone, 0 to 1000m thick - a basal red sandstone which is coarse, feldspathic and conglomeratic along the margin with the Chaillu Block and has associated dolomite, anhydrite and gypsum, passing upwards into a pyritic and asphaltic facies;

Bangombe Shales, 600m thick - calcareous and carbonaceous pelites with alternating jaspers and some sideritic and manganese rich iron formations and lenticular channel sandstones, including a lower mafic volcano-sedimentary unit;

Djoumou Shales, up to 50m thick - a marker unit of mostly thickly banded chert and massive dolomite which oversteps onto the basement;

Bambai Black Shales with associated ignimbritic tuffs at the top;

Lepaka Sandstones - the uppermost unit comprising alternations of sandstone and shale.

The age of deposition is limited by the 2700 Ma dates on the Chaillu Block basement and 2050 Ma of the contained uranium mineralisation.

The Franceville uranium orebodies are hosted by the upper sections of the Palaeoproterozoic Mabinga Sandstone and sections of the overlying basal Bangombe Shale, within the fault controlled Franceville Basin which was developed over the Archaean North Gabon Craton.

The host sequence commences with fluviatile and deltaic sediments comprising initial fluviatile coarse, poorly sorted and conglomeratic sandstones, including interbedded red radioactive conglomerates (averaging around 9 ppm U and containing thorite and uranothorite) with heavy mineral concentrations.   The most common cements in the sandstones are evaporitic sulphates (gypsum and anhydrite) and dolomite.   The fluviatile sediments grade upwards into deltaic facies comprising finer sandstones, siltstones and lesser shales.   These sandstones are transgressively overlain by carbonaceous marine shales carrying kerogen and asphalt/bitumen.

All of the uranium deposits of the basin appear to be located in tectonic structures characteristic of petroleum traps.   These deposits are located in the upper levels of the basin, capped by the impermeable marine black carbonaceous shales that overlie the fluviatile and deltaic sandstones.   The lower sandstones and conglomerates are red, and are followed by a green zone within the upper part of the sandstones, whose boundary transgresses stratigraphy. This green zone is capped by a black hydrocarbon rich zone which hosts the ore within sandstones, immediately below the black shales.   The uranium always accompanies hydrocarbon (kerogen or bitumen) and has associated sulphides (chiefly pyrite and galena, with minor chalcopyrite, digenite and covellite).   There are two types of ore, the 'common ore' which is from 0.1 to 1% U and 'high grade' lenses (5 to 20 m long x 5 to 10 m wide x 0.3 to 2 m thick) with grades of 1 to 10% uranium. The main phase of U mineralisation has been dated at from 1700 to 2100 Ma. See Gauthier-Lafaye and Weber 1989.

The 'common ore' consists of black silicified sandstone containing secondary silica with varying chlorite and illite and abundant sulphides in the form of pyrite and galena, with minor marcasite, chalcopyrite, digenite and covellite.   Uranium is present as pitchblende, sometimes enveloped by coffinite.   The deposit contains significant vanadium.   Pitchblende is also associated with the high carbon sections of the ore.

The 'high grade' ore generally occurs in more fractured sandstones with a clay gangue believed to represent hydraulic breccias associated with tectonic features such as faults and jointing.   It is found in black sandstones with variable silicification, but containing hematitic spots.   Uranium is present mainly as pitchblende with some coffinite but is never included within the organic material.   It is restricted to secondary porosity created by the corrosion of detrital quartz and is associated with sulphides (mostly pyrite and galena) within a matrix containing chlorite and some illite.   High grade ore is interpretted to result from oxidation-reduction processes in strongly hydro-fractured 'common ore'.

Sections of the orebody at Okla contain areas with 20 to 60% U (compared to the normal 0.2 to 1%) which are believed to have initiated natural fission reactions in the geologcal past.

For detail on these deposits consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 1989.    
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:
Dymkov Y, Holliger P, Pagel M, Gorshkov A, Artyukhina A,  1997 - Characterization of a La-Ce-Sr-Ca aluminous hydroxy phosphate in nuclear zone 13 in the Oklo uranium deposit (Gabon) : in    Mineralium Deposita   v32 pp 617-620
Evins L Z, Jensen K A and Ewing R C,  2005 - Uraninite recrystallization and Pb loss in the Oklo and Bangombe natural fission reactors, Gabon: in    Geochimica et Cosmochimica Acta   v69 pp 1589-1606
Gauthier-LaFaye F, Weber F  1989 - Natural fission reactors of Oklo: in    Econ. Geol.   v84 pp 2286-2295
Gauthier-LaFaye F, Weber F  1989 - The Francevillian (Lower Proterozoic) Uranium ore deposits of Gabon: in    Econ. Geol.   v84 pp 2267-2285
Mossman D J, Gauthier-Lafaye F and Jackson S E,  2005 - Black shales, organic matter, ore genesis and hydrocarbon generation in the Paleoproterozoic Franceville Series, Gabon: in    Precambrian Research   v.137 pp. 253-272


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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