Geological Information
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Deposit Type
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Sandstone - Tectonic/Lithologic
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Geological Setting
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Uranium deposits of the Mounana region are hosted in Lower Proterozoic sediments of the intracratonic Franceville Basin in south eastern Gabon. The basin, which overlies Archean crystalline formations, was filled by unmetamorphosed coarse to conglomeratic fluviatile sandstones of the « FA » Formation (100 to 1000m thick), overlain by organic shales of the « FB » Formation and other units, the « FC » formation (massive dolomite and cherts interbedded with black shales), the « FD » and « FE » formations mainly composed of ignimbrites and epiclastic sandstones with interbedded sandstones.
Most deposits are located at the periphery of the basin. All the mineralisation is located in silicified FA sandstone in contact with the overlying FB shales, either conformably (Oklo, Okelobondo) or in fault contact (Mounana, Boyindzi, Mikouloungou). The highest uranium contents are associated with microfracture network, more or less parallel to the bedding.
The Mounana deposit is located between the faulted contact with the basement (F1 fault, footwall) and the F3 fault in the metasediments. It correspond to a steeply dipping (80°) lense following the perturbed bedding .
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Age of Mineralization
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- sedimentation : 2150 Ma,
- early diagenesis : 2099-2036 Ma,
- mineralisation : 2050 Ma,
- criticality of the high grade mineralisation : 1970 Ma
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Mineralization
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Two types of ore are distinguished in the uranium deposits : a commonly occuring low grade ore with a uranium content between 0.1 and 1.0% and a high grade ore with a uranium content ranging from 1.0 to 10%.The high grade ore is restricted to areas associated with tectonic structures and is located in bodies about 5 to 20m in lengh, 5 to 10m wide and 0.3 to 2m thick. The economic character of the mineralisation is due to the common ore, but the high grade ore is important because it is the ore in which fission reactions started.
Mineralisation of the Mounana deposit was separated into two levels :
- on top, starting at the surface, an oxidised zone with abundant and various secondary uraniferous minerals,
- below, an area with black ore. Uranium is associated to phyllites and organic matter with high concentrations of vanadium. Pitchblende and coffinite are dominant.
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MetallogenicAspects
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Ore Mineralogy: pitchblende with some coffinite surrounding the pitchblende. Pitchblende is always associated with hydrocarbon material of high catagenic rank. The mineralised organic matter consists of solid bitumen.
Sandstone matrix consisting largely of secondary silica, with chlorite and illite in varying proportions.
Abundant sulfides dominated by pyrite and galena with minor marcassite and copper sulfides (chalcopyrite, digenite and covellite).
Ore Mineralogy: chalcocite-chalcopyrite-galène-greigite-marcassite-pyrite-sphalérite-bariandite-duttonite-goethite-karelianite-montroséite-nolanite-pechblende-mounanaïte-mottramite-chervétite-shubnélite-lenoblite-uranopilite-zippéite-johannite-torbernite, ...
The Mounana deposit is characterised by the abundance of secondary uranium minerals. Five uranium vanadates were described as new species : francevillite, curientite, mounanaite, vanuralite and metavanuralite.
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Age of Mineralization
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- sedimentation : 2150 Ma,
- early diagenesis : 2099-2036 Ma,
- mineralisation : 2050 Ma,
- criticality of the high grade mineralisation : 1970 Ma
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DepositShape
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Steeply dipping lense developped at the intersection of two faults
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DepositDimensions
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Deposit Width: 40m
Deposit Length: 150m
Deposit Depth Bottom: 400m
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GeologicalRemarks
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Metallogenic Aspects
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Ore Mineralogy: pitchblende with some coffinite surrounding the pitchblende. Pitchblende is always associated with hydrocarbon material of high catagenic rank. The mineralised organic matter consists of solid bitumen.
Sandstone matrix consisting largely of secondary silica, with chlorite and illite in varying proportions.
Abundant sulfides dominated by pyrite and galena with minor marcassite and copper sulfides (chalcopyrite, digenite and covellite).
Ore Mineralogy: chalcocite-chalcopyrite-galène-greigite-marcassite-pyrite-sphalérite-bariandite-duttonite-goethite-karelianite-montroséite-nolanite-pechblende-mounanaïte-mottramite-chervétite-shubnélite-lenoblite-uranopilite-zippéite-johannite-torbernite, ...
The Mounana deposit is characterised by the abundance of secondary uranium minerals. Five uranium vanadates were described as new species : francevillite, curientite, mounanaite, vanuralite and metavanuralite.
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