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Karroo Basin Uranium - Ryst Kuil, Rietkuil, Beaufort Group |
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South Africa |
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U
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Super Porphyry Cu and Au
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Uranium mineralisation is widespread within the Permo-Triassic Karro Basin of southern Africa where it in the Beaufort and Stormberg groups in most parts of the basin within South Africa but predominantly in the south-west.
The Adelaide Subgroup of the Beaufort Group comprises at least four upward-fining megacycles reflecting tectonic pulses in the source areas to the south. The known uranium mineralisation has been found to be preferentially developed within the basal sandy members of each cycle, which were deposited under more reducing conditions.
Uranium is contained within both 'tabular' and 'ribbon' channel sandstones, although the mineralised lenses are commonly thicker, narrower and more continuous in the latter. Braided stream sediments with low overall uranium grades are present in the north-western part of the basin.
Uranium occurs in the tetravalent form in the reduced zone and as hexavalent uranium minerals in the weathered zone. Molybdenum, copper, and arsenic are the main associated elements, with calcite as a common gangue mineral. Organic carbon acted as a reducing agent to precipitate the uranium.
Remobilization of the uranium into permeable zones during diagenesis led to the formation of ore grade deposits, a process possibly hastened by the Cape Orogeny, which elevated the temperature of groundwaters in the area.
The larger ore deposits have a grade of around 1 kg/t U3O8. Representative examples include the Ryst Kuil, Rietkuil
Ryst Kuil
This deposit is located about 60 km south-east of the town of Beaufort West, and was discovered early in 1975. It is hosted by a thick sandstone a few thousand metres above the basal contact of the BeaufortGroup and the underlying Ecca Group in an area of gentle folding. On its northern margin mineralisation is terminated by a normal fault with a displacement of about 30 m, preserving a large part of the original deposit on its downthrow side over a distance of abouth 10 km in a south-westerly direction. The host sandstone reaches a maximum thickness of 60 m and is approximately 3 km wide. It was deposited by a low-sinuosity river which flowed in a north-easterly direction. All the characteristics of a fluvial-sand system, such as point-bars, abandoned channels, channel lag conglomerates, etc., are present. The sandstone comprises at least two major sedimentary cycles, with the uranium generally concentrated within the older.
The host rock is light-to-dark-grey, fine-grained quartz, feldspar greywacke with rock fragments. Elevated-background mineralisation is found almost continuously over a wide area, although ore grade pods are isolated and of varying thickness, lateral extent, and grade. The best mineralisation is in organic-rich tabular pods, which may be found in mud-pebble conglomerate, very fine-grained sandstone lenses, or in silty horizons. The principal ore mineral is coffinite, with minor amounts of urano-organic compounds and rare uraninite. Pyrite, arsenopyrite, and molybdenum sulphide occur in association with the uranium, while small amounts of copper and vanadium have been detected in assays.
The source of the uranium is thought to be tuffaceous material deposited contemporaneously with the sediments, or the matrix and rock fragments of the sandstone itself.
Rietkuil
The Rietkuil deposit, which was discovered in 1970, lies below an escarpment approximately 50 km west of Beaufort West. The host sandstones were deposited by high-sinuosity rivers developing aggradational channels, with characteristic trough and ripple cross-bedding, horizontal lamination, and massively bedded sedimentary structures. The sedimentary units are arranged in upward-fining cycles. The host sandstone, an arkosic-wacke, is generally 10 to 20 m thick, lenticular, and elongated in the mean direction of the palaeocurrent. It is moderately sorted, green to grey in colour, and are composed of quartz, orthoclase, and detrital matrix and rock fragments thought to be related to volcanic activity. Disseminated carbonaceous material is fairly abundant.
Uranium mineralisation is concentrated at the base of the host sandstone, in trough cross-bedded or horizontally laminated sandstone, and mud-pebble conglomerate with abundant carbonaceous material. Higher grade mineralisation occurs preferentially in the thicker and more continuous sandstones which show higher sandstone/ mudstone ratios. The ore body is tabular in form and appears to be slightly dished at its margins, where the grade of the uranium commonly increases.
There is a close spatial relationship between uranium and molybdenum (jordisite) and copper, although higher values of copper and molybdenum are apparently offset from the higher concentrations of uranium. Minor arsenic, lead, zinc, and iron-bearing sulphides are also found. The primary uranium minerals are uraninite and coffinite, which have replaced carbonaceous material, calcite and matrix. Oxidation of the primary minerals produced metatorbernite, uranospinite and uranophane.
A volcanic source for the uranium is favoured on account of the high percentage of feldspar and rock fragments in the sandstone. The ore-forming solutions were probably slightly alkaline and rich in carbon dioxide. Mineralisation preceded folding, deep burial, and dolerite intrusion.
Matfieskloof
The Matfieskloof uranium anomaly is situated on the slopes of Groottafelberg at the base of the Nuweveld Escarpment in the Fraserburg District of the Cape Province, west of Beaufort West. It lies within, or immediately adjacent to, the Poortjie Member of the Teekloof Formation, Beaufort Group. The deposit is associated with a succession of fluvial tabular sandstones interbedded with red, purple, and green mudstones and siltstones.
Uraninite and coffinite mineralisation with minor molybdenum is Uavan type, occurring as pods within abandoned loops of a meandering channel system. The geometry of the orebody suggests coalescence between sandstones played a major role in the location of ore pods. The ore resource amounts to about 1 Mt.
DR-3 Anomaly
The DR-3 anomaly near Laingsburg (and west of Beaufort West), Cape Province, occurs some 500 m lower in the stratigraphy than other known large sandstone uranium deposits in the lower Beaufort Group. The deposit occurs in a discrete, linear, east-trending palaeochannel, within an upward-coarsening sequence interpreted as being part of a short-lived, rapidly prograding, delta lobe. The channel, which is considered to represent an abandoned distributary channel, supports two zones of mineralisation, up to 12 m apart, each having similar geometry to that of the channel itself. Overall reserves of the two zones are estimated as >1 Mt averaging >0.5 kg/t U3O8, including a small reserve with a grade in excess of 3.0 kg/t U3O8.
Molybdenum values of up to 0.05% may occur, usually in the hanging wall of The uranium mineralisation is predominantly uraninite and may have associated 0.05% Mo, usually in its hanging wall.
Mineralization is controlled primarily by channel geometry, but also by the abundance of organic detritus, and by the coalescence of two major sandstone sub-units within the channel.
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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Selected References:
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Duane M J, Welke H J, Allsopp H L and Wilsher W A, 1989 - U-Pb isotope systematics, ages and genesis of Karoo uranium deposits, South Africa : in S. Afr. J. Geol. v92 pp 49-64
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le Roux J P and Toens P D, 1986 - A review of the uranium occurrences in the Karroo sequence, South Africa: in Anhaeusser C R, Maske S, (Eds.), 1986 Mineral Deposits of South Africa Geol. Soc. South Africa, Johannesburg v2 pp 2119-2134
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le Roux J P, 1991 - Flume experiments on permeability and organic matter as related to the genesis of uranium deposits in the Beaufort Group: in S. Afr. J. Geol. v94 pp 212-219
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le Roux J P, 1990 - Flume study on the concentration of ilmenite in fine-grained sand and implications concerning uranium mineralization in the Beaufort Group: in S. Afr. J. Geol. v93 pp 785-794
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Stuart-Williams V and Taylor C M, 1986 - The Matjieskloof uranium anomaly, Fraserberg District: in Anhaeusser C R, Maske S, (Eds.), 1986 Mineral Deposits of South Africa Geol. Soc. South Africa, Johannesburg v2 pp 2135-2139
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Wadley R G and Hoffmann J, 1986 - A case history of the DR-3 uranium anomaly, Laingsburg District: in Anhaeusser C R, Maske S, (Eds.), 1986 Mineral Deposits of South Africa Geol. Soc. South Africa, Johannesburg v2 2141-2147
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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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