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Kayelekera |
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Malawi |
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U
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Super Porphyry Cu and Au
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The Kayelekera sandstone hosted uranium deposit is located in far northern Malawi, 35 km to the west of Karonga.
Much of Northern Malawi is underlain by the pre-Karoo Malawi Basement Complex metamorphic and igneous rocks, composed predominantly of gneisses and intrusives of the Misuku Belt representing the southeastern extension of the ~2000 Ma Ubendian Mobile belt in southwestern Tanzania and Malawi. This basement was affected by four episodes of largely brittle deformation in the Neoproterozoic Irumide and early Palaeozoic Mozambique orogenies. The subsequent extended period of erosion of the Misuku belt ended in the early Permian with deposition on a subdued but irregular topography by the Karoo sequence, commencing with glacial and periglacial sediments. The Karoo sedimentation, which was controlled by faulting and subsidence, ended with the commencement of the Gondwana erosion cycle in the Lower Jurassic. Subsequent faulting and erosion of the areally extensive Karoo sequence has left several partially or totally fault-bounded basins remnants.
The Kayelekera deposit lies within the north-south elongated 50 x 6.5 km North Rukuru Basin which contains a >1500 m thick sequence of Karoo sedimentary rocks preserved in a semi-graben. This basin is ~35 km to the west of and broadly parallel to, the Lake Malawi segment of the East African Rift system.
The North Rukuru Basin contains a thick succession of clastic sediments which are divided into two distinctly different formations, namely the glacial and glacio-lacustrine Basal Beds Formation, comprising diamictite (tillite) horizons overlain by flaggy sandstone and varved shale horizons, followed by a thick series of alternating arkosic sandstones and mudstones of the North Rukuru Sandstone and Shale Formation, representing an environment of subsiding basin with lakes, braided and meandering river.
The rocks of the North Rukuru Basin generally dip at up to 35° to the east, although, adjacent to the eastern basin margin fault often dip at 10 to 20°W.
The Kayelekera uranium deposit is developed in the uppermost part of the preserved North Rukuru Sandstone and Shale Formation and lies entirely within the Kayelekera Member which, within the deposit area, has a maximum thickness of ~150 m and contains a total of eight separate arkosic units with intervening silty mudstones and mudstones in an approximate 1:1 ratio. The base of the Kayelekera Member is a mottled dark grey/chocolate brown mudstone that is underlain by at least 70 m of silty, chocolate brown mudstones with thin, discontinuous, poorly-sorted, muddy, hematitic arkose intercalations forming the top of the Muswanga Red Beds Member.
The Kayelekera Member succession indicates cyclic sedimentation within a broad, shallow, intermittently subsiding basin. Each cyclothem generally passes upwards from coarse reduced facies arkose through oxide-facies 'red-bed' mudstone into reduced facies, grey-black carbonaceous silty mudstones, with thin coaly horizons in some cyclothems. The individual arkose units may contain several upward fining sequences grading from quartz-feldspar pebble conglomerate to medium, or more rarely, fine-grained micaceous arkosic sandstone. Current bedding is common, with a consistent current direction from the southwest. Carbonaceous debris is commonly present in association with pyrite in reduced facies arkose, occuring as layers on cross-stratification surfaces, disseminations and as individual 'woody' fragments several cm in length.
The 'red-bed' mudstone within the cyclothems is a typically red to chocolate-brown, homogenous fine-grained sediment with no discernible bedding. Pale green patchy 'reduction zones' may be present, while calcareous, concretionary nodules and calcite veining are common in the lower units. These sediments appear to have accumulated in an oxygenated, subaerial environment as extensive floodplain mud flats following deposition of the coarse arkosic sediments.
The grey carbonaceous mudstone units are more variable than the 'red beds' and include light to dark grey homogenous mudstones, grey silty mudstones containing discrete quartz grains, sometimes with calcite veining, silty mudstones with multi-coloured angular mud clasts, laminated bedded carbonaceous pyritic black shales, fine grained ripple cross-stratified carbonaceous sandstone and 'coal' shales.
The Muswanga Member of the North Rukuru Sandstone and Shale Formation is folded into gentle synclinal structures by drag against the Eastern Boundary Fault. The Kayelekera Member and the uranium deposit occupy the core of one of these synclinal structures which has an axial trend of 330°, parallel to the Eastern Basement Fault. The regional structure is dominated by the Eastern Basement Fault, which strikes NW-SE and dips steeply SW. Faults within the Karoo basin are predominantly steep normal structures trending either parallel with or normal to the Eastern Basement Fault. The Kayelekera syncline occurs within a downfaulted block bounded by NNW trending normal faults, while a series of steep, close spaced normal faults with a cumulative throw of >100 m define the eastern margin of the deposit. The transverse Chimpanji fault, cuts across this structure to the north of the deposit resulting in a dip reversal and the creation of a basin structure bounded by faults on its north, east and west margins.
Lenses of uranium mineralisation occur within arkose units to a depth of 100 m. These lenses are stacked vertically in a trend approximately parallel to the synclinal axis of the fault-bounded structure. Mineralisation is offset, but is not confined, by the fault structures, although the mineralisation is limited by the surface topography which truncates the host lithologies. The majority of the mineralisation is contained within the arkose units, although some secondary mineralisation occurs in the mudstones immediately below the mineralised arkose, particularly adjacent to the Chimpanji fault. Individual arkose units are readily correlated between drill holes due to the continuity of the stratigraphy and the presence of two coaly-shale marker horizons.
Three types of mineralisation are recognised, based on visual identification of the redox state of the hosting lithology, namely: i). reduced, ii). oxidised and iii). transition (mixed oxide and reduced) facies.
Six principal mineral lenses contain the bulk of the uranium mineralisation.
Coffinite is the main primary uranium mineral in the reduced facies, often associated with organic debris and/or pyrite, occurring as very finely intergrowths with chlorite/clay which fills the interstices of the arkosic sandstones. Minor amounts of extremely fine-grained uranium oxide, probably uraninite, have been identified in some reduced and transitional zone mineralisation from all the main horizons. A uranium-titanium mineral, possibly betafite or tanteuxenite has also been identified in minor quantities. Several yellow-green secondary uranium minerals have been identified in the oxide facies, all of which result from the oxide weathering of the primary uranium minerals. The dominant secondary uranium minerals are metaautunite and boltwoodite with minor but ubiquitous uranophane.
The uranium contained in the Kayelekera deposit is interpreted to have originally been derived from the sub-aerial erosion of basement granitic rocks to the SW and subsequently deposited with the arkosic sandstones and mudstones of the North Rukuru sandstone and Shales Formation. Where un-influenced by surface weathering processes or circulation of oxidised groundwater, the arkoses of the Kayelekera Member comprise a low grade protore associated with carbonaceous rich zones. Rift faulting and associated folding resulted in the uranium being mobilised in oxidised ground waters and re-deposited as ore grade concentrations at the redox front within a preserved structural basin of reduced arkose. The basinal structure at Kayelekera has led to the accumulation of uranium in a structural low to form a series of stacked arkose horizons.
At a 300 ppm U3O8 cutoff, the total measured + indicated reource (2006) is calculated as 15.3 Mt @ 0.09% U3O8 comprising 13 630 tonnes of U3O8, with a further 3.40 Mt @ 0.06% U3O8 comprising 2 040 tonnes of U3O8 as Inferred Resource under the JORC code.
The title to the deposit is owned by Paladin Resources Ltd through Paladin (Africa) Ltd.
The most recent source geological information used to prepare this decription was dated: 2007.
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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Bowden R A and Shaw R P, 2007 - The Kayelekera uranium deposit, Northern Malawi: past exploration activities, economic geology and decay series disequilibrium : in Trans. IMM (incorp. AusIMM Proc.), Section B, Appl. Earth Sc. v116 pp 55-67
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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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