Summary:
The Project area is situated within the Karoo Supergroup, which comprises thick terrestrial sedimentary strata deposited during the Carboniferous to late Triassic and is widespread across much of southern Africa.
The primary uranium mineralisation in the Karoo rocks conforms to sandstone-hosted fluvial channel type deposit (Nash et al., 1981; Turner, 1988).
The Karoo basins of sub-Sahara Africa comprise what may be the world’s largest sandstone-hosted uranium province. Compared to the well-known uranium-bearing sandstone basins of the western US, the area of the Karoo basins is about 30% greater, but their known uranium content as of 2003 was only about 7% of that in the US basins. Whereas both areas contain broadly similar, little deformed, predominantly non-marine strata, mainly of Mesozoic age, the order of magnitude lower apparent uranium content of the Karoo basins indicates that they are relatively underexplored (Roux, 1998; Bowell et al., 2009).
At the Muntanga Project, all of the known uranium mineralization occurs within the Escarpment Grit, a 400 m-thick series of continental arenaceous siliciclastic sediments with interbedded mudstones and fine-grained sandstones as well as grits and conglomerates. The Escarpment Grit consists of two informal members thought to represent a change in fluvial style; a lower “Braided Facies” member is interpreted as braided stream deposits and the overlying “Meandering Facies” is much more extensive and thought to represent point-bar and flood plain deposits. The Escarpment Grit unconformably overlies the Madumabisa Mudstone that appears to have acted as an impermeable barrier controlling the base of the mineralization.
Within the Muntanga uranium deposit, the Escarpment Grit Formation comprises at least 120 m of sandstone and conglomerates with occasional mudstones and silts. It overlies the Madumabisa Mudstone Formation, which comprises silty mudstone, with a dark red hematised layer 2-3 m below the contact representing either oxidising groundwater or a sub-aerial surface. Dibbwi East occurs predominantly within the Escarpment Grit Formation and specifically, the uraniferous mineralization is hosted by the relatively un-faulted “Meandering Facies”. Generally, uranium mineralization occurs in four different associations: (i) as disseminated mineralization where grades vary considerably; (ii) associated with mudstones and siltstones; (iii) fracture-hosted uranium mineralization and (iv) mineralization associated with pyrite.
Mutanga, Dibwe and Dibwe East Mineralisation
Mineralisation appears to be later than at least some of the normal faults which cut the Escarpment Grit Formation. This is evident from the good correlation of the radiometric logging data between adjacent holes within the Mutanga mineral deposit separated by interpreted faulting (Lusambo, V. 2011).
The source of the uranium is believed to be the surrounding Proterozoic gneisses and plutonic basement rocks. Having been weathered from these rocks, the uranium was dissolved, transported in solution and precipitated under reducing conditions in siltstones and sandstones. Post lithification fluctuations in the groundwater table caused dissolution, mobilization and redeposition of uranium in reducing, often clay-rich zones and along fractures.
Mineralization is not strictly associated with a particular unit in the stratigraphic section. It is observed to occur in both the fine-grained and coarser material and in mudstones, especially where fractures and mud balls occur. Some mineralization occurs in association with manganese oxide or disseminated with pyrite. Mineralization in some bore holes is seen to occur where there was grey alteration, limonite and feldspar alteration and in dark grey mudstones (Sakuwaha, 2011). The strata dip in the south-easterly direction and mineralization seems to occur along dip.
Uranium mineralization occurs in a number of different associations:
• Disseminated uranium mineralization.
Occurs in sandstones, conglomerates, and within mud layers, mud balls and mud flakes. Uranium is present as interstitial fine-grained crystals or small amorphous masses constituting less than 1% by volume. Grades vary considerably between zones of disseminations, from approximately 20 to 2000 ppm U3O8 in mineralization thought to be solely of a disseminated nature. The presence of sulfides alongside uranium oxides may indicate a transitional zone and/or preferential replacement/reduction of uranium compounds by one chemical route over another (such as decaying organic matter over oxidation of sulfides) as uraniferous groundwaters moved through the lithologies.
• Uranium mineralization associated with mudstones and siltstones.
Muddy lithologies include mud balls (within sandstones), flakes and interbeds. In some cases, mud balls may be completely replaced by uranium mineralization. The degree of replacement varies from fully replaced mud balls to those with a thin selvage of mineralization, whilst others are unmineralized. This is attributed to different ground water chemistry, differing volumes of reducing matter within the mud (fully replaced material may have been a peat-like material), and porosity of the muddy lithology during the influx of uraniferous ground water.
• Fracture hosted uranium mineralization
Uranium mineralization is seen as crystal coatings on surfaces and as concentrations close to surfaces. Most notably at the Dibbwi-Muntanga-Dibbwi corridor, these fractures are coated with black Fe/Mn oxides which in turn may be coated with secondary uranium phosphate mineralization (Autunite, meta-Autunite and selenite).
• Primary uranium mineralization
Outside of the overlying oxidised zone, the mineralization is associated with redox fronts within sandstone layers, where the interface can clearly be seen by a change in colour from pale grey-white to darker grey and the presence of pyrite. It is interpreted that mineralized fluids move along the layers as opposed to the Oxidised zone, where fluid movement is vertical. Other controls on mineralization appear to be the permeability differences where finer-grained sediments and “dirty” sandstone are better hosts to uranium due to the presence of reductants such as organic matter or sulfides, but also reduce the flow rate of groundwater such that reduction reaction can happen. The mineralization is considered primary and consists mostly of Pitchblende, Uraninite or Coffinite.
Njame and Gwabi
At Njame the uranium mineralization occurs at the interface between siltstones and sandstones at redox boundaries. Approximately 25% of the Njame mineralization is siltstone-hosted, with the balance in coarser-grained sandstones and grits.
Similarly to Njame, the uranium mineralization at Gwabi is also related to the redox front; there is one main mineralized horizon which appears to be controlled by both lithology and the redox boundary. It is hosted by the coarse-grained sediments that are interpreted to be the along-strike continuation of the Escarpment Grits which host the Njame uranium mineralization. Uranium mineralization at the Gwabi deposit occurs in red, oxidised, coarse-grained sandstones, grits and pebble conglomerates which overlie a green, non-mineralized, reduced silty-shale horizon. This is interpreted to represent a major redox boundary and may in fact be the regional unconformity between the Upper and Lower Karoo.