Summary:
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).
Mineralisation
Mutanga, Dibwe and Dibwe East
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.
Mineralisation is not strictly associated with a particular unit in the stratigraphic section. It was observed to occur in both the fine-grained and coarser material and mudstones especially where fractures and mud balls occur. Some mineralisation occurred in association with manganese oxide or disseminated with pyrite. Mineralisation in some bore holes was 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 mineralisation seems to occur along dip.
Uranium mineralisation occurs in a number of different associations:
• Disseminated mineralisation
Occurs in sandstones, conglomerates, and within mud layers, mud balls and mud flakes. Uranium present as interstitial fine grained crystals or small amorphous masses constituting less than 1% by volume. Grades vary considerably between zones of disseminations, approximately 20 to 2000 ppm U3O8 in mineralisation 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.
• Mineralisation 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 mineralisation. The degree of replacement varies from fully replaced mud balls to those with a thin selvage of mineralisation, 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 mineralisation
Mineralisation is seen as crystal coatings on surfaces and as concentration close to surfaces. Most notably at the Dibwe-Mutanga-Dibwe corridor, these fractures are coated with black Fe/Mn oxides which in turn may be coated with secondary uranium phosphate mineralisation (Autunite, meta- Autunite and selenite).
• Uranium mineralisation associated with pyrite
Mineralisation may be elevated in some (relatively) pyrite rich zones. Presence of sulfides in close proximity to 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.
Njame, Njame South and Gwabe
At Njame and Njame South, the uranium mineralisation occurs at the interface between siltstones and sandstones at redox boundaries. Approximately 25% of the Njame mineralisation is siltstone hosted, with the balance in coarser grained sandstones and grits.
Drilling conducted by AFR (AFR, March 2008; April 2012) identified two main mineralised horizons; the thickest, most consistent and highest grade is the lower horizon within the second sequence from the base. Drilling was carried out along the entire length of the 5 km long system, with uranium mineralisation encountered along the entire length. Unlike the high energy sandstone and grit horizons, which show very rapid changes over several tens of meters, the siltstone horizons are generally laterally continuous for hundreds of meters, except where younger grit/sandstone channels have cut through them. There is a clear stratigraphic control on mineralisation at deposit scale, although structural control may be present on a larger scale.
Similarly to Njame, the uranium mineralisation at Gwabe is also related to the redox front; there is one main mineralised 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 mineralisation. Uranium mineralisation at the Gwabe deposit occurs in red, oxidised, coarse grained sandstones, grits and pebble conglomerates which overly a green, non-mineralised, 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.