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).
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.
At the Muntanga uranium deposit, three stratigraphic zones (“Packages”) were historically identified. The stratigraphic sequence for these packages commences with Package A as the Basal Zone, overlain by Package B, and Package C at the top. The three packages are detailed as follows:
Package A ‘Package A’ is approximately 24 m thick. Overlying the Madumabisa Mudstone Formation, it is a thick, dark grey mudstone coarsening upwards into pyritic, coarse-grained sandstones. Occasional iron oxides are noted. ‘Package A’ is capped by an approximately 5 m thick, coarse matrix-supported conglomerate.
Package B ‘Package B’ is approximately 70 m thick. Overlying ‘Package A’ is a sequence of repeated fining-up cycles that coarsen upwards. Each fining-up unit starts with a very coarse-grained sandstone or conglomerate and fines up to a mudstone or siltstone. The units contain a variety of sedimentary structures including trough and tabular crossbedding and laminations. Sulfides are observed to be within an approximate depth of 50 m from the surface. Above this depth, oxidisation and weathering are evidenced by reddish brown and orange iron oxides and the breakdown of micaceous and feldspathic minerals.
Package C ‘Package C’ is approximately 25 m thick. ‘Package C’ comprises bedded, generally very coarse-grained sandstones with occasional conglomerates.
The Dibbwi uranium deposit is located approximately 10 km to 15 km west of the Muntanga area. Mineralisation in the Dibbwi area appears to be hosted by relatively un-faulted "Meandering Facies” units of the Escarpment Grit Formation.
The Dibbwi East mineral deposit is predominantly composed of Escarpment Grit Formation. The surface geology is characterised by a few scattered sandstone outcrops. At Dibbwi East a clear interface can be observed between surface oxidation to a depth of approximately 40 m, where the sedimentary sequence is bleached with red iron oxide horizons, usually at the interface between mudstones and sandstones. Underlying this oxidised sequence, the sedimentary pile could be considered fresh with sulfides present in areas. Strata dip at about 8° to 15° in the south-easterly direction and strike in the northeast-south-westerly direction. Sandstone layers 10 m to 50 m thick tend to alternate with 2 m to 5 m thick mudstones and siltstones.
Mutanga, Dibwe and Dibwe East Mineralisation
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.
• 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.
• 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. 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. 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. It is hosted by the coarse-grained sediments. 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.