Summary:
The primary uranium mineralization in the Karoo rocks of the Muntanga Project conforms to a sandstone hosted fluvial channel type deposit (Nash et al., 1981; Turner, 1988).
The Project area is situated within the Karoo Supergroup, which comprises thick, Carboniferous to late Triassic age, terrestrial sedimentary strata and is widespread across much of what is now southern Africa.
The Karoo Supergroup in the Project area consists of three formations within the Lower Karoo; the Siankondobo Sandstone Formation, overlain by the Gwembe Coal Formation, which itself is overlain by the Madumabisa Mudstone Formation.
Muntanga, Dibbwi and Dibbwi East
The Escarpment Grit Formation sequence at the Muntanga uranium deposit comprises at least 120 m of sandstone and conglomerates with occasional mudstones and silts. The Escarpment Grit Formation overlies the Madumabisa Mudstone Formation which comprises a grey to dark grey silty mudstone, with a dark red hematised layer representing either oxidising groundwater or a sub-aerial surface. The mudstone forms an impermeable unit and is thought to have prevented uranium mineralization from moving further down through the stratigraphy. The contact between the Madumabisa Mudstone Formation and overlying Escarpment Grit Formation is between two and three metres above the dark red hematised layer.
Muntanga Geology
Three stratigraphic zones (“Packages”) were historically identified from core logging and utilised as geological boundaries during the resource evaluation phase at Muntanga. The stratigraphic sequence for these packages commences with Package A as the Basal Zone, overlain by Package B, and Package C at the top.
Dibbwi and Dibbwi East Geology
The Dibbwi uranium deposit is located approximately 10 to 15 km west of the Muntanga area. The Dibbwi East mineral deposit is predominantly composed of Escarpment Grit Formation. The surface geology is characterised by a few scattered sandstone outcrops. Two major units can be distinguished in core, the “Braided Facies” member of the lower Escarpment Grit Formation and the “Meandering Facies” member of the upper Escarpment Grit Formation which appear to be transitional from one another. Most of the Dibbwi East mineralization occurs in the “Meandering Facies”. 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.
Strata dip at about 8° to 15° in the south-easterly direction and strike in the north-east-south-westerly direction. The sandstones are predominantly massive looking with cross beddings indicating that they are channel deposits. Cross-bed foreset orientations are variable suggesting high sinuosity (meandering) river deposition. Sandstone layers 10-50 m thick tend to alternate with 2-5 m thick mudstones and siltstones. Mudstones can be laterally continuous for hundreds of metres.
Manganese nodules are common at the surface. These manganese nodules are composed of pyrolusite and hollandite and usually contain uranium mineralization. The uranium is homogeneously distributed within the host manganese and phosphatic minerals.
Mudballs are also present in drill core. These are rounded clasts of clay which bind sediments and minerals to their surfaces.
Muntanga, Dibbwi and Dibbwi East Mineralization
The source of the uranium is believed to be the surrounding Proterozoic gneisses and plutonic basement rocks.
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.
• 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 (Uranium Mineralization Associated with Pyrite). 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. The mineralization is considered primary and consists mostly of Pitchblende, Uraninite or Coffinite.
Njame Mineralization
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.
Drilling conducted by AFR (AFR, March 2008; April 2012) identified two main mineralized 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 mineralization encountered along the entire length. Unlike the high energy sandstone and grit horizons, which show very rapid changes over several tens of metres, the siltstone horizons are generally laterally continuous for hundreds of metres, except where younger grit/sandstone channels have cut through them.
Gwabi Mineralization
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.