Geologically, the Letlhakane uranium mineralisation is hosted within shallow, flat lying sedimentary rocks of the Karoo Super Group. These Permian to Jurassic aged sediments were deposited in a shallow, broad, westerly dipping basin, generated during rifting of the African continent. The source area for the sediments was the extensively weathered, uranium-bearing, metamorphic rocks of the Archaean Zimbabwe Craton which outcrop in the eastern portion of our license. The deposit contains two main resource areas, Gojwane in the north and Serule in the south. The Gojwane resource hosts most of the mineralisation across the Mokobaesi, Gorgon, Gorgon South, Gorgon West and Kraken prospects and the Serule resource contains the Serule North West, Serule West and Serule East prospects. The mineralisation is still open towards the west.
Schematic cross section of the mineralisation in relation with lithology and basin shape (Looking Northeast).
The sandstone hosted mineralisation has roll front characteristics, where the uranium was precipitated at redox boundaries. Three ore types have been identified; Primary Ore, Secondary Ore and Oxide Ore. The most abundant is the Primary ore.
The second style of primary uranium mineralisation is generated by the migration of uranium bearing groundwater (uranium sourced from basement) through the Karoo sediments and subsequent re-precipitation of uranium in suitably reductive environments. This has resulted in the development of narrow high grade (up to 1500ppm U3O8), tabular zones of mineralisation within fine sandstones. These horizons are typically enveloped with mineralised (150 – 450ppm U3O8) organic rich mudstones. This primary mineralisation is thought to have been developed relatively soon after the Karoo deposition and lithification.
Oxide Mineralisation: Where the primary mineralisation moves into the active weathering environment, the rock becomes oxidized and the uranium mineralogy is altered. This mineralisation is referred to as the ‘oxide’ portion of the resource and has a similar distribution, in terms of both host lithology and grade, to the primary mineralisation. Importantly because of the oxidation of the uranium minerals, the metallurgy of the oxide mineralisation is high with recoveries of up to 87%.
Secondary Mudstone Mineralisation: Dissolution and remobilisation of the primary mineralisation along zones of fracture permeability resulted in the development of secondary uranium (VI) minerals in the near surface weathering environment. The uranium minerals (predominantly uranium vanadates) occur as fine, yellow, powdery coatings on fracture surfaces and bedding planes. This secondary phase of mineralisation occurred recently in geologic history.
Secondary Calcrete Mineralisation: The youngest phase of mineralisation is the result of supergene remobilisation and re-precipitation of uranium minerals from the secondary and primary zones into surficial pedogenic calcrete. The most dominant uranium mineral observed in this style of mineralisation is carnotite.
Calcrete-hosted secondary mineralisation
Uranium mineralisation is controlled by the shape of paleo-channels and the main accumulations occur along the edges of these channels. The majority of the mineralisation occurs within the primary zone, which is mostly un-oxidised. The remainder of the resource occurs as an oxidised zone and a secondary oxidised zone. The secondary oxidised zone occurs in calcretes and mudstone near or at current land surface.