The geology of the Letlhakane Uranium Project is relatively simple. Uranium mineralisation occurs within the flat-lying sedimentary rocks of the Karoo Supergroup and the entire resource is within 70m of the surface. Importantly some of the highest-grade mineralisation occurs in the secondary zone close to surface ensuring early access to high grades and low cost mining from the commencement of production.
Based on exploration and the geology logged from extensive trenching of the Mokobaesi area, four distinct styles of mineralisation have been identified:
Primary Mineralisation: Two dominant styles of primary uranium mineralisation have been defined within the Letlhakane project area. The earliest phase of mineralisation consists of uranium, which is hosted by fine-grained, organic rich mudstones (with minor coaly interbeds) which are developed on the margins of sandy river channels. The carbonaceous mudstones, and hence the mineralisation, tend to be sub-horizontal and laterally continuous (particularly in the Kraken Resource area) over large areas. The average uranium grade in the carbonaceous mudstones is around 150ppm, but most units have grades exceeding 1000ppm in narrow bands at the top and bottom of the sequences.
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.