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AMIS0095 Uranium, Karoo sandstone, 119ppm U, Letlhakane Project, Botswana
Origin of Material: This material was supplied by A-Cap Ltd from their Lethakane Uranium Project 85 km south of Francistown in Botswana.
The uranium mineralisation is hosted within the basal sediments 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 Achaean Zimbabwe Craton which outcrop in the eastern portion of the license.
Mineral and Chemical Composition: The geology of the Letlhakane Uranium Project is relatively simple. Uranium mineralisation occurs within flat-lying sedimentary rocks of the Karroo Supergroup and the entire resource is within 70m of the surface. Importantly some of the highest-grade mineralisation occurs in a secondary zone close to surface. 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 over large areas. 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, tabular zones of mineralisation within fine sandstones. These horizons are typically enveloped with mineralised organic rich mudstones. This primary mineralisation is thought to have been developed relatively soon after the Karoo deposition and lithification.
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.
Dissolution and remobilisation of the primary mineralisation along permeable fracture zones resulted in the development of secondary uranium minerals in the near surface weathering environment. These uranium minerals (predominantly uranium vanadates) occur as fine, yellow, powdery coatings on fracture surfaces and bedding planes. 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.
Recommended Concentrations and Limits
(at two Standard Deviations)
U M/ICP 119 ± 14 ppm
U XRF 118 ± 7 ppm
Specific Gravity 2.43 ± 0.20