The Mothae kimberlite is situated on the southern edge of the Kaapvaal Craton, which extends through central, eastern and north-eastern South Africa, into southern Zimbabwe and southeastern Botswana, and incorporates most of Swaziland.

As the diamondiferous Northern Lesotho Kimberlite Field is in the Kaapvaal Craton, it conforms to ‘Clifford’s Rule’, which states that diamondiferous kimberlites tend to occur in geological regions that have been tectonically stable since the Archaean.

The Archaean basement in Lesotho is entirely covered by the flat-lying Paleozoic to Mesozoic Karoo Supergroup which reaches a thickness of approximately 4 km in Lesotho.

The surface geology within the Mothae license area comprises amygdaloidal and nonamygdaloidal Mesozoic (180 Ma) Drakensberg Group flood basalt, into which the Mothae kimberlite has intruded. The average elevation of the Mothae kimberlite is approximately 2,900 mamsl and the thickness of the basalt into which it is emplaced is estimated to be of the order of 1,000 m, although basalt thickness on the property may locally reach up to 1,400 m. Basalts are underlain by Beaufort Group sediments of the Karoo Supergroup.

Kimberlite emplacement during the Cretaceous Period was widespread throughout southern Africa and was probably associated with tectonic triggers during the break-up of Gondwana (Bailey, 1992).

The Mothae kimberlite consists of a main southern pipe-like lobe (South Lobe) connected to a smaller northern lobe (North Lobe) by an elongate central kimberlite body (Neck). The South Lobe has a surface expression of 5.05 ha and the three areas combined form a total surface area of 8.81 ha. Wall rock contacts for the North and South Lobes have been delineated by geophysical data, mapping and drill core intercepts. The contact between the kimberlite and the basalt is typically sharp and steep with localised zones of wall rock breccia.

The kimberlite itself comprises almost entirely of massive volcaniclastic kimberlite (“VK”) of different types. The different kimberlite types have been ‘fingerprinted’ in terms of their Kimberlite Indicator Mineral (“KIM”) content and petrographic characteristics as a control on bulk sampling; this being important as each has a different diamond grade and revenue.

The Mothae kimberlite intrusion is a kimberlite diatreme, or pipe, which was the feeder to a now eroded kimberlite volcano. [p.33]

Lucapa envisages mining the kimberlite in two phases. Phase 1 will include the weathered kimberlite material at the top of the pipe while mainly the underlying harder and unweathered kimberlite is planned to be mined during Phase 2. [p. 138]

Mining by conventional open pit methods such as drill and blast followed by load and haul will be employed. Drilling and blasting will be performed on 10 m benches, as will loading of the blasted material. Where possible in the near surface weathered zone, “free dig” mining will be carried out (i.e. without drilling and blasting). Ripping by bulldozer may also be employed in transitional kimberlite to reduce the quantity of drilling and blasting required. [p. 141]

The envisaged scale of mining at the Mothae deposit is relatively small with a peak total material movement of 8 Mtpa to 9 Mtpa. The annual processing plant feed requirement is approximately 1.1 Mtpa (2019) ramping up to approximately 2.2 Mtpa (2022) until end of life of mine.

The new 1.1 Mtpa Mothae diamond plant commissioned in January 2019 [2019 AR, p. 4]

Lucapa envisages building a new plant for the initial Phase 1 processing of weathered kimberlite. For Phase 2 mining of the mainly unweathered and harder kimberlite Lucapa plans to further modify the plant to ensure that optimal treatment processes are used for the two different types of kimberlite material.

Process Description
Receiving Bin
The plant will receive -450 mm ROM material via a 5 m 3 feed hopper, fitted with a static grizzly to scalp at 450 mm. The material is then extracted from the bin via a variable speed vibrating grizzly feeder (“VGF”), supplied with 100 mm aperture grizzly bars. A self-cleaning magnet positioned above the feeder, will remove tramp iron prior to the refurbished Sandvik Hybrid crusher. The VGF undersize and hybrid crusher product will discharge onto the primary crusher product transfer conveyor, which in turn will discharge onto the sc ........