The Karowe Mine is based on the AK6 kimberlite pipe, which is part of the Orapa Kimberlite Field (“OKF”) in Botswana. The bedrock of the region is covered by a thin veneer of wind-blown Kalahari sand and exposure is very poor. Rocks close to surface are often extensively calcretised and silcretised due to prolonged exposure on a late Tertiary erosion surface (the African Surface) which approximates to the present day land surface.

The OKF includes at least 83 kimberlite bodies, varying in size from insignificant dykes to the 110 ha AK1 kimberlite which is Debswana’s Orapa Mine. All kimberlite intrusions are of post-Karoo age. Of the 83 known kimberlite bodies, five (AK1, BK9, DK1, DK2 and AK6 which is the Karowe Mine) have been or are currently being mined, and a further four (BK1, BK11, BK12 and BK15) are recognized as potentially economic deposits.

The country rock at the Karowe Mine is sub-outcropping flood basalt of the Stormberg Lava Group (approximately 130 m thick on the Karowe property) which is underlain by a condensed sequence of Upper Carboniferous to Triassic sedimentary rocks of the Karoo Supergroup (approximately 245 m thick on the Karowe property). The Karoo sequence overlies granitic basement.

AK6 is a roughly north-south elongate kimberlite body with a near surface expression of ~3.3 ha and a maximum area of approximately 7 ha at ~120 m below surface. The body comprises three geologically distinct, coalescing pipes that taper with depth into discrete roots. These pipes are referred to as the North Lobe, Centre Lobe, and South Lobe.

The AK6 kimberlite is an opaque-mineral-rich monticellite kimberlite, texturally classified primarily as fragmental volcaniclastic kimberlite with lesser macrocrystic hypabyssal facies kimberlite of the Group 1 variety. The nature of the kimberlite differs between each lobe, with distinctions apparent in the textural characteristics, relative proportion of internal country-rock dilution, and degree or extent of weathering. The South Lobe is considered to be distinctly different from the North and Centre Lobes which are similar to each other in terms of their geological characteristics. The North and Centre Lobes exhibit internal textural complexity (reflected in apparent variations in degree of fragmentation and proportions of country-rock xenoliths) whereas the bulk of the South Lobe is more massive and internally homogeneous.

The upper parts of all three lobes contain severely calcretised and silcretised rock. This zone is typically approximately 10 m in thickness, but can be up to 20 m in places. Beneath the calcrete and silcrete, the kimberlite is highly weathered. The intensity of weathering decreases with depth with fresh kimberlite generally intersected at about 70 m to 90 m below present day surface.

The geological model and list of geological units are presented in Figure 1-2 and Table 1-1 respectively. A unit within the South Lobe (a variety of M/PK(S)) has been found to be hard, and to produce a very large DMS concentrate primarily as a consequence of an abundance of fresh olivine in the kimberlite. Plant upgrades are underway at Karowe to be able to effectively process this material.

The North Lobe is predominantly infilled by a light greenish-grey, medium-grained (4 to 32 mm), matrix-supported, poorly sorted, massive fragmental volcaniclastic to superficially magmatic kimberlite (Hanekom et al., 2006). Basalt is the dominant country-rock xenolith type with lesser basement and Karoo sedimentary rock fragments. Two broad textural groups in the kimberlite of the North Lobe were identified: rocks with a matrix consisting of both serpentine and calcite, and samples with a matrix consisting predominantly of serpentine with minor calcite. No clear spatial distinction between the two groups could be resolved and the fragmental kimberlite was modeled as a single unit and domain.

The Centre Lobe is infilled by kimberlite that bears a superficial resemblance to the kimberlite from the North Lobe in that both lobes include non-fragmental, apparent magmatic material as well as fragmental volcaniclastic kimberlite (Hanekom et al., 2006). Macroscopically, colour and texture variations are common within Centre Lobe, but contacts between texturally distinct zones are generally gradational. Kimberlite textures locally alternate between superficially nonfragmental and more fragmental (volcaniclastic), similar to that of the North Lobe. The most consistent recognisable difference between the Centre Lobe and North Lobe kimberlite infill is a higher carbonate content in some samples from the Centre Lobe relative to North Lobe; a feature that could reflect varying hydrothermal alteration processes (e.g. Stripp et al., 2006). Two main units of fresh kimberlite are recognised in the Centre Lobe.

The fresh infill in the upper part of Centre Lobe comprises a medium-grained (4 to 32 mm), matrix-supported, poorly-sorted and massive, carbonate-rich fragmental kimberlite. Basalt represents the dominant country-rock xenolith type with lesser basement and Karoo sedimentary rock fragments present. Microscopically, the majority of samples show carbonate infilling of voidspace, highlighting the potential fragmental texture of the kimberlite. Point counting data reported by Hanekom et al. (2006) on a very limited sample suite suggest that the carbonate-rich fragmental kimberlite generally contains higher concentrations of olivine macrocrysts and lower country-rock xenolith concentrations than those of the fragmental kimberlite unit. The groundmass opaque-mineral content is also slightly higher, although overlap occurs.

The remaining fresh kimberlite within the Centre Lobe comprises matrix-supported, poorly sorted and massive fragmental kimberlite which is distinct from CFK(C) due to an apparent relative decrease in carbonate content. Hanekom et al., (2006) noted that samples showing clay alteration and thin magmatic selvages around olivine grains and country-rock xenoliths, i.e. a more volcaniclastic appearance, are generally but not exclusively associated with areas of increased country-rock xenolith content. This material is often greenish in colour and characterised by the presence of large blocks of basalt. Basalt breccia units in the Centre Lobe also occur within the fragmental kimberlite unit rather than in the carbonate-rich fragmental kimberlite unit. Basalt represents the dominant country-rock xenolith type with lesser basement and Karoo sedimentary rock fragments.

The South Lobe is dominantly infilled by medium–grained to coarse (4 to >32 mm), matrixsupported, poorly-sorted and massive, macrocrystic magmatic/pyroclastic kimberlite. The name of this unit reflects the initial uncertainty with respect to the textural classification of the kimberlite. The kimberlite exhibits textures consistent with a magmatic or hypabyssal kimberlite (HK), but also exhibits subtle textures suggesting a possible pyroclastic origin (PK). Macroscopically the kimberlite is grey in colour and contains approximately 5% to 10% thermally metasomatised/altered country-rock xenoliths. Olivine grains are relatively fresh and abundant opaque minerals are present. Fresh monticellite is present and increases in abundance with depth. Country-rock xenoliths are predominantly basalt with lesser basement and Karoo sedimentary rocks, but the overall proportion of crustal dilution is very low (typically <10%), rarely ranging up to a maximum of 25%. Minor zones of crude layering are locally apparent, defined by accumulations of olivine macrocrysts and sub-horizontal preferentially oriented crustal xenoliths. These zones range from 0.16 m to 1.5 m in thickness.

The Karowe Mine is an existing open pit mine located in Central Botswana that has been in production since 2012 and has extracted approximately 20 Mt of ore to date. Conventional open pit drill and blast mining with diesel excavators and trucks provide an average annual 2.6 Mt of kimberlite feed to the mill, plus additional ore to surface stockpiles.

The open pit mine operation is expected to terminate mid-2025 at an elevation of approximately 710 masl. The mine currently has approximately two years of stockpiled reserves, which will be increased through the life of the open pit and then consumed according to value through the end of the mine life.

All open pit mining operations are performed by mine contractors working year-round on two 12-hour shifts. The on-site mining contractor is currently performing load and haul operations with a Caterpillar 6015 Hydraulic Shovel and Caterpillar 777E/G Haul Truck pairing. The mining contract has a mixed fleet of additional production, support, and ancillary equipment available on-site.

The average total (ore and waste) open pit mine production rate in 2020 is approximately 21,300 t/d. Production rates will decrease until the end of the mine life, as no further pushbacks are planned, and the strip ratio will be reduced at depth.

There are substantial resources remaining below the economic extents of the open pit that may be extracted by underground mining methods.

Crushing
Previous mill simulations and associated mass balances indicated that to achieve a head feed rate of 350-500 t/h processing hard ore, a secondary crushing stage is required ahead of the mill. The secondary crushing section stabilizes and reduces the mill load as well as the pebble crusher load. It also assists with top size feed control to the downstream milling section.

ROM material is delivered to the ROM tip by means of articulate and non-articulated trucks and first stage crushing in the form of a primary jaw crusher reduces ore to an acceptable feed envelope size ahead of the secondary crusher section.

Depending on the material treated, a proportion or the entire primary crushed ROM stream is diverted and processed through the secondary crusher circuit. Feed to the secondary crusher is scalped of undersize on the MDR screen while the oversize removed on the same screen is partially sent to the crusher depending on a diverter setting. In addition, a portion (or all or none) of the MDR tails can be sent to the secondary crusher. The secondary crusher product is reintroduced onto the mill stockpile feed conveyor with the screen undersize and bypass stream.

The +80 mm mill screen product and the 32 x 80 mm LDR XRT tailings are processed through the existing pebble crusher. The pebble crusher product is sized at 32 mm with all the +32 mm material reporting to the mill feed conveyor. A portion of the -32 mm material bypasses the mill with the split balance of the -32 mm bleed screen undersize reporting directly to the mill feed conveyor. The bleed is required and balanced operationally to reduce mill loading.

The 20 x 32 mm tailings from the XRT bulk sorters are processed through a wet flush tertiary crusher circuit to liberate diamonds in this particular size fraction. The tertiary crusher product is reintroduced back into the circuit via a bulk sorter sizing screen and reports to the relevant downstream process based on the crushed product size envelope.

Comminution – Milling, Bleed Screening & Pebble
Crushing Fresh mill feed is introduced into the mill from the feed stockpile along with a variable portion of the pebble crusher product directly. A bleed screen has been installed on the pebble crusher product stream, so that a portion of the – 32 mm pebble crusher product can be bled out of the mill feed and report directly to downstream processes, thereby alleviating and balancing mill loading. The current AG Mill discharge grate incorporates Turbo Pulp Lifter technology to improve discharge and grate efficiency as well as withdrawal of material out of the mill.

The Karowe processing plant was designed by DRA Mineral Projects for operations beginning in 2012. It consisted of a diamond milling, Dense Media Separation (DMS) and recovery plant, and associated crushing, screening and thickening systems. It was designed to process 2.5 Mt of run-of-mine (ROM) material per year with a single 200 t/h DMS module. The concentrate material from the DMS was subsequently treated through a 2.5 t/h wet x-ray recovery system for material reduction and diamond winning. This circuit was designed with adequate space to accommodate future expansions.

The Karowe plant was upgraded in 2015 with the inclusion of XRT machines installed ahead of the DMS in order to recover large diamonds. This upgrade included the construction and commissioning of a new secondary (gyratory) crusher, XRT sizing and XRT diamond recovery circuits.

In 2017, the Mega Diamond Recovery Project was completed – which included adding XRT sorting technology ahead of the A ........