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 mine is a conventional drill and blast; truck and shovel operation. Ore is typically mined from three benches simultaneously, and a second pushback known as Cut 2 is underway and waste is currently being mined from Cut 2 to access the deeper ore. Ore is mined and either fed directly to the primary crusher or stockpiled for future treatment. Waste is mined and hauled ex-pit and dumped on a waste dump to the west of the pit, in a manner that permits concurrent rehabilitation.

Mining to date has focussed mainly on the North Lobe which is the smallest portion of the pipe and which has become very confined. In 2013 mining has shifted to the Central and Southern Lobes where bench access and operating width are much easier.

Currently the mine is utilising Toe to Crest angles. While Toe to Crest angles represent the “real” angle over a specific height, berm widths will vary as the stack height increases, while a Crest to Crest angle has the berm widths at the same size. For a 96 m stack height, the stack angle will vary from 62° for a Crest to Crest slope, to 65° for a Toe to Crest slope. The consequence of this evaluation is that if a Toe to Crest angle is used in the design, berm widths decrease as the slope height is decreased, resulting in a berm width that is non-functional. To this end it was recommended that a Crest to Crest angle be used in updates to the pit design and that the design berm width be incorporated in the mining plan.

In the QP’s opinion, the mine is doing well in waste tonnes mined but the ore tonnes are 10% under budget. This is not unusual for a new open pit. The ratio of ore tonnes down while waste tonnes up, for a small pit of this nature is healthy from a practical point of view as it demonstrates that the pit has been effectively stripped and should have adequate working space.

The life of mine stripping ratio is 2.46:1 waste/ore to the planned closure in 2027.

The upper 70 m of the AK6 kimberlite is significantly weathered, and most of this material has already been processed through the existing plant. It is anticipated that the fresher ore, particularly some of the ore from the South Lobe, is significantly harder than the weathered ore that has been already processed.

Current process circuit methodology is as follows. The Run of mine (ROM) ore is either fed directly to the process plant front-end, or stockpiled for later treatment. The process facility, with a nameplate capacity of 2.5mtpa, comprises a comminution section of a primary crusher, which feeds onto scalping screen which permits all or a portion of plus 75mm ore to proceed to a secondary gyratory crusher. The scalping screen underflow and gyratory crusher product proceed onto a crushed ore stockpile as a storage buffer ahead of the AG (Autogenous grinding) mill. Mill feed is via a single conveyor running from the coarse ore stockpile to the AG mill. AG mill discharge ........