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 is screened at 90mm with oversize proceeding to a pebble crusher and the underflow proceeding to a bulk sorter sizing screen where it is further screened at 8mm. Underflow (-8mm ore) proceeds to a dense media separation circuit (“DMS”). Overflow (+8mm) ore proceeds to an XRT sizing screen where it is split into size fractions appropriate to the X-ray Transmission machines(“XRT”). Pebble crusher product may be routed either back to the fine ore stockpile or alternatively to a 28mm bleed screen. When pebble crusher product is directed to the bleed screen all or a portion of the screen underflow is directed to the bulk sorter sizing screen feed belt. The oversize and the residual portion of the undersize rejoin the mill feed.

The XRT circuits consist of five machines dealing with three size fractions. The largest size fraction +35-90mm is processed through the Large Diamond Recovery (“LDR”) XRT machine. The +14-35mm faction is processed through two coarse XRT machines, and the finer +8-14mm fraction is processed through two middles XRT machines. All XRT concentrate is directed to a high security ‘Red Area’ directly below the XRT machines, and into separate, individual sorting glove boxes, where hand sorting takes place. This area is joined to the main recovery secure area by a secure corridor and all sorted diamonds and glove box tailings are transported to the main recovery area using a hands-off vacuum transport system for a validation and accounting prior to being shipped to Gaborone. Glovebox tailings are periodically audited to ensure efficient sorting.

LDR XRT tailings rejoin the mill discharge screen overflow and proceed to the pebble crusher. The tailings from the coarse XRT size fraction are screened at between 20mm and 28mm with the screen underflow going to coarse residue disposal via an XRT audit circuit. The overflow is fed to a tertiary crusher which operates at a 14mm closed size setting. The tertiary crusher product returns to the bulk sorter sizing screen for further processing. Middles XRT tailings proceed to the coarse residue disposal via the XRT audit circuit. The XRT audit circuit consists of an XRT machine designed to treat ore in the +8-28mm size range at a rate of up to 50 tonnes per hour. A portion of the tailings from the middles and coarse XRT recovery machines are continuously diverted to the XRT audit circuit as an online quality control check on the upstream XRT recovery circuit.

Oversize discharged from the mill (+90mm) is combined with rejects from the LDR and sent to the pebble crusher which operates at a 35mm closed side setting. Product from the pebble crusher is diverted either to the crusher ore stockpile or to the product sizing screen where specific sizes and volumes are diverted back to the AG Mill to optimize performance.

The bulk sorter sizing screen underflow (-8mm) ore fraction proceeds to the DMS circuit, with DMS floats proceeding to coarse residue disposal, and DMS sinks, the heavy minerals, which include diamonds are fed to a highly secure Main Recovery section where the diamond rich concentrate from the DMS is further concentrated utilizing two stages of magnetic separation and three stages of x-ray recovery technology followed by secure hands off sorting to recover the diamonds. The final product is then shipped to Gaborone where all diamonds are sorted into specific size and quality categories and sold via a sealed bid tender process. The discharge from the mill, which is less than 1.25mm is fed to a de-grit plant and thickener. Grits are disposed of on a Course Tailings Stockpile, along with the low density material from the DMS plant. The slimes are thickened and disposed of in a fine tailings impoundment. Decant water from this facility is returned for re-use in the process plant along with water recovered from the thickener.

The existing process plant has undergone an upgrade in 2015 (the Phase 2 capital project) which was intended to:
* Protect and enhance recovery of large diamonds.
* Enhance comminution performance to maintain design throughput with harder kimberlites.
* Minimise recovery yields when treating these harder kimberlites.

Whilst these process enhancements should cater for the expected increase in kimberlite hardness, with the change from open pit to underground mining and increasing depth. Hardness may increase further putting additional stress on the comminution circuit and recovery yields. Additional information on kimberlite metallurgical characteristics that will affect plant throughput and recoveries needs to be collected and assessed during the ongoing geotechnical drilling campaign. Any additional comminution requirements would lead to an increase in energy requirements.

Phase 3 process upgrades have been completed and have been in operation since Q3 2017. These upgrades include an XRT circuit treating +50-125mm material, prior to milling, and enables the recovery of larger diamonds as early as possible in the process in addition to reducing the risk of diamond damage. A new XRT circuit has also been introduced to treat the 4-8mm fraction, previously sent to DMS, thus reducing the load on the DMS to cater for higher yield material expected in future.