The Karowe Mine is exploiting the AK6 kimberlite which is part of the Orapa Kimberlite Field (OKF) in the Central District of Botswana. The OKF includes at least 83 kimberlite bodies of post-Karoo age. Three of these (AK1, BK9, and AK6) have been, or are currently being mined and four (BK1, BK11, BK12 and BK15) are recognized as potentially economic deposits. The Karowe Mine is one of the world’s most significant producers of large and high-value diamonds including Type IIa and coloured diamonds.

The OKF lies on the northern edge of the Central Kalahari Karoo Basin along which the Karoo succession dips very gently to the south-southwest and off-laps against Precambrian rocks that occur at shallow depth within the Makgadikgadi Depression. The country rock at Karowe is sub-outcropping flood basalt of the Stormberg Lava Group (~130 m thick), underlain by a condensed sequence of Upper Carboniferous to Triassic sedimentary rocks of the Karoo Supergroup (~345 m thick), below which is the granitic basement

AK6 is a roughly north-south trending elongate kimberlite body with a surface expression of ~3.3 ha and maximum area of ~8 ha at approximately 120 m below surface. It comprises three geologically distinct, coalescing pipes known as the North, Centre and South Lobes that taper with depth into discrete roots. The kimberlite in each lobe is different, in terms of its textural characteristics, relative proportion of internal country rock dilution, degree of weathering and alteration, as well as the characteristics of mantle-derived components including the diamond populations. The South Lobe is the largest of the three lobes and is distinctly different from the North and Centre Lobes which are similar in terms of their geological characteristics. The South Lobe is broadly massive and more homogeneous than the North and Centre Lobes which exhibit greater textural complexity and more variable and higher proportions of internal country rock dilution.

The kimberlite in each lobe has been grouped into mappable units based on its geological characteristics and interpreted grade potential. Units occurring in more than one lobe (e.g. BBX, CKIMB, WK) were modelled as separate domains for each lobe (denoted by N, C or S suffix) in the geological model. The calcretized and weathered horizons in the upper portions of the lobes have now been mined out. Zones of high country rock dilution (termed breccias) are present in all three lobes, and in the South Lobe these appear to be largely restricted to the upper now-depleted portion. The South Lobe additionally comprises two volumetrically dominant units, Magmatic / Pyroclastic Kimberlite (M/PK(S)) and Eastern Magmatic / Pyroclastic Kimberlite (EM/PK(S)), and six volumetrically minor units, one of which (KIMB3) becomes more prevalent with increasing depth in the pipe, particularly below 400 masl. M/PK(S) forms the dominant pipe infill above 600 masl, below which EM/PK(S) increases in volume at the expense of M/PK(S) to become the dominant infill below 500 masl. EM/PK(S) has now been drilled to 66 masl (~935 metres below surface (mbs)). The names applied to the two dominant units reflect the uncertainty historically regarding their textural classification (magmatic (M) or pyroclastic (P) kimberlite). The M/PK(S) and EM/PK(S) are broadly massive, olivine-rich and country rock xenolith-poor phlogopite monticellite kimberlites; they exhibit features suggesting they were formed extrusively and can be described as having clastogenic or apparent coherent texture (Scott Smith et al., 2017). The North and Centre Lobes are each infilled by single volumetrically dominant kimberlite units.

The Karowe Mine is an existing open pit operation, which has been in production since 2012. 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. All open pit mining activities are performed by Botswanan mine contractors working 365 days per year on three, eight-hour shifts in the pit and two, 12-hour shifts in the processing facility. The open pit mine operation is expected to terminate mid-2025, ending at an elevation of approximately 700 masl.

There are substantial resources remaining below the economic extents of the open pit that may be extracted by underground mine methods. A 7,200 t/d shaft operation utilizing long hole shrinkage mining (a form of fully-assisted caving) is proposed to provide an additional 13 years of mine life to the Karowe operation after a five-year construction period commencing in 2020.

The Karowe resource contains three distinct coalescing pipes, referred to as the North, Centre, and South Lobe. All lobes are outcropping, dip vertically, and vary in diameter and depth. The South Lobe is the largest of the three, and its Indicated Resources extend approximately 760 mbs (from 1,010 masl to 250 masl). The North and Centre Lobes extend below the open pit limit but have been excluded from the planned underground mine as they are inferred at depth and are of low value.

The long hole shrinkage method (LHS) is planned to systematically drill and blast the entire lobe on a vertical retreat basis. The method can be thought of conceptually as a fully assisted cave. In LHS, the blasted muck is left in the excavation during stoping to stabilize the host rock with only the swell extracted / pulled during the drill and blast phase. Mucking takes place from draw points at the bottom of the mine on the 310 Level (L) (310 masl). As ore is blasted, it swells beyond its in-situ volume, and this volume is mucked / pulled from the draw points to maintain a blasting void within the excavation. Once the ore is fully blasted to the bottom of the open pit, the South Lobe is drawn empty by mucking the draw points.

Access to the underground mine will be from a 767 m deep production shaft, 7.5 m in diameter, sunk from surface to 245 masl. The shaft will be equipped with two 21-t skips for production hoisting and a service cage for man and material movement through the mine. This shaft will also serve as the main fresh air intake to the mine. A second shaft, 6.0 m in diameter, 717 m deep, driven from surface to 295 masl, will be equipped with a heavy lift hoist for moving large equipment throughout the mine and hoisting development waste during pre-production. This shaft will serve as the main exhaust route and secondary egress for the mine.

There will be a total of eight working levels in the mine, six of which will be accessed by a shaft station. Levels are named by their elevation in masl. The 310 L will serve as the primary working level and provide access to the main underground infrastructure including production draw points, crusher, and maintenance facilities. Above this level will be four drilling horizons: 380 L, 480 L, 580 L, and 680 L; where production equipment will work to drill and blast stopes. The 380 L will be accessed by ramp from the 310 L. The 480 L and 680 L will be accessed by a dedicated shaft station. The 580 L will be accessed by ramping down from the 680 L through the kimberlite to avoid development in the less competent carbonaceous shale hosted between 520 masl and 650 masl. Near the main 310 L will be the conveyor station at 335 masl, shaft load out station at 285 masl, and the production shaft bottom at 245 masl.

The underground lateral development will be driven by three development jumbos, initially mobilized to the 310 L. After the majority of the development is complete on the 310 L, one jumbo will be sent up to the 480 L and another up to the 680 L. The last jumbo will remain on the 310 L for any rehabilitation work that will need to be completed throughout the mine life. During preproduction, a total of 15 km of development will be driven.

A pyramidal sequence is proposed for the drilling and blasting of the stopes at Karowe. This blasting sequence will create a dome shape at the top of the blasted volume to maintain stability of the stope back. Stopes will be blasted sequentially upwards in 17.5 m increments until a 30 m sill pillar is left between the drill panel and the stope back. A final 30 m blast will wreck this sill pillar and terminate access to the drill panel at that location. The drill will relocate to the next above drill horizon and repeat the process until the lobe is fully blasted.

Through areas of weaker host rock above the granite, a 15 m skin of kimberlite will be left temporarily around the walls of the lobe to prevent dilution and unraveling. This skin will be recovered later through drilling and blasting during final draw down of the muck pile.

Five ITH drills will be utilized to drill and blast approximately 21,000 t/d in order to supply 7,200 t/d of swell to the draw bells for the first six years of operations. Peak broken inventory occurs in year five for a total of 18.9 Mt. After six years, the South Lobe will be fully blasted, and mucking will continue at a constant rate of 7,200 t/d until the underground reserves are depleted at the end of year thirteen. It is important to note that the combination of the kimberlite skin and mining the first half of the stope (200 vertical metres) in granite host rock keeps dilution to a minimum during the first years of underground mining.

The extraction level will be made up of five panels that are driven 31.5 m apart and run the entire length of the lobe. Each panel will access one of 54 draw points driven 18 m x 12 m in a herringbone pattern. The extraction level will contain one perimeter drive to allow traffic to go around panels in the event of a blockage or maintenance at the draw points. At the northwest side of the extraction level, the five panels will access a 1,000 mm static grizzly from three sides. Re-muck bays will be located near the grizzly to allow for continued mucking during crusher maintenance periods and a quick re-handle once the crusher returns to normal operation. There will be approximately 34,000 t of muck storage capacity on the extraction level, equal to 4.7 days of production, and another 66,000 tonnes of available storage elsewhere in the mine. Three 21-t loaders will be required to maintain production at the draw bells. In addition, two 17-t development loaders will be made available to assist with mucking during periods of re-handle or increased haul distances due to panel rehabilitation.

Material dumped onto the grizzly will feed a 1.3 m x 1.5 m (50” x 60”) underground jaw crusher with 960 t/h capacity located 26 m below the extraction level. The jaw crusher discharge conveyor will feed material onto the skip feed conveyor for transport to the 335 L shaft station. The skip feed conveyor will discharge onto a reversible transfer conveyor which will deposit into one of two crushed ore storage bins, each with a capacity of 3,500 t.

The storage bins will discharge onto a skip loadout conveyor which will direct material to one of two 21-t skips. Skips will cycle to surface every two minutes and dump into an elevated bin for direct truck loading. 55-t trucks will load at the shaft and tram ore to the plant or waste to the waste rock storage facility, some two km away.

The underground mine will be contract developed and owner operated. Contractors will be utilized for shaft sinking, pre-production lateral development, and raisebore development.

Existing surface stockpiles will be consumed at about 100 kt/y during underground operations and then will be fully exhausted when all mining stops and stockpile processing capacity comes available.

Underground Crushing and Conveyance
LHDs will tram ore from the drawpoints directly to a single stage crushing plant. The crusher will process 450 t/h or 7,200 t/d of material, operate 16 hours per day based on a utilization of 65% and produce a final product P80 of 150 mm.

Material will be dumped onto a 1,000 mm static grizzly above the crusher dump pocket. The material will discharge through the static grizzly into the 200-t crusher feed hopper. Oversized material from the static grizzly will be size reduced using a rock breaker mounted beside the static grizzly.

An apron feeder will draw material from the dump pocket to feed the vibrating grizzly feeder at a rate of 450 t/h. The vibrating grizzly oversized material will feed directly into a 1,270 mm x 1,524 mm (50” x 60”) jaw crusher with an installed power of 250 kW. The undersized -120 mm material will bypass the crusher and feed directly onto the crusher discharge conveyor. The primary crushing stage will produce a product P80 of approximately 150 mm and an F100 of 228 mm at a crusher closed side setting (CSS) of 152 mm.

The crusher discharge conveyor will pass through a magnet to retrieve rock bolts and other metalliferous material that may cause damage to the main conveyor and hoisting system. Scrap metal will be pulled aside and disposed of.

The crusher discharge conveyor will feed material onto the skip feed conveyor for transport to 335 L. The skip feed conveyor discharges onto the skip reversible transfer conveyor which feeds one of two crushed ore storage bins, each with a capacity of 3,500 t.

The crushing area is equipped with a 35-t crane for maintenance, compressed air, dust collection and a self-cleaning belt magnet.

Karowe processing plant
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 Underground Project at Karowe will include the use of existing and new infrastructure at the Karowe Mine. Project infrastructure is designed to support the operation of a 2.6 Mt/a mine and 2.7 Mt/a processing plant.

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 an ........