Summary:
Karowe Diamond Mine(KDM) 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. KDM 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 where 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 KDM 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 area 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 current geological model was first presented in Doerksen et al. (2019) as an update to the Nowicki et al. (2018) model based on the 2018/2019 FS drilling program and no additional updates have been made. The 2019 update involved revisions to the pipe margin to reflect mining gains in all three lobes, and changes to the pipe shell and internal domain model of the South Lobe based on 2018/2019 core drilling. The most significant changes were extension of the base of the model by 190 m (from 256 to 66 masl), reduction in the volume of M/PK(S) below 500 masl, and modelling of an additional internal domain encompassing the areas where drilling to date indicates KIMB3 is most prevalent. The pipe shells of the North and Centre Lobes were also updated based on the 2018/2019 core drilling.
The upper ~70 to 100 m of calcretized and weathered kimberlite and country rock breccia units, which are now mined out. Some domains are rendered transparent to display the internal domains.