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Karowe, AK6 |
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Botswana |
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Super Porphyry Cu and Au
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The Karowe diamond mine, exploiting the composite AK6 kimberlite pipe, is located ~25 km south and 23 km west of the Orapa and Letlhakane diamond mines respectively in north central Botswana, ~350 km north of Gabarone and 200 km west of Francistown (#Location: 21° 29' 59" S, 25° 28' 16" E).
The AK6 kimberlite was discovered by De Beers in 1969, but based on early work, was originally considered to be small and low grade. Reassessment starting in 2003 suggested that the kimberlite was larger and had a higher grade than previously estimated. In December 2011, the AK6 Project was renamed the Karowe Mine and construction of the mine was substantively completed by the end of March 2012 with the first production diamonds recovered in April of the same year by Lucara Diamond Corp.
The AK6 kimberlite pipe is part of the Orapa Kimberlite Field. This field contains at least 83 kimberlite bodies, all of which are of post-Karoo age, ranging from insignificant dykes to the 110 ha AK1 kimberlite exploited by Debswana's Orapa Mine. Of the 83 known kimberlite bodies, five (AK1/Orapa, BK9/Damtshaa, AK6/Karowe, DK1-DK2/Letlhakane) have been or were currently being mined in 2015, and a further four (BK1, BK11, BK12 and BK15) recognised as potentially economic.
Geological Setting
The Orapa Kimberlite Field is situated on the northern margin of the Central Kalahari Karoo Basin where the Karoo succession has a very gentle SSW dip and laps onto the Precambrian basement that occurs at shallow depth within the Makgadikgadi Depression. The Karoo succession is condensed, with a total thickness of ~600 m, best preserved in WNW-ESE oriented grabens. The large AK1 kimberlite was preserved within such a graben (Coates et al., 1979).
The bedrock of the region is covered by a thin veneer of wind-blown Kalahari sand and exposure is very poor. Near surface rocks are often overprinted by extensive calcrete and silcrete due to prolonged exposure on the 'African Surface', a late Tertiary erosional peneplain which approximates to the present day land surface.
The main country rock sub-outcropping at Karowe is the locally ~130 m thick flood basalt of the Stormberg Lava Group, underlain by a condensed sequence of ~245 m of the Upper Carboniferous to Triassic Karoo Supergroup sedimentary rocks, which in turn, overlie granitic basement of the Palaeoproterozoic Magondi mobile belt, just east of the Archaean Zimbabwe craton. The basalts, which are very extensive and underlie much of central Botswana, are dated at 180 Ma in the Jurassic, and lie unconformably on the sedimentary succession, but are stratigraphically part of the Karoo Supergroup.
The regional stratigraphy is as follows, from the basement upwards:
Basement - granite gneiss and amphibolite, encountered at a depth >480 m at Karowe.
unconformity
Karoo Supergroup
Ecca Group
Mea Arkose Formation - coarse, white micaceous sandstone and dark shales. Absent at Karowe.
Tlapana Formation - black carbonaceous shale and coal, which is ~120 m thick at Karowe
Tlhabala Formation - reddish grey non-carbonaceous siltstone, mudstone and shale. Weathers red, green or khaki and is ~105 m
thick at Karowe
unconformity
Lebung Group which is ~120 m thick at Karowe
Mosolotsane Formation
Upper member - red mudstones
Lower member - red and green sandstones
Ntane Sandstone Formation - Aeolian sandstone
unconformity
Stormberg Lava Group (Drakensberg Group) - extensive flood basalts, which is ~127 m thick at Karowe
Kimberlite intrusions - which intrude the Stormberg Lavas
unconformity
Kalahari Group - Wind blown sand overlying duricrusts, which is ~8 m thick at Karowe
Kimberlite Geology
The composite AK6 pipe is a roughly north-south elongate kimberlite body with a near surface expression of ~3.3 ha, and a maximum area of ~7 ha at ~120 m below surface. It comprises three geologically distinct, north-south aligned pipes that coalesce near the surface, but individually taper with depth into discrete roots. These 'pipes' are referred to as the North, Centre and South Lobes.
The AK6 kimberlite is an opaque mineral rich monticellite kimberlite, which has texturally been classified as principally being a fragmental volcaniclastic kimberlite with lesser macrocrystic hypabyssal facies kimberlite of the 'Group 1' variety (Field, 1989). Each lobe has a different nature, distinguished by textural characteristics, relative proportion of internal country-rock dilution, and degree or extent of weathering. The South Lobe is distinctly different to the North and Centre Lobes which have similar geological characteristics. The latter two lobes have significant textural complexity (reflected in variations in the degree of fragmentation and amounts of country-rock xenoliths) whilst much of the South Lobe is more massive and internally homogeneous.
The upper sections of all three lobes have been subjected to severe calcrete and silcrete development over a zone that is typically ~10 m, but can be locally up to 20 m in thickness. Below the calcrete and silcrete, the kimberlite is highly weathered, the intensity of which decreases with depth, with fresh kimberlite generally encountered at ~70 m to 90 m below the present surface.
Kimberlite rock types identified in the AK6 kimberlite (encompassing true kimberlite as well as internal breccias dominated by basalt xenoliths) have grouped into mappable units based on their geological characteristics and interpreted grade potential by De Beers, based on drill core logging, petrographic studies, analysis and interpretation of groundmass spinel composition, and whole-rock geochemical analysis (Stiefenhofer and Hanekom, 2005; Hanekom et al., 2006; Tait and Maccelari, 2008). The main geological features of each unit are summarised below:
• Calcretised kimberlite - The upper parts of all three lobes have been subjected to severe calcrete and silcrete development, typically to ~10 m, but locally extending up to 20 m in depth.
• Weathered kimberlite - the upper 30 to 50 m of all three lobes is highly weathered, the intensity of which decreases with depth with fresh kimberlite generally encountered at ~70 to 90 m below the surface. Whilst the primary mineralogical and textural features of the kimberlite are obliterated in the upper levels, there is a transition into the underlying fresh kimberlite units in each lobe.
• Basalt breccia - each lobe contains discontinuous zones of brecciated basalt, mixed with variable, but generally minor amounts, typically <10%, of kimberlite. The basalt breccias comprise large, metre-sized, to smaller basalt clasts surrounded by a matrix of kimberlite. The basalt clasts are variably fractured and carbonate-veined and comprise vesicular and non-vesicular varieties. The majority of the breccias are found close to the wall-rock contacts in each lobe. A second breccia that is broadly similar to that described above, has lower levels of country-rock dilution (50 to 90%), and appears to be interbedded and/or spatially associated with the main breccia domains. Tait and Maccelari (2008) interpreted the latter deposits as either talus-type slump deposits or as deposits of possible pyroclastic origin (based on their higher kimberlite content).
• North Lobe kimberlite - the North Lobe is predominantly composed of a fragmental kimberlite, 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). The dominant country-rock xenolith type is basalt, with lesser basement and Karoo sedimentary rock fragments. Two broad textural groups have been distinguished: i). rocks with a matrix of both serpentine and calcite, and ii). those with a matrix consisting predominantly of serpentine with minor calcite. No clear spatial distinction has been resolved between the two groups and the fragmental kimberlite is mapped as a single unit.
• Centre Lobe kimberlite - this lobe is infilled by kimberlite that has a superficial resemblance to the kimberlite of the North Lobe in that both include non-fragmental, apparent magmatic material as well as fragmental volcaniclastic kimberlite (Hanekom et al., 2006). Colour and texture variations are macroscopically common in the Centre Lobe, although contacts between texturally distinct zones are generally gradational. Kimberlite textures locally alternate between superficially non-fragmental and more fragmental (volcaniclastic), similar to that of the North Lobe. A higher carbonate content of kimberlite infill in the Centre Lobe relative to North Lobe is the most consistent recognisable difference between the two lobes. This is taken to reflect varying hydrothermal alteration processes (e.g. Stripp et al., 2006). Two main units of fresh kimberlite are recognised in the Centre Lobe, namely:
Carbonate-rich fragmental kimberlite - the upper part of Centre Lobe is infilled by a medium-grained (4 to 32 mm), matrix-supported, poorly-sorted and massive, carbonate-rich fragmental kimberlite. Basalt is the dominant country-rock xenolith type, with lesser basement and Karoo sedimentary rock fragments. Much of this unit has carbonate infilling of void space, highlighting the fragmental texture of the kimberlite. This carbonate-rich fragmental kimberlite appears to generally contain 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.
Fragmental kimberlite - the remainder of the unweathered kimberlite in this lobe is composed of matrix-supported, poorly sorted and massive fragmental kimberlite which is distinguished from the carbonate-rich fragmental kimberlite by an apparent relative decrease in carbonate content. Hanekom et al., (2006) noted that this unit contains clay alteration and thin magmatic selvages around olivine grains and country-rock xenoliths, i.e. it has a more volcaniclastic appearance, and is generally, but not exclusively, associated with areas of increased country-rock xenolith content. It is material is often greenish in colour and characterised by the presence of large blocks of basalt. Basalt breccia units in the Centre Lobe are found occur within the fragmental kimberlite unit rather than the carbonate-rich fragmental kimberlite unit. Basalt is the dominant country-rock clast type, with lesser basement and Karoo sedimentary rock fragments.
• South Lobe kimberlite - the South Lobe kimberlite has a character that is distinct in comparison to that of the North and Centre Lobes (Hanekom et al., 2006; Stiefenhofer, 2007; Stiefenhofer and Hanekom, 2005). The upper-western part of the South Lobe is dominantly a weathered basalt breccia, with an underlying unweathered basalt breccia unit, but also includes a large block ('floating reef') of solid basalt.
Magmatic/pyroclastic kimberlite - which is the dominant infill of the South Lobe, comprising medium-grained to coarse (4 to >32 mm), matrix-supported, poorly-sorted and massive, macrocrystic magmatic/pyroclastic kimberlite. It exhibits textures consistent with a magmatic or hypabyssal kimberlite, but also has subtle textures suggesting a possible pyroclastic origin. Macroscopically it has a grey colour and contains approximately 5 to 10% thermally metasomatised/altered country-rock xenoliths. Opaque minerals are present. Olivine grains are relatively fresh and abundant, whilst fresh monticellite increases in abundance with depth. Xenoliths of country-rock are predominantly basalt with lesser basement and Karoo sedimentary rocks, although the overall proportion of crustal dilution is very low, typically <10%, and rarely as high as 25%. Minor crude, 0.16 to 1.5 m thick layering zones are locally apparent, defined by accumulations of olivine macrocrysts and sub-horizontal, preferentially oriented, crustal xenoliths.
Eastern diluted magmatic/pyroclastic kimberlite - occurring as a pipe-shaped internal kimberlite unit along the eastern section of the lobe, composed of coarse to medium-grained (4 to >32 mm), matrix-supported, poorly sorted and largely massive magmatic kimberlite. This unit is primarily distinguished from the magmatic/pyroclastic kimberlite by an apparent increase in small (typically <1 cm) country-rock fragments. Hanekom et al. (2006) recorded that this unit contains fewer olivine macrocrysts in comparison with the remainder of the South Lobe and contains abundant coarse microlitic diopside. Perovskite is apparently slightly more abundant in the diluted zones and the groundmass has a greenish colour, possibly due to serpentinisation. Country-rock clasts are primarily basalt, and less commonly basement and Karoo sedimentary rock, although basement fragments may locally be more abundant than in the magmatic/pyroclastic kimberlite. Crustal dilution ranges from 3 to 10%, and greenish serpentinised zones are common.
Eastern diluted magmatic/pyroclastic kimberlite - which occurs as a pipe-shaped internal kimberlite unit, with characteristics that distinguish it from those of the other two units of the lobe. It comprises a greenish-grey, medium-grained (4 to >32 mm), matrix-supported, poorly sorted, massive magmatic kimberlite, and is macroscopically distinct in colour due to its apparent altered nature. It is also differentiated by its whole rock geochemistry and rock density. Olivine is serpentinised and locally completely weathered/absent in drill core. Basalt is the dominant country-rock clast lithology, with less common basement and rare black shale xenoliths. Crustal dilution ranges from 7 to 36%.
Mineralisation
Karowe produces diamonds of relatively high average value, in part due to the presence of Type IIa diamonds, which are distinguished by their infra-red (IR) spectra, with Type IIa stones characterised by their very low (<20 ppm) nitrogen contents. The Type IIa stones often have top quality white colours (D-G), a consequence of their low nitrogen contents. They include the largest gem diamond ever found, the 3106 ct Cullinan, recovered from the Premier Mine, South Africa, as well as gems like the legendary 793 carat Koh-i-noor diamond from India.
The mine has yielded the 1109 carat Lesedi la Rona diamond (~66.4 x 55 x 42 mm), the second-largest gem-quality rough diamond ever discovered, which was recovered in November 2015. Previously, during 2014 and 2015, other large white diamonds of 813, 374, 342, 336 and 204 carats have been reported (Lucara Diamond Corp. releases 2014, 2015).
Reserves and Resources
Published mineral resource and ore reserve estimates are as follows (Lynn et al., 2013):
Mineral resources at October, 2013
Indicated resource
North Lobe - 0.74 Mm3 = 1.83 Mt @ 16 cpht for 0.30 Mct
North Centre Lobe - 2.53 Mm3 = 6.49 Mt @ 20 cpht for 1.27 Mct
North South Lobe - 13.50 Mm3 = 37.89 Mt @ 16 cpht for 5.89 Mct
North Working stockpile - 0.33 Mm3 = 0.62 Mt @ 13 cpht for 0.08 Mct
North Life of Mine stockpile - 0.66 Mm3 = 1.24 Mt @ 6 cpht for 0.07 Mct
TOTAL indicated resource - 17.76 Mm3 = 48.07 Mt @ 16 cpht for 7.61 Mct
Inferred resource
Centre Lobe - 0.08 Mm3 = 0.21 Mt @ 15 cpht for 0.03 Mct
South Lobe - 7.01 Mm3 = 20.79 Mt @ 14 cpht for 3.01 Mct
TOTAL Inferred resource - 7.09 Mm3 = 21.00 Mt @ 14 cpht for 3.04 Mct.
Ore reserves (included in mineral resources) at December, 2013
Probable reserve
North Lobe - 0.991965 Mt @ 18.4 cpht for 0.182217 Mct
Centre Lobe - 5.998544 Mt @ 18.4 cpht for 1.105729 Mct
South Lobe - 25.261762 Mt @ 15.1 cpht for 3.803981 Mct
Life of Mine stockpile - 0.873059 Mt @ 5.7 cpht for 0.049912 Mct
TOTAL Probable reserve - 33.125330 Mt @ 15.5 cpht for 5.141839 Mct.
Note: Mm3 = million cubic metres; Mt = million tonnes; cpht = carats per hundred tonnes; Mct = million carats.
This summary is drawn, and paraphrased from: "Lynn, M., Nowicki, T., Valenta, M., Robinson, B., Gallagher, M., Bolton, R. and Sexton, J., 2013 - Karowe Diamond Mine, Botswana; an NI 43-101 Independent Technical Report prepared for Lucara Diamond Corp., 169p."
The most recent source geological information used to prepare this decription was dated: 2013.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Karowe
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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