Tsodilo's Gcwihaba diamond project is located in the north-west of Botswana, where it's wholly owned Botswana subsidiary company, Gcwihaba Resources (Pty) Ltd, has seven diamond prospecting licenses, covering a total area of approximately 6,796 km2. (Figure 1) The ground covers an area with relatively thin Kalahari cover (see, Figures 2 and 3).
UNEXPLAINED G10 / MACRODIAMOND KIM ANOMALIES IN NAMIBIA
Two important KIM anomalies in northeast Namibia, located to the south of Tsumkwe and in the Omatako drainage respectively, were originally identified during the course of prospecting work carried out by De Beers in the 1970's. Their positions are shown in Figure 2, and in detail in Figure 3. These anomalies have never been satisfactorily explained, and the source kimberlites represent important exploration targets.
Tsumkwe garnet anomaly
A northwest - southeast swath of kimberlitic garnets, concentrated in an area roughly 40 kilometres by 15 kilometres, was identified close to the village of Tsumkwe (Figure 3). A significant proportion of the garnets are reported to have G10 compositions, and a number of macro-diamonds have also been recovered in the area. The deposit appears to be associated with basal Kalahari palaeo-channels with headwaters located to the east.
Omatako garnet anomaly
Prospecting carried out by de Beers in the early 1980's identified a scatter of kimberlitic garnets, which include G10 compositions, in the ephemeral Omatako River in the extreme northeast of Namibia. Follow-up work showed that these garnets were derived from sandstones, probably part of the Kalahari sequence, which are exposed in the riverbank. The Omatako anomaly is therefore clearly secondary, but the ultimate source of the kimberlitic minerals has never been located.
Implications of these secondary KIM anomalies
Unexplained kimberlitic heavy mineral anomalies provide classic pathfinder "lead-ins" to undiscovered kimberlites. Diamonds, and G10 garnets associated with such anomalies indicate derivation from diamondiferous, and potentially economic, kimberlites. Following the paleochannels back to the kimberlite source is a fully justified exploration strategy.
GEOMORPHOLOGICAL SETTING OF THE TSODILO GROUND
The geomorphological setting of northwest Botswana identifies the Tsodilo ground as an important potential source of the unexplained Tsumkwe and Omatako KIM anomalies.
The main Tsumkwe G10 garnet and diamond anomaly is located at the edge of the Etosha sub-basin (see Figure 3). The geomorphological setting of this KIM anomaly is therefore the shoreline of the former inland lake, probably close to the mouth of a major river - in other words, an environment that would favour the concentration of heavy minerals, including KIMs. Because high ground was located to the east, the source must have been located to the east.
The Omatako anomaly in the extreme northeast of Namibia is also closely associated with the 30 to 60 metre Kalahari isopachs, suggesting that this is also a shoreline heavy-mineral concentration on the ancient Etosha Lake (Figure 3).
Kalahari sand cover is relatively thin (0 to 30 metres) over much of the Tsodilo ground, but thickens to the north, west and east into three distinct sub-basins, each with greater than 240 metres of sediment infill. These are referred to as the Angolan, Etosha and Okavango-Makgadigadi basins respectively. The Tsodilo licenses therefore cover a ridge of high ground that provided sediment to the three basins and is therefore the "water-shed" from which the oldest Kalahari stream systems drained into the basins. Rivers draining off this high ground in northwest Botswana would provide sediment to the Etosha basin to the west.
This interpretation suggests that the source kimberlites are located within the Tsodilo property.
TECTONIC SETTING OF THE GCWIHABA LICENSES
The basement to the area covered by the Gcwihaba licenses was originally considered to be part of the late Proterozoic Damara-Ghanzi foldbelt; however, on the basis of an aeromagnetic survey commissioned by the Botswana Government in the mid-1990's, it was concluded that this area is underlain by a southern extension of the Angolan-Congo craton, and that the younger Damara rocks are overthrust onto this craton. This is supported by field evidence that shows that tectonic fabrics in the Damara rocks dip to the southeast, while folds close to the northwest. The tectonic setting is therefore a craton, overthrust by a late Proterozoic foldbelt.
The Tsodilo licenses are transected by a major southeast - northwest trending dyke swarm. The Gcwihaba Licenses are located along the southern edge of this dolerite dyke swarm (see in the "Imagery > Dolerite Dyke Swarm" section) that traverses Namibia, Botswana, northern South Africa and Zimbabwe. Although the significance of the association between this major crustal feature and kimberlites in not well understood, some 25 million carats (16%) of the world's annual diamond production (150 million carats) comes from the Orapa and Venetia clusters, which are also located on the southern edge of this same dyke swarm.
During 2004, 53 airborne magnetic anomalies were selected from the Gcwihaba license areas for further exploration using indicator mineral sampling.
Sampling teams collected 3 samples from each anomaly, one from the centrepoint, one 100m west and one 100m east of the centrepoint. The 159 samples yielded positive KIM's over 1 anomaly, 2021 B4, part of a cluster of five magnetic anomaly targets. A singleton eclogitic, diamond-inclusion garnet was recovered.
The cluster of magnetic anomalies was followed up by ground geophysical survey using both magnetic and gravity techniques.
In late 2004, the 2021 B4 anomaly was drilled using mud rotation and RC methods to penetrate the thick, loose Kalahari sand cover at this location. The basement rock, a magnetite quartzite, was intersected at 158m depth and explained the magnetic anomaly, but failed to explain the eclogitic garnet recovered here.
There is an abundance of geological evidence that leads one to conclude that the Gcwihaba project area is worthy of further kimberlite exploration -
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