Summary:
The Central Kalahari region is covered by the vast expanse of unconsolidated to semi-consolidated Kalahari Bed sediments. The bedrock in the area of concern / interest (Gope) lies approximately 75 m below the topographic surface, and comprises a suite of rocks belonging to the Karoo Supergroup. The lithostratigraphy of the Karoo is complex but well known in Botswana (Smith, 1986), with the specifics of the lithologies underlying the site being no exception. The entire Karoo sedimentary sequence, inclusive of the basaltic lava cap is known to occur in the Central Kalahari region.
The Karoo Supergroup has been divided into several lithostratigraphic groups, including (in decreasing age) the Dwyka Group, the Ecca Group, Lebung and Stormberg Groups. For the GDMP only the Lebung and Stormberg Groups need to be considered. The Lebung Group comprises two lithostratigraphic units, referred to as the Mosolotshane and the Ntane Sandstone Formations, comprising intercalated reddish medium to fine grained sandstones, siltstone and mudstone (Mosolotshane Formation) having been laid down in a terrestrial fluvitile environment, while the Ntane Sandstone, which overlies the Mosolotshane Formation, comprises medium to fine grained Aeolian sandstones (wind derived).
Both of these formations are essentially sandstone units, although the transitional member is more silty, and has layers and streaks of mudstone and siltstone as distinguishing marker horizons.
The Stormberg lava that overlies the Ntane Formation is the top most unit of the Karoo Supergroup, and comprises tholeiitic basalt lava that has extruded over an undulating (disconformities) Ntane weathered surface.
The Kalahari Beds that occur in the vicinity of Gope Project Site are between 75 m and 80 m thick and overlie Karoo bedrock. Two broad units are recognised in the area, including the upper fine grained, unconsolidated Aeolian sand unit, and the lower, partially consolidated unit that is often calcified and/or silicified.
A total of five Kimberlite facies have been identified during the exploration undertaken at the Gope Project Site. These four facies make up the Kimberlite pipe that is proposed to be exploited at the GDMP.
The five facies include a tuffisitic Kimberlite, a hypabyssal Kimberlite, a basaltic breccia with tuffisitic Kimberlite, and a basaltic breccia hypabyssal Kimberlite.
In turn, the Gope pipe is one of pipes that make up the Kimberlite field in the Central Kalahari area.
Of significance to the area of concern is the abundance of large faults that transect the area. These are not apparent at surface due to the extremely thick sand cover. However, the exploration drilling has exposed a strong West-north-west—East-south east and North-north-east—South-south-west trend to the dominant fault zones. These are significant features in that the Kimberlite intrusions have taken advantage of these zones of weakness. The area is understood to be tectonically stable now, but would have shown movement along the fault and fracture zones in geological history.
Venmyn reported that “below the pre-Kalahari surface, the wall rocks of the kimberlite consist of Karoo basalt to 391 m and Ntane sandstone from 391 m to an unconfirmed depth. The surface topography is very flat with a maximum elevation difference over the kimberlite of ~5m. The sub-cropping area of the pipe is 10.8 ha. This area decreases to 6.3 ha at a depth of 500 m. The pipe has an almost elliptical shape in plan view. While the orebody has been defined to a depth of 524 m, Venmyn believes that the orebody extends well beneath this depth.”
“The three volcaniclastic kimberlite (VK) facies types are distinguished essentially by differences in grade and, to a lesser extent, by density. The BXVK facies represents a VK facies type with a high (average of 44 %) basalt breccia content. The BXVK facies consequently has the lowest diamond grade of the VK facies rocks. The VKSE and VKMain facies appear to contain similar kimberlite components but different quantities and distributions of basalt, and therefore grade. The coherent magmatic kimberlite (CK) facies types are more variable, harder, have a higher density and are more resistant to weathering than the VK facies types. The BXCK facies is differentiated from the CK facies type by its high (average of 48 %) basalt breccia content and consequent lower diamond grade.”
VKSE is the mine’s first target. Thus far this is the highest grade portion of the orebody at 28.45 ct/ 100 t (cpht).