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PL 3/2000 Alluvials ProjectThe present day Letlhakane River is a remnant of a formerly much larger system which has drained the Orapa kimberlite field, 100 km to the north-west, for millions of years. Most of the Orapa kimberlites have suffered significant erosion and the liberated diamonds, which should amount to tens of millions of carats, have never been found. The Orapa kimberlites were emplaced approximately 90 million years ago and though there is little erosion at present, this has not always been the case. The early- Tertiary period (~65 mya) is thought to have been a time during which erosion was especially vigorous. Block faulting in the late-Cretaceous, resulting from regional uplift related to the opening of the Atlantic Ocean, gave way to graben in-filling in the early Tertiary. The climate was tropical and the rainfall high. The Motloutse river channel, near the South African border in the east of Botswana, was about 4 times wider and 3 times deeper than it is today. This active erosion began to slow down by the mid-Miocene (15mya) and there has been very little erosion or sedimentation in central Botswana since then. Desert conditions developed during the last 2 million years and the Kalahari sands were deposited, masking the earlier topography. As a consequence, the search for the missing Orapa alluvials needs to focus on these early-Tertiary, buried river channels.
The company had previously used freely available, high resolution altimetry data to identify depressions in the present day topography that may betray the presence of buried palaeo-channels. A number of detailed gravimetric traverses were completed at selected points across these channels in 2002 and 2003. The area selected for the gravity survey was previously covered by a regional aeromagnetic survey. The results from this aeromagnetic survey indicated that the area is traversed by large dolerite dykes, giving rise to changes in bedrock geology. This environment is believed to be favourable for the development of trap sites and the deposition of diamonds. Gravity surveys measure minute changes in the Earth's gravitational field caused by differences in the density of underlying rocks. Buried river channels are likely to be filled with sediments that are less dense than the surrounding rocks and should therefore correspond with a small decrease in the observed gravitational field. This approach appears to have worked with several traverses showing pronounced gravity lows in the vicinity of the Letlhakane channel. The results of the gravity surveys strongly suggest that there is more than one channel within the area investigated. The projected channels appear to represent different ages of stream channel development.In October 2003, a 36-hole drill programme totalling 1471 metres was completed. The objectives of this programme were twofold;
Samples were collected at one metre intervals in every hole drilled and subsequently processed to concentrate any heavy minerals present. The resulting heavy mineral concentrate was sorted by technicians skilled at identifying heavy minerals derived from kimberlites, so called Kimberlitic Indicator Minerals (KIMs). To date, over 900 samples have been analysed in this manner and many samples have yielded kimberlitic indicator minerals. Over 500 kimberltic mineral grains have been recovered to date. Most are kimberlitic ilmenite, but significant numbers of kimberlitic garnets, spinels and diopsides have also been recovered. Map 5 summarises these results. As can be seen, a few holes have yielded significant numbers of kimberlitic indicators. For instance, Hole 6 on Line 3, at the lowest point of the profile shown in Figure 2, yielded large numbers of KIMs. These results strengthen the hypothesis that these channels may contain pockets of alluvial diamonds. The potential of the area will be further investigated by high resolution geophysical surveys to be carried out during 2005. Kimberlite Exploration
The chemical composition of this garnet population is also distinct to those from Orapa kimberlites, and is characterized by an absence of eclogitic type garnets and the presence of a significant proportion of high chrome and diamond inclusion type, 'G10' garnets. Some 20% of the garnets analysed from this area to date have a G10 composition - a high proportion that is indicative of a prospective source. This block of licences lies on the southern flank of a major, west-north-west trending dolerite dyke swarm that traverses northern Botswana and marks a 'failed rift' zone. The dolerite dyke swarm marks a major zone of weakness in the Earth's crust that was also exploited, at various points along its length, by many kimberlite volcanoes in the region, including those that comprise the Orapa kimberlite field. The regional aeromagnetic data shown in Map 5 vividly depicts the WNW-trending lineaments caused by dolerite dykes, as well as the typical noisy pattern caused by sub-cropping Karoo basalts. The particular dykes that are related to the anomaly of kimberlitic indicator minerals in PL 3/2000 can be seen to extend north westwards into PL 50/2004. These areas will be further investigated by high-resolution, airborne magnetometry survey in 2005. BACK |
Map 5 Overview of Alluvial Project prospecting License 