EXPLORATION TARGET SELECTION MODEL
NGAMILAND DIAMOND PROJECT, NORTHWEST BOTSWANA
Tsodilo's Ngamiland diamond project is located in the extreme northwest of Botswana,
where its 81% owned subsidiary company, Newdico (Pty) Limited ("Newdico"), holds 18 diamond prospecting licenses covering a total area of approximately 16,800 km2. The location of the Newdico license blocks are shown in Figure 1. The Kalahari ground cover in this area is relatively thin (see Figures 2 and 3).
PREVIOUS DIAMOND EXPLORATION WORK WITHIN THE TSODILO PROPERTY
A portion of the ground covered by the Newdico licenses was investigated by Ashton Mining in joint venture with Reunion Mining in 1997 and 1998. Their exploration program relied primarily on the newly published government aeromagnetic data for the selection of kimberlite targets. Soil samples were only collected over magnetic anomalies selected for follow-up, but not on a systematic regional grid. Nineteen kimberlites were discovered, in the vicinity of the Nxau Nxau village. The largest of these has an estimated diameter of 10 hectares, three have estimated surface areas of 5 to 7 hectares, while the remainder were 3 hectares or smaller. Small numbers of micro-diamonds were recovered from four of these pipes, but none carried G10 garnets and it was concluded that they would all be sub-economic.
In addition to the work carried out in the Nxau Nxau area, Ashton collected soil samples over a number of magnetic targets located in the eastern portion of the ground currently held by Newdico. A singleton G9 pyrope garnet was recovered over one of these targets, and singleton ilmenites over a number of the others sampled. One magnetic target sampled produced three ilmenites. Published partial chemical analyses are only available for some of the ilmenites recovered in this eastern area of the property. However, these cannot be matched with the fingerprints of ilmenite populations of the Nxau Nxau kimberlites for which data are available and this means that they were derived from a different kimberlite or cluster of related kimberlites.
A small number of magnetic targets in this eastern area were drilled, but none reported intersecting kimberlite. The source of the diffuse regional kimberlite indicator mineral anomaly in the east of the Newdico property (designated the Guma Anomaly) remains unexplained.
Despite the initial technical success of the Ashton joint venture in locating kimberlites, the exploration program was terminated. Reasons given for their decision to terminate are listed in their final report and include the following:
Two important KIM anomalies in northeast Namibia, located to the south of Tsumkwe and in the Omatako drainage, were originally identified during the course of prospecting work carried out by DeBeers in the 1970's. The positions of the anomalies are shown in Figure 2, and in detail in Figure 3. These anomalies and their feeder paleo drainage channels are currently being explored in Namibia but to date their origin is unexplained. The source kimberlites represent important exploration targets and since the Tsodilo / Newdico ground covers the watershed of these paleo drainage channels, it is our contention that there is a strong probability that they are located within the Newdico licenses.
Tsumkwe Garnet Anomaly
A northwest - southeast swath of kimberlitic garnets, concentrated in an area roughly 40 kilometers by 15 kilometers, was identified close to the village of Tsumkwe (Figure 3). Scattered garnets were recovered outside the main area of concentration. 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. A small proportion of the garnets are angular, but the greatest percentages are described as well worn. The Tsumkwe diamond and G10 KIM anomaly was originally identified by DeBeers in the late 1970's and early 1980's. Further diamonds and G10 garnets were recovered when Rio Tinto investigated the ground in the late 1990's, and ongoing work in this area continues to recover further diamonds (including macro-diamonds) and G10 garnets. Several of these appear to have originated from basal Kalahari infill in paleo drainage channels with headwaters located to the east.
A three hectare kimberlite was discovered some 10 kilometers to the east of the main garnet-diamond anomaly, but this appears to be barren or low grade. While angular garnets in the Tsumkwe area may be derived from this kimberlite, it is considered unlikely to be the source of the well-worn garnets that dominate this important, unexplained, KIM anomaly.
Omatako Garnet Anomaly.
Prospecting carried out by de Beers in the early 1980's identified a scatter of kimberlitic garnets, which reputedly 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. Recent work has traced the paleo drainage channel feeders to this secondary deposit as being from the south-east; the trace of the paleo drainage channels disappears some 10 - 15 kilometers west of the Botswana border as a result of subsequent erosion.
Characteristics of Primary and Secondary KIM Anomalies
Unexplained kimberlitic heavy mineral anomalies provide classic pathfinder "lead-ins" to undiscovered kimberlites. Diamonds, and / or garnets with compositions matching diamond inclusions (G10 garnets) associated with such anomalies indicate derivation from diamondiferous, and potentially economic, kimberlites. Anomalous KIM concentrations may be closely associated with the source kimberlites as a primary or proximal anomaly. However, secondary or distal KIM concentrations may also accumulate at considerable distances from the source rock, where there is a favorable environment for concentrating dense minerals. One example of such an environment is where a river enters the sea or a lake. Waves and currents selectively winnow and remove the lighter minerals deposited by the river, resulting, over time, in a build-up of those with greater densities. Thus secondary anomalies represent sinks where dense KIM's derived from a variety of different source rocks are concentrated. The most spectacular example of such a secondary anomaly is the concentration of diamonds along the west coast of southern Africa, some 800 kilometers distant from the presumed source pipes in the interior of southern Africa.
It is clearly crucial to establish whether an anomaly is primary or secondary. While this is not always straightforward in practice, the processes responsible for heavy mineral concentration leave characteristic imprints that are useful in understanding their origin. For example, when minerals are transported, relatively lighter silicates such as garnets will travel further than the markedly denser ilmenite and other oxides. The latter therefore tend to form a lag concentration close to the source rocks, while the garnet / ilmenite ratio typically increases with greater dispersion away from the primary source. However, some kimberlites lack ilmenite. Garnet-dominated anomalies may therefore indicate the presence of ilmenite-free kimberlites.
With travel, minerals will also be abraded, producing a worn surface. Secondary anomalies therefore tend to have a high proportion of worn KIM's. They will also form in a geomorphologic environment that favors the concentration of dense minerals.
GEOMORPHOLOGIC SETTING OF THE TSODILO / NEWDICO GROUND
The geomorphologic setting of northwest Botswana identifies the Newdico ground as an important potential source of the unexplained Tsumkwe and Omatako KIM anomalies. This area is blanketed by partially consolidated sand of the Kalahari formation, which extends from South Africa in the south to the Democratic Republic of Congo to the north. The regional variation in the thickness of this sand cover is illustrated in Figure 2, with greater detail for the Tsodilo project area and surroundings shown in Figure 3. Sand cover is relatively thin (0 to 30 metres) over much of the Newdico 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 Newdico property therefore covers 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.
The age of the Kalahari formation is not well constrained. However, while the basal sediments of the major Etosha basin may be as old as upper Cretaceous (80 to 90 million years), those in the Okavango-Makgadigadi basin appear to be Miocene in age (25 to 5 million years), and thus much younger. This suggests that the latter basin developed significantly later than the commencement of sedimentation in the Etosha basin. During the initial stages of sedimentation into the Etosha basin, but prior to subsidence to form the Okavango - Makgadigadi basin, the area of high ground covering northwest Botswana therefore probably extended further to the east. Rivers draining off this high ground in northwest Botswana would have provided sediment to the Etosha basin to the west.
The main Tsumkwe G10 garnet / diamond anomaly is located between the 30 to 60 metre Kalahari formation isopach (thickness) contours 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. Differences in garnet wear indicate that there could be more than one source for the Tsumkwe anomaly.
Figure 3 also illustrates major magnetic lineaments (from a regional Botswana Government aeromagnetic survey) over the Tsodilo project area. These reflect structural trends, which may have influenced early drainage lines into the early Etosha basin. A major northeast-southwest set of lineaments, related to the late-Proterozoic Damara - Ghanzi foldbelt, is oriented directly towards the Tsumkwe anomaly. Rivers controlled by this structural trend could therefore have contributed heavy minerals to the Tsumkwe anomaly.
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). Magnetic lineaments in the northwest of the Tsodilo property are oriented southeast-northwest, directly towards the Omatako drainage line. Rivers draining the high ground covering the Tsodilo property, controlled by these latter structural trends could therefore have been the source of the Omatako anomaly. This interpretation suggests that the source kimberlites are located within the Tsodilo property.
A majority of the kimberlites previously discovered by Ashton in the Tsodilo property have preserved crater sediments, indicating that there has been minimal erosion since their emplacement. As kimberlites are typically cone-shaped, widening upwards, this geomorphological observation is important as it flags the Tsodilo ground as a favourable area for the discovery of large kimberlites. For example, the Orapa kimberlite in central Botswana (Figure 1), which also has preserved crater sediments, has a surface area of 110 hectares.
TECTONIC SETTING OF THE TSODILO PROPERTY
The basement to the area covered by the Newdico property was originally considered to be part of the late Proterozoic Damara-Ghanzi foldbelt (Figure 1). 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 had been overthrust onto this craton from the south-east. 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 tectonic setting of the Tsodilo / Newdico property is closely comparable to that of the economic 110 hectare Orapa kimberlite in central Botswana. This pipe is located at the margin of the Zimbabwe craton, where it is overthrust by the early Proterozoic Magonde foldbelt (Figure 1). In such geological settings, ocean floor basalt subducted beneath the craton would be converted to eclogite in the upper mantle. This can account for the high proportion of eclogite-derived diamonds in the Orapa kimberlite. It is noteworthy that the Ngami kimberlites have a higher than normal proportion of eclogitic garnets. Further, many of the ilmenites in the Nxau Nxau kimberlites match compositions that have been reported in eclogite xenoliths recovered from the Orapa kimberlite. By analogy with the latter pipe, the Nxau Nxau kimberlites could have a markedly high proportion of eclogite-derived diamonds.
The Tsodilo property is transected by a major southeast - northwest trending dolerite dyke swarm (Figure 3 & 4). Both the Orapa kimberlite cluster and the Venetia kimberlites are located immediately to the south of this same dyke swarm. Some geologists draw attention to the close association between kimberlites and major dyke swarms in the Northwest Territories, Canada, and suggest that the latter provide a favourable geological environment for locating kimberlite emplacement. While such a link remains speculative, the area covered by the dyke swarm represents a major portion of the Tsodilo property where the subtle kimberlite magnetic anomalies are masked by the short amplitude dolerite dyke background noise. If kimberlites are present in this magnetically noisy area innovative methods must be applied to discover them. Tsodilo has researched magnetic survey strategies for overcoming the masking effect of the dyke swarm and hopes to perform a trial airborne survey in mid-2006 to address the problem.
The mantle geothermal gradient or the rate of temperature increase with depth in the earth's mantle is important in determining whether kimberlites are likely to carry diamonds. In cratonic areas, where most economic kimberlites are found, the gradient is relatively low - typically around 40mW/m2.
A variety of techniques have been developed to estimate temperatures and pressures of clinopyroxenes and garnets recovered from kimberlite concentrates in order to determine the local geothermal gradient. A majority of the clinopyroxene from concentrates recovered from the Nxau Nxau kimberlites straddle a typical cratonic geotherm of 40mW/m2. In contrast, the higher temperature clinopyroxenes define a much steeper, perturbed geotherm. However, these are undoubtedly derived from a suite of mantle rocks termed sheared lherzolites, which occur in a majority and possibly all kimberlites. They are, for instance, present in economic kimberlites such as the Kimberley and Premier pipes in South Africa, and the high grade Udachnaya pipe in Siberia, where they also appear to define perturbed geotherms. This suggests that, while both the origin and significance of these high temperature sheared rocks are controversial, they have no bearing on whether or not the underlying mantle is favourable for the preservation of diamonds.
Equilibration temperatures and pressures determined on garnets recovered from the Nxau Nxau kimberlites define a temperature-pressure array that is very closely comparable to published data for the pipes in the Kimberley area of South Africa. This is consistent with the clinopyroxene data, and highlights the Tsodilo ground as a favourable geological environment for the discovery of economic kimberlites.
A recent study (see Technical Reports - Mineral Chemistry) by the Euclid Geometrics Laboratory of the chemistry of ilmenite, garnet and chrome diopside grains recovered from the A15/1 kimberlite drillhole concluded that:
In-house screening by Tsodilo Resources of aeromagnetic data covering the Nxau Nxau kimberlite field has identified a large number of classic bulls-eye magnetic targets that were not drilled during the course of earlier exploration work carried out by Ashton. In addition, many of these targets were not covered by either geophysical surveys or soil sampling. Based on their geophysical signature, it is estimated that these untested targets show a wide range in size, from less than 100m in diameter to several 100metres in diameter. An important aspect of target selection is to prioritise this large number of targets for follow-up work and possible drilling.
Much of southern Africa was covered by a basalt carapace at the time of eruption of the upper Cretaceous pipes. Recent research has demonstrated that this basalt carapace played a major role in controlling the characteristic upward-flaring carrot shape of many southern African kimberlites. In contrast, the champagne glass shape of many of the Australian pipes appears to be related to their emplacement into water-saturated sediments.
In southern Africa, there also appears to be a positive correlation between kimberlite size and grade for pipes within the same cluster. In other words, if there are economic kimberlites within a cluster, these tend to be the ones with the largest surface area. This is well illustrated for the Orapa kimberlite field of northern Botswana, where the two richest pipes, AK1 or Orapa and DK1 or Letlakane are also the two largest. As yet, the reason for this correlation remains poorly understood, or why kimberlites within a given cluster may show a wide range in size. However, on the basis of in-house studies carried out by Tsodilo Resources, it does appear that many of the world's large, high grade pipes are composite bodies. Thus, of the two biggest producing kimberlites in Botswana, Orapa (110 hectare) is composed of two feeders that coalesce on the surface, while Jwaneng consists of three coalescing feeders with a combined surface area of 43 hectare. The 45 hectare Argyle pipe in Australia consists of at least three pipes, with coalescing surface tuffs, and the 160 hectare Camafuca pipe in Angola comprises five coalescing feeder pipes. This suggests that large economic pipes within a cluster reflect a localized area characterized by more intense kimberlite volcanic activity.
A critical aspect of target selection in the Tsodilo Ngamiland project will be to understand the geological setting at the time of kimberlite emplacement. Of particular importance will be to determine whether there was a capping of Karoo basalts at this time, and its thickness. Karoo basalts do occur some 50km to the north of the Nxau Nxau cluster, but it has not yet been established whether there was a basalt capping over the Nxau Nxau area at the time of kimberlite eruption. The presence of a basalt capping would suggest that the South African model is more appropriate for selection of follow-up pipes - in other words that attention be focussed on the larger targets. Nevertheless, it will also be necessary to investigate a cross section of the numerous smaller targets in particular those which are grouped close together to represent a local focus of kimberlite volcanic activity.
There is an abundance of geological evidence that leads one to conclude that the Newdico Ngamiland project offers a wealth of potential kimberlite targets. Tsodilo is confident that it has the in-house technical capability to rank these targets for follow-up investigation and successfully turn these targets into economic diamond mines.
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