A progress report on the geophysical exploration of the Kalahari in Botswana

Geoexpforafion, 20 (1982) 209-224 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

209

A PROGRESS REPORT ON THE GEOPHYSICAL EXPLORATION OF THE KALAHARI IN BOTSWANA

C.V. REEVES and D.G. HUTCHINS

Paterson, Grant & Watson Ltd., Toronto (Canada)

Botswana Geological Survey, Lobafse [Botswana)

(Accepted for publication March 30, 1982)

ABSTRACT

Reeves, C.V. and Hutchins, D.G., 1982. A progress report on the geophysical exploration of the Kalahari in Botswana. Geoexploration, 20: 209-224.

The mineral wealth of southern Africa as a whole gives considerable incentive to the exploration of Botswana, despite the cover of Kalahari sand that totally obscures the solid geology underlying the greater part of the country. Nationwide geophysical survey coverage has been undertaken during the past decade and has served to indicate the thickness of the cover rocks and to delineate a number of distinct tectono-metamorphic provinces within the Precambrian basement. The results of these surveys have prompted a number of more regional programmes of exploration which are presently in progress, employing geophysics, geochemistry and drilling. The paper demonstrates the role of geophysics in such a logically-developing programme of exploration, which, it is hoped, will eventually provide the necessary basis for the search for ore bodies at a local scale.

INTRODUCTION

Botswana is a land-locked country whose immediate neighbours - South Africa, Namibia, Zambia and Zimbabwe - all enjoy a considerable degree of mineral wealth, derived in large part from the Precambrian rocks found there. Botswana however, occupies the central region of the 1000 m high continental plateau of southern Africa, where a veneer of waterborne and aeolian deposits .- the Kalahari sands -- have accumulated in Tertiary to Recent times, almost totally obscuring the older rocks (Fig.1). As a result, only about 20% of the country, predominantly the eastern part, is mappable in the conventional geological sense. Even so, important mines - for nickel, copper, diamonds and coal have been brought into production in this region during the past decade. During the same period, the Geological Survey of Botswana has been actively pursuing a programme of regional geophysical exploration in an attempt to extend knowledge of the basement geology into the remainder of the country (Hepworth et al., 1972).

It is the purpose of this paper to review this programme to date and to set

0016-7142/82/0000-0000/$02.75 0 1982 Elsevier Scientific Publishing Company

BOTSWANA

,.....,,

Fig.1. Botswana in relation to the major geological elements of southern Africa. 1, Kalahari Cover in central area, Cretaceous and post-Cretaceous cover in eastern coastal area; 2, Karoo cover; 3, Cape Fold Belt; 4, Damaran Belt (- 550 Ma); 5, Namaqua-Natal Belt (N) and Irumide Belt (I) (- 1000 Ma); 6, Limpopo Belt (L) and Kheis (K) (- 2700 Ma); 7, Archaean Cratons, Kaapvaal (KV) and Rhodesia (R) (> 2800 Ma). Sedimentary cover predating the Karoo is not shown.

it in the context of a logically developing exploration strategy which will, it is hoped, eventually lead to the discovery of new mineral wealth concealed below the Kalahari sands.

NATIONWIDE SURVEYS

The first phase of the programme involved nationwide geophysical reconnaissance surveys which have recently been reviewed in some detail elsewhere (McEwen, 1979; Hutchins and Reeves, 1980).

The national gravity survey of Botswana was undertaken during 1972 and 1973 by the staff of the Botswana Geological Survey, supported by a grant from the British Government. A network of gravity base stations, connected to International Gravity Standardization Net (IGSN) base stations in neigh- bouring countries, was established to bring most points in Botswana within a 200-km radius of a gravity base. Two LaCoste and Romberg gravimeters

211

A ,.‘;2~ 32’

,->,,-, + + +

I

TECTONIC FRONT OLDER

AHCHEAN CRATONIC AREAS PROVINCE ON TICKED SIDE

Fig.2. An interpretation of tectono-metamorphic provinces in southern Africa based on published gravity data (after Reeves, 1977). K-K’, Kalahari Line; M--M’, Makgadikgadi Line.

and a light aircraft were employed in this phase of the work. Subsequently over 2000 gravity observations, made by truck and helicopter over the whole country, were tied into these bases. The featureless nature of the terrain covered necessitated the use of innovative techniques of position fixing. The exceedingly low topographic relief and the stable climatic pattern afforted a high degree of accuracy for altitude determinations by altimetry. The results of the survey are tabulated, and a Bouguer anomaly map of Botswana at a 1 : 1000,000 scale are presented in Reeves and Hutchins (1976).

A reconnaissance aeromagnetic survey of the 80% of Botswana lying west of 27”E was undertaken with a grant from the Canadian International De- velopment Agency (CIDA) during a six-month period (1975-76). A total

212

of 150,000 line kilometres of medium-sensitivity magnetometer data was acquired along north--south flight lines spaced at 4-km intervals. Again, the featureless nature of the terrain required the implementation of some novel techniques of survey execution (Paterson et al., 1979). The survey was flown and compiled by Terra Surveys Ltd., Ottawa, and the resulting maps were published by the Botswana Government in 1977. A detailed interpretation of the data has since been published (Reeves, 1978a).

In addition to these two national surveys, several other studies of regional significance have been undertaken. A study of the seismicity of Botswana (Reeves, 1972) highlighted the high level of seismic activity in the northwest of Botswana, which has subsequently been studied in greater detail (Scholz et al., 1976). A more detailed regional gravity survey of this area (Reeves, 1978b), a seismic refraction survey (Greenwood and Carruthers, 1973) and a geomagnetic deep sounding study (De Beer et al., 1975) have also been carried out in the northwest. An intensive multidisciplinary attempt to produce a geological map of the subcropping basement was undertaken over a 300 X 50 km strip of central Kalahari terrain thought to be relatively favourable for surface exploration (Coates et al., 1979). A photogeological study of the whole country, based on LANDSAT imagery, has been published (Mallick et al., 1980) and, more recently, a programme of radiometric dating has been executed over the small number of basement outcrops known within the Kalahari (Key and Rundle, 1980).

Taken together, the results of these investigations have provided a great deal of new information on the geology of the Kalahari.

GEOLOGICAL RESULTS - COVER ROCKS

The Kalahari sand itself has been shown by large numbers of depth determinations on anomalies arising from magnetic markers at its base to be generally less than 100 m in thickness over most of the country, a figure that has been confirmed locally by water-well drilling. The depth increases to in excess of 200 m in some localities in the north of Botswana, principally as a result of the infilling of the seismically active rift zone in the Okavango Delta of northwest Botswana. In other regions, however, the thinner cover admits the possibility of using conventional geophysical and geochemical techniques to explore the underlying rocks. The economic potential of the Kalahari deposits themselves is considered to be largely confined to industri- al uses and their role as an aquifer.

In much of Botswana, the Kalahari sand is underlain by more or less flat- lying rocks of the Karoo Supergroup. These comprise a sedimentary sequence, late Carboniferous to Jurassic in age, which culminates in a thick pile of volcanic basaltic rocks. Late- or post-Karoo dykes, sills and (locally- diamondiferous) kimberlite pipes intrude the Karoo sequence. The approxi- mate distribution of Karoo rocks was estimated from their occurrence in widely spaced water wells (Green, 1966). The distribution of the Karoo

213

basahs is rmw better defined as a result of the aeromagnetic survey, though the relatively weak magnetic signature of the volcanic rocks, especially where they are thin and have been deeply weathered prior to Kalahari deposition, is by no means always clear. The somewhat younger dykes and sills, on the other hand, are well defined by the magnetic data and their distribution ap pears to be causally related, at least in part, to the break-up of Gondwana- land (Reeves, 1978e).

-.”

lB” “.---

_-‘--,

#.-----

J I Eastern limit of

I aeromognetic survey area

2p* 28’ /

Fig.3. Outline interpretation of the aeromagnetic survey of Botswana. Provinces of the Precambrian basement (solid letters): A, Archaean (Kaapvaal-Limpopa- Rhodesian); P, deep basins of Proterozoic-Paleozoic sediments; G, Ghanzi-Chobe Fold Belt (1000 Ma and/or 550 Ma); D, Damaran (550 Ma). Basement lineaments (solid iines): I(-%, Kalahari Line; M-M*: Makgadikgadi Line; Z-Z’, Zoetfontein Fault. Precambrian intrusions (solid black): T, Tshane Complex; X, Xade Complex. Karoo features (broken lines): N, Northern Karoo Basin; C, Central Karoo Basin, S, South- western Karoo Basin; DS, post-Karoo dyke swarm.

214

The bulk of the Karoo sequence rocks is non-magnetic, however, and the depth of the underlying (Precambrian) magnetic basement may be estimated from the magnetic survey data by comparing each observed anomaly with theoretical anomalies in the computer, and hence estimating the most prob- able depth for the basement below each anomaly. The extent of overlying Karoo rocks both in area and thickness may thus be inferred. Three large basins are indicated in the north, centre and southwest of the country, respectively (Fig.3). Of these, the first two are thought not to exceed 1000 m in thickness, for the most part; they both include basalts for which a thickness in excess of 300 m has been proven by drilling at a few localities, but which is often much less. By contrast, the southwestern Karoo basin con- tains no basalt, and the magnetic basement below it is estimated to be at a depth of over 10,000 m. Non-magnetic Proterozoic sediments probably occupy much of this thickness, below the Karoo; one drillhole, in Namibia, has passed through 886 m of Karoo before entering older formations.

The Karoo sediments contain large tonnages of coal which are presently being evaluated for the world market, and hydrocarbon potential in the un- explored southwestern Karoo basin cannot be ruled out. Much more detailed magnetic surveys are necessary to show the small circular anomalies that can be associated with kimberlite pipes in Botswana - surveys which have been undertaken with considerable success by a number of mining interests.

GEOLOGICAL RESULTS - BASEMENT ROCKS

Exploration for base and precious metals would naturally be directed towards the Precambrian rocks below the Karoo and Kalahari cover. The geophysical surveys have enabled an attempt to be made to extrapolate into the Kalahari the provinces of Precambrian rocks, known and described in the countries surrounding Botswana. This was first attempted by the com- pilation of the gravity data for Botswana with that published previously for the surrounding countries (Reeves, 1977) to provide a uniform coverage of gravity data for the whole of Africa south of 18”s (except Zimbabwe and Mozambique). It is clear that the Archaean rocks of the Kaapvaal and Rho- desia cratons and the Limpopo Mobile Belt lying east of the Kalahari must give way, at some point below the Kalahari, to the Proterozoic rocks of the Namaqua-Natal and Irumide Belts (1000 Ma) and to the late Proterozoic/ early Phanerozoic Damaran Belt (550 Ma), which are recognized to the south-west, west and north of Botswana (Fig.1).

Examination of the Bouguer gravity anomaly contours for southern Africa revealed two lineaments separating areas of distinct gravity anomaly signature (Fig.2). The first, called the Makgadikgadi Line, bisected Botswana in a NE-SW direction and was thought to represent the front of Proterozoic metamorphism and tectonism evident in the isolated rock exposures of NW Botswana. This front coincided with a line of weak seismicity, quite distinct from the epicentres in the Okavango Delta, reported previously (Reeves,

215

1972). The second lineament known as the Kalahari Line, followed longitude 22”E across the southern half of Botswana and into the northern Cape Province of South Africa; it was thought to represent the western limit to Archaean rocks of the Kaapvaal Craton.

The magnetic signatures revealed by the aeromagnetic coverage of Bots- wana coincided closely with the divisions made on the basis of the gravity data, and naturally added a great deal of definition in excess of that provided by the quite widely-spaced gravity data (Fig.3). Major magnetic bodies (the Tshane Complex) of presumed basic or ultrabasic composition, were identified to occur along the Kalahari Line and were tentatively ascribed to an ophiolite suite abducted onto the continental block at a convergent plate boundary (Reeves and Key, in prep.). Other mafic bodies of considerable dimension were identified near the Makgadikgadi Line, the largest of these being called the Xade Complex. Further large mafic bodies are indicated near the southern boundary of Botswana in the Molopo Farms region (25’S, 23-24”E).

On the basis of the national geophysical coverage it is thought, therefore, that approximately half of Botswana, i.e., that part lying southeast of the Makgadikgadi Line and east of the Kalahari Line, is underlain by Archaean rocks of Kaapvaal-Limpopo-Rhodesian association. Apart from their area of exposure along the eastern border regions of Botswana, they lie mostly below Kalahari and Karoo rocks of up to 1000-m thickness. Exploration of the Archaean rocks is therefore unpromising in the Kalahari, except for two regions: (1) in the area of a major late- or post-Karoo dyke swarm in the Makgadikgadi where Archaean rocks reach the surface in a broad upwarp between the Central and Northern Karoo basins, and (2) south of the Zoet- fontein fault which may be extrapolated WSW using the magnetic data, from its known expression in the northern Transvaal of South Africa. The fault appears to have a post-Karoo vertical displacement of several hundred metres (upthrown to the south) in southern Botswana. It therefore cuts out the Karoo rocks and brings Archaean granite-greenstone terrain, overlain by Proterozoic platform sediments, close to the surface (below only Kalahari cover) in southern Botswana.

The nature of the Kalahari Line and the pre-Karoo succession to the west of it in SW Botswana remains the subject of investigation.

Immediately northwest of the Makgadikgadi Line, thick accumulations of Proterozoic sediments appear to lie below Kalahari and some Karoo cover. The latter does not appear to be extensive in the NW of Botswana; outliers containing a very abbreviated succession are known south of the Okavango Delta, and thin Karoo basalts, lying directly on Precambrian basement, occur in the northernmost parts of the country. Elsewhere in NW Botswana only Kalahari sediments cover Precambrian rocks, adding to the potential for exploration there. Tightly folded Proterozoic rocks form a belt (the Ghanzi- Chobe Fold Belt) about 100 km in width which extends from Namibia in the west to Zambia in the north. These rocks outcrop locally and consist

216

of detrital sediments folded together with extrusive and/or intrusive igneous rocks dated at about 980 Ma. Their correlation with known rocks in Namibia and Zambia is unclear, but they contain significant stratiform horizons of Cu-Ag mineralisation at certain localities in NW Botswana.

The youngest Precambrian rocks in Botswana are those of the Damaran Belt which appear to be confined to the northwesternmost corner of the country. By analogy with the mineral occurrences in the Damaran rocks of northern Namibia, contiguous to the west, their economic potential must also be considered significant.

There would therefore appear to be a division of the Precambrian rocks of Botswana into a number of provinces, each with its own tectonic and metamorphic history and mineral potential. Some of these will have evolved separately, prior to their present juxtaposition, if suturing along the Kalahari Line has occurred. On the other hand, an interpretation of the geophysical data by Pretorius (1979) envisages a unified origin and a perpetual cyclicity of tectonic development for the southern part of Africa as a whole, in a pattern radially disposed around the ancient nucleus of the Rhodesian Craton. While this differs from the interpretation favoured by the present authors, the renewed interest in the large scale tectonics of this area occa- sioned by the advent of national geophysical data coverage is evident.

REGIONAL MINERAL EXPLORATION

The exploration of the Kalahari, which commenced with the national geophysical surveys, is now entering the second phase of more detailed regional surveys and ground truth investigations. These programmes, brief- ly outlined below, will continue over the next 2-5 years. The primary ob- jective is the verification of the interpretations of the geophysical data and the identification of prospective mineral targets with a view to encouraging private sector involvement in the mineral exploration of Botswana.

An exploration strategy has been devised in which the Government, with the assistance of aid agencies, will undertake verification of the aeromagnetic interpretation and location of possible mineralised targets (Jones, 1979). The prime aim of these investigations is to encourage the involvement of the private sector by reducing the risks involved in prospecting, and to develop exploration techniques suited to prospecting in the Kalahari environment. A provisional prospectivity map of Botswana (Fig.4) indicates possibly highly prospective areas at various depths. These areas have been selected on the basis of the aeromagnetic interpretation and current geological knowledge, both within Botswana and in the surrounding territories. This map has provided a basis for assessing priorities and apportioning areas for exploration projects conducted by the government and in various bilateral aid projects.

A ground truth investigation to verify the interpretation of the aeromagnetic survey, involving ground surveying of 29 targets followed by a total

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EXPLANATION

Possibly highly p~as~cti~e:~e~~s 6001~ q Prospectivity uncertain

Possibly highly prospective:depthe 500-l 0001~ •nnll

Possibly prospective for oil/gas:depths up to 1OOOOm

III :.::‘-‘i:-:: : i..- .. Possibly highly prwie:deptbs 13 OWJm

Fig.4. Prospectivity map of Botswana (alter Jones, 1979).

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2oc I* 2-p 13’ 24’ 25” 28”

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AEROMAGNETIC FOLLOW-UP HYDROGEOLOGICAL INVESTIGATIONS

0 CANADA

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UNITED KINGDOM WEST GERMANY

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Fig. 5. Areas of current bilateral technical assistance projects.

219

of 8000 m of confirmatory drilling at selected sites, commenced in mid 1979. Technical Assistance and finance are being provided by CIDA, as a continuation of the aeromagnetic survey project. The location of the survey targets is shown in Fig.5; magnetic surveying of target anomalies was completed early in 1981 and follow-up diamond drilling has already commenced. Despite the remote location of some of the sites, coupled with the poor map and photographic coverage over much of the Kalahari, target anomalies have all been successfully located using geophysical techniques on the ground.

The Metal Mining Agency of Japan has undertaken geological mapping, geochemical sampling and airborne geophysics (INPUT) in an area of North- east Botswana, to the east of the Makgadikgadi Pans (Fig.5). Ground geophysical follow-up of airborne geophysical anomalies has been completed and drilling is due to commence mid-1981.

The Federal Institute for Geoscience and Natural Resources (West Ger- many) conducted car-borne magnetometer traverses across aeromagnetic features in western Botswana as a preliminary phase of exploration. Two areas have been selected for detailed investigation which will involve geological mapping, geochemical sampling, airborne and ground geophysics, followed by confirmatory drilling.

The Institute of Geological Sciences (United Kingdom) is conducting a detailed helicopter-borne gravity survey of the Molopo Farms area. The survey, coupled with sand sampling for heavy-mineral concentrates, aims to trace concealed layered ultrabasic and basic rocks which have been tentatively correlated with the Bushveld Complex of the Transvaal.

In addition to these activities, the Botswana Government is pursuing active exploration of the Kalahari. A reconnaissance ground geophysical traverse across the Xade Complex (Fig.3) was conducted early in the follow- up programme. The Xade Complex is a ‘Y ‘-shaped magnetic feature, consist- ing of a series of magnetic anomalies, parallel to which a strong lineament is recognised on both aerial photography and Landsat imagery (Mallick et al., 1980). Magnetic and gravity profiles across the stem of the ‘Y’ (Fig.G), have been interpreted as either a layered basic lopolith with zones of variable magnetisation, or alternatively, as a synformal ‘greenstone’ belt (Hutchins et al., 1979). Exploratory drilling is planned as part of the CIDA programme and additional ground magnetics have been conducted over the complex.

In eastern Botswana, drilling on circular magnetic highs within the Gaborone granite where it is concealed by shallow Kalahari sands has indicated gab- broic intrusions at a depth of 150 and 180 metres. These depths are in good agreement with those estimated from the aeromagnetic survey (120 and 200 m respectively).

Further geophysical surveys are planned, including a detailed gravity survey of the Makgadikgadi Pans, to locate possible basement schists beneath a thin veneer of pan and Karoo sediments. The abundance of late- and post-Karoo dykes and the saline nature of the pan deposits preclude the use of electromagnetic, electrical and magnetic methods in an area where base-

Fig.6. A. Ground magnetic, and B. gravity profiles of the Xade Complex with theoretical values calculated for the 2-D combined gravity and magnetic model shown in section at C. (After Hutchins et al., 1979.)

ment is regarded as being relatively shallow, from both geological and geophysical evidence (Reeves, 1978a; Coates et al., 1979).

Despite the vast wealth of geological information gleaned from the regional geophysical surveys there remain two major problems which are restricting mineral exploration except in high-value commodities (diamonds). The first is the location and evaluation of ore bodies beneath Kalahari and Karoo cover and the high cost to the private sector of such exploration. The second, and equally important, is the availability of adequate water supplies to support mining development once economic deposits have been located.

A five-year programme of ground water evaluation commenced in 1976, financed by a bilateral aid agreement between the British and Botswana

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governments. The project has concentrated on the quantitative and qualita- tive evaluation of the groundwater resources of the eastern portion of the central Karoo Basin. A number of quantitative investigations have been completed (Fig.5); in all cases geophysical surveys have been applied to assist the evaluation and exploration of potential aquifers. Peart (1979) outlined the application of various geophysical techniques in the exploration of groundwaters in Botswana and concluded that electrical resistivity (both sounding and traversing) combined with magnetics proved the most effi- cient exploration method in Botswana. Consequently these techniques, with the addition of gravity, are being used extensively in groundwater investigations. The results of these studies will be published during 1980 and plans are well advanced to undertake further research studies in the central Kalahari.

The Kalahari cover and the lack of reliable surface water supplies naturally impose restraints on exploration by the private sector. The remoteness of the Kalahari and the lack of infrastructure are problems which could be relatively easily overcome once ore bodies and adequate water supplies are proven to exist. The location of ore bodies, or perhaps more correctly, the location of possible mineral provinces, and the evaluation of groundwater supplies are thus the main priorities of the current phase of Kalahari exploration. In both instances the deployment of geophysical exploration techniques suited to the Kalahari environment are essential.

BROADER PERSPECTIVE

Finally, it might prove informative to set the regional geophysical exploration of Botswana into a broader context by summarising similar programmes in the other countries of southern Africa (south of 10”s). The availability of published regional gravity and aeromagnetic survey data is indicated in Table I. The table is compiled from publications in the hands of the authors plus a limited literature search. We cannot claim, therefore, that the list is exhaustive, but it is unlikely that any large body of data has been omitted. In addition, there is certainly a large amount of proprietary data in the possession of private companies, but these are not thought to include large regional coverage of otherwise unsurveyed areas. Many regional aeromagnetic surveys include scintillometer or spectrometer data. For the sake of simplicity, neither these radiometric data nor the large number of relatively confined areas of airborne electromagnetic surveys (which usually include magnetic observations) have been considered in Table I.

It will be seen from this table that almost two-thirds of southern Africa is now covered by reconnaissance gravity survey with better than 15 stations per 1000 km2. The main omissions in this coverage are Angola, Zimbabwe and Mozambique, where political emergencies have made field work impos- sible in recent years. Problems of accessibility have naturally led to voids being left in the coverage of most of the other countries, and further opera-

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TABLE I

The status of regional gravity and aeromagnetic survey coverage in southern Africa, 1980

Country

Angola

Zambia

Malawi

Zimbabwe

Mozambique

Namibia

Botswana

South Africa

Lesotho

Swaziland

Gravity survey

None known.

1700 stations cover entire country. Bouguer anomaly map at scale 1 : 1,500,OOO (Mazak, 1974).

2000 stations cover entire country. Map at 1 : l,OOO,OOO scale (Andrew, 1974).

Wide-ranging survey of Great Dyke undertaken (Podmore, 1970) but no national coverage as such.

Limited coverage in southern part of country.

1300 stations cover entire country. Map at 1:l ,OOO,OOO scale (S. A. Geol. Surv., 1964; Kleywegt, 1966).

2100 stations cover entire country. Map at 1 : l,OOO,OOO scale (Reeves and Hutchins, 1976).

6300 stations originally published as nationwide coverage (Smit et al., 1962). Substantial fill-in subsequently. 1 : l,OOO,OOO scale map updated to 1970 (S.A. Geol. Surv., 1970).

Essentially unsurveyed, apart from work of Smit et al. (1962).

2500+ stations cover entire country (Burley et al., 1970). Maps at 1 : 250,000.

Aeromagnetic survey -

None known in onshore areas.

Entire country surveyed; maps published at 1 : 250,000 scale (Money and Saviaro, 1977; Saviaro, 1979).

6% of country covered by UN helicopter-borne survey, 1972.

None known.

None known in onshore areas

Only limited areas (less than 5%) of country published to date.

80% of country covered, principally W of 27”E. Maps at 1 : 500,000 scale released 1977. Canadian aid, 1975-77.

Coverage of entire country nearing completion. Most areas released as 1 : 250,000 scale maps. 1971-present.

About 30% coverage, principally lowland areas. Canadian aid C. 1976.

Entire country covered. Maps at 1 : 250,000 (Bacon, 1970).

tions will be needed to fill these gaps. It is also certain that intensification of the data coverage will be necessary in almost all areas if the maximum amount of geological information is to be derived from the gravity survey method. Such intensification has already been undertaken in selected areas of several countries.

223

Aeromagnetic survey coverage is generally less well advanced and is presently available for less than half the area under consideration. If the pattern followed elsewhere in Africa is pursued, coverage funded by international aid agencies can be anticipated as a precursor of systematic mineral exploration programmes in the years ahead.

To be effective, such a strategy requires a thorough interpretation of the aeromagnetic data, and to date this has not always been undertaken, even where such data is presently available. Rapid publication of ‘the survey data at least permits individuals and companies to undertake their own interpretations, but we feel that any such interpretation is much elevated in stature by being closely wed to the existing geological data-base for the country in question. In the context of southern Africa, this presupposes the active participation of the national geological survey authorities. We hope we have demonstrated in the present paper the effectiveness of this approach in the drive for new mineral discoveries in the case of Botswana.

ACKNOWLEDGEMENTS

We thank the Department of Geological Survey and the Ministry of Mineral Resources and Water Affairs of the Republic of Botswana for the opportunity to be involved in the studies we have outlined. We acknowledge the en- couragement of David Hastings in the preparation of this paper and thank Dr. G.C. Clarke, Director of the Department of Geological Survey, for critical- ly reading the manuscript.

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Documents

A progress report on the geophysical exploration of the Kalahari in Botswana