Extension of the Archaean Madibe-Kraaipan granite-greenstone terrane in southeast Botswana:...

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Journal of African Earth Sciences 123 (2016) 39e56

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Journal of African Earth Sciences

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Extension of the Archaean Madibe-Kraaipan granite-greenstoneterrane in southeast Botswana: Constraints from gravity and magneticdata

Calistus D. Ramotoroko a, *, Rubeni T. Ranganai a, Peter Nyabeze b

a Department of Physics, University of Botswana, P. Bag, UB0704, Gaborone, Botswanab Geophysics Division, Council for Geosciences, P. Bag, X112, Silverton, Pretoria, South Africa

a r t i c l e i n f o

Article history:Received 14 December 2015Received in revised form20 June 2016Accepted 21 June 2016Available online 23 June 2016

Keywords:Kaapvaal cratonSoutheast BotswanaGranite-greenstone terranePotential field dataGravity modellingGeotectonic evolution

* Corresponding author.E-mail address: ramotorokoc@biust.ac.bw (C.D. Ra

a b s t r a c t

The main goal of this study is to use suitably processed potential field data between longitude 24�E to26�E and latitude 25�S to 26.5�S to gain a better geological and structural understanding of the extensionof the Madibe-Kraaipan granite-greenstone terrane in southeast Botswana. Specifically, 150 new gravitymeasurements at 2e4 km intervals are reduced and later merged with existing gravity data in Botswanaand South Africa for an integrated crustal interpretation with regional aeromagnetic data.

Gravity and aeromagnetic anomalies of the region present partly coincident medium to high ampli-tude regions alternating with low zones. Analysis of the data revealed the existence of relatively narrowNeS trending rocks of dense and high magnetic intensity extending to the village of Mmathethe, cor-responding to the northern extent of the Kraaipan greenstone belt. Much of the north-central area formsa broad magnetic low and gravity high implying Kraaipan metavolcanic rocks are more extensivelydeveloped in this area than previously recognized, under a blanket of Kalahari sediments that are ~55 mthick as indicated by borehole data. The whole area lies within an ENE-trending Pre-Transvaal dykeswarm visible on the regional aeromagnetic data and much clearer on (proprietary) high resolutionaeromagnetic data. The derivative and analytic signal techniques applied for both gravity and magneticdata spatially map the greenstone belt and multiple granite plutons very well.

Depth estimates obtained by the 3D Euler method in combination with two-dimensional powerspectrum technique locate the high magnetic intensity horizon at around 4.0 km. The depths werefurther confirmed by gravity model results along two profiles across the granite-greenstone terrane inBotswana and South Africa in a W-E direction. The models show generally steep-sided bodies of com-parable width with a maximum depth extent of 4.7 km for the greenstones and 4.4 km for the youngerplutons. The distribution and configuration of the greenstones and plutons suggest assembly of thisKaapvaal craton western domain by plate tectonic processes. Episodic intrusion of granitoids intoKraaipan greenstone successions led to deformation of the belt with significance to Au and Au-PGEmineralization. The mapped structures could be important for groundwater exploration in this dryfarming terrane.

1. Introduction

The Archaean granite-greenstone terranes comprise some of theEarth’s oldest rocks and host various important mineral depositsbut the complex mechanism of formation and stabilization of theseearly continents is still incompletely understood (e.g., Peschler

motoroko).

et al., 2004; Benn et al., 2006; Groves and Bierlein, 2007; Dill,2010; Gallardo and Thebaud, 2012; Anhaeusser, 2014). The Kaap-vaal Craton of South Africa is a typical example with several oper-ating mines producing economically significant amounts of gold,base metals, platinum (PGE/M), diamonds, asbestos, coal, etc.(Anhaeusser and Viljoen, 1986; de Wit et al., 1992; Schmitz et al.,2004; Anhaeusser, 2014). Its western domain extends into south-ern Botswana but the geological outcrop is largely obscured bycover of Tertiary-Recent sediments and Kalahari calcretes (Meixnerand Peart, 1984; Mapeo et al., 2004; Poujol et al., 2008). The

C.D. Ramotoroko et al. / Journal of African Earth Sciences 123 (2016) 39e5640

Madibe-Kraaipan granite-greenstone terrane occurs on the centralpart of this domain in both countries outcropping sporadically forover 200 km (Fig. 1; Anhaeusser and Walraven, 1999; Mapeo et al.,2004). In South Africa, the greenstone belts host operating gold andplatinum mines (Fig. 1; Hammond and Moore, 2006; Lewins et al.,2008), and several geological, geochronological, petrological andsome preliminary geophysical studies have been undertaken(Stettler et al., 1990, 1997; Anhaeusser and Walraven, 1999; Poujolet al., 2002, 2008) with the view to better understand its tectonic

Figure 1. Simplified regional geology map of the Madibe-Kraaipan granite-greenstone terWalraven, 1999; Hammond and Moore, 2006). The bold box outlines the study area. Thshowing other greenstone belts in South Africa (after Schmitz et al., 2004). A- Amalia, K- K

evolution and full mineral potential. However, the geophysicalstudies in South Africa have been largely qualitative in nature,while to date the Botswana segment of this important terrane isless investigated, and its mineral potential under-estimated, mainlydue to the Kalahari cover. Only limited geological mapping,geochemistry, geochronological and geophysical studies which areisolated and restricted to areas of outcrop exist (Mapeo, 1990, 1998;Gould et al., 1987; Mapeo et al., 2004; Walker et al., 2010). There isno regional synthesis of geoscience data to complement the studies

rane, southeast Botswana and adjacent South Africa (Modified after Anhaeusser ande inset shows the location of the study area (box) within the Kaapvaal craton, alsoraaipan, M- Madibe, and S- Stella.

C.D. Ramotoroko et al. / Journal of African Earth Sciences 123 (2016) 39e56 41

in South Africa.The main aim of this study is to close these gaps and get a better

insight into the characteristics of the area between 24�E to 26�Eand 25�S to 26.5�S (Fig. 1) from an overall view of its gravity andmagnetic fields. This area roughly equates to the ‘Kraaipan northerndomain’ of Anhaeusser and Walraven (1999); north of Delareyvilleplus the rest of the Botswana sector (Fig. 1). In particular, the po-tential field data are processed and enhanced to: 1) map the areal/spatial extent (regional distribution) of the greenstone belts withspecial emphasis in Botswana, 2) derive a structural interpretationof the region, 3) determine the geometry or configuration of thegreenstone belts and plutons from (quantitative) modelling, and 4)assess/evaluate the tectonic evolution of the region. Understandingthe geology and tectonics of a region is crucial to mineral explo-ration area selection. Tectonic settings are related to the majorgeological features, which can be associatedwith economicmineraldeposits depending on the styles and types of mineralization(Mitchell and Garson, 1984; Hutchison, 1985; Goleby et al., 2004;Groves and Bierlein, 2007). This information can be obtainedfrom potential field geophysical data at moderate expense, as theyrespond to density and magnetization contrasts at both the surfaceand subsurface (e.g., Jaques et al., 1997; Gibson and Millegan, 1998;Wellman, 2000). The methods can be used to identify rock unitsand major structures by applying various filter enhancements todisplay anomalies as images with improved information content(Milligan and Gunn, 1997; Reeves et al., 1997; Lyatsky et al., 2005).

2. Regional geologic and tectonic setting

The Archaean Kaapvaal Craton is believed to extend westwardsunderneath the Kalahari rocks probably up to the northerlytrending Kalahari line in southwestern Botswana (Meixner andPeart, 1984; Schmitz et al., 2004, Fig. 1). The Madibe-Kraaipangranite-greenstone terrane constitutes the western central part ofthe Kaapvaal Craton which comprises greenstones, felsic gneiss,migmatite and unfoliated granite. Important (partly mapped)regional geological units in the study area include the Stella-Kraaipan-Madibe greenstone belts, Mosita granite, the Mma-thethe granite, the Transvaal Supergroup (Kanye Volcanic Forma-tions) as well as Tertiary-Recent Kalahari sediments (Mapeo, 1998;Anhaeusser and Walraven, 1999; Mapeo et al., 2004).

There are several published reports regarding the geology in theMadibe-Kraaipan terrane and neighbouring areas; a number ofgeological reports and geological maps at 1:125 000 scale havebeen published by many investigators from Department ofGeological Surveys (DGS) and Council for Geosciences (CGS). Keyand Ayres (2000) compiled the geology of Botswana, inclusive ofMadibe-Kraaipan area at 1:1250000 scale. Aldiss (1985), Gouldet al. (1987), Mapeo (1990, 1998), Anhaeusser and Walraven(1999), Mapeo et al. (2004), Poujol et al. (2002, 2008) and Walker

Table 1Summary lithostratigraphic table of the study area.

Rock type Age (Ma)

Segwagwa-Masoke Igneous Complex 2054 ± 9Late-stage granites 2665 ± 1.8Kanye Volcanic formation 2783.8 ± 1.1Mmathethe granite 2775.2 ± 7.4Mosita adamelite 2749 ± 3 to

Kraaipan granodiorite 2846 ± 22 to

Basement gneisses 2816 ± 16 to

Kraaipan Group BIF 3410 ± 61/64

et al. (2010), have undertaken regional and local geological map-ping in the study area, and readers more interested in the detailscan refer to these publications.

The generalized lithostratigraphy in the area is summarised inTable 1. Rock sequences begin with Archaean metamorphosedmafic volcanic rocks interbedded with ferruginous and phylliticmetasedimentary rocks, which outcrop intermittently over a dis-tance of 200 km from north of Vryburg in South Africa to Mma-thethe in southern Botswana (Fig. 1; Mapeo, 1998; Anhaeusser andWalraven, 1999). The greenstones consist of three narrow NNW-striking belts (Stella, Kraaipan, Madibe; Fig. 1) dominated bymafic metavolcanic rocks interlayered with ferruginous and sili-ceous metasedimentary rocks, mainly BIF and ferruginous chert(Mapeo et al., 2004; Hammond and Moore, 2006). In Botswana,banded iron formation outcrops have been discovered over theNeS striking Mosi ridge which stretches approximately 20 kmwithsome minor outcrops in the Molopo River (along the border) to thesouth (Aldiss, 1985).

The surrounding granitoid rocks include tonalitic and trondh-jemitic gneisses (TTG), granodiorites and adamellites (Anhaeusserand Walraven, 1999). The granitoids associated with the Kraaipangreenstone belts yielded ages between 3250 and 2735 Ma(Anhaeusser andWalraven, 1999) and recent SHRIMP UePb ages of2929 ± 9 Ma and 2943 ± 9 Ma for granites adjacent to the Madibe-Kraaipan Belt of southeast Botswana are believed to date the as-sembly of the Kaapvaal Craton (Mapeo et al., 2004).

The regional structure of the study area incorporates the Kaap-vaal cratonic block, which forms the core of the basement geology ofthe region. The Kraaipan Group ranks amongst the oldest rock unitsin southern Africa and shows a distinctively NeS trend bothgeologically and geophysically in contrast to the general ENE-WNWtrend in the east of the craton (Fig. 1 and Table 1; deWit et al., 1992;Stettler et al., 1997; Anhaeusser and Walraven, 1999; Mapeo et al.,2004). The Kraaipan rocks are thought to belong to a complex syn-clinal structure with a minor central anticline (Anhaeusser andWalraven, 1999). Gravity and magnetic data show two major NeStrending belts (Stella and Kraaipan) forming part of an eastwarddipping syncline that closes southwards (horseshoe shape, Fig. 1)where it forms theAmalia belt (Stettler et al.,1990,1997). The terraneis intruded by the late (Neoarchaean) granitic suites that include A-type granitoids, anorthosites, rhyolites and subsidiary mafic rocks(Anhaeusser and Walraven, 1999; Poujol et al., 2002, 2008), repre-sented by the Mmathethe and Mosita granites in the study area(Mapeo et al., 2004). It has beenproposed that theKraaipan volcano-sedimentary greenstone belt and associated granitoids evolved as aresult of episodic accretion of juvenile crust on to the westernboundaryof theKaapvaalCratonbetweenca. 3010and2790Ma (e.g.,Anhaeusser and Walraven, 1999; Mapeo et al., 2004; Poujol et al.,2008). The mode of assembly and regional architecture of thisterrane remain important questions to be answered.

Source/reference

Mapeo and Wingate (2009)Stettler et al. (1997)Mapeo et al. (2004)Mapeo et al. (2004)

2791 ± 8 Anhaeusser and Walraven (1999)Poujol et al. (2002)

2915 ± 15 Anhaeusser and Walraven (1999)Poujol et al. (2002)

3070 ± 7 Anhaeusser and Walraven (1999)Poujol et al. (2002)Anhaeusser and Walraven (1999)

3. Methodology

3.1. Aeromagnetic data collection and merging

The aeromagnetic data used in this study were obtained fromthe DGS (Botswana) and CGS (South Africa). Regional aeromagneticsurveys in Botswana have been performed since 1962 by interna-tional companies contracted by the DGS at different times, some-times with different parameters, technologies and acquisitionspecifications. The Botswana data for the study area come from asurvey flown and compiled by Geosurvey International G.m.b.H in1990. It was carried out at a mean terrane clearance of 150 mwith anominal NeS traverse line spacing of 1 km and control linesrunning from E-W (15 km apart). The magnetic data weremeasured using a Caesium Vapour (Scintrex Y201) magnetometer(sensitivity 0.01 gamma) mounted in the aircraft tail and recordeddigitally using PICODAS data logging system. The flight navigationsystem used was the Global Positioning System (GPS) supple-mented by the Doppler system. A video camera was used tocontinuously record the path of the aircraft. Barometric and radaraltimeters were used for altitude determination. A base stationcaesium vapour magnetometer was installed in a magneticallyquiet environment to monitor the diurnal variations.

The situation is quite similar in neighbouring South Africa withthe aeromagnetic data obtained from the CGS as a grid in the

Figure 2. Reduced to the pole aeromagnetic survey map of the study area. The data valuewavelength core field (IGRF) removed. Solid black line is the border between South Africa

Transverse Mercator 25 system compiled frommany surveys by theCGS or contracted companies and stretching in time from 1965 to1980 (E Stettler, pers. Comm., 2000). Flight directions were E-WandNeS, approximately perpendicular to the dominant geologicaltrends in each area (that of greenstone belts). The nominal flyingheight was 100 ± 15 m, line spacing 1 km, tie lines 10 km, andproton magnetometer measuring at 1 s (or ~ every 63 m at flyingspeed of 250 km per hour). The different data sets were epochcorrected to 1 July 1975 (Stettler, pers. comm., 2000).

The data sets from the countries partially overlapped but vary inflight-line spacing and elevation. Due to the changing main fieldfrom the Earth’s core, and due to differences in quality andcoverage, combining these data to a consistent regional magneticanomaly grid was challenging. Integrationwas therefore performedthrough several procedures including continuation between gen-eral surfaces (e.g., Barritt, 1993; Finn, 1999). The levelling of indi-vidual surveys was performed using a combined computer-manualmethod by first subtracting the International Geomagnetic Refer-ence Field (IGRF) for the year of the survey for each survey grid.Then the difference at the boundary of adjacent surveys wasremoved using a low-order polynomial, with the remaining errorslocally smoothed out where required (cf. Dumont et al., 1997). Thelevelled data were re-gridded in the UTM co-ordinate system at250 m cell size, i.e. equal to ¼ of the nominal line spacing, using abidirectional algorithm (Smith and Wessel, 1990). They were then

s are total field amplitude (in nanoTesla, nT) of Earth’s magnetic field, with the long-in the south and Botswana in the north.

reduced to the pole (RTP, Fig. 2) to correct for the effect of themagnetic inclination using algorithms that cater for both high andlow magnetic latitudes (Geosoft, 2013). The RTP filter reconstructsthe magnetic field of a data set as if it were at the pole (verticalmagnetic inclination and a declination of zero). Since the study areahas an inclination of about �65�, the reduction to the pole may berelatively accurate. RTP greatly simplifies the interpretation ofmagnetic data as it produces anomaly maps that can be morereadily correlated to the near surface geology (Blakely, 1995, p330;Milligan and Gunn, 1997); similar to our targets.

3.2. Gravity survey, data reduction and merging

The gravity data used in this study includes two major sets; thefirst pre-existing obtained from the DGS and CGS together with theaeromagnetic data, and the second acquired in this study. The firstset includes data acquired over several years of gravity surveys inBotswana and South Africa by government agencies and supple-mented in certain areas by several minerals exploration companies(e.g. Stettler et al., 1990, 1997; McMullan et al., 1995; de Beer andStettler, 2009). Most of these earlier data were acquired alongexisting roads and motorable tracks, with station elevations beingdetermined by barometric altimeters and/or autonomous GPS.Height network closure errors were adjusted by themethod of leastsquares, resulting in a precision ranging from 5 m to 2 m (e.g.McMullan et al., 1995). Station positioning was made using 1:50 000 topographic map sheets with an uncertainty of 50 m orautonomous GPS (20 m). The data were reduced to Bougueranomaly values using the 1967 International Gravity Formula(Moritz, 1984) and a reduction density of 2670 kg m�3. Terraincorrections were applied where the topography was deemed veryrugged (e.g., de Beer and Stettler, 2009). Although variable, the totalaccuracy of the calculated gravity anomalies in this set is placed at 2mGal, being the accuracy of the least precise older surveys.

The gravity observation stations in South Africa are in a randompattern, and the spacing varies between 3 and 5 km (Stettler et al.,1990, 1997), with a few, more detailed profiles across the green-stone belts (Fig. 3). The older reconnaissance and regional gravitysurveys encompassing the study area in Botswana are generallybased on the DGS gravity-data standard of one station per 10 squarekm (e.g., McMullan et al., 1995), with the exception of the densecoverage over the Molopo Ultramafic Complex (Fig. 3; Gould et al.,1987; Walker et al., 2010). They were considered too widely spaced(Fig. 3) to provide reliable interpretation at 1: 50 000 scale; they arenot adequate for the modelling and defining the subsurface struc-ture and configuration of the greenstone belt required in this study.A gravity survey was therefore undertaken to improve the datadistribution in Botswana. A software-controlled Scintrex CG5Autograv gravimeter was used for gravity data acquisition alongroads and motorable tracks in the area. The fieldwork was con-ducted in two two-week field expeditions from Gaborone, with theIGSN71 station in Lobatse as the primary base. In order to be able toreduce the gravity values to the same datum as countrywide surveyand to tie this present survey to the previous surveys in the studyarea, and also to correct for long term drift effects several readingswere taken at each of these base stations at the commencement,during and the end of each daily survey.

Traverses across the greenstone belt were 50 kme80 km long,with station spacing of 2 km over the belts and 3e4 km over thegranitic terrain (Fig. 3). The survey navigation was primarily byvisual means from geological maps (1: 100 000) and topographicmaps (1: 250 000; 1: 50 000). With the use of vehicle odometer,station spacings were approximated. Physical features such as pansand rivers, fences and gates, road intersections or junctions, set-tlements and schools and major roads were also used to fix station

locations established. The elevation of the gravity stations weredetermined simultaneously with the gravity readings by deployingDifferential GPS (DGPS) using trigonometrical beacons as controlpoints. Two different units were used for ‘correlation’; a Trimble5700/5800 GPS system and Mobile Mapper 120, both designed forhigh precision survey (submeter; < 1 m), positioning and naviga-tional applications. The positional accuracy was maintained at ±2 m and elevation accuracy was maintained at ± 50 cm with post-processing.

Corrections were made during data reduction to remove theeffects of latitude, changes in elevation and topographic effects.Bouguer anomalies were calculated against the InternationalGravity Formula 1967 (Moritz, 1984) and referred to the Interna-tional Gravity Standardisation Net 1971 station at the DGS inLobatse. Data were reduced using a Bouguer correction density of2670 kg m�3. Terrain corrections were not applied as they wereestimated to be small (cf. Gould et al., 1987). A total of one hundredand fifty (150) new gravity data points collected were merged withother existing gravity data from Botswana and South Africa toproduce a satisfactory data distribution (Fig. 3).

Finally a grid of the Bouguer anomaly was produced at 3.0 kmcell size using a minimum curvature algorithm (Smith and Wessel,1990) for detailed interpretation and modelling of some geologicalfeatures in the study area. One of the advantages of properlyreferenced (IGSN71) gravity data is the ease of integration of oldand new data; and the result also lends itself to simpleenhancements.

3.3. Potential field data processing

Gravity and magnetic anomalies remaining after appropriatecorrections have been applied to represent the superimposed effectof magnetization and density changes at various depths. Thesmoothness or apparent wavelength of the anomaly is generally anindication of the depth of the source of the anomaly; the smootherthe anomaly, the deeper the source (Telford et al., 1990, p26). In thisstudy it is necessary to separate these longer wavelength anomaliesfrom the sharper, shorter wavelength anomalies that are due tonear surface geology. Gravity data are commonly used to determinethe major crustal architecture necessary to establish a suitableregional framework, while shorter wavelength magnetic anomaliesreflect near-surface heterogeneity (e.g., Jaques et al., 1997;Wellman, 2000; Lyatsky et al., 2005; Aitken et al., 2008; Gwavavaand Ranganai, 2009; Gallardo and Thebaud, 2012). A number ofcommonly applied potential field transformations and image pro-cessing techniques were applied to both data sets to enhance bothshallow, short wavelength features for lithological contact andstructural mapping as well as mediumwavelengths for the purposeof regional crustal structure (Lyatsky et al., 2005). These includeshaded relief imaging, vertical and horizontal derivatives, analyticsignal, apparent density/susceptibility mapping, upward continu-ation, automatic gain control, directional cosine filter, 3D Eulerdeconvolution and spectral analysis (e.g., Blakely, 1995, p303;Milligan and Gunn, 1997; Reeves et al., 1997). Each technique helpsto display the edges of source bodies and lateral contrasts inphysical property, which are mainly caused by lithological andstructural changes in the buried basement. Only those techniqueswith maps/figures presented or discussed are briefly describedbelow.

Shaded relief maps treat the potential field data as topographyilluminated from different selected directions, thus highlightingsome of the structural details perpendicular to the illuminationdirection (e.g., Broome, 1990). Another style of presentation is tooverprint the shaded relief onto the colour map to produce acombined colour shadow map. These maps are particularly

Figure 3. Distributions of gravity stations in the study area (þrepresents existing data in Botswana, circle represents gravity points in South Africa and red triangle representsgravity data acquired in this study). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

effective as they contain information on both anomaly amplitude(colour) and anomaly gradient (relief); the latter also relates to thedepth of burial of the causative structures (Jaques et al., 1997;Reeves et al., 1997). The combined maps also display both shortwavelength (shaded relief) and long wavelength (colour raster)features enabling themapping of nonlinearmagnetic features, suchas irregular anomalies associated with plutons and/or sills (c.f.Reeves et al., 1997).

Derivatives are particularly useful as they remove or suppressthe regional trends in the data, and can also be used for locating thedetailed structural geology (lineaments) of the area (e.g., Lyatskyet al., 2005; Bierlein et al., 2006). The analytic signal of magneticand gravity anomaly is a combination of the vertical and horizontalderivatives, and for magnetic data, it has the useful property ofbeing independent of the magnetization direction of the causativebody (Roest et al., 1992; MacLeod et al., 1993; Blakely, 1995, p355).Furthermore, as the peaks of analytic signal function are symmet-rical and occur directly over the edges of wide bodies and directlyover the centres of narrow bodies, interpretation of analytic signal

maps and images should, in principle, provide simple, easily un-derstood indications of dense or magnetic source geometry (e.g.,Jaques et al., 1997). Standard 3D Euler deconvolution also calculatesfrom the magnetic/gravity gradients in the x, y, and z direction theboundary of a causative unit and the depth to the boundary (Reidet al., 1990), thus fully locating the unit. However, care should betaken in data preparation and selection of processing parameters(Reid et al., 2014; Reid and Thurston, 2014).

The logarithm of the radial average of the energy spectrumgenerated in the Fourier transformation process (Blakely, 1995,p303) can be plotted against the radial wavenumber for furtherdepth analysis. The slope of the linear segments of the 2D radiallyaveraged spectrum curve provides information about the depth tothe top of an ensemble of magnetic or gravity bodies (Spector andGrant, 1970). For wavenumbers in cycles/km, the depth h is thencalculated from the relation: slope ¼ �4 h (Spector and Grant,1970). A typical energy spectrum for magnetic data may exhibitthree parts; a deep source component, a shallow source componentand a noise component.

Finally, in addition to the structural, lithological (contact) and/orlocation mapping, quantitative modelling is undertaken to deter-mine the possible depth, shape, size, and magnetization or densityof local anomalies. Theoretically, two reversed operations are per-formed sequentially (Telford et al., 1990, p46); the first is a directmodelling process and the second is an inverse modelling process.The direct modelling process transforms the variations reflected bypotential field data in an area of study. The inverse modellingprocess matches the calculated potential effects resulting from theinferred potential models with the observed one. In this study23/4D modelling was performed using Geosoft’s GM-SYS softwarebased on algorithms described by Won and Bevis (1987).

4. Results and interpretation

4.1. Aeromagnetic data analysis

In this paper all aeromagnetic results are displayed as digitalimage maps. The colour-shaded residual magnetic intensity,

Figure 4. Aeromagnetic colour-shadow map of the study area (inclination and declination 4Stella and Kraaipan belts. MUC: Molopo Ultramafic Complex.

vertical derivative and analytic signal maps are discussed below, inthat order.

The data in Fig. 4 has the IGRF removed and RTP applied, andthus reflect near-surface geology. Magnetic intensity level rangesfrom around �330.00 nT to over 200.00 nT. The map shows rathercomplex crustal magnetization pattern with different locations ofhigh and low magnetic intensities. It exhibits some different typesof positive (magenta and red colours) and negative (light and darkblue colours) anomalies.

Themap can generally be subdivided into threemagnetic zones;a western half of much higher signatures that culminate in a largepositive over the Molopo Ultramafic or Igneous Complex (MUC) inthe NW corner of the mapwith some lows in-between, followed bya large area of subdued signatures with significant lows over thenorth-central part, and finally a small area of moderate/interme-diate to high values in the southeast. There is suggestion of apossible subsurface connection between the Stella belt and MUC;this has important implications on the lithology of the former asdiscussed later. Linear, relatively narrow, magnetic highs correlate

5�). Conspicuous ENE-trending dykes and NeS trending highs over the location of the

well with the BIF/magnetite quartzite outcrop pattern, furtherrevealing sub-outcropping sections. A NeS striking elliptical (sub-oval/elongate) high is observed over the Segwagwa Complex whichis genetically related to the Bushveld Igneous Complex but mainlycomprising granitic, syenitic and dioritic rocks (Mapeo andWingate, 2009). Structurally, there are several ENE-trendinglinear magnetic highs in the southern half of the map, a fewWNW-to-EW-trending highs in the east-central part, and NNW-trending highs in the northeast, all probably corresponding todykes? Comparison of the aeromagnetic and geology maps bringsout some interesting observations: only two NeS trending linearmagnetic highs coincide with the Stella and Kraaipan belts whilethe Madibe belt is not showing/represented; probably alsoimplying different lithology from the BIF/magnetite quartzite. Alsonotable is a magnetic low over the mapped outcrop of the Mositagranite/adamelite south of Mabule (Fig. 4), which is not surprisingas it reportedly consists mainly of non-magnetic minerals such asquartz, perthite, microline, albite with accessory amounts of chlo-rite, zircon and magnetite (Anhaeusser and Walraven, 1999).Accordingly, the western and the central parts of the map area areoccupied by two major magnetic zones which need to be furtherinterpreted.

Several kinds of derivative maps are applied in this study andthe first vertical derivative is shown in Fig. 5, revealing more

Figure 5. Aeromagnetic survey map of the sur

significant magnetic patterns such as linear structures or linea-ments. The lithological unit or zone boundaries are quite clearwhen using the first vertical or horizontal derivatives. The twomajor NeS trending linear structures across the centre of the mapcorrelating with the Stella and Kraaipan belts are now moredistinct. As shown in Fig. 5, the banded iron formation (BIF) bodies(Fig. 1) are clearly distinguished from other linear patterns. SeveralWNW, ENE and E-W trending lineaments cross-cut the major NeStrending anomalies in the central part of the mapped area. Goodexamples are between Digawane and Kanye and around Phitshane-Molopo-Mafikeng villages (Fig. 5). Some NNW-to-NW trendinghighs are observed in the northeast from Digawane to Kanye, withsimilar trends in the southeast (NW of Delareyville, Fig. 5).

In general, the regional geological lithologies and structuresshown in the IGRF map are quite more distinct in the vertical de-rivative map, including continuity between the Stella belt and theMolopo Complex. NNW-trending structures in the northeast andsoutheast corners of the map could correlate to the Madibe belt, orpost-Kanye volcanic dykes. However, there is a bit of noise on thismap suggesting that care should be taken when working with (i.e.,processing) merged datasets that combine surveys with differentacquisition specifications.

The NeS to NNW-trending structures, as well as the ENE-trending presumed dykes, are more outstanding on the analytic

vey area showing first vertical derivative.

signal map (Fig. 6). The map elucidates two narrow majoroutstanding magnetic belts trending in the NeS direction over theStella and Kraaipan belts, with a total length of about 180 km fromSouth Africa to Tshaane and Gasita through Mabule and Phitshane-Molopo respectively. Also, positive anomalies trending approxi-mately NNWoccupyingmainly the northeastern part are nowmoresignificant to those of the RTP and derivative maps (Figs. 4 and 5).Interestingly, the rim of the Segwagwa Complex shows highs onASA and this could be related to the BIF mapped (Mapeo andWingate, 2009) in these areas, or other ferromagnesian minerals(hornblende, amphibole, magnetite) present. The rest of the ana-lytic signal map is covered with relatively lowmagnetic intensities.

4.2. Gravity data analysis

Gravity and aeromagnetic data are generally complementary asthe former delineate the position of boundaries at deeper crustallevels than the latter (e.g., Jaques et al., 1997; Wellman, 2000;Lyatsky et al., 2005; Gwavava and Ranganai, 2009). In this paperthe gravity data are displayed as Bouguer gravity anomaly map,

Figure 6. Analytic signal (total gradient amplitude) image of total magn

residual gravity anomaly map and analytic signal.The Bouguer gravity anomaly map (Fig. 7) reveals well defined

gravity highs and lows of varying dimensions and relief, and largelysupports the aeromagnetic maps (Figs. 4e6) with the NeS regionaltrend. However, the two main roughly NeS trending linear gravityhighs which dominate the central part of the study area correlatewith the Stella and Madibe belts, while the location of the Kraaipanbelt coincides with a gravity low in the south (centred at Kraaipan,Fig. 7). This implies that the Kraaipan ‘granodiorite/adamelite’(Anhaeusser and Walraven, 1999) is much more extensive thanpreviously recognized as its associated gravity low overshadowsthe greenstone response in that area. Further, the whole northernhalf of the study area is associated with a general gravity high, withpeak values reaching over �100 mGal, which is the strongestanomaly within the study area, in the central area around Tswaa-neng (Fig. 7). An apparent Bouguer gravity-high belt is associatedwith these areas; at Phitshane (Molopo) and Tswaaneng villagesthe gravity anomaly map is composed of oval highs. The SegwagwaComplex is also associated with a NeS elliptical gravity high, whichis unusual for the largely granitic complex, which probably points

etic intensity of the study area showing the major magnetic units.

Figure 7. Simple Bouguer gravity anomaly map of study area also showing the location of the modelled profile sections (cf Fig. 3) A-A0 in Botswana and BeB0 in South Africa (section4.5). Note the general gravity high in the northern half of the map and low in the south with two distinct NeS highs over Stella and Madibe belts.

to larger proportions of syenite. In general, the pattern of alter-nating elongated NeS trending gravity highs and lows in SouthAfrica essentially end up with the approximately E-W high inBotswana, as previously noted by other workers who observed a‘high plateau’ in the border area (e.g., Stettler et al., 1990). There arealso several gravity lows with peak magnitudes of less than �140mGal recognized in this map. The map reveals prominent gravitylows in circular and oval-to-elongated shapes distributed at thecentral and southern parts of the mapped area. For example, aconspicuous circular gravity low (typical of granitic plutons?) isnoticed at Sekhutlane village, and a lesser one southeast ofMmakgori, the latter correlating with the Kraaipan granodiorite onthe geology map. The southeast corner of intermediate/moderatemagnetic signatures (Figs. 4 and 5) is here associated with a sig-nificant gravity low, suggesting existence of a K-rich pluton. Thesouthwest corner also reflects a gravity low and a small NNW-trending low over the Mosita granite east of Piet Plessis (Fig. 7).

The above observations are partly amplified in the residualgravity map (Fig. 8) obtained by subtracting a regional of �120mGal from the Bouguer gravity map. The regional value wasdetermined by upward continuing the Bouguer data to 5000 mbased on the average greenstone belt depth extent in South Africa(Stettler et al., 1997; de Beer and Stettler, 2009) and taking the

mean of the data. The two linear highs over the greenstone belts aremuch clearer and both show sharp gradients implying sub-verticalcontacts with the granites. The gravity high associated with theMadibe belt is shifted further east to encompass the Mafikeng area,beyond the ‘mapped’ extent (cf Fig. 1). The Stella and Kraaipan beltsappear connected at depth in the north, around Mabule-Sedibeng-Tswaaneng, separated by a pluton southeast of Mmakgori. Here,both the Stella and Kraaipan belts appear to be ‘resolved’ aroundthe Kraaipan granodiorite. The Stella belt is further linked to theMUC in the subsurface but broken by pluton intrusion at Sekhut-lane (associated with a distinctive circular low), which could againimply similar lithology in the two units. The Madibe belt ends as alarge zone of gravity high between Lobatse and Gasita (though datacontrol is not very good, Fig. 3). There is evidence of dense but non-magnetic rocks in most of the north-central part as this area cor-relates with a general magnetic low.

Well-defined gravity lows are associated with several (multiple)granitic intrusive bodies. The residual gravity low over Mositagranite/adamelite adjacent to the Stella belt now extends intoBotswana, with ‘minimum’ coincidingwithmapped outcrop area inthe south (east of Piet Plessis, Fig. 8). Furthermore, the gravity lowaround Kraaipan in the Bouguer map (Fig. 7) now appears to beresolved into two separate anomalies, a large part with a NeS trend

to the northeast and a smaller ‘lobe’ NNE-trending to the south-west. The ‘break’ between these low gravity values approximatelycoincides with the Kraaipan BIF outcrop (cf Fig. 1). The SegwagwaComplex gravity high in the north is further highlighted, and sug-gesting a NNE trend than the NNW trend on the magnetic map.

The analytic signal amplitude was calculated in order to try andimprove the definition of dense rocks better than the Bouguer andresidual gravity maps. It should be ‘noted’ that the gradientamplitude peaks at edges of ‘wide’ bodies and centres of narrowbodies (e.g., Blakely, 1995, p355; Reeves et al., 1997). The analyticsignal gravity map (Fig. 9) outlines all the three main linear massiveunits of the Madibe-Kraaipan granite-greenstone terrane as well asthe large body around Tswaaneng, and best shows the extent of thegreenstone belt in Botswana. However, the Kraaipan and Madibebelts are not clearly resolved, while Stella is well outlined.

4.3. Lithological and structural analysis

In this study, extraction of the lithological boundaries, geolog-ical structures and smaller-scale lineaments was based oncombining multiple data attributes from the vertical and horizontalgradients, analytic signal and several other maps. By combining theobservations from various attribute maps, an interpretation map(Fig. 10) depicting the three major greenstone limbs and many

Figure 8. Residual gravity anomaly map obtained by subtracting a mean regional of �110 mGcontinuation of the Bouguer values to 5 km (average greenstone belt depth extent, Stettler

cross-cutting lineaments was prepared.To correlate magnetic anomalies with rock units, it is note-

worthy that sedimentary rocks are generally non-magnetic,whereas igneous rocks rich in iron and magnesium (mafic to ul-tramafic) tend to be very magnetic (Telford et al., 1990, p74).Granite intrusions and hornfels contact aureoles can also be mag-netic (e.g., Reynolds et al., 1990; Clark, 1997). On the other hand,gravity reflects density variations from larger volume and deepercrustal levels but have lower resolution (e.g., Stettler et al., 1997;Wellman, 2000; Gwavava and Ranganai, 2009). Accordingly, grav-ity evidence shows that the Kraaipan greenstone rocks are exten-sively developed under Kalahari cover in the north-central partthan previously recognized. Predominantly low magnetic signa-tures suggest that these are metavolcanics rather than BIF/magnetite quartzite. This is also supported by their wide/broaddistribution (spatial extent) unlike the latter that tend to occur asnarrow linear features (cf Stettler et al., 1997). The major exceptionis the NeS trending Segwagwa Complex with coincided gravity andmagnetic highs, explained by presence of hornblende-biotite sye-nite-diorite and BIF rim reported by Mapeo and Wingate (2009).The BIF outcrop (Morsi ridge, Aldiss,1985) fromPitshane-Sedibeng-Metlobo constitutes the northern extension of the Kraaipan belt.The subsurface link between the Stella belt and MUC apparent inboth gravity and magnetic maps suggests that the former is

al from the simple Bouguer gravity values. The regional value was obtained by upwardet al., 1997; de Beer and Stettler, 2009).

Figure 9. Analytic signal (total gradient amplitude) of the gravity data of the survey area.

predominantly mafic/ultramafic in composition. Similarly, possiblelink betweenMadibe and zone of gravity plateau, coupled with lowmagnetic signatures for both implies metalvolcanics dominate. Onthe other hand, K-rich plutons generally show up as high magneticand low gravity regions while Na-rich plutons are associated withlow-to-no gravity/magnetic anomalies (e.g., Clark, 1997; Ranganaiet al., 2008; Ranganai, 2013). Cross-cutting structures are alsousually either dykes with magnetic highs as opposed to faults withmagnetic lows (cf Jaques et al., 1997; Ranganai and Ebinger, 2008).

4.4. Euler depth and power spectrum analysis

The 3D Euler deconvolution method was used in an attempt toimprove the mapping of the Madibe-Kraaipan granite-greenstoneterrane at depth, for delineating contacts between the sedimentaryrocks and basement rocks, and rapid depth estimation. The qualityof the depth estimation depends mostly on the choice of the properstructural index and adequate sampling of the data (Reid et al.,1990, 2014). Structural indices SI ¼ 0, 1 and 2 and window size of12 grid cells (3000 m) were used for interpreting contacts, faultsand other curvilinear source types. The Euler solutions maps (e.g.,Fig. 11), reveal that the depth to the basement increases to just over2.0 km throughout the survey area. The majority linear solutionscorresponding to greenstone belt and major dykes have a1.5e2.0 km depth range. As the Euler method relies on the gradient

of the magnetic field, the resulting depth readings relate primarilyto the areas of basement heterogeneities, and thus only the localmaxima in the resulting depth maps should be taken into consid-eration (cf Reid et al., 2014). The boundaries of the primary base-ment elements and major domains were not recognizable in theEuler depth map.

The 2-D radially averaged power spectra of all geophysical datasets over the study area were calculated and analysed. A radiallyaveraged two-dimensional power spectrum curve for the magneticdata is shown in Fig. 12. Based on the appearance of the spectrum,(i.e. change in the slope of the spectrum curve), the spectrum isdivided into three components, the deep or regional componentdominates the low wavenumber and the shallow or near-surfacecomponent dominates the high wavenumber.

Threemain average ensemble interfaces at depths of 0.7 km (S3),1.99 km (S2) and 4.8 km (S1) below themeasuring level are revealedfor shallowest layer, intermediate sources and for the deep ensemble(base of greenstone belt?) respectively. The shallowest depth is themost significant in this case as it probably relates to depth to top ofmagnetic rocks below the approximately 0.5 km thick Kalahari sed-iments. Possible magnetic basement is at 8.2 km for the entire area.

Quantitative results of the above two methods, together withavailable geological and petrophysical information, are used to helpbuild 2¾D structural models to help in understanding the subsur-face structure of the study area.

4.5. 23/4D gravity modelling

The configurations of the main geological units in the study areawere determined along two selected regional profiles (see Fig. 7)with data sampled from the Bouguer gravity map along detailed

Figure 10. Geophysical interpretation map showing major geologica

field traverses (~2 km spacing). The profiles A-A0 (Botswana) andBeB0 (South Africa) are crossing the central gravity anomaly causedby the greenstone belts.

The density contrast values for the various rock units and thebasement were estimated from Telford et al. (1990), p16, Stettler

l formations and structures in the Madibe-Kraaipan study area.

Figure 11. Classified symbol plot of 3D Euler deconvolution solutions (SI ¼ 1, consistent with a thin dike or sill) overlain on the IGRF corrected map of the survey area.

et al. (1997), Mare and Oosthuizen (2000) and Peschler et al. (2004).Regional gravity structural cross-sections (Figs. 13 and 14) weremodelled using the 23/4D-forward modelling technique. The grav-ity field was calculated iteratively for these geological models, untila good fit was reached between the observed (dots) and calculated(line) profiles. Different density values (within the standard devi-ation of ±30 kg/m3) were tested but these did not significantlychange the shapes and depth extents of the units (cf Peschler et al.,2004; Gwavava and Ranganai, 2009). The two gravity modelsproduced for the greenstone belts and plutons have, in general,similar geometries with steep-sided bodies within the graniticcrust (Figs. 13 and 14). The models show different shapes for the

Figure 12. Radially averaged power spectrum of the IGRF corrected magnetic data. Thedepth to a statistical ensemble of sources is determined from the expression:slope ¼ �4ph.

units, and a maximum depth extent of 4.7 km for the greenstonesand 2.0 km for the plutons in profile A-A0 in Botswana, and 4.5 kmand 4.4 km respectively in profile BeB0 in South Africa.

In bothmodels, the Stella and Kraaipan belts were modelled as asingle unit with Madibe separate in accordance with Bouguer andresidual gravity maps. For BeB0, Stella-Madibe shows relatively flatbase whereas A-A0 has three ‘roots’, probably corresponding to thelinear magnetic horizons. Joint gravity/magnetic modelling isimportant (cf Gallardo and Thebaud, 2012) but previous workindicated presence of remanent magnetization on the BIFs(Anhaeusser and Walraven, 1999), thereby complicating and pre-cluding the exercise. The plutons are either relatively wide and flat-bottomed or narrow diapirs (cylindrical form) of varying thickness(cf Ranganai, 2013).

The configuration of the greenstone belts and adjacent plutonsrevealed in this study allow us to examine different geodynamicmodels and/or theories explaining their geotectonic evolution.

5. Discussion

Geophysical aspects of granite-greenstone terranes havereceived little attention when compared to the voluminousgeological accounts of these regions (Stettler et al., 1997;Anhaeusser, 2014). However, to explore covered areas, we need torely on modern techniques to redefine the tectonic elements and toidentify target areas for mineral exploration. The present studyaddresses this situation with particular reference to the westerndomain of the Kaapvaal Craton in southern Africa. In this regard,gravity and magnetic data have helped elucidate the subsurface

Figure 13. Madibe-Kraaipan Greenstone Belt 23/4 D Gravity model along Profile A-A0 (Botswana) of the Bouguer anomaly map. The numbers inside the model bodies are densities inkg m�3. Densities not shown are for sediments 2500 kg m�3, granitic pluton 2620 kg m�3 and granitic gneiss 2750 kg m�3. Vertical Exaggeration (V.E) ¼ 11.36.

crustal structure and geotectonic evolution of the Kraaipan terranewhich is largely under cover.

Clearly evident in both datasets (Figs. 4e9) is the prominent/dominant NeS trend defined by alternating highs and lows up tothe village of Mmathete in southeast Botswana under Kalaharicover; with cross-cutting dykes and faults throughout the area. Thegeophysical data outline the three main greenstone belts, Stella,Kraaipan, and Madibe from west to east, and several intrusiveplutons very well. The greenstone belts have contrasting

Figure 14. Madibe-Kraaipan Greenstone Belt 23/4 D Gravity model along Profile BeB’ (Sodensities in kg m�3. Densities not shown are for sediments 2500 kg m�3 and granitic pluto

geophysical signatures. The Stella belt displays coincident gravityand magnetic highs, consistent with BIF (magnetite-quartzite) ormafic/ultramafic, with the latter most likely as the belt appears tobe connected to theMolopo Igneous (ultramafic) Complex at depth.The Kraaipan belt coincides with a distinctive magnetic high alongthe entire central area, a significant gravity high over the outcroparea over the Morsi ridge in Botswana but does not have adiscernable gravity high over the outcrop area in the south. Instead,a NNE-trending gravity low coinciding with the Kraaipan

uth Africa) of the Bouguer anomaly map. The numbers inside the model bodies aren 2620 kg m�3. Vertical Exaggeration (V.E) ¼ 11.36.

granodiorite is conspicuous in the south, overshadowing the ex-pected gravity high over the narrow BIF outcrop. The relativelylinear granodiorite/adamelite most probably extends to greatdepths.

In contrast, the Madibe belt correlates with a distinctive gravityhigh much wider than on the few outcrops (for example from thetown of Madibe to just beyond Mafikeng, Figs. 7 and 8), but doesnot correlate with an obvious magnetic signature (Figs. 4e6). InBotswana in the north, the linear NeS Madibe gravity high mergeswith a wide/broad E-W trending gravity high between Lobatse andGasita. This area generally correlates with a magnetic low, serve forthe linear cross-cutting highs presumably due to dykes and a sub-oval/elongate magnetic high over the Segwagwa Complex. The twoareas are thus interpreted to be predominantly underlain by dense,non-magnetic metavolcanics (Fig. 10). Possibility of relatively densemafic gneisses (~2900 kg m�3; e.g., Telford et al., 1990, p16;Peschler et al., 2004) is ruled out by the low magnetic responses.The high gravity plateau in Botswana therefore constitutes anextension of the Madibe-Kraaipan greenstone belt up to the villageof Mmathete (Figs. 7e10).

The above analysis is partly corroborated by the known and/orsuspected mineralization in the three greenstone belts. The Kraai-pan hosts medium size goldmines with only small, minor workingsexist in the other two, while Stella hosts Au-PGE/M prospects(Hammond and Moore, 2006; Lewins et al., 2008), supportingmafic/ultramafic nature than BIF for the latter? In Botswana, theMabule area (in line with Stella belt, Figs. 4e6) is currently underlicense for Au-PGE (DGS/Discovery Metals pers. comm., 2013). Thesubsurface connection between the Stella and the MUC is onlybroken by the intrusion of the newly discovered pluton atSekhutlane.

With regards to the granites, petrological and geochronologicaldata (e.g., Anhaeusser and Walraven, 1999; Poujol et al., 2002;2008, Mapeo et al., 2004) has managed to separate or link thevarious multiple/episodic intrusions. The geophysical data hasdemonstrated the widespread nature of the plutons in the sub-surface, including their areal and depth extents. Most young plu-tons are K-rich and they generally correlate with gravity lows andmagnetic highs (cf Anhaeusser and Walraven, 2009; Ranganai,2013). The Mosita granite and Kraaipan granodiorite appear to belarger than their surface exposures, while other discovered plutonsoccur in the southwest and southeast corners of the study area. TheMmathete granite has been genetically linked to the Mosita granite(Poujol et al., 2002) but the two have contrasting geophysical sig-natures, with the former geophysically indistinguishable from thesurrounding TTGs.

The present size and/or shape of the greenstones and plutonsare often used to infer their geotectonic evolution (e.g., Stettleret al., 1997; Gibson and Millegan, 1998; Wellman, 2000; Peschleret al., 2004; Benn et al., 2006; Ranganai et al., 2008; de Beer andStettler, 2009), while granite intrusion and subsequent green-stone deformation are related to mineralization (e.g., Hutchison,1985; Duuring et al., 2007; Groves and Bierlein, 2007; Lin andBeakhouse, 2013; Anhaeusser, 2014). The tectonic evolution ofgranite-greenstone terranes is generally/widely discussed in termsof two contrasting models where vertical tectonics or horizontalforces (compression and shortening) dominate; the former in-volves gravity driven diapirism and its variants while the latter issimilar to modern-style plate tectonics (e.g., Peschler et al., 2004;Hamilton, 2011; Van Kranendonk, 2011; Anhaeusser, 2014 andreferences therein). Other views content that ‘the dichotomy be-tween vertical and horizontal tectonics is false’, gravity pulls down,heat pushes up, and all tectonics including horizontal follow fromthat; vertical forces drive horizontal (H. Lyatsky, pers. comm., 2015;see also Hamilton, 2011). On the basis of global geochemical data,

Furnes et al. (2015) content that 85% of greenstone belts aresubduction-related generated in backarc to forearc tectonic envi-ronments, with plate-tectonic-like processes extending to the earlyArchaean. The remainder subduction-unrelated belts include thosedeveloped through continental rifting, rift-drift tectonics, seafloorspreading and/or plume magmatism (Furnes et al., 2015).

The subsurface configuration of the greenstones and plutons inthe study area has been investigated along two profiles by 2¾Dgravity modelling which shows the units to be steep-sided withdepth extents not exceeding 5 km. The present results are relativelysimilar to many in the southern Africa region, including othergreenstone belts in South Africa (Stettler et al., 1997; Ranganai et al.,2008; de Beer and Stettler, 2009; Gwavava and Ranganai, 2009;Ranganai, 2013). However, the lengths and composition of theMadibe-Kraaipan appear to be atypical of other southern Africaexamples, particularly older BIF domination. The intruding youngerplutons (Table 1) do not appear to deform the older BIF which is themain part of the greenstone belt. Probable equivalence have beenreported in the Yilgarn and Abitibi cratons in Australia (cf Stettleret al., 1997; Wellman, 2000; Peschler et al., 2004; Gallardo andThebaud, 2012; Anhaeusser, 2014), and possibly the Buhwa beltin the Zimbabwe craton (see Ranganai et al., 2008). Peschler et al.(2004) cautiously attribute the continuous linear nature to accre-tion at plate margins by plate-tectonic processes rather than crustaldiapirism or partial convective overturn. This could well be the casefor the Madibe-Kraaipan greenstone belt, as also proposed for thePietersburg belt (Stettler et al., 1997), but this is not conclusive.Schmitz et al. (2004) content that subduction and terrane collisionbetween 2.93 and 2.88 Ga stabilized this segment of the Kaapvaalcraton. The depth extents of the Kraaipan greenstones andmultiplepluton components of batholith/granite, though providing a 3Dview of the units, are not enough to deduce their respective com-plex geotectonic evolution process. More detailed structural (ki-nematic) analysis is required to complement the presentgeophysical studies which are also rather incomplete. The naturalcomplementarity of gravity and magnetic is ‘hampered’ by lack ofdirect structural and/or lithological constraints due to cover leadingto uncertainty in aeromagnetic structural interpretation (cf Gunnet al., 1997; Aitken et al., 2008). In this regard, the methodinvolving joint inversion of gravity and magnetic data with nodirect geological or petrophysical constraint recently proposed byGallardo and Thebaud (2012) appears to be the best, especially inareas of poor rock exposure such as present.

Finally, we comment on the socio-economic potential of theMadibe-Kraaipan granite-greenstone terrane. As noted by Jaqueset al. (1997) and others, a modern geoscientific knowledge basedeveloped through systematic mapping is a major component ofsustainable development strategies, underpinning mineral (andgroundwater) resource assessment and exploration. The structural/lineament pattern and 3D structure of the upper crust is the mostimportant in terms of economic mineral deposits because ofexploration and accessibility from a mining point of view, respec-tively (e.g., Goleby et al., 2004; Bierlein et al., 2006; Groves andBierlein, 2007; Dill, 2010; Holden et al., 2012). There are alsophysical barriers hampering access to mineral deposits, such asmining depth controlled among others by the geothermal gradientswith 5 km unlikely to be crossed by exploitation methods in theforeseeable future (Dill, 2010). The depth extent of the greenstonesin this study appear optimum, the late (younger) K-rich graniteintrusions are significant to mineralization potential as theymobilise fluids (e.g., Duuring et al., 2007; Lin and Beakhouse, 2013)while further (cross-cutting) structural control is important (e.g.,Blenkinsop et al., 2000; Bierlein et al., 2006; Holden et al., 2012).

The tectonic evolution of the Madibe-Kraaipan granite-green-stone terrane has not been considered in detail. It is therefore

recommended that this should be undertaken in future based oncomplete crustal structure, including basement faulting and blockstructure, and synthesis with geochronological data and boreholeinformation from exploration companies. It is suggested thatdetailed gravity traverses across the greenstone belt and plutons beconducted at 2.0 km spacing with regional coverage elsewhere at2e4 km spacing. Further, the extent and form of poorly magnetizedshear zones and faults within the banded iron formation undersediments is difficult to determine from the present regionalaeromagnetic data, and recently acquired high resolution aero-magnetic (HRAM) and airborne electromagnetic (AEM) data insome areas (‘confidential’ private mining companies data) shouldbe integrated to investigate possible gold, Ni and platinum miner-alization. In addition, regional structural features associated withfaulting and dyke intrusion within the older surrounding gneissesand younger geological formations like the Kanye volcanics andsandstones also constitute important targets for groundwaterexploration in this cattle ranching area (e.g., Dietvorst et al., 1991;Ranganai and Ebinger, 2008; Terblanche and Stroebel, 2013).

6. Summary and conclusions

Regional Bouguer gravity and aeromagnetic anomaly mappingof the extension of the Madibe-Kraaipan granite-greenstone beltcovering the region approximately between 24�E to 26�E and 25�Sto 26.5�S resulted in a continuous geological framework of theArchaean basement rocks and Proterozoic dykes in southeastBotswana. Several processing and enhancement techniques resul-ted in new attribute maps sensitive to the regional crustal andstructural elements of the survey area and reveal the complexgeological structure of the area. Derivative and analytic signal mapsfor both gravity andmagnetic data clearly bring out the edges of thegreenstone belts and granite plutons and linear features.

The Madibe-Kraaipan granite-greenstone terrane, consisting ofmetamorphosed volcano-sedimentary rocks and associated gran-itoids crops out sporadically for about 300 km from the Amalia/Vryburg in the south to Mmathete, Botswana in the north (Fig. 1;Anhaeusser andWalraven, 1999; Mapeo et al., 2004). Evident is theregional NeS trend, and Kraaipan metavolcanic rocks are moreextensive in the north-central part (in SE Botswana) than previ-ously recognized. The geophysical maps show the three major NeStrending massive and/or magnetic units of the Stella-Kraaipan-Madibe greenstone belts in the central part of the survey area.The high gravity/magnetic values are mainly related to mafic/ul-tramafic, banded iron formations and metavolcanics of theArchaean Kraaipan Group, respectively, largely covered by Kalaharisand. Several cross-cutting linear features in magnetic maps of thesurvey area are attributed to the dykes. A map showing theimproved lithological and structural mapping has been produced,which shows the extent of the greenstone belts for several kilo-metres from South Africa to the villages of Mmathethe andMetloboin southeast Botswana. The information obtained from gravity datamodelling suggests that the dense and magnetic greenstone beltshave a maximum depth extent of 4.7 km and the low densityplutons are approximately 4.4 km thick. Spectral analysis of themagnetic data indicates the magnetic basement at about 0.7 kmdepth under recent cover.

A tentative geotectonic evolution of the Madibe-Kraaipangranite-greenstone terrane involves accretion by plate-tectonicprocesses rather than crustal diapirism nor partial convectiveoverturn. However, this should be considered in detail in future,and together with a more detailed analysis of the mineral andgroundwater prospectivity of the area constitute continuing tasks,building on the cumulative geoscientific knowledge base devel-oped thus far. This maywarrant the general improvement of gravity

data distribution in Botswana in the north (at 2e4 km spacing),followed by joint inversion of gravity/magnetic data.

Acknowledgements

Sincere thanks to Department of Geological Survey of Botswanaand Council for Geoscience, South Africa for providing the existingaeromagnetic and gravity data. Thanks to staff at Department ofSurveys and Mapping, Ministry of Lands and Housing for infor-mation on trigonometric beacons in the survey area. Geophysicalequipment for field work was provided by Departments of Geologyand Physics, University of Botswana. The work was financiallysupported by Office of Research and Development (ORD) of Uni-versity of Botswana e Grant R0699 to RTR. The reviewers’ criticalcomments and the editor’s suggestions are greatly appreciated asthey improved the paper.

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Extension of the Archaean Madibe-Kraaipan granite-greenstone terrane in southeast Botswana:...

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