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Gondwana Research 15 (2009) 354–372

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Gondwana Research

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A Kaapvaal craton debate: Nucleus of an early small supercontinent or affected by an enhanced accretion event?

Patrick G. Eriksson a,⁎, Santanu Banerjee b, David R. Nelson c, Martin J. Rigby a, Octavian Catuneanu d, Subir Sarkar e, R. James Roberts a, Dmitry Ruban a, Mtimkulu N. Mtimkulu a, P.V. Sunder Raju f a Department of Geology, University of Pretoria, Pretoria 0002, South Africa b Department of Earth Sciences, Indian institute of Technology Bombay, Powai, Mumbai-400076, India c Intierra Resource Intelligence Ltd., 57 Havelock Street, West Perth WA 6005, Australia d Department of Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Sciences Building, Edmonton, Alberta, Canada T6G 2E3 e Department of Geological Sciences, Jadavpur University, Kolkata-700032, India f National Geophysical Research Institute, Hyderabad, India article info abstract

Article history: Incorporation of the Kaapvaal craton within a speculative Neoarchaean–Palaeoproterozoic supercontinent Received 10 June 2008 has long been debated, and this idea provides a potential solution to solving the apparently enigmatic Received in revised form 22 August 2008 provenance of the huge quantities of gold within the famous Witwatersrand auriferous deposits of Kaapvaal. Accepted 25 August 2008 Within a framework of a postulated Neoarchaean “Kenorland” (“northern”; present-day reference) Available online 4 September 2008 supercontinent, we examine possible “southern” cratons that may have been contiguous with Kaapvaal: Pilbara, Zimbabwe, Dharwar, São Francisco, Amazon, Congo. Brief reviews of their basic geology and inferred Keywords: – Witwatersrand evolution in syn-Witwatersrand basin times (c. 3.1 2.8 Ga) show no obvious support for any such Gold supercontinental amalgamations. An alternative idea to explain a measure of gross similarity amongst Neoarchaean supercratons several Neoarchaean cratons is through global events, such as a c. 3125–3000 Ma cratonic-scale erosive event Global plume events interpreted for both Pilbara and Kaapvaal, and a much more widespread magmatic event at c. 2760–2680 Ma. Pilbara We postulate that a global superplume event at c. 3.0 Ga included a plume beneath the Kaapvaal cratonic Zimbabwe nucleus, thus halting any subduction around that terrane due to the thermal anomaly. Such a speculative Dharwar global magmatic event is assumed to have enhanced production of juvenile oceanic crust at mid-ocean São Francisco ridges, including those “offshore” of the thermally elevated Kaapvaal nucleus. Intra-oceanic obduction Amazon Congo cratons complexes may have built up fairly rapidly under such conditions, globally, and once the plume event had abated, “normal” plate tectonics would have resulted in composite (greenstone-tonalite, possibly also including granite) terranes accreting with nuclei such as Kaapvaal. This enhanced plume-related cratonic growth can be seen as a rapid accretion event. Formation of the envisaged ophiolite complexes possibly encompassed deformation-related ﬁrst-order concentration of gold, and once accretion occurred around Kaapvaal's nucleus, from north and west (present-day frame of reference), a second-order (deformation- related) gold concentration may have resulted. The third order of gold concentration would logically have occurred once placer systems reworked detritus derived from the orogens along the N and W margins of Kaapvaal. Such conditions and placer gold deposits are known from many Neoarchaean cratons. The initial source of gold was presumably from the much hotter Mesoarchaean mantle and may have been related to major changes in Earth's tectonic regime at c. 3.0 Ga. The unique nature of Kaapvaal is probably its early stabilization, enabling formation of a complex ﬂexural foreland basin system, in which vast quantities of placer sediments and heavy minerals could be deposited, and preserved from younger denudation through a unique post-Witwatersrand history. © 2008 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction concomitant faunal and ﬂoral adaptations (e.g., Aspler and Chiar- enzelli, 1998). A major debate is the question of the antiquity of this The supercontinent cycle is one of the fundamental concepts cycle and the veracity of various categories of data to unequivocally within geology, and is generally accepted as being one of the major support the operation of the cycle from an approximate point in controls on Earth's geological evolution, palaeoclimatic changes and Precambrian time. Two major models for supercraton formation were originally proposed: (1) the coalescence of a large number of cratons ⁎ Corresponding author. Tel.: +27 12 4202238; fax: +27 12 3625219. (cf. continental fragments) concomitant with closure of the relatively E-mail address: [email protected] (P.G. Eriksson). small oceans separating them, and numerous sutures formed along

P.G. Eriksson et al. / Gondwana Research 15 (2009) 354–372 355 colliding cratonic margins (Unrug, 1992; Bleeker, 2003); (2) that a breakups of several supercontinental amalgamations (Rogers, 1996). limited number of large continental masses, extant since at least More recently, the supercontinent cycle is also seen as being directly c. 1.5 Ga, have been repeatedly reorganized through assemblies and related to mantle superplumes and superplume events (to use the

Fig. 1. (A) Schematic sketch map of the Kaapvaal craton (upper map shows geographic locality), illustrating its early southeastern nucleus (shaded), major Archaean greenstone belts and the preserved Witwatersrand and Pongola basins. Note Colesberg magnetic lineament, representing the suture of the Witwatersrand and Kimberley blocks, and the Limpopo mobile belt (Central Zone thereof) to the north of the Kaapvaal craton (modiﬁed after de Wit et al., 1992; Cheney, 1996; Tinker et al., 2002; Eriksson et al., 2005). (B) Schematic sketch map of the Kaapvaal craton, showing “tectonic escape” (to the southeast) model (Stanistreet, 1993) for the Witwatersrand basin after accretion of northern and western composite terranes (modiﬁed after Catuneanu, 2001). Author's personal copy

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sutures formed during the widespread c. 1.0 Ga Grenville orogeny (Rogers and Santosh, 2002). Although there is still debate on the configuration of Rodinia (e.g., Weil et al., 1998; Meert and Torsvik, 2003; see also Meert and Lieberman, 2008), its c. 0.3 Ga successor, Pangea, is relatively well understood (Lottes and Rowley, 1990). Data used to support the various proposed Precambrian supercontinents encompass geochronology, correlatable impact ejecta/ fallout units (see Glikson, 2008 for a recent example), the matching up of basin-fill stratigraphies or inferred analogous mobile belt segments on separate cratonic blocks, and palaeomagnetic techniques (e.g., Personen et al., 2003). The latter provide the only really quantitative means of testing postulated relationships between Proterozoic continental blocks within any of the proposed assemblages. Meert (2002) stressed the need in such studies to separate primary and secondary magnetizations and to have relatively accurate age data for palaeomagnetic poles, and summarized the two main palaeomagnetic methods of evaluating supercontinental reconstructions and config- urations. In the first, palaeomagnetic poles are rotated to a common reference frame and apparent polar wander paths are made for known segments of the inferred supercontinent. In the second method, individual palaeomagnetic poles are utilized to place continental blocks in a “correct” palaeolatitudinal location and to evaluate the relationships between blocks based on a best fit; it should be noted that no palaeolongitudinal control can be derived from palaeomagnetic studies (Meert, 2002). In complexly deformed Precambrian Fig. 1 (continued). successions, reconstructing palaeopoles is generally challenging, and there is also uncertainty about the alignment of magnetic and geographic poles during the Precambrian. In addition, palaeomagnetic terminology of Larson, 1991; see also Condie, 2004a,b; Condie et al., data are not indicative of a specific hemisphere (e.g., Hartz and 2001; Maruyama et al., 2007; Rino et al., 2008; Santosh et al., 2009- Torsvik, 2002). Meert (2002) examined in detail, the quantity and this issue). Zhong et al. (2007) discuss two modes of alternating quality of palaeomagnetic data available to evaluate the configuration mantle convection within the Earth, that control the large scale of Columbia, and concluded that whereas the data set only has limited assemblage and breakup of supercontinents related to hemispheric potential in this regard, Columbia probably post-dated c. 1.77 Ga. upwellings and downwellings, specifically for those amalgamations Clearly, thus, the application of the palaeomagnetic technique to younger than c. 1.0 Ga. inferred supercontinents older than c.1.8 Ga will not reap much reward, Within the philosophy of the second older model of super- and for such postulated amalgamations, reliance must rather be placed continent formation discussed above, Rogers (1996) has postulated on geological (especially geochronology) and particularly structural several ancient continental masses (cf. the “ancient nuclei” of Meert, data without any real quantitative methodology (cf. Meert, 2002). A 2002), of varying antiquity, as follows (Rogers and Santosh, 2002, more reliable approach for analyzing possible fragments of ancient 2009-this issue): supercontinents than palaeomagnetism is to compare the timing of geological events recorded on those cratons (fragments). Nelson (2008) (1) “Ur”, c. 3.0 Ga and centred on India, encompassing Kalahari, utilized probability distribution plots of precise geochronological data to Western Dronning Maud, the Dharwar, Bhandara and Singhb- analyze the geological evolution and relationship between the Pilbara hum cratons of India, as well as the Napier and Pilbara cratons; and Yilgarn cratons during the Archaean, and extension of this (2) Ur is postulated to have evolved into “expanded Ur” (prior to c. methodology to Kaapvaal and other ancient Precambrian cratons offers 1.5 Ga) as younger cratons became accreted to the original great scope for an improved understanding of their history. An nucleus: Zimbabwe, Madagascar, Bundelkhand, Aravalli, Yil- additional problem that arises with all methods of examining ancient garn, Kimberley, Gawler, and the Eastern Australian terranes; supercontinents, and which is also relevant to the suggestions made (3) The c. 2.5 Ga Arctica supercontinent comprising the Aldan, later in this paper, is that the preservational products from geological Anabar/Angara, Slave, Rae, Greenland, Hearne, Nain, Superior events of truly global scale (e.g., mantle superplume events (Condie and Wyoming cratons; 2004a) or global glaciations (Kopp et al., 2005; Maruyama and Santosh, (4) Arctica expanded at c. 1.5 Ga to form “Nena”, through addition 2008)) can be confused with those reflecting events common to more of the Baltica continent (itself of Palaeoproterozoic age; Ruban, than one daughter fragment of a pre-existing supercontinent (e.g., 2007) as well as most of East Antarctica; discussions in Eriksson et al., 2004). (5) “Atlantica”, c. 2.0 Ga, and made up of West Africa, Congo/Kasai, A number of relatively speculative reconstructions of Neoarchaean Amazon, São Francisco and Rio de la Plata cratons. supercontinents have been proposed (e.g., Button, 1976; Piper, 1983; The existence of a Palaeo-Mesoproterozoic supercontinent was Gaál, 1992; Stanistreet, 1993; Cheney, 1996). Aspler and Chiarenzelli first suggested by Piper (1976), based largely on palaeomagnetic data (1998) discussed the merits of “Kenorland” (first postulated by (see also Piper, 2007). Rogers and Santosh (2002) discuss such an Williams et al., 1991, for the assembled cratonic blocks of North ancient Precambrian supercontinent which they termed “Columbia” America), and argued in favour of such a Neoarchaean amalgamation, (see Hou et al., 2008, for a review of its configuration), and which based largely on chronological data. In addition, they suggested that comprised an assembly of most of the cratonic blocks then existing, at an “expanded Kenorland” may have also included the Baltic and some time within the interval ~1.9–1.5 Ga. Breakup of this began at Siberian shields, and speculated on a possible second, or “southern” about 1.6 Ga and continued until c. 1.4 Ga, and subsequent drifting and (in modern latitudinal context) Neoarchaean–Palaeoproterozoic rotations of most of the component blocks was followed by assembly supercontinent, including the “Zimvaalbara” (cf. assembled Zim- of supercontinent Rodinia (e.g., Dalziel, 1997), characterized by babwe, Kaapvaal and Pilbara cratons; Stanistreet, 1993) block as well Author's personal copy

P.G. Eriksson et al. / Gondwana Research 15 (2009) 354–372 357 as the São Francisco, and cratons from present-day India. The craton soon after Witwatersrand sedimentationwas complete. However, “Zimvaalbara” concept has enjoyed some debate in the literature, the still extant debate on the age of the Limpopo orogeny rather favours a with Cheney (1996) discussing lithostratigraphic, sedimentary facies- much younger age for this event, as discussed later in this paper. based and sequence stratigraphic arguments in favour of, particularly, Relating the widely accepted foreland basinal model applied to the a Kaapvaal–Pilbara (“Vaalbara”) connection, although he also includes Witwatersrand succession to the inferred accretion of northern and the Zimbabwe craton and Grunehogna province from Antarctica. It western terranes onto the SE nucleus, points to a complex (cf. “double”) should be pointed out that Cheney's (1996) use of “sequence foreland depository, encompassing the compressive convergence of two stratigraphy” is one of unconformity-bound units (sensu Sloss, 1963) oblique stress fields at an angle of c. 100° to each other (Catuneanu, rather than the modern techniques more familiar today (e.g., recent 2001). The latter author demonstrated that a retroarc flexural foreland compilation by Catuneanu, 2006). system model can also be applied to the Pongola Supergroup (a coeval This paper attempts to contribute to the debate on early Precambrian andcorrelated[Beukes and Cairncross, 1991] basin-fill in the SE of supercontinents by looking specifically (and briefly) at the Kaapvaal the Kaapvaal craton; see Fig. 1). Within this inferred scenario, the craton in the Neoarchaean time period. We choose this craton for several Witwatersrand depository occupies the foredeep basin and the Pongola reasons: it is one of the oldest cratons; it contains Earth's most ancient the back-bulge basin, separated by the flexural forebulge (Fig. 2). The known reasonably-sized sedimentary basin (the famous auriferous emergent forebulge reflects low rates of subduction, and the con- Witwatersrand depository, c. 3.1–2.8 Ga); the Pilbara–Kaapvaal concomitant forebulge unconformity was apparently present throughout nection has enjoyed a measure of discussion in the literature; assuming the lifespan of the “greater Witwatersrand” (cf. Witwatersrand and a greenstone source for the gold in this basin, the present-day extent of the Kaapvaal craton appears to be inadequate as a hinterland. We will thus briefly review the geological history of Kaapvaal over this chosen time coincident with Witwatersrand sedimentation, and will discuss the merits and demerits of various possible cratons which potentially might have been joined to Kaapvaal and which do not fall within the inferred bounds of Kenorland. Finally, we will speculate on an altogether different model for addressing the size or richness of the gold-bearing hinterland to the Witwatersrand basin.

2. Geological framework: Kaapvaal craton and Witwatersrand basin

The c. 3.0–2.8 Ga Witwatersrand depocentre is thought to have been coeval with ongoing terrane amalgamation and accretion, as well as granitisation (Robb and Meyer, 1995), related to the genesis of the Kaapvaal craton (cf. De Wit et al., 1992). Recent modeling of the evolution and sequence stratigraphy of the Witwatersrand basin-fill supports a flexural foreland basin setting (e.g., Catuneanu, 2001, 2004), a general tectonic setting also inferred by many earlier workers (e.g., Burke et al., 1986; Winter, 1987; Robb et al., 1991; Stanistreet and McCarthy, 1991). Craton-scale tectonics is thus inferred by many to have been active during formation of Witwatersrand sequence stratigraphic architecture and basin-fill geometry. Such a viewpoint also falls within the debate on the character of Early to Middle Archaean tectonics and mantle convection patterns, on a much hotter Earth (e.g., Trendall, 2002; Eriksson and Catuneanu, 2004). De Wit et al. (1992) postulate that an early Kaapvaal cratonic nucleus (now preserved in the S–SE parts of the craton; Fig. 1) had formed by c. 3.1 Ga, through the amalgamation of oceanic and arc terranes, and granitoid magmatism. The c. 3.1–2.8 Ga Witwatersrand basin was formed during a second stage of cratonic evolution, from c. 3.0–2.7 Ga, when Cordilleran-style accretion of composite terranes (e.g., Poujol and Robb, 1999) on the northern and western margins of the nucleus are inferred within the De Wit et al. (1992) model. “Tectonic escape” (Stanistreet, 1993) of the Witwatersrand basin to the southeast likely followed (Fig. 1B). Within the assembled larger Fig. 2. Sketch map (top) and schematic profile through inferred Witwatersrand foreland system (below). The preserved Witwatersrand basin equates to the foredeep Kaapvaal craton, the sutured nucleus and northerly accreted terranes depozone, with area “B” representing an area of subsequent erosion of these foredeep are referred to by some as the Witwatersrand block, with the westerly strata. The two solid line half-circles, centred on the areas of maximum loading accreted terranes being termed the Kimberley block (De Wit et al., (numbered “1” and “2” for accreting northern and western composite terranes, 1992; Tinker et al., 2002)(Fig. 1A). respectively), outline the approximate distribution of the foredeep depozone. The forebulge developed outside the area covered by these two half-circles, with its apex, The Witwatersrand depository thus had a provenance made up of “ ” fi − N b point A (see also pro le, below) enclosed by the 130 mgal isoline of the gravity older( 3.1Ga)granitoidcrust(nucleus)andthe 3.1Gaaccretedterranes field. The three dashed circles suggest contour lines of the foreland system centred (RobbandMeyer,1995).Anumberofpulsesofgranites(termed“Randian” around the forebulge apex, A, with the outermost such circle marking the position of granites) accompanied the accretion of the b3.1 Ga composite terranes, the back-bulge axis (which equates with the depo-axis of the Pongola Supergroup andsomeof thesemighthaveledtoconcomitantpulsesofsedimentation basin), as suggested in theoretical flexural profile models (cf. Catuneanu, 2001 and references therein). In the case of the greater Witwatersrand basin, the forebulge withintheevolvingbasin(RobbandMeyer,1995).WithintheDeWitetal. remained emergent, separating discrete foredeep (fill = Witwatersrand Supergroup) (1992) model,the CentralZoneof the Limpopobelt(joiningthe Kaapvaal and back-bulge (fill = Pongola Supergroup) basins. The cross-sectional profile 2–2′ on and Zimbabwe cratons; Figs. 1 and 6) amalgamated with the Kaapvaal the map is shown below in the profile. Modified after Catuneanu (2001). Author's personal copy

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Fig. 3. (A) Geological sketch map of the preserved Witwatersrand basin, showing the lower West Rand and upper Central Rand Groups, Witwatersrand Supergroup. Modiﬁed after Catuneanu (2001). (B) Lithostratigraphy of the Witwatersrand Supergroup and inferred depositional systems, as well as correlation between the West Rand Group and the Mozaan Group of the Pongola Supergroup. Note that the Dominion Group locally forms a partial basement to the Witwatersrand Supergroup. Modiﬁed after Eriksson et al. (2005). Author's personal copy

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Pongola Supergroups) basin; the flexural wavelength is inferred to have volcanism (3074–3086 Ma) and subsequent hiatus for about 100 my been much shorter compared to many Phanerozoic equivalent deposi- before the marine sedimentation (West Rand and Mozaan Groups) tories (Catuneanu, 2001). began (Robb and Meyer, 1995)(Fig. 4) are compatible with an Fig. 3 summarizes the broad-scale lithostratigraphy and inferred alternative explanation, that the flexural tectonic framework during depositional palaeoenvironments of the Witwatersrand Supergroup an early phase of basin evolution led to starved foredeep and shallow in the foredeep basin. The Dominion Group (c. 60 m of fluvially- back-bulge settings within which the marine deposits accumulated. deposited sandstones followed by c. 2.25 km of andesitic lavas and This postulate can be related to the sedimentary character of the pyroclastic rocks overlain by predominant acid volcanic rocks, which preserved deposits in the two flexural basins of the foreland system. locally form basement to the Witwatersrand; Tankard et al., 1982)is These deposits accumulated from c. 2970 Ma until soon after correlated (cf. Beukes and Cairncross, 1991) with the volcanic Nsuze 2914 Ma (Fig. 4), with up to 7.5 km of preserved mudrocks, mature Group of the Pongola Supergroup (forms partial basement to the back- sandstones and minor lavas (West Rand Group in the foredeep) forming bulge sedimentary basin-fill; Fig. 2). While some see this initial within a continuum of settings from marginal fluvial braidplains, volcanic component as signaling a proto-basinal stage (e.g., Stanis- through littoral and to distal shelf milieus (cf., Eriksson et al.,1981; Burke treet and McCarthy, 1991), others suggest lithospheric stretching and et al., 1986; Stanistreet and McCarthy, 1991; Beukes, 1996)(Fig. 3). thermal collapse to accommodate these thick lavas (e.g., Robb and An analogous succession of arenaceous–argillaceous sediments reflect- Meyer, 1995). Thermal subsidence following volcanism would ing interbedded fluvial braidplain and inner to outer shelf deposits logically have accommodated the marine deposits (Fig. 3) of the (Beukes and Cairncross, 1991) is preserved within the Mozaan Group West Rand (foredeep basin) and Mozaan (back-bulge basin; Fig. 2) back-bulge basin. The inferred craton-margin location of the Mozaan Groups. However, the relatively short duration of the Dominion back-bulge basin within the greater Witwatersrand flexural foreland

Fig. 4. Geodynamic history chart for the “greater Witwatersrand basin” (=Witwatersrand and correlated Pongola Supergroups; see also Figs. 1A and 2). Chronology and stratigraphy of the Witwatersrand Supergroup are shown at left, with major granitic events affecting this basin and its hinterland in the central column (modiﬁed after Robb and Meyer, 1995, and references therein). The second column from the right shows major terrane accretion and amalgamation events for the Kaapvaal craton (after de Wit et al., 1992; Tinker et al., 2002). The right hand column gives ﬂexural retroarc foreland basin system stages (from Catuneanu, 2004) for the greater Witwatersrand depository. Figure after Eriksson et al. (2005). Author's personal copy

360 P.G. Eriksson et al. / Gondwana Research 15 (2009) 354–372 model (Figs. 2 and 1) would logically have subjected this basin to open 2.0 Ga Vredefort bolide impact (Robb et al., 1997; Frimmel et al., 1999; ocean waves, and can thus explain the higher energy character of these Frimmel, 2005). deposits compared to the coeval West Rand Group foredeep basin-fill, The source areas of the gold and associated sediment particles are where tidal processes predominated (Eriksson et al., 2005). inferred to have been the Archaean granitoid–greenstone terranes Central Rand Group sedimentation followed in the foredeep basin, that presumably largely made up the Kaapvaal craton at the time of from N2894–c. 2780 Ma (Robb and Meyer, 1995)(Fig. 4), there being the Central Rand Group deposition (e.g., Robb and Meyer, 1995; no Pongola equivalent unit in the back-bulge depository. A maximum Frimmel, 2005). Wronkiewicz and Condie (1987) document geo- of 2.88 km of mainly sandstones and subordinate auriferous chemical evidence for an ultramafic–mafic greenstone belt contribu- conglomerates accumulated in the foredeep basin, essentially within tion, which is corroborated by abundant detrital PGE and chromite a braided fluvial setting (Robb and Meyer, 1995; Els, 1998a,b; Fig. 3). minerals in the auriferous conglomerates. Pebble compositions There is a still widely-held perception that alluvial fans were mainly support a strong Archaean granitoid contribution (Frimmel, 2005 responsible for deposition of these famous gold-bearing sediments, and references therein). Although mafic greenstone lithologies are based largely on a well entrenched earlier literature. However, seminal traditionally seen as the primary source of the detrital gold, Robb and studies by Els (1998b) indicate clearly that such deposits are absent in Meyer (1995) discuss evidence for a second Au source, in the granitic the preserved Central Rand record, and that braided rivers predomi- intrusions (see Fig. 4) that accompanied evolution of the Witwaters- nated (with minor aeolian influences) right up to an epeiric sea, rand depository; U is seen as having a similar origin. Inherent in all reflecting the lack of vegetation; here, braid-deltas rather than deltas discussions of the source of the gold in the Witwatersrand basin is the sensu stricto developed, due to limited quantities of suspended “volume problem”, whereby it is difficult to resolve the quantities sediment, a lack of well-defined river mouths, intermittent sediment deposited and those mined over a long time period, with known supply generally, and epeiric littoral reworking processes (Els and greenstone (major preserved greenstone belts of Kaapvaal are Meyer, 1993, 1998). Beach conglomerates in the Central Rand illustrated in Fig. 1) and possibly additional granitic Au sources of succession are uncommon, which is ascribed to low palaeoslopes the Kaapvaal craton (see discussions by many authors, useful and small channel depths within the coastal braided channel systems summaries being provided by Robb and Meyer, 1995; Frimmel, (Els, 1998a). These relatively coarse sediments were deposited late in 2005, 2008). Consideration of the possibility of a Neoarchaean the evolution of the flexural foreland basin system postulated in the “supercontinent” could also provide an element to this debate, as Catuneanu (2001) model, and point to an overfilled basin (i.e., discussed in the next section of the paper. sedimentation rate≫subsidence rate) character. 4. Possible cratonic assemblies with the Kaapvaal craton 3. The Witwatersrand gold provenance: the “volume problem” Although any attempted reconstructions of postulated Neoarch- It is beyond the purpose of this paper to investigate the gold deposits aean supercontinent(s) (e.g., Button, 1976; Piper, 1983; Gaál, 1992; of the Witwatersrand basin in detail, and the reader is referred to an Stanistreet, 1993; Cheney, 1996; Aspler and Chiarenzelli, 1998) are excellent review of this topic by Frimmel (2005),aswellasearlier fraught with uncertainty, and necessarily premised on commonly seminal work on divergent interpretations of gold genesis (e.g., Minter, equivocal geological data, a brief examination of possible amalgama- 1978, 1981; Minter et al.,1993; Pretorius,1981,1991; Phillips and Myers, tions between Kaapvaal and a set of potentially adjoined cratons is 1989; Robb and Meyer, 1990, 1991; Phillips and Law, 1994, 2000). The deemed sensible here, particularly if there is evidence against such debate on syngenetic versus epigenetic models for the Witwatersrand postulated cases. Care must also be taken with such speculations, as gold deposits is well over a century old (e.g., Mellor, 1916; Graton, 1930; the preserved products from truly global geological events (for Pretorius, 1991; Phillips et al., 1997), but irrespective of genetic example, mantle superplumes) may be confused with those formed interpretational preference, vast amounts of gold (and uranium) due to events which affected more than one daughter fragment of an would have been needed for the formation of these deposits (e.g., inferred supercontinent prior to breakup. Robb and Meyer,1995; Frimmel, 2008). A realistic mass balance suggests Aspler and Chiarenzelli (1998) in their speculative “southern” two main scenarios: a source which was exceptionally rich in these two Neoarchaean–Palaeoproterozoic supercontinent, discuss the merits of elements (e.g., Robb and Meyer, 1990), or placer sedimentation amalgamations of “Zimvaalbara” (cf. assembled Zimbabwe, Kaapvaal processes acting on “normal” Archaean basement (e.g., Loen, 1992)to and Pilbara cratons; Stanistreet,1993) with Indian cratons, as well as the a degree capable of producing the known volumes of these minerals São Francisco; Cheney (1996) adds the Grunehogna province to this (Robb and Meyer,1995). However, a further solution would be provided possible assembly. We will examine briefly the possible merits or by a greater provenance area than just the then-extant Kaapvaal craton, otherwise of contiguity of the components of “Zimvaalbara”,andof such as that which, hypothetically, could be provided by a Neoarchaean other possible “southern” cratons such as Congo, the São Francisco and “supercontinent”, as discussed later in this paper. Amazon cratons of South America, and of the Dharwar craton of India, all Frimmel (2005), in his review of these famous gold deposits, with respect to being joined to the Kaapvaal craton. The presence of contrasts the evidence in favour of a primary placer origin (see also, thick Neoarchaean–Palaeoproterozoic BIF successions on the Pilbara, Minter, 2006) with that suggesting a secondary hydrothermal genesis, Kaapvaal, São Francisco, Dharwar and Amazon cratons has stimulated and finds in favour of what many before him have termed the past discussion of possible amalgamations amongst these crustal “modified placer model”. The essence of often strongly-held opinions fragments (e.g., Button, 1976; Cheney, 1996; Nelson et al., 1999), while on the genesis of the gold rests on the fact that a majority of the gold the Zimbabwe and Congo cratons are logical candidates for potential shows evidence for hydrothermal processes and is commonly contiguity with Kaapvaal during these times (e.g., Eriksson et al., 1999). distributed along microfractures; however detailed work, particularly from microtextural, mineralogical, geochemical and isotopic investi- 4.1. Pilbara gations, clearly shows that the “hydrothermal gold” was formed from remobilization of primary particulate or placer deposits (Frimmel, A possible Pilbara–Kaapvaal amalgamation (“Vaalbara” or even the 2005 and references therein). This remobilization is ascribed to expanded “Zimvaalbara”) certainly has its proponents (e.g., Stanis- several geological events subsequent to Central Rand Group deposi- treet, 1993 and Cheney, 1996, more recently). This postulate is tion: extrusion of the c. 2.7 Ga Ventersdorp Supergroup lavas commonly premised on seemingly closely matched post-Witwaters- (Pretorius, 1991), burial due to c. 2.6–2.1 Ga Transvaal Supergroup rand-aged lithostratigraphies, either volcanic (c. 2.7 Ga Ventersdorp sediments, intrusion of the c. 2.0 Ga Bushveld Complex and the c. on Kaapvaal; c. 2.7 Ga Fortescue on Pilbara) or sedimentary (c. 2.6– Author's personal copy

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2.1 Ga Transvaal on Kaapvaal; c. 2.6–2.2 Ga Hamersley and Turee Creek and Superior cratons. On the basis of these, they argued that rapid and successions on Pilbara) (Fig. 5). However, neither precise zircon age large scale convective overturn of the Earth's mantle (with con- data (Nelson et al., 1999) or palaeomagnetic investigations (Wingate, comitant enhanced magmatic activity globally) may have occurred in 1998) support the idea. Correlation of impact ejecta layers (e.g., this period, as a global scale event affecting most if not all extant Glikson, 2008) also offers support for Kaapvaal–Pilbara contiguity, cratons rather than a more restricted event affecting a possible although such ejecta layers can also be widespread across part of or Neoarchaean supercontinent. the entire globe. Possible correlations discussed by Glikson (2008) are for units either older or younger than the Witwatersrand basin 4.2. Zimbabwe interval discussed in this paper. Nelson et al. (1999) provided an exhaustive review of available The Limpopo Belt of southern Africa, comprising a Central Zone, reliable age data, and found significant differences in the chronologies of Northern and Southern Marginal Zones (Fig. 6), has traditionally been magmatic events in the granite–greenstone crusts of the two cratons: regarded as a high-grade terrane whose metamorphic and deforma- e.g., 2985 Ma and 2782 Ma events on Kaapvaal are absent in the Pilbara, tional characteristics can be attributed to an Alpine–Himalayan style while a 2760 Ma event comprising granitic intrusions of Pilbara as well continental collision event that occurred between the Zimbabwe and as lower Fortescue flood basalts, was not identified on Kaapvaal. Nelson Kaapvaal cratons at c. 2.7–2.55 Ga (Roering et al., 1992; Van Reenen et al. (1999) went further, and suggested that first-order and second- et al., 1992; Treloar et al., 1992). However, in recent years the relative order sea level cyclicity within the c. 2.6–2.1 Ga sedimentary successions timing, style and extent of the magmatic and tectono-metamorphic on the two cratons reflect global eustasy rather than any possible activity has come in question by numerous authors (e.g., Rigby et al., contiguity of these ancient cratonic blocks. Their postulate of global 2008 and references therein). events rather than contiguity being the prime influences on the A Neoarchaean age for the Limpopo Belt is based largely on U–Pb Kaapvaal and Pilbara cratons, is reinforced by reliable age data geochronological data obtained from zircons in “late tectonic” supporting a measure of similarity in the history of the two cratons: granitoids, which appear to cross-cut an earlier deformational fabric (1) similar duration of granite–greenstone crust formation, from 3650– and thus provide a minimum age constraint for the contractional 3100 Ma; (2) cratonic-scale erosion on both, from c. 3125–3000 Ma; phase of orogenesis (Barton and Van Reenen, 1992; McCourt and (3) major flood basalts from 2760–2680 Ma; (4) chemical sedimenta- Armstrong, 1998; Kröner et al., 1999). There are, however, a number of tion (c. 2630–2440 Ma) succeeded by a transition to clastic deposition problems associated with the interpretation of these data. Firstly, the (2440–2200 Ma) on Kaapvaal and Pilbara (Nelson et al., 1999). apparent “late tectonic” granitoids exhibit a widespread deforma- Nelson et al. (1999) noted analogous volcanic events in the 2760– tional fabric (e.g., Kröner et al., 1998) implying that there was a post- 2680 Ma time period (#3 above) on the São Francisco, Amazon and Neoarchaean tectonic event. Secondly, the geochronological data were Karnataka cratons, as well as ultramafic magmatic events on Yilgarn determined from analyses of zircon “cores”, yielding dates that can

Fig. 5. Geological sketch map of the Pilbara craton, showing basement lithologies (greenstone belts and granitoid rocks; c. 3650–3100 Ma), as well as the Fortescue (c.2.7 Ga), Hamersley and Turee Creek Groups (together spanning c. 2.6–2.2 Ga). Modiﬁed after Nelson et al. (1999). Author's personal copy

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Fig. 6. Geological sketch map of the Limpopo mobile belt separating the Kaapvaal craton (KC, south) from the Zimbabwe craton (ZC, north). The belt is divided into three main zones, Central (CZ), Northern Marginal (NMZ) and Southern Marginal (SMZ). only be interpreted as the crystallization age of the igneous body. permitting deﬁnition of northern and southern blocks; the former block Holzer et al. (1998), Kröner et al. (1999) and Zeh et al. (2007) is further subdivided into the “Eastern Dharwar Craton” (EDC) presented additional geochronological evidence to support the and “Western Dharwar Craton” (WDC) (Fig. 7), based on high prevalence of Neoarchaean magmatism; however, this is not necessa- temperature and low temperature metamorphic mineral assemblages, rily indicative of a continental collision event. Conversely, by dating a respectively, and separated by a major mylonite belt (Sarangi et al., single, high-grade metamorphic event one can conclude that crustal 2007). The Dharwar Supergroup refers to the c. 2900–2600 Ma convergence took place. (Chadwick et al., 2000 and references therein) supracrustal rocks of Droop (1989) and Zeh et al. (2004, 2005) present compelling the WDC, made up of lower Bababudan and younger Chitradurga Groups petrological arguments that suggest the Limpopo Belt underwent a (Sarangi et al., 2007). In the south of the WDC, many workers argue single, clockwise P–T evolution that is consistent with contempora- for an older, N3.0 Ga supracrustal unit, known as the Sargur Group (3.13– neous burial and heating in a collisional regime. The timing of 2.96 Ga; U–Pb; Nutman et al., 1992), and inferred to form part of the the metamorphism was bracketed by Jaeckel et al. (1997), who dated Dharwar Supergroup basement, which is collectively known as the metamorphic zircons from an undeformed melt vein in a metapelite of Peninsular Gneiss (c. 3.3–2.9 Ga; Chadwick et al., 2000). However, the Beit Bridge Complex (supracrustal rocks of the Central Zone of others see the Sargur lithologies rather as highly metamorphosed and the Limpopo Belt; Fig. 6), which yielded a Pb207/Pb206 evaporation migmatised Dharwar Supergroup enclaves (Sarangi et al., 2007,and mean age of 2026±7 Ma. A Palaeoproterozoic age is further supported references therein). The WDC has been intruded locally by c. 2.6 Ga by Pb stepwise leaching data obtained by Holzer et al. (1998) from granites (Taylor et al., 1984; Rogers, 1988). The EDC is characterized by a garnet and titanite, which yield ages of 2010±17 Ma and 2007±5 Ma, Neoarchaean dioritic–granitic basement, with linear schist belts respectively. Moreover, a recent study by Buick et al. (2006) of analogous lithology to those of the Dharwar Supergroup, but precise demonstrated that previously determined c. 2.69 Ga ages, obtained age data are unavailable (Chadwick et al., 1996, 2000; Nutman et al., from igneous zircons, contained metamorphic overgrowths that yield 1996). Emplacement of the large scale Dharwar batholith in the EDC a Palaeoproterozoic age of 2022±11 Ma. Collectively, the geochrono- took place at c. 2750–2510 Ma (Chadwick et al., 2000). logical and petrological data indicate that crustal convergence, which The Dharwar Supergroup in the WDC comprises lithologies (now was initiated by the collision of the Kaapvaal and Zimbabwe cratons, preserved as schist belts, of which the Chitradurga belt is the largest) only occurred during the Palaeoproterozoic. This strongly mitigates varying from clastic (conglomerates, orthoquartzites and greywackes) against any contiguity of the Zimbabwe and Kaapvaal cratons during to chemical metasediments (marbles and BIF), to maﬁc and felsic evolution of the Witwatersrand basin. volcanic rocks (Gnaneshwar Rao and Naqvi,1995; Sarangi et al., 2007). Sm–Nd whole rock ages from volcanic rocks in the lowest formation of 4.3. Dharwar the Bababudan Group date at c. 2.911 Ga, with stratigraphically higher units giving c. 2.848 Ga; in the succeeding Chitradurga Group, The Dharwar craton of southern India is a relatively typical Archaean volcanics are dated at 2.747 Ga (Kumar et al., 1996). Trendall et al. “basement” assemblage of linear “schist belts” within widespread (1997a) obtained precise SHRIMP ages on single zircon crystals of gneissic rocks, intruded by younger granites (e.g., Swami Nath and 2720±7 and 2718±6 Ma, from inferred ashfall tuff beds from the Ramakrishnan, 1981; Halls et al., 2007). Metamorphic grades increase uppermost formation of the Bababudan; using the same technique, from north (greenschist) to south (granulite) (Halls et al., 2007), Trendall et al. (1997b) found an age of 2601±6 Ma for Chitradurga Author's personal copy

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Fig. 7. Geological sketch map of the northern part of the Dharwar craton, India. Note the Western and Eastern Dharwar craton subdivisions, separated by the Closepet Granite. The Dharwar Supergroup essentially equates with the greenstone belts (cf. “schist belts”) of the Western Dharwar craton.

Group rhyolites, similar to Nutman et al.'s (1996) age of 2614±8 Ma minimal BIF; carbonates are lacking, and in the upper Central Rand on the same formation. Group, auriferous conglomerates become important. In contrast, the Chadwick et al. (2000) interpret the WDC as the foreland to an Dharwar is characterized by an association of clastic and chemical accretionary arc (Dharwar batholith and undated linear schist belts in sedimentary lithologies and significant volcanic rocks, and conglomer- EDC) which became sutured in the late Neoarchaean, with con- ates are neither common nor auriferous. Major granitisation events of comitant NE–SW shortening and transcurrent displacements in the the Dharwar craton do not appear even broadly coeval with those WDC. The schist belts in the EDC are seen as intra-arc basins formed known from Kaapvaal; detailed chronological data are anyway lacking during the accretion, and oceanic lithosphere is thought to have been from Dharwar to enable detailed chronological comparisons such as over-ridden by the WDC foreland. those made by Nelson et al. (1999) between Pilbara and Kaapvaal. Their The basal part of the Bababudan Group overlying the Dharwar craton overlapping ages and apparently analogous large scale geodynamic basement comprises oligomictic quartz pebble conglomerates (e.g., history (with accretion of terranes being implicit in both Kaapvaal and Kumar et al., 1996), and there are other ruditic horizons within the c. Dharwar) may be favourably compared; however, this does not 2.9–2.6 Ga Dharwar Supergroup succession, including those whose necessarily imply contiguity. In any case, in the Neoarchaean, accre- geochemistry indicates chromite-rich source rocks, most likely of tionary processes appear to have been common on many cratons, and greenstone belt origin (e.g., Chatterjee and Das, 2004). Although this may rather reflect global magmatic influences (cf. Nelson et al., 1999). provides an immediate analogy with the c. 2970–2780 Ma (Robb and Meyer, 1995) Witwatersrand succession on Kaapvaal, the Dharwar 4.4. São Francisco supracrustals have several significant differences that do not support contiguity of the two cratons. The Witwatersrand succession In broad terms, this craton comprises an Archaean–Palaeoproterozoic is dominated by clastic sedimentary rocks, with minor lavas and basement (of medium- to high-grade metamorphic lithologies, low- Author's personal copy 364 ..Eiso ta./Gnwn eerh1 20)354 (2009) 15 Research Gondwana / al. et Eriksson P.G. – 372

Fig. 8. (A) Geological sketch map of Brazil, showing the São Francisco craton, and the Guiana and Brazil Shields together comprising the Amazon craton. Modiﬁed after Barbosa and Sabaté (2004). (B) Geological sketch map of the São Francisco craton, showing basement rocks and supracrustal lithologies as well as the surrounding Brasiliano orogenic belts. Modiﬁed after Martins-Neto et al. (2001). Author's personal copy

P.G. Eriksson et al. / Gondwana Research 15 (2009) 354–372 365 grade greenstone remnants, and Palaeoproterozoic granite–syenite– (Carajás Formation), and an upper mixed metavolcanic–metasedi- uncommon mafic plutonic intrusions), overlain by an essentially mentary unit, this in turn being succeeded by a largely sandstone undeformed Mesoproterozoic (mainly siliciclastic) and Neoproterozoic succession (Olszewski et al., 1989). The Grão Pará Group is thought to (largely carbonate sedimentary rocks) cover sequences (e.g., Barbosa reflect deposition upon an extending preceding continental crust, and Sabaté, 2004)(Fig. 8). These authors have studied the four important with the lower volcanic unit dated (U–Pb zircon) at 2758±39 Ma crustal segments (cf., Barbosa and Dominguez, 1996) that make up the (Olszewski et al., 1989). More recent U–Pb dating of zircons indicates basement of the São Francisco craton in their area of best exposure, Bahia the Carajás Formation to have been deposited between c. 2750 and (large area of basement, west of Salvador on Fig. 8B). The first crustal 2740 Ma, and the uppermost sandstone-dominated unit contains segment, the Gavião block, has a 3.4–3.2 Ga trondhjemite–tonalite– syndepositional volcanic rocks with zircons dated at 2681±5 Ma granodiorite (TTG) nucleus surrounded by a c. 3.2–2.9 Ga gneissic– (Trendall et al., 1998). amphibolitic terrane, with c. 3.2–2.9 Ga greenstone belts (Barbosa and Olszewski et al. (1989) note a potential correlation between the Sabaté, 2004). The second crustal segment, the Serrinha block, has a Grão Pará volcanics and those of the Fortescue Group of the Pilbara largely gneissic–amphibolitic–granodioritic basement of c. 3.5–2.9 Ga craton, dated by precise zircon chronology to c. 2765–2687 Ma (Arndt age, with much younger greenstone belts, of c. 2.0–2.1 Ga (Padilha and et al., 1991; Nelson et al., 1999). The latter authors also note an overlap Melo,1991; Barbosa and Sabaté, 2004 and references therein). The Jequié in the geochronology of the Fortescue and Ventersdorp (Supergroup, block (Barbosa,1990; Barbosa and Sabaté, 2000) or third crustal segment Kaapvaal) volcanics, although eruption of the latter lavas began at mainly comprises c. 2.7–2.6 Ga granulite-grade enderbitic–charnockitic least 30 my after the onset of Fortescue volcanism. Despite these likely lithologies (Barbosa and Sabaté, 2004). The fourth segment consists of correlations of extensive volcanic suites on the Amazon, Pilbara and two separate segments (in NE and Sa Bahia), together making up the Kaapvaal cratons at c. 2.76–2.64 Ga, and the analogous nature of the Itabuna–Salvador–Curaça granulite belt; lithologies are predominantly widespread BIF deposits on the same three cratons after this tonalitic–charnockitic, with basic–ultrabasic enclaves and lesser supra- volcanism (Nelson et al., 1999), this cannot be used as definitive crustal rocks (Barbosa and Sabaté, 2004). support for contiguity of Kaapvaal with either or both of the Pilbara In the southern part of the São Francisco craton, the Palaeoproter- and Amazon cratons during deposition of the Witwatersrand ozoic Minas Supergroup, famous for its BIF and iron deposits, Supergroup sediments on Kaapvaal. Although there is a certain unconformably overlies c. 3.2–2.9 Ga crystalline basement rocks and gross similarity in the geodynamic history of the Kaapvaal craton, greenstone belts of the c. 2.78–2.61 Ga Rio des Velhas Supergroup with composite terranes amalgamating with the Kaapvaal nucleus at (Chemale et al., 1994; Alkmim and Marshak, 1998). The basal Caraça c. 3.0–2.7 Ga (De Wit et al., 1992; Poujol and Robb, 1999), and the Group of the Minas succession, has a lowermost formation comprising inferred accretion of the ensimatic northern Guiana Shield with the alluvial coarse-grained quartzite, conglomerate, fine-grained marine southern, essentially continental Brazil Shield, as discussed above (cf. quartzite, and phyllites (Villaça, 1981), and is unconformably Olszewski et al., 1989), the age of the latter event must post-date c. succeeded by an essentially argillaceous formation (Spier et al., 2.25–2.1 Ga. 2007). The Caraça is thought to be a transgressive deposit overlying a peneplained substrate, with the lower formation reflecting paralic to 4.6. Congo stable shelf environments (Dorr, 1969; Spier et al., 2007). The famous BIF and iron deposits occur in the succeeding c. 2.6–2.4 Ga Itabira Schlüter (2006) has reviewed the early Precambrian geology of the Group (Spier et al., 2007, and references therein). These overlap in Congo craton. This large and poorly exposed craton encompasses four broad lithological terms and in overall geochronology with parts of widely separated main areas where early Precambrian basement rocks the Transvaal Supergroup of Kaapvaal; however, such potential and supracrustal successions are exposed: (1) the North Gabon massif support for possible contiguity in post-Witwatersrand times (cf., and the Chaillu massif to the south, (2) Kasai massif, (3) NE Angolan Cheney, 1996, for Zimvaalbara) does not necessarily have any bearing shield, and (4) NE Congo craton (Bomu and West Nile gneissic for the Witwatersrand basin period on Kaapvaal. complexes) (Fig. 9). Essentially, these terranes comprise extensive Any possible correlation based on lithostratigraphy or analogous gneissic rocks, greenstone belts (particularly in the NE part of the geological evolutionary histories between the São Francisco and craton) and granitoids. Intrusion of the latter is generally taken to Kaapvaal cratons at the time of the deposition of the Witwatersrand indicate stabilization of the evolving cratonic terranes, with this being Supergroup appears unlikely. The predominantly gneissic–green- achieved by c. 2.7 Ga for (1), c. 2560 Ma for (2) and (3) and stone–granitic lithologies of the four crustal blocks discussed by approximately 2430 Ma for (4) above (Cahen et al., 1984; Petters, Barbosa and Sabaté (2004) for the Bahia region, although showing 1991). some chronological overlap with Witwatersrand basin evolution, are In Gabon, the Chaillu massif is dominated by c. 2.8–2.6 Ga granitoid essentially cratonic basement assemblages rather than supracrustal rocks including a large charnockitic body, and relatively rare pods and basin-fill successions. The transgressive marine deposits at the base of lenses of greenstone lithologies (Schlüter, 2006 and references the Caraça Group, although superficially comparable to Witwaters- therein). The North Gabon massif is characterized by a greater variety rand sedimentary rocks, are younger than c. 2.61 Ga and thus post- of granitoid basement lithologies, including charnockites and granu- date the Kaapvaal sediments. lites, and in its southern portions has many small greenstone belts characterized by itabirites, ferruginous quartzites and schists (Schlü- 4.5. Amazon ter, 2006). The c. 2.0 Ga Franceville Supergroup succeeds these terranes. The Amazon craton comprises a southern Brazil Shield (where In the Kasai and NE Angola shield areas, poorly exposed gneisses Archaean basement and succeeding weakly deformed and low-grade and migmatites are characteristic (Schlüter, 2006 and references metamorphic supracrustal rocks are known), and a northern Guiana therein); c. 3.4 Ga Upper Luanyi granite gneisses and pegmatites are in Shield (comprising essentially c. 2.25–2.1 Ga ensimatic Palaeoproter- apparent structural contact with the poorly dated (possibly 3.4– ozoic greenstone belts), with the inferred age-terrane boundary 2.82 Ga) Kanda Kanda tonalites and granodioritic gneisses. In addition between these two not being well exposed or understood (Olszewski to the latter rocks, are the Kasai–Lomami gabbro-norite and et al., 1989)(Fig. 8A). The supracrustal rocks of the Grão Pará Group in charnockite assemblage, thought to reflect high-grade metamor- the eastern part of the central Brazil Shield overlie the Xingu Complex phosed (c. 2.8 Ga granulite-grade event) deep-seated to possibly Archaean basement gneissic–granitic lithologies unconformably, and effusive mafic rocks, and acidic rocks of possible sedimentary heritage consist of a lower basaltic–felsic metavolcanic unit, a middle BIF unit (Schlüter, 2006). Author's personal copy

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Fig. 9. Outline map of the present-day African continent showing the Congo craton and the four “shields/massifs” that outcrop at its approximate corners: NE Congo craton, Kasai massif, NE Angolan shield and the North Gabon massif/de Chaillu massif. Modiﬁed after Eriksson et al. (1999).

In the NE part of the Congo craton, there is a very large region of cursors to the c. 2.8 Ga high-grade metamorphic rocks of the Kasai Archaean rocks, consisting of: (a) basement gneisses (c. 3.5 Ga Bomu area, cannot be considered in any way equivalent to early clastic and West Nile gneissic complexes); (b) two generations of c. 3.2– sedimentary deposits of the Witwatersrand Supergroup in terms of 2.6 Ga greenstone belts, known as the Ganguan greenstone belt in the their gross geodynamic setting. Once again, thus, there is no west and the Kibalian in the east; (c) two major generations of convincing evidence to support the Congo and Kaapvaal cratons granitoids that were intruded between c. 2.9 and 2.7 Ga (Schlüter, being joined together during the period of Witwatersrand basin 2006 and references therein). The c. 3.2 Ga Ganguan greenstone belt evolution. comprises quartzites, phyllites and schists, whereas the younger Kibalian belt consists largely of maﬁc–intermediate volcanics (eastern 5. Discussion facies) and BIF's (western facies). The latter belt is divided stratigraphically into lower Kibalian (c. 2.89 Ga) and upper Kibalian For each of the cratons discussed above, as possibly having been (c. 2.5 Ga), and an overall island arc model has been applied to these joined to the Kaapvaal craton during the lifetime of the Witwatersrand greenstones (Schlüter, 2006). Two generations of widespread gran- basin, there is no convincing evidence to support any such contiguity, itoid rocks are dated at c. 2.84 and 2.46 Ga (Schlüter, 2006 and nor the existence of any small supercratons with Kaapvaal as an older references therein). nucleus thereof. This also has an impact on possible solutions for the The essentially basement lithologies of the Congo craton in the c. source of the Witwatersrand gold and solving the “volume problem”. 3.5–2.5 Ga period are not supportive of any contiguity with the Three main options for this were discussed previously in the paper: Kaapvaal craton. Possible sedimentary rocks which were the pre- (1) a source which was exceptionally rich in Au and U; (2) an Author's personal copy

P.G. Eriksson et al. / Gondwana Research 15 (2009) 354–372 367 extremely efficient placer “engine” operating on “normal” Archaean the Archaean remains problematic (Ernst et al., 2004, 2005). The latter greenstone–granitoid (source) terranes; (3) a relatively small super- authors recognize that the oldest demonstrable flood basalt provinces craton to increase the potential source area for option #2. The lack of are the Fortescue of Pilbara and the Ventersdorp lavas of Kaapvaal (see any strong evidence for contiguity of the cratons discussed in Section 4 also, Eriksson et al., 2002), both correlatable with the c. 2.7 Ga LIP of the paper mitigates against either option (2) or (3) above. This event, while observing that the predominant volcanic style of the leaves option (1) as a possibility, but this is not supported by Archaean is one of greenstone belts. Although short duration volcanic preserved greenstone remnants on the Kaapvaal craton (see, however, events of significant stratigraphic thickness are identified in this Frimmel, 2008). This therefore leads us to consider an alternative greenstone belt record, these belts essentially comprise fault-bounded explanation for certain similarities noted amongst these various packages of volcanic beds with problematic correlation of such cratons, and discussed previously, namely the possibility of global “events” over large lateral distances (Ernst et al., 2004). The same events and the postulate of enhanced mantle influences during the authors, however, note that the LIP record appears to be fairly period of Witwatersrand basin evolution. continuous over time, barring some significant gaps, amongst them one at c. 3.0–3.3 Ga. 5.1. Global events: craton-scale erosion and formation of a residual gold- Relating the inferred craton-wide erosive event (c. 3125–3000 Ma) rich sedimentary cover on Kaapvaal on the Kaapvaal to a mantle plume/LIP origin is thus not supported, but significant weathering of exposed cratons in the Archaean is to be Nelson et al. (1999) proposed global events as the major influences expected under the presumed prevailing aggressive reducing and on the Pilbara and Kaapvaal cratons, in order to explain their apparent greenhouse-type palaeoatmospheric conditions then extant (e.g., similarities. They summarized reliable age data to support broadly Donaldson and de Kemp, 1998; Corcoran and Mueller, 2004), analogous such events: (1) comparable granite–greenstone crust irrespective of freeboard. However, postulating a significant LIP of formation, from c. 3650–3100 Ma; (2) an erosional event of global context at about 3.0 Ga (concomitant with a recent analysis of apparently cratonic scale between c. 3125 and 3000 Ma; (3) major the LIP record as far as it is known; Ernst et al., 2004, 2005) can flood basalt events from 2760 to 2680 Ma; (4) c. 2630–2440 Ma provide a possible mechanism to form composite terranes rich in Au chemical sedimentation succeeded by a transition to clastic deposition for subsequent accretion to the Kaapvaal nucleus within the known (c. 2440–2200 Ma). Comparable and approximately coeval volcanic time constraints of the evolution of the Witwatersrand basin (see events noted on the São Francisco, Amazon and Karnataka (cf. Fig. 4). Abbott and Isley (2002), in an earlier review of superplume Dharwar) cratons, allied to ultramafic magmatism on Yilgarn and history, suggested a superplume event at 3024–3022 Ma (albeit a Superior cratons, all in the c. 2760–2680 Ma interval, led Nelson et al. relatively minor event, following the c. 3.3–3.0 Ga gap in the plume (1999; following Condie, 1998) to postulate rapid and large scale record) and found a prominent increase in superplume activity just convective overturn of Earth's mantle, with accompanying enhanced prior to 3.0 Ga, with a peak at c. 2.9 Ga. If a relatively minor global magmatic activity globally in this time period, as a planetary scale superplume event (Abbott and Isley, 2002) is inferred at c. 3.0 Ga, and event affecting most and possibly all existing cratons at that time. if it is further postulated that one of the plumes impinged beneath the The cratonic-scale erosive event (c. 3125–3000 Ma) on Kaapvaal, Kaapvaal craton, then the resultant thermal anomaly would logically acting on granite–greenstone–gneissic rocks, would logically have have shifted the locus of any subduction–arc complexes “offshore” of provided a heavy mineral (Au and U)-rich clastic supracrustal deposit the Kaapvaal nucleus (see Fig. 10 for a schematic representation of this on this craton, prior to the onset of tectonic compression presumably postulated model) for a fairly brief period (a few million years; Condie, responsible for the widely accepted foreland basin model applied to 2004a). Alternatively, the Kaapvaal craton may have migrated across a the Witwatersrand depository (basin-fill c. 2970–c. 2780 Ma; see hot spot (cf. Zhao, 2007) or the head of a single plume, unrelated to a Fig. 4). Such an existing supracrustal auriferous heavy mineral deposit superplume event, given the lack of strong evidence for the postulated may have provided some of the source material for a second c. 3.0 Ga superplume event. While potential subduction beneath the generation of more efficient placer concentration during the evolution Kaapvaal nucleus would have been rendered unfeasible under such and filling of the foreland basin (this would argue in favour of an postulated conditions, mid-ocean ridge “push” through continued unusually Au and U-rich provenance, as discussed under #1 in the first production of juvenile oceanic crust would have continued (Fig. 10). If paragraph of this discussion). However, it is likely that the major there had been a global superplume event at c. 3.0 Ga, even a weak source of the gold was from the accreted composite terranes along one, such oceanic crustal growth would have been amplified (cf., both the northern and western (modern frame of reference) margins Vaughan and Livermore, 2005). Under such a situation, it is not of the earlier Kaapvaal nucleus (see Fig.1B), as envisaged in the De Wit unreasonable to expect that intra-oceanic obduction tectonics (cf. De et al. (1992) model for Kaapvaal cratonic evolution. Frimmel (2008) Wit et al., 1992) could have ensued, “offshore” of the Kaapvaal stresses the importance of volcanic arc-related gold deposits, within a nucleus, forming komatiitic basalt-rich arc and ophiolite complexes. framework of Earth's crustal gold endowment. Presumably, such ophiolite complexes reflected subduction rollback rather than an ocean closure-related genesis (cf. Robinson and Zhou, 5.2. Mantle superplume event or single plume affecting Kaapvaal at 2008). The likelihood of low angle subduction in the Archaean (e.g., c. 3.0 Ga?: enhanced crustal growth and consequent accretion of gold- Abbott and Hoffmann, 1984; Martin, 1994; Pollack, 1997) probably bearing composite terranes also advanced the formation of such envisaged ophiolite complexes, thus supporting the “enhanced accretion event” concept in the title of Nelson et al. (1999) do not provide an explanation for the craton- this paper. In addition, hydration and serpentinisation at subaqueous wide erosive event noted for both Pilbara and Kaapvaal in the c. 3125– (De Wit and Hynes, 1995) mid-ocean ridges would have produced 3000 Ma period, other than to refer to the possibility of global eustasy. more buoyant upper oceanic lithosphere, again favouring ophiolite Eriksson et al. (2006), in a review of continental freeboard over formation (Robinson and Zhou, 2008). Precambrian time, note specifically for Kaapvaal that freeboard was Such intra-oceanic ophiolite complexes would also have encom- apparently elevated for this craton for the c. 3.1–2.7 Ga period; they passed subduction and partial melting of tholeiitic lithologies to postulate that this may reflect thermal–magmatic influences, possibly produce tonalites (cf. Arkani-Hamed and Jolly, 1989; Windley, 1995; plume-related. The global plume or LIP (“large igneous provinces”; however, as a caveat, tonalites could also have formed through e.g., Ernst et al., 2004, 2005) record supports the likelihood of a global underplating of mafic lower crust by plume-related basalts), thus superplume event at c. 2.7 Ga, in post-Witwatersrand times (Condie, forming composite terranes with time, as required by the De Wit et al. 2004a), but the extrapolation of the LIP record into the earlier part of (1992) model of Kaapvaal (and Witwatersrand foreland system; Author's personal copy

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Fig. 10. Schematic sketch illustrating postulated model for accretion of intra-oceanic obduction complexes to the Kaapvaal nucleus, and how these might have been related to a mantle plume impinging beneath the nucleus. In the ﬁnal stage of the model, as the composite terrains accreted onto the nucleus (from west and north — see Fig. 1B), the “greater Witwatersrand basin” comprising foredeep (Witwatersrand Supergroup) and back-bulge (Pongola Supergroup) sub-basins was formed (see also Fig. 2).

Catuneanu, 2001) evolution. The deformation and metamorphism composite terranes occurred (after dissipation of the thermal anomaly inherent in the evolution of these composite (greenstone) terranes associated with the plume impacting beneath Kaapvaal), high degrees may already have begun concentration of minerals such as Au (derived of geodynamism would have provided an even higher initial order of from orogenic quartz vein deposits in the greenstones) within Au concentration (as found for the Pietersburg greenstone belt, structural elements such as fold hinges etc. Assuming a primary northern Kaapvaal craton; De Wit et al., 1993) within uplifted foreland source for gold within the hotter Mesoarchaean mantle (cf., Zhang and basin source areas. It is plausible that the accretion of already Au- Hu, 2002; De Wit, 1998), possibly also related to major change in enriched greenstone–tonalitic terranes, combined with an already Au- Earth's tectonic regime, intra-oceanic subduction–arc–obduction bearing supracrustal clastic cover (formed from aggressive Archaean terranes would have provided signiﬁcant potential gold provenances weathering) on the cratonic nucleus, provided the “enriched Au (cf., Frimmel, 2008). Once accretion of these postulated Au-rich source” envisaged in the ﬁrst paragraph of this discussion. This would Author's personal copy

P.G. Eriksson et al. / Gondwana Research 15 (2009) 354–372 369 also remove any necessity for an expanded gold provenance area in of the Ventersdorp Supergroup on the Kaapvaal craton (e.g., Eriksson addition to Kaapvaal itself, and would thus be compatible with the idea et al., 2002). Kaapvaal rode high during this event (Eriksson et al., already expressed, that there is no convincing evidence to support 2006), and neither accretion of composite greenstone terranes nor contiguity of Kaapvaal with any other “southern” (reference frame that foreland basin formation could have been accommodated within this of Aspler and Chiarenzelli, 1998) craton. Assuming the validity of the setting. postulated maximum age (at time of recycling) of subducted Archaean oceanic lithosphere, of c. 30 my (Windley, 1995), accretion of these 6. Conclusions inferred composite terranes could have taken place by about 2970 Ga, following upon a (relatively weak) c. 3.0 Ga plume event (or even a In this paper we have briefly reviewed the geology of the Kaapvaal single plume beneath Kaapvaal), thus satisfying the approximate time craton (South Africa) pertinent to the evolution of the c. 3.1–2.8 Ga constraints of both cratonic erosive event (Nelson et al., 1999) and the Witwatersrand basin, the oldest large depository known, and one onset of Witwatersrand foreland-related sedimentation (see Fig. 4 for famous for its placer gold deposits. Within a framework of relatively the latter). speculative possible Neoarchaean supercontinents, we have examined the basic geology and inferred evolution of a set of six “southern” (cf., 5.3. Foreland basin evolution and placer sedimentation: reworking of Aspler and Chiarenzelli, 1998; as opposed to the “northern” [present- different gold sources during Central Rand Group deposition day frame of reference] Kenorland supercontinent) cratons, which might hypothetically have been linked to Kaapvaal to form a small The Witwatersrand foreland basin encompassed an earlier under- supercontinent: Pilbara, Zimbabwe, Dharwar, São Francisco, Amazon filled stage (West Rand Group — WRG) followed by an overfilled stage and Congo. The possible location of such cratons contiguous to (Central Rand Group — CRG), as found with most classic foreland Kaapvaal may help explain the apparently inadequate (e.g., Robb and basins of all ages (Catuneanu, 2001). It is possible that placer gold Meyer, 1995; Frimmel, 2005, 2008) Archaean granite–greenstone concentration may already have begun in the proximal source areas to source areas for the vast gold deposits, and thus address what is often the WRG, and that the heavy minerals only reached the basin during termed the Au “volume problem”. its final, overfilled CRG stage. A first order of gold concentration may However, the basic syn-Witwatersrand geology and inferred thus have begun, by initial magmatic (Frimmel, 2008) and subsequent evolution of all six of these chosen cratons provides no convincing structural means in the evolving intra-oceanic ophiolite (later support for any such potential supercratons. In contrast, the composite) terranes, and by erosion on the Kaapvaal nucleus prior alternative explanation for similarities in overall geology of cratons to accretion; a second order thereof (both structural and placer- of this age, and particularly relevant for a possible Kaapvaal–Pilbara related) probably occurred during accretion itself, with a third order connection, namely that of global events affecting separated cratons (purely placer; reworking of Au from the accreted terranes and from analogously, seems to be a more plausible explanation (cf. Nelson earlier first- and second-order placers) possibly being related to the et al., 1999). The latter authors discuss a possible global erosion event two stages of flexural foreland basin evolution as envisaged by (c. 3125–3000 Ma) as well as a younger global magmatic event (c. Catuneanu (2001) with placer processes being enhanced by an 2760–2680 Ma). Within this broad framework, we postulate that a aggressive palaeo-atmosphere (Minter, 2006). global mantle plume event at c. 3.0 Ga included a plume which The inferred (albeit weak) superplume event affecting Earth at impinged directly beneath the Kaapvaal craton, and that the c. 3.0 Ga, or even a single plume head beneath the craton, may thus concomitant thermal anomaly halted subduction beneath the nucleus have led to an enhanced rate of mid-ocean ridge juvenile crust of Kaapvaal. However, with continued mid-ocean ridge “push” being production (with anomalous Au contents; cf., Frimmel, 2008), and likely under the influence of a global plume event, we suggest that concomitantly to higher rates of intra-oceanic-related arc–ophiolite intra-oceanic thrust-stacking of ophiolite complexes would have been complex formation within envisaged composite terranes “offshore” of stimulated in many parts of the globe, including “offshore” of the Kaapvaal. Together, these can be seen as beginning a period of thermally elevated Kaapvaal craton. Along with intrusion of tonalitic enhanced accretion, which through its influence also on gold magmas in such evolving arc-subduction complexes, it is possible that concentration prior to the inferred Witwatersrand placer “engine”, a first order of gold concentration through initial magmatic sources might have obviated the need for a (relatively small) “supercontinent” (Frimmel, 2008) and subsequently by structural means during centred on Kaapvaal to provide an adequate gold source for this deformation, may already have begun in these composite terranes. famous auriferous basin. Two obvious questions which arise here are: The earlier “global erosive event” might have led to a parallel Au if a weak plume/superplume event at c. 3.0 Ga could have played such concentration through placer reworking of exposed granite–green- a pivotal role in forming the Witwatersrand basin and its gold stone terranes on Kaapvaal's cratonic nucleus. deposits, why were such depocentres and rich ores not relatively Once the relatively short-lived mantle plume beneath Kaapvaal common in the Neoarchaean across the globe?; and, why did the very subsided as the global LIP event passed, “normal” plate tectonic significant c. 2.7 Ga superplume event (e.g. Condie, 2004a; see also regimes would have dominated over plume influences once again, and Condie et al., 2009-this issue) not also result in a similar scenario accretion of such composite auriferous terranes with Kaapvaal may developing? What makes Kaapvaal (the oldest craton) unique is its logically have followed, according to the well-known De Wit et al. relative stability at c. 3.1–2.8 Ga already, which enabled formation of (1992) model for Kaapvaal evolution. The resultant orogenesis would the Witwatersrand flexural foreland system (Catuneanu, 2001). have produced a flexural foreland basin setting for Witwatersrand Frimmel (2005) emphasizes the importance of this basin genesis in sedimentation, along with a likely second order of Au concentration in the concentration of most of the gold. Furthermore, it is the the uplifted source areas, along the northern and western margins of preservation of this unique (and oldest) basin from later denudation; this craton. As the highly efficient placer system operating under the post-Witwatersrand deformation, particularly the Vredefort impact influence of an aggressive Neoarchaean palaeo-atmosphere (e.g., event at c. 2023 Ma (Kamo et al., 1996) played a critical role in this. In Corcoran and Mueller, 2004) was stimulated by these already Au-rich conclusion, thus, it is not that gold-rich placer deposits (later and highly deformed, elevated proximal regions, a third-order remobilized by dynamic events) developed on the Kaapvaal craton concentration possibly resulted, producing the world's greatest that was a unique occurrence, but that they were able to accumulate known gold deposits. on such a scale and to be preserved to such a degree that formed this However, the question remains: what made the Witwatersrand unique mineral deposit. The c. 2.7 Ga superplume event resulted in the gold deposits so unique in terms of their size? Gold-bearing granite– fully subaerial lavas and fully continental rift-related detrital deposits greenstone terranes are common in many Archaean cratons, mantle– Author's personal copy

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