Vredefort South Africa

The most prominent progenetic ore deposits on Earth related to impact are the gold deposits of the Vredefort-Witwatersrand Basin system. The Vredefort impact structure (Fig. 1) is centered on the Vredefort Dome at about latitude 27° S and longitude 27o30'E in north-central South Africa, in the heart of the Witwatersrand Basin.

The origin of the Vredefort Dome was controversial for most of the 20th century (see review by Gibson and Reimold 2001a). Since Du Toit (1954) published a tectonic model involving large-scale thrusting for the origin of the Vredefort Dome, ideas linking the origin of the Vredefort Dome with tectonic processes have been repeatedly proposed (e.g., Colliston 1990; Coward et al. 1995). Early workers, such as Shand (1916) and Hall and Molengraaff (1925), on the other hand, recognized the enigmatic rock deformations associated with the structure and already then related them to a 'catastrophic event'.

Witwatersrand Basin

Fig. 1. The locality of the Vredefort Dome in the geographically central area of the Witwatersrand Basin between the Johannesburg Dome in the northeast and the Welkom (or Free State) goldfield to the southwest. Inset indicates the position of the Witwatersrand basin on the southern African subcontinent. The Dominion Group has been dated at 3.07 Ga, providing an upper age limit for the deposition of the economically important Witwatersrand Supergroup (WSG). The lower age limit for the WSG is given by the 2.7 Ga age of the Ventersdorp Supergroup (Armstrong et al. 1991; Gibson and Reimold 2001a).

Fig. 1. The locality of the Vredefort Dome in the geographically central area of the Witwatersrand Basin between the Johannesburg Dome in the northeast and the Welkom (or Free State) goldfield to the southwest. Inset indicates the position of the Witwatersrand basin on the southern African subcontinent. The Dominion Group has been dated at 3.07 Ga, providing an upper age limit for the deposition of the economically important Witwatersrand Supergroup (WSG). The lower age limit for the WSG is given by the 2.7 Ga age of the Ventersdorp Supergroup (Armstrong et al. 1991; Gibson and Reimold 2001a).

First thoughts about a possible meteorite impact origin were published by Boon and Albritton (1937). A decade later, Daly (1947) concluded that, in the absence of any other conclusive hypothesis, the impact hypothesis

Vredefort Meteorite Impact Diagram
Fig. 2. Schematic geology of the Vredefort Dome (modified after Gibson and Reimold 2001b). CG - Central Granite, see text for further detail.

deserved to be further investigated. In 1961, Dietz discussed the possibility that the Vredefort Dome, which he compared with the Sudbury Structure, could also contain shatter cones, as identified a year earlier at Sudbury. Shortly thereafter, Hargraves (1961) recognized this conical fracture phenomenon in rocks of the Vredefort Dome, providing the first hard evidence for impact at Vredefort. Carter (1965, 1968) described planar deformation lamellae in quartz from Vredefort rocks, which at the time were investigated at a number of impact structures (see papers in French and Short 1968). The Vredefort 'lamellae', though, remained controversial for a long time (e.g. Alexopoulos et al. 1988; Grieve et al. 1990; Reimold 1990), until Leroux et al. (1994) showed conclusively that they indeed constituted bona fide shock deformation features (basal Brazil twin lamellae). Strong evidence in favour of an impact origin for the Vredefort Dome was also produced by Martini (1978), who discovered coesite and stishovite in the structure.

The Vredefort Structure is 2.02 Ga old (Kamo et al. 1996) and deeply eroded. Allogenic breccias and sheets of melt rocks, therefore, are absent. Furthermore, the Vredefort Dome has been subject to a widespread postimpact thermal overprint, which has obscured or modified many of the impact-diagnostic shock features. Pseudotachylitic breccias are common and locally abundant (Reimold and Colliston 1994). Impact melt breccias in the deeply eroded structure are only represented by a series of narrow dikes of the so-called Vredefort Granophyre (see review in Gibson and Reimold 2001a).

Koeberl et al. (1996a) applied the very sensitive Re-Os isotopic method to Vredefort Granophyre samples in comparison to various country rock types and established that the Granophyre contained a very small - only 0.2 % - meteoritic component. Shock deformation in zircon from rocks of the Vredefort Dome (Kamo et al. 1996; Gibson et al. 1997; Leroux et al. 1999; Reimold et al. 2002b) and in feldspar (Gibson et al. 2001) has been discovered, and just recently, Buchanan and Reimold (2002) identified -for the first time - shock deformation in lithic inclusions in the Granophyre. The Vredefort Dome is well known for another, regionally unique deformation phenomenon: ubiquitous veinlets and massive dikes or network breccias, for which Shand (1916) coined the term "pseudotachylyte" (modern spelling 'pseudotachylite'). In the past decades many such occurrences have been described from impact structures, whereby, however, different types of breccias have not been carefully distinguished. Reimold (1995, 1998) proposed to use the non-genetic term "pseudotachylitic breccia" where an origin by impact melting, friction melting, or combination of these processes can not be determined with certainty, and to reserve the expression "pseudotachylite" for bona fide friction melt rock. The largest occurrence of pseudotachylitic breccia observed by our group in the Vredefort Dome measures 2.6 km in length and 50 m in width (Dressler and Reimold 2004). Such breccias also occur in great abundance in the northern, well explored and heavily mined, part of the Witwatersrand Basin and have mostly been linked to the Vredefort impact event (Fletcher and Reimold 1989; Killick and Reimold 1990; Killick 1993; Reimold and Colliston 1994; Trieloff et al. 1994; Reimold et al. 1999b; Hayward et al. 2003), but some older occurrences have also been identified (Berlenbach and Roering 1992; Reimold and Colliston 1994).

Was this article helpful?

0 -1

Post a comment