Post Impact Ore Mineralization in the Vredefort Witwatersrand Structure

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At Vredefort, a curiously complicated ore geological situation prevails. As explained earlier, the Archean Witwatersrand Supergroup strata of exceptional gold and uranium concentration were impacted by the Vredefort bolide some 700-900 Ma after their deposition. The current erosion level has been estimated for geological and metamorphic reasons (McCarthy et al. 1986; Gibson et al. 1998; Gibson and Reimold 2000) at ca. 7-10 km. Consequently, only a deep cross-section through the central uplift is currently accessible for investigation, and a relatively deep level through the surrounding ring basin (compare Fig. 10). Hydrothermal deposits that could be unequivocally related to the Vredefort impact event or a post-impact thermal/hydrothermal overprint have not been identified to date. However, isotopic work by a number of researchers (Trieloff et al. 1994; Reimold, 1994; Reimold et al. 1995; Friese et al. 1995, 2003) and mineralogical and chemical studies (Gibson and Wallmach 1995; Gibson et al. 1998; Gibson and Reimold 1999; Reimold et al. 1999b) have indicated that the whole area of the Vredefort Dome and environs has experienced late overprint at about impact times (2 Ga), as well as significantly later, during Kibaran (Grenvillian) times around 1 Ga ago.

The West Rand Group rocks exposed in the inner part of the collar of the Dome exhibit mid-amphibolite facies mineral assemblages, whereas the regional metamorphic grade in the surrounding basin is much lower at greenschist facies level. Detailed petrographic analysis of recent years (Gibson and Reimold 2000, 2001b; Gibson et al. 1998, 2001, 2002; Foya 2002) has shown that the Vredefort Dome experienced not one but two post-Archean metamorphic events. The first event is attributed to the 2.06 Ga (Walraven et al. 1990; R.E. Harmer, personal communication to RLG) Bushveld regional magmatic event, during which intraplating of lower- to mid-crustal mafic and ultramafic magmas raised the regional crustal geotherm to ca 40 oC/km. The lower greenschist facies metamorphism in the gold fields and in the Central Rand Group of the outer collar of the Dome is also related to this event. The close textural association between the silicate metamorphic minerals, authigenic pyrite and gold in the Witwatersrand reefs suggests that gold was mobile during this event. The second metamorphic event was centered on the Vredefort Dome and generated temperatures between 1000 and 1400 oC in the center of the Dome (Gibson 2002; Gibson et al. 2002), and between 300 and 500 oC in the Witwatersrand Supergroup strata in the Dome (Gibson et al. 1998; Foya 2002). Hydrothermal effects (T ~ 300 oC) related to this event extend to the gold fields along the northwestern margin of the Witwatersrand basin (Frimmel and Gartz 1997; Foya 2002). The extreme temperatures in the center of the Dome reflect a combination of uplift of hot rocks from mid-crustal levels and shock heating induced by release of elastic strain energy within mineral lattices following passage of the shock wave. Farther from the Dome, heat from a large impact melt sheet lying in the crater may have contributed, to some degree, to the generation and circulation of hydrothermal fluids.

Chemical alteration and isotopic resetting observed in Witwatersrand strata including gold mineralized reef horizons have both been related to the Vredefort event (e.g., Frimmel et al. 1993; Frimmel and Minter 2002; Reimold et al. 1995, 1999b; Foya et al. 1999; Foya 2002). In both the Vredefort Dome and surrounding Witwatersrand Basin, the abundant pseudotachylitic breccias (Reimold and Colliston 1994 and references therein; Gibson and Reimold 2001b) provide an excellent time marker, since it has been established that at least the majority of these occurences must be related to the impact event. Detailed ore mineralogical investigations, such as those by Foya (2002), Foya et al. (1999), Reimold et al. (1999b, 2002b), Frimmel and Gartz (1997), Gartz and Frimmel (1999) and Hayward et al. (2003), established that the present ore textures were developed at post-impact times (with the impact event characterised by basin-wide brittle deformation and pseudotachylitic breccia development), involving gold remobilization and redeposition throughout the basin.

Current thinking by some Witwatersrand workers (e.g., Gibson 2002; Gibson et al. 2002) involves a thermal/hydrothermal system that raised temperatures in the currently exposed inner parts of the central uplift to at least 1000 oC and in the surrounding collar of Witwatersrand Supergroup strata to between 300 and 500 oC. Associated hydrothermal effects at temperatures of about 300 oC extended throughout the Witwatersrand Supergroup in gold fields surrounding the Vredefort Dome at a distance of 40-60 km from the center of the impact structure (e.g., Gibson and Reimold 2001a,b). Under these conditions, much of the gold in important economic strata, such as the Ventersdorp Contact Reef or the Kimberley Reefs, has been redissolved and then redeposited, as a direct consequence of the impact event. It is not clear whether this event has, thus, caused local enrichment of the pre-existing ore, or whether it only resulted in redistribution and textural changes on small scales - but within a large ore province.

A conceptual model for hydrothermal processes in a Vredefort sized impact structure, in general agreement with many of the findings of Naumov (2002), is presented in Figure 10. A schematic complex impact structure is shown, with an original blanket of hot impact melt. An erosion level consistent with the Vredefort-Witwatersrand situation is also indicated. The central uplift region is made up of hot mid-crustal rocks that have been subjected to high shock pressures. Along a profile radially outward from this hot central uplift, temperature decreases continuously, and the same effect is noted along a profile downward from the upper impact melt layer. Outside of the central uplift region, lithostatic pressure increases with depth. One must also assume that impact-generated porosity (= permeability) decreases away from the central parts of the impact structure.

Because of elevated temperatures near the overlying impact melt body and in the central uplift region, in combination with a decrease of porosity away from the center of the impact structure and a decrease of lithostatic pressure in the uppermost strata, it can be concluded that fluid flow will be largely laterally away from the central uplift and will largely be confined to upper stratigraphic levels. As discussed, the Vredefort-Witwatersrand region has suffered extensive erosion since impact at about 2 Ga ago, with strata of several kilometer thickness having been removed. If erosion of 57 km is assumed for these outer parts of the Vredefort impact structure, it appears that the currently mined Witwatersrand strata would have been located in, or perhaps in the lower part, of the assumed flow regime. This implies that gold grade at even lower depths might be less for two reasons: (a) the strata would be more distal with regard to the assumed source regions for gold to the north/northwest of the Witwatersrand Basin (based on paleocurrent directions), and (b) less gold could have been remobilised from the Archean sediments below the favored flow regime for impact-mobilised fluids.

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