Relation to Archean Witwatersrand Gold and Uranium Ore Deposits

The Vredefort impact structure encompasses the bulk of the Witwatersrand Basin (Fig. 3). The synclinal structure of the supracrustal sequence from the Witwatersrand to the Transvaal Supergroup in the environs of the Vredefort Dome represents effective downwarping of the stratigraphic units including those of the Witwatersrand Supergroup (McCarthy et al. 1990). This entire succession containing the gold- and uranium-rich conglomerate horizons (known locally as "reefs") of the Witwatersrand Supergroup could have been removed due to erosion since 2 billion years, if it had not been for the Vredefort impact event.

The Witwatersrand Basin represents the world's richest gold province. Some 40-50 % of all the gold ever mined on Earth has been produced from the basin (Robb and Robb 1998), which is roughly an amount of 45 000 to 50 000 tons, at an estimated value of more than 50 billion US$ at a gold price of US$ 310 per ounce. The remaining gold reserves of the Witwatersrand Basin are estimated at 45% of the world's known total reserves. Currently mined strata in the northern part of the Witwatersrand Basin generally dip between 15 and 20 degrees south, towards the Vredefort dome. With many mines already operating at mining depths between 2 500 and 4 000 m, it does seem unlikely, though, that much of the remaining resource will be accessible to mining. However, there are distinct extension possibilities to previously and currently mined areas, such as the so-called Argonaut region adjacent to and to the south/southwest of the Central Rand goldfield (Fig. 5), and the main mining-houses with large-scale operations in the Witwatersrand goldfields do not hesitate to emphasize that they foresee continued gold-mining in this region for many years to come. Although sustainability depends hugely on prevailing gold prices.

Most of the Witwatersrand gold has been mined from the Witwatersrand Supergroup, especially its upper succession, the Central Rand Group. In addition, subordinate amounts of gold have been obtained from Transvaal Supergroup strata, mainly in the region to the northwest of Johannesburg and along the base of the Transvaal Supergroup, the Black Reef Quartzite Formation, to the east of Johannesburg. Minor gold mining activity has also taken place in the 3.07 Ga Dominion Group strata that form the base onto which the Witwatersrand Supergroup was deposited, in an area to the west of the town of Klerksdorp (Fig. 5), along the western margin of the Witwatersrand Basin. In addition to these vast amounts of gold, some 150 000 tons of uranium have been mined, mainly as a by-product of gold mining, but also from several comparatively gold-poor reef horizons. The value of this mined product has been estimated at another 4 billion US$, and estimated remaining uranium reserves are of the order of 475 000 tons.

Mining in the Witwatersrand has been the mainstay of the South African economy for more than a century, and by supporting tens of thousands of migrant workers, has been beneficial for the economies of other southern African countries as well. It is estimated that several million people, directly or indirectly, still owe their livelihood to the Witwatersrand industry today.

Witwatersrand Golden Arc

Fig. 5. The Golden Arc, the annular pattern of Witwatersrand goldfields around the Vredefort dome, extending from the Evander Basin (1) and East Rand (2), via the Central Rand (3) and the Carletonville goldfield ((4) - also known as West Wits Line), and the West Rand (5) goldfields to the southwestern Welkom (6) goldfield. The location of the hamlet of Venterskroon, the focus of a minor goldrush into the Vredefort Dome is denoted (7).

Fig. 5. The Golden Arc, the annular pattern of Witwatersrand goldfields around the Vredefort dome, extending from the Evander Basin (1) and East Rand (2), via the Central Rand (3) and the Carletonville goldfield ((4) - also known as West Wits Line), and the West Rand (5) goldfields to the southwestern Welkom (6) goldfield. The location of the hamlet of Venterskroon, the focus of a minor goldrush into the Vredefort Dome is denoted (7).

The major mining camps of the Witwatersrand Basin are located within a semi-circular pattern at a distance of approximately 30 to 70 km from the outer edge of the Vredefort Dome, known as the Golden Arc (Fig. 5). They are arranged at a distance from the Vredefort Dome that is determined by the depth to the major reefs that can be exploited with current technology.

A long-standing debate has centered on the origin of the Witwatersrand ores. As recently reviewed in great detail by, for example, Robb et al. (1997), Robb and Robb (1998), Phillips and Law (2000), and Frimmel and Minter (2002), one school of thought has favored an origin of the Witwatersrand gold by accumulation as purely detrital ore (e.g., Minter 1999; most recently, Kirk et al. 2002, and references therein; Jolley et al. 2004), whereas others (e.g., Barnicoat et al. 1998; Phillips and Law 2000, and other work comprehensively cited therein) have subscribed to a purely hydrothermalist view. In addition, a large number of Witwatersrand workers, especially those that in recent years carried out detailed petrographic studies on samples from the Witwatersrand reefs, have supported the so-called "modified placer theory" that depends on combined elements of the detrital ore hypothesis as well as the view that post-sedimentary thermal overprint affected the original placer deposits (e.g., Stevens et al., 1997; Robb et al. 1997; Frimmel 1997b; Frimmel and Gartz 1997; Frimmel and Minter 2002; Foya et al. 1999; Gibson and Reimold 1999; Reimold et al. 1999b; Foya 2002; Hayward et al. 2003). The timing of these growths of authigenic ore mineralization is, however, debated, with phases at 2.7, 2.5, 2.3-2.15, 2.06, and 2.02 Ga having been favored by different workers at different times. In addition, a number of workers have established that at post-Vredefort times thermal metamorphism and hydrothermal overprint affected the basin region at around 1.2-1.0 Ga (e.g., Reimold et al. 1995, 1999b; Friese et al. 1995, 2003). Hydrothermal overprint on the Ventersdorp Contact Reef and - to be precise - pseudotachylitic breccia along the reef was related by Reimold et al. (1999b) to autometasomatism of these materials as a direct and immediate consequence of the impact event. Hayward et al. (2003) reported results of a detailed petrographic study of numerous ore samples from all major gold-bearing Witwatersrand conglomerates and goldfields. These authors discussed basin-wide observed mineral deformation (cataclasis) and permeability increase, late in the evolution of the Witwatersrand ores, that was followed by an even later stage of authigenic sulfide-gold mineralization. Hayward et al. (2003) made a case for the most likely deformation event of basin scale having been the Vredefort impact at 2.02 Ga ago.

A number of workers have recognized a low-pressure hydrothermal, lower-greenschist metamorphic overprint in the gold ores throughout the expanse of the Witwatersrand Basin. This aspect of the Vredefort-Witwatersrand system is discussed in the section on epigenetic mineralization related to impact structures. There it is also emphasized that the lower-greenschist metamorphism and related mineralization can be related to the Vredefort event and do not have to be considered as of Ventersdorp (2.7 Ga) or Transvaal (2.6-2.15 Ga) age - as still favoured by many Witwatersrand workers.

In the Vredefort Dome itself, gold mining has taken place at various times in different geological settings. In the latter decades of the 19th century, shortly after the first white settlers had come to the region in the 1840s/1850s, small deposits of gold- and also silver-bearing vein quartz were exploited in the greenstone terrane of the southeastern part of the core (Nel 1927). Gold in strata of the Upper Witwatersrand Supergroup was mined or explored during two periods: In the years prior to the Anglo-Boer War (1899-1902), the Kimberley Reefs - locally known as the Amazon Reef - were mined in the Venterskroon gold field (also known as the Rooderand gold field) in the western collar of the Dome (Fig. 5). Mining was proclaimed in 1887, but, due to unsatisfactory results, operations were discontinued already in 1911 (Borchers 1964). Main mining operations in the early 1900s included the Amazon Mine which was opened in 1910 on farm Rooderand in the Transvaal Republic, and a mine on farm Elandslaagte No. 28 located in the then Orange Free State Republic. Resurgence in mining occurred in the 1930s when the Great Western Mining Company re-opened the mine on Elandslaagte and extended its operations into the Transvaal. In numerous places around the collar of the Dome can one still observe the remnants of gold exploration trenches and adits in the strata of the Kimberley Reefs (Turffontein Subgroup of the Upper Witwatersrand Supergroup, UWS) and also in conglomeratic units of the Johannesburg Subgroup of the UWS. Altogether, these operations never brought a thorough success. The Great Western Mining Company ceased production in 1937. The total amount of gold extracted from Vredefort sites is estimated at no more than some 130 kg - a trivial amount in comparison to the gold production from the entire Witwatersrand Basin.

In conclusion, the Vredefort impact event at 2.02 Ga secured one of the world's largest ore deposits, namely the Witwatersrand gold and uranium province. Based on the ample evidence for sedimentary structures related to gold mineralization, textural evidence that strongly supports a detrital origin of Witwatersrand gold, and, in essence, widespread occurrence of evidence of sedimentary controls on gold mineralization for all auriferous reefs in the basin (e.g., Robb and Robb 1998; Foya 2002; Minter 1999; Minter et al. 1993; Frimmel and Minter 2002), there can be no doubt that much of this deposit was in place prior to the impact event. This event itself caused preservation of these progenetic Archean deposits along the ring syncline of the impact structure, and the ejecta assumed to have covered the gold-bearing strata contributed to their protection from erosion since 2.02 Ga. Thus, it must be concluded that this ore deposit is primarily of a progenetic nature, according to the classification by Grieve and Masaitis (1994). By improving the understanding of the deformation imparted on the strata in the Witwatersrand region by the impact event, beneficial output will be obtained with regard to working effectivity and safety, besides the benefit that the gold-mining industry will reap from improved structural geological understanding of the Witwatersrand Basin.

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  • marta
    How does witwatersrand basin affect sustainability?
    7 years ago

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