7.0 Witwatersrand basin, worlds’ largest deposits of gold (Fig. 1a)

The almost 300 km diameter Vredefort Structure rim is surrounded on all sides except the south-east by the world’s largest accumulation of gold mines. This can be seen on the Geoscience Gold map (Vorster 2001). One hundred and thirty years of mining from the Witwatersrand goldfields has produced more than half of the worlds’ gold, over 56,000 tons (Chamber of Mines 2017). They still have the largest reserves in the world although current levels of newly mined gold accounts for only 5% of worldwide production. Now that the deepest mines are below 4 kilometres, the costs of mining limit the depth of gold extraction.

vredefort meteorite structure

Within the Vredefort Impact Structure, gold, mainly as fine particles less than 0.5 mm, is found embedded with 25 mm round translucent pebbles in a matrix of quartz (Whiteside, et al. 1976). This is associated with the entire structure cavity excluding the interior of the rebound at Vredefort itself. The gold reef lies on the inside of the Vredefort Structure cavity, sloping at about 60 degrees to the south at the surface of the Main Reef in the vicinity of Johannesburg, decreasing to about 15 degrees at a depth of three kilometres (Whiteside, et al. 1976). The walls of the entire original ‘jelly bowl’ from Welkom to Klerksdorp to Johannesburg and Springs are lined with fine gold It is probable that the original floor at 15 km deep is also lined with gold as this metal occurs on some of the outside wall of the remnant rebound core (Vorster 2001).

When the meteor cluster penetrated the Earth’s atmosphere at a shallow angle from the current south-east, the smaller fragments of gold were already heated from friction. After penetrating the Earth with other larger, higher melting point minerals they melted first and were vapourised with the underlying strata forming a huge underground chamber as described above. When the roof of this chamber collapsed in a ring at the edge of the transient cavity the vapourised gold was blasted out with other shattered rock and ejecta. The rebound lifted the supracrustal strata forming an upturned collar around the dome and, as the transient wall collapsed, the outer ring of strata slumped into the newly formed crater (Fig. 1b).

Minutes after the crater formation, the ring of fire, with its mushroom cloud and radiant energy, 1400 times greater than received from the sun, melted all the exposed, steeply angled strata surfaces lining the crater, turning silica rock into molten quartz. A shower of hot ejecta pebbles rolled into this melt combined with fine grains of precipitated gold to become imbedded, forming the Witwatersrand Gold Reef, the richest in the world.

An example of metals precipitating after impact is Meteor Crater in Arizona where an engineer, Daniel M. Barringer, searched for nickel and iron under the crater floor without success (Barringer 1964). However, a geologist, Harvey Nininger, discovered fine particles, less than one millimetre in diameter, of nickel and iron in the surrounding area that had precipitated out of a cloud of vapourised metal (Nininger 1949). This was estimated to total between 10,000 to 15,000 tonne.

In South Africa some of the vapourised gold was deflected to the north and north-east by the remains of the surge wave from the initial explosion causing fine particles to rain down in the Mpumalanga and Limpopo provinces as well as north into Zimbabwe. To this day you can still find Zimbabweans panning for those small specks of gold in the streams. It is well known historically that this area was a great source of surface and alluvial gold that was exported via the Mozambique coast by Arab and then Portuguese traders. Hans Merensky’s missionary/mapmaker father located the area of the lost city, Great Zimbabwe, by reading tales passed on to early Portuguese Captains (Lehmann 1959). This great stone complex was known as a gold trading centre from 700 to 1100 AD (Gayre of Gayre 1972).

Geologists still write about gold that was placed in reefs on the walls of the Witwatersrand Basin Lake by age old river deltas as paleoplacer deposits or welled up out of the crust as hydrothermal deposits (Pretorius 1991; Therriault, Grieve & Reimold 1996). Pretorius also said that the long held belief that the source of gold and uranium from the very old greenstone granite was not possible as the mineral composition was not the same and there was just not enough to be a source of the rich Witwatersrand Gold Reef.

I believe that the reason for overlooking a meteoritic source of gold was that until about 1960 the Vredefort Dome was considered to be the full size of the meteorite crater. Such a simple crater did not match up with the Witwatersrand Gold Reef and therefore could not be considered. No one has returned to the enlarged complex crater to examine why the gold coats the inner walls of the rim as well as the outer walls of the rebound in such a rich, uniform manner.

I propose that the gold came from core particles of the Vredefort Meteor cluster. This gold came with the chrome-platinum-mineral rich meteorite fragments that had penetrated under Vredefort. The kilometer’s thick Transvaal Group strata slumped into the crater at 60 degrees, forming a new wall, the surface of which melted in the extreme heat from the fireball. Some of the quartz ejecta pebbles tumbled down the steep sides until they became imbedded in the molten walls with minute particles of precipitation from vapourised gold. Millions of years of sedimentation followed by coal formation and then more sedimentation topped up the remnant crater (Chapter 15).




Rev 20180907 Copyright (c) 2018 dave (at) howcroft.co.za