There should be significant reserves of gold north of the Main Reef line, buried beneath the built-up areas of Johannesburg and Langlaagte, among others, posits historian Rod Kruger in his paper, The Origins of the Original Witwatersrand Gold Fields.
“When George Harrison discovered the Main Reef of gold on the Langlaagte farm in 1886, it was believed by many to be an old river that had been turned on its side. They therefore believed the gold could not continue for more than 30 m to 50 m underground,” Kruger comments.
He notes that many early theories were disproved, and that the picture of how the Witwatersrand goldfields developed only became clearer once imagery from space became available.
He states that, millions of years ago, a great inland sea or lake existed on what has since become the Witwatersrand. This inland sea stretched out toward and across the Free State.
“Along the shoreline, mostly along what would become the Witwatersrand, stretching away towards Potchefstroom, [there] seemed to be a series of hot springs . . . there may have also been genuine volcanic lava vents.”
Kruger states that these springs carried iron-rich mud dense enough to lift various minerals, such as copper, silver, uranium and others from the earth’s core. He says it is presumed that these hot springs vented in a great semicircle of about 150 km.
“The springs seem seasonal. In times of rain, as cool water reached the hot rock, it cracked the underlying rock, allowing [minerals] to boil out in huge quantities across the low-lying ground to the south . . . these minerals, mixed with quartz rock and other debris, formed layers of iron-rich conglomerate.”
He suggests that, in dry seasons, the waters dried up and sand and dust blew across the lake, hardening the successive layers into the quartzite found today. “This can be seen at the Roodepoort Botanical Gardens, where a cliff face exposes these layers.”
At some far-off point in time, a huge meteor fell at what is now known as Vredefort. Kruger says: “This was so big it is reported by University of the Witwatersrand (Wits) [School of Geoscience head] Professor Roger Gibson, as having an impact equalling 400 000 megatons of TNT.” It caused a crater over 100 km wide and tens of kilometres deep.
“Looking at the impact site from space – using Google Earth – it is clear that the Witwatersrand was formed as part of this impact, the semi-circular shape of the ridge forming an outer rim of the crater.”
Kruger adds that it is evident that the meteor struck at an angle, from the south, demonstrated by the Witwatersrand ridge being most disturbed and deformed north of the impact. “This will also explain why some of the gold mines in the Randfontein area had stopes that were vertical.”
He states that the gold- and mineral-rich layers were displaced downward, with the ‘hinge being along the Witwatersrand ridge and sloping downwards at angles of about 30º at surface and curving out as deep as 6 km underground.
Kruger explains that, on the surface, following the impact, a huge tidal wave rushed northwards, “taking the tops of the volcanic vents, or pipes, with it”.
This wave carried debris and pipes that piled up along a roughly east-to-west line to form the Witwatersrand Ridge, about 120 km from the initial impact. “Today, if one studies the formation of the ground between the Main Reef and the Witwatersrand Ridge, one realises that the ground slopes gradually up [and] then drops dramatically over the ridge – in fact, there are two ridges in most places, becoming more extreme towards Krugersdorp and Randfontein.”
He suggests that the tops of the pipes came to rest along the Witwatersrand Ridge, “mostly along the southernmost, north-facing ridge”.
Kruger states that, when the Struben brothers began prospecting the Witwatersrand ridge around 1884, they found gold in small quantities in the streams quartzes. “When Fred Struben discovered the Confidence Reef, he wrote in his diary: ‘I was finding a grain of gold for every grain of sand’.’’ Kruger sourced this information from the Fred Struben diaries found in the Oppenheimer Library at Wits.
Kruger adds that many others tried mining into the Witwatersrand ridge, only to find gold on the surface that petered out at about 30 m. The pipes also, “almost all dipped towards the south”.
He recalls that Fred Struben’s diary illustrated this – Struben lamenting: “I was chasing a pipe, but lost it and could not find it again.”
Kruger believes that the gold Struben and others found was the tops of the vents, displaced north by the meteor impact and sheared off from the parent pipes.
He adds: “It is therefore probable that [more of] these pipes still exist north of the Main Reef and that their existence was never suspected by early miners, concerned only in mining on the Main Reef series, which dipped south.”
He knows that it is probably impossible to discover the Johannesburg pipe, given that the city and its surrounding suburbs are built on top of it. “It lies with future prospectors to prove or disprove these pipes,” Kruger concludes.