The Witwatersrand Basin in South Africa and how it acquired its gold reefs

Written by Dr Carina Lemmer
Published: 25 May, 2020

The Witwatersrand (Wits) Basin was created about three billion years ago in the centre of current South Africa.  It was an inland sea at the time with beaches all around the inner edges.  It is known worldwide for the spectacular gold mineralization that was concentrated on these beaches.  Many theories have been proposed as to what created this unique phenomenon.

Recent studies of aeromagnetic maps revealed that mantle plumes (hotspots) followed a path along an ancient spreading ridge that is today along the eastern coast of Africa.  The very first hotspot came down from the north following in this spreading ridge in a sea that covered everything, creating lava flows around the ridge.  At its southern extreme the spreading ridge has a fork where it swings sharply to the west.  This fork is roughly underneath the town of Newcastle in the province of KwaZulu-Natal today. 

When the first hotspot reached this fork it followed along to the west within the ridge, but soon afterwards its track curved to the north as it tried to reach the spreading ridge again for the northward journey.  Its track thus created a loop over Southern Africa.  Subsequent hotspots tried to follow in its footsteps but their tracks had to accommodate the built up volcanic material in the ridge as best they could, and thus they started to deviate from the spreading ridge per se.  Nevertheless, the latest hotspot almost four billion years later still aimed for the fork underneath NewCastle as a turning point.

And so it happened that a hotspot with lava that contained plentiful gold and uranium made the turn and completed the loop before its track crossed over the spreading ridge again on its northward journey.  In the process of making the loop the erupting volcanoes of the hotspot of course built a mountain range all along the loop.  This captured an inland sea within the loop, with lava containing gold and uranium all around the edges.

A hotspot punches up against the crust repeatedly before it punches through to create a volcano. Each punch creates uplift of the crust, not only at the site of the soon-to-be volcano, but also at the previous site where the volcano was. This lifted the beaches of the inland sea around each punch and water washed away the light particles.  The heavy particles settled down in place.  It so happens that gold and uranium particles are very heavy, and therefore this process concentrated the gold and uranium particles on the beaches further with every punch.

The miracle for gold in the Witwatersrand Basin was that the next hotspot followed almost exactly the same loop around its beaches.  There was thus a second opportunity at concentrating the gold mineralization further, although this second hotspot did not contain gold itself. In all the millennia that followed, the Wits Basin itself moved several times, so that it is no longer near Newcastle, but the gold was sealed and preserved by subsequent lava flows.