The Witwatersrand Basin (Wits Basin) is a 6m-thick geological sequence of thin sedimentary layers that stretches from just east of Johannesburg in the north, to the southern Free State in the south west, in the shape of a shallow elliptical dish. It is approximately 300km long and 160km wide and outcrops along its northern and northwestern edges, tilting down into the earth as it extends further south.
It is estimated to have formed between 2.6 and 3.0 billion years ago. Geologists believe that several braided rivers flowed into a huge inland sea that extended across this area at this time. These rivers carried eroded materials down from the surrounding highlands and deposited them in alluvial deltas around the sea, at their respective entry points.
Heavy materials, such as granules, pebbles and heavy minerals were deposited first, building up mineral-rich deltas around the shoreline, while finer sand, clay and silt would have been carried further into the sea. This was happening over millions of years, during which time, various geological changes caused movements that shifted the deposit locations, causing gravel layers to be covered by sand or clay layers, only for those to be covered in turn by later levels of gravel, and again by layers of sand or silt. These deposits piled up over time, layer upon layer, until eventually the lake became silted up.
Several volcanic and metamorphic events are also known to have occurred in the millions of years that followed this, which would have thrust the sequence up in some places, caused it to slip down in others, and also deposited various lava flows or flood basalts, across parts of its surface.
Then approximately two billion years ago, the earth was struck by a massive meteor, which hit a nearby area today known as Vredefort. The impact shattered and lifted an estimated 70km³ of rock, and the debris thrown up by this event covered the whole Witwatersrand with a thick blanket of debris.
The Wits Basin were therefore preserved as mineralised reefs and hardened under these layers, which provided the heat and pressure necessary to transform the unconsolidated sediments into sedimentary rock.
The Wits Basin gold occurs almost exclusively within quartz pebble conglomerates. While the origins of the sedimentary layers of the Wits Basin are generally agreed upon, there remains much contention as to how the gold itself got there.
There are two leading hypotheses: one called the placer model, which holds that the tiny grains of gold were carried down by the rivers from gold-bearing regions to the north and east of the Witwatersrand Basin, and deposited together with the heavier quartz pebbles in the gravel-rich deltas.
The competing theory is called the hydrothermal model, and this advocates that hot water containing dissolved gold ions, emanating from deep within the earth’s crust, was pushed up through the fractures and pores in the sedimentary rock, long after it had been deposited. These then precipitated when they came into contact with the hydrocarbons stored in the rock.
Both sides claim different pieces of evidence in favour of their respective theories, but there is still no conclusive proof as to which theory is correct, and this is primarily as a result of the fact that there is still an enormous amount that we do not know about the world’s richest ever goldfield.
Sources:
- Chamber of Mines of South Africa
- American Scientist November-December 2003; “The Origin of Gold in South Africa”
- City of Jo’burg