Witwatersrand

on which a large portion of South Africa rests. (See illustration below.) To place this diagram into its broader context, see the diagrammatic geological cross section through the Vredefort impact structure at the end of the article. The Witwatersrand plateau consists of a layer of mainly sedimentary rocks laid down over a period of about 260 million years, starting approximately 2.97 billion years ago. The entire series of rocks, known as the "Witwatersrand Supergroup", consists of very hard erosion resistant quartzites, banded ironstones and some marine lava deposits, interspersed with softer, more easily eroded tillites, mudstones and conglomerates. The oldest rocks (laid down 2.97 billion years ago) form the northern scarp of the Witwatersrand plateau; the youngest (laid down 2.71 billion years ago) are those that form the southern edge of the plateau. Gold is found in the conglomerate strata of the younger members of the Supergroup, locally referred to as banket. The abundance of this gold is without a natural equal anywhere else in the world. Over have been mined from these rocks since this precious metal was first discovered here in 1886. This accounts for approximately 22% of all the gold that is accounted for today. Gold mining in these buried portions of the Witwatersrand Supergroup is sometimes carried out at depths of below the surface. Witwatersrand Basin and uraninite. This is a paleoplacer deposit, part of an ancient alluvial fan succession. . The Witwatersrand Basin is a largely underground geological formation which surfaces in the Witwatersrand. It holds the world's largest known gold reserves and has produced over , which represents about 22% of all the gold accounted for above the surface. Geological origin The Witwatersrand basin was created during the Archean Eon, and is therefore amongst the oldest geological structures on Earth. It was laid down in two stages, over the course of 260 million years starting just short of 3 billion years ago. The first phase, lasting 60 million years, consisted of sedimentary deposits in a shallow sea, conveniently named the "Witwatersrand Sea". The resulting 2500–4500 m thick layer of sediments is termed the "West Rand Group" of Witwatersrand rocks. The second phase, which lasted for 200 million years, followed on from the first phase, with on-land deposits, resulting from the retreat of the Witwatersrand Sea, leaving a wide almost flat coastal plain over which rivers from the north formed wide braided river deltas, into some of which rich deposits of gold were deposited. The resulting 2500 m thick layer of rock is termed the "Central Rand Group". The "West Rand Group" and "Central Rand Group" of rocks together form the "Witwatersrand Supergroup", the full horizontal extent of which is termed the Witwatersrand Basin. , outlined in red, in relation to present-day Southern Africa. The blue area depicts the portion of the craton that subsided below the "Witwatersrand Sea", about 3 billion years ago. It is in this sea that the sediments accumulated and would ultimately form the "West Rand Group" portion of the "Witwatersrand Supergroup" of rocks. The younger "Central Rand Group" of rocks accumulated on the low, flat coastal plain (see diagram below) after the Witwatersrand Sea had retreated southwards as a result of uplifting of the craton, especially in the north. CT indicates Cape Town, D Durban, B Bloemfontein, J Johannesburg, and K Kimberley. , Klerksdorp, Carletonville, West Rand, East Rand and Evander. but island arcs did form. It was the coalescence of several of these island arcs that led to the formation of the Kaapvaal craton, one of the first microcontinents to form on Earth about 3.9 billion years ago. Within 60 million years, up to 4500 m of sediment had accumulated on the granite base, to become the "West Rand Group" of rocks that contribute over 60% of the total thickness of the Witwatersrand Supergroup. The gold in the Witwatersrand Basin area was deposited in Archean river deltas having been washed down from surrounding gold-rich greenstone belts to the north and west. Rhenium-osmium isotope studies indicate that the gold in those mineral deposits came from unusual 3 billion year old mantle-derived intrusions known as komatiite, present in the greenstone belts. ==Consequences of mining the ancient Witwatersrand rocks==

Apart from the obvious hollowing out of the rocks below southern Johannesburg, causing unpredictable sinkholes, surface instabilities and earth tremors, the bringing to the surface of rocks that had been laid down in oxygen-free conditions had unforeseen effects. Iron pyrite (FeS2), which is relatively plentiful in the gold ores of the Witwatersrand, oxidises to insoluble ferric oxide (Fe2O3) and sulfuric acid (H2SO4). Thus, when mine waste comes into contact with oxygenated rainwater, sulfuric acid is released into the ground water. Acid mine drainage, as the phenomenon is called, has become a major ecological problem, because it dissolves many of the heavy elements, such as the uranium, cadmium, lead, zinc, copper, arsenic and mercury found in the mine dumps, facilitating their passage into surface water and ground water. The tailings ponds contain an average of 100 mg/kg of U3O8, and uranium levels are measureable in human hair. Sulfuric acid also erodes concrete and cement structures, resulting in structural damage to buildings and bridges. ==History==

Although gold had been discovered in various locations in South Africa, such as Barberton and Pilgrim's Rest, as well as at several sites near the Witwatersrand, these were alluvial concentrates in contemporary rivers, or in quartz veins, in the form that gold had always been found elsewhere on earth. When George Harrison, probably accompanied by George Walker, found gold on the farm Langlaagte, west of what would become the city of Johannesburg, in an outcrop of conglomerate rocks, in February 1886, they assumed that this was alluvial gold in an old riverbed, that had been tilted as a result of earth movements. The park is on the busy Main Reef Road, immediately west of Nasrec Road. In 1887 Cecil John Rhodes registered "The Gold Fields of South Africa" in London, South Africa's first mining house, with a capital of . His brother Thomas was the first chairman. ==See also==