Gold's atomic number of 79 makes it one of the higher atomic number elements that occur naturally. Although traditionally, gold is thought to have formed by supernova nucleosynthesis., a new theory suggests that gold and other elements heavier than iron are made by the collision of neutron stars instead. Either way, satellite spectrometers in theory detect the resulting gold, "but we have no spectroscopic evidence that [such] elements have truly been produced." These gold nucleogenesis theories hold that the resulting explosions scattered metal-containing dusts (including heavy elements such as gold) into the region of space in which they later condensed into our solar system and the Earth. Because the Earth was molten when it was just formed, almost all of the gold present on Earth sank into the core. Most of the gold that is present today in the Earth's crust and mantle is thought to have been delivered to Earth later, by asteroid impacts during the Late Heavy Bombardment. A schematic diagram of a NE (left) to SW (right) cross-section through the 2.020 billion year old Vredefort impact crater in South Africa and how it distorted the contemporary geological structures. The present erosion level is shown. Johannesburg is located where the Witwatersrand Basin (the yellow layer) is exposed at the "present surface" line, just inside the crater rim, on the left. Not to scale. The asteroid that formed Vredefort crater 2.020 billion years ago is often credited with seeding the Witwatersrand basin in South Africa with the richest gold deposits on earth. However, the gold-bearing Witwatersrand rocks were laid down between 700 and 950 million years before the Vredefort impact. These gold-bearing rocks had furthermore been covered by a thick layer of Ventersdorp lavas and the Transvaal Supergroup of rocks before the meteor struck. What the Vredefort impact achieved, however, was to distort the Witwatersrand basin in such a way that the gold-bearing rocks were brought to the present erosion surface in Johannesburg, on the Witwatersrand, just inside the rim of the original 300 km diameter crater caused by the meteor strike. The discovery of the deposit in 1886 launched the Witwatersrand Gold Rush. Nearly 50% of all the gold ever mined on earth has been extracted from these Witwatersrand rocks.

• published: 16 Feb 2015
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