1. Almost all the gold present in Earth’s crust today was delivered by asteroid impacts four billion years ago. Since then, it has been buried under lava flows and other rock strata.
2. Gold is one of the least reactive elements, but it will dissolve very slightly in water. There are about 20g of gold dissolved in every cubic kilometre of seawater.
3. This dilute gold solution seeps into many rock formations as groundwater. However the concentration, just 10 parts per quadrillion, is far too low to make mining economical.
4. Earthquakes force open cracks and the sudden pressure drop precipitates out the gold. Over millions of years, repeated earthquakes build up veins. Eighty per cent of the world’s gold deposits were formed this way.