Gold is often found in underground veins of quartz and, less frequently, in other minerals such as pyrite, granite and mica slate. The veins can be anything from half an inch to several feet wide. 80% of the gold that’s produced today is mined from such sources (the rest is ‘alluvial’ gold – see below). Knowing where to mine involves ‘chasing’ the gold-bearing veins on the surface, back underground. It’s been estimated that in the past 500 years, about 100, 000 tonnes of gold have been mined, but even this amount would only fill a cube with 17 m sides. The world’s biggest gold producer is South Africa, where gold mines are sunk over 3,000 metres deep into the earth.
There are also large deposits of gold above the ground. So-called ‘alluvial’ gold is found as small yellow grains and flakes, or even small nuggets, on the beds of fast-flowing rivers and streams. Natural erosion, flooding, glacial movement and weathering also play their part in freeing the gold from mountain rocks. As it’s carried downstream, the gold-bearing rock is broken up into increasingly smaller pieces, thereby releasing the gold from the quartz and mineral veins. The released gold is itself broken up into smaller and smaller fragments. The further it’s carried downstream, the smaller the gold particles become. By the time it reaches the sea, any gold is in the form of very small specks or grains.
You’re more likely to find gold deposits on the inside of bends in a river, where the water flows less quickly. As the water slows down, the heavy gold particles fall through the gravel on the riverbed, and work their way down through the soil underneath, eventually settling on the riverbed’s clay bottom. The gold can be separated from the rest of the riverbed soil and clay by panning (panning involves scooping up a sample of the sand, gravel and clay on the riverbed and swirling it with water in a shallow, wok-shaped dish), a process that relies on the extreme heaviness of gold. Any gold, being so much heavier than the rest of the minerals in the pan, will settle to the bottom, while the lighter material is washed out of the pan by the swirling motion. Any gold in your panned sample will stand out clearly as tiny, yellow flecks or small grains, a process that relies on the extreme heaviness of gold.
Gold is also often found deposited where a river widens, as well as on the downstream side of large boulders and rocks. The flow of the water slowing down allows any gold to fall to the riverbed under its own weight.
To locate gold on a riverbed, you need to take soil samples by digging down to the darker layers. The darker layer indicates the presence of heavy deposits like magnetite (iron oxide, Fe3O4, a black, magnetic iron ore). The samples are panned in order to discover whether they contain any viable gold. Because of the flow of the river, the gold is often deposited in a line along the riverbed. You can take further samples up and downstream to trace the line that the gold has followed.