![]() You’re more likely to find gold deposits on the inside of bends in a river, where the water flows less quickly. By the time it reaches the sea, any gold is in the form of very small specks or grains. The further it’s carried downstream, the smaller the gold particles become. The released gold is itself broken up into smaller and smaller fragments. 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. Natural erosion, flooding, glacial movement and weathering also play their part in freeing the gold from mountain rocks. 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. ![]() There are also large deposits of gold above the ground. The world’s biggest gold producer is South Africa, where gold mines are sunk over 3,000 metres deep into the earth. 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. Knowing where to mine involves ‘chasing’ the gold-bearing veins on the surface, back underground. ![]() 80% of the gold that’s produced today is mined from such sources (the rest is ‘alluvial’ gold – see below). The veins can be anything from half an inch to several feet wide. Gold is often found in underground veins of quartz and, less frequently, in other minerals such as pyrite, granite and mica slate. For these purposes it’s usually alloyed with other metals such as silver, copper and zinc. It will scratch easily, and it’s therefore unsuitable in its pure state for use as coinage or jewellery. Gold is also extremely ductile (something is ductile when it is capable of being drawn out as a wire under tension without breaking). Just 1g of gold (the size of a grain of rice) can be beaten into a thin film covering 1 square metre. Gold is the most malleable (something is malleable when it is easily beaten into a thin film) element there is. Its heaviness plays a crucial part in many of the physical methods used to extract gold from its various sources. The density of lead, by comparison, is only 11.4 g cm-3. Gold is also extremely heavy, with a density of 19.4 g cm-3. Consequently, gold jewellery can survive essentially unchanged for thousands of years. Gold is extremely unreactive and doesn’t tarnish like most other metals. They are both relatively easy to reclaim from the rocks in which they’re found, and are easy to work. Most metallic artefacts recovered by archaeologists are fashioned from either gold or silver, which are thought to be the first metals to be worked by humans. The current price of gold (October 2002 prices) is $322 per troy ounce, compared with platinum at $566, and rhodium at $690. However, gold is by no means the scarcest or even the most expensive metal. Gold is valuable simply because it’s scarce and difficult to extract. You would need at least 250 twenty-tonne trucks full of the earth in order to recover just 20g of gold that’s a cube of gold with approximately 1cm sides. Gold is a relatively rare element, making up only 0.000 000 4% of the Earth’s crust (by mass). Silver and copper are the only other metals naturally found in their elemental form. Gold is an unusual metal in that it predominantly exists in the Earth’s crust as the element (so-called ‘native’ gold) that is, it’s not chemically combined with other elements. Find out more about The Open University's Science courses and qualifications
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