Science: After the gold rush: a new quest

When Columbus landed in the Americas, he reputedly found local inhabitants fishing using hooks crafted from gold. Early cultures recognised some of gold's physical properties - it is easy to work, and resistant to corrosion - as well as its decorative appearance. But for most people, gold is still the metal of jewellery, or the gold bars gathering dust in vaults around the world.

Nobody knows who was the first human being to value gold, but it is likely to have been someone in central Europe before 4000BC; somebody with time on their hands to notice the dense, bright-yellow nuggets that collected with the coarse sand in fast-flowing rivers draining the mountains. In South America, too, there was an early gold culture; the Chavin Indians of Peru wrought gold artefacts as early as 1200BC. The early Egyptians mined and used gold for ornamentation.

King Croesus was the first to link the metal directly with wealth by having his head stamped on the first gold coins, dating from the sixth century BC. Subsequent empires, including the Roman, were built in the quest for gold.

The search for gold became democratised with various popular gold-rushes in the mid to late 19th century. These rushes resulted in new colonies of migrant miners in places as far afield as the Yukon, California and Western Australia. The gold rush still continues in many parts of the world, driven largely by multinational companies looking for large deposits, but also by large numbers of artisan miners in areas of South America, Africa and Asia that still employ age-old, labour-intensive techniques.

The challenge to miners is that the Earth's rock normally contains only an average of five-1,000ths of a gram of gold per ton of rock, while the world's oceans contain less than a 200th of that concentration. A troy ounce of gold, the weight at which the gold price is fixed daily in London and New York, is 31.1 grams and would make about three modest 14-carat wedding-rings.

Modern mines today are profitable working rock, which to miners is known as "ore", containing less than three grams per ton, which means that to make the average ring, three tons of ore need to be excavated. The Morro do Ouro mine in Brazil makes a profit from ore containing one-seventh of that proportion. Given this, it is not surprising that during the First World War German chemists even considered extracting gold from sea water, since the oceans of the world contain gold to the value of about pounds 125,000bn at today's prices.

Economic concentrations of gold are rare in nature; they were usually formed by the movement of dissolved gold into fracture "veins" during geological events. Scientists find clues as to how they formed by studying gold deposits that are forming close to the surface today in active geo-thermal areas such as Japan and New Zealand, and at the deep hydrothermal vents that are developed where new oceanic crust is forming deep on the sea-floor.

In the western Pacific, one of these sea-floor vents is now being investigated to assess the feasibility of mining gold-rich ore one kilometre beneath the ocean.

In hard-rock gold deposits, gold occurs largely as the metal itself, or alloyed with another metal such as silver or copper. There are also a few natural compounds with other elements. Although it is relatively soft, metallic gold is highly durable and resistant to attack by most chemicals, hence at the surface of the Earth it can become physically concentrated in soils and rivers.

Historically, much gold has been panned and sluiced from rivers where it is found as nuggets, but even the Romans followed gold underground to mine it from the rock. Despite more than 6,000 years of gold production, only 125,000 tons have ever been mined; about 40 per cent of this has been extracted from the rocks of the Witwatersrand region, which lies in and around Johannesburg in South Africa.

To put this amount of gold into perspective, all the gold ever mined could be fashioned into a cube that would fit between the two sets of stumps on a cricket pitch. Although Britain and Ireland have produced gold in the past, the recorded total production of the British Isles is less than one of the South African gold- mines might produce in a month. British and Irish gold is periodically investigated, but is not likely to compete with the major gold-producing regions, which are largely centred around the earth's oldest rocks (formed more than 2.5 billion years ago) in Canada, Australia, Africa and Brazil, as well as the younger volcanic regions of the Pacific rim and some older, equivalent areas in the Central Asian republics of the former Soviet Union.

The recent upheavals in the gold markets, which have mainly been caused by the wrangle over sales of institutional gold stocks, have been matched by large changes in the geographical spread of where gold is currently being mined. Thirty years ago, South Africa produced 79 per cent of the open-market gold production; now its mines account for only 20 per cent of world production.

The mines of South Africa are deep and labour intensive and their mining costs are spiralling towards the current price of gold, making them less economic than previously.

The new generation of gold-miners come increasingly from the Pacific rim - the Americas, Australia and South-east Asia - where shallow, large, low-cost mines producing low-grade ore are being opened using new technology.

Gold demand from industry is still on the increase, and since 1985 gold usage in the manufacture of jewellery has more than doubled. In addition, hi-tech usage of gold has also nearly doubled. Gold is an excellent conductor of electricity and, because it will not corrode, is perfect for wiring modern electronic circuitry. Pure gold is naturally yellow, betraying its high reflectance of light towards the red end of the visible spectrum. This makes it particularly useful as an infrared reflector and as a result it is an essential part of the shielding system of, among other things, satellites and other space probes, military weapon systems and aircraft such as the US Air Force One (one of the presidential jets).

The Franciscan friar Bartholomaeus Anglicus observed that "gold comforteth lymmes", as far back as 1250, foretelling the use of gold compounds in a modern treatment for arthritis. The inertness of metallic gold in the human body makes it ideal for use in dentistry, and as a tracer for other medical procedures.

The need to reduce mining costs in the face of declining or static prices has meant that miners have looked to advances in metal extraction technology to cut costs. The 19th-century discovery by two Scottish chemists that gold was soluble in dilute cyanide solution was applied to new deposits of low-grade ores discovered in Nevada, where traditional methods of extraction were uneconomic.

In other low-grade ores, where gold is encapsulated in sulphide minerals from which cyanide is unable to leach it out, biotechnology provides an even more innovative extraction tool. The humble bacterium Thiobacillus ferroxidans was known to be active in rotting waste heaps at metal mines where iron and copper sulphide minerals are digested under the right conditions.

Important uses for the mineral-munching abilities of this and other bacteria was recognised by mineral processors, who applied them to complex ores where gold is locked up in sulphide grains. Exposing the gold-containing grains to a bacterial culture results in dissolution of the mineral grain, and the indigestible gold grain being left behind.

The bacteria even appear preferentially to attack the regions where gold may be hidden. The resulting slurry of iron oxides can then be treated with cyanide to dissolve the now-liberated gold. By experimentation it has been found that a cocktail of bacteria, under controlled conditions of acidity and oxygen, improves the efficiency, as does raising the temperature of the culture to about 40C. Processors are even experimenting with bacteria that produce their own cyanide, which may lead to a totally biological extraction technique.

Researchers in New Zealand are taking the biotechnology even further, proposing the use of plants to concentrate gold. Trials using Indian mustard (Brassica juncea) grown on low-grade gold ore piles have shown that the gold is taken up and concentrated, to levels in the dried plants that are comparable to the content in a high-grade ore from a mine.

Given further trials, this bio-mining may become an efficient way to mine in a sustainable way, and since the method also shows some promise with other, more toxic metals, there is potential for plants to be used to clean up contaminated sites.

Until recently gold appeared to be moving, as silver has done in the past, from being considered an intrinsic part of the monetary system towards being a market commodity like most of the other metals. Recent promises to end the central bank sell-off of gold may have put off that moment. Despite this, the apparent strong demand for jewellery and in technology should ensure that the yellow metal has a bright, indeed golden, future, and it would seem that mankind continues to find tools to produce gold at what we consider to be an economic price.

Even if gold runs out on Earth, which is not likely, the metal is an intrinsic part of other planetary bodies in the Solar System. Given this and the speculation about hydrothermal vents on places such as Europa, Jupiter's moon, where gold deposits may be forming today, it is highly likely that there are other Klondikes out there in space.

Dr Richard Herrington works as a mineralogist at the Natural History Museum in London. His recent book, `Gold', is published by the Museum