...Three operations are necessary before a deposit of ore yields pure gold. The ore must be removed from the ground, or mined; the ore must be treated, or dressed, to extract the gold and its alloyed metals; and the gold must be refined.

Taking Ore from the Earth...

...Surface or near-surface deposits whether they are placers or veins are the least difficult to mine. All that is required is to scoop up the ore, extract the gold, and discard the worthless material, or gangue. The open-pit method may be employed, as at Carlin, Nev. Where placers occur near water, all operations may be carried out within one huge machine, called a dredge (see Dredge and Power Shovel). Buried placers or veins are mined by conventional methods shafts or tunnels are dug (see Mine and Mining). Gold mines may reach phenomenal depths. In the Witwatersrand, or Rand, in South Africa, one shaft descends more than 12,000 feet. The Homestake Mine at Lead, S.D. the largest gold mine in the United States has reached a depth of more than 6,000 feet.

Ore Dressing...

...Depending upon their composition, gold ores are treated by a variety of methods, either singly or in combination. The processes commonly used are gravity separation, amalgamation, cyanidation, and flotation. Much gold is also recovered in the smelting of other ores, such as copper.

Some gold ores may also be smelted, and others may be roasted to ready them for further treatment. Gravity-separation processes rely upon the fact that gold is heavier than the minerals with which it is mixed. They are most widely used in working placer deposits. Simple gravity-separation devices, which most commonly included the pan, the rocker, and the sluice, though handy decades ago are now tools of the past. The prospector swirled water and gold-bearing sand in the hand-held pan.

The motion separated the particles; the gold settled to the bottom; and the water carried the gangue over the side. The pan yielded only small amounts of gold; it was and is more a prospecting tool than a production device. The rocker, or cradle, resembled an old-fashioned child's cradle.

As it was rocked from side to side in a stream of water, gravel was shovelled onto a screen in its top. Small particles passed to the bottom, where water and motion separated them. Light material washed away, while the heavier gold settled behind crosswise riffles, or ridges. The sluice was a long, downward-sloping trough. Water ran from the high end to the low, carrying along gravel shovelled in at the top.

Gold settled from the mixture as it flowed, lodging behind riffles. In hydraulicking, banks of gold-bearing gravel were broken down and driven through sluices by jets of water. Hydraulicking, however, is highly destructive to land downstream from the mine. Conservation laws prohibit it in many regions. In a device akin to the sluice, a sand-water mixture may be flowed across sheets of corduroy fabric.

The corrugations entrap the descending gold while the gangue flows on. The Golden Fleece of legend, sought by the Argonauts, was probably a sheepskin that had been similarly employed. Gold amalgamates with mercury on contact. Further, mercury is strongly attracted to itself. These properties are joined in the amalgamation process.

First, ore is finely ground to free the gold mechanically from the gangue. Water is added to form a pulp. Then mercury is added to the pulp, and the fluid is passed over mercury-coated copper plates. The mercury unites with the gold in the pulp, and the amalgam adheres to the plates. At intervals the accumulation is removed. Most of the mercury can be squeezed from the amalgam; the rest is removed by heat. Mercury was often placed behind the riffles in sluices to improve gold collection.

The cyanide process, invented near the end of the 19th century, was first employed to treat the gangue, or tailings, from amalgamation plants. It is now the primary process for treating gold ores. Finely ground ore is worked in a solution of sodium cyanide or calcium cyanide. The cyanide dissolves the gold, but not the gangue. The gold-cyanide solution is filtered and washed from the gangue. When powdered zinc is added, the gold is precipitated. In certain ores, gold and gangue may be separated by flotation. Finely ground ore is placed in a solution of chemicals and water. Some constituents of the ore are wetted by the solution and sink; others are not wetted and float in a froth on top.

Gold Refining...

...The gold yielded by ore-dressing processes remains alloyed with silver and trace impurities. It may be refined, or purified, by several methods. Electrolysis is employed at all of the United States mints. Anodes of crude gold from the mines and cathodes of pure gold are suspended in an electrolyte of gold chloride and hydrochloric acid. When current is applied, gold dissolves from the anodes and deposits on the cathodes. Impurities remain behind. Although a slow process, electrolysis yields nearly pure gold as much as 999.9 fine. In chlorination, chlorine gas is passed through molten crude gold. The chlorine combines with impurities, and the resulting compounds rise to the top.

The gold begins to react with the chlorine only when virtually all the impurities are gone. In other refining processes, gold is treated with acids that react with impurities but not with gold itself. Finally, refined gold is melted and cast into bars, or bricks. Those that back United States currency usually weigh about 28 pounds (12.7 kilograms).

GOLD PRODUCTION...

...In the last 500 years, only about 80,000 tons of gold have been taken from the Earth a quantity that could be contained in a cube measuring about 50 feet (15 meters) on a side. (The amount of steel produced every day in the United States is about two times this total.) World reserves of gold economically recoverable by present methods may total only about 32,000 tons.

Where the Gold Is...

...Since gold is both durable and carefully guarded, most of the gold that has been taken from the Earth still exists. Much of it has been buried again in underground vaults, where it is held in government monetary reserves. In 1990 the bullion reserves of the free world were estimated to total some 43,000 tons. Of this, the United States held 11 percent. Gold reserves of South Africa were estimated at some 20,000 tons. Despite the prevalence of antihoarding laws, another 50,000 tons were believed to be privately held.

People are willing to take enormous risks and short-term financial losses to hoard gold against the possibility of fiscal inflation (see Money). The rest of the world's accumulated gold was held in official stocks by central banks or was industrially employed or lost.

Today gold may be bought and sold on many markets. The largest is in London, England. Others exist in several nations of continental Europe, in the Middle East, and in Asia.

Gold Output Today...

...The day of the lonely prospector is largely past. Large-scale mining operations, requiring heavy investment, yield most of the gold produced today. In 1990 world gold production approached 2,000 tons of refined gold a year. About 35 percent of the gold that is mined each year comes from the deep, rich mines of South Africa. United States production is believed to total about 295 tons a year.

Mines in Australia yield about 242 tons of gold a year. Canada, in fourth place, produces about 165 tons of gold a year. Brazil produces about 78 tons of gold a year. Before World War II, United States gold production reached some 5 million ounces a year.

Classified during the war as "nonessential," gold mines were forced to close. Many indeed, most failed to reopen after the war because costs had so risen that the mines could no longer be worked at a profit. Today mines in the United States produce some 295 tons of gold a year. The majority of this comes from the Homestake Mine in South Dakota. The Homestake, the Carlin mine in Nevada, and a copper mine in Utah have traditionally accounted for about half of all United States production. Arizona, Washington, California, Alaska, Colorado, and Montana also yield significant quantities of gold. Annual United States consumption of gold in jewellery and in manufacturing exceeds annual domestic production by a margin of two or more to one.