Gold Fact Sheet

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Introduction

Gold, the only yellow metal, has the chemical symbol Au, which is derived from the Latin word for gold - aurum. It has a density nearly twice that of lead, is a good conductor of electricity and heat and is so malleable that it can be rolled thin enough to allow light to pass through. Common acids will not dissolve gold but 'aqua regia' (a mixture of nitric and hydrochloric acids) will, as will alkaline cyanide solutions.

Gold had a significant historical role in Australia, which had its first gold rush in 1851 after the mineral was found near Bathurst in New South Wales. The Bathurst gold rush was followed by discoveries in Victoria. Gold fever drew tens of thousands of immigrants from many parts of the world to the Australian colonies. Ballarat and Bendigo in Victoria became sites of major rushes. Later, in the early 1890s, great finds were at Coolgardie and Kalgoorlie in Western Australia.

Within 10 years of the rushes to Bathurst, Ballarat and Bendigo, Australia's population trebled to more than one million people. Gold discoveries spurred the development of inland towns, communications, transport and foreign trade. Although gold boosted Australia's development, its importance declined during most of the 20th century as other minerals became of greater economic significance. It underwent a resurgence in the 1980s and 1990s when the application of new technology allowed lower grade ores to be processed economically.

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Occurrence

Gold usually occurs in its metallic state, commonly associated with sulphide minerals such as pyrite, but it does not form a separate sulphide mineral itself. The only economically important occurrence of gold in chemical combination is with tellurium as telluride minerals.

Most gold mined in Australia today cannot be seen in the rock. It is very fine grained and mostly has a concentration of less than 5 grams in every tonne of rock mined. Primary gold deposits are formed from gold-bearing fluids at sites where the chemistry and physical characteristics permit gold deposition. Primary deposits are often modified by weathering, but secondary deposits are formed only after the complete breakdown of the host rock has occurred. Liberated gold is concentrated in alluvial (placer) deposits.

Examples of primary deposits include those mined at Kalgoorlie in the Super Pit, Granny Smith, St Ives, Norseman and Mount Magnet (WA), Gympie and Ravenswood (Qld), Callie (NT), Stawell (Vic), Cadia (NSW), Henty (Tas) and Challenger(SA). At Olympic Dam (SA) gold occurs and is mined with copper and uranium. Secondary (alluvial) deposits which sparked the rushes of the 1850s, are no longer major sources in Australia. Gold is found also as a minor component in many base metal deposits and is recovered as a by-product at some operations such as Rosebery (Tas).

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Australian Resources and Deposits

Australia's economic gold resource is just over 5,200 tonnes.

About 60% of Australia's resources occur in Western Australia, with the remainder in all other States and the Northern Territory. Virtually all resources occur in primary deposits, many of which have undergone some degree of weathering. Weathered primary deposits are important to the gold industry because they are usually easier and cheaper to mine and the gold is easier to recover.

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Australia in the World

Australia has about 10% of world economic gold resources and is ranked third after South Africa and USA. It is the world's third largest producer, after South Africa and USA and accounts for about 11% of world output.

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Mining and Processing

Most of Australia's gold production comes from open-cut mines. Large capacity earth-moving equipment is used to remove waste rock from above the ore body and then to mine the ore. Waste and ore are blasted to break them into sizes suitable for handling and transport to waste dumps or, in the case of the ore, to the crusher.

Underground mining is used where the depth of ore below the surface makes open-cut mining uneconomic. Vertical shafts and declines (spiral tunnels) are used to move people and equipment into and out of the mine, to provide ventilation and for hauling the waste rock and ore to the surface.

Deep extensions of deposits mined by open pit methods may be mined later by underground methods beneath the old open pit.

Processing

Coarse gold may be removed by gravity concentration. The processing required to recover fine gold from crushed ore is determined by the free-milling or refractory nature of the ore. Free-milling ore is ore from which gold can be recovered by crushing, grinding and cyanidation (treatment with a dilute cyanide solution) without additional processing. Free-milling oxide ores are suitable for direct cyanidation of the crushed and ground ore.

In refractory ore, gold is locked in the sulphide minerals so that, to achieve satisfactory levels of gold recovery, additional processing such as roasting or biological leaching is required before cyanidation. Sulphide minerals in refractory ores are converted to oxides by either roasting or biological leaching to release the gold. In biological leaching the oxidation is caused by the action of specific bacteria on the ore. The tonnage of refractory ore to be roasted or leached is greatly reduced by first producing a finely ground concentrate.

Ground ore or treated concentrate is placed in a weak solution of sodium cyanide, which dissolves gold and forms a slurry of gold-bearing solution and barren solids. Some ores may be treated by heap-leaching. This involves sprinkling a weak cyanide solution over an open pile of ore stacked on an impervious base. The solution percolates through the ore, leaching gold as it goes, and is drawn off at the base before being treated to recover the gold.

In both cases, the gold is recovered from the gold-bearing solution in a process in which pellets of activated carbon made from charred coconut husks are added to the slurry and the gold-bearing ions are adsorbed onto the pellet surface. The pellet load is moved through a number of linked tanks containing slurry in a direction opposite to the slurry movement.

The pellets loaded with gold are removed and the gold is stripped from them by washing in a solution of hot cyanide. The carbon used in the process is recycled and an electric current is passed through the new solution, depositing the gold on a steel wool cathode. The gold laden cathode is treated with hydrochloric acid to dissolve any residual steel and the gold sludge is filtered and dried, ready for smelting. At this stage the gold-bearing material may contain silver and base metals.

Gold is smelted in a crucible furnace to produce unrefined bullion. In smelting, base metal impurities are oxidised and absorbed, leaving the precious metals to be poured into ingot moulds. Smelted gold is then refined.

Refining

Several refining processes are used in Australia - chlorination, electrolytic and aqua regia.

In the chlorination process (Miller process), chlorine is introduced to melted bullion in a crucible furnace. The gas reacts with silver and any remaining base metals to form chlorides, bubbles of which rise to the surface of the molten bullion and are removed. The molten, refined gold is then cast into bars. The electrolytic process (Wohlwill process) involves dissolving gold from the bullion (anode) in a chloride solution and redepositing the gold on a pure gold or titanium cathode. Silver remains on the anode. The cathodes are melted and cast. In the third process the unrefined bullion is dissolved in aqua regia and silver is precipitated as silver chloride. Sulphur dioxide gas is passed through the remaining solution and gold is precipitated as a fine metallic powder. The gold is then melted and cast.

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Uses

Gold has long been a medium of exchange and investment. It is widely used to produce coins such as the Australian Nugget. Although Governments of most countries include holdings of gold as part of their monetary reserves many are starting to reduce their holdings. Financial institutions and individuals also use gold as a store of wealth.

The main uses of gold are jewellery, dentistry, the adornment of buildings and for artistic purposes. In jewellery, gold is often mixed with other metals to produce alloys of different colours. White gold is an alloy of gold with silver, palladium, nickel and copper. Yellow, green and red golds are produced by alloying gold with copper and silver in different proportions. The gold content of jewellery is expressed as a number of carats - pure gold is said to be 24 carat gold; 18 carat gold contains 18/24 or 75% pure gold.

High electrical conductivity, malleability and ductility favour the use of gold in electronic and computer circuitry, radar equipment and satellites. Because of gold's heat reflecting properties it was used as a film coating on the Apollo 14 lunar module, the vehicle which first landed man on the moon. An important and growing use is in the mechanism and circuitry of safety air bags in motor vehicles.

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Suggestions for Further Reading

  • Berkman, D.A. and Mackenzie, D.H. (Eds) 1998 Geology of Australian and Papua New Guinean Mineral Deposits, Monograph 22, The Australasian Institute of Mining and Metallurgy, Melbourne.
  • Blainey, G.L. 1993 The Rush that never ended: A history of Australian Mining, 4th edition, Melbourne University Press, Melbourne.
  • Geological Survey of New South Wales 1986 Gold in New South Wales, Department of Mineral Resources, Sydney.
  • Australia's Identified Mineral Resources 2007, Geoscience Australia, Canberra.
  • Geoscience Australia, Gold Downunder.
  • Hughes, F.E. (Ed) 1990 Geology of the Minerals Deposits of Australia and Papua New Guinea, Volumes 1 and 2. The Australasian Institute of Mining and Metallurgy, Melbourne.
  • Woodcock, J.T. and Hamilton, J.K. (Eds) 1993 Australasian Mining and Metallurgy : The Sir Maurice Mawby Memorial Volume, Volume 2, The Australasian Institute of Mining and Metallurgy, Melbourne
  • World Gold Council Internet site has a variety of informative articles at http://www.gold.org.
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