gold, silver, and copper deposits

gold, silver, and copper deposits Gold, silver, and copper were the first metals used by humans. Under suitable geological conditions each of these metals can be found in the native, or elemental state, and thus can be gathered for use without having to resort to smelting. Our ancestors had learned to work these metals into useful and ornamental objects by at least seven to eight thousand years ago.

Gold and silver are present in the Earth's crust at extremely low levels of concentration. The average geochemical abundance of silver is approximately 80 parts per billion (ppb), and gold is even lower at about 2.5 ppb. Various geological processes can lead to the localized concentrations of gold and silver that are called ore deposits. Gold must reach a concentration of about 1 part per million (ppm) for mining to be considered, and silver must reach a level of 50 to 60 ppm to be profitably recovered. The several circumstances under which such concentrations can occur in nature determine the various kinds of gold and silver deposits.

Gold deposits

Gold occurs primarily as the native metal, but it almost always has some silver admixed by atomic substitution. If the silver content is 20 per cent or more by weight, the name electrum is used. Gold also forms telluride minerals, such as calaverite (AuTe₂), but tellurides are rare. Although much of the gold that is mined is recovered from mines worked specifically for gold, it is also present in small amounts in many kinds of deposits, and as a result a great deal of gold is also recovered as a by-product from the mining of copper, lead, and zinc.

Gold deposits can be divided into lode and placer deposits. Lode deposits are primary: that is they are emplaced in the hard rocks of the Earth's crust. Placer deposits are secondary: that is, they are derived from lode deposits and are found in the zone of weathering and sediment accumulation at the Earth's surface.

The formation of lode deposits requires the collection and transport of gold in hot, aqueous solutions moving through the crust. Although gold is a noble metal (that is, it is chemically unreactive), it can form soluble halide complexes, thiosulphate complexes, and other chemical complexes; it is in these complexed forms that it is transported in solution during ore-forming processes.

Ore-forming solutions can either be released by crystallizing magmas, or they can evolve from deeply circulating meteoric waters. As the solutions move through fractures in the crust, the gold carried in solution can be precipitated as a result of cooling, boiling, or chemical reactions with enclosing rocks.

Many of the most ancient gold mines, and some of the more famous ones from modern times, were simple gold-bearing quartz–pyrite veins in faults and other fractures in rocks. Examples of gold–quartz–pyrite vein deposits are the Mother Lode in California, the Hollinger deposit in Ontario, Canada, and the Bendigo and Ballarat deposits of Victoria, Australia.

In the classification of mineral deposits proposed by Waldemar Lindgren (1860–1939), deposits that are formed under conditions of low to intermediate temperatures (150–350°C) and low-pressure, near-surface conditions are called epithermal deposits. They are of two general kinds. First, and most common, are epithermal deposits formed as a result of subaerial volcanism of the kind associated with convergent margins of tectonic plates. Such deposits tend not to extend to great depths but to be exceptionally rich and deserving of the term bonanza. Examples are Cripple Creek, Colorado; Goldfield, Nevada; and El Oro, Mexico.

A second kind of epithermal deposit has been recognized only in relatively recent times because the gold is present in tiny grains. The deposits are accordingly referred to as ‘invisible gold deposits’, or ‘no-see-em’ deposits. Examples of such deposits are Carlin, Cortez, Jerritt Canyon, and Getchell, all in Nevada. The deposits, which are of relatively low grade and are mined on a very large scale, were formed from relatively low-temperature solutions (250°C or less) that circulated through large volumes of fractured rock and deposited the tiny grains of very finely dispersed gold.

Because gold is inert to chemical action by rainwater and air, or nearly so, grains of gold persist and concentrate in the soil as other minerals are weathered away. When the products of weathering are removed by flowing water and redeposited as sediments, gold can become concentrated in placer deposits. Gold is malleable and does not fracture as it is tumbled about in running water. With a density of 15 to 19 g cm⁻³, depending on the silver content, gold becomes concentrated by flowing water as less dense grains of sand (density 2.6–3.0 g cm⁻³) are winnowed out. Tens of thousands of placer gold deposits have been discovered in various parts of the world. Indeed, placer gold is so widespread that it has been won on every continent and in almost every country.

It was surely placer gold that was first exploited by our ancestors, and an important amount of gold is still recovered today from placers in Alaska, the Yukon in Canada, Russia, and elsewhere. More important than young placers, however, are the vast palaeoplacers, estimated to be about 2.7 billion years old, of the Witwatersrand in South Africa. The source of the gold in the Witwatersrand placers is a geological puzzle, but the size and importance of the deposits are not in question. It is estimated that about 40 per cent of all the gold ever mined has come from the Witwatersrand mines, and even though production is now declining, South Africa is still the world's leading gold producer.

Silver deposits

Unlike gold, silver forms many minerals by combining with sulphur arsenic, antimony, and other chemical elements. Native silver is a relative rarity and most of the world's silver is recovered from minerals such as argentite (Ag₂S) and proustite (Ag₃AgS₃), or from tetrahedrite (Cu₁₂Sb₄S₁₃), in which silver replaces some of the copper in the atomic structure.

Like gold, a great deal of silver is recovered as a by-product of bulk mining of copper, lead, zinc, and gold—indeed far more silver is produced as a by-product than is produced from what are primarily silver mines. Deposits that are mined largely or entirely for their silver content are mostly epithermal veins. The bonanza silver deposits of Potosi, Bolivia; Guanajuato, Mexico; Creede, Colorado; and Tonopah, Nevada, are classic bonanza-type epithermal deposits. The Andean volcanic province, together with the Mexican and western North American volcanic provinces, are host to the world's largest and richest epithermal silver deposits, and more than half the silver ever mined has come from the region. Indeed, silver was essentially a rare metal in most parts of the world until the Spanish conquistadors invaded South and Central America and silver started to flow from the vast silver stocks of the Incas and Aztecs.

Brian J. Skinner