5 Occurrence
Due to its relative chemical inertness gold is usually found as the native metal sometimes as large nuggets, but usually as minute grains in minerals, typically quartz (usually as veins), or sulfide minerals most commonly, pyrite, chalcopyrite, galena, sphalerite, arsenopyrite, stibnite and pyrrhotite or associated with these minerals. Gold more rarely occurs with tellurium in the minerals petzite, calaverite, sylvanite, muthmannite , nagyagite and krennerite .
Gold is widely distributed in the earth's crust at a background level of 0.03 g/1000 kg (0.03 ppm), hydrothermal ore deposits of gold occur in metamorphic rocks and igneous rocks, alluvial deposits and placer deposits originate from these sources.
The primary source of gold is usually igneous rocks or surface concentrations. A deposit usually needs some form of secondary enrichment to form an economically viable ore deposit: either chemical or physical processes like erosion or solution or more generally metamorphism, which concentrates the gold in sulfide minerals or quartz. There are several primary deposit types, common ones are termed reef or vein. Primary deposits can be weathered and eroded, with most of the gold being transported into stream beds where it congregates with other heavy minerals to form placer deposits. In all these deposits the gold is in its native form. Another important ore type is in sedimentary black shale and limestone deposits containing finely disseminated gold and other platinum group metals.
Gold ore grades may be as little as 0.5 g/1000 kg (0.5 ppm) on average in large easily mined deposits, typical ore grades in open-pit mines are 1 - 5 g/1000 kg (1-5 ppm), ore grades in underground or hard rock mines are usually at least 3 g/1000 kg (3 ppm) on average. Ore grades of 30 g/1000 kg (30 ppm) are usually needed before gold will be visible to the naked eye, therefore even in gold mines you will often not see any gold.
Gold is extracted from alluvium ores by techniques of placer mining and from hard rock ores by initially crushing and grinding the ore and then by one or more of the following techniques; gravity separation , flotation, cyanidation, amalgamation, roasting, bioleaching or pressure oxidation (autoclaving). Refining of the metal is frequently accomplished by electrolysis. Gold occurs in sea water at 0.1 to 2 mg/1000 kg (0.1 - 2 ppb) depending on sample location. However, as of 2004 there is no profitable method for recovering gold from sea water.
Since the 1880s South Africa has been the source for about two-thirds of the world's gold supply. The city of Johannesburg was built atop the world's greatest gold finds. Gold fields in the Orange Free State and the Transvaal were deep and require the world's deepest mines. The Boer War of 1899– 1901 between the British and the white Boers was at least partly over the rights of miners and possession of the gold wealth in South Africa. Other major producers are Canada, United States and Western Australia. Mines in South Dakota and Nevada supply two-thirds of gold used in the United States.
