TYPES OF ORE (below)
Gold occurs principally as a native metal, usually alloyed to a greater or lesser extent with silver (as electrum), or sometimes with mercury (as an amalgam). Native gold can occur as sizeable nuggets, as fine grains or flakes in alluvial deposits, or as grains or microscopic particles embedded in other rocks. Ores in which gold occurs in chemical composition with other elements are comparatively rare. They include calaverite, sylvanite, nagyagite, petzite and krennerite.Gold is extremely rare. According to geological experience essentially all gold is found only in low concentrations in rocks.
Gold is rarer even than platinum, although because of platinum's more even dispersion in the Earth's crust it is actually harder to find commercial deposits of platinum.
Gold is more frequently deposited in the concentrations which make gold mining viable.
Gold's average concentration in the Earth's crust is 0.005 parts per million. The technology of extraction is expensive primarily because the process always requires gold mining companies to manipulate large physical quantities of ore for small results. The energy required to heave, grind and process ore is itself valuable, as are the chemicals used in the process, and this places a lower limit on the quality of ore which can be profitably worked in the gold mining process.
At different points concentration of minerals within the earth's crust varies from their average, and it is those variations which produce workable ores for gold mining. Iron, for example, accounts for an average 5.8% of the content of the Earth's crust. It needs to be concentrated by natural variations to about 30% to be considered an ore, indicating a required geological concentration of about 5 times. A lower grade gold ore would contain something like 5 grams per tonne (5 parts per million). So gold ore needs to be concentrated by about 1,000 times above its average dispersion to become viable for gold mining.
The process of gold concentration happens both above and below the surface of the Earth. On the surface there is alluvial gold which has been concentrated by the effects of running water, usually rivers. Because of its extreme density metallic gold will readily fall out of suspension as water slows down. So where a river cuts through gold bearing rock, and then slows down as it hits a flatter/wider river bed, gold will concentrate in a 'placer' deposit, allowing extraction of gold particles by panning and the modern day industrial gold mining equivalents.
Underground gold veins or 'lodes' are produced in association with various metallic deposits, often including sulphides and pyrites. Gold concentration may occur as other minerals are leached away over a long period. Ore of sufficient yield to support gold mining is very rare.
Relatively raw gold is purified in two main ways. The cheaper first stage of purification is the Miller process which uses chlorine gas and reaches purification of 99.5%, and then there is the more expensive Wohlwill process which electrolyses gold to purities of 99.99%.