Gold has a very small content in the nayural world, and because of the uneven distribution in the Crustal, gold enriched in some areas, forming valuable mining deposits.
Generally, gold is insoluble. It is usually remained and enriched in the oxidation zone or weathered, denuded and then redeposited, forming alluvial gold.
Gold is pro-sulfur, so gold minerals are often associated with pyrite, arsenopyrite and other sulfide minerals under native conditions. However, gold doesn't form a sulfide with sulfur, and is not easy to be oxidized. As a result, gold exists as native gold.
There are over 20 kinds of gold minerals, mainly include:
(1) Natural gold (Au). Natural gold often shapes as granular, flake or other irregular shapes. It can be larger than tens kilograms or small as a highly dispersed gold particles in the ore, whose size can be as small as 0.1 microns or finer. Natural gold minerals are not pure, often contains Ag, Cu, Fe, Te, Se and other impurities. But with the increased proportion of lower impurity content, the proportion of native gold is 15.6 ~ 18.3 (19.3 for pure gold), hardness is 2 to 3.
Normally, impurities in natural gold include Ag, Cu and Fe: Ag takes 5%～30%, Cu takes 1.5%, and Fe takes 2%. Although natural gold contains Cu, Ag and other impurities, it is not real alloy, and it has a special structure and inhomogeneity.
(2) Gold minerals with silver. Since the atomic radius of gold and silver are similar, the lattice structure type and chemical properties are teh same, and if the content of silver reaches a certain degree, the mineral can be called silver-gold mineral.
(3) Minerals with gold and platinum group elements. If considerable amount of platinum group elements are mixed in gold, it can form palladium gold deposit (containing 11.6% palladium), platinum mine (containing 10% platinum), iridium ore (containing 30% iridium ), platinum silver gold mine, palladium-copper-gold, etc.
(4) Bismuth gold mine. It is a combination of gold and bismuth under certain geological conditions. If bismuth content is less than 4%, it is a solid solution; while bismuth content is more than 4%, it is a mixture of bismuth and nature, whose characteristics are similar with nature gold.
(5) Combined minerals of gold and tellurium. These combined minerals includ tellurium gold (AuTe2), tellurium bright gold and silver ore needle tellurium, tellurium and silver mines and leaf gold and other tellurium.
(6) Gold-antimony mine (AuSb2). Gold and zinc compounds.
In all, main gold mineral is natural gold, followed by silver and gold mineral, telluride gold, etc.