The element gold is one of the least reactive metals and is found in nature mostly as a free metal or with silver as a gold-silver alloy. It is also found in some mineral compounds, particularly with tellurium. These compounds are usually formed by melting and crystallization of magmas. The formation of such gold compounds requires predominance of the siderophile element magnesium over the chalcophile element sulfur in the melt.
Refractory gold concentrates typically contain submicroscopic amounts of gold encapsulated within the crystal matrix of iron sulfide minerals such as pyrite, pyrrhotite and arsenopyrite. These host minerals must be broken down chemically by roasting, pressure oxidation or bacterial leaching to liberate the gold for recovery by cyanide leaching. Much attention is given to the oxidation of the sulfides to elemental sulfur, sulfur dioxide gas or sulfate ions in these processes. Less attention is given to the deportment of the iron into basic iron sulfate, ferric sulfate or jarosite, which can have a profound effect on gold liberation and processing costs in the subsequent cyanidation process.
The crystal structure of gold(II) sulfate is quite different from the structure of coinage metal(II) sulfates such as CuSO4 or AgSO4. Central to this unusual behavior is an arrangement of the Au24+ ion that makes very close contacts with the octahedral Au atoms. This feature is not reflected in periodic density functional theory calculations that are used to predict the structures of other metal(II) sulfates.