Arsenic sulphides

The most important minerals in this class are arsenopyrite, orpiment and realgar.

Arsenopyrite (FeAsS₂) is a common sulphidic host for gold, second only to pyrite. Gold concentrations as high as 15.2 gr/t have been found in a sample from France. Gold associations are similar to those for pyrite/gold assemblages. In arsenical ores formed at high temperature, gold may be incorporated into the lattice of arsenopyrite either in solid solution or on the crystal faces. On cooling there is a distortion in the structure due to the gold content. The concentration of solid solution gold can be much higher in arsenopyrite than pyrite, due to its better matched atomic spacing, crystal chemistry and similar formation temperature to gold. When sulphides are subjected to thermal event gold can migrate to grain surfaces and fractures, thereby increasing liberation.

Orpiment (As₂S₃) contains 61% As and is very minor component in several commercial gold deposits. It has yellow color and a specific gravity of 3.5. Orpiment tends to form under oxidizing conditions, e.g. in the oxidized zone of mineral veins associated with igneous intrusions. Orpiment is readily soluble in alkaline solutions, which leads to some dissolution during gold Cyanidation.

Realgar (AsS) is associated with orpiment to which alters on weathering. It has a red or orange color and has a density of 3.6. it is less soluble than realgar in alkaline cyanidation solutions and has less significance on cyanidation.