Iron sulphides

In this class of ores gold is principally unliberated in an iron sulphide matrix, or the behavior of iron sulphides affect process selection or operating conditions. The more important iron sulphides minerals are pyrite, marcasite, and pyrrhotite.

Pyrite is the sulphide mineral most commonly associated with gold. It is very common throughout the world and is ubiquitous in sulphide ore bodies. Pyrite is not usually an accessory mineral in primary igneous rocks, but is common in ore veins and metamorphic ores.

Marcasite has the same composition as pyrite but it has an orthorhombic rather than cubic crystal system. It is formed at lower temperatures than pyrite and usually occurs in secondary rocks. Although marcasite is less common than pyrite, it frequently occurs intergrown with pyrite in sulphide ores and may constitute up to 30% of the total iron sulphides in a pyritic gold ore.

Pyrrhotite is the name for iron sulphides given the formula Fe_(1-x)S, in which x can vary between 0 and 0.2. Pyrrhotite is stable under more reducing conditions than pyrite and hence it tends to oxidize more readily. Monoclinic pyrrhotite has a relatively high magnetic susceptibility. Gold/pyrrhotite composite particles occur predominantly in greenstone belt gold ores, e.g. in several west Australian ores and in some Canadian deposits. The main impact on gold recovery is that pyrrhotite consumes cyanide and oxygen in Cyanidation.