What is a gossan in geology ?

Gossan : definition

The gossan is the surface zone highly enriched in iron oxides and hydroxides (limonite, goethite, hematite, etc...) of a sulphide deposit. It was formed by the transformation of sulphides into oxides under the action of oxidizing atmospheric agents. Since the dominant sulphide of these deposits is pyrite, iron oxides will predominate, giving the gossan a characteristic rusty or red tint.

The identification of oxidized minerals (oxides, phosphates, sulphates, carbonates ...) present in a gossan is an excellent indicator of the metals present in the underlying sulphide deposit. The gossan remains the site of intense dissolutions : a part of the elements is dragged downwards by the infiltration water and will reprecipitate in depth in the cemantation zone of the deposit.