Gossan is defined as an Iron bearing capping over a Sulphide deposit or it can be defined as It is a ferruginous and porous looking residue that forms a superficial cover over an oxidized sulfide deposit. It is formed by the oxidation and leaching of sulphide minerals, leaving hydrated Iron Oxides such as Limonite and Goethite, along with Manganese Oxide. Hematite and Jasper may also form. The porous nature of gossans is caused by the removal of the soluble materials. The residual limonite of gossans usually shows a honeycomb structure called “boxwork“. Gossan is formed in zone of Oxidation above the water table and is sometime referred to as an “Iron hat“.Gossan is especially obvious at the surface in arid and semi arid regions, often forming bold outcrops.
Often, a gossan contain cavities left by sulfide minerals that have been leached. Cubic and triangular cavities usually indicate the former presence of Sulfide minerals. These cubic and triangular cavities in gossan could be a bad sign because Gold bearing sulfide usually form as small crystals that would not create large cavities. Gossan is commonly found in areas where there are Copper, Gold ore bodies. There may be enough gold in gossan to form placer Gold deposits. Even the presence of unaltered pyrite, Chalcopyrite or other Sulfites in gossan is a bad sign because it indicates the leaching is incomplete.
Color of Gossan
Color is another potentially useful characteristics , if used with caution. As the color and structure shows the underneath type of mineral content.
- Massive Dark Brown to Black color is formed by leaching of Pyrite.
- Reddish gossan indicate the presence of Chalcopyrite.
- Brown to Red color indicates presence of Iron.
- Brick Red color indicates the presence of Hematite.
- Blue and Indigo colors indicate presence of Copper and Silver.
World Class gossan are found at Rajpura-Dariba Pb-Zn-Cu-Ag deposit in India.