Veins of sulfide minerals come from deep underground where they are in a chemically reduced form. When they are exposed to the water and air at the surface, the minerals proceed to oxidize and liberate their ingredients. The metals in the sulfides may move away along with the sulfur compounds, which generally oxidize to sulfuric acid (H2
), but oxides of iron and aluminum tend to stay put. A large sulfide deposit, as it ages at the surface, may gradually gain a thick cap of iron oxide mineralsa gossan. "Gossan" may also mean the red oxide material itself.
The word "gossan" comes from the Cornish language and refers to the red color of the oxidized iron minerals. It may also be called an iron hat or the French chapeau de fer or the German eisenhut.
Gossans are primarily iron oxides such as hematite and goethite, or the intimate mixture of both called limonite. In the old days these might have been of interest as iron ore, but their real value to prospectors was as a sign of ore deposits underneath. And in addition to iron, gossans concentrate native gold and silver.
Also Known As: capping, iron hat, chapeau de fer, eisenhut
Alternate Spellings: gozzan