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Gossan (eiserner hut oreisenhut) is intenselyoxidized,weathered or decomposed rock, usually the upper and exposed part of anore deposit or mineralvein. In theclassic gossan oriron cap all that remains isiron oxides andquartz, often in the form ofboxworks (which are quartz-lined cavities retaining the shape of the dissolved ore minerals). In other cases, quartz and iron oxides,limonite,goethite, andjarosite, exist as pseudomorphs, replacing thepyrite and primary ore minerals. Frequently, gossan appears as a red "stain" against the background rock and soil, due to the abundance of oxidized iron; the gossan may be atopographic positive area due to the abundance oferosion-resistant quartz and iron oxides. Although most gossans are red, orange, or yellow, black gossans frommanganese oxides such aspyrolusite,manganite, and especiallypsilomelane form at the oxidized portion of manganese-rich mineral deposits.
In the 19th and 20th centuries, gossans were important guides to buried ore deposits used byprospectors in their quest for metal ores.[1] An experienced prospector could read the clues in the structure of the gossans to determine the type of mineralization likely to be found below theiron cap.
The namegossan appears to come from Cornish miner's slang.[2] The terms "iron cap" and "iron hat", common in America,[2] are direct translations from the German "Eiserner Hut" or "Eisenhut", which term was also used in America.[2]
