Apr 29 2014
Gossan refers to the rust-colored capping or staining of a mineral deposit, which is mostly formed due to the oxidation or alteration of iron sulfides.
Gossan generally appears as a red stain against the background rock and soil, due to the presence of a large quantity of oxidized iron. It gets its name from a Cornish miner's slang “gōs”, meaning "blood". Gossan is known as "chapeau de fer" in France, and "eisenhut" in Germany.
Gossan is an intensely oxidized, weathered, or decomposed rock that is found on the upper and exposed part of an ore deposit or mineral vein. When gossan is exposed to water and air at the surface, the minerals tend to oxidize and liberate their contents. The iron oxides in them include hematite and goethite.
The classic gossan contains mainly iron oxides and quartz, frequently in the form of boxworks and quartz lined cavities. The other varieties of gossan contain quartz and iron oxides, goethite, limonite, and jarosite, which form as pseudomorphs.
Although mostly red in color, gossans also appear orange, yellow, or black. Black gossans contain manganese oxides, such as pyrolusite, manganite, and psilomelane.
Geologists have been using gossans as important guides to buried metal ore deposits. An experienced ore prospector will be able to read the clues in the structure of gossans to establish the type of mineralization likely to be found below it.
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