is intensely Oxidized , Weathered or decomposed rock, usually the upper and exposed part of an Ore deposit or mineral vein. In the ''classic'' gossan or ''iron cap'' all that remains is Iron Oxide s and Quartz often in the form of ''boxworks'', quartz lined cavities retaining the shape of the dissolved ore minerals. In other cases quartz and iron oxides, Limonite , Goethite , and Jarosite , exist as pseudomorphs replacing the Pyrite and primary ore minerals. Frequently gossan appears as a red ''stain'' against the background rock and soil due to the abundance of oxidized iron and the gossan may be a Topographic positive area due to the abundance of Erosion resistant quartz and iron oxides.
In the 19th and early 20th centuries gossans were important guides to buried ore deposits used by Prospectors in their quest for metal ores. An experienced prospector could read the clues in the structure of the gossans to determine the type of mineralization likely to be found below the ''iron cap''.
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