GOSSAN

also spelled Gozzan, rust-coloured oxide minerals (usually limonite) that cap an ore deposit; they thus may be used as clues to the existence of subsurface ore deposits, especially if distinctive boxworks are present. In addition to limonite (a mixture of hydrous iron oxide minerals), gold and silver in the native (natural, nearly pure) state and various sulfate, carbonate, and silicate minerals occur in gossans. The limonite occurs as the residuum when sulfide minerals are dissolved from the outcrops; it is either indigenous (i.e., fixed at the site of the original sulfide mineral) or transported. Indigenous limonite indicates the presence of copper, whereas transported limonite indicates its absence or its presence in very low proportion to iron. See also boxwork.