Gossan (
eiserner hut or
eisenhut) 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 oxides 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. Although most gossans are red, orange, or yellow, black gossans from
manganese oxides such as
pyrolusite,
manganite, and especially
psilomelane form at the oxidized portion of Mn-rich mineral deposits.