Gossan (eiserner hut or eisenhut) is intensely

oxidized Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a de ...

, weathered or decomposed rock, usually the upper and exposed part of an ore deposit or mineral

vein Veins are blood vessels in humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenate ...

. In the ''classic'' gossan or iron cap all that remains is

iron oxide Iron oxides are chemical compounds composed of iron and oxygen. Several iron oxides are recognized. All are black magnetic solids. Often they are non-stoichiometric. Oxyhydroxides are a related class of compounds, perhaps the best known of w ...

s and

quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...

, often in the form of

boxwork In geology, boxwork is defined as a honeycomb-like structure that can form in some fractured or jointed sedimentary rocks. If the fractures in the host rock are mineralized, they can become more resistant to weathering than the surrounding rock, ...

s (which are quartz-lined cavities retaining the shape of the dissolved ore minerals). In other cases, quartz and iron oxides,

limonite Limonite () is an iron ore consisting of a mixture of hydrated iron(III) oxide-hydroxides in varying composition. The generic formula is frequently written as FeO(OH)·H2O, although this is not entirely accurate as the ratio of oxide to hydrox ...

goethite Goethite (, ) is a mineral of the diaspore group, consisting of iron(III) oxide-hydroxide, specifically the "α" polymorph. It is found in soil and other low-temperature environments such as sediment. Goethite has been well known since ancient ...

, and jarosite, exist as pseudomorphs, replacing the

pyrite The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic luster and pale brass-yellow hue giv ...

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 a

topographic Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the land forms and features themselves, or a description or depiction in maps. Topography is a field of geoscience and planetary sci ...

positive area due to the abundance of

erosion Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is di ...

-resistant quartz and iron oxides. Although most gossans are red, orange, or yellow, black gossans from

manganese Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy u ...

oxides such as

pyrolusite Pyrolusite is a mineral consisting essentially of manganese dioxide ( Mn O2) and is important as an ore of manganese.. It is a black, amorphous appearing mineral, often with a granular, fibrous, or columnar structure, sometimes forming reniform c ...

, manganite, and especially

psilomelane Psilomelane is a group name for hard black manganese oxides including hollandite and romanechite. Psilomelane consists of hydrous manganese oxide with variable amounts of barium and potassium. Psilomelane is erroneously, and uncommonly, known as ...

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 by prospectors in their quest for metal ores.Johnson, Joseph Colin Francis (1897) ''Getting Gold: A Practical Treatise for Prospectors, Miners, and Students'' Lippincott, Philadelphia, Pennsylvania
page 15
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''.

Name

The name ''gossan'' appears to come from Cornish miner's slang."gossan" '' Century Dictionary and Cyclopedia'' Vol. III, page 2581 The terms "iron cap" and "iron hat", common in America, are direct translations from the German "''Eiserner Hut''" or "''Eisenhut''", which term was also used in America.

Notes

References

*Guilbert, John M. and Charles F. Park Jr (1986) ''The Geology of Ore Deposits,'' W. H. Freeman, pp 799–830, Economic geology Mineralogy Cornish words and phrases {{mineral-stub