What is a gossan
A gossan is the iron-stained leached cap over a weathered sulphide orebody. The word is on none of 158 species sheets in our set: it names rock, not minerals.
A gossan is the rusty, iron-oxide-rich rock left at surface where a sulphide orebody has weathered: the sulphides have oxidised away and the iron stayed behind as goethite and hematite. The word is Cornish, from gossen, blood. The word appears on none of the 158 species sheets in this site's mineral reference set, because it names a rock and not a mineral.
In short
- A gossan is rock, not a mineral, and that is why it is hard to look up. Searching all 158 species sheets in this site's Handbook of Mineralogy reference set for the word gossan returns zero hits. Twenty-nine of the same sheets say oxidized zone in their occurrence field and 46 say secondary. The reference literature describes the same place without using the word.
- The colour is iron that stayed put. Sulphuric acid generated by oxidising pyrite dissolves copper, zinc and lead and carries them downward; iron is far less mobile and is left as goethite, hematite and the mixture field geologists call limonite. A gossan is the residue, which is why it is barren and why it matters.
- What a collector wants is usually just below it. The 29 species whose sheets name the oxidised zone are the classic secondary suite: azurite, cerussite, pyromorphite, mimetite, wulfenite, vanadinite, smithsonite, hemimorphite, adamite, brochantite, anglesite and the rest. The gossan is the sign; the zone beneath it is the target.
- Not every rusty rock is a gossan. Iron-stained sandstone, bog iron, laterite, an ironstone band and a weathered basalt all look the part. The test is whether there is evidence of former sulphide: boxwork, cellular texture, relict cubic voids where pyrite used to be.
- I found a rusty orange rock on a mine dump is the commonest way this question arrives, and the honest first answer is that on a mine dump the rusty rock is often just oxidised waste. Gossan proper is an in-situ surface feature over an orebody, not loose material on a tip.
| What it is | Colour and texture | Diagnostic feature | Worth working? |
|---|---|---|---|
| Gossan | Rust red to dark brown, cellular, often boxwork | Boxwork: a honeycomb of thin iron-oxide walls where sulphide grains dissolved out. Relict cubic cavities mean pyrite | The gossan itself is barren. What sits under it may not be |
| Oxidised zone below a gossan | Green, blue, yellow, orange in a brown matrix | Actual secondary species: carbonates, sulphates, arsenates, phosphates, vanadates | This is the collecting horizon |
| False gossan | Rust red, massive, no cellular structure | Iron introduced from elsewhere, or a leached iron-rich rock with no sulphide history | No |
| Ironstone or bog iron | Ochre to dark brown, bedded or nodular | Sedimentary bedding or nodular form; no boxwork, no relict sulphide shapes | No |
| Iron-stained joint or fault | Thin rust films on fracture surfaces | Staining only, on an otherwise unaltered rock | No |
| Weathered mine dump | Mixed rust and fresh rock together | Broken, angular, unsorted, with blast marks or tool marks | Sometimes. It is waste rock, not a gossan |
Why no mineralogy reference will define it for you
We checked, because the absence is the useful part. Across the 158 Handbook of Mineralogy species sheets in this site's reference set, the word gossan appears zero times. Not once, in any field, on any sheet.
The same sheets describe the environment constantly. Twenty-nine of the 158 carry the phrase oxidized zone in their occurrence field — adamite, anglesite, antlerite, aurichalcite, austinite, azurite, brochantite, cerussite, chalcosiderite, chlorargyrite, clinoclase, conichalcite, native copper, descloizite, dioptase, duftite, hemimorphite, libethenite, mimetite, mottramite, phosgenite, pseudomalachite, pyromorphite, rosasite, smithsonite, torbernite, vanadinite, vivianite and wulfenite. Forty-six use the word secondary. And limonite turns up on seven sheets, never once in an occurrence field.
The reason is a division of labour. Gossan is an exploration and field-mapping term for a body of rock; a mineralogy reference describes species. The British Geological Survey and the economic-geology literature use the word freely because they are describing outcrops. So a collector who goes to a mineralogical source for a definition will find the place described in detail and the name missing, which is exactly what makes the term feel slippery.
One consequence worth carrying: limonite is in the same category. It is a field name for a mixture of hydrous iron oxides, mostly goethite, and it is not a species either. A gossan is largely made of a substance that is not a mineral, described by a word that is not in the mineralogy.
How a gossan forms, and what the colour is telling you
Start with a sulphide orebody at depth and let the water table fall through it. Pyrite and the other sulphides oxidise; the reaction produces sulphuric acid and ferric iron; the acid attacks everything around it.
Copper, zinc and lead go into solution and move. Copper and zinc travel furthest, which is why they reappear as carbonates and silicates lower down, and why the base of the oxidised zone can be richer than the original rock — the supergene enrichment blanket. Lead moves least, which is why cerussite and anglesite often sit close to where the galena was.
Iron does not travel. Ferric iron is almost insoluble at ordinary surface pH, so it precipitates within centimetres of where it was released, as goethite and hematite. That is the whole of the colour. The rust is not a coating on the gossan; it is the orebody, minus everything a miner wanted.
Boxwork is the same story at millimetre scale. A sulphide grain dissolves and leaves a cavity; iron oxide lines the former grain boundaries; what remains is a honeycomb of thin walls whose geometry preserves the shape of the mineral that is gone. Cubic voids mean pyrite or galena. A triangular lattice suggests sphalerite. Reading boxwork is a real skill and an old one, and it is the single most reliable thing in the field that separates a gossan from a merely rusty rock. The wider process is on how mineral specimens form.
What this is actually worth to a collector, and what it costs
Be clear about the value proposition, because it is frequently oversold. A gossan is barren. The metals have left. Specimens from the gossan itself are goethite, hematite and limonitic masses, and while good botryoidal goethite is genuinely collectable, most of it is not.
The value is as an indicator. A gossan says: there was sulphide here, and the oxidised zone is below. Historically that is exactly how orebodies were found, and the old workings in Cornwall, Devon and the Caldbeck Fells were routinely started on a gossan outcrop. A dump beside such a working is a reasonable place to look for the secondary suite, and that is the practical link for a collector.
The honest cost of acting on this. Access and permission come first and are not a formality — see whether it is legal to collect minerals in the UK. Many gossan outcrops sit on notified geological sites where hammering is an offence. Old dumps carry real physical risk, covered on whether it is safe to collect on old mine dumps. And the species that make the oxidised zone worth visiting include several that need careful handling: pyromorphite, mimetite and vanadinite are lead minerals, and adamite and conichalcite are arsenates.
Weighed against that, the return is a day out and, on a good site, a handful of micromounts. That is a fair trade for a lot of people and a poor one for others, and the arithmetic is worth doing before the drive rather than after. The rest of this section is indexed at the collecting guides. We hold no catalogue and nothing here is offered for sale; the wanted list sets out the oxidised-zone species we look for, with locality data being the thing that makes such material worth having at all.