Gossan

A rust-coloured, intensely weathered rock outcrop, formed where sulphide minerals in a mineral vein are broken down by air and water — long used by prospectors as a visible surface clue to buried ore.

Historical usage

Gossans were a standard prospecting guide wherever mineral veins reached the surface, read by experienced prospectors as a sign of sulphide mineralisation underneath — sometimes called 'the lighthouses of prospectors' for exactly this reason.

How it worked

Where a sulphide-bearing vein is exposed to air and rainwater, the sulphides oxidise, releasing sulphuric acid that dissolves and carries away the metal content, leaving behind only iron oxides (haematite and goethite) and quartz. The result is a rust-red, orange, yellow or sometimes black capping rock, often riddled with 'boxworks' — small quartz-lined cavities left where the original sulphide crystals have dissolved away entirely.

Why it was used

A gossan's colour, texture and the shape of its boxwork cavities could tell an experienced prospector a good deal about what kind of mineralisation lay beneath, before any digging began — the same basic surface-reading skill costeaning (see this site's own term page) was used to test more directly.

Risks & limitations

A gossan only marks where a sulphide vein has been exposed and weathered at surface; it says nothing about how rich, wide or persistent the ore might be at depth, so a promising gossan still had to be followed up with real underground work — or, more recently, drilling — before a deposit's true worth was known.

Regional variation

The term and the phenomenon are universal to hard-rock mineral exploration worldwide, not specific to Britain, and gossans remain a genuine tool in modern mineral exploration alongside satellite and geochemical surveying.

Sources

1. (secondary) Gossan — Wikipedia
2. (secondary) Gossans: Critical Indicators for Successful Mineral Exploration — Discovery Alert
3. (secondary) Gossans: Rusty Clues to Hidden Ore Beneath the Surface — OzGeology

Record created: 21 August 2026 · Last researched: 21 August 2026