Greisen and Pegmatite Deposits

A zone of intensely altered granite, converted by high-temperature fluids escaping from the granite's own crystallising magma into a coarse, quartz-and-mica rock hosting tin and tungsten ore — the deposit type behind some of Cornwall's and Devon's most important tungsten mines, and the last stage of a single continuous process that starts deep inside a cooling granite intrusion.

How it forms

As a body of granite magma crystallises, most elements lock into ordinary rock-forming minerals, but a distinctive suite of rarer elements — tin, tungsten, fluorine, lithium, boron and others — tend to stay concentrated in the last remaining pockets of fluid-rich melt, especially where the parent granite is itself unusually rich in these elements to begin with. As the granite finishes crystallising, this fluid becomes trapped and pressurised near the top of the intrusion, in structural highs called cupolas, and is eventually released as a very hot, aggressive fluid that attacks the surrounding solid granite: feldspar and mica are broken down and largely replaced by new mica, leaving a pale, sugary, quartz-rich rock — greisen — often laced with veins of the same origin carrying cassiterite (tin ore) and wolframite (tungsten ore).

Why minerals concentrate here

The same fractionation process that concentrates tin and tungsten into the last dregs of the crystallising magma also concentrates them into the fluid that later attacks the granite roof, so greisen zones and their associated veins end up carrying ore grades far above anything found in the ordinary granite around them — a direct result of the magma's own chemistry being progressively refined as it solidified, rather than of any later, unrelated fluid arriving from elsewhere.

Typical shape of the deposit

An irregular zone or cap of altered rock at or near the top of a granite cupola, commonly shot through with a stockwork of narrow, closely spaced quartz veins carrying the ore minerals — distinct from a single clean vein, and better pictured as a broad halo of alteration and mineralisation around the granite's apex than as one discrete body.

What this means for mining

Because ore in a greisen system is spread through a zone of altered rock and a network of small veins rather than concentrated along one clean fracture, greisen deposits often support bulk, lower-grade extraction methods — like Hemerdon's large open pit — that would make little sense applied to a single narrow vein, even though the underlying geology is closely related to the vein deposits described elsewhere on this site. Where greisen instead formed as a narrower, higher-grade zone, as at Cligga Head, more selective working closer to conventional vein mining was viable instead.

Associated commodities

Tin (Cassiterite is a characteristic greisen and associated-vein ore mineral.), Tungsten (Wolframite is the characteristic ore mineral of greisen deposits.)

Related geology

Hydrothermal Deposits, Kaolinization and China Clay Deposits

Related Mining Terms & Methods

Vein (Greisen zones are commonly shot through with a stockwork of narrow quartz veins carrying the ore.)

Images

MineArchive examples

Included only where MineArchive's own research gives a reasonable evidential basis — not every mine where this geology might plausibly apply.

• Cligga Head — Cornwall (well documented example)
  The mine's own recorded geology explicitly identifies greisen — mineralised, altered granite — hosting tin and tungsten ore.
• Hemerdon Mine — Devon (well documented example)
  The mine's own recorded geology explicitly describes a tungsten-tin skarn/greisen body associated with the Dartmoor granite.

Sources

1. (primary) Vein and Greisen Sn and W Deposits — United States Geological Survey
2. (secondary) Greisen — Wikipedia

Record created: 22 August 2026 · Last researched: 22 August 2026