Greisen
The top of a granite chemically rebuilt by its own escaping vapours into quartz and mica — the classic host of tin, and of the tungsten and molybdenum that accompany it.
Greisen is a granite that has been attacked by fluids released from its own interior. As the last of a granite crystallises it expels a hot vapour rich in fluorine, boron and water, and that vapour rises into the already-solid roof of the intrusion and destroys the feldspar there, leaving a rock of quartz and coarse mica with topaz and fluorite scattered through it.
The metals come with the vapour. Tin, tungsten, molybdenum and bismuth are all carried in it and deposited as the fluid cools, which is why greisen and the veins around it are where hard-rock tin has been mined for three thousand years.
How it forms
The chemistry is specific. Fluorine-bearing vapour breaks feldspar down into mica and quartz, releasing the potassium and sodium into solution, and the fluorine that drove the reaction is left behind in the topaz and fluorite that characterise the rock. That is why a greisen is recognisable by what it lacks — feldspar — as much as by what it contains.
Alteration is concentrated at the roof of the intrusion because that is where the vapour collects, so greisen forms a cap or a set of stockwork veins in the uppermost part of a granite. The practical consequence is that a greisen tin deposit tells you which way is structurally up, and that the granite below it is likely to be barren.
Cassiterite is the reason the vapour matters commercially: tin travels as a fluoride complex and drops out as the oxide when the fluid cools or reacts, which is why it concentrates in and around greisen rather than being dispersed through the granite.
Economic significance
Cornwall is the type example and the historical one. Its tin was worked from greisen and the associated veins from the Bronze Age until 1998, and the district's zoning — tin at depth, copper above it, lead and zinc further out — became the model for how metals distribute around a cooling granite.
Most tin now comes from placer deposits derived from weathered greisen rather than from the rock itself, because cassiterite is dense and durable and survives being washed out of it. That does not make the greisen irrelevant: the placer is only as good as the source that fed it.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
is composed of
- Quartz — mineral · 30–60% · one of the two minerals the alteration leaves behind, feldspar having been destroyed
- Muscovite — mineral · 20–50% · the coarse mica that replaces feldspar, and the feature that makes greisen recognisable in hand specimen
- Topaz — mineral · crystallised from the fluorine in the altering vapour, and a signature of the process rather than an ore
- Fluorite — mineral · the other fluorine mineral left behind by the reaction that destroyed the feldspar
- Cassiterite — mineral · the tin oxide, dropped out of the vapour as it cooled — the ore the rock is mined for
- Molybdenite — mineral · with the tin and tungsten, from the same vapour and often at a slightly different level
- Arsenopyrite — mineral · common in the veins around a greisen, and the mineral that makes tin and tungsten concentrates awkward to smelt
- Bismuthinite — mineral · the bismuth that travels with tin and tungsten, and is recovered as a by-product of them
is sourced from
- Granite — rock · the roof of a granite, chemically rebuilt by vapour escaping from the granite below it
Sources
- Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
- Material WorldOur own writing