Greisen

Greisens appear to be restricted to intrusions which are emplaced high in the crust, generally at a depth between 0.5 and 5 km, with upper aureoles which are sealed shut to prevent fluids escaping. This is generally required, as the boiling to produce greisenation cannot occur deeper than about 5 kilometres. They are also generally associated only with potassic igneous rocks; S-type granite, not I-type granodiorite or diorite. Greisens are prospective for mineralisation because the last fluids of granite crystallization tend to concentrate incompatible elements such as tin, tungsten, molybdenum and fluorine, as well as metals such as gold, silver, and occasionally copper. Tectonically, greisen granites are generally associated with generation of S-type suites of granites in thick arc and back-arc fold belts where subducted sedimentary and felsic rock is melted.

Greisens are formed by endoskarn alteration of granite during the cooling stages of emplacement. Greisen fluids are formed by granites as the last highly gas- and water-rich phases of complete crystallisation of granite melts. This fluid is forced into the interstitial spaces of the granite and pools at the upper margins, where boiling and alteration occur.

Examples of greisen are: Tin and tungsten deposits of Cornwall Ardlethan, Lachlan Fold Belt, Australia (tin-antimony greisen) Timbarra, Lachlan Fold Belt, Australia (gold greisen deposit) Anchor Mine, Tasman Fold Belt, Australia (tin greisen) Pitinga topaz granite, Brazil (tin, topaz, beryl) Lost River, Alaska, USA (tin greisen) Sisson Brook, Burnt Hill and other deposits, New Brunswick, Canada (tin-tungsten-molybdenum greisen) Erzgebirge, Czech Republic (tin greisen) Tungsten deposit at Panasqueira Mine, Portugal