Greisen

Greisen is a highly altered granitic rock or pegmatite. Greisen is formed by autogenic alteration of a granite and is a class of endoskarn.

Greisens appear as highly altered rocks, partly coarse, crystalline granite, partly vuggy with miarolitic cavities, disseminated halide minerals such as fluorite, and occasionally metallic oxide and sulfide ore minerals, borate minerals (tourmaline) and accessory phases such as sphene, beryl, topaz, etcetera.

Petrogenesis

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 crystalisation 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.

Alteration facies

* Incipient greisen (granite): muscovite ± chlorite, tourmaline, and fluorite.
* Greisenized granite: quartz-muscovite-topaz-fluorite, ± tourmaline (original texture of granites retained).
* Massive greisen: quartz-muscovite-topaz ± fluorite ± tourmaline (typically no original texture preserved). Tourmaline can be ubiquitous as disseminations, concentrated or diffuse clots, or late fracture fillings. Greisen may form in any wallrock environment, typical assemblages developed in aluminosilicates.

Greisen environments

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 potassium, 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.

Distribution

Typical greisen deposits include
* Tin 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)
* Erzgebirge, Czech Republic (tin greisen)

ee also

* List of rock textures
* Metasomatism
* Granite; specifically for S-type and I-type distinction
* Ore genesis

External links

* [http://pubs.usgs.gov/bul/b1693/html/bull217y.htm USGS descriptive Sn greisen model]
* [http://sbgeo.org.br/rgb/vol30_down/3002/3002238.pdf Lenharo, S.L.R., Pollard P.J., Born H., "Matrix rock texture in the Pitinga Topaz Granite, Amazonas, Brazil", Brazilian Geoscience Reviews, vol 30, 2000 (pdf)]

References

Evans, A.M., 1993. "Ore Geology and Industrial Minerals, An Introduction.", Blackwell Science, ISBN 0-632-02953-6

Reed, B.L., 1982, "Tin greisen model", in Erickson, R.L., ed., "Characteristics of mineral deposit occurrences: U.S. Geological Survey Open-File Report" 82-795, p. 55-61.

Taylor, R.G., 1979, "Geology of tin deposits": Elsevier, Amsterdam, 543 p.