Greisen
Greisen is a highly altered granitic rock or pegmatite. Greisen is formed by self-generated 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 or topaz.
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 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.
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 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
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
See also
- List of rock textures – List of rock textural and morphological terms
- Metasomatism – Chemical alteration of a rock by hydrothermal and other fluids
- Granite – Type of igneous rock; specifically for S-type and I-type distinction
- Ore genesis – How the various types of mineral deposits form within the Earth's crust
References
- Evans, A.M., 1993. Ore Geology and Industrial Minerals, An Introduction., Blackwell Science, ISBN 0-632-02953-6
- Reed, B.L., 1986, Descriptive model of Sn greisen deposits, in Dennis P. Cox and Donald A. Singer, eds, Characteristics of mineral deposit occurrences: U.S. Geological Survey Bulletin 1693, http://pubs.usgs.gov/bul/b1693/html/bull217y.htm
- Taylor, R.G., 1979, Geology of tin deposits: Elsevier, Amsterdam, 543 p. ISBN 0-444-41805-9