Greisen is a highly altered granitic rock or pegmatite, usually composed predominantly of quartz and micas (mostly muscovite). Greisen is formed by self-generated alteration of a granite and is a class of moderate- to high-temperature magmatic-hydrothermal alteration related to the late-stage release of volatiles dissolved in a magma during the solidification of that magma.
Granite (light) with sheeted veins of greisen (dark) at Cligga Head, Cornwall
Greisens are usually variably altered rocks, grading from coarse, crystalline granite, commonly vuggy with miarolitic cavities, through to quartz and muscovite rich rocks, which may be locally rich in topaz, tourmaline, cassiterite, fluorite, beryl, wolframite, siderite, molybdenite and other sulfide minerals, and other accessory minerals. They may occur as small to large veins, or large zones in the roof of some granites. The rocks can sometimes be mined as ores of tin and other minerals.
Greisens are formed by endogenous 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 through the interstitial spaces of the granite into veins and pools at the upper margins, where boiling and rock alteration occur.
Greisenized granite: quartz-muscovite-topaz-fluorite, ± tourmaline (some 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, but typically in granites and metamorphic rocks.
Greisen environmentsedit
Greisens appear to be restricted to intrusions which are emplaced high in the crust, generally at a depth between 0.5 and 5 km, as the hydrous fluid separation from granite to produce greisenation cannot occur deeper than about 5 kilometres. The roof or upper aureole is mostly sealed shut to prevent most fluids escaping. This sealing is largely due to hornfelsing and silicification of the overlying rocks, and fracturing of these rock typically forms greisen veins.
They are generally associated mostly with potassic plutonic rocks; Alkali feldspar granite, and are rare in less potassium rocks like granodiorite or diorite. Greisens are prospective for mineralisation because the last fluids of granite crystallization tend to concentrate incompatible metals such as tin, tungsten, molybdenum and beryllium, and in places other metals such as tantalum, gold, silver, and copper.
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
Referencesedit
^Lilley, Hamish C.; Scott, James M.; Schwartz, Josh J.; Turnbull, Rose E.; Tulloch, Andy J. (2021-01-11). "Cretaceous tungsten-tin mineralisation in the Tin Range, Stewart Island, New Zealand". New Zealand Journal of Geology and Geophysics. 65 (2): 283–298. doi:10.1080/00288306.2020.1855216. ISSN 0028-8306. S2CID 234272962.
Evans, A.M., 1993. Ore Geology and Industrial Minerals, An Introduction., Blackwell Science, ISBN0-632-02953-6
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. ISBN0-444-41805-9
Mustard, R. 2004. Textural, mineralogical and geochemical variation in the zoned Timbarra Tablelands pluton, New South Wales. Australian Journal of Earth Sciences, 51.
Richardson, G.C. (2016) Lottah and the Anchor: the history of a tin mine and a dependent town: North East Tasmania. Tasmania Forty South Publishing Pty Ltd, Hobart, 265 pages and maps.
External linksedit
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)
December 18, 2023
greisen, highly, altered, granitic, rock, pegmatite, usually, composed, predominantly, quartz, micas, mostly, muscovite, formed, self, generated, alteration, granite, class, moderate, high, temperature, magmatic, hydrothermal, alteration, related, late, stage,. Greisen is a highly altered granitic rock or pegmatite usually composed predominantly of quartz and micas mostly muscovite Greisen is formed by self generated alteration of a granite and is a class of moderate to high temperature magmatic hydrothermal alteration related to the late stage release of volatiles dissolved in a magma during the solidification of that magma Granite light with sheeted veins of greisen dark at Cligga Head CornwallGreisens are usually variably altered rocks grading from coarse crystalline granite commonly vuggy with miarolitic cavities through to quartz and muscovite rich rocks which may be locally rich in topaz tourmaline cassiterite fluorite beryl wolframite siderite molybdenite and other sulfide minerals and other accessory minerals They may occur as small to large veins or large zones in the roof of some granites The rocks can sometimes be mined as ores of tin and other minerals Contents 1 Petrogenesis 1 1 Alteration facies 2 Greisen environments 3 Distribution 4 See also 5 References 6 External linksPetrogenesis editGreisens are formed by endogenous 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 through the interstitial spaces of the granite into veins and pools at the upper margins where boiling and rock alteration occur Alteration facies edit nbsp Thin section of greisen containing fluorite black from the Ore MountainsIncipient greisen granite addition of muscovite chlorite topaz tourmaline and fluorite original texture of granites retained Greisenized granite quartz muscovite topaz fluorite tourmaline some 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 but typically in granites and metamorphic rocks Greisen environments editGreisens appear to be restricted to intrusions which are emplaced high in the crust generally at a depth between 0 5 and 5 km as the hydrous fluid separation from granite to produce greisenation cannot occur deeper than about 5 kilometres The roof or upper aureole is mostly sealed shut to prevent most fluids escaping This sealing is largely due to hornfelsing and silicification of the overlying rocks and fracturing of these rock typically forms greisen veins They are generally associated mostly with potassic plutonic rocks Alkali feldspar granite and are rare in less potassium rocks like granodiorite or diorite Greisens are prospective for mineralisation because the last fluids of granite crystallization tend to concentrate incompatible metals such as tin tungsten molybdenum and beryllium and in places other metals such as tantalum gold silver and 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 edit nbsp Dark greisen veins of cassiterite and arsenopyrite in granite Ore MountainsExamples of greisen are Tin and tungsten deposits of Cornwall Ardlethan New South Wales Australia tin antimony greisen Timbarra gold mine New South Wales Australia gold greisen deposit Anchor Mine Lottah Tasmania Australia tin copper topaz 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 Ore Mountains Czech Republic tin greisen Panasqueira Mine Portugal Tin and Tungsten deposit The Tin Range Tungsten Tin deposit Stewart Island Rakiura New Zealand 1 See also editList 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 crustReferences edit Lilley Hamish C Scott James M Schwartz Josh J Turnbull Rose E Tulloch Andy J 2021 01 11 Cretaceous tungsten tin mineralisation in the Tin Range Stewart Island New Zealand New Zealand Journal of Geology and Geophysics 65 2 283 298 doi 10 1080 00288306 2020 1855216 ISSN 0028 8306 S2CID 234272962 This article includes a list of general references but it lacks sufficient corresponding inline citations Please help to improve this article by introducing more precise citations March 2021 Learn how and when to remove this template message 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 Mustard R 2004 Textural mineralogical and geochemical variation in the zoned Timbarra Tablelands pluton New South Wales Australian Journal of Earth Sciences 51 Richardson G C 2016 Lottah and the Anchor the history of a tin mine and a dependent town North East Tasmania Tasmania Forty South Publishing Pty Ltd Hobart 265 pages and maps External links editLenharo 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 Retrieved from https en wikipedia org w index php title Greisen amp oldid 1174606419, wikipedia, wiki, book, books, library,
