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Greisen

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A medium-temperature metasomatic rock characterized by the presence of quartz and white mica, commonly with topaz, fluorite, tourmaline and locally with amazonite, orthoclase, andalusite and diaspore. Typically greisens may host Be, W, Mo, Sn and Ta mineralization. They are associated with high-level late-orogenic leucogranites and form as replacements either in the granite body and/or in a wide range of country rocks. Zoning may be present.


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Classification of GreisenHide

Sub-divisions of GreisenHide

Mineralogy of GreisenHide

Essential minerals - these are minerals that are required within the classification of this rock:
MicaA petrological term for Mica Group.
Silica > QuartzSiO2
Non-essential minerals - these minerals are common, sometimes major components, but are not always present:
AmazoniteK(AlSi3O8)
AndalusiteAl2(SiO4)O
DiasporeAlO(OH)
Feldspar > Alkali Feldspar > OrthoclaseK(AlSi3O8)
FluoriteCaF2
Mica > MuscoviteKAl2(AlSi3O10)(OH)2
TopazAl2(SiO4)(F,OH)2
TourmalineA(D3)G6(T6O18)(BO3)3X3Z
ZinnwalditeKLiFe2+Al(AlSi3O10)(F,OH)2
Accessory minerals - these are minor components, often far less than 1% of the total mineral material:
CassiteriteSnO2
MolybdeniteMoS2
Wolframite(Fe2+)WO4 to (Mn2+)WO4

Age informationHide

Age range:
Cisuralian : 284 ± 4 Ma to 275 ± 6 Ma - based on data given below.
Sample ages:
Sample IDRecorded ageGeologic TimeDating method
1275 ± 6 MaCisuralianK/Ar
2279 ± 4 MaCisuralianRb/Sr
3284 ± 4 MaCisuralian
Sample references:
IDLocalityReference
1Cligga Head, Perranporth, Perranzabuloe, St Agnes District, Cornwall, England, UKHalliday, A. N. (1980). The timing of early and main stage ore mineralization in Southwest Cornwall. Economic Geology, 75(5), 752-759.
2Lower Bostraze China Clay Pit, Tregeseal Valley, St Just, St Just District, Cornwall, England, UK  "  "
3St Michael's Mount, Marazion, Mount's Bay District, Cornwall, England, UK  "  "

References for GreisenHide

Reference List:
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Fettes (et al.), Metamorphic Rocks - A Classification and Glossary of Terms (2007) Cambridge University Press

Internet Links for GreisenHide

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