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 moderate- to high-temperature magmatic alteration related to release of volatiles dissolved in a magma during the solidification of that magma.

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 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 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

This article uses material from the Wikipedia article  Metasyntactic variable, which is released under the  Creative Commons Attribution-ShareAlike 3.0 Unported License.

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Greisen

Source : alexstrekeisen.it

 

Hydrothermal alteration is a chemical replacement of the original minerals in a rock by new minerals, where a hydrothermal fluid delivers the chemical reactants and removes the aqueous reaction products. Reed ( 1997 ).

The nature of the alteration assemblage formed in a given granitoid depends on various factors, such as temperature, the fluid’s composition (e.g. pH, salinity) and the water : rock ratio. The pervasiveness of alteration depends largely on the rock’s microstructure, e.g. on grain size, mineral cleavage and the intensity of fracturing in the rock (which assists fluid penetration). At relatively low temperatures, alteration of igneous rocks may resemble low - grade (greenschist facies) metamorphism, with the appearance of minerals like epidote and chlorite.

The term Greisen derive from German, probably a dialect word, from greiszen, “to split”. This term traditionally refers to hydrothermally metamorphosed granitic rock that consist of F- and B-bearing minerals, quartz and mica, and typically cassiterite, pyrite, wolframite and sphalerite. Greisen, commonly, are granitoid rocks in which >90% of the feldspar is repleaced by quatz, mica, topaz, cassiterite and other minerals.

Greisen, sometimes capped by pegmatite, forms near the roof of some granite plutons, and commonly hosts workable - though not always currently economic - Sn - W - Mo ore bodies. The type of metal deposit correlates closely with the type of granite and the source rocks from which the magma formed. Greisen is formed by self-generated alteration of a granite and is a class of endoskarn; It means that greisen in most cases formed inside the granitic pluton itself that provided heat and hydrothermal fluids to initiate the greisenization. Greisen occurs typically in belts or veins that intersect granite, and it passes into granite at the outer edges of these. The transition between the two rocks is gradual, indicating that the greisen has been produced through alteration of the granite by vapours or fluids rising through fissures.

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 vein cutting a granite. The narrow vein contains black tourmaline and some tin oxide.

Greisen vein cutting a granite. The narrow vein contains black tourmaline and some tin oxide. From Geomartin .

Wolframite-bearing greisen veins cutting a granite stock. Cligga Head, Cornwall. From Kevin Walsh.

Bibliography

• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Le Maitre, R. W., Streckeisen, A., Zanettin, B., Le Bas, M. J., Bonin, B., Bateman, P., & Lameyre, J. (2002). Igneous rocks. A classification and glossary of terms, 2. Cambridge University Press.
• Middlemost, E. A. (1986). Magmas and magmatic rocks: an introduction to igneous petrology.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.

Photo

Zinnwaldite (micaceous habit), topaz (high relief, colorless) and quartz in a greisen. Zinnwald, Saxony, Germany. PPL image, 1x (Field of view = 9mm)	Zinnwaldite (high interference colors), topaz (I order gray) and quartz in a greisen. Zinnwald, Saxony, Germany. XPL image, 1x (Field of view = 9mm)	Topaz (high relief, colorless) and quartz in a greisen. Zinnwald, Saxony, Germany. PPL image, 1x (Field of view = 9mm)
Topaz (I order gray) and quartz in a greisen. Zinnwald, Saxony, Germany. XPL image, 1x (Field of view = 9mm)	Zinnwaldite (micaceous habit), topaz (high relief, colorless) and quartz in a greisen. Zinnwald, Saxony, Germany. PPL image, 1x (Field of view = 9mm)	Zinnwaldite (high interference colors), topaz (I order gray) and quartz in a greisen. Zinnwald, Saxony, Germany. XPL image, 1x (Field of view = 9mm)
Zinnwaldite (micaceous habit), topaz (high relief, colorless) and quartz in a greisen. Zinnwald, Saxony, Germany. PPL image, 1x (Field of view = 9mm)	Zinnwaldite (high interference colors), topaz (I order gray) and quartz in a greisen. Zinnwald, Saxony, Germany. XPL image, 1x (Field of view = 9mm)	Zinnwaldite (micaceous habit), topaz (high relief, colorless) and quartz in a greisen. Zinnwald, Saxony, Germany. PPL image, 1x (Field of view = 9mm)
Zinnwaldite (high interference colors), topaz (I order gray) and quartz in a greisen. Zinnwald, Saxony, Germany. XPL image, 1x (Field of view = 9mm)	Zinnwaldite (micaceous habit), topaz (high relief, colorless) and quartz in a greisen. Zinnwald, Saxony, Germany. PPL image, 1x (Field of view = 9mm)	Zinnwaldite (high interference colors), topaz (I order gray) and quartz in a greisen. Zinnwald, Saxony, Germany. XPL image, 1x (Field of view = 9mm)
Zinnwaldite (micaceous habit), topaz (high relief, colorless) and quartz in a greisen. Zinnwald, Saxony, Germany. PPL image, 1x (Field of view = 9mm)	Zinnwaldite (high interference colors), topaz (I order gray) and quartz in a greisen. Zinnwald, Saxony, Germany. XPL image, 2x (Field of view = 7mm)	Zinnwaldite (high interference colors), topaz (I order gray) and quartz in a greisen. Zinnwald, Saxony, Germany. XPL image, 2x (Field of view = 7mm)
Zinnwaldite (high interference colors), topaz (I order gray) and quartz in a greisen. Zinnwald, Saxony, Germany. XPL image, 2x (Field of view = 7mm)	Zinnwaldite (micaceous habit), topaz (high relief, colorless) and quartz in a greisen. Zinnwald, Saxony, Germany. PPL image, 1x (Field of view = 9mm)	Zinnwaldite (high interference colors), topaz (I order gray) and quartz in a greisen. Zinnwald, Saxony, Germany. XPL image, 2x (Field of view = 7mm)
Zinnwaldite (micaceous habit) and quartz in a greisen. Zinnwald, Saxony, Germany. PPL image, 2x (Field of view = 7mm)	Zinnwaldite (high interference colors) and quartz in a greisen. Zinnwald, Saxony, Germany. XPL image, 2x (Field of view = 7mm)	Zinnwaldite (micaceous habit) and quartz in a greisen. Zinnwald, Saxony, Germany. PPL image, 2x (Field of view = 7mm)
Zinnwaldite (high interference colors) and quartz in a greisen. Zinnwald, Saxony, Germany. XPL image, 2x (Field of view = 7mm)	Zinnwaldite (high interference colors) and quartz in a greisen. Zinnwald, Saxony, Germany. XPL image, 2x (Field of view = 7mm)	Zinnwaldite (high interference colors), topaz (I order gray) and quartz in a greisen. Zinnwald, Saxony, Germany. XPL image, 2x (Field of view = 7mm)
Zinnwaldite (high interference colors) and quartz in a greisen. Zinnwald, Saxony, Germany. XPL image, 2x (Field of view = 7mm)	Zinnwaldite (high interference colors) and quartz in a greisen. Zinnwald, Saxony, Germany. XPL image, 2x (Field of view = 7mm)	Topaz (I order gray) and quartz in a greisen. Zinnwald, Saxony, Germany. XPL image, 10x (Field of view = 2mm)
Topaz (I order gray), Zinnwaldite (high interference colors). Zinnwald, Saxony, Germany, XPL image 10x (Field of view = 2mm)	Topaz (I order gray), Zinnwaldite (high interference colors). Zinnwald, Saxony, Germany, XPL image 10x (Field of view = 2mm)	Topaz (I order gray), Zinnwaldite (high interference colors). Zinnwald, Saxony, Germany, XPL image 10x (Field of view = 2mm)