Greisen

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.

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








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Greisen vein cutting a granite at Cligga Head, Cornwall. Greisen consists of quartz and muscovite replacing feldspar of the granite adjacent to a narrow vein that contains black tourmaline and some cassiterite (tin oxide).








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Greisen bordered veins at Cligga Head; Cornwall, UK.








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Zinnwaldite greisen. Gairnshiel at the centre of the Glen Gairn granite complex.








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Grained albite-amazonite Li-F granite, Etyka Ta deposit, Transbaikalia. Kyrgyzstan







Bibliography



• Eric A.K.Middlemost (1985): Magmas and Magmatic Rocks. Longman, London
• Ron H. Vernon (2004): A pratical guide to rock microstructure. Cambridge editore
• K.G.Cox, J.D.Bell & R.J Pankhurst (1979): The interpretetion of igneous rocks. George Allen&Unwin editori.


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