Geol Resources Lecture 15 (Greisen SN-W and Pegmatites)
Geol Resources Lecture 15 (Greisen SN-W and Pegmatites)
Geol Resources Lecture 15 (Greisen SN-W and Pegmatites)
Lecture 15
Greisen (German “greissen” - to split)
Definition: Hydrothermal alteration assemblage in granitic rock. Specifically
quartz-muscovite with one of more F- or B-bearing minerals (most commonly
fluorite, topaz or tourmaline).
Hydrothermal system is developed in the upper few hundred metres of a granite batholith, around
a cupola and its associated wall rock. Sheeted and stockwork veins are present above the cupola.
Quartz-tourmaline veins in granite (Cligga Head)
Quartz-tourmaline veins in granite (Cligga Head)
Alteration and metal zonation in greisens
Maps and sections of tungsten vein deposits illustrating mineral and alteration zoning. (A) Chicote Grande
deposit, Bolivia; (B) Xihuashan, China (from Cox and Bagby, 1986).
Metal districts and the Cornubian batholith
Settings of mineralization in Cornwall
Dines (1934) provided field evidence against this, noting that the various zones were
considerably flatter than the granite contact and that the higher the zone, the greater
its lateral extent. Dines also suggested that the appearance of certain minerals in
particular zones was temperature dependent and determined by the temperature
gradient between the granite and the surface.
Mineral zones of the St Agnes district
Tourmaline
Wolframite-bearing quart-feldspar-tourmaline
pegmatite (South Crofty mine, Cornwall)
Section showing the relative position of gossan, supergene and hypogene ores. Adapted from the Camborne-Redruth
mining district. (After Hosking (1988).
Weathered greisenized granite
KEY:
1— coarse-and medium-biotite granite;
2—fine-and medium-grained muscovitized granite
3—aplite-like, aplite;
4 — oligoclase-microcline pegmatite
5—graphic pegmatite (qtz-K feldspar)
6—microcline in blocks with beryl and columbite
7—microcline-albite with tantalite, cassiterite, beryl
(rarely spodumene),
8—albitic (productive) with tantalite, cassiterite,
beryl, pollucite, spodumene, lepidolite
9—albite (cleavelandite)-spodumene, with tantalite,
amblygonite, petalite, polychromatic tourmaline;
10—faults;
11, 13—direction of the fluid movement;
12 —ore-controlling deep fault.
Minerals from rare metal pegmatites
Lepidolite (Li-mica)
Topaz
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Further Reading
• Mineralization in Cornwall – The Cornubian orefield
(http://myweb.tiscali.co.uk/geologyofcornwall/Mineralisation.htm).
• Elliot et al. (2000) Vein and greisen Sn and W deposits. USGS Report.