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PEGMATITES
Introduction
PEGMATITESare extremely coarse grainedGranitic rocks. They form a separate clan
within granites and are described assilica over saturated coarse intrusive granites. The term
pegmatite is named after its characteristic texture probably derived from the Greek term
pegmawhich literally means adhesion of large or coarse grains.The term 'pegmatite' is
strictly applicable to granitic rocks. The textural term: 'pegmatitic' which refers only to the
grain size can be applied to other rocks, e.g. pegmatitic syenite, pegmatitic gabbro,etc.,
withoutany genetic relation to granitic rocks.
Mineralogy
Granitic pegmatitesare principally composed offeldsparandquartz. The type of quartz
ranges from rock crystal to smoky to milky varieties. The feldspar is commonly alkali feldspar
: Ba K Na Feldspars. These feldspars may be perthitic to antiperthitic. When plagioclase is
present it ranges in composition from albite to oligoclase.
The accessory minerals include the following groups:
Silicates: Micas Biotite,Muscovite,Li Micas, and others.
Pyroxenes Augite, Diopside, and Spodumene.
Amphiboles Hornblende, Uralite, and others.
Garnets Andradite, Almandine,etc.
Rare silicates: Beryl,Tourmaline,Zircon,Topaz,Chrysoberyl, Allanite(epidote), Axinite,Phenakite,
etc.
Non-silicates: Calcite,Columbite-Tantalite,Samarskite,Apatite,Monazite,Fluorite, U & Th
oxides, Opaque oxides, and others.
The type and abundance of accessory minerals in pegmatites are variable.
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Mode Of occurrence
Pegmatites mainly occur asdiscordant plutonsof varying sizes,shapes, and widths. Most
pegmatites aretabularbodies emplaced within fractures and fracture systems. They may be
localized along breccia zones, rock cleavages, fold hinges, bedding contacts, and lineations.
Leucosomes of some migmatites may be pegmatitic in composition and texture. Pegmatites
are generally distributed around granitic batholiths (as regionally chemically zoned bodies)
and their overlying sedimentary basins. The pegmatite dikes nearer to the parent batholith
are less fractionated (proximal pegmatites) while those farther away are most fractionated
(distal pegmatites) in terms of incompatible elements and REE.
Regional Zonation of Pegmatite
The swarms of pegmatite dikes around a parental batholith may be described as'interior
pegmatites'which occur within the batholith body,'marginal pegmatites'which have no
physical connection to the batholith, and'exterior pegmatites'which are farthest from the
batholith. Gneisses, schists, migmatites, and meta-sedimentary rocks are their common
host rocks in Cratons and Shield areas. Their radiometric age ranges from Precambrian to
Cenozoic.Fringe zone pegmatiteoccurs along the marginal contacts of intermediate
intrusions (syenite or diorite).
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Internal Zoning in Pegmatite
Most pegmatites display some form of internal zoning from the wall rock to the centre of
the pegmatite. This zoning is evidenced by the arrangement, distribution, and abundance of
the constituent minerals from wall rock to the centre or core of the pegmatite. Each zone is
described as a'shell' or 'layer'of rock with distinct mineralogy. The zones are:border
zone,wall zone,intermediate zone and core. Contact between the zones is sharp and
gradational. Zones may merge along the length of the pegmatite body. The zones may
sometimes be incomplete or discontinuous. In most cases the zonal character may be
noticed only after detailed mapping. The common minerals in a zoned pegmatite is shown in
the diagrams. The centre or core portions of most zoned pegmatites is quartz followed by an
outward assemblage of:Quartz+ Feldspar+Micas (massive) ----> Quartz + Albite + Aplite or
Graphic pegmatite ----> Fine grained granitic aplite or non granitic host rock.The core
quartz may contain miarolitic cavities orgeodesorgem pocketswith nearly euhedral
minerals.
Thestyle of zoningmay be'symmetrical'with a central quartz core or'asymmetrical'
with a marginal or sub central quartz core with an aplitic base. The margin of asymmetrically
zoned pegmatites may be granitoid in character. Some asymmetrically zoned pegmatites
may be a complex composite pegmatite. Zoning may result by the wall to core mineralization
where quartz is the last to mineralize (forming the core). Fracture fillings and replacement
features are often observed in zoned pegmatites. Thedirection of zoningmay be along the
lengthor along thewidthof the pegmatite body.
In the pegmatite outcrop zoning may be observed as follows: the border zone is composed
of aplite, the wall zone graphic granite, intermediate zone large feldspars, and core
quartz with or with out gem pockets. Mineralogy of typically zoned pegmatites is shown
below:
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Vertically zoned pegmatite.
Vertically zoned pegmatite.
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Asymmetrically Zoned Pegmatite (vertical section).
Vertical zonation in Pegmatite (outcrop scale)
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Classification of Pegmatites
Several schemes have been proposed. They are summarized below.
Jahn's Scheme (Richard Jahn (1985)).
This scheme is based on the mineralogy of pegmatites. Under this scheme, pegmatites
may be grouped as aSIMPLE PEGMATITEwhen quartz and feldspar are the essential
minerals with limited to type and distribution of accessory minerals. Other pegmatites may
be grouped as COMPLEX PEGMATITESwhen they contain a variety of accessory mineral
including metallic ores.
Cerny' Scheme (Petr Cerny (1991)).
This is a detailed scheme based on the whole rock geochemistry, metamorphic
environment, depth, structural features of the pegmatite and its petrologic relation to
batholithic granite. The different classes of pegmatites and their characters are provided in
the following table.
Class Family Spatial Relation to
Granitebody or batholith
Structural
Features
Metamorphic
Environment
Typical Minor
Elements
Abyssal - None (segregations ofanatectic leucosome).
Conformable tomobilized crosscutting veins
UpperAmphibolite toLow to High PrGranulite Facies
U,Th,Zr,Nb,Ti,Y,REE,Mo.Poor tomoderatemineralization
Muscovite - None,(anatectic bodies)to marginal to exterior.
Quasi conformable tocross cutting.
High PrBarrovianAmphiboliteFacies (kyanite -
sillimanite)
Li,Be,Y,REE,Ti,U,Th,Nb>Ta.Poor tomoderate
mineralization.Micas andceramicminerals.
RareElement
LCT Li Cs Ta Interior to marginal toexterior.
Quasi conformable tocross cutting.
Low Pr AbukumaAmphibolite toUpperGreenschistFacies.(andalusite-sillimanite)
Li,Rb,Cs,Be,Ga,Nb Ta,Sn,Hf,B,P,F.Poor toabundantmineralization,gemstock andindustrial
minerals.NYF- Nb Y F Interior to marginal. Interior pods to
conformable toVariable. Y,REE,Ti,U,Th,
Zr,Nb>Ta,F.
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cross cuttingexterior bodies.
Poor toabundantmineralization,ceramicminerals.
Miarolitic NYF Interior to marginal. Interior pods tocross cuttingdikes.
Shallow to sub Be,Y,REE,Ti,U,Th,Zr,Nb >Ta,F.Gemstock.
Varlamoff's Scheme (Varlamoff (1972)).
This scheme is based on the granite type which hosts or is associated with the pegmatite.
The scheme contains the following types and is a field classification.
Type or
Group
Granite Type Mineralogy
1 I Type Granite Microcline,plagioclase,biotite,magnetite.
2 I Type Granite Microcline,plagioclase,biotite,magnetite,quartz.
3 I Type Granite Microcline,plagioclase,biotite,quartz,black tourmaline.
4 I Type Granite Microcline,plagioclase,biotite,muscovite,black tourmaline.
5 I to S Type Granite Microcline,quartz,muscovite,beryl,albitization of potash feldspar.
6 A Type Beryl,amblygonite,spodumene,columbite-tantalite,quartz,albitized andgreisenised feldspars. Zoned pegmatite.
7 A Type Partially or completely albitized,quartz,spodumene,muscovite,greisen with
subordinate cassiterite,columbite-tantalite,white beryl.8 S Type Quartz veins with large microcline,muscovite,cassiterite.
9 S Type Quartz veins with muscovite and cassiterite.
10 S Type Quartz veins with cassiterite,wolframite ,scheelite.
Petrogenetic Scheme (after Cerny and Ercit 2005)
A petrogenetic scheme was proposed by Cerny & Ercit (2005) incorporating the
geochemical characters, tectonic association,and source lithologies. The salient features are
given in the following table:
Family PegmatiteSubclass
GeochemicalSignature
Pegmatite bulkcomposition
AssociatedGranites
Granite bulkcomposition
Sourcelithologies
LCT Rareelement(REL) +REE +LiMiarolitic +-Li
Li,Rb,Cs,Be,Sn,Ga,Ta>Nb (B,P,F)
Peraluminous tosubaluminous
Synorogenicto lateorogenic (toanorogenic);largelyheterogeneous.
Peraluminous,S,I or mixed S+Itypes
Undepletedupper tomiddle crustsupra-crustal andbasementgneisses.
NYF REL + REE Nb>ta,Ti,Y,Sn,R Subaluminous to Syntectonic Peraluminous Depleted
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MiaroliticREE
EE,Zr,U,Th,F. metaluminous (tosubalkaline)
,late,post, tomainlyanorogenic:partlyhomogeneous
tosubaluminousandmetaluminous;A & I types
middle tolower crustalgranulite or
juvenilegranitoids.
Mixed Cross bred:LCT & NYF
Mixed Metaluminous tomoderatelyperaluminous
Post orogenicto anorogenic;heterogeneous
Subaluminousto slightlyperaluminous
Mixedprotoliths orassimilationof supracrustal byNYF granites
Petrographic features
Pegmatites are extremely coarse grained and variable. All the grains are randomly
oriented and interlocking. Grain size varies from 10 cm to more than 10 meters for a single
crystal irrespective of length or width. Grain shape varies from euhedral to anhedral. The
contact zones are also coarse grained.
Textural types
The common texture ishypidiomorphic granularin normal (not internally zoned)
pegmatites. In zoned pegmatites the textures include:apliticin aplite layers or veins,
perthitic texturein massive feldspar,poikilitic texturesinvolving aggregates of accessory
minerals within quartz or feldspar,graphic texturein graphic granite layers, dendritic or
snowflake texture, spherulitic texture, and rarecumulate textureinvolving feldspars.
Contact zones of different layers may displayseriateandgradedtextures.
Gross Structures
Zoned pegmatites displaycrude layeringorconcentric zoningof minerals.Veinsof aplite
or quartz and a variety offracture fillingsalso occur. The style of mineralization in these
structures may be symmetrical (wall to centre). Severalrare element pegmatitescontain
gem pocketsoften associated withgeodesormiarolitic cavitiesorvughs.The size and shape
of gem pockets is variable. The mineralogy of the gem pockets depends on the type or class
of pegmatite.
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Geochemical Characters of Pegmatites
The geochemical characters of pegmatites are similar or equivalent to granite. They are
evolved rocks. TheSiO2content varies from 60 to greater than 70%.The amount of
K2O,Na2O,Al2O3is high. CaO is subordinate. The other oxides are minor in volume. Most
pegmatites are derived from crustal granitic melts and hence their trace element content is
higher thanPMandCIindicating crustal involvement in their genesis. Some elements may
be elevated:Li,Be,Sr,Ba,and Rb(particularly crustal lithophile elements). The distribution of
incompatible trace elements is similar to granitic rocks.Enrichment of incompatible trace
elements is common.TheLREETis highwith verylow to minute HREET.REE profiles plot
above CI range (100 times of CI values). The trend of the profiles indicate moderate to high
fractionation from source granitic melt. The slope of REE profile is steep. Eu anomaly is
negative as pegmatite generally contains negligible calcic plagioclase. The distribution of
minor elements is given in the classification table proposed by Cerny.
Petrogenesis of Pegmatites
Several models have been proposed to explain the generation of pegmatites from granitic
melts. The models have been derived from the results of experimental petrology of granitic
systems.
Jahns & Burnham's Model (1969)
In this model pegmatites crystallize out from a water and volatile rich magma which are
saturated in alkalis and silica. These magmas result by the generation of residual fluids from
a differentiating parental crustal magma. Most incompatible elements, ore metals, and REE
tend to accumulate in the generated residual volatile rich fraction of crustal magmas.
Jahns and Burnham proposed their model based on experiments using anartificial volatile
rich granite magmas. They discovered that dissolvedvolatilesin the host magma play a role
in pegmatite genesis. The volatiles affect the fractionation byreducing the number of
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crystalsthat can form. This allows the already growing crystalsto grow to large sizesin a
near fluidic environment. The crystals can grow bigger without interfering with each other.
The volatiles furtherreduce the viscosityof the host magma so that the chemical elements
and molecules move more freely in the fluidic condition. This supplies the growing crystals
with the chemicals needed to grow larger. Thesaturation of volatilesleads to their
separation as watery bubbles (resurgent boiling effect) surrounded by normal liquid magma.
Crystals growsimultaneously from both the liquid and the volatile gas bubbles, with the
larger ones forming from the bubbles. This explains thecoarse grain sizeof pegmatites.
Thelocalization of volatilesform geodes, vughs, and miarolitic pods which accumulate rare
metals to form gem pocketsand nearly euhedral quartz and feldspars and a variety of
accessory silicates. The crystallization of rare silicates such as beryl,topaz, ores; monazite,
columbite tantalite, REE monazite,etc., within gem pockets or as fracture veins is due to
the incompatible elements, ore metals, and REE already present the volatile rich fraction. The
volatiles eventually form fluorine, chlorine or water-bearing minerals: micas,amphiboles,
etc.,as the pegmatite fluid finally consolidates.
Any remaining volatiles escape by wall or host rock mineralization (metasomatic
exchange). Pegmatites are often contain broken and re-healed crystals that have been
broken by progressive sudden releases (explosive pressure) of some of the volatiles.
The volatile poor fluidic fraction which often remains after the formation of the main
pegmatite body forms'aplite'veins with a fine grained saccaroidal texture. This is due to the
'dry' and rapid crystallization of the host magma after the sudden removal of volatiles.
Economic Mineralization related to Pegmatites
Pegmatites may or may not contain economic mineralization or exploitable ore bodies.
Pegmatites can be thus grouped as'barren'pegmatite: with poor or no economic
mineralization within it, and as'fertile'pegmatites when some form or viable economic
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mineralization occurs within it for future exploitation.
The type of economic minerals generally associated with pegmatites includes the
following:
Ore Minerals:
Uraninite, thorianite, allanite, pyrochlore, columbite-tantalite, cassiterite, molybdenite, rare
galena, rutile, ilmenite, REE monazite, REE apatite, wolframite,scheelite and magnetite.
Industrial Minerals:
Micas Li,biotite,muscovite micas. Feldspar,Quartz, Kaolin (in altered feldspar of miarolitic
pegmatites).
Gem Minerals:
Beryl (all species), Tourmaline (several species), Feldspar microcline, albite, orthoclase
moon stone, Topaz, Corundum (peraluminous pegmatites), Quartz amethyst, smoky, citrine,
Epidote, Spodumene (Li pyroxene), Kyanite,Andalusite, Apatite, Fluorite, Kunzite, Sphene,
Zircon, Garnet, and others.