Pegmatite Full

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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.

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