Pegmatite Full

PEGMATITES Introduction PEGMATITES are extremely coarse grained Granitic rocks. They form a separate clan within granites and are described as silica over saturated coarse intrusive granites. The term pegmatite is named after its characteristic texture probably derived from the Greek term “pegma” which 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., without any genetic relation to granitic rocks. Mineralogy Granitic pegmatites are principally composed of feldspar and quartz. 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.

Mode Of occurrence Pegmatites mainly occur as discordant plutons of varying sizes,shapes, and widths. Most pegmatites are tabular bodies 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 pegmatite occurs along the marginal contacts of intermediate intrusions (syenite or diorite).

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 or geodes or gem pockets with nearly euhedral minerals. The style of zoning may 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. The direction of zoning may be along the length or along the width of 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:

Vertically zoned pegmatite.

Vertically zoned pegmatite.

Asymmetrically Zoned Pegmatite (vertical section).

Vertical zonation in Pegmatite (outcrop scale)

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 a SIMPLE PEGMATITE when quartz and feldspar are the essential minerals with limited to type and distribution of accessory minerals. Other pegmatites may be grouped as COMPLEX PEGMATITES when 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 Granite body or batholith

Abyssal

-

None (segregations of anatectic leucosome).

Muscovite

-

Rare Element

Structural Features

Metamorphic Environment

Typical Minor Elements

Conformable to mobilized cross cutting veins

Upper Amphibolite to Low to High Pr Granulite Facies

U,Th,Zr,Nb,Ti,Y,R EE,Mo. Poor to moderate mineralization

None,(anatectic bodies) Quasi – to marginal to exterior. conformable to cross cutting.

High Pr Barrovian Amphibolite Facies (kyanite sillimanite)

Li,Be,Y,REE,Ti,U,T h,Nb>Ta. Poor to moderate mineralization. Micas and ceramic minerals.

LCT– Li Cs Ta

Interior to marginal to exterior.

Quasi – conformable to cross cutting.

Low Pr Abukuma Amphibolite to Upper Greenschist Facies. (andalusitesillimanite)

Li,Rb,Cs,Be, Ga,Nb Ta, Sn,Hf,B,P,F. Poor to abundant mineralization, gemstock and industrial minerals.

NYF- Nb Y F

Interior to marginal.

Interior pods to conformable to

Variable.

Y,REE,Ti,U,Th, Zr,Nb>Ta,F.

cross cutting exterior bodies.

Miarolitic

NYF

Interior to marginal.

Interior pods to cross cutting dikes.

Poor to abundant mineralization,c eramic minerals. 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 and greisenised 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 Pegmatite Subclass

Geochemical Signature

Pegmatite bulk composition

Associated Granites

Granite bulk composition

Source lithologies

LCT

Rare Li,Rb,Cs,Be,Sn,G Peraluminous to element a,Ta>Nb (B,P,F) subaluminous (REL) +REE + Li Miarolitic +Li

Synorogenic Peraluminous,S Undepleted to late ,I or mixed S+I upper to orogenic (to types middle crust anorogenic); supralargely crustal and heterogeneou basement s. gneisses.

NYF

REL + REE

Syntectonic

Nb>ta,Ti,Y,Sn,R Subaluminous to

Peraluminous

Depleted

Mixed

Miarolitic REE

EE,Zr,U,Th,F.

metaluminous (to subalkaline)

,late,post, to mainly anorogenic: partly homogeneous

to subaluminous and metaluminous; A & I types

Cross bred: LCT & NYF

Mixed

Metaluminous to moderately peraluminous

Post orogenic Subaluminous to anorogenic; to slightly heterogeneou peraluminous s

middle to lower crustal granulite or juvenile granitoids. Mixed protoliths or assimilation of supra crustal by NYF 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 is hypidiomorphic granular in normal (not internally zoned) pegmatites. In zoned pegmatites the textures include: aplitic in aplite layers or veins, perthitic texture in massive feldspar, poikilitic textures involving aggregates of accessory minerals within quartz or feldspar, graphic texture in graphic granite layers, dendritic or snowflake texture, spherulitic texture, and

rare cumulate texture involving feldspars.

Contact zones of different layers may display seriate and graded textures. Gross Structures Zoned pegmatites display crude layering or concentric zoning of minerals. Veins of aplite or quartz and a variety of fracture fillings also occur. The style of mineralization in these structures may be symmetrical (wall to centre). Several rare element pegmatites contain gem pockets often associated with geodes or miarolitic cavities or vughs. The size and shape of gem pockets is variable. The mineralogy of the gem pockets depends on the type or class of pegmatite.

Geochemical Characters of Pegmatites The geochemical characters of pegmatites are similar or equivalent to granite. They are evolved rocks. The SiO2 content varies from 60 to greater than 70%. The amount of K2O,Na2O,Al2O3 is 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 than PM and CI indicating 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. The LREE T is high with very low to minute HREE

T.

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 an artificial volatile rich granite magmas. They discovered that dissolved volatiles in the host magma play a role in pegmatite genesis. The volatiles affect the fractionation by reducing the number of

crystals that can form. This allows the already growing crystals to grow to large sizes in a near fluidic environment. The crystals can grow bigger without interfering with each other. The volatiles further reduce the viscosity of 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. The saturation of volatiles leads to their separation as watery bubbles (resurgent boiling effect) surrounded by normal liquid magma. Crystals grow simultaneously from both the liquid and the volatile gas bubbles, with the larger ones forming from the bubbles. This explains the coarse grain size of pegmatites. The localization of volatiles form geodes, vughs, and miarolitic pods which accumulate rare metals to form gem pockets and 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

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.