The pegmatite problem

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THE mTI7R3 AND ORIGIN 0? PEGMATITE CONTEKTS INTRODUCTION . ; DEFINITION OF THE TERM 'PEGMATITE* I#

THE NATURE OF PEGMATITE Size and Shape of Pegmatites llineralogical end Chemical Composition of Pegmatites Texture of pegmatites Internal Structures. Aplites and Acid Pegmatites Association of Pegmatite with Veins and Masses of Quartz Distribution of pegmatites Attitude of Pegmatites Nature of Contacts Inclusions in pegmatite

II. DISCUSSION OF SONE CHARACTERISTIC FEATURES OF PEGMATITES V^ITH SPECIAL REFERENCE TO MODE OF ORIGIN A.

Formation of large Crystals

B.

The Concentration of Unusual Minerals, often containing Rare Elements

C.

The Origin of Zonal Structures

D.

Graphic-texture

E.

The Significance of perthite Feldspar

F.

The Association of Aplite with Acid Pegmatite

G.

The Relationship of Pegmatite to Veins end Masses of Quartz

/

/ H* , J*

Comments on the Nature of Pegiriatite-contacts The Interpretation of Chemioal Réactions at Pegmatlte-contaots

K.

Inclusions In Pegmatite

1,

Rellct-struotures

III. MODE OF £MPLAG£MMiT OF FSGMÂTITS! IV. , THE POSSIBLE GEKETIQ RELATIONSHIP BET^^EN. PEGhUTXTE. PLUTONIC ÏETRHSION COUNTRY-ROCK V.

THE TEMPERATURE OF FORIMTION OF PEGMATITES '

VI.

THEORIES CONCERNING THE GENESIS OF PEGMATITE

StJtlMARY AND CONCLtJSIONS APPENDIX

- 1 -'

iNfROIXJCTIOy Because of their unusual features and their economic importance pegmatites have attracted the attention of geologists since a very early date# Eady, writing at the beginning of the 19th century, first gave the rocks their present name and since then a vast volume of literature has accumulated without providing a conclusive answer to the question of origin and mode of formation#

To a certain extent this

position appears to be due to overextrapolation and generalization by investigators who have studied only one area restricted either in space or time, or who have considered only certain aspects of the rocks in their region*

An attempt is therefore made here to

provide a concise picture of present day knowledge of pegmatites#

It is not proposed to give an historical I survey of pegmatite literature, since this would not

in any way help the method of treatment adopted in this paper, but would be a mere repetition of chapters from earlier writers* The known facts about the occurrence and nature of pegmatites are outlined in brief in this communication, including their relationship to aplites, quartz-veins and other rocks with which they are commonly associated#

Although views on the origin

of pegmatites vary considerably in detail, they fall

2 -

into one of two ruain groups; (1) The formation of pegmatites associated with the emplacement of magma;

(2)

their formation through complete reconstitution of pre* existing rooks without magmetlo intervention# The arguments put forward by these two schools of thought are considered critically and upon the validity and relative importance of these arguments a comprehensive and ordered view of pegmatites and their origin isr presented in the hope that such will guide and facilitate further research on problems connected with pegmatites.

- 3 -

DEFINITION OF THl'l TERM «PSCMATITS* • Before going itirther a definition of the term , *pegEiatite V seems appropriate*

As originally applied

hy EaSy It related only to a rather special phenomenon the graphic intergrowth of quartz and feldspar*

At a =

later date the Century Dictionary defines pegmatite as a coarsely crystallised granite”*

Schaller (1925,p.269/

writes: "The term pegmatite is used in the ordinary sense as meaning a coarse-grained dyke rock consisting essentially of quartz and feldspar".

Hess (1933) offers this definition;

"A general name for rooks with coarsely and unevenly crystallized and segregated minerals occurring as dikes, veins or metamorphic masses formed from the aqueous ; solutions of a freezing magma or from the combination of the solutions with previously existing minerals".

Landes

wrote in the same year that pegmatite is "an intrusive holoorystalline rock composed essentially of rock-making minerals which are developed in part in individuals larger than the grains of the same minerals occurring in the normal plutonic equivalent" J* Geikle (1940) st€ites that "pegmatites are coarse grained, dyke-like end sill­ like apophyses associated with many deep seated intrusions* They are grouped in and around the parent intrusion, cutting it and the country rook*

They are notable in that, though

like the parent mass, they are more acid in composition and often carry rare minerals".

Chamber’s Technical

" 4 -

Dictionary (1947) defines pegmatites thus:

"a term .....

now applied to Igneous rocks of any composition but cf particularly coarse grain* occurring as off-shoots from, or veins in, larger intrusive rock bodies, representing a flux rich residuum of the original magma”. Wehlstrom (1947, p.273) writes;

"Pegmatite is a very coarse-grained

rock. 'The term Implies a mode of formation and a geologic occurrence somewhat different from those of ordinary igneous reeks.**

He goes on to say that when

the term "pegmatite" is used alone, it implies granitepegmatite;

but the term is also used in a second sense to

describe very coarse-grained igneous rocks of any type. The above are but a few ex&mples of the views held by authors who try to attach a definite genetic significance to the name. It seems to the writer more appropriate to restrict the name to a type of rock as advocated by 0. Andersen (193$.

He suggests that in view

of the uncertain genesis of pegmatite, no theory, such as the assumption of an igneous origin, should be taken for granted.

Based on observable facts only he proposes.the

following definition as being stringent enough to have meaning, loose enough to be comprehensive, and reasonably detached from petrogenetio theories:

"Pegmatites are

mineral associations crystallized in situ, decidedly more coarse-grained than similar mineral associations in the form of ordinary rocks and differing from these in having a more irregular fabric of the mineral aggregates ... "

- 5 «

H© then proceeds to state that "this definition would include ordinary granite pegmatites;

all other types

of pegmatites hearing oomagmatio relations to definite families of igneous rooks, like syenite and gabbro pegmatites 1.^ " T o the writer this statement seems to transgress, or at any rate tends to transgress the rule be'lays 'down,^^^^

" " ' '" ' ' ''

'

"

âs a preliminary it is suggested by the writer that pegmatites should be defined simply as rooks of exceptionally coarse-grained nature of any composition from acid to basic#

6 “

I.

THIS MATURE OP PEGMATITE

Size pnd Shape of Pe&mmtltes

.

’ . FegïTiatltes are generally described as finite bodies varying greatly in size from minute dimensions to a few miles long and some hundreds of feet wide* Adams and Barlow (1910) describe a pegmatite in the Haliburton-Bankroft area of Ontario 250 feet wide, another with an area of 4 miles long*

square miles and a third

Because, by definition, pegmatites are

coarse-grained rocks. It may be questioned whether those described as mere stringers, only a fraction of an inch wide, are correctly named* It is generally agreed that pegmatites differ from dyke-intrusions in having no great extent, either vertically or horizontally* Their shapes are irregular and very varied* Some pegmatites are tabular with parallel wells like those of dykes, veins and sills;

Many ere thinly

lenticular, and may be inclined in any direction* Other pegmatites have been described as circular or elliptical in plan, with their longest axes vertical or steeply inclined*

Ihese are the so-called

♦pegmatite-pipes*. The ktta pegmatite in the Black Hills, S* Dakota, has been described as funnel-shaped*

- 7 -

âdems end Barlow bave coined the term ‘’eplashes’ for seme irregularly-shaped pegmatites In Ontario, and lehîiienn (1884) has described similar ones as *flames* (flammen).

The shapes of some pegmatites have been

referred to as *tear-drops’. Pegmatites may brahch, appear merely as irregular segregations, or exist as coarse­ grained phases of normal rocks.

I

The terms ’dyke’ end ’sill* are often used loosely to differentiate between pegmstite-bodles with a cross-cutting or with a concordant attitude with regard to the surrounding rock*

It appears that when

pegmatites are dyke-like, they remain parallel-walled only for short distances and the economically more important pegmatites appear to be almost uniformly lens-shaped, discontinuous bodies, with a tendency to

;

swelling and pinching# Mlneralo^ioal and Chemical Ooianosltion of Pegmatites

|

pegmatites range in composition from some more acid than granite to others, more basic then gabbro# ültrebaslç pegmatites are known#

Those of acid

composition appear to be far more common and more widespread than basic types# Mineralogically pegmatites range from very simple, almost monomineralio rooks (e#g# the cryolite

! : ^

deposits of Eastern Greenland) to rocks of very complex mineral composition.

A feature which distinguishes many pegmatites

from other crystalline rocks is their relatively high

I

j

.- 8 -

content of rate elements. ’

Grenlte-pegmatites consist In the main of

anhearal quartz end anhedral or subhedral potseh-feldspar which is often microcline or microoline-perthite*

Biotite,

muscovite end albite are common constituents of some pegmatites*

Albite is usually present as the low-'

temperature form clevelandi te- with platy habit when in idiomorphic crystals.

Muscovite and biotite generally

develop as book-like masses.

Tourmaline, beryl and

topaz ere locally abundant in pegmatites {Wahlstrom 1947, p. 273.). as ’simple*.

Such pegmatites have been designated

Those containing'in addition rare and

unusual'minerals have been distinguished as ’complex* pegmatites*

The rarer minerals in gi’anite-pegmatites

may be compounds of lithium, fluorine, beryllium, boron, phosphorus, graphite, rare-earths and ore minerals. ''

"

Intermediate pegmatites may contain the same minerals as syenite, monzonite or diorlte sometimes with rare elkalineminerals, caloite, radio-active minerals end sulphideores.

''

'

'

- Basic pegmatites may correspond in minerelcomposltion with gebbro, anorthosite, pyroxenite, etc., but may contain additional calcite, apatite, phlogopite end platinum, e.g. the platinum - bearing dunitepegmatites of South Africa.

- 9 - %

When minerals that can appear in more than one form are present in pegmatites, they are usually represented hy" their low-temperature forms,, e.g. mloroellne, olevelandite,o^-quartz, etc. Indalusite, oordlerlte, epidote, corundum, etc., usually regarded as metamorphic minerals, have been recorded as large crystals in stmie pegmatites. A few chemical analyses of pegmatites are given in Washington’s Tables, but complete analyses are not often available, because of the large size of the individual minerals and the consequent difficulty in obtaining representative samples.

Some chemical

analyses were used in assessing the amount of similarity between a particular pegmatite, end a larger plutonic/ body to which it appeared genetically related.

From

the figures it seems that the pegmatite is usually the more silicic of the two rocks aid that its soda-content may be appreciably higher (Brammall and Harwood, 1924, P.205). Texture of Hemaatltes The texture of pegmatites is characterised by the large-size of the constituent minerals, their subhedral form and their tendency to intergrowth.

One of the

largest crystals on record is a crystal of spodumene from the Black Hills, 8, Dakota, which measured 47 ft. Microcline

"

10 -

crystals tip to 53 ft. have been repeatedly found in Ontario and elsewhere.'' â crystal Of beryl, 18 ft, long and weighing 18 tons, was fohnd in kalne, and miba often develops as' book-llke masses' several feet 'in diameter. •

j

iB characteristic of pegmatites that the'

-.

large crystals ere often more or less completely surrounded by minerals of much smaller size. ‘ In granitic pegmatites grephld-intergrowth of quartz and feldspar is &/ common feature, and rosettes and large ' nodes of radially-arranged feldspar crystals ere often observed (Hess, 1925).

Radiating'iaeEses of quartz, "’ ''

iiore than one foot in diameter, have been seen by the * present writer at YUerby, Sweden.

They were distributed'

irregularly amongst graphic-intergrowths of quartz end feldspar m d massive perthite and It seems possible that they represent a special form of runic texture. Internal Structures ' ^ . /iithough simplè pegmatites may be of homogeneous structure, it is more common, particularly for complex pegmatites, to show zoning of various kinds. ^ The common types of zoning may be'divided into two general groups; (1) Symmetrical zoning, (21 Asymmetrical zoning. (1)

Symmetrical'zoning is a bended structure

consisting of layers which differ in mineral composition, in texture, or both.

The zones are generally parallel to

"'

-li­ the v/slls of the pegiustite body end in this respect seem to be related to its external form.

The Junction

between successive shells in usually transitional and indeterminate, although sharp boundaries do occur, particularly if one of the zones concerned is aplitio, Cameron, Jahns, McNair end Page (1949), after an exhaustive study of pegmatite structures claim to have established a definite mineralogical zone-sequence, from the well inwards;

(1) Plegioclase, quartz, muscovite;

(2) Plegioclase, quartz; (3) Quartz, perthite, plagloclese, with or without muscovite and/or biotite;

(4) perthite,

quartz; (5) Perthite, quartz, plegioclase, emblygonite, èpodumene; (6) Plegioclase, quartz, spodumene; (7) Quartz, spodumene; (8) Lepldolite, plegioclase, quartz; (9) Quartz, microcline; (10) Microcline, plagioclese, lithia, mica, quartz; and (11) Quartz.

Few, if any, pegmatites contain

all these eleven zones. Of much more common occurrence is a zoning which comprises an outer zone of graphic intergrowth of quartz end feldspar, followed inwcrds by coarsely mixed individuals or quartz end feldspar, and a central core of quartz.

The

Intendedlate zone frequently contains muscovite along its outer margin.

Where lithia-bearing and other rare minerals

are present, they comionly occur along the inner margin of this zone,

A decrease in the proportion of the anorthite

molecule in the plagiocl&se from the margin of the pegmatite

M»CA

SCM>&Tfe

"An d

^CiNEISSESI ■•

--- ---- M ICA-- SCHISTS— AND — — C|NEISSE S---- ------/afctrl^oy,Stiarma,♦G«ttbpadhw»y) T ig . 1. ^ Qwl.Mag.H»9 ' G eneralistd P lan o[ M ica-pegm atite, M- muscovita:

B " beryl.

G* g a r n e t ,

A* a p a tite .

T * to u rm a lin e ..

Crofcfc-ruled areas (5) Individual crystals microcline. (G) Quarte, (afkr O.Andera»«n. Nor»k.(fleol.TWük. 1951)

Sketch showing Vertical section through p egm atite near ,

A rn evik , T lo sta , Norway.

: . _ , ; . • • . :: ■ G"gabbro; (I) plagioclase graphic-granite with magnetite; (2) plagiocLase graphic-granite with biotite) (5)m ici^line graphic- granite with biotite ) (A)plagioclase graphic-granite with b io tite .

- 12 - inwerds Is a feature' frequently observed. . '

•■Although Individual pegraatites may vary in their

liiinerel-oomposltlon/ thls fundamental zonal arrangement Is rema rkàbly constant wherever granite pegmatites oc cur. ■

- = Hoy, Shsriiia and Chattopadhyey (1929) picture a

typical mica^besring pegmatite from India (Flg.l,), This lens-shaped body consists of an outer zone of graphic granite with marginal tourmaline which lies partly within the pegmatite, partly just outside it In the country rook.

Considerable quantities of book-form ,

muscovite lie 'along the contact with the Intermediate zone which consists of an aggregate of coarse quartz and alhite-oligoolese.

The lens^shapsd central core -

is filled with quartz.

Along the inner margin of the

intermediate zone more large books of muscovite are fOund associated with spodumene, lepiolitolite, apatite,- etc., and well-developed large individuals of beryl, some of which the are shared in part by^quartz core. ”- ;

A similar zoning has been described by 0, Anderson (1921) from/Arnevik, Plosta, Norway.

There a granite- ^

pegmatite cutting gabbro consists of different layers of graphic granite followed inward by individual crystals of microcline and quartz as shown in the sketch. (i?ig.2. ) To report one similar occurrence from America, , the Branchville, Connecticut, pegmatite is chosen (Shelnin, 1946). T h i s body is made up of a border zone of quartz

— 13 “ end oligoclase, a querter inch in grain size, followed by a muscovite*quartz zone, clevelandite-quertz unit, çieveiandite-spodmïiene zone and quartz core.

Here

again a decrease in the anorthite content of the plagloclase from An 14 at the margin to almost pure

., ,

soda-feldspar at the centre is quoted. Grout (1918) describes a most striking ,

^

example at Duluth where the zoning takes the form of.

y

a continuous change from gabbroic wells to a granite

A frequent structural variation is shown by pegmatites which show a central cavity instead of e quartz core.

^

•

,.

These opeh spaces may range from minute

,

voids to large cavities, some of which ere more or less filled with well-developed crystals. ,

.. .

(2 ) Asyminetric a 1 zoning may either take ,the form of a sequence of differing layers or maybe caused by a ^one­ sided development of a special facies.

„

A good example of the simpler type is afforded by the group of pegmatites associated with the Godolphin granite where the upper portion of sill-like bodies is .y formed by pegmatite, while the footwell is aplitic. .

,

Sohaller (1925) describes e typical; ’dyke* from Pale Co, California. attitude in gabbro.

This pegmatite has a sill-like

:

The upper half of the body consists of

graphic granite grading into the ‘pay streak*, a zone

:iï: ’contaihing litiiiuiu'mineral-sggregstôs end other rare elements.

Of the lower half he says

is a fine­

grained rock, definitely aplltio'in texture'. " It is composed essentially of. albite end quartz with many straight or wavy bands of varying thickness of brownish

shaubu in his studies of pegmatites at Mewry, Maine 11940) describes a beautiful example of asymmetrical zoning.

The principal pegmatite of the group shows fine-?

grained albite along the contact with the hanging wall* '

«

This is followed by a band consisting‘chiefly ;of"quartz, with some albite and muscovite, and a further zone of muscovite and albite. the/footwall.

There is no repetition of this banding along There a layer of medium or fine-grained

albite more than 1 foot thick is followed without sharp demarcation by clevelandlte of gradually increasing coarse­ ness which continues into the body of the pegmatite. In general it appears that symmetrical zoning occurs in lens-shaped pegmatites while an asymmetrical arrangement is more common in sill-like and dyke-like bodies where there is an absence of swelling and pinching. The common orientation of the component minerals more or less perpendicular to the contact surfaces in

'

border zones is another characteristic of many pegmatites. This also applies to the borders of inclusions within a ‘ -^1. pegmatite.

- 15 Oolites end Aold Pe&metites

.

Much coraEient has been made on the apparently paradoxical association of fine-grained aplites end coarse pegmatites in many parts of the world, " ïhus Daly (1914) writes; "Aplite and pegmatite are known to be syngenetlc at thousands of localities end may often be seen forming parts of the same dyke or sill." In 8 m e ce ses ©plite may form zones marginal to a pegmatite as in ©n example described by Uspensky (1943) where a pegmatite-veln showed bilateral symmetry of structure. An irregular line of drusy cavities along

the middle of

the vein was bordered on each side by a zone consisting of coarse-grained aggregate of quartz and feldspar, and , this in turn was followed outward by true graphic intergrowth of quartz and feldspar. margined externally by eplite.

The graphic zone was

The contact between the

last two zones was quite sharp and so was the contact between the aplite and surrounding granite.

This example

is representative of many others. In other occurrences, however, aplite is found forming a central zone or core.

Andersen (1931) observed

—

that when aplite tnd pegmatite occur together in one body in the granite-pegmatites of Southern Norway, the borderzones are usually a pegmatitic aggregate, while the central pert is an aplite.

He states that the reverse is exceptional.

— 16 — Similarly Ghosh (1926) observed a coarse marginal facies in small aplite dykes in the Bodmin Koor Granite. S. Hall (1920) described sill-like pegmatites, associated with the Godolphin granite, the lower portion of which consisted of aplite, while the upper portion was pegmatitic with the mineral-constituents often perpendicular to the upper contact.

Similar to this is

the arrangement observed by Schaller (1925) from pala County, California, end described previously. Aplites and pegmatites may occur separately in the same area and may or may not out one another.

In

some cases the aplite, and in others the pegmatite, appears to be the earlier. Gevers and Frommurze (1930) found that at psukwaie and Erongo-ICanona and numerous other localities the very numerous aplites were invariably cut and faulted by the pegmatites.

Derry (1921) observed in Manitoba that the

aplites were usually of earlier age, but he points out that this is by no means universal,

J.H.L.Vogt has

recorded observations in Southern Horway where the pegmatite is usually the younger body*

0. Andersen has,

however, noted examples of the opposite relationship In the same area. ' The aplites referred to so far are all acid rocks associated with granitic pegmatites.

Aplites

associated with less acid pegmatites are of rare occurrence.

— 17 —

Syenite *-8plltes ere described by Hatch and Wells (1949) as being exceedingly rare, but they consider bostonlte-dylces as possible examples. An occurrence of dolerite-aplite may be found in Merionethshire, North Wales, In the form of veins and clots of highly felsic material in ©rdovician dolerites. . Acid segregations in Tertiary and Carboniferous basalts, described by W,(^.Kennedy (1923) are considered by Hatch and Wells (1949, p.370) to be of a similar nature.

These

occurrences of aplite, however, lie outside the scope of this dissertation because of the absence of associated pegmatites. Of greater interest is the occurrence of associated dykes of gabbro-pegmatite, gabbro and fine-grained ♦black dykes’ on the Lizard penisula, Cornwall.

In this

area a large number of gabbro-dykes, varying from normal gabbro to extremely coarse gebbro-pegmatlte are found f cutting serpentine.

These were succeeded by a swarm of

•black dykes’ which include a type described as beerbachite#. The Geological Survey (1912) describes this rock as containing feldspar in small square and rounded crystals and augite In rounded or anhedral grains not moulded upon the surfaces of the feldspars.

The Memoir considers

that ’in fact most of the structural and mineralogical features of the coarse gabbro are repeated in these rocks on a smaller scale.”

A possible connection between these

- 18 -

two series of injections is suggested by the present writer and will be discussed later. Association of Pegmatites with Veins and Messes of Quartz The association of quartz-veins end more or less irregularly-shaped bodies of quartz with pegmatites is a matter of common observation and has frequently been described.

Some authors think it significant end normal

that the quartz-bodies occur marginally to the pegmatites or form gradational lateral variations or end-phases of pegmatite-bodies*

Adams end Barlow (1910), for instance,

found that quartz becomes very abundant at the extremities of pegmatite dykes where these grow narrow, and state that ”in fact, cases can be observed where the feldspar is represented only by a few individuals here and there, and the dyke develops into a mass of quartz."

'They conclude that

if only this portion were seen it would not suggest that the hidden part were other then a quartz-vein.

According to

J, Geikie (1940, p.265) it is occasionally possible "to trace a pegmatite outward from the parent mass end to find that it passes gradually into a quartz vein carrying simple sulphides and gold./..*. In contrast with the above there are observations and views such as those of Gevers and Fromiiiurze (1930). According to these authors pegmatites, aplites, and masses of quartz, pegmatitic in texture, have been found occurring together In the same locality, both near end at some distance

,

■

* 19

—

from the oontect of granite with oountry-rooke#

Ko

transition from pegmatite directly or by way of aplite, to quartz was observed.

In addition, numerous veins of

quartz were found to be intrusive into ordinary pegmatite and were therefore considered to belong to a'later period of intrusion, v;

:

; 0. Andersen (1931) found in Southern Norway that masses of quartz, some of which could be celled quartzites, others quartz-pegmatite or quartz-veins, were frequently found, associated with pegmatites, in various rocks such as in gneisses and other metamorphic rocks, and also in gabbro, but rarely in granite. Distribution of pegmatites '

Although pegmatites are widely distributed in space

their distribution in geologic-iime is somewhat restricted* They are found most commonly either in Pre-Cambrian terraines or amongst formations associated with orcgenlc; periods* Their mode of occurrence can be divided into three types: (1) within igneous or igneous-looking bodies; (2) marginal to such bodies; and (3) in metamorphic rocks, more or less remote from such bodies.

Ho pegmatite

occurrence has been recorded in unmetamorphbsed sedimentary rocks or in volcanic rocks.

In connection with the first

type it is interesting to note that within granite-messes, thick masses of pegmatites are extremely rare while

— so "■ thin ones are very frequent, as Vogt observed in S. Norway The letter ere described as being either shapeless massed grading into the "normal granite* or as dyke-shepad bodies showing sharp boundaries with the’granite*

In the Bodmin

Moor granite Ghosh has described pegmatites occurring "generally as irregular and lenticular pocket-infillings of no great extent in the body of the main granite .

Grout also finds an

absence of larger pegraatite-bodies within the Duluth gabbro mass, but records pegmatitic patches in which the minerals of the gabbro have grown large#

The borders of these

patches are ill-defined. Pegmatites of the second type occur either as marginal modifications of the 'igneous* bodies or just outside them, embedded In country rock, but clearly related to the igneous rocks#

Pegmatites of the third

group, embedded in rocks with which they have no obvious

I

connection, have been described as being utterly remote from any igneous bodies to which they could be related j

(e.g. 0. Andersen).

-

The correctness of such observations

is open to considerable doubt .since it must be difficult in most cases to ascertain whether or not a subjacent

i

igneous body is present at some distance below the pegiuatltes.

It seems more than probable that groups 2

and 3 ere one end the same.

All the large and economically

important pegmatite-bodies that have been described seem to occur in the country-rock adjacent to igneous or igneous-looking rocks.

| ;

21 Gevers and Frommiorze (1930) suggest that there may he a relation between distribution and size of pegmatlte*hodies*

They observed, in the Erongo tinfield

of S*W* Africa, that the pegmatites were widest at the granite margin and decreased in width with Increasing distance from the granite* Some writers stress that large pegmatite-masses are always found embedded in certain types of rock, in particular in mica-or hornblende-sehlsts and gneisses* Cameron, Jahns, McNair and Page write;

**most of the

pegmatites from which valuable minerals have been discovered occur in metamorphic rocks, chiefly In mica-schist and gneiss, hornblende-schist and gneiss, and quartzlte"*

Roy, Shama and Chattopaothyay in their

investigation of the Kodarma region, India, find that workable mica-pegmatites are always associated with micabearing gneiss and schist*

Higazy (1949) describes

perthite-pegmatites from the Black Hills, 6* Dakota, as occurring in perthite-schist*

In yet another example

A*Ii*Anderson (1933) describes jp-ica-pegmatites in muscovlteand biotite-schists and micaeeous-quartzitea* Other writers, however, do not support this observation but emphasize the persistence of an individual pegmatite through several strongly contrasted country-rocks< Gabbro and amphibolite are fcund to be common hosts to granite- and syenite-pegmatites in Southern Norway, whi&h in New England mica-bearing pegmatites have been found

22 •*■

to OQcur, in quartz-monzonit©, grenite, apllte, hornblendeschist and even calc-silioate gneiss end cele-sillcate marble# ■'■'' Certain types of country-rock appear to be less conducive then others to pegmatite formation*

Gevers end

^Tommurze (1930) found pegmatite abundantly developed at the Margins of granite and even more strongly developed in the surrounding schists#

where the^country-rock is

crystalline limestone pegmatites mere rare* H.H.Head (1923) describes a pegmatite at Ellon, Aberdeenshire, as a normal biotite-pegmatite while traversing schists, but when traced into limestone it becomes fine-grained and diopsidic*

It must be pointed

out, however, that observations such as the above refer to granitic pegmatites only.

Pyroxenite-pegmatites in

Ontario and Quebec occur in limestone, and similar examples are known in Madagascar and Russia.

Furthermore,

Eskola (1932) remarks that pegr^atit ©-dykes are very numerous inmost limestone areas which are out by granite and gives examples in Western Massachusetts, Finland and elsewhere*

-

./

' I

Attitude of Pegmatite A study of statements regarding the relationship to and boundaries between pegmatites and country rook reveals a considerable diversity in observations* ' Cameron, Jahns, McNair and Page observe that: "the

\

\

— 25 —

Ehepes of îiisiiy pegraatites ar@ influenced greatly by the type of wall-rock .

In competent rooks such as

weekly-foliated gn@ias0 , granite end qu&rtzite pegmatites are often discordant, tabular or branching and occupying joints and fractures.

In the more strongly-foliated

schists pegmatites are more commonly concordant, and lenticular*

Gevers end Frommurze describe the well-

defined belts of tin-bearing pegmatites with their orientation determined by the strike of the schists.

They

examined 1710 pegmatites and found 7 # of them to be parallel to the strike of the schists.

They point out,

however, that ^It is a noteworthy fact that dykes obviously somewhat later and cuttings parallel dykes,are usually cross or. oblique dykes*.

âs examples of

conformable pegt;&tlte-bodies,mey be mentioned those in Koderma, India,(Roy, etc. 1939), in Latha Co., Idaho : (A.l.Anderson 1933), end granite-pegmatites in the lewisian of L.

1 exford

Highlands of Scotland.

end.other localities in the îî.ïï# 0. Andersen describes lens-

shaped pegm&tlte-bodies running approximately parallel with the schistosity of foliated rocks, while pegmatites in non-foliated rocks are generally small and cross­ cutting.

It may be concluded that no special significance

should be attached to the attitude of pegmatites, but that their emplacement follows lines of least resistance. Heture of contacts , Many authors have comruented on the nature of the

oontect between pegmatites and neighbouring rooks.

A,L.

Anderson (1933) remarks that the contacts between some pegmatites and schists is sharp, but that it is gradational in others, particularly when the pegmatites contain minerals other than feldspars.

In contrast/

pegmatites composed only of a graphic intergrowth of quartz and microc1 1 ne have been observed to show sharp boundaries.'

Hess (1932) differentiates between

♦insoluble^ host-rock, where boundaries are sharp, and 'soluble* rocks where pegmatlte-boundaries are not sharp. I. Hordensk^ôld(1908-9), in a description of the Ytterby pegmatite, suggests that the junction between hornblendegneiss and pegmatite footwall is a continuous gradation. B.C.King (1948) points out that although some contacts may appear sharp at first sight, they may be quite gradational when examined more closely.

In support of

this statement he describes a small pegmatite-dyke from Osi in which the marginal feldspar crystals are shared by both pegmatite and enclosing rock.

[

Contact-feetures may provide^important clues as to mode of emplacement of pegmatites.

Effects of pegmatites

on their wall rocks can be divided into two groups: mechanical and chemical.

These may occur together or

separately. Mechanical effects pegmatites emplaoed in foliated or schistose rocks are commonly stated not to have deformed the structure of

- 25 *

their hosts*

0. Andersen (1931), for instance* described

siTiall pegmatIte-*lense8 T/lth boundaries that did not

‘

conform to the foliation of the surrounding gneiss, but he could find no indication of any disturbance of the foliation of the host rock#

He added that these particular

bodies mostly consisted of almost pure feldspar, largely mioroeline-perthite*

Fig.l shows a similar example in

India, where a lens-shaped, zoned pegmatite cuts across mica-schist and gneiss without visible disarrangement of their structural features*

0

* Andersen also records the

very opposite and shows a photograph of a pegmatite-lens occurring in a strongly-foliated gneiss on the island of Floats*

There the foliation of the gneiss Is clearly

seen to bend around the lens-shaped pegmatite.

Higazy

(1949) reports interesting instances of deformation and pushing aside of the folia of schist by pegmatitio feldspar crystals*

A survey of a large number of pegmatite

occurrences In the H*S.A* (Cameron, etc* 1949) indicates that Undisturbed wall-rock predominates but exceptions are frequent*

They say:

secondary foliation is commonly

induced in schist along contacts with pegmatite*"

This

foliation is found associated with both concordant and discordant bodies*

A further statement from the same

survey may be quoted; "During emplacement of some pegmatites the well rocks were deformed end pre-existing foliation, bedding, and folds were deformed and crumpled*...

Some of

—

26

""

these folds show that both walls of the pegmatite have been dragged upward in the same direction.” Ho detailed evidence is offered, however, in support of these : statements#

^

Chemical Effects Observations reported by various investigators seem to cover a wide range of theoretical possibilities.

it

one extreme B.C. King (1948) refers to "the unusual absence of signs of contamination” of granite pegmatites and eplites in the Osi area of Nigeria#

At the other

extreme K.K. Landes observes "pegmatite metamorphism at the contact is not only exomorphic but also endomorphic, for in many localities wall-rock contamination has produced abnormal mineralization within the borders of the pegmatite."

. .' -

V?'. Long before B.C.King, Dr. Box observed the absence of any thermal effects of mioa-pegmatites on the contact walls of enclosing schists at jorasemar, India. A considerable number of records stress the presence in the country-rock of material introduced by or with the pegmatite.

The pegmatites, however,

are stated to show little evidence of introduction of matter from the country-rock.

Bor example, according

to Hess fl925) "the outward movement is exemplified in Idaho where great quantities of schist surrounding a

-» 27 «" pegmatite are so altered that they look like granite.*» A,L.Anderson (1923) who investigated pegmatites in this Sana region found that the metamorphism of the enclosing Eohists and micaceous quartzites was much more pronounced in the vicinity of the pegmatites than elsewhere, with the exception of the immediate contact with the Idaho hetholith or its outliers*

As a consequence of the

metemorphism the micaceous rocks are studded in many places with pink garnets and, locally, much black tourmaline* According to Uspensky (1943) considerable quantities of brown mica occur disseminated in the granite close to the pegmatites of Transbaikalia.

Ghosh (1926) observed

occasional mineralisation of the Bodmin Moor granite at its contact with pegmatite-veins. Dealing next with contamination of the pegmatite itself J.J* Runner (1943) writes?

»*In places garnet is

found in pegmatite near schist inclusions and metalimestones, suggesting an assimilation product.

**

He does not state

if the country-rock shows any sign of alteration by the pegmatite*

A granite-pegmatite at Roche, just outside the

margin of the St. Austell granite, Cornwall, contains pinite pseudomorphs after cordierite indicating contamination by the surrounding killas. R.W.Webb (1943) describes two I andj^alusite-bearing pegmatites from California, one a lens-shaped body in granite-gneiss, the other a biotite* microcline-quartz pegmatite in gabbro*

A feature of some

26

-j

• *

" X

interest Is the pccurrenpe; of oorunduiii, dlasettinated in the /1

Goodspeed (1940) and B.C,King (1948) approached

the subject of mode of emplacement in a different way*

Good-

speed recognised, at Cornucopia, Oregon, two categories of ; dykes;

those emplaced by dilation and others by replacement

of the host-rock.

He showed that emplacement by dilation,

that is the forcing apart and widening of a potential or actual joint or. fissure, would cause lateral displacement of an earlier dyke by a later one where two dykes cross at oblique angles.

The amount of off-settings would be of

definite magnitude and direction depending on the width of the dykes and the angle of intersection*

He,also showed

that such laterial displacement is often absent, end that the disconnected portions of the earlier dyke ere in complete allpament*

Goodspeed points out that it is most unlikely for

the later dyke to be of dilational type in such a case since it would demand the postulation of a lateral displacement that precisely corresponds to the expected offsetting.

In

those dykes which intersect without offsetting the later dyke is regarded to owe its ,origin to replacement *

\

It may be doubted whether lack of offsettings alone would justify the rejection of one mode of emplacement in favour of another.

There ere e number of factors, such

as the possibility of vertical movement,:which should be taken into consideration.

-,

B.v.King examined pegmatite and aplite dykes in the Osi area of Nigeria and observed the absence of appropriate offsetting^ in many instances*

In addition to recordin^^

-

74

*

these observations he gives the following reasons for regarding the dykes as being due to replacement rather than dilation,

(a)

The coiïanon highly-irregular form of the

bodies, with ’pinches’ and ’swells’ that produce no effect on the structure of the adjacent host-rock;

(b) the

enclosure of undisturbed patches of the invaded rook, in many places so extensive that they occupy the greater part of the width of the dyke;

(c) the transition of

regular dykes into a series of tongues or into a body with indefinite margins; and (d) the truncation of inclusions end basic patches in the host-rock by the pegmatite-boundery and the absence of the ’cut-off’ portion of such inclusions in a corresponding position on the opposite contact.

Because of the irregularity

and ’closed’ nature of the bodies. King does not favour a process of stoping for the displacement of the wall-rock to make room for pegmatites or aplites,

Goodspeed also

thinks stoping improbable in the case of pegmatites# King remarks on the usual absence of ’contamination’ which would accompany bodily removal of the invaded rook, end would be especially conspicuous where host and dykerock are of contrasting composition.

He concludes that ’the

transformation must have been effected in terms of ultimate c crystal units by processes of metasomatlc replacement**, which he proposes to term **pegmatization** and **aplitizatlon**. the particular cases under investigation no considerable introduction of material took place.

In

-

75

»

other writers concede that some pegmatites are enlarged by wall-rock replacement but

they maintain that

dilation-injection is the primary mechanism for dykefomation.

In this connection it is pointed out by

King that two entirely different processes are invoked to form one uniform rook.

- 75 - ,

rV.

THE POSSIBLE GENETIC RSLATIOTfSHIP B'KTOECT PEO:'ATITB. PLuTO HIG IN T R U S IO N S

Am

OOtJHTaY-HOGK

.

The apparently close relationship of pegmatite to Igneous Intrusions, and the absence of similarity between pegmatite end country-rock:, have been stressed repeatedly as evidence for a magmmtic origin of pegmatite* Those Instances where any similarity between country-rock and pegmatite exists* have been regarded as of minor importance* Minerelogical and chemical resemblances are suggested to have arisen from the introduction of material by pegmatite magma into the country-rock, accompanied by a certain amount of contamination of the pegmatite by assimilation* J.ndersen(1 0 2 1 ) pointed out that the crowding of granitepegmatites around large bodies of granite, for instance, gives support to the idea of a genetic association between pegmatites and definite magmas.

Or to quote Brogger’e words

(1890) in free translation? "The composition of the pegmatites agrees generally, as far as the major constituents are concerned, over long distances with their associated igneous rocks*".: It is quite probable that .a genetic relationship exists between pegmatites and deep-seated large bodies of chemically similar crystalline rocks;

but it seems illogical

to conclude that the pegmatites must therefore be magmatic, because it is not accepted today that the "parent bodies" are necessarily of magmatlo origin, at least so far as those of granitic, syenitic and dioritio composition are

-

.concerned.

77

-

, There exists considerable evidence that the

similarity between pegmatites and surrounding rocks Is of genetic significance and many Investigators believe in 8 close relationship between pegmatites and countryrock* even In oases where no obvious mineraloglcal similarity exists. \ TTnder the heading "Relation of Type of Pegmatite to Type of Wall Rook* Cameron, etc. (1949) consider that since most of the economically-important sheet-mica-bearing pegmatites in the ITIS.A. occur in mica-schists and micabearing quartzites it may be surmised that the mica- content of the pegmatites is due in some way to the incorpora­ tion and redeposition of aluminous material.

This suggestion

is not supported by other authors who believe that the mica in the well-rock has been derived from the pegmatite. Furthermore, sheet-mloa-bearing pegmatites do occur in other types of rocks as stated earlier in this paper. Similarity between pegmatite end host-rock was also discussed by 0. Andersen (1931).

His group

of "pegmatites endemic in metamorphic rooks", as exemplified by plegloclase-oordierite-pegmetltes in plagioclasecordierite gneisses, the garnet-pegmatite in garneti^ferous gneisses, and the epidote-microcline-quartz pegmatites in epldote-bsaring mlcrooline-quartz-mioa-schist, is regarded ad having largely the same peragenesis as the surrounding

-

78

-

rooks, i.e. they seem to owe their origin to the same genetic processes that produced the final metamorphism of the rocks.

In such occurrences Andersen finds plain

demonstration that processes of metamorphism and pegmatiteformation may he closely related phenomena.

He thinks

it unnecessary that any appreciable amount of material was supplied but regards it essential that solutions were active as fluxes during reorystallisation. In the contrasting case where granitio-pegmatites end masses of quartz are found in gabbros and einphibolites, a common occurrence in S. Norway, Andersen visualises a very different mode of formation, namely a differentiation of e parent-magKia into gabbro and residual pegmatite.

In

yet a third type of occurrence, where gr&nltio-pegmatltes are associated with large granlte-intrusions, an

even

simpler co-magmatic relationship is suggested. However plausible his explanations may be, it must be noted that he suggests a number of fundamentally different processes to account for the formation of similar rock-types. Further views on possible relationships between pegmatites end other rooks are very closely bound up with theories of pegm&tite-formatlon and will be more conveniently considered along with other theories of origin.

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T.

79

-

THE TEMPERATURE 'OF FORMATION OF PEGMATITES '

The temperature of formation of pegmatites forms as interesting a field for spéculation as that of any other Plutonic rock, but in the writer's opinion there is no conclusive evidence in the matter,

A few temperature-

^

limits may he inferred from the presence or absence of certain mineral-phases, but even these are of doubtful value in view of the lack of information about the type end quantity of volatiles and fluxes that may have been present originally# Alpha and beta quartz are often recorded side by side in pegmatites and it is inferred that the temperature at the time of formation was neither much below, nor much above the inversion point of quartz, viz. 573^0•

Certain phenacite-quartz-fluorite veins

Switzer (1939) considers to have formed at a temperature perhaps as low as lOO^C because these minerals appear to be later than adularia with which they are associated, the temperature of formation of adularia being about 230®C# Landes (1925) finds some evidence for a hydrothemal, i.e. low temperature, origin of pegmatite in the presence of many minerals which are also found in hydrothermal veins. It has been pointed out by Ramberg (1949), and general agreement exists among other authors, that magnetic melts are confined to temperatures above 600°C - 900^0, the

wide variation being due to the effect of water and other Volatiles* in the sillcate-systera# ; Below these tempe rature i a melt may he substituted for by saturated but dilute aoqueous fluids*

According to Landes and others the

presence of water has the effect of lowering temperatures, but does not influence the course of crystallisation.