CONCEPTS
OF
PEGMATITES
THE INTERNAL
AND
PEGMATITES
THEIR
OF
STRUCTURE
ApPLICATIONS
SOUTH WEST
OF
GRANITIC
TO
CERTAIN
AFRICA
By Eugene N. Cameron
[PLATES
VI-VIII]
ABSTRACT
Detailed mapping and structural analysis of pegmatite bodies in the United States
and Brazil during and since World War II have indicated that mineral distribution in
pegmatites is closely related to the internal structure of these bodies. The concepts
developed have served as a basis for exploration of pegmatites and for estimation of
tonnage and grade of pegmatite mineral deposits.
A brief examination of some 60 pegmatites in the Karibib-Omaruru and Warmbad
areas of South West Africa indicates that many of these pegmatites have internal
structures comparable to those described from the United States. It therefore appears
that structural analysis could be successfully applied to problems of prospecting,
exploration, and mining of pegmatite mineral deposits of the two areas.
INTRODUCTION
During July of 1951, the writer visited the Karibib-Omaruru and Warmbad
areas of South West Africa, the pegmatites of wbich have become known the
world over through the able investigations of T. W. Gevers, H. F. Frommurze,
and their fellow workers. The visit was brief, but it gave the writer an opportunity
to compare some of the pegmatites of these areas with those studied in detail
by him and by many others in the United States during the past twelve years.
During this period certain concepts of the internal structure and distribution
of minerals in pegmatite bodies have been developed. The writer has attempted
to· determine, in a preliminary way, whether these concepts are applicable to
prospecting and exploration of pegmatite bodies in South 'Vest Africa. The
present paper offers the results for consideration by his colleagues in South
Africa.
The writer wishes to express his appreciation to all those whom he met
during his visit or in connection with it, for their kind assistance and for many
courtesies. In particular, thanks are due to Mr. Peter Weidner, Warmbad,
South West Africa; Mr.]\ Tinschmann, Karibib; and Mr. G. K. Joubert, of
the Geological Survey of South Africa at Pretoria.
CONCEPTS OF THE INTERNAL STRUCTURE OF GRANITIC PEGMATITES
General statement.-The urgent demand for beryl, sheet nlica, and other
pegmatite minerals during World War II led to intensive investigation of large
numbers of pegmatites in various districts of the United States and in several
areas of Brazil. The purpose of these investigations was to assist the production
of pegmatite minerals, and the work soon resolved itself into an effort to
D
46
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
determine whether any pattern of internal structure could be found in pegnlatites
and used as a key to the distribution of valuable minerals in them. Thousands
of pegmatites were mapped and studied in detail in the course of the investigations. Studies in Brazil have been summarized by Johnston (19) and by Pecora
and others (25). The cardinal results of the investigations in the United States
were described in 1949 by Cameron, Jahns, :McNair and Page (4) and are briefly
summarized below. Descriptions of pegmatites in various districts have been
given in a series of pUblications (3, 5, 11, 12, 16, 17, 18, 24).
Types of pegmatites and units found in them.-Two general types of pegmatite
bodies have been recognized on the basis of detailed studies. One type consists
of pegnlatite bodies that are essentially uniform mixtures of theil' component
minerals. These have been called homogeneous pegmatites. In every district
studied, this type of pegmatite has numerous representatives, but in the United
States few have been commercially productive. The other type, the inhomogeneous pegmatites, comprises pegmatite bodies each of which consists of two
or more structural and lithologic units differing in mineralogy or texture, or
both. All but a small fraction of the production of pegmatite minerals in the
United States has come from bodies of this type. They are therefore of particular
economic interest, and the kinds of structural units within thenl have been
scrutinized with care.
Three general kinds of structural and lithologic units within inhomogeneous
pegmatites have been recognized: zones, fracture-fillings, and replacement
bodies. * The relationships of these three types of units are shown diagrammatically in Fig. 1. Of the three types of units, zones are by far the most
important, both quantitatively and economically, in the pegmatites of the
United States, and most of the pegmatite minerals produced in America have
come from zones.
Zones.-In ideal development, zones are successive shells symmetrically
developed with respect to an innermost zone or core. Many zones, however,
are incomplete or discontinuous shells or, even if complete, are thicker in some
parts than in others. Figs. 2 and 3 are diagrammatic illustrations of some
common variations in zone thickness and continuity, and Figs. 4 and 5 show
examples from one of the pegmatite districts of the United States. In some
pegmatites, these variations show a close relationship to structural features of
the wallS, such as the keel or crest of a pegmatite lens or bulges in the pegmatite
walls.
Zones have been classified into border zones, wall zones, intermediate
zones, and cores (Fig. 1). This classification was originally made chiefly for
purposes of convenient deseription, but experience has shown that each type
has certain characteristics that are significant both practically and genetically.
The border zone of a pegmatite is the outermost zone. Border zones are
commonly fine-grained and a few inches or less in thickness, but border zones a
foot or more in thickness are known. The zone next inside the border zone is
* The existence of structural and lithologic units within pegmatites has been
recognized since the time of Bragger, though systematic investigation of these units in
large numbers of pegmatites has come only in recent years. For a full discussion of this
subject, the reader is referred to Cameron, Jahns, McNair, and Page (4).
CONCEPTS OF THE INTERNAL STRUCTURE
47
termed the wall zone. Wall zones are mostly coarse-grained and range up to
10 feet or even more in thickness. The core of a pegmatite is the innermost
zone or nucleus; any zones present between it and the wall zone are termed
intermediate zones. A given pegmatite may contain as many as five or more
intermediate zones. Intermediate zones are commonly coarse or very coarsegrained, and some consist of crystals many feet in length.
Of the four types of zones, border zones and wall zones are in general more
continuous and more constant in thickness than intermediate zones or cores.
Intermediate zones are commonly variable in thickness, discontinuous, or
FIG. 1.
Idealized diagram to show types of pegmatite units.
asymmetrically developed with respect to the core. In a pegmatite in which
valuable minerals are concentrated in one or more intermediate zones, this can
be an important consideration.
As shown in Fig. 6, one cannot be certain of the full zonal structure of a
pegmatite unless the body is adequately exposed in three dimensions. If the
crest of a zoned pegmatite lens has only just been stripped of its cover of enclosing
rock, the pegmatite will appear to be homogeneous and to consist only of the
material of the border zone. As erosion penetrates deeper into a pegmatite
body, more and more of the zonal structure will be exposed until the core finally
is revealed. For this reason, surface exposures are of limited value as indications
48
TRANSAOTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRIOA
of the three-dimensional internal structure of a zoned pegmatite. This is a
serious matter, and failure to allow for it may result in an erroneous appraisal
of the mineral potentialities of a pegmatite.
Mineral assemblages in zoned pegmatites.-During the wartime period of
intensive studies of pegmatites in the United States it became apparent that the
mineral assemblages composing the various units of zoned pegmatites studied
FIG. 2.
Diagram showing two common types of asymmetric zone development in pegmatites.
In pegmatite A the intermediate zone (stippled) is thicker along the keel of the core
(black) than along the flanks. In pegmatite B the intermediate zone forms a hoodlike
body over the core.
occur in an astonishingly regular sequence from the walls of pegmatites inward.
Individual pegmatites differ in the number of assembll1ges present. In some
pegmatites a given mineral assemblage forms two or IrlOre zones differing in
texture or mineral proportions. Pegmatites are also kno"\Vn in which a single
zone contains the minerals of two or more adjacent menlbers of the sequence.
In any given pegmatite, however, the order in which the assemblages present
occur from the walls inward is the same as the order of the corresponding
assemblages in other pegmatites. Comparison of sequences of mineral assemblages
CONCEPTS OF THE INTERNAL STRUCTURE
49
in various districts led finally to compilation of a general sequence of mineral
assemblages which is the summation of the sequences found in various districts
studied. This general sequence is as follows : 1.
2.
3.
4.
5.
Plagioclase-quartz-muscovite.
Plagioclase-quartz.
Quartz-perthite-plagioclase, with or without muscovite, with or without biotite.
Perthite-quartz.
Perthite-quartz-plagioclase-amblygonite-spodumene.
FIG. 3.
Variations in zonal structures. In pegmatite A, the intermediate zone (black) is
discontinuous. In pegmatiteB, the core (stippled) is discontinuous, likewise the
intermediate zone. In pegmatite C, the core and intermediate zone form podlike
bodies scattered through the central part of the pegmatite.
6.
7.
8.
9.
10.
11.
Plagioclase-quartz-spodumene.
Quartz-spodumene.
Lepidolite-quartz-plagioclase.
Quartz-microcline.
Microcline-plagioclase-lithia mica-quartz.
Quartz .
.Exceptions to this sequence in pegmatites studied in various American districts
appear to be quite rare.
It should be emphasized that the sequence given above is not theoretical,
but empirical; i.e., it is simply a summary. of observations. Its value as an aid
to prospecting and exploration is therefore independent of the uncertainties
50
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
that attach to theories of pegmatite origin. The sequence of mineral assemblages
is of practical use because it enables the prospector to locate himself mineralogically within a zoned pegmatite and to some extent to predict whether the
minerals sought occur within a given pegmatite. Thus, if the wall zone of a
pegmatite consists of perthite-quartz pegmatite, the plagioclase-quartz-muscovite
assemblage, an important source of commercial sheet muscovite, will not be
found. On the other hand, an intermediate zone or core containing lithium
minerals may exist. Similarly, assemblage No. 4 is represented in many
Branton
o
40
SCALE
80
I60Feet
SCALE
80 Feet
'----'-----'
o
FIG. 4.
Geologic plans of vario~s pegmatites of the Bryson City district, North Carolina,
showmg shapes and arrangements of zones.
51
CONCEPTS OF THE INTERNAL STRUCTURE
pegmatites by abundant graphic granite, which in the United States has been
a source of No.2 grade potash feldspar. The same assemblage, on the other
hand, may also be represented by a zone consisting largely of very coarse to
giant crystals of perthite free of intergrown quartz. Such zones are the principal
American sources of high-grade potash feldspar. They invariably lie inside
whatever zone or zones are composed of assemblage No.3.
If the prospector is successful in locating the zone in which the minerals
he seeks are concentrated, he can use the relationships of zone shapes and
~--------------------------------------------------------------~
EXPLANATION
~P'flhlt.-qU.'tlP"1III0tlt.
~"lddl.ll'lt.'",.dlal"OI'l'
<::2>
Contoct of p.gmotlt, with woll rocks
0-
Limit of "l'Ido",01l1'l4 .orkin,
FIG. 5.
Isometric plate diagram of the pegmatite at the Deep Creek No.1 mine, Bryson City
district, North Carolina, showing arrangement of zones.
attitudes to the overall form and attitude of a pegmatite as a further guide to
exploration and d~velopment ..
The general sequence of assemblages is expressed in terms of the essential
minerals composing each assemblage. Along with changes in the essential
minerals from assemblage to assemblage, however, certain systematic changes
in the character of accessory minerals have been noted. Thus, beryl from
assemblages 1 to 4 is characteristically green, whereas beryl from assemblages
5 to 10 is characteristically blue, golden, white, or pink. '¥hite beryl is particularly characteristic of the assemblages rich in lithium minerals. .As the
52
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
changes in colour correspond to variations in alkali content, the BeO content
of beryl varies from assemblage to assemblage. In general, beryl from numbers 1
to 4 of the general sequence is high in BeO (12 to 14 per cent.), beryl from
assemblages 5 to 10 is lower in BeO and may contain only 10 per cent. or even
less. Similarly, the gem varieties of green and pink tourmaline characteristically
occur with the lithia-bearing mineral assemblages. Columbite-tantalite likewise
shows variations in composition from assemblage to 'assemblage within some
pegmatites; this partly explains why different specimens of columbite-tantalite
from a pegmatite may show a considerable range of Cb-Ta ratio. Specimens
from a single zone, on the other hand, commonly show little variation in
composition.
A
B
FIG. 6.
Two stages in the erosion of a pegmatite body. In the first stage only the border
zone (not shown) and the wall zone are exposed. In the second stage the intermediate
zone has been exposed and forms the apparent core. The true core is still concealed.
Fracture fillings.-Fracture fillings (Fig. 1) are bodies formed by the filling
of fractures that cut across pre-existing pegmatite. Most fracture fillings are
simple tabular bodies, but a variety of forms is possible, dependent on the
pattern of fracturing. Fracture fillings are mostly small, and their economic
importance in the United States is slight indeed.
Replacement bodies.-Replacement bodies (Fig. 1) are bodies formed by
the replacement of pre-existing pegmatite, commonly along 'fractures. Replacement bodies may form at the expense of either homogeneous or zoned pegmatites.
In most pegmatites thus far mapped in detail, replacement bodies are small
and of little economic importance, but they constitute sizeable portions of some
pegmatites (Jahns, 16). They show a wide range of shapes and relationships
to previously formed zones and to the flanks, crests, or keels of pegmatite bodies.
The simplest are tabular bodies that are formed by replacement of the walls
CONCEPTS OF THE INTERNAL STRUCTURE
53
of fractures cutting homogeneous pegmatites or cutting zonal structures ill
inhomogeneous pegmatites, but there are all gradations between these simple
bodies and complex bodies formed along intersecting fractures, and between
bodies of simple form and bodies that are highly irregular. Irregular bodies
may have no apparent relation to fractures. Replacement of a zoned pegmatite
may be selective, in that one or more zones may be replaced partly or entirely,
whereas the remainder of the pegmatite is little affected. In certain districts,
the pattern of replacement deposits is rather consistent. Thus, in the Petaca
district of New Mexico, Jahns (16) reports that replacement bodies in zoned
pegmatites in general have developed upward from the plunging keels of
pegmatite lenses. In the keel itself, the pre-existing zoned structure has been
partly or completely obliterated in the course ,of replacement.
The detection of replacement bodies is frequently a difficult task and can
ordinarily be done only on the basis of detailed study and mapping. Evidence
that one mineral has been replaced by another is not necessarily evidence that
a replacement body is present. Replacements may be effected during the
development of zones in consequence of reaction between rest liquids and
previously formed minerals, or, if one argues that pegmatites develop in open
system, by reaction between early-formed minerals and progressively changing
solutions. In short, we must · recognize that replacement can occur at many
different stages of pegmatite development.
Most replacement bodies in pegmatites in the United States have proved
to be either too small or too poor in valuable minerals to warrant commercial
exploration, but as statistics of occurrence do not make mines, the possibility
of important replacement bodies must be considered carefully in each new study
of a pegmatite or a pegmatite district.
Relation of fracture fillings and replaceme:n t bodies to zones .-Detailed mapping
of pegmatites has repeatedly demonstrated that of the three types of units
discussed above, zones are the el1rliest. Fracture fillings and replacement bodies
formed after the development of zones was complete and in effect are superimposed upon whatever zonal structure was developed in the early stage. The
evidence for this statement obtained during studies of pegmatites in the United
States 'has been discussed by Oameron and others (4, pp. 101-2). A consequence
of this time relation is that extensive development of replacement bodies results
in partial or complete obliteration of zones. In most pegmatites of the United
States, however, the fraction of the total pegmatite body involved in replacement
is very small, a fact that Landes (21) long ago pointed out for certain pegmatites
of Maine.
INTERNAL STRUCTURES AND MINERAL DISTRIBUTION IN CERTAIN
PEGMA TITES OF SOUTH WEST AFRICA
General statement.-Some 60 pegmatites were examined in the course of
the writer's visit to South West Africa. Forty-six of these are scattered over
the area included in the Karibib sheet (7) and the Omaruru sheet (13) and south
of Karibib on Okongava Farm. One pegmatite on Donkerhoek Farm, southwest of the Karibib-Omaruru area, was also examined. The remainder are
54
TRANSACTIO~S
OF THE GEOLOGICAL SOCIETY OF SOUTH AI<'RICA
located on Umeis Farm, south of Warmbad, on the north side of the Orange
River (14). The Warmbad district is the northeastern portion of the Orange
River pegmatite area. The locations of these areas are shown in Fig. 7.
Pegmatites occur in each in great numbers, hence those examined can scarcely
be considered fully representative. They constitute a random sample of those
1St
~
__
~
________
~
______
~L-
-r__-+______ ZSO
__________
~----~~----------~---------r-----3~
100
!
zoo
MIL.ES
I
SCALE
ZOO
INDEX MAP OF SOUTH WEST AFRICA
FIG. 7.
Index map of South West Africa, showing pegmatite areas visited.
pegmatites that were currently productive or had recently been prospected,
together with such other pegmatites as were noted in their immediate vicinities.
In both areas, the pegmatites occur in folded pre-Cambrian metamorphic rocks
that have been invaded by various igneous rocks, including pre-Cambrian
granitic rocks of at least two ages. The pegmatites are considered to be related
to the younger group of granitic intrusions in each area.
CONCEPTS OF THE INTERNAL STRUCTURE
55
Characteristics of the pegmatites.-The pegmatites examined show a wide
range of mineral compositions. At one extreme are those consisting essentially
of quartz, plagioclase, and perthite, with accessory muscovite or .biotite or
both, and traces of garnet or tourmaline. At the other are those containing in
addition, beryl, lepidolite, amblygonite, columbite-tantalite, cassiterite, topaz,
triphylite, spodumene, microlite, bismuth, and perhaps other minerals in
various combinations and proportions. Some of the pegmatites examined
appeared to be of the homogeneous type .. None of these appeared to have been
productive, although a few had evidently been prospected at one time or another.
l\Iost of the pegmatites examined are of the inhomogeneous type, consisting
of two or more contrasting lithologic units, and the large majority of these
.pegmatites show zonal structures. Zonal structures were observed in three
pegmatites examined on the Gossow claims, in a series of pegmatites on the
Van der Made property in the Erongo Schlucht, in two pegmatites being worked
or prospected by Mr. G. J. Vroom a few miles east of Karibib, in pegmatites
mined or prospected by Mr. F. Tinschmann, one on Farm Davib Ost and two
on Farm Okongava, in the tantalite-bearing pegmatite on the J. H. Viljoen
claim, Farm Okongava, in two pegmatites on the same farm being worked by
South West Africa Ores (Pty.), Ltd., in the pegmatite then being worked
by Mr. M. H. C. Brockmann on Farm Donkerhoek, in two pegmatites
formerly worked on Farm Davib West, and in a series of pegmatites on Farm
Umeis. The pegmatites range from those in which zonal structure is poorly
developed to those in which it is strikingly developed. The following examples
illustrate the zoned structures observed.
Pegmatite at the Donkerhoek Mine.-The pegmatite at the Donkerhoek
Mine is an irregular lens that strikes approximately N. 20° W. and is exposed
intermittently over a length of about 250 feet (Fig. 8). The pegmatite ranges
from 12 feet to nearly 50 feet in width. The walls vary markedly in dip within
short distances, and as the workings are shallow, the overall dip of the body is
undetermined. The pegmatite has a well developed zonal structure. The border
zone is 1 to 2 inches thick and consists of fine-grained quartz, plagioclase, and
muscovite. The wall-zone is mineralogically similar but is much coarser grained,
containing abundant wedge muscovite books up to one foot in diameter and
4 inches thick. The zone is 1 to 2 feet thick. Inside this is an intermediate zone
conSisting of intergrown coarse massive plagioclase and quartz, plumose
plagioclase, and traces of perthite. In the northern part of the pegmatite, this
zone contains scattered spodumene crystals as much as 1 foot in length. The
zone is poorly exposed in the southern part of the pegmatite but appears there
to be barren of spodumene.
The core of the pegmatite is discontinuous, consisting of a northern and
a southern segment. The southern segment consists of quartz that locally has
been replaced by albite along fractures. The northern segment is essentially
similar. The keel of this segment is exposed, plunging gently in a direction
sOlllewhat west of south. On the underside of the keel there is an inner intermediate zone consisting of quartz with scattered crystals of spodumene. Owing
to the gentle plunge of the keel, the zone appears on the map as a long tapering
body extending north from the north end of the core.
56
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
Tantalite, together with scattered crystals of beryl, is apparently concentrated in the inner part of the outer internlediate zone along the north
segment of the core. No evidence of extensive replacement of any of the zones
by later material was seen, though the northern segment of the core is veined
6
SECTION ALONG LINE 6-8'
QUARTZ PEGMATITE
QUARTZ-SPODUMENE
PEGMATITE
1># ~
..j>
t>'''''
I
f\ 5/.~ >vl
PLAGIOC.LASE -QUARTZSPODUMENE PEGMATITE
PLAGIOCLASE-QUARTZ
PEGMATITE
~ PLAG,OCLASE-QUARTZ-
~ MUSCOVITE PEGMATITE
1i.92J
./,~
BIOTITE GNEISS
CONT/-.CT OF PEGMATITE
WITH WALL ROCK
INFERRED CONTACT
CONTACT BETWEEN ZONES
Of PEGMATITE
.
INFERRED CONTACT BETWEEN
ZONES OF PEGMATITE
RIM Of EXCAVATION
o
I
20
40
,
I
60 FEET
---------'
APPROXIMA TE SCALE
FIG. S.
Pace-compass map of pegmatite on Farm Donkerhoek.
CONCEPTS OF THE INTERNAL STRUCTURE
57
by late albite locally, and cleavelandite present in places in the outer intermediate
zone and along the margin of the core may likewise have formed by replacement
after development of the zones.
Pegrnatites on the Van der Made property.-A series of pegrnatites is exposed
on the property of Mr. G. H. Van der Made, on the west wall of the Erongo
Schlucht, Karibib sheet. The pegmatites range from those that are composed
essentially of quartz, perthite,and plagioclase, with minor amounts of muscovite
or biotite, or both, to those in which lepidolite, amblygonite, columbite-tantalite,
cassiterite, topaz, tourmaline, and beryl occur in various combinations and
proportions. The pegmatites of simpler composition show varying degrees of
dpvelopment of zonal structure. A common arrangement consists of a thin
border zone of quartz and feldspar, a wall zone composed of quartz and feldspars
with accessory mica, an intermediate zone composed of coarse, blocky perthite
with interstitial quartz, and a core of quartz. The core is commonly discontinuous, consisting of a series of medial lenses.
Zonal structures are evidently present in the pegmatites of more complex
mineralogy, but the writer had time to examine only one in sufficient detail to
work out the zonal arrangement. This is a pegmatite that crops out a short
distance above the Van der Made house. The pegmatite strikes N. 15° E. to
N. 20° E., dips 70° to 80° W., and is exposed almost continuously from its north
end southward for a distance· of about 1,450 feet, where it passes beneath
overburden. The pegmatite is enclosed in Salem granite (7). Time did not
permit detailed mapping of the pegmatite. In ]'ig. 9, successive horizontal
sections at intervals along the length of the pegmatite are used to show the
relationships of the principal units. The fundamental structure of the pegmatite
is an asymmetrical arrangement of zones. The most striking zone is the quartz
core (Plate VI, Fig. 1), which is continuous from a point 25 feet from the north
end of the pegmatite southward for a distance of approximately 950 feet. The
zones flanking this core are less well exposed. The border zone is the usual
inch to two inch-thick fine-grained mixture of quartz and feldspars. The wall
zone, 6 inches to 2 feet thick, consists of plagioclase, quartz, muscovite, and
striking black prisms of tourmaline that are oriented perpendicular or subperpendicular to the contact with granite (Plate VI, Fig. 2). The zone is first
exposed along the footwall at a point about 390 feet from the north end, and
scattered exposures suggest that it is continuous from this point to the south
end of the pegmatite. The zone js exposed along the hanging wall southward
at intervals from. a point 700 feet from the north end of the pegmatite. The
zone thickens southward and near the southern end is marked by prisms of
tourmaline as much as two feet in length. In the northern half of the pegmatite
the outer intermediate zone is a coarse-grained mixture consisting essentially
of quartz and plagio~lase with minor perthite and varying amounts of muscovite.
Inside this is a thin, discontinuous zone composed of large crystals of perthite
with interstitial quartz. Along the hanging wall side of the pegmatite this zone
is directly in contact with the core. Along the footwall side a zone of gray,
massive lithia miea, with varying amounts of plagioclase and quartz, begins
about 350 feet from the north end of the pegmatite and extends to 645 feet.
Over this distance it separates the perthite-quartz zone from the core.
Ol
Cf)
o~
60
I
I
1
393'
~;J3'
700'
910'
I
I
I
I
lifO'
1275'
1-3
1455'
I
I
~
I>
~
r:n
I>
a
1-3
H
0
~
r:n
~i~u
III
0
~
1-3
I:I:
- ...... _----. ......
t;j
80'
Q
t;j
0
~
0
Q
100
,
200
,
300
,
H
a
400
, F"EET
I>
SCALE
" ,
82]
,- "
; ,',:.
r;r;xl
~
~
~
I(-!\'I
QUARTZ PEGMATITE
1;. . .- ,. - 1
11'\-'1
LEPIDOLITE PEGMATITE
1'\ '>- >-1
MUSCOVITE PEGMATITE
PERTHITE-QUARTZ PEGMATITE
N
<
./.
<"
t--
~
PERTHITE-QUARTZ-AMBLYGONITE
TOPAZ PEGMATITE
~ PLAGIOCLASE-QUARTZ-t.lUSCO-
EXPOSED
~ VITE PEGMATITE
1
CONTACTS
BETWEEN
PLAGIOCLASE-QUARTZBERYL PEGMATITE
~
PLAGIOCLASE-QUARTZ-PERTHITEMUSCOVITE PEGMATITE
...
EXPOSED
CONTACT
j
__ , ••• 7'"
INFERRED
PEGMATITE UNITS
OF PEGMATITE
/
//
~ PLAGIOCLASE-QUARTZ
~ PEGMATITE
APPROXIMATE
WITH WALL
7--0
/7/
INFERRED
~I'-~I-\
ROCK
SECTIONAL PLAN OF VAN DER MADE PEGMATITE
~I'-UE t.\
r:n
0
aH
t;j
1-3
~
0
~
r:n
0
d
1-3
I:I:
I>
~
FIG. 9.
Sectional plan of Van der Made pegmatite, Erongo Schlucht.
~
a
I>
CONCEPTS OF THE INTERNAL STRUCTURE
5it
As the quartz core pinches out, the perthite-quartz zone thickens and in
general forms the central part of the southern portion of the pegmatite. At the
south end, however, a small segment of the quartz core is present. Between
1,000 feet and 1,100 feet from the north end of the pegmatite, a series of
intermediate zones appears between the perthite-quartz zone and the wall zone.
These are shown in the diagram. The most important is a beryl-plagioclasequartz zone 1 foot to 3i feet thick containing crystals of white beryl up to
38 inches in length and 20 inches in diameter. This zone was traced along both
footwall and hanging wall sides of the pegmatite for more than 300 feet. Inside
it is a zone composed essentially of plagioclase, quartz, amblygonite, and blue
topaz, the topaz forming stout crystals up to 6 inches in length and 3 inches:
in diameter. The zone is remarkable both for its topaz content and for its
occurrence between the wall of the pegmatite and the perthite-quartz zone.
Reference to the general sequence of mineral assemblages discussed earlier will
indicate that this is not the normal position for amblygonite, which commonly
occurs with perthite, plagioclase, quartz, and spodumene in a zone inside the
perthite-quartz zone.
Several other units are shown in the diagram. In general they are small or
poorly exposed or both, and time did not permit deciphering the relationships
satisfactorily. Several, particularly those characterized by abundant muscovite,
may be replacement bodies. Replacement after development of the zones may
likewise be responsible for the presence of sugary to platy albite that is abundant
in places in the intermediate zones. It seems evident, however, that the basic
structure of the pegmatite is a zonal one. There is nothing to indicate that
beryl has formed by replacement. It appears to be a normal component of the
zone in which it is concentrated. The lithia mica unit is likewise a zone.
The pegmatite shown in Fig. 9 appears to be the same as one described
by P. J. Rossouw (7, pp. 107-109, 113, 114). Certain zonal units in the pegmatite
are discussed and illustrated in this report, although the overall relationships
of the zones are not given. The lithia mica is reported by F. C. Partridge
(loe. cit., p. 113) to be zinnwaldite.
Tantalite was not seen in place in the pegmatite by the writer, but according
to Rossouw it occurred along the contact between two thin zones included in
Fig. 9 in the outer intermediate zone (plagioclase-quartz pegmatite).
Pegmatite on the Gossow elaim.-The most prominent pegmatite on this
claim, which is located on the Karibib sheet, has been described briefly by
Frommurze, Gevers, and Rossouw (7, p. 104). The pegmatite underlies part
of a low hill, and a body of quartz within the pegmatite forms a conspicuous
white knob at the summit (Plate VII, Fig. 1). The quartz body is about 125 feet
long and about 50 feet in surface width. It strikes northeasterly, parallel to the
apparent trend of the pegmatite as a whole. The southeast margin of the quartz
body dips steeply north, and several shallow pits have been excavated
along it. These follow what is evidently a narrow, tantalite-bearing zone of
muscovite, plagioclase, and quartz. This zone separates the quartz body from
the bulk of the pegmatite, which is a mixture composed essentially of perthite,
graphic granite, plagioclase, and quartz.
The overall pegmatite has an
apparent width of about 600 feet. This includes partings of schist; hence a.
60
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
series of parallel pegmatites may be involved. The pegmatite is at least 1,000 feet
in length.
The pegmatite described above is apparently the same one described by
Frommurze and others (7, .p. 104). Judging from descriptions by these same
authors, occurrences of columbite-tantalite at Sandamap and at the Davib
mine (p. 106-108) are comparable to the occurrence in the Gossow pegmatite.
The latter appears to be a clearcut example of a type of pegmatite that is
characterized by a well developed quartz core but is otherwise poorly zoned.
Two other pegmatites nearby appear similar.
Tinschmann Mine, Farm Davib Ost.-Most of the beryl produced from
this mine has come from eluvium on the slopes of a low hill that is apparently
underlain by a series of pegmatite lenses enclosed in a granite-schist complex.
At the crest of the hill, however, recent work has partly exposed a highly
irregular pegmatite body. This appears to consist largely of plagioclase-quartzperthite pegmatite enclosing scattered lenses of quartz that are rimmed or
partly rimmed by giant perthite crystals. Beryl has been found adjacent to
these pods, but the manner of association was not shown at the time of visit.
The pegmatite has numerous counterparts in the United States. They are
interpreted as cases of poorly developed zonal structure, in which isolated lenses
of quartz take the place of a continuous quartz core, and the perthite zone,
likewise discontinuous, forms complete or incomplete shells around the quartz
lenses. Beryl is often present in the perthite-quartz shells. As in many American
occurrences, graphic granite is present in the Tinschmann pegmatite immediately
surrounding the perthite shells.
Pegmatites on Farm Okongava.-Certain pegmatites on Farm Okongava
show a similar arrangement but with better developed zoning. The cores of
these pegmatites consist of giant perthite crystals and interstitial quartz. These
are separated from the pegmatite walls by plagioclase-quartz-perthite pegmatite.
The contacts between the two units in certain pegmatites are marked by a.
narrow zone carrying abundant coarse "wedge" muscovite, with or without
beryl. Beryl also occurs, however, as crystals scattered through the core.
One of the pegmatites examined on Farm Okongava consists of a lepidolitequartz-plagioclase core enveloped in outer zones consisting of plagioclase,
muscovite, and quartz.
Other beryl, tantalite, and lithia pegmatites of the Karibib-Omaruru area.Various other pegmatites mined for beryl, tantalite, or lepidolite in the KaribibOmaruru area were examined. Small replacement bodies of albite or albite and
muscovite were seen in some of the pegmatites, but concentrations of the
valuable minerals appear to be unrelated to these bodies. Instead, the distribution
of the minerals is governed by zonal structures, and the minable deposits appear
merely to be zones rich in one or more valuable minerals. The one possible
exception to this statement is a tantalite-bearing unit composed of granular
muscovite (lithia muscovite ~), plagioclase, and quartz seen on the Viljoen
claim, Farm Okongava. The pegmatite containing the unit is complex, and
the internal structure could not be worked out in the brief time available for
examination.
CONCEPTS OF THE INTERNAL STRUCTURE
61
Cassiterite pegmatites.-Cassiterite pegmatites are numerous in the KaribibOmaruru area. According to published reports (e.g., 9, p. 111), the tin deposits
of the Erongo region occur chiefly in three east-west belts, the southern one
crossing the Karibib sheet, the central one crossing the Omaruru sheet, and the
third lying north of the Omaruru sheet. Descriptions given, for example those
of two pegmatites at the Humdigams Mine (13, pp. 46-48) suggest that some
of the deposits are cassiterite-rich zones, but others would seem to have the
characteristics of true replacement bodies. Such is definitely the case for some
of the tin deposits on Ameib Farm, the only cassiterite deposits examined by
the writer. Here banded, finely granular replacement bodies of cassiteritebearing plagioclase-quartz-muscovite-tourmaline material cut across the zonal
!Structures of pegmatite bodies composed originally of plagioclase, quartz, and
perthite. The pegmatites appear to be rather clearcut examples of the formation
of replacement bodies at the expense of inhomogeneous pegmatite.
Pegmatites on Umeis Farm.-The writer had the pleasure of visiting a
series of peg!llatite bodies on Umeis Farm in the Orange River area in company
with Mr. Peter Weidner, owner and operator. The farm is in the southeastern
part of the Unleis sheet (14). The pegmatites show a considerable range of
mineral composition and internal structure, but a few bodies will serve as
illustrations.
Beryl-bearing pegmatite on north branch of TantaUte Valley.-This pegmatite
is about 200 feet in length at the surface and about 75 feet in width. It strikes
approximately N. 50° W. A sketch plan of the pegmatite is given in Fig. 10.
The country rock consists of amphibolite and diorite. The exposures suggest
an undulating lens that plunges gently southeast. At the northwest end, its
crest passes beneath a steep hillside. To the southeast it appears to plunge
beneath wall rock again but must underlie the surface at shallow depth.
The pegmatite shows four principal units. Along the contact there is a
fine-grained border zone of plagioclase, quartz, and muscovite. Inside this is
a wall zone (the outermost zone of Fig. 10) composed essentially of coarse
plagioclase, quartz, and large books of muscovite, with accessory garnet,
scattered masses of graphic granite, and scattered crystals of beryl. Inside this
is an intermediate zone, up to 6 feet in thickness, composed of plagioclase,
muscovite, quartz, and beryl, with accessory perthite, and scattered small
crystals of columbite-tantalite. A small amount of native bismuth has been
reported from this zone. The beryl forms golden to yellow-green crystals 1 inch
to nearly 1 foot in length and ~- inch to 8 inc.hes in diameter. Muscovite forms
large books of the wedge-herringbone type. This zone was being worked chiefly
for beryl at the time of visit. The apparent core of the pegmatite consists of
giant crystals of perthite embedded in coarse quartz.
The workings thus far have been confined to the part of the intermediate
zone overlying the apparent core. Exposures north of the workings are poor.
but perthite-quartz' pegmatite is exposed over a small area to the north, and
the pits immediately north and south of it expose the beryl-bearing intermediate zone. The zone may well extend under the hillside to the north.
E
62
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
Pegmatites neaT the wOTkeTs' quaTteTs.-Two snlall lenses of pegmatite
exposed up valley from the workers' quarters are shown in sketch plan in
Fig. 11. The smaller pegmatite strikes N. 30° W. and the larger strikes about
N. 10° W. on the average. Both lenses dip steeply east. The smaller pegmatite
has three zones. The border zone, t inch to 2 inches thick, is fine-grained and
consists of quartz with abundant small muscovite books and subordinate
SECTION
AL.ONG
LINE
A'A'
,', · '.
.• ... ..
fZiJJ
OVERBURDEN
I!;~'I
PERTHITE-QUARi
~
PLAGIOCLASE-QUARTZ -MUSCOVITEBERn. PEGMATITE
~ ',~ :',.:; ;
~
<
v
[";""Vl>
~
PEGMATITE
PLAGIOCLASE-QUARTZ-MUSCOVITF:
PEGMATITE
DIORITE AND AMPHIBOLITE
IN SECTION
CONTACT, WITH DIP. DASHED
WHERE INFERRED
CONTACT BETWEEN ZONES
OF PEGMATITE. WITH DIP
INFER REO CONTACT BETWEEN
ZONES OF PEGMATITE
DIRECTION OF PLUNGE OF
CREST OF PEGMATITE
R 1M OF EXCAVATION
20
110
60 FEET
'---A':"':P""'P':'-:RO=-X-IM"O":AT=-E-=SC""'A-LE:---'
PIG. 10.
Plan of beryl-bearing pegmatite on north branch of Tantalite Valley, Warmbad area.
plagioclase. Inside this is a wall zone composed essentially of plagioclase,
quartz, and lnuscovite, with 'accessory perthite. The apparent core of the
pegmatite consists of quartz with large, well-formed crystals of perthite. No
beryl was observed in this pegmatite.
The larger pegmatite is 175 feet long and appears to plunge northward.
This pegmatite has a border zone 1 to 8 inches thick which is best exposed
along the east side of the pegmatite (Plate VII, Fig. 2). It consists of quartz,
IIluscovite books up to 8 inches in diameter, stout blocky crystals of plagioclase,
CONCEPTS OF THE
L~TERN.AL
63
STRUCTURE
and beryl crystals up to 5 inches in length and 1 to 2 inches in diameter. Both
the muscovite books and beryl crystals are oriented perpendicular or subperpendicular to the walls. The outer portion of this zone is fine-grained and
richer in quartz than the inner portion. The percentage of beryl is estimated
roughly as between 0·5 and 1·0 per cent. A few scattered crystals of beryl
occur in the wall zone.
I~x ~I
QUARTZ-PERTHITE
WJ
PERTHITE P£G~ATlTE
-< 7".A
~
PLAGIOCLASE-QUARTZPERTHITE PEGMATITE
PEGMATITE
PLACIOCLASE-QUARTZ~USCOVITE
PEG~ATITE
GZJ
PLAGIOCLASE -QUARTZMUSCOVITE PEGMATITE
(BORDER ZONES)
A-
CONTACT OF' PEC~ATITE AND
WALL ROCK. WITH DIP
."
-"
Aw
0
I
CONTACT BETWEEN ZONES
OF PEC~ATlTE
DIRECTION OF' PLUNC.E OF
CREST (LEFT) OR KEEL (RIGHT)
OF' PEG~ATlTE
20
I
40
I
60
I
8.0 FEET
SCALE
FIG. 11.
Plan of pegmatites near workers' quarters, Umeis Farm.
The wall zone of the pegmatite is developed only at the ends of the pegmatite.
It is a coarse-grained mixture of blocky plagioclase, quartz, and muscovite.
The intermediate zone is similar but contains perthite in addition to the other
minerals. The apparent core is represented by a medial lens composed of large
crystals of perthite with massive quartz.
The concentration of beryl in the border zone of this pegmatite is noteworthy. Recovery from interior zones has been negligible. It would appear
64
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
that either mining must be made to pay on the basis of the beryl content of the
border zone or the deposit must be abandoned.
Pegmatite at bend of Krom River.-On the east wall of the Krom River,
in the southern part of Farm Umeis, a pegmatite striking N. 50°-60° W. is
exposed over a distance of about 300 feet, its full length. The dip of the pegmatite
ranges from vertical to steeply east. At its northwest end, the pegmatite splits
irregularly into three fingers, and in the northwestern half of the pegmatite
inclusions of gneiss are present.
The pegmatite has an asymmetrical internal structure. In general, it consists
of a perthite-quartz zone enclosing a quartz core with sparsely scattered perthite
crystals (Plate VIII). Between 172 and 208 feet from the northwest end,
however, the pegmatite has the structure shown in Fig. 12. Along the hanging
NE.
SW.
0
x
x
QUARTZ PEGMATITE
[IJ
PERTHITE
~
.r" -
PERTHITE -QUARTZ
PEGMATITE
PLAGIOCLASE-QUARTZMUSCOVITE-BERYL PEGMATITE
[BJ
A
..<...
0
I
10
I
I
20
I
PLAGIOCLASE-QUARTZPERTHITE PEGMATITE
30
I
GRANITIC GNEISS
SCALE IN FEET
FIG. 12.
Structure section of pegmatite at bend of Krom River, Umeis Farm.
wall side, the perthite-quartz zone is represented only by masses and single
large crystals of perthite. Outside this is a zone of blocky plagioclase, wedge
muscovite, and quartz 1 to 2 feet thick. This zone is rich in greeniSh-yellow
beryl crystals t inch to 4 inches long and. up to 12 inches in length. Platy crystals
of columbite-tantalite are likewise present in this zone. The outermost zone
on the hanging-wall side consists of plagioelase-quartz-perthite pegmatite. Both
the outer zone and the core in places have been sheared roughly parallel to the
walls of the pegmatite, and along the shear surfaces small books of muscovite
have developed. There is nothing to indicate, however, that the wedge muscovite
in the beryl-bearing zone has developed by replacelllent.
Pegmatite near boundary of Farms Umeis and Kinderzitt.-A strikingly
zoned pegmatite is exposed in the walls of a deep ravine on Farm Umeis near
the boundary with Farm Kinderzitt. The pegmatite is a tabular body that
strikes about N. 42° W., and dips 27° SW., and is enclosed in diorite, biotite
schist, and amphibolite. It is ll10re than 1,000 feet in length, and lllay be divided
into two sections with reference to its intersection with the stream. The section
65
CONCEPTS OF THE INTERNAL STRUCTURE
northwest of the streaIll was only partly examined. It shows a fine-grained
border zone composed essentially of quartz and feldspar. Inside this is a zone
consisting of quartz, plagioclase, and minor amounts of perthite. The middle
of the pegmatite is marked by the presence of lenses consisting of large perthite
crystals and quartz. In one lens these minerals are intermingled, and part of
the quartz is interstitial, part graphically intergrown with perthite. A.nother
consists of quartz bordered by large crystals of perthite. One of the lenses is
40 feet long and 6 to 8 feet thick. The total thickness of the pegmatite body in
the parts examined north of the river ranges from 12 to 22 feet.
Southeast of the stream the slope is nearly parallel to the contacts of the
pegmatite. The hanging wall country rock has been stripped away, and erosion
has cut into the pegmatite body itself to varying depths (Fig. 13) . .As a result,
NE.
SW.
I~
A
x
x.x.1
QUARTZ PEGMATITE
r--71
LEPIDOLITE-ALBITE
~ PEGMATITE
I~
. \ '. "I
~ " .'
1'::-: :'-',1
.. ,1... ' , .. "
9
QUARTZ-ALBITE
PEGMATITE
PYROXENITE
'..L..90_ _ _
2..L..90_ _---I3pO FEET
L ___
APPROXIMATE SCALE
FIG. 13.
Structure section of pegmatite near boundary between Farms Umeis and Kinderzitt.
the outcrop width of the pegmatite ranges up to 240 feet, although the pegmatite
body is probably nowhere more than 30 feet thick. The pegmatite here shows
other zones. The core, exposed for 300 feet along strike, consists of quartz.
Flanking this is a zone of lepidolite, albite, and quartz, parts of which are rich
in granular lepidolite. This zone is discontinuous, being developed in places
only along the hanging wall side of the core, in places only along the footwall
side. The quartz core itself, however, is discontinuous, consisting of a series of
tabular lenses of quartz, and in a few places segments of the core are completely
enclosed in lepidolite. Outside the lepidolite zone is another discontinuous
zone (not shown in Fig. 13) marked by crystals of perthite up to 6 feet in length.
This zone corresponds to the perthite that borders quartz of one of the pods
northwest of the stream. Outside the perthite zone, on each side of the core, is
a wall zone 3 to 8 feet .thick conSisting essentially of albite ·and quartz, with
minor perthite.
66
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
The above description is believed to cover the main elements of the structure
of this interesting pegmatite, but only detailed mapping and study could do
full justice to it. Microlite has been found since the writer's visit (P. Weidner,
personal communication) in the outer part of the lepidolite zone, and beryl is
also reported by Mr. Weidner to occur in the pegmatite. Hemispheres of
cleavelandite may represent small replacement bodies developed at the expense
of parts of the inner zones.
Pegmatite on the hill above Alluvial Block ..Vo. 2.-Several pegmatites are
exposed on the crest of the hill overlooking Alluvial Block No.2, in the northern
part of Farm Umeis. Zonal structures, more or less well-developed, are present
in all. Only one could be examined in detail sufficient for purposes of description,
but others are similar. The pegmatite strikes about N. 65° W. and is exposed
over a distance of 240 feet. It ranges from 5 to 33 feet in outcrop width. The
dips of the walls could not be determined, but the trace of the pegmatite across
the topography suggests a steep dip. The wall rock is amphibolitized pyroxenite
and diorite.
The pegmatite consists of the usual fine-grained border zone, a coarsegrained wall zone, two intermediate zones, and a diseontinuous core. The wall
zone consists of plagioclase, perthite, and quartz, with minor muscovite. The
core is of quartz, containing scattered crystals of tantalite and rare beryl.
Patches of granular albite, probably replacement bodies, also occur in the core,
but neither beryl nor tantalite is associated with them. The largest segment of
the core is 20 feet long. It is bordered by giant perthite crystalS that form an
inner intermediate zone, and outside this is a nearly complete outer intermediate
zone composed essentially of graphic granite.
Other pegmatites on Farm Umeis.-Other zoned pegmatites were seen on
}farm Umeis, but time available was insufficient for describing them.
DISCUSSJON OF THE OBSERVATIONS
Pegmatites were long notorious for the supposed erratic distribution of
component minerals, and from the standpoint of estimating tonnage and
grade of portions containing economic minerals, pegmatites have been considered completely unpredictable. FrOIIl time to time, however, geologists of
many countries have noted the occurrence of definite lithologic units in pegmatites, have described them as bands, layers, lenses, veins, or zones, and have
discussed the relations between these units and various economic minerals
(for a review of the literature, see reference 4). Bragger (2), for example, clearly
recognized the presence of zonal structures in his classic work on the pegmatites
of southern Norway. In America, the zonal structure of the famous Etta
pegmatite in the Black Hills of South Dakota was described as early as 1885 (1).
Opinions as to the significance of these units, however, have undergone
broad changes with time, and these have reflected changing opinion as to the
origin of pegmatites. The views of. the majority of geologists during the period
1900 to 1924 were summarized by J. F. Kemp (20). Kemp discussed zonal
structures and ascribed them to differentiation of pegmatitic magma in situ,
leading to deposition of layers of contrasting composition successively from
th~ir
CONCEPTS OF THE INTERNAL STRUOTURE
67
the walls inward. Beginning in the middle 1920's, however, Hess (15), Landes (21),
Schaller (27), Cook (6), Miillbauer (23) and many others called attention to the
evidence of successive replacements in pegmatites, and on this basis new concepts
of the development of pegmatites arose. The views of W. T. Schaller (27) may
be taken as an example. Schaller divided pegmatites into two groups: simple
and complex. The simple pegmatites he defined as those consisting essentially
of perthite and quartz, with only minor amounts of other minerals. Such
pegmatites were considered to have developed through crystallization of
pegmatite magma. Complex pegmatites he defined as characterized by the
abundance of other minerals, and he considered that they developed by
successive hydrothermal replacements of the original pegmatite. Similar ideas,
with various modifications, were widely discussed in the late 1920's and 1930's.
For the pegmatites of South West Africa and N amaqualand, the concepts of
origin involved were ably expounded by Gevers and Frommurze. (9) and by
Gevers (8). Gevers (8, p. 351) stated:" Among the long list of minerals occurring in the pegmatites of Namaqualand, all species, outside primary feldspar (orthoclase, micro cline, and subordinate
albite), !3-quartz, zircon, small crystals of apatite and spessartite . . . were
shown by field evidence and laboratory examination to have originated by
replacement."
Gevers regarded the minerals mentioned above as constituting a pegmatitic
quartz-feldspar base formed during an early, essentially magmatic stage of
crystallization. The other minerals he regarded as developed during later
pneumatolytic and hydrothermal stages by successive replacements of the
pegmatitic base.
The concept that such minerals as beryl, tourmaline, spodumene, lepidolite,
tantalite, bismuth, and cassiterite form by replacement of pre-existing solid
pegmatite inevitably leads to certain inferences regarding the distribution of
economic mjnerals within these bodies. The inferences are of great practical
significance. Gevers expressed them with regard to the pegmatites of N amaqualand as follows (8, p. 342) : " Since most mineral occurrences in pegmatites are the result of more or
less intense pneumatolytic and hydrothermal alteration of the pegmatitic
quartz-feldspar base at fairly high temperatures, the metallic and other elements
being introduced initially by fluids of high vapour tension and great irruptibility,
all pegmatite mineral occurrences are characterised by great irregularity and
patchiness. "
Gevers recognized that many concentrations of beryl and other minerals
are associated with the margins of centrally disposed bodies of quartz, but
attributed this to pneumatolytic or hydrothermal alteration and replacement
of the bordering pegmatite.
Studies of pegmatites in America and elsewhere in recent years have not
supported either the inferences on mineral distribution discussed above or the
concepts of pegmatite formation on which they are based. In America, detailed
mapping on scales of 20 to 40 feet to 1 inch was carried on during the war
concurrently with mining operations. In many pegmatites, zones continuous
68
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
and reasonably constant in grade over distances of 100 feet to.nl0re than 1,000 feet
along strike were found. Their arrangement with respect to one another and
with respect to the walls of pegmatites was found to be systematic, and estimates
of tonnage and grade in terms of these units were found to be practicable. For
a considerable number of pegmatites, inferences from surface mapping were
used successfully as a guide to diamond drilling and development work.
structural concepts drawn from this work have been summarized in a previous
section of this paper. The question is: Are they applicable to the pegmatites
of South West Africa ?
The writer's observations suggest that they can be widely applied to these
pegmatites and that detailed mapping and structural analysis of the pegmatites
can be used in exploration, development, and evaluation of many pegmatite
mineral deposits in South West Africa. Even during so brief a visit, all degrees
of development of zonal structure were seen. In some mines, zones were actually
being followed by the then operators. In others, marked variations in the grade
of ore mined were due to the fact that mining was not confined to the zone or
zones in which the minerals sought were concentrated.
The above applies particularly to beryl and lithiurn minerals. If the mines
visited are at all representative, then the major portion of the production of
beryl from pegmatites (apart fronl eluvial deposits) in South West Africa is
from zoned pegmatites, and the workable deposits consist of zones in which
this mineral is concentrated. Noone mineral assemblage or zone can be selected
as the sole source of beryl. The writer observed concentrations of beryl in wall
zones, border zones, intermediate zones, and apparent cores. The beryl-bearing
mineral assemblages represented included quartz-plagioclase-muscovite, quartzperthite, quartz-amblygonite-spodumene-plagioclase, and quartz. With the sole
exception of the topaz-bearing zone of the Van del' Made pegmatite, the sequence
of mineral assemblages from the walls of the pegmatites inward is consistent
with the general sequence presented on a previous page. Lepidolite concentrations occur flanking quartz cores or themselves form the apparent cores of
the pegmatites; this is their characteristic occurrence in the pegmatites of the
Black Hills of South Dakota and in the pegmatite districts of the southwestern
part of the State of Maine. Microlite is present with the lepidolite in some
pegmatites, or occurs in the zone next outside. Beryl in lithia-free pegmatites
or zones is green, whereas in zones carrying spodumene, amblygonite, or lepidolite
the beryl is characteristically blue, white, pink, or golden. The zones in which
these various minerals are concentrated have systematic relationships to the
walls of the pegmatites. Spodumene and amblygonite occur characteristically
in intermediate zones.
Replacement bodies are undoubtedly present in some of the pegmatites
visited. Masses of cleavelandite and of intergrown muscovite and quartz
(greisen) present in the Van del' Made pegmatite, for example, may be of
replacement origin. The works of Gevers and others cite clearcut examples of
replacement bodies, particularly in the tin-bearing pegmatites. The writer
suggests, however, that the conclusion that all minerals save quartz, feldspars,
and certain minor accessory minerals are formed by replacement of an original
pegmatite base needs review. Detailed studies elsewhere (4, pp. 98-106) suggest
CONCEPTS OF THE INTERNAL STRUCTURE
69
rather that zones have formed, as Kemp concluded, from the walls inward by
successive deposition of layers of contrasting composition or texture or both.
In this connection, one must recognize that replacement of one mineral
by another is quite as likely to result by reaction of an early-formed mineral
with rest liquid as by later hydrothermal replacement. The fact that one mineral
replaces another is not proof that it formed during a hydrothermal stage. The
existence of replacement bodies can only be accepted if it can be shown that the
distribution of a particular mineral or assemblage of minerals is related to
structures, such as fractures or faults, that transect pre-existing solid pegmatite.
A review of the criteria used for recognizing replacement in pegmatites is
likewise needed. It has been the writer's experience that textural features such
as veins and pseudomorphs that constitute proof of relative age of minerals
are less common in pegmatites than a perusal of the literature would suggest
and that many of the textural relations among minerals in pegmatites are
capable of more than one interpretation.
The writer does not expect that the views expressed above will be accepted
by his South African colleagues without searching scrutiny. He would suggest,
however, that the concepts that have proved so fruitful in America, Brazil, and
lately in Canada (26) be tried out, through the medium of detailed mapping
and structural analysis of representative pegmatites of South West Africa. The
concept of systematic distribution of economic minerals is one of great potential
value to those concerned with the prospecting, development, and evaluation of
pegmatite deposits. On the basis of this concept, the geologist in particular
can contribute significantly to the economic recovery of pegmatite minerals.
CONCLUSIONS
Zonal structures are present in certain pegmatites of the Karibib-Omaruru
and Warmbad districts of South West Africa.
2. The distribution of spodumene, lepidolite, amblygonite, beryl, mica, and
columbite-tantalite is governed in most of the pegmatites observed by zonal
structures, and minable deposits ·of these minerals consist of zones in which
one or more of the minerals are concentrated.
3. In general, sequences of mineral assemblages from the walls of the pegmatites
inward agree with those found during detailed studies of pegmatites in the
United States, Brazil, and Canada.
4. Analysis of the internal structures of the pegmatites can be used to guide
prospecting, exploration, development, and estimation of tonnage and grade.
l.
BIBLIOGRAPHY
(1) BLAKE, W. P. (1885). 'fin: Mineral resources of the U.S., 1883-84, pp. 606-607.
(2) BRaGGER, W. C. (1890).
Die Minera.lien der Syenitpegmatitgange der sudnorwegischen Augit und Nephelinsyenite. ZeUschr. f. Mineralogie, 16, pp. 215-235.
(3) CAMERON, E. N. (1951). Feldspar deposits of the Bryson City district, North
Carolina. N. Carolina Dept. of Conservation and Development, Bull. 62, 100 pp.
(4) CAMERON, E. ~., JAHNS, R. H., McNAIR, A. H., and PAGE, L. R. (1949). Internal
structure of granitic pegmatites. Econ. Geol., Mon. 2, 115 pp.
70
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
(5) CAMERON, E. N., and others (1954).
{6)
(7)
(8)
(9)
{l0)
(11)
(12)
((3)
(14)
(15)
(16)
(17)
(18)
(19)
(20)
(21)
(22)
(23)
{24)
(25)
(26)
(27)
Pegmatite investigations 1942-1945: New
England. U.S. Geol. Survey, Prof. Paper 255, 352 pp.
COOK, C. W. (1925). The molybdenite deposits near New Pass, Nova Scotia.
Econ. Geol., Vol. 20, pp. 185-188.
FROMMURZE, H. F., GEVERS, T. W., and Rossouw, P. J. (1942). Geology and
Mineral Deposits of the Karibib Area, S.W. Africa.
Union of S. Africa, Geol.
Survey, 180 pp., Expl. Sheet 79.
GEVERS, T. W. (1936). Phases of mineralization in Namaqualand pegmatites.
Geol. Soc. of S. Africa, Trans., Vol. 39, pp. 331-377.
GEVERS, T. W., and FROMMURZE, H. F. (1929). The tin-bearing pegmatites of the
Erongo area, South-West Africa. Geol. Soc. of S. Africa, Trans., Vol. 32, pp. 111-149.
GEVERS, T. W., PARTRIDGE, F. C., and JOUBERT, G. K. (1937). The pegmatite area
south of the Orange River in Namaqualand. Union of S. Africa, Geol. Survey,
Memoir 31, 172 pp.
HANLEY, J. B., HEINRICH, E. Wm., and PAGE, L. R. (1950). Pegmatite investigations in Colorado, Wyoming, and Utah, 1942-44. U.S. Geol. Survey, Prof. Paper
227, 125 pp.
HANLEY, J. B. (1951). Economic Geology of the Rincon pegmatites, San Diego
County, California. Calif. Division of Mines, Report 7-B, 24 pp.
HAUGHTON, S. H., FROMMURZE, H. F., GEVERS, T. W., SCHWELLNUS, J. M., and
Rossouw, P. J. (1939). Geology and Mineral Deposits of the Omaruru Area,
S.W. Africa, 160 pp., Union of S. Africa, Geol. Survey Expl. Sheet 71.
HAUGHTON, S. H., and FROMMURZE, H. F. (1936). The geology of the Warmbad
district, South West Africa. South West Africa Dept. of Mines, Memoir II, 64 pp.
HESS, F. L. (1925). The natural history of the pegmatites. Eng. and Mining
Jour. Press, Vol. 120, pp. 289-298.
JAHNS, R. H. (1946). Mica deposits of the Petaca district, Rio Arriba County,
N. Mexico. N. Mex. Bureau of J.l!lines and Mineral Resources, Bull. 25, 294 pp.
JAHNS, R. H., and others (1952-53). Mica deposits of the southeastern Piedmont.
U.S. Geological Survey, Prof. Paper 248, parts A to G.
JAHNS, R. H. (1951). Gem- and lithium-bearing pegmatites of the Pala district,
San Diego County, California. Calif. Div. of Mines, Special Report 7-A, 72 pp.
JOHNSTON, W. D., Jr. (1945). Beryl-tantalite pegmatites of northeastern Brazil.
Geol. Soc. Amer., Bull., Vol. 56, pp. 1015-1070.
KEMP, J. F. (1924). The pegmatites. Econ. Geol., Vol. 19, pp. 697-723.
LANDES, K. K. (1925). Paragenesis of the pegmatites of central Maine. A mer.
~Mineralogist, Vol. 10, pp. 355-411.
LANDES, K. K. (1933). Origin and classification of pegmatites. Amer. Mineralogist,
Vol. 18, pp. 33-56, 95-103.
MULLBAUER, F. (1925).
Die phosphatpegmatite von Hagendorff, Bayern.
Zeitschr. f. Kristallographie, Vol. 61, pp. 318-336.
PAGE, L. R., and others (1953). Pegmatite investigations 1942-45, Black Hills,
South Dakota. U.S. Geol. Survey, Prof. Paper 247, 228 pp.
PECORA, W. T., and others (1950). Mica deposits in Minas Gerais, Brazil. U.S.
Geol. Survey, Bull. 964-C, pp. 205-305. .
ROWE, R. B. (1953). Pegmatitic beryllium and lithium deposits, Preissac-Lacorne
region, Abitibi County, Quebec. Geol. Survey of Canada, Paper 53-3, 35 pp.
SCHALLER, W. T. (1933). Pegmatites: Ore deposits of the western states. Am. Inst.
Min. Met. Eng., New York (1933), pp. 144-151.
UNIVERSITY OF WISCONSIN
MADISON, WISCONSIN, U.S.A.
Accepted for publication by the Society
on 2nd July, 1955.
'l'R ..tA',...". GEOL. SOC. S.£1 .. VOL. LVIii
PLATE VI
FIG. J.
()utcrops of part of the quart7; core of the Van del' Made pegmatite, Erongo
Schlucht.
Fw. 2.
Contact of Van der Made pegnlatite (left) with granite (r-ight). The wall zone
of the p0g1l1aUtc is characterized by black tourmaline crystals oriented
perpendiclIla,'
snb-per'pcndicula," to the eontaet. '('he largest crystals are
nearly two feet in length.
0"
'TRANS. GEOL. SOC. S.A., VOL. LV LI I
PLA'I'H VIE
Fro. 1.
Outcrop of the quart.z core of the Gossow pegmatite, Karibib-Omaruru area.
FJU. 2.
Pegm.atite on Farm Umeis, Wal'lllbad area, above the wOl'kel's' quarters.
'fhe dark, smooth surfaces opposite the nath-e's feet are part of the hanging
wall slllface of the pegmatite, stripped of waH rock by erosion. 'rhe d:u'keJ'
patch to the right is the beryl-bearing border zone, with books of muscovite
that stand out as dark knobs on the weathered surface.
TRANS. GE'OL. SOC. S.A., V()L I. V III
PLA'I'H V III
Pegmatite at bend of Krom Hiver,F'arm Umeis. 'fhe prorninent ledge running
from left to right is the core of the pegmatite, consisting of q lIal'tll, with
scattered crystals of perthite. The beJ'yl,bearing wne is in the shadmvs
beneath the core.