BUSHVELD GRANITES IN ZAAIPLAATS TIN MINING AREA
47
THE BUSHVELD GRANITES IN THE ZAAIPLAATS TIN MINING AREA.
By C. A. Strauss, D.Se., and F. C. Truter, M.A., Ph.D.
(Published by permission of the Hon. the Minister of Mines.)
[PLATES
I-X.]
ABSTRACT.
The distribution, modes of occurrence, interrelations, petrographical features and
variations of Bushveld granites of three different ages are described. The Bushveld
granophyre is contrasted with the Rooiberg pseudo-granophyre. The significance of this
new information in relation to the problems of mineralisation and the general theory of
the Bushveld Complex is briefly discussed. Six new analyses of Bushveld granites are
given.
INTRODUCTION.
It is well known that the acid plutonic phase of the Bushveld Complex
comprises several granitic and allied types which, although more or less comparable in chemical and mineralogical composition, are distinctly different in
textural design~ This diversity is not so noticeable in those parts of the Complex
in which the topographic relief is not particularly marked. However, in the
mountainous region to the west and north-west of Potgietersrust, where the
entire succession is represented in a strip of rnountainous country barely six miles
wide, this feature is very pronounced and is all the more striking because of the
more regular arrangement of the constituent granite types into successive and
easily recognisable sheets..
This phenomenon was observed and recordedas far back as 1907 by the late
H. Kynaston (1) who distinguished no less than five zones in the granite of the
Zaaiplaats tin mining area. From below upwards these were referred to as:(1) Zone of fine-grained granites, aplites and diorites;
(2) Zone of coarse massive granite;
(3) Zone of medium-grained granulitic granite;
(4) Zone of pegmatite; and
(5) Zone of red granophyre. His conclusions regarding the relationships
of the different varieties of granite to one another were as follows : "They probably do not represent distinct intrusions, separated by any
very marked interval of time, but for the most part may be regarded as modifications of the normal type, the textural and structural variations being due to
differences in the conditions of consolidation. Possibly the red granophyre,
which overlies the pegmatite zone, may represent·a slightly later phase of the
main intrusion.. . . The fine-grained acid bands, occasionally found intersecting
the coarse granite, may also belong to a slightly later stage of the main intrusion,
but are more of the nature of contemporaneous veins than later dykes."
Whilst Kynaston's subdivision represented a signal advance in our knowledge of the Bushveld granites, it could not be profitably utilized for correlative
purposes as it was based more on lithological distinctions than on mutual
relationships in space and time. The same applies more or less to Wagner's (2)
48
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
classification of the granites in the Zaaiplaats region into (i) a thick "sheet of
granophyric rocks" intrusive" in a succession of felsites and shales that here
formed the roof of the Bushveld Complex," and (ii) " a great composite sheet of
red granite" which was "apparently slightly later," because, although he
emphasised the composite character of the" main granite sheet,." he nlade no
mention of the relationships of its constituent types. According to him, these
consist from below upwards of (a) porphyritic granite, (b) alternations of coarsegrained pink and red granite and aplitic granite, (0) a thick zone of miarolitic
granite which merges into (d) fine-grained aplitic granite which in turn is limited
above by (e) the pegmatite zone.
Daly (3) recognized the possibility of the existence of Bushveld granites
of diverse ages when he suggested that, whereas the :lVIagnet Heights granite is
indisputably post-norite, the "Red Granite" may have been "erupted
immediately before the norite." Hall (4) preferred to leave this question open.
CLASSIFICATION OF THE GRANITES.
From a geological point of view, the most striking features of the Zaai.,.
plaats area are the magnificent display of a multiplicity of granite types more or
less zonally arranged, the occurrence about half-way up in the granite succession
of a concordant and remarkably persistent band of pegmatite, and the localization of the cassiterite and scheelite deposits below this pegmatite instead of in
the roof of the granite mass. As a result of an intensive survey of this area
it was disclosed, firstly, that the fine-grained granites and aplites of zone No.1
of Kynaston's classification are, indeed, in part at least, granitized Rooiberg
sediments; and secondly, that while certain granite types have gradational
relationships, others have sharp intrusive contacts. The information collected
served to establish : (1) That granites belonging to three different ages of intrusion are
represented;
(2) That each of these has its own variational or derivative facies; and
(3) That mineralization is genetically associated only with the youngest
of the granites and occurs in its roof.
To facilitate reference the granites of the different intrusions have been given
local or descriptive names and, arranged in chronological order, are referred
to as:(1) Main granite,
(2) Porphyritic granite, and
(3) the Bobbejaankop granite with its associated bodies of Lease microgranite and pegmatite.
In Column A. of Table I they are presented in their proper order of injection
denoted by the numbers 1 to 3, while their varieties or derivatives are arranged
in their true topographic sequence. For comparison, the classifications of Wagner
and Kynaston are given in columns Band C respectively. To emphasise the
difference between their schemes of classification and the one adopted in this
paper, the type-rocks of each have been rearranged so as to conform in position
49
BUSHVELD GRANITES IN ZAAIPLAATS TIN MINING AREA
with their equivalents in Column A. The numbers in Column B denote the order
of injection of Wagner's intrusive sheets, while the bracketed letters refer to
his zones in the" main granite sheet." The numbers in Column C are those of
Kynaston's zones.
'fABLE
B.
Wagner's Classification.
A
Present Classification.
Pegmatite
..
..
(in part) aplitic granite
Lease microgranite .. ~
Fine-grained
aplitic
granite
..
..
2 (b) (in part) coarse-grained
Bobbejaankop granite
pink and red granite
l2 (c) Miarolitic granite
rPegmatite
3~
l
2
1.
..
..
(e)
r222 (d)
(b)
f
Porphyritic granite ..
r2
~
(a) Porphyritic granite ..
2 (b) (in part) aplitic granite
C.
Kynaston's Classification.
(4) Pegmatite.
(3) Medium - grained
granulitic granite.
(2) Coarse massive granite
(in part).
(1) Fine-grained granites,
etc. (in part),
l
Contact mic.rogranite
(sometimes
absent)
Granophyric .granite
succeeded by or intercalated with granite
1 ~ porphyry and granophyre proper
Coarse gray Ieucocratic granite
..
Coarse gray mesotype
19ranite
intrusive contact
Metamorphosed Rooiberg sediments
..
r
l
~
J
1
Granophyric rocks
2 (b)( in part) coarse-grained
pink and red granite ..
(5) Red granophyre.
(2) Coarse massive
granite (in part).
(1) Fine-grained
granites,
etc. (in part).
DESCRIPTION OF THE GRANITES.
Except for tongue-like offshoots in the norite* and older rocks farther to
the east, the Bushveld granites are restricted to the strip of country; about
six miles wide, which is bounded in the east by the norite and remnants of
Rooiberg sediments and in the west by Rooiberg felsites and Waterberg sediments. Thus they build the major portion of the Macapaans mountain range
which, with its foot-hills of Rooiberg sediments, rises scarp-like from the norite
flats in the east to approximately 1,500 feet above them and then descends in
gentle dip-slopes to the Sterk River in the west. By far the greater part of this
granite terrain is occupied by the Main granite. Exceptions to this are afforded
in the Sterk River basin from Groenvley northwards, and in the oblong area
between Zaaiplaats and SolOlnons Temple, where the erosion of the Main granite
has progressed sufficiently far to expose the younger and deeper seated Bobbejaankop granite and its associated bodies of Lease microgranite. The Porphyritic granite mostly hugs the eastern escarpment or occurs in its foot-hills or on
the flats below. (See geological map at end of paper, Plate X.)
*Although the term" norite" is no longer accepted as a collective term for the basic
rocks of the Bushveld Complex, the authors retained it here on account of its longstanding application to these rocks in this area.
5'0
TRA:NSAOTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
1.
THE MAIN GRANITE.
The name" Main granite" was given to this type of granite because of its
great areal extent. It occurs as a sheet-like mass, approximately 8,000 feet
~hick, intruded more or less concordantly between the overlying Rooiberg
felsites along the. Sterk River in the west, and the underlying, highly metamorphosed Rooiberg sediments which cap the Bushveld norite just west of the
Magalakwin River in the east. It comprises a remarkable number of lithelogically distinct varieties which grade into one another, usually almost imperceptibly, but occasionally fairly abruptly. Oollectively these variational facir-s
form a crude pseudo-stratification which conforms roughly to the floor and roof
of the sheet and dips at 10 degrees to 20 degrees to the south-west. This southwestern inclination conforms to the regional post-Waterberg structure. From the
floor upwards these varieties are : -
(a) The Coarse, Gray, Mesotype Granite.-This variety is developed at the
base of the Main granite where the latter is in contact with the Rooiberg sediments. Good exposures of it occur along the base of the escarpment in the foothills on the farms De Hoogedoorns 706 and Sterkwater 296. There it is intrusive
into the sediments as big apophyses and intricately anastomosing veins and
dykes.
In hand-specimen this granite is a 'coarse, gray rock with abundant mafic
minerals. It weathers into smooth domical faces, and supports a prolific vegetation. On weathered surfaces the felspars stand out prominently, while the
quartz is inconspicuous. On fresh fractures the felspars are grayish-white and
have a brilliant lustre, but in places they have a bluish tinge and a horny
appearance.
In thin section the rock is allotriomorphic, inequigranular, and consists of
big a:p.hedral to subhedral grains of turbid felspar and round grains of quartz
of varying size (Fig. 3, B). Most of the felspar grains are optically continuous
oligoclase, the continuity of the twinning lamellae being interrupted only by
scattered areas of turbid orthoclase. The felspars will be described more fully
in a later section. They seem to have had a protracted period of crystallization,
beginning before and continuing until after the quartz. Thus, some felspar
crystals have euhedtal faces on which quartz is moulded, while the same crystals
may, at their other ends, insinuate themselves in between the round quartz
grains, enclose, and also penetrate and replace them. Green hornblende occurs
interstitially and shows incipient alteration to brown biotite. The quartz and
felspar are intensely cracked, and the cracks are filled with hornblende and biotite continuous with the large hornblende grains (Plate 1, No.1). The hornblende encloses sphene and zircon. Some magnetite is also present.
The hornblende obviously formed at a very late stage. It probably originated
as a result of the assimilation of material from the sediments. This suggestion is
strengthened by the fact that small rounded inclusions of amphibolite are
often found in this granite. The amount of hornblende decreases upwards, so
that the granite passes gradually into a coarse, gray, leucocratic variety.
BUSHVELD GRANITES IN ZAAIPLAATS TIN MINING AREA
51
(b) The Coarse, Gray, Leucocratic Granite.-This granite forms the major
portion of the eastern escarpment of the Macapaans mountain range. In handspecimen it is identical with the nlesotype granite, except for the virtual absence
of dark minerals. Although usually gray, it is sometimes red, especially in the
vicinity of diabasic or other intrusions. On weathered surfaces felspar is again
the conspicuous mineral. A porphyritic facies, consisting of large felspar insets
embedded in a coarse granitic groundmass, is also developed in places, but is
not so well represented in this area as in other parts of the Bushveld Complex.
In thin section the predominant minerals are large anhedral to subhedral
crystals of turbid felspar and smaller round grains of quartz. Green hornblende
is still occasionally seen, but it is mostly altered to green chlorite, with brown
biotite apparently as an intermediate stage. (Fig. 3, B). Magnetite, mostly
altered to limonite, seems to have been liberated in this process. Sphene,
zircon and fluorspar are usually present.
Upwards, this granite becomes progressively finer-grained. This diminution
in grain-size is attended by the appearance of micropegmatite, so that the
normal granite passes imperceptibly into a granophyricvariety with its numerous
variational facies.
(c) The Granophyric Granite.-This granite tends to weather into small,
smooth, round boulders which lie thickly strewn about. It is, furthermore,
typically devoid of the thick bush which characterises the coarse granite below
and, instead, is thickly overgrown with grass interspersed with sparsely scattered
proteas and "stamvrugte" (Chrysophyllum). These features make outcrops
of this granite recognizable from a distance. In places it has, in addition, a
markedly platy structure concordant to the regional dip, so that it resembles
well-bedded sediments (Plate VI, No.1). This can be seen to advantage next
to the road from the Zaaiplaats battery to the mine offices. The upper portions
of the granophyric granite, near the contact with the overlying felsites, are very
fine-grained and massive, and resemble the Lease granite as well as some of the
altered felspathic sediments of the Rooiberg series on the farms Lisbon 2366
and Blinkwater 707.
Near the felsites the colour of the granophyric granite is usually a brick red,
but lower down the colour varies between gray and red. The red colour is usually
confined to the vicinity of later intrusions, but sometimes it also occurs in irregular
patches where no intrusive rocks are exposed.
At the contact with the felsites a segregation pegmatite is often formed
in the granophyric granite. This pegmatite consists of coarse felspar and
quartz which occur as blobs and streaks in the fine granite. The zone of pegmatitisation has a thickness of about 3 feet and is laterally discontinuous.
Inclusions of felsite in the granite are often found in this horizon.
In thin section the granophyric granite consists of micropegmatite as well
as of coarse quartz and felspar. While probably the bulk of the micropegmatite
resulted from the simultaneous crystallisation of quartz and felspar, some of it
has equally certainly been formed by the replacement of quartz by felspar. This
replacement structure, or pseudo-micropegmatite, is characteristic of the altered
quartzo-felspathic sediments of the Rooiberg series and will be referred to again
52
TRANSACTioNS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
later. In the granophyric granite it is present in every thin section. A typical
example is shown in Plate II, No. 1. The same sections also show what appears
to be true micropegmatite. Thus, in Plate II, No.2, the large inset of quartz is
marginally cracked and centripetally replaced by felspar, while the surrounding
area is occupied by fine, apparently true niicropegmatite which forms the bulk
of the rock. ]'rom the evidence of optical continuity it is clear that the quartz
in the outlined area in Plate II, No. 2,originally formed one grain which was
marginally replaced by felspar, resulting ina pseudo-graphic structure.
In the field the distinction between granophyric granite on the one hand
and granite and granophyre on the other is of necessity arbitrary, since it is based
entirely on the relative amounts of micropegmatite present as estimated roughly
by means of a hand-lens. During the present investigation rocks were classed
as granite if micropegmatite was absent, or present only in very subordinate
amounts, and as granophyre if micropegmatite was predominant and individualised felspar and quartz were comparatively infrequent. The nanle granophyric
granite was reserved for rocks which, although essentially granitic, contained
an appreciable amount of micropegmatite. Actually the amount of micropegmatite varies between wide limits and all gradations occur from slightly
granophyric granite to granophyre proper.
(d) The Bushveld Granoph:ljre.-The Bushveld granophyre, as distinct
from the Rooiberg pseudo-granophyre to be described later, occurs as concordant,
lenticular, sheet-like masses in the granophyric granite, as on Solomons Temple
1014, or between the granophyric granite and the felsites 1 as on Welgevonden
777 where inclusions of felsites are found in it. The transitions from granophyric granite to granophyre may be very gradual, or it may be fairly abrupt
and complete within a few feet. On Solomons Temple this transition can be
studied in several good exposures in dry water-courses. In one place near the
corner beacon between Blinkwater, Kromdraai and Welgevonden, the contact
of the granophyre with the. granite is fairly sharp, and along this contact the
former is full of small cavities filled with fluorspar, quartz and chlorite. This
suggests that in places the granophyre solidified before the granite and was
metasomatised by it along the contact. This view finds support in the observation that in one locality on SololllOns Temple the granophyre is cut by a small
vein of granophyric granite.
The granophyre is a red, conlpact rock composed essentially of micropegmatite and varying amounts of free quartz. Sometimes free quartz is absent
and the intergrowth of felspar and quartz is so fine as to be hardly recognizable
even with the aid of a hand-lens. In such cases the rock very closely resembles a
syenite. Another variety lllay be designated as porphyritic granophyre in view
of the fact that it contains isolated insets of quartz and turbid felspar distributed
through a matrix of fine micropegmatite (compare Plate II, No.2, with
Plate IV).
In thin section the graphic intergrowth of quartz and felspar follows definite
patterns. The felspar grains are anhedral to subhedral, and the quartz intergrown with them are optically continuous throughout one grain, but does not
pass over into neighbouring felspar grains. Similarly one felspar grain seldom
contains more than one optically continuous set of quartz rods. Although small
BUSHVELD GRANITES IN
ZAAIPJ~AATS
T:J:N MINING AREA
53
areas of quartz optically continuous with that in the felspar, often occur interstitially to the felspars, large grains of free quartz are seldom seen in the typical
granophwre. The felspar often contains euhedral cores of another felspar, and
although the quartz is optically continuous throughout such a felspar unit, the
shap~ and arrangement of the quartz rods vary in the different sectors (Plate III,
No.2). The quartz sometimes shows unduloE!e extinction, and the extinction
position~ in different parts of a big felspar oikocryst differ by 2 degrees to 3
degrees. Although no micrometric measurements of these fine intergrowths were
made, it ~eems that the amount of quartz in the individual felspar grains varies
only bet"Veen narrow limjts, and that this variation may be due to differences
in orientation of the planes of section of the different oikocrysts. In the grano..
phyre there is no suggestion of replacement of quartz by felspar, and the graphic
texture ~erns to be due to simultaneous crystallization of the two minerals.
In the transitional facies and in the slightly granophyric granite, on the other
hand, there is, in addition to obvious pseudo-micropegmatite, also fine micropegmatite which may have had either mode of origin. This raises the possibility
that even the typical Bushveld granophyre may owe its texture entirely to a
replacement process.
(e) The Granite Porphyry.-Like the Bushveld granophyre, the granite
prophyry occurs as flat concordant lenticular bodies, either in direct contact
FIG. ~·-'t'hin section of the dark type of granite porphyry (fades pf the Main granite),
sho,,:mg ellhedral crystals of quartz (white) and turbid plagioclase (P), surrounded by
turbId orthoclase (Or) in a fine-grained matrix of quartz, chlorite, felspar and magnetite.
Solomons Temple; camera lucida, x 13.
54
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
with the felsites, or in the granophyric granite some distance below the felsite
contact. Its relationship with the granophyric granite is similarly gradational,
the transition from the one to the other being complete within a short distance
and sometimes within a few feet. Its colour varies from red to dark gray. The
red porphyry contains insets of quartz and red felspar set in a fine, brownish,
almost felsitic matrix. In the gray porphyry insets of quartz and felspar are
distributed in a dark greenish, felsitic groundmass. 'Vith the aid of a hand-lens
the felspar is seen to consist of pink, euhedral crystals which, together with
quartz, are enclosed. in large areas of gray felspar.
:M:icroscopically the porphyry is composed of euhedral crystals of quartz
and turbid felspar, mostly oligoclase, enclosed in felspar which appears to be
orthoclase (Fig. 1). The latter forms large continuous areas, but, from its extinction between crossed nicols, it appears to be radially fibrous and to be arranged
in fan-shaped sectors around the enclosed euhedral crystals. Immediately around
the latter it is usually fairly clear,but farther away from them it becomes
progressively more turbid (Fig. 1). The quartz and plagioclase idiomorphs
show fracturing and replacement by the surrounding orthoclase. In the red
type of porphyry the matrix consists practically only of quartz with a little
felspar and magnetite. In the dark porphyries the fine matrix is very rich in
chlorite and quartz, felspar and magnetite being only sparingly present. The
matrix corroded the orthoclase.
.
(f) The Contact Microgranite.-In some places the granophyric granite
.and its several variational facies are separated frorn the roof of Rooiberg felsites
by a narrow zone of rocks which are best described as contact microgranite.
~Iegascopically it is of an allotriomorphic granular nature, and alt9gether or
practically devoid of a granophyric texture. In hand-specimen it closely resembles
both the Lease microgranite and the aplitic facies of the Porphyritic granite.
Along the contact with the felsites it is sometimes so exceedingly fine-grained
that it is not easily distinguishable from the felsites, except where it develops a
contact pegmatite, which is a fairly frequent phenomenon. Downwards it
becomes progressively .. coarser-grained and passes into granophyric granite.
North-west of Warmbaths, however, it passes directly into the Q'oarse, gray,
leucocratic variety of the Main granite. In the latter region, as well as in some
parts of the Moos. River area, it is especially well developed and, in addition,
is often porphyritic due to the dissemination of .quartz and felspar insets in the
microgranitic base. But for its relation to the Main granite, this porphyritic
facies of the microgranite would .be indistinguishable from the Porphyritic
granite. This microgranite is, no doubt, the chill phase. of the lVIain granite.
Applica,tion of the Zoning of the Main Gr,anite.-As far as the writers are
aware, the arrangement of the various facies of the J\'Iain granite into zones,
as above described, is a feature characteristic of the granite of the Bushveld
Complex as a whole. This. recognition is of great practical importance, for it
will be readily appreciated that these zones can be used like stratigraphic markers.
A detailed knowledge of their regional distribution and attitude will, therefore,
be helpful in determining whether masses of older rocks associated with the
granites are exposures of the floor, xenoliths or roof-pendants. Furthermore, it
will help to decipher the structure and shape of the intrusion of Bushveldgranite
in a way whieh has hitherto been impossible.
BUSHVELD GRANITES IN ZAAIPLAATS TIN MINING AREA
2.
55
'l'HE PORPHYRITIC GRANITE.
In outcrop this granite is confined to the escarpment and foot-hills of the
Macapaans mountain range and to the flats to the east. It occurs as a slightly
transgressive zone of irregular sills rising to successively higher horizons from
the south-east to the north-west (see map). Thus, in the extreme south-eastern
corner of the area it is intrusive into the Bushveld norite as dykes and sills.
Dykes of it in the norite can be seen to advantage in the bed of the Rooisloot
(Plate IX, No. 2) just below the bridge about 5 miles from Potgietersrust (not
shown on map) on the road to Zaaiplaats. Farther to the north-west, on De
Hoogedoorns 706 and Sterkwater 296, it cuts through the hornfels and quartzites of the Rooiberg series as a network of anastomosing sills and dykes which
could only be represented schematically on the accompanying map. On Groenfontein 871 it traverses the coarse facies of the Main granite in equally complex
fashion, while on :M:ooihoek 727 and farther north-westwards it rises to the horizon
of the fine granophyric facies of the lVIain granite. In general the sills dip to the
south,.west in conformity with the regional structure, but occasionally they also
cut across the zoning of the Main granite (geological map, section C-D). Good
exposures of the sharp contacts with the Main granite are numerous on Sterkwater,
Groenfontein and Mooihoek where iIi many places it is intrusive into the latter
as networks and encloses large blocks of it. A.long these contacts local dev~lop­
ments of pegmatite is a frequent phenomenon. The best evidence of its intrusive
relation to the Main granite is found on Sterkwater where the coarse, gray,
mesotype variety of the lVIain granite is injected into the fine-grained Rooiberg
quartzites as a network of veinlets, while blocks of these quartzites, complete
with veinlets of Main granite, are enclosed in the Porphyritic granite.
.
Typically this granite is a porphyritic rock with a fine-grained aplitic
matrix through which insets of both quartz and felspar are scattered. The size
and frequency of the insets vary between wide limits. In some places the insets
are few and inconspicuous and the rock is typically aplitic and, in hand-specimen,
difficult to distinguish from the Lease granite. In others the fine-grained matrix
only fills the interstices between closely spaced insets, and the rock is almost a
normal granite, often very similar to some of the coarse varieties of the
Main granite. The distribution of the aplitic and porphyritic facies in the same
sill generally shows a vertical zonal arrangement in a way which strongly suggests
gravitative crystal settling. Thus, in many of the sills the uppermost portion
is devoid of, or poor in insets, while lower down the amount of insets increases
until in the basal portions the sill is almost a coarse granite with very little interstitial aplitic material. This is well illustrated by the sill on Groenfontein. In
the hill behind the Groenfontein Cash Store the Porphyritic granite along the
upper part of the sill is so poor in insets that it is almost identical with the Lease
granite, while down at the farmhouse it is a coarse gray, almost equigranular
rock difficult to distinguish from the coarse Main granite. Similarly another
exposure of it east ,of the Groenfontein Hotel is practically free of interstitial
material, and, as it is red, it is almost indistinguishable from the Bobbejaankop
granite. The dykes in the Norite at the Rooisloot bridge are also of this coarse
variety. The typically porphyritic facies of this granite is also easily confused
with some of the Rooiberg felspathic sediments containing porphyroblasts of
felilpar, such as those along the lVlagalakwin River on Bellevue 360. Usually,
06
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
however, the two can be distinguished with the aid of a hand-lens, for the insets
of the granite tend to be idiomorphic and show resorption bays, while the
porphyroblasts are spherical and often enclose numerous small quartz grains
around their peripheries. In addition, pseudo-micropegmatite is common in the
sediments and rare in the Porphyritic granite.
In the field the Porphyritic granite is mostly pink, but very often it is gray
or pinkish-gray. Where the colour is gray the insets of felspar are white and have
a somewhat horny appearance. The pinkish-gray colour is due to the presence
of both red and white felspars. Mafic minerals are usually present.
B
One Foot
EIG. 2.~(A), Ve~nlet of aplo-granite cutting the Porphyritic granite; (B), patches without
insets forming irregular streaks in the Porphyritic granite.
In several places, such as opposite the farmhouse on Groenfontein, the
Porphyritic granite is cut by narrow veins. These are usually from 1 to 4 inches
wide, and are of fine aplitic material identical to the matrix of the contiguous
granite. Field relations suggest that these represent auto-injections of residual
interstitial material into cracks formed in the granite during the later stages of
its consolidation. Distinct from these veins are irregular patches and veinlike
streaks of aplo-granite free of insets. These seem to be due to an irregular permeation of the granite by its'own residual fluids, or to a streaming and eddying of
these through the granite while the latter was still in a semi-fluid state (compare
A and B, Fig. 2).
Ill. thin section the Porphyritic granite shows insets of quartz and turbid
felspar distributed in an allotriomorphic matrix of felspar and quartz. The
insets show acerta.in amount of fracturing and penetration by the matrix
(J;>late V, No.1). The quartz grains of the m.atri~ are usually ll~lund and Several
of them are often .enclosed poikilltically bya single irregular crystal of felspar..
BUSHVELD GRANITES IN ZAAIPLAATS TIN MINING AREA
57
Replacement of quartz by felspar, giving rise to pseudo-micropegmatite, is
sometimes seen. Biotite, partly or completely altered to chlorite, is usually
present, while magnetite, zircon, and fluorspar are always present in small
amounts. The felsparis often intensely chloritized and sometimes calcified.
3.
(a)
THE BOBBEJAANKOP GRANITE.
This is the youngest and economically by far the most important of the
Busllveld granites as all the cassiterite and scheelite deposits are genetically
connected with it. It is named after the bald, domical hill on Roodepoort opposite
the Zaaiplaats mill. It outcrops in three separate areas, the most southerly of
which is oblong in shape and extends from Zaaiplaats 236 in a south-easterly
direction to Solomons Temple 1014. This is the main tin-producing area in the
Potgietersrui:lt district. The second exposure is very small and occurs in the
south-western corner of Delagoa 245. The largest exposure extends from Groenvley 610 northwards to beyond the limits of the area mapped. This last area is of
economic import~nce on account of the deposits of cassiterite and molybdenite
on Groenvley and Appingendarn 469.
In the Zaaiplaats-Solomons Temple exposure the Bobbejaankop granite is
bounded in the south-west by the Lease microgranite and in the other segments
by the coarse facies of the Main granite in the lower, and by the granophyric
granite in the higher horizons. Along its eastern margin its contact with the Main
granite is vertical or steeply inclined to the north-east. In view of the red colour
of t4e Main granite alongside of instrusions, the contact is rather ill-defined
and the line of demarcation between the two types of granites is generally seen
best from a distance due to the difference in the density and type of vegetation
which each supports. The transition is, however, fairly sharp and the contact
is usually marked by the development along it of pegmatite, quartz-specularite
rocks, or of quartz-tourmaline bodies, the last being often of pipe-like habit.
Except at the corner beacon common to Groenvley, Delagoa, Eckstein and
Appingendam, the Bobbejaankop granite shows no chilling against the ~Iain
granite. Its contact with the Lease granite in the south-west dips south-westwards at angles varying from 5 degrees to about 15 degrees and is usually fairly
sharp. In the other two areas the attitude of the Bobbejaankop granite and its
relations to the other granites are the same as in the southern exposure, except
that on Eckstein it also cuts the Porphyritic granite and that on Leyden 580
and Klipplaatdrift 277 it is directly overlain by the Waterberg basal conglomerate
with a sedimentary contact.
Typically this granite weathers into prominent bald domes by which outcrops of it are usually recognizable from a distance. Its outcrops are often strewn
with huge spherical boulders, and for this reason it is sometimes referred to as
" ball granite" (Plate VI. No.2).
Megascopically the Bobbejaankop granite is a coarse" red or dark pink rock
composed essentially of quartz and felspars with variable amounts of chlorite
and some biotite. The quartz grains, mostly well separated from the felspar, are
often coalesced to form chains which stand out conspicuously on weathered
surfaces. The felspars are characterized by their red colour and dull, feltlike
appearance as a result of which they reflect light rather feebly.
.FIG. 3.-(A) Thin section of Bobbejaankop granite from Roodepoort, showing its typical
texture: The quartz (white) and the felspar are well s~gregated into large continuous
areas, the felspar tending to be interstitial to the quartz. Camera lucida, x 13.
B. Thin section of the coarse gray variety of the Main granite from Groenfontein, showing
its typical texture: Quartz grains of varying size (white) are partly or completely enclosed
in the larger felspar grains. It also shows some magnetite (black), a large area of chlorite
(Ch) after hornblende (cross-ruled), and biotite (B), apparently as an intermediate prodllct.
Camera lucida, x 13.
BUSHVELD GRANITES IN ZAAIPLAATS TIN MINING AREA
59
The potential association of important economic minerals with the Bobbejaankop granite renders its ready recognition a primary necessity in connection
with prospecting.. 'iVhile it is easy to distinguish it from the Porphyritic granite
and the fine to medium-grained facies of the Main granite, its distinction from
the coarse varieties of the lVlain granite is sometimes a difficult matter. A somewhat full account of its principal diagnostic features will therefore be given.
These are:(i) Its Coarse Texture.-East of the Groenfontein Hotel outcrops of the
coarse red, non-porphyritic facies of the Porphyritic granite are found which at
first sight resemble the Bobbejaankop granite. Olose examination of handspecimens shows, however, that the Porphyritic granite is not only finer-grained
but that it also has small irregular patches of aplitic Inaterial which do not occur
in the Bobbejaankop granite.
(ii) Its Red Colour.-The Bobbejaankop granite is invariably of a deep red
colour, even to the deepest levels of the mines; the other granites are, on the
contrary, more of a brick-red colour and are more often gray than red. Thus, if a
coarse granite is red in one place and gray a few yards away, itis sure not to be
the Bobbejaankop granite. This makes the term" red granite" as a collective
name for all the acid plutonic rocks of the Oomplex very undesirable and the
designation" Bushveld granites" seems to be more conlmendable.
(iii) The Nature of its Quartz.-In the Bobbejaankop granite the quartz
is well segregated from the felspar and forms little chains and clusters which
stand out prominently on weathered surfaces (Plate IX, No.1). In the coarse
Main granite, on the other hand, the quartz usually occurs as small round grains
enclosed by the coarse felspar and is thus inconspicuous on weathered surfaces
which appear to be composed of felspar only. This segregation of the quartz
and the felspar in the Bobbejaankop granite may be diagnostic even in thin
section. (Compare Fig. 3, A, with Fig. 3, B.)
This criterion is very useful in the field, for, although the Main granite
sometinles weathers like the Bobbejaankop granite, it more often shows surfaces
on which felspars are most strikingly displayed. It must thus be stressed that,
although the Main granite sometimes resembles the Bobbejaankop granite, the
reverse is never the case, and any outcrop which resembles the latter in one
place and the former a few feet away is therefore usually not the Bobbejaankop
granite.
(iv) The Nature of its Felspar.-In the Bobbejaankop granite the felspars
are usually deep pink or red in colour and have a dull, earthy appearance on fresh
fractures. In the other granites the felspars 011 unweathered surfaces look quite
fresh and, even when red, have a bright lustre.
(v) Small cavities of about 2 to 6 rnillimetres in diameter into which the
surrounding quartz and felspar project euhedral crystal faces are common in
the Bobbejaankop granite.
(vi) Small spherulitic clusters of tourmaline are scattered in the Bobbejaankop granite on Appingendam, Leyden, Eckstein and Klipplaatdrift and
constitute a very useful criterion.
60
TRANSACTIONS· OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
(vii) That the Bobbejaankop granite is younger and the Main granite
older than the Porphyritic granite makes their distinction and the delineation
of the contact between them fairly easy in places where the last-named granite
is present, as on Eckstein.
If, despite these criteria, an observer familiar with the granites is still
doubtful about the identity of a particular outcrop, it may be assumed with a
reasonable amount of certainty that it is not Bobbejaankop granite.
In thin section* the Bobbejaankop granite is composed of coarse, wellsegregated quartz and turbid felspar. The latter again seems to have started
crystallising before and to have continued till after the quartz. Thus, some
felspars show euhedral faces on which quartz is moulded, while others enclose
quartz and also occur interstitially to it (Fig, 3, No.1). The quartz is crowded
\vith small bubble-like inclusions arranged in curved lines, as in the coarse :l\fain
granite (Plate I, No.1), and also enclose long thin needles, possibly of rutile.
The felspar is usually intensely replaced by fine chlorite. Big plates of brown
biotite, partially or wholly altered to green chlorite, are often present. Magnetite,
ilmenite, fluorspar and zircon, the last partly altered to malacon, are common
accessories.
3.
(b)
THE LEASE MICROGRANITE.
The rocks here under review have been named "Lease microgranite"
from the fact that they are fine-grained and that at Zaaiplaats the old tin workings, known as the" Lease Workings," are situated in then1. The term" IJease
granite" for the same rocks is, however, used for the sake of brevity.
So far the Lease granite has only been found in association with the Bobbejaankop granite in which it occurs as veins, dykes, sills and irregular bodies.
Predominantly, however, it occurs as sheet-like masses between the Bobbejaankop granite and the overlying granophyric variety of the Main granite. These
last occurrences are not only the most important economically but also have
by far the largest areal extent, and will be described first.
Considering the southern exposure of Bobbejaankop granite first, it will be
noted from the map that along its south-western margin this granite is.sepal'ated
fronl the overlying granophyric granite by a strip of Lease granite which extends
continuously from Zaaiplaats to Solomons Temple, but is entirely absent along
the other segments of the Bobbejaankop granite outcrop. This strip of microgranite has the shape of a lenticular sheet and both its upper and lower contacts
dip to the south-west at angles ranging from 5 degrees to 15 degrees. It attains
its maximum, development on Roodepoort and Groenfontein where it is fully
400 feet thick. Traced along its strike~ it thins out rapidly towards the southeast and north-west and finally peters out on Zaaiplaats in the north and on
Solomons Temple in the south.
The contact between the sheet of microgranite and the underlying Bobbejaankop granite is always fairly sharp and well-defined, Along its upper linlits
the sheet is in intrusive relation to the granophyric granite, the line of junction
being marked by the development of a persistent zone of pegmatites. Although
* The megascopjc and microsc9pic descriptions of both the Bobbejaankop and Lease
granites given here apply only to the normal rocks unaffected by alterations connected
with mineralisation.
PIG. 4.-Schematical composite representation of the pegmatite zone at the contact of the
Lease microgranite (A) with the Main granite (B), based on sketches made at the" Groenfontein" mine: (C) quartz-felspar pegmatite, (d) oriented quartz, (E) oriented
microcline, (F) vug with copper sulphiede (black), (G) pegmatite stringers with copper
sulphides, (H) sinuous bands of pegmatite, possibly indicative of contortion of the platy
Main granite during the emplacement of the Bobbejaankop granite. Scale: 1 inch
=±2 feet.
62
TRANSAOTIONS OF THEGEOLOGIOAL SOCIETY OF SOUTH AFRIOA
this boundary is clearly visible from a distance by the abrupt change in vegetation (Plate VII, No.2) the actual contact is sometimes very difficult to demarcate.
This is the case where the pegmatite is absent, or where a thin development of
Lease granite is present above the pegmatite horizon, and is due to the fact
that in some plac~s the microgranite not only grades into, but also cuts the granophyric granite as ramifying veinlets which make it difficult· to decide upon its
upper limits (Fig. 4). Along such transitional contacts the granophyric granite
is rich in secondary fluorspar, calcite, sericite and chlorite. In the mines irregular
patches of typical granophyric granite are occasionally found in or below the
pegmatite zone.
?~.:,
./:.', ',' . ::"
[illaObbe;aankOP granite
... '.
~.'.-
,•••
:1
..
: ".
'
.;.
..
Pegmatite
r:<:..-:. ]
. . ... :...... ':
'
':.:.:
Lease granite
FIG. 5.-The "roots" of the Lease microgranite : Eyes and stringers of pegmatite
coalesce with veinlets of Lease microgranite which unite and eventually merge with the
main body of Lease microgranite. Natural section, Zaaiplaats. Scale: 1 inch=±10 feet.
The pegmatite zone is composed of lenticular and laterally discontinuous
bodies of pegmatite, or of several layers of pegmatite, fronl a few inches to 30
feet thick, arranged one above the other with several feet of Lease granite in
between. The zone as a whole reaches its maximum thickness at the" Groenfontein" mine where, at the No. 23 shaft, no less than seven bands of pegmatite
BUSHVELD GRANITES INZAAIPLAATS TIN MINING AltEA'
'63
are found, varying from 6 inches to 6 feet in thickness and separated from one
another by 3 to 30 feet of Lease granite, the whole zone having a thickness of 90
feet. The intimate relation between the pegmatite and the Lease microgranite
is shown by their sympathetic thinning out in north-westerly and south-easterly
directions, and by the fact that the disappearance of the former coincides with
or only slightly precede& that of the latter.
The pegmatite bodies are well-banded concordantly to their lower and
upper contacts which are always sharply defined. The banding is due to alternating layers of quartz and red microcline. These minerals are always elongated
and oriented normal to the banding (Fig. 4). The banded structure is further
accentuated by concordant joints and long, flat, concordant vugs with quartz,
carbonates and copper sulphides usually lining the walls. Between two layers
of pegmatite, or where a pegmatite body disappears laterally, the Lease granite
ofteI); contains isolated, oriented crystals of quartz which show euhedral basal
sections. on surfaces parallel to the banding of the pegmatites (Plate VIII, No.2).
In many places in the mines and on the surface the pegmatitic quartz and felspar
are seen to replace the Lease granite. It may be stated here that the pegmatites
are practically devoid of primary cassiterite and scheelite.
The exposures of Bobbejaankop granite on Delagoa and'in the Sterk River
basin on Groenvley and Appingendam are again accompanied by similar developments of Lease granite and pegmatite, and there too they fringe the western
margins of these intrusions and dip to the south-west. On the last farm the Lease
granite has its maximum thickness at Lone Tree Hill where its angle of dip is
only 10 degrees to the south-west. From there it curves round to the south-east
in which direction it thins out rapidly and steepens in dip to a maximum of
85 degrees before it finally disappears.
In addition to the main sheets of Lease granite there are also numerous
of it in the Bobbejaankop granite itself in the form of sills,
dykes and veins. These occur not only on the surface but are also found in the
deepest levels of the mines. Followed from the main body of microgranite downwards, the dykes and veins bifurcate and become narrower. At the same time
eyes of pegmatite appear in them at intervals. These become more frequent so
that the veins pass over into stringers of pegmatite and finally fade out (Fig. 5).
The dykes and veins link up the sills with one another and with the main mass of
microgranite. They usually have a medial parting of pegmatite (Fig. 6, section 1).
The sills similarly have a thin, concordant development of pegmatite, either in
their centre or near the upper contact. Where they make monoclinal steps,
the pegmatite is absent in their steeply dipping portions (Fig. 6, section 1).
Where they make domical bulges, there is a big development of pegmatite in
the apical portion of the bulge (Fig. 6, section 2). Occasionally flat vugs occur
in these minor pegmatites too (Fig. 6, section 2).
root~like extensions
In grain-size the Lease granite varies between fairly wide limits. Along its
upper contact the granite proper, as distinct from its pegmatitic phase, is mostly
very fine-grained. Downwards it becomes gradually coarser until at its junction
with the ,Bobbejaankop granite it is sometimes a medium-grained granite rather
than a microgranite. This is the case in the Sterk River valley on Groenvley
64
TRANSACTIONS OF THE
GEOI~OGICAL SOCIETY OF SOU'l'H AFRICA
and in some places on Zaaiplaats. It often has patches of small euhedral pegmatitic felspar and quartz. Upwards it sometimes becomes drusy, especially
as the pegmatite horizon is approached, and is frequently honeycombed with
cavities into which euhedral quartz and felspar crystals project and which are
filled with chlorite, sericite, tourmaline, quartz, calcite and sometimes cassiterite.
Sometimes it is white, especially near big tourmaline bodies, but in thin section
it is still identical to the ordinary red variety, except that the turbid portions of
the felspars are white instead of red in reflected light. In hand-specimen the
Lease granite is difficult to distinguish from the fine-grained, non-porphyritic
variety of the Porphyritic granite. As this distinction is important from the
economic aspect, it is fortunate that the two granites are easily distinguishable
in thin section.
Section I
Section
[ [ ] Bobbejaankop granite
~
Lease granite
n
1:'11V'tj Pegmatite
FIG. 6.-Nat:ural. sections on R?odepoort, sh~wing sills and dykes of Lease microgranite
and pegmatIte III the BobbeJaankop gramte. Note the vug in the pegmatite in
section 11.
BUSHVII}LD GRANITES IN ZAAIPLAATS TIN MINING AREA
65
Under the microscope euhedral crystals of quartz and felspar are
characteristic of the Lease granite. Its texture is inequigranular, hypidomorphic
to panidiomorphic, while that of the Porphyritic granite is typically allotriomorphic (compare Plate V, Nos. 1 and 2). The quartz of the Lease granite is full
of" bubble-trains" and long needles, possibly of rutile. Pseudo-micropegmatite
is often present, and what appears to be true micropegmatite is sometimes
abundant. The sections containing the latter may, however, have been cut from
a ,specimen of a partially altered remnant of granophyric granite. A little
chlorite and s(jricite are usually present.
The Origin of the Lease Granite.-The occurrence of the main body of Lease
granite along the upper contact of the Bobbejaankop granite, its intrusive relation to the overlying granophyric granite and its persistently fine- to mediumgrained texture, are facts which suggest that it may be a chilled phase of the
Bobbejaankop granite. As such it has, indeed, been regarded by Wagner (5).
This possibility is, however, ruled out by the observation that numerous veins
of it cut the Bobbejaankop granite, and, that in its distribution it is confined to
certain segments of the Bobbejaankop granite intrusion only. Since it is therefore
clearly of later formation than the Bobbejaankop granite, the Lease granite may
represent either a paulopost reaction product formed in situ, or a later intrusion.
For a clear understanding of the mode of origin of the Lease granite it is
necessary first to reconstruct the intrusion of Bobbejaankop granite to its preWaterberg shape and disposition. The Zaaiplaats-Solomons Temple outcrop
will be considered first.
This area is affected by three post-Waterberg, and possibly even postKarroo, step faults, trending roughly S.W.-N.E., with a total downthrow of
about 550 feet to the S.E. Two of these faults are occupied by dykes of Karroo
dolerite. The fault planes all form conspicuous valleys or grooves across the
topography (See map). A N.W.-S.E. section normal to these faults is shown in
Fig. 7, NO.1, and the same section with the formations restored to their original
position in Fig. 7, NO.2, from which it is clear that in that direction the roof of
the Bobbejaankop granite was arched, with the apex on the Roodepoort-Groenfontein boundary, where the" Groenfontein" tin mine is situated. The Lease
granite also has its maximum development of about 400 feet at the" Groenfontein" mine, while down the flanks of the arch it decreases in thickness and
finally peters out on Zaaiplaats and Solomons Temple.
Both the upper and lower contacts of the Lease granite dip at about 15
degrees to the S.W. in conformity with the regional post-Waterberg structure
(Fig. 8, No.1), while the contact between the Bobbejaankop granite and the
coarse Main granite is practically vertical. If the formations are restored to their
pre-Waterberg disposition the Lease granite becomes horizontal, and the contact
between the Bobbejaankop and the Main granites assumes a dip of about 75
degrees to the S.W. (Fig. 8, No.2), and it is significant that the Lease granite is
restricted to the upper contact of the Bobbejaankop granite and abruptly
peters out at the point where the contact begins plunging.
~
~
S.E.
N.W
~
GROENFONTEIN
ZAAIPL.AATS
+
--
'I
_ -
-
-
-
-:
_
_
_
SOL.OMONS
_ _
-----~::-- - ;~-~-~-~LI-~-~~~-~-,-~-q-f-N~n~f1-9-t-F-fi-~-~-~I
TEMPL.E
~
Ul
I
a>
~
o
r-T_",,-_
~
Ul
o
Section I
~
~l:'j
.,.
+
+
~
+
+
+
+
+
t
+
+
+
+
+
+
~
+
+
+
+ -.4I'l
-
+
+
To
+
+
+
t
+
+
+
+
+ ............ +
t'
1-
+
+
+
+
+
1-
~
+
.,.
+
1-
+
t
+
+
+
+
~
+-
T
+
+
i-
+
+
+
+
+
+
t
+
+
+
+
+
1-
+
+
+
EJ
Alluvium
~.
Post- Waterberg diabase
~
Lease granite
[[[J] Bobbejaankop granite
,. . +1 Main
+
+
++++
+
..
+
+
+
+
+
+
+
+
+
+
+
+
~
~
n
+
+
+
+
+
~
+
+
Section
•
+
+
+
~~t
~
+
T
+
...
...... ++++
"t
+
+
+
+-
+
+
'+
+
Horizontal scale: 1 inch = 6~200 feet; Vertical scale exaggerated.
o
~
t"'
Ul
o
~
~
o
+
1-
~
"
~
~
Pegmatite
horizon
FIG. 7.-(1) Section across the Zaaiplaats-Solomons Temple exposure of Bobbejaankop granite; and (2) the same section restored
to its pre-Waterberg disposition.
t"'
~
Ul
o
"d
~
>
granite
~ Rooiberg fe/SItes
~
~
o
~
67
BUSHVELD GRANITES IN ZAAIPLAATS TIN MINING AREA
In the Appingendam-Groenvley area the Lease granite likewise has its maximum development at Lone Tree (See map) where the formation has only the
regional dip to the S.W., while to the south, where it assumes a progressively
steeper original dip to the south, it also becomes progressively thinner and
eventually peters out where the contact between the Bobbejaankop and the
Main granites becomes practically vertical.
S.W.
... ---------::-----....
i-
~~If ~~
~
<- <-
rf
-t-.,.+-T
.,..
+
+
+
-r
+
+-
+
-t
i
+
+
~+
~I
+ :
+ ~
Section I
-t-
+
,,~+'
DYKE
+ + -i
1 + +
+
'1'-
+
.... --
~
N.E.
~
-t-
+
+
+
+
+-
T
+
+
-r
+
-+-
+
-t-
+
+
-t-
T
+
+
~~~~ +
1"'0
+
+
+
+
)++++ +
V+
V
+
+
-+
+
Section JI
[I]
Bobbejaankop granite
~
Lease granite
Q
Main granite
,
.i'
'Pegmatite horizon
"
FIG. 8 -(1) Section across Zaaiplaats; and (2) the same section restored to its pre-
Waterberg disposition.
Scale: 1 inch = ±l,OOO feet.
It appears thus that the main bodies of Lease granite are confined to arches
in the roof of the Bobbejaankop granite intrusion, and reach their maximum
thickness in the apical portions of such arches. The Lease granite is furthermore
closely connected genetically with the pegmatites. As regards development the
Lease granite and the pegmatites also vary sympathetically.
These phenomena, taken in conjunc.tion with the sills and dykes of Lease
granite and pegmatite cutting the Bobbejaankop granite, and the fact that
xenoliths of the Porphyritic granite enclosed by the Bobbejaankop granite are
impregnated with banded pegma'tite and partially altered to Lease granite,
suggest that the latter is a paulopost reaction product. This process may have
68
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
been more or less as follows: As the mass of Bobbejaankop granite was solidifying
the volatiles migrated upwards and collected under the solid roof of granophyric
granite. In this way the upper, partially solidified Bobbejaankop granite became
highly charged with volatiles, under whose influence it was reconstituted to form
Lease granite. The stringers, dykes and sills of Lease granite and pegmatite
then appear to han been the paths along which the segregated volatiles migrated
upwards through the solidifying upper portions of the Bobbejaankop granite.
They thus appear to be the" roots" of the main body of Lease granite.
The volatiles eventually migrated beyond the contact, and caused the same
transformation in the granophyric granite. When the temperature became low
enough the volatiles started replacing the already formed Lease granite and
crystallized as pegmatite. The flat open vugs in the pegmatites suggest that,
assisted by the platy cleavage of the Main granite, the volatiles also formed flat
concordant solution cavities in which they eventually crystallized as coarse
pegmatite (Fig. 4). The rhythmically banded structure of the pegmatite is possibly
due to a Leisegang effect. The flat concordant jointing often seen in the pegmatite and upper portions of the Lease granite in the mines is possibly a palimpsest structure inherited from the platy granophyric granite. As solidification
advanced downwards, more volatiles collected and migrated upwards so that it
could be expected -that Lease granite dykelets of later formation should cut
upwards through the already existing body of Lease granite, and this is exactly
what is found on Roodepoort and elsewhere. As most of the volatiles would
collect in the apical portions of the arches in the roof, the Lease granite could be
expected to attain its maximum thickness there. The Lease granite may thus
have had a mode of formation analogous to that of the "red rock" of
Ontario (6).
Alternatively the bodies of Lease granite and pegmatite may represent the
attenuated fluid residuum of the Bobbejaankop granite magma which was
injected as veins, dykes and sills along real or potential cracks in the first formed
crust of the Bobbejaankop granite, and as thicker sheets along its upper contact
with the Main granite. Such residual fluids would not only have high penetrative and ascensive ability, but, being highly charged with volatiles and fluxes,
would also be capable of enlarging their conduits by solution of the existing
rocks. The fine grain size could be attributed to the small dimensions of the
injections and the lower temperature of the rocks into which they were introduced. The diminution in grain-size upwards would correspond roughly to the
arrangement of the isogeotherms. As the main sheets of Lease granite were
cooling from above downwards, their volatiles would have migrated upwards and,
by reaction with or replacement of the already consolidated portion of the Lease
granite, probably along actual or potential endo~inetic joints, crystallized as
pegmatites.
The second hypothesis, however, does not satisfactorily explain the shape of
the main body of Lease granite and its restriction to the apical portions of the
arches in the roof of the Bobbejaankop granite intrusion. Likewise it does not
account for the impregnation with pegmatite and the partial reconstitution to
Lease granite of the xenoliths of Porphyritic granite in the Bobbejaankop granite.
BUSHVELD GRANITES IN ZAAIPLAATS TIN MINING AREA
69
The absence of any marked difference in chemical composition between the
Lease granite and the Bobbejaankop granite also militates against the last
hypothesis.
The Shape of the Bobbejaankop Granite Intrusion.-The shape of the roof
of the intrusion both in the Zaaiplaats and Groenvley areas has been described
as gently arched. The dips suggest that these two arches are connected as shown
in Section A-B of the attached map. The continuity from Zaaiplaats to Groenvley
is further rendered probable by the small outcrop about midway on Delagoa.
As regards the floor the evidence is more limited. As shown in Fig. 8, No.2,
it seems that the intrusion may have a floor of Main granite. This is also suggested
on Eckstein, where the Bobbejaankop granite is intrusive into the Porphyritic
granite on a grand scale. On encountering the sills of the latter on Groenvley the
former at first follows the upper contact, but on Eckstein it is transgressive,
and encloses numerous xenoliths of Porphyritic granite. It is significant that the
xenoliths lying deep in the body of Bobbejaankop granite with evidently a
great thickness of the latter below them, are strongly impregnated with bands of
pegmatite and partially altered to Lease granite, while those near the eastern
limit of the intrusion are practically unaltered, indicating a scarcity of rising
volatiles during the intrusion, and therefore a small thickness of underlying
Bobbejaankop granite. This suggests that here also the contact between the
Bobbejaankop and the Main granite dips towards the west or south-west, and
thus that the former may have a floor of the latter (See Section E-F on geological
map.)
It seems therefore that the intrusion of Bobbejaankop granite is not batholithic, but rather an irregular laccolithic body with domical arches in its roof
where the volatiles collected and formed the Lease granite and pegmatite, and at
a later stage also the cassiterite-scheelite deposits.
RELATION OF THE GRANITES TO THE BUSHVELD NORITE AND THE
ROOIBERG SERIES.
Thus far attention was devoted mainly to the relationships between the
various types of granites. For the sake of a proper appreciation of the geological
setting it is necessary to consider in addition the relation of the Bushveld
granites to the other formations in this area, viz., the Bushveld norite and the
Rooiberg series.
The Main granite is intrusive into both the volcanic and sedimentary rocks
of the Rooiberg series. Concerning its relation with the norite there is no direct
evidence in this area, but from observations in other parts of the Bushveld
Complex it is known to be intrusive into the norite.
The Porphyritic granite is intrusive into the Main granite as well as into the
norite and the sedimentary rocks of the Rooiberg series, but, being confined to
the eastern margin of the Main granite, it is nowhere in contact with the felsites
overlying the Main granite.
Although the Bobbejaankop granite is younger than both the Main and
Porphyritic granites there is no evidence that it pierced them and came into
contact with either the norite or the rocks of the Rooiberg series.
70
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
A.n interesting feature of the Rooiberg sediments is their extreme metamorphism as a result of their invasion by both the Bushveld norite and granite.
A.part from recognisable quartzites and various types of hornfels, the Rooiberg
series along the eastern escarpment also comprises rocks which in the field
resemble felsites and which at first were regarded as such (7). Subsequent
more extensive field work and microscopic study revealed that they are actually
exceedingly fine felspathie quartzites, the grains averaging about 0·05 millimetres in diameter. Overlying these is a thick development of coarse granophyre which shows a well-developed platy or bedded structure in the field.
Farther to the south-east, on Lisbon 2366 (not shown· on the map), this granophyre is intercalated with still recognizable quartzites. In addition, the latter are
intercalated on the same farm with fine red rocks which resemble aplitic granites.
The absence of megascopic features comparable with those of the Bushveld
granites, the concordant intercalation of the granophyres and granitic-looking
rocks with the sediments, and the bedded nature of the granophyres, suggested
in the field already that these rocks were reconstituted sediments. Heavy
residues were prepared of the granite..,like rocks, the granophyres, undoubted
~uartzites of the same succession, the Porphyritic granite and the Bobbejaankop granite. Mere inspection under the microscope. of the abundant zircons in
the residues already showed a close similarity between the granite-like rocks,
the granophyre and the quartzites, and a distinct difference between these and the
two granites. In the first three the zircons very seldom show euhedral pyramidal
terminations and are almost invariably brown and turbid or even opaque, while
in the last two they are mostly fresh and clear, and consist of long prisms terminated by sharp, euhedral pyramid8. In the granites incipient alteration of the
zircons to malacon is only occasionally seen and is mostly confined to minute
cracks. The length-breadth ratios of the zircons were plotted according to the
method described by Smithson (8), and used later by Coetzee in his study of rocks
in Namaqualand (9). The zircons of the first three rocks mostly fell well within
the sedimentary field, with pnly a small number falling in the igneous field, while
those of the granites were with few exceptions restricted to the igneous field.
In thin section it is further possible to recognIze a series ranging from felspathic
quartzites to granophyre. The additional facts that the Bushveld granites are
intrusive into these rocks and that the recognizable quartzites in places grade
into rocks similar to the granite-like rocks on Lisbon, leave little room for doubt
that these rocks as well as the granophyres are metamorphosed sediments. The
term " pseudo-granophyre " is suggested for the Rooiberg granophyres.
In all these sediments an outstanding feature in thin section is the replacement of quartz by turbid plagioclase. Prior to replacement the quartz was
cracked, but the fragments were not sensibly displaced, hence they are still
optically continuous over large areas, or else their extinction positions differ
only by i degree to 2 degrees. The replacement of the quartz by the felspartook
place along these cracks and sometimes gave rise to" exploded bomb" structures
(Plate 1, No.2), but more often it resulted in pseudo-micropegmatite which, as
previously mentioned, is also characteristic of the granophyric granite. In
the sediments this process, when carried to completion throughout the body. of
the rock, gave rise to the Rooiberg pseudo-granophyre. The possibilty that even
BUSHVELD GRANITES IN ZAAIPLAATS TIN MINING AREA
71
the typical Bushveld granophyres may represent the end-point of such replacement processes cannot therefore be ignored. This inevitably raises the implication that the Bushveld granophyre itself may be a completely reconstituted
sediment. Until more information on this question is available, however, it se~ms
best to classify the granophyres as proposed in this paper, since this sub-division
may throw some light on the confusion which still prevails regarding the relation
between the granophyres and the Bushveld granites, as summarized by Hall (10).
In hand-specimen the Rooiberg pseudo-granophyre differs from the previously, described granophyres, which are taken to be facies of the Main type of
Bushveld granite, in being coarser and in usually containing much hornblende.
Thin sections clearly show that its texture is entirely due to the process of
replacement described above. There is usually much coarse quartz in optical
continuity with the quartz intergrown with the surrounding felspar (Plate III,
No.1). The remnants of one grain of quartz are usually intergrown with several
of the surrounding grains of felspar, while one grain of felspar is often intergrown
with several of the surrounding ~reas of quartz. The grains of qliar.tz in the
pseudo-micropegmatite are of various sizes and shapes and lack reg~larity of
design.
THE FELSPARS IN THE BUSHVELD GRANITES.
The norms of these granites show about equal amounts of orthoclase and
oligoclase, but on account of the general turbidity of the felspars the relationships
between these two 'could not be sat~sfactorily established by microscopic study.
It seems, however,that the term ': microperthite" which is usually assigned to
the felspars of the Bushveld granites is not applicable to the felspars of any of
the granites described above.
In· all the sections examined, not one
example of undoubted microperthite·was seen. In all the granites replacement
of one felspar by another seems to illave been the general process, although
it is usually impossible to decide which felspar is of later formation. In. some
fine intergrowths, which may be the result of exsolution, the potash-felspar seems
to have separated, giving rise to antiperthite.
Most of the felspar grains are oligoclase of which the optical continuity is
interrupted only by irregular areas of orthoclas~~j'The oligoclase is usually
fairly clear, while the orthoclase is invariably turbia: .T;be colour of the granites
depends on the colour of the turbid orthoclase which,.in reflected light, IS red in
red granite and white in gray granite. With higl;1 magnification the red colour
is seen to be due to minute specks of what appears to be linlonite or hematite.
The areas of turbid orthoclase are usually irregular in shape and distribution
(Fig. 9, No.2), and in places seem to be continuous across the boundary between
two plagioclase grains, although the turbidity makes this last observation
doubtful owing to the difficulty of observing the extinction 'positions of the
orthoclase. Often, however, these areas are arranged in regular patterns in
conformity with the plagioclase twinning lamellae (Fig. 9, No.1 and Plate V,
No.2). In simple twins the plagioclase and orthoclase are generally arranged in
herring-bone pattern, while, where the 010 cleavage is seen, they are arranged
as irregular bands making an angle of about 30 degrees with the traces of the
cleavage (Fig. 9, Nos. 3 and 6). Where the cleavage is not seen the arrangement
ishaphazard (Fig. 9, No.5). These intergrowths all seem to be due to replacement,
but it is generally impossible to decide which felspar was introduced last.
72
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
Oligoclase is sometimes seen to have replaced orthoclase, as in Fig. 9, No.7,
where it passes over the boundary between two orthoclase grains, the upper
one of which contains in addition another system of differently oriented oligoclase. Occasionally oligoclase of later formation can be seen to have replaced
both orthoclase and earlier formed oligoclase (Fig. 9, No.4). In the grain shown
in Fig. 9, No.4, the orthoclase seems to have replaced the earlier oligoclase,
but in neighbouring grains where orthoclase is the predominant felspar, the
reverse seems to be the case. Occasionally grains of plagioclase showing normal
zoning are seen and these no doubt crystallized as such. It is significant that in
these usually no orthoclase is present.
2
PI.
PI.
~
Or.
.~.
#I
,
PI.
3
Or. 5
I1I1111TTIlln!IlJI5II"~~1mlI1lf'l\ PI.
Or.
4
FIG. 9.
THE COMPOSITION 0]' THE GRANITES.
Quantitative data on the composition of the granites is still scanty. There
are numerous analyses. of Bus~veld ?ranites, but as the specimens analysed
cannot be correlated wIth certaInty With the types of granites described ,above
six new analyses of specimens representative of the various types were made:
These analyses are given in Table 2 and the corresponding norms in Table 3.
73
BUSHVELD GRANITES IN ZAAIPLAATS TIN .MINING AREA
TABLE 2.
6
2
73·30
12·18
1·75
1·87
0·51
1'57
3·14
5·00
0·67
0·03
0·02
()'21
0·04
73'48
12'36
1·75
l'58
0'50
1'18
3'22
5'OU
0·50
0·10
0'10
0·21
0'05
tr
0'18
0'06
0'05
0'007
tr
tr
.....
0·24
0-05
0'06
0'007
tr
tr
73'68
12·92
1·5!)
1'44
0·69
0'90
3·00
5-00
0·79
0'09
0·10
0·25
0·05
0·03
75'42
11'80
1·27
1'44
0'53
0'84
3-08
4'80
0'75
()'04
0·13
0-24
0'04
0·03
72'56
13'36
0'95
1'87
0·61
1·63
3'22
5'05
0'80
0·09
0'14
0·06
0'03
0'097
0'23
0'06
0'02
0·022
tr
tr
tr
0'06
tr
0·04
0·62
0'05
0'004
0'029
tr
tr
72'67
13 -16
0·80
1·44
0·79
2·13
3'09
4'37
0·88
0·06
0'45
tr.
0·03
tr
0·05
0·65
0'05
0·005
0·20
0'07
-----------------I------:-----I------~··---I_----Ir_----
F-O
100'5871100'537
0'08
0-10
100·857
0·06
100'742
0-10
100 '507
100 ·797
100 ·642
----------------1------1---
100 -437
100,943
0·26
100'825
0'27
-I----~-----
100 '683
100'555
CrsO. was determined, but was absent in all cases.
Analyst: Mr. C. J. Liebenberg, Division of Chemical Serviees, Pretoria.
F-determination: Dr. P. J_ Hamersma.
TABI,E 3.
2
3
4
5
6
31·80
29'47
26·72.
4'45
31-38
30·02
27·25
4·45
34·32
29·47
25·15
3'11
1-22
36'60
28-36
26·20
2·50
0-61
29·76
30-02
27·25
5·84
0·41
33'18
26'13
26·20
5'56
1·22
1'16
0'50
0'66
0·80
1·06
2·55
0-46
0·23
0'12
0'06
0'06
1·34
1'14
2·55
0·46
0'23
0·20
1·70
0·92
2'32
0·61
0'16
0'20
1·30
1'32
1·86
0'46
0·23
0-30
1'50
2·64
1·39
2·00
2'11
1·16
0-62
0'62
1'00
99·26
·657
99'18
1·057
99·74
·882
~.
1---
Quartz
Orthoclase
Albite ...
Anorthite
Corundum
{
C~SiO. :::
:MgSiO .
FeSiO a .
J MgSiO.
Hypersthene L FeSiO.
Magnetite
Ilmenite
Fluorite
Calcite
Diopside
- - - - - -----~----I----.......:.._.-- - - - -
---~---------------I------
99'86
-767
Water, etc.
-----------------I--------~.
100-627
No.1
No.2
No.3
No.4
No.5
No.6
99·917
99-43
1'019
99'18
1'295
1-------1-----
100'237
100'622
100'449
100'475
= Coarse, gray, mesotype facies of the Main granite behind the Groenfontein Cash Store.
=Medium-grained, gray facies of the Main granite, Lone Tree Hill, Appingendam.
=Fine, red, granophyric facies of thc Main granite, Roodepoort.
= The Porphyritic granite intrusive into No_ 1, behind the Groenfontein Cash Store.
= Bobbejaankop granite, ZaaipJaats mine.
= Lease granite, " Giant Quarry," " Groenfontein" mine.
The results show no significant chemical difference between the various
granites. Thus, the difference in composition betw~en the Bobbejaankop granite
and the coarse Main granite is smaller than the difference between the latter
and its granophyric facies. Similarly the Lease granite differs but slightly from
the Bobbejaankop granite. If the vast bodies of pegmatite, of tourmaline and
quartz, and of pyrite, sericite,. fluorite, scheelite and cassiterite contained by the
Lease and Bobbejaankop granites are taken into account it is clear that the
Bobbejaankop granite magma must have differed sensibly from the magma
of the other Bushveld granites as regards the volatiles and minor constituents.
74
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
On the whole it appears that the chemical differences are so small that they
could only be established by making series of analyses, and that the different
textural modifications are mainly due to different physical conditjpns prevailing
during consolidation.
On account of the coarse texture of some of the granites and the fine intergrowths in others, micrometric determinations of the mode in normal-sized
sections were found useless.
MINERALISATION IN THE GRANITES.
Owing to limitations of space this aspect can only be dealt with in very brief
outline. The following are the main facts and conclusions pertaining thereto : All the mineralization connected with the Bushveld granites is genetically
related to the Bobbejaankop granite and occurs either in arches in its roof, or
as epigenetic fissure lodes in the Main granite and Rooiberg sediments and felsites. Thus, on Zaaiplaats 236 and Roodepoort 813 disseminated cassiterite
and scheelite occur in the upper portions of the Bobbejaankop granite, while
disseminated cassiterite occurs in the Lease granite on Roodepoort and Groenfontein 871. Pipes and other high-grade bodies of cassiterite and scheelite occur
in both the Bobbejaankop and Lease granites from Zaaiplaats to Solomons
Temple. On Sterkwater 296 there is a pipe of high-grade tin-copper-zinc ore in
the Lease granite. On Groenvley (nO and Appingendam 469 irregular bodies of
cassiterite, molybdenite, arsenopyrite and bastnaesite are found in both the
Lease and Bobbejaankop granites. The existence of epigenetic lodes in the Main
granite and felsites and their restriction to certain areas, as on Welgevonden
777 and Welgelegen 357, suggest as yet unexposed arches in the roof of the Bobbejaankop granite, as shown schematically in Section A-B on the geological map.
So far only the outcrop areas of the Bobbejaankop granite have been mined
and explored, butif the exploration is extended beyond these, it is quite possible
that our tin-mining industry may be greatly expa~ded. It is suggested that the
information concerning the connection of mineralization with the Bobbejaankop
granite, and the phenomenon of zoning in the Main granite, helping as it does to
decipher the structure of the Bushveld granites, could be utilized to advantage
in further prospecting.
ACKNOWLEDGMENTS,
The authors wish to thank Mr. H. J. Nel, of the Geological Survey
Laboratory, for his ready assistance in connection with the microphotography,
and Messrs. 1\1. J. Steyn and A. J. Besaans, of the Drawing Office, for preparing
the map and text-figures. They further wish to express their gratitude to Mr.
D. 1\IcWilliam, Manager of the Zaaiplaats mine, Mr. T. S. Haldane, ex-Manager
of the " Groenfontein " tin mine, and Mr. A. Gilbertson, present tributer of the
" Groenfontein" mine for all information and assistance so generously given.
BUSHVELD GRANITES IN ZAAIPLAATS TIN l\IINING AREA
75
REFERENCES.
(1) KYNASTON, H.: "Report on a Preliminary Examination of the Cassiterite
Deposits of Zaaiplaats." Pret., Ann. Rep. geol. surv. Transvaalfor 1907, pp. 93-102.
" The Geology of the Country North and North-West of Potgietersrust." Pret.,.
Ann. Rep. geol. surv. Transvaal for 1908, pp. 11.....23 .
. (2) WAGNER, P. A.: "Handbuch der Regionalen Geologie." 27 I;[eft. VII Bd. 7
a. Abt.: The Union of South Mrica, Heidelberg, 1929, pp. 194-195.
(3) DALY, R. A.: "Bushveld Igneous Complex of the Transvaal." Rochester, N. Bull.
geol. soc. Amer., xxxix, 1928, pp. 703-768.
(4) HALL, A. L.: "The Bushveld Igneous Complex of the Central Transvaal." Mem.
28, geol. surv., Union of B.A., 1932, pp. 377-385.
(5) WAGNER, P. A. : Op. cit., p. 195.
(6) BASTIN, E. S.: "Aplites of Hydrothermal Origin Associated with Canadian CobaltSilver Ores." Jour. econ. geol., vol. XXX, No.7, 1935, pp. 715-734.
(7) STRAUSS, C. A.: "Notes on Rehomorphic Breccias North of Potgietersrust."
Trans. geol. soc. B.A., vol. LXVI, 1943, p. 40.
(8) SMITHSON, F.: "Statistical Methods in Sedimentary Petrology 11." Geol. Mag.,
vol. LXXVI, 1939, p. 351.
(9) COETZEE, C. B.: "The Petrology of the Goodhouse-Pella Area, Namaqualand,
South Mrica." Trans. geol. soc. B.A., vol. XLIV, 1941, pp. 192-193.
(10) HALL, A. L.: Op. cit., p. 379.
76
TRANSAOTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
EXPLANATION OF PLATES.
PLATE I.
No. I.-Photo-micrograph of the coarse, gray, mesotype variety of the main granite,
showing the interstitial nature of the hornblende (dark) which also fills the cracks
in the quartz and felspar, where it is mostly altered to biotite. Note the" bubbletrains" in the quartz, and the sphene enclosed by hornblende on the left side of the
photograph. Sterkwater.
Ordinary. light, X 30.
(Photo by C. A. Strauss
and H. J. Nel.)
No. 2.-Photo-micrograph of reconstituted felspathic quartzite showing the fracturing
of the quartz (light) and its replacement by felspar, resulting in an" exploded bomb"
structure which resembles micropegmatite. Lisbon. Crossed nicols X 95. (Photo
by C. A. Strauss and H. J. Nel.)
PLATE II.
No. I.-Photo-micrograph of the fine-grained facies of the Main grani~e near its contact
with the felsites. The quartz (light) is fractured and intensely replaced by felspar so
that fine pseudo-micropegmatite is formed (near the top of the photo). The quartz
shown is all optically continuous, and is evidently the remnants of one original
grain. Solomons Temple. Crossed nicols X 30. (Photo by C. A. Strauss and
H. J. Nel.)
No. 2.-Photo-micrograph of the porphyritic-granophyric facies of the Main granite near
its contact with the felsites. The quartz (white) in the outlined area is optically
continuous and evidently formed one grain which was marginally cracked and
then replaced by felspar, resulting in a pseudo-graphic structure. In the surrounding
area fine-grained and apparently true micropegmatite can be seen. Welgevonden
Crossed nicols x30. (Photo by C. A. Strauss and H. J. Nel.)
PLATE III.
No. 1.-Photo-micrograph of the Rooiberg pseudo-granophyre, showing the replacement
of quartz by turbid plagioclase giving rise to coarse pseudo-micropegmatite. Sterkwater, crossed nicols x30. (Photo by C. A. Strauss and H. J. Nel.)
No. 2.-Photo-micrograph of the Bushveld granophyre, showing the fine grain and the
different shapes of the rods of optically continuous quartz in the different sectors
of one felspar unit. The euhedral felspar forming the core could not be determined.
The surrounding felspar is turbid acid plagioclase. Solomons Temple. Crossed nicols,
x75. (Photo by C. A. Strauss and H. J. Nel.)
PLATE
IV.
Photo-micrograph of the porphyritic-granophyric facies of the Main granite, showing
insets of quartz (light) and turbid felspar (dark, mottled) in a matrix of granophyre.
Roodepoort. Crossed nicols X 30. (Photo by C. A. Strauss and H. J. Nel.)
PLATE
V.
No. I.-Photo-micrograph showing the typical texture of the Porphyritic granite with
insets of quartz (light) and turbid felspar (not appearing in photo) in an allotriomorphic groundmass of quartz and turbid felspar. Groenfontein. Crossed nicols,
X 13·5. (Photo by C. A. Strauss and H. J. Nel.)
No. 2.-Photo-micrograph of the Lease granite showing the typically hypidiomorphic
texture with subhedral to euhedral grains of quartz (white) and felspar (mottled).
Note the red, turbid orthoclase areas (black) in the plagioclase crystals. Sterkwater.
Crossed nicols x30. (Photo by C. A. Strauss and H. J. Nel.)
PLATE VI.
No. I.-The platy cleavage of the granophyric granite just above the pegmatite horizon
Zaaiplaats. (Photo by C. A. Strauss.)
,
,
No. 2.-The typical mode of weathering of the Bobbejaankop granite, Groenfontein.
(Photo by C. A. Strauss.)
BUSHVELD GRANITES IN ZAAIPLAATS TIN MINING AREA
77
VII.
PLATE
No. I.-View of Bobbejaankop, the prominent bald dome on Roodepoort just opposite
the Zaaiplaats battery. The hill consists of the youngest, tin-bearing granite which
is named after it. (Photo by C. A. Strauss.)
No. 2.-Showing the contact between the Lease granite and the granophyric granite
sharply marked by the vegetation and mode of weathering. The Lease granite forms
bare, rocky outcrops, while the granophyric granite breaks into small blocks and is
thickly overgrown with grass and bush. (Photo by C. A. Strauss.)
PLATE
VIII,
No. I.-The banded contact-pegmatite between the Lease granite and the overlying
granophyric granite. "Lease Workings," Zaaiplaats. (Photo by C. A. Strauss.)
No. 2.-The basal sections of the scattered, oriented quartz crystals (light) in the Lease
granite in the pegmatite horizon. Groenfontein. (Photo by C. A. Strauss.)
PLATE
IX.
No. I.-A typical weathered surface of the Bobbejaankop granite with the quartz (light)
standing out conspicuously. (Photo by C. A. Strauss.)
No. 2.-A dyke of the Porphyritic granite cutting the Norite in the bed of the Rooisloot,
just below the bridge, five miles from Potgietersrust. (Photo by C. A. Strauss.)
PI,ATE
X.
Geological Map of the Potgietersrust Tin Mining Area 3{ld Surrounding Country.
'['llANs. GBOL. SOO. S.A., VOL. XLVII.
No. 1.
No.2.
PLATE 1.
TRANS. GEOL. soC. S.A., VOL. XLVII.
No. 1.
Xo.2.
PLATE 11.
TRANS. GEOL. SOC. S.A., VOL. XLVII.
No. 1.
No.2.
PLATE III.
TRANS. GEOL. SOC. S.A., VOL. XLV II.
PLATE IV.
TRANS. GEOL. SOC. S.A., VOL. XLVII •
.No. 1.
.No.2.
PLATE V.
TRANS. GBOL. SOC. S.A., VOL. XLVII.
No.1.
~o.
2.
PLATE VI.
TRANS. GEOL. SOC. S.A., VOL. XLVII.
No. 1.
:No.2.
PLATE VII.
TRANS. GEOL. SOC. S.A., VOL. XLVII.
No. 1.
No.2.
PLATE VIII.
TRANS. GEOL. sao. B.A., VOL. XLVII.
No. 1.
No.2.
PLATE IX.
PLATE X.
TRANS. GEOL. SOC. B.A., VOL. XLVII.
o
GEOLOGICAL MAP
OF THE
POTGIETERSRUST TIN MINING
AREA
AND SURROUNDING COUNTRY
By C.A. Strauss D.Se.
o
2
3
4 MILES
A====t==;::jlss:::::::====s======:i:I=======3'======::IJ
1
--I
'
} Superficial Deposits
Soli, sand, ferricrete etc.
Post -Woterberg diobase
.~ Basal conglomerote
} Waterberg System
(J)
1"1
o
~ Lease granite
DJllII] Bobbeiaankop
~
o
z
o
granite
I
+
+
Porphyritic granite
Bushveld Granite
+
+
=
+
Main granite
11
~
If
v
V)
+
+
Bushveld oorite
+
+
v Quartzites, hornfels, leptites,
v
v
v rheomorphic breccios etc.
~
+
(U
+
+
v
II-
+
+
Granite porphyry
~
+
+
Coarse gray to (ine
red granite
Bushveld granophyre
o
+
+
I------,'("T~:'\'N.+)
~
~ .......
-
Rooiberg Series
Pseudo - granophyre
Felsites and intercaloted
agglomerates and sholes
CJ)
.1"1
o
+
- f - Fault (Post-Karroo?1
~
(5
a
o
- - - -Dyke (Post-Karroo?l
-
+
J- Fault and Dyke (Post- Karroo?l
~ - Dyke
~
Z
):-
I
OJ
+
(Post- Waterbergl
+
Workings
.-
+
, ,+
J
+
I-------K.i~
)
+
+
+
+
I.
t------nw,J
,
1======1.,:
1:::=====1::
t--------\ , •
~========::=1: ::
1-------/, ,
11--------1, '
t-----~.'
t------~;:
~
~:
!1l
....
~
~
..
~.
1"
en
PI
0
....
0
Z
PI
+
I
"
t---
'<\)
+
----4,~!
J------cu.lX
~-----I:~
+
~.
I-------~c'j jo,
...a..
1