Download FOUND (Read 16th March, 1908). By Hans Merensky.

THE EOCE:S :aELONGING TO TilE :BUSIlVELD GEANITE COMPLEX.
TIlE
ROCKS
BELONGING
TO
TIlE
AREA
OF
THE
25
l)USHVELD
GHANITE COMPLEX, IN WHICH TIN MAY BE EXPECTED,
WITH
DESCRIPTIONS
OF
THE
DEPOSITS
FOUND
(Read 16th March, 1908).
ACTUALLY
By Hans Merensky.
In oonsideration of the great number of lay members of this
Society whose warm interest in our doings is a constant source of
satisfaction to us, and further in consideration of criticism of the
theory ~s to the genesis of tin deposits which my observations in the
Bushveld tin fields supply, I shall first give a brief sketch of our
presen~ day knowledge of tin deposits and their genesis.
Deposits of tin ore are with two exceptions connected with granitic
rocks intrusive in palaeozoic strata. In Mexico and Bolivia-the
exceptio~al cases-the tin is associated with acid eruptive rocks of
upper tertiary age, belonging to the family of rhyolites and dacites.
Some granites carry tin-stone as a primary cons1tituent, yet
in such minute percentage that they are not to my knowledge payable
in any locality. Only secondary ores resulting' from subsequent enrichments form deposits of economic value.
'The origin of these enrichments is traced back to pneumatolytic
action which takes place as an immediate after-effect of the eruptions
of the magma. 'With the solidification of the rocks in the upper horizons, cracks begin to appear in the granite as contraction-cracks, and
also more regular fissures which may be prolonged into the adjacent
sedimentary and other rocks.
Through all these p&ssages, there rise gases and vapours, composed
mainly of silica, boron, fluorine and tin. Through mutual chemical
reactions minerals and ores are formed; they are chiefly quartz, followed quantitatively by the boro-fluoric silicate: tourmaline; the three
fluor minerals: topaz, fluprspar and fluorapatite; then the tin ores,;
cassiterite and in a lesser degree stannite.
,26
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
In addition to these mine1rals, compounds of lithium are almost
always found, in the form of lithium-bearing muscovite, true lithia
mica (lepidol~te), and also, though rarely, ,s,podumene, all, as I may
mention, of commercial value.
Other ores besides tin-stone occur, viz., arsenical and oopper
pyrites, bornite, metallic bismuth, bismuth-glance, molybdenite, and the
wolfram compounds: wolframite and scheelite, which are specially
typical of tin deposits though not exclusively confined to them; iron
ores are generally rare except in the Malay States, where they occur
in considerable quantity.
All these minerals are formed not only in the open cracks and
fissures, but to a still higher degree in the adjacent country rocks, both
granitic and sedimentary. In the granites the felspar usually succumbs
first to destruction and then the other constituents. Very often the
metamorphism has reached such a degree that identification of the
original rock is almost impossible. Such rock is designated "greisen,"
if it consists of quartz, t.opaz, tin ore and lithia mica~ The prototype of
this rock comes from Zinnwald in Saxony.
In other localities, for example in Cornwall, quartz and tourmaline
prevail next to tin, topaz being less common. The term "greisen"
has lately been extended to these rocks also.
Where regular fissures are missing, the minerals often form
irregular bodies, such as pockets, lenses and 'bands in the neighbourhood
of the contraction cracks. The latter increase in number sometimes
to such an extent that a network is formed through large masses of the
rock, which is then more or less impregnated with tin ore. Such an
occurrence is termed a stockwork.
The foregoing theory has been somewhat modified by Prof. Bergeat
of Clausthal, who states that the mineralisation is in immediate oonnection with the solidification of the rocks'. and results at first from
gas mixtures by means of pneumatolysis and then continues through
hydatogenic* (thermal) action.
Prof. Beck of Freibergt goes even further and attributes the genesis
of tin deposits to the coni.pound action of hot solutions and vapours
with a great preponderance of the former, that is thermal or hydatogen ic action.
In the light of the foregoing introduction I shall now proceed to
examine the ore deposits in the Bushveld red granite.
The plutonic series ,of the Bushveld which cover,s' the major part
of the central rrransvaal, consists to a large degree of acid, and to a less
extent of intermediate and basic rocks, viz., syenites and gabbro-norites.
The latter need not be described here, because only the acid rocks are
probable tin carriers. Among them the Bushveld red granite greatly
* By "hydatogenic" (German, hydatogen) is implied a genesis in which the
action of water plays a part.
"\' He gives his reasons for this statement on p.p. 431-432 of his book on
"Ore Deposits."
TIlE itoCKS BELONGING TO THE BUSHVELD GRANITE COMPLEX.
27
prevails over the felsite and other types. 'rhe granite consists, as is
well known, of red orthoclase, quartz and either some hornblende or
biotite; it is well characterised by a beautiful graphic structure.
'rhe
felsites show great varia'hon in their petrographical structure" the
porphyritic type, however, predominating.
'rhe sedimentary ro.c1\:.s in the area belong to the Black Reef,
while
DolOlnite, and Pretoria series, and to the Waterberg system;
there are a few outliers of the Kanoo system.
For the present purposes I shall take the railway line from Pretoria
to Pietersburg as dividing the whole territory into an ea,stern and
a western section.
As regards the eastern portion, Messrs. Kynaston, Hall and Mellor*
have shown in various papers that great portions of the felsites are
volcanic sheets, etc., contemporaneous with the lower Waterberg series
and that the granite is intrusive into the strata of the Pretoria and
of the Waterberg series, whilst the Karroo'formation' is more
recent than the eruptive rocks. It is therefore possible for tin ore
deposits to occur in the felsite and the granite as well as in all the
sedimentary strata with the exception of those belonging to the Kanoo
formation.
I shall now deal with the question, whether these conditions also
apply to the area west of the Pietersburg railway.
The granite covers vast tracts to the north-west of Pretoria with
uninteresting uniform plains, ,on which lie some large isolated patches
of Kanoo formation. The granite disappears more and more from
observation in a broad contact zone which forms a curve open to the
north-west; extending from the Rooiberg in a north-eastern
direction towards Warmbad; continuing dose to, but always west
of the line into the Nylstroom district and finally at Potgietersrust
swinging sharply round to the north-west.
'rhe rocks found in this contact belt are almost everywhere felsitic
masses and in a lesser degree red granite. There are some remnants
of the Pretoria ,series and dolomite, especially in a strip extending
from Warmbad to the north-west; for the rest there are the large masses
of "Waterberg strata, first in detached patches broken and tilted, and
then in .a huge continuous mass which may extend as far as the
Crocodile (or Limpopo) river. vVe have very little' direct knowledge
or authentic information of this practically unknown portion of the
'Vvaterberg district; but I have been informed that there is another
boss of red granite ,somewhere near the Palala river.
I know nothing
of its dimensions or character and shall therefore not deal with it in
the following remarks.
Remnants of Kanoo formation are also to be f.ound in the contact
belt in the vicinity of the railway line. r:ehey are easily recognised even
from afar, for, being always horizontal, they form a marked contrast with
the outer fringe of the vVaterberg strata.
* Trans. Geol. Soc. S.A., 1905·1907, and Reports of Geological Surve.lJ, Transvaal.
28
TRANsAc~rroNS OF TIlE <lEOtO<1ICAt SOCI:BTY OF souTH AFRICA.
rrhe Rooiberg range forms a circle of prominent felsite or felsitic porphyry hills enclosing sandstones, quartzites and shales. In a long trench on
Kwaggafontein No. 996 the contact of the felsite and granite has been
exposed, dipping slig11tly to the west, the granite disappearing beneath
the felsite.
rrhis does not, however, afford any conclusive evidence as to which
rock is the more recent. From observations I was able to make on the
farm Leeuwpoort No. 938, it is, however, indisputable that the granite
is younger than the sedimentary rocks. There a distinct apophysis of
granite has broken through the sedimentary rocks close to the central
beacon on the western line. As far as I know, the age of this stratified
formation has been taken to oorrespond with the Pretoria series, a view
requiring 'Some additional investigation, since there certainly occur to the
north-east of this locality, various rocks with a greater or lesser resem·
blance to typical Waterberg formation.
If followed from the Hooiberg to the north-east, the oontact zone
is well exposed in the Nylstroom district. For instance, the greater
part of the farm Magalakween zyn Oog, about 15 miles north of Nylstroom, is taken up by felsitic masses which often show a bedded structure.
The ground mass of these rocks consists mostly of a fine grained
mixture of quartz and felspar, in which occur larger individuals of
quartz and plagioclase, with some limonite and occasionally epidote. A
conglomerate, the pebbles in which are not fully rounded, is contained
in the felsite on the northern portion of the farn~; it is of varying
width and of short longitudinal extension. It is followed by a quartzporphyry of well developed structure, several hundred yards in thickness,
and then by a huge conglomerate with well rounded pebbles and finally
by quartzite. These strata bel,ong undoubtedly to the Waterberg formation, the lowest portion of which consists of contemporaneous acid
flow,s and sedimentary rocks, as Mr. Mellor found in the Middelburg
district.
The same felsitic rocks occur further west on the farm Doornkom
No. 1807, where a granitic apophysis cuts through them. This is oonelusive evidence that the granite must be intrusive at least into the lower
'Vaterberg series.
Apart from the felsites just described, there are
on the farm Geelhoutkloof, to the north of Doornkom, veins or dykes
of a felsite-porphyry which are undoubtedly more recent than the former.
These dykes penetrate the VVaterberg series right up to the higher
horizons.
I met with very interesting exposures again OIl the most northern
part of the oontact zone on the farms Salomonstempel and Verdoornsdraai, north-west of Potgietersrust. In the contact belt which is there
as a rule a few miles wide, a strip of granite is followed to the west
by .a strip of felsite-porphyry and then by the Waterberg formation.
The contacts of the various rocks are generally hidden from observation
by masses of detritus; at one spot, however, the contact of the felsite
and granite is well exposed. rrhis place lies in the bed of the only
spruit flowing westward on Salomonstempel, where it widens to a
THE ROCKS BELONGING TO THE BUSHVELD GRANITE COMPLEX.
29
small pool with a fringe of reeds about one thousand yards from the
boundary line with the farm Grootrivier. The contact line i8 partly
straight and distinct, showing plainly that the granite dips westward
at about 30 degrees under the felsite in the same way as in the Rooiberg and partly zigzag int{) the felsite, with a well marked granite apophysis, t-inch wide.
The rocks are so fresh and well prewrved that I obtained several excellent specimens of the felsite containing this apophysis,
which under the microscope shows the true character of Bushveld granite.
Further north I found in a prospecting hole on the farm Verdoornsdraai No. 2251 between the prospector's camp and the boundary of
Appingadam No. 187Q a mass of sandstone eml.:edded in felsite;
this sandstone belongs to the lower W' aterberg strata, which dip gently
to the west, are grey in colour and very rich in particles of felspar.
The felsite here oontains leaders of haematite, and through contactmetamorphism has influenced the enclosed sandstone in such a way that
it is now partly transformed into a haematitic quartzite.
r.rhe above goes to prove that this felsite is more re0ent than the
Waterberg formation i~ which in this instance the contemporaneous acid
flows seem to be missing. The felsite correlates therefore with the younger
fissures on Geelhoutkloof No. 1482. I feel inclined also to inchlde in this
recent group the Rooiberg felsite on acoount of its similar petrographical character. Whatever the case may be, the granite is younger
than these felsites and is intrusive even into the higher horizons of the
Vvaterberg formation. I cannot yet endorse the suggestion recently
made to assign it an independent position between Waterberg and
Karroo formations on account of the following additional observations
on the same farm Verdoornsdraai.
Between the prospector's camp
and the Duiker spruit there rests horizontally on the felsite a rather
small patch of white and red banded sandstone. It contains enclosures
of this felsite and of a tin bearing tourmaline-sericite rock.
This
rock, unlike the grey sandstone just described, must therefore be younger
than the felsite. Felspars are missing in it; it consists almost exclusively of well rounded tightly packed grains of quartz. 1'hese two
peculiarities alone characterise this rock as a member ,of the Vvaterberg
and not of the Karroo formation, which latter consists of more sharp
edged grains only loosely coherent (Kynaston).
In my opinion these tourmaline-sericite enclosures lJelong to the
granitic period, as I shall show more fully later on, and arc, therefore, of the same value as a true piece of granite in determining the
geological age.
It would be rash to draw too far reaching conclusions from
occurrence of such a sandstone; but should this observation
firmed at other places, the necessity may arise to accept also
upper series of *the Waterberg formation, inthe same way, as Dr.
has proposed for the lower, that the upper Waterberg series
* Presidental Address Geol. Soc. S.A., January, 1908.
a single
be oonfor the
Joriss~m
was not
30
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH
APincA.
formed continuously during one single geological period, but that a
break ,occurred in the continuity above a certain horiZiOn.
The strata formed after such a break could be includ.::~d with the
upper Waterberg series, but quite an independent name and position
would have to be as,signed to them between the Waterberg and K::"rroo
formations. 1'hese strata would then be more recent than the eruptive
rocks. If this is correct, there would be no possibility of the existence
of tin deposits in these upper strata.
THE TIN ORE DEPOSITS.
I shall again begin with the eastern half of the granite area and its
ore deposits on Enkeldoorn and Vlaklaagte, which are well known to
most of you and which have been repeatedly dealt with by various writel's.
There are, howeve,r, some points connected with these deposits which
have remained unexplained, so that to render this paper compJete, the
following remarks may be excused : On these farms, be,sides the Bushveld granite and the felsite, there
exists, in a very much lesser degree a fine grained granite, red in
oolour but with occasional transitions into grey. It ,occurs in irregular
bodies as well as in perfectly defined veins sometimes only a few inches
in width. I did not hesitate to designate this granite as an independent
eruption (UN achschub") and as the ore bringer. rrhis term has been
often misconstrued to mean that tin had to occur in this finely grained
granite and nowhere else, which of course is not what I meant.
The
term "ore bringer" signifies only that the formation of ore deposits
came as a sequence to the eruption of this granite, with the result that
ore bodies may have been formed, not only in the fine grained granite
itself, but also in the coarse~grained, and in every other rock that
existed prior to this event.
The practical application of this theory would be for prospectors
and other interested parties to look for the fine grained granite.
The theory wa,s not generally accepted, having been opposed principally by Molengraaff and Hall, in the belief that the fine grained granite represented nothing but a special facies in the cooling of the red Bushveld
granite and that it could not therefore be the ,ore bringer. Hall more or
less withdrew this opinion later on; while Prof. Beck of Freiberg proved
positively that the aplitic structure of t\his fine grained granite could never
result from the cooling process of a coarse grained granite with a typical
micro-pegmatitic structure, ahd that these two must theref,ore be accepted
as separate rocks.
We may now consider this question as definitely ,settled; the fine
grained granite which I may briefly refer to as aplite, represents a
more recent "Nachschub," being the third acid eruptive rock of the
Bushveld. I havel to recur repeatedly in the following remarks to the
question whether it is responsible for the mineralisation or not.
THE ROCKS BELONGING TO THE BUSHVELD GRANITE COMPLEX.
31
The fissures on Enkeldoorn strike N. W. by N.; they are filled
with quartz which often grows from both sides of a fissure towards
its centre and forms then distinct crystals, sometimes several inches
in length. These conditi.ons recall Zinnwald in Saxony. The ores
are a fine grained darkish cassiterite, arsenical and copper pyrites,
copper-glance and in some fissures red oxide of iron.
The walls of the
fissures are partly altered to kaolin, partly transformed into greenish
sericitic ma1sses, and are impregnated with tin-stone.
Chlorite and
epidote are also present.
Acce3sory minerals, such as fluorspar, topaz,
tourmaline, etc., elsewhere so typical, are missing.
Vlaklaagte shows a fairly well defined zone' of quartz fissures containing some tin-stone. The walls of these fissures are hardly altered;
but parallel to them irregular bands and bodies of true greisen are
found. The greisen is of medium grain and in appearance white with
many dark dots. The white ground masS' consists of quartz and topaz;
the dark portions are cassiterite of a resinous eolour or, in
lieu of it, dark mica.
Alongside of the bands and bodies of
greisen, sericite and in a lesser degree some tourmaline, fluorspar, copper
pyrites and molybdenum glance occur scattered throughout the granite
or filling small druses. Greisen occurs-replacing coarse grained as
well as fine grained granite.
Two facts are remarkable: the abundance of topaz, and the entire
absence ()f lithium in the mica. My early statements* to this effect
were contested, but have since been fully confirmed by tho investigations
of Prof. Beck. t
The greisen is ,sometimes rich in tin-stone, ore yielding up to 40 per
cent. having been found. The distribution of tin ore in the greisen is
often very erratic, as extensive greisen bodies sometimes contain just a;
few blocks of rich ore while the re3t does not yield more than traces. The
great number of greisen outcrops of this nature was responsible for overestimation, for hopes which were not to be fulfilled, and for the temporary
cessation of all work.
On other farms in the vicinity of Enkeldoorn and Vlaldaagte
smaller deposit,s of tin ore were found. These need not be referred to
specially, but two occurrences on the farm Houtenbek deserve mention,
as frDHl their mineral oomposihon they :belong in my opinion to the tin
age. One is a band, two feet in width, consistil1g of almost pure
arsenical pyrites, some fluorspar and large leaves of molybdenite. The
other similar band shows a1'3ono pyrite, fluorspar and splendid large
crystals ,of monazite, all closely intorgrown. The monazite is in colour
greenish grey to darkish red. Some of the ore was reported to have
an average yield of 5 per cent. of thorium. The tin bearing pegmatite
veins in the grey granite in Swaziland also contain monazite and
euxenite.
It is well-known that monazite occurs in acid eruptive
rocks, but, as far a,s I know, it has not been observed in direct concurrence with tin deposits.
---------- ---------0)',
t
B.A. llf.£nes, 1904, p. no.
Zeitschl'ift, f. p. G. June, 1906.
32
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
Turning now to the western area of the Bushveld granite and
omitting a few lesser ()ccurrences, the following sub-divisions suggest
themselve~ for the contact-belt, which I described in my general remarks.
There are four separate districts:
A. The Rooiberg.
B.
IVal'mbad.
C. Nylstroom.
D.
A.
Potgietersr'ltst.
'Phe RooibeTg District:
Tin deposits are found in the inner circle of the Rooiberg, i.e.,
within the area of the sedimentary rocks.
The ores occur in two ~ones running north-.south, the eastern
of which begins on the farm Olivenbosch and then crosses southwards
through Hartebeestfontein into Vellerfontein.
The other wne passes through the farms Doorndraai and Wegnek
(Weynek), adjacent to the west.
The ,country, though fairly level on Olivenbosch and Hartebeestfontein, rises towards the other three farms, so that their surface is
several hundred feet above the level of the former two.
The eastern wne, attaining now and then a hundred yards and
more in width, contains great quartz-tourmaline occurrences which protrude in the shape of bands and lenses from the arenaceous masses.
I had no opportunity of visiting the underground workings; but
from communications received and from the examination of the ore
dumps it appears that fairly regular bands of tourmaline and quartz
intersect the strata more or less vertically, whilst others follow the almost
horiwntal bedding planes of the sandstones and quartzites. Corresp.()Ilding occurrences have been described from other tin fields such a,s Banca
and Billiton.
'l'he tourmaline, dark in colour, sometimes shows well shaped,
lustrous columns, but forms generally peculiar intertwined aggregates.
The rock then resembles less a tourmaline-fels than a matted actinolitic
hornblende rock, of the texture typical of contact deposits.
To the naked eye the tin-stone is dark in appearance and intimately
intergrown with the tourmaline. This observation is corroborated by
the microscopical examination of slides which shows :-tourmalina' in
needle-like aggregates.; cassiterite in good crystals with copious twinning,
in colour light yellow to dark brown, some crystals consisting of alternating light and dark layers parallel to the crJ'stal surfaces; then quartz,
THE ROCKS BELONGING TO THE BUSHVELD GRANITE COMPLEX.
33
topaz, sericite, traces 'Of fluorite, chaloopyrite and traces of bornite.
Cassiterite and tourmaline were fo-rmed simultaneously, because cassiterite crystals enclose and cross those of tourmaline, and vice versa.
Topaz and chaloopyrite are more recent; the latter crosses the tOUT'
maline fels in tiny stringers and also surrounds completed crystals of tinstone and tourmaline.
Similar formations occur 'On Hartebeestfontein; but, in addition
to the aforedescribed minerals and ores, large quantities 'Of a more
recent red felspar exist. Such ore deposits are common, as foOl' example,
at Graupen in the Erzgebirge. Besides the felspar, calcite occurs which
fully encloses tin crystals.
The continuation of this tourmaline zone with some tin is found
on the higher levels of Vellerfontein. The presence of the tourmaline
is often indicated by nothing but black dots which cause the sandstone
to be dark coloured without otherwise changing its structure.
To the
west of and parallel toO this tourmaline zone, another occurs· which
is especially characterised by the columnar development of the quartz
and by a higher percentage of arsenical pyrites together with traces of
molybde:nite. On the eastern portion of Vellerfontein a fissure-zone
is found with copper ores, some iron pyrites and a gangue matter consisting of quartz, and refatively much calc- and fluorspar; the latter
being white, green and violet in colour and generally massive in texture.
It is an open question if these formations on the higher geographical
level of Vellerfontein owe their diversity from the occurrence 'On Hartebeestfontein to differences of primary horizons ("primaere Teufenunterschiede"). Whether these deposits will change or not at depth into tin
ores, especially when attaining the horizon of the other farms, will be
demonstrated by the progress of development.
I found 'On dump heaps on Olivenbosch and Vellerfontein masses of a
rock which shows a great resemblance to the aplite of Enkeldoorn and
Vlaklaagte. I felt disposed to look 'On this rock as an aplite. Messrs.
Recknagel and J orissen, however, who are intimately acquainted with
the Rooiberg district, maintain from their observations underground
that this rock must be taken for a metamorphosed sedimentary rock.
I have reaSDns to believe that these two gentlemen are about to present
us with valuable papers on the ore deposits and geological features
of this portion of the country. In view of this I abandoned my intention of a second visit tD this district for the purpose of more exhaustive
personal study, and I shall be glad to leave to the said gentlemen
the solution of this and several other interesting problems. I may add
here for myself that the ore deposition is unquestiDnably connected
with the granite, which is to be expected at depth. vVe have seen that
it is intrusive into the sedimentary strata on Leeuwpoort and that)t
disappears .under the felsites on Kwaggafontein in the direction of the
ore deposits. The aplitic metamorphosed rlock on Olivenbosch and
Vellerfontein indicates further the close proximity o,f a related eruptive
rock.
34
B.
TRANSACTIONS OF THE GEOLOGICAL SOCIETY O:P SOUTH AFRICA,
The Warm bad District.
A few miles west of Warmbad a fairly broad strip of Bushveld
granite runs across 'Several farms in a north-western direction and
penetrates right into the oontact zone. It forms here a chain of hills
and rises therefore perceptibly above its usual horizon in the Bushveld
generally. I could trace intrusions in the fine grained aplite on the
farm Zwartkloof, No. 2126, where some quartz bodies of irregular
dimensions contain in places coarse cassiterite, whilst scattered impregnations are found here and there, in preference on the contact of granite
and aplite; sericite, some fluorspar and molybdenite also occur. Eluvial
and alluvial tin occurs on the slopes of these hills and in the beds of the
several spruits.
A few miles further north-west on the farm Elandsfontein No. 1782,
some other intrusions of aplite are found, with tin more or less in the
eontact zone. The tin occurs in quartz and also as impregnations in a light
green sericite mas,s.
The cassiterite crystals are small with a badly
developed prism; the ,small dark pyramids, which protrude from the
ground mass lend to specimens the appearance of chrome-iron ore in a
serpentine base. Now and then particles of tourmaline darken the pale
sericite. The ore bodies occur in bands and pockets, generally of a massive
and less often of a banded structure. In Home not a trace of the felspar,
nor even of the quartz of the original granite is observable.
The aplite of Zwartkloof and Elandsfontein corresponds in every
detail to the same rock on Enkeldoorn and Vlaklaagte.
C.
The
Nylstroorrr~
District.
On proceeding from Elandsfontein to the Nylstroom oountry, the
granite disappears completely, whilst enormous chains of mountains
are formed alternately by felsites and Waterberg strata. It is only
16 miles north-east of Nylstroom that the granite is exposed again on
the southern half of the farm Doornhoek. The most important ore
deposits, so far proved, lie on the western portion of the farm, where
rocks are found of the family of the quartz- and felsite porphyri83,
containing quartz, felspar, limonite and muscovite. In these rocks
,shales are embedded, over a width of several hundred yards and with an
east-west strike. As frequent transitions from the one class of rocks to the
other are observable, at least the major portion of these ,shales belongs
to the contemporaneous acid rocks of the lower Waterherg series. The
" shales" are often very highly laminated and disclose metamorphic influences. The rock slide of one specimen showed quartz, microsoopical
as well as coarse-grained, some zircon and well defined tourmaline.
Repeatedly bodies carrying tin-stone are intercalated in the shales.
The main occurrences, however, are connected with one well marked
fi,s.sure, which intersects the shales at acute angles and which has been
already prospected over a few hundred yards. The filling of this fissure
is often clayey; now and then its course is indicated by quartz veins.
THE ROCKS BELONGING TO THE BUSHVELD GlUNITE COMPLEX.
35
In the region adjacent to the fissure, the !shales are crushed and
sheared, and impregnated with tin ore, the yield of which increases
up to 50 per cent. of tin, and more.
The cassiterite and its
associated minerals {1re as a rule of the finest grain and indistinct; the ore is therefore of insignificant appearance, red and earthy,
whilst fresh cleavages show a lively red to dark green colour. The
most frequent of the associated minerals is again tourmaline, then quartz,
sericite, and here and there fluorspar.
Hock slides of the richer ore
showed : (1) The major portion of the mass tourmaline;
the cassiterite
light to dark yellow, reddish brown, bright, and grey violet;
quartz and limonite.
(2) Almost entirely cassiterite of yellow, grey-violet and dark brown
colour; some, tourmaline and remnants of a mixture of fine
grained quartz and felspar.
In other pieces some sericite, quartz and chlorite were also found,
and in one portion of the fissure beautiful tetrahedra and octahedra of
magnetite. The structure of Ule tin-stone is seldom bedded, but usually
granular; the great variety of colours is interesting. Here also the
often observed fact is corroborated that the new formations of tin-stone
are correlated in their structure to that of the original rock. Although
I could not detect here the fine grained aplite, I alP- convinced that the
genesi.S' of the ore is to be traced to a granite hidden at depth, and not
to the eruption of the felsite itself.
D.
The Potgietersrust District.
rrhere remains now only the fourth and so far the most important
tin district, that of P,otgietersrust. I may repeat that the granite whi.ch
forms here a high range, is a few miles wide; it disappears to the
w,est with a slight dip under the felsite. Going from east to west it shows,
in a cross section, first a coarse, then a medium grained structure with
which the manner of its weathering corresponds, huge spheres from the
coarse grained, smaller ones from the medium grained rock.
The
transition from one to the other is relatively sudden, the medium grained
not seldom beginning with a prominent comb. rrhe observer i~ frequently left with the impression that the coo.Iing magma adopted
configurations which dip gently to the west. Not far to the east of
this comb a number of contraction fissures occur in a zone which is
several hundred feet wide and which strikes approximately N.W. by
N., and dips also gently to the west. These fissures which are only
up to a few inches wide, are sometimes ,so indistinct that just traces
of them may be perceived here and there over considerable distances.
They are occasionally filled with an aplite which resembles point for
point the aplite of Warmbad, Enkeldoorn and Vlaklaagte. The micro-
36
THANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH A~'RICA.
soope shows commonly quartz, plagioclase, orthoclase, partly altered
into sericite, fluorite and limonite. It is here as a rule obvious that
they are after-eruptions ("Nachschuebe"). Otherwise these contraction
cracks are filled with quartz, showing but little m.ineralisation.
The tin deposits which are confined to the environments of these
cracks take the shape of impregnation zones; some of these are of a
very interesting and peculiar character. 1'hey all occur practically
at the same height and extend for four miles over the farms Salomons~
tempel, Sterkrivier, Groenfontein, Hoodepoort and on to Zaaiplaats.
The chief minerals, besides cassiterite, are quartz and tourmaline,
then sericite and fluorspar and occasi,onally topaz; calcspar and wme
chalcopyrite, bornite, arsenical- and iron pyrites, and as a last product of
the mineralisation, galena and zinc blende. The bulk of the tin is
found in peculiar cylindrical bodies, sometimes. likened to chimneys
and polonies; personally I can think of no better comparison, than a
pliable india rubber pipe. They vary in diameter from a few inches
to many feet and dip generally fairly flat to the north-west, 'Sometimes
in a straight, more often in a bent line, but also with sharp kinks
when they start vertically and then turn. Frequently they gather into
groups, the several members of which join in depth one after the other
to one main body. rrhey are therefore not placed in one plane, i.e.,
along one of the fissures or cracks as for instance the pipes or chimneys of
the Old East Wheal Lovell Mine in Cornwall, but are orientated without
any regularity. Th~ best comparison is with a storm bent tree trunk and
its main branches.
The mineralisation of these pipes may be considered rich; it falls
in no instance below 20 per cent., but reaches even as much as 50
per cent. of cassiterite. Their ore contents show individual varieties
and peculiarities.
On Salomonstempel the central mass of the pipes is highly siliceous
and therefore hard: it is white with innumerable dark dots of cassiterite impregnations; in addition a little sericite and tiny needles
of tourmaline are found. Towa.rds the circumference tourmaline becomes more distinct and its needles are grouped almost without exception in small rosettes. The outer ring is black and consists exclusively
of tourmaline rosette~ with interpolated quartz. It is seldom possible
in a loose specimen of such a pipe to detect its origin from the granite.
An entire piece was found weathered out, lying amongst the
detritus on the surface; it is. about 20 inches long 12 inches
in diameter and perfectly cylindrical so that it resembles the fragment
of a broken column. I am glad to hear from Mr. Dlaine of Salomonstempel, to whom I am greatly indebted for kindness shown, that this piece
will be presented to the Geological Museum.
The ore occurs in granite as well as in aplite, with the difference
that in the latter the crystals of cassiterite are in a remarkable degree
smaller in grain than in the former. Also here the structure of the
ore reflects the structure of the original r.ock. In some p.ortions of
fine grained bodies the single tin crystals are packed so tightly that
THE ROOKS BELONGING TO THE BUSHVELD GRANITE COMPLEX.
37
other mineral,s hardly find room in the interstices; such ore then
resembles a dark quartzite in appearance.
A typical sample from
Salomonstempel showed under the microscope: quartz; tourmaline; cassiterite, light, yellow and brown; sericite and topaz. I found on the
dump at a prospecting hole on this farm loose lumps containing felspar
and green mica, the latter giving the flame of the blowpipe the distinct
carmine colour of lithium. This is the only instance where I succeeded
in proving the presence of lithium in the Bushveld.
On the farms Sterkrivier, Groenfontein and Roodepoort the central
portions of the pipe'S, i.e., the mineralised cores, differ in 'so far from
those on Salomonstempel that the silicification is generally not so advanced. In many places the red felspar and the primary quartz of
the granite appear still intact. The microscope only demonstrates the
beginning of the metamorphosis of the felspars into sericite.
The tourmaline ring is again always of a substantial thickness.
l'he crosssections of some pipes are no longer circular, nor oval, but assume the
outlines of a pear.
On Za,aiplaats the silicification appears to the naked eye st~ll
less pronounced. Nevertheless the original granitic structure is as a
rule strongly obliterated, being altered into a brittle mass, now bright,
then dark greyish green in colour. Not only does the surrounding
tourmaline ring never exist., with but one slight exception, but this, so
far most important, accessory mineral is' also missing from the ore
itself j in its place enter very much sericite, less fluorsrp.ar, then
topaz and some chloritoid, the latter previously not recognised. Arsenical
pyrites usually appear as the accompanying ore; in one pipe, even in
almost equal proportions with the tin-stone.
All these points are corroborated by rock slides, the cassiterite being always older than the
arsenical pyrites; quartz, although not apparent to the eye, is abundant.
Of great interest are some portions with a fine grained calcite full
of ca,slsiterite. The microscopical. examination proves that this calcite,
in oonjunction with sericite, replaces the felspars.
In addition are
found: quartz, cassiterite, chlorite, topaz and arseno-pyrite.
Such a
metamorphosis of rocks associated with tin deposits has never before
come to my notice, although this transformation is common enough
with the salbands of fissure veins in consequence of the actions of
thermal waters.
Lindgren for example describes a common alteration of
the walls of certain gold-reefs occurring in the granite of Idaho a's follows:
"The coloured bi-silicates and felspars are attacked first of all
the normal constituents, and changed into aggregates of sericite and
calcite, the former sometimes with a chloritic intermediate stage. Then
follows the alteration of the quartz.
Besides the fine grained, a coarse grained crystalline, calcspar is
found as a more recent formation, in tiny veins, or in druses and cavities;
these never carry tin. In addition to the aforementioned minerals the
pipes often contain also copper and iron pyrites, galena and zinc blende.
One fissure contains pure galena about four inches wide.
On the
38
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
hanging side of the principal chimney a wne of druses occurs over a
great length, into the cavities of which crystals of quartz and fluorspar
protrude together with aggregates of galena and beautiful yellow zinc
bIende. These ores are always younger than the cassiterite j they are
also. known in other tin fields.
Development on Salomonstempel has not yet reached any great
depth, but on the farms Sterkrivier, Groenfontein and R'oodepoort
some pipes were opened and mined down to 300 feet. Out of one pipe
alone about 250 tons of ore were recovered with an average yield of
30 per cent. of tin. At the said depth the mineralisation decreases
fairly rapidly, being replaced by tourmaline. In other words, the tintourmaline pipes turn at depth into pure tourmaline columns. Only
by continuing development work can it be found out if a reversion to the
former condition sets in again at greater depth.
On Zaaiplaats the principal chimney has been mined to an approximate depth of 300 feet, but neither in yield nor in appearance of the
ore has a change yet occurred. Here also it is deeper development only
that will solve the question of equal ,scientific and practical interest
whether or not differences in primary horizons (primaere Teufenunterschiede) exist.
Opinions vary greatly with regard to the relative age of tin-stone
and tourmaline. It appears to me that the formation of the pipes
pr.ogressed concentrically from the core towards the periphery, so that
the tourmaline ring is more recent that the tin ore column. Were
cassiterite the younger mineral, at least fissures or vents had to be found
in the tourmaline below the 300 foot level, where the tin gases would
have had to ascend through the already formed tourmaline. My views are
corroborated by microsoopical examination, when the tourmaline is seen
to crowd round the crystal'S of cassiterite, and is therefore younger.
Thi,s constitutes an interesting difference from the Rooiberg occurrence where both minerals were recognised to be contemporaneous. Topaz
and arseno-pyrite are still younger.
Fluorspar and sericite continue
right into the most recent formation.
In addition to the pipe-shaped ore columns zones are now and
then
found
speckled
with
isolated
crystals
of
cassiterite,
gathered occasionally in little pockets. rrhey may but rarely give rise
to direct remunerative mining, although they were the main source
for the patches ~f shed tin.
One of these zones on Salomollstempel which might be compared
to a small Istockwork, shows peculiar features.
Two parallel tourmaline
bands are outcropping about 50 yards apart and seem to converge;
in between these the coarse grained granite is bleached to such an extent
through the ore-bringing actions that it as,s,umes a palish grey colour.
It is alive with dark tin crystals of fairly large size, but the naked
eye detects otherwise no alteration except the loss of its colour.
The
microsoope, however, reveals the beginning metamorphism of most
felspars into sericite and not seldom an advanced pseudomorphosis into
cassiterite.
THE ROCKS BELONGING TO THE BUSHVELD GRANITE COMPLEX.
39
On Groenfontein a third and quite unique type exists which by
the way led to the discovery of the tin fields in this district. In the
granite a lenticular segregation of very coarse and highly quartzose
graphic granite is found a few yards in width and about 50 yards long,
dipping flat to the west.
Along the lower side of this inclined rock
gases were. pressing and ores and alterations resulted which resemble
those already described.
These gases also penetrated the oomplex of
the graphic granite over numerous clefts, approximately at right angles
to the dip, and transformed chiefly its fel,spars into larger tin crystals.
I found that the grains of ore are not only crowding round the large
quartz prisms, but in several instances there ar'e beautiful and complete
pseudomorphic crystals of cassiterite after quartz. Such pseudomorphs
have, as far as I am aware, not yet been mentioned in geological literature.
The most northern farm on which the zone of contraction fissures
has been proved, i~ Appingadam which lies quite 700 feet below the
level of the other farms. The granite is here frequently bleached .to
a grey colour in the same way as described in the occurrence on Salomonstempel; prospecting has not yet advanced to any extent, but some slight
impregnations with tinstone and its accompanying minerals have been
already encountered. The south-east corner ma~es so far an exception,
as here a very rich, though small, lens has been' opened, the ore of which
shows remarkable differences from all the other occurrences. Whilst the
cassiterite retains its granular structure, the gangue matter adopts
a well developed fibrous texture which is caused by a parallel arrangement of the slender elongated quartz and topaz oolumns, which occur in
almost the same proportions., Of the other accessory minerals, tourmaline
and fluorspar are miss,ing.
In addition to these a slide revealed tUle
presence of sericite, chalcopyrite and traces of bornite, limonite and
mica.
About a mile to the west of this discovery a vein of aplite occurs,
and in close connection with it a well defined band of tourmaline which
is abundantly inter~rown witharseno-pyrite, fluo,rspar and molybdenite. The tourmaline occurs here as often in the well known rosettes
as in excellent lustrous crystals; the molybdenite is in fine scales and
shows well formed hexagonal leaves, which rest in thick layers one
upon the other. Its oxidation product, the yellow molybdenum ochre,
is not uncommon and contrasts strikingly with the pitch black tourmaline
on which it restsl.
In one prospecting hole bright yellow zinc blende was met with,
a vein of which, about one inch thick, crosses the tourmaline band;
it has to be attributed therefore to the most recent depositions. My attention was first drawn to the occurrence of this mineral by von Des,smIer.
Away from the principal mineral 21one, so far dealt with, a seoond
one exists which deserves to be mentioned, although it has not yet
attained any economical importance. This zone is about two miles
further west and rUIis almost parallel to the main zone. It is usually
found in the felsite where the latter' disappears below the Waterberg
40
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
series. I have traced this mineralised zone from Salomoll's.tempel over
Appingadam to Verdoornsdraai, the only place where even scanty prospecting work had been done. This zone correlates with the one described
at
Doornhoek,
but
it
is
here
very
much
narrower.
It is undoubtedly a brecciated felsite, the fragments of which are recemented by quartz and tourmaline. 'rhe tourmaline occurs here in
very much greater bulk and with a more distinct crystallisation than
on Doornhoek.
There is also much sericite and some cassiterite of
the finest grain.
Its yield, on Verdoornsdraai, I was told, is
about 0.5 per cent. of metallic tin over a width of several
yards.
It is from this ore deposit that the fragments were
derived which I found as enclosures in the banded sandstone of Verdoorns,draai and which induced me inter alia to support the theory
for an eventual separation of these stratified r·ocks from the general
Waterberg formation.
I am of opinion that the genesis of this ~one also has unquestionably
to be traced back to the influence of the red granite, which, as we have
seen earlier, must occur below the felsite; firstly this zone runs parallel
to the main zone, and secondly it contains the same minerals.
In conclusion I beg to summarise the main points of this paper
as follows:
'rhe acid eruptive rocks of this area of the Bushveld are in order
of their geological age: the felsite in relatively considerable m.asses; then
the Bushveld red granite with a graphic structure; and whilst this rock
prevails, the third and youngest type, the fine grained red granite,
occurs only to a subordinate extent in bodies and fissures as "Nachschuebe," and shows an aplitic structure.
The aplite also maintains in the western area its very close connection with ore formations; it was never found without associated ore
deposits. This cannot be merely accidental. It is the ore bringer.*
Older than the eruptive rocks are the sedimentary rocks occurring
here, the :strata of the Black reef, Dolomite, Pretoria and Waterberg series.
Parts of the upermost Waterberg series contain detritus of the eruptive
rocks 0.£ the granite period; eventually therefore they ought to be given ap
independent geological position.
Younger too than the acid eruptions and their lust manifestations
are the remnants of the Karroo formation; they cannot contain therefore
any tin deposits, but may even cover and hide existing occurrences.
In all other rocks tin ore deposits may be expected; in fact such
have already been exposed in the granites of Enkeldoorn and Vlaklaagte,
of the Warmbad and Potgieter:.srust districts; in the felsites of Nylstroom
* At one of our recent meetings Mr. J. P. Johnson contested this statement, but
it is obvious from his remarks that he failed to recognise the aplite or confounded
the medium grained granite with it. His deductions rest therefore on a wrong
baRis.
THE ROCKS BELONGING TO THE BUSHVELD GRANITE COMPLEX.
41
and of the second ore wne of Potgietersrust; in the sandstones and
quartzites of the Rooiberg.
The typical "Leit" minerals in respect of the eastern area are
quartz, topaz and sericite, for the western area, quartz, tourmaline
and siericite.
We see hom all this evidence that our local conditions correspond
well in all essential points with those in other parts of the world.
There are three eruptive periods, the ore deposits resulting from the
last one. Prof. Reck referred to the close correlation of the eruptive
rocks here and in the Erzgebirge.
The local eruptive rocks are also intrusive in palaeozoic strata,
at least right into the higher horizons of the Waterberg series which
are taken to correspond to the Devonian age.The composition of the gangue matter is as a rule not complicated.
Tourmaline and quartz are the most prominent - minerals in the
western area.
Here topaz recedes comparatively, being always younger
than tourmaline and cassiterite, an observation which probably applies
to almost all the other tin fields, interr alia to the Erzgebirge. Of accesisory
ores arsenical pyrites are oommon, but not ubiquitous, then copper
pyrites and molybdenite. Lithium bearing mica was found only once,
at Salomonstempel.
Especially remarkable is the absence of stannite, bismuth and wolfram ores, and also of fluorapatite.
A further deviation from the general conditions here, is the frequent
occurrence of a greenish sericite, massive and in fine scales. It is never
missing and forms next to tourmaline the most important " Leit "
mineral. Sericite is found in the Bushveld in the most intimate connection with tin-stone and also in some places with calcspar.
These
three minerals may then completely replace the primary constituents
of the granite without anyone of the other accessory minerals, even
the tourmaline, having participated in this metamorphosis.
The 'siericite stands elsewhere by itself or in its association with
calcspar as typical for the alteration of rocks on the salbands of fissures
which do not belong to the tin formation. They are the chief representatives of the minerals which are formed by thermal metamorphism.
We must therefore admit that here again the deposition of the ore
is to a great extent due to the action of hot ,solutions. These thermal
deposits are found in conjunction with those of pneumatolytic origin,
such as tourmaline and topaz.
The above would support the theory of Prof. Bergeat, when he says
that the formation of tin ore deposits took place first through pneumatolysis from gas mixtures and later on through hydatogenic-thermal action,
were it not that these actions are sometimes separated. On Enkeldoorn
for insltance the pneumatolytic minerals are completely missing, the formations are thermal only; similar oonditions rule in respect of some of
the sericite lenses on Elandsf'ontein.
Against this may be placed the exclusively pnenmatolytic formations
in the greisen bodies of Vlaklaagte.
42
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
According to numbers the pneumatolytic and mixed types predominate; the theory that the action of hot solutions prevailed, which
Prof. Beck seems to favour, finds its only oorroboration on Enkeldoorn.
For the country in question therefore it is evident from the foregoing
that tin ores may originate exclusively from solutions as well as exclusively
from pneumatolytic actions, but that a relatively greater number of
transitions and modifications of various types do occur between thes1e
two extremes.
I beg to record my appreciation of the valuable assistance rendered me by
Mr. L. Leger with the microscopical work, and by Messrs. J. J. Busschau and
N. Weigel in the field.