ON SOME MINERAL DEPOSITS IN THE ROOIBERG DISTRICT.
ON
SOME
MINERAL
DEPOSITS
IN
THE
ROOIBERG
83
DISTRICT
(Read 20th July, 1908).
By R. Recknagel.
[Plates VI. .XVI.]
Intr od'll ction.
The object of this paper is to draw the attention of the Society to a
district hitherto undescribed, which, on account of its variety of ore
deposits and of the unusual, often unique, features of the individual
deposits, is of the greatest scientific interest, and which, at the same time,
promises to become a very important district economically.
Although acquainted with the district and some of its deposits for
nearly three years, I have hesitated until now to publish my observations,
hoping that new facts being revealed almost weekly, by the development
-work going on there, would solve some of the many problems and enable
me to give a clear description and explanation of the facts observed.
The consciousness growing upon me that even now we are only in the
beginning of our knowledge, and that for years to come new facts will
be revealed, solving some old problems, and presenting new ones instead,
has prompted me to collect my data and present them to the Society at
the present moment.
Any interesting new development of the deposits known at present,
or any new discoveries I shall bring to the notice of the Society in due
time.
84
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
Topography.
The Rooiberg, which gives its name to the district, is situated about
40 miles due west of Warmbaths. It is a group of hills which rises from
undulating, or almost level, country eastward and southward t.o a height
of about 700 feet. Towards the north, undulating country connects the
Rooiberg with the Elandsberg, and westward a lower ridge of hills forms
the connection with the Boschof! Mountains.
The ore deposits to be described are situated mainly on the northern
slope of the Rooiberg and its western extension, on the farms Haartebeestpoort, Blaauwbank, Haartebeestfontein, Olievenhosch and Onverwacht.
Besides some deposits will be mentioned situated on the farms
Quaggafontein and Vellefontein, east and west of the Rooiberg.
Quite
a number of other farms in the district are known to be mineral bearing,
as Hartebeestvlei, vVeynek and others, but not having had the same opportunities of examining them, I confine my remarks to the deposits on the
first mentioned farms.
Geology.
As nothing has been published yet on the Geology of the district, and
a knowledge of it is necessary for the full understanding of the problems
involved by the ore deposits, I propose giving first a brief sketch of the
geology of the district based on my own observations. I am glad to know
that the Geological Survey is extending its most useful work in that
directioQn, and that before long we shall have a reliable description of
the whole district and a correlation of the doubtful sedimentary formation to be mentioned with the recognised formations elsewhere.
Both
igneous and sedimentary rocks are well represented.
Sedt'mentary Formation.-A sedimentary formation eonsisting of a
series of beds of quartzite, shales, and thin layers of conglomerate extends uniformlv and undisturbed over the whole north-western half of
the area unde~' discussion. The general strike of the strata is N.W.S.E. to N.N.W'.-S.S.E., and their dip from 5 to 10 degrees, to the northeast.
In the eastern part of the district on the northern slopes oQf the
Rooiberg the same sedimentary formation occurs, but strike and dip are
less uniform and everywhere quite different from that further west. The
strike in the eastern part is mainly. east-west with a southerly dip of
about 30 degrees, but also a more northerly strike with a dip to the southwest has been observed. The line of disturbance which can be assumed
to exist from the facts mentioned can be observed in several places, in
prospecting trenches on Blaauwbank and Vellef,ontein, and it appears
to strike in a north-easterly direction from near the common beacon of
the farms Blaauwbank, Hartebeestfontein and Vellefontein.
The thickness of this sedimentary formation is very considerable,
amounting to several thousand feet.
The southern slope of the Elandsberg alone shows a section ahout 600 feet, and from there southward for
ON SOME MINERAL DEPOSITS IN THE ROOIBERG DISTRICT.
85
abou't SIX miles a continuous s.eri'es ()f sedimentary strata underlying
uniformly the Elandsberg strata is found outcropping.
By far the
greatest number of these .strata consist of hard fine-grained pink and
white quartzites, but in several places in the Elandsberg section, as well
as in the southern portion of Hartebeestfontein, greyish blue, thinly
bedded shales form part of the series. In some of the beds of clay shales
outcropping on the slope of an isolated hill on Olievenbosch, clo~e to tIle
Elandsberg', well shaped disc-like nodules occur. On account of their
great hardness they are found lying about on the surface, while the
strata which once enclosed them are found in a decomposed clayey state
only.
Apart from occasional larger pebbles on the bedding planes of the
quartzite, thin small-pebbled conglomerates are found in several places
as, for example, in the bed of the main creek on Vaalwater.
A coarse
conglomerate must :exist in the south-eastern portion of Hartebeestfontein, judging from the coarse angular and rounded pieces of quartzite
and sha1es foreign to the spot where they are found, but it has not been
met with in situ.
Some petrographical notes on the quartzite will be given later in
connection with the description of the ore deposits.
As this sedimentary formation has not been found within the district
in contact with other S!2dimentary formatiiOns of known age, it must
remain doubtful for the present if it is part of the Pretoria Series, which
it resembles, or not.
There is no doubt whatever that it forms the
northern continuation of the quartzite found and described by Mr. Humphrey as extending from the undoubted Pretoria Series near the Aapies
River northward as far as Knoppieskraal.
Igneous Rocli:s.-The igneous rocks are represented by red granite,
granophyre, felsite, diabase and dolerite.
The red granite is found in the eastern part of the district forming
the flat or undulating country east and south of the Rooiberg and is
found there in contact with felsites and quartzites.
In the western
portion red granite forms part of the Boschoff Mountains, where it is
also found underlying the quartzites. rrhe distance between the eastern
and western outcrops of the red granite is at least ten miles, and the
whole area between them and north of the Rooiberg is covered by the
sedimentary formation.
No dykes of ordinary granite have been
observed, but the contact phenomena of granite and quartzites as observed
on Hartebeestfontein make it certain that the granite is intrusive into
the sedimentary formation.
Granophyre.-An exceptionally fine type of granophyre, shewing
beautifully the intergrowth of felspar and quartz, has been observed as
a dyke about 100 feet in width, breaking through the felsite on the farm
Welgevonden.
Some loose pieces of rock of very similar nature have
been picked up on \Veynek, close to the boundary of Hartebeestfontein,
hut the dyke from which they are derived has not been found yet.
86
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
Felsite.-This rock occurs in bosses as well as in dykes. The Ro-oiberg proper is formed by it, and so is the northern portion of the Elandsberg.
In both instances the felsite is overlying the quartzite formation.
In the Rooiberg the quartzites are forming mainly the northern range of
foothills, striking approximately N.E. - S.W., but disappear below the
felsites of the main range.
In the Elandsberg the highest points, about
700 feet above the Vaal water creek, consist of quartzites, or shew just
a thin layer of felsite.
Going north" however, the thickness of the
felsite increases, the height of the hills remaining the same, but the
quartzite formation on account of its strike N.W. and dip N.E. gradually decreasing and coming down to the level of the Vaalwater River, on
the farm Welgevonden. A few felsite dykes have been observed on W·eynek and Hartebeestfontein, breaking through the sedimentary formation.
As they are to be found within the mineral bearing area, some peculiar
features of theirs will be mentioned when .discussing the probable origin
of the deposits.
Diabase.-Dykes and sills of diabase are found in several places. A
peculiar "blind ending" of a dyke in the sedimentary form.ation, as
observed in a creek bed near the western boundary of Hartebeestpoort,
is illustrated by Plate No. VI.
A. few diabase sills interbedded with shales and quartzites are outcropping on the slopes of the Elandsberg on the farm Onverwacht.
Dolerite.-Various kinds of this rock have been observed, all formdyl~es in the sedimentary 'formatiou.
One variety, which is found as a dyke in the midst of the tin lodes
on Olievenbosch, is coarsely crystalline and consisting of plagioclase and
a monoclinic pyroxene without shewing any ophitic structure, it might
more appropriately be called a gabbro. A finer crystalline dyke met
with underground on Olievenbosch consists of labradorite and augite, the
latter enclosing a serpentinised mineral which originally was olivine most
likely. The rock appears to be, therefore, an olivine-dolerite.
Another dyke observed on the farm Onverwacht, and which, on
account of its contact phenomena, will be mentioned later on, is nlacroUnder the microscope small
scopically dense and almost black in colour.
felspars with a great deal of pyroxene are to be noticed.
The rock
appears to be a quickly cooled dolerit-e. (See Plate XV. and Plate XIV.,
Fig. 5.)
Regarding the relationship between sedimentary and igneous rock,
the following general statement may be made : ing
1. The red granite is intrusive into the quartzite formation.
2. The felsite and basic dykes are all younger than the quartzites.
3. The granophyre is younger than the felsite.
. 4. The relationship between red granite and felsite appears doubtful.
The felsite is overlying the red granite on Quaggafontein, and, further north, the quartzite, but tthe contact be-
87
ON SOME l\HNERAL DEPOSITS IN THE ROOIBERG DISTRICT.
tween red granite and felsite, on account of weathering, is so
indistinct as to make it impossible to ascertain if the granite
is intrusive or if the felsite has covered the sedimentary
formation and the pre-existing granite.
5. It is doubtful what relationship exists between the granophyre
and the red granite, and no oonclusion can be drawn from
the fact that the granophyre appears as a dyke in the felsite.
The granophyre mayor may not be a dyke contemporaneous
with the granite intrusion.
(Plate VII., Figs. 1 and 2, represent diagramatic sections through
the eastern and western portion of the district, and illustrate the statements just made.)
Ore Deposits.
Ore deposits occur in the igneous as well as in the sedimentary rocks
of the district, and are too numerous to be all described in detail, but I
shall at -least mention all which have come under my observation.
1.
Ore deposits in igneous rocl(,s.
(a) In irregulal' poc7i:ets; concretions. Examples: tin ore in
the red granite on the farm Quaggafontein, and specular
iron in the felsite of the Rooiberg.
(b) In mineral veins, Copper pyrites, and its derivatives, copper glance and malachite, with quartz and fluorite as
gangue minerals in the lodes in diabase ·on the farm
Quaggafontein, and most likely on Hartebeestpoort; also
specular iron and malachite in quartzose zones of interbedded, shaley diabase on the south-western slope of the
Elandsberg.
2. Ore deposits in sedimentary formation.
(a) As veins proper, with iron, tin, copper, cobalt
gold as their chief metallic contents.
(b) As bedded im1J1'egnation
~:n
01'
llickel-
quartzite: tourmaline, tin.
Of these deposits, those under 2 (a) are by far the most interesting
and important, and will be described in detail. A few words, however,
may first be said about the tin deposit under 1 (a).
Tin ore occurrence in the red granite on Quaggafontein.-Cassiterite
occurs there within either specially quartzose or specially felspathic portions of the granite.
In th.e first case large crystals of quartz, up to
several inches in length, are seen to be connected by a fine grained, or
homogenous, mass of quartz, coloured red by hematite, and the cassiterite
in crystalline grains is found either in the dense quartzose mass between
88
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
the large quartz crystals, or it follows in string-like aggregates the outlines of the large crystals.
In the other instance, cassiterite is found in crystalline aggregates
within a granite in which the quartz is almost wholly absent, and which
consists almost e,ntirely of med'i urn sized orthoclase crystals.
Both
occurrences are quite irregular, and have nothing to do with a lode formation. They appear to be due to a differentiation of the granitic magma,
the felspar occurrence representing more or less a type of impregnation,
whereas the quartz cassiterite occurrence may be considered as an orebearing pegmatite. Both occurrences are close to each other, and not far
from the contact of granite and felsite.
As they are both of small size,
it will depend on the discovery of many more and larger pockets to make
the occurrences economically valuable. The granite in which these concretions occur does not differ from the usual type of red granite, and
only the presence of fluorspar at several places away from the tin oreoccurrences points to its mineralisation.
Copper Lode.-A close relationship genetically appears to
exist between the pegmatitic tin ore occurrence III the red
granite and an undoubted vein formation which carn~s copper
as its chief metallic constituent.
This vein occurs in the
quartzite formation on Vellefontein, close to the S.E. beacon
of Hartebeestfontein.
The gangue minerals are most uncommon, and
consist of quartz, orthoclase, muscovite, siderite, or another iron-bearing
carbonate, and fluorite.
The original copper mineral is copper pyrites,
which, near the surface) has been changed into erubescite, copper glance,
malachite an9- azurite.
Besictes copper pyrites, iron pyrites occurs.
Old workings have obscured the outcrop Of \the lode where recently a
development of the l,ode in depth has been started, but some distance further south the same lode crops outJ and appears as a typical pegmatite
vein, consisting of very coarse crystalline orthoclase and quartz. On the
dumps near the old workings copper ore with siderite, quartz, orthoclase
and fluorite can be observed, and I have no doubt that this ore deposit
is a copper-bearing pegmatitic vein connected with the red granite which
is most likely underlying the quartzites in which it occurs. The nearest
outcrop of red granite occurs about three miles east of it.
Iron Lodes.-BeJ:ore describing' the most important vlein deposits, a few
words· may be said about the occurrence of specular iron in quartz veins.
There are a great numLer of such lodes, and in a number of places they
have been worked by nativ!es, not for the specularite as a high grade iron
,ore, but as a pigment which even at the present moment is largely used.
Such old workings have been noticed specially on Blaauwbank, in the
foothills of the Hooiberg, and on the western slope of the Elandsbergen.
On the farm Onverwacht reefs which have been worked for specular iron
shew many of the 'characteristics of the typical tin lodes to be described,
and there is a possiLility that the iron lodes, or at least a number of
them, are closely related to the tin lodes.
ON SOME :MINERAL DEPOSITS IN THE ROOIBERG DISTRICT.
89
l'in Lodes.-The tin lodes, a description of which forms the main
object of this paper, are situated on the farms Hartebeestfontein and
Olievenbosch.
They owe their discovery to the existence of very extensive old workings, which were assumed to be old iron workings, but
which, without any doubt, represent an{lient tin mines. It can easily
be understood that casual observers have mistaken the old workings for
iron workings, as tin ore is found in very small quantities only in the
dumps, and as moreover layers of ferruginous rock are found close to
the surface in the area of the old workings.
In fact, in soine instances
the old workings have broken through these layers of iron rock, and are
surrounded by them.
rrhis iron rock, the "ouklip" of the Boers, is
mainly a surface breccia in which subangular fragments of rock outcropping in the neighbourhood, quartzite, tourmaline rock, etc., have
been cemented by limonite.
In places cassiterite in form of small
isolated crystals has been found in the "ouklip."
These cassiterite
crystals undoubtedly formed part of tin lodes originally.
They were
set free and spread over the surface when the outcrops of the lodes decom··
posed, and, together with rock fragments lying on the surface, were
cemented by limonite.
In a few isolated cases "ouklip" seems to have been formed by
decomposition and alteration of quartzite or lode outcrops in sit'u.
In
such instances which correspond to the formation of laterite the "ouklipi'
does not contain angular fragments of undecomposed rock, but appears
cavernous, part of the quartzite having been leached out, and the rest
changed into limonite.
Old Workings.-The accompanying plan (Plate VIII.) shews the
grouping of the main old workings true to scale, which, perhaps, better than anythiug else, conveys a proper idea of
the very large amount of work done by the ancient miners.
The area oovered by these old workings IS over 70 acres,
and within that area there is very little virgin ground, most
of it having been turned over, leaving behind innumerable irregular
holes and excavations, separated by dumps, or in some instances by ribs
of country rock. At present most of these holes are shallow, from a few
to about ten feet, but it is certain that most of them were much deeper
originally, and have been filled up mostly by drifting sand. In the
course of prospecting, old workings, thirty feet deep, have been encountered, which at the surface shewed a trench-like depression in the ground
of about eight feet depth only, and it may be that even deeper ones
exist, and will be found in course of further exploitation.
It is certainly most interesting to know that ancient tin mines exist in this country, and possibly they, in course of time, may be of help in tracing the
historical development of the sub-continent. At present no explanation
can be given either of their age, or of the people who are responsible for
them.
It is certain that the present-day Kaffir is unconnected with
them.
rrhere is no tradition amongst them of the time when the workings were made, and there is no record besides that any of the Kaffir
tribes knew the metal tin and have made use of it.
These facts point
90
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
certainly t,o the greater age ,of the old workings, but, at the same time,
there is no reason whatever to connect them with ancient history in our
sense; in fact, there are reasons to consider them of moderate age only.
In a number of places remains of old kraals are to be noticed within
the mining area, and, although. in some of the old workings fairly large
trees are growing, others are far too fresh-looking to make an age of
thousands of years probable.
Hegarding the people who have done the
mining, it appears certain from the crude way the work has been done,
and from the primitive tools used, viz., stone hammers, scrapers made of
bone, small iron gads used as chisels, or picks, etc., that· the old miners
were natives of this country, and not more civilised foreigners. While
the meagre and contradictory facts known at present do not allow of
our tracing the history of the old miners, the following assumptions,
however, which explain away certain incongruities, may possibly at the
same time interpret the facts correctly, viz.: The ancient miners were
natives of the Bantu race who, as slaves, were working for foreigners,
to whom all the tin won had to be delivered: rrhe frequent native wars
and the consequent migration of tribes may be responsible for the fact
that the present-day Kaffir living in the district lias no memory left of
the time when the mines were worked.
It is to be mentioned that tin-smelting was carried on in connection
with the old mines. In two localities old smelting places; with large slag
heaps, broken pieces of tuyers and smelting pots, have been discovered.
A few tin ingots, weighing about 31bs. apiece, and shaped like the hollow
of the hand, were found a few years ago in an abandoned Kaffir kraal
some fifteen miles north of the mines, and I believe they came from these
smelting places, or at least were smelted out of ore found in the mining
area under discussion.
A great many features of the old workings are
identical with those of the large old copper workings near the Limpopo,
and I quite believe that both copper and tin mines have been worked by
the same native people, about the same time and under coercion of the
same foreigners.
The bewildering number of irregular holes within the old mining
area made it quite impossible to form any definite idea of the character
of the deposits worked. It was clear, however, that they were not simple,
but it was scarcely expected that they were as complicated as subsequent
prospecting work has proved them to be. In giving a description Qf
them, I cannot help generalising, although I am conscious that future
development may possibly modify my genera] views even to a very considerable extent.
A great amount of prospecting and development worl\
has been done already on which I can base my conclusiQns, but, compared to the whole mining area proved by Qld workings, work dQne up
to the present has been confined to a few small places only, and the
exploration of the whole area will undoubtedly add many new facts to
our present knowledge, besides proving the existence of many more deposits of the same or similar kind as those disc-overed and developed
so far.
J~ode 8y,stems.-;-The whole mining field may be likened to a
gigantic "stock-work/' in which the tiny cracks, veinlets and impregna-
ON SOME MINERAL DEPOSITS IN THE ROOIBERG DISTRICT.
91
tions of the stock-work proper are represented by fissures and lodes of
large dimensions, accompanied by impregnations of corresponding magnitude.
rfhese lodes strike in all directions, but it seems that several
lode systems, each containing a number of lodes wiith approximately the
same strike, may be distinguished, which include the majority of the
existing lodes. Up to the present the existence of at least two of such
systems has been established, one containing lodes with an approximately
north-south strike, and the other whose lodes strike west-north-west. It
is impossible at present to state over what distance these lodes are likely
to extend 2 as the old workings do not allow of the tracing of any individual lode beyond a few hundred feet, and as, besides, it has been proved
already that the lodes extend far beyond the area worked, and that lodes
are in existence below a sandy surface, where neither old workings nor
outcrops indicate their presence.
In consequence, only underground
development will establish facts in this respect, but there is no question
whatever that lodes will be found in existence over much longer distances
than has been established up to the present, and that is about 1,200 feet
already.
TY1Jes of hades (Plate IX. ).~It is very difficult to describe
a few words the vari,ous types of lodes encountered, as
there is scarcely any type unrepresented.
There are fissures
of small width, mere cracks with no vein filling except some
clay; similar fissures with irregular pockets of vein minerals
attached; two or three ,of such fissures close together, embracing between them impregnated mineralised country rock; lodes with
one well defined wall, shewing brecciated structure and enclosing vughs
of large dimensions; and lodes which shew both walls well defined over
long distances.
The task of conveying a proper idea regarding the
various types is made even more difficult by the fact that in most lodes
various types are represented. 'rhe acoompanying diagramatic sketches
shew the more common types of deposits, and some further reference will
be made to them when describing some of the lodes further on.
111
hode Fi1ling.-The filling of the lodes consists in part of (1800mposed cOUlltry rock and clay, but mainly of the following gangue
minerals :-'rourmaline, quartz, carbonates of iron and lime, and orthoclase.
As an accessory: gangue mineral found in small quantities only
white, yellow and mauve fluorspar has to be mentioned. 'fhese gangue
minerals enclose, besides cassiterite as the main metallic mineral, smaller
quantities of iron pyrites, haematite, specular iron, limonite, copper
pyrites, copper glance, malachite, chrysocolla, galena, zince hlende, and,
as a rarity found by panning only, gold.
Gangue ilhnerals.-The most frequent gangue mineral and, in many
places, the only one, is tourmaline. It occurs mostly in a fibrous state,
and very often forms radial fibrous aggregates which, in places, are the
only filling of ,vide veins. In some of the lodes the tourmaline crystals
form, with quartz or iron lime carbonates, a dense felt-like mixture,
which, only under the microscope, reveals its true nature.
Its most
92
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
common colour is dark green to brownish green, but under the microscope, also light and dark blue tinges can be observed.
The dichroism
is most pronounced, pink and green usually being observed. The interstices between the fibrous aggregates are sometimes filled with ore, cassiterite or copper pyrites, but more often with one of the gangue minerals.
It is worth special mention that tourmaline occurs not only in intermixture with quartz, but also withfelspar and iron-calcium carbonate.
Quartz in crystalline white masses is, next to tourmaline, the most
abundant gangue mineral. It occurs in well developed crystals in vughs
of some of the lodes.
Carbonates of Iron and of Calcium have been mentioned before in
connection with other tin deposits, but have usually been described as
secondary minerals.
In the Rooiberg tin lodes we find such carbonates
in many places as the real gangue mineral of the cassiterite, and undoubtedly of contemporaneous origin.
In a few places the carbonate
appeared greenish-yellow, like some varieties of siderite, f,or which it
has been mistaken, but, in most instances, it is pure white, and on
that account had been considered to be calcite, until a rapid discolouration under the action of atmospheric agencies revealed the existence of
iron carbonate to a considerable extent even in the pure wh-ite mineral.
An analysis, which Mr. E. H. Croghan made for me, gave the following
results : 1'10
Insoluble residue (quartz, etc.)
48'13
Protocarbonate of iron
47'93
Carbonate of lime
2'27
Carbonate of magnesia
Alumina
Manganese dioxide, undetermined, but very
small.
Total
per cent.
per cent.
per cent.
per cent.
rrrace.
99'43 per cent.
It will be seen from this that the mineral is to all extent a compound
of protocarbonate of iron and carbonate of lime in even percentage,
corresponding to dolomite as an intermediary between calcite and magnesite. As far as I am aware, an intermediary mineral between siderite
and calcite has not been described as yet, and I propose for this new
mineral the name of sidero-calcite.
In one locality in connection with worked out large pockets of high
class tin ore some pseudomorphs of limonite after this carbonate have
been observed shewing beautifully flat lens-like rhomhohedrolls. As an
additional and most interesting feature of these groups of crystals must
be considered that on some of them crystals of cassiterite are f-ound
which are apparently of later age than the original carbonate.
This
carbonate is to be found apparently in everyone of the tin lodes, and
often in large quantities.
It is, in my opinion, the iron mineral which
has furnished the large quantities of iron now being found as " ouklip
within and beyond the mining area.
ON SOME MINERAL DEPOSITS IN THE ROOIBERG DISTRICT.
93
Orthoclase, mostly pink in colour, rarely white, forms the gangue
mineral in several lodes, at least in part, and is usually ass>ociated with
tourmaline. In most instances, crystals of tourmaline are found disseminated through the crystalline mass of orthoclase, but also distinct
layers of tourmaline between layers of orthoclase without tourmaline can
be observed.
The felspar appears to be usually quite fresh to the
unaided eye, but in places can be seen changing into a yellowish green
sericite. It is doubtful, however, if all the sericite has originated from
orthoclase, and it appears possible, or even likely, that some sericite
found in small irregular masses within the lodes shewing no felspar,
but quartz and carbonate as gangue minerals, has been deposited as
sericite originally.
Cassiterite.-This mineral occurs either by itself, forming crystalline
masses of smaller extent within the lodes or associated with either of the
gangue minerals, forming coarse to fine grained mixtures, or as impregnations in the country rock.
The size of the individual crystals is from
a pea downwards.
Very coarse crystalline masses are uncommon, and
so are the specially fine grained varieties.
As a rule, there is no difficulty whatever in distinguishing the cassiterite crystals by the naked
eye.
Its oolour is uniformly dark-brown to black j under the microscope light yellow, or dark yellow to brownish yellow.
Regarding the other metallic minerals, it may be said that hematite
and iron pyrites are of frequent occurrence, the latter in very coarse
crystals, up to the size of ~in., especially when associated with siderocalcite and tourmaline.
Hematite is undoubtedly in part the product
of decomposition of iron pyrites, but it is probable that in some places
it was deposited as an oxide originally.
Specular Iron Ore has been observed in a few places, and in small
quantities only.
Copper Pyrites oecurs in most lodes in minute quantities, but in a
few instances larger quantities have been met with forming horizontal
layers within a mass of tourmaline.
Associated with such an occurrence
zinc bIende, as a group of indistinct crystals, has been ·found. Galena
has been met with in a few instances only, in the shape of minute crystalline masses within the carbonate gangue.
Country Rock.-Before giving a descripti.on 'of some representative lodes, a few remarks may be made about the country rock in which
the lodes occur.
It has been stated that it is a fine grained quartzite,
and that it forms part of a sedimentary formation of wide extent and
great thickness.
Judging from small specimens only, or even from
microscopical investigations of some special kinds, one might easily be
misled in taking the country rock for an igneous rock of the aplite type.
Even macroscopically the existence of felspar as a main constituent of
the rock besides quartz is evident, and under the microscope the importanceof the felspar is even more apparent. Out of a -large number of
slides examined, only one shewed the constitution of a proper quartzite,
94
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
all others shewed a mosaic of more or less idiomorphic grains of orthoclase and plagioclase, besides fragments of quartz.
But the most regular strike and dip of these layers of rocks over large areas, their interstratification with shales and conglomerates and the existence of ripple
marks on some of the bedding planes, leave no doubt that the country
rock is sedimentary, either a quartzite with felspar as an original con~
stituent, or an arkose or a greywacke.
So far sh~les have been encountered underground in very narrow
and much decomposed layers only, 6in. to 8in. thick, between the hard
quartzite beds.
They are noteworthy, however, as they seem to have
influenced the formation of fissures to a remarkable degree.
The
fissures, when passing from quartzites into these shale beds, lose their
continuity in many places, and the lodes often appear to be cut off or
displaced by these clay seams. It has been proved, however, that the
fissures continue on the other side of these seams within the quartzite,
at least in some places.
I believe that the facts may be interpreted in
the following way, which is illustrated by Plate X. 'rile clay searns,
on account of their elasticity, acted like cushions between the quartzite
heds at the time the fissures were formed.
Below and above them the
rocks were parted uninterruptedly over long distances, but within the
shale beds most of the renting force was lost in friction, and only in
some places and over shorter distances was the stress great enough to tear
the shales and form a connection between the fissures in the quartzite
above and below.
Description of
will now be given.
lodes.-A
description of three types of lodes
Type No. I.-This type is characterised by comparatively narrow
fissures from which an irregular impregnation of the country rock has
taken place.
These impregnations form ore pockets from very small to
very large sizes, and as they are often extremely rich, have been mined
extensively by the ancient miner.
They are most interesting with regard to their structure as well as their mineral contents.
Small pockets
are usually filled with fibrous or pulverulent tourmaline, with or without
cassiterite, and with some iron pyrites crystals intermixed, shewing no
special structure.
But the large pockets shew a most distinct parallel
arrangement of their various minerals, and these bands or layers are
parallel not to the fissure, but to the bedding planes of the wall rock.
The ending of these pockets towards the country rock is often indIstinct,
but sometimes quite sharp, the contact planes in such cases l!>efng spnerlcalor elliptical.
Old working's on such pockets appear now as huge
irregular cavities extending up to twenty and more feet away from the
fissures.
In some cases where the pocket had a sharp oontact with the
country rock, the old workings are exactly like pot-holes in a river bed.
The minerals forming these pockets, apart from the prevalent tourmaline
already mentioned, are mainly orthoclase, sider.a-calcite, cassiterite, copper and iron pyrites. Quartz has been noticed in small quantities only so
far.
Everyone of these minerals occurs in places in almost pure state
ON SOME
MINEl~AL
DEPOSITS IN THE HOOIBERG DISTRICT.
95
in the form of horizontal layers within these pockets, and some of the
richest tin ore so far discovered containing 40 to 50 per cent. met. tin
has been found in such layers. As a rule, however, two or ·three minerals
mixed form the bands or layers mentioned; among them orthoclase,
cassiterite, tourmaline, and iron-lime carbonate; cassiterite and tourmaline are the most frequent mixtures.
In one instance most beautiful
alternating layers of copper pyrites and tourmaline w~th and without
cassiterite have been discovered.
(See Plate XVI., Fig. 1.)
Apart
from these mineral bands considerable portions of some of the large
pockets are formed by rich impregnations of cassiterite int.o the country
rock.
rrhese . impregnations are also layer-like, and are sometimes
directly connected with the mineral layers mentioned.
Type No. 2.-rrhe second type of lode is, in many respects, the counterpart of the first, and is characterised by great regularity in form and
size of the lodes, as well as in the nature of their contents.
Both walls
are often well developed, and the width of the lode is, as a rule, from
2 to 3 feet, with extremes of about one foot and six feet.
The lode
contents are an intimate fine mixture of tourmaline, cassiterite and
quartz, which appears dense to the unaided eye.
Occasionally small
grains of quartz and iron-lime carbonate can be noticed within the dense
mineral mixture, and stringers and veins of either mineral are also of
frequent occurrence.
A peculiarity of this type of lodes is the presence
of a stiff clay with or without tourmaline and cassiterite within the lode.
Small quantities of it are found in most places, but occasionally the
quantities increase considerably, and form a greater portion of the lode.
The cassiterite appears to be distributed through the lode matter, more
or less evenly, and concentrations of it forming ore of similar richness
as that from the pockets described above are not known.
Nevertlleless, rich ordinary ore, containing, say, 15 per cent. metallic tin, has
been found in large quantities.
rrhe regularity of form and contents
of this type make lodes belonging to it most valuable economically, a's a
few of them will form a safe basis for even a large mining enterprise.
Type No. 3.-This type, a mixed type, is best represented by a'lode
called Main North Lode. As it has been developed to greater depth and
over longer distances than any other lode, revealing most interesting
features, a morc detailed description appears justified. (See Plate XI.)
On the surface thi's lode is seen outcropping in several places between
old workings and beyond-one of the very few instances wijiere such outcrops can be noticed.
The outcrops consist of large masses' of tourmaline
and quartz with very little cassiterite.
At a depth of about 30 feet,
where the same lode was struck in a shaft, its character had changed and
orthoclase and tourmaline were the prevalent minerals. Orthoclase,
however, soon gave way to quartz again, and down to a depth of about
one hundred feet, where driving was started, quartz tourmaline and cassiterite with some hematite remained the lode minerals.
'1'he lode which,
at and near the surface, was very poor in tin, increased in value rapidly
lower down, and shewed itself to be an exceptionally high grade ore body.
96
TRANSAC'rIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
Its width on the average was found to be about two feet. The ore appeared "frozen" to the walls, which were not well defined. In driving
westward on the lode, values were found to continue over long distances
at about the same grade, but the physical character of it changed very
considerably; from a compartively simple lode it turned into a brecciated
lode, in which the angular fragments were either pieces of the country
rock or pieces of dense tourmaline-quartz-cassiterite ore, the broken-up
remnants of a pre-existing vein.
The interstices between the pieces were
filled by crystalline masses of quartz, or iron-lime carbonate, or both,
with irregular or string-like arranged aggregates of crystalline cassiterite.
Occasionally the spaces between the fragments of country rock
were not filled completely, the empty cavities being lined with crystals of
quartz.
Further on the character of the lode changed completely, and
turned into a large fissure with one wall well defined, but filled almost
completely with clay. The other wall was most irregular, and vein-like
offshoots following the bedding planes of the country rock for several
feet were of common occurrence.
Some very rich tin are, cassiterite and tourmaline, or tourmaline
only, was found in the clay, but generally the lode was poor when clayey.
Some distance further in, the character changed back into what it was
before it turned into clay, and remained so as far as it has been driven on.
Going eastward, a very decided change took place within fifty feet from
the shaft, the lode changing very suddenly into a small fissure of ironlime carbonate with occasional pockets of the same mineral, and some
tourmaline and cassiterite.
While the changes first mentioned took
place without any apparent cause, the last mentioned change may be
attributed to one of the cross-lodes, the change taking place almost
directly after passing through it.
Considering the great and rapid changes in character of the lode
just described, it appears well possible that type No.1 does not represent
a definite type at all, and that only portions of lode's are represented by
it, and that type No.3, the mixed type, besides type No.2, include all
the existing lodes.
Bedded Impregnations .--Apart from the impregnations olf
tourmaline in quartzite, which are clearly connected with the lode formations just described, numerous tourmaline impregnations are found in
the Rooiherg District, which are confined to one or a few particular beds
of quartzite, and which, on account of the fiat dip of the strata in places,
shew most conspicuous outcrops, acres of land being covered with blocks
of the dark ooloured tourmalille-quartz rock. In localities where the dip
of the beds is steeper, the outcrops of such impregnations can be followed
for hundreds of yards, and can be studied for what they are--bedded
impregnations.
On the farm Hartebeestpoort the impregnated zone is
from one inch to about twenty-four inches wide, and while a narrow
impregnation often changes its position within this width, divides itself
into two or more branches or increases in width, the impregnation doe:;;
not seem to extend beyond the particular strata of quartzite.
In this
case the width of the strata affected by impregnation is not more than
ON SOME MINERAL DEPOSITS IN THE ROOIBBRG DISTRICT.
97
about two feet, but in other places their width is much larger. Besides
such continuous bedded impregnations, most irregularly shaped impregnations, also confined to certain beds, are met with. (See Plate XII.)
In cases where these impregnations are of spherical or cylindrical form,
they appear just like concretions within an igneous rock, and this fact,
besides the other one, that the quartzites consist to a large extent of
grains of felspar, may easily create the impression, a'S it has done in my
own case, that the country rock of the lodes and impregnations were not
a sedimentary formation at all, but an aplite.
Between the impregnations following uninterruptedly the same stratum for long distances, and the most irregular isolated concretion-like
impregnation, all sorts of intermediary stages can be observed, and I
As an
have no doubt that they all owe their origin to the same causes.
extreme case of impregnation must be considered the presence of tourmaline in minute crystals, more or less evenly distributed through certain
beds of quartzite.
These impregnations, though often noticeable near
the lode area, are by no means oonfined to it, but can be noticed miloR
away from any known lode.
Altogether it is most surprising over what
a large area tourmaline impregnations are met with, and to what extent
the district is mineralised. On most of the farms in the district I have
observed them, and I believe that they occur on every farm where the
sedimentary formation is in existence.
Regarding the mineralogical character of these impregnations, it has
to be stated that besides tourma.line, which is the prevalent mineral,
quartz, felspar, and iron-lime carbonate are met with; that cassiterite
is often present, but as a rule in very small quantities only, and that
iron pyrites is of frequent, and copper pyrites, or malachite, of rare
occurrence.
Little has to be said about the structure of these impregnations.
In the case of bedded impregnations proper no special arrangement or
the various minerals can be observed. The tourmaline is in a massive
crystalline state, enclosing small quantities of other minerals. But with
the concretion-like impregnations, a certain concentric arrangement of
minerals can sometimes be observed, the centre consisting of a mixture
of tourmaline ·and quartz, or fe]spar, which is surrounded by a ring of
tourmaline, and this again by a white quartz ring.
Such white quartz
rings are also often seen surrounding very irregular tourmaline pockets,
following their contours in every detail, and having a width of about a
quarter to half an inch.
Special m~mtion must be made of a felspar-tourmaline impregnation
which, on the farm Hartebeestpoort, appears 'to be in connection with a
There an eight feet wide,
bedded impregnation of the ordinary type.
and apparently quite irregular, impregnation of felspar and tourmallne
in the white quartzite was found outcropping oontaining trace's of caSSIterite which, by trenching, was found to change at one side to a narrow
bedded impregnation of the ordinary kind. It was further proved that
the felspar-tourmaline impregnation was, at the same time, the outcrop
of a few vertical fissures, consisting likewise of tourmaline and felspar.
98
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
Small quantities of a yellowish white mica were found in the impregnation where it consisted almost entirely of pink felspar.
It appears scarcely doubtful that the bedded impregnations are only
extreme cases of the mineralised branches which the lodes are sending off
along tho bedding planes.
Such branches are, as a rule, a few feet
only in length, but side offshoots up to 15 feet in length have been observed, and.! in the case of the felspar-tourmaline impregnation on Hartebeestpoort, the bedded impregnation has been followed down even much
further. This leads one to attribute the formation of the bedded and
other impregnations 'to the 'Same causes, which, I believe, have brought
about the existence of the lodes, and the following discussion on the origin
of the lodes will include also that of the impregnations.
Origin of Lodes,
Formatl~on of Pissures .-It is certainly most uncommon to find an
area of sedimentary formation shewing such even undisturbed stratification with a dip often approaching the level and rising rarely over 10
degrees, fi'Ssured to such an extent as it is in this case.
rrIlere are no
intrusions of large bodies of igneous rocks, bending the directly overlying strata, and breaking those further away from it, which could be
connected with the Rooiberg lodes, and neither is there any evidence that
at any time the fissured region might have been the crest of a huge anticline. The only rupture of the sedimentary formation of such an extent
as to cause the fissuring of the strata far away from it is the zone of disturbance which 'Strikes N.E. through the farm Blaauwbank, and which
has been described above.
It is quite possible that the fissures near
that zone, like the copper lode on Vellefontein, or tho nickel lode on
Blaauwbank, to be mentioned further on, are contemporaneous with that
main break, and due to the same causes. But the main centre of fissuring, as far as we know at present, is the area of old workings of the tin
lodes on Hartebeestfontein and Olievenbosch, and thi'S area is about
three miles away from it.
Besides, the lode systems known so far do
not agree in strike with that disloeation. I consider it, therefore, excluded that the fissuring which resulted in the formation of the tin lodes
is connected with the zone of dislocation on Blaauwbank. In looking for
possible causes of the fissuring observed, the theory of torsional movement suggests itself, but, while it has to be admitted that torsional
motion of the strata may have caused all the rupturing we observe, I
have to leave the solution of the problem to the future.
rrhe mere statement that such a possibility exists, without being able to state the causes
of 'Such movement, does not advance our knowledge, and cannot be considered a satisfactory explanation.
It is to be hoped that a more detailed investigation of the geological conditiolls of the district, together
wit.h an increased knowledge of the character of the fissures gained by
additional underground development, will in future allow a satisfa,ctory
explanation of what at present must remain a problem.
ON SOME MINERAL DEPOSITS IN THE ROOIBERG DISTRICT.
99
Regarding the character of the fissures, it has already been established that most of them must have 'been originally mere rents, or cracks,
and that open fissures were the exceptions, and not the rule. Further,
that few, if any, of the fissures were simple ruptures, the majority of
them being connected with parallel or side fissures, f'orming shattered
zones along main ruptures. It has further to be considered that, according to our present knowledge, the fissures belonging to the different systems were oontemporaneous, and that dislocations of any magnitude are
not known.
Slickensides have been met ,,",ith in many places underground, and prove that a certain movement has been going on, but whereever it has been possible to ascertain the amount of it, it has been found
quite insignificant, inches instead of many feet.
In one instance, however, where a wide flat lode, striking east-west, crosses another lode
striking somewhat north of west, that is, under a very acute angle, a
lateral displacement of about 10 feet of the latter seems to have taken
place. It has been impossible so far to ascertain if any movement has
taken place on the dip at the same time.
It has been further established that a l'e-opening or re-rupturing of many fissures has taken place
after they were filled with lode minerals, as the brecciated lodes, including angu'lar fragments of pre-existing lode filling, prove oonclusively.
T-ode P1·ZZing.-r rhe prevalence of impregnations in connection with
many of the fissures, the imperceptible gradation between gangue
minerals and enclosed fragments of the country rock in some instances,
and the existence of a white 'silica ring around fragments of red quartzite
in other instances, together with the fact that hom the physical characte.,r
of the fissures the formation of the ore bodies by the filling of pre-existing cavities~ was in most instances excluded, suggested to me at an early
date that the ore bodies were formed mainly by replacement of the country.rock.
Further investigations confirmed this opinion, and the microscopical examinations of the country rock adjoining the lodes has put
beyond doubt that at least some of the quartzite has been replaced by
the various lode minerals.
The term "impregnation" is often used in the description of ore
deposits without a clear definition of what is meant by it.
It is quite
appropriate to use it for an originally porous country rock which has
its pores filled subsequently by a new mineral, and this is the original
meaning of the term, but I believe in most instances the impregnations
described are not of this kind, but are really replacements of special constituents of ~he country rock by metallic minerals.
The impregnations
which are met with in the Rooiberg lodes are distinctly of the latter
kind, and the chief replacing minerals there are tourmaline and cassiterite.
Better than any description in words will be the microphoto
(Plate XIV., Fig. 1), shewing the replacement of felspar and quartz
grains of the quartzite by tourmaline, convey a proper idea of the
manner in which replacement takes place.
Impregnations in the 'sense mentioned are only different in degree
from replacement-deposits, as the term is generally used.
The former
is confined to a part of the country rock only, but may be found a con-
100
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
siderable distance away from the channel from where the invading
minerals are derived, whereas, in the latter instance, a large part, if not
the whole, of the country rock has been replaced, and the replacement has
taken place in the immediate neighbourhood of the fissures.
Replacement of Felspar by Quartz.-A replacement of this kind is
responsible for the formation of the white bands around the pink
quartzite fragments in the brecciated lodes, or of the walls of the lodes
and irregular pockets.
The country rock has been silicified by the
replacement of both othoclase and plagioclase by quartz.
The mode ,of
replacement appears to be a gradual growth of the original quartz grains
at the cost of the felspar grains. Microphotos (Plate XIII., Figs. 1 and
2) taken from the same slide, shew the ordinary quartzite and the
silicified quartzite. (See also Plate XVI., Fig. 3.)
Replacement of Quartzite by Iron-lime Carbonate.-One of the most
interesting facts in connection with the tin deposits, which I have to
state is the replacement of the quartzite by 'iron-lime carbonate.
Replacement of quartz by calcite or siderite is rare under all ci f·curnstances, but have been described already by Lindgren* in oonnection with
the Coors d'Alene Silver-Lead deposits, and by J. Kuntz and Prof. Youngt
in connection with Rand bankets and quartzites, but I believe it has been
quite unknown up to the present that the same extraordinary replacement
takes place also in connection with tin deposits.
Microphoto (Plate XIII., Figs. 3 and 4) shews clearly hO"Wi the felspar, as well as the quartz grains, have been attacked and totally, or
partially, changed into the carbonate. In other slides the carbonisation
takes place from minute cracks which are filled with carbonate.
(See
Plate XIII., Fig. 5.)
Replacement of Q'uartz by Felspar-Sericite.-An almost still more
remarkable replacement can be observed in the country rock, adjoining
the lodes where orthoclase is the g.angue mineral, and which is to be
studied in the microphoto (Plate XIV., Fig. 2).
It is clear from the
crystalline outlines that the replacing mineral was felspar, at least in
part, and that sericite is mainly a product of alteration of. the felspar.
Paragenesis.-A few remarks on the paragenesis of the lode-forming
minerals may find a place here, which were purposely omitted when
enumerating their minerals.
The massive or brecciated structure of the
lodes is unfavourable for the observatioOn of distinct succession of
minerals if they should exist, and little can be said that is based on
microscopical evidence. In the instances where vughs have been formed,
quartz crystals always line the cavities, having been deposited last. Rut
as the minerals on which these quartz crystals rest are quartz and tourmaline mostly, forming an irregular mixture without any line of demarcation towards the quartz crystals, it appears most likely that the qua'rtz
* Metasomatic processes in fissure veins.
Geo. Soc. S. A ., vol. x., p. 27.
t Trans.
Trans. Am. 1. Mg. Eng., vol. xxxi.
ON SOME MINERAL DEPOSITS IN THE ROOIBERG DISTRICT.
101
does not belong to a different generation at all, but is in both instances
of the same age as the tourmaline.
Tourmaline is in many pla.ces the
only lode mineral, while in other parts of the same lode tourmaline, quartz
and cassiterite, or tourmaline sidero-calcite and cassiterite, or felspar
tourmaline and cassiterite form the lode filling. These oonditions allow one
explanation only, viz., that all these lode forming minerals were deposited
simultaneously.
Special attention has beon paid to the relationship of
quartz and sidero-calcite, as it had to be considered possible that siderocalcite might have replaced the quartz of an older lode-filling.
My
observations, however, make it certain that sidero-calcite is as
much an original ~ode mineral as quartz, and that cassiterite,
tourmaline and sulphides were formed at the same time and
in the same lode, where in other parts quartz, tourmaline and
cassiterite were deposited.
(See Plate XVI., Figs. 2 and 3.) This
holds also true regarding the first lode formations which, by subsequent
re-opening of the 'same fissure, were fra,ctured, and are forming now,
together with pieces of oountry rock, the main bodies of the brecciated
parts of the lodes.
These first fillings are almost homogeneous to the
naked eye, but under the microscope it can be seen that they consist
either of quartz or of sidero-calcite, as the matrix in which large masses
of crystalline tourmaline and cassiterite are imbedded.
The cement
between these fragments of the second lode filling, also consists either of
quartz, or of sidero-calcite, or of a mixture of Loth with cassiterite, tourmaline, etc.
It is further to be stated that the impregnations in the quartzite,
although shewing sometimes tourmaline or quartz, or sidero-calcite, or
cassiterite only, as a rule are formed by two or three of these minerals,
which have been formed 'by replacement at the same time. Although
I have not been able to' prove by microscopic study beyond doubt that all
the lode-forming minerals are contemporaneous, I am now strongly inclined towards this opinion.
When examining the first few slides, I
found often what I considered sure signs of replacement of the first
formed minerals by later ones, as, for example, vein quartz replaced
by tourmaline, cassiterite by tourmaline, tourmaline by sulphides, etc.
In other slides, however, just the reverse replacement could he noticed,
and sometimes both replacements in the same slide, thus shewing that no
replacement whatever had taken place J that these minerals were formed
at the same time, impeding their mutual growth, and each enveloping
parts of the other minerals.
Tho accompanying microphoto (Plate XIV., Fig. 3) may give an
illustration of these conditions.
It certainly seems at first as if a
remarkable pseudomorph of tourmaline, cassiterite and mica had been
formed after felspar, parts of the original felspar being still visible
between the pseudomorphous minerals.
Other parts of the same slides,
and other slides (see Plate XIV., Fig. 4), prove that these minerals also
OCClolr as fillings between the felspar crystals, either single or together,
and the apparent pseudomorph after felspar is nothing else than the
filling of an exceptional large interstice bet" een felspar crystals, both
filling and felspar having been deposited at the same time.
102
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
While in the foregoing instances the various lode minerals were
formed simultaneously by replacement of the country rock, microphoto
(Fig. 6, Plate XIII.) shews quartz, sidero-calacite, cassiterite and tourmaline deposited simultaneously in an open fissure.
Mode of Lode Filling.-It has been customary to attribute the formation of tin deposits to pneumatolytic processes, which were supposed to
have followed, and to be caused by the intrusion of large granite masses.
It follows from the description of the minerals forming the ore deposits,
and from the replacement processes, that in our case such an explanation
is excluded.
Everything points to the same origin, which is common
to most ore deposits, the hydro-thermal origin.
The chemical nature
of the solutions from which ore deposits have been formed has always
been a much disputed subject, and a great deal more has to be known
before a satisfactory explanation of the intricate processes implied can
be given.
This is true as far as the ordinary ore deposits are concerned, but it is more so where minerals come into question, like tourmaline and tin, which were always considered as having been formed by
pneumatolysis. I cannot venture to give even a suggestion of a possible
explanation at present, but have to leave this, with either prohlems, to
the future.
Taking the hydro-thermal origin for granted, the process of lode
formation appears to have gone on in the following way.
The mineralbearing solutions rose through the many fissures to the surface, depositing the lode minerals wherever the spaces left open were wide enough to
allow of such precipitati.on, but mainly depositing them at the same rate
as the solutions dissolved the adjoining country rock, or the fractured
pieces of country rock enclosed between a num11cr of fh;sures.
The
solutions must have been of 'snch a nature as to allow of the formation
of the various gangue minerals in one and the same fissure, the physical
nature of the various portions of a fissure, and surrounding country
rock, besides other unknown causes, determining the precipitation of one
kind of gangue mineral throughout, or of various kinds of gangue
minerals in different parts of the same fissnre. Apart from the more or
less vertical fissures, fissures following a certain bed of the country rock
and also many of the bedding planes, formed part of the system of underground circulation, an'd the solutions acted there in the same way as in
the vertical fissures, changing country rock into mineral bodies.
In
this way I consider that the bedded impregnations and the irregular
nodular impregnations, have originated.
The ore bodies formed along
bedding planes close to vertical fissures, and the peculiar pockets of ore
shewing an arrangement of their mineral contents in bands parallel to
the bedding planes, i.e., more or less horizontal, represent an intermediatei:?tage between the first two types. (See Plate XVI., Fig. 1.)
Origin of the Ore-forming Solutions.,-On account of the usual connection of tin deposits with granite eruptions, an eager search was made
for the granite which could have caused the Rooiberg tin deposits. rrhe
search, however, proved futile, and it has to be admitted that there is
no granite in existence, either as boss or dyke, within less than four or
ON SOME MINERAL DEPOSITS IN THE ROOIBERG DISTRIC'f.
10~
five miles fr.om the centre of the lode district-from the mining area on
Hartebeestfontein and Olievenbosch.
Considering, however, the fact that
the red granite is underlying the sedimentary formation east, west and
south of the lode area, the conclusion seems justified that these granite
masses are connected with each other ,and that' granite is underlying the
lode area in some unknown depth.
Taking these conclusions for
granted. I believe that the tin occurrence on Quaggafontein, the oopper
lode on Vellefontein and the tin lodes on Hartebeestfolltein and Olievenbosch are dosely connected, and represent different phases only of one
process, viz., that of the formation of pegmatite veins.
The Quaggafontein seems to represent the incipient beginning of such a formation
within the granite itself, the Vellefontein lode, a pegmatite vein, a distance away from the granite, where quartz and felspar are often still
precipitated together, and the Rooiberg lodes, still further away from the
granite, where quartz and felspar are deposited apart from each other.
Some contact phenomena observed on HarteLeestpoort, where the
granite intrudes into the quartzites, appears to strengthen this theory.
There, about 20 feet above the contact, vein and lens-like mineral bodies
consisting mostly of quartz and felspar, mostly separate, but sometimes
also together in coarse pegmatite intergr{l'wth, are seen in the quartzite
approximately parallel to the bedding planes.
There are besides some
small tourmaline impregnations to be noticed.
Nearer to the granIte
a pegmatite vein is in existence between the normal quartzite, and a finegrained quartzite much affected by the intruding granite.
It is evident
that both kinds are pegmatitic offshoots of the main granite mass,
although just at the contact the granite does not shew a pegmatitic
structure.
(See Plate VII., Fig. 3.)
In speaking -of the copper and tin lodes as pegmatite veins, I, of
course, use the term in a wider sense than it is used in petrography. But
as I believe that many of the pegmatite veins proper have been formed
from aqueous solutions, and that pegmatite veins m~y gradually change
into mineral veins, I consider it permissible to use the term also for
mineral veins proper, which, although very different in structure as well
as mineral contents, are likely to be connected with ordinary pegmatite
veins, and owe their existence to the same causes.
'l'ourmalinisation.-One of the difficulties which has to be explained
away by accepting the theory mentioned is the fact that all the lodes,
as well as the impregnations, shew tourmaline as their chief mineral, and
in large masses. It could he concluded from this that the igneous rock
which is the source of the tourmaline, would also shew tourmaline freely.
The fact, however, seems to be that the granite near the quartzite and
felsite contact is not tourmaline bearing at all: tourmaline is riot in
existence in the tin area on Quaggafontein, and it has been observed near
the contact on Hartebeestpoort only in the form of small impregnations
in the quartzite, although the latter is only a few hundred yards away
from the bedded impregnations and felspar-tourmaline impregnation
described above.
The difficulty could be got over by assuming that certain areas only of the granite were tourmaline bearing, and that these
104
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
area were confined to the immediate neighbourhood of the lodes and
impregnations, and, therefore, not observable.
But there are also other
facts which makes the theory advanced less plausible than it might appeaI
at first. Even granted that the granite is tourmaline bearing, it has
to be taken into account also that tourmaline is not confined to granite,
but occurs also in connection with felsite, and even with a dolerite dyke.
On Weynek, near the boundary of Hartebeestfontein, a felsite dyke
occurs about two feet in width, and, near by, a narrower dyke of about
four inches, presumably a branch of the former. While the larger dyke
shews no special features; the narrower branch shews near its ending a
strong tourmalinisation. A similar dyke, which penetrates the quartzite
within the area of the old tin workings, shews no signs of tourumliuisation, neither has tourmalinisation been observed with the large felsite
masses of the Rooiberg and Elandsberg. The tourmalinisation of the felsite
dyke on Weynek is at present an isolated occurrence, and though it proves
that tourmalinisation has taken place in connection with the eruption,
or intrusion, of igneous rocks other than granite, it is scarcely admissible
on· that account to bring the formation of the ore deposits in direct connection with eruptions of the felsite.
A more remarkable occurrence even is the following.
In connection
with a narrow dolerite dyke on Onverwacht, a strong tourmalinisation
of the quartzite on both sides has taken place. rrhe microphotos (Plate
XIV., Figs. [) and 6), of the dyke and tourmalinised quartzite, shew no
relationship whatever between them. There is no t·ourmaline in the dolerite and the impregnations, consistiug of tourmaline and quartz only,
shew no signs of the adjoining dyke. Can it be that the coutact planes
between dyke and quartzite formed channels like the fissures in the
mining area for the flow of the solutions carrying the tourmaline-forming
compounds and depositing that mineral by replacement of the quartzite 1
It certainly does not seem likely that the dolerite magma could have
contained all the elements for the formation of tourmaline, and have
acted on the wall rock to so remarkable a degree, changing it to a tourmaline quartz rock without shewing a trace of tourmaline within the
dyke itself. (See Plate XV.)
I am looking f{}rward to the evidence forthcoming when igneous
rO(~ks are met by underground workings, as I believe that such evidence
is likely to give us valuable hints towards a satisfactory explanation of
the problems inv<>lved.
'Pourmaline-'l.lin-Cobalt Lodes.-About a mile south of the tin lodes
described another group of lodes has been -discovered, which, although
closely related to the former generally, have a certain number {}f features
{}f their own deserving special mention.
These lodes, too, have been formed mainly by replacement from narrow fissures, and are filled by tourmaline as the prevalent gangue mineral.
Besides tourmaline, felspar is often noticeable, whereas quartz and carbonates are rare, and met with in small quantities only.
The tin contents are low~ and the cassiterite not visible to the naked eye, but the
presence of a few old workings make it very probable that high grade tin
ore also occurs in these lodes.
As remarkable features of these lodes
ON SOlllE MINERAL DEPOSITS IN THE ROOIBERG DISTRICT.
105
have to be mentioned the occurrence of wolfram and cobalt ores, both of
which are unknown in the main group of lodes.
The wolfram ore is
generally scheelite, hut wolframite has also been met with. Remnants of
the latter have been observed within the scheelite, thus proving that
wolframite is the parent mineral of scheelite.
The scheelite,
when fresh, is greyish white, but whenever encountered so far,
that is near the surface, it appears covered with a green
product of decomposition which gives the reaction for copper
and tungsten, and is the rare mineral cupro-tungstate of lime or
cupro-scheelite.
The scheelite itself gives the reaction for copper, and
so does the wolframite.
In one place an intimate connection of oopper
carbonates and wolframite can be observed, the carbonates surrounding
the wolframite.
In this case, however, the copper has not derived from
the wolframite, but from another copper ore! most likely copper pyrites,
which, however, has not yet been met with.
The~e is also cuprite and
native oopper to be noticed as products of alteration of the same oopper
mineral.
The other remarkable occurrence is the occurrence of cobaltite in
irregular pockets within the mass of crystalline tourmaline.
The ee"
existence of cobalt ore and tourmaline in the same lode has been mentioued by 0. Stutzer as occurring in Chili*, but it is undoubtedly very
rare.
In another lode cobalt-bearing arsenical pyrites within a tourmaline quartz gangue has been observed.
Occurrence of Nickel Ore on Blaauwbank.-A very remarkable
nickel l,ode has been discovered on the farm Blaauwbank. It occurs near
the zone of dislocation mentioned above, and is a flat dipping fissure
from a few inches to six feet in width, which carries as gangue iron-lime
carbonate, and, as metallic minerals, gersdorphite and niccolite or copper
nickel (Ni. As.) in solid masses.
Near the surface the gangue has been
decomposed to limonite, and the nickel ores to nickel bloom (annabergite).
'rhis mineral, however, does not occur as a film on the undeoomposed
arsenical minerals, as is usually the fashion, but as a solid body, up to
two feet wide, the whole of the original nickel ore having been changed
into nickel bloom.
rrhe occurrence of this mineral in bulk appeared so
remarkable that an analysis was made giving the following result:Hygroscopic moisture
Water of constitution
Quartz sand
Ferric oxide
Protoxide of nickel, Ni
Arsenic pentoxide, AS 2 0 5
Calcium oxide
°
0·60
19·88
0·91
1·20
37·25
39·87
Small
per cent.
per cent.
per cent.
per cent.
per cent.
per cent.
trace.
27·67 per cent. Ni.
99·71 per cent.
Total
thus shewing that it
satisfies almost exactly the requirements of the
* Zeitschr. f. pract.. Geol. 1906, p. 294
106
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
theoretical foOrmula, Ni 3 As 2 • Os + 8 H 2 0. By panning a little free
gold was discovered in the nickel bloom corresponding to about 3 or 4
dwts. to the ton. A fire assay, made on account of the free gold, shewed,
however, remarkably high gold contents, aoout three ounces to the ton.
I am at present noOt able to say in what state the gold occurs which is not
shewing in panning. rrhe disoovery of this nickel lode is of quite recer t
date, and not enough work has been done on it oOr in its neighbourhood
tA) allow a conclusion as to its origin, and it must lemain doubtful at
present whether this lode is in any way connected with the tourmaline
(~ohalt lodes, or tourmaline-tin lodes further west.
It appears possible,
however, that such a connection exists, as a number of lodes of tourmaline with some cassiterite have been discovered in the close neighbourhood
of the nickel lode.