FLUORSPAR, LEAD AND ZINC DEPOSITS OF THE WESTERN TRANSVAAL.
THE FLUOHSPAH, LEAD AND
"\V ESTERN
Znw
5
DEPOSITS OF THE
T HANS V AAL.
•
(Read 24th January, 1927.)
By W. l(upferburger,
M.Sc.
INTRODUCTION.
Location.-The area containing these deposits lies in the southwestern portio>n of the MaricO' District in the \Vestern r:rransvaa1. It
is about 150 miles west of Johannesburg, and not far from the towns
of Zeerust and Ottoshoop.
As far as communications go, it is served by the MafekingJohannesburg railway line, and ' by fairly good trunk roads between
the towns Maieking, Zee'r ust and Lichtenburg, and by the road from
Zeerust to Johannesburg. Of historic interest is the so-called Jameson
Road, which passes right thro>ugh the mineralised area., being the route
followed by J a.llleSOn in entering the Transvaal in his raid on the
Republic in 1896. (Figs. 1 and 2.)
Topography.-Topographically the a,rea can be divided into two
portions: a southern one vvhich is a generally flat or undulating
region and is pa,rt of the great roughly triangular pla.teau formed by
the Dolomite Series of the rrransvaal System, the importance of which
as a water bearing forma,t ion · is seen in the streams which have their
sources on all sides a.r ound it.
r:ehis has been fully described by
Harger l in a, paper to the S.A. Geographical Society.
Except for the lVla,lmani River flowing fro>m lVIalmani Oog (101)
N.N.\Vestwards, and the Moiopol River flowing from Molopos Oog
westwards there is no surface drainage, which, however, is typical of
do>lomite and limestone regions generally. Both the Molopo and Malmani Rivers issue from fairly strong springs or "eyes," and then
continue a.1ong the surface in a series of marshes.
The
" eyes" have been explained by Humphrey 2 as due to the damming
up o>f the underground water circulation by one of a, series 0'£ quartz
reefs running in as. S. W. direction through the dolomite. The flow
o>f the ]Vlalmani River has evidently varied considerably, and the history
of the stream has been described by Humphrey in his report on the
Geology of the S .W . portion of the l\fa,rico District, and also in the
explana,tory pa.mphlet to the Ttansvaal Geological Survey Maps,
1 "Underground
Erosion in the S.-W. Transvaal Dolomite."
Geographical Journal, Vol. V., pp. 55-65.
2 Annual Report Geological Survey. T1'onsvaal, 1908, p. 143.
S.A.
6
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
Sheets 5 and 6, by A. L. Hall and W. A. Humphrey. The country
occupied by the Dolomite Series is generaUy fiat, but broken here and
there by low ridges and kopjes capped by chert. A good example of
the latter is the low conical hill carrying the trigonometrical survey
beacon at the meeting point of the three farms Witkop, Kaalplaats
and Doornplaats. Low chert-ca,p ped ridge:;; occur in many places,
notably Ka,a lplaats, Na,auwpoort, Strydfontein, and further eastwards.
I
'.' . '.
·.B:.~·
Pr~t(J)"la
Dolomite
SerieS} Trans- III!1IDJ
','
Black R~ef"
,
vaal
SYS,t:
m1J
Noritej Bushveld Igneous
Complex
Venteradorp System.
~
Granlt~
Scale - I: IpOOPOO
FIG. I.
Over the greater part the area is grass veld relieved in places by
clusters of trees and bush on the slopes of the ridges. Sometimes the
trees and bush run in long straight lines of about twenty to thirty feet
across for several hundreds of yards, two such lines of bush and trees
occasionally intersecting each other. This is speciaUy noticeable on
parts of Na,auwpoort, Malmani Oog and Strydfontein. The explanation of this feature is probably that these lines ma.rk joint planes along
which there has been greater decomposition of rock into soil than
elsewhere, and also tha,t along these lines there might be some
.circulation of undergTound water.
7
FLUORSPAR, LEAD AND ZINC DEPOSITS OF 'l.'HE WESTERN TRANSVAAL.
'.
..
.
~
'. . .
.'
.
,;
i
I-DO
8
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
Topographically the northern portion, i. e., that albng the conta.ct
of the dolomite and the Pretoria Series, and also on the la,tter forrnation, is much more diversified. A number of small streams issue from
the dolomite, very often from strong "eyes," and run northwards over,
the rocks of the Pretoria Series to give rise eventually to the Little Marico
River. 'rhe Groot lVIa,r ico River has its source in a simila,r manner in
the extreme western part of the area from strong" eyes " on Bokkraal,
Rhenosterhoek or Grootfontein. Erosion on the uppermost beds of
the dolomite, including a thick band of chert, and on the shales and
quartzite of the lower Pretoria Ser'ies has produced much more hilly
country. The slopes of the hills are usually well covered with bush
which include v.arious varieties of acacia, stinkhoutboom, oliewenhout,
taaibos, etc. This region forms a distinct contrast to the southern
region with its level grassland, and whereas the southern plateau
region merges eastwards into the Highveld region of the Transvaal,
the northern region corresponds to what is known as the Middleveld,
and further northwards beyond Zeerust merges into the true Bushveld.
HISTORY OF MINING ACTIVITIES.
Gold.-The district attracted a great deal of prospecting and a
certain amount of mining in the early days before the Witwatersrand
gold fields had been discovered and opened up. This activity was due
to the occurrence of a number of gold-bearing quartz reefs cutting
through the dolomite in a N.N.W.-S.S.E. direction in the neighbourhood of Ottoshoop (Fig. 2). The history of the Malmani gold field, as
it was called, from the lYlalmani Hiver running close to the reefs and
intersecting them in places, dates back to 1875. Shortly afterwards
at one time over 2,000 miners were working on the field; whereas in
1908 the numbers could be counted on one hand, and the output for
that year was only 50'417 ounces of fine gold valued at £214.3 The
total production of the field must have been quite considerable, as the
upper portions of the reef are considered to have been of fairly high
grade, but unfortunately, no definite records of the output are available.
The reefs, however, were eventually abandoned on account of (1) the
diminishing of values below water level; (2) The difficulty of coping
'with the large quantities of water met with in the dolomite; (3) the
discovery and opening up of other richer and more extensive gold
deposits elsewhere, e.g., the De Kaap Valley and the \Vitwatersrand.
The reefs are usually narrow and almost vertically inclined quartz
reefs often accompanied by calcite and siderite, and in places carrying
visible gold. It is, however, not the purpose of the present paper to
describe these veins, and they will, therefore, be passed over for the
present.
.
Lead and Zinc.-Occurrences of lead and zinc have been known
for a considerable time in this district, and according to Humphrey,
3Humphrey.
Gp. cit., page 160.
FLUOl~SPAR, LEAD AND ZINC DEPOSITS OF THE WESTERN TRANSVAAL.
9
the most authentic account of their discovery places the> time in the
early _'seventies when John Volschenk accidentally discovered galena
on the Farm Rhenosterhoek 211. Subsequently lead was discovered
on some of the ,adjoining farms, and galena was mined at what is now
called Old Lead Mine on Bhenosterhoek 211, on the abovementioned
farm. The ore was smelted on the farm, and the lead was chiefly
used for the manufacture of bullets. rrhe production, however, could
only have' been small, and the work was subsequently stopped on
account of the remoteness of the district and the attendant transport
difficulties.
\Vith the opening of the railway from Krugersdorp to Zeerust in
1907 interest was revived in this district.
Active prospecting took
place, with the result that lead and zinc was discovered on a further
number of farms, and a certain amount of galena, has since been
produced intermittently from deposits on Bhenosterhoek 211, Doornhoek 32, and a few other farms (see page 39), and zincblende from
\Vitkop 288.
FluoTspaT.--The principal occurrences of fluorspar are those on
Buffelshoek, \Vitkop and Malmani Oog, and were known for a long
time before its commercial value was realised, and before active
exploitation commenced in 1917. It had been used for a long time by
the farmers in the neighbourhood as a decorative covering for graves.
r:rhey obtained the fluorspar by digging and scraping shallow pits into
the outcrops. Subsequently, fluorspar was discovered on a number of
other farms, such as Kaalplaats, Strydfontein, Bhenosterfontein, etc.,
and large quantities of this mineral are being obtained from this district.
r:rhe production of fluorspar to date from this district has been in the
neighbourhood of 40,000 tons, the greater portion of which has been
obtained from two large quarries, one on Malmani Oog, and the other
on Buffelshoek. Quantities totalling perhaps a few hundred tons or
less have been produced from some of the other farms previously
mentioned. The whole of the output, with the exception of a few
tons per annum for local consumption, is e~ported to U.S.A.
(For the above particulars the writer is indebted to Messrs. B.
Owen-Jones, Limited, Southern Life Buildings, Johannesburg, the
brokers handling the exportation of this mineral.)
Concerning mining activities at present, it appears, therefore, from
the foregoing statements, that mining for gold and zinc has come to a
standstill, and that lead mining is stiill carried on to a small extent.
r:rhe discovery and development of the fluorspar deposits, however, has
given a decided filljp to the interest in the district, and during the years
1924 to 1926 two large quarries were being worked and prospecting was
being carried out on a large number of farms.
The fact that the production and export of fluorspar from the
Ottoshoop area had assumed such considerable proportions, as well as
the rather unusual mode of occurrence of the mineral, caused the
writer, at the suggestion of Dr. P. A. \Vagner, of the Geological Survey
of South Africa, to undertake a geological investigation of the area.
10
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
Eive weeks of the summer of 1924-1925, and a further three weeks
in the summer of 1925-1926, were spent in the field studying the various
deposits. The following farms were visited, and the descriptions given,
and conclusions arrived at in this paper, axe based on the mineral
occurrences developed on the farms mentioned.
\Velgedacht ...
l\lalmani Oog..
Doornhoek.. '"
Witkop ... ...
Zeekoevallei ...
Paardevallei ...
Karreebosch ...
Naauwpoort ...
Rhenosterfontein..
(100)
(101)
(32)
(288)
(81)
(62)
(108)
(102)
(50)
'Buffelshoek ...
Stinkhoudboom
Hhenosterhoek
Kaalplaats
Strydfontein .. .
Winterhoek .. .
Doornplaat .. .
Zendlingspost
Bokkraal ... . ..
(284)
(269)
(211)
(97)
(267)
(287)
(99)
(268)
(300)
In carrying out the work, the writer's thap.ks are due to Messrs.
J. G. Gubbins, F. T. Blane and J. J. McArthur, for their hospitality
and kindness in many respects, as well as for such useful information.
The writer also wishes to thank Drs. R. B. Young and P. A. Wagner
for many helpful suggestions, to the authorities ,of the University of
the Witwatersrand for the use of apparatus and other facilities in
pursuing this work, and to the Research Grant Board for a monetary
grant towards travelling and field expenses.
GEOLOGICAL STRUCTURE.
The rocks of the Transvaal System of this area form part of the
S.\V. portion of the great syncline containing the rocl{s of the Bushveld
Igneous Complex (Fig. 6). To the N. \V., in the direction of Moilo' s
IJocation, the outcrop of the Dolomite Series is about seven or eight
miles in width with dips of low angles to the east. The rocks of the
overlying Pretoria Series have a similar structure. To the south of
Zeerust, however, one finds minor folding with axes oblique to the
margin of the syncline. This folding is seen best in the rocks of the
Pretoria Series south of Zeerust, but it must have affected the underlying rocks of the Dolomite Series as well, for the dolomite outcrop
widens to over thirty miles and extends in a southerly direction to
just beyond the town of Lichtenburg (Figs. 1 and 2). Over the
greater part of the area occupied by the dolomite the surface is very
flat, and from the surface outcrops alone it is a difficult matter to
. make out the geological structure of the formation as a whole. The
interbedded chert bands afford a clue to the direction of the dip, but
determination of the structure is complicated by the occurrence of
so many bands that are obviously replacements along joint planes and
occur in an altogether haphazard fashion through the dolomite.
One set of such parallel chert bands might be taken as indicating
the stratification, and then a few yards further another set of similar
chert bands may be seen inclined at quite a different direction, and
FLUORSPAR, LEAD AND ZINC DEPOSITS OF THE WESTERN TRANSVAAL.
11
perhaps intersecting bands of the previously mentioned set. \Vhile
no persistent dips could be made out, the general impression gained
from a few exposures, such as the railway cutting near Ottoshoop,
and a few outcrops along the l\1almani River, and from the fluorspar
quarries on Malmani Oog, Buffelshoek and elsewhere, is that the strata
are disposed in gentle folds with a general dip to the N.E.
STRATIGRAPHICAL GEOLOGY.
The area under discussion is shown in the shaded portion of
Map (Fig. 1). It is underlain almost entirely by the rocks of the
Dolomite Series, and in the N.E. portion by the rocks of the Pretoria
Series of the Transvaal System. The Black Reef Series underlying
the dolomite comes to the surface some distance to the west of the
area in question, and will, therefore, not come up for discussion.
Quite considerable areas of the dolomite are covered by superficial
deposits of calcareous tufa, and gravel. The formations may be tabulated a.s follows:Surface deposits ...
r Pretoria
Transvaal System:
Series
1 Dolomite Series
Calcareous tufa. and gravel.
Shales .and Qua,rtzites.
Dolomite and Chert.
Surface Deposits.-Calcareous Tufa of the Sheet Limestone type
covers considerable portions of the area in the western part· between
Ottoshoop and Mafeking on the Farms Karreelaagt,e 51, \Velgedacht
100, Karreebosch 108. Smaller isola.ted areas of Vlei Limestone
occurs on Stinkhoutboom 96, Kaalplaats 97, Buffelshoek 284. These
occurrences have been described in detail by \V. Wybergh 4 in his
memoir on the Limestone Resources of the Union of South Africa.,
In plflrces, notably on Karreelaagte 51, gravel beds up to four feet
thick, and composed chiefly of blue and white chert pebbles and
rounded brown ferruginous nodules, were noticed.
Pretoria Series.- In this area only the very lowest portion of
this series of rocks is met with. The base of the Pretoria Series is
usually taken as the layer of Chert Conglomerate found overlying the
Dolomite Series. This rock has been variously described as a Chert
Conglomerate and a Chert Breccia.
According to the writer's
experience, the latter term is the more suitable, at any rate for the
rock a.s found in this portion of the Western rrr.ansvaal. It consists
mostly of decidely angular fragments; the average size of a fragment
appears to be about one inch in length, but angular slabs and
fragments several inches in length, even up to a foot, have been
4
Geological S1trvey Memoir II., Vol. 1., Chapter VII1., pp. 61-75.
12
TRANSACTIONS OF 'l'IIE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
noticed. Some fragments are slightly rounded or sub-angular. rThe
chert of the fragments is of three kinds: dark bluish grey, white and
white with fine parallel grey markings. All three are types of chert
which are fDund in place in the DolDmite Series below. rThe fragments
are closely pa,cked together, and there is very little matrix compareo.
to chert fragments. 'The matrix is also siliceous, often with impurities
giving it a grey cDlouration. Frequently small cubes of limonite, or
cubical cavities which presumably contained limonite, are to be seen,
and in places in great abundance. Sometimes, also, in the matrix
small drusy cavities are noticed, partly filled up with chalcedony
and opal. This Chert Breccia varies considerably in thickness, ,and
in places is missing altogether, although it is fairly persistent over
the whDle area: on Bokkraal 300 it is about 30 feet thick, and is
underlain by a band of grit about two feet thick, the fragments of the
grit being well rounded and abDut l/lOth to 1/30th of an inch in
dia,meter, and also composed of chert. The siliceous matrix enclosing
them is almDst indistinguishable from the rounded grains, except that
it is of slightly finer grain.
The .occurrence of this chert, breccia, indicates, as suggested by
Hall,5 the existence of a slight, though fairly persistent, unconformity
between the DolDmite Series and the Pretoria Series. On the tDP of
the Dolomite Series there occurs a fairly massive band of chert. It
is conceivable that the exposure to air and weathering of such a rock
would give rise to angular or slightly rounded fragments, by disintegration in situ such as are found in the chert breccia above. Furthermore, where there is a cDnsiderable thickness .of this chert breccia,
the underlying massive chert bed is relatively thin, as, for instance,
on Bokkraal 300.: whereas, further westwards, on Zendelingspost 268
and Buffelshoek 284 and \Vinterhoek 287, where the massive chert is
of considerable thickness, the chert breccia overlying it is .only a few
feet thick, or entirely absent, so that· the shales of the Pretoria
Series follow directly .on the massive chert. 'The chalcedonic matrix
and the drusy cavities in the rock also lend support to this view.
Outcrops .of the dolDmite and chert at present are covered by a cherty
rubble very similar in charaeter, and the tendency of the chert to
break into slabs and angular fragments can be well seen in many
IDcalities. A specimen .of this rock in the Geological collection of the
University of the Witwatersrand, and labelled Western Transvaal,
shows distinct round water.-worn chert pebbles, an inch to two inches
in diameter. Local .occurrences of the rock with water-worn pebbles
would be expected, and probably mark channels alDng which there
had boen actual transportation and attrition .of the chert fragments.
At the present time the stream CDurses cutting through the upper
chert contains similar rounded pebbles of chert.
5 The
Geology of th€i Country between Zeerust and Swartruggens.
Ann. Report, Geol. Survey, Transvaal, 1908, p. 129.
FLUOHSPAR, LEAD AND ZINC DEPOSITS OF THE WESTERN TRANSVAAL.
13
Nothing approaching the rock reeently described by \Vagner 6 as
pseudn-conglomerate occurring in similar cherty rocks in the Central
Transvaal has been noticed in this area.
Following on the top of the chert breccia one finds the shales of
the Pretoria Series.
They are usually red weathering ferruginous
shales, well laminated and occasionally with bands rich in magnetite.
The further west one goes in the area the less ferruginous and more
sandy do these overlying shales a,ppear to become.
On the Farm
Zendelingspost 268, a few bands of oxidised asbestos, presumably
crocidolite, were seen in some t,renches below the railway line in the
banded ferruginous shales overlying the upper chert of the Dolomite
Series. The lower quartzites of the Timeball Hill Beds occur throughout the area a short distance above the shales, and give rise to
characteristic escarpments facing south and south-west.
Dolomite' Series .-As elsewhere in the Transvaal, this series is
made up chiefly of dolomite and chert. In places in this area it also
contains layers of quartzites and thin beds of shales. In places the
uppermost layer of the series is massive white chert up to 30 feet in
thickness. It is well developed in the western portion of the area,
but diminishes in thickness eastwards.
Where the massive chert
layer does not occur there is a gradual passage from the dolomite into
the shales of the Pretoria Series. In the upper portion of the Dolomite
Series thin shale bands are found intercalated with the dolomite, foreshadowing the change in the character of the sedimentation from
calcareous to argillaceous.
The thickness of the Dolomite Series has been estimated, and is
generally accepted, as 3,500 feet.7 In some plaees it is even thicker,
and in t,he Potchefstroom District Mellor has computed its thickness
as 5,500 feet. 8 In this area the dolomite is undoubtedly that of the
uppermost 1,000 or 1,500 feet.
Wherever the Dolomite Series is
found in the Transvaal, it has been recognised that chert and other
non-calcareous layers are more common in the upper portion of the
series.
Here, also, siliceous rocks are found abundantly in the
dolomite.
As far south as the Malmani River, near its source on
Malmani Oog, well-defined quartzite layers are found interbedded
with the dolomite, and dipping at about five degrees N.N.E. These
quartzites appear to be lenticular intercalations rat,her than uniformly
interbedded layers, as they cannot be followed for .any great distances
along the direction of their strike. It is, nevertheless, true that
qua.rtzitic rocks and chert are well developed along certain horizons,
'1'mnsactions, Geol. Soc. S.A., Vol. XXIX., 1926, pp. 47-58.
"Geological Structure of t11e Union," A. "V. Rogers. Explanation to
Geological Map of the Union of S.A. Geol. Survey of S.A.
8 E.
T. :Mellor, "Geology of Central Portion of The Potchefstroom
·District." A nn11al Report, Geological Survey of the Transvaal, 1907.
6
7
14
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
separated by varying thicknesses of dolomite. Similar quartzitic beds
are found further to the north, and apparently at a higher horizon.
One prominent horizon of these quartzite and chert beds forms a series
of low ridges and kopjes, and can be traced from Stinkhoutboom 269
through Windheuvel 216, Stinkhoutboom 96, Doornplaat 99, Kaalplaats 97, Halmani Oog 101, Wonderfontein 78, Christina's Home 329.
Another horizon, inside the first, further north and closer to the
contact between the Dolomite and Pretoria Series, can be traced along
Stinkhoutboom 268, Buffelshoek 284, Witkop 288, Strydfontein 267,
Witrand 317, Doornplaats 320. The upper portion of the Dolomite
Series of this area is, therefore, very similar to that of the Eastern
Transvaal, as described by Von Dess,auer,9 Hall 10 and others: really
a complex series of dolomite, chert, quartzite and shale, and not one
of uniform monotony.
Dolomite.-The dolomite itself is the typical fine-grained rock,
blue OIn fresh surfaces and greyish-brown on weathered surfaces.
It weathers with char'acteristic "elephant-skin" surface over
generally rounded outcrops. In the upper portionO'f this series the
dolomite is distinctly carbonaceous and darker in colour than is
generally the case. Powdered and hea,ted in a closed tube, it gave
off inflammable gases, and a tarry sublimate settled on the cold part
of the tube. ' In thin section the rock is seen to be crowded with
inclusions of some indefinite, finely-divided black substance, shiny
and like gra,phite when viewed by reflect.ed light. Occurring underneath sUITa,ce limestone, the dolomite exposed in the lime pits on
Ka,rreela,a.gte 51 is much lighter in colour and coa,rser in grain, and
probably belongs to' a, lower horizon in the series.
The mck is practically a normal dolomite, but is often siliceous
due to the occurrence of quartz as a metasomatic replacement of th€
dolomite. (See analyses given hereunder.)
A. Average sample of dolomite from quarry of White Limes,
Limited, opposite OUoshoop Railway Station, on Zeekoevallei
81. Analysis by Heyman's Laboratories, Johannesburg (by
courtesy of Messrs. White Limes, Limited).
B. Odd sample of dolomite from same locality as A. Analysis
by 1. B. McCrae, University of \Vitwatersrand.
C. "Knox Dolomite," Morrisville, Alaba,ma, U. S.A. Analysis by
W. F. Hillebrand. Given by F. W. Clarke, in the" Data of
Geochemistry" as tha,t of a typical dolomite.
D. Dark brown pellets from surface of dolomite. Analysis by
H. G. Weall.
9 A. Von Dessauer, "Dolomites of Pilgrims Rest."
Trans., Geol. Soc. of
S.A., Vol. XII., 1909, p. 78.
10 A. L. Hall, "Geology of the Pilgrims Rest Gold Mining District."
Tvl. Geol. Survey Memoir!, No.5, p. 64.
FLUORSPAR, LEAD AND ZINC DEPOSITS OF 'rHE WESTERN TRANSVAAL.
Si0 2
...
... ...
Fe,O, ......... }
PeO .............
A1 2 0 3 . . . . . . . . .
MnO .........
CaO ... ... ... ...
lVlgO ... ... . .. . ..
CO 2 ... ... ...
80 2 ... ... ... ...
P205 ... ... ... ...
Loss on ignition
Nloisture ... ...
r
A.
1'4
1'70
l 0'12
1·10
30·01
20·12
45·53
trace
99·98
B.
11'78
C.
3·24
0·23
D.
19·1
33'3
0'17
10·3
21·6
3'9
1'05
15
1·68
0'01
27'78
18·78
41'0
trace
0·04
101'08
29·58
20'84
45·54
trace
0'30
1'05
2·65
99·90
92'95
Analysis A approaches very closely to that of ,a typical dolomite,
whereas B is distinctly high in silica,. That is explained by the
presence of quartz in the rock replacing the dolomite as can be seen
in thin section.
Dolomite surfaces are generally poorly covered with soil, but in
places pockets extending for several feet downwards from the surface
are found filled with a finely-divided dark brown earth. The fact that
the weathering of the dolomite takes place chiefly by solution is borne
out by the almost entire absence on the surfaces of detrital material
made up of dolomite fragments.
As elsewhere on the dolomite
surfaces the detritus consists chiefly of chert fragments. On flat
surfaces of the dolomite, however, one invariably finds an accumulation of dark chocolate-brown nodules and pellets. The larger nodules
often have botryoidal surfnces, whereas the smaller pellets are smooth,
shiny, and roughly spherical. The nodules and pellets, as well as the
brown earth mentioned above, are undoubtedly composed of the
residual insoluble matter of the dolomite left after the calcium and
magnesium carbonates have been removed by solution. According to
the analyses aforementioned, this residual matter consists chiefly of
silica, alumina and iron and manganese oxides or hydroxides. The
shape of the nodules and pellets suggests that the material composing
them had first been dissolved and then precipitated as colloids.
Replacement of the dolomite by silica as noted by Youngll in
the dolomite from Griqualand West, has been frequently noticed in
this area. Thin sections of apparently normal dolomite frequently
revealed grains of secondary quartz which have metasomatic ally
replaced the dolomite. In a thin section of a fine-grained dark-blue
dolomite quarried at the White Limes Quarry, small irregular patches
11 R. B. Young, "The Calcareous Rocks of Grigua'land West."
Geol. Soc. of S.A., Vol. IX., 1906, p. 57.
Trans.,
16
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
of quartz are to be seen throughout the dolomite (cf. Analysis B)
On Buffelshoek and Malmani Oog some dolOimite with peculiar
wa,rt-like excrescences on the wea,thered surface was seen to. be
replaced by silica. in near1y spherical masses.
The quartz
occurs as a fine-grained mosaic as in a quartzite, and in places residual
grains of dolomite can be seen. The dolomite in the vicinity of these
quartz areas is recrystallised to a much coarser grain and is free from
the carbonaceous inclusions of the original dolomite. In other thin
sections of dolomite nearly all showed the presence of quartz either
as scattered grains or as patches of small interlocking grains. The
dolomite grains occurring in or on the contact of the replacing quartz
frequently show rhombic crystal outlines.
A thin section of chert from Malmani Oog occurring as a typical
interbedded layer showed that it contained a large number of rhombs
of dolomite scattered through the quartz. These rhombs are sometimes well formed, but often appear to be partly replaced by the
quartz. On outcrop this kind of chert usually has a pitted appearance
due to the solution of these dolomite crystals leaving small rhombshaped ca,vities.
On Karreebosch 108 a ripple-marked surface of chert was
seen.
This appears tOi be clearly a case of repla,cement of
dO'lOimite along the ripple-marked bedding plane because a, thin
section cut at right angles to the surface showed that the upper layers
were composed purely of quartz to a depth of about half-an-inch; after
that the chert contained a large number of rhombs of dolomite. This
in turn was succeeded by dolomite with a fair amount of quartz grains
scattered through it, and lower down by pure dolomite. It appears,
therefore, that silicification of the dolomite is extremely widespread,
and it is possible that apart from purely mechanical siliceous beds
deposited contemporaneously all the so-called chert bands in the
dolomite may be of secondary origin, by metasomatic replacement
of the dolomite by quartz. This must have taken place subsequently
to the dolomitisation if the rock was originally a limestone, or
subsequently to the recrystallisation of the dolomite if the rock was
deposited as such.
On Malmani Oog 101 a band of pisolitic dolomite was found very
similar to one of the rocks described by Young (ll) (op. cit.) from the
Campbell-Rand Series in Griqualand West. In this rock the spherical
pisolitic grains were still composed mainly of dolomite, but the matrix
replaced by quartz grains conta,ining sOime remaining dOilomite grains.
Other pisolitic structures, more or less silicified with the pisolites
running in bands, were seen in other localities. The pisolites were
O'ften up to an inch or more in diameter but usually smaller. In one
locality a chert breccia was found in which some O'f the fragments were
pisolitic.
On Karreebosch 108 round spherical structures about a foot in
diameter were noticed in an outcrop of dolomite, but as these were
FLUORSPAR, LEAD AND ZIXC DEPOSITS OF THE WESTERN TRANSVAAL.
17
isolated occurrences it is more likely that they are concretionary
structures in the dolomite. In this case there appeared to be no
silicification. A pisolite about an inch in diameter found in a chert
band was cut through and treated with dilute HCl, and revealed a
concentric structure showing considerable though not complete silicification. In another case a chert pisolite of about an inch in diameter
""as found imbedded in brown manganese earth, and on being cut
through proved to be merely a shell of siliceous material surrounding
a core of more soft manganese earth.
This no doubt reveals the
former presence of dolomite around and inside the siliceous shell.
Siliceous Rock~ .-The siliceous rocks found in the dolomite can
be divided into two groups: (a) Cherts, (b) Quartzites.
(a) Chert is almost invariably found with the dolomite in this
area. It occurs either as definitely interbedded layers in the dolomite
varying in thickness from a fraction of an inch to several inches
(PIa te 1., Figs. 3, 5), or as very irregular and often branching masses,
apparently as replacements of the dolomite in isolated patches or along
joint planes. No definite nodules have been observed. Two kinds of
chert are commonly found: (1) a blue very fine grained flinty rock
with subconcoidal fracture. In thin section it is seen to be composed
of small interlocking grains of quartz 'I mm. across with irregular
and generally curving outlines. (2) A white sugary-looking rock, in
which the quartz grains [,re slightly larger than in the previous type.
This is the rock which usually contains residual grains of dolomite
as previously described. Small cubes of limonite are also frequently
seen in this variety of chert.
(b) Siliceous rocks are also found which are undoubtedly quartzites
derived from sandstones. They are fine-grained, but still coarser than
the chert, and small rounded grains of quartz can be recognised
through a lens. In thin section they show the recrystallised interlocking mosaic of quartz, and often the outline of the original rounded
grains of sand can still be recognised. In some cases sericite and
limonite occur between the quartz grains, and in some cases are
present in quite large amounts indicating original rather ferruginous
and argillaceous sandstones. Hed and brmvn coloul'ation on the outcrop
is also common, and in some cases ripple-marked surfaces have beer;.
seen. On Strydfontein a remarkable white quartzite layer about four
feet thick containing black pellets was found. These black masses were
usually round or oval, but sometimes curved or flattened, and varied
in size from a fraction of an inch to three inches in length. They occur
in layers which conform roughly to the bedding. In thin section the
black patches "vere seen to be formed of a fine-grained dark indeterminable material with scattered angular grains of quartz very much
smaller in size than that of the enclosing white quartzite. The dark
material forming the pellets lost most of its colour, becoming bluishgrey on being strongly heated, indicating the presence of organic
18
TRANSACTIO~S OF TIlE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
matter. These rounded black masses are, therefore, probably of the
nature of clay pellets included in an otherwise sandy sediment. This
explanation is supported by the fact that in thin section the quartz
grains surrounding the pellets are seen to dent the outline and to be
partly ernbedded in it.
It is clear from the above statements that rocks which have
been described generally as chert often display distinctly different
lithological characters. The dense, fine-grained varieties occurring as
interbedded layers or along joint planes or irregular masses in the
dolomite are undoubtedly replacements of the dolomite by quartz.
Some interbedded layers, especially those of several feet in thickness,
such as the thick bed of chert at the top of the series, ma,y be due to
direct precipitation of silica contemporaneously with the deposition of
dolomite" Even this type of rock is in places distinctly quartzitic in
appe,arance, and its origin is not quite clear. It may be stated that
in thin sections of these rocks no trace of anything resembling organic
structures could be found. Lastly, there are t·hose rocks that are
without a doubt quartzites derived from ordinary detrital sandy sediments.
Shale.-r:ehin beds of shale, sometimes distinctly carbonaceous
or graphitic, interbedded with the dolomite, chiefly near the top of
the series, were noticed on Buffelshoek, Witkop nnd Zendelingspost.
Layers of shale sometimes indurated were, hGwever, noticed in the
southern portion of Ka,alplaats and on Karreelaagte as well.
Intrusive Rocks.-With the single exception of some diabase
outcrops in the village of Ottoshoop, next to the :Main Road to. Zeerust
just before it crosses the Malmani River, no intrusive rocks were
noticed in the Dolomite Series over the whole area under discussion.
In the overlying l)retoria Series, intrusive sheets of diabase are a
common feature. The nature of this diabase occurrence could not be
ascertained, as nothing but, a,pparently loQose round boulders were seen.
There was no doubt, however, about their being in place, and they
are probably p~t of" a concealed dyke. The rock appears to be a
quartz diabase 9£ dol$rite. In thin section all the minerals are fairly
fresh. It shows the typical ophitic relation between the feldspars and
augite. Quartz is present in small interstitial grains, and also in
micrographic intergrowth with feldspars.
The augite is frequently
twinned, and in places intergrown with greenish-brown hoQrnblende.
The hornblende sometimes shows distinct pleochroic haloes. ChloQrite
in grass green aggregates is seen to have developed from both horn··
blende and augite. The feldspars are sometimes slightly decomposed,
and generally show twinning on both the albite and pericline laws.
The extinction measured with reference to the albite twinning lamellae
is 30 degrees, and that makes the feldspar a bytownite with the
composition of Ab. 23 per cent., An. 77 per cent., according toO the
curve given by Iddings.
Magnetite is abundant.
Apatite is also
present in large amounts, usually in long thin prisims.
FL"CORSPAR, LEAD AXD ZIXC DEPOSITS OF THE WESTERX TRANSVAAL.
IU
METAMORPHISM OF 'rIlE DOLOMITE.
Over a large part of the area the dolomite is notably different
from the normal type found elsewhere, in that it shows signs of
metamOorphism. New minerals, such as tremolite, talc and brucite.
are developed in the dolomite over iarge areas, not directly connected
\-vith ore deposits, as well as locally in ,association with the various ore-'
deposits found in the region.
In M1e upper layers of the dolomite, not far below the contact with
the overlying beds of the Pretoria Series, tremolite is found extensively. The occurrence of tremolite has been previously mentioned
by Hall, Humphrey and others, and was observed on the farms
Paarde-vallei 62, Stinkhoutboom 269, Zendelingspost 268, Buffelshoek
284, Winterhoek 287, Rhenosterfontein 50, and, according to Hall/ 2 '
is to be found all along the contact for a considerable distance on
either side of these farms. The tremolite is found in bundles, and
radiating aggregates of acicular crystals throughout the dolomite. The
needles vary from a fraction of an inch up to three inches in length.
They genera.lly occur sporadically distributed through the rock, but
in placAs constitute fully as much as a third of the rock. In thin
section the tremolite is clearly seen to replace the dolomite.
Locally, also, tremolite is found in the dolomite, associated with
deposits of le,ad and zinc, minerals on Buffelshoek, \Vitkop and Stryd-fontein.
vVollastonite has been mentioned by Hall (op. cit.) as one of the
metamorphic mineraJs in the dolomite. It has,however, not been
noticed by the writer in any of his thin sections.
Further outwards, away from the dolomite immediately underlying the Pretoria Series, and beyond the tremolite zone, there is
a remarkable development of talc throughout the dolomite. The"
talc occurs in small flakes, ,a bout 2 mm. in diameter, disseminated'
more or less evenly throughout the rock. Sometimes, however, the
flakes are clustered together, or are seen to run in lenses or curving
lines. In a few places a distinct circular and concentric arrangement
of thesf. lines of talc have been observed, the circles varying from a'
foot to several feet in diameter. On weathered surfac-es th'3 talc flakes
stand out white and are easily seen, as they afford a distinct contrast
to the dark grey surface of the dolomite.
Talc is not developed
uniformly throughout the dolomite , but occurs in greater abundance
1n som.e la,yers than in others.
It is, nevertheless, genera.l1Y
present.
In thin section the talc IS seen to replace the
dolomite.
SectiOons parallel, or nearly so, to the basal plane
show low order interference colours, and usually undulatory extinction.
Those inclined to the hasal plane are generally long thin sections, and
show high interference colours and a wavy, silky surface.
These
12 A.
L. Hall, "Contact Metamorhpism in the "\VeBtern TransvaaL :..
Trans., C,t:ul. Soc. SA., 1909, p. 119.
:W
'l'lUNSACTIO~S OF 'fIlB GBOLOGICAL SOCIETY OF SOuTH AFRICA.
sections give straight extinction.
The dolomite is generally finegrained, but contains patches of coarser and apparently recrystallised
material. Sometimes there, is a border of recrystallised dolOomite grains
round the talc flakes. '1'he talc is biaxial and optically 'negative.
2E is variable but always small, soo that the interference figures
appears practically like that of a uniaxial mineral. The refractive
indices are: a = 1'542, ~ = 1·573, 'Y = 1·573.
Of the metamorphic minerals, talc is the mOost widely distributed,
and was seen in the dolomite on practically all the farms that were
visited.
As in the case of tremolite, talc is found extensively
developed in places apparently not connected with any ore deposit.
In such cases is often found together with .secondary calcite and
fluOorspar.
vVhere these three minerals occur tOogether, fluorspar is
seen to replace both calcite and talc, and is consequently the latest
mineral to be formed. On the other hand, it is found definitely associated with ore depo8its such as the large fluorspar deposits on
lYralmani Oog as a contact mineraI, and extensively developed. in the
meta.n10l'phosed zone of dolomite containing veins and impregnations
of zincblende and fluorspar on Buffelshoek.
\Yhere talc and tremOolite occur in the same rock, the talc seems
to be the older of the hyc, minerals.
Brucite is another mineral which is found in the metamorphosed
dolomite. It is not so common as talc or tremolite, and seems to
be more directly connected with the ore deposits. It Ooccurs in the
metamorphosed dolomite containing the metallic sulphides and
fluorspar on Malmani Oog, Buffelshoek, Witkop and Doornhoek It
is usually present in small flakes of irregular shape, occurring interstitially between other minerals. Such ramifying grains often extinguish
at the same time, showing that they are all part of the crystal. Where
it occurs together ,,,,ith talc and tremolite, it appears to be of later
formation, and encloses crystals of those minerals. It is only a, minor
mineral when present in the rock with tremolite and talc. In a, few cases,
however, it was found to be the only metamorphic mineral present,
as in a dark, fine-grained dolomite from Buffelshoek and a similar
rock from Doornhoek frhe rock from Doornhoek was found exposed
in the open workings Oof the lead deposit, and is a dark, fine-grained
rQc;k ,,,,ith glistening flakes of brucite up to 3 mm. in diameter. The
crystals of brucite are tabular in habit, and exhibit a pearly lustre
on cleavage faces. In thin section the dolomite was seen to be
crowded with carbonaceous Inatter, and contained the flat tabular
erystals of brucite.
In basal sections, this mineral shows verY'
low interference colours, no cleavage cracks, and has rather
indistinct outlines.
On these sections it gives a distinct uniaxial
interference figure, and is optically positive. Sections inclined to
t he basal plane show higher interference cOolours, distinct cleavag2
cracks, and straight extinction. The mineral was isolated and the
FLUORSPAR, LEAD AN"D ZIXC DEPOSITS OF THE WESTERN TRANSVAAL. 21
refractive indices f~und to be: e = 1'582, W = 1·560, and these
correspond with brucite, as do all the other previously-mentioned
optical properties.
Analyses of some of the metamorphosed types of dolomite are
given below:A. Average analysis of typical unaltered dolomite, given previously, but included here for comparison.
E. Talc-calcite rock on contact of fluorspar deposit, Malmani Oog.
F.Dark talcy dolomite, not associated with any fluorspar,
Doornplaat.
G. Dark greenish-black metamorphosed dolomite from fluorspar
quarry, 1\lalmani Oog.
(Analyses E.F.G. by H. G. \VeaH, Govt. Chern. Labs.,
Johannesburg.)
A.
E.
F.
G.
Si0 2 . . . . . . . . . . . .
1'4
22·55
25·5
43'45
Fe 2 0 3 ... " , ••.
1'45
2'4
0'8
FeO ........... . :::} 1'70
0'12
A1 2 0 3
0'35
0'35
24'4
0·55
MnO ....... ..
1·10
1·6
0·15
30·9
10·3
30·01
31'85
CaO ....... ..
14·35
20·12
MgO ........ .
15'7
2'8
45·53
23·7
27·6
CO 2 . . . . . . . . .
7·2
S03 ............. .. trace
trace
1·0
Ti0 2 • . • • . • • . • • • . • • ,
trace
Loss on ignition ...
1·95
2·8
1·2
0-2
2\1oisture ...
0·2
0·4
99·98
100·85
101·55
93·3
ECONOMIC GEOLOGY.
Besides the Fluorspar, Lead and Zinc deposits already referred
t(), the district contains valuable deposits of surface limestone. This'
is burned for the manufacture of white lime in a few places, and is
used for the manufacture of Portland Cement by the Pretoria Portland
Cement Co., Ltd., at Slurry. The dolomite itself is also utilised for
the manufacture of blue lime, chiefly by the \Vhite Limes Limited,
at Ottoshoop.
rrhe Gold quartz veins of the area have previously been mentioned.
The three minerals, Fluorspar, Galena and Zincblende are widely
distributed over the area, and are confined in occurrence to the rocks
of the Dolomite Series. These three minerals are always closely
associated with each other, although in the different deposits to be
described, it will be seen that large masses of one or t\VO of the
minerals may occur almost to the exclusion of the others.
These
deposits are,. hO\vever, undoubtedly all related to each other genetically.
22
TRANSACTIOXS OF TIlE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
Associated with the above mineral deposits' there are, on some
farms, large quantities of low grade manganese earth, which may .at
some future date be exploited commercially.
FLOURSPAR.
Fluorspar occurring in the dolomite has been observed from surface
indications on the farms Malmani Oog, Naauwpoort, Strydfontein,
])oornhoek, Hhenosteriontein, Winterhoek, Buffelshoek, \Vitkop, Kaalplaats, Doornplaat, Stinkhoutboom, Zendelingspost, and it probably
occurs on some of the adjoining farms as well.
The fluorspar occurrences can be classed into hvoO main types:1. Gash-veins and impregnations of the dolomite.
2. Large bodies comprising thousands of tons of practically pure
fluorspar.
Type i.-Occurrences of fluorspar of this type are extremely
common, and can be seen .on outcropSoOf the dolomite standing out
white against the dark grey surface of the dolomite. Quartz veins,
calcite and talc are also often met with, and they are sometimes
associated with the flourspar, but more often the latter occurs by itself.
These surface indications of fluorspar appear to be concentrated in
definite areas, which sometimes appear to run in lines. Throughout
the area, however, no definite orientation of these mineralised lines
could be made out, as, for instance, is the case ,,,ith the auriferous
quartz veins with their persistent N.N.\V.-S.S.E. strike.
fThe
dimensions of the fluorspar veins vary from fractions of an inch to
one or two feet across.
In prospecting operations these fluorspar indications have, in many
localities, been opened up toO a depth varying from a few feet to oOver
twenty feet, and, consequently, could be well studied.
The open
workings usually reveal a branching net-work of fluorspar veins cutting
through the dolomite, and sometimes the chert layers as well. The
veins are of the gash-vein type, and are extremely irregular in size
and extent. frhey are often lenticular and, starting as mere thread-like
veins, they may suddenly bulge out to a thickness of a foot or moOre.
There is apparently noO control by bedding planes, and whilst most
of the veins are transverse to the bedding, some lie parallel to it.
The difference in the layers of the dolomite affected can sometimes
be seen in the occurrence of a large number of small veins in one
band, and then fmver, but larger, veins in the following layer of
dolomite.
The fluorspar also occurs as irregular replacements of the dolomite
ancl chert. In some cases only a few scattered crystals or patches
of fluorspar occur in the dol.omite, and then again one finds that
replacement has taken place to such an extent that there is more
fluorspar than dolomite. Somet,imes the replacement is of such a
nature that the original dolomite is represented only by thin curving
and branching bands betvveen the fluorspar. frbis is very ,veIl seen on
FLl:ORSPAR, LEAD AND ZINC DEPOSITS OF 'l'HE WESTERN TRANSVAAL.
23
parts of Strydfontein and \Vitkop. The 'iveathering of dolomite, with
such a net-work of fluorspar veins, sometimes results in the complete
removal of the dolomite, leaving a, .crusty, cellular mass of fluorspar,
stained brown or black by the residual iron and manganese oxides of
the dolomite.
Associated with the fluorspar occurring in the small gash-veins
other minerals, such as galena, zincblende, calcite, quartz and talc,
are sometimes found.
Zincblende and galena are usually seen
occurring in the inner portions of the small veins.
The fluorspar occurrences of this type, although very widespread
Dver the district, have, however, not produced very large amounts of
the mineral. Quantities varying from a few tons to a few hundred
tons have been taken from some workings, but, on the wh.ole, they
have proved rather disappointing. Outcrops of flourspar, in some
cases quite as promising as those which led to the opening up of the
bigger deposits, have often proved to be merely ., flats" or veins of
the fluorspar parallel to the stratification of the dolomite.
La,rge
vertical veins have also .often been found to pinch out very rapidly
in depth, and resolve themselves into a branching net-work of smaller
veins running through highly altered dolomite. Some occurrences on
,Vitkop, Strydfontein, Malmani Oog and Kaalplaats are examples of
such favourable surface indications that have proved on opening up
to be disappointing.
In places the dolomite is silicified and traversed by quartz veins
several inches thick.
'rhe silicified dolomite often contains large,
'ivell-formed, rhomb-shaped crystals of siderite and sulphides, such as
pyrrhotite, pyrite and zincblende. The zincblende and pyrrhotite occur
in scattered grains and aggregates, and the pyrite usually in crystals
of octahedral habit, frequently with striations indicating an oscillatory
combination with the pyritohedron. In places a talcy mineral, pale
greenish-yellow in colour, occurs in flaky aggregates, with round exterior
outlines, in cavities in this silicified rock. This mineral, on optical
and chemical examination, proved to be penninite.
Fluorspar also occurs in veins lined by quartz crystals, the quartz
cryst,als projecting into the inner part of the vein, which is filled with
fluorspar. This has been noticed in several localities on Malmani Oog,
in a small pit next to the main quarry and elsewhere, and also on
Strydfontein, Doomhoek and Kaalplaats.
Type .B.-The principal deposits of fluorspar are those on 11almani
Oog 101, Buffelshoek 284 and Witkop 288, worked by ·Messrs. J. G.
Gubbins and The \Vestern Quarries Limited, respectively.
J.\1 almani Oog .-The deposit on 1VIalmani Oog is situated right
next to the road from Lichtenburg to Zeerust, and about 100 yards
S.W. of the point where the road to Ottoshoop turns from the abovementioned main road. This point is near the drift over the Malmani
24
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
River, where there is an old water-driven mill and, a little further on,
the residence of :Mr. J. G. Gubbins.
The deposit is actually on the farm Naauwpoort 102, according
to the new sub-division of the farms, but the old name, Malmani
Oog, will be used during the present description, in view of the fact
that the older farm boundaries are the only ones marked on theGeological Maps.
The shape of the deposit is roughly oval in plan view, with a
The shape of the body,
long axis running almost due E.W.
however, is such that it has a dome-shaped roof, and the floor of the
deposit also rises in the centre, as is shown in the sketch map and
section of the deposit (Fig. 3).
SKdcl-r :M~-2f the F'luors~:.\r Dc?oSit
~f1alm:ani Oo~.
A
FIG. 3.
FLUORSPAR, LEAD AND ZI~C DEPOSITS OF THE WESTERN THANSVAAL.
25
The original outcrop, according to verbal informa,tion from Mr.
J. G. Gubbins and others, was approximately circular and about 10
feet in diameter. One or two small masses of the original outcrop
are still to be seen on the edge of the present quarry, and they are
composed of fluorspar, stained brown, and showing the well-developed
octahedml cleavage, accentuated by weathering.
The country rock immediately surrounding the deposit, as seen
exposed on the walls of the quarry and on the surface around, is
dolomite and chert.
It is worthy of notice that small veins of
fluorspar are not common in the surface rocks surrounding the deposit.
To t.he west and south of the deposit the rock is entirely chert and
dolomite, but eastwards, in the direction of the l\lalmani River, and
on the other side of it, there occur beds of stratified siliceous rocks,
often stained red or brown, and having the appearance of quarzites
on outcrop. In thin sect,ion they appear to be made up almost entirely
of fine quartz grains, but in some flakes of a micaceous mineral occur
interstitially to the quartz grains. The dip of these beds is about five
degrees N.E.
The deposit has been worked to a depth of about 85 feet as an
open qu any , a.s is shown on the sketch map (Fig. 3). On the east side,
pillars of fluorspar have been left and excavation carried on behind them,
lea.ving a covering supporting roof of fluorspar. The fluorspar mass, in
spite of the cleavage of the mineral, is massive and coherent enough
to form a good hanging wall, and even to keep out ground water.
rrhis was shown in the deepest part of the working on the east side,
when, on penetrating the fluorspar boundary, an underground water
channel was tapped, which caused t·he flooding of the Im,ver levels
of the war kings.
The waIl of country rock is exposed at various places along the
contact, which is generally clea.r1y defined and distinct, but with small
veinlets OIr fluorspa,r penetra,ting the country rock. Small points and
ledges of dolomite and chert are often found projecting into the fluorspar mass.
On the east side, the uppermost thirty feet exposed in the open
side of the quarry is made lip of chert bands of a few inches thick,
separated by rather thicker bands of soft brown earth. This material
can be called manganese earth or wad. In the wad there are always
numerous flakes of talc and small quartz grains. The talc is of the
same character as that which occurs disseminated in the dolomite
throughout the whole area, and, therefore, these layers of wad were
obviously originally bands of dolomite alternating with the chert layers,
and which have since been converted into wad by the removal of all
the carbonate matter, leaving behind the insoluble talc and quartz
and the residue of iron and manganese and aluminum oxides (see
Analysis H.).
26
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
H. Analysis of manganese earth from east wall of fluorspar
quarry on Malmani Oog (by H. G. Wean, Govt. Chem. Labs,
Johannesburg).
H.
SiO.,
58'55
Fe 2 0 3 l
11'4
FeO
f
4'6
A1 2 0 3
11:nO
5'2
0'35
CaO
12'05
:MgO
3'85
CO.,
2'4
Lo;s on ignition
1'3
Moisture
99·7
There has, consequently, probably been a great deal of shrinkage
and diminution of volume because the soft residual wad would not
occupy the same volume as the unaltered dolomite from which it was
derived. Evidence of this decrease in volume and consequent readjustment can be seen in the chert layers, which are often slightly
disturbed and sag downwards in places. On the southern face, there
seems to have been more disturbance, resulting in the fracturing and
overthrusting of the chert layers.
The chert is usually white and sugary, and the surfaces pitted
by minute rhomb-shaped depressions, which suggest the former
presence of crystals of dolomite, and in a thin section of a piece of
fresh chert such cryst,als of dolomite were observed. rrhey seemed
to be residual, and these chert bands, therefore, like those previously
described, are undoubtedly replacements of the dolomite along its planes
of stratifica1tion. rrhe chert also contains small cubes of limonite in
places.
From the upper 30 feet downwards the rocks exposed along the
contact of the fluorsp,ar mass are both dolomite and chert. The fluorspar
cuts across both types of rock, but it seems to have replaced the dolomite
somewhat more readily than the chert. Small ridges along the contact
and ledges projecting into the fluorspar usually mark the occurrence of
the chert layer. This can be well seen in an overhanging roof
rendant of the country rock exposed on the northern face of the quarry.
The chert does not seem to be visibly altered by its contact with
the fluorspar, but close to the contact it is penetrated by numerous
veinlets of pale violet fluorspar. In thin section the chert again shows
itself to be a replacement of dolomite, inasmuch as all sections of chert
show a considerable amount of residual carbonate grains. The quartz
in the chert is generally fine-grained, but patches of larger irregular
grains also occur. The fluorspar veinlets are seen to replace the
FL"CORSl!AR, LEAD AND ZINC DEPOSITS OF THE WESTERN TRANSVAAL.
27
rock, irrespective of the occurrence of dolomite and of coarse or fine
grains of quartz.
In the chert the fluorspar frequently shows a
tendency towards the development of cubic outlines along tw.o or more
sides of the grain. r:rhis was also noticed much more strikingly in a rock
fr0'm one of the wOl'kings 0'n Strydfontein. The flu0'rspar often contains
small residual grains of quartz enclosed in it.
The dolomite, on the other hand, in contact with the fluorspar,
shows many traces of metamorphism. It is generally recrystallised
into larger grains, ,vith very irregular sutured outlines, and contains,
in addition, the minerals talc, penllinite, brucite, jefferisite and sphene
in places. In some cases the dolomite is charged to a light-coloured
rock, made up of talc and calcite in about equal proportions. \Vhere
the original dolomite contained quartz, this is seen to ha,ve been partly
replaced by the calcite and talc, and to occur as small residual grains
included in both the Jatter minerals. This type of rock usually occurs
where pieces of dolomite project into the fluospar.
On the N.W.
side of the quarry, a mass of dolomite, about five fEet high and three
feet in diameter, and sunounded on all sides by fluorspar, was
,converted into this type of light-coloured rock, composed of talc and
calcite. At several other places along the contact this type of rock
was also notjced. Elsewhere the dolomite is darker in colour, and in
thin section is seen to contain talc and brucite. The talc occurs in
small flakes, as previously described, and the brucite in irregular
grains, usually enclosing, in poikolitic fashion, small crystals of talc
and dolomite. On the east side, near the top of the fluorspar rim,
a rock was found containing a rather shiny brown mineral, which
proved to be str.ongly pleochroic from paIe yellow to brown, and closely
resembled a brown mica. On determination of its refractive indices,
and by its blow-pipe reactions, it proved to be .one of the vermiculite
group of minerals, jefferisite. It is biaxial and negative with a very
small axial angle, 2E about three degrees. The refractive index for
B and I is 1·563 ± 0·003, and the birefringence about 0·02. These
properties agree very closely with those of jef-ferisite from vVest Chester,
Fa., U.S.A., quoted by Larsen. I3 It is probably the result of the
alteration of some other micaceous mineral. It occurs in the dolomite
and chert alike.
On the southern side, near the top of the guany, a peculiar
dark-coJoured rock was found. It appears not to be interbedded, but
rather to cut across the dolomite and chert surrounding the fluorspar.
It is traversed in places by veins of deep violet fluorspar and calcite,
and in places contains large isolated crystals or groups of quartz
crystals. When the fluorspar and calcite occur together in a vein,
calcite appears to be the earlier mineral, and is on the outside, while
the fluorspar is in the middle of the vein. The rock varies in detail
from places to place, but it is usually dark in colour with greenish
13 E. S. Larsen, "lHicrc.scopic Determination of Non-opaque Minerals."
1].8.G.8. Bull., G79, p. 93.
28
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
patches. In places small black areas can be seen, which appear to
have a distinct cleavage. On wetting the surface, numerous wedgeshaped crystals of sphene can be seen. Examined under the microscope, the rock proved to be composed of ft fine, flaky ground mftss, and
the pale green patches seen ftt the surface proved to be the chloritic
mineral penninite.; whereas the small black areas marked tJ:1e position
of irregular crystals of brucite enclosing some of the flaky ground mass.
The penninite is occasionally clustered together in spherical aggregates~
but m0're commonly: occurs as fine flakes, with no particular 0'rientation.
The mineral is biaxial and P0'sitive with a small axial angle. 2E
about five degrees. The refractive index for ex and ~ is 1'573 ± 0'003.
It exfoliates strongly when heated before the bl0'\v-pipe. The ground
mass is made up of very fine fine-grained indeterminate flakes. The
analysis of the rock (Analysis G) sh0'WS that it is high in alumina and
silica, and relati vely low in magnesia, and, therefore, some aluminium
silicate like pyrophylitte probably makes up a considerable portion of
the ground mass. In pla.ces the flakes of the ground mass, as well as
the spherical aggregates of penninite, are enclosed poikolitically by
larger crystals of calcite, brucite or fluorspar.
Sphene occurs in
typical wedge-shaped crystals and irregular grains, and is rather in a
decomposed state. Sometimes the rock contains drusy cavities, which
are lined with apple-green botryoidal aggregates of penninite. Quartz,
where it occurs, appears to be of later origin than the other minerals.
A little distance away from the. quarry, just at the meeting-point of
the three roads, there occurs an outcrop of a very similar rock. It
appears to run in the saddle of a very small anticline, with n0'rmal
chert and talcy dolomite above and below it. It is black in colour,
and in places has the bright green patches of penninite.
Microscopically it showed the same mineral associations as the rock
described from near the quarry. In the brecciated floor of the dep0'sit
a similar rock was found, and it probably represents an extreme phase
of alteration of the dolomit·e.
rrhe fluorspar body itself is of considerable size. So far about
25,000 tons of high-grade fluorspar have been taken out of the quarry,
and a considerable tonnage still remains. The fluorspar is massive and
crystalline, transparent with a vitreous lustre, and exhibits very
distinctly the octahedral cleavage of the mineral. The fluorspar mass
is compact as a whole, and no cavities or open fissures have been
found.
Consequently, the beautiful crystals, with well-developed
outlines, which are a feature of so many other fluorspar occurrences.
are conspicuous by their absence. Occasionally in the fluorspar mass,
and also along its contact with the dolomite pockets filled with l0'ose,
incoherent wad, which usually contained numerous flakes of talc
were found. These pockets, however, were never lined by fluorspar
crystals, and they are, therefore, apparently residual masses of dolomite
from which the Ca. and Mg. carbonates have been removed, leaving
the more insoluble impurities of thA dolomite and talc behind.
FLUORSPAR, LEAD AXD ZINC DEPOSITS OF 'l'HE WESTERN TRANSVAAL.
29
The colour of the fluorspar varies considerably.
It is usually
colourless to pale blue or green. It is alsG white and clGudy or grey
i'1 patches.
Small quantities of clear, colourless and ~ransparent
" optical" fluorspar have been found.
N ear the contact of the
dolomite, a deep violet colouration is characteristic, and sometimes
a brownish-red \vine coloured variety was found. All varieties appear
toO become colourless on exposure toO the sun for a long period, as the
fine fragments of fluorspar seen on the dumps around the quarry are
all colourless. The colQura,tion of the deepest violet, and also of all
the other colours, disappear on heating to redness within a few minutes.
On the floor of the deposit, as well as in the fluorspar of the lower
part of the deposit, sulphide minerals were found in a line running,
roughly, E.\V. through the centre (Fig. 3).
The minerals were
pyrite, pyrrhotite, chalcopyrite and zincblende. Galena was also found
in places, but usually independently of the other minerals. Pyrite,
pyrrhotite and chalcopyrite are more abundant than zincblende and
gelena. The former minerals occur in the brecciated floor of the deposit
and in places the rock containing them has been completely silicified.
Zincblende occurs in small grains, only det-ected in polished sections
under the microscope. Zincblende and galena als.o occur alone in the
fluorspar, usually close to the walls of the deposit. The texture of
these sulphide minerals is coarse-grained, and with the exception of
pyrite, none displayed crystal outlines.
The pyrite, though oft·en
intergrQwn with pyrrhotite and massiye, was sometimes seen toO have
drusy clwities, which in places were lined with imperfectly-formed
pyritohedral crystals. Pyrite was undoubtedly the first mineral toO be
formed, and it is seen to be replaced by pyrrhQtite.
Chalcopyrite
occurs in irregular masses, replacing pyrrhotite where it is in contact
with it, and often cutting through the latter in minute veins. The
zincblende is obviously of later formation than the chalcopyrite, as it
can distinctly be seen to replace and invade the latter. It sometimes
contains small rounded inclusions of chalcQPyrite. Galena was not
seen in polished sections in assQciation with the above minerals. It
occurs hy itself, however, in the fluorspar, and in polished sectiQns
\vas seen to contain minute inclusions of chalcopyrite.
The galena
carries silver to the extent of 17·80 ounces to the ton of 2,000 lbs.
(Assay of a single specimen by D. Millin, University of the \Vitwatersrand.) A pale yellow hard mineral, which is possibly either pyrite or
marcasite, is seen to cub through and replace pyrite and pyrrhotite
in small beins and sometimes as rounded grains. A grain of earlier
formed pyrite is cut through by several veinlets of this mineral.
It
is freqnently seen associated with small veins of fluorspar, which
traverse through all the previously formed minerals up to the
zincblende.
The relation between these veins and galena, was
not observed. In places the veinlets are composed of fluorspar,
zincblende and this yellow mineral, and ha.ve a, distinct
symmetrical banded structure, with the fluorspRr on the outside,
then a thin streak of the yenow mineral and the centre filled
30
TRANSACTIONS OF 'l'HE GEOLOGICAl, SOCIETY OF SOU'l'H AFRICA~
with fluorspar and zincblende.
Sometimes the zincblenc1e is
missing. rrhe accurate identification of this mineral by chemical
means was not possible on account of its occurrence in such small
veins, so that enough material of assured purity could not be obtained
for testing. In polished se,ctions under the microscope it has a pale
yellow appearance, is very hard, and its reactions with various
etching reagents correspond to either marcasite or pyrite. Examination
by polarised light with crossed nieols did not throw any light on its
identity, as both the pyrite and this mineral remained of the same
colour and degree of illumination on a complete rotation of the stage.
According to Schneiderh()hn, it is possible to distinguish pyrite and
marcasite by means of polarised light, but the anisotropic behaviour
of marcasite is seen only in definitely crystalline material, and with
col1oida 1 amorphous marcasite nO' distinguishing effect is obtained with
polarised light.
If the mineral is marcasite, then its intimate
association with fluorspar would seem to indicate that it is of hypogene
orlgm. On the other hand, if it is pyrite, thm} its occurrence indicates
two distinct periods of pyrite ,formation. Tolman and Rogers describe
a very similar mineral occurring in the ore from the Friday Mine,
San Diego, CalHornia. "A brass yellow mineral occurs in veinlets
and reticulate masses as a replacement of pyrrhotite.
This was
called pyrite by Calkins, but more probably it is marcasite. The
marcasite gives the impression of being a very late mineral. It is
usually extensively developed along calcite veinlets, which occur as
a. net of intersecting stringers cutting all the other minerals. "11,
Although the small veinlets of fluorspar and 7;Jncblende are seen
to cut through larger grains of zincblende, and are consequently of
later formation, it is possible that the formation of the zincblende,
galena and fluorspar could not have been "widely separated, and ill
places appears to have been contemporaneous.
In hand specimens
showing galen a and fluorspar, the galena is seen to run in veins
through the fluorspar. Also in the small gash-veins of fluorspar found
in the dolomite, galena and zincblende are seen to replace the fluorspar.
In some oxidised specimens of these sulphides, the minerals limonite
and covellite occur as secondary minerals replacing the other sulphides.
The deposit, on Buffelshoek is situated in the S.W. part of the
farm, and about half a mile to the east of the road between Zeerust
and Mafeking, and about six miles from Ottoshoop (Pig. 2).
The country rock is principally dolomite, chert being much less
conspicuous than in the case of the l\ialmani Oog deposit, but a, few
bands of shale occur in the dolomite. The deposit is situated on the
southern slope of a low hill, at the top of which is found the massive
white chert occurring at the top of the Dolomite Series. In places
the chert breccia also occurs, but not prominently. Further north
the Pretoria shales overlie the Dolomite Series. Near the quarry and
14
1916.
c.
]i'.
p. 40.
To1man and A. Ii'. Rogel'S, "Stndy of Magmatic Sulphide Ores." .
FLL'"ORSPAR, LEAD A~D ZINC DEPOSITS OF THE WESTERN TRANSVAAL.
31
between it and the residence of :Mr. OOF. 'r. Blane, there is a small
conical kopje, which is capped by white chert, and chert breccia similar
to that found elsewhere at the· top of the dolomite. It also occurs
as the capping layer of a few small kopjes further south on \Vitkop
and Kaalplaats. l"ollowing the chert breccin, there occurs a dark black
shaly rock.
It is typically laminated and splits into slabs on the
surface. These shales are found in severnl places in the neighbourhood
of the quarry. In prospecting shafts, and in the quarry itself, this
shale is seen to be underlain by blcy dolomite "vith hardly any chert
bands in it. The shales 'ivere first thought to be remnants of the
overlying Pretoria Series, but the section exposed on the small kopje
already referred to, where the shales are covered by the chert breceia
which is taken as the base of' the Pretoria Series, proves that these
shales are in the Dolomite Series. Similar shales can be seen also
on the walls of No. :2 Qmlrry on \Vitkop, and nre clearly seen to be
thin layers interbedded with dolomite.
On the floor of the No. 1
Quarry on Buffelshoek a similar shale brecciated and cemented with
quartz was encountered. Around the qnarry there is just the suggestion
0'£ a circular rim continuous except on the S. VV. side.
The slight
circular elevation is due to a thin covering of shale, and it is fmmd to
be underlain both inside and outside by normal dolomite.
The deposit has been worked as an open quarry to a depth of
about sixty feet, and almost completely cleared of the overlying
does not appear to be so much altered as in the case of the deposit
(Fig. 4). r:rhe average dimensions of the deposit, as at present revealed
in the quarry, are: -Lengi h 100 feet, breadth 60 feet, depth 60 feet.
The quarry has produced to date approximately 15,000 tons of highgrade fluorspar. 'rhe shape of the deposit is roughly rectangular, and
control of the outlines by joint planes can be very clearly seen on
some parts of the exposed walls. r:rhe contact between the dolomite
and the fluorspar is usually clear cut, and often along vertical faces
in a terraced fashion.
At the present stage of development, this
feature can be best seen in the western side of the quarry. Here the
contact waH of dolomite runs along a large smooth joint face about
twenty feet in thickness, and below it the fluorspar extends inwards
almost at right angles for a distance of ten feet. According to verbal
information from 1fr. F. T. Blane, the contact, in the upper portion of
the quarry was of a similar terraced nature.
The country rock exposed along the wal]s of the quarry general]y
sho\v8 about ten to fifteEm feet of decomposed dolomite and manganese
earth with large residual boulders of dolomite. This is followed by
massive dolomite for about 35 to 40 feet, frequently showing well
defined vertical joint faces.
The rocks exposed on the floor
of the quany are talcy dolomite with fluorspar and sulphides
in veins, and in places a black rock which proved to be a,
shale or slate breccia cemented by quartz and containing zincblende
and fluorspar veinlets. As in the previously described deposit, thp
32
'rHANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
Fluol"sp'ar nep-0"'~
on
~Llffe.lshDC.K.
(from 1"1 an L,y F. T B\-.te E.s~., "'i~\. ad.hbons)
"plan.
~c:ornpo.sed J)olorn',te
tofa?gar7C:4oC t.arth WI~lT
:Bcu.klar..
.at1d
0\
a-rd
large.
'Do\ol'1'!ite
mUll\-' .It,,,,<;Q "Do Of'nlte
F'h,lOf".sr-" 01\' c;:onrad·
?;; Cherty
:Dolo,'" lie.
F"toor ~t ~I~.r&d J)oIOfT1·,t-..
'I!'\f,h'
f1'Jorar-" arrd
Q.
s.,.. I",
1·
.L:;.o,_~_","-_ _ _~_ _ _......
'Z.rf'tt::lal~e.
P.. t
FIG. 4.
fluorspar cuts vertically across the country rock and has apparently
completely replaced the dolomite. Sometimes along the contact there
is a selvage of schistose talcy material. The dolorriite country rock
cloes not appear to be so much altered as in the case of the deposit
on l\falmani Oog, and the highly altered types of dolomite ~mch as
the talc and calcite rock, and the dark-green rock with talc, brucite,
peninite and sphene have not been found here. The dolomite is,
FIXORSPAR, LEAD AXD ZINC DEPOSITS OF rfHE WESTERN TRANSVAAL.
03
however, distinctly recrystallised and contains some flakes of talc aI~d
brucite in places. It frequently contains sulphides, such as pyrite, In
cubic crystals up to a quarter of an inch across, and pyrrhotite and
L-incblende in small irregular grains, and is also traversed by small
veins of fluorspar in places.
Pyrrhotite and pyrite occur in a few places in the fluorspar mass
as well as in the surrounding dolomite. Galena also occurs in the
fiuorspa;, but zincblende is always found in the country rock or in the
fiuorspar very close to the contact. At the bottom of the quarry, near
the centre of the deposit and extending for some distance up into the
fluorspar mass, these sulphides were found. They consisted chiefly
of galena, pyrite, pyrrhotIte and marcasite. The galena was sometimes
found alone as small isolated crystals, or in larger masses cutting
through the fiuorspar in veins. Sometimes it was coarse-grained, but
more often finely crY8tallised. Even where it was coarsely crystalline.
the crystals seemed to have been partly crushed and deformed, and
the finely granular galena is undoubtedly derived from originally
coarse-grained materia1. Specimens were obtained, which show every
stage in the change from the coarse. to the fine-grained condition. The
silver content of a specimen of this galena was found to pe 6'4 ounces
per ton of 2,000 lbs. (assayed by D. Millin, University of the
\Vitwatersrand). Examined in polished section, zincblende and chalcopyrite were also discovered occurring as small grains with the other
sulphides. The relation of the minerals to each other is similar to
that of the sulphides of the previously described deposit on Malmani
Oog. Pyrite is again the earliest mineral to be formed, and is replaced
by pyrrhotite, chalcopyrite, zincblende and galena. Galena can be
seen very clearly in small veins cutting through pyrite and pyrrhotite.
A pale yellow mineral, which in this case has been definitely identified
as marcasite, occurs in masses up to half an inch across, replacing
pyrite and pyrrhotite. It appears to be of later formation than the
galena as well, for the latter mineral has not been seen to cut through
or invade the marcasite. Contacts between marcasite and galena are
hardly ' ever seen, as the marcasite has replaced principally the pyrite
and pyrrhotite. ffhe borders of the marcasite are usually marked by
a depression as if something had been removed. Open spaces usually
occur in the mineral as weJl. This could not have been produced in
the process of grinding and polishing of the. sect-ion, because the feature
can be noticed in hand specimens which have not been treated in any
"vay. It frequently shows a spherical structure with concentric
markings, and in places is seen to cut through the pyrrhotite in smaJI
veins from which at intervals these spherical masses are given off.
This structure suggests that the marcasite was deposited as a gel
and that shrinkage in volume on crystallisation would give rise to the
open spaces around and in the mineral, and also to the concentric
markings.
Cutting through the marcasite as well as through the.
previously formed sulphides, there are small veins and irregular-
34
TRANSACTI9NS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
reticulate replacements of a carbonate mineral which has been identified as mangano-siderite. This occurrence of marcasite is of interest
in view of the recent controversy regarding the p.ossible hypogene origin
of some occurrences of ma,rcasite.
"In recent years there has been a general tendency
am.ongst students of ore-deposition to consider marcasite as
a mineral of supergene origin, and to have been deposited
from circuluting ground water, cold and acidic in nature. The
presence of earbonates, however, indicates that these solutions
forming marc.asite could certainly not have been very strongly
acidic in nature. "15
The terms hypogene and· supergene are used here in accordance
with Hansome's definition, to denote (a) origin by hot ascending
solutions (b) origin by cold descending solutions. 16
Newhouse (op. cit.) describes a very similar occurrence of ma,rcasite
from the Michigan :Mine, Kokomo, California, and attributes to it a
hypogene origin.
"The last ebbing phase of hypogene solutions
carrying carbonat·e are then conceived to be the medium whieh formed
the mareasite."
The fluor:spar from this Buffelshoek occurrence is generally white,
but grey and green varieties aTe alsO' common.
frhe violet and
wine-coloured varieties common in the Malmani Oog deposit were
not not,iced here.
Two kinds of fluorspar are found, a vitreous
cleavable variety and a massive non cleavable variety with a splintery
fracture. The latter has a rather dull .appearance, and the fractures
sometimes correspond imperfectly to the octahedral cleavage, but
more often the cleavage directions are not indicated at all. In the
well-marked terrace on the N.E. side, the fluorspar for two feet along
the contact with the dolomite is of this massive type, and has a
rather cherty appearance, and lower down it merges gradually into
the ordinary cleavable variety. This massive splintery variety was
also noticed at some other places, notably at the No. 2 Quarry on
Witkop. From its cherty appearance it was at first thought that
such fluorspar had resulted from the replacement of chert, whereas
the cleavable variety from. the replacement of dolomite.
This,
however, was found not to be the case, for in the deposit on M.almani
Oog, where ehert is more prevalent in the country rock, this variety
was not found. Microscopic examination, moreover, revealed no traces
of residual rock minerals.
Along cracks in the fluorspar in some pla.ces there occurred t,hin
The
films of fibrous material of the nature of mountain leather.
massive fluorspar was sometimes traversed by joints, with thin films
of this material occurring 0long the cracks. It was probably deposited
subsequently to the formation of the fluorspar by solutions carrying
magnesia and silica percolating through the fluorspar.
Econ. Geol., Vol. XX.,
Jan.-Feb., 1925.
16 F. L. Ransome,
"Contributions to Economic Geology." U.S.G.S.
null., 540, p. 152.
15
No. 1.
'V. H. Newhouse, "Paragenis of Marcasite."
FLUORSPAR, LEAD AND ZINC DEPOSITS OF THE WESTERN TRANSVAAL.
35
None of the pockets of loose manganese earth found in the deposit
at 11almani Oog were noticed here. Along the floor the occurrence
of talcose dolomite and the bla,ck brecciated rock with veins of fluorspar,
zincblende and other sulphides cutting through them indicates, as in
the case of the previously described deposit, that the mineralising
solutions must have come up from below along zones of fracturing.
Witlwp.-The deposit worked as the No.2 Quarry, by the Western
Quarries, Ltd., is situated on the western portion of the farm Witkop
and close to the northern boundary. It is in line with, and due
south of, the No. 1 Fluorspar Quarry and the Zinc and Fluorspar
Deposit on Buffelshoek (Fig. 2).
It is still in a rather early stage of development, but promises
to yield a considerable tonnage of fluorspar. It has been opened up
to an average depth of 10 feet over an almost circular area of about
150 feet in diameter.
A crescentic mass of fiuorspa,r of
several thousand tons was exposed on the northern and
western sides, and it appeared to dip away outwards underneath
dolomite. Elsewhere in the qua,rry the fluorspar occurs in irregular
branching veins up to three feet across, cutting shale and talcose
dolomite. Four shafts have been sunk on the periphery of the quarry
and one in the centre, and they reve.al a zone of rather brecciated
dolomite cut by veins of fluorspar surrounded by more or less normal
dolomite.
Shale bands occur along the top of the quarry, and they are
intercalated layers in the dolomite of the same horizon as those found
near the No. 1 Quarry on Buffelshoek. In places the fluorspar is
seen to cut across these shale layers, but otherwise the shales formed
an unbroken cover over the deposit.
Galena and a little zincblende is found gener.ally near the outer
limits of the deposit.
The zincblende a,pparently repla,ces the
galena. rrhe galena carries silver to the extent of 4'32 ounces
to the ton of 2,000 lbs. ,(Assay by D. Millin, University of
the \Vitwa.tersrand.) The fluorspar of this quarry is generally white,
colourlAss or grey, and both the cleavable and compact splintery
varieties are found. It is of a similar high degree of purity as that
of the other large deposits of the mineral; that is, 98 per cent. Ca F2
and over.
THE FLUORSPAR.
Two types of fluorspar are found: (1) a massive crystalline
and dea.vable variety, which is usually transparent, except in the
deeply coloured varieties, and has a vitreous lustre; (2) a massive,
compact variety without any definite cleavage, having a splintery
fracture and generally dull in appearance. The latter is not nearly
so common as the former. It occurs chiefly in the fluorspar quarry
on Buffelshoek, and in the No. 2 Quarry on Witkop, and in some
other small workings on the same farm. It is generally associated
with the cleavable variety.
In one of the smaller workings on
\Vitkop, normal cleavable fluorspar on the outcrop passed into friable
36
TRANSACTIONS OF 'l'HE GEOLOGICAL SOCIETY OF SOU'l'H AFRICA.
inc.oherent powdery fluorspar a few feet below the outcro.P.
On
o.utcropping surfaces the cleavable variety usually shows up with ·
its cleavage cracks accentuated by weathering, and is often stained
br.own or grey. The massive, splintery variety on the surface turns
white, and has the appearance of unglazed porcelain. Under the
micro.scope this variety was seen to be traversed by numerous small
fractures, but no cleavage cracks. According to analysis, both varieties
are pure calcium fluoride.
T'he absence .of well-defined crystals has already been mentioned.
The range of colours represented in the fluo.rspar of the various deposits
is considerable. In the cleavable varieties the following colours were
noticed : (1) Co.lourless.
(2) White.
(3) Pale blue and bluish-green.
(4) Yellowish-green to sea-green.
(5) Pale violet to deep purple.
(6) Brownish claret colour.
In the massive variety:(1) White.
(2) Grey.
(3) Pale mauve.
The water-clear, flawless "optical" fluorspar is comparatively
rare.
Usually this variety contains numerous small white cloudy
spots, which proved t.o be due to concentration of numerous small
liquid inclusio.ns. The different varieties of fluorspar were all examined
micro.sc.opically, with a view t.o ascertaining the possible cause of the
. colour9,tion. This method of examination, however, did not throw
any light on the matter, and in most cases the colours were such
that thin fragments and sections appeared colourless.
In some
specimens from the main qua.n-y at l\1almani Oo.g some violet splashes
could be seen in otherwise colourless flourspar. In these spots the
colouration was sufficiently intense still to be visible in thin section,
and were seen to. be irregular cloudy haloes around a central cavity,
which probably contained a crystal of some mineral, but in grinding
the sertion this was ground away, and the distinctly circular outline
of the cavity was all that could be seen.
.
The subject of the colouration of fluorspar, as well as that of
other minerals, such as quartz, halite, and certain gem stones, like
the diam.ond, topa,z, sapphire and others, has long been discussed
by many workers. A great deal of experimental work and examination
by chemical and physical means has been done in order to explain
the various colours that are met with in the above-mentioned minerals.
The coloura,tion has been variously attributed to minute quantities of
inorgaDic o.xides and to hydrocarbon compounds. The presence of
these substances in the rock, which is replaced, especially if it is a
limestone or dolomite, is well kno.wn. As far as hydrocarbon compounds
are concerned, the dolomite in this area contains a fair amount of
FLUORSPAR, LEAD AND ZINC DEPOSITS OF THE WESTERN TRANSVAAL.
37
carbonaceous matter, which give the rock a dark colour, and which
are visible in thin section.
Iron and manganese oxides are also
common impurities in the dolomite. (See Analyses previously given.)
A great deal of experimental data about the behaviour of the
.colour of fluorspar when heated, either alone or in the presence of
various gases and solutions, has been collected. The influence of
various electric rays, 'and of rays given off from radio-active compounds,
has also been noticed. Nevertheless, up to the present, no definite
proof of the cause of the colouration has been arrived at. \Vyrouboff
(Bul. Soc. Chem., Paris, 1866, pp. 5, 16,334) (N. Jarb., 1867, p. 473;
1869, p. 235) found a loss of weight on heating and decolourisation,
and detected the presence of hydrocarbons in the gases given off.
He concluded, therefore, that the colouring matter was various
,hydrocarbon compounds, probably derived from the replacement of
carbonaceous limestone which constituted the material out of which
thefiuorspar \vas formed. Pearsall (Pogg. Ann., 1831, p. 22 and
581) found that fluorspar decolourised by heat was. recoloured under
the influence of electric discharges, the colours produced being usually
blue or liJac. According to Goldstein (Sitzb. Ak., Berlin, 1891, p. 225)
exposure to cathode rays had a similar effect of recolouring fluorspar
decolourised by heat.
The influence of rays given off from radium compounds was
investigated by Berthelot (Compt. Bnd., 1906, p. 43 and 477) (N.
Jarb., 1908, p. 2, 170). He found that t,he original colours were
clumged or rendered more intense, whereas fluorspar decolourised by
heating became recoloured again, but not necessarily to the original
colour. Generally, the colours produced in this. way were not very
lasting. DoeIter (Zitzb. Ak. Wiss. Wien., 1908, p. 1312, and Radium"
Dresden, 1910, pp. 76, 119, 127) points out that the behaviour of
fluorspar decolourised by heat under the influenee of emanations from
radium compounds could hardly be attributed toO the presence of
hydrocarbons. rrhe analagous behaviour on glasses containing calcium
fluoride indicated that the colouration of natural fluorspar could not
be produced by manganese compounds or organic substances unless
the colouring matter were in chemical combination with the CaF'2.
As the most probable cause for the colouration, DoeIter considers a
colloidal colouring matter,probably Ca, or also a kind of sub-fluoride
or some other colloidal calcium compound.
The fluorspa,r in most cases contains numerous inclusions,
some of which are spherical and stand out with a very
distinct border, when examined in thin fragments under the
microscope.
These a,re undoubtedly either liquid or gaseous
inclusions, and in some. of them small, highly refracting spots could
be seen, which are presumably small crystals within the inclusions.
These inclusions often run in lines and series of parallel lines, but
do not seem to occur along any particular crystallographic directions.
The liguid inclusions are sometimes grouped together in clusters,
.and give rise to the cloudy spots frequently seen in the fluorspar.
38
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
rrhe size of these inclusions varies from, that of 1j100th of an inch
to ultramicroscopic dimensions. The small solid inclusions appear to
be quartz, and the insoluble residue obtained by dissolving crushed
fluorspar in acid was found to be made up entirely of quartz grains.
All the varieties of fluorspar were found to be phosphorescent
in some degree, the colourless as well as the coloured varieties, and
the fluorspar from the surface. as well as that from depth in the
quarries. The most highly phosphorescent varieties were the pale
brownish wine-coloured specimens from Malmani Oog, the dark violet
varieties from the same place, and the yellowish-green variety from
No. 1 Quarry on Buffelshoek. rrhe other varieties were all feebly
phosph orescent.
All varieties lost their colour on henting between 300 and 400
degrees C. and then also lost the property of phosphorescence. None
of the varieties are phosphorescent below 100 degrees.
On heating the fluorspar decrepitated violently, especially the
deep violet variety, which often shattered with explosive violence.
This decrepitation has often been attributed to the expansion of gases
in the small liquid or gaseous inclusions, but this is not the case, as
all varieties were found to contain these inclusions, and on being
examined after heating to red heat, the fluorspar fragments were found
still to contain the inclusions in an apparently unchanged condition.
r:rhe decrepitation is undoubtedly connected with the cleavage, as it
was found that all the cleavable varieties decrepitated on sudden
heating, whereas the massive non-cleavable varieties could be heated
to high temperatures without any cracks developing in the mineral.
A remarkable fact about the fluorspar from this district,. especially
that of the larger deposits, is its extraordinary purity. The material
exported , and all the tonnages given for production are for mineral
which analyses over 98 per cent. CaF2. In the case of t,h e larger deposit,s,
except for the occasional presence of sulphide minerals, the bodies
were of pure solid fluorspar, and could be quarried and transported
directly and with hardly any sorting. The foreign materials, such
as the sulphides and bit.s of country rock attached to lumps of fluorspar
can easily be recognised even by nat,ive workers and discarded. In
the case of the fluorspar derived from the smaller vein-like deposits,
admixture with the country rock is much more likely, small fragments
sticking to, and being included in, the fluorspar; consequently, more
careful sorting and hand-picking is necessary. In spite of this,
however, the min~ral from these sources sometimes fails to give the
necessary percentage of CaF2 for truly high-gmde fluorspar required
for export purposes.
As stated previollsly, the two main quarries in the district have
produced roughly 40,000 tons of high-grade fluorspar. The visible
sources of supply are, however, now practicnlly exhausted, as far as
can be judged from the present state of development in the respective
~uarries. As they have not been cleaned out completely, t,he fluorspar
III these quarries may, on development, be found to continue either in
depth or as lateral off-shoots.
FLUORSPAR, LEAD AND ZINC DEPOSI'l'S OF THE WESTERN TRANSVAAr..
30
A large number of surface indications of small fluorspar gash"eins or impregnations have been opened up, but have so far not led
to the discovery of further large massive deposits. This, however,
does not prove their non-existence, and it is quite within the bounds
of possibjJity that· other such large massive deposits are yet to be
discovered. The application of geophysical methods, electrical or gravitational, of prospecting would undoubtedly be extremely useful in this area.
'1'here are, nevertheless, still large tonnn,ges of fluorspar available in
the widespread irregular replacements and vein-like occurrences in the
dolomite. These will naturally entail higher costs of working, but vdll
nevertheless undoubtedly be profitably exploited in the future.
LEAD AND ZINC DEPOSITS.
The occurrences of lead and zinc respectively in small quantities
has already been mentioned in connection with fluorspar deposits.
Galena and zincblende is associated with the fluorspar in .both the
large deposits of the latter mineral, and also on several farms in
connection with the smaller vein-like occurrences of fluorspar: for
instance, on Malmani Oog, Strydfontein, Kaalplaats, Winterhoek,
Hhenosterfontein, Paardevallei, etc. The galena and zincblende occurs.
with fluorspar in small veins, and usually occupies the centre of the
vein. In a few places these minerals occurred in silicified dolomite
associated with pyrite, pyrrhotite and siderite.
On Zendelingspost
and also Paardevallei, a small quartz vein with galena in the middle
of it was found cutting through the dolomite. Similar quartz-galena
veins occur on Klaarstroom, according to Humphrey. 17
Other deposits of zincblende and galena occur on the farms
\Vitkop, Buffelshoek, Doornhoek, Bokkraal, Rhenosterhoek.
The
deposit on \Vitkop was worked bYLhe Blane-Witkop Zinc Co., Ltd.,
from 1909-1911, with a production of 200 tons of zincblende per month;
and later again by the F. T. Blane .1VIining Co., during 1921 and 1922,
cluril\g which period 4,000 tons of high-grade concentrates were shipped
to the European market.
From Hhenosterhoek and Doornhoek intermittent production took
place, but no accurate statistics are available. The former has novv
been closed down for a considerable period, and the latter is still being
worked on it small scale. Small quantities of galena have been taken
from a number of other farms, such as Strydfontein, Kuilfontein,
Rietspruit, Kaalplaats, at various times, but no actual systematic and
continuous mining has been carried on.
The galena from KaalpJaats 97, where it occurs in soft manganese
earth in the dolomite, is said to contain from ten to hvelve ounces of
silver to t,he t'on of 2,000 Ibs. (from statement by F. rr. Blane, Esq.,
Buffelshoek).
17
\-Y. A. Humphrey.
Ann. HepoTt Geol. SUL, Tvl., 1908, p. 154
40
TRANSACTIO~S OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
ZINC DEPOSITS.
lVitlwp.-The occurrence on \Vitkop is particularly interesting
The shape of the deposit is roughly conical, with a surface diameter
of 90 feet, and widening gradually in depth. The other walls of the
-cone consist of tremolite dolomite, which passes outwards into less
highly altered and EWidently into normal dolomite. The rim of tremolitic dolomite is only a few feet in thickness. The material filling the
interior of the cone ·is a kemolitic dolomite brecci8, cemented together
with c8lcite.
The chief ore mineral of the deposit is zincblende,
:although in the upper levels a certain amount of galena was also
found.
Associated with these minerals there occurs also pyrite,
pyrrhotite, chalcopyrite, and Anderson 18 reports marcasite and arsenopyrite as well. In the upper oxidised zone, secondary lead and zinc
minerals occur. The zincblende and other sulphides occur principally
along the periphery of the cone, the width of the ore body varying from
-a few inches to a few feet. Occasionally, zincblende veins are given
off from the outer body into the cone in a radial direction, and in a
few instances these radial veins were found to expand into large ore
·chambers containing considerable tonnages of zincblende.
The deposit has been opened up by vertical shafts and circular
drives at 50-ft. levels to a depth of 250 feet.
At the time of the
writer's visit, however, on account of the mine having been closed
for several years, all but the upper levels were flooded , and con-sequently. the deposit could not be studied directly in depth. A large
number of representative specimens were, however, collected from the
dumps and afforded material for microscopic investigation to determine
the relations of the ore minerals to each other and to the country rock.
The outcrop of the deposit is marked by a circular rim of cellular
brown gossan rock projecting in places about two 01' three feet above
-the general level of the ground. The oxidised zone extended to an
average depth of only ten feet, although in places secondary lead and
zinc minerals were found to a depth of fifty feet. The outcropping
gossan ",vas composed largely of zinc silicate hemimorphite and the
zinc carbonate smithsonite or calamine.
These minerals formed a
c.ellular brown porous rock impregnated with limonite, both as the
powdery earthy variety, and as the more compact variety with typical
botryoidal surfaces. Smithsonite was also found in crystalline aggregates and in masses 'with rounded botrvoidal surfaces. The smithsonite
crystals were usually imperfectly form~d, and often w'ith rough curving
faces. In one specimen, however, distinct scalenohedral crystals,
although with slightly rough faces, could be made out. Crystals of
hemimorphite were not observed. Le.a d oxidation minerals were not
abundant, although some specimens of galena, partly altered to anglesite and encrusted with cerussite "vere noticed. From the collection
in the Geological Museum of the University of the 'Vitwatersrand, a
18 W. Ande<l'son.
118-128.
Tmns., Geol. Soc. of S.A., Yol XVIII., ]915, pp.
FLUORSPAR, LEAD AND ZINC DEPOSITS OF THE WESTERN TRANSVAAL.
41
specimen of galena was obtained, which was covered on the outside
by the red oxide of lead, minium, and encrusted on top of that with
beautiful crystals of cerussite.
The Country Rock.-1'he vvalls of the brecciated cone are composed of tremolitic dolomite, which is dark blue in colO'ur.
The
tremolite needles are usually frm)]. half an inch to an inch in length,
but locally radia,ting clusters of tremolite crystals up tOt four inches.
long were observed. rrhey are scattered irregularly whether singly or
in radia,ting bunches 0'1' tuHs through the rock. Microscopic examination shows that the tremolite needles obviously replace the dolomite,
1lP, elsewhere the dolomite is carbonaceous and is often crowded with
small black inclusions.
Brucite also
occurs in the rO'ck in
irregular grains, often enclosing dolomite and tremolite grains in
poikolitic fashion. Sulphides, chiefly pyl'l'hotite and sometimes pyrite,
are common in tremolitic dolomite, especially within a foot or so from
the contact of the ore body. It is notev\'odhy that brucite usually
'Occurs in close association with the pyrrhotite, often enclosing the
latter completely or occurring very close to it. This feature was also
noticed at the No.1 Fluorspar Quarry on Buffelshoek.
The Breccia.-The breccia filling up the interior of the cone is
made up of angular fragments and larger blocks of the dark coloured
tremolitic dolomite, whereas the cementing material is pure white
calcite, the fragments of the filling material, consequently, displaying
,1 striking contrast.
rrhe fragments of dolomite vary considerably in
size from a fraction of an inch to large blocks more than a foot across.
The larger fragments are ahvays distinctly angular, and their
boundaries clearly defined. \Vhen the fragments are sman they sometimes seem to have lost their angularity, and seem to have been partly
absorbed by the calcite.
The calcite is usually fine-grained, but
sometimes large crystals with cleavage surfaces several inches across
were noticed. In hand sections and also in microscopic sections it can
be seen that some of the tremolite needles extend from the dolomite
into the calcite.
Tremolite even O'ccurs independently in the
calcite, but not nearly so abundantly as in the dolomite fragments. The dolomite fragments are indistinguishable from. the
dolomite of the outside of the cone. Brucite occurs in the dolomite
fragments of the breccia as well, and often in small grains along the
contact of the dololnite fragment and calcite.
The occurrence of
tremolite in the calcite cementing breccia indicates that the metamorphism causing the formation of the tremolite must have been
subsequent to the formation of the brecciated cone. The cementing
material is pure calcite, and, therefore, during the process 'of metamorphism some magnesium and silica-bearing solutions must have
penetrated the dolcite to some extent. The dolomite, with its chert
bands and secondary quart.z, would contain the necessary elements
for the production of tremolite, but, nevertheless, there must have
been a marked redistribution of Inaterials, especially of silica.
42
rI'RANSAC'l'IONS OF THE GEOLOGICAL SOCIETY OF SOUrI'll AFRICA.
The altered tremolitic zone extends outwards only from about
four to ten feet round the cone.
The tremolite crystals gradually
diminish in size and number, and although outcrops are not continuous
nor very numerous, the dolomite forty feet away from the deposit is
more or less normal dolomite without any metamorphic minerals.
Occasionally in depth thin bnnds of shaly rock, often carbonaceous
and graphitic, have been encountered on the wall of the brecciated
cone. The carbonaceous character of the dolomite, and also the
occasional shale bands might be the source of the material which gave
rise to the peculiar smooth and shiny nodules of the carbonaceous
mineral which occurs locallv in the breccia. The mineral occurs in
the cementing calcite of th; breccia, and has been fully described by
C. Anderson. 19 It was probably formed from hydrocarbons derived
by distillation from the dolomite during the hydrothermal metamorphism of the brecciated cone.
The Ore.-The ore minerals occur principally along the periphery
of the 00ne. The zincblende is found completely around the cone,
although it varies in width from a few inches to a few feet, and
occasjonally widens out into large chambers.
It is massive and
dark golden-brown in colonr, translucent in thin splinters, and has
the typical resinous lustre. Some darker and almost black zincblende
is also met with in places. It can be seen to have replaced both
the tremolitic dolomite on the outside as well as the breccia, and
specimens can be obtained showing zincblende in tremolitic dolomite
as well as in the calcite of the breccia. In thin sections it can be
seen replacing the tremolite needles of the dolomite, although dolomite
and caloite seem to have been more readily replaced. In some
specimens the zincblende can be seen to have replaced the dolomite
only, and the needles of tremolite are left projecting into the
zincblende.
Other sulphides noticed were pyrite, pyrrhotIte,
chalcopyrite and galena. The writer was unable to obtain any material
showing arsenopyrite and marcasjte, mentioned by Humphrey and
Anderson (17 and 18 op. cit.) . The polished sections of specimens
showing these minerals in association with one another, the following
order of succession was made out: pyrrhotite, pyrite, chalcopyrite,
galena and zincblende.
In the polished sections the pyrrhotite is
seen to replace the dolomite, often lea,ving the hemolite unaffected.
The pyrrhotite usually occurs in irregular masses, whereas the
pyrite sometimes assumes distinct crystal outlines.
Pyrite or
pyrrhotite often occur as a border between the zincblende 0'1'
galena and the country rock. In specimens showing both zincblende
and galena, the former is of apparently later formation, as it is seen
to replace the galena, and to. run through it in streaks and veinlets.
Chalcopyrite is not found in large amounts, and is neVel'
seen in hand specimens.
In all cases, however, the polished
sections showed inclusions of chalcopyrite in zincblende.
The
total absence of fiuorspa,r as a gangue materjal is a remarkable feature
19
T1·ans., Geol. Soc. S.A., 1915, Vol. XVIII., p. 129.
FLUORSPAR, LEAD AND ZINC DEPOSITS OF THE WESTERN TRANSVAAL.
43
of this deposit, and strongly differentiates it from the occurrence of
zincblende on Buffelshoek.
Buffelshoek.-On the S.W. portion on Buffelshoek 284, and only
half a mile from the main road between Ottoshoop and Zeerust,
zincblende occurs extensively as a dissemipated replacement of the
dolomite, associated with large quantities of fluorspar. The deposit
has been opened up by several vertical shafts, and development
work has been done, which has exposed considerable quantities of
low grade ore. A costly concentrating plant was erected and completed
in 1914. Shortly afterwards, after only a trial rUll, the mine was
closed down, and no actual production of zincblende took place.
The deposit appears to be distinctly different from the Witkop
occurrence, in that it has apparently no distinct structural boundaries,
but is simply an impregnated zone of dolomite with zincblende and
fluorspar. In character it resembles many of the sma.lles occurrences
of fluorspar, some of which occasionally have zincblende together with
fluorspar. The surface area over which indications of fluorspar and
z:ncblende are met with is of no particular shape, but could possibly
be included in a circle 300 feet in diameter. On part of this area
there is a slightly elevated circular rim of black hard r0'ck, often with
large cubical crystals of pyrite.
This rim or circle does not,
however, extend in depth, nor does it seem to have any
structural significance. The impregnated zone of dolomit-e is bound
both within ~nd without this rim. The rocks examined in thin section
under the microscope are seen to be indurated shale with much
da,rk carbonaceous matter, and in places it becomes a fine-grained
dark siliceous dolomite, with occ.asional crystals of brucite and talc.
A very similar rock is found close to the No. 1 Fluorspar Quarry
on the same farm. The dolomite in the impregnated zone is fairly
normal, although hemolitic in places. The dolomite in which the
fluorspar and zincblende are found is more highly altered and full of
flakes of talc. The zincblende is seen to replace both the minerals,
but the dolomite more readily than the talc, and jn. some specimens
zincblende can be seen enclosing flakes of talc, having replaced the
interstitial dolomite.
Fluorspa,r occurs either independently 0'1'
together with zincblende, sometimes in irregula.r patches but
more often in a branching net-\vorkl of gash-veins.
In S0'me
places the rock has the a,ppearance of a dolomite breccia cemented
together with zincblende. A small amount of galena and pyrite was
also found in places. Flourspar occurred in branching veins as in
other deposits, and in places veins two feet thick have been
encountered.
Some secondary quartz and calcite is also associated
with the fluorspar and zincblende
LEAD DEPOSITS.
Of the deposits in which lead predominates, those on Doornhoek
32 and Rhenosterhoek 211 are the most important,. T'he deposits
have been previously described by Humphrey and Anderson (17 and
18 op. cit.)
They exhibit many common features. In both cases
the deposits are situated in the dolomite close to its contact with
44
TRANSACTIOXS OF TIlE GEOLOGICAL SOCIE'rY OF SOUTH AFRICA.
the shales and quartzites of the overlying Pretoria Series. In both
cases the galena is found in large masses of manganese ea.rth, which
seem to occur in the dolomite along a fairly persistent horizon about
50 to 100 feet below the Pretoria Series.
This manganese earth
horizon has been found on Rhenosteriontein 50, adjoining Doornhoek
Doornhoek.-The deposit on DoorIihoek is situa,ted on the S.W.
slope. of a hill, on the top of which the rocks of the Pretoria Series
still occur as a covering layer. It was originally worked from adits,
or rath8r incline shafts, from the side of the hill, but on a.ccount
of the treacherous nature of the loose incoherent manganese earth,
this method of working became impossible, as the workings proceeded
into the side of the hill. Since then a large amount of manganese
earth has been removed, and the deposit mined as an' open working.
The tremendous quantities of loose ma,terial in proportion to the ore
obtained, however, still remain a serious obstacle in the exploitation
of the deposit. It is known locally as the Hammerkop Lead Mine
Galena is the chief ore mineral, although small quantities of zincblende
\vere also found. The galena occurs usually in curiously-sha,ped lumps,
varying in weight from a few pounds to several hundred pounds. In
places the galena. is found running in more or less well-defined veins
through the manganese earth. rJ.lhe occurrence of these lumps and
veins of galena is, however, extremely erratic, and the nature of the
mining, for the greater part, has simply been a process of grubbing
for galena in the soft, loose manganese earth. The manganese earth
is of chocolate-brown coJour, and a loose, friable, porous material.
It is, nevertheless, quite firm and coherent in places, yet soft enough
so that- one can put the sharp end of a, hammer or pid{ into it
without much force. In such cases, the firmness is probably due
j:o the seconda,ry calcium carbonate, which acts as a binder.
Thin
seams of calcite are sometimes seen running through the manganese
earth. Where exposed on the side of the open cutting for at least
50 feet, it is seen to exhibit some degree of stratification, and in
plFtces chert bands are seen running through the manganese earth.
Large blocks of dolomite are occasionally met with in the loose earth,
n,nd these usually show rounded surfaces.
'The dolomite blocks
are generally quite fresh in the interior, but on the outside
there is a la,yer where the
dolomite is partly decomposed
and earthy. From the .nature of the manganese earth, that is, the
stratification, the chert bands running through it., the occasional
residual boulders of dolomite, it is clear that it has been derived
from the dolomite.
In the masses of unchanged dolomite occuring as loose boulders in
the manganese earth, and also in the unaltered dolomite met with
at the furthest end of the workings, galena was found oceurring as
irregular replacement of the dolomite, and also in veins with calcite
and qllartz as gangue minerals. The dolomite also contaiL.S patches
of calcite, and small quantities of zincblende, pyrrhotite and pyrite,
2nd in places small net-like aggregates. of talc crystals. Fluorspar
FLUORSPAR, LEAD AXD ZINC DEPOSITS OF THE WESTERN TRANSVAAL. ~15
was only met with in a single instance, and that as a seam about
four inches thick cutting through the dolomite on the floor of the
deposit. In thin section, the dolomite containing these sulphides
is seen to be fine-grained, and sometimes full of dark carbonaceous
matter. \Vhere it c.onta.ins grains of pyrite and pyrrhotite and
zincblende, the dolomite is usually coarser and apparently recrystallised.
All these sulphides have clearly been formed at the expense .of the
carbonate. Brucite is developed in small flakes near .or at the c.ontact
of the sulphides with the dolomite. Except for these minerals, the
dolomite is not highly metam.orphosed. At the furthest end of the
w.orking, a dark-col.oured rock, containing numer.ous glistening flakes
of brucite , two to three m.m. in diameter, ,,, as encountered .
On the sout,hern side of the deposit. there is a very muc:h
broken and disturbed z.one, in which the minerals occur in an
oxidised state. ITrom this part of the working specimens ,\-vere obtained,
which were composed of 'galena, with anglesite and cerussite,
pyromorphite and vanadinite, and often the red and yellow oxides of
The galena in the
lead, minimum and massicot, and also limonite.
manganese earth was sometimes f.ound with a c.oating of minium.
Pyromorphite occurs chiefly as greenish-yellow ' incrustations .on
dolomite, galena and other minerals, whereas vanadinite is usually
in beautiful red hexag.onal crystals , sh.owing the pri sm basal pinacoid
and small pyramid faces.
Rhenoste1'hoek.-The deposit .on Rhen.osterhoek is situated about
half a mile south .of the so-called Lead Mines Road running between
Zeerust and Kost,er. This r.oad is now the route of a S.A.H. motor
'bus service. The deposit lies on the S.\V. slope of a· hill, along one·
of the main head streams of the Gro.ot. Marico River. It was the
first deposit to be worked in the district, and is known loca.lly as
the 01(1 Lead Mine, or Brays Lead Mine. It has pr.obably yielded
more galena than any of the other deposits, but on account of the
intermittent nature .of the mining activities, no accurate statistics are
availa ble. As in the case of D.oornhoek, the galena is found in leose
mangan8se earth. At the time of t,he writer's visit, it had been
closed down for several years, and consequently the actual "vorking
faces could not be inspected. The deposit had been worked, first as
an open w.orking, and later from an adit into the side of the hill,
and by drives in vari.ous directions up to 500 feet from phe entrance.
AU the workings were in seft manganese earth or dolomite partly
altered to manganese earth, in which the galena occurred in lumps,
and sometimes in vein-like bodies, as in the case .of Doornhoek. The
manganese earth at the entrance of the adit was s.oft and porous,
though coherent enough to form a geod face, and showed traces of
stratific8.tion. In places cherty layers were seen in it running parallel
to the direction .of stratification.
According to verbal informa,tion
from Mr. F. T. Blane, the workings stopped everywhere a.t the end
of the drives in s.oft, pa.rtly-altered dolomite, containing spots and
replacements of galena, zincblende, calcite and qua.rtz. Apparently
46
TRANSACTIONS OF 'I'HE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
some unaltered dolomite was passed through, as specimens collected
from the old sorting dump were similar to the dolomite met with in
the Doornhoek deposit, and contained a similar association of minerals.
The mine was eventually closed down owing to the difficulty of
preventing the fall of the loose manganese earth in the deeper portions
of the drives. The galena obtained was of good quality, and carried
silver to the extent of 9 to 15 ounces to the ton of 2,000. 20
Boklaaal.-A deposit containing both lead and zinc ores is situated
on the northern portion of Bokkraal 300. It is situated on the S.-W.
slope of a prominent valley, running in a N.N.\V. direction, at the
head of which there are several strong springs rising out of the dolomite.
They constitute oneot the principal sources of the Groot Ma,rico River.
The valley is cut through the lowest shales of the Pretoria Series,
chert breccia and the upp'e l'most beds of the Dolomite Series. In the
Pretoria shales there is considerable local folding, indicated by dips
of 20 degrees and less in varying directions within short distances
on the surface. This is probably due to the sinking and adjustment
on account of the existence of caverns in the underlying dolomite.
The chert breccia or conglomerate occurs between the shales and the
dolomite, and varies considerably in thickness. On the N.E. slope
of the valley, it is only about 10 feet thick, whereas on the S.W.
side of the valley it is fully 30 feet thick, and the crest of the hill
on that side is composed entirely of chert breccia outcrops.
The
dolomite underlying it contains a great deal of secondary quartz in
irregular replacements, and is also cut by a series of thin parallel
quartz stringers, running in a constant direction 30 degrees east of
north (Magnetic). This seems to indicate lines of weakness in this
direction. rrhe ore deposit is situated on the S.\V. slope, not far
from the head of the valley . The main 'bus road from Zeerust
crosses this valley, and the deposit is about a mjle from the Lead
Mines Post Office and Store, but on the opposite side of the valley.
The deposit is in dolomite, quite close to the, contact with the
overlying chert breccia (Fig. 5). On the surface in the neighbourhood
D i a:s ram m a h c,
HI/. E
_~ho"'ln:'l the Lead - Zinc.
Sec tiP n
Dep...!sil' on Bokkraal
(.30'1)
s:..w
Dolomite.
.5 ..... 1(" 1~~"_ _ _........._ _ _'-,-'OO_ _ _....::.;2000F'ccr
FIG. 5 .
. 20 'V. VetTsfeld, "Base Metal Resources of S.A."
Mmes and Ind., Pret., 1919, p. 45.
Memo. No.1.
Dept.
FLUORSPAR, LEAD AND ZINC DEPOSITS OF THE WESTERN 'l'RANSVAAL.
47
t here are a few outcrops of galena, where it occurs as a replacement
of the dolomite, and sometimes with quartz and siderite in distinct
veins, with the same prevailing dil'ection of strike as previously
mentioned. 'These veins extend downwards, and some of them have
been found to give lateral off-shoots in the plane of stratification,
Hnd to open up into ore bodies .. 'rhe deposit has been opened up by
three 'i'ertical shafts to 100 feet, and these have been connected with
drives at two levels. An adit was also driven into the side of the
hill to connect up with the upper level. The ore exposed in the
workings is in fiat, lense-like bodies with some vertical connecting
"eins. The ore so far exposed is mostly composed of lead and zinc
·o xidation products. It is genera.l1y a cellular mass of angle site and
cerussite, smithsonite and hemimorphite, together with limonite,
psiJomelane and occasionally minium. In some specimens, psilomelane
with typical botryoidal surfaces were seen to be encrusted with
radiating aggregates of colourless flattened prismatic crystals of
hemimorphite. These were in turn covered in places by white flattened
l'homboh9dral crystals of smithsonite. In this oxidised portion of the
dolomite is also largely converted into manganese earth, and
{)(Jcasionally lumps of galena occur in it. No zincblende was noticed
in the upper level. In the lower levels both zinc bIen de and galena
'were met with, but the ore was, nevertheless, still in an oxidised
condition. No doubt, at lower levels, bodies of unoxidised zincblende
and galena will be met with, and it is possible that, as in the case
of ,\Tit·lmp, zincblende "vill be the predominat.ing mineral in the deeper
levels. In the quartz-siderite reefs galena usually occurs in the quartz.
In places the quartz shows crystal outlines, as if it had crystallised
in open spaces. In such cases galena. is occasionally found in the
interstices between the crystals. The quartz is coarsely crystallised
milky-ve.in quartz with rhombohedral crystals of siderite. The dolomite
in contact and close to these veins does not seem to have been
greatly effected. It is the normal dark blue rock, which in thin
sections showed none of the metamorphic minerals met with in the
other deposits. In places in the dolomite there are fissures filled with
dad\" chocolate-brown cellular material full of open spaces lined with
small glistening crystals of calcite.
HESUl\fE AND GENETIC CONSIDERATIONS.
There is a mineralised area at least 150 square miles in extent
in the Dolomite Series of the Marico District, \Vestern Transvaal. In
this area lead and zinc minerals and fluorspar occur as veins, pipes,
and irregular replacement deposits. 'rhe deposits are found principally
in a belt about three miles wide in the dolomite, along its contact
with the overlying Pretoria Series from Bokkraal 300 in the S.-E.
to Klaarstroom 55 in the N. W. South of Zeerust and west of
Ottoshoop this belt widens and extends southwards for about ten
miles from the contact as far as Malmani Oog 101 (Fig. 2). The
extension southwards of the mineralised area is coincident with the
48
'l'RA);TSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
widening of the dolomite outcrop in the same direction discussecl'
previously under the heading of " Geological Structure."
The dolomite in this area has undergone hydrothermal metamorphism causing the extensive. development of tremolite, talc and
brucite .. This metamorphism "vas followed soon aftenvards by the
introduction of solutions carrying sulphides of lead and zinc, iron and
copper and fluorine and silica.
The occurrence of tremolite in the upper layers of dolomite has
been explained by Hall (op. cit. l l ) as due to distant contact metamorphism resulting from the intrusion of the Bushveld Igneous
Complex into the rocks of the 'Transvaal System.
The contact·
metamorphic effect on the rocks of the Pretoria Series is seen as feU'
awav from the contact as the chiastolite slates below the Timebnll
Qua~tzite immediately north of Zeerust. Tremolite is, however, also
found locally in the zone of metamorphosed dolomite around the leadzinc deposits on \Vitkop, and also on Buffelshoek and Strydfo_ntein.
'11he close association of these occurrences of tremolite and of the
other t\VO minerals, talc and brucite, with the various ore deposits,
and their wide-spread distribution in the dolomite throughout the
mineralised area, make it certain that their formation must be due
to hydrothermal metamorphism just preceding and accompanying the
introduction of the metallic sulphides and fluorine.
The deposits occurring in the belt along the contact, such as those
on Bokkraal, Rhenosterhoek and Doornhoek, are in the upper layers
of the dolomite, usually within a 100 feet from the top of the series.
They are principally lead deposits. Galena is found in veins and as
an irregular metasomatic replacement of the dolomite, accompanied
usually by small quantities of other sulphides and some gangue
minerals. Zincblende, pyrite, pyrrhotite, quartz, calcite and fluoride
in the case of the Doornhoek deposit, and. zincblende, pyrite, quart7.
and siderite in the case of the Bokkraal deposit. The other deposits,
such as those on Buffelshoek, \Vitkop amI 1falmani Oog, which lie
·further to the south, are chiefly zinc and fluorspar deposits. Galena
is usually present, but in quite subordinate amounts.
Although
undoubtedly at a lower horizon Hum the lead deposits, they are still
at least in the uppermost thousand feet of the Dolomite Series. Of
these deposits the group occurring fairly close together on Buffelshoek
and Witbank are again on a higher hori7.on than the deposit on
lVIalmani Oog.
In this latter deposit galena and zincblende were
present in only very small amounts, whereas chalcopyrite and pyrrhotite and pyrite were found in the deeper parts of the workings. The
vertical zoning in the occurrence of the different metals is interesting,
and conforms to that of the normal order of succession of metals
found in deposits of other parts of the vvorld. 21 The relative distribu21 R. H. Rastall, "Metallogenetic Zones."
Econ. Geol., March, 192~~,
Vol. XVIII. , No.2, pp. 105-121.
" The Geology of l\[etallifclrous Deposits. ".
Cambridge University Press, 1923, Chapter X., pp. 181-201.
FLUORSPAR, LEAD A~D ZINC DEPOSITS OF THE WESTERN TRANSVAAL.
49
tion of the various minerals in the different deposits can be best seen
in the accompanying diagram (Fig. 6). The deposits are tabulated as
far as possible in the order of the horizon in "\vhich they occur.
~
i:~~.:r·~_~
II!~
t/tJ:i5I!1
RJt«:" •• t& ... hQ~Ic.. dt
fum!!!!'
Z",,,L/end.
Cha/.. ~.'I~'
II
f~~
l':IdoM..
fusujw:
Q~
~tlP ,
':./clt..
I
D.~ ..,r
B"tf~/.h .. "
Zi"c
n../!!!".t
~ff./6"O' k
M~
L..atw-
JI.':::!..1o.!.!I.
Z'".C D.. ~M!t
~
II
~I! .. r.lk~
PI .. lma,,; 00,
FIG. 6.
From the order of succession found in these deposits, it would
seem probable that on deeper development the proportion of chalcopyrite would increase.
\Vhether this mineral exists in payable
quantities in depth naturally remains to be proved by actual
prospecting development.
In the area west of Ottoshoop, the Pretoria Series overlying the
dolomite has been completely removed, by erosion, and is found only
as far south as the tongue of these rocks extending into Buffelshoek
from the north' (Fig. 2).
As pointed out in the previously given
description of the Dolomite Series, however, the uppermost thousand
feet or so contains several horizons of interbedded chert, quartzite
and shale layers. It is significant that the deposits on Malmani Oog,
and some small occurrences of fluorspar, galena and zincblende, on
the southern part of Kaalpluats 97, occur just south, that is, below
a prominent horizon of these cherty and quartzitic layers. ' This is very
well seen neor the main fluorspar quarry on ]Vfalmani Oog. The
quartzites and chert layers lie about seventy-five yards N.B. of the
quarry on both sides of the ~Inlmani HiveI'. Thev are definitely interbedded with the dolomite, and are overlain by dolomite to the N.E.
rrhey are seen to dip at angles of about five degrees N.E.
In the group of deposits on Buffelshoek and "Witkop in practically
all cases interbedded layers of shale can be seen near or partly overlying the deposits.
In places they are broken through by the
deposit.
\Vhere erosion has partly removed the overlying shale
there is often ft slight depression with a, low circular rim usually
broken on one side.
This feature has been pointed out in the
individual description of some of the deposits. These slight circular
depressions in the case of the Witkop and I3uffelshoek deposits, have
50
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
given rise to the local prospecting belief that deposits of these
nlinerals are to be expected jn all circular depressions. This has,
unfortunately, not been borne out by experience.
The deposits that have been opened up some depth all reveal a
floor in which "the dolomite, chert or shale beds have been brecciated,
and the interstices filled up with secondary silica, fluorspar and sulphides such as zincblende, pyrite, pyrrhotite and chalcopyrite. In other
cases the deposits re801 ve themselves in depth into ramifying veins of
fluorspar, occasionally with zincblende, cutting through metamorphosed
dolomite.
All these facts indicate that the solutions carrying the
fluorine and sulphides must have come from below and ascending
along lines of brecciation, fault,s, or joint planes, replaced the dolomite
"where favourable conditions existed.
The \Vitkop zinc deposit is the only case where favourable conditions for the ascent of solutions can be definitely proved, that is, the
existence of a brecciated pipe or chimney prior to the mineralisation.
:F'or the other deposits definite structural lines, such as intersecting
systems of faults which would have acted as channels of ascent, and
which would explain the location of the various deposits, have not been
made out. There can be no doubt, however, that the crustal movements which gave rise to the folding and faulting in the Pretoria
Series, along the bend where the trend of the formation changes from
E.S.E. to N.N.\\!. south of Zeerust, also effected the dolomite in
a similar manner.
A system of fractures and zones of brecciation
would be formed as a result of such crustal movements, and even the
development of joint planes and other mirior fractures would be
accentuated. Such a disturbed condition of the rocks would naturally
be favourable to the ascent and penetration of magmatic solutions.
The occurrence of interbedded and covering layers of relatively
impervious strata at intervals in and on top of the dolomite would
act as barriers to the solutions, and provide favourable conditions for
the metasomatic replacement of the dolomite. The actual shape and
outlines of the deposits would be determined by local structural conditions. The shape of the \Vitkop deposit was undoubtedly determined
by that of the brecciated chimney. In the No.1 Fluorspar Quarry on
Duffelshoek joint planes seem to have played an important role. The
fluorspar deposit on Malmani Oog has the shape somewhat like that
of an asymmetrical mushroom, and is probably due to replacement
laterally along the crest of a shallow dome-fold in the dolomite
overlain by impervious layers.
Concerning the ultimate origin of the ore-bearing solutions one
can only speculate. It has been previously mentioned that with the
single exception of some diabase outcrops in Ottoshoop, no intrusion
of igneous rocks are known to exist in the area under discussion. The
nearest intrusive rocks into the ~rransvaal System are those of the
Bushveld Igneous Complex. In a direct line" the centre of this area
is at least thirty miles away from the norite of the Bushveld lopolith.
It is unlikely that the deposits of this area have any direct connection
FL"CORSPAR, LEAD AND ZINC DEPOSITS OF THE WESTERN TRANSVAAL.
51
\vith either the noritic 01' granitic phase of the main mass of the
Bushveld lopolith found in the northern portion of the Marico district
and in the Hustenburg district. Hed granite and norite are, however,
found in several places intrusive into rocks of the Ventersdorp and
'Transvu8J Systems outside and north of the syncline of Transvaal
rocks containing the Bushveld Complex, in the area included in Sheets
No. 14 (\Vitfontein) and No. 15 (Crocodile Pools) of the Transvaal
Geological Survey Maps. It is not unlikely, therefore, that intrusive
rocks of the Bushveld age, exist elsewhere outside of the great basin
formed by the rocks of the Transvaal System in the Central Transvaal,
and have not yet been uncovered by erosion. On the S.W. side of
the basin the outcrop of the Transvaal rocks is particularly wide, and ·
only very slightly dissected by erosion compared with the rocks on the
N.E. side of the basin in the Pilgrims Rest and Lydenburg area. In
the Western Transvaal, the country covered by the dolomit8 is a
plateau of an average elevation of close on 5,000 fe et above sea-level,
whereas proceeding northwards in the }\1:arico district the level of
erosion becomes progressively lower. The presence of such an intrusion, although highly speculative, is possible, and could be a source
of ore-bearing solutions. The deposits, on the other hand, may be
connected to some intrusion such as the alkaline intrusives of Secucuniland , the li'ranspoort line, and the Pilandsberg. Those intrusions
me known to have been rich in fluorine as shown by the occurrence of
fluorspar cemented breccias at· Leeuwfontein and the Pilandsberg.
These deposits, therefore, possibly belong to the crypto-batholithic
type of deposits derived from solutions emanating from hidden igneous
intrusions . 22
The temperatures o£ the solutions producing the metamorphism
of the dolomite, and responsible for the introduction of the ore
minerals, could not have been extremely high, for the minerals talc
and brucite are both hydrated compounds. The talc was undoubtedly
formed directly, and not by hydration of some previously existing
magnesium silicate. Brucite is usually formed by the hydration of
periclase, which is a high temperature mineral. The occurrence of
brucite in this case as the latest mineral, and occurring as interstitial
grains enclosing the other minerals, gives the impression that it was
also formed directly as brucite. Tremolite cannot be regarded a.s a
high temperature mineral either, as amphiboles are generally unstable
at high temperatures, and the presence of water is necessary for
their formation.
The presence of pyrrhotite, on the other hand, in the majori~y
of deposits, even at Doornhoek on .the uppermost horizon, indica.tes
that tbe temperature cannot have been very low. Pyrrhotite is never
found formed a,t low temperatures, and is generally a characteristic
sulphide of high temperature deposits. Taking the above facts into
22 'V. G. Emmo.ns, "Relations. t<? Metalliferous Lo~le Systems to Igneous
Intl'u,:;ives." AmerIcan Inst. of Mmlllg and Met. Engmeers. Paper read at
New York meeting, Feb., 1926.
52
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOU'rH AFRICA.
consideration, it appears that the temperature of formation of the
lead, zinc and fluorspar deposits, and the accompanying metamorphism
of the dolomite, must, have been caused by solutions of intermediate
temperatures. The terms "low, "intermediate ,. and "high" in
connection with temperature are used in the sense given them by
Lindgren in his classification of mineraI deposits.
Origin oj Manganese Earth.-The large masses of so-called
manganese earth found in association with some of the deposits,
notably those on Doornhoek and Rhenosterhoek, is an interesting and
characteristic feature of the deposits. Large masses of dolomite have
apparen,tly had the greater proportion of their ca,rbona,te matter leached
.out, Ip,aving only the remaining insoluble silica, alumina, iron and
manganese oxides and minerals, such as talc, brucite or tremolite,
where they exist in the dolomite. The leaching was probably gradual,
as the main structures, such as the stratification of the dolomite, ha,ve
been preserved. Unaltered chert layers are also still found running
through the manganese earth. The manganese earth is generally soft,
light and porous. Usuany it is loose and friable, but sometimes
coherent and firm, and in that case usually contains a certain amount
of secondary calcite. The specific gravity of the normal unaltereddolomite is 2'8, whereas that of the coherent porous variety of the
leached rock is 1·5.
rrhe leaching of the dolomite to produce the manganese earth
could hardly have been affected by ordinary ground water circulation.
A possible explanation is the action on the dolomite of decending acid
waters derived from the weathering and decomposition of sulphides
near the surface. Such weakly acidic solutions would leach out the
Oa and Mg carbonates, and at the sa,me time libera,te ca,rbon dioxide,
which, with water, might still further aid in the task of disolving
out the carbonate minerals.
It is unlikely, however, that this
process could have been very effective, except" perhaps, locally,
because of the fact that no large bodies. of oxidised sulphide ores exist,
at any rate not large enough to have brought about such a widespread
leaching of the dolomite. The weathering of the known ore deposits
would not have been sufficient for this purpose.
The nature of the manganese earth, and its association with the
ore deposits, ha,ve lead the writer to the conclusion that it must
be due to extensive leaching of the dolomite by ascending solutions
rich in carbon dioxide rising from depth at a late period along
similar channels as those along which the ore-bea,ring solutions travelled
previously. It is well known that in the final stages of the consolidation
of igneous magmas water vapour, -carbon dioxide and other acid gases
are given off in large quantities long after an other activities have
ceaRed. This would give rise to solutions weakly acidic in nature,
and probably slightly heated, ascending through the rocks above and
surrounding the intrusion. The effect of such solutions on dolomite
would be to dissolve out the Oa, and J\1g carbonates: and to leave
behind the othor insoluble matter. Not all of the carbonate ,vould
II
FLUORSPAR, LEAD AND ZINC DEPOSITS OF THE WESTERN TRANSVAAL.
53
be dissolved, and some of it would undoubtedly be redeposited as
secondary calcite. The Mg salts being the more soluble, would be
carried further away in solution. It is significant that in several
places on Doornhoek, Rhenosterfontein, Bokkraal and elsewhere,
fissures extending downwards have been notieed, along which the
dolomite is changed to a da,rk brown porous rock, often containing
a large amount of secondary calcite as a glittering incrustatiDn
lining the open spaces in the rock. These fissures nD doubt represent
channels of ascent of these carbonated waterswhieh brought about
the leaching, and which eventually must have become saturated wIth
Ca.C03, leading to the deposition of seconda,rycalcite. It is, a local
prospecting maxim that these fissures filled with dark porous rock
are "lead indications."
This has some measure of justification,
because when followed downwards the fissures have in some cases led
to large masses of manganese earth containing galena" but, obviously,
the association is incidental, and, consequently, not infallible. The
greatest development of manganese earth is to be found at the top
of the Dolomite Series not far below the covering impervious layers
of Pretoria shales. This, would be due to the spreading .out of the
ascending solutions .on coming in contact with the impervious layers,
and the eventual slow escape along the plane of contact, or through
occasional fissures in the covering layers.
In that way widespread
leaching of the dolomite would be brought about, giving rise to the
large masses of manganese eart,h in the dolomite.
The residual soil or earth formed at the surface by the weathering
.of the dolomite, as previously des'cribed, is composed of more or less
the same constituents, but the nature of the material is quite different
to thFtt of the manganese earth associated with the ore deposits. SDme
analyses of the different types of manganese earth a,re given below:D.
1.
J.
H.
K.
Si0 2 . . . . . . . . . . ..
19'1
22'9
85'78
58'55
5'3
Fe 2 3
33'S
3'0
3'8
11'4
5'1
FeO
...
°}
A1 2 0 3
l\lnO
'CaO ........ .
MgO ......... '"
CO 2 . . . . . . . . . '"
Loss on ignition .. .
~oisture ........ .
10'3
21'6
3'9
1'05
1'05
2'65
92'95
D.
1.
3'0
0'9
35'5
4'5
29'8
1'65
0'65
101'4
1'7
2'8
1'35
0'9
1'4
0'4
0'75
98'85
4'6
5'2
0'35
12'05
3'85
2'4
1'3
99'7
0'7
2'7
47'6
1'0
38'65
0'6
0'45
102'1
Dark brown pellets from dolomite surfaee Doornplaat.
Brown crust from weathered surface of dolomite.
This is
practically an impure calcareous tufa with included quartz
grains giving the high Si0 2 percentage.
54
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
J:
Firm coherent variety of Mn. earth from Rhenosterhoek. This
must have been derived from a rather siliceous type of dolomite.
H. Loose, friable Mn. earth associated with fluorspar deposit on
Malmani Oog.
The comparatively high Si0 2 and MgO percentage is due to the residual talc flakes and quartz grains in
the 1\1n. earth.
K. Brown porous leached dolomite from fissure on Bokkraal. The
high CaO and CO 2 percentage is due to the large amount of
secondary calcite.
Analyses by II. G. \Veall, Government Chemical Laboratories,.
Johannesburg.
The sequence of events in the geological history of the lead, zinc
and fluorspar deposits of this area can be indicated by the following
stages: (1) Earth movements in the S.-W. limb of the great Bushveld
syncline of the Transvaal System giving rise to folding and
fracturing in the area south of Zeerust. The result of these
earth movements are visible in the Pretoria Series, and are
believed to extend into the Dolomite Series as well, producing gentle folding, faulting and local brecciation. These
earth movement,s probably took place contemporaneously
with the intrusion of the rocks of the Bushveld Igneous
Complex.
(2) Contact and hydrothermal metamorphism of the rocks of
the Pretoria and Dolomite Series giving rise in the dolomite
to the formation of tremolite, talc and brucite.
(3) The introduction shortly after (2) of ore-bearing solutions
from some deep-seated igneous source probably related to
the granitic phase of the Bushveld Complex, giving rise to
the formation of lead, zinc and fluorspar deposits along
various horizons in the dolomite. .
(4) The leaching of large masses of dolomite, to form the
so-called manganese earth, by late CO" bearing magmatic
solutions.
~
(5) Erosion and weathering of dolomite and ore deposits to
present condition accompanied by the production of
characterist,ic oxidation effects in the lead and zinc deposits.
COMPARISON WITH DEPOSITS IN OTHER PARTS OF THE WORLD.
Lead and Zinc ores occur in many parts of the world under
widely different geological conditions. They may be classified into
three main types of deposits from the genetic standpoint.
(1) Deposits whose origin seemed to be entirely independent of
igneous. activity.
(2) Deposits which are definitely connected with igneous
intrusions
FIXOHSPAI~, LEAD AXD ZINC DEPOSITS OF THE WESTERi'< TRANSVAAL.
55
(3) Deposits whose origin is rather doubtful, and for 'which an
obvious genetic connection 'with igneous rocks cannot be
traced, but which neverthe1ess exhibit 1nine1'al associations
characteristic of type (2).
Type I.-The best examples of this type are the famous Lead
and .zinc Deposits of the l\1ississippi Valley ,occurringchiefiy in the
States of Missouri, Arkansas and Oklahoma. Although the matter of
their origin is still highly controversial, the prevailing view among the
Amerjcan Geologists seems to be that they 'were formed by concentration from surrounding rocks hy an artesian circulation of meteoric
water aud then' deposited in an underlying limestone. 23 According to
Lindgren, they represent a type of world-\vide distribution of
remarkably constant characteristics.
They occur principally in
limestones, dolomites and cherts (derived from limestone) or calcareous
shales. The mineral association is comparatively simple; galena and
zincblend0 ai'e essential constituents, with their tnljn of oxidised
mineraJs near the surface, sulphates, carbonates, silicat.es, pyrite,
marcasite and chalcopyrite. Among the gangue minerals, dolomite is
the most common. Quartz in crystals is not common, but secondary
chert is typical of many occurrences. Barite is found, but is not
characteristic. Lindgren states that fiuorite is rare in these deposits,
but he includes in this type the deposits of the Alpine Trias in
Austria, such as that of Bleiberg, in which fluorite oecurs as a gangue
mineral.
The ores usually lie in zones of local brecciation or in
crevicAs (gash-veins) or in joints whic~l have heen enlarged by solution;
less commonly they occupy fault fissures; sometime~ they are purely
metasmatic, the minerals occurring disseminated in limestone and
dolomite and closely following certain sedimentary horizons. The ores
usually lie within a few hundred feet of the surface, and are oxidised
in the vicinity of the water level. Frequently they are found below
impervious shale beds.
Type 2.-Examples of this type nre the Cornish Lodes, of which
Hastall states "that there is no reasonahle doubt that they are to
be referred to the same cause as the tin, \volfl'um and copper lodes;
that is, with the int-rusion of the post Carboniferous granites."
Numerous other examples of this type occur elsewhere in the world,
for example, 'Vood Hiver, Coeur d'Alene, Park City, in the U.S.A.;
Przibram, Bohemia; Freiberg, Saxony; Broken Hill, New South 'Vales;
Bawdin l\/[ines, Burma.
Types 3.-As examples of this type can be quoted the Welsh,
Pennine, Lake Distrid and Scotch Deposits. These deposits are
not in obvious genetic connection with any definite igneous intrusions.
They have been variously explained as having been formed by
extraction of the material from surrounding rocks and by infilling from
above. Some of the deposits, notably those of Derbyshire, contain
23
Siebenthal.
Rull. 606, U.S.C.S., 1916.
56
THAXSACTIO)JS OF TIlE GEOLOGICAL SOCIETY OF SOU'l'Il AFRICA.
large quantities of fluGrspar, which suggest a, deep-seated origin for
the lode material, since it is known tha.t fluorine is a characteristic
constituent of late maglnatic va.pours and ore solutions. " The grea,test
objection to both theories, percolation and extraction from surrounding
rocks, lies in the explanation of the SGurce of the meta.ls and of the
fluorine.
Till this explanatiGn is forthcoming, it seems most
satisfactory to conclude that the source of the vein material was below"
(H. H. Hastall, op. cit.).
The depGsits described in this paper, therefore, show many
simila.rities to those of the third type just mentioned. "\¥hile no
definite genetic connection can be established with kn.own igneous
intrusions, the nature of the deposits, and the presence of minerals
such as fluorite and pyrrhotite acc.ompanying. the lead and zinc .ores,
make it certain that the origin .of the vein-f.orming solution was from
belGw as emanations from some hidden and unknown igneous intrusion.
Concerning the origin of the l\:1ississippi Valley deposits, Lindgren
"Hites as follows:" \Ve find here the same conflict of opiniGn between the
ascensionists and the descensionists. The problem is not yet
sol ved, but looking beyGnd these cGntroversies, we cannGt deny
that in many countries transition types appear which seem to
connect these appa.rently distinct non-igneous deposits with those
of igneous affiliations."
The deposits .of the OHoshGGp area are undoubtedly among the
so-called transition types, and may consequently throw some light
on other depolsits vvhose origin is still problema.tical.