KIMBERLEY AND ITS DIAMOND MINES
The famous diamond mines in the neighbourhood are Kimberley, De
Beers, Dutoitspan, Bultfontein, and Wesselton (Fig. 2). They
are situated in latitude 28° 43´ South and longitude 24° 46´
East. Kimberley is practically in the centre of the present
diamond-producing area. Besides these mines others of some
importance of the Orange River Colony are known as Jaggersfontein
and Koffyfontein, Lace, and Monastery, besides two new mines, the
Roberts-Victor and the Voorspoed.
The areas of the mines are:
Kimberley 33 acres
De Beers 22 acres
Dutoitspan 45 acres
Bultfontein 36 acres
In 1907 the total number of carats raised from these mines was more
than two million and a half, the sales of which realised £6,452,597.
The most important mine outside the Kimberley group is the new
Premier Mine, about 20 miles West-North-West of Pretoria, where the
famous Cullinan diamond was found.
Other diamond mines are the Frank Smith, Wesselton, the Kamfersdam,
the Kimberley West, the Newlands, and the Leicester Mine.
The surface of the country round Kimberley is covered with a
ferruginous red, adhesive, sandy soil, which makes horse traffic
very heavy. Below the red soil is a basalt, much decomposed and
highly ferruginous, from 20 to 90 feet thick, and lower still
from 200 to 250 feet of black slaty shale containing carbon and
iron pyrites. These are known as the Kimberley shales; they are
very combustible, and in a part of the De Beers Mine where they
were accidentally fired they smouldered for over eighteen months.
Then follows a bed of conglomerate about 10 feet thick, and below
the conglomerate about 400 feet of a hard, compact rock of an
olive colour, called “Melaphyre,” or olivine diabase. Below the
melaphyre is a hard quartzite about 400 feet thick. The strata
are almost horizontal, dipping slightly to the north; in places
they are distorted and broken through by protruding dykes of trap.
There is no water nearer than the Vaal River, about 14 miles away,
and formerly the miners were dependent on rain-water and a few
springs and pools. Now, however, a constant and abundant supply of
excellent water is served to the town, whilst good brick houses,
with gardens and orchards, spring up on all sides. To mark the
rate of progress, Kimberley has an excellent club and one of
the best public libraries in South Africa. Parts of the town,
affectionately called “the camp” by the older inhabitants, are not
beyond the galvanised iron stage, and the general appearance is
unlovely and depressing. Reunert reckons that over a million trees
have been cut down to supply timber for the mines, and the whole
country within a radius of 100 miles has been denuded of wood with
the most injurious effects on the climate. The extreme dryness of
the air, and the absence of trees to break the force of the wind
and temper the heat of the sun, probably account for the dust
storms so frequent in summer. The temperature in the day frequently
rises to 100° in the shade, but in so dry a climate this is not
unpleasant, and I felt less oppressed by this heat than I did in
London the previous September. Moreover, in Kimberley, owing to the
high altitude, the nights are always cool.
The approach to Kimberley is deadly dull. The country is almost
treeless, and the bare veldt stretches its level length, relieved
only by distant hills on the horizon.
THE PIPES OR CRATERS
The five diamond mines or craters are all contained in a circle 3½
miles in diameter. They are irregularly shaped round or oval pipes,
extending vertically downwards to an unknown depth, retaining about
the same diameter throughout (Fig. 3). They are said to be volcanic
necks, filled from below with a heterogeneous mixture of fragments
of the surrounding rocks, and of older rocks such as granite,
mingled and cemented with a bluish-coloured, hard clayey mass, in
which famous blue clay the imbedded diamonds are hidden.
[Illustration: FIG. 3. KIMBERLEY MINE. THE “PIPE.”]
[Illustration: FIG. 4. SECTION OF KIMBERLEY MINE.
To face p. 18.]
The craters or mines are situate in depressions, which have no
outlets for the water which falls upon the neighbouring hills. The
watersheds of these hills drain into ponds, called pans or vleis.
The water, which accumulates in these ponds during the rainy
season, evaporates during the dry months, only one of them holding
water throughout the dry season. The rocks which surround the
craters are capped by red soil or calcareous tufa, and in places by
both, the red soil covering the tufa.
The diamantiferous breccia filling the mines, usually called “blue
ground,” is a collection of fragments of shale, various eruptive
rocks, boulders, and crystals of many kinds of minerals. Indeed,
a more heterogeneous mixture can hardly be found anywhere else
on this globe. The ground mass is of a bluish green, soapy to
the touch and friable, especially after exposure to the weather.
Professor Maskelyne considers it to be a hydrated bronzite with a
little serpentine.
The Kimberley mine is filled for the first 70 or 80 feet with what
is called “yellow ground,” and below that with “blue ground” (Fig.
4). This superposed yellow on blue is common to all the mines.
The blue is the unaltered ground, and owes its colour chiefly to
the presence of lower oxides of iron. When atmospheric influences
have access to the iron it is peroxidised and the ground assumes
a yellow colour. The thickness of yellow earth in the mines
is therefore a measure of the depth of penetration of air and
moisture. The colour does not affect the yield of diamonds.
Besides diamonds, there have been detected more than eighty species
of minerals in the blue ground, the more common being magnetite,
ilmenite, garnet, bright green ferriferous enstatite (bronzite),
a hornblendic mineral closely resembling smaragdite, calc-spar,
vermiculite, diallage, jeffreysite, mica, kyanite, augite, peridot,
eclogite, iron pyrites, wollastonite, vaalite, zircon, chrome
iron, rutile, corundum, apatite, olivine, sahlite, chromite,
pseudobrookite, perofskite, biotite, and quartz. The blue ground
does not show any signs of passing through great heat, as the
fragments in the breccia are not fused at the edges. The eruptive
force was probably steam or water-gas, acting under great pressure,
but at no high temperature. According to Mr. Dunn, in the Kimberley
Mine, at a depth of 120 feet, several small fresh-water shells were
discovered in what appeared to be undisturbed material.
A selection of thin sections of some of these rocks and minerals,
mounted as microscopic objects and viewed by polarised light, are
not only of interest to the geologist, but are objects of great
beauty.
The appearance of shale and fragments of other rocks testify that
the _mélange_ has suffered no great heat in its present condition,
and that it has been erupted from great depths by the agency of
water vapour or some similar gas.
The rock outside the pipes and encasing them is called “reef.”
Inside some of the mines occur large masses of “floating reef,”
covering an area of several thousand square feet. In the De Beers
Mine is what is called “the snake,” a dyke of igneous rock taking a
serpentine course across the mine, and standing like a vein nearly
vertical, varying in thickness from 2 to 7 feet. The main body of
the blue ground is entirely analogous to the snake rock, naturally
more decomposed, but in essential points the microscopic appearance
of the blue ground and of the “snake” is in an extraordinary degree
alike. Mr. Gardner Williams supposes that the “snake” is a younger
eruptive formation coming from the same volcanic source as the blue
ground. No diamonds have been found either in the “snake” or the
floating reef. The ground, however, is generally richer in diamonds
in the neighbourhood of the floating reef.
Before the discovery of the mines there was nothing in the
superficial appearance of the ground to indicate the treasures
below. Since the volcanic ducts were filled with the
diamantiferous ground, denudation has planed the surface and the
upper parts of the craters, and other ordinary signs of volcanic
activity being smoothed away, the superficial and ubiquitous red
sand covered the whole surface. The Kimberley Mine seems to have
presented a slight elevation above the surrounding flat country,
while the sites of other mines were level or even slightly
depressed. The Wesselton Mine, within a mile of Dutoitspan, has
only been discovered a few years. It showed a slight depression
on the surface, which had been used as a shoot for dry rubbish.
There are other diamantiferous pipes in the neighbourhood, but they
are small and do not contain stones in payable quantities. More
recently another diamantiferous pipe has been discovered about
40 miles off, near Klipdam, and is now worked as the Leicester
Mine. Other hoards of diamonds may also be near; where there are
no surface signs, and the pipe itself is hidden under 10 or 20
feet of recent deposits, it is impossible to prospect the entire
country. Accident has hitherto been the chief factor in the
discovery of diamond mines.
How the great pipes were originally formed is hard to say. They
were certainly not burst through in the ordinary manner of volcanic
eruption, since the surrounding and enclosing walls show no signs
of igneous action, and are not shattered or broken up even when
touching the “blue ground.” It is pretty certain these pipes were
filled from below after they were pierced and the diamonds were
formed at some previous time and mixed with a mud volcano, together
with all kinds of debris eroded from the rocks through which it
erupted. The direction of flow is seen in the upturned edges of
some of the strata of shale in the walls, although I was unable to
see any upturning in most parts of the walls of the De Beers Mine
at great depths.
THE KIMBERLEY MINE IN OLD DAYS
According to Mr. Paterson, who examined the diamond fields of
Kimberley soon after their discovery, “Wherever the diamond is
obtained perfect in form and smooth in finest smoothness of
surface, without depression, hump, or twist of any kind, such
diamonds were ever found in their own little moulds of finest
limey stuff,[2] and as if such mould of lime had been a necessity
to their perfect formation. And further, where the splinters of
diamonds, or boarty stuff, were chiefly met by the diggers, there
was much less presence of limey matter in the claim at the section
of it where such broken or fragmentary diamonds were found; and
that chiefly from among what the diggers termed ‘clay-ballast,’ or
‘burnt brick,’ were unearthed the bits or undeveloped crystals so
plentiful at New Rush.”[3]
In the first days of diamond mining there was no idea that
diamantiferous earth extended to any particular depth, and miners
were allowed to dig holes at haphazard and prospect where they
liked. When the Kimberley Mine was discovered a new arrangement was
made, and in July, 1871, it was cut up into about 500 claims, each
31 feet square, with spaces reserved for about fifteen roadways
across the mine. No person at first could hold more than two
claims--a rule afterwards modified.
The following quotation from a description of a visit to Kimberley
in 1872, by Mr. Paterson, taken from a paper read by him to the
Geologists’ Association, gives a graphic picture of the early days
of the Kimberley Mine:
“The New Rush diggings (as the Kimberley Mine was at first called)
are all going forward in an oval space enclosed around by the trap
dyke, and of which the larger diameter is about 1000 feet, while
the shorter is not more than 700 feet in length. Here all the
claims of 31 feet square each are marked out with roadways of about
12 feet in width, occurring every 60 feet. Upon these roadways, by
the side of a short pole fixed into the roadway, sits the owner of
the claim with watchful eye upon the Kafir diggers below, who fill
and hoist, by means of a pulley fixed to the pole above, bucketful
after bucketful of the picked marl stuff in which the diamonds are
found.
“Many of the claims are already sunk to a depth of 100 feet,
and still the diamonds continue to be found as plentifully as
ever. From the roadway above the marl is carted away to the
sorting-tables, outside the range of the diggings, among mounds of
marl stuff which seem like little hills. Here, amidst such whirls
of dust as are nowhere else seen, the marl stuff is pounded, sifted
from the finest powder of lime and clay, and from the residue put
on the sorting-tables, the diggers, with a piece of zinc 9 inches
long by 4 inches in breadth, search out in the successive layers
taken from the heap the precious gems. I need not tell you that
the search is by no means very perfect, or that perhaps as many
diamonds escape the digger’s eye as are discovered and taken out
by him, but you will perhaps confess with me that their aptness
in picking out the diamonds is by no means to be despised, when I
tell you that in one six months from the date of opening New Rush
diggings, little short of a million sterling in diamonds has been
extracted from them. At close of day the diggers take daily stock
of their finds, and between five and six o’clock in the afternoon
are to be seen hundreds and hundreds moving through the main street
of New Rush on visits to the tents of the buyers, seated behind
their little green baize tables, with scales all ready, and bags of
gold and silver and piles of banknotes, to buy the little gems.”
It may help to realise the enormous value of the Kimberley Mine if
I say that two claims, measuring together 62 by 31 feet and worked
to a depth of 150 feet, yielded 28,000 carats of diamonds.
The roadways across the mine soon, however, became unsafe.
Claims were sunk 100 or 200 feet each side of a roadway, and the
temptation to undermine roadways was not always resisted. Falls of
road frequently took place, followed by complete collapse, burying
mine and claims in ruin. At that time there were probably 12,000 or
15,000 men at work in the mine, and then came the difficulty how to
continue working the host of separate claims without interference
with each other. A system of rope haulage was adopted.
The following description of the work at the Kimberley Mine at this
stage of its history is given by Mr. Reunert:[4]
“A succession of tall, massive timber stagings was erected round
the margin of the mine. Each staging carried two or three platforms
one above the other, every platform serving as an independent level
from which to communicate with the claims below. Stationary ropes
were then stretched from the different levels of the stagings to
the claims, the ropes being anchored to the ground at both ends:
the upper platforms communicated with the claims in the centre of
the mine, the lower platforms with those nearer the margin. The
hauling ropes were attached to windlasses worked by Kafirs on the
several platforms, on which grooved guide wheels for the ropes were
also fixed, the buckets being swung from the stationary ropes by
little overhead runners and crooks. Arrived at the level of the
platform the bucket was tipped into a narrow shoot, down which the
ground ran into a bag held ready to receive it, in which it was
conveyed away to be sorted. The din and rattle of these thousands
of wheels and the twang of the buckets along the ropes were
something deafening, while the mine itself seemed almost darkened
by the thick cobweb of ropes, so numerous as to appear almost
touching. This mode of haulage continued in vogue during the whole
of 1873, and if the appearance of the mine was less picturesque
than when the roadways existed, it was, if anything, more unique.
By moonlight, particularly, it was a weird and beautiful sight.”
The mine was now threatened in two other quarters. The removal of
the blue ground took away the support from the walls of the pipe,
and frequent falls of reef occurred, not only covering up valuable
claims with rubbish, but endangering the lives of workers below.
Moreover, as the workings deepened, water made its appearance,
necessitating pumping. In 1878 one quarter of the claims were
covered by reef, and in 1879 over £300,000 were spent on removing
reef and water. In 1881 over £200,000 were thus spent, and in 1882
more than half a million sterling was needed to defray the cost of
reef removal. So matters went on until four million cubic yards
of reef had been removed, at a cost of two millions sterling, and
still little good was done, for out of 400 claims in the mine only
about fifty could be regularly worked. Ultimately, in November,
1883, the biggest fall of reef on record took place, estimated at
250,000 cubic yards, surging half across the mine, where the bulk
of it lies to this day. It became evident that open workings could
not be carried on at such depths, and after many experiments the
present system of underground working was devised.
During this time of perplexity, individual miners who could easily
have worked one or two claims near the surface could not continue
work in the face of harassing difficulties and heavy expenses.
Thus the claims gradually changed hands until the mine became the
property first of a comparatively small number of capitalists, then
of a smaller number of limited liability companies, until finally
the whole of the mines have practically become the property of the
“De Beers Consolidated Mines, Limited.”