Springs and run-off water have
been responsible for some extraordinary mineral concentrations. The
gold deposit of the Witwatersrand, the greatest subterranean treasure
so far found by man, was created by water action, although the
details of how this actually happened are complex and controversial.
It seems most likely that there
was, about 1000 million years ago, an inland sea or lake on the site
of the Witwatersrand. It was to become a strange dream sea of gold.
Into this sea, rain washed a rich spoil of erosion. .Included in the
spoil, apart from pebbles and silt were carbon, uranium, iron
pyrites, chromite, gold and silver, as well as small green diamonds.
This mixture was deposited on the
sea floor in layers. Their nature varied because of changes in
climate, water currents, and the material available as spoiI for the
eroding water. The sea level also changed. Eventually, the basin was
filled and dried up. The bed solidified. Over the ages it was
subjected to irnmense pressures. It was twisted, tilted, faulted and
exposed to extremes of heat and cold. Lava from subterranean deeps
intruded into it. Its minerals were partly dissolved and redistributed.
The whole was transformed, to the
extent that if there was indeed an ancient sea it has vanished
completely, but has left behind a shroud of gold and other
precious things - a shroud below which no life ever made a home.
Something like 1000 million years
passed, with this strange series of rocks and sediments, like a huge
saucer, buried deep beneath a later overburden of volcanic and
sedimentary material. The saucer tilted. Its rim reached the surface
to form the Witwatersrand (ridge of white waters).
This was the lid of a real
treasure chest of nature. Prehistoric man lifted it a little and
extracted iron. Pioneer Europeans found specks of gold in streams,
but their origin was a mystery.
An 1801 map of the South African
interior showed the Witwatersrand, and marked it as rich in gold, but
prospectors searched without success. The 73 families who farmed
along the ridge eked out a living, isolated frorn markets for their
produce, their nondescript homes huddling in hollows for shelter from
winter's cold, dry, frosty winds.
The man who found the answer to
the riddle of the specks of gold arrived in December 1885. In that
summer month, when the Witwatersrand was green and warm, a
shabby-looking man who spoke with an Australian accent, George
Harrison, tramped up to the Witwatersrand and secured a job building
a house for Johan Oosthuizen, on the farm Langlaagte (the Long dale).
Harrison was on his way to prospect in the Eastern Transvaal but,
short of money, he took on this constructional work to help him on
his way.
Harrison, taciturn and almost
illiterate, fossicked around during his spare hours. He never
said anything then or afterwards about how it happened, but some time
in March 1886 he crushed, as others had done before, a sample of the
conglomerate. The sample he panned came from the Main Reef, at one of
the few places where it was exposed. He panned it in one of the
streams and without great hope looked down into the battered black
pan and saw the dirt dissolve and swirl away. Suddenly, there was a
marvellous glitter. The treasure chest was open. Harrison made little
from his find. He registered it, and received the usual free
discoverer's claim but, an alluvial miner at heart, sold it for
£10 when the great rush started, and vanished. There is a legend
that he passed that way some years afterwards, looked at the boom
city of Johannesburg in disgust and opined that he was sorry he had
ever done it.
The extent of the Main Reef
is still unknown. The deepes gold mines in the world probe down
into it. The E.R.P.M. (East Rand Proprietory Mine) shaft at
Boksburg goes down 3240 m, and ore is known to extend deeper than 15
000 m.
The limits of recovery are set,
not by the extent of the reef, but by technical problems and costs.
Everyday, 40 000 tons of ice are used in the E.R.P.M. shaft to
provide a reasonable temperature. Apart from gold, the Witwatersrand
reefs are prolific producers of silver, to the extent of 1 part
silver to every 10 parts of gold; and of iron pyrites, a pale,
brass-yellow coloured mineral composed of 53,4 percent sulphur and
46,6 per cent iron. Once known as fool's gold because it was
thought to be valueless, iron pyrites is today used to make
sulphuric acid. Uranium oxide (uraninite) is also abundant.
The basin containing the
Witwatersrand System is about 300 km by 150 km and is tilted
southwards, deep below the Karoo System. Using magnetometers,
geophysicists have traced the gold-bearing reefs to the west of the
Rand, where they are buried under such thick masses of
water-saturated dolomite that to reach them miners have to
resort to the most extraordinary a feats of applied
science. At Blyvooruitzicht, one of the world's richest gold mines,
they had to plug an influx of nearly two million litres of water an
hour which suddenly burst through a fissure 500 m below the
surface as the shaft was being sunk.
In the Orange Free State, the
gold-bearing reefs, known there as the Basal Reefs, were traced as a
result of the sinking of more than 500 boreholes.
More than 100 brought significant
quantities of gold to the surface. One, on the farm Geduld
(Patience), produced from a depth of 1300 m a sample which contained
nearly 20 per cent of pure gold. Its assay of 23 037 inch-dwt
staggered the mining world, as it was the richest ever found.
There are very
few gold specimens that ever come out of any of these mines. I met a
miner who mined at one of the richest gold mines who told me that he
never saw physical gold in any of the rocks in 15 years that he
worked on the mine. I once saw gold specimens from the Eersteling
mine at Pietersburg. The gold there was caused by hydrothermal water
action. It is illegal to own raw gold in South Africa.
