Geology Reference
In-Depth Information
Central and West Rand groups were deposited between 2.8 and 3.0 Ga at the margin
of an inland sea in a system of braided rivers that eroded material from the
surrounding highlands. Fluctuations in sea level repeatedly changed the position
of coastline, building up a ˜ 5 km-thick sequence of deltaic sands, shales and
conglomerates. The orebodies are located in lenses of pebbly arenite at several
different stratigraphic levels in the Witwatersrand Supergroup. Other deposits or
this type include those in the ca. 2.1 Ga Tarkwaian System of Ghana and at
Jacobina, Bahia, Brazil.
Box 5.2 Exercise - Geological, Economic and Ethical Aspects of Gold
Mining
Ore mined from the Witwatersrand deposits averages 10 ppm Au, 30 ppm Ag
and 280 ppm U. In normal samples of ore in this deposit, as with most other
modern gold deposits, the gold is invisible to the naked eye. 10 ppm is the
equivalent to 10 g/t - a tonne of rock must be dug out and treated in order to
extract a piece of gold the size of a sugar cube! Mining the gold produces
enormous holes in the ground and vast waste dumps at the surface. If not
properly maintained the dumps leak toxic metals into surrounding rivers and
although they are usually replanted and transformed into grass- or tree-
covered hills, the remediation process is long and not always effective.
On the other hand the total surface of land used in gold mining is not very
large, the size of a large American or Australian wheat field. The farms are
not environmentally benign - they have replaced the original forest of the
area, they are ecologically sterile in that they support only one species, and
they too generate pollution in the form of nitrates and pesticides.
The amount of wealth generated by the gold mine is many orders of
magnitude greater than that produced by the farm. For example, the mine
site, tailings dumps and all other infrastructure of Telfer, a moderately sized
gold mine in Western Australia, cover about 30 km 2 . The mine produces
about 1,000,000 oz of gold/year, which, at $1,200/oz, is worth 1.2 billion
dollars. Several big wheat fields cover about the same area (30 km 2 ) and
produce about 800 t of wheat/km 2 /year, a total of 24,000 t. The wheat price is
about $100/t giving a production worth 2.4 million dollars. In other words,
the wealth produced by the gold mine is 500 times greater that that from the
wheat field.
The Telfer mine employs about 1,000 workers, the large wheat fields about
50.
Some 200 miners lose their lives each year in South African gold mines
and the health of thousands of others is permanently damaged by silicosis and
other diseases contracted during the life down the mines. Artisanal mining
pollutes streams and rivers through South America.
Some of the gold produced in the mines is indeed used in industry (the
computers we are using to write this topic each contain about half a gram), but
most newly mined gold is used in jewelry, particularly in India where to metal
(continued)
 
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