Geology Reference
In-Depth Information
Box 4.7 Discovery of the World's Biggest Mine
In 1975 geologists and geophysicists from Western Mining Corporation, then
a medium-sized Australian mineral-exploration and mining company, were
exploring for copper deposits in the Gawler Block of South Australia. The
target was elusive, in the desert, hidden beneath 300 m of younger sediment
formations, and 100 km away from the closest known mineralization. Doug
Haynes, a geologist just out of his PhD, had developed the idea that copper
deposits might form from a basaltic source via the oxidation of magnetite.
The first hole they drilled intersected a magnetite breccia containing a small
concentration of copper - a tantalizing hint - but then holes 2-9 found
nothing. Finally persistence (and an unusual level of support from the
Melbourne head office) paid off and hole number 10 intersected 200 m of
ore containing 2% copper and significant tenors of gold and uranium. The
team had discovered one of the richest ore bodies in the world and an entirely
new type of ore deposit.
The Olympic Dam deposit contains almost eight billion tons of copper-
uranium-gold ore: it is the world's biggest uranium resource, the fifth largest
gold deposit, and one of the biggest copper deposits. When expansion
planned in coming years is completed, it is expected to become the world's
biggest mine. Polymetallic deposits such as Olympic Dam are particularly
attractive to mining companies because the prices of metals normally do not
vary in unison - the gold price, for example, tends to increase during periods
of recession thereby protecting the companies in times of trouble. The
discovery of Olympic Dam set off exploration programs for similar deposits
throughout the world and led geologists to take a new look at many existing
deposits which were subsequently reclassified as iron-oxide copper gold
(IOCG) deposits.
(For a more complete account of the Olympic Dam discoverey, see
http://
www.science.org.au/scientists/interviews/w/woodall.html#9
)
Due to the very recent discovery of the deposit type, theories of ore formation are
subject to continual revision; most call on large-scale magmatic events that drive
large-scale flow of oxidized probably magmatic hydrothermal fluids into mid to
upper crustal levels along fault zones. Mixing of these fluids with near surface
meteoritic fluids or brines is commonly invoked as the ore-forming process.
4.4.4 Gold Deposits
“Well, have you found any gold?” This question comes up when any geologist or
student on a field excursion talks to a local farmer. Even in Europe, a continent with
few gold deposits of any size (at least that have so far been discovered), the
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