Page 89 - Handbook of Gold Exploration and Evaluation

70 Handbook of gold exploration and evaluation

world, the sixth largest copper deposit and contains one of the most significant
gold±silver resources. Interpreted rocks of the region include granite, granitic
gneiss, layered intrusions and sills. The Olympic Dam deposit was formed in a
volcanic environment about 1,500 million years ago when magma was rising
from the deep molten part of the Earth's upper crust. Some crystallising magma
reached the surface before erupting explosively from volcanoes. Super-hot
water-rich fluids were released containing dissolved iron together with copper,
uranium, gold and silver. These fluids were under extreme pressure and caused
fracturing and brecciation of the crust. The area was eroded flat by the scouring
action of glaciers and covered by thick layers of sedimentary rocks including
shale deposited on the ocean floor. A further uplift exposed the shales to erosion
and the formation of the present land surface. The ore body was discovered
350 m below the surface in 1950 when boreholes identified copper and uranium
mineralisation. On average, each tonne of ore contains about 13 kg copper,
0.4 kg uranium and 0.5 g gold.
The younger Proterozoic placers may be associated with Archaean proven-
ances but are more likely linked with high-grade supracrustal gneisses having an
ultramafic/plutonic affinity and greenstone belts with sedimentary, chemical and
conglomerate affinities. The gold is generally subordinate to sulphide minerals
in massive or stratiform deposits linked with major non-conformities involving
high-grade sediments. Surviving remnants of Proterozoic mobile belts appear to
be cores of old mountain chains that were formed as a result of continental drift
and plate tectonic processes similar to those that became widespread in the
Phanerozoic.
The distribution of landmasses at any time in the geological past is recon-
structed by investigating such features as the distribution of rock types, fossils
and geological structures, palaeomagnetism and radioactive age dating. Several
lines of evidence suggest that plate motions at the present day and in the
geological past can be correlated through changes in magnetic lineation on a
worldwide basis, and linked to a geological timescale. With few exceptions only
those parts of the above geological cycle that relate to Archaean sialic crust are
preserved and the possible existence of small remnants of oceanic crust locked
up in Archaean cratons is uncertain. Existing continental shield areas (cratons)
consist of Precambrian rocks arranged in a series of elongate structures similar
to lineal mobile belts. The structures are progressively younger with distance
from the central craton, thus suggesting that over time, the attachment of mobile
belts to continental belts could have obscured evidence of the mineralisation of
those times.

The supercontinental cycle
Since the beginning of Earth's accretionary stage, the continents have been
progressively joining together and drifting apart sending small pieces of