Page 88 - Handbook of Gold Exploration and Evaluation

Geology of gold ore deposits 69

metamorphic terraines have been repeatedly deformed and metamorphosed at
temperatures as high as 900 ëC and at pressures corresponding to depths of 20±
50 km in the continental crust. Under these conditions the supracrustal materials
would have lost their primary textures through recrystallisation and their relations
to the granitoids would have been obliterated.

2.1.3 Proterozoic eon (2.6Ga±580mya)

Transition from the Archaean to the Proterozoic, 2.6 billion years ago was
associated with gradual changes in overall composition of the upper crust,
evolution of the hydrosphere-atmosphere and extensive sediment deposition in
broad shallow seas. Many of the continental masses, with the exception of
Australia, were apparently located near to or around the North and South Poles;
widespread evidence of glaciation is found in late Archaean continental systems.
The landmasses had grown significantly in extent by the accumulation of felsic
rock produced in subduction zones. Changes in the geological framework would
probably have occurred due to chemical overprints of metamorphism and
hydrothermal alteration, combined with uncertainties of the geothermal gradient
and the scale of the tectonic systems.
Basic, intermediate-felsic volcanic piles appear to be prominent from mid-
Archaean times into the Proterozoic (Hutchinson, 1983, 1987). Deposits asso-
ciated with these piles include zinc-copper and lead-zinc-copper-silver sulphides
with lesser gold. Proterozoic sequences are dominated by collisional and obducted
material, and by sediments in long-lived passive margins of the craton system.
During periods of continental fragmentation and dispersal wide continental
margins provided conditions suitable for the development of extensive non-clastic
basins and a surge in BIF (banded iron formation) and Mn sedimentation.
Hydrothermal Au-U-Pt group assemblages were formed in extensive fractured
cratonic basins in Australia (e.g. Jabiluka) and Canada (e.g. Rabbit Lake) from
about 1.7 Ga (Solomon and Shen, 1997). Important metallogenic events associated
with the African continent include volcanogenic massive-sulphide and
mesothermal gold mineralisation occurred during 900±600 Ga in opiolite and
island arc terraines of the Arabian-Nubian Shield (Al-Shanti, 1979).
Deposition of the Witwatersrand gold-uranium deposits commenced at the
end of the Archaean and continued through the early Protozoic. Similar deposits
are known in other Proterozoic intracratonic basins such as the Archaean
Yilgarn Craton of Western Australia. The Yilgarn Craton covers 20% of the
Australian continent. Exposure to bedrock is extremely poor and most deposits
occur either at or adjacent to sparse outcrops. Nevertheless, the craton produces
two-thirds of the gold recovered from Australian mines. One of the most
important deposits is the Roxby Downs Olympic Dam gold±uranium±copper ore
deposit of South Australia. Olympic Dam, which is estimated to contain more
than 2.3 million tonnes of ore and is the largest known uranium deposit in the