Page 96 - Handbook of Gold Exploration and Evaluation

Geology of gold ore deposits 77

Cainozoic mountain belts are manifested in two broadly curved chains of
volcanoes. One of these, the Circum-Pacific belt, rings the Pacific Ocean in what
is known as the Pacific Ring of Fire. The other, the Eurasian-Indonesian belt
commences in the west at the Atlas Mountains in North Africa and runs through
the Near East to the Himalayas and thence to Indonesia where it joins the
Circum-Pacific Belt. Gold-bearing ore zones are located within the igneous and
metamorphic rocks of these settings at various depths ranging from near surface
(epithermal) vein systems, to deep-seated (mesothermal) deposits at depths of
10 km and more. Deposits of economic significance are found mainly in
stratovolcanoes and rhyolitic centres in continental and marine volcanic centres.
Extensive sub-tropical weathering during the Mesozoic favoured the formation
of lateritic and alluvial gold placer types.

Case history ± the Chillagoe story

A scientific description of major geological processes that may have contributed
to the formation of gold orebodies over long periods of geological time is
provided by the geological history of the Chillagoe gold field (Fig. 2.8), as
summarised by Ian Plimer in his book A Journey through Stone (1997). The
book is a compilation of the work of geologist John Nethery who, together with
his team, revolutionised the thinking on the regional geology of Chillagoe and
its orebodies. In his book Plimer shows how the simultaneous action of all the
contributing processes of landscape evolution and denudation is a balance over
time between tectonic forces that create mountains and form orebodies, and the
contributing processes of denudation that act to wear them down. The story
serves as a general model to explain how present surface exposures of
mineralised zones are controlled by factors related to continental drift and plate
tectonic processes, reactivity of the host and country rocks to hydrothermal
processing, and climate.
Many of the sediments and volcanics were formed while Australia formed
part of Gondwanaland together with landmasses that are now parts of Africa,
India, Antarctica and South America. In north-eastern Australia intense crustal
forces were beginning to build up deep in the crust and the Dargalong
Metamorphics first started to stretch and then broke. Some 1,570 million years
ago, a very large folded mountain chain (similar to the Himalayas) was formed
in a rift valley to the east of Chillagoe. The direction of compression changed
about 100 million years later and the still hot and bent rocks were again pushed
up into mountains. Five hundred million years later the third deformation of the
Dargalong Metamorphics bent the rocks into a corrugated shape.
An ice age, which covered the Australian continent about 750 million years
ago, left behind retreating glaciers, large masses of rock debris that yielded lake
sediments containing thousands of layers of winter mud and summer sand.
Intensification of crustal forces and stresses deep in the crust followed melting