Page 90 - Handbook of Gold Exploration and Evaluation

Geology of gold ore deposits 71

continent across the ocean to collide and form other larger continental masses.
Each new evolutionary trend generated new continental crust, the continents
becoming larger each cycle. Fichter (1999) suggests that this cycle of break-up
and collision has been a constant and evolutionary trend for some 4 billion years
requiring about half a billion years for each cycle. Other estimates of elapsed
time between each fully assembled supercontinent range between a few hundred
million years to 2,000 million years (Gurnis, 1988). Worsley et al. (1985)
recognised 500 million years tectono-eustatic cyclicity for the post Archaean.
Megacycles of 210±325 million years are proposed by Vail et al. (1977) for the
Phanerozoic. Hoffman (1989) interprets megacycles of similar length for the
North American Proterozoic and Australian Archaean respectively.
Conceptually the supercontinental cycle begins with continental joining on
one side of the Earth balanced on the other side by a superocean in which new
oceanic crust is being created at spreading centres. In this scenario the formation
of a supercontinent follows sequentially through:

· creation of small island arcs by convection cells and subduction zones
· volcanic arc collisions and evolution of protocontinents through many
individual subduction zones
· volcanic-arc-protocontinent collisions to form microcontinents
· collision zones in which the microcontinents join together to form continents
such as North and South America, Siberia and Australia
· further continental edge subductions leading to the formation of a
supercontinent (Fichter, 1999).

The theory then suggests that the evolution of a major supercontinent is followed
by a build-up of heat in the mantle; upwelling at hot spots, and subsequent
break-up into separate continental masses. Metal deposits are associated with
both the periods of orogenic supercontinent aggradation and the initial stages of
break-up.

Rodinia
The oldest-known supercontinent, `Rodinia' is thought to have been assembled
about 1100 million years ago with Laurentia (Proto-North America) as its core.
At that time, although the exact size and configuration of the supercontinent is
uncertain, North America was adjacent to western South America on the east
while Australia and Antarctica lay next to it on the west. Rodinia is thought to
have split into two halves about 750 million years ago opening up the
Panthalassic Ocean. North America rotated southward towards the ice-covered
South Pole. The northern part of Rodinia, which comprised Antarctica,
Australia, India, Arabia and a group of continental fragments that would one
day meld and form China, rotated northwards in a clockwise direction across the
North Pole. A third continent, the Congo craton, which was made up of much of