Page 94 - Handbook of Gold Exploration and Evaluation

Geology of gold ore deposits 75

1.8±1.6 Ga resulted in the preservation of large marine platforms and intra-
continental basins (e.g. the Athabasca Basin). Anorogenic magmatism preceded
the break up of this supercontinent and renewed orogenic activity was
accompanied by a period of continental aggregation between 1.3 and 1.0 Ga.
A gradual fragmentation took place during the Mesozoic era (235±280 mya).
Laurasia and Gondwanaland drifted apart about 180 mya in the Jurassic Period,
thus opening the way for the formation of the South Indian and North Atlantic
Oceans. The South Atlantic Ocean was formed at the spreading centre between
Africa and South America; the latter probably remained connected to part of
Africa, Antarctica and Australia until the end of the Cretacious. The Urals were
weathered during the Triassic and the first phases of Alpine and Andean folding
introduced the great mountain ranges of modern times about 100 million years
ago.

The supercontinent Gondwanaland
Evidence that the continents of New Zealand, Antarctica, Australia, Africa,
India and South America were joined together as the supercontinent `Gond-
wanaland' about 200 million years ago is shown by the geological make-up of
the continents themselves. Fossil records of plants such as the fern Glossopteri
found in the Karoo System in India, Africa and South America are very rare in
the northern Hemisphere. Geologically, Upper Proterozoic to Cambrian rocks of
the Adelaide orogenic belt continue into the Ross orogen of Antarctica and
tillites, which occur all over the southern Hemisphere, contain many similarities
in their sediments. Physically, the bulge of South Africa fits the impression in
the western coast of Africa when fitting the individual continents together.
Fragmentation of the major southern continent Gondwana resulted in splitting
South America and Africa as a separate landmass away from the remainder of
Gondwana (Antarctica, Australia, and India). Figures 2.6 and 2.7 respectively,
depict the postulated configuration of Gondwanaland after its split from
Laurentia and the present configuration of the continents and plate margins
(Plimer, 1997). The initial separation of Australia from Antarctica took place at
a rate of about 12 cm/year and its current movement is 7 cm/year northwards
away from Antarctica. The Pacific Ocean is becoming smaller along subduction
zones under North and South America and Japan as North America and Japan
get closer together. Ultimately Asia and America will collide to form another
supercontinent. Judging from the past, this continent will also be comparatively
short-lived in a geological sense.

Cainozoic mountain belts
The Cenozoic era commenced about 65 mya at the end of the Cretacious with
the northward movement of Africa and creation of the Mediterranean Ocean;