Page 99 - Handbook of Gold Exploration and Evaluation

80 Handbook of gold exploration and evaluation

Another great crisis in the history of life on Earth occurred 245 million years
ago, when 96% of all marine species on Earth became extinct. For some
unknown reason, while almost all of the marine animals suddenly disappeared,
very few terrestrial species were similarly affected. There was no evidence of
any great climate or sea level change at that time. A massive Siberian volcanic
eruption that may have triggered the Featherbed volcanic eruption added to the
sulphurous gases that would have been released during that time but there is no
evidence of land plants becoming extinct. Other possibilities, which have
received little general acceptance, include a supernova bombardment from space
and a global viral disease pandemic.
The geological history of Chillagoe is a continuing process. Weathering and
erosion are still taking place; the land is rising in response and being reshaped by
the elements. Climate is fluctuating between greenhouse and icehouse
conditions. In time the plate tectonic cycle will move onto its next phase and
small Earth tremors that result from release of stresses along fault lines will
again be reactivated by further changes to the Earth's crust, oceans and climate.

2.2 Tectonic elements of plate movements

Specific variations in metallogeny associated with the aggregation and break-up
of continents were not recognised until a few decades ago when it became
evident that any explanations not involving plate tectonics were in most cases
less satisfactory than those that do. In plate tectonic terms, the crust and upper
mantle of the Earth consist of a number of lithospheric plates, comprising
oceanic or oceanic plus continental crust (sima and sial) together with the
adjacent upper mantle. The plates tend to move as rigid units, with most of the
resulting deformation arising from interactions with other plates along their
margins.
Plate boundaries show a variety of characteristics, which in part are due to the
nature and motion of the adjacent plates. Some variation must also arise by the
speed and angle of collision. However, differences in the features by which plate
interaction is recognised are developed along the subducting boundaries, where
earthquake foci define a zone of slippage dipping to depths as great as 700 km at
angles between 15ë and near vertical. Lines of evidence that indicate plate
motions throughout geological time include magnetic lineations on the ocean
floor that are the result of basaltic magma emplaced at spreading axes crystal-
lising and becoming magnetised according to Earth's prevailing magnetic field.
Reversals of this field produce marked changes of the remnant magnetism of
successive parallel zones, thus recording the Earth's magnetic history and
serving to date sections of the seafloor. Arc systems consisting of deep elongate
oceanic trenches and volcanic islands are developed at the surface, on the
majority of plate boundaries. Plate tectonic activity is best known in the
Phanerozoic and to a lesser extent in Proterozoic settings. Some authors also