Page 183 - Handbook of Gold Exploration and Evaluation

Gold deposition in the weathering environment 159

levels rose globally during the late Jurassic. Subduction of the oceanic plate
beneath the margin of North America commenced during the Mesozoic era. The
resulting orogeny, which is still active, has led to the continued accumulation of
intrusive and extrusive igneous rocks of the Sierra Nevada Mountains and to the
formation of chains of folded mountains along the Pacific coast of the Americas,
from Alaska in the north to Chile in the south.
Surface temperatures were essentially warm and equable during the Creta-
ceous and Stanley (1986) suggests a possible connection between global
temperature, water circulation and the presence or absence of black muds in the
sedimentary record. He notes that these muds form at levels where bottom
waters are depleted of oxygen. The widespread accumulation of black muds
provides evidence of the particular warmth of mid-Cretaceous times; not even
the deep-sea waters were cold. Cooling of the atmosphere in the late Cretaceous
resulted in extinction of the dinosaurs and marked a major transition in Earth
history.

Ice ages
It is known from the fossil record that since the development of an oxygenated
atmosphere, global climates have passed through great many cyclic and non-
cyclic changes. Long-term patterns of climatic change are represented by major
glacial epochs (ice ages or glaciations) with mean annual global temperatures of
5±7 ëC. Development of an ice age is characterised by the growth of massive ice
sheets (glaciations), which advance and retreat over great expanses of the Earth's
surface. A succession of alternating glacial and deglacial intervals interspersed
by interglacial stages over time intervals of up to several millions of years
constitute an ice age. Global temperatures return to favourable metabolic condi-
tions during interglacials with average temperatures of around 25 10±12 ëC
(Fairbridge, 1987). The Earth is currently within an interglaciation following a
deglaciation that set in about 15,000 years ago.
However, while periodicity of ice ages is thought by most geologists to be
mainly controlled by rotation of the galaxy, many other possible causes also
strongly affect climate and its cyclicity. To name a few, these causes and effects
include tectonically induced topographic relief, continental drift, the periodic
arrival of continental crust at polar positions and changing patterns of the
circulation of warm and cold oceanic currents. Ice sheets cover 13 million km 2
of the Antarctic landmass, which is presently centred on the South Pole. By
contrast, no land exists over the North Pole where only sea ice accumulates. The
Greenland ice sheet is centred some distance from the pole at latitude 75ë N.
Furthermore, evidence from glacial samples of atmospheric CO 2 trapped in
Greenland ice appears to offer a close linkage between CO 2 levels in the
atmosphere and periodicity of ice ages (Hay, 1987). Possible connections are
also being made with major tectonic upheavals, which result in downwind