Page 171 - Handbook of Gold Exploration and Evaluation

Gold deposition in the weathering environment 149

nature of weathering patterns developed on slopes facing in opposite directions
in sub-polar and periglacial conditions. Slopes facing north in high northern
latitudes remain snow-covered long after melting occurs on slopes facing south.
This promotes increased chemical weathering and biological activity on the
southern aspect, thereby encouraging animal grazing on the warmer side of the
hill and adding to mass-wasting along that sector. North-facing hillsides are
weathered mainly by frost action, slopes become steeper and more prone to
landslides. The effects of insolation on slopes in southern latitudes are mirror
images of those in the north.

The greenhouse effect
Gases in the troposphere include dry gases, which are fundamental to life, and
others such as water and industrial gases. The natural `greenhouse effect', which
prevents temperatures dropping excessively when the sun goes down or in
wintertime in middle and high latitudes, is largely due to the presence of
concentrations of water vapour, carbon dioxide, methane, CFCs, halons and
nitrous oxide in the lower levels of the atmosphere. Rapid condensation of water
vapour provides precipitation in the form of rain, hail, snow and sleet. Fresh
water is supplied for plant and animal life, and preserves the global water
balance between that leaving the land and ocean surfaces by evaporation and
that falling on the land and oceans by precipitation. Impurities cause environ-
mental problems, however. Human activities result in the production of myriad
microscopic dust particles from forest and grass fires, from industrial processes
involving fuel combustion and from the vaporisation of meteors as they
encounter the heat barrier in the upper atmosphere. Residues of minute crystals
of salt also contribute to the troposphere as water lifted from the ocean dries
out; more heat energy is retained in the troposphere and global temperatures
rise.
According to scientists working on the Intergovernmental Panel on Climatic
Change (IPCC), global warming over the 20th century has resulted in an average
increase in air temperature of about 0.6 ëC and a corresponding mean sea level
rise of 10±20 cm. Computer models suggest that global temperatures and sea
levels will continue to rise during the 21st century although possibly at lower
rates. These changes have occurred largely because of the additional amount of
heat energy trapped by the absorption and emission of some of the infra-red
radiation and warming of the lower atmosphere during the latter part of the 18th
century. CO 2 appears to be the main contributor to the enhanced greenhouse
levels as shown in Fig. 3.9, the CSIRO graph of the sharp increase of CO 2 levels
in the atmosphere since AD 1000.
Another important influence on the enhancement of greenhouse levels is the
presence of an ozone layer within the stratosphere, which provides a concen-
tration of oxygen molecules at an altitude of about 15 km and extends upward to