Page 178 - Handbook of Gold Exploration and Evaluation

Gold deposition in the weathering environment 155

much as 7 kpm in shallow waters of the continental shelf before eddying out into
the Tasman Sea. According to Wen ju Cai, a senior researcher at the CSIRO: `A
shift in wind systems is affecting circulation in the Pacific, Indian and Atlantic
ocean basins and this is leading to a rate of warming in the South-west Tasman
Sea that is fastest in the Southern Hemisphere.' The resulting strong seasonal
pulse of warm water has increased water temperatures in the Tasman Sea by two
degrees during the past 60 years whereas global temperatures have risen by only
half a degree. The EAC is being pushed further towards Tasmania by a
southward shift in the westerly weather systems.

Atmospheric circulation
The principal factors determining the pattern of global circulation are the greater
retention of solar heat in low rather than high latitudes, and the rotation of the
Earth. Equatorial heating induces comparatively shallow low-pressure systems
in contrast to the formation of shallow high-pressure systems in polar regions.
Flow patterns are produced as a result of these differences. Masses of hot air,
displaced in the tropics and moving poleward, are displaced to the right in the
Northern Hemisphere and to the left in the Southern Hemisphere, so that winds
in the upper troposphere tend to be westerly. Air masses have a characteristic
zoning of temperature and humidity for which values are closely constant at all
points in a horizontal plane. Weather fronts are formed when westerlies of
middle latitudes and masses of cold air from the poles collide.
Individual air masses reflect the temperature and humidity of the surfaces
over which they pass. For each temperature there is a saturation point at which
precipitation from water vapour to droplets of water must occur (Table 3.4). The
air is in constant motion and clouds form by condensation of the water vapour
when its relative humidity reaches 100% in its upwards rise. Hot winds blowing
across oceans become increasingly humid until they reach saturation point for
the conditions of flow. Precipitation occurs when winds laden with moisture are
swept up over steep and high mountains. Rapid cooling gives rise to precipita-
tion on the seaward side, thereby promoting favourable climatic conditions for

Table 3.4 Saturation point for precipitation (after Barlow and Newton,
1974)

Temperature (ëC) Maximum water vapour capacity
3
(g/m )
0 4.8
10 9.1
20 17.3
30 30.0
40 50.9