Page 190 - Handbook of Gold Exploration and Evaluation

Gold deposition in the weathering environment 165

molecules of liquid water are also held together by hydrogen bonding, but some
of these bonds are broken when ice melts and the remainder are too few to keep
the water molecules in a regular arrangement. When heat is added to water at
0 ëC, the water contracts until it reaches 4 ëC after which it slowly expands until it
reaches boiling point and turns to steam. In the gaseous state above 100 ëC, water
vapour consists of water molecules moving independently of one another. Water
is most destructive in a solid state and most reactive chemically when heated.

Water as ice
Ice occurs naturally in glaciers as masses of frozen water that move under the
influence of gravity. They are composed of mixtures of snow, firn and ice and
are fed in areas of snow accumulation above the mean snow line. Freshly fallen
snow consists of snowflakes of average density 0.08 g/cc. Prolonged periods of
high snowfall and increased snow depths result in the transformation of snow to
firn (density 0.1±0.9) and then to glacial ice (density 0.92). The continued
increase in density occurs by packing, melting and freezing during which air
entrapped between ice particles is partly squeezed out and the final compression
of ice takes place in deep sections of the glacier.

Water as a fluid

Water falls onto a land surface in the form of rain, hail or snow. Run-off at the
ground surface takes the form of sheet flow, streams and rivers. In arid regions,
where rainfall is only occasional and evaporation rates are high, infiltration is
negligible and surface run-off may occur only once in several years. Infiltration
and run-off are both seasonally heavy in monsoon regions, but flow rates fall
away sharply for the rest of the year. Rivers that drain alpine regions receive
little run-off from melt-waters during the winter months, but typically swell to
flood proportions from the melting of snow and ice in spring and early summer.
Only in climates such as those of the British Isles and Tasmania is rain abundant
throughout most of the year and even these areas are subject to occasional
flooding and periods of drought.
As already indicated, water as a weathering agent is a solvent for many
reactive gases, particularly those which dissociate and form ions. Rainwater thus
carries various constituents of the air into parts of the ground not in direct
contact with the atmosphere. Weathering processes are becoming increasingly
destructive with the growth of fossil fuel-dependent industrial applications.
Close to cities, where industrial gases discharge directly into the atmosphere,
rainwater acidities have been recorded as high as pH 4.4 (from Douglas, 1977).
Conversely, in highly vegetated terrain, impurities are filtered out of the air and
rainwater reaching the ground may be slightly alkaline, hence much less
destructive.