Page 187 - Handbook of Gold Exploration and Evaluation

162 Handbook of gold exploration and evaluation

· Change from arid to semi-arid conditions involves higher rates of precipita-
tion and run-off, increased chemical weathering and more extensive fanning
out of detritus from weathering.
· Change from semi-arid to temperate climatic conditions increases the extent
of weathering and transport of detritus in source areas; it thus provides
conditions more conducive to placer formation on a large scale than those
that are related to the predominantly mechanical type breakdown of rocks in
drier climates.
· Change from temperate to humid tropic conditions usually results in
increased chemical weathering but lower rates of sediment production;
exceptions may be found in higher ground where slopes are steep and
gravitational forces can more easily overcome the frictional forces resisting
movement.
· Change from humid tropic to periglacial conditions is followed by reduced
chemical weathering and a dramatic increase in mechanical disintegration,
particularly in rocks already weakened by previous chemical action;
mechanical freeze-thaw processes are dominant in alpine regions where
brief periods of sunshine in late spring and early summer bring melting of the
snow and short-lived but violent fluvial activity.
· Change from periglacial to glacial conditions produces glaciation of valleys
and recycling of any placer deposits not protected from glaciation by their
location, e.g. in incised bedrock features. In some cases this involves a radial
dispersion of pay streaks across the valley, in others the telescoping of
auriferous gravels down valley into the boreal system and, for a few, glacial
transportation of the gold-bearing materials into coastal settings as at Nome,
Alaska.
· Change from glacial to humid tropic conditions is dominated by climatic
change; usually this involves recycling Tertiary age placers through repeated
cycles of Quaternary climatic change and glacio-fluvial transport of the gold-
bearing material downstream into either a boreal environment or in low relief
areas into a jungle setting.

3.3 Agents of weathering
In any area of gold mineralisation, the vertical entrenchment of valleys will
establish a general lowering of land surfaces so that eventually, the gold source
rocks will be unroofed and exposed to weathering. Primary deposits all have a
different gold-bearing potential and in order to form alluvial gold concen-
trations, the rocks must then break down in such a way as to release the
contained gold. Ultimately, this means rock disintegration down to the scale of
individual grains. There must also be a means of removing the detritus. Fine
materials are removed from the surface by sheet wash and deflation; soluble
salts are carried away by percolating ground waters and rock waste shed from