Page 197 - Handbook of Gold Exploration and Evaluation

172 Handbook of gold exploration and evaluation

are slightly chloritised and so on. This slight alteration takes place in igneous
rocks during the deuteric stage of crystallisation with chemical changes taking
place along structure planes of the mineral, e.g. cleavage traces. The resulting
clay-type minerals will cause slight expansion, thus aiding the subsequent
weathering process.
Rainwater contains dissolved gases capable of setting up a series of chemical
reactions. Nitrogen and carbon dioxide are the more reactive of these gases. In
addition there are traces of NH 4 , HNO 3 , H 2 SO4, NaCl and Cl which, though
present in minor amounts, are so reactive over a period of time to be considered
important in rock decomposition. Humid conditions accelerate decomposition
and areas of low relief allow the reactants a longer time for the effective
reactions to proceed to completion. Rapid run-off of surface water is effective in
removing the products of chemical change but militate against reaction time.
Chemical weathering processes involve the complete removal of some
substances (e.g. limestone) and the creation of secondary, more stable minerals
to replace constituents that are less stable in the weathering environment. The
principal forces acting together to break rocks down to their decomposition
products are oxidation, carbonation and hydrolysis. The presence of dissolved
oxygen in water promotes oxidation of metallic elements (Mg, Fe, Al, Ca, etc.)
that are abundant in silicate minerals. Carbon dioxide in solution forms weak
carbonic acid, which assists in the decomposition of granite and gneiss, both of
which contain an abundance of feldspar minerals. The water itself combines
with certain rock minerals to form insoluble precipitates (clay minerals).
The various silicate minerals decompose at different rates, the order of
increasing susceptibility to weathering being the same as the sequence of
crystallisation from a melt, i.e., olivine±pyroxene±amphibole±plagioclase±
biotite±K-feldspar±vermiculite±smectite±muscovite±kaolinite±quartz.
Notwithstanding the susceptibility of minerals such as olivine, pyroxenes and
amphiboles to chemical weathering they do occur in placers, often quite
plentifully. In part, this may be accounted for by prior mechanical disintegration
followed by rapid transport to the site of deposition with insufficient time for
complete oxidation to take place.
The effects of chemical and biological weathering are most pronounced in the
humid conditions of tropical rain-forest areas. Vegetation cover is at a maximum
and a proliferation of plant roots enlarges openings in the surface layers, thereby
enhancing water penetration and chemical reactiveness. Destruction of organic
matter by decomposition results in the production of carbon dioxide and organic
acids for chemical weathering and the rate of decay is further increased by the
action of termites, bacteria and fungi. Surface attack by biological agents is less
significant in temperate climates, where tree roots may penetrate to greater
depths in their search for water, but cause much less lateral disturbance.
The role of micro-flora varies with changing ecological conditions (Douglas,
1977). The formation of living matter from natural organic compounds is largely