Page 196 - Handbook of Gold Exploration and Evaluation

Gold deposition in the weathering environment 171

Two most important agents of in-situ mechanical weathering are the crystal growth
of ice and salts (freeze-thaw), and thermal expansion-contraction.

Freeze-thaw
Freeze-thaw involves repetitive growth and melting of ice crystals in cracks and
pores in the rock. Frost action occurs at low elevations in high latitudes and in
cool inland areas of lower latitudes where temperatures fluctuate below and
above freezing point. In its frozen state, water expands by 9.2% in volume at
atmospheric pressure. At various stages of confinement, freezing exerts pres-
sures up to a theoretical 2,000 kg/sq.inch in joints, cracks, etc., well in excess of
forces holding rocks together. The action of freeze-thaw is enhanced in near
surface rocks that are opened up by dilatation, so allowing water, bacteria and
plant roots to penetrate the openings and wedge rock fragments further apart.
Stresses created by the crystallisation of salts such as sodium chloride, calcite
and gypsum may enter pores and other openings in rocks in dissolved form. On
drying and crystallisation they expand and set up similar disruptive effects as
those caused by ice during its stage of crystal growth. Crystallisation has been
observed to occur against pressure as great as 47 bars, at least twice the tensile
strength of many rocks (Bryant, 1976). The growth of salt crystals may create
honeycombing of rocks that is sometimes thought to be due wind abrasion or
chemical weathering.

Thermal expansion and contraction
The effects of diurnal and seasonal heating and cooling of surface rocks on their
physical disintegration is the subject of many debates. Some believe that normal
diurnal temperature changes cause rocks to exceed their thermal expansion
limits thus making the outer skins of boulders peel away from their inner cores.
Others suggest that the differential heat absorption of dark- and light-coloured
rocks in a multi-coloured rock (e.g. granite) should provide sufficient stress to
shatter such rocks. Excessive day temperatures in some desert areas appear to
have this effect on some rocks. A possible contributing factor is the varied
content of atmospheric water, some of which is always present. In most cases,
however, a better case can be made for shattering by a combination of weather-
ing forces, the dominant factor being the cumulative effect of innumerable
repetitions of expansion and contraction in rocks already weakened by other
weathering processes.

3.4.2 Chemical weathering
Many rock-forming minerals show incipient alteration characteristics prior to
atmospheric weathering; feldspars are often slightly kaolinised, mafic silicates