Page 204 - Handbook of Gold Exploration and Evaluation

Gold deposition in the weathering environment 179

nature of the movement and susceptibility of slope materials to the action of
freeze-thaw processes appears to depend upon the soil grain size distribution,
permeability of the soil, rate of freezing and depth of frost penetration and
availability of water. With increasing levels of saturation the development of
positive pore pressure decreases cohesive resistance, which ultimately becomes
negligible when the soil approaches complete saturation.
Movement by solifluction is usually restricted to a few metres each year
when seasonal thawing of the mantle in periglacial areas causes masses of
material to break away, typically forming an eroded rock basin surrounding an
armchair-like depression (cirque). Provided that movement first takes place on
steeper slopes solifluction may occur on slopes as flat as two degrees or even
less. The process is typical of periglacial weathering in arctic and alpine regions
but may also occur in humid mountainous areas of the tropics. Andersson (1982)
coined the term solifluction in his studies of the `mud glaciers' of Bear Island in
the North Atlantic and `stone rivers' in the Falkland Islands. He described
solifluction as slow flowing from higher to lower ground of soil or earth
saturated with water, thereby distinguishing the process from the surface creep
of unsaturated soils.

Mud/debris flow
Rapid mass-movement by mud/debris flow may occur on almost any slopes
wherever the rainfall is seasonal or heavy. Periods of torrential rain cause
weathered slope material to become saturated and behave more as a fluid than as
a solid. This may take place in:
· humid tropic areas where slope materials typically comprise unconsolidated
debris of glacial or volcanic origin
· frigid areas where the wedging action of freezing water disintegrates the
near-surface rocks
· humid temperate to tropic regions where source rocks, prior to uplift, are
subjected to long periods of deep chemical weathering.

3.6 Low-temperature aqueous geochemistry

In order to understand the various chemical transformations involved in low-
temperature aqueous solutions, the particular properties of the matrix and of the
chemical composition of the gold must be known. Two important solution
properties are acidity (pH) and the oxidation potential Eh. Acidity is usually
measured as pH (the negative logarithm of the H + concentration) and is
controlled by the nature of the underlying geology, acidic or basic (see Section
+
ÿ
2ÿ
1.1.2). Carbonates (CO 3 ), protons (H ) and bicarbonate (HCO 3 ) all exist in
solution in controlled proportions. This control is defined in terms of equili-