Page 202 - Handbook of Gold Exploration and Evaluation

Gold deposition in the weathering environment 177

3.5.2 Movement on slopes

The course of erosion can be expressed in terms of a ratio of mechanical to
chemical weathering. In terrains of high relief mechanical weathering is
dominant because of the steepness of slopes and lack of vegetation, and the
spontaneous downward movement of the regolith in response to the influence of
gravity with or without the dynamic action of a moving fluid. Large masses of
unsorted debris may be washed rapidly (up to 10±15 km/h) downslope, settling
finally into accumulations of partly worked debris fans wherever the supply of
weathered material is greater than the capacity of local streams to carry it away.
Ground surfaces are significantly above the base level towards which the flow
processes are working (e.g., sea level) and as the detritus is in constant
downslope movement more fresh rock is continuously exposed to chemical and
physical change. Vegetation is sparse and weathering relates only to the present
environment. The rate of movement is variable and sometimes abrupt depending
upon the angle of slope, the nature of the slope materials, intensity and type of
precipitation and climate. No significant accumulation of heavy minerals
including gold takes place in such settings.
However, as time passes and land surfaces are reduced in altitude, chemical
weathering processes become more important and both the depth of weathering
in the eluvial-colluvial ore zone and thickness of the regolith increase. The
stability of the slope material increases in response to the decreasing amount and
size of the material in transport and because more time is available for it to attain
equilibrium with its environment. The rate of movement is directly related to the
slope angle, the relationship between soil-water content and the applied pressure.
The infiltration rate is a function of the properties of the soil and the rate of supply
of water to the soil surface. Small amounts of water play a subordinate role by
providing lubrication between grains and under most types of climate alternate
wetting and drying of the soil will allow the detritus to move very slowly by soil
creep in close grain to grain contact on very low slope gradients.
Gravity, as the fundamental force causing movement relates directly to the
slope on which it acts. Natural surface slopes are about 35ë in loose scree
material but may exceed 45ë if silty. Landslides tend to occur when cliff faces
steepen to as much as 70ë. Gravity acts at its maximum value on vertical faces
and declines as the sine of the angle of slope to zero on the horizontal plane.
However, although gravity is the ultimate cause of downslope movement and
the rate of movement is directly related to the slope angle, the relationship
between soil-water content and the applied pressure is also a major cause of
mass movement. The development of positive pore pressure, with increasing
levels of saturation, decreases cohesive resistance, which becomes negligible
when the soil approaches complete saturation.
Small amounts of water play a subordinate role by providing lubrication
between grains and under most types of climate this will allow the detritus to