Page 152 - Handbook of Gold Exploration and Evaluation

130 Handbook of gold exploration and evaluation

an elevation of 6 km in about a million years less what might be lost in that time
by erosion. Francis (1983) summarises known erosion rates within the context of
volcanic terrains, citing downcutting rates varying between 0.1±1.0 m per
thousand years in areas of high relief.

2.6.2 Denudation
Denudation is the sum total of the combined efforts of weathering, mass wasting
and erosion involved in the lowering of the Earth's surface and transport of
sediments to the sea. The preservation potential of orebodies depends to a large
degree upon the stage of denudation reached at the time of their exposure to the
atmosphere. On a geological timescale, magmatic arcs and collision type
mountain belts are rapidly exhumed and eroded away. Erosion rates of modern
gold-rich porphyry copper deposits, which form in volcano-plutonic arcs in
subduction-related settings, are also high. Epithermal ore bodies are shallowly
emplaced in tension fractures in the rocks and, although values are often
laterally extensive, they appear to be restricted to vertical depths of about 600 m
(Hutchison, 1985). Being closest to the surface, these deposits are exposed to
atmospheric weathering and erosion during the most violent stages of uplift and
derived sediments thus have low preservation potential, particularly after long
exposure to landscape denudation. Climatic conditions play an important role in
chemical decay of rock, as does topography. Humid conditions accelerate
decomposition and areas of low relief allow the reactants a longer time to
proceed to completion. Rapid run-off of surface water is effective in removing
the products of chemical change but militates against reaction time. For all of
these reasons, gold deposits derived from epithermal ores are preserved mainly
in Tertiary and younger sedimentary successions.
Large-scale fracturing in intrusive bodies stems from stresses set up during
cooling where the force of rising magma below causes the roof or hood zone to
yield; such stresses are transmitted to the immediate country rock. Concomitant
tectonic forces also play a significant role on a regional scale causing all rock
formations to fracture over a wide area. Intrusive rocks themselves exist within a
uniform stress field, in addition to external or regional forces, when confined by
surrounding rocks prior to erosion. As erosion progresses the stresses within the
rock are dissipated. Each type of rock-forming mineral has a different modulus
of elasticity and this leads to the development of miriads of microfractures and
to the loosening of the constituent grains ± rock disintegration on a small scale
(grain by grain) is initiated. Deposits, which form much deeper in the Earth's
crust, have a much longer preservation potential. There is some evidence to
suggest that gold-rich deposits, possibly of porphyry style, may be widespread in
Archaean greenstone belts (Symonds et al., 1987).
The course of erosion can be expressed in terms of a ratio of mechanical to
chemical weathering. In high rugged terrain mechanical erosion is dominant