Page 156 - Handbook of Gold Exploration and Evaluation

3

Gold deposition in the weathering environment

Alluvial and lateritic gold deposition in the weathering environment involves a
complex and interrelated sequence of chemical, physical and biological
weathering processes within the regolith of waste in situ, above or in the
immediate vicinity of gold source rocks. Climate and topographic relief govern
the rates of operation of the different weathering processes at the atmosphere-
lithosphere interface; various rock types are progressively exposed to changing
environmental conditions and as time passes and relief diminishes zones of
mineralisation undergo complex sequences of change. The gold grains are
liberated and modified by largely chemical means at various depths in the
weathering zone and on slopes. Long-term changes and reversals of climate are
accompanied by changing water tables and deep-seated chemical weathering.
Eluvial and colluvial gold placers, as short-lived products of weathering, are
related to processes of the present environment in terrains of moderate to low
relief. Saprolitic and lateritic placer types are formed on flat slopes by processes
that relate to past climatic regimes of long duration in deeply weathered regoliths.

3.1 The plate tectonic rock cycle
The plate tectonic rock cycle asserts that the Earth's physical/chemical evolution is
driven by energy from the interior and that every geological feature is formed under
a specific set of tectonic conditions. Minerals and rocks are stable only under the
conditions at which they form and as conditions change so too do the rocks change
in order to achieve equilibrium with the new conditions. Igneous rocks evolve into
sedimentary rocks when exposed to weathering and erosion at the Earth's surface,
sedimentary rock changes under stress into metamorphic rocks, and metamorphic
rocks eventually melt to once more become igneous rocks.
A fundamental concept is that all rocks are related to one another, and can be
transformed each to the other. Figure 3.1 suggests how these transformations
may take place, distinguishing between a surface environment of low pressures
and temperatures (sedimentation and weathering processes) and a deep environ-