Page 157 - Handbook of Gold Exploration and Evaluation

Gold deposition in the weathering environment 135

3.1 Cycle of rock transformation (after Strahler and Strahler, 1992).

ment of high pressures and high temperatures (a realm of igneous intrusion and
metamorphism). In its complete form this diagram stresses that mineral matter is
continually recycled through the three major rock classes as an introduction to
the dynamic geological system (plate tectonics), which provides an additional
perspective to the cycle of rock transformation. Occurrences of detrital gold and
primary gold are now seen as integral parts of a cycle in which the rocks are
related to one another and can be transformed one to the other.
The plate tectonic rock cycle, as illustrated in Fig. 3.2 is a complete summary
of the processes that lead to the evolution of the Earth. The cycle begins with the
generation of mafic oceanic crust by fractional melting of ultramafic magma at
an oceanic or any other divergent rift zone. A rising mantle convection cell
brings the ultramafic parent magma to the surface creating a mafic melt, which
forms the oceanic crust, leaving behind an ultramafic residue rich in Cu, Ni and
olivine. The parent mafic magma is designated the komatite suite. The major
paths of flow through the cycle are arrowed. The komatite suite passes through
the tholeiite >> calc-alkaline suites, to the sedimentary processes through the
Barrovian metamorphism (greenschist >> amphibolite >> granularite), and turns
back towards the calc-alkaline and alkaline suites.
As illustrated diagrammatically in Fig. 3.3 the tectonic rock cycle appears to
operate in a neverending cycle with each round of the cycle increasing the