Page 162 - Handbook of Gold Exploration and Evaluation

140 Handbook of gold exploration and evaluation

basalts, suggest that some association with an intermediate granitic stage may be
necessary to their formation. The melting of some mixtures of crustal rocks and
sediments in a subduction zone gives melts of granitic origin.
Residual products of weathering of volcanic rocks especially basaltic and
andesitic rock types include a variety of ultra-fine sediments such as clay
minerals and silt. Difficulties in predicting the hydraulic behaviour of such
particles pose many problems in commercial mining operations (see Chapters 7
and 8). Size is only one consideration. Physical properties such as dilatance and
plasticity affect such problems as rates of carry over of finely divided gold
particles and the provision of ample space for predicted volumes of slimes
disposal. Dilatance relates to wave motions set up during settling. Plasticity
affects the rheology of the slime and the ease with which the solid/water
mixtures deform. Because of differences in such properties, slimes fractions
derived from andesitic rocks tend to have higher settling rates than basaltic
slimes thus causing fewer plant operational problems.

3.1.2 Sedimentary rocks
Although igneous and metamorphic rocks together comprise about 95% of the
total volume of rock types in the Earth's crust, they are predominantly covered
by sedimentary rocks and thin surface layers of sediment. About three-quarters
of the surface of continental platforms and almost all of the marine basins are
covered by sediments that have been reworked chemically or mechanically from
pre-existing rocks of any origin. The processes involve weathering of source
material, transportation by water and airflows, deposition and lithification.
The components of sedimentary rocks are simpler than the rocks from which
they derive but are still variable in their resistance to forces imposed by
weathering, erosion and transport. These forces are mainly dependent upon
topography, climate and geochemistry. Topography is a major factor influencing
the nature of sediment movement from source to the area of deposition. The depth
of weathering is a function of the rate of in-situ chemical weathering compared to
the rate of mechanical removal. Organic acids that promote the chemical
breakdown of rock materials are released by the decay of plant and animal waste.
Elevated temperatures and the availability of adequate water for chemical activity
and the mobilisation of waste products enhance most of the chemical reactions.
Two separate types of sediment (continental and marine) reflect the
sometimes rapidly fluctuating depositional conditions of their environments of
deposition. Continental sediments grade gradually from rapid mass movements
of mud and debris flow down to the slow accumulation of sands and muds on
river flats. Marine sediments grade from the shallow granular deposits of
beaches and deltas and continental slopes, to deposits of wind-blown dust and
chemical sediments flooring the ocean basins.
The clastic end products of weathering of pre-existing rocks are quartz sands,