Page 163 - Handbook of Gold Exploration and Evaluation

Gold deposition in the weathering environment 141

clays and hydrated iron oxides, which eventually pass into shales, sandstones
and occasional ironstone. Chemical rock types, which originate as chemical
precipitates are formed either as supersaturated solutions or by the biochemical
actions of marine organisms. Chemical precipitates are roughly homogeneous in
composition and are characterised by an interlocking crystal texture in which the
crystal sizes vary widely. The components of sedimentary rocks are simpler than
the rocks, from which they derive, but still vary in their resistance to weathering.
The most important textural property is the size of the individual particles.
Based upon particle size, sedimentary particles are divided into conglomerate
(2±4 mm) gravel, (larger than 2 mm), sandstone (0.0625±2 mm), siltstone
(0.0004±0.0625 mm), and claystone (smaller than 0.0004 mm). Sedimentary
rocks containing grains of different sizes are classified on the basis of the
dominant particle size. Conglomerate is the name given to a consolidation of
water-worn pebbles in a matrix of constituent rock and mineral fragments,
agglomerates and fine cementing materials such as carbonates and iron oxide.
Of the various sedimentary rocks, shales and sandstones take up water most
easily and are susceptible to mechanical breakdown by frost action. High density
jointing in some sandstone promotes rapid weathering by both chemical and
physical means. Being weakly compacted, they break down into irregular
fragments and eventually revert to clays and silts. Basal conglomerates
associated with each fresh cycle of erosion are the most important deposits in
many alluvial gold areas. Notable examples are the Banket conglomerates of the
Witwatersrand, South Africa, which have contributed more than one-half of the
gold produced in the world today. Figure 3.4 is a sedimentary classification of
rocks and their derived sediments.

3.1.3 Metamorphic rocks

Metamorphic rocks comprise igneous and sedimentary rocks that have been
altered from their primary states at higher temperatures and greater pressures
than are normally present at the Earth's surface. They represent products of both
mechanical shearing and crushing for which chemical and mineralogical
changes are negligible and recrystalised rocks in which new crystals are formed
although hindered in their growth by old minerals. Their structures reflect the
physiochemical environment in which they form and thereby the genesis and
history of the metamorphic rock (Barlow and Newton, 1974).
Metamorphism may result from orogeny and the intrusion or extrusion of
magma, or by interaction with migrating fluids from an external source. All
gradations of change are displayed according to differences in the type and
intensity of the metamorphic processes:
· Dynamic metamorphism due to pressure along dislocations in the Earth's
crust, is local and restricted in occurrence.