Page 219 - Handbook of Gold Exploration and Evaluation

190 Handbook of gold exploration and evaluation

and high sulphide contents. Gold associated with tellurides or held in the lattice
of the sulphides and other minerals may be released, but free gold remains
immobile due to the absence of complexing ligands.
Saprolite is an upward extension of saprock, but one in which more than 20%
of the weathered materials have been altered. Fine fabrics of weathered bedrock
are retained but saprolitic materials tend to become more massive upwards as
the proportion of clay increases. Cementation by secondary silica, carbonates,
aluminosilicates and especially iron oxides is not uncommon. Gold deposits,
formed either within the weathered ore zone or laterally dispersed into the wall
rock consist primarily of secondary gold with residual primary gold confined to
the lode system. Upgrading of a saprolite is similar to upgrading in an eluvial
gold deposit in that original fabrics of the source rock are retained while mobile
constituents of the rock are lost.

Pedolith
The four principal zones of the pedolith are the arenose zone, mottled zone,
ferruginous zone (laterite) and soil. Although intensely weathered major struc-
tural features such as quartz veins may be preserved, usually with some change
in orientation (dip). In the arenose or plasmic zone, massive clays or sandy clays
are developed. The mottled zone is characterised by blotches of Fe oxides,
which may develop with further mobilisation into nodules and other secondary
structures containing tubular voids. These nodules increase in number upward
and in the ferruginous zone may be cemented together to form a duricrust.

Post-lateritic modification
Modifications of the pre-existing lateritic regoliths may occur by partial truncation
or by cementation as they adjust to complex sequences of environmental change
(Butt and Anand, 1997). Gold grains are gradually liberated and/or modified until
ultimately, two different styles of gold may be present in the weathering profile,
primary and supergene. If gold-silver alloys are more soluble than gold, primary
gold enrichment will be preserved unless extreme conditions prevail. A marked
depletion of gold at depths of about 5 m to 15 m over most mineralised units and
lodes follows the repeated strong leaching of the upper saprolite. This has
occurred even when the profile has been truncated (Butt, 1988).

Partial truncation
Erosion typically follows episodes of climatic change or drainage rejuvenation
following uplift. It results in removal of part or all of the upper layers of the
profile and may expose the lower horizons and unweathered rock to surface
weathering. These fresh surfaces may themselves become the parent material of