Page 220 - Handbook of Gold Exploration and Evaluation

Gold deposition in the weathering environment 191

newly formed soils or be overlaid by transported sediment. Aridity also
promotes erosion although most of the debris is usually redeposited locally on
lower ground.

Cementation

Secondary accumulation of alkali and alkaline earth elements in ground waters
and their precipitation in the regolith commonly results in the precipitation of
secondary sulphates and carbonates in the soils and upper horizons. Cementation
is an important modification to the lateritic profile, the most important
cementing agents being iron oxides (ferricrete), silica (hardpan), Ca and Mg
carbonates aluminosilicates and gypsum (Butt and Anand, 1997). Modifications
to lateritic regoliths due to changes in tectonic and climatic conditions are
summarised in Table 3.9.

Table 3.9 Modification to lateritic regoliths due to changes in tectonic and climatic
conditions (after Butt and Zeegers, 1992)

A. Tectonic activity
Uplift
· increased erosion
· lowering of the water table
· irreversible dehydration and hardening of ferruginous and siliceous horizons
· increased leaching of upper horizons under more oxidising conditions.
Downwarping
· decreased erosion, increased sedimentation in valleys
· waterlogging of lower parts of the landscape and imposition of reducing
conditions.

B. Climatic change
To a more humid climate
· decreased erosion (due to thicker vegetation)
· increased leaching and deeper soil development.
To a less humid climate
· increased erosion (due to less vegetation)
· lowering of the water table
· irreversible dehydration and hardening of ferruginous and siliceous horizons
· decreased leaching.
To a semi-arid or arid climate
· increased erosion from uplands, with sedimentation on plains and in valleys
· lowering of the water table
· irreversible dehydration and hardening of ferruginous and siliceous horizons
· decreased leaching
· salinisation of ground water
· retention and precipitation of silica, alkaline earths and alkalis.