Page 214 - Handbook of Gold Exploration and Evaluation

Gold deposition in the weathering environment 187

Deeply weathered profiles are products of long histories of weathering under
widely varying climatic conditions and the full sequence of horizons is not
developed until a considerable interval of time has passed. Climatic cyclicity is
an important part of the genetic model; a basic requirement appears to be
repeated wetting and drying and a fluctuating water table. Most lithologies are
affected and regoliths exhibit great variation in fabric and origin even within the
same profile or sequence (Butt and Anand, 1997). Factors determining the
distribution of elements in regoliths are the stability of their primary host
minerals, the presence or absence of secondary host minerals and the effects of
changing climates on the chemical mobilities of the elements.

Lateritisation

Ferruginous zone. The ferruginous zone (laterite) is composed principally of
secondary oxides and oxyhydroxides of iron (e.g., goethite, haematite) and
hydroxides of aluminium (gibbsite). The term lateritic residuum is used by
Anand et al. (1989) as a collective term embracing units such as loose gravels
and duricrust that have a close genetic and/or compositional relationship with
the substrate. Lateritic gold deposits contiguous with the ferruginous zone
contain both Ag-rich primary gold and Au-rich secondary gold.
Thiosulphate ion, humic acid, and possibly cyanide ion are the most active
ligands capable of complexing and dissolving the gold. A schematic
representation (Fig. 3.25) is presented by Smith et al. (1999) of the conditions
under which lateritic deposits may be formed (a) at or near the surface, and in (b)
where laterite is likely to occur in the sub-surface. The principal units of the
lateritic profile are `saprolith' and `pedolith'.

Saprolith. The base of the saprolith marks the effective onset of weathering,
though Fe oxide staining may occur along joints, shears and veins to con-
siderable depths. Two saprolith horizons are recognised, saprock and saprolite.
Saprock is a partly weathered rock with less than 20% of the weatherable
minerals altered. The base of the saprock marks the onset of weathering
(weathering front) which occurs along mineral boundaries and intra-mineral
fissures, shears and fracture planes. Upper and lower boundaries are gradational
and vary in depth and thickness over short distances. Weathering usually com-
mences with the oxidation of primary sulphides, which are highly susceptible to
weathering and only persist higher in the profile if enclosed within vein quartz.
Carbonates are also chemically unstable and elements hosted by them, e.g. Ca,
Mg, Sr, are similarly affected by leaching in the weathering zone. Gruz, a
fragmental disintegration product of largely unweathered granitic rock is similar
to saprock in many ways but is more friable.
During weathering, oxidation of the weathering front deep beneath the water
table produces neutral to acid conditions, with lower pH favoured by felsic rocks