Page 24 - Handbook of Gold Exploration and Evaluation

Nature and history of gold 7

countries. Although the cost of gold is only a small fraction of end-product costs
in the electronics industry, consumers try to economise in its use by methods of
spot plating, strip plating and reduction in the thickness of plated surfaces.
Despite these economies, the rapid expansion of the electronics industry in
keeping with increased demand for electronic appliances is expected to increase
the overall demand for gold in that industry.

1.1.2 Gold geochemistry
Two important aspects of the aqueous geochemistry of gold are its chemistry
and the particular properties of the matrix solution (e.g. acidity, pH and
oxidation potential, Eh). Salinity can arise from various processes including rock
weathering and dissolution of previously deposited halite, evaporation, seawater
and aerosol deposition of seawater. Acidity, which is usually measured as pH
and factors such as pH, Eh, and salinity have major effects on the speciation and
solubility of gold. Eh (electrical conductivity) values less than 200 mV indicate
reducing solutions, which tend to be rich in reduced species such as Fe 2‡ or
ÿ
SH . Values higher than 500 mV indicate oxidising solutions, which generally
2‡ ÿ
contain high concentrations of oxidised species e.g., UO 2 or AuCl 4 (Gray,
1997a). The oxidation of pyrite and other sulphide minerals plays an important
role in the generation of hydrogen ions (acidity) during weathering.
Particular complexing anions and/or solution processes are required to enable
ground water mobility and the various complexes to become important under
different chemical conditions as listed by Gray (1997b) in Table 1.4. Specific
complexes are thio-complexes, halide complexes and organic complexes.

Thio-complexes
ÿ
‡
Sulphur forms a number of species with varying oxidation states from 2 to 6.
Depending upon the concentration of reduced sulphur, the most important
species for gold mobilisations appear to be (from lowest to highest oxidation-
state):
ÿ
· hydrogen sulphide (SH )
· solid sulphur, which does not mobilise gold
2ÿ
· thiosulphate (S 2 O 3 )
2ÿ
· sulphite (SO 3 )
2ÿ
· sulphate (SO 4 ), which does not complex gold.
The most important sulphur species for Au mobilisation appear to be hydrogen
sulphide and thiosulphate.
Sulphide is oxidised to sulphate in the presence of oxygen, although the
intermediate product, thiosulphate may be formed during pyrite oxidation under
neutral to alkaline weathering conditions. Mann (1984a) calculates that 400±800