Page 25 - Handbook of Gold Exploration and Evaluation

8 Handbook of gold exploration and evaluation

Table 1.4 Potentially important aqueous species (after Gray, 1997b)

Species Possible origin Solubility range
ÿ
Au(OH) 2 Oxidative dissolution of gold under Oxidised pH > 8
alkaline conditions
ÿ ÿ
AuCl 2 /AuCl 4 Oxidative dissolution of gold under Oxidised/saline/acidic
acid/saline conditions
ÿ
Au(HS) 2 Dissolution of gold by reduced waters Reduced/neutral
during early supergene alteration, or Eh < ÿ0.1V pH 6±9
by reducing solutions generated by total sulphur > 0.02 M
biological action
3ÿ
Au(S 2 O 3 ) 2 Weathering of gold/pyrite in neutral Alkaline to weakly acid
to alkaline solution
ÿ
Au(CN) 2 Interaction of cyanide with gold Limited to cyanide
content
Au ± organic Interaction of organic phases with Not certain
matter gold
Colloidal gold Formed during reduction of gold by Not certain
organic matter

grams of CaCO 3 are required for every gram of FeS 2 to maintain alkaline conditions
for thiosulphate production, and hence significant Au thiosulphate mobilisation.
Under highly reducing conditions gold bisulphide complex [Au (HS) 2 ] is
ÿ
particularly important for the hydrothermal transport of gold (Seward, 1973, 1982;
Boyle, 1979) but has only a restricted occurrence in the supergene zone (Gray,
1997b). In the vicinity of sulphide deposits, sulphide may exist in solution at 10±
ÿ
1000 mg/L. [Au (HS) 2 ] solubility is greatest in neutral reducing conditions and
assuming a total dissolved content of 2 10 ÿ6 M (700 mg/L) under optimum
conditions, total dissolved gold will equal 6 10 ÿ6 M (1200 g/L).

Halide complexes
The dissolution of Au chloride (AuCl 2 ) requires highly acid, saline and oxidising
conditions:
1
ÿ
2Au (S) + 4Cl + O 2 + 2H $ 2AuCl 2 + H 2 O 1.1
ÿ
‡
2
Laboratory simulations of weathering in the presence of manganese dioxide
have produced Au concentrations in acid (pH < 4) and highly oxidising (Eh >
680 mV) solutions containing one mole/litre chloride i.e., about twice seawater
concentration (Cloke and Kelly, 1964; Lakin et al., 1974). In this chemical
2‡
environment the oxidation potential is controlled by the Mn /Mn oxide redox
couple as observed in water from an ore deposit (Panglo) near Kalgoorlie,
Western Australia which has an Eh high enough for Au chloride dissolution.