GOLD
The chemical solubility of gold
This is a very complex issue that can't be resolved with a certainty in an overall approach. What follows is only a broad vision of some specific conditions that can play part in the oxidation of primary gold deposits.
Gold occurs in pyrite in three forms:
- (1) Sub-microscopic, perhaps in the lattice of pyrite
- (2) As native gold in small blebs, droplets and ramifying masses
- (3) As irregular masses, plates and grains along fractures, slips and seams or coating of pyrite crystals
The aqueous chemistry of gold is essentially that of complex ions since the ions Au+1 and Au+3 are unstable in water because of their high oxidation potentials.
Complexing agents such as (S2O3)-2 or (CN)- or an excess of Cl- give soluble complexes of the type [Au(S2O3)]- [Au(CN)2]- and [AuCl2]- which render gold highly mobile in solution. Some humic complexes have the same facility.
Acidity (pH) of water plays important role in solubility of gold complexes.
Under acid conditions, such as where pyrite, pyrrhotite and other sulphides are undergoing oxidation, gold is more soluble and complexes with [AuCl2]-.
Under alkaline conditions gold complexes [Au(S2O3)]- and [Au(CN)2]- are mobile.
The pH seems to have little effect on the mobility of gold in the organic horizons.
The mobility of gold is decreased by the presence of H+ in alkaline environments and OH- in acid environments. The presence of Fe2+ and Mn2+, H2S or S2- may precipitate the sulphide or the free metal, thus greatly reducing the mobility of gold.
Where a high Fe3+ concentration is present gold is retained in the solution and hence mobile state. It should be noted that this applies to iron in solution; once hydrated oxides of hydroxides are precipitated these no longer play a part in the oxidation-reduction reactions.
Soluble manganous salts probably also play a role similar to those of ferrous salts in slightly acid, neutral and mildly alkaline solutions.
Where a high Fe3+ concentration is present gold is retained in the solution and hence mobile state. It should be noted that this applies to iron in solution; once hydrated oxides of hydroxides are precipitated these no longer play a part in the oxidation-reduction reactions.
Soluble manganous salts probably also play a role similar to those of ferrous salts in slightly acid, neutral and mildly alkaline solutions.
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