GOLD

Oxidation and Mobility of gold

General principles

During oxidation processes in gold-bearing deposits gold may migrate in the oxidizing waters:
1. As the metal Auo apparently in solution or in a dispersed state as a colloid protected by a variety of other colloids including silica, hydrous iron oxides, hydrous manganese oxides, etc. Extremely finely divided gold and lattice gold released as a result of the oxidation of auriferous pyrite, pyrrhotite, arsenopyrite, chalcopyrite, stibnite, tetrahedrite, the various gold tellurides and aurostibite would seem to be especially susceptible to migration in these forms.
2. As hydroxocomplexes of the type [Au(OH)], [Au(OH)₂]^-, [Au(OH)₄]^- and
[Au(HS)(OH)]^-. Roslyakov et al. (1972) claim that some of these complexes are relatively stable in oxidized zones.
3. As various dissolved sulphur (thio) species. During the oxidation of sulphides and sulphosalts the sulphur component may yield a variety of species, including sulphide ion, thiosulphate, sulphite, polythionate and sulphate depending upon the Eh and pH.

Metal sulphide—> S^2- —> (S₂O₃)^2- —> (SO₃)^2- —> (SnO₆)^2- —> (SO₄)^2-
(n= 2-6)

A number of other complexes may also be formed such as HS^-, HSO^3- and probably many other H-S-O species, making the oxidation of a sulphide a most intricate process. A number of these complexes render gold (and silver) soluble, in the neutral and alkaline pH range including particularly HS^-,
(S₂O₃)^2-, and (SO₃)^2-. The complexes formed are of the types [AuS]^-,
[Au(HS)₂]^-, [Au(S₂O₃)₂]^3- and [Au(SO₃)₂]^3-. Where arsenic and antimony are abundant in gold-bearing sulphide deposits gold arsenothio and gold antimonothio complexes of the type [Au(AsS₃]^- and [Au(Sb₂S₄)]^- may be responsible for the migration of gold. That such complexes are probable is suggested by analogy with silver, the presence of secondary (supergene) pyrargyrite and proustite being relatively common in some silver deposits.

Gold (III) salts of oxy-anions are not very stable, but complex auric sulphates of the type [Au(SO₄)₂]^- are known. Such complexes may be present where the oxidation potential is high in oxidizing sulphide zones and where H₂SO₄ and an oxidant such as MnO₂ are present. This mechanism may partly account for the relatively high migration capacity of gold in some oxidizing sulphide bodies.

Roslyakov et al. (1972) consider that the solubility of gold as sulphate complexes is improbable from thermodynamic considerations, the standard electrode potential for the formation of the [Au(SO₄)₂]^- complex being unfavorable, even under strongly oxidizing conditions.

It seems probable that much of the migration of gold in sulphide deposits is the result of solution of gold by sulphur, arsenic and antimony complexes particularly the thiosulphate, sulphite and sulphide species. Complex auric sulphates may also be a factor in the mobility of gold since we have observed that finely divided gold is slightly soluble in ferric sulphate solutions (Boyle et al., 1975).

Listova et al. (1966) carried out a number of experiments involving solutions formed during the oxidation of various natural Pb, Zn and Fe sulphides. They found that gold was dissolved in these solutions, especially when CaCO3 was present and concluded that the metal was complexed under weakly alkaline conditions by thiosulphates and polythionates formed during reaction of carbonates with the products of the oxidizing sulphides.

4. As various dissolved halide species, mainly chloride complexes. Gold has long been known to be soluble as chloro complexes:
2Au^o + 2H⁺ + 4Cl^- —> 2[AuCl₂]^- + H₂
Provided an oxidant such as Fe³⁺ is present, gold may have a relatively high mobility in oxidized zones where chloride occurs in the waters; It has been considered for many years that the generation of the reactive chlorine that solubilizes the gold is due to the following reaction:
MnO₂ + 2Cl^- + 4H⁺ = Mn²⁺ + 2H₂O + Cl₂

Gold in: Primitive Classic Medieval Renaissance post-Renaissance period.

Gold: Deposits Transport 1 2 3 4 5 6

Rafal Swiecki, geological engineer email contact
February, 2006

This document is in the public domain.