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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—> S2- —> (S2O3)2- —> (SO3)2- —> (SnO6)2- —> (SO4)2-
(n= 2-6)

A number of other complexes may also be formed such as HS-, HSO3- 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:
2Auo + 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

Since all of these reactants may be present in some oxidation zones it seems probable that some gold migrates as the soluble chloride complexes. There have, however, been critics of the MnO2 mechanism mainly because of the lack of this compound or because of unfavorable pH conditions. Eddingfield (l913a) for instance notes that the influence of manganese applies mainly to noncalcareous deposits. In the Philippines where abundant manganiferous calcite occurs in the veins the oxidizing waters are neutral or alkaline, and this prevents the formation of any free chlorine. He attributes the near-surface enrichment in the gold deposits of the Philippines mainly to the removal of gangue elements. Other critics have doubted that all of the necessary reactants are brought together at the most propitious time to effect solution of gold. Another possibility obtains, however, in oxidized zones, and this would seem to be the most important. Ferric salts such as the sulphate solubilize gold in the presence of dissolved chlorides under acid conditions. The ferric ion maintains the gold in an oxidized state that is then capable of binding with chloride to form soluble complexes. The simplified ionic reactions probably run as follows in an acidic and complexing environment:
Fe3+ + Auo —> Fe2+ + Au+
Au+ + 2Cl- —> [AuCl2]-
Since ferric sulphate is one of the most common compounds; in oxidizing zones containing sulphides and dissolved salts such as NaCl, KC1, etc. are present in some oxidizing waters, it follows that gold may be solubilized as suggested. Lakin (l969b) observed that when NaCl was added to synthetic leach solutions approximating the composition of solutions resulting from the commercial leaching of dumps of porphyry copper deposits gold was dissolved in the presence of manganese dioxide. The test solution of ferric sulphate and cupric sulphate (pH 1.5) contained 0.5 wt. per cent NaG. Leaf gold was added to two 200 ml portions of solution, and 0.5 g MnO2 was added to one of these portions. Air was bubbled through both solutions for 3 days, and the solutions were then filtered. The filtrate of the portion that had been in contact with MnO2 contained 400 micrograms of gold, the other portion only 4 micrograms.

Supergene gold is frequently found in association with the silver halides, chlorargyrite, bromargyrite and iodargyrite in a manner suggesting that the two precious metals were carried as the chloride, bromide or iodide in the supergene waters.

5. As various organic complexes. Gold is readily soluble as cyanide and thiocyanate complexes of the type [Au(CN)2]-, [Au(CN)4]- and [Au(CNS)4]-. Such complexes may be present in oxidized zones where large amounts of organic matter are present in the overlying soils and eluvium. Humic substances may also solubilize or carry gold in an adsorbed form under similar conditions. Actually we have few data on the efficiency of these particular types of organic compounds to carry gold in oxidizing zones. Cyanide complexes if present would seem to be only transitory because of the relatively low pH in zones where sulphides are undergoing oxidation. Similarly, the stability of humic complexes would not seem to be particularly high under acidic conditions.

6. Adsorbed to various organic and inorganic colloids. The possibility of this mode of transport by humic colloids has been alluded to above. It may be important where highly humic conditions prevail. Transport as an adsorbed constituent on inorganic colloids such as hydrous iron and manganese oxides and silica is highly probable in the oxidized zones of most types of gold-bearing deposits. In these substances (colloids and gels) we have repeatedly found enriched amounts of gold (and silver) in gold-silver deposits.

The pH and Eh of the oxidizing solutions and environment affect the mobility and precipitation of gold in a number of ways. Many of the soluble complexes of gold including the various sulphide species such as [Au(HS)]-, [Au2(HS)2S]2-, the thiosulphate, sulphite, cyanide and the thioarsenic and thioantimony species are stable only in neutral or alkaline solutions. Acidification of such solutions breaks up the complexes and native gold is precipitated. On the other hand the chloro complexes are stable only in acid and near neutral media. Increase in pH precipitates native gold. The pH and Eh also affect the mobility of gold in other ways, the principal ones being the effect on the ferrous-ferric and manganous-manganese dioxide couples which in turn markedly influence the solubility and precipitation of gold as we shall see.

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Rafal Swiecki, geological engineer email contact

This document is in the public domain.

March, 2011