GOLD

Oxidation and Mobility of gold

General principles

The pH of oxidizing solutions in gold deposits is controlled mainly by the presence of various acids, especially H2SO4, which derives directly from the oxidation of pyrite and other sulphides or the hydrolysis of certain sulphates, mainly ferric sulphate, and by the neutralizing effect of carbonate rocks or gangue. Thus, where the gangue is mainly quartz, pyrite and other sulphides and the wall rocks unreactive such as slate, greywacke, quartzite, gneiss or schist the conditions are commonly acid, and the waters are only very slowly neutralized. If chlorides are present in the environment, gold may show considerable migration as the chloro complexes. In addition there is less tendency for absorption of gold on hydrous iron oxides (limonite) and hydrous manganese oxides (wad). In fact, the latter may not be precipitated at all because of the acidic conditions. On the other hand where the gangue or wall rocks are carbonate bearing the solutions are quickly neutralized, and the alkaline complexes of gold are stable thus imparting a relatively high mobility to the element. However, abundant hydrous iron and manganese oxides are formed under slightly acid, neutral and alkaline conditions, and these tend to adsorb gold strongly in this pH range (5-8). The mobility of gold is, therefore, a rather complicated phenomenon, particularly so when one encounters various mixtures of gangue minerals and a variety of host rocks. Gold deposits tend to be individual, and the factors that have led to the supergene mobility and precipitation of gold have to be worked out for each deposit. To generalize one can say that gold is particularly mobile in an acidic or strongly alkaline environment, and hence the most favorable conditions for secondary enrichment are weakly acid, neutral and weakly alkaline conditions. This is also the general view of Roslyakov (1976).

With respect to silver and the base metals, gold generally has a relatively low mobility during oxidation processes. In the absence of humic complexing, studies carried out by the writer indicate that the generalized sequence of mobility is

Zn>Cd=Hg>Ag>Cu=Mo>Co=Ni>Au>Pb>Sn=W=Bi.

Morris and Lovering (1952) found a somewhat similar sequence in their study of the dispersion of heavy metals in the Tintic district, Utah. As with all generalized sequences there are commonly local modifying influences. When humic matter is present in the system the order of the mobility of the various elements may be greatly modified, depending on the type of humic complexes present and other factors. Gold appears to have a relatively high mobility in humic waters; its mobility is probably about the same as that of Ag, Cu, Hg and Mo.

A number of mechanisms for the precipitation of gold from downward migrating supergene solutions are possible. Some of these are:

1. Increase or decrease in the pH of the solutions due to oxidation of pyrite with the consequent production of H₂SO₄, hydrolysis of ferric sulphate, which produces H₂SO₄ and reactions with gangue minerals and wall rocks, which tend to neutralize or make the solutions alkaline. Increase in pH destroys the chloro complexes of gold and precipitates the native metal. Decrease in pH has a similar effect on the alkaline complexes such as [AuS]^-, [Au(HS)₂], thiosulphate, sulphite, etc., and native gold is precipitated.

2. Precipitation of native gold from solution by ferrous ion. Where gold-bearing solutions encounter ferrous ion as in zones where a low oxidation prevails native gold is precipitated.
Fe²⁺ + Au⁺ <=> Au^o + Fe³⁺.
Commonly this reaction is characteristic of the deeper zones of oxidizing deposits, but in some veins and lodes the phenomenon is marked in the near-surface zones. This mechanism of precipitation seems to be one of the most important in deposits containing pyrite, pyrrhotite, arsenopyrite, siderite and other readily oxidized iron minerals. It is probably the main mechanism of precipitation of gold in many gossans; also in the zone of reduction (zone of supergene sulphides).

Experimentally, Machairas (1967) has shown that auriferous pyrite and arsenopyrite yield secondary gold as a result of reduction by FeSO4. He recognized three stages in the process:
(1) oxidation of pyrite and arsenopyrite to yield H₂SO₄ + Fe₂(SO₄)₃ which solubilizes the primary gold;
(2) reduction of the dissolved gold by FeSO₄ to give secondary gold; and
(3) hydrolysis reactions, which precipitate the iron oxides (limonite). These results agree with those carried out in our laboratories and reported elsewhere (Boyle et al., 1975).

3. Precipitation of gold from solution by manganous ion in feebly acid, neutral or alkaline solutions:
2Au⁺ + Mn²⁺ + 4OH^- —> 2Au^o + MnO₂ + 2H₂O

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.