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

This commonly takes place in the deeper parts of gossans and oxidized gold deposits. The native gold is commonly adsorbed or absorbed by the hydrous manganese oxide in a very finely divided form and is rarely visible (see 4. below).

4. Adsorption and/or coprecipitation negatively charged gold complexes and colloids by positively charged gels such as hydrous ferric oxides (limonite). This mechanism appears to be particularly effective since most gelatinous iron oxides in or near gold-bearing deposits are generally enriched in gold. Gelatinous manganese oxides, likewise, adsorb and/ or coprecipitate gold, but the mechanism is not entirely clear. Since the gold complexes and colloids are negatively charged the negative hydrous manganese oxides should repel them. However, these gels invariably contain considerable iron and other elements that may reverse the overall charge giving the complex a positive charge, which is effective in precipitating the negative gold complexes and colloids. Alternatively, anionic gold complexes may be adsorbed on the positively charged hydrous manganese (II) oxides.

The writer's observations on natural gold- and silver-bearing hydrous iron and manganese oxide gels suggest that initially the reaction is essentially an adsorption phenomenon, that is, the gels exist as a random arrangement of micelles of the oxides with adsorbed gold and silver ions. No gold or silver can be observed in these dried gels even under the highest powers available with the optical microscope. With aging of the gel some of the gold and silver may be desorbed and split out of the complex yielding small spangles and filaments of native gold in the vicinity of the oxides (limonite and wad). In other cases much of the gold and silver is retained by the oxides as an adsorbed phase or in an extremely finely divided form inextricably mixed with the oxides.

Clay minerals and gelatinous silicate complexes also tend to markedly concentrate gold. One frequently finds seams, bunches and patches of clay minerals in the oxidized zones of gold deposits greatly enriched in gold. Gouge and puggy clay along fractures and faults, likewise, tends to concentrate gold in the oxidized zones. The reason for this is evidently largely due to adsorption processes.

Numerous other natural gel-like substances tend to precipitate gold readily. Among these may be mentioned humic gels, humic-limonitic gels, humic-limonite-wad-alumina gels, bismuth ochres, tellurium ochres and amorphous antimony and arsenic ochres. The precipitation mechanism is evidently largely due to adsorption on these substances.

5. Coagulation and/or precipitation of gold colloids by various charged ions, sols and gels. Metallic gold colloids carry a negative charge and hence positively charged clay minerals, hydrous iron oxide and various other positively charged ions, sols and gels should effectively coagulate or precipitate the gold. The importance of this mechanism is unknown since we have no data on the transport of gold as a colloid in the oxidized zones of gold deposits.

6. Precipitation due to the presence of various natural reductants. Various field and laboratory observations show that native gold is readily precipitated from gold-bearing solutions by practically all sulphides, silicates, carbonates, native metals, clay minerals, carbonaceous substances, and a great variety of supergene hydrous oxides, arsenates, antimonates, etc. The gold is only rarely found coating these various substances; more generally the dissolved metal is precipitated on nearby minute nuclei of native gold, and these continue to grow to give relatively coarse platelets, wires, sprigs, flakes, slugs and nuggets in the oxidized zones. In some veins, however, there is a general precipitation of gold in the form of finely divided particles throughout such materials as disintegrated sulphides, limonite, wad, puggy clay and various ochres in the oxidized zones.

7. Coprecipitation and/or adsorption by numerous supergene minerals. Among these may be mentioned bismutite, chlorargyrite, the jarosites, beudantite, bindheimite, malachite, azurite, gypsum, native sulphur, pyromorphite, scorodite, and various other phosphates, vanadates, arsenates and antimonates. The exact nature of gold in some of these materials is unknown. Some gold is evidently in lattice positions in these minerals, but most is probably present in a very finely divided form (dust). In some occurrences in chlorargyrite and the analogous bromide and iodide the gold may be coarse and often well crystallized.

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Rafal Swiecki, geological engineer email contact

This document is in the public domain.
March, 2011