GOLD

Oxidation and Mobility of gold

General principles

Gold deposits, subjected to oxidation, may become enriched near the surface both as a result of chemical processes involving the migration of gold and by the removal of soluble gangue and sulphides. Where silver and lead occur in quantity in auriferous deposits in argentite, tetrahedrite, galena or other sulphides, there is also a parallel enrichment of these elements as a result of oxidation processes. In contrast to copper and silver, however, gold exhibits less the tendency to be enriched in secondary sulphide zones in veins and other deposits.

The mobility, migration, concentration or dispersion of gold during the oxidation of gold deposits are strongly influenced by the following factors:
1. The type of deposit and its mineral composition. The nature of the gangue, types of sulphides and primary gold bearing minerals are the main considerations. The first two control to a large degree the nature and presence of solubilizing and precipitating constituents in the oxidizing waters. They also control to a varying degree the various reactions between the solid components of the veins and the oxidizing waters.
2. Types of enclosing rocks, specifically their mineral and chemical composition. These have some control on the nature and presence of solubilizing and precipitating constituents in the oxidizing waters. They also control to a varying degree the various reactions between the oxidizing waters and the solid components in the wall rocks.
3. Degree of fracturing, crushing or shearing of the deposit and its enclosing rocks.
4. Climatic factors.
5. Position of the water table.
6. Organic agencies. Gold is soluble as certain organic compounds, or it may be complexed and rendered soluble by the products of these agencies, e.g., H₂S produced by bacteria that gives rise to [AuS]^-.

All these factors interact to produce a variety of complex chemical systems that are impossible to follow in detail here. The discussion will be restricted to some of the general features of oxidized gold deposits and to a relatively simple view of the chemical processes involved.

Here we shall deal mainly with the course of gold during oxidation processes. The principal hypogene gold-bearing minerals are native gold, aurostibite, fischesserite and the various tellurides, e.g., sylvanite. Auriferous pyrite, pyrrhotite and arsenopyrite should also be included because although they are not gold minerals per se they provide a large proportion of the gold in a number of districts. There is only one supergene mineral of gold namely the native metal. In some auriferous lodes there is a suggestion that aurostibite and some of the tellurides may be of supergene origin, but the evidence is equivocal. Supergene native gold occurs in a great variety of forms ranging from irregular slugs, nuggets, platelets, dendrites, wires, sprigs, tufts, hairs, filaments, filligrees, spongy masses, mossy aggregates and foils to very finely divided metal commonly called paint or mustard gold. Much of the gold in the oxidized zones of auriferous deposits occurs in relatively discrete particles and aggregates perched on, attached to or coating the various products of oxidation or in some cases intimately intergrown with minerals such as secondary calcite, gypsum, sulphur, kaolinite and opal. In addition there is commonly much microscopic and submicroscopic gold in. mineral aggregates such as limonite and wad. Some of this exists in discrete microcrystals and masses; the remainder is in an adsorbed or chemically combined form.

Native gold is relatively inert and in many of its deposits, especially those with a paucity of sulphides and sulphosalts, the mineral passes into the oxidized zones in essentially the same condition, as it is present in the primary ore. Under other conditions it is dissolved or migrates in a variety of forms discussed in some detail subsequently.

The fate of gold during oxidation processes is greatly affected by the nature and size of the gold particles in the primary deposits. The solubility of gold is much greater when the element is released in an ionic form from the lattices of pyrite, arsenopyrite, etc. or when the gold is released in a submicroscopic (colloidal) form from these host minerals. Much of this gold evidently migrates either in solution, as a colloid or in suspension. When the gold is present in particles greater than about 100 µ in diameter, its solubility is greatly restricted. Most of this gold migrates in a physical form as platelets, spangles or small nuggets. Gold in this form also tends to restrict the migration of ionic gold, since it forms readily available nuclei for precipitation of the dissolved gold. Al'bov (1952) has claimed that the size of the gold basically determines its behavior in the zone of oxidation.

Oxidation of gold tellurides may yield soluble gold and silver or finely divided native gold (containing some silver) and tellurite or a variety of tellurites and tellurates. Native tellurium may also be of supergene origin in the oxidized zones of some auriferous telluride deposits, although it is often difficult to prove that this is in fact the case. In the zones of reduction of certain auriferous telluride deposits, however, there is little doubt that at least some of the native tellurium is supergene in origin. In some deposits tellurite and/or paratellurite is a product of the oxidation of sylvanite, calaverite, etc.
AuTe₂ + 2O₂ —> Au + 2TeO₂

In others, insoluble iron tellurites such as emmonsite, mackayite and blakeite are precipitated mainly by reaction of Fe³⁺ with the tellurous acid formed during the oxidation of the tellurides.
AuTe₂ + 2O₂ + 2H₂O —> Au + 2H₂TeO₃

In still other places, tellurates may be precipitated by reactions involving telluric acid formed by the oxidation of the tellurides under very high oxidation potentials.
AuTe₂ + 3O₂ + 6H₂O —> Au + 2H₆TeO₆

Relatively little is known about the oxidation of aurostibite, AuSb₂. My observations indicate that the common products of oxidation are spongy masses of gold and antimony ochres, probably the oxides cervantite, senarmontite and valentinite although they give no X-ray patterns. The oxidation reaction can be written empirically as:
2AuSb₂ + 3O₂ —> 2Au + 2SbO₃

Presumably under certain conditions the gold may be liberated in a soluble form. Similarly the Sb (III) and Sb (V) oxides may be dissolved under alkaline conditions and may precipitate various antimonites and antimonates. The oxidation of gold-bearing tetrahedrite is somewhat similar to that for aurostibite.

The oxidation of the silver-gold selenide, fischesserite, probably yields native gold enriched in silver and various selenites and/or selenates depending on the magnitude of the oxidation potential.

The oxidation of auriferous pyrite, pyrrhotite and arsenopyrite yields various soluble iron and arsenic compounds and gold. The latter may be released in a soluble form or as finely divided, spongy or mustard gold.

The fate of primary gold and finely divided native gold released by the oxidation of auriferous pyrite, pyrrhotite, arsenopyrite, chalcopyrite, stibnite, various other sulphides and sulphosalts, tellurides, aurostibite and fischesserite is varied and depends on many factors operating in the oxidized zone. My observations show that much of the primary gold and finely divided gold released from sulphides is incorporated into the cementing minerals of the gossans where it is inextricably intermixed with limonite, wad and other mineral aggregates. Some of this gold also finds its way into the eluvium and ultimately into alluvial placers. The very finely divided gold (flour gold), however, may retain considerable mobility in the water and be moved downstream in the drainage system for great distances. Where the oxidation zones are highly porous and loose, much of the fine gold moves physically downward simply by gravity or in downward percolating waters. Finally, the gold may migrate in a variety of chemical forms under certain conditions. We do not know the actual chemical forms of gold in the oxidizing waters of gold deposits for no one has measured these directly. However, we can infer certain possible forms from the known chemistry of the element and from the chemistry of groundwaters and mine waters. Some of the mechanisms of chemical migration follow.

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Rafal Swiecki, geological engineer email contact

This document is in the public domain.
March, 2011