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.

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, 2008

This document is in the public domain.