Oxidation and other hydrothermal alteration process lead to the breakdown of rock structure, resulting in increase permeability. This often allows high leaching extractions to be achieved by heap leaching of run-of-mine ore, even though the ore particle size is very coarse.
A detrimental feature of rock oxidation and alteration is the formation of considerable amounts of hydrated, amorphous and poorly crystalline silica, sulphate, oxide and hydroxide gangue phases. These phases have relatively high solubilities in comminution and cyanide leaching, and may act as strong cyanicides, due to the generation of extremely large and fresh surface areas with high sorption potential.
Gold usually occurs either liberated or in the alteration products of pyrite and other sulphidic minerals. The most common of these are the iron oxides, such as hematite, magnetite, goethite and limonite, although gold can be associated with manganese oxides/hydroxides. Generally the degree of gold liberation is increased by oxidation; however, in some cases protective coatings of secondary and hydrated oxides on gold are commonly encountered.
Oxidized ores differ from primary ores as a large proportion of fines are often generated on grinding, or during heap leaching and minerals such as clays are more abundant. The presence of clays, such as pyrophyllite, talc, kaolinite and montmorillonite can have important process implications, for example: decreased heap leaching pad permeability, increased slurry viscosities in processing and hence increased energy required for slurry mixing, blinding of activated carbon in CIL or CIP processes.
Carbonate minerals such as calcite, dolomite and siderite are more common in oxidized ores and affect pH control, especially in oxidative pretreatment processes. In the oxidized zone of vein deposits, native gold generally has a lower silver content, due to the great solubility of silver. The distribution of gold in supergene enriched and lateritized deposits is shown in Fig. 2.3. Gold is present as unaltered liberated or partially liberated grains and as redistributed secondary gold at depth, with some material mobilization. A zone of supergene enrichment may exist near the primary ore body and below a depleted region.
The deposits of Yilgarn region (Australia) have been subjected to prolonged and extensive oxidation to depths of 50-100 m. these ores respond well to heap leaching or agitated cyanide leaching and overly primary sulphidic deposits. Gold occurs as residual primary gold and redeposited supergene mineralization in iron oxide/silicate gangue. This secondary gold occurs as fine grains or with sponge texture, has low silver content and is closely associated with iron oxides. The proposed deposition mechanism is by dissolution of gold in saline groundwater and redeposition under reducing conditions.
Oxidized ores may also contain various oxide copper minerals, many of which dissolve in cyanide and may interfere with gold leaching and recovery processes.