Many gold deposits around the world present a certain degree of oxidation, which has an impact on the performance of the recovery process. Oxidation changes sulphide, arsenide, antimonite, telluride minerals and allied compounds into oxides, native metals, and salts containing oxygen, which, according to conditions of solubility and local precipitants, either remain in situ, or migrate with or without gold to be redeposited under favorable conditions lower down in the deposit. Access by oxidizing solutions is the controlling factor in this process, which reaches its most complete development in deposits carrying abundant sulphides, which through solution and removal, become progressively more porous under the action of the solutions and so helps to bring the process to completion.
Sulphides that are locked up as small grains within massive gangue minerals are rarely altered by oxidation, unless intense post-mineral shattering affords access to the solutions. Oxidation tends to obscure primary structural relationships, and also to segregate the newly formed gold bearing minerals into larger masses, either of enriched oxides or enriched sulphides. Oxidation, through the more ready circulation of solutions, is likely to proceed to greater depths along gold veins than through the mass of the enclosing rocks; the country rock, therefore, is likely to exhibit signs of oxidation in the vicinity of gold veins, which is a valuable guide in underground exploration.
Gold prospectors have realized that the order in which the various sulphides are attacked by oxidation varies according to their relative abundance, and also according to structural conditions and the character of associated gold bearing minerals. The general order of attack by oxidation is directly as the relative affinities of the several metals for oxygen, and inversely as their affinities for sulphur. The order, in which the common primary sulphides are attacked, therefore, is arsenopyrite, pyrite, chalcopyrite, blende and galena. Chalcopyrite in certain occurrences is quite resistant to oxidation, and pyrite varies in this respect markedly in different localities, occasionally remaining quite fresh for many years in mine dumps that are completely permeated by sulphate solutions, while elsewhere it has so strong an affinity for oxygen as to become highly heated when wetted. Tellurides appear to be ready subjects of oxidation.
Regarding gold-copper deposits with presence of secondary copper sulphide, chalcocite is readily dissolved by acid solutions, as is illustrated in the various leaching processes. Tetrahedrite is a complex mineral of variable composition, and is irregular in its susceptibility to oxidation and solution. Most silver minerals are readily, attacked by sulphate solutions, but become fixed in the presence of chlorine or its compounds. The results of oxidation are likely to differ widely according to the mineral associations present, and are to a large extent subject to the influence of the enclosing rocks; some gold deposits are likely to contain carbonates as oxidized ores, while silicates or oxides are likely to predominate in acid rocks. During the oxidation of sulphide deposits in limestone, acid solutions reacting with this rock produce gypsum, a relatively soluble but strongly combined compound that resists change or precipitation; it is probable that the soluble products of oxidation that are not precipitated leave but little trace for present observation. Contact minerals are quite resistant to the attack of oxidizing solutions, and frequently enclose unaltered sulphides at the surface, while associated bodies of originally more compact sulphides are oxidized to great depths.