Hydrometamorphism is the alteration of rocks of a gold deposit; and this process has influence in the formation of ores and gold bearing minerals by atmospheric waters. In its broadest sense, it includes the varied processes of weathering, oxidation, hydration, the leaching of rocks and ores, and the solution, migration and enrichment of gold. Rain water, as it strikes the earth, is practically pure water, except for small quantities of dissolved atmospheric gases; upon coming in contact with ore minerals, especially sulphides, however, it is transformed into solutions of great chemical activity, which attack, transform and rearrange the minerals of gold orebodies, and so effect enrichments of the greatest economic importance.
Primary gold deposits so low in grade as to be without commercial value are frequently thus transformed into deposits of commercial importance. It is necessary in the examination of any gold area, therefore, to determine whether the gold is of primary or of secondary origin; if primary, its values may be expected to continue indefinitely in depth to the zone of primary impoverishment; if secondary, the values are known to be controlled by surface agencies, and gold may be expected to continue to such depths only as have been reached by surface waters. The decomposition and weathering of rocks, unlike the changes undergone by ore-bodies, are accomplished by water carrying as dissolved constituents chiefly atmospheric oxygen and carbon dioxide, together with vegetable acids. The chemical changes wrought are principally hydration, oxidation and the formation of carbonates.
Essentially, the tendency is toward the solution of the more easily dissolved minerals, which is attended by the formation and a surface concentration of the more resistant minerals, among which are quartz, aluminous clays and limonite; complex minerals are thus broken down into simpler constituents more resistant to weathering, which remain at the surface. The weathering of rocks is characteristically regular over large areas, and is rarely mistaken for the results of hydrothermal alteration or kaolinization. The residual minerals from rock decomposition are commonly soft, and are readily carried away by erosion, which is likely constantly to present fresh rock surfaces to the attack of the chemical alteration above outlined. Even in regions of heavy rainfall, however, a combination of slight slopes and abundant and active vegetable acids may result in deep accumulations of residual gold bearing minerals, as frequently occurs in tropical countries.
In this way, weathering attended by slight or partial decomposition results chiefly in the staining of the surface rock red, brown or yellow by iron oxides set free by the alteration of basic silicates, or magnetite, and in the alteration of amphibolitic rocks to serpentine. Where atmospheric waters percolate to depths considerably below the surface, their oxidizing power is diminished, and hydration is the principal alteration; the minerals thus formed through rearrangement of original constituents, and to a less extent through introduced substances, are epidote, chlorite, serpentine, pyrite, and quartz. Leaching, usually accompanied by kaolinization, is a process of solution exercised by surface waters by which the soluble minerals are removed and the resistant minerals left as residual products. The minerals resistant to leaching and which commonly make up the leached upper parts of ore-bodies are quartz, kaolin, limonite and oxide of manganese. The completeness with which minerals are removed from the leached zone depends upon their relative solubility and also upon the presence or absence of effective precipitants in that zone, which latter may cause residual masses of oxidized ores to form in the leached zone at the expense of the enrichments below.
In many gold deposits the presence of active oxidizing precipitants causes the metals to be precipitated immediately or shortly after undergoing solution, a process known as precipitation in situ; where this process predominates, secondary enrichments are prevented or are of relatively slight importance. Under certain conditions limonite is completely removed, and the leached zone, white in color, is made up of quartz, kaolin and unchanged sericite. Where oxidation and solution of the accessory minerals proceed more rapidly than the solution and migration of the valuable metal, as often happens in deposits containing gold, the reduction in mass may result in a residual concentration; this form of enrichment is not uncommon, and is very different from the secondary enrichment due solution and migration.