It is important to indicate that the upper parts of gold deposits are commonly richer than the underlying primary ores. This residual enrichment in the oxidation zone is influenced by the solution and migration of associated minerals, the reduction in mass giving rise to a concentration of the undissolved gold. A further factor in this concentration is the actual solution, migration and enrichment of gold. The solution and migration of gold takes place more slowly in most cases than the migration of associated metals; this results in the zone of gold enrichment being left at a horizon above the enrichments of associated metals.
Gold prospectors have noted that the minerals that commonly occur with gold in the upper parts of oxidized ore-bodies are quartz and limonite; the limonite is often removed by solution, leaving the gold in a porous mass of quartz, the result being a still further concentration of the gold with respect to the containing gangue. A surface enrichment frequently takes place in gold deposits during the disintegration of the outcrop on weathering; the lighter grains of gangue are washed or blown away and the gold particles tend to work downward into superficial cracks, forming enrichments that usually extend for very short distances only below the surface. Where erosion or glaciation proceeds more rapidly than oxidation, the soft upper parts of ore-deposits are removed and primary ores outcrop at the surface.
Strictures of residual gold ores must be considered as surface phenomena. Their greatest dimension is more likely to be horizontal than vertical, being confined to the zone above the water level, or the depth to which oxidation has penetrated. Silver ores are affected in various ways by oxidation. The silver sulphate is readily soluble, migrates freely, and may precipitate as sulphide enrichments at the water level. Precipitation of silver as chloride is frequent, however, in the zone of oxidation, especially in arid regions where the residual ores are likely to be the most important: finally, silver chloride is slightly soluble, and, migrating through relatively short distances, is likely to produce local enrichments in the oxidized zone. A residual concentration of silver as chloride and native silver may also take place through the removal by solution of relatively soluble gangue minerals. Minerals that are frequently associated with Cerargyrite in oxidized ores are native silver, the bromide and the iodide. The residual ores of silver, being confined to the zone of oxidation, commonly show a distribution related to the topography.
Manganese oxides are frequently found as residual concentrations in the oxidized zone. Indeed, it is sometimes difficult to explain their abundance in the oxidized parts of certain deposits whose primary ore contains but little of this element. The oxides of iron and of manganese frequently occur with residual concentrations of both silver and of gold. Manganese oxide usually is most abundant in the upper part of the oxidized zone. The relative abundance of manganese in the oxidized zone as compared with the primary zone often gives an idea in regard to depth of leaching and erosion necessary to produce such concentrations.