It has been noted that an oxidized gold zone may be characterized by the precipitation process of gold bearing minerals, base metals sulphides and gangue minerals. In gold deposits in which the secondary zones are well-defined a layer of rich oxidized gold ore is frequently found to immediately overlie the enriched sulphides, from which it is derived by direct oxidation in place. In gold deposits that contain oxidizing precipitants particles and masses of residual oxidized ore are likely to be scattered through the leached zone, having been precipitated during migration before reaching the zone of enrichment. This precipitation hinders secondary enrichment and in extreme cases prevents the formation of such concentrations.
According to experienced gold prospectors, residual ores are precipitated by various reagents. Carbonates are formed through the action of the carbonic acid contained in surface waters, and also directly through the replacement of calcite, especially where the containing rock is limestone. Native metals, such as copper, gold, and silver, are formed by the action of reducing agents, among which organic matter and ferrous sulphate are prominent. Kaolin, gouge, and certain shales occasionally act as powerful precipitants through adsorption. Silver is often found as residual chloride, formed through precipitation by the chlorine contained in surface waters. Residual ores are often the result of incomplete solution, the relatively insoluble minerals being left behind during the migration of associated metals; incomplete oxidation and solution often leave residual masses of unaltered, or partly altered, sulphides in the oxidized zone. Such residual particles or masses have commonly been enriched by additions from circulating, metal-bearing solutions.
Also, through oxidation under conditions that permit re-precipitation, such as the oxidation of sulphide deposits in limestone, a scattered primary mineralization is probably often segregated into masses of rich oxidized minerals without important vertical migration. There are several gold-copper deposits with different degree of oxidation and the presence of high level of oxidized copper minerals is a serious problem, more even when the leaching option may be an attractive option to recover gold. In other cases the flotation process could have a low performance. Native copper is the la.st stage in the reduction of copper compounds, and is frequently found as pellets or films in the upper oxidized zones of some gold-copper deposits. It is commonly associated with cuprite, from which it is probably derived in most cases. Cuprite is often found just above the chalcocite zone, where it is formed from the chalcocite by direct oxidation. Both native copper and cuprite are commonly indicative of long and thorough oxidation, and are frequently found above important chalcocite enrichments. Tenorite is intimately related and associated with chalcocite. Chrysocolla is a common residual ore of copper, and is more likely to be abundant in siliceous rocks than in limestone, where carbonates are likely to prevail. The silicate of copper is frequently associated with manganese in black compounds of indefinite composition commonly called copper-pitch ores. Malachite and azurite are among the most important oxidized copper minerals; they are relatively resistant to oxidizing processes and are frequently found as residual ores. Azurite appears to be the more resistant of the two. Malachite and azurite frequently occur with limonite, an association that is explained on the supposition that siderite was formed with the copper carbonates, but subsequently, being more susceptible to alteration, decomposed to limonite. Brochantite is occasionally an important oxidized ore of copper, being often a transitional step in the formation of carbonates. In desert climates, atacamite may constitute an important ore, though its easy solubility confines it to regions of extreme aridity.