It is not possible to predict the depths at which the occurrence of gold is important. It is possible and of great practical importance to recognize the zone exposed, and so to appreciate the changes that may reasonably be expected with deeper exploration. The relative importance of the several gold zones depends in large measure upon the minerals and metals of the primary gold ore. For example, in a copper deposit, copper is frequently completely leached from the surface zone and is found as chalcocite enrichments without overlying residual ores. Where oxidation proceeds more rapidly than solution and enrichment is a gold deposit, the oxidized ore commonly contains the bulk of the values of the primary ore, perhaps in concentrated form, while the zone of enriched sulphides is poorly developed, or lacking. The upper part of the enriched zone is commonly the richest horizon of a secondary deposit.
According to several geologists, the completeness with which any orebody is rearranged by oxidation is proportional to the relative solubility of the metals it contains in the presence of existing precipitants, and the degree of post-mineral fracturing, or of porosity due to abundant sulphides as compared with resistant gangue minerals. In the upper zones of deposits that have been subject to oxidation and enrichment, kernels, or residual masses, are likely to be found that represent the character of the zones below. Residual nodules of oxidized ores are likely to occur in the leached surface zone, nodules of enriched sulphides in the oxidized zone, and masses of residual primary ore in the zone of sulphide enrichment. A study of such residual masses may show information in regard to the character of the lower gold zones not yet exposed by exploration.
A free milling oxidized gold ore may contain masses of unaltered and refractory pyritic ore, indicating that such will be the nature of the primary ore when reached. Residual kernels of galena frequently carry higher values in silver than does this mineral in the primary ore. In some deposits, the gold vein has suffered post-mineral movement and much of the ore is fractured, permitting ready access to percolating surface waters. The primary ore may consist of pyrite, arsenopyrite, tetrahedrite, and tennantite, with some galena and sphalerite. Sparingly scattered through this ore may be found small specks of pyaragyrite, proustite, and small quantities of some sulphides such as realgar and orpiment. The primary ore carries from 5-10 oz/t of silver and 0.2 – 0.8 g/t of gold. The gangue is quartz and rhodochrosite. The secondary or enriched sulphide ore, resting upon the primary ore may comprise bands of quartz and rhodochrosite alternating with bands of rich sulphides, comprising argentite, proustite, pyrargyrite, tetrahedrite, tennantite, pyrite and arsenopyrite. Probably more than one half of the silver in this ore is present as dark ruby silver, or pyrargyrite, with some proustite, occurring in minute veins or seams filling cracks in the vein, as films on the outside of crushed vein material, or as relatively large crystals lining cavities.
The enriched oxidized gold ore may occur just above the enriched sulphide ore; may be composed of quartz stained by the oxides of iron and manganese, and less commonly by copper carbonates. Cerargyrite and native silver may be present as thin seams cutting through the quartz and as films on the outside of quartz fragments. Associated with this ore are small quantities of argentite, and residual galena, pyrite and chalcopyrite are of local occurrence.