Supergene (geology)
In ore deposit geology, supergene processes or enrichment are those that occur relatively near the surface as opposed to deep hypogene processes. Supergene processes include the predominance of meteoric water circulation with concomitant oxidation and chemical weathering. The descending meteoric waters oxidize the primary (hypogene) sulfide ore minerals and redistribute the metallic ore elements. Supergene enrichment occurs at the base of the oxidized portion of an ore deposit. Metals that have been leached from the oxidized ore are carried downward by percolating groundwater, and react with hypogene sulfides at the supergene-hypogene boundary. The reaction produces secondary sulfides with metal contents higher than those of the primary ore. This is particularly noted in copper ore deposits where the copper sulfide minerals chalcocite Cu2S, covellite CuS, digenite Cu1.8S, and djurleite Cu31S16 are deposited by the descending surface waters.
All such processes take place at essentially atmospheric conditions, 25 °C and atmospheric pressure.
Different zones can be identified at different depths. From the surface down they are gossan cap, leached zone, oxidized zone, water table, enriched zone (supergene enriched zone) and primary zone (hypogene zone).
Pyrite FeS2 is generally abundant, and near the surface it oxidises to insoluble compounds such as goethite FeO(OH) and limonite, forming a porous covering to the oxidized zone known as gossan or iron hat. Prospectors take gossan as an indication that there might be reserves of ore underneath.
The groundwater contains dissolved oxygen and carbon dioxide, and as it travels downwards it leaches out the minerals in the rocks to form sulfuric acid, and other solutions that continue moving downwards.
...
Wikipedia