Oxidation Zone of Mineral Deposits

a zone found near the earth’s surface that forms as a result of chemical decomposition of unstable minerals under the action of surface waters and groundwater, as well as the oxygen and carbon dioxide present in the air and dissolved in these waters. The change is caused mainly by the process of oxidative transformation. Metal sulfide ores and deposits of sulfur, salts, and coal undergo the most vigorous change. The metal sulfides are replaced by oxides, hydroxides, carbonates, and sulfates of the same metals. Accumulations of alunite and gypsum form where sulfur is present. Gypsum also accumulates in deposits of rock salt. The decomposition of coal seams increases the content of moisture, ash, and oxygen in the coal, with a simultaneous decrease in the amount of carbon and hydrogen. The coverings of oxidation products that form under such circumstances at mineral deposits are called mineral caps. So-called gossan, which is deposited primarily by bog iron ore, is distinguished in the case of sulfide ore deposits; for salt and sulfur deposits, the “gypsum cap” is distinguished.

The oxidation zone extends down to the level of the groundwater and is usually a few meters to a few dozen meters thick, but sometimes at particular parts it is up to a few hundred meters thick (measuring from the earth’s surface). In the oxidation process part of the mineral material of a deposit may be dissolved by the groundwater, percolate downward, and be redeposited below the groundwater level, forming a zone of secondary mineral enrichment. A secondary enrichment zone is peculiarly characteristic of some copper, uranium, gold, and silver deposits.