Supergene (geology)
In
All such processes take place at essentially atmospheric conditions, around room temperature (25 °C) and standard atmospheric pressure (1 atm).[2]
Zones
Distinct zones of supergene processes can be identified at various depths. From the surface down they are the gossan cap, leached zone, oxidized zone, water table, enriched zone (supergene enriched zone) and primary zone (hypogene zone).[3]
Gossan cap
Pyrite (FeS2) has oxidised to form goethite (FeO(OH)) and limonite (FeO(OH)·nH2O),[2] which form a porous covering over the oxidized zone known as a gossan cap or iron hat.[4] Prospectors use gossan as an indication of ore reserves.
Leached zone
Groundwater contains dissolved oxygen and carbon dioxide. As it travels downwards it oxidizes primary sulfide minerals, concomitant with forming sulfuric acid and solutions of oxidized metals.[5] For example, groundwater commonly interacts with pyrite (FeS2) to form an oxidized iron (FeO(OH)) and sulfuric acid (H2SO4), portrayed in the idealized chemical reaction below (intermediate steps omitted):
- 4 FeS2 + 12 H2O + 15 O2 → 4 FeO(OH) + 8 H2SO4
An intermediate in this process is ferric sulfate (Fe2(SO4)3), which oxidizes pyrite and other sulfide minerals.[6]
Oxidized zone
Above the water table the environment is
Solutions traveling downward from the leached zone react with other primary minerals in the oxidised zone to form secondary minerals[5] such as sulfates and carbonates, and limonite, which is a characteristic product in all oxidised zones.[3]In the formation of secondary carbonates, primary
Water table
At the
Enriched zone
The net effect of these supergene processes is to move metal ions from the leached zone to the enriched zone, increasing the concentration there to levels higher than in the unmodified primary zone below, possibly producing a deposit worth mining.
Primary zone
The primary zone contains unaltered
Mineral alterations
Chalcopyrite CuFeS2 (primary) readily alters to the secondary minerals bornite Cu5FeS4, covellite CuS and brochantite Cu4SO4(OH)6.[5]
Galena PbS (primary) alters to secondary anglesite PbSO4 and cerussite PbCO3.[2][5]
Sphalerite ZnS (primary) alters to secondary hemimorphite Zn4Si2O7(OH)2.H2O, smithsonite ZnCO3 and manganese-bearing willemite Zn2SiO4.[2][5]
Pyrite FeS2 (primary) alters to secondary melanterite FeSO4.7H2O.[5]
If the original deposits contain arsenic and phosphorus bearing minerals, secondary arsenates and phosphates will be formed.[5]
Etymology
The word supergene is derived from the Latin root super meaning 'above' and the Greek root -gene (-γενής) meaning 'born' or 'produced'. The terms supergene and hypogene refer to the depth at which they occur.
See also
References
- ISBN 0-7167-1456-6
- ^ a b c d Manual of Mineralogy (1993) Klein and Hurlbut. Wiley
- ^ a b c d e Understanding Mineral Deposits (2000). Kula C Misra. Kluwer Academic Publishers
- ^ a b The Encyclopedia of Gemstones and Minerals (1991). Martin Holden. Publisher: Facts on File
- ^ a b c d e f g h Field Guide to North American Rocks and Minerals (1992) The Audubon Society. Alfred A Knopf
- ^ Harraz, Hassan Z. (2012), Topic 9: Supergene enrichment, Tanta University
- ^ a b c d John Rakovan (2003) Rocks & Minerals 78:419