Hypogene

Inoredepositgeology,hypogeneprocesses occur deep below the Earth's surface, and tend to form deposits ofprimary minerals,as opposed tosupergeneprocesses that occur at or near the surface, and tend to form secondary minerals.

At great depth the pressure is high, and water can remain liquid at temperatures well above 100 °C. Hot aqueous solutions originating frommagmas,deep sedimentary basins, or areas of elevated geothermal gradients can contain metals and otherionsderived from the magma itself or fromleachingofsedimentary,igneous,ormetamorphicrocks. Hypogene deposition processes include crystallization from the hot aqueous solutions flowing through the Earth's crust, driven by temperature and pressure gradients, as well as topographic, orogenic, and structural changes and/or controls.

Major dissolved components arechlorine,sodium,calcium,magnesiumandpotassium,and other important components includeiron,manganese,copper,zinc,lead,sulfur(asSO²⁻
₄orS²⁻or both)carbon(asHCO⁻
₃andCO₂) andnitrogen(asNH⁺
₄). Most ore fluids containchlorideas the dominantanion.

As the solutions rise the temperature and pressure fall. Eventually a point is reached where the minerals start to crystallise out.Minerals formed in this way are called primary, or hypogene, minerals.Sulfuris a common component of the fluids, and most of the common ore metals,lead,zinc,copper,silver,molybdenumandmercury,occur chiefly assulfideandsulfosaltminerals. Examples of primary minerals formed in this way include the sulfide mineralspyrite(FeS₂),galena(PbS),sphalerite(ZnS), andchalcopyrite(CuFeS₂).

Etymology

The wordhypogeneis derived from the Greek rootshypo-(ὑπο-) meaning 'under' and-gene(-γενής) meaning 'born' or 'produced'. The termshypogeneandsupergenerefer to the depth at which they occur.

See also

• Supergene (geology)