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Hydrothermal and aqueous solutions

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Hydrothermal and Aqueous Solutions

A large number of economically valuable ores and mineral deposits exist as tabular or columnar bodies called veins.  These veins occur in a wide variety of igneous, metamorphic and sedimentary host rocks.  Typically, the veins form through the precipitation of minerals from high- to low-temperature aqueous or hydrothermal solutions in pre-existing fractures, fracture zones, and solution cavities.  

Cassiterite Most high-temperature hydrothermal waters are related to igneous activity or metamorphism, either as the source of heat or water.  The classic hydrothermal vein consists of minerals precipitated from the last dregs of igneous magmas.  If igneous differentiation continues past the pegmatitic stage, the last remaining liquids become highly enriched in water and volatile components such as fluorine, chlorine, sulfur, and metals such as tin, tungsten, gold, silver, uranium, and other elements that could not be incorporated into the structures of earlier-formed minerals because of their unusual sizes or ionic charges.  These hydrothermal solutions may crystallize fluorine-bearing minerals such as fluorite, topaz, or tungsten and tin  

Realgar (red), Orpiment (yellow), Stibnite (black), and Calcite (white) oxides (cassiterite photograph to left), iron, copper, lead, zinc, silver, antimony, arsenic sulfides, including pyrite, chalcopyrite, chalcocite, tetrahedrite, galena, sphalerite, and stibnite, realgar (photograph on right), and even native metals such as gold, silver, and copper, or simple quartz-, calcite- and fluorite-rich veins without sulfides or oxides.   Metamorphism may produce dehydration reactions releasing water and dissolved metals or minerals from previously hydrated minerals.  Hydrothermal solutions generated by magmatic and tectonic process have supplied much of the world's gold, silver, copper (porphyry copper deposits), and other metals.  Volcanic activity typically heats the mineral-precipitating water in hot springs and geysers. 

Fuorite from Missississippi Valley Type Deposit, Harden County, IllinoisMeteoric waters (groundwater) or salty water-rich brines derived from sedimentary basins occur as low-temperature aqueous solutions.  Ground waters may dissolve and later precipitate simple carbonate, silica, and sulfate minerals (calcite, chalcedony, gypsum, barite, celestite) or precipitate more complex sulfide minerals at greater depths through supergene enrichment processes (see below).  Low-temperature sedimentary brines are capable of scavenging metals and other ions in solution and later precipitating galena, chalcopyrite, pyrite, sphalerite, fluorite (photograph on left), and other minerals in Mississippi Valley Type deposits after reaction with carbonate sedimentary rocks has occurred.  


Supergene Enrichment of Ore Minerals  Primary, or hypogene, sulfide ore minerals such as pyrite, chalcopyrite, galena and sphalerite, often are altered near the Earth's surface to secondary or supergene minerals by  a complex process called "supergene enrichment{".  Low-temperature, oxygenated meteoric waters oxidize and dissolve the unstable primary sulfide minerals near the Earth's surface.  The dissolved metals are carried downward in aqueous solution, eventually producing a leached zone at a greater depth.  The dissolved metals may precipitate out to form two zones of secondary enrichment, one above and the other below the water table.  Secondary oxidized minerals such as malachite, azurite, cuprite, smithsonite, and hemimorphite precipitate out or crystallize above the water table in the zone of oxidized enrichment.   Beneath the water table, in the zone of supergene enrichment, secondary minerals such as covellite, chalcocite, and native copper may precipitate from solution.  In this way, the ore is enriched and concentrated.

Whenever one of the components dissolved in an aqueous or hydrothermal solution exceeds its solubility limit, mineral precipitation occurs.  Factors such as an increase in the concentration of a component in the aqueous solution, a decrease in temperature or pressure, reaction with surrounding rocks or minerals, and mixing with other solutions can initiate precipitation.  Typically, the specific chemical reactions occurring in mineral dissolution, transport, and precipitation are extremely complex and still not well understood.  Because crystals precipitated from aqueous solutions crystallize in water-filled spaces, they have the potential to be well-formed and extremely beautiful.

Primary or hypogene ore minerals Zone of oxidized enrichment minerals Zone of supergene enrichment ore minerals
Chalcopyrite (CuFeS2)
Bornite (Cu5FeS4)
Tetrahedrite (Cu12Sb4S13)
Tennantite (Cu12As4S13)
Enargite (Cu3AsS4)
Malachite (Cu2(CO3)(OH)2)
Azurite (Cu3(CO3)2(OH)2)
Rosasite ((Cu,Zn)2CO3(OH)2)
Cuprite (Cu2O)
Olivenite (Cu2(AsO4)(OH))
Covellite (CuS)
Chalcocite (Cu2S)
Native copper (Cu)
Pyrite (FeS2)
Marcasite (FeS2)
Pyrrhotite (Fe1-xS)
Goethite (aFeO.OH)
Limonite (mineraloid of hydrated iron oxides)
Galena (PbS)  Anglesite (PbSO4)
Cerussite (PbCO3)
Wulfenite (PbMoO4)
Vanadinite (Pb5(VO4)3Cl)
Mimetite (Pb5(AsO4)3Cl)
Pyromorphite (Pb5(PO4)3Cl)
Descloizite (PbZn2(VO4)(OH))
Sphalerite (ZnS) Smithsonite (ZnCO3)
Aurichalcite ((Zn,Cu)5(CO3)2(OH)6)
Adamite (Zn2(AsO4)(OH))
Descloizite (PbZn2(VO4)(OH))



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