|
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.
 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
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.
 Meteoric
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)) |
|
