Hydrothermal mineralization
Hydrothermal mineralization is a geological process where minerals precipitate from heated water, often influenced by tectonic and volcanic activities. This phenomenon occurs both in submarine and terrestrial environments, playing a vital role in the formation of economically important mineral deposits. The process begins when liquids containing minerals are heated by magmas or hot rocks, leading to changes in pressure, chemistry, and temperature that cause minerals to fall out of solution. Key locations for hydrothermal mineralization include mid-ocean ridges and subduction zones, where tectonic plates interact, creating conditions favorable for hydrothermal activity.
Submarine hydrothermal vents, characterized by either "black smokers" or "white smokers," release mineral-laden water, resulting in the formation of various deposits. Terrestrial hydrothermal vents, like those found in Yellowstone National Park, also contribute to mineral formation through steam and gas emissions. The minerals deposited can include metals such as copper and zinc, valuable both scientifically and economically. Beyond their economic significance, hydrothermal deposits offer scientists insights into tectonic processes, sediment transport, and the Earth's magnetic history. Overall, hydrothermal mineralization is a fascinating intersection of geology, resource management, and Earth science.
Hydrothermal mineralization
Hydrothermal mineralization occurs when mineral particulates precipitate from water. This process is closely tied to tectonics and volcanic activity, which create the heat and convection necessary for the transportation and deposition of minerals. Hydrothermal mineralization can occur in submarine or terrestrial environments. Hydrothermally created mineral deposits are economically important, and they provide data that help geologists to understand Earth processes.
Hydrothermal Mineralization: The Big Picture
Liquids frequently contain minerals, and when the liquids are heated, the minerals tend to fall from solution (precipitate). An everyday example of this is the mineral layer that sometimes forms on the bottom of tea kettles or in coffee makers.
In geologic terms, hydrothermal mineralization means that water is heated by magmas or hot rocks, and as the water travels, it leaves behind the minerals it was holding in solution. Other factors that affect precipitation are chemical reactions, pressure, oxygen levels, acidity, and rate of cooling.
To understand how water becomes heated and chemically active, it is necessary to understand wider geologic forces. Humans are familiar with the earth’s crust because it is the layer of inhabitation, but beneath the crust are the lithosphere (composed of tectonic plates), the asthenosphere, and the mantle, which is hot. Convection causes the plates to move. Sites where two tectonic plates crash together are called convergent boundaries. Sites where they are moving apart are called divergent boundaries.
Movement of tectonic plates creates seismic activity and facilitates the release of heat and hot magma from the earth’s interior. Evidence of this process comes in the form of volcanoes. Hydrothermal activity occurs at divergent boundaries (often midocean ridges) because new, hot material comes to the surface as the plates spread apart. Hydrothermal processes also occur at subduction zones (associated with convergent boundaries), where surface crust slides under another plate as they collide and descends to an area of increased heat and pressure. The rock in the crust then melts and is less dense than surrounding material, allowing the rock to again rise to the surface.
Because hydrothermal activity is associated with plate boundaries and hot spots (localized areas where a column of magma rises from the mantle), one can infer that hydrothermal mineralization occurs at divergent boundaries such as the East African Rift, the Mid-Atlantic Ridge, the East Pacific Rise, and the Red Sea Rift. Hydrothermal mineralization also occurs at subduction zones, such as the ones along the West Coast of the Americas. Hydrothermal activity also occurs at oceanic hot spots such as the Hawaiian Islands and at continental hot spots such as the Yellowstone National Park region of the United States.
Hydrothermal Processes and Mineral Deposition
Hydrothermal vents—fissures in which water heated by magmatic activity rises and escapes to the surface—can occur in submarine or terrestrial environments. The types of mineral deposits associated with these vents depend upon the composition of the nearby rock and magma. Magma from divergent plate boundaries is made of a high percentage of iron and magnesium, but at convergent plate boundaries, the magmas tend to be higher in silicon and oxygen because they are mixing with continental sediments rich in these elements.
At submarine vents, ocean water heated to temperatures nearing 350 degrees Celsius (662 degrees Fahrenheit) rises, leaching metals from the basaltic rocks it passes through. The metals later precipitate from the water, creating mineral deposits. These deposits can occur in the form of black smokers, which are tower-like structures built from iron-rich sulfides. White smokers, which have lower temperatures, contain calcium sulfate and silica deposits. Some silver, lead, zinc, and copper sulfides are produced near submarine hydrothermal vents, too.
Deep ocean deposits are neither large enough nor accessible enough to mine economically. However, in the future, resource scarcity and improved mining technology may make these deposits more attractive to exploit. Some exploitable ore deposits can be traced to ancient seafloor spreading centers that have moved great distances in time and have been lifted by plate collision. An example can be seen in the island nation Cyprus, where copper and other minerals, formed eons ago, are now near the surface.
Silicates, including feldspar, quartz, and mica, are not as economically valuable as metal deposits, but they are also formed by hydrothermal processes. Continental hydrothermal deposits are often rich in silica because igneous rocks that make up continental crust are high in silica. Silica-rich igneous rock that cools and solidifies underground is called granite, but silica-rich igneous rock that cools at the surface is known as rhyolite. These rock compositions are important because hydrothermal fluids pass through the rock and pick up minerals from them.
Hydrothermal fluids seep into existing spaces and precipitate minerals called cavity-filling deposits. Sometimes, however, the hydrothermal fluids can react with the rocks they pass through and alter the rock to form a deposit made from both hydrothermal precipitates and host-rock material. This process is called hydrothermal replacement.
Terrestrial hydrothermal vents occur in several forms associated with volcanic activity. Examples can be seen in and around Yellowstone National Park, which is located at a hot spot. A fumarole is a type of gas- and steam-producing vent that occurs in volcanic craters or along their sides. Deposits form when the vent’s gases cool. The Yellowstone area is rich in rhyolite, so the hydrothermal fluids there are silica-rich. Hot springs or geysers frequently contain silicon dioxide and calcium carbonate. Precipitation of these minerals can be aided by the biologic activity of organisms that live in and around hot springs.
Other continental hydrothermal deposits were formed in ancient subduction zones where metals were liquefied as they made their way down toward the mantle then rose into fractures in the crust. As the fluids flowed through, such metals as copper, molybdenum, silver, gold, lead, and zinc were left behind. These metals are called porphyry metal deposits because they occur in porphyritic igneous rock associated with hydrothermal activity. Deposits of this type have been found in North America, and are being mined.
Reading the Earth
Even when hydrothermal deposits have no economic value, they have scientific value. Because of the association of seafloor spreading centers with hydrothermal activity and the creation of mineral deposits, scientists can use deposits and rock samples from and near oceanic ridges to learn about cycles of spreading, subduction, and sediment transport. In addition, because humans cannot descend into the mantle to gather data about it, the material brought to the crustal layer through geologic events is a valuable extrapolative tool.
Hot hydrothermal fluids emerge from point sources (vents) and begin to spread out and cool as they mix with cold ocean water. Scientists can track the movement of these fluids by using a method called isotopic tracing, which uses isotopes (forms of an element with the same number of protons but differing numbers of neutrons) to provide information about the vertical and horizontal movement of water. Isotopes also can be used to determine the age of particular rock samples or the conditions under which they formed. This is helpful in establishing patterns in tectonic plate movement or the rate of seafloor spreading.
Because many of the hydrothermal deposits and magmas at seafloor spreading centers contain iron, which can be magnetized, they also provide a record of Earth’s magnetism. The needle of a compass will invariably indicate north because that is where the earth’s magnetic pole is located. This has not always been true. Earth’s magnetic poles reverse approximately every few hundred thousand years. The process occurs gradually and does not follow strict cycles, but because seafloor spreading occurs with magmatic activity, and because magma becomes magnetized at a specific point during cooling, scientists can construct a relatively precise timeline of the earth’s magnetism based on the magnetism of rocks and deposits near seafloor spreading centers.
One of the other benefits of studying mineral deposits at seafloor spreading centers is to compare magnetic and other qualities of the deposits on each side of the ridge. Minerals on one side of the ridge should have a matching counterpart on the other side of the ridge. By looking for similarities, scientists can see plate movement outward from a spreading center, but they also can track horizontal movement, such as the type of movement at transform fault boundaries.
Principal Terms
black smoker: a columnar structure formed deep in the ocean when sulfides precipitate from hydrothermal fluids
crust: the outermost layer of the earth; crust may be continental or oceanic
divergent boundary: an area where two tectonic plates are spreading apart instead of crashing together or sliding against each other
fumarole: a gas- and steam-producing vent at volcanic craters
hot spot: a column of magma that rises from the mantle, remaining in one place as the lithospheric plate moves over it; also known as a mantle plume
hydrothermal vent: a fissure through which magmatically heated fluids escape to Earth’s surface
isotopes: atoms of an element that contain the same number of protons but different numbers of neutrons
magma: hot fluid or semifluid from beneath the earth’s crust; later forms igneous rock
mantle: a hot layer of the earth below the crust, asthenosphere, and lithosphere; surrounds the inner core of the planet
silicate: a compound containing silica; includes mica, feldspar, and quartz
sulfide: a compound made with sulfur; an iron sulfide is a compound of iron and sulfur
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