Hydrothermal Metamorphism

Hydrothermal metamorphism, also called metasomatism, may take place across wide regions of rock, thereby constituting a variant of regional metamorphism. It may alternately may take place in a limited, localized area and constitute a variant of local metamorphism.

Hydrothermal metamorphism takes place when hot, volatile solutions percolate into and react with the protolith, or the original rock. The heat of the intrusive igneous body and the hot volatile fluids serves to catalyze metamorphic reactions in the host rock. The incident fluids enhance ion mobility in the system and are highly reactive.

Both the intrusive magma and the associated volatile fluids may introduce elements which were not present in the protolith into the reaction process. The incident volatile fluids may also dissolve and remove elements originally abundant in the host rock. The chemical composition of the host rock may be extensively altered during the process of hydrothermal metamorphism. Hydrothermal metamorphism is therefore a type of allochemical metamorphism.

The hot, volatile fluids responsible for hydrothermal metamorphism may be derived from several sources. Such fluids may consist of volatiles escaping directly from the intrusive magma; they may also be composed of meteoric groundwater heated by the igneous intrusion or by the geothermal gradient.

High temperatures and pressures can expel water from hydrated minerals such as gypsum, talc, chalk, and clay. These sources of fluid are typically associated with local metamorphism. When an oceanic plate is subsumed at a boundary between two plates, water may be transported along with it and may subsequently take part in any metamorphic processes which occur. Seawater may also seep into the crust as two oceanic plates move apart. Such sources of volatile fluids are associated with regional metamorphism and the movement of crustal plates.

Much hydrothermal metamorphism occurs at the boundaries of oceanic plates. Plates which are moving apart allow seawater to percolate through the oceanic crust. As the seawater migrates, it heats and reacts with the host rock.

Large quantities of metals such as iron, cobalt, nickel, silver, gold, and copper are dissolved from the crust. When the hot, metal-laden fluid later comes into contact with cold seawater, sulfide and carbonate minerals precipitate to form deposits of metal ores. The copper ore which has been mined on Cyprus for several thousand years is thought to have been deposited in this way.

The presence of previously unavailable elements and the catalyzation provided by heat and volatile fluids enables a wide variety of mineral species to form during hydrothermal metamorphism. Many of these species contain elements not originally present in the host rock.

The process of hydrothermal metamorphism thus produces a greater variety of minerals than may be formed by thermal metamorphism alone. Silicates which are found in hydrothermal metamorphic deposits include quartz, garnet, wollastonite, olivine, topaz, and tourmaline. The halide fluorite may be present. Sulfide ores such as pyrite, chalcopyrite, sphalerite, and molybdenite and oxides such as magnetite, hematite, spinel, and corundum may also occur.

Local hydrothermal metamorphic deposits may grade into hydrothermal vein deposits. A vein forms when hot, mineral-bearing fluids deposit new materials along the walls of a preexisting fissure or crack in the host rock.

In the case where minerals merely exsolute from the aqueous solution to crystallize along the walls of the fissure, leaving the wall rock intact and unchanged, no metamorphism is involved. However, the minerals of the fissure walls often react with the passing volatile fluids. Some may be dissolved and transported away; some may recrystallize or crystallize as new mineral species. The formation of this type of vein deposit, called a hydrothermal replacement deposit, does constitute hydrothermal metamorphism.

How does hydrothermal metamorphism work?

Dissolution: Minerals in the protolith susceptible to the new conditions dissolve, releasing their constituent ions into the fluid.

Metasomatism: Ions in the fluid interact with each other and dissolved elements, forming new mineral assemblages stable under the altered pressure and temperature.

Recrystallization: Hot fluids can cause existing minerals to grow larger or form new crystals with distinct shapes and sizes.

Vein formation: New minerals introduced by the fluid can fill cracks and fractures, creating veins or veinlets within the rock.

What are the effects of hydrothermal metamorphism?

Hydrothermal metamorphism can have a variety of effects on rocks, including:

Changes in mineral composition: New minerals can form, existing minerals can dissolve or be replaced, and the overall mineral assemblage of the rock can be significantly altered.

Changes in texture: The original texture of the rock may be preserved, or it may be completely replaced by a new texture, such as banded or fractured.

Vein formation: The fluids can deposit minerals in cracks and fissures, forming veins of quartz, calcite, or other minerals.

Mineralization: Hydrothermal fluids can be enriched in valuable metals such as gold, silver, copper, and zinc, making them important for mineral exploration.

Where does hydrothermal metamorphism occur?

Hydrothermal metamorphism leaves its mark on a variety of rock types, including basalts, granites, and even sedimentary rocks. Some iconic locations showcasing its effects include:

Mid-ocean ridges: Seawater heated by newly formed oceanic crust percolates through the basaltic rocks, generating mineral-rich hydrothermal vents and altering the surrounding rock.

Geothermal regions: Hot springs and geysers in areas like Yellowstone National Park, driven by geothermal activity, trigger hydrothermal metamorphism in nearby rocks.

Subduction zones: As oceanic plates plunge beneath continental plates, they carry water deep into the Earth's crust, leading to extensive hydrothermal metamorphism within the subduction zone.

Economic Implications of Hydrothermal Metamorphism

What are some economic implications of hydrothermal metamorphism?

Hydrothermal metamorphism can play a role in the formation of valuable mineral deposits, including:

Gold, silver, copper, zinc, and lead: These metals are often dissolved by hydrothermal fluids and deposited in veins or disseminated throughout the rock.

Garnet, tourmaline, and other gemstones: These minerals can be formed during hydrothermal metamorphism and are prized for their beauty and durability.

Lithium: Hydrothermal fluids can concentrate lithium, a crucial element for batteries and other electronic devices.

What are some examples of rocks formed by hydrothermal metamorphism?

Greenschists, serpentinites, and some ore deposits are common examples of rocks formed by hydrothermal metamorphism.

What is the difference between hydrothermal metamorphism and other types of metamorphism?

Unlike regional metamorphism which involves large areas of rock under high pressure and temperature, and contact metamorphism which occurs near hot igneous intrusions, hydrothermal metamorphism is driven by the chemical reactions between hot fluids and the rock.

In conclusion, hydrothermal metamorphism, a complex interplay of heat, water, and chemical reactions, continues to shape our planet and leave its mark on the rocks that tell the Earth's story. By unraveling its scientific principles, we gain a deeper appreciation for its transformative power and its wider implications across various fields.