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Metasomatism is the chemical alteration of a rock by hydrothermal fluids. It is also known as "alteration".
Metasomatism can occur via the action of hydrothermal fluids from an igneous or metamorphic source.
In the igneous environment, metasomatism creates skarns, greisen, and may affect hornfels in the contact metamorphic aureole adjacent to an intrusive rock mass.
In the metamorphic environment, metasomatism is created by mass transfer from a volume of metamorphic rock at higher stress and temperature into a zone with lower stress and temperature, with metamorphic hydrothermal solutions acting as a solvent. This can be envisaged as the metamorphic rocks within the deep crust being squeezed dry of fluids and dissolved mineral components, with this fluid percolating up into the shallow levels of the crust to chemically change and alter these rocks.
This mechanism implies that metasomatism is open system behaviour, which is different from classical metamorphism which is the in-situ mineralogical change of a rock without appreciable change in rock chemistry. In nature, because metamorphism requires water in order to facilitate metamorphic reactions, metasomatism and metamorphism nearly always occur together.
Further, because metasomatism is a mass transfer process, it is not restricted to the rocks which are changed by addition of chemical elements and minerals or hydrous compounds. In all cases, to produce a metasomatic rock some other rock is also metasomatised, if only by dehydration reactions with minimal chemical change. This is best illustrated by gold ore deposits which are the product of focused concentration of fluids derived from many cubic kilomatres of dehydrated crust into thin, often highly metasomatised and altered shear zones and lodes. The source region is often largely chemically unaffected compared to the highly hydrated, altered shear zones, but both must have undergone complementary metasomatism.
Types of Metasomatites
Metasomatic rocks can be extremely varied. Often, metasomatised rocks are pervasively but weakly altered, such that the only evidence of alteration is bleaching, change in colour or change in the crystallinity of micaceous minerals.In such cases, characterising alteration often requires microscope investigation of the mineral assemblage of the rocks to characterise the minerals, any additional mineral growth, changes in protolith minerals, and so on.
In some cases, geochemical evidence can be found of metasomatic alteration processes. This is usually in the form of mobile, soluble elements such as barium, strontium, rubidium, calcium and some rare earth elements. However, to characterise the alteration properly, it is necessary to compare altered with unaltered samples.
When the process becomes extremely advanced, typical metasomatites can include;
- Chlorite or mica whole-rock replacement in shear zones, resulting in rocks in which the existing mineralogy has been completely recrystallised and replaced by hydrated minerals such as chlorite, muscovite, and serpentine.
- Skarn and skarnoid rock types, typically adjacent to granite intrusions and adjacent to reactive lithologies such as limestone, marl and banded iron formation.
- Greisen deposits within granite margins and cupolas.
Alteration assemblages
Investigation of altered rocks in hydrothermal ore deposits has highlghted several ubiquitous types of alteration assemblages which create distinct groups of metasomatic alteration effects, textures and mineral assemblages.- Propylitic Alteration is caused by iron and sulfur bearing hydrothermal fluids, and typically results in epidote-chlorite-pyrite alteration, often with hematite and magnetite facies
- Albite-epidote alteration is caused by silica bearing fluids rich in sodium and calcium, and typically results in weak albite-silica-epidote
- Potassic Alteration, typical of lode gold deposits, results in production of micaceous, potassic minerals such as biotite in Fe-rich rocks, muscovite mica or sericite in felsic rocks, and orthoclase (adularia) alteration, often quite pervasive and producing distinct salmon pink alteration vein selvages.
Rarer types of hydrothermal fluids may include highly carbonic fluids, resulting in advanced carbonation reactions of the host rock typical of calc-silicates, and silica-hematite fluids resulting in production of jasperoids, mantos and pervasive zones of silicification, typically in dolomite formations.
See also
"Metasomatism"