hydrothermal alteration


Metasomatism (from the Greek μετά ''metá'' "change" and σῶμα ''sôma'' "body") is the chemical alteration of a
rock Rock most often refers to: * Rock (geology) A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its Chemical compound, chemical composition and the way in w ...
hydrothermal Hydrothermal circulation in its most general sense is the circulation of hot water (Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC ...
and other fluids. It is the replacement of one rock by another of different mineralogical and chemical composition. The minerals which compose the rocks are dissolved and new
mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks of which it is composed, and the proces ...

formations are deposited in their place.
Dissolution Dissolution may refer to: Arts and entertainment Books * Dissolution (Forgotten Realms novel), ''Dissolution'' (''Forgotten Realms'' novel), a 2002 fantasy novel by Richard Lee Byers * Dissolution (Sansom novel), ''Dissolution'' (Sansom novel), a 2 ...
and deposition occur simultaneously and the rock remains solid. Synonyms to the word metasomatism are metasomatose and metasomatic process. The word metasomatose can also be used as a name for specific varieties of metasomatism (for example ''
-metasomatose'' and ''
-metasomatose''). Metasomatism can occur via the action of hydrothermal fluids from an
igneous Igneous rock (derived from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the p ...
or metamorphic source. In the
igneous Igneous rock (derived from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the p ...
environment, metasomatism creates
skarn Skarns or tactites are hard, coarse-grained metamorphic rocks that form by a process called metasomatism Metasomatism (from the Greek μετά (change) and σῶμα (body)) is the chemical alteration of a rock by hydrothermal and other fluids. ...

greisen Greisen is a highly altered granitic A granitoid is a generic term for a diverse collection of coarse-grained Igneous rock, igneous rocks that consist predominately of quartz, plagioclase, and alkali feldspar. Granitoids range from plagioclase-ric ...
, and may affect
hornfelsImage:Hornfels.jpg, A sample of banded hornfels, formed by contact metamorphism of sandstones and shales by a granite intrusion Hornfels is the group name for a set of Metamorphism#Contact .28thermal.29, contact metamorphic rocks that have been bake ...

in the contact metamorphic aureole adjacent to an
intrusive rock , an igneous ''intrusion'' exposed when the surrounding softer rock eroded away Intrusive rock is formed when magma penetrates existing rock, crystallizes, and solidifies underground to form '' intrusions'', such as batholiths, dikes, sills, ...
mass. In the metamorphic environment, metasomatism is created by
mass transfer Mass transfer is the net movement of mass from one location, usually meaning stream, phase, fraction or component, to another. Mass transfer occurs in many processes, such as absorption Absorption may refer to: Chemistry and biology *Absorption ...
from a volume of
metamorphic rock , a type of metamorphic rock Metamorphic rocks arise from the transformation of existing rock (geology), rock to new types of rock, in a process called metamorphism upright=1.35, Schematic representation of a metamorphic reaction. Abbreviat ...

metamorphic rock
at higher stress and
temperature Temperature ( ) is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Thermal energy refers to several distinct physical concept ...

into a zone with lower stress and temperature, with metamorphic hydrothermal solutions acting as a
solvent A solvent (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the ...

. This can be envisaged as the metamorphic rocks within the deep crust losing fluids and dissolved
mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks of which it is composed, and the proces ...

components as hydrous minerals break down, 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 upright=1.35, Schematic representation of a metamorphic reaction. Abbreviations of minerals: act = actinolite; chl = Chlorite group">chlorite; ep = epidote; gt = garnet; hbl = hornblende; plag = plagioclase. Two minerals represented in the figure ...
which is the in-situ mineralogical change of a rock without appreciable change in the chemistry of the rock. Because metamorphism usually requires
water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for all known forms of , even though it provide ...

in order to facilitate metamorphic reactions, metamorphism nearly always occurs with metasomatism. Further, because metasomatism is a mass transfer process, it is not restricted to the rocks which are changed by addition of
chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, eleme ...
s and minerals or hydrous
compound Compound may refer to: Architecture and built environments * Compound (enclosure), a cluster of buildings having a shared purpose, usually inside a fence or wall ** Compound (fortification), a version of the above fortified with defensive structu ...
s. 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 kilometres 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. Metasomatism is more complicated in the Earth's mantle, because the composition of peridotite at high temperatures can be changed by infiltration of carbonate and silicate melts and by carbon dioxide-rich and water-rich fluids, as discussed by Luth (2003). Metasomatism is thought to be particularly important in changing the composition of mantle peridotite below island arcs as water is driven out of ocean lithosphere during subduction. Metasomatism has also been considered critical for enriching source regions of some normative mineralogy, silica-undersaturated magmas. Carbonatite melts are often considered to have been responsible for enrichment of mantle peridotite in incompatible elements.

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 group, 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 group, 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 cupola (geology), cupolas. * Rodingite typical of ophiolites particularly their serpentinized mafic dykes, containing grossular-andradite garnet, calcic pyroxene, vesuvianite, epidote and scapolite. * Fenite, as a variant of metasomatism associated with strongly alkaline or carbonatitic magmatism introducing a variety of feldspars, sodic pyroxenes or amphiboles and often unusual minerals (such as chevkinite or columbite) comprising ordinarily incompatible elements that do not readily become incorporated into a crystal lattice i.e. niobium, zirconium * Albitite, from replacement of plagioclase by albite (albitization) Effects of metasomatism in mantle peridotite can be either modal or cryptic. In cryptic metasomatism, mineral compositions are changed, or introduced elements are concentrated on grain boundaries and the peridotite mineralogy appears unchanged. In modal metasomatism, new minerals are formed. Cryptic metasomatism may be caused as rising or percolating melts interact with surrounding peridotite, and compositions of both melts and peridotite are changed. At high mantle temperatures, solid-state diffusion can also be effective in changing rock compositions over tens of centimeters adjacent to melt conduits: gradients in mineral composition adjacent to pyroxenite dikes may preserve evidence of the process. Modal metasomatism may result in formation of amphibole and phlogopite, and the presence of these minerals in peridotite xenoliths has been considered strong evidence of metasomatic processes in the mantle. Formation of minerals less common in peridotite, such as Dolomite (mineral), dolomite, calcite, ilmenite, rutile, and armalcolite, is also attributed to melt or fluid metasomatism.

Alteration assemblages

Investigation of altered rocks in hydrothermal ore deposits has highlighted 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 group, 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 porphyry copper and lode gold deposits, results in production of micaceous, Potassium, potassic minerals such as biotite in iron-rich rocks, muscovite mica or sericite in felsic rocks, and orthoclase (adularia) alteration, often quite pervasive and producing distinct salmon-pink alteration vein selvages. *''Sericitic alteration, Quartz-sericite-pyrite alteration'', in which these minerals can be deposited both in Veins, metallic and mineral, veins and in a disseminated manner; sericite in particular replaces plagioclase and biotite. This is common in Porphyry copper deposit, porphyry copper and porphyry molybdenum deposits. *''Argillic alteration'', commonly present in the distal areas of porphyry deposits, is a low-temperature assemblage that converts feldspars and some other minerals into clay minerals such as kaolinite and illite. It can overprint older, higher-temperature alteration assemblages. 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, manto ore deposits and pervasive zones of silicification, typically in Dolomite (rock), dolomite strata. Stressed minerals and country rocks of granitic plutons are replaced by porphyroblasts of orthoclase and quartz, in the Papoose Flat quartz monzonites.Dickson, F. W., 1996, Porphyroblasts of barium-zoned K-feldspar and quartz, Papoose Flat California, genesis and exploration implications. In Coyner,A.R., Fahey, P.I., eds. Geology and Ore Deposits of the American Cordillera: Geological Society of Nevada Symposium Proceedings, Reno/Sparks, Nevada, April 1995, p. 909-924. Dickson, F. W., 2000, Chemical emplacement of magma, v. 30, p.475-487. Dickson, F. W., 2005, Role of liquids in irreversible processes in earth and replacement in Papoose Flat pluton, California. In Rhoden, R. H., Steininger, R. C., and Vikre, R.G., eds: Geol. Soc. Nevada Symposium 2005: Window to the World, Reno, Nevada May, 2005, p. 161-178.


* Luth, R. W., ''Mantle volatiles -- distribution and consequences.'' In ''The Mantle and Core'' (ed. R. W. Carlson) Volume 2 Treatise on Geochemistry (editors H. D. Holland and K. K. Turekian), Elsevier-Pergamon, Oxford, pages 319-361 (2003).

See also

* * * * * * {{Authority control Metamorphic petrology Geological processes