Skarn is a metamorphic rock that is usually variably colored green or red, occasionally grey, black, brown or white. Metamorphic rock is the result of the transformation of an existing rock type the protolith, in a process called Metamorphism, which means "change
It usually forms by chemical metasomatism of rocks during metamorphism and in the contact zone of magmatic intrusions like granites with carbonate-rich rocks such as limestone or dolostone. Metasomatism is the chemical alteration of a rock by Hydrothermal and other fluids Metamorphism can be defined as the solid state recrystallisation of pre-existing rocks due to changes in heat and/or pressure and/or introduction of fluids i Magma (Plurals magmas and magmata) is molten rock that sometimes forms beneath the surface of the Earth (or any other Terrestrial planet Granite (ˈɡrænɪt is a common and widely occurring type of intrusive, Felsic, igneous rock. In Chemistry, a carbonate is a salt or Ester of Carbonic acid. In Geology, rock is a naturally occurring aggregate of Minerals and/or Mineraloids The Earth's outer solid layer the ‘ Lithosphere Limestone is a Sedimentary rock composed largely of the Mineral Calcite ( Calcium carbonate: CaCO3 Dolostone is a sedimentary Carbonate rock that contains a high percentage of the Mineral Dolomite.
Skarns in the igneous environment are associated with hornfels, marble hornfels and wider zones of calc-silicate rocks. Hornfels ( German, meaning "hornstone" is the group designation for a series of contact metamorphic rocks that have been baked and indurated by the heat Marble is a nonfoliated Metamorphic rock resulting from the Metamorphism of Limestone, composed mostly of Calcite (a crystalline form of
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Petrology and types
Skarns are in their broadest sense formed by mass and chemical transport and reactions between adjacent lithologies. A metamorphic reaction is a Chemical reaction that takes place during the Geological process of Metamorphism in an amalgamate of Minerals that They need not be igneous in origin; two adjacent sedimentary layers such as a banded iron formation and a limestone may react to exchange metals and fluids during metamorphism, creating a skarn. Igneous rocks (etymology from Latin ignis, fire are rocks formed by solidification of cooled Magma (molten rock Sedimentary rock is one of the three main rock types (the others being igneous and Metamorphic rock) Banded iron formations (also known as banded ironstone formations or BIF s are a distinctive type of rock often found in primordial ( Precambrian) Sedimentary Limestone is a Sedimentary rock composed largely of the Mineral Calcite ( Calcium carbonate: CaCO3
However, the widest use of the word is in describing the metasomatised zones of wall rock adjacent to granites. Granite (ˈɡrænɪt is a common and widely occurring type of intrusive, Felsic, igneous rock. Skarns which are created by reaction between metamorphic-sedimentary layers are also known as chemical skarns or skarnoids. Skarns must also be distinguished from calc-silicate hornfels, usually by field relationships. Hornfels ( German, meaning "hornstone" is the group designation for a series of contact metamorphic rocks that have been baked and indurated by the heat
Skarns of igneous origin are classified as exoskarns or endoskarns. Exoskarns occur at and outside the granite which produced them, and are alterations of wall rocks. Endoskarns, including greisens form within the granite mass itself, usually late in the intrusive emplacement and consist of cross-cutting stockworks, cooling joints and around the margins and uppermost sections of the granite itself. Greisen is a highly altered granitic rock or Pegmatite. Greisen is formed by autogenic alteration of a granite and is a class of endoskarn. In Geology, a stockwork is a complex system of structurally controlled or randomly oriented veins.
Typical skarn minerals include pyroxene, garnet, idocrase, wollastonite, actinolite, magnetite or hematite, and epidote. A mineral is a naturally occurring substance formed through geological processes that has a characteristic chemical composition a highly ordered atomic structure and specific The pyroxenes are a group of important rock-forming Silicate minerals found in many Igneous and metamorphic rocks. The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives Vesuvianite, also known as idocrase is a green brown yellow or blue silicate mineral. For the community in Canada see Actinolite Ontario. For the Sailor Moon character Magnetite is not to be confused with Magnesite or Maghemite. Magnetite is a ferrimagnetic Mineral with chemical Hematite, also spelt hæmatite, is the Mineral form of Iron(III oxide (Fe2O3 one of several Iron oxides Epidote is a Calcium Aluminium Iron sorosilicate Mineral, Ca2Al2(Fe3+Al(SiO4(Si2O7O(OH Because skarns are formed from incompatible-element rich, siliceous aqueous fluids a variety of uncommon mineral types are found in the skarn environment, such as: tourmaline, topaz, beryl, corundum, fluorite, apatite, barite, strontianite, tantalite, anglesite, and others. Tourmaline is a Crystal Silicate mineral compounded with elements such as Aluminium, Iron, Magnesium, Sodium, Lithium Topaz is a Silicate mineral of Aluminium and Fluorine with the Chemical formula Al 2 Si[[oxygen O]]4( The Mineral beryl is a Beryllium Aluminium cyclosilicate with the Chemical formula Be3Al2(SiO36 Corundum (from Tamil kurundam குருந்தம் or kuruvindam குருவிந்தம் is a Crystalline form of Fluorite (also called fluorspar) is a Mineral composed of Calcium fluoride, Ca[[Fluorine F2]] Baryte ( Ba[[Sulfur S]] O 4 is a Mineral consisting of Barium sulfate. Strontianite ( Sr[[Carbon C]] O 3 is a Mineral consisting of Strontium carbonate, named after the village of Strontian, Tantalite, ] Mn) Ta 2 O 6] is a Mineral that is close to Columbite. Anglesite is a Lead Sulfate Mineral, PbSO4 It occurs as an oxidation product of primary lead sulfide ore Galena. Often, feldspathoids and rare calc-silicates such as scapolite are found in more marginal areas. The feldspathoids are a group of tectosilicate Minerals which resemble Feldspars but have a different structure and much lower Silica content Scapolite (Gr asairos, rod stone is a group of rock-forming Silicate minerals composed of Aluminium, Calcium and Sodium Silicate
Formation
Skarns are a class of calc-silicate rocks and are intimately associated with granite intrusions, usually of sedimentary-metamorphic origin (S-type). Granite (ˈɡrænɪt is a common and widely occurring type of intrusive, Felsic, igneous rock. Skarns are rarely seen with other types of granites, because of the fluid chemistry and crystallization behaviour of M-type (mantle origin) and I-type (igneous-metamorphic origin) granites. The mantle is a part of an Astronomical object. The interior of the Earth, similar to the other Terrestrial planets, is Chemically divided S-type granites are more prone to generating late-stage fluid rich in silica, incompatible elements and halides because they are generally more potassic, oxidised and hydrous. The Chemical compound silicon dioxide, also known as silica or silox (from the Latin " Silex " is an Oxide A halide is a Binary compound, of which one part is a Halogen Atom and the other part is an element or radical that is less Potassium (pəˈtæsiəm is a Chemical element. It has the symbol K (kalium from qalīy Atomic number 19 and Atomic mass 39
Exoskarns are formed when fluids left over from the crystallisation of the granite are ejected from the mass at the waning stages of emplacement. When these fluids come into contact with reactive rocks, usually carbonates such as limestone or dolostone, the fluids react with them, producing alteration (metasomatism). Limestone is a Sedimentary rock composed largely of the Mineral Calcite ( Calcium carbonate: CaCO3 Dolostone is a sedimentary Carbonate rock that contains a high percentage of the Mineral Dolomite.
Because these fluids carry dissolved silica, iron, metals, halides and sulfur, the resulting rock is usually a highly complex combination of calcium, magnesium and carbonate rich minerals.
Uncommon types of skarns are formed in contact with sulfidic or carbonaceous rocks such as black shales, graphite shales, banded iron formations and, occasionally, salt or evaporites. Shale (also called mudstone) is a fine-grained Sedimentary rock whose original constituents were Clay minerals or Muds It is characterized by The Mineral graphite, as with Diamond and Fullerene, is one of the Allotropes of carbon. Banded iron formations (also known as banded ironstone formations or BIF s are a distinctive type of rock often found in primordial ( Precambrian) Sedimentary Salt is a Dietary mineral composed primarily of Sodium chloride that is essential for Animal life but toxic to most land plants Evaporites (iˈvæpəraɪt are water-soluble Mineral sediments that result from the Evaporation of bodies of surficial Water. Here, fluids react less via chemical exchange of ions, but because of the redox-oxidation potential of the wall rocks. Redox (shorthand for reduction-oxidation reaction describes all Chemical reactions in which atoms have their Oxidation number ( Oxidation state
Endoskarns are rarer, generally because the fluids created by a granite are usually formed in equilibrium with the minerals of the granite. In a Chemical process, chemical equilibrium is the state in which the chemical activities or Concentrations of the reactants and products have no net change Endoskarns seem to form in granites which lose earlier, more dilute hydrous fluids, thereby creating a less dilute last spurt of exsolved fluids. Boiling of the exsolved fluid is also considered important, as this creates a highly saline, incompatible-element-rich fluid phase and a highly volatile gas phase.
Ore deposits
Skarns are often hosts for copper, lead, zinc, iron, gold, molybdenum, tin, and tungsten ore deposits. Copper (ˈkɒpɚ is a Chemical element with the symbol Cu (cuprum and Atomic number 29 Characteristics Lead has a dull luster and is a dense, Ductile, very soft highly Zinc (ˈzɪŋk from Zink is a Metallic Chemical element with the symbol Zn and Atomic number 30 Iron (ˈаɪɚn is a Chemical element with the symbol Fe (ferrum and Atomic number 26 Gold (ˈɡoʊld is a Chemical element with the symbol Au (from its Latin name aurum) and Atomic number 79 Molybdenum (məˈlɪbdənəm from the Greek word for the metal " Lead " is a Group 6 Chemical element with the symbol Mo Tin is a Chemical element with the symbol Sn (stannum and Atomic number 50 Tungsten (ˈtʌŋstən also known as wolfram (/ˈwʊlfrəm/ is a Chemical element that has the symbol W and Atomic number 74 An ore is a volume of rock containing components or Minerals in a mode of occurrence that renders it valuable for mining Examples of economic skarn deposits include:
- Pine Creek mine (tungsten), Inyo County, California, USA
See also
References
Einaudi M. Inyo County is located in east-central California in the southwestern United States, on the east side of the Sierra Nevada Mountains and south of Yosemite The various theories of ore genesis explain how the various types of mineral deposits form within the Earth's crust. Metamorphism can be defined as the solid state recrystallisation of pre-existing rocks due to changes in heat and/or pressure and/or introduction of fluids i Metasomatism is the chemical alteration of a rock by Hydrothermal and other fluids Granite (ˈɡrænɪt is a common and widely occurring type of intrusive, Felsic, igneous rock. T. , & Burt D. M. , 1982. Introduction, terminology, classification and composition of skarn deposits. Economic Geology, 77, pp. 745-754.
Evans, A. M. , 1993. Ore Geology and Industrial Minerals: An Introduction 3rd ed. Blackwell Publishing ISBN 0632029536
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