Skarn is a metamorphic rock Metamorphic rock is the result of the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change. The protolith may be sedimentary rock, igneous rock or another older metamorphic that is usually variably colored green or red, occasionally grey, black, brown or white.
It usually forms by chemical metasomatism In the igneous environment, metasomatism creates skarns, greisen, and may affect hornfels in the contact metamorphic aureole adjacent to an intrusive rock mass of rocks during metamorphism Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Both mineralogical, chemical and crystallographic changes can occur during this process and in the contact zone of magmatic Magma is molten rock that is found beneath the surface of the Earth, and may also exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and gas bubbles. Magma often collects in a magma chamber inside a volcano. Magma is capable of intrusion into adjacent rocks, extrusion onto the surface as lava, and intrusions like granites Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite has a medium to coarse texture, occasionally with some individual crystals larger than the groundmass forming a rock known as porphyry. Granites can be pink to dark gray or even black, depending on their chemistry and mineralogy. Outcrops of granite tend to with carbonate In chemistry, a carbonate is a salt or ester of carbonic acid-rich rocks In geology, rock is a naturally occurring solid aggregate of minerals and/or mineraloids such as limestone Limestone is a sedimentary rock composed largely of the mineral calcite . The deposition of limestone strata is often a by-product and indicator of biological activity in the geologic record. Calcium (along with nitrogen, phosphorus, and potassium) is a key mineral to plant nutrition: soils overlying limestone bedrock tend to be pre-fertilized or dolostone Dolostone or dolomite rock is a sedimentary carbonate rock that contains a high percentage of the mineral dolomite. In old U.S.G.S. publications it was referred to as magnesian limestone. Most dolostone formed as a magnesium replacement of limestone or lime mud prior to lithification. It is resistant to erosion and can either contain bedded layers. Skarns in the igneous environment are associated with hornfels Hornfels is the group designation for a series of contact metamorphic rocks that have been baked and indurated by the heat of intrusive igneous masses and have been rendered massive, hard, splintery, and in some cases exceedingly tough and durable. Most hornfels are fine-grained, and while the original rocks (such as sandstone, shale and slate,, marble Marble is a nonfoliated metamorphic rock resulting from the metamorphism of limestone, composed mostly of calcite (a crystalline form of calcium carbonate, Ca hornfels and wider zones of calc-silicate rocks.
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Petrology and types
Skarns are in their broadest sense formed by mass and chemical transport and reactions A metamorphic reaction is a chemical reaction that takes place during the geological process of metamorphism in an amalgamate of minerals that helps determine the final stable state of the resulting metamorphic rock between adjacent lithologies. They need not be igneous Igneous rock is one of the three main rock types (the others being sedimentary and metamorphic rock). Igneous rock is formed by magma (molten rock) being cooled and becoming solid. They may form with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. This magma can be in origin; two adjacent sedimentary Sedimentary rock is one of the three main rock types . Sedimentary rock is formed by deposition and consolidation of mineral and organic material and from precipitation of minerals from solution. The processes that form sedimentary rock occur at the surface of the Earth and within bodies of water. Rock formed from sediments covers 75-80% of the layers such as a banded iron formation Banded iron formations are a distinctive type of rock often found in primordial (Precambrian) sedimentary rocks. The structures consist of repeated thin layers of iron oxides, either magnetite (Fe3O4) or hematite (Fe2O3), alternating with bands of iron-poor shale and chert. Some of the oldest known rock formations, formed around three thousand and a limestone Limestone is a sedimentary rock composed largely of the mineral calcite . The deposition of limestone strata is often a by-product and indicator of biological activity in the geologic record. Calcium (along with nitrogen, phosphorus, and potassium) is a key mineral to plant nutrition: soils overlying limestone bedrock tend to be pre-fertilized may react to exchange metals and fluids during metamorphism, creating a skarn.
However, the widest use of the word is in describing the metasomatised zones of wall rock adjacent to granites Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite has a medium to coarse texture, occasionally with some individual crystals larger than the groundmass forming a rock known as porphyry. Granites can be pink to dark gray or even black, depending on their chemistry and mineralogy. Outcrops of granite tend to. 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 Hornfels is the group designation for a series of contact metamorphic rocks that have been baked and indurated by the heat of intrusive igneous masses and have been rendered massive, hard, splintery, and in some cases exceedingly tough and durable. Most hornfels are fine-grained, and while the original rocks (such as sandstone, shale and slate,, usually by field relationships.
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 Greisen is a highly altered granitic rock or pegmatite. Greisen is formed by autogenic alteration of a granite and is a class of endoskarn form within the granite mass itself, usually late in the intrusive emplacement and consist of cross-cutting stockworks In geology, a stockwork is a complex system of structurally controlled or randomly oriented veins. Stockworks are common in many ore deposit types and especially notable in greisens. They are also referred to as stringer zones, cooling joints and around the margins and uppermost sections of the granite itself.
Typical skarn minerals A mineral is a naturally occurring solid formed through geological processes that has a characteristic chemical composition, a highly ordered atomic structure, and specific physical properties. A rock, by comparison, is an aggregate of minerals and/or mineraloids, and need not have a specific chemical composition. Minerals range in composition include pyroxene The pyroxenes are a group of important rock-forming silicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems. Pyroxenes have the general formula XY2O6 (where X represents calcium, sodium, iron+2 and, garnet The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. The name "garnet" may come from either the Middle English word gernet meaning 'dark red', or the Latin granatus , possibly a reference to the Punica granatum ("pomegranate"), a plant with red seeds similar in shape,, idocrase Vesuvianite, also known as idocrase is a green, brown, yellow, or blue silicate mineral. Vesuvianite occurs as tetragonal crystals in skarn deposits and limestones that have been subjected to contact metamorphism. It was first discovered within included blocks or adjacent to lavas on Mount Vesuvius, hence its name, wollastonite Wollastonite is a calcium inosilicate mineral (Ca , actinolite Actinolite is an amphibole silicate mineral with the chemical formula Ca25Si8O22(OH)2, magnetite Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part wüstite (FeO) and one part hematite (Fe2O3). This or hematite Hematite, also spelled as hæmatite, is the mineral form of Iron oxide (Fe2O3), one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and as corundum. Hematite and ilmenite form a complete solid solution at temperatures above 950°C, and epidote Epidote is a calcium aluminium iron sorosilicate mineral, Ca2Al2(SiO4)(Si2O7)O(OH), crystallizing in the monoclinic system. Well-developed crystals are of frequent occurrence: they are commonly prismatic in habit, the direction of elongation being perpendicular to the single plane of symmetry. The faces are often deeply striated and crystals are. 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 Tourmaline is a crystal silicate mineral compounded with elements such as aluminium, iron, magnesium, sodium, lithium, or potassium. Tourmaline is classed as a semi-precious stone and the gem comes in a wide variety of colors. The name comes from the Sinhalese word "turamali" or "toramalli", which applied to different gemstones, topaz Topaz is a silicate mineral of aluminium and fluorine with the chemical formula Al2Si , beryl The mineral beryl is a beryllium aluminium cyclosilicate with the chemical formula Be3Al26. The hexagonal crystals of beryl may be very small or range to several meters in size. Terminated crystals are relatively rare. Pure beryl is colorless, but it is frequently tinted by impurities; possible colors are green, blue, yellow, red, and white. The, corundum Corundum is a crystalline form of aluminium oxide and is one of the rock-forming minerals. It is one of the naturally clear transparent materials, but can have different colors when impurities are present. Transparent specimens are used as gems, called ruby if red, while all other colors are called sapphire, fluorite Fluorite is a mineral composed of calcium fluoride, Ca , apatite Apatite is a group of phosphate minerals, usually referring to hydroxyapatite, fluorapatite, chlorapatite and bromapatite, named for high concentrations of OH−, F−, Cl− or Br−ions, respectively, in the crystal. The formula of the admixture of the four most common endmembers is written as Ca106(OH, F, Cl, Br)2, and the crystal unit cell, barite The radiating form, sometimes referred to as Bologna Stone, attained some notoriety among alchemists for the phosphorescent specimens found in the 1600s near Bologna by Vincenzo Cascariolo, strontianite Categories: Lochaber | Strontium minerals | Carbonate minerals | Aragonite group | , tantalite Tantalite, [(Ta,Nb)2O6], is a mineral that is close to columbite. In fact, the two are often grouped together as a semi-singular mineral called columbite-tantalite in many mineral guides. However, tantalite has a much greater specific gravity than columbite (8.0+ compared to columbite's 5.2). Iron-rich tantalite is the mineral ferrotantalite and, anglesite Anglesite is a lead sulfate mineral, PbSO4. It occurs as an oxidation product of primary lead sulfide ore, galena. Anglesite occurs as prismatic orthorhombic crystals and earthy masses, and is isomorphous with barite and celestine. It has a high specific gravity of 6.3 due to its lead content, 74% by mass; its hardness is 2.5 - 3. Color is white,, and others. Often, feldspathoids The feldspathoids are a group of tectosilicate minerals which resemble feldspars but have a different structure and much lower silica content. They occur in rare and unusual types of igneous rocks and rare calc-silicates such as scapolite are found in more marginal areas.
Formation
Skarns are a class of calc-silicate rocks and are intimately associated with granite Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite has a medium to coarse texture, occasionally with some individual crystals larger than the groundmass forming a rock known as porphyry. Granites can be pink to dark gray or even black, depending on their chemistry and mineralogy. Outcrops of granite tend to intrusions, usually of sedimentary-metamorphic origin (S-type). Skarns are rarely seen with other types of granites, because of the fluid chemistry and crystallization behaviour of M-type (mantle The mantle is a part of an astronomical object. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core. Earth's mantle is an about 2,970 km thick rocky shell that constitutes about 84 percent of Earth's volume. It is origin) and I-type (igneous-metamorphic origin) granites. S-type granites are more prone to generating late-stage fluid rich in silica The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with a chemical formula of Si , incompatible elements and halides 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 electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, or astatide compound. Many salts are halides. All Group 1 metals form halides with the halogens and they are white solids because they are generally more potassic Potassium is a chemical element. It has the symbol K (Latin: kalium, from Arabic: القَلْيَه al-qalyah “plant ashes”, cf. Alkali from the same root), atomic number 19, and atomic mass 39.0983. Potassium was first isolated from potash. Elemental potassium is a soft silvery-white metallic alkali metal that oxidizes rapidly in air and, oxidised and hydrous.
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 Limestone is a sedimentary rock composed largely of the mineral calcite . The deposition of limestone strata is often a by-product and indicator of biological activity in the geologic record. Calcium (along with nitrogen, phosphorus, and potassium) is a key mineral to plant nutrition: soils overlying limestone bedrock tend to be pre-fertilized or dolostone Dolostone or dolomite rock is a sedimentary carbonate rock that contains a high percentage of the mineral dolomite. In old U.S.G.S. publications it was referred to as magnesian limestone. Most dolostone formed as a magnesium replacement of limestone or lime mud prior to lithification. It is resistant to erosion and can either contain bedded layers, the fluids react with them, producing alteration (metasomatism).
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 The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Greek γραφειν : "to draw/write", for its use in pencils, where it is commonly called lead, as distinguished from the actual metallic element lead. Unlike diamond (another carbon allotrope), graphite is an electrical shales, banded iron formations Banded iron formations are a distinctive type of rock often found in primordial (Precambrian) sedimentary rocks. The structures consist of repeated thin layers of iron oxides, either magnetite (Fe3O4) or hematite (Fe2O3), alternating with bands of iron-poor shale and chert. Some of the oldest known rock formations, formed around three thousand and, occasionally, salt Salt is a dietary mineral composed primarily of sodium chloride that is essential for animal life, but toxic to most land plants. Salt flavor is one of the basic tastes, an important preservative and a popular food seasoning or evaporites. Here, fluids react less via chemical exchange of ions, but because of the redox-oxidation potential of the wall rocks.
Endoskarns are rarer, generally because the fluids created by a granite are usually formed in equilibrium with the minerals of the granite. 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. Examples of economic skarn deposits include:
- Pine Creek Mine (tungsten), Inyo County, California, USA
- Ok Tedi Mine (gold and copper), Western Province (Papua New Guinea)
- Avebury Mine (Nickel), Zeehan, Tasmania, (Australia)
See also
References
Einaudi M.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
Categories: Economic geology | Metamorphic rocks
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