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Skarn

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Skarn is coarse-grained metamorphic rocks that forms by a metasomatism. Also called tactites. Skarn tend to be rich in calcium-magnesium-iron-manganese-aluminium silicate minerals that also regerred to calc-silicate minerals. Skarn, in geology, metamorphic zone developed in the contact area around igneous rock intrusions when carbonate sedimentary rocks are invaded by large amounts of silicon, aluminum, iron, and magnesium. Many skarns also include ore minerals; several productive deposits of copper or other base metals have been found in and adjacent to skarns. Granitic and dioritic magmas are most commonly associated with skarns. Skarns can form by regional, or contact metamorphism and therefore form in relatively high temperature environments. The hydrothermal fluids associated with the metasomatic processes can originate from either magmatic, metamorphic, meteoric, marine, or even a mix of these.The resulting skarn may consist of a variety of different minerals which are highly dependent on the original composition of both the hydrothermal fluid and the original composition of the protolith.

Name origin: Skarn names came from old Swedish mining term is silicate gangue, or waste rock, associated with iron-ore bearing sulfide deposits

Colour: Black, Brown, Colourless, Green, Grey, White

Grain size: fine or course grains rock

Group: Metamorphic rock

Texture: They are fine, medium or coarse grained.

Alterations: Hydrothermal alteration

Minerals: Mostly garnets and pyroxene with a wide variety of calc-silicate and associated minerals. Skarn minerals is include pyroxene, garnet, idocrase, wollastonite, actinolite, magnetite or hematite,

Economically Important Minerals: Tungsten, manganese, gold, copper, zinc, nickel, lead, molybdenum and iron

Classification of Skarn

Skarns can be divided into sub-sections according to certain criteria:

Skarn can be classified according to its Protolith. If sakarn protolith is sedimentary origin, it can be reffered to as an exoskarn. If the protolith is ignouse rock, it can be called an endoskarn.

Skarn also classification can be made based on the protolith by observing the skarns dominant composition and the resulting alteration assemblage. If the skarn has a Olivine, Serpentine, Phlogopite, magnesium Clinopyroxene, Orthopyroxene, Spinel, Pargasite, and minerals s from the Humite group, are characteristic of a dolomitic protolith and can be classed as a magnesian skarn.

Calcic skarns are  replacement products of a limestone protolith with dominant mineral assemblages containing Garnet, Clinopyroxene, and Wollastonite.

Skarn deposits have typical skarn Gangue minerals but also contain ore minerals in abundance which are of economic importance. Skarn deposits are therefore classified by their dominant economic element, such as copper (Cu) skarn deposit, or molybdenum (Mo) skarn deposit to name a few.

Fe (Cu, Ag, Au) skarn deposits

The tectonic setting for calcic Fe skarns tends to be the oceanic island arcs. The host rocks tend to be gabbros to syenite associated with intruding limestone. The tectonic setting for magnesium Fe skarns tends to be the continental margin. The host rocks tend to be granodiorite to granite associated with intruding dolomite and dolomitic sedimentary rocks. Magnetite is the principal ore in these types of skarn deposits which its grade yields from 40 to 60 %. Chalcopyrite, bornite and pyrite are the minor ores.

Cu (Au, Ag, Mo, W) skarn deposits

The tectonic setting for Cu deposits tends to be the Andean-type plutons intruding older continental-margin carbonate layers. The host rocks tend to be quartz diorite and granodiorite. Pyrite, chalcopyrite and magnetite are typically found in higher abundances.

Types of Skarn Deposits

A descriptive skarn classification can be based on the dominant economic minerals.

1.   Iron Skarns

The largest skarn deposits, with many over 500 milliion tonnes. They are mined for their magnetite. Minor amounts of Ni, Cu, Co and Au may be present, but typically only Fe is recovered. They are dominantly magnetite, with only minor silicate gangue.

2.   Gold Skarns

Most gold skarns are associated with relatively mafic diorite – granodiorite plutons and dyke/sill complexes. Some large Fe or Cu skarns have Au in the distal zones. There is the potential that other skarn types have undiscovered precious metals if the entire system has not been explored.

3.   Tungsten Skarns

These are found in association with calc-alkaline plutons in major orogenic belts. They are associated with coarse grained, equigranular batholiths (with pegmatite and aplite dykes), surrounded by high temperature metamorphic aureoles. This is indicative of a deep environment.

4.   Copper Skarns

These are the world’s most abundant type and are particularly common in orogenic zones related to subduction both in continental and oceanic settings. Most are associated with porphyritic plutons with co-genetic volcanic rocks, stockwork veining, brittle fracturing, brecciation and intense hydrothermal aleteration. These features are all indicative of a relatively shallow environment. The largest copper skarns can exceed 1 billion tonnes and are associated with porphyry copper deposits.

5.   Zinc Skarns

Most occur in continental settings associated either with subduction or rifting. They are also mined for lead and silver, and are high grade. They form in the distal zone to associated igneous rocks.

6.   Molybdenum Skarns

Most are associated with leucocratic (lacking ferromagnesian minerals) granites and form high graade, small deposits. other metals are also commonly associated, the most common being Mo-W-Cu skarns.

7.   Tin Skarns

These are almost exclusively associated with high silica granites generated by partial melting of continental crust. Greisen alteration by fluorine produces a characteristic yellowish mica.

Composition of Skarn

Skarn is composed of of calcium-iron-magnesium-manganese-aluminum silicate minerals. Skarn deposits are economically important valuable sources, which metals such as tungsten, manganese, gold, copper, zinc, nickel, lead, molybdenum and iron.

A skarn is formed by consists of various metasomatic processes during the metamorphism between two adjacent lithological units. Skarn can form in almost any lithology type such as shale, granite and basalt but the majority of skarns are found in lithology containing a limestone or a dolomite. It is common to find skarns near plutons, along faults and major shear zones, in shallow geothermal systems, and on the bottom of the sea floor. The mineralogy of skarn rock usually highly related to the protolith.

Skarn minerals are mostly garnets and pyroxene with a wide variety of calc-silicate and associated minerals. Skarn minerals is include pyroxene, garnet, idocrase, wollastonite, actinolite, magnetite or hematite, epidote and scapolite. 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.

Skarn Formation

In general, there are two main tyoes of skarns that are exoskarns and endoskarns.

Exoskarns are more common than endoskarns. Exoskarns form  on the except intrusive body than comes into contact with carbonate units.They are formed when fluids left over from the crystallisation of the intrusion are ejected from the mass at the waning stages of emplacement. When these fluids come into contact with reactice rocks, generally carbonates rock such as limestone or dolomite, the fluids reacting them, so producing alteration. Also other name of this production infiltration metasomatism

Endoskarns forming within the intursive body where fracturing, cooling joints, and stockworks have been produced, which results in a permeable area. The permeable area can incorporate from the carbonate layer. The magmatic hydrothermal fluids that  were transported or created by the intrusion interacts with the carbonate material and forms the endoskarn. Endoskarns are considered to be rare. Both the composition and the textures of protolith strongly play a role in the formation of the resulting skarn

On the other hand reaction skarn is formed from isochemşcak metamorphism occuring on thinly interlayered sedimentary lithology units that involves a small scale (perhaps centimetres) metasomatic transfer of components between adjacent units.

Skarnoid is a calc-silicate rock than is fine-grained rock but iron poor. It founded lies between a hornfels and a coarse-grained skarns. Generally the skarnoid tends to reflect the composition of the protolithic rock.

Skarn deposit mostly experience a transition from  early metamorphism which forms hornfels, reaction skarns, and skarnoids to late metamorphism which forms relatively coarser grained, ore-bearing skarns. The magma intrusion triggers contact metamorphism in the region where sedimentary rocks are present and form as a result.The recrystallization and phase change of a hornfel reflect the composition of the protolith. After the formation of a hornfel, a process called metasomatism occurs which involves hydrothermal fluids associated with magmatic, metamorphic, marine, meteoric or even a mix of these. This process is called isochemical metamorphism and can result in the production of a wide range of calc-silicate minerals that form in impure lithology units and along fluid boundaries where small-scale metasomatism occurs (argillite and limestone, and banded iron formation).

An antiskarn is a calc-silicate rock that formed by direct metasomatism of silicate rocks by a carbonatite magma.These rocks are characterised by a high-temperature assemblage of diopsidic clinopyroxene, olivine and wollastonite, and an apparent lack of igneous calcite.

Skarn Uses

A variety gemstones have been founded skarn deposits, with garnet, ruby, and sapphire being common occurrences in skarn.

Creating Artwork, Gemstone, Jewelry, Metallurgical Flux, Source of Magnesia (MgO)

As a Flux in the Production of Steel and Pig Iron, As a Sintering Agent in Steel Industry to process Iron Ore, As Dimension Stone, Gold and Silver production, Manufacture of Magnesium and Dolomite Refractories

As Building Stone, As Facing Stone, Garden Decoration, Paving Stone

Other Important Information About Skarn

Rocks that contain garnet or pyroxene as major phases, are fine-grained, lack iron, and have skarn-like appearances, are generally given the term skarnoid. Skarnoid therefore is the intermediate stage of a fine-grained Hornfel and a coarse-grained skarn.

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. Here, fluids react less via chemical exchange of ions, but because of the redox-oxidation potential of the wall rocks

References

  • Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
  • Wikipedia contributors. (2019, April 29). Skarn. In Wikipedia, The Free Encyclopedia. Retrieved 22:48, May 7, 2019, from https://en.wikipedia.org/w/index.php?title=Skarn&oldid=894634204
Cite this article as: Geology Science. (2019). Skarn. [online] Available at: http://geologyscience.com/rocks/skarn/ [8th December 2019 ]
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