View source for Skarn

{{short description|Hard, coarse-grained, hydrothermally altered metamorphic rocks}}
[[File:Skarn2.jpg|thumb|Microscopic view of skarn under crossed polarizers]]
[[File:Skarn Alta Stock.jpg|thumb|Hand sample of skarn containing [[serpentinite]] from the edge of the Alta Stock, [[Little Cottonwood Canyon]], Utah]]
'''Skarns''' or '''tactites''' are coarse-grained [[Metamorphic rock|metamorphic rocks]] that form by replacement of carbonate-bearing rocks during regional or contact [[metamorphism]] and [[metasomatism]]. Skarns may form by metamorphic recrystallization of impure carbonate protoliths, bimetasomatic reaction of different lithologies, and infiltration metasomatism by magmatic-[[Hydrothermal fluid|hydrothermal fluids]].<ref name=":6">{{Citation |last1=Einaudi |first1=M. T. |title=Skarn Deposits |date=1981 |url=https://doi.org/10.5382/AV75.11 |work=Seventy-Fifth Anniversary Volume |access-date=2023-07-14 |publisher=Society of Economic Geologists |doi=10.5382/av75.11 |last2=Meinert |first2=L. D. |last3=Newberry |first3=R. J.|isbn=978-1-9349-6953-3 |url-access=subscription }}</ref> Skarns tend to be rich in [[calcium]]-[[magnesium]]-[[iron]]-[[manganese]]-[[aluminium]] [[Silicate mineral|silicate minerals]], which are also referred to as [[Calc–silicate rock|calc-silicate minerals]].<ref name=":0">{{Cite journal|last1=Einaudi|first1=Marco T.|last2=Burt|first2=Donald M.|title=Introduction; terminology, classification, and composition of skarn deposits|journal=Economic Geology|volume=77|issue=4|pages=745–754|doi=10.2113/gsecongeo.77.4.745|year=1982|bibcode=1982EcGeo..77..745E }}</ref><ref name=":1">Ray, G.E., and Webster, I.C.L. (1991): An Overview of Skarn Deposits; in Ore Deposits, Tectonics and Metallogeny in the Canadian Cordillera; McMillan, W.J., compiler, B. C. Ministry of Energy, Mines and Petroleum Resources, Paper 1991-4, pages 213-252.</ref><ref name=":2">Meinert, L.D., 1992. Skarns and Skarn Deposits; Geoscience Canada, Vol. 19, No. 4, p. 145-162.</ref><ref name=":3">Hammarstrom, J.M., Kotlyar, B.B., Theodore, T.G., Elliott, J.E., John, D.A., Doebrich, J.L., Nash, J.T., Carlson, R.R., Lee, G.K., Livo, K.E., Klein, D.P., 1995. Cu, Au, and Zn-Pb Skarn Deposits, Chapter 12; United States Geological Survey: Preliminary Compilation of Descriptive Geoenvironmental Mineral Deposit Models: https://pubs.usgs.gov/of/1995/ofr-95-0831/CHAP12.pdf.</ref> These minerals form as a result of alteration which occurs when hydrothermal fluids interact with a [[protolith]] of either [[Igneous rock|igneous]] or [[Sedimentary rock|sedimentary]] origin. In many cases, skarns are associated with the intrusion of a [[Granitoid|granitic]] [[pluton]] found in and around [[Fault (geology)|faults]] or [[Shear zone|shear zones]] that commonly intrude into a [[Carbonate rock|carbonate]] layer composed of either [[Dolomite (rock)|dolomite]] or [[limestone]]. Skarns can form by [[Regional metamorphism|regional]] or [[Contact metamorphic|contact]] metamorphism and therefore form in relatively high temperature environments.<ref name=":0" /><ref name=":1" /><ref name=":2" /><ref name=":3" /> The hydrothermal fluids associated with the metasomatic processes can originate from a variety of sources; [[Magmatic water|magmatic]], metamorphic, [[Meteoric water|meteoric]], [[Marine water|marine]], or even a mix of these.<ref name=":2" /> The resulting skarn may consist of a variety of different minerals which are highly dependent on both the original composition of the hydrothermal fluid and the original composition of the protolith.<ref name=":2" />

If a skarn has a respectable amount of ore mineralization that can be mined for a profit, it can be classified as a '''skarn deposit'''.<ref name=":0" /><ref name=":1" /><ref name=":2" />

==Etymology==
''Skarn'' is an old Swedish mining term originally used to describe a type of silicate [[gangue]], or waste rock, associated with iron-ore bearing sulfide deposits apparently replacing [[Palaeoproterozoic]] age [[limestone]]s in Sweden's [[Persberg]] mining district.<ref>{{cite journal|last1=Burt|first1=Donald M.|year=1977|title=Mineralogy and petrology of skarn deposits|journal=Societa Italiana Mineralogia Petrolgia Rendiconti|volume=33|issue=2|url=https://rruff.info/rdsmi/V33/RDSMI33_859.pdf|pages=859–873}}</ref>

==Petrology==
Skarns are composed of calcium-iron-magnesium-manganese-aluminum silicate minerals. Skarn deposits are economically valuable as sources of metals such as [[tin]], [[tungsten]], [[manganese]], [[copper]], [[gold]], [[zinc]], [[lead]], [[nickel]], [[molybdenum]] and [[iron]].<ref name=":3" />

A skarn is formed by a variety of metasomatic processes during metamorphism between two adjacent lithologic units. Skarns can form in almost any rock type such as [[shale]], [[granite]], or [[basalt]] but the majority of skarns are found in carbonate rocks containing limestone or 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.<ref name=":2" /> The specific mineralogy of skarns are highly related to the mineralogy of the protolith.<ref>{{Cite journal|last1=Jolis|first1=E. M.|last2=Troll|first2=V. R.|last3=Harris|first3=C.|last4=Freda|first4=C.|last5=Gaeta|first5=M.|last6=Orsi|first6=G.|last7=Siebe|first7=C.|date=2015-11-15|title=Skarn xenolith record crustal CO2 liberation during Pompeii and Pollena eruptions, Vesuvius volcanic system, central Italy|url=http://www.sciencedirect.com/science/article/pii/S0009254115300255|journal=Chemical Geology|language=en|volume=415|pages=17–36|doi=10.1016/j.chemgeo.2015.09.003|bibcode=2015ChGeo.415...17J |issn=0009-2541|url-access=subscription}}</ref>

Skarn mineralogy is dominated by [[garnet]] and [[pyroxene]] with a wide variety of calc-silicate and associated minerals, including [[idocrase]], [[wollastonite]], [[actinolite]], [[magnetite]] or [[hematite]], [[epidote]] and [[scapolite]]. Because skarns are formed from silica-rich aqueous fluids replete with [[Incompatible element|incompatible elements]], a variety of uncommon mineral types are found in skarns, such as: [[tourmaline]], [[topaz]], [[beryl]], [[corundum]], [[fluorite]], [[apatite]], [[Baryte|barite]], [[strontianite]], [[tantalite]], [[anglesite]], and others.<ref>{{Cite web|url=http://www.geol-amu.org/notes/b3-3-4.htm|title=Hydrothermal and Skarn Deposits|website=www.geol-amu.org|access-date=2018-03-29}}</ref>

==Classification==
Skarns can be subdivided depending on specific criteria. One way to classify a skarn is by its [[protolith]]. If the protolith is of sedimentary origin, it can be referred to as an exoskarn and if the protolith is igneous, it can be called an endoskarn.<ref name=":1" /><ref name=":2" />

Further classification can be made based on the protolith by observing the skarn's dominant composition and the resulting alteration assemblage. If the skarn contains minerals such as [[olivine]], [[Serpentine subgroup|serpentine]], [[phlogopite]], magnesium [[clinopyroxene]], [[orthopyroxene]], [[spinel]], [[pargasite]], and minerals from the [[humite]] group, it is characteristic of a [[Dolomite (rock)|dolomitic]] protolith and can be classed as a magnesian skarn. The other class, called calcic skarns, are the replacement products of a [[limestone]] protolith with dominant mineral assemblages containing [[garnet]], clinopyroxene, and [[wollastonite]].<ref name=":1" />

Rocks that contain garnet or pyroxene as major phases, and that are also fine-grained, lack iron, and have skarn-like appearances, are generally given the term "skarnoid". Skarnoid  is therefore the intermediate stage of a fine-grained [[hornfels]] and a coarse-grained skarn.<ref name=":1" /><ref name=":2" />

=== Skarn ore deposits ===
Metal ore deposits that have skarn as [[gangue]] are called skarn deposits and can form by any combination of closed metamorphism or open system metasomatism, although most skarn deposits are thought to be related to magmatic-hydrothermal systems.<ref name=":6" /> Skarn deposits are classified by their dominant economic element, e.g.,  a copper (Cu) skarn deposit or a molybdenum (Mo) skarn deposit.<ref name=":0" /><ref name=":1" /><ref name=":3" />

===Fe (Cu, Ag, Au) skarn deposits===

The [[tectonic setting]] for calcic Fe skarns tends to be the [[Island arc|oceanic island arcs]]. The host rocks tend to range from [[gabbro]] to [[syenite]] associated with intruding limestone layers. The tectonic setting for magnesium Fe skarns tends to be the [[Passive margin|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]] constitute minor ores.<ref name=":4">{{Cite journal|last1=Nadoll|first1=Patrick|last2=Mauk|first2=Jeffrey L.|last3=Leveille|first3=Richard A.|last4=Koenig|first4=Alan E.|date=2015-04-01|title=Geochemistry of magnetite from porphyry Cu and skarn deposits in the southwestern United States|journal=Mineralium Deposita|language=en|volume=50|issue=4|pages=493–515|doi=10.1007/s00126-014-0539-y|bibcode=2015MinDe..50..493N|s2cid=128816207 |issn=0026-4598}}</ref><ref name=":5">{{Cite journal|last1=Soloviev|first1=Serguei G.|last2=Kryazhev|first2=Sergey|title=Geology, mineralization, and fluid inclusion characteristics of the Chorukh-Dairon W–Mo–Cu skarn deposit in the Middle Tien Shan, Northern Tajikistan|journal=Ore Geology Reviews|volume=80|pages=79–102|doi=10.1016/j.oregeorev.2016.06.021|year=2017|bibcode=2017OGRv...80...79S }}</ref>

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

The tectonic setting for Cu deposits tends to be the [[Continental arc|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.<ref name=":4" /><ref name=":5" />

==Formation==
Generally, there are two types of skarns that form, exoskarns and endoskarns.<ref>{{Cite journal|last1=Whitley|first1=Sean|last2=Halama|first2=Ralf|last3=Gertisser|first3=Ralf|last4=Preece|first4=Katie|last5=Deegan|first5=Frances M.|last6=Troll|first6=Valentin R.|date=2020-10-18|title=Magmatic and Metasomatic Effects of Magma–Carbonate Interaction Recorded in Calc-silicate Xenoliths from Merapi Volcano (Indonesia)|url=https://academic.oup.com/petrology/article/61/4/egaa048/5822871|journal=Journal of Petrology|language=en|volume=61|issue=4|doi=10.1093/petrology/egaa048|issn=0022-3530|doi-access=free}}</ref>

Exoskarns are more common and form on the outside of an intrusive body that comes into contact with a reactive rock unit. They are formed when fluids left over from the crystallisation of the intrusion are ejected from the mass at the waning stages of emplacement, in a process called boiling. When these fluids come into contact with reactive rocks, usually carbonates such as limestone or dolomite, the fluids react with them, producing alteration (infiltration [[metasomatism]]).<ref name=":2" />

Endoskarns form within the intrusive body where fracturing, cooling joints, and [[Stockwork|stockworks]] have been produced, which results in a permeable area. This permeable area can be altered by fluids originally sourced from the intrusion itself, after interacting with surrounding rocks ([[protolith]]). Thus, both the composition and the textures of protoliths strongly play a role in the formation of the resulting skarn. Endoskarns are considered to be rare. <ref name=":2" />

Reaction skarns are formed from {{Linktext|isochemical}} metamorphism occurring on thinly interlayered sedimentary units, via small-scale{{Efn|(on the order of a few centimetres)}} metasomatic exchange between adjacent units.<ref name=":2" /><ref>{{cite journal|last1=Zarayskiy|first1=G. P.|last2=Zharikov|first2=V. A.|last3=Stoyanovskaya|first3=F. M.|last4=Balashov|first4=V. N.|year=1987|title=The experimental study of bimetasomatic skarn formation|journal=International Geology Review|volume=29|issue=6|pages=761–858|publication-date=29 June 2010|doi=10.1080/00206818709466179|bibcode=1987IGRv...29..629Z |url=https://www.tandfonline.com/doi/abs/10.1080/00206818709466179|url-access=subscription}}</ref>

Skarnoids are calc-silicate rocks that are fine-grained and iron poor. Skarnoids tend to be found between hornfels and coarse-grained skarn.<ref>{{cite journal|last1=Korzhinskii|first1=D.S.|year=1948|title=Petrology of the Tur'insk skarn deposits of copper|place=Academy nauk SSSR|publisher=Institute of Geology Nauk Trudy|volume= 68|others=Ser. Rundnykh Mestorozhdenii|issue= 10|page=147}}</ref><ref>{{cite journal|last1=Zharikov|first1=V. A.|year=1970|title=Skarns (Part I)|url=https://www.tandfonline.com/doi/abs/10.1080/00206817009475262 |journal=International Geology Review|volume=12|issue=5|doi=10.1080/00206817009475262|publication-date=7 September 2009|pages=541–559|bibcode=1970IGRv...12..541Z |url-access=subscription}}</ref><ref>{{cite journal|last1=Zharikov|first1=V. A.|year=1970|title=Skarns (Part II)|url=https://www.tandfonline.com/doi/abs/10.1080/00206817009475270 |journal=International Geology Review|volume=12|issue=6|doi=10.1080/00206817009475270|publication-date=7 September 2009|pages=619-647,760-775|bibcode=1970IGRv...12..619Z |url-access=subscription}}</ref> Skarnoids commonly reflect the composition of the protolith.<ref name=":2" />

Most large skarn deposits 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 surrounding region, forming hornfels as a result. The recrystallization and phase change of hornfels reflects the composition of the protolith. After the formation of hornfels, metasomatism occurs involving hydrothermal fluids from a source that is 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]]).<ref name=":0" /><ref name=":1" />

The skarn deposits that are considered economically important for containing valuable metals are a result of large-scale metasomatism, where the composition of fluid controls the skarn and its ore mineralogy. They are relatively coarser grained and do not strongly reflect the composition of protolith or surrounding rocks.<ref name=":1" /><ref name=":2" />

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 [[Evaporite|evaporites]]. Here, fluids react less via chemical exchange of ions, but because of the [[Redox potential|redox-oxidation potential]] of the wall rocks.<ref name=":2" />

==Ore deposits==
The major economic metals that are sourced from skarn deposits are [[copper]], [[tungsten]], [[iron]], [[tin]], [[molybdenum]], [[zinc]]-[[lead]], and [[gold]].<ref name=":0" /><ref name=":1" /><ref name=":2" /><ref name=":3" /> Other minor economic elements include [[uranium]], [[silver]], [[boron]], [[fluorine]], and [[rare-earth element]]s.<ref name=":2" />

Some examples of the major economic skarn deposits, both current and historical, are:
* Iron skarns: Dashkesan Mine, Azerbaijan

* Copper skarns: [[Bingham Canyon Mine]], Utah, U.S.A

* Tungsten skarns: [[Sangdong mine]], South Korea

* Gold-bearing skarns: [[Hedley Mascot Mine]], British Columbia, Canada

* Zinc-lead skarns: [[Santa Eulalia, Chihuahua]], Mexico

* Nickel skarns: Avebury Mine, Zeehan, Tasmania (Australia)

* Molybdenum skarns: Yangchiachangtze mine, China

==See also==
* {{annotated link|Ore genesis}}
{{Commonscat}}

== Notes ==
{{notelist|30em}}

==References==
{{Reflist}}

==External links==
{{Wiktionary|skarn|tactite}}
{{Rock type}}
[[Category:Metamorphic rocks]]