Skarn

  Skarn is a metamorphic rock that is usually variably colored green or red, occasionally grey, black, brown or white.

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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.
Skarns in the igneous environment are associated with hornfels, marble hornfels and wider zones of calc-silicate rocks.

Petrology and types

Skarns are in their broadest sense formed by mass and chemical transport and reactions between adjacent lithologies. 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.

However, the widest use of the word is in describing the metasomatised zones of wall rock adjacent to granites. 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.

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.

Typical skarn minerals include pyroxene, garnet, idocrase, wollastonite, actinolite, magnetite or haematite, and epidote. Because skarns are formed from incompatible-element rich, sliceous 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. Often, feldspathoids 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 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 origin) and I-type (igneous-metamorphic origin) granites. 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.

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).

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. 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.

Skarns are often hosts for copper, lead, zinc, iron, gold, molybdenum, tin, and tungsten ore deposits.

See also

• Ore genesis
• Metamorphism
• Metasomatism
• Granite

Evans, A.M., 1993. Ore Geology and Industrial Mineras: An Introduction 3rd ed. Blackwell Publishing