Skarn is a non-foliated metamorphic rock created by metasomatism. An interesting fact about skarn is that its chemical or mineral composition does not define it; the formation process defines skarn.
Some metamorphic rocks are known as metasomatic rocks. Metasomatism is the process of alteration of rocks by hot and chemically active fluids that diffuse and flow through the rocks. This movement changes the rock composition and forces crystallization.
Skarn is also known as tacite. Skarn is a Swedish term. It is a mining term that describes silicate gangue or waste rock.
Skarns are usually composed of calcium-iron-magnesium-manganese-aluminum silicate minerals. These minerals are also known as calc-silicate minerals. The minerals form due to alteration when hydrothermal fluids flow through rocks.
Skarns are usually classified by their parent rock. Skarns with a sedimentary parent rock are known as exoskarn, and those with an igneous parent rock are termed endoskarns.
Further classification is done by studying the dominant composition and how the crystals have altered after the formation process.
Here are three common classifications for skarns.
It contains minerals like magnesium clinopyroxene, spinel, pargasite, orthopyroxene, olivine, serpentine, phlogopite, magnesium, and other minerals from the humite group.
These skarns have a limestone protolith with a dominant mineral assemblage of garnet, wollastonite, and clinopyroxene.
These are fine-grained rocks that lack iron and look like skarns. These rocks are an intermediate stage of fine-grained hornfels and coarse-grained skarn. These rocks contain garnet or pyroxene as major phases.
Skarns form when magma bodies intrude carbonate rocks like limestone, marble, or dolostone. This intrusion can happen via metasomatism or contact metamorphism. The heat generated from contact metamorphism is the primary change agent in the alteration process.
The magma then starts to cool and releases hot and acidic silicate rich-fluids. Magmas also contain dissolved water in varying concentrations. This water carries heat and chemical fluids into the country’s rock through pores, fractures, and mineral grains.
The hot, acidic, and metal-laden water dissolves, replaces and recrystallizes the minerals when it comes in contact with carbonate rocks. The acidic water neutralizes, and the temperature falls when it comes in contact with carbonate rocks. As a result, large amounts of calc-silicate minerals appear in the carbonate rocks.
It is not necessary that metasomatism create skarns from only carbonate rocks. Many rocks like granite, basalt, tuff, shale, and conglomerate can also act as protoliths for skarns.
Exoskarn, a type of skarn, is formed when the original chemistry of a rock mass is changed. This happens when hot fluids of incompatible chemistry move through the rock. The distance between the magma body and the rock mass can change the intensity of alteration and the types of minerals that will form in the rock.
Skarns can also form in the following environments.
- seafloor hydrothermal systems
- Sheer zones and faults
- Regional metamorphism areas
- Above subduction zones
It can also form with different water inputs
- Magma water
- Shallow groundwater
- Deep brines
Endoskarns vs. Exoskarns
Skarns usually form on the boundary between a magma body and its surrounding rock mass. Skarns on the igneous side of the contact are called endoskarns, and those formed on the country-rock side are called exoskarns.
Where is Skarn Found?
Skarn deposits are linked with economic metals. The major metals that make up skarn deposits include copper, iron, tungsten, molybdenum, zinc-lead, tin, and gold.
Skarn deposits are ore deposits and are found at the following places.
- Copper skarns: Bingham Canyon Mine, Utah, U.S.A
- Tungsten skarns: Sangdong mine, South Korea
- Gold-bearing skarns: Hedley Mascot Mine, British Columbia, Canada
- Iron skarns: Dashkesan Mine, Azerbaijan
- Nickel skarns: Avebury Mine, Zeehan, Tasmania (Australia)
- Molybdenum skarns: Yangchiachangtze mine, China
- Zinc-lead skarns: Santa Eulalia, Chihuahua, Mexico
- Skarn is used to obtain valuable metals like copper, gold, lead, zinc, molybdenum, tin, and tungsten.
- It is also a source of a wide range of valuable gemstones like garnet, ruby, and sapphire.