Skarn is a type of silicate gangue or waste rock associated with iron ore bearing sulphide. It is also expanded to refer to a rock containing calcium bearing silicate mineral. “Tactite” term in America is synonymously used with skarn. Basically, Skarn is a metamorphic zone developed in the contact area around Igneous Intrusion when carbonate sedimentary rocks are invaded by a large amount of silicon, aluminum, iron, and magnesium-rich granitic magma. These fluids mix in the contact zone, dissolve calcium-rich carbonate rock and convert the host carbonate roc to skarn deposit (figure below)
The minerals commonly present in skarn are Pyroxene, Garnet, Wollastonite, Actinolite, Hematite or Magnetite and epidote. A variety of uncommon minerals found in skarns (tourmaline, beryl, corundum, fluorite, apatite, barite, etc.) is because when the skarns are formed by the incompatible element rich siliceous aqueous fluids.
Skarns deposits are economically valuable as sources of metal such as tin, tungsten, manganese, copper, gold, zinc, lead, nickel, molybdenum and iron.
Skarns are subdivided according to several criteria, the most being their mineralogy and their enclosing rock type:
- EXOSKARNS– the skarns that are developed in the sedimentary rocks surrounded by thermal source.
- ENDOSKARNS– the skarns that are developed within the igneous intrusion.
- Other types are :
- MAGNESIUM AND CALCIUM SKARNS– can be used to describe the dominant composition of the original rock resulting skarn mineral. (example- Garnet, Orthopyroxene, Wollastonite).
- SKARNOID– it is the intermediate stage between the fine-grained hornfels and coarse-grained skarn. Contain garnet or pyroxene as major phases and are fine-grained with lack of iron.