Skarn

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. A skarn is formed by a variety of metasomatic processes during metamorphism between two adjacent lithologic 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 is highly related to the protolith. Skarn minerals are mainly garnets and pyroxene with a wide variety of calc-silicate and associated minerals. Typical skarn minerals 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.

Generally, there are two types of skarns that form, exoskarns and endoskarns. Exoskarns are more common and form on the outside of an intrusive body that comes into contact with a carbonate 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. 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). Endoskarns form within the intrusive body where fracturing, cooling joints, and stockworks have been produced, which results in a permeable area. The permeable area can incorporate material from the carbonate layer. The magmatic hydrothermal fluids that were transported or created by the intrusion interact with the carbonate material and form 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. Reaction skarn is formed from isochemical metamorphism occurring 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 that is fine-grained and iron poor. It lies between hornfels and coarse-grained skarn. Skarnoid tends to reflect the composition of the protolith. 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 region where sedimentary rocks are present, and forms hornfels as a result. The recrystallization and phase change of hornfels reflects the composition of the protolith. After the formation of hornfels, 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). 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 reflect the composition of protolith or surrounding rocks. 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.

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. A skarn is formed by a variety of metasomatic processes during metamorphism between two adjacent lithologic 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 is highly related to the protolith. Skarn minerals are mainly garnets and pyroxene with a wide variety of calc-silicate and associated minerals. Typical skarn minerals 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 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 limestones in Sweden's Persberg mining district.