thumb|Microscopic view of skarn under crossed polarizers
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 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 fluids. Skarns tend to be rich in calcium-magnesium-iron-manganese-aluminium silicate minerals, which are also referred to as calc-silicate minerals. These minerals form as a result of alteration which occurs when hydrothermal fluids interact with a protolith of either igneous or sedimentary origin. In many cases, skarns are associated with the intrusion of a granitic pluton found in and around faults or shear zones that commonly intrude into a carbonate layer composed of either dolomite or limestone. Skarns can form by regional or contact metamorphism and therefore form in relatively high temperature environments.
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.
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 elements, a variety of uncommon mineral types are found in skarns, such as: tourmaline, topaz, beryl, corundum, fluorite, apatite, barite, strontianite, tantalite, anglesite, and others.
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.
Cu (Au, Ag, Mo, W) skarn deposits
The tectonic setting for Cu deposits tends to be the 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.
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).
Skarnoids are calc-silicate rocks that are fine-grained and iron poor. Skarnoids tend to be found between hornfels and coarse-grained skarn. Skarnoids commonly reflect the composition of the protolith.