Skarn

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. ==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. ==Formation==

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 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. 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 surrounding region, forming hornfels as a result. The recrystallization and phase change of hornfels reflects the composition of the protolith. After the formation of hornfels, metasomatism occurs involving hydrothermal fluids from a source that is 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 strongly 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. ==Ore deposits==

The major economic metals that are sourced from skarn deposits are copper, tungsten, iron, tin, molybdenum, zinc-lead, and gold. Other minor economic elements include uranium, silver, boron, fluorine, and rare-earth elements. Some examples of the major economic skarn deposits, both current and historical, are: • Iron skarns: Dashkesan Mine, Azerbaijan • Copper skarns: Bingham Canyon Mine, Utah, U.S.A • Tungsten skarns: Sangdong mine, South Korea • Gold-bearing skarns: Hedley Mascot Mine, British Columbia, Canada • Zinc-lead skarns: Santa Eulalia, Chihuahua, Mexico • Nickel skarns: Avebury Mine, Zeehan, Tasmania (Australia) • Molybdenum skarns: Yangchiachangtze mine, China ==See also==