Skarns are silicate rocks produced by the interaction between the hot hydrothermal fluids released by crystallizing magma with carbonate rocks – a process known as metasomatism. The metasomatic fluids emanating from a magmatic system, commonly enriched in Si, Fe, and Mn and with temperatures between 400 and 900 °C, react with Ca and/or Mg-bearing carbonate rocks, causing decarbonation of carbonate minerals (hence the release of CO2) and producing a wide range of Ca-Mg-Fe-Mn silicates and in some cases even alkali feldspar and feldspathoids. These silicate parageneses can form at the expense of carbonate rocks – and in this case are termed exoskarns – or the magmatic rocks themselves (endoskarns), since Ca and Mg can also migrate from the parent carbonate rocks into igneous or silicate-rich rocks nearby. Skarns can occur at the contact between carbonate and silicate rocks (contact skarns) or as veins or dikes crosscutting the carbonate rocks (vein skarns). Skarns are commonly associated with sulphide ore bodies containing Fe, Cu, Ag, Au, Mo, and W, among other commercially valuable elements (B, Be, rare earth elements). Indeed, the term ‘skarn‘ is an old Swedish mining term that was used to describe the sterile ‘wasterock’ associated with such deposits.
According to their composition, skarns can also be distinguished into magnesian skarns, produced by alteration of Mg-rich dolomites and in turn containing several Mg-bearing silicates, and calc-skarns, formed by metasomatism of Ca-rich carbonate rocks and rich in calcsilicates. Typical assemblages of magnesian skarns include Mg-rich olivine (forsterite), diopside, magnesian spinel, periclase, clinohumite, phlogopite, and pargasite, as well of several other minerals like enstatite or fassaitic pyroxene, monticellite, åkermanite, and merwinite. Calc-skarns are typically characterized by ugrandite garnet, Fe-rich or Mn-rich ortho- and clinopyroxenes (hedenbergite, ferrosalite, johannsenite), wollastonite and/or other pyroxenoids, in addition to other minerals that may form under specific conditions (e.g. epidote, vesuvianite, ilvaite, rhodonite, bustamite…). Skarns may also contain carbonate minerals like magnesite, dolomite, and calcite, alkali feldspars (e.g. adularia), scapolite, and quartz. Furthermore, high-temperature skarns can easily be replaced by a wide range of late hydrothermal minerals such as chlorite, serpentine, tremolite-actinolite amphibole, talc, carbonates, and brucite.
• magnesian skarn: • forsterite • diopside • spinel • periclase • clinohumite • phlogopite • pargasite
• calc-skarn: • ugrandite garnet • hedenbergite • johannsenite • wollastonite or Mn-rich pyroxenoids • ilvaite • epidote
• endoskarn: produced by metasomatism of magmatic or other silicate rocks
• exoskarn: produced by metasomatism of carbonate rocks
The presence of all these different minerals is governed not only by the pressure and temperature of skarn formation, but also by the activity of chemical elements and the composition of the metasomatizing fluid, the distance from the (parent) igneous rocks, and the composition of the rocks involved. All these variables produces and extensive zoning in skarn minerals and typically many different mineral parageneses, some formed at higher temperature and closer to the parent magma (high temperature skarns), others forming in the distal parts or overgrowing high-temperature skarns during cooling of the system (low-temperature skarns). To summarize, most skarns on Earth are governed by several local factors and their origin as well as the conditions of their formation are unique to each specific study case.
Field images and skarn samples
Wollastonite skarn (Norsi, Elba, Italy)
Calc-skarn containing fibrous aggregates of wollastonite surrounded by recrystallized calcite (originally a limestone). Wollastonite grew because silica was supplied by external fluids and calcium by the decarbonation of carbonate minerals. [BLOG POST]
Calcsilicates overgrowing limestone, formed at the limestone/shale contact, which was infiltrated by super-hot hydrothermal fluids.
Limestone lens replaced by calcsilicates. The wollastonite skarn formed in the core, where only Ca and Si are available. The rims contain greenish Fe-Mg-Ca silicates (emphibole, epidote, chlorite, hydrogrossular) produced by elements released by the carbonate and the surrounding shales.