Skarn

Skarns may form by metamorphic recrystallization of impure carbonate protoliths, bimetasomatic reaction of different lithologies, and infiltration metasomatism by magmatic-hydrothermal fluids.

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.

[2][3][4][5] The hydrothermal fluids associated with the metasomatic processes can originate from a variety of sources; magmatic, metamorphic, meteoric, marine, or even a mix of these.

Skarn deposits are economically valuable as sources of metals such as tin, tungsten, manganese, copper, gold, zinc, lead, nickel, molybdenum and iron.

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.

[7] 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.

The other class, called calcic skarns, are the replacement products of a limestone protolith with dominant mineral assemblages containing garnet, clinopyroxene, and wollastonite.

[3] Rocks that contain garnet or pyroxene as major phases, and that are also fine-grained, lack iron, and have skarn-like appearances, are generally given the term "skarnoid".

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.

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).

[4] The major economic metals that are sourced from skarn deposits are copper, tungsten, iron, tin, molybdenum, zinc-lead, and gold.