exoskarn

Exoskarn is a term used in geology and in speculative literature to describe a calc-silicate metamorphic zone that forms at the contact between carbonate rocks and an external intrusion or metasomatic source. The name combines exo-, meaning external, with skarn, the classic end-member metamorphic assemblage produced by heat and metasomatic fluids during contact metamorphism.

Formation and characteristics: Exoskarn is hypothesized to develop when heat and reactive fluids originate from an external intrusion—such as a silicate magma body, impact-related breccia, or extraterrestrial material—migrate into surrounding carbonate rocks. The interaction drives metasomatic replacement by calc-silicate minerals, yielding garnet, pyroxene, wollastonite, epidote, and calcite, often with sharp reaction rims. Compared to conventional skarn, exoskarn assemblages may show enrichment patterns tied to the exogenous source, including metals like copper, iron, zinc, or tungsten, and distinctive zoning reflecting the external fluid source.

Geology and deposits: Exoskarn footprints are expected to occur near ancient external intrusions or impact-related settings. In real-world geology, the concept is largely theoretical or used in fiction, with exploration implications similar to classic skarn deposits but focusing on external-fluid signatures and isotopic compositions.

See also: Skarn, metasomatism, calc-silicate rocks, ore genesis.