skarns

Skarns are metasomatic, calc-silicate rocks formed by contact metamorphism between an igneous intrusion and carbonate rocks such as limestone or dolostone. They develop where silica- and metal-bearing fluids from the intrusion alter the carbonate country rock, dissolving carbonate and precipitating new minerals in a calc-silicate assemblage.

Formation typically occurs at contact zones where granitic to dioritic intrusions invade carbonate sequences. Temperature and fluid activity promote metasomatism and mineral replacement, producing a zoned aureole in which calcium, silicon, iron, and aluminum are redistributed. The resulting rock commonly contains garnet, diopside, hedenbergite, wollastonite, epidote, vesuvianite, calcite, and quartz, often with accessory magnetite or sulfide minerals.

Two main skarn types are recognized: endoskarn, which forms within the intrusion itself or its immediate margins, and exoskarn, which develops in the surrounding carbonate country rock adjacent to the contact. Calc-silicate skarns are the most common, though other variants may include hornfelsic or carbonate-rich phases.

Economic importance is a hallmark of skarns. They host major ore deposits of copper, iron, zinc, lead, tungsten, molybdenum, tin, and gold, with sulfide minerals such as chalcopyrite, sphalerite, galena, and cassiterite frequently present. Skarn-related ores are commonly associated with granitic intrusions and faulted carbonate terrains, making skarn deposits a focus of mineral exploration in orogenic belts.

Geographically, skarns occur worldwide wherever carbonate rocks have been intruded by igneous bodies, with notable occurrences in orogenic belts where granitoids contact carbonate sequences.