Skarn

Skarn is a coarse-grained metamorphic rock formed by metasomatism, the chemical alteration of carbonate rocks (limestone or dolomite) by hot, silica- and metal-rich fluids, usually derived from a nearby cooling magma. It is dominated by calc-silicate minerals such as garnet (grossular-andradite), pyroxene (diopside-hedenbergite), epidote, and wollastonite.

Skarns are colorful and mineralogically diverse, and are economically vital because they host major deposits of tungsten, copper, iron, zinc, molybdenum, and even gold. The term originally came from Swedish miners describing the gangue around ore.

Skarns form at the contacts between intrusions and carbonate country rock worldwide.

Skarn forms in two stages. First, contact metamorphism bakes carbonate rock near an intrusion, recrystallizing it. Then, as hydrothermal fluids exsolve from the cooling magma, they react with the carbonate, introducing silica, iron, and metals and removing carbon dioxide. This metasomatic exchange grows the distinctive calc-silicate minerals.

Prograde skarn forms at high temperatures, producing anhydrous garnet and pyroxene, while later, cooler retrograde fluids add hydrous minerals like epidote, amphibole, and chlorite, often along with ore minerals. Skarns develop at the contacts of granitic to dioritic intrusions with limestone and dolomite, and are mined worldwide, including in China, the western United States, and Peru.

Skarn is typically coarse-grained, hard, and colorful, with conspicuous garnet (often brown, green, or red), green to black pyroxene, and sometimes pale wollastonite or epidote. The mineralogy is variable and patchy, reflecting fluid pathways.

Look for an association with both a carbonate host and a nearby igneous intrusion. Distinguish skarn from marble (which is dominated by calcite and fizzes in acid) and from ordinary calc-silicate hornfels (finer grained, formed without major metasomatism). The presence of abundant coarse garnet and pyroxene, often with metallic ore minerals, in a former limestone setting is the hallmark of skarn.

Skarns are extremely important as ore deposits, supplying much of the world's tungsten, plus significant copper, iron, zinc, lead, molybdenum, tin, and gold. Mining skarn ore is a major economic activity in many regions.

The colorful garnet- and pyroxene-rich rock is also collected for its mineral specimens, and gem-quality grossular, andradite (demantoid), and diopside can be recovered from some skarns. Wollastonite from skarn is used industrially in ceramics and as a filler. The rock has little metaphysical following; its importance is overwhelmingly economic and scientific.