Skarn

Skarns or tactites are calcium-bearing calc–silicate rocks. Skarns are most often formed at the contact zone between intrusions of granitic magma bodies and carbonate sedimentary rocks such as limestone and dolostone. Hot fluids derived from the granitic magma are rich in silica, iron, aluminium, and magnesium. These fluids mix in the contact zone, dissolve calcium-rich carbonate rocks, and convert the host carbonate rock to skarn deposits in a metamorphic process called metasomatism. The resulting metamorphic rock may consist of a very wide variety of minerals dependent largely on the original composition of the magmatic fluids and the purity of the carbonate sedimentary rocks.

Skarns are sometimes associated with mineable accumulations of metallic ores of iron, copper, zinc, lead, gold, and several others. In such cases these deposits are called "skarn deposits".

Skarn is an old Swedish mining term originally used to describe a type of silicate gangue, or waste rock, associated with iron-ore bearing sulfide deposits apparently replacing Palaeoproterozoic age limestones in Sweden's Persberg mining district.

Skarns are, in their broadest sense, formed by mass and chemical transport and reactions between adjacent rock units. They need not be igneous in origin; two adjacent sedimentary layers such as a banded iron formation and a limestone may react to exchange metals and fluids during metamorphism, creating a skarn.

...
Wikipedia