Metasomatism

Metasomatism is the chemical alteration of a rock by hydrothermal and other fluids. The minerals of the original rock are dissolved and new minerals are deposited in their place, with dissolution and deposition going on at the same time while the rock stays solid. The fluids come from igneous or metamorphic sources and carry chemical elements in and out, so metasomatism is open system behaviour, unlike classical metamorphism, in which a rock changes mineralogy without appreciable change in its chemistry. In the igneous setting it produces skarns and greisen next to intrusions. It also operates in the mantle, where infiltrating melts and carbon dioxide-rich or water-rich fluids change the composition of peridotite, and many gold ore deposits are the product of focused metasomatic fluid flow in shear zones.