metasomatism

Metasomatism is a chemical alteration of a rock by hydrothermal or other externally derived fluids that introduce or remove chemical components, resulting in a new mineral assemblage and often a change in rock composition. It involves mass transfer between the rock and circulating fluids, with dissolution of some minerals and crystallization of new minerals as the fluid moves through porous or fractured rock. Metasomatism can affect small veins or large rock volumes and is commonly linked to magmatic activity, hydrothermal systems, and subduction zones.

The process relies on fluid flow carrying dissolved ions, complexing agents, and sometimes volatile components such as water and carbon dioxide. As fluids react with the host rock, original minerals can dissolve while new minerals precipitate, leading to replacement textures, zoning, and the development of characteristic mineral assemblages and alteration halos. Temperature, pressure, fluid composition, and rock permeability influence the extent and style of metasomatism. Common metasomatic products include chlorite, sericitized or albitized feldspars, epidote, calc-silicate minerals (such as wollastonite and garnet in skarns), and various carbonates and silicates.

Skarn metasomatism is one well-known form, occurring when carbonate rocks interact with intruding magmas and fluids to form calc-silicate mineral assemblages. Other forms include albitization, saussuritization, silicification, and chloritization. Metasomatism is economically important because it can create ore-bearing alteration zones associated with copper, tungsten, iron, and other mineral deposits, and it provides key records of crustal fluid flow and chemical budgets in tectonically active regions.