Metasomatism

Metasomatism is a geological process involving the chemical alteration of a rock by hydrothermal and other fluids. It is a form of metamorphism that results in the change of mineral composition of the rock without melting. This process is driven by the infiltration of fluids that introduce or remove chemical components from the rock. Metasomatism plays a crucial role in the formation of various mineral deposits, including those of precious metals, gemstones, and other economically important minerals.

Process

Metasomatism occurs when a rock is exposed to a fluid with a different composition than the rock itself. These fluids, often rich in volatiles such as water (H2O), carbon dioxide (CO2), and fluorine (F), can initiate chemical reactions that alter the mineralogy of the rock. The process can take place at various scales and geological settings, from the alteration of rocks by fluid flow along fractures and faults to the large-scale exchange of elements between the mantle and crust.

The two main types of metasomatism are:

• Skarn Formation: Occurs at the contact between igneous intrusions and carbonate rocks, leading to the formation of calcium-rich silicates.
• Greisenization: Associated with the late stages of granite crystallization, resulting in the replacement of feldspar by quartz and mica.

Types of Metasomatic Processes

Metasomatism can be classified based on the direction of fluid flow or the nature of the chemical exchange:

• Infiltration Metasomatism: Fluids introduce new components into the rock.
• Outflow Metasomatism: Fluids remove components from the rock.
• Isochemical Metasomatism: Involves the redistribution of elements within the rock without significant addition or removal.

Mineral Deposits

Metasomatism is responsible for the formation of various types of mineral deposits, including:

• Skarn Deposits: Rich in iron, copper, tungsten, and gold.
• Greisen Deposits: Typically contain tin, tungsten, and molybdenum.
• Hydrothermal Vein Deposits: Formed by the precipitation of minerals from hydrothermal fluids within fractures.

Environmental and Economic Significance

Metasomatism has significant environmental and economic implications. It is a critical process in the formation of mineral resources, many of which are essential for modern technology and industry. Understanding metasomatism can also provide insights into fluid flow in the crust, the cycling of elements between the Earth's surface and interior, and the thermal history of specific geological settings.

See Also

• Metamorphic rock
• Hydrothermal circulation
• Mineralogy
• Petrology