Metasomatism (mineralogy)
Introduction
Metasomatism is a geological process involving the chemical alteration of a rock by hydrothermal and other fluids. This process results in the introduction or removal of chemical components, leading to the formation of new minerals and the modification of existing ones. Metasomatism plays a crucial role in the formation of various mineral deposits and the evolution of the Earth's crust.
Mechanisms of Metasomatism
Metasomatism occurs through several mechanisms, including diffusion, advection, and infiltration. These processes facilitate the movement of ions and molecules within the rock matrix, enabling chemical reactions that alter the mineralogical composition.
Diffusion
Diffusion is the process by which ions and molecules move from areas of high concentration to areas of low concentration. In the context of metasomatism, diffusion occurs at the microscopic level, allowing chemical components to migrate through the rock's pore spaces and crystal lattices.
Advection
Advection involves the transport of chemical components by fluid flow. Hydrothermal fluids, which are often rich in dissolved minerals, can infiltrate rock formations and introduce new chemical species. This process is particularly significant in the formation of ore deposits.
Infiltration
Infiltration refers to the penetration of fluids into a rock body. These fluids can be of various origins, including magmatic, metamorphic, or meteoric sources. The interaction between the infiltrating fluids and the host rock leads to metasomatic alteration.
Types of Metasomatism
Metasomatism can be classified into several types based on the nature of the fluids involved and the resulting mineralogical changes.
Skarn Metasomatism
Skarn metasomatism occurs when carbonate rocks, such as limestone or dolomite, are chemically altered by hydrothermal fluids. This process typically results in the formation of skarn minerals, including garnet, pyroxene, and wollastonite.
Greisen Metasomatism
Greisen metasomatism is associated with the alteration of granitic rocks by fluids rich in volatiles, such as fluorine and boron. This type of metasomatism leads to the formation of greisen, a rock composed predominantly of quartz and muscovite, with accessory minerals like topaz and tourmaline.
Serpentinization
Serpentinization is the process by which ultramafic rocks, such as peridotite, are altered by hydrothermal fluids to form serpentine minerals. This type of metasomatism is common at mid-ocean ridges and subduction zones, where seawater interacts with the mantle-derived rocks.
Mineralogical Changes in Metasomatism
Metasomatism results in significant mineralogical changes within the affected rock. These changes can be categorized into two main processes: mineral replacement and mineral growth.
Mineral Replacement
Mineral replacement involves the dissolution of pre-existing minerals and the subsequent precipitation of new minerals. This process is driven by the chemical disequilibrium between the infiltrating fluids and the host rock. An example of mineral replacement is the transformation of feldspar to sericite in hydrothermally altered granites.
Mineral Growth
Mineral growth occurs when new minerals precipitate from the infiltrating fluids without the complete dissolution of the original minerals. This process can lead to the formation of large, well-formed crystals, such as the growth of tourmaline in greisenized granites.
Geochemical Aspects of Metasomatism
The geochemical aspects of metasomatism involve the study of element mobility, fluid-rock interaction, and the thermodynamic conditions that drive the chemical reactions.
Element Mobility
Element mobility refers to the ability of chemical elements to move through the rock matrix during metasomatism. Elements such as sodium, potassium, calcium, and silica are highly mobile and can be transported over significant distances by hydrothermal fluids.
Fluid-Rock Interaction
Fluid-rock interaction is a critical aspect of metasomatism, as it determines the extent and nature of the chemical alteration. The composition of the infiltrating fluids, the temperature and pressure conditions, and the mineralogy of the host rock all influence the outcome of the metasomatic process.
Thermodynamic Conditions
The thermodynamic conditions, including temperature, pressure, and fluid composition, play a crucial role in metasomatism. These conditions dictate the stability of minerals and the direction of chemical reactions. For example, the formation of garnet in skarn deposits requires specific temperature and pressure conditions to stabilize the mineral assemblage.
Economic Significance of Metasomatism
Metasomatism is of great economic significance due to its role in the formation of mineral deposits. Many valuable ore deposits, including those of gold, copper, and tin, are associated with metasomatic processes.
Gold Deposits
Gold deposits are often formed through metasomatic processes, particularly in hydrothermal systems. The interaction of gold-bearing fluids with reactive host rocks leads to the precipitation of gold and the formation of ore bodies.
Copper Deposits
Copper deposits, such as porphyry copper deposits, are commonly associated with metasomatic alteration. The introduction of copper-rich fluids into the host rock results in the formation of copper minerals, including chalcopyrite and bornite.
Tin Deposits
Tin deposits, particularly those associated with granitic intrusions, are often formed through greisen metasomatism. The alteration of granitic rocks by tin-bearing fluids leads to the concentration of tin minerals, such as cassiterite.
Metasomatism in the Earth's Crust
Metasomatism plays a significant role in the evolution of the Earth's crust. It contributes to the formation of new mineral assemblages, the modification of existing rock compositions, and the redistribution of chemical elements.
Crustal Metasomatism
Crustal metasomatism involves the alteration of rocks within the Earth's crust by fluids derived from various sources, including magmatic, metamorphic, and meteoric origins. This process can lead to the formation of economically important mineral deposits and the modification of crustal compositions.
Mantle Metasomatism
Mantle metasomatism refers to the chemical alteration of mantle rocks by fluids and melts. This process is significant in the formation of kimberlite pipes, which are the primary source of diamonds. The interaction of mantle-derived fluids with peridotite results in the formation of metasomatic minerals, such as phlogopite and amphibole.
Analytical Techniques in Metasomatism Studies
The study of metasomatism involves various analytical techniques to characterize the mineralogical and geochemical changes in altered rocks.
Petrographic Analysis
Petrographic analysis involves the examination of thin sections of rocks under a microscope to identify mineral assemblages and textures. This technique is essential for understanding the mineralogical changes associated with metasomatism.
Geochemical Analysis
Geochemical analysis involves the determination of the chemical composition of rocks and minerals. Techniques such as X-ray fluorescence (XRF), inductively coupled plasma mass spectrometry (ICP-MS), and electron microprobe analysis are commonly used to quantify the elemental concentrations and trace element distributions in metasomatized rocks.
Isotopic Analysis
Isotopic analysis involves the measurement of stable and radiogenic isotopes to understand the sources and processes involved in metasomatism. Isotopic systems, such as oxygen, hydrogen, and strontium isotopes, provide insights into the origin of fluids and the conditions of metasomatic alteration.
Case Studies of Metasomatism
Several well-documented case studies provide valuable insights into the processes and outcomes of metasomatism.
The Skarn Deposits of the Cornubian Batholith
The Cornubian Batholith in southwest England is renowned for its extensive skarn deposits. The interaction of granitic intrusions with surrounding carbonate rocks has led to the formation of economically significant tin and tungsten mineralization.
The Greisenized Granites of the Erzgebirge
The Erzgebirge region in Germany is famous for its greisenized granites, which host significant tin and tungsten deposits. The alteration of granitic rocks by volatile-rich fluids has resulted in the formation of greisen and associated mineralization.
The Serpentinized Peridotites of the Mid-Atlantic Ridge
The Mid-Atlantic Ridge is a prime location for studying serpentinization. The interaction of seawater with mantle-derived peridotites has led to the extensive formation of serpentine minerals, providing insights into the processes occurring at mid-ocean ridges.