November 30, 2020

Metasomatism : Metasomatic (Replacement) Deposits

What is Metasomatism?

Metasomatism is defined as the formation of new minerals due to the metamorphic process by the mechanism of chemical replacement of the pre-existing minerals under the influence of chemically active fluids. Many metamorphic reactions are usually considered as the isochemical. The bulk chemistry of the chemical remains unchanged but the elements that get in touch with chemically active fluid which derived from the igneous magmas.

A common thing that observed in the case of metamorphism is that the total volume of the rocks remains unchanged after the process is completed. Further, the changes can take place over a wide range of temperatures and pressure like any other metamorphic processes are completed essentially in solid-state.

Metamorphic reactions involve the release of fluids as minerals change and chemical reactions with locally-derived fluids. However, if a large amount of externally-derived fluid—such that supplied by magma —is flushed through the system at the high pressures and temperatures characteristic of metamorphism, it can substantially alter the chemical composition of the rock. This type of hydrothermal alteration is called metasomatism.

The gases and hot solutions involved emanate from an igneous source, and the effects of
metasomatism are often particularly notable about an intrusion of granitic character. Indeed,
there is a greater concentration of volatiles in acid than in basic magma.

Metasomatic action, especially when it is concentrated along fissure zones and veins, may bring about severe alteration of certain minerals. For instance, feldspars in granite or gneiss may be highly kaolinized as a result of metasomatism, and limestone may be reduced to a weakly bonded granular aggregate.

What causes Metasomatism?

Metasomatism is the process in which recrystallization and other metamorphic changes done but the bulk chemistry of the rock remains unchanged. If other elements are introduced into the rock by circulating fluid derived from an igneous rock then the resulting metamorphism is called the “metasomatism”. sometimes it is also referred to as the type of contact metamorphism in which much material is added to the rock by the hydrothermal fluids.

During metasomatism, the composition of the parent rock is changed substantially but its volume value remains unchanged, the texture and structure of the original rock are usually preserved.

During the contact metamorphism, it is particularly effective. The products found through the metamorphism is named as the “Skarns”. This made because of the contact of the granite with the limestone. When magma which is high in water and other volatile components comes in contact with limestone, a variety of minerals like magnetite, Garnet, and enstatite are formed. Some metalliferous deposits like lead, zinc, copper, and iron also found with “skarns”.

The susceptibility of rock to metasomatism depends partly on its mineralogical and structural characteristics and partly on the characteristics of the fluid (i.e., temperature, pressure, pH, oxygen-reduction potential, and
water/rock ratio). Hydrothermal activity and metasomatism often go hand in hand. In many cases, the dissolved ions that precipitate to form hydrothermal mineral deposits are derived from the
interaction of these waters and the rocks through which they have moved.

A special type of metasomatism takes place where a hot pluto intrudes into carbonate rock such as limestone. Magmatic fluids rich in silica, calcium, magnesium, iron, and other elements dramatically change the chemistry of the limestone, forming minerals that would not normally exist in either the igneous rock or limestone. A rock called skarn results, containing minerals such as garnet, epidote, magnetite, and pyroxene, among others.

Where does Metasomatism occur?

The metasomatic replacement of minerals takes place generally at the atomic level and in solid-state. The availability of the chemically active fluid is the most important aspect of metasomatic replacement to be done.

There are 2 main places where it actually occurs.

  1. Inside the rock – The metasomatism can happen in the pore fluids, in which end metasomatic replacement could form the new mineral. The total composition of the rock-forming minerals usually remains the same. This is also known as the mineral metasomatism.
  2. Outside the rock – When the magmatic solutions get mixed with the groundwater then it is capable to form the new minerals. The composition of the change in the mineral composition is bulk but the volume remains the same. Metasomatic changes that happen outside the rock are also referred to as the “Rock Metasomatism”.

The process of the metasomatism further distinguished in the following terms.

  1. Hydrothermal – When the fluids are in the form of solutions.
  2. addictive – When the net result of the process is the addition of the new constitutes.
  3. Expulsive– When some component gets moved from the original composition of the rocks.

The original minerals in these rocks have been altered to new minerals by chemical replacement through interaction with the ore-forming hydrothermal fluids. An understanding of the nature of metasomatic alteration is useful because it can yield information about the origins and physical conditions of the ore-forming fluids.

What is the difference between metamorphism and metasomatism?

Metamorphism and metasomatism both processes are involved in change the structure of the chemical. Both work on the same principle but still, they have some differences. We will decide this in deep.

Metamorphism is the process in which the structure and mineral composition of the earth is changed by the agents of metamorphism like temperature, pressure, and chemically active fluids, while the metasomatism is the process in which the mineral bodies to get replaced by the chemically active fluids which main sources are igneous magma.

It is the process in which also some metamorphic changes are done in the chemical structure but the bulk chemical chemistry remains the same. So the basic difference between metamorphism and metasomatism is in metamorphism whole structure gets changed but in metasomatism changes done but its chemical composition remains constant.

When they change because of an increase in the pressure and/or temperature of their surroundings, it is
called metamorphism. When metamorphism is accompanied by a change in the chemistry of a rock, the rock is said to have been metasomatized.

Metasomatic (Replacement Deposits)

Deposits that are formed due to the process of the metasomatism or you may call the actions of the chemically active fluids in a host or country rock is termed as the metasomatic deposits. The basic difference between the magmatic and replacement deposits is in the process of the replacement of the original component by a new component in a solid-state change.

Replacement deposits usually occur in the forms of the veins, lodes, or zones. These are commonly termed as the sheet lodes and sheet zones. The factors which are responsible for the creation of metasomatic deposits are. These types of deposits contain some minerals that have a high economic value.

  1. Preservation of original rock structure.
  2.  Presence of the small-sized fragments of the original rock.
  3. Typical outlines of the ore bodies.

During the contact metamorphism process, the Metasomatism process is more active at that time. The scarns are the products of the metasomatic replacement which formed due to the contact of the Granites with the Limestones. When granitic magma which is usually rich in the magma mixed water and volatile components comes in contact with limestones, then a variety of the minerals like magnetite, Garnet, etc are formed. Some metals which generally have a high economic value like lead, zinc, copper, and iron also formed due to the metasomatic replacement process.

Metasomatic deposits contain some useful metals and minerals like Lithium, thorium, tin, tungsten, Uranium, Quartz, Flourite, Garnet, Topaz, etc.



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