GENERAL GEOLOGYUncategorized

Origin Of Ore Deposits | Processes of Ore Formation

Natural concentration of an ore mineral in an massive rock body is defined as an ore deposit. It may have different shapes and forms but size is of real importance. A small patch or layer of iron, say one meter thick and a kilometer long will not make an ore deposit. The deposit has to be of considerable size and volume, often in terms of million tons, to qualify as an ore deposit.

On the basis of their time of formation relative to host rock, the ore deposits are divided into two groups:

Syngenetic Ore Deposit: The ore deposits that are formed at the same time as the enclosing rock are called “Syngenetic ore deposits”. Sedimentary ore deposits are the examples of syngenetic deposits.

Epigenetic Ore Deposits: The ore deposits are formed later than the enclosing rock are called “Epigenetic ore deposits”. Hydrothermal ore deposits are the examples of epigenetic deposits.

Classification Of Ore Deposits

The ore deposits are formed in many different ways. Depending upon the process that may operate to produce them, the ore deposits may be classified as follows.

1. Magmatic Ore Deposits

The magmatic ore deposits are the magmatic products which crystallize from magmas. They have close relationships with the intrusive igneous rocks. The magmatic ore deposits can be classified as follows.

Early magmatic deposits

Early magmatic deposits are formed during the early stages of the magmatic period. In this case the ore minerals crystallize earlier than the rock silicates. The minerals of nickel, chromium and platinum are usually found as early magmatic deposits. The early magmatic deposits can be subdivided into groups:

  1. Dissemination deposits:- When a magma crystallize under deep seated conditions, a granular igneous rock formed. In such rock early formed crystals of ore minerals may occur in dissemination. Hence grains of ore are found scattered more or less evenly throughout the rock mass.
  2. Segregation deposits:- Early magmatic segregation deposits are formed as a result of gravitation crystallization differentiation.  In such cases, the ore minerals which crystallize early, get concentrated in a particular  part of the igneous mass the ore deposits thus formed are called “Segregation deposits”.

Late magmatic deposits

The ore deposits which are formed towards the close of the magmatic period are called “late magmatic deposits”. The late magmatic deposits contain those ore minerals which have crystallized at rather low temperature from a residual magma. The magma which is left after crystallization of the early formed rock silicates, is called “residual magma”. This magma frequently contains many ore minerals. The late magmatic deposits include most of the magmatic deposits of iron and titanium ores. These deposits are almost always associated with mafic igneous rocks.

The late magmatic deposits have been classified into four groups

  1. Residual Liquid Segregation:– In a magma, particularly the basic magma which is undergoing differentiation, the residual liquid may become enriched in iron and titanium. This heavy residual liquid may segregate and crystallize within the parent igneous mass.
  2. Residual Liquid Segregation:- The iron rich residual liquid accumulated as a result of differentiation of mafic magma, may get injected into the country rock. The ore deposits of magnetite and ilmenite formed in this way.

2. Hydrothermal Deposits

This is another distinct group of economic minerals, which has been formed from cooling of gaseous and liquid solutions in cavities, fissures or pore spaces of the rock, that is wherever these solution find a place to enter. The solvent in such cases is very often superheated steam, emanating from magmas towards the final stages of Crystallization. The hydrothermal solution move through cracks and openings present in the rock and deposit their dissolved minerals there. The epigenetic ore deposits formed by hydrothermal solutions are called “hydrothermal ore deposits”. The ore deposits which are commonly formed by the hydrothermal process are gold, silver, copper, lead, zinc and mercury.

Classification of Hydrothermal Deposits

1. On the bases of temperature of deposition, Lindgren has classified the hydrothermal deposits into three groups

  • Hypothermal Deposits:- These are high temperature deposits which are formed close to the intrusive body. Hence the temperature ranges between 300 to 500 degrees. The chief minerals which are commonly found in hypothermal deposits are arsenopyrite, wolframite, native gold and chalcopyrite.
  • Mesothermal Deposits:- These are the intermediate temperature deposits which are formed at some distance outward from the intrusive igneous mass. Here the temperature ranges between 200 to 300 degrees. The chief ore minerals of mesothermal deposits are native gold, bornite, sphalerite, balena and argentite.
  • Epithermal Deposits:- These are the low temperature deposits formed very much away from the intrusive body. Their temperature of formation ranges between 50 to 200 degrees. The  ore minerals which commonly occur in epithermal deposits are ruby, stibnite, and cinnabar.

2. On the bases of mode of formation the hydrothermal deposits have been classified into two groups

  • Cavity Filling Deposits:- These deposits are formed when hydrothermal solution deposit their dissolved minerals in the various types of opening present  in the rocks. In this deposition no replacement is involved. Such a deposition takes place by change of temperature and pressure of the hydrothermal solution. Most of the epithermal deposits are cavity filling type.
  • Replacement Deposits:- These deposits are formed due to chemical interaction between the hydrothermal solution and the country rock. The ore minerals are deposited from a mineral bearing solution in the place of the country rocks, the later being dissolved and removed in solution.

3. Oxidation And Supergene Enrichment Deposits

If an ore deposit is exposed to the ground surface, it undergoes weathering. The surface water oxidizes and dissolves the ore minerals from the weathered zone and carries them downwards. The dissolved minerals are redeposited just below the groundwater table. In this way a low grade primary ore deposits may be enriched to form valuable ore deposit. Such ore deposits are called “supergene enriched deposits”. Three zones have been recognized in the supergene enrichment deposits

  1. Zone of Oxidation:- The oxidized part of the ore deposit is called the “zone of oxidation”. This zone extends from the ground surface upto the water table. The ore minerals form this part of the ore body are dissolved and removed.
  2. Supergene Enrichment Zone:- This zone lies below the water table. In this zone the dissolved ore minerals are precipitated in the form of secondary sulfides.
  3. Primary Zone:- The lower unaffected part of the ore body is called the “primary zone”.

Gossan

Also See- What is Gossan? 

4. Pegmatite Deposits

These are sometimes classified as a distinct group of magmatic deposits formed towards the end of crystallization process and as such necessarily occurring close to the roofs of magmatic masses. The pegmatites may be either of simple type or of complex type. In the simple type the rock is made up primarily of potash feldspars and quartz whereas in the complex pegmatites different zone can be distinguished in the rock which are indication of differentiation having taken place as per a definite pattern. In their form, pegmatites occur as dikes, veins, lenses and nests of various sizes and dimensions. Pegmatites associated with basic and ultrabasic rocks are also known. Pegmatites of a recrystalline type called Metasomatic Replacement Pegmatites are also of common occurrences.

5. Metamorphic Deposits

Metamorphism, as described elsewhere, is a natural process of change brought out in rocks of all types subjected to changed conditions of temperature and pressure and chemically active fluids. In this process, pre- existing rocks and minerals of economic value may undergo metamorphic changes. Such deposits may be simply called as metamorphic deposits. In a truly metamorphic deposit however new minerals or rocks of some economic value are formed as a result of metamorphic change. Example of this type of deposits are marble, shales and refractory minerals.

6. Sedimentary Deposits

Sedimentation deposits are the syngenetic ore deposits which are formed at the same time as the enclosing rocks. They occur as beds in the sedimentary rocks. Some of the important sedimentary deposits are Iron ore , Manganese ore, Copper ore, Phosphates, limestone, etc. These deposits are formed by the process of sedimentation.

During weathering, the materials are released from the source rock. In this process the valuable minerals constituents are taken into solution. The chief solvents are carbonated water, organic acids and sulfate solutions. Most of the valuable substances are transported either in suspension or in solution by means of river water to the sea. In the sea, the valuable material is deposited mechanically, chemically or biochemically. The chemical precipitation of materials in solution is controlled largely by the pH and Eh of the environment. The pH is responsible for the acidic or alkaline conditions and the Eh for the oxidation- reduction potential.

 

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