As we know, an ore is composed of ore minerals and gangue, which can be utilised for a profitable extraction of one or mere metallic compounds or metals. The entire crust of the earth consists of minerals. They occur as solid masses, or rocks of which the earth’s crust is composed, or as local accumulator s of varying size, such as veins, pockets or impregnations in rocks.
The processes of formation of mineral deposits are grouped into three main types:
Each type of these processes includes a number of subsidiary processes associated with them. Mineral deposits formed due to the verious processes associated with magmatic activities are called ‘Primary-Mineral Deposits’. These are also called ‘hypogene-deposits’ Mineral deposits arising out of the processes of weathering, and activities of several geological agents are called ‘Secondary Mineral Deposits’. These are closely associated with the sedimentary processes of formation. Metamorphic mineral deposits are the outcome of metamorphic processes acting upon an earlier formed mineral deposits or rocks.
(A) The Magmatic process of formation of mineral deposits include the following processes:
1. Magmatic concentration.
2. Pegmatite (pneumatolytic).
3. Contact-metasomatic process.
4. Hydrothermal processes.
1. Magmatic concentration:
As we know, magma consists of a multitude of constituents, which are in mutual solution. As the magma approaches the earth’s surface its temperature and the external pressure drop, with the result of crystallization and differentiation of minerals in a definite sequence. The formation temperature of different magmatic deposits varies from 1500’C to 300C°.
The magmatic deposits are classified into two major groups, viz., (i) Early magmatic deposit, and (ii) Late magmatic deposit. The early magmatic deposits are believed to have been formed simultaneously with the host-rock, whereas the late magmatic deposits are formed towards the close of the magmatic deposits.
The early magmatic deposits are usually formed by
(a) Simple crystallization without concentration.
(b) Segregation of early formed crystals.
(c) Injection of material concentrated elsewhere by differentiation.
It involves simple crystallization whereby early formed crystals are found disseminated throughout the host- rock.
Diamond pipes of South Africa, the Uranium minerals in the Singhbhum granites in Bihar (Jaduguda) are the examples.
This type of magmatic concentration is often due to the gravitative crystallization of early formed heavy minerals, e.g., Bushveld chromite deposits (South Africa), chromite deposits of Keonjhar (Orissa).
In this case, the metallic concentrates instead of remaining -at the place of their original accumulation, get injected into the adjacent solid rock-masses It occurs at the residual magmatic stage, e.g., Magnetite deposits of Kiruna (Sweden).
The late magmatic deposits are the consolidated parts of the igneous fractions that remained after the crystallization of the early formed rock-silicates. These deposits are formed by the following processes:
(a) Residual liquid segregation:
Basic magmas undergoing differentiation may sometimes become enriched in iron and titanium. This residual liquid may drain out from the crystal interstices and consolidate without further movement. The host rocks are usually anorthosite, norite, gabbro etc, e.g.. Titaniferous magnetite bands of Bushveld complex.
(b) Residual liquid injection:
In such cases, residual liquid be squeezed out towards places of less pressure into the neigh- ourmg rockmass or the interstitial liquid may be filter pressed out forming late magmatic injections, e.g., Titaniferous magnetite deposits Adirondack region of New York,
(c) Immiscible liquid segregation:
Sometimes magma of an ore-and-silicate composition breaks down during cooling into two immiscible fractions which accumulate to form liquid segregation deposits e.g. Sulphide minerals usually associated with platinum, gold, silver copper etc.
The immiscible liquid accumulations before consolidation when subjected to disturbances, get injected into the surrounding rocks, forming immiscible liquid-injection. A nickeliferous sulphide deposit of Sudbury (U.S.A.) is an important example of this type.
2. Pegmatitic deposits:
These are formed towards the very end of consolidation of the magma, in which the residual fraction is highly enriched with volatile constituents. Pegmatitic liquids may be squeezed out to fill in the cracks and fissures in the parent igneous body or the adjoining country rocks, and form pegmatite veins or dykes. These are usually formed between 500°C to 800″C.
Deposits of mica, feldspar, beryl, lithium minerals and tin mineral like cassiterite are included in the pegmatite deposits. Besides, columbium, tantalum, thorium are the examples.
3. Contact metasomatic deposits:
In the neighbourhood of invading magmas, alteration and replacement of the country rocks due to invasion of magmatic emanations may sometimes lead to the development of mineral deposits of economic importance. This process of formation of mineral deposits has been described as pyrometasomatism by Lindgren and as contact-metasomatism by Bateman.
In this case, the enclosing country rock is altered by the heat and other chemical constituents of the invading intrusive magma forming new minerals under conditions of high temperature and pressure. The deposits are usually resulted in calcareous rocks. The temperature of formation ranges from 400°C to 10008C.
The gangue minerals in these deposits comprise an assemblage of high temperature metamorphic minerals, called ‘skarn’, which are usually silicates of iron, magnesium, calcium and aluminium, depending upon the nature of country rock.
Thus deep seated batholithic masses of intermediate composition occurring within pure or impure carbonate country rocks serve as the most suitable locations, where the process of contact metasomatism can operate efficiently and lead to the development of mineral deposits of economic importance. Thus contact metasomatic deposits are formed in nature. Examples are deposits of cassiterite, zinc, magnetite, graphite and sulphides of copper, iron, lead etc.
4. Hydrothermal deposits:
As we know, a magma due to the alteration of physicochemical conditions gradually cools down, producing rock-forming silicate minerals under different conditions of temperature and pressure. It also gives rise to a segregation of residual solutions enclosed within that parent rock which is obvious form the Bowen’s reaction series.
Thus towards the end of the process of crystallization, the once widely dispersed gases and metals have collected near the top of the intrusive body. In moving up, the gases ooze and stream through the magma and collect some of the metals in their journey. At this stage pressure may force the gases and their dissolved rare elements to leave the magma chamber and to move along zones of weakness towards the surface. Such fluids may begin their journey upwards as liquids or a gas which later becomes liquid and this hot water solution is known as hydrothermal solution.
Such hydrothermal solutions are important in the formation of certain kinds of mineral deposits, as they carry out metals from the consolidating intrusive to the site of deposition.
They are mostly epigenetic deposits. Since hydrothermal solution is originated from the magma, its temperature is about 350°C and it is under very high pressure. It is acidic in nature containing non-metallic and volatile constituents mostly. Metallic constituents ot iron, zinc, nickel, gold, silver, lead etc. also find their places in ‘he hydrothermal solution.
Causes of deposition:
1. Changes in the temperature.
2. Changes in the pressure of the system.
3. Exchange reactions between the substances in the solution.
4. Exchange reactions following mixing of solution.
5. Exchange reaction between solution and wall rocks.
6. Changes in the pH of the medium (which determines the acidity/alkalinity of the medium).
7. Coagulation of the colloids, which is brought about by exchange reaction, by breaking down of complexion, by the action of electrolytes arising from exchange reactions and sometimes by supersaturation or super-cooling of the solution.
8. Filtration effect, which helps in the precipitation of components when hydrothermal solutions filter through poorly permeable rocks and mineralize the rocks in front of such barriers.
Classification of Hydrothermal Deposits:
On the basis of the following two factors the hydrothermal .deposits are classified:
(a) Temperature of formation.
(b) Mode of formation.
(a) On the basis of temperature and pressure, the depth of formation and the distance from the magmatic source, the hydro- thermal deposits are of the following types:
(i) Hypothermal deposits:
These deposits are formed at great depths, near the intrusive and within the temperature range of 300°C to 5000C
(ii) Mesothermal deposits:
They are formed at a depth of 1500-4000 metres below the surface and within the temperature range of 200-300°C. The pressure ranges from 140 to 400 atmosphere.
(iii) Epithermal deposits:
These are formed at shallow depths (further away from the surface). The temperature range is from 50oC to 200°C.
(iv) Telethermal deposits:
These are formed under low temperature and pressure, far away from the parent igneous body with which their genetic relationship is not well established.
(v) Xenothermal deposits:
These are formed by high-temperature ore forming fluids expelled from huge igneous rock masses, which have intruded into shallow depths. Thus they are characterise by high temperature, shallow depth of formation and rapid cooling.
(b) On the basis of the mode of formation, the hydrotherma deposits are of two types:
(i) Cavity-filling deposits,
(ii) Replacement deposits.
(i) Cavity-filling deposits:
Cavities occur naturally in the s fractures or fractured zones along which the crust has been a sheared or displaced. In the cavities, the metallic minerals are carried by the hydrothermal solution get deposited under thermal condition.
Cavities are of two types:
1. Original cavities, which include pore spaces, crystal lattices, vesicles, lava drain channel, cooling cracks, bedding plane and igneous breccia cavity.
2. Induced cavities, which include volcanic pipes, shear-zone cavities, solution caves, collapse breccia, tectonic breccia and cavities due to folding and warping.
Deposition of layers of different minerals upon the walls of a cavity leads to the development of crustification. Layers of crystals which are developed within the cavities, give rise to what is called comb structure. The open spaces left after the filling up of the cavities with mineral deposits are known as vugs or drosses. If cavity filling deposits are composed of one mineral only and devoid of crustification, they are said to be massive.
The cavity-filling deposits often include the fillings of fissure veins which range in shape, size and form between wide limits, The terms like stock-work, saddle-reef, ladder-vein, gash veins, pitches and flats etc. are used accordingly.
Masses of country-rocks are often enclosed within the fissure vein deposits and are known as horses.
(ii) Metasomatic replacement deposits form, when the hydro- thermal solutions react with some mineral or substances in the crust, dissolving one substance and replacing it with the ore condition Mostly these deposits are formed under hypothermal condition
The chemical composition and physical characteristics of the host rocks or minerals and the composition, temperature and pressure of the invading mineralising solutions determine the
Metasomatic replacement deposits are characterised by:
(a) Presence of remnants of the country-rock.
(b)Presence of pseudomorphs of replacing minerals after the replaced ones.
(c) Absence of crustification etc.
This process is generally associated with volcanism and fumaroles. These are sublimates deposited at or near the surface at low temperature and pressure due to the sudden cooling of the vapours emanating from volcanoes or fumaroles. These deposits are generally very small and rather superficial and are seldom useful from the economic point of view. The sulphur deposits are the best examples of deposits formed by this process.