ADVERTISEMENTS:
In this article we will discuss about:- 1. Introduction to Ore Deposits 2. Origin of Ore Deposits 3. Types.
Introduction to Ore Deposits:
Minerals form chief source of all the industrial metals and non-metals. The commonly used metals like aluminum, copper, iron, lead, zinc etc. are all manufactured using minerals as raw materials. Similarly non-metals like glass and refractories are also made from minerals. Minerals are also source of energy.
Coal, petroleum (in a general sense, called mineral oil) and radioactive minerals like uranium and radium have been and still remain the agents of change in our industrial civilization. In fact the role played by the mineral deposits in the overall progress of any country can hardly be overemphasized.
ADVERTISEMENTS:
Ore mineral is defined as a mineral which contains a metallic element in a quantity that can be exploited and extracted for use at an economical cost. Clays are rich in aluminum but since metal cannot be extracted from ordinary clays at an economical cost, these are not included among the ores of aluminum.
Bauxite, an oxide of aluminum, however, is an ore of aluminum, since the metal Aluminium can be recovered from bauxite by electrolytic refining at economic cost. The definition of an ore is, therefore, technology dependent. If a technology is developed to extract aluminum from clays at an economically viable cost, at a later date, these could also be included among the ores of aluminum.
Ore Deposit:
Natural concentration of an ore mineral in a massive rock body is defined as an ore deposit. It may be of any shape and form but its size is of real importance. A small patch or layer of bauxite, 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 millions of tons, to qualify as an ore deposit. Hence the definition of an ore deposit is also size dependent.
Gangue Minerals:
ADVERTISEMENTS:
A few non-metallic minerals are often found associated with an ore mineral. These have to be separated from the ore before the same is processed for extraction of the metal. These associated minerals are called gangue minerals and are worthless. Quartz (SiO2) is a common gangue mineral associated with many metallic ore minerals.
Origin of Ore Deposits:
Mineral deposits may occur independently forming layers and bodies of different shapes extending for many kilometers in areal extent and upto many meters in depth. Sedimentary deposits of iron oxides and hydrous aluminum oxide are of this type. In other cases, the ore deposits (or more precisely ore minerals) may be present in an enclosing rock, called the country rock or the host rock.
In such cases, the host rock itself is removed in the first stage of mining. The ore minerals are then separated at the second stage of exploitation.
ADVERTISEMENTS:
On the basis of their time of formation relative to the host rock, the ore deposits are divided into two groups:
(i) Syngenetic and
(ii) Epigenetic.
(i) Syngenetic deposits are such deposits that have formed simultaneously with the enclosing rock. These are either of igneous or of sedimentary types.
(ii) Epigenetic deposits are those deposits that were formed subsequent to the formation of the host rock. Epigenetic deposits of igneous, sedimentary and metamorphic types are known.
In another common method of classification, ore minerals are divided into hypogene (also termed primary) deposits and supergene (also known as secondary) deposits. The secondary deposits are formed as alteration products of primary deposits through weathering and related processes.
An ore deposit may be either of igneous or of sedimentary origin. Some deposits are related to metamorphic processes and are as such classified as deposits of metamorphic origin. Within these three broad modes of origin, a mineral deposit of economic value might have originated due to set of simple or complex processes.
Types of Ore Deposits:
1. Igneous Deposits:
These deposits are formed by crystallisation of magmatic melts in different environments.
Principal types are:
ADVERTISEMENTS:
a. Magmatic Deposits and
b. Hydrothermal Deposits.
a. Magmatic Deposits:
During the crystallisation of a magmatic melt, many minerals of economic value also get formed depending on the original composition of the melt. These minerals may get concentrated towards the bottom or margins or other places within the cooling body of the magma because of some factors operating at the time of crystallisation. These concentrations are sometimes of such size and nature that their economic exploitation is feasible. Magmatic deposits form major source of some important metals as gold, chromium, titanium and uranium.
Concentration of the economic magmatic minerals may take place during the earlier or the later stages of the process of crystallisation. Accordingly, these are often distinguished as early or late magmatic crystallizations. Their stage of formation is easily revealed by their relationship with the enclosing rocks.
Magmatic ore deposits commonly occur in the following forms:
i. Segregations,
ii. Disseminations and
iii. Injections.
i. Segregations:
In magmatic segregations, economic minerals get concentrated more or less at the place of their formation such as along bottom or margins of an igneous body. In the earlier stages of crystallisation, when most of the magma is still in a molten state, minerals that happen to form sink to and settle at the bottom due to gravity.
This is naturally referred as gravitative settling. Similarly, towards the later stage, when most of magma has crystallized into solid rock, some left over melt containing economic minerals crystallises along the margins of the solidified magma. This process is referred as residual magmatic segregation.
Chromium and titanium ores are examples of early and late segregations respectively.
ii. Disseminations:
In magmatic disseminations, the economic minerals are dispersed at random in a cooled igneous body. In some cases the dispersion is within a limited space whereas in other cases it is over a large volume when concentration is economically not exploitable. When the mineral of economic value is dispersed only in a limited space in the body of igneous mass and the trend is common over large area, whole of the rock mass is mined for exploitation and extraction of the valuable mineral.
Diamond disseminations are a typical example. This is its mode of occurrence in the famous diamond pipes of South Africa where the mineral occurs as dispersed crystals in ultrabasic rocks called Kimberlites. In India, the Panna diamond mines of Madhya Pradesh are also similar in nature to Kimberlites of South Africa. The main diamondiferous pipe is of volcanic origin and has a width of more than 350 metres, made up primarily of serpentine rock.
iii. Magmatic Injections:
These are thin, lenticular or slightly tabular bodies of magmatic minerals that appear as having been intruded or injected into the host rocks. When magma rises up with sufficient hydrostatic force, it may intrude into available fissures and fractures in the surrounding rocks where eventually it may cool down. Injection deposits may result both as early and late stages of crystallization.
2. 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 pegmatites, the rock is made up primarily of potash felspars (e.g. orthoclase) and quartz whereas in the complex pegmatites, different zones can be distinguished in the rock which is indication of differentiation having taken place as per a definite pattern.
In their form, pegmatites occur as dikes, veins, lenses and nests of variable sizes and dimensions. Some pegmatites are known for the exceptionally large-sized crystals they have yielded. Crystals of mineral beryl, a typically pegmatite mineral, as big as 6 m long, 1.3 m in diameter and 18 tons in weight have been obtained at Maine, at Abany in U.S.A.
These rocks (pegmatites) are often associated with granites (granite-pegmatite). Pegmatites associated with basic and ultra-basic rocks are also known. Pegmatites of a recrystallised type called Metasomatic Replacement Pegmatites are also of common occurrence.
Pegmatites are source of mica, quartz and many strategic minerals such as beryl, lithium minerals, rare earths and numerous gems.
3. 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 rocks, that is, wherever these solutions 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 three essential conditions for the formation of hydrothermal deposits are – highly active and enriched fluids, suitable pathways (cracks, fissures etc.) for their migration through the rocks, and suitable physico-chemical environment for their deposition to take place.
As regards superheated steam, it is an essential component of many magmatic melts and remains dispersing around the magma body till it cools down to a solid mass. The steam is capable of containing within it, in dissolved form, metallic elements like gold, copper, tungsten, molybdenum and to some extent silver, lead and zinc etc. These elements are crystallized out as the enriched solutions pass upwards and suffer cooling and loss of pressure.
The fissures or fractures through which these fluids travel may be of primary or secondary character. Bedding planes, porosity and jointing structures are examples of primary spaces. Faults, solution cavities, fissures and jointing caused by stresses in the host rocks after their formation are examples of common spaces of secondary origin.
The question of describing the physico-chemical environment responsible for deposition of economic minerals from the hydrothermal solution remains yet unanswered. Many hypotheses have been put forward. The problem is of a very complex nature and solution may lie in one or more of the following possibilities.
Deposition may take place due to:
(i) A change in temperature or pressure (or both, of the ascending solutions);
(ii) Ensuring chemical reactions between various components present within these liquid-gas solutions;
(iii) Chemical reactions between the components of the solutions and those of the country rock.
Such factors as change in the pH of the solutions, coagulation of colloids (a form in which elements might be in transport) and effects of natural electric fields have also been mentioned as (possibly) being responsible for deposition of mineral deposits from the hydro-thermal solutions.
Hydrothermal deposits occur in a variety of shapes and sizes.
The most common broad based forms are:
(a) Veins and
(b) Cavity fillings.
(a) Veins:
These are narrow, elongated or tabular bodies of economic minerals occurring within a host rock of entirely different composition and origin.
These are further distinguished into following types:
(i) Fissure-Veins:
These may be defined as mineral bodies of elongated or tabular shape deposited in pre-existing fissures. The original fissures may be parallel or intersecting, radial or fan-shaped in pattern and that is also the shape of the resulting ore bodies.
(ii) Ladder-Veins:
These are commonly found in igneous rocks such as dikes and similar bodies and consist of transverse, roughly regularly spaced fractures that are filled with deposits of economic minerals. These veins give a ladder-like impression in vertical section, hence the name
(iii) Gash-Veins:
These are narrow, sloping and thick-bodies deposits of minerals deposited in solution-fissures or cracks of the host rock which is generally a carbonate rock like limestone.
(iv) Stock Works:
The term is used for hydrothermal deposits that occur in veins or fissures of exceptionally small size, but in good number, within a limited space, traversing the body of rock profusely. Mining in such cases involves removal of the entire rock body containing the stockwork.
(b) Cavity Fillings:
They are such ore bodies that get deposited from the hydrothermal fluids in well- defined open spaces (rather than along fractures or cracks) available in the host rock. A small or big cavity (caused due to any reason) occurring in a rock when got filled with a mineral deposit would qualify as a cavity filling. It may be regular or irregular in outline. The resulting deposit from cavity fillings may be quite massive.
Another interesting example of filling is provided by the so-called saddle-reefs. These are mineralized cavities associated with the folded rocks comprising beds of competent and incompetent nature. In such cases hydrothermal solutions find pathways in the weak zones along axial regions and deposit their minerals in those zones.
4. Metasomatic (Replacement) Deposits:
These deposits are formed essentially by an action of chemically active fluids (gases and liquids) involving simultaneous solution of pre-existing component(s) from a rock and replacement of the same by new mineral(s).
The basic difference between the pure hydrothermal (magmatic) deposits and metasomatic deposits lies in the process of replacement of some original component by a new component in a solid state change.
The metasomatic (replacement) deposits of economic minerals are important source of many metals and non-metals.
The process of metasomatic replacement may be quite simple or complex in nature. Both liquid and gaseous solutions may be individually or jointly responsible for the change to take place. Liquid solutions are, however, credited with a major role in the formation of the ore deposits.
Temperature and pressure are not binding factors for replacement to occur; replacement deposits formed at surface temperature and pressure are well known. But the fact remains that at elevated temperatures, the process of replacement is accelerated in many cases.
Further, though every rock may be susceptible to replacement process, it has been observed to occur commonly in rocks of suitable composition only which belong to the carbonate group, i.e. limestones and dolomites. Igneous rocks have also been found altered by replacement (metasomatic) process. Sandstones, quartzites and argillaceous rocks like clays and shales are least susceptible.
The replacement deposit may occur in the form of veins, lodes or zones. These are commonly termed as sheet lodes or sheet zones.
The metasomatic replacement deposits may be differentiated from the magmatic and hydrothermal deposits by virtue of replacement evidences which include:
(i) Preservation of original rock structure;
(ii) Presence of small sized fragments of the original rocks (still unaltered);
(iii) Typical outlines of the ore bodies.
Metasomatic replacement deposits have been the source of:
(i) Metals – like beryllium, caesium, lithium, niobium, rubidium, tantalum, thorium, tin, tungsten and uranium;
(ii) Non-metals – quartz and fluorite;
(iii) Gemstone – amethyst, aquamarine, garnet and topaz.
5. Sedimentary Deposits:
Some very important deposits of ores of iron, copper, gold, phosphates and coal are of sedimentary origin.
Their accumulation takes place due to one or more of the following processes:
(a) Weathering:
Atmospheric gases and water vapour are incessantly reacting with surface rocks of suitable composition whereby they get decomposed and altered. The process of change is called weathering of rocks. The altered, disintegrated rock product is called mantle of waste.
This mantle is the source material for development of economic mineral deposit which can take place by either of the following two ways:
First:
Most of the useless (economically speaking) materials from the loose mantle is leached away or removed by subsurface waters leaving behind residual material that has economic importance. Such concentrations form Residual Deposits. Bauxite, the well-known ore of aluminium, is a typical residual accumulation in most cases.
Ores of iron like limonite, goethite and hematite and of manganese like psilomelane and pyrolusite are other examples of residual deposits. Similarly, deposit of silicate nickel ore of residual origin is also known. The kyanite deposits of Singhbhum (India) occur in the form of residual boulders.
Second:
The soluble components of mantle of waste are taken into solution by percolating waters and re-deposited at lower levels during the process of infiltration. These deposits, when of economic value and size form Infiltration type of deposits. Among important deposits of this type may be mentioned ores of uranium, silicate-carbonate group ores of iron, some types of bauxite and also hydroxide of iron, native sulphur and hydrous calcium sulphate (gypsum).
(b) Sedimentary Genesis:
Such deposits originate due to set of natural processes in almost an orderly fashion which involves:
(i) Physical and chemical disintegration of rocks by weathering and erosion into sediments.
(ii) Transport of these sediments by wind, water and ice to the suitable environment of deposition.
(iii) Deposition, followed by diagenesis of these sediments into sedimentary formations.
The deposits of pure sedimentary origin occur very commonly in the form of layered formations of extensive areal extent and considerable depth. In economic value they range from non-metallic deposits like sandstones and limestones (useful as building stones), clays, salts of sodium, potassium and magnesium to those of important metals like iron, manganese, aluminium, copper, uranium and vanadium etc. Coals also form a distinct group of sedimentary deposits in which biochemical changes have also taken place during the process of deposition and diagenesis.
(c) Placer Deposits:
These may be broadly described as a special group of weathering deposits that have been accumulated in varied environments by virtue of some particular qualities of the minerals involved. Among such qualities a high density and chemical stability are most important. Similarly good strength and hardness against abrasive actions are also essential qualities.
These economic minerals or substances once released from the original rock masses due to processes of physical and chemical weathering are transported to places of deposition by such factors as gravity, wind, water and ice.
Accordingly, placer deposits are often distinguished on the basis of the agency involved in their transport and deposition, such as:
(i) Deluvial placers, when gravity is the agency involved; such placers would be found along the foot hills and talus slopes;
(ii) Aeolian placers, when sorting and transport is carried out by the agency of wind;
(iii) Alluvial placers, where running water such as of streams and rivers is involved in their transport and deposition;
(iv) Beach placers are deposits of economic value accumulated by waves along shores of water bodies like seas, lakes and oceans.
Placers deposits found resting on the rocks from which they are generally derived are often called eluvial placers.
The formation of placer deposits is primarily due to sorting power of the transporting medium and the inherent physical properties of ore or other minerals. Heavier, stronger and stable minerals have better chances of accumulation and preservation as deposits whereas lighter and chemically unstable minerals get further disintegrated and dispersed.
Placer deposits are the source of such minerals and metals as:
(i) Gold
(ii) Diamond
(iii) Platinum
(iv) Tungsten
(v) Titanium
(vi) Tin
(vii) Garnet
(viii) Magnetite
(ix) Monazite
(x) Zircon
The coastal placers of Australia and beach placers of South India are known to be rich source of many heavy and rare minerals such as monazite, rutile, limonite and zircon. The coastal tracts of Kerala, Tamil Nadu, Andhra Pradesh and Orissa contain extensive deposits of monazite associated with reserves of ilmenite and rutile.
6. Metamorphic Deposits:
Metamorphism is a natural process of change brought out in rocks of all types subjected to changed conditions of temperature, 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 metamorphosed igneous or sedimentary deposits. In a truly metamorphic deposit, however, new minerals or rocks of some economic value are formed as a result of metamorphic change.
Examples of this type include the marble deposits which are formed from metamorphism of limestones, slate deposits formed from shales and refractory minerals like kyanite, sillimanite and andalusite, the insulating minerals like muscovite and asbestos and abrasives like corundum and garnet. Obviously metamorphism generally results in minerals of non-metallic nature, all of which have their own significance in economic terms.