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BGYCT-133: Crystallography, Mineralogy and Economic Geology

BGYCT-133: Crystallography, Mineralogy and Economic Geology

IGNOU Solved Assignment Solution for 2023

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Assignment Solution

Assignment Code: BGYCT-133/TMA/2023

Course Code: BGYCT-133

Assignment Name: Crystallogarphy, Mineralogy and Economic Geology

Year: 2023

Verification Status: Verified by Professor


Part A


1. Write short notes on the following:


a) Parts of a crystal (5)

Ans) Faces: Faces are the flat parts of a crystal that hold it together. In the crystal structure, the faces are parallel to the net planes. In most of the cases, the faces are flat but sometimes they are curved, e.g., siderite, dolomite, diamond, etc. If all the faces of a crystal are similar, the crystal is said to have ‘like faces,’ e.g., galena. On the other hand, a crystal with "unlike faces," like zircon, has faces that are not the same. The only thing that makes them different is how they look. You should remember that the fact that the sizes of similar faces on natural crystals can vary does not mean that the faces are different. All the six faces of a cube for example galena are like faces and all the faces possess similar properties. It is called "single form" if the crystal has only faces that are the same. It is called a "combination form" when there are both like faces and different faces.


Edges: The edges are made when two faces touch each other. The edge is naturally straight, and where it goes on a crystal depends only on where the other faces are.


Solid Angle: When three or more than three faces come together, they make a solid angle. Another name for a solid angle is a vertex. The formula, shows how these elements in a crystal are linked to each other.

f + c = e + 2 where,

  1. number of faces

  2. number of solid angles

  3. number of edges


Zone and Zone Axis: The study of crystals shows that many of their faces are set up so that when they meet, the edges that are made are parallel to each other. Each group of edges like these is part of a group of faces. A zone is made up of a set of faces whose intersection makes parallel edges. The edges of a zone are all parallel to one imaginary line in a crystal. This line is chosen. The line or axis that runs through these edges is called the "zone axis." A quartz crystal has edges and faces that run along the c-axis. These edges and faces make a zone, and they are all parallel to the zone's axis, which in this case is the c-axis. In a crystal, there may be many zones and zone axes.


b) Development of mineral classification scheme (5)

Ans) Mineral classification was primarily non-scientific before. Georgius Agricola, the "Father of Mineralogy," released "Agricola's De re metallica" after his death, the first scientific mineral classification.


More accurate physical property observations were improving mineral classifications based on physical attributes. In the early 19th century, substantial chemical research of natural materials led to hybrid mineral classification based on both physical and chemical features. Chemical categorization supplanted hybrid classification as knowledge increased.


Most mineralogists classified minerals into chemical families at that time. Two and a half centuries later, Swedish chemist and mineralogist Jöns Berzelius created the modern categorization system. In 1824, J.J. Berzelius introduced chemical composition classification. Minerals' major anion group determines their Berzelian classification. All minerals in a class share the same anion group, making them chemically identical. Physical properties, kind of cations, water or hydroxyl anions, and mineral structure are used to further separate mineral classes. Minerals are classed into native elements, sulphides and sulfosalts, oxides and hydroxides, halides, carbonates, nitrates, borates, sulphates, phosphates, and silicates.


Later in the nineteenth century, American mineralogist James Dwight Dana refined Berzelius's classification system, which American geologists Brian Mason and L.G. Berry simplified. In 1854, Yale University Professor J.D. Dana proposed a chemical composition-based mineral classification system, which is now used. The Dana classification system is most popular. This approach divides minerals into eight chemically distinct classes. These are native elements, silicates, oxides, sulphides, sulphates, halides, carbonates, phosphates, and mineraloids.


Although amended and modified, the Dana system of mineral categorization remains the core. Later, it was discovered that chemical composition alone cannot classify a mineral. British physicist Lawrence Bragg discovered X-rays, which allowed crystalline substance atomic arrangement to be determined. Bragg and Norwegian mineralogist Victor Goldschmidt grouped silicate minerals by atomic structure. Professor Hugo Strunz proposed chemical-structural mineral categorization in 1941, combining crystallography and chemical composition. Strunz classification. He classified minerals into eleven groups: elements; sulphides and sulphosalts; halides; oxides; carbonates; borates; suplhates; phosphates, arsenates, and vanadates; silicates; and organic compounds. In his classification system, minerals are sorted into families based on their intrinsic atomic structure and then subdivided into comparable families.


The current version of the Strunz categorization system, Nickel-Strunz, has been modified in light of recent crystalstructure measurements. Minerals are classified into ten compositional classes: elements; sulphides; halides; oxides; nitrates, carbonates; borates; sulphates; phosphates; silicates; and organic compounds. Chemical composition and crystal structure separate these classes into divisions, families, and groups.


2. Define symmetry and describe the three elements of symmetry with the help of suitable diagrams. (10)

Ans) The concept of symmetry explains how similar objects (known as motifs) are repeated systematically in space to produce ordered structures where all objects have specific and predictable positions. Symmetry operations "act" on a given object to produce sets of identical objects in prescribed positions.


Elements of Symmetry


Plane of Symmetry: The imaginary plane of symmetry separates the crystal into two mirror-image halves. Cube symmetry is 9-fold. The cube faces identify them. Their dissected planes clearly indicate their relative positions. Cubic system has 9 planes of symmetry, 3 axial and 6 diagonal. Diagonal symmetry planes can be horizontal or vertical.


Axis of Symmetry: The Axis of Symmetry is the rotation axis if a crystal rotates 360°(n#1) and occupies the same location twice. Four symmetry axes exist:

  1. Diad axis of two-fold symmetry, where n = 2, which means that the same view is in the same place once every 180° on rotation axes. So, in one full turn of 360°, the same position is reached twice.

  2. Triad axis of three-fold symmetry, where n = 3, which means that the same view is in the same place once every 120° on rotation axes. So, in one full turn of 360°, the same position happens three times.

  3. Tetrad axis of four-fold symmetry, where n=4: that is, the same view is in the same place on rotation axes once every 90°. So, the same position happens four times in one full turn around 360°.

  4. Hexad axis of six-fold symmetry, where n=6: i.e., the same view is in the same place on rotation axes once every 60°. So, the same position happens six times in one full turn around 360°.


There is no such thing as five-fold symmetry in nature.


Symmetry elements have a special relationship with the way the atoms inside a crystal are arranged. Because of this, they are the basis for putting crystal systems into 32 different classes of symmetry.


Now that you know what axes of symmetry are, let's look at another cube to see how they work. There are 13 axes of symmetry in a cube. Three of them are fourfold (tetrad), four are threefold (triad), and six are twofold (diad).

Centre of Symmetry: The centre of symmetry is the third part of symmetry. The symmetry with respect to a point is the symmetry at the centre of symmetry. A crystal is said to have a centre of symmetry when the edges of its faces are in the same place on opposite sides of its centre. If you draw a line through this point in the middle, points that are the same on both sides are the same distance away. When two faces are next to each other on opposite sides of a crystal, this is called the Center of symmetry. For a crystal to have a centre, all of its faces must be in parallel pairs. A regular cube has a centre of symmetry as an example. If we take away one corner of a cube, there is no longer a centre of symmetry. In the same way, there is no centre of symmetry in a tetrahedron


3. Discuss the crystallographic axes, symmetry elements and forms of normal class of hexagonal crystal system with the help of neat well labeled diagrams. (10)

Ans) Crystallographic Axes: Minerals with a hexagonal crystal system have three crystallographic axes that cross each other at an angle of 120o. The fourth "c" axis is vertical, not the same length as the other three, and perpendicular to them. It goes in the opposite direction of the plane with the horizontal axes. Both happy and sad endings are marked. The front end of a2 is negative.


Because the hexagonal system has four axes, each symbol will have four digits.

If you hold the crystal and turn it along the "C" axis, you will see that the rectangular face appears six times. This is what is called "one axis of six-fold symmetry." All hexagonal crystals have a single axis of rotation that can turn in six directions.


  1. Basal Pinacoid: It has two faces and an open shape. It is written as 0001. Each face is parallel to three horizontal crystallographic axes and cuts along a vertical axis. Since it is an open form, it is often used with other forms.

  2. Hexagonal Prism: It has six sides that are all open. Each face is parallel to the vertical axis and cuts all three horizontal axes, one at unit and the other two at twice the distance. The horizontal axes meet the vertical faces in the middle.


Beryl is made up of a hexagonal prism (100) and a pinacoid at the bottom (0001).

4. Describe the physical properties of minerals belonging to quarts and garnet groups of minerals giving suitable examples. (10)

Ans) Quartz and garnet are two distinct mineral groups that possess different physical properties. The physical properties of minerals belonging to the quartz and garnet groups are:


Quartz Group:

The quartz group of minerals is a group of silicate minerals that includes quartz, chalcedony, opal, and agate. The physical properties of minerals belonging to the quartz group are:

  1. Hardness: Minerals belonging to the quartz group are known for their high hardness. Quartz, for example, has a hardness of 7 on the Mohs scale, which means that it is scratch-resistant and can be used to scratch other minerals.

  2. Colour: The colour of minerals in the quartz group can vary widely, depending on the impurities present in the mineral. Quartz, for example, can be colourless, white, grey, brown, or pink, depending on the impurities present.

  3. Transparency: Minerals in the quartz group are typically transparent to translucent, meaning that they allow light to pass through them. Quartz, for example, is transparent to translucent, which makes it a popular gemstone.

  4. Cleavage: Minerals in the quartz group do not exhibit cleavage, which means that they do not break along planes of weakness. Instead, they fracture along irregular planes.


Garnet Group:

The garnet group of minerals is a group of silicate minerals that includes almandine, pyrope, spessartine, and grossular. The physical properties of minerals belonging to the garnet group are:

  1. Hardness: Minerals belonging to the garnet group are also known for their high hardness. Almandine, for example, has a hardness of 7.5 on the Mohs scale, which makes it a scratch-resistant mineral.

  2. Colour: The colour of minerals in the garnet group can vary widely, depending on the impurities present in the mineral. Almandine, for example, is typically reddish-brown to brownish-red in colour.

  3. Transparency: Minerals in the garnet group are typically translucent to opaque, meaning that they do not allow light to pass through them. Almandine, for example, is opaque.

  4. Cleavage: Minerals in the garnet group exhibit poor to absent cleavage, which means that they do not break along planes of weakness. Instead, they fracture along irregular planes.


In conclusion, the physical properties of minerals belonging to the quartz and garnet groups differ in terms of hardness, colour, transparency, and cleavage. Quartz minerals are typically transparent to translucent, have a hardness of 7 on the Mohs scale, and do not exhibit cleavage. Garnet minerals, on the other hand, are typically translucent to opaque, have a hardness of 7.5 on the Mohs scale, and exhibit poor to absent cleavage. Examples of minerals in the quartz group include quartz, chalcedony, opal, and agate, while examples of minerals in the garnet group include almandine, pyrope, spessartine, and grossular.

5 Describe the physical properties of minerals based on senesces and forces. (10)

Ans) The physical properties of minerals based on senesce are:


a) Feel

Some minerals can be identified by how they feel in our hands, as well as by their colour, shape, and size. When you touch a mineral with your bare hands, you can feel this property. Some minerals have their own unique qualities, like:

1.Soapy: Some minerals, like talc, have a soapy feel to them.

2. Greasy: When you touch some minerals, like graphite, they feel like grease.

3. Smooth and Rough: Some minerals, like chromite, feel smooth to the touch, while others, like talc, feel rough.


b) Taste

You can tell which minerals dissolve in water by how they taste. But it could be dangerous to test this property in a classroom or lab because some minerals are poisonous. So, you should not put minerals on or in your mouth. Based on how minerals taste, Gribble (1991) has called them the following:

1.Saline: Some minerals such as halite (NaCl) taste salty which is common salt.

2.Alkaline: Potash and soda taste alkaline.

3. Cooling: Nitre or potassium chlorate give cooling taste.

4. Astringent or Puckering: The taste of green vitriol, which is hydrated iron sulphate, is astringent, while the taste of alum is sweet and astringent.

5. Bitter: Some minerals taste salty and bitter, like sylvite (KCl) and epsom salt (hydrated magnesium sulphate).

6.Sour: The taste of sulphuric acid is sour.


c) Odour

Most minerals have no smell, but when they are rubbed, hit, heated, or breathed on, they give off a smell that could be used to identify them.


The physical properties of minerals based on forces are:



Minerals change when heated. Some minerals melt at low temperatures and others at high temperatures at atmospheric pressure. Each mineral has a melting point, the temperature or heat required to melt. German mineralogist Wolfgang Xavier Franz Ritter von Kobell developed a fusibility scale. Mineralogy uses this scale to determine mineral fusibility (fusion temperature). The scale consists of six minerals listed in order of their estimated melting temperatures (Gribble 1991).

1.Stibnite (525° C)

2.Natrolite (965° C)

3.Almandine garnet (1,200° C)

4. Actinolite (1,296° C)

5.Orthoclase (1,300° C); and

6.Bronzite (1380° C).



Magnetized minerals attract or repel magnetic objects. This could identify these minerals. Most iron-containing minerals are magnetic, but not all. Magnetism is not connected to iron content either. Minerals are non-magnetic, somewhat magnetic, or very magnetic. It is crucial to recognise the differences between magnetic types, even when they're hard to distinguish.


Magnetism classifies minerals into the following categories:

  1. Diamagnetic (non-magnetic): Minerals like quartz, calcite, and most minerals that do not attract a magnetic field.

  2. Paramagnetic: Minerals that are attracted to a magnetic field as long as it is there. Paramagnetic minerals can also be broken down into:

  3. Strongly Magnetic/Ferromagnetic: These minerals, like magnetite and native iron, are the most magnetic.

  4. Moderately Magnetic: These minerals, like ilmenite, siderite, hematite, and chromite, are not very magnetic.

  5. Weakly Magnetic: Some examples of these minerals are tourmaline, monazite, and some types of hematite.


c) Electricity

Some minerals can conduct electricity, which distinguishes them. Most minerals that appear like native metals (such copper, silver, and gold) or sulphides transmit electricity, except for sphalerite, which has a non-metallic brilliance. Non-conductors, semiconductors, and conductors are mineral classes based on electrical conductivity.


It has been noticed that some minerals get an electric charge.

  1. when they are put under stress, like when quartz is put under stress.

  2. When they get hot, like tourmaline, they become electrically charged.



Uraninite, pitchblende, thorianite, and autunite contain radioactive elements. Radioactive U and Th isotopes like U238, U235, and Th232 break down into daughter elements and release energy as alpha, beta, and gamma radiation. Geiger-Müller counters, scintillometers, and radon detectors measure radiation.


e)Solubility in Acid or Reaction to Acid

A drop of watered-down hydrochloric acid (HCL) on fresh mineral surfaces can also identify them. Calcite, aragonite, strontianite, and other carbonate minerals bubble or effervesce, making this test valuable. Hot HCL bubbles dolomite, rodochrosite, magnesite, and siderite.

Part B


6. Explain the oxidation and supergene enrichment processes of ore formation giving suitable examples. (10)

Ans) Ore formation is a complex geological process that involves a combination of physical, chemical, and biological processes. Two important processes of ore formation are oxidation and supergene enrichment are:



Oxidation is a process that involves the reaction of minerals with oxygen and water to produce new minerals. This process is commonly observed in sulphide deposits, where sulphide minerals such as pyrite (FeS2) react with oxygen and water to form new minerals such as iron oxide (Fe2O3) and sulfuric acid (H2SO4).


For example, the La Escondida copper deposit in Chile is a sulphide deposit that has undergone extensive oxidation. The original sulphide minerals, which included chalcopyrite (CuFeS2) and bornite (Cu5FeS4), reacted with oxygen and water to form new minerals such as malachite (Cu2CO3(OH)2) and azurite (Cu3(CO3)2(OH)2).


Supergene Enrichment:

Supergene enrichment is a process that involves the leaching of minerals from the weathered surface of a deposit and their concentration in the underlying bedrock. This process is commonly observed in deposits that contain copper, gold, and silver.


For example, the Butte copper deposit in Montana is a deposit that has undergone extensive supergene enrichment. The original deposit contained chalcocite (Cu2S), which was leached from the weathered surface of the deposit and deposited in the underlying bedrock as chalcopyrite (CuFeS2).


Another example of supergene enrichment is the formation of gold nuggets in placer deposits. Placer deposits are formed when minerals such as gold are eroded from their original source and deposited in streams and rivers. Over time, the gold nuggets undergo supergene enrichment as they are exposed to oxygen and water, which dissolve the surrounding minerals and concentrate the gold.


In conclusion, oxidation and supergene enrichment are important processes of ore formation that involve the reaction of minerals with oxygen and water. These processes can result in the formation of new minerals and the concentration of minerals in the underlying bedrock. Examples of oxidation and supergene enrichment include the La Escondida copper deposit and the Butte copper deposit, as well as the formation of gold nuggets in placer deposits.

7. Write short notes on the following:


a) Optical properties of calcite (5)

Ans) Calcite is a mineral that belongs to the carbonate group and has a chemical formula of CaCO3. It is widely distributed in nature and is commonly found in sedimentary rocks, such as limestone, marble, and chalk. Calcite exhibits a range of interesting optical properties due to its unique crystal structure.


One of the most noticeable optical properties of calcite is its birefringence or double refraction. This means that when a beam of light enters calcite, it is split into two separate beams, each with a different refractive index. These two beams travel through the crystal at different speeds and in different directions. The amount of birefringence in calcite is dependent on the thickness of the crystal and the orientation of the crystal structure relative to the direction of light propagation.


Another important optical property of calcite is its high dispersion or ability to separate white light into its component colors. This is because the refractive index of calcite varies with the wavelength of light. When a beam of white light enters a calcite crystal, it is split into its different colors, which are then separated and visible as a spectrum.


Calcite also exhibits pleochroism, meaning that it can show different colors when viewed from different angles. This property is due to the crystal structure of calcite, which allows it to absorb light differently depending on the polarization direction and wavelength. When viewed under a polarizing microscope, calcite can display a range of colors, from yellow to blue to green. Calcite is also fluorescent, meaning that it emits light when exposed to certain wavelengths of light. The fluorescence of calcite can range from blue to red, depending on the impurities present in the crystal.


In addition, calcite is anisotropic, meaning that it has different optical properties in different directions. This is due to its crystal structure, which has three axes of symmetry. When light passes through calcite in different directions, its polarization and direction of propagation are altered in a unique way. Overall, the optical properties of calcite make it a fascinating and important mineral in the fields of geology, mineralogy, and optics. Its unique properties have been used for a variety of applications, such as in the manufacture of polarizing filters, as a gemstone, and in the study of crystallography.


b) Isotropic and isotropic minerals (5)

Ans) Isotropic substances: This group is made up of things whose refractive index does not change depending on which way light is travelling. The isotropic substances have a single refractive index that stays the same for all wavelengths. Minerals that form in the isometric/cubic system have the most symmetry because all three axes are equal and can be switched around. This means that minerals that form crystals in a cubic or isometric system are isotropic. Isotropic materials include things like glass, liquids, gases, and solids that don't have crystals. Since air is a gas, it is the same everywhere.

Usually, the index of refraction of air is 1.0. If you put a blank glass slide on a mount and look at it under plane-polarized light, the field of view will be bright. It will stay bright when you turn the stage. When you bring the analyzer in to cross the nicol and look at it again, the field of view will be completely black. Even if the stage turns 360°, the darkness will stay the same. The glass is isotropic, so it doesn't do anything to light and has double refraction. Minerals that have a cubic structure and the bottom parts of uniaxial minerals act like glass. So, when nicols are crossed in isotropic minerals, the field of view looks completely dark and stays that way even if the stage is turned.


Anisotropic substances: This group includes all crystals except those with an isometric system. Anisotropic minerals have tetragonal, orthorhombic, monoclinic, trigonal, hexagonal, or triclinic systems. Because they have double refraction, anisotropic minerals behave differently when crossed nicols are put in front of them. They let light travel in different directions and at different speeds. In anisotropic substances, the speed of light changes depending on the direction of the crystals. When light goes through an anisotropic crystal, it splits into two polarised rays, called O-ray and E-ray.


These two rays move in planes that are not parallel to each other. So, a crystal has two indices of refraction, one for each polarised ray in a certain direction. Anisotropic materials can be further broken down into uniaxial minerals and biaxial minerals. Minerals that are uniaxial have two refractive indices, while minerals that are biaxial have three refractive indices. When you turn the stage through 360 degrees, you can make two important observations, such as how colours polarise or disappear. You'll notice that when the stage turns four times, a mineral disappears, and between the two places where it disappears, it shows a wide range of polarisation colours.


8. Discuss the optical properties of minerals studied under plain polarised light. (10)

Ans) Minerals have various optical properties that can be studied using a microscope under plain polarised light. The optical properties of minerals studied under plain polarised light are:

  1. Refractive index: The refractive index of a mineral is the ratio of the speed of light in a vacuum to the speed of light in the mineral. The refractive index of a mineral can be determined by measuring the angle of refraction of a light beam passing through the mineral. The refractive index is an important property that helps to identify minerals.

  2. Birefringence: Birefringence is the difference between the refractive indices of a mineral for light polarised in two perpendicular directions. Birefringence is a result of the anisotropic nature of minerals, which means that their physical and chemical properties vary with direction. Birefringence can be observed under plain polarised light as double refraction, where the light passing through the mineral is split into two rays with different refractive indices.

  3. Pleochroism: Pleochroism is the property of minerals to show different colours when viewed along different crystallographic directions. This property is a result of the absorption of different wavelengths of light along different directions in the crystal. Pleochroism can be observed under plain polarised light by rotating the mineral and observing the colour changes.

  4. Extinction angle: Extinction angle is the angle between the crystallographic axis of a mineral and the direction of vibration of polarised light at which the mineral appears dark. The extinction angle can be used to determine the crystallographic orientation of a mineral and to identify its mineral species.

  5. Relief: Relief is the difference in the apparent height of a mineral and the surrounding medium when viewed under a microscope. Relief is a result of the difference in refractive index between the mineral and the surrounding medium. Minerals with higher relief appear to be more elevated than those with lower relief.


The optical properties of minerals studied under plain polarised light include refractive index, birefringence, pleochroism, extinction angle, and relief. These properties are important for identifying minerals and determining their crystallographic orientation. The use of plain polarised light allows for the observation of these optical properties, which provides valuable information for mineral identification and geological studies.


9. Describe chief ores, processes of formation and geographical distribution of manganese ores in India with the help of a neat map. (10)

Ans) Manganese makes up only 0.095% of the Earth's crust, which is much less than iron. So, there are not as many manganese deposits as there are iron ore deposits. Manganese is a metal that is essential for making steel. India is thought to have 93 million metric tonnes of manganese in its reserves.


Chief Ores

Manganese is found as oxides, carbonates, and silicates in many places in the Earth's crust. Manganese doesn't happen naturally in its pure form, but it can be made by electrolysis or by reducing its oxides. It is a hard, brittle, gray-pink metal that melts at about 1260oC. In nature, manganese can be found as an oxide, a carbonate, or a silicate. Manganese ores like pyrolusite and psilomelane are common.

Processes of Formation

Manganese is found in many different types of deposits across a wide range of the Earth's geological history. Manganese deposits are made by hydrothermal activity, sedimentation processes, and the weathering of continental rocks (Roy, 1981). Most of the manganese that is used right now comes from sedimentary and residual deposits. Manganese can also be found in the deep ocean floor of the Indian Ocean as ferromanganese nodules. Deep-sea nodules could be a future source of manganese and other important metals like cobalt, nickel, copper, and others. Manganiferous sediments are changed into bedrock by a process called metamorphism. The manganiferous rocks of the Dharwar Group were changed on the surface, which led to the lateritic deposits. The deposits in India are either beds (both syngenetic and supergenetic) or laterites. The most important manganese ore deposits are sedimentary, stratified metamorphic deposits found in rocks from the Proterozoic era called Gondite and Kodurite.

Distribution in India

Indian manganese ore deposits are mostly metamorphosed bedded sedimentary deposits associated with Gondite Series of Madhya Pradesh in Balaghat, Chhindwara and Jhabua Districts, Maharashtra in Bhandara and Nagpur Districts, Gujarat in Panchmahal District, Odisha in Sundergarh District, and Kodurite Series of Odisha in Ganjam and Koraput Districts and Andhra Pradesh in Srikakulam and Visakha India's states' manganese ore production for the year 2016-17. How manganese ore is spread out in India.


10. Explain the nature and morphology of ore bodies with the help of neat well labelled diagrams.

Ans) Syngenetic deposits are ore deposits that originated at the same time as the enclosing rock, such as an iron-rich sedimentary horizon. Epigenetic deposits, like veins, arise after the host rock. After formation, these ore minerals were added to country rock.


Discordant Ore Bodies


a)Regularly Shaped Ore Bodies


Tabular ore Bodies: These two-dimensional bodies have limited third-dimensional development. Lodes and fissure-veins are here. Infilling vacant gaps caused veins. Lodes replace host rock extensively.

Rock layers deposit ore veins. As they ascend or descend a stratigraphical sequence, veins squeeze and swell. Lode is a metallic ore deposit in a rock fracture. Stringer lode is a metallic ore deposit with many irregular branching and anastomosing stringers and small veinlets. The entire bulk of ore and host rock is extracted (as it is inseparable from the country rock).


Tubular Ore Bodies: Tubular ore bodies are two-dimensional. They are 3D. Pipes or chimneys are vertical or subvertical. Mantos (Spanish for "blanket") are horizontal or subhorizontal tubular ore deposits. Pipes and mantos branch and interconnect. They often occur with feeder pipes. Pipe connections can transport mantos between beds. Some tubular deposits generated by sub-horizontal mineralising fluid flow also form discontinuous pod-shaped entities.


b) Irregular Shaped Ore Bodies


Disseminated Deposits: Disseminated deposits contain ore minerals across the host rock. Zircon and apatite in igneous rocks are analogous to them. Disseminated deposits can create a stock work network along close-spaced veinlets cutting the host rock. This mineralisation fades gradually into sub-economic mineralisation, and the ore body's boundaries are assessed regardless of manner of occurrence. Ore bodies may span geological boundaries and be irregular. Some are cap-shaped and cylindrical. Stock works typically cut over the contact into country rocks in felsic to intermediate plutonic igneous intrusions. Copper and molybdenum are dispersed worldwide.


Irregular Replacement Deposits: Carbonate-rich sediments replace pre-existing rocks to generate ore deposits (e.g., magnesite deposits). High-temperature replacement processes with medium-to-large igneous intrusions occur. These deposits are called contact metamorphic or pyrometasomatic.

Concordant Ore Bodies

Base metals and iron ore deposits depend on sedimentary rocks with concordant ore bodies. They match the bedding. Like Phanerozoic ironstones, they can be stratigraphically important. They may represent epigenetic pore fillers or replacement ore bodies. These ore bodies form mostly parallel to the bedding and seldom perpendicular to it. Thus, stratiform deposits. These deposits are in volcano sedimentary and sedimentary rock formations. Contrast with stratabound. Stratabound ore bodies, concordant or discordant, are limited to a stratigraphic column.


a) Sedimentary Host Rocks

1.Limestone Host Rock: Base metal deposits often host limestone. Dolomitisation or fracture increases permeability in them. Solubility and reactivity make limestone good mineralization horizons.

2.Argillaceous Host Rock: Shale, mudstone, argillite, and slate host concordant ore deposits. German Kuperschiefer deposit hosts Upper Permian shale. Sullivan, British Columbia's Late Precambrian argillites house the world's largest lead-zinc ore deposit.

3.Arenaceous Host Rock: The Zambian Proterozoic Mufulira copper deposit is in altered feldspathic sandstones

4.Audacious Host Rock: Recent and ancient placer deposits include alluvial gravels and conglomerates. Precambrian deposits in South Africa's Witwatersrand goldfields contain the world's gold. By-products include uranium.

5.Chemical Sedimentary Host Rocks: Iron, manganese, evaporates, and phosphorite create broad beds across the stratigraphical column.


b)Igneous Host Rocks


Volcanic Host Rock: Volcanic rock deposits are mostly:


ii)Massive volcanic sulphide deposits.


Volcanic-associated huge sulphide deposits are more common than vesicular filling deposits. They are stratiform, lenticular to sheet-like formations formed at volcanic unit or volcanic sedimentary interfaces.


Plutonic Host Rock: Mafic plutonic igneous intrusions have rhythmic layering. Mafic and felsic minerals alternate to produce layers. Such layered complexes may contain mineable veins of commercial minerals like chromite, magnetite, and ilmenite. In South Africa's Bushveld Complex, chromite seams are stratiform and extend for kilometres.


c)Metamorphic Host Rocks

Metamorphic irregular replacement deposits. Wollastonite, andalusite, garnet, and graphite can form in contact metamorphic aureoles.ASSIGNMENT INFORMATION

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