ECONOMIC GEOLOGY.pptx

Economic geology - processes

1. ECONOMIC GEOLOGY

2. Economic geology is a branch of geology which deals with different aspects of economic minerals being utilized by mankind to fullfill his various needs. The economic minerals are those which can be extracted profitably. The mineral deposits of a country are its natural wealth upon which depends its development and prosperity. There are mainly two types of ore deposits – Metallic and Non-Metallic. ECONOMIC GEOLOGY

3. Metallic mineral deposits Metallic mineral deposits are ore deposits from which metals they contain, are extracted. In dealing with metalliferous mineral deposits, concept of ore, ore mineral, gangue and tenor of ore are required to be understood. An ore is the mined out material which is profitably used. An ore mineral is the desired metal held in chemical or physical combination with other elements. For example, chalcopyrite which is mined for copper, occurs in combination with iron and sulphur besides copper. Ore minerals may also be found in free state, of which gold and platinum are examples.

4. The gangue is the unwanted or discarded material of ore and it is mainly composed of non-metallic materials or enclosing rock. Thus, an ore is made up of both ore minerals and gangue. Ore = Metal + Gangue Bauxite = Aluminium+Laterite Gold = Gold + Quartz Tenor of ore – it is the metal content of the ore. In order to declare the ore economically workable, a deposit must contains a certain percentage of metal in it and the lowest admissible limit of metallic content of an ore is called its ‘tenor’. The tenor of an ore depends on the price of the metal obtained from it.

5. Nonmetallic mineral deposits The materials for nonmetallic mineral deposits may be solid, liquid or gas. Coal, petroleum, sulphur are some of the examples. The term ore and ore minerals are not used in the case of non metallic minerals. The undesired material if any in the deposit, is generally reffered to as ‘waste ’ and not ‘gangue’. This includes enclosing rocks and discarded non-metallic substances unfit for use due to certain defects. The waste material is removed by handsorting, simple mechanical concentration, or washing.

6. Industrial minerals They are mined for their commercial value which are not fuel minerals but are used in industries based on their physical and chemical properties. Mainly used in constructions, paints, electronics, glass, paper, detergent etc. Eg: graphite (carbon), gypsum, kaolin, talc, mica, limestone.

7. Processes of formation of mineral deposits The mineral deposits may be classified on the basis of their genesis or process that operate to form them. The main geological process that yield mineral deposits are as follows; Magmatic concentration Sedimentation inclusive of Evaporation Metamorphism Contact metasomatism Hydrothermal process Oxidation and supergene sulphide enrichment Sublimation Residual and mechanical concentration Pegmatic deposits The ore deposits that formed at the same time as the rock that enclose them, it may be referred to as syngenetic deposits. The ore deposits that formed later than the rock that contain them, it may be referred to as epigenetic deposits .

8. 1. Magmatic concentration Magmatic deposits are formed during different stages of magmatic crystallization. There are early magmatic deposits and late magmatic deposits. Certain metallic oxides, sulphides and native minerals were formed in the early stages of magmatic crystallization and became segregated by crystal separation to form early magmatic mineral deposits , while others crystallized later than the host rock and accumulated at the original site or injected elsewhere to give rise to late magmatic deposits like pegmatitic deposits.

9. Early magmatic deposits The early formed ore minerals in the deposits may occur as below: Disseminated deposits – this deposit occur in the shape of a dyke, pipe or small stock like mass. Here, grains of ore are found scattered more or less evenly throughout the rock mass. Eg: diamond in kimberlite of Wajrakarur pipe, Andra Pradesh and Majhagawan pipe, Madhya Pradesh. The diamonds might have crystallized early and were transported with the enclosing magma and perhaps even continued to grow before final consolidation took place in the present pipes. Segregation deposits – early magmatic segregation deposits are formed as a result of gravitative crystallization differentiation. Eg: stratiform and banded deposits of chromite in Orissa. The early magmatic segregation may be due to sinking of heavy early formed crystals to the lower part of the magma chamber or by marginal accumulation. These deposits are generally lenticular in shape and relatively small size. Injection deposits – it occurs as veins and lenses within ultrabasics and gabbro. Eg: Vanadiferous magnetite deposits,singhbhum district in Bihar. The early formed ore mineral, magnetite, did not remain at the site of original accumulation but have been injected into ultrabasic rock of Iron ore formation .

10. Magmatic Dissemination Magmatic Segregation

11. Late magmatic deposits The ore deposits which are formed towards the close of the magmatic periodis called late magmatic deposits. These contain those ore minerals which have crystallised at rather low temperature from residual magma. The magma which left after crystallization of the early formed rock silicates is called residual magma. Late magmatic deposits may be accounted for; Residual liquid segregation – in a magma which is undergoing differentiation, the residual liquid may become enriched in titanium and iron which on crystallization segregated to form titaniferous magnetite deposit. Such ore bodies commonly occur in the form of parallel bands and may form valuable ore deposits. Eg ; titaniferous magnetite of Hassan district, Karnataka.

12. Residual liquid injection which takes place due to earth’s disturbance like igneous intrusion. The residual liquid rich in iron accumulated as a result of magmatic differentiation, may get injected into the surrounding rocks to form magnetite deposits, eg; magnetite dyke rock of Kasipatanam, Visakhapatanam district, Andra Pradesh. Such deposits usually occurs in the form of dyke, sills or veins.

13. Immiscible liquid segregation in which certain salts in magma under certain conditions separate out an unmixed solutions like oil and water and segregate to form important mineral deposits. When a mafic magma cools, the sulfide rich immiscible liquid separates out and accumulates at the bottom of the igneous body. Upon consolidation, it give rise to sulfide segregation deposits. These deposits occur commonly at the bottom of the differentiated mafic intrusives as disconnected bodies. Eg; lead-zinc-copper-sulphide deposits, Bihar.

14. Immiscible liquid injection when the sulphide rich immiscible fraction which separates out during the differentiation of magma may squirted out before consolidation towards the places of less pressure such as shear zones. They are injected into the older rocks and enclose brecciated fragments of host and foreign rocks. On consolidation it give rise to immiscible liquid injection deposit. Such deposits are usually irregular or dyke like form. Eg; Nickeliferous chalcopyrite of Singhbhum copper belt.

15. 2. Sedimentation inclusive of Evaporation Sedimentation Sedimentation deposits are syngenetic deposits, occur as beds in sedimentary rocks. Sedimentary rocks with valuable mineral deposits like iron, manganese, copper, phosphate, coal, clay, sulphur etc are formed due to process of sedimentation. The minerals in a rock are the important source of sedimentary mineral deposit. For example, the iron bearing minerals, hornblende, pyroxene, mica etc, in a rock, on oxidation and weathering yield materials for the sedimentary iron ore deposits.

16. This process may be summarized as follows; During weathering, the materials are released from the source rock. The weathered materials go into solutions of different kinds. The carbonated water, humic and other organic acids and sulphate solutions are the chief solvents. Humic and other organic acids are derived from decomposition of vegetation, while oxidation of pyrite produces sulphuric acid. Most of the valuable substances are transported either in suspension or in solution by means of river water to sea. The materials in solution may be deposited mechanically, chemically or biologically whether in a sea or in a swampy basin or elsewhere. The chemical precipitation of material solution is controlled by pH and Eh of the environment.

17. Evaporation Evaporation is one of the important agencies which brings about deposition of many valuable minerals like common salt, gypsum, unhydrate, potash, nitrates and many other non-metallic minerals. Warm and arid climate is essential to cause evaporation whether in ocean water, lake water or groundwater. The main source of evaporation deposits are seawater. The concentration of the soluble salts in the bodies of water takes place by evaporation and when the supersaturation of any salt is reached, that salt is precipitated. The least soluble salt is precipitated first, and the most soluble last.

18. 3. Metamorphism Metamorphism is an important process to give rise to many new mineral deposits by altering the earlier deposits. It is by recrystallisation and reconstitution of preexisting rock forming minerals that have some valuable mineral deposits are formed. Heat, pressure and water play an important role in bringing about metamorphism. The original texture and structure are obliterated; with the some of the ore minerals may exihibit streaked, banded and smeared appearences with indistinct boundaries between minerals of different colours. Many of the non-metallic mineral deposits of importance are formed in this way. Eg: deposits of asbestos, graphite, talc, soapstone, andalusite, silimanite, kyanite etc. For eg, graphite is the product of regional or contact metamorphism and carbon of the graphite may be derived by the sediments from the carbonate rocks. The coal beds which have been altered to graphite are also the result of metamorphism, by which volatile matters of the coal have been removed and the residual carbon is changed to graphite.

19. 4. Contact metasomatism Contact metasomatism is a process of formation of new mineral by reaction between the contact rock and the escaping high temperature gaseous emanations with other important materials from the magma chamber. For the deposits of this type, the magma must contain the ingredients of mineral deposits and must be intruded at depth at the contact of reactive rocks. It differs from contact metamorphism in which only effect of heat is involved and role of accession from magma chamber is negligible. The process of contact metasomatism starts with recrystallization and recombination of rock minerals in the contact zones, eg. Limestone or dolomite converts to marble, shale to hornfels.

20. The ore-minerals in such cases result from accession from the magma. Large volume of material may be added and subtracted from the invaded rocks. The reaction apparently commences just after intrusion and continues until well after consolidation of the outer part. The temperature for contact metasomatism may possibly be ranging from 400° to 800°C, or even higher. It take place under high pressure and high temperature and are generally discontinuous and irregular in shape. They are in general of comparitively small size.

21. 5. Hydrothermal deposits Hydrothermal deposits are epigenetic ore deposits which are formed by high temperature solutions . The main ore deposits formed by this process are gold, silver, copper, lead, zinc, mercury etc. The term hydrothermal means hot water with possible temperature of 500° C to 50°C. Hydrothermal solutions - The fluid left during the later stages of crystallization of intrusive magma after the crystallization of main rock forming minerals, mainly called as residual fluid. Metal present in the magma concentrate in this fluid. This fluid which in a hot watery solution containing mineralised liquids derived from an intrusive magma is called hydrothermal solution. They move through cracks and openings present in the rocks and deposit their dissolved minerals there, in its journey through the rocks it loses heat and metal contents with increased distance.

22. On the basis of temperature, hydrothermal deposits are classified into 3; Hypothermal deposits – high temperature deposits formed close to the intrusive body. Temperature ranges between 300- 500°C. Chief ore minerals are chalcopyrite. Mesothermal deposits (200- 300°C) – intermediate temperature deposits, formed at some distance from intrusive body. Chief ore minerals are sphalerite (ZnS), galena (PbS), bornite. Epithermal deposits – low temperature deposits, formed away from intrusive body. Temperature ranges between (50- 200°C). Chief ore minerals are silver, stibnite, zinnabar.

23. On the basis of the mode of formation, hydrothermal deposits are classified into ; Mode of formation Cavity filling deposits Replacement deposits 1. Fissure vein deposits 2. Shearzone deposits 3. Stockworks 4. Saddle reefs 5. Ladder veins 6. Cave deposits 7. Breccia filling deposits 8. Solution filling deposits 9. Porespacefilling deposits 10. Vesicular filling deposits 2. Massive deposits 1. Replacement lode deposits 3. Disseminated replacement deposits

24. Cavity filling deposits – The precipitation of minerals from mineralising solution in the cavities or the open spaces in rock forms cavity filling deposits. Here, no replacement is involved. Deposition takes place due to changes in temperature and pressure of the solution. The walls of the cavity are first lined by the first mineral to be deposited. The minerals usually grow inward with the development of crystal faces pointed towards the supplying solution in the form of comb structures. Successive crusts of different minerals may be precipitated upon the first one until filling is complete. This give rise to crustification and if the cavity is a fissure, a crustified vein is formed. Symmetrical crust may result with similar precipitation on both the walls of the vein, and asymmetrical with unlike crustification on each side. When a vein is not filled completely with mineral matter, open spaces are left in the centre forming vug. In general, cavity filling deposits occur at lower temperature and pressure.

25. Types of cavity filling deposits – Depending upon the nature of openings, cavity filling deposits occur in different shape and size. They are classified into; 1. Fissure vein deposits - It is a tabular type of deposit, involving formation of fissure itself by stresses operating within the earth’s crust and ore forming process. A fissure filled with ore is called fissure vein. Fissure veins may be massive or crustified. Pinches and swells produced by movement along irregular fissures may ocur. Several minerals, both ore and gangue, may fill in the fissure. Fissure may occur in group and may have formed at the same time or may be of different ages. The depth of fissure vein is quite variable. Fissure veins with large concentration of ore is called oreshoot. Main fissure deposits are gold, silver, gypsum, platinum, lead, zinc, mercury etc.

26. Fissure veins are classified into – Simple fissure vein – single fissure whose walls are nearly straight and parallel. Composite vein – it is a wide zone of nearly parallel fissures connected by diagonals. Sheeted veins – group of closely spaced parallel veins, very small thickness. Dialated/lenticular vein – characterized by lensoid shaped body. The pattern of an inclined lensoidal body is known as en-echelon veins. Chambered veins- the veins whose walls are irregular and brecciated. Such veins branch and join again enclosing countryrocks inside it, seen in highly fractured areas.

27. 2. Shearzone deposits – when two tectonic blocks are moved to each other, usually leads to the formation of shearzones. They contain thin closely spaced fractures. The opening between the shear zone is very ideal for the accumulation of economic minerals since they contain number of openings. It is an ideal site for the hydrothermal deposits. A shear zone with sheet like connected openings, and large exposed surfaces serves as excellent channel ways for mineralising solutions and precipitation take place as thin plates of minerals or in the form of fine grains, eg; Singhbhum shear zone deposits. 3. Stockworks – it is a mass of rock transversed by a network of small ore bearing veins. Each vein is very small in thickness. These veinlets have a thickness of 1 cm or less and length wise it will be few metres. This space in between veinlets may vary. Stockworks are formed when hydrothermal solutions percolate through the vertical zones of highly shattered rocks or igneous intrusions. Here, full rockmass is mined for the ore.

28. 4. Saddle reefs – during folding, openings will form between rocks at the crust of the anticlines. Mineralization will occur in these openings that will lead to ore deposition known as saddlereefs. 5. Ladder veins – these are commonly formed in dykes. They are short, transverse, roughly parallel fractures that are held with ore because they appear like a ladder, they are called ladder veins. 6. Cave deposits – solution cavities in the form of caves, galleries and gash veins may contain deposits of lead, zinc, copper, mercury etc. They occur only in soluble rocks like limestone. Gashveins are veins which occur along joints and bedding planes in limestone. 7. Breccia filling deposits – the breccias offer opening spaces in between the angular fragments for deposition by the mineralising solution traversing through them. 8. Solution cavity filling – certain solution formng rocks like limestone give rise to this type of deposits. 9. Porespace fillings – many mineral deposits occur as pore space fillings, in sandstone. 10. Vesicular fillings – the vesicular lava flows being permeable, form channel ways for mineralising solution and sites of mineral deposits.

29. Stockwork

30. Metasomatic replacement deposits Metasomatic replacement is defined as a process of simultaneous solution and deposition by which earlier formed mineral is replaced by new one. It take place when the mineralising solution comes in contact with mineral which is unstable in its presence. The resulting mineral occupies the same volume as that of the replaced mineral and tends to retain the original shape, size and structure. The replacement is by diffusion. The rate of replacement is dependent upon the rate of supply of new mineral and removal of dissolved material. The replacement take place in stages. The first formed minerals are replaced by the later ones. The sequence formed in common primary metallic minerals is pyrite, shalerite, chalcopyrite etc. Pyrite and arsenopyrite are the first formed metallic minerals and are replaced by later formed minerals. In the case of rocks, ferro- magnesian minerals are replaced first, followed by feldspar and quartz.

31. The ore deposits formed by replacement can be subdivide into – Massive deposits – massive replacement ore deposits commonly occur in limestones. The ore terminates abruptly against the host rock. The massive deposits are characterized by great variations in size and extremely irregular form. Replacement lode deposits – the lode deposits are formed when the replacement is localized along thin beds or fissures. In this case, the fissure walls are replaced by the ore. The lode deposits resemble fissure veins in form, however they are wider than fissure veins and their walls are wavy and irregular. Disseminated replacement deposits – these are low grade ore deposits where grains of ore minerals are found scattered throughout the host rock. The size of disseminated ore deposits is generally large.

32. 6. Oxidation and supergene sulphide enrichment The processes of oxidation and supergene enrichment give rise to many large and rich ore-deposits. Both the processes, in general, occur together. If an oredeposit is exposeed to groundsurface, it undergoes weathering. The surface water oxidizes and produce solvent that dissolves the ore minerals from the weathered zone and carries them downwards. The leaching solution as proceeds downward looses a part of their metallic content within the zone of oxidation as oxidised ore. The ore minerals in solution which are not precipitated in the zone of oxidation, trickle down below the watertable where they are deposited as secondary sulphides. The existing metal in the zone is, thus, enriched by addition from above. This is known as the zone of supergene sulphide enrichment. Below this zone is the primary or hypogene zone which remains altogether unaffected.

33. Oxidation is operative in the upper part of the ore-deposits above the ground water table, called zone of oxidation. The process in ideal situation gives rise to a zone of gossan in the topmost part of the oxidation zone, followed by a supergene enrichment zone and then a primary zone.

34. Three zones are recognized in this process; they are- Zone of oxidation It is the oxidized part of the ore deposit. This zone extends from groundsurface upto watertable. The ore minerals from this part of the ore body are disssolved and removed. Gossan – gossan is the cap rock of an ore deposit in the form of cellular mass of limonite and gangue formed due to oxidation of an ore and points to the nature of underlying hidden deposits. Limonite which is ubiquitous in the Gossan of the oxidized zone occurs in a variety of colours and structures. It may have formed from iron-bearing sulphides or iron-bearing rock silicates. In general, seal brown, maroon andd orange colours of limonite in the cappings signify copper, while yellow and brick red indicate pyrite. Ocherous orange is suggestive of galena, tan to brown of sphalerite and tan to maroon of molybdenite. The residual limonite of gossans usually shows a honeycomb structure called boxwork. Gossan sometimes serve as an indication for the occurrence of useful ore deposits.

35. 2 . Supergene enrichment zones – this zone lies below water table. In this zone dissolved ore minerals are precipitated in the form of secondary sulphides. 3. Primary zone – the lower unaffected part of the orebody. The primary requisites for supergene enrichment are – Oxidation Presence of suitable primary minerals to yield neccessary solvents. Permeability of rock to enable the solution to penetrate the oxidation zone. Absence of precipitants in the oxidised zone. Zone of no available oxygen where secondary sulphides may be deposited. Presence of hypogene minerals below the water table to cause precipitation of secondary sulphides.

36. 7. Sublimation Sublimation is a process of mineral deposits associated with volcanism, thermal springs and fumaroles where from volatilised matter is redeposited at lower temperature and pressure in the same form. Sulphur and borax of Puga area, Ladakh are examples of such deposits. Sodium chloride (common salt ) is also formed to some extent by this process. Several other sublimates, such as chlorides of copper, iron, zinc etc., ammonium and various salts of alkali metals formed in this way are quickly washed away.

37. 8. Residual and mechanical concentration The process of residual and mechanical concentration, yielding mineral deposits, is based on weathering of rock containing economic minerals. The weathering causes mechanical disintegration and chemical decomposition of rock constituents. The unstable minerals undergo chemical decay and are removed in solution, while the insoluble residues remain as such and concentrate. The stable minerals like gold, cassiterite, quartz etc are freed from the enclosing matrix and mechanically concentrated by moving water or air into placer deposits.

38. Residual concentration It is the process of accumulation of valuable minerals after removal of undesired material by weathering . The undesired material are removed in solution leaving behind the concentration of valuable mineral at the site of the original rock. The pre-requisites for the process are: Presence of rocks containing valuable minerals Climate conditions which promote chemical decay Country with no too great relief to wash away valuable residues Long continued crustal stability to faciliate residual concentration and prevent destruction of deposits by erosion . There should be adequate rain to carry away in solution the soluble products of weathering.

39. Mechanical concentration Mechanical concentration is a process by which heavy minerals are separated from light ones by moving water or air and concentrated in the form of placer deposits . The process is; The oreminerals are released from the rock by weathering and disintegration. The disintegrated material are carried downslope by water, air etc. In moving water / air, the heavier placer mineral sink to the bottom while yhe lighter materials are carried further. Thus the heavier material are separated from the lighter ones. In this way, the heavier minerals get concentrated in particular localities to form placer deposits. A continuous supply of placer minerals is essential for mechanical concentration. The placer minerals have high specific gravity, and are durable and resistant to weathering.

40. Type of placer deposits Elluvial placers – deposits along hill slope. Here, minera concentration is caused by gravity. When debris produced due to weathering of rocks moves downslope, heavier particles move more slowly than lighter ones. In this way, heavier minerals get concentrated to form eluvial placer deposits. Beach placers - when the concentration is on beaches . The mineral concentration is caused by wave action. Beach sands of kerala contains mineral deposits of monazite and illmenite. E olian placers – occur in arid region where the mineral concentration is by wind action. Stream or alluvial placers – occur at various places along a stream. Here, the mineral concentration is caused by running water. Weathered materials travels with stream water and concentration of heavier materials occur in those places where velocity of water slackens. The place where this occurs are; In potholes and plugepools which forms at the base of waterfall and rapids. In sandbars that occurs in the inner curve of the meander. Just downstream o the junction of tributary to the main stream. On the river bed. In the riffles ( streams flowing across vertically or steeply inclined beds have uneven floors called riffles).

41. Placer deposits

42. The condition necessary for the formation of the placer deposits are; There must be a primary source such as an ore deposits, a dissemination deposit or a low grade deposit. It must be exposed to weathering on a slope from where the disintegrated material may be carried away by water, air etc. Ore mineral in the deposit must be of such chemical composition that resist weathering. For a mineral to be concentrated as a placer deposit , it must have a higher density than the worthless material with which it occurs.

43. 9. Pegmatic deposits Certain magmas such as those which form granites, contain certain percentages of water dissolved in them. When a granitic magma cools, the first mineral to crystallise tend to be unhydrous, so an increasingly water rich residue remains. Certain rare elements like litjium, beryllium become concentrated on the water rich residual magma. If crystallisation process occurs at a depth of 5 km or greater, water rich residual magma may migrate and form small bodies of igneous rock called pegmatities. The late residual magma which is left in the last stage of crystallization, commonly alkalies, water, carbondioxide and concentrations of rare elements and metals. Pegmatites are formed when this residual magma gets injected into the enclosing rocks. Many such pegmatites form in valuable mineral deposits. The economic mineral which commonly occur in pegmatites are mica, corandum, gemstones and feldspar. Pegmatities of economic importance are mostly found associated with felsic igneous rocks such as quartz dioritic rocks.