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.

9. 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;
1. Magmatic concentration
2. Sedimentation inclusive of Evaporation
3. Metamorphism
4. Contact metasomatism
5. Hydrothermal process
6. Oxidation and supergene sulphide enrichment
7. Sublimation
8. Residual and mechanical concentration
9. 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.

10. 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.

12. 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.

13. Magmatic Dissemination
Magmatic Segregation

14. 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.

15.  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.

16.  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.

17. 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.

18. 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.

19.  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.

20. 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.

21. 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.

22. 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.

23.  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.

24. 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.

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

27.  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

28.  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.

29. 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.

30.  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.

31. 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.

32. 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.

33. Stockwork

34.  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.

35.  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.

36. 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.

37.  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.

38.  Three zones are recognized in this process; they are-
1. 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.

39. 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.

40. 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.

41. 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.

42. 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.

43. 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.

44. 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.
• Eolian 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).

45. Placer deposits

46. • 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.

47. 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.